Controlled tuning of radiative-nonradiative transition via solvent perturbation: Franck-Condon emission vs. aggregation caused quenching.

Organic molecules with tunable fluorescence quantum yield are attractive for opto-electronic applications. A fluorophore with tunable fluorescence quantum yield should be a better choice for a variety of applications that demand fluorophores with different quantum yields. Here organic emitters with a continuous bell-shaped fluorescence yield profile would be promising in view of sustainability and reusability; however, fluorophores with these properties are rarely reported. A bis-indole derivative, 3,3'-bisindolyl(phenyl)methane (BIPM), was synthesised and found to undergo a unique 'rise-and-fall' profile in fluorescence yield with a compositional change of the 1,4-dioxane (DiOx)-H 2 O solvent system. A predominant interplay of two contrasting factors, (a) polarity and proticity induced emission enhancement and (b) aggregation caused fluorescence quenching, on either side of a crossover solvent composition (∼50% f W ), resulted in a continuous bell-patterned fluorescence yield profile. Interestingly, these two factors could be observed individually or simultaneously by adjusting the H 2 O fraction. Detailed spectroscopic, electron microscopic and computational studies have been performed to substantiate the photophysics behind the solvent regulated modulation of fluorescence quantum yield.