4 4 bipyridine

Viologens have been explored as alternative dyes in photoelectrochromic cells due to their ability as redox mediators to lead electronic charge transfer in the presence of prokaryotic light-harvesting proteins^{25 (link)}. Here we incorporated the phosphonated viologen (PV) N-(Diethylphosphono-2-ethyl)-4,4′-bipyridinium bromide as both a redox mediator and a light absorber. The PV was synthesized according to previous reports^{61 (link)}. Briefly, 1.5 g of 4,4′-bipyridine (Sigma-Aldrich) was diluted in 5.25 mL of 97% diethyl 2-bromoethylphosphonate (AllScience Colombia) and 10 mL of Dimethylformamide (DMF) and subsequently maintained for 24 h at 50 °C under continuous agitation. The synthesis product was cooled down, washed with 20 mL of diethyl-ether and dried out in a low-vacuum rotary evaporator for 36 h. The resulting product was collected and finally stored at room temperature until further use.
EEG thin-films deposited on ITO were immersed in 4%(w/v) of PV dispersed in dH₂O. They were let to react for 18 h at 50 °C under magnetic stirring. Reactants excess was removed by additional dipping in dH₂O and the obtained thin-films were dried out in a low-vacuum chamber at 718 mbar. Samples were stored at 4 °C until further use.

The following chemicals were all sourced from Sigma Aldrich and used without modification or purification unless stated otherwise: trisodium citrate dihydrate (Na₃C₆H₅O₇), chloroauric acid (HAuCl₄·3H₂O), l-ascorbic acid (C₆H₈O₆), silver nitrate (AgNO₃), sodium chloride (NaCl), sodium hydroxide (NaOH), 11-mercaproundecanoic acid (C₁₁H₂₂O₂S), methylene blue (C₁₆H₁₈ClN₃S), pyridine anhydrous (C₅H₅N), 4,4′-bipyridine (C₁₀H₈N₂), polystyrene sulfonate (PSS) ∼10 000 MW, and indium tin oxide (ITO) coated glass slides (surface resistivity of 30–60 Ω, 1-inch square). Poly(diallyl dimethylammonium chloride) (PDDA) ∼8500 MW was sourced from Polysciences Europe. All glassware and PTFE-coated stirrer bars used for nanoparticle synthesis experiments were soaked for at least 2 hours in aqua regia before being rinsed with water several times. Millipore MilliQ purified water was used in all experiments.

4,4′-Bipyridine and 1,4-dioxane were obtained from Sigma-Aldrich; 1-bromobutane and lithium bis(trifluoromethylsulfonyl)amide were purchased from Alfa Aesar. All solvents were purified employing standard drying agents prior to use.
CHNS elemental analyses were made using a 5E-CHN2200 elemental analyzer taking 20 mg sample. ¹H and ¹³C NMR, using a Bruker AM-270 (270 MHz) and Bruker 400 MHz spectrometers, and ¹⁹F NMR using a Bruker AV-400 (376.5 MHz) spectrometer were employed to confirm the structures and check the purity of the synthesized ligand salt and complex. ESI MS was used to determine the molecular ion mass of the ligand and complex using Bruker Micro TOF. Bromide and chloride estimation was conducted taking 30 mg sample and dissolving in 40 mL distilled water. Excess AgNO₃ solution was added for the formation of AgBr and AgCl precipitate. Then, the cruddy white precipitate formed was filtered and dried in an oven, and the amount of bromide and chloride was calculated from the weight difference. Conductivity of 3 × 10⁻⁴ M ethanolic solution of the complex was also investigated using a Bante901P portable pH/conductivity/TDS meter at room temperature.

Zn-trisporphyrin conjugate 1 and monomeric Zn-porphyrin 2 were synthesized according to our previous report [34 (link)]. All other chemicals (pyridine, 4,4-bipyridine, and 1,3-di(4-pyridyl)propane) were purchased from Sigma-Aldrich and used without further purification. Steady-state UV-vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer. For UV-vis titrations, we prepared the host and guest solutions in toluene at room temperature. The porphyrin (host) concentration was fixed at 1 μM, and ligand (guest) concentration was varied from 0.001 to 0.1 M. UV-vis titration experiments were performed by recording a spectrum of the host (porphyrin) in toluene at 298 K and adding the guest (ligand) solution in increments. After each addition, a new UV-vis spectrum was obtained. The data obtained from the UV-vis spectrophotometric titrations were analyzed by fitting the entire spectral series at 1 nm intervals using the SPECFIT software [39 ], as mentioned in earlier reports [32 (link),33 (link)]. To determine the binding constants, the titration results were fitted using the nonlinear regression method within the Origin 9 software and generated the following equation:

A mixture of Ni(NO₃)₂·6H₂O (14.5 mg, 0.05 mmol; purchased from Sigma-Aldrich), 4,4’-bipyridine (8 mg, 0.05 mmol; purchased from Sigma-Aldrich), H₂VTTF, (23.5 mg, 0.05 mmol; prepared as previously reported procedure^{38 (link)}) in DMF (7.5 mL)/MeOH (2.5 mL) was added to a 20 mL glass vial. The vial was capped tightly and placed in an oven at 80 °C for 48 h to yield block-shaped green crystals. The contents of the vial were allowed to cool at room temperature. Crystals were filtered and washed with 5 mL of DMF for three times. Calculated yield was found to be >60%.