Method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a thin coating, conducting thin coating containing polycyclic aromatic hydrocarbons and solution of polycyclic aromatic hydrocarbons in a molecular form

Abstract

Method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a coating, using aromatic precursors and the process of their oxidation, is characterised in that the electrochemical oxidation of naphthalene is carried out in a three-electrode system, where the working electrode is a platinum electrode with a surface of 0.5-5 cm2, preferably 2 cm2, the reference electrode is a silver electrode Ag|0.001M Ag+, (C4h9)4N+, PF6 II with a potential of 0.197 V against ferrocene, and the counter electrode is a platinum mesh located in a separate electrode space filled with a supporting electrolyte and closed with a porous PTFE electrolytic key, with a surface area corresponding to half the surface area of the working electrode, where the geometry of the system ensures parallelism of the working electrode and the PTFE membrane closing the space of the counter electrode and their positioning opposite each other, and the distance of the working electrode from the PTFE membrane is 0.2-2.0 cm, preferably 0.5 cm, and the distance between the working electrode and the counter electrode is 0.5-2.5 cm, preferably 1 cm, the solvent is dry acetonitrile with a water content below 10 ppm, stored in an inert, anhydrous and oxygen-free atmosphere, and a flow of dry protective gas is used, preferably argon with a purity higher than 5.0, passed through a scrubber filled with dry acetonitrile with a water content below 10 ppm, and the supporting electrolyte is tetrabutylammonium hexafluorophosphate at a concentration of 0.1 mol / l in dry acetonitrile, wherein the initial concentration of naphthalene in acetonitrile is 1-10 mmol / l, preferably 6 mmol / l, and a potential is imposed in the range of 1.3-2.0 V vs RE, preferably 1.6 V vs RE, and then the oxidation is carried out under potentiostatic conditions by passing a charge Q = 1-20 C / cm2 of the working electrode surface, preferably 5 C / cm2, at room temperature under normal pressure for 20-400 minutes, preferably for 100 minutes, as a result of which a smooth and tight coating containing polycyclic aromatic hydrocarbons is obtained on the surface of the working electrode, which coating adheres tightly to the electrode surface and has a thickness in the range of about 10-100 nm, preferably about 30 nm, conducts electric current and has an energy gap in the range of 2.0-2.5 eV, and in the solution polycyclic aromatic hydrocarbons are obtained in a molecular form with masses reaching about 2400 Da, composed of phenyl fragments with a mass of 74 Da, which are obtained in a powder form by evaporating the solvent, and exhibit an energy gap in the range of 2.0-2.5 eV. Coating containing polycyclic aromatic hydrocarbons is characterised in that it is deposited on a platinum surface, it is smooth and continuous, has a thickness in the range of 10-100 nm, preferably about 30 nm, conducts electric current and has an energy gap in the range of 2.0- 2.5 eV. Polycyclic aromatic hydrocarbons in a molecular form are characterised that they have a mass reaching about 2400 Da and are composed of phenyl fragments with a mass of 74 Da, exhibit solubility in dry acetonitrile, and have an energy gap in the range of 2.0-2.5 eV. Polycyclic aromatic hydrocarbons in the form of smooth conducting coatings, either solution or powder, are prepared under normal conditions, without the need to use high temperatures, reduced pressure or strong oxidants, which significantly reduces production costs and is environmentally friendly.

Description

[0001] Method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a thin coating, conducting thin coating containing polycyclic aromatic hydrocarbons and solution of polycyclic aromatic hydrocarbons in a molecular form

[0002] The subject of the invention is a method of preparing polycyclic aromatic hydrocarbons in the form of a coating on an electrode and in a molecular form in solution, as well as polycyclic aromatic hydrocarbons prepared by this method.

[0003] Polycyclic aromatic hydrocarbons (PAHs) are planar materials in which carbon atoms occur in sp2hybridization and can also be classified as nanographene materials. However, unlike graphene that has a semimetallic character, the energy gap of PAHs depends on the size of their molecules [1], Due to their unique semiconducting properties, PAHs are considered for use in optoelectronic and electronic devices, such as photovoltaic cells, transistors and lightemitting diodes [2-5],

[0004] Direct synthesis of substituted molecular PAHs is known, which requires the presence of strong Lewis acids, acidic environment and temperatures of the order of 150°C [6], There is also a known method of synthesis of the molecular PAHs under drastic conditions in the presence of strong oxidants [7,8], However, these methods are cumbersome due to the necessity of operating at high temperatures using strong oxidants. Diels-Alder type reaction protocols are also known that in combination with subsequent aromatization reactions using CuCh / AICh and 2,3-dichloro-4,5-dicyanoquinone (DDQ) can lead to the production of the PAHs in a molecular form [9,10], However, these procedures are complicated and include many stages, which is associated with nuisance and low overall yields. There are also known methods for the synthesis of the PAH molecules based on intramolecular photocyclization of stilbene-type compounds [11,12] or olefin metathesis with ring closure using transition metals as reaction catalysts [13,14] which allow for the synthesis of the PAHs in a molecular form under relatively mild conditions, however are cumbersome due to the need to use expensive and complicated reaction precursors.

[0005] Considering the possible applications related to the electrical properties of these materials, the formation of thin PAH films is the most advantageous. Thin films of large PAHs or graphene sheets can be formed on the surfaces of some transition metals, such as copper or nickel, from methane, toluene or small PAH units, such as coronene (molecular weight = 300 Da). There is a known method of producing thin PAH films by chemical vapor deposition (e.g. deposition in ultra-high vacuum), however these methods require extremely low pressures and / or high temperatures, reaching 1000 K [15,16], There is also a known method consisting in deposition of PAHs from solution, however the poor solubility of PAHs makes it necessary to functionalize the aromatic rings with alkali groups in order to improve the solubility of PAHs [17,18],

[0006] The use of electrochemical techniques is known as an alternative to the use of strong oxidants, extreme pressures and temperatures to create new C-C bonds in aromatic compounds. The advantage of electrochemical methods is the possibility of precise control of electrosynthesis conditions (charge, time, current and potential) and the possibility of directly preparing coatings on the electrode surface.

