Tethered tertiary amines as solid-state n-type dopants for solution-processable organic semiconductors

Tertiary amines covalently tethered to electron-deficient aromatic molecules by alkyl spacers enable solid-state n-doping.


MATERIALS AND METHODS
All reagents from commercial sources were used without further purification unless otherwise stated. N,N′-(1-hexyl)-1,4,5,8-naphthalenetetracarboxydiimide (NDI-Control), and 3,6-bis(5-bromo-2-thienyl)-2,5-bis(2-hexyldecyl)-2,5-dihydro-pyrrolo [3,4-c]pyrrole-1,4-dione (DPP-Control) were purchased from Aldrich. [6,6]-Phenyl C61 butyric acid methyl ester (PCBM-Control) was purchased from Solenne. 6-(Dimethylamino)hexylamine was purchased from Matrix Scientific. Deuterated solvents were obtained from Cambridge Isotope Laboratories, Inc. NMR spectra were recorded using a Varian 500 or 600 MHz spectrometer. All 1 H NMR experiments are reported in δ units, parts per million (ppm), and were measured relative to the signal for residual chloroform (7.26 ppm) in deuterated solvent. All 13 C NMR spectra were measured in deuterated solvents and are reported in ppm relative to the signals for residual chloroform (77.16 ppm) or 1,1,2,2tetrachloroethane (73.78 ppm). Mass spectrometry was performed on a Micromass QTOF2 quadrupole/time-of-flight tandem mass spectrometer (ESI) or a Waters GCT Premier time-of-flight mass spectrometer (EI). MALDI spectra were obtained on a Bruker Microflex series MALDI-TOF using a matrix of dithranol saturated chloroform. IR spectra were recorded on a Perkin Elmer Spectrum 100 with a Universal ATR sampling accessory. Toluene was dried by passage through two columns of alumina and degassed by argon purge in a custom-built solvent purification system. at 120 o C; PDI-NMe2 and PDI-C6/PDI-NMe2 composite samples were annealed for 4hrs. All samples were then inserted into 4 mm-diameter quartz EPR tubes. These tubes were sealed with plastic caps and Teflon tape inside the glovebox. They were then transferred outside of the glovebox and their EPR spectra were measured within two hours. Spin concentrations were determined by comparing the integrated signal intensity of a sample with the integrated signal intensity of a standard of known concentration. In this experiment, the spin concentration calculations were complicated by the variation in film thickness that resulted from the dropcast. The thickness of each sample was measured using profilometry. The film was scraped off in three locations down to the substrate and the average difference in thickness across these sites was quoted as the average thickness of the dropcasted film. To calculate the spin concentrations, the normalized, integrated intensity of the samples was compared to the normalized, integrated intensity of a standard material. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) was used as the standard sample and a calibrator for determining the spin concentrations of each material. All samples were measured in triplicate.

NDI-NMe2, NDI-Control, DPP-NMe2, and DPP-Control
EPR samples were prepared as discussed above and were all annealed for ~4hrs at 120 °C inside a glovebox prior to measurement.

PCBM-NMe2 and PCBM-Control
For enhanced signal-to-noise, solutions were case and dried directly inside the quartz EPR tubes, which were then annealed for 4 hrs at 150 °C in a glovebox prior to measurement.

Functionalized Perylene Diimide Derivatives
General procedure for the synthesis of dimethylamino perylene diimides Perylene-3,4,9,10-tetracarboxylic dianhydride and the alkyl amine (4 equiv) were combined with imidazole in a round bottom flask equipped with a stir bar and sealed with a septum. The reaction vessel was purged with argon and subsequently heated with stirring at 130 °C for the indicated amount of time. At the conclusion of the reaction, the vessel was allowed to cool to room temperature, the contents were suspended in methanol, and the solid was collected by filtration using a 0.8 μm nylon membrane. The solid was washed with methanol and dried under vacuum to afford the pure product.

General procedure for the quaternization of dimethylamino perylene diimides
The dimethylamino substituted perylene diimide and iodomethane (4 equiv) were dissolved in chloroform in a round bottom flask equipped with a stir bar and condenser and refluxed for the indicated amount of time. At the conclusion of the reaction, the mixture was cooled to room temperature and filtered through a 0.8 μm nylon membrane. The solid was washed consecutively with chloroform, diethyl ether, hexane, and ethanol and dried under vacuum to afford the pure product.

PDI-OH preparation via counterion exchange
Synthesized PDI-I variants were dissolved in DI water at concentrations of ~2-3mg/ml and slowly eluted through a counterion exchange column (DOWEX 550A, Sigma). The resulting solution was deep purple in color and was used without further treatment.

PDI-Alkene
N,N'-bis(hex-5-enyl)perylene-3,4,9,10-tetracarboxylic diimide Perylene-3,4,9,10-tetracarboxylic dianhydride (610 mg, 1.55 mmol) and 1-amino-5-hexene 1 (661 mg, 6.66 mmol) were combined with imidazole (7.4 g) in a 50 mL round bottom flask equipped with a stir bar and sealed with a septum. The flask was purged with argon for approximately 5 min and heated with stirring in an oil bath maintained at 120 °C. After 2 h, the flask was cooled and the contents were suspended in 2M HCl and filtered using a 0.45 μm nylon membrane. The solid was washed with water, methanol, and acetone and dried under vacuum to yield 860 mg (quant) of the desired product as a red solid after drying under vacuum.

