Picosecond Time-Resolved Spectroscopic Studies of Phototriggers and Polyhalomethanes

Abstract

Polyhalomethanes are important sources of reactive halogens in the natural environment and it is important to understand their photochemistry in both gas and condensed phase environments. While the gas phase photochemistry has been extensively studied, there is much less corresponding condensed phase work. Here, we report picosecond time-resolved resonance Raman (ps-TR3) experiments done to investigate the photochemistry of polyhalomethanes in aqueous environments. We found that photolysis of polyhalomethanes in the condensed phase leads to fast geminate recombination of the radical fragments to form some high energy isomers that can undergo O-H insertion reactions with water.[1] Efficient phototrigger compounds are needed for the very fast and localized release of biological stimulants that can be employed for real-time monitoring of physiological responses in biological systems. Here, we report a study of the photorelease reactions of pHP phototriggers in order to better understand the mechanism(s) for the photodeprotection reactions that occur in aqueous environments. Time-resolved resonance Raman (TR3) as well as time-resolved transient absorption (TA) and timer-resolved fluorescence (TRF) spectroscopies were used to study the structure and dynamics of the photophysical and photochemical processes involved in the photodeprotection reactions of interest.[2,3] The ps-TR3 and TA experiments found the triplet decay kinetics are highly solvent and leaving group dependent for the p-hydroxyphenacyl (pHP) phototrigger compounds.[2,3] The TR3 method was used to monitor the product formation dynamics for the first time.[3] Our results indicate the cleavage and product formation occur in a step-wise manner in these compounds and we have elucidated a mechanism for the photorelease mechanisms these compounds. References [1] W. M. Kwok, C. Zhao, Y.-L. Li, X. Guan, and D. L. Phillips, J. Am. Chem. Soc. 126, 3119-3132 (2004). [2] C. Ma, W. M. Kwok, W. S. Chan, P. Zuo, J. T. W. Kan, P. H. Toy and D. L. Phillips, J. Am. Chem. Soc. 127, 1463-1472 (2005). [3] C. Ma, W. M. Kwok, W. S. Chan, J. T. W. Kan, P. H. Toy and D. L. Phillips, J. Am. Chem. Soc. 128, 2558-2570 (2006).