| Abstract
| - Fluorescence upconversion experiments were performed on monomer solutions of a number of structurallysimilar thiacarbocyanine dyes. The results from these time-resolved experiments combined with the dataobtained from steady-state absorption and emission and 1H NMR spectra allowed us to assign the componentswith different decay times to distinct isomers of the thiacarbocyanine dye molecules. Furthermore, it waspossible to relate the substitution pattern of the dye molecules to their photophysical properties. For all meso-substituted dyes, a short-lived component of either 3.5 or 5.7 ps could be assigned to a mono-cis conformationof the thiacarbocyanines. This component emits at longer wavelengths. A second component ranging from15 to 71 ps and emitting at shorter wavelengths was attributed to the all-trans conformation of the meso-substituted trimethine dyes. Time-resolved emission spectra revealed in a very clear and direct way the evolutionof the contribution of the two conformations to the spectra. The size and/or solvation of the N,N‘-substituentshave 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-livedcomponent.
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