Cross Conjugated Polyenes Derived from 2‐Vinyl‐butadiene: Electronic States of their Radical Cations, and Triplet Energy

The doublet states of the radical cations of the cross conjugated polyenes 4,4‐dimethyl‐1‐methylidene‐2,5‐cyclohexadiene 2 and its bis‐derivative 1 have been investigated by photoelectron spectroscopy and by electronic spectroscopy of \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{+\kern-4pt {.} } $\end{document} + , prepared at 77 K in an electron scavenging matrix by γ‐irradiation. Simultaneous consideration of the spectral results shows \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{+\kern-4pt {.} } $\end{document} to be the second hydrocarbon molecular cation (after 2,2‐dimethyl isoindene) which possesses a first excited doublet state (D 1 ) of non‐ Koopmans nature ( 2 B 3g ). The first Koopmans ‐type excited state ( 2 B 2g ) expected from PE. spectroscopy lies, however, very close in energy. In addition T 1 of 1 was observed by electron energy loss spectroscopy at 2.0±0.1 eV. Application of the ‘SDT‐equation’ predicts for this state only 1.05 eV; there is at present no reasonable explanation for this failure.