Photoinitiation and Inhibition under Monochromatic Green Light for

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Photoinitiation and Inhibition under Monochromatic Green Light for Storage of Colored 3D Images in Holographic Polymer Dispersed Liquid Crystals Guannan Chen, Mingli Ni, Haiyan Peng, Feihong Huang, Yonggui Liao, Mingkui Wang, Jintao Zhu, Vellaisamy A. L. Roy, and Xiaolin Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13129 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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ACS Applied Materials & Interfaces

Photoinitiation and Inhibition under Monochromatic Green Light for Storage of Colored 3D Images in Holographic Polymer Dispersed Liquid Crystals Guannan Chen,† Mingli Ni,† Haiyan Peng,*, † Feihong Huang, § Yonggui Liao, † Mingkui Wang, § Jintao Zhu,† V.A.L. Roy, ‡ Xiaolin Xie*,† †

Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of

Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. §

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan 430074, China. ‡

Department of Physics and Materials Science, City University of Hong Kong, Tat Chee

Avenue, Kowloon Tong, Hong Kong SAR, China. KEYWORDS: photopolymerization, inhibition, holography, green light, rose bengal, anticounterfeiting

ABSTRACT: Holographic photopolymer composites have garnered a great deal of interest in recent decades, not only because of their advantageous light sensitivity, but also due to their

1 ACS Paragon Plus Environment


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attractive capabilities of realizing high capacity three-dimensional (3D) data storage that is long-term stable within two-dimensional (2D) thin films. For achieving high performance holographic photopolymer composites, it is of critical importance to implement precisely spatiotemporal control over the photopolymerization kinetics and gelation during holographic recording. Though a monochromatic blue light photoinitibitor has been demonstrated to be useful for improving the holographic performance, it is impractical to be employed for constructing holograms under green light due to the severe restriction of the First Law of Photochemistry, while holography under green light is highly desirable considering the relatively low cost of laser source and high tolerance to ambient vibration for image reconstruction. Herein, we disclose the concurrent photoinitiation and inhibition functions of the rose bengal (RB)/N-phenylglycine (NPG) system upon green light illumination, which result in significant enhancement of the diffraction efficiency of holographic polymer dispersed liquid crystal (HPDLC) gratings from zero up to 87.6±1.3%, with an augmentation of the RB concentration from 0.06 × 10-3 to 9.41 × 10-3 mol L-1. Interesting, no detectable variation of the 𝜙 " # 𝑘% 𝑘&" # , which reflects the initiation efficiency and kinetic constants, is given when increasing the RB concentration. The radical inhibition by RBH • is believed to account for the greatly improved phase separation and enhanced diffraction efficiency, through shortening the weight-average polymer chain length and subsequently delaying the photopolymerization gelation. The reconstructed colored 3D images that are easily identifiable to the naked eye under white light demonstrate great potential to be applied for advanced anticounterfeiting.

1. INTRODUCTION Holography of Nobel Prize fame,1 with the unique capability of simultaneously reconstructing the whole (both amplitude and phase) information of coherent optical beams, represents a valuable technology for realizing astonishing glasses-free colored 3D display


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under white light within two-dimensional (2D) thin films.2-4 This distinct attribute enables holography to prevent any illegal copying or scanning,5 which is of particular importance for a simplest anticounterfeiting protocol to defeat the ever-increasing fake products that have caused severe threats to the public security, including the authentication integrity, wealth privacy and life safety. The high storage capacity (~1012 bits cm-3), fast data transfer speed (~109 bits s-1), and page-wise readout characteristic,3,6 also enable holography to be the ideal solution for high-density data storage,7-9 holographic memory,10 autostereoscopic 3D displays,11,12 and updatable telepresence.13 Typical holography, known as two-phase holography, is implemented by the intersection of two coherent plane waves (one is the object beam and another is the reference beam), with sinusoidal interference patterns generated.14,15 Upon exposure to these interference patterns, an alteration of the chemical or physical properties in the recording media occurs,16 thus resulting in holographic information storage. In turn, the two-phase holography not only enables the spatiotemporal understanding of photochemical and photophysical processes of materials upon light exposure,17 but also offers a precise maskless control of molecule or nanoparticle placement, orientation, patterning, and so forth, for the construction of well-structured functional assemblies and devices,18-21 such as solar energy concentrators,22,23 anisotropic ion conducting channels,24 optical sensors with ordered structures,25 and optical polarizers.26 Nevertheless, despite the astonishing advantages of holography in anticounterfeiting,27,28 the current widely applied 2D embossed holograms are readily counterfeited, and it remains challenging to build genuine 3D authentication tags for daily optical anticounterfeiting use. Holographic photopolymers, which rely on photopolymerization and polymerization induced phase separation upon exposure to interference patterns,18,19 are regarded as the most promising recording media for permanent holographic storage in the two-phase holography.29,30 Photopolymers have also been well-recognized for their high light sensitivity, 3 ACS Paragon Plus Environment


