Evaluation of photodynamic sensitizer photophysics through the transient grating method

Application of the transient grating technique to the study of potential photodynamic therapy agents yields signals whose dependence on the presence of oxygen in solution is profound. A theoretical model describing the time evolution of the grating is derived for photodynamic sensitizers based on a simple three-level kinetic mechanism having two distinct contributions to the diffracted light signal: a density and an excited state population contribution. Here, three zinc phthalocyanines with varying numbers of fluorine substituents and perfluoro-isopropyl groups on the phthalocyanine macrocycle are studied by the transient grating method as potential photodynamic sensitizers. The triplet state lifetime and quantum efficiency for singlet oxygen production of the three phthalocyanines are determined by an analysis of the thermal mode of the transient grating. Data from experiments in acetone are fit to the model and discussed. The advantages of the transient grating method compared to alternative detection techniques of photophysical parameters of photodynamic sensitizers is discussed.