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

Abstract

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, 1Δg, 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 GaTTP 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 τs in the presence of oxygen are 3.2–4.5 ns at 293 K; fluorescence quantum yields φ1 vary from 0.1 to 0.2 depending on the solvent; phosphorescence quantum yields φ1 are of t order 10−5; T1 state lifetimes τT = 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 laserexcitation the quantum yields of 1Δg generation by chlorins have been measured: φΔ = 0.35−0.68. In this case values of φ1 and φΔ depend strongly on the solve probably because of the formation of aggregates. For TPPM, TPPMF and Ga-TPP the φΔ values measured are higher (0.87–0.98) and are explained by the higher intersystem crossing S1 → T1 quantum yields.