A Combined Experimental and Theoretical Study on Circular

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

A Combined Experimental and Theoretical Study on Circular Dichroism and Circularly Polarized Luminescence of Configurationally Robust D-Symmetric Triple Pentahelicene 3

Hiroki Tanaka, Yuka Kato, Prof. Michiya Fujiki, Yoshihisa Inoue, and Tadashi Mori J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05247 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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The Journal of Physical Chemistry

A Combined Experimental and Theoretical Study on Circular Dichroism and Circularly Polarized Luminescence of Configurationally Robust D3Symmetric Triple Pentahelicene

Hiroki Tanaka,† Yuka Kato,‡ Michiya Fujiki,‡ Yoshihisa Inoue,† and Tadashi Mori†,*

† Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan †‡ Graduate School of Materials Science, Nara Institute of Scienc eand Technology, Ikoma, Nara 630-0101, Japan * E-mail: [email protected].

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ABSTRACT. Pentahelicene (PH) exhibits the largest absorption (gabs) and luminescence (glum) dissymmetry factors amongst the helicene family, but is configurationally and (photo)chemically labile, encumbering its application to chiroptical materials. To bypass the pitfalls, three PH units are

merged

in

a

single

molecule

to

build

D3-symmetric

triple

pentahelicene,

hexabenzotriphenylene (HBT), which attains indeed the configurational and (photo)chemical robustness through equilibrium with a C2-symmetric conformer that interrupts the racemization and photocyclization. UV-vis, circular dichroism (CD) and circularly polarized luminescence (CPL) spectral examinations reveal the significantly larger gabs and glum values for HBT than for any of configurationally robust single [n]helicenes (n ≥ 6) and C2-symmetric triple pentahelicene, trinaphthotriphenylene (TNT). Theoretical calculations precisely reproduce the main features of the experimental CD and CPL spectra of PH, HBT, and TNT, and the relevant electric and magnetic transition moments and their mutual angles well rationalize the relative CD and CPL intensities of all the single and triple pentahelicenes.


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Introduction Helicene, an ortho-fused helical array of aromatic rings, is unique in shape and property and has attracted tremendous interests.1,2,3,4 The helicene family finds various applications in chirality sensing,5,6,7 asymmetric catalysts,8,9,10 and chiroptical materials.11,12 Recent advances in organic synthesis have enabled to incorporate multiple helices in a single molecule, as exemplified by double,13,14,15,16,17,18,19,20,21 triple,22,23 and even quadruple24 helicenes reported to date. In one of such studies,5 we have demonstrated that the photophysical properties and in particular the chiroptical responses in the ground and excited states, i.e., circular dichroism (CD) and circularly polarized luminescence (CPL), are greatly enhanced in chiral S- and X-shaped double helicenes. In the present study to expand the scope to more sophisticated multiple helicenes, we rationally designed, prepared, and optically resolved the enantiomers of D3-symmetric triple pentahelicene, and theoretically investigated the chiroptical responses of this and other related single and triple pentahelicenes. For carbo[n]helicenes (n ≥ 6), the absorption dissymmetry factor (gabs = ∆ε/ε, where ∆ε denotes the molar CD and ε the molar extinction coefficient) at the main (1Bb) transition is known to increase with increasing n but the increment (per benzene unit) gradually diminishes at larger n.25 This means that simply extending the helix is not a cleaver, or cost-efficient, strategy for achieving higher gabs. Intriguingly, molar CD and gabs factor for the lowest-energy (1Lb or 1La) transition of pentahelicene (PH) are larger than any of the higher homologues.25 This fact leads us to an idea that PH may serve as a promising CPL molecule, since the luminescence dissymmetry (glum) factor is known to correlate with the gabs factor for a various type of chiral molecules including helicene.26 Recently, the CPL behavior of helicene and helicenoids has attracted considerable 3 ACS Paragon Plus Environment


