Circular Dichroism of (Di)methyl- and Diaza[6]helicenes. A

Article pubs.acs.org/JPCA

Circular Dichroism of (Di)methyl- and Diaza[6]helicenes. A Combined Theoretical and Experimental Study Yoshito Nakai, Tadashi Mori,* and Yoshihisa Inoue* Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: Circular dichroism (CD) and relevant chiroptical properties of (di)methyl- and diaza[6]helicenes were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations, results of which were compared with the corresponding experimental data obtained for newly synthesized enantiopure helicenes. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97D/TZVP level accurately reproduced the experimental CD spectra in both excitation energy and rotational strength. The electric and magnetic transition dipole moment vectors for the helical sense-responsive 1Bb and the substitution-sensitive 1Lb bands were compared with those for parent carbo[6]helicene, from which the effects of methyl and nitrogen introduced at different positions upon the experimental CD spectra were discussed to separately evaluate the electronic and steric consequences of the substitution to the chiroptical properties. The electronic effects of substitution on CD spectra were further investigated theoretically by employing a series of 3,3-disubstituted [6]helicenes. This first systematic investigation allows us not only to accurately reproduce the experimental CD spectra of known substituted helicenes but also to directly envisage the chiroptical properties of unknown helicenes.

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the 1Bb band was established as a sensible parameter for defining the nature of helix in helicene geometry. Although parent carbo[n]helicenes are the most pristine and solid models for theoretical investigations of the CD spectra of helicenes, substituted helicenes are definitely more useful as catalysts in asymmetric synthesis,8 receptors/sensors in molecular recognition, and components of supramolecular architecture.9 Very recently, it was revealed that the chiroptical properties of helicenes can be engineered by introducing lateral organometallic substituents. This result has highlighted the vital role of substituents incorporated in the helical π-system in determining its chiroptical properties, which were traditionally considered to be controlled primarily by the number and helical pitch of fused aromatic rings.10 The chiroptical properties of substituted helicenes, nevertheless, have neither attracted much attention nor been investigated systematically so far. Table 1 collects all of the numerical CD spectral data reported in the literature for the 1Bb band of substituted[6]helicenes, excepting more complex derivatives of the listed helicenes. This small, fragmental list of the substituted helicenes and the lack of comprehensive understanding of the substitution effects on the helicene structure and CE prompted us to systematically investigate the chiroptical properties of a series of substituted [6]helicenes particularly at the helix-responsive 1Bb band and

INTRODUCTION Helicenes are planar-chiral polyaromatics with a unique helical array of ortho-fused benzene rings (Chart 1).1 As a consequence of the conformationally fixed helical arrangement of π systems, helicenes display unique chiroptical properties, in particular exceptionally strong circular dichroism (CD) spectra. Thus, carbo[6]helicene (CH[6]) exhibits extremely strong bisignate Cotton effect (CE) peaks in the 1Ba and 1Bb transition regions with molar CD intensities (Δε) of −267 M−1 cm−1 (at 246 nm) and +259 M−1 cm−1 (at 324 nm) in dramatic contrast with the very weak Δε of −0.3 M−1 cm−1 observed for the 1Lb band (410 nm).2,3 Despite the increasing recent interest in the preparation, optical resolution, and (chir)optical properties of helicenes,4 only a limited number of studies have been devoted to the CD spectra of helicenes.5−7 For this reason, we have recently performed the combined theoretical and experimental studies on the chiroptical properties of a series of carbo[n]helicenes (CH[n]).2 The theoretical inspection of the electric and magnetic transition dipole moments (μe and μm) calculated for CH[n] revealed that the observed CD spectral features are not assignable to a single parameter. Nevertheless, this first systematic study unequivocally disclosed that the anisotropy (g = Δε/ε) factor of the 1Bb transition (as well as the specific rotation) is linearly correlated with 1/n (with a kink at n = 6) through the mutual compensation between the electric and magnetic moment vectors, and the discontinuity is attributable to the abrupt change of the helical pitch upon completion of the first helix structure at n = 6. As a consequence, the CE at © 2012 American Chemical Society

Received: October 21, 2012 Revised: December 2, 2012 Published: December 3, 2012 83

dx.doi.org/10.1021/jp3104084 | J. Phys. Chem. A 2013, 117, 83−93

The Journal of Physical Chemistry A

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Chart 1. Structures of (Di)methyl- and Diaza[6]helicenes Examined in This Study, as Well as the Parent Carbo[6]helicene CH[6]

Table 1. Experimental CD Spectral Data for the 1Bb Transitions of Substituted [6]Helicenes helicenes 1-aza[6]helicene N-oxide 2-bromo[6]helicene 2-aza[6]helicene 2-aza-15-methoxy[6]helicene 3-(8-phenylocta-1,7-diyn-1yl)-4-aza[6]helicene 2,15-dimethoxy[6]helicene 2,15-dicyano[6]helicene 2,15-bis(hydroxymethyl)[6] helicene bis(2-methylpropyl)[6] helicene-2,15-dicarboxylate 1,1,1′,1′-tetrakis(4methylphenyl)-1,1′-([6] helicene-2,15-diyl) dimethanol hexahelicene (CH[6]) a

λ/ nm

Δε/ M−1 cm−1

solvent

ref

340 328 325 332 330

+198 +240 +177 +66 +200

acetonitrile a a dichloromethane dichloromethane

12 13 13 14 15

336 340 329

+235 +222 +250

a a n-hexane

5 5 16

340

+220

n-hexane

16

331

+225

n-hexane

16

324

+259

acetonitrile

2

energy and in rotational strength, as was the case with CH[n] (Chart 1). We will neglect both the vibrational contribution and the solvent effect in this study as these effects on the theoretical chiroptical properties have been found marginal in the case of CH[n].2 In this study, we will first show the experimental and theoretical investigations on the CD spectra of various C2-symmetric dimethyl[6]helicenes (1,1′DM[6], 2,2′DM[6], 3,3′DM[6], 4,4′DM[6], and 8,8′DM[6], Chart 1). The substitution effect observed for DM[6] is composed of the electronic and steric parts, the former of which is associated with the positive inductive effect and/or the substitution pattern of two methyl groups, whereas the latter is related to the geometry change in helical structure. These two parts can be separated by theoretical calculations on model compounds (vide infra). We will then briefly discuss the effect of symmetry using unsymmetrical dimethyl[6]helicene (1,3′DM[6]) and (mono)methyl[6]helicene (3M[6]) (Chart 1). It is also interesting to examine the chiroptical properties of Nincorporated aza[6]helicenes, the coordination ability of which may find a number of prospective applications.11 We thus expanded the scope of the study to C2-symmetric diaza[6]helicenes (1,1′DA[6], 2,2′DA[6], 3,3′DA[6], and 4,4′DA[6]), where the steric perturbations become negligible and pure electronic effects are to be extracted (Chart 1). Finally, we will theoretically examine the effects of substitution on the CD spectra of (mostly unknown) helicenes by employing a series of 3,3-disubstituted [6]helicenes (Chart 2). Through these systematic investigations, we will provide a comprehensive picture of the substitution effect on the chiroptical properties of helicenes.

