Synthesis of nanographenes by DPEX

In the case of DBATT and its derivatives, pyrene was brominated with bromine in chloroform, yielding 1,6-dibromopyrene 1 on a decagram scale, which was purified by a single recrystallization step from xylenes (see Supplementary Methods)41,42. Two-fold Suzuki-Miyaura coupling of 1 with 2-formylphenylboronic acid 2 gave the diarylated pyrene precursors, 1,6-bis(2-formylphenyl)pyrene 3, which was obtained in pure form and good yields after silica gel plug filtration and precipitation with hexanes from dichloromethane. The preparation of BPc-precursor 4, was achieved analogously by the coupling with 3-formylnaphthalene-2-boronic acid pinacol ester 5, which was obtained in three steps from 1,2,4,5-tetrabromobenzene (see Supplementary Methods). As proof of principle, regioselective 3,8-functionalization of 3 could be easily achieved by bromination to 6 which can be either directly condensed to dibromo DBATT (bBr-DBATT) via DPEX (see Supplementary Methods) or converted to the respective aryl derivative by Suzuki-Miyaura coupling with e.g., phenylboronic acid to 3a, yielding respective diaryl DBATTs (see also compound 16 and pyridinyl substituted compound S5). This route avoids the earlier reported formation of non-selective 1,2 and 1,4-Michael addition products30,43. For small scale brominations, the 1:2 hexamethylentetramine-bromine complex (HMTAB) was found to be particularly useful, however not mandatory for the selective bromination44. Starting from 1,3,6,8-tetrabromopyrene 7, four-fold-substituted pyrenes 8 and 9 were obtained in equal manner. An overview on the synthetic scheme is shown in Fig. 2. Precursors 3, 3a, 4, 8, and 9 were subjected to the elaborated DPEX conditions (vide infra, see also Supplementary Table 1), giving the target NGs in close to quantitative yield, without any sign of side-product formation, as determined by HPLC and MS. Supplementary Table 2 shows all synthesized precursors and Supplementary Table 3 shows all nanographenes accessed by DPEX (DBATT, bPh-DBATT, bBr-DBATT, TTc, BPc, TPc, 11, 13, 17, and S7). Even though DPEX takes place already in neat H 2 SO 4 in moderate yields (see Supplementary Table 1), the best possible conditions were first refined by the conversion of 3 to DBATT on an analytical scale. The reaction outcome was followed by quantitative HPLC analysis.

Fig. 2 Synthesis of zig-zag NGs. a CHCl 3 , Br 2 ; b 2:1 toluene/MeOH, 2.5% Pd(PPh 3 ) 4 , K 2 CO 3 , 80 °C, N 2 (3: 61%, 4: 65%); c DPEX: CH 2 Cl 2 , 2 vol% sat. SnCl 2 •2H 2 O/i-PrOH, 1 vol% conc. H 2 SO 4 , rt, (quant.); d nitrobenzene, Br 2 ; e 2:1 toluene/MeOH, 4% Pd(PPh 3 ) 4 , K 2 CO 3 , 80 °C, N 2 (8: 81 %, 9: 91 %); f CH 2 Cl 2 , 1.6 equiv. HMTAB, rt (quant.); g 2:1 toluene/MeOH, 2.5 % Pd(PPh 3 ) 4 , K 2 CO 3 , 80 °C, N 2 (3a: 66 % with phenylboronic acid); HMTAB: 1:2 hexamethylentetramine-bromine complex Full size image

