Photothermal effects and fluorescence spectra of tetrapyridylporphyrins

Abstract

The generation of triplet state of tetrapyridylporphyrin and the Zn complex of this dye was established by laser induced optoacoustic spectroscopy (LIOAS). The influence of prolonged illumination by light from Soret band and Q bands regions on fluorescence spectra of dyes solutions were measured. Two concentrations of dyes in chloroform solution were investigated. The yield of fluorescence of investigated dyes was very low and the yields of singlet→triplet intersystem crossing (ISC) for higher dye concentrations were about 0.73 for both TPyP and ZnTPyP, whereas the lifetime of the triplet states for these concentrations (4×10−6M for TPyP and 7×10−6M for ZnTPyP) were 2.08 and 1.74μs, respectively. At about two times lower concentrations, only weak thermal deactivation components with decay time of about 0.5μs were observed. Probably in this case, besides these components and “prompt” thermal deactivation, some very slow decays also occur. The most other spectral properties of TPyP were similar at both concentrations, suggesting that this dye is predominantly in monomeric state. At higher concentration for ZnTPyP solution, the additional maximum fluorescence spectrum located at 631nm is observed. It is related to Qx(0, 0) absorption band at about 625nm. These maxima belong probably to some dimeric form of dye being much more sensitive to photodestruction than monomeric form of ZnTPyP. The appearance of various forms of dyes with different photosensitivity and various ability of triplet state generation has to be investigated in order to establish the right time of illumination of tissue stained by the dye in photodynamic treatment.