Magnetic Circularly Polarized Luminescence in the Photoexcited States of Racemic [n]Helicenes (n=3-5,7) in Tetrahydrofuran and Dimethyl Sulfoxide Solutions.

This paper reports the first magnetic circularly polarized luminescence (MCPL) characteristics of racemic helicenes, including four unsubstituted [n]helicenes (n=3,4,5,7) and two [4]helicene derivatives bearing methoxy substituents, in tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) solutions. The value of |gMCPL | was calculated to be of the order of 10-3  T-1 within 350-430 nm under the north-up (N-up) and south-up (S-up) Faraday geometries in an external magnetic field of 1.6 T. The [n]-dependent MCPL signs were altered by the N-up and S-up geometries.

Remarkable Effects of External Magnetic Field on Circularly Polarized Luminescence of EuIII (hfa)3 with Phosphine Chirality.

Herein, magnetic circularly polarized luminescence (CPL) (MCPL) spectroscopy was conducted to analyze an EuIII (hfa)3 complex with three chiral PIII -ligands. Resultantly, (R)-chirality luminophores with S-up orientation and (S)-chirality luminophores with N-up orientation were observed to possess symmetrical mirror image spectra, i. e., they were enantiomers. Similarly, the (R)-chirality luminophores with N-up orientation and the (S)-chirality luminophores with S-up orientation were also enantiomers. Contrarily, (R)-S-up and (S)-S-up were diastereomers and did not possess a mirror-image relationship. Likewise, (R)-N-up and (S)-N-up were diastereomers. The J-dependency of gMCPL and gCPL datasets suggested that the N-up/S-up external magnetic field, with the aid of chiral PIII -ligands, increased the gMCPL values by two- to sixteen-fold and modulated the gMCPL signs at J=1-4. Additionally, the origins of the nonideal mirror-symmetric CPL and MCPL spectral characteristics of EuIII (hfa)3 with three chiral PIII -ligands were discussed in terms of parity (space-inversion, P)-symmetry, time-reversal (T)-symmetry, and PT-symmetry laws.

Deep-red circularly polarised luminescent C70 derivatives.

Optically active fullerenes, including C60 and C70 derivatives carrying organic substituents, are used in a range of applications because of their unique spectroscopic, catalytic, and chiral recognition properties. However, their inherent photoexcited chirality is yet to be elucidated because of their very poor fluorescence quantum yield (Φf). We synthesised a new chiral C70 derivative, X70A, with 20% yield, by reacting bis-borylated xanthene with C70 in a one-step double addition reaction, followed by a successful optical resolution. The isolation of two separate X70A enantiomers was confirmed by mirror-image circular dichroism spectroscopy in the range of 300-750 nm. In toluene, the enantiomeric pair of X70A clearly revealed mirror-image circularly polarised luminescence (CPL) spectra with a high |glum| value of 7.0 × 10-3 at 690 nm. The first fullerene-based deep-red CPL of X70A should provide a new guideline for the design of chiral nanocarbon materials.

Magnetic Circularly Polarized Luminescence from PtII OEP and F2 -ppyPtII (acac) under North-up and South-up Faraday Geometries.

The magnetic circularly polarized luminescence (MCPL) and photoluminescence (PL) spectra of achiral (2,3,7,8,12,13,17,18-octaethylporphyrinato)platinum(II), PtOEP, and [2-(4',6'-difluorophenyl)pyridinato-N,C2' ]platinum(II) acetylacetonate-O,O, F2 -ppyPt(acac), in toluene and dichloromethane solutions were recorded under an external magnetic field of 1.6 T with N-up and S-up Faraday geometries. The MCPL signs of PtOEP and F2 -ppyPt(acac) were controlled solely by changing the N-up and S-up geometries. The MCPL/PL wavelengths of F2 -ppyPt(acac) in solutions were varied by the ratio of the monomeric and excimeric species.

Sign dependence of MCPL spectra on type and position of substituent groups of pyrene and phenanthrene derivatives.

