Stereoisomers of an azine-linked donor–acceptor conjugated polymer: the impact of molecular conformation on electrical performance

Abstract

We herein report the synthesis of a pair of azine linked donor–acceptor type conjugated polymers by the use of palladium(II)-based direct arylation. The two stereoisomers of the azine molecule were synthesized, separated, characterized, and further incorporated with a thiophene–phenylene–thiophene-based fused lactam (TPTL) acceptor molecule to form the donor–acceptor skeletons. The effects of the azine linkage isomerism on the polymer skeleton were studied both theoretically and experimentally. Semiconducting properties of these polymers were also evaluated in thin film transistors.

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Introduction

The synthesis of organic semiconductors has been extensively developed in recent years due to the ability to fabricate low-cost and/or large-area electronic components.^1,2 Among the various organic molecules used as semiconductors, conjugated polymers with donor–acceptor (D–A) architectures are of particular interest because the donor–acceptor systems facilitate partial intramolecular charge transfer (ICT), and the deployment of these systems hence enables the manipulation of electronic structure, leading to low-bandgap semiconductors with relatively high charge-carrier mobilities.^3–10 Hybridization of the HOMO on the donor moiety and the LUMO on the acceptor moiety provides a means to tune the electronic and optoelectronic properties of these donor–acceptor systems for device applications.^11–15 The development of new donors/acceptors that undergo this type of chemistry is thus a compelling goal. We have recently developed a new acceptor molecule based on thiophene–phenylene–thiophene fused bislactam (TPTBL) and its donor–acceptor copolymers as novel OTFT materials.¹⁶ The TPTBL molecule has a highly planar structure with a strong electron-withdrawing amide group, and has thus been recognized as a promising acceptor material.

Azine-based polymers, i.e., polyazines (PAZs), constitute an important class of conjugated polymers with a –C=N–N=C– backbone, which is isoelectronic with the diene structure, i.e., –C=C–C=C–.^17–26 When a π-conjugated dialdehyde or diketone compound reacts with an equimolar amount of hydrazine, PAZs with extended π-conjugation along the polymer main chain can be formed. Compared with their carbon counterparts, PAZs are more thermally stable and air-stable, and have drawn interest since the 1980's mainly as electrically conductive polymers. Other interesting properties such as non-linear optical phenomena have also been reported for some PAZs.^27–31

Here we report the synthesis and the photophysical and electrochemical properties of a new type of D–A material made of conjugated polymers based on the TPTBL molecule and azine-based 2-lauroyl-substituted 3,4-ethylenedioxythiophene (EDOT) (Fig. 1). We also investigated the effects of isomerism in the azine fragment on the optoelectronic properties of the corresponding polymers. Although isomerism in azine-based molecules has been well studied,^31–33 the current work is the first example, to the best of our knowledge, of the preparation and characterization of each isomeric form of a polyazine, i.e., anti (E,E) (2a) and gauche (E,Z) (2b).

Fig. 1 The molecular structures of the two isomeric polyazines.

Experimental

All commercially available reagents were used without further purification.

The thiophene–phenylene–thiophene fused lactam (TPTL) and its brominated form (Br-TPTL) 3 was synthesized by a previously reported procedure.¹⁶

All the experimental techniques involving ¹H, ¹³C, FT-IR, mass, and UV-vis absorption spectra, together with cyclic voltammetries, were carried out using the same methods as previously described.¹⁶

Fabrication of bottom-gate top-contact OFETs, analysis of the films using XRD and AFM techniques and device characterization were performed also following the same procedure as previously reported.^16,34

Synthesis of 2-lauroyl EDOT (1)

