Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization.
Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization.
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DOI：
10.1039/b611877a
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发表时间：
2007-02
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通讯作者：
W. Fuß;W. Schmid;Kumbil Kuttan Pushpa;S. Trushin;T. Yatsuhashi
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作者：
W. Fuß;W. Schmid;Kumbil Kuttan Pushpa;S. Trushin;T. Yatsuhashi
4-氨基苯甲腈衍生物有两个类似能量的激发态：除了类苯的L（B）态（也称为“局部激发”或LE态）外，一个具有电荷转移（CT）特性，在孤立分子中略高。适当极性的溶剂可以降低CT态，从而在它们中可以观察到双重荧光。尽管氨基扭曲是一个广泛的假设，但这种状态被置换的坐标沿着是有争议的。我们研究了一些这样的化合物在气相中的瞬态电离，最初激发较高的躺L（a）状态（S（2））。在这里，我们简要回顾了以前的结果4-（二甲氨基）苯甲腈（这类分子的原型），4-哌啶基-，吡咯烷基-和吡咯基-苯甲腈，并比较它们与新的结果4-氨基苯甲腈和桥衍生物N-甲基-6-氰基-1，2，3，4-四氢喹啉（NMC 6）。虽然在后两种分子中CT态以前从未被检测到过，但我们发现所有化合物的弛豫路径都是相同的：从S（2）开始，波包通过一个圆锥交叉点（CI）;从那里，它的一部分直接到达S（1）（L（B））状态，而另一部分临时填充CT状态（也在匪C 6中），从那里它也围绕CI到L（B）井。直接到达L（B）阱的波包在那里沿着坐标振荡，涉及氨基扭曲和摇摆或分子扭曲和奎尼向畸变。这些坐标必须是CI位移向量的分量。涉及苯环的键长交替的振动归因于由CI中的电子对称性变化引起的动量，即，非绝热耦合矢量。此外，CT状态涉及氨基扭曲，从相应信号的各向异性中得出结论。NMC 6中的六元脂族环阻碍了扭曲，并将CT状态提高到仍然低于L（a）状态的能量，因此它可以在无势垒过程中暂时填充。同样在氨基苯甲腈中，CT状态在L（a）和L（B）之间，并且从L（a）到达而没有势垒。扭曲是合理的振动相互作用与更高的状态，这是π-反键之间的氨基和芳环。
4-Aminobenzonitrile derivatives have two excited states of similar energy: besides the benzene-like L(b) state (also termed "locally excited" or LE state) one with charge-transfer (CT) character that is slightly higher in the isolated molecules. The CT state can be lowered by solvents of suitable polarity, so that dual fluorescence can be observed in them. It is controversial along which coordinate this state is displaced, although the amino-group twist is a wide-spread assumption. We investigated a number of such compounds by transient ionization in the gas phase, initially exciting the higher-lying L(a) state (S(2)). Here we briefly review the previous results on 4-(dimethylamino)benzonitrile (the prototype of this class of molecules), 4-piperidino-, pyrrolidino- and pyrrolyl-benzonitrile and compare them with new results on 4-aminobenzonitrile and on the bridged derivative N-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6). Although in the latter two molecules the CT state has never been detected before, we find the same relaxation path for all compounds: From S(2), the wave packet passes through a conical intersection (CI); from there part of it reaches the S(1) (L(b)) state directly, whereas another part temporarily populates the CT state (also in NMC6), from where it goes around the CI also to the L(b) well. The wave packet directly reaching the L(b) well oscillates there along coordinates involving amino-group twist and wagging or molecular arching and a quinoidal distortion. These coordinates must be components of the CI displacement vector. A vibration involving bond-length alternation of the benzene ring is ascribed to a momentum caused by the electronic symmetry change in the CI, i.e., to the nonadiabatic coupling vector. Also the CT state involves amino-group twist, as to conclude from the anisotropy of the corresponding signal. The six-membered aliphatic ring in NMC6 hinders the twist and raises the CT state to an energy that is, however, still below the L(a) state, so that it can be temporarily populated in a barrierless process. Also in aminobenzonitrile the CT state is between L(a) and L(b) and is reached from L(a) without a barrier. The twist is rationalized by vibronic interaction with a higher state that is pi-antibonding between the amino group and the aromatic ring.