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Dynamic Domino Inversion Revealed by Combined Experimental and Theoretical Circular Dichroism Studies ... Publication Date (Web): February 16, 2016 ...
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Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes. Dynamic Domino Inversion Revealed by Combined Experimental and Theoretical Circular Dichroism Studies Tomoyo Kosaka, Yoshihisa Inoue, and Tadashi Mori* Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan S Supporting Information *

ABSTRACT: Hexaarylbenzenes (HABs) have greatly attracted much attention due to their unique propeller-shaped structure and potential application in materials science, such as liquid crystals, molecular capsules/rotors, redox materials, nonlinear optical materials, as well as molecular wires. Less attention has however been paid to their propeller chirality. By introducing small point-chiral group(s) at the periphery of HABs, propeller chirality was effectively induced, provoking strong Cotton effects in the circular dichroism (CD) spectrum. Temperature and solvent polarity manipulate the dynamics of propeller inversion in solution. As such, whizzing toroids become more substantial in polar solvents and at an elevated temperature, where radial aromatic rings (propeller blades) prefer orthogonal alignment against the central benzene ring (C6 core), maximizing toroidal interactions.

H

Scheme 1. Propeller Chirality of HAB

exaarylbenzene (HAB) has been utilized as a scaffold for attaining an unusual, characteristic electron delocalization,1 which is aptly described as toroidal interaction. This unique interaction has been extensively studied in conjunction with electron hopping and π-delocalization, the degree of which can be precisely modulated by the degree of electron coupling among radial aromatic rings in HAB.2−4 The analysis of such interactions in hexagonal toroids will provide insights into the properties of not only toroidally but also linearly π-stacked aromatic wires, that is, an infinite array of cofacial π-systems. As such, HAB can be employed as an ideal model for elucidating the mechanism and factors that control electronic interactions occurring among accumulated π-systems.5−7 Indeed, a large number of symmetrical and unsymmetrical HABs with electron-donating and -withdrawing aromatic rings have been described to date.8−13 Recently, a modular synthesis of HABs with all different aryl substituents has also been reported.14 Aromatic rings in biphenyl and terphenyl are tilted to certain degrees but not perpendicular to each other.15 Most of the studies on HAB, nevertheless, either ignore such tilting behavior of radial aromatic rings or simply assume that they are all (nearly) orthogonal to the central C6 core. Theory predicts, on the contrary, the ideal tilt angles in HAB to be ∼60° for balancing steric hindrance between the adjacent (cofacial) aromatic rings and π-conjugation with the central aromatic ring (vide infra). Therefore, two possible conformations should be possible as a consequence of the atropisomerism of each propeller blade against the C6 core. Avoiding the steric congestion, all six radial aromatic rings tilt in one direction, provoking, as a whole, propeller chirality either in a clockwise (C) or counterclockwise (CC) manner (Scheme 1). In this study, we focused on this propeller chirality of HAB in © XXXX American Chemical Society

the condensed phase. We will show how such chiral propellers can be enriched and how the dynamic propeller inversion affects observed (chir)optical properties of HAB. To this end, we employed uni- and omni-chiral HABs H1 and H6, in which a small point-chiral substituent introduced at the periphery (para position) of radial aromatic ring(s) effectively triggered the induction of propeller chirality (Chart 1).16 Omni-chiral H6 as well as nil-chiral reference compound (hexa-p-anisylbenzene, H0) was prepared by the Co2(CO)8-induced trimerization of the corresponding acetylene derivatives, while unichiral H1 was synthesized by the Diels−Alder addition of tetra-p-anisylcyclopentadienone to the unichiral aryl acetylene derivative and subsequent decarbonylation. Detailed synthesis procedures are described in the Supporting Information. First, we briefly inspected the structural characteristics of the present and related HABs theoretically and experimentally. The geometry optimizations of H0, H1, and H6 were carried out by dispersion-corrected density functional theory at the DFTReceived: January 26, 2016 Accepted: February 16, 2016

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DOI: 10.1021/acs.jpclett.6b00179 J. Phys. Chem. Lett. 2016, 7, 783−788

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Table 1. Calculated and Experimental Tilt Angles of Radial Aromatic Rings in HABsa

