Intramolecular interactions and optical properties of indole-containing alkaloids extracted from Arundo donax L.

Abstract

Three optically active natural alkaloids were computationally investigated. The electronic ground state was found to be characterised by numerous intramolecular interactions, among which a hydrogen bond. These interactions are responsible for the different rate constants of the kinetics of conversion from one atropisomer to the other. The complex molecular electrostatic potential reveals a strong polar nature of these compounds, which strengthens when the electronic excited states are populated. The absorption spectra were interpreted and the intramolecular charge transfer nature of the Sn ← S0 transitions was revealed. The ECD spectra for the different atropisomers of these compounds indicate that the assignment proposed in the literature for 1 should be revised. The geometry of the excited state shows dramatic changes with respect to the ground state; some intramolecular interactions get lost. The fluorescence spectrum, proposed for the first time, shows de-excitation energies in the blue-green region (488 nm to 518 nm). Internal Conversion and intramolecular Electronic Energy Transfer may be fluorescence quenching channels.