Biomolecular E ﬃ cient access to conjugated 4,4 ’ -bipyridinium oligomers using the Zincke reaction: synthesis, spectroscopic and electrochemical properties †

The cyclocondensation reaction between rigid, electron-rich aromatic diamines and 1,1 ’ -bis(2,4-dinitro-phenyl)-4,4 ’ -bipyridinium (Zincke) salts has been harnessed to produce a series of conjugated oligomers containing up to twelve aromatic/heterocyclic residues. These oligomers exhibit discrete, multiple redox processes accompanied by dramatic changes in electronic absorption spectra.

Our own work in this area has previously concentrated on the synthesis of 4,4′-bipyridinium-containing macrocycles such as 4 (Scheme 1). We synthesized a family of related macrocycles by exploiting the Zincke reaction, 59 in which the condensation of nucleophilic amines with N-(2,4-dinitrophenyl)pyridinium salts resulted in efficient access to N-substituted pyridinium residues. 60 These macrocycles underwent one-electron reduction both chemically [e.g., by triethylamine (TEA)] and electrochemically, to yield the corresponding radical cations such as 5. 60 The unpaired spin density was found to be delocalized over four aromatic/heterocyclic rings, but was prevented from more extensive delocalization by the break in conjugation as a consequence of the ether linkages in these macrocyclic systems.
Here we report the syntheses and spectroelectrochemical analyses of a series of redox-active oligomers containing progressively increasing numbers of 4,4′-bipyridinium units, culminating in the synthesis of a hexa-cationic trimer containing twelve aromatic/heterocyclic residues.
Higher molecular-weight oligomers in the series were accessed through intermediate 8 that was produced in 59% yield in two steps from known precursor 9, 62 using simple extraction and crystallization procedures for purification. Condensation of two equivalents of 8 with the electron-rich aromatic diamine 10 resulted in the formation of dimer 2, in 77% yield. In contrast, reaction of three equivalents of 10 with 6 gave diamine 11 (95%) that was converted smoothly to trimer 3 by reaction with two equivalents of precursor 8 (72.5%), which was isolated as its hexafluorophosphate salt. Significant quantities of all three oligomers (e.g., ca. 1 g of trimer 3) could be obtained using these protocols, without recourse to column chromatography until the final step.
Electronic absorption spectra of 1, 2 and 3 in dimethylformamide (DMF) are presented in Fig. 1. The main difference between the spectra of 1 and 2 is the new absorption band of the dimer at ca. 380 nm, tailing into the visible region. Its intensity increases markedly with oligomer length. At the same time the dominant UV absorption at ca. 300 nm shifts slightly to higher energy.
We next studied the chemically reversible redox process using TEA and trifluoroacetic acid (TFA), as described previously for the macrocyclic systems. 60 Fig. 2(A) shows the 1 H NMR spectrum of an orange solution of trimer 3, which exhibits clearly resolved signals in the aromatic region (7.5 to 10.0 ppm), as well as two distinct resonances from the methoxy-ether and methoxy-ester residues (3.8 and 4.0 ppm).
The protons of the pyridinium residues at the termini of the oligomer, closest to the electron withdrawing ester groups, resonate at lower field (H a at 9.80 ppm) compared to the signals from the bipyridinium residue in the central section ( protons H e , at 9.52 ppm). Thus, the three bipyridinium residues are in two distinct environments: two at the termini of the oligomer and one in the centre.
