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. 2025 Jul 28;27(31):8668–8673. doi: 10.1021/acs.orglett.5c02610

Substantial Effects of Ring Size on the Structure and Properties of 2,9-Anthrylene Cyclic Oligomers

Yuki Shigihara 1, Eiji Tsurumaki 1, Masahiro Yamashina 1, Shinji Toyota 1,*
PMCID: PMC12340951  PMID: 40717408

Abstract

Cyclic oligomers consisting of unsymmetrical anthracene units were designed as novel aromatic compounds. The Suzuki–Miyaura coupling of a 2-boryl-9-bromoanthracene precursor afforded the cyclic trimer and tetramer. X-ray crystallographic analysis revealed that the trimer took a highly strained propeller-like structure with short H···H contacts. In contrast, the tetramer took a saddle-shaped structure with minimal strain. Effects of ring size on UV–vis and fluorescence spectra and dynamic behavior are discussed in relation to the molecular structures.

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Arenes are versatile building units for constructing three-dimensional structures by modifying the kind and number of aromatic units and their connection patterns. For phenylene oligomers, the aromatic units can be linked through three connection patterns, the ortho, meta, and para positions, at variable angles. For example, cyclo-p-phenylenes (CPPs), in which all of the benzene units are connected at para positions, have been extensively studied as carbon nanotube analogues. We have adopted anthracene as an arene unit to design novel π-conjugated macrocyclic compounds. This rectangular aromatic unit can be connected at various positions, resulting in different directions and distances. The 9,10-anthrylene unit (A 9,10 ; hereafter, the x,y-anthrylene unit is abbreviated as A x,y ) and A 2,6 unit are linear or nearly linear, , whereas A 1,8 and A 2,7 units are nonlinear (Figure a). We synthesized hexagonal cyclic hexamer [6]­CA 2,7 by connecting V-shaped A 2,7 units directly and found that this molecule functions as a macrocyclic aromatic host for including fullerene guests. In contrast, unsymmetrical anthracene units are much less popular than symmetrical anthracene units, mainly because of the limited synthetic approach to such building units. For example, the A 2,9 unit is a bent aromatic unit, in which the two bonds extend at an angle of 60°. However, the two bonds are positioned farther apart than those in an o-phenylene unit. Some acyclic 2,9-diaryl or diethynylanthracene derivatives with two A 2,9 units have been synthesized from various starting materials. , However, the synthetic methods are not always applicable to the synthesis of other oligomers. Therefore, in this study, we designed cyclic oligomers [n]­CA 2,9 consisting of n A 2,9 units, which were synthesized from a general precursor through a coupling reaction (Figure b). We obtained the cyclic trimer [3]­CA 2,9 and cyclic tetramer [4]­CA 2,9 , which have characteristic structures dependent on their ring size. These structures are related to cyclic quinoline oligomers TQ and TEQ reported by Kumagai et al. (Figure c). These heteroaromatic compounds have N atoms in the inner region, whereas [n]­CA 2,9 oligomers have H atoms pointing toward the center. As expected from the molecular models, these H atoms would result in severe steric hindrance for the cyclic trimer. This steric environment is comparable to that in recently reported cyclic fused anthracene C­[3]­HA, which has very short H···H nonbonding contacts (ca. 1.65 Å). By contrast, cyclic tetramer [4]­CA 2,9 should have a saddle-shaped structure similar to that of quinoline cyclic tetramer TEQ. We report herein the structures, properties, and dynamic behavior of unsymmetrically substituted anthracene cyclic oligomers [3]­CA 2,9 and [4]­CA 2,9 and discuss the substantial effects of ring size on these compounds.

1.

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(a) Shapes of typical anthrylene units, (b) target anthrylene unit and cyclic oligomers, and (c) related anthracene and quinoline cyclic compounds (substituents have been omitted from TEQ). Mes is mesityl (2,4,6-trimethylphenyl).

We prepared 2-boryl-9-bromoanthracene 1 as a building unit for the Suzuki–Miyaura coupling through the route shown in Figure a. 1,8-Dimesitylanthracene (2) was brominated with NBS to give bromoanthracene 3. This compound was borylated by Hartwig–Miyaura reaction with 1.24 equiv of bis­(pinacolato)­diboron (Bpin)2 to give a mixture of mono- and diborylated products. The desired compound 1 was separated in 50% isolated yield by chromatography on silica gel treated with boric acid to avoid unfavorable tailing. Suzuki–Miyaura coupling of 1 was performed with PdCl2(dppf)·CH2Cl2 and K2CO3 to give a mixture of several coupling products. Separation of the crude products by recycling GPC afforded cyclic trimer [3]­CA 2,9 as a dark red solid in 8.4% yield and cyclic tetramer [4]­CA 2,9 as a yellow solid in 7.2% yield. The difference in color between the two compounds is notable. These cyclic oligomers were reasonably characterized on the basis of their NMR and mass spectra (Supporting Information).

