A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems

Abstract We designed a straightforward synthetic route towards a full‐fledged family of π‐extended helicenes: superhelicenes. They have two hexa‐peri‐hexabenzocoronenes (HBCs) in common that are connected via a central five‐membered ring. By means of structurally altering this 5‐membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In‐depth physico‐chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded‐off by crystal structure analyses. Mixed stacks of M‐ and P‐isomers led to twisted molecular wires. Using such stacks, charge‐carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.


Introduction
PA Hs have attracted tremendous interest in recent decades both in terms of potential novel materials, [1] as well as ab enchmark for carbon allotropes (e.g. graphene,c arbon nanotubes or fullerenes). [2] To this end, many different planar and non-planar PA Hs (e.g.b uckybowls,h elicenes or other curved structures) have been synthesized, studied, and applied. Among these,c onformationally stable p-extended [n]-helicenes (with n ! 5) have lately received increased attention. [3] Such helicenes commonly consist of large planar segments,that is,t he "p-extension" and one or more twisted parts,t hat is,t he "helicene".
[n]-helicenes feature unique characteristics that differ strongly both from planar PA Hs as well as from non p-extended helicenes.T heir properties are paired with their inherent chirality,afeature that many PA Hs lack.
Apart from potential applications,w hich are well-documented for PA Hs in general, [1] chirality opens up additional possibilities ranging from enantioselective catalysis [4] to applications based on circular polarized light absorption and emission such as advanced display technologies, [5] chiral molecular probes and sensors, [6] improved security inks, [6a,7] and chiroptical switches. [8] Spin-filtering properties of chiral compounds,k nown as the chirality induced spin selectivity effect (CISS-effect), [9] render helicenes intriguing targets for spin dependent applications [10] like spintronics.F inally,h elicity is au seful tool to fine-tune the parameters for organic materials,asmixtures of enantiomers in different ratios lead to varying solid-state packings and, therefore,d ifferent properties. [11] Form any of these purposes,such as optical and electronic applications,the p-extension of helicenes is beneficial due to the strong visible light absorption and fluorescence,t unable semiconducting properties,and aggregation of large p-systems.
In the context of future developments,the synthesis of pextended helical compounds might also pave the way towards the design of novel helical carbon allotropes or so-called graphene spirals,for which outstanding optical and electronic properties have been predicted. [12]

Results and Discussion
Anovel structural motif,that is, p-extended helicenes was recently published by our group: [13] Oxa- [7]-superhelicene. Inspired by its unique structure together with its striking features,w eaimed at, on one hand, enlarging this family of superhelicenes and, on the other hand, gathering ad eeper understanding of p-extended helicenes.C onsequently,t he next step was to vary the nature of the heteroatoms in the central five-membered ring. To this end, we synthesized eight new superhelicenes either directly or indirectly via postfunctionalization ( Figure 1). Such av ersatility is rather uncommon and renders our superhelicenes to stand out from the vast majority of published p-extended helicenes. [3] In the original synthesis of oxa- [7]-superhelicene, [13] we started from adiphenyl ether precursor.Inthe final step of the synthesis,it was necessary not only to close the hexaarylbenzenes,but also the furan ring to obtain the extended helicene.T he ring closure in the last step towards oxa- [7]-superhelicene proved to be difficult. It only proceeded cleanly under certain Scholl conditions.T hus,wefocused in our current work on employing precursors that have already ac losed, central, fivemembered ring. Precursors 2,8-dibromofluorenone 1, [14] 2,8dibromofluorene 2, [14] 2,8-dibromodibenzofuran 3, [15] 2,8dibromodibenzothiophene 4 [16] and 2,8-dibromocarbazole 5 [17] were synthesized following adapted literature procedures starting from 9,10-phenanthrenquionene,d ibenzofuran, dibenzothiophene,and carbazole,respectively.The brominated precursors were then converted into the corresponding helicenes in an efficient three-step synthesis.F irst, ad ouble Sonogashira reaction with 4-tert-butylphenylacetylene was performed to provide the necessary triple bonds for the subsequent Diels-Alder reactions.T hese were conducted with 2.5 equiv.o f2,3,4,5-tetrakis[4-(tert-butyl)phenyl]cyclopenta-2,4-diene-1-one 24 