Optimum conjugation length in donor–acceptor molecules for third-order nonlinear optics

We report on the third-order nonlinear optical properties of a series of molecules, first published in Chem. Eur. J. 19, 12693-12704 (2013), where a donor and an acceptor group are separated by a linear conjugated system consisting of only carbon atoms building triple bonds (0 to 5 acetylene spacers). We determine how the wavelength-dependent complex third-order polarizabilities of these molecules vary with the number of acetylene spacers between donor and acceptor groups. We find that both peak two-photon absorption cross sections and off-resonant third-order polarizabilities reach a maximum for 3 spacers and that their corresponding intrinsic values decrease for longer conjugation lengths. At the same time, the observed third-order polarizabilities are two orders of magnitude larger than those seen in similar molecules of similar size but without donor-acceptor substitution. There is also a resonant enhancement of the real part of the third-order polarizability that only appears for the longer molecules, and reaches ~ 5 times the off-resonant value for a molecule with 4 spacers. This enhancement is found at the long-wavelength side of the two-photon absorption where the ratio between real and imaginary parts of the third-order polarizabilities still exceeds 10.