Ma Chensheng, Ou Yue-Qun, Chan Chris Tsz-Leung, Wong Allen Ka-Wa, Chan Ruth Chau-Ting, Chung Bowie Po-Yee, Jiang Chao, Wang Ming-Liang, Kwok Wai-Ming
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong, P. R. China.
Phys Chem Chem Phys. 2018 Jan 3;20(2):1240-1251. doi: 10.1039/c7cp05140a.
As a case study of the interplay and the consequence of the interplay between intramolecular charge transfer (ICT) and intermolecular hydrogen (H)-bonding, a combined femtosecond time-resolved fluorescence (fs-TRF) and density functional theoretical (DFT) and time-dependent DFT (TDDFT) study has been conducted on methyl dimethylaminobenzoate (MDMABA) largely in a water solvent. Direct observation of the broadband spectra, anisotropy, and kinetic decays of fs-TRF from photo-excited MDMABA revealed a rapid ICT reaction occurring with a time constant of ∼0.7 ps from an initial locally excited (LE) state identified to have the Lππ* character; this produced a weakly emissive ICT state featuring radiative rate constant decreased by more than two orders of magnitude. The fluorescence of the ICT state is strongly quenched exhibiting a decay time of ∼49.7 ps, unusually faster than the nanosecond range lifetime in a polar aprotic solvent when intersystem crossing (ISC) is the major deactivation channel. This, according to the study of the solvent kinetic isotope effect, is identified to originate from an instantly enhanced strong solute-solvent H-bonding induced by the ICT reaction which allows elimination of the ISC, and enables the nonradiative decay to proceed almost entirely through the otherwise inaccessible internal conversion from the ICT state. The enhancement of H-bonding is verified by the calculation which presents theoretical evidence for not only the binding site and binding energy of the H-bonding configuration but also the electronic and structural characterization, lending support to the twisted ICT (TICT) description of the photo-excited MDMABA. This study contributes a prominent example for the extraordinary ability of water and a decisive role of ICT promoted H-bonding in offering a highly effective molecular mechanism for rapid elimination of the electronic excitation energy. The results contain an important insight for the in-depth understanding of the excited state H-bonding dynamics, and also have significant implication for clarifying the "sunscreen controversy" of the DMABA type of UVB sunscreen molecule.
作为分子内电荷转移（ICT）与分子间氢键（H）相互作用及其后果的一个案例研究，我们主要在水溶剂中对甲基二甲基氨基苯甲酸酯（MDMABA）进行了飞秒时间分辨荧光（fs - TRF）、密度泛函理论（DFT）和含时密度泛函理论（TDDFT）的联合研究。对光激发的MDMABA的fs - TRF宽带光谱、各向异性和动力学衰减的直接观察表明，从初始确定具有Lππ*特征的局域激发（LE）态发生了快速的ICT反应，时间常数约为0.7 ps；这产生了一个弱发射的ICT态，其辐射速率常数降低了两个多数量级。ICT态的荧光被强烈猝灭，衰减时间约为49.7 ps，异常快于极性非质子溶剂中当系间窜越（ISC）是主要失活通道时的纳秒级寿命。根据溶剂动力学同位素效应的研究，这被确定源于ICT反应瞬间增强的强溶质 - 溶剂氢键，它允许消除ISC，并使非辐射衰减几乎完全通过原本无法实现的从ICT态的内转换进行。氢键增强通过计算得到验证，计算不仅给出了氢键构型的结合位点和结合能的理论证据，还给出了电子和结构特征，支持了光激发MDMABA的扭曲ICT（TICT）描述。本研究为水的非凡能力以及ICT促进的氢键在提供快速消除电子激发能的高效分子机制中所起的决定性作用提供了一个突出的例子。这些结果为深入理解激发态氢键动力学提供了重要见解，也对阐明DMABA型UVB防晒分子的“防晒争议”具有重要意义。
