PHOTOREFRACTIVE EFFECT-GENERAL INTRODUCTION
Photorefractive (PR) effect- History and definition
In 1966, the observation of optically-induced inhomogeneity in the refractive index of LiNbO3 marked a significant development in nonlinear optics Initially deemed detrimental to the functionality of nonlinear devices and referred to as optical damage, this phenomenon was thought to hinder light frequency-doubling applications However, it was later recognized that this "damage" could replicate original intensity variations through changes in the dielectric constant, making it valuable for holographic recording and other uses Until 1990, the primary materials for photorefractive applications were inorganic crystals like BaTiO3.
Bi12SiO20 and SrxBa1-xNbO3 are examples of inorganic photonic materials, but their complex crystal growth limits their application in photonic technologies In contrast, polymeric organic materials offer significant advantages, including low cost and ease of processing and fabrication Additionally, the ability to modify chemical structures or components in composites allows for tunable photonic response properties The first organic polymer composite for photonic applications was reported in 1991, featuring a nonlinear optical epoxy polymer, bisA-NPDA, combined with 30% of the hole transporting agent DEH Since that initial development, there have been substantial advancements in the performance and understanding of organic photonic materials.
PR organics have been extensively discussed in various review articles According to W E Moerner, the PR effect involves the spatial modulation of the refractive index due to charge redistribution in optically nonlinear materials This phenomenon occurs when charge carriers, generated by spatially modulated light intensity, undergo separation through drift and diffusion, leading to a non-uniform space-charge distribution The internal electric field created by this space-charge then modulates the refractive index, forming a grating that can diffract light This comprehensive definition encapsulates the entire process within PR materials, including charge generation in illuminated areas, the movement of positive charges to darker regions, and the subsequent modulation of the refractive index.
Photorefractive mechanism
The generation of photorefractive (PR) gratings occurs in two main stages: first, the formation of a spatially non-uniform space charge field, and second, the alteration of the refractive index influenced by the combined effects of the space charge field and an externally applied electric field The mechanism behind the PR effect is illustrated in Figure 1, with each process detailed in the following sections.
The optical interference pattern is created when two coherent lasers intersect in photorefractive (PR) materials This interaction, combined with non-uniform illumination and an external electric field, generates an internal space-charge field The development of this space-charge field involves a complex process of charge carrier generation, the transportation of more mobile charges—typically holes in PR materials—and charge trapping in darker areas Consequently, this process results in a redistribution of charge carriers, establishing a spatial difference in charge concentration between the dark and bright regions.
Under non-uniform laser illumination, charges are primarily generated in areas of higher light intensity within the interference pattern In pure photoconductive polymers, the photo-generation of charge carriers is inefficient due to the prevalent charge-recombination, which occurs when electrons are excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) To improve the generation of charge carriers, a sensitizer is introduced For p-type photoconductive materials, this sensitizer possesses a significantly lower HOMO level, indicating a smaller ionization potential compared to the charge-transport manifold.
The Onsager model is the most widely utilized framework for understanding charge photo-generation Initially, a photon absorption creates an excited state characterized by a hot localized electron-hole pair Subsequently, the electron and hole undergo thermalization, leading to a charge-transfer state with a mean separation distance, r₀ The likelihood of this excited state transitioning to a charge-transfer state is quantified by the primary quantum yield, φ₀ The final phase involves dissociation, where the charge-transfer state is divided into a free electron and a free hole The rate of charge separation competes with recombination to achieve equilibrium, necessitating the overcoming of the Coulomb interaction between the electron and hole, with the Coulomb radius defined as rₖ.
The photo-generation of charge carriers in organic photoconductive polymers exhibits a strong dependence on the applied electric field When the distance between a charge carrier and its counter charge approaches or exceeds the Coulomb radius, the carrier can escape and become a free charge This process is influenced by several factors, including the elementary charge (e), the relative dielectric constant (ε), vacuum permittivity (ε₀), Boltzmann constant (kₐ), and temperature (T) The application of an external electric field enhances charge separation, facilitating the generation of free carriers.
5 generation efficiency ( ) E is a product of the primary quantum yield 0 and the dissociation probability Ω
Figure 2 Schematic of charge transport manifold in PR polymer
Once free charge carriers are generated, they migrate towards areas of lower light intensity through diffusion or an external field Unlike the crystalline structures found in inorganic materials, most organic photorefractive (PR) materials possess an amorphous structure that lacks long-range order, preventing the formation of energy bands As a result, the transportation of carriers cannot be accurately represented by traditional band models.
Transport in PR polymers occurs through intermolecular hopping of carriers between neighboring molecules, characterized as an electron transfer reaction Electron transport takes place at the LUMO level, while hole transport occurs at the HOMO level Consequently, photoconductivity is influenced not only by energy levels but also by the distance and orientation between hopping sites In most PR polymers, charge transport is effectively described using disorder formalism.
6 states in polymer are completely localized and the distribution of the hopping sites is assumed to be Gaussian (Figure 2)
The mobility difference between hole and electron carriers is crucial in photorefractive (PR) materials In efficient PR materials, holes serve as the primary charge carriers, necessitating their higher mobility compared to electrons to establish a space charge field However, at elevated applied fields, electron transport becomes significant Research by Banerjee et al demonstrates electron transport at high applied fields through the analysis of time dynamics in self-pumped reflection gratings within PR composites.
After holes are generated and transported to a dark area, they become trapped at specific sites, although the true nature of these trap sites remains uncertain Charge transport occurs through hopping between localized sites, where each site has the potential to act as a trap, contributing to the overall transport mechanism It has been proposed that traps may arise from molecular conformation or ionic impurities In hole-conducting systems, any component with a higher HOMO level than the charge-transport manifold is likely to function as a hole trap When a sensitizer absorbs photon energy and injects a hole into the charge-transport manifold, it forms a radical anion To prevent electron migration that could disrupt the space-charge field, the sensitizer's LUMO level must be the lowest in the system Strategies to enhance space-charge field formation, such as incorporating electron trap compounds, have proven effective.
The Kukhtarev model, introduced in 1979, provides a framework for understanding the internal space-charge field in inorganic materials by applying the band model, where electrons are assumed to play a key role in the photorefractive (PR) effect.