[0007] Condensation reactions of naphthalene rings are known, leading to obtaining 1,4- polynaphthalene in a powder form, which is not a PAH-type compound [20-23], Electrooxidation of naphthalene is also known [24-26], which leads to obtaining granular deposits on electrodes with a poorly defined or insufficiently studied structure. Studies conducted in the literature were carried out in naphthalene solutions at a concentration of the order of 0.5 mol / l, using both platinum and glassy carbon electrodes, in solvents such as acetonitrile or nitrobenzene, or dichloroethane. The oxidation reaction of a fluoranthene derivative is also known, which leads to the formation of two C-C bonds between two naphthalene monomers with the expansion of the conjugated multiple bond system to the perylene system

[0027] ,

[0008] DC deposition of PAH coatings using polyphenyl precursors on ITO electrodes made of dichloromethane or acetonitrile at a potential of 1.65 V against the silver electrode is known

[0019] , However, this method requires the use of complicated polyphenyl precursors, for example, (1,3,5-tris(2'-(4"-phenyl)biphenyl)benzene), (3',3"',4',4"',5',5"',6',6"'-octaphenyl- 1 ,1 ':2',1 ":4",1 "':2"',1 ""-quinquephenyl), (5"-(3',6'-diphenyl-[1 ,1 ':2’,1 "-terphenyl]-4'-yl)-

[0009] 3"',4',4"',5',5"',6'-hexaphenyl-1,T:2',1":3",T":2"',1""-quinquephenyl), the synthesis of which is in itself very laborious and complicated. There is an unmet need for a new, single-stage, simple and low-cost method of preparing polycyclic aromatic hydrocarbons, both as a coating on electrode materials and in a molecular form in solution.

[0010] Summary of the Invention

[0011] Method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a coating, using aromatic precursors and the process of their oxidation is characterised in that the electrochemical oxidation of naphthalene is carried out in a three-electrode system, where: the working electrode is a platinum electrode with a surface of 0.5-5 cm2, preferably 2 cm2, the reference electrode is a silver electrode Ag|0.001M Ag+, (C4H9)4N+, PF6~II with a potential of 0.197 V against ferrocene, and the counter electrode is a platinum mesh located in a separate electrode space filled with a supporting electrolyte and closed with a porous PTFE electrolytic key, with a surface area corresponding to half the surface area of the working electrode, where the geometry of the system ensures parallelism of the working electrode and the PTFE membrane closing the space of the counter electrode and their positioning opposite each other, and the distance of the working electrode from the PTFE membrane is 0.2-2.0 cm, preferably 0.5 cm, and the distance between the working electrode and the counter electrode is 0.5-2.5 cm, preferably 1 cm, the solvent is dry acetonitrile with a water content below 10 ppm, stored in an inert, anhydrous and oxygen-free atmosphere, and a flow of dry protective gas is used, preferably argon with a purity higher than 5.0, passed through a scrubber filled with dry acetonitrile with a water content below 10 ppm, the supporting electrolyte is tetrabutylammonium hexafluorophosphate at a concentration of 0.1 mol / l in dry acetonitrile, wherein the initial concentration of naphthalene in acetonitrile is 1-10 mmol / l, preferably 6 mmol / l, a potential in the range of 1.3-2.0 V vs RE , preferably 1.6 V vs RE, is imposed, and then the oxidation is carried out under potentiostatic conditions by passing a charge Q = 1-20 C / cm2of the working electrode surface, preferably 5 C / cm2, at room temperature under normal pressure for 20-400 minutes, preferably for 100 minutes, as a result of which a smooth and tight coating containing polycyclic aromatic hydrocarbons is obtained on the surface of the working electrode, which coating adheres tightly to the electrode surface and has a thickness in the range of about 10-100 nm, preferably about 30 nm, conducts electric current and has an energy gap in the range of 2.0-2.5 eV, and in the solution polycyclic aromatic hydrocarbons are obtained in a molecular form with masses reaching about 2400 Da, composed of phenyl fragments with a mass of 74 Da which are obtained in a powder form by evaporating the solvent, and exhibit an energy gap in the range of 2.0-2.5 eV.

[0012] According to the invention, acetonitrile dried and purified according to the following procedure conducted in a protective atmosphere of argon with a purity not less than 5.0 is used, where the subsequent stages provide for the following: 24 hours of pre-drying using 3A molecular sieves to remove water, 6 hours of ozonolytic purification to remove unsaturated hydrocarbons, 24-hour argon purging to eliminate HCN, purification by adsorption of ozonolysis products and other impurities with dry AI2O3, simple distillation to remove higher- boiling impurities remaining after ozonolysis, distillation from above P2O5 to remove residual water, distillation from above CaH2 to remove traces of water, wherein the dry (anhydrous) acetonitrile thus prepared has a water content below 10 ppm and is stored in a protective atmosphere of argon with a purity of not less than 5.0, under anhydrous and oxygen-free conditions.

[0013] A coating containing polycyclic aromatic hydrocarbons, characterised in that it is deposited on a platinum surface, it is smooth and continuous, has a thickness in the range of 10-100 nm, preferably about 30 nm, conducts electric current and has an energy gap in the range of 2.0- 2.5 eV.

[0014] Polycyclic aromatic hydrocarbons in a molecular form, characterised in that they have a mass reaching about 2400 Da and are composed of phenyl fragments of a mass 74 Da, exhibit solubility in dry acetonitrile, and have an energy gap in the range of 2.0-2.5 eV.

