Controlling Circularly Polarized Luminescence Using Helically Structured Chiral Silica as a Nanosized Fused Quartz Cell

Circularly polarized luminescence (CPL) is typically achieved with a chiral luminophore. However, using a helical nanosized fused quartz cell consisting of chiral silica, we could control the wavelength and helical sense of the CPL of an achiral luminophore. Chiral silica with a helical nanostructure was prepared by calcining a mixture of polyhedral oligomeric silsesquioxane (POSS)-functionalized isotactic poly(methacrylate) (it-PMAPOSS) and a small amount of chiral dopant. The chiral silica encapsulated functional molecules, including luminophores, along the helical nanocavity, leading to induced circular dichroism (ICD) and induced circularly polarized luminescence (iCPL). Because chiral silica can act as a helical nanosized fused quartz cell, it can encapsulate not only the luminophore but also solvent molecules. By changing the solvent in the luminophore-containing nanosized fused quartz cell, the wavelength of the CPL was controlled. This method provides an effective strategy for designing novel CPL-active materials.

S ilica-based chiral materials have attracted considerable attention in various fields such as catalysis, templating, and chiral recognition.Chiral silica precursors are typically prepared via a sol−gel reaction using tetraethoxysilane with a chiral surfactant 1−3 or a mixture of polyethylenimine and tartaric acid. 4Chiral silica prepared from a chiral surfactant forms a well-ordered helical structure.Although chiral silica prepared from polyethylenimine/tartaric acid does not form a specific helical structure, it does exhibit chiral optical properties. 5Because silica-based materials do not absorb light in the UV and visible regions, the helical nanocavities in chiral silica are expected to act as "nano quartz cells" for the circularly polarized luminescence (CPL) of encapsulated solvent and luminophore molecules.Photoluminescence (PL) and CPL are strongly affected by the solvent used.Consequently, if chiral silica can encapsulate both achiral luminophores and solvents within the helical nanocavity, the wavelength of the CPL could be controlled, and selfaggregation-caused quenching (ACQ) could be prevented.Although chiral silica has been used as a host for CPL , accommodating both solvent and luminophore molecules within the chiral silica has not yet been achieved.Helically structured chiral silica typically ranges in length from a few micrometers to ∼100 nm, with helical pores 2−3 nm in diameter. 3,6However, this size may be unsuitable for accommodating solvent molecules due to the lack of a nanoscale confinement effect.
−10 Various ordered bulk and thin-film nanostructures have been prepared using POSS-containing block copolymers, 11−14 giant molecules, 15 and supramolecular compounds. 16POSS-containing block copolymers are used to obtain conventional lamellar, cylindrical, and spherical nanostructures, whereas giant POSScontaining molecules are used to prepare gyroid and Frank− Kasper phases.Although natural polymers, including DNA and RNA, form helical structures, the arrangement of POSS in a helical configuration remains challenging.We first reported the control of the preferred-handed helical conformation using an isotactic POSS-functionalized methacrylate polymer (it-PMA-POSS) in the presence of small amounts of chiral dopants.The helical structure was maintained during the calcination process, resulting in chiral silica with a helical structure comprising subnanometer-diameter structures. 17The small helical nanocavity of the it-PMAPOSS-based chiral silica may act as a nanosized fused quartz cell and enable the encapsulation of both luminophore and solvent molecules.Herein we report the control of the CPL wavelength using chiral silica to encapsulate an achiral luminophore and solvent molecules in a helical nanosized quartz cell.
Figure 1a shows the chemical structure of it-PMAPOSS.The number-average molecular weight (M n ) and polydispersity index (Đ) were 8000 and 1.24, respectively.The stereoregularity of it-PMAPOSS was evaluated by using 13 C NMR spectroscopy.The signals at ∼45 ppm were assigned to meso− meso (mm), meso−racemo (mr), and racemo−racemo (rr) couplings from low to high magnetic field values, respectively.The ratio of mm, mr, and rr in it-PMAPOSS was 97:2:1.To control the helical structure, it-PMAPOSS and (S)-(−)-or (R)-(+)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-bi-2-naphthol (BN) were mixed at a MAPOSS/BN molar ratio of 0.1 in toluene.The solution was annealed at 90 °C for 2 h, and the toluene was subsequently slowly evaporated.The solid sample was calcined at 620 °C, resulting in chiral silica. 17Figure 1b,c show the transmission electron microscopy (TEM) image and vibrational circular dichroism (VCD) spectra, respectively, of the chiral silica prepared from it-PMAPOSS with enantiomeric BNs.The helical structure was evident from the TEM image.In the VCD spectrum, a clear split-type Cotton effect was observed for the peak between 1000 and 1240 cm −1 , which was assigned to the Si−O−Si stretching vibration.The Cotton effect between the it-PMAPOSS/enantiomeric BN chiral silicas exhibited a mirror relationship.Moreover, no other organic peaks were observed, indicating that chiral silica with a helical structure was obtained. 17The chiral silicas prepared from it-PMAPOSS/(S)-BN and it-PMAPOSS/(R)-BN are denoted as S-chiral and R-chiral silica, respectively.