HAB

φav/degb

individual angles/deg

note

H0

58.7 59.0c 71.5 58.6 58.7f 58.4 58.7f 62.2

(C6-symmetry) (D3-symmetry) 65.7/57.0/76.8/89.3/73.8/66.2d 58.4/58.6/58.7/58.5/58.7/58.8e 58.6/58.6/58.7/58.7/58.6/58.8e (C6-symmetry) (C6-symmetry) 58.3/67.1/63.6/59.7/59.2/65.1g

calcd calcd X-ray calcd calcd calcd calcd X-ray

H1 H6

a Figures: optimized structures of H0, H1, and H6 in the most stable conformations are overlapped, emphasizing the similar tilt angles of radial aromatic rings (left), and ORTEP drawings of the X-ray structures of H0 (middle) and H6 (right) with 30% probability ellipsoids (123 K). Hydrogens are omitted for clarity. bAveraged tilt angle of radial aromatic rings against the central C6 core. The calculated values were obtained from the DFT-D3(BJ)-TPSS/def2-TZVP-optimized geometries for the C (clockwise) isomer with alkoxy groups being aligned in the same direction (i.e., uuuuuu) and the conformation of chiral groups being fixed at the Tg− conformation, unless otherwise stated. cThe value for the alternate conformation (i.e., ududud) in terms of orientation of methoxy groups. dThe orientation of methoxy groups: uuuudd. eNote that the last value is the angle for the radial aromatic ring with a chiral substituent. fThe values for the less stable CC (counterclockwise) isomer. gThe orientation and conformation of chiral alkoxy groups: u(Tg−)/u(Tg−)/d(Tg−)/d(G+g+)/u(Tt)/d(Tt).

Figure 1. (a) Comparison of the experimental and calculated CD spectra of HABs with propeller chirality. The calculated spectrum (red) was obtained by assuming a 96.5:3.5 mixture of a C and CC array of anisyl groups in propeller conformation using H0 as a model, which is compared with the experimental spectra of H6 at −150 °C in isopentane (blue solid line) and at 25 °C in methylcyclohexane normalized at 290 nm (dashed line). (b) Energy diagram for the inversion of propeller chirality in H6 estimated by the SCS-MP2/def2-TZVPP calculation with the COSMO solvation model. The inset describes the proposed domino inversion of propeller chirality (see the text for details). 784

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Figure 2. CD spectral investigations on the propeller chirality of HABs. (a) UV−vis and CD spectra of H1 and H6 in methylcyclohexane at 25 °C. Due to the poor solubility, the UV−vis spectrum of H0 was obtained in dichloromethane and was normalized at 260 nm. (b) Solvent and temperature dependences of the CD spectra of H6. The dotted, dashed, and solid lines indicated by “alkanes” are for n-octane, n-hexane, and npentane, respectively. The temperature dependence was examined mostly in methylcyclohexane, while isopentane was employed for the spectrum at −150 °C. (c) Temperature dependence of the excitation energy (absorption maximum) of the 1Lb transition (Emax) and the intensity of CE at the extremum (Δεext) for H1 and H6 in methylcyclohexane. (d) Temperature dependence of the anisotropy factor (g = Δε/ε) at the extremum for H6 (296 nm) in three representative solvents.

89°. Due to the shallow potentials for the inversion of radial aromatic rings (vide infra), these angles are susceptive to small perturbations such as packing forces. The crystal structure of hexakis(p-hydroxyphenyl)benzene, for instance, was found to be significantly affected even by the existence of a cocrystallized solvent molecule, showing distinct structures with tilt angles of up to 103°.18 The energy profiles for possible chirality inversion processes through rotation of radial aromatic ring(s) were verified by using hexa-p-anisylbenzene H0 as a model. Synchronous inversion, where all six cofacial anisyl groups rotate simultaneously (the sterically most feasible process), turned out to proceed with a fairly large rotation barrier of 8.4 kcal mol−1 (at the DFT-D3(BJ)-TPSS/def2-TZVP level). When a single radial aromatic ring was solely rotated to invert its local axial chirality (while the rest were fixed), a much reduced energy of 4.2 kcal mol−1 was required to reach orthogonal, but further rotation (or inversion) was again unlikely (8.8 kcal mol−1 at 100°). These results led us to a domino inversion mechanism, where the one ring rotation induces small alterations in tilt angles of the adjacent rings on both sides and such a motion propagates to all rings, achieving the overall propeller inversion (see Table S1 and Figure S11 in the Supporting Information for more details). Hence, the energetic profile of the propeller chirality inversion of omni-chiral H6 was assessed at the SCS-MP2/def2-TZVPP level, incorporating the COSMO solvation model for methylcyclohexane, to afford

