Electrochemical Route to Solution-Processable Polymers of

Nov 18, 2013 - Dod), respectively and characterized by gel permeation chromatography (GPC) and 1H NMR. The selenophene containing polymer possesses ...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Electrochemical Route to Solution-Processable Polymers of Thiophene/Selenophene Capped Didodecyloxybenzo[1,2‑b:4,3‑b′]dithiophene and Their Optoelectronic Properties Anjan Bedi and Sanjio S. Zade* Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, PO: BCKV Campus main office, Mohanpur 741252, Nadia, West Bengal, India S Supporting Information *

ABSTRACT: Two new solution-processable polymers P1 and P2 are being reported here, which were prepared by electrochemical polymerization of thiophene and selenophene capped 7,8-didodecyloxybenzo[1,2-b:4,3-b′]dithiophene (BdTDod), respectively and characterized by gel permeation chromatography (GPC) and 1H NMR. The selenophene containing polymer possesses lower band gap than the thiophene analogue. Density functional theory (DFT) calculation showed the highly curved structure of the polymers and reproduced the trend in their optical band gaps. P2 showed larger bathochromic shift in the absorption spectrum from solution to film state compared to that of P1, which indicates better π-stacking interaction in the solid state for P2. In spite of having highly curved chains, the polymers successfully exhibited electrochromic switching. The exchange of the end-caps from thiophene to selenophene have manifested with higher electrochromic switching ability and better polaronic and bipolaronic features in spectroelectrochemical measurement of P2 than that of P1. Kinetic study on the polymer films using chronoamperometry revealed that the selenophene containing polymer P2 afforded Δ%T of ∼60 in the visible region with a coloration efficiency of 100 cm2 C−1. Electrochemical polymerization of BdT-Dod using different solvent/electrolyte systems was unsuccessful.

■

INTRODUCTION Conjugated polymers have attracted considerable interests because of their tunable electronic properties and wide range of the applications in organic electronic devices such as photovoltaic devices, light emitting diodes (LEDs), field effect transistors (FETs), sensors and electrochromic devices (ECDs).1 In pursuit of finding out suitable thiophene based materials for organic electronics, benzo[1,2-b:4,5-b′]dithiophene (BDT, fused aromatic system) was prepared and reported as donor unit in high performance conjugated donor− acceptor (D−A) copolymer for polymer solar cells (PSCs).2,3 The systems with fused aromatic rings are chosen over its analogous nonfused structures (with more rotational degrees of freedom) in many electronic applications4 as fused aromatic rings impart rigidity5 and shorten the effective conjugation length6 of the polymeric backbone. In particular, for the application in the OFET devices, the fused rings imply better stacking between the neighboring polymer chains to afford higher mobility compared to its monocyclic analogue.7 Incorporation of fused aromatic rings into the repeating unit to impart quinoidal character is one of the important strategies for tuning the band gaps of conjugated polymers. Other © 2013 American Chemical Society

strategies include (i) synthesis of polymers having alternate (D−A) units8 and (ii) atomistic approach.9 On the other hand benzo[1,2-b:4,3-b′]dithiophene (BdT) (which is another isomer of BDT) based polymers are rarely studied. Sasaki and co-workers10 reported the fabrication of OLEDs using 1,2-dithienylethylene derivatives, including the ortho-fused heterocyclic compound benzo[1,2-b:4,3-b′]dithiophene (BdT). A series of metal-free organic dyes comprising BdT as the central π-spacers was reported to show very good power conversion efficiency (PCE), when applied in dye sensitized solar cell (DSSC).11 Mullen and coworkers12 have reported the polymers having repeating units of thiophene capped benzodithiophene isomers (five possible isomers) to study the effect of varying degree of backbone curvature on the solubility, the electronic levels and the morphology in a film. It was observed that with increase in backbone curvature solubility increases, however, decreases the order in the film. Li and co-workers13 have obtained a PCE as Received: September 5, 2013 Revised: November 4, 2013 Published: November 18, 2013 8864

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

Scheme 1. Synthetic Route to the Comonomers and Polymers

energy levels, optical band gaps (Eopt g ), charge carrying abilites and nature of electrochromic switching.