7 to be the only carrier that can be transported The amplitude of the space-charge field (Esc) based on Kukhtarev model is given by
(3) where m is the fringe visibility:
The local maximum and minimum intensity values, I max and I min, are crucial in understanding the intensity values of the writing beams, I 1 and I 2 For s- (p-) polarized beams, the parameter p is set to 0, and the internal angle between the two writing beams is represented by 2θ int Additionally, E 0 denotes the component of the applied field along the grating wave vector, while the diffusion field E D plays a significant role in the overall analysis.
The magnitude of the grating wave vector (|k|) is determined by the grating spacing (Λ) of the interference pattern, the refractive index (n) of the material, the optical wavelength (λ) in a vacuum, and the internal angles of incidence (θ1 and θ2) of the two writing beams Additionally, the Boltzmann constant (kB), temperature (T), and elementary charge (e) are also relevant factors in this context.
The limiting space-charge field is given by
K (5) where ε is the dielectric constant, ε 0 is the vacuum permittivity constant and N eff is the effective density of traps
The phase-shift between the space-charge field and the original interference pattern is given by
Figure 3 Two trapping site model
In organic materials, electronic states are highly localized, making the band model inapplicable Unlike inorganic materials, the trap density in organics varies with factors such as electric field, light exposure, and temperature Additionally, most organic photorefractive (PR) materials function as p-type semiconductors, where holes are the primary charge carriers Consequently, the Kukhtarev model requires modification to accurately describe the space-charge field in organic systems.
PR materials The Kukhtarev model has been refined by Schildkraut et al 12 to include of the rate equation of trap density and field dependence of photo-generation efficiency as
9 well as carrier mobility Cui et al considered the erased dynamic of the space-charge field
The most comprehensive model to date, developed by Singer and Ostroverkhova, is a two-trapping site model based on the Schildkraut model This model introduces two types of traps: shallow and deep, where deep traps have a thermal de-trapping rate that is significantly lower than that of shallow traps, yet still possess a non-zero probability for de-trapping When a photon is absorbed by a neutral sensitizer molecule, an electron is transferred to the sensitizer, resulting in the injection of a free hole into the transport manifold This hole then hops between transport sites until it either gets trapped or recombines with ionized acceptors The model effectively explains pre-illumination phenomena in certain photorefractive materials and includes a set of equations that describe the dynamics of space-charge field formation.
(7) where ρ is the free charge (hole) density
N A is the total density of acceptors i
N A is the density of ionized acceptors
M1, M2, MT1, and MT2 represent the densities of filled shallow traps, filled deep traps, and the total densities of shallow and deep trapping sites The parameters γT1 and γT2 denote the trapping rates, while β1 and β2 indicate the detrapping rates for shallow and deep traps, respectively Additionally, γ signifies the recombination rate in this context.
I is the incident light intensity
J is the current density à is the charge carrier drift mobility s is the total cross-section of the photogeneration process ξ is the diffusion coefficient ( k T B
2.2 Formation of refractive index grating and orientational enhancement effect
Material types
To fabricate photorefractive (PR) materials, essential components include a photoconductive medium for charge generation, a sensitizer to enhance photo-induced charge transfer, and a nonlinear optical (NLO) molecule that facilitates changes in refractive index under an electric field Additionally, trapping sites are crucial for the functionality of these materials, although their specific identities remain largely unknown These components are integrated into various types of materials to achieve the desired photorefractive properties.
Figure 5 Examples of PR polymer composites components: NLO chromophore; 18,19 sensitizer; 4 Photoconductive polymer 20,21 NLO chromophore Sensitizer Photoconductive polymer
The most effective materials for this application utilize a photoconductive polymer as a medium, combined with a nonlinear optical (NLO) chromophore for refractive index modulation and a sensitizer to facilitate charge generation Additionally, a plasticizer is incorporated to lower the glass transition temperature, enhancing the orientation of the chromophore Figure 5 illustrates the chemical structures of each component that demonstrate high photorefractive (PR) performance.
One significant drawback of polymer composites is that a high concentration of highly polar nonlinear optical (NLO) chromophores can lead to phase separation in non-polar photoconductive polymers, often occurring within days or weeks To address this issue, researchers have developed monolithic or fully functionalized photoconductive (PR) polymers, where both the photoconductive and NLO chromophore components are integrated into a single polymer chain An example of this fully functionalized polymer type is illustrated in Figure 6.
Figure 6 Chemical structure of PR monolithic polymer 22
This article discusses a specific type of PR materials that utilize a glass-forming molecule exhibiting both photoconductivity and nonlinear optical (NLO) properties By minimizing the inert volume, the concentration of NLO and photoconductive components is maximized However, challenges such as the synthetic modification of the glass-forming molecule and a slow response time present significant drawbacks Examples of this type of material are illustrated in Figure 7.
Figure 7 Chemical structure of small molecular weight glass 4,23
This material features a high concentration of polymer and liquid crystalline molecules that phase separate into droplets The polymer contributes to photoconductivity, enabling space-charge field formation, while the liquid crystalline molecules impart orientational nonlinear optical (NLO) properties for refractive index modulation A key advantage is that these liquid crystalline molecules can be oriented with significantly lower electric fields compared to polymer and composite materials However, a notable drawback is their high scattering, which limits the production of thin films and typically results in a grating that operates in the Raman-Nath regime, characterized by low diffraction efficiency.
Applications
Figure 8 Recording and reading a hologram
Rewritable holography and holographic data storage represent the pioneering applications of photorefractive (PR) materials Utilizing two coherent laser beams—an object beam and a reference beam—a refractive index grating is formed, allowing for the recording of three-dimensional (3D) images within the PR material The hologram is subsequently read by illuminating the sample at the specific angle of the reference beam's recording Notably, the hologram can be effortlessly erased by applying a single laser beam in the presence of an electric field The dynamic characteristics of PR polymers and composites pave the way for innovative applications, including updatable holograms, which enhance the potential for 3D communication A straightforward process for recording and reading holograms is visually represented in Figure 8, showcasing the versatility of PR materials in capturing various object images.
Using angle multiplexing, 18 images can be recorded within the same material volume by varying the recording angle for each image or object Each image can be accessed by selecting the appropriate recording angle.
This application utilizes the principle that the diffracted signal serves as a conjugate of the object beam As illustrated in Figure 9, the concept of a phase-conjugate mirror is demonstrated using photorefractive (PR) materials An image of the object, significantly distorted by a medium, is recorded in the PR medium alongside a reference beam Subsequently, a reading beam, which acts as the conjugate of the reference beam, is directed counter to the reference beam's path The resulting diffracted beam effectively represents a conjugate of the distorted object's image.
