Triphenyl group containing molecular glasses of azobenzene for photonic applications

Optical Materials 53 (2016) 146–154

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Triphenyl group containing molecular glasses of azobenzene for photonic applications Elmars Zarins a,⇑, Andrejs Tokmakovs b, Valdis Kokars a, Andris Ozols c, Peteris Augustovs c, Martins Rutkis b a b c

Riga Technical University, Institute of Applied Chemistry, 3/7 Paul Walden Str., LV-1048 Riga, Latvia Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., LV-1063 Riga, Latvia Riga Technical University, Institute of Technical Physics, 3/7 Paul Walden Str., LV-1048 Riga, Latvia

a r t i c l e

i n f o

Article history: Received 5 November 2015 Received in revised form 8 January 2016 Accepted 22 January 2016

Keywords: Molecular glasses NLO Holographic gratings Azobenzene Amorphous dyes

a b s t r a c t D-p-A type organic molecules have attracted considerable attention of scientists due to their potential applications in nonlinear optics and holographic data storage as light, flexible and low-cost photonic materials. To provide a better understanding on the relation between the compound chemical structure and their physical properties necessary for the mentioned purposes, eight glassy triphenyl group containing derivatives of azobenzene with incorporated 5,5-dimethylcyclohex-2-enylidene or 4H-pyran-4ylidene structural fragments and dicyanomethylene, indene-1,3-dione and pyrimidine-2,4,6 (1H,3H,5H)-trione acceptor groups have been synthesized and investigated. Thermal stability of synthesized glasses is no lower than 250 °C and glass transition in higher than 70 °C which both further increases (up to 120 °C) by additional number of attached triphenyl-moieties and incorporated structural fragments. Almost all of the synthesized azodyes form good optical quality transparent amorphous films from volatile organic solvents with their light absorption in thin solid films in the range of 400–660 nm. Azocompounds with sterically small cyclohex-2-ene-1-ylidene fragment in their molecules proved to be most efficient materials for holographic data storage and nonlinear optics with diffraction efficiency up to 20.40%, self diffraction efficiency up to 12.94% and NLO coefficient d33 up to 125.7 pm/V. Azodyes with no additionally incorporated structural fragments and indene-1,3dione electron acceptor group were least efficient materials for these purposes, however may show potential as photoactive components in organic solar cells due to their remarkable light absorption properties in the solid state. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction In the last two decades the small organic molecules where the electron acceptor fragment is bonded through p-conjugated system with electron donating fragment have been studied due to their potential applications as a new generation optical materials for nonlinear optics (NLO) and holographic information data storage due to their flexibility, light weight and low-cost fabrication possibility [1–8]. Standard design for such materials with nonlinear optical activity requires the incorporation of chromophore fragments with high hyperpolarizability, thermal and chemical stabilities [1,2,8–15]. Organic materials with azobenzene fragment in their structures makes them possible to form holographic volume and diffraction ⇑ Corresponding author. E-mail address: [email protected] (E. Zarins). http://dx.doi.org/10.1016/j.optmat.2016.01.044 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.

gratings due to a reversible trans-cis-trans photoisomerization processes after exposure to laser radiation, which could be useful for information storage [10,12,16–21]. Various synthetic modification possibilities for organic compounds has provided numerous approaches regarding the placement of azobenzene fragment containing or NLO active chromophores, most commonly in polymer [1,2,8–21], dendrimeric [22,23] and host-guest systems [12,24]. For NLO applications inorganic crystals were widely used, however their production is expensive and complicated [1,2,8]. Organic polymers obtained through synthesis are light, flexible and simple to produce, but posses several other drawbacks. The repeated synthesis of the exactly the same polymer is a challenging task [1,2,12] as the obtained products always will have different physical properties. In case chromophore molecules are doped in a polymer matrix, phase separation problem may occur. This resulting in decrease of optical transparency in the polymer composite films.

E. Zarins et al. / Optical Materials 53 (2016) 146–154

Recently it was demonstrated [25–27] that by the incorporation of bulky triphenyl groups in the low molecular mass organic compounds enables their excellent solubility in non-polar organic solvents and enhances the ability to form good optical quality transparent glassy films by solution processing. Such organic compounds with nonlinear optical [25,26] and holographic information storage properties were obtained [27]. The mentioned approach and have a certain and well defined structure and good synthetic repeatability. The investigated triphenyl group containing molecular materials could be used either for NLO or optical information storage but never for both purposes at the same time. Therefore to provide with a better understanding on the relation between the compound chemical structure and their physical properties, eight glassy triphenyl group containing derivatives of azobenzene with incorporated different structural fragments and functional acceptor groups have been synthesized and investigated in this work for potential applications in NLO and holographic information storage. 2. Experimental All necessary reagents were purchased as commercial products from Acros Organics, Sigma-Aldrich and Alfa-Aesar. Organic solvents (pyridine, dichloromethane, chloroform, triethylamine and piperidine) were dried by refluxing with calcium hydride and distilled. 2.1. Syntheses of compounds Full synthesis and characterization and/or relevant references of all organic compounds is presented and available in Supplementary Information. 2.1.1. General method of synthesis of azoaldehydes 4a-b [21] To a suspension of 2.0 g (16.50 mmol) 4-amino-benzaldehyde (2) [28] in 34.0 mL distilled water a 4.0 mL of concentrated hydrochloric acid was slowly added in a period of 30 min while the solution was cooled down to 0. . .5 °C temperature. Afterwards a 1.20 g (16.70 mmol) NaNO2 dissolved in 4.0 mL distilled water were added over a period of another 30 min. The obtained solution was stirred at a 0. . .5 °C temperature over 40–60 min until the suspension fully dissolves. To the afforded clear solution a 16.30 mmol of respective N,N-disubstituted aniline was added and the reaction mixture was stirred further over a period of 24 h, then poured into 300 mL distilled water and neutralized with KHCO3 until pH  7. The formed solid were filtered, washed and dried. 2.1.2. General method of synthesis of azocompounds 6a-b [29] To a solution of indene-1,3-dione (5) (0.49 g, 3.37 mmol) and (E)-4-((4-(ethyl(2-hydroxyethyl)amino)phenyl)diazenyl)benzalde hyde (4a) (1.00 g, 3.37 mmol) in dry ethanol (30.0 mL) was added piperidine (0.43 mL, 4.37 mmol). The reaction mixture was refluxed for 8 h, then cooled overnight to the room temperature. The afforded solid was filtered, washed and dried. 2.1.3. General method of synthesis of azocompounds WE-1, WE-2, WE-3 and aldehydes 7a-b [29] To a solution of trityl-chloride or triphenylsilyl-chloride (1.50 mmol or 3.00 mmol) in dry pyridine (5.0 mL) was added triethylamine (1.50 mmol, 0.21 mL) and the respective hydroxyl group containing azocompound (1.00 mmol). Depending on the reactants the reaction mixture was either refluxed for 2 h or stirred for 24 h at a room temperature. The formed slurry was poured either in 200 mL ethanol or distilled water. The afforded solid were filtered, dried and then purified.

