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We have studied the influence of solvent acidity on the aggregation/protonation behaviour of two charged tetraphenylporphyrins: positively charged tetrakis(N,N,N-trimethylanilinium-4-yl)porphyrin (TAP) and negatively charged tetrakis(4-sulfonatophenyl)porphyrin (TPPS4). Spectroscopic measurements (absorption, fluorescence, excitation and photoacoustic) have been made to follow the radiative and nonradiative relaxation processes of excited dye molecules at various pH values. The ability of these porphyrins to exist in aggregated and protonated forms was also investigated. It has been shown that TPPS4 exists in at least three spectroscopic forms: monomer (M1), dication (D1) and aggregated dication (AD), whereas TAP exists in two forms: monomer (M2) and dication (D2). These forms are characterised by different absorption and fluorescence properties. The short wavelength forms of the dyes (M1 and M2) were assigned to the monomeric forms and D1 and D2 were assigned to the protonated forms of TPPS4 and TAP, respectively; AD was identified as the aggregate of the protonated form of TPPS4. Each form shows deactivation of its excited state by nonradiative pathways-particularly high thermal deactivation was observed for the aggregates of the protonated form. Energy transfer between the monomeric form and the dication form has been demonstrated and Förster radii have been estimated (R0 = 31 and 44 A for TAP and TPPS4, respectively). Since protonation and aggregation of photoreceptors can strongly affect their photosensitizing effects (e.g. cellular uptake, singlet oxygen production) the results presented here may be important in the study of the function of photosensitizers in tumour tissues because, as is already known, the microenvironment in cancerous tissue is more acidic than in healthy cells.

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Positively charged tetra(4-N,N,N,N-trimethylanilinium) porphyrin (TAP) and negatively charged tetrakis(4-sulphonatophenyl) porphyrin (TPPS 4 ) and their mixtures with dopa melanin in polyvinyl alcohol have been investigated. On the basis of the absorption and fluorescence spectra it has been established the formation of TAP-dopa melanin complex which fluorescence is quenched whereas TPPS 4 fluorescence behaviour is only slightly changed in the presence of dopa melanin. Photovoltaic effects and photocurrent generation in a photoelectrochemical cell consisting of a layer dye solutions sandwiched between the semiconducting and gold electrodes have been investigated. The dependence of photovoltage signal on excitation wavelength (photovoltage action spectrum), kinetics of photocurrent rise and decay in the second time scale have been presented. Electric parameters of the photoelectrochemical cell (conductivity and capacitance) with porphyrin dyes, dopa melanin and their mixtures in the dark and upon illumination have been estimated. It was shown that all investigated dyes are able to generate photosignal but the values of photocurrent generated depend on the dye used in the experiment and the presence or absence of dopa melanin.

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