high as 3.4% from PSCs fabricated using 7,8-dioctyloxybenzo[1,2-b:4,3-b′]dithiophene based polymers. Previous effort in electrochemical polymerization of unsubstituted BdT resulted in a polymer having a low electroconductance of 10−11 S/cm, which lacks solution processability.14 Reynolds and co-workers15 prepared two solution-procesable BdT based polymers, PBDT-Oct and PBDT-Ethex by chemical polymerization, which exhibited different electrochemical properties based on the substitution due to varying propensity to form aggregates by π−π interaction. However, no BdT based polymer has yet been prepared by electrochemical polymerization and reported with any electrochromic switching study. Stable electroactive moieties with electrochromic switching ability between the oxidized and neutral states have potential applications in smart cards, electronic identification tags and switching element in flat panel displays.1b To examine the electrochromic properties of BdT based polymers, we have synthesized two new chalcogenophene capped BdT-Dod comonomers and electropolymerized to get two new solution-processable fluorescent polymers. The effect of the heteroatoms has been discussed in terms of electronic

■

RESULTS AND DISCUSSION Synthesis. The comparative study on the effect of selenophene/thiophene attachment with BdT-Dod unit involved synthesis of two new comonomers, 4 and 5. The comonomers were prepared by Suzuki coupling of the dibromo compound 116 with boronate derivatives 217 and 3.18 The pure products, 4 and 5 were obtained after column chromatography. Electropolymerization of the Comonomers, 4 and 5. Conjugated polymers, obtained by electrochemical polymerization often lack solution-processability.19,20 Here, we report the preparation of two new solution-processable conjugated polymers by electrochemical polymerization and their further characterization by gel permeation chromatography (GPC). As compound 4 and 5 are insoluble in acetonitrile (ACN), the cyclic voltammetry (CV) experiments were done using a threeelectrode system, comprising of a Pt-disk as working electrode, a Pt-wire as counter and a Ag/AgCl reference electrode, ACN/ dichloromethane (DCM) (9:1) as solvent system and tetrabutylammonium perchlorate (TBAPC) as electrolyte. In anodic scan, compound 4 (Figure 1a) and 5 (Figure 2a) 8865

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

Figure 1. (a) CV and (b) multisweep electropolymerization of 4 on a Pt electrode in ACN/DCM (9:1) and 0.1 M TBAPC at 50 mV s−1 vs Ag/AgCl wire. (Inset) CV of P1 in compound 4-free ACN and 0.1 M TBAPC as a function of scan rate.

Figure 2. (a) CV and (b) multisweep electropolymerization of 5 on a Pt electrode in ACN/DCM (9:1) and 0.1 M TBAPC at 50 mV s−1 vs Ag/AgCl wire. (Inset) CV of P2 produced in compound 5-free ACN and 0.1 M TBAPC as a function of scan rate.

Figure 3. CV of (a) P1 and (b) P2, using ACN/TBAPC solvent/electrolyte system.

s−1. The current density was increased with the number of scans. It can be attributed to the gradual deposition of increasing amount of polymers by repetitive scans with increase in the thickness of the deposited polymer layer on the Pt-disk electrode. The polymer films were tested for their scan rate dependence at 50−300 mV s−1. Both the polymers exhibited linear scaling of the anodic and cathodic currents with increasing value of scan rate (inset of Figure 1b and 2b), as

exhibited oxidation potentials at 1.01 and 1.02 V, respectively. In cathodic scan, reduction potentials of −1.30 and −1.31 V were observed for 4 and 5, respectively. So, the replacement of the two thiophene rings of 4 by selenophene rings to get 5 had negligible effect on the HOMO and LUMO energy levels. P1 (Figure 1b) and P2 (Figure 2b) were obtained on Pt-disk electrode by repetitive CV cycles ranging from −0.2 to +1.2 V and from −0.2 to +1.1 V, respectively, at a scan rate of 50 mV 8866

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

Figure 4. (a) CV of BdT-Dod in ACN/DCM (9:1) and 0.1 M TBAPC on a Pt electrode. (b) 20 cycles of BdT-Dod in ACN/DCM (9:1) and 0.1 M TBAPC at 50 mV s−1 vs Ag/AgCl wire.