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interest in fundamental science and practical application and thus has been investigated extensively. 27,28,29,30 Nevertheless, the glum factors reported so far for helicenes stay low and rarely reach the order of 10-2, at least in part due to the lack of comprehensive theoretical scrutinization. In this combined experimental and theoretical study, we investigated the CD and CPL behaviors of D3-symmetric triple pentahelicene (i.e., hexabenzotriphenylene, HBT) and parent pentahelicene (PH), the structures of which are illustrated in Scheme 1. Theoretical calculations were employed not only to reproduce the CD and CPL spectra but also to better analyze the origin of the strong chiroptical responses observed. We also compared the theoretical and recently reported experimental

31

CD spectra of C3-symmetric triple pentahelicene,

trinaphthotriphenylene (TNT).32 Our study revealed for the first time that PH emits CPL with the largest glum factor among the pristine helicene family. Crucially, HBT constructed by merging three PH units in D3 symmetry is made chemically stable and configurationally robust, i.e., free from the undesirable racemization and degradation, under the ambient experimental conditions employed. These results demonstrate that the new design principle based on the helical structure is advantageous for developing more promising CPL materials.


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Scheme 1. Parent pentahelicene (PH) and triphenylene-based triple helicenes HBT and TNT incorporating three PH units in D3 and C3 symmetry, respectively.



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Methods Electronic absorption (UV-Vis), circular dichroism (CD), fluorescence (FL), and circularly polarized luminescence (CPL) spectra were measured in non-degassed solution placed in a conventional quartz cell (light path 1 cm) fitted with a temperature controller. UV-vis spectra were recorded on JASCO V-650 or V-670 spectrometer under the following conditions: slitwidth, 1 nm; scan rate, 100 nm min-1; response, medium; data interval, 0.2 nm. CD spectra were recorded on a JASCO J-820YH spectropolarimeter under the following conditions: slitwidth, 2 nm; scan rate, 50 nm min-1; response, 4 sec; accumulation, 4 times; sensitivity, standard; data interval, 0.2 nm. FL spectra were recorded on a JASCO FP-6500 spectrofluorimeter under the following conditions: excitation slitwidth, 5 nm; emission slitwidth, 5 nm; scan rate, 100 nm min-1; response, 1 sec; sensitivity, medium; data interval, 0.2 nm. CPL spectra were recorded on a JASCO CPL-200 spectrofluoropolarimeter under the following conditions unless otherwise stated: scattering angle, 0°; excitation slitwidth, 5 nm; emission slitwidth, 5 nm; scan rate, 100 nm min-1; response, 4 sec; accumulation, 8 times (unless otherwise stated); data interval, 0.5 nm. Fluorescence lifetimes were determined by the timecorrelated single-photon-counting method on a Hamamatsu Quantaurus-Tau C11367-01 under the following conditions excitation (LED) wavelength, 365 nm; peak count, 2000. Optical rotations were measured in a cylindrical 10 cm cell at the sodium-D line (589.3 nm). All calculations were performed on Linux-PCs by using TURBOMOLE 7.2 33 or Gaussian 0934 program suite. Geometries of molecules were fully optimized using appropriate symmetry constraint (D3 for HBT and C3 for TNT) 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, [3s2p1d]; C, [5s3p2d1f]) at the TPSS-D3/def2-TZVPP level. 35,36,37,38


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The resolution of identity (RI) approximation was employed and the corresponding auxiliary basis-sets were taken from the Turbomole basis-set 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. The center of mass was set at the origin of the coordinate system. The UV-vis and CD spectra were calculated by the time-dependent, second-order approximate coupled-cluster singles and doubles model (RI-CC2 method) 39 , 40 , 41 and by the algebraic diagrammatic construction scheme that combines perturbation theory with configuration interaction (RI-ADC(2) method), 42 both in conjunction with the resolution-of-identity method using the def2-TZVPP basis-sets. The calculation for TNT with the RI-CC2 method was unsuccessful due to the convergence problem. The calculation was also verified for HBT by the TD-DFT method with several selected functionals using def2-TZVP basis-sets. The solvent effect was examined by the TD-DFT method with M06-2X functional using the COSMO solvation model43 for dichloromethane with def2-TZVP basis-sets. The calculated rotational strengths in length gauge were expanded by Gaussian functions and overlapped where the width of the band at 1/e height is fixed at 0.5 eV. Optical