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2.1.3.1. (E)-2-(4-((4-(ethyl(2-(trityloxy)ethyl)amino)phenyl)diazenyl) benzylidene)-1H-indene-1,3(2H)-dione (WE-1). Yield: 73%; m.p. 175 °C; IR (KBr) t, cm 1: 3468, 2924, 1719, 1698, 1683, 1605, 1583, 1557, 1516, 1350, 1316, 1259, 1222, 1204, 1192, 1162, 1137. 1H-NMR (200 MHz; CDCl3) d, ppm.: 1.15 (3H, t, 3 J = 7.69 Hz), 3.33 (2H, t, 3J = 5.13 Hz), 3.43–3.58 (4H, m), 6.63 (2H, d, 3J = 9.0 Hz), 7.10–7.40 (15H, m), 7.70–8.00 (9H, m), 8.57 (2H, d, 3J = 8.5 Hz). Elemental analysis: calcd. for WE-1 (C45H37N3O3): C, 80.94; H, 5.58; N, 6.29, found C, 80.12; H, 5.58; N, 5.97. 2.1.3.2. (E)-2-(4-((4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl) benzylidene)-1H-indene-1,3(2H)-dione (WE-2). Yield: 90%; m.p. 239 °C; IR (KBr) t, cm 1: 3083, 3055, 2922, 2871, 2859, 1724, 1682, 1598, 1582, 1557, 1513. 1H-NMR (400 MHz; CDCl3) d, ppm.: 3.30 (4H, t, 3J = 5.46 Hz), 3.62 (4H, t, 3J = 5.5 Hz), 6.55 (2H, d, 3J = 9.4 Hz), 7.10–7.40 (30H, m), 7.75 (3H, m, 3J = 8.6 Hz), 7.97 (2H, m), 8.56 (2H, d, 3J = 8.6 Hz). Elemental analysis: calcd. for WE-2 (C64H51N3O4): C, 83.00; H, 5.55; N, 4.54, found C, 83.27; H, 5.57; N, 4.62. 2.1.3.3. (E)-2-(4-((4-(bis(2-(triphenylsilyloxy)ethyl)amino)phenyl)diazenyl)benzylidene)-1H-indene-1,3(2H)-dione (WE-3). Yield: 41%; m.p. 236 °C; IR (KBr) t, cm 1: 3065, 2929, 2860, 1724, 1683, 1599, 1581, 1557, 1514, 1429. 1H-NMR (400 MHz; CDCl3) d, ppm.: 3.47 (4H, t, 3J = 5.9 Hz), 3.86 (4H, t, 3J = 6.3 Hz), 6.32 (2H, d, 3 J = 9.4 Hz), 7.20–7.50 (30H, m), 7.62 (2H, d, 3J = 9.0 Hz), 7.76 (2H, m), 7.88 (3H, m, 3J = 8.2 Hz), 7.97 (2H, m), 8.56 (2H, d, 3 J = 8.6 Hz). Elemental analysis: calcd. for WE-3 (C62H51N3O4Si2): C, 77.71; H, 5.36; N, 4.39, found C, 76.70; H, 5.31; N, 4.43. 2.1.3.4. (E)-4-((4-(ethyl(2-(trityloxy)ethyl)amino)phenyl)diazenyl) benzaldehyde (7a). Yield: 76%; m.p. 171 °C; IR (KBr) t, cm 1: 3435, 3061, 3024, 2980, 2966, 2928, 2914, 2871, 2835, 2740, 1687, 1594, 1560, 1518, 1491, 1425, 1407. 1H-NMR (200 MHz; CDCl3) d, ppm.: 1.14 (3H, t, 3J = 5.5 Hz), 3.32 (2H, q, 3J = 6.3 Hz), 3.44–3.52 (4H, m), 6.62 (2H, d, 3J = 9.4 Hz), 7.10–7.40 (15H, m), 7.78 (2H, d, 3J = 9.4 Hz), 7.85–7.97 (4H, m), 10.00 (1H, s). 2.1.3.5. (E)-4-((4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl)benzaldehyde (7b). Yield: 52%; m.p. 225 °C; IR (KBr) t, cm 1: 3084, 3054, 3021, 2945, 2921, 2891, 2876, 2849, 2259, 1694, 1592, 1560, 1518, 1490. 1H-NMR (200 MHz; CDCl3) d, ppm.: 3.29 (4H, t, 3 J = 7.0 Hz), 3.6 (3H, t, 3J = 5.5 Hz), 6.5 (2H, d, 3J = 9.4 Hz), 7.10– 7.40 (30H, m), 7.73 (2H, d, 3J = 7.4 Hz), 7.85–7.97 (4H, m), 10.00 (1H, s). 2.1.4. General method of synthesis of azocompounds IWK-2M and IWK-2D [29,30] To a solution of 3,5,5-trimethylcyclohex-2-enone (8) (1.84 g or 2.0 mL, 13.30 mmol) and malononitrile (9) (0.87 g, 13.30 mmol) in 3.0 mL DMFA was added piperidine (0.20 mL) and catalytic amount of acetic acid (2 drops) and acetic anhydride (1 drop). The reaction mixture was stirred at room temperature for one hour and at a 70 °C for another hour. To the heated reaction mixture the respective trityl group containing azoaldehyde (7a or 7b) (2.50 mmol) and additional 4.0 mL DMFA were added. The obtained solution was stirred under reflux for one hour, then cooled to the room temperature and poured in 200 mL ethanol. The afforded solid were filtered, dried and then purified. 2.1.4.1. 2-(3-(4-((E)-(4-(ethyl(2-(trityloxy)ethyl)amino)phenyl)diazenyl)styryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile (IWK-2M). Yield: 21%; m.p. 188 °C; IR (KBr) t, cm 1: 3448, 3082, 3032, 2969, 2957, 2927, 2872, 2217, 1600, 1563, 1518, 1406. 1 H-NMR (300 MHz; CDCl3) d, ppm.: 1.01 (6H, s), 1.13 (3H, t,

