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 bet aine (M-trans) and its conjugate acid MHtrans+ in aqueous solution is studi ed by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primary photodynamics is determined to be k = 1.1 x 10(12) s(-1) for M-trans at pH 10 and 0.8 x 10(12) s(-1) for MHtrans+ at pH 6. This was ass igned to either conformational changes or a simple vibrational relaxation b efore the actual isomerization takes place. Wavelength excitation studies g ive support for the former assignment. These results an discussed in terms of the recent results found for the primary processes of retinal in bacteri orhodopsin. Time-resolved transient measurements show that no excited-state deprotonation of MHtrans+ occurs in aqueous solutions at pH 6 or pH 0, sug gesting that the deprotonation occurs on a longer time scale than the picos econd time domain. This is in agreement with present theories of intermolec ular proton-transfer reactions, which require solvent reorganization as wel l as the time of deprotonation estimated from the pK(a) value of this molec ule in the excited state. The results of our MO calculations on the electro nic structure of these two compounds could account for the fact that, while MHtrans+ photoisomerizes, its deprotonated form does not.