Abstract
Nanographenes, finite models of graphene sheets, are endowed with intriguing optical, electronic, and spintronic features. So-called heteroatom-doping, where one or more carbon is replaced by non-carbon light atoms has been proved effective in tuning the properties of nanographenes. Here we extend the concept of heteroatom nanographene doping to include metal centers. The method employed involves the use of a dipyrromethene fragment as an auxiliary ligand that is directly linked to the bay area of the model nanographene hexa-peri-hexabenzocoronene (HBC) to give a dipyrromethene-fused nanographene-type hybrid ligand (HBCP). HBCP has a corrole-like trianionic core that is capable of coordinating group 11 metal cations, including trivalent Cu, Ag and Au. These cations are introduced into the cavity with atomic precision to give metal complexes (HBCP-M; M = Cu, Ag, Au). The electronic structure and photophysical properties of HBCP and its metal complexes are investigated by steady-state and fs-transient spectroscopies, as well as DFT calculations. The ligand and metal complexes are also characterized via single crystal X-ray diffraction analyses. This work paves the way towards the precise metal doping of nanographenes within the carbon network, as opposed to the synthetic appendage of an independent chelating group, such as a fused tetrapyrrolic moiety.
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Introduction
Nanographenes have garnered increasing attention due to their intriguing structures and potential utility in nanoelectronics, optoelectronics, and spintronics1,2,3,4,5,6,7,8. Incorporating heteroatoms into the otherwise all-carbon framework is an effective strategy for tuning the electronic structure of nanographenes9,10. To date, bottom-up organic syntheses have allowed the preparation of structurally defined heteroatom-doped nanographene analogs containing boron, nitrogen, oxygen, sulfur, and even selenium with perfect control over the size, edge structure, substituent, and even the doping concentration and position11,12,13,14,15,16,17,18,19,20,21,22. However, achieving this level of precision has proved challenging in the case of metal-doped nanographenes. Currently, metal-nanographene complexes with metal-carbon π23,24,25,26 or σ27 bonding, complexes between ruthenium28,29 and rhenium30, and peripheral nitrogen-doped nanographenes, as well as some tetrapyrrolic metalloporphyrin fused-nanographene systems31,32,33 are known. However, strategies that would allow molecular-level metal doping within a nanographene carbon network are still lacking. Here, we report a nanographene platform based on the iconic hexa-peri-hexabenzocoronene (HBC) framework that allows the precise complexation of three representative trivalent metal cations with direct metal-carbon bonding. This precision of metal insertion arises from the synthetic incorporation of a carbaporphyrin unit into the HBC skeleton that, in turn, provides an elaborated macrocyclic adj-CCNN coordination environment.
Carbaporphyrins represent porphyrin analogs where one or more nitrogen donor atoms in the core is replaced by a carbon atom34. The introduction of carbon donors makes carbaporphyrin a versatile coordination motif capable of complexing a wide range of metal ions34,35. In addition to classic carbaporphyrins, such as N-confused porphyrins, benziporphyrins, and azuliporphyrins, systems incorporating polycyclic aromatic hydrocarbons (PAHs) have begun drawing attention. To date, PAHs such as naphthalene36,37, anthracene38,39, phenanthrene40,41, triphenylene42, pyrene43,44,45, dibenzo[g,p]chrysene46,47,48 have been introduced into carbaporphyrin systems. Our groups recently reported carbaporphyrin trimers and tetramers with cyclo-meta-phenylene cores49. We have also synthesized an expanded carbaporphyrin incorporating two HBC and bridging dipyrromethene units. This tweezers-like system was found to possess a figure-of-eight carbaoctaphyrin (1.1.1.0.1.1.1.0) core and proved capable of coordinating BF2 to form a BODIPY-type complex50. This finding led us to consider whether dipyrromethene and HBC subunits could be combined to produce a small, macrocyclic tetradentate coordination site, and whether the resulting adj-CCNN core would allow metal complexation. To test this possibility, a new dipyrromethene-fused nanographene ligand (HBCP) was synthesized. It possesses a corrole-like adj-CCNN trianionic cavity and, as detailed below, was found to form stable complexes with Cu(III), Ag(III), and Au(III).
