Charge-transfer emission in nonplanar three-coordinate organoboron compounds for fluorescent sensing of fluoride
Charge-transfer emission in nonplanar three-coordinate organoboron compounds for fluorescent sensing of fluoride
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DOI：
10.1002/anie.200601286
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发表时间：
2006-01-01
影响因子：
16.6
通讯作者：
Wang, Suning
中科院分区：
文献类型：
--
作者：
Liu, Xiang Yang;Bai, Dong Ren;Wang, Suning
三配位有机硼化合物在硼中心上具有空的p π轨道，因此当存在适当的电子供体时，具有显示强烈的分子内电荷转移跃迁的趋势。这种给体-受体电荷转移特性使得三配位硼化合物在材料化学中的许多重要应用成为可能，例如非线性光学材料，[1]电荷传输材料和有机发光器件（OLED）中的发光体。[2]最近，几个研究小组已经证明，通过利用硼中心上的空p π轨道，三配位硼化合物也可以用作有效的比色、荧光或比率传感器来检测氟化物。[3]氟化物的选择性检测是当前的兴趣，因为它们对人类健康的重要性，它们对环境的影响，以及它们与神经毒剂的关联。[4，5]以前报道的基于三配位硼化合物的荧光传感器的工作原理是，氟化物与硼中心的结合破坏或干扰硼中心与芳族发色团之间的p π-π共轭，从而诱导荧光信号的变化。[3d]虽然三配位硼化合物的给体-受体电荷转移性质已被广泛用于非线性光学和OLED中的应用，但令人惊讶的是，很少有人对其在荧光传感器应用中的效用进行研究。我们的初步研究表明，三配位硼化合物的电荷转移荧光对氟化物非常敏感和选择性，并且可以通过操纵分子中供体和受体的几何形状来制造氟化物的“开启”荧光传感器。所有先前报道的产生强电荷转移荧光的三配位硼化合物都是平面共轭体系[1 - 3]，其中一个例子是我们最近报道的BNPB。[2k]BNPB具有通过4，4 ′-联苯基连接的N（Ph）（1-萘基）供体和B（均三甲苯基）2受体基团（参见方案2）. BNPB产生强烈的溶剂依赖性的荧光发射（例如lem = 492 nm，Fp = 0.67，在CH2Cl2中）源自氨基和硼中心之间的电荷转移。[2k]我们观察到，向BNPB溶液中添加氟化物会导致荧光猝灭，因为来自Fà的电子占据了硼p π轨道，从而有效地阻止了分子内电荷转移，从而导致发射强度下降。氟与BNPB中硼中心结合的直接证据来自我们对用BNPB滴定的（nBu4N）F溶液（TBAF）的19F NMR研究：光谱显示结合和未结合氟化物的不同19F化学位移。我们还已经确定，即使在其他卤化物如Cl2或Br2存在下，BNPB也仅与氟化物结合，当加入BNPB溶液时，这些卤化物不会引起荧光的显著变化。这种高选择性的氟代溴化铵与以前报道的氟化物传感器的基础上的三配位有机硼化合物，其中硼中心的邻位取代基的保护是一致的。[3]虽然BNPB对选择性检测氟代磷酸有潜在的用途，但它是一种"关闭"传感器;也就是说，荧光在氟化物存在下被淬灭。对于实际应用，信号检测在"开启"传感器中更有效。更好的是“开启”传感器，显示不同的颜色变化，快速和有效的传感。[3f]在我们寻找基于分子内电荷转移的"开启"有机硼传感器时，
Three-coordinate organoboron compounds possess an empty pπ orbital on the boron center and hence have a tendency to display intense intramolecular charge-transfer transitions when an appropriate electron donor is present. Such donor–acceptor charge-transfer properties have enabled a number of important applications of three-coordinate boron compounds in materials chemistry such as nonlinear optical materials,[1] charge-transport materials, and emitters in organic light emitting devices (OLEDs).[2] Recently it has been demonstrated by several research groups that three-coordinate boron compounds can also be used as effective colorimetric, fluorescent, or ratiometric sensors for the detection of fluoride by utilizing the empty pπ orbital on the boron center.[3] Selective detection of fluoride is of current interest because of their importance to human health, their impact on the environment, and their association with nerve agents.[4, 5] Previously reported fluorescent sensors based on threecoordinate boron compounds operate on the principle that the binding of fluoride to the boron center disrupts or perturbs the pπ–π conjugation between the boron center and the aromatic chromophore, thus inducing a change in fluorescent signal.[3d] Although the donor–acceptor chargetransfer properties of three-coordinate boron compounds have been exploited extensively for applications in nonlinear optics and OLEDs, surprisingly little investigation has been done on their utility in fluorescent sensor applications. Our preliminary investigation indicates that charge-transfer fluorescence of three-coordinate boron compounds can be very sensitive and selective for fluoride, and that “turn-on” fluorescent sensors for fluoride can be made by manipulating the geometry of the donor and acceptor in the molecule. All previously reported three-coordinate boron compounds that produce intense charge-transfer fluorescence are planar conjugated systems,[1–3] an example of which is BNPB, reported by us recently.[2k] BNPB has an N (Ph)(1-naphthyl) donor and a B (mesityl) 2 acceptor group that are linked by a 4, 4о-biphenyl group (see Scheme2). BNPB produces intense solvent-dependent fluorescent emission (eg lem= 492 nm, Fp= 0.67 in CH2Cl2) originating from the charge transfer between the amino and the boron centers.[2k] We have observed that the addition of fluoride to a solution of BNPB causes fluorescent quenching due to the occupation of the boron pπ orbital by electrons from FÀ, which effectively blocks intramolecular charge transfer, thus causing decrease in emission intensity. Direct evidence for FÀ binding to the boron center in BNPB comes from our 19F NMR study of a (nBu4N) F solution (TBAF) titrated with BNPB: the spectra revealed distinct 19F chemical shifts for bound and unbound fluoride. We have also established that BNPB binds to fluoride exclusively even in the presence of other halides such as ClÀ or BrÀ, which do not cause a significant change in fluorescence when added to the BNPB solution. This high selectivity for FÀ by BNPB is consistent with previously reported fluoride sensors based on three-coordinate organoboron compounds in which the boron center is protected by substituent groups at the ortho positions.[3] Although BNPB is potentially useful for selective detection of FÀ, it is a “turnoff” sensor; that is, fluorescence is quenched in the presence of fluoride. For practical applications, signal detection is more effective in “turn-on” sensors. Even better are “turn-on” sensors that display distinct color changes for fast and efficient sensing.[3f] In our search for “turn-on” organoboron sensors based on intramolecular charge-transfer …