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

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.