Dithienopyrrole-based monomers as an acceptor unit building for synthesis of donor – acceptor conjugated polymers

A new monomer of N-benzoyl dithieno[2,3-b:2’,3’-d]pyrrole (BDP), has been successfully prepared via copper-catalyzed amidation. Then, this monomer was brominated to form 2,6-dibromo-n-benzoyl dithieno[2,3b:2’,3’-d]pyrrole (DiBDP) monomer. The structures of monomers were confirmed via the nuclear magnetic resonance (1HNMR) and Fourier transform infrared (FT-IR). BDP and DiBDP monomers will be used as monomers for Suzuki polycondensation reaction to synthesize the donor-acceptor (D-A) conjugated polymers.


INTRODUCTION
Polymer solar cells (PSCs) have attracted great interests in both academic and industry because of their various distinctive advantages including flexibility, simple manufacturing techniques, ability to incorporate other technologies, low material cost [1].At the meantime, despite of their advantages, PSCs have some drawback and other technical limitations that they have low stability, low power conversion efficiency and short lifetime [2].Consequently, enormous efforts have been devoted to overcome these weaknesses as well as to improve the operated efficiency of PSCs.An effective way to broaden absorption of PSCs is to narrow their band gaps.Recently, one of the most concerned research direction to do so is to alternatively bind an electron-rich units (D) and an electron-deficient units (A) into the same Trang 100 polymer backbone [3].For this kind of polymers, the interactions between the donor segments and the acceptor segments will form a new higher HOMO level and a new lower LUMO level.Through the interaction between pushpull driving forces, the electrons will redistribute from the initial orbitals (before interacting) to the new hybridized orbitals of the polymer.As a result, the magnitude of the band gap will be reduced.The degree of band gap reducing depends much on the strength of the donor, acceptor units imposed in the polymer backbone.
Therefore, judicious selection of donor, acceptor segments can allow to adjust the band gap magnitude to the expected value.
Experimentally, it is recognized that the narrower the optical band gap, the stronger the electron-withdrawing ability of acceptor unit in the copolymer [4].Besides that, the combinations of medium/strong donor units and medium/strong acceptor units usually result in good photovoltaic performances (PCE > 5 %) [5][6][7][8][9][10][11]. From that, medium and strong acceptor units are believed to be a good choice for effective D-A conjugated polymer [12,13].Therefore, in recent years, the N-acyl dithieno[2,3-b:2',3'd]pyrrole (DTP) moieties have been received considerable interest due to their good planar crystal structure, strong electron-withdrawing ability and symmetrical chemical structure with the side chain at the bridging unit [14][15][16]

Characterization
Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectra were recorded using BIO-RAD Excalibur spectrometer equipped with an ATR Harrick Split PeaTM. 1 HNMR spectra of the compounds were recorded in deuterated chloroform (CDCl3) with a 500 MHz spectrometer -Bruker AMX500 apparatus, and the chemical shift are given relative to tetra methyl silane (TMS).
Then, toluene was added to the reaction mixture and the solution was stirred for 30 minutes.
Benzamide (12 mmol) was added, followed by 3,3'-dibromo-2,2'-bithiophene (3.24 g, 10 mmol).The reaction mixture was stirred for 24 hours at 110 o C. The reaction was cooled to the room temperature in the next step, washed with distilled water (3 x 20 mL) and extracted with chloroform (3 x 20 mL).The organic phase was dried by anhydrous K2CO3.The solvent was removed by rotary evaporation.The crude product was purified by silica gel column chromatography (with the eluent as following -4 n-heptane: 1 ethyl acetate) to give the isolated product as a white crystalline solid (3.82 g, Rf = 0.75, yield = 45.29%).hours.After that, the mixture was washed with distilled water (3 x 20 mL) and extracted with chloroform (3 x 20 mL).The organic phase was dried by anhydrous K2CO3.The solvent was removed by rotary evaporation.The crude product was purified by silica gel column chromatography (the eluent as following -4 nheptane: 1 ethyl acetate) to give the isolated product as a white crystalline solid (1.172 g, Rf = 0.57, yield: 74.34%).

RESULTS AND DISCUSSION
The synthesis of monomers BDP and DiBDP are shown in Scheme 1. Monomer BDP was synthesized by using copper-catalyzed amidation 3,3'-dibromo-2,2'-bithiophene at the reflux temperature, in presence of CuI as the catalyst, DMEDA as the ligand for coupling amides with thiophene rings, toluene as solvent and K2CO3 as the base.The reaction proceeded with the formation of a dark-blue complex of copper (I) iodide and DMEDA and a subsequent brown mixture after 24 hours.After completion of reaction, the monomer was attained by extracting with chloroform, washing with distilled water and purification via column chromatography using the eluent of n-heptane and ethyl acetate (v/v : 4/1).It is necessary to brominate of BDP monomer as intermediated product for forthcoming reactions such as conjugated oligomerization and conjugated polymerization.
So that the achieved BDP monomer reacted with 2 equivalent of NBS based on the nucleophilic substitution mechanism to form DiBDP monomer.The reation was carried out in chloroform as a solvent at 0 o C in 24 hours.Then, the reaction mixture was extracted with chloroform, washed with distilled water several times and purified by column chromatography to give a pale yellow solid in high yield (74.34%).In the 1 HNMR spectrum of monomer BDP (Fig. 3.2), the doublet peak at 7.73 ppm, the triplet peak at 7.65 ppm and the triplet peak at 7.55 ppm respectively corresponded to the five protons on the benzene ring, in particular, two at positions 'd', one at position 'f' and two at positions 'e'.The doublet peak at 7.1 ppm corresponded to the two protons on the thiophene rings 'peak b'.The broad singlet peak at 6.85 ppm was assigned to two protons left on the thiophene rings 'peak a'.The chemical shifts along with the integrals of obtained signals were suitable with the structural formula of this monomer.These results indicated that copper-catalyzed amidation reaction successfully forming the desired monomer.

Figure 3 . 1 .
Figure 3.1.FT-IR spectrum of monomer BDP The ATR FT-IR spectra of monomer BDP (Fig 3.1) displayed several peaks between 2921 and 3109 cm -1 which contributed to C-H stretching vibrations of the benzyl groups.The peak at 1680 cm -1 , which was ascribed to the C=O stretching vibrations clearly proved for the existence of the N-acyl group in this monomer structure.The peaks at 1482/1443 cm -1 and the bands in range of 656 to 974 cm -1 are assigned to the aromatic C-C stretching vibrations and aromatic C-H deformation vibrations respectively.Whereas the bands from 1307 to 1384 cm -1 are assigned to the aromatic C-N stretching vibrations of the pyrrole units.In