Thiazole fused S,N-heteroacene step-ladder polymeric semiconductors for organic transistors

Ladder-type thiazole-fused S,N-heteroacenes with an extended π-conjugation consisting of six (SN6-Tz) and nine (SN9-Tz) fused aromatic rings have been synthesized and fully characterized. To date, the synthesis of well-defined fused building blocks and polymers of π-conjugated organic compounds based on the thiazole moiety is a considerable synthetic challenge, due to the difficulty in their synthesis. Acceptor–donor building blocks M1 and M2 were successfully polymerized into ladder homopolymers P1–P2 and further copolymerized with a diketopyrrolopyrrole unit to afford step-ladder copolymer P3. The optical, electronic, and thermal properties, in addition to their charge transport behavior in organic thin-film transistors (OTFTs), were investigated. The results showed an interesting effect on the molecular arrangement of the thiazole-based ladder-type heteroacene in the crystal structure revealing skewed π–π-stacking, and expected to possess better p-type semiconducting performance. The polymers all possess good molecular weights and excellent thermal properties. All the polymer-based OTFT devices exhibit annealing temperature dependent performance, and among the polymers P3 exhibits the highest mobility of 0.05 cm2 V−1 s−1.


Introduction
In recent years, there has been a signicant drive towards changes in the molecular design and device engineering of semiconductors to optimize their charge-carrier mobilities. 1-3 Ideally, high mobility can be achieved when polymer units orient themselves in a coplanar conguration favourable for charge delocalization along a p-conjugated backbone. 4,5 Conjugated ladder-type small molecules and macromolecules, which feature coplanar and rigid p-conjugated backbones, have emerged as an intriguing class of new organic materials, due to their unique electrical, physical and chemical properties. [6][7][8][9][10][11] In contrast to conventional conjugated materials which tend to adopt a non-zero dihedral angle conformation as a result of thermal uctuation or torsional strain, 12 the fused rings in a pconjugated ladder-type backbone have a low degree of bond rotation, thus leading to a linear and torsion-free planar conformation. As a result, this leads to the reduction of the re-organizational energy during charge transfer and promotes p-p electron delocalization, enabling materials with higher chargecarrier mobilities in the condensed phase. [13][14][15] Several examples of such conjugated ladder-type copolymers with impressive high mobilities are composed of donor or acceptors units including pentacyclic indacenodithiophene (IDT), [16][17][18][19] indacenodiselenophene (IDSe), 20 indacenodithiazole (IDTz), 21 and bithiophene imide (BTI). 22 Another interesting class of fused ladder-type building blocks with promising electronic and optical properties are based on heteroacenes. [23][24][25][26][27][28][29][30][31][32][33][34] Among the various reported building blocks, S,N-heteroacenes consisting of fused thiophene and pyrrole rings, a structural analogue of the electrondonating dithieno[3,2-b:2 0 ,3 0 -d]pyrrole (DTP), unit are of particular interest. [35][36][37][38] Introducing solubilizing substituents on the sp 2 -hybridized nitrogen atom of the pyrrole moiety, enhancing the intramolecular charge transfer (ICT) interactions, and tuning the energy levels are promising strategies for inuencing the properties required to achieve high performance organic thin-lm transistors (OTFTs). Mitsudo et al. reported the rst example of a fused aromatic S,N-heteroacene (SN5) consisting of ve membered-rings. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the small molecules were tuned by the functional groups on the nitrogen atoms. 39 Extending the structure to six-fused aromatic rings afforded S,N-heterohexacene (SN6) (Fig. 1), which was reported by Bäuerle and co-workers. 40 Vacuum-deposited lms of the acceptor-capped SN6 oligomers exhibited a mobility of 0.021 cm 2 V À1 s À1 . The preparation of S,N-heteroacenes was further extended from SN8 to a stable SN13 tridecamer, providing data with an interesting structure-property relationship. 41,42 Most recently, Wong and co-workers reported the synthesis of donor-acceptor (D-A) alternating copolymers containing pentacyclic S,N-heteroacene building blocks. OTFT devices fabricated using the step-ladder copolymer exhibited a hole mobility of 0.1 cm 2 V s À1 . 43 However, all current reported examples are based on S,Nheteroacenes consisting of fused thiophene and pyrrole units in the backbone. Replacing the anked thiophene units with the more electron-decient thiazole moieties is a promising approach to increase the ionization potential and improve the oxidative stability. Such an example as depicted in Fig. 1 includes fused pyrrolo[3,2-d:4,5-d 0 ]-bisthiazole (PBTz). 34,[44][45][46] In addition, the anking thiazole units can provide potential anchoring points for the formation of non-covalent bonds or can enhance the van der Waals interaction with neighbouring units to further rigidify and coplanarize the conjugated system. 47 Until now there have been no reported examples of thiazole-fused S,N-heteroacene building blocks, which can be attributed to the difficulty in their synthesis.