Diamagnetic achiral pyrene and phenanthrene derivatives substituted with electron-donating hydroxyl/methoxy groups and electron-withdrawing carboxylic acid groups exhibited clear magnetic circularly polarised luminescence (MCPL) spectra at 360-460 nm in dilute solvents upon the application of N-up and S-up Faraday geometries under an external magnetic field of 1.6 T. Their MCPL signs were also susceptible upon application of the same Faraday geometry.

Sign inversion of magnetic circularly polarized luminescence in Iridium(iii) complexes bearing achiral ligands.

Optically inactive, paramagnetic Ir(iii)(ppy)3 and Ir(iii)(ppy)2(acac) (ppy: 2-phenylpyridinate and acac: acetylacetonate) showed nearly mirror-symmetric magnetic circularly polarised luminescence (MCPL) spectra in dilute dichloromethane and dimethyl sulfoxide under N-up and S-up geometries in a 1.6-T magnetic field. However, the MCPL signs of Ir(iii)(ppy)3 and Ir(iii)(ppy)2(acac) under the same N-up (or S-up) Faraday geometry were opposite to each other when one ppy was replaced with an acac. This ligand exchange approach provides facile control of the MCPL sign, irrespective of the Faraday geometry.

Sign inversion in magnetic circularly polarised luminescence of fused aromatics with 1.6 T N-up/S-up Faraday geometry.

Although 12 diamagnetic fused aromatics with or without substituents exhibit mirror-symmetric magnetic circularly polarised luminescence (MCPL) through N-up and S-up Faraday geometries under a magnetic field intensity of 1.6 T, their signs (single and multiple) and magnitudes depend strongly on either the aromatic structures or the peripheral positions of the substituents.

Stereospecific Synthesis of Cyclic Sulfite Esters with Sulfur-Centered Chirality via Diastereoselective Strategy and Intramolecular H-Bonding Assistance.

Stereospecific synthesis of several cyclic sulfite esters containing three stereogenic centers from enantiopure 1,1,4,4-tetraarylbutanetetraols was achieved. Chiral sulfur centers were constructed stereospecifically via a diastereoselective reaction with the assistance of an intramolecular H-bonding interaction. The absolute configuration of the S atom was elucidated by using the corresponding single-crystal X-ray diffraction analysis of the synthesized monochloride cyclic sulfite esters. Furthermore, a crystallographic evidence of the specific intramolecular C(sp3)-H···CAr weak H-bondings was presented, and its dramatic effect on the 1H NMR spectral properties was revealed. This intriguing behavior was unambiguously rationalized by different shielding effects of the neighboring phenyl rings. Additionally, the theoretical results obtained on the basis of MP2 calculations fully supported the existence of intramolecular hydrogen bonding interactions being responsible for the observed unique chemical and spectral properties.

Corrigendum: Chirogenesis and Pfeiffer Effect in Optically Inactive EuIII and TbIII Tris(ß-diketonate) Upon Intermolecular Chirality Transfer From Poly- and Monosaccharide Alkyl Esters and α-Pinene: Emerging Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD).

[This corrects the article DOI: 10.3389/fchem.2020.00685.].

Chirogenesis and Pfeiffer Effect in Optically Inactive EuIII and TbIII Tris(ß-diketonate) Upon Intermolecular Chirality Transfer From Poly- and Monosaccharide Alkyl Esters and α-Pinene: Emerging Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD).

