Solvent and Temperature Effects on Dynamics and Chiroptical

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Solvent and Temperature Effects on Dynamics and Chiroptical Properties of Propeller Chirality and Toroidal Interaction of Hexaarylbenzenes Tomoyo Kosaka, Satono Iwai, Yoshihisa Inoue, Toshiyuki Moriuchi, and Tadashi Mori J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06535 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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The Journal of Physical Chemistry

Solvent and Temperature Effects on Dynamics and Chiroptical Properties of Propeller Chirality and Toroidal Interaction of Hexaarylbenzenes

Tomoyo Kosaka, Satono Iwai, Yoshihisa Inoue, Toshiyuki Moriuchi, and Tadashi Mori*

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan * E-mail: [email protected].

Dedication This work is dedicated to the memory of Professor Rajendra Rathore.

ACS Paragon Plus Environment

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ABSTRACT. Because of the unique interaction of radial aromatic blades, propeller-shaped hexaarylbenzenes (HABs) attract much research interest and find various practical applications. By introducing a small point-chiral group at the tip of aromatic blade(s), HAB becomes propeller-chiral to exhibit strong Cotton effects. Due to the dynamic nature of propeller chirality, the chiroptical properties of HAB critically responded to minute changes in environment. Using a series of chiral HABs with one to six (R)-1-methylpropyloxy substituent(s) introduced at the blade tip, we elucidated how the smallest chiral auxiliary at the HAB periphery progressively and cooperatively boost the overall chiroptical properties, and also how subtle changes in temperature and solvent structure affect the propeller dynamics and thus the chiroptical responses. The unique features of propeller-chiral HABs further enabled us to switch on/off their circularly polarized luminescence.


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Introduction Hexaarylbenzenes (HABs) are sterically congested and hence their radial aromatic blades are twisted to form a non-planar propeller structure. In most HABs, all the π-orbitals of radial aromatic rings interact with each other to achieve the so-called toroidal interaction, where effective electron delocalization and/or hopping are known to occur.1,2,3 Accordingly, a huge number of HABs have been prepared to investigate the correlation of the π-delocalization with the electron-coupling intensity among radial aromatic rings.4,5 Recently, detailed quantitative studies have also been performed to better understand the nature of toroidal interaction.6,7 Furthermore, the unique structure of HABs has been employed as an effective scaffold for sensors and receptors, 8,9,10 redox 11,12 and charge-transfer systems, 13,14 catalysts, 15,16 as well as supramolecular architectures.17,18 Recently, pyrene-19 and porphyrin-appended20 HABs have been demonstrated to show aggregation-induced emission and strong near-infrared emission in the solid state, respectively. It is also to emphasize that the selective synthesis of diversely substituted HABs, circumventing formation of undesired regioisomeric products, has become available in recent years.21,22 In our previous study, we have investigated the dynamic nature of the propeller chirality of HABs.23 Because the radial aromatic rings in HAB conflict with each other to tilt as was the case with biphenyl and terphenyl,24 HABs are intrinsically chiral but exist as an equimolar mixture of the enantiomeric clockwise (C) and counterclockwise (CC) propellers in equilibrium in the absence of internal or external chiral elements (Figure 1). By introducing a small point-chiral group at the periphery of HAB, the C- and CC-propellers become diastereomeric to each other and the energy difference between them induces the preference for C or CC through a shift of the equilibrium to provoke strong Cotton effects (CEs) in circular dichroism (CD) spectrum. More


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interestingly, the whizzing toroids, ensemble of the intermediate geometries populated between the C and CC conformers on the potential surface along the tilt angle (φ), become essential for interpreting the UV-Vis and CD spectral behaviors observed at elevated temperatures and in polar solvents.23 Despite the numerous reports on HABs, the studies on HAB chirality are still limited in number and depth, excepting those on the solid-state chirality.25


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Figure 1. Schematic illustrations of the propeller chirality and the toroidal interaction in hexaarylbenzene (HAB). The propeller blades are tilted by dihedral angle φ or 180° – φ in the C or CC geometry, respectively, but synchronously changing φ in the whizzing toroids (and hence HAB becomes essentially achiral when average φ = 90°).


