Spectroscopic and photochemical study of inclusion complexes of dimethoxybenzenes with cyclodextrins

The effect of addition of the cyclodextrins (CDx) α-CDx and β-CDx on the photophysics and photochemistry of the dimethoxybenzenes (DMB) 1,4-DMB and 1,2-DMB in aqueous solution has been investigated by means of absorption, circular dichroism, fluorescence, and nanosecond transient absorption spectroscopies. The experimental results are discussed in the light of model calculations on the structure and the circular 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 β-CDx, 1,4-DMB forms 1:1 complexes with a high association constant (K = 630 M^-1) in spite of incomplete inclusion. Complexation of 1,4-DMB with α-CDx strongly affects the photophysical properties of the guest: fluorescence is enhanced and triplet state decay slowed by a factor of 10 or more, depending on the decay reaction mode. Fluorescence enhancement is mainly due to a reduction of the rate constant for internal conversion. The effect of complexation on triplet decay was exploited for a detailed study of the association mechanism 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 temperatures (T < 50°C), 1:2 complex formation predominates. These findings are supported by the circular dichroism spectra and the model calculations. There is no significant effect of complexation with either α- or β-CDx on hydrated electron ejection from 1,4-DMB, but the e_aq^- decay is accelerated in the presence of β-CDx. This is tentatively attributed to spatial correlation between e_aq^- and radical cations induced by complexation.