Not reported.

the substituent-sensitive 1Lb band. To the best of our knowledge, no such attempt has hitherto been reported. In the present study, we choose hexahelicene as the basis, because this is geometrically minimal in forming a single welldefined helical turn and the experimental data of substituted helicenes, though limited, are available (Table 1) for comparison with the results of theoretical calculations. As will be shown below, the approximate coupled cluster linear response theory calculations successfully reproduced the experimental UV−vis and CD spectra for all (di)methyl- and diaza[6]helicenes examined (1,1′DM[6], 3,3′DM[6], 1,3′DM[6], 3M[6], 1,1′DA[6], and 3,3′DA[6]) both in excitation

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COMPUTATIONAL METHODS All calculations were performed on Linux-PCs by using the TURBOMOLE 6.3 program suite or newer versions.17 Details 84



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The selected geometrical parameters for all the (di)methyland diaza[6]helicenes examined are summarized in Table 2. The dihedral angles of the inner helix27 and the helical pitch28 (rp, obtained by least-squares-fitting to the equation: z = rp × sin−1[y/(x2 + y2)0.5]) are the most frequently used critical parameters for the evaluation of helical structure. The detailed explanation of these and other parameters may be found in our previous report.2 We confirmed the validity of our DFT-D method in geometry optimization of substituted [6]helicenes with 1,1′-dimethyl-29 and 1,1′-diaza[6]helicenes,30 X-ray structures of which have been reported in the literatures. In short, the nonbonded C/N(1)−C/N(16) and C/N(2)−C/ N(15) distances (r1 and r2) were found slightly overestimated by theory the differences in distance found between crystal and theory were only ∼3%. The circumferential dihedral angles of inner helix around ring B or C were also well reproduced by theory. The replacement of aromatic C−H unit(s) by nitrogen(s), affording (di)aza[6]helicenes, caused less significant structural changes than anticipated. Thus, the interplane angles were only slightly altered (≪1°). The helical pitch and nonbonded distances were decreased (leading to a bit smaller helix) in 1,1′DA[6] by introducing smaller-sized N atoms (instead of C−H) at the steric bulk-conscious 1-position. The geometrical changes found in 2,2′DA[6] were not very straightforward (see the dihedral angle change), most probably due to the electronic influence of nitrogen atoms (and/or lone pairs), mostly on the substituted rings. The structures of 3,3′DA[6] and 4,4′DA[6] were almost intact. At all events, the geometrical deviations from CH[6] were rather marginal in diaza[6]helicenes. Except for the decrease of the nonbonded distance in 2,2′DM[6], the position of methyl substitution did not make significant differences in structure and the helical pitch was also unaffected for 2,2′DM[6], 3,3′DM[6], and 4,4′DM[6]. This result indicates that the deformation or strain caused by introducing methyls at the C2-positions is not distributed over the whole helicene structure but is rather localized at the terminal benzene rings. In contrast, the introduction of methyls at 1-position significantly altered the geometry of helicene, obviously due to the steric crush. Thus, the mean-plane angles between rings A and B as well as the dihedral angles around ring B were significantly increased in both 1,1′DM[6] and 1,3′DM[6] by 3.8° and 3.3°, respectively (note that two terminal rings are not identical in 1,3′DM[6]). Interestingly, the change was not very apparent for the other mean-plane angles and the dihedral angles of the inner rings, demonstrating again that the distortion rarely propagates from the terminal rings to the inner ones. Although the deviations appears to be comparable between 1,1′DM[6] and 1,3′DM[6] (despite that the number of methyl group(s) at the steric bulk-sensitive C1position is different), a closer inspection of the nonbonded distance and the helical pitch revealed that the effect of methyl group is rather additive and the inner helical pitch gradually increases from 3.10 (CH[6]) to 3.27 (1,3′DM[6]) and then to 3.34 Å (1,1′DM[6]). This structural change is consistent with the additivity observed for their CD spectra (vide infra). Introduction of methyl groups at inner benzene rings (8,8′DM[6]) led to a larger deformation at the inner rings (the mean-plane angle of the inner rings was increased by 5.5°) but afforded a modest effect on the helix geometry (inner pitch =3.29 Å). Comparison of Theoretical and Experimental CD Spectra. In our previous studies on a series of carbo[n]-

Chart 2. Structures of 3,3′-Disubsituted [6]Helicenes Examined in This Study

of the theoretical calculations for UV−vis and CD spectra of helicenes have been described in our earlier reports.2,18 Briefly, geometry optimization was performed at the DFT-D2-B97-D/ ZTVP level19 using appropriate symmetry constraint (C1 for 1,3′DM[6] and 3M[6], C2 for all other symmetrical helicenes). All excited-state calculations were then performed by using the time-dependent, second-order approximate coupled-cluster singles and doubles model, in conjunction with the resolution-of-identity method and the basis-set of TZVPP quality (i.e., RI-CC2/TZVPP level).20 Forty excitations were considered in all the [6]helicenes calculated in this study. The UV−vis and CD spectra were simulated by overlapping the individual Gaussian functions for each transition, where the bandwidth at 1/e height is fixed at 0.5 eV. Dipole moments can be drawn in vector manner, as the values are found in the output file of ricc2 calculation module, unit(s) of which are converted appropriately. Thus, the calculated dipole operator (r) in relatively robust length expression and the angular momentum (L) were numerically converted to the electric (μe) and the magnetic (μm) transition dipole moments, respectively.21 The optical (specific) rotations at the sodium-D line (589.3 nm) were calculated by the time-dependent density functional theory calculations (TD-DFT) with the BH-LYP22 and B3-LYP23 functionals using the Dunning’s aug-cc-pVDZ basis-set.24 For clarity, both the theoretical and experimental chiroptical properties are reported for (P)-(+)-isomers throughout the work (whereas the sign was inverted if the literature data was available only for the antipodal (M)(−)-enantiomer). Note that the theoretical spectra at the RICC2/TZVPP level were not adjusted (no scaling or energyshift applied) and were directly compared with the corresponding experimental data.

