Theoretical study of solvent effects on the intramolecular charge transfer of a hemicyanine dye

4-[2-(4-Dimethylaminophenyl)ethenyl]-1-methylpyridinium iodide (HR) is a solvatochromic dye and is widely used in Langmuir-Blodgett (LB) film preparations. Quantum chemical calculations combined with a cylindrical solvation model are used to investigate the possibility of TICT (twisted intramolecular charge transfer) state formation by rotating part of the molecule around several different chemical bonds. The results account for most of the experimental observations and partially confirm the assumption that the structure of HR is a compromise between benzenoid and quinoid resonance forms. It is revealed that the TICT state can be formed by rotating the dimethylamino group to the perpendicular position in polar solvents. But this probability should be very small due to the relatively high energy barrier to the rotation in the excited state. Rotations of the pyridyl ring and the aniline ring enhance the charge transfer in the first excited state. There is a small energy barrier to the rotation of the pyridyl ring in the first excited state. In polar solvents, then is no barrier to rotation of the aniline ring in the excited state. The high-energy barrier to rotation around the central double bond is the cause of the low photoisomerization yield of HR.