Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

Photo-optical properties of amorphous and crystalline films ofazobenzene-containing photochromes with bent-shaped molecularstructure

Alexey Bobrovskya,*, Valery Shibaeva, V�era Hamplováb, Alexej Bubnova,Vladimíra Novotnáa, Miroslav Kašpara, Alexey Piryazevc, Denis Anokhinc,d,Dimitri Ivanovc,e

a Faculty of Chemistry, Moscow State University, Leninskie Gory, Moscow 119991, Russiab Institute of Physics, Academy of Sciences of the Czech Republic, 182 21 Prague 8, Czech Republicc Faculty of Fundamental Physical and Chemical Engineering, Moscow State University, Leninskie Gory, Moscow 119991, Russiad Institute for Problems of Chemical Physics RAS, Semenov Prospect 1, Chernogolovka, Moscow region, 142432, Russiae Institut de Sciences des Matériaux de Mulhouse, CNRS UMR 7361, 15 rue Jean Starcky, Mulhouse, 68057, France

A R T I C L E I N F O

Article history:Received 10 July 2015Received in revised form 20 October 2015Accepted 25 October 2015Available online 28 October 2015

Keywords:Bent-shaped azobenzene-containingcompoundsE–Z isomerizationThin filmsPhotoinduced phase transitionPhotoorientation

A B S T R A C T

A comparative study of the photo-optical properties and the photo-orientation processes in thin films ofthe bent-shaped azobenzene-containing compounds with different molecular structure was performed.For this purpose three new compounds were designed and prepared: two of these compounds with asymmetric bent-shaped molecular structure incorporate two azobenzene chromophores with differentlength of alkyl chains (6 and 8 methylene units), the third bent-shaped compound possesses only oneazobenzene fragment. The effect of molecular and supramolecular structure, thermal prehistory of thefilms, wavelengths of the excitation light on the photo-optical properties, photoorientation processes andfilms morphology was revealed. It was shown that UV-irradiation leads to E–Z isomerization ofazobenzene fragments in both amorphousized and crystalline films of the synthesized substances. Thisprocess is partially suppressed in crystalline films, but, nevertheless, UV-irradiation of thebichromophoric compounds results in transition from crystalline to amorphous state and decrease inthe surface roughness. Irradiation of amorphousized films with polarized visible and UV-light inducesthe photoorientation of chromophores in direction perpendicular to the polarization plane of theincident light. Relatively high values of photoinduced dichroism in the bent-shaped compounds(D � 0.60–0.65) are comparable with other azobenzene-containing rod-shaped systems. Photoinducedisothermal melting and photoorientation processes could be served for the photo-optical data recordingand storage.

ã 2015 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journal homepa ge: www.elsev ier .com/ locate / jphotochem

1. Introduction

The photoorientation process is a powerful and effective tool forthe local manipulation of molecular orientation and controlling ofthe optical properties of photochromic compounds [1,2]. The originof this process is associated with the selective polarized lightexcitation of the chromophores aligned along the polarizationplane (or electric field of light wave). Subsequent angular-selectivephotochemical reactions of chromophores, in most cases E–Zisomerization, causes preferred orientation chromophores in

* Corresponding author. Fax: +7 95939 0174.E-mail address: bbrvsky@yahoo.com (A. Bobrovsky).

http://dx.doi.org/10.1016/j.jphotochem.2015.10.0211010-6030/ã 2015 Elsevier B.V. All rights reserved.

direction perpendicular to the polarization plane of the incidentlight.

There are a large number of papers devoted to the study ofphotoorientation processes in azobenzene-containing low-molar-mass and polymer systems. New amazing possibilities of applica-tion of this phenomenon for optical data storage, holographicrecording, photo-actuation, block-copolymers self-assembly,nanoparticles ordering were demonstrated recently [3,4]. Mostof the published papers in this field are devoted to the polymericsystems because they can exist in rigid glassy state enablingfixation of the photoinduced molecular orientation [1,3,5–7].Nevertheless, there are only a few publications demonstrating agreat potential of photoorientation process in low-molar-massglass-forming substances [8,9].

76 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

Among different types of compounds the bent-shaped mole-cules present a special interest because due to their unusualmolecular configuration they could form unique polar mesophaseswith promising electro-optical properties [10–13]. This class offunctional materials can be also successfully used as dopants formodification of properties of conventional liquid crystals. A specialinterest presents the bent-shaped substances with azobenzenechromophores capable of E–Z isomerizing [14–22]. In some recentpapers the E–Z photoisomerization process has being used for themanipulation of the liquid crystalline order [15–18] and modifica-tion of electro-optical properties [19,20].

Up to now there is only one paper devoted to the study ofphotoorientation processes in films of bent-shaped substances[22]. Two studied compounds consist of one or two bent-coremesogen fragments connected with azobenzene chromophoresthrough the aliphatic linkage (dimer and trimer, respectively).Irradiation of films of these substances with polarized UV (365 nm)and visible light (473 nm) induces a photo-orientation process ofthe chromophores in a direction perpendicular to the polarizationplane of the incident light. For a trimeric compound bearing anazobenzene bridge between both bent-core mesogenic units thisprocess is associated with cycles of the E–Z–E isomerization of theazobenzene groups followed by rotational diffusion of molecules inthe films. The E–Z isomerization in the azobenzene-containingdimer films is strongly suppressed and the observed photo-induced orientation process proceeds through a rotational diffu-sion mechanism.

The present work is devoted to the first study of the photo-optical properties and photoorientation processes in films of bent-shaped substances with one and two azobenzene chromophoresincorporated into the tails of bent aromatic core (Scheme 1).

Molecules of 6WAVI and 8WAVI have symmetrical structureconsisting of two azobenzene chromophores including into thetails of the bent aromatic core and containing aliphatic groups ofthe different length, 6 and 8 carbon atoms, respectively. 8BVIABrmolecules are asymmetric and contain only one azobenzenechromophore in one tail of the bent structure. It is noteworthy, thatthe structure of azobenzene chromophore and type of itssubstituents, namely the methoxy group placed close to thecentral benzene ring, are completely the same for all threecompounds.