We report emerging circularly polarized luminescence (CPL) at 4f-4f transitions when lanthanide (EuIII and TbIII) tris(ß-diketonate) embedded to cellulose triacetate (CTA), cellulose acetate butyrate (CABu), D-/L-glucose pentamethyl esters ( D-/ L-Glu), and D-/L-arabinose tetramethyl esters ( D-/ L-Ara) are in film states. Herein, 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate (fod) and 2,2,6,6-tetramethyl-3,5-heptanedione (dpm) were chosen as the ß-diketonates. The g lum value of Eu(fod)3 in CABu are +0.0671 at 593 nm (5 D 0 → 7 F1) and -0.0059 at 613 nm (5 D 0 → 7 F2), respectively, while those in CTA are +0.0463 and -0.0040 at these transitions, respectively. The g lum value of Tb(fod)3 in CABu are -0.0029 at 490 nm (5 D 4 → 7 F6), +0.0078 at 540 nm (5 D 4 → 7 F5), and -0.0018 at 552 nm (5 D 4 → 7 F5), respectively, while those in CTA are -0.0053, +0.0037, and -0.0059 at these transitions, respectively. D-/ L-Glu and D-/ L-Ara induced weaker g lum values at 4f-4f transitions of Eu(fod)3, Tb(fod)3, and Tb(dpm)3. For comparison, Tb(dpm)3 in α-pinene showed clear CPL characteristics, though Eu(dpm)3 did not. A surplus charge neutralization hypothesis was applied to the origin of attractive intermolecular interactions between the ligands and saccharides. This idea was supported from the concomitant opposite tendency in upfield 19F-NMR and downfield 1H-NMR chemical shifts of Eu(fod)3 and the opposite Mulliken charges between F-C bonds (fod) and H-C bonds (CTA and D-/ L-Glu). An analysis of CPL excitation (CPLE) and CPL spectra suggests that (+)- and (-)-sign CPL signals of EuIII and TbIII at different 4f-4f transitions in the visible region are the same with the (+)-and (-)-sign exhibited by CPLE bands at high energy levels of EuIII and TbIII in the near-UV region.

Mirror-image magnetic circularly polarized luminescence (MCPL) from optically inactive EuIII and TbIIItris(ß-diketonate).

Five optically inactive EuIII(hfa)3, TbIII(hfa)3, EuIII(acac)3·Phen, TbIII(acac)3·Phen and EuIII(hfa)3·BDPB (hfa: hexafluoroacetylacetonate, acac: acetylacetonate, BDPB: 2,2'-bis(diphenylphosphino)biphenyl and Phen: phenanthroline) complexes showed clear mirror-image magnetic circularly polarised luminescence (MCPL) spectra in CHCl3, acetone, and DMF solutions, in a poly(methyl methacrylate) film, a KBr pellet and bulk powder at room temperature under 1.6 T as an external magnetic field with Faraday geometry. The signs of the MCPL signals were determined via N-up and S-up geometries. The gMCPL values of EuIII(hfa)3 and TbIII(hfa)3 were ±(0.046-2.0) × 10-2 T-1 at the 5D0 → 5FJ transitions (J = 1-4) in the range of 590 nm and 703 nm and ±(0.0081-1.0) × 10-2 T-1 at the 5D4 → 7FJ transitions (J = 3-6) in the range of 480 nm and 630 nm, respectively. Interestingly, the photoluminescence spectra of EuIII(hfa)3 and TbIII(hfa)3 in CHCl3 and acetone under 1.6 T considerably redshifted by 10-114 cm-1, while those of EuIII(acac)3·Phen and TbIII(acac)3·Phen did not reveal detectable redshifts.

Circularly polarised luminescence from planar-chiral Phanephos/Tb(iii)(hfa)3 hybrid luminophores.

Planar-chiral Phanephos, containing the coordinatable P(iii), formed P(iii)/Tb(iii)(hfa)3 hybrid luminophores that successfully emitted characteristic circularly polarised luminescence (CPL) due to 5D4→7F5 transitions in solution. On the other hand, BINAP, containing P(iii)[double bond, length as m-dash]O as axially chiral ligand, exhibited no detectable CPL with Tb(iii)(hfa)3.

Torsional chirality generation based on cyclic oligomers constructed from an odd number of pyrenes.

A Ni(0)-mediated coupling reaction of a 1,8-dibromopyrene gave a series of cyclic pyrene oligomers CPn up to nonadecamer. CP5 and CP7 represent a unique class of asymmetric compounds with no stereogenic chiral centers in terms of their global chirality, which are achieved because of an odd number of pyrenes.

Aggregation-induced chiroptical generation and photoinduced switching of achiral azobenzene-alt-fluorene copolymer endowed with left- and right-handed helical polysilanes.

