rus
Issue #7-8/2024
A.A.Piryazev, I.E.Kuznetsov, A.A.Rychkov, A.F.Akhkiamova, A.Y.Konyakhina, M.A.Gorbunova, A.V.Akkuratov, D.A.Ivanov
DESIGN OF A CELL FOR STUDYING DOMAIN FORMATION PROCESSES IN CONJUGATED DONOR-ACCEPTOR SYSTEMS AT A GIVEN TEMPERATURE GRADIENT
DESIGN OF A CELL FOR STUDYING DOMAIN FORMATION PROCESSES IN CONJUGATED DONOR-ACCEPTOR SYSTEMS AT A GIVEN TEMPERATURE GRADIENT
DOI: https://doi.org/10.22184/1993-8578.2024.17.7-8.464.475
The actively developing field of organic semiconductor electronics requires not only development of new conjugated donor-acceptor compounds, but also the creation of new methods of sample preparation at the stage of creating devices based on them. It is known that improving the mutual packing of molecules can significantly increase the efficiency of devices. In this paper, a cell was proposed that allows creating a thermal gradient in the process of structure formation, allowing the study of structures obtained at different annealing temperatures within a single experiment. Using the example of an organic compound with an irreversible phase transition, the processes occurring during such annealing were studied using atomic force microscopy, X-ray structural analysis with a sliding beam, and polarization optical microscopy.
The actively developing field of organic semiconductor electronics requires not only development of new conjugated donor-acceptor compounds, but also the creation of new methods of sample preparation at the stage of creating devices based on them. It is known that improving the mutual packing of molecules can significantly increase the efficiency of devices. In this paper, a cell was proposed that allows creating a thermal gradient in the process of structure formation, allowing the study of structures obtained at different annealing temperatures within a single experiment. Using the example of an organic compound with an irreversible phase transition, the processes occurring during such annealing were studied using atomic force microscopy, X-ray structural analysis with a sliding beam, and polarization optical microscopy.
Теги: atomic force microscopy differential scanning calorimetry organic electronics x-ray diffraction analysis атомно-силовая микроскопия дифференциальная сканирующая калориметрия органическая электроника рентгеноструктурный анализ
INTRODUCTION
Organic conducting materials of various compositions are used in many fields of science and technology, while having a number of advantages over traditional materials based on silicon or metals [1–3]. Such materials can be used to produce products with flexibility, cheaper and more environmentally friendly manufacturing, transparency, and a number of other properties [4, 5]. However, such products have a number of disadvantages, including low efficiency of the devices [6–8]. A rather popular approach is the one that allows, by controlling structure during sample preparation or post-processing, to change orientation of molecules with respect to the substrate plane by varying the domains size, the crystallinity degree, and anisotropy of transport characteristics, etc. [9–11]. There are several main ways to physically attack the sample, notably thermal annealing [12], solvent vapour annealing [13], UV exposure [14, 15] or chemical attack [16, 17]. By using one of these methods or combination of them, it is largely possible to improve performance of the final devices [18].
Existing methods for controlling structure and thermophysical properties often require a significant number of experiments to select the optimum conditions for sample preparation, however, conducting such a large number of experiments can be labour and time intensive, which, combined with the need to test a large number of new compounds, can significantly reduce effectiveness of this approach. This problem can be solved by reducing the number of experiments, e.g. by performing thermal annealing at different temperatures on the same sample, using a temperature gradient, followed by monitoring the structure formed by such an exposure. The experimental techniques used may include scanning electron, atomic force and optical microscopy and X-ray structural analysis in a sliding beam geometry.
As part of this work, a thermal cell was constructed to allow thermal annealing of a sample deposited as a thin film on a substrate using a temperature gradient. This cell was tested on a sample obtained by spin-coating of an organic conjugated donor-acceptor system, which has a promising application in organic electronics. This approach may have prospects for systems with a series of phase transitions, such as liquid crystalline compounds and composites, which makes this method quite universal. In addition, it is possible to integrate this cell into synchrotron stations infrastructure for fast X-ray beam scans. The cell can also be integrated into Raman and IR microscopes to study orientation effects.
