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

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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.