Synthesis and some properties of three stereoisomers of 1,4-diethoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene

The title electroluminescent macromonomers were synthesized starting from hydroquinone in four following steps: O -ethylation, C -chloromethylation, reaction with triphenylphosphine and Wittig’s condensation of the resulting salt with 2-thienylcarbaldehyde. The product was separated into three stereoisomers. Structures of the stereoisomers were studied by spectroscopy and rentgenography as well as by electrochemical methods.


Introduction
Interesting optical properties, among them electroluminescence, of conjugated macromonomers and polymers of linear structure have recently focused a wide attention to those molecules. 1,2For that type of substances a correlation between the molecular structure and both optical and electrical properties is fundamental, when any applications of these systems are considered in e.g., optoelectronics or molecular electronics. 2,3Macromonomers that have exactly defined structure are excellent model systems that can be used to study basic properties of the corresponding polymers.In this paper we demonstrate a synthesis of three stereoisomers of 1,4diethoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene and an effect of the stereo-structure of the newly synthesized compounds on theirs some properties.Data on electrochemical and optical behavior of the macromonomers synthesized by us and products of theirs electro-polymerization have been disclosed elsewhere. 4It is worth mentioning that in other papers concerning similar compounds the most stable (EE) isomers or mixtures of stereoisomers were only reported and studied.

Results and Discussion
The title macromonomers were synthesized in four steps starting from hydroquinone according to slightly modified known general respective procedures used in each step.(Scheme) Hydroquinone (1) was O-alkylated using diethyl sulfate.The obtained diether 2 was chloromethylated and the reaction product 3 was then treated with triphenylphosphine to produce the corresponding salt 4, which was directly used for the Wittig's reaction 5 with 2thienylcarbaldehyde to afford 1,4-diethoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene (5) as a mixture of 5EE, 5EZ and 5ZZ stereoisomers (proportions 2:5:3 from 1 H NMR spectra).Optimized overall yield of the four-step synthesis was 26%.The mixture contained three types of crystals, which were preliminarily separated by hands followed by fractional recrystallization from hexane.Overall yield of the separation was 78%.Pure stereoisomers were characterized by m.p., EA, 1 H and 13 C NMR, UV-VIS as well as MS spectra.All the analytical data were consistent with the proposed structures.Additionally, the structures of 5EE and 5ZZ isomers were confirmed by x-ray measurements (Fig. 1 and 2).Polycrystalline 5EZ isomer was not suitable for crystallography. 13C NMR spectra of the three isomers were also recorded in the solid state.The results of detailed analysis of those spectra and the spectra obtained in solutions will be compared with the data from x-ray crystallography in a separate paper.In the monocrystals of 5EE and 5ZZ isomers the thiophene substituents were disordered over two orientations.Attempts were made in order to distinguish between the two possibilities: the partially disordered mode as shown in Figure 1 or 2 and the structure of lower symmetry, however it did not lead to better results.Other authors have observed similar phenomena for some thiophene derivatives too.Basic structural checkCIF/Platon 6 was obtained for the results of crystal structures determination of 5EE and 5ZZ isomers.No syntax error was found.Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications no.257687 (for 5EE) and 257688 (for 5ZZ).Some more important data concerning crystal parameters and structure refinement are also disclosed in an experimental section (Table ).UV-VIS absorption spectra clearly indicated different optical properties of the stereoisomers studied [λ max 5EE = 402 (in CH 2 Cl 2 ), 402.6 (in CH 3 CN) nm, λ max 5EZ = 377 (in CH 2 Cl 2 ), 374.0 (in CH 3 CN) nm and λ max ZZ = 345 (in CH 2 Cl 2 ), 347.4 (in CH 3 CN) nm].The highest absorbance at the longest wavelength was recorded for 5EE molecule (see the experimental part).The photoluminescence spectra were obtained in dichloromethane solutions at concentrations of 0.05 mM under excitation at the wavelength of the main absorption band and are published elsewhere 4 .In each spectrum two main bands at 460 nm and 490 nm were present with intensities decreasing in the following order 5EE, 5EZ and 5ZZ.

Conclusions
Concluding, a model macromonomer namely 1,4-diethoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene (5) was obtained in three pure stereoisomeric 5EE, 5EZ and 5ZZ forms.The structures of 5EE and 5ZZ isomers were confirmed using x-ray measurements.Three isomers have photoluminescence properties.They undergo electrochemical polymerization. 4perimental Section General Procedures.Melting points (not corrected) were determined on Boetius HMK apparatus.NMR spectra were taken in CDCl 3 with TMS as an internal reference by Varian XL-300 at 300 MHz for 1 H and at 75.5 MHz for 13 C. UV-VIS spectra were recorded using Shimadzu UV-2102 PC recording spectrophotometer in acetonitrile solution using kuvetes 1 cm long.MS spectra were obtained from HPLC-MS Integrity Systems with Termabeam Mass Detector (EI, 70 eV); samples (in methanol solutions) were introduced to the spectrometer using direct inlet.Xray diffraction data were collected on KUMA KM4 four-circle diffractometer at 295 K.
Step 2. A mixture of crude 2 (6 mmol), glacial acetic acid (5.45 cm 3 ), concentrated hydrochloric acid (6.36 cm Step 4. Salt 4 and 2-thienylcarbaldehyde 5 were used for the Wittig reaction performed in the inert anhydrous atmosphere.Sodium methoxide solution in methanol (obtained by dissolving 9.0 mmol of sodium in 15 ml of methanol) was added gradually within 1 h without heating to a stirred mixture of 2-thienylcarbaldehyde (9.1 mmol), phosphonium salt 4 (4.4 mmol) and anhydrous methanol (45 ml).The reaction mixture was refluxed for 7 h and then left without heating for 18 h.The solvents were evaporated to dryness and the residues were extracted with benzene-hexane (1:1, v/v).Triphenylphospine oxide was removed from the obtained solution by chromatography.Evaporation of the remaining solution gave a mixture (72%) of almost pure 5EE, 5EZ and 5ZZ stereoisomers of 1,4-diethoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzenes (2:5:3 from 1 H NMR measurements).Overall yield of the four-step synthesis of 5 was c.a. 26%.The mixture contained three types of crystals, which were preliminarily separated by hands followed by two fractional crystallizations from hexane.Overall yield of the separation was 78%.Pure stereoisomers were characterized by m.p.s, elementary analyses, 1 H, 13

X-ray measurements
Monocrystals of 5EE and 5ZZ were obtained by a slow evaporation of hexane solutions of the preliminary separated isomers.Diffraction data were collected using MoKα radiation (0.71073 Å) on a four-circle KUMA KM-4 diffractometer with a graphite monochromator at 295 K using ω-2θ scan.Cell parameters, reflections collection and theirs reduction were done using KUMA KM4 software. 10No absorption correction was introduced.The structures were determined by means of direct methods and refined by the full-matrix least squares technique. 11Position of non -hydrogen atoms were determined using anisotropic thermal parameters.Most of hydrogen atoms were found in subsequent difference Fourier maps and isotropically refined; thiophene hydrogen atoms were added.Data concerning crystal and structure refinement is disclosed in Table .Crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.For examples of thiophene ring disorders over two orientations see e.g., ref. 12