Functionalized polymers with strong push-pull azo chromophores in side chain for optical application

Optical Materials 85 (2018) 391–398

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Functionalized polymers with strong push-pull azo chromophores in side chain for optical application

T

B. Derkowska-Zielinskaa,∗, L. Skowronskib, M. Sypniewskaa, D. Chomickia, V. Smokalc, O. Kharchenkoc, M. Napartyb, O. Krupkac a

Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100, Torun, Poland Institute of Mathematics and Physics, UTP University of Science and Technology, S. Kaliskiego 7, 85-796, Bydgoszcz, Poland c Taras Shevchenko National University of Kyiv, 64/13 Volodymyrska St., 01601, Kyiv, Ukraine b

A R T I C LE I N FO

A B S T R A C T

Keywords: Azo polymers Optical properties Refractive index Extinction coefficient Optical conductivity Thin films

In this paper, we show that the optical properties such as extinction coefficient, refractive index, optical conductivity and optical energy band gap of aminoazobenzene and pseudostilbene-type azobenzene polymers can be effectively manipulated by changing or adding various substituents in their polymeric system. We found that the absorption spectrum of aminoazobenzene polymer thin film is shifted to blue compared to pseudostilbene-type azobenzene polymer thin films. Therefore, the optical energy band gap for aminoazobenzene polymer thin film is higher than for pseudostilbene-type azobenzene polymers. We also noticed that the studied azo dyes polymer thin films exhibit high photoresponse proper for use in the optoelectronic applications.

1. Introduction

isomers have different physical and chemical properties, because their molecules have different interatomic distances [12–14]. It is well known that the trans azobenzene absorption spectrum is characterized by an intense, symmetrical π-π∗ ultraviolet transition band and lowintensity, low-energy n-π∗ pass band in the visible region. In contrast, in the absorption spectrum of cis azobenzene, there is a more intense n-π∗ electron transition band. The studied functionalized polymers can be attributed to the aminoazobenzene-type and pseudostilbene-type by Rau classification [15]. The pseudostilbene-type molecules are push-pull molecules, which contain an electron-acceptor (A) group and an electron-donor (D) unit. These groups are connected with each other by a π-conjugated organic backbone. Such structure (D-π-A) provides a good transport of charge between electron-donor and electron-acceptor groups D→A charge transfer (CT), which gives a high value of dipole moment [16]. Aromatic azo derivatives containing intramolecular D-π-A CT show excellent photo-physical properties since they have extensive π-systems delocalized between the acceptor and donor units across the azo linkage. With appropriate electron-donor/acceptor ring substitution, the π-electron delocalization of the extended aromatic structure can yield high optical nonlinearity [17–20]. Among the pseudostilbene-type molecule, a very commonly used is Disperse Red 1 (DR1). In this azo-compound, the n-π* and π-π* transitions overlap, and it has been indicated that the latter is the lowest-

Azo compounds are a very important class of chemical compounds receiving attention in scientific research. They are used in many practical applications such as coloring fiber, printing systems, liquid crystal displays, optical storage technology, and photoelectronic applications [1–4]. They have also attracted attention due to their interesting electronic features in connection with their application for molecular memory storage, nonlinear optical elements and organic photoconductors [5–8]. Azobenzenes are the π-conjugated compounds, where two or more phenyl rings are bridged by the azo (-N]N-) linkage. This π-conjugation gives rise to strong absorption at ultraviolet and visible (UV-VIS) wavelengths, as well as is greatly sensitive to substitutions influence [1,9,10]. Their most attractive properties are that the azobenzene units can exist in two isomeric forms: trans and cis. Their photo-physical properties can be modificated by proper irradiation. Under the influence of irradiation with the appropriate wavelength, the stable form of trans-azobenzene transforms into a less stable cis form [2]. Since the azo group has a free pair of electrons on both nitrogen atoms, it can be isomerized according to two possible mechanisms: to pass n-π∗ electrons with the inversion of the atom and to pass π-π∗ that follow the rotational mechanism [11]. It has been intensively investigated by different computational and experimental methods that the trans and cis