The main goal of this work is the comparative study of thephotoorientation processes in thin films of these compoundsprepared by spin-coating method. Special attention is paid to the

Scheme 1. Molecular structure of bent-sha

investigation of the influence of molecular structure, thermalprehistory of the films, the wavelength of excitation light on thephoto-optical properties, photoorientational processes and filmsmorphology.

2. Experimental

2.1. Synthesis and purity

The molecular structure of the compounds and their puritywere checked using standard analytical methods. Elementalanalyses were carried out on Elementar vario EL III Instrument.1H NMR spectra were acquired on a spectrometer Varian 300 MHz,deuterochloroform serving as solvent and the signals of the solventwere used as internal standards. The chemical purity of thematerials was checked by high pressure liquid chromatography(HPLC), which was carried out using a silica gel column (Bioshere Si100–5 mm, 4 � 250, Watrex) with a mixture of 99.9% of toluene and0.1% of methanol as an eluent, and detection of the elutingproducts by a UV–vis detector (l = 290 nm). The chemical purity ofthe synthesised compound was found within 99.5–99.9%.

2.2. Mesomorphic behaviour

The characterization of the mesomorphic properties of newphotochromic bent-shaped compounds was done using polarizingoptical microscope (POM) and differential scanning calorimetry(DSC). The observation of the characteristic textures and theirchanges in POM were carried out on 2–12 mm thick glass cells,which were filled with LC material in the isotropic phase by meansof capillary action in darkness. The inner surfaces of the glass platesare covered by Indium-Tin-Oxide electrodes and polyimide layersunidirectionally rubbed, which ensures planar alignment of themolecules, e.g. bookshelf geometry in the smectic phase. ALINKAM LTS E350 heating/cooling stage with TMS 93 temperatureprogrammer was used for temperature control, which enabledtemperature stabilization within �0.1 K. Phase transition temper-atures, melting point (m.p.), clearing point (c.p.) and phasetransition enthalpies (DH) were determined by DSC (PyrisDiamond PerkinElmer 7) on the samples of 4–8 mg hermeticallysealed in aluminum pans in cooling/heating runs in a nitrogenatmosphere at a heating/cooling rate of 5 K min�1. The temperaturewas calibrated on extrapolated onsets of melting points of water,

ped azobenzene-containing molecules.

A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87 77

indium and zinc. The enthalpy change was calibrated on enthalpiesof melting of water, indium and zinc.

The polarizing optical microscope investigations of the result-ing thin films were performed using LOMO P-112 polarizingmicroscope equipped by Mettler TA-400 heating stage.

The grazing-incidence wide-angle X-rays scattering (GIWAXS)measurements were performed at the ID10 beamline of ESRF(Grenoble, France) using photon energy of 10 keV. The 2D diffractionpatterns were recorded at different temperatures using a Pilatus300k detector with a sample-to-detector distance of approx. 24 cm.For all experiments incidence angle was 0.16�. A Linkam heatingstage adopted for the GIWAXS geometry was used for high-temperature experiments. The modulus of the scattering vector s(s = 2sinu/l, where u is the Bragg angle) was calibrated using severaldiffraction orders of silver behenate. The indexation of 2D GIWAXSpatterns corrected for geometrical distortions was performed withhome-made routines written in Igor Pro software (WavemetricsLtd.). Thin films for GIWAXS measurements were fabricated on Sisubstrates by spin-coating using solutions of 20 mg/ml concentra-tion in chloroform with a SCS 6800 (SCS, USA) spin coater.

2.3. Photo-optical investigations

Thin films (100–200 nm) of the photochromic substances forphoto-optical experiments were obtained by spin-coating tech-nique using photochromes solutions in chloroform (c � 20 mg/ml).Quartz plates were used as substrates. In order to completelyremove any traces of chloroform the spin-coated films were kept atroom temperature during one day.

Photochemical investigations were performed using an opticalset up equipped with a DRSh-350 ultra-high pressure mercurylamp and MBL-N-457 diode laser (457 nm, CNI Laser). To preventheating of the samples due to the IR irradiation of the mercurylamp, a water filter was introduced in the optical set-up. To assurethe plane-parallel light beam, a quartz lens was applied. Using theinterference filters (Lambda Research Optics Inc, center wave-length 366.6 nm with bandwidth 10.6 nm and center wavelength

Scheme 2. General synthetic proc

437.6 nm with bandwidth 10.4 nm) a light with the wavelengths365 nm and 436 nm was selected. The intensity of light wasmeasured by LaserMate-Q (Coherent) intensity meter and wereequal to �2.0 mW/cm2 (365 nm), �1.0 mW/cm2 (436 nm) for lampand �0.3 W/cm2 for laser.

Spectral measurements were performed using UnicamUV-500 UV–vis spectrophotometer.

The studies of photoorientation processes were performedusing polarized UV/vis spectroscopy. For this purpose the angulardependence (with a step-width of 10�) of the polarized lightabsorbance was measured using a photodiode array UV/visspectrometer TIDAS (J&M) equipped with rotating polarizer(Glan–Taylor prism controlled by a computer).

AFM images were obtained using scanning multi-microscopeSMM-2000 (Proton-MIET).

2.4. Synthesis

2.4.1. Synthetic procedure for nWAVI compoundsSynthesis of nWAVI compounds has been done according to

general procedure presented in Scheme 2. 4-Alkoxy-40-hydrox-yazobenzene I was prepared from p-acetamidophenol by usualmethod (alkylation, hydrolysis using dilute sulphuric acid,diazotation and copulation with phenol). The products werereacted with protected vanilic acid chloride in pyridine/dichloro-methane mixture. The reaction mixture was boiled under reflux for8 h and then held overnight at room temperature. The mixture waswashed with dilute HCl, filtered and evaporated in vacuum. Theprotecting group was removed by following amonolysis inchloroform/ethanol 1:1 mixture at room temperature; thenammonia hydroxide was added in small portions under stirring.The precipitate II was washed with water and dried in vacuum.Total yield was 40%.

