Giant Star-Shaped Nitrogen-Doped Nanographenes.

Star-shaped nanographenes (SNGs) are large monodisperse polycyclic aromatic hydrocarbons that are larger than a nanometer and have shown a lot of promise in a wide range of applications including electronics, energy conversion, and sensing. Herein, we report a new family of giant star-shaped N-doped nanographenes with diameters up to 6.5 nm. Furthermore, the high solubility of this SNG family in neutral organic solvents at room temperature allowed a complete structural, optoelectronic, and electrochemical characterisation, which together with charge transport studies illustrate their n-type semiconducting character.

Nanographenes (NGs) are large monodisperse polycyclic aromatic hydrocarbons that extend in size beyond the nanometer and have shown a lot of promise in a wide range of applications including electronics, photonics, and energy. [1] Atomicallyprecise control over the NG structure is crucial to fully exploit their potential. For instance, by controlling the number of rings, their arrangement, heteroatom-doping and substitution, it is possible to fine-tune their energy levels and therefore, modulate electron affinities, ionization potentials, energy gaps, absorption and emission properties, among others.
Among these, planar threefold symmetric star-shaped nanographenes (SNGs), such as starphenes, cloverphenes and their extended derivatives (Figure 1), [2] have shown a prominent position as materials for charge transport, energy conversion and storage, light-emitting and sensing applications. [3] However, even if, there have been very impressive advances in recent years and large SNGs diameters reaching 3.0 nm (55 condensed rings) have been reported by solution synthesis, [2c-e, 2i-k] the largest soluble SNG that has been fully characterised presents a diameter of 2.8 nm (16 condensed rings). [2l] This is because of the lack of solubility of planar π systems that extend in two dimensions, which tend to aggregate strongly by π-stacking in solution ( Figure 1). In fact, the synthesis of extended SNGs is still a challenging task that requires dealing with insoluble intermediates and products, which overall makes synthesis, purification, characterisation and processing difficult, slowing down the exploration of their fundamental properties and the development of potential applications. Herein we report a new family of giant SNGs that show diameters exceeding those of the largest SNGs (3 nm) and that have been fully characterised thanks to their high solubility ( Figure 1). For instance, the first generation of this family (SNG-G 1 ) shows a diameter of 4.1 nm (34 condensed rings), and remarkably, the second generation (SNG-G 2 ) shows a diameter of 6.5 nm (55 condensed rings). As we show below, the synthesis of this SNG family is not trivial and requires the careful design of a key C 3symmetrical precursor (SNG-G 0 ), from which the aromatic core can be then extended radially. Furthermore, the high solubility of this SNG family in neutral organic solvents at room temperature allowed a complete structural, optoelectronic and electrochemical characterisation that together with charge transport studies illustrate their n-type semiconducting character.
On a first approach, we aimed at synthesising SNGs by cyclocondensation of precursor A [4] to the commercially available cyclohexane-1,2,3,4,5,6-hexaone (HKT) (Scheme 1a), which has been broadly used for the synthesis of hexaazatriphenylenes and hexaazatrinaphthylenes. [3a, 5] We selected precursor A, which consists on a dibenzodiazatetracene core with terminal protected ketones in the pyrene end and diamino groups at the quinoxaline end that enables an iterative reaction scheme, as it can be assembled with itself by a set of cyclocondensation/deprotection reactions. In addition, A possesses a combination of tert-butyl and tri-iso-butylsilyl (TIBS) groups that have proven to render large NGs soluble. However, when we carried out the cyclocondensation between building block A and HKT in the solvent mixtures typically used for this type of cyclocondensations, the reaction did not provide the expected cycloadduct B and instead, yielded an inseparable mixture of compounds. We exposed this mixture of compounds to Bunz oxidation conditions (MnO 2 ), which have proven to be useful to aromatise mildly incomplete dihydro intermediates of azaacenes and their derivatives, [6] and the mixture evolved to a major compound. Structural characterisation of such compound confirmed the formation of linear C 2 ribbon-like condensation adduct C with 13 fused aromatic rings (Scheme 1a). Therefore, we can safely attribute the formation of C to the early formation of a mixture of dihydro species of the dicondensation product that was fully aromatised afterwards by MnO 2 , in line with several cases that have been noted in recent literature. [7] The formation of the C 2 instead of the C 3 condensation was surprising since some of us reported the synthesis of the structurally equivalent hexa(TIBS)acetylene-substituted hexaazatrinaphthylene [8] and the reaction proceeded without problems to the desired product. Nevertheless, the formation of the C 2 condensation product can be rationalised in terms of both nucleophilicity and steric hinderance because of the lower nucleophilicity of the diamines on the quinoxaline residue of A in comparison to phenylenediamine derivatives, which are unable to overcome the steric hinderance of the C 3 condensation product. At this stage, we changed strategy (Scheme 1b) and designed a new C 3 symmetrical precursor in which the three sets of diones are far apart from each other and at the same time are close to tert-butyl solubilizing groups that should not interfere sterically with the TIBS groups present at A during the triple cyclocondensation step. We therefore condensed pyrene diketone D [9] that can be obtained in 4 steps from pyrene and 1,2,3,4,5,6-hexaaminobenzene (HAB) [5] that can be obtained in 3 steps from 1,3,5-trichlorobenzene. The cyclocondensation proceeded without problems and yielded the hexaone-protected precursor SNG-G 0 in a good yield (45%). The deprotection of the terminal o-diones in the presence of TFA and water yielded the hexaone-terminated precursor SNG-G 0 -Q (70%). The cyclocondensation reaction between A and SNG-G 0 -Q proceeded without any problems and yielded SNG-G 1 after chromatographic purification (32%). The deprotection of the terminal diones in water/TFA, followed by cyclocondensation with A yielded the desired SNG-G 2 (38%).