The image can pass through the distorting medium once more, allowing for the reconstruction of the object's image This is made possible by the phase-conjugate replica characteristic of the diffracted beam, which effectively cancels out the distortion.
Figure 10 Image amplification using PR materials
This application utilizes the TBC effect, which facilitates energy exchange between two writing beams When a weak object's image is combined with a stronger pump beam, the energy transfer occurs through non-local grating, enhancing the object's image after it passes through the sample Figure 10 illustrates the process of image amplification using photorefractive (PR) material.
This study
During the last decade, a tremendous improvement of organic PR materials has been reported 4,23-25 There are various PR material with interesting properties 18,20 Following the
This dissertation aims to introduce a novel approach to achieving desirable piezoelectric properties through advanced material design, building on the objectives of previous studies that sought innovative materials.
The most effective and widely used PR materials are based on composites, chosen for their ability to easily tune properties by adjusting component concentrations Typically, these composites consist of a photoconductive polymer, a nonlinear optical (NLO) dye, a plasticizer, and a sensitizer The polymer serves as a crucial dispersive matrix and photoconductive environment for the composite This dissertation focuses on developing new photoconductive matrices, synthesizing various photoconductive polymers for PR composites The design of these polymers must fulfill several criteria, including optical clarity, strong photoconductivity, and compatibility with NLO chromophores Additionally, the glass transition temperature (Tg) of the polymer is critical, as it should be close to the operating temperature to enhance orientation Therefore, selecting an appropriate Tg for the polymer is essential.
PR composites with low Tg polymers struggle to maintain mechanical strength and withstand high electric fields in thin films To enhance the performance of PR composites using high Tg polymer matrices, techniques such as incorporating a significant amount of plasticizer and minimizing polymer concentration are essential Other strategies to adjust Tg include utilizing internal plasticizers and extending the alkyl chains linking the photoconductive moiety to the polymer backbone However, these methods must be approached cautiously to avoid diluting the photoconductive component, which can negatively impact PR performance Additionally, the high viscosity of photoconductive matrices associated with high Tg polymers poses challenges in the fabrication of PR devices For applications like hologram displays, the size of the PR device should be prioritized, especially when selecting the appropriate polymer matrix.
To create a polymer from a monomer containing a photoconductive moiety, various polymerization methods can be employed, including ionization polymerization and atom-transfer radical polymerization (ATRP) Among these techniques, free-radical polymerization in solution is favored due to its simplicity, as it involves dissolving the monomer and initiator in a solvent and initiating the polymerization process through heating.
A meticulous purification process is essential for eliminating catalysts in ATRP or ionization polymerization, as well as surfactants in suspension or emulsion polymerization However, this method allows for the production of highly pure polymers due to the minimal use of initiators.
To achieve optimal optical clarity, an amorphous polymer is essential for the photoconductive matrix The use of acrylate monomers facilitates the formation of such polymers through free-radical polymerization, which results in a flexible backbone structure and a lower glass transition temperature (Tg) compared to main-chain or styrene-type polymers One significant advantage of this approach is the ability to modify the side chain moiety with minimal impact on the polymerization process, allowing for the incorporation of various photoconductive and nonlinear optical (NLO) moieties This study focuses on synthesizing acrylate side-chain polymers via free-radical polymerization to meet the requirements of photoconductivity, an amorphous structure, and an appropriate Tg Additionally, the research will explore different photoconductive moieties and propose methods to enhance compatibility with NLO chromophores by modifying the polymer matrix structure.
METHODOLOGY AND INSTRUMENTATIONS
Material characterization
All synthesized products were characterized using various analytical techniques, including mass spectroscopy (MS) with a MicroTOF from BrukerDaltonics, proton nuclear magnetic resonance (1H NMR) spectroscopy on an AV-300 from BrukerBioSpin, and infrared (IR) spectroscopy with a SpectrumGX from Perkin Elmer The melting point was determined using a MP-500P melting point apparatus from Yanaco, while ultraviolet-visible (UV-visible) absorption spectroscopy was conducted with a UV-2101PC from Shimadzu Weight-average molecular weight (Mw) and number-average molecular weight (Mn) were assessed through gel permeation chromatography (GPC) using Shodex GPC KF-805 + KF-803 columns and tetrahydrofuran (THF) as an eluent, calibrated against polystyrene standards The glass transition temperature (Tg) was measured using a DSC 2920 from TA Instruments at a heating rate of 10 °C min-1, and the ionization potential (IP) was estimated in air with an AC-1 from Riken Keiki.
Absorption coefficient
Determining the linear absorption of PR composites is crucial, as the sensitizer absorbs photons to generate charges and allows for estimating the percentage of laser beam loss transmitted through the PR sample The absorption coefficient can be measured using UV-Visible absorption spectroscopy, which utilizes two arms: one for the sample and one for the reference In this setup, the PR composite film is positioned in the sample arm, while ITO-coated glass serves as the reference Results are typically recorded as absorbance (A) or optical density (OD).
(13) where α is the absorption coefficient, d is the thickness of the PR film
Photorefractive characterization
The polymer was combined with NLO dye and sensitizer in varying compositions within THF, and the mixture was stirred overnight It was then cast onto a hot plate at 70 °C for 24 hours before being dried in a vacuum oven at 60 °C for an additional 24 hours The resulting composites were sandwiched between two indium-tin-oxide (ITO) coated glass plates using the melt-pressing method, with the film thickness regulated by Teflon spacers The fabrication process for the PR sample is illustrated in Figure 11.
3.2 Degenerated four-wave mixing (DFWM)
Figure 12 Degenerated four-wave mixing (DFWM) geometry
The diffraction efficiency of the PR sample was evaluated using a degenerate four-wave mixing (DFWM) technique, where holographic gratings were created by two intersecting s-polarized beams A weak p-polarized probe beam from the same source, traveling in the opposite direction of one writing beam, was diffracted by the refractive index gratings within the sample film This diffracted signal then moved against the second writing beam and was reflected by a beam splitter, with detection performed by a photodiode Additionally, another photodiode detected the probe beam passing through the sample film A geometric configuration for the DFWM setup is illustrated in Figure 12.