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RESULTS AND DISCUSSION Structures of Substituted [6]Helicenes. The structural changes of [6]helicenes upon substitution by methyl group(s) or replacement with nitrogen atom(s) were considered by the DFT-D2-B97-D/TZVP level of theory.19 It has been shown that reasonable geometries can be obtained in a cost-effective manner by the dispersion-corrected density functional theory calculations for a series of carbo[n]helicenes.2,25 As usual, we used the C2-symmetry constraint for the calculations of symmetrically substituted [6]helicenes to further reduce the time required for calculations. Although the X-ray structures of helicenes are usually deviated from the perfect C2-symmetry,26 theoretical CD calculations based on C2-symmetric geometries have shown to successfully reproduce the experimental spectra.2 85


helicenes,2 the state-of-the-art RI-CC2/TZVPP20,31 method was found most reliable for calculating CD spectra of helicenes. Before discussing the effects of type, position, and number of substituent on the CD spectrum of [6]helicene, we first examined how suitable the same theoretical method is in reproducing the CD spectra of methyl- and aza-substituted helicenes. Figure 1 compares the experimental and theoretical

Mean-plane angles (rings A−C were defined from the terminal), average deviation from the nonweighted mean-plane of six atoms in a benzene, inner helix C−C−C−C dihedral angle, beginning from C(1) carbon (circumference around ring B or C), nonbonded C/N(1)−C/N(16) and C/N(2)−C/N(15) distances (r1 and r2), and the helix pitch at the inner carbon atoms (rp1) and mean-center of rings (rp2) are compared. bValues for the X-ray structures in the parentheses.29 cValues for the X-ray structures in the parentheses.30

rp2 rp1

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Figure 1. Comparison of experimental and theoretical CD spectra of dimethyl[6]helicenes (1,1′DM[6], 3,3′DM[6], and 1,3′DM[6]), methyl[6]helicene (3M[6]), and diaza[6]helicenes (1,1′DA[6] and 3,3′DA[6]). Black: experimental spectra in acetonitrile at 25 °C. Blue: theoretical spectra calculated at the RI-CC2/TZVPP//DFT-D2-B97D/TZVP level.

CD spectra of six representative substituted helicenes; for the corresponding UV−vis spectra, see Figure S8 in the Supporting Information. The experimental CD spectra of enantiomerically pure (>99% ee) samples were obtained in acetonitrile at 25 °C for three dimethyl[6]helicenes (1,1′DM[6], 3,3′DM[6], and 1,3′DM[6]), monomethyl[6]helicene (3M[6]), and two diaza[6]helicenes (1,1′DA[6] and 3,3′DA[6]), after the optical resolution with chiral HPLC of racemic samples (see Figure S1, Supporting Information, for details). As a consequence of the C2-symmetric structure, the main transitions of symmetrically substituted [6]helicenes observed in the UV−vis and CD spectra can be categorized into A (C2-axis polarized) and B (perpendicular to the A-axis) symmetry groups. Fairly strong bisignate negative and positive CEs were observed at around 250 and 320 nm in (P)-(+)-helicenes, which are assigned as 1Ba and 1Bb transitions, respectively, after the Platt’s nomenclature.32 An additional weak CE was observed (not always

a

4.06 (4.03) 4.09 4.24 4.23 4.37 4.10 4.23 3.92 (3.82) 4.17 4.28 4.24 4.24

r2 r1

3.39 (3.37) 3.07 3.12 3.13 3.20 3.23 3.11 2.89 (2.79) 3.11 3.11 3.12 3.12 24.2 (23.0) 27.6 27.6 27.6 29.8 26.4/24.4 27.4/27.6 27.3 (26.5) 26.8 28.1 27.6 27.6

ring C ring B

29.2 (31.5) 14.8 14.8 15.7 13.7 27.0/17.7 14.9/15.0 12.6 (11.8) 16.1 13.1 14.8 14.8

ring C

0.643 0.451 0.395 0.422 0.262 0.540/0.483 0.439/0.361 0.192 0.426 0.424 0.843 0.467

ring B

12.7 13.1 13.2 13.2 18.6 12.9 13.2 12.5 12.7 13.1 13.2 13.1

0.311 0.273 0.233 0.256 0.184 0.305/0.235 0.298/0.194 0.175 0.252 0.256 0.484 0.265

ring A

0.122 0.068 0.067 0.086 0.109 0.116/0.077 0.092/0.056 0.117 0.094 0.068 0.136 0.074

∠CC′ ∠BC

11.8 12.7 12.7 12.7 11.9 12.8/11.0 12.8/12.6 12.7 12.4 13.1 12.7 12.7

∠AB

12.9 9.2 9.2 9.1 8.9 12.4/12.9 9.1/9.3 9.2 9.8 9.0 9.2 9.1

helicenes

1,1′DM[6]b 2,2′DM[6] 3,3′DM[6] 4,4′DM[6] 8,8′DM[6] 1,3′DM[6] 3M[6] 1,1′DA[6]c 2,2′DA[6] 3,3′DA[6] 4,4′DA[6] CH[6]

nonbonded distance/Å dihedral angle/deg standard deviation from common plane/Å mean-plane angles/deg

Table 2. Structural Parameters Calculated for (Di)methyl- and Diaza[6]helicenes (DM[6]s and DA[6]s) at the DFT-D2-B97-D/TZVP Levela

3.43 3.10 3.10 3.10 3.29 3.28 3.10 3.05 3.10 3.11 3.10 3.10

pitch/Å

3.42 4.08 4.08 4.08 4.19 3.76 4.08 4.02 4.08 4.07 4.08 4.08


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Table 3. Theoretical CD Spectral Parameters for (Di)methyl- and Diaza[6]helicenes (compared with those of CH[6]) at the 1Bb and 1Lb Bands Calculated at the RI-CC2/TZVPP Levela helicenes

