Solvent-Mediated Control of Twisted Intramolecular Charge Transfer in 7-(Diethylamino)coumarin-3-carboxylic Acid.

Understanding the influence of solvent environments on the excited-state charge transfer process remains a fundamental question in molecular photophysics and photochemistry. While twisted intramolecular charge transfer (TICT) is crucial in determining fluorescence efficiency and photostability, the combined effects of solvent polarity and hydrogen bonding interactions are still elusive. Here, we employ steady-state and femtosecond transient absorption (fs-TA) spectroscopy with density functional theory (DFT) calculations to investigate the excited-state dynamics of 7-(diethylamino)coumarin-3-carboxylic acid (7-DCCA) in different solvents. Our findings reveal that in highly polar solvents with strong hydrogen-donating and hydrogen-accepting capabilities, 7-DCCA undergoes significant TICT formation, resulting in fluorescence quenching. Conversely, in environments with low polarity or weak hydrogen-bonding interactions, this transformation is largely suppressed. Quantitative correlation analysis utilizing the Kamlet-Taft and Catalán four-parameter models further elucidates the synergistic role of solvent polarity and specific hydrogen-bonding parameters in modulating the steady-state spectral behavior of 7-DCCA. This study provides microscopic insights into solvent-charge transfer interactions and establishes a general framework for enhancing the luminescence efficiency and structural robustness of organic optoelectronic materials through strategic solvent engineering.