Introduction

Two-dimensional sheet-like nanostructures have been made by organic and/or inorganic materials, for example, graphene, supramolecules, polymers, covalent organic frameworks, metal organic frameworks, and hexagonal boron nitride, and interested in wide fields such as electronics, optoelectronics, catalysts, energy storage, energy generation, sensors, separation, and biomedicines due to their unique physical and chemical properties1,2,3. Supramolecular assembling systems are progressive strategy for preparing well-ordered nanostructures4,5,6. In nature, one of the supramolecular sheet-like nanostructures have been observed in major light-harvesting antenna systems of photosynthetic green bacteria, called chlorosomes7,8,9. Chlorosomes are egg-like, ellipsoidal, extramembranous antenna apparatuses (Fig. 1a), and one of the most efficient photofunctional nanodevices in nature, which have attracted attention from the viewpoints of their functions with photophysical properties, supramolecular structures, and nanostructures10,11,12,13. A single chlorosome contains a large amount of bacteriochlorophyll-c, d, e, and f pigments (Fig. 1b), magnesium complexes of 31-hydroxy-131-oxo-chlorin, which are surrounded by lipid monolayers and form J-aggregates in hydrophobic environments without any protein assistance14,15. Their self-assembly is mainly organized by coordination bonding (31-O∙∙∙Mg), hydrogen bonding (31-O–H∙∙∙O=C-13), and π–π stacking of chlorin skeletons16,17. Their supramolecular (nano)structures are still in discussion, but suggested to be rods, tubes, and/or lamellar sheets.

Figure 1
figure 1

Natural and artificial chlorosomal systems. (a) Schematic of photosynthetic apparatuses in photosynthetic green bacteria. (b) Molecular structures of bacteriochlorophyll-c–f molecules. (c) Synthetic chlorophyll derivatives reported as models of bacteriochlorophyll-d.

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Model compounds of such chlorosomal bacteriochlorophyll-c–f pigments have been synthesized from naturally occurring chlorophyll-a18,19. Their rod-, tube-, and/or lamellar-shaped supramolecular nanostructures have been prepared using model molecules in solution and solid states as well as on substrates (Fig. 1c)20,21,22,23,24. Recently, we have reported that a zinc 31-hydroxy-131-oxo-chlorin having amide and urea groups in the 17-substituent self-assembled to form rod-like supramolecular nanostructures with a spacing of 1.4 nm25. In this study, we synthesized zinc 31-methoxy-chlorophyll derivatives lacking the 31-hydroxy group, which was requisite for chlorosomal J-aggregation through specific hydrogen bonding (31-O–H∙∙∙O=C-13). The zinc 31-methoxy-131-oxo-chlorin having amide and urea groups in the 17-substituent gradually formed chlorosomal J-aggregates in the solid state, and the supramolecular nanostructures of the resulting solids were mainly nanosheets that mimic the natural bacteriochlorophyll-c–f supramolecules in chlorosomes. This is the first report of biomimetic supramolecular nanosheets of chlorosomal J-aggregates being prepared from a synthetic zinc chlorophyll derivative.

Results and Discussion

Zinc chlorophyll derivatives Zn-1 and Zn-2 (Fig. 2a) were synthesized from methyl 3-devinyl-3-methoxymethyl-pyropheophorbide-a, which was prepared from naturally occurring chlorophyll-a extracted from commercially available Spirulina powders according to reported procedures (see synthetic details and spectral data in Supporting Information). The Zn-1 and Zn-2 molecules have ester and amide/urea groups, respectively, in the substituents at the 17-position, which are designed as without and with hydrogen bonding moieties. The UV-Vis absorption spectra of Zn-1 and Zn-2 in tetrahydrofuran (THF) at a concentration of 10 μM showed their monomeric states with Qy/Soret maxima at 649/426 nm (Fig. 2b, black line). The self-assembly of the zinc 31-methoxy-chlorins in a hydrophobic hexane-based solution was examined. A solution of the zinc chlorin in THF was added to 99-times the volume of hexane (total concentration: 10 μM). The UV-Vis-NIR absorption spectrum of Zn-2 exhibited red-shifted Qy/Soret maxima at 667/439 nm (Fig. 2b, blue line), which are ascribable to dimeric species, while Zn-1 gave only a monomeric state similar to its THF solution. Additionally, excitonically coupled circular dichroism (CD) signals were observed in the Qy region for the hexane-based solution of Zn-2 (Fig. 2c, blue), indicating that the molecular dipole moments of the chlorin π-systems were aligned along the y-axis. These results indicate that Zn-2 primarily form the dimeric species by intermolecular hydrogen bonding of its amide and urea groups in the 17-substituent, which readily induce stacking of the chlorin π-systems as well as coordination of the 31-O and central zinc in a dimer.

