Cationic π-electron systems with high quadratic hyperpolarisability

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Abstract
Cationic NLO-chromophores based on tolane π-systems in which conventional electron donors are combined with ionic pyridinio or triorganoammonio and -phosphonio substituents, respectively, perform much better than conventional donor/acceptor-substituted (D/A-substituted) tolanes concerning their hyperpolarisability–transparency trade-off. This effect occurs because ionic acceptors do not enlarge the π-system in contrast to conventional acceptors. The same holds true for benzene-type chromophores. Despite their different electronic nature, the extrapolated maximal high-energy absorption of the ionic chromophore series as well as of the conventional donor/acceptor-tolanes coincide at the absorption energy of unsubstituted tolane. This proves that the maximal blue transparency of a given series of substituted chromophores is governed by the absorption maximum of the unsubstituted parent chromophore. In this way chromophores have been designed with much higher quadratic hyperpolarisability than e.g.p-nitroaniline at about the same absorption wavelength. By applying the same concept, a two- and a three-dimensional highly efficient octupolar NLO-chromophore assembly has also been synthesised.