Article pubs.acs.org/JPCC
Molecular Origin of Isomerization Effects on Solid State Structures and Optoelectronic Properties: A Comparative Case Study of Isomerically Pure Dicyanomethylene Substituted Fused Dithiophenes Zhihua Wang,†,‡,§ Anjaneyulu Putta,†,§ Jeffery D. Mottishaw,† Qiang Wei,† Hua Wang,‡ and Haoran Sun*,† †
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng, 475004, China
‡
S Supporting Information *
ABSTRACT: Introduction of a strong electron-withdrawing dicyanomethylene (−CH− (CN)2) group onto a fused bithiophene frame is a useful strategy to convert fused bithiophene derivatives from p-type organic semiconductor materials into n-type materials. Here, through systematic studies of isomerically pure 7-dicyanomethylene7H-cyclopenta-[1,2-b:4,3-b′]dithiophene (1), 4-dicyanomethylene-4H-cyclopenta[2,1b:3,4-b′]dithiophene (2), and 7-dicyanomethylene-7H-cyclopenta[1,2-b:3,4-b′]dithiophene (3) as well as their oligomers and polymers, we report that isomerization has the potential to fine-tune the optoelectronic properties of these materials including band gap (Eg), electron affinities (EAs), ionization potentials (IPs), electrochemical polymerization behaviors, and the solid state molecular packing, all of which are important for the performance of semiconductor devices. The monomers of these isomers exhibit noticeable difference in maximum absorption energies; and the oligomers and polymers composed of these monomers exhibit increased band gap difference as predicted by DFT calculation. Furthermore, the isomer 2 exhibits better electrochemical polymerization behavior as well as profound electrochromic switching in the near to middle infrared region. X-ray diffraction and quantum mechanical calculations reveal that the difference of dipole and quadrupole moments in these isomers is likely responsible for the difference in the solid state packing and subsequent polymer assembly.
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INTRODUCTION
(ca. 0.8 eV) reported to date for a fused thiophene-based system.29 Its isomers, 7-dicyanomethylene-7H-cyclopenta[1,2-b:4,3b′]dithiophene (1) and 7-dicyanomethylene-7H-cyclopenta[1,2-b:3,4-b′]dithiophene (3), have not attracted as much attention because of synthetic difficulties (Chart 1).32 Given the lower band gap and the good electron transfer character of compound 2, we were initially interested in the preparation of this compound, with the goal to study the isomerization effects on optoelectronic properties. Compounds 1 and 3, also bearing the strong electron-accepting dicyanomethylene group at the
Fused thiophenes, one of the most popular organic semiconductor materials, have attracted enormous interest because of their extended planar backbone framework, which can improve π−π intermolecular interactions due to the large πconjugation found in these compounds.1−8 In the past few decades, polythiophene9−17 and fused polythiophene18−20 based p-type semiconductor materials have been extensively investigated for applications in organic solar cells (OSCs) and organic field-effect transistors (OFETs). However, the development of fused thiophene-based n-type organic semiconductor materials has still largely lagged behind p-type semiconductors,21−25 particularly for practical n-type organic semiconductor materials with high mobility and air stability.26,27 The electron-rich nature of the thiophene ring precludes its derivatives from being air-stable n-type semiconductor materials unless one can effectively introduce strong electronwithdrawing groups onto the conjugated thiophene or fused thiophene cores.21,28−31 Ferraris and Lambert first reported that poly-4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (compound 2) has one of the lowest band gaps © 2013 American Chemical Society
Chart 1
Received: April 3, 2013 Revised: July 14, 2013 Published: July 29, 2013 16759
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Scheme 1. Synthetic Routes for Compounds 1, 2, and 3
Reagents and conditions: (i) n-BuLi (1.1 equiv), Et2O, −78 °C; CuCl2 (1.1 equiv); (ii) NBS (2.1 equiv), HOAc−CHCl3 (1:1), r.t.; (iii) LDA (2.3 equiv), Et2O, 0 °C; TMSCl (5.0 equiv), −78 °C to r.t., overnight; (iv) n-BuLi (2.2 equiv), Et2O, −78 °C; Dimethyl carbamyl chloride (1.0 equiv), −55 °C to r.t., overnight; (v) TFA, CHCl3; (vi) CH2(CN)2 (3.0 equiv), HOAc:pyridine = 1:1 (cat.), reflux, overnight. (vii) CH2(CN)2 (3.0 equiv), HOAc:pyridine = 1:1 (cat.), reflux, 2 h. (viii) CH2(CN)2 (3.0 equiv), HOAc:pyridine = 1:1 (cat.), reflux, 4 h. a
