Symmetry-based rational design for boosting chiroptical responses

Tanaka, Hiroki; Ikenosako, Mina; Kato, Yuka; Fujiki, Michiya; Inoue, Yoshihisa; Mori, Tadashi

doi:10.1038/s42004-018-0035-x

Cited by 188 publications

(215 citation statements)

References 60 publications

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“…It was found that, for many helicenes, the electric and magnetic transition dipole moments ( μ e and μ m , respectively) are usually not perfectly aligned in their ideal parallel or antiparallel orientation, which reduces their rotatory strength . To overcome this limitation, double helicenes with the appropriate orientation were recently introduced and were shown to display superior chiroptical properties …”

Section: Introductionsupporting

confidence: 87%

Chiroptical Properties of Twisted Acenes: Experimental and Computational Study

Bedi

Gidron

2019

Chemistry A European J

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Acenest hat are twisted out of planarity are expected to display chiroptical properties. However,t he effect of twisting on the chiroptical properties of acenes has not been investigated computationally or experimentally.H erein, we present ac omputational investigation of the chiroptical properties of anthracenes to pentacenes, combined with an experimental study using as eries of helicallyl ocked acenes, twisted to different torsional angles in their enantiopure form.T he lowest energy transition, which is relatively weak in acenes, becomes dominanti nt heir circular dichroism spectra upon twisting. We find that the rotational strength of acenes consistently increases with increasing twist. The experimental data obtainedf rom enantiopure tethered twistacenes showt he same trend as the calculatedr esult, with as trong Cotton effect and anisotropy factor,r endering twisted acenes as excellent chiroptical materials.

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Section: Introductionsupporting

confidence: 87%

Chiroptical Properties of Twisted Acenes: Experimental and Computational Study

Bedi

Gidron

2019

Chemistry A European J

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“…Method of obtention To explain the enhancement of CPL in double azahelicenes, Mori et al proposed in 2018 a protocol for rationally aligning multiple chiral units to boost the chiroptical responses. 135 They used hexahelicene as a prototype; they aligned two hexahelicenes in various orientations and examined by theoretical calculations which orientation resulted in the highest chiroptical performance from either X-shaped (101, Figure 15) or S-shaped (102) double hexahelicenes. Compound 101 and 102 exhibited more than a twofold increase in intensity of circular dichroism and circularly polarized luminescence.…”

Section: Compoundmentioning

confidence: 99%

“…These diastereomers were configurationally very stable at room temperature and no racemization was observed after heating the diastereomers at 150 °C for 12 hours. Other enantioenriched derivatives bearing bromo (135) and carboxaldehyde (136) Bedekar et al reported the synthesis of aza [7]helicene in 2014, 168 and aza [9]helicene in 2016, 169 bearing a central N-butyl-carbazole unit (Scheme 35). Upon classical oxidative photocyclization process, the "angular-angular" compound 139a was obtained together with other isomeric structures named "angular-linear" 139b, 'linear-linear" 139c and "linear-fused-angular" 139d.…”

