synthesis and investigations of novel ...synthesis and investigations of novel alkenylporphyrins and...

SYNTHESIS AND INVESTIGATIONS OF

NOVEL ALKENYLPORPHYRINS AND

BIS(PORPHYRINS)

A thesis presented to

THE QUEENSLAND UNIVERSITY OF TECHNOLOGY

In fulfilment of the requirements for the degree of

Doctor of Philosophy

Submitted by

Oliver Brett Locos

Bachelor of Applied Science (Chemistry/Forensics)

Synthesis and Molecular Recognition Program

School of Physical and Chemical Sciences

April 2006

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ABSTRACT

Twelve porphyrin dyads linked by an ethene bridge were synthesised as model

systems for conjugated polymers. The extent of interporphyrin interaction was

investigated for meso-meso and meso-β linked homo- and heterobimetallo-porphyrin

dyads. To complement these dyads, model monomers with alkenyl substituents were

also studied. Once the synthesis of these compounds was achieved, the extent of

interaction was studied using UV-visible and fluorescence spectroscopy and

molecular modelling.

In order to gain a true indication of the extent of interaction in a dyad, the effect of

the bridge as a substituent must be accounted for. This was achieved by studying the

series of monomers by UV-visible and fluorescence spectroscopy. The increased

conjugation resulting from mono- and bis-alkenyl substituents results in a red shift of

the origin of transition energies in the absorption spectrum which is accompanied by

a broadened and less intense Soret band and an increase in the intensity of the Q

bands. The emission of these compounds also displays an increase in Stokes shift and

a loss of vibronic coupling due to the increased conjugation.

The serendipitous synthesis of three asymmetric meso-β ethene-linked porphyrin

dyads was achieved by the use of palladium-catalysed Heck coupling of meso-

ethenyl- with meso-bromoporphyrins. A possible mechanism for this meso to β

rearrangement was proposed. A series of nine meso-meso ethene-linked dyads was

synthesised by palladium-catalysed Suzuki coupling of meso-(2-iodoethenyl)- with

meso-borolanylporphyrins. All of these dyads were characterised by 1D and 2D

NMR as well as MS analysis. The absorption spectra of ethene-linked dyads exhibit

a split Soret band and a red-shifted and intensified HOMO-LUMO band. In the

meso-β dyads, the degree of splitting in the Soret band is sufficient only to generate a

shoulder on the red edge, whereas in the meso-meso dyads two separate bands

appear. The extent of splitting is believed to be an indication of the amount of

porphyrin-porphyrin interaction.

The fluorescence profiles of the dyads change dramatically depending upon the

central substituents in the porphyrins and the wavelength used for irradiation, which

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suggests that different conformations of these compounds give rise to different parts

of their absorption and emission profiles. The fluorescence profiles of the dyads also

do not reflect their absorption profiles, and therefore the excitation of the dyad is

believed to be accompanied also by a change in geometry. All ethene-linked dyads

exhibited an anti-Stokes shift, and the excitation spectra of the different parts of the

fluorescence envelope also support the possibility of different conformers

contributing to the fluorescence spectra.

Molecular mechanics and time-dependent quantum mechanical calculations were

performed on seven ethene-linked porphyrin dyads. These calculations further

support the proposal of different conformations contributing to the physical

properties of ethene-linked dyads. Electronic structure calculations also show

considerable electron density on the alkene for the meso-meso ethene-linked dyads,

which highlights the important influence of this bridge upon the electronic nature of

these conjugated diporphyrins.

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LIST OF PUBLICATIONS

Papers:

“meso-Iodo- and meso-iodovinylporphyrins via organopalladium porphyrins and the

crystal structure of 5-iodo-10,20-diphenylporphyrin”

Atefi F., Locos, O. B., Senge M. O., Arnold D. P. Journal of Porphyrins and

Phthalocyanines (in press)

“The Heck reaction for porphyrin functionalisation: synthesis of meso-alkenyl

monoporphyrins and palladium-catalysed formation of unprecedented meso-β

ethene-linked diporphyrins”

Locos, O. B., Arnold D. P. Org. Biomol. Chem., 2006, 4, 902 – 916

Posters:

“Novel functionalised porphyrins via metal-catalysed coupling”

D. P. Arnold , F. Atefi, L. J. Esdaile, O. B. Locos

International Conference on Porphyrins and Phthalocyanines (New Orleans, July

2004)

“Palladium Catalysis: A new approach to alkenyl-linked oligoporphyins”

O. B. Locos, D. P. Arnold

RACI National Conference: Conference for Organic Chemistry (RACIOC)/

International Symposium for Macrocyclic Chemistry (ISMC) (Cairns, June 2004)

“For the Heck of it: A Palladium-Catalysed Approach to Alkenyl-linked Dimers”

O. B. Locos, D. P. Arnold

Australian Organometallics Forum 2 (OZOM2) (Adelaide, January 2003)

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TABLE OF CONTENTS

ABSTRACT ............................................................................................... i

LIST OF PUBLICATIONS....................................................................iii

TABLE OF CONTENTS........................................................................ iv

LIST OF TABLES ................................................................................xiii

LIST OF FIGURES ............................................................................... xv

LIST OF SCHEMES ............................................................................xix

ABBREVIATIONS ..............................................................................xxii

DECLARATION.................................................................................. xxv

ACKNOWLEDGEMENTS................................................................xxvi

CHAPTER 1 ................................................................................................1

INTRODUCTION.................................................................................... 1 1.1. Structure and Basic Properties of Porphyrins .......................................... 1

1.1.1. Basic Structure and Roles in Nature ....................................................... 1

1.1.2. Electronic Absorption Spectra of Porphyrins ......................................... 7

1.1.3. Excitonic Coupling Theory................................................................... 10

1.1.4. Emission Spectra of Porphyrins ............................................................ 13

1.2. Uses of Multiporphyrin Arrays................................................................. 16

1.2.1. Molecular Electronics ........................................................................... 16

1.2.2. Nonlinear Optical Materials.................................................................. 20

1.2.3. Models for Photoinitiated Charge Transfer........................................... 21

1.2.4. Photodynamic Therapy73....................................................................... 22

1.3. Classes of Covalently Linked Porphyrins ................................................ 23

1.3.1. Directly Linked ..................................................................................... 23

1.3.2. Saturated Linkages ................................................................................ 25

1.3.3. Alkene Linkages ................................................................................... 27

1.3.4. Alkyne Linkages ................................................................................... 33

1.3.5. Aromatic Linkages ................................................................................ 37

1.3.6. Fused/Fused Aromatic Linkages........................................................... 43

1.3.7. Hybrid Linkages.................................................................................... 45

1.4. Outline of Project ....................................................................................... 51

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CHAPTER 2 ..............................................................................................54

SYNTHESIS OF MONOMERIC PORPHYRINS WITH ETHENYL

SUBSTITUENTS.................................................................................... 54 2.1. Synthesis of Porphyrin Precursors ........................................................... 54

2.1.1. Synthesis of Diaryl- and Triphenylporphyrin ....................................... 54

2.1.2. Halogenation of Porphyrins .................................................................. 56

2.1.3. Formylation of Diaryl- and Triphenylporphyrin................................... 58

2.2. Literature Survey of Porphyrins with Ethenyl Substituents ................. 59

2.2.1. Wittig and Knoevenagel Condensations ............................................... 59

2.2.2. Nucleophilic Addition with Grignard Reagents.................................... 62

2.2.3. Stille Coupling ...................................................................................... 63

2.2.4. Heck Coupling ...................................................................................... 65

2.2.5. Dehydrohalogenation ............................................................................ 66

2.3. Synthesis of Alkenyl Porphyrins............................................................... 67

2.3.1. Synthesis of Unsubstituted Ethenylporphyrins ..................................... 67

2.3.2. Synthesis of Alkenylporphyrins by Pd-Catalysed Heck Coupling using

Bromoporphyrins ............................................................................................ 72


Dibromoporphyrins ......................................................................................... 77


Ethenylporphyrins ........................................................................................... 79

2.4. Summary..................................................................................................... 82

CHAPTER 3 ..............................................................................................83

SYNTHESIS OF PORPHYRIN DYADS LINKED BY A trans-

ALKENE................................................................................................. 83 3.1. Literature Survey of Porphyrins Linked by a trans-Alkene .................. 83

3.1.1. Dehydrogenation of Alkanes................................................................. 83

3.1.2. Wittig Reaction ..................................................................................... 85

3.1.3. Reductive Dimerisation Using Low-Valent Titanium Complexes ....... 86

3.1.4. Stille Coupling ...................................................................................... 88

3.2. Synthesis of Porphyrin Dyads using Heck Alkenylation........................ 90

3.2.1. Heck Coupling and Optimisation.......................................................... 90

3.2.2. Mechanistic Studies .............................................................................. 94

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3.3. Alkene Metathesis .................................................................................... 102