Δ|Δε|

Δ|μe|

Δ|μm|

Δε/Δε0

|μe|/|μe0|

|μm|/|μm0|

|cos θ|/|cos θ0|

1

Bb

1,1′DM[6] 1,1′DH[6] 2,2′DM[6] 3,3′DM[6] 3,3′DH[6] 4,4′DM[6] 8,8′DM[6] 8,8′DH[6] 1,3′DM[6]b 3M[6] 1,1′DA[6] 2,2′DA[6] 3,3′DA[6] 4,4′DA[6]

−81 −50 +6 +21 ±0 −31 −43 −6 −53 +7 −61 −48 −48 −30

−1.2 −0.8 −0.8 −0.2 ±0 +0.1 −0.8 −0.8 −1.9/−3.2 −0.6 −1.7 −0.3 −0.2 −1.2

−0.9 −0.2 +1.0 +1.3 ±0 −0.1 −4.0 −2.6 −2.8/−6.0 −0.8 −3.6 −0.6 −1.5 −3.0

+0.68 +0.81 +1.02 +1.08 +1.00 +0.88 +0.83 +0.98 +0.79 +1.00 +0.76 +0.81 +0.81 +0.88 1

1,1′DM[6] 1,1′DH[6] 2,2′DM[6] 3,3′DM[6] 3,3′DH[6] 4,4′DM[6] 8,8′DM[6] 8,8′DH[6] 1,3′DM[6] 3M[6] 1,1′DA[6] 2,2′DA[6] 3,3′DA[6] 4,4′DA[6]

+2.0 +1.3 +1.7 +1.1 ±0 +1.3 +0.5 +0.3 +2.0 −0.52 +9.8 +1.6 +0.08 +9.5

+0.03 −0.01 +0.02 −0.02 ±0 −0.02 −0.01 ±0 ±0 −0.01 +0.07 +0.01 +0.3 +0.09

0.72 0.81 0.81 0.95 1.00 1.02 0.81 0.81 0.56/0.26 0.86 0.60 0.93 0.95 0.72

0.89 0.98 1.12 1.16 1.00 0.99 0.52 0.69 0.66/0.28 0.90 0.57 0.93 0.82 0.64

0.90 0.91 1.08 1.02 1.00 0.87 1.11 1.06 0.86/0.88 0.96 0.99 0.96 0.91 1.19

Lb −19 −12 −16 −10 +1.0 −12 −4.0 −2.0 −19 +6.2 −97 −15 +0.3 −94

+0.05 −0.02 +0.05 −0.01 ±0 −0.01 −0.01 ±0 +0.02 ±0 +0.2 ±0 +0.1 +0.2

2.5 0.30 2 0.1 1 0.1 1 1 1 0.3 5 2 14 6

3.5 0.10 6 0.4 1 0.2 ∼0 1 3 1 16 1 11 24

1.0 4.6 0.6 2.2 1.0 3.7 4.3 1.5 0.9 2.4 4.5 5.4 0.03 4.2

a

Differential molar circular dichroism, relative electric and magnetic transition dipole moments, and relative cosine value of dipole moments, compared to the corresponding values calculated for CH[6]. For more detailed theoretical values, see Table S1 in the Supporting Information. bTwo transition dipoles were evaluated by theory.

geometry. Not only the excitation energies but also the rotational strengths (or molar circular dichroism, Δε) calculated for 1Bb band were remarkably less deviated from the experimental values (the deviation in Δε < 3%), excepting the 1,3′DM[6] case, for which the less-symmetrical structure seems responsible (more detailed discussion will be given later). Accordingly, we safely make use of the theoretical Δε values calculated for the 1Bb band of unknown [6]helicenes (especially C2-symmetric ones) to further explore the effects of substituent(s) on the CD spectral behavior of helicenes (vide infra). b. 1La and 1Lb Transitions. The forbidden 1La and 1Lb transitions are known to be less useful for the analysis of the CD spectra of pristine carbo[n]helicenes.2 Indeed, the 1La band of substituted [6]helicenes was also inept for the analysis of the CD spectra, as the rotational strength is much smaller than the allowed 1Ba/1Bb transitions and usually hidden under those bands. The similarly weak 1Lb band, however, may become valuable (at least qualitatively) if this lowest-energy transition is well separated from the nearby transitions, and this is the case for some substituted [6]helicenes. Briefly, the negative CE for the 1Lb band of parent CH[6] was correctly reproduced by theory (Δε410 nmexp = −0.3 versus Δε393nmtheory = −0.1 M−1 cm−1). The sign became positive for two symmetrical dimethyl[6]helicenes (1,1′DM[6] and 3,3′DM[6]), which

apparent, but usually) at the longer wavelength region, which can be assigned as the 1Lb transition. The same band assignments were applied to quasi-C2-symmetric 1,3′DM[6] and 3M[6] for simplicity throughout the work, because essentially the same CE patterns were observed for these quasi-C2-helicenes. a. 1Ba and 1Bb Transitions. The strong bisignate CEs observed at the 1Ba and 1Bb bands were reproduced by theory quite satisfactorily. One should note that our theoretical spectra were neither shifted in energy nor scaled in rotational strength. A shoulder at the 1Bb band, considered vibrational in origin,33 was not reproduced appropriately, as we did not include such effects in our calculations. The more precise theoretical treatment of (chir)optical properties of helicenes incorporating such effects remains to be elucidated in a future study but is obviously beyond the scope of this study.34 Overall, slight underestimations were observed in excitation energy and the deviation in energy was larger for the 1Ba band, in particular for diaza[6]helicenes (Figure 1), which may be attributed to the mixed nature of the 1Ba band region (composed of several transitions) and also to the incompleteness of the basis-set employed in this study. In contrast, the 1Bb band, which is usually derived from a single transition, was theoretically much better reproduced, and therefore we will mainly discuss the CE of this band, which is also more sensitive to the changes in helix 87



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Figure 2. Comparison of the theoretical CD spectra of (di)methyl- and diaza[6]helicenes calculated at the RI-CC2/TZVPP//DFT-D2-B97-D/ TZVP level. Left: Dimethyl[6]helicenes. Middle: Unsymmetrical (di)methyl[6]helicenes. Right: Diaza[6]helicenes.