Figure 5. UV−vis absorption spectra of (left) P1 and (right) P2 in solution and film states.

expected from an electroactive and nondiffusive polymer film. The electrochemically obtained polymers were further characterized by GPC by dissolving in THF to know their molecular weights. The number-average molecular weights (Mn) of P1 and P2 were found to be 8 and 12 KDa, respectively, which indicate degree of polymerization of 12−15. Weight-average molecular weights (Mw) were found to be 13 and 26 KDa to afford polydispersity indices (PDI) of 1.6 and 2.2 for P1 and P2, respectively. Strongly curved polymers are found to exhibit lower molecular weights than that of polymers without curvature in the backbone, possibly due to relatively less accessibility of the reactive chain ends of the intermediate oligomers as they have tendency to fold back.12 So, the electrochemical polymerization proved to be successful to provide quite similar Mn values reported for PBDT-Oct and PBDT-Ethex, which were prepared by chemical polymerization method.15 The 1H NMR spectra (Figure S5 and S6, Supporting Information) of P1 and P2 clearly indicated that the electropolymerization reactions were advanced via α−α coupling. The formation of the unwanted cross-linked polymers via α−β or β−β coupling was not observed. So, the difference in molecular weights between P1 and P2 may be attributed to the better charge carrying capacity of selenophene rings compared to that of thiophene rings. To know the HOMO and LUMO energy levels and the stability of the polymers in charged states, the polymers were examined by CV. Electrochemistry of the polymers was done in comonomer free solutions using three-elctrode cell with ACN/

TBAPC as solvent/electrolyte system, where Pt-disk working electrode was precoated with the polymers (Figure 4, Table 3). Onsets of the oxidation potentials (Eox onset) of P1 (Figure 3a) and P2 (Figure 3b) were found to be 0.87 and 0.82 V, which correspond to the HOMO levels of −5.58 and −5.53 eV, respectively.21 The LUMO levels, estimated from the HOMO and the onsets of the absorption were −3.42 and −3.54 eV for P1 and P2, respectively. Lower LUMO of P2 can be attributed to the presence of more polarizable selenium atom of selenophene rings.22 P1 and P2 can be considered as donor polymers,23 based on the fact that they showed well-defined oxidation peaks and poor reduction peaks. Several attempts to polymerize BdT-Dod in ACN/DCM (9:1) using TBAPC or tetrabutylammonium hexafluorophosphate as electrolyte (Figure 4b) produced a black material on Pt-disk electrode as reported earlier for unsubstituted BdT.14 This black mass peeled off from the Pt electrode to the solution during electrochemistry. However, the capping of BdT-Dod by thiophene or selenophene provided ability to the comonomers 4 and 5 to polymerize smoothly under the identical electrochemical conditions. Spectroelectrochemistry. In solution, P1 (yellow in color) exhibited a wavelength for maximum absorption (λmax) at 437 nm, which shifted to 453 nm in the film prepared in situ by electrochemical polymerization (Figure 5a). P2 exhibited a λmax of 462 nm in solution, which shifted to 492 nm in thin film (Figure 5b). P1 and P2 exhibited bathochromic shifts of 16 and 30 nm in λmax, respectively from solution to film state. This 8867

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

Figure 6. Spectroelectrochemistry of (a) P1 (between −0.2 and 1.2 V) and (b) P2 (between −0.2 and +1.1 V) in a comonomer free, 0.1 M TBAPC/ACN solution.

