Charge-Transfer Triplet Chromophores: Adding Efficiency to Organic Photosensitisers

光诱导的化学转化在自然界中普遍存在，是许多生命过程的中心功能；细菌或植物的光合作用和植物或真菌的向光性，等等，都是长期感兴趣和重要的两个例子。在这些情况下，光吸收分子被光激发，并在将多余的能量(或电子)转移到衬底以进行后续的化学或物理过程之前，暂时存储这种能量。由于后一种过程中的活性物质通常不能与太阳辐射直接相互作用，因此这些原本不活跃的成分被光吸收分子“敏化”。太阳能作为最终的能量输入不仅表明了这些过程的可持续性，而且也暗示了这样一个事实，即光激发可以提供充足的能量来实现高势垒、高耗能的过程--光催化裂解水制氢作为太阳能燃料就是最近引起人们关注的一个例子，因为对可持续化学的需求不断增加。受这种可能性的启发，在温和的条件下使用光敏剂进行合成化学、光催化废物/毒素分解、光动力学治疗和光伏发电得到了广泛的应用。然而，由于过渡金属基材料的长寿命和强的激发态氧化还原能力，光敏剂的选择在很大程度上是由过渡金属基材料主导的。虽然毒性较小的有机光敏剂的开发将增强这一过程的可持续性，以及能量和结构的多样性，但它对优化过渡金属材料中的有利动力学和热力学参数提出了相当大的挑战。金属光敏剂的流行主要源于三重态激发态的有效形成，即不同分子轨道上的两个电子具有平行的自旋取向。三重态和基态(反平行自旋，单重态)的自旋组态的不同阻碍了能量的耗散，从而为分子间能量(或电子)从激发光敏剂中转移提供了足够的时间。为了释放无金属有机光敏剂的全部潜力，我在这个项目中提出了一个基于基本量子化学考虑的更新的分子设计。我将在商业染料中引入一个小的官能团来偏向它们的高能电子的轨道取向。我还将设计分子骨架上的电子密度，使电子在基态和激发态之间的分布不同。这两个特征将保证快速形成三重态，同时将自旋翻转过程的能量损失降至最低。我将定量分析光激发后的动力学过程，以测量在没有和存在反应底物的情况下新型光敏剂的能量转换效率。将特别注意确保能量调谐和实际应用的高光稳定性。此外，由于在这些激发光敏剂中存在几个小能量分离的自旋态，我将阐明附近能级对自旋翻转机制的影响。这项建议是建立在我在光化学和电化学活性材料的合成、分析和理论方面的研究专长的基础上的。该项目的成果将立即提供新的光催化工具，以提高反应效率。对能量转换机制的基本了解将有助于深入了解任何涉及自旋的过程，如电致发光、光伏和自旋电子学。

Light-induced chemical transformation is ubiquitous in nature and central to the function of many life processes; photosynthesis of bacterial or plant and phototropism of plant or fungi, among many others, are two examples of longstanding interest and importance. In these events, light-absorbing molecules are photo-excited and store this energy transiently before transferring the excess energy (or electron) to the substrates for subsequent chemical or physical processes. Since the reactive species in the latter process often cannot interact with solar radiation directly, these otherwise inactive components are said to be 'sensitised' by the light-absorbing molecules. The involvement of solar power as the ultimate energy input not only suggests the sustainability of these processes, but also hints at the fact that photoexcitation can supply an ample amount of energy to enable high barrier, energy-demanding process to occur--photocatalytic water splitting to generate hydrogen as solar fuel is one example that attracts much recent attention due to the increasing demands for sustainable chemistry.Inspired by this possibility, the use of photosensitizers under mild conditions for synthetic chemistry, photocatalytic waste/toxin decomposition, photodynamic therapy and photovoltaics has found widespread applications. The selection of photosensitizers, however, is largely dominated by the transition-metal-based materials due to the long lifetime and strong redox power in their excited states. While the development of less toxic organic photosensitizers would enhance the sustainability of this process, along with the energetic and structural diversity, it presents considerable challenges for optimising the favourable kinetic and thermodynamic parameters seen in transition-metal materials. The popularity of metal-based photosensitizers stems primarily from the efficient formation of the triplet excited state, where two electrons in different molecular orbitals have a parallel spin orientation. The difference between the spin configurations in the triplet excited state and the ground state (antiparallel spins, singlet) impedes energy dissipation, thus allowing sufficient time for inter-molecular energy (or electron) transfer from excited photosensitizers to occur.To unlock the full potential of metal-free organic photosensitisers, I present in this project a renovated molecular design based on fundamental quantum chemical considerations. I will introduce a small functional group into commercial dyes to bias the orbital orientation of their high-energy electrons. I will also engineer the electron density across the molecular skeleton such that the electron distribution will be dissimilar between the ground and excited states. These two features would guarantee fast triplet formation with minimal energy loss from the spin flipping process. I will quantitatively analyse the kinetic process after photoexcitation to gauge the energy conversion efficiency of the new photosensitizers in the absence and presence of reactive substrates. Special attention will be paid to ensure energy tuning and high photostability for practical applications. Furthermore, since several spin states of small energy separation are present in these excited photosensitisers, I will elucidate the influence of the nearby energy levels on the spin flipping mechanism. This proposal is built upon my research expertise in the synthetic, analytical, and theoretical aspects of photo- and electrochemically active materials. The outcome of the project will immediately offer new photocatalysis tools to enhance reaction efficiency. The fundamental understanding of the energy conversion mechanism will provide insight into any spin-involving processes, such as electroluminescence, photovoltaics and spintronics.