2.

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(a) Synthesis of cyclic oligomers [3]­CA 2,9 and [4]­CA 2,9 by Suzuki–Miyaura coupling of 1. (b) 1H NMR (CDCl3), (c) UV–vis (CHCl3), and (d) fluorescence (CHCl3) spectra of [3]­CA 2,9 and [4]­CA 2,9 . Spectra of 2 are compared in panels c and d.

The 1H NMR spectra of [3]­CA 2,9 and [4]­CA 2,9 were measured in CDCl3 (Figure b). Both compounds gave only one set of anthracene signals: three singlets (H1, H3, and H10) and one ABC system of signals (H6–H8). Signals due to the inner H1 atoms were observed at 10.74 and 8.64 ppm for [3]­CA 2,9 and [4]­CA 2,9 , respectively. The significant deshielding of the former signal is attributable to the steric compression effect , and the ring current effect. The observed chemical shifts are supported by the calculated chemical shifts by the GIAO (gauge-independent atomic orbital) method (Figure S8). For [3]­CA 2,9 , the two singlets at ca. 6.9 ppm were assigned to m-H atoms in the 4- and 5-Mes groups. In contrast, the corresponding signals were observed as two sets of two singlets for [4]­CA 2,9 , although two of the singlets overlapped (Figure S25). This means that the two m-H atoms in each Mes group are nonequivalent on the NMR time scale. A similar pattern was also observed for the o-Me groups. This difference will be discussed below in relation to the molecular structures.

The UV–vis and fluorescence spectra of the cyclic oligomers were measured in CHCl3 (Figure c). The UV–vis spectra showed broad absorption bands in the p-band region, with absorption maxima at 431 and 393 nm for [3]­CA 2,9 and [4]­CA 2,9 , respectively. These bands were red-shifted compared to that of 1,8-dimesitylanthracene (2), indicating the extended π-conjugation due to the coupling of the anthracene units. This tendency is more significant for the trimer than for the tetramer and is consistent with the dihedral angles between the anthracene units in the molecular structures mentioned below. The fluorescence spectra showed broad emission bands at 488 and 463 nm for [3]­CA 2,9 and [4]­CA 2,9 , respectively (Figure d). Notably, the fluorescence quantum yield of the tetramer (Φf = 0.66) is much larger than those of the other compounds.

X-ray crystallographic analysis was performed using single crystals of [3]­CA 2,9 and [4]­CA 2,9 . For the structural analysis of [3]­CA 2,9 , H atoms were refined anisotropically using NoSpherA2 analysis to determine reliable positions. Trimer [3]­CA 2,9 takes a propeller-like C 1 symmetric structure rather than the ideal C 3 symmetric structure (Figure a–c). The C9a–C9–C2–C1 torsion angles along the single bonds connecting the three anthracene units (X, Y, and Z) are θ 1 = +48.6°, θ 2 = −44.9°, and θ 3 = −32.9° (|θ|ave = 42.1°). We designate this conformation as PMM′, where the primed symbol means an angle with the same sign close to the coplanar conformation. Each mesityl group takes a nearly bisecting conformation relative to the anthracene moiety to minimize steric hindrance. The side view reveals that the aromatic framework is deformed significantly out of plane, forming a strained cyclic structure. The aromatic moieties around the C9 X and C2 Y atoms are particularly deformed, with torsion angles along the aromatic rims reaching ca. 24° (0° or 180° for a planar structure) (Figure S3). The inner region of the cyclic structure is congested owing to the presence of three H atoms in the inner 12-membered ring. Two of them (H1 X and H1 Z ) point upward, and one atom (H1 Y ) points downward with respect to the inner 12-membered ring (Figure b). Notably, the interatomic H1 X ···H1 Z distance, 1.74(3) Å, is much smaller than the sum of the van der Waals radii of the hydrogens (2.40 Å). This short nonbonding H···H contact is comparable to that observed in C­[3]­HA and other compounds. , This steric hindrance results in the nonplanar and strained aromatic framework, in contrast to the nearly planar TQ that lacks inner H atoms.