sp2 carbon bridging the α and α′ positions, have the same chemical components as compound 2, but with different isomeric structures. We are interested in how these simple changes in structure affect the physicochemical properties of these materials such as band gap (Eg), electron affinities (EAs), ionization potentials (IPs), electrochemical polymerization behaviors, and the solid state structures of their monomers and polymers. The answers to these questions have the potential to lead us to the discovery of new design principles for organic semiconductor materials, in which intermolecular interaction plays an important role.33−39 The structure−property relationship of compounds 1, 2, and 3, and their polymers is of significant interest in the design of new organic semiconductor materials at the molecular level. Such design requires that one can use molecular structure information to predict the properties of solid monomericmolecular crystalline and polymeric states. These isomers differ only in the location of sulfur within each molecule, thus providing an excellent example to study how the position of the thiophene ring affects the isomers’ properties in the solid and polymeric states. Here we report a comparative study of these three isomeric compounds through spectroscopic, electrochemical, crystallographic, and quantum mechanical calculations, along with electrochromic measurements. We found that the isomerization leads to interesting differences in physicochemical properties of both monomeric and polymeric materials. For example, polymers from compound 2 exhibit high intensity absorbance and excellent electrochromic switching properties ranging from the visible region continuously to the infrared region; however, polymers from compounds 1 and 3 show less profound absorption in the near-infrared region with no electronic absorption in the middle infrared region. The underlying causes for these differences are further
discussed from the electronic structures and the solid state structures of these isomeric materials, with the ultimate goal to provide insight to the design of high-performance, low bandgap, air-stable n-type organic semiconductor materials.
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RESULTS AND DISCUSSION Compound 1 can be synthesized from 3-bromothiophene through four steps without TMS protection or six steps with TMS protection (Scheme 1). The former synthetic route contains fewer steps than the latter one; however, the reactive α position of compound 7 leads to low yield of compound 4 if there is no TMS protection. The TMS-protecting group prevents the possible side reactions from occurring at the reactive positions and avoids the solubility problem during the Br/Li-exchange reaction and subsequent homocoupling.8 Compound 1 was obtained in overall ca. 20% isolated yields in six steps from 3-bromothiophene. Compound 2 was synthesized by a modified reported procedure.40,41 Compound 3 was prepared with a Knoevenagel condensation reaction, similar to the procedure described for preparation of compounds 1 and 2, from the ketone (compound 1032) and malononitrile in anhydrous ethanol. Detailed synthetic procedures and characterization data are given in the Supporting Information (SI). The structures of compound (TMS) 2-4 (Supporting Information Figure S1), compound 1 (Figure 1), and compound 2 (Figure 2) were confirmed by single crystal Xray analysis. The single crystals of compounds (TMS)2-4, 1, and 2 suitable for X-ray structure analysis were obtained by slow evaporation of solution of a mixture of CHCl3 and CH3OH. Unfortunately, we were not able to obtain a single crystal of compound 3 suitable for X-ray structure analysis, though microcrystalline material of this compound was 16760
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(Figure 1A). There are no short S···S contacts in the molecular packing of sulfur-rich 1. The S···N interactions are clearly observed between molecules within the adjacent columns, and the distance of S2···N2 is 3.148 Å. Isomer 1 formed π···π stacking structure along the a axis, where the interplanar π···π stacking distance is 3.437 Å (Figure 1B) with relatively large overlaps between neighboring molecules (Figure 1C). Though the synthesis and characterization of isomer 2 has been reported in the literature, its crystal structure has not been reported. Isomer 2 exhibits a similar planar structure compared to isomer 1, but with a different packing