Article pubs.acs.org/Macromolecules
Oligo- and Polyfluorenes Meet Cellulose Alkyl Esters: Retention, Inversion, and Racemization of Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD) via Intermolecular C−H/OC Interactions Sibo Guo,† Nozomu Suzuki,‡ and Michiya Fujiki*,† †
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan Department of Chemistry, College of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan
‡
S Supporting Information *
ABSTRACT: 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 n = 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 n = 1−7, CTA and CABu commonly induced (+)-CD signals, whereas when n ≥ 47, they induced (+)- and (−)-CD signs, respectively. In the photoexcited state, when n ≥ 3, CTA and CABu induced (+)- and (−)-CPL signs, respectively. Upon comparing the ground and photoexcited states, when n = 2−7, CABu induced (+)-CD and (−)-CPL signs, whereas when n ≥ 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/OC interactions between H−C (due to the methylene group of the dioctylfluorenes) and OC (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 13CO (δC = 170.6 ppm, CTA) and the finding CH2 protons (δH = 2.55 ppm, fluorene with n = 201) represented the shortest C−1H/O13C 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 n increases and remains unchanged when n ≥ 47.
■
INTRODUCTION
However, the intermolecular chirality and/or helicity transfer mechanisms remain unclear. In particular, elucidating chiral and/or helical interactions between noncharged polymers is one of the most difficult challenges due to the unlimited rotational freedom of the main and side chains. Previously, circular dichroism (CD)-silent, noncharged, semiflexible poly(n-hexyl isocyanate) was found to become CD-active in nonracemic chlorinated solvents, possibly because of chiral dipole−dipole interactions.28 Even a mixture of noncharged, nonhelical it- and st-poly(methyl methacrylate) with a 2-to-1 ratio formed triple helices on highly ordered pyrolytic graphite, although the helices were racemic.29,30
Understanding the noncovalent chiral and/or helical interactions among molecules and polymers is crucial to synthesizing helical molecules,1−3 supramolecules,4−7 polymers,8−12 and others.13−15 Several attractive intermolecular interactions involving CO/H−N, Y−H/π(Y = O, N), cation/π, C−H/ Z (Z = F, O, N, H−C), π/π, C−F/Si, Coulombic and chargetransfer interactions, and London dispersion forces16,17 aid in the spontaneous organization of these chiral/helical architectures.18−20 Detecting these invisible interactions experimentally and theoretically is vital to the design of elaborate chiralityorigin functions involving high-throughput molecular chirality recognition and resolution,21,22 efficient asymmetric catalytic reactions,23,24 and emerging and enhancing chiroptical signals from achiral/nonhelical building blocks.25−27 © XXXX American Chemical Society
Received: December 23, 2016 Revised: February 9, 2017
A
DOI: 10.1021/acs.macromol.6b02762 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Among the many artificial polymers, soluble polysaccharides are the most versatile semiflexible (stiff) chain-like platform and consist of chiral D-glucose residues.31,32 For example, porous silica gels functionalized by cellulose and amylose with aromatic carbamates exhibited excellent activity as chiral stationary phases (CSPs).33−45 With the aid of high-performance chromatography, the energy difference between enantiomers interacting with CSPs is generally of the order of 0.01−0.1 kcal mol−1; this difference is responsible for the molecular chirality recognition capability.37 Moreover, molecular mechanics (MM)/molecular dynamics (MD) simulations of cellulose tris(phenylcarbamate) and tris(3,5-dimethylphenylcarbamate) suggested the importance of π−π interactions between the polysaccharides and enantiomers, whereas intramolecular hydrogen bonding between CO and N−H groups led to the emergence of an internal helical groove, which plays a crucial role in the CSP performance.33−45 In recent years, sophisticated circularly polarized luminescence (CPL)-functional materials with a high gCPL values in which the sign and wavelength of the CPL signals are facilely controlled are attracting increasing interest for several potential applications in synthetic chemistry, chiroptical science, supramolecular chirality, polymer chemistry, photochemistry, and photophysics. Additionally, understanding the chirality transfer and amplification mechanisms