Decoding structural rigidity and charge-transfer polarization in barbituric-acid-based donor–π–acceptor chromophores

Abstract

Two barbituric-acid-based donor–π–acceptor chromophores were systematically investigated using density functional theory (DFT), time-dependent DFT, and solvent-dependent polarizable continuum model (PCM) calculations to elucidate the interplay between molecular structure, excited-state electronic redistribution, reactivity, and photovoltaic relevance. Excited-state geometry optimizations revealed that compound 1 retained nearly invariant bond lengths and strict planarity upon S₀ → S₁ excitation, indicating minimal structural relaxation with a predominantly locally excited (LE) character. In contrast, compound 2 showed pronounced excitation-induced bond-length modulation along the donor–acceptor axis, which was consistent with significant intramolecular charge transfer (ICT). Mulliken charge analysis, electrostatic potential mapping, and frontier molecular orbital distributions further confirmed the delocalized electronic excitation in 1 and strong donor-to-acceptor charge migration in 2. Solvent-dependent studies demonstrated a weak dielectric sensitivity for 1, whereas 2 exhibited enhanced polarization, solvent-tunable HOMO–LUMO gaps, and greater stabilization in polar media. Quantitative photovoltaic descriptors, including electron-injection driving force, excited-state dipole moment, light-harvesting efficiency, and reorganization energy, revealed that 2 possessed a favorable energetic alignment for electron injection into TiO₂, large excited-state polarization, and superior absorption efficiency, establishing its suitability as a dye sensitizer for dye-sensitized solar cell (DSSC) applications. In contrast, 1 exhibited marginal electron-injection capability and limited excited-state charge separation, restricting its photovoltaic applicability.