<title>Energetics of chlorins: a potent photosensitizer of PDT</title>

Abstract

A number of derivatives of chlorin e6 (mono, di and trimethyl ethers) and purpurin-18 (monomethyl ether) potential sensitizers of PDT have been prepared and spectral-kinetic properties of their lowest electronic singlet and triplet states have been studied. The extinction of Q-band of the studied chlorins is much higher than convenient porphyrin photosensitizers and varies between 3 - 6 (DOT) 104 M-1 cm-1. The highest degree of polarization is 0.45 and related to S1-S0 transition. Quantum yields of fluorescence (rho) purpurin-18 in diethyl ether is lowest among the other studied chlorins and consists of 0.10 and decreases with increasing concentration as a result of the aggregation. The chlorin e6 derivatives have close values of quantum yields of fluorescence (0.16 - 0.18). Appreciable quantum yield of fluorescence is promising for application of chlorins for tumor detection due to photosensitizer retention in tumor tissues. The quantum yield of triplet formation (gamma) measured flash photolysis method was 0.70 - 0.76. The energy of the lowest triplet level estimated by maximum of phosphorescence band of chlorins is between 830 - 900 nm. The rate constant of oxygen quenching of triplets in solutions is one order less than diffusion rate constant. The luminescence of singlet oxygen has been observed as a result of energy transfer from triplet state of chlorins to molecular oxygen in organic solvents and deuterated water. Relative intensity of singlet oxygen luminescence 1272 nm photosensitized by different chlorins correlated with quantum yields of its triplets formation. The difference 0.05 - 0.20 between the unit and the sum of (rho) plus (gamma) for the number of chlorins has been assigned to the channel of internal conversion. The internal conversion was eliminated by deuteration of an NH group of chlorins showing that high frequency NH vibrations are responsible for radiationless deactivation of electron energy of the S1 state of chlorins. The comparison of photodynamic action chlorins and photofrin on cells level and tumors in vivo demonstrates that chlorins are more potent photosensitizers of PDT.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.