Highly Sensitive Formation of Stable Surface Relief Structures in

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Highly Sensitive Formation of Stable Surface Relief Structures in Bisanthracene Films with Spatially Patterned Photopolymerization Takashi Ubukata, Megumi Nakayama, Taishi Sonoda, Yasushi Yokoyama, and Hideyuki Kihara ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07943 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 12, 2016

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ACS Applied Materials & Interfaces

Highly Sensitive Formation of Stable Surface Relief Structures in Bisanthracene Films with Spatially Patterned Photopolymerization Takashi Ubukata,*‡ Megumi Nakayama,‡ Taishi Sonoda,‡ Yasushi Yokoyama,‡ and Hideyuki Kihara§ ‡Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan §Research Institute for Sustainable Chemistry (ISCHEM), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan KEYWORDS: surface relief structure, photo-triggered mass migration, photopolymerization, anthracene, photodimerization

ABSTRACT: A facile method for the fabrication of a highly sensitive surface relief is demonstrated, which operates on the principle of spatially patterned photopolymerizationinduced mass transport in the amorphous films of a series of bisanthracene compounds. The stability of the resultant colorless transparent relief structure is dramatically improved owing to the polymerization of the bisanthracene.

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Euglena has a positive phototaxis to perform photosynthesis efficiently. Earthworms are thought to have a negative phototaxis to avoid the surface unsuitable for their survival. Other such responses to light are observed in nature. Moreover, a system to control the motion of artificial organic material by light has attracted widespread attention in recent years. 1-4 As a microscopic photoresponsive motion, the photoformation of surface relief gratings (SRGs) has received much attention because of its basic phenomenological interest and technological applications.5-8 When a solid, thin azobenzene-containing film is irradiated by light with a periodic intensity and/or polarization pattern, the material begins to move away from highintensity areas, resulting in the formation of SRGs. Owing to the simple, one-step approach to produce SRGs, a significant number of potential applications have been proposed in the field of fabrication of various optical elements.9 However, the use of azobenzene compounds limits the wavelength of the light for optical elements because they have a strong absorption band in the visible region. Therefore, research on a new class of SRG-forming materials other than azobenzene has commenced and several papers have been published to date.10-18 In particular, colorless SRGforming materials are desired to broaden the application field of SRGs as optical elements. From a practical viewpoint, it is desirable for SRG formation to be highly photosensitive and for the resultant SRG structure to be highly stable. For azobenzene-containing materials, Seki and coworkers reported a two-step strategy to solve the problem by using a soft liquid crystalline polymer based on azobenzene with crosslinkable groups.19-21 Highly sensitive SRG formation was achieved during the phase-transition and the resultant SRG was fixed via crosslinking. However, there have been no reports on SRG-forming materials other than azobenzene that satisfy both criteria, which are generally in a trade-off relationship. Therefore, as azobenzene-


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based materials absorb visible light, we began to search for alternatives. Recently, we discovered that SRGs were formed on relatively low molecular weight polystyrene films via spatially patterned UV irradiation, which causes the molecular weight of polystyrene to increase upon UV-light irradiation during the SRG formation process.22 This result prompted us to find materials in which the molecular weight increases with irradiation of light. In this letter, we propose newly designed, photopolymerizable low-molecular-weight materials that can form SRG structures. It is anticipated that highly sensitive SRG formation is possible owing to their soft and flexible properties, which we ascribed to their low-molecular-weight. Moreover, it is also anticipated that the resultant SRG structure would be stable since the polymerization occurs during the patterned light irradiation. For photopolymerizable materials, we focused on bisanthracene molecules; these are molecules in which two anthracene moieties are connected with a spacer. It is known that two anthracene molecules react to form a dimer because of [4+4] cycloaddition when they are irradiated with long wavelength UV light (ca. 365 nm) causing π−π* excitation.23,24 The reverse reaction is also possible upon exposure of the photodimer to short wavelength UV light (< 300 nm) or upon heating. Although when one bisanthracene molecule undergoes photodimerization intramolecularly its molecular weight does not change, when two bisanthracene molecules undergo photodimerization intermolecularly, it doubles. Therefore, the successive intermolecular photodimerization of bisanthracene molecules finally forms a polymer.25-27 In this letter, we report on the potential use of bisanthracene compounds for novel SRG-forming materials and on their photopolymerization effect. We have designed two bisanthracenes, 1 and 2, which have different spacers as well as monoanthracene, 3, as a model compound, as shown in Scheme 1. In bisanthracenes, the


