Governing efficiency and thermoresponsivity of luminescence in dirhenium(v) molecules by a highly tunable emission mechanism

Abstract

The search for strategies that can create photoluminescent (PL) materials linking high emission efficiency with sensitivity to external stimuli is important for applications in light-emitting devices and sensors. We report experimental and theoretical studies on achieving the conjunction of PL of room-temperature quantum yields (RT-QYs) to 71% with optical thermometry of relative thermal sensitivity (S_r) to 5.2% K⁻¹ at RT. This was realized using molecular luminophores based on dirhenium(V) molecules whose properties are tuned by the organic ligand linking two Re(V) centers. We report molecular materials of the formula of (PPh₄)₄{[Re^V(CN)₄(N)]₂(L)} (Re₂–L) where N³⁻ is a nitrido ligand and L is a bridging ligand, i.e., 1,2-diaminoethane (Re₂–en), 4,4′-bipyridine (Re₂–bpy), 1,2-bis(4-pyridyl)acetylene (Re₂–bpac), 1,2-bis(4-pyridyl)ethylene (Re₂–bpee), 1,2-bis(4-pyridyl)ethane (Re₂–bpen), 1,4-bis(4-pyridyl)benzene (Re₂–bpb), and 4,4′-bis(4-pyridyl)biphenyl (Re₂–bpbp). They exhibit multi-color green-to-red PL whose origins range from metal-to-ligand charge transfer (³MLCT) to metal-centered d–d (³MC, ³[(d_xy)¹(d_π*)¹], dπ* = d_xz, d_yz) excited states. Materials of group A (Re₂–en, Re₂–bpen, Re₂–bpbp) reveal the critical role of ³MC states leading to higher energy emission with RT-QYs of 64–71%, while materials of group B (Re₂–bpy, Re₂–bpac, Re₂–bpee, Re₂–bpb) show ³MLCT emission of lower energy and RT-QYs of 34–41%. Materials of both groups exhibit distinct optical thermometry exploiting such parameters as the wavelength position, integrated intensity and width of emission peaks, correlated color temperature (CCT), Delta u,v (Duv), and emission lifetime (τ). Materials of group B reveal much better optical thermometry with high S_r values up to 5.2% K⁻¹ (300 K) and 2.29% K⁻¹ (170 K) for τ and Duv, respectively. We prove that efficient PL and optical thermometry can be achieved using metal–organic dirhenium(V) molecules, and further tuned by optimizing the organic linker expansion and character favoring less efficient but stronger thermoresponsive ³MLCT emission or more efficient but weaker thermoresponsive ³MC-type PL.