Manipulating spatial alignment of donor and acceptor in host-guest MOF for TADF‡

Thermally activated delayed fluorescence (TADF) was achieved when electron-rich triphenylene (Tpl) donors were confined to a cage-based porous metal-organic framework (MOF) host (NKU-111) composed of electron-deficient 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine (Tpt) acceptor as the ligand. The spatially separated donor and acceptor molecules in a face-to-face stacking pattern generated strong through-space charge transfer (CT) interactions with a small energy splitting between the singlet and triplet excited states (similar to 0.1 eV), which enabled TADF. The resulting Tpl@NKU-111 exhibited an uncommon enhanced emission intensity as the temperature increased. Extensive steady-state and time-resolved spectroscopic measurements and first-principles simulations revealed the chemical and electronic structure of this compound in both the ground and low-lying excited states. A double-channel (T-1, T-2) intersystem crossing mechanism with S-1 was found and explained as single-directional CT from the degenerate HOMO-1/HOMO of the guest donor to the LUMO+1 of one of the nearest acceptors. The rigid skeleton of the compound and effective through-space CT enhanced the photoluminescence quantum yield (PLQY). A maximum PLQY of 57.36% was achieved by optimizing the Tpl loading ratio in the host framework. These results indicate the potential of the MOFs for the targeted construction and optimization of TADF materials. A host-guest MOF strategy has been raised to achieve through-space charge transfer based TADF and modulable PLQY, which has been verified by spectroscopic measurements and first-principles simulations.

Automated summary

Thermally activated delayed fluorescence was achieved by spatially separating donor and acceptor molecules in a metal-organic framework host. The resulting material exhibited enhanced emission intensity and high photoluminescence quantum yield, which increased with temperature.