Abstract

Frenkel Excitons (FEs) as collectivized electronic excitations are treated using two models of helical two-strand regular polymers with B-DNA geometry. As basis excitations we consider two excitations of a pair of complementary bases, not the excitations of separate bases (due to the permanent pairing, e.g. A-T, of complementary bases and small inter-bases distance). The studied regular structures contain combination of one pair only ordered as follows: A. Complementary homopolymers, i.e. pairing of two strands each with one type of base (A or T) B. Self-complementary copolymers in which each strand is supposed to contain alternatively ordered A-T-A:… (and each step of double helix contains the pair A-T). The linear absorption and circular dichroism (CD), correspondingly the tensor of dielectric permittivity and of gyration are calculated near the frequencies of FEs, using method of Green functions at T=0 (T-temperature). Two types of FEs appear in the helices: a) two branches of FEs with transition dipole moment perpendicular to the helical axis (like strong π→π* transitions) do not couple each other; b) two branches with parallel to the axis n→π* transitions dipoles are mixed and this modulates linear absorption spectra but for antiparallel DNA strands phenomenon CD in model B does not appear near n→π* maxima. Exciton spectra and their splitting (like Davydov splitting) are exposed, as well as the approximate connection between FEs of the pair and FEs of separate bases which compose the pair.