Unraveling the effect of solvents on the excited state dynamics of C540A by experimental and theoretical study

Abstract

In this work, the excited-state dynamics including intramolecular charge transfer (ICT) and the redshift of C540A have been investigated in a series of solvents on the basis of the Kamlet-Taft solvatochromic parameters (π*, α, β) using femtosecond transient absorption spectra and systematic theoretical calculation. We demonstrate that the redshift of the emission peak has a linear relationship with the α and π* scales and the effect of the π* scale is slightly stronger than that of the α scale. Meanwhile, the ICT rates can be suggested as relevant to not only the α scale but also the π* scale. Additionally, C540A-AN has proved that the excited state molecules have a unique inactivation mechanism because of the dark feature of the S₁ (CT) state. The valuable mechanistic information gleaned from the excited-state dynamics by the experimental and theoretical study would facilitate the design of organic materials for prospective applications in photochemistry and photobiology.

Fig. 1. The transient absorption spectra of C540A in AN (a), MeOH (b), and FA (c) solvents after excitation at 400 nm. The TA kinetic curves and fitting curves of C540A (d)–(f).

Fig. 2. The ESP maps of C540A (a), MeOH (b), AN (c), and FA (d) molecules.

Fig. 3. Optimized geometric structures for the C540A-FA complex in S₀ (a) and S₁ (b) at the B3LYP/TZVP/IEF-PCM (FA) level.

Fig. 4. NCI isosurface of the C540A-MeOH complex in S₀ (a), S₁ state (b), and the C540A-FA complex in S₀ (c), S₁ state (d). Plots of sign(λ₂)ρ versus RDG(r) for hydrogen-bonded complexes C540A-MeOH and C540A-FA in the S₀ (e), (g), and S₁ state (f) and (h), respectively.

Fig. 5. Frontier molecular orbitals and electronegativity changes (Coulomb: C) in the hydrogen-bonded complexes C540A-MeOH, C540A-FA, and C540A-AN.

Fig. 6. Deactivation mechanism of the C540A-AN complex in the excited states.