Reaction dynamics of photochromic dithienylethene derivatives Original Research Article

The reaction dynamics of the photochromic ring-opening reaction of 1,2-bis(5-formyl-2-methyl-thien-3-yl)perfluorocyclopentene (CHO-BMTFP) in dichloromethane solution was investigated using femtosecond transient absorption spectroscopy. The data were analyzed in terms of a model potential and single-electron density matrices, which were calculated using the collective electronic oscillator (CEO) approach and the INDO/S semiempirical Hamiltonian. The S0–S1 and S0–S2 transitions of the closed isomer were resonantly excited using 120 fs pump pulses at 610 and 410 nm, respectively. A temporally delayed white light continuum probe pulse monitors the decay of the S1 or S2 state as well as the recovery of the S0 state. Within the first picosecond after excitation, CHO-BMTFP was observed to undergo a fast structural relaxation along the S1 potential energy surface into a minimum constituting a precursor of the ring-opening process. The rather long lifetime of the precursor, τ2=13 ps, was consistent with the calculated potential barrier in front of the conical intersection with the S0 potential energy surface, which may arise from stabilization of the nearly planar closed isomer by an efficiently delocalized π-electron system.