New Developments in Second-Order Nonlinear Optical Polymeric Materials
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symmetry is broken due to the external DC-field. Berkovic et al. [10] observed SHG in the monolayers of several diacetylenes monomers and polymers spread at the air-water interfaces in a Langmuir Blodgett(LB) trough. They believed that the SHG signals arise mainly from the sidegroups on the PDAs since the diacetylene core is centrosymmetric. Garito et al. [11] demonstrated the significant electric-field-induced SHG from PDAs containing two different, highly asymmetric substituents (one is NLO active chromophore). They concluded that the magnitude of the macroscopic NLO response in PDAs critically depend on the nature of the microscopic charge correlated iT-electron states and also on the symmetry and the degree of the structural order. Recently we have reported a soluble, asymmetrically substituted PDA, viz. poly((8-butoxy carbonyl)methyl urethanyl) 1-(5-pyrimidyl)octa- 1,3-diyne(BPOD), in which an aromatic substituent is directly attached to the main backbone [12].
0.6• 0.5-
(B)
(A)
` 0.4-
(0.3
D)
S0.2S0.1 0-
300
400
500 600 700 800 Wavelength (NM) Figure 1. Absorption spectra of poly(4-BCMU) and poly(BPOD). (A) and (B): in chloroform (C) and (D): in chloroform/hexane, respectively.
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Poly(BPOD) is highly soluble in chloroform and this good solution shows a Xmax at 510nm(red)(figure 1-B). Upon the addition of nonsolvents such as hexane and ethanol, poly(BPOD) shows a dramatic color transition from red to purple(599nm)(figure l-D). Absorption spectra of poly(BPOD) in good and poor solutions are shown in figure 1 and are compared with those of poly(4-BCMU). The only structural difference in poly(BPOD) from that of poly(4-BCMU) is that the former has one of the flexible urethane sidegroups replaced by a rigid aromatic pyrimidyl group. The differences in the Lmax between poly(4-BCMU) and poly(BPOD) in the poor solutions as well as good solutions clearly indicate that the effective conjugation in poly(BPOD) is enhanced through conjugation between the main backbone and the aromatic sidegroups. Poly(BPOD), with its enhanced conjugation length, is an ideal candidate for the investigation of NLO properties since various forms of the polymer can be obtained such as single crystal, solution, and optical quality spin on films. In this paper, we present for the first time the second harmonic generation (SHG) in an amorphous spin coated films of poly(BPOD). External electric fields were not employed to produce the poled alignment. We speculate that the SHG signals are mainly due to the spontaneous orientation of the sidegroups during the film formation process. This type of process dependJent orientation has been observed in spin coated films of polyimides [13,14]. SHG from the LB monolayer and multilayers of poly(BPOD) are also reported. EXPERIMENTAL Thick film formation Synthesis and solid state polymerization of diacetylene monomer BPOD has been reported elsewhere [12]. Single crystals of poly(BPOD) were dissolved in chloroform(- 1% w/v) and the films were obtained either by spin coating or casting the polyme
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