A Hexabenzocoronene‐Based Helical Nanographene

Abstract A synthetic route towards a novel hexabenzocoronene‐based helical nanographene motif was developed. A hexaphenylbenzene precursor was therefore designed, which cannot undergo, due to steric restrictions, a complete planarization reaction. This precursor was transformed under oxidative cyclodehydrogenation conditions to a π‐extended [5]helicene, which was fully characterized including X‐ray diffraction analysis.

nene (HBC) type chemistry.P recise control over the formation of the helical unit should be gained by the help of steric repulsive effects that prevent complete planarizationd uring the final oxidative cyclodehydrogenation reactions tep. This strategy and all synthetic detailsa re presented in the following paper.
First, as uitable tolane, which is capable of formingahelical HBC-based product in the upcoming reactions teps, had to be designed. For that, tetra-tert-butyl-tolane 4 was selected. Although as ynthesis of 4 is already literature known, [25] we employedadifferent approach,w hich easily allows to prepare gram quantities of tolane 4.W ith this procedure, no precious reagents or expensive purification techniques, for example, columnchromatography,w ere necessary.Atf irst, 3,5-di-tert-butylbenzaldehyde was reacted under McMurry conditions [26] to stilbene 2 (Scheme 1) using zinc powder as the reducing agent and TiCl 4 as the titanium species. After aqueous workup and recrystallization from CH 2 Cl 2 /MeOH, product 2 was obtained in suitable yields of 69 %. Stilbene 2 was subsequently brominated [27] using Br 2 ,w hich yielded the double brominated compound 3 after aqueous workup in almost quantitative yields (95 %). Next, ad ouble HBr elimination [27] under strongb asic conditions (KOtBu in THF) generated the desired tolane 4, whichw as isolated after aqueous workup and recrystallization from CH 2 Cl 2 /MeOHi n7 8% yield. With tolane 4 and tetracyclone 5 as tandard [4+ +2] Diels-Alder reaction was performed. The reaction proceeded smoothly at 260 8Ci nt he microwave reactor,w hich yieldedh exaphenylbenzene (HPB) 6 after recrystallization in 81 %y ield. Finally,a no xidative cyclodehydrogenation reaction was appliedt of orm an extended aromatic psystem.H owever,d ue to the steric demand of the tert-butyl groups attached to the phenylr ings drawn in red (Scheme 1), Scheme1.Synthesis of HBC-based [5]  it seemed impossible to form all six CÀCb onds as it is usually the case for HBCs and relatedm aterials. As aresult,the twisted nanographene 7,i nw hich five CÀCb onds were formed, was generatedu nder standard reactionc onditions (FeCl 3 ,C H 3 NO 2 / CH 2 Cl 2 )a nd isolated in 80 %y ield. The reactionp roceeded smoothly without any side product formation (followed by TLC).
Product 7 as well as all of the precursor compounds were carefullyc haracterized by NMR spectroscopic and mass spectrometrict echniques (for detailss ee Supporting Information). X-ray crystallography of 2, 4, 6 and 7 confirmed furthermore the successful synthesis of these compoundsa nd allowedt o get an insighti nto the solid-state behavior.G enerally,m olecules within this projects howedahigh tendency to form crystalline structures, as some of the singlec rystals, for example, the ones of 7,g rew within 24 h. The structures of 2, 4 and 6 are depicted in the Supporting Information (Figures S1-S4) and ad etailed structural analysis of the target compound 7 is shown in Figure 1. HBC-based [5]helicene 7 crystallizes in a monoclinic crystal system with the space group P2 1 /n. Thehelical character of 7,w hich is defined by the sum of torsion angles (81.58)a nd the interplanar angle (42.28), [28] was unambiguously confirmed by the crystal structure. The helicity values of 7 significantly differ to the ones of previously reported [5]helicenes,f or example, to ac arbon [5]helicene (sum of torsion angles:6 5.9-67.88;i nterplanar angle:4 7.3-51.38). [29][30] For more details see Ta bles S6 and S7 in the Supporting Information. This deviation can be explained by two major structural variations that affect the helicalp roperties of the HBC-based [5]helicene. On the one hand, the rigid aromatic backbone of 7 (Figure 1a,d rawn in grey) reduces the flexibility,e specially of the inner rings B, Ca nd D, and therefore limits the overall twisting capability of the helix. However,o nt he other hand, the bulky tert-butyl groups counteract, due to steric repulsion, to the effects of the stiff backbone and are leading to an increasedd istortion of the helicalu nit, particularly of the outer rings Aa nd E. The sum of these two effects lead to ah elical unit with an unevenly distributed helical character with areas of in-and decreased degree of distortion. This is also supported by the large bond length variations of 1.41-1.48 between the inner helical carbon atoms (Figure 1b). The crystal packing (Figure 1d,e) reveals that both enantiomers (M)a nd (P)a re present, therefore formingaracemic crystal.T he molecules are, compared to the crystal packing of, for example, hexa-tertbutyl-HBC [31] relatively loosely packed with distances of ! 5 betweent he aromatic planeso ft he molecules.H ence, no p-p interactions and only CH-p as well as London dispersion interactions were found, which seem to be the driving force for the preferential formation of all shown structures in the crystal. Interestingly,l ayers of as ingle chirality are found in the crystal of 7,g enerating an alternating pattern of (M)a nd (P)e nantiomer layers (compare Figure S5).
The differences in the spectroscopic properties between the HBC-based [5]helicene 7 and ap lanar reference compound, hexa-tert-butyl-HBC, [32] were studied by UV/Vis absorption and emission spectroscopy.T he UV/Vis absorption spectrumo f7 (Figure 2a)f eatures ar edshifted maximum at 367 nm (p-band) with as ignificantly decreased molare xtinction coefficient compared to the reference HBC. Furthermore,areduced resolution of the fine structure, due to ag eneral broadening of all absorptionb ands, was observed. This is also true for the fluorescence spectrum (Figure 2b), which shows, aside from broadened signals,h ypsochromically shifted emission maximaa t4 75 and 506 nm for 7.T herefore, the implementation of ah elical unit into an anographene has clear effects on the photophysical properties, such as on the positionb ut also on the shape of the respective absorption and emissionbands.
In conclusion, we have presented an ew procedure for the synthesis of aH BC-based twisted nanographene that contains a [ 5]helicalu nit. The helicity was introduced with the help of sterically demanding groups that preventacomplete closure to planar PAHs during the oxidative cyclodehydrogenation step. These results complement our previous findings,inw hich related nitrogen-doped HBC-like [5]helicenesw ere formed, however,c ontrolled by electronic effects. [33] The chiral separation and characterization as well as the usage of the twisted nanographene motif of 7 as ap latform for furtherf unctionalized hybrid materials, for example, analogousa rchitectures to Figure 2. a) UV/Vis absorption spectraofH BC-based [5]helicene 7 and reference compound hexa-tert-butyl-HBC [34] in CH 2 Cl 2 ;b)normalized steady state fluorescence emission spectra upon irradiation of the absorption maximum. our previously reported porphyrin-HBCs, [34][35][36][37][38][39][40][41][42] are currently under investigation.

Experimental Section
Crystallographic data:D eposition number 1990061 contains the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.