Weakened Triplet-Triplet Annihilation of Diiodo-BODIPY Moieties without Influence on Their Intrinsic Triplet Lifetimes in Diiodo-BODIPY-Functionalized Pillar[5]arenes.

The triplet-triplet annihilation (TTA) effect of sensitizers themselves can lead to the additional quenching of lifetimes of triplet states; therefore, how to weaken the TTA effect of sensitizers is an urgent issue to be resolved for their further applications. Besides, it remains a tremendous challenge for constructing supramolecular systems of photosensitizers based on photosensitizer-functionalized pillararenes because there have been very few investigations on them. Thus, 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (DIBDP) and ethoxy pillar[5]arene (EtP5) were utilized to synthesize a DIBDP-functionalized pillar[5]arene (EtP5-DIBDP), a cyano-containing DIBDP (G) used as a guest molecule was also prepared, and they were used to investigate the electron-transfer mechanism between EtP5 and DIBDP moieties and weaken the TTA effect of DIBDP moieties. The theoretical computational results of frontier molecular orbitals and isosurfaces of spin density preliminarily predicted that the cavities of the EtP5 moiety had influence on the fluorescence emission of DIBDP units but not on their triplet states in EtP5-DIBDP. The fluorescence emission intensities in a variety of solvents with different polarities and electrochemical studies revealed that there was electron transfer from EtP5 to the DIBDP units, and the electron-transfer process had influence on the fluorescence emission but not on the triplet states of DIBDP moieties in EtP5-DIBDP, which verified the results of density functional theory calculations. The triplet state lifetimes of EtP5-DIBDP were longer than those of DIBDP and G and the photooxidation abilities of EtP5-DIBDP were better than those of DIBDP and G at a high concentration (1.0 × 10-5 M) in various solvents; in contrast, the intrinsic triplet state lifetimes and singlet oxygen quantum yields (ΦΔ) of DIBDP, G, and EtP5-DIBDP were very similar. This was because the steric hindrance of EtP5 moieties could weaken the TTA effect of DIBDP moieties without influencing their intrinsic triplet state lifetimes in EtP5-DIBDP.