Phase-sensitive fluorescence study of mono-L-aspartyl chlorin e6

Phase-resolved fluorescence studies of a photosensitizer for photodynamic therapy, mono-L-aspartyl chlorin e6 (ME2906), have been carried out. The experiments were done for its water solutions in the concentration range from 3.13×10^-7 to 8.00×10^-5 M, and some photophysical parameters have been experimentally determined for the lowest singlet excited state of ME2906. It was confirmed that ME2906 molecules were in the isolated molecular state below 1.00×10^-5 M. It was also confirmed that the fluorescence in this concentration range was ascribed to the electronic transition from the lowest singlet excited state to the ground state. It was found that, above 1.00×10^-5 M, a part of ME2906 molecules form dimers in water solution, which causes the red shift of the fluorescence spectrum and the enhancement of fluorescence in the 700-750 nm wavelength region. Semiempirical molecular orbital calculation revealed that the sodium aspartate attached to the tetrapyrrole ring through the ethanoic acid group was remarkably bent with respect to the tetrapyrrole plane, which seems to hinder the formation of Me2906 dimers up to 1.00×10^-5 M.