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A Concise Synthetic Strategy for Accessing Ambient Stable Bisphenalenyls Towards Achieving Electroactive Open-Shell #-Conjugated Materials Caleb M. Wehrmann, Ryan T. Charlton, and Mark S. Chen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13300 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Journal of the American Chemical Society

A Concise Synthetic Strategy for Accessing Ambient Stable Bisphenalenyls Towards Achieving Electroactive Open-Shell π-Conjugated Materials Caleb M. Wehrmann, Ryan T. Charlton, and Mark S. Chen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015-3102, United States

ABSTRACT: Open-shell, π-conjugated molecules represent exciting next-generation materials due to their unique optoelectronic and magnetic properties and their potential to exploit unpaired spin densities to engineer exceptionally close π-π interactions. However, prior syntheses of ambient stable, open-shell molecules required lengthy routes and displayed intermolecular spin-spin coupling with limited dimensionality. Here we report a general fragment-coupling strategy with phenalenone that enables the rapid construction of both biradicaloid (Ph2-sIDPL, 1) and radical [10(OTf)] bisphenalenyls in ≤ 7 steps from commercial starting materials. Significantly, we have discovered an electronically stabilized π-radical cation [10(OTf)] that shows multiple intermolecular closer-than-vdW contacts (< 3.4 Å) in its X-ray crystal structure. DFT simulations reveal that each of these close π-π interactions allows for intermolecular spin-spin coupling to occur, and suggests that 10(OTf) achieves electrostatically enhanced intermolecular covalent-bonding interactions in two dimensions. Single crystal devices were fabricated from 10(OTf) and demonstrate average electrical conductivities of 1.31 × 10-2 S/cm. Overall, these studies highlight the practical synthesis and device application of a new π-conjugated material, based on a design principle that promises to facilitate spin and charge transport.

INTRODUCTION Research in π-conjugated molecules continues to grow as new materials emerge with properties well-suited for application to electronic and spintronic technologies.1 Amongst these newly characterized compounds those with open-shell character, where spin densities are at least partially unpaired, are especially intriguing candidates. The presence of unpaired spin densities not only leads to unique optoelectronic and magnetic properties,2 but also introduces a powerful strategy for enhancing spin/charge transport efficiency through engineering exceptionally close π-π interactions.3 Although some materials have been shown to possess unpaired spin densities within their π-system,4 there are still few rationally designed ground state open-shell materials due to challenges related to their synthesis and characterization.5 As unpaired spin densities are prone to dimerize via σ-bond formation, kinetic suppression of side reactivity is often achieved by installing sterically bulky substituents.6 However, steric crowding hinders intermolecular π-π interactions – phenomena that impart some of the most interesting optoelectronic properties. Alternatively, open-shell character can be thermodynamically stabilized via spin/charge delocalization, which is often achieved with planar, highly conjugated systems.7 Several compound classes that achieve persistent open-shell character by this design include: indenofluorenes,8 zethrenes,9 anthenes,10 quinodimethanes,11,9e and bisphenalenyls.12 Bisphenalenyls are distinctive, even amongst open-shell compounds, for their exceptional ability to achieve small intermolecular π-stacking distances (≤ 3.16 Å). This close packing arises from the

C3-symmetry of phenalenyl moieties that are very effective at delocalizing spin/charge and therefore enable multicenter “pancake bonding.”13 By harnessing the ability of phenalenyls to promote intermolecular covalent-bonding interactions, unusual physical properties have been observed by simply linking two moieties in different orientations: spiro through boron,14 or coplanar through a π-conjugated subunit.15 One particular π-conjugated bisphenalenyl, Ph2-s-IDPL (1), has been well-studied for its unique optoelectronic and solid-state properties and was first synthesized by Kubo et. al. (Figure 1a).16 Its structure, comprising two phenalenyls fused to an s-indacene core, enables 1 to access two resonance structures in the ground state – one represented by a quinoidal Kekule structure (1), and the other by an aromatic biradical structure (1’). Prior synthesis of 1 began with a [4+2] cycloaddition of 2 and 3 (neither of which is commercially available), followed by oxidation, to provide heptacycle 4. The two outermost rings of the target biradicaloid were then installed by a 7-step sequence from 4 to afford 1. We hypothesized that a shorter, alternate route might be realized if phenalenyls could be installed as pre-constructed tricyclic fragments from commercial phenalenone (5) (Figure 1b). Specifically, Ph2-s-IDPL (1) might be prepared by dual cyclizations of bisphenalenone 7 that itself could be generated from a 2:1 coupling reaction of 5 and 6 via conjugate arylations. Such a strategy builds complexity quickly and offers generality for accessing new structural analogs like 10(OTf). Herein we report the short and practical synthesis of two bisphenalenyls [1, 10(OTf)] in ≤7 steps from commercial starting materials, through fragment coupling with 5. While 1 is a neutral biradicaloid that is identical in characterization to previous reports, 10(OTf) is a new π-radical cation that displays

