Using automated synthesis to understand the role of side chains on molecular charge transport

The development of next-generation organic electronic materials critically relies on understanding structure-function relationships in conjugated polymers. However, unlocking the full potential of organic materials requires access to their vast chemical space while efficiently managing the large synthetic workload to survey new materials. In this work, we use automated synthesis to prepare a library of conjugated oligomers with systematically varied side chain composition followed by single-molecule characterization of charge transport. Our results show that molecular junctions with long alkyl side chains exhibit a concentration-dependent bimodal conductance with an unexpectedly high conductance state that arises due to surface adsorption and backbone planarization, which is supported by a series of control experiments using asymmetric, planarized, and sterically hindered molecules. Density functional theory simulations and experiments using different anchors and alkoxy side chains highlight the role of side chain chemistry on charge transport. Overall, this work opens new avenues for using automated synthesis for the development and understanding of organic electronic materials.

In the coupling module, a 40 mL I-Chem vial with Teflon septa equipped with a rare earth, Teflon coated stir bar (10 mm diameter) is charged with alkyl dihalide (0.1 mmol), XPhos 2nd generation palladacycle (0.01 mmol, 8 mg, 10 mol%), and K3PO4 (1.8 mmol, 382 mg) To this cartridge is added 3 mL of a thoroughly argon sparged 5:1 Dioxane:water mixture. The THF solution of freshly deprotected boronic acid is split evenly and added via syringe pump to four different reaction vials. At the end of the addition, the reaction is heated to 100 °C and stirred for 16 hours. Products were then purified by automated MPLC with ethyl acetate/hexanes eluent.
General automated procedure II -Deprotection: To a 10g, polypropylene Luknova column, outfitted with a polyethylene frit and charged with starting MIDA boronate (1.9 mmol, 442 mg) and NaOH (5.7 mmol, 228 mg) is added 10 mL THF followed by 3 mL water. This solution is then agitated by bubbling argon through the solution for 20 minutes at room temperature. After agitation, 3 mL aqueous potassium phosphate buffer (pH=6, 0.5 M) and 5 mL Et2O are added. The layers are briefly mixed (again, via argon sparging) before being allowed to separate. The aqueous layer is disposed of using a syringe pump pulling from the bottom of the column. Then, 3 mL 50% saturated aqueous NaCl is added, the layers are mixed, and allowed to separate. Again, the aqueous layer is disposed of using a syringe pump. The solution is then transferred to a Luknova column charged with celite (800 mg) and MgSO4 (2.5 g). The solution is dried by repeated cycles of withdrawal and injection into this column (20 repetitions) The solution is then transferred to a Luknova column charged with celite (300 mg, tapped into pellet) and activated molecular sieves (3.6g, powdered, 4 Å) and further dried by repeated withdrawals and injections to this column. (20 repetitions). The solution is then passed into an empty Luknova column with the frit removed before washing the MgSO4 and molecular sieve drying agents sequentially with 6 mL THF and adding the wash to the cartridge S4 containing boronic acid mixture. This organic solution is then concentrated to 10 mL (evaporating most of the Et2O) before washing the drying agents with a further 6 mL THF. The organic solution (now only THF) is concentrated to 10 mL. This deoxygenated, dry solution is used directly in the subsequent coupling reaction.
In the coupling module, a 40 mL I-Chem vial with Teflon septa equipped with a rare earth, Teflon coated stir bar (10 mm diameter) is charged with bifunctional MIDA boronate (0.63 mmol, 303 mg), XPhos 2nd generation palladacycle (0.0032 mmol, 2.5 mg, 5 mol%), and K3PO4 (5.7 mmol, 1.2 g) The contents of this vial are placed under argon and dissolved in anhydrous THF (3 mL) before being magnetically stirred. The THF solution of freshly deprotected boronic acid is then added via automated syringe pump to this vial. At the end of the addition, the reaction is heated to 60 °C and stirred for 16 hours.
In the purification module, 3 mL of the crude reaction mixture is added to 20 mL of hexanes in a magnetically stirred Precipitation Cartridge (containing 250 mg aminopropyl functionalized silica gel and 150 mg Celite) and connected to a Silica Gel Plug, precipitating the MIDA boronate. The solvent is removed from the cartridge, loading any crude reaction product onto the Silica Gel Plug ("catch"). This process is performed a total of 5 times, using 3 mL THF to wash the Reaction Cartridge twice. Then, 12 mL of 1.5% MeOH in Et2O is added and the solvent is removed three times (36 mL total). Then, 12 mL of Et2O is added and the solvent is removed 3 times (36 mL total). Finally, 12 mL THF is added and slowly reverse eluted through the Silica Gel Plug ("release"), giving a purified solution of MIDA boronate.

S.4 NMR and UV-vis Dilution Experiment
For the NMR dilution experiment, solutions of R6 were prepared at concentrations of 10 mM, 1 mM, and 0.1 mM in C6D6. No shifting of proton signals corresponding to R6 upfield or downfield was observed in the 1 H spectra (500 MHz, CB500 instrument with cryoprobe) at these critical concentrations, suggesting no in-solution aggregation phenomena visible on the NMR timescale. For UV-Vis dilution experiments, we studied R6 at 0.1 mM, 0.05 mM, and 0.01 mM in 1,2,4trichlorobenzene using a Nanodrop UV-Vis instrument with a quartz cuvette (10 mm path length). In general, concentrations larger than 0.5 mM saturated the UV/Vis detector and did not provide accurate UV/Vis measurements. From these data, we observe no clear differences in the UV/Vis spectra between alkyl chain containing terphenyl molecules that display the high conductance mode in the STM-BJ experiments and those that do not. In particular, we observe no clear aggregation-related red-shifting between 0.01 mM and 0.1 mM, and there is no apparent spectral shifting occurs above the background absorption of the solvent. However, we note that the spectral cutoff (background absorption) for 1,2,4trichlorobenzene (TCB), the solvent used in this experiment and the STM-BJ experiments, is quite high (~304 nm) and may obscure absorbance features of the terphenyl molecules below this wavelength. We believe that these results are consistent with the NMR dilution experiments, which provides evidence against in-solution aggregation driving conductance behavior.