RESEARCH METHODS
The structure experiments by large-angle X-ray scattering with sliding beam were carried out on a Xenocs diffractometer (France), with a GeniX3D source (λ = 1.54 Å), with a beam size of 300 × 300 μm on the sample. Two-dimensional diffractograms were recorded using an Eiger1M detector located at distance of 75 mm from the sample. The wave vector modulus s (s = 2sinθ/λ, where θ is the Bragg angle) was calibrated using several diffraction orders of AgBe. One-dimensional diffractograms obtained by integration of two-dimensional diffraction patterns were analysed using a software package developed by the authors of the paper in Igor Pro (Wavemetrics Inc.).
Phase transitions were studied by differential scanning calorimetry using a Mettler Toledo DSC 3+ analyser (FRS 6+ sensor) in a nitrogen atmosphere at a temperature scanning rate of 10 °C/min. Data processing was carried out in the Star software by Mettler Toledo.
The surface morphology was studied on a JPK NanoWizard® ULTRA Speed 2 atomic force microscope (Bruker, USA). Measurements were performed on scanning areas of 6 × 6 μ2 and 18 × 18 μ2 in semi-contact mode in air. RTESPA-300 cantilevers (Bruker, USA) with resonance frequencies from 200 to 400 kHz and a picture sweep frequency of 1 Hz were used in the experiment. FemtoScan software (version 2.4.10) was used for standard image processing and data presentation.
The structure was studied by scanning electron microscopy using a Carl Zeiss Crossbeam 550 scanning electron microscope. Parameters of magnification and accelerating voltage are given on the corresponding microphotographs. A secondary electron detector was used to record the micrographs. The accelerating voltage was 800 V and the current was 80 pA.
The optical texture and film structure of the synthesised samples were studied using a Carl Zeiss Axioscope A1 microscope equipped with crossed polarizers. The measurements were carried out using ×50 and ×100 objective lenses in the “on reflection” mode. A halogen lamp was used as an illuminator.
Synthesis of S1 sample
All starting reagents and solvents were purchased from Macklin Inc. and used without further purification.
A four-step synthesis was carried out to obtain sample S1 (Fig.1). In the first step, 2,2’-(9,9-didecyl-9H-fluorene-2,7-diyl)dithiophene (1) was prepared from 2,7-dibromo-9,9-dodecyl-9H-fluorene and trimethyl(2-thienyl)stannane by Stille cross-coupling reaction. Next, compound 1 was lithiated with n-butyllithium and the resulting product was treated in-situ with trimethylchlorostannane. As a result, ((9,9-didecyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(trimethylstannane) was obtained. In the next step, 11-bromodibenzo[a,c]phenazine (3) was synthesised by condensation of phenanthrene-9,10-dione with 4-bromobenzene-1,2-diamine. The target product S1 was prepared by the Stille reaction from compounds 2 and 3.
Synthesis of compound 1
The 0.5 g (0.83 mmol) of 2,7-dibromo-9,9-didecyl-9H-fluorene, 0.4 g (1.66 mmol) of trimethyl(2-thienyl)stannane, 8 mg (6.9 μmol) of tetrakis(triphenylphosphine)palladium (0) and 35 mL of toluene were placed in a 100 mL two-neck flask with a reflux condenser and stirring anchor. The mixture was cooled in a liquid nitrogen bath and degassed by vacuuming and filling the vessel with argon (three cycles). The reaction mixture was then boiled for 24 hours and solvent was distilled off at a rotary evaporator. Compound 1 was purified by column chromatography on silica gel (60 Å) using a mixture of hexane (0.7 volume parts) and toluene (0.3 volume parts) as eluent. Fractions with purity more than 98% were combined, and solvent was distilled off at a rotary evaporator. The yield of 2,2’-(9,9-didecyl-9H-fluorene-2,7-diyl)dithiophene was 72%. 1H NMR (500 MHz, CDCl3, δ, m. e.): 7.67 (e, 2H); 7.58–7.60 (dd, 2H); 7.54 (s, 2H); 7.36 (e, 2H); 7.27 (d, 2H); 7.10 (t, 2H); 1.98–2.01 (m, 4H); 1.03–1.24 (m, 32H); 0.81 (t, 6H).