[0015] Polycyclic aromatic hydrocarbons, in the form of smooth conductive coatings, either solution or powder, are prepared under normal conditions, without the need to use high temperatures, reduced pressure or strong oxidants, which significantly reduces production costs and is environmentally friendly. The method according to the invention makes it possible to prepare a very good quality PAH coating directly on an electrode in a single-stage process, which in the context of the use of this semiconductor material in electronics is a key advantage. The method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a coating, the conductive coating containing polycyclic aromatic hydrocarbons and the solution of polycyclic aromatic hydrocarbons in a molecular form are described in detail below, with reference to the attached drawing, in which

[0016] Fig. 1 shows five voltammetric cycles (CV) of a 6 mmol / l naphthalene solution in dry acetonitrile with a water content below 10 ppm (seven-stage purified) in a protective atmosphere of argon 5.0, with 0.1 M TBAPF& as the supporting electrolyte, in a three- electrode system (WE: 2 cm2Pt sheet, CE: Pt mesh, RE: Ag|Ag+) recorded in the range of 0-2.3 V vs. RE (Ag|0.001M Ag+, (C4H9)4N+, PFs Il; 0.197 V against ferrocene), with a visible PAH deposition wave in the range of 1.3-2.0 V;

[0017] Figure 2 shows the chronoamperometric (CP) curves of PAH formation from a 6 mmol / l naphthalene solution in dry acetonitrile with a water content below 10 ppm (seven-stage purified), in a protective atmosphere of argon 5.0, with 0.1 M TBAPF6 as the supporting electrolyte, in a three-electrode system (WE: 2 cm2Pt sheet, CE: Pt mesh, RE: Ag|Ag+), recorded at a potential of 1.6 V (blue curve) and 1.9 (red curve) versus RE (Ag|0.001 M Ag+, (C4H9)4N+, PFt'H; 0.197 V against ferrocene), with a deposition charge Q = 10 C (5 C / cm2WE);

[0018] Fig. 3 shows a diagram of an exemplary electrochemical system for the production of the PAHs in a molecular form and in the form of a coating on a working electrode, with a geometry ensuring operation under linear diffusion conditions, consisting of the working electrode (WE) in the form of a platinum plate, the reference electrode (RE) in the form of a silver electrode Ag|0.001 M Ag+, (C4H9)4N+, PF6-II (0.197 V against ferrocene), the counter electrode (CE) in the form of a Pt mesh, with a porous electrolytic key made of PTFE;

[0019] Fig. 4 shows SEM photographs of

[0020] A. the platinum working electrode covered with smooth and continuous WWE film, prepared from 6 mmol / l naphthalene solution in dry acetonitrile with water content below 10 ppm (seven-stage purified), in a protective atmosphere of argon 5.0, with 0.1 M TBAPF& as the supporting electrolyte, in three-electrode system (WE: Pt sheet 2 cm2, CE: Pt mesh, RE: Ag|Ag+), at a potential of 1.6 V vs. RE (Ag|0.001 M Ag+, C4H9)4N+, PFfe ll; 0.197 V against ferrocene), with a charge of 3 C (1.5 C / cm2); B. cross-section through the platinum working electrode covered with a smooth and continuous WWE film, shown in Fig. 4.A, where the coating thickness of about 30 nm is visible;

[0021] Fig. 5 shows a set of voltammetric (CV) curves of the platinum working electrode coated with the PAH coating prepared from a 1 mmol / l naphthalene solution in dry acetonitrile with water content below 10 ppm (seven-stage purified) in a protective atmosphere of argon 5.0, with 0.1 M TBAPF& as the supporting electrolyte, in a three- electrode system (WE: 2 cm2Pt sheet, CE: Pt mesh, RE: Ag|Ag+), prepared at a potential of 1.4 V vs. RE (Ag|0.001 M Ag+, (C4H9)4N+, PFfe ll; 0.197 V against ferrocene), deposition charge Q = 1.3 C (0.65 C / cm2WE), where a system of quasi-reversible oxidation-reduction processes characteristic of the PAHs with conjugated phenyl rings is visible, and the reference spectrum of the clean electrode working in the supporting electrolyte is shown in black;

[0022] Fig. 6 shows a set of voltammetric (CV) curves of the platinum electrode in dry acetonitrile with water content below 10 ppm (seven-stage purified) in a protective atmosphere of argon 5.0, with 0.1 M TBAPF& as the supporting electrolyte, with perylene added at a concentration of 1 mmol / l, in a three-electrode system (WE: 2 cm2Pt sheet, CE: Pt mesh, RE: Ag|Ag+), recorded in the range of 0-1.3 V vs. RE (Ag|0.001 M Ag+, (C4H9)4N+, PFfe ll; 0.197 V against ferrocene), where a system of quasi- reversible oxidation-reduction processes characteristic of perylene is visible;

[0023] Fig. 7 shows the APCI MS spectrum (in three m / z ranges: 150-600, 650-1800, 1500- 2500) of a dry acetonitrile solution with water content below 10 ppm (seven-stage purified) sampled after the deposition of the PAH coatings on the platinum working electrode from a 6 mmol / l naphthalene solution in this solution, in a protective atmosphere of argon 5.0, with 0.1 M TBAPF& as the supporting electrolyte, in a three- electrode system (WE: 2 cm2Pt sheet, CE: Pt mesh, RE: Ag|Ag+), at a potential of 1.9 V versus RE (Ag|0.001 M Ag+,C4H9)4N+,PF6-ll; 0.197 V against ferrocene), with a charge Q = 10 C (5 C / cm2), where the fragmentation with a mass progression of 74 Da is visible, characteristic of phenyl fragments in the nanographene structure

[0031] ; Fig. 8 shows the APCI MS spectrum (in two m / z ranges: 125-600, 600-1200) of a dry acetonitrile solution with water content below 10 ppm (seven-stage purified) with water added, sampled after the attempt to deposit the PAH coating on the platinum working electrode from a 10 mmol / l naphthalene solution in this solution, in a protective atmosphere of argon 5.0, with 0.1 M TBAPF& as the supporting electrolyte, in a three-electrode system (WE: 2 cm2Pt sheet, CE: Pt mesh, RE: Ag|Ag+), at a potential of 1.6 V versus RE (Ag|0.001M Ag+,C4H9)4N+,PF6-ll; 0.197 V against ferrocene), with a charge of 10 C (5 C / cm2), where the fragmentation is visible with a mass progression of 16 Da and 126 Da, characteristic of OH- groups and naphthalene fragments.

[0024] Detailed description of the invention

[0025] The invention relates to an electrochemical method of preparing polycyclic aromatic hydrocarbons in the form of a coating on a platinum working electrode and in a molecular form in solution or in a powder form after separation from this solution. The method, according to the invention, is unique since in a single-stage reaction it leads to obtaining a smooth, continuous and conductive coating containing the PAHs, exhibiting nanographene properties, without the need to use extreme conditions such as high temperatures, ultra-low pressures or strong oxidants. For the first time, the present invention describes the simultaneous preparing PAHs in the electrooxidation process of naphthalene both in the form of thin coatings on the electrode and as molecular products in solution.