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3

J = 7.4 Hz), 2.43 (2H, s), 2.55 (2H, s), 3.30 (2H, t, 3J = 6.2 Hz), 3.49 (4H, m), 6.61 (2H, d, 3J = 9.1 Hz), 6.81 (1H, s), 7.01 (2H, m), 7.10– 7.40 (17H, m), 7.54 (2H, d, 3J = 8.5 Hz), 7.77 (4H, m). Elemental analysis: calcd. for IWK-2M (C48H45N5O): C, 81.44; H, 6.41; N, 9.89, found C, C, 81.30; H, 6.47; N, 9.86. 2.1.4.2. 2-(3-(4-((E)-(4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl)styryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile (IWK-2D). Yield: 22%; m.p. 140 °C; IR (KBr) t, cm 1: 3449, 3061, 3034, 2958, 2925, 2871, 2218, 1597, 1560, 1512. 1H-NMR (400 MHz; CDCl3) d, ppm.: 1.04 (6H, s), 2.44 (2H, s), 2.56 (2H, s), 3.29 (4H, t, 3J = 5.9 Hz), 3.62 (4H, t, 3J = 5.9 Hz, 3J = 5.5 Hz), 6.55 (2H, d, 3J = 9.4 Hz), 6.82 (1H, s), 7.02 (2H, d, 3J = 8.6 Hz), 7.10– 7.40 (30H, m), 7.56 (2H, d, 3J = 8.6 Hz), 7.71 (2H, d, 3J = 9.4 Hz), 7.80 (2H, d, 3J = 8.6 Hz). Elemental analysis: calcd. for IWK-2D (C67H59N5O2): C, 83.29; H, 6.15; N, 7.25, found C, 83.54; H, 6.19; N, 7.30. 2.1.5. General method of synthesis of azocompounds ZWK-2TB, DWK2TB and JWK-2TB [31,32] A solution of 4H-pyrane derivative (11a-c) (0.75 mmol) and (E)4-((4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl)benzaldehyde (7b) (0.61 g, 0.75 mmol) in dry pyridine (7.0 mL) was refluxed for 8 h. After the reaction solution was cooled to the room temperature and poured in 100 mL ethanol. Obtained solids were filtered and purified. 2.1.5.1. 2-(2-(4-((E)-(4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl)styryl)-6-tert-butyl-4H-pyran-4-ylidene)-1H-indene-1,3(2H)dione (ZWK-2TB). Yield: 13%; m.p. 205 °C; IR (KBr) t, cm 1: 3521, 3081, 3026, 2961, 2924, 2868, 2854, 2784, 2735, 1655, 1640, 1600, 1532. 1H-NMR (300 MHz; DMSO-d6) d, ppm.: 1.43 (9H, s), 3.23 (4H, s), 3.76 (4H, s), 6.83 (2H, d, 3J = 8.5 Hz), 7.10–7.40 (32H, m), 7.50– 7.70 (6H, m), 7.84 (2H, d, 3J = 8.5 Hz), 8.00 (2H, d, 3J = 8.6 Hz), 8.37 (1H, s), 8.45 (1H, s). Elemental analysis: calcd. for ZWK-2TB (C74H63N3O5): C, 82.73; H, 5.91; N, 3.91; found C, 82.96; H, 6.03; N, 4.03. 2.1.5.2. 2-(2-(4-((E)-(4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl)styryl)-6-tert-butyl-4H-pyran-4-ylidene)malononitrile (DWK2TB). Yield: 15%; m.p. 220 °C; IR (KBr) t, cm 1: 3421, 3064, 3025, 2969, 2955, 2926, 2871, 2783, 2734, 2210, 2196, 1646, 1621, 1600, 1561, 1508. 1H-NMR (300 MHz; DMSO-d6) d, ppm.: 1.38 (9H, s), 3.21 (4H, s), 3.76 (4H, s), 6.50 (1H, s), 6.83 (2H, s, 3J = 9.2 Hz), 6.99 (1H, s), 7.10–7.40 (32H, m), 7.49 (1H, d, 3J = 16.3 Hz), 7.64 (1H, d, 3J = 16.3 Hz), 7.74 (2H, d, 3J = 9.6 Hz), 7.84 (2H, d, 3 J = 9.6 Hz), 7.92 (2H, d, 3J = 9.2 Hz). Elemental analysis: calcd. for DWK-2TB (C68H59N5O3): C, 82.15; H, 5.98; N, 7.04; found C, 82.38; H, 6.01; N, 6.92. 2.1.5.3. 5-(2-(4-((E)-(4-(bis(2-(trityloxy)ethyl)amino)phenyl)diazenyl)styryl)-6-tert-butyl-4H-pyran-4-ylidene)pyrimidine-2,4,6 (1H,3H,5H)-trione (JWK-2TB). Yield: 10%; m.p. 276 °C; IR (KBr) t, cm 1: 3456, 3187, 3115, 3078, 3020, 2971, 2925, 2851, 1712, 1671, 1621, 1599, 1514. 1H-NMR (300 MHz; DMSO-d6) d, ppm.: 1.40 (9H, s), 3.23 (4H, s), 3.76 (4H, s), 6.82 (2H, d, 3J = 8.2 Hz), 7.10–7.40 (31H, m), 7.75 (3H, m), 7.78 (2H, d, 3J = 9.0 Hz), 8.85 (1H, s), 8.90 (1H, s), 10.64 (2H, s). Elemental analysis: calcd. for JWK-2TB (C69H61N5O6): C, 78.46; H, 5.82; N, 6.63; found C, 78.39; H, 5.89; N, 6.59. 2.2. Sample preparation for optical investigation For optical absorption spectra investigations the solid state thin films were deposited on quartz glass by spin-coating technique. Before the deposition of the layers, the quartz glass substrates

