Spectral study of photodynamic processes in the C$_{60}$-($n$-methylpyrrolidone)-oxygen system

The processes of photoexcitation and energy transfer in an air-saturated solution of fullerene C$_{60}$ in $n$-methylpyrrolidone are studied. With femtosecond laser pumping at a wavelength $\lambda(p)$ = 520 nm, transient absorption spectra were obtained in the range 470–750 nm. Broadband absorption is observed at $\lambda>$ 650 nm as a result of aggregation of fullerene molecules, characterized by a short decay time of the picosecond scale. The analysis of the time dependencies indicates the presence of several processes. Within the proposed interpretation, we state a remarkable reduction in the time of singlet-triplet intersystem crossing with an increase of nanocluster size while the rapid quenching of the singlet state varies only slightly. The quantum yield of the triplet state can therefore increase, reaching 0.14. The phosphorescence spectra of singlet oxygen with a maximum at $\lambda$ = 1276 nm were measured with LED excitation at various wavelengths in the range 370–625 nm. The lifetime and quenching rate constant of singlet oxygen in $n$-methylpyrrolidone are determined. The combined quantum yield of triplet fullerene and singlet oxygen in $n$-methylpyrrolidone is estimated as 0.74. The efficiency spectra of the singlet oxygen photosensibilization in the C$_{60}$-($n$-methylpyrrolidone)-oxygen system are obtained, indicating the increasing role of large nanoclusters in this process during aging of the solution.