Time-Resolved and Pulse EPR Study of Triplet States of Alkylketones in beta-Cyclodextrin
Résumé
The triplet states of deoxybenzoin (DOB) and benzophenone (BP) molecules in randomly methylated beta-cyclodextrin (CD) cavity are studied by time-resolved (TR) and pulse electron paramagnetic resonance (EPR). The observed TR EPR spectrum of DOB in beta-CD at 30 K is close to the spectrum measured in polar solvent trifluoroethanol, revealing strong hydration by water molecules. At the same time, TR EPR spectrum of BP in beta-CD corresponds to nonpolar surrounding of the CO-group. The electron spin relaxation times T (1) and T (2) of triplet BP at 30 K measured by pulse EPR are found to be different in beta-CD compared to nonpolar toluene glass. The observed increase of T (2) by up to a factor of four in beta-CD is caused by the lower vibration amplitude of CO-bond of BP due to the confinement in beta-CD. The influence of beta-CD with covalently attached nitroxide on the triplet states of DOB and BP is principally different: the excited triplet states could not be observed by TR EPR due to the efficient quenching of the excited states by nitroxide.