Temperature/component-dependent luminescence in lead-free hybrid metal halides for temperature sensor and anti-counterfeiting

Hybrid metal halides (HMHs) have emerged as a promising platform for optically functional crystalline materials, but it is extremely challenging to thoroughly elucidate the electron transition coupled to additional ligand emission. Herein, to discover sequences of lead-free HMHs with distinct optically active metal cations are aimed, that is, Sb${}^{3+}$ (5s${}^2$) with the lone-pair electron configuration and In${}^{3+}$ (4d${}^{10}$) with the fully-filled electron configuration. (Me${}_2$NH${}_2$)${}_4$MCl${}_6$·Cl (Me = -CH${}_3$, M = Sb, In) exhibits the superior temperature/component-dependent luminescence behaviors resulting from the competition transition between triplet-states (T${}_n$-S${}_0$) self-trapped excitons (STEs) of inorganic units and singlet-state (S${}_1$-S${}_0$) of organic cations, which is manipulated by the optical activity levels of [SbCl${}_6$]${}^{3-}$ and [InCl${}_6$]${}^{3-}$. The bonding differences between Sb${}^{3+}$/In${}^{3+}$ and Cl${}^{-}$ in terms of electronic excitation and hybridization are emphasized, and the different electron-transition mechanisms are established according to the PL spectra at the extreme temperature of 5 to 305 K and theoretical calculations. By fine-tuning the B-site Sb${}^{3+}$/In${}^{3+}$ alloying, the photoluminescence quantum yield (PLQY = 81.5%) and stability are optimized at 20% alloying of Sb${}^{3+}$. This research sheds light on the rules governing PL behaviors of HMHs, as well as exploring the optical-functional application of aviation temperature sensors and access-control systems. (© 2024 Wiley‐VCH GmbH)