1. Introduction

Polarized luminescence of organic materials has been attracting great attentions for both electroluminescence and photoluminescence applications [1,2]. Such anisotropy can be obtained by orientational order of chromophore molecules in a variety of forms. Various methods have been employed to orient chromophores into a specific direction, resulting in uniaxially anisotropic organic films with dichroic absorption and emission behaviors. Polymeric molecules can be aligned by mechanical stretching and rubbing with a cloth or glass rod [3–6]. Langmuir-Blodgett deposition is also a useful method for a highly oriented thin molecular film [7–10]. Specific substrates can be adopted for a surface-mediated epitaxial growth of molecular order [11]. Especially, mesomorphic self-organization of molecules with shape anisotropy provides very efficient routes to induce orientatial order for both low molar mass and polymeric materials [12–14]. Liquid crystalline chromophore, forming a mesomorphic phase, spontaneously organizes into a uniaxially oriented phase. Macroscopic orientation of a chromophore, in this case, can be more easily controlled by the surface anchoring and applied electric field [15]. Luminescent chromophores can compose liquid crystalline systems as either a host or guest. Luminescent molecules can be doped to reactive mesogens (RM) as a host and then molecular order, controlled by the host LC, is frozen into solidified networks by polymerizing RMs to form a solid matrix [16–19]. On the other hand, a small amount of luminescent RMs and large amount nonluminescent RMs, dissolved in a chiral nematic host, can be copolymerized and phase separated into bulk solid networks with circulary polarized luminescence [20]. As a result of molecular orientational order, materials exhibit uniaxial anisotropy in optical behaviors such as birefringence, dichroic absorption and polarized luminescence. Uniaxially oriented photoluminescent films, exhibiting highly linearly polarized absorption and emission, has been integrated into liquid crystal displays for enhanced device brightness, contrast, light efficiency, and viewing angle [21]. Polarizing energy transfer in photoluminescent materials has also been demonstrated for remarkably enhanced light efficiency by combining randomly oriented sensitizers and uniaxially aligned luminescent materials [22]. The isotropic sensitizer absorbs unpolarized incident light and then efficiently transfer the energy to uniaxially aligned luminescent materials, resulting in a linearly polarized emission with a high isotropic-to-polarized conversion efficiency.

Synthesis of reactive and nonreactive analogues of nematic mesogens has been reported for the fluorescent π-extended fluorene compounds [23]. The fluorene-core has been symmetrically extended by thiophene and benzene at opposite ends. Acrylate has been chosen for a photopolymerizable group by considering its superior reactivity over other groups. Synthesized compounds show mesomorphic behaviors and strong blue fluorescence peaked at 438 nm. Quantum yields of the fluorene derivatives with π-extended core, polymerized in the LC host, have recently been reported [20].

In this report, uniaxially aligned copolymer films have been created by in situ photopolymerization and their anisotropic absorption and emission properties have been examined. Photopolymerization of the fluorescent RM and conventional RM in a uniformly aligned unreactive nematic LC host results in orientationally ordered thin polymer films at surfaces. The film forming process and properties of polarized fluorescence in both monomer and polymer states are investigated by polarized optical microscopy (POM), polarized fluorescence microscopy (PFM), polarized fluorescence spectroscopy (PFS), and field emission electron scanning microscopy (FE-SEM).

2. Experiment

Materials: MLC 15600-100 (Merck) has been used for a nematic LC host as a reaction medium. Chemical structures and phase transitions of the reactive mesogens are shown in Fig. 1. The fluorescent π-extended fluorene RM (FRM) in Fig. 1(b), 2,7-bis[5-(4-(6-acryloyloxyhexyloxy) phenyl) thien-2-yl]-9,9-dimethylfluorene, is a diacrylate with six-methylene-spacer groups symmetrically at opposite end of the fluorene-core. The methylene-spacer groups lower crystal to nematic transition temperature (T_Cr-N) while the shorter alkyl groups at 9,9-position and extended π-conjugation of the fluorene-core enhance a lateral intermolecular interaction [23]. The fluorescent RM exhibits enantiotropic mesophase transition: T_Cr-N = 130 °C and T_N-Cr = 109 °C. The FRM retains its mesomorphic behaviors until it is thermally polymerized at above 250 °C. Refer to the reference [23] for more details. Conventional RM-257 in Fig. 1(c), 1,4-bis [3(acryloyloxy) propyloxy]-2-methybenzene, has been used as a comonomer for polymerization. Irgacure 651 (2-2-dimethoxy-2-phenyl acetophenone, Ciba Additive) shown in Fig. 1(d) has been added as a photoinitiator. The reaction mixtures have been prepared by adding RMs and photoinitiator to the LC host. The homogeneous mixtures have been obtained by agitating in the isotropic phase and cooled down to room temperature.