The measurement sequence involves applying voltage in a dark environment for 10 minutes, followed by activating one of the writing beams for 3 seconds The measurement begins when the remaining writing beam is turned on, necessitating precise synchronization of the transmitted and diffracted beam signals, which are detected by two separate detectors To ensure accurate timing, an Oscilloscope with a trigger signal is employed, although its resolution may be limited Consequently, Agilent digital multimeter devices are utilized for improved accuracy A schematic representation of the system is illustrated in Figure 13.
Figure 13 Schematic illustration of DFWM with a trigger signal
In the experimental setup illustrated in Figure 13, a beam passing through a shutter is divided by a beam splitter into two components: one functions as a writing beam, while the other is directed to detector D3 The output from D3 serves as a trigger signal for two digital multimeters, ensuring they begin recording simultaneously when the shutter opens This arrangement allows for synchronized data collection from detectors D1 and D2, with the starting point automatically established.
From the intensity of the transmitted beam (I t ) and the diffracted beam (I d ), the diffraction efficiency (η) was calculated with equation:
The internal diffraction efficiency which is taken into account the absorption lost of the probe beam after the sample can be experimentally determined by the equation: int
The response time for the composite material was assessed using consistent geometry, with the grating build-up over time accurately modeled by a modified single exponential function based on the Kohlrausch-Williams-Watts (KWW) equation.
(16) where τ is the response time, β (0 < β ≤ 1) is the fitting parameter related to a width of time constant distribution or bi-exponential function
(17) where τ 1 , τ 2 are time constants with weighing factor of m (0 ≤ m ≤ 1) and (1-m), respectively
Figure 14 illustrates the increase in diffraction efficiency over time, with the raw data represented by circles and the fitting result depicted by a red curve.
Figure 14 An example of diffraction efficiency growth as the function of time with fitting curve
Figure 15 Two-beam coupling (TBC) measurement
The optical gain values of the sample were measured using a two-beam coupling (TBC) technique, employing the same geometric configuration as the degenerate four-wave mixing (DFWM) method This setup involved two equal intensity p-polarized beams crossing at the thin film sample, but without a probe beam The transmitted intensities of the two beams through the sample were detected using photodiode detectors, allowing for the evaluation of optical gain through a specific equation.
In the study of sample films, the thickness (d) plays a crucial role, while θ A and θ B represent the internal angles of two beams relative to the normal Additionally, I A and I B denote the intensities of these beams after passing through the sample film.
An example of data detected by TBC technique is shown in Figure 16 The intensity of beam 1 is increased while the intensity of beam 2 is decreased by the same amount
Figure 16 An example of asymmetric energy transfer in TBC measurement
3.3 Calculation of PR parameters using Kogelnik’s coupled-wave theory and
According to Kogelnik’s coupled-wave theory, 29 diffraction efficiency for p- polarized probe beam is related to the refractive index modulation using an equation of sin [ 2 cos( )] p K n B A
, n is the refractive index modulation, θ A ,θ B are the internal angles of two beams with respect to the normal, is the wavelength of laser beam, d is
31 the film thickness n is calculated with diffraction efficiency p obtained from DFWM measurement with p-polarized reading beam
It is possible to calculate the phase shift from the values of diffraction efficiency and optical gain in DFWM and TBC measurement, respectively Optical gain is related to
n with the equation as described above with 4 n sin m
, where is the optical gain,
The wavelength of the laser beam is denoted as λ, while Φ represents the phase shift between the refractive index modulation and the illumination pattern The refractive index modulation, indicated as Δn, was determined using the calculations from section 3.3.1, and the phase shift was calculated based on this modulation alongside the optical gain, Γ, obtained from TBC measurements.
3.3.3 Trap-limited space-charge field and number density of trap
The Kukhtarev model, despite being rooted in inorganic materials, proves valuable for estimating trap density in organic materials By applying this model, the trap-limited space-charge field \( E_q \) can be determined using the phase-shift \( \Phi \) calculated through the following series of equations.
The tilt angle of the grating wave vector, denoted as Ѱtilt, is measured concerning the direction of the applied electric field E Using the value of E q, one can calculate the effective trap density, N eff, using the appropriate equation.
MONOLITHIC POLYMER BASED ON CARBAZOLE AS A
Introduction
A typical PR composite consists of a photoconductive polymer, a nonlinear optical (NLO) chromophore, a plasticizer, and a sensitizer To enhance refractive index modulation, it is beneficial to increase the concentration of the NLO chromophore However, higher levels of this highly polar chromophore can lead to phase separation within the less polar photoconductive polymer Previous studies have shown that NLO chromophore concentrations often remain below 35 weight percent (wt.%) Phase separation can occur rapidly, sometimes within days or weeks after sample fabrication To achieve long-term stability against phase separation, various types of monolithic structures are being explored.
Recent advancements in PR polymers have been made; however, their high glass transition temperature (Tg) results in reduced flexibility compared to guest-host composites Therefore, there is a need for long-term stable PR materials that feature a high concentration of NLO chromophores while maintaining a low Tg.
This chapter presents a composite material that integrates low glass transition temperature (Tg) and high nonlinear optical (NLO) concentration within a monolithic polymer The compatibility between the NLO chromophore and the photoconductive polymer matrix is crucial for maintaining stability and preventing phase separation.
Poly(vinyl carbazole) (PVK) has been widely used as a photoconductive host for
PR composite materials are gaining traction, with PVK emerging as a leading polymer due to its exceptional PR performance This study focuses on synthesizing a monolithic PR polymer using carbazole, a photoconductive moiety, combined with poly(methacryloyl chloride) through a polymer-analogous reaction Free-radical polymerization is employed for its efficiency in incorporating functional groups as side chains Optimizing the polymerization conditions for vinyl compounds linked to the nonlinear optical (NLO) and photoconductive moieties is essential for successful polymerization and copolymerization processes.
The polymerization process is influenced by factors such as initiator and monomer concentration, reaction temperature, and solvent type NLO chromophores and photoconductive materials with effective photorefractive (PR) properties often contain multiple functional groups, which can hinder free-radical polymerization To circumvent these challenges, a polymer analogous reaction is preferred for its ease of use The resulting monolithic polymer is combined with the plasticizer carbazoylethylpropionate (CzEPA), the sensitizer 2,4,7-trinitro-9-fluorenone (TNF), and an additional NLO chromophore identical to that in the polymer's side-chain The PR performance of these composites is evaluated, with a focus on their long-term stability against phase separation when a significant amount of NLO chromophore is incorporated.
