Structural implications on the excitation dynamics of fluorescent 3H-indolium cations

Fluorescent 3H-indolium cations are valuable components for the realization of activatable fluorophores for bioimaging applications. Their relatively poor fluorescent quantum yields in organic solvents, however, appear to be in contradiction to their good performance in analytical methods based on single-molecule detection. The elucidation of the structural factors governing the excitation dynamics of these compounds is, therefore, essential to rationalize these effects and possibly guide the future design of activatable probes with improved performance. In this context, the structural, photochemical and photophysical properties of a model compound, consisting of coumarin and 3H-indolium heterocycles separated by a [C-CC-C] bridge, were characterized with a combination of experimental and theoretical analyses. These studies demonstrate that the fast rotation about the [C-C] bond adjacent to the coumarin component competes with the radiative deactivation of the excited state in nonviscous environments. This geometrical change dislodges the coumarin and 3H-indolium cations out of planarity to allow the population of a weakly-emissive twisted intramolecular charge-transfer (TICT) state and produce fluorescence with low quantum yield. In viscous environments, the conformational change is slow and cannot compete effectively with the radiative deactivation of the excited state, which instead produces fluorescence with high quantum yield. These results indicate that structural modifications aimed at the restriction of the rotation of this [C-C] bond are essential to improve considerably the fluorescence quantum yield of this chromophoric platform. Should a synthetic strategy for the implementation of these design guidelines be identified, activatable fluorophores, based on the 3H-indolium platform, with improved brightness will ultimately emerge.