UV-induced self-assembly of the inclusion complexes formed between a long-chain photochromic spiropyran and cyclodextrins

Photochromic spiropyran with a long chain alkyl substitute can form axial c omplexes with alpha-, beta-, and gamma-cyclodextrin, respectively. The comp lexes show normal photochromism. The novel property of the colored forms of the inclusion complexes is that they can assemble into dimers at relativel y low concentration or J-aggregates at relatively high concentration. For a lpha-, beta-, and gamma-cyclodextrin, gamma(max) of the J-aggregates appear at 700 650, and 630 nm, respectively. The sizes of the cavities of cyclode xtrins have very little effect on the spectra and decoloration kinetics of the dimers, but have great effects on the spectra of the J-aggregates. Unli ke the charge transfer complex of Krongauz, the decoloration process of the dimers or J-aggregates cannot be described by an exponential or a two-expo nential kinetics, but obey half-order kinetics very well. Another result th at can be deduced from the kinetic analysis is that unlike the dimers forme d in apolar solvents or in polymers, which consist of a colored molecule (B ') and a colorless molecule (A') with a composition of A'B', in the present system, the dimer of the inclusion complex consists of two colored molecul es (B). All these results can be interpreted by the proposed structure mode l and decoloration mechanism.