were cleaned in chloroform. The compound saturated solutions of dry dichloromethane (1 mg/mL) were spin-coated on glass substrates for 40 s at 1000 rpm and 1000 rpm/s acceleration. For nonlinear optical property investigation, the films from their diluted solution of chloroform (1 mg/mL) were spin-coated with a Laurell WS-400B-NPP/LITE spin-coater on indium tin oxide covered glass substrates. For the diffraction grating investigation, the obtained compounds were spin-coated on glass substrates for 180 s at 250 rpm and 500 rpm/s acceleration from their saturated solutions of chloroform (10 mg/mL). 2.3. Measurement systems Differential scanning calorimetry (DSC) measurements were carried out by using Mettler Toledo DSC 1/200W equipment under nitrogen atmosphere [33]. Three thermo cycles are performed for the determination of glass transition temperature (Tg). The first scan was done within the temperature range from +25 °C to +250 °C at a heating rate of 10 °C/min. After the first heating scan samples of the glassy compounds were cooled to 25 °C at a rate of 50 °C/min and heated for a second time from +25 °C to +250 °C at a rate of 10 °C/min. The Tg values were obtained from the second heating scan. Thermogravimetric analysis (TGA) measurements were made by using Simultaneous Thermal Analyzer STA 6000 [33]. Thermal decomposition temperatures (Td) of glassy compounds are determined in the temperature range from +30 °C to +510 °C at a heating rate of 10 °C/min at the level of 5% weight loss. Optical images of the samples were obtained by high resolution optical microscope Nikon ECLIPSE L150. Absorption (UV–VIS) spectra of the thin films of the azobenzene derivatives were measured by an Ocean Optics DT-MINI-2-G spectrometer. It contains halogen and deuterium lamps as well as Ocean Optics UV–Vis optical fiber. The 1H-NMR spectra were obtained on a Bruker UXNMR/XWIN-NMR spectrometer (300 MHz) and Varian VRX-Unity spectrometers (200 MHz and 400 MHz). The element analysis of azobenzene derivatives were obtained with ‘‘Euro Vector 300” element analyzer. The NLO properties of the thin films of amorphous azocompounds were characterized by means of the Maker fringe technique [34] in corona poled glassy thin films. The poling procedure and the custom build corona triode setup was identical to one described previously [25]. In the Maker fringe setup, a Qswitched DPSS Nd:YVO4 laser NL640 by EKSPLA was used as an excitation source. Typically excitation pulse characteristics were – energy 16 lJ, width 10 ns, repetition rate 40 kHz. The detailed descriptions of the experimental procedures and setup can be found in our previous paper [26]. The NLO coefficients d33, d31 and d15 were obtained by a least square fit of the experimental Maker fringe curves to a theoretical approximation. Theoretical values of SH intensity were calculated according to Herman and Hayden approach [35], using previously measured refractive indices at 532 nm and 1024 nm, absorption values and a thickness of the films. Setup response function was calibrated after each thin film sample measurement using an x-cut quartz crystal as a reference (d11 = 0.3 pm/V) [36]. The extrapolation to the zero frequency d33(0) according to the two level model [37] was employed to benchmark different chromophores. The temperature corresponding to the half vanished NLO activity (TSHI50) was evaluated from NLO activity measurements with temperature scans at 10 °C/min. The holographic set-up used for inscribing diffraction gratings is described elsewhere [27,38]. During experiments it was mounted on STANDA holographic table with the vibration isolation system. Holographic gratings with the period of 2 lm were recorded by KLASTECH DENICAFC 532-300 diode pumped solid state laser (k = 532 nm) and Melles Griot 25LH928-230 He–Ne gas laser (k = 632.8 nm). Linear p-p recording beam light polarizations were used for experiments.

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3. Results and discussion 3.1. Synthesis and characterization 4-Amino-benzaldehyde (2) was used as the staring reactant, which was obtained in 64% yield from 4-nitro-toluene (1) [28] (see Fig. 1). Its diazotation and further azocoupling reactions with N,N-disubstituted anilines (3a-b) afforded azobenzene fragment containing aldehydes 4a-b in 79–83% yields. Strong electron acceptor fragment was incorporated in their Knoevenagel condensation reactions with indene-1,3-dione (5) resulting in formation of azocompounds 6a-b which were obtained in satisfactory yields (59–69%). Further tritylation or silylation reactions of indene-1,3dione acceptor group containing intermediates 6a-b afforded target compounds WE-1, WE-2 and WE-3 with bulky triphenylmethyl or triphenyl-silyl moieties in 41–90% yields. In order to obtain the glassy azobenzene group containing derivatives with cyclohex-2-ene-1-ylidene or 4H-pyran-4-ylidene fragments, at first it was necessary to synthesize triphenymethylgroup containing aldehydes 7a-b which were obtained in tritylation reactions from previously acquired azobenzenes 4a-b in 52–76% yields (see Fig. 2). Glassy cyclohex-2-ene-1-ylidene fragment containing derivatives of azobenzene IWK-2M and IWK-2D were obtained in ‘‘one pot synthesis” [29,30] from isophorene (8), malononitrile (9) and aldehydes 7a-b in 21–22% yields. Glassy 4H-pyran-4-ylidene fragment containing derivatives of azobenzene ZWK-2TB, DWK-2TB and JWK-2TB were obtained in 10–15% yields from 4H-pyranes 11a-c with incorporated electron acceptor groups [18] in their Knoevenagel condensation reactions with aldehyde 7b (see Fig. 3). Different synthetic strategy and complicated purification process of bulky triphenyl group containing azodyes with additionally

incorporated cyclohex-2-ene-1-ylidene and 4H-pyran-4-ylidene structural fragments (IWK-2M, IWK-2D, ZWK-2TB, DWK-2TB and JWK-2TB) could be the main reasons why they were obtained in lower yields (10–22%) than glasses WE-1, WE-2 and WE-3 (yields 41–90%) (see Table 1). 3.2. Thermal and glass-forming properties Molecular glasses WE-1, WE-2 and WE-3 are thermally stable (Td from 250 °C to 282 °C) and showed glass transition in the second DSC heating cycle (see Table 1). The highest thermal stability was observed for two triphenymethyl- (WE-2) or two triphenylsilyl (WE-3) group containing azodyes with almost identical structure, however the glass transition of WE-2 (Tg = 98 °C) was higher by 23 °C than of WE-3 (Tg = 75 °C). This could be explained by the larger diameter of Si atoms in triphenylsilyl- (Si-Ph) group of WE-3 in relation to C atoms in triphenymethyl- (C-Ph) groups of WE-2 which leads to more rotational conformations of phenyl groups and reduced glass transition of WE-3 similar as reported in [25]. In comparison to glassy azobenzenes WE-1, WE-2 and WE-3, the thermal stability of cyclohex-2-ene-1-ylidene fragment containing azodyes IWK-2M and IWK-2D (Td from 288 °C to 289 °C) (see Table 1) is not influenced by the number of incorporated triphenyl groups and showed higher glass transition (Tg from 90 °C to 105 °C). However, higher glass transition temperature was observed for IWK-2 (Tg = 105 °C) and WE-2 (Tg = 98 °C) with two incorporated triphenymethyl- groups then for glasses IWK-2M (Tg = 90 °C) and WE-1 (Tg = 70 °C) with only one triphenymethylgroup. 4H-Pyran-4-ylidene fragment containing azodyes ZWK-2TB, DWK-2TB, JWK-2TB showed slightly increased thermal stability (Td from 289 °C to 320 °C) and also increased glass

O H 3C

1

EtOH 64%

NO 2

1. NaNO2

H

NaOH, Na 2S*9H 2O, S

HCl/H2O 79-83%

NH 2

2

R1

2.