reported in several polymer materials and asymmetric photochemical reactions in the photoexcited states is emerging as an active area of research.46−61 For example, soluble polycarbohydrates and chiral solvents act as efficient scaffoldings and enable the transfer of their chirality and/or helicity to other achiral/nonhelical semiflexible π- and σ-conjugated oligomers and polymers.57−61 By complexation with schizophyllan and curdlan,55−59 CD-silent/CPLsilent polythiophene with cationic charges and poly(fluorenealt-m-phenylene) with anionic charges generated their corresponding CD-active/CPL-active macromolecular complexes. The cationically charged curdlan formed a CD-active permethylsilane oligomer.55−59 Alternatively, noncharged chiral solvents, including limonene, α-pinene, and alcohols facilitated the instantaneous generation of CD-active/CPL-active π-/σconjugated polymers as aggregates, despite their CD-silent/ CPL-silent nature.50−54 Recently, we demonstrated that noncharged rodlike helical polysilanes with right- and lef t-helix senses could become ultraviolet (UV)-light scissable helix scaffolding, allowing the instant generation of CD-active/CPL-active poly(di-n-octylfluorene) as aggregates (PF8, Chart 1).61 This result led us to propose that noncharged, nonpolar CD-silent/CPL-silent oligomers and polymers may become CD-active/CPL-active upon becoming embedded into polysaccharide filmseven noncharged, less-polar polysaccharides carrying alkyl esters through intermolecular attractive forces. Regarding this topic, several questions remain to be answered: (i) Which D-glucose chirality or main-chain helicity is crucial for the saccharideinduced chirality transfer capability? (ii) How does the molecular weight of the guest molecule affect the chirality and/or helicity transfer capability? (iii) Do the magnitudes and signs of the induced chiroptical activity remain unchanged over time? To answer these questions, we chose cellulose triacetate (CTA) and cellulose acetate butyrate (CABu) (Chart 1). These celluloses are readily soluble in chloroform and nonchromophoric chiral polymers. Although CTA and CABu share D-
Chart 1. Chemical Structures of CTA, CABu, and a Family of 9,9-Dialkylfluorene Derivatives
glucose repeat units with a β(1→4) linkage, they prefer lef tand right-helix senses, respectively.62−64 As chromophoric and luminophoric probes, we highlighted CD-silent/CPL-silent 9,9dialkylfluorene oligomers and polymers (Chart 1) because dialkylfluorenes are well-established semiflexible π-conjugated molecules that can enable us to systematically investigate the molecular weight dependency of their (chir)optical properties in the ground and photoexcited states.65−67 Herein, we found that CTA and CABu commonly generate a series of CD-active/CPL-active 9,9-dialkylfluorenes as Monomer, Dimer, Trimer, Pentamer, Heptamer, and polymers (PF6 and PF8) (Chart 1) by embedding into cellulose thin films. Both a preference for the helicity of the cellulose and D-glucose residue chirality are crucial, as demonstrated by plotting their CD/UV−vis and CPL/photoluminescene (PL) spectra as a function of the fluorene repeating number (n). Moreover, solidstate 1H−13C heteronuclear correlation (HETCOR) nuclear magnetic resonance (NMR) spectral analysis indicated the existence of intermolecular C−H/OC interactions between the n-octyl side chains of PF8 and the oxygen atoms of the acetyl groups in the CTA framework. This was further supported by density functional theory (DFT) at the B3LYP/ 6-31G(d) level. The multiple C−H/OC interactions near the chiral centers of the D-glucose framework associated with flexible C−O−C and C−C−O−C single bonds were assumed to be responsible for emerging CD-active/CPL-active fluorenes in CTA and CABu.
■
EXPERIMENTAL SECTION
Materials. CTA (Wako Pure Chemicals Industories Ltd. [WAKO], Osaka, Japan, first grade, Mn = 60 000 as per catalogue), CABu (Sigma-Aldrich Corp. St. Louis, MO, now as subsidiary of Merck KGaA (Darmstadt, Germany), Mn = 75 000 as per catalogue), 9,9dimethylfluorene (Monomer) (Tokyo Chemical Industry Co. [TCI], Tokyo, Japan), and 9,9-di-n-propylfluorene dimer (Exalite 384, Dimer, Excition, Inc. [Dayton, OH]) were used without further purification. 9,9-Di-n-hexylfluorene trimer (Trimer), 9,9-di-n-hexyl-fluorene penB
DOI: 10.1021/acs.macromol.6b02762 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Specimens were prepared on a Si-crystal substrate by a multiple dropcasting technique from chloroform solutions of CTA and CABu.
