The influence of meso-substitution on the photophysical behavior of some thiacarbocyanine dyes in dilute solution

Fluorescence upconversion experiments were performed on monomer solutions of a number of structurally similar thiacarbocyanine dyes. The results from these time-resolved experiments combined with the data obtained from steady-state absorption and emission and H-1 NMR spectra allowed us to assign the components with different decay times to distinct isomers of the thiacarbocyanine dye molecules. Furthermore, it was possible to relate the substitution pattern of the dye molecules to their photophysical properties. For all mesosubstituted dyes, a short-lived component of either 3.5 or 5.7 ps could be assigned to a mono-cis conformation of the thiacarbocyanines. This component emits at longer wavelengths. A second component ranging from 15 to 71 ps and emitting at shorter wavelengths was attributed to the all-trans conformation of the mesosubstituted trimethine dyes. Time-resolved emission spectra revealed in a very clear and direct way the evolution of the contribution of the two conformations to the spectra. The size and/or solvation of the N,N ' -substituents have a parallel influence on the decay time of the fast and the slow decaying component. Besides the N,N ' -substituents also the meso-substituent has an influence on the decay time and amplitude of the longer-lived component.