Anhydrous aluminum chloride (0.94 g, 7.03 mmol) was added in portions to a mixture of EDOT (1.00 g, 7.03 mmol) and lauroyl chloride (1.53 g, 7.03 mmol) in dichloromethane, under nitrogen atmosphere at room temperature. The above mixture was stirred for three hours at room temperature, and after completion of the reaction (confirmed by TLC) the whole mixture was poured into ice-cold water. The combined organic layer was extracted with ethylacetate, dried over anhydrous MgSO₄ and filtered. The excess solvent was evaporated, and the crude product was purified by using silica gel column chromatography to yield compound 1 as a colorless oily liquid (1.70 g, 75%); R_f 0.25 (5 : 1, Hex : EtOAc); ¹H NMR (400 MHz, CDCl₃, δ) 6.66 (1H, s, ArH), 4.38 (2H, m, (–O–CH₂–)), 4.24 (2H, m, (–O–CH₂–)), 2.83 (2H, t, J = 7.4, (CH₂)), 2.35 (2H, t, J = 7.4, (CH₂)), 1.63 (4H, m, 2 × (CH₂)), 1.28 (12H, m, 6 × (CH₂)), 0.88 (6H, t, J = 6.8 Hz, (CH₃)).

Synthesis of azine molecules 2a and 2b

A mixture of 2-lauryl EDOT (600 mg, 1.84 mmol) and hydrazine monohydrate (47.0 mg, 0.92 mmol) was stirred at 50–60 °C in ethanol and acetic acid (5 mL/1 mL) for 3 h. After the reaction (monitored by TLC) was completed, water was added and the organic layer was extracted with ethylacetate, dried over anhydrous MgSO₄ and filtered. The solvent was evaporated and the crude product purified by using silica gel column chromatography with 5 : 1 hexane–ethyl acetate as the eluent. Both of the isomers were separately collected and characterized.

Azine 2a (upper spot) as a yellow oily liquid (280 mg, 45%); R_f 0.37 (5 : 1, Hex : EtOAc); ν_max (KBr)/cm⁻¹ 2917, 2850, 1641, 1467, 1390, 1238, 1060 and 844; ¹H NMR (400 MHz, CDCl₃, δ) 6.66 (2H, s, 2 × ArH), 4.37 (4H, d, J = 5.4 Hz, 2 × (O–CH₂–CH₂)), 4.24 (4H, d, J = 5.4 Hz, 2 × (O–CH₂–CH₂–O)), 2.83 (4H, t, J = 7.0 Hz, 2 × (–CH₂–)), 1.68 (4H, m, 2 × (CH₂–)), 1.26 (32H, multiplet containing quintet, J = 6.3 Hz, 2 × (CH₂)₈) and 0.88 (6H, t, J = 6.8 Hz, 2 × (CH₃)); ¹³C NMR (100 MHz, CDCl₃, δ) 160.57, 140.60, 140.27, 118.11, 101.21, 63.80, 63.22, 30.92, 28.69, 28.27, 26.26, 21.68 and 13.11; m/z (FAB) 645 (100%), 489 (40%) and 322 (50%) [found M⁺ 645.3757. C₃₆H₅₇Br₂N₂O₄S₂ requires M, 645.3760].

Azine 2b (lower spot) as a pale yellow solid (205 mg, 40%); mp 70–72 °C; R_f 0.22 (5 : 1, Hex : EtOAc); ν_max (KBr)/cm⁻¹ 2917, 2850, 1641, 1467, 1390, 1238, 1060 and 844; ¹H NMR (400 MHz, CDCl₃, δ) 6.35 (2H, s, 2 × ArH), 4.28 (4H, d, J = 5.4 Hz, 2 × (O–CH₂–CH₂)), 4.23 (4H, d, J = 5.4 Hz, 2 × (O–CH₂–CH₂–O)), 2.99 (4H, t, J = 7.0 Hz, 2 × (–CH₂–)), 1.54 (4H, m, 2 × (CH₂–)), 1.23 (32H, multiplet containing quintet, J = 6.3 Hz, 2 × (CH₂)₈) and 0.87 (6H, t, J = 6.8 Hz, 2 × (CH₃)); ¹³C NMR (100 MHz, CDCl₃, δ) 192.74, 144.25, 141.67, 120.13, 108.13, 65.22, 63.93, 41.34, 31.91, 29.51, 24.32, 22.68 and 14.11; m/z (FAB) 645 (100%), 489 (50%) and 325 (75%) [found M⁺ 645.3760. C₃₆H₅₇Br₂N₂O₄S₂ requires M, 645.3760].