motif. The molecular structure of 2 possesses an orthorhombic P2(1)2(1)2(1) space group with four molecules in the unit cell. The two thiophene rings and center ring are coplanar, and the torsion of S1−C4− C5S2 is only 0.5(0)° (Figure 2). Multiple S···S, S···N, π···N, and π···π intermolecular short contacts are found between the neighboring rows and neighboring layers (SI, Figure S3). There are Csp2-H···N interactions between adjacent molecules with H···N distance around 2.64 Å. Compound 2 formed π···π stacking structure along the a axis, where the interplanar π···π stacking distance is 3.471 Å with similar molecular overlapping as compound 1. The crystal packing of these two isomers is different, though the interplanar π−π distances are similar (SI, Figures S2 and S3). Multiple S···S, S···N, π···N, and π···π intermolecular short contacts result in the herringbone crystal packing of isomer 2, whereas isomer 1 does not have this feature. Molecular packing in solid state plays a very important role in the organic semiconductor and optoelectronic properties.42−45 In order to explore how isomer geometry affects intermolecular interaction energy, we performed quantum mechanical calculations on intermolecular interaction energies for both isomers at MP2/ aug-cc-pVDZ (with BSSE correction) and dispersion-corrected density functional theory (DFT-D) calculations using B97D/ TZV level of theory.46−48 Initial geometries of dimers were extracted from the crystal structures and single-point energy computations were performed. The noncovalent intermolecular interaction energies were calculated by subtracting the individual energies of the monomers from the energy of the dimer structures. The DFT-D results demonstrated that the isomer 2 was more stable than isomer 1 by 1.5 kcal/mol (−12.9 kcal/mol for isomer 2 vs −11.4 kcal/mol for isomer 1). This is perhaps due to a greater interaction between sulfur and π electrons of the thiophene rings in isomer 2 than that in isomer 1. MP2 calculation results give a similar trend of dimerization energy, the dimer of isomer 2 is more stable than the dimer of isomer 1 by 0.7 kcal/mol, though the MP2 calculation seem to overestimate the dimerization energy (dimerization energies are −19.2 kcal/mol and −18.5 kcal/mol for isomers 2 and 1, respectively). Due to the lack of crystal structure data, the intermolecular interaction energy of isomer 3 could not be obtained. The electrochemical properties of these three isomers were studied by cyclic voltammetry in 1,2-difluorobenzene (DFB) solution with 0.1 M TBAPF6 as the supporting electrolyte. Due to the oxidative polymerization of these two isomers, a freshpolished Pt electrode was used to study the reduction of the compounds. Because of the fast following chemical reaction (i.e., polymerization) after the oxidation of the compound, we recorded the first cycle of the cyclic voltammogram to identify the first oxidation potential shown here. We chose Ep/2 to represent estimated oxidation potential for all three compounds.
Figure 1. Crystal structure of compound 1: (A) top view with thermal ellipsoids set at the 50% probability level except hydrogen atoms; (B) side view of two overlapped molecules; (C) top view of two overlapped molecules.
Figure 2. Crystal structure of compound 2: (A) top view with thermal ellipsoids set at the 50% probability level except hydrogen atoms; (B) side view of two overlapped molecules; (C) top view of two overlapped molecules.
obtained after recrystallization. Detailed crystallography data including crystallographic tables, cif files, and checkcif results for all these crystals are given in the Supporting Information. The crystal structure of compound 1 possesses an orthorhombic P2(1)2(1)2(1) space group with four molecules in the unit cell. Two thiophene rings and the center ring are coplanar, and the torsion of C2−C3−C7−C8 is 0.9(2)° 16761
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Figure 3 shows cyclic voltammograms of compounds 1, 2, and 3 with a Pt working electrode. The cathodic cycle was
