JOURNAL OF LUMINESCENCE EISEVIER Journal of Luminescence 76&77 (1998) 90-94 Femtosecond transient absorption studies on photochromism of dithienylethene derivates J. Em”, A.T. Bensb, A. Bock”, H.-D. Martinb, C. Kryschi”,* a Lehrstuhl ftir Festkiirperspektroskopie (IPkM), Heinrich-Heine-Uniaersitiit, D-40225 Diisseldo$ Germany b Institut fu’r Organ&he Chemie und Makromolekulare Chemie I, H&rich-Heine-Universitiit, D-40225 Diisseldocf; Germany Abstract The kinetics of the photochromic ring-opening reaction of benzoylLphenylLetheny1 substituted 1,2-bis[2-methyl- thien-3-yllperfluorocyclopentene (bpe-BMTFP) in solution was investigated using fs transient absorption spectroscopy. While resonant excitation of the So-S1 transition of the closed isomer is performed with a 100 fs pump pulse, the decay of the S1 state as well as the recovery of the So state were monitored using a white-light continuum probe pulse. Within the first 5OOfs after excitation bpe-BMTFP is observed to undergo fast structural relaxations into two minima of the SI potential-energy surface which differ significantly in their lifetimes ~~ and ‘rdis. While the long lifetime of about 5dir = 9 ps is assigned to the potential minimum reached by the disrotatory twist motion, the other minimum with 5L = 1.9 ps is ascribed to the prestate of the ring-opening process. JC~m 1998 Elsevier Science B.V. All rights reserved. Keywords: Photochromism; Reaction kinetics; Femtosecond laser spectroscopy; Dithienylethene derivates 1. Introduction Diarylethene derivates developed by Irie et al. [1,2] and Lehn with coworkers [3,4] constitute a new and technologically promising class of photochromic compounds. The photochromic re- action following the Woodward-Hoffmann rule occurs photochemically in a conrotatory mode. Fig. 1 shows the absorption spectra of the open and closed isomer of benzoyl-phenyl-ethenyl sub- stituted 1,2-bis[2-methyl-thien-3-yllperfluorocyclo- pentene (bpe-BMTFP) synthesized by us. The structural formulae of these isomers are depicted in *Corresponding author. Tel.: +492118114813; fax: + 49 2118114850; e-mail: ckryschi@clio.rz.uni-duesseldortde. the inset of Fig. 1. The assignment of the absorp- tion bands to the singlet states is based on a semiempirical AM1 calculation of the geometry- optimized closed isomer of BMTFP [S]. Upon UV irradiation the open isomer undergoes a ring-clos- ure reaction, while the ring-opening reaction is in- duced by excitation with visible light (550-700 nm). Previous time-resolved studies of this reaction type have shown that both ring-opening and ring- closure processes occur in the ps time domain [6-lo]. Until now, reaction kinetics of only two diethienylethene derivates, namely 1,2-bis(2,4,5-tri- methyl-3-thienyl) maleic anhydride and a thiophene oligomer with a dithienylethene structure, have been investigated on the ps and fs timescale, respec- tively [9,10]. The ps transient absorption spectro- scopy studies on the maleic anhydride derivate in 0022-2313/98/$19.00 ($2 1998 Elsevier Science B.V. All rights reserved PI1 SOO22-23 13(97)00269-X