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ORTEP drawings of X-ray structures with thermal ellipsoids at the 50% probability level. [3]­CA 2,9 : (a) full structure, (b) expanded structure around anisotropically refined inner H atoms, and (c) side view without Mes groups. [4]­CA 2,9 : (d) full structure, (e) top view without Mes groups, and (f) side view without Mes groups. The numbering is shown in panels a and e. Solvent molecules, CH2Cl2 for [3]­CA 2,9 and PhCl for [4]­CA 2,9 , have been omitted for the sake of clarity.

In contrast, tetramer [4]­CA 2,9 takes a saddle-shaped structure of approximately S 4 symmetry (Figure d–f). All anthracene units are nearly planar, and the torsion angles along single bonds θ 1–θ 4 are alternately ca. 68° and −68° (|θ|ave = 68.1°), namely, the MPMP form. Because these values are comparable to the corresponding angle in 2,9′-bianthracene BA (±72.1° (Figure S11)), the torsional strain between the anthracene units should be small in the cyclic structure. Compared with the trimer, this conformation is close to the bisecting conformation, thereby reducing π-conjugation across the single bonds. In the saddle-shaped framework, the two m-H atoms in each Mes group are in different environments within the macrocyclic structure. The observed NMR spectra are consistent with those of this structure.

The structures of cyclic oligomers [3]­CA 2,9 and [4]­CA 2,9 and their Mes-free compounds ([3]­CA 2,9 -H and [4]­CA 2,9 -H, respectively) were optimized by the DFT method at the B3LYP-D3/6-31G­(d,p) level. The calculations reproduced the macrocyclic frameworks in the observed structures involving the dihedral angles and the deformation of anthracene units (Figures S4 and S5). We utilized the optimized structures of the Mes-free compounds to visualize intermolecular interactions through noncovalent interaction (NCI) analysis (Figure a). In the NCI plot of [3]­CA 2,9 -H, there are wide isosurfaces, indicating interactions between the inner H atoms. By contrast, the isosurfaces in the inner region are insignificant in the NCI plot of [4]­CA 2,9 -H. The strain energies of the two molecules were evaluated by the two methods. For the homodesmotic reaction approach, we adopted the reaction shown in Figure b. The calculated enthalpies of reaction (ΔH) at the B3LYP-D3/6-31G­(d,p) level are −152.5 and −9.3 kJ mol–1 for [3]­CA 2,9 -H and [4]­CA 2,9 -H, respectively (Table S6). Assuming that BA and TA are free of strain, the −ΔH values approximately correspond to the strain energies of the cyclic oligomers. Therefore, the large −ΔH value (152.5 kJ mol–1) of [3]­CA 2,9 -H indicates the presence of severe strain. We also adopted StrainViz analysis to visualize molecular strain (Figure c and Figures S9 and S10). In [3]­CA 2,9 -H, the strain is mostly distributed around one of the C–C bonds, as indicated by the red color. The calculated strain energy using this method is 134 kJ mol–1, comparable to that of C­[3]­HA (126 kJ mol–1) calculated by the same method. These values are smaller than those of [2.2]­paracyclophane (156 kJ mol–1) and [10]­CPP (240 kJ mol–1) as typical isolable strained cyclic compounds. On the other hand, the strain is small for [4]­CA 2,9 -H (20 kJ mol–1).

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(a) Noncovalent interaction (NCI) plots, (b) homodesmotic reactions, and (c) StrainViz analysis of [3]­CA 2,9 -H and [4]­CA 2,9 -H. Calculations were carried out at the B3LYP-D3/6-31G­(d,p) level.

The molecular orbitals and energies of [3]­CA 2,9 and [4]­CA 2,9 were calculated at the PBE0/6-31G­(d,p)//B3LYP-D3/6-31G­(d,p) level (Figure a). At the HOMO (H) and LUMO (L) levels of the two cyclic compounds, the orbital lobes spread over all of the anthracene moieties. The H–L gap decreased in the following order: 2 (3.86 eV (Figure S6)) > [4]­CA 2,9 (3.35 eV) > [3]­CA 2,9 (2.66 eV). The narrow gap of [3]­CA 2,9 resulted from the increased H and decreased L levels caused by the less twisted conformation between the anthracene units. Time-dependent (TD) DFT calculations of [3]­CA 2,9 and [4]­CA 2,9 at the same level reasonably reproduced the observed UV–vis spectra (Figure S7). For [3]­CA 2,9 , the H → L (S0 → S1) excitation was nearly forbidden, and weak absorption due to H – 1 → L (S0 → S2) at 471 nm was predicted. For [4]­CA 2,9 , the observable absorption band at the longest wavelength was assigned to H → L + 1/L + 2 and H – 2/H – 1 → L excitations at 410–420 nm.