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T-dependent paramagnetism in solution and optoelectronic properties that suggest the presence of intermolecular covalent-bonding interactions. These interactions are observable in X-ray crystal structures and DFT simulations further support that the solid-state π-assembly of 10(OTf) is a rare example of an open-shell, πconjugated molecule that achieves electrostatically enhanced intermolecular covalent-bonding interactions in two dimensions. Average electrical conductivities of 1.31 × 10-2 S/cm were measured from devices fabricated from single crystals of 10(OTf). Altogether, these studies suggest that delocalization may occur through an entire two-dimensional (2D) array of 10(OTf) rather than only a single π-stack (1D) or π-dimer (0D), which is highly desirable in developing new materials for spin and charge transport applications.

capable of coupling with 5 (2 equiv.), that when treated with DDQ furnished bisphenalenone 7.18 In order to construct the s-indacene core, we simply needed to promote two C-C bond formations between the central phenyl ring and C1/C1’ of each phenalenone moiety. Reaction with various oxidative (i.e. FeCl3, TiCl4) and reductive (i.e. CrCl3, SmI2) reagents was attempted with 7 but none provided the desired five-membered rings. We postulated that reactivity at the desired C1/C1’ sites was diminished by conjugation with the α,β-unsaturations, therefore saturated ketones (12) were obtained by CuH-catalyzed conjugate reductions of 7.19 Yet even 12 was resistant to common acid-catalyzed cyclization methods, likely due to its conformational rigidity, so we surmised that a less classical approach might be needed.20 In particular, Rh(II)carbenoid chemistry appeared to be an attractive method for promoting cyclizations through C-H functionalization of the central ring.21 In three steps from 12, we effected dual aromatic C-H insertions with catalytic Rh2(OAc)4 to furnish 13 in 69% yield (based on 12). Instability of the hydrazone and diazo intermediates complicated their isolation, so both species were used with minimal purification. Lastly, oxidation with DDQ restored full π-conjugation to provide Ph2-s-IDPL (1) as a dark green solid. Scheme 1. Syntheses of bisphenalenyls 1 and 10(OTf) by fragment coupling with phenalenone (5).

Figure 1. Comparison of strategies for synthesizing bisphenalenyls. (a) Prior synthesis of 1 required [4+2] cycloaddition of 2 and 3, followed by 7 steps from 4 to complete the outer rings of the phenalenyls. (b) This work employs a retrosynthesis that dissects 1 into bisphenalenone species 7, which is constructed by coupling phenalenone (5) and terphenyl 6. This strategy also provides access to an electronically stabilized, bisphenalenyl π-radical cation [10(OTf)].

