Theoretical insights into the π → π ^* transition and the large Stokes shift of a curcumin-based chromophore

This theoretical study investigates the photophysical properties and excited-state dynamics of the monocarbonyl curcumin derivative (1E,4E)-1,5-bis(4-dimethylaminophenyl)penta-1,4-dien-3-one (CCM) in aprotic solvents. Using DFT/TD-DFT with the CAM-B3LYP functional and PCM solvation modeling, we analyze the solvent-dependent absorption spectra, π → π ^* π → π ∗ transition, and the large Stokes shifts observed experimentally. The S₀ → S₁ S 0 → S 1 transition demonstrates a mixed contribution from locally excited (LE) and intramolecular charge transfer (CT) characters, showing minimal sensitivity to solvent polarity. The CT component accounts for 68–69% of the electronic reorganization, with charge transfer from the N,N-dimethylaniline (donor) units to the divinyl ketone (acceptor) core, as indicated by natural transition orbitals (NTOs) and Hirshfeld population analysis. XMS-CASPT2 multiconfigurational calculations corroborate the TD-DFT picture, revealing a bright ¹(π π ^*) 1 ( π π ∗ ) state containing a mixture of LE and CT character. Gas-phase excited-state optimizations show that the lowest excited state is an ¹(nπ ^*) 1 ( n π ∗ ) dark state with negligible oscillator strength, while the bright ¹(π π ^*) 1 ( π π ∗ ) state lies slightly above it; consequently, fluorescence originates from the relaxed ¹(π π ^*) 1 ( π π ∗ ) minimum. Large Stokes shift (3607 to 5773 cm ⁻¹ - 1 ) has been linearly correlated with solvent polarity by Lippert-Mataga plot (R = 0.985), driven by dipole moment changes ( Δ μ ~ 3.1-3.6 Δ μ ∼ 3.1 - 3.6 Debye) upon excitation. Calculations reproduce experimental absorption and emission trends, that is, the redshift in polar solvents, and validate CAM-B3LYP for curcumin derivatives. This work elucidates the mixed ICT-LE character in various solvents and provides design insights for fluorophores in sensing and optoelectronics.