Effect of inclusion in sulfobutylether-β-cyclodextrin on the rate of merocyanine-spirobenzopyran reversible photochromic transformations

Abstract

To gain insight into the effect of host-guest binding on the negative photochromic behavior, the reversible interconversion between a merocyanine dye and its spirobenzopyran (SP) isomer was studied in the cavity of sulfobutylether-β-cyclodextrin having on average 6.4 degrees of substitution (SBE6.4βCD). Depending on the acidity of the solution, trans-merocyanine (trans-MC) or its protonated form (trans-MCH+) were the thermodynamically most stable species both in water and in the interior of SBE6.4βCD. The association constant of 1:1 encapsulation significantly diminished in the series of SP > trans-MCH+ > trans-MC. Inclusion complex formation accelerated the visible-light-induced conversion to SP and efficiently slowed down the thermal back reaction in acidic and slightly basic medium alike. A good linear correlation was found between the logarithm of the frequency factors and the activation energies for the SP transformation to the colored most stable isomer within various types of macrocycles.