tamer (Pentamer), and 9,9-di-n-hexylfluorene heptamer (Heptamer) were purchased from American Dye Source, Inc. (Baie d’Urfé, Quebec, Canada). Poly(9,9-di-n-hexylfluorene) (PF6) (Sigma-Aldrich, Mw = 69 400 [DPw = 208], Mn = 15 700 [DPn = 47]) was used without further purification. A fractionated weight sample (Mw = 141 120 [DPw = 362] and Mn = 78 400 [DPn = 201])61 of poly(9,9-di-noctylfluorene) (PF8, Sigma-Aldrich) was used without further purification. The purity of PF8, CTA, and CABu was characterized by elemental analysis and 1H/13C NMR spectra (see Supporting Information, Figures S3−S8). PF6 was subjected to 1H NMR only (Figure S9). First, 10 mg of a fluorene derivative and 10 mg of a cellulose derivative (CTA or CABu) were dissolved separately in 1.0 mL of CHCl3 at 50 °C for 5 h and then were left for 6 h at room temperature. After the solutions were completely dissolved, they were mixed at room temperature to generate the desired combinations of the fluorenes with CTA and the fluorenes with CABu (CTA or CABu/ fluorenes/CHCl3 = 10 mg/10 mg/1 mL by w/w/v), and the mixed solutions were kept in glass vials in the dark. The hybridized film was deposited onto a polished circular quartz plate (diameter of 25 mm and thickness of 1 mm) by coating with a spin coater (MIKASA, model MS-B100 [Tokyo, Japan]). To reduce the formation of unfavorable defects and voids, 40 μL of the solution was placed onto the center of the plate and spun at 2000 rpm for 60 s. Instrumentation. UV−vis absorption and CD spectra were simultaneously obtained with a JASCO (Hachioji-Tokyo, Japan) J820 spectropolarimeter with a bandwidth of 2.0 nm, a scanning rate of 100 nm min−1, and a sampling point of 0.5 or 1.0 nm interval. A film sample was placed on the cylindrical quartz plate in a sample housing at room temperature. CPL and PL spectra were recorded at room temperature on a JASCO CPL-200 spectrofluoropolarimeter with bandwidths of 10 nm for excitation and emission, a scanning rate of 100 nm min−1, and a data sampling point of 0.5 nm. The absolute CPL amplitude was calibrated with fresh ethanol solutions (0.4% w/v) of D-/L-camphores (Sigma-Aldrich) at bandwidths of 3000 μm/3000 μm for excitation and emission, respectively. The hybridized polymers were characterized by one-dimensional (1D) cross-polarization magic angle spinning (CP/MAS) solid-state (SS) 13C NMR (contact time 2 ms, 550 scans, relaxation delay 5 s, spinning 8 kHz, repetition time 5.05 s) and HETCOR 1H−13C NMR (contact time 2 ms, 3072 scans, relaxation delay 5 s, spinning 8 kHz, repetition time 5.05 s) spectra in the solid state at room temperature with a 400-MHz JEOL (Akishima-Tokyo, Japan) ECX-400P FT-NMR spectrometer. The CTA-PF8 film with the ratio of 1:1 (w/w) was prepared by drop-casting their CHCl3 solution onto glass substrate, followed by pulverizing the drop-casted thick film into powders by immersion in liquid nitrogen and used for 1D CP/MAS SS 13C NMR and 2D HETCOR SS 1H/13C NMR measurements. All of the model structures simulated in this work, which permitted us to explore all possible intermolecular interactions between PF8 and CTA, were obtained by MD using the Forcite module with a universal force field (UFF) in Materials Studio, ver.7 (Accelrys, now, BIOVIA, San Diego, CA) and DFT calculations with B3LYP using the 631G(d) basis set in the Gaussian09 package (Gaussian, Inc., Wallingford, CT).68 Twenty transition states (singlet) of trimer with 9,9-dimethyl substituents, that is, a model of Trimer with 9,9-di-nhexyl groups, were obtained using the time-dependent (TD)-DFT method with the B3LYP and 6-31G(d) basis sets. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were evaluated by gel-permeation chromatography (GPC) using a Shimadzu (Kyoto, Japan) A10 chromatograph with a Varian-Agilent PLGel mixed B column (length of 25 cm and i.d. of 4.6 mm) and special-grade tetrahydrofuran (Wako Pure Chemical, Osaka, Japan) as the eluent at 40 °C using calibration with polystyrene standards (Varian-Agilent). Wide-angle X-ray diffraction (WAXD) data were collected using a Rigaku RINT-TTR III/NM instrument (AkishimaTokyo, Japan) at an X-ray wavelength of 1.5418 Å using Cu Kα radiation with an Ni filter, a 2θ = 0.05° scan interval, and a scanning speed of 2° min−1. The instrument was operated at 40 kV and 25 mA.
■
RESULTS AND DISCUSSION Helicity and/or Chirality Transfer Capabilities. First, the chiroptical CD and CPL signs, magnitudes, and wavelengths of the fluorenes induced by CTA and CABu are discussed below in line with quantitative chiroptical analysis.69,70 The helicity and point chirality transfer capabilities of CTA largely depend on the fluorene repeating number. We compared the UV−vis absorption and CD spectra of three thin filmsCTA-Trimer, CTA-Heptamer, and CTA-PF6as shown in Figure 1. Trimer, Heptamer, and PF6 all possess n-hexyl side chains at the 9,9position on each fluorene ring.
