1. Introduction

Polydiacetylene (PDA) crystal is one of the most promising materials for high-speed optical switching devices in the next generation because of excellent third-order nonlinear optical (NLO) properties with fast response owing to well-developed π-conjugated main chain [1]. The π-conjugated polymer crystal can be obtained through topochemical solid-state polymerization of corresponding monomer (diacetylene (DA)) crystal. Especially, in the case of bulk crystal, γ-ray is frequently used as a more penetrating and effective irradiation to induce solid-state polymerization. A DA derivative, 1,6-di(N-carbazolyl)-2,4-hexadiyne (DCHD), is considered as one of the most stable derivatives with almost complete conversion to polymer [2,3]. The corresponding polymer, polyDCHD, also exhibits remarkably thermal and light resistance against so-called blue-to-red transition inducing the disorder of π-conjugated system, because strong π-stacking among carbazolyl side groups contributes to rigid crystal lattice [2,3]. The mechanical strength of bulk PDA crystal, however, is not high enough to reproducibly cleave or polish bulk single crystal into high-optical quality thin film for optical devices, like most organic crystals. In addition, due to the difference in lattice parameter between monomer and polymer crystals, epitaxially-grown DA film, which is tightly adhered to substrate, often results in low conversion and the formation of many cracks during the solid-state polymerization. To overcome the above problems, DA nanocrystals (NCs) were fabricated as a dispersed system and quantitatively polymerized in the free standing state without any substrate, and then their electrostatic-adsorption thin films were successfully fabricated instead of a single crystalline thin film with less light scattering loss [4]. In a plane of the thin film, however, PDA NCs were randomly orientated, so that we have not fully utilized optical anisotropy of PDA crystal so far. Hence, the alignment of PDA crystalline nanofibers (NFs) can be expected as fabrication method of a pseudo single crystalline thin film.

In this paper, we report the fabrication method of highly-oriented polyDCHD-NFs thin films as a pseudo single crystalline thin film and discuss their optical properties.

2. Experimental

2.1 Materials

1,6-Di(N-carbazolyl)-2,4-hexadiyne (DCHD), synthesized according to the literature [5], was used as a diacetylene monomer. Sodium dodecyl sulfate (SDS) as an anionic surfactant was purchased from Wako Pure Chemical Industries, and used without further purification. Water used was purified up to 18.2 MΩ·cm using a laboratory water purification system (sartorius, 611UV).

2.2 Fabrication of PDA NFs

PDA NFs were fabricated by the nanocrystallization of DA using the reprecipitation method and subsequent solid-state polymerization as shown in Fig. 1. A 1 mL of acetone dissolving both 5 mM DA and 1 mM SDS was rapidly injected into 50 mL of vigorously stirred water at 60°C in order to precipitate DA monomer NFs on the principle of the difference in solubility of DA to acetone and water. Then, DA monomers epitaxially grew up to be monomer NFs within an elapsed time of 30 min. Under the present condition, SDS works as a shape controller of DA NCs [6]. After cooling down to the room temperature, the resulting DA monomer NFs were converted to PDA NFs by irradiation of UV light (λ = 254 nm, ca. 1 mW) for 30 min. During the solid-state polymerization, colorless aqueous dispersion of DA NFs turned into bright blue one of PDA NFs. Here, it is noted that DA NFs can be polymerized quantitatively even by UV light because small NFs are highly transparent with less surface reflection and light scattering losses, compared with bulk crystal.

 

Fig. 1 (a) Solid-state polymerization of DCHD and (b) the fabrication procedure of PDA NFs using the reprecipitation method.

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3. Results and discussion

The resulting PDA NFs were more than 10 μm in length and ca. 50 nm in width. In addition, the resulting NFs have almost same crystal structure as bulk crystal in the measurement of powder X-ray diffraction (XRD). Since no signal derived from DA monomer NFs was observed in the powder XRD pattern, we were able to confirm that solid-state polymerization proceeded quantitatively even in DA crystalline NFs.

Then, a cleaned glass substrate, of which length is 6 cm and width is 2.5 cm, was immersed into an aqueous dispersion liquid of PDA NFs with a given inclination angle θ as shown in Fig. 2, and then dried it up gradually at 10°C, which was performed on an anti-vibration table. PDA NFs at a contact line were immobilized in the direction parallel to the contact line on the glass substrate through the so-called convective assembly [7,8]. The hydrodynamic flow, induced by a solvent evaporation near the contact line, carries the PDA NFs to the contact line. When NFs arrive at the contact line, the flow directions along the axis change to parallel to the contact line because of a flow-induced torque on the NFs pinned one of their ends by the contact line.

