The excited states of biphenylene and its negative and positive ions

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Abstract
The mononegative, dinegative and monopositive ions of biphenylene have been formed in solution. The electronic spectra of the negative ions (in tetrahydrofurane and dimethoxyethane solution) have been measured in the range 10 000–44 000 cm-1, and that of the positive ion (in H2SO4 solution) in the range 10 000–35 000 cm-1. The presence of paramagnetic ions in the solutions of the singly-charged species has been confirmed by measurement of electron spin resonance absorption. The long wavelength electronic spectra of the monopositive and mononegative ions are closely similar, which confirms the essentially ‘aromatic’ character of the biphenylene structure. Assignments are proposed for the principal bands in the spectra of the ions and the neutral molecule. Explicit calculation of the energies of the excited states by an approximate self-consistent field molecular orbital method yields results in reasonably good overall agreement with the experimental energy spectra. There is evidence from relative intensities of some of the bands that the ‘pairing’ of bonding and antibonding sets of molecular orbitals which is characteristic of benzenoid alternant hydrocarbons is partially removed in biphenylene. This may result from a difference of electronegativity of the carbon atoms in the 4-membered ring from that of the remaining carbon atoms.