[0026] Method of preparing polycyclic aromatic hydrocarbons

[0027] According to the invention, the PAHs are prepared electrochemically by electrooxidation of naphthalene on a platinum electrode. The process is carried out in a three-electrode system, where the working electrode (WE) is a platinum electrode (e.g. a platinum sheet or a plate covered with a platinum coating), the reference electrode (RE) is a silver electrode, and the counter electrode (CE) is a platinum mesh.

[0028] The platinum electrode with a surface of 0.5-5 cm2, preferably 2 cm2, is used as the working electrode (the specified surface area corresponds to the surface of two sides of a sheet, i.e. a square platinum sheet with a side of 1 cm is preferably used). According to the invention, it is possible to use smaller and larger platinum electrodes, however the sizes presented here are optimal for laboratory operations.

[0029] A silver electrode with an internal electrolyte containing a supporting electrolyte in acetonitrile used as a reaction medium (Ag|0,001 M Ag+, (C4H9)4N+, PFs’ll) is used as a reference electrode. The reference electrode has a constant potential of 0.197 V determined against ferrocene. According to the invention, it is also possible to use another reference electrode, exhibiting a stable potential in acetonitrile.

[0030] A standard platinum mesh is used as the counter electrode, placed in a separate cathode space filled with an acetonitrile solution of the supporting electrolyte. According to the invention, it is also possible to use other counter electrodes, the material of which is chemically inert under the conditions of the experiment, for example, electrodes made of glassy carbon. The cathode space is separated by a porous membrane made of PTFE in the form of a flat membrane, usually circular in shape (or close to circular), for example, with a surface area corresponding to half the surface of the working electrode.

[0031] The supporting electrolyte is tetrabutylammonium hexafluorophosphate at a concentration of 0.1 mol / l in acetonitrile. The supporting electrolyte was selected to stabilize the naphthalene cation radical that is an intermediate stage in the electrooxidation process of naphthalene. It is possible to use another supporting electrolyte, provided that it also stabilizes the naphthalene cation radical.

[0032] The geometry of the electrochemical system ensures parallelism of the working electrode and the PTFE membrane, closing the space of the counter electrode, and ensures their positioning opposite each other, which enables uniform distribution of the electric field lines and enables uniform growth of the PAH film. The distance of the working electrode from the PTFE membrane is 0.2-2.0 cm, preferably 0.5 cm, and the distance between the working electrode and the counter electrode is 0.5-2.5 cm, preferably 1 cm. It should be noted that the PAH coating grows in the manner described in the present invention only from the side facing the PTFE membrane and the counter electrode. The coating also grows on the other side of the working electrode, but not as uniformly and at a slower rate. According to the invention, it is also possible to uniformly deposit coatings on both sides of the working electrode, using two counter electrodes, one on each side of the electrode. Dry acetonitrile with a water content below 10 ppm is used as the solvent. The absence of water and the high purity of the solvent are essential for the possibility of conducting the process, as even small amounts of water in the system cause the generation of hydroxyl radicals under electrooxidation conditions (instead of naphthalene cation radicals, which means that the naphthalene coupling process does not take place and nanographene is not formed (see comparison of Fig. 7 and Fig. 8). Moreover, hydroxyl cation radicals react with naphthalene molecules, thereby building themselves into the structure of the emerging polymer, changing its properties. The electrooxidation process in the presence of water leads to the preparation of a material that fragments in the APCI MS chamber with the cleavage of naphthalene groups (mass 126 Da) or hydroxyl groups (mass 17 Da), while the material prepared from dry acetonitrile exhibits masses reaching 2400 Da and fragments in the APCI MS chamber with a cleavage of phenyl groups (mass 74 Da), which is characteristic of nanographene.

[0033] In order to obtain satisfactory results of electrooxidation of naphthalene, according to the invention, a specially developed method of purification and drying of acetonitrile should be used. According to the invention, acetonitrile is dried and purified according to the following seven-stage procedure that is carried out in a protective atmosphere of argon with a purity of not less than 5.0:

[0034] - the first stage is a 24-hour pre-drying of acetonitrile using 3A molecular sieves (dried at 200°C) to roughly remove most of the water;

[0035] - the second stage consists in a 6-hour purification of acetonitrile by its ozonolysis method to remove unsaturated hydrocarbons that would otherwise be difficult to separate in a distillation process due to similar boiling point to that of acetonitrile;

[0036] - the third stage is a 24-hour argon purging (rinsing) to remove HCN dissolved in acetonitrile;

[0037] -; the fourth step is to purify the acetonitrile of ozonolysis products and other impurities by adsorbing them onto dry AI2O3.

[0038] - the fifth step is a simple distillation of acetonitrile to remove the higher boiling impurities remaining after ozonolysis.; - the sixth stage consists in distilling acetonitrile from above P2O5 to remove any remaining water;

[0039] - the seventh stage is the distillation of acetonitrile from above CaHi to remove any traces of water.

[0040] The dry (anhydrous) acetonitrile prepared in this way is characterised by a water content below 10 ppm (water content determined by the Karl Fisher method). In order to maintain its properties, the dry acetonitrile thus prepared should be stored in a protective atmosphere of argon with a purity of not less than 5.0, under anhydrous and oxygen-free conditions, preferably in a glove box (O2 < 0.1 ppm; H2O < 0.1 ppm). It is extremely important that commercially available, nominally anhydrous acetonitrile (even HPLC grade) does not allow obtaining PAHs exhibiting nanographene features. According to the invention, in order to prepare the PAHs with nanographene properties, it is necessary to use acetonitrile subjected to the purification, as described above.

[0041] During electrooxidation, a dry protective gas flow is used, preferably argon with a purity of not less than 5.0. To prevent the acetonitrile from evaporating and the reactor from drying out, the protective gas is passed through a scrubber filled with the dry acetonitrile with a water content below 10 ppm, of the same purity class as the solvent used in the reaction.

