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

Three substituted tetraazaporphyrins, octa-(4-tert-butylphenyl)-tetrapyrazinoporphyrazine, tetra-(4-tert-butyl)phthalocyanine and tetra-(4-tert-butyi)phthalocyanatomagnesium (t₄-PcMg), were spectroscopically checked in solutions and liposomes with respect to suitability as potential sensitizers of a possible new start 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 (¹δ₂)-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 (φ_fi= 0.84) and a very low crossing to the triplet (φ_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 properties similar to those in solution.