∗

Corresponding author. Institute of Physics, Nicholas Copernicus University, Grudziadzka 5/7, 87-100, Torun, Poland. E-mail address: beata@fizyka.umk.pl (B. Derkowska-Zielinska).

https://doi.org/10.1016/j.optmat.2018.09.008 Received 23 April 2018; Received in revised form 18 July 2018; Accepted 3 September 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

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cyanophenylazo)aniline are obtained by azo coupling reaction which includes initial diazotization of an aromatic amine at low temperature and then diazonium salt (weak electrophiles) reacts with an electron rich aromatic nucleophile having electron donor groups. The substitution reaction takes place at the para position to the electron donor group [27]. The initial azo chromophores N-ethyl-N-(2-hydroxyethyl)4-(4-nitrophenylazo)aniline, 4-[4-(phenylazo)-1-naphthylazo]phenol, 2-[4-(2-chloro-4-nitrophenylazo)-N-ethylphenylamino]ethanol were commercially available (Aldrich). The monomers of azobenzene chromophores were prepared as previously reported [27,28]. 4-((2-Methacryloyloxyethyl)ethylamino)-4-(4-nitrophenylazo)-azobenzene: Dark purple crystals yield 60%, mp 160 °C. 1H NMR (500 MHz, CDCl3): δ 8.40 (d, 2H, Ar-H), 8.12–7.92 (m, 8H, Ar-H), 6.85 (d, 2H, ArH), 6.12 (s, 1H, CH2), 5.61 (s, 1H, CH2), 4.38 (t, 2H, OCH2), 3.75 (t, 2H, NCH2), 3.55 (q, 2H, NCH2CH3), 1.97 (s, 3H, CH3), 1.28 (s, 3H, CH3). UV–vis (THF): λ = 340, 502 nm. 4-((2-Methacryloyloxy)-4-(phenylazo)-1-azonaphthalene: Red solid residue yield 75%, mp 90 °C. 1H NMR (500 MHz, DMSO‑d6): δ 9.01–8.98 (m, 2H, naphthalene), 8.14 (d, 2H,Ar-H), 8.05 (d, 2H, Ar-H), 7.92 (s, 2H, naphthalene), 7.85 (d, 2H, Ar-H), 7.78–7.81 (m, 2H, naphthalene), 7.63–7.58 (m, 3H, Ar-H), 6.35 (s, 1H, CH2), 5.9 (s, 1H, CH2), 2.07 (s, 3H, CH3). UV–vis (THF): λ = 325, 428 nm. 4-(N-ethyl-N-2-methacryloxyethylamino)-2-chloro-4-nitroazobenzene: Red solid residue yield 72%, mp 85 °C. 1H NMR (500 MHz, CDCl3): δ 8.31 (s, 1H, Ar-H), 8.07 (d, 1H, Ar-H), 7.87 (d, 2H, Ar-H), 7.70 (d, 2H, Ar-H), 6.76 (d, 2H, Ar-H), 6.03 (s, 1H, CH2), 5.52 (s, 1H, CH2), 4.34 (m, 2H, OCH2), 3.67 (m, 2H, NCH2), 3.49 (m, 2H, NCH2), 1.87 (s, 3H, CH3), 1.2 (m, 3H, CH3). UV–vis (CHCl3): λ = 475 nm. 4-[(2-Methacryloyloxyethyl)ethylamino]-4-cyanoazobenzene: Red solid residue, yield 87%. 1H NMR (500 MHz, CDCl3):δ 7.89, 7.87 (d, 4H, Ar), 7.75, 7.73 (d, 2H, Ar), 6.82, 6.8 (d, 2H, Ar), 6.1 (s, 1H,CH2), 5.6 (s, 1H, CH2), 4.36 (m, 2H, OCH2), 3.7 (m, 2H, NCH2), 3.55 (m, 2H, NCH2), 1.94 (s, 3H, CH3), 1.25 (m, 3H, NCH2CH3). 4-((2-Methacryloyloxyethyl)ethylamino)-4-nitroazobenzene: Dark red crystals, yield 80%, mp 83 °C. 1H NMR (500 MHz, CDCl3): δ 8.35 (d, 2H, Ar-H), 7.92 (t, 4H, Ar-H), 6.85 (d, 2H, Ar-H), 6.1 (s, 1H, CH2), 5.6 (s, 1H, CH2), 4.38 (m, 2H, OCH2), 3.75 (m, 2H, NCH2), 3.56 (m, 2H, NCH2), 1.94 (s, 3H, CH3), 1.24 (m, 3H, CH3). UV–vis (THF): λ = 475 nm. The copolymers B1-B5 were synthesized by free radical polymerization using azobenzene methacrylic monomers and MMA in 10% DMF solution with AIBN as radical initiator at 80 °C (argon atmosphere) as previously reported [27,28]. The glass-transition temperatures were measured by differential scanning calorimetry to be 140 °C, 110 °C, 125 °C, 120 °C, 128 °C for the copolymers B1, B2, B3, B4 and B5, respectively. The structures of the B1-B5 copolymers (see Fig. 1) were confirmed by 1H NMR spectroscopy and a reasonable accord was found between the observed n/m values in the polymers and the respective amounts of both monomers which were introduced 1:3.

