Narrowing the band of green emission in manganese hybrids by reducing the hydrogen bond strength and structural distortion

Abstract

Zero-dimensional (0D) tetrahedral Mn(II) hybrids as narrow-band green emitters show great potential for the backlighting of liquid crystal displays. However, the factors influencing the full-width at half-maximum (fwhm) of green emission in the 0D Mn hybrid systems still need to be understood in depth. In this work, the regulation of the fwhm of green-emitting tetrahedral Mn(II) hybrids has been investigated from the structural distortion and hydrogen bond strength points of view. Two 0D Mn-based OIMHs with cyclic organic cations, (MDPA)₂MnBr₄ (1, MDPA = N-methyldiphenylaminium) and (MTPP)₂MnBr₄ (2, MTPP = methyltriphenylphosphonium), have been synthesized. Both of them contain isolated [MnBr₄]²⁻ tetrahedra and show green emission at 540 and 518 nm upon 450 nm excitation. It is found that the fwhm of compound 2 (48 nm) is much narrower than that of 1 (62 nm), and we demonstrate that it is associated with the smaller distortion of the Mn(II) tetrahedron in 2. Furthermore, Hirshfeld surface calculations suggest that this smaller distortion could be contributed by the weak hydrogen bonds between the organic cations and the [MnBr₄]²⁻ units in 2 compared to that in 1. We found that the other tetrahedral Mn(II) bromides reported in previous literature also show this behavior. Meanwhile, the triboluminescence emission is observed in both 1 and 2, ascribed to the polarized charges on the crystal surface. A white light-emitting diode device was fabricated using compound 2 with a high PLQY (55.66% upon 450 nm). Our work not only provides an insight to understand in depth the role of structural distortion and hydrogen bond strength in regulating the fwhm of the Mn(II) tetrahedron, but also provides feasible rules to explore the new narrow-band green-emitting OIMHs.