N

3a-b

OH

O

O N

O

5

R1 N

Piperidine O EtOH 59-69%

N OH

N

N H

OH

4a-b

6a-b Ph 3SiCl Et3N Py

Ph 3CCl Et3N Py 73-90%

R1

N

O

(a) R 1 = -Et (b) R1 = -CH2CH2OH

41% Ph

O O

N

R2 N

O Ph

N O

WE-1 : R = -Et WE-2 : R = -CH 2CH 2OC(Ph)3

O Si Ph

Ph

O

N N

N

Ph Ph O Si Ph

Ph

WE-3

Fig. 1. Synthesis of indene-1,3-dione group containing molecular glasses of azobenzene.

Ph

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R2

R1 O

N

N

Ph 3CCl , Et 3N

O

Py 52-76%

H

N H

OH

N

O

(7a) R 2 = -Et (7b) R 2 = -CH 2CH 2OC(Ph) 3

(4a) R = -Et (4b) R1 = -CH 2CH 2OH

N

O

H3 C

N

CH 3

Piperidine DMFA

N

Ph Ph

N

H3 C

CH 3

H 3C

9

8

Ph

N

1

H 3C

N

10

7a-b

Piperidine

DMFA 21-22%

N

Ph

N

O N H 3C

Ph Ph

N R2

N CH3

IWK-2M : R2 = -Et IWK-2D : R2 = -CH 2CH 2OC(Ph) 3 Fig. 2. Synthesis of cyclohex-2-ene-1-ylidene fragment containing molecular glasses azobenzene.

Ph O

A N

O

H 3C

N

O

CH 3

H

O

7b

CH 3

Piperidine

Ph

N

H3 C

Ph Ph

Py 10-15%

Ph Ph

11a-c

O

A

O

H 3C H3 C

;

A = 11a and ZWK-2TB

O CH3 N

ZWK-2TB, DWK-2TB, JWK-2TB

H

N

N O

N

Ph O Ph

Ph Ph

Ph Ph

O N

; N

11b and DWK-2TB

O N H

O

11c and JWK-2TB

Fig. 3. Synthesis of 4H-pyran-4-ylidene fragment containing molecular glasses of azobenzene.

transition (Tg from 115 °C to 120 °C) for DWK-2TB and ZWK-2TB, unfortunately no glass transition was observed for JWK-2TB which indicates on the low kinetical stability of it amorphous phase. Among the investigated glassy compounds, dicyanomethylene acceptor group containing azodye DWK-2TB was found to be most thermally stable (Td = 320 °C). Almost all of the glassy azodyes – WE-1, WE-2, WE-3, IWK-2M, IWK-2D, ZWK-2TB, DWK-2TB, except JWK-2TB form good optical

quality transparent amorphous films from volatile organic solvents (see Fig. 4) by ‘‘spin-coating” approach. The crystalline solid state of the JWK-2TB film indicates on a strong aggregate formation between molecules which is even more enhanced by the hydrogen bond formation of the N–H groups in pyrimidine-2,4,6(1H,3H,5H)-trione electron acceptor. Due to the poor film formation of JWK-2TB compounds was not used in further physical property studies.

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E. Zarins et al. / Optical Materials 53 (2016) 146–154 Table 1 Yields and thermal properties of synthesized glassy azocompounds. Compound

Yield (%)

Tg (°C) 1. Heating

Tg (°C) 2. Heating

m.p. (°C)

Td (°C)

WE-1 WE-2 WE-3 IWK-2M IWK-2D ZWK-2TB DWK-2TB JWK-2TB

73 90 41 21 22 13 15 10

– – – – 108 – – –

70 98 75 90 105 120 115 –

175 239 236 188 140 205 220 276

250 280 282 289 288 289 320 291

Tg – glass transition temperature, m.p. – melting point, Td – thermal decomposition temperature at the level of 5% weight loss.

3.3. Optical absorption properties The light absorption of investigated compounds in the solid state is in range from 400 nm to 660 nm (see Fig. 5) in the visible part of the spectra. The absorption of 4H-pyran-4-ylidene fragment containing azodyes is blue-shifted by 10–20 nm comparing with glasses WE-1, WE-2, WE-3, IWK-2M and IWK-2D which could

be attributed to the additional small conjugation caused by the oxygen and the double bond in the molecules of ZWK-2TB and DWK-2TB. Azodyes with 4H-pyran-4-ylidene (ZWK-2TB, DWK2TB) and cyclohex-2-ene-1-ylidene (IWK-2M, IWK-2D) fragments have extended conjugation and therefore in their solid state may have more possible rotational conformations as well as cis- or trans- configurations comparing to glasses WE-1, WE-2 and WE-3. Naturally the absorption of the dye will be a mixture of all configurations and conformations in the solid state and therefore several maxima can be observed in the absorption spectra of IWK-2M, IWK-2D, ZWK-2TB and DWK-2TB films. Nevertheless, the absorption maxima of almost all of the investigated compounds of the pure films is close to 532 nm therefore a laser with such wavelength could be used for inscribing holographic gratings. Investigated azodyes still had some absorption at 632.8 nm therefore a laser with the mentioned wavelength could be used for the readout of the obtained diffraction gratings for determination of holographic data storage properties. Among the investigated compounds – thin films of indene,1-3-dione electron acceptor group containing glasses WE-1, WE-2, WE-3 show relatively high light absorption in their solid state and could be further studied as potential photoactive components in organic solar cells.

IWK-2D

IWK-2M

ZWK-2TB

WE-3

WE-2

WE-1

DWK-2TB

JWK-2TB

Fig. 4. Optical microscope images (magnification 1000) of the pure films of the glassy azocompounds. Dots on the pure film surface represent compound crystalline state while the remaining smooth area shows amorphous solid state similar as shown in [25].

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E. Zarins et al. / Optical Materials 53 (2016) 146–154

Absorption coefficient * 10-4 (cm -1)

7

532 nm laser

6 5

Table 2 Nonlinear optical property investigation results.