A solution of 5 mmol of isophtalic dichloride in 100 ml ofdichloromethane was added under stirring to a solution of10 mmol of mesogenic phenol II in 100 ml of dry dichloromethaneand 10 ml of pyridine and heated under reflux for 24 h. Cold

edure for nWAVI compounds.

78 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

reaction mixture was poured into dilute hydrochloric acid (3%) andthen extracted by dichloromethane. The extract was washed withdilute HCl, twice with water and the solvent was removed usingrotavapor (Buchi Rotavapor R-205 with Vacuum Controller V-800).The crude product was purified by column chromatography onsilica gel under light protection using mixture of dichloromethaneand acetone (99.5:0.5) as an eluent. Then crystallization was madefrom ethanol/acetone mixture (1:1).

1H NMR of intermediate II (CDCl3, 300 MHz): 7.95 dd (4H, orthoto N¼N); 7.85 d (1H, para to ��OCH3); 7.70 s (1H, ortho to ��OCH3);7.35 d (2H, ortho to ��OCO); 7.00 m (3H, ortho to RO- and meta toOCH3); 6.80 brs (1H, OH); 4.05 t (2H, CH2OAr); 4.00 s (3H, OCH3);1.20–1.80 m (8H, CH2); 0.93 t (3H, CH3).

1H NMR of final product III (n = 6) (CDCl3, 300 MHz): 9.10 s (1H,isophtal. ortho to ��COO); 8.52 d (2H, isophtal. ortho to ��COO);7.90–8.00 m (12H, ortho to N¼N and ortho and para to ��OCH3);7.75 t (1H, isophtal. meta to ��COO); 7.35 m (6H, ortho to ��OCO);7.02 d (4H, ortho to ��RO); 4.05 t (4H, CH2OAr); 3.95 s (6H,CH3OAr); 1.85 quint (4H, CH2CH2OAr); 1.22–1.60 m (12H, CH2);0.95 t (6H, CH3). 13C NMR (CDCl3, 300 MHz, see figure below fornumbering): 164.52 (C-16), 164.50 (C-9), 161.6 (C-1), 152.41 (C-5),

151.33 (C-12), 150.63 (C-8), 146.60 (C-4), 144.12 (C-13), 135.30 (C-18), 132.17 (C-20), 129.12 (C-19), 128.49 (C-10), 124.61 (C-6), 123.50(C-3), 123.40 (C-14), 123.03 (C-15), 122.20 (C-7), 114.55 (C-2),114.02 (C-11), 68.21 (CH2O), 56.21 (CH3O), 31.89 (CH2CH2CH3),29.16 (CH2CH2O), 25.71 (CH2(CH2)2O), 22.68 (CH2CH3), 14.12(CH2CH3). Elemental analysis of 6 WAVI: for C60H58N4O12 calc. C70.16, H 5.69, found C 70.03, H 4.81%.

1H NMR of final product III (n = 8), (CDCl3, 300 MHz): 9.10 s (1H,isophtal. ortho to ��COO); 8.52 d (2H, isophtal. ortho to ��COO);7.90–8.00 m (12H, ortho to N¼N and ortho and para to ��OCH3);7.76 t (1H, isophtal. meta to ��COO); 7.35 m (6H, ortho to ��OCO);7.00 d (4H, ortho to ��RO); 4.05 t (4H, CH2OAr); 3.96 s (6H,CH3OAr); 1.85 quint (4H, CH2CH2OAr); 1.20–1.60 m (16H, CH2);0.95 t (6H, CH3). 13C NMR (CDCl3, 300 MHz): 164.52 (C-16), 164.50(C-9), 161.6 (C-1), 152.41 (C-5), 151.33 (C-12), 150.63 (C-8), 146.60(C-4), 144.12 (C-13), 135.30 (C-18), 132.17 (C-20), 129.12 (C-19),128.49 (C-10), 124.61 (C-6), 123.50 (C-3), 123.40 (C-14), 123.03 (C-15), 122.20 (C-7), 114.55 (C-2), 114.02 (C-11), 68.21 (CH2O), 56.21(CH3O), 31.85 (CH2CH2CH3), 29.29 (CH2(CH2)3O), 29.32(CH2(CH2)2CH3), 29.18 (CH2CH2O), 26.03 (CH2(CH2)2O), 22.65(CH2CH3), 14.01 (CH2CH3). Elemental analysis of 8 WAVI: forC64H66N4O12 calc. C 70.96, H 6.14, found C 70.61, H 5.33%.

2.4.2. Synthetic procedure for 8BVIABr compoundSynthesis of 8BVIABr compound has been done according to

general procedure presented in Scheme 3. The design of mesogenicphenol I and acid chloride II has been described in details in our

recent works [32,33]. Synthesis of mesogenic phenol III and finalproduct 8BVIABr was described recently [34].

1H NMR of 8BVIABr (CDCl3, 300 MHz): 9.07 s (1H, isophtal orthoto ��COO); 8.50 d (2H, isophtal. ortho to ��COO); 7.90–8.00 m (6H,ortho to N¼N and ortho and para to ��OCH3); 7.73 t (1H, isophtalmeta to ��COO); 7.50–7.60 dd (4H, ortho to ��Ar); 7.40 m (5H, orthoto ��OCO); 7.00 m (4H, ortho to ��OCH2); 4.05 m (4H, CH2OAr);3.96 s (3H, OCH3); 3.40 t (2H, CH2Br); 1.20–1.90 m (20H, CH2); 0.92t (3H, CH3). 13C NMR (CDCl3, 300 MHz, see figure below fornumbering): 164.56 (C-16), 163.97 (C-23), 163.24 (C-9), 161.68 (C-31), 158.79 (C-1), 152.07 (C-24), 151.33 (C-12), 150.58 (C-27), 149.71(C-8), 146.76 (C-28), 144.12 (C-21), 138.85 (C-13), 135.30 (C-18),135.16 (C-20), 132.60 (C-4), 132.02 (C-22), 130.10 (C-21), 129.71 (C-17), 129.20 (C-19), 128.49 (C-10), 128.09 (C-3), 127.76 (C-6), 124.82(C-26), 123.82 (C-29), 123.39 (C-14), 123.05 (C-15), 122.20 (C-25),121.85 (C-7), 114.78 (C-2), 114.68 (C-30), 113.96 (C-11), 68.08(CH2O), 68.05 (CH2O), 56.21 (CH3O), 33.80 (CH2Br), 32.63(CH2CH2Br), 31.81 (CH2CH2CH3), 28.94–29.39 (CH2), 27.89(CH2(CH2)2Br), 26.05 (CH2(CH2)2O), 25.26 (CH2(CH2)2O), 22.66(CH2CH3), 14.11 (CH2CH3). Elemental analysis of 8 WAVI: forC54H56N2O9Br calc. C 67.85, H 5.80, found C 66.41, H 4.88%.