 

Fig. 1 Chemical structures and phase transition temperatures for the materials used in the study: (a) host nematic LC, (b) fluorescent reactive mesogen FRM, (c) conventional reactive mesogen RM-257, and (d) photinitiator Irgacure 651. Numbers in parenthesis correspond to the transition temperature observed from cooling.

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Cell preparation: For uniaxially aligned planar cells, PI-6514 (Nissan Chemical Industries, LTD) has been spin casted to indium-tin-oxide (ITO) coated glass substrates. The substrates have been uniaxially rubbed and assembled into antiparallel cells. Tape spacers with 10 µm thickness have been used to maintain a cell gap. The reaction mixtures are injected into the cells at isotropic temperature of the LC mixture. The cells are cooled to room temperature for characterizations and polymerization. For polymerization reaction, the Spot Cure Model SP-9 (Ushio Inc.) has been used. Polymerization of the fluorescent RM has been initiated by irradiating unpolarized UV-light. Unfiltered light from 150 Watt xenon lamp is used at the intensity of 40 mw/cm² for one hour.

Characterizations: Alignment of mesomorphic reaction mixtures and orientational order of polymerized RM-networks have been confirmed by polarized optical microscopy (POM, Nikon Eclipse LV 100 POL equipped with Nikon DS-Ri1 CCD camera). Photoluminescent properties of the cell have been analyzed by polarized fluorescence microscopy (PFM) and polarized fluorescence spectroscopy (PFS). For excitation, shortpass filter with cut-off wavelength at 380 nm is adopted for fluorescence microscopy (Nikon Eclipse TE 2000-U equipped with Nikon Digital Sight DS-U1 CCD camera) in a transmission mode. For fluorescence spectroscopy, linearly polarized 400 nm wavelength of light is incident at + 45° to the substrate plane for excitation and resulting emission spectra have been taken at ̶ 45° by using JASCO FP-6500 spectrofluorometer. POM, PFM, and PFS have been performed before and after polymerization of the RMs for the polarized excitation light parallel or perpendicular to the rubbing direction (i.e., LC director), respectively.

To investigate morphologies of the bare polymer networks, the polymerized cells have been immersed into excess amount of hexane for four days to preferentially dissolve and remove unreacted components of the reaction mixture. After removal of LCs, the cells have been carefully disassembled to examine individual top and bottom surfaces separately. After complete drying, both substrates have been investigated separately by using POM, PFM, PFS, and FE-SEM. A thin platinum layer is deposited by ion sputter (E-1010-Hitachi) to avoid charge accumulation. The surface morphologies of the polymerized RM- networks have been imaged by using Hitachi 54800 FE-SEM.

2. Results and discussion

As expected, the reaction mixtures loaded into liquid crystal (LC) cells exhibit uniformly aligned nematic texture under POM. Figure 2 shows various POM textures observed from the mixture with 1.8 wt% RM-257 and 0.2 wt% FRM in a host MLC 15600-100 nematic LC. Irgacure 651 has been doped by 0.1 wt% as a photoinitiator. Nematic director of the reaction mixture is aligned uniaxially along the surface rubbing direction. As seen in Fig. 2(a), POM images exhibit complete extinction and brightest states when the rub-direction is parallel and 45° to the polarizer, respectively. Thus it is evident that the optic axis lies parallel to the rub-direction in the substrate plane. Figures 2(b) and 2(c) display POM textures after polymerization of the RMs. Overall dark and bright optical appearances of the sample are similar to thoseobserved from the reaction mixture prior to polymerization. However, much more grainy texture in Fig. 2(b), obtained from the rub-direction aligned slightly off to the polarizer, strongly indicates that significant amount of RM-networks are formed during polymerization. The grainy texture also indicates some degrees of local disorder of the polymer networks. Overall similarity of birefringent properties of the cell, however, clearly reveals optical anisotropy of the polymer networks templated by nematic order of the host LC. POM images of the bare polymer networks, obtained after removal of LCs, further elucidate orientationally ordered polymer networks in the molecular level.

