Converting aggregation-induced emission of N-BF2 merocyanines to colour-tunable ultralong room temperature phosphorescence via Dexter triplet energy transfer: circumventing restriction of intersystem crossing

Abstract

The current research developed a pure π–π* transition-based guest design strategy, which mostly or solely utilizes Dexter triplet energy transfer to light up the guest triplet excitons, neglecting the reliance on guest's intrinsic intersystem crossing (ISC). In this work, the hybridization of conventional cationic cyanines with difluoroboron β-diketonate (BF₂bdk)-based anionic cyanines yielded a new group of neutral N-heteroaromatics-BF₂bdk (N-BF₂) merocyanines (NBMCy-A series). The pure (π,π*) character of the T₁ state, embedded intramolecular interactions, and high structural diversity were expected to endow these guest emitters with long τ_Ps, good Φ_Ps, and phosphorescence colour tunability. The photophysical properties of NBMCy-A1–A4 showed that these guests alone were typical AIEgens with excellent solid-state fluorescence properties. However, when they were doped in a benzophenone (BP) matrix in 1000 : 3 molar ratio, the resulting doped materials exhibited emerald to red room temperature phosphorescence with ultralong phosphorescence lifetimes ranging from 115 ms to 524 ms. Both experimental and theoretical investigations confirmed that the RTP of A1/BP–A4/BP was generated via the TTET pathway. The key factors including triplet–triplet energy gaps, reverse TTET, and interference from guest's absorption and other energy transfer processes were studied via theoretical calculations. Furthermore, with the assistance of molecular dynamics (MD) simulations and QM/MM calculations, the effects of molecular packing and orbital contact on Dexter triplet energy transfer, the proportions of the (π,π*) configuration of the T₁ state, and the intramolecular interactions of A1–A4 in the doped systems were also revealed.