photoswitchableacids and bases - chemistry · photochromism: molecules and systems; 1 ed.; rau, h.,...

Photoswitchable Acids and

BasesBases

Georgina A. Carballo

November 5, 2008

Why Should We Care about Photogenerated Acids or Bases?

Photoresist

Base insoluble Base soluble

Ito, H.; Willson, C. G.; Frechet, J. M. J.; Farrall, M. J.; Eichler, E. Macromolecules 1983, 16, 510-517.

Can distinguish between irradiated and non-irradiated areas - photolithography

Applications of Photoresists

Light

1. Remove mask

2. Wash exposed

photoresist

Etch oxide layer

Remove

unexposed

photoresist

Silicon Wafer

Oxide Layer

Photoresist

Mask

(Plasma)

photoresist

Microchip Fabrication

http://www.advantivtech.com/wafers/patterned-test-wafers.html

80nm Line and Space Pattern

Photogenerated Acids

1,8-Naphthalimide sulfonate photoacid generators

Malval, J.-P.; Suzuki, S.; Morlet-Savary, F.; Allonas, X.; Fouassier, J.-P.; Takahara, S.; Yamaoka, T. J. Phys. Chem. A 2008, 112,

3879-3885

Ito, H.; Willson, C. G.; Frechet, J. M. J.; Farrall, M. J.; Eichler, E. Macromolecules 1983, 16, 510-517.

Photogenerated Base

• Light induced generation of a free amine• Light induced generation of a free amine

• 3’,5’-dimethoxybenzoin carbamates as the protecting group

• Benzofuran cyclization photo-product is major

• Applications in polymer crosslinking, coatings and

microlithography

Cameron, J. F.; Willson, C. G.; Frechet, J. M. J. J. Am. Chem. Soc. 1996, 118, 12925-12937.

Acid

Switch

Acid

Empty p Orbital

Switch

Externally Regulated Acidity and Basicity

hν

Switch

Switch

hν

Photochromism

Reversible transformations of a single chemical species upon electromagnetic radiation where the two states show distinguishable absorption spectra

(UV)

Photochromism: Molecules and Systems; 1 ed.; Rau, H., Ed.; Elsevier, 1990; Vol. 40.

http://www.athenseyecare.com/EyeGlassLenses.cfm

(vis)

(UV)

Light Dark

Photochromic Molecules

Diarylethenes Spirooxazines

Fulgides

AzobenzeneSpiropyrans

Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100, 1741-1754.

Matsuda, K.; Irie, M. J. Photochem. Photobiol., C 2004, 5, 169-182.

Yokoyama, Y. Chem. Rev. 2000, 100, 1717-1740.

Photoswitchable Base

• Photoswitching ability given by azobenzene chromophore

• Z isomerization allows access to piperidine group

Peters, M. V.; Stoll, R. S.; Kühn, A.; Hecht, S. Angew. Chem., Int. Ed. 2008, 47, 5968-5972.

Photoisomerization of Azobenzene

Hexanen,π* 432 nm (εmax =4 x 102 M-1 cm-1)π,π* 318 nm (ε =2.2 x 104 M-1 cm1)

Hexanen,π* 430 nm (εmax = 1500 M-1 cm-1)

π,π* 260 nm

0.99nm 0.55 nm

• Reduced conjugation and planarity of cis-isomer

• Trans isomer thermally stable by 12-14 Kcal/mol

• Change in dipole 0� 3.1D

• Change in length of molecule

Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.; Orlandi, G. J. Am. Chem. Soc. 2004, 126, 3234-3243.

Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol., A 2006, 182, 250-261.

Crecca, C. R.; Roitberg, A. E. J. Phys. Chem. A 2006, 110, 8188-8203.

Fujino, T.; Arzhantsev, S. Y.; Tahara, T. Bull. Chem. Soc. Jpn. 2002, 75, 1031-1040.

π,π* 318 nm (εmax =2.2 x 104 M-1 cm1)max

π,π* 260 nm

Simplified Energy Level for Azobenzene - π��π*

n

*

n

*

*

weakenedπ bond

Torsion about the N=N bond


S0 S2

Simplified Energy Level for Azobenzene – n��π*

n

*

n

*

n *

π bond intact

In plane inversion

S0 S1

intact


Accepted Mechanisms for Azobenzene Isomerization


In plane inversionIn plane inversion


Rotation about N=N Bond

• S2 accessed by π�π*

• Single bond character

for N-N in S2

• Rotation about the N-N• Rotation about the N-N

bond possible in S2

Lednev, I. K.; Ye, T. Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V. R.; Umapathy, S.; Hester, R. E.; Moore, J. N. Chem.