 

Fig. 2 Polarized optical images for various samples: (a) reaction mixture confined by LC cell prior to polymerization, (b)/(c) cell after polymerization at different azimuthal orientations with respect to a polarizer, (d) cell after removal of LC, and separate substrates for (e) near UV-side substrate and (f) far UV-side substrate. Crossed arrows and thiner arrows represent polarizer directions and surface rub-direction (i.e., nematic director of a host LC), respectively. For all samples, uniaxial optic axes are aligned parallel to the nematic director.

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Birefringence of the polymer films and orientationally organized polymer bundles precisely aligned along the nematic director have been manifested by the POM images from both a combined cell shown in Fig. 2(d) and dismantled substrates shown in Figs. 2(e) and 2(f). It is evident that the RM-networks are bundled with orientational order in a molecular level and macroscopically aligned along the nematic director. Both microscopic and macroscopic orientational orders have been efficiently templated by the nematic reaction medium to the RM-networks. Majority of the polymer networks have been observed on a near UV-side substrate. This is probably due to the gradient of UV-light intensity across the cell during polymerization [24]. Due to the intensity gradient of incident UV-light, polymerization preferentially occurs near UV-side and thus forms bundled networks on the substrate. Much smoother and thinner polymer layer is observed on the opposite substrate as in Fig. 2(f). Figure 2(f) has been taken under much stronger backlight owing to its much reduced thickness.

Fluorescence properties of the cells at different conditions have been examined by using a polarized fluorescence microscope and the results are displayed in Fig. 3. Double-ended white arrows denote the direction of incident light polarization for excitation and thin yellow arrows indicate surface rub-direction (i.e., nematic director direction). No analyzer is used in-between sample cell and detector. Microscopic fluorescence images in Figs. 3(a) and 3(b) have been taken from the reaction mixture contained in the LC cell before polymerization at room temperature. The images clearly show dichroic excitation and consequent emission anisotropy. The excitation light polarized along the nematic director results in strongest fluorescence as in Fig. 3(a). Fluorescence intensity sinusoidally changes by rotating the sample and reaches to its minimum at 90° angle between polarization direction of excitation light and the surface rub-direction. Essentially the same behaviors are observed by placing a single polarizer in-between the sample and detector, indicating polarized emission of fluorescence under unpolarized light excitation. It is evident that homogeneously mixed fluorescent RM molecules are aligned along nematic LC molecules and forms nematic LC phase together. Therefore, the fluorescent RM possesses uniaxial orientational order and exhibits consequent anisotropy in phtoluminescence.

 

Fig. 3 Fluorescence microscope images of the FRM and polymerized FRM with single polarized: (a)/(b) reaction mixture confined by the LC cell prior to polymerization, (c)/(d) cell after polymerization, and (e)/(f) templated polymer networks formed on the near UV-side substrate. White and yellow arrows denote the polarization direction of incident excitation light and surface-rub direction, respectively. No analyzer is used.

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After polymerization of RMs, the sample cell retains its anisotropy in both excitation and emission of fluorescence light as seen in Figs. 3(c) and 3(d). This discloses embedded orientational order of the fluorescent RMs in the cross-linked copolymer networks. Polymer networks consist mainly of the RM-257 with approximately 10 wt% of fluorescent RMs. Orientational order of both RM-257 and fluorescent RM in the polymer networks has been confirmed by POM (i.e., birefringence) and PFM (i.e., polarized photoluminescence). As a result of orientational anisotropy in a molecular level, the fluorescent RM-networks exhibits strong polarized fluorescence. After removal of LCs, the cell has been dismantled. Majority of the polymer networks has been found on the near UV-side substrate. Figures 3(e) and 3(f) demonstrate polarized fluorescence of the bare polymer networks. Based on all these observations, it is obvious that fluorescent RMs can be efficiently templated by the nematic order of host LCs into orientationally ordered polymer networks. As a result, the polymer layer exhibits dichroic excitation and polarized fluorescence.

For more quantitative analysis of the photoluminescent behaviors, polarized fluorescence spectroscopy has been performed for the cells before and after polymerization. The emission spectra have been measured in 400 nm to 750 nm range with excitation wavelength at 400 nm. A single polarizer has been placed in-between a light source and sample. The samples exhibit fluorescence in 420 nm to 590 nm range with three merged peaks and with emission maximum at ~471 nm. The fluorescent monomer and polymer with nematic order reveal a significant dichroic anisotropy. The degree of excitation and emission anisotropy varies at different states.