Herein, we report the rst example of ladder-type thiazolefused S,N-heteroacenes with an extended p-conjugation in the backbones consisting of six and nine fused aromatic rings (Fig. 1). The acceptor-donor building blocks were successfully polymerized into ladder homopolymers and copolymerized with a diketopyrrolopyrrole (DPP) unit to afford a step-ladder copolymer. Their optical, electronic, and thermal properties and charge transport behavior in OTFTs were investigated. The results showed an effect on the molecular arrangement of the thiazole-based ladder-type heteroacene in the crystal structure revealing a skewed p-p-stacking at a higher order. The polymerbased OTFT devices exhibit annealing temperature dependent performance with the highest mobility of 0.05 cm 2 V À1 s À1 obtained for P3.
We postulate that the formation of 4 can be a result of a radical initiation that takes place by the homolytic ssion of monomer 3 into R1 and R2 radicals (Fig. 2). The radical R1 then can either recombine with R2 to generate monomer 3 or react with itself to form 4, and this was only conrmed once the sample was characterized using single crystal XRD analysis (Fig. 3a, Tables S4 and S5). † In addition, the homo-dimer of R2 can also be observed in small traces as evaluated by NMR spectroscopy.
Buchwald-Hartwig amination of the crude precursor 3 containing trace amounts of 4 (used without further purications) with hexadecyl amine or 2-n-octyl-1-dodecylamine in a sealed microwave vial at 100 C in the presence of tris(dibenzylideneacetone)dipalladium(0) as the catalyst under basic conditions afforded triisopropylsilyl (TIPS) protected six membered fused ladder monomers 6a and 6b, and nine-membered ladder monomers 5a and 5b from precursor 4. Subsequently, TIPS deprotection of monomers 5a/b and 6a/b with TBAF afforded 7a/b and 8a/b in good overall yield. Initial attempts to dibrominate monomers 7a and 7b using N-bromosuccinimide (NBS) at room temperature did not yield the desired products. Successful bromination using NBS at a lower temperature (À15 C) in anhydrous chloroform afforded the desired monomers M1 and M2 (Schemes 1 and 2) in 94% and 82% yield, respectively.
Interestingly the six thiazole fused S,N-heteroacene monomer 7b exhibits a blue emission in chloroform solution under 365 nm UV light, while the nine-heterocyclic ring system 8b exhibits green emission (Scheme 1), which can be attributed to the extension of the heterocyclic ring system. Single crystal XRD analysis conrmed the structure of the hexadecyl ladder monomer 7a (Fig. 3b, Tables S6 and S7), † which crystallizes in the monoclinic crystal system, exhibiting a centrosymmetric  space group of P2 1 /c at the point of inversion. In comparison to the thiophene-fused SN6 single crystal structure reported by Bäuerle, 40 which has two equivalent molecules in the unit cell, the thiazole-fused 7a also exhibits two equivalent molecules in the unit cell, with a 2-fold screw axis of symmetry; a ¼ 25.4850(5), b ¼ 14.7657(3), and c ¼ 5.59660(10)Å; a ¼ 90.00, b ¼ 94.742 (2), and g ¼ 90.00 (additional information such as bond lengths and torsion angles is summarized in Tables S6 and S7). † Interestingly, the ordered non-covalent self-assembly established in the crystal packing of the thiophene based system SN6 was reported to be 3.37Å, with a p-p interaction at a distance of 3.55Å. 40 However, the thiazole containing moiety 7a along the caxis through the anking thiazole sulfur-sulfur (S-S) interactions was measured to have a slightly shorter contact of 3.35Å, which is lower than the sum of van der Waals radii (1.8Å for one S) (Fig. 3d). The molecular arrangement of 7a in the crystal structure reveals a skewed p-p stacking with a distance of 3.63 A in the columns. The bond angle of S-C-N in the thiazole moiety is 116.6 , while for the thiophene fused SN6 it was reported to be 113.8 which is slightly lower. The presence of the nitrogen atom in the thiazole moiety also has an effect on the bond length (1.305Å) in comparison to the thiophene counterpart (1.366Å) in SN6. Intermolecular charge transfer is closely related to the orientation and molecular packing in the backbone. Based on the single crystal structure of 7a, charge transfer integrals between two p-p stacked molecules were calculated. A large hole charge transfer integral between two molecules is found at 0.27 eV, which indicates efficient hole transfer in the single crystal. However, the electron transfer is less efficient with a transfer integral at 0.13 eV, and thus, the polymers derived from thiazole-fused S,N-heteroacenes are expected to possess better p-type semiconducting performance. The good intermolecular charge transfer could be attributed to the co-facial orientation between molecules, which is promoted by the ladder-type backbone structure. As a comparison, the calculated hole and electron charge transfer integrals in compound 2 are lower (0.08 and 0.03 eV) than that of 7a, 0.27 and 0.12 eV, respectively. This can be attributed to the twisted stacking orientation present in the molecule ( Fig. S2 and Table S8 †).

