Doubling the Length of the Longest Pyrene-Pyrazinoquinoxaline Molecular Nanoribbons.

Molecular nanoribbons are a class of atomically-precise nanomaterials for a broad range of applications. An iterative approach that allows doubling the length of the longest pyrene-pyrazinoquinoxaline molecular nanoribbons is described. The largest nanoribbon obtained through this approach-with a 60 linearly-fused ring backbone (14.9 nm) and a 324-atoms core (C276 N48 )-shows an extremely high molar absorptivity (values up to 1 198 074 M-1  cm-1 ) that also endows it with a high molar fluorescence brightness (8700 M-1  cm-1 ).

Graphene nanoribbons (NRs)—nanographenes that extend in one‐dimension—have shown much promise in a broad range of electronic, photonic and spintronic applications. The electronic, optical and magnetic properties of NRs depend on a series of structural variables such as edge structure, width, length and heteroatom doping. For this reason, the synthesis of NRs with atomic precision is key to establish their fundamental properties, validate theoretical predictions, and also to target the specific properties required by different applications.

Organic synthesis has recently emerged among the different top‐down[ , ] and bottom‐up[ , , , , , , , , , , , , ] approaches to prepare NRs. This is because, it can provide control over all the structural variables that dominate the properties of NRs. The synthesis of different families of increasingly longer molecular NRs[ , , , , ] has revealed how some of their properties can be influenced by the length. In particular, increasing molar absorptivities have been observed with increasing NR lengths. For instance, the longest molecular NR with a 53 linearly‐fused rings backbone of 12.9 nm has shown record molar absorptivities as high as 986 100 M−1cm−1. Such high molar absorptivities are highly desirable for photovoltaic and photodetector applications, but also endow fluorescent nanomaterials with an enhanced brightness for biomedical and energy applications. Yet, despite the remarkable advances, the synthesis of longer molecular NR remains challenging, as their synthetic routes require an increasing number of synthetic and purification steps that need to be optimized individually.

In 2018, we reported the first iterative approach for the synthesis of molecular NRs, which was aimed at simplifying their synthesis. This approach consists on the consecutive assembly of building block A into a NR by repeating the same set of reactions that allows incorporating 10‐ring‐long segments per iteration (Figure 1a). Through this approach, we were able to synthesise NR‐30 —with a 30 linearly‐fused rings backbone of 7.7 nm—after three iterations. NR‐30 remains to this date the largest pyrene‐pyrazinoquinoxaline NR. Even if, it should be possible to obtain longer NRs by increasing the number of iterations, we found it difficult to run more than three iterations, because of the challenging purification of this type of NRs that led to reduced isolated yields after each iteration.

Figure 1

Iterative synthetic methods of pyrene‐pyrazinoquinoxaline molecular NRs.

Herein, we describe an iterative methodology that allows doubling the length of the longest pyrene‐pyrazinoquinoxaline molecular NRs. This methodology relies on the introduction of a new building block (AA) with an aromatic backbone that doubles the length of its predecessor (Figure 1b). Consequently, by running iteration rounds with building block AA, 20‐ring‐long segments are incorporated into the NR per iteration. As such, a series of pyrene‐pyrazinoquinoxaline molecular NRs (NR‐20, NR‐40 and NR‐60) with a 20‐, 40‐ and 60‐ring backbone has been achieved after one, two, and three iteration rounds, respectively (Scheme 1). The length of NR‐40 (10.1 nm) already surpasses the current length record for pyrene‐pyrazinoquinoxaline NRs. Whereas, the length of NR‐60 (14.9 nm) does not only double the current length record for pyrene‐pyrazinoquinoxaline NRs, but also sets a new record as the longest molecular NR. Remarkably, NR‐40 and NR‐60 also show a high solubility in common organic solvents at room temperature, which allows establishing their structure and fundamental properties by a broad range of structural, optoelectronic and redox characterization techniques. This new family of NRs show extremely high molar absorptivity values that increase with the length reaching, in the case of NR‐60, up to 1 198 074 M−1 cm−1 that surpass those observed in the largest molecular NRs and in single‐chromophore nanographenes. Furthermore, the high molar absorptivity endows NR‐60 with a high molar brightness of 8700 M−1 cm−1.

Scheme 1

Iterative synthesis of NR‐20, NR‐40 and NR‐60. The lettering indicates the proton assignments on the 1H NMR spectra and in the Supporting Information. The molecular formula indicates only the atoms in the aromatic core (highlighted in grey).

