Dipole moment determination of 4-[N-(5,6,7,8-tetrahydroisoquinolinium-5-ylidene)methyl]-N,N-dialkylaniline iodides in solution.

Electronic absorption and fluorescence spectra of eight hemicyanine dyes were recorded at room temperature in several solvents of different polarity. The spectral data were analyzed using the theory of solvatochromism, based on a dielectric continuum description of the solvent and the classical Onsager cavity model. They were used to evaluate the excited state dipole moment using methods applied by McRae, Lippert, Mataga and Bakhshiev. DFT calculations were carried out to estimate the ground state dipole moment and Onsager cavity radius. The difference in the excited and ground state dipole moments (μe-μg) of the molecule under study is positive. It means that the excited states of the dyes are more polar than the ground state. The increase in dipole moments upon excitation is explained in terms of the nature of the excited state and its resonance structures.