Second-order nonlinear optical responses of heptahelicene and heptathiahelicene derivatives

⿢βof helicenes built from fused benzenes and thiophenes result from subtle effects.⿢Without substituents the first hyperpolarizabilities (β) are very small.⿢βis enhanced when acceptor or donor groups are placed at terminal positions.⿢When these are NO2 groups β is enhanced when helicenes contain few benzene rings whereas in the case of NMe2 groups the enhancement can be damped by three effects.⿢These calculations suggest to replace thiophene by furan rings to achieve even larger β.

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.

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