从第一性原理修订分子内光致电子转移（PET）,Accounts of Chemical Research

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从第一性原理修订分子内光致电子转移（PET）
Accounts of Chemical Research ( IF 16.4 ) Pub Date : 2016-08-30 00:00:00 , DOI: 10.1021/acs.accounts.6b00299
Daniel Escudero ₁

Affiliation

光致电子转移（PET）在化学的许多领域都发挥着重要作用，包括光伏中的电荷分离过程，自然和人工光合作用以及光致发光传感器和开关。与其他许多光化学情况一样，结构和能量因素在确定PET及其竞争性光失活过程的速率和效率方面起着相关的作用。特别地，在荧光传感器和开关领域，认为分子内PET（在许多情况下没有令人信服的实验证据）是造成荧光猝灭的原因。对荧光团的分子设计以及实现最佳激发/发射光谱，激发系数和荧光量子产率（主要用于生物成像目的）的实验兴趣越来越高，但是较少的精力投入到基础力学研究上。在此报告中，我将使用最先进的量子化学工具来修改其中一些系统中的荧光猝灭的起源。这些研究超出了分析边界轨道能量图和执行PET热力学计算的通用策略。取而代之的是，使用时变密度泛函理论（TD-DFT），完整的有源空间自洽场（CASSCF）计算以及来自辐射的和非辐射的衰减率，来探索最低激发态的势能面（PES）。使用热振动相关函数形式从第一原理计算出所涉及的激发态。通过这种策略，这项工作揭示了荧光猝灭的真正起源，本文称为暗态淬灭。暗状态（不吸收或不发光的状态）通常难以进行实验，因此，计算研究可以为实际的光失活机理提供新颖的见解。对于包括质子，阳离子和阴离子靶在内的多种荧光探针，证明了暗态猝灭机理的成功。此外，这种机理提供了荧光猝灭的一般情况，其中包括分子内电荷转移（ICT），比率猝灭，以及那些认为是由PET引发的无辐射机理。最后，该帐户首次提供了一种计算协议，可以定量地估计这种现象，并根据第一原理为荧光探针的最佳设计提供了要素。暗状态（不吸收或不发光的状态）通常难以进行实验，因此，计算研究可以为实际的光失活机理提供新颖的见解。对于包括质子，阳离子和阴离子靶在内的多种荧光探针，证明了暗态猝灭机理的成功。此外，这种机理提供了荧光猝灭的一般情况，其中包括分子内电荷转移（ICT），比率猝灭，以及那些认为是由PET引发的无辐射机理。最后，该帐户首次提供了一种计算协议，可以定量地估计这种现象，并根据第一原理为荧光探针的最佳设计提供了要素。暗状态（不吸收或不发光的状态）通常难以进行实验，因此，计算研究可以为实际的光失活机理提供新颖的见解。对于包括质子，阳离子和阴离子靶在内的多种荧光探针，证明了暗态猝灭机理的成功。此外，这种机理提供了荧光猝灭的一般情况，其中包括分子内电荷转移（ICT），比率猝灭，以及那些认为是由PET引发的无辐射机理。最后，该帐户首次提供了一种计算协议，可以定量地估计这种现象，并根据第一原理为荧光探针的最佳设计提供了要素。

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Revising Intramolecular Photoinduced Electron Transfer (PET) from First-Principles

Photoinduced electron transfer (PET) plays relevant roles in many areas of chemistry, including charge separation processes in photovoltaics, natural and artificial photosynthesis, and photoluminescence sensors and switches. As in many other photochemical scenarios, the structural and energetic factors play relevant roles in determining the rates and efficiencies of PET and its competitive photodeactivation processes. Particularly, in the field of fluorescent sensors and switches, intramolecular PET is believed (in many cases without compelling experimental proof) to be responsible of the quench of fluorescence. There is an increasing experimental interest in fluorophore’s molecular design and on achieving optimal excitation/emission spectra, excitation coefficients, and fluorescence quantum yields (importantly for bioimaging purposes), but less efforts are devoted to fundamental mechanistic studies. In this Account, I revise the origins of the fluorescence quenching in some of these systems with state-of-the-art quantum chemical tools. These studies go beyond the common strategy of analyzing frontier orbital energy diagrams and performing PET thermodynamics calculations. Instead, the potential energy surfaces (PESs) of the lowest-lying excited states are explored with time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations and the radiative and nonradiative decay rates from the involved excited states are computed from first-principles using a thermal vibration correlation function formalism. With such a strategy, this work reveals the real origins of the fluorescence quenching, herein entitled as dark-state quenching. Dark states (those that do not absorb or emit light) are often elusive to experiments and thus, computational investigations can provide novel insights into the actual photodeactivation mechanisms. The success of the dark-state quenching mechanism is demonstrated for a wide variety of fluorescent probes, including proton, cation and anion targets. Furthermore, this mechanism provides a general picture of the fluorescence quenching which englobes intramolecular charge-transfer (ICT), ratiometric quenching, and those radiationless mechanisms believed to be originated by PET. Finally, this Account provides for the first time a computational protocol to quantitatively estimate this phenomenon and provides the ingredients for the optimal design of fluorescent probes from first principles.

更新日期：2016-08-30

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