Photophysical and photochemical properties of potential porphyrin and chlorin photosensitizers for PDT

Structural and optical properties as well as photophysical and photochemical parameters (excited S1 and T1 state lifetimes at 77 K and in the presence of O2 in solution at 293 K; efficiencies of singlet oxygen, 1Δ(g), generation) are presented for porphyrins and chlorins with potential for the PDT of cancer: chlorin p6 and its trimethyl ester, chlorin e6 and its Na3 and K3 salts, purpurin-18 and its monomethylester, 5,10,15,20-tetrakis(3-methoxyphenyl)porphyrin (TPPM), 5,10,15,20-tetrakis(2,4-difluoro-3-methoxyphenyl)porphyrin (TPPMF) and GaTPP in different solvents (ethanol, toluene, pyridine and buffer pH 7.4) at 77-300 K. It has been shown that for monomeric chlorin e6, chlorin p6 and its derivatives the photophysical parameters are similar, as follows: fluorescence lifetimes τ(s) in the presence of oxygen are 3.2-4.5 ns at 293 K; fluorescence quantum yields ∅(f) vary from 0.1 to 0.2 depending on the solvent; phosphorescence quantum yields ∅(f) are of the order 10-5; T1 state lifetimes τ(Υ) = 1.5-2.0 ms at 77 K and 250-390 ns at 293 K in the presence of O2. By use of the direct kinetic measurement of singlet oxygen emission at 1.27 μm on laser excitation the quantum yields of 1Δ(g) generation by chlorins have been measured: ∅(Δ) = 0.35-0.68. In this case values of ∅(f) and ∅(Δ) depend strongly on the solvent, probably because of the formation of aggregates. For TPPM, TPPMF and Ga-TPP the ∅(Δ) values measured are higher (0.87-0.98) and are explained by the higher intersystem crossing S1 → T1 quantum yields.