D3(BJ)-TPSS/def2-TZVP level. Due to the greater steric crowding, tilt angles of radial aromatic rings against the C6 core were found to be much larger than those in sterically lessdemanding simple biaryls. As depicted in Table 1, the average tilt angle was consistently ∼59° for all of the examined HABs, the geometries of which were essentially superimposable with each other, while the angle was shown to be much smaller at ∼40° for biphenyls. The effects of the conformation of the peripheral chiral methylpropyloxy group turned out to be minor but are briefly discussed below. First, the most stable Tg− conformation was adopted as in previous investigations.8,17 The D3-symmetrical conformer of H0 (in which the alkoxy groups are oriented alternately up and down; i.e., ududud) was slightly more stable (ΔE < 0.1 kcal mol−1) than the C6-symmetrical one (i.e., uuuuuu), but the calculated UV−vis and/or circular dichroism (CD) spectra for these conformers did not appreciably differ from each other (see Figure S10 in the Supporting Information). An X-ray crystallographic study revealed the conformationally well-behaved structure of H6 with an averaged tilt angle of radial aromatic rings against the C6 core of 62 ± 3° (however, the conformation of the peripheral chiral groups was more variable; see the Supporting Information). It was shown that the (R)-point chirality in the alkoxy group at the periphery induces the C-propeller chirality in radial rings, which agreed with the theoretical prediction (Figure 1a). In contrast, the tilt angles found in crystalline H0 varied more widely from 55 to 785

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Figure 3. Comparison of fluorescence spectra of H6 at 25 and −120 °C in methylcyclohexane. (a) Normalized emission spectra obtained by excitation at 290 nm. The peak at ∼320 nm is the Raman scattering. (b) Excitation spectra monitored at 370 nm, normalized at peaks around 270 nm. Dotted lines are the corresponding normalized absorption spectra.

an activation energy of 2.1 kcal mol−1 (via the domino enantiomerization) from the C- to CC-enantiomer with an energy difference of 0.81 kcal mol−1 (Figure 1b). For unichiral H1, the energy difference was found to be as low as 0.14 kcal mol−1. Such a shallow potential well explains the diversity of the tilt angles found in the crystal structures of various HABs. The theoretical analyses, together with the X-ray crystallographic investigations, imply that the radial aromatic rings are not fixed at certain stable angles but exist as an ensemble of whizzing (or even rotating) arms with varying angles, and they definitely perturb each other to some extent. The Cotton effect (CE) observed for an aryl ether having the same chiral alkyl group is as small as −0.4 M−1 cm−1, while the CD of 4,4′- and 2,2′-biphenyls having the same chiral group shows only marginally larger CEs of −0.5 and −1.8 M−1 cm−1, respectively.8 In contrast, the CE intensities observed for the chiral HABs with the same chiral auxiliary were found to be considerably stronger (Figure 2a). The details of the CD spectral measurements are described in the Supporting Information. Briefly, the spectra were measured for the sample solutions of typically 20 μM concentration placed in a temperature-controlled 1 cm quartz cuvette. At the 1Lb band, the Δε values were found to be −7.7 M−1 cm−1 for H1 and −35.1 M−1 cm−1 for H6 (in methylcyclohexane at 25 °C). The CE for H6 is approximately four times stronger in intensity than that for H1. However, such large CEs were observed only in less polar solvents (Figure 2b). Indeed, the CDs in acetonitrile and diethyl ether were much smaller. Remarkably, some additional solvent parameter(s) other than polarity appear to play a significant role as the observed CD was much larger in decalin than that in linear alkanes. A direct correlation, however, was not found with any single solvent parameter such as viscosity. The effect of concentration (i.e., aggregation) was excluded (Figure S12 in the Supporting Information), in accord with their three-dimensional doughnut shapes. In general, the CE intensity is enhanced to some extent with decreasing temperature in conformationally flexible systems.19 As such, the enhanced CEs for H6 by lowering temperature were somewhat anticipated, but the enhancement factor observed for HABs was surprisingly large. Thus, the Δε value was augmented up to −200 M−1 cm−1 at −150 °C (in isopentane). The overall enhancement factor from the simple aryl ether to HAB may be calculated to be as large as 80 (∼200/0.4/6). Optical rotation is a more complex function of properties but worth mentioning here. Briefly, the enhancement factor of the optical rotation was also considerably larger