3. Results and discussion

3.1. Phase behaviour of bent-shaped photochromic compounds

At room temperature both substances are the crystallinepowders with melting transition temperatures 181 �C, 174 �C

and 183 �C for 6WAVI,8WAVI and 8BVIABr, respectively. Athigher temperatures 6WAVI and 8WAVI form LC nematic phase,whereas 8BVIABr melts into isotropic liquid. In this paper, wedo not consider details of the LC behaviour of the synthesizedbent-shaped compounds because all investigations of photo-optical properties were performed for crystalline and amorphou-sized films of these substances or dilute solutions. The preliminaryresults of the LC properties of the synthesized compounds arepresented in Supporting Information (Table S1, Figs. S1–S3).

Variable-temperature GIWAXS measurements reveals acomplex thermotropic behavior of the initially amorphousspin-coated films of these compounds during heating. Fig. 2presents the thin-film 2D GIWAXS patterns of 6WAVI measuredat different temperatures. During heating of a disordered as-castfilm a smectic structure with the lattice parameter of 51.4 Ådevelops at 110 �C (Fig. 2a and b). At higher temperature a smectic-to-crystal transition occurs approximately at 140 �C (Fig. 2c). Thislow-temperature phase was indexed to a pseudo-monocliniclattice (symmetry group P21) with the following parameters:a = 34.98 Å, b = 10.44 Å, c = 6.35 Å, a = b = g = 90�. The indexation of

Scheme 3. Synthetic procedure for 8BVIABr.

Fig.1. Absorbance spectra of 6WAVI (a) and 8BVIABr (b) in dichloroethane solutionand spin-coated films before and after annealing at 120 �C (a), 150 �C (b). Spectrawere normalized at 245 nm.

A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87 79

the reflections is presented in Table S2. The a-axis is orientednormal to the substrate. Further, at 170 �C the orthorhombic phasetransforms to a monoclinic one (a = 40.46 Å, b = 9.83 Å, c = 6.32 Å,b = 105.2�, cf. Table S3). This phase reveals a highly-ordered texturewith sharp reflections (Fig. 2d). The increase of a-parameter isprobably related to a decrease of the molecule tilt with respect tothe normal direction, which is typical for liquid-crystallinesystems with a complex molecular architecture [35–37]. Above180 �C all reflections disappear from the diffractogram indicatingtransition to the nematic phase (cf. Fig. S3a). It is noteworthy thatthe observed phase transitions are reversible on cooling, which isin good agreement with the DSC data given in Table S1.

The thermal behaviour of 8WAVI thin films are similar to that of6WAVI (Fig. 3, Fig. S3b). At room temperature the elements of adisordered smectic phase with the lattice parameter of 48.2 Å isobserved. (Fig. 3a). Above 100 �C the smectic phase graduallyevolves in a pseudo-monoclinic crystal phase with the followingparameters: a = 54.07 Å, b = 11.05 Å, c = 6.55 Å, a = b = g = 90�, sym-metry group P21. (cf. Fig. 3b and Table S4). In turn, at 140 �C thisphase transforms to a highly-ordered monoclinic structure(a = 38.68 Å, b = 10.57 Å, c = 6.17 Å, b = 93.7�, Table S5) (Fig. 3c). Incontrast to 6WAVI, prior to transition to the nematic phase at170 �C the diffractograms show formation of an ordered smecticphase with the characteristic distance of 46.1 Å (Fig. 3d). Theevolution of 1D-reduced X-ray patterns with temperature ispresented in Fig. S4b. The temperatures of the phase transitions arein good agreement with DSC data (cf. Fig. S2).

Fig. 2. 2D diffraction patterns of 6WAVI obtained during heating from 25 to 140 �C: (a) 25 �C, (b) 110 �C, (c) 140 �C, (d) 170 �C.

Fig. 3. 2D diffraction patterns of 8WAVI obtained during heating from 25 to 200 �C: (a) 25 �C, (b) 120 �C, (c) 150 �C, (d) 170 �C.

80 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

Fig. 4. Absorbance spectra of 8WAVI (a) and 8BVIABr (b) films at differenttemperatures measured during heating. In (b) spectra were recorded with step 5�

during heating.

20 40 60 80 100 12 0 14 0 16 0

0.3

0.4

0.5

0.6

0.7

0.8

8BVIABr

8WAVI

Abs

orba

nce

T / oC

Fig. 5. Temperature dependence of absorbance of spin-coated films at maximum ofp–p* electronic transition of azobenzene chromophore under slow heating scan(1 �/min).

A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87 81

3.2. Effect of thermal prehistory on the phase behaviour, absorbancespectra and thermochromism of the substances films

Let us consider the influence of thermal prehistory on theabsorbance spectra in films of the bent-shaped compounds understudy. Absorbance spectra of all studied compounds consist ofintensive peaks in UV-range; one of them corresponds to p–p*electronic transition of azobenzene chromophore (Fig. 1). Due tothe very similar structure of azobenzene chromophore and type ofthe substituents for all compounds maximum of this peak indichloroethane solution is located at 355 nm.

Spin-coating method enables to prepare the thin (100–200 nm)amorphousized films. Isotropic structure of these films isconfirmed by polarizing optical microscopy (POM) and bycomplete absence of birefringence and light scattering.