difference could be attributed to the improved extent of πstacking in solid state compared to that in solution. The higher red shift in case of P2 compared to P1 may be due to the Se··· Se interaction24 between selenium atoms from the neighboring chains, which could induce better ordering of the film to allow several improved intermolecular interactions (like, π-stacking and van der Waals interactions) in the solid state. Additionally, this Se···Se interaction may play a role in maintaining planarity of the polymer main chain to increase the extent of π-stacking between two interacting polymer chains. The onset values of the thin film absorption spectra correspond to the optical band gaps (Eopt g ) of 2.16 and 1.99 eV for P1 and P2, respectively. So, the attachment of two additional chalcogenophene rings to the central BdT-Dod unit has enabled the polymers to attain lower band gaps than chemically synthesized PBDT-Oct and PBDT-Ethex15 by extending the conjugation length. Again, incorporation of the selenophene rings in place of the thiophene units has successfully lowered the band gap in the resulting polymer. The higher polarizability of the selenium atom than sulfur broadened the bandwidth of the absorption spectra of P2 in both the states compared to that of the sulfur analogue, P1. Spectroelectrochemistry was performed to probe the optical changes in polymer films upon doping majorly with application of positive potential as they displayed distinct oxidation peaks in CV but almost negligible reduction peaks.23 Thin films for P1 and P2 were deposited on indium tin oxide (ITO) coated glass slides at constant potentials of 1.1 and 0.9 V, respectively. The spectroscopic changes were investigated using a UV−vis− NIR spectrophotometer in a comonomer free, 0.1 M TBAPC/ ACN solution with increasing the applied potential from −0.2 to +1.2 V (Figure 6). Table 1 depicts the corresponding colors of P1 and P2 in neutral and oxidized states. With increase in potential, a gradual decrease in the intensity of the UV−vis absorption bands (arises due to the π−π* transition in the neutral polymer) for both the polymers was observed. Meanwhile, two new absorption bands originated at 640 nm and extending from 1500 nm due to the formation of singly charged species (polaron) and doubly charged species (bipolaron), respectively, for P1. Similarly, in P2 the polaronic (710 nm) and bipolaronic species (1600 nm) were found to generate at lower potential than that was observed for P1. It is evident that the selenium atom can carry more charge than sulfur atom upon doping.24,25 The chalcogenophene caps

Table 1. Corresponding Changes in Color from Neutral to Oxidized States of the Polymer Films

improved the spectroelectrochemical behavior of P1 and P2 compared to that reported for PBDT-Oct and PBDT-Ethex.15 Luminescent Properties. Solution spectra of compounds 4 (Figure 7a) and 5 (Figure 7b) (10−5 M solution in chloroform) exhibited absorption maxima (λmax) at 343 and 369 nm, respectively. In solid state (comonomers were spin coated on ITO-coated glass) λmax shifted to 355 and 395 nm for 4 and 5, respectively. Fluorescence was studied for compound 4, 5, P1 and P2 in solution and in the thin films using the similar conditions used for recording absorption spectra. Compound 4 showed emission at 435 and 481 nm in solution and thin film, respectively when excited at absorption λmax. Compound 5 displayed corresponding emission at 450 and 506 nm. Compound 4 and 5 showed bright-ocean and blue fluorescence, respectively, when excited at 366 nm in solution. In the thin films the comonomers 4 and 5 showed cyan and turquoise-blue fluorescence, respectively. P1 showed brightgreen fluorescence in solution, while in thin film it appeared as orange under excitation at 366 nm (Table 2). Upon similar excitation condition, P2 showed pale green and red emission in solution and solid state, respectively. In both the absorption and emission spectra significant red shift in λmax were observed from solution to thin film state. The emission spectra of 8868

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

Figure 7. UV−vis absorption and photoluminescence (PL) spectra of (a) 4 and (b) 5 in solution and thin film.

