The coupling of excitons in π-conjugated molecules to high-frequency vibrational modes, particularly carbon–carbon stretch modes (1,000–1,600 cm⁻¹) has been thought to be unavoidable^1,2. 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 (680–800 nm) from charge-transfer excitons. We show that these systems have substantial coupling to vibrational modes only below 250 cm⁻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.

π共轭分子中的激子与高频振动模式的耦合，特别是碳-碳拉伸模式（1,000-1,600 cm ^-1）被认为是不可避免的^1,2。这些高频模式会加速非辐射损耗并限制发光二极管、荧光生物标记物和光伏器件的性能。在这里，通过结合宽带脉冲振动光谱、第一原理建模和合成化学，我们探索了一系列 π 共轭分子中的激子-振动耦合。我们发现了两条将激子与高频振动解耦的设计规则。首先，当激子波函数具有空间不相交的电子和空穴密度的显着电荷转移特征时，我们发现高频模式可以局域于供体或受体部分，因此它们不会显着扰动激子能量或其空间分布。其次，可以选择使参与的分子轨道具有对称性非键合特征的材料，从而与调节 π 键序的高频振动模式解耦。我们通过创建一系列自旋自由基系统来例证这两个设计规则，这些系统具有来自电荷转移激子的非常有效的近红外发射（680-800 nm）。我们表明，这些系统与仅低于 250 cm ^{-1 的}振动模式有很大的耦合，该频率太低而无法实现快速非辐射衰减。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近两个数量级。我们的结果表明，由于高频模式耦合造成的损耗不一定是这些系统的基本属性。