Electrochemically estimated highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO− LUMO) gaps, the energy differences between the first reduction potential and first oxidation potential, are 2.19, 2.01, and 2.10 V for compounds 1, 2, and 3, respectively. The oxidation reaction occurs irreversibly, which is likely due to the following chemical reactions of the electrochemically formed radical cations that are the key intermediates responsible for the electrochemical polymerization.50−53 All three compounds were polymerized at the electrode surface through oxidation processes with multicycle potential sweeps.54,55 As shown in Figure 3B, compound 1 polymerized onto the electrode surface after oxidation, showing a new redox couple at Ep/2 = 0.62 V and an irreversible reduction peak at Ep = 0.87 V. However, the current of this new redox couple only slightly increases followed by repetitive potential sweeps. This phenomenon implies that the polymerization slows down after initial polymer formation at the electrode surface, which eventually blocks further polymerization at the electrode surface. The fresh monomer 1 has an oxidation peak at Ep/2 = 1.45 V that is not observed after polymer formation at the electrode surface. This observation demonstrates that the polymeric film of compound 1 somewhat blocks, or at least slows down significantly, the electron transfer reaction at the electrode−electrolyte interface. In contrast, compound 2 shows significant current increase as the number of repetitive potential sweep cycles increase.29,56 Compound 3 was polymerized after the first oxidation similar to what we observed in compounds 1 and 2. The electrochemical polymerization of compound 3 shows a new reduction wave at Ep/2 = 0.39 V corresponding to the first oxidation wave, and the oxidation peak at Ep/2 = 1.48 V shifts to more positive, followed by the current increases caused by increasing the number of potential sweep cycles. The smaller current observed with compound 1 reflects the fact that a smaller amount of electroactive materials was deposited onto the electrode surface than that of compounds 2 and 3. This difference of electrochemical polymerization behavior implies that the polymer structures and electron transfer kinetics of these compounds may be different. Polymer films of compounds 1, 2, and 3 show broad oxidation peaks at much lower oxidation potential than the monomers. Though the polymers still maintain clear reduction waves for the dicyanomethylene group, the reduction potentials also moved in the positive direction significantly (Figure 3C). These results are in line with our calculated IPs and EAs (Table 2) for the oligomers, which show that IPs decrease and EAs increase as the degree of polymerization increases. The electron absorption spectra of 1, 2, and 3 are shown in Figure 4. The maximum absorption of 1 is red-shifted in comparison with 2 and 3 and exhibits two absorption peaks at ca. 379 and 398 nm (Figure 4A). Compounds 1, 2, and 3 also display an absorption band in the range of 450−700 nm. The low-energy maximum absorption of trans-isomer 2 is at ca. 600 nm (2.067 eV); whereas, compound 1 displays its maximum at 530 nm (2.340 eV) and compound 3 displays its maximum at 552 nm (2.246 eV). These results are in good agreement with the electrochemical HOMO−LUMO gaps measured in DFB solution. TD-DFT calculation (at B3LYP/6-311G(d,p) level of theory) results indicate that these maximum absorption peaks are attributed to the intramolecular charge transfer (ICT) from the electron-rich sulfur atom to the electron deficient dicyanomethylene group.57−61 DFT calculated HOMO− LUMO gaps (Table 2) in the gas phase are higher than that
Figure 3. (A) Cyclic voltammograms (first cycle of a fresh polished Pt electrode) of compounds 1, 2, and 3 (1.0 mM) in 1,2-difluorobenzene (DFB) containing 0.1 M TBAPF6 at 100 mV/s. (B) Cyclic voltammograms (100 mV/s) recorded during the formation of polymer film on a Pt electrode in the same solution under potential sweeping conditions. (C) Cyclic voltammograms at 100 mV/s of polymer-1, polymer-2, and polymer-3 modified Pt electrode in DFB solution containing 0.1 M TBAPF6.
performed first to avoid contamination of the electrode surface by the polymer formed during anodic cycling. Compound 1 exhibits two reversible one-electron reduction reactions at E1/2 = −0.96 and −1.60 V. The oxidation reaction occurs irreversibly at EP/2 = 1.44 V. Compound 2 shows two reversible one-electron reduction reactions at E1/2 = −0.99 and −1.62 V. The oxidation reaction occurs irreversibly at EP/2 = 1.17 V, which is 0.27 V less positive than the oxidation of isomer 1. Isomer 3 shows two reversible one-electron reduction reactions at E1/2 = −0.98 and −1.62 V. The oxidation reaction occurs irreversibly at EP/2 = 1.27 V, which is 0.17 V less positive than the oxidation of isomer 1 and 0.10 V more positive than the oxidation of isomer 2. These experimental results (Table 1) are consistent with the empirically estimated redox potentials based upon DFT calculated EA and IP values (vide infra).49 Table 1. Redox Potential Data of Isomersa