[0042] According to the invention, the initial naphthalene concentration in acetonitrile is 1-10 mmol / l, preferably 6 mmol / l. This value is ten times lower than the concentrations of reagents used in similar processes in the state of the art. Such a low concentration is crucial for preparation of smooth, uniform and continuous PAH coatings with nanographene properties. In the state of the art, to achieve satisfactory synthesis effects, precursor concentrations at a level of 10 times higher are used. There have been a consensus that the use of precursor concentrations of 50-100 mmol / l (and higher) was necessary to achieve the condensation to PAHs. To date, there have been no reports of attempts to electrooxidize naphthalene at concentrations as low as 1-10 mmol / l, so there is no prior art in this field.

[0043] The electrooxidation process of the naphthalene, according to the invention, is carried out under constant potential conditions, imposing a potential in the range of 1.3-2.0 V, preferably 1.6 V. At lower potential values, no oxidation of the naphthalene is observed on the working electrode, while for higher potentials, decomposition of the supporting electrolyte is observed (see Fig. 1). What is extremely important, during the cyclic voltammetry experiment, an increase in current is observed in the subsequent cycles, in the region of the naphthalene electrooxidation wave, which indicates the formation of a coating of the conductive material, i.e. PAHs, on the surface of the working electrode. It is also possible to carry out the electrooxidation of the naphthalene by cyclic voltammetry, provided that the cycles cover the potential range of 1.3-2.0 V, preferably 1.6 V, omitting the PAH decomposition potential. However, it is much more advantageous to carry out the synthesis of PAHs under chronopotentiometric conditions.

[0044] Electrooxidation is carried out under constant potential conditions by passing a charge of 1- 20 C / cm2per the working electrode surface, preferably 5 C / cm2. During the PAH coating deposition experiments, it was observed that the coating grows smoothly and evenly over the entire surface of the working electrode (facing the counter electrode). Good quality coatings were prepared in the entire range of 1-20 C / cm2, i.e. the coatings were smooth, continuous and adhered well to the substrate even in vacuum conditions. Nevertheless, a deposition charge value of 5 C / cm2seems to be optimal, as it provides coatings thick enough to be clearly visible both to the naked eye (the change in the colour of the electrode) and under an electron microscope, and at the same time, the layer is thin enough to avoid peeling off. The process is carried out at room temperature under normal pressure.

[0045] The electrooxidation is carried out for 20-400 minutes, preferably for 100 minutes. The deposition time in the chronopotentiometric process is the result of the assumed fixed deposition charge and the surface of the working electrode used.

[0046] The method according to the invention allows for the production of PAHs both in the form of a coating on a working electrode and in a molecular form in solution.

[0047] The smooth and tight coating containing the PAHs, produced under the experimental conditions (passing the favourable charge of 5 C / cm2) has a thickness of about 30 nm (not greater than 35 nm, see Fig. 4B). Additionally, the coating conducts electric current and has an energy gap in the range of 2.0-2.5 eV. The coating does not block the surface of the electrode that can still be used electrochemically. However, it is possible to prepare coatings of a different thickness using other process parameters. For example, deposition with a lower charge leads to obtaining thinner coatings, while deposition with a higher charge leads to obtaining thicker coatings.

[0048] The PAHs in a molecular form are prepared simultaneously with the coating that covers the surface of the working electrode and it is assumed that the PAHs with a similar mass range are prepared both in solution and in the form of a coating. The PAHs produced in solution can be prepared in a powder form by evaporating the solvent. In solution, the PAHs composed of phenyl fragments with a mass of 74 Da are obtained. In the APCI MS chamber, signals with a mass progression of 74 Da are observed in the range from about 741 m / z to about 2371 m / z (see Fig. 7), with a broad population maximum in the range of about 889-1483 m / z. This means that there are PAH molecules in the solution which contain from 10 to 32 phenyl units, wherein the most numerous are the PAHs with molecules counting from 12 to 20 phenyl units. Based on the known dependence of the energy gap size on the number of phenyl fragments in a PAH molecule [1], it can be estimated that the material prepared by the method according to the invention, composed of the PAHs containing from 10 to 32 phenyl fragments, is characterised by an energy gap in the range of 2.0-2.5 eV. The value of 2.5 eV corresponds to 10 phenyl units (60 n electrons) in the paper by Rieger and Mullen (fig.7) [1], and for 32 phenyl units (192 n electrons) the value of 2.0 eV was assumed, being the asymptotic (fig.7. ) value in the pointed publication[1]. The energy gap value of a specific coating or sample of the prepared PAHs depends on the specific population size distribution of the obtained mixture of PAH molecules and is usually an average value of this range.

[0049] The method of preparing PAHs in the form of smooth conductive coatings, either solution or powder, according to the invention, is obtained under normal conditions, without the need to use high temperatures, reduced pressure or strong oxidants, which significantly reduces production costs and is environmentally friendly. In addition, the presented electrochemical method allows for precise control of the synthesis conditions (potential, current, charge), which allows for minimizing the amount of generated waste in comparison with other PAH synthesis methods. Moreover, the method according to the invention makes it possible to obtain a very good quality PAH coating directly on an electrode in a single-stage process, which in the context of the use of this semiconductor material in electronics is a key advantage. Polycyclic aromatic hydrocarbon conductive coating

[0050] The coating containing the PAHs, prepared by the method of the invention described above, covers the platinum electrode (for example, a platinum sheet), used in the synthesis process as a working electrode. Under the experimental conditions (anhydrous acetonitrile, 0.1 mol / l (C4H9)4NPF6, 1-10 mmol / l naphthalene, WE: Pt sheet, CE: Pt mesh, RE: Ag|Ag++0.197 V against ferrocene, deposition potential of 1.3-2.0 V, deposition charge of 0.5-5 C / cm2WE), a smooth and continuous coating of about 30 nm thickness (not greater than 35 nm, see Fig. 4B) is formed, which does not block the electrode surface and allows for electrochemical use of the PAH-coated electrode. The coating conducts electric current and has an energy gap in the range of 2.0-2.5 eV.