excited singlet state with the strongest absorption because of its pushpull archetype. DR1 molecular motifs lead to a structureless absorption band in the trans isomer. The presence of the trans form of DR1 is characterized by the absorption band centred at about 490 nm. Excitation of a molecule to the first excited state by a photon of suitable energy leads to its subsequent decay to one of two possible ground states: the stable (trans) form or metastable (cis) form. The latter, at room temperature, relaxes spontaneously to the trans form with an environment-dependent rate. In the trans form, molecules are strongly anisotropic (rod-like), while in the cis form they are bent and their anisotropy is less pronounced [21]. In respect to photoreactive units molecular systems containing two or more azobenzene units are interesting for the fields of dyes, pigments and advanced materials due to their multiphotochromic nature. The main advantage that such compounds can exist in many different states (up to 2n, where n is the number of photochromic units). This kind of behavior may be useful for storage and information processing at molecular level. In such bis-azo compounds, the cooperation of the different photoisomerisable units can produce an overall amplification of the geometrical changes related to the trans-cis transformation, leading to new light-induced functions [22]. The compounds containing two azobenzene units have been studied by other research groups [23,24]. In most cases, the two azobenzenes were found to be basically noninteracting, giving rise to a behavior determined by the simple superimposition of the properties of the two isolated units [25,26]. In a recent paper, we had quantified both the second- and the third-order nonlinearity for push-pull side chain bis azobenzene polymers [27]. In this paper, the refractive index, the extinction coefficient, the optical conductivity and the optical energy band gap of selected bis-azo dye attached polymer thin films are presented in comparison with a mono-azo dye based polymers. The goal of this work was to present the influence of various substituents of azo dyes on their optical properties. These optical properties were measured using spectroscopic ellipsometry (SE) combined with transmittance measurements (T). Additionally, atomic force microscopy (AFM) was used to examine the surface topography of the studied thin films. It should be mention that the studied functionalized polymers are promising candidates for applications in optical data storage, information storage, surface relief gratings, photoswitching, optical elements and nonlinear optics [2,4,27]. 2. Experimental 2.1. Materials 2.1.1. Characterization methods 1 H NMR (500 MHz) spectra were measured on a Bruker Avance DRX-500 spectrometer (Bruker Corp., Karlsruhe, Germany). Chemical shifts are given in ppm from the internal standard tetramethylsilane (TMS). The glass transition temperatures (Tg) of all polymers are determined by Differential Scanning Calorimetry with Q20 Differential Scanning Calorimetry model (TA Instruments, New Castle, PA, USA) with a continuous N2 purge. The sample was initially stabilized and after the first scan was made at a heating rate of 10 °C/min up to 200 °C then cooled to 20 °C, a second scan was performed with the same parameters to obtain the values of Tg.

2.2. Thin film preparation BK7 glass slides were cleaned in a solution of 3:1 H2SO4:H2O2, then thoroughly rinsed in deionized water. Finally, the solutions of the polymers B1-B5 in 1,1,2 trichloroethane were spin coat on the substrates at 1000 rpm for 60 s. The same polymer concentration of 83 mM was used. Immediately after the deposition, the films were cured in an oven at 50 °C and for 120 min in order to eliminate any remaining solvent.

