Nozomu Suzuki

Creation of Circularly Polarized Luminescence from an Achiral Polyfluorene Derivative through Complexation with Helix-Forming Polysaccharides: Importance of the meta-Linkage Chain for Helix Formation

A circularly polarized luminescence (CPL) material has been created by polymer-polymer complexation between a helix-forming polysaccharide, schizophyllan (SPG), and a meta-phenylene-linked polyfluorene derivative (mPFS). Computational modeling revealed that mPFS can adopt a helical structure although a conventional polyfluorene derivative with a para-phenylene linkage tends to enjoy a rigid rodlike conformation. Our detailed experimental examination showed that mPFS forms a chiral nanowire complex through cohelix formation with SPG. We have found, as expected, that this cohelical complex emits highly efficient CPL even in an aqueous solution. The appearance of the high CPL property is due to 1) a high quantum yield of the fluorene unit and 2) immobilization of the helically twisted conformation of mPFS in an isolated manner through cohelix formation with SPG. One can propose, therefore, that the SPG/mPFS complex acts as a new high-performance CPL material with a solvent-dispersible nanowire structure.

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-(S)-2-methylbutylsilane]s (achiral alkyl side chains = ethyl, n-propyl, n-butyl, n-pentyl, n-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 n-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.

Limonene induced chiroptical generation and inversion during aggregation of achiral polyfluorene analogs: structure-dependence and mechanism

Chirality transfer from biological agents to π-conjugated polymers in the solid state is an attractive method to generate, switch, and amplify chirality, especially when one considers several potent device applications. However, the polymer-structure dependence on the solvent-induced chirality transfer mechanism is not well understood. To elucidate the structural–chiroptical property relationship of the polymer aggregates, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PF8), poly(9,9-di-n-octylsila-fluorenyl-2,7-diyl) (PSi8), poly(9-(1-octylnonyl)-9H-carbazole-2,7-diyl) (PCz8), P(F8-alt-Si8), P(F8-alt-Cz8), and P(Si8-alt-Cz8) were synthesized. The limonene chirality was efficiently transferred to PF8, PSi8 and P(F8-alt-PSi8) aggregates in CHCl3–limonene–CH3OH tersolvent, but the optical activity was not observable for PCz8, P(F8-alt-PCz8), and P(Si8-alt-PCz8). The reason for the absence of chiroptical activity in Cz8-containing polymers could be that those polymers have stronger polarity compared to limonenes, and thus the interaction between limonene and polymer chains was too weak compared to the interaction among Cz8-containing polymers. Surprisingly, chiroptical inversion in CD spectra between PF8 and PSi8 aggregates was found. More surprisingly, chiroptical inversion between CD and CPL spectra of PSi8 aggregates was observed. The unique chiroptical inversion was attributable to the opposite Mulliken charges between 9-Si in Si8 and 9-C in F8 unit and between Cipso(1) in Si8 and Cipso(1) in F8 unit. Another possible reason for this unexpected chiroptical inversion behaviour is the opposite direction of dipole moments in three stable rotational isomers of the equatorial limonene rotamer.

Chiroptical generation and amplification of hyperbranched π-conjugated polymers in aggregation states driven by limonene chirality

In this contribution, a series of hyperbranched π-conjugated poly(9,9-di-n-octylfluorene)s (HPF8s) with hexaoctyltruxene (HT) as branching units were successfully prepared by a Suzuki polycondensation reaction. We present the first generation of optically active HPF8s in aggregation states with strong circular dichroism (CD) and circularly polarised luminescence (CPL) properties. The optically active aggregates of HPF8s were efficiently produced by CD-/CPL-silent HPF8s with the help of solvent chirality transfer from chiral limonene at room temperature. The emergence of CD and CPL signals of HPF8s aggregates results from the weak forces (π/π, van der Waals, CH/π and OH/π interactions) in a tersolvent system of chloroform (a good solvent), alkanol (a poor solvent), and (R)-(+)-/(S)-(−)-limonene (1R/1S, a chiral source). The hyperbranched polymer structure (degree of branching), poor solvent type, volume fraction of methanol and 1R/1S, limonene enantiopurity and polymer concentration have obvious effects on the magnitude and sign of the CD signals. The obtained HPF8s aggregates have relatively strong UV-vis absorption and CD signals in the range of 300–500 nm as well as CPL emission (420–460 nm) under optimised conditions. The current study will open up a new avenue for efficiently producing ambidextrous chiroptical hyperbranched polymeric materials without any specific chiral catalysts or substituents under mild conditions.

Chiral optofluidics: gigantic circularly polarized light enhancement of all-trans-poly(9,9-di-n-octylfluorene-2,7-vinylene) during mirror-symmetry-breaking aggregation by optically tuning fluidic media

