The Dark State, Vibronic Coupling, and Viscosity Sensing of Methoxylated Benzoyl Coumarins

The 8/6-methoxylated 3-benzoyl coumarins (BC8 and BC6) are synthesized and demonstrated potential as a viscosity-sensitive fluorescent probe. To date, the mechanism of viscosity sensing has predominantly been attributed to a twisted intramolecular charge transfer effect. However, the observed fluorescence quenching is better ascribed to the presence of a dark charge transfer (¹CT) state. Fluorescence enhancement in high-viscosity media is driven by an emissive hybridized local and charge-transfer (¹HLCT) state and the suppression of the ¹HLCT → ¹CT transition and configuration conversion. Furthermore, vibronic fluorescence emission spectra at 77 K are computed using an intense adiabatic Hessian model and Voigt line shape functions to probe the contributions of Franck-Condon/Herzberg-Teller effects. The fluorescence emission and internal conversion rate at both 298 and 77 K are also theoretically determined, enabling the calculation of a fluorescence quantum yield that agrees well with experimental findings and further reconfirming the presence of ¹CT configuration. Thus, this study not only advances understanding of the dark ¹CT state of methoxylated benzoyl coumarins but also provides computational protocols for modeling ¹HLCT → ¹CT transition and vibronic emission spectra. Additionally, it offers a new molecular design and mechanism for the development of viscosity-sensitive fluorophores.