For all substances the maximum of p–p* electronic transitionof azobenzene moieties in the films is shifted to the short-wavelength spectral region and has smaller intensity (in compari-son with their solutions). In the case of bichromophoriccompounds 6WAVI and 8WAVI this shift is more pronounced(28 nm), whereas the difference between maxima position for8BVIABr is only 17 nm. The phenomenon of such hypsochromicshift is associated with H-aggregates formation and was observedmany times for the azobenzene-containing substances [23–25].

An annealing of the films results in the significant absorbancespectra changes. For the bichromophoric substances 6WAVI and8WAVI slight bathochromic shift and decrease in absorbance arefound. This shift is accompanied by an appearance of the lightscattering, visible as absorbance in whole visible range (Fig. 1a). Asdiscussed in the previous section, heating of these films inducessmectic phase formation followed by crystallization. However, incontrast to 6WAVI and 8WAVI POM observations of annealed filmsof 8BVIABr do not reveal ordered phase formation probably due tothe small size of crystallites or their low birefringence.

As seen from spectrum in Fig. 1a for 6WAVI the absorbancepeak with maximum at 364 nm also consists of two “shoulders”with maxima at �350 nm and �380 nm. Probably, absorbance withlmax� 350 nm corresponds to nonaggregated form of chromo-phores, whereas long-wavelength “shoulder” (lmax� 380 nm)could be related to J-aggregates formation characterizing byhead-to-head chromophores arrangement [23]. Intriguingly, theappearance of J-aggregates is very rarely observed for theazobenzene chromophores; there are only a few examplesdescribed in literature and most of them are related to Langmuir-–Blodgett films [26–29]. Seemingly, bent-shape of the bichromo-phoric compounds and molecular packing in crystalline phaseprovoke the head-to-head chromophores arrangement andJ-aggregation. Crystallization of the 8BVIABr films improvesH-aggregates formation; large hypsochromic shift (48 nm) takesplace and accompanied by the increase of its intensity as seen inFig. 1b.

We have studied in detail the temperature dependence ofabsorbance spectra for amorphousized spin-coated films andobserved remarkable thermochromic properties. Heating of thefilms results in gradual changing of shape of absorbance peakcorresponded to p–p* electronic transition of azobenzenechromophores (Fig. 4, Figs. S4 and S5). It is interesting to notethat mono- and bichromophoric bent-shaped substances demon-strate completely different thermochromic behaviour. For 6WAVIand 8WAVI heating induces bathochromic shift of absorbance peak(Fig. 4a), whereas in 8BVIABr films strong hypsochromic shift wasfound (Fig. 4b).

Analysis of temperature dependence of values of absorbance atmaximum of p–p* electronic transition presented in Fig. 5 andFig. S5 enables to determine glass transition of spin-coated films ofall three substances. At room temperature films of all substances

are in stable glassy state and characterized by the frozen molecularmobility preventing crystallization and molecular ordering. Heat-ing of the fresh spin-coated films leads, at the first, to a slightdecrease in absorbance. Then, at �100 �C for 6WAVI, �90 �C for8WAVI and �80 �C for 8BVIABr noticeable changes of absorbancevalues are observed associated with the more pronouncedaggregates and smectic phase formation followed by

0 20 0 400 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4A

bsor

banc

e

Time / s

Solution:UVVis

Fil m: UV Vis

a

0 100 200 8000 1000 00.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce

Time / s

before aft er anneali ng

b

Fig. 6. (a) Comparison of kinetics of absorbance changes for solution and spin-coated film of 6WAVI during UV-irradiation (365 nm, 4 mW/cm2) and subsequentvisible light irradiation (436 nm, 2 mW/cm2). (b) Comparison of kinetics ofabsorbance changes for film of 6WAVI before and after annealing during UV-irradiation. Figures present the values of absorbance corresponding to maximum ofp–p* electronic transition.

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4

0.5

Abso

rban

ce

Time / s

film solution

Fig. 7. Kinetics of absorbance changes of 8BVIABr dichloroethane solution(0.0128 mg/ml) and film during UV-irradiation (365 nm, 4 mW/cm2).

82 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

crystallization processes. A difference in glass transition temper-atures is associated with the length of alkyl tails (6 and 8 methyleneunits in 6WAVI and 8WAVI, respectively). An additional decreasein glass transition temperature for 8BVIABr is caused by shorterlength of aromatic core.

Subsequent cooling of the crystallized films results in onlyslight changes in absorbance values (Fig. S5), because a relativeposition of molecules and their mobility in crystalline state areslightly dependent on temperature.

Summarizing the observed spectral phenomena we maysuppose that the molecular structure and ordering in thecrystalline phase predetermine spectral properties of the solutionand films of the synthesized compounds as well as types of theaggregate formation and interchromophores interactions. Inparticular, mono- and bichromophoric bent-shaped substancesdemonstrate completely different thermochromic behaviour,bichromophoric substances demonstrate formation of J-aggre-gates, whereas monochromophoric substance forms H-aggregatesduring annealing and crystallization.

3.3. E–Z photoisomerization in solutions and films of bent-shapedsubstances

UV-irradiation leads to E–Z photoisomerization of azobenzenechromophores accompanied by large reversible spectral changes.Figs. S6–S10 show the changes of absorbance spectra of substancesin dichloroethane solution and their films during UV-irradiation(365 nm) and subsequent visible light irradiation (436 nm).

For the bichromophoric compounds 6WAVI and 8WAVI therates of the forward and back E–Z and Z–E isomerization insolutions and fresh amorphous films are comparable (Fig. 6a,Fig. S7), whereas the rate of E–Z isomerization for monochromo-phoric 8BVIABr and relative concentration of its Z-isomer inphotostationary state in films is much less (Fig. 7). It is noteworthy,that in the case bichromophoric compounds three isomers, EE, EZand ZZ, coexists during photoisomerization process.

Crystalline phase formation after films annealing stronglysuppresses E–Z isomerization for all investigated compounds(Fig. 6b). This phenomenon is explained by a small available freevolume in crystalline phase because the transition from rod-like E-isomer to bent shaped Z-isomer is only possible in systems havingthe sufficient free volume [29,30].