Here we report the first chiral optofluidic system that enables the emergence and enhancement of chiroptical signals of μm-sized polymer particles from achiral all-trans-poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) during mirror-symmetry-breaking aggregation due to the optically active fluidic media consisting of (R)-limonene (1R) and (S)-limonene (1S), chloroform and methanol. These results were proven by refractive index (RI) and specific rotation (SR) of the media as well as circular dichroism (CD), optical rotatory dispersion (ORD), circular polarised luminescence (CPL), UV-vis and photoluminescence spectral characteristics of PFV. For comparison, a triple bond linker poly[(9,9-di-n-octylfluoren-2,7-diyl)-alt-yleneethynylene] (PFE) aggregate, which is an analogue of PFV, did not show any CD-signals in the π–π* transition. Gaussian 03 (TD-DFT, B3LYP, 3-21G basis set) calculations of PFV and PFE trimer models suggested that PFV is CD-/CPL-silent helix due to an equal proportion of P- and M-helices in a double-well with a small barrier height, conversely, that PFE is inherently optically inactive due to non-helix conformation in a single-well. Under optimised chiral optofluid with specific RI values of 1.38–1.39, the μm-sized PFV particles showed a great enhancement in Kuhns dissymmetry amplitudes by ∼1500 times compared to the computed value in helical models of PFV; the gCD at 473 nm reached +0.078 (1S) and −0.104 (1R), and the gCPL at 474 nm attained +0.056 (1S) and −0.077 (1R), while maintaining a high quantum yield (ΦPL) of 75–88% with a short PL lifetime of ∼0.5 ns. For comparison, as for six π-conjugated molecular aggregates with/without H–H repulsions, including trans-stilbene and diphenylacetyelene, with the help of the 1R-/1S-based media, any detectable CD signals were not observed. These results led to the idea that optically active π-conjugated polymers carrying longer alkyl side groups in a double-well potential may be needed to efficiently generate the corresponding optically active aggregates in the chiral tersolvents.

Photon magic: chiroptical polarisation, depolarisation, inversion, retention and switching of non-photochromic light-emitting polymers in optofluidic medium

Circularly polarised (CP)-photons at six different energies (six wavelengths) were irradiated to non-photochromic, highly photoluminescent poly[(9,9-di-n-octylfluoren-2,7-diyl)-alt-bithiophene] (PF8T2) as μm-sized particles in chloroform–methanol cosolvents. This approach led to the capability of photophysically controlling all chiroptical polarisation, depolarisation, inversion, retention and switching in the ground and/or photoexcited states, as proven by circular dichroism (CD), optical rotation dispersion (ORD) and circularly polarised luminescence (CPL) spectroscopy. By optofluidically tuning the refractive index of the cosolvents at 589 nm (nD), these CP-photon induced CD signals were resonantly enhanced at a specific nD of ≈1.40. Surprisingly, regardless of the same l- (and r-) CP hand, the choice of higher and lower CP-photon energies led to a swapping of signs in chiroptical polarisation of the particles. The CP-photon hand, l- and r-, was not a deterministic factor for the induced chiroptical sign of the PF8T2 particles. This anomaly originates from CD inversion characteristics between lower and higher energy regions, supported by ZINDO calculations of PF8T2 model oligomers. Finally, controlled CP-photon sense and irradiating energy successfully allowed CPL-silent PF8T2 particles (quantum yield of 15%) to change to CPL-active particles of the order of |gCPL| = (1.9–4.3) × 10−3 at ≈535 nm whilst almost keeping the quantum yield of 8%.

Optically Active Conjugated Polymer from Solvent Chirality Transfer Polymerization in Monoterpenes

Disubstituted acetylene monomers [1,2-diphenylacetylenes (DPAs: DPA-pC1, DPA-mC1, DPA-pC8); 1-phenyl-2-hexylacetylene (PHA-pC1)] are tested for asymmetric polymerization in chiral monoterpenes used as solvents and compared with the corresponding monosubstituted acetylene monomer [1-phenylacetylene (PA-pC1)]. DPA-pC1 containing a trimethylsilyl group in the para-position of the phenyl ring produces an optically active polymer with a large Cotton effect, despite the absence of a stereogenic center. The polymer sample obtained by polymerization in 87% ee (-)-α-pinene shows the strongest CD signal (gCD value at 385 nm: ∼3.2 × 10⁻³). The Cotton bands of the polymers obtained in (-)- and (+)-α-pinenes show the opposite sign in the CD signals. Theoretical calculations show that only the cis-cisoid model adopts a helical conformation. A time-correlated single photon counting experiment shows that the emission of the chiral polymer originates from a virtually single excited species with a 98% component fraction. This polymer solution does not show any significant decrease in gCD value over a wide temperature range of 20 to 80 °C. No noticeable decrease in the gCD value is detected when the polymer solution is kept at relatively low temperatures for a prolonged period (35 d). In contrast, the other polymers show no CD signal.

Gigantic chiroptical enhancements in polyfluorene copolymers bearing bulky neomenthyl groups: importance of alternating sequences of chiral and achiral fluorene units

An alternating copolymer consisting of achiral and chiral units emits remarkably efficient CPL on photo-excitation. The main-chain twist bias is enhanced by thermal annealing by the factor of 104 in the ground state. An anisotropy factor of the polymer in the excited state is greater by approximately an order of magnitude compared with that in the ground sate.

Mirror symmetry breaking and restoration within μm-sized polymer particles in optofluidic media by pumping circularly polarised light

The first circularly polarised light (CPL)-pumped mirror symmetry breaking experiment with achiral fluorene-alt-azobenzene copolymer as polymer particles dispersed in optofluidic organic solvents was demonstrated. CPL-pumped chiroptical generation, racemisation, inversion and retention were effectively achieved by finely tuning the media and the wavelength, pumping time and ellipticity of the CPL. The absolute gCD value was ≈6 × 10−2 for chloroform/n-pentanol and chloroform/i-propanol cosolvents. For comparison, neither the magnetochiral birefringence effect nor CPL-induced chirality in molecular particles of azobenzene was detected under similar conditions.

Solid-state circularly polarised luminescence and circular dichroism of viscous binaphthyl compounds

The chiroptical properties of two viscous chiral fluorophores, (R)-2,2′-hepthoxy-1,1′-binaphthyl [(R)-1] and its antipode [(S)-1], dispersed in inorganic (KBr and KCl) and organic polymer (PMMA and PS) matrices are studied in the photoexcited and ground states. Both (R)-1 and (S)-1 emit circularly polarised luminescence (CPL) with a high quantum efficiency in these solid matrices (15–58%) and in chloroform solution (14%).