Building block AA comprises a tetrabenzobenzohexaazanonacene core equipped with o‐diamines at one end, and diacetal‐protected o‐dione functionalities at the other end (Scheme 1). o‐Diamines and o‐diones have been selected as they converge through the formation of a pyrazine ring by an imine‐type cyclocondensation reaction, which provide an efficient mean to interconnect building blocks. The o‐dione functionalities have been protected to avoid the self‐condensation of building block AA, and thus, enable the controlled condensation of one building block after the other. tert‐Butyl and tri‐iso‐butylsilyl solubilizing groups were used because of their relatively small size and large solubilising power. The synthesis of building block AA was achieved following the route set out in Scheme 2. Compound A was obtained in 12 steps using a procedure previously described by some of us, whereas compound C was obtained in 6 steps (details in the Supporting Information). Condensation between the free amines of compound A and the free diones of compound C yielded thiadiazolotetrabenzohexaazanonacene D (96 %), which was subsequently reduced in the presence of LiAlH4 into AA (quantitative).

Scheme 2

Synthetic route for building block AA. Complete details are given in the Supporting Information.

The cyclocondensation of 2.5 equivalents of AA with 2,7‐di‐tert‐butylpyrene‐4,5,9,10‐tetraone B gave NR‐20 in a 67 % yield after chromatographic purification (Scheme 1). Then, the terminal diones of NR‐20 were deprotected in the presence of TFA/H2O and the crude was subsequently cyclocondensed with 2.5 equivalents of AA to give NR‐40 in a 22 % yield over the two steps after purification by preparative gel permeation chromatography. The same deprotection/condensation process was repeated on NR‐40 by deprotecting the terminal diones in TFA/H2O that were subsequently cyclocondensed in the presence of 5 equivalents of AA to provide NR‐60 in a 26 % yield over the two steps after purification by preparative gel permeation chromatography. NR‐20, NR‐40 and NR‐60 are soluble in common chlorinated solvents, such as CHCl3, CH2Cl2 and o‐dichlorobenzene (ODCB).

The structure of the NRs was confirmed by 1H and 13C NMR at room temperature in CDCl3 (Figure 2 and Supporting Information). The structural (and also optoelectronic and redox) data of NR‐20 prepared from AA is identical as that of NR‐20 prepared from A. The NMR signals in the aromatic and aliphatic region are consistent with the structure and most importantly with the molecular weights of the different NRs. For instance, the integration in the 1H NMR spectra can be used to establish unambiguously the length of the NR, as signal of the inner pyrene protons a (the assignments correspond to the lettering shown in Scheme 1) integrates 12, 28 and 44 protons, respectively for NR‐20, NR‐40 and NR‐60, whereas the signals of the protons b, c, and d at the terminal pyrene show a constant integration of 4, 4 and 16 in all NRs independently of the length of the NR (Figure 2). Matrix‐assisted laser desorption/ionization time of flight mass spectrometry (MALDI‐TOF MS) show the expected molecular ion peaks for NR‐20 and NR‐40. In the case of NR‐60, while the structure has been confirmed unambiguously by NMR, we have not been able to detect the molecular ion peak. Similar observations have been reported for another type of high molecular weight pyrene NRs.

Figure 2

1H NMR spectra of NR‐20, NR‐40 and NR‐60 in CDCl3 at room temperature. Stars designate solvent residual peaks.