Experimental Section
2.1 Synthesis of PR monolithic polymer
Scheme 1 Synthetic route for PR monolithic polymer (5)
The synthetic procedure outlined in Scheme 1 involves the synthesis of the NLO chromophore 2 and carbazole 3 moieties, following established literature methods These moieties are then integrated into poly(methacryloyl chloride) 4 through a polymer analogous reaction, resulting in the formation of the PR monolithic structure.
2.1.1 Synthesis of 4-[4-(2-Hydroxyethyl)piperidin-1-yl]-benzaldehyde (1)
A mixture of 4-piperidineethanol, p-fluorobenzaldehyde, and dried potassium carbonate was heated at 90 °C in dimethyl sulfoxide for 24 hours After cooling, the mixture was diluted with deionized water, and the yellow precipitate was collected via vacuum filtration The solid was purified using silica gel column chromatography with ethyl acetate as the eluent and recrystallized from chloroform/hexane, yielding 3.95 g of a crystalline product (65% yield, melting point 106.4–107.1 °C) The product was characterized by TOF/MS (ESI) with m/z found at 256.14 [M + Na + ] and 1H NMR (300 MHz, CDCl3) showed peaks at 9.75 (s, 1H, CHO), 7.73 (d, 2H, ArH), 6.9 (d, 2H, ArH), and 3.96 – 1.30 (m, 14H, Ar–piperidine–C2H4–OH).
1H NMR spectrum of 4-(4-(2-hydroxyethyl)piperidin-1-yl)benzaldehyde (1) was shown in Figure 17
Figure 17 300MHz 1 H NMR(CDCl3) of 4-(4-(2-hydroxyethyl)piperidin-1- yl)benzaldehyde (1)
2.1.2 Synthesis of 4-(4-(2-hydroxyethyl)piperidin-1-yl)benzaldehyde malononitrile
In a controlled reaction, three drops of piperidine were added to a methanol solution of malononitrile (0.57 g, 8.6 mmol) Subsequently, a methanol solution of compound 1 (2.0 g, 8.6 mmol) was gradually introduced, resulting in a color change from yellow to red The mixture was stirred at room temperature for 24 hours, then refrigerated overnight to facilitate precipitate formation A red precipitate was collected via vacuum filtration and recrystallized from chloroform/hexane, yielding 1.14 g of crystalline product (44% yield) with a melting point of 96.1–98.4 °C TOF/MS (ESI) analysis indicated a mass-to-charge ratio of m/z: found 304.12 [M + Na + ].
1H NMR (300 MHz, CDCl3, δ): 7.78 (d, 2H, ArH), 7.45 (s, 1H, –CH =C(CN)2), 6.83 (d, 2H, ArH), 3.96 –1.30 (m, 14H, –Ar–piperidine–C2H4–OH) 1 H NMR spectrum of HE- PDCST (2) was shown in Figure 18 Elemental analysis calcd for C17H19N3O: C 72.57, H 6.81, N 14.94; found: C 72.29, H 6.87, N 15.00
Figure 18 300MHz 1 H NMR(CDCl3) of 4-(4-(2-hydroxyethyl)piperidin-1- yl)benzaldehyde malononitrile (HE-PDCST) (2) ppm (t1)
Table 1 compares the absorption properties of HE-PDCST (2) and 4-azacycloheptyl-benzylidenemalononitrile (7-DCST), highlighting the UV-Visible absorption measurements of 7-DCST, a widely utilized NLO chromophore in PR composites, alongside HE-PDCST (2) in CHCl3, as illustrated in Figures 19 and 20.
Table 1 Comparison between HE-PDCST and 7-DCST
Extinction Coefficient (M -1 cm -1 ) at peak
Figure 19 UV-VIS absorption of 7-DCST with CHCl3 as a solvent
Figure 20 UV-VIS absorption of HE-PDCST (2) with CHCl3 as a solvent
2.1.3 Synthesis of 6-(9H-Carbazol-9-yl)hexan-1-ol (3)
A potassium hydroxide solution (4.4 g, 31.8 mmol) in dimethylformamide (20 mL) was stirred at room temperature for 15 minutes Carbazole (2.6 g, 15 mmol) was then gradually added, and the mixture was stirred for 2 hours Chlorohexan-1-ol (2.3 g, 17 mmol) was added dropwise, followed by stirring at room temperature for 4 hours and then at 40 °C for 48 hours The mixture was poured into cold water, and the resulting precipitate was filtered and purified using a silica gel column.
40 chromatography (eluent, ethylacetate/hexane, 1/1) to give a crude solid After recrystallization from chloroform/hexane, needle-like crystals (0.97 g) were obtained Yield: 23% mp 123.8–126.7 °C TOF/MS (ESI) m/ z: found 290.13 (M + Na + ) 1 H NMR
(300 MHz, CDCl3, δ): 8.10 (d, 2H, ArH), 7.44 (m, 4H, ArH), 7.23 (m, 2H, ArH), 4.31 (t, 2H, –OCH2–), 3.59 (t, 2H, –NCH2), 1.96–1.36 (m, 8H, –NCH2–(CH2)4–CH2–OH), 1.15 (– OH) 1 H NMR spectrum of 6-(9H-carbazol-9-yl)hexan-1-ol (3) was shown in Figure 21 Elemental analysis calc for C18H21NO: C 80.86, H 7.92, N 5.24; found: C 81.10, H 8.05,
Figure 21 300Mhz 1 H NMR(CDCl3) of 6-(9H-carbazol-9-yl)hexan-1-ol (3) ppm (t1)
Methacryloyl chloride was distilled under ambient pressure to eliminate the inhibitor prior to the reaction A mixture of 10 mL methacryloyl chloride and 0.17 g azoisobutyronitrile (AIBN) in 11 mL anhydrous 1,4-dioxane was carefully degassed and stirred at 50 °C for 48 hours under a nitrogen atmosphere The resulting viscous solution was then diluted with 1,4-dioxane, and the polymer was precipitated into a large volume of dried hexane, yielding 46% The infrared spectroscopy analysis (IR KBr) showed characteristic peaks at ν 000 and 2945.
