Second-order nonlinear optical responses of heptahelicene and heptathiahelicene derivatives

Using quantum chemistry methods, structure-NLO property relationships are investigated for families of 7-ring helicenes built from thiophene and benzene rings. In absence of donor or acceptor substituents, the first hyperpolarizability (beta) is small. On the other hand, with NO2 groups in terminal positions beta increases, especially for helicenes containing few benzene rings. For NMe2 substituents the enhancement is damped by the less polarizable benzene rings, the cross-conjugated all-thiophene segments, and the auxiliary acceptor character of the thiophenes. From this analysis, we selected three helicenes and conjectured larger beta upon replacing the thiophenes by furans. This hypothesis has been verified. (C) 2015 Elsevier B.V. All rights reserved.