Figure 1. Normalized (a) CD/UV−vis and (b) CPL/PL spectra of CTA thin films on a quartz substrate at 293 K: Trimer (red curves), Heptamer (green curves), and PF6 (blue curves).
At the ground state chirality of the CTA-Trimer, a broad UV absorption band at 350 nm is assigned to the S0 → S1 transition characteristic of Trimer (Figure 1a). Evidently, a bisignate weak CD signal arose, exhibiting a (+) sign at the first Cotton band (gCD = +0.08 × 10−3 at 375 nm) and a (−) sign at the second Cotton band (gCD = −0.04 × 10−3 at 330 nm), associated with the zero-cross-point at 360 nm (Figure 1a). The CD spectral characteristics of the other CTA films hybridized with Monomer, Dimer, Pentamer, Heptamer, PF6, and PF8 showed similar tendencies and the same chiroptical (+)- and (−)-signs at the first and second Cotton bands, respectively (Figure 1a and Figure S1). Among these fluorene oligomers and polymers, PF6 afforded the largest gCD values at the first and second Cotton bands (Figure 1): gCD = +0.59 × 10−3 at 407 nm and gCD = −0.36 × 10−3 at 361 nm. These induced CD signals from all of the fluorene derivatives clearly demonstrate that the efficient induction of D-β-glucose chirality and/or its main chain helicity is possible in a family of achiral (or CD-silent) fluorene derivatives via intermolecular interactions. Given that CTA, which possesses five chiral C
DOI: 10.1021/acs.macromol.6b02762 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
films, except for those of Monomer, PF6, and PF8 for unknown reasons. PF6 and PF8 exhibited oppositely bisignate CD profiles, with (−)- and (+)-signs at the first and second Cotton bands, respectively. Nevertheless, the shorter fluorene oligomers (n = 2, 3, 5, 7) in CTA and CABu commonly led to the (+)- and (−)-signs at the first and second Cotton bands, respectively, regardless of the opposite helix sense of CTA and CABu. In the photoexcited state, the representative CPL and PL spectra of Trimer, Heptamer, and PF6 in CABu are depicted in Figure 2b, and those of the Monomer, Dimer, Pentamer, and PF8 in CTA films are given in Figure S2. All of the fluorenes adopted a similar twisted conformation with a preferred helicity in the photoexcited state. Notably, the CPL sign of CABu film hybridized with fluorene oligomers (n = 2, 3, 5, and 7) was the opposite of the CD sign at the first Cotton CD band. Moreover, the |gCPL| and |gCD| values in CTA and CABu tend to be nonlinearly enhanced when the fluorene repeating unit, n, increases and approach constant values (Figure 3a). The helixand/or chirality-inducing capabilities that depend on the fluorene repeating unit are assumed to be enhanced by the
centers per pyranose ring, prefers lef t-handed helicity in dilute CHCl3,62−64 we assumed that the three acetate groups per pyranose attached to these chiral centers provided the fluorenes with chirality/helicity transfer capability. Although the acetate groups have no chirality individually, their restricted rotational freedom along the C−O−C and C−C−O−C bonds (Chart 1, shown as red bold lines for CTA) is expected lead to the twisted geometry of the fluorene main chain in a preferred screw sense, as proven by the bisignate CD spectra.60,61 In the photoexcited state chirality, the representative CPL and PL spectra of Trimer, Heptamer, and PF6 in CTA are depicted in Figure 1b, and for comparison, those of Monomer, Dimer, Pentamer, and PF8 in CTA films are given in Figure S1. As shown in Figures 1a and 1b, the CPL and CD signals at the first Cotton bands of the fluorene oligomers and polymers have an identical (+)-sign, except for Dimer. The |gCPL| values of all fluorenes are enhanced by 1.7−3.5-fold compared to their corresponding |gCD| values. When the photoexcited chirality is identical to the ground-state chirality, the CPL sign at the first Cotton band (or the shortest CPL extremum wavelength) should be identical to the CD sign at the first Cotton band, and their absolute gCPL and gCD amplitudes, |gCPL| and |gCD|, should be identical. These findings indicate the occurrence of significant change in the main chain structure in the photoexcited state and the maintenance of the helix sense in the ground states. In contrast, Dimer swaps its main chain twisting between the photoexcited and ground states. For comparison, with respect to the ground-state chirality in the CABu film, the UV−vis and CD spectra at the S0 → S1 transitions of Trimer, Heptamer, and PF6 and, for comparison, those of Monomer, Dimer, Pentamer, and PF8 are shown in Figure 2a and Figure S2a, respectively. Similarly, the right-hand helicity63,64 and/or D-glucose chirality of CABu efficiently transferred to these fluorenes. The bisignate CD profiles associated with the (+)- and (−)-signs at the first and second Cotton bands in CABu are quite similar to those in the CTA
Figure 3. (a) gCD and gCPL values at the first Cotton band of the CD and CPL spectra as a function of the fluorene ring number (n) in CTA and CABu films. (b) λmax (S0 → S1 transition) and λem values (S1 → S0 transition) in CTA, CABu, and chloroform solution as functions of the fluorene ring number. (c) Optical bandgap, Eg,opt, obtained with the λmax and λem values in CTA, CABu, and chloroform solutions as a function of reciprocal n−1. Arrows indicate the Stokes shift.