resolution

of

D3-symmetric

dibenzo[f,j]phenanthro[9,10-s]picene,

(hexabenzotriphenylene, HBT). Racemic D3-HBT was prepared according to the literature procedure.44 This was optically resolved by preparative chiral HPLC on a Daicel Chiralpak IA column. Starting with 0.18 mg of the racenmic mixture, 0.07 mg (39%) and 0.07 mg (39%) of the first and second elutes were collected. Analysis of each resolved sample by analytical chiral HPLC (Figure S1 in the Supporting Information) revealed the optical purity higher than 99%. The first elute was assigned to the (P,P,P)-enantiomer by direct comparison of the experimental versus theoretically calculated CD spectra (vide infra). (P,P,P)-HBT: [α]D25 (c 0.00051, MeCN) 7 ACS Paragon Plus Environment


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= +1600 ± 100 deg cm3 g-1 dm-1. (M,M,M)-HBT: [α]D25 (c 0.00063, MeCN) = -1700 ± 100 deg cm3 g-1 dm-1.


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Results and Discussion Structure of Triple versus Parent Pentahelicene. Before discussing the chiroptical properties of triple versus single pentahelicene, we briefly compare the structures of two triple helicenes (HBT and TNT) with that of parent PH. The structures of pristine and modified PHs have been thoroughly discussed and the X-ray structure of PH is also available.45,46 Interestingly, three independent geometries exist in crystalline PH. The shortest C1-C14 and second-shortest C2-C13 non-bonded distances (r1 and r2) between the terminal benzene units of PH measure 2.92-2.95 Å and 4.59-4.66 Å, respectively. The dihedral angle of four successive inner-helix carbons is 16.0-20.2° (θ1) at the periphery but increases to 29.8-32.7° (θ2) at the center. These structural features were well reproduced by the geometry optimization; thus, the dispersion-corrected density functional theory at the DFT-D3(BJ)TPSS/def2-TZVP level35-37 afforded the comparable θ1 and θ2 and the slightly underestimated r1 and r2 shown in Table 1.



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Table 1. Experimental and Calculated Structural Parameters for Single and Triple Phentahelicenes in the Ground and Excited States a

Helicene

r1 / Å b

r2 / Å b

θ1 / ° c

θ2 / ° c

2.97

4.69

21.2

33.4

[2.97] d

[4.66] d

[20.0] d

[34.6] d

2.99

4.71

21.2 / 21.6

33.2

3.01

4.77

20.6 / 20.5

34.5

3.06

4.83

23.0 / 21.3

36.4

Calculation

2.90

4.57

20.7 / 15.3

33.4

X-ray f

2.91

4.58

18.2 / 18.2

33.6

2.93

4.66

19.2 / 14.9

34.8

2.94

4.63

19.5 / 17.9

34.5

2.90

4.55

18.2

29.4

[2.98] d

[4.60] d

[18.4] d

[31.5] d

2.92

4.59

16.4 / 20.2

29.8

2.95

4.65

16.0 / 17.7

32.1

2.95

4.66

16.8 / 17.3

32.7

Theory/ Experiment

HBT

Calculation

X-ray e

TNT

PH

Calculation

X-ray g

a

Geometries in the ground and excited states optimized at the DFT-D3(BJ)-TPSS/def2-TZVPP and at the RI-CC2/def2-TZVPP levels, respectively. b Non-bonded C(1)-C(14) (r1) and C(2)C(13) (r2) distances in a pentahelicene unit. c C-C-C-C dihedral angles at periphery (θ1) or center (θ2) of inner helix in a pentahelicene unit. d Values calculated for the excited state. e Due to crystal packing, the structure was not completely symmetrical to give three sets of parameters. See also ref. 48. f Due to crystal packing, the structure was not completely symmetrical to give three sets of parameters. See also ref. 31. g Three independent structures exist in a unit cell. See ref. 45, 46. 10 ACS Paragon Plus Environment

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Racemic D3-symmetric triple pentahelicene HBT has been already described.