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In this study, we prepared a series of HABs with zero to six chiral blade(s) (H0-H6, Figure 2) in order to elucidate how the smallest alkyl chirality introduced at the blade tip cooperatively propagates to the entire propeller, and also how and to what extent the chiroptical properties thus induced are manipulable by the environmental factors, such as temperature and solvent. Because the propeller structure is dynamic in nature and hence the chiroptical properties of HAB are expected to critically respond to the (micro)environment, we first closely examine the temperature and solvent effects on the UV-vis and CD spectra of selected HABs (H4 and H6) to elucidate how subtle changes in solvent parameters, such as polarity, viscosity, and size/shape, affect the propeller dynamics and thus the observed CDs. Then, we will demonstrate that the circularly polarized luminescence of chiral HABs can be turned on and off by properly manipulating the propeller dynamics.


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Figure 2. Structures of chiral HABs with different numbers of chiral blade(s) (H1-H6) and reference compound H0 employed in this study.


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Experimental Section 1

H-NMR (400 MHz) and 13C-NMR (100 MHz) spectra were obtained in chloroform-d on JEOL

GX-400 or JNM-ECS-400 and chemical shifts are reported in ppm relative to Me4Si (0 ppm for 1

H) or residual solvent peak (77.16 ppm for 13C). Coupling constants (J values) are reported in

Hz. Electronic absorption (UV-vis) and circular dichroism (CD) spectra were measured in a conventional quartz cell (light path length 1 cm) fitted with a temperature controller. Decalin (ca. 1:1 mixture of cis and trans isomers of decahydronaphthalene) was distilled prior to use. The spectroscopic grade solvents were used as obtained for the rest of spectroscopic studies. UV-vis spectra were recorded on a JASCO V-650 spectrometer under the following conditions: slit width, 1 nm; scan rate, 100 nm min 1; response, medium. CD spectra were measured typically for −

a ca. 20 μM solution on a JASCO J-820 Spectropolarimeter under the following conditions: slit width, 4 nm; scan rate, 50 nm min 1, response, 8 sec; accumulation, 4 times. Aqueous solution of −

(+)-ammonium camphorsulfonate-d10 (0.06%) was used for a calibration of the spectrometer sensitivity and wavelength (θ = 0.1904° at 290.5 nm). Circularly polarized luminescence (CPL) spectra were recorded on a JASCO CPL-300 spectrometer in methylcyclohexane (ca. 25 μM) fitted with a Unisoku temperature controller under the following conditions: excitation wavelength, 260 nm, slit width, 10 nm; scan rate, 50 nm min 1, response, 8 sec; accumulation, 8 −

times. All calculations were performed on Linux-PCs by using the Turbomole 6.6 (or later) program suite.26 Geometries were fully optimized at the dispersion-corrected density functional theory (3rd generation, DFT-D3 with BJ dumping), with AO basis-set of valence triple-ξ quality (in standard notation: H, [3s1p]; C/O, [5s3p2d1f]) at the TPSS-D3/def2-TZVP level, 27,28,29 employing appropriate symmetry constraint. The resolution of identity (RI) approximation was employed and the corresponding auxiliary basis-sets were taken from the Turbomole basis-set


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library. The numerical quadrature grid m5 was employed and the convergence criterion for the optimization regarding the change of total energy between two subsequent optimization cycles was set to 10–7 Eh.


