Comparison of the photoisomerization and relaxation processes studied by time resolved spectroscopic methods in two polymethine dyes with the same polymethine chromophor

A streptocyanine dye (BMC) and an oxadicarbocyanine (DODCI) were studied in the picosecond and millisecond time region. The comparison showed that for both dyes the following parameters are nearly equal: the radiative lifetime, the difference between the absorption maxima of the stable form and the photoisomer, the activation energy and the frequency factor for the thermal backreaction and even the energy barrier in the excited state.The dyes show strongly different kinetics in the excited state. BMC relaxes faster by about 2 orders of magnitude than DODCI and shows a much higher photoisomerization rate. In accordance with Eyring's theory the higher relaxation and isomerization rate of BMC is connected with an entropy increase in the transition state.