Theoretical and experimental investigations of photophysics of the nile red molecule and its protonated structures

Results of quantum-chemical studies of the nile red (NR) molecule and its protonated structures by the INDO/S method are presented. It is demonstrated that the best agreement between the calculated and experimental data is obtained for the flat molecule in the ground electron state. Energies of the strongest singlet and triplet electronic states, molecular nature of these states, transition oscillator force, dipole moments in the ground and excited states, electron density distribution around atoms and molecular fragments in the S 0 and S 1 states, and rate constants of radiative, internal, and intercombination conversion are presented for the NR molecule and its protonated structures. The most probable NR protonation centers are analyzed using the molecular electrostatic potential (MESP) method. It is established that the reaction of proton addition to the NR molecule proceeds at the cyclic nitrogen atom. As demonstrated the results of quantum-chemical calculations, low fluorescent properties of the protonated NR structures (with a quantum yield of 4%) are due to a high rate of the S 1 – T 4 intercombination conversion.