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Results and Discussion Structure of chiral HABs. First, we briefly discuss the structural features of the chiral HABs employed in this study. The geometry optimizations of H0-H6 were carried out at the DFT-D3(BJ)-TPSS/def2-TZVP level. 27,28,29

This level of theory, which implements the dispersion correction for augmenting the

pairwise -C6/R6 potentials in order to describe the van der Waals interactions, has been successfully applied, as a cost-efficient alternative to the more demanding electron correlation methods, to a number of molecular systems, where the weak inter- and intramolecular noncovalent interactions play significant roles. 30,31,32,33,34,35 For simplicity, we assumed that the peripheral alkoxy groups in H1-H6 are alternately up and down oriented (i.e., the ududud conformer), as was the case with H0.23 For the same reason, we employed the most stable transgauche (Tg-) conformation for the (R)-1-methylpropyloxy group(s) in H1-H6.23 For all chiral HABs H1-H6, the C-propeller conformer was found slightly more stable than the corresponding CC-conformer and the energy difference between the C- and CCpropeller increased with increasing number of chiral auxiliaries (Table S1 in the Supporting Information). As illustrated in Figure 3, the core structures of H0 and the C-conformers of H1H6 were practically superimposable with each other, excepting some small deviations. Table 1 compares the tilt angles (φ) of radial aromatic blades against the central benzene ring for H0 and the C-conformers of H1-H6. All the examined HABs gave essentially the same average φ of 58.8 ± 0.2°, which is much larger than those reported for simple biaryls (≈40°).24 Because the CD intensity of HAB is a critical function of φ and known to maximize at φ ≈ 70°, the tilt angles calculated for H1-H6 (φ = 58-59°) promise strong CD responses.23


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A single crystal of H2P obtained by slow evaporation of an aqueous dichloromethane solution of H2P was subjected to X-ray diffraction analysis to give the structure shown in Figure 4; note that two independent structures exist in an asymmetric unit cell. The crystal structures of H0 and H6 have been already described and are also shown in Figure 4 for comparison.23 Due to the packing requirement, the alkoxy groups were not alternatingly oriented in the ududud manner, implying small energy differences among the orientational conformers (Table 1). In all the crystal structures, the C-propeller was favored (excepting one irregularly oriented blade in each unit cell of H2P crystal) when the (R)-point chirality was introduced to the periphery, which is in good agreement with the theoretical prediction. The tilt angles in the crystal were comparable to or significantly larger than the theoretical predictions and spread over a wide range (φ = 57-90° for H0, 67-111° for H2P, and 58-68° for H6). In H2P, one blade was oppositely oriented (φ > 90°), most probably due to the intermolecular interactions between the mutually penetrated blades.


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Figure 3. Structures of the most stable conformers (i.e., clockwise, ududud, and all-Tgconformation) of H0-H6 optimized at the DFT-D3(BJ)-TPSS/def2-TZVP level.


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Figure 4. Molecular structures of (a) H0, (b) H2P, and (c) H6. Note that two independent molecules exist in the asymmetric unit of H2P and one of them is shown here for clarity (see also Figure S1 in the Supporting Information). Structures of H0 and H6 were reported in ref. 23.


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Table 1. Calculated and Experimental Tilt Angles (φ) of Radial Aromatic Blades in H0-H6 a HAB

method

orientation

φava / °

individual φ / °

H0

DFTb

ududud

59.0

(D3-symmetry)

X-ray

uuuudd

71.5 ± 4.2

65.7/57.0/76.8/89.3/73.8/66.2

H1

DFTb

ududud

58.6 ± 0.1

58.8/58.4/58.6/58.7/58.5/58.7

H2

DFTb

ududud

58.9 ± 0.1

58.9/58.8/58.9/58.8/58.8/58.9

H2P

DFTb

ududud

58.9 ± 0.2

59.1/58.5/58.5/59.1/59.0/59.0

X-rayc

uuuddu uudddd

80.3 ± 5.7 76.8 ± 4.6

72.9/71.4/76.8/110.6/79.8/70.2 99.0/79.5/67.1/67.3/69.3/78.6

H4

DFTb

ududud

58.9 ± 0.1

59.0/58.8/58.8/59.0/58.8/58.7

H6

DFTb

ududud

58.4

(C6-symmetry)

X-ray

uuddud

62.2 ± 1.4

58.3/67.1/63.6/59.7/59.2/65.1

a

Average tilt angle of radial aromatic rings against central benzene ring. b The calculated values were obtained from the geometries optimized by the DFT-D3(BJ)-TPSS/def2-TZVP method for the more stable clockwise isomer with the alkoxy groups being fixed in the Tgconformation. c Two different geometries were found in a unit cell.


