Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers

Abstract

Two π-conjugated radicals, fluorenyl (C13H9) and benzhydryl (C13H11), as well as the carbene diphenylmethylene (C13H10) were studied by imaging photoelectron-photoion coincidence spectroscopy using VUV synchrotron radiation. The reactive intermediates were generated by flash pyrolysis from 9-bromofluorene and α-aminodiphenylmethane (adpm), respectively. Adiabatic ionization energies (IEad) for all three species were extracted. Values of 7.01 ± 0.02 eV for fluorenyl and 6.7 ± 0.1 eV for benzhydryl are reported. For the triplet diphenylmethylene, an IEad of 6.8 ± 0.1 eV is found. The dissociative photoionization of 9-bromofluorene, the precursor for fluorenyl, was also studied and modeled with an SSACM approach, yielding an appearance energy AE0K(C13H9(+)/C13H9Br) of 9.4 eV. All experimental values are in very good agreement with computations. For fluorenyl, the IEad agrees well with earlier values, while for the benzhydryl radical, we report a value that is more than 0.6 eV lower than the one previously reported. The geometry change upon ionization is small for all three species. Although individual vibrational bands cannot be resolved, some vibrational transitions in the threshold photoelectron spectrum of fluorenyl are tentatively assigned based on a Franck-Condon simulation. In addition, the dimerization products of fluorenyl and the benzhydryl radical were detected. Ionization energies of (7.69 ± 0.04) and (8.11 ± 0.04) eV were determined for C26H18 and C26H22, respectively. On the basis of the ionization energies, we identified both molecules to be the direct dimerization products, formed in the pyrolysis without further rearrangement. Both dimers might be expected to play a role in soot formation because the radical monomers do appear in flames.

Formation of polycyclic aromatic hydrocarbons from bimolecular reactions of phenyl radicals at high temperatures

“…12 We emphasize that the degree of secondary reactions in the reactor can be controlled by several parameters, such as the precursor concentration, lengths of the SiC tube, pyrolysis temperature, and backing gas pressure. While in a continuous expansion at low pressure significant dimerization takes place, 20,24 it can in principle be suppressed in a pulsed jet. However, in the present experiments we deliberately maintained conditions that favour bimolecular reactions and observed them by choosing a suitable delay between laser and pulsed valve.…”

“…More detailed information on reaction products is obtained, when ions and photoelectrons are detected in coincidence. [24][25][26] We recently showed that IR/UV ion dip spectroscopy is a powerful, alternative tool to study high temperature reactions in a flow reactor, since it combines the structural sensitivity of infrared spectroscopy with mass information. 27,28 In this technique, molecules are ionised by [1+1] resonance-enhanced multiphoton ionisation (REMPI) with UV laser radiation.…”

Slow photoelectron velocity-map imaging spectroscopy of the C9H7 (indenyl) and C13H9 (fluorenyl) anions

“…ESR spectra have been measured for both radicals in solution. 34,35 Fischer and co-workers characterized the indenyl 36 and fluorenyl 37 radicals and their cations by threshold photoionization spectroscopy. In an inert gas matrix, visible absorption and emission of indenyl have been observed but not assigned, 38 while a combined Raman, FTIR, and UV/Vis study was carried out on fluorenyl.…”

Photochemical Generation of 9H‐Fluorenyl Radicals

“…Carbon‐centered radicals are ubiquitous species involved in numerous chemical reactions such as combustion, radical‐induced polymerization and photochemistry . Their paramagnetic properties allow for their direct observation by EPR spectroscopy, as well as other spectroscopic techniques such as UV/visible spectroscopy for conjugated systems incorporating such centers .…”