Synthesis of compound 2
Solution of compound 1 (0.55 g, 0.9 mmol) in 25 mL anhydrous tetrahydrofuran was placed in a 100 mL two-neck flask. The flask was purged with a weak argon current for 10 minutes. The mixture in the flask was cooled in an acetone bath to -70 °C and 0.72 mL of a solution of n-butyllithium in hexane (2.5M, 1.8 mmol) was added from a syringe through a silicone septum while stirring. The mixture was stirred at –70 °C for 1 hour and then 5 mL of a solution of trimethylchlorostannan (0.36 g, 1.8 mmol) in anhydrous tetrahydrofuran was added. The mixture was then warmed to room temperature and poured into a separating funnel containing 100 mL distilled water. The product was extracted three times with 25 mL diethyl ether. The organic extract was dried from traces of water over anhydrous magnesium sulfate, filtered through a paper filter and the ether was distilled off at a rotary evaporator. The yield of ((9,9-didecyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(trimethylstannane)was 68%. Compound 2 was used in the next step without further purification. 1H NMR (500 MHz, CDCl3, δ, m. e.): 7.67 (dd, 2H), 7.63 (s, 2H), 7.61 (e, 2H), 7.47 (e, 2H), 7.21 (e, 2H), 2.06 (e, 2H), 1.10–1.35 (m, 32H), 0.92 (t, 6H), 0.44 (s, 18H).
Synthesis of compound 3
The 0.5 g of phenanthroquinone (2.4 mmol), 0.45 g of 4-bromobenzene-1,2-diamine (2.4 mmol) and 50 mL of ethanol were placed in a 100 mL round bottom flask equipped with a reflux condenser. The mixture was boiled for 10 hours, cooled to room temperature and resulting precipitate was filtered off on a paper filter. The product was washed with hot ethanol until the filtrate was light coloured. The yield of 11-bromodibenzo[a,c]phenazine was 80%. 1H NMR (500 MHz, CDCl3, δ, m. e.): 9.32 (m, 2H); 8.48–8.54 (m, 3H); 8.14 (e, 1H); 0.96 (e, 1H); 7.79 (t, 2H); 7.74 (t, 2H).
Synthesis of compound S1
Synthesis of S1 was carried out under similar conditions described for compound 1 using compound 2 – 0.4 g (0.43 mmol) and compound 3 – 0.31 g (0.85 mmol). The product was purified by recrystallisation from chlorobenzene. The yield of 11,11’-((9,9-didecyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))didibenzo[a,c]phenazine was 52%. 1H NMR (500 MHz, CDCl3, δ, m. e.): 9.42-9.47 (m, 4H); 8.60–8.62 (m, 6H); 8.35 (e, 2H); 8.22 (e, 2H); 7.74–7.83 (m, 12H); 7.67–7.70 (m, 4H); 7.52 (e, 2H); 1.62–1.65 (m, 4H); 1.13–1.32 (m, 32H) 0.89 (t, 6H).
The film was prepared by spin-coating onto a silicon substrate from a solution in chloroform with a concentration of 20 mg/ml. A 50 μl drop was applied to the substrate pre-cleaned in the laboratory low-pressure plasma unit Diener electronic Zepto for 1 minute and then washed with chloroform immediately before applying the drop. The substrate rotation speed was 1000 rpm and the drying time before stopping was 1 minute.
