Olesya A. Krumkacheva†, Vitaly R. Gorelik†,

Elena G. Bagryanskaya†, Natalia V. Lebedeva‡ and

Malcolm D. E. Forbes‡

Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

† International Tomography Center, Institutskaya 3a, Novosibirsk 630090, Russia‡ Caudill Laboratories, Department of Chemistry, CB #3290, University of North

Carolina, Chapel Hill, North Carolina 27599-3290

The Host-Guest systems as “nanocontainers” for the photochemical reaction

It can be used for the directional synthesis

hν

The application the Chemical Induced Dynamic Nuclear Polarization (CIDNP) and TR EPR methods allows to obtain information about the mechanism of photochemical reaction inaccessibleby other methods.

Carrying out photochemical reaction in organized media can lead to the product structure changing

Alteration of the re-encounter statistics and average

distance between partners in radical pair

Alteration the nature of the excited states

Reactivity between host and guest

TR EPR method

nonequilibrium population of spin level

EPR signal

Laser pulse

Direct observation of the short-lived paramagnetic intermediates (triplet molecules, radicals) formed during photolysis

Mh

R1(H)••R2 T R1(H)••R2

SM

*SM

T

R1(H)• •R2

absorption

emission

Investigation of the radical’s mobility

Mh

R1(H)••R2 T R1(H)••R2

S

In-cage products

M*S

MT

R1(H)• •R2Escape products

CIDNP method

9.6 3 2 1

9

88'

7

6

5

4

3

2

1

, ppm

emission

enchanced absorption

Advantages:

1) Investigation of the mechanisms of photochemical radical

reactions

2) Possibility to detect product with low yield (10-6 M/l)

3) Detection of intermediate product

Investigation of the influence of β-cyclodextrin presence on the mechanism of the ketones photolysis:

dibenzyl ketone (DBK), deoxybenzoin (DOB) and benzophenone (BP)

For the Complex formation : NMR and UV spectroscopy

For the Mechanism of photochemical reaction: TR EPR and CIDNP methods

Goal:

Research Methods:

Ketone’s photolysis

326 328 330 332 334 336

2.8 mT

b)

a)

1.1 mT

c)

5.7 mT

Магнитное поле, мТ

Guest

NMR spectrum

OH

PhCOPh

CD

CIDNP spectrum

Observed TR EPR spectrum

Ketyl radical

CD radical

Magnetic field, mT

Photolysis of -CD / benzophenone complex

hydrogen abstraction from CD cavity interior

Strong exchange interaction in radical pair

location of ketyl radical inside CD cavity

increasing of the Radical Pair lifetime

H

H O

H H

OH

PhCH2COPh

(PhCH2)2

Magnetic field, mT

Ketyl radical

Observed TR EPR spectrum

NMR spectrum

CIDNP spectrum

Guest

α-cleavage and hydrogen abstraction

Photolysis of -CD / deoxybenzoin complex

Strong exchange interaction in radical pair

location of ketyl radical inside CD cavity

formation of rearrangement product

Increasing of radical pair lifetime

O

CH2

H

-CD radical

CD

NO! TR EPR Signal

fast radicals recombination inside CD ( < 100 ns)

H

H

(PhCH2)2CO (PhCH2)2

NMR spectrum

CIDNP spectrum

O

Guest

α-cleavage

O

CH2

H

Photolysis of -CD / dibenzyl ketone complex

(PhCH2)2CO

Photophysics and photochemistry of DOB, DBK, and BP in β-CD inclusion complexes had been examined in detail. The DOB triplet state undergoes both reactions whereas the DBK triplet shows exclusively -cleavage and the BP triplet shows exclusively H-atom abstraction.

Results

Observation of rearrangement product of the radicals from -cleavage implies that there is substantial mobility of the radicals into the CD interior

It was found that there is a fast radicals recombination inside CD in the case of the -cleavage reaction (t < 100 ns)

It was shown that there is a strong exchange interaction between the ketyl and CD radicals, due to location of ketyl radical inside CD.

Langmuir, 2010, 26 (11), pp 8971–8980

Thank you for your kind attention !

Olesya A. Krumkacheva

International Tomography Center SB RAS

olesya@tomo.nsc.ru

Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

Photochemistry in Octa Acid capsulesGuest H

H

H

H

O

10 9 8 7 6 5 4 3 2 1 0 -1

PhCH2C

6H

4CH

3

ppm

PhCH2C(O)C

6H

4CH

3

Guest CH3 CH3

O

H H

10 9 8 7 6 5 4 3 2 1 0 -1

ppm

Ph-C(H)(CH3)-C(H)(CH

3)-Ph

H

H

O H

H

H

HH

H

O

H

H

O

CH3

H

CH3

O H

H3C

CH3

H

CH3

H

Radical, rotating in the capsule

Radical, fixed into the capsule

Varying the correlation between host and guest sizes we can manage the chemical reaction way

NMRmethod

CIDNPmethod

CIDNPmethod

326 328 330 332 334 336

2.8 mT

b)

a)

1.1 mT

c)

5.7 mT

Магнитное поле, мТ

2.2 mT

1.9 mT

a)

b)

330 332 334 336 338 340 342 344

Магнитное поле, мТ

5.7 mT

c)

ХПЭприфотолизе кетонов в водном раствореЦД

ДБК ДОБ

Нет сигнала ХПЭ

Быстрая рекомбинация радикалов в ЦД

Кетильный радикал (a) + радикал от ЦД (c)

БФ

S-T- механизм формирования поляризации

O

CH3