Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, United States.
J Phys Chem B. 2012 Sep 6;116(35):10522-34. doi: 10.1021/jp3020645. Epub 2012 May 4.
4-(Dimethylamino)benzonitrile (DMABN) has been one of the most studied photoinduced charge-transfer (CT) compounds for over 50 years, but due to the complexity of its excited electronic states and the importance of both intramolecular and solvent reorganization, the detailed microscopic mechanism of the CT is still debated. In this work, we have probed the ultrafast intramolecular CT process of DMABN in methanol using broad-band transient absorption spectroscopy from 280 to 620 nm and ultraviolet femtosecond stimulated Raman spectroscopy (FSRS) incorporating a 330 nm Raman pump pulse. Global analysis of the transient absorption kinetics revealed dynamics occurring with three distinct time constants: relaxation from the Franck-Condon L(a) state to the lower locally excited (LE) L(b) state in 0.3 ps, internal conversion in 2-2.4 ps that produces a vibrationally hot CT state, and vibrational relaxation within the CT state occurring in 6 ps. The 330 nm FSRS spectra established the dynamics along three vibrational coordinates: the ring-breathing stretch, ν(ph), at 764 cm(-1) in the CT state; the quinoidal C═C stretch, ν(CC), at 1582 cm(-1) in the CT state; and the nitrile stretch, ν(CN), at 2096 cm(-1) in the CT state. FSRS spectra collected with a 400 nm Raman pump probed the dynamics of the 1174 cm(-1) CH bending vibration, δ(CH). Spectral shifts of each of these modes occur on the 2-20 ps time scale and were analyzed in terms of the vibrational anharmonicity of the CT state, calculated using density functional theory. The frequencies of the δ(CH) and ν(CC) modes upshift with a 6-7 ps time constant, consistent with their off-diagonal anharmonic coupling to other modes that act as receiving modes during the CT process and then cool in 6-7 ps. It was found that the spectral down-shifts of the δ(CH) and ν(CN) modes are inconsistent with vibrational anharmonicity and are instead due to changes in molecular structure and hydrogen bonding that occur as the molecule relaxes within the CT state potential energy surface.
4-（二甲氨基）苯甲腈（DMABN）是研究时间超过 50 年的最具代表性的光诱导电荷转移（CT）化合物之一，但由于其激发电子态的复杂性以及分子内和溶剂重组的重要性，CT 的详细微观机制仍存在争议。在这项工作中，我们使用从 280 到 620nm 的宽带瞬态吸收光谱和结合 330nm 拉曼泵浦脉冲的紫外飞秒受激拉曼光谱（FSRS）研究了甲醇中 DMABN 的超快分子内 CT 过程。瞬态吸收动力学的全局分析表明，动力学发生在三个不同的时间常数：从 Franck-Condon L(a)态到较低的局域激发（LE）L(b)态的弛豫在 0.3 ps 内完成，在 2-2.4 ps 内发生内部转换，生成一个振动热 CT 态，以及在 6 ps 内 CT 态中的振动弛豫。330nm FSRS 光谱确定了三个振动坐标上的动力学：在 CT 态中，764cm(-1)处的环呼吸伸展，ν(ph)；在 CT 态中，1582cm(-1)处的醌式 C═C 伸展，ν(CC)；以及在 CT 态中，2096cm(-1)处的腈伸展，ν(CN)。用 400nm 拉曼泵收集的 FSRS 光谱探测了 1174cm(-1)处 CH 弯曲振动，δ(CH)的动力学。这些模式的每个模式的光谱位移都发生在 2-20 ps 的时间尺度上，并根据 CT 态的振动非谐性进行了分析，这是使用密度泛函理论计算得出的。δ(CH)和 ν(CC)模式的频率随时间常数为 6-7 ps 而上移，这与它们与作为 CT 过程中接收模式的其他模式的非对角非谐耦合一致，然后在 6-7 ps 内冷却。结果发现，δ(CH)和 ν(CN)模式的光谱下位移与振动非谐性不一致，而是由于分子在 CT 态势能表面内弛豫时分子结构和氢键的变化所致。