47

Interestingly, C2-symmetric isomer was also prepared under the kinetically controlled conditions.44 This isomer, however, was shown to gradually isomerize to the thermodynamically more stable D3-symmetric isomer (HBT). Photochemical stability has not been discussed. The crystal structure of racemic HBT was also reported, revealing a highly twisted propeller-like structure.48 Three sets of parameters are provided in Table 1, as HBT was not completely D3symmetry in the crystal due to the packing requirement as well as the effects of multiple CH-π and π-π interactions. The deviations among the three pentahelicene units are insignificant and comparable to the differences found for the three independent structures of PH in the crystal. The geometry optimization at the DFT-D3(BJ)-TPSS/def2-TZVP level well reproduced all of the structural parameters (r1, r2, θ1 and θ2) and confirmed the D3-symmetric structure as the energy minimum. More importantly, all the parameters are significantly augmented in HBT relative to PH, for which the accumulated twisting distortion of the benzene ring shared by two or three merged pentahelicene units is responsible. Recently, the synthesis of another triple pentahelicene TNT was reported, together with its X-ray structure.31 Again, the crystal structure is not completely C3-symmetric, but the deviations among the pentahelicene units are almost negligible. The theoretical calculation at the same level confirmed the C3-symmetric structure for TNT and the deformation of each pentahelicene unit was similar to that of parent PH, revealing that HBT is much more deformed than TNT and PH. The deviation from planarity and its relation to aromaticity have been extensively discussed for various benzenoid hydrocarbons.49 The structural changes upon photoexcitation of HBT and PH from the ground (S0) state to the first excited singlet (S1) state were also investigated by the time-dependent approximate 11 ACS Paragon Plus Environment


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second-order coupled cluster singles and doubles model in conjunction with the resolution-ofidentity (RI-CC2) method with basis-sets of def2-TZVPP quality.40,41 The helical pitch was found slightly increased in the S1 state, the degree of which was less apparent in HBT than in PH. The overall structural difference between the S0 and S1 states was trivial (≤0.08 Å in r and ≤2.1° in θ) in both HBT and PH. This is rather unexpected from the behavior of homologous hexahelicene, which noticeably shrinks in the excited state to release the structural strain mainly located in the central part of molecule.13 However, this particular structural feature that the minor conformational relaxation upon excitation is required for HBT and PH is advantageous for carrying over the inherently strong CD response (gabs) in the ground state to the excited state and eventually emitting strong CPL with high glum factor.

Circular Dichroisms of D3- and C3-Symmetric Triple Pentahelicenes HBT and TNT. Although several multiple helicenes are known,13-24 the chiroptical properties are not much available for those with pentahelicene units. Optical resolution of parent PH has been already described in detail.25 As pristine PH easily racemizes and/or photocyclizes,50 the CD spectrum of (P)-PH was measured in a hexane-2-propanol mixture (98 : 2) immediately after chiral HPLC separation at 0 °C (Figure S2 in the Supporting Information) to show a strong positive Cotton effect (CE) at the 1Bb transition (ca. 310 nm), which is characteristic to the helicene chromophore. Also, a weak negative CE was observed at ca. 400 nm, which is assignable to the 1

La or S0-S1 transition.25 In this study, the optical resolution of HBT was achieved by chiral HPLC on a Daicel

Chiralcel IA column (Figure S1 in the Supporting Information). Figure 1 shows the experimental CD spectrum of the first-eluted HBT enantiomer in dichloromethane at 25 °C, together with the 12 ACS Paragon Plus Environment

