Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, USA.
J Phys Chem B. 2010 Nov 18;114(45):14646-56. doi: 10.1021/jp1023982. Epub 2010 Jun 22.
Femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption have been used to probe the photoinduced charge transfer (CT) dynamics of 4-(dimethylamino)benzonitrile in methanol and n-hexane. Through a combined analysis of temporal changes in the Raman modes and transient absorption kinetics, a more complete picture of the reaction coordinate of the intramolecular charge transfer process has been established. FSRS spectra of the phenyl C═C stretching mode (Wilson mode 8a) at 1607 cm(-1), which shifts to 1581 cm(-1) in the CT state, and transient absorption measurements ranging from 360 to 700 nm support internal conversion from the locally excited to the charge transfer state in 4-5 ps and then a subsequent vibrational relaxation within the CT state manifold on a 6-8 ps time scale. Dramatic shifting and narrowing of the 1581 cm(-1) quinoidal C═C stretch (ν(8a)) on the ∼7 ps time scale indicates that the quinoidal distortion is an important probe of the CT reaction dynamics. The cause of the spectral shifts is determined by comparing the observed shifts in the vibrational spectrum to anharmonic couplings computed for the benzonitrile radical anion by density functional theory (DFT) and with quantitative theoretical models of the solvent induced vibrational peak shifts. The DFT calculations indicate that the 10 cm(-1) downshift of the C═C stretch is most likely attributable to significant vibrational excitation in nontotally symmetric modes that are strongly anharmonically coupled to the C═C stretch.
飞秒受激拉曼光谱（FSRS）和飞秒瞬态吸收已被用于探测 4-（二甲氨基）苯甲腈在甲醇和正己烷中的光诱导电荷转移（CT）动力学。通过对拉曼模式和瞬态吸收动力学的时间变化的综合分析，建立了分子内电荷转移过程反应坐标的更完整的图像。在 CT 态下，苯基 C═C 伸缩模式（Wilson 模式 8a）的 FSRS 光谱从 1607 cm(-1)移至 1581 cm(-1)，瞬态吸收测量范围从 360 到 700 nm，支持在 4-5 ps 内从局部激发态到 CT 态的内转换，然后在 6-8 ps 的时间尺度上在 CT 态谱中进行后续的振动弛豫。在约 7 ps 的时间尺度上，醌式 C═C 伸缩（ν（8a））的急剧移动和变窄表明醌式畸变是 CT 反应动力学的重要探针。通过将观察到的振动谱中的位移与通过密度泛函理论（DFT）计算的苯甲腈自由基阴离子的非谐耦合以及溶剂诱导振动峰位移的定量理论模型进行比较，可以确定光谱位移的原因。DFT 计算表明，C═C 伸缩的 10 cm(-1)下移最可能归因于强烈非谐耦合到 C═C 伸缩的非全对称模式中的强烈振动激发。
