Roy Khokan, Kayal Surajit, Ravi Kumar Venkatraman, Beeby Andrew, Ariese Freek, Umapathy Siva
Department of Inorganic and Physical Chemistry, Indian Institute of Science , Bangalore, 560012, India.
Department of Chemistry, University of Durham , South Road, Durham DH1 3LE, U.K.
J Phys Chem A. 2017 Sep 7;121(35):6538-6546. doi: 10.1021/acs.jpca.7b03893. Epub 2017 Aug 24.
Excited state ultrafast conformational reorganization is recognized as an important phenomenon that facilitates light-induced functions of many molecular systems. This report describes the femtosecond and picosecond conformational relaxation dynamics of middle-ring and terminal ring twisted conformers of the acetylene π-conjugated system bis(phenylethynyl)benzene, a model system for molecular wires. Through excitation wavelength dependent, femtosecond-transient absorption measurements, we found that the middle-ring and terminal ring twisted conformers relax at femtosecond (400-600 fs) and picosecond (20-24 ps) time scales, respectively. Actinic pumping into the red flank of the absorption spectrum leads to excitation of primarily planar conformers, and results in very different excited state dynamics. In addition, ultrafast Raman loss spectroscopic studies revealed the vibrational mode dependent relaxation dynamics for different excitation wavelengths. To corroborate our experimental findings, DFT and time-dependent DFT calculations were carried out. The Franck-Condon simulation indicated that the vibronic structure observed in the electronic absorption and the fluorescence spectra are due to progressions and combinations of several vibrational modes corresponding to the phenyl ring and the acetylenic groups. Furthermore, the middle ring torsional rotation matches the room-temperature electronic absorption, in stark contrast to the terminal ring torsional rotation. Finally, we show that the middle-ring twisted conformer undergoes femtosecond torsional planarization dynamic, whereas the terminal rings relax on a few tens of picosecond time scale.
激发态超快构象重组被认为是促进许多分子系统光诱导功能的重要现象。本报告描述了乙炔π共轭体系双（苯乙炔基）苯（一种分子导线模型体系）中环和端环扭曲构象异构体的飞秒和皮秒构象弛豫动力学。通过依赖激发波长的飞秒瞬态吸收测量，我们发现中环和端环扭曲构象异构体分别在飞秒（400 - 600飞秒）和皮秒（20 - 24皮秒）时间尺度上弛豫。向吸收光谱的红边进行光化泵浦主要导致平面构象异构体的激发，并导致非常不同的激发态动力学。此外，超快拉曼损耗光谱研究揭示了不同激发波长下依赖振动模式的弛豫动力学。为了证实我们的实验结果，进行了密度泛函理论（DFT）和含时密度泛函理论（TD-DFT）计算。弗兰克 - 康登模拟表明，在电子吸收光谱和荧光光谱中观察到的振转结构是由于对应于苯环和炔基的几种振动模式的进展和组合。此外，中环扭转旋转与室温电子吸收相匹配，这与端环扭转旋转形成鲜明对比。最后，我们表明中环扭曲构象异构体经历飞秒扭转平面化动力学，而端环在几十皮秒的时间尺度上弛豫。
