Thianthrene-based oligomers as hole transporting materials

The thianthrene–arylene conjugated units have been designed and synthesized via Suzuki or Stille coupling reaction. The structures and properties of the synthesized compounds were characterized by 1 HNMR, 13 CNMR, MS, UV-Vis absorption spectroscopy, fluorescence spectroscopy as well as electrochemical measurements. The luminescent studies demonstrate that thianthrenes are good chromophores. Also the electrical properties of obtained films confirm the applicable potential of these novel aryl-based π-conjugated polymers for the development of various electrical and electrochemical solid-state devices


Introduction
Molecular organic materials with conjugated π-electron systems have found widespread use due to their special optical and electronic properties.Among them, polycyclic aromatic hydrocarbons, such as derivatives of tetracene are frequently applied, for example in organic electronic devices.
In the past, conjugated polymers based on chelating ligands have been prepared, but their solubility is often low.For example, polybipyridinevinylenes, like polybipyridines, are only soluble in formic acid 1 unless metallated or substituted with solubilizing groups. 2 Polyfluorenes are known to emit very efficiently under PL or EL conditions with good hole mobility. 3,4The large band-gap gives efficient blue emission and potential to transfer its energy to dopant emitting molecules with lower band-gaps.The solubilizing groups in poly(9,9-dialkylfluorene) materials lie perpendicular to the conjugated chain, so do not hinder planarity. 5Recently, the versatile Suzuki coupling reaction has been utilized to introduce metal binding groups into fluorene alternating copolymers. 6ur interest in the design of reversible multi-redox systems contains the synthesis of functionalized thianthrenes. 7Thianthrene with π-donor properties is one of the expected central organic unit for the construction of new type of donor systems with structural and redox properties: a/ oxidation of thianthrene to the π radical cation species occurs reversibly, 8 b/ the thianthrene radical cation is thermodynamically stable, c/ thianthrene and oxidized thianthrene units constitute highly ordered arrays with intermolecular interactions involving both π-π stacking and S-S contact. 9hianthrenes in general have attracted strong interest due to the fascinating physical and chemical properties of numerous sophisticated derivatives.The electron-rich, heterocyclic thianthrene seems to be a good example for charge transporting materials, since it has shown a reversible oxidation behavior at low potential in cyclic voltammetry. 10n the present article, we report on the synthesis, spectroscopic and electrochemical properties of new properly functionalized thianthrenes.The properly functionalized thianthrene derivatives (4-7) were assembled from building block 3. The starting compound for obtaining 3 was thianthrene (1), which was lithiated and then unstable compound 2 was subsequently boronated into corresponding derivatives 3, (Scheme 1).Finally, compound 3, under the adapted Suzuki coupling conditions, 11,12 was converted to the desired thianthrene derivatives 4-7 (Scheme 2a) in moderate yields (around 50%).Whereas synthesis of compounds 9-13 was based on Stille coupling 13 between different arylene-derivatives and dibromothianthrene (8) (Scheme 2b).Stannanes were obtained by direct ortho-metalation of suitable arylene groups followed by transmetalation with tributyltin chloride.Dibromothianthrene was synthesized directly from commercially available 1 by bromination in acetic acid.
The electrochemical and optical properties of these compounds were measured with attempts made to correlate to their respective π-electron conjugation lengths as well as their molecular conformations.The most efficient optical materials seem to be the ones with various electron-donor (D) and electron-acceptor (A) moieties attached symmetrically to a conjugate linker.The chemical structures of 3-13 were confirmed by 1 H and 13 C NMR as well as MS and UV spectroscopy for selected compounds.

Synthesis
We have designed a group of monomers containing a thianthrene entity and elaborated short routes for their synthesis (Schemes 1,2).-78 °C -rt, 15h; 54%
All the synthesized compounds except 6c were isolated after the synthesis as oily products.Molecules 9-13 were synthesized by the use of palladium catalyzed Stille-coupling reactions as follows (Scheme 2b).Starting from dibromothianthrene (8), 14 which was condensed with appropriate tributyltin derivative of thiophene, 3,4-ethylenedioxythiophene, furan, oxazole and pyridine, the products 9-13 were obtained. 15The cross coupling reactions were performed in toluene under reflux conditions in the presence of palladium catalyst (Pd(PPh3)4), (Scheme 2b).