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theoretical spectrum calculated at the RI-CC2/def2-TZVPP level. Comparison of the two spectra allows us to unambiguous assign the first elute to the (P,P,P)-enantiomer. Note that HBT was stable during the spectral examinations at ambient temperatures, without showing any signs of racemization or photocyclization. The experimental31 and theoretical CD spectra of TNT are also compared in Figure 1. We employed the algebraic diagrammatic construction scheme that combines the perturbation theory with configuration interaction (RI-ADC(2) method) 51 for the calculation of TNT, due to the technical problem in applying the RI-CC2 method to this highly symmetrical molecule. The comparable accuracy for the RI-CC2 and RI-ADC(2) methods was verified using HBT as an illustrative example. For further comparison of various computations, the theoretical CD calculations for HBT were also performed at the TD-DFT level using several representative functionals (Figure S3 in the Supporting Information). Briefly, among the various functionals examined, the global hybrid functional M06-2X and the asymptotically corrected functional CAM-B3LYP outperformed in reproducing the experimental CD spectra. Although the importance of vibrational coupling in the CD and CPL spectra of helicenes has been emphasized recently, 52,53 this will be ignored throughout this study. Nevertheless, most of the important features (besides the fine-structures) have been well reproduced by our calculations as detailed below.



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Figure 1. Comparison of experimental (solid lines) and theoretical (dotted lines) CD spectra of (P,P,P)-HBT and (P,P,P)-TNT. Experimental spectrum of TNT was traced from ref. 31.


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The chiroptical response is quantified by rotational strength (R). According to the Rosenfeld equation, R is defined as the imaginary part of the scalar product of relevant electric (µe) and magnetic (µm) transition dipole moments.54,55

= Im ( ∙ ) = | || | cos The enhanced CEs at the 1Bb band of HBT (∆ε = +274 M-1 cm-1 at 300 nm) as well as TNT (∆ε = +342 M-1 cm-1 at 281 nm) in comparison to that of parent PH (∆ε = +161 M-1 cm-1 at 310 nm) is ascribable, at least in part, to the relative orientation of the relevant electric and magnetic transition dipole moments (Figure 2; for the numerical details, see Table S1 in the Supporting Information). Because of the C3-symmetry element along the helical axis in both HBT and TNT, the electric and magnetic transition dipole moments are parallel with each other (and hence θ = 0) to maximize the cos θ term in the Rosenfeld equation to unity. In contrast, the same term for PH is as small as cos 88° = 0.03. In addition, the magnitudes of both dipole moments are larger for HBT, which also enhances the CE intensity at the 1Bb band. Accordingly, the absorption dissymmetry (gabs) factor of HBT amounted to +0.0104, which is 2.5-fold larger than that of PH (+0.0042). An extremely weak negative CE of ∆ε = -1.0 M-1 cm-1, accompanied by vibrational fine structures, was also noticeable for HBT at around 440 nm, which corresponds to the 1La band of PH (∆ε = -1.3 M-1 cm-1). Our theoretical calculation successfully estimated this weak transition occurring at 2.90 eV (427 nm), but with a very weak rotational strength below the numerical limit of calculation (Table S1 in the Supporting Information). Due to the intrinsically weak nature of this 1La transition, we were unable to quantitatively compare the corresponding electric and magnetic transition dipole moments of HBT and PH.



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Figure 2. Calculated electric (µe, indigo) and magnetic (µm, magenta) transition dipole moments for the 1Bb bands of triple versus single pentahelicene with the magnitudes relative to those of PH.


