Study on the triplet states of N-phenyl carbazoles. Transient spectra and singlet oxygen generation

Here we report a study on the photophysics of a series of N-phenyl-carbazoles (N-Ph-Czs) in homogenous solvents and micellar media. The compounds investigated were TCz (N-tolyl-carbazole), TCz-TCz (a 3,3'-linked dimer), and CzPh-Ch(2)-PhCz (a methylene-bridged N-phenyl-carbazole). Absorption and fluorescence measurements were carried out in ethanol, N,N-dimethyl-formamide and n-decane. Aqueous micellar solutions of SDS, CTAB and Tween-80 were also employed to explore the effect of the organized medium on the photophysics of the N-Ph-Czs. The triplet-triplet absorption spectra of the compounds were characterized by transient absorption spectroscopy. The absorption and fluorescence experiments revealed that although the compounds showed lithe solvent effects, an unexpected twisting out of planarity of the dicarbazolyl one (TCz-TCz) was observed in micelles. Quantum yields of triplet states and singlet oxygen generation were determined. The high values obtained point to the potential use of these compounds as sensitizers in photodynamic processes, which highlights the potential pharmacological role of these carbazoles. The quenching of the triplet states by electron-acceptors was evaluated and rate constants were near to the diffusion limit. The present results might be relevant for future applications of these compounds in diverse fields.