in minimal amounts of toluene in pressure vials at high temperatures of 220 8 8C. Bis-hexaarylbenzenes 11-15 were reacted under classical Scholl conditions with FeCl 3 /MeNO 2 or DDQ/triflic acid in dichloromethane to afford superhelicenes 16-20.Despite the closure of 12 bonds in as ingle step,t he reaction proceeds cleanly.I ts hould be noted that 20 decomposes fast (within several hours) under ambient conditions.T his might be attributed to the strong + Me ffect of the NÀHm aking the HBC subunits very electron rich and reactive.Asimilar instability was observed by Müllen and co-workers for amino-HBC. [18] Fluorenone-based 16 and thiophene-based 19 were used for post-functionalizations.The ketone functionality of 16 was utilized for the conversion to 21 via aK noevenagel condensation with malononitrile or to 22 with tetrabromomethane and triphenylphosphine,which is known to be the first step in the Corey-Fuchs-Ramirez reaction. Notably, 22 slowly decomposes (within several weeks) under ambient conditions.E xcept for 20 and 22,a ll helicenes are long time stable under ambient conditions. 19 was oxidized to sulfonederivative 23 with meta-chloroperoxybenzoic acid in dichloromethane. [19] Detailed physico-chemical investigations corroborated the tunability of our superhelicenes.A ss uch, this is remarkable as only the five-membered, central ring was altered. In the following,d etailed information is given for 16, 17, 19, 21 and 23. 20 and 22 are excluded due to the low stability of 20 and due to the fact that 22 was only synthesized for later postfunctionalization. For 18,detailed information is given in our previous publication. [13] Thea bsorptions of 16, 17, 19, 21,a nd 23 resemble the absorption features of the recently published oxa-[7]-superhelicene 18. [13] In particular, PA H-centered b-and p-bands are discernable between 300 and 440 nm, while the a-bands are observed between 440 and 550 nm (Supporting Information, Table S1). Similar to our previous results,t he a-bands are intense due to symmetry lowering in the bent HBC structure. Thes ame holds for 17, 19,a nd 23.I nt oluene, 16 exhibits an additional absorption, which is red-shifted relative to the abands to 584 nm. Finally, 21 features a5 71 nm maximum, as houlder at 624 nm, and aw eak, red-shifted absorption at around 750 nm.
In general, the absorption spectra reveal in solvents of different polarities only aminor solvatochromism ( Figure 2). For 23 in PhCN,a na dditional low-intense absorption band appears red-shifted at 588 nm, which indicates an additional transition in the low energy regime.M ore drastic are, however, the changes for 16 and 21.F or example,i nP hCN rather than toluene,the absorptions of 16 red-shift by 15 nm. For 21,o verlaying absorptions render it, nevertheless,m uch harder to discern any underlying shifts.T herefore,multipeak analyses were applied to dissect the different features (Figure S1). In toluene, 21 exhibits absorptions at 571, 623, and 738 nm. When changing to PhCN,r ed-shifts of 16, 13, and 27 nm, respectively,evolved. Compared to the most intense aband, which 4.5 nm shifts,t he strong solvatochromic effects substantiates an underlying charge-transfer character. [20] Thef luorescence features support the conclusions from absorption spectroscopy ( Figure S2). 17, 19,a nd 23 show fluorescence,w hich is similar to that reported previously for 18.F or example, 17 exhibits intense fluorescence at 537 and 570 nm. In toluene,t he fluorescence quantum yield is 86 %. By switching to PhCN,the quantum yield is somewhat lower with 69 %. 19 and 23 both possess very similar fluorescence features.F or 23 in PhCN,t he fluorescence red-shifts moderately by 13 nm ( Figure S2). This points to aw eak CT character of 23.T heir quantum yields are moderate with 36 %a nd 61 %i nt oluene.A sa forementioned, the quantum yields decrease with increasing solvent polarity (Table 1). In stark contrast, the fluorescence of 16 is shifted and broadened. Here,the maximum is located at 631 nm and is subject to apolarity-dependent red-shift of 35 nm, when, for example, switching from toluene to PhCN.T he quantum yields are distinctly low with 6.6 %, 6.9 %, and 1.8 %i nt oluene,T HF, and PhCN,r espectively. 21 reveals fairly weak fluorescence, Theb road tunability of p-extended helicenes is ag reat asset. On one hand, strongly fluorescent helicenes are realized with quantum yields as high as 86 %in, for example, 17.S uch av alue is exceptionally high for helicenes and/or nanographenes.Onthe other hand, broad absorptions all the way up to 850 nm were recorded for 21.H ere,t he combination of al arge p-extended helicene and an electron acceptor evokes panchromic absorptions.P anchromaticity is uncommon for helicenes and, in turn, only scarcely investigated.
