Femtosecond Dynamics of a Simple Merocyanine Dye:  Does Deprotonation Compete with Isomerization?

Abstract

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 × 10 12 s -1 for M trans at pH 10 and 0.8 × 10 12 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.

Comparison of the Dynamics of the Primary Events of Bacteriorhodopsin in Its Trimeric and Monomeric States

“…Femtosecond pump-probe spectroscopy system has been described previously (Logunov et al, 2001;Burda et al, 2000). Briefly, an amplified Ti:sapphire laser system (CPA 1000, Clark MXR, Dexter, MI) was pumped by a diode-pumped, frequency-doubled Nd:Vandate laser (Verdi, Coherent, Palo Alto, CA).…”

Steady-state fluorescence and photo-isomerization study of 1-methyl-4′-(p-N,N dimethyl-amino styryl) pyridinium iodide

“…(5) Burda et al shows that the isomerization of M tr H + in water occurs in a very short picosecond time scale at pH 6 or 0 without any protolytic dissociation but the deprotonation process (pk a is 8.54 and pk n is 1.9 [43]) occurs on a longer time scale than the picosecond time domain (nanosecond or microsecond scale) [44]. This means that the isomerization process of n M tr H + represents the main channel of deactivation processes in water.…”

Influence of water on photophysical properties of N-bromobenzyl- or nitrobenzyl derivatives of substituted 4-hydroxystilbazolium hemicyanines

“…23 Note also that simple hemicyanine dyes like 1-methyl-4-(4¢hydroxystyryl)pyridinium salts undergo photoisomerisation from trans-to cis-isomer. 24 This process can be difficult to monitor, especially because both isomers may absorb in the same wavelength range, being only distinguishable by the values of their molar absorption coefficient. Therefore, we always checked whether the absorption spectra in the beginning and in the end of the experiments were the same, noting no isomerization for compounds 1-9.…”