Prev. Article Next Article Table of Contents

Article

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

C. Burda,^† M. H. Abdel-Kader,*^‡ S. Link,^† and M. A. El-Sayed*^†

Contribution from the Laser Dynamics Laboratory, Department of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and National Institute for Laser Enhanced Science, Cairo University, Egypt

J. Am. Chem. Soc., 2000, 122 (28), pp 6720–6726

DOI: 10.1021/ja993940w

Publication Date (Web): June 22, 2000

†

Georgia Institute of Technology.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

‡

National Institute for Laser Enhanced Science.

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 × 10¹² s^-1 for M_trans at pH 10 and 0.8 × 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.