Temperature and solvents dependence of excited state dynamics in molecular excitons of bodipy oligomers

We studied how oligomer length, solvent polarity, and temperature affect the excited-state behavior of a BODIPY chromophore and four of its stable oligomers.[1] These molecules combine a rigid BODIPY core with a substituted aniline group to induce a reductive photoinduced electron transfer (rPET). Longer oligomers shift absorption and emission to lower energy and narrow the bands, showing exciton stabilization by delocalization. Femtosecond pump–probe experiments reveal two pathways: an excitonic state and a charge-transfer (CT) state. In toluene, spectra show only excitonic features, including ground-state bleaching, stimulated emission, and high-energy ESA. In THF and benzonitrile, a new low-energy absorption band appears, assigned to the CT state, whose population increases with solvent polarity and is strongest in the dimer. Temperature-dependent measurements in 2Me-THF confirm this behavior: cooling stabilizes the CT state, reduces fluorescence, and introduces a new low-energy emissive band. Pump–probe data show CT absorption growth and excitonic ESA decrease. Longer oligomers reduce CT population because the excitonic state remains more stable. Combining steady-state spectroscopy, pump–probe measurements, and temperature studies, we mapped how excitonic and CT states shift relative to each other. Oligomer length stabilizes excitons, while solvent polarity and cooling stabilize the CT state, enabling controlled tuning of emission across the series. Overall, BODIPY oligomers emerge as promising environment-responsive chromophores with tunable excited-state pathways for optoelectronic and light-harvesting applications.