Experimental and computational insights into the electronic structures and absorption-emission characteristics of coumarin, C-6H, C-153, and C-343 dyes.

This paper explores the electronic structure and spectral characteristics of coumarin (C), C-6H, C-153, and C-343 in the protic polar solvent acetonitrile, combining computational methods via Density Functional Theory (DFT) and time-dependent Density Functional Theory (TD-DFT) with experimental analysis of UV-Vis and fluorescence spectra. The optoelectronic features of C, C-6H, C-153, and C-343 are primarily utilized in the solution phase for various applications, such as lasers and dye-sensitized solar cells. Computational studies were conducted using four different Modal Chemistry methods [MC1: CAM-B3LYP/6-311++G(d.p), MC2: CAM-B3LYP/6-31 + G(d.p), MC3: B3LYP/6-311++G(d.p), and MC4: B3LYP/6-31 + G(d.p)]. The excited state features were investigated based on TD-DFT/Polarizable Continuum Model-Linear Response and TD-DFT/Polarizable Continuum Model-State Specific formalisms. Molecular orbital configurations, molecular electrostatic potentials, and electron density difference isosurface of the dyes were analyzed to uncover the factors influencing the absorption and emission properties. The decomposed UV-Vis and fluorescence spectra of compounds indicate that emission characteristics are complex and contribute to low-lying energy transitions. The state-specific solutions provide more reliable estimates for smaller molecular structures with less intramolecular charge transfer, whereas the linear response approach excels when more electron-donating functional groups are present. The effect of the basis set in determining both absorption and emission features is almost negligible compared to Hartree-Fock exchange contributions in DFT functionals. B3LYP appears to provide satisfactory results for systems where long-range HF exchange is not as crucial.