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Effect of the number and position of chiral auxiliary on propeller chirality. CD spectra of H1-H6 were first recorded in methylcyclohexane and also in dichloromethane at 25 °C (Figure 5); the corresponding UV-vis and absorption dissymmetry factor (gabs = Δε/ε) spectra can be found in Figure S2 in the Supporting Information. All the chiral HABs, possessing (R)-1-methylpropyl substituent(s), exhibited trisignate negative-positive-negative CEs for the 1Lb, 1

La, and 1Bb transitions, respectively, the shapes of which were essentially superimposable with

each other (see the normalized CD spectra in Figure S2, inset, in the Supporting Information), suggesting that analogous propeller geometries are evoked irrespective of the number and position of introduced chiral auxiliary. However, the CD intensity was progressively augmented with increasing number of the chiral auxiliaries, while the corresponding UV-vis spectra showed much smaller changes in shape and intensity. Intriguingly, the CD intensities of all the chiral HABs were significantly solventdependent to give nearly five-fold smaller values in dichloromethane than in methylcyclohexane, which is rationalized mainly by solvation. In dichloromethane, each alkoxyphenyl blade should be solvated to increase its effective size, which drives the propeller blades to further tilt against the central benzene ring, leading to the reduction of CD intensity (which becomes nil if φ = 90°). In methylcyclohexane, the molar CD (Δε) at the 1Lb band was as high as -7.7 M-1 cm-1 even for H1 and doubled for H2 and H2P to finally achieve the 4.6-fold larger value of -35.1 M-1 cm-1 for H6, which is an enormous enhancement if compared with the Δε values of -0.4 and -0.5 M-1 cm-1 observed

for

(R)-1-methylpropylbenzene

and

4,4’-bis((R)-1-methylpropyl)biphenyl,

respectively.24 This sudden increase of molar CD for the chiral HABs relative to the reference compounds may be taken as experimental evidence for the formation of chiral propeller driven by the synchronized unidirectional domino twisting of six radial aromatic blades. Interestingly,


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the CD intensities were almost the same for H2 and H2P in methylcyclohexane, while the former showed appreciably stronger CEs than the latter in dichloromethane. This can be explained by assuming that the cooperative effect, or the chiral information, effectively propagates over the entire blades in methylcyclohexane but is interrupted by solvation in polar dichloromethane.


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Figure 5. Circular dichroism spectra of H1-H6 obtained in methylcyclohexane (left) and in dichloromethane (right) at 25 °C. Note that no reliable spectrum was obtained for H1 in dichloromethane, due to the very weak CD signals relative to the signal-to-noise ratio.


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Since the CD spectra, and the UV-vis spectra (to a much lesser extent), were affected by the number of introduced chiral blades, a quantitative comparison of the molecular dissymmetry of H1-H6 was performed using their absorption dissymmetry factors (gabs = Δε/ε). In Figure 6, the gabs factors for the three extrema at 229, 266, and 292 nm in methylcyclohexane and for the minimum at 294 nm in dichloromethane are plotted against the number of chiral auxiliaries to give very similar profiles. Thus, the gabs factor increased with increasing number (n) of the chiral units until n = 2 (i.e., H1, H2, and H2P), but gradually saturate at larger n (H4 and H6). In the present chiral HAB system, the C- and CC-propellers are in equilibrium and the energy difference between them is progressively augmented with increasing n to shift the equilibrium to the favored propeller conformation (i.e., the C-propeller if (R)-alkoxy is incorporated), which eventually enhance the CD intensity. It is to note that the number of chiral auxiliaries rather than the relative position is more important in forming the chiral propeller, particularly in less polar solvent. These observations indicate that the effects of chiral modification at the tip of propeller blade are not restricted to the adjacent blade but further propagate to more distant blades through the domino motion.