Results
Synthesis and characterization of free base nanographene ligand
The synthesis of HBCP is shown in Fig. 1 and given in detail in the Supplementary Information. Briefly, the bicarbinol 150 was condensed with pyrrole in dichloroethane (DCE) at reflux to form the bipyrrole intermediate 2. A BF3·OEt2-catalyzed condensation reaction between 2 and pentafluorobenzaldehyde, followed by oxidation with excess 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), then yielded the hybrid nanographene HBCP in 20% yield. Alternatively, HBCP can be also obtained by condensing 1 with a pentafluorophenyl substituted dipyrromethane directly in 10% yield under similar conditions. Silica gel chromatography followed by gel permeation chromatography (GPC) allowed the target compound HBCP to be isolated in the form of a black solid. Benefit from the multiple peripheral mesityl and tert-butyl substituents, HBCP (including its metal complexes HBCP-M as detailed subsequently) shows good solubility in organic solvents such as dichloromethane (DCM) and toluene, facilitating their characterizations in solution. HBCP was characterized by 1H, 19F, 2D-correlation spectroscopy (COSY), UV−vis−NIR spectroscopies, high-resolution MALDI-TOF mass spectrometry, and a single-crystal X-ray diffraction analysis.
Conditions: (i) pyrrole and BF3•OEt2 in dry DCE, reflux, 8 h, 64%; (ii) pentafluorobenzaldehyde and BF3·OEt2 in dry CH2Cl2, room temperature, 2.5 h, then DDQ, room temperature, 0.5 h, 20%; (iii) dipyrromethane and BF3·OEt2 in dry CH2Cl2, room temperature, 2.5 h, then DDQ, room temperature, 0.5 h, 10%; Mes, t-Bu and C6F5 denote mesityl, tert-butyl, and pentafluorophenyl groups, respectively.
Diffraction grade single-crystals of HBCP were grown from toluene/methanol. The resulting X-ray structure confirmed the formation of the dipyrromethene-fused nanographene ligand. The crystal cell contains four crystallographically independent molecules (Supplementary Fig. 9). One of these molecules is shown in Fig. 2A, B. The dipyrromethene unit is tilted, with the angle between the dipyrromethene plane (defined by all the 11 atoms of the dipyrromethene backbone) and the HBC plane (defined by all 42 carbon atoms of the HBC backbone) being 28.5°. The length of the nanographene ligand defined by the distance from the centroid of atoms C12 and C15 to C32 is 14.23 Å. The C1-C37 and C26-C27 bond lengths are 1.468 Å and 1.469 Å, respectively, supporting the notion that the HBC moiety is bound to the dipyrromethene unit through what are predominantly single bonds. The non-hydrogen atoms in the backbone appear to be sp2 hybridized, leading to the conclusion that the HBCP structure is fully conjugated. Two individual HBCP molecules adopt a back-to-back orientation (Supplementary Fig. 9), with the two HBC planes separated by a distance of about 3.545 Å (from one HBC center to the other HBC plane). We thus propose that π–π interactions play a role in promoting the stacking seen in the solid state.
A Front and B Side views of HBCP. Thermal ellipsoids are scaled to the 50% probability level. Solvent molecules and hydrogen atoms are omitted for clarity. C Partial 1H NMR spectrum of HBCP recorded in CD2Cl2 at room temperature (293 K, 600 MHz). Asterisks indicate residual solvent peaks. The labeling refers to the features for only one set of proton signals due to the symmetry.