2.1.2. Materials synthesis Standard distillation procedures were performed for triethylamine and THF just prior to use. 2,2′-Azobis (isobutyronitrile) (AIBN) was recrystallized twice from absolute methanol. Methacrylic chloride was vacuum-distilled, immediately before use. Methylmethacrylate (MMA) was washed with aq NaOH to remove inhibitors, dried with CaCl2 under nitrogen at reduced pressure. The azobenzenes 4-(N-ethyl-N-(2-hydroxyethyl)amino)-4-(4-nitrophenylazo)-azobenzene and N-ethyl-N-(2-hydroxyethyl)-4-(4-

2.3. Measurements Transmission measurements (T) were performed using spectrophotometer Cary 5000 (Agilent). While, the absorption spectra 392

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Table 1 The values of roughness parameter (Ra, Rq) and thickness (L) of the B1-B5 azo polymer and PMMA thin films.

Ra [nm] Rq [nm] L [nm]

B1

B2

B3

B4

B5

PMMA

0.286 0.512 35 ± 1

0.179 0.230 559 ± 2

0.256 0.330 137 ± 1

0.210 0.284 294 ± 1

0.192 0.242 453 ± 1

0.28 0.37 854 ± 1

Fig. 1. Molecular structures of azobenzene-functionalized polymethacrylates aminoazobenzene (B2) and pseudostilbene-type (B1, B3-B5).

irradiated by the mercury lamp were measured using spectrophotometer CCD (Spectra Laser) and the halogen lamp. Ellipsometric azimuths (Ψ and Δ) were measured using the V-VASE device (J.A.Woollam Co., Inc.) for three angles of incidence (65°, 70° and 75°). All optical measurements were performed in the spectral range 275–1000 nm at room temperature. The optical constants and thickness of azo polymer thin films were determined using three medium optical model of the sample (glass/polymer layer/ambient). The optical constants of azo polymer films were parameterized using Gauss-shape and Sellmeier-type dispersion relations in the absorption and transparent regimes, respectively [9,10,29–31]. The model fits were performed taking into account both ellipsometric azimuths (Ψ and Δ) and transmittance spectra (T) [9,10,29]. Analytical formulas and parameters of particular absorption bands are presented in Appendix A. The AFM Innova device from Bruker was used to determine the surface topography of azo polymer thin films. The imaging tapping mode (with standard Si tips) was used during the measurements. The scan size was 2 μm × 2 μm. The roughness parameters (Ra - average roughness and Rq – root mean square roughness) were determined using the NanoScope Analysis software (ver. 1.40) [32].

Fig. 3. Transmission spectra of the B1-B5 azo polymer thin films and PMMA thin film. Inset: Changes in the absorption spectra of the B1-B5 azo polymer thin films: before – 0 s and after 900 s of irradiation using the mercury lamp (HBO 50 Narva, 50 W) at room temperature.

thin films. The roughness parameters (Ra and Rq) were obtained to characterize the surface quality of the studied azo polymers thin films. These parameters are shown in Table 1. From Fig. 2 and Table 1, one can notice that the surfaces of all studied azo polymers are very smooth. The values of Ra and Rq quantities are in the range from 0.18 nm to 0.29 nm and from 0.23 nm to 0.51 nm, respectively, similar to those obtained for PMMA layer (Ra = 0.28 nm and Rq = 0.37 nm). These relatively low values of roughness parameters justify the omission of the rough layer in the optical model of the sample. The values of the thickness of studied azo polymers thin films are vary from about 35 nm to almost 600 nm (see Table 1). Fig. 3 presents the transmission spectra of azo dyes containing sidechain polymer thin films and polymer thin film deposited on glass substrate using spin-coating method. We can see that PMMA is transparent from about 350 to 1000 nm [33]. Whereas, for all studied azo

3. Results and discussion Fig. 2 presents the AFM images of selected studied azo polymers

Fig. 2. The thin film surface topography of B2 and B5 azo polymers obtained from AFM measurements. 393