electro-deposited onto ITO-coated glass having dimensions of 3 × 0.7 cm2 at a constant potential of 0.9 V vs Ag/AgCl and passing a charge of 50 mC. To investigate the switching time of these polymers and the stability of their electrochromic changes, double potential pulse switching experiments were performed using ACN/TBAPC solvent/electrolyte system. Switching was studied between 0 and 1.1 V for P1 (Figure 9) and 0 and 1.0 V for P2 (Figure 10) vs Ag/AgCl. P2 showed percent transmission (%T) of 30 in neutral (yellow) state and 90 in bleached state (transmissive blue) to result in a percent optical contrast (Δ%T) as high as ∼60, which is measurable at 492 nm. At 714 nm (polaron) and 1600 nm (bipolaron) the polymer achieved the Δ%T of 20 and 17, respectively. For P1 a lower value of Δ%T was obtained. It also exhibited Δ%T of 25, 16, and 15 at 453 (neutral), 640 (polaron) and 1500 nm (bipolaron), respectively. The higher Δ%T of P2 than that of P1 in visible range is supported by the spectroelectrochemical results as the peak at 453 undergoes more changes in intensity than the peak at 492 nm for P2. Both the polymers exhibited switching time of less than 2 s. The coloration efficiency of the P2 film at 492 nm was found to be 100 cm2 C−1, whereas that of P1 was 66 cm2 C−1 at 453 nm. However, the coloration efficiencies and switching speeds are much less that the PEDOT,26 PProDOT27 and PEDOS28 based conjugated polymers. So, it is notable that P1 and P2 yield an overall homogeneous and high quality films and favorably stable with their optical properties offering reasonable switching time. Theoretical Calculation. To obtain more structural insight, frontier energy levels of the polymers and to correlate those with the experiments, DFT calculations were carried out on P1 and P2 (dodecyloxy chains were replaced by methyloxy groups) using Gaussian 09 program29 at B3LYP/6-31G(d) level with application of periodic boundary condition (PBC).30 The optimized geometries (Table S1) hinted toward a quasi coplanar chain structure for P1 with dihedral angle between

Table 2. Fluorescence of 4, 5, P1, and P2 As Observed under Excitation at 366 nm

polymers P1 and P2 (Figure 8) showed a more distinct vibronic fine structure compared to the corresponding comonomers 4 and 5 (Figure 7). It can be ascribed to the self-aggregation of the polymer chains in presence of interdigitation of alkoxy chains. When PL spectra of P1 and P2 in chloroform solution were recorded at elevated temperature (60 °C), the shoulder peaks were disappeared (Figure S8). Switching Properties. Spectroelectrochemical experiments have demonstrated the ability of P1 and P2 to switch between its neutral and doped states (i.e., coloring/bleaching process of P1 and P2) with a significant change in transmittance at 453 and 492 nm for P1 and P2, respectively. The polymers were Table 3. Optical and Electrochemical Properties of the Polymers λmax(abs) (nm)

λmax(em) (nm)

compounds

soln

film

soln

film

a Eox onset (V)

b Ered onset (V)

EHOMOc (eV)

ELUMOd (eV)

e Eopt (eV) g

P1 P2

437 462

453 492

507, 542 531, 565

622 634

0.87 0.82

−0.85 −0.82

−5.58 −5.53

−3.42 −3.54

2.16 1.99

21 d e Obtained from CV. bObtained from CV. cDetermined by EHOMO = −4.71 − (Eox Determined by ELUMO = EHOMO − Eopt onset). g . Determined from opt Eg = hc/λonset.

a

8869

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

Figure 8. Fluorescence spectra of (a) P1 and (b) P2 in solution and film.

Figure 9. (a) Change in %T of the film of P1 on ITO in 4-free 0.1 M TBAPC/ACN solution at the visible and NIR region during switching between 0 and 1.1 V. (b) expansion of part a.

Figure 10. (a) Change in %T of the film of P2 on ITO in 5-free 0.1 M TBAPC/ACN solution at the visible and NIR region during switching between 0 and 1.0 V. (b) Expansion of part a.

repeating unit of ∼14° and a planar structure for P2 with the presence of curvature in both the polymer chains. Thiophene capped oligomer of unsubstituted BdT reported to possess the strongest curvature among the series of benzodithiophene based oligomers.12 This kind of curvature is supposed to result in higher band gap and less ordering in the bulk material. However, the curvature provides improved solubility to the polymer chains.

The electron density in HOMO and LUMO is delocalized over the whole polymer chain length and is nearly same for both the polymers. From DFT, the band gaps were obtained to be 2.42 and 2.30 for P1 and P2, respectively. The decrease in band gap with change in substitution from thiophene to selenophene is expected as selenophene rings increase the overall conjugation.31 The trend in lowering of band gap from P1 to P2 matches well with the experimentally obtained Eopt g of 8870

