Li
Meng
ab,
Zhong-Qiu
Li