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Circularly Polarized Luminescence of HBT and PH. Table 2 summarizes the photophysical and chiroptical properties of HBT, TNT, and PH. Fluorescence spectra of HBT and PH obtained in dichloromethane are shown in Figure 3 (bottom), together with the UV-vis spectral tails highlighting the lowest energy transitions. Fluorescence behavior of PH has been briefly documented earlier.56 The fluorescence spectrum of HBT with the peak maximum at around 480 nm was found structured but slightly more broadened when compared with that of PH. It is to note that special care was executed during the fluorescence measurement to avoid the oxidative photocyclization of PH to benzoperylene, whilst HBT was totally stable under the conditions employed. Fluorescence quantum yields were estimated relative to that of hexahelicene in dioxane (Φfl = 0.041).57 The quantum yield of 0.018 determined for HBT is smaller than those of TNT (0.026) and PH (0.057), but is still strong enough to obtain the reliable CPL spectrum. The fluorescence lifetimes showed a general tendency similar to that for the quantum yields. Thus, the lifetime was the shortest 3.6 ns for least fluorescent HBT, but gradually increased to 5.2 ns for TNT and then to 12.2 ns for most fluorescent PH. The small Stokes shift for HBT (1040 cm-1) is compatible with the rigidity of the molecule, but is larger than the value for PH (440 cm-1). All of these observations lead us to a conclusion that the helical twist and overall distortion in each pentahelicene unit of HBT are larger than those in PH, but are released to allow HBT a more extensive global relaxation in the excited state.


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Table 2. Photophysical and Chiroptical Properties of Triple and Single Pentahelicenes a

Helicene

λabs / nm

λfl / nm

Φfl

τfl / ns

Stokes shift / cm-1 b

glum / 10-3

gabs / 10-3

HBT

427, 460 sh

483

0.018

3.6

1040 (2700) c

-1.3

-1.8

TNT d

352

469

0.026

5.2

– e (7100) c

–e

– e (-1.1) f

PH g

356, 398 sh

406

0.057 (0.04 h)

12.2

440 (3500) c

-2.7 i

-7.6

a

Peak wavelengths of UV-vis (λabs) and fluorescence (λfl) spectra, fluorescence quantum yield (Φfl) and lifetime (τfl), as well as dissymmetry factors of luminescence (glum) and absorption (gabs) at 25 °C. b For HBT and PH, the 0-0 excitation energy was estimated from the shoulder peak observed at the longest wavelength in the UV-vis spectrum (Figure 3, bottom). This was not possible for TNT due to the less-resolved spectrum. c Value in the parentheses is the energy difference between absorption and fluorescence peak maxima. d Ref. 31. e Not reported. f Because the 0-0 band was not well resolved, the gabs value at 352 nm is shown in the parentheses. g See also ref. 25. h Ref. 56. i At 0 °C. Also see the text.


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Figure 3. Experimental (top) and theoretical (middle) CPL as well as experimental fluorescence (bottom) spectra of (P,P,P)-HBT (green) in dichloromethane at 25 °C and (P)-PH (black) in dichloromethane at 0 °C. Dotted lines show the corresponding UV-vis spectra normalized at the 0-0 band.



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Finally, we measured the CPL spectra of HBT and PH. To the best of our knowledge, this is the first experimental investigation of the CPL of pentahelicenes. In the CPL measurement of PH in dichloromethane at 0 °C, the absorbance and CD intensity decreased by 3% and 5%, respectively, after a single scan (duration: 3 min; slit width: 5 nm) on a JASCO CPL-200 instrument. In a separate experiment under the comparable conditions, PH was found totally stable for a much longer period of time in the dark, revealing that the deterioration of UV-vis and CD signals upon CPL measurement is attributable to the racemization and photocyclization of PH induced by the excitation light of the spectrometer, a phenomenon unavoidable for a photolabile molecule. Subsequently, to obtain more reliable CPL spectrum of PH, the measurement was repeated eight times using a fresh PH solution at lower temperature (0 °C) each time, and all the spectra thus obtained were numerically averaged afterwards. The CPL spectrum of HBT in dichloromethane at 25 °C was obtained by averaging eight repeated scans without changing the sample solution, and no decomposition or racemization was detected after the measurement. This photostability of HBT is attributable to the intervention of a C2symmetric isomer, which is produced as a less stable intermediate upon stereoinversion of one of the pentahelicene units (i.e., partial racemization) in HBT but spontaneously goes back to the original D3-symmetric enantiomer without driving the stereoinversion of adjacent hexahelicene units to complete the total racemization. Details of the conformational isomerization including the structure of transition states have been thoroughly discussed previously with an aid of computation.44 Unfortunately, no CPL behavior of TNT has been reported in the literature.31 We recently reported that S- and X-shaped double hexahelicenes emit CPL with comparable glum factors of -2.1 × 10-3 and -2.5 × 10-3, respectively.13