Figure 2. Normalized (a) CD/UV−vis and (b) CPL/PL spectra of CABu thin films on a quartz substrate at 293 K: Trimer (red curves), Heptamer (green curves), and PF6 (blue curves). D
DOI: 10.1021/acs.macromol.6b02762 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
semiflexible chain with a persistent length (q) of 7−9 nm in solution, which corresponds to n ≈ 10.65−67 Oligofluorenes (n = 2−7) and polyfluorenes (n ≤ 10) should behave as rigid rod chains while maintaining the rotational freedom along the C−C bonds between the fluorene rings. Fluorenes shorter than n ≈ 10 are typically susceptible to the D-glucose chiralities of CTA and CABu. However, a sufficiently long fluorene (n ≥ 10) should act as a random coil and be susceptible to the helicity of CTA and CABu to a greater degree than the D-glucose chiralities. The cellulose-dependent swapping of the CD and CPL signs is believed to arise from the degree of the conflict between the helicity and D-glucose chiralities in the ground and photoexcited states. For CTA, the D-glucose chiralities and lef t-hand helicity cooperatively make the fluorene rings twist strongly in a preferred screw sense, leading to the maintenance of the CD-/ CPL-signs in the ground and photoexcited states. In contrast, for shorter fluorenes in CABu, the D-glucose chiralities and right-hand helicity competitively induce the opposite twist sense in the fluorene rings, leading to swapping between the CD- and CPL-signs in the ground and photoexcited states. This conflict led to a significant decrease in the |gCD| values in CABu by 2−15-fold relative to those in CTA for the same n number (Figure 3a). The right-hand helicity of CABu became a dominant factor for longer PF6 and PF8 over D-glucose chiralities in the ground state, and as a result, a negligibly small |gCD| value was achieved. Our successful helicity and/or chirality transfer based on CTA and CABu is believed to arise from the invisible weak intermolecular interactions79,80 existing between the fluorenes and D-glucose residues substituted with three achiral alkyl esters. This is discussed in the MM/MD calculations and CP/ MAS 1D 13C NMR and 2D HETCOR 1H/13C NMR spectral analysis presented below. Origin of the Bisignate CD and Evidence for Twisted Fluorene Rings. Previously, Chen et al.81 achieved greatly amplified bisignate CPL signals by embedding a 9,9-di-npropylfluorene trimer with p-tert-butylphenyl termini (Exalite 428, Chart 1) in glass-forming cyclohexane-based chiral nematic liquid crystalline films. Recently, Pietropaolo et al.82 conducted theoretically and experimentally a chirality switching in free-energy landscape and bisignate CD signals on fluorene dimer/decamer and 43-mers of PF8. They indicated an importance of chain assembling onto an amorphous surface of silica and elaborated the possibility of an opposite helicity transfer capability to PF8. These excellent reports promoted us to conduct a grid scan method for possible global and local minima of the trimer without phenyl termini (Chart 1) using DFT calculation as a function of two dihedral angle sets with an increment of 20°. This facilitated the prediction of the CD/ UV−vis spectra of the rotational isomers. The trimer and all other 9,9-dialkylfluorenes should exist as rotational isomers because of the inherent rotational freedom along the C−C single bonds between fluorene rings restricted by multiple C− H/H−C repulsions (Chart 1, shown as a bold red line). Next, the potential energy surface scan method, followed by structural optimizations at the dihedral angles, was conducted by DFT calculation at the B3LYP/6-31G(d) level.83−85 The calculated potential energy surfaces associated with all the transition dipole and magnetic moments as a function of the dihedral angles are given in Figure S10. The calculated CD/ UV−vis spectra of the trimer arise from the twisted fluorene units (Figure S11). The four stable rotamers with the lowest