3.3.1. Introduction to Grubbs’ Catalyst......................................................... 102

3.3.2. Metathesis of Ethenylporphyrins using Grubbs’ Catalyst .................. 103

3.4. Synthesis of Porphyrin Dyads using Suzuki Coupling ......................... 104

3.4.1. Synthesis of Porphyrin Suzuki Electrophiles...................................... 105

3.4.2. Synthesis of Porphyrin Suzuki Boronates........................................... 109

3.4.2. Suzuki Coupling to Synthesise Porphyrin Dyads ............................... 112

3.5. Summary................................................................................................... 117

CHAPTER 4 ............................................................................................119

UV-VISIBLE, FLUORESCENCE AND NMR SPECTRA OF

PORPHYRINS WITH ALKENYL SUBSTITUENTS AND

ETHENE-LINKED PORPHYRIN DYADS...................................... 119 4.1. Introduction to UV-Visible Spectra of Alkenylporphyrins and Ethene-

linked Porphyrin Dyads.................................................................................. 119

4.1.1. Porphyrin Monomers with Alkenyl Substituents in the meso- and β-

positions ........................................................................................................ 119

4.1.2. Introduction to Porphyrin Dyads Linked by Alkenes ......................... 122

4.2. UV-Visible Spectra of Porphyrins with Alkenyl Substituents and

Ethene-linked Porphyrin Dyads .................................................................... 127

4.2.1. Porphyrins with Alkenyl Substituents................................................. 127

4.2.2. Ethene-linked Porphyrin Dyads .......................................................... 135

4.3. Fluorescence Spectra of Porphyrins with Alkenyl Substituents and

Ethene-linked Porphyrin Dyads .................................................................... 142

4.3.1. Fluorescence Properties of 1,2-trans-Arylethenes.............................. 142

4.3.2. Porphyrin Monomers with Alkenyl Substituents................................ 143

4.3.2. Fluorescence Spectra of Ethene-linked Porphyrins Dyads ................. 149

4.4. NMR Spectra of Porphyrins with Alkenyl Substituents and Ethene-

Linked Porphyrin Dyads ................................................................................ 161

4.4.1. Chemical Shifts of Alkenyl Protons in trans-Alkenyl porphyrins...... 161

4.4.2. Assignment of the chemical shifts in the NMR spectrum of 1,1-

phenylethenylporphyrinatonickel(II) 192 ..................................................... 162

4.4.3. Assignment of the chemical shifts in the 1H NMR spectra of meso-β

ethene-linked dyads....................................................................................... 164

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4.4.4. Assignment of the NMR spectra of meso-meso ethene-linked

heteroporphyrin dyads .................................................................................. 168

4.4.5. Assignment of the chemical shifts in the NMR spectra of meso-meso

ethene-linked triphenylporphyrin dyads ....................................................... 171

4.5. Summary................................................................................................... 174

CHAPTER 5 ............................................................................................177

THEORETICAL STUDY OF ETHENE-LINKED PORPHYRIN

DYADS .................................................................................................. 177 5.1. Density Functional Theory...................................................................... 177

5.1.1. Basis Sets352 ........................................................................................ 180

5.2. Geometry Calculations on Ethene-linked Porphyrin Dyads................ 181

5.2.1. Molecular Mechanics Calculations on the Ethene-linked Porphyrin

Dyad 245 from Different Starting Conformations ........................................ 181

5.2.2. Molecular Mechanics Calculations on meso-meso and meso-β Ethene

Linked Porphyrin Dyads ............................................................................... 183

5.2.3. Electronic Structures of meso-meso and meso-β Ethene-linked Dyads187

5.2.4. Time-Dependent Calculations for Ethene-linked Porphyrin Dyads ... 195

5.3. Summary................................................................................................... 202

CHAPTER 6 ............................................................................................204

EXPERIMENTAL ............................................................................... 204 6.1. General ...................................................................................................... 204

6.2. Synthesis of Diaryl- and Triphenylporphyrin Precursors ................... 205

6.2.1. 2,2’-Dipyrromethane (86) ................................................................... 205

6.2.2. 3,5-Di-tert-butylbenzaldehyde (88) .................................................... 206

6.2.3. 5,15-Diphenylporphyrin (89) .............................................................. 206

6.2.3. 5,15-Bis(3,5-di-tert-butylphenyl)porphyrin (90) ................................ 207

6.2.5. Phenyllithium ...................................................................................... 208

6.2.6. 5,10,15-Triphenylporphyrin (91) ........................................................ 208

6.2.7. 5,10,15-Triphenylporphyrinatonickel(II) (95) .................................... 209

6.2.8. 5,15-Bis(3,5-di-tert-butylphenyl)porphyrinatonickel(II) (97) ............ 209

6.2.9. 5,10,15-Triphenylporphyrinatozinc(II) (99) ....................................... 210

6.2.10. 5-Formyl-10,15,20-triphenylporphyrinatonickel(II) (103) ............... 210

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6.2.11. 5-Formyl-10,20-bis(3,5-di-tert-butylphenyl)porphyrinatonickel(II)

(104) .............................................................................................................. 211

6.3. Synthesis of Haloporphyrin Precursors ................................................. 212

6.3.1. 5-Bromo-10,15,20-triphenylporphyrin (92)........................................ 212

6.3.2. 5-Bromo-10,15,20-triphenylporphyrinatonickel(II) (96).................... 212

6.3.3. 5-Bromo-10,15,20-triphenylporphyrinatozinc(II) (100)..................... 213

6.3.4. 5-Iodo-10,15,20-triphenylporphyrin (94)............................................ 213

6.3.5. 5,15-Dibromo-10,20-bis(3,5-di-tert-butylphenyl)porphyrin (93)....... 214

6.3.6. 5,15-Dibromo-10,20-bis(3,5-di-tert-butylphenyl)porphyrinatonickel(II)

(98) ................................................................................................................ 214

6.3.7. 5,15-Dibromo-10,20-bis(3,5-di-tert-butylphenyl)porphyrinatozinc(II)

(102) .............................................................................................................. 215

6.4. Synthesis of Alkenyl and Haloalkenylporphyrins................................. 215

6.4.1. 5-Ethenyl-10,15,20-triphenylporphyrin (155) .................................... 215

6.4.2. 5-Ethenyl-10,15,20-triphenylporphyrinatonickel(II) (153) ................ 216

6.4.3. 5-Ethenyl-10,15,20-triphenylporphyrinatozinc(II) (152).................... 217

6.4.4. 5-Ethenyl-10,20-bis(3,5-di-tert-butylphenyl)porphyrinatonickel(II)

(154) .............................................................................................................. 217

6.4.5. (E)-5-(2-Bromoethenyl)-10,15,20-triphenylporphyrin (228).............. 218

6.4.6. (E)-5-(2-Iodoethenyl)-10,15,20-triphenylporphyrin (229) ................. 219

6.4.7. (E)-5-(2-Iodoethenyl)-10,15,20-triphenylporphyrinatonickel(II) (225)

and (Z)-5-(2-Iodoethenyl)-10,15,20-triphenylporphyrinatonickel(II) (226). 220

6.4.8. (E)-5-(2-Iodoethenyl)-10,15,20-triphenylporphyrinatozinc(II) (230) 221

6.4.9. (E)-5-(2-Iodoethenyl)-10,20-bis(3,5-di-tert-

butylphenyl)porphyrinatonickel (II) (224).................................................... 221

6.5. Synthesis of Heck-Coupled Porphyrins ................................................. 222

6.5.1. General Procedure for Heck Coupling using Mono-haloporphyrins .. 222

6.5.2. (E)-5-(2-Methoxycarbonylethenyl)-10,15,20-triphenylporphyrin (156)223

6.5.3. (E)-5-(2-Methoxycarbonylethenyl)-10,15,20-

triphenylporphyrinatonickel (II) (157).......................................................... 223

6.5.4. (E)-5-(2-Methoxycarbonylethenyl)-10,15,20-triphenylporphyrinatozinc

(II) (158)........................................................................................................ 224

6.5.5. (E)-5-(2-Phenylethenyl)-10,15,20-triphenylporphyrin (159).............. 224

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6.5.6. (E)-5-(2-Phenylethenyl)-10,15,20-triphenylporphyrinatonickel(II)

(160) .............................................................................................................. 225

6.5.7. (E)-5-(2-Phenylethenyl)-10,15,20-triphenylporphyrinatozinc(II) (161)225

6.5.8. (E)-5-(2-Cyanoethenyl)-10,15,20-triphenylporphyrin (162) and (Z)-5-

(3-cyanoethenyl)-10,15,20-triphenylporphyrin (163)................................... 226

6.5.9. (E)-5-(3-Cyanoethenyl)-10,15,20-triphenylporphyrinatonickel (II)

(164) and (Z)-5-(3-cyanoethenyl)-10,15,20-triphenylporphyrinatonickel (II)

