Excited-state dynamics of spiropyran-derived merocyanine isomers

Merocyanine (MC) isomers that are formed after absorption of a UV photon by 1',3'-dihydro-1',3'-3'-trimethyl-6-nitrospiro2H-1-benzopyran-2',2'-(2H)-indole] were studied. Several, predominantly TTC and TTT, merocyanine isomers are present in toluene solution ("T" and "C" indicate trans and cis conformations of the C-C bonds in the methine bridge). Excitation in the MC visible absorption band (at 490, 550, and 630 nm) with 100 fs laser pulses was used to study MC excited-state dynamics. Internal conversion on the picosecond time scale was found to be the dominant relaxation pathway. Excited-state isomerization reactions were also observed. Excitation at 630 nm (assigned to TTC isomer excitation) leads to formation of a third isomer (either CTC or CTT). Excitation at 490 nm (assigned to TTT isomer excitation) leads to more complex excited-state relaxation, including formation of two isomers: TTC (absorption at 600 nm) and CTC or CTT (absorption at 650 nm).