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dyes polymer thin films we can see one wide absorption band in the spectra range 350–600 nm. This band is assigned to π-π* electronic transitions due to the strong internal charge transfer (ICT) character of azo dyes, and occurring in aggregate and free azobenzene chromophores. Whereas, the absorption band established as n-π* is hidden. From Fig. 3, one can also notice that the maximum of the absorption peak for DR1 containing side chain copolymer (i.e. B5) appear at 466 nm. Whereas, adding the chlorine (Cl) as a substituent in orthoposition of the benzene ring of DR1 (i.e. B3 polymer) causes the bathochromic shift of the absorption peak. This effect is due to presence of additional Cl group. It should be mentioned that the Cl group in B3 is an electron-withdrawing substituent, and its presence raises the oxidation potential [29]. When we add additional phenylazo group into DR1 (B1 system) the absorption band is also red shifted of about 38 nm. Furthermore, changing the nitro group in the DR1 fragment of copolymer on cyano one (B4 polymer) results in the hypsochromic shift. The B2 compound belongs to the polymer with azo disperse dyes in side chain, which are based on diazobenzene structures i.e. dye with two azo groups. And its absorption band is blue shifted comparing to the B5 compound due to higher conjugation with additional π-molecular orbital, half bonding and half antibonding. Thus, one can observed that the absorption peak changes as follows: λmax,B1 > λmax,B3 > λmax,B5 > λmax,B4 > λmax,B2. The inset of Fig. 3 presents the optical absorption spectra of B1-B5 azo polymers thin films before and after irradiation using the mercury lamp at room temperature. We can notice one absorption region (300–600 nm). After irradiation of azo polymers thin films using the mercury lamp we can see decrease in absorbance assigned to the π-π* transition. The azobenzene moieties in the azo polymers undergo transto-cis photoisomerization, which is indicated by the decrease of the trans absorption band. In the same time thermal cis-trans isomerization of polymers with electron-rich (donor/push) and electron-deficient (acceptor/pull) moieties as known as push-pull polymers B1, B3, B4, B5 is much faster than azobenzene type of polymer B2 which have not strong electron-rich substituted amine NR2 fragment. In this connection the photoisomerization of pseudo-stilbene containing polymers B1, B3, B4, B5 has also impact of the rapid thermal cis-trans isomerization [2,15]. However, irradiation of B2 polymer by UV-light gives possibilities of the photo-Fries reaction besides of trans-cis photoisomerization process as compared with of push-pull polymers B1, B3, B4, B5 (see Fig. 3). All available evidence on the mechanism of the photo-Fries reaction suggests that the rearrangement is intramolecular. It has been demonstrated that the relative quantum yields for the formation of o-hydroxyphenone groups from solid poly (phenylacrylate) via the photo-Fries rearrangement reaction do not depend on the temperature and the wavelength of excitation [34]. Fig. 4 shows the examples of experimental ellipsometric azimuths (Ψ, Δ) with the best fits of result obtained from optical model for B3

Fig. 5. The extinction coefficient (k) of the B1-B5 azo polymer thin films as well as PMMA thin film.

sample. We can see that the model fit is well matched to the experimental data. These ellipsometric parameters (Ψ, Δ) express the amplitude ratio and phase difference between p- and s-polarizations, respectively. Therefore, the variation of light reflection with p- and s-polarizations is measured as the change in polarization state. In particular, the amplitude ratio Ψ is characterized by the refractive index (n), while Δ represents light absorption described by the extinction coefficient (k). In other hand, the two values (n, k) can be determined directly from two ellipsometry parameters (Ψ, Δ) obtained from a measurement by applying the Fresnel equations [30]. Therefore, ellipsometric quantities (Ψ, Δ) defined from the ratio of the amplitude reflection coefficients for p- and s-polarizations can be expressed using the following fundamental equation [30]:

ρ͠ = tan Ψ eiΔ =

r͠ p r͠ s

(1)