RESULTS AND DISCUSSION Synthesis of neutral biradicaloid Ph2-s-IDPL (1). Our synthesis began with selective arylation of 11 via Suzuki-Miyaura cross-coupling to provide terphenyl 6 (Scheme 1). Although the coupling of organometallic species with phenalenone (5) had been shown before, addition of two equivalents of 5 was unprecedented.17 Additions of 5 to monometalated species generated sequentially only led to complex mixtures, so we deduced that dimetalation was essential. Amongst dimetalation procedures, reaction with sBuLi/TMEDA was uniquely effective for generating a species

Reagents and conditions: (a) Pd2dba3/P(otol)3, PhB(OH)2, K2CO3, 96%; (b) sBuLi, TMEDA, then 5, then DDQ, 68%; (c) CuCl/BINAP, NaOtBu, PMHS, iPrOH, 98%; (d) N2H4; (e) Pb(OAc)4; (f) Rh2(OAc)4, 69% (3 steps); (g) DDQ, 65%; (h) PdPPh4, PhB(OH)2, K2CO3, 78%; (i) NBS, NH4NO3, 86%; (j) sBuLi, TMEDA, then 5, then DDQ, 65%; (k) TfOH, TCE, 90%; (l) Na2S2O4, 93%.

Synthesis of π-radical cation (Ph2-PCPL)(OTf) [10(OTf)]. Encouraged by our success with 1, we applied our strategy to constructing an analog that replaced s-indacene with a pyrano[2,3g]chromene core [10(OTf), Scheme 1]. O-Substitution of the πsystem enables electronic stabilization of spin/charge, and introduces polarizable functionality to facilitate self-assembly.22 We attempted to generate the pyranochromene core by either heating or


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Journal of the American Chemical Society UV irradiating 7 in the presence of oxidants (i.e. DDQ, PhI(OAc)2) to promote two sequential oxidative 6π-oxoelectrocyclizations; however, monopyrylium intermediates proved resistant to a second cyclization.23 Therefore, in order to circumvent unfavorable oxidations, we pursued a route where paraoxygenated functionalities were pre-installed (14). Like 1, our synthesis began with Suzuki-Miyaura cross-coupling of 14 with PhB(OH)2 to provide terphenyl 15. Dibromide 8 was obtained with NBS via methoxy-directed bromination of the central phenyl ring. Dilithiation of 8 with sBuLi/TMEDA, followed by addition of 5, and then DDQ dehydrogenation, furnished 9. Treatment with trifluoromethanesulfonic acid (TfOH) easily promoted methyl ether deprotection, but subsequent dual cyclization to generate dipyrylium 16 required stirring in 1,1,2,2-tetrachloroethane (TCE) at 120°C. Given that 16 is dicationic, we hypothesized that reduction to a neutral species might be quite facile. However, even in the presence of strong reducing reagents (i.e. Na, K, Zn) we were unable to isolate neutral 10. From all reductions tried, even with mild Na2S2O4, the only isolable and characterizable product was a dark blue solid, (Ph2-PCPL)(OTf) [10(OTf)], that is ambient stable for multiple (ca. 7) days. 1H NMR spectroscopy reveals that 10(OTf) exhibits subtle T-dependent paramagnetism, with an effective magnetic moment (μeff) of 0.753 μB (0.252 e- per molecule) at 298 K and μeff = 0.591 μB (0.161 e- per molecule) at 243 K (Figure S1). The inverse relationship between paramagnetism and T suggests that lower T promotes spin-spin coupling that can result from intermolecular interactions. EPR characterization of solid 10(OTf) provided further insight into this phenomenon by showing an isotropic signal (ge = 2.004) at 11K (Figure S2). Therefore, even at low T that may promote higher order spin-spin coupling via aggregation, 10(OTf) remains paramagnetic.

Meanwhile cyclic voltammetry of a 0.1 mM solution of 10(OTf) in TCE reveals two quasi-reversible oxidations (E1/2ox = -0.60 V, -0.24 V) and one irreversible reduction (E1/2red = -1.08 V) (Figure 2b). Oxidations that are quasi-reversible versus irreversible when comparing 10(OTf) to 1, highlight how O-substitution stabilizes higher oxidation states. The first oxidation (-0.60 V) likely indicates the formation of a relatively stable dication: in agreement with a reversible transition and our ability to isolate 16. The second oxidation at higher potential is more curious though, since further oxidation of a dication is unlikely to be reversible. However, if bisphenalenyl units can achieve electronic coupling intermolecularly, then the redox state of one can influence another, like in mixedvalence systems.26 Voltammograms obtained from 10(OTf) as a thin film further suggest that its electronic properties are affected by intermolecular interactions. While two quasi-reversible oxidations (E1/2ox = -0.44 V, -0.05 V) are still observed, the reduction (E1/2red = -1.46 V) is now quasi-reversible and occurs at a higher potential. Since greater potentials are required to both oxidize and reduce 10(OTf), these data suggest that intermolecular interactions in the solid-state help to stabilize the radical cation.

