The energy spectra of three classes of polybenzenoid hydrocarbons with a large number N (N ≈ 10 3 ) of carbon atoms have been studied theoretically. It is shown that in the asymptotic case N f ∞ the energy gap (EG) ∆E(Nf∞) is different from zero if the electron correlation is taken into account; that is, the π systems calculated should possess semiconductor properties. The results for the ∆E(Nf∞) * 0 of the hydrocarbons are in qualitative agreement with the results calculated for the EG of three classes of 1-D ladder polymers, which can be considered as models of quasi-1-D graphite. With increasing width (L) of the polymers, the band gap ∆E(Lf∞) approaches a value different from zero. The problem of the existence of defect states of hydrocarbons with vacancies is briefly discussed.

The structure and energy spectra of four classes of polybenzenoid hydrocarbons with different edge structures, a large number N (N ∼ 10 4 ) of carbon atoms, and different types of defect states (Tamm, Schottky and chemisorption states) have been studied theoretically. Several types of (probably) stable monoradicals and triplet biradicals with singly occupied MOs in the energy gap (molecular analogues of semiconductors with defect states) are characterized which could be used in the molecular electronics. A new concept for viewing high-spin π systems with ferromagnetically coupled electrons within the half-filled band of NBMOs in term of point defects (vacancies) was developed. The NBMOs can be considered as surface states (Tamm or Schottky).

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The energy spectra and the electric and magnetic properties of one-dimensional (1D) stacks consisting of conjugated π-electron systems (polycyclic aromatic hydrocarbons and polymethines) with Tamm defects are investigated theoretically by means of the many-electron band theory. The conditions for the relative arrangement of the stacks in terms of the slip parameters for yielding different magnetic and electric ground-state properties are studied.

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The nature of the spin exchange interaction within the half-filled band of one-dimensional stacks of 1-D mixed molecular radical crystals (MMRC) with the general formula:  ······PAH····R • ·····PAH····R • ·····PAH····R • ·····PAH····R • ·····(R • , radical; PAH, diamagnetic polycyclic aromatic hydrocarbon) was investigated theoretically. The various contributions to the Heisenberg effective exchange integral, the direct (Coulomb), kinetic, and indirect spin exchange for the MMRCs, are evaluated quantitatively, using band theory, and are compared with those of the corresponding parent molecular radical crystals (MRC, without hydrocarbons). The nature and the magnitude of the exchange interaction in MMRCs are significantly different in comparison with the parent MRCs:  the ferromagnetic interaction in MRC changes into antiferromagnetic in MMRCs and vice versa.