[0051] The PAH coating covering the electrode surface, similarly to the PAHs in a molecular form found in solution, is composed of conjugated phenyl fragments exhibiting nanographene characteristics. This is evidenced by the results of the voltammetric measurements performed in a pure supporting electrolyte for the PAH-coated electrode, where a system of quasi- reversible oxidation-reduction peaks of the coating can be observed, characteristic of materials such as fullerene (see Fig. 5)

[0028] , To confirm this hypothesis, the control CV measurements were performed using a clean platinum electrode and a pure stock solution (anhydrous acetonitrile, 0.1 mol / l (C4H9)4NPF6) with the addition of perylene (PAH with a conjugated phenyl ring system), which gave qualitatively identical results, i.e. a quasi-reversible oxidationreduction peak system of perylene was obtained (see Fig. 6).

[0052] It is assumed that the PAHs deposited as a coating on a working electrode and those formed in solution in a molecular form, have the same mass range. Based on the known dependence of the energy gap size on the number of phenyl fragments in PAH molecules, it can therefore be estimated that the coatings prepared in the present invention, composed of the PAHs containing from 10 to 32 phenyl fragments, are characterised by an energy gap in the range of 2.0-2.5 eV. The value of 2.5 eV corresponds to 10 phenyl units (60 n electrons) in graph 7 in publication [1], while 32 phenyl units (192 n electrons) were assumed to be 2.0 eV, which is the asymptotic value in graph 7 in publication [1], The energy gap value of a specific coating composed of PAHs depends on the specific population size distribution of the obtained mixture of PAH molecules and is an average value of this range. The electrode coated with a PAH coating is a modified electrode and can be treated as a new electrode material and can be used in many reactions, such as, for example, electroreduction reaction of CO2.

[0053] Polycyclic aromatic hydrocarbons in a molecular form

[0054] The PAHs in a molecular form, prepared simultaneously with the deposition of the PAH coating, covering the surface of the working electrode, can be used in solution or isolated in a powder form after stripping off the solvent.

[0055] The PAHs, according to the invention, are composed of conjugated phenyl groups, forming systems with nanographene properties. In the APCI MS spectra of the PAH solution sampled after the coating deposition under the above-described experimental conditions (anhydrous acetonitrile, 0.1 mol / l (C4H9)4NPF6, 1-10 mmol / l naphthalene, WE: Pt sheet, CE: Pt mesh, RE: Ag|Ag++0.197 V against ferrocene, deposition potential of 1.3-2.0 V, deposition charge of 0.5- 5 C / cm2WE) a set of peaks separated by 74 Da, is visible, which confirms the fact that the molecular structure of the PAHs consists of conjugated phenyl units, as in the nanographene structure

[0029] (see Fig. 7). The PAHs have a mass reaching about 2400 Da, which corresponds to 32 phenyl units in the molecular structure. For comparison (see Fig. 8), the sample containing aromatic hydrocarbons not exhibiting nanographene features, obtained from a solution contaminated with water, exhibits the presence (mass differences between the MS spectrum peaks) of hydroxyl groups (17 Da) and naphthalene groups (126 Da).

[0056] The PAHs in a molecular form, prepared in anhydrous acetonitrile solution, as mentioned above, are composed of phenyl fragments with a mass of 74 Da. In the APCI MS chamber, signals with a separation of 74 Da are observed in the range from about 741 m / z to about 2371 m / z (see Fig. 7), with a broad population maximum in the range of about 889-1483 m / z. This means that there are the PAH molecules in the solution which contain from 10 to 32 phenyl units, wherein the most numerous are the PAHs with molecules counting from 12 to 20 phenyl units. Based on the known dependence of the energy gap size on the number of phenyl fragments in a PAH molecule, it can be estimated that the material prepared by the method according to the invention, composed of the PAHs containing from 10 to 32 phenyl fragments, is characterised by an energy gap in the range of 2.0-2.5 eV. The value of 2.5 eV corresponds to 10 phenyl units (60 n electrons) in graph 7 In publication by Rieger and Mullen [1], and 32 phenyl units (192 n electrons) were assumed to be 2.0 eV, which is the asymptotic value in graph 7 in publication [1], The energy gap value of a specific sample of the prepared PAH depends on the specific population size distribution of the obtained mixture of PAH molecules and is usually an average value of this range.

[0057] A PAH solution in acetonitrile (for example, a coronene solution) can be used as a reference material in MS measurements. The PAHs in a powder form can be used as a semiconductor material.

[0058] The method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a coating, the conductive coating containing polycyclic aromatic hydrocarbons and the solution of polycyclic aromatic hydrocarbons in a molecular form, are described in detail below in the embodiments.

[0059] Example 1. (preparation of anhydrous acetonitrile) Laboratory glassware was cleaned in fresh Caro's acid and then thoroughly rinsed with distilled water and dried under vacuum at 200°C for 1 hour. A seven-stage drying and purification of a commercially available acetonitrile solution was carried out in a protective atmosphere of argon with a purity not less than 5.0. In the first stage, a 24-hour pre-drying of the acetonitrile was carried out using 3A molecular sieves (dried at 200°C) to roughly remove most of the water. This was followed by a 6-hour ozonolysis to remove unsaturated hydrocarbons from the acetonitrile, which would otherwise be difficult to separate by distillation due to their similar boiling point to that of acetonitrile. After the ozonolysis was completed, the prepared solution was subjected to a 24-hour purging (rinsing) with argon 5.0 to remove HCN dissolved in the acetonitrile. Then, the acetonitrile was then purified from ozonolysis products and other impurities by adsorbing the impurities on dry AI2O3, which was added to the acetonitryle. Then, a simple distillation of the acetonitrile was carried out to remove higher-boiling impurities remaining after the ozonolysis. Finally, two distillations were carried out to remove traces of water from the acetonitrile: first distillation from above P2O5, and then the distillation from above CaHi was performed. The acetonitrile thus prepared was then determined for the presence of water using the Karl Fisher method and the result was obtained below 10 ppm H2O. In order to maintain the properties (purity and low water content), the acetonitrile thus prepared should be stored in a protective atmosphere of argon with a purity of not less than 5.0, inside a glove box with a water-free and oxygen-free atmosphere (O2 < 0.1 ppm; H2O < 0.1 ppm). Example 2. (determination of PAH formation potential) A 50 ml three-necked flask (reactor) was cleaned in fresh Caro's acid and then thoroughly rinsed with distilled water and dried under vacuum at 200°C for 1 hour. The experiment was performed in an electrochemical cell inside a glove box (O2 < 0.1 ppm; H2O < 0.1 ppm) in an argon atmosphere with a purity of 5.0 in a three-electrode system, the diagram of which is shown in Fig. 3. The working electrode (WE) was a platinum electrode in the form of a square sheet with a side equal to 1 cm, annealed in a hydroxide burner flame. The reference electrode (RE) was a silver electrode (Ag|0.001 M Ag+, (C4H 4N+, PFfe ll; 0.197 V against ferrocene), the potential of which was freshly determined against ferrocene. The counter electrode (CE) was a platinum mesh cleaned in Caro's acid, rinsed with water and dried in a vacuum at 200°C. The anode and cathode parts were separated by a PTFE membrane, rinsed several times with fresh portions of the anhydrous acetonitrile. The anhydrous acetonitrile used throughout the experiment was the one purified in the seven-stage procedure described in Example 1. The cathode part with the counter electrode was a tight FEP cylinder filled with the supporting electrolyte, closed at one end with a porous PTFE membrane in the form of a circle with an area of about 1 cm2, parallel and positioned opposite the working electrode at a distance of about 0.5 cm (the distance between WE and CE was about 1 cm). The anode part was the remaining part of the reactor (three-necked flask). In a protective atmosphere of argon 5.0, the 50 ml threenecked flask was tightly secured with silicone septa against the ingress of outside air, and then 10 ml of a 0.1 mol / l tetrabutylammonium hexafluorophosphate (TBAPF&) solution in anhydrous acetonitrile was introduced into the reactor prepared in this way, using a syringe. In order to check the purity of the system, several CV cycles were recorded in a wide range of potentials from -2.5 V to +3.0 V vs. RE (in the case of contamination of the system with water, significant reduction currents are observed in voltammograms in the range of negative potentials). Then a concentrated solution of naphthalene in acetonitrile was added to prepare a naphthalene concentration in the electrochemical cell of 6 mmol / l (the solutions were prepared in a glove box in an argon atmosphere with a purity of 5.0).

