We synthesized imino-substituted bipyrrole derivatives BPI-1–4, containing a 1,4-phenylene spacer according to the previously reported procedures Scheme 1 (Setsune et al., 2015). BPF bearing formyl groups at pyrrolic α-positions were synthesized in advance. Then, the formyl groups were converted to the corresponding imines bearing various functional groups on the nitrogen atom through condensation reactions with the amine in the presence or absence of an acid catalyst (isolated yields 75–96 %).

We initially attempted the synthesis of 1D-CPs with BPI-1, bearing N-benzyl groups, and transition metal salts such as Zn(OAc)₂, Cu(OAc)₂, and Pd(OAc)₂. BPI-1 and metal salt were mixed in a 1:1 ratio in a mixture solution of CH₂Cl₂/CH₃OH (5:1) and stirred overnight at room temperature. In these experiments, only the sample solution containing BPI-1 and Pd(OAc)₂ provided a precipitate, which is slightly soluble into organic solvents such as CHCl₃ and CH₂Cl₂, likely due to the complexation. ¹H NMR spectrum of BPI-1 in CDCl₃ provides characteristic singlet signals corresponding to the protons on imine, phenylene, and benzyl groups at δ = 8.27, 7.49, and 4.73 ppm, respectively (Figure 2). These peaks become broad and are shifted to the higher magnetic field region at 8.01, 7.33–6.97, and 3.36 ppm, respectively, upon complexation with Pd(II) ion. Similar changes were observed for other peaks corresponding to the phenyl and ethyl exteriors. These ¹H NMR spectral features indicate the formation of the certain polymers (Berl et al., 2002).

On the other hand, in UV-Vis absorption spectroscopy, BPI-1 showed a large red-shift of its lowest energy absorption band from λ_max = 368 nm to 419 nm in CH₂Cl₂ upon complexation with Pd(II) (Figure 3), which may mainly originate from exciton coupling of BPI-1 array, owing to the electric transition dipole for the long-axis polarized π–π^* transition (vide infra) (Ahmad et al., 2009; Li et al., 2019). Further, the emission spectrum of BPI-1, observed at 400–600 nm with λ_max = 426 and 450 nm, disappeared upon the complexation. This observed quenching behavior may be ascribed to the heavy atom effect due to Pd ion (Drzewiecka-Matuszek et al., 2005). These observed spectral features strongly indicate the formation of a metal-organic CP. However, its low solubility into organic solvents does not allow measurement of the average molecular weight in SEC.

We then synthesized BPI-2, having N-dodecyl chains, to increase the solubility of the product into organic solvents. BPI-2 was mixed with 2 equiv. amounts of Pd(OAc)₂ in CH₂Cl₂, and stirred for 2 days at room temperature to give the complex soluble into organic solvents such as CHCl₃ and CH₂Cl₂. The BPI-2/Pd complex was thus isolated through extraction with CH₂Cl₂ and water to give a film-like solid after evaporation of the solvent (Figure 4A). Spectral changes of BPI-2 in ¹H NMR, UV-Vis absorption, and fluorescence spectroscopies upon complexation with Pd(II) were almost the same as those observed in BPI-1 as described above (Supplementary Figures 1, 2). BPI-2/Pd is stable in solutions and was subjected to the SEC measurements to estimate average molecular weight. SEC was conducted with CHCl₃ as an eluent, and it revealed that BPI-2/Pd complex has shorter retention time (13.4 min) than BPI-2 monomer (23.5 min) (Figure 4B). The average molecular weight of BPI-2/Pd complex was estimated to be M_w = 106,000 (corresponding to 130 mer) and M_n = 18,000, M_w/M_n = 5.88 compared to polystyrene standards. As an experimental reference data, dynamic light scattering (DLS) measurement of BPI-2/Pd complex provided an average particle size of 372 nm with a size distribution of 143–2,114 nm in CHCl₃ (Supplementary Figure 3). These results indicate that BPI/Pd complexes have polymeric structures.

