Understanding Degradation Dynamics of Azomethine-containing Conjugated Polymers

Understanding the influence of chemical environments on the degradation properties of conjugated polymers is an important task for the continued development of sustainable materials with potential utility in biomedical and optoelectronic applications. Azomethine-containing polymers were synthesized via palladium-catalyzed direct arylation polymerization (DArP) and used to study fundamental degradation trends upon exposure to acid. Shifts in the UV–vis absorbance spectra and the appearance/disappearance of aldehyde and imine diagnostic peaks within the 1H NMR spectra indicate that the polymers will degrade in the presence of acid. After degradation, the aldehyde starting material was recovered in high yields and was shown to maintain structural integrity when compared with commercial starting materials. Solution-degradation studies found that rates of degradation vary from 5 h to 90 s depending on the choice of solvent or acid used for hydrolysis. Additionally, the polymer was shown to degrade in the presence of perfluoroalkyl substances (PFASs), which makes them potentially useful as PFAS-sensitive sensors. Ultimately, this research provides strategies to control the degradation kinetics of azomethine-containing polymers through the manipulation of environmental factors and guides the continued development of azomethine-based materials.

2.54 x 2.54 cm glass slide from a 15-20 mg/mL solution in CHCl 3 using an Ossila spin coater set at 1400 rpm for 30 s. Prior to spin-coating, the glass sides were cleaned via untrasonication for 5 min in actetone then isopropyl alcohol (IPA) before being dried with compressed air.Differential pulse voltammetry (DPV) was measured using an EG&G Princeton Applied Research Potentiostat/Galvanostat Model 263A under CorrWare control in a three-electrode cell configuration, using indium tin oxide (ITO) coated glass (Delta Technologies, 7 × 50 × 0.7 mm, SiO 2 passivated, sheet resistance (R s ) = 8-12 Ω/sq) as the working electrode, an Ag/AgCl reference electrode (calibrated vs. the Fc/Fc + redox couple, E 1/2 = 363.5 mV), and a Pt flag as the counter electrode, with a step time of 0.1 s, step size of 2 mV, and amplitude of 25 mV.An electrolyte solution of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF 6 , 98%) in anhydrous acetonitrile (ACN) was used for all electrochemical measurements.The polymer film was deposited onto an ITO-coated glass slide from a 3 mg/mL CHCl 3 solution via spray-coating using an Iwata airbrush at 20 psi.The ITO slide was cleaned by sequential sonication in acetone and IPA before being dried with compressed air.Polymer molecular weight is reported relative to polystyrene (PS) standards and was estimated via size-exclusion chromatography (SEC) using a Tosoh EcoSEC high temperature GPC operated at 130 °C using 1,2,4-trichlorobenzene (TCB) as the eluent with a flow rate of 1 mL/min.Samples were prepared by dissolving 3 mg of polymer in 1 mL of TCB and stirring at 120 °C for 3 h before filtering through a 0.45 m PTFE membrane filter.Elemental analyses were performed by Atlantic Microlab Inc.All pictures are presented without manipulation except for cropping.All azomethine-containing monomers were synthesized following a literature procedure reported by Lei et al. 2 with slight modifications.Br 2 Az was synthesized by dissolving 4bromobenzaldeyde (529.2 mg, 2.86 mmol), 4-bromoaniline (492.0 mg, 1 eq.), p-TSA (27.2 mg, 5 mol%), and anhydrous CaCl 2 (400 mg, ~ 1.2 eq.) in toluene (100 mL, 0.1 M) in a 250-mL 3-neck round bottom flask.The flask was rendered inert via vacuum/refill cycles (3x) with Ar and the reaction mixture was stirred with a magnetic stir bar for 48 h at reflux in an oil bath thermostatted at 110 °C.After the allotted reaction time, dry K 2 CO 3 (40 mg, 10 mol%) was added and the reaction was stirred for 30 min while refluxing.The reaction mixture was cooled to r.t.before being filtered and washed with toluene to remove insoluble salts and drying agents.The solvent was evaporated via rotary evaporation and the crude product was dried under vacuum.The crude product was recrystallized from chloroform and collected via vacuum filtration.