Electrochemical properties
Electrochemical investigation of the compounds in dichloromethane has shown that all molecules undergo multi-electron oxidation processes but the initial potentials depend on the molecular structure and were found as 0.93 V for 6a, 1.10 V for 5b, 1.21 V for 4a, and 1.25 V for 7c, respectively as shown in Figure 1 and Table 1.The first oxidation peak of compounds occurs at 1.16 V 6a, 1.47 V 7c, 1.50 V 5b, 1.51 V 4a.Oxidation of all monomers was reversible in the low oxidation potential region (up to first oxidation peak) (Figure 1 insets).When the oxidation potential is much higher than the first oxidation wave, the next peaks are less reversible and probably products of degradation may occur on the electrode.During the successive scans, there are no significant changes in the voltammograms, and lack of growing current suggested absence of an electropolymerization process in the solution as well.
Thianthrene monomer with diphenylamine core (6a) has the lowest oxidation potential equal to 1.16 V, and is responsible for oxidation of diphenylamine unit.Oxidation potential near 1.40 V in all investigated compounds is responsible for oxidation of thianthrene group.Differential pulse voltammetry (DPV) spectra (Figure 2) exibit that oxidation potential peaks are similar for all thianthrene derivatives.Redox processes in the potential range of 1.4 V -1.75 V concerns the first and second oxidation step of thianthrene group.In respect to that, the peak of 6a located at potential 1.10 V is due to the presence of the diphenylamine unit, when carbazole 7c and fluorene 5b group have similar redox peaks at a potential near 1.6 V.The phenothiazine 4a unit has an oxidation potential around 1.75 V because of irregular shape of oxidation peak.
DPV spectra (Figure 2) show absolute peaks of oxidation and reduction processes.In correlation to the molecular band, the oxidation peak is related to the HOMO band and the reduction peak is related to the LUMO band.The electrochemical band-gap value is the range between those peaks (Table 1).Comparing 7a and 7c where there is only difference in alkyl chain (Figure 3), we observe only small deviation between oxidation peaks.From a practical point of view, compound 7c has a longer alkyl chain and has much better solubility.From electrochemical point of view, it is clear that 7c is more stable in redox process than 7a.In CV's of 7a we found lowering current in next cycle what suggests that the electrode surface is blocked.

Optical properties of thianthrene derivatives. Absorption and luminescence characteristic
The photophysical characteristics of thianthrene derivatives (6a, 4a) were investigated by ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) in dilute dichloromethane solution (4x10 -6 mol/L).The optical data are also summarized in Table 2.A plot of extinction coefficient versus wavelength is depicted in Figure 4.
The one-photon absorption (OPA) spectra of 6a and 4a consist of three bands; in case of 6a one high-intensity, high-energy band at 260 nm and a low-energy 352 nm.It was clear that the absorption bands of 4a appeared at 231 (high-energy) nm, 263 nm and 318 (low-energy) nm.For 6a, the absorption bands red-shifted compared with 4a, the bands were located at 260, 320, 352 nm.Except that, much larger extinction coefficient of the absorption bands of 6a rather than those 4a were obtained (Table 2).The OPA spectrum of 4a is little different from those of 6a.Such differences may be caused by a stronger donor core used as the π-bridge in the chromophore 6a.  a One-photon absorption maxima (nm) and coefficients (ε was in unit of dm 3 mol -1 cm -1 ).b Onephoton emission fluorescence maxima (nm).
The one-photon excited fluorescence (OPEF) spectra of 6a and 4a were similar, which both feature an intense luminescence at 452 -504 nm with a shoulder at 384 -412 nm.As shown in Figure 5 and Table 2, both chromophores emit in the blue region, and the emission maximum exhibits a 52-nm red shift from 6a to 4a.The observed red shift is probably because of enlarging of the conjugation in case of 4a.The fluorescent quantum yield was measured with quinine (a) (b) sulfate as the standard (Table 2), and it increased from 6a to 4a.The φf of 4a was very close to phenothiazine-based polymers. 16The di(aryl)thianthrenes are interesting because of their blue emission properties as well as bis(thianthrene)derivatives.The diluted solutions of obtained derivatives (9, 10, 12) have maximum of emission in the region 450 -481 nm (Figure 6).