b. Comparison of Theoretical CD Parameters of Substituted [6]Helicenes with CH[6]. We performed the RICC2/TZVPP calculations for all the substituted[6]helicenes listed in Chart 1. The theoretical (and experimental, if available) excitation wavelengths (λ), molar extinction coefficients (ε), molar circular dichroisms (Δε), and anisotropy (g = Δε/ε) factors at the 1Ba, 1Bb, 1La, and 1Lb bands are listed in Tables S1 and S2 in Supporting Information. Note that the experimental values for weak 1La and 1Lb bands are apparent ones, the sign (and magnitude) of which would be affected or altered by the nearby transitions. The intensity of CE is determined by a scalar product of electric (μe) and magnetic (μm) transition dipole moments,21 which are readily evaluated by theory through electric dipole operator (r) and angular momentum (L). Therefore, the magnitudes of these dipole moments and their mutual angles (θ), as well as the assignment of molecular orbital configurations, are also included in the tables. To facilitate ready assessment of the substituent effects, the deviations of the Δε values at the 1Bb and 1Lb bands from those of parent CH[6] and relative μe, μm, and cos θ values (against the corresponding values for CH[6]) are tabulated in Table 3. c. Comparison of CD Spectra of Isomeric Dimethyl[6]helicenes at the 1Bb Band. For our systematic investigation of substitution effect on the CD spectra of [6]helicenes, we chose a series of C2-symmetric dimethyl[6]helicenes as the initial model. We will primarily focus on the strong 1Bb band that can be used as a sensible measure of the helix structure changes.2 Substitution at the 1-, 2-, 3-, 4-, or 8-position causes distinct structural changes that depend on the position of methyl groups, whereas the electronic inductive effect also independently operates in different directions. For (P)-helicenes, the tangential transition dipoles of the 1Bb state are combined to give a counter-clockwise charge rotation, which determines the direction of electric transition dipole moment (μe), and eventually the CE intensity. Thus, the inductive effect of methyl at the terminal ring is expected to give considerably large influence on the intensity and direction of mutual μe. The theoretical CD spectra of symmetrical dimethyl[6]helicenes (1,1′DM[6], 2,2′DM[6], 3,3′DM[6], 4,4′DM[6], and 8,8′DM[6]) are compared in Figure 2a; the corresponding UV−vis spectra in Figure S9 in Supporting Information. A comparison of the numerical data for the 1Bb (and 1Lb) bands with those of CH[6] is provided in Table 3. It is to note that the model compound 3,3′DH[6], which has the same geometry as 3,3′DM[6] but the two methyls are replaced by hydrogens, possesses essentially the equivalent helical structure

was also correctly reproduced by our calculations (Figure 1). The CEs were displayed mostly due to the electronic rather than geometric consequences as judged from the examination of model compounds (1,1′DH[6] and 3,3′DH[6], vide infra). On the contrary, The signs were not correctly reproduced in unsymmetrically methylated [6]helicenes (1,3′DM[6] and 3M[6]). This can be ascribable, at least in part, to the considerable overlap of nearby (stronger) transitions, but the exact reason remained to be elucidated. The sign of 1Lb band was opposite between 1,1′DA[6] and 3,3′DA[6] but correctly reproduced by theory, whereas the intensity showed larger inconsistency, again due to the overlap of nearby transitions. In addition, the vibrational features were not reproduced as anticipated (vide supra). At any rate, the sign and intensity of the 1Lb band are critical functions of the angle (and magnitude) of the relevant electric and magnetic transition dipole moments, and thus sensitive to the electronic perturbation (i.e., the values are critically affected by substitution). Therefore, the 1Lb transition can be used as a sensible measure of the electronic nature of helicene, if the possible perturbation of nearby transitions is (intentionally or unintentionally) eliminated by substitution. From the calculation point of view, it is recommended to directly inspect the calculated rotational strength of the 1Lb transition, rather than merely compare the Gaussian function-expanded spectra (in the current case, compare using data in Table S1, Supporting Information rather than referring Figure 1). Effect of Substitution on CD Spectra of [6]Helicenes. a. Insight from the Literature. As mentioned above, the literature CD data available for substituted helicenes are rather limited (Table 1). The Δε values reported for the 1Bb band of substituted [6]helicenes and aza[6]helicenes are mostly in a narrow range of 180−250 M−1 cm−1, except for 15-methoxy-2aza[6]helicene (Δε = 66 M−1 cm−1). Interestingly, all of the substituted [6]helicenes consistently afford more or less smaller Δε values than parent CH[6] (Δε = 259 M−1 cm−1), irrespective of the electronic nature of the substituent. Among the substituted [6]helicenes listed in Table 1, 2,15bis(hydroxymethyl)[6]helicene gave the largest Δε, whereas aza[6]helicenes the smallest. With this limited, unsystematic list of the data, we were initially unable to find any direct relationship between the magnitude of CE at the 1Bb band and the nature of substitution. However, the present study to calculate the CD spectra of a series of methyl- and azasubstituted [6]helicenes allowed us to rather straightforwardly explain the magnitude of CE by a combination of the steric (geometrical) and electronic factors (vide infra). 88



