Dynamics of the excited states of chromenes studied by fast and ultrafast spectroscopies

In this paper, the photodynamics of three chromenes (2,2-spiro-adamantyl-7,8-benzo(2H)chromene, 2,2-diphenyl(2H)chromene and 2,2-diphenyl-5,6-benzo(2H)chromene) has been investigated by nano- and femtosecond time resolved techniques in hydrocarbon solutions at room temperature. Using pump–probe techniques, ultrafast steps of molecular dynamics characterizing the photoresponse of chromenes have been investigated: the breakage of the C–O bond was found to occur within a few hundred femtoseconds producing a short-lived transient that converts to the cisoid-cisopen form in less than one picosecond and then to the metastable transoid-cisstructure within a few picoseconds. The effect of different excitation wavelengths on the formation rate of the first shortest-lived transient of 5,6DPBC is in agreement with the model previously proposed to explain the wavelength dependence of the reaction and emission quantum yields in photostationary experiments. The results obtained in the nanosecond time domain show that the triplet marginally participates in the photoreaction for only one of the studied compounds (2,2-spiro-adamantyl-7,8-benzo(2H)chromene).