Near-Infrared Emitting Cu(I) Cluster-Based Metal–Organic Frameworks with Thermally Activated Delayed Fluorescence

Reaction of 4-methylpyrimidine-2-thione (Hmpymt) with excess CuCl or CuCN at 150 °C gave two [Cu6(mpymt)6] cluster-based metal–organic frameworks {(CuCl)4[Cu6(mpymt)6]}n (1) and {(CuCN)4[Cu6(mpymt)6]·2Me2CO}n (2). The solid-state structures of 1 and 2 consisted of three-dimensional frameworks in which quasi-hexagonal [Cu6(mpymt)6] units are connected by [Cu4Cl4] units in 1 and [CuCN]n chains in 2. Compounds 1 and 2 exhibited strong near-infrared emission (λem = 737 and 744 nm) with photoluminescence quantum yields of 18.4% and 25.2% at ambient temperature, respectively. Fitting of the temperature-dependent lifetimes yielded model-derived singlet–triplet energy gap estimates of 0.060 and 0.077 eV for 1 and 2, respectively, with model-derived thermally activated delayed fluorescence contributions of approximately 72% and 66% at 300 K. The corresponding room-temperature emission lifetimes were 3.60 and 4.85 μs. A near-infrared phosphor-converted light-emitting diode device fabricated from 2 demonstrated promise for night-vision and potential near-infrared imaging applications.