Pb2+-Doping-Activated Localized Luminescence of Organic-Inorganic Cobalt (II) Bromides

Transition-metal Co2+ ions in halide systems are generally regarded as inefficient emitters due to rapid nonradiative relaxation of their d-d excited states. Here, we prepared two zero-dimensional (0D) organic-inorganic hybrid cobalt halides, benzyl triphenyl phosphonium bromide cobalt ((BTP)2CoBr4) and triphenyl phosphonium bromide cobalt ((PPh3)2CoBr4), composed of [CoBr4]2− tetrahedra embedded in distinct organic frameworks. Upon Pb2+ incorporation, both compounds exhibit visible photoluminescence composed mainly of two emission bands centered at approximately 450 and 500 nm, with decay lifetimes on the nanosecond timescale. The emission at 450 nm originates from d-d transitions associated with short-range Co-Co ferromagnetic (FM) interactions, whereas the 500 nm emission can be attributed to donor-acceptor pair (DAP) transitions involving Co and Pb. At low temperatures, an additional broadband emission centered at ~620 nm emerges in Pb2+-doped (PPh3)2CoBr4, with lifetimes reaching the microsecond scale, while such emission is not observed in (BTP)2CoBr4. Low-temperature lattice contraction in the PPh3 framework further shortens the Co-Co distance, thereby enhancing short-range FM interactions and leading to the formation of a magnetic bipolaron for emission. Overall, this work reveals the role of organic ligand structure and dopant modulation in regulating the Co-centered excited states, providing deeper insight into the interactions between cobalt ions and organic ligands in low-dimensional halide systems.