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large data capacity, good reproducibility, great processing suitability, and long term stability,8 since the first disclosure in 1969 at Hughes Research Laboratories.31 To enable predesigned high performance holographic photopolymers, nanoparticles with high refractive index and controlled dispersion,4,32-36 liquid crystals (LCs),2,3,19,35-45 and photopolymeric multifunctional dendrimer9 have been employed as functional additives. Besides, orthogonal strategies involving ring-opening reaction,46,47 thiol-ene14 and thiol-yne15 click reactions have also been demonstrated for aims of depressing the volumetric shrinkage and dimension instability of photopolymers. Among all of them, LC-based holographic photopolymer composites exhibit superior response capabilities upon external stimulation such as electrical pulse, optical illumination and temperature variation, and thus afford holograms the added functionality. The LC-based holography can be dated back to 1992 when Zhang and Sponsler employed the reactive LC monomers for holographic recording, in which unreacted LC monomers or chemical inert LCs were left in the destructive (dark) regions for electric switching.19,42 But the diffraction efficiency was quite low because of the low refractive index difference between the bright and dark areas. Only one year later (in 1993), another independent paradigm was proposed by Sutherland and Natarajan and their co-workers, in which holograms with 100% of diffraction efficiency were generated by coherently illuminating the homogeneous mixture comprising low refractive index acrylates and high refractive index LCs.41 The named holographic polymer dispersed liquid crystals (HPDLCs) later have sparked a storm of popular research interest till today. During the formation of HPDLCs, photosensitizers absorb photons and then transform to their excited state upon exposure to laser interference patterns. The excited sensitizers (usually in their triplet states) then react with co-initiators through photoinduced electron transfer and subsequent proton transfer, generating radicals to initiate photopolymerization in the constructive (bright) regions, while squeezing the LCs into the destructive (dark) areas from the bright regions, eventually forming periodically phase


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separated structures. Despite the longstanding recognition that the spatial, temporal and spectral control of the photopolymerization kinetics and gelation plays a critical role for precise shaping of the predesigned HPDLC gratings and tuning the electro-optical performance of HPDLCs,38,39,48,49 till two years ago, inspired by the two-color photoinitiation and photoinhibiton,50 a monochromatic blue light photoinititibitor,2 was developed as a useful protocol for improving the phase separation of HPDLCs, which implemented the function by simultaneously generating one initiating radical and one inhibition radical upon light exposure. Furthermore, a numerical equation was successfully built to describe the relation between the segregation degree and the ratio of gelation time to mixture viscosity. This useful equation worked very well for both holographic photopolymer/nanoparticle composites and HPDLCs.3,4 In addition, concurrent increase of the diffraction efficiency and decrease of switching voltage can be enabled using the photoinitibitor.3,35 Nevertheless, it is impractical to employ the monochromatic blue light photoinitibitor for constructing holograms under green light due to the severe restriction of the First Law of Photochemistry, while holography under green light is of critical importance considering the relatively low cost of laser source and high tolerance to ambient vibration for image reconstruction.51 Meanwhile, despite the wide applications of the rose bengal (RB)/N-phenylglycine (NPG) photoinitiating system in green laser holography,41,52-56 the photoinitiation and inhibition functions of RB/NPG system as well as its control capability over the formation of HPDLCs remain unclear. Furthermore, it is reported to be hard to obtain bright 3D images in HPDLCs due to the limited resolution,57 and colored 3D images identifiable to the naked eye under white light have not been reconstructed using the RB/NPG photoinitiating system. Herein, we disclose the concurrent photoinitiation and inhibition upon green light illumination of the RB/NPG system, which significantly improve the diffraction efficiency of HPDLC gratings from zero up to 87.6±1.3%, with an augmentation of the RB concentration from 0.06 × 10-3 to 9.41 × 10-3 mol L-1. Distinct from previous reports,



