BODIPY-Perylene Charge Transfer Compounds; Sensitizers for Triplet-Triplet Annihilation Up-conversion

Abstract

BODIPY heterochromophores, asymmetrically substituted with perylene and/or iodine at the 2 and 6 positions were prepared and investigated as sensitizers for triplet-triplet annihilation up conversion (TTA-UC). Single-crystal X-ray crystallographic analyses show that the torsion angle between BODIPY and perylene units lie between 73.54 and 74.51, though they are not orthogonal. Both compounds show intense, charge transfer absorption and emission profiles, confirmed by resonance Raman spectroscopy and consistent with DFT calculations. The emission quantum yield was solvent dependent but the emission profile remained characteristic of CT transition across all solvents explored. Both BODIPY derivatives were found to be effective sensitizers of TTA-UC with perylene annihilator in dioxane and DMSO. Intense anti-Stokes emission was observed, and visible by eye from these solvents. Conversely, no TTA-UC was observed from the other solvents explored, including from non-polar solvents such as toluene and hexane that yielded brightest fluorescence from the BODIPY derivatives. In dioxane, the power density plots obtained were strongly consistent with TTA-UC and the power density threshold, the I^th value (the photon flux at which 50 % of ΦTTAUC is achieved), for B2PI was observed to be 2.5x lower than of B2P under optimal conditions, an effect ascribed to the combined influence of spin-orbit charge transfer intersystem crossing (SOCT-ISC) and heavy metal on the triplet state formation for B2PI.

Keywords: BODIPY; TTA-UC; sensitizer; up-conversion.