[0060] The subsequent voltammetric cycles (CV) of the naphthalene solution prepared in the procedure presented above were recorded. Fig. 2 shows the first 5 CV cycles. A clear wave of naphthalene oxidation (and PAH formation) was observed in the range of 1.3-2.0 V vs. RE with a maximum at 1.6 V vs. RE. It was also observed that in the range of naphthalene oxidation (1.3-2.0 V vs. RE), the current increased in the subsequent cycles, which indicates the formation of a coating of naphthalene electrooxidation products or a PAH containing coating on the electrode. It was also observed that, in the naphthalene oxidation range (1.3-2.0 V vs. RE), the current increased over successive cycles, which indicates the building up of a coating on the electrode from the electro-oxidation products of naphthalene, which is a coating containing PAHs.

[0061] Example 3. (potentiostatic preparation of the PAHs) In the electrochemical system, as in Example 2, the PAHs were produced under potentiostatic conditions at a potential of 1.6 V vs RE and at a potential of 1.9 V vs RE, when imposing a limit on the flowing charge (10 C). The obtained results are shown in Fig. 2. After the reactions, each working electrode was thoroughly rinsed in pure acetonitrile, and the cathode solutions were centrifuged 5 times, using fresh portions of the anhydrous acetonitrile to prepare the product from the solution in the form of a powder without a significant amount of the supporting electrolyte. A colour change of the surface was observed on the electrodes, which indicated the deposition of the PAH coating. After evaporation of the solvent, a solid residue was obtained, that is the PAHs in a molecular form.

[0062] Example 4. (SEM examination of the PAH coating) After the PAH coating deposition was completed at a potential of 1.6 V vs RE, carried out in Example 3, the working electrode was subjected to an examination of the surface morphology under an electron microscope. The obtained SEM images are shown in Fig. 4. A smooth, continuous PAH containing coating was obtained, well adhered to the surface of the working electrode (Fig. 4A). The cross-section through the electrode shows the coating with an average thickness of about 30 nm (readings: 27.05 nm and 32.46 nm; Fig. 4B).

[0063] Example 5. (CV study of the PAH coating) In the electrochemical system as in Example 2, a voltammetric study of the PAH coating, covering the working electrode, prepared as in Example 3, was carried out, with the difference that electrooxidation was carried out at a potential of 1.4 V vs RE, and the total charge was Q = 1.3 C. The results of the experiment are shown in Figure 5. A system of quasi-reversible oxidation-reduction processes characteristic of the PAHs with conjugated phenyl rings with nanographene properties was observed, wherein the voltammetric curve for the clean working electrode in the supporting electrolyte is shown in black. A reference experiment was performed in an identical electrochemical system, consisting in recording voltammetric curves on a clean working electrode in the supporting electrolyte with the addition of perylene at a concentration of 1 mmol / l. The results of the experiment are shown in Figure 6. A system of quasi-reversible oxidation-reduction processes characteristic of perylene was observed, qualitatively similar to the curves recorded for the PAH coatings prepared above. This confirms the occurrence of the PAH compounds in both cases.

[0064] Example 6. (MS study of the PAHs in a molecular form) The PAH solution in the anhydrous acetonitrile, prepared in Example 3 at a potential of 1.9 V vs RE, was subjected to APCI MS analysis. The mass spectrum obtained in three m / z ranges: 150-600, 650-1800 and 1500-2500 has been collected in Fig. 7. The recorded mass spectrum characterised by a sequence of peaks with a mass progression of 74 Da, which is characteristic of the PAHs with the nanographene structure

[0029] , differing structurally by one phenyl ring, the mass of which is 74 Da. Based on the spectrum, it was estimated that the electrooxidation process in Example 3 resulted in the formation of the molecular PAHs with masses ranging from about 741 m / z to about 2371 m / z (see Fig. 7), with a broad population maximum in the range of about 889-1483 m / z. This means that there are PAH molecules in the solution which contain from 10 to 32 phenyl units, wherein the most numerous are the PAHs with molecules counting from 12 to 20 phenyl units. Based on the known dependence of the energy gap size on the number of phenyl fragments in a PAH molecule [1], it was estimated that the material prepared in Example 3 (assuming that there are PAHs with a similar mass distribution in the coating and in the solution), composed of the PAHs containing from 10 to 32 phenyl fragments, has an energy gap in the range of 2.0-2.5 eV. The limiting value of 2.5 eV corresponds to 10 phenyl units (60 n electrons) in graph 7 the paper by Rieger and Mullen [1], and for 32 phenyl units (192 n electrons), a limiting value of 2.0 eV was adopted, which is the asymptotic value in graph 7 in publication [1],