Absorption and emission spectra of the FRM in dichloromethane are shown in Fig. 4(a). Each graph shows absorption maximum at 393 nm and emission maximum at 438 nm. Since it is dissolved in an isotropic solvent, its absorption and emission are completely isotropic. Figure 4(b) shows the fluorescence emission spectra of the reaction mixture in the LC cell prior to polymerization. The red and blue lines correspond to fluorescent emissions excited by the incident 400 nm light polarized parallel and perpendicular to the surface rub-direction (i.e., nematic director), respectively. Although emission profiles are essentially the same, the intensity is significantly decreased with the incident polarization perpendicular to the nematic director. This implies dichroic absorption of incident polarized light for excitation. As a result, orientationally aligned chromophores emit polarized fluorescent light. The results are corroborative to the PFM observations discussed in Fig. 2. Polarized fluorescence has been confirmed by placing polarizer in-between the sample and detector with unpolarized excitation light. Fluorescence with a single polarizer, either in front of or after the sample, exhibits essentially the same behaviors. Both result in a polarized fluorescence. The dichoric ratio at emission maximum (~471 nm) corresponds to 5.1 for the reaction mixture before polymerization.

 

Fig. 4 UV-vis absorption spectrum and polarized fluorescence spectra of the FRM and polymerized FRM: (a) absorption and emission spectra of the FRM dissolved in methylene chloride, (b) reaction mixture confined by the LC cell prior to polymerization, (c) cell after polymerization, and (d) and templated bare polymer networks formed on the near UV-side substrate.

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The behaviors of polarized fluorescence after polymerization of RMs are shown in Fig. 4(c). It shows similar emission profile and polarized properties to those in Fig. 4(b). However, both fluorescence intensity and dichroic ratio have been remarkably decreased. The maximum fluorescence reduces to approximately 30% of the maximum measured for unpolymerized fluorescent monomer in LCs. The dichroic ratio has also been diminished to 3.6 after polymerization. This can be caused by the reduced order of chromophore in the polymerized networks as evidenced in Fig. 2(b). The grainy domains in Fig. 2(b), formed during polymerization, indicate a reduced long range orientational order of the polymer networks. It is known that birefringence of reactive mesogens decreases during polymerization in a LC solvent due to the same reason.

Therefore, measured photoluminescence in Fig. 4(c) could be originated from both unreacted residual FRM dissolved in a host LC and copolymerized FRM with the conventional reactive mesogen RM-257. The properties of polarized fluorescence for solidified copolymer networks have been examined by using the bare polymer network aftercomplete removal of LC host. Figure 4(d) shows the fluorescence spectra of the bare polymer networks formed on the near UV-side substrate. The emission intensity has been further decreased. Dichroic ratio for the emission also decreases to 2.5 for the bare networks. Compared to Fig. 4(c) with a LC host, the intensity reduction may be attributed to either the absence of unreacted FRM in LCs or the increase in light scattering of the bare networks. However, the reduction in anisotropy of polarized fluorescence should somehow be related to the orientational order of fluorophore before and after polymerization. This will be further discussed below together with morphology of the networks.

Figure 5 shows the FE-SEM micrographs of the copolymer networks formed by FRM and conventional RM-257. Figures 5(a)/(b) and 5(c) correspond to the network images from near and far UV-side surfaces, respectively. Although larger amount of networks have been conveyed to the near UV-side substrate, morphologies of the networks on both substrates commonly show a uniaxial orientational order along the nematic director. In Fig. 5, white arrows denote the surface rub-direction, corresponding to a nematic director of the reaction mixture. Even though polymer bundles exhibit more or less molten and fused states, orientational order of the bundles is clearly seen in the micrographs. Uniaxial order in a molecular level has already been confirmed by the POM results as discussed in Fig. 2. Order parameter of the fluorophore, calculated by the polarized fluorescence spectra [25], has been curtailed from S = 0.57 for the reaction mixture prior to polymerization to S = 0.32 for the bare polymer networks after polymerization and removal of unreacted LC molecules. These observations may provide additional information on the formation of templated networks during polymerization.

 

Fig. 5 SEM micrographs of the copolymer networks templated on the uniaxially aligned nematic host: networks on the (a)/(b) near UV-side and (c) far UV-side substrates. Arrows represent the direction of a nematic director of a host LC.