The 1H NMR spectrum of HBCP was recorded in CD2Cl2 (Fig. 2C). The proton resonance signals were readily assigned on the basis of the COSY spectrum (Supplementary Figs.16, 17). The signals at 6.27 and 5.77 ppm are ascribed to the pyrrolic β-CH protons. The resonance at 14.68 ppm was assigned to the CHs within the carbaporphyrin ring (inner CHs, labeled with H38 and H39). The chemical shifts of the other CH signals (H2 to H25) on the HBC backbone range from 7.69 to 9.20 ppm. The NH proton signal is observed at 15.27 ppm, which was confirmed by a D2O exchange experiment (Supplementary Fig. 18). The average chemical shift differences between the pyrrolic NH and β-CH protons (Δδ), and the inner CH and outer CH protons (Δδ’) were 9.25 and 6.99 ppm, respectively. These values are larger than those of non-aromatic adj-dicarbacorrole51 and slightly smaller than those of our reported antiaromatic dibenzo[g,p]chrysene(DBC)-fused bis-dicarbacorrole46 (Δδ and Δδ’ = 10.3 and 7.8 ppm, respectively). Thus, HBCP is best classified as weak antiaromatic. The signals of H2, H25 (7.69 ppm) and H5, H22 (8.34 ppm) show an upfield shift relative to the CH signals of a typical HBC; for instance, the skeleton CH protons of hexa-tert-butylhexabenzocoronene resonate at 9.30 ppm in CDCl323. This upfield shift is attributed to the shielding effect of the paratropic ring current of the antiaromatic carbaporphyrin ring.
The UV−vis-NIR spectrum of HBCP was recorded in toluene (Fig. 3A). Intense and sharp absorption bands are seen in the short-wavelength region (300–500 nm). In contrast, weak and ill-defined absorption bands are observed at longer wavelengths (600–900 nm). Analogous spectral features were reported for phenanthriporphyrin40 and DBC-fused bis-dicarbacorrole46, where a paratropic ring current arises from macrocyclic π-conjugation within the phenanthriporphyrin framework. In particular, faint absorption features in the long wavelength region, ascribed to an optically forbidden lowest electronic transition with a small energy gap52, are observed for all three systems. Thus, the spectral concordance between HBCP and these two recognized antiaromatic systems provides support for the initial assessment that HBCP is weak antiaromatic drawn on the basis of the 1H NMR spectral studies above.
A UV−vis−NIR spectrum of HBCP in toluene with calculated electronic transitions (B3LYP/6-311 g(d,p)). Major MO contributions to the two lowest electronic transitions are marked with black arrows. B ACID plot and C NICSZZ(1) 2D map of HBCP. The ACID plot is drawn with an isovalue of 0.045, where the magnetic field is toward the viewers. D MO energy diagram and distribution of electron densities from HOMO-2 to LUMO + 2 in HBCP.
The electronic structure of HBCP was interrogated in detail by quantum chemical calculations. Anisotropy of the induced current density (ACID) plots, which allow the magnetically-induced current to be visualized53, revealed a paratropic ring current (Fig. 3B and Supplementary Fig. 3). Nucleus-independent chemical shift (NICS) calculations, which predict idealized NMR chemical shifts54, revealed positive values within the cavity. These ACID and NICS results are sharply contrasting to those of non-aromatic adj-dicarbacorrole51 (Supplementary Figs. 3, 4). Moreover, the harmonic oscillator model of aromaticity (HOMA), which estimates molecular aromaticity by comparing bond lengths with an ideal aromatic molecule (benzene)55, showed a higher HOMA value of 0.561 for HBCP than 0.496 of the adj-dicarbacorrole. Taken in concert, these two calculations lead us to suggest that the weak antiaromatic features of HBCP derive primarily from the carbaporphyrin-like ring (Fig. 3C). An MO analysis provided support for this conclusion. For instance, in the HOMO and LUMO, the π-electron density is mainly distributed on the carbaporphyrin subunit (Fig. 3D). Calculations made using time-dependent density functional theory (TD-DFT) revealed that the HOMO-to-LUMO transition is a major contributor to the lowest electronic transition (Fig. 3A). Considered in conjunction with the 1H NMR and UV−vis−NIR spectral data, the calculations reveal that the electronic structure of HBC is effectively perturbed in HBCP by the fusion of the dipyrromethene moiety into an overall fully sp2 hybridized structure. Here, in contrast to a structural distortion caused by the ortho-protons of directly linked phenyl groups in the adj-dicarbacorrole51, the HBC moiety of HBCP results in the planarized structure with a large perturbation of the local electronic structure of benzene unit56. These effects appear to favor the formation of a weakly antiaromatic nature.
