Dinuclear Metal Complexes for Near-Infrared Organic Light Emitting Diodes

该项目旨在创造出高效率地发出明亮的深红色和/或近红外(NIR)光的新设备。近红外(NIR)指的是电磁光谱中略高于人眼所感知的红光的区域。典型的定义是波长范围700-1400 nm。近红外辐射的能量比可见光低。虽然肉眼不可见，但近红外光谱是光谱中一个重要的技术区域，使用广泛可用的仪器很容易检测到；例如，低成本的硅探测器工作在1000 nm左右，峰值在850 nm左右。近红外通常用于电信、夜视系统的功能，并可应用于指纹技术等安全设备。它也特别适合于光用于疾病诊断或治疗的应用--因为生物组织对这一光区域是最透明的。在过去的20年里，可见光发射技术取得了巨大的进步。其中，有机发光二极管(OLED)因其节能、灵活、重量轻、易于大规模生产、适用于大面积显示器而特别有吸引力。金属络合物在这方面发挥着关键作用。它们的自旋-轨道耦合(SOC)提供了一种利用三重态的方法，否则由于自旋选择规则，这些三重态是不发射的。在器件中进行电荷组合时，三重态的形成与单重态的比例高达3：1，因此诱导三重态发射的能力提供了很大的效率收益。例如，在移动电话中，这直接转化为更低的功耗。而且电池充电的间隔时间更长。然而，用于近红外区域的OLED非常少，到目前为止所研究的大多数OLED效率都很低。许多因素共同降低了在深红色和近红外区域发射低能量的分子材料的发光量子产率。由于相关振动能级的Franck-Condon重叠较大，通过电子激发态与基态较高振动能级的耦合而产生的非辐射衰变变得更加有效，因为激发态能量降低--即所谓的“能隙定律”。对于金属有机发光体，由于处于激发态的金属特征的数量随着配体共轭的增加而趋于减少，因此红色/近红外发光体的磷光速率常数通常较低。因此，我们寻求解决的挑战是：(1)设计和合成红色/近红外发光分子，使其振动非辐射衰减通道最小化；(2)开发策略，通过该策略可以使此类分子的SOC途径更有效，从而磷光可以被促进并有效地与非辐射衰减竞争。我们将合成含有两个或两个以上金属离子的目标分子，旨在应对上述挑战。在研究了它们在所需光谱区域的发射特性后，我们将使用它们来制备深红色和近红外发光OLED，试验不同的器件架构以最大化效率，并设计出系统评估在该区域运行的器件的方法。到项目结束时，我们的目标将是：(1)制备一系列显示低能量发射的新的多核配合物；(2)对多核配合物中的SOC路径有清晰的了解；(3)获得工作在NIR区域的磷光OLED，其效率大大高于迄今报道的任何其他器件(我们的目标是超过40%的效率)。

This project aims to create new devices that brightly emit deep red and / or near infrared (NIR) light with high efficiency.The near-infrared (NIR) refers to that region of the electromagnetic spectrum that is just beyond what the eye perceives as red light. A typical definition is the wavelength range 700-1400 nm. NIR radiation has lower energy than visible light. Though invisible to the eye, the NIR is a technologically important region of the spectrum, readily detectable using widely available instrumentation; e.g. low-cost silicon detectors work to around 1000 nm and peak at about 850 nm. The NIR is commonly used in telecommunications, features in night vision systems, and can be applied to security devices such as fingerprint technology. It is also particularly well-suited to applications where light is used in the diagnosis or treatment of disease - since biological tissue is most transparent to this region of light.There have been huge advances in visible light-emitting technology over the past 20 years. Amongst them, organic light emitting diodes (OLEDs) are proving particularly attractive, as they are energy-efficient, flexible and light-weight, amenable to mass production, and well-suited to large-area displays. Metal complexes have a key role to play here. Their spin-orbit coupling (SOC) offers a means of harnessing triplet states that are otherwise non-emissive due to the spin-selection rule. Triplet states are formed in ratios as high as 3:1 relative to singlet states upon charge combination in a device, so the ability to induce triplet emission offers large gains in efficiency. In a mobile phone, for example, this directly translates into less power consumption ... and longer time intervals between charging of the battery.Yet, there are very few OLEDs for the NIR region, and most investigated to date have low efficiency. A number of factors conspire to reduce the luminescence quantum yield of molecular materials that emit at low energy - in the deep red and NIR regions. Non-radiative decay through coupling of the electronic excited state with higher vibrational levels of the ground state becomes more efficient as excited-state energy decreases, owing to greater Franck-Condon overlap of pertinent vibrational levels - the so-called "energy gap law". For organometallic emitters, this is compounded by typically lower phosphorescence rate constants in the red / NIR, since the amount of metal character in the excited state tends to decrease with increasing ligand conjugation.The challenges we seek to address are thus, simultaneously:(1) to design and synthesise red / NIR-emitting phosphorescent molecules in which vibrational non-radiative decay channels are minimized;(2) to develop strategies by which SOC pathways can be made more efficient for such molecules, so that phosphorescence can be facilitated and compete effectively with non-radiative decay. We will synthesise target molecules containing two or more metal ions, designed to meet the above challenges. Having studied their emission properties in the desired region of the spectrum, we will then use them to prepare deep red and NIR-emitting OLEDs, experimenting with different device architectures for maximization of efficiency, and devising methods for the systematic evaluation of devices operating in this region.Our goals by the end of the project will be to have:(1) prepared a diverse range of new multinuclear complexes showing low-energy emission;(2) developed a clear understanding of SOC pathways in multinuclear complexes;(3) obtained phosphorescent OLEDs operating in the NIR region that have efficiencies substantially higher than any others reported to date (our target is to exceed 40% efficiency).