Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Photosensitizers that display "unusual"emissionfrom upper electronically excited states offer possibilities for initiatinghigher-energy processes than what the governing Kasha's rulepostulates. Achieving conditions for dual fluorescence from multiplestates of the same species requires molecular design and conditionsthat favorably tune the excited-state dynamics. Herein, we switchthe position of the electron-donating NMe2 group aroundthe core of benzo[g]coumarins (BgCoum) and tune theelectronic coupling and the charge-transfer character of the fluorescentexcited states. For solvents with intermediate polarity, three ofthe four regioisomers exhibit fluorescence from two different excitedstates with bands that are well separated in the visible and the near-infraredspectral regions. Computational analysis, employing ab initio methods,reveals that the orientation of an ester on the pyrone ring producestwo conformers responsible for the observed dual fluorescence. Studieswith solid solvating media, which restricts the conformational degreesof freedom, concur with the computational findings. These resultsdemonstrate how "seemingly inconsequential" auxiliarysubstituents, such as the esters on the pyrone coumarin rings, canhave profound effects leading to "anti-Kasha" photophysicalbehavior important for molecular photonics, materials engineering,and solar-energy science.