SPECTROSCOPIC PROPERTIES OF POTENTIAL SENSITIZERS FOR NEW PHOTODYNAMIC THERAPY START MECHANISMS VIA 2-STEP EXCITED ELECTRONIC STATES

Three substituted tetraazaporphyrins, octa-(4-tert-butylphenyl)-tetrapyrazinoporphyrazine, tetra-(4-tert-butyl)phthalocyanine and tetra-(4-tert-butyl)phthalocyanatomagnesium (t4-PcMg), were spectroscopically checked in solutions and liposomes with respect to suitability as potential sensitizers of a possible new star-t mechanism for photodynamic therapy (PDT) from a stepwise excited higher singlet state. This PDT start mechanism was recently proposed to overcome the problem of O2 (1DELTA2)-caused cutaneous phototoxicity in PDT. By means of absorption and fluorescence measurements as well as nonlinear absorption investigation, transient spectroscopy and lasing experiments, compound t4-PcMg was found to have the most appropriate properties: Based on a high fluorescence quantum yield (PHI(FI) = 0.84) and a very low crossing to the triplet (PHI(ISC) = 0.05), two higher excited states can be effectively populated both by two stepwise absorption transitions at 674 nm and consecutive absorption transitions at 674 and 710 nm. Moreover, t4-PcMg incorporates into liposomes very well with spectroscopic proper-ties similar to those in solution.