Theoretical insights into the $$\pi \rightarrow \pi ^*$$ transition and the large Stokes shift of a curcumin-based chromophore

Abstract 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, $$\pi \rightarrow \pi ^*$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>π</mml:mi> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> </mml:mrow> </mml:math> transition, and the large Stokes shifts observed experimentally. The $$S_0 \rightarrow S_1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>S</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>→</mml:mo> <mml:msub> <mml:mi>S</mml:mi> <mml:mn>1</mml:mn> </mml:msub> </mml:mrow> </mml:math> 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 $$^1(\pi \pi ^*)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>1</mml:mn> </mml:mmultiscripts> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>π</mml:mi> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> state containing a mixture of LE and CT character. Gas-phase excited-state optimizations show that the lowest excited state is an $$^1(n\pi ^*)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>1</mml:mn> </mml:mmultiscripts> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>n</mml:mi> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> dark state with negligible oscillator strength, while the bright $$^1(\pi \pi ^*)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>1</mml:mn> </mml:mmultiscripts> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>π</mml:mi> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> state lies slightly above it; consequently, fluorescence originates from the relaxed $$^1(\pi \pi ^*)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>1</mml:mn> </mml:mmultiscripts> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>π</mml:mi> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> minimum. Large Stokes shift (3607 to 5773 cm $$^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:math> ) has been linearly correlated with solvent polarity by Lippert-Mataga plot (R = 0.985), driven by dipole moment changes ( $$\Delta \mu \sim 3.1-3.6$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>μ</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>3.1</mml:mn> <mml:mo>-</mml:mo> <mml:mn>3.6</mml:mn> </mml:mrow> </mml:math> 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.