Photoinduced Heterocyclic Ring Opening of Furfural: Distinct Open-Chain Product Identification by Ultrafast X-ray Transient Absorption Spectroscopy
Photoinduced Heterocyclic Ring Opening of Furfural: Distinct Open-Chain Product Identification by Ultrafast X-ray Transient Absorption Spectroscopy
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DOI：
10.1021/jacs.8b07155
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发表时间：
2018-10-03
影响因子：
15
通讯作者：
Leone, Stephen R.
中科院分区：
文献类型：
--
作者：
Bhattacherjee, Aditi;Schnorr, Kirsten;Leone, Stephen R.
紫外诱导的五元杂环光化学反应中，开环是一个重要的激发态弛豫途径。然而，这种特殊的光诱导机制的鉴定对许多实验方法提出了挑战。我们发现，在碳k边（类似于284 eV）的飞秒x射线瞬态吸收光谱提供了核价光谱指纹，能够明确识别有机杂环的开环异构体。在x射线光谱中，环中与氧结合的碳原子与开环产物中与氧分离的碳原子在电子结构上的独特差异是显而易见的。在266nm光激发的糠醛中，通过C-O键裂变形成的超快开环在大约350fs内发生，证明了指纹核（碳1s）在283.3 eV时电子跃迁到开链碳中间体的非键轨道。在286.4 eV时，糠醛中1s pi*基态耗竭没有恢复，表明向基态的内部转换是一个次要通道。这些实验结果，加上最近在碳k边缘产生孤立阿秒脉冲的进展，将为未来探测开环锥形交叉动力学铺平道路。
The ultraviolet-induced photochemistry of five-membered heterocyclic rings often involves ring opening as a prominent excited-state relaxation pathway. The identification of this particular photoinduced mechanism, however, presents a challenge for many experimental methods. We show that femtosecond X-ray transient absorption spectroscopy at the carbon K-edge (similar to 284 eV) provides core-to-valence spectral fingerprints that enable the unambiguous identification of ring-opened isomers of organic heterocycles. The unique differences in the electronic structure between a carbon atom bonded to the oxygen in the ring versus a carbon atom set free of the oxygen in the ring-opened product are readily apparent in the X-ray spectra. Ultrafast ring opening via C-O bond fission occurs within similar to 350 fs in 266-nm photoexcited furfural, as evidenced by fingerprint core (carbon 1s) electronic transitions into a nonbonding orbital of the open-chain carbene intermediate at 283.3 eV. The lack of recovery of the 1s pi* ground-state depletion in furfural at 286.4 eV indicates that internal conversion to the ground state is a minor channel. These experimental results, augmented by recent advances in the generation of isolated attosecond pulses at the carbon K-edge, will pave the way for probing ring-opened conical intersection dynamics in the future.