Asymmetric unit cell polymers with large first hyperpolarizabilities

Large first hyperpolarizabilities (β) can result from the combination of donor and acceptor groups separated by a conjugated linker but also from the use of asymmetric unit cell polymers. Quantum chemistry methods including electron correlation effects are used to assess the chain length dependence of β of polymethineimine (PMI), a prototype of this kind of polymers. The evolution of β of PMI upon modification of the bond length alternation is investigated. It is demonstrated that PMI presents a very large β in comparison with usual push-pull molecules.