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 (β) is small. On the other hand, with NO₂ groups in terminal positions β 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 β upon replacing the thiophenes by furans. This hypothesis has been verified.