Study of photophysical properties of a series of polymethine dyes by femtosecond laser photolysis

• Excited-state dynamics of 12 polymethine dyes was studied with pump–probe spectroscopy.
• 4 groups of compounds with different terminal heterocycles were studied.
• The influence of several structural factors on the photophysical properties was evaluated.

The excited-state dynamics of 12 symmetric polymethine dyes in acetone was studied with femtosecond pump–probe spectroscopy. Four groups of compounds with different terminal heterocycles were studied: indoles, indoles with cyclic group attached to the nitrogen atom, benzimidazoles and benzo[cd] indoles. Within each group, the length of polymethine chain between the terminal groups was varied. The influence of electron-donating ability of the terminal groups and the length of the polymethine chain on the excited-state dynamics of the compounds was evaluated. The length of the polymethine chain was found to have an obvious effect both on the distance between the S1 ← S0 and Sn ← S1 transition bands and the lifetime of the excited state. The latter parameter is largely affected by the nature of terminal groups, most importantly, their electron-donating ability. In addition to experimental results, quantum chemical calculations were performed to provide with complementary insights on the excited states of investigated compounds.