Polymer synthesis
As depicted in Scheme 2 homopolymers P1 and P2 were prepared via microwave-assisted Stille coupling of M1 and M2 with bis(tributyltin) in xylene using tetrakis(triphenylphosphine) palladium(0). Copolymer P3 was synthesized by copolymerization of monomer M1 with the stannylated DPP monomer. All the polymers were precipitated in acidied methanol and puried via Soxhlet extraction with a sequence of reuxing methanol, acetone, and n-hexane. Finally reuxing chloroform was used to extract the polymers. Aer removing the solvent, homopolymers P1 and P2 were isolated as dark blue solids, while the copolymer P3 was isolated as a dark green solid. All the polymers possessed good solubility in common chlorinated solvents such as chloroform and chlorobenzene. The number-average molecular weight (M n ) and the polydispersity index (Đ) were determined via gel permeation chromatography (GPC) in chlorobenzene solution at 85 C using polystyrene standards as the calibrants. The M n of homopolymers P1 and P2 was measured to be 11.7 and 13.6 kDa, respectively, with a narrow Đ in the range of 1.8-2.0, while copolymer P3 exhibited an M n of 20.1 kDa with a Đ of 2.1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under an inert atmosphere were used to investigated the thermal properties of polymers P1-P3 ( Fig. S3a and b). † The thermal decomposition temperatures (T d ) at 5% weight loss for the three polymers P1-P3 were measured to be above 300 C. Homopolymers P1 and P2 having different alkyl side chains (linear vs. branched) and the copolymer P3 presented similar features with the onset weight loss corresponding to the elimination of alkyl side chains. The DSC data did not reveal any pronounced thermal transitions in the range of 25 to 300 C, as a result of the low degree of order in the polymer thin lm.

