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

Olesya A. Krumkacheva † , Vitaly R. Gorelik † , Elena G. Bagryanskaya † , Natalia V. Lebedeva ‡ and Malcolm D. E. Forbes ‡. † International Tomography Center, Institutskaya 3a, Novosibirsk 630090, Russia

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

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  1. Olesya A. Krumkacheva†, Vitaly R. Gorelik†, Elena G. Bagryanskaya†, Natalia V. Lebedeva‡ and Malcolm D. E. Forbes‡ † 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 Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

  2. The Host-Guest systems as “nanocontainers” for the photochemical reaction Carrying out photochemical reaction in organized media can lead to theproduct structure changing hν It can be used for the directional synthesis • Alteration of the re-encounter statistics and average distance between partners in radical pair • Alterationthe nature of the excited states • Reactivity between host and guest The application the Chemical Induced Dynamic Nuclear Polarization (CIDNP)andTR EPR methods allows to obtain information about the mechanism of photochemical reaction inaccessible by other methods.

  3. TR EPR method EPR signal S Laser pulse R1(H)••R2 T h M *S M M R1(H)••R2 T •R2 R1(H)• absorption emission nonequilibrium population of spin level Direct observation of the short-lived paramagnetic intermediates (triplet molecules, radicals) formed during photolysis Investigationof the radical’s mobility

  4. CIDNP method S R1(H)••R2 T h M *S M M R1(H)••R2 T •R2 R1(H)• Escape products In-cage products enchanced absorption emission • Advantages: • Investigation of the mechanisms of photochemical radical reactions • Possibility to detect product with low yield (10-6 M/l) • Detection of intermediate product

  5. Goal: Investigation of the influence of β-cyclodextrin presence on the mechanism of the ketones photolysis: dibenzyl ketone (DBK), deoxybenzoin (DOB) and benzophenone (BP) Research Methods: For the Complex formation : NMR and UV spectroscopy For theMechanism of photochemical reaction: TR EPR and CIDNP methods

  6. Ketone’s photolysis

  7. Photolysis of b-CD / benzophenonecomplex Guest Ketyl radical Observed TR EPR spectrum NMR spectrum CD radical CIDNP spectrum PhCOPh CD Magnetic field, mT • hydrogen abstraction fromCD cavity interior • location of ketyl radical inside CD cavity • increasing of the Radical Pair lifetime • Strong exchange interaction in radical pair

  8. Photolysis of b-CD / deoxybenzoincomplex Ketyl radical NMR spectrum Guest Observed TR EPR spectrum (PhCH2)2 CD PhCH2COPh b-CD radical CIDNP spectrum Magnetic field, mT • α-cleavage and hydrogen abstraction • location of ketyl radical inside CD cavity • Increasing of radical pair lifetime • Strong exchange interaction in radical pair • formation of rearrangement product

  9. Photolysis of b-CD / dibenzyl ketonecomplex Guest NMR spectrum NO! TR EPR Signal (PhCH2)2 (PhCH2)2CO CIDNP spectrum (PhCH2)2CO • α-cleavage • fast radicals recombination inside CD • (t < 100 ns)

  10. Results • 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 a-cleavage and the BP triplet shows exclusively H-atom abstraction. • Observation of rearrangement product of the radicals from a-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 a-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

  11. Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation Olesya A. Krumkacheva International Tomography Center SB RAS olesya@tomo.nsc.ru Thank you for your kind attention !

  12. Photochemistry in Octa Acid capsules Guest Guest NMR method CIDNP method Radical, rotating in the capsule Varying the correlation between host and guest sizes we can manage the chemical reaction way CIDNP method Radical, fixed into the capsule

  13. ХПЭ при фотолизе кетонов в водном растворе ЦД ДБК ДОБ БФ Нет сигнала ХПЭ Быстрая рекомбинация радикаловв ЦД Кетильный радикал (a) + радикал от ЦД (c) S-T-механизм формирования поляризации

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