Electronic screening in second order optical polarizabilities of elongated Donor/Acceptor polyenes

Optical polarizabilities of a family of donor/acceptor substituted polyenes are calculated using collective electronic normal modes obtained by solving equations of motion for the single-electron density matrix combined with the INDO/S semiempirical hamiltonian. Size-scaling of static polarizabilities χ=α,β,γ is analyzed using the exponents b_χ≡dlnχ/dlnn where n is the number of repeat units. We find that for long chains b_α and b_γ tend to 1 whereas b_β approaches 0. Two-dimensional plots of the dominant collective electronic modes show that α and γ are generated along the entire chain whereas β only originates from the donor and acceptor chain regions. Electronic screening makes these regions neutral. The size of this boundary screening region is ~15-17 double bonds, and β saturates at ~30-40 double bonds, depending on the type of donor/acceptor substitution.