Synthesis and characterization of metal-nanographene complexes
Although Cu carbaporphyrins with adj-CCNN cores have been extensively studied46,49,51,57,58, the corresponding Ag and Au complexes have received much less attention. To our knowledge, structurally characterized Ag and Au carbaporphyrin complexes with adj-CCNN core have only been stabilized with doubly N-confused porphyrin57 and hexaphyrins59,60 as ligands, where the pyrrolic β-carbon atoms act as donors. This may reflect the low reactivity of the phenyl CH protons. Against this background, we elected to explore the coordination chemistry of HBCP using salts of the group 11 metals, Cu, Ag, and Au (Fig. 4).
Conditions: (i) Cu(OAc)2 in CHCl3/CH3CN(v/v 6/4), reflux, 48 h, 75%; (ii) Ag2CO3 in o-dichlorobenzene, reflux, 24 h, 11%; (iii) NaAuCl4, AgOTf and NaOAc in CH3COOH, reflux, 12 h, 24%.
The synthesis of the Cu complex of HBCP proved quite straightforward. Treating HBCP with Cu(OAc)2 in a mixture of chloroform and acetonitrile at reflux gave HBCP-Cu in 75% yield. However, synthesis of the corresponding Ag nanographene complex proved to be more challenging. Several conventional methods were attempted, including the use of silver salts, such as AgOAc57 and AgOCOCF361,62, as well as carrying out the reaction in pyridine, CHCl3, and CH2Cl2. Unfortunately, none of these attempts yielded the desired silver complex. Fortunately, HBCP-Ag could be obtained an 11% yield after heating HBCP in o-dichlorobenzene at reflux in the presence of Ag2CO3. For the Au complex, reagents such as NaAuCl459 or NaAuCl4/AgOTf60 were tested, along with different solvents, including CH2Cl2/methanol and toluene; again, these conditions failed to yield HBCP-Au. Finally, HBCP-Au was obtained in a 24% yield by treating HBCP with a mixture of NaAuCl4, AgOTf, and NaOAc in acetic acid at reflux.
Diffraction grade single-crystals of HBCP-Cu and HBCP-Au were grown from a mixture of dichloromethane/methanol, while those of HBCP-Ag were grown from toluene/methanol. The resulting crystal structures served to confirm the metal complexation of HBCP with Cu, Ag, and Au, respectively (Fig. 5A–C). Compared to the structure of HBCP, these three metal complexes are relatively flat. The calculated mean plane (defined by all the 56 atoms of the HBCP-M backbone) deviations are 0.179, 0.085, and 0.146 Å for HBCP-Cu, HBCP-Ag, and HBCP-Au, respectively. The longest dimension of HBCP-Cu, HBCP-Ag, and HBCP-Au, corresponding to the distance between C32 and the centroid of the C12 and C15 atoms, was estimated to be 14.486, 14.629, and 14.644 Å for these three complexes, respectively. The metal ions are located within the adj-CCNN cores bound in approximate square-planar coordination geometries with no axial ligands being seen in the crystal structure. In HBCP-Cu, the Cu-C and Cu-N bond lengths are 1.949, 1.945, 1.917, and 1.917 Å, which are similar to those reported for the copper complexes of bis-dicarbacorrole46 and phenanthriporphyrin58. The metal-carbon bond lengths for HBCP-Ag and HBCP-Au are also similar to those found in the respective metal complexes of N-confused porphyrins57 and hexaphyrins59 with similar CCNN coordination modes. No axial ligands or counter ions are observed in the crystal structures. We thus conclude that the metal cations are bound in their formal trivalent states (i.e., Cu(III), Ag(III), and Au(III)).
A–C Single-crystal X-ray diffraction structures of HBCP-Cu, HBCP-Ag, and HBCP-Au; top views are on the top, and side views are at the bottom. Selected bond lengths (Å) are shown in the top views. Thermal ellipsoids are scaled to the 50% probability level in HBCP-Cu and HBCP-Au, and the 30% probability level in HBCP-Ag. Solvent molecules, hydrogen atoms, meso-aryl substituents, and tert-butyl groups are omitted for clarity. D Partial 1H NMR spectra of HBCP-M recorded in CD2Cl2 at room temperature (293 K, 400 MHz for HBCP-Cu and HBCP-Ag, 600 MHz for HBCP-Au). Asterisks indicate residual solvent peaks. E UV−vis−NIR absorption spectra of HBCP-Cu (orange), HBCP-Ag (blue), and HBCP-Au (purple) recorded in toluene.