632,8 nm laser

4 3

WE-2

WE-1

2

450

500

550

600

650

Absorption coefficient * 10-4 (cm -1)

7

532 nm laser

6

632,8 nm laser

5 4

IWK-2D IWK-2M

2 1 400

450

500

550

600

650

700

Wavelength (nm)

Absorption coefficient * 10-4 (cm -1)

8

532 nm laser

7

632,8 nm laser

DWK-2TB

6

d33 (532) (pm/V)

d31 (532) (pm/V)

d14 (532) (pm/V)

d33 (0) (pm/V)

IWK-2M IWK-2D ZWK-2TB

74 84 100

78 90 108

62.8 125.7 66.2

40.6 75.4 28.7

35.3 48.4 19.9

2.8 6.1 7.3

4 3 2 1 400

450

higher in relation to IWK-2M and IWK-2D by 26 °C and 16 °C. The increased NLO efficiency values and higher thermal characteristics of IWK-2D in comparison to IWK-2M could be attributed to the incorporation of two bulky triphenylmethyl groups in the azodye IWK-2D which enhances possibility of reorientation of the molecules during corona poling in their pure films. Obtained results of measurements clearly show that the possibility to acquire low-molecular materials as a NLO active media strongly depend on the presence of certain structural fragments, which also serve as a part of the p-conjugated system. The influence on the incorporated electron acceptors is also important. Successfully combining different promising fragments of the molecules (donors, acceptors, structural fragments, p-systems, bulky amorphous phase promoting groups, etc.) through organic synthesis presents huge perspective to obtain new materials with even more improved NLO properties. 3.5. Holographic data storage properties

ZWK-2TB

5

0

TOR (°C)

700

Wavelength (nm)

0

TSHI50 (°C)

TSHI50 – by reaching this temperature value the material looses half of it second harmonic generation efficiency. TOR – sample orientation temperature. d33, d31, d14 – measured nonlinear optical coefficients. d33(0) – value obtained by extrapolation to zero frequency according to two level model [37].

1 0 400

3

Compound

WE-3

500 550 Wavelength (nm)

600

650

700

Fig. 5. Material absorption properties in thin solid films and their holographic investigation possibilities with 532 nm and 632.8 nm lasers.

3.4. Nonlinear optical properties In advance of the NLO property investigation, thin film samples of azodyes WE-1, WE-2, WE-3, IWK-2M, IWK-2D, DWK-2TB and ZWK-2TB were spin-coated on ITO and subjected to a thermoassisted electrical field poling procedure (corona poling) which is described in detail elsewhere [7,25,26]. However, by heating the samples as well as by applying an additional electrical field a rapid crystallization was observed for azodyes WE-1, WE-2, WE-3 with indene-1,3-dione acceptor part and for 4H-pyran-4-ylidene fragment containing dye DWK-2TB with dicyanomethylene acceptor. Nevertheless, excellent pure films of cyclohex-2-ene-1-ylidene fragment containing azodyes IWK-2M, IWK-2D and 4H-pyran-4ylidene fragment containing glass ZWK-2TB bearing indene-1,3dione as electron acceptor were obtained after the procedure. The measured NLO parameters [7,25,26] are summarized in Table 2. Cyclohex-2-ene-1-ylidene fragment containing dyes IWK-2M, IWK-2D showed higher nonlinear optical coefficients d33, d31, d14 then azodye ZWK-2TB with 4H-pyran-4-ylidene fragment, but temperature of 50% NLO intensity decay TSHI50 for ZWK-2TB is

To investigate the ability for synthesized glassy compounds to form holographic volume and diffraction gratings due to the trans-cis-trans photoisomerization of azobenzene fragment after exposure to laser radiation [1,2,12,27,38], their thin films samples were spin-coated on the glass substrate. The thickness of each film was found by using the light transmission and absorption at 632.8 nm. Obtained holographic data storage parameters [27,38] – self diffraction efficiency (SDEt, %, measured with 532 nm laser), diffraction efficiency (DEt, %, measured with 632.8 nm laser), specific writing energy (Wmax, J) and recording efficiency factor (REFmax, (%cm)2/kJ) were determined in the transmitted light and are shown in Tables 3 and 4. From azodyes WE-1, WE-2, WE-3 bearing indene-1,3-dione as an electron acceptor group, the highest self-diffraction (SDEt = 1.23%) and diffraction efficiencies (DEt = 4.70%) were observed for WE-1 which could be explained by the larger amount of chromophore molecules in the same volume comparing to the azodyes WE-2 and WE-3 as WE-1 contain only one triphenymethyl group. No significant change in diffraction properties was observed

Table 3 Holographic grating recording investigation results with 532 nm laser. Sample

Thickness of the film (lm)

t (min)

WE-1 WE-2

0.5 3.9

12 9

1.23 0.03

WE-3 IWK-2M IWK-2D ZWK-2TB DWK-2TB

0.2 0.9 0.7 0.2 0.2

6 8 24 10 9

0.24 3.77 12.94 1.53 1.07

SDEt (%)

Wmax (J/(%cm)2)

REFmax (%cm)2/kJ)

196.93 7680.12

0.0062 Less than 0.0001 0.0005 0.0882 0.3475 0.0117 0.0062

512.01 42.67 37.24 131.29 0.0020

t – time of the irradiation. SDEt – self-diffraction efficiency. Wmax – specific writing energy. REFmax = SDEt/Wmax – recording efficiency factor.

E. Zarins et al. / Optical Materials 53 (2016) 146–154 Table 4 Holographic grating recording investigation results with 632.8 nm laser. Sample

Thickness of the film (lm)

t (min)

DEt (%)

Wmax (J/(%cm)2)

REFmax (%cm)2/kJ)

WE-1 WE-2 WE-3 IWK-2M IWK-2D ZWK-2TB DWK-2TB

0.5 3.9 0.2 0.9 0.7 0.2 0.2

12 12 8 9 24 12 10

4.70 1.40 1.30 9.00 20.40 0.40 1.00

4.51 15.13 10.86 1.77 2.08 52.94 17.65

1.4300 0.0930 0.0120 5.1000 9.8300 0.0076 0.0567

t – time of the irradiation. DEt – diffraction efficiency. Wmax – specific writing energy. REFmax = DEt/Wmax – recording efficiency factor.

for azodyes WE-2 and WE-3 by substituting bulky triphenylmethyl groups (WE-2) with bulky triphenylsilyl groups (WE-3). The holographic data storage parameters for glasses WE-1, WE-2, WE-3 overall were not high which could be explained by difficult transcis-trans photoisomerisation process caused by the sterical interaction of indene-1,3-dione and triphenyl groups. Among the glasses IWK-2M and IWK-2D with incorporated cyclohex-2-ene-1ylidene fragment, IWK-2D showed highest self-diffraction (SDEt = 12.94%) and diffraction efficiencies (DEt = 20.40%), but the time required for the holographic recording is 2–3 times longer than for IWK-2M. Increased holographic data storage parameters (SDEt and DEt) indicate that by substituting indene-1, 3-dione group with sterically smaller dicyanomethylene and cyclohex-2-ene-1-ylidene fragments enhances the trans-cis-trans photoisomerisation process of the molecules. By analyzing the photorefractive property investigation results of glassy 4H-pyran4-ylidene fragment containing azodyes ZWK-2TB and DWK-2TB (SDEt = 1.53% and DEt = 1.00%), no significant improvement of DEt and SDEt values have been noted comparing to compounds WE-1, WE-2, WE-3 which points to the difficult trans-cis-trans photoisomerization of azodyes ZWK-2TB and DWK-2TB. Higher diffraction and self-diffraction efficiencies could be achieved by applying laser beams with different polarization as well as obtaining several samples with varying thicknesses of the same films. The obtained measurement results clearly indicate that the small sterical dimensions of the structural fragments as well as for the electron acceptors are the most influential factors for the successful formation of holographic diffraction gratings in low molecular mass azobenzene derivatives and make them suitable for optical and holographic information storage. The mentioned observation show potential for successful applications in organic synthesis of new materials with even higher diffraction and self-diffraction efficiencies.