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Figure 6. Correlation of the absorption dissymmetry (gabs) factor with the number of chiral blades in H1-H6 at the extrema 229 nm (light blue), 266 nm (red), and 292 nm (blue) in methylcyclohexane and at 294 nm (blue open circle) in dichloromethane. Note that the absolute gabs factors for H2P are always slightly larger than those for H2


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Variable temperature CD spectra of H4 and H6 in various solvents. To obtain further insights into the C-CC propeller equilibrium dynamics and its chiroptical consequences, we measured the variable temperature CD spectra of H4 and H6 to observe relatively strong CEs, especially in less-polar solvents, as shown in Figure 7. The corresponding UV-vis and gabs factor spectra are provided in Figure S3 in the Supporting Information. Although the absolute CD intensity was consistently smaller for H4 than for H6 in all the solvents examined, their temperature-dependence behaviors were quite similar to each other. By decreasing the temperature from 25 °C to -120 °C, the CD intensities of H4 and H6 in methylcyclohexane were dramatically enhanced from -24.5 and -35.1 M-1 cm-1 to -140.4 and 164.2 M-1 cm-1, respectively. Such extraordinary CD enhancements caused by reducing temperature are not explainable by the conformational freezing alone. It is also to note that lowering temperature did not cause peak-sharpening but induced considerable red-shifts of the CD extremum with appreciable band-broadening but without accompanying any clearly defined isodichroic points, suggesting operation of multiple conformational equilibria independently affected by the temperature variation. The Eyring-type analysis of temperature-dependent equilibrium shift is often employed as an effective means to quantitatively elucidate the dynamic aspects of the system. In Figure 8 (left), the CD intensities at the 1Lb band of H4 and H6 in methylcyclohexane were plotted against the reciprocal temperature to afford two distinct linear correlations for each HAB, suggesting that the major contributor is switched at a critical temperature (i.e., the inflection point) of Tc ≈ -50 °C. These observations are well explained by assuming the whizzing toroids (i.e., ensemble of transient conformers of intermediate geometries in upper rotational and/or vibrational levels) in addition to the C- and CC-propeller geometries at the potential minima (Figure 1). Thus, in the temperature domain below Tc, the whizzing toroids


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are less populated and hence the observed CD changes are mostly attributable to a shift of the CCC propeller equilibrium. Thus, the CD intensity is determined by the relative population of C and CC propellers in this temperature regime. In the temperature domain above Tc, the CD intensities of both H4 and H6 were more rapidly decreased by increasing the temperature (Figure 8, left). The contribution of whizzing toroids, which is maximized at the perpendicular blade angle, becomes more substantial at higher temperatures with increasing population to the transient conformers in upper rotational and/or vibrational levels. The contribution of whizzing toroids seems comparable for H4 and H6, as both the HABs show parallel temperaturedependence behaviors in methylcyclohexane (Figure 8, left). In dichloromethane, the CD intensities were significantly reduced and the temperaturedependence behaviors, though rather sluggish, were much less pronounced in the range of variation, for which the following mutually linked changes in conformation and energy are likely to be responsible: (1) the larger tilt angles caused by the size-expanding solvation to blades, (2) the equilibrium shift to the CC propeller, and (3) the increased contribution of whizzing toroids, the latter two of which are caused by the destabilization of both the C- and CC-propeller conformations that reduces the energy difference between them; see below for more detailed discussion. In this regard, it has been reported that the radial aromatic blades in HABs are nearly perpendicular to the central ring in chloroform at an ambient temperature, at least in the NMR timescale.36 The excitonic coupling theory predicts that the chiroptical responses essentially vanish when two chromophores (more precisely, two transition moments) are perpendicular to each other, 37 which is also applicable to the chiroptical behaviors of chiral HABs in dichloromethane. It has been already demonstrated that the dynamics of molecule, along with the


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stable conformation, play a substantial role in determining the observed shape and intensity of CDs in the case of relatively flexible molecules.38,39


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Figure 7. Variable temperature CD spectra of H4 (top) and H6 (middle and bottom) in dichloromethane, methylcyclohexane, hexane, and/or 1:1 hexane-methylcyclohexane mixture. Note that the vertical scale is 10-fold expanded for the spectra in dichloromethane.