Optical and electrochemical characterization of bisphenalenyls. We confirmed a low energy absorption band centered at 750 nm in solutions of 1 that matches previous reports (Figure S3).12b In thin films this band redshifts into the nearinfrared (NIR) region (λmax = 1515 nm) due to assembly into a onedimensional (1D) π-stack. In this arrangement, adjacent phenalenyl moieties achieve multicenter “pancake bonds” by maximizing overlap of carbons bearing unpaired spin density.24 Since two electrons are shared intermolecularly, uniquely close πspacings arise through intermolecular covalent bonding interactions. These intermolecular interactions combine with intramolecular covalency to create long-range π-conjugation in the solid-state.25 Absorption spectra of 10(OTf) show similar behavior, where bands redshift from solution to solid-state (Figures 2a and S4). In solution there are several significant spectral features: one in the visible (λmax = 616 nm), and two in the NIR (λmax = 1445 and 1859 nm). Given their low energy, the NIR absorptions likely result from SOMO-related transitions or intermolecular electronic interactions. Noticeably all these bands redshift and broaden in films, even the lowest energy band shifts to λmax = 2008 nm. Peak broadening may arise from disordered phases in the film, but redshifting strongly suggests that 10(OTf) achieves some type of long-range πconjugation via intermolecular covalent-bonding interactions. Electrochemical characterization also alludes to the presence of strong intermolecular interactions in bisphenalenyls 1 and 10(OTf). Cyclic voltammetry of 1 as a thin film shows one irreversible oxidation (Epaox = -0.16 V, versus Fc/Fc+ couple) and one quasi-reversible reduction (E1/2red = -1.23 V), which correspond to a HOMO-LUMO gap of approximately 1.07 eV (Figure S6).16

Figure 2. Optical and electronic characterization of (Ph2PCPL)(OTf) [10(OTf)]. (a) UV-vis-NIR absorption spectra in TCE solution (blue) and drop cast film on glass (green). (b) Cyclic voltammograms of 10(OTf) in TCE solution (0.1 mM, blue) and thin film on the electrode (green). (c) Spectroelectrochemical difference spec-



tra of 10(OTf) at varying potentials to identify changes in absorption upon oxidation (-0.14 V) and reduction (-1.20 V, versus Fc/Fc+ couple).

Having characterized the redox behavior of 10(OTf), we postulated that spectroelectrochemical analysis might allow us to generate and observe neutral 10 by bulk electrolysis. We first confirmed the validity of this approach by exposing a solution of 10(OTf) in CH3CN to an oxidative potential (-0.14 V versus Fc/Fc+ couple) that would generate dication 16. A strong blueshifted absorption (λmax = 530 nm) was recorded that matched the spectrum obtained from purified 16 (Figures 2c and S5). We then subjected a fresh solution of 10(OTf) to a reductive potential (-1.20 V) and observed a redshifted absorption (λmax = 727 nm) that we tentatively assigned to neutral 10. Despite the use of degassed solvent in an unsealed cuvette under N2 atmosphere, this new peak shifted back to a peak centered at ca. 619 nm within minutes. A minor absorption peak at 740 nm is also observed when 16 is reduced by Na2S2O4 in the presence of TfOH; however, its intensity rapidly diminishes within 2 minutes when exposed to ambient atmosphere (Figure S7). These data provide further evidence that neutral 10 is unstable and rapidly oxidizes to 10+ under ambient conditions, which would make it non-ideal for materials applications.