Fig. 5 presents the extinction coefficient (k) of azo dyes containing side-chain polymer thin films extracted from spectroscopic ellipsometry (SE) measurements, which is related to absorption coefficient (α). In the spectra range from 400 to 530 nm, the maximum value of k can be observed for particular layers. These absorption features are associated with π-π* transitions. Therefore, these results are in good agreement with the data obtained from transmission measurements (see Fig. 3). In the k spectra (275–350 nm) the additional absorption band is visible for all compounds (except B4). This band was not observed in the transmission spectra (see Fig. 3) due to the absorption caused by the substrate (glass). We can also see that extinction coefficients of all azo dyes polymer thin films are higher compared to PMMA thin film. Fig. 6 shows the refractive index (n) of studied azo polymers, a fundamental parameter used in the design of optoelectronic devices, which was also got from SE measurements. From 275 nm to 600 nm, the spectral behavior of n exhibits an anomalous dispersion, which is directly associated with shape of the k spectra by the Kramers-Kronig relation. Whereas, from 620 nm to 1000 nm, one can see that the n decreases with increases of the wavelength. It means that we have the normal dispersion in this spectral range. Additionally, one can notice that for the selected wavelengths (620–1000 nm), the refractive index changes as follows: nB3 > nB2 > nB5 > nB1 > nB4. It means that the lowest value of the refractive index possess the compound with cyano group (B4). Whereas, adding the chlorine (Cl) as a substituent in orthoposition of the benzene ring of DR1 (i.e. B3) results in an increase of the value of n. Additionally, we can see that refractive indexes of all azo dyes polymer thin films are higher compared to PMMA thin film in the

Fig. 4. Experimental Ψ and Δ azimuths for three angles of incidence (65°, 70° and 75°) and their model fits for sample B3. 394

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Cl (B3) or the phenylazo group (B1) into DR1. Whereas, by exchanging nitro group in DR1 (B4) or by creating the disazo structure (B2) we can increase the value of Eg. It should be note that the push–pull pseudostilbene-type polymers have π bond polarized sequence B1 > B3 > B5 > B4 and aminoazobenzene-type B2 hasn't noticeably π bond polarization. The optical response of the material is the most conveniently studied in terms of the optical conductivity (σ), which can be determined in units of frequency (s−1) from the relation [37]:

σ=

(3)

where c is the velocity of light, α is the absorption coefficient and n is the refractive index. Fig. 8 shows the optical conductivity (σ) as a function of photon energy for azo polymers thin films. The optical conductivity directly depends on the absorption coefficient and the refractive index of the material. From Fig. 8, one can see that the σ changes with changing photon energy. Moreover, the transmission dips associated with the presence of the π-π* transitions of azo polymers (Fig. 3) and the associated variations in the experimentally determined values of n (Fig. 6) cause enhancement of σ at the corresponding photon energies. We can also notice that the values of optical conductivity of all azo dyes polymer thin films are significantly higher compared to PMMA thin film. One can see that the magnitude of the optical conductivity for most studied thin films is high (1013 s−1-1014 s−1). This is due to the high absorbance of azo polymers thin film and may be due to electron excitation by photon energy. Therefore, these compounds present the high photo-response. And this suggests that studied compounds are suitable for optoelectronic applications [38].

Fig. 6. The index of refraction of the B1-B5 azo polymer thin films as well as PMMA thin film.

spectral range 500–1000 nm. One of the most important factors governing the optical properties is the optical energy band gap (Eg). From transmission data, the optical energy band gap (Eg) may be determined using the Tauc method [35] by plotting the function (α hν )1/ n versus hν and using the following equation:

(α hν ) = B (hν − Eg )n

αnc 4π

(2)

where α is the absorption coefficient (cm−1), hν is the photon energy (eV), B is the band tailing parameter, and n = 1/2 for a direct allowed transition, n = 3/2 for a direct forbidden transition, n = 2 for an indirect allowed transition and n = 3 for an indirect forbidden transition. This relationship assumes that the densities of the electron states in the valence and the conduction bands, near the band gap, have a parabolic distribution and, also, that the matrix elements for the interband transitions associated with the photon absorption are equal for all the transitions [36]. Fig. 7 shows Tauc plots of azo polymers, which were used to determine the optical energy band gap (Eg). By fitting a straight line to the data, the intercept at α = 0 determines the optical energy band gap (in eV) through Eq. (2). The Eg of azo polymers determined by the Tauc method are summarized in Table 2. One can observe that the value of optical energy band gap of studied azo polymers changes in the following way: Eg(B2) > Eg(B4) > Eg(B5) > Eg(B3) > Eg(B1). In this connection, the value of optical energy band gap can be reduced by adding