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Figure 8. Left: Temperature dependence of the molar circular dichroism (Δε) at the 1Lb band of H6 (closed circle) and H4 (open circle) in methylcyclohexane (blue), hexane (red), dichloromethane (green), and 1:1 hexane-methylcyclohexane (orange). Right: CD spectral changes as a function of the fraction of methylcyclohexane in hexane for H6 at 25 (open circle) and -90 °C (closed circle).


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Solvent effect on the CD spectra of H4 and H6. In the previous study,23 we have noticed that the CD spectrum of H6 suffers substantial, but somewhat puzzling, solvent effects. In this study, we more systematically and closely examined the solvent-dependent CD spectral behaviors of HABs by employing H4 and H6 and various solvents. The solvents employed may be classified into four categories: (1) a series of linear alkanes (from pentane to octane) for examining the effect of chain length and viscosity, (2) (bi)cyclic alkanes (methylcyclohexane and decalin) for examining the effects of molecular volume and viscosity, (3) slightly polar solvents of similar size (dichloromethane and diethyl ether) for examining the effects of polarity and solvation, and (4) more polar protic and aprotic solvents (methanol and acetonitrile) for examining the effect of polarity, solvation, and hydrogen-bonding interaction. As shown in Figure 9, the CD spectra of H4 and H6, despite the obvious difference in absolute intensity due to the larger population to the C-propeller in the latter, behaved quite analogously in all the solvents examined, suggesting a common single mechanism operative in the two HABs. As a major source of the dramatic solvent effects on CD, we postulate the change in tilt angle, which inevitably accompanies a shift of the C-CC propeller equilibrium as well as a change in the toroidal interaction but these are subordinate functions. This idea is also compatible with the following chiroptical features shared by the two chiral HABs: (1) As a global trend, the absolute gabs factors of both H4 and H6 decrease with decreasing solvent viscosity to exhibit a loose positive correlation with the reciprocal viscosity (Figure 9, bottom), although other parameter(s) and mechanism(s) may also play minor roles in each category of solvent as discussed below. A possible reason for this global trend would be the reduced contribution of whizzing toroids in viscous solvents due to the increased rotational


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barrier and energy level-spacing that hamper the transient population to the upper vibrational levels. (2) In the series of straight chain alkanes, the gabs factors are ca. 2-3 fold smaller than those in (bi)cycloalkanes and slowly decrease with increasing chain length or solvent viscosity (apparently opposing to the global trend mentioned above) (Figure 9, bottom). These behaviors would be rationalized by the penetration of solvent molecule(s) into the void space between the aromatic blades (as the para positions of two neighboring phenyls are separated by 5.7 Å, which is enough to accommodate at least one alkane molecule). The alkane penetration, if it occurs, should drive the blades to further tilt and reduce the gabs factor to such a degree that depends on the size of penetrating alkane molecule. The appreciably blue-shifted (by ca. 1 nm) and somewhat broadened CD spectrum in pentane, relative to those in methylcyclohexane (Figure 7), would also support the idea that the aromatic blades are (rather randomly) more tilted and hence less conjugated with the central benzene ring. (3) In mono- and bicyclic alkanes, the gabs factors of both H4 and H6 are significantly augmented to reach the largest in decalin, which is too bulky (with a radius of ca. 4.8 Å) to penetrate into the inter-blade void space and simply interacts with the peripheral alkoxy groups by London dispersion to fix the blade conformation. As such, the blades are kept at or near the theoretically predicted tilt angle to give the maximal gabs factor. The smaller gabs factors in methylcyclohexane are attributable to the partial penetration of branching methyl into the interblade space. (4) The polar solvents most significantly suppress the gabs factor. This is due to the volume-expanding solvation to the entire blades and the penetration into the void space, both of which cooperatively increase the tilt angle to afford the smallest, and somewhat variable, gabs