4. Conclusions Various physical properties and their relation with chemical structure of eight glassy dicyanomethylene, indene-1,3-dione and pyrimidine-2,4,6(1H,3H,5H)-trione acceptor groups containing derivatives of azobenzene with incorporated 5,5dimethylcyclohex-2-enylidene or 4H-pyran-4-ylidene structural fragments and triphenyl moieties, have been synthesized and investigated in this report. Synthesized dyes generally form good quality transparent amorphous films by applying wet-casting methods, however this ability can be disabled by introducing a hydrogen-bond containing pyrimidine-2,4,6(1H,3H,5H)-trione electron acceptor group in the molecules. The increase of thermal stability of the obtained organic glasses is mostly influenced by the number of attached triphenyl-moieties and slightly influenced by incorporation of additional structural fragments. Increased glass transition of azodyes was equally influenced by the number of

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additional triphenyl-moieties, insertion of structural fragments as well as substituting bulky triphenyl-silyl groups with triphenylmethyl groups. Azodyes IWK-2M and IWK-2D with incorporated sterically small cyclohex-2-ene-1-ylidene fragment in their molecules which can be successfully obtained in ‘‘one pot” synthesis not only showed the highest holographic data storage parameters (SDEt = 12.94% and DEt = 20.40% for IWK-2D) but also were found to be most perspective for NLO applications (d33 up to 125.7 pm/V for IWK-2D). Although azodye ZWK-2TB bearing 4H-pyran-4ylidene fragment and indene-1,3-dione group showed nonlinear activity, it holographic data storage parameters were not high due to sterical interaction of it functional groups. Despite not being useful for nonlinear optical applications, indene-1,3-dione acceptor group containing azodyes (WE-1, WE-2, WE-3) with no additional structural fragment show remarkable light absorption properties in their solid state which may make them useful other fields of photonics, for example, as photoactive components for organic solar cells. Successfully combining perspective structural fragments and effective functional groups of the molecules with different sterical dimensions presents good perspective for obtaining new generation optical materials suitable for NLO applications and holographic data storage. Acknowledgements This work has been supported by the Latvian State Research Program on Multifunctional Materials IMIS2. Authors are grateful to Kristine Lazdovica for thermal and IR property measurements, Zane Kalnina for assistance on nonlinear property measurements and to Dr. chem., Kaspars Traskovskis for useful discussion. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.optmat.2016.01. 044. References [1] O. Ostroverkhova, W.E. Moerner, Organic photorefractives: mechanisms, materials, and applications, Chem. Rev. 104 (2004) 3267–3314. [2] K.G. Yager, C.J. Barret, Light-induced nanostructure formation using azobenzene polymers, Polym. Nanostruct. Appl. (2006) 1–38. [3] J.C. Min, H.L. Jeong, S.H. Chang, H.K. Jae, S.L. Ho, H.C. Dong, A tricyanopyrrolinebased nonlinear optical chromophore bearing a lateral moiety: a novel steric technique for enhancing the electro-optic effect, Dyes Pigm. 79 (2008) 193– 199. [4] R. Andreu, S. Franco, E. Galįn, J. Garin, N.M. De Baroja, C. Momblona, J. Orduna, R. Alicante, B. Villacampa, Isophorone- and pyran-containing NLOchromophores: a comparative study, Tetrahedron Lett. 51 (2010) 3662–3665. [5] G. Koeckelberghs, L. De Groof, J.P. Moreno, I. Asselberghs, K. Clays, T. Verbiest, C. Samyn, Synthesis and nonlinear optical properties of linear and K-shaped pyranone-based chromophores, Tetrahedron 64 (2008) 3772–3781. [6] J. Hua, J. Luo, J. Qin, Y. Shen, Y. Zhang, Z. Lu, New nonlinear optical chromophores exibiting good transparency and large nonlinearity: synthesis and characterization of chromophores with stilbene ring-locked triene as a combined conjugation bridge, J. Mater. Chem. 12 (2002) 863–869. [7] E. Jecs, J. Kreicberga, V. Kampars, A. Jurgis, M. Rutkis, Novel azobenzene precursors for NLO active polyuretanes: synthesis, quantum chemical and experimental characterization, Opt. Mater. 31 (2009) 1600–1607. [8] J. Xu, S. Semin, T. Rasing, A.E. Rowan, Organized chromophoric assemblies for nonlinear optical materials: towards (Sub) wavelength scale architectures, Small 11 (9–10) (2015) 1113–1129. [9] A.-L. Roy, C. Bui, I. Rau, F. Kajzar, B. Charleux, M. Save, D. Kreher, A.-J. Attias, Well-defined second-order nonlinear optical polymers by controlled radical polymerization, via multifunctional macromolecular chain transfer agent: design, synthesis, and characterizations, Polymer 55 (2014) 782–787. [10] M.C. Spiridon, K. Iliopoulos, F.A. Jerca, V.V. Jerca, D.M. Vuluga, D.S. Vasilescu, D. Gindre, B. Sahraoui, Novel pendant azobenzene/polymer systems for second harmonic generation and optical data storage, Dyes Pigm. 114 (2015) 24–32. [11] M. Lanzi, L. Paganin, New photosetting NLO-active polythiophenes with enhanced optical stability, Eur. Polym. J. 45 (2009) 1118–1126.

