It has been demonstrated that chlorin e<sub>6</sub> form molecular complex with polyvinylpyrrolidone. The binding constant and
the number of binding sites have been determined as Κ= (2.5 ± 0.5)•10<sup>4</sup> M<sup>-1</sup> and N = 2.1 ± 0.5, respectively. Complex in comparison with free chlorin e<sub>6</sub> possesses increased fluorescence quantum yield, triplet and singlet state lifetimes, slightly modified intersystem crossing quantum yield, and also more intensive and red shifted absorption band locating in the "phototherapeutic window" of biological tissues. The photophysical properties of chlorin e<sub>6</sub> incorporated into the polymer are insensitive to pH. The formation of the complex leads to the disruption of chlorin e<sub>6</sub> aggregates at acidic pH.
Binding to polymer only slightly improves photophysical properties of photosensitizer and should not provoke fundamental changes in PDT efficiency.