multiple noncovalent intermolecular interactions aided by helical and/or chiral CTA and CABu. On the basis of the bisignate CD spectral sign induced by CTA and CABu, we assume that the multiple chiral centers in the D-glucose framework are a deterministic factor of the twist sense in shorter fluorene oligomers (n = 2−7) with (+)-/(−)-CD signs in the ground states. In contrast, a preference for the helix sense in the longer D-glucose sequence is more crucial for the longer polyfluorenes (n > 47) with (−)-/(+)-CD signs than for the local chiral centers of the Dglucose units. Conversely, the opposite helicity between CTA and CABu induced the opposite CPL signs in the fluorene oligomers, although PF6 and PF8 exhibited the same (−)-CPL sign. Here, we demonstrate a versatile approach to obtain isolated π-conjugated chains of CD-active/CPL-active oligomers and polymers50−61,71−74 in 1 min at room temperature when CTA and CABu are used as host matrices, which possess a helix preference and/or multiple chiral centers with (S) or (R) chirality when appropriate achiral source materials are chosen. In the case of the fluorene oligomers with n ≈ 10, a (+)- or (−)-CPL sign can be designed by appropriately choosing the helix preference of CTA or CABu, although the (+)-CD sign at the first Cotton band was commonly observed to remain unchanged, possibly because the D-glucose chirality is the dominant factor. When a π-electron in π-conjugated molecular wires delocalizes, the S0 → S1 transition energy decreases. The degree of twisting in the π-conjugated wires is expected to be responsible for the charge carrier mobility in lower optical bandgap (Eg,opt) polymers, as demonstrated by the red-shifts of the PL and UV−vis bands.75,76 The Eg,opt values obtained with the λmax and λem values in the series of dialkylfluorene oligomers and polymers in CTA and CABu films and chloroform decreased almost linearly as a function of the reciprocal fluorene ring number (n−1), except for that of Monomer with n−1 = 1.0 (Figure 3c). Moreover, the Stokes shift between Eg,opt from λmax and Eg,opt from λem decreased as the n value increased. Thus, a minimal reorganization of the fluorene main chain in the photoexcited and ground states occurs when the number of fluorene rings increases.77,78 The Eg,opt values from the λem values of PF6 and PF8 in the CTA and CABu films are considerably red-shifted (by ≈0.16 eV [≈1300 cm−1]) compared to those in chloroform (Figure 3c), indicating substantial suppression of the twisted distortion of fluorene rings in these films, even in the photoexcited state. Although the planarization of the fluorene rings in the photoexcited state causes such red-shifts, the existence of multiple inherent C−H/H−C repulsions between the rings should prevent this planarization in the photoexcited and ground states (Chart 1, shown as a red bold line for 9,9substituted dimeric fluorene). The absolute gCD value, |gCD|, associated with the (+)-sign at the first Cotton band in CTA and CABu commonly increases as the n value increases from 2 to 7 (Figure 3a). Contrarily, the gCD sign at the first Cotton band in CABu swaps between n = 8 and n ≥ 47 (Figure 3a). The gCPL sign in CABu is opposite that in CTA when n ≥ 3, and the gCPL sign in CABu is opposite the gCD sign in CABu when n = 2−7. The gCPL sign in CTA swaps between n = 2 and 3, whereas the gCPL sign in CABu remains unchanged when n ≥ 3. One possible explanation for this n-dependency is that poly(dialkylfluorene)s in the ground state behave as a E
DOI: 10.1021/acs.macromol.6b02762 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
from the lowest S1 state at 385 nm, which is in line with the scenarios of Jablonski’s diagram and Kasha’s rule.86−89 The origin of the bisignate CD signals is thus not the exciton couplet of chirally assorted structures. Based on the inherent C−H/H−C repulsion between the fluorene rings, the twist rotamers can be induced by embedding in polysaccharides that exhibit a conflict between the main-chain helicity and point chiralities at the D-glucose residues. Photoinduced Racemization. To answer the third questioni.e., whether the helical fluorenes induced by CTA in the photoexcited state can stably existthe first photoinduced racemization experiment of CPL-active/CD-active fluorene derivatives in CTA (0.2 cm2 in area) was designed under aerobic conditions and continuous irradiation from an unpolarized UV-light source. This was achieved by utilizing a Xe−Hg arc lamp equipped with the CPL-200 spectrometer that used monochromated unpolarized light with a bandwidth of 10 nm (320 nm for Dimer, 350 nm for Trimer, 360 nm for Heptamer, and 370 nm for PF6). The changes in the gCPL values of Dimer, Trimer, Heptamer, and PF6 in CTA as a function of the photoirradiation time up to 60 min with depolarized UV-light irradiation are displayed in Figure 5. The time course of the gCPL magnitude was evaluated at a CPL extremum.69,70 It was found that the photoinduced racemization rate depends strongly on the number of fluorene rings.