The whole SNG series were remarkably soluble in toluene, and chlorinated solvents at room temperature and we were able to establish unambiguously their structure by 1 H-NMR and 13 C-NMR spectroscopy and matrix-assisted laser desorption/ionization time of flight high-resolution mass spectrometry (MALDI-TOF HRMS). The 1 H-NMR and 13 C-NMR spectra showed remarkably sharp signals that allowed confirming the structure in all the SNGs, illustrating the high solubility of the whole series. For instance, the integration of 1 H-NMR signals are in agreement with the structures of the SNGs (Figure 2a), integrating 6 protons for the aromatic signals a and b in SNG-G 0 , SNG-G 1 and SNG-G 2 , 6 protons for the aromatic signals e and f in the case of SNG-G 1 and SNG-G 2 , 12 protons for the aromatic signal j in the case of SNG-G 2 (the assignments correspond to the lettering in Figure 1). While in SNG-G 0 , SNG-G 1 and SNG-G 2 , 12 protons for the terminal diketal signals c were observed in all cases, which is also consistent with the structure. [10] Further evidence of the successful preparation of the SNGs came from MALDI-TOF HRMS that show ion peak masses (M+Ag) + of 1553.5692, 4211.1790, and 6868.7884 Da, respectively for SNG-G 0, SNG-G 1 and SNG-G 2 , that matched with the expected mass. The isotopic distributions could be only recorded for SNG-G 0 and SNG-G 1 due to the high molecular weight of SNG-G 2 , which is at the detection limit of the technique that illustrates the extremely high molecular weight of these monodisperse systems.
Since we were not able to obtain single crystals suitable for X-ray diffraction, semiempirical quantum mechanics were used to investigate the structure of SNGs with the GFN-xTB method (Geometry, Frequency, Non-covalent, eXtended Tight-Binding) that allows computing efficiently high molecular weight systems with thousands of atoms. [4,11] The simulations show that the SNG series can adopt a plethora of slightly twisted conformations as the result of the bulkiness and the TIBS groups ( Figure S1), but that given the inherent flexibility of the iso-propyl substitutents the energies for interconversion between conformations is very small, which gives rise to nearly planar structures on average. The simulations show that the SNGs possess stable disk structures with diameters of 1.7, 4.1 and 6.5 nm respectively for SNG-G 0, SNG-G 1 and SNG-G 2 . on pyrene-fused systems. [4, 11b, 12] The spectra showed that while the α band remains at almost invariable energies, the β and the ρ bands are increasingly shifted towards lower energies as a result of the radial extension of the π-system. Remarkably, also the molar absorptivity (ε) increases together with the diameter of the SNG, as exemplified by comparing the ρ bands of SNG-G 1 (448,985 Lmol -1 cm -1 ) and SNG-G 2 (629,021 Lmol -1 cm -1 ). The experimental electronic absorption spectra are in agreement with the calculated ones, which do not only corroborate the electronic structure but also shine light on the nature of the electronic transitions ( Figure S2 and Table S1). In fact, time-dependent density functional theory (TD-DFT) and frontier orbital energies were computed with the 6-31g ( (Tables S2 and S3). The HOMO levels (E HOMO ) have been calculated from the difference between E LUMO and E g and were the following: -6.02, -5.96 and -5.97 eV for SNG-G 0 , SNG 1 and SNG-G 2 , respectively.
To assess the charge transporting properties of SNGs, we performed time-resolved microwave conductivity measurements (TRMC) [15] directly on the solids powders ( Figure 2e and Table S4). TRMC allows calculating the pseudo-photoconductivity values (φΣµ max ), which can be considered the intrinsic or minimum charge carrier mobility of the material, without the need of contacts. For instance the φΣµ max values correspond to the sum of the hole and electron mobilities (Σµ) times the quantum yield (φ). We obtained nearly invariable φΣµ max values in the order of 10 -4 cm 2 V -1 s -1 with average φΣµ max values of 1.05 x 10 -4 , 0.98 x 10 -4 and 1.11 x 10 -4 cm 2 V -1 s -1 for SNG-G 0 , SNG-G 1 and SNG-G 2 , respectively, which are similar to those obtained for ribbon-like NGs, [4] π-gels [16] and conjugated polymers. [17] Similarly, nearly invariable half lifetimes (τ 1/2 ) of 0.45, 0.65 and 0.50 µs for SNG-G 0 , SNG-G 1 and SNG-G 2 , respectively, were measured. The nearly invariable φΣµ and τ 1/2 values observed are consistent with observed localized states.
In this work, we have reported the synthesis of highly soluble SNGs with diameters up to 6.5 nm (55 condensed rings), which doubles those of the largest SNGs. Their synthesis has been achieved by a careful design and synthesis of a C 3 -symmetrical precursor from which the SNG core is then extended radially. Most importantly, this approach provides highly soluble SNGs, which has allowed their synthesis and purification by solution methods and also a full characterisation (  [5] D. Z. Rogers, J. Org. Chem. 1986, 51, 3904-3905. [10] 1 H-NMR does not show any evidence of the presence dihydro species during the cyclocondensation. For instance when SNG-G 2 was exposed to Bunz oxidation conditions no change of colour was observed, which confirms that the aromatic core is fully aromatised. [12] E. Clar, The Aromatic Sextet, Wiley, London, 1972.
[14] Estimated using oxazine in MeOH as a reference. [

