Enhanced Triplet-Triplet Energy Transfer and Upconversion Fluorescence through Host-Guest Complexation

Perylene-tethered pillar[5]arenes and C₆₀-boron-dipyrromethene (BODIPY) dyads were synthesized acting as emitters and organic triplet photosensitizers, respectively, for the purpose of improving the efficiency of triplet-triplet annihilation upconversion (TTA-UC). The photophysical properties of the sensitizers (guests) and the emitters (hosts) were not greatly influenced by the chemical modifications except for a notable decrease in the fluorescence quantum yields of the perlyene emitters due to the high local concentration. The perylene-tethered pillar[5]arenes form stable 1:1 complexes with a nitrile-bearing C₆₀-BODIPY dyad, showing association constants as high as 4.0 × 10⁴ M^-1. Through host-guest complexation, the efficiencies of both triplet-triplet energy transfer and TTA were significantly enhanced, which overcompensated for the loss of the fluorescence quantum yield of the emitters (hosts). Thus, an improved TTA-UC efficiency of 3.2% was observed even at a diluted concentration of 6 × 10^-5 M, demonstrating for the first time the effectiveness of the supramolecular motif for enhancing TTA-UC without varying the inherent photophysical properties of sensitizers and emitters.