Figure 2
figure 2

Supramolecular assembly of zinc 31-methoxy-chlorophyll derivatives. (a) Molecular structures of Zn-1 and Zn-2. (b) UV-Vis-NIR absorption and (c) CD spectra of Zn-2 (10 μM) in THF (black), and THF/hexane (1:99, vol/vol) (blue). (d) UV-Vis-NIR absorption and (e) CD spectra of Zn-2 solids prepared from THF/hexane (5:95, vol/vol) (100 μM) just after preparation (blue) and after standing in solution for 1 week (red).

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At a 10-fold higher concentration (100 μM) in THF/hexane (5:95, vol/vol), Zn-2 was precipitated immediately, while Zn-1 was dissolved in the solution as the monomeric state (Fig. S1). The resulting Zn-2 solids just after preparation were adsorbed on a quartz substrate. The UV-Vis-NIR absorption spectrum exhibited Qy/Soret maxima at 667/437 nm (Fig. 2d, blue line), which was similar to that diluted in the hexane-based solution (Fig. 2b, blue line). After standing the precipitates in the above mixed solvent for 1 week in the dark at room temperature, the color of the Zn-2 solids was changed to dark green from blue-green, while Zn-1 was still dissolved in the mixed solvent to give no color change (Fig. S1). The resulting dark green solids were adsorbed onto a quartz substrate. The UV-Vis-NIR absorption spectrum showed further red-shifted Qy/Soret maxima at 728/439 nm (Fig. 2d, red line), and the excitonically coupled CD signals in the Qy region were enhanced (Fig. 2e, red line). These spectroscopic data indicate that the Zn-2 solids were gradually transformed from dimeric or small oligomeric species into chlorosomal J-aggregates, in which the molecular dipole moments of the Zn-2 molecules were arranged along the y-axis in a well-ordered manner. Thus, Zn-2 molecules form dimeric species kinetically and transform into more thermodynamically stable J-aggregates in the solid states. The resulting Zn-2 chlorosomal J-aggregates on a quartz substrate exhibited a 752-nm fluorescence emission peak at the excitation of the Soret band (Fig. S2).

The Zn-2 solids produced in THF/hexane (5:95, vol/vol) after standing for one week were dispersed by ultrasonication and the resulting suspension was drop-cast onto a substrate to analyze their supramolecular nanostructures using tapping-mode atomic force microscopy (AFM). The AFM height image of the Zn-2 self-assemblies on a highly oriented pyrolytic graphite (HOPG) substrate showed sheet-like supramolecular nanostructures with heights of 15–25 nm (Figs. 3a and S3a). The cross-sectional analysis showed that the surface of the nanosheet was flat over a submicrometer range (Figs. 3b and S3b–e). The nanosheet had a step of 6 nm (Fig. 3b), suggesting a cleavage of a single layer of the nanosheet. The Zn-2 solids produced in THF/hexane (5:95, vol/vol) just after preparation were also examined using tapping-mode AFM. The AFM images showed mainly particle-like nanostructures with heights of 1–25 nm and partially nanosheets with heights of 13–21 nm (Fig. 4). Thus, the results indicate that nanosheets of Zn-2 J-aggregates are more thermodynamically stable nanostructures and that they are gradually formed from amorphous particle-like aggregates of kinetically favored dimeric species.

Figure 3
figure 3

Supramolecular nanosheets of Zn-2 self-assemblies formed upon standing in THF/hexane (5:95, vol/vol) for one week. Tapping-mode AFM analysis of Zn-2 J-aggregates on an HOPG substrate, (a) AFM height image and (b) cross-section analysis. Cryo-TEM analysis of Zn-2 on a carbon-coated copper grid, (c) and (d) cryo-TEM images and (e) electron beam diffraction.

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Figure 4
figure 4

A tapping mode AFM height image of Zn-2 just after preparation in THF/hexane (5:95, vol/vol, 100 μM); the sample was dispersed by ultrasonication and drop-cast on an HOPG substrate.