Synthesis of PAz-I copolymer

The brominated lactam 3 (100.0 mg, 0.122 mmol), azine 2a (79.00 mg, 0.122 mmol), Pd(OAc)₂ (3.41 mg, 0.015 mmol), and potassium acetate (35.95 mg, 0.366 mmol) were all transferred into a Schlenk flask under nitrogen atmosphere. Anhydrous DMAc (3 mL) was then added, and the mixture was stirred at 130 °C for 12 h. The resultant mixture was poured into 100 mL of methanol. A resulting purple precipitate was filtered and further purified by carrying out a Soxhlet extraction using methanol, acetone and hexane to remove any remaining salts or oligomers and to give the desired polymer PAz-I as a purple powder (90 mg, 55%); GPC (CHCl₃, RI) Da, M_n = 9.5 × 10³, M_w = 40.2 × 10³; TGA(%/^oC) 90/345, 80/408; ¹H NMR (400 MHz, CDCl₃, δ) 7.71 (2H, broad signal, 2 × ArH), 7.60 (2H, broad signal, 2 × ArH), 4.44 (12H, broad signal, 2 × (N–CH₂), 2 × (O–CH₂CH₂–O)₂), 2.81 (4H, broad signal, 2 × (–CH₂)), 1.85 (4H, broad signal, 2 × (–CH₂)), 1.10 (36H, broad signal, 2 × (–CH₂)₉) and 0.84 (12H, broad signal, 4 × (CH₃)).

Synthesis of PAz-II copolymer

PAz-II copolymer was prepared essentially in the same manner as was PAz-I, except for the use of azine 2b (79.00 mg, 0.122 mmol). PAz-II was obtained as a purple powder (75 mg, 50%); GPC (CHCl₃, RI) Da, M_n = 10.2 × 10³, M_w = 20.1 × 10³; TGA(%/^oC) 90/345, 80/408; ¹H NMR (400 MHz, CDCl₃, δ) 7.71 (2H, broad signal, 2 × ArH), 7.52 (2H, broad signal, 2 × ArH), 4.39 (12H, broad signal, 2 × (N–CH₂), 2 × (O–CH₂CH₂–O)₂), 2.98 (4H, broad signal, 2 × (–CH₂)), 1.80 (4H, broad signal, 2 × (–CH₂)), 1.49 (4H, broad signal, 2 × (–CH₂)), 1.25 (36H, broad signal, 2 × (–CH₂)₉) and 0.86 (12H, broad signal, 4 × (CH₃)).

Results and discussion

Synthesis of the azine monomers

We first prepared the EDOT-based azines 2a and 2b from the 2-lauryl-substituted EDOT molecule 1 as shown in Scheme 1. The azines 2a and 2b were formed by a condensation reaction between compound 1 and hydrazine monohydrate in the presence of acetic acid in an ethanol solution. The reaction was monitored by TLC analysis and two spots appeared after full conversion of the starting compound 1. These spots were successfully separated by column chromatography and each product was characterized by acquiring ¹H NMR, ¹³C NMR (Fig. S1–S4 in the ESI†) and high-resolution mass spectra.

Scheme 1 Synthesis of the two isomeric azine molecules: anti-isomer 2a (E,E) and gauche-isomer 2b (E,Z).

The mass spectra of the azine products showed equal molecular weights for both molecules, indicating that they were isomers. The ¹H NMR spectra of the two isomers were very similar, and only a slight difference between their chemical shifts was observed. After several attempts to identify the exact structures of the two compounds, we presumed that the compound with a higher R_f value in the TLC, and hence having the less polar nature, might be the anti-isomer (2a).

Single crystals, grown by the slow evaporation of solution (dissolved in a mixture of methanol and chloroform) of both molecules 2a and 2b, were further analyzed by X-ray diffraction in order to verify the isomeric structure (Fig. 2). The results of this analysis confirmed that compound 2a had the anti configuration (E,E), whereas compound 2b had the gauche configuration (E,Z) (Fig. 2, S5 and S6 in the ESI†).

Fig. 2 Molecular structures of the two isomeric azines 2a and 2b, along with the atom numbering schemes used for these structures.