Despite to the strong suppression of photoisomerizationprocess, UV-irradiation of the annealed films of 6WAVI and

Fig. 8. POM photo of annealed film of 8WAVI after UV irradiation through the mask(380 nm, �8 mW/cm2, 30 min).

Fig. 9. AFM scans and cross-sections of 8WAVI film after spin-coating (a), after annealing at 150 �C (b) and after UV irradiation (380 nm, 4 min) (c).

A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87 83

8WAVI results in isothermal melting of crystalline phase thatappears as drop in birefringence clearly visible by POM observa-tions (Fig. 8). It is important to note that back thermal Z–E

isomerization at room temperature does not lead to samplecrystallization due to suppressed molecular mobility in the glassystate.

Fig. 10. Molecular models showing Z-isomers of 6WAVI and 8BVIABr. (HyperCh-emTM 8.0.8 for Windows.)

Fig. 11. Polarized absorbance before and after 2 min of irradiation (a) and polar diagramvisible light (457 nm, �300 mW/cm2) for spin-coated film of 6WAVI.

84 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

The surface topography of the substances films just after spin-coating, after annealing, and after UV-irradiation was studied bymeans of AFM. Films of substances just after preparation havesmooth surface (Fig. 9a) which transforms to the crystalline platesunder annealing (Fig. 9b, Fig. S11). UV-irradiation of bichromophoriccompounds 6WAVI and 8WAVI leads to irreversible melting of thecrystalline structure accompanied by smoothing of the surfacetopography (Fig. 9c). Nevertheless, the irradiated films still havelarge variations in relief height that is explained by the “traces” ofthe melted crystallites (see topography cross-section in Fig. 9c). It isnoteworthy, as was mentioned above, the amorphous state iscompletely frozen at room temperature; thus, topography of theirradiated samples remains stable at room temperature. Annealingof the films leads to completely recovery of the crystalline state.Thus, light action allows one not only to change spectral propertiesand ordering of the synthesized substances but also enables toreversibly control the surface roughness of their films.

In contrast to 6WAVI and 8WAVI, monochromophoric substance8BVIABr does not undergo isothermal melting under UV-irradia-tion (Fig. S11), probably, due to the small changes in anisometryduring E–Z isomerization (Fig.10) and suppression of isomerizationprocess in the annealed films, as was discussed above.

of absorbance (b) during photoorientation and reorientation using the polarized

0 50 10 0 150 200 250 300

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Dic

hroi

sm

Time / s

8WAVI 6WAVI

a

0 100 0 2000 30 00 4000 5000

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

6WAVI

Dic

hroi

sm

Time / s

8WAVI

b

Fig. 12. Kinetics of dichroism changes during photoorientation (a) and reorienta-tion (b) induced by polarized visible light (457 nm, �300 mW/cm2) for spin-coatedfilms of 6WAVI and 8WAVI.

0 200 0 400 0 600 0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

6WAVI

8BV IABr

Dic

hros

im

Tim e / s

a

0 100 200 30 0 40 0 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

6WAV I

8BVIABr

Dic

hroi

sm

Time / s

b

A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87 85

Thus, light action and thermal treatment of the films of theinvestigated compounds enable to realize the control of spectralproperties, as well as topography of the films surface (for thebichromophoric compounds).

3.4. Comparative study of photoorientation phenomena in spin-coatedfilms of bent-shaped compounds

Irradiation of the bent-shaped substances films with polarizedvisible light induces the rather large anisotropy in polarizedabsorbance spectra (Fig. 11, Fig. S12). This effect is associated withlight-induced orientation of chromophores in direction perpen-dicular to the polarization plane of the excitation light during therepeated cycles of E–Z–E isomerization of azobenzene fragments.(POM observations also reveal an appearance of uniaxial orienta-tion of chromophores which appears as a film birefringence.)Photoinduced anisotropy is very stable at room temperature; allirradiated films preserve the birefringence for a long time, at leastone year. Changing direction of polarization allows one to inducethe reorientation process (Fig. 11b).

The dichroism values, D, of the spin-coated irradiated filmswere calculated from the polarized absorbance spectra using

Eq. (1).

D ¼ Ak � A?Ak þ A?

� �ð1Þ

where Ak is the absorbance of light polarized along the preferredchromophore orientation direction and A? is the absorbanceperpendicular to this direction.

Figs. 12–14, Figs. S12c, S13 demonstrate the kinetics of thedichroism growth under the irradiation with polarized UV(365 nm) and visible (457 nm) light. It is noteworthy, that kineticsof dichroism growth for bichromophoric compounds (6WAVI and8WAVI) is very similar, whereas rate of reorientation processes andmaximum achievable dichroism is smaller for 8WAVI (Fig. 12). Thepresence of two azobenzene fragments in 6WAVI and 8WAVI inthe case of polarized visible light action predetermines higher rateand maximum values of dichroism in fresh and annealed films incomparison with monochromophoric substance 8BVIABr (Fig. 13,Fig. S13).

For the annealed crystalline films of all compounds photo-orientation process is prohibited when the rather moderateintensity light is used (�0.3 W/cm2). Increasing light intensitytwice, nevertheless, allows one to realize slow growth in dichroism(Fig. S13); in other words, the photoorientation process hasthreshold character in respect to light intensity. In the case of highlight power the possible effect of films heating could induce

Fig. 13. Comparison of kinetics of dichroism growth during photoorientation bypolarized visible light (457 nm, �300 mW/cm2) for spin-coated film of 8BVIABr and6WAVI. (b) Shows first 500 s of irradiation.

0 200 0 400 0 60 00 800 0 1000 00.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40D

ichr

oism

Time / s

8BV IAB r 6WAV I

Fig. 14. Kinetics of dichroism changes for spin-coated film of 8BVIABr and 6WAVIduring photoorientation by polarized UV light (365 nm, 0.7 mW/cm2).

86 A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87

melting of the samples followed by crystallization with uniaxiallyaligned crystallites. This phenomenon is the subject of futureinvestigations.

