Spherical nanoparticle formation of DPZnT
The synthesis of porphyrin tripods was conducted by a copper-catalysed click reaction between 1,3,5-tris(azidometyl)benzene and ethynyl-bearing porphyrin derivatives. As control compounds for DPZnT, the freebase and copper-coordinated forms of porphyrin tripods (DPFBT and DPCuT, respectively; Fig. 1) were prepared to investigate the effect of axial coordination. Several structural fragments of DPZnT, DPPZn, TB, m-TB, and TC were also prepared for control experiments. The details of the synthetic procedures and characterisation of porphyrin tripods and compounds used in this study are summarised in the Supplementary Information (Supplementary Scheme 1).
The Lewis acidic zinc porphyrins generally show a strong binding affinity to the lone pair electrons in triazolic nitrogen to form axial coordination complexes.20 The binding of axial ligands causes a bathochromic absorption shift of zinc porphyrins; however, the Soret absorption band of DPZnT appeared at approximately 412 nm, which is consistent with that of DPPZn (Fig. 2A). Therefore, the UV/vis absorption spectroscopic observation of DPZnT indicated the absence of axial coordination interactions between the triazole and porphyrin units. UV/vis titrations of TB and m-TB to DPPZn in toluene were performed to elucidate the absence of coordination interactions between the triazole and zinc porphyrin units in DPZnT. When TB was added to DPPZn, the absorption did not change even after the addition of 13,000 equivalents of TB (Supplementary Fig. 1A). In contrast, upon the successive addition of m-TB (0–9,600 eq.), the Soret absorption band of DPPZn exhibited a significant bathochromic shift with the existence of clear isosbestic points at 414.5, 543.5, and 575.0 nm (Supplementary Fig. 1B-C). The computer aided modeling of TB showed coplanar geometry between the triazole and the neighbouring phenyl group (Fig. 2B). These observations indicate that the triazole moiety with the neighbouring phenyl group cannot be coordinated to zinc porphyrin because of steric hindrance. Although DPZnT did not show a bathochromic absorption shift, a relatively large shoulder at approximately 400 nm was observed owing to the π-π stacking of the porphyrin units.21, 22 However, the intensity of the shoulder near 400 nm increased when a small amount of n-hexane (15%) was added to the solution of DPZnT in toluene, which could be attributed to the enhanced molecular interaction in DPZnT (Fig. 2A). The solution of DPZnT in toluene was spin-coated onto a freshly cleaved mica surface, and atomic force microscopy (AFM) was performed to observe the morphological aspects of DPZnT. The AFM image showed spherical nanoparticles with a height of approximately 3 nm (Fig. 2C). The formation of nanoparticles was also observed when DPZnT was spin-coated onto highly oriented pyrolytic graphite (HOPG) (Supplementary Fig. 2). However, the average height of the nanoparticles was slightly shorter than that observed in the AFM specimen prepared on the mica substrate. The flattening of nanoparticles can explain the decreased average height of the nanoparticles on HOPG because of the high affinity of the alkyl chains to HOPG.23, 24 Grazing-incidence wide-angle X-ray scattering (GIWAXS) was conducted on the drop-cast films of DPZnT. Thus, the in-plane and out-of-plane diffraction of DPZnT overlapped well (Fig. 2D), indicating the absence of angular dependency in the wide-angle region owing to the formation of spherical nanoparticles.
Because DPZnT formed spherical nanoparticles, the morphological aspects of DPFBT and DPCuT, non-coordinatable derivatives of DPZnT, were again measured by AFM. The AFM images of both DPFBT and DPCuT also showed the formation of spherical nanoparticles (Supplementary Fig. 3), indicating that the coordination interaction does not contribute to the formation of spherical nanoparticles. Similar to DPZnT, the UV/vis absorption spectra of both DPFBT and DPCuT exhibited a shoulder in the blue-shifted region of the Soret absorption of DPPFB and DPPCu, respectively, indicating that the driving force for the spherical particle formation of porphyrin tripods is π–π interactions among the porphyrin units.
