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

玻恩-奥本海默近似是量子力学的基石。它假设，由于它们各自质量的巨大差异，电子对核位置的任何变化的反应都是瞬时的。因此，这两个物种(电子和原子核)被独立地考虑。然而，当分子处于电子激发态时，这种近似就失效了，从而引起了原子核运动(振动)和电子运动之间的耦合。这被称为振动耦合。这种耦合允许电子激发态之间的通信，对于理解绝大多数光物理过程是至关重要的。它的效应，如非辐射衰减，在包括有机发光二极管(OLED)在内的各种应用中，对于寻求利用激发态特性的应用的效率起着至关重要的作用。事实上，很明显，如果人们希望从用于OLED的分子中提取尽可能多的光，那么了解非辐射衰变以及由此产生的振动耦合是至关重要的，然后通过合成新的络合物来消除它。显然，为了设计基于振动耦合的分子系统，首先观察其影响并使其合理化是至关重要的。在这个提议中，我们发展了宽带脉冲振动光谱学的实验基础设施和理论方法。这是利用热激活延迟荧光(TADF)作为第一个具有明显工业影响的例子来研究振动耦合对第三代OLED性能的影响。通过确定促进辐射和非辐射衰减的振动模式，以及最重要的是能够实现三重态捕获并产生高效TADF OLED的反向系间交叉机制，我们将建立对影响TADF分子体系光物理性能的关键相互关联因素的详细了解，这些因素可以随后发展到分子设计中。从第一个例子中，我们的目标是展示如何对涉及电荷转移激发态的许多不同现象进行建模。我们还想证明这种结合的方法是理解所有激发态光学现象的前进方向。最后，我们想要证明，有效的TADF可能是一个动态过程的例子，其中振动运动使分子在两个状态之间切换，一个状态具有有效的系统间反向交叉，另一个状态具有快速辐射衰减。这将是我们在理解复杂激发态现象方面向前迈出的重要一步。

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.