Fused electron deficient semiconducting polymers for air stable electron transport

Conventional semiconducting polymer synthesis typically involves transition metal-mediated coupling reactions that link aromatic units with single bonds along the backbone. Rotation around these bonds contributes to conformational and energetic disorder and therefore potentially limits charge delocalisation, whereas the use of transition metals presents difficulties for sustainability and application in biological environments. Here we show that a simple aldol condensation reaction can prepare polymers where double bonds lock-in a rigid backbone conformation, thus eliminating free rotation along the conjugated backbone. This polymerisation route requires neither organometallic monomers nor transition metal catalysts and offers a reliable design strategy to facilitate delocalisation of frontier molecular orbitals, elimination of energetic disorder arising from rotational torsion and allowing closer interchain electronic coupling. These characteristics are desirable for high charge carrier mobilities. Our polymers with a high electron affinity display long wavelength NIR absorption with air stable electron transport in solution processed organic thin film transistors.

A microwave vial was charged with (E)-1,1'-dibutyl-[3,3'-biindolinylidene]-2,2'-dione (300 mg, 0.80 mmol), 1-butylindolin-2-one (152 mg, 0.80 mmol) and triethyl amine (0.03 mL,0.24 mmol). The vial was sealed, evacuated and charged with argon three times, before anhydrous toluene (6 mL) was injected into the flask, degassed for 1 hour and heated at 120 o C for 16 hours. The Michael addition product above was not observed by proton NMR or LC-MS, only starting materials were observed. Compound S1 was obtained according to literature procedure. 2 Compound S1: 5-aminonaphthalene (1.82 g, 12.7 mmol) was dissolved in 20 mL of glacial acetic acid and heated to reflux. To the resulting purple solution was added a solution of diethyl ketomalonate (4.0 mL, 26 mmol) in glacial acetic acid (20 mL) dropwise over 30 minutes. The resulting suspension was heated at refluxed for 18 hours. The acetic acid was removed in vacuo and the resulting red solid was dissolved in 1 M NaOH to a final solution with pH of 11-12. The resulting yellow suspension was heated at refluxed with sparging air for 5 hours. The solution was then poured onto ice and acidified to pH 0 with aqueous 6 M HCl. The resulting bright red solid was collected by suction filtration, washed with water and dried in vacuum to yield the crude product as a red solid (1.6 g, 64%). The crude product was used without further purification.

Supplementary Figure 2: Single crystal of NIID.
Here is the single crystal structure of NIID. The dihedral angle of the two lactams is 12.8 o . Crystal data: C32H30N2O2, triclinic P-1, a: 15.3052 (5)

Synthesis of Phenyl Core Monomers
The synthetic procedure for the phenyl core monomers were adapted from literature. 1

Synthesis of N,N'-(1,4-phenylene)bis(N-(2-decyltetradecyl)-2-oxoacetamide)
Under argon atmosphere, oxalyl chloride (0.31 mL, 3.89 mmol) was diluted with DCM (4 ml) and cooled to -78 o C. A solution of DMSO (0.28 mL) in DCM (4.2 mL) was added to the reaction flask at -78 o C. The reaction flask was stirred for 20 minutes before a solution of N,N'-benzene-1,4-diylbis[N-(2-decyltetradecyl)-2-hydroxyacetamide] (1.45 g, 1.62 mmol) in 7 mL DCM was injected drop wise into the flask. The reaction mixture turns aqua green. After 1.5 hours at -78 o C, trimethylamine (2.26 mL, 16.2 mmol) was added slowly. The reaction was then stirred at -78 o C for 4 hours before it was warmed to room temperature slowly. The reaction was quenched with saturated NaHCO3 solution and the phases separated. The aqueous phase was extracted three times with DCM and the combined phases washed with brine, dried over MgSO4, filtered and the solvent removed under vacuum to yield brown oil, 0.61 g, which was used immediately.
The naphthalene bis-oxindole was synthesized according to literature. 3

Synthesis of N 1 ,N 5 -dihexadecylnaphthalene-1,5-diamine
The N,N'-(naphthalene-1,5-diyl)dihexadecanamide (12.0 g, 18.90 mmol) was suspended in dry THF (160 mL) and cooled to 0 o C. 1 M lithium aluminium hydride (75.6 mL, 75.6 mmol) was added cautiously to the reaction flask and the reaction was refluxed for 72 hours. To quench, the reaction was cooled to 0 o C, prior to a slow addition of 1 M NaOH. The quenched mixture was then concentrated under reduced pressure before addition of water and extraction of the aqueous phase with CHCl3. Combined organic phases were dried over MgSO4, the salts filtered and the solvent evaporated under reduced pressure. The brown residue was used without further purification (6.8 g, 60 % yield). MS TOF ES+: C42H74N2, calculated, 606.59, [M+H] + found 607.5931.

