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A suite of zwitterionic pyridylidene-based merocyanines that contain no interconnecting p-bridge between the donor and acceptor rings has been synthesised and their second-order NLO properties evaluated largely by semi-empirical computational methods (MOPAC 97/AM1). Contrary to expectation, increasing the degree of inter-ring twist (h), at least up to 55u, in these new pyridylideneazolone chromophores is found to have little or no effect on the figure of merit [mb( 0 )]. An X-ray crystallographic appraisal of one of these chromophores, 14, reveals however that the twist angle (albeit in the solid state) is greater than that predicted by computation and that all other features are consistent with the highly zwitterionic nature of these systems. In spite of this, a combination of factors-insufficient acceptor strength, insufficient extent of conjugation and perhaps insufficient twist angle, in particular-clearly leads to the low values of the quadratic hyperpolarisabilities. The trade-off between targeting a more modest b from a minimum of p-conjugating framework between D and A (and therefore synthetic expediency) and seeking a moderate-to-high dipole moment has therefore resulted in only low figures of merit for these systems.Calculations performed on a suite of readily accessible, isoelectronic cyanines, in which the acceptor is a stabilised cyclopentadienide carbocycle rather than a heterocycle, have revealed the potential that these systems, exemplified by 27, have as NLO chromophores. Representative polymer-tetherable derivatives of this system have been prepared as have the corresponding TDI-based polyurethanes.

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We present measurements of thermal and optical properties of poly(methyl methacrylate) (PMMA), PMMA modified by the addition of ethanol (MPMMA), and copolymers of methyl methacrylate with 2-hydroxyethyl methacrylate [P(HEMA:MMA)]. Spectral transmission of the polymers is excellent (alpha = 0.5 cm(-1) at 400 nm, decreasing to 0.04 cm(-1) at 633 nm). Measured laser damage thresholds of MPMMA and P(HEMA:MMA) show at least a twofold increase over PMMA. Thermal lensing measured in these hosts doped with Rhodamine 6G is shown to be similar (f = -450 mm for pump power of 200 mW in a 2-mm-diameter spot, scaling with pump intensity). Compared with MPMMA, P(HEMA:MMA) offers an improved surface quality and a more uniform dye distribution.

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