A comprehensive study on the effect of substitution position and solvent effect on absorption and emission characteristics of n-methylindoles via linear response and State-Specific formalisms
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Abstract
The light absorption emission characteristics of n-methylindoles (n = 1-7) have been explored thoroughly by taking solvatochromic, substitution position, and solvation model effects into account. Experimental UV-Vis and Fluorescence spectra were recorded between 200 and 400 nm, and 250-800 nm, respectively, in cyclohexane, dichloromethane, and acetonitrile. Molecular geometry optimizations and excited state calculations were conducted using DFT and TD-DFT methods with a long-range corrected hybrid exchange-correlation functional CAM-B3LYP. Absorption emission characteristics were determined based on the polarizable continuum model accompanied with Linear-Response and State-Specific formalisms. Absorption spectra were resolved using Franck-Condon analysis for a more accurate representation of experimental data and underlying vibronic transitions were determined. Overall, although the Linear-Response and State-Specific approaches provide a quantitative explanation of experimental spectra, the approach used for the calculations can affect the solvent polarity-dependent shifts by nearly an order of magnitude. Besides, the State-Specific model provides slightly better results in the determination of Stokes shifts. The shouldered fluorescence peak observed in weakly polar solvents diminishes at higher polarities indicating the elimination of solute-solute interactions and domination of solute-solvent interactions with increasing solvent polarity.