The 1H NMR spectra of HBCP-M (M = Cu, Ag, and Au) were recorded in CD2Cl2 (Fig. 5D). No discernible NH and inner CH signals were observed for any of the complexes. This finding is consistent with the conclusion that the metal cations are coordinated within the carbaporphyrin cavity. The observation of well-resolved 1H NMR spectra further supports the +3 oxidation state assignment for the coordinated metal cations in HBCP-M. Metalation causes little change in the chemical shifts of the protons on the HBC backbone. In contrast, the pyrrolic β-CH protons resonate at a lower field in HBCP-M as compared to HBCP. Such shifts are reminiscent of what was seen in the case of the copper(III) complexes of phenanthriporphyrin58 and bis-dicarbacorrole46. These observations are well reflected in their ACID and NICS results (Supplementary Figs. 3,7). Compared to HBCP, the paratropic ring current on the carbaporphyrin unit is reduced in the ACID plots of HBCP-M. Concurrently, a counterclockwise current flow on the metallacyclopentadiene unit is evident. These features are well reproduced in the NICS results. Analogous metalation effect was reported in the asymmetric carbaporphyrin pseudo-dimer and its hetero-complex63, where a metal-ligand interaction by the Cu coordination causes a decrease of its antiaromaticity. Differences amongst the present set of congeneric complexes were seen. For instance, the downfield shift in the pyrrolic β-CH proton signals of HBCP-Au (6.68 and 6.38 ppm) was more pronounced than what was seen in the case of HBCP-Cu (6.52 and 6.13 ppm) and HBCP-Ag (6.48 and 6.16 ppm) (Fig. 5D). While further study is required, this difference is ascribed to a greater deshielding effect being produced by the Au(III) center than either Cu(III) or Ag(III)64.
The UV−vis-NIR spectra of HBCP-M (M = Cu, Ag, and Au) were recorded in toluene and are shown in Fig. 5E. Compared to HBCP, not only were the spectral features red-shifted, but additional absorption bands were apparent in the absorption spectra of HBCP-M as compared to HBCP with the weak absorption bands at longer wavelengths (600–900 nm) being particularly well-resolved. These spectral changes mirror those seen when phenanthriporphyrin40,58 and bis-dicarbacorrole46 are converted to their corresponding Cu(III) complexes. In all cases, an increase in planarity and rigidification of the structure is thought to enhance the metal-ligand interactions associated with metal complexation65. TD-DFT calculations revealed that the π-electron density in the three metal complexes of this study is concentrated predominantly on the carbaporphyrin unit in both the HOMO and LUMO (Supplementary Fig. 5). Here, the metal dπ orbitals, which effectively form nodes with the ligand π-orbital, are observed in H-2, H-1, and LUMO, which leads that their weak absorption bands in the longer wavelength region originate from the dominant transition from HOMO–H-2 to LUMO. Furthermore, the distribution of π-electron density estimated from occupied π-MOs well describes the effective metal-ligand interaction of HBCP-M (Supplementary Fig. 6), where the significant π-electron density on the metallacyclopentadiene unit aligns well with the intense current density on that in their ACID plots. This finding helps account for the antiaromaticity-decreasing nature of HBCP-M that arises from the effective metal-ligand interaction and the accompanied perturbation in the π-electronic structures.
