Step and terrace structure has been observed in an area of 1 mm  1 mm on the cleaved surface of KCl-KBr solid-solution single crystal by scanning near-field optical microscope (SNOM) with a small sphere probe of 500 nm diameter. Lateral spatial resolution of the SNOM system was estimated to be 20 nm from the observation of step width and the scanning-step interval. Vertical spatial resolution was estimated to be 5-2 nm from the observation of step height and noise level of photomultiplier tube (PMT). With applying a dielectric dipole radiation model to the probe surface, the reason why such a high spatial resolution was obtained in spite of the 500 nm sphere probe, was understood as the effect of the near-field term appeared in the radiation field equations. r

Nonlinear *-conjugated polymers which include poly-(diacetylene),' polyacetylene,2 and poly(pheny1enevi-nylene13in relation to a potential use in the optoelectronic field have attracted much attention. Among them, polydiacetylenes exhibit the highest nonlinear susceptibility (x3) of the order of 10-6 esu in the resonant region for poly-PTS4 [poly(2,4-hexadiyne-1,6-diol bis(p-toluenesulfonate))] and poly-DCH6 [poly[l,6-bis(N-carbazolyl)-2,4-hexadiynel] single crystals. However, polymers such as poly(phenyleneethyny1ene) containing triple bonds in the main chain still remain undeveloped due to the lower solubilitp despite the unique *-conjugation system. Soluble poly(aryleneethyny1ene) type polymers have recently been prepared by the incorporation of pyridine and a long alkyl-substituted thiophene moiety into the main hai in.^^^ On the other hand, the introduction of the m-phenylene structure into the main chain in the poly-(phenyleneethynylene) may provide high processability and the maximum absorption shift to a shorter wavelength due to the suppression of the insoluble rigid linear structure.This paper describes the synthesis and the nonresonant third-order nonlinearity of a new type of poly(pheny1eneethynylene) which is soluble in common organic solvents and does not include heterocyclic components.The Pd-catalyzed coupling reaction of ethynyl compounds with aryl bromide or iodide is widely used for the preparation of ethynylarene~.~ A similar reaction of 1,4diethynylbenzene with l-alkoxy-2,4-dibromobenzene (n = 15, 17, and 21)lO in triethylamine led to the desired soluble polymers (1)" with conversions of 60, 80, 89%, respectively (SchemeI). Particularly, I (n = 21) was found to be superior to the shorter alkyl-substituted polymers in processability. The film cast from a chloroform solution has a maximum absorption at 335 nm, quite different from other third-order poly(diacety1enes) which have strong absorptionsaround 600 nm. The x3(w) of I (n = 21), which was determined by the nanosecond time-resolved DFWM (degenerate four wave mixing) method in a benzene solution at 532 nm, is 1.8 X leg esu. This value is comparable to the largest nonresonant x3(w) (3 X le9 esu) of poly-PTS.l Scheme I / \ n --PdClAPPh3)z CUI, Et3N (I) : R =CH3(CH2),, n= 15,17, 21 Kim, K.; Miyazaki, Y.; Maruyama, T.; Kubota, K.; Yamamoto, T. Chem. Lett. 1993,913. (8) Yamamoto, T.; Takagi, M.; Kizu, K.; Maruyama, T.; Kubota, K.; Kanbara, H.; Kunihara, T.; Kaino, T. J. Chem. SOC., Chem. Commun. 1993,797. (9) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.Synthesis 1980,627. (10) l-Alkoxy-2,4-dibromobenzene was obtained by the reaction of 2,4-dibromophenol with sodium hydride in DMF followed by alkyl bromide (n = 15, 17, and 21) treatment at 55 OC. (11) In a general procedure, 1,4-diethynylbenzene (4.8 mmol) and l-(docosyloxy)-2,4-dibromobenzene (4.8 mmol) were refluxed for 6 h in triethylamine (70 mL) in the presence of PdC12-(PPh&/CuI (0.014/0.026 mmol). After removal of insoluble material and the solvent, the residue was dissolved ...

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