1788 (COCl), 1484, 1449 cm −1 IR spectrum of this polymer was shown in Figure 22 1 H NMR (300 MHz, CDCl3, δ): 2.5 –2.1 (CH2–C(CH3)OCl), 1.7 –1.2 ( α-CH3) 1 H NMR spectrum of Poly(methacryloyl chloride) (4) was shown in Figure 23
Figure 22 IR spectrum (KBr pellet) of poly(methacryloyl chloride) (4)
Figure 23 300Mhz 1 H NMR(CDCl3) of poly(methacryloyl chloride) (4)
2.1.5 Poly((6-(9H-carbazol-9-yl)hexyl methacrylate) - co - (2-(1-(4-(2,2- dicyanovinyl)phenyl)piperidin-4-yl)ethyl methacrylate) - co - (methacrylic anhydride)) (5) ppm (f1)
A novel PR polymer was synthesized through a polymer analogous reaction, utilizing HE-PDCST 2 as the nonlinear optical (NLO) component and the carbazole derivative 3 as the photoconductive moiety The reaction involved dissolving HE-PDCST 2 (1.0 g, 3.55 mmol) and 6-(9H-carbazol-9-yl)hexan-1-ol 3 (0.95 g, 3.55 mmol) in pyridine (15 mL), followed by the addition of poly(methacryloyl chloride) 4 (0.682 g, 6.52 mmol) in pyridine (20 mL) and stirring the mixture at 100 °C The progress of the reaction was monitored using IR spectroscopy, which indicated the complete disappearance of the acryloyl chloride peak at 1788 cm −1 after 24 hours, along with the emergence of ester (1727 cm −1) and anhydride groups (1803 and 1760 cm −1) in the resulting polymer.
The polymer's main chain features a newly formed anhydride group due to a polymer analogous reaction conducted at reflux temperature, as proposed by Strohriegl et al Characterization results align with previous reports, culminating in the precipitation of the final polymer into methanol, which was then filtered and vacuum-dried for two days, yielding 1.27 g of a yellow polymer powder (5) with a yield of 58% and a glass transition temperature (Tg) of 111 °C The 1H NMR analysis (300 MHz, DMSO-d6) revealed chemical shifts indicative of various functional groups, including aromatic hydrogens and aliphatic chains The molar ratio of the polymer components was determined to be n1 : n2 : n3 = 0.4 : 0.32 : 0.28, with the empirical formula (C22H25O2N)0.4(C21H23O2N3)0.32(C8H10O3)0.28, corresponding to calculated values of C 73.75, H 7.04, and N 6.588.
Figure 24 IR spectrum (KBr pellet) of monolithic PR polymer (5)
Figure 25 Mechanism of anhydride formation in polymer analogous reaction proposed by P.Strohgriel 38
Figure 26 300Mhz 1 H NMR (DMSO – d6) of monolithic PR polymer (5)
The UV-visible absorption spectrum of the monolithic PR polymer 5 in THF solution reveals five distinct peaks at 239, 263, 293, 346, and 432 nm, indicative of π − π∗ transitions The peaks at 239, 263, 293, and 346 nm correspond to the carbazole moiety, while the peak at 432 nm is attributed to the NLO chromophore moiety This spectrum clearly demonstrates the successful attachment of both functional groups to the polymer chain, as illustrated by the green and red lines representing the carbazole and NLO chromophore moieties, respectively.
Molecular weights (M_w and M_n) of the polymer were determined through Gel Permeation Chromatography (GPC) in THF, utilizing polystyrene as a standard The polymer 5 exhibited M_w and M_n values nearing 79,000 and 26,000, respectively It demonstrated excellent film-forming capabilities, resulting in high-quality thin films of PR composites.
The UV-VIS absorption spectra of monolithic PR PR polymer (5) were analyzed alongside carbazole (3) and the nonlinear optical chromophore HE-PDCST moiety (2) The study utilized THF as the solvent, with THF solution serving as the reference for comparison.
PR polymer ( 5 ) HE-PDCST ( 2 ) Carbazole ( 3 )
Figure 28 Chemical structure of each component using in the composite
Figure 28 illustrates the chemical structures and abbreviations of the materials used The sample fabrication process, detailed in CHAPTER 2:3.1, involved a melting temperature of 120 °C PR composites were created with a consistent polymer concentration of 35 wt.%, while varying the concentration of HE-PDCST at 24, 32, 40, and 50 wt.%.
Figure 29 Photographs for composite (a) PVK/7-DCST/CzEPA/TNF, (b) Polymer 5/7- DCST/CzEPA/TNF, (c) Polymer 5/HE-PDCST/CzEPA/TNF after aging experiment.(d),
(e), (f):the schematic illustrations for the respective state
The primary objective of utilizing a composite based on a monolithic PR polymer is to enhance stability against phase separation To assess the thermal stability of the composite, an accelerated aging experiment was conducted Photographs in Figure 29(a)-(c) illustrate the composites with varying components after exposure to ambient conditions and elevated temperatures Specifically, Figure 29(a) depicts a typical composite comprising PVK, 4-azacycloheptylbenzylidenemalononitrile (7-DCST), CzEPA, and TNF, revealing that the PVK-based composite experienced phase separation and subsequent chromophore degradation.
49 aggregation after 30 days due to incompatibility between the PVK and the 7-DCST even at room temperature The photograph (Figure 29b) shows a composite containing Polymer
5/7-DCST/CzEPA/TNF At 60 C, the composite also exhibited the phase separation and chromophore aggregation after 1 day However, the composite based on polymer 5 and
HE-PDCST has demonstrated remarkable transparency and stability, with no phase separation occurring even after two weeks at 60 °C, as illustrated in Figure 29(c) This stability is consistent across all compositions, maintaining transparency even at high NLO chromophore concentrations of up to 50 wt.% The findings suggest a strong compatibility between the polymer matrix and the NLO chromophore, attributed to intermolecular hydrogen bonding between hydroxyl and nitrile groups within the composite This interaction indicates that HE-PDCSTs are effectively dispersed, preventing phase separation due to weak hydrogen bonding among themselves and the NLO chromophore moieties in polymer 5 While the current PR composites exhibit lower diffraction efficiency, optical gain, and response time compared to previously reported composites, they offer enhanced stability against phase separation These results highlight the potential of hydrogen bonding for improving long-term performance in composite materials.