3 and the other nanographene precursors show relatively good solubility in CH 2 Cl 2 and THF compared to other tested solvents like e.g., hexanes, toluene, and ethyl acetate, which subsequently determined the tested reaction media for the DPEX reaction. However, only CH 2 Cl 2 as solvent provides the best reaction outcome. As reducing agent, we focused on common SnCl 2 •2H 2 O. The addition of 2 vol% of a saturated solution of SnCl 2 •2H 2 O, dissolved in iso-propanol is superior over all other investigated reduction systems. Interestingly, using methanol instead, which dissolves SnCl 2 •2H 2 O by far better than iso-propanol, the conversion appears to be faster, however the reaction is accompanied by the formation of unidentified side products. Although the side products are formed in trace amounts, as indicated by HPLC analysis, further post synthetic purification appears to be difficult due to the low solubility. On the other hand, the addition of iso-propanol significantly slows down the conversion but remarkably improves the selectivity and the reaction outcome; used as mere solvent however, the reaction is inhibited completely. The addition of 1 vol% conc. H 2 SO 4 initiates the reaction, which is indicated (in case of 3 and 3a) by a rapid purple coloration of the mixture, accompanied by the formation of a white precipitate. Supplementary Fig. 1 shows a pictured illustration of the single reaction steps. Hereby, the empiric ratio of 2:1 of the SnCl 2 /i-PrOH–solution and the H 2 SO 4 plays a crucial role for a successful outcome. After work-up with aqueous hydrochloric acid and extraction, the product is precipitated with MeOH, which gives pure DBATT in close to quantitative yields. The usage of stronger acids like e.g., trifluoromethanesulfonic acid (TfOH), or 34% oleum shows good performances, however significantly worse than conc. H 2 SO 4 . On the other hand, weaker acids like e.g., trifluoroacetic acid (HTFA) and acetic acid, show no conversion at all. As mentioned above, reactions carried out in neat H 2 SO 4 and thus lacking a reducing agent were found to give DBATT in moderate yields. Since the required two-electron reduction process cannot stem from H 2 SO 4 , we surmise a disproportionation reaction between two intermediate molecules, leading to DBATT and oxidized derivatives. This assumption is further supported by the fact that the yields of DBATT decreased upon lowering the concentration of 3 and never exceeded yields of 50%. Further details can be extracted from Supplementary Table 1.

With respect to the scope of the DPEX protocol, the reactivity is demonstrated using the less reactive naphthalene core as the central aromatic unit. The respective precursor molecules 10 and 12 are obtained from 1,4-dibromonaphthalene and 1,5-diiodonaphthalene by standard two-fold Suzuki-Miyaura coupling reactions (see Supplementary Methods). Despite the lower activity of the naphthalene core, the DPEX cyclization results the desired benzo[rst]pentaphene 11 and dibenzo[b,def]chrysene 13 in moderate yields (Fig. 3a). Interestingly, in both cases the cyclisation is only successful in the presence of SnCl 2 , indicating that SnCl 2 plays a crucial role in the DPEX process and participates already in the first reaction step. Thus, the mechanism of DPEX appears to be more complex than the intuitively assumed two-step domino reaction. This is also supported by the lack in formation of undesired pentagons under DPEX conditions, which otherwise would be expected for protonated forms of aldehydes. This transformation however, is completely suppressed as demonstrated by the attempt to synthesize indeno[1,2-b]fluorene 15 from p-terphenyl dicarbaldehyde 14 (Fig. 3b). Surprisingly, compound 14 remains completely intact under DPEX condition, pointing out that the aldehyde functionality tolerates the reaction conditions if being misplaced. In other words, the aldehyde group shows activity only if it is placed in the formal cove region of the PAH. In order to support further this claim, the para-formylphenyl DBATT precursor 16, bearing two aldehyde groups in active, and two in inactive positions was prepared, starting from dibromo precursor 6. As shown in Fig. 3c, 16 reacts under typical DPEX conditions selectively, yielding the desired DBATT derivative 17 in 92% isolated yield. This unprecedentedly high regioselectivity of DPEX provides essential flexibility in design and facile access to complex functional PAHs. Regarding other functionalities, DPEX shows to be tolerant towards keto-groups, which undergo no transformation (compare compound S6 in the Supplementary Methods); heterocycles like pyridine substituted precursors do not affect the outcome of DPEX and nearly quantitative conversions are obtained (see compound S7 in the Supplementary Methods).

Fig. 3 The scope of the DPEX protocol. a Reaction with less reactive naphthalene core units; b No formation of five-membered rings; c Tolerance towards misplaced formyl groups Full size image

Noteworthy, the solvents were neither degassed, pre-distilled or purified, and used as obtained from the suppliers, which underpins the applicability of this protocol. Moreover, the reactions were performed under ambient atmosphere at room temperature. The only pre-caution taken was the avoidance of direct light irradiation, since DBATT is known to undergo light-induced oxidative decomposition39. In order to proof that this reductive-condensation protocol is suitable for the preparative–scale production, we carried out the reaction on a 0.50 g scale of 3, which allowed us to isolate DBATT in 0.44 g as pure dark blue solid (isolated yield 96%). Supplementary Fig. 1 shows a detailed and pictured illustration of the single reaction steps. Accordingly, TTc was prepared on a 100 mg scale and BPc and TPc on a 20 mg scale.

Structure elucidation

Structure analysis of highly insoluble pristine NGs is evolving as major problem in modern nanographene sciences. Therefore, solubilizing groups are typically attached to the PAH’s skeleton, leading inevitably to an alteration of its original characteristics28,29,30,32. Spectroscopically at the border of solubility for NMR analysis, we elucidated the 1H NMR spectrum of DBATT, obtained at 100 °C in o-DCB (Fig. 4a). The peaks were assigned by the correlation with its computed NMR spectrum (RB3LYP 6–311 + G(d,p) GIAO). The absorption and emission spectra of DBATT are shown in Fig. 4b. The lowest energy absorption maximum is found at λ max = 586 nm; it is absorption onset of λ onset = 600 nm corresponds to the HOMO–LUMO transition of 2.07 eV and compares well to its computed HLG (vide infra). The small Stoke’s shift of 4 nm gives an emission maximum at 600 nm and is characteristic for the rigid carbon skeleton. For the irrefutable structure determination, we took advantage of the stability of our zzNGs towards thermal sublimation. In case of DBATT, we were able to grow crystals in shape of dark blue needles (see Supplementary Fig. 2) by sublimation at 310 °C at 10–5 mbar, suitable for single crystal X-ray diffraction (see Fig. 4c). Unlike the examples from substituted DBATT structures28,29,30,32, the aromatic skeleton of pristine DBATT remains flat. However, the lack of substituents becomes most apparent in view of the crystal packing. While e.g., threefold substituted triisopropylsilylethynyl-DBATT30, packs in a pseudo-sandwich herringbone motif with a π–π distance of 3.61 Å, pristine DBATT assembles in slipped co-facially aligned columnar stacks with an interlayer distance of 3.49 Å (Fig. 4d, e). This particular arrangement is especially favorable for energy efficient exciton splitting processes, known as singlet fission, as the eclipsed conformation maximizes the frontier molecular orbital overlap of the HOMO and the LUMO45,46.

Fig. 4 Spectroscopic analysis and structure elucidation of DBATT. a 1H NMR (o-DCB-D 4 , 100 °C, 400 MHz); proton assignments were correlated with computed NMR spectra at the DFT RB3LYP 6–311 + G(d, p) GIAO level of theory; inset shows HPLC chromatogram after reaction work-up; b absorption (black) and emission (red) in THF at rt; c single crystal X-ray structure depicted as ORTEP model with 50% thermal ellipsoids, independent C–C bond lengths are indicated; d View onto the (101) face of the crystal (lattice) structure; depicted as balls and sticks model; e columnar crystal packing motif of DBATT with an interlayer distance of 3.49 Å; depicted as space filling model Full size image

With 10, 12, and 16 annulated benzene rings respectively, the actual highlight-NGs – TTc, BPc and TPc – could not be brought into solution without decomposition (boiling 1,2,4-trichlorobenzene). In that respect, laser desorption ionization mass spectrometry, as shown in Fig. 5a–c, indicate the high selectivity and full completeness of the DPEX process. No starting material, intermediates and other side products can be detected by LDI-MS which gave the first evidence for the constitutional integrity of the NGs. In the case of BPc a small intensity signal corresponding to the oxygen adduct can be detected, indicating slow oxidation of the compound under ambient conditions (no special precautions were taken during the MS preparation and analysis).

Fig. 5 Structure elucidation of larger NGs – TTc, BPc and TPc. a–c LDI-MS, insets show the calculated and measured isotope pattern, respectively; d–f STM images of TTc on Au(111) at 77 K, BPc on Au(111) at 4.7 K and TPc on Ag(111) at 4.7 K. The perfect fit of the superimposed DFT models corroborates the unambiguous identification of the NGs. The TPc’s are surrounded by bright protrusions that are assigned to halogens, which are residues from the synthesis (see SI). Scale bars: 2 nm and 1 nm for the insets with the DFT model overlaid images, respectively. Tunneling conditions: 100 pA/1 V, 50 pA/-500 mV, and 100 pA/-500 mV Full size image

The structural proof was unambiguously obtained by low-temperature scanning tunneling microscopy (STM). The NGs were sublimed in ultra-high vacuum between 300 and 395 °C onto a Au(111) or Ag(111) surface, which was kept at room temperature. The STM results shown in Fig. 5d–f reveal that the size and shape of the NGs fit perfectly to the superimposed structural models, which were obtained by DFT. BPc and TPc adsorb as single molecules on Au(111) and Ag(111) respectively, while TTc forms a self-assembly on Au(111) (detailed analysis of the self-assembly is discussed in the Supplementary Discussion and depicted in Supplementary Fig. 14). This is the first report on the successful preparation and characterization of such extended zig-zag nanographenes.

Computations

In order to shed light into the electronic properties of the NGs, we carried out DFT calculations. We determined the HOMO–LUMO levels at the B3LYP-6–311 + G(d,p) level of theory; the theoretical diradical character y (y = 0 pure closed-shell; y = 1 pure open-shell) was determined according to the equation in the inset in Fig. 647, from the occupation number of the frontier molecular orbitals (σ HOMO , σ LUMO ), at the UHF 6–31 + G(d,p) level of theory48,49. The values were compared to the parent linear acenes (tetracene, pentacene, and hexacene) and are depicted in Fig. 6. With respect to the HOMO–LUMO gap the cross-shaped NGs TTc and TPc allocate values of 2.23 eV and 2.35 eV between tetracene and pentacene, respectively. At the first glance counterintuitive, TPc reveals a larger gap than TTc. However, this can be attributed to the stabilizing effect of Clar’s sextets in the outer benzene rings, which are more dominant in TPc and thus lower the HOMO level due to an increased aromatic stabilization energy. Interestingly, from broken symmetry calculations, no mixing of the highest occupied natural orbital (HONO) and lowest unoccupied natural orbital (LUNO), and therefore no diradical character (y = 0) for TTc and TPc can be observed. Even though the bandgap of tetracene is bigger than of the respective fourfold fused NGs, it shows already a diradical character of y = 0.27. Unlike the cross-shaped NGs, the bis-fused compounds DBATT and BPc show, with an increasing number of annulated benzene rings, a decreasing HOMO–LUMO gap (HLG). With a computed gap of 2.05 eV, which is in good agreement with its absorption spectrum, DBATT can be located between pentacene and hexacene; with y = 0.44 it shows a slightly higher diradical character than pentacene. However, in the solid-state DBATT shows to be kinetically much more stable than pentacene, which tends to undergo [2 + 2] cycloaddition reactions rapidly and requires typically stabilizing groups like e.g., triisopropylsilylacetylene. With a gap of only 1.62 eV, which corresponds to a theoretical transition at 765 nm, BPc shows the smallest HLG of the herein discussed NGs and compares well to notoriously unstable heptacene50. Even though a significant diradical character of y = 0.60 is computed for BPc, its persistence during the synthesis, thermal sublimation conditions, and storage in the solid state is remarkable.