Calculation of static (Hyper)polarizabilities for π-conjugated donor/acceptor molecules and block copolymers by the elongation finite-field method

The elongation finite-field (elongation-FF) method is applied to calculate static (hyper)polarizabilities of π-conjugated chains, with donor and/or acceptor substituents, as well as a heterol block copolymer. Substituent effects on the linear polarizability (α) and second hyperpolarizability (γ) are modest, but a large first hyperpolarizability (β) can be induced. The dependence of β on chain length is rationalized on the basis of quasi-symmetry and donor/acceptor strength. Our copolymer consists of alternating blocks containing pyrrole rings and silole rings, which generates a quantum well. Copolymerization induces a fairly small β that oscillates in sign from block to block. The effect on α and γ is to enhance the contribution of the pyrrole blocks and diminish, by a larger amount, the contribution of the silole blocks.