By means of time-correlated single photon counting (TCSPC), the radiative lifetimes were determined upon photo-excitation at, for example,4 80 nm (Table 2a nd Figure S39). Measurements with 19 reveal lifetimes in the range from 1.3 to 1.4 ns in toluene,THF,and PhCN.Compared to 23 and 17,w hich exhibit lifetimes of 2.8-3.2 ns and 3.2-3.8 ns, respectively,t he fluorescence of 19 is,h owever, rather shortlived. Thec harge transfer state of 16 radiates with substantially lower quantum yields,but longer lifetimes all the way up to 4.4 ns. [21] Next, the oxidations and reductions,w hich were determined using at hree-electrode electrochemical setup,a re considered. In dichloromethane,t he first and second oxidations evolved for all superhelicenes at around + 1.0 and + 1.2 V, respectively ( Table 3). Assignments of the reductions are far more complex. Except for 21,a ll superhelicenes undergo reductions at around À1.5 V. In 17 and 19,t his is, however, the first reduction, while in 16 and 23 the first reduction occurs at À1.1 V. In the case of 21,t he lowest reduction occurs at À0.7 V. Thes hift in reduction for 16, 21, and 23 leads to alowering of the band gap.T his matches the trend seen in the steady-state absorption measurements,i n which the absorption features are red-shifted. Theb ands at 583 nm for 16 and 572-743 nm for 21 are too far red-shifted for HBC typical a-bands and point to the presence of ac harge-transfer state stemming from ar edistribution of charge-density between the p-extended helicenes and the functional groups at the five-membered central ring. DPVs and CVs are depicted in Figures S9-S11.
Spectroelectrochemical measurements were carried out to oxidize 16,17,19,21,and 23 at apotential of 1.0 V. Handin-hand with the oxidation is ad ecrease of the a-, b-a nd pabsorptions and the concomitant formation of new characteristics between 400 and 500 nm and between 550 and 800 nm. Here,local maxima are seen at 560, 700, and 750 nm. For 17, the broad absorption with am aximum around 700 nm is missing. All differential absorption spectra are shown in Figure S38.
Tr ansient absorption experiments were carried out on the femtosecond (fs-TAS) and nanosecond (ns-TAS) timescales in toluene,THF,and PhCN.Upon fs-TAS photo-excitation at 550 nm, 19 exhibits in toluene ac haracteristic ground-state bleaching with minima at 492 and 545 nm, together with as timulated emission minimum at 600 nm. All of the aforementioned is rounded off by maxima at 435, 630, 800, and 958 nm. At longer time delays,t he minima at 492 and 545 nm and the transient absorption at 958 nm blue-shift. At the same time,t he stimulated emission minimum at 600 nm disappears and is replaced by amaximum around 550 nm. In THF and PhCN,the excited state characteristics and dynamics are very similar (Figures S20-S25). By employing global analysis,weidentified two excited state species in the fs-TAS. Thefirst state,for which the lifetime is 1.22 ns,isassigned to the singlet excited state,which deactivates to the ground state by fluorescence and to the triplet excited state by intersystem crossing. This is the second species and it deactivates in 11.4 mst othe ground state. [22] Immediately after 550 nm fs-TAS photo-excitation, 16 shows in toluene as trong ground-state bleaching at 475 nm, positive transients at 575, 659, 737, and 757 nm, and ab road shoulder between 800 and 900 nm. On the time scale of fs-TAS, neither complete transformation nor deactivation of the excited state species was noted. Therefore,w et urned to ns-

Research Articles
TAStomonitor the complete deactivation. Immediately after ns-TAS photo-excitation, excited state species are observed, which agree well with those in the fs-TAS experiments,b ut with apoorer fine structure.Within the first nanoseconds,the excited state features are replaced by maxima at 550 and 640 nm, which then decay to the ground state.W ee mployed global analysis to identify two different excited state species in fs-TAS with lifetimes of 1.04 ps and > 5ns( Figure S12). In ns-TAS,t wo different excited state species were also concluded ( Figure S15). Important is hereby that the first ns-TAS species is identical to the second fs-TAS species and lives for 5.11 ns.T he second, long-lived excited state species decays with 21.03 ms. We assign the 1.04 ps-lived species to ah igher unrelaxed singlet excited state.I nl ine with the results from steady-state and time-resolved fluorescence measurements, the 5.11 ns-lived species,w hose ground-state bleaching slightly blue-shifts with time,i sa scribed to ac harge-transfer state.I td eactivates in parallel to either the ground-state as am ajor pathway or to the triplet excited state as am inor pathway.From the latter,the ground-state is recovered within 21.03 ms. In THF,t he same trend is observed. Negligible are the changes associated with the three different states (Figures S13 and S16). Quite different is,h owever, the picture in PhCN.Here,the first lifetime increases to 18.49 ps,while the second lifetime is as short as 2.42 ns.T he third species deactivates in 53.31 ms( Figures S14 and S17). 