The Threshold Photoelectron Spectrum of Fulvenone: A Reactive Ketene Derivative in Lignin Valorization

Unveiling reaction mechanisms by isomer‐selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c‐C5H4=C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2‐methoxy acetophenone. The high‐resolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well‐resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadiene moiety. The adiabatic ionization energy was determined to be 8.25±0.01 eV and is assigned to the X˜+ 2A2←X˜ 1A1 transition. A broad and featureless band arising at 9 eV is associated with the A˜+ 2B1←X˜ 1A1 excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from the A˜+ into the X˜+ state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis. Fulvenone, a reactive ketene in lignin catalytic pyrolysis, was synthesized and characterized in situ using threshold photoelectron spectroscopy with synchrotron radiation. Two cationic states, strongly coupled via a conical intersection, contribute to the spectrum.