More complicated behaviour of kinetic curves was found whenfilms are irradiated by polarized UV light (Fig. 14). For thebichromophoric compounds the fast increase in dichroism at first�7 min of irradiation is followed by its noticeable decrease underprolonged irradiation. This observation is probably associated withhigh concentration of Z-isomer disrupting uniaxial photoinducedorder; but this effect is negligible for the monochromophoricsubstance 8BVIABr. This observation is also supported by acomparison of the molecular models of Z-isomers of mono- andbichromophoric compounds (Fig. 10): Z-isomer of monochromo-phoric 8BVIABr has higher anisometry and less pronouncedinfluence on the photoinduced order. Instead of this, two stages,slow and fast, are clearly seen in the kinetic curve for 8BVIABr. Thephotoinduced uniaxial orientation of the films completelydisappears under annealing at temperatures higher than glasstransition temperature.

Summarizing the results of the investigation of the photo-orientation processes in films of the studied compounds we mayconclude, that despite their significant difference in the molecularstructure the values of the photoinduced dichroism are high andcomparable with the photoinduced dichroism in well-studiedazobenzene-containing side chain polymers [6,7,28,31].

4. Conclusions

One mono- and two bichromophoric compounds with bent-shaped molecular structure have been obtained and thin films ofthese compounds have been studied. Investigation of photo-optical properties has revealed a noticeable influence of theirchemical structure and phase state of the films on aggregationphenomena, kinetics of E–Z isomerization and photoorientationprocesses. It was found that spin-coated films of all compoundspossess the thermochromic properties and heating of the filmsinduces H- and J-aggregates formation accompanied by largespectral changes. Investigations of the photoorientation processesshowed that the presence of two azobenzene chromophoresincreases the rate and maximal achievable dichroism values duringthe photoorientation. This type of the novel bent-shaped com-pounds could be considered as promising substances for thephotonic applications.

Acknowledgements

This research was supported by the Russian Foundation ofFundamental Research (13-03-00648, 13-03-12071 and 13-03-12456), Czech Science Foundation CSF 13-14133S and MEYSLD14007. The investigations of the structure of thin films byGIWAXS measurements were performed under financial supportof the Russian Science Foundation (Project 14-13-00379: A.Bobrovsky, V. Shibaev).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.jphotochem.2015.10.021.

References

[1] Y. Zhao, T. Ikeda, Smart Light-responsive Materials: Azobenzene-containingPolymers and Liquid CrystalS, Wiley & Sons, Hoboken, NJ, 2009.

[2] V. Chigrinov, V. Kozenkov, H.-S. Kwok, Photoalignment of Liquid CrystalsMaterials: Physics and Application, Wiley-SID series, 2008.

[3] G. Cipparrone, P. Pagliusi, C. Provenzano, V.P. Shibaev, Reversible photoinducedchiral structure in amorphous polymer for light polarization control,Macromolecules 41 (2008) 5992–5996.

[4] H. Yu, T. Ikeda, Photocontrollable liquid-crystalline actuators, Adv. Mater. 23(2011) 2149–2180.

[5] T. Fujii, S. Kuwahara, K. Katayama, K. Takado, T. Ube, T. Ikeda, Moleculardynamics in azobenzene liquid crystal polymer films measured by time-resolved techniques, Phys. Chem. Chem. Phys. 16 (2014) 10485–10490.

[6] A. Bobrovsky, V. Shibaev, A. Bubnov, V. Hamplová, M. Kašpar, M. Glogarova,Effect of molecular structure on chiro-optical and photo-optical properties ofsmart liquid crystalline polyacrylates effect of molecular structure on chiro-optical and photo-optical properties of smart liquid crystalline polyacrylates,Macromolecules 46 (2013) 4276–4284.

[7] A. Bobrovsky, V. Shibaev, A. Bubnov, V. Hamplova, M. Kašpar, D. Pociecha, M.Glogarova, Effect of molecular structure and thermal treatment on photo-optical properties of photochromic azobenzene-containing polymer films,Macromol. Chem. Phys. 212 (2011) 342–352.

[8] P. Wolfer, H. Audorff, K. Kreger, L. Kador, H.-W. Schmidt, N. Stingelin, P. Smith,Photo-induced molecular alignment of trisazobenzene derivatives, J. Mater.Chem. 21 (2011) 4339–4345.

[9] A. Ozols, V. Kokars, P. Augustovs, D. Malinovskis, K. Traskovskis, E. Zarins, G.Ivanovs, Self-enhancement of scalar and vector holographic gratings inazobenzene molecular glassy films, Opt. Photonics J. 4 (2014) 143–152.

[10] N. Gimeno, J. Barbera, J.L. Serrano, M.B. Ros, M.R. de la Fuente, I. Alonso, C.L.Folcia, Terminal chains as a tool to modulate the properties of bent-core liquidcrystals, Chem. Mater. 21 (2009) 4620–4630.

[11] A. Eremin, U. Kornek, S. Stern, R. Stannarius, F. Araoka, H. Takezoe, H. Nadasi, W.Weissflog, A. Jakli, Pattern-stabilized decorated polar liquid crystal fibers,Phys. Rev. Lett. 109 (2012) 017801.

[12] N. Gimeno, J. Vergara, M. Cano, J.L. Serrano, M.B. Ros, J. Ortega, C.L. Folcia, S.Rodríguez-Conde, G. Sanz-Enguita, J. Etxebarria, Janus-type dendromesogens:a tool to control the nanosegregation and polar organization of bent-corestructures janus-type dendromesogens: a tool to control the nanosegregationand polar organization of bent-core structures, Chem. Mater. 25 (2013) 286–296.

[13] A. Eremin, A. Jakli, Polar bent-shape liquid crystals—from molecular bend tolayer splay and chirality, Soft Matter 9 (2013) 615–637.

[14] S.K. Prasad, G.G. Nair, K.L. Sandhya, D.S.S. Rao, Photoinduced phase transitionsin liquid crystalline systems, Mol. Cryst. Liq. Cryst. 436 (2005) 83–105.

[15] L. Rahmana, S. Kumar, C. Tschierske, G. Israel, D. Ster, G. Hegde, Synthesis andphotoswitching properties of bent-shaped liquid crystals containingazobenzene monomers, Liq. Cryst. 36 (2009) 397–407.

