Photolithographic patterning of viologens containing styrene groups

A simple method for the patterning of styrene derivatives for electrochromic applications is presented. Novel viologen derivatives containing styrene groups were used in the formation of patternable electrochromic films. The patterning was done via photopolymerization and it shows the possibility of the use of styrene derivatives for the preparation of electrochromic patterns.


Experimental procedures
Spectroscopy Spectroscopic measurements consisted in performing NMR spectra using the Bruker Avance 600 MHz apparatus. The tests were performed in deuterated solvents from Deutero GmbH, and the spectra were calibrated against the solvent signals (CDCl3 = 7.26 ppm, CD3CN = 1.94 ppm and CD3OD = 4.78 ppm). HR-ESI-MS spectra were measured on a QTOF mass spectrometer (Impact HD, Bruker) in the positive ion mode.

Electrochemistry
Electrochemical measurements were done using a Bio-Logic VSP potentiometer. Monomers were dissolved in anhydrous acetonitrile electrolyte solution 0.1 M TBAPF6. A platinum electrode was used as a working electrode, a platinum wire as the counter electrode, and a silver wire electrode was used as the pseudoreference electrode. The polymers were measured using the ITO electrode as the working electrode, the pseudoreference electrode was silver wire and the platinum wire as counter electrode.

Spectroelectrochemistry
Spectroelectrochemical measurements were recorded using a potentiostat and a UV-vis-NIR spectrometer (Jasco V770) by stepwise increasing the applied potential and simultaneously measured the absorption spectra. The solution of monomers with a supporting electrolyte were tested using gold honeycomb electrode in quartz cuvette with the silver wire as a reference electrode. The thin film of the polymers on ITO electrode were measured in anhydrous acetonitrile electrolyte solution 0.1 M TBAPF6 where silver wire was used as a reference electrode and platinum wire as the counter electrode.

Preparation of polymer thin films
ITO glasses (5 × 0.7 cm) or glass cover slides (5 × 2.5 cm) were previously sonicated in water and 2-propanol for 15 min and then blowed it with an airbrush to dry. 1 Afterwards they were exposed to an UV-ozone atmosphere for 20 min. The compound 1 or 2 (4 mg) and 2,2dimethoxy-2-phenylacetophenone (~10% mol) were dissolved in a small amount of anhydrous acetonitrile. The solution was spray-coated on the substrate and the substrates were irradiated with a UV lamp at 365 nm for 5 min. To obtain homogenous irradiation four LED panels were used and the light intensity was set to be 100% at every time. Then the resulting films on glasses were rinsed with acetonitrile and acetone and they were air-dried.

Preparation of polymer pattern
The glass slide with the spray-coated thin layer of monomer 1 or 2 was placed between two glass plates with the marked inscription. Self-adhesive tape was used to hold the three glass cover slides overlapping each other and this setup was placed in the photoreactor and then irradiated at 365 nm for 5 min. To obtain homogenous irradiation four LED panels were used and the light intensity was set to be 100% at every time. The formed polymer pattern was left to dry after washing the glasses with acetonitrile and acetone to remove the monomer residue and other impurities. 2,3 For preparing of smaller patterns, the same regularly repeating design with different sizes was prepared and photopolymerization was done. The patterning of square at the different sizes was used as a photomask to obtain photopatterns photolithographically immobilized on a substrate.

Investigation of polymer after irradiation with light of different wavelengths
The same amount of monomer 1 was spray-coated at the same time on the seven glass slides. Afterwards the each of spray-coated sample was irradiated with light of different colors according to the previously described procedure: UV (365 nm), violet (395 nm), blue (457 nm), cyan (500 nm), green (523 nm), amber (595 nm) and red (625 nm). The substrates containing the polymer were washed with dichloromethane and immersed in the solution of TBAPF6 to ensure the anion exchange. After washing the substrates, the absorbance of obtained polymers were measured using UV-Vis.
All the electrochemical and spectroelectrochemical measurements as well as polymerizations were done several times to ensure reproducibility of the results. 4 4-pyridylboronic acid (0.96 g, 7.81 mmol), 4,7-dibromobenzothiadiazole (1.00 g, 3.40 mmol) and potassium carbonate (1.88 g, 13.60 mmol) were solubilized in 1,4-dioxane/water mixture (40 mL, 3/1, v/v) under an argon atmosphere. The solution was degassed with argon for 15 min and then tetrakis(triphenylphosphine)palladium(0) (0.39 g, 0.34 mmol) was added. The mixture was stirred and heated under reflux at 90°C in an inert atmosphere for 96 h. After cooling, the reaction mixture was extracted with dichloromethane and washed with water and brine. The organic phase was collected, dried over magnesium sulfate and the solvent was evaporated under vacuum. The crude product was purified by column chromatography on silica gel using dichloromethane/acetone (9:1) to give a product as a pale yellow solid (0.60 g, 61%). 1