PICOSECOND TIME-RESOLVED FLUORESCENCE SPECTROSCOPY OF THE PHOTOCHROMIC REACTION OF SPIROPYRAN IN LANGMUIR-BLODGETT-FILMS

The photochromic reaction of spiropyran <--> merocyanine has been studied with 1'-octadecyl-3',3'-dimethyl-6-nitrospiro-[2H-1-benzopyran-2,2'-indoline] (SSP) incorporated in Langmuir-Blodgett (LB) films prepared in a mixed film with stearic acid, arachidic acid, and tripalmitine. Time-resolved fluorescence spectra and decay curves of SSP and its photoisomer, merocyanine (SMC), were measured with a picosecond time-correlated, single-photo-counting apparatus. When SSP in LB films was excited at 360 nm, two fluorescence bands were observed: one has a maximum at 660 nm, and the other has a relatively weak band at 530 nm. We assigned them respectively to the fluorescence band of SMC and an intermediate X as a precursor of SMC. The rigidity of the medium surrounding photochromic molecules is an important factor in determining the rate of reversible photoconversion of SSP <--> SMC.