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are comparable to θ of 69.0° for CH[6]), which is in stark contrast to the behavior of 1Lb transition (Figure 3b). The maximal total effect of the angle change on the CE may amount to as large as ∼10%. d. Effect of Helicene Geometry. CD Spectra of Model Compounds DH[6]s. Experimental CD spectra of all substituted [6]helicenes were remarkably well reproduced by the theoretical calculations at the RI-CC2/TZVPP level (vide supra). Thus, the effects of electronic and geometric factors observed in the experimental CD spectra were further examined by using model compounds DH[6]s with the aid of theoretical calculations. Thus, 1,1′DH[6], 3,3′DH[6], and 8,8′DH[6] were constructed by using the geometries of the corresponding dimethyl[6]helicenes (i.e., 1,1′DM[6], 3,3′DM[6], or 8,8′DM[6]), but the two methyl groups were replaced by hydrogen atoms (consequently these models are formally parent carbo[6]helicene but in slightly different geometries). By calculating the CD spectra for these model compounds, one can sort out the pure structural effect from the electronic effect of methyl groups. Figure 4 compares the CD spectra of the models and their parent dimethyl[6]helicenes (for UV−vis spectra, see Figure S10 in Supporting Information) and numerically compared in Table 3. The comparison of 1,1′DM[6], 1,1′DH[6], and CH[6] revealed that both the electronic and geometric factors almost equally contribute to the observed CD spectrum of 1,1′DM[6]. Inspection of the parameters for the model showed that, by introducing inductive methyl groups, both electric and magnetic moments are decreased (i.e., the absolute values of μe and μm are governed mostly by electronic effect), whereas θ is almost unaffected by the electronic effect (i.e., geometrical change plays more important role in determining θ). In μe, an additional effect was observed, due to the structural factors. In contrast, all the effects appear to be electronic in nature for 3,3′DM[6] as the CD spectrum of the model is identical to that of CH[6]. Comparison of the CD spectral features of 8,8′DM[6]/DH[6] with those of 1,1′DM[6]/DH[6] led us to a conclusion that the CE is more sensitive to the substitution at the terminal rings than at the inner rings. Although the electric transition dipole moment is reduced comparably for 1,1′DM[6] and 8,8′DM[6] (strictly speaking, slightly larger for the former than for the latter due to the greater geometrical changes), the magnetic moment is more severely affected in 8,8′DM[6]/DH[6]. However, the overall CEs observed are canceled out by the preferable change of the angle (θ) in 8,8′DM[6]. Accordingly, the observed CE is more seriously altered in 1,1′DM[6] than in 8,8′DM[6]. Such a tendency is seen also in model compound 8,8′DH[6], where the CE is apparently unaffected but the parameters are substantially affected in a similar way, which is in sharp contrast to the 1,1′DH[6] and 3,3′DH[6] cases. Finally, we wish to briefly comment on the 1Lb band parameters calculated for the model compounds. All the parameters μe, μm and their angle θ are almost unaffected in both 8,8′DH[6] and 8,8′DM[6], but these parameters are incredibly affected (in the opposite directions) in 1,1′DH[6] and 1,1′DM[6]. Therefore, it is safe to conclude that the CE at the 1Lb band is more sensitive to the substation at the terminal rings (than the central ring substitution). e. Comparison of CD Spectra of Unsymmetrically Methylated [6]Helicenes. In this section, we briefly describe the CD spectral behavior of unsymmetrically substituted [6]helicenes. Naturally, the unsymmetrical helicenes are more

to parent CH[6]. As for the 1Bb transition, sterically more congested 1,1′DM[6] afforded considerably red-shifted positive CE (Δλ = 19 nm) with a reduced intensity, whereas small energy shifts (and strengths) of similar magnitudes (Δλ ≤ 5 nm) were found for 2,2′DM[6], 3,3′DM[6], and 4,4′DM[6], which may be ascribed essentially to the pure electronic effect of methyl groups. The shift was moderate for 8,8′DM[6] (Δλ = 9 nm). The intensity of CE at the 1Bb band was also affected by the introduction of methyl groups. Thus, the CE was slightly enhanced for 2,2′DM[6] and 3,3′DM[6] but reduced for 4,4′DM[6] and 8,8′DM[6]. The effect was more evident for 1,1′DM[6], where the observed Δε value was reduced to ∼70% of CH[6]. This apparently puzzling behavior of the effect of introducing methyls can be more clearly explained by using the parameters μe and μm. Thus, the electric dipole moment gradually decreases from 4.4 for 4,4′DM[6], to 4.1 for 3,3′DM[6], then to 3.5 for 2,2′DM[6], and finally to 3.1 for 1,1′DM[6] by shifting the methyls from the outer 4- to the inner 1-position. This order is in nice agreement with the expectation that the dipole moment is composed of the tangential transition of each benzene ring and hence becomes smaller by moving the methyls from the outer to inner positions. In contrast, the magnetic moment displays very different behavior upon methylation. The μm value is enhanced by 12−16% for 2,2′DM[6] and 3,3′DM[6], presumably as a result of the horizontal expansion of molecule size and electron distribution but shows no change for 4,4′DM[6] due to the lack of such elongation in the helical direction. The small (11%) reduction in μm found for 1,1′DM[6] is also attributable to the contracted effective size of electron distribution. Remarkably, the decrease in μm is substantial (48%) for 8,8′DM[6]. The angle between μe and μm is of additional importance in determining the CE intensity (Figure 3a). However, this angle was found not very sensitive to methyl substitution; the θ values of 67.3−71.8° for terminal methylation and 66.6° for inner methylation (8,8′DM[6])

Figure 3. Schematic drawings of the calculated electric transition dipole moments of dimethyl- and diaza[6]helicenes at the 1Bb and 1Lb transitions. Arrow colors are matched to those of the spectra in Figure 2. 89



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Figure 4. Comparison of the theoretical CD spectra of (di)methyl[6]helicenes and their model carbo[6]helicenes calculated at the RI-CC2/ TZVPP//DFT-D2-B97-D/TZVP level. Left: 1,1′-Dimethyl[6]helicene and its model. Middle: 3,3′-Dimethyl[6]helicene and its model. Right: 8,8′Dimethyl[6]helicene and its model.

moments are reduced by aza-substitution in the order CH[6] > 2,2′DA[6] ∼ 3,3′DA[6] > 4,4′DA[6] > 1,1′DA[6]. Although the introduction of electronegative nitrogen (or lone pair) by itself reduces the magnitudes of these parameters, the alignment perpendicular to the helical geometry is of additional influence on such effect. The angles between dipoles (θ) in diaza[6]helicenes are schematically shown in Figure 3a. Again, the angle is not very much affected as is the case in dimethyl[6]helicenes. The angle for the 1Lb transition was found very sensitive to the position of introduced nitrogen(s) (Figure 3b). Thus, the angle gradually increased from 69.2° (1,1′DA[6]) to 69.8° (2,2′DA[6]) and then to 70.9° (3,3′DA[6]) but suddenly decreased to 64.7° for 4,4′DA[6], which affected the observed CE intensity in the opposite way. Despite the fact that the angle θ between the two moments (μe and μm) and their magnitudes independently contribute to the observed CE, mutual compensation resulted in the apparent ordering of the CE intensities at the 1Bb band of 1,1′DA[6] to 4,4′DA[6]. g. Comments on the Specific Rotations. A variety of chiroptical methods (other than electronic CD) have been employed for elucidating configuration and conformation of chiral molecules.36 Indeed, the specific rotations of helicenes are outsized and the signs are also useful as a (Supporting Information) tool for determining the absolute configuration.37 Although the accurate prediction of specific rotations by quantum chemical calculation is rather challenging in general, we also addressed it briefly by using the TD-DFT method. Advanced TD-DFT studies on optical rotation (OR) have appeared recently for relatively small helicenes38 and for a series of carbo[n]helicenes.2 Our calculations with the standard protocol (B3-LYP/aug-cc-pVDZ) overestimated the specific rotations of (di)methyl- and diaza[6]helicenes, whereas the use of BH-LYP/aug-cc-pVDZ method led to somewhat better agreement with the experimental values (Table S3 in Supporting Information). However, we were unable to find any general rule for directly explaining the specific rotation values of these helicenes, despite the Kramers−Kronig relationships transform the absorptive CD features to the dispersive OR values. The substitution effect on the specific rotation of helicene remains an issue to be elucidated. h. Effect of Substituent(s) Elucidated by the CD Spectral Investigation of 3,3′-Disubstituted[6]helicenes. Finally, we further extended our study of substituent effects on the CD spectrum of helicene to a variety of 3,3′-disubstituted [6]helicenes (Chart 2). The choice of 3,3′-positions relies on the fact that the introduction of substituents at these positions

