Curved π-conjugated corannulene dimer diradicaloids

Curved π-conjugated diradicaloids based on bridged corannulene dimers were synthesized, showing 3D spin delocalization and global aromaticity for their dianions.


Introduction
Recently there have been increasing research interests on openshell singlet diradicaloids due to their unique magnetic activity and potential applications in organic electronics, non-linear optics and spintronics. 1 So far, most reported open-shell singlet diradicaloids are based on planar p-conjugated molecules or their oligomers, and there are very rare examples based on non-planar systems. 2 Our particular interest here is to exploit stable diradicaloids based on curved p-conjugated molecules, which may exhibit unique three-dimensional (3D) spin distribution nature and intra-and intermolecular magnetic exchange interactions. Bowl-shaped corannulene is a very good candidate given its curved structure. It was reported that the radical anions of corannulene and its derivatives showed effective spin delocalization throughout the p-conjugated framework but they were still highly reactive and some of them tended to form intermolecular C-C s-bond linked dimer or polymer. 3 On the other hand, stable neutral radicals-substituted corannulenes such as Cor-Ver (verzazyl), Cor-IN (iminonitroxide), Cor-PhO (phenoxyl) and Cor-t BuNO (tert-butylnitroxide) (Fig. 1a) were prepared and in all cases, the spin can be partially delocalized to the corannulene bowl. 4 The phenalenyl-fused corannulene anion was synthesized and fully characterized, but the property of its neutral radical Cor-PLY (Fig. 1a) was not reported, presumably due to its poor stability. 5 The corannulene-bridged phenoxyl diradical Cor-2PhO showed signicant intramolecular magnetic interaction with a singlet-triplet energy gap (DE S-T ¼ 2Jk B À1 ) of À810 K (À1.61 kcal mol À1 ). 6 Calculations reveal that the unpaired electronic spin in the triplet state is delocalized onto the corannulene skeleton from the phenoxyl moieties. However, so far, corannulene-based radicals with more than one corannulene unit have never been reported. Herein, we report the rst two stable p-conjugated corannulene dimer diradicaloids, Cor-D1 and Cor-D2, in which the two corannulene units are annulated with a para-quinodimethane (p-QDM) and 2,6-naphthoquinodimethane (2,6-NQDM) moiety, respectively (Fig. 1b). The central pro-aromatic quinodimethanes have irresistible wish to become diradicals by recovering aromaticty, 1d and one can expect a signicant contribution of the open-shell diradical form to the groundstate electronic structure. Therefore, they are ideal open-shell singlet diradicaloids with a unique 3D curved p-conjugated structure and the spin is supposed to be largely delocalized onto the two corannulene bowls (Fig. 1b).

Ground-state geometry
Single crystals of Cor-D1 and Cor-D2 suitable for X-ray crystallographic analysis were grown by slow solvent evaporation from chlorobenzene solution. 11 Both molecules adopt a doublecurved geometry, with the two corannulene bowls bent to opposite directions (Fig. 2). The bowl depth (d) of Cor-D1 is measured to be 0.89Å, which is slightly larger than that of parent corannulene (  Å ) between neighboring corannulene bowls are found in the crystallographic structure of Cor-D2, and there are additional [CH/p] interactions between the methyl groups and the corannulene bowls (distance: 2.736Å), both drive the molecules into a densely packed structure (Fig. 2b). The central p-QDM unit in Cor-D1 has a large bond length alternation (BLA) (Fig. 2a), indicating that the quinoidal structure is more important. However, the exo-methylene bond, C1-C2 (1.388(13)Å), is signicantly longer than that of typical olens (1.33-1.34Å), implying certain contribution of diradical form. The central 2,6-NQDM unit in Cor-D2 also has signicant BLA, with the exo-methylene C1-C2 bond being elongated (1.389 (8) A) (Fig. 2b). Natural orbital occupation number (NOON) calculations at UCAM-B3LYP/6-31G(d,p) level based on the X-ray structures suggest that Cor-D1 and Cor-D2 have a diradical character (y 0 ) of 5.4% and 16.9%, respectively. Interestingly, a dihydro-product (Cor-D2-2H), the anti-isomer of 7b, was found during the crystal growth of Cor-D2 by slow evaporation of chlorobenzene at 60 C. The structure was conrmed by X-ray crystallographic analysis ( Fig. S60 in ESI †), and the formation can be explained by the hydrogen abstraction from solvents at the most reactive sites (C1). Variable-temperature (VT) 1 H NMR spectra of Cor-D1 in CDCl 3 (245-335 K) revealed splitting of the protons for both the central benzenoid ring, the OMe group, and the ortho-methyl groups in the mesityl substituents below the coalescent temperature T c (z280 K for the central benzene ring protons) (Fig. S1 in ESI †). Due to the bowl-shaped corannulene units, there would be two isomers of Cor-D1: one with two terminal bowls bent to the opposite directions (anti-form) and the other with two terminal bowls bent to the same directions (syn-form). Our calculations (B3LYP/6-31G(d,p)) predict that the syn-form is 0.538 kcal mol À1 higher in energy than the anti-isomer (Table S13 in ESI †), and thus the species with higher NMR integration can be correlated to the anti-isomer. The bowl inversion rate constant k at different temperatures was estimated by careful line-shape analysis 13 and tting of the data by Eyring equation (ln(k/T) ¼ ÀDH ‡ /R Â 1/T + ln(k B /h) + DS ‡ /R) gave thermodynamic parameters DH ‡ ¼ 19.8 AE 0.6 kcal mol À1 and DS ‡ ¼ 21.9 AE 2.3 cal (mol À1 K). Accordingly, DG ‡ was estimated to be 13.3 AE 1.3 kcal mol À1 at 298 K ( Fig. S2 and Table S1 in ESI †). This barrier is larger than the parent corannulene ($11.5 kcal mol À1 ) but slightly smaller than that of the corannulene annulated with a -CH 2 -CE 2 -CH 2 -(E ¼ COOCH 3 ) ring ($15.5 kcal mol À1 ). 12b Cor-D2 showed similar behavior with slightly lower T c (z270 K for central naphthalene ring protons) (Fig. S3 in ESI †) and line-shape analysis gave DH ‡ ¼ 19.7 AE 1.4 kcal mol À1 , DS ‡ ¼ 22.5 AE 5.3 cal (mol À1 K À1 ) and DG ‡ (298 K) ¼ 13.0 AE 3.0 kcal mol À1 (Fig. S4 and Table S2 in ESI †). Calculations show that the anti-and syn-isomers of Cor-D2 have nearly identical energy (Table S13 in ESI †), and thus both isomers exist equally in solution.

Optical, electrochemical and magnetic properties
Compound Cor-D1 is blue while Cor-D2 is green in dichloromethane (DCM). Cor-D1 shows an intense absorption band with maximum (l max ) at 612 nm (Fig. 3a), which can be correlated to HOMO / LUMO electronic transition based on timedependent (TD) DFT calculations (see ESI †). Cor-D2 displays a similar intense absorption band with l max at 690 nm, but with one weak shoulder at 743 nm. The appearance of such weak shoulder at long wavelength is characteristic for many openshell singlet diradicaloids, which can be mainly correlated to the H,H / L,L double excitation. 14 This is in accordance with the calculated moderate diradical character for Cor-D2. Our TD DFT calculations show that the anti-and syn-isomers show almost the same absorption spectra (ESI †) and thus one cannot simply distinguish the contribution of the individual isomers.
Cor-D1 showed sharp NMR spectra in THF-d 8 /CS 2 , CDCl 3 and D-toluene even at 373 K. Its solid powder displayed a weak broad electron spin resonance (ESR) signal (Fig. S5 in ESI †). Heating led to increase of the ESR intensity due to thermal population from the diamagnetic singlet state to the paramagnetic triplet state. Fitting of the IT-T data by Bleaney-Bowers equation 15 gave a large singlet-triplet energy gap (DE S-T ) of À8.4 kcal mol À1 (Fig. S6 in ESI †). Such a large gap is in accordance with its very small diradical character as well as the observed sharp NMR spectra. Cor-D2 also exhibited sharp NMR spectra in THF-d 8 /CS 2 and CDCl 3 (from 245 K to 335 K). However, NMR spectral broadening in toluene-d 8 was observed along with the elevated temperatures from 333 K to 373 K (Fig. S10 in ESI †). This phenomenon is consistent with its larger diradical character. The powder of Cor-D2 exhibited an intense one-line ESR signal with g e ¼ 2.0027 at room temperature ( Fig. S7 in ESI †), indicating existence of thermally populated paramagnetic species. Superconducting quantum interference device (SQUID) measurement on the powder sample of Cor-D2 revealed that the product of molar magnetic susceptibility and temperature (c M T) increased aer 150 K and tting of the data using Bleaney-Bowers equation gave a moderate DE S-T ¼ À3.0 kcal mol À1 (Fig. 4). The calculated spin-density distribution maps for the triplet diradical of Cor-D2 in both anti-and syn-forms show that the unpaired electronic spin is delocalized onto the two corannulene bowls, with the largest spin density distributed at the zigzag edges (inset in Fig. 4 and S15 in ESI †).
Cyclic voltammetry measurements of Cor-D1 and Cor-D2 were carried out in dry 1,2-dichlorobenzene (1,2-DCB) under heating (50 C) which can suppress molecular aggregation and give better resolved redox waves (Fig. 3b). Cor-D1 exhibited two oxidation waves with half-wave potential E ox 1/2 ¼ 0.23 and 0.55 V, and two reduction waves with half-wave potential E red 1/2 ¼ À1.42 and À1.64 V (vs. Fc + /Fc). The HOMO and LUMO energy levels were calculated to be À5.03 and À3.38 eV from the onset of the rst oxidation/reduction wave, respectively, and an electrochemical energy gap (E EC g ) of 1.65 eV was estimated. Cor-D2 showed two reduction waves at E red 1/2 ¼ À1.28 and À1.56 V, with a LUMO energy level of À3.52 eV. Due to strong aggregation, clear oxidation wave could not be observed. Both compounds can be chemically oxidized into radical cation and dication by NO$SbF 6 in dry DCM, and reduced into radical anion and dianion by sodium anthracenide (NaAn) in anhydrous THF (Fig. 5 and S11-S12 in ESI †). In both cases, the radical anion and radical cation show similar band structure, indicating a similar electronic structure, but the corresponding bands of radical anion are red-shied compared to the radical cation (l max ¼ 1490 nm vs. 1417 nm in Cor-D1, and l max ¼ 1808 nm vs. 1700 nm in Cor-D2). The dianion and dication exhibit similar band structure and trend (l max ¼ 892 nm vs. 844 nm in Cor-D1, and l max ¼ 1030 nm vs. 960 nm in Cor-D2). Clear 1 H NMR spectra of the dianions of Cor-D1 and Cor-D2 can be recorded by in situ reduction with NaAn in anhydrous THF-d 8 and most of the aromatic protons appear above 7 ppm despite their electronrich character (Fig. S13-S14 and S59 in ESI †), indicating that they are strongly aromatic compounds.
To better understand the electronic structure, anisotropy of the induced current-density (ACID) plot 16 and nucleus independent chemical shi (NICS) 17 calculations were conducted at (U)B3LYP/ 6-31G(d,p) level (see Fig. 6 and more images in Fig. S16-S25 in ESI †). The neutral anti-Cor-D1 shows clear diamagnetic/paramagnetic ring current ow along the outer/ inner ring of each corannulene bowl, like the parent corannulene ( Fig. S26 in ESI †), while there is no ring current circuit at the central p-QDM unit, indicating its non-aromatic character. In accordance with this, the average NICS(1) zz value 18 of the central benzene ring is near zero (À0.34 ppm). However, its dianion shows diamagnetic ring current along the periphery of the whole p-conjugated framework. At the same time, the average NICS(1) zz value of the central benzene ring becomes largely negative (À30.66 ppm), and even the inner cyclopentaring in the each corannulene unit shows signicantly negative NICS(1) zz value (À14.75 ppm), indicating a strong global aromatic system. Apparently, 38 p electrons can be counted along the periphery, which satises Hückel's [4n + 2]   aromaticity rule. Its dication exhibits similar global aromaticity with 34 p electrons delocalized along the periphery. The NICS(1) zz value of the central benzene ring (À12.45 ppm) is less negative than that in dianion, and that of the cyclopenta-ring becomes quite positive (+34.13 ppm), indicating relatively weak global aromaticity. In the anti-Cor-D2, the two corannulene bowls remain their individual aromatic character similar to that in Cor-D1, but diamagnetic ring current was found in the central naphthalene unit (NICS(1) zz : À10.18 ppm), indicating signicant contribution of the open-shell diradical form. Its dianion also show global aromaticity with 42 p-electrons delocalized along the periphery and the NICS(1) zz value in the central naphthalene ring is shied to À26.98 ppm. Its dication does not display global aromaticity as Cor-D1 2+ , and the localized aromaticity at the central naphthalene unit dominates (NICS(1) zz : À12.74 ppm), making the two corannulene units somehow segregated. The radical anions and radical cations of both compounds with unpaired electron show partial global aromatic character ( Fig. S22-S25 in ESI †).

Conclusions
In summary, rigid, p-QDM and 2,6-NQDM bridged corannulene dimers Cor-D1 and Cor-D2 were synthesized in stable form, which represent a new type of curved p-conjugated open-shell singlet diradicaloids. Cor-D1 has a small diradical character and behaves more like a closed-shell hydrocarbon at room temperature, while Cor-D2 possesses a moderate diradical character. Both compounds display magnetic activity at elevated temperatures. Electronic spins are distributed onto both corannulene bowls in Cor-D2, and stronger intermolecular interactions and more ordered packing are observed in its single crystals. Notably, their dianions show prominent global aromaticity with [4n + 2] p electrons delocalized along the periphery. Our studies reveal the unique electronic structure of such kind of bowl-shaped p-conjugated polycyclic hydrocarbons, which implies their potential applications for organic electronic and spintronic devices in the future.

Conflicts of interest
There are no conicts to declare.