Solvatochromism of novel donor–π–acceptor type pyridinium dyes in halogenated and non-halogenated solvents

Donor–π–acceptor type pyridinium dyes that have been newly synthesized showed bathochromic shift of the absorption band with decreasing dielectric constant (εr) of solvent (negative solvatochromism), and the bathochromic shifts in halogenated solvents were larger than those in non-halogenated solvents of similar εr values. The influences of halogenated solvents on the large bathochromic shifts are studied on the basis of 1H NMR measurements, semi-empirical molecular calculations (AM1 and INDO/S using the SCRF Onsager model), and absorption spectral measurements of the dyes adsorbed on nanocrystalline TiO2 film immersed in solvents.