## Main

Homoaromatic molecules display characteristics of aromatic stabilization yet possess an interrupted cyclic conjugated π system1. The concept of homoaromaticity, resting on through-bond or through-space homoconjugation2,3 (the interaction between disconnected π systems, usually interrupted by one or more sp3-hybridized carbon atoms or cyclopropane units1,4), has been highly controversial in the past1. The difficulty in identifying clear criteria for the assignment of molecules as homoaromatic1,5, as well as the small number of known homoaromatic molecules, has spurred discussions in this field.

The generally accepted criteria1,4 for homoaromaticity are as follows: (1) one or more homoconjugative interactions (through-space or through-bond), (2) electron delocalization of (4n + 2) π electrons in a cyclic structure due to effective overlap of π orbitals, (3) bond equalization of single and/or double bonds, (4) magnetic characteristics associated with ‘normal’ aromaticity and (5) resonance stabilizing energy. It should be noted that these criteria have to be regarded as a whole, and structural changes such as consideration of bond lengths alone (which some researchers challenge as a criterion for or against aromaticity6,7,8), without investigation of the magnetic properties of a compound, do not suffice for analysis of homoaromaticity9,10. The term ‘equalization’ is somewhat misleading, as, according to this term, only the shortening of single bonds and elongation of double bonds is referenced. However, in homoaromatic compounds, the single and double bond lengths rarely converge to exactly the same length.

Several ionic homoaromatic compounds have been disclosed11,12,13. However, examples for neutral homoaromatic molecules (especially purely organic ones) remain extremely rare14,15,16,17,18,19,20,21,22 and have generally been heteroatom-containing molecules14,15,16,20. There is only limited experimental and computational evidence of neutral homoaromatic hydrocarbons, spurring the comment in the leading review that ‘there remains a dearth of neutral homoaromatics’1. A good example of such a controversial discussion about homoaromatic character in neutral organic compounds relates to the formal ‘homobenzene’ 1,3,5-cycloheptatriene (1) and its valence tautomer norcaradiene (2). In 1, interactions between the terminal π orbitals of the triene moiety account for the through-space homoconjugation (Fig. 1a). For an explanation of through-bond (cyclopropyl) homoconjugation in 2, the Walsh orbitals of the cyclopropane moiety are generally considered23. Both types of homoconjugation can be visualized via the calculated highest occupied molecular orbitals (HOMOs) of 1 and 2 (Fig. 1a and Supplementary Fig. 40). For the through-space homoconjugation in 1, a bridging lobe of the HOMO between the terminal carbon atoms of the π system can be observed, whereas typical Walsh orbitals are obtained for the cyclopropanyl moiety of 2. A large number of theoretical studies, including some experimental evidence in support of and against a potential homoaromatic character of 1 or 2, are available1,3,22,24,25,26,27, with the most recent investigations supporting a homoaromatic character in 1 (refs. 22,27). Underlining the controversy, one of these studies refers to 1 and 2 as bearing the ‘unique distinction of being the subject of the highest number of claims and counter-claims about its homoaromatic stabilization’27.

Similarly, elassovalene (3)28,29, a potential neutral 10π-electron homoaromatic, has been the topic of long-standing debate. After independent preparation of the generally unstable 3 by the groups of Paquette and Vogel, evaluation of the spectral data and magnetic criteria, and finally comparison to the related aromatic annulenes, it was concluded that elassovalene 3 is not a neutral homoaromatic hydrocarbon, based on 1H NMR analysis28,30,31,32,33,34. The group of Quast came close to isolating a homoaromatic hydrocarbon when they found that in barbaralane derivative 4, the homoaromatic transition state is markedly stabilized in select polar solvents35,36,37,38. However, in the crystal structure of 4, only limited bond-length equalization was observed (Fig. 4, below), rendering this molecule the closest candidate for a neutral homoaromatic hydrocarbon so far.

The above-mentioned examples underscore the requirement for combined computational and experimental evidence for the assignment of a homoaromatic electronic structure. However, structural information is difficult to access, because neutral homoaromatics are often transient species1,18,19,37 and not crystalline, hampering the identification of bond-length equalization as one of the key criteria for homoaromaticity6,7,8. In this Article we present the synthesis of a class of stable neutral homoaromatic hydrocarbons, homoannulenes 6, and discuss their homoaromaticity, supported by experimental data (chemical shifts from NMR spectroscopy and bond-length comparison from X-ray structure elucidation) and calculations. We further show that compounds 6 act as a structurally novel photochemical switch through an unprecedented reversible photoinduced [1, 11] sigmatropic rearrangement, forming a novel neutral 10π homoaromatic.

## Results and discussion

### Synthesis of stable neutral homoaromatics

Our working hypothesis for the design of stable and isolable neutral homoaromatics originates from elassovalene 3 (Fig. 1c). Extending the bridging unit from a semibullvalene to a barbaralane core, as in 5 (which has been judged not to be homoaromatic based on circumstantial evidence28,32), we anticipated that an additional carbon atom in the bridge would create less ring strain compared to elassovalene 3. This would avoid fast degradation (see the strain energy analysis in Supplementary Discussion Section 1.8.5) and would also facilitate the interaction of both ends of the π system through space to allow for a homoconjugative interaction39,40. At the same time, the target molecule would in principle bear both a potential 6π as well as a 10π homoaromatic system with enhanced homoaromatic interactions. To the best of our knowledge, no 10π homoaromatic has been reported previously in the literature, which makes the presented example the largest homoaromatic π system that has been characterized1.

We targeted the synthesis of a potential 6π- or 10π-electron homoaromatic homoannulene 6 with a barbaralone-derived framework (Fig. 2). For the related bridged aromatic annulenes41,42,43, semibullvalenes28,43 and elassovalene28 3 (Fig. 2a), the synthesis starts from tetrahydronaphthalenes 7 or hexahydroanthracenes, respectively, using the central alkene(s) of hydrogenated polyaromatic compounds such as tetrahydronaphthalene as the synthetic linchpin for a key cyclopropanation (Fig. 2a, inside → outside). We envisioned an inverse approach (outside → inside), utilizing first the outer disubstituted alkene of the related dihydronaphthalene 8, leaving the benzene ring untouched. In this manner, we could circumvent the inherent higher reactivity of the central tetrasubstituted alkene for the key cyclopropanation. This approach requires a challenging intramolecular dearomative Buchner reaction as the central methodology downstream in the synthesis (Fig. 2b)33.

In this vein, we first reduced naphthalene 9 to 1,4-dihydronaphthalene 8, making the outer double bond accessible for the subsequent rhodium(II)-catalysed cyclopropanation44, giving cyclopropane 10 as a diastereomeric mixture of trans-10 and cis-10 (2:1). Separation of the two isomers, trans-10 and cis-10, was achieved by iterative temperature-dependent saponification with KOH, exploiting the innate reactivity difference of the diastereomeric cyclopropyl esters trans-10 and cis-10. A first saponification with KOH at room temperature selectively transformed trans-10 to the trans-11 acid and thus facilitated removal of unwanted trans isomer. Raising the temperature from room temperature to reflux afforded the cis-carboxylic acid cis-11 through subsequent saponification at elevated temperature. In cis-11, the carboxylic acid is aligned suitably towards the benzene ring, a key necessity for the downstream intramolecular Buchner reaction. It should be noted that this particular Buchner reaction is impeded by the fact that acetoacetate-derived diazo compounds generally display significantly lower reactivity in cyclopropanation reactions33. We continued by activation of all-cis-11 and chain elongation by Claisen condensation to form acetoacetate 12. A subsequent diazo transfer reaction led to the corresponding diazo compounds 13. The following intramolecular Buchner reaction, vital for our outside → inside strategy, required extensive catalyst optimization. After investigation of a variety of Rh(II) and Cu(II) complexes, we identified Cu(hFacac)2 as a suitable catalyst for the desired Buchner dearomatization to give triasteranes 14 (for the optimization see Supplementary Information). Only a few of the catalysts employed gave the desired product 14 at all, whereas the Buchner reaction catalysed by Cu(hFacac)2 (hFacac, hexafluoroacetylacetonate) did not show any traces of otherwise competitive C–H insertions. Finally, dehydrogenative oxidation using DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) gave homoannulene ester 15. To support the connectivity and stereochemistry of the desired products, crystal structures were obtained for 11, 12b14a,b and 15. This allowed the analysis of bond lengths to predict any potential homoaromaticity (see section ‘Bond-length comparison as structural evidence for homoaromaticity’).

When exploring the general chemical reactivity of 15, we found that saponification conditions unexpectedly led to the formation of the rearranged homoannulene carboxylic acid 16 (a proposed mechanism is outlined in Supplementary Fig. 5). This turned out to be a fortunate discovery, as 16 not only represents yet another neutral homoaromatic compound, but, with the X-ray crystal structure of 16 available, it also serves as key reference compound for the following investigation of the magnetic properties of the neutral homoaromatics 15 and 16.

### Magnetic characteristics of neutral homoaromatics

Homoannulene ester 15 and the rearranged homoannulene carboxylic acid 16 turned out to be ideal probes for the investigation of magnetic properties (induced ring current) indicative of a homoaromatic interaction1. The magnetic anisotropy and susceptibility of homoaromatic compounds are verifiable by NMR spectroscopy via marked chemical shifts1,9,45,46,47,48. Comparing the 1H NMR spectra of 14a, 15 and 16 (Fig. 3), we observed that 15 and 16 display a significant downfield shift of hydrogen atoms 5 and 6 of ~1.00 ppm as compared to 14a, giving a first indication of the presence of a ring current in the former two. Furthermore, the α-carbonyl methine group of homoannulene ester 15, facing away from the potential homoaromatic system, displays an expected chemical shift of δ = 3.81 ppm (Fig. 3, middle). In stark contrast, the α-carbonyl methine 1H NMR resonance of homoannulene carboxylic acid 16, placed directly above the homoaromatic system, is significantly shifted upfield to δ = 0.89 ppm, despite being placed directly adjacent an electron-withdrawing ketone (Fig. 3, bottom). Even taking the anisotropic effect of the double bonds into account24, this unusually high upfield shift of Δδ = 2.92 ppm unambiguously indicates the presence of a diamagnetic ring current47,49, which shields the methine hydrogen atom of 1624,41. Both key spectral properties (downfield shift of hydrogen atoms 5 and 6 and the shift of the bridgehead methine hydrogen atom) are missing in elassovalene (3)28,31, whose analogous methine hydrogen atom appears at δ = 1.77 ppm (and Csp2–H appear at 6.30 and 6.64 ppm, respectively). Homoannulene ester 15 and homoannulene carboxylic acid 16 possess an interrupted cyclic π system. The observation of a strong ring current therefore indicates the existence of through-space interaction of the π system, leading to a delocalization of π electrons in 15 and 16 (ref. 2) (calculations of the magnetic properties of 15, which support the experimental data, are shown in section ‘Computational investigation of photoswitchable homoaromatics’). Therefore, the fused barbaralone framework prepared in this study seems to structurally enable a more efficient homoconjugation by an elongated bridge that facilitates the through-space interaction of carbon atoms C(4)···C(9) through steric congestion. Combined, these findings serve as an important indication that homoannulene ester 15 and homoannulene carboxylic acid 16 are neutral homoaromatic compounds.

### Bond-length comparison as structural evidence for homoaromaticity

One of the paramount characteristics for homoaromatic compounds is bond-length equalization of the single and double bonds involved1,2,6,7,8. With the X-ray structures of 14a,b, 15 and 16 available for detailed analysis, we observed significant shortening of the single bonds and elongation of the double bonds (Fig. 4). As limits, the bond lengths of isolated C–C single (1.54 Å) and C=C double bonds (1.34 Å), as well as the ‘ideal’ bond-length equalization of benzene (1.39 Å), are employed for comparison50. These values are compared to the longest C–C single bond and the shortest C=C double bond, respectively, of the respective neutral homoaromatic molecules 14a, 15 and 16 (see Supplementary Fig. 3 for a detailed analysis of all bond lengths).

For homoannulene ester 15, a homoaromatic cycloheptatrienyl fragment seems to be present, as indicated by the bond-length equalization (the shortest double bond is 1.364(2) Å (Fig. 4, red arrows) and the longest single bond is 1.420(2) Å (Fig. 4, blue arrows)). The formal vinyl substituents in the barbaralone substructure show alternating (single and double) bond lengths (~1.45 and ~1.34 Å, respectively), indicating a 6π homoaromatic compound with two vinyl substituents. Also, for homoannulene carboxylic acid 16, equalized bond lengths are observed (1.428(2) and 1.360(2) Å for the longest C–C single and shortest C=C double bond in the cycloheptatriene moiety). In contrast, non-oxidized triasterane 14a displays the expected bond-length alternation for a 1,3-diene and lacks any indication for homoconjugation. We compared our observations to barbaralane 4 (refs. 36,37,38) (Fig. 4), which to our knowledge is considered the example for a neutral homoaromatic compound (1.459(3) and 1.347(3) Å for the longest C–C single and shortest C=C double bond) closest to exhibiting homoaromatic character in terms of bond lengths1. Homoannulene ester 15 and carboxylic acid 16 show significantly higher bond-length equalization than barbaralane 4, as reflected in the harmonic oscillator model of aromaticity (HOMA) values, which are used for the description of bond-length equilibration. Indeed, the bond-length equalization observed for 15 and 16 is rather comparable to that of methano[10]annulene 17 (ref. 41) (1.373(3)–1.419(4) Å (ref. 51)), which is considered aromatic41,52. The bond-length equalization of 15 and 16, in addition to the ring current observation discussed above, lead us the conclusion that 15 and 16 are indeed neutral and stable homoaromatic compounds.

### Photoswitching behaviour of stable neutral homoaromatics

Extended conjugated π systems—especially aromatic ones—express particular reactivity and show the ability to absorb energy in the ultraviolet visible (UV–vis) range. To further elucidate the homoaromatic character of 15, we investigated its behaviour upon irradiation with light, as has been done for several other homoaromatic compounds28,35,36,53. We found that homoannulene ester 15 undergoes a photoinduced rearrangement of the barbaralone framework, forming ester 18 (305 nm light-emitting diode (LED) irradiation for 80 s of a dilute cyclohexane or acetonitrile (MeCN) solution of 15; Fig. 5a). The thermal stability of photoproduct 18 was assured by heating the solution at the photostationary state (PSS) at 305 nm in MeCN at 55 °C for 9.25 h (Supplementary Figs. 17 and 18). Thermal stability allowed the separation of the two compounds 15 and 18, present in the PSS, using preparative HPLC. Full characterization of 18 using NMR spectroscopy and high-resolution mass spectrometry analysis allowed the unambiguous determination of its structure. The formation of 18 can be rationalized by an unprecedented [1, 11] sigmatropic rearrangement from 15 to 18, which is suprafacially allowed under photochemical conditions in accordance with Woodward–Hoffmann rules54. The defined isosbestic points in the UV–vis spectra indicate a distinct photochemical rearrangement, supported by HPLC (Fig. 5b,c). Integration of the peak areas of the UV–vis/HPLC traces revealed a 15/18 ratio of 16:84 at the PSS (Supplementary Fig. 14). This photochemical reaction was found to be independent of solvent polarity. It was observed that it occurs at equally rapid rates in both cyclohexane and in MeCN (PSS composition of 15/18 of 20:80), which indicates a non-polar transition state as typically observed for pericyclic reactions (Supplementary Figs. 812).

Photochromic systems are of special interest in the development of functional materials55. The prediction and rational design of molecules bearing new motifs that can be reversibly switched from one state into another by an external stimulus is challenging. Attempting the back-reaction of 18 to 15, we found that the photochemical [1, 11] sigmatropic rearrangement is reversible upon irradiation with a 455-nm LED light source. This renders homoannulene ester 15 a structurally novel photochemical switch56,57,58. The photochromic system can be classified as a p-type photoswitch (back-reaction photochemically activated).

We performed multiple cycles of irradiation by alternating between 305-nm and 455-nm LEDs (Fig. 5b). Although the irradiation repeatedly led to the formation of 18 in the PSS and back-formation of 15, analysis of the UV–vis absorption maxima at 390 nm over several cycles indicated a moderate switching fatigue caused by the formation of trace side products during the photoswitching process (Supplementary Fig. 15). We hypothesized that the formation of rearranged 18 is driven by a homoaromatic stabilization effect and thus turned our attention to the computational electronic and magnetic structure analysis of 15 and 18.

### Computational investigation of photoswitchable homoaromatics

The induced ring currents of (homo)aromatic systems by an external magnetic field were probed computationally as a measure for the homoaromaticity of 15 and 18. In conjunction with (indirect) experimentally determined diatropicity via 1H NMR spectroscopy for the adjacent hydrogen atoms, magnetic indices can be used to theoretically correlate the degree of aromaticity. Two such magnetic indices that are widely used are nucleus-independent chemical shifts (NICS)59,60,61,62,63,64 and anisotropy of the induced current density (ACID)65,66, which were calculated for 15 and 18 (Fig. 6 and Supplementary Discussion Section 1.8).

To simplify the calculations, the methyl ester groups in 15 and 18 were replaced by methyl groups (now Me-15, Me-18). A geometric overlay of Me-15 with Me-18 showed slight differences in their homoaromatic skeleton (Supplementary Discussion Section 1.8). Besides the unsymmetrically aligned bridge in Me-18, we found through-space distances of 2.31 Å (6π current) and 2.47 Å (10π current) for Me-15, whereas these C–C distances slightly converge for Me-18 to 2.37 Å (6π current) and 2.44 Å (10π current). Remarkably, elassovalene 3 shows a 9% longer C–C through-space distance of 2.45 Å for its (6π current) when compared to Me-15. Due to the non-planar π-ring geometry, we calculated NICSzz values at various distances greater than 1.7 Å below the ring centroids. As a result of this non-planar geometry, the distances from the calculated NICSzz values to the molecule are not fixed. A distance greater than 1.7 Å was chosen to minimize the effect of the σ-bond electrons on the π-electron system61,62. These one- and two-dimensional NICSzz scans of Me-15 (Fig. 6a,c) reveal a diatropic ring current of the C(4) to C(9) fragment of Me-15, with NICS(1.8)zz values reaching down to −18.6 ppm. In contrast, the adjacent ring current in the C(2–4)···C(9–11) fragment is only moderately diatropic, with less negative NICS(1.8)zz values of −12.5 ppm. Upon [1, 11] sigmatropic rearrangement to 18, the diatropic ring currents are redistributed, with NICS(1.8)zz values of −11.3 ppm and −15.2 ppm for the six- and seven-membered fragments, respectively. We interpret this as a shift of the local homoaromatic character from the C(9–11) fragment 6π system in Me-15 to a more evenly distributed 10π system in Me-18, within the error of this challenging analysis of non-planar π systems61. In comparison, cycloheptatriene (1) and norcaradiene (2) show a substantially lower degree of homoaromaticity, with NICS(1.8)zz values of −10.7 ppm and −9.3 ppm, respectively, whereas benzene shows a NICS(1.8)zz value of −20.2 ppm at the same level of theory. Finally, the plotted ACID results unambiguously underscore the conjugation paths of the ring currents (Fig. 6e,f)65,66. A defined homoaromatic through-space interaction for Me-15 can be observed between atoms C(2) and C(11) at a critical isosurface value (CIV) of 0.034, supporting a global 10π homoaromatic character, in agreement with the NICS analysis of 15 (in comparison, the CIV for non-planar cycloheptatriene 1 is 0.027 at the same level of theory65,66). A second through-space interaction between C(4) and C(9) is observed at a CIV of 0.026, indicating a local 6π homoaromatic ring current in 15.

For 18 the situation is changed again, with a through-space interaction observed between C(1) and C(10) at a CIV of 0.040, indicating an enhanced global 10π homoaromaticity. In contrast, no through-space interaction can be observed for the local 6π system between C(4) and C(9), even at a CIV as low as 0.01 (Supplementary Fig. 36). We conclude that both 15 and 18 are globally 10π homoaromatic, with an enhanced local homoaromaticity in the 6π system of 15 and an evenly distributed 10π homoaromatic state in 18. The latter is due to geometric constraints originating from the rearranged scaffold in 18, allowing for efficient homoconjugation between C(1) and C(10) that facilitates a 10π-electron ring current.

## Conclusion

Assigning molecules as homoaromatic has been a long-standing problem, because molecules with sufficient stability and lifetime have been elusive. Here we have described the synthesis of homoannulene ester 15 and the related acid 16, stable neutral homoaromatic hydrocarbons. Their homoaromatic state was evidenced by 1H NMR spectroscopic data, indicating a substantial ring current in addition to a high degree of bond-length equalization in the X-ray crystal structures. A detailed computational analysis by means of NICS-XY-scans scans and ACID isosurface plots revealed the ring currents and conjugation pathways. The local 6π homoaromatic state of homoannulene ester 15 can be photoswitched at 305 nm to a global 10π homoaromatic state by an unprecedented photochemical [1, 11] sigmatropic rearrangement to ester 18. This photoreaction is reversible, and irradiation of 18 at 455 nm yields back homoannulene 15. Therefore, 15/18 is a photoswitch based on a sigmatropic rearrangement that operates based on different homoaromaticity states. Notably, both 15 and 18 exhibit high thermal stability; the photochemical switch does not suffer from thermal background reactions, as these are thermochemically not allowed according to the Woodward–Hoffmann rules. We hope that interconvertible homoannulenes such as 15$${\leftrightarrows}$$18 may find use as molecular photoswitches based on homoaromatic stabilization effects.