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The aforementioned suspension of Zn-2 J-aggregates was similarly drop-cast onto a carbon-coated copper grid, and analyzed using cryo-transmission electron microscopy (cryo-TEM). Cryo-TEM images also mainly showed supramolecular nanosheets (Fig. 3c) similarly as in AFM analysis (rod-like nanostructures were partially visible in the cryo-TEM images of Zn-2 J-aggregates in Figs. 3c and S4). Moreover, black stripes with a spacing of 2.9 nm were visible in the magnified cryo-TEM image (Figs. 3d and S5). Taking a closer look at this image, it can be seen that one black stripe consists of a pair of black lines (Fig. 3d, inset). Since the cryo-TEM image was obtained without stain, the black lines in the Fig. 3d image were based on the zinc atoms of Zn-2 molecules. The electron beam diffraction of the nanosheet showed an orthogonal pattern (Fig. 3e). Assuming an orthorhombic unit cell (a = 6 nm, b = 2.9 nm, c = 0.69 nm) from Fig. 3e as well as Fig. 3b, there is a systematic absence of 00 l (l: odd) reflections by extinction rule. This indicates the presence of the 21 screw axis along the c-axis. The 0.69-nm distance between adjacent zinc chlorophyll molecules was similar to previously reported results for microcrystalline of a cadmium BChl-d analog26 as well as self-assemblies of Zn-325 (Fig. 1c, bottom), the 31-hydroxylated analog of Zn-2. Zn-3 formed a large rod-shaped supramolecular nanostructure through intermolecular hydrogen bonding between 31-OH and 13-C=O, and its striped orientation was parallel to the long axis of the rod. In the present study, 31-methoxylated Zn-2 forms no hydrogen bonding between 31-OCH3 and 13-C=O and also weakens the coordination ability of 31-O to central zinc due to the sterically bulky 32-methyl group, inducing less chlorosomal J-aggregation and relatively enhancing the intermolecular hydrogen bonding of the amide and urea groups in the 17-substituents. The balance of intermolecular interactions is the driving force for the slow growth of two-dimensional Zn-2 J-aggregates as major supramolecular nanostructures under the present conditions.

The two-dimensional Zn-2 nanosheets were investigated in terms of their anisotropy to further elucidate their supramolecular structure. The solid Zn-2 nanosheets were drop-cast onto a quartz optical waveguide and their Vis-NIR absorption spectra were measured using polarized light, perpendicular (p-light) and parallel (s-light) to the surface (Fig. 5a). The Vis-NIR absorption spectrum of the nanosheet without polarization showed Qy/Soret maxima at 736/446 nm (Fig. 5b, black line), which was similar to those measured using conventional transmission mode (Fig. 2d, red line). The p- and s-light absorption spectra showed Qy/Soret maxima at 733/441 and 743/454 nm, respectively (Fig. 5b, blue and red lines). These observations revealed that the molecular dipole moments of Zn-2 molecules along the y-axis were arranged in two situations with both vertical and horizontal fashions. The Qy and Soret absorption peaks of the nanosheets under s-light were observed at longer wavelength regions than those under p-light, indicating that Zn-2 molecules stacked more strongly in parallel direction to the optical waveguide surface than that in perpendicular one. This suggests that Zn-2 molecules form linearly stepped oligomers favorably orientated parallel to the quartz surface. It is consistent with the microscopic analyses, which reveal a striped orientation in the nanosheet parallel to the surface of the substrate. Thus, a single striped layer is ascribable to the linearly stepped oligomeric Zn-2 molecules.

Figure 5
figure 5

Vis-NIR absorption spectra of Zn-2 nanosheets using optical waveguide. (a) Schematic of the measurement and (b) Vis-NIR absorption spectra of Zn-2 nanosheets on an optical waveguide using p- (blue) and s-light (red) as well as non-polarized light (black).

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The self-assembly of Zn-2 molecules is schematically represented in Fig. 6. The Zn-2 molecules kinetically formed a dimeric species based on its intermolecular hydrogen bonding of amide and urea groups in a hydrophobic environment (Fig. 6, step i). The dimeric species immediately formed nanoparticles (Fig. 6, step ii)23,24, which gradually transformed into the more thermodynamically stable supramolecular nanosheets of chlorosomal J-aggregates (Fig. 6, step iii). Appropriate crystal structure model of Zn-2 molecules in nanosheets consistent with the cryo-TEM data and the extinction rule is shown in middle and bottom of Fig. 6. Two arrays of zinc atoms along c-axis in bc-plane (a-axis projection) corresponds to the black lines in cryo-TEM image (Fig. 3d). It is noted that the supramolecular structures of bacteriochlorophyll-c–f assemblies are still in discussion23,24,27,28,29, and the supramolecular structure of Zn-2 molecules in the nanosheet and the ligation of the central zinc (α or β) as well as the configuration between the 31-methoxy moiety and 17-methylene group [syn (same side) and anti (opposite side)] are unclear. The Zn-2 supramolecular nanosheets had a striped pattern, which is reminiscent of a lamellar sheet nanostructure of natural chlorosomal bacteriochlorophyll assemblies. A natural chlorosome consists of epimeric mixtures of bacteriochlorophyll-c–f homologs and a variety of carotenoids, quinones, and waxes, which are surrounded by a lipid monolayer. Although all these components should affect their supramolecular nanostructures of bacteriochlorophyll-c–f assemblies, their essential supramolecular (nano)structures are controlled by themselves. For the present Zn-2 J-aggregates, supramolecular nanosheets are simply constructed by intermolecular interactions of coordination (31-O∙∙∙Zn) and hydrogen bonding (C=O∙∙∙H–N) as well as π–π stacking of the chlorins in the solid state.

