Spectroscopic and quantum chemical characterization of the ground and lowest electronically excited singlet and triplet states of halo- and nitro-harmines in aqueous media

Halogenated and nitro beta-carboline (beta Cs) alkaloids have garnered increasing interest for their role in a broad range of biological, pharmacological and biotechnological processes. Addressing their spectroscopic and photophysical properties provide tools to further explore the presence of these alkaloids in complex biological matrices. In addition, these studies help to elucidate processes where these alkaloids are involved. The UV-visible and steady-state room temperature fluorescence of bromo- and nitro-harmines in an aqueous environment at different pHs, low-temperature phosphorescence (at 77 K) and quantum yields of singlet oxygen production are reported herein. Singlet (S-0 and S-1) and triplet (T-1) electronic states are further analyzed using density functional theory (DFT) and the results compared with experimental data. Data are discussed in the framework of potential biotechnological applications of these beta C alkaloids.