<title>Energetics of chlorins as potent photosensitizers of PDT</title>

Abstract

A number of derivatives of chlorin e<SUB>6</SUB> (mono, di and trimethyl ethers), purpurin-18 (monomethyl ether) and tetracarboxyphenylporphyrin -- potential sensitizers of PDT have been prepared and spectral-kinetic properties of their lowest electronic excited singlet and triplet states have been studied. The extinction of Q-band of studied chlorins is much higher than conventional porphyrin photosensitizers and varies between 3 - 6 multiplied by 10<SUP>4</SUP> M<SUP>-1</SUP> cm<SUP>-1</SUP>. The highest degree of polarization is 0.45 and related to S<SUB>1</SUB> - S<SUB>0</SUB> transition. Quantum yields of fluorescence (rho) purpurin-18 in diethyl ether are lowest among other studied chlorins and consists of 0.10 and decreases with increasing concentration as a result of the aggregation. The chlorin e<SUB>6</SUB> 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 a number of sensitizers has been assigned to the channel of internal conversion. The internal conversion was eliminated by deuteration of NH group of macrocycle showing that high frequency NH vibrations are responsible for radiationless deactivation of electron energy of the S<SUB>1</SUB> state of sensitizers. The comparison of photodynamic action chlorins and photofrin on cells level and tumors in vivo demonstrates that chlorins are more potent photosensitizers of PDT.