The photorefractive (PR) properties of the synthesized composites were analyzed using Dual-Frequency Wave Mixing (DFWM) and Transmission Beam Coupling (TBC) measurements Holographic gratings were created in the samples by intersecting two He-Ne laser beams (wavelength of 632.8 nm, power of 3 mW, intensity of 1.5 W/cm²) at incidence angles of 40.54° and 59.46° in air.
All PR properties of each compositions are listed in Table 2
Table 2 PR properties of the composite films
Polymer 5/HE-PDCST/CzEPA/TNF (wt.%) η a) (%) τ a) (ms)
4 35/50/14/1 30.7 4912 21.9 -4 a) Applied electric field is 45 Vμm -1 b) Measured by DSC with the heating rate of 10 o C/min
Conclusions
A novel monolithic polymer incorporating photoconductive carbazole and nonlinear optical (NLO) moieties has been successfully synthesized, achieving a weight-average molecular weight of 79,000 through polymer analogous reactions The polymeric composites demonstrated exceptional stability against phase separation, even at a high dye concentration of 50 wt.%, due to hydrogen bonding interactions between the NLO chromophore and the polymer host Notably, a diffraction efficiency exceeding 30% was attained at a relatively low electric field of 45 Vμm⁻¹ However, challenges such as slow response times and limited optical gain were observed, with discussions highlighting potential interruptions in the conducting pathways along the polymer chain and the trapping effects of the NLO chromophore within the composites.
The next chapter will concentrate on reducing conducting interruptions and fine-tuning the ionization potential values of each component to enhance both response time and optical gain.
PHOTOREFRACTIVE COMPOSITE BASED ON POLY(4- (DIPHENYLAMINO)BENZYL ACRYLATE) AND OPTIMIZATION FOR REAL-
Experimental section
2.1 Synthesis of poly (4-(diphenylamino)benzyl acrylate) (PDAA)
Scheme 2 Synthetic route for poly (4-(diphenylamino)benzyl acrylate) (PDAA)
A synthetic route for poly(4-(diphenylamino)benzyl acrylate) is shown in Scheme
The synthesis of (4-(diphenylamino)phenyl)methanol (6) was achieved through the reduction of 4-(N,N-diphenylamino)benzaldehyde using sodium borohydride (NaBH4) Following this, compound (6) underwent esterification with acryloyl chloride, resulting in the formation of the monomer 4-(diphenylamino)benzyl acrylate (7) Subsequently, the polymer (PDAA) was synthesized via free radical polymerization, utilizing azobisisobutyronitrile (AIBN) as the initiator.
4-(N,N-diphenylamino)benzaldehyde (10 g, 36 mmol) was added to a solution of NaBH4
(1.4 g, 37 mmol) in dichloromethane (20 mL) and methanol (5 ml) The mixture was
59 stirring for 2 h at room temperature and then water (25 mL) was added and stirred continuously for 24 h The mixture was concentrated in vacuum and saturated aqueous
A 30 ml solution of NH4Cl was combined with a mixture that was subsequently extracted using dichloromethane The organic layer was dried over anhydrous MgSO4, and the solvent was completely evaporated using a rotary evaporator, yielding a crude solid This solid was purified through recrystallization from ethanol, resulting in 9.1 g of the crystalline product (4-(diphenylamino)phenyl)methanol (6) with a yield of 90% and a melting point of 92.8-94.0 °C The TOF/MS (ESI) analysis indicated a mass-to-charge ratio of m/z: 298.09 (M + Na +) The UV-visible absorption spectrum is illustrated in Figure 31, along with the 1H NMR data.
(300 MHz, CDCl3, δ): 7.27-6.94 (m, 14H, ArH), 4.55 (d, 2H, -CH2-OH), 1.84 (s, 1H, -
CH2-OH) The NMR spectrum of (4-(diphenylamino)phenyl)methanol (6) is also shown in Figure 32
Figure 31 UV-VIS absorption spectrum of solution of 4-(diphenylamino)phenyl methanol (TPAOH) (6) in CHCl3
Figure 32 300MHz 1 H NMR(CDCl3) of (4-(diphenylamino)phenyl)methanol (6)
A solution of 6 (3.0 g, 10.9 mmol), triethylamine (2 ml, 14.3 mmol) in dichloromethane (30 mL) was cooled to 0 C by an ice bath Acryloyl chloride (1 ml, 12.36 ppm (f1)
A solution containing 61 mmol was gradually added to a stirring mixture, which was maintained at 0 °C for 2 hours before being allowed to reach room temperature The reaction progress was monitored using thin layer chromatography (TLC), and upon completion, 100 ml of water was introduced to the mixture The resulting solution underwent extraction with dichloromethane, followed by drying the organic layer over anhydrous MgSO4 and removing the solvent via rotary evaporation The product was purified through column chromatography using an eluent of ethyl acetate and hexane in a 2:1 ratio, yielding 2.54 g of white crystals of (7) with a 70% yield The 1H NMR spectrum (300 MHz, CDCl3) displayed chemical shifts at δ 5.1 (2H, COO-CH2-Ar), 5.84 (1H, CHcisHtrans=CH-COO), 6.16 (1H, CH2=CH-COO), 6.45 (1H, CHcisHtrans=CH-COO), and 6.9-7.2 (14H, ArH), as illustrated in Figure 33 for 4-(diphenylamino)benzyl acrylate (7).
Figure 33 300MHz 1 H NMR(CDCl3) of 4-(diphenylamino)benzyl acrylate (7) ppm (t1)
2.1.3 Poly(4-(diphenylamino)benzyl acrylate) (PDAA)
A mixture of monomer (7) (1.0g, 3 mmol), AIBN (5 mg, 0.03 mmol), and THF (2 mL) was carefully degassed and heated to 60 °C for 24 hours The resulting polymer was isolated through precipitation in methanol, yielding 0.97 g of polymer, which corresponds to a yield of 95% The polymer exhibited a molecular weight (M_w) of 11,000, a molecular weight distribution (M_w/M_n) of 1.86, and a glass transition temperature (T_g) of 75 °C.