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photopolymerization occurs via successive intermolecular photodimerization of anthracene units. In contrast, photoreaction of 3 is surely limited to photodimerization.

h

O O

R

O

O

R

O O

R=

O O

h ',

n

1

C6H12 C5H10O

2

OC5H10

O O

O

CN

3

Scheme 1. Photopolymerization of bisanthracene compounds and anthracene derivatives used in this study. Esterification of 9-anthracenecarboxylic acid with 1,6-dibromohexane gave the desired material 1 in 50% yield. Bisantharacene 2 was obtained via the reaction of biphenyl-4,4′-diol with 5-bromopentyl 9-anthracenecarboxylate in 68% yield (see SI). The structures of the synthesized compounds were confirmed by NMR and mass spectroscopy (MS). The synthesis of 3 is described in the literature.28 Differential scanning calorimetry (DSC) measurements of anthracene derivatives 1, 2, and 3 displayed the materials' glass transition temperatures (Tgs) at 292, 304, and 291 K, respectively, using the second heating process.


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Transparent amorphous thin-films (ca. 0.1 µm thickness) were prepared via spin coating on a cleaned glass or quartz substrate from chloroform solutions (1.1 wt%). The stability of the resulting amorphous glass depends considerably on the molecular structure and Tg; additionally, material 2 proved to be superior to 1 since it remained amorphous after one month while 1 crystallized partially after 8 h. To improve the amorphous properties of the thin-films, films of the anthracene derivatives 1 and 3 were irradiated with uniform UV light (365 nm, 1.2 mW cm−2, 5 s) at room temperature under a nitrogen atmosphere prior to SRG formation (preirradiation). Though partial crystallization was observed in films of materials 1 and 3 at room temperature without preirradiation within 8 and 1 h, respectively, the crystallization period was extended to at least 1 week and 1 day, respectively, with radiation. Before the SRG experiments, the photodimerization properties of the anthracene derivatives were investigated. The films of the anthracene derivatives were irradiated with UV light (365 nm, 0.14 mW cm−2) at 338 K in a nitrogen atmosphere. Figure 1(a) illustrates the change in the UV-visible absorption spectra (normalized using the initial absorbance at 369 nm) of the bisanthracene 1 film. The gradual decrease of the absorption band at 230–420 nm, which is ascribed to the anthracene moiety, indicates the consumption of anthracene moieties to form the photodimer upon UV light irradiation. To verify the intermolecular dimerization between the anthracene moieties, we performed gel permeation chromatography measurements using chloroform as a solvent in which the anthracene films were immersed. Figure 1(b) shows gel permeation chromatography charts for solutions of 1 after different irradiation times (normalized to the peak intensity of the original molecule 1 observed at the retention time of ca. 35 min). As irradiation time increased, components with shorter retention times than that of the original peak appeared and increased. This result indicates that dimerization and further oligomerization of the


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bisanthracene molecules occurs upon UV light irradiation, clearly revealing the occurrence of the intermolecular dimerization reaction. Similar chromatogram changes were also observed for the samples from 2. Conversely, only dimerization was observed in monoanthracene 3 films, as expected.