4. Conclusions The extinction coefficient, refractive index, optical conductivity and optical energy band gap of selected aminoazobenzene and pseudostilbene-type azobenzene polymer thin films were determined using spectroscopic ellipsometry (SE) combined with transmittance measurements in the spectral range 275–1000 nm at room temperature. We observed the blue-shift of absorption spectrum for aminoazobenzene polymer thin films compared to pseudostilbene-type azobenzene thin films. In the case of to pseudostilbene-type azobenzene thin films, the shift of absorption band depends on the attached or different substituents in the system. And for instance, adding the chlorine (Cl) as a substituent in the benzene ring of DR1 (B3) or changing the nitro on cyano group in DR1 (B4) result in the red-shift or blue-shift, respectively. And in consequence, the similar behavior is observed for the optical energy band gap of studied azo polymers: Eg(B2) > Eg(B4) > Eg(B5) > Eg(B3) > Eg(B1). In the case of the refractive index in the non-absorbing spectral range, the changes for the appropriate wavelength are the following: nB3 > nB5 > nB4 and nB2 > nB5 > nB1 for mono- and bis-azo dyes polymer thin films (with B5 polymer indicated as reference material), respectively. The produced azo dyes polymer thin films exhibit high photo-response, which is suitable for optoelectronic applications. Concluding, one can effectively manipulate the optical properties by changing or adding the different substituents in the azo polymeric system. Acknowledgements Financial support to instrumentation was obtained from European Regional Development Fund (Stage 2 of the Regional Centre of Innovativeness) and Polish Ministry of Science and Higher Education.

Fig. 7. The Tauc plot of the B1-B5 azo polymers thin films. 395

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Table 2 The values of the optical energy band gap (Eg) of the B1-B5 azo polymer thin films.

Eg [eV]

B1

B2

B3

B4

B5

1.99 ± 0.04

2.46 ± 0.06

2.13 ± 0.04

2.38 ± 0.02

2.25 ± 0.03

Fig. 8. The optical conductivity (σ) as a function of photon energy for the B1-B5 azo polymers thin films as well as PMMA thin film.

Appendix A The thickness and optical constants of the polymer films prepared were determined by means of spectroscopic ellipsometry and spectrophotometric (transmittance) techniques [30,31]. The analytical expressions used to describe the complex refractive index can be written in the following form:

∼2 N = (n + ik )2 = ε͠ = ε∞ + εP + ε͠ G (A1) ∼ ∼ where N is a complex refractive index, n is the real part of N , k is the extinction coefficient, ε͠ is the complex dielectric function and ε∞ is the highfrequency dielectric constant (was set as 1). The mathematical formulas of particular oscillators used to describe the complex dielectric functions of the films are given by Refs. [30,31]: A0 E02 − E 2

(A2)

ε͠ G (E ) = ε1G (E ) + iε2G (E )

(A3)

εP (E ) = and

In (Eq. (A2)) E0 is the pole oscillator position and A0 is its magnitude, and in (Eq. (A3)) the quantities ε1G and ε2G are real and imaginary parts of the Gaussian-type dispersion relation of the complex dielectric function, respectively and are defined as [30,31]:

ε1G (E ) =

∞

∫ ξξε22−G (Eξ )2 dξ

2 P Π

(A4)

0

and 2

2

⎧ ⎡ E + Ej ⎞ ⎤ ⎫ ⎡ E − Ej ⎞ ⎤ ε2G (E ) = Aj exp ⎢−⎛⎜ − exp ⎢−⎛⎜ ⎟ ⎟ ⎨ σj ⎠ ⎥ σj ⎠ ⎥ ⎬ ⎝ ⎦⎭ ⎦ ⎣ ⎝ ⎣ ⎩