Synthesis of polymers PAz-I and PAz-II

Donor–acceptor-type conjugated polymers were synthesized by reacting azine monomers 2a and 2b each with a brominated TPTBL (thiophene–phenylene–thiophene fused bislactam) (3) acceptor monomer (Scheme 2). The polymerization reactions were carried out under palladium-based direct C–H arylation reaction conditions. Unlike the traditional metal-catalyzed cross-coupling reactions, including the Suzuki, Stille, Negishi, and Kumada reactions, in which organometallic compounds are necessary, the direct arylation reactions, in which the organometallic component is replaced by a simple unfunctionalized (hetero)arene, involve coupling through C–H bond cleavage; direct arylation reactions thus eliminate the need to carry out the numerous synthetic steps to prepare the corresponding organometallic reagents.^35,36

Scheme 2 Syntheses of polymers PAz-I and PAz-II.

After completion of the reactions, a dark blue powder was obtained in each case, and was precipitated in methanol and further purified by applying the Soxhlet extraction method. The structure was then analyzed by ¹H NMR spectroscopy (Fig. S7 and S8 in the ESI†). The molecular weights of the polymers were determined using GPC, and rather low molecular weights were obtained for both polymers (Table S11 in the ESI†). The TGA graphs of the obtained polymers showed good thermal stability. The characteristic degradation of azine bonding with liberation of nitrogen gas at 345 °C was also observed in the TGA diagram (see Fig. S9 in the ESI†).

Optical, electrochemical and vibrational properties

The UV-visible spectra of the polymers were recorded in both solution and film states (Fig. 3). The two types of isomeric linkages yielded similar absorption profiles. However, in the solution state, a slight red shift of about 5–8 nm was observed in the case of PAz-I compared to PAz-II. This bathochromic shift in the absorption profile of PAz-I was more visible in the film state, together with an obvious vibronic peak. The absorption maximum of the PAz-I thin film was at about 555 nm, whereas PAz-II yielded an absorption maximum at approximately 545 nm. These results were attributed to the greater planarity of PAz-I provided by the anti conformation of the azine linkage. These results are in good agreement with the theoretical evaluation, done using DFT calculations (vide infra). Finally the optical band gaps of the PAz-I and PAz-II polymers were estimated from the absorption edge value to be 1.78 eV and 1.80 eV, respectively (Table 1).

Fig. 3 UV-visible absorption spectra of PAz-I and PAz-II in chloroform solution (a) and as a thin film (b).

Table 1 Optoelectronic properties of the polyazines

Polymer	λmax^a (soln)	λmax film	HOMO (eV)	LUMO (eV)	E^optg^b (eV)
a Solution: in chloroform.b E^optg: optical bandgap estimated from the band edge (λonset) of the absorption spectra.
PAz-I	540	555	−5.40	3.62	1.78
PAz-II	535	545	−5.30	3.50	1.80

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In order to estimate the frontier orbital energy levels of the PAz-I and PAz-II polymers, we carried out cyclic voltammetric experiments by depositing the thin film of the studied polymers on a carbon electrode in a 0.1 M solution of TBAPF₆ in acetonitrile and scanned vs. Ag/Ag⁺ (Fig. S10 in the ESI†). The oxidation potential of the PAz-I polymer was found to be slightly higher than that of PAz-II, indicative of a lower HOMO energy level for PAz-I. This lower HOMO energy level of PAz-I suggests its higher oxidative stability, compared to that of PAz-II, due to the greater planarity provided by its anti conformation.

FT-Raman spectra were also recorded for the PAz-I and PAz-II polyazines in order to determine the effects of their different conformations on their molecular and electronic structures, and the results are shown in Fig. 4. Surprisingly, the spectral profiles of both systems were observed to be basically identical, with only small changes in the relative intensities of the most intense bands of the spectra. Theoretical eigenvectors (see ESI†) indicated these vibrations to be delocalized over the conjugated skeleton, with a stronger contribution from the EDOT unit.

Fig. 4 FT-Raman spectra recorded for the PAz-I and PAz-II polymeric systems.