energies were assigned as ID33, ID38, ID83, and ID88 (Figures S16−S19). Among these rotamers, the ID88 data set simulated CD and UV−vis spectra with a full width at half-maximum of 0.10 eV (Figure S11) because the inherent C−H/H−C repulsions between fluorene rings are responsible for the twisted geometry. Evidently, the simulated CD spectra of trimer as a model of Trimer in CTA and CABu exhibit a bisignate CD profile with gCD = +0.31 × 10−3 at the first Cotton band at 362.47 nm and gCD = −9.34 × 10−3 at the second Cotton band at 306.81 nm. These CD magnitudes associated with the sign agree qualitatively with the experimental CD results that are in CTA, gCD = +0.08 × 10−3 at 375 nm and gCD = −0.04 × 10−3 at 330 nm (Figure 1a) and, in CABu, gCD = +0.04 × 10−3 at 378 nm gCD = −0.08 × 10−3 at 333 nm (Figure 2a). The (+)-sign CD and intense UV π−π* bands at 362.47 nm are ascribed to the electric dipole allowed, magnetic dipole allowed S0 → S1 transition with an oscillator strength of f = 2.0107. These transitions occur parallel to the long axis of the trimer and from HOMO (antisymmetric wave functions to the pseudo-σh plane) to LUMO (symmetric to the σh) (98%), as displayed in Figure 4 and Figure S18.
Figure 5. Changes in the gCPL values monitored at CPL λext wavelengths of Dimer (orange), Trimer (red), Heptamer (green), and PF6 (blue) in CTA films at 293 K on a quartz substrate under unpolarized light irradiation. The raw data were obtained using a builtin program of the JASCO CPL-200 spectrofluoropolarimeter and plotted as gCPL = ellipticity (mdeg)/14320/PL (DC in volts).
Among the four fluorenes, PF6 exhibited the greatest photostability in the photoexcited state, maintaining the initial gCPL amplitude after prolonged irradiation. By comparison, Dimer caused rapid racemization (chiroptical depolarization), and its gCPL value decreased to ≈45% of its initial value after 60 min of photoirradiation. The maintenance of the gCPL value in CTA depended strongly on the n-number as follows: PF6 ≫ Heptamer > Trimer > Dimer. Thus, we hypothesized that the photophysical stability of the optically active fluorenes in CTA increases as n increases and remains constant when n is sufficiently high. Computational Study for Modeling. In the previous section, we demonstrate the helicity and/or chirality transfer of PF8, which led to the retention and inversion in the CD and CPL signals influenced by the nature of CTA and CABu. Herein, we investigate the induction mechanism of optical active PF8, which depends on the dihedral angles of the main chain and a possible intermolecular interaction between PF8
Figure 4. Top: an ID88 model with dihedral angles of 140° and −40°. Bottom: isosurfaces of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs).
In contrast, the (−)-sign CD but weak UV π−π* bands at 306.81 nm are ascribed to the electric dipole forbidden, magnetic allowed transitions with a phantom S0 → S2 transition with f = 0.0008. These transitions are perpendicular to the long axis and occur from HOMO−1 (symmetric) to LUMO (symmetric) (57%) and from HOMO (antisymmetric) to LUMO+1 (antisymmetric) (38%). Our experimental CD and UV−vis spectra of Trimer and other longer oligomers and polymers indicate that trimer is a valid computational model by the TD-DFT calculation.83−85 The twisted trimer with dihedral angles (e.g., 140° and −40°) agree well with the apparent bisignate CD bands at 375 and 300 nm of Trimer, leading to CPL emission with a single sign F
DOI: 10.1021/acs.macromol.6b02762 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules and CTA, by the MD method using the Materials Studio Forcite module and a UFF; a more reliable PCFF was not available to perform this calculation.90−93 CTA contains three achiral methyl ester groups near the chiral centers of D-glucose residues that may contribute to pseudochiral intermolecular C− H/OC hydrogen bonds. The chiral hydrogen bonds should lead to certain helical structures. The current simulations were designed to act as an enormous, realistic hybridized system consisting of CTA (12 repeating units) and PF8 (12 repeating units) to fully interpret the intermolecular interactions in the real system, followed by the induction of helical and/or chiral motifs, as shown in Figure S12. The optimized CTA (12 units) and PF8 (12 units) were obtained by Forcite Geometry Optimization. Two structures were connected to form a polymer hybrid system. After generating the initial hybrid