N,1-bis(4-bromophenyl)methanimine (Br 2 Az):
White solid, 405.0 mg (42%). 1  (350 mg, 2.2 eq.) in toluene (50 mL, 0.1 M) in a 100-mL 3-neck round bottom flask.The flask was rendered inert via vacuum/refill cycles (3x) with Ar and the reaction mixture was stirred with a magnetic stir bar for 48 h at reflux in an oil bath thermostatted at 110 °C.After the allotted reaction time, dry K 2 CO 3 (20 mg, 10 mol%) was added and the reaction was stirred for 30 min while refluxing.The reaction mixture was cooled to r.t.before being filtered and washed with toluene to remove insoluble salts and drying agents.The solvent was evaporated via rotary evaporation and the crude product was dried under vacuum.The crude product was recrystallized from a 1:1 mixture of hexanes and ethanol (EtOH) and collected via vacuum filtration.

Figure S7. 1 H
Figure S7.1 H NMR of a solution of polyAzProDOT dissolved in CDCl 3 before (red) and 30 min after the addition of TFA (blue).The box from 8.4 ppm < δ < 8.6 ppm highlights the disappearance of the imine peak with addition of TFA and the box from 9.9 ppm < δ < 10.1 ppm highlights the increase in intensity of the aldehyde peak.

Figure S8 .
Figure S8.GPC elugram of polyAz 2 ProDOT using trichlorobenzene as the eluent at 130 °C with a flow rate of 1 mL/min.M n = 8.8 kg/mol, M w /M n = 2.1.

Figure S9 .
Figure S9.Photographs of a spin-coated polyAz 2 ProDOT film before (left) and after (right) being exposed to TFA vapors.

Figure S10 .
Figure S10.Photographs of a spray-coated polyAz 2 ProDOT film prior to oxidation, after oxidative doping with NOPF 6 , and after reduction with N 2 H 2 back to the neutral film (left to right) to demonstrate the reversible chemical doping of the polymer.

Figure S11 .
Figure S11.DPV trace for a polyAz 2 ProDOT film spray-casted onto an ITO/glass working electrode in a 0.1 M TBAPF 6 /ACN electrolyte solution with a step time of 0.1 s, a step size of 2 mV, and amplitude of 25 mV.

Figure S12 .
Figure S12.Photographs of polyAzProDOT in CHCl 3 before, immediately, 1 hour, and 24 hours after addition of TFA.Photographs of polymer solutions irradiated with UV-light before and after degradation are also shown (left to right).

Figure S13 .
Figure S13.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAzProDOT dissolved in CHCl 3 after the addition of 3 drops of a 0.4 M TFA/CHCl 3 solution.

Figure S14 .
Figure S14.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in CHCl 3 after the addition of 50 μL of a 0.4 M TFA/CHCl 3 solution.

Figure S15 .
Figure S15.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in THF after the addition of 50 μL of a 0.4 M TFA/THF solution.

Figure S16 .
Figure S16.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in toluene after the addition of 50 μL of a 0.4 M TFA/EtOH solution.

Figure S17 .
Figure S17.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in toluene after the addition of 50 μL of a 0.4 M (COOH) 2 /EtOH solution.

Figure S18 .
Figure S18.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in toluene after the addition of 50 μL of a 0.4 M p-TSA/EtOH solution.

Figure S19 .
Figure S19.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in THF after the addition of 50 μL of a 0.4 M H 3 PO 4 /THF solution.

Figure S20 .
Figure S20.UV-vis absorbance spectra as a function of time of a 0.01 mg/mL solution of polyAz 2 ProDOT dissolved in THF after the addition of 50 μL of a 0.4 M HCl/THF solution.