When dissolved, NR‐20, NR‐40 and NR‐60 give rise to fuchsia solutions similar to the colour of the solids (inset Figure 3a). The electronic absorption features of the NR series in chloroform are consistent with their structure and with previous observations on other members of this pyrene‐pyrazinoquinoxaline NR family (Table S1), which further support the structural assignments. The electronic absorption spectra show a small band at 605 nm, a set of bands with clear vibronic features in‐between 400–600 nm and another set in the UV region, which were assigned to the α, ρ, and β bands from longer to shorter wavelengths, respectively (Figure 3a and Table S2). The wavelength of the α band (with maxima at 604 nm in all cases) is independent of the length of the NR, while the ρ (with maxima at 545, 550 and 553 nm, respectively for NR‐20, NR‐40 and NR‐60) and the β bands (with maxima at 383, 385 and 387 nm, respectively for NR‐20, NR‐40 and NR‐60) show a clear bathochromic shift with increasing lengths. Remarkably, the molar attenuation coefficients of NR‐20 (ϵ =425 358 L mol−1 cm−1), NR‐40 (ϵ =789 164 L mol−1 cm−1), and NR‐60 (ϵ =1 198 074 L mol−1 cm−1) increase with increasing lengths as an effect of the more extended π‐system. The maximum molar attenuation coefficient value for NR‐60 surpasses the largest values reported for giant nanographene chromophores (986 100 M−1 cm−1 for C296N24S2; 844 000 M−1 cm−1 for C186 ). The experimental HOMO–LUMO gap (E gap) values were estimated from the onset of the longest wavelength transition. The E gap values of NR‐20 (1.96 eV), NR‐40 (1.96 eV) and NR‐60 (1.96 eV) are the same, independently of the length of the NR. This is consistent with previous observations that show how the E gap values of pyrene‐quinoxaline NRs decrease rapidly for NRs with a backbone from 6 to 10 linearly‐fused rings (E gap 6‐rings=2.80 eV, E gap 8‐rings=2.40 eV, and E gap 10‐rings=1.97 eV ) and then for backbones >10 rings the E gap values saturate and remains almost invariable. This trend is consistent with that observed on other types of narrow NRs.[ , ]

Figure 3

a) Absorption and b) fluorescence (λ ex=415 nm) spectra in CHCl3 (insets show a solution of NR‐60 under natural and UV light, respectively) and c) cyclic voltammograms in n‐Bu4NPF6/ODCB (20 mV s−1) of NR‐20, NR‐40, and NR‐60. d) Bond length analysis and NICS(0) values of NR‐60‐H. Bonds are rendered in a color continuum ranging from red (1.31 A) to white (1.40 A) to blue (1.49 A) so that Clar's aromatic sextets are lighter/whiter colors and localized double and single bonds are red and blue, respectively. e) ACID plots of NR‐60‐H. f) Representative B3LYP‐6‐31G(d,p) frontier orbitals for NR‐60.

In contrast to most soluble giant nanographenes, which generally show weak or no fluorescence, solutions of NR‐20, NR‐40 and NR‐60 show a bright magenta fluorescence upon exposure to UV light (inset Figure 3b). The fluorescence spectra of NR‐20, NR‐40 and NR‐60 in chloroform are almost superimposable and show a vibronically‐resolved fluorescence band (with maxima at 623, 623 and 625 nm, respectively) that span from 600 to 800 nm (Figure 3b and Table S2). This is in agreement with the florescence spectra reported for pyrene‐pyrazinoquinoxaline NRs with >10‐ring backbones that also show the same fluorescence band as it emerges from the α absorption band that remains at the same energy independently of the length (Table S1). The fluorescence quantum yields (Φ) of NR‐20, NR‐40 and NR‐60 show values of 0.18, 0.16 and 0.11, respectively, which are remarkable considering the size of their aromatic core. These quantum yield values are comparable to those of current state‐of‐the‐art carbon quantum dots in the same spectral region.[ , ] Considering their high molar absorption, the molar fluorescence brightness (ϵ Φ) and mass fluorescence brightness values ((ϵ Φ)/molecular weight) of NR‐20 (5534 M−1 cm−1 and 1.3 g−1 L cm−1, respectively), NR‐40 (9950 M−1 cm−1 and 1.3 g−1 L cm−1, respectively) and NR‐60 (8700 M−1 cm−1 and 0.8 g−1 L cm−1, respectively) surpass those reported for carbon quantum dots (≈4000 M−1 cm−1 and ≈0.2 g−1 L cm−1, respectively).[ , ]

The redox properties of NR‐20, NR‐40 and NR‐60 were studied by cyclic voltammetry. All the three NRs exhibited three waves on the cathodic side that overlap in some cases (Figure 3c and Table S2) and that are consistent with the structure and with previous reports on shorter pyrene‐pyrazinoquinoxaline NRs (Table S1). The first reduction half‐wave potentials (E 1/2) of the NRs were very similar (E 1/2 NR‐20=−1.14 V, E 1/2 NR‐40=−1.16 V, E 1/2 NR‐60=−1.05 V vs Fc/Fc+) and confirm that all the NRs are electron‐deficient materials. The electrochemical LUMO or electron affinities (E LUMO) were estimated from the onset potential of the first reduction wave of the cyclic voltammograms. The E LUMO values (−3.62, −3.72 and −3.74 eV, respectively for NR‐20, NR‐40 and NR‐60) are very similar and tend to decrease with the increasing the size of the NR's backbone.

