Dinuclear Metal Complexes for Near-Infrared Organic Light Emitting Diodes

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).

People and Knowledge:

The most direct impact on people is through the training and research work of the post-doctoral researchers to be appointed. Our laboratories offer excellent research and training environments for development of skills beyond PhD level, with a nurturing ethos pervading our Departments. The PDRAs will benefit from the multidisciplinary nature of the work: each will develop a broader appreciation of different factors involved in device optimisation beyond their specialist day-to-day activities. Coupled with the research management, data analysis and communication aspects, it will ensure that three well-rounded researchers emerge at the end of the project, well-placed to tackle diverse research and technological challenges in industry or academia.

The research will generate data on the absorption and emissive properties of a range of molecular materials. Rich insight is anticipated into pathways influencing the performance characteristics of low-energy phosphorescence, both at the molecular and device level. Synthetic design strategies will emerge that could equally be applied to multimetallic compounds for other applications, such as molecular magnetism and catalysis.

Economic Impact:

This proposal is rooted in fundamental research, but its outcomes have the potential to impact specialist industries; e.g., lighting and displays, security, sensing, bio-imaging, and therapy. These are areas where the UK has economic activity, yet they are fast-moving and subject to intense competition from elsewhere: access to new systems and to new insight is important to staying competitive. Companies with UK bases, to whom the research will be relevant and with whom the applicants have worked, include Merck, Samsung Cambridge Display Technology (OLED materials); FScan (imaging); Polyphotonix (phototherapy); Procter and Gamble (including interests in security and imaging). The applicants thus have a good starting point from which to initiate transfer of technology for economic impact. We will also liaise with manufacturers and suppliers of spectroscopic instrumentation, such as Horiba and Edinburgh Instruments, on methods for assessment of NIR-emitting materials and the definition of improved standards for this troublesome region of the spectrum. For example, materials may emerge that could be used as standards for NIR photoluminescence quantum yields, which could be amenable to marketing. Phosphorescent materials from the PI's laboratory have previously been supplied to Sigma for marketing as stains for cell imaging.

Academic and Societal impact:

To maximise the dissemination of our findings, we will invite UK academic and industrial researchers to a 1-day "sandpit-style" event early in the project and to an "NIR Symposium" towards the end. The first will aim to discuss needs and exchange ideas; the latter will showcase the key findings of our research, with presentations from the applicants and/or the PDRAs, along with selected invited lectures from overseas researchers with an interest in this spectral region. This dissemination route will be complemented by prompt publication of key results in leading high-visibility journals.

The PI has also been an instructor in a number of research-led summer schools on inorganic spectroscopy (e.g. the Dalton Summer School held in Edinburgh and similar events of the Polish Academy of Sciences and of Universities in Region Bretagne, France). Participation at such events - invariably well-received - allows results to be communicated to the next generation of researchers currently at PhD or PDRA stage of their careers.

The applicants are keen to engage with the wider public about their research. We will work on the design of a submission for the Durham Lumiere Festival (a biennial light show largely funded by Durham County Council, with Durham University as a partner) for an exhibit on light-emitting materials in the event anticipated in Nov 2021.