Phys. Lett. 1998, 290, 68-74.

In Plane Inversion

• S1 accessed by n�π*

• Double bond character

of S1

•

• Excitation with visible

light to S1 will result in in light to S1 will result in in

plane inversion

• Relaxation of S2 �S1 will

also result in inversion


Lednev, I. K.; Ye, T. Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V. R.; Umapathy, S.; Hester, R. E.; Moore, J. N. Chem.

Phys. Lett. 1998, 290, 68-74.

Which Mechanism is Preferred?


In plane inversion

• Need to look at the transition states for both mechanisms

• Vibrational spectroscopy

• N-N ~1000 cm-1, -N=N- 1420-1410 cm-1


Lambert, J.; Shurvell, H.; Lightner, D.; Cooks, G. Organic Structural Spectroscopy; 1 ed.; Prentice Hall: New Jersey, 1998.

Raman Spectroscopy-Detecting the Structures of Excited States

• Inelastic collisions of photons with molecules

• Scattering spectroscopy

• Ti:sapphire laser pump 1.8 ps pulses

• 0 ps delay after the pulse

Lambert, J.; Shurvell, H.; Lightner, D.; Cooks, G. Organic Structural Spectroscopy; 1 ed.; Prentice Hall: New Jersey, 1998.

Fujino, T.; Tahara, T. J. Phys. Chem. A 2000, 104, 4203-4210.

Picosecond Raman Spectroscopy- N=N Stretching

• Photoexciting (273 nm)

from S0 � S2 � S1

• transient Raman detected

at 0 ps delay (410 nm)

• Evidence of double bond 15N

15N

14N14N

N=N stretching

• Evidence of double bond

character of NN at S1

supports in-plane

inversion mechanism


Fujino, T.; Tahara, T. J. Phys. Chem. A 2000, 104, 4203-4210.

14N14N

Quantum Yield - Efficiency of Photochemical Reactions

Measures the efficiency of a particular light induced process

For efficient photochemical process Φ≈1

Anslyn, E.; Dougherty, D. Modern Physical Organic Chemistry; University Science Books: Sausalito, California, 2006.

hν

For efficient photochemical process Φ≈1

Photostationary State

• Analogous to thermal equilibrium

• Particular proportion of isomers that does not change upon further irradiationirradiation

• Importance of the absorbance

Anslyn, E.; Dougherty, D. Modern Physical Organic Chemistry; University Science Books: Sausalito, California, 2006.

[T]

[C]=

c c

t t

Where ε = efficiency of absorptionΦ= quantum yield

Rotationally Locked Azobenzenes

trans,trans trans,cis cis,cis

Testing the photisomerization mechanism via rotationally restricted

azobenzene derivatives

• Cyclic array of

Azobenzenophanes (ABP)

restricts rotation about the N=N

bond

• Isomerization more plausible via

inversion

• Increase in Φπ,π* might indicate

prior relaxation to n,π*

Rau, H.; Lueddecke, E. J. Am. Chem. Soc. 1982, 104, 1616-1620.

trans,trans trans,cis cis,cis

Irradiation at 366 nm to the

photostationary state

PhotoReactionAzobenzene ABP

Φ t�c Φt,t�t,c

S1 � S0 0.23 0.24

S2 � S0 0.1 0.21

Structural Basis for Photoswitchable Base

E-isomer Z-isomer


Blocked

Features of Photoswitchable Bases

4a R= Me, R'= t-Bu

Accessible basic Site

Blocked basic site

• Conformationaly restricted

• Steric shielding of active site

• Orthogonal positioning of the chromophore

4b R= t-Bu, R'= t-Bu

4c R= t-Bu, R'= 2,6-Me2C6H3


OFF ON

Photoisomerization of base vs. time

• Isomers or base have different absorption spectra.

• E�Z photoisomerization can be followed overtime with UV-visspectroscopy.