Guest-induced supramolecular polymerisation of DPZnT
The guest-induced supramolecular polymerisation of DPZnT was investigated (Fig. 3A). Considering the three zinc porphyrin units in DPZnT, we envisaged that DPZnT could also form a host-guest complex with 1,3,5-tris(4-pyridylbenzene) (Py3B; Fig. 1) through axial coordination interactions.25-27 The UV/vis titration of DPZnT on the addition of Py3B showed distinct spectral shifts with isosbestic points at 412, 541, and 575 nm (Fig. 3B). The binding isotherm recorded at 417 nm showed that the absorption reached saturation upon adding 1 equiv. of Py3B, indicating a strong binding affinity of Py3B toward DPZnT (Fig. 3C). Although it is difficult to estimate the binding constant between Py3B and DPZnT because pristine DPZnT forms spherical nanoparticles, the apparent binding constant exceeds the upper limit (K > 108), which can be measured by the absorption changes.28 Analysis using the continuous variation method (modified Job’s plot analysis) suggested the formation of a 1:1 host-guest complex between DPZnT and Py3B (DPZnT•Py3B) (Supplementary Fig. 4).29-31 Energy-minimised molecular modelling indicates that DPZnT•Py3B adopts a cone-shaped geometry, wherein the porphyrin units of DPZnT are exposed to the outer environment. Therefore, the host-guest complex could undergo further aggregation to avoid unfavourable exposure to the exterior solvent molecules, similar to our previous study.32 Transmission electron microscopy (TEM) images showed the formation of fibrous supramolecular polymers of ca. 4.0 nm width (Fig. 4D). In addition, AFM images of DPZnT with 1 equiv. of Py3B demonstrated fibrous supramolecular polymer formation (Supplementary Fig. 5). DPZnT, non-coordinatable DPFBT, and DPCuT exhibited no absorption spectral changes upon adding Py3B (Supplementary Fig. 6). Therefore, the binding of Py3B to DPZnT plays a critical role in the formation of fibrous supramolecular polymers. The formation of supramolecular polymers of DPZnT•Py3B was further confirmed by 1H diffusion-ordered spectroscopy (1H DOSY) NMR experiments. DPZnT•Py3B showed concentration-dependent changes in the diffusion coefficient (D) value (Supplementary Fig. 7A), indicating the concentration-dependent elongation of the supramolecular polymers.
In contrast, DPZnT formed networked supramolecular polymers on adding Cl-. DPZnT has two types of anion-binding sites. The first type is the triazole groups generated by the click reaction, and the second type are Lewis acidic zinc atoms in the porphyrin wings. Anionic species can bind to triazolic C-H and zinc atoms in porphyrin wings through C-H∙∙∙X- hydrogen bonding and axial ligation, respectively.33, 34 The binding of anionic species to triazolic C-H and zinc porphyrin can be monitored by 1H NMR spectroscopy. However, the 1H NMR measurements of DPZnT are not eligible because DPZnT forms spherical nanoparticles. Therefore, the 1H NMR spectral change of TC, a structural fragment of DPZnT, was monitored upon adding Cl- to confirm the binding of halide ions to the triazole C-H group (Supplementary Fig. 8). Thus, the triazole C-H peak (Ha) of TC was downfield shifted from 7.23 to 7.38 ppm when Cl- was added in the form of tetrabuthylammonium salt, indicating the binding of Cl- to TC through the C-H∙∙∙Cl- hydrogen bonding. The UV/vis absorption of DPPZn also showed a bathochromic shift upon adding Cl- through axial coordination complex formation (Supplementary Fig. 9). The absorption spectra exhibited a similar bathochromic shift with clear isosbestic points at 417.5, 547.0, and 580.0 nm when Cl- was added to DPZnT (Fig. 3E). The binding isotherm obtained from the absorption changes at 413 nm suggested that the absorption reached saturation after adding 20 equiv. of Cl- (Fig. 3F). A toluene solution of DPZnT with 10 equiv. of Cl- (DPZnT•Cl-) was subjected to AFM after spin-coating onto a freshly cleaved mica surface to observe the morphological aspects. Therefore, we observed the formation of a fibrous network structure (Fig. 3G). Unlike pristine DPZnT, the in-plane and out-of-plane diffractions of the GIWAXS signals did not overlap with each other (Supplementary Fig. 10). The angular dependency on WAXS indicates that the packing structure of the mixture system of DPZnT with Cl- differs from that of the spherical particles of pristine DPZnT. The D values obtained from 1H DOSY NMR of DPZnT with 10 equiv. of Cl- in toluene-d8 confirmed the concentration-dependent elongation of the network structure. The D values gradually decreased with increasing concentration, indicating the elongation of the supramolecular polymer and formation of a fibrous network (Supplementary Fig. 7B).35
