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Within a tight-binding Su–Schrieffer–Heeger model, the elementary excitations and optical absorption of a polyacene chain are investigated. The polyacene chain composed of a number of aromatic rings is considered as two strongly coupled polyacetylene chains with an open boundary condition. First of all we found an interchain-coupled neutral soliton in a pristine chain as a consequence of odd-number sites in each chain. There are two localized electronic states accompanying the soliton and appearing in the conduction and valence bands, respectively. Moreover, an injected electron or hole will induce a polaron-like deformation mixed with the interchain soliton, while two extra electrons or holes will result in three separate solitons, among which one is doubly charged and other two are neutral. The optical absorption due to these elementary excitations are obtained.

In this Part 1 of our two part study, 1D-linear oligomeric phthalocyanine zinc(II) with fully annulated (fused) phthalocyanine macrocycles are described. By semi-empirical and DFT computational methods the UV-vis and near infrared spectra, including energy gaps, are thoroughly evaluated. The long-wavelength Q band of the oligophthalocyanines is compared with the long wavelength absorption band of analogous oligoacenes consisting of annulated benzene rings. In addition, graphene nanoribbons are considered. The aromaticity is especially interesting when considering Clar’s rule. Therefore it is the aim to get information about the extent of $\pi$-electron delocalization in linear or semi-linear systems. Finally, the results of linear oligophthalocyanines are extrapolated to estimate the energy gap of 1D linear polyphthalocyanine by DFT, and the result is compared to the DFT calculation of the periodic structure with a plane-wave basis set. Part 2 will describe non-linear oligo- and two-dimensional polyphthalocyanines.