[16] C.L. Folcia, I. Alonso, J. Ortega, J. Etxebarria, I. Pintre, M.B. Ros, Achiral bent-coreliquid crystals with azo and azoxy linkages: structural and nonlinear opticalproperties and photoisomerization, Chem. Mater. 18 (2006) 4617–4626.

[17] N. Gimeno, I. Pintre, M. Martinez-Abadia, J.L. Serrano, M.B. Ros, Bent-coreliquid crystal phases promoted by azo-containing molecules: from monomersto side-chain polymers, RSC Adv. 4 (2014) 19694–19702.

[18] A. Zep, K. Sitkowska, D. Pociecha, E. Gorecka, Photoresponsive helicalnanofilaments of B4 phase, J. Mater. Chem. C 2 (2014) 2323–2327.

[19] G.G. Nair, S.K. Prasad, U.S. Hiremath, C.V. Yelamaggad, Effect of light on thepolarization of a banana-shaped achiral compound doped with a photoactiveazobenzene material, J. Appl. Phys. 90 (2001) 48–52.

[20] A. Jákli, V. Prasad, D.S.S. Rao, G. Liao, I. Jánossy, Light-induced changes ofoptical and electrical properties in bent-core azo compounds, Phys. Rev. E 71(2005) 021709.

A. Bobrovsky et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 75–87 87

[21] M. Alaasar, M. Prehm, C. Tschierske, Influence of halogen substituent on themesomorphic properties of five-ring banana-shaped molecules withazobenzene wings, Liq. Cryst. 40 (2013) 656–668.

[22] M.-G. Tamba, A. Bobrovsky, V. Shibaev, G. Pelzl, U. Baumeister, W. Weissflog,Photochromic azobenzene functionalised banana–calamitic dimers andtrimers: mesophase behaviour and photo-orientational phenomena, Liq.Cryst. 38 (2011) 1531–1550.

[23] M. Kasha, H.R. Rawls, A. El-Bayoumi, The exciton model in molecularspectroscopy, Pure Appl. Chem. 11 (1965) 371–392.

[24] V. Czikkely, H.D. Försterling, H. Kuhn, Extended dipole model for aggregates ofdye molecules, Chem. Phys. Lett. 6 (1970) 207–210.

[25] J.M. Kuiper, J.B.F.N. Engberts, H-aggregation of azobenzene-substitutedamphiphiles in vesicular membranes, Langmuir 20 (2004) 1152–1160.

[26] O. Haruta, K. Ijiro, Application of oligonucleotide as a template for theassembly of nucleoamphiphile bearing azobenzene at the air–water interface,J. Nanosci. Nanotechnol. 7 (2007) 734–737.

[27] B.V. Shankar, A. Patnaik, J-aggregates in matrix stabilized two-dimensionalazobenzene derivatives, J. Colloid Interface Sci. 302 (2006) 259–266.

[28] I. Zebger, M. Rutloh, U. Hoffmann, J. Stumpe, H.W. Siesler, S. Hvilsted,Photoorientation of a liquid-crystalline polyester with azobenzene sidegroups: effects of irradiation with linearly polarized red light afterphotochemical pretreatment, Macromolecules 36 (2003) 9373–9382.

[29] J.G. Victor, J.M. Torkelson, On measuring the distribution of local free volume inglassy polymers by photochromic and photophysical techniques,Macromolecules 20 (1987) 2241–2250.

[30] Y. Tateishi, K. Tanaka, T. Nagamura, Kinetics of photoinduced e to zisomerization of azobenzene in polystyrene films: thickness, molecularweight and temperature effects, J. Phys. Chem. B 111 (2007) 7761–7766.

[31] A. Bobrovsky, V. Shibaev, Polarised light-induced orientation and reorientationprocesses and unexpected ‘memory effect’ in side-chain azobenzene-containing LC polymers, Liq. Cryst. 39 (2012) 339–345.

[32] M. Kašpar, V. Hamplová, V. Novotná, M. Glogarová, P. Vanek, New banana-typeliquid crystal with a methoxy group substituted near the central ring, J. Mater.Chem. 12 (2002) 2221–2224.

[33] V. Novotná, M. Kašpar, V. Hamplová, M. Glogarová, L. Lej9cek, J. Kroupa, D.Pociecha, Ferroelectric-like behaviour of the SmCP phase in liquid crystallinecompounds with asymmetrical bent-core molecules, J. Mater. Chem. 16 (2006)2031–2038.

[34] A. Bobrovsky, V. Shibaev, V. Hamplová, V. Novotná, M. Kašpar, Photoinducedmelting of LC gels based on banana-shaped photochromic gelator, RSC Adv. 5(2015) 56891–56895.

[35] X. Zhu, A. Mourran, U. Beginn, M. Möller, D.V. Anokhin, D.A. Ivanov, Self-assembled structures formed by a wedge-shaped molecule in 2D and 3D: therole of flexible side chains and polar head groups, Phys. Chem. Chem. Phys. 12(2010) 1444–1452.

[36] M. Defaux, F. Gholamrezaie, J. Wang, A. Kreyes, U. Ziener, D.V. Anokhin, D.A.Ivanov, A. Moser, A. Neuhold, I. Salzmann, R. Resel, D.M. de Leeuw, S.C.J.Meskers, M. Moeller, A. Mourran, Solution-processable septithiophenemonolayer transistor, Adv. Mater. 24 (2012) 973–978.

[37] D.V. Anokhin, M. Defaux, A. Mourran, M. Moeller, Y.N. Luponosov, O.V.Borshchev, A.V. Bakirov, M.A. Shcherbina, S.N. Chvalun, T. Meyer-Friedrichsen,A. Elschner, S. Kirchmeyer, S.A. Ponomarenko, D.A. Ivanov, Effect of molecularstructure of a,a0-dialkylquaterthiophenes and their organosilicon multipodson ordering, phase behavior and charge carrier mobility, J. Phys. Chem. C 116(2012) 22727–22736.