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。它假定，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，这两种物质（电子和原子核）被认为是独立的。然而，当一个分子处于电子激发态时，这种近似就失效了，这导致原子核运动（振动）和电子运动之间的耦合。这就是所谓的振动耦合。这种耦合允许电子激发态之间的通信，并且对于理解绝大多数光物理过程具有重要意义。它的影响，如非辐射衰变，在确定包括有机发光二极管（oled）在内的各种应用中寻求利用激发态特性的应用效率方面起着关键作用。事实上，很明显，如果希望从用于OLED的分子中提取尽可能多的光，就必须了解非辐射衰变，从而了解导致它的振动耦合，然后通过合成新的复合物来消除它。显然，为了设计基于振动耦合的分子系统，首先观察和合理化其影响是至关重要的。在本提案中，我们发展了宽带脉冲振动光谱的实验基础和理论方法。这被应用于研究振动耦合对利用热激活延迟荧光（TADF）的第三代oled性能的影响，作为具有实际工业影响的第一个例子。通过确定负责促进辐射和非辐射衰变的振动模式，以及最重要的反向系统间交叉机制，使三重态收获和生产高效的TADF oled，我们将建立对影响TADF分子系统光物理性能的关键互连因素的详细了解，这些因素随后可以发展为分子设计。从第一个例子开始，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方法。最后，我们想表明，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种状态之间切换，一种状态具有有效的反向系统间交叉，另一种状态具有快速的辐射衰变。这将是我们对复杂激发态现象的理解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.