Oligo- and Polyfluorenes Meet Cellulose Alkyl Esters: Retention, Inversion, and Racemization of Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD) via Intermolecular C–H/OC Interactions

Detecting chiral/helical interactions among noncharged molecules and polymers is difficult due to their unlimited intra- and intermolecular rotational freedom. To clarify the chirality and/or helicity transfer from a chiral polymer to noncharged achiral molecules, we chose stiff cellulose triacetate (CTA) and cellulose acetate butyrate (CABu) as nonchromophoric helical/chiral polymers. Here, we highlighted stiff 9,9-dialkylfluorene oligomers and polymers (repeating number <i>n</i> = 1, 2, 3, 5, 7, 47, 201) as achiral chromophoric luminophores. These fluorenes revealed clear circularly polarized luminescence (CPL) and bisignate circular dichroism (CD) signals when embedded into CTA and CABu films. In the ground state, when <i>n</i> = 1–7, CTA and CABu commonly induced (+)-CD signals, whereas when <i>n</i> ≥ 47, they induced (+)- and (−)-CD signs, respectively. In the photoexcited state, when <i>n</i> ≥ 3, CTA and CABu induced (+)- and (−)-CPL signs, respectively. Upon comparing the ground and photoexcited states, when <i>n</i> = 2–7, CABu induced (+)-CD and (−)-CPL signs, whereas when <i>n</i> ≥ 3, CTA induced the same (+)-CD and (+)-CPL signs. A conflict between the d-glucose chirality and main-chain helicity was assumed to be responsible for these anomalies because CTA and CABu, despite being common frameworks of β(1→4)-linked d-glucose residues, prefer left- and right-handed helicities, respectively. Molecular mechanics/molecular dynamics simulations suggested intermolecular C–H/OC interactions between H–C (due to the methylene group of the dioctylfluorenes) and OC (due to the acetyl group attached to the d-glucose of CTA). This simulation was confirmed by the first detection of a clear cross-peak at ¹³CO (δ_C = 170.6 ppm, CTA) and the finding CH₂ protons (δ_H = 2.55 ppm, fluorene with <i>n</i> = 201) represented the shortest C–¹H/O¹³C distance according to the phase-modulated Lee–Goldburg homonuclear decoupling of solid-state ¹H–¹³C HETCOR NMR spectroscopy. Moreover, the first photoinduced change in the real-time CPL/PL amplitude measurement of optically active fluorenes in CTA revealed that the stability of the chiroptical state increases as <i>n</i> increases and remains unchanged when <i>n</i> ≥ 47

Noticeable Chiral Center Dependence of Signs and Magnitudes in Circular Dichroism (CD) and Circularly Polarized Luminescence (CPL) Spectra of <i>all</i>-<i>trans</i>-Poly(9,9-dialkylfluorene-2,7-vinylene)s Bearing Chiral Alkyl Side Chains in Solution, Aggregates, and Thin Films

Effects of chiral alkyl side chains in poly­(9,9-dialkyl-fluorene-2,7-vinylene)­s [PFVs, (<i>S</i>)-3-methylpentyl (3mpe), (<i>S</i>)-4-methylhexyl (4mhex), (<i>S</i>)-5-methylheptyl (5mhep), (<i>S</i>)-6-methyloctyl (6moct), and (<i>S</i>)-3,7-dimethyloctyl (dmo)] toward aggregation-induced circular dichroism (AICD) and circularly polarized luminescence (AICPL) and CD/CPL spectra in solution and in thin film have been explored. The (<i>all</i>-<i>trans</i>) PFV samples with similar conjugation repeat units containing well-defined (vinyl) end groups were prepared by acyclic diene metathesis polymerization using Ru catalyst. The PFV aggregates, prepared <i>in situ</i> in a mixed solution of CHCl₃/MeOH, showed clear CD signals ascribed to AICD, whereas these samples showed CD-silent in the CHCl₃ solution. The absolute magnitude (<i>g</i>_CD value) was affected by the chiral side chains (without obey the even–odd rule) and increased in the order 3mpe, 5mhep < 4mhex < dmo < 6mcot. Both the 6-moct and dmo aggregates showed clear CPL signals ascribed to AICPL, whereas signals of the others were not obvious. The λ_max values in the UV–vis spectra red-shifted depending upon kind of alkyl side chains due to formation of <i>J</i>-type aggregates. These results suggest that the optically active aggregates adopt certain helical supramolecular ordered structures induced by an interpolymer interaction through chain entanglement. The <i>g</i>_CD values in the drop casted thin film (prepared from the CHCl₃ solution) were lower than those in the aggregates, and the value increased in the order 5mhep, dmo < 4mhex, 6mcot < 3mpe. The λ_max values in their UV–vis spectra red-shifted but were not affected by the side chain. These results suggest that supramolecular structures formed by aggregate and film are different, and the formation in film could be induced by an interpolymer π-stacking. In contrast, the basic characteristics were preserved in the thin film prepared from the PFV-6moct aggregate (CHCl₃/MeOH); the film showed high both <i>g</i>_CD and <i>g</i>_CPL values close to those in the original aggregate