Optical and electrochemical properties
The optical properties of the novel thiazole fused heteroacene SN6-Tz 7b and SN9-Tz 8b monomers were investigated using Ultraviolet-visible (UV-vis) and uorescence spectroscopy as depicted in Fig. 4 and summarized in Table 1. Typically, multiple vibronic splitting absorption bands are observed in S,N-heteroacenes, and this is due to the rigidity of the highly ordered, planar and fused molecular p-backbone. SN6-Tz (7b) exhibits vibronically resolved absorption bands (Fig. 4a), at l max ¼ 334 and 321 nm corresponding to p-p* electronic transitions perpendicular to the molecular axis and an energy band at l max ¼ 371 and 390 nm attributed to the p-p* electronic transitions along the molecular axis. The lower energy transitions correspond to the HOMO-LUMO energy gap in the conjugated psystem. 42,48 In comparison, the thiophene fused SN6 monomer Scheme 2 Synthesis of homopolymers P1 and P2 and copolymer P3. exhibited energy absorption bands at l max ¼ 362 and 380 nm. Thus, replacing the thiophene units with the thiazoles in the ladder moiety slightly lowers the optical band gap from Eg opt ¼ 3.12 eV in SN6 to 3.03 eV in SN6-Tz (7b). SN6-Tz with an A-D-A architecture is expected to result in a smaller band gap in comparison to the SN6 counterpart. However, the polarized nature of the thiazole ring makes the heterocyclic thiazole unit act as both a donor and an acceptor. 49 The stronger electrondonating ability of the two neighbouring substituents makes SN6-Tz a weaker D-A than expected, and as a result only a small difference of $0.12 eV is seen in the band gap. As depicted in Fig. 4a when increasing the conjugation length from six units in SN6-Tz (7b) to SN9-Tz (8b) the absorptions bands are signicantly red shied by 41-46 nm (l max ¼ 412 and 436 nm), thus lowering the optical band gap further. Increasing the aromatic fused units from three in the DTP moiety (SN3), to ve (SN5), six (SN6), and nine (SN9) fused heterocycles, or even in the case of the thiazole moieties SN6-Tz and SN9-Tz leads to the main absorption maximum to gradually red-shi. On exciting SN6-Tz at 335 nm, a strong emission at 425 nm was observed (Fig. 4b), which is slightly red shied in comparison to the thiophene SN6 (411 nm) counterpart. The thiazole fused heteroacene SN6-Tz (7b) displayed a slightly higher Stokes shi of 2111 cm À1 compared to its thiophene fused SN6 counterpart (1985 cm À1 ), due to a higher dipole moment present in the excited state as a result of the electron withdrawing effect of thiazole units present at the peripherals. Similar results were observed for the extended nine membered SN9-Tz (8b) ladder monomer, exhibiting strong emission at 451 nm, with a broad shoulder at 490 nm. In addition, SN9-Tz (8b) also showed a Stokes shi of 792 cm À1 .
The UV-vis absorption spectra of polymers P1-P3 in chloroform solution and thin lms are depicted in Fig. 5 and the optical and electrochemical properties are summarized in Table  1. Both homopolymers P1 and P2 in solution display a HOMO-LUMO absorption band represented by p-p* transitions in the visible region peaking at around 560 to 580 nm, while the n-p* energy bands are observed at around 400 nm. By incorporating the strong accepting DPP unit into the copolymer P3, the LUMO energy level is lowered and the absorption peak is red-shied to 750 nm.
Going from solution to the solid state, P1 shows a slight blue shi and broadening of the maximum absorbance peak (l max ) from 561 nm to 550 nm, while P2 shows a slight red shi from 584 nm to 592 nm, indicating the different optical behaviour of the polymer as a result of side chain engineering. The copolymer P3 shows a blue shi from 750 to 745 nm with broadening of the absorption and a pronounced shoulder peak visible around 855 nm, and this can be attributed to aggregation with strong interchain p-p stacking, which is benecial for regular structural organization of copolymer backbones in the solid state. [50][51][52] The optical band gaps (Eg opt ) of P1 and P2 calculated from the onsets of the absorption spectra in the thin lms are 1.49 and 1.51 eV, while the absorption onset of P3 is 1030 nm, resulting in a lower Eg opt of 1.22 eV. Cyclic voltammetry (CV) was used to measure the oxidation and reduction potentials of polymers P1ÀP3 in anhydrous acetonitrile under a nitrogen atmosphere. The HOMO and LUMO energy levels of the polymers were calculated using reduction and oxidation peaks with onset potentials relative to the ferrocene/ferrocenium (Fc/Fc + ) redox potential as shown in Fig. 5c. All the polymers show distinct oxidation and reduction bands indicating both electron donating and electron accepting characteristics. Homopolymers P1 and P2 exhibit distinct quasi-reversible oxidation bands with onset potentials (E ox ) at 0.51 and 0.49 V and an irreversible reduction band with onset potentials (E red ) at À0.84 and À0.83 V, respectively. The calculated HOMO/LUMO energy

DFT computational studies
To evaluate the effect of replacing the fused thiophene with the fused thiazole unit, a comparative DFT calculation (B3LYP/6-311G+(d,p)) was performed based on the structures of SN6 and SN6-Tz. Structure optimization was performed at the same level for SN9 and SN9-Tz, while all alkyl groups were simplied with methyl groups to shorten calculation times. Compared to the thiophene end groups in SN6 and SN9, the electron withdrawing thiazole units show slightly lower electron density (Fig. 6) on the HOMO and higher electron density on the LUMO, and thus as a result decrease both HOMO and LUMO energy levels. The thiazole groups pull the electron density from the heterocyclic SN-p-system, leading to a donor-acceptor system, and thus the stabilization of the frontier molecular orbital energies is achieved with narrowing of the band gaps. The extended SN9-Tz heteroacene shows an obviously lower calculated energy band gap (3.09 eV) than SN6-Tz (3.61 eV). However, the calculated results are both slightly at a higher end than the experimental Eg opt values; this could be attributed to the solvation effect.

OTFT charge transport characterization
The charge transport properties and electrical behavior of P1-P3 polymers were analysed by fabricating bottom-gate/bottomcontact (BG/BC) OTFTs (Table 2). Gold (Au) and titanium (Ti) layers were thermally evaporated onto a SiO 2 (300 nm)/p++-Si substrate using a shadow mask to form source and drain contacts, and this was followed by octadecyl trimethoxy silane (OTS) treatment. Fig. 6 Optimized geometries of thiophene fused SN6, thiazole fused SN6-Tz, thiophene fused SN9 and thiazole fused SN9-Tz molecules and spatial electron distributions of frontier molecular orbitals (FMOs) and their energy levels.

