Specific solvatochromism of D–π-A type pyridinium dyes bearing various counter anions in halogenated solvents

Donor–π-acceptor type pyridinium dyes OD1–OD4 bearing various counter anions (X−=Cl−, Br−, I−, or BPh 4 − ) have been designed and developed to gain greater insight into the influences of the electronic structures of D–π-A type pyridinium dye on the specific solvatochromism in halogenated solvents: the intramolecular charge transfer absorption bands of the dyes OD1–OD4 in halogenated solvents show a large bathochromic shift compared with those in the non-halogenated solvents. This study revealed that the halogenated solvents can significantly enhance the intramolecular charge transfer characteristics of D–π-A type pyridinium dye with soft counter anion, leading to large bathochromic shift of absorption band in halogenated solvents, which is in good agreement with an increase in the energy of halogen–halide anion interaction for the type of the halogen atom of halogenated solvents.