Influence of halogen substitution on aggregation-induced near infrared emission of borondifluoride complexes of 2′-hydroxychalcones

Abstract

The generation of organic dyes displaying intense red and near infrared fluorescence emission in solution and in the solid state is a topic of intense current interest. In this study, we present the synthesis and investigation of nine new derivatives of borondifluoride complexes of 2′-hydroxychalcones: four homo- and five hetero-substituted compounds X-X and X-Y (X, Y = F, Cl, Br, and I), where two halogen atoms are attached ortho and para (X and Y respectively in X-Y) to the 2′-hydroxyl group. UV-vis absorption fluorescence spectra in solution and in the solid state, DFT calculations, and analyses of single-crystal structures were performed. For all compounds, a fluorescence emission enhancement was clearly observed when passing from the solution (DCM) to the crystal (up to more than 10 fold). These molecules thus exhibit aggregation-induced enhanced emission (AIEE) with fluorescence maxima ranging from 654 nm (F-F) to 807 nm (I-I) in the solid state. The highest fluorescence quantum yield value within the series is reached for Cl-Cl (24% at 730 nm) whilst I-Br shows a rather good efficiency (2.5% at 806 nm) despite the presence of the two heavy iodine and bromine atoms. This study sheds light on the role of the halogen substitution on crystalline packing and solid-state emission properties of the chalcone-BF2 dyes. We outline that using halogen atoms provides a useful cocktail of steric, electrostatic and photophysical ingredients upon which to base the generation of near infrared emitting organic solids.