Figure 34 Chemical structures of PR components
In this study, the PDAA polymer served as the host photoconductive matrix, while 2-(4-(azepan-1-yl)benzylidene)malononitrile (7-DCST) acted as the NLO chromophore and phenyl-C61-butyric acid methyl ester (PCBM) functioned as the sensitizer To lower the glass transition temperature (Tg), benzyl n-butyl phthalate (BBP) was incorporated The composite, with a composition of PDAA/7-DCST/BBP/PCBM at 55/40/4/1 weight percent, was prepared by sandwiching it between two ITO-coated glass substrates at 130°C Differential Scanning Calorimetry (DSC) measurements revealed that the synthesized PDAA had a Tg of 75°C, which is beneficial for the composite Notably, despite maintaining a high polymer concentration of 55% by weight and only adding 4% by weight of BBP, the Tg of the composite was reduced to approximately 25°C, creating an optimal environment for chromophore orientation with a NLO chromophore concentration of 40%.
T g was obtained without additionally diluting the polymer concentration, which could lead to a lower photoconductivity
The article discusses various PR samples of different sizes, including a large device with an 8 cm diameter designed for hologram movies, a medium-sized device intended for hologram displays, and a test piece used to measure essential PR parameters such as diffraction efficiency, optical gain, and response time.
Using a low viscosity polymer matrix of PDAA, large photorefractive (PR) samples were successfully fabricated, as illustrated in Figure 35 The largest device, sample (a), has a diameter of 8 cm and is intended for hologram movies, while sample (b) is a medium-sized device designed for hologram displays Sample (c) serves as a test piece to measure fundamental PR responses, including diffraction efficiency, optical gain, and response time All samples were prepared through a simple gentle pressing method after melting the composite at 130°C, eliminating the need for special equipment or high pressure due to the composite's low viscosity Additionally, the optical clarity of the samples was maintained for several weeks.
Storing samples at 5°C in a refrigerator maintained their stability for months, while raising the temperature to 60°C led to chromophore recrystallization within days The use of PDAA did not provide long-term stabilization against recrystallization, in contrast to the monolithic polymer composite discussed in Chapter 3.
When selecting a wavelength for hologram applications, it is crucial to consider the writing beam wavelength, as the photorefractive (PR) performance and holographic properties are influenced by the absorption coefficient This study investigated the PR effect on a composite using four specific wavelengths: 532 nm, 561 nm, and 594 nm from a DPSS laser (Cobolt, 25 mW), along with 633 nm from a He-Ne laser (Melles-Griot, 10 mW) Holographic gratings were created on the sample using two intersecting laser beams at incidence angles of 40° and 55° in air.
Figure 36 Diffraction efficiency as a function of applied electric field with writing beam at 532, 561, 594 and 633 nm of the composite film PDAA/7-DCST/BBP/PCBM
The diffraction efficiency results from the DFWM measurement were analyzed as a function of the applied electric field, revealing high efficiencies across all laser wavelengths with moderate electric field strengths Specifically, under 532 nm illumination, a peak diffraction efficiency exceeding 80% was recorded at an electric field of 40 V μm -1 This sinusoidal relationship between diffraction efficiency and electric field can be elucidated through Kogelnik’s coupled-wave theory, where the diffraction efficiency (η) is defined by the refractive index modulation, as described in equation (19): η = sin(2K nΔ cos(θB - θA)).
The film thickness (d), laser beam wavelength (λ), internal angles of the beams (θ A and θ B), and refractive index modulation (Δn) are critical parameters in understanding the behavior of photorefractive (PR) polymer composites Δn, which combines orientational birefringence and electro-optic contributions, is significantly influenced by the electric field within the composite As the external electric field increases, the induced internal space-charge field between dark and bright areas enhances Δn The diffraction efficiency (η) exhibits a sine-squared relationship with Δn, peaking around Δn of approximately 2 x 10^-3 Notably, when shorter wavelength lasers are employed, this diffraction efficiency peak shifts to a lower applied electric field, indicating that the changes in diffraction efficiency are directly linked to variations in Δn The calculation of Δn can be performed using Kogelnik's coupled-wave theory, utilizing η values derived from the DFWM results.
Figure 37 Refractive index modulation as a function of applied electric field with writing beam at 532, 561, 594 and 633 nm of the composite film PDAA/7-DCST/BBP/PCBM
Figure 37 illustrates the relationship between refractive index modulation values and the applied electric field, revealing that the maximum Δn occurs at the shortest operating wavelength for each specific electric field This trend may be attributed to varying absorption levels at different wavelengths Additionally, the UV-visible absorption spectrum of the composite with a thickness of 100 µm is presented in Figure 38.
Figure 38 UV-Visible absorption spectrum of composite film PDAA/7-
The absorption spectrum exhibits a broad distribution in the visible region, with absorption coefficients increasing as the wavelength decreases The UV-Visible absorption spectra of triphenylamine and 7-DCST demonstrate minimal absorption at the investigated wavelengths of 532-633 nm Consequently, the observed growth in absorption of the PR composite may facilitate the formation of charge-transfer complexes, which are essential for charge generation This charge generation was corroborated by photocurrent measurements, as detailed in Table 4 To minimize the impact of strong absorption, the photocurrent sample was kept thin, with a thickness of 10 µm, allowing for the production of more charge carriers at shorter wavelengths As a result, under specific electric fields, these charge carriers could migrate to darker areas and become trapped, leading to the formation of a stronger space-charge field at lower electric fields and a greater modulation of the refractive index.
At each operating wavelength, increasing the applied electric field enhances the refractive index modulation However, it is important to note that higher electric fields also elevate the risk of electrical breakdown.
Table 4 Absorption coefficient, photocurrent, PR quantities at different operating wavelength
633 76 0.58 733/0.7 a Measured at 70 àm -1 b Measured at 45 V àm -1
For optimal hologram applications, achieving a large refractive index modulation without increasing the applied electric field is essential Instead of exploring alternative materials, enhancing diffraction efficiency at lower electric fields can be accomplished by adjusting the wavelength of the writing beams Although shorter wavelengths with strong absorption may lead to reduced external diffraction efficiency, they can still provide valuable insights into the material's internal diffraction efficiency for hologram displays However, significant absorption at shorter wavelengths raises concerns about transparency and beam attenuation, which could negatively impact the performance of photorefractive (PR) materials Therefore, careful consideration of the operating wavelength is crucial.
Figure 39 Diffraction efficiency growth as the function of time with writing beam at 532,
561, 594 nm of the composite film PDAA/7-DCST/BBP/PCBM (55/40/4/1) at 45 Vàm -1