Reinvestigation on the photoinduced aggregation behavior of photochromic spiropyrans in cyclohexane

Three photochromic spiropyrans (SP-1, SP-16, SP4-SP) are investigated at carefully controlled condition. Special attentions are paid to the aggregation behavior and its effect on thermal decay of the colored form which is formed under UV irradiation. The three compounds show normal photochromism in CHCl3. In cyclohexane solution, the spectrum of colored form of SP-16 has structures in the visible region. Detail analysis indicates that its photoinduced merocyanine form cannot form aggregate and the structures of the spectrum are believed to come from the cis/trans isomers of merocyanine form. For SP-1 in cyclohexane solution, it is found that only the dimer is formed under N-2 laser irradiation at 8 degrees C, apart from the merocyanine monomer form. From E-diagrams and decoloration kinetics, it is known that only one linearly independent process exists in thermal decay of the colored form and there is an equilibrium between merocyanine monomer (B), colorless form (A) and the dimer (AB). In the decoloration process, the decoloration of B causes the shift of the equilibrium and which can further cause the dissociation of AB. This is the main route for the decoloration of AB. And all these results provide direct evidences that the composition of dimer is AB but not BB. From the effect of A concentration on the decoloration rate of the system, the formation constant (K) of AB is estimated to be 3.2 x 10(4). For a bis-indolino-spirobenzopyran supramolecular system SP-4-SP, the result indicates that it can promote the formation of dimer and retard the decoloration of the dimer thus formed efficiently. (C) 1998 Elsevier Science S.A.