Energy-Transfer Pathways for Photoluminescence in Mn²⁺-Doped 2D Hybrid Lead Bromide Systems

Mn²⁺ doping in hybrid perovskite-derived two-dimensional (2D) lead bromide systems exhibits efficient Mn²⁺ emission and enhanced environmental stability compared to their 3D analogue perovskites. Here, we investigate the sensitization of Mn²⁺ emission in doped 2D BA₂PbBr₄ (BA = butylammonium) through comprehensive temperature-dependent PL analyses. We establish a near-resonant energy transfer from the host excitonic state to the higher-lying Mn²⁺ multiplet (⁴T_2g (D)), followed by successive multiphonon-assisted de-excitations through the intermediate multiplets of Mn²⁺ between the originally populated ⁴T_2g (D) and the first excited multiplet, ⁴T_1g (G) state. Infrared (IR) spectroscopy identifies the highest-energy phonon mode at 59.2 meV in BA₂PbBr₄, more than twice that of the previously analyzed 3D perovskite CsPbCl₃ nanocrystals, pointing to a significantly more efficient multiphonon relaxation and suppressed phonon bottleneck effects in the 2D case, explaining the efficient Mn²⁺ emission observed even at low temperatures. The contrasting behaviors are further accentuated by the significantly higher exciton binding energy in the 2D case compared to the 3D ones. We derive temperature-dependent rate equations to quantitatively explain host-to-dopant energy-transfer dynamics, identifying the role of dark excitonic states in near-resonant energy-transfer and back-transfer processes, establishing a comprehensive understanding of Mn²⁺ emission in such 2D hybrid lead bromide systems.