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

coated glass slide, the counter electrode was a platinum wire and nonaqueous Ag/AgCl was used as the reference electrode. Density functional theory (DFT) studies were carried out using Gaussian 09.29 The polymers were geometrically optimized using the periodic boundary conditions (PBC) with Becke’s three-parameter exchange functional combined with the LYP correlation functional (B3LYP) and the 6-31G(d) basis set (PBC/B3LYP/6-31G(d)).32 Synthesis of 4. To a solution of compound 1 (250 mg, 0.35 mmol) and Pd(dppf)Cl2 (9 mg, 0.01 mmol, 2.5 mol %) in dry toluene (15 mL) were added a methanol solution (15 mL) of 2-thienylboronic acid (2) (180 mg, 1.4 mmol) and 132 mg (2.25 mmol) of KF. The resulting mixture was heated at 70 °C for 20 h. The reaction mixture was concentrated and then diluted with DCM (20 mL) and washed with water (2 × 20 mL), dried (Na2SO4), and concentrated in vacuo to give a brown solid as crude product. Purification by silica gel column chromatography (Hexane/EtOAc = 99.5/0.5), afforded a pale yellow solid (240 mg, 95%): mp 45−50 °C. 1H NMR (400 MHz, CDCl3, ppm, Figure S1) δ: 7.66 (s, 2H), 7.32−7.29 (m, 4H), 7.08 (m, 2H), 4.27 (t, J = 6.88 Hz, 4H), 1.88−1.81 (q, J = 6.88 Hz, 4H), 1.51 (m, 4H), 1.26 (br, s, 36H), 0.89 (t, J = 6.84 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm, Figure S2) δ: 142.1, 137.6, 136.9, 132.8, 131.6, 127.9, 125.1, 124.7, 118.0, 73.7, 31.9, 30.4, 29.7, 29.67, 29.64, 29.4, 29.3, 26.0, 22.6, 14.1. HRMS (M + Na) + : calculated for C42H58NaO2S4, 745.3217; found, 745.3245. Synthesis of Compound 5. To a solution of compound 1 (50 mg, 0.07 mmol) and Pd(dppf)Cl2 (3 mg, 0.002 mmol, 2.5 mol %) in dry toluene (10 mL) were added a methanol solution (10 mL) of 2selenylboronic acid (3) (48 mg, 0.3 mmol) and KF (26 mg, 0.45 mmol). The resulting mixture was heated at 70 °C for 26 h. Following the identical work up procedure used for 4, a dark solid was obtained as crude. Purification by silica gel column chromatography (Hexane/ EtOAc = 99/1) resulted in a yellow solid (60 mg, 60%): mp 55−60 °C. 1H NMR (400 MHz, CDCl3, ppm, Figure S3) δ: 7.96 (d, J = 5.32 Hz, 2H), 7.60 (s, 2H), 7.47 (s, J = 3.8 Hz, 2H), 7.31 (m, 2H), 4.27 (t, J = 6.88 Hz, 4H), 1.88 (q, J = 6.88 Hz, 4H), 1.51 (m, 4H), 1.26 (br, s, 36H), 0.89 (t, J = 6.08 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm, Figure S4) δ: 142.6, 142.1, 139.0, 132.8, 131.6, 130.39, 130.36, 126.8, 118.8, 73.6, 31.9, 30.4, 29.7, 29.6, 29.4, 29.3, 26.0, 22.6, 14.1. HRMS (M + Na)+: calculated for C42H58NaO2S2Se2, 841.2106; found, 841.2123.

the polymers. After replacement of thiophene by selenophene the HOMO energy level nearly remained unchanged; however, the LUMO was stabilized by 0.1 eV in P2 compared to that of P1 (Table S2).

