近日，英国剑桥大学的Akshay Rao及其研究团队取得一项新进展。经过不懈努力，他们从有机分子高频振动中解耦激子。相关研究成果已于2024年5月8日在国际权威学术期刊《自然》上发表。

本文通过结合宽带脉冲振动光谱学、第一性原理建模和合成化学，探讨了π共轭分子中的激子-振动耦合。研究人员发现两个设计规则，将激子从高频振动中解耦。首先，当激子波函数具有明显的电荷转移特征，且电子和空穴密度在空间上不相连时，研究人员发现高频模式既可以定位于供体部分，也可以定位于受体部分，因此它们不会显著干扰激子能量或激子能量的空间分布。其次，可以选择这样的材料，使参与的分子轨道具有对称强加的非键合特性，从而与调制π键序的高频振动模式解耦。研究人员通过创建一系列具有非常有效的，近红外发射(680-800nm)电荷转移激子的自旋自由基系统来举例说明这两种设计规则。

研究表明，这些系统与振动模式的耦合仅低于250cm^-1，频率太低，无法允许快速非辐射衰变。与具有相似带隙的π共轭分子相比，这使得非辐射衰减率被抑制了近两个数量级。这项研究结果表明，由于耦合到高频模式的损耗不一定是这些系统的基本性质。

据悉，π共轭分子中激子与高频振动模式的耦合，特别是碳-碳伸缩模式(1,000-1,600cm^-1)被认为是不可避免的。这些高频模式加速了非辐射损耗，限制了发光二极管、荧光生物标志物和光伏器件的性能。

附：英文原文

Title: Decoupling excitons from high-frequency vibrations in organic molecules

Author: Ghosh, Pratyush, Alvertis, Antonios M., Chowdhury, Rituparno, Murto, Petri, Gillett, Alexander J., Dong, Shengzhi, Sneyd, Alexander J., Cho, Hwan-Hee, Evans, Emrys W., Monserrat, Bartomeu, Li, Feng, Schnedermann, Christoph, Bronstein, Hugo, Friend, Richard H., Rao, Akshay

Issue&Volume: 2024-05-08

Abstract: The coupling of excitons in π-conjugated molecules to high-frequency vibrational modes, particularly carbon–carbon stretch modes (1,000–1,600cm^-1) has been thought to be unavoidable. These high-frequency modes accelerate non-radiative losses and limit the performance of light-emitting diodes, fluorescent biomarkers and photovoltaic devices. Here, by combining broadband impulsive vibrational spectroscopy, first-principles modelling and synthetic chemistry, we explore exciton–vibration coupling in a range of π-conjugated molecules. We uncover two design rules that decouple excitons from high-frequency vibrations. First, when the exciton wavefunction has a substantial charge-transfer character with spatially disjoint electron and hole densities, we find that high-frequency modes can be localized to either the donor or acceptor moiety, so that they do not significantly perturb the exciton energy or its spatial distribution. Second, it is possible to select materials such that the participating molecular orbitals have a symmetry-imposed non-bonding character and are, thus, decoupled from the high-frequency vibrational modes that modulate the π-bond order. We exemplify both these design rules by creating a series of spin radical systems that have very efficient near-infrared emission (680–800nm) from charge-transfer excitons. We show that these systems have substantial coupling to vibrational modes only below 250cm^-1, frequencies that are too low to allow fast non-radiative decay. This enables non-radiative decay rates to be suppressed by nearly two orders of magnitude in comparison to π-conjugated molecules with similar bandgaps. Our results show that losses due to coupling to high-frequency modes need not be a fundamental property of these systems.

DOI: 10.1038/s41586-024-07246-x

Source: https://www.nature.com/articles/s41586-024-07246-x