(165) .............................................................................................................. 226

6.5.10. (E)-5-(3-Cyanoethenyl)-10,15,20-triphenylporphyrinatozinc (II) (166)

and (Z)-5-(3-cyanoethenyl)-10,15,20-triphenylporphyrinatozinc (II) (167). 227

6.5.11. General Procedure for Heck Coupling using Dihaloporphyrins ....... 228

6.5.12. (E)-5-(2-Methoxycarbonylethenyl)-10,20-bis(3,5-di-tert-butylphenyl)

porphyrin (171) and (E,E)-5,15-Bis(2-methoxyycarbonylethenyl)-10,20-

bis(3,5-di-tert-butylphenyl)porphyrin (172) ................................................. 228

6.5.13. (E)-5-(2-Methoxycarbonylethenyl)-10,20-bis(3,5-di-tert-butylphenyl)

porphyrinatonickel(II) (173) and (E,E)-5,15-Bis(2-methoxycarbonylethenyl)-

10,20-bis(3,5-di-tert-butylphenyl)porphyrinatonickel(II) (174)................... 229

6.5.14. (E,E)-5,15-Bis-(2-Methoxycarbonylethenyl)-10,20-bis(3,5-di-tert-

butyl phenyl)porphyrinatozinc(II) (175) ....................................................... 230

6.5.15. (E)-5-(2-Phenylethenyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrin

(176) and (E,E)-5,15-bis(2-phenylethenyl)-10,20-bis(3,5-di-tert-

butylphenyl)porphyrin (177)......................................................................... 231

6.5.16. (E)-5-(2-phenylethenyl)-10,20-bis(3,5-di-tert-

butylphenyl)porphyrinato nickel(II) (178) and (E,E)-5,15-bis(2-

phenylethenyl)-10,20-bis(3,5-di-tert-butyl phenyl)porphyrinatonickel(II)

(179) .............................................................................................................. 231

6.5.16. (E,E)-5,15-Bis(2-phenylethenyl)-10,20-(3,5-di-tert-butylphenyl)

porphyrinatozinc(II) (180) ............................................................................ 232

6.5.17. (E)-5-(2-cyanoethenyl)- 10,20-bis(3,5-di-tert-

butylphenyl)porphyrinatonickel(II) (181), (Z)-5-(2-cyanoethenyl)- 10,20-

bis(3,5-di-tert-butylphenyl) porphyrinatonickel(II) (182), (E,E)-5,15-Bis-(2-

cyanoethenyl)-10,20-bis(3,5-di-tert-butylphenyl) porphyrinatonickel(II)

(183), (E,Z)-5,15-Bis-(2-cyanoethenyl)-10,20-bis(3,5-di-tert-

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butylphenyl)porphyrinatonickel(II) (184) and (Z,Z)-5,15-Bis-(2-

cyanoethenyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrinatonickel(II) (185)233

6.5.18. (E)-5-(2-Cyanoethenyl)-10,20-bis(3,5-di-tert-

butylphenyl)porphyrinatozinc (II) (186), (E,E)-5,15-bis-(2-cyanoethenyl)-

10,20-bis(3,5-di-tert-butylphenyl) porphyrinatozinc(II) (187) and (E,Z)-5-15-

bis-(2-cyanoethenyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrinatozinc(II)

(188) .............................................................................................................. 235

6.5.19. General Procedure for Heck Coupling using Ethenylporphyrins ..... 236

6.5.20. 5-(1-Phenylethenyl)-10,15,20-triphenylporphyrinatonickel(II) (192)236

6.5.21. 5-(1-Phenylethenyl)-10,20-bis(3,5-di-tert-

butylphenyl)porphyrinatonickel (II) (193) and (E)-2-(1-Phenylethenyl-10-20-

bis(3,5-di-tert-butylphenyl porphyrinatonickel(II) (194).............................. 237

6.5.22. (E)-5-(2-Anthracen-9-ylethenyl)-10,20-bis(3,5-di-tert-butylphenyl)

porphyrinatonickel(II) (195) and (E)-2-(2-Anthracen-9-ylethenyl)-10,20-

bis(3,5-di-tert-butylphenyl) porphyrinatonickel(II) (196) ............................ 238

The β-alkenylanthracence 196 could not be fully characterised by NMR due

to the number of overlapping peaks in the spectrum. ................................... 238

6.5.23. General Procedure for Heck Coupling of Porphyrin Dimers............ 238

6.5.24. (E)-1-[Bis(3,5-di-tert-butylphenyl)porphyrin-2-ylatonickel(II)]-2-

(10,15,20-triphenylporphyrin-5-yl)ethene (214)........................................... 239


[10,15,20-triphenyl)porphyrin-5-ylatonickel(II)]ethene (215) ..................... 240


[10,15,20-triphenylporphyrin-5-ylatozinc(II)]ethene (216).......................... 240

6.6. Borylation of Bromoporphyrins ............................................................. 241

6.6.1. General Procedure for Borylating Haloporphyrins ............................. 241

6.6.2. 5-(4, 4, 5, 5-Tetramethyl-[1,3,2]dioxaborolan-2-yl)-10,15,20-triphenyl

porphyrin (234) ............................................................................................. 242


porphyrinatonickel(II) (235) ......................................................................... 242


porphyrinatozinc(II) (236) ............................................................................ 243

6.7. Synthesis of Suzuki Coupled Porphyrins............................................... 243

6.7.1. General Procedure for Suzuki Coupling of Porphyrin Dimers ........... 243

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6.7.2. [(10,15,20-Triphenyl)porphyrin-5-yl]-[(10,15,20-triphenyl)porphyrin-

5-yl] (250) and (E)-1-(10,15,20-triphenylporphyrin-5-yl)-2-[10,15,20-

triphenylporphyrin-5-yl-atonickel(II)]ethene (244) ...................................... 244

6.7.3. (E)-1,2-bis[10,15,20-Triphenylporphyrin-5-ylatonickel(II)]ethene (245)245

6.7.4. [(10,15,20-Triphenyl)porphyrin-5-ylatozinc(II)]-[(10,15,20-triphenyl)

porphyrin-5-ylatozinc(II)] (252) and (E)-1-[10,15,20-triphenylporphyrin-5-yl-

atonickel(II)]-2-[10,15,20-triphenyl-5-ylporphyrinatozinc(II)]ethene (246) 245

6.7.5. (E)-1,2-bis(10,15,20-triphenylporphyrin-5-yl)ethene (247) ............... 246

6.7.6. (E)-1-[10,15,20-Triphenylporphyrin-5-ylatozinc(II)]-2-(10,15,20-

triphenyl porphyrin-5-yl)ethene (248) .......................................................... 247

6.7.7. (E)-1,2-bis[10,15,20-triphenylporphyrin-5-ylatozinc(II)]ethene (249)247


(10,15,20-triphenylporphyrin-5-yl)ethene (241)........................................... 248


[10,15,20-triphenylporphyrin-5-ylatonickel(II)]ethene (242)....................... 249

6.7.9. (E)-1-[Bis(3,5-di-tert-butylphenyl)porphyrin-5ylatonickel(II)]-2-

[10,15,20-triphenylporphyrin-5-ylatozinc(II)]ethene (243).......................... 249

6.8. Synthesis of Alkynyl Porphyrins ............................................................ 250

6.8.1. 5,10,15-triphenyl-20-trimethylsilylethynylporphyrinatozinc(II) (237)250

6.8.2. 5,10,15-triphenyl-20-ethynylporphyrinatozinc(II) (238) .................... 251

6.9. Synthesis of Anthracene Derivatives ...................................................... 251

6.9.1. 9-Anthraldehyde (169) ........................................................................ 251

6.9.2. 9-Ethenylanthracene (170) .................................................................. 252

6.9.3. 9-Bromoanthracene (191) and 9,10-dibromoanthracene (270)........... 253

6.10. Synthesis of Palladium Catalysts .......................................................... 254

6.10.1. Tetrakis(triphenylphosphine)palladium(0) (253).............................. 254

6.10.2. Bis[1,2-(diphenylphosphino)ethane]palladium(0) (254) and Bis[1,3-

(diphenylphosphino)propane)palladium(0) (255)......................................... 254

6.10.3. Dichlorobis(triphenylphosphine)palladium(II) (269) ....................... 254

CHAPTER 7 ............................................................................................256

CONCLUSIONS AND FUTURE WORK......................................... 256 7.1. Conclusions ............................................................................................... 256

7.2. Future Work............................................................................................. 259

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REFERENCES..................................................................................... 263

APPENDIX I......................................................................................... 272

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LIST OF TABLES

Table 1.1. Emission properties of metalloporphyrins at room temperature43 ............. 15

Table 2.1. Yields of alkenyl porphyrins from mono-bromoporphyrins...................... 73