[0065] Example 7. (study of the effect of water on PAH formation) In a system as in Example 2, the PAHs were produced under potentiostatic conditions as in Example 3 at a potential of 1.9 V vs RE, with the difference that the dry acetonitrile prepared in Example 1 was used, to which 200 ppm of water was added. The obtained mass spectrum in two m / z ranges: 125-600 and 600-1200 has been collected in Fig. 8. The recorded mass spectrum is characterised by a sequence of peaks with a mass progression of 16 Da and 126 Da, characteristic of OH- groups and naphthalene fragments, which indicates hydroxylation of the prepared polymers and the lack of conjugation of phenyl rings, and thus the lack of formation of the nanographene structure.

[0066] Example 8. (optimisation of PAH production parameters - potential) In the system as in Example 2, the PAHs were produced under potentiostatic conditions as in Example 3, with the difference that the following potentials were systematically tested: 1.3 V vs RE, 1.4 V vs RE, 1.5 V vs RE, 1.6 V vs RE, 1.7 V vs RE, 1.8 V vs RE, 1.9 V vs RE and 2.0 V vs RE. In all cases, the PAH coating was prepared both on the surface of the platinum working electrode and in the solution. The optimal conditions for coating growth were obtained for the potential of 1.6 V vs RE, corresponding to the maximum of the oxidation wave shown in Fig. 1.

[0067] Example 9. (optimisation of PAH production parameters - charge) In the system such as in Example 2, the PAH production tests were carried out at a constant potential as in Example 3, at a potential of 1.6 V vs RE, with the difference that the electrooxidation charges were systematically tested: 1 C / cm2, 3 C / cm2, 5 C / cm2, 8 C / cm2, 10 C / cm2, 15 C / cm2and 20 C / cm2. In all cases, the PAH coating was prepared both on the surface of the platinum working electrode and in the solution. For the extreme charge values, coatings of thickness of about 10 nm and 100 nm were prepared, respectively. The optimal conditions for coating growth were obtained for a charge of 5 C / cm2, which allows a coating with a thickness of about 30 nm to be deposited, as shown in Fig. 4.

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Claims

Claims1. Method of preparing polycyclic aromatic hydrocarbons in a molecular form and in the form of a coating, using aromatic precursors and the process of their oxidation, characterised in that the electrochemical oxidation of naphthalene is carried out in a three-electrode system, where:- the working electrode is a platinum electrode with a surface of 0.5-5 cm2, preferably 2 cm2,- the reference electrode is a silver electrode Ag|0.001M Ag+, (C4H9)4N+, PFs-!! with a potential of 0.197 V against ferrocene,- the counter electrode is a platinum mesh located in a separate electrode space filled with a supporting electrolyte and closed with a porous PTFE electrolytic key, with a surface area corresponding to half the surface area of the working electrode,- the geometry of the system ensures parallelism of the working electrode and the PTFE membrane closing the space of the counter electrode and their positioning opposite each other, and the distance of the working electrode from the PTFE membrane is 0.2-2.0 cm, preferably 0.5 cm, and the distance between the working electrode and the counter electrode is 0.5-2.5 cm, preferably 1 cm,- the solvent is dry acetonitrile with a water content below 10 ppm, stored in an inert, anhydrous and oxygen-free atmosphere,- a flow of dry protective gas is used, preferably argon with a purity higher than 5.0, passed through a scrubber filled with dry acetonitrile with a water content below 10 ppm,- the supporting electrolyte is tetrabutylammonium hexafluorophosphate at a concentration of 0.1 mol / l in dry acetonitrile,- the initial concentration of naphthalene in acetonitrile is 1-10 mmol / l, preferably 6 mmol / l,- a potential in the range of 1.3-2.0 V vs RE , preferably 1.6 V vs RE, is imposed,- the oxidation is carried out under potentiostatic conditions by passing a charge Q = 1-20 C / cm2of the working electrode surface, preferably 5 C / cm2,- at room temperature under normal pressure for 20-400 minutes, preferably for 100 minutes, as a result of which- a smooth and tight coating containing polycyclic aromatic hydrocarbons is obtained on the surface of the working electrode, which coating adheres tightly to the electrode surface and has a thickness in the range of about 10-100 nm, preferably about 30 nm, conducts electric current and has an energy gap in the range of 2.0-2.5 eV,- and in the solution polycyclic aromatic hydrocarbons are obtained in a molecular form with masses reaching about 2400 Da, composed of phenyl fragments with a mass of 74 Da, which are obtained in a powder form by evaporating the solvent, and exhibit an energy gap in the range of 2.0-2.5 eV.

2. Acetonitrile dried and purified according to the following procedure conducted in a protective atmosphere of argon with a purity not less than 5.0 is used:- 24 hours of pre-drying using 3A molecular sieves to remove water,- 6 hours of ozonolytic purification to remove unsaturated hydrocarbons,- 24-hour argon purging to eliminate HCN,- purification by adsorption of ozonolysis products and other impurities with dry AI2O3,- simple distillation to remove higher-boiling impurities remaining after ozonolysis,- distillation from above P2O5 to remove residual water,- distillation from above CaH2 to remove traces of water, wherein the dry (anhydrous) acetonitrile thus prepared has a water content below 10 ppm and is stored in a protective atmosphere of argon with a purity of not less than 5.0, under oxygen-free conditions.

3. A coating containing polycyclic aromatic hydrocarbons, characterised in that it is deposited on a platinum surface, it is smooth and continuous, has a thickness in the range of 10-100 nm, preferably about 30 nm, conducts electric current and has an energy gap in the range of 2.0-2.5 eV.

4. Polycyclic aromatic hydrocarbons in a molecular form, characterised in that they have a mass reaching about 2400 Da and are composed of phenyl fragments of a mass 74 Da, exhibit solubility in dry acetonitrile, and have an energy gap in the range of 2.0-2.5 eV.

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