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 (CT) state. Fluorescence enhancement in high-viscosity media is driven by an emissive hybridized local and charge-transfer (HLCT) state and the suppression of the HLCT → CT 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 CT configuration. Thus, this study not only advances understanding of the dark CT state of methoxylated benzoyl coumarins but also provides computational protocols for modeling HLCT → CT transition and vibronic emission spectra. Additionally, it offers a new molecular design and mechanism for the development of viscosity-sensitive fluorophores.