Upon addition of excess triethylamine, all the signals corresponding to the protons of the aromatic and heterocyclic rings disappear, as a consequence of the formation of a para- magnetic tris(radical cation) (Fig. 2B). This change in the 1 H NMR spectrum is accompanied by a transformation in the color of the solution to a deep green, which is characteristic of the formation of radical cationic chromophores of this type. 63 However, the 1 H NMR signals corresponding to the non-conjugated methoxy groups remain visible at ca. 3.8 and 4.0 ppm. Addition of TFA to the tris(radical cation) not only regenerates the original color of the solution but also restores the missing signals in the aromatic region of the 1 H NMR spectrum ( Fig. 2C; see also the ESI - Fig. S1 and S2 † for 1 and 2, respectively). The radical cationic redox states of compounds 1, 2 and 3 were investigated by electron paramagnetic resonance (EPR) spectroscopy in acetone at room temperature. The EPR spectrum of 1 •+ centred at g = 2.0034(3) is characteristic of a viologen radical cation, showing well-resolved hyperfine splittings arising from a pair of equivalent 14 N nuclei and multiple groups of equivalent 1 H nuclei (Fig. 3). 64,65 The bis(radical cationic) form of dimer 2 and the tris(radical cationic) form of trimer 3 are also EPR-active. This indicates that the electron spins of the dimer biradical do not exclusively pair to form a diamagnetic singlet state, though a half-field transition characteristic of a triplet state was not observed. The EPR signal of 2 2(•+) exhibits the same broad envelope as 1 •+ , but the hyperfine structure is apparent only through weak shoulders. The EPR spectrum of 3 3(•+) also has unresolved hyperfine shoulders, though the peak-to-peak linewidth has narrowed. The changes in the spectral shape from unimer to dimer to trimer are characteristic of successive broadening of the hyperfine spectrum as may arise from increasingly rapid Heisenberg spin exchange or electron transfer processes. 66 The electrochemical reduction of 1, 2 and 3 was studied by cyclic voltammetry (CV) at a polished glassy carbon disc electrode, using anhydrous DMF as solvent, containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as supporting electrolyte. Ferrocene was used as internal reference. Cyclic voltammograms of compounds 1, 2 and 3 are shown in Fig. 4 and Table 1. Two reversible one-electron cathodic waves for unimer 1 occur at E 1/2 = −0.63 and −0.84 V vs. Fc/Fc + . For     The data indicate that the two viologen moieties in tetracationic dimer 2 are electronically independent, as revealed by the identical first reduction potentials of 2 and dicationic compound 1, resulting from lack of conjugation through the twisted, central biphenyl rings. The second reduction potential of 2 is shifted to a more negative value compared to that of 1. This difference can be attributed to increased electronic conjugation in the more planar bis(radical cationic) form of 2 generated at the first cathodic wave.
Hexacationic compound 3 is reduced at only a slightly more negative potential than 1 or 2 (E 1/2 = −0.66 V, ΔE p = 100 mV). As observed for 2, all three viologen units in 3 are reduced to their corresponding radical cations at the same electrode potential, suggesting that the viologens are twisted with respect to the chain direction and are therefore electronically independent. The cyclic voltammogram of 3 indicates a strong influence of adsorption phenomena at this scan rate (resolved cathodic waves and diminished anodic counter-waves). Although the resolution of the CV improved slightly at higher scan rates (v = 3 V s −1 ; Fig. S8 in the ESI †), the rather unusual second cathodic step which ultimately converts the tris(radical cationic) form of 3 to the neutral species, remains indistinct.
Thus, to further investigate the electrochemical properties of 3, a square wave voltammogram was acquired. Under these conditions three distinct reduction events were resolved (signals A, B and C in Fig. 5 and Table 2). Although the peak current of square wave voltammetry (SWV) signals can be influenced by solubility effects and molecular re-organisation during reduction, it would appear that reduction steps for the single central (signal B, Fig. 5) and two terminal bipyridinium units (signal C, Fig. 5) to the neutral, quinoidal species can, nevertheless, be resolved during this experiment.
Infra-red spectroelectrochemistry (IR-SEC) was carried out on unimer 1 within an OTTLE cell 67 (Fig. S6 in the ESI †) to gain insight into the changes in the conjugated structure throughout the redox cycle. Formation of the viologen radical cation was accompanied by the appearance of a strong ν(CvC) band at 1639 cm −1 associated with this species. There was, however, negligible change in the wavenumber of the ν(CvO) band at 1734 cm −1 arising from the terminal ester groups. This observation suggests that the ester groups are not significantly conjugated with the aromatic system, so that their influence on the reduction potential will only be minor. Unfortunately, IR-SEC experiments with 2 and 3 were precluded by poor solubility of the reduced species in DMF at the high concentrations required to give reasonable absorbance values.