Electrochemical properties
The electrochemical properties of HBCP and HBCP-M (M = Cu, Ag, and Au) were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Fig. 6 and Supplementary Fig. 1). Two reversible oxidation and reduction waves were observed for both HBCP and HBCP-M. The first oxidation and reduction potentials of HBCP were at 0.03 and −1.68 V (vs Fc/Fc+), respectively, corresponding to an electrochemical HOMO − LUMO gap of 1.71 eV. All redox waves were significantly shifted anodic in the corresponding HBCP-M metal complexes. This shifting is ascribed to the electron-withdrawal effect of the bound trivalent metal cations. Accordingly, the shift was most dramatic in the case of HBCP-Au, where the first oxidation and first reduction waves were shifted to 0.40 and −1.34 V, respectively66. Nevertheless, the electrochemical HOMO-LUMO gaps proved roughly comparable within the HBCP-M series, with gaps of 1.73, 1.68, and 1.74 eV being calculated for HBCP-Cu, HBCP-Ag, and HBCP-Au, respectively.
Cyclic voltammograms (CVs) of HBCP and HBCP-M were recorded in dry CH2Cl2 containing 0.1 M TBAPF6. The scan rate is 50 mV·s−1.
Excited state dynamics
To investigate their excited state dynamics, transient absorption (TA) measurements were carried out on HBCP, and HBCP-M (M = Cu, Ag, and Au) (Fig. 7 and Supplementary Fig. 2). Two limiting sets of behavior were seen. Following photoexcitation at 400 nm, double-exponential decay behavior was seen in the TA signals of HBCP. The observation of both fast and relatively slow signal decay is consistent with the overall antiaromaticity proposed for this system. Considered in conjunction with the weak and ill-defined NIR signals seen in the absorption spectrum, this double-exponential decay feature is considered consistent with the existence of an optically forbidden S0-to-S1 transition with a small energy gap as commonly found in antiaromatic porphyrinoids52. In contrast, the TA signals of HBCP-M were characterized by a relatively long mono-exponential decay during hundreds of ps without fluorescence emission. The absence of fluorescence during the relatively long time can be attributed to their effective metal-ligand interaction that is reflected with the well-resolved NIR absorption bands. Based on the ligand-metal interaction, the photoexcitation of HBCP-M leads to a shift of electron density from ligand-dominated states to metal-dominated states, so-called ligand-to-metal charge transfer (LMCT), and subsequent non-radiative d-d transition relaxation65,67. Analogous decay features have been observed in metal complexes that support LMCT68,69. Moreover, the relaxation time becomes longer from HBCP-Cu to HBCP-Ag to HBCP-Au (75, 93, and 130 ps, respectively). The elongated decay can also be comprehended with LMCT. As the size of the metal increases, metal d-orbitals undergo the larger ligand field splitting70. This results in a large d-d energy gap and decelerates the d-d transition relaxation. Here, this sharp contrasting excited state dynamics of HBCP-M compared to HBCP also elaborates that the effective metal-ligand interaction leads to a significant perturbation of the π-electronic structure of the ligand.
Femtosecond transient absorption spectra and decay profiles of HBCP (A, B) and HBCP-Cu (C, D) recorded in toluene with a photoexcitation at 400 nm.
Discussion
In summary, we synthesized and characterized a hybrid nanographene ligand, HBCP, which is an embedded hybrid of a nanographene fragment and a carbaporphyrin. HBCP contains an adj-CCNN core that acts as an effective trianionic ligand for the group 11 trivalent metal cations Cu(III), Ag(III), and Au(III). Complexation of these cations results in the generation of what, to our knowledge, is the set of precisely metal-doped nanographene analogs, namely HBCP-M (M = Cu, Ag, Au). Our approach allows for the controlled insertion of metal cations of different sizes into a nanographene network while showing that the electronic structure of nanographenes may be perturbed through metalation. As such, it sets the stage for the preparation of new nanographene analogs with controlled structure, electronic features, and chemical properties.