N

O

O

Me

N

N

tBu

tBu

E

Z

UV-Vis spectrum of E and Z Isomers

Z

E�� Z isomerization365 nm for 13 min

Z�� E isomerization400 nm for 4 min


Parameters Measured for Photoswitchable Bases

4a R= Me, R'= t-Bu

4b R= t-Bu, R'= t-Bu

4c R= t-Bu, R'= 2,6-Me2C6H3

PSS (Z/E) t 1/2 (h) pKa E pKa Z


Hall, H. K. J. Am. Chem. Soc. 1957, 79, 5441-5444.

PSS (Photostationary state) irradiation at 365 nm

Half life of Z isomer at 20°C in the dark

PSS (Z/E) t 1/2 (h) pKa E pKa Z

4a 90:10 268 NR NR

4b 90:10 286 15.9 ± 0.1 16.7 ± 0.1

4c >90:10 466 16.0 ± 0.1 16.7 ± 0.1

R NO

R2

NO2

O

R1R

Henry Reaction

CTA-Cl72%

R NO2

R NO2

B

B H

R2

NO2

OHR1

R

O

R2R1

LA

LA

R1 R2

O

Ballini, R.; Bosica, G. J. Org. Chem. 1997, 62, 425-427.

Luzzio, F. A. Tetrahedron 2001, 57, 915-945.

Henry Reaction Catalyzed by a Smart Base – NMR Kinetics

Catalytic Activity with 10 mol% base

k [10 s ] k rel

Base

k [10-5 s-1 ] k relexp. 100% Z isomer 1.4 35.9

Z��E back rxn. 1.2 30.8Z-isomer at PSS 1.1 28.2

E-isomer 0.039 1E-azobenzene 0.0024 0.062

no catalyst 0.039 1


PSS= Photostationary State 90:10

A Photoswitchable Lewis Acid

• Photoswitchable• Photoswitchable

• Lewis acidity “turned OFF” by

electron donation from N to B

• Lewis acid “ON” when donor is

away from B

N N

B

O

O

Yoshino, J.; Kano, N.; Kawashima, T. Tetrahedron 2008, 64, 7774-7781.

Dative Bond

Substituent Effects on the Strength of the B-N bond

Stronger B-N Weaker B-N

1a 1b 1c

N N

B

O

O

F

N N

B

O

O

OMe Electron Donating

ElectronWithdrawing


1.8247 Å

Stronger B-N1.773 Å

Weaker B-N1.8947 Å

Photoisomerization of 2-(phenylazo)-phenylboranes

N N

B

O

O

F

N N

B

O

O

OMe

Substituent at 4' position

Bond Length (Å)

Photoisomerization Irradiation at 360 nm or

431 nm to the


position Length (Å) 360 nm E/Z 431 nm Z/E

H 1.8247 65:35 98:2

OMe (EDG) 1.773 100:0 --

F (EWG) 1.8947 54:46 94:6

431 nm to the

photostationary state

Electron donating groups

strengthen the B-N

interaction

Electron withdrawing

groups weaken the B-N

interaction

Measured with NMR in C6D6

EDG=Electron donating group

EWG = Electron withdrawing group

Isomerization 2-(Phenylazo)-catecholboranes at 360 nm

I II III

Isomerizes No isomerization Isomerizes

Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett. 2005, 7, 3909-3911.

Compound E:Z ratioBond

Length Å

I 65:35 1.8247

II 100:0 1.721

III 69:31 --

Trend is reversed

EWG strengthens B-N interaction

EDG should weaken B-N

interaction

Testing Lewis Acidity Followed by 11B NMR

11B δ 21.8 ppm 11B δ 30.8 ppm

11B δ 18.0 ppm 11B δ 13.8 ppm

Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett. 2005, 7, 3909-3911.

E-isomer does not act as a

Lewis acid

Z-isomer shows Lewis acidity

Attempts to Diversify 2-(Phenylazo)-Phenylboranes

(HO)2B N N

HO OH

B N N

O

O

R1

R2

R3

HO

HON N

(E)-6 R1=R3=Me, R2=H

(E)-7 R2=R3=Cl, R1=H

HOHO

HO OH

H2N

NH2

(E)-12(E)-5a

B

O

O R1

R2

R3

N N

R' R' O

N N

B

(E)-9, R'=H(E)-10, R'=PH

N NBHN

NH

(E)-8

(E)-11

B

O

O

O

OO

R'R'


Photoswitchable Lewis Acid

BO O

Resonance stabilization associated

with dioxaborole ring system

Claimed to be a 4n+2 system

ONOFF

Letsinger, R. L.; Hamilton, S. B. J. Org. Chem. 1960, 25, 592-595.

