The algebraic form of the perimeter model for nonaromatic cyclic pi-electron systems developed in parts 1-4 of this series is used to analyze the previously reported magnetic circular dichroism (MCD) of biphenylene (1) and its aza analogues, to classify its excited states, and to relate them to those of other nonaromatic cyclic,7 systems. The observed MCD signs are interpreted in terms of relative sizes of orbital energy differences and the resulting configuration energy ordering. These require deviations from the alternant pairing associated with the simplest classical description, which are attributed to the increased negative magnitude of the diagonal resonance integrals in the four-membered ring. The interpretation of the UV and MCD spectra of I is confirmed by the observed effects of aza substitution, and predictions for other types of substitution follow. The magnetic field induced state mixing deduced from the perimeter model is supported by computations by the linear combination of orthogonalized atomic orbitals (LCOAO), time-dependent density functional theory (TD DFT), and symmetry-adapted cluster configuration interaction (SAC-CI) methods.