21 contrasts the excited-state deactivation eluded for 19 and 16.I mmediately after fs-TAS photo-excitation, as trong ground-state bleaching develops at 500 nm together with transients at 430, 450, 720, 757, and 870 nm. It is within afew picoseconds that the transients decay to the ground state.No notable changes and no appreciable population of any triplet excited state are,h owever,s een ( Figure S18). By means of global analysis,amono-exponential decay was found, by which the ground state is recovered. In toluene,the lifetime is 53 ps.I nT HF ( Figure S19) and PhCN (Figure 3a nd 4), the decays are even faster with 38 and 32 ps,r espectively.T he lifetimes found for 21 are assigned to charge recombination from acharge-separated state rather than deactivation of the charge-transfer state.S upport for our assignment came from the fact that the transient maxima at 430, 450, and 720 nm match the fingerprints of the differential absorption spectra for 21 upon applying oxidative conditions ( Figure S38). [23] Accordingly,w ep ostulate that the initially formed chargetransfer state transforms in less than 200 fs into ac hargeseparated state.
For 16,w ep resent the first high quality crystal structure for one of our superhelicenes ( Figure 5). Thes ize of the distorted p-extended helicene is without the carbonyl group approximately 20.8 10.3 .T his underlines the fact that 16 is ar eal helical nanographene.I n16,t he sum of the 5 torsion angles of the [7]helicene core is 96.88 8 with interplanar/ dihedral angles of 32.88 8.Overall, the central, helical part is far more twisted compared to its parent structure,t hat is, fluorenone [7]helicene.I nt he latter the sum of the 5t orsion angles is 82.88 8 and the dihedral angle is 26.88 8. [24] We postulate that this distortion is due to the bulky tert-butyl groups,which point inwards and, in turn, push the HBC-plates apart.
Thec rystal packing is dominated by mixed stacks of Mand P-isomers forming twisted molecular wires.T hey consist of two different dimers based on p-p interactions between the HBC plates.T he p-p distances are around 3.27 and 3.34 . Thes urrounding tert-butyl groups prevent direct p-p interactions between the single wires but keep them together presumably by weak dispersion interactions.
To gain insight into the capability for charge transport of these wire-like columns of 16,wecalculated electron and hole mobilities within the crystal structure,a ssuming ah opping regime of charge transport as described by semi-classical Marcus Theory. [25] 16 gives rise to ah igh hole mobility,b ut virtually no electron mobility in all directions.T he negligibly small electron mobility of 16 with low maximum electron transfer integrals (J max = 16 meV) could be related to electron being localized on the molecule,w hich is suggested by our visualization of the LUMO ( Figure S59). If the electron  wavefunction is strongly localized in ap art of al arge molecule,t hen wavefunction overlap between neighboring molecules can be reduced. Thepromising hole mobility of 16 is ac ombination of high hole transfer integrals between neighboring pairs of the molecules within the crystal (J max = 95 meV) ( Figure S42) and ar elatively low inner reorganization energy l intra (hole) = 100 meV.T aking the outer reorganization energy l outer = 0.30 eV,t his leads to an average hole mobility of 1.56 cm 2 V À1 s À1 and am aximum hole mobility of 3.75 cm 2 V À1 s À1 (Figure 6) along the bc plane.For comparison, from equivalent mobility calculations with higher outer reorganization energies (l outer = 0.5 eV), hole mobilities for TIPS-pentacene (0.33 cm 2 V À1 s À1 )a nd TES-pentacene (0.28 cm 2 V À1 s À1 )a re lower than for 16. [26] In TIPS-Pn the lower hole mobility can be attributed to the lower hole transfer integral (J max = 65 meV) and the increased inner and outer l. In TES-Pn, J max is equal to J max in 16;the lower hole mobility is largely related to ahigher l (l inner = 142 meV). The maximum hole mobility of 3.75 cm 2 V À1 s À1 in 16 exceeds all hole mobilities for helicenes previously computed (i)a za- [6]helicene regioisomers (l outer = 0.30 eV) 0.02-0.26 cm 2 V À1 s À1 ; [27] (ii)c arbo [6]helicene (l outer = 0.14 eV) 0.00-2.00 cm 2 V À1 s À1 ; [11b,c] (iii)( rac)-aza [6]helicene 0.06 cm 2 V À1 s À1 and (+ +)-aza [6]helicene 0.03 cm 2 V À1 s À1 . [11a] Those helicenes generally show smaller hole transfer integrals and larger l intra (hole) as would be expected for smaller  molecules.T herefore,w eh ave reason to believe that the crystal structure of 16 could be the basis for good holetransporting materials.