The schematic diagram of the assembled cell for forming a thermal gradient is shown in Fig.2. On the system, designed for convergence and increase between the parts of the cell (3), are placed movable heating (5) electric element and cooling (6) element consisting of a hollow plate with a cavity of complex geometry, connected to a pump (4) pumping liquid nitrogen vapour. On top of these elements there are heat exchanger plates (1, 2) equipped with thermocouples. On these plates there is an experimental sample (7).
The cell design is based on a module for forming a temperature gradient, which consists of two elements, the working faces of which are located in the same plane. One of the elements is a heater, the other is a refrigerator. The heater is a parallelepiped with a resistive element inside. Heating is carried out by passing current through a resistive element, temperature control and regulation can be carried out using a thermocouple and PID regulator. The second element of the module (the cooler) is designed as a hollow heat exchanger through which liquid nitrogen vapour is pumped. The temperature of the working edge of the cooler and the heater can be controlled and regulated by means of a thermocouple and PID regulator.
RESULTS
To select the temperature range of the heater and cooler of the gradient cell, measurements of the thermophysical behaviour of the S1 sample were carried out. For this purpose, a differential scanning calorimetry experiment was carried out. The data are shown in Fig.3.
When the sample is heated, a step transition corresponding to the glass transition is observed, as well as a recrystallisation peak and a melting peak. On cooling, only transition corresponding to glass transition is visible. The enthalpy and temperature changes of the phase transitions are given in Table 1.
Thus, based on the DSC data, the temperatures of phase transitions in the sample were determined and heater and cooler temperatures were selected above the end of the melting process and below the end of the crystallisation process in order to cover the entire range structure formation occurs. The selected temperatures were 25 °C and 250 °C for the heater and cooler, respectively.
To compare the results obtained using the experimental cell, XRD structure experiments were carried out for a sample of the original film and the film heated to temperature above the melting point. The results are presented in Fig.4.
It can be observed that the film structure changes upon heating, which agrees well with the DSC data and confirms the hypothesis about necessity of selecting the sample preparation parameters described in introduction to this work. The initial structure can be characterised as crystalline, the peak parameters and possible indexing are given in Table 2, but the small number of peaks makes it difficult to calculate the cell parameters more accurately. The structure obtained after heating the film differs significantly from the initial one: the diffractogram shows a set of reflexes at 11.2, 6.8, and 4.8 Å, as well as an equatorial peak at 3.5 Å, characteristic of the π-π interaction. Based on orientation of the peaks, we can conclude about vertical orientation of the long axes of the molecules in the sample, as well as possible horizontal charge transport in the direction of π-π interaction. Upon annealing, more regularly arranged crystals are formed, which can be evidenced by narrower and orientated peaks, as well as the absence of peaks corresponding to mixed indices.
Figure 5 shows optical micrographs of the sample prepared using the experimental cell. These images were obtained using crossed polarisers, the frames obtained on three regions: warmed, initial and middle zone are given. The sample is optically active; however, in the warmed film region, more regularly arranged needle-like crystallites with average sizes of about 3 µm in length and 1 µm in width can be seen.
Figure 6 shows the microphotographs obtained by electron scanning microscopy.
All photographs were obtained at magnification ×5000. The microphotographs show that after heating of the film substance is organised in the form of larger needle-like crystals lying on the silicon surface, while in the crystals of initial film are arranged chaotically. The characteristic crystallite size in the heated region is 2 μm, while in the original one the crystal size is larger on average, of the order of 3 μm, and the width is 0.3 μm for both regions.
Figure 7 shows the image obtained by atomic force microscopy.
In the presented microphotographs one can notice elongated objects representing crystals of substance S1. The sizes of the objects in image A are more regular and their arrangement is less dense. It can be noticed that during film heating orientation of crystals changes quite strongly: almost all crystals lie in the substrate plane, while in the image of the initial film the crystals are arranged with different orientation due to denser packing. When studying the transition region, one can notice a gradient transition from lying crystals to more densely packed crystals.
Fig.8 shows one-dimensional diffractograms obtained by scanning the X-ray beam across the sample, plotted as a function of the beam shift from the film edge.
