Charge-transfer emission involving three-coordinate organoboron: V-shape versus U-shape and impact of the spacer on dual emission and fluorescent sensing.

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 pi-pi* 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 BN 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 BN 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.