# From Stilbenes to carbo-Stilbenes: an Encouraging Prospect

Chongwei Zhu, Alix Sournia-Saquet, Valérie Maraval, Christian Bijani, Xiuling X Cui, Albert Poater, Remi R Chauvin

► To cite this version:

Chongwei Zhu, Alix Sournia-Saquet, Valérie Maraval, Christian Bijani, Xiuling X Cui, et al.. From Stilbenes to carbo-Stilbenes: an Encouraging Prospect. Chemistry - A European Journal, 2024, 30 (26), pp.e202400451. 10.1002/chem.202400451. hal-04532883

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# From Stilbenes to carbo-Stilbenes: an Encouraging Prospect

Chongwei Zhu,[a, b] Alix Saquet,[c] Valérie Maraval,[c] Christian Bijani,[c] Xiuling Cui,[d] Albert Poater,[*e] and Remi Chauvin*[a, d]

*Dedicated to Prof. Miquel Solà for his 60th Anniversary*

Beyond previously described carbo-naphthalene and carbo-biphenyl, a novel type of bis-carbo-benzenic molecules is envisaged from the stilbene parent. The synthesis, structure, spectroscopic and electrochemical properties of two such carbo-stilbenes are described at complementary experimental and computational DFT levels. In the selected targets, the bare skeletal carbo-mer of carbo-stilbene is decorated by 8 or 10 phenyl groups, 0 or 2 tert-butyl groups, and 2 n-octyl chains, the later substituents being introduced to compensate anticipated solubility issues. As in the parent stilbene series, isomers of the phenylated carbo-stilbenes are characterized. The cis- and trans-isomers are, however, formed in almost equal amounts and could not be separated by either chromatography or crystallization. Nevertheless, due to a slow interconversion at the NMR time scale (up to 55 °C) the ¹H NMR signals of both isomers of the two carbo-stilbenes could be tentatively

assigned. The calculated structure of the cis-isomer exhibits a helical shape, consistent with the observed magnetic shielding of phenyl *p*-CH nuclei residing inside the shielding cone of the facing C₁₈ ring. The presence of the two isomers in solution also gives rise to quite broad UV-vis absorption spectra with main bands at *ca* 460, 560 and 710 nm, and a significant bathochromic shift for the decaphenylated carbo-stilbene vs the ditert-butyl-octaphenylated counterpart. Square wave voltammograms do not show any resolution of the two isomers, giving a reversible reduction wave at -0.65 or -0.58 V/SCE, and an irreversible oxidation peak at 1.11 V/SCE, those values being classical for most carbo-benzene derivatives. Calculated NICS values (NICS(1) = -12.5 ± 0.2 ppm) also indicate that the aromatic nature of the C₁₈ rings is not markedly affected by the dialkynylbutatriene (DAB) connector between them.

## Introduction

The C₁₄ skeleton of stilbene (C₆H₅-CH=CH-C₆H₅) occurs in numerous natural and synthetic products with various prospects of applications in life, chemical, and materials science,[1] thanks to their therapeutic,[2] chromophoric, and photo-isomerization properties,[3] respectively.

Within the context of the recent focus of carbo-mer[4a] chemistry[4b–9] on derivatives containing two annulated or catenated C₁₈ carbo-benzenic rings at least (carbo-naphthalene skeleton A,[5] carbo-biphenyl skeleton B[6]), the C₄₄ skeletal carbo-mer of stilbene C (C₁₈H₅-C≡C-CH=C = C=CH-C≡C-C₁₈H₅) emerges as an imperious target to be studied by both experimental and computational methods (Figure 1a). Contrary to toluene (C₆H₅-C≡C-C₆H₅) and carbo-biphenyl B (C₁₈H₅-C≡C-C₁₈H₅), the so-called fictitious carbo-stilbene C is likely to exhibit the same stereoisomerism as stilbene in the Eliel's sense,[7] under either cis- (Z) or trans-(E) configuration through the [3]cumulene linker. From a general standpoint, acyclic [n]cumulenes R¹R²C = (C)ₙ₋₁ = CR¹R² were long studied for their bond-length alternation (BLA), and their cumulenic rotation barrier (CRB) corresponding to R/S enantiomerization for even *n* and *cis(Z)/trans(E)* diastereo-conversion for odd *n*.[8,9] Given a substituent set, both BLA and CRB are anticipated to decrease vs *n* and parity of *n* due to the relative stabilization of the open shell diradical form R¹R²C*-(C)ₙ₋₁-C*R¹R²*.[10] In the odd series, the [3]cumulene (or butatriene) motif has attracted special attention due to its second position after the alkene

<footnote>[a] Dr. C. Zhu, Prof. R. Chauvin
Faculté Science et Ingénierie - Département de Chimie. Unsaturated molecules for physics, biology and chemistry group. Université Toulouse III - Paul Sabatier, 118 route de Narbonne, 31-062 Toulouse Cedex 09, France
E-mail: remi.chauvin@univ-tlse3.fr</footnote>

<footnote>[b] Dr. C. Zhu
Key Laboratory of Functional Molecular Solids, Ministry of Education, and
School of Chemistry and Materials Science, Anhui Normal University, Wuhu
241002, China</footnote>

<footnote>[c] Dr. A. Saquet, Dr. V. Maraval, Dr. C. Bijani
LCC-CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France</footnote>

<footnote>[d] Prof. X. Cui, Prof. R. Chauvin
Engineering Research Center of Molecular Medicine of Ministry of Education,
Key Laboratory of Fujian Molecular Medicine, Key Laboratory of Xiamen
Marine and Gene Drugs, School of Biomedical Sciences, Huaqiao University,
Xiamen 361021, China</footnote>

<footnote>[e] Dr. A. Poater
Institut de Química Computacional i Catàlisi, Departament de Química,
Universitat de Girona, c/ Mª Aurèlia Capmany 69, 17003 Girona, Catalonia,
Spain
E-mail: albert-poater@udg.edu
Homepage: http://iqcc.udg.edu/wordpress/portfolio/albert-poater/</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202400451</footnote>

<footnote>© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_2.png" /></figure>

# From Stilbenes to carbo-Stilbenes: an Encouraging Prospect

Chongwei Zhu,[a, b] Alix Saquet,[c] Valérie Maraval,[c] Christian Bijani,[c] Xiuling Cui,[d] Albert Poater,[*e] and Remi Chauvin*[a, d]

Dedicated to Prof. Miquel Solà for his 60th Anniversary

Beyond previously described carbo-naphthalene and carbo-biphenyl, a novel type of bis-carbo-benzenic molecules is envisaged from the stilbene parent. The synthesis, structure, spectroscopic and electrochemical properties of two such carbo-stilbenes are described at complementary experimental and computational DFT levels. In the selected targets, the bare skeletal carbo-mer of carbo-stilbene is decorated by 8 or 10 phenyl groups, 0 or 2 tert-butyl groups, and 2 n-octyl chains, the later substituents being introduced to compensate anticipated solubility issues. As in the parent stilbene series, isomers of the phenylated carbo-stilbenes are characterized. The cis- and trans-isomers are, however, formed in almost equal amounts and could not be separated by either chromatography or crystallization. Nevertheless, due to a slow interconversion at the NMR time scale (up to 55 °C) the ¹H NMR signals of both isomers of the two carbo-stilbenes could be tentatively

assigned. The calculated structure of the cis-isomer exhibits a helical shape, consistent with the observed magnetic shielding of phenyl *p*-CH nuclei residing inside the shielding cone of the facing C₁₈ ring. The presence of the two isomers in solution also gives rise to quite broad UV-vis absorption spectra with main bands at *ca* 460, 560 and 710 nm, and a significant bathochromic shift for the decaphenylated carbo-stilbene vs the ditert-butyl-octaphenylated counterpart. Square wave voltammograms do not show any resolution of the two isomers, giving a reversible reduction wave at -0.65 or -0.58 V/SCE, and an irreversible oxidation peak at 1.11 V/SCE, those values being classical for most carbo-benzene derivatives. Calculated NICS values (NICS(1) = -12.5 ± 0.2 ppm) also indicate that the aromatic nature of the C₁₈ rings is not markedly affected by the dialkynylbutatriene (DAB) connector between them.

## Introduction

The C₁₄ skeleton of stilbene (C₆H₅–CH=CH–C₆H₅) occurs in numerous natural and synthetic products with various prospects of applications in life, chemical, and materials science,[1] thanks to their therapeutic,[2] chromophoric, and photo-isomerization properties,[3] respectively.

Within the context of the recent focus of carbo-mer[4a] chemistry[4b–9] on derivatives containing two annulated or catenated C₁₈ carbo-benzenic rings at least (carbo-naphthalene skeleton A,[5] carbo-biphenyl skeleton B[6]), the C₄₄ skeletal carbo-mer of stilbene C (C₁₈H₅–C≡C–CH=C = C=CH–C≡C–C₁₈H₅) emerges as an imperious target to be studied by both experimental and computational methods (Figure 1a). Contrary to toluene (C₆H₅–C≡C–C₆H₅) and carbo-biphenyl B (C₁₈H₅–C≡C–C₁₈H₅), the so-called fictitious carbo-stilbene C is likely to exhibit the same stereoisomerism as stilbene in the Eliel's sense,[7] under either cis- (Z) or trans-(E) configuration through the [3]cumulene linker. From a general standpoint, acyclic [n]cumulenes R¹R²C = (C)ₙ₋₁ = CR¹R² were long studied for their bond-length alternation (BLA), and their cumulenic rotation barrier (CRB) corresponding to R/S enantiomerization for even *n* and *cis(Z)/trans(E)* diastereo-conversion for odd *n*.[8,9] Given a substituent set, both BLA and CRB are anticipated to decrease vs *n* and parity of *n* due to the relative stabilization of the open shell diradical form R¹R²C*–(C)ₙ₋₁–C*R¹R².[10] In the odd series, the [3]cumulene (or butatriene) motif has attracted special attention due to its second position after the alkene

<figure><img src="image_3.png" /></figure>

Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202400451

<figure><img src="image_4.png" /></figure>

© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

<footnote>[a] Dr. C. Zhu, Prof. R. Chauvin
Faculté Science et Ingénierie - Département de Chimie. Unsaturated molecules for physics, biology and chemistry group. Université Toulouse III - Paul Sabatier, 118 route de Narbonne, 31-062 Toulouse Cedex 09, France
E-mail: remi.chauvin@univ-tlse3.fr</footnote>

<footnote>[b] Dr. C. Zhu
Key Laboratory of Functional Molecular Solids, Ministry of Education, and
School of Chemistry and Materials Science, Anhui Normal University, Wuhu
241002, China</footnote>

<footnote>[c] Dr. A. Saquet, Dr. V. Maraval, Dr. C. Bijani
LCC-CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France</footnote>

<footnote>[d] Prof. X. Cui, Prof. R. Chauvin
Engineering Research Center of Molecular Medicine of Ministry of Education,
Key Laboratory of Fujian Molecular Medicine, Key Laboratory of Xiamen
Marine and Gene Drugs, School of Biomedical Sciences, Huaqiao University,
Xiamen 361021, China</footnote>

<footnote>[e] Dr. A. Poater
Institut de Química Computacional i Catàlisi, Departament de Química,
Universitat de Girona, c/ Mª Aurèlia Capmany 69, 17003 Girona, Catalonia,
Spain
E-mail: albert-poater@udg.edu
Homepage: http://iqcc.udg.edu/wordpress/portfolio/albert-poater/</footnote>
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

b Known acyclic n-DABs:

<figure><img src="image_4.png" /></figure>

1: n = 1, R¹ = alkynyl

2: n = 2, R¹ = aryl, alkynyl

3: n = 3, R¹ = phenyl

Known endocyclic n-DABs:

<figure><img src="image_5.png" /></figure>

n = 2-3

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

carbo-benzene 4:

R², R³ =

H, aryl, alkynyl, alkyl

<figure><img src="image_8.png" /></figure>

carbo-1,3-cyclobutadiene 5:

R² = Ph

carbo-1,4-cyclobutadiene 6:

R⁴ = CF₃

<figure><img src="image_9.png" /><figcaption>**Figure 1.** a) Exemplified bis-carbo-benzenic structures A, B, and carbo-stilbene target C; b) known acyclic and cyclic oligo-dialkynylbutatrienes (n-DABs, *n* = 1-3).</figcaption></figure>

motif, and its specific nature as the carbo-mer of the latter.[7,11]
In the carbo-stilbene structure C, the two carbo-benzene rings
are conjugated through a particular [3]cumulene connector, i.e.
a dialkynyl-butatriene (DAB) unit. In contrast to the ethenylene
C₂ connector of stilbene, the C₈ DAB unit is known to undergo
facile cis $\rightleftharpoons$ trans isomerization, with small thermodynamic
preference for either isomer due to the absence of steric
interaction between remote substituents.[12,13,14]

Regardless of stereochemistry, the kinetic stability of acyclic DABs 1 was shown to increase with the substituent bulkiness (Figure 1b). [14,15,16,17] In fused acyclic oligo-DABs (n-DABs), carbo-butadienes 2 (n = 2) were also shown to be quite stable when protected by bulky silyl groups,[2c] while the carbo-hexatriene 3 (n = 3) could be isolated and stored in the solid state only, as an unfrozen cis-trans stereochemical mixture.[18,19] The cis – > trans isomerization is however frozen in endocyclic n-DABs, such as aromatic carbo-benzenes 4 (n = 3) where the alternating butatriene and butyne edges of the equivalent Kekule structures provide the three DAB units with a "hemi-butatriene" character.[9] In non-aromatic cyclic n-DABs, the two butatriene units of carbo-1,3-cyclohexadienes 5 are π-conjugated through the fusing triple bond (as in acyclic carbo-butadienes 2),[20] while the two π-independent DAB motifs of carbo-1,4-cyclohexadienes 6 (n = 2)[21] or carbo-barrelene 7 (n = 3)[22] are homoj conjugated through two sp³-C atoms (Figure 1).

As the *n*-DABs **1–7** were shown to be chemically quite
stable, and sp²C-unsubstituted DABs (R–C≡C–CH = C=C
CH–C≡C–R) and carbo-benzene (C₁₈H₆) remain unknown,[4,23]
per-substituted derivatives of *C* appeared as reasonable
objectives. Considering the large size and synthesis efficiency of

tetraphenyl-carbo-benzenes,[⁴e⁻⁹] the carbo-stilbenes **8a-b** were
devised as optimal first targets (Scheme 1).

**Results and Discussion**

The preparation of carbo-stilbenes **8** was envisaged from the
[6]pericyclynedione **9**[24] by successive nucleophilic additions, as
previously experienced for the synthesis of *n*-octyl-substituted
carbo-biphenyl (B, Figure 1) and carbo-terphenyl derivatives.[6]
All the starting materials or products described below were
used or obtained as statistical mixtures of stereoisomers.

In a first attempt (Scheme 1), addition of the triyne **10a**[24] or **10b**[25] to two equivalents of the known [6]pericyclynone **11**, obtained by addition of one equivalent of *n*-octylmagnesium bromide to the diketone **9**,[6] gave only trace amounts of the double adduct **12a** or **12b**. An alternative strategy was thus considered (Scheme 2).

Addition of the mono-bromomagnesium salts of the triyne
**10a–b** (generated from 1.5 equivalent of triyne and 1.1
equivalent of EtMgBr) to a single equivalent of diketone **9** was
found to be an efficient method to obtain the monoadduct

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Scheme 1. Carbo-stilbene targets **8**, and first attempt at synthesis thereof.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 1.** a) Exemplified bis-carbo-benzenic structures A, B, and carbo-stilbene target C; b) known acyclic and cyclic oligo-dialkynylbutatrienes (n-DABs, *n* = 1-3).</figcaption></figure>

motif, and its specific nature as the carbo-mer of the latter.[7,11]
In the carbo-stilbene structure C, the two carbo-benzene rings
are conjugated through a particular [3]cumulene connector, i.e.
a dialkynyl-butatriene (DAB) unit. In contrast to the ethenylene
C₂ connector of stilbene, the C₈ DAB unit is known to undergo
facile cis $\rightleftharpoons$ trans isomerization, with small thermodynamic
preference for either isomer due to the absence of steric
interaction between remote substituents.[12,13,14]

Regardless of stereochemistry, the kinetic stability of acyclic
DABs 1 was shown to increase with the substituent bulkiness
(Figure 1b).[14,15,16,17] In fused acyclic oligo-DABs (n-DABs), carbo-
butadienes 2 (n = 2) were also shown to be quite stable when
protected by bulky silyl groups,[2c] while the carbo-hexatriene 3
(n = 3) could be isolated and stored in the solid state only, as an
unfrozen cis-trans stereochemical mixture.[18,19] The cis – > trans
isomerization is however frozen in endocyclic n-DABs, such as
aromatic carbo-benzenes 4 (n = 3) where the alternating
butatriene and butyne edges of the equivalent Kekule struc-
tures provide the three DAB units with a "hemi-butatriene"
character.[9] In non-aromatic cyclic n-DABs, the two butatriene
units of carbo-1,3-cyclohexadienes 5 are π-conjugated through
the fusing triple bond (as in acyclic carbo-butadienes 2),[20] while
the two π-independent DAB motifs of carbo-1,4-cyclohexa-
dienes 6 (n = 2)[21] or carbo-barrelene 7 (n = 3)[22] are homo-
conjugated through two sp³-C atoms (Figure 1).

As the *n*-DABs **1–7** were shown to be chemically quite
stable, and *sp*²*C*-unsubstituted DABs (*R*—C≡C—*CH* = C=C
*CH*—C≡C—*R*) and *carbo*-benzene (*C*₁₈*H*₆) remain unknown,[4,23]
per-substituted derivatives of *C* appeared as reasonable
objectives. Considering the large size and synthesis efficiency of

tetraphenyl-carbo-benzenes,[4e-g] the carbo-stilbenes 8a-b were
devised as optimal first targets (Scheme 1).

**Results and Discussion**

The preparation of carbo-stilbenes **8** was envisaged from the
[6]pericyclynedione **9**[24] by successive nucleophilic additions, as
previously experienced for the synthesis of *n*-octyl-substituted
carbo-biphenyl (B, Figure 1) and carbo-terphenyl derivatives.[6]
All the starting materials or products described below were
used or obtained as statistical mixtures of stereoisomers.

In a first attempt (Scheme 1), addition of the triyne **10a**[24] or **10b**[25] to two equivalents of the known [6]pericyclynone **11**, obtained by addition of one equivalent of *n*-octylmagnesium bromide to the diketone **9**,[6] gave only trace amounts of the double adduct **12a** or **12b**. An alternative strategy was thus considered (Scheme 2).

Addition of the mono-bromomagnesium salts of the triyne
**10a–b** (generated from 1.5 equivalent of triyne and 1.1
equivalent of EtMgBr) to a single equivalent of diketone **9** was
found to be an efficient method to obtain the monoadduct

<figure><img src="image_7.png" /><figcaption>Scheme 1. Carbo-stilbene targets **8**, and first attempt at synthesis thereof.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Scheme 2.** Synthesis of the *carbo*-stilbenes **8a** and **8b**.</figcaption></figure>

13a–b as major product, accompanied with minor quantities of the diadduct **14a–b**, along with unreacted diketone **9** and triyne **10a–b** (Scheme 2). The monoketone **13a–b** was then treated with *n*-octyllithium to give the diol **15a–b**, which was subsequently di-O-methylated to the octaether **16a–b**. Finally, addition of **16a–b** to the monoketone **11** in the presence of LiHMDS afforded the key bis-hexaoxy[6]pericylyne **17a–b** with 46–55% yields. Ultimate acidic reductive treatment of **17a–b** with SnCl₂/HCl allowed isolation of the *carbo*-stilbene **8a–b** with 46–53% yield, i. e. with 89–91% average yield for each of the seven butatriene edges formed in **8a–b**. The two *carbo*-stilbenes were isolated as black solids and found to be stable in solution, albeit poorly soluble despite the presence of two *n*-octyl chains, and two extra *tert*-butyl groups in **8b**, both types of substituents being previously reported to increase the solubility of *carbo*-benzene derivatives.[6,25]

The low solubility of both the *carbo*-stilbenes **8a** and **8b** prevented their full characterization, in particular by ¹³C NMR spectroscopy. ¹H and ¹H-¹H COSY NMR spectra could however be recorded, and ¹H-¹³C HSQC and HMBC experiments (at 600 MHz, 45 °C) could also be recorded for the slightly more soluble *carbo*-stilbene **8b**. The results allowed a tentative assignment of all the ¹H NMR signals, in spite of their duplication due to the simultaneous presence of the *cis*- and *trans*- isomers, the diastereoisomeric ratio being *ca* 52:48 for **8a**, 56:44 for **8b** (Figure 2 and S.I.).

<figure><img src="image_14.png" /><figcaption>**Figure 2.** a) Aromatic region of ¹H NMR spectra of the *carbo*-stilbenes **8a** (*R* = Ph, *bottom*) and **8b** (*R* = ¹Bu, *top*), in CDCl₃ solution (500 MHz, 55 °C). b) DFT-calculated equilibrium structures of *cis*-**8a** and *trans*-**8a** (see Table S4), possibly accounting for magnetic shielding effects on the *p*-C¹*H* nuclei of phenyl substituents made chemically inequivalent by restricted rotation of the C₁₈ rings with respect to the central DAB connector.</figcaption></figure>

From systematic variable temperature NMR experiments at 500 MHz, noteworthy is the absence of signal coalescence between 25 and 55 °C, suggesting that the stereo-conversion barrier should be greater the 20 kcal/mol. Owing to the extremely poor solubility of **8a** and **8b** in solvents other than CD₂Cl₂ or CDCl₃ and known thermal sensitivity of non-endo-aromatic butatriene derivatives, recording spectra at higher temperatures would not be feasible.

The most striking observation for one of the isomers (and only one, assigned to *cis*-**8a** or *cis*-**8b**) is the degeneracy lift of pairs of phenyl substituents, suggesting a blockage of the free rotation about the DAB–C₁₈ single bonds, which is allowed in the *trans*-isomer only. This is actually markedly evidenced in Density Functional Theory (DFT)-calculated isomers of **8a** and **8b** (see discussion below, Figure 2b, and legend of Figure 2).

As usually observed for centrosymmetric tetraphenyl-*carbo*-benzenes, the *p*-C¹*H* (and *m*-C¹*H*) nuclei of adjacent phenyl substituents have identical chemical shifts. Such an equivalence was however unexpected for the internal phenyl groups of *cis*-**8a** (and *cis*-**8b**), whose *p*-C¹*H* nuclei both resonate at 6.55 ppm in spite of their different positions from the facing C₁₈ ring in the calculated structure. This structure being rigid, the observed average equivalence at 55 °C could be tentatively attributed to flexibility in solution incidentally subjecting the two phenyl rings to identical average magnetic anisotropy effects. Moreover, using a classical software, such as NMR predict from Mestrenova 14.1.2-25024, the simulated NMR spectra of **8a** and **8b** are differentially shifted to high field (see S.I.), due to the
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Scheme 2.** Synthesis of the *carbo*-stilbenes **8a** and **8b**.</figcaption></figure>

13a–b as major product, accompanied with minor quantities of the diadduct **14a–b**, along with unreacted diketone **9** and triyne **10a–b** (Scheme 2). The monoketone **13a–b** was then treated with *n*-octyllithium to give the diol **15a–b**, which was subsequently di-O-methylated to the octaether **16a–b**. Finally, addition of **16a–b** to the monoketone **11** in the presence of LiHMDS afforded the key bis-hexaoxy[6]pericylyne **17a–b** with 46–55% yields. Ultimate acidic reductive treatment of **17a–b** with SnCl₂/HCl allowed isolation of the *carbo*-stilbene **8a–b** with 46–53% yield, i. e. with 89–91% average yield for each of the seven butatriene edges formed in **8a–b**. The two *carbo*-stilbenes were isolated as black solids and found to be stable in solution, albeit poorly soluble despite the presence of two *n*-octyl chains, and two extra *tert*-butyl groups in **8b**, both types of substituents being previously reported to increase the solubility of *carbo*-benzene derivatives.[6,25]

The low solubility of both the *carbo*-stilbenes **8a** and **8b** prevented their full characterization, in particular by ¹³C NMR spectroscopy. ¹H and ¹H-¹H COSY NMR spectra could however be recorded, and ¹H-¹³C HSQC and HMBC experiments (at 600 MHz, 45 °C) could also be recorded for the slightly more soluble *carbo*-stilbene **8b**. The results allowed a tentative assignment of all the ¹H NMR signals, in spite of their duplication due to the simultaneous presence of the *cis*- and *trans*- isomers, the diastereoisomeric ratio being *ca* 52:48 for **8a**, 56:44 for **8b** (Figure 2 and S.I.).

<figure><img src="image_10.png" /><figcaption>**Figure 2.** a) Aromatic region of ¹H NMR spectra of the *carbo*-stilbenes **8a** (*R* = Ph, *bottom*) and **8b** (*R* = ¹Bu, *top*), in CDCl₃ solution (500 MHz, 55 °C). b) DFT-calculated equilibrium structures of *cis*-**8a** and *trans*-**8a** (see Table S4), possibly accounting for magnetic shielding effects on the *p*-C¹*H* nuclei of phenyl substituents made chemically inequivalent by restricted rotation of the C₁₈ rings with respect to the central DAB connector.</figcaption></figure>

From systematic variable temperature NMR experiments at 500 MHz, noteworthy is the absence of signal coalescence between 25 and 55 °C, suggesting that the stereo-conversion barrier should be greater the 20 kcal/mol. Owing to the extremely poor solubility of **8a** and **8b** in solvents other than CD₂Cl₂ or CDCl₃ and known thermal sensitivity of non-endo-aromatic butatriene derivatives, recording spectra at higher temperatures would not be feasible.

The most striking observation for one of the isomers (and only one, assigned to *cis*-**8a** or *cis*-**8b**) is the degeneracy lift of pairs of phenyl substituents, suggesting a blockage of the free rotation about the DAB–C₁₈ single bonds, which is allowed in the *trans*-isomer only. This is actually markedly evidenced in Density Functional Theory (DFT)-calculated isomers of **8a** and **8b** (see discussion below, Figure 2b, and legend of Figure 2).

As usually observed for centrosymmetric tetraphenyl-*carbo*-benzenes, the *p*-C¹*H* (and *m*-C¹*H*) nuclei of adjacent phenyl substituents have identical chemical shifts. Such an equivalence was however unexpected for the internal phenyl groups of *cis*-**8a** (and *cis*-**8b**), whose *p*-C¹*H* nuclei both resonate at 6.55 ppm in spite of their different positions from the facing C₁₈ ring in the calculated structure. This structure being rigid, the observed average equivalence at 55 °C could be tentatively attributed to flexibility in solution incidentally subjecting the two phenyl rings to identical average magnetic anisotropy effects. Moreover, using a classical software, such as NMR predict from Mestrenova 14.1.2-25024, the simulated NMR spectra of **8a** and **8b** are differentially shifted to high field (see S.I.), due to the
<figure><img src="image_15.png" /></figure>

fact that specific magnetic anisotropy effect of carbo-aromatic
rings are not (yet) implemented.

In both ¹H NMR spectra, deshielded doublet signals above
9 ppm are characteristic of o-C¹H nuclei of phenyl substituents
of carbo-benzenes (Figure 2a). At the opposite, a shielded triplet
signal at 6.55 ppm is here attributed to p-C¹H nuclei of
particular phenyl groups of the cis-isomer: for steric reason, one
of phenyl substituents of each C₁₈ ring in ortho position to the
DAB linker is indeed expected to lie in the shielding cone of the
facing C₁₈ ring, and vice-versa, in agreement with the DFT-
calculated equilibrium structures of 8a and 8b (see legend of
Figure 2 and S.I.). This feature being observed for both 8a and
8b, the shielded signals at 6.55 pm cannot be assigned to the
phenyl substituents of the DAB connector of 8a.

DFT calculations of **8a** and **8b** were thus performed at the B3PW91-D3/6-311G(d,p)-pcm//B3PW91/6-31G(d,p) level, by enforcing *C*₁ symmetry and *C*₂ symmetry for the *trans* and *cis* stereoisomers, respectively. The calculated Gibbs energy at 298.15 K of the *cis* $\rightleftharpoons$ *trans* equilibrium opted by -0.8 and 0.9 kcal/mol for **8a** and **8b**, respectively. The *cis* $\rightleftharpoons$ *trans* transition state failed to be located, but energy profiles vs incremental rotation did suggest an energy barrier of at least 20.0 and 18.9 kcal/mol for **8a** and **8b**, respectively (see Table S4).

The *trans*-isomers exhibit classical structural features for
both the tetraphenyl-octyl-*carbo*-benzene moieties and their
DAB connector. The equilibrium structure of **8a** thus exhibits a
roughly planar diphenyl-DAB core (with central and lateral
*sp*C–*sp*C bond distances of 1.248 and 1.222 Å, respectively, and
torsion angles of the phenyl substituents of *ca* 5.6°), along with
*quasi*-planar C₁₈ rings (with deviation from the mean plane in
the range 0.001-0.043 Å) of slightly dissymmetric hexagonal
shape (with n-OctC···C(DAB) and PhC···CPh diagonal through-
space distances of 8.07 and 7.82 Å, respectively).

As expected from the intramolecular steric hindrance, the cis-isomers exhibit a helical structure, with a distorted DAB core (with a global torsion of 9.4°) along with slightly more planar C₁₈ rings (with deviation from the mean plane in the range 0.001-0.039 Å) of slightly dissymmetric hexagonal shape (with n-OctC···C(DAB) and PhC···CPh diagonal distances of 7.796 Å and 8.087 ± 0.009 Å, respectively). Although the internal phenyl substituents at *ortho* positions of the two C₁₈ rings roughly overlap by parallel mean planes (see Figure 2b), no π stacking could be evidenced, the shortest through-space distances to the facing C₁₈ ring and to the symmetric phenyl substituent being C1···C2=4.567 Å (where C1 denotes a phenyl *m*-C atom, C2 denotes a *sp*C atom of the C₁₈ ring) and C1′···C2′=4.773 Å (where C1′ and C2′ denote *o*-C atoms of each phenyl ring), respectively.

Calculations were also made after truncating the octyl
chains to methyl groups in simplified versions **8aS1** and **8bS1**
(see Table S4), to check possible steric effects on the relative
thermodynamic stability or spectroscopic properties. No signifi-
cant differences were however obtained.

With the view to appraising specific effects of the carbo-
meric rings and DAB bridge, all the octyl and phenyl
substituents were replaced by H atoms in simplified versions

**8S2**, giving rise to almost isoenergetic *cis*- and *trans*-isomers,
but still with 0.1 kcal/mol in favor of the *trans* isomer (see
Table S4). Nevertheless, the isomerization would still require a
minimum of 16.9 kcal/mol.

In an ultimate simplification, the C₁₈ rings and phenyl DAB substituents of **8a** were replaced by hydrogen atoms in a bare DAB **8S3**, leading to an isomeric preference of 3.3 kcal/mol, while the barrier corresponding to the characterized transition state is reduced to 5.0 kcal/mol.

In conclusion, the isomerization is not impeded by the
central butatriene motif itself, but by the carbo-benzene rings
and substituents thereof.

Knowing the stronger magnetic aromatic character of C₁₈
rings with respect to C₆ phenyl rings from previous studies (as
indicated by comparison of ¹H NMR spectra at low field),[6,22,25]
the local aromaticity of carbo-stilbenes was assessed using the
nucleus-independent chemical shift (NICS), a metric introduced
by Schleyer and colleagues, serving as a general scalar
descriptor of algebraic magnetic (anti)aromaticity.[26] NICS is
defined as the negative absolute shielding value computed at a
ring center (NICS(0)), or 1 Å above or below it (NICS(1) and
NICS(−1)). Rings exhibiting substantial negative NICS values are
identified as aromatic.[27] NICS values were determined utilizing
the gauge-independent atomic orbital method (GIAO). The
magnetic shielding tensor was computed for ghost atoms
positioned at the center of the rings (or 1 Å above or below it),
as determined by the non-weighted mean of the heavy atom
coordinates.[28] Calculated NICS values of carbo-stilbenes sug-
gest a minimal impact of the DAB connector on the aromatic
nature of the C₁₈ rings. For **8a**, the NICS(1) values at the C₁₈
rings supports a slightly higher aromaticity in the *trans* isomer
than in the *cis*-isomer (−12.5 and −12.4 ppm respectively, to be
compared with a similar or identical value of −12.5 ppm for the
*carbo*-diphenyl **18**, all being significantly less aromatic than
unsubstituted *carbo*-benzene C₁₈H₆ (−16.8 ppm). Qualitatively,
the trend is the same with the classical NICS(0) index, again not
differentiating between the *cis*- and *trans*-isomers for **8a**. The
values for *cis*- and *trans*-**8b** are rather similar, but slightly larger
in absolute value (−12.7 and −12.6 ppm for the *cis* and *trans*
isomers, respectively), correlating with the known π-insulating
effect of *tert*-butyl groups with respect to phenyl groups when
conjugated to a C₁₈ ring,[25] even through a [3]cumulene linker
in the present case. In addition, comparison of the NICS(1)
values (Table S3) show that the DAB phenyl substituents of *cis*
and *trans* - **8a** (≈−9.8 ppm) are more aromatic by ca 1 ppm
than those of the C₁₈ rings. Changing two Ph groups by two
*tert*-Bu groups from **8a** to **8b**, the average NICS(1) values of the
peripheral phenyl and C₁₈ rings decrease by ca 0.2 ppm.

UV-vis absorption spectra of **8a** and **8b** were recorded in chloroform solutions. The two *carbo*-stilbenes exhibit comparable UV-vis spectra with two quite broad intense absorption bands at 459±4 and 550±3 nm, followed by a weaker band at longer wavelength (Figure 3). These spectral profiles are similar to that of the *carbo*-biphenyl **18**,[6] in which the two C₁₈ rings are conjugated by a C₂ ethynyl linker instead of the C₈ DAB linker: as expected, the more π-extended DAB linker induces a bathochromic shift. The stronger effect is observed for the small
<figure><img src="image_11.png" /></figure>

fact that specific magnetic anisotropy effect of carbo-aromatic
rings are not (yet) implemented.

In both ¹H NMR spectra, deshielded doublet signals above
9 ppm are characteristic of o-C'H nuclei of phenyl substituents
of carbo-benzenes (Figure 2a). At the opposite, a shielded triplet
signal at 6.55 ppm is here attributed to p-C'H nuclei of
particular phenyl groups of the cis-isomer: for steric reason, one
of phenyl substituents of each C₁₈ ring in ortho position to the
DAB linker is indeed expected to lie in the shielding cone of the
facing C₁₈ ring, and vice-versa, in agreement with the DFT-
calculated equilibrium structures of 8a and 8b (see legend of
Figure 2 and S.I.). This feature being observed for both 8a and
8b, the shielded signals at 6.55 pm cannot be assigned to the
phenyl substituents of the DAB connector of 8a.

DFT calculations of **8a** and **8b** were thus performed at the B3PW91-D3/6-311G(d,p)-pcm//B3PW91/6-31G(d,p) level, by enforcing C<sub>1</sub> symmetry and C<sub>2</sub> symmetry for the *trans* and *cis* stereoisomers, respectively. The calculated Gibbs energy at 298.15 K of the *cis* <i>↔</i> *trans* equilibrium opted by -0.8 and 0.9 kcal/mol for **8a** and **8b**, respectively. The *cis* <i>↔</i> *trans* transition state failed to be located, but energy profiles *vs* incremental rotation did suggest an energy barrier of at least 20.0 and 18.9 kcal/mol for **8a** and **8b**, respectively (see Table S4).

The *trans*-isomers exhibit classical structural features for
both the tetraphenyl-octyl-*carbo*-benzene moieties and their
DAB connector. The equilibrium structure of **8a** thus exhibits a
roughly planar diphenyl-DAB core (with central and lateral
*sp*C–*sp*C bond distances of 1.248 and 1.222 Å, respectively, and
torsion angles of the phenyl substituents of *ca* 5.6°), along with
*quasi*-planar C₁₈ rings (with deviation from the mean plane in
the range 0.001-0.043 Å) of slightly dissymmetric hexagonal
shape (with n-OctC···C(DAB) and PhC···CPh diagonal through-
space distances of 8.07 and 7.82 Å, respectively).

As expected from the intramolecular steric hindrance, the cis-isomers exhibit a helical structure, with a distorted DAB core (with a global torsion of 9.4°) along with slightly more planar C₁₈ rings (with deviation from the mean plane in the range 0.001-0.039 Å) of slightly dissymmetric hexagonal shape (with n-OctC···C(DAB) and PhC···CPh diagonal distances of 7.796 Å and 8.087 ± 0.009 Å, respectively). Although the internal phenyl substituents at *ortho* positions of the two C₁₈ rings roughly overlap by parallel mean planes (see Figure 2b), no π stacking could be evidenced, the shortest through-space distances to the facing C₁₈ ring and to the symmetric phenyl substituent being C1···C2=4.567 Å (where C1 denotes a phenyl *m*-C atom, C2 denotes a *sp*C atom of the C₁₈ ring) and C1′···C2′=4.773 Å (where C1' and C2' denote *o*-C atoms of each phenyl ring), respectively.

Calculations were also made after truncating the octyl
chains to methyl groups in simplified versions **8aS1** and **8bS1**
(see Table S4), to check possible steric effects on the relative
thermodynamic stability or spectroscopic properties. No signifi-
cant differences were however obtained.

With the view to appraising specific effects of the carbo-
meric rings and DAB bridge, all the octyl and phenyl
substituents were replaced by H atoms in simplified versions

8S2, giving rise to almost isoenergetic cis- and trans-isomers,
but still with 0.1 kcal/mol in favor of the *trans* isomer (see
Table S4). Nevertheless, the isomerization would still require a
minimum of 16.9 kcal/mol.

In an ultimate simplification, the C₁₈ rings and phenyl DAB substituents of **8a** were replaced by hydrogen atoms in a bare DAB **8S3**, leading to an isomeric preference of 3.3 kcal/mol, while the barrier corresponding to the characterized transition state is reduced to 5.0 kcal/mol.

In conclusion, the isomerization is not impeded by the
central butatriene motif itself, but by the carbo-benzene rings
and substituents thereof.

Knowing the stronger magnetic aromatic character of C₁₈
rings with respect to C₆ phenyl rings from previous studies (as
indicated by comparison of ¹H NMR spectra at low field),[6,22,25]
the local aromaticity of carbo-stilbenes was assessed using the
nucleus-independent chemical shift (NICS), a metric introduced
by Schleyer and colleagues, serving as a general scalar
descriptor of algebraic magnetic (anti)aromaticity.[26] NICS is
defined as the negative absolute shielding value computed at a
ring center (NICS(0)), or 1 Å above or below it (NICS(1) and
NICS(−1)). Rings exhibiting substantial negative NICS values are
identified as aromatic.[27] NICS values were determined utilizing
the gauge-independent atomic orbital method (GIAO). The
magnetic shielding tensor was computed for ghost atoms
positioned at the center of the rings (or 1 Å above or below it),
as determined by the non-weighted mean of the heavy atom
coordinates.[28] Calculated NICS values of carbo-stilbenes sug-
gest a minimal impact of the DAB connector on the aromatic
nature of the C₁₈ rings. For 8a, the NICS(1) values at the C₁₈
rings supports a slightly higher aromaticity in the *trans* isomer
than in the *cis*-isomer (−12.5 and −12.4 ppm respectively, to be
compared with a similar or identical value of −12.5 ppm for the
*carbo*-diphenyl **18**, all being significantly less aromatic than
unsubstituted *carbo*-benzene C₁₈H₆ (−16.8 ppm). Qualitatively,
the trend is the same with the classical NICS(0) index, again not
differentiating between the *cis*- and *trans*-isomers for **8a**. The
values for *cis*- and *trans*-8b are rather similar, but slightly larger
in absolute value (−12.7 and −12.6 ppm for the *cis* and *trans*
isomers, respectively), correlating with the known π-insulating
effect of *tert*-butyl groups with respect to phenyl groups when
conjugated to a C₁₈ ring,[25] even through a [3]cumulene linker
in the present case. In addition, comparison of the NICS(1)
values (Table S3) show that the DAB phenyl substituents of *cis*
and *trans* - **8a** (≈−9.8 ppm) are more aromatic by ca 1 ppm
than those of the C₁₈ rings. Changing two Ph groups by two
*tert*-Bu groups from **8a** to **8b**, the average NICS(1) values of the
peripheral phenyl and C₁₈ rings decrease by ca 0.2 ppm.

UV-vis absorption spectra of **8a** and **8b** were recorded in chloroform solutions. The two *carbo*-stilbenes exhibit comparable UV-vis spectra with two quite broad intense absorption bands at 459±4 and 550±3 nm, followed by a weaker band at longer wavelength (Figure 3). These spectral profiles are similar to that of the *carbo*-biphenyl **18**,[6] in which the two C₁₈ rings are conjugated by a C₂ ethynyl linker instead of the C₈ DAB linker: as expected, the more π-extended DAB linker induces a bathochromic shift. The stronger effect is observed for the small
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 3.** Normalized UV-visible absorption spectra of the carbo-stilbenes **8a** and **8b** and carbo-biphenyl **18** (in degassed CHCl₃).</figcaption></figure>

absorption bands in the 630–740 nm region, with a red shift of 74 nm from the carbo-biphenyl **18** to the carbo-stilbene **8a** suggesting the largest π-conjugation extent including the two phenyl substituents of the DAB bridge as compared to **8b**.

By comparison to carbo-biphenyl **18** (Figure 3), the UV-vis absorption spectrum of **8b** is slightly broader but not markedly resolved for the cis and trans isomers, tending to preclude significant photochromic behavior (see discussion in ref[1c] and references therein). For **8a**, while the second and third absorption bands of the cis and trans forms remain unresolved (at 553 and 735 nm), the maximum band at 455 nm is accompanied by a shoulder red-shifted by $\Delta\lambda = +20$ nm at 475 nm. This shoulder could actually be attributed to cis-**8a** by reference to the TD-DFT calculated values $\lambda_{calcd}(trans-8a) = 470$ nm and $\lambda_{calcd}(cis-8a) = 484$ nm, with $\Delta\lambda_{calcd} = 14$ nm (Table S1). Indeed, at the CAM–B3LYP–D3/6-311G(d,p)-smd(chloroform)//B3PW91/6-31G(d,p) time dependent DFT level, calculated absorption spectra reproduced the experimental ones in Figure 3, in particular the bathochromic displacement of the highest wavelength band for **8a** and **8b** with respect to **18** (see Table S1). This band is actually mainly associated to the HOMO→LUMO transition. The corresponding gap decreases from 3.42 eV for **18** to 3.34 and 3.20 eV for **8b** and **8a**, respectively, averaging the cis- and trans-conformations to take into account the coexistence of both. Going into detail, with respect to **18**, the HOMO of **8a** and **8b** destabilizes by 0.13 and 0.11 eV, respectively. The LUMO shows a disparate behavior: for **8b** it also destabilizes, by 0.03 eV, while for **8a** it stabilizes by 0.09 eV (see Table S2). This last factor accounts for the accentuated bathochromic shift of **8a** vs **8b**.

The electrochemical behavior of the carbo-stilbenes **8b** and **8b** was also studied by square wave voltammetry (Figure 4 and S.I.). Their low solubility did not allow satisfactory results in cyclic voltammetry. For still poor solubility reasons, the measurements were performed after deposition on the working electrode by dipping in a chloroform solution of **8b** or **8a**. Because of this unconventional method, only the first oxidation and reduction potentials could be determined (Table 1 and S.I.). In these conditions, a quasi-reversible reduction was observed at $E_{1/2} = -0.63$ V/SCE (**8b**) or $E_{1/2} = -0.53$ V/SCE (**8a**), and an irreversible oxidation was evidenced at 1.06 V/SCE (**8b**) or $E_{1/2} = 1.04$ V/SCE (**8a**). These results are in agreement with those previously obtained for other tetraphenyl-carbo-benzene derivatives, and in particular the carbo-biphenyl **18**, exhibiting a first reduction potential at $-0.58$ V/SCE.[21] The low reduction

<figure><img src="image_18.png" /><figcaption>**Figure 4.** Square-wave voltammograms (reduction scan) of **8a** and **8b** deposited on Pt electrode in electrolytic solution: CHCl₃ (10 mL) + 0.1 M ["Bu₄N][PF₆] at 0.1 V·s⁻¹. Oxidation scan in the S.I.</figcaption></figure>

<table><caption>**Table 1.** Electrochemical data for **8a** and **8b** deposited on a Pt microdisk at room temperature in CHCl₃, 0.1 M ["Bu₄N][PF₆] as supporting electrolyte. Potential values in V/SCE.</caption><thead><tr><th></th><th>E<sub>1/2</sub><br>red 3</th><th>E<sub>1/2</sub><br>red2</th><th>E<sub>1/2</sub><br>red1</th><th>E<sub>1/2</sub><br>ox1</th><th>E<sub>1/2</sub><br>ox2</th></tr></thead><tbody><tr><td><b>8b</b></td><td>-1.19</td><td>-1.04</td><td>-0.63</td><td>1.06</td><td>(1.22)**</td></tr><tr><td><b>8a</b></td><td>(-1.19)**</td><td>-0.99</td><td>-0.53</td><td>1.04*</td><td>1.28*</td></tr></tbody></table>

* shoulder. ** Uncertain values (see Figures 4 and S4.2).

potentials of both **8b** and **18** are consistent with their extended π-conjugated systems.

## Conclusions

In spite of many attempts, the failure to obtain single crystals of the carbo-stilbenes **8a** and **8b** with a suitable quality for X-ray diffraction analysis can be attributed to the presence of quasi-equal amounts of the cis- and trans-isomers, which could not be separated by either fractional crystallization or preparative chromatography. The absence of crystallographic characterization was however superseded by DFT calculations at suitable level allowing mimicry of experimental conditions, in particular by using the B3PW91-D3 and CAM–B3PYP–D3 functionals taking into account dispersive effects, and the PCM method taking into account a dichloromethane solvent medium.

The disclosed results extend the knowledge on oligo-carbo-benzenic derivatives, illustrating further the basic relevance of carbo-mer chemistry, and paving the way to future developments.

For example, in spite of the fact that the exemplified cis- and trans-carbo-stilbenes are quasi-isoenergetic, a next challenge would be to achieve some stereochemical control, e.g. by photo-isomerization at proper wavelengths, as in the case of parent stilbenes.[1,3]

Although the Oki temperature[29] of the first carbo-stilbenes **8a–b** remains unknown, it should be possible to trigger the cis
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 3.** Normalized UV-visible absorption spectra of the carbo-stilbenes **8a** and **8b** and carbo-biphenyl **18** (in degassed CHCl₃).</figcaption></figure>

absorption bands in the 630–740 nm region, with a red shift of 74 nm from the carbo-biphenyl **18** to the carbo-stilbene **8a** suggesting the largest π-conjugation extent including the two phenyl substituents of the DAB bridge as compared to **8b**.

By comparison to carbo-biphenyl **18** (Figure 3), the UV-vis absorption spectrum of **8b** is slightly broader but not markedly resolved for the cis and trans isomers, tending to preclude significant photochromic behavior (see discussion in ref[1c] and references therein). For **8a**, while the second and third absorption bands of the cis and trans forms remain unresolved (at 553 and 735 nm), the maximum band at 455 nm is accompanied by a shoulder red-shifted by $\Delta\lambda = +20$ nm at 475 nm. This shoulder could actually be attributed to cis-**8a** by reference to the TD-DFT calculated values $\lambda_{calcd}(trans-8a) = 470$ nm and $\lambda_{calcd}(cis-8a) = 484$ nm, with $\Delta\lambda_{calcd} = 14$ nm (Table S1). Indeed, at the CAM–B3LYP–D3/6-311G(d,p)-smd(chloroform)//B3PW91/6-31G(d,p) time dependent DFT level, calculated absorption spectra reproduced the experimental ones in Figure 3, in particular the bathochromic displacement of the highest wavelength band for **8a** and **8b** with respect to **18** (see Table S1). This band is actually mainly associated to the HOMO→LUMO transition. The corresponding gap decreases from 3.42 eV for **18** to 3.34 and 3.20 eV for **8b** and **8a**, respectively, averaging the cis- and trans-conformations to take into account the coexistence of both. Going into detail, with respect to **18**, the HOMO of **8a** and **8b** destabilizes by 0.13 and 0.11 eV, respectively. The LUMO shows a disparate behavior: for **8b** it also destabilizes, by 0.03 eV, while for **8a** it stabilizes by 0.09 eV (see Table S2). This last factor accounts for the accentuated bathochromic shift of **8a** vs **8b**.

The electrochemical behavior of the carbo-stilbenes **8b** and **8b** was also studied by square wave voltammetry (Figure 4 and S.I.). Their low solubility did not allow satisfactory results in cyclic voltammetry. For still poor solubility reasons, the measurements were performed after deposition on the working electrode by dipping in a chloroform solution of **8b** or **8a**. Because of this unconventional method, only the first oxidation and reduction potentials could be determined (Table 1 and S.I.). In these conditions, a quasi-reversible reduction was observed at $E_{1/2} = -0.63$ V/SCE (**8b**) or $E_{1/2} = -0.53$ V/SCE (**8a**), and an irreversible oxidation was evidenced at 1.06 V/SCE (**8b**) or $E_{1/2} = 1.04$ V/SCE (**8a**). These results are in agreement with those previously obtained for other tetraphenyl-carbo-benzene derivatives, and in particular the carbo-biphenyl **18**, exhibiting a first reduction potential at $-0.58$ V/SCE.[21] The low reduction

<figure><img src="image_14.png" /><figcaption>**Figure 4.** Square-wave voltammograms (reduction scan) of **8a** and **8b** deposited on Pt electrode in electrolytic solution: CHCl₃ (10 mL) + 0.1 M ["Bu₄N][PF₆] at 0.1 V·s⁻¹. Oxidation scan in the S.I.</figcaption></figure>

<table><caption>**Table 1.** Electrochemical data for **8a** and **8b** deposited on a Pt microdisk at room temperature in CHCl₃, 0.1 M ["Bu₄N][PF₆] as supporting electrolyte. Potential values in V/SCE.</caption><thead><tr><th></th><th>E<sub>1/2</sub><br>red 3</th><th>E<sub>1/2</sub><br>red2</th><th>E<sub>1/2</sub><br>red1</th><th>E<sub>1/2</sub><br>ox1</th><th>E<sub>1/2</sub><br>ox2</th></tr></thead><tbody><tr><td><b>8b</b></td><td>-1.19</td><td>-1.04</td><td>-0.63</td><td>1.06</td><td>(1.22)**</td></tr><tr><td><b>8a</b></td><td>(-1.19)**</td><td>-0.99</td><td>-0.53</td><td>1.04*</td><td>1.28*</td></tr></tbody></table>

* shoulder. ** Uncertain values (see Figures 4 and S4.2).

potentials of both **8b** and **18** are consistent with their extended π-conjugated systems.

## Conclusions

In spite of many attempts, the failure to obtain single crystals of the carbo-stilbenes **8a** and **8b** with a suitable quality for X-ray diffraction analysis can be attributed to the presence of quasi-equal amounts of the cis- and trans-isomers, which could not be separated by either fractional crystallization or preparative chromatography. The absence of crystallographic characterization was however superseded by DFT calculations at suitable level allowing mimicry of experimental conditions, in particular by using the B3PW91-D3 and CAM–B3PYP–D3 functionals taking into account dispersive effects, and the PCM method taking into account a dichloromethane solvent medium.

The disclosed results extend the knowledge on oligo-carbo-benzenic derivatives, illustrating further the basic relevance of carbo-mer chemistry, and paving the way to future developments.

For example, in spite of the fact that the exemplified cis- and trans-carbo-stilbenes are quasi-isoenergetic, a next challenge would be to achieve some stereochemical control, e.g. by photo-isomerization at proper wavelengths, as in the case of parent stilbenes.[1,3]

Although the Oki temperature[29] of the first carbo-stilbenes **8a–b** remains unknown, it should be possible to trigger the cis
<figure><img src="image_19.png" /></figure>

‡ trans interconversion photochemically, or even thermally above 80 °C. Under such conditions, or by a natural kinetic control during the last step of the synthesis (17a–b + 7 SnCl₂ – > 8a–b + 7 SnCl₂X₂, X = OH, OMe, see Scheme 2), a way to favor the *trans*-isomer would be to implement “bulky phenyl substituents” at the *carbo*-benzene rings (e.g. mesityl groups), as recently illustrated for a related geometrical control of helical polyanthracenes.[30] Alternatively, the *cis*-isomers might be favored by hydrogen bonding between hydroxylated phenyl substituents.

An ultimate way to favor *cis*-isomers would require the challenging synthesis of dissymmetric *carbo*-stilbenes with the view to enhancing π-stacking of complementary aryl substituents by charge transfer, e.g. between C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub> and C<sub>6</sub>H<sub>4</sub>NMe<sub>2</sub> groups. Even in the *trans* geometry, such donor-acceptor *carbo*-stilbenes could exhibit significant values of quadratic hyperpolarizability for applications in nonlinear optics, such as second harmonic generation, just as earlier predicted for donor-acceptor-substituted *carbo*-benzenes.[31]

## Acknowledgements

A.P. is a Serra Húnter Fellow and ICREA Academia Prize 2019, and thanks the Spanish Ministerio de Ciencia e Innovación for project PID2021-127423NB-I00 and the Generalitat de Catalunya for project 2021SGR623. C.Z. thanks the China Scholarship Council for his Ph.D. scholarship. V.M. thanks The Centre National de la Recherche Scientifique (CNRS). The work was partly performed thanks to funding from the Agence Nationale de la Recherche, which is acknowledged for supporting the related projects ANR- 11-BS07-016-01 and ANR-18-CE06-00029-01.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** aromaticity · *carbo*-benzene · DFT calculations · *carbo*-stilbene · stilbene

[1] Fore recent reviews on stilbene derivatives, see: a) *Stilbenes: Applications in Chemistry, Life Sciences and Materials Science*, G. Likhtenshtein, Wiley-VCH, Weinheim, 2009; b) E. A. Abourashed, *J. Nat. Prod.* **2017**, *80*, 577; c) D. Villaron, S. J. Wezenberg, *Angew. Chem. Int. Ed.* **2020**, *59*, 13192–13202; d) F. Doraghi, F. Yousefnejad, S. Farzipour, S. P. Aledavoud, B. Larijani, M. Mahdavi, *Org. Biomol. Chem.* **2023**, *21*, 1846–1861, and references therein.

[2] See for example: a) ref [1a] pp. 189–223; b) Y. Wang, Y. Jiang, X. Fan, H. Tan, H. Zeng, Y. Wang, P. Chen, M. Huang, H. Bi, *Toxicol. Lett.* **2015**, *236*, 82–89; c) E. Giacomini, S. Rupiani, L. Guidotti, M. Recanatini, M. Roberti,

Curr. Med. Chem. **2016**, *23*, 2439–2489; d) H. M. Wahedi, S. Ahmad, S. W. Abbasi, *J. Biomol. Struct. Dyn.* **2021**, *39*, 3225–3234; e) J.-Y. Chen, X. Lian, Y.-W. Fan, Z.-Y. Ao, W. J. Zhang, Y.-C. Pan, L.-P. Chen, J. Yuan, J.-W. Wu, *J. Nat. Med.* **2023**, *77*, 858–866; f) C.-G. Duta-Bratu, G. M. Nitulescu, D. P. Mihai, O. T. Olaru, *Plants* **2023**, *12*, 2935; g) M. Kluska, J. Jablonska, W. Prukala, *Molecules* **2023**, *28*, 4482, and references therein.

[3] See for example: a) ref [1a] pp. 159–188; b) M. Cao, X. Chen, K. Yi, D. Wei, *J. Phys. Chem. C* **2017**, *5*, 9597–9601; c) E. Kukkonen, E. Lahtinen, P. Myllyperkio, M. Haukka, J. Konu, *New J. Chem.* **2021**, *45*, 6640–6650; d) D. Zhang, H. Zhu, X. Sheng, *Phys. Chem. Chem. Phys.* **2023**, *25*, 7508–7518; e) Y.-G. Lee, J.-H. Choi, *Appl. Sci.* **2023**, *13*, 5543 (1-14); f) J. Rouillon, C. Andraud, C. Monnereau, *ChemRxiv.* **2023**, doi:10.26434/chemrxiv-2023-b9x37, and references therein.

[4] a) The term “carbo(–)m(M)er” has actually 3 different namesakes and spellings: (i) as understood in this article, an etymology-based name for a category of chemical structures defined in 1995,[4b] which was hyphenated in 2003,[4h] before being spelled, in 2004,[4i] with an italicized “carbo–” descriptor similar to standard Greek prefixes (*mono-*, *oligo-*, *poly-*, *iso-...*), non-figure locants (*ortho-*, *meta-*, *para-*, *α-*, *β-*, *γ-*, *ω-...*) or stereochemical indicators (*cis-*, *trans-*, *syn-*, *anti-*, *meso-*, *threo-*, *erythro-...*); (ii) a commercial trademark with the one-piece spelling “carbomer”, coined for hydrophilic polyacrylic gels used as emulsifying or thickening agents in cosmetics or toiletry products; (iii) an industrial company name with the doubly capitalized spelling “CarboMer”, leader in carbohydrate and polymer technology with extensive expertise in materials sciences (https://carbomer.com/); for early papers and reviews, see: b) Y. Kuwatani, N. Watanabe, I. Ueda, *Tetrahedron Lett.* **1995**, *36*, 119–122; c) R. Chauvin, *Tetrahedron Lett.* **1995**, *36*, 397–400; d) R. Suzuki, H. Tsukuda, N. Watanabe, Y. Kuwatani, I. Ueda, *Tetrahedron* **1998**, *54*, 2477–2496; e) V. Maraval, R. Chauvin, *Chem. Rev.* **2006**, *106*, 5317–5343; f) K. Cocq, C. Lepetit, V. Maraval, R. Chauvin, *Chem. Soc. Rev.* **2015**, *44*, 6535–6559; g) K. Cocq, C. Barthes, A. Rives, V. Maraval, R. Chauvin, *Synlett* **2019**, *30*, 30–43; h) C. Lepetit, M. B. Nielsen, F. Diederich, R. Chauvin, *Chem. Eur. J.* **2003**, *9*, 5056–5066; i) C. Lepetit, V. Peyrou, R. Chauvin, *Phys. Chem. Chem. Phys.* **2004**, *6*, 303–309.

[5] a) Cocq, N. Saffon-Merceron, Y. Coppel, C. Poidevin, V. Maraval, R. Chauvin, *Angew. Chem. Int. Ed.* **2016**, *55*, 15133–15136; b) J. Poater, J. Heitkämper, A. Poater, V. Maraval, R. Chauvin, *Eur. J. Org. Chem.* **2021**, 6450–6458.

[6] C. Zhu, A. Poater, C. Duhayon, B. Kauffmann, A. Saquet, V. Maraval, R. Chauvin, *Angew. Chem. Int. Ed.* **2018**, *57*, 5640–5644.

[7] E. L. Eliel, S. H. Wilen, M. P. Doyle, *Basic Organic Stereochemistry*, 16th Ed, Wiley, 2001.

[8] a) J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski, *Angew. Chem. Int. Ed.* **2013**, *52*, 1817–1821; b) D. Wendinger, R. R. Tykwinski, *Acc. Chem. Res.* **2017**, *50*, 1468–1479.

[9] J. E. Barquera-Lozada, *Chem. Eur. J.* **2020**, *26*, 4633–4639.

[10] a) Assuming a singlet spin state, the Pauli exclusion principle repels the unpaired electrons to the ends of the chain; b) The CRB of a [2 m]cumulene is much higher than that of the shorter [2 m-1]cumulene due to the 4-electron octet-deficient carbenic form R¹R²C⁻-(C=C)m⁻–C⁻:C*R¹R² vs the 2-electron octet-deficient form R¹R²C⁻-(C=C)m⁻–C⁻:C*R¹R².

[11] L. Leroyer, V. Maraval, R. Chauvin, *Chem. Rev.* **2012**, *112*, 1310–1343.

[12] P. D. Jarowski, F. Diederich, K. N. Houk, *J. Phys. Chem. A* **2006**, *110*, 7237–7246.

[13] P. Gawel, Y.-L. Wu, A. D. Finke, N. Trapp, M. Zalibera, C. Boudon, J.-P. Gisselbrecht, W. B. Schweizer, G. Gescheidt, F. Diederich, *Chem. Eur. J.* **2015**, *21*, 6215–6225.

[14] A. Auffrant, B. Jaun, P. D. Jarowski, K. N. Houk, F. Diederich, *Chem. Eur. J.* **2004**, *10*, 2906–2911.

[15] J.-D. van Loon, P. Seiler, F. Diederich, *Angew. Chem. Int. Ed.* **1993**, *32*, 1187–1189.

[16] A. Auffrant, F. Diederich, C. Boudon, J.-P. Gisselbrecht, M. Gross, *Helv. Chim. Acta* **2004**, *87*, 3085–3105.

[17] V. Maraval, L. Leroyer, A. Harano, C. Barthes, A. Saquet, C. Duhayon, T. Shinmyozu, R. Chauvin, *Chem. Eur. J.* **2011**, *17*, 5086–5100.

[18] A. Rives, V. Maraval, N. Saffon-Merceron, R. Chauvin, *Chem. Eur. J.* **2012**, *18*, 14702–14707.

[19] A. Rives, V. Maraval, N. Saffon-Merceron, R. Chauvin, *Chem. Eur. J.* **2014**, *20*, 483–492.

[20] A. Rives, I. Baglai, C. Barthes, V. Maraval, N. Saffon-Merceron, A. Saquet, Z. Voitenko, Y. Volovenko, R. Chauvin, *Chem. Sci.* **2015**, *6*, 1139–1149.

[21] C. Barthes, A. Rives, V. Maraval, E. Chelain, T. Brigaud, R. Chauvin, *Fr.Ukr. J. Chem.* **2015**, *3*, 60–65.
‡ trans interconversion photochemically, or even thermally above 80 °C. Under such conditions, or by a natural kinetic control during the last step of the synthesis (17a–b + 7 SnCl₂ – > 8a–b + 7 SnCl₂X₂, X = OH, OMe, see Scheme 2), a way to favor the *trans*-isomer would be to implement “bulky phenyl substituents” at the *carbo*-benzene rings (e.g. mesityl groups), as recently illustrated for a related geometrical control of helical polyanthracenes.[30] Alternatively, the *cis*-isomers might be favored by hydrogen bonding between hydroxylated phenyl substituents.

An ultimate way to favor *cis*-isomers would require the challenging synthesis of dissymmetric *carbo*-stilbenes with the view to enhancing π-stacking of complementary aryl substituents by charge transfer, e.g. between C₆H₄NO₂ and C₆H₄NMe₂ groups. Even in the *trans* geometry, such donor-acceptor *carbo*-stilbenes could exhibit significant values of quadratic hyperpolarizability for applications in nonlinear optics, such as second harmonic generation, just as earlier predicted for donor-acceptor-substituted *carbo*-benzenes.[31]

## Acknowledgements

A.P. is a Serra Húnter Fellow and ICREA Academia Prize 2019, and thanks the Spanish Ministerio de Ciencia e Innovación for project PID2021-127423NB-I00 and the Generalitat de Catalunya for project 2021SGR623. C.Z. thanks the China Scholarship Council for his Ph.D. scholarship. V.M. thanks The Centre National de la Recherche Scientifique (CNRS). The work was partly performed thanks to funding from the Agence Nationale de la Recherche, which is acknowledged for supporting the related projects ANR- 11-BS07-016-01 and ANR-18-CE06-00029-01.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** aromaticity · *carbo*-benzene · DFT calculations · *carbo*-stilbene · stilbene

[1] Fore recent reviews on stilbene derivatives, see: a) *Stilbenes: Applications in Chemistry, Life Sciences and Materials Science*, G. Likhtenshtein, Wiley-VCH, Weinheim, 2009; b) E. A. Abourashed, *J. Nat. Prod.* **2017**, *80*, 577; c) D. Villaron, S. J. Wezenberg, *Angew. Chem. Int. Ed.* **2020**, *59*, 13192–13202; d) F. Doraghi, F. Yousefnejad, S. Farzipour, S. P. Aledavoud, B. Larijani, M. Mahdavi, *Org. Biomol. Chem.* **2023**, *21*, 1846–1861, and references therein.

[2] See for example: a) ref [1a] pp. 189–223; b) Y. Wang, Y. Jiang, X. Fan, H. Tan, H. Zeng, Y. Wang, P. Chen, M. Huang, H. Bi, *Toxicol. Lett.* **2015**, *236*, 82–89; c) E. Giacomini, S. Rupiani, L. Guidotti, M. Recanatini, M. Roberti,

Curr. Med. Chem. **2016**, *23*, 2439–2489; d) H. M. Wahedi, S. Ahmad, S. W. Abbasi, *J. Biomol. Struct. Dyn.* **2021**, *39*, 3225–3234; e) J.-Y. Chen, X. Lian, Y.-W. Fan, Z.-Y. Ao, W. J. Zhang, Y.-C. Pan, L.-P. Chen, J. Yuan, J.-W. Wu, *J. Nat. Med.* **2023**, *77*, 858–866; f) C.-G. Duta-Bratu, G. M. Nitulescu, D. P. Mihai, O. T. Olaru, *Plants* **2023**, *12*, 2935; g) M. Kluska, J. Jablonska, W. Prukala, *Molecules* **2023**, *28*, 4482, and references therein.

[3] See for example: a) ref [1a] pp. 159–188; b) M. Cao, X. Chen, K. Yi, D. Wei, *J. Phys. Chem. C* **2017**, *5*, 9597–9601; c) E. Kukkonen, E. Lahtinen, P. Myllyperkio, M. Haukka, J. Konu, *New J. Chem.* **2021**, *45*, 6640–6650; d) D. Zhang, H. Zhu, X. Sheng, *Phys. Chem. Chem. Phys.* **2023**, *25*, 7508–7518; e) Y.-G. Lee, J.-H. Choi, *Appl. Sci.* **2023**, *13*, 5543 (1-14); f) J. Rouillon, C. Andraud, C. Monnereau, *ChemRxiv.* **2023**, doi:10.26434/chemrxiv-2023-b9x37, and references therein.

[4] a) The term “carbo(–)m(M)er” has actually 3 different namesakes and spellings: (i) as understood in this article, an etymology-based name for a category of chemical structures defined in 1995,[4b] which was hyphenated in 2003,[4h] before being spelled, in 2004,[4i] with an italicized “carbo–” descriptor similar to standard Greek prefixes (*mono-*, *oligo-*, *poly-*, *iso-...*), non-figure locants (*ortho-*, *meta-*, *para-*, *α-*, *β-*, *γ-*, *ω-...*) or stereochemical indicators (*cis-*, *trans-*, *syn-*, *anti-*, *meso-*, *threo-*, *erythro-...*); (ii) a commercial trademark with the one-piece spelling “carbomer”, coined for hydrophilic polyacrylic gels used as emulsifying or thickening agents in cosmetics or toiletry products; (iii) an industrial company name with the doubly capitalized spelling “CarboMer”, leader in carbohydrate and polymer technology with extensive expertise in materials sciences (https://carbomer.com/); for early papers and reviews, see: b) Y. Kuwatani, N. Watanabe, I. Ueda, *Tetrahedron Lett.* **1995**, *36*, 119–122; c) R. Chauvin, *Tetrahedron Lett.* **1995**, *36*, 397–400; d) R. Suzuki, H. Tsukuda, N. Watanabe, Y. Kuwatani, I. Ueda, *Tetrahedron* **1998**, *54*, 2477–2496; e) V. Maraval, R. Chauvin, *Chem. Rev.* **2006**, *106*, 5317–5343; f) K. Cocq, C. Lepetit, V. Maraval, R. Chauvin, *Chem. Soc. Rev.* **2015**, *44*, 6535–6559; g) K. Cocq, C. Barthes, A. Rives, V. Maraval, R. Chauvin, *Synlett* **2019**, *30*, 30–43; h) C. Lepetit, M. B. Nielsen, F. Diederich, R. Chauvin, *Chem. Eur. J.* **2003**, *9*, 5056–5066; i) C. Lepetit, V. Peyrou, R. Chauvin, *Phys. Chem. Chem. Phys.* **2004**, *6*, 303–309.

[5] a) Cocq, N. Saffon-Merceron, Y. Coppel, C. Poidevin, V. Maraval, R. Chauvin, *Angew. Chem. Int. Ed.* **2016**, *55*, 15133–15136; b) J. Poater, J. Heitkämper, A. Poater, V. Maraval, R. Chauvin, *Eur. J. Org. Chem.* **2021**, 6450–6458.

[6] C. Zhu, A. Poater, C. Duhayon, B. Kauffmann, A. Saquet, V. Maraval, R. Chauvin, *Angew. Chem. Int. Ed.* **2018**, *57*, 5640–5644.

[7] E. L. Eliel, S. H. Wilen, M. P. Doyle, *Basic Organic Stereochemistry*, 16th Ed, Wiley, 2001.

[8] a) J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski, *Angew. Chem. Int. Ed.* **2013**, *52*, 1817–1821; b) D. Wendinger, R. R. Tykwinski, *Acc. Chem. Res.* **2017**, *50*, 1468–1479.

[9] J. E. Barquera-Lozada, *Chem. Eur. J.* **2020**, *26*, 4633–4639.

[10] a) Assuming a singlet spin state, the Pauli exclusion principle repels the unpaired electrons to the ends of the chain; b) The CRB of a [2 m]cumulene is much higher than that of the shorter [2 m-1]cumulene due to the 4-electron octet-deficient carbenic form R¹R²C⁺–(C=C)m⁻–C⁻:C⁺R¹R² vs the 2-electron octet-deficient form R¹R²C⁻–(C=C)m⁻–C⁻R¹R².

[11] L. Leroyer, V. Maraval, R. Chauvin, *Chem. Rev.* **2012**, *112*, 1310–1343.

[12] P. D. Jarowski, F. Diederich, K. N. Houk, *J. Phys. Chem. A* **2006**, *110*, 7237–7246.

[13] P. Gawel, Y.-L. Wu, A. D. Finke, N. Trapp, M. Zalibera, C. Boudon, J.-P. Gisselbrecht, W. B. Schweizer, G. Gescheidt, F. Diederich, *Chem. Eur. J.* **2015**, *21*, 6215–6225.

[14] A. Auffrant, B. Jaun, P. D. Jarowski, K. N. Houk, F. Diederich, *Chem. Eur. J.* **2004**, *10*, 2906–2911.

[15] J.-D. van Loon, P. Seiler, F. Diederich, *Angew. Chem. Int. Ed.* **1993**, *32*, 1187–1189.

[16] A. Auffrant, F. Diederich, C. Boudon, J.-P. Gisselbrecht, M. Gross, *Helv. Chim. Acta* **2004**, *87*, 3085–3105.

[17] V. Maraval, L. Leroyer, A. Harano, C. Barthes, A. Saquet, C. Duhayon, T. Shinmyozu, R. Chauvin, *Chem. Eur. J.* **2011**, *17*, 5086–5100.

[18] A. Rives, V. Maraval, N. Saffon-Merceron, R. Chauvin, *Chem. Eur. J.* **2012**, *18*, 14702–14707.

[19] A. Rives, V. Maraval, N. Saffon-Merceron, R. Chauvin, *Chem. Eur. J.* **2014**, *20*, 483–492.

[20] A. Rives, I. Baglai, C. Barthes, V. Maraval, N. Saffon-Merceron, A. Saquet, Z. Voitenko, Y. Volovenko, R. Chauvin, *Chem. Sci.* **2015**, *6*, 1139–1149.

[21] C. Barthes, A. Rives, V. Maraval, E. Chelain, T. Brigaud, R. Chauvin, *Fr.Ukr. J. Chem.* **2015**, *3*, 60–65.
<figure><img src="image_20.png" /></figure>

[22] C. Zhu, A. Poater, C. Duhayon, B. Kauffmann, A. Saquet, V. Maraval, A. Rives, R. Chauvin, *Chem. Eur. J.* **2021**, *27*, 9286–9291.

[23] R. Chauvin, *Tetrahedron Lett.* **1995**, *36*, 401–404.

[24] L. Leroyer, C. Zou, V. Maraval, R. Chauvin, C. R. Chim. **2009**, *12*, 412–429.

[25] D. Listunov, C. Duhayon, A. Poater, S. Mazeres, A. Saquet, V. Maraval, R. Chauvin, *Chem. Eur. J.* **2018**, *24*, 10699–10710.

[26] a) P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. van Eike-ma Hommes, *J. Am. Chem. Soc.* **1996**, *118*, 6317–6318; b) Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta, P. v. R. Schleyer, *Chem. Rev.* **2005**, *105*, 3842–3888.

[27] Examples of NICS applications in different nature and ring sizes, where NICS values are not normalized: a) P. D. Listunov, O. Hammerich, I. Caballero-Quintana, A. Poater, C. Barthes, C. Duhayon, M. H. Larsen, J.-L. Maldonado, G. Ramos-Ortiz, M. B. Nielsen, V. Maraval, R. Chauvin, *Chem. Eur. J.* **2020**, *26*, 10707–10711; b) A. Poater, S. Moradell, E. Pinilla, J. Poater, M. Solà, M. A. Martínez, A. Llobet, *Dalton Trans.* **2006**, 1188–1196; c) A. Poater, X. Ribas, A. Llobet, L. Cavallo, M. Solà, *J. Am. Chem. Soc.* **2008**, *130*, 17710–17717; d) R. Credendino, A. Poater, F. Ragone, L. Cavallo, *Catal. Sci. Technol.* **2011**, *1*, 1287–1297; e) J. Poater, M. Gimferrer, A. Poater, *Inorg. Chem.* **2018**, *57*, 6981–6990; f) J. Poater, S. Escayola, A. Poater, F. Teixidor, H. Ottosson, C. Viñas, M. Solà, *J. Am. Chem. Soc.* **2023**, *145*, 22527–22538.

[28] F. Feixas, E. Matito, J. Poater, M. Solà, *J. Comput. Chem.* **2008**, *29*, 1543–1554.

[29] M. Oki in *Topics in Stereochemistry*, Vol. 14 (Eds.: N. L. Allinger, E. L. Eliel, S. H. Wilen), Wiley, New York, 1983, pp. 1–81.

[30] K. Suzuki, H. Fukuda, H. Toda, Y. Imai, Y. Nojima, M. Hasegawa, E. Tsurumaki, S. Toyota, *Tetrahedron* **2023**, *132*, 133243.

[31] a) J.-M. Ducere, C. Lepetit, P. G. Lacroix, J.-L. Heully, R. Chauvin, *Chem. Mater.* **2002**, *14*, 3332–3338; b) C. Lepetit, P. G. Lacroix, V. Peyrou, C. Saccavini, R. Chauvin, *J. Comput. Methods Sci. Eng.* **2004**, *4*, 569–588.

Manuscript received: January 31, 2024

Accepted manuscript online: February 26, 2024

Version of record online: ■■, ■■
<figure><img src="image_15.png" /></figure>

[22] C. Zhu, A. Poater, C. Duhayon, B. Kauffmann, A. Saquet, V. Maraval, A. Rives, R. Chauvin, *Chem. Eur. J.* **2021**, *27*, 9286–9291.

[23] R. Chauvin, *Tetrahedron Lett.* **1995**, *36*, 401–404.

[24] L. Leroyer, C. Zou, V. Maraval, R. Chauvin, C. R. Chim. **2009**, *12*, 412–429.

[25] D. Listunov, C. Duhayon, A. Poater, S. Mazeres, A. Saquet, V. Maraval, R. Chauvin, *Chem. Eur. J.* **2018**, *24*, 10699–10710.

[26] a) P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. van Eike-ma Hommes, *J. Am. Chem. Soc.* **1996**, *118*, 6317–6318; b) Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta, P. v. R. Schleyer, *Chem. Rev.* **2005**, *105*, 3842–3888.

[27] Examples of NICS applications in different nature and ring sizes, where NICS values are not normalized: a) P. D. Listunov, O. Hammerich, I. Caballero-Quintana, A. Poater, C. Barthes, C. Duhayon, M. H. Larsen, J.-L. Maldonado, G. Ramos-Ortiz, M. B. Nielsen, V. Maraval, R. Chauvin, *Chem. Eur. J.* **2020**, *26*, 10707–10711; b) A. Poater, S. Moradell, E. Pinilla, J. Poater, M. Solà, M. A. Martínez, A. Llobet, *Dalton Trans.* **2006**, 1188–1196; c) A. Poater, X. Ribas, A. Llobet, L. Cavallo, M. Solà, *J. Am. Chem. Soc.* **2008**, *130*, 17710–17717; d) R. Credendino, A. Poater, F. Ragone, L. Cavallo, *Catal. Sci. Technol.* **2011**, *1*, 1287–1297; e) J. Poater, M. Gimferrer, A. Poater, *Inorg. Chem.* **2018**, *57*, 6981–6990; f) J. Poater, S. Escayola, A. Poater, F. Teixidor, H. Ottosson, C. Viñas, M. Solà, *J. Am. Chem. Soc.* **2023**, *145*, 22527–22538.

[28] F. Feixas, E. Matito, J. Poater, M. Solà, *J. Comput. Chem.* **2008**, *29*, 1543–1554.

[29] M. Oki in *Topics in Stereochemistry*, Vol. 14 (Eds.: N. L. Allinger, E. L. Eliel, S. H. Wilen), Wiley, New York, 1983, pp. 1–81.

[30] K. Suzuki, H. Fukuda, H. Toda, Y. Imai, Y. Nojima, M. Hasegawa, E. Tsurumaki, S. Toyota, *Tetrahedron* **2023**, *132*, 133243.

[31] a) J.-M. Ducere, C. Lepetit, P. G. Lacroix, J.-L. Heully, R. Chauvin, *Chem. Mater.* **2002**, *14*, 3332–3338; b) C. Lepetit, P. G. Lacroix, V. Peyrou, C. Saccavini, R. Chauvin, *J. Comput. Methods Sci. Eng.* **2004**, *4*, 569–588.

Manuscript received: January 31, 2024

Accepted manuscript online: February 26, 2024

Version of record online: ■■, ■■
<figure><img src="image_21.png" /></figure>

carbo-stilbene synthesis & stereoconversion

Dr. C. Zhu, Dr. A. Saquet, Dr. V. Maraval, Dr. C. Bijani, Prof. X. Cui, Dr. A. Poater*, Prof. R. Chauvin*

1 – 8

From Stilbenes to carbo-Stilbenes:
an Encouraging Prospect

<figure><img src="image_22.png" /></figure>

Two novel bis-carbo-benzenic
molecules, derived from the stilbene
parent, are explored beyond carbo-
naphthalene and carbo-biphenyl. The
synthesis, structure, and properties of
these carbo-stilbenes are investigated
using experimental and computation-
al methods. The molecules exhibit iso-

merization, with *cis*- and *trans*-isomers, and the calculated *cis*-isomer shows a helical structure. The UV-vis spectra display broad bands, and vol-tammograms indicate classical behavior for *carbo*-benzene deriva-tives.
<figure><img src="image_16.png" /></figure>

carbo-stilbene synthesis & stereoconversion

Dr. C. Zhu, Dr. A. Saquet, Dr. V. Maraval, Dr. C. Bijani, Prof. X. Cui, Dr. A. Poater*, Prof. R. Chauvin*

1 – 8

From Stilbenes to carbo-Stilbenes:
an Encouraging Prospect

<figure><img src="image_17.png" /></figure>

Two novel bis-carbo-benzenic
molecules, derived from the stilbene
parent, are explored beyond carbo-
naphthalene and carbo-biphenyl. The
synthesis, structure, and properties of
these carbo-stilbenes are investigated
using experimental and computation-
al methods. The molecules exhibit iso-

merization, with *cis*- and *trans*-isomers, and the calculated *cis*-isomer shows a helical structure. The UV-vis spectra display broad bands, and voltammograms indicate classical behavior for *carbo*-benzene derivatives.