154

E. Zarins et al. / Optical Materials 53 (2016) 146–154

[12] S.K. Yesodhaa, C.K.S. Pillaia, N. Tsutsumi, Stable polymeric materials for nonlinear optics: a review based on azobenzene systems, Prog. Polym. Sci. 29 (2004) 45–74. [13] F.A. Jerca, V.V. Jerca, F. Kajzar, A.M. Manea, I. Rau, D.M. Vuluga, Simultaneous two and three photon resonant enhancement of third-order NLO susceptibility in an azo-dye functionalized polymer film, Phys. Chem. Chem. Phys. 15 (2013) 7060–7063. [14] H.Y. Woo, H.-K. Shim, K.-S. Lee, Synthesis and optical properties of polyurethanes containing a highly NLO active chromophore, Macromol. Chem. Phys. 199 (1998) 1427–1433. [15] A. Carella, F. Borbone, U. Caruso, R. Centore, A. Roviello, A. Barsella, A. Quatela, NLO behavior of polymers containing Y-shaped chromophores, Macromol. Chem. Phys. 208 (2007) 1900–1907. [16] M.C. Spiridon, F.A. Jerca, V.V. Jerca, D.S. Vasilescu, D.M. Vuluga, 2-Oxazoline based photo-responsive azo-polymers. Synthesis, characterization and isomerization kinetics, Eur. Polym. J. 49 (2013) 452–463. [17] V.V. Jerca, F.A. Jerca, I. Rau, A.M. Manea, D.M. Vuluga, F. Kajzar, Advances in understanding the photoresponsive behavior of azobenzenes substituted with strong electron withdrawing groups, Opt. Mater. 48 (2015) 160–164. [18] F.A. Jerca, V.V. Jerca, D.F. Anghel, G. Stinga, G. Marton, D.S. Vasilescu, D.M. Vuluga, Novel aspects regarding the photochemistry of azo-derivatives substituted with strong acceptor groups, J. Phys. Chem. C 119 (19) (2015) 10538–10549. [19] O.N.O. Jr, J. Kumar, L. Li, S.K. Triphaty, Surface-relief grantings on azobenzene films, Photorefract. Org. Thin Films 14 (2002) 429–486. [20] E. Schab-Balcerzak, H. Flakus, A. Jarczyk-Jedryka, J. Konieczkowska, M. Siwy, K. Bijak, A. Sobolewska, J. Stumpe, Photochromic supramolecular azopolyimides based on hydrogen bonds, Opt. Mater. 47 (2015) 501–511. [21] H. Nakano, T. Takahashi, T. Tanino, Y. Shirota, Synthesis and photoinduced surface relief grating formation of novel photo-responsive amorphous molecular materials, 4-[bis(9,9-dimethylfluoren-2-yl)amino]-4’cyanoazobenzene and 4-[bis(9,9 dimethylfluoren-2-yl)-amino]-4’nitroazobenzene, Dyes Pigm. 84 (2010) 102–107. [22] W. Zhang, J. Xie, W. Shi, X. Deng, Z. Cao, Q. Shen, Second-harmonic properties of dendritic polymers skeleton-constructed with azobenzene moiety used for nonlinear optical materials, Eur. Polym. J. 44 (2008) 872–880. [23] J. Ortíz-Palacios, E. Rodríguez-Alba, G. Zaragoza-Galan, J.R. Leon-Carmona, A. Martínez, E. Rivera, Incorporation of novel azobenzene dyes bearing oligo (ethylene glycol) spacers into first generation dendrimers, Dyes Pigm. 116 (2015) 1–12. [24] A. Vembris, M. Rutkis, V. Zauls, E. Laizane, Stability of the functional NLO polymers–optically induced depoling of the DMABI molecules in sPMMA matrix, Thin Solid Films 516 (2008) 8937–8943. [25] K. Traskovskis, I. Mihailovs, A. Tokmakovs, A. Jurgis, V. Kokars, M. Rutkis, Triphenyl moieties as building blocks for obtaining molecular glasses with nonlinear optical activity, J. Mater. Chem. 22 (2012) 11268–11276.

[26] K. Traskovskis, E. Zarins, L. Laipniece, A. Tokmakovs, V. Kokars, M. Rutkis, Structure-dependent tuning of electro-optic and thermoplastic properties in triphenyl groups containing molecular glasses, Mater. Chem. Phys. 155 (2015) 232–240. [27] A. Ozols, V. Kokars, P. Augustovs, K. Traskovskis, A. Maleckis, G. Mezinskis, A. Pludons, D. Saharov, Green and red laser holographic recording in different glassy azocompounds, Opt. Mater. 32 (2010) 811–817. [28] E. Campaigne, W.M. Budde, G.F. Schaefer, p-Aminobenzaldehyde, Org. Synth. 31 (1951) 6, http://dx.doi.org/10.15227/orgsyn.031.0006. [29] E. Zarins, V. Kokars, A. Ozols, P. Augustovs, Synthesis and properties of 1,3dioxo-1H-inden-2(3H)-ylidene fragment and (3-(dicyanomethylene)-5,5dimethylcyclohex-1-enyl)vinyl fragment containing derivatives of azobenzene for holographic recording materials, Proc. SPIE 8074 (2011). 80740E-1–80740E-6. [30] S. Wang, S.-H. Kim, Photophysical and electrochemical properties of D-p-A type solvatofluorchromic isophorone dye for pH molecular switch, Curr. Appl. Phys. 9 (2009) 783–787. [31] E. Zarins, A. Vembris, E. Misina, M. Narels, R. Grzibovskis, Valdis Kokars, Solution processable 2-(trityloxy)ethyl and tert-butyl group containing amorphous molecular glasses of pyranylidene derivatives with lightemitting and amplified spontaneous emission properties, Opt. Mater. 49 (2015) 129–137. [32] Y.S. Yao, J. Xiao, X.S. Wang, Z.B. Deng, B.W. Zhang, Starburst DCM-type redlight-emitting materials for electroluminescence applications, Adv. Funct. Mater. 16 (2006) 709–718. [33] A. Vembris, E. Zarins, J. Jubels, V. Kokars, I. Muzikante, A. Miasojedovas, S. Jursenas, Thermal and optical properties of red luminescent glass forming symmetric and non symmetric styryl-4H-pyran-4-ylidene fragment containing derivatives, Opt. Mater. 34 (2012) 1501–1506. [34] P.D. Maker, R.W. Terhune, M. Nisenhoff, C.M. Savage, Effects of dispersion and focusing on the production of optical harmonics, Phys. Rev. Lett. 8 (1962) 21– 23. [35] W.N. Herman, L.M. Hayden, Maker fringes revisited: second-harmonic generation from birefringent or absorbing materials, J. Opt. Soc. Am. B 12 (1995) 416–427. [36] J. Jerphagnon, S.K. Kurtz, Optical nonlinear susceptibilities: accurate relative values for quartz, ammonium dihydrogen phosphate, and potassium dihydrogen phosphate, Phys. Rev. B 1 (1970) 1739–1744. [37] J.L. Oudar, D.S. Chemla, Hyperpolarizabilities of the nitroanilines and their relations to the excited state dipole moment, J. Chem. Phys. 66 (1977) 2664– 2668. [38] A. Ozols, V. Kokars, P. Augustovs, I. Uiska, K. Traskovskis, G. Mezinskis, A. Pludons, D. Saharov, Polarization dependance of holographic recording in glassy azocompounds, Lith. J. Phys. 50 (1) (2010) 17–25.