Improved third-order nonlinear optical properties of polydiacetylene derivatives

In order to improve third-order nonlinear optical properties of polydiacetylenes (PDAs), we have been investigating modification of PDA structures. From the point of view of PDA molecular design, three series of monomers for ladder- type PDAs were prepared. Oligoyne monomers having more than five conjugated acetylenes gave polymers which showed broad bands at longer wavelength than their excitonic absorption bands. This implies production of (pi) -conjugated ladder polymer having two PDA backbones linked by acetylenic groups in each repeating unit. From monomers with two butadiynes connected by an alkylene group, ladder-type PDAs were obtained when the carbon number of the alkylene group is more than four. However, monomers with two butadiynes connected by an arylene group gave only single-chain PDAs. From the point of view of PDA morphological engineering, PDA microcrystal water dispersions were applied to evaluate nonlinear optical susceptibilities by z-scan method together with PDA thin films. High-density microcrystal deposited film prepared by layer-by-layer deposition technique showed more than three orders of magnitude enhanced (chi) (3) than the dispersion state. Polycrystalline thin films compose of ladder-type PDAs were also found to have large (chi) (3). The quite large nonlinear refractive index of -41 cm2/GW was attained just near excitonic absorption maximum for one of the polymers.