Table 2.2. Yields of mono- and bis-alkenyl porphyrins from dibromoporphyrins..... 78

Table 2.3. Yields of alkenylporphyrins from ethenylporphyrins................................ 80

Table 3.1 Effect of catalyst loading upon yield of 215 ............................................... 92

Table 3.2 Effect of phosphine ligand upon yield of 215............................................. 92

Table 3.4 Effect of base upon yield of 215 ................................................................. 92

Table 3.3 Effect of temperature upon yield of 215 ..................................................... 92

Table 3.5. Yields of β-meso Heck-coupled dyads....................................................... 94

Table 3.6. Yields of products from Suzuki Coupling ............................................... 114

Table 4.1. Shift of origin upon the introduction of alkynyl substituents to the meso-

position of porphyrins ............................................................................................... 120

Table 4.2. Shift of origin upon the introduction of alkenyl substituents to the β-


Table 4.3. Shift of origin upon the introduction of alkenyl substituents to the meso-


Table 4.4. UV-visible spectral characteristics of OEPs linked by an ethane or ethene123

Table 4.5. UV-visible spectral characteristics of ethene-linked and ethyne-linked

meso-arylporphyrins.................................................................................................. 124

Table 4.6. UV-visible spectral features of β-β ethene-linked CuTPP dimer and its

parent monomer ........................................................................................................ 126

Table 4.7. Comparison of selected features of the UV-visible spectra of porphyrin

monomers with alkenyl substituents. ........................................................................ 128

Table 4.8. Features of the UV-visible spectra of ethene-linked porphyrin dyads..... 136

Table 4.9. Stokes shift and quantum yields of ethenyl substituted porphyrins......... 145

Table 4.10. Stokes shift and quantum yields of ethene-linked porphyrin dyads ...... 150

Table 4.11. Chemical shifts of the alkenyl protons in trans-alkenyl porphyrins ...... 161

Table 5.1. Angles between the mean planes of the porphyrins and between each

porphyrin and the plane of the alkene from various starting geometries of the dyad

245............................................................................................................................. 182

Table 5.2. Calculated eigenvalues and orbital label for the triphenylporphyrin dyads190

Table 5.3. Calculated eigenvalues and orbital label for the triphenylporphyrin dyads191

-xiv-

Table 5.4. Angles between the planes of the porphyrins and the plane of the alkene

and energy splitting between the excited state interacting orbitals........................... 193

Table 5.5. TDDFT calculated excitation energies, one-electron transitions and

oscillator strengths for optical transitions in the gas phase (λ ≥ 375 nm, f ≥ 0.01)... 197

-xv-

LIST OF FIGURES

Figure 1.1. The structure of porphine and the IUPAC numbering system. .................. 1

Figure 1.2. Molecular representation of the special pair of bacteriochlorophyll

molecules10.................................................................................................................... 3

Figure 1.3. Schematic representation of the photosynthetic reaction centre isolated

from Rhodopseudomonas viridis, showing the chain of electron transfer from the

primary electron donor to the final electron acceptor9.................................................. 4

Figure 1.4. The relative positions of bacteriochlorophyll b in a) LH-I protein and b)

LH-II in bacteria.16 ........................................................................................................ 6

Figure 1.5. The inner 16-membered ring responsible for maintaining the conjugation

pathways for the 18π-electrons that generate the porphyrin optical spectrum ............. 7

Figure 1.6. Absorption spectra of a) NiDAP and b) H2DAP ........................................ 8

Figure 1.7. The orbitals involved in the Gouterman four-orbital theory25 and the

promotion of an electron from the HOMO to the LUMO for porphyrins with D4h

symmetry....................................................................................................................... 9

Figure 1.8. Exciton band structure in dimeric molecules with various geometrical

arrangements28............................................................................................................. 11

Figure 1.9. Definitions for the geometrical parameters in equation (1)...................... 13

Figure 1.10. Decay scheme for singlet and triplet relaxation. The radiation processes

are shown as straight lines; radiationless processes are shown by wavy lines ........... 14

Figure 1.11. General fluorescence profile of fluorescent porphyrins ......................... 15

Figure 1.12. General structure of coordinated porphyrin arrays................................. 19

Figure 4.1. Components related to the splitting in the Soret transition for ethene-

linked porphyrin dyads135.......................................................................................... 125

Figure 4.2. Magnitudes of CI and wavenumbers of origins for free-base alkenyl

porphyrins ................................................................................................................. 129

Figure 4.3. Magnitudes of CI and wavenumbers of origins for NiII alkenyl

porphyrins ................................................................................................................. 130

Figure 4.4. Magnitudes of CI and wavenumbers of origins for ZnII alkenyl

porphyrins ................................................................................................................. 132

Figure 4.5. UV-visible spectra of NiII porphyrins with increasing number of

conjugated substituents ............................................................................................. 133

-xvi-

Figure 4.6. UV-visible spectra of free-base alkenyl porphyrins with electron-

withdrawing substituents........................................................................................... 134

Figure 4.7. UV-visible spectra of meso-β and meso-meso ethene-linked dyads in

comparison to phenylethenyl-NiTriPP...................................................................... 135

Figure 4.8. Comparisons between the red-shift of the HOMO-LUMO transition and

Soret band splitting of homo-metal dyads ................................................................ 138

Figure 4.9. Comparisons between the red-shift of the HOMO-LUMO transition and

Soret band splitting of heterobimetallic dyads.......................................................... 140

Figure 4.10. UV-visible spectra of the meso-meso ethene-linked dyads, 244, 247 and

248............................................................................................................................. 141

Figure 4.11. Fluorescence profiles of H2TPP at different excitation wavelengths .. 143

Figure 4.12. Fluorescence profiles of ZnTPP at different excitation wavelengths.. 144

Figure 4.13. Absorption and fluorescence spectra of the alkenyl monomers 155 and

156............................................................................................................................. 146

Figure 4.14. Absorption and fluorescence spectra of the alkenyl monomers 159 and

177............................................................................................................................. 147

Figure 4.15. Absorption and fluorescence spectra of the ethene-linked dyad 247 ... 151




Figure 4.19. Fluorescence spectra of the ethene-linked dyad 247 at different

wavelengths of excitation.......................................................................................... 155

Figure 4.20. Excitation spectra of the ethene-linked dyad 247 at different

wavelengths of emission ........................................................................................... 155

Figure 4.21. Fluorescence spectra of the ethene-linked dyad 248 at different

wavelengths of excitation.......................................................................................... 156



Figure 4.23. Emission spectra of the ethene-linked dyad 249 at different wavelengths

of excitation............................................................................................................... 157



Figure 4.25. Emission spectra of the ethene-linked dyad 244 at different wavelengths

of excitation............................................................................................................... 158

-xvii-



Figure 4.27. General conformations of an ethene-linked dyad which may contribute

to the fluorescence spectrum..................................................................................... 160

Figure 4.28. NOESY spectrum of the ethenyl porphyrin 192................................... 163

Figure 4.29 a) Chemical shifts of 192; b) Illustration of the expected geometry of 192164

Figure 4.30. DQF COSY spectrum of the meso-β ethene-linked dyad 215.............. 165

Figure 4.31. NOESY spectrum of the meso-β ethene-linked dyad 215 .................... 165

Figure 4.32. Assigned chemical shifts for the meso-β ethene-linked dyads ............. 167

Figure 4.33. DQF COSY of the ethene-linked dyad 241.......................................... 169

Figure 4.34. NOESY spectrum of the ethene-linked dyad 241................................. 169

Figure 4.35. Assigned chemical shifts for the meso-meso ethene-linked

heteroporphyrin dyads 241, 242 and 243.................................................................. 170

Figure 4.36. 1H NMR spectrum of the dyad 245 ...................................................... 171

Figure 4.37. Assignment of the chemical shifts of the homo-metal triphenylporphyrin

dyads 245, 247, 249 .................................................................................................. 171

Figure 4.38. DQF COSY spectrum of the meso-meso ethene-linked dyad 244 ....... 172

Figure 4.39. NOESY spectrum of the meso-meso ethene-linked dyad 244.............. 172

Figure 4.40. Assigned chemical shifts for the meso-meso ethene-linked mixed metal

dyads. ........................................................................................................................ 173

Figure 5.1. Equilibrium geometry calculated for dyad 245 starting from a) PM3

Optimised; b) 0°,0°; c) 0°,90° and d) 90°,90°........................................................... 182

Figure 5.2. Equilibrium geometries obtained for ethene-linked triphenylporphyrin

dyads and the angles between the mean planes of the porphyrin and between the

mean planes of each porphyrin and the plane of the alkene. .................................... 184

Figure 5.3. Equilibrium geometries obtained for ethene-linked diaryl-

triphenylporphyrin dyads and the angles between the mean planes of the porphyrin

and between the mean planes of each porphyrin and the plane of the alkene. ......... 185

