Annihilation delayed fluorescence of indole and carbazole in liquid solutions at room temperature

The spectra of long-lasting luminescence of indole and carbazole in neutral (cyclohexane) and polar (ethanol) solvents are obtained. The spectra of long-lasting luminescence of indole in these solvents are shifted to longer wavelengths with respect to the fluorescence spectra. The spectra of long-lasting luminescence of carbazole in both solvents coincide with the fluorescence spectra. The long-lasting glow of indole is interpreted as the annihilation delayed fluorescence (ADF) of dimers, while the long-lasting glow of carbazole is attributed to the ADF of monomers. It is suggested that the indole dimers are formed due to the dipole–dipole interaction of molecules in the excited singlet and ground states appearing as a result of the triplet–triplet annihilation. The ADF kinetics is measured, and the lifetimes of the triplet states of indole in cyclohexane are estimated. It is established that the lifetime is determined by quenching of the triplet states by molecules in the ground state. The self-quenching rate constant is found to be 6.2 $\times$ 10$^{7}$ M$^{-1}$s$^{-1}$.