Dinuclear Metal Complexes for Near-Infrared Organic Light Emitting Diodes

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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).