Figure 5.4. Aligned and offset conformations of a meso-β ethene-linked porphyrin

dyad........................................................................................................................... 186

Figure 5.5. The eight frontier orbitals calculated for the di-zinc dyad 249 .............. 188

Figure 5.6. The eight frontier orbitals calculated for the di-nickel dyad 245Error! Bookmark not de

Figure 5.6. Graphic representation of the splitting in the lowest excited singlet state193

-xviii-

Figure 5.7. Absorption and excited state transition data of the ethene-linked

triphenylporphyrin dyads .......................................................................................... 199

Figure 5.8. Absorption and excited state transition data for the ethene-linked diaryl-

triphenylporphyrin dyads .......................................................................................... 199

Figure 5.9. Comparison of the conformations between the calculated equilibrium

geometry of 249 and the crystal structure of 213...................................................... 202

Figure 7.1. TDDFT calculations for zinc dyad 249 and its fused analogue as per

Scheme 7.1. ............................................................................................................... 261

-xix-

LIST OF SCHEMES

Scheme 1.1. Types of directly-linked porphyrin oligomers........................................ 23

Scheme 1.2. Mechanism for atropisomerism of cis-ethene-linked OEP dimer .......... 29

Scheme 1.3. Atropisomerism for ethene-linked OEP trimers..................................... 30

Scheme 1.4. Thiophene-linked porphyrin dimers ....................................................... 43

Scheme 1.5. Porphyrin dimers linked by phenylene/alkyne and phenylene/alkene

bridges ......................................................................................................................... 47

Scheme 1.6. Porphyrin dimers linked by alkene/alkyne bridges ................................ 50

Scheme 1.7. Series of monomers containing a double bond in the meso-position ..... 52

Scheme 1.8. Series of dimers linked by an ethenyl bridge in the meso-position of

both porphyrins ........................................................................................................... 53

Scheme 2.1. Synthesis of 5,15-diarylporphyrins ........................................................ 55

Scheme 2.2. Synthesis of 5,10,15-triphenylporphyrin................................................ 56

Scheme 2.3. Synthesis of bromo- and iodoporphyrins ............................................... 57

Scheme 2.4. Metallation of free-base porphyrins ....................................................... 57

Scheme 2.5. Formylation of NiII porphyrins............................................................... 59

Scheme 2.6. First synthesis of alkenyl NiII porphyrins via the Wittig condensation .. 60

Scheme 2.7. Synthesis of β-ethenyl-meso-arylporphyrins via the Wittig condensation61

Scheme 2.8. Synthesis of meso-ethenyl-meso-arylporphyrins via the Wittig

condensation................................................................................................................ 61

Scheme 2.9. Synthesis of alkenyl porphyrins via Knoevenagel condensations.......... 62

Scheme 2.10. Synthesis of ethenylporphyrins by the use of Grignard reagents ......... 63

Scheme 2.11. Synthesis of ethenyl ZnII porphyrins by Stille coupling....................... 64

Scheme 2.12. Synthesis of ethenyl free base porphyrins by Stille coupling............... 64

Scheme 2.13. Synthesis of ethenylporphyrins by the Heck reaction using

mercurioporphyrins ..................................................................................................... 65


bromoporphyrins ......................................................................................................... 65


protoporphyrin IX ....................................................................................................... 66

Scheme 2.16. Synthesis of ethenylporphyrin by dehydrohalogenation ...................... 66

Scheme 2.17. Synthesis of ethenylporphyrins 153 and 154 ....................................... 67

-xx-

Scheme 2.18. Attempted synthesis of ethenylporphyrins using vinylmagnesium

bromide ....................................................................................................................... 69

Scheme 2.19. Synthesis of ethenylporphyrin 155 and subsequent metallation .......... 70

Scheme 2.20. Mechanism and palladium cycle for the palladium-catalysed Stille

coupling....................................................................................................................... 71

Scheme 2.21. Synthesis of ethenylporphyrins by Heck coupling with


Scheme 2.22. Mechanism and palladium cycle for the palladium-catalysed Heck

reaction........................................................................................................................ 75

Scheme 2.23. Synthesis of 9-vinylanthracene............................................................. 76

Scheme 2.24. Synthesis of bis-ethenylporphyrins via the Heck reaction using

dibromoporphyrins...................................................................................................... 77

Scheme 2.25. Synthesis of alkenylporphyrins via the Heck reaction using 5-

ethenylporphyrins 153 and 154................................................................................... 79

Scheme 2.26. Expected oxidised products of 153 from the palladium catalysed

Wacker process ........................................................................................................... 81

Scheme 3.1. Synthesis of ethene-linked H2OEPs by dehydrogenation of ethane-

linked H2OEPs ............................................................................................................ 83

Scheme 3.2. Synthesis of ethane-linked H2OEPs ....................................................... 84

Scheme 3.3. Synthesis of alkenyl-linked porphyrins via Wittig condensation........... 85

Scheme 3.4. Synthesis of ethene-linked porphyrins via reductive dimerisation using

low-valent titanium ..................................................................................................... 86

Scheme 3.5. Synthesis of porphyrin dyads by reductive dimerisation using McMurry

conditions .................................................................................................................... 87

Scheme 3.6. Synthesis of cis-ethene-linked porphyrin oligomers by Stille coupling. 88

Scheme 3.7. Synthesis of trans-ethene-linked porphyrins by copper assisted Stille

coupling....................................................................................................................... 89

Scheme 3.8. Products from the Heck coupling of ethenylporphyrins and


Scheme 3.9. Possible pathways to generate a β-ethene link from a meso-

ethenylporphyrin ......................................................................................................... 95

Scheme 3.10. Example of 1,4-palladium migration involving iodo aromatic

compounds .................................................................................................................. 96

Scheme 3.11. Control reactions for Heck coupling .................................................... 96

-xxi-

Scheme 3.11. Using d2-ethenylporphyrin to investigate the formation of meso-β Heck

coupled ethene-linked porphyrins ............................................................................... 97

Scheme 3.12. Speculated mechanism for the synthesis of meso-β Heck-coupled

ethene-linked porphyrins............................................................................................. 99

Scheme 3.13. Intramolecular cyclisation involving β-H activation.......................... 100

Scheme 3.14. Attempted synthesis of ethene-linked porphyrin dyads via alkene

metathesis using Grubbs’ catalyst............................................................................. 104

Scheme 3.15. Retrosynthesis of ethene-linked (bis)porphyrins to Suzuki synthons 105

Scheme 3.16. Synthesis of iodoethenylporphyrins by Takai iodoalkenation ........... 106

Scheme 3.17. Synthesis of iodoethenylporphyrins by palladium facilitated iodination

of bromoethenylporphyrins....................................................................................... 107

Scheme 3.18. Borylation of bromoporphyrins.......................................................... 110

Scheme 3.19. Attempted synthesis of borolanylethenylporphryins.......................... 111

Scheme 3.20. Porphyrin dyads synthesised by Suzuki coupling .............................. 113

Scheme 7.1. Synthesis of planar ethene-linked porphyrin dyad ............................... 260

Scheme 7.2. Azo-linked porphyrin dyad .................................................................. 261

-xxii-

ABBREVIATIONS

ADMET acyclic diene metathesis

ATP adenosine triphosphate

B3LYP Becke's three parameter hybrid using the Lee, Yang and Parr correlational

functional

BAHA Tris(4-bromophenyl)aminium hexachloroantimonate

BCl bacteriochlorophyll

BPh bacteriophytin

calc. calculated

CI configuration interaction

CM cross metathesis

CuTPP 5,10,15,20-tetraphenylporphyrinatocopper(II)

d doublet

dba dibenzylideneacetone

dd double doublet

DDQ 2,3-dichloro-5,6-dicyanobenzoquinone

DFT density functional theory

DMAM dimethylaminomethyl

DME dimethoxyethane

DMF N,N'-dimethylformamide

dppe 1,2-bis(diphenylphosphino)ethane

dppf 1,1’-bis(diphenylphosphino)ferrocene

dppp 1,3-bis(diphenylphosphino)propane

DQF COSY double-quantum filtered correlation spectroscopy

ESI electrospray ionisation

GGA generalised gradient approximation

H2DAP free-base 5,15-diarylporphyrin

H2DPP free-base 5,15-diphenylporphyrin

H2OEP free-base octaethylporphyrin

H2TriPP free-base 5,10,15-triphenylporphyrin

HF Hartree-Fock

HK Hohenberg-Kohn

-xxiii-

HOMO highest occupied molecular orbital

HpD haematoporphyrin derivative

KS Kohn-Sham

LB Langmuir-Blodgett

LC-TOF liquid chromatography - time of flight

LDA localised density approximation

LDI laser desorption ionisation

LUMO lowest unoccupied molecular orbital

LYP Lee, Yang and Parr correlation functional

m multiplet

m.p. melting point

MALDI matrix assisted laser desorption ionisation

MM molecular mechanics

MS mass spectrometry

NADP+ nicotinamide adenine dinucleotide phosphate, oxidised form

NADPH nicotinamide adenine dinucleotide phosphate, reduced form

NBS N-bromosuccinimide

Ni(acac)2 bis(acetylacetonato)nickel(II)

