Uncovering the mask of sensitizers to switch on the TTA-UC emission by supramolecular host-guest complexation

Pt(II)−salophen complexes (S-1∼S-4) and 9,10-diphenylanthracene (DPA) tethering pillar[5]arene derivatives (A-1 and A-2) were synthesized to act as sensitizers and annihilators for triplet-triplet annihilation upconversion (TTA-UC), respectively. It turned out that the pyridine cation served as a mask for the excited state of the sensitizer, the triplet states of S-2 and S-3 were significantly quenched by photo-induced electron transfer (PET) with phosphorescence quantum yield quenched from 24.4% for S-4 to 9.3% for S-3, and therefore, both S-2 and S-3 led to negligible UC emissions when traditional annihilator DPA was used as the annihilator. Delightfully, when supramolecular annihilator A-1 and A-2 were employed to include the pyridine cation, PET was significantly inhibited and the triplet states of the sensitizers were activated, TTA-UC emission was therefore boosted. The UC quantum yield of A-2/S-3 system was up to 130 times higher than that of DPA/S-3 system, and the UC emission was switchable by the addition of competitive guests.