Tertiary Amine Functionalized C60
C60-OH was synthesized according to literature methods by reduction of PCBM. 1 The product was purified using column chromatography on silica gel with 9:1 toluene to ethyl acetate as the eluent. C60-OH was produced with a yield of 70%. Synthesis of C60-OTs. Under argon atmosphere, C60-OH (0.068 mmol, 1 eq.), p-toluenesulfonic acid (0.272 mmol, 4 eq.), and DMAP (0.068 mmol, 1 eq.) were dissolved in chloroform (1.50 mL) and carbon disulfide (1.50 mL). DIPEA (0.08 mL, 7 eq.) was added via syringe. The reaction was allowed to stir for 15 hours. The reaction mixture was diluted with dichloromethane and washed three times with water. The organic layer was dried over sodium sulfate and the solvent removed under reduced pressure. The product was purified using column chromatography on silica gel starting with an eluent mixture of 2:1 hexane to chloroform and ending with a mixture of 1:2 hexane to chloroform. C60-OTs was produced (0.042 g) with a yield of 60.%. 1

PCBM-NMe2
Synthesis of PCBM-NMe2. Under argon atmosphere, C60-OTs (0.055 mmol, 1 eq.) was dissolved in THF (2 mL) and chloroform (2 mL). Dimethylamine (2 M in THF, 1.65 mmol, 30 eq.) was added via syringe. The reaction was heated to 60°C and allowed to stir for 20 hours. The reaction mixture was diluted with chloroform and washed three times with dilute aqueous potassium hydroxide. The organic layer was dried over sodium sulfate and the solvent removed under reduced pressure. The product was purified using column chromatography on silica gel starting with chloroform as the eluent and ending with chloroform plus 5% methanol. C60-NMe2 was produced (0.036 g) with a yield of 72%. 1

NDI-NMe2
N,N'-bis(6-dimethylamino)hexyl-1,4,5,8-naphthalene tetracarboxylic diimide 1,4,5,8-Naphthalenetetracarboxylic dianhydride (3.73 mmol, 1 eq.) was suspended in anhydrous dimethyl acetamide (15 mL) and the contents were sparged with nitrogen for 15 min. N,N-dimethylethylenediamine (9.3 mmol, 2.5 eq.) was added via a syringe and the reaction mixture was stirred at 80˚C for 15 h. Upon cooling, the crude reaction mixture was poured into hexanes/isopropyl alcohol mixture (200 mL, 1:1) cooled to 0˚C. The resulting suspension was stirred and allowed to warm to room temperature. The solids were collected via filtration and subsequently washed with copious amounts of water. The product was purified over a short silica gel plug with ethyl acetate as the eluent. The solution was concentrated in vacuo to yield a beige solid (1.0 g, 65% yield). 1 [3,4-c]pyrrole-1,4(2H,5H)-dione (534 mg, 1.78 mmol) was dissolved in DMF (20 mL) in a 100 mL round bottom flask equipped with a stir bar and sealed with a septum. The solution was cooled to 0 °C in an ice bath and sodium hydride (137 mg, 5.71 mmol) was added portionwise. The reaction was stirred for 10 minutes followed by the addition of 1,6-dibromohexane (1.64 mL, 10.7 mmol) via syringe. The reaction was allowed to warm slowly to room temperature and was stirred overnight. The reaction was diluted with water (100 mL) and extracted with dichloromethane (3 x 100 mL). The combined organic fractions were dried over MgSO4, filtered through a short plug of celite, and concentrated with silica gel. The crude mixture was purified by column chromatography, eluting with a gradient of 50-100% dichloromethane in hexanes. The product was precipitated with methanol and collected by filtration to yield a dark purple solid after drying under vacuum (329 mg, 29%).   Figure S1: X-ray photoelectron spectroscopy analysis. (a) Comparison of the XPS nitrogen signal distribution between quaternary ammonium, imide, and tertiary amine environments for PDI-NMe2 and PDI-OH (annealed for 0 hrs, 20min, 1hr, 4 hrs, and 16hrs) is shown. (b) Evolution of the fractional ammonium signal (in red) and the resulting tertiary amine signal in PDI-OH with annealing. The dashed lines represents the fractional tertiary amine signal (blue) and fractional ammonium signal (red) measured for PDI-NMe2 in XPS. Presence of ammonium signal in annealed PDI-NMe2 thin films may arise from radical cations formed upon charge donation from the tertiary amine groups. Radical cations along with unreacted quaternary ammonium functional groups are likely compounded in the remaining ammonium signal observed in PDI-OH thin films with extended annealing.

Estimation of Charge-Transfer Energetics in Self-Doping PDI Solid-State Thin Films
To estimate the charge transfer energetics associated with self-doping PDIs in the solid-state, we consider an alkylamine as a representative Donor (D) and the perylene dimide core as the Acceptor (A), which is shown in Figure S4. For the charge transfer to happen spontaneously, as is observed experimentally, the overall free energy change (∆GCT) must be negative. We can determine ∆GCT as IPD -EAA -EC, where IPD is the solid state ionization potential of the donor and EAA is the solid state electron affinity of the acceptor, and EC is the Coulombic stabilization energy. We use reported solid-state values for the electron affinity of perylene diimide (3.9 eV) [5] and ionization potential of alkylamines (6.5 eV) [6] . In solid-state, energetic stabilization effects, including electronic polarization [7] and Coulombic stabilization, aid in reducing the energy barrier. Polarization effects are already taken into account when using solid state EA and IP values.
The Coulombic stabilization can be roughly approximated as, C = − Therefore, even with the aid of Coulombic stabilization, we estimate ∆GCT ~ +1.4 eV. This energy barrier is well beyond the ~25-40 meV of thermal excitation that would be present at the temperatures considered in this study. While the doping effect were observed even when processing films in the dark, it is possible that rapid visible light photo-induced excitation during sample transfer was sufficient to aid the sample doping.