The left and right helicities of azobenzene (Azo)-containing main-chain polymer (PF8Azo) were successfully controlled with an enantiomeric pair of rigid rod-like helical polysilanes carrying (S)- and (R)-2-methylbutyl groups (PSi-S and PSi-R, respectively) as their hetero-aggregates in a mixture of chloroform and methanol solvents and in the solid state. Optimizing the good and poor cosolvents and their volume fractions showed that the molar ratio of PF8Azo to PSi-S/-R and the molecular weight of PF8Azo were crucial to boost the CD amplitudes of PF8Azo/PSi-S and PF8Azo/PSi-R hetero-aggregates. The photoresponsive trans-cis transformation caused noticeable changes in the sign and magnitude of the chiroptical behavior due to the hetero-aggregates. Moreover, the optically active PF8Azo homo-aggregates were produced by complete photoscissoring reactions at 313 nm, which could be assigned to the Siσ-Siσ* transitions of PSi-S and PSi-R.

Correction: Fujiki, M., et al. Questions of Mirror Symmetry at the Photoexcited and Ground States of Non-Rigid Luminophores Raised by Circularly Polarized Luminescence and Circular Dichroism Spectroscopy: Part 1. Oligofluorenes, Oligophenylenes, Binaphthyls and Fused Aromatics. Molecules, 2018, 23, 2606.

The authors wish to make the following correction to their paper [...].

Questions of Mirror Symmetry at the Photoexcited and Ground States of Non-Rigid Luminophores Raised by Circularly Polarized Luminescence and Circular Dichroism Spectroscopy: Part 1. Oligofluorenes, Oligophenylenes, Binaphthyls and Fused Aromatics.

We report experimental tests of whether non-rigid, π-conjugated luminophores in the photoexcited (S1) and ground (S0) states dissolved in achiral liquids are mirror symmetrical by means of circularly polarized luminescence (CPL) and circular dichroism (CD) spectroscopy. Herein, we chose ten oligofluorenes, eleven linear/cyclic oligo-p-arylenes, three binaphthyls and five fused aromatics, substituted with alkyl, alkoxy, phenyl and phenylethynyl groups and also with no substituents. Without exception, all these non-rigid luminophores showed negative-sign CPL signals in the UV-visible region, suggesting temporal generation of energetically non-equivalent non-mirror image structures as far-from equilibrium open-flow systems at the S1 state. For comparison, unsubstituted naphthalene, anthracene, tetracene and pyrene, which are achiral, rigid, planar luminophores, did not obviously show CPL/CD signals. However, camphor, which is a rigid chiral luminophore, showed mirror-image CPL/CD signals. The dissymmetry ratio of CPL (glum) for the oligofluorenes increased discontinuously, ranging from ≈ -(0.2 to 2.0) × 10-3, when the viscosity of the liquids increased. When the fluorene ring number increased, the glum value extrapolated at [η] = 0 reached -0.8 × 10-3 at 420 nm, leading to (⁻)-CPL signals predicted in the vacuum state. Our comprehensive CPL and CD study should provide a possible answer to the molecular parity violation hypothesis arising due to the weak neutral current mediated by the Z°-boson.

Combined Experimental and Theoretical Study on Circular Dichroism and Circularly Polarized Luminescence of Configurationally Robust D3-Symmetric Triple Pentahelicene.

Pentahelicene (PH) exhibits the largest absorption ( gabs) and luminescence ( glum) dissymmetry factors among 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]helicene ( 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.

Solvent-sensitive signs and magnitudes of circularly polarised luminescence and circular dichroism spectra: probing two phenanthrenes as emitters endowed with BINOL derivatives.

A C2-symmeric binaphthyl framework bearing phenanthrene as the emitter exhibited circularly polarised luminescence (CPL) in dilute solutions. The CPL and circular dichroism signs of the luminophores were altered by solvents (chloroform, methanol, acetonitrile, and dimethylformamide). DFT and TD-DFT calculations indicated that the dihedral angle between the phenanthrene and naphthyl rings was responsible for the apparent sign inversion. The role of solvent molecules in the ground and photoexcited states was discussed based on Hansen solubility parameters (δ, δd, δp and δh).