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factors. The CD extrema observed in polar solvents are more or less blue-shifted (by 2-3 nm) relative to that in decalin, implying larger tilt angles caused by solvation.


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Figure 9. Top: CD spectra of H6 (left) and H4 (right) at 25 °C in decalin (red solid), methylcyclohexane (red dashed), pentane (black solid), hexane (black chain), heptane (black dotted), octane (black dashed), dichloromethane (blue solid), diethyl ether (blue chain), methanol (blue dotted), and acetonitrile (blue dashed). Bottom: Plots of absorption dissymmetry (gabs) factors against the reciprocal viscosity of solvent.


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In order to elucidate the origin of the unexpected difference in CD intensity observed in linear versus (bi)cyclic alkanes, we further examined the CD spectra of H6 in a 1:1 mixture of hexane and methylcyclohexane at various temperatures (Figure 7) and compared the result with those obtained in pure hexane and methylcyclohexane. Taking into account the lower solubilities of H6 in hexane and the mixed hydrocarbon solvent at reduced temperatures, we performed the following experiments at a lower concentration of 10 μM. As shown in Figure 8 (left), the plot of molar CD at the extrema (Δεext) in the mixed solvent against reciprocal temperature also gave a bent line with Tc ≈ -70 °C, which was however not a simple average of the two lines obtained for hexane and for methylcyclohexane but was much closer to that for methylcyclohexane. The observation in the mixed solvent was still somewhat puzzling and hence we more closely investigated the effects of solvent composition on the CD intensity of H6 at 25 and -90 °C to obtain the very contrasting behaviors shown in Figure 8 (right). At 25 °C, the CD intensity showed a slightly concave dependence on the fraction of methylcyclohexane in hexane, indicating that methylcyclohexane progressively replaces hexane occupying the void space but its affinity is appreciably lower than that of hexane at that temperature, most likely as a result of an interplay of the enthalpy and entropy factors.40 At -90 °C, where the entropy factor plays a less important role, the CD intensity rapidly increased at low fractions of methylcyclohexane, being driven by a higher affinity of methylcyclohexane to the void space at this temperature. As such, the effect of solvent on the propeller chirality is not simply derived from the bulk solvent parameters such as viscosity and polarity, but is rather related to the size and shape of solvent molecule that determine its affinity to the void space. Accordingly, a small fraction of methylcyclohexane in the mixed solvent can have a strong impact on the observed chiroptical


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responses by manipulating the tilt angles of propeller blades to eventually increase CD intensity particularly at lower temperatures.

Circularly polarized luminescence of H6 turned on and off by propeller dynamics. Circularly polarized luminescence (CPL) of organic molecules have attracted glowing attention in recent years.41,42 Nevertheless, no CPL behavior has been reported for propeller chiral HABs. We have recently reported the strong CPL response of a structurally related compound, i.e., fully arylated boron dipyrromethenes (BODIPY) derivative. 43 As a quantitative measure of CPL dissymmetry, we employ the luminescence dissymmetry factor: glum = 2 × (IL - IR) / (IL + IR), where IL and IR refer to the intensities of left and right circularly polarized spontaneous emission, respectively. In methylcyclohexane at 25 °C, H6 emitted moderately strong fluorescence at around 340 nm as reported previously,23 but no reliable CPL signals were detected (Figure 10), indicating that the glum factor is smaller than the detection limit of our CPL instrument (

Solvent and Temperature Effects on Dynamics and Chiroptical

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