Material
Method m ave (cm 2 V À1 s À1 ) a m max (cm 2 V À1 s À1 ) a V th (V) I On /I Off

Crystallinity and morphology
Thin lms of polymers P1-P3 were fabricated and measured using XRD in order to analyse the relationship between the chemical structure of the polymers and their crystallinity behaviour, and the lamellar/p-p stacking d-spacing values are summarized in Table 2. As depicted in Fig. 8, all polymers exhibit a similar degree of crystallinity with lamellar 2q peaks in the range 3.7 to 4.6 and p-p stacking peaks ranging from 21 to 24 . With the branched side chains, P2 presented a larger lamellar packing distance (23.86Å) compared to P1 (21.53Å) and P3 (19.19Å), which possess linear side chains. Similarly, the p-p stacking distance of P2 (4.50Å) is also larger than that of P1 (3.77Å) and P3 (3.69Å). In addition, small lamellar reection at 2q ¼ 7.2 was observed for P3. These results suggest that copolymer P3 presents better packing and higher crystallinity than homopolymers P1 and P2, which is in good alignment with the solid-state UV-vis absorption data and as a result could contribute to higher mobility performance in OTFT devices. Additionally, morphological properties in the solid-state thin lms for P1-P3 were evaluated using atomic force microscopy (AFM). As shown in Fig. 9 the AFM images of P1 and P3 with linear side chains present slightly better homogeneity, in comparison with P2, which exhibits some nonuniform regions  on the surface (Fig. 9a). P2 has a rougher surface with a root mean square (RMS) roughness of 1.04 nm, while P1 and P3 show slightly lower values of 0.80 and 0.75 nm, respectively. In addition, P2 presents bigger grain sizes as depicted in the phase image of Fig. 9b, in comparison with P1 and P3. These small differences were easily detected by 3D-height topography as shown in Fig. 9c (represented by the same scale for the polymers).

Conclusion
In summary, we have reported the design, synthesis and characterization of four novel S,N-heteroacene based thiazole endfused ladder monomers. Linear and branched alkyl groups on the nitrogen atoms afforded six fused aromatic ladder-type molecules M1 and M2. We further extended the conjugation length to afford ladder-type 5a-b with nine-fused rings in excellent solubility. In comparison to the thiophene-fused analogue SN6, the thiazole-fused molecules show a different molecular arrangement in the crystal structure and have a slightly enhanced coplanarity in the backbone. In addition, SN6-Tz exhibits highly structured and vibronically resolved absorption bands, which are red-shied in comparison to the thiophene fused SN6 monomers exhibiting slightly lower absorption bands. Thus, replacing the thiophene units with the thiazoles in the ladder moiety slightly lowers the optical band gap from 3.12 eV to 3.03 eV. When increasing the conjugation length from six units in the thiazole ladder monomer to nine, the absorptions bands are signicantly red shied by 41-46 nm, thus further lowering the optical band gap. Furthermore, the thiazole-fused heteroacene SN6-Tz displayed a slightly higher Stokes shi compared to its thiophene fused SN6 counterpart.
Ladder homopolymers P1 and P2, and step-ladder copolymer P3 constructed from thiazole-fused S,N-heteroacene moieties present good solubility and high thermal stability up to 300 C. All the polymers exhibit narrow band gaps ranging from 1.28 and 1.35 eV. The presence of the strong accepting DPP unit in the copolymer P3 backbone increases the p-p interactions and promotes the packing of polymer chains in the solid-state, which was investigated by thin-lm absorption and XRD characterization. The absence of the DPP unit in the homopolymers P1 and P2 results in lower OTFT mobilities in the order of 10 À3 and 10 À4 cm 2 V À1 s À1 , respectively. Meanwhile P3 exhibits mobility values as high as 0.05 cm 2 V À1 s À1 . Overall, this study has demonstrated that step-ladder polymers consisting of thiazole-fused S,N-heteroacene building blocks can be suitable candidates for the fabrication of semiconductor OTFTs.

Data availability
The datasets supporting this article have been uploaded as part of the ESI material. †

Author contributions
S. Attar carried out the synthesis; R. Yang, Z. Chen & Y. Liu carried out OFET measurements; X. Ji did the DFT computational studies; M. Comi XRD and AFM measurements; S. Banerjee, and L. Fang edited and contributed in the writing of the manuscript with M. Al-Hashimi taking the led in with idea and nialzing.

Conflicts of interest
There are no conicts of interests to declare.