Spectroscopy and photophysics of 9-methylalloxazine. Experimental and theoretical study

We present a systematic study of spectroscopy and photophysics of 9-methylalloxazine using experimental and theoretical approaches. Fluorescence lifetimes and quantum yields were measured in a range of solvents. The singlet state decay is dominated by non-radiative processes. The non-radiative relaxation rate and the radiative rate undergo notable changes when going from non-polar solvents to polar and hydrogen-donor solvents, due to the level inversion phenomena: the lowest excited state is of the n,π* character in non-polar solvents, and of the π,π* character in hydrogen-donor solvents. The S0→Sn and the T1→T n absorption spectra were calculated, using the TD-DFT ab initio approach. The theoretical results for this and other similar molecules demonstrate an overall consistency and reproduce the experimental spectra quite well. The calculations confirmed the existence of a weak n,π* absorption band close to each of the two lowest-energy π,π* bands, of low oscillator strength and thus not observable in the absorption spectra. The remaining difference between experimental and calculated absorption bands, of 1000-2000 cm-1, was attributed to the condensed-phase intermolecular interactions, disregarded in the present calculations, made for isolated molecules. The singlet oxygen emission measurements demonstrated that 9-methylalloxazine is an efficient singlet oxygen sensitizer, especially in non-polar solvents. © 2004 Elsevier B.V. All rights reserved.