general, but the calculation of the CD spectrum becomes more demanding. As mentioned earlier, the assignment of 1Bb (and other) bands was done by direct comparison of the observed spectra with those of related C2-symmetric helicenes. The theoretical CD spectra of 1,3′DM[6] and 3M[6] are compared with that of CH[6] (Figure 2b). The difference between 3M[6] and CH[6] seems quite insignificant from the figure, but the two important parameters (i.e., energy shift and a small increase of CE intensity) indicate the intermediate nature of 3M[6] between CH[6] and 3,3′DM[6] (Table 3). This demonstrates the additive nature of the substitution effect on the CE of helicenes at the 1Bb band. Similarly, the CD spectrum of 1,3′DM[6] can be explained quite satisfactory as a hybrid of 1,1′DM[6] and 3,3′DM[6], supporting the additivity of substitution effect. One should note that without using C2constraint, our theoretical calculations were also able to reproduce the experimental spectra with reasonable agreement for both 3M[6] and 1,3′DM[6]. f. Comparison of CD Spectra of Diaza[6]helicenes at the 1 Bb Band. To expand the scope of this study to cover the effects of heteroatom incorporation on the chiroptical properties of helicene, we next examined the CD spectra of C2-symmetric diaza[6]helicenes. Azahelicenes have been extensively employed recently in a number of application studies in supramolecular recognition and asymmetric synthesis.11 Besides, 1-aza[6]helicene is currently commercially available in milligram quantities (or can be readily prepared). The CD spectra of 1- and 2-aza[6]helicenes, as well as several aza[5]helicenes, have been reported,35 but the chiroptical data are rather fragmentary and the effect of incorporating nitrogen atom in π-skeleton was not well elucidated. In view of the present status of azahelicene chemistry, the use of symmetrical azahelicenes is expected to facilitate the analysis of the substitution effect on CD spectra, and the results can be extended to unsymmetrical ones. We therefore compared the experimental CD spectra of C2-symmetric diaza[6]helicenes (1,1′DA[6] and 3,3′DA[6]) with the theoretical results (vide supra). The identical method employed above successfully reproduced the experimental spectra of these azahelicenes. The original helical structure of CH[6] was almost intact in diaza[6]helicenes, which allowed us to directly address the effect of aza-substitution. The CE intensity of the 1Bb band is consistently smaller for diaza[6]helicenes than for parent CH[6] and gradually decreases in the order 4,4′DA[6] > 3,3′DA[6] ∼ 2,2′DA[6] > 1,1′DA[6] (Figure 2c). Closer inspection of the μe and μm values revealed that both the electric and magnetic dipole 90



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Table 4. Theoretical CD Spectral Parameters for Symmetrical 3,3′-Disubstituted [6]Helicenes at the 1Bb Banda substituent at 3/3′ positions

λ/nm

ε/M−1 cm−1

Δε/M−1 cm−1

g factor/103

|μe|

|μm|

θ/deg

H (CH[6]) Me (3,3′DM[6]) t-Bu CN −CCH −CHCH2 (s-cis)b -CHCH2 (s-trans)b Ph F Cl Br OH OMe NH2

317 321 327 324 327 332 333 333 319 322 323 325 328 338

32 300 31 400 26 500 32 500 35 000 32 800 34 500 35 300 29 700 32 000 32 300 25 700 26 200 19 700

+257 +278 +231 +310 +339 +384 +264 +338 +259 +289 +298 +250 +240 +209

+8.0 +8.9 +8.8 +9.5 +9.7 +11.7 +7.6 +9.6 +8.7 +9.0 +9.1 +9.7 +9.2 +10.5

4.3 4.1 3.5 4.3 4.3 3.4 4.0 4.0 4.0 4.3 4.4 2.7 3.0 1.4

8.3 9.6 13.5 8.6 9.6 11.3 8.4 13.1 8.1 8.4 8.4 6.7 7.5 4.3

69.0 68.6 73.7 64.6 64.0 59.3 68.0 66.9 67.9 66.3 65.8 63.3 66.6 54.1

a Excitation wavelengths (λ), molar extinction coefficients and circular dichroism (ε and Δε), anisotropy factors (g = Δε/ε), electric (μe) and magnetic (μm) transition dipole moments in atomic unit, and their mutual angle (θ) calculated at the RI-CC2/TZVPP level. bNote that the s-cis isomer was found more stable than the s-trans isomer by 1.5 kcal mol−1 based on the calculations at the DFT-D2-B97-D/TZVP level.

and smaller angle θ due to the increased effective electron densities of larger halogen atoms.