C8C6’, while the opposing phenalenyl edges are spaced as far as 4.662(3) Å (Figure S11). The second close π-stacking mode is observed in a head-to-head arrangement that achieves the shortest intermolecular C-C distances (3.065-3.282 Å) in the π-assembly, and resembles “pancake bonding” observed with other phenalenyl compounds (Figures 3c and S12). The third close π-stacking mode occurs within a tail-to-tail arrangement where C11’ on adjacent 10+ units are only spaced 3.297(2) Å apart (Figure 3d). The C11’C11’ and C8C6’ contacts are certainly not as close as the “pancake bonding” contacts; however the distances are similar to the inter-plane spacings (3.269 to 3.347 Å) observed with rylene-based radical cations that also achieve spin-spin interactions.28 Additionally, on the opposing face of this C11’C11’ contact, another phenalenylcentered interaction is present; however, it appears to be governed by dispersion forces since all interatomic distances (3.453-3.871 Å) are greater than 3.4 Å (Figure S13).

Single crystal X-ray diffraction of bisphenalenyls. X-ray diffraction of a Ph2-s-IDPL (1) single crystal reproduces the planar fused ring system that shows minimal bond length alternation, which is characteristic of the molecular biradicaloid (Figures S8 and S9).16 In a single crystal of 10(OTf), X-ray diffraction reveals that one triflate counterion is associated with each cationic bisphenalenyl (10+, Figure S10). Noticeably, each 10+ unit is nonplanar and desymmetrized despite being fully π-conjugated and C2-symmetric. Like 1 and other bisphenalenyls, bond length alternation is minimal through the pyranochromene core: all of the bonds within the central six-membered ring range from 1.399(6) to 1.427 (10) Å (Figure 3a). Additionally, the bonds that connect the core to the phenalenyl moieties are longer than a typical olefin C=C (1.33-1.34 Å), measuring 1.454(7) and 1.446(7) Å. Altogether these bond lengths suggest that a quinoidal resonance form does not contribute significantly to the ground state electronic structure. The dissymmetry of 10+ is most apparent by the difference in bayregion torsion angles (9.26° versus 14.04°) and in bond lengths between C6O1 [1.361(6) Å] and C6’O1’ [1.345(6) Å], but both show intermediate bond order (~1.5) that suggest non-bonding O lone pairs delocalize into the π-system. This resonance-based donation of electron density likely stabilizes the cationic π-system in 10(OTf), but consequently causes neutral 10 to be highly susceptible to oxidation. Since analysis of a single 10+ unit did not explain why 10(OTf) displays molecular dissymmetry in the solid-state, we inferred that this phenomenon might be due to molecular packing. Significantly, there are three π-stacking modes that demonstrate intermolecular distances less than the sum of C-C van der Waals radii (3.4 Å), and therefore suggest the presence of intermolecular covalent-bonding interactions. The first close π-stacking mode occurs within a headto-tail arrangement that is centered on the pyranochromene core (Figure 3b). Head-to-tail arrangement of π-conjugated radical ions is often observed since it optimizes electrostatic and dipole-dipole interactions to facilitate multicenter covalent π-bonding.27 Neighboring π-systems are not completely coplanar in 10(OTf) though, as the closest intermolecular distance [3.302(2) Å] occurs between

Figure 3. Single crystal X-ray structures of (Ph2-PCPL)(OTf) [10(OTf)]. (a) ORTEP depiction of 10+ that includes bond lengths and bay-region torsion angles. (b) Head-to-tail π-stacking with a closer-than-vdW contact between C8 and C6’. (c) Head-to-head π-


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Journal of the American Chemical Society stacking that shows closer-than-vdW contacts between overlapping phenalenyl carbons. (d) Tail-to-tail π-stacking that shows a closerthan-vdW contact between C11’ of adjacent phenalenyls. Interatomic distances are shown in angstroms (Å).