Lemieux, V.; Spantulescu, M. D.; Baldridge, Kim K.; Branda, Neil R. Angew. Chem., Int. Ed. 2008, 47, 5034-5037.

Prevention of Irreversible Oxidation (aromatization)

Aromatic

Kellogg, R. M.; Groen, M. B.; Wynberg, H. J. Org. Chem. 1967, 32, 3093-3100.

Oxidation is prevented by the addition of substituents at the 2-positions

Aromatic

π-Orbital Symmetry

Woodward-Hoffmann Rules

Thermally Photochemicaly

4n conrotatory disrotatory

4n+2 disrotatory conrotatory

ConrotatoryDisrotatory

Woodward, R. B.; Hoffmann, R. J. Am. Chem. Soc. 1965, 87, 395-397.

Alignment of Orbitals Prior Cyclization

Antiparallel

Prior E��Z isomerization

Mallory, F. B.; Wood, C. S.; Gordon, J. T. J. Am. Chem. Soc. 1964, 86, 3094-3102.

Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63, 1311-1315.

AntiparallelParallel

Ring Closed Isomer

Features of Dithienylethene Photoswitches

• Prevention of irreversible oxidation to aromatic compound

• Thermal irreversibility

• Sensitivity to irradiation

• Fatigue resistance

Matsuda, K.; Irie, M. J. Photochem. Photobiol., C 2004, 5, 169-182.

Thermal Stabilization of Diarylethenes of Ring-Closed Isomer

Thermal Reversion

Lifetime 3 min at 30°C Thermal Reversion

Lifetime 12h at 80°C

Thermal irreversibility is

achieved by using

substituents with low

aromatic stabilization

energies

Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985-996.

Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803-808.

Aromatic stabilization Energies in Kcal/mol

Features of Lewis Acid

OFF ON

• Lewis acidity is “OFF” due to delocalization of π electrons that partially

occupy the Boron p orbital.

• Lewis acidity is “ON” when electrons are crossconjugated to the polyene

system.

OFF ON


Photocyclization of Lewis Acid

OFF

Photostationary state

285 nm

Photostationary state

reached in 12s with 81% of

ring closed isomer

Reverts to open ring isomer

after irradiation with visible

light for 5 min

535 nm


370 nm

Calculated LUMO of Simplified Model

O OB

H

S SH H

Lower in energy by

19 Kcal/mol

Node at the B

Node


Available p-orbital

Lewis acidic

Probing Lewis Acidity with Pyridine Followed by 1H NMR

1H NMR δ will not change

δ= 8.1 ppm

1H NMR δ will shift

ONOFF

• Open ring isomer will not bind to pyridine

• Closed ring isomer has Lewis acid reactivity in the presence of

pyridine (Lewis base)


Binding of Closed Ring Isomer to Pyridine

• Lewis Acidity : “ON”8.05

8.1

8.15

8.2

1H NMR • Lewis Acidity : “ON”

• Ring Closed Isomer Binds

with pyridine

• Chemical shift decreases

upon binding

•1H δ consistent with 1:1

stoichiometry


7.85

7.9

7.95

8

8.05

0 1 2 3 4

1H NMR Chemical

Shift (ppm)

Equivalents of Pyridine

Summary

• Acids and bases were given the ability to be “turned on” and

“turned off” by attaching a molecular switch.

• Azobenzene and dithienylethene were shown to be effective

at photoswitching acidity and basicity.

• The applications of photoswitchable acids and bases are yet

to be explored.

Acknowledgements

• Dr. Baker

• Qin, Sampa, Hui, Tom

• Dr. Jackson

• Dr. Hecth

• Dr. Blanchard

• Dr. Wulff• Dr. Wulff

• Babak

• Karrie, Luis S., Wynter, Heyi, Yiding, Wen, QuanXuan, Scott

photoswitchableacids and bases - chemistry · photochromism: molecules and systems; 1 ed.; rau, h.,...

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