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Based on all these results, the process of forming thin polymer networks with polarized photo-luminescence is summarized in Fig. 6. A small amount of fluorescent RM and conventional RM-257 have been homogeneously dissolved in a host LC and form the nematic mesophase. The reaction mixture has been uniaxially aligned in an LC cell as schematically illustrated in Fig. 6(a). As confirmed earlier by POM, PFM and PFS, the fluorescent monomers are evenly distributed through an entire cell and aligned along the nematic director. Blue and white ellipses represent fluorescent and conventional RMs, respectively. Unreactive host LC molecules are not presented for simplicity. Color gradient in green across the cell designates an intensity of UV-light used for polymerization.

 

Fig. 6 Schematic illustration of the process for forming thin polymer networks with polarized photo-luminescence: (a) polymerization of comonomers in a uniformly aligned nematic host, (b) surface localization of the networks under the gradient of UV-light intensity, (c) uniaxially aligned localized copolymer networks after completion of polymerization, and (d) illustration of uniaxially aligned fluorescent RM copolymer networks with the conventional RM-257. Color gradient in green indicates UV-light intensity. Blue and white ellipses represent fluorescent and non-fluorescent acrylate monomers. Double ended arrow on the SEM image denotes the surface-rub direction.

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The strong localization of polymer networks on a UV-side surface is mainly ascribed to an intensity gradient of UV-light inside the cell. The intensity of polymerization initiating UV-light exponentially decays, as illustrated by the green in Figs. 6(a) and 6(b), due to a strong absorption by the fluorescent RM in the range of 350 ~420 nm wavelength as discussed in Fig. 4(a). Consequently, a selective initiation of photopolymerization, followed by monomer diffusion, forms polymer networks preferentially on the near UV-side substrate (Figs. 6(b) and 6(c)) [24].

In Fig. 6(b), the curved arrows indicate a diffusion of RMs and polymer aggregates formed during UV-light irradiation. In Figs. 6(b) and 6(c), the yellow layers at both surfaces specify the localized copolymer networks. After completion of polymerization, the networks are localized at the surfaces. No evidence for the networks formed in a bulk has been noticed. Figure 6(c) demonstrates localized networks and orientational order of fluorophore in the networks. The SEM micrograph reveals morphological anisotropy of the network. The polymer bundles fused into a thin film and fibrils aligned along the nematic director denoted by the white arrow in the inset. Figure 6(d) schematically illustrates the molecular order of polymer networks in a molecular level. In addition to the morphological anisotropy of the networks, a uniaxial orientational order has been already confirmed by the POM, PFM, and PFS. Although fluorophore retains its nematic order during a network-forming process, its degree of order is significantly curtailed by the network formation as discussed earlier in Fig. 4. It may also indicate that a nematic order of the host LC is not directly duplicated into polymer networks. Instead, intermediate polymer aggregates are formed initially and subsequently fused into bundled networks under the influence of nematic LCs. More detailed mechanism is currently under investigation.

This approach for forming a thin surface layer with polarized photoluminescence has advantages over other methods previously reported. Since it forms stable thin solid films at surfaces, dynamic switching of LCs are not affected by the additional component. The uniaxially templated film exhibits relatively higher dichroic ratio and uniformity, compared to the luminescent polymer films aligned by mechanical rubbing [5,6]. Although mechanically stretched films show very high dicroic ratio (i.e., luminescent polarization efficiency), incorporating premade film into device is an unavoidable drawback [21]. On the contrary, demonstrated in situ approach provides a great deal of flexibility to integrate anisotropic luminescent films into the cell.

3. Conclusion

The fluorene-based fluorescent reactive mesogen has been synthesized and used to form crossed-linked copolymer networks with a polarized photoluminescence. The fluorescent and conventional RMs dissolved in a nematic LC host have been copolymerized into thin surface-localized networks. The nematic order of a host LC as a reaction medium has been successfully duplicated to the networks, hence exhibiting a uniaxial orientational order. Orientational anisotropy and polarized photoluminescence of the RMs before and after polymerization have been characterized by using POM, PFM, and PFS. The nematic order of the host LC facilitates the formation of orientationally ordered polymer networks. As a result, thin-layered polymer networks exhibits polarized photoluminescence. This in situ creation of anisotropic films may provide a useful approach to the in-cell integration of polarized photoluminescence for future display devices with enhanced light efficiency and color gamut.