Synthesis of Thieno[3,2-b]thiophene-3,6-dicarboxylic acid
To a mixture of dimethyl thieno[3,2-b]thiophene-3,6-dicarboxylate (4.79g, 18.69 mmol, 1.0 e.q.) in ethanol/tetrahydrofuran/water (300 mL/300 mL/30 mL) was added sodium hydroxide (9.33 g, 233.2 mmol, 12.5 e.q). After the reaction mixture was refluxed overnight, the solvent was evaporated under vacuum to about half of its original volume. Water (300 mL) was added to the mixture and the solution was treated with concentrated hydrochloric acid until white precipitates formed. The precipitate was filtered and then washed with water to give thieno[3,2b]thiophene-3,6-dicarboxylic acid as a white solid which was dried in a vacuum oven and then used in the next step without further purifications (3.80 g, 89% yield

DFT Calculated Absorption Spectra and Hole and Electron
Wavefunctions in the S1 State

DFT Calculated Torsional Barriers
The torsional potentials between adjacent aromatic units of the various oligomers calculated at the OT-ωB97XD/6-31G(d,p) level of theory.

Supplementary Figure 3: Torsional potential profile of the polymers with changing dihedral angles.
A comparison of the torsional potential profiles for the dihedral angle between two adjacent aromatic cores in the PP, NN, and TN oligomer chains. All values are calculated at the OT-ωB97XD/6-31G(d,p) level of theory. The red line represents 1 kT energy (0.6 kcal/mol).
The optimal confirmations appears around 12º, 18º, and 0º for the PP, NN, and TN oligomers, respectively. It is worth stressing that the double-like character of the interunit bonds leads to very large barriers to full rotation, on the order of 25 kcal/mol, which is much higher than the values typical of chains with single-like inter-unit bonds (in the range of 2 to 8 kcal/mol).
A closer look of the torsional potentials (from 0º to 30º) between adjacent aromatic units of the oligomer series to highlight the smoothness of PES around the local minima.

Name
Backbone structure PP Phenylene-Phenylene NN Naphthalene-Naphthalene TN Thienothiophene-Naphthalene The vertical excited-state energies were evaluated at the TD-DFT OT-ωB97XD/6-31g(d,p) level of theory. The simulated absorption spectra calculated for the isolated oligomers ("in the gas phase") are shown below.

Supplementary Figure 5: Simulated absorption spectra for the PP, NN, and TN
The simulated absorption spectra of isolated oligomers were calculated at the TD-OT-ωB97XD/6-31G(d,p) level of theory. The spectra are simulated based on a Gaussian-function convolution (FWHM=0.3 eV) of the oligomer vertical excitation energies and oscillator strengths.

Supplementary Figure 6: Calculated hole and electron wavefunction of PP, NN, TN
Illustration of the hole and electron wavefunctions (isovalues 0.02 a.u.) determined at the TD-OT-ωB97XD/6-31G(d,p) level of theory for the S0 to S1 transitions in the PP, NN, and TN oligomers. The contributions of the various hole-electron configurations are given in percentages.

Supplementary Figure 7: Calculated hole and electron wavefunction of PP, NN, TN for S0 -Sn.
Illustration of the hole and electron wavefunctions (isovalues 0.02 a.u.) determined at the TD-OT-ωB97XD/6-31G(d,p) level of theory for the S0 to Sn transitions in the PP, NN, and TN oligomers. The contributions of the various hole-electron configurations are given in percentages.

Supplementary Table 1: The simulated oligomer reorganisation energy, ionisation potential and electron.
Ionization potentials and electron affinities of the isolated oligomers calculated at the tuned-ωB97XD/6-31G(d,p) level of theory. Reorganization energies in the anionic state of the oligomers, calculated at the tuned ωB97XD/6-31G(d,p) level of theory. The anionic oligomer chains and polaron wavefunctions (isovalues of 0.02 electrons 1/2 /bohr 3/2 ) were obtained at tuned ωB97XD/6-31G(d,p) level of theory.

UV-Vis and NIR Spectra of Polymers
Supplementary Figure 9: UV-Vis near infra-red absorption spectra of P1.
Normalised UV-Vis NIR spectra of polymer P1 in chlorobenzene and thin film spun from chlorobenzene 5 mg/mL.

Supplementary Figure 10: UV-Vis near infra-red absorption spectra of P2.
Normalised UV-Vis NIR spectra of polymer P2 in chlorobenzene and thin film spun from chlorobenzene 5 mg/mL. There is insignificant changes to the absorbance upon heating from 20-90 o C. A slight blue shift, as excepted, is observed. This polymer was measured because it has mobility of 0.03 cm 2 V -1 s -1 .

Synthesis of Polymer P3
(N-(2-octyltetradecyl))-napthalene bisoxindole (41.80 mg, 0.0523mmol), (N-(2octyltetradecyl))-napthalene bisisatin (43.26mg, 0.0523mmol) and PTSA monohydrate (4 mg, 0.3eq) were placed into a dry 2mL microwave vial which was capped and evacuated with argon. 0.5mL of degassed dry toluene was added and the mixture was heated at 120 o C for 2 hours to give a purple solid. Chlorobenzene was added and the polymer was precipitated into methanol. Successive soxhlet extractions with methanol, acetone, hexane and finally DCM gave a single major polymer fraction, which was reduced to minimum volume and precipitated into methanol to give 47 mg of a purple solid (56%yield). GPC (chlorobenzene, 80 °C): Mn 214 kDa, Mw 677 kDa.