Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer.
Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer.
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DOI：
10.1021/acs.jpca.9b10840
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发表时间：
2020-01
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通讯作者：
Martin Pižl;A. Picchiotti;M. Rebarz;Nils Lenngren;Yingliang Liu;S. Záliš;M. Kloz;A. Vlček
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文献类型：
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作者：
Martin Pižl;A. Picchiotti;M. Rebarz;Nils Lenngren;Yingliang Liu;S. Záliš;M. Kloz;A. Vlček
在40-2000（4000）cm-1的宽光谱范围内，测量了典型有机金属光敏剂/光催化剂ReCl（CO）3（2，2 '-bipyridine）的时间分辨飞秒受激拉曼光谱（FSRS）.理论的基态和激发态拉曼光谱通过非谐振动分析，使用二阶振动微扰理论的谐波近似计算的振动DFT优化的结构。一个很好的匹配与非谐计算的振动频率允许分配实验拉曼功能，以特定的振动。激发时观察到的频率偏移（ReCl（ReCl）和ReCl（CC环间）振动向上; ReCl（CC，CN）和ReCl（Re-C）向下）与激发中涉及的HOMO和LUMO的成键/反键特征一致，并支持最低三重态的离域公式为ReCl（CO）3 π bpy电荷转移。FSRS光谱显示了特定模式的时间演化，提供了深入了解系统间交叉（ISC）机制和随后的弛豫。大多数的拉曼特征存在于~40 fs处，并且随着时间的推移表现出小的位移和强度变化。1450-1600 cm-1的带组由于CC，CN和CC（环间）伸缩振动在40和~150 fs之间经历了广泛的重组，随后是频率上移和双指数（0.38，21 ps）面积增长，表明在最低三重态的形成和弛豫过程中进行电荷分离。早期（40-150 fs）的重组也观察到在低频范围内的Re-Cl（Re-Cl）和Re-C-O（Re-C-O）的振动，推测激活ISC。详细描述和讨论了用于同时测量低能和高能拉曼特征的FSRS实验创新。
Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO)3(2,2'-bipyridine) were measured in a broad spectral range ~40-2000 (4000) cm-1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at DFT-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation ((ReCl) and (CC inter-ring) vibrations upwards; (CC, CN) and (Re-C) downwards) are consistent with the bonding/antibonding characters of HOMO and LUMO involved in the excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO)3bpy charge-transfer. FSRS spectra show a mode-specific temporal evolution, providing insight into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ~40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm-1 group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ~150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for (Re-Cl) and (Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high- energy Raman features simultaneously are described and discussed in detail.