Structure of rhodanine cyanine dyes, spectroscopy and performance in photographic emulsions

Abstract

The synthesis is reported of eight pyridine–, benzothiazole– and benzimidazole–rhodanine zeromethine merocyanines as blue–sensitising dyes (1–8), and of three benzoxazole-rhodanine dimethine merocyanines as green-sensitising dyes (9–11). The compounds were characterised from UV–VIS, mass spectroscopic and NMR data, and the purity of the products was analysed using HPLC. Oxidation potentials were measured using cyclic voltammetry. Ionisation potentials and electron affinities were obtained by semi-empirical MOPAC calculations. The charge-density distributions of the pyridine and benzothiazole rhodanine dyes are in agreement with assignments in NMR spectroscopy. A linear dependence is found between the oxidation potential and ionisation potential, as well as between the reduction potential and electron affinity. Reflection spectra of 6 and 7 coated on photographic emulsions showed J-aggregate absorption at λ= 450 and 465 nm, respectively. The sensitising properties of the merocyanine dyes were evaluated in actual photographic T-grain emulsions.