CC BY
6ISSN 1991-3966. Жидк. крист. и их практич. использ. /Liq. Cryst. and their Appl. 2016, 16 (4), 22-26
UDC 547.595+535.012
A. A. Knyazev1, A. S. Krupin1, W. Haase2, Yu. G. Galyametdinov1
POLARIZED LUMINESCENCE OF ALIGNED FILMS BASED ON NEMATIC BLENDS
OF PFO AND LANTHANIDOMESOGEN
1Kazan National Research Technological University, 68 K. Marx St., Kazan, 420015, Russian Federation 2Institute of Physical Chemistry, Darmstadt University of Technology, 5 Karolinenplatz, Darmstadt, 64289, Germany E-mail: [email protected]
This work reports on the first research of the aligned film based on nematic blend of LC polymer PFO (poly(9,9-di-n-octylfluorenyl-2,7-diyl)) and lanthanidomesogen. Alignment of a blend was carried out by its deposition on a quartz substrate modified with orientant (Nylon 6). The film has polarized luminescence properties with twofold change of intensity due to film alignment.
Key words: nematic blend, polymer PFO (poly(9,9-di-n-octylfluorenyl-2,7-diyl)), lanthanidomesogen, aligned film, polarized luminescence.
DOI: 10.18083/LCAppl.2016.4.22
А. А. Князев1, А. С. Крупин1, В. Хаазе2, Ю. Г. Галяметдинов1
ПОЛЯРИЗОВАННАЯ ЛЮМИНЕСЦЕНЦИЯ ОРИЕНТИРОВАННОЙ ПЛЕНКИ НЕМАТИЧЕСКОЙ СМЕСИ НА ОСНОВЕ ПОЛИ(9,9-ДИ-н-ОКТИЛФЛУОРЕНИЛ-2,7-ДИИЛА)
И ЛАНТАНИДОМЕЗОГЕНА
1 Казанский национальный исследовательский технологический университет,
ул. К. Маркса, д. 68, 420015 Казань, Россия 2Институт физической химии, Дармштадтский технологический университет, пл. Кэролайн, д. 5, 64289 Дармштадт, Германия E-mail: [email protected]
В данной работе сообщается о первом исследовании ориентированных пленок на основе нематической смеси ЖК-полимера ПФО (поли(9,9-ди-н-октилфлуоренил-2,7-диил)) и лантанидомезогена. Ориентацию смеси проводили путем ее нанесения на модифицированную ориентантом (нейлон 6) кварцевую подложку. Пленка обладает поляризованной люминесценцией, интенсивность которой изменяется в 2 раза за счет ориентации.
Ключевые слова: нематические смеси, полимер ПФО (поли(9,9-ди-н-октилфлуоренил-2,7-диил)), лантанидомезоген, ориентированные пленки, поляризованная люминесценция.
© Knyazev A. A., Krupin A. S., Haase W., Galyametdinov Yu. G., 2016
Introduction
Liquid crystal (LC) complexes of lanthanides or lanthanidomesogens are promising polyfUnctional materials for optoelectronics [1, 2]. In such materials, liquid crystalline behavior provide anisotropy of physical properties. The presence of lanthanide ions in liquid crystal molecules allows creating new media with polarized monochromatic luminescence [3, 4]. Following a general trend of development of organized defectless structures for molecular electronics, publications in the scientific literature report about materials with polarized emission which are based on liquid crystalline conducting polymers [5-8] and their blends with lanthanide complexes [9, 10]. Ligand environment in lanthanide complexes is a kind of "antenna" which is able to absorb energy and transfer it to an ion. Organic "envelope" in lanthanide complexes also reduces nonradiative relaxation [11]. In lanthanide complexes for molecular electronics, P-diketones and various Lewis bases are quite efficient as such "antennas" [12, 13].
Until recently, the progress in creating lanthanide-containing luminescent liquid crystals with controlled alignment was restrained by the complexity of synthesis of calamite lanthanidomesogens with nematic mesophase. The number of scientific literature reports about nematic lanthanidomesogens with polarized luminescence is limited [14, 15]. The substances described in [14] decompose during transition to an isotropic state. Therefore, it is impossible to make these compounds aligned and develop respective anisotropic materials. A significant advantage of complexes described in [15, 16] is their ability to undergo glass transition without changing supramolecular packing obtained in mesophase. Due to this feature, application of these compounds is possible both in mesophase interval and at lower temperatures.
Due to their thermal and chemical stability, polymer directors (such as polyimide and nylon) offer the simplest solution for the synthesis of ordered layers of liquid crystals with uniaxial alignment of molecules. However, significant differences between the structure of lanthanidomesogens and the structure of the majority of organic liquid crystals make it difficult to select appropriate directing polymer.
This paper is focused on the original study of creation of substrate-oriented films from lantha-nidomesogen - LC polymer nematic blends. The influence of alignment on luminescence is demonstrated.
Experimental section
Polymer PFO (poly(9,9-di-n-octylfluorenyl-2,7-diyl)); director Nylon 6; Europium(III) chloride hexahydrate (EuCl36H2O) were purchased from Sigma Aldrich. Liquid crystal properties were studied by polarized optical microscopy (POM). Textures and phase transition temperatures were determined by "Olympus BX51" polarized microscope equipped with "Linkam" heating system. Luminescence spectra were taken by the original laboratory unit with the 90° geometry. This device is described in [17]. Nitrogen laser was used as an excitation source, the line Xex= 337 nm. The composition of the synthesized LC complex was determined by elemental analysis.