Section: Carbazoles From Oxidative Photocyclizationmentioning

confidence: 99%

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

2019

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In this review, we discuss the rich chemistry of helicenes and helicenoids containing maingroup elements. Enantioenriched helicenic derivatives containing main-group elements B, Si, N and P, either incorporated within the helical backbone or grafted to it, will be thoroughly presented. We will describe their synthesis, resolution, and asymmetric synthesis, their structural features, electronic and chiroptical properties, emission, together with other photochemical properties and applications. OUTLINE 14 The absorption spectrum of 19c recorded in CH 2 Cl 2 solutions exhibited a maximum at 376 nm which is significantly red shifted compared with that of double [5]helicene 19a (310 nm) due to the more extended π-conjugation in 19c. According to TD-DFT calculations, the low-energy absorption band from 400 to 500 nm is assignable to the HOMO→LUMO transition corresponding to a π−π* transition of the fully conjugated bis-helical system. DFT calculations revealed a very high theoretical isomerization barrier of 45.1 kcal/mol for the (P,P)/(M,M)-19c stereoisomer, i.e. 4.4 kcal/mol higher than the (P,M) stereoisomer (not observed experimentally). The high configurational stability of 19c enabled to obtain the pure (P,P)and (M,M)-19c by chiral HPLC (Daicel Chiralcel IE column; EtOAc/MeOH = 9:1 as eluent) and to study their chiroptical properties. The ECD spectrum of (P,P)-19c in CH 2 Cl 2 displayed two strong positive bands at 275 and 320 nm, a strong negative one at 375 nm and a broad and less intense one between 425-475 nm. 52 Note that the isomerization barrier of the OBO-fused double [7]helicene is much higher than for oxabora[6]helicene 9 (vide supra) and higher than carbo[7]helicene (42.0 kcal/ mol), indicating the advantage of double helicenes over monohelicenes in terms of conformational stability. Indeed no racemization was observed upon heating enantiomer (P,P) and (M,M)-19c for 24 h up to 200 °C. 52 Note that in 2017 a longer double OBO-helicene analogue was deposited on a Au(111) under UHV conditions and its surface-assisted cyclodehydrogenation into a planar OBO-perihexacene was observed by STM. 54 Similarly to 6 and 9, the strong bond alternation found in the BOC4 rings of 19a 51 may account for the small NICS value of 1.3 while the surrounding C6 rings, including the distorted central benzene ring, show large negative NICS values (Scheme 3c). For comparison, in the all-carbon analogue tetrabenzo-[a,f,j,o]perylene shown in Scheme 3c, the central C6 ring shows a positive NICS value, indicating substantial antiaromatic character. Notably, molecular orbital calculations of 19a indicate that the HOMO and LUMO are spread over the C6 rings rather than on the boron and oxygen atoms, accounting for the substantial stability of 19a (Scheme 3c). Borahelicenes are efficient blue fluorophores. Indeed, azabora[6]helicene 6 displays blue fluorescence at 447 nm (Table 2), while smaller achiral [4]helicenic systems were successfully used as host materials in phosphorescent OLEDs with efficiencies better than the classical 4,4'...

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“…Planar and helical chiralities, as found in helicenes, were highlighted to promote important dissymmetry factors because of the larger magnetic transition dipole moments induced by aromatic distortion. Hence, important efforts were made to rationalize this observation with the calculation of

and

and the determination of the θ angle (Sato et al, 2017 ; Ito et al, 2018 ; Tanaka et al, 2018b , c ). The outstanding | g lum | value of 0.152 reported for the single-wall carbon nanotubes (Dhbaibi et al, 2019 ) cyclo-2,8-chrysenylene by Isobe and Sato was attributed to the unusually important magnetic transition dipole moment originating from a cyclic aromatic structure which allows and facilitates a magnetic field-induced current loop (Sato et al, 2017 ).…”

Section: Physical Basis Of Cpl and Strategies To Achieve Strong Cpl Ementioning

confidence: 99%

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

2020

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Chiral molecules are essential for the development of advanced technological applications in spintronic and photonic. The best systems should produce large circularly polarized luminescence (CPL) as estimated by their dissymmetry factor ( g lum ), which can reach the maximum values of −2 ≤ g lum ≤ 2 when either pure right- or left-handed polarized light is emitted after standard excitation. For matching this requirement, theoretical considerations indicate that optical transitions with large magnetic and weak electric transition dipole moments represent the holy grail of CPL. Because of their detrimental strong and allowed electric dipole transitions, popular chiral emissive organic molecules display generally moderate dissymmetry factors (10 −5 ≤ g lum ≤ 10 −3 ). However, recent efforts in this field show that g lum can be significantly enhanced when the chiral organic activators are part of chiral supramolecular assemblies or of liquid crystalline materials. At the other extreme, chiral Eu III - and Sm III -based complexes, which possess intra-shell parity-forbidden electric but allowed magnetic dipole transitions, have yielded the largest dissymmetry factor reported so far with g lum ~ 1.38. Consequently, 4f-based metal complexes with strong CPL are currently the best candidates for potential technological applications. They however suffer from the need for highly pure samples and from considerable production costs. In this context, chiral earth-abundant and cheap d-block metal complexes benefit from a renewed interest according that their CPL signal can be optimized despite the larger covalency displayed by d-block cations compared with 4f-block analogs. This essay thus aims at providing a minimum overview of the theoretical aspects rationalizing circularly polarized luminescence and their exploitation for the design of chiral emissive metal complexes with strong CPL. Beyond the corroboration that f–f transitions are ideal candidates for generating large dissymmetry factors, a special attention is focused on the recent attempts to use chiral Cr III -based complexes that reach values of g lum up to 0.2. This could pave the way for replacing high-cost rare earths with cheap transition metals for CPL applications.

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Symmetry-based rational design for boosting chiroptical responses

Cited by 188 publications

References 60 publications

Chiroptical Properties of Twisted Acenes: Experimental and Computational Study

Chiroptical Properties of Twisted Acenes: Experimental and Computational Study

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

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