The electrochemical properties, especially for oxidation, of the metal-organic 1D-CP, were studied by cyclic voltammetry (CV) in CH₂Cl₂ containing 0.1 M tetrabutylammonium perchlorate (Bu₄NClO₄) as supporting electrolyte. The cyclic voltammograms (V vs. Fc/Fc⁺) of BPI-2 monomer and BPI-2/Pd polymer are shown in Figure 5. BPI-2 showed an irreversible oxidation peak at around 0.8 V, where the oxidation may bring about the chemical reaction and/or decomposition of the imine groups. Differential pulse voltammetry (DPV) also provided the corresponding peak at 0.78 V (Supplementary Figure 4A). However, in sharp contrast, BPI-2/Pd polymer provided reversible multi redox peaks in CV at around E_1/2 = 0.35 and 0.5 V, where the electrochemical properties of imino group may be changed upon complexation with Pd(II) ion. DPV also provided the corresponding two peaks at 0.33 and 0.50 V, respectively (Supplementary Figure 4B). This observed multi-redox behavior of BPI-2/Pd polymer could be explained by the π-electronic interactions between bipyrrole ligands via Pd(II) ion (Hildebrandt et al., 2011). The positive charge generated upon first oxidation on the polymer backbone withdraw π-electrons of other BPI ligands via the metal ions, resulting in a positive shift of their oxidation potential to give the multi redox profile.

BPI can attach a variety of substituent groups on imino-nitrogens. We have previously synthesized N-hexyl BPI and found in the crystal structure that two pyrrole components adopt a twisted geometry with an anti-folded confirmation through the phenylene spacer (Ie et al., 2015). The dihedral angle of pyrrole and phenylene components is 33.3° on average, owing to the steric repulsions between the ethyl group attached to the pyrrole-β and phenylene ring (Li et al., 2020). This observed twisted structure allows our expectation that BPI/Pd polymer can form a helical CP by attachment of chiral substituent groups on imino-nitrogens (Huang et al., 2018; Wang et al., 2019; Zhou et al., 2020).

We then synthesized BPI-3(R) and BPI-3(S), having chiral alkyl chains on imino-nitrogens. BPI-3, upon mixing with Pd(OAc)₂, also formed the coordination polymer under the same conditions of synthesizing BPI-2/Pd polymer (Supplementary Figure 5). However, their average molecular weights [BPI-3(R)/Pd, M_w = 10,900, M_n = 6,800, M_w/M_n = 1.60; BPI-3(S)/Pd, M_w = 8,600, M_n = 4,700, M_w/M_n = 1.80] are clearly lower than that of BPI-2/Pd polymer (Supplementary Figure 6). The 1-methyl group attached to the N-alkyl chain may increase steric crowdness around metal-ligand coordination bonds to decrease the polymerization degree of the CP (Guzei and Wendt, 2006; Talarico and Budzelaar, 2008). Here, BPI components may adopt a chiral conformation in the resulting polymer structure.

In circular dichroism (CD) spectroscopy, BPI-3(R) and BPI-3(S) ligands showed weak CD spectra with λ_max at 425 nm (Figure 6). However, interestingly, strong CD responses with Cotton effects appeared in their coordination polymers. BPI-3(R)/Pd and BPI-3(S)/Pd showed negative to positive and positive to negative Cotton effects, respectively, at the lowest energy absorption band to give the mirror-imaged CD profiles in each other. These observed spectral features may originate from exciton-coupled CD owing to the helical orientation of BPI-3 ligands in the 1D-polymeric structure as schematically illustrated in Figure 7. Their Cotton effects allow empirical estimation of their absolute configurations (Gonnella et al., 1982; Furo et al., 2005; Telfer et al., 2011). BPI-3(R)/Pd and BPI-3(S)/Pd may dominantly adopt (S)- and (R)-configuration, which allows the formation of P- and M-helical coordination polymer, respectively.

Next, we synthesized a coordination copolymer (CCP) with a 1:5 mixture of chiral BPI-3(R) and achiral BPI-4 upon mixing with Pd(OAc)₂ through the similar procedures to the synthesis of the above polymers. The polymer was obtained in 72% yield and characterized using ¹H NMR, UV-Vis absorption, and CD spectra and SEC. The SEC profile provided only a broad single peak, a typical polydispersion profile of the polymer, indicating random copolymerization. Its average molecular weight is estimated to be M_w = 11,100 and M_n = 6,300, M_w/M_n = 1.76 compared to polystyrene standards in SEC. Hence, its sample solution in CH₂Cl₂ was CD active, and the spectral pattern observed was essentially the same as that of BPI-3(R)/Pd polymer. However, since the spectral intensity is ~4–5 times smaller, it seems that the local chiral twisted conformations occurred in BPI-3(R)/Pd complexation in the copolymer is not amplified on the resulting polymer backbone. The p-phenylene-spacer of BPI unit, which provides a distance between its substituent groups, likely allows the polymerization without steric repulsions and/or interactions of the chiral metal-coordination moieties.