Variable dual luminescence in d/f hybrid complexes by control of energy transfer

发光分子（即发光的）用于各种各样的应用中，其中两个最重要的是显示设备（有机发光二极管或OLED）和传感器（其中发射光的强度/颜色对被分析的特定物质的存在敏感）。在几乎所有的情况下，发光化合物都含有一个发光中心，从而发出单一颜色的光。这意味着，例如，白光显示器需要含有红/蓝/绿发光化合物的混合物，这些化合物可能以不同的速率缓慢降解或对温度具有不同的敏感性，使得很难获得清晰的白色发光。（ii）基于Eu（III）的红色发射镧系元素片段;和（iii）连接它们的共轭桥。其工作原理是，光被过渡金属片段选择性地吸收，过渡金属片段在UV和可见光区具有通常的强烈电荷转移吸收，而Eu（III）片段不吸收光。这将产生过渡金属单元的激发态，该激发态将部分地随着蓝/绿色光的发射（辐射衰减）而塌陷，并且部分地通过将其激发能通过桥接配体转移到Eu（III）单元而塌陷，Eu（III）单元又将产生红色发光。两种发光成分（蓝色/绿色和红色）之间的平衡将非常精确地取决于能量转移步骤的效率：如果它是缓慢的，来自过渡金属的蓝色/绿色发光将占主导地位，因为只有一小部分Eu（III）中心将被能量转移激发。如果能量转移快，则几乎没有蓝色/绿色发光，但大部分是红色发光，因为几乎所有的激发能量都将转移到Eu（III）中心。因此，可以通过控制沿桥连配体沿着的能量转移程度来微调发光的颜色。这可以通过以下两种机制之一来实现：控制两个苯环之间的扭曲程度，这将改变金属末端之间的电子耦合;或者使用溶剂化变色或金属变色现象来改变过渡金属组分的激发态能量。以这种方式对来自单个分子的发光颜色的这种控制和变化性本身是新颖的，并且具有许多可能的应用。例如，来自两种组分的仔细平衡的贡献将允许白光产生（以正确比例的红色和蓝色/绿色的组合）形成单个分子，这一结果在白色发光显示器件的制造中提供了实质性的优点。此外，调制蓝色/绿色和红色发光组分的能力将允许络合物用作比率传感器，其中不同波长的组分之间的平衡取决于金属离子的存在或不存在，所述金属离子或者改变构象柔性桥的扭转角，或者通过金属变色效应改变过渡金属组分的能量，在任一情况下控制能量传递事件。结果是特定金属离子的存在会导致在可见光谱区从蓝色到红色的宽范围内发生明显的颜色变化，这是非常容易检测到的。总之，合成，详细的电子物理研究和能量转移的计算研究的组合将被用于开发可切换的双发射配合物，其具有作为OLED中的白光发射体，和双色比率传感器，基于过渡金属通过桥接配体向镧系元素能量转移的相同潜在控制。

Luminescent molecules (i.e. that emit light) are used in a wide variety of applications, two of the most important being display devices (organic light-emitting diodes, or OLEDS) and as sensors (where the intensity / colour of the emitted light is sensitive to the presence of a particular species being analysed). In virtually all cases the luminescent compounds contain one slight-emitting centre such that a single colour of light is emitted. This means, for example, that white-light displays need to contain a mixture of red/blue/green-emitting compounds which may slowly degrade at different rates or have different sensitivity to temperature, making it very difficult to get clear white luminescence.This proposal is to make a series of complexes that contain (i) a blue- or green-emissive transition metal fragment based on Ir(III) or Pt(II); (ii) a red-emissive lanthanide fragment based on Eu(III); and (iii) a conjugated bridge connecting them. The principle of operation will be that light is absorbed selectively by the transition metal fragment, which will have the usual intense charge-transfer absorptions in the UV and visible region, whereas the Eu(III) fragment does not absorb light. This will generate the excited state of the transition metal unit which will collapse partly with emission of blue/green light (radiative decay) and partly by transferring its excitation energy through the bridging ligand to the Eu(III) unit which will in turn generate red luminescence. The balance between the two luminescence components (blue/green and red) will depend very precisely on how efficient the energy-transfer step is: if it is slow, blue/green luminescence from the transition metal will dominate because only a small proportion of the Eu(III) centres will be excited by energy-transfer. If energy-transfer is fast, there will be little blue/green luminescence but mostly red luminescence as nearly all of the excitation energy will be transferred to the Eu(III) centre. Thus the colour of the luminescence can be fine-tuned by controlling the degree of energy-transfer along the bridging ligand. This can be done by one of two mechanisms: controlling the degree of twist between two phenyl rings, which will modify the electronic coupling between the metal termini; or altering the excited-state energy of the transition metal component using the phenomena of solvatochromism or metallochromism. Such control and variability of the colour of luminescence from a single molecule in this way is in itself novel and has many possible applications. For example, carefully-balanced contribution from the two components will allow white-light generation (combination of red and blue/green in the correct proportions) form a single molecule, a result which offers substantial advantages in the preparaton of white-luminescent display devices. In addition, the ability to modulate the blue/green and red luminescence components will allow the complexes to be used as ratiometric sensors, in which the balance between the components at different wavelengths depends on the presence or absence of a metal ion which either varies the twist angle of the conformationally flexible bridge, or alters the energy of the transition metal component via the metallochromic effect, in either case controlling the energy-transfer event. The result will be that the presence of specific metal ions results in an obvious colour change over a wide range from blue to red in the visible region of the spectrum, which is exceptionally easy to detect.In conclusion a combination of synthesis, detailed photophysical studies, and computational studies on energy-transfer will be used to develop switchable dual-emissive complexes with potential applications as both white-light emitters in OLEDS, and two-colour ratiometric sensors, based on the same underlying control of transition-metal to lanthanide energy-transfer through the bridging ligand.