In particular, while PAz-II yielded two comparably intense FT-Raman bands at 1488 and 1476 cm⁻¹, the former (1488 cm⁻¹) was observed to be more intense than the latter for the anti conformer, i.e., PAz-I. Using DFT calculations, we predicted the theoretical Raman activity for both systems (Fig. 5) and the results obtained are in good agreement with the experimental data (Fig. 4). That is, the higher frequency band gains intensity in the anti polymer, becoming the most intense of the spectrum.

Fig. 5 DFT-predicted Raman spectra for a monomeric unit of the anti polymer PAz-I (top) and the gauche polymer PAz-II (bottom).

DFT calculations

The two different polymeric conformers were predicted theoretically using the DFT level of calculation as implemented in Gaussian.³⁷ In order to reduce the computational cost, only one or two monomeric subunits were optimized to mimic the polymeric systems. In a first approach, a syn and an anti conformer were considered. However, DFT calculations predict a completely unstable structure for the syn conformer. On the contrary, the gauche configuration, as predicted by XRD data, is also stable.

The DFT quantum-chemical calculations, using the hybrid functional B3LYP^38,39 and the 6-31G** basis set,^40,41 predict the anti conformation to be 2.48 kcal mol⁻¹ and 5.73 kcal mol⁻¹ more stable than the gauche conformation for the one monomeric unit and dimer approach, respectively. However, both configurations are stable. The optimized geometrical structures differ substantially for both conformers. In fact, while the anti conformer, PAz-I, present a basically planar skeleton, the gauche polymer shows a dihedral angle of the azine unit of approximately 20° and therefore it is expected to present an helicoidal structure, as evidenced in Fig. 6.

Fig. 6 DFT/B3LYP/6-31G** optimized geometries for the monomer and dimer model systems of the polymers under study. (a) Front view of the monomeric model and (b) lateral view of the dimeric model.

The twist angles measured experimentally for the crystal structures of 2a and 2b are in between 110 and 130° (see values in Tables S3 and S8 in the ESI†). These data are in nice agreement with the values predicted theoretically for the optimized geometries of the monomers. The wB97XD functional⁴² was also tested to predict the molecular conformations. In this case, although the trend coincides with the one found for the B3LYP functional, it predicts a higher difference between anti and gauche conformers, being the anti one more stable by 19.67 kcal mol⁻¹. Furthermore, this functional predicts a more severe distorsion for the gauche derivative (see Fig. S11 and S12 in the ESI†). Nevertheless, since the trends are comparable for both functionals, the B3LYP will be the one used in the following discussions.

Using the TD-DFT approach, the electronic spectra of both conformers (anti and gauche), corresponding respectively to PAz-I and PAz-II, were predicted. As found experimentally, a redshift of the most intense band is observed upon going from the gauche conformer to the anti one (see Fig. S13 in the ESI†). Note that this red shift is not very pronounced, with only a 5 nm change, but it undoubtedly states that the anti conformer (PAz-I) is slightly more conjugated due to their planar conformation. However, one would expect a more drastic change of their electronic properties with such a dramatic change of the molecular conformation.

Therefore, the aforementioned data points out quite similar electronic scenarios for both conformers; however, a different crystal packing should be expected that may influence charge transport. Other important factor influencing the transport properties is internal molecular reorganization energy. The intramolecular reorganization energy, λ, is a parameter accounting for the structural changes needed to accommodate added charge, being small reorganization energies desirable for efficient charge transport. In this sense, while the anti conformer PAz-I shows hole and electron reorganization energies of 299 meV and 246 meV, respectively, these values are slightly incremented for the gauche conformer, PAz-II (λ_h = 324 meV, λ_e = 291 meV).

Thin film transistor fabrication and characterization

Bottom-gate top-contact OTFTs were fabricated to investigate the charge transport properties of the two polymers. The semiconductor layers were deposited by spin coating a 5 mg mL⁻¹ polymer solution in chloroform under ambient conditions on hexamethyldisilazane (HMDS)- and octadecyltrichlorosilane (OTS)-treated p-doped Si wafers with 300 nm thermally grown SiO₂ dielectric layers.

