Exciton-Like and Charge-Transfer States in Cyanine−Oxonol Ion Pairs. An Experimental and Theoretical Study

Abstract

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 S3 (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.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

“…124 The polymethine family is extremely broad and also includes anionic structures (e.g. oxonols), [125][126][127] neutral D-A merocyanines (e.g. 32), [128][129][130][131][132] zwitterionic squaraine (e.g.…”

Investigation of Low-Temperature Fluorescence and Absorption Spectra of Highly Stable Mero-Cyanines Derivatives of 2, 6-Di-Tert-Butyl-4-[R]-Cyclohexa-2, 5-Dienone

“…The neutral merocyanines as donor-acceptor -electron conjugated molecules are well-known highly sensitive sensors on solvent polarity [1,2]; and hence they are used upon quantumchemical modelling of the solvatochromism [2,3]. Recent growing interest in the merocyanines is connected with use of the similar compounds as highly effective materials for non-linear optics [4].…”

Energy Transfer and Fluorescence Quenching in Complexes of Polymethine Dyes with Human Serum Albumin¶

“…Anionic and cationic polymethine dyes were used as the reactants which, as shown earlier (8), can efficiently bind to HSA. We found recently (8) the following essential features of such binding: (1) complexation with HSA leads to a red shift of polymethine dye absorption spectra and, in most cases, to a growth of the fluorescence quantum yield; (2) the binding constants for anionic polymethine dyes are much higher than those for the cationic dyes and are around (5–6) × 10 7 M −1 ; (3) interaction of meso substituted anionic thiacarbocyanines with HSA results in cis → trans isomerization of the dye monomer and a steep rise of dye fluorescence because of rigidity of the trans ‐form; and (4) in many cases, binding to HSA is accompanied by formation of nonfluorescent aggregates of the dyes (at low [HSA]). The aggregates formed by the dyes alone are decomposed in the presence of HSA.…”

“…To avoid this problem, the donor and acceptor molecules can be brought to close distance, in particular, by electrostatic attraction. This was successfully achieved in the case of cationicanionic polymethine dyes, in which EET and PET between the dye counterions were detected in low‐polarity solvents, where the dyes form ion pairs (1–3). Another possibility is found in biological systems, where photosynthetic plants and bacteria use EET and PET for the collection, conversion and storage of light energy (4).…”