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.