J-Aggregates of BODIPYs: Heat-Induced Fluorescence Enhancement via Polarity-Modulated Photoinduced Electron Transfer

: The ability to switch emission between bright and dark states through external stimuli is a key requirement for the design of adaptive optoelectronic materials. Here we demonstrate that covalently linked oligo-BODIPYs exhibit an unusual competition between exciton delocalization and polarity-driven reductive photoelectron transfer (rPET). While the nonfluorescent monomer dissipates excitation energy through rPET between the BODIPY core and a meso aniline substituent, J-aggregation in the oligomers enhances radiative decay in nonpolar environments. Using temperature-dependent fluorescence and ultrafast transient absorption spectroscopy, we show that solvent polarity and temperature finely regulate the population transfer between the bright exciton state and the dark charge-transfer (CT) state, with a pronounced dependence on oligomer chain length. Remarkably, lowering the temperature in moderately polar solvents leads to a dramatic decrease in emission intensity as the concomitantly increasing dielectric constant stabilizes the CT state. Our findings establish a general design principle for developing new environment-responsive chromophoric assemblies.