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The CPL spectra thus obtained for pentahelicenes are shown in Figure 3 (top). Both (P)-PH and (P,P,P)-HBT afforded negative CPL responses at around 410 and 480 nm, respectively, which accord in sign with the 1La transitions observed in the CD spectra of these (P)pentahelicens, as well as (P)-hexahelicenes.58 The theoretical calculation for CPL properties has rarely been reported and is usually limited to the TD-DFT level on much smaller systems.59 Our calculation at the RI-CC2/def2-TZVPP level40,41 reproduced the main features of the CPL spectra of both PH and HBT. While the CPL intensity predicted for highly symmetrical HBT was considerably smaller and the excitation energies were slightly overestimated, the general trend, i.e., the signs and the relative intensities and excitation energies, were well reproduced by the calculation (Figure 3, middle). The glum value was found -1.3 × 10-3 for HBT, which is smaller than that of PH (-2.1 × 10-3). However, the glum/gabs ratio of 0.72 for HBT, as a measure of the excited-state relaxation, is substantially larger than the global average (0.61) for all the reported helicenes and helicenoids,26 confirming the advantage of this triple helicene skeleton that increases the rigidity and circumvents the undesirable and racemization photocyclization.



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Conclusions In this combined experimental and theoretical study to elucidate the CD and CPL behaviors of parent PH and triple pentahelicenes HBT and TNT, we have shown that the pentahelicene unit exhibits such absorption and luminescence dissymmetry factors (gabs and glum) for the main 1Bb and the lowest-energy 1La transition, respectively, that are intrinsically larger than those of higher homologues. Thus, PH emits strong CPL of glum = -2.7 × 10-3 at a low temperature, which is however about a half value of the gabs. Due to the photolabile nature, PH is not suitable as a chiral motif to be incorporated in chiroptical materials. However, such undesirable properties/reactivities can be excluded by merging three PH units into HBT. Thus, the inherently high g factors of parent PH is intact in HBT to give the glum and gabs factors as high as -1.3 × 10-3 and -1.8 × 10-3, respectively, with a high glum/gabs ratio of 0.72 indicative of insignificant excitedstate relaxation. The theoretical calculations provide further insights into the improved chiroptical responses at the main band (1Bb transition) in the triple pentahelicenes HBT and TNT, which is primarily ascribable to the symmetric nature of these triplehelicenes. Unfortunately, our calculation either at the RI-CC2 or ADC(2) level only reproduced the qualitative feature of the first (i.e., 1La) transition, which is responsible for the CPL response. Nevertheless, our study detailed above will be of pronounced significance in designing advanced CPL materials incorporating helical chirality.

ASSOCIATED CONTENT Supporting Information.


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The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acs.jpca.xxxxxx. Experimental and theoretical details and extended spectral data (PDF)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Fax: +81-6-6879-7923 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial supports by Grant-in-Aids for Scientific Research, Challenging Exploratory Research, Promotion of Joint International Research (Fostering Joint International Research), and on Innovative Areas "Photosynergetics" (Grant Numbers JP15H03779, JP15K13642, JP16KK0111, JP17H05261, JP18K19077, and JP18H01964) from JSPS and by the Asahi Glass Foundation are greatly acknowledged.

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A Combined Experimental and Theoretical Study on Circular

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