Photophysical characterization of free-base corroles, promising chromophores for light energy conversion and singlet oxygen generation

Abstract

A detailed photophysical characterization of a series of six free-base corroles with different substitution patterns at meso-positions is presented. In air-free toluene at 295 K, the luminescence quantum yields range from 0.13 to 0.22 and lifetimes vary from 4.1 ns to 6.3 ns whereas in air-saturated toluene the lifetimes range from 3.8 ns to 5.6 ns. In glassy toluene at 77 K the lifetimes have values in the range 5.2 ns to 7.9 ns. Transient absorption spectra of the singlet excited states have positive bands art λ < 550 nm and at λ > 680–700 nm. The transient absorption spectra of the lowest triplet display a band around 460–470 nm and bleaching features in the Q bands region, no absorption above 700 nm. The triplet lifetimes in air-free toluene are in the interval 50–150 μs. The lowest excited states react with oxygen and sensitized singlet oxygen (¹Δ_g) yields of 0.51–0.77 are determined via its luminescence. Electron and energy transfer from the excited states to molecular oxygen is discussed. Effect of polar solvents on the luminescence parameters indicate that charge transfer (CT) excited states play a very minor role. Thermal and photo stability is excellent for most of the examined compounds in toluene and dichloromethane.