Ternary Exciplexes for Enhanced Two-Photon Excited Fluorescence by the Synergy Effect of Dual Reverse Intersystem Crossing Channels

A nonlinear optical (NLO) ternary exciplex is developed using a two-photon absorbing acceptor material tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB), a two-photon absorbing donor material 4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC), and a linear optical acceptor material 2,4,6-tris(1,1′-biphenyl)-1,3,5-triazine (T2T). In the ternary mixture, the high-energy exciplex TAPC:3TPYMB and the low-energy exciplex TAPC:T2T generate an effective synergy effect on the basis of their reverse intersystem crossing (RISC) channels to achieve further utilization of triplet excitons, giving rise to significantly enhanced one- and two-photon excited fluorescence properties. The highest photoluminescence quantum yield (PLQY) of the ternary exciplex 3TPYMB:TAPC:T2T measured in air atmosphere is ∼77.8%, which is greatly improved compared to that of the binary exciplex TAPC:T2T (∼39.6%) and the binary exciplex TAPC:3TPYMB (∼3.7%). More interestingly, the ternary exciplex also exhibits enhanced two-photon excited fluorescence under long-wavelength laser excitation. In comparison, no improvement in PLQY is observed in another ternary exciplex 3TPYMB:4,4′,4″-tris(carbazole-9-yl)triphenylamine (TCTA):T2T, which lacks an effective synergy effect between its high- and low-energy exciplex RISC channels. Our synergistic dual-channel RISC strategy may be helpful for the design of multicomponent thermally activated delayed fluorescence (TADF) NLO materials and facilitate the development of NLO applications.