a
cpds
Ered2 1/2 /V
Ered1 1/2 /V
EOX p/2/V
1 2 3
−1.60 −1.62 −1.62
−0.96 −0.99 −0.98
1.44 1.17 1.27
Potential data vs Ag/AgCl reference electrode. 16762
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Table 2. DFT Calculated HOMO, LUMO Energies, HOMO−LUMO Gap (Eg), IPs, and EAsa of Oligomers of Three Isomers Isomer 1 b
Eg
IP
EA
2.777 2.024 1.800 1.703 1.654 1.622 1.602 1.576 1.558 2
8.001 7.272 7.020 6.879 6.785 6.713 6.657 6.574 6.494
1.978 2.956 3.350 3.568 3.712 3.815 3.893 4.007 4.117
HOMO
LUMO
1 2 3 4 5 6 7 9 12
−6.358 −6.100 −6.070 −6.069 −6.075 −6.080 −6.085 −6.091 −6.098
−3.581 −4.076 −4.270 −4.366 −4.421 −4.458 −4.483 −4.515 −4.540 Isomer
no.b
HOMO
LUMO
1 2 3 4 5 6 7 9 12
−6.152 −5.665 −5.500 −5.420 −5.378 −5.352 −5.336 −5.319 −5.307
−3.608 −3.845 −3.989 −4.065 −4.115 −4.149 −4.174 −4.207 −4.235 Isomer
no.b
HOMO
LUMO
Eg
IP
EA
1 2 3 4 5 6 7 9 12
−6.252 −5.890 −5.827 −5.816 −5.820 −5.834 −5.852 −5.901 −5.897
−3.595 −3.953 −4.111 −4.200 −4.256 −4.294 −4.323 −4.355 −4.372
2.657 1.937 1.716 1.616 1.564 1.540 1.529 1.546 1.525
7.892 7.070 6.800 6.670 6.592 6.543 6.512 6.488 6.445
1.986 2.783 3.131 3.339 3.484 3.592 3.677 3.798 3.706
no.
Eg
IP
EA
2.544 1.820 1.511 1.355 1.263 1.203 1.162 1.112 1.072 3
7.783 6.840 6.443 6.223 6.081 5.983 5.911 5.810 5.714
1.996 2.599 3.071 3.284 3.427 3.531 3.609 3.721 3.829
Figure 4. UV−visible-near infrared (NIR) spectra of compounds 1 (black line), 2 (red line), and 3 (blue line). Monomers in CH2Cl2 solution and TD-DFT results (vertical bars) (A), monomers at solid states (B), and their polymers on FTO glass (C).
5). These polymers show similar diffraction patterns, although polymer 2 on FTO shows lower intensity diffraction signals and
a
All units are in electron volts (eV). All calculations were done with DFT at B3LYP/6-31G level of theory. Vertical IPs and EAs were calculated. bNumber of repeating monomer unit in the oligomer.
measured according to the maximum absorption measured in solution, indicating that solvation62−66 and possibly dimerization,67−69 play an important role in the light absorption of these compounds. The solid state absorption spectra measured by integrating sphere show a red-shift of onset optical band gap around 200 nm for all three compounds 1, 2, and 3, a clear indication of the π−π stacking contribution to the absorption spectra. The onset optical bandgap for compound 1 is about 1.49 eV, more than 1 eV lower than the value calculated by DFT in gas phase. Compound 2 exhibits an optical onset bandgap of 1.24 eV. Compound 3 exhibits an optical onset bandgap of 1.33 eV. These results demonstrate the important role of solid state molecular packing in tuning the optoelectronic properties.70−76 After compounds 1, 2, and 3 electrochemically polymerized onto the FTO glass, the maximum absorption of the polymer films all red-shifted to the near-infrared region, which is a common phenomenon for the conjugated polymers owing to the aggregation of the conjugated polymer main chains and π−π stacking in the solid films. This is in line with the X-ray diffraction measurement of the thin films on FTO glass (Figure
Figure 5. Powder XRD results of electrochemically polymerized compounds 1 (black line), 2 (red line), and 3 (blue line) on FTO glass. (FTO background was given in SI Figure S21).
fewer observable diffraction signals. This is perhaps due to the low crystalline quality of the thin film which is possibly caused by a faster polymer growing process than the other two as observed in polymerization cyclic voltammogram in Figure 3B.77−79 The polymer of compound 1 exhibits a broad absorption band starting from about 2500 nm (0.50 eV) with a maximum absorption at 1500 nm (0.827 eV). The polymer of compound 3 exhibits similar absorption to compound 1. Compared to the other two isomers, the isomer 2 polymer shows an even longer wavelength absorption starting from the middle infrared range 16763
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the absorption spectra of the monomers and polymers. We plotted the HOMO−LUMO gaps against the reciprocal of number of repeating monomer units of fused thiophene oligomers (Figure 7). Oligomers’ band gaps of compounds 1 and 3 possess saturation behavior as the result of increasing the chain length of these two oligomers, whereas isomer 2 shows linearity.85
(>3000 nm or