Near-Ultraviolet Circular Dichroism of Achiral Phenolic Termini Induced by Nonchromophoric Poly(l,l‑lactide) and Poly(d,d‑lactide)

Herein, we present the first induced chirality of vanillin and its phenolic analogs attached to the chain ends of poly­(l,l-lactide) and poly­(d,d-lactide). Vanillin analogs were used as chromophoric and luminophoric, but achiral, ring-opening initiators of corresponding chiral cyclic lactides. Induced chirality was evident from clear circular dichroism bands at 270–320 nm due to π–π* and n−π* transitions at the vanillin moiety. However, no circularly polarized luminescence band was detected. Density functional theory (DFT) and time-dependent DFT calculations suggested the existence of multiple through-space intramolecular CH/O interactions between the <i>ortho</i>-methoxy moiety of vanillin and nearest-neighbor lactic acid units. The terminus sensitively indicated whether the main-chain chirality was l or d

Chiroptical Inversion in Helical Si–Si Bond Polymer Aggregates

To elucidate the factors involved in the chiroptical properties of polymer aggregates composed of helical building blocks, a series of rigid rod helical poly­[alkyl-(<i>S</i>)-2-methylbutylsilane]­s (achiral alkyl side chains = ethyl, <i>n</i>-propyl, <i>n</i>-butyl, <i>n</i>-pentyl, <i>n</i>-hexyl) have been investigated. It was found that the chiroptical sign in the circular dichroism (CD) spectra of the polysilane aggregates depends on the achiral side chain length and cosolvent fraction. Concerning the achiral side chains, the <i>n</i>-propyl group was of a critical length for solvent-dependent chiroptical inversion on aggregation. This unique side chain length-dependent chiroptical inversion was theoretically predictable by using the novel approach of combining the cholesteric hard-core model and exciton chirality method. The latter was also investigated theoretically by Gaussian 03 (TD-DFT, B3LYP, 6-31G­(d) basis set) calculations applied to two spatially arranged helical Si–Si bonded decamer models

Circularly Polarized Light with Sense and Wavelengths To Regulate Azobenzene Supramolecular Chirality in Optofluidic Medium

Circularly polarized light (CPL) as a massless physical force causes absolute asymmetric photosynthesis, photodestruction, and photoresolution. CPL handedness has long been believed to be the determining factor in the resulting product’s chirality. However, product chirality as a function of the CPL handedness, irradiation wavelength, and irradiation time has not yet been studied systematically. Herein, we investigate this topic using achiral polymethacrylate carrying achiral azobenzene as micrometer-size aggregates in an optofluidic medium with a tuned refractive index. Azobenzene chirality with a high degree of dissymmetry ratio (±1.3 × 10^–2 at 313 nm) was generated, inverted, and switched in multiple cycles by irradiation with monochromatic incoherent CPL (313, 365, 405, and 436 nm) for 20 s using a weak incoherent light source (≈ 30 μW·cm^–2). Moreover, the optical activity was retained for over 1 week in the dark. Photoinduced chirality was swapped by the irradiating wavelength, regardless of whether the CPL sense was the same. This scenario is similar to the so-called Cotton effect, which was first described in 1895. The tandem choice of both CPL sense and its wavelength was crucial for azobenzene chirality. Our experimental proof and theoretical simulation should provide new insight into the chirality of CPL-controlled molecules, supramolecules, and polymers

Chiral Self-Assembly of Designed Amphiphiles: Optimization for Nanotube Formation

Four amphiphiles with l-aspartic acid headgroups (Asp) and a diphenyldiazenyl group (Azo) contained within the hydrophobic tails were designed and synthesized for self-assembly into helically based nanotubes. The amphiphiles of the form <i>R</i>′<i>-</i>{4-[(4-alkylphenyl)­diazenyl]­phenoxy}­alkanoyl-l-aspartic acid (where <i>R</i>′ is 10 or 11) varied only in alkyl chain lengths either side of the azo group, having 4, 7, or 10 carbon distal chains and 10 or 11 carbon proximal chains (<i>R</i>-Azo-<i>R</i>′-Asp, where <i>R</i> denotes the number of carbons in the distal chain and <i>R</i>′ denotes the number of carbons in the proximal chain). Despite the molecular similarities, distinct differences were identified in the chiral order of the structures self-assembled from hot methanolic aqueous solutions using microscopy and spectroscopic analyses. This was reflected in dominant thermodynamic aggregate morphologies that ranged from amorphous material for 10-Azo-10-Asp, through twisted ribbons (196 ± 49 nm pitch) for 7-Azo-11-Asp, to the desired helically based nanotubes for 4- and 7-Azo-10-Asp (81 ± 11 and 76 ± 6 nm diameters, respectively). Another key variable in the self-assembly of the amphiphiles was the use of a second method to precipitate aggregates from solution at room temperature. This method enabled the isolation of thermodynamically unstable and key transitional structures. Helical ribbons were precursor structures to the nanotubes formed from 4- and 7-Azo-10-Asp as well as the wide, flattened nanotube structures (587 ± 85 nm width) found for 4-Azo-10-Asp. Overall, the results highlighted the interplay of influence of the headgroup and the hydrophobic tail on self-assembly, providing a basis for future rational design of self-assembling amphiphiles