It is worth highlighting that the high hole mobility is relatively isotropic,m eaning that the average mobility is the same in all 3D directions and, thus,the material is not limited to a2Dplane of good transport. Thecalculated poor electron mobility (m max = 0.031 cm 2 V À1 s À1 )o f16 is ar esult of ah igh inner reorganization energy l intra (electron) = 213 meV and alow J max = 16 meV.
Thec harge carrier mobilities for the previously reported 18 [13] were calculated as well but are unreliable due to ahighly disordered crystal structure.F or further details see the Supporting Information, Section 5.
Enantiomeric separation via analytical HPLC (Figures S47-S48) was possible and 16, 17, 18,a sw ell as 19 were separated successfully.Circular dichroism (CD) spectra of the separated isomers were measured. By comparison with the spectra obtained from time-dependent density-functional (TDDFT) [28] calculations,t he M-and the P-isomers could be assigned (Figures S50-S57;T able S10).
From the CD spectra, the g-factors for the isomers were determined (Figure 7). Forclarity reasons only the g-factor of the P-enantiomers are displayed. Themirror image spectra of the respective M-enantiomers are shown in the supporting information ( Figure S58). Them aximum g-factors (Table 4) are around 10 À3 ,which is relatively common for small organic molecules.I nterestingly,t he spectra of 18 and 19 are quite similar with the peaks of 19 slightly red shifted. However,g max of 19 is approximately twice as high as g max of 18.Asthe only difference is that in 19 sulfur is present compared to oxygen in 18 (both chalcogenides) it could be argued that the presence of the heavier atoms increases the g-factor.

Conclusion
We have presented afull-fledged family of superhelicenes, whose structures and, in turn, physico-chemical characteristics are freely tunable.P lease note that this has never been shown in such av ersatile way for p-extended helicenes. Particularly noteworthy are the very high fluorescence quantum yields of up to 80 %i n17, 18, 19,a nd 23. Remarkable is also the fact that in 16 and 21 the absorption covers nearly the entire visible region. All of the aforementioned was independently supported by excited-state deactivation pathways that differed as af unction of the functionalization at the 5-membered ring. We did document that the superhelicenes are tolerant to post-functionalization reactions to gather an even larger library of superhelicenes.A detailed solid-state crystallographic analysis was presented for 16.B ased on this analysis,w eu sed calculations to demonstrate the unique potential of 16 as ahole transporting material. In light of the latter, further research, which, for example,will include thin-film and/or single-crystal measurements needs to be carried out to verify the results experimentally.W esucceeded in separating the enantiomers of 16, 17, 18,a nd 19 via analytical HPLC over ac hiral stationary phase and the features were assigned to the respective enantiomers.After separation, first insights into their chiroptical properties were derived from the CD spectra with respectable g Abs -values of around 10 À3 .E ven though many more aspects of superhelicenes,onwhich we did not touch in the current work, but are subject to future studies,w ed id show that these superhelicenes are av ersatile,e asily tunable platform to obtain agreat variety of p-extended helicenes.We like to mention here detailed chiroptical properties,m ore efficient enantiomeric separation, and spintronic applications. We are confident that our work will help to unleash the full potential of chiral, helical nanographenes and bring them as tep closer to applications via their on-demand tunability.