 

Fig. 2 (a) Experimental setup and (b) the schematic of convective assembly method controlling the inclination angle θ.

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The resulting thin film on the glass substrate was characterized by UV-Vis polarized absorption spectroscopy and SEM observation. From UV-Vis polarized absorption spectra, we evaluated orientation degree as order parameter S,

 

Fig. 3 (a) The dependence of order parameter S of PDA NFs on inclination angle θ of a glass substrate and (b) SEM images of PDA-NFs thin films with different order parameter S. The broken line indicates a trend line.

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Fig. 4 UV-Vis polarized absorption spectra of the most highly orientated PDA-NFs thin film with order parameter S = 0.87 and dichroic ratio D = 21.8 at inclination angle θ = 30 degree.

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Finally, third-order NLO susceptibility χ⁽³⁾ of the resulting PDA-NFs thin films was evaluated by third harmonic generation (THG)/Maker-fringe method [12]. The THG measurements were performed using a Q-switched yttrium aluminum garnet (Nd:YAG) laser (< 0.2 mJ per pulse, 5 ns pulse duration, λ = 1064 nm and 10 Hz). The sample was set on a motor-controlled rotation stage to monitor the frequency-tripled light (λ = 355 nm) as a function of the incidence angle. The incidence angle of the fundamental light was varied from –60° to + 60 degree. The fundamental light beam was polarized linearly and set to be parallel or perpendicular to the plane of incidence (p or s light, respectively) and the p-light component of TH light to incident light was monitored using a photomultiplier and a boxcar averager system. In the configuration, the p- and s-polarization directions of incident beam are also parallel and perpendicular to the orientation direction of PDA-NFs. A fused quartz crystal with 0.3 mm in thickness was used as a reference. In the evaluation of χ⁽³⁾ values of PDA-NFs thin films, absorption at 355 nm was taken into account. Figure 5(a) shows typical THG/Maker fringe patterns of PDA-NFs thin film and fused quartz. Although the TH intensity for fused quartz showed some characteristic maxima and minima corresponding to consecutive optical pathlength variations equal to one and two coherence lengths, respectively, that from PDA-NFs thin film was a monotonic function of incidence angle. This is because the thickness of PDA-NFs thin films (40 – 80 nm evaluated with a surface profiler (Dektak)) was much smaller than the coherence length l_c of PDA crystal (l_c = 370 nm and 560 nm in the direction parallel and perpendicular to π-conjugated main chain in bulk crystal, respectively [13]). The correlation between χ⁽³⁾ values and order parameter S is summarized in Fig. 5(b). The χ⁽³⁾ values were estimated to be ca. 1.3 x 10⁻¹² esu in randomly oriented sample (S = ca. 0) and ca. 6.3 x 10⁻¹² esu in highly orientated sample (S = 0.87). In theoretical calculation, χ⁽³⁾ for uniaxially orientated sample in a plane parallel to the substrate is expected to be 8/3 times as much as that for randomly orientated sample in a plane parallel to the substrate [14,15]. In this study, the ratio of χ⁽³⁾ values was larger than that for theoretical calculation, because number density of PDA NFs in random orientation was smaller than that in uniaxial orientation as shown in Fig. 3. This fact clearly indicates that highly orientated and dense PDA-NFs thin film was successfully fabricated as a pseudo single crystalline thin film.

 

Fig. 5 (a) Typical THG/Maker fringe patterns of PDA-NFs thin film and fused quartz as a reference, and (b) the correlation between order parameter S and χ⁽³⁾ value in the configurations of p- and s-polarizations.

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4. Summary and conclusion

We successfully fabricated highly orientated PDA-NFs thin films as a pseudo single crystalline thin film by the convective assembly method controlling inclination angle of a glass substrate. The resulting thin film exhibited high order parameter S = 0.87 at inclination degree θ = 30 degree, which contributed to five times as high χ⁽³⁾ value as that of randomly orientated PDA-NFs thin film. By developing highly orientated and dense NFs thin film, we were able to fully utilize optical anisotropy of PDA crystal.