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 <i>D</i>-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 (<i>g</i>_CPL) from the polymer film showed <i>g</i>_CPL = +2.1 × 10^–3 at 429 nm. Conversely, when dichloromethane (DCM) was used as the solvent, the CPL sign was inverted to <i>g</i>_CPL = −2.4 × 10^–3 at 429 nm. The dissymmetry ratio of Cotton CD band (<i>g</i>_CD) from the THF solution was <i>g</i>_CD = +3.2 × 10^–3 at 392 nm; conversely, from the DCM, the CD sign inverted to <i>g</i>_CD = −0.8 × 10^–3 at 371 nm. The sign and magnitude of the <i>g</i>_CD values were interpreted to a London dispersion term (δ_d) of Hansen solubility parameter (δ) of the casting solvents rather than a dipole–dipole interaction term (δ_p) and a hydrogen bonding interaction term (δ_h) 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

Time-Resolved Observation of Chiral-Index-Selective Wrapping on Single-Walled Carbon Nanotube with Non-Aromatic Polysilane

In the present paper, we ascertain two novel findings on chiral-index-selective binding/separating of single-walled carbon nanotubes (SWNTs) with a nonaromatic polymer, poly­(dialkylsilane) (PSi). PSi is a typical σ-conjugated polymer, composed of alkyl side chains attached to the silicon (Si)-catenated main chain. First, PSi’s with linear alkyl side chains showed significant diameter-selective wrapping for SWNTs with ca. 0.9 nm in diameter, resulting in the selective separation of (7,6) and (9,4) SWNTs. Its driving force was demonstrated to be cooperative CH−π interactions among the alkyl side chains of PSi’s and the curved graphene of SWNTs. Second, the dynamic wrapping behavior of PSi’s onto SWNTs was elucidated with time-resolved UV spectroscopy. Highly anisotropic UV absorption of PSi along the Si main chain was utilized as a “chromophoric indicator” to monitor the global/local conformations, which enabled us to track kinetic structural changes of PSi’s on SWNTs. Consequently, we concluded that upon wrapping, flexible/helical PSi with an average dihedral angle (φ) of 145° and Kuhn’s segment length (λ^–1) of 2.6 nm interconverted to the more stiffer/planar conformation with 170° and λ^–1 of 7.4 nm. Furthermore, through kinetic analyses of the time-course UV spectra, we discovered the fact that PSi’s involve three distinct structural changes during wrapping. That is, (i) the very fast adsorption of several segments within dead time of mixing (<30 ms), following (ii) the gradual adsorption of loosely wrapped segments with the half-maximum values (τ₁) of 31.4 ms, and (iii) the slow rearrangement of the entire chains with τ₂ of 123.1 ms, coupling with elongation of the segment lengths. The present results may be useful for rational design of polymers toward chiral-index-selective binding/separating of desired (<i>n</i>,<i>m</i>) SWNTs

Chiral Self-Assembly of Designed Amphiphiles: Influences on Aggregate Morphology

A series of novel amphiphiles were designed for self-assembly into chiral morphologies, the amphiphiles consisting of a glutamic acid (Glu) headgroup connected through an 11-carbon alkoxy chain to a diphenyldiazenyl (Azo) group and terminated with a variable length alkyl chain (R-Azo-11-Glu, where R denotes the number of carbons in the distal chain). TEM imaging of amphiphile aggregates self-assembled from heated, methanolic, aqueous solution showed that chiral order, expressed as twisted ribbons, helical ribbons, and helically based nanotubes, increased progressively up to a distal chain length containing eight carbons, and then decreased with further increases in distal chain length. TEM and CD showed that the chiral aggregations of single enantiomers were influenced by the molecular chirality of the headgroup. However, the assembly of d,l-10-Azo-11-Glu into nanotubes demonstrated that chiral symmetry breaking effected by the azo group was also relevant to the chiral organization of the amphiphiles. The chiral order of aggregate morphologies was additionally affected by the temperature and solvent composition of assembly in a manner correlated to the mechanism driving assembly; i.e., d,l-10-Azo-11-Glu was sensitive to the temperature of assembly but less so to solvent composition, while l-14-Azo-11-Glu was sensitive to solvent composition and not to temperature. FTIR and UV–vis spectroscopic investigations into the organization of the head and azo groups, in chiral and achiral structures, illustrated that a balance of the influences of the hydrophilic and hydrophobic components on self-assembly was required for the optimization of the chiral organization of the self-assembled structures

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-<i>n</i>-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 λ_max values red-shifted due to the formation of <i>J</i>-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