By achieving these close π-stacking modes, the solid-state assembly of 10(OTf) differs significantly from 1 in terms of dimensional order. Assembly of 1 displays close π-stacking interactions with a minimum intermolecular C-C distance of 3.129(3) Å (Figure 4a).16 Each molecule achieves intermolecular covalent-bonding interactions with two other molecules through a slipped stacking arrangement that is described as a one-dimensional (1D) wire. In contrast, the various π-stacking interactions in 10(OTf) allow for every 10+ unit to achieve closer-than-vdW contacts with three other 10+ ions (Figure 4b). Correspondingly, while 1 and other bisphenalenyls have demonstrated 1D intermolecular covalentbonding interactions, these data suggest that 10(OTf) achieves similar electronic interactions across a 2D network.

model each of the close π-stacking modes, as a preliminary study into the intermolecular effects on electronic structure. All three of the model dimers display a HOMO that is fully delocalized across both 10+ units, which agrees with intermolecular spin-spin coupling occurring in each interaction. In the head-to-tail dimer, the HOMO shows the largest intermolecular overlap between two carbons (C8 and C6’) that bear high SOMO density in the single 10+ unit and demonstrate a close intermolecular contact of 3.302(2) Å (Figure 5b). Similarly, the head-to-head dimer shows that the largest overlap in the HOMO occurs at carbons that coincide with high SOMO density and are involved in close intermolecular contacts (i.e. C4C10, C6C12, C8C14) (Figure 5c). Therefore this π-π interaction is analogous to previous cases of phenalenyl-based “pancake bonding”: where a 6-center, 2-electron bond is achieved through SOMO-SOMO overlap. The HOMO of the tail-to-tail dimer also indicates that intermolecular spin-spin coupling occurs by showing large orbital overlap between C10’ and C12’ of neighboring 10+ units (Figure 5d). While this overlap might be unexpected since the C10’C12’ contact measures 3.748(2) Å in our crystal structure, it simply results from the HOMO symmetry. Orbital overlap that corresponds with the close C11’C11’ contact [3.297(2) Å] is observed; however, it is displayed in a lower lying occupied MO where it is symmetry-allowed (Figure S18).

Figure 4. Solid-state π-assemblies of bisphenalenyls 1 and 10(OTf) determined by single crystal X-ray diffraction. (a) π-Stacking arrangement of 1 that displays closer-than-vdW contacts in one dimension. (b) π-Stacking arrangement of 10(OTf) that displays closer-than-vdW contacts in two dimensions. Interatomic distances are shown in angstroms (Å).

DFT simulation of the electronic structure of 10(OTf). Given that phenalenyl π-pimer (radical-cation) versus π-dimer (radical-radical) interactions are stronger due to electrostatics, one can postulate that π-stacking variation in 10(OTf) results from a dissymmetric electronic structure where spin and charge are localized to separate phenalenyl moieties.13g In order to investigate this possibility, we performed DFT calculations at the UB3LYP/6311G(d,p) level to simulate a single 10+ unit based on our single crystal X-ray data. Notably, we found that the SOMO and all other frontier molecular orbitals are symmetric and fully delocalized, which is consistent with the lack of hyperfine coupling in the EPR spectrum (Figures 5a and S14). These data challenge a model where each phenalenyl is a distinct radical or cation, and instead suggest that radical cation character is delocalized across both moieties and polarizable. Consequently, we can infer that π-stacking is not driven solely by intermolecular spin-spin coupling, but is also likely enhanced by electrostatic effects, similar to those present in π-pimer interactions. Furthermore, although 10+ is not intrinsically desymmetrized, electrostatic and steric biases introduced by solidstate packing can induce electronic polarization, which likely causes the variation in π-stacking observed in the single crystal. Due to the complexity of simulating the entire solid-state assembly, DFT calculations were performed on dimers of 10+, chosen to

Figure 5. DFT simulations of Ph2-PCPL+ (10+) and representative dimers that model the three close π-stacking modes, including interaction energies obtained by counterpoise calculations. (a) SOMO of 10+ that shows high delocalization across the π-system. (b) HOMO delocalized across the head-to-tail dimer that shows large overlap between C8 and C6’. (c) HOMO delocalized across the head-to-head dimer



that displays multicentered “pancake bonding.” (d) HOMO delocalized across the tail-to-tail dimer that shows symmetry-allowed intermolecular orbital overlap.