Based on peaks intensity, it can be seen that the region corresponding to structure changes is located at a distance of 5 mm from the edge of the film on the heating element.
CONCLUSIONS
Using the developed cell, the structure studies in-situ of a novel conjugated donor-acceptor in-situ material have been carried out. Different domain zones with different structures on one sample were obtained and characterised by a number of experimental methods. The paper shows possibility of optimising the number of experiments to study the temperature annealing effect on the sample structure. In addition, the phase transitions and the obtained structures are characterised, and assumptions about the possible type of charge transport in the obtained film are performed. Thus, performance of the experimental cell and the proposed measurement technique is shown. The obtained results will make it possible to optimise the process of obtaining thin films with optimised structure on the basis of organic materials, in particular, to select the temperature and time of post-treatment of polymer films.
ACKNOWLEDGEMENTS
The study was performed within the framework of the state assignment of Federal State Budgetary Institution of Science Federal Research Centre for Problems of Chemical Physics and Medical Chemistry of RAS, No. FFSG-2024-0017, FFSG-2022-0004 and FFSG-2024-0010.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Organic conducting materials of various compositions are used in many fields of science and technology, while having a number of advantages over traditional materials based on silicon or metals [1–3]. Such materials can be used to produce products with flexibility, cheaper and more environmentally friendly manufacturing, transparency, and a number of other properties [4, 5]. However, such products have a number of disadvantages, including low efficiency of the devices [6–8]. A rather popular approach is the one that allows, by controlling structure during sample preparation or post-processing, to change orientation of molecules with respect to the substrate plane by varying the domains size, the crystallinity degree, and anisotropy of transport characteristics, etc. [9–11]. There are several main ways to physically attack the sample, notably thermal annealing [12], solvent vapour annealing [13], UV exposure [14, 15] or chemical attack [16, 17]. By using one of these methods or combination of them, it is largely possible to improve performance of the final devices [18].
Existing methods for controlling structure and thermophysical properties often require a significant number of experiments to select the optimum conditions for sample preparation, however, conducting such a large number of experiments can be labour and time intensive, which, combined with the need to test a large number of new compounds, can significantly reduce effectiveness of this approach. This problem can be solved by reducing the number of experiments, e.g. by performing thermal annealing at different temperatures on the same sample, using a temperature gradient, followed by monitoring the structure formed by such an exposure. The experimental techniques used may include scanning electron, atomic force and optical microscopy and X-ray structural analysis in a sliding beam geometry.
As part of this work, a thermal cell was constructed to allow thermal annealing of a sample deposited as a thin film on a substrate using a temperature gradient. This cell was tested on a sample obtained by spin-coating of an organic conjugated donor-acceptor system, which has a promising application in organic electronics. This approach may have prospects for systems with a series of phase transitions, such as liquid crystalline compounds and composites, which makes this method quite universal. In addition, it is possible to integrate this cell into synchrotron stations infrastructure for fast X-ray beam scans. The cell can also be integrated into Raman and IR microscopes to study orientation effects.
RESEARCH METHODS
The structure experiments by large-angle X-ray scattering with sliding beam were carried out on a Xenocs diffractometer (France), with a GeniX3D source (λ = 1.54 Å), with a beam size of 300 × 300 μm on the sample. Two-dimensional diffractograms were recorded using an Eiger1M detector located at distance of 75 mm from the sample. The wave vector modulus s (s = 2sinθ/λ, where θ is the Bragg angle) was calibrated using several diffraction orders of AgBe. One-dimensional diffractograms obtained by integration of two-dimensional diffraction patterns were analysed using a software package developed by the authors of the paper in Igor Pro (Wavemetrics Inc.).
Phase transitions were studied by differential scanning calorimetry using a Mettler Toledo DSC 3+ analyser (FRS 6+ sensor) in a nitrogen atmosphere at a temperature scanning rate of 10 °C/min. Data processing was carried out in the Star software by Mettler Toledo.
