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 (D₁) of non-Koopmans nature (²B_3g). The first Koopmans-type excited state (²B_2g) expected from PE. spectroscopy lies, however, very close in energy. In addition T₁ 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.