Förster and Dexter energy transfer boosted and weakened respectively by host−guest complexations between cyano-containing perylene diimide and BODIPY/diiodo-BODIPY functionalized pillar[5]arenes

How the intensities of host−guest interactions influence on FRET/TTET processes based on fluorophore-functionalized pillararenes is a key scientific issue to be resolved for designing novel FRET/TTET-based supramolecular fluorescent materials and devices by using fluorophore-functionalized pillararenes. Herein, 1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (BODIPY)-functionalized pillar[5]arene (EtP5-BDP), diiodo-BODIPY-functionalized pillar[5]arene (EtP5-DIBDP), and cyano-containing perylene diimide (PBI-CN) were prepared respectively. With the increasing of solvents’ polarities and the reducing of their molecular sizes (toluene→CHCl3→acetone→CH3CN), the host−guest interactions between EtP5 moieties and the solvents became stronger, which induced the host−guest interactions between EtP5-BDP⊃PBI-CN and EtP5-DIBDP⊃PBI-CN became weaker so that more free hosts and guests could neither be kept in the effective ranges (1–10 nm), nor diffuse to these effective ranges for FRET owing to the extremely short fluorescence lifetimes of them (only a few nanoseconds), which induced Stern–Volmer fluorescence quenching constants, FRET rate constants and efficiencies of FRET decreased. Meanwhile, in the order of toluene, CHCl3, acetone and CH3CN, the weakening of host−guest interactions between EtP5-DIBDP and PBI-CN enabled more free EtP5-DIBDP and PBI-CN to diffuse for colliding and completing a TTET process more efficiently as a result of the long-enough triplet lifetimes of EtP5-DIBDP in various solvents (>100 μs). Conversely, in the formed stable host−guest complexes of EtP5-DIBDP⊃PBI-CN in low-polar and large molecular size solvents, the steric hindrance of EtP5 moiety in EtP5-DIBDP⊃PBI-CN could not make more free molecules diffuse for colliding and completing an efficient TTET process, therefore, Stern–Volmer triplet quenching constants, TTET rate constants and efficiencies of TTET were all boosted.