Understanding and Design Beyond Born-Oppenheimer using Time-Domain Vibrational Spectroscopy

The Born-Oppenheimer approximation is the cornerstone of quantum mechanics. It assumes, due to the large differences in their respective masses, that the response of the electrons to any changes in nuclear position is instantaneous. Consequently the two species (electrons and nuclei) are considered independently. However, this approximation breaks down when a molecule is in an electronically excited state, giving rise to coupling between the motion of the nuclei (vibrations) and that of the electrons. This is called vibronic coupling. This coupling permits communication between electronically excited states, and is of fundamental importance for understanding the vast majority of photophysical processes. Its effects, such as nonradiative decay, play a critical role in determining the efficiency of applications seeking to exploit excited state properties within a diverse range of applications including organic light-emitting diodes (OLEDs). Indeed, it is clear that if one wishes to extract as much light as possible from a molecule to be used in an OLED, it is essential the nonradiative decay and therefore vibronic couplings responsible for it are understood and then removed through the synthesis of new complexes.
Clearly in order to design molecular systems based upon vibronic couplings it is critical to first observe and rationalise its effects. In this proposal we develop the experimental infrastructure and theoretical methodology for broadband impulsive vibrational spectroscopy. This is applied to study the effect of vibronic coupling on the performance of 3rd generation OLEDs exploiting thermally activated delayed fluorescence (TADF) as a first example having tangible industrial impact. By determining the vibrational modes responsible for promoting radiative and non-radiative decay and most importantly the reverse intersystem crossing mechanism that enables triplet harvesting and yields efficient TADF OLEDs, we will establish a detailed understanding of the key interconnecting factors influencing the photophysical performance of TADF molecular systems, which can subsequently be developed into molecular design. From this first example, we aim to show how it will be possible to model many different phenomena that involve a charge transfer excited state. We want also to demonstrate this combined approach is the way forwarded in understanding all excited state optical phenomena. Lastly we want to show that efficient TADF may be an example of a dynamic process where vibrational motion switches the molecule between two states, one which has efficient reverse intersystem crossing and the other, fast radiative decay. This would be a major step forward in our understanding of complex excited state phenomena.

Knowledge Generation
The main outcomes from the current proposal are expected to be i) a new state-of-the-art setup for performing Broadband Impulsive Vibrational Spectroscopy, which will be optimized for the study of charge transfer excited states including the development of long delays between excitation and pump probe to enable transient pathways to be studied. ii) A rigorous framework for the computation of experimental signals based upon solutions of the time-dependent Schrödinger equation. iii) A detailed understanding of the role of molecular vibrational modes on the photophysical performance of TADF emitters of 3rd generation OLEDs.

People
This will have the most impact on the two Postdoctoral Research Assistant (PDRA) funded by this grant. The multidisciplinary and strongly collaborative components of the project will be hugely beneficial for their research and the skills they obtain will greatly increase their employability in the UK economy. This area's encompassed by this research (laser spectroscopy, computational, programming) will also enhance important skill sets of importance to the UK. Research performed and the setups realized are also expected to contribute to numerous PhD's and Master's students extending the scope of the impact far beyond those directly employed on the project. It will also impact the PI and CoI by continuing their ability to perform high quality novel research. Given the latter (TP) is an early career scientist, this proposal will have increased impact by providing significant research independence and opportunities for career development by embracing scientific leadership roles.

Society
The PI and CoI are strongly committed to the communication of scientific results and strategies to the general public. The School of Chemistry at Newcastle University has a strong outreach program, delivering a wide range of quality outreach activities including laboratory sessions, spectroscopy visits, presentations, revision workshops and science showcases. The schools dedicated outreach officer delivers this. Such events have a positive educational benefit to society. An additional societal impact arises with the successful application of TADF emitters for efficient OLEDs. While a longer term potential impact, this opens the possibility for significantly improved and cheaper devices used within a variety of everyday display and lighting appliances. Consequently it has the potential for a broad impact to the general public, leading to a significant economical and societal impact.

Economy
In the short term (<3 years), the economy will benefit via the employment of newly trained early-career scientists (PDRA's). In the longer term, economic impact will be achieved through the development of new efficient OLED devices based upon the research ideas outlined by this proposal. All of the work outlined in the present proposal is related to applied research funded by industrial partners. Indeed, the AM group is currently involved in three major European TADF projects, collaborating with 8 chemistry groups across Europe, three SME OLED materials companies including Novaled and also a separate research program with Merck. TP's group has a close collaboration with Cynora GmBH