Charge-Transfer Emission Involving Three-Coordinate Organoboron: V-Shape versus U-Shape and Impact of the Spacer on Dual Emission and Fluorescent Sensing

Graphical Abstract

V-Shape is better than U-shape in “turn-on” sensing of fluoride by charge-transfer emission (see figure). V-shaped molecules with a boron acceptor and a nitrogen donor group linked together by an organosilicon group have been found to display dual emission and much higher affinity towards fluoride ions than the corresponding U-shaped molecules, in which the donor and the acceptor groups are linked together by a 1,10-naphthyl group.

Abstract

New V-shaped bifunctional organosilicon compounds that contain an electron acceptor, B(Mes)₂, and an electron donor, N(1-naph)Ph, with the formulae Ph₂Si{p-C₆H₄B(Mes)₂}{p-C₆H₄N(1-naph)ph)} (1), Ph₂Si{p-C₆H₄(Mes)₂}{p-biphenyl-N(1-naph)ph} (2), and Si{p-C₆H₄B(Mes)₂}₂{p-C₆H₄N(1-naph)ph)}₂ (3) have been synthesized as model compounds for the investigation of through-space charge-transfer emission involving triarylboron and triarylamino centers. The photophysical properties of the new bifunctional organosilicon compounds are compared to two U-shaped compounds sBN and BN in which the boron acceptor and the amino donor groups are linked together by a rigid 1,10-naphthyl group. The results of our investigation establish that dual emission pathways, namely through-space donor-acceptor charge transfer and π–π* transitions coexist in the V-shaped molecules 1–3, while charge transfer emission is dominant in the U-shaped molecules. It is found that depending on the geometry of the linker and the B⋅⋅⋅N separation distance, the compound either displays dual emission bands simultaneously or single emission band. In addition, the dual emission pathways in these molecules can be selectively switched on or off by using fluoride ions. The sensitivity of response to fluoride ions by these molecules is also found to be highly dependent on the geometry of the linker and the B⋅⋅⋅N separation distance. The V-shaped molecules are found to be “turn-on” sensors to fluorides with a much higher sensitivity than the U-shaped molecules.