In order to study the redox-induced changes in electronic spectra of these systems, thin-layer ultraviolet-visible spectroelectrochemical (UV-vis SEC) measurements were carried out at 293 K with 0.2 mM 1, 2 or 3 in DMF/0.1 M TBAPF 6 ( Fig. S11 in ESI, † 6 and 7). The observed spectral changes for all three species exhibit isosbestic points, excluding the possibility of side-reactions on the timescale of the experiment. All cathodic steps were fully reversible and parent electronic absorption spectra were recovered upon reoxidation. Fig. 6 show the UV-vis spectral monitoring of the electrochemical reduction of unimer 1 and dimer 2 at the two well-defined cathodic waves shown in Fig. 4. The tetracationic species shows two absorption bands at 290 and 375 nm (Fig. 6A). After completion of the first reduction step, the new absorption bands at 450, 600 and 715 nm are indicative of the bis(radical cation) (Fig. 6A  and B). 18 Continuation of the cathodic sweep results in a second transformation in the electronic absorption spectrum due to formation of the neutral redox form absorbing at 380 and 503 nm ( Fig. 6B; see also the ESI - Fig. S11 † for 1).
Spectroelectrochemical (UV-vis) data for the four stable redox forms of trimeric species 3 were recorded at different reduction potentials corresponding to the three cathodic waves measured previously ( Table 2). Conversion of the hexacationic species to the tris(radical cation) results in a spectrum similar to that observed for the analogous radical cation of 1 and bis-(radical cation) of 2, with absorption maxima at 450, 600, 650 and 715 nm (Fig. 7A). Continued decrease in the applied cathodic potential results in the formation of a second stable species associated with absorption bands at 384 and 508 nm,   which confirm the formation of fully reduced, quinoidal heterocycles. The absorption bands of the radical cation above 600 nm are diminished but still evident (Fig. 7B). This spectrum is consistent with a mixed oxidation state species containing both radical cationic and quinoidal heterocyclic species. Further lowering of the applied potential delivers the final species with intense absorption bands at 385 and 510 nm and no significant absorption bands above 600 nm (Fig. 7C), indicative of a fully reduced, neutral species. Fig. 8 shows an overlaid plot of the UV-vis spectra of the four stable, spectroscopically distinct species in the redox cycle of 3. The increase in intensity of the absorption band of the quinoidal bipyridinium species at 515 nm as the molecule becomes reduced from the tris(radical cation) to the mixed oxidation state system is 870 au (Arrow A). This compares to a change in intensity of 1800 au when moving from the mixed oxidation state species to the fully reduced, neutral trimer (Arrow B). These data suggest that the intermediate species, which is formed at approximately −0.9 V (Table 2), contains a single quinoidal bipyridinium species compared to the three quinoidal species present in the ultimate, neutral form of 3. Data from the SWV (Fig. 5) and the UV-vis SEC ( Fig. 7 and 8) of 3, supported by thin-layer cyclic voltammetry carried out during the latter experiment (see S14 in the ESI †), prove that the reduction of the three bipyridinium groups in the hexacationic trimer 3, giving a tris(radical cation), occurs by the addition of three electrons at the same potential (−0.65 V). However, the subsequent reduction of the tris(radical cation) to the neutral species occurs in a stepwise manner, by the Fig. 6 Reversible UV-vis spectral changes accompanying the stepwise 1e reduction of tetracationic dimer 2 to the corresponding bis(radical cation) (spectrum A) and the neutral quinonoid form (spectrum B). Spectra recorded in anhydrous DMF/0.1 M TABPF 6 , using an OTTLE cell. 67 Fig. 7 Reversible UV-vis spectral changes accompanying the stepwise reduction of hexacationic 3 to the corresponding tris(radical cation) (spectrum A) and the mono(radical cation) (spectrum B) and the neutral quinoidal form (spectrum C), recorded in anhydrous DMF/0.1 M TABPF 6 using an OTTLE cell. 67 Fig. 8 UV-vis spectral changes accompanying the stepwise reduction of hexacationic 3 to a neutral species. The change in intensity of the band at 515 nm associated with the formation of quinoidal bipyridinium residues occurs in the ratio 1 : 2 (arrows A and B respectively) during two-step reduction of the tris(radical cationic) form of 3 to the neutral species.

