Low-temperature luminescence and curcumin-sensitized singlet oxygen generation

Detailed comparative studies of spectral and fluorescent properties of curcumin in solutions at room temperature (293 K) and in rigid glass matrices of 2-methyltetrahydrofuran and ethanol at liquid nitrogen temperature (77 K) were carried out. It was shown that the transition to low temperature, accompanied by a decrease in the efficacy of non-radiative processes, manifests itself (depending on the solvent) in a 3- to 7-fold increase in the fluorescence quantum yield and fluorescence decay time, a sharp decrease in the Stokes shift, and the appearance of a vibrational structure in the fluorescence and fluorescence excitation spectra of curcumin. Rigid glass matrices were used to record phosphorescence, the attribution of which to curcumin was confirmed for the first time by the correspondence of its excitation spectrum to the fluorescence excitation spectrum at 77 K. The phosphorescence maximum of curcumin was shown to be located at ${\lambda }_{\mathrm{fl}}^{\max }$  = 635 nm, which differs significantly from the data ( ${\lambda }_{\mathrm{fl}}^{\max }$ = 730 nm) of the most cited work on curcumin photonics. The quantum yield of curcumin phosphorescence and its decay times were estimated for each solvent. By monitoring the luminescence of curcumin-sensitized singlet oxygen generation in the 1270 nm region, its quantum yield (φ_Δ) was determined in a number of organic solvents. The obtained values φ_Δ are 1.4-2.0 times higher than the literature data from 30 years ago.