Probing ground and low-lying excited states for indole derived compounds of potential herbicide action

Abstract

In this work, we present a study on a series of recently synthesized indole derived compounds that were previously suggested by experimental tests as having herbicidal action. Density functional theory (DFT) and its time-dependent formalism (TD-DFT) were used for investigating ground and excited state properties, respectively. The M06-2X and CAM-B3LYP exchange-correlation functionals were used with the aug-cc-pVTZ basis set. Computations were performed in the gas-phase, water, and dimethyl sulfoxide. Solvation was found to play minor effects on the ground state structure of any given compound. On the other hand, solvation was found to cause a decrease in the excitation energies of the lowest-lying excited singlet states of almost all the compounds. More importantly, all the indole derived compounds presented excitation energies that are considerably lower than those determined for their parent molecule. For instance, the lowest-lying excited singlet was probed at 3.45 eV in the case of 6-chloro-8-nitro-2,3,4,9-tetrahydro-1H-carbazole (compound 15a) versus 5.00 eV for the corresponding state of the indole molecule, both values determined at the TD-DFT/M06-2X/aug-cc-pVTZ level in water. In addition, compound 15a along with 5-chloro-2,3-dimethyl-7-nitro-1H-indole (15b) presented three excited states (each) that are likely to be accessed (having oscillator strength ≳0.1 at the TD-DFT/M06-2X/aug-cc-pVTZ level of theory in water) among the five respective lowest-lying excited singlets. The existence of various excited states accessible at lower energies may provide competing alternative paths to the chlorophyll-solar radiation interaction, in detrimental manner to the photosynthetic process; an observation in agreement to the previous experimental findings. Frontier orbital analysis suggested electron delocalization (advent from structural modifications) to be responsible for lowering the excitation energies.