NiDAP 5,15-diarylporphyrinatonickel(II)

NiOEP octaethylporphyrinatonickel(II)

NiTriPP 5,10,15-triphenylporphyrinatonickel(II)

NiTPP 5,10,15,20-tetraphenylporphyrinatonickel(II)

NMR nuclear magnetic resonance

NOESY nuclear overhauer effect spectroscopy

OEP octaethylporphyrin

PPh3 triphenylphosphine

RC reaction centre

RCM ring closing methathesis

ROMP ring opening metathesis polymerisation

s singlet

SCF self-consistent field

t triplet

TDDFT time dependent density functional theory

-xxiv-

TEA triethylamine

TFA trifluoroacetic acid

THF tetrahydrofuran

TLC thin layer chromatography

TPP 5,10,15,20-tetraphenylporphyrin

UV ultraviolet

ZnTriPP 5,10,15-triphenylporphyrinatozinc(II)

-xxv-

DECLARATION

The work contained in this thesis has not been previously submitted for a degree of

diploma at any higher educational institution. To the best of my knowledge and

belief, this thesis contains no material preciously published or written by another

person except where due reference is made.

Oliver Brett Locos

August 2006

-xxvi-

ACKNOWLEDGEMENTS

The author would like to acknowledge and sincerely thank:

Firstly, my principal supervisor, Dr. Dennis P. Arnold, for providing inspiration,

guidance and above all surviving my candidature with a seemingly inexhaustible

supply of patience, particularly with my spelling of the most integral word,

“porphryin” (or more commonly known as porphyin). His support and confidence in

my abilities and the opportunities he has provided, both directly and indirectly, have

been vital towards my understanding of porphyrin chemistry, completing this project

and becoming a better chemist in general.

My associate supervisor, Assoc. Prof. Steven Bottle, for being an exceptionally

astute problem solver when it comes to laboratory and equipment issues, and also

pushing me towards the finish line when I needed it.

Professor Ken-ichi Sugiura, for supplying the ruthenium and palladiumII catalysts,

which were integral in this work, and upon his visits to our laboratory he provided

many valuable insights to the work at hand and always “wished me luck”.

Dr. John Bartley, for his knowledge and training in the NMR which provided

assistance in the pursuit of characterising new compounds, and for the many

discussions and lamentations about playing the clarinet and how difficult it is to get

over the bridge.

Dr. Martin Johnston (Central Queensland University) and Dr. Roger Meder, for their

invaluable assistance with NMR, especially regarding the NOESY experiments on

porphyrin dyads.

Mr. Greg Wilson (soon to be Dr. Wilson) for his immeasurable help with performing

the theoretical calculations, the discussions (and grievances) about both of our

projects, and taking the place of Ms. Helen Eldridge as coffee buddy since her

departure from QUT in early 2002.

-xxvii-

Dr. Anthony Rasmussen for ensuring the supercomputer was always up to “scratch”

for running Gaussian 03, and being extremely efficient in notifying us of any

problems with calculations.

All the technical staff of the School of Physical and Chemical Sciences, Queensland

University of Technology, who have always been ready and willing to provide help

when I have requested it. Special thanks must go to Dr. Chris Carvalho who, in the

middle of juggling ten things, managed to squeeze in time to help with instrumental

problems EVERY time it was needed. His constant searching for new software has

made handling data easier to manage while his “Carvalhoisms” have made the quest

for knowledge a most enjoyable one.

The Queensland University of Technology, for providing financial support for the

past four years.

All of the postgraduate community, past and present, for their friendship and

discussions which on many occasions provided insight into some of the smaller

details of this project.

Lastly, but not least, my family, for their interminable support, interest,

understanding, patience and love throughout the highs and lows of the last four years.

Introduction

-1-

CHAPTER 1

INTRODUCTION

1.1. Structure and Basic Properties of Porphyrins

1.1.1. Basic Structure and Roles in Nature

The class of organic compounds collectively known as porphyrins has been the focus

of research in many fields of science for over a hundred years. The common

structural feature of porphyrins is a tetrapyrrole macrocyclic ring that consists of an

18-π electron system. The parent compound, known as porphyrin (or porphine),

consists of four pyrrole rings which are connected through one-carbon methine

bridges. The structure of porphyrin and numbering of the ring positions are shown in

Figure 1.1.1

HNN

NH N12

3 4 5 6 78

910

11

12

13141617

18

1920 21 22

2324

15 Figure 1.1. The structure of porphine and the IUPAC numbering system.

Positions 5, 10, 15 and 20 are usually referred to as the meso-positions and positions

2, 3, 7, 8, 12, 13, 17 and 18 are usually referred to as the β-positions. The two central

pyrrolenine nitrogen atoms are capable of accepting protons to form a dication, while

the two NH groups are capable of losing protons to form a dianion. The porphyrin

dianion has the potential to form metalloporphyrins which contain at least one bond

between one of the central nitrogen atoms of the porphyrin and a metal ion.

Porphyrins have been combined with almost all metals and some semi-metals to

form complexes with a variety of geometries, for example, in-plane, out-of-plane or

bimetallic complexes.2,3

The main function of porphyrins in Nature is to chelate to metals, and these

complexes subsequently play an integral role in biochemical processes.4 Complexes

involving porphyrinic ligands and the elements magnesium, manganese, iron, cobalt,

Introduction

-2-

nickel, copper, vanadium and zinc are known to occur in Nature. Two very well-

known biological molecules which contain the FeII protoporphyrin IX derivative, 1,

are haemoglobin and cytochrome c.1,5 In haemoglobin, the iron acts as a reversible

binding site for oxygen, allowing it to be transported throughout an organism.

Cytochrome c makes use of the +2 and +3 oxidation states of the iron while

performing the function of electron transfer in cell respiration.

2

NN

N N

Co

NH

O

O

P O

-OO

OHO

HON

N

NH2

O

OH2N

O

H2N

O

H2N

NH2O

O

NH2

R = CN, OH, CH3, deoxyadenosyl

O

HOHO

NN

NN

N N

Fe

HO2C CO2H

1 Adenosylcobalamin, or vitamin B12 2, is the only naturally occurring organometallic

compound identified so far.1,5 It is based on a trivalent cobalt/corrin complex and

when derivatised in the body, is a cofactor in the methylation of DNA and in the

production of haemoglobin. The corrin ring, although it possesses similar properties,

differs from the porphyrin ring in that it contains one less methine bridge and

reduced pyrroles which result in a loss of planarity and aromaticity within the

macrocycle.

Chlorophyll, 3, is the green pigment in higher plants, algae and cyanobacteria which

is responsible for a majority of light absorption in the red and blue portions of the

electromagnetic spectrum. Chlorophyll is based on a magnesium/chlorin complex

which contains a saturated bond between the 7 and 8 positions on the macrocyclic

ring.1,6 The substituents around the macrocycle serve to optimise the light absorption

characteristics of the molecule and assists in anchoring the chromophore within the

light-harvesting proteins. The metal also serves to fine-tune light-absorbing and

energy-transfer characteristics of the chlorophyll, while acting as a centre for binding

water, which is the source for electron replacement.7-9 The long phytyl hydrocarbon

chain assists in anchoring the molecule in a hydrophobic environment.

Introduction

-3-

3

3N

N

N

N

Mg

O

O

O

R

O

O

The mechanism of photosynthesis and the assembly of pigments involved in natural

light-harvesting have been the subject of intense study for the past two decades. The

reaction centre (RC) contains a special pair of overlapping bacteriochlorophyll

molecules which are shown in Figure 1.2. It is the overlap of these two molecules

and their near perfect two-fold symmetry that allow them to act as the primary

electron donors in the initial stage of photosynthesis.10

Figure 1.2. Molecular representation of the special pair of bacteriochlorophyll molecules10

A molecular ball and stick representation of the photosynthetic reaction centre,10-15 as

isolated from the purple bacterium Rhodopseudomonas viridis, is shown in Figure

1.3. When excited, an electron is transferred from the special pair to the

bacteriophytin (BPh), which is facilitated by an intermediate bacteriochlorophyll

molecule (BCl) in close proximity to the primary electron donors. The excitation is

further relayed to the menaquinone and then to the final electron acceptor,

ubiquinone, which is facilitated by a histidine-glutamine iron complex.

R = CH3 – Chlorophyll a R = CHO – Chlorophyll b

hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library

Introduction

-4-

Figure 1.3. Schematic representation of the photosynthetic reaction centre isolated from

Rhodopseudomonas viridis, showing the chain of electron transfer from the primary electron

donor to the final electron acceptor9


Introduction

-5-

The BCl, BPh and quinones form two almost symmetrical arms, L and M, from the

special pair which span a cytoplasmic membrane. Due to slight conformational

differences, however, the L side is highly favoured in the chain of electron transfer.