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no detectable variation of the ϕ" # k % k "& # , which reflects the initiation efficiency and kinetic constants, is given when increasing the RB concentration. The greatly improved diffraction efficiency of holographic gratings allows for the storage of colored 3D images, and these colored 3D images are easily recognizable by the naked eye under white light. In practice, the virtual 3D effect of these images prohibits any illegal copying, and thus shows great potential to be applied in advanced authentication. 2. MATERIALS AND METHODS 2.1. Materials. Rose bengal (RB, purity: 85%) was purchased from Acros Organics. Nphenylglycine (NPG, purity: 97%) and acetonitrile (purity: ≥99.8%) were obtained from Aladdin. N,N-Dimethylacrylamide (DMAA, purity: 98%) was received from J&K Scientific. Hyperbranched monomer 6361-100 was donated by Eternal Chemical Co., Ltd. The LC mixture named P0616A (∆n = 0.20, no(589 nm, 293 K) = 1.52, clearing point: 331 K) were procured from Shijiazhuang Chengzhi Yonghua Display Material Co., Ltd. (China). 2.2 Ground state absorption. Ground state absorption was acquired by UV-Vis spectroscopy (Thermo Fisher Evolution 220 UV-Vis spectrometer). The concentration of RB was 1.6 × 10-5 mol L-1. 2.3 Transient absorption. Laser flash photolysis with transient absorption was conducted on a nanosecond pulse laser flash photolysis system (LP920, Edinburgh, UK) at room temperature. Solutions with RB (2.0 × 10-5 mol L-1) and NPG (0, 3.3 × 10-5, 3.3 × 10-4, 1.7 × 10-3 mol L-1, respectively) were sealed into quartz sample cells in an oxygen-free glove box, and then excited by 532 nm nanosecond pulse laser (Surelite I10, Continuum Inc.), simultaneously probed by a pulsed xenon lamp (Xe920) and measured by a Hamamatsu R928 photomultiplier (450 -800 nm) after guiding the light beam through a monochromator.


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2.4 Preparing the mixtures for holography. The mixtures for holography are required to be homogeneous and visible light transparent. RB, NPG, DMAA, 6361-100 and LCs P0616A were added into a brown glass bottle with a sealed cap, and kept mixing by ultrasonication at 313 K for 15 min. The employment of polar monomer DMAA was critical to dissolve RB and NPG completely, discarding the previous consecutive two-step mixing procedure.3 The components for each entry are displayed in Table 1. To understand the photoinitiation and inhibition, the ratio of DMAA, 6361-100, LCs P0616A and NPG was fixed at 30:15:26.4:1 by weight, while allowing the RB content to vary from 0.06 × 10-3 to 9.41 × 10-3 mol L-1. Table 1. Chemical Components for Holography Entry

DMAA (g)

6361-100 (g)

P0616A (g)

NPG (g)

RB (g)

[RB]/10-3 mol L-1

1

8.40

4.20

7.40

0.280

0.0013

0.06

2 3 4 5 6 7 8 9 10 11 12 13

6.30 4.20 1.26 0.84 0.63 0.42 0.42 0.42 0.42 0.42 0.42 0.42

3.15 2.10 0.63 0.42 0.32 0.21 0.21 0.21 0.21 0.21 0.21 0.21

5.55 3.70 1.11 0.74 0.56 0.37 0.37 0.37 0.37 0.37 0.37 0.37

0.210 0.140 0.042 0.028 0.021 0.014 0.014 0.014 0.014 0.014 0.014 0.014

0.0012 0.0014 0.0014 0.0014 0.0014 0.0014 0.0028 0.0042 0.0056 0.0070 0.0084 0.0098

0.08 0.14 0.45 0.68 0.90 1.35 2.71 4.05 5.40 6.74 8.07 9.41

2.5 Photopolymerization kinetics. The photopolymerization kinetics was characterized by photo-differential scanning calorimetry (TA Q2000 DSC). A pair of short pass filters centered at the wavelength of 532 nm were used to filter the optical beams respectively from the two separate light guides, providing an intensity of 0.3 mW cm-2 for each beam. One beam was used to irradiate the sample, while another was employed to expose the reference. The light intensity was the highest for our current setup. Into one aluminum pan around 10 mg of homogeneous mixture was added, and the pan was placed on top of one DSC sample holder. The reference was another vacuum aluminum pan. The samples were kept static for 5 min at



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303 K, and purged by nitrogen gas at a rate of 50 mL min-1, and then isothermally irradiated for 20 min. The exothermal heat flow during photopolymerization was recorded to calculate the polymerization rate and double-bond conversion.2,37 The exothermal heat flow, 𝑑𝐻 𝑑𝑡, is generated by the consumption of C=C bond, thus the polymerization rate, 𝑅2 , and double-bond conversion, 𝛼, are calculated as follows, 𝑅2 = 𝑑𝐻 𝑑𝑡 𝛼 =

? >: 𝑑𝑡 @ >?

56 786 ∆:;

Photoinitiation and Inhibition under Monochromatic Green Light for

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