The surface morphology was studied on a JPK NanoWizard® ULTRA Speed 2 atomic force microscope (Bruker, USA). Measurements were performed on scanning areas of 6 × 6 μ2 and 18 × 18 μ2 in semi-contact mode in air. RTESPA-300 cantilevers (Bruker, USA) with resonance frequencies from 200 to 400 kHz and a picture sweep frequency of 1 Hz were used in the experiment. FemtoScan software (version 2.4.10) was used for standard image processing and data presentation.
The structure was studied by scanning electron microscopy using a Carl Zeiss Crossbeam 550 scanning electron microscope. Parameters of magnification and accelerating voltage are given on the corresponding microphotographs. A secondary electron detector was used to record the micrographs. The accelerating voltage was 800 V and the current was 80 pA.
The optical texture and film structure of the synthesised samples were studied using a Carl Zeiss Axioscope A1 microscope equipped with crossed polarizers. The measurements were carried out using ×50 and ×100 objective lenses in the “on reflection” mode. A halogen lamp was used as an illuminator.
Synthesis of S1 sample
All starting reagents and solvents were purchased from Macklin Inc. and used without further purification.
A four-step synthesis was carried out to obtain sample S1 (Fig.1). In the first step, 2,2’-(9,9-didecyl-9H-fluorene-2,7-diyl)dithiophene (1) was prepared from 2,7-dibromo-9,9-dodecyl-9H-fluorene and trimethyl(2-thienyl)stannane by Stille cross-coupling reaction. Next, compound 1 was lithiated with n-butyllithium and the resulting product was treated in-situ with trimethylchlorostannane. As a result, ((9,9-didecyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(trimethylstannane) was obtained. In the next step, 11-bromodibenzo[a,c]phenazine (3) was synthesised by condensation of phenanthrene-9,10-dione with 4-bromobenzene-1,2-diamine. The target product S1 was prepared by the Stille reaction from compounds 2 and 3.
Synthesis of compound 1
The 0.5 g (0.83 mmol) of 2,7-dibromo-9,9-didecyl-9H-fluorene, 0.4 g (1.66 mmol) of trimethyl(2-thienyl)stannane, 8 mg (6.9 μmol) of tetrakis(triphenylphosphine)palladium (0) and 35 mL of toluene were placed in a 100 mL two-neck flask with a reflux condenser and stirring anchor. The mixture was cooled in a liquid nitrogen bath and degassed by vacuuming and filling the vessel with argon (three cycles). The reaction mixture was then boiled for 24 hours and solvent was distilled off at a rotary evaporator. Compound 1 was purified by column chromatography on silica gel (60 Å) using a mixture of hexane (0.7 volume parts) and toluene (0.3 volume parts) as eluent. Fractions with purity more than 98% were combined, and solvent was distilled off at a rotary evaporator. The yield of 2,2’-(9,9-didecyl-9H-fluorene-2,7-diyl)dithiophene was 72%. 1H NMR (500 MHz, CDCl3, δ, m. e.): 7.67 (e, 2H); 7.58–7.60 (dd, 2H); 7.54 (s, 2H); 7.36 (e, 2H); 7.27 (d, 2H); 7.10 (t, 2H); 1.98–2.01 (m, 4H); 1.03–1.24 (m, 32H); 0.81 (t, 6H).
