Impact of water hydrogen-bonding on structural and electronic properties of indole derived compounds

We examined the ground and low-lying excited states of four indole derived compounds: 2,3,4,9-tetrahydro-1H-carbazole (compound 3a), 6-chloro-2,3,4,9-tetrahydro-1H-carbazole (compound 3b), 6-chloro-8-nitro-2,3,4,9-tetrahydro-1H-carbazole (compound 15a), and 5-chloro-2,3-dimethyl-7-nitro-1H-indole (compound 15b), focusing on the effects of water microsolvation. The density functional theory (DFT) and time-dependent DFT (TD-DFT) were used with the M06-2X exchange–correlation functional and the aug-cc-pVTZ basis set. A polarizable continuum model was used for taking into account the implicit component of the solvation. In general, the presence of hydrogen-bond interactions contributed to the stabilization of the water-clustered systems. In terms of the excited states, a direct comparison involving the present results and those corresponding available in the literature yielded interesting findings, suggesting the hydrogen-bond interactions between the H₂O molecules and compounds 3a, 3b, 15a, and 15b as playing a significant role in their excitation energies. For instance, the lowest-lying singlet excited state of the compound 15a$\cdots$H₂O cluster was determined at 3.34 eV (when hydrogen-bonding is present at its NO₂ moiety) while the corresponding state was found to be located at 3.45 eV in the case of the compound 15a, at the TD-DFT/M06-2X/aug-cc-pVTZ in water (IEF-PCM). Overall, a marked decrease in the energetic cost regarding the photoexcitation of all the indolic compounds can be noted due to hydrogen-bonding with the H₂O molecules.