We have not been able to obtain crystals suitable for X‐ray diffraction, so we relied on calculations (B3LYP‐6‐31G(d,p)) to get an insight into the structure of the NRs. Calculations were carried out on NR‐20‐H, NR‐40‐H and NR‐60‐H, in which the tert‐butyl and tri‐iso‐butylsilyl solubilizing groups have been substituted by hydrogen atoms for computational efficiency. The models of NR‐20‐H, NR‐40‐H and NR‐60‐H illustrate a virtually flat backbone with lengths of 5.3, 10.1 and 14.9 nm, respectively. The dominant resonance structure in all the NRs is best represented by Clar structures (as shown in Scheme 1) with a biphenyl group (2 sextets) in the pyrene residues, and an anthracene group (1 sextet) in the pyrazinoquinoxaline residues. This is consistent with bond‐length alternation analysis (Figures 3d and S1) that show nearly aromatic distances on the off‐linear pyrene rings and on the pyrazinoquinoxaline residues, whereas distances that approximate to single bonds are observed on the pyrene rings on the linear backbone. This is also consistent with the negative NICS(0) values (Figures 3d and S1) found on almost all the off‐linear rings of the pyrene and of the pyrazinoquinoxaline residues (shown in red), whereas positive values (shown in blue) were observed on the pyrene rings on the linear backbone. The anisotropy of the induced current density (ACID) plots of NR‐20, NR‐40 and NR‐60 (Figures 3e and S2) are also consistent with this assignment and shows a diamagnetic current that goes around the NR periphery (red trace) and that has been described for other types of NRs.[ , ]

To shine additional light on the optoelectronic and redox properties, we carried out DFT calculations (B3LYP‐6‐311+g(2d,p)‐Chloroform/B3LYP‐6‐31G(d,p)) that reveal a set of quasidegenerate HOMO orbitals and a set of quasidegenerate LUMO orbitals in all NRs (Table S3). The near degeneracy increases with increasing lengths (4, 8 and 12 quasidegenerate HOMO orbitals and 4, 8 and 12 quasidegenerate LUMO orbitals, respectively for NR‐20‐H, NR‐40‐H and NR‐60‐H). The computed E gap values (2.26, 2.23, and 2.22 eV, respectively for NR‐20‐H, NR‐40‐H and NR‐60‐H) and E LUMO values (−3.32, −3.36, −3.37 eV, respectively for NR‐20‐H, NR‐40‐H and NR‐60‐H) correlate very well with the experimental ones and also show the same trend. The quasidegenerate HOMOs and LUMOs show electron densities mostly localised in specific segments of the different NR (Figures S3–S5), which in some cases can spread over as many as 28 consecutive rings along the linear backbone (representative HOMO, LUMO, LUMO+1 and LUMO+8 of NR‐60 are shown in Figure 3f). TD‐DFT reveals that the α band of the electronic absorption spectra corresponds in all cases to the transitions between the HOMO‐1/HOMO and all the quasidegenerate LUMO orbitals (Table S4). Whereas the ρ band shows a major contribution from the lowest quasidegenerate HOMO (HOMO‐4, HOMO‐8 and HOMO‐12, respectively for NR‐20‐H, NR‐40‐H and NR‐60‐H) and the LUMO (Table S4). The energies of the computed absorption bands (Figure S6 and Table S4) show the same trends observed experimentally, where the energy of the α band is independent of the length and the energy ρ band decrease with increasing lengths.

Overall, we have reported a new iterative synthetic approach that allow preparing molecular NRs with lengths that double the current length record of the pyrene‐pyrazinoquinoxaline family and also sets a new length record as the longest molecular NR. The synthesis relies on the iterative coupling of a double sized building block that allows to double the length of the resulting NRs after each iterative round. The NRs are soluble in common organic solvents at room temperature, which made possible their purification and also their structural, optoelectronic and redox characterization in solution. Optoelectronic characterization evidence the remarkable light absorbing capabilities of the NRs that increase with the length of the backbone. In the case of NR‐60, the molar absorptivities (up to 1 198 074 L mol−1 cm−1) surpass the largest values reported for other giant nanographene chromophores[ , ] and endows it with a high fluorescence brightness (8700 M−1 cm−1). Overall, this work provides a new approach for the synthesis of giant molecular NRs with remarkable optoelectronic properties that derive form their large dimensions. This work also paves the way for the synthesis of other types of giant NRs and nanographenes that may find applications in sensing, energy and biomedicine.

Conflict of interest

The authors declare no conflict of interest.

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