Methods
Materials and characterization
Chemicals were purchased from commercial sources and used without further purification unless otherwise indicated. Analytical pure solvents, including CH2Cl2, chloroform (CHCl3), acetonitrile (CH3CN), petroleum ether (PE), and toluene were purchased from Bei Jing Tong Guang Fine Chemicals Company. CH2Cl2, when used as a solvent, was distilled over CaH2 under nitrogen. NMR spectra (1H NMR spectra at 400 MHz or 600 MHz, 13C NMR spectra at 100 MHz or 150 MHz, 19F NMR spectra at 376 or 565 MHz, two-dimensional NMR spectra at 400 or 600 MHz) were recorded on a JEOL NMR spectrometer at room temperature (293 K). High-resolution MALDI-TOF mass spectrometric analyses were carried out using a Bruker Autoflex speed mass spectrometer. UV−vis−NIR spectra were measured on a UV-3600i Plus spectrometer from Shimadzu. Recycling preparative gel permeation chromatography (GPC) was performed with an LC-5060 P2 instrument from Japan Analytical Industry Co., Ltd. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were carried out on a CH Instrument Model 660E electrochemical system utilizing a three-electrode configuration consisting of glassy carbon (working electrode), platinum wire (counter electrode) and SCE (saturated calomel electrode; reference electrode) in a solution of 0.1 M TBAPF6 in dry CH2Cl2 at a scan rate of 50 mV s−1 in a nitrogen-filled cell. Crystallographic data were collected using a Rigaku Oxford Diffraction XtaLAB Synergy diffractometer equipped with a HyPix-6000E area detector at 100 K using Cu Kα (λ = 1.54184 Å) from a PhotonJet micro-focus X-ray source.
Transient absorption spectroscopy
Femtosecond transient absorption data were obtained using a lab-built femtosecond absorption spectrometer, which consisted of optical parametric amplifiers (Palitra, Quantronix) pumped by a Ti:sapphire regenerative amplifier system (Integra-C, Quantronix) operating at 1 kHz repetition rate and an optical detection system. The OPA pulses generated in this way had a pulse width of ~100 fs and an average power of 100 mW in the range 280–2700 nm. These were used as pump pulses. White light continuum (WLC) probe pulses were generated using a sapphire window (3 mm thickness) by focusing a small portion of the fundamental 800 nm pulses, which were picked off by a quartz plate before entering into the optical parametric amplifiers. After the measurements, the absorption spectra were carefully checked to detect if there were artifacts due to degradation or photo-oxidation of the samples.
Data availability
The authors declare that the data supporting the findings of this study are available within the article and Supplementary Information file, or from the corresponding author upon request. The X-ray crystallographic data of corresponding structures reported in this study have been deposited to the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2341606, 2341609, 2341610, 2341611 (HBCP, HBCP-Cu, HBCP-Ag, HBCP-Au). These data can be obtained free of charge from CCDC via www.ccdc.cam.ac.uk/data_request/cif. All the other supplementary data are available from the article and its Supplementary Information files or available from the corresponding authors upon request. Source Data are provided with this manuscript. Source data are provided with this paper.
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Acknowledgements
The work at Beijing Normal University (BNU) was supported by the National Natural Science Foundation of China (grant no. 22275020 to X.-S.K.) and BNU startup funding (grant no. 312232114 to X.-S.K.). The work at Yonsei University was supported by the National Research Foundation of Korea (NRF) funded by the Korea Government (MSIT) (No. RS-2023-00240624 to J.K.). The work at Soonchunhyang University was supported by the NRF and funded by the Korean Government (MSIT) (No. RS-2024-00343229 and 2021R1A6A1A03039503 to J.O.). The work in Austin was supported by the Robert A. Welch Foundation (F-0018 to J.L.S.) and by the National Science Foundation (CHE-2304731 to J.L.S.) subsequent to Nov. 1, 2023.
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X.-S.K. conceived the project. H.H. performed the compound syntheses and characterization, grew the single crystals, and measured the absorption spectra. N.L. conducted the electrochemical studies. J.L. and Y.N. carried out the theoretical calculations and fs-TA measurements. Z.Z. and V.M.L. performed X-ray crystallography. J.O., J.K., J.L.S., and X.-S.K. coordinated and supervised the project. All authors contributed to the manuscript.
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The authors declare the following competing interest(s): X.-S.K., H.H., and Z.Z. have an invention disclosure lodged with Beijing Normal University based on this work. The remaining authors declare no competing interests.
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He, H., Lee, J., Zong, Z. et al. Precisely metal doped nanographenes via a carbaporphyrin approach. Nat Commun 16, 1534 (2025). https://doi.org/10.1038/s41467-025-56828-4
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DOI: https://doi.org/10.1038/s41467-025-56828-4
