Efficient NIR-Light Emission from Solid-State Complexes of Boron Difluoride with 2′-Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2'-hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor-acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5b, which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the pi-stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene-containing dye 4, for example, a fluorescence quantum yield of about 15% with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3a and acetonaphthone- based compounds 1b, 2b, and 3b showed no evidence of degradation as solutions in CH2Cl2 that contained EtOH. In particular, solutions of brightly fluorescent compound 3a (brightness: epsilon x Phi(f) = 45 000 M (1)cm (1)) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid-state NIR fluorophores for applications in materials science.