Influence of hydrogen bonding on twisted intramolecular charge transfer in coumarin dyes: an integrated experimental and theoretical investigation

Abstract

Twisted intramolecular charge transfer (TICT) is a critical mechanism influencing the emission efficiency and stability of fluorescent materials, thereby playing a pivotal role in the design of highly fluorescent and stable dyes. Although substantial research has concentrated on the role of intermolecular hydrogen bonding in excited-state dynamics, the impact of intramolecular hydrogen bonding has not been thoroughly investigated. To elucidate the solvent polarity dependence of C7 and C30, we employed the Kamlet-Taft and Catalán 4P models in conjunction with steady-state and transient absorption spectroscopy, complemented by time-dependent density functional theory (TDDFT) calculations. Our findings demonstrate that C30 exhibits a pronounced TICT process in both solvents. Conversely, C7, stabilized by intramolecular hydrogen bonds, retains a planar configuration of its benzimidazole and benzopyrone moieties, effectively preventing the TICT process. Moreover, in MeOH, the intermolecular hydrogen bonding in C30 significantly extends the lifetime of the TICT state compared to ACN. Theoretical analyses of electrostatic potential, molecular geometry, and frontier molecular orbitals further corroborate these observations. This work provides valuable insights into the design of fluorescent dye molecules and the selection of solvents, laying a foundation for advancing the photophysical and photochemical understanding of coumarin dyes.

Fig. 1. Molecular structure diagram of C7 (a) and C30 (b).

Fig. 2. Steady-state absorption (a) and (c) and fluorescence emission (b) and (d) spectra of C7/C30 in Cy, THF, ACN and MeOH (λ_ex = 375 nm).

Fig. 3. 2D fs-TA spectra of C7/C30 in ACN (a and c) and MeOH (b and d) (λ_ex = 375 nm).

Fig. 4. Fs-TA spectra of C7/C30 in ACN (a and c) and MeOH (b and d) after excitation at 375 nm.

Fig. 5. TA kinetic curves of C7/C30 in ACN (a and c) and MeOH (b and d) (λ_ex = 375 nm).

Fig. 6. Geometric configurations of the C7–MeOH/C30–MeOH in S₀ (a and c) and S₁ (b and d).

Fig. 7. The potential energy surface of the hydrogen bond complex formed by C7 and C30 in MeOH solvent at S₁ state (relative θ = 0° refers to the optimized dihedral angle in the ground state).