Reaction dynamics of photochromic dithienylethene derivatives

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.