Direct emission from quartet excited states triggered by upconversion phenomena in solid-phase synthesized fluorescent lead-free organic-inorganic hybrid compounds

Abstract

We report, for the first time, the solid-phase gram-scale synthesis of two lead-free, zero-dimensional (0D) fluorescent organic–inorganic hybrid compounds, [Bu4N]2[MnBr4] (1) and [Ph4P]2[MnBr4] (2), achieved by grinding the organic and inorganic precursor salts. The solid-phase synthetic route has several advantages for modulating molecular dimensionalities. During grinding, organic cations and Mn2+ cations are co-crystallized together in the solid-state, forming a 0D assembly at the molecular level where each individual metal center is surrounded by organic cations. Both compounds exhibit an emission peak at 520 nm and a photoluminescence (PL) quantum yield (QY) of 47%. Here, we also report, for the first time, the upconversion phenomena which trigger different emission energies occurring in different quartet states of Mn, 4T1(4G), 4T2(4G), 4A1(4G), 4E(4G), 4T2(4D), 4E(4D), and 4T1(4P). These optical properties are unusual phenomena which break Kasha's rule of emission. Single particle imaging and low-temperature PL measurement are performed to obtain a deeper insight into these ground products. These results pave a new path to develop highly fluorescent non-toxic hybrid compounds with remarkable optical properties.