Understanding the unconventional effects of halogenation on the luminescent properties of oligo(phenylene vinylene) molecules.

It is commonly known that halogenation tends to decrease the luminescence quantum yield of an organic dye, owing to the high electronegativity and heavy-atom effect of the halogen atom. However, based on an investigation of the effects of halogenation on the luminescence of the oligo(phenylene vinylene) (OPV) framework, we demonstrate that halogenation can have positive impact on the solid-state fluorescence and electrochemiluminescence (ECL) properties of OPV derivatives. The chlorinated OPV exhibits a very high solid-state fluorescence quantum yield (91%), whilst the brominated analogue gives the highest ECL emission intensity. Time-dependent density functional theory calculations, natural bond orbital analysis, and natural transition orbital analysis were performed to assist the understanding of the origin of these positive halogenation effects, which provide insight into the rational design of highly luminescent halogenated organic materials for solid-state devices and ECL applications.