Modification of photochemical reactivity by cyclodextrin complexation: alteration of photochemical behavior via restriction of translational and rotational motions

The photochem. behavior of alkyldeoxybenzoins $PhCOCHPhCH_2CH_2R$ (R = H, Me, Et, hexyl, decyl) has been investigated in isotropic org. solvents, in aq. cyclodextrin solns., and bound to cyclodextrin in the solid state. Norrish type I and type II reactions occur in these media and the product distribution resulting from these primary processes are dependent on the medium. In org. solvents, type I and type II products are obtained in equal amts. In aq. cyclodextrin soln., type II products are formed in large excess. In the solid state, type II products constitute more than 90% of the product distribution. Ratios of products resulting from elimination and cyclization from the type II 1,4-diradical are also altered by the host cyclodextrin. Conformational and super-cage effects have been invoked to rationalize the dramatic alteration of the photobehavior of alkyldeoxybenzoins by cyclodextrin