Picosecond and nanosecond photo-dynamics of a naphthopyran merocyanine

For the first time the photochemistry of the trans, trans (TT) to trans, cis (TC) isomerisation for merocyanines (MCs) of a chromene (CHR) compound has been investigated using steady-state nanosecond and time-resolved femtosecond irradiation. The dominant photo-transformation of the TT isomer by these irradiations is conversion to TC. The yield of the TT to TC photoreaction is inherently low in all solvents studied due to efficient ground state recovery in competition with inter-system crossing, but it is lowest of all in polar solvents. Several intermediate states have been identified in the first 6 ns of the reaction that we tentatively assigned, including a possible twisted MC T₁ state. We assign other distinct states to TT S₁, TT T₁ and a further species to TC T₁. The full photochemical conversion from TT to TC is a slower process than 6 ns. The presence of oxygen marginally increases the overall bleaching of the TT isomer and slightly enhances TC formation.