(A5)

where Aj and Ej are the amplitude and energy of the j-th absorption band, respectively. The quantity σj is directly related to the broadening (Brj) of this oscillator [31]:

σj =

Br j (A6)

2 2 ln 2

Real part of the dielectric function in form presented in (Eq. (A4)) was calculated providing the Kramers–Kronig consistency of ε1G and ε2G. To perform the fit procedure the WVASE32 software (J.A. Woollam Co., Inc.) was used. The model quantities were varied to minimize the reduced mean squared error, χ2, defined as [31]:

χ2 =

1 NΨΔ − P

2

j

2

exp mod − Δjexp ⎞ ⎡ Ψ jmod − Ψ j ⎞ ⎛ Δj +⎜ ⎟ ⎟ σΨj σΔj ⎢ ⎠ ⎠ ⎝ ⎣⎝

∑ ⎢ ⎛⎜

⎤ 1 ⎥+ NT − P ⎥ ⎦

2

T jmod − T jexp ⎞ wT σTj ⎟ ⎠ ⎝

∑ ⎛⎜ j

396

(A7)

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where NΨΔ,T and P are the total number of data points and the number of fitted model parameters, respectively. The quantities σΨ, σΔ and σT are standard deviations of Ψ, Δ and T data, respectively. Whereas, wT is the weighting of T spectra. The quantities with superscripts and mod are experimental and model parameters, respectively. Table A1 summarizes the parameters of the particular oscillators used to describe the optical response of the analyzed polymer thin films, their thickness and value of χ2. Table A1 Thickness (L), parameters of Sellmeier-type/Pole (A0, E0) and Gaussian-type (Aj, Ej, Brj) oscillators used to describe the optical response of the analyzed polymer thin films and value of χ2. Parameter

L [nm] A0 [eV2] E0 [eV] A1 [-] E1 [eV] Br1 [eV] A2 [-] E2 [eV] Br2 [eV] A3 [-] E3 [eV] Br3 [eV] A4 [-] E4 [eV] Br4 [eV] A5 [-] E5 [eV] Br5 [eV] χ2 [-] ∗

Sample B1

B2

B3

B4

B5

35 ± 1 160.0 ± 0.4 11∗ 0.17 ± 0.04 2.57 ± 0.07 0.50 ± 0.08 0.126 ± 0.002 3.70 ± 0.02 1.65 ± 0.07 0.156 ± 0.07 2.32 ± 0.02 0.36 ± 0.03 – – – – – – 2.979

559 ± 2 51.8 ± 2.7 11∗ 0.19 ± 0.36 2.50 ± 0.10 0.36 ± 0.14 0.14 ± 0.10 2.86 ± 0.06 0.55 ± 0.29 0.003 ± 0.001 3.25 ± 0.33 0.30 ± 0.18 0.10 ± 0.02 3.81 ± 0.37 1.05 ± 1.13 – – – 1.86

137 ± 1 247.4 ± 1.1 11∗ 0.05 ± 0.02 2.27 ± 0.01 0.28 ± 0.02 0.09 ± 0.06 2.48 ± 0.02 0.40 ± 0.07 0.09 ± 0.04 2.70 ± 0.09 0.55 ± 0.07 0.051 ± 0.007 4.27 ± 0.09 1.43 ± 0.12 – – – 5.93

294 ± 1 96.6 ± 1.3 11∗ 0.10 ± 0.06 2.54 ± 0.03 0.30 ± 0.03 0.02 ± 0.01 2.81 ± 0.04 0.42 ± 0.16 0.004 ± 0.002 3.19 ± 0.12 0.36 ± 0.12 – – – – – – 4.124

453 ± 1 93.4 ± 4.4 11∗ 0.33 ± 0.25 2.33 ± 0.05 0.25 ± 0.04 0.02 ± 0.01 2.48 ± 0.07 0.24 ± 0.10 0.06 ± 0.03 2.63 ± 0.13 0.43 ± 0.22 0.08 ± 0.06 2.83 ± 0.15 0.60 ± 0.09 0.07 ± 0.01 4.48 ± 0.11 1.29 ± 0.11 5.84

Fixed parameter.

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