The importance of Cl- coordination on zinc porphyrin in DPZnT was supported by control experiments using DPFBT and DPCuT. Unlike DPZnT, DPFBT and DPCuT did not exhibit a bathochromic shift in the absorption spectra (Supplementary Fig. 11), indicating the absence of axial coordination of Cl- to porphyrin units in both DPFBT and DPCuT. However, 1H NMR spectral studies showed that triazole units in DPFBT interact with Cl- through C-H∙∙∙Cl- hydrogen bonds; the triazolic C-H (Ha) signal was downfield shifted from 8.17 to 8.33 (Δd = - 0.16 ppm) (Supplementary Fig. 12). The morphological aspects of DPFBT and DPCuT were observed by AFM after adding 10 equiv. of Cl-. AFM images showed that the shape of the spherical particles did not change; however, the size of the particles significantly increased. Therefore, we concluded that both C-H∙∙∙Cl- hydrogen bonding and axial coordination interactions simultaneously contribute to the formation of networked supramolecular polymers.
Reversible supramolecular depolymerisation
Because the binding of Py3B or Cl- to DPZnT induced supramolecular polymer formation, the removal of Py3B or Cl- could lead to the dissociation of the supramolecular polymers. First, we attempted to remove Py3B from the fibrous supramolecular polymers. Pyridyl group could be removed by adding copper ions as they form stable metal-coordination complexes with pyridyl ligands.36 As aforementioned, DPZnT showed a bathochromic absorption shift during the formation of a 1:1 host-guest complex with Py3B. The absorption spectrum almost recovered to that of pristine DPZnT (Fig. 4A) when Cu(ClO4)2 was added to DPZnT•Py3B,. After the solution was filtered to remove the insoluble precipitates, morphological aspects were observed by AFM. The AFM results indicated the formation of spherical nanoparticles in pristine DPZnT (Fig. 4B). For the networked supramolecular polymers, Cl- was removed by AgNO3 treatment.37 With the addition of AgNO3 to DPZnT•Cl- solution in toluene, the Soret absorption band at 423.5 nm completely recovered to the absorption of the pristine DPZnT (Fig. 4C). After adding AgNO3, the insoluble salt was removed by filtration, and the filtrate solution was spin-coated onto the mica surface for AFM measurements. The AFM results also indicated the recovery of the original spherical nanoparticles formed by pristine DPZnT (Fig. 4D). The UV/vis absorption spectral changes were monitored upon the addition of Cl- and AgNO3 (Fig. 4E) observe the reversibility of this process. The absorption changes at both 413 and 423.5 nm upon the successive treatment of Cl- and AgNO3 supported the reversible changes in supramolecular polymerisation and depolymerisation (Fig. 4F).
In summary, we prepared a triazole-bearing tripodal porphyrin, DPZnT, that formed spherical nanoparticles. UV/Vis titration with structural fragments of DPZnT and molecular modelling revealed that axial coordination of the triazole groups to the zinc porphyrin units was prevented owing to steric hindrance. The addition of Py3B resulted in the formation of a 1:1 host-guest complex between Py3B and DPZnT, and this host-guest complex was further aggregated to form a linear fibrous supramolecular polymer. The removal of Py3B from the host-guest complex resulted in the formation of spherical nanoparticles by the reversible depolymerisation of linear fibrous supramolecular polymers. In contrast, the spherical nanoparticles of DPZnT were transformed into networked supramolecular polymers through the binding of Cl-. The original spherical nanoparticles of DPZnT were recovered by the reversible depolymerisation of networked supramolecular polymers when AgNO3 was added to remove Cl-. Because DPZnT has successfully undergone supramolecular polymerisation and depolymerisation upon the treatment with Py3B/Cl- and Cu2+/Ag+, respectively, the results provide insight into a better understanding of molecular-level association processes in natural systems that exhibit structural transformation.