■

CONCLUSION We have synthesized two new benzo[1,2-b:4,3-b′]dithiophene and thiophene/selenophene comonomers (4 and 5) by Pd (II)catalyzed Suzuki coupling. Incorporation of two end-capping chalcogenophene rings to benzo[1,2-b:4,3-b′]dithiophene not only eased the oxidation of the comonomers and their polymerization, but also enabled the polymers to attain a lower band gap than benzo[1,2-b:4,3-b′]dithiophene homopolymer. The electropolymerization of BdT-Dod monomer using chronoamperometry/CV was unsuccessful, whereas that of comonomers, 4 and 5 produced corresponding solutionprocessable polymers. 1HNMR spectra clearly demonstrated the formation of the polymers specifically, via α−α coupling of the corresponding comonomers. The fused BdT-Dod unit provided high oxidative stability to both the polymers by lowering the HOMO levels, which avoids any chance of aerial doping. Replacement of the nonfused thiophene rings by selenophene afforded the polymer P2 with better optical and electronic properties. From DFT, P2 was found to be planar but P1 was quasi coplanar structure (with a torsional angle of ∼14°). Selenophene containing polymer, P2 showed higher molecular weight, low band gap and broader absorption bands than that of P1. P2 showed better optical contrast (Δ%T = 60 at 492 nm) and coloration efficiency (100 cm2 C−1) than that of P1.

■

EXPERIMENTAL SECTION

n-BuLi (1.6 M in hexane) was purchased from Neo-Synth. Selenophene, [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II), and tetrabutylammonium perchlorate (TBAPC) were obtained from Aldrich and they were used as received. Toluene was distilled from sodium/benzophenone under an atmosphere of dry nitrogen. ACN (Merck, India) was distilled from P2O5 (Spectrochem, India) under dry nitrogen atmosphere. DCM (Merck, India) was dried on calcium hydride and distilled off before experiment under dry condition. The other chemicals reagents used as received without any further purification. All reactions were carried out under nitrogen. NMR spectra were recorded on JEOL ECS 400 MHz spectrometer as a solution in CDCl3 with tetramethylsilane (TMS) as the internal standard and chemical shifts (δ) are reported in parts per million. Columns for column chromatography were prepared with silica gel (230−400 mesh, Merck, India). Nonaqueous Ag/AgCl wire was prepared by dipping silver wire in a solution of FeCl3 and HCl. All electrochemical measurements were referenced against standard ferrocene sample from Aldrich. Electrochemical studies were carried out with a Princeton Applied Research 263A potentiostat using three-electrode cell, consisted of a platinum (Pt) disk electrode as the working electrode, a platinum wire as counter electrode and an AgCl coated Ag wire as the reference electrode, which was directly dipped in the electrolyte solution. Pt-disk electrodes were polished with alumina, water and acetone followed by drying with nitrogen gas before use for the removal of any incipient oxygen. Electropolymerization was done on both Pt-disk and indium tin oxide (ITO) coated glass as working electrode, separately with 0.1 M of TBAPC in DCM/ACN (1/9). Before examining the optical properties of polymer films, the thin films were rinsed with DCM/ ACN (1/9). UV−vis-NIR spectra were recorded on HITACHI U4100 UV−vis−NIR spectrophotometer. CV experiments of the polymers were performed against Ag/Ag+ reference electrode. In spectrochemical measurements, the working electrode was an ITO-

■

ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra of 4 and 5 and optimized structures, absolute energies, PL spectra, picture of HOMO and LUMO, and coordinates for optimized structure of polymers P1 and P2. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: (S.S.Z.) [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS A.B. acknowledges research fellowship from CSIR, India. S.S.Z. thanks DRDO, India for financial support. REFERENCES

(1) (a) Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: New York, 1997, Vol. 2. (b) Handbook of Conducting Polymers, 3rd edn.; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press: Boca Raton, FL, 2007. (c) Handbook of Thiophene-based Materials: Applications in Organic Electronics and Photonics; Perepichka, I. F., Perepichka, D. F., Eds.; John Wiley & Sons: New York, 2009, Vol. 1 and 2. 8871