Terthiophene Functionalized Conjugated Triarm Polymers Containing Poly(fluorene-2,7-vinylene) Arms Having Different Cores-Synthesis and Their Unique Optical Properties.

Optical properties of three types of terthiophene (3T) functionalized conjugated triarm (star-shaped) polymers consisting of poly(9,9-di-n-octyl-fluorene-2,7-vinylene) (PFV) arms and different [2,4,6-tri(biphenyl)benzene (TBP), 1,3,5-tri(benzyl)benzene (TBB), and triphenylamine (TPA)] cores, prepared by combined olefin metathesis with Wittig coupling, have been studied. Relative intensities [increases in the higher vibronic bands, (0, 1) fluorescence (FL)] of the fully conjugated TPA-core polymers, TPA(PFV-3T)3, in the fluorescence (FL) spectra in tetrahydrofuran (toluene) solution were higher than those in the other triarm polymers, TBP(PFV-3T)3, TBB(PFV-3T)3, whereas no significant differences were observed in their UV-vis spectra; notable temperature dependences were not observed in the UV-vis and FL spectra (at -5, 25, and 55 °C). Remarkable differences were not observed in the spectra in these polymer thin films, whereas λmax values red-shifted due to the formation of J-type aggregates. The observation for the time-resolved study well corresponds to results for the steady-state fluorescence measurements. The observed unique emission by the star-shaped (triarm) polymer containing the TPA core would be assumed to be due to a difference in nature of the core (higher coplanarity) compared to that of the others.

Ambidextrous Chirality Transfer Capability from Cellulose Tris(phenylcarbamate) to Nonhelical Chainlike Luminophores: Achiral Solvent-Driven Helix-Helix Transition of Oligo- and Polyfluorenes Revealed by Sign Inversion of Circularly Polarized Luminescence and Circular Dichroism Spectra.

We investigated whether helicity and/or chirality of cellulose tris(phenylcarbamate) (CTPC) can transfer to noncharged, nonhelical oligo- and polyfluorenes when CTPC was employed as a solution processable homochiral platform of a D-glucose-skeletal polymer. Noticeably, CTPC revealed the solvent-driven, ambidextrous intermolecular helicity/chirality transfer capability to these fluorenes. The chiroptical inversion characteristics of circularly polarized luminescence (CPL) and the corresponding CD spectra were realized by solely choosing a proper achiral solvent and/or achiral cosolvent. When the solution of PF6 and CTPC in tetrahydrofuran (THF) was cast on a quartz substrate, the dissymmetry ratio of CPL (gCPL) from the polymer film showed gCPL = +2.1 × 10-3 at 429 nm. Conversely, when dichloromethane (DCM) was used as the solvent, the CPL sign was inverted to gCPL = -2.4 × 10-3 at 429 nm. The dissymmetry ratio of Cotton CD band (gCD) from the THF solution was gCD = +3.2 × 10-3 at 392 nm; conversely, from the DCM, the CD sign inverted to gCD = -0.8 × 10-3 at 371 nm. The sign and magnitude of the gCD values were interpreted to a London dispersion term (δd) of Hansen solubility parameter (δ) of the casting solvents rather than a dipole-dipole interaction term (δp) and a hydrogen bonding interaction term (δh) of the δ values and dielectric constant (ε). Analysis of solvent-driven changes in FTIR spectra, wide-angle X-ray diffraction profiles, and differential scanning calorimetry diagrams indicated that solvent driven on-off switching of multiple hydrogen bonds due to three urethane groups of CTPC play the key for the inversion. Intermolecular CH/π and π-π interactions among phenyl rings and alkyl groups were assumed to be crucial for helicity/chirality transfer capability based on molecular mechanics and molecular dynamics simulations of PF6-CTPC hybrids. These chiroptical inversion characteristics arose from solvent-driven order-disorder transition characteristics of the CTPC helix rather than a helix-helix transition of CTPC itself.