The electron transfer from the special pair to the quinone promotes an extremely

long-lived charge-separated state, facilitated by the relatively large distance between

the redox sites. The small progressive steps in charge-separation are sufficiently

energy-releasing that the adverse process of recombination is effectively reduced,

giving the process a quantum efficiency of ≈ 1. With the complementary H+ gradient

in the NADP+ → NADPH cycle, the ensuing electron gradient from the charge-

separated state is responsible for the production of ATP. The product from the initial

excitation, the positively-charged oxidised dimer, is reduced again by neighbouring

cytochrome c proteins. Between the different species of photosynthetic bacteria and

higher plants, the type of chlorophyll in the special pair, BCl and BPh, as well as the

reducing moieties are slightly different, however the mechanism of photosynthesis is

believed to be the same.

By itself, the special pair is capable of absorbing light to generate electronic

excitation, although it is not sufficient to saturate its maximum turnover rate.16 The

remaining energy is supplied by light harvesting proteins, and indeed, most

chlorophyll molecules serve as light-harvesting antennae capturing the sunlight and

funnelling the electronic excitation towards the RC. The organisation of the proteins

responsible for bacterial photosynthesis has been deduced by x-ray crystallography,

electron microscopy and molecular modelling.16-18

For Rhodopseudomonas viridis, the antennae usually consist of a number of small

light harvesting proteins, LH-II, which contain two orthogonal rings of

bacteriochlorophyll b molecules. The first ring contains an octameric aggregate of 16

chlorophyll molecules while the second slightly larger ring contains 8 chlorophyll

molecules, as shown in Figure 1.4a.

Introduction

-6-

Figure 1.4. The relative positions of bacteriochlorophyll b in a) LH-I protein and b) LH-II in

bacteria.16

Typically, the LH-II protein is in close proximity to a much larger light harvesting

protein, LH-I; the LH-I and LH-II proteins are co-planar which allows the maximum

amount of electronic interaction between the proteins. The LH-I protein, as shown in

Figure 1.4b, consists of 32 chlorophyll molecules that form a ring which

encompasses the RC and its complementary accessory chromophores. The resulting

planar pigment organisation appears to be optimal for energy transfer from the outer

light-harvesting complexes. Excitation transfer occurs in the order LH-II8 → LH-II16

→ LH-I → RC. The resulting energy cascade from the outer light harvesting proteins

fuels the RC to generate an electron for photosynthesis approximately 1000 times per

second.

a)

b)



Introduction

-7-

1.1.2. Electronic Absorption Spectra of Porphyrins

Porphyrins are well known for the intense colours they possess which is a

consequence of their extensive macrocyclic conjugation and is the key feature in

their applications. The UV-Visible absorption spectrum of porphyrins consists of two

distinct regions.19 In the violet region, an extremely intense absorption known as the

Soret or B band has a typical extinction coefficient of approximately 1 x 105 M-1cm-1.

In the visible region, a number of absorptions known as the Q bands are present,

which possess extinction coefficients of approximately 1 x 104 M-1cm-1.

As shown in Figure 1.5, the inner 16-membered ring of a porphyrin is responsible for

maintaining the major conjugation pathway for the 18 π-electrons that generate the

general porphyrin-type optical spectra.20 This ring is susceptible to perturbation by

various chemical modifications to the basic structure. One modification which has an

effect on the optical spectrum of a porphyrin is the central substituent.

N

NH N

HNN

NH N

HN

Inner 16-membered ring 18 π−electron pathway

N HN

NNH

Figure 1.5. The inner 16-membered ring responsible for maintaining the conjugation pathways

for the 18π-electrons that generate the porphyrin optical spectrum

Effect of the Central Substituent upon Porphyrin Spectra

Figure 1.6 shows an example of the spectral changes that occur when the central

substituent is NiII (NiDAP) or is two hydrogens (H2DAP) in a diarylporphyrin. It has

been well established that the optical spectra of porphyrins are attributed to π- π*

transitions within the ring, with the central substituent contributing small electronic

perturbations.20

Metalloporphyrins often have two major absorption bands in the visible region.19,20

The lower energy band, Q(0,0), is attributed to excitation to the first excited state.

Introduction

-8-

The higher energy band, Q(1,0), is a result of vibrational transitions interacting with

the electronic transitions of the porphyrin, and has been referred to as a vibronic

overtone of the Q(0,0) band. The Soret band is attributed to excitation to the second

excited state, and in well resolved spectra a vibronic overtone, B(1,0), may also be

observed.

Figure 1.6. Absorption spectra of a) NiDAP and b) H2DAP

The vibronic overtone in the Soret region is attributed to symmetric vibrations

interacting with the electronic transitions.20 The effect of this interaction is the

progression of the overtone to higher energy in accordance with the Franck-Condon

principle; the spacing between the overtone and the Soret band is equal to the

frequency of the vibrations in the excited electronic state. The vibrational transitions

involved in Q(1,0), however, have been attributed to vibronic borrowing from the

Soret state.

The spectra of metalloporphyrins are influenced by the type of metal ion chelated to

the porphyrin.7 Transition metals possess dπ (dxz, yz) orbitals with eg symmetry which

overlap with the porphyrin π* orbitals. As the metals increase in atomic number, the

d π orbitals decrease in energy. The d8 metal ions have approximately the same

energy as the porphyrin orbitals. For the d1-d5 metal ions, the partially occupied d π

300 400 500 600Wavelength (nm)

500 600

300 400 500 600Wavelength (nm)

500 600

NHN

NH N

t-Bu t-Bu

t-Bu t-Bu

NN

N N

t-Bu t-Bu

t-Bu t-Bu

Ni

Soret

Q(0,0)

Q(1,0)

Qx(0,0)

Qy(0,0) Qx(1,0)

Qy(1,0)

Soret

Abs

orba

nce

Introduction

-9-

orbital allows the possibility of porphyrin to metal charge transfer. These transitions

are observed but are less intense than the porphyrin π→ π* transitions. For the d6-d9

metal ions, metal to ligand backbonding occurs as the filled d π orbital can interact

with the vacant π* orbitals, which effectively stabilises the metal d π orbitals but

raises the energy of the porphyrin π* orbitals. Therefore the energy of π→ π*

transitions will be shifted to higher energy. Metal ions with a completed d-electron

shell (d10) have d π orbitals which lie well below the porphyrin π orbitals.

Compared to metalloporphyrins, free-base analogues possess lower symmetry due to

the inner-protons.19,21 As a result, the degenerate Q(0,0) transition is replaced by

transitions polarized along each of the inequivalent axes, Qx(0,0) and Qy(0,0). As in

metalloporphyrins, the Qx(0,0) and Qy(0,0) bands also possess vibronic overtones,

Qx(1,0) and Qy(1,0) respectively, resulting in four bands in the visible region.

The Four-Orbital Theory

Gouterman developed a four-orbital model which has been used extensively to

explain the absorption and emission spectra of metalloporphyrins.21-25 The four-

orbital model is generated from the cyclic polyene theory and takes into account the

four frontier orbitals of the porphyrin macrocycle which are shown in Figure 1.7.

Figure 1.7. The orbitals involved in the Gouterman four-orbital theory25 and the promotion of

an electron from the HOMO to the LUMO for porphyrins with D4h symmetry


Introduction

-10-

For a metalloporphyrin with D4h symmetry, a 16-membered cyclic polyene model is

applied. The highest filled orbitals are singly degenerate and denoted a1u and a2u,

while the lowest empty orbitals to which electrons can be promoted are doubly

degenerate and denoted eg. The transitions seen in the absorption spectra of

porphyrins are due to excitation of an electron from a2u or a1u to eg (Figure 1.7). As

the porphyrin ring has 4-fold symmetry, the symmetry of the excited states resulting

from the two transitions is the same, resulting in their overlap and consequent

interaction (configuration interaction, CI) with one another. This interaction, known

as inner-configuration, is the interaction between anti-symmetrised products which

are degenerate for symmetry reasons,26,27 and can occur in two ways: constructively

or destructively. Constructive interaction leads to the more intense Soret bands,

whereas the destructive interaction leads to the weaker Q bands. These two

transitions are split about the origin of transitions for the porphyrin.

For a free-base porphyrin possessing D2h symmetry across the plane of the ring, the

opposing protons stabilise an 18-membered cyclic polyene, with each proton causing

a one electron perturbation of the macrocycle. This perturbation causes the split in

the absorption bands in the visible region but not in the ultraviolet.20 The splitting

about the origin of transitions for porphyrins of lower symmetry arise from the

mixing of anti-symmetrised products which are nearly or accidentally degenerate,

and is known as inter-configurational interaction.26,27

1.1.3. Excitonic Coupling Theory

The excitonic coupling theory was originally described as the treatment of resonance

interaction between excited states of weakly coupled systems.28 It has been used to

describe the spectral properties observed in crystalline molecular solids, molecular

aggregates such as multilayered laminae and monolayers, polymers, and simpler

systems such as dimers and trimers.