Synthesis of compound 2
Solution of compound 1 (0.55 g, 0.9 mmol) in 25 mL anhydrous tetrahydrofuran was placed in a 100 mL two-neck flask. The flask was purged with a weak argon current for 10 minutes. The mixture in the flask was cooled in an acetone bath to -70 °C and 0.72 mL of a solution of n-butyllithium in hexane (2.5M, 1.8 mmol) was added from a syringe through a silicone septum while stirring. The mixture was stirred at –70 °C for 1 hour and then 5 mL of a solution of trimethylchlorostannan (0.36 g, 1.8 mmol) in anhydrous tetrahydrofuran was added. The mixture was then warmed to room temperature and poured into a separating funnel containing 100 mL distilled water. The product was extracted three times with 25 mL diethyl ether. The organic extract was dried from traces of water over anhydrous magnesium sulfate, filtered through a paper filter and the ether was distilled off at a rotary evaporator. The yield of ((9,9-didecyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(trimethylstannane)was 68%. Compound 2 was used in the next step without further purification. 1H NMR (500 MHz, CDCl3, δ, m. e.): 7.67 (dd, 2H), 7.63 (s, 2H), 7.61 (e, 2H), 7.47 (e, 2H), 7.21 (e, 2H), 2.06 (e, 2H), 1.10–1.35 (m, 32H), 0.92 (t, 6H), 0.44 (s, 18H).
Synthesis of compound 3
The 0.5 g of phenanthroquinone (2.4 mmol), 0.45 g of 4-bromobenzene-1,2-diamine (2.4 mmol) and 50 mL of ethanol were placed in a 100 mL round bottom flask equipped with a reflux condenser. The mixture was boiled for 10 hours, cooled to room temperature and resulting precipitate was filtered off on a paper filter. The product was washed with hot ethanol until the filtrate was light coloured. The yield of 11-bromodibenzo[a,c]phenazine was 80%. 1H NMR (500 MHz, CDCl3, δ, m. e.): 9.32 (m, 2H); 8.48–8.54 (m, 3H); 8.14 (e, 1H); 0.96 (e, 1H); 7.79 (t, 2H); 7.74 (t, 2H).
Synthesis of compound S1
Synthesis of S1 was carried out under similar conditions described for compound 1 using compound 2 – 0.4 g (0.43 mmol) and compound 3 – 0.31 g (0.85 mmol). The product was purified by recrystallisation from chlorobenzene. The yield of 11,11’-((9,9-didecyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))didibenzo[a,c]phenazine was 52%. 1H NMR (500 MHz, CDCl3, δ, m. e.): 9.42-9.47 (m, 4H); 8.60–8.62 (m, 6H); 8.35 (e, 2H); 8.22 (e, 2H); 7.74–7.83 (m, 12H); 7.67–7.70 (m, 4H); 7.52 (e, 2H); 1.62–1.65 (m, 4H); 1.13–1.32 (m, 32H) 0.89 (t, 6H).
The film was prepared by spin-coating onto a silicon substrate from a solution in chloroform with a concentration of 20 mg/ml. A 50 μl drop was applied to the substrate pre-cleaned in the laboratory low-pressure plasma unit Diener electronic Zepto for 1 minute and then washed with chloroform immediately before applying the drop. The substrate rotation speed was 1000 rpm and the drying time before stopping was 1 minute.
The schematic diagram of the assembled cell for forming a thermal gradient is shown in Fig.2. On the system, designed for convergence and increase between the parts of the cell (3), are placed movable heating (5) electric element and cooling (6) element consisting of a hollow plate with a cavity of complex geometry, connected to a pump (4) pumping liquid nitrogen vapour. On top of these elements there are heat exchanger plates (1, 2) equipped with thermocouples. On these plates there is an experimental sample (7).
The cell design is based on a module for forming a temperature gradient, which consists of two elements, the working faces of which are located in the same plane. One of the elements is a heater, the other is a refrigerator. The heater is a parallelepiped with a resistive element inside. Heating is carried out by passing current through a resistive element, temperature control and regulation can be carried out using a thermocouple and PID regulator. The second element of the module (the cooler) is designed as a hollow heat exchanger through which liquid nitrogen vapour is pumped. The temperature of the working edge of the cooler and the heater can be controlled and regulated by means of a thermocouple and PID regulator.
RESULTS
To select the temperature range of the heater and cooler of the gradient cell, measurements of the thermophysical behaviour of the S1 sample were carried out. For this purpose, a differential scanning calorimetry experiment was carried out. The data are shown in Fig.3.