Paper
Organic & Biomolecular Chemistry

Conclusion
In this study we report an efficient synthesis of 4,4′-bipyridinium-based aromatic oligomers (1-3) and their spectro-electrochemical properties. Specifically, we use a controlled stepwise synthesis to produce single molecules with three 4,4′-bipyridinium residues containing, for the first time, twelve conjugated aromatic/heterocylic rings. All the compounds undergo reversible stepwise two-electron reduction of each viologen moiety. Addition of the first electron to each viologen to form a radical cation produces a more planar structure with significantly greater electronic communication between the aromatic/ heterocyclic groups along the length of each oligomer. Consequently, the tris(radical cation) of trimer 3 is reduced to the neutral, quinoidal form in two well-resolved cathodic steps. Combined results from CV and UV-vis SEC demonstrate that, in these three oligomers, their ground states are not planar, which results in a twisted configuration between the phenyls and viologens. When going to the radical cationic state, because of the greater extent of π conjugation induced by their planar structure, the second E 1/2 tends to a slightly more negative value. Spectroscopically, the UV-vis SEC result of 3 demonstrated that the apparently more electron rich, central radical cationic bipyridinium moiety is reduced to the quinoidal, neutral form before the two terminal bipyridinium units which appear comparatively electron poor. In support, computational studies of related conjugated oligomers and polymers have also shown that that maxima for their HOMO/LUMO coefficients are found at the centre of the molecule. 68,69 If then the highest occupied spin orbital (HOSO) of the tris(radical cation) form of 3 is also localised in the centre of the molecule then the central 4,4′-bipyridinium would be reduced prior to the higher energy terminal radical cationic units. In addition, the UV-vis SEC measurements have highlighted the potential of these new molecules to find application in display technologies where dramatic and reversible colour changes are required. Work in our laboratory is continued towards integrating conjugated 4,4′-bipyridinium residues into more extended materials systems such as MOFs, mechanically interlocked molecules and high-MW polymers.

General methods
Starting materials were purchased from Sigma Aldrich and Alfa Aesar and used without further purification unless otherwise stated. Anhydrous N,N-dimethylformamide (DMF) (Alfa Aesar, 99.8%, packaged under argon) was used as received. Tetra-n-butylammonium hexafluorophosphate (TBAPF 6 ) was recrystallized twice from absolute ethanol and dried at 80°C under vacuum overnight.
Compounds 6 61 and 9 62 were synthesized as described previously. All reactions were carried out in oven-dried glassware under dry nitrogen. 1 H and 13 C NMR were acquired on a Bruker DPX-400 spectrometer operating at 400 MHz and 100 MHz respectively or on a Bruker AV-700 spectrometer operating as 175 MHz for 13 C nuclei. Residual 1 H signals from the solvent were used as internal calibrants. Infrared spectra were recorded on a Perkin Elmer 17 20-X spectrometer from thin films cast from acetone, and major absorption bands are reported in wavenumbers (cm −1 ). For spectroelectrochemical (SEC) measurements, IR spectra were recorded on a Bruker Vertex 70v FTIR spectrometer. UV-Vis spectra were acquired on a Scinco S-3100 diode-array spectrophotometer.
Cyclic voltammetry (CV) measurements were performed on 0.2 mM solution of the compound 1, 2 and 3 in anhydrous DMF containing 0.1 M TBAPF 6 as supporting electrolyte with a single-compartment three-electrode cell equipped with glassy carbon disc (d = 2 mm) working, coiled platinum wire auxiliary and coiled Ag wire pseudoreference electrodes. Spectroelectrochemical measurements were carried out at 293 K, using an optically transparent thin-layer electrochemical (OTTLE) cell equipped with Pt minigrid working and auxiliary electrodes, a silver microwire pseudoreference electrode, and CaF 2 windows. The course of the spectroelectrochemical experiment was monitored by thin-layer cyclic voltammetry conducted with an EmStat 3 (PalmSens BV) potentiostat. The applied potentials stated in the UV-vis SEC are approximated from the CV of each compound. All the melting points were determined on a Gallenkamp melting point apparatus. Mass spectra were recorded Thermo Scientific LQT Orbitrap XL under conditions of electrospray ionisation. X-band EPR spectra were acquired on a Bruker EMX spectrometer using a TM 110 cylindrical mode resonator (ER 4103TM). Due to the high dielectric loss of the Scheme 3 The structures of the stable intermediates observed during the electrochemical interconversions of trimer 3: tris(dicationic), tris-(radical cationic), di(radical cationic) and neutral, quinoidal forms (top to bottom respectively).