Figure 1. Photoreaction of bisanthracene 1 in film upon irradiation with UV light (365 nm, 0.14 mW cm−2) at 338 K in a nitrogen atmosphere. (a) Change in normalized UV-visible absorption spectra of bisanthracene 1 in film. Absorbance is normalized using the initial absorbance at 369 nm. (b) Change in normalized gel permeation chromatograms of the chloroform solution of film 1 after different irradiation durations. Normalization is performed using the peak intensity of the original bisanthracene 1. Anthracene films were then irradiated with patterned UV light to generate surface relief structures. Figure 2(a) shows the atomic force microscopy (AFM) image of the surface relief structure produced in the bisanthracene 1 film by the spatially patterned UV light irradiation, which used a grating photomask with periods of 6 µm and which was carried out in a nitrogen


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atmosphere at 333 K. When the film was irradiated with 0.12 mW cm−2 for 10 min, regular surface modulation was produced, and its spatial period was coincident with that of the photomask. To investigate the direction of material transfer, irradiation experiments through a photomask possessing one slit line with a width of 3 µm were performed (Figure 2(b)). When we examined the cross-sectional topography, we found that after irradiation, compared with the initial surface, the top of the convex part of the film was higher while the bottom of the depression was lower. This result clearly shows that lateral material transfer occurs from the shaded areas to the irradiated areas. Similar surface relief structures were observed for the films of 2 and 3, and the same material transfer direction (toward irradiated areas) was observed in those films.

Figure 2. SRG formation of bisanthracene 1 via spatially patterned UV (365 nm, 0.12 mW cm−2) irradiation for 10 min. at 333 K in a nitrogen atmosphere. (a) Topographical AFM image of SRG produced by UV irradiation through a photomask, which had a stripe pattern with a period of 6 µm. (b) Height profile of surface relief structure produced by UV irradiation through a photomask with pattern of a single 3-µm-wide slit.


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The efficiency of the transfer of materials strongly depends on the temperature of the films during the spatially patterned UV light irradiation, which is similar to observations made in experiments using spirooxazine16 and polystyrene22 films. Figure 3(a) shows the temperature dependence of the resulting relief heights of the anthracene films. The height of the relief produced on the anthracene films increased gradually to a maximum and then decreased with increasing temperature. This temperature dependence may be caused by the competition between increasing molecular mobility and the smoothing effect of surface tension with increasing temperature. The optimal temperature for each film to achieve its highest relief height relates to its glass-transition temperature (Tg). In the bisanthracene films, the optimal temperature was observed to be about 50 K higher than its bulk Tg (292 K for 1 and 304 K for 2). The difference between the optimal temperature and Tg was large for the bisanthracene films, compared with the monoanthracene 3 and spirooxazine films. This fact must be ascribed to the occurrence of polymerization in the exposed regions. The larger half-value width observed for the bisanthracene films than for the monoanthracene film must also be caused by polymerization in the bisanthracene films.

Figure 3. Plots of relief height (mask period: 6 µm) as a function of film temperature and irradiation time. (a) The relief heights were measured after UV light (0.12 mW cm−2 for 1 and 3;


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0.15 mW cm−2 for 2) irradiation for 10 min. (b) The relief heights of 1 film were measured after UV light irradiation with different intensities at 332 K. Surprisingly, the exposure energy to generate the surface relief on the bisanthracene films is dramatically decreased compared with previously examined materials. To investigate the sensitivity of relief formation, we examined the SRG height produced with different irradiation times using different irradiation intensities. Figure 3(b) shows the growth of SRGs in the bisanthracene 1 film as a function of irradiation time with different intensities of the recording light. Initially, the amplitude of SRGs increased nearly proportionally to the irradiation time before saturating for all intensities. It is interesting that the saturation level is gradually increased with decreasing irradiation intensities (0.57 to 0.053 mW cm−2). To our knowledge, this is the first report of SRG formation with such a low light intensity. With a medium-strength irradiation intensity of 0.12 mW cm−2, a relief height of more than 50 nm was attained within 10 min of continuous irradiation, which corresponds to an exposure energy of 72 mJ cm−2. Such small exposure energy is comparable to the highest sensitivity SRG formation system reported in phase-transition type liquid crystalline azobenzene polymer films.8 This highly sensitive SRG formation in the bisanthracene film is attributable to the large mobility of the low molecular weight bisanthracene above the Tg during irradiation and the sufficient confinement of molecular motion caused by the photopolymerization of the bisanthracene molecule in the irradiated region. We propose that the surface undulation is formed along with the stiffening of the bisanthracene film caused by the successive dimerization of bisanthracene molecules by spatially patterned UV irradiation. Another important requirement for SRGs is that the shape remains stable in long-term storage and is durable at higher temperatures. The photomodulated structure in the film of 2 remained


