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| - Femtosecond Dynamics of a Simple Merocyanine Dye: DoesDeprotonation Compete with Isomerization?
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| - The primary photochemistry of the trans isomer of a simple merocyanine dye of the stilbazoliumbetaine type 1-methyl-4-(4‘-hydroxytyryl)pyridinium betaine (Mtrans) and its conjugate acid MH+trans in aqueoussolution is studied by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primaryphotodynamics is determined to be k = 1.1 × 1012 s-1 for Mtrans at pH 10 and 0.8 × 1012 s-1 for MH+trans atpH 6. This was assigned to either conformational changes or a simple vibrational relaxation before the actualisomerization takes place. Wavelength excitation studies give support for the former assignment. These resultsare discussed in terms of the recent results found for the primary processes of retinal in bacteriorhodopsin.Time-resolved transient measurements show that no excited-state deprotonation of MH+trans occurs in aqueoussolutions at pH 6 or pH 0, suggesting that the deprotonation occurs on a longer time scale than the picosecondtime domain. This is in agreement with present theories of intermolecular proton-transfer reactions, whichrequire solvent reorganization as well as the time of deprotonation estimated from the pKa value of this moleculein the excited state. The results of our MO calculations on the electronic structure of these two compoundscould account for the fact that, while MH+trans photoisomerizes, its deprotonated form does not.
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