SPECTROSCOPIC AND PHOTOCHEMICAL STUDY OF INCLUSION COMPLEXES OF DIMETHOXYBENZENES WITH CYCLODEXTRINS

The effect of addition of the cyclodextrins (CDx) alpha-CDx and beta-C Dx on the photophysics and photochemistry of the dimethoxybenzenes (DM B) 1,4-DMB and 1,2-DMB in aqueous solution has been investigated by me ans of absorption, circular dichroism, fluorescence, and nanosecond tr ansient absorption spectroscopies. The experimental results are discus sed in the light of model calculations on the structure and the circul ar dichroism spectra of the DMB-CDx complexes. The association of 1,2- DMB with both CDxs is much weaker than that of 1,4-DMB owing to steric hindrance. With beta-CDx, 1,4-DMB forms 1:1 complexes with a high ass ociation constant (K = 630 M(-1)) in spite of incomplete inclusion. Co mplexation of 1,4-DMB with alpha-CDx strongly affects the photophysica l properties of the guest: fluorescence is enhanced and triplet state decay slowed by a factor of 10 or more, depending on the decay reactio n mode. Fluorescence enhancement is mainly due to a reduction of the r ate constant for internal conversion. The effect of complexation on tr iplet decay was exploited for a detailed study of the association mech anism in terms of the consecutive formation of 1:1 and 1:2 complexes. Temperature-dependent measurements showed that both association steps are controlled by enthalpy-entropy compensation, the first step being entropy-driven and the second one enthalpy-driven. At lower temperatur es (T < 50 degrees C), 1:2 complex formation predominates. These findi ngs are supported by the circular dichroism spectra and the model calc ulations. There is no significant effect of complexation with either a lpha- or beta-CDx on hydrated electron ejection from 1,4-DMB, but the e(aq)(-) decay is accelerated in the presence of beta-CDx. This is ten tatively attributed to spatial correlation between e(aq)(-) and radica l cations induced by complexation.