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unchanged for at least 3 weeks at room temperature in a dark place. We then evaluated the stability of the SRG structure upon heating. Figure 4 shows the changes in relief height for the inscribed anthracene film after the heating process. The anthracene film was heated in a stepwise fashion (each heating period was 10 min), after which the film was quenched and the relief height was investigated using AFM. For the monoanthracene film, the surface relief was decreased drastically around 320 K, which is 30 K higher than its Tg. However, for the film of bisanthracene 1, the collapse temperature was observed at around 360 K; thus, an improvement of thermal stability by 40 K was observed. We ascribe this effect to the polymerization of bisanthracene compounds instead of the dimerization of monoanthracene molecules. This improvement of the thermal stability was further enhanced by subsequent uniform irradiation with UV light (0.12 mW cm−2, room temperature) as indicated by other symbols in Figure 4 (square: 5 min, diamond: 10 min, and triangle: 20 min). These results were mainly caused by the polymerization and dimerization of anthracene compounds in the unirradiated region during the patterned irradiation process. Temporal increases in the relief amplitude were observed before the collapse of the SRG structure in both anthracene films of 1 and 3. Such film deformation to form periodic relief structure during the heating process was reported in the photoembossing system.29,30 By taking the reported results into consideration, the observed phenomenon is probably caused by enhancement of molecular mobility during the heating process. Thus, we are considering that the present system is related to the photoembossing system to some extent.


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Figure 4. Changes in relief height of the SRGs without (circle) and with subsequent uniform irradiation of UV light (0.12 mW cm−2, room temperature) for 5 min (squre), 10 min (diamond), and 20 min (triangle) when SRGs were heated in a stepwise fashion (each heating period was 10 min): (a) Initial SRG was produced in the bisanthracene 1 film with patterned light (mask period: 6 µm) irradiation of UV light (365 nm, 0.12 mW cm−2) for 10 min at 333 K. (b) Initial SRG was produced in the monoanthracene 3 film with patterned light (mask period: 6 µm) irradiation of UV light (365 nm, 0.12 mW cm−2) for 10 min at 312 K. In conclusion, we have demonstrated efficient photo-triggered SRG formation in amorphous thin-films of bisanthracene compounds. The total photon dose required for SRG formation was smaller than 0.1 J cm−2, which is comparable to that of the previously reported most sensitive SRG formation system. This high sensitivity was attained via the high molecular mobility in the bisanthracene state. The resultant SRG structure was stable and maintained its structure when it was stored below 360 K. This structure stability was achieved by the stiffening of the polymerized bisanthracene film. We believe that this novel bisanthracene-based system is a highly promising candidate for dynamic optical patterning applications. Detailed studies on the variation of the spacer structure linking the two anthracene units and reversibility of the resulting surface relief are currently underway in our laboratories.


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ASSOCIATED CONTENT Supporting Information. Experimental details including the syntheses of 1 – 2

AUTHOR INFORMATION Corresponding Author *[email protected] Funding Sources JSPS KAKENHI Grant Numbers JP23750153, JP25410089, JP26107009, and JP16K13976, MST Foundation, JGC-S Scholarship Foundation, and Tonen General Foundation. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was financially supported by the JSPS KAKENHI Grant Numbers JP23750153, JP25410089, JP26107009, and JP16K13976, MST Foundation, JGC-S Scholarship Foundation, and Tonen General Foundation. The authors thank the Instrumental Analysis Center, Yokohama National University, for the use of the NMR and MS spectrometers. The authors would like to thank Enago for the English language review.


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