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872

Macromolecules

Article

(2) Chen, H.-Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photon. 2009, 3, 649. (3) (a) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. J. Am. Chem. Soc. 2009, 131, 15586. (b) Huang, Y.; Huo, L. J.; Zhang, S. Q.; Guo, X.; Han, C. C.; Li, Y. F.; Hou, J. H. Chem. Commun. 2011, 47, 8904. (c) Price, S.; Stuart, A.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011, 133, 4625. (4) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. Adv. Mater. 2006, 18, 3029. (5) Mcculloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macdonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M.; Kline, R.; McGehee, M.; Toney, M. Nat. Mater. 2006, 5, 328. (6) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078. (7) Boudreault, P.-L. T.; Wakim, S.; Blouin, N.; Simard, M.; Tessier, C.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2007, 129, 9125. (8) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2008, 130, 732. (9) Gibson, G. L.; McCormick, T. M.; Seferos, D. S. J. Am. Chem. Soc. 2012, 134, 539. (b) Das, S.; Pati, P. B.; Zade, S. S. Macromolecules 2012, 45, 5410. (10) Tanaka, K.; Osuga, H.; Tsujiuchi, N.; Hisamoto, M.; Sasaki, Y. Bull. Chem. Soc. Jpn. 2002, 75, 551. (11) Gao, P.; Tsao, H. N.; Grätzel, M.; Nazeeruddin, M. K. Org. Lett. 2012, 14, 4330. (12) Rieger, R.; Beckmann, D.; Mavrinskiy, A.; Kastler, M.; Müllen, K. Chem. Mater. 2010, 22, 5314. (13) Zhang, M.; Sun, Y.; Guo, X.; Cui, C.; He, Y.; Li, Y. Macromolecules 2011, 44, 7625. (14) Berlin, A.; Pagani, G. A.; Sannicolo, F. J. Chem. Soc. Chem. Commun. 1987, 18, 157. (15) Brookins, R. N.; Berda, E.; Reynolds, J. R. J. Mater. Chem. 2009, 19, 4197. (16) Fritzsche, M.; Bohle, A.; Dudenko, D.; Baumeister, U.; Sebastiani, D.; Richardt, G.; Spiess, H. W.; Hansen, M. R.; Höger, S. Angew. Chem. 2011, 123, 3086. (17) Burrell, A. K. J. Org. Chem. 2003, 68, 8974. (18) Shabana, R.; Galal, A.; Mark, H. B., Jr.; Zimmer, H.; Gronowitz, S.; Hörnfeldt, A.-B. Phosphorus, Sulfur Silicon Relat. Elem. 1990, 48, 239. (19) Durmus, A.; Gunbas, G. E.; Camurlu, P.; Toppare, L. Chem. Commun. 2007, 3246. (20) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477. (21) (a) Zhou, E.; Tan, Z.; He, Y.; Yang, C.; Li, Y. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 629. (b) Bedi, A.; Senanayak, S. P.; Narayan, K. S.; Zade, S. S. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4481. (22) Das, S.; Bedi, A.; Ramakrishna, G.; Reddy, C. M.; Zade, S. S. Org. Biomol. Chem. 2011, 9, 6963. (23) Sun, Q.; Wang, H.; Yang, C.; Li, Y. J. Mater. Chem. 2003, 13, 800. (24) Patra, A.; Bendikov, M. J. Mater. Chem. 2010, 20, 422. (25) Bedi, A.; Senanayak, S. P.; Narayan, K. S.; Zade, S. S. Macromolecules 2013, 46, 5943. (26) (a) Sankaran, B.; Reynolds, J. R. Macromolecules 1997, 30, 2582. (b) Gaupp, G. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R. Chem. Mater. 2002, 14, 3964. (27) (a) Krishnamoorthy, K.; Ambade, A. V.; Kanungo, M.; Contractor, A. Q.; Kumar, A. J. J. Mater. Chem. 2001, 11, 2909. (b) Reeves, B. D.; Grenier, C. R. G.; Argun, A. A.; Cirpan, A.; McCarley, T. D.; Reynolds, J. R. Macromolecules 2004, 37, 7559. (28) (a) Li, M.; Sheynin, Y.; Patra, A.; Bendikov, M. Chem. Mater. 2009, 21, 2482. (b) Li, M.; Patra, A.; Sheynin, Y.; Bendikov, M. Adv. Mater. 2009, 21, 1707. (29) Gaussian 09, Revision C.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;

Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc.: Wallingford CT, 2010. (30) Zade, S. S.; Bendikov, M. Org. Lett. 2006, 8, 5243. (31) Zade, S. S.; Bendikov, M. Chem.Eur. J. 2009, 15, 8613. (32) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory; Wiley-VCH: New York, 2000.

8872

dx.doi.org/10.1021/ma401853q | Macromolecules 2013, 46, 8864−8872