Exciton-like and charge-transfer states in cyanine-oxonol ion pairs. An experimental and theoretical study

Absorption and fluorescence emission properties of cyanine-oxonol mixed dyes, i.e., salts formed by a cationic cyanine with an anionic oxonol as counterion, were investigated both theoretically and experimentally in order to probe the effects of ion pairing occurring in low-polarity solvents. We analyzed, in particular, three model systems (S1, S2, and S3) built combining thiacarbo- and thiadicarbocyanine (C1, C2) with two vinylogous oxonol chromophores (A1/A1F, A2). in systems S1 (C1-A1) and S2 (C2-A2), where the visible absorption bands of the individual ions are almost superimposed, the formation of ion pairs gives rise to marked spectral alterations traceable to interchromophore resonance interactions. On the contrary, in system 53 (C2-A1F), whose components absorb widely apart, the spectrum of the contact ion pair and that of the dissociated form differ only for the relative band intensities. In both cases, however, contact ion pairing results in complete quenching of the emission of the chromophoric units. Such behaviors, emphasized by absorption and fluorescence emission and excitation spectra of both the mixed dyes and their components in solvents of different polarities, were the subject of a theoretical study based in particular on the calculation of structures and electronic spectra of the contact ion pairs. Molecular dynamics (MD) simulations and local full geometry optimizations led to two types of structures characterized by almost parallel and orthogonal arrangements of the long molecular axes. CS INDO SCI calculations using both arrangements emphasized the role of the exciton coupling between the local HOMO-LUMO excitations of the two chromophoric units. The most striking spectral characteristics in low-polarity solvent turned out to be explainable in terms of parallel type arrangements, even if an appreciable contribution of the orthogonal type structure was to be invoked for a complete interpretation of the S1 spectral properties. In all contact ion pairs, independently of the structure, the lowest excited singlet is a forbidden anion --> cation charge transfer (CT) state explaining why no fluorescence emission was observed in such systems.