Tris[1-phenyl-3-(4-(4-propylcyclohexyl)phenyl) propane-1,3-dionato]-[5-heptadecyl-5 '-methyl-2,2 '-bipyridine]europium. The synthesis of a complex was carried out by the procedure described in literature [18-20]. An alcoholic solution of 0.01 mmol of EuCl3 6H2O was slowly dropped into a stirred hot alcoholic solution containing 0.03 mmol of P-diketone [21], 0.01 mmol of 5-heptadecyl-5'-methyl-2,2'-bipyridine [22, 23] and 0.03 mmol of KOH. The suspension was stirred during 10 minutes. The light yellow precipitate that formed was isolated by hot filtration, washed with hot alcohol, and dried under vacuum over P2O5. Further, the product was dissolved in toluene, and the obtained solution was filtered off and evaporated under vacuum. The melting temperature: 180 °C. The yield: 0.166 r (62 %). Experimental values (%): C, 78.97; H, 7.09; N, 1.84. c:00h:07n2o2eu. Calculated values (%): C, 78.49; H, 7.30; N, 1.77.
The surface and structure of films were studied by Solver P47 PRO NT-MDT atomic force microscope and EVO 50 XVP Carl Zeiss scanning electron microscope.
Results and Discussion
The following substances were used as blend components: a conjugated liquid crystalline polymer PFO (poly(9,9-di-n-octylfluorenyl-2,7-diyl)) (Fig. 1) and the LC adduct Eu(III) (Fig. 2); their ratio was 1:1. These substances demonstrate nematic mesomorphism in the 147-186 °C and 160-280 °C temperature ranges, respectively.
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MudK. Kpucm. u ux npaKmm. ucnonb3. /Liq. Cryst. and their Appl. 2016, 16 (4)
Fig. 1. The structure of PFO polymer
To obtain a homogeneous blend, the structure of the synthesized complex was selected in a way to provide coincidence temperature ranges of its and the polymer mesophases. The nematic mesomorphism temperature range of the prepared blend is within 170-270 °C. The type of mesophase was determined by polarized optical microscopy (Fig. 3).
Fig. 3. The Schlieren texture of a blend in polarized microscope at 175 X
Alignment of a blend was carried out by its deposition on a quartz substrate modified with director (Nylon 6). The director film was spin-coated from the 1 % blend solution in acetic acid and then modified by velvet fabric polishing. The atomic force microscopy photo of the modified support is shown in Fig. 4. The photo demonstrates regular texture of grooves formed by nylon on the entire polymer surface. The depth of these grooves is about 10-15 nm (Fig. 5).
Fig. 2. The structure of Eu(CPDk3-Ph)3Bpy17-1 complex
[jm
0 —i-1-1-i-1-\- 0
0 2 4 6 8 10
Fig. 4. The nylon surface polished by velvet fabric
0.5 1.Û 1.5 2 Û 2.5
Fig. 5. Nylon surface profile after velvet fabric polishing
The next step was preparation of thin films (with ~100 nm thickness) in aligned and nonaligned states by spin-coating on quartz and modified quartz substrate with further heating up to 200 and cooling. The surface of films was studied by scanning electron microscopy (Fig. 6, 7).
Fig. 6. Scanning electron microscopy image of an aligned film surface
0-
-50-
-100-
d -150-
-200-
St -250-
w
a) -300-
с
-350-
-400-
-450-
-500-
JL—
550 600 650
Wavelength, nm
Fig. 8. The spectrum of polarized luminescence of a blend's film with parallel (—) and perpendicular (—) alignment relative to the polarization plane of an excitation source
Conclusion
The aligned films based on nematic LC polymer - lanthanidomesogen blends have been produced for the first time. A twofold increase of the film's luminescence intensity is observed when this aligned film undergoes transition from a perpendicular to a parallel position relative to the excitation source.
Fig. 7. Scanning electron microscopy image of a nonaligned film surface
As we can see from Fig. 6, the aligned film surface is more homogeneous and its alignment is predominantly directed according to microscopic grooves formed on nylon surface. Figure 7 represents the surface made from the same blend by its deposition on a quartz support without a director. According to Fig. 7, the film is non-uniform, contains microfractures, and no predominant alignment of its layers can be observed.
To determine the influence of alignment on luminescent properties, the aligned film was studied by polarized luminescence (Fig. 8). Irradiation by a laser with 337 nm wavelength results in intensive red luminescence of a film with the maximum at 612 nm wavelength. Figure 8 represents luminescence spectra of samples with parallel (—) and perpendicular (—) alignment relative to the polarization plane of an excitation source. We can see from the spectrum that the film has polarized luminescence properties with twofold change of luminescence due to film alignment.
Acknowledgment
The work was supported by the Ministry of Education and Science of the Russian Federation (project № 4.323.2014/K)
A.A. Knyazev thanks the grant of the President of the Russian Federation № MD-6102.2016.3 for supporting synthesis of the individual complexes.
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Поступила в редакцию 7.11.2016 г Received 7 November 2016.