causes only minimal steric effects on the helix structure or structural deviations from CH[6] (Table 3) and hence pure electronic effects are expected to be extracted. We theoretically tested the effects of steric bulk (methyl and tert-butyl), conjugation (ethynyl, cyano, vinyl, and phenyl), and inductive halogens and heteroatoms (oxygen and nitrogen). The CEs at the 1Bb band, together with the transition moment vectors (μe and μm), calculated at the RI-CC2/TZVPP level are compared in Table 4. The effect of substitution on the transition energy (absorption wavelength) of the 1Bb band was substantial to cause significant bathochromic shifts of 2−21 nm for the 3,3′disubstituted [6]helicenes examined, irrespective of the type of substituent. Such substituents that are attached to helicene with a carbon atom (which possesses positive inductive effect) generally enhance the molar circular dichroism (Δε), except for tert-butyl, for which the larger deviation in dihedral angle θ and the reduction of electric moment μe are thought to be jointly responsible. Interestingly, the molar extinction coefficient (ε) of di-t-Bu-[6]helicene is simultaneously reduced to afford an anisotropy (g) factor of +8.8 × 10−3, which is comparable to those of 3,3′DM[6] (+8.9 × 10−3) and CH[6] (+8.0 × 10−3). Inspection of the parameters revealed that the alkyl substitution affects more strongly the magnetic moment μm but slightly reduces the electric moment μe, and that such changes in transition moments also affect angles θ. The conjugating groups, such as phenyl, vinyl, ethynyl, and cyano, further enhanced the Δε value. For example, the predicted Δε value became as large as +384 M−1 cm−1 (a 1.5-hold enhancement compared to CH[6]) for helicenes substituted by vinyl groups (note that the s-trans conformer is energetically unfavorable by 1.5 kcal mol−1 than s-cis). For this compound, the g factor is expected to reach +1.2 × 10−2, a value 2 orders of magnitude larger than the typical values for allowed transitions.39 The introduction of negatively inductive oxygen or nitrogen (i.e., substitution by OMe, OH, or NH2) reduced the μe, and accordingly Δε. Such substituents, however, also reduced the extinction coefficient due to the increased forbidden nature of the relevant transition, eventually leading to the appreciably enhanced g factors of 9.2−10.5. A systematic increase in Δε was observed for a series of halogenated helicenes. This steady increase is a consequence of the larger moments (μe and μm)

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CONCLUSIONS Since the first report in 1956,40 helicenes have been a target of intensive theoretical and experimental studies in various areas of science and technology, owing to their unique helical chirality. In-depth understanding of the chiroptical properties, in particular the circular dichroism, of helicenes was certainly one of the main targets of these foregoing studies. Although we have recently reported a comprehensive study of the chiroptical properties of parent [n]helicenes,2 no systematic investigation of the effects of substitution on the CD spectra of helicenes has been done. Parent helicenes are structurally beautiful and theoretically pristine but need to be derivatized for the use in practical applications by introducing substituent(s), which inevitably alters the (chir)optical properties of substituted helicenes. In the present study, we have carried out the state-ofthe-art theoretical calculations of the CD spectra for a series of (di)methyl- and diaza[6]helicenes, as well as a variety of 3,3′disubstituted [6]helicenes. This combined theoretical and experimental study has promoted our understanding of the CD spectra of substituted helicenes in particular in the following aspects: (1) By introducing methyl group(s) to [6]helicene, we examined the two distinctive roles of substituent: i.e., electronic and steric effects. The steric effect of the methyl groups introduced at 1,1′- or 8,8′-positions significantly deforms the helix structure, which, however, does not propagate but is localized within the substituted benzene ring. This local deformation causes a significant reduction of CE at the 1Bb band, the magnitude of which is larger for the helicene substituted at the terminal, rather than central, rings in nice agreement with the degree of structural change. (2) The electronic effect of methyl substitution is positiondependent, affording enhanced CE at the 1Bb band of 2,2′DM[6] and 3,3′DM[6] (positive inductive effect) but reduced CE for 4,4′DM[6]. The electronic effect on CE is not immediately clear for deformed 1,1′DM[6] and 8,8′DM[6] due to the associated steric effect. However, the use of model compounds, 1,1′DH[6] and 91



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Sumitomo Foundation, the Shorai Foundation for Science and Technology, and the Kurata Memorial Hitachi Science and Technology Foundation are gratefully acknowledged.

8,8′DH[6], allowed us to offset the steric part of the substitution effect and eventually reveal the contribution of significant negative electronic effect for the former and a much less effect for the latter. (3) The studies on the unsymmetrically methylated [6]helicenes revealed that the effect of methylation is nearly additive, affording intermediate CE intensities for 3M[6] when compared with the values for 3,3′DM[6] and CH[6]. Similarly, the CE intensities of 1,3′DM[6] can be well explained as a hybrid of 1,1′DM[6] and 3,3′DM[6]. (4) The aza-substitution reduces the CE intensity at the 1Bb band, due to the negative inductive effect. The effect depends on the position of substitution, the Δε values being decreased in the order CH[6] > 4,4′DA[6] > 3,3′DA[6] ∼ 2,2′DA[6] > 1,1′DA[6]. Most of the CD spectral changes observed for diaza[6]helicenes are electronic in origin, whereas the steric effect is minimal. (5) The effects of substituent were further surveyed by theoretical calculations on a series of 3,3′-disubstituted [6]helicenes. This confirmed the positive (by carbon) and negative (by nitrogen and oxygen) inductive effect on the CD spectra of [6]helicenes. Addition of conjugating substituents such as phenyl, vinyl, ethynyl, and cyano groups, further intensifies the observed CEs. Substitution by a series of halogens also leads to a progressive increase of Δε with increasing size of halogen atom. (6) The CE at the 1Lb band is more sensitive to the substitution in [6]helicene. However, the observed CEs frequently overlap with strong nearby transitions. Therefore, caution should be exercised in analyzing the signs and magnitude of CE at the 1Lb band, which is difficult without the aid of quantum chemical calculations. This systematic study has elucidated the steric and electronic factors that control the CD spectral behavior of substituted helicenes and further underscored the above-mentioned general guidelines for analyzing the CEs of various substituted helicenes. These findings provide us with trustful insights into the CD behavior when we design new helicenes and related molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Details of preparation, enantiomer separation, spectroscopic data, and theoretical calculations for (di)methyl- and diaza[6]helicenes: HPLC traces, NMR and UV−vis spectra, CD spectral parameters, specific rotations, and optimized geometries. Complete refs 4i and 26b. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this research by a Grant-in-Aid for Scientific Research (No. 23350018, 24655029, and 21245011) from JSPS, the Mitsubishi Chemical Corporation Fund, the 92



Article

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Circular Dichroism of (Di)methyl- and Diaza[6]helicenes. A

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