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 perfluoro- . cyclopentene CHO-BMTFP in dichloromethane solution was investigated using femtosecond transient absorption spec- troscopy. 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 INDOrS semiempirical Hamiltonian. The S -S and 01 S -S transitions of the closed isomer were resonantly excited using 120 fs pump pulses at 610 and 410 nm, respectively. A 02 temporally delayed white light continuum probe pulse monitors the decay of the S or S state as well as the recovery of the 12 S state. Within the first picosecond after excitation, CHO-BMTFP was observed to undergo a fast structural relaxation 0 along the S potential energy surface into a minimum constituting a precursor of the ring-opening process. The rather long 1 lifetime of the precursor, t s 13 ps, was consistent with the calculated potential barrier in front of the conical intersection 2 with the S potential energy surface, which may arise from stabilization of the nearly planar closed isomer by an efficiently 0

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