for H6. Thus, the specific rotations of H1 and H6 were found to be −19 and −120° (at sodium D-line in chloroform at 25 °C), respectively, while the corresponding chiral alcohol, (R)-1methylpropanol, shows an [α]D of −13°. Along with the increase of CEs, the absorption was also blueshifted gradually with lowering temperature. Figure 2c plots the changes in excitation energy and CE intensity for H1 and H6 in methylcyclohexane against reciprocal temperature. All of the plots commonly gave bent lines with abrupt points, or critical temperatures (Tc), at around −50 °C, indicating switching of the major contributor to the (chir)optical properties. We rationalize this unusual temperature dependence as follows. Below Tc, where the C−CC equilibrium becomes slow, the (chir)optical properties can be approximated by a static equilibrium mixture of the C and CC isomers. Therefore, the 93% enantiomeric excess (ee) for the C/CC mixture of H0 calculated from the 96.5:3.5 Boltzmann distribution for ΔE of 0.81 kcal mol−1 roughly reproduced the magnitude of CE intensities (as well as the pattern) observed in the experimental CD spectrum of H6 at −150 °C in isopentane (Figure 1a). The relative preference for more stable C over CC isomer explains the small increase of CD intensity for H6 in the lower temperature region. This trend was not seen for H1, and rather, the opposite slope was found at lower temperatures. This may be ascribed, at least qualitatively, to the fact that the C isomer possesses smaller tilt angles (for which a smaller CD intensity is predicted) than the CC isomer, and this factor overwhelms the C/CC equilibrium factor in H1 (Table 1 and Figure S11 in the Supporting Information). At temperatures higher than Tc, the CD intensity showed more dramatic dependence for both H1 and H6, and the CEs almost vanished at high temperature. Such an unusual behavior is well explained by assuming the influence of whizzing toroids, in which the toroidal interaction is considered more effective. We assume that the whizzing toroids are better described as an ensemble of intermediate geometries between the C and CC isomers populated on the tilt angle surface (depicted in Figure 1b, inset) rather than a single discrete conformation having six orthogonal radial aromatic rings. As the temperature increases, a larger number of such structures contribute to the total (chir)optical properties. Polar solvents such as dichloromethane and methanol seem to favor the whizzing toroids (Figure 2d). Such dynamic trajectories have been shown to contribute to the overall shape and intensity of CD.20−22 The following observations in absorption and fluorescence spectra also affirm the contribution of such whizzing toroids. 786

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Izumi Science and Technology Foundation is greatly acknowledged.

Thus, apparent blue shifts in absorption spectra of HABs in polar solvents and at higher temperatures are indicative of the H-type association of radial aromatic rings, where the propeller blades are more or less aligned face-to-face. In contrast, the partially π-overlapped J-type association becomes favored at lower temperatures in nonpolar solvents, as evidenced by the bathochromic shift (Figure S13 in the Supporting Information).23−25 Unlike bi- or terphenyl,26 the absorption spectra of HABs exhibited split peaks at the 1Lb band (and bisignate CEs in CD), and the relative intensity of these peaks was also compatible with such a switching from J- to H-association by increasing the temperature. The H-type association becomes most feasible when the tilt angles of radial aromatic rings to the central C6 core become ∼90°, maximizing the toroidal interactions. A reproduction of relative UV shifts by theoretical calculations on H0 with different tilt angles reinforced such toroidal interactions (Figure S14 in the Supporting Information). Fluorescence spectra of H6 were again blue-shifted when measured at higher temperatures (Figure 3a). The relative contribution of the H- against J-type exciton (thus propeller) alignment becomes apparent at higher temperatures, as shown in the excitation spectra of H6 (Figure 3b). The analysis of fluorescence decay of H6 also supported the existence of two distinct species, assignable to the C/CC isomers and whizzing toroids (see Figure S15 in the Supporting Information for more details). In summary, propeller chirality in HABs can be effectively conjured by a small axial twist of radial aromatic ring(s), which is induced by minute point chirality at its periphery. The contribution of whizzing toroids becomes substantial in polar solvents and at higher temperatures to impact the observed (chir)optical properties. Such propeller inversion dynamics has to be taken into account in fine-tuning the structure and properties of HABs and other face-to-face π-stacked arrays such as molecular wires. Further research on the propeller chirality of various HABs and related systems is now in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00179. Experimental and theoretical details and extended spectral data(PDF) Crystallographic data for H0 (CIF) Crystallographic data for H6 (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-6-6879-7923. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by Grant-in-Aids for Scientific Research, Challenging Exploratory Research, and on Innovative Areas “Photosynergetics” (Nos., 15H037790, 15K13642, and 15H010870) from MEXT/JSPS, the Matching Planner Program from JST (No. MP27215667549), the Shorai Foundation for Science and Technology, the Kurata Memorial Hitachi Science and Technology Foundation, the Tokuyama Science Foundation, Toyota Physical and Chemical Research Institute, Iketani Science and Technology Foundation, and 787

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