I. Baraldi et al., Exciton-like and charge-transfer states in cyanine-oxonol ion pairs. An experimental and theoretical study, J PHYS CH A, 105(19), 2001, pp. 4600-4610
Absorption and fluorescence emission properties of cyanine-oxonol mixed dye
s, i.e., salts formed by a cationic cyanine with an anionic oxonol as count
erion, were investigated both theoretically and experimentally in order to
probe the effects of ion pairing occurring in low-polarity solvents. We ana
lyzed, in particular, three model systems (S1, S2, and S3) built combining
thiacarbo- and thiadicarbocyanine (C1, C2) with two vinylogous oxonol chrom
ophores (A1/A1F, A2). in systems S1 (C1-A1) and S2 (C2-A2), where the visib
le absorption bands of the individual ions are almost superimposed, the for
mation 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 in
tensities. In both cases, however, contact ion pairing results in complete
quenching of the emission of the chromophoric units. Such behaviors, emphas
ized by absorption and fluorescence emission and excitation spectra of both
the mixed dyes and their components in solvents of different polarities, w
ere the subject of a theoretical study based in particular on the calculati
on 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 arrang
ements of the long molecular axes. CS INDO SCI calculations using both arra
ngements 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 te
rms of parallel type arrangements, even if an appreciable contribution of t
he orthogonal type structure was to be invoked for a complete interpretatio
n 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 obse
rved in such systems.