The water effect on the spectral properties of porphyrin sensitizers

Porphyrin derivatives, which played a key role in photodynamic therapy (PDT), exhibit different photophysical and photochemical properties in aqueous solution of organism and organic solvents. Limited by the poor water-soluble, present studies on porphyrin derivatives covered the real performance in aqueous solution of organism. In this study, the water effect on the properties of porphyrin derivatives was investigated by the water proportion evolution spectroscopy. The variation in fluorescence intensities of hematoporphyrin monomethyl ether (HMME) in solvents with different proportions of methanol and water were analyzed. Corresponding absorption spectra were measured, and the absorption intensity at the excitation wavelength is found to be unchanged. Utilizing Perrin-Jablonski energy level structure, as well as related rate equations, the fluorescence variation is supposed to be origin from both the decreased intersystem crossing rate (k_ISC) and increased non-fluorescence transition rate (k_nF). The decrease of k_ISC is induced by the raise of the lowest vibrational energy level of the first singlet excited state (S₁), which can be verified by the blue-shift of the absorption and fluorescence spectra. The increase of the k_nF was caused by the high-energy vibrations in aqueous solutions. The fluorescence lifetimes are also measured, which consist with our analysis. Our research not only uncovered the water effect on the properties of HMME, but also provided a way to insight the PDT behavior of porphyrin derivatives in the aqueous solution of organism.