Ultrafast charge transfer via a conical intersection in dimethylaminobenzonitrile
Ultrafast charge transfer via a conical intersection in dimethylaminobenzonitrile
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DOI：
10.1039/b111678a
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发表时间：
2002-04-01
影响因子：
3.1
通讯作者：
Trushin, SA
中科院分区：
文献类型：
--
作者：
Fuss, W;Pushpa, KK;Trushin, SA
4-（二甲氨基）苯甲腈的类L-α S-2态（2A）在130 ℃的气相中在267 nm处被泵浦。在800 nm处的非共振多光子电离与质量选择性检测，然后探测随后的过程。而在弗兰克-康登几何电离只产生了母离子，碎片增加运动的电荷转移（CT）状态。这种有用的差异归因于离子中的几何依赖性共振。发现的时间常数解释为超快（约68 fs）松弛通过一个圆锥交叉CT和L-b型S-1状态（1B）。然后，在1 ps内，粒子数在这两种状态之间达到平衡。从那里，分子在90 ps内弛豫到只能是三重态（T-n）的较低激发态，然后在300 ps内分解。以前的实验要么只研究了1B->CT弛豫--由于能量原因，它不会发生在气相或非极性溶剂中--要么从S-2激发开始，通常没有足够的时间分辨率（>1 ps）来检测暂时的电荷转移。仅最近在非极性溶剂中发现了CT状态的临时群体（Kwok等人，J Phys.Chem.A，2000，104，4188），这一结果与我们的机理完全一致。我们还表明，S-2-->S-1松弛不发生垂直，但涉及一个中间强的几何畸变，通过一个圆锥形的交叉。
The L-a-like S-2 state (2A) of 4-(dimethylamino)benzonitrile was pumped at 267 nm in the gas phase at 130 degreesC. Nonresonant multiphoton ionization at 800 nm with mass-selective detection then probed the subsequent processes. Whereas ionization at the Franck-Condon geometry only gave rise to the parent ion, fragmentation increased on motion towards the charge-transfer (CT) state. This useful difference is ascribed to a geometry-dependent resonance in the ion. The time constants found are interpreted by ultrafast (approximate to68 fs) relaxation through a conical intersection to both the CT and the L-b-type S-1 state (1B). Then the population equilibrates between these two states within 1 ps. From there the molecule relaxes within 90 ps to a lower excited state which can only be a triplet state (T-n) and then decomposes within 300 ps. Previous experiments either investigated only 1B-->CT relaxation-which does not take place in the gas phase or nonpolar solvents for energetic reasons-or, starting from S-2 excitation, typically had insufficient time resolution (>1 ps) to detect the temporary charge transfer. Only recently temporary population of the CT state was found in a nonpolar solvent (Kwok et al., J Phys. Chem. A, 2000, 104, 4188), a result fully consistent with our mechanism. We also show that S-2-->S-1 relaxation does not occur vertically but involves an intermediate strong geometrical distortion, passing through a conical intersection.