X-Ray structure determination of bis(thiophene)thianthrene
The structure of single molecule of 9 is depicted in Figure 7 together with the numbering system adopted.The molecule possesses the following stereochemically characteristic features.a/ The fused thianthrene and thiophene rings are almost butterfly-like structures because the thianthrene derivatives are folded along the S-S axis.b/ The thiophene rings are twisted out of the plane of the thianthrene by the dihedral angle of 14.2-20.3o .
Interestingly, molecules of 9, have a geometry with no S … S contacts, where the sulfur atoms of thiophenes are far from each other.The molecular long axis of 9 is perpendicular to the ab plane in the unit cell.The crystal of 9 showed no S … S intermolecular interactions.
We were pleased to find that the crystal growing method yielded a single-crystal structure of 9 stabilized through hydrogen bonding.Crystallographic analysis found the crystal system as orthorhombic with space group P212121.The unit-cell dimensions and other X-ray data are shown in Table 3. From the crystallographic analysis is clear that most of molecules of compound 9 are presented as displayed in Figure 7a.
Since the torsion angles between the thianthrene and two thiophene rings are not large enough to disconnect the π-conjugation through the three rings, the π-electron system apparently denotes the UV absorptions and, therefore, the fluorescence.

Conclusions
New derivatives of thianthrene containing different arylene cores were synthesized according to Suzuki or Stille condensation and their optical, electrochemical properties were studied (Figure 8).The lowest energy absorption edges and the lowest ionization potentials were observed for diphenylamine thianthrene derivatives.The obtained semiconducting units as viable luminance and high hole-transporting one, exhibit good solubility in common organic solvents, thermal stability and luminescence in blue region and can be cast into uniform films.They possess good fluorescence quantum.The band gap values of synthesized compounds are in the adequate range (2.62-3.95eV) for testing as semiconductors.
In the investigation of functional organic electroluminescent materials, the relationship between the organic molecular structure and the optoelectronic property should be always established.Now, the investigation of the organic molecular structure and the optoelectronic function is still at the stage of registering data.
Among the three basic colors, blue, red and green, for full color display, while the efficiency, color purity and life-time of the former two have met the requirements of commercialization, the blue light is still lagging behind.Therefore, we succeeded in establishing a new type of organic hybrid molecules with blue luminescence.Further studies will be aimed at mapping the precise relationship between the doping level and the electrochemical properties of the polymers.

Experimental Section
General.All chemicals, reagents and solvents were used as received from commercial sources without further purification, except toluene and tetrahydrofuran (THF), which was distilled over sodium/benzophenone. 1 H NMR and 13 C NMR spectra were recorded in CDCl3 and DMSO on a Brücker 300 spectrometer.Chemical shifts are denoted in δ unit (ppm) and were referenced to internal tetramethylsilane (0.0 ppm).The splitting patterns are designated as follows: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), quin (quintet), and m (multiplet).Mass spectra were recorded on a Waters GCT Premier spectrometer operating at an ionization potential of 70 eV and on a LCT Premier XE with ESI+ ionization.Preparative column chromatography was carried out on glass columns of different sizes packed with silica gel Merck 60 (0.035-0.070 mm).Absorption spectra were gathered with UV-Vis HP 8452A diode array spectrophotometer.Fluorescence spectra were collected with a Hitachi F-2500 fluorescence spectrophotometer.

Electrochemical measurement
The solutions of the synthesized compounds with the concentration of 1.0 mM were used for cyclic voltamperometry measurements.Electrochemical studies were conducted in 0.1 M solution of Bu4NBF4 (Sigma Aldrich 99%) in anhydrous dichloromethane at room temperature.The electrochemical investigations were carried out using Eco Chemie Company's Autolab potentiostat "PGSTAT20".The results were collected using GPES (General Purpose Electrochemical System) software.The electrochemical cell comprised platinum wire with 1 mm diameter of working area as working electrode, Ag/AgCl reference electrode and platinum coil as auxiliary electrode.Cyclovoltamperometric measurements were conducted at 50 mV/s potential rate.

Figure 3 .
Figure 3.Comparison of a) cyclic voltammetry data and b) DPV of thianthrene-carbazole derivatives with different alkyl chains; scan rate 50 mV/s, Ag/AgCl -reference electrode.

Figure 8 .
Figure 8.General pathway of synthesis and application of thianthrenes.

Table 1 .
Electrochemical band-gap and oxidation potential data

Table 2 .
Photophysical data of 6a and 4a