When the sample is heated, a step transition corresponding to the glass transition is observed, as well as a recrystallisation peak and a melting peak. On cooling, only transition corresponding to glass transition is visible. The enthalpy and temperature changes of the phase transitions are given in Table 1.
Thus, based on the DSC data, the temperatures of phase transitions in the sample were determined and heater and cooler temperatures were selected above the end of the melting process and below the end of the crystallisation process in order to cover the entire range structure formation occurs. The selected temperatures were 25 °C and 250 °C for the heater and cooler, respectively.
To compare the results obtained using the experimental cell, XRD structure experiments were carried out for a sample of the original film and the film heated to temperature above the melting point. The results are presented in Fig.4.
It can be observed that the film structure changes upon heating, which agrees well with the DSC data and confirms the hypothesis about necessity of selecting the sample preparation parameters described in introduction to this work. The initial structure can be characterised as crystalline, the peak parameters and possible indexing are given in Table 2, but the small number of peaks makes it difficult to calculate the cell parameters more accurately. The structure obtained after heating the film differs significantly from the initial one: the diffractogram shows a set of reflexes at 11.2, 6.8, and 4.8 Å, as well as an equatorial peak at 3.5 Å, characteristic of the π-π interaction. Based on orientation of the peaks, we can conclude about vertical orientation of the long axes of the molecules in the sample, as well as possible horizontal charge transport in the direction of π-π interaction. Upon annealing, more regularly arranged crystals are formed, which can be evidenced by narrower and orientated peaks, as well as the absence of peaks corresponding to mixed indices.
Figure 5 shows optical micrographs of the sample prepared using the experimental cell. These images were obtained using crossed polarisers, the frames obtained on three regions: warmed, initial and middle zone are given. The sample is optically active; however, in the warmed film region, more regularly arranged needle-like crystallites with average sizes of about 3 µm in length and 1 µm in width can be seen.
Figure 6 shows the microphotographs obtained by electron scanning microscopy.
All photographs were obtained at magnification ×5000. The microphotographs show that after heating of the film substance is organised in the form of larger needle-like crystals lying on the silicon surface, while in the crystals of initial film are arranged chaotically. The characteristic crystallite size in the heated region is 2 μm, while in the original one the crystal size is larger on average, of the order of 3 μm, and the width is 0.3 μm for both regions.
Figure 7 shows the image obtained by atomic force microscopy.
In the presented microphotographs one can notice elongated objects representing crystals of substance S1. The sizes of the objects in image A are more regular and their arrangement is less dense. It can be noticed that during film heating orientation of crystals changes quite strongly: almost all crystals lie in the substrate plane, while in the image of the initial film the crystals are arranged with different orientation due to denser packing. When studying the transition region, one can notice a gradient transition from lying crystals to more densely packed crystals.
Fig.8 shows one-dimensional diffractograms obtained by scanning the X-ray beam across the sample, plotted as a function of the beam shift from the film edge.
Based on peaks intensity, it can be seen that the region corresponding to structure changes is located at a distance of 5 mm from the edge of the film on the heating element.
CONCLUSIONS
Using the developed cell, the structure studies in-situ of a novel conjugated donor-acceptor in-situ material have been carried out. Different domain zones with different structures on one sample were obtained and characterised by a number of experimental methods. The paper shows possibility of optimising the number of experiments to study the temperature annealing effect on the sample structure. In addition, the phase transitions and the obtained structures are characterised, and assumptions about the possible type of charge transport in the obtained film are performed. Thus, performance of the experimental cell and the proposed measurement technique is shown. The obtained results will make it possible to optimise the process of obtaining thin films with optimised structure on the basis of organic materials, in particular, to select the temperature and time of post-treatment of polymer films.
ACKNOWLEDGEMENTS
The study was performed within the framework of the state assignment of Federal State Budgetary Institution of Science Federal Research Centre for Problems of Chemical Physics and Medical Chemistry of RAS, No. FFSG-2024-0017, FFSG-2022-0004 and FFSG-2024-0010.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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