Femtosecond dynamics of a simple merocyanine dye: Does deprotonation compete with isomerization?

The primary photochemistry of the trans isomer of a simple merocyanine dye of the stilbazolium betaine type 1-methyl-4-(4-hydroxytyryl)pyridinium betaine (M(trans)) and its conjugate acid MH ⁺(trans) in aqueous solution is studied by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primary photodynamics is determined to be k = 1.1 x 10¹² s^-1 for M(trans) at pH 10 and 0.8 x 10¹² s^-1 for MH(trans)⁺ at pH 6. This was assigned to either conformational changes or a simple vibrational relaxation before the actual isomerization takes place. Wavelength excitation studies give support for the former assignment. These results are 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 aqueous solutions at pH 6 or pH 0, suggesting that the deprotonation occurs on a longer time scale than the picosecond time domain. This is in agreement with present theories of intermolecular proton-transfer reactions, which require solvent reorganization as well as the time of deprotonation estimated from the pK(a) value of this molecule in the excited state. The results of our MO calculations on the electronic structure of these two compounds could account for the fact that, while MH⁺(trans) photoisomerizes, its deprotonated form does not.