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Bhattacherjee, Aditi; Schnorr, Kirsten; Oesterling, Sven; Yang, Zheyue; Xue, Tian; Vivie-Riedle, Regina de; Leone, Stephen R. (2018): Photoinduced Heterocyclic Ring Opening of Furfural: Distinct Open-Chain Product Identification by Ultrafast X-ray Transient Absorption Spectroscopy. In: Journal of the American Chemical Society, Vol. 140, No. 39: pp. 12538-12544
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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.