Both PAz-I and PAz-II polymers present p-type characteristics in an OTFT device (Fig. 7); however their performance is quite limited, possibly due to low conjugation of the azine linkage.^43,44 The results are summarized in Tables 2 and 3. Note that both on HMDS- and OTS-treated surfaces, PAz-I performed better than PAz-II. Taking in consideration that the electronic characteristics of both systems are similar, the differences in the device performance may be ascribed to changes in the film morphology/crystallinity, due to their completely different polymeric structure.

Fig. 7 Output plots of the OTFT devices: (a) output plot for PAz-I polymer deposited on OTS-treated substrates and annealed at 110 °C and (b) output plot for PAz-II polymer deposited on HMDS-treated substrates and annealed at 110 °C.

Table 2 OFET characteristics for PAz-I and PAz-II polymers. The polymeric films were annealed at 110 °C before device fabrication

Polymer	Substrate treatment	Mobility (cm^2 V^−1 s^−1)	VT (V)	ION/IOFF
PAz-I	HMDS	3.0 × 10^−5	−26	3 × 10^2
PAz-II	HMDS	1.9 × 10^−5	−30	6 × 10^1
PAz-I	OTS	4.3 × 10^−5	−28	9 × 10^2
PAz-II	OTS	1.7 × 10^−5	−34	5 × 10^1

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Table 3 OFET characteristics for PAz-I films deposited on HMDS-treated substrates and annealed at the indicated temperatures

Substrate temperature	Mobility (cm^2 V^−1 s^−1)	VT (V)	ION/IOFF
110 °C	3.0 × 10^−5	−26	3 × 10^2
150 °C	3.2 × 10^−5	−33	7 × 10^2
180 °C	6.1 × 10^−5	−39	4 × 10^3
210 °C	7.0 × 10^−5	−33	2 × 10^2

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Table 3 summarizes the OFET performance for PAz-I-based devices as a function of the annealing temperature. Note that the devices performances gradually increase with higher annealing temperatures; however, they still show modest figures of merit.

XRD spectra show a completely amorphous film for the gauche conformer, PAz-II, while a broad peak is observed for PAz-I films (Fig. 8). This indicates that even while both polymers are poorly crystalline, the anti configuration is preferred in terms of crystallinity towards the gauche one. This is in good agreement with the results recorded for the OTFT devices.

Fig. 8 XRD spectra of selected films.

Conclusions

We have synthesized and characterized two stereoisomerically related azine molecules, obtained by the reaction between 2-lauroyl EDOT and hydrazine. A palladium(II)-based direct arylation method was used to combine one or the other of these isomeric molecules with an electron-acceptor subunit consisting of a thiophene–phenylene–thiophene-based fused lactam (TPTBL) to form either the anti PAz-I or gauche PAz-II azine-linked donor–acceptor-type conjugated co-polymer. The effects of the different azine linkage isomers on the polymer skeleton were studied both theoretically and experimentally. While the two isomers were expected to form quite different polymeric structures, their electronic properties were not predicted to differ drastically, according to DFT calculations. Consistent with these calculations, their Raman spectra only showed slight differences in relative intensities.

Semiconducting properties of the anti PAz-I and gauche PAz-II polymers were also evaluated in thin film transistors. Although the semiconducting characteristics were shown to be modest for both systems, possibly due to the low level of conjugation provided by the azine linkage, the anti conformer systematically performed better than the gauche conformer. This difference is in accordance with the improved π-conjugation and lower reorganization energies of the anti conformer. Furthermore, even though both polymers yielded poorly crystalline films, the crystallinity resulting from the use of the anti conformer was better.

Acknowledgements

Research at Incheon National University was supported by the IT R&D program of MKE/KEIT(10044962). Research at University of Malaga was supported by MINECO (CTQ2012-33733) and Junta de Andalucia (P09-4708). R. P. O. and I. A.-M. thank the MINECO of Spain for a "Ramón y Cajal" research contract and for a predoctoral fellowship, respectively. For single crystal structure, Dr. Yonghwi Kim at the Center for Self-assembly and Complexity, Institute for Basic Science (IBS) POSTECH is acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available: Details of the NMR spectra, single crystal data, TGA and CV diagrams. CCDC 1453352 and 1453353. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra07389a

‡ These authors contributed equally to this work.

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