Simple zwitterionic merocyanines as potential NLO chromophores

A suite of zwitterionic pyridylidene-based merocyanines that contain no int erconnecting pi -bridge between the donor and acceptor rings has been synth esised and their second-order NLO properties evaluated largely by semi-empi rical computational methods (MOPAC 97/AM1). Contrary to expectation, increa sing the degree of inter-ring twist (theta), at least up to 55 degrees, in these new pyridylideneazolone chromophores is found to have little or no ef fect on the figure of merit [mu beta (0)]. An X-ray crystallographic apprai sal of one of these chromophores, 14, reveals however that the twist angle (albeit in the solid state) is greater than that predicted by computation a nd that all other features are consistent with the highly zwitterionic natu re of these systems. In spite of this, a combination of facto rs-insufficie nt acceptor strength, insufficient extent of conjugation and perhaps insuff icient twist angle, in particular-clearly leads to the low values of the qu adratic hyperpolarisabilities. The trade-off between targeting a more modes t beta from a minimum of pi -conjugating framework between D and A (and the refore synthetic expediency) and seeking a moderate-to-high dipole moment h as therefore resulted in only low figures of merit for these systems. Calculations performed on a suite of readily accessible, isoelectronic cyan ines, in which the acceptor is a stabilised cyclopentadienide carbocycle ra ther than a heterocycle, have revealed the potential that these systems, ex emplified by 27, have as NLO chromophores. Representative polymer-tetherabl e derivatives of this system have been prepared as have the corresponding T DI-based polyurethanes.