In order to evaluate the total interaction energies associated with each π-stacking mode, counterpoise calculations were performed on the three model dimers. Stabilization energies for all of the dimers are notably on the same order of magnitude despite their variation in packing distance and arrangement. The most stabilizing arrangement is the head-to-tail interaction (54.44 kcal/mol), followed by the head-to-head, “pancake bonding” interaction (38.22 kcal/mol), and then the tail-to-tail interaction (32.52 kcal/mol). Noticeably, these calculated values trend with decreasing parallel displacement between π-surfaces rather than decreasing interfacial distance, which suggests that other factors like electrostatics contribute significantly to the total dimer energies. Altogether these data suggest that the π-assembly of 10(OTf), where all close contacts are ≤ 3.3 Å, is governed by electrostatically enhanced intermolecular covalent-bonding interactions. Although a few molecular systems have shown similarly close packing motifs, 10(OTf) is a unique example of an open- rather than closed-shell molecule that demonstrates carbon-centered, closer-than-vdW contacts in two dimensions.29 Such solid-state order suggests that 10(OTf) or related compounds can potentially overcome the anisotropic limitations of most organic materials by achieving electronic delocalization with greater dimensionality. Just as π-π interactions with 2D versus 1D order marked a breakthrough in the evolution of acene-based materials, realizing 2D intermolecular covalent-bonding interactions is a significant achievement for crystal engineering π-conjugated materials.30 Solid-state electrical conductivity. Single crystal device studies reveal that 10(OTf) can function as an electrically conductive molecular material. Conductivity was measured through a two-contact probe device by mounting individual crystalline needles of 10(OTf) on a glass slide with conductive carbon paste fully covering each end of the crystals (Figure S19).31 By measuring across the long crystal axis at room temperature, we found the resistance to be 53.4 kΩ, which corresponds to a conductivity (σRT) of 1.31 × 10-2 S/cm based on the crystal dimensions (Figure S20). This conductivity is significant because even with minimal optimization, 10(OTf) demonstrates values within an order of magnitude of known organic conductors (≤ 0.3 S/cm)14 and even as-cast commercially available conducting polymers like PEDOT:PSS (< 1.0 S/cm).32 We anticipate that through further optimization of compounds like 10(OTf), which achieve intermolecular spin-spin coupling, materials can be developed that significantly enhance phenomena reliant on π-π interactions like spin and charge transport.33

CONCLUSION Given the need for molecular materials that exhibit rationallyengineered π-π interactions in organic electronic and spintronic technologies, there is exciting potential for the application of openshell compounds. Towards acquiring such materials, we have discovered a novel and concise synthetic strategy that provides access to both neutral (1) and ionic [10(OTf)] bisphenalenyls. Newly characterized π-radical cation 10(OTf) displays intermolecular covalent-bonding interactions, an already uncommon phenomenon, with 2D order in the solid-state, and achieves average electrical conductivities of 1.31 × 10-2 S/cm in single crystal devices. Overall, this practical strategy for accessing ambient stable, open-

shell molecules, elucidating their unique solid-state order, and demonstrating conductivity in a device represent major advances towards the development of a new generation of π-conjugated materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures, characterization data, details of experimental procedures, supplemental figures (PDF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for 10(OTf) (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. S. C.)

ORCID Mark S. Chen: 0000-0001-5415-4660 Caleb M. Wehrmann: 0000-0002-1030-541X

Notes C. M. W., R. T. C., and M. S. C. are inventors on a U.S. provisional patent filing by Lehigh University covering the herein-described synthetic process and example compounds.

ACKNOWLEDGMENTS The authors thank M. Neidig at the University of Rochester for obtaining EPR characterization, L. Fredin for assisting with computational analysis, D. Paliwoda and W. Breyer for assistance in structural refinement of the X-ray diffraction data, and R. A. Flowers for discussion of the manuscript. Portions of this research were conducted with research computing resources provided by Lehigh University. In addition, this work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (ACI-1548562). Specifically, it used allocation TG-CHE180067 on the Bridges system, which is supported by NSF (ACI-1445606), at the Pittsburgh Supercomputing Center (PSC). Financial support for this research comes from Lehigh University and the Lehigh University Class of ’68 Research Fellowship, in addition to the Charles E. Kaufman New Investigator Award (KA2015-79201) of The Pittsburgh Foundation.

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