An exciton is a neutral, bound electron-hole pair which has properties that differ

from those of the excited state of an isolated chromophore.29 These differences result

from a combination of electrostatic and electron-exchange interactions with other

identical chromophores in the vicinity. Excitons can be treated in two limiting cases,

Introduction

-11-

depending upon the ability of a material to resist the formation of an electric field

within it.30

When a substance possesses a small resistance, or small dielectric constant, the

Coulombic interaction between the electron and the hole is strong and the excitons

tend to be localised almost exclusively on a single chromophore. Such a tightly-

bound electron-hole pair is referred to as a Frenkel exciton.31 The interactions

between chromophores in such a system are mainly electrostatic in origin, and

energy is transferred non-radiatively from the excited state of one molecule to the

next by a long range dipole-dipole coupling mechanism. This mechanism of energy

migration, which is referred to as Förster transfer,32 can result in “photonic-wire”

characteristics of one-dimensional arrays.

The effect of Frenkel excitons upon the optical properties of dimers has been

described by Kasha et al.28,33 using a quasi-classical vector model for the electrostatic

interaction of dipoles. The energy of the exciton state in a dimer is dependent on

whether the dipole interaction is attractive or repulsive, and an exciton state is only

allowed if the dipoles are in-phase. The orientation of the dipoles and how they can

interact depends upon the orientation of the monomeric units. Examples of this are

shown in Figure 1.8.

Figure 1.8. Exciton band structure in dimeric molecules with various geometrical

arrangements28


Introduction

-12-

(i) Face-to-face dimers

Dimers that are face-to-face have parallel transition dipoles as shown in Figure 1.8a.

The dipole arrangement which is out-of-phase correlates to an attraction of the

dipoles and a consequent lowering of energy in the exciton state E′. As the

interaction is out-of-phase, the transition from the ground state to E′ is forbidden and

therefore not seen. The interaction which is in-phase constitutes a repulsion between

the dipoles and a consequent raising of energy in the exciton state E″ compared to

the excited state of the monomer. The result is an overall blue-shift in the absorption

spectrum.

(ii) Head-to-tail dimers

Dimers which are aligned head-to-tail possess in-line transition dipoles as shown in

Figure 1.8b. In this instance, the transition to E′ is allowed as the dipoles are

arranged in-phase and as the interaction is attractive, the energy of the transition is

lowered relative to that in the monomer. The E″ transition correlates to an

arrangement where the dipoles are repelled from one another, raising the transition to

a higher energy, but is forbidden as it is out-of-phase. As the allowed transition is

lower in energy, the effect is a red-shift in the absorption spectrum.

(iii) Oblique dimers

Dimers which possess oblique transition dipoles are represented in Figure 1.8c. In

this case, the in-phase arrangement of the transition dipoles is attractive, resulting in

a lowering of energy for the transition E′, while the out-of-phase interaction is

repulsive, resulting in the raising of energy for transition E″. Unlike the previous

dimeric geometries, from vector analysis of the dipoles, neither transition results in a

complete cancelling of the dipoles, therefore both transitions are allowed.

Consequently, dimers which are oblique in geometry will show two transitions, one

at higher and one at lower energy in comparison to the monomer.

The exciton splitting energy from a Frenkel exciton, is directly proportional to the

square of the transition moment and inversely proportional to the cube of the

Introduction

-13-

intermolecular distance between the two chromophores. This relationship is defined

in equation 1.

( )γβα coscos3cos2 32

+=Δr

EM

(1)

M = transition moment for the singlet-singlet transition in the monomer

r = distance between the centres of the two molecules in the dimer

The angles in equation 1 are defined in Figure 1.9.

αr

βγ

Figure 1.9. Definitions for the geometrical parameters in equation (1)

If a molecule possesses a large dielectric constant, the interaction between electrons

and holes is limited and therefore the hole and the electron forming the exciton are

not necessarily localised on the same chromophore. This exciton, referred to as a

Mott-Wannier exciton,34,35 is relevant when the electronic interaction between

chromophores is mediated by a direct overlap of the π-orbitals or by the π-orbitals of

an intervening bridge, allowing electrons to exchange with relative ease between the

chromophores.

Excitonic coupling theory has been applied to the spectral phenomena seen in

porphyrin dyads,36,37 aggregates38 and multiporphyrin arrays.29 Examples of this

application will be discussed in Section 1.3.

1.1.4. Emission Spectra of Porphyrins

Figure 1.10 shows an energy level diagram that is applicable for most aromatic

compounds with singlet ground states.39 Excitation from the ground state S0 to any

excited state, Sx, leads to very fast radiationless decay to the lowest excited singlet

state S1. From S1, the molecule can emit fluorescence radiation S1→S0 with a rate kf,

Introduction

-14-

can radiationlessly decay S1→S0 with a rate k1 or can internally convert to the lowest

triplet state S1→T1 with rate k2.

S0

S1A

bsor

ptio

n S 0

S

1*

Fluo

resc

ence

S1*

S 0

nonr

adia

tive

deac

tivat

ion

nonr

adia

tive

deac

tivat

ion

intersystem crossing T1

Abs

orpt

ion

S 0

T1

Phos

phor

esce

nce

T 1

S 0

Figure 1.10. Decay scheme for singlet and triplet relaxation. The radiation processes are shown

as straight lines; radiationless processes are shown by wavy lines

From T1 the molecule can emit phosphorescence radiation T1→S0, radiationlessly

decay T1→S0 or be re-excited to the first excited singlet T1→S1. In porphyrins, the

luminescence is affected by the spin-orbit perturbations of the central substituent,

which is observed as dramatic changes to the quantum yields of fluorescence and

phosphorescence and the decay time for phosphorescence.20

Effect of the Central Substituent upon Emission Spectra

The luminescence of porphyrins can be organised into four different categories.

Table 1.1 shows the types of emission that can be observed from various metallated

and free-base porphyrins at room temperature in solution.20,40-43 For

metalloporphyrins, the phosphorescence and fluorescence generally occur at shorter

wavelengths than their free-base analogues as a consequence of possessing higher

singlet and triplet-state excitation energies. Metalloporphyrins also experience a

general decrease in fluorescence and an increase in phosphorescence, which has been

partially attributed to the heavy atom effect. Due to low-lying d-d transitions, NiII

complexes do not exhibit any luminescence.42 Interestingly, all other d8 transition

metals exhibit fluorescence and/or phosphorescence.

Introduction

-15-

Table 1.1. Emission properties of metalloporphyrins at room temperature43

Fluorescence Spectrum of Porphyrins

Two properties which are commonly observed from the fluorescence data are the

Stokes shift and quantum yields. Figure 1.10 shows the general profile expected from

the fluorescence of a porphyrin when excited at the Soret band.

Figure 1.11. General fluorescence profile of fluorescent porphyrins

The fluorescence spectrum usually consists of two peaks, the emission from the first

excited singlet state, Q(0,0), and a vibronic overtone Q(0,1) which are mirror images

of those seen in the absorption spectrum.20 The Stokes shift is the energy which is

dissipated during the lifetime of the excited state before the return to the ground

state. The energy dissipated, ΔE, can be calculated from the subtraction of the energy

of emission from the energy of excitation, as shown in equations 2 and 3.39

emissionexcitation EEE −=Δ (2)

430 530 630 730 830

Wavelength (nm)

Q(0,1)

Q(0,0)

hallaThis table is not available online. Please consult the hardcopy thesis available from the QUT Library

Introduction

-16-

1000

10022.6 23××⎟⎟

⎠

⎞⎜⎜⎝

⎛−=Δ

emex

hchcEλλ

kJ mol-1 (3)

where h is Planck’s constant, c is the speed of light and λex and λem are in metres. The

quantum efficiency Φ denotes the ratio of the total energy emitted by any molecule

per quantum of energy absorbed:

efficiencyoryieldquantumabsorbedquantaofnumberemittedquantaofnumber

==Φ (4)

The value of Φ can be determined by a quantitative comparison to a standard which

has known quantum efficiency:

unk

std

unk

std

std

unkstdunk A

Aqq

FF

××Φ=Φ (5)

Where F is the relative fluorescence determined by the integration of the area

beneath the corrected fluorescence spectrum, q is the relative photon output of the

source at the excitation wavelength and A is the absorbance.

1.2. Uses of Multiporphyrin Arrays

Porphyrin arrays have found application in a number of areas. Three of these are

molecular electr

synthesis and investigations of novel ...synthesis and investigations of novel alkenylporphyrins and...

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