State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China.
Acc Chem Res. 2012 Mar 20;45(3):404-13. doi: 10.1021/ar200135h. Epub 2011 Nov 9.
Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.
由于氢键在许多科学领域的基础重要性，它是当代研究的热点。化学家、物理学家和生物学家已经广泛地从实验和理论两个方面研究了氢键在基态的物理和化学性质。然而，氢键在电子激发态中的作用也很重要，它在许多光物理过程和光化学反应中起着重要作用，却很少被研究。当氢键系统通过光被电子激发时，由于不同电子态之间的电荷分布差异很大，氢键的供体和受体分子必须在电子激发态中重新排列。氢键的电子激发态动力学主要由氢键供体和受体基团的振动运动决定，通常发生在超快时间尺度的几百飞秒内。因此，先进的飞秒时间分辨振动光谱被用于直接监测氢键在电子激发态中的超快动力学行为。需要注意的是，激发态氢键动力学与电子激发相关联。幸运的是，飞秒时间分辨光谱学与激发态的精确量子化学计算相结合，在激光实验中解决了这个问题。通过比较氢键复合物与基态和电子激发态中分离的氢键供体或受体，获得了激发态氢键的结构和动力学信息。此外，我们还证明了氢键在许多光物理过程和光化学反应中的重要性。在本报告中，我们回顾了我们在电子激发态氢键动力学方面的最新进展，以及电子激发态氢键在内部转换 (IC)、电子光谱位移 (ESS)、光致电子转移 (PET)、荧光猝灭 (FQ)、分子内电荷转移 (ICT) 和金属-配体电荷转移 (MLCT) 中的重要作用。各种光谱实验与理论计算的结合，为激发态氢键研究带来了巨大的进展。我们首先证明了电子激发态中分子间氢键大大增强了香豆素发色团，而减弱了硫代羰基发色团。我们还澄清了分子间氢键的增强和减弱分别对应于电子光谱的红移和蓝移。此外，通过氢键相互作用调节电子态，可以显著影响无辐射衰减（通过 IC、PET、ICT、MLCT 等）。因此，在氢键环境中的发色团的荧光通过氢键被猝灭或增强。我们的研究通过描绘氢键与光子之间的相互作用，扩展了对氢键本质的理解，为光物理、光化学和光生物学中的激发态氢键研究提供了基础。