Figure 6
figure 6

Schematic drawing of the Zn-2 chlorosomal J-aggregation process.

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Conclusion

Zinc 31-methoxy-chlorophyll derivatives Zn-1 and Zn-2 were synthesized from naturally occurring chlorophyll-a. Zn-2 having amide and urea groups in the 17-substituent kinetically formed a dimeric species that gradually transformed into the more thermodynamically stable chlorosomal J-aggregates in the solid state, while Zn-1 bearing a dodecyl at the 17-propionate residue formed neither dimers nor large aggregates. The dimerization and J-aggregation of Zn-2 molecules are driven by its intermolecular hydrogen bonding of amide and urea moieties in the 17-substituents as well as the coordination (31-O∙∙∙Zn) and π–π stacking of chlorin π-systems. The structural transformation of the chlorophyll supramolecules was first observed in the solid state using spectroscopy. The major supramolecular nanostructures of the more thermodynamically stable Zn-2 J-aggregates were nanosheets, which were first constructed. The present two-dimensional nanomaterial photophysically and nanostructurally mimics chlorosomal bacteriochlorophyll-c–f J-aggregates. The supramolecular nanosheets (the present study) and nanotubes20,21,22, which are reminiscent of covalently-bonded graphene and carbon nanotube, respectively, could be prepared using zinc chlorophyll derivatives. The bioinspired supramolecular nanosheets of chlorosomal J-aggregates have various potential applications, such as artificial light-harvesting antennas and photosynthesis, dye-sensitized solar cells, catalytic reactions, and photodynamic therapy.

Methods

Samples

Chlorophyll-a was extracted from commercially available cyanobacterial spirulina powders (DIC LIFETEC). Synthetic procedures and spectral data for Zn-1 and Zn-2 are described in Supplementary Information. The samples in a solution at 10 μM for UV-Vis-NIR absorption measurements were prepared as follows. The samples were weighed on Ultra-Microbalance XPR2UV (Mettler Toledo), and dissolved in THF, then an aliquot of the solution was dried in vacuo. The residue (20 nmol) was dissolved in only THF (2 mL), or first dissolved in THF (0.02 mL) that was then added to hexane (1.98 mL) in a 1-cm quartz cuvette. The samples at 100 μM were prepared as follows. The samples (1 μmol) were first dissolved in THF (0.5 mL) and diluted with hexane (9.5 mL). The resulting suspension of Zn-2 in THF/hexane (5:95, vol/vol) was stored at room temperature in the dark for one week to prepare two-dimensional nanosheets. The solid samples of Zn-2 in THF/hexane (5:95, vol/vol) were dispersed using ultrasonication, and a quartz plate was dipped in this suspension and then dried more than 10 times to adsorb self-assemblies of Zn-2 onto the quartz surface for UV-Vis-NIR absorption measurements performed using conventional transmission mode. The Zn-2 solids were drop-cast onto a quartz optical waveguide (System Instrument) for the measurement of Vis-NIR absorption spectra in total reflection mode in air. For AFM measurements, the solid samples of Zn-2 in the above hexane-based solution were dispersed by ultrasonication for 30 seconds and the resulting suspensions were drop-cast onto HOPG substrates (Bruker) and dried immediately by purging nitrogen gas. For the cryo-TEM measurements, a suspension of Zn-2 was drop-cast onto a carbon coated copper grid. The solvents used for the optical and microscopic measurements were purchased from Nacalai Tesque as reagents prepared specifically for spectroscopic experiments.

UV-Vis-NIR absorption spectroscopy

UV-Vis-NIR absorption spectra in transmission mode were measured at room temperature on a Hitachi U-3500 spectrophotometer. Vis-NIR absorption spectra in a total reflection mode using an optical waveguide were measured at room temperature with a System Instruments SIS-50 spectrophotometer.

CD spectroscopy

CD spectra were measured at room temperature on a Jasco J-720W spectropolarimeter.

AFM

AFM was performed with a Bruker Multimode 8 system in tapping mode in air. An HOPG substrate was freshly cleaved before drop-casting the samples. A silicon cantilever (Bruker, MPP-11100–10) was used.

Cryo-TEM

Cryo-TEM was performed with a JEOL JEM-2100F (G5) microscope operated at the acceleration voltage of 200 kV and measured at liquid helium temperature (4.2 K).

Open access

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