To evaluate the encapsulation behavior of the helical nanocavity in chiral silica, phenol was introduced.Chiral silica was immersed in a phenol/hexane solution for 14 h and then collected by centrifugation.The resulting chiral silica was washed several times with hexane to remove unencapsulated phenol from the helical silica nanocavity.The silica was dispersed in a MeOH/H 2 O mixture, and electronic circular dichroism (ECD) measurements were subsequently performed.Figure 2 shows the ECD spectra of chiral silica/ phenol, chiral silica, and phenol.For the chiral silica/phenol sample, a clear split-type Cotton effect was observed in the broad UV absorption peaks at 210−220 and 260−280 nm, which corresponded to the π−π* electronic transition of phenol.In contrast, the Cotton effect was not observed in this UV region for chiral silica and phenol individually (Figure 2b,c).The UV peaks in the 210−220 and 260−280 nm regions were broadened and slightly shifted to higher wavelengths (Figure 2d,e).These results indicated that the phenol molecules were placed in the helical nanocavities via Jaggregation, as shown in Figure 2f.Achiral chromophores can be placed along helically structured chiral silica, leading to induced ECD (iECD) in the ground state.To confirm the universality of the chirality behavior, an S-silica sample was immersed in 0.5 wt % pyranine/H 2 O for 4 days and subsequently rinsed twice with pure H 2 O.The induced negative Cotton effect observed at 280 nm was attributed to the characteristic absorption band along the short axis of the molecular plane of the pyrene moiety between 250 and 300 nm 18−20 (Figure S3a).These results indicated that the S-silica encapsulated the pyranine molecules within its helical nanocavity (see the Supporting Information).In addition, the chiral silica exhibited no absorption peak in the UV region (Figure 2b).Thus, chiral silica has the potential to act as a nanosized fused quartz cell.
Because achiral chromophores can be placed in a helical nanocavity, the induced chirality of electronically excited fluorophores is also expected to occur, resulting in induced circularly polarized luminescence (iCPL).Achiral pyranine is known to exhibit specific fluorescence emissions depending on its acid dissociation state and was selected as the achiral PL molecule (Figure 3a).Pyranine was dissolved in MeOH and H 2 O to obtain 0.5 wt % clear solutions, and the chiral silica was immersed in these solutions for 2 days at room temperature.The chiral silica immersed in the pyranine/MeOH and pyranine/H 2 O solutions exhibited blue and green emissions, respectively, upon UV irradiation at 340 nm (Figure 3b).CPL measurements were performed to evaluate the effect of the solvent on the iCPL.Figure 3c shows the CPL spectra of the enantiomeric chiral silicas in the pyranine/MeOH and pyranine/H 2 O solutions.Symmetrical mirror-image CPL spectra were obtained for the enantiomeric chiral silica/ pyranine in MeOH and H 2 O.This suggested that the achiral luminophores were placed along the helically structured chiral silica, leading to iCPL in the excited state.The chiral silica immersed in the pyranine/H 2 O solution exhibited a green CPL at 520 nm, whereas that in the pyranine/MeOH solution exhibited a blue CPL at 436 nm.In contrast, pyranine itself exhibited no CPL when dissolved in H 2 O or MeOH (Figure S1).The OH group in pyranine does not dissociate in MeOH, whereas dissociation occurs when pyranine is dissolved in H 2 O.The emission peaks at 436 and 520 nm were assigned to the pyranine−OH and pyranine−O − , respectively. 21,22nsidering these results and those reported in the literature, the chiral silica clearly acted as a helical nanosized fused quartz cell that encapsulated the pyranine and solvent molecules.To investigate stability of the solution within the nanosized fused quartz cell, S-silica with pyranine/MeOH was rinsed twice with fresh H 2 O, and the CPL dissymmetry factor (|g lum |) was evaluated (Figure 4a).The value at 436 nm increased from ∼0.0013 to ∼0.026 after two consecutive solvent replacements, which corresponded to a 20-fold enhancement.These results indicated that pyranine and MeOH molecules were accommodated within the chiral silica and that unassociated pyranine molecules could be removed by rinsing with fresh H 2 O.
Fluorophores typically undergo self-aggregation in the solid state, leading to ACQ (Movie S1).Therefore, the PL and CPL of chiral silica and pyranine in the solid state were evaluated.Chiral silica was immersed in a 1 wt % pyranine/H 2 O solution for 2 days at room temperature and then drop-cast onto quartz substrates; CPL measurements were subsequently performed (Figure 4b).Symmetrical mirror-image CPL spectra were observed for solid-state enantiomeric chiral silica.This indicated that the chiral silica acted as a nanosized fused quartz cell and regulated the self-aggregation of the pyranine molecules, thereby regulating the ACQ.
In conclusion, the optical activities in the electronic ground and excited states of a novel chiral silica prepared from it-PMAPOSS were investigated.The chiral silica acted as a nanosized fused quartz cell to encapsulate functional materials and solvents along the helical nanocavities without any surface modification.The chiral silica with functional molecules exhibited ICD and iCPL.The wavelength of the CPL between 436 and 520 nm could be controlled by changing the solvent in the achiral luminophore-containing helical nanosized fused quartz cell.This finding enables the control of the wavelength in CPL using a simple and cost-effective approach that can

Figure 1 .
Figure 1.(a) Chemical structure of it-PMAPOSS.(b) TEM image of chiral silica prepared from (R)-BN.The inset shows a highly magnified image.(c) VCD spectra of chiral silica prepared from it-PMAPOSS with enantiomeric BN.

Figure 2 .
Figure 2. (a−c) ECD spectra of (a) chiral silica/phenol, (b) chiral silica, and (c) phenol measured in MeOH/H 2 O solution.(d, e) Highly magnified UV−vis spectra of chiral silica/phenol and phenol in MeOH/H 2 O solution.(f) Schematic of the J-aggregation state of phenol in a chiral silica helical nanocavity.

Figure 3 .
Figure 3. (a) Chemical structure of pyranine.(b) Photographs of chiral silica/pyranine in MeOH (left) and H 2 O (right) upon UV irradiation at 340 nm.(c) CPL and corresponding PL spectra of chiral silica associated with pyranine under excitation at 340 nm.