structure, the geometry of the polymers was refined at a pressure of 0.1 MPa until the calculations converged. To optimize by energy minimization, the hybrid system was annealed at high temperature by Forcite Anneal Dynamics, and the model was finally equilibrated at 300 K and 1 atm for 150 ps using the NPT method via classical MD.92 C−H/OC Interactions between PF8 and CTA. The numbering of the atoms in the main and side chains is shown in Figure S13. The aim was to consider the effects of the helicity and chirality transfer and the twisted polyfluorene, and the CTA backbone generally exhibited three unique CO of acetyl groups. MM/MD simulations revealed the existence of intermolecular C−H/OC interactions between CTA and PF8. The distance between H and O ranges from 2.49 and 2.86 Å (Table S1-1), which is significantly shorter than the sum of their van der Waals contacts (3.0 Å).79,80 In a real system, huge numbers of intermolecular C−H/O C interactions between PF8 and CTA are responsible for transferring the helicity and/or chirality, followed by the induction of left−right imbalance. These phenomena are reflected by the emergence of optically active PF8 detected by measuring the CD and CPL signals (Figure 1). This concept is also applicable to other 9,9-dialkylfluorene oligomers and PF6. Although a similar scenario may exist between CABu and the fluorenes, the capability of helicity and/or chirality transfer is expected to be rather weak because the C−H/OC interactions between the n-alkyl C−H and n-butylate OC groups may be suppressed by the bulkier n-butylate groups and the mixture of n-butylate and acetyl groups. The differences in the alkyl substituents of the polysaccharides are reflected in the WAXD profiles of CTA and CABu films deposited on Si substrates (Figure 6). CTA had two distinct diffraction peaks at 10.818 and 15.133 Å and two broad peaks at 4.152 and 5.1613 Å, whereas CABu had two broad peaks at 14.368 and 4.557 Å. The long d-spacing (14−15 Å) is ascribed to the interchain distance of the polysaccharides. Actually, the similar liquid crystalline chitosan-substituted phenylcarbamate had a d-spacing of 15−17 Å,64,94 although a shorter d-spacing of 4−5 Å has not been observed. CTA may form a fairly ordered film, whereas CABu adopts an ill-ordered, glassy state. This subtle discrepancy is responsible for the substantial reductions in the |gCD| and |gCPL| values of the fluorene derivatives in CABu relative to those in CTA (Figure 3a). Possible Conformations of the Intermolecular C−H/ OC Interactions. We employed 1D solid-state (SS) CP-
Figure 6. Left: comparison of the WAXD profiles of CTA (red) and CABu (blue) on a Si substrate casted from chloroform solution in air. Right: computer-generated dimer model of CTA 12 units with Materials Studio (ver.7).
MAS95−97 and SS 2D 1H−13C HETCOR98−103 NMR experiments for solid less-crystalline polymers to demonstrate the intermolecular C−H/OC interactions between CTA and PF8, as predicted by the MD/MM calculations (Figure 7, top). SS CP-MAS 13C NMR spectra were analyzed by considering the CTA95−97 and PF8104 structures. The predicted interactions were detected by integrating the phase-modulated Lee− Goldburg (PMLG) homonuclear decoupling technique in 2D 1 H−13C HETCOR NMR,99−102 which measures the 13C−1H magnetic dipolar coupling between 13CO and 1H−C groups attributable to the nuclear Overhauser effect.104 This method was established to address internuclear distances and C−H/O angles and determine molecular structures.103 The PMLG 2D SS CP/MAS 1H−13C HETCOR NMR technique allowed to detect intermolecular CH/π interactions between polyethylene and crystalline aromatic host.102 Also, PMLG 2D SS CP/MAS 1H−13C technique detected intramolecular C−H/OC interactions of two anomers existing maltose molecular solids.103 However, 2D solid-state NMR experimental studies to detect intermolecular C−H/OC interactions between two noncrystalline polymers are not reported yet. The PMLG 2D 1H−13C HETCOR NMR spectrum of the CTA-PF8 hybrid provides clear evidence of an intense crosspeak between the methylene protons of the n-octyl group of PF8 (δH = 2.55 ppm) and 13CO (δC = 170.6 ppm, achiral acetyl of CTA) shown in the red region of Figure 7, bottom. Because the polarization transfer from the 1H to 13C atoms occurs at distances shorter than 5.0 Å,103 the intense cross-peak, which obeys a scaling law of the interatomic distance d−6,106 directly supports the close distances (