Synthesis and characterisation
Commercial chemicals and solvents were used as received. Analytical thin layer chromatography (TLC) was carried out using aluminum sheets (20x20 cm) pre-coated with silica gel RP-18W 60 F254 from Merck. Column chromatography was carried out using Silica gel 60 (40-60 µm) from Scharlab. NMR spectra in solution were recorded on a Bruker Avance 400 MHz or 500 MHz spectrometer at 298 K using partially deuterated solvents as internal standards. and SNG-G 2 , that matched with the expected mass. The isotopic distributions could be only recorded for SNG-G 0 and SNG-G 1 due to the high molecular weight of SNG-G 2 , which is at the detection limit of the technique that illustrates the extremely high molecular weight of these monodisperse systems.

Steady-state electronic absorption
Absorption spectra were recorded on a Perkin-Elmer Lambda 950 spectrometer.

Photoluminescence
Photoluminescence spectra were recorded on a LS55 Perkin-Elmer Fluorescence spectrometer.

Electrochemistry
Electrochemical measurements were carried out on a Princeton Applied Research Parstat 2273 in a 3-electrode single compartment cell with glassy carbon disc working electrode, a platinum wire counter electrode and a silver wire pseudoreference electrode. All the potential values are reported versus the redox potential of the ferrocene/ferrocenium couple.

Time-resolved microwave conductivity (TRMC)
A film on a quartz substrate was set in a resonant cavity and probed by continuous microwaves at ~9.1 GHz. The third harmonic generation (THG; 355 nm) of an Nd:YAG laser (Continuum Inc., Surelite II, 5-8 ns pulse duration, 10 Hz) was used as an excitation source (incident photon density, I0 = 9.1 × 1015 photons cm -2 pulse -1 ). The photoconductivity transient Δσ was converted to the product of the quantum yield (φ) and the sum of the charge carrier mobilities, Σµ = (µ h + µ e ) by φΣµ = Δσ(eI 0 F light ) -1 , S11 where e and F light are the unit charge of a single electron and the correction (or filling) factor, respectively.