Spectroscopic and Theoretical Investigation of a Series of Pyridinium-Betaine Dyes

Pyridinium-betaine dyes are a fascinating class of molecules displaying a marked solvatochromism due to the presence of low-lying intramolecular charge-transfer (ICT) excited states. The remarkable reduction of the dipole moment on going from the ground state to the excited state, which has been used for the quantitative determination of solvent polarities, also induces large first-order hyperpolarizability and thus large non-linear optical (NLO) response. The chemistry, physicochemical properties and applications of pyridinium-betaine dyes have been recently reviewed [1]. Among all the molecules exhibiting photoinduced ICT properties, 2-(1-pyridinio)benzimidazolate (3 in Fig. 1) has received much attention because of its unusually large dipole moment (+10.33 D for the ground state in dioxane) and due to a large hypsochromic shift of the predominant absorption band in polar solvents [2,3]. We report here the study of a series of pyridinium-betaine dyes based on a combination of steady-state and femtosecond transient absorption spectroscopic measurements in solution, supported by DFT, TD-DFT and CASSCF calculations. The theoretical investigation allows the identification of the excited states involved in the ICT process, analysed in terms of charge distribution and evolution of dipole moments upon photoexcitation. The results of the theoretical investigation are compared to steady-state spectroscopic measurements. The investigation of excited state dynamics by time-resolved femtosecond transient absorption spectroscopy allows verifying if the photoinduced ICT properties of 3 are still present in the parent compounds 2 and 1.