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(a) Frontier orbital diagrams and energies of [3]­CA 2,9 and [4]­CA 2,9 calculated at the PBE0/6-31G­(d,p)//B3LYP-D3/6-31G­(d,p) level. Additional data are provided in Figure S6. (b) Proposed mechanism of the dynamic behavior of [3]­CA 2,9 -H calculated by the NEB method and the GFN2-xTB method (ALPB: CHCl3). (c) Energy profile during flipping processes around the mechanism. GM represents the global minimum, TS the transition state, and Int the intermediate.

The dynamic behavior of the cyclic oligomers was analyzed using theoretical methods for the Mes-free compounds. Transition states (TSs) were searched using the nudged elastic band (NEB) method and the GFN2-xTB method. A proposed mechanism of the dynamic processes of [3]­CA 2,9 -H is shown in Figure b. The PMM′ form (GM1), one of the global-minimum (GM) structures, is converted into the P′MP form (GM2) via concurrent rotation of the three single bonds and the downward flipping of the H1 X atom (red). Alternatively, GM1 is similarly converted into the PP′M form (GM6) via the downward flipping of the H1 Z atom (green). Because GM2 and GM6 are enantiomers of the original GM1, these are enantiomerization processes that can occur continuously, resulting in circular conformational exchanges among six structures, GM1–GM6, which are topomers orenantiomers. Each time a process occurs, the combination of closely contacting inner H atoms, indicated by red, blue, and green spheres, changes. Figure c shows the energy profile of the overall processes, where all of the TSs are less stable by 29 kJ mol–1 than the GM structure at 298 K. Notably, the energy change curve for each step is not symmetric; the clockwise and counterclockwise processes have different energy gradients. The barrier is so low that the flipping process should occur rapidly on the NMR time scale. In fact, the fluxional nature of [3]­CA 2,9 is supported by the symmetric NMR signal pattern even at −50 °C in CDCl3 (Figure S24).

We also searched the TS of the flipping process of [4]­CA 2,9 -H, which exchanges one saddle form (MPMP) with another form (PMPM), using the same method (Figure S13). We found a TS, which was less stable by ca. 200 kJ mol–1 than the GM. The very high barrier means that the saddle-to-saddle flipping process is highly unlikely under ordinary conditions. Within the rigid framework, the two m-H atoms in each Mes group are diastereotopic: one is inside, and the other is outside. This signal pattern was retained even at 135 °C in the 1H NMR spectra of [4]­CA 2,9 (Figure S25). Therefore, both the flipping and the rotation of the Mes group (>200 kJ mol–1) are very slow on the NMR time scale.

In conclusion, we synthesized the 2,9-anthrylene cyclic trimer and tetramer through Suzuki–Miyaura coupling of the 9-bromo-2-borylanthracene precursor. The cyclic trimer had a considerably strained cyclic structure and sterically congested inner H atoms, undergoing facile flipping among several energy-minimum structures. In contrast, the cyclic tetramer had a rigid saddle-shaped structure without flipping. The extended π-conjugation for the cyclic trimer is explained by the difference in the conformations of the anthracene units. Therefore, the structure, properties, and dynamic behavior of the cyclic oligomers are significantly influenced by the number of aromatic units owing to geometrical constraints. We can design other novel aromatic compounds by using the unsymmetrical bent anthracene unit. Further studies on the synthesis of large cyclic oligomers and their chiral derivatives as well as their supramolecular behavior and optical and chiroptical properties are underway.

Supplementary Material

ol5c02610_si_001.pdf (8.3MB, pdf)

Acknowledgments

This work was partly supported by JSPS KAKENHI Grants JP23H01944 and JP23K26637 (S.T.) and JP22K05077 (E.T.). The X-ray crystallographic analysis was performed under the approval of the Photon Factory Program Advisory Committee (Proposal 2024G540). The authors thank the Institute of Science Tokyo Core Facility Center for performing high-resolution mass spectrometry and Prof. Kimihisa Yamamoto (Institute of Science Tokyo) for fluorescence lifetime analysis.

The data underlying this study are available in the published article and its Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.5c02610.

  • Experimental details, X-ray data, computational data, and NMR and mass charts (PDF)

The authors declare no competing financial interest.

Published as part of Organic Letters special issue “π-Conjugated Molecules and Materials”.

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Associated Data

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Supplementary Materials

ol5c02610_si_001.pdf (8.3MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

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