Photothermal effects and fluorescence spectra of tetrapyridylporphyrins

The generation of triplet state of tetrapyridylporphyrin and the Zn complex of this dye was established by laser induced optoacoustic spectroscopy (LI OAS). The influence of prolonged illumination by light from Soret band and Q bands regions on fluorescence spectra of dyes solutions were measured. Tw o concentrations of dyes in chloroform solution were investigated. The yiel d of fluorescence of investigated dyes was very low and the yields of singl et --> triplet intersystem crossing (ISC) for higher dye concentrations wer e about 0.73 for both TPyP and ZnTPyP, whereas the lifetime of the triplet states for these concentrations (4 x 10(-6) M for TPyP and 7 x 10(-6) M for ZnTPyP) were 2.08 and 1.74 mus, respectively. At about two times lower con centrations, only weak thermal deactivation components with decay time of a bout 0.5 mus were observed. Probably in this case, besides these components and "prompt" thermal deactivation, some very slow decays also occur. The m ost other spectral properties of TPyP were similar at both concentrations, suggesting that this dye is predominantly in monomeric state. At higher con centration for ZnTPyP solution, the additional maximum fluorescence spectru m located at 631 nm is observed. It is related to Q(x) (0, 0) absorption ba nd at about 625 nm. These maxima belong probably to some dimeric form of dy e being much more sensitive to photodestruction than monomeric form of ZnTP yP. 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. (C) 2001 Elsevier Science B.V. All rights re served.