Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol.
Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol.
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DOI：
10.1039/c3cp51108a
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发表时间：
2013-04
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通讯作者：
A. Chatterley;J. D. Young;D. Townsend;Justyna M. Żurek;M. Paterson;G. Roberts;V. Stavros
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文献类型：
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作者：
A. Chatterley;J. D. Young;D. Townsend;Justyna M. Żurek;M. Paterson;G. Roberts;V. Stavros
利用超快时间分辨速度图离子成像(TRVMI)和时间分辨离子产额(TRIY)方法，全面揭示了光激发邻苯二酚(1，2-二酚)的电子态驰豫动力学。在激发到280.5(S1起源带，S1(v=0))到243nmS_1((1)ππ*)态之后，观察到这种态的布居数通过耦合到S_2((1)πσ*)态而衰变，S_2((1)O_(*))态相对于非氢键的‘自由’O-H键(标记为O(1)-H)是解离的。这一过程通过在5-11ps的时间框架内在S1/S2锥形交点(CI)下的隧道作用发生，导致O(1)-H键沿S2分裂。与平动激发的H原子相一致，伴随着基态过氧化氢自由基(C6H5O2(X))的形成发生在与S1态布居衰变相同的时间尺度上。在254-237 nm范围内，还观察到对S_2态的直接激发，表现为H原子的超快(~100飞秒)形成，具有较高的动能释放。根据这些测量，我们确定S1/S2CI位于S1(v=0)能级上方约3700-5500 cm(-1)，这表明从S1(v=0)→S2到隧道的势垒高度与相关的基准物种苯酚(苯酚)的观察到的高度相当。我们讨论了高度“振动增强”的隧道机制是如何导致邻苯二酚的隧道速率比先前测定的苯酚(>；1.2 ns)的隧道速率提高两个数量级的，尽管势垒高度相似。这种现象是邻苯二酚中非平面的S1激发态最小结构(C1对称性)的直接结果，这反过来又为S1(v=0)→S2的振动耦合带来了松弛的对称性约束-这是苯酚所不存在的情况。这些发现提供了一个很好的例子，说明了即使是对基本的、具有生物学意义的UV发色团进行简单的化学修饰(邻羟基取代)，也可以对随后的激发态动力学产生深远的影响。
Ultrafast time-resolved velocity map ion imaging (TR-VMI) and time-resolved ion-yield (TR-IY) methods are utilised to reveal a comprehensive picture of the electronic state relaxation dynamics in photoexcited catechol (1,2-dihydroxybenzene). After excitation to the S1 ((1)ππ*) state between 280.5 (the S1 origin band, S1(v = 0)) to 243 nm, the population in this state is observed to decay through coupling onto the S2 ((1)πσ*) state, which is dissociative with respect to the non-hydrogen bonded 'free' O-H bond (labelled O(1)-H). This process occurs via tunnelling under an S1/S2 conical intersection (CI) on a timeframe of 5-11 ps, resulting in O(1)-H bond fission along S2. Concomitant formation of ground state catechoxyl radicals (C6H5O2(X)), in coincidence with translationally excited H-atoms, occurs over the same timescale as the S1 state population decays. Between 254-237 nm, direct excitation to the S2 state is also observed, manifesting in the ultrafast (~100 fs) formation of H-atoms with high kinetic energy release. From these measurements we determine that the S1/S2 CI lies ~3700-5500 cm(-1) above the S1(v = 0) level, indicating that the barrier height to tunnelling from S1(v = 0) → S2 is comparable to that observed in the related 'benchmark' species phenol (hydroxybenzene). We discuss how a highly 'vibrationally-enhanced' tunnelling mechanism is responsible for the two orders of magnitude enhancement to the tunnelling rate in catechol, relative to that previously determined in phenol (>1.2 ns), despite similar barrier heights. This phenomenon is a direct consequence of the non-planar S1 excited state minimum structure (C1 symmetry) in catechol, which in turn yields relaxed symmetry constraints for vibronic coupling from S1(v = 0) → S2- a scenario which does not exist for phenol. These findings offer an elegant example of how even simple chemical modifications (ortho-hydroxy substitution) to a fundamental, biologically relevant, UV chromophore, such as phenol, can have profound effects on the ensuing excited state dynamics.