Photochromic reaction and fluorescence of dithienylethenes in the solid state

Photochromic cyclization reaction efficiency and fluorescence of two dithienylethenes in two solid states, colloidal solutions and amorphous films, were studied. Colloidal solutions of dithienylethenes were prepared by dilution of concentrated methanol solution into hot water. The diameter of particles of the colloidal solutions was about 1 mu m according to observation by a scanning electron microscope. Photochromic cyclization reaction efficiency of colloidal solution of cis-1,2-dicyano-1,2-dithienylethene (1) was as high as that of hexane solution of 1. An amorphous film of 1 also shows high efficiency. On the other hand, neither colloidal solution nor polycrystal of 2,3bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (2) was photochromic; only an amorphous film of 2 was colored very slightly by UV irradiation. For the dithienylethenes in the solid states, long-wavelength fluorescence which can be assigned to the colored closed-ring form was observed, while deeply colored hexane solution did not give any long-wavelength fluorescence. Fluorescence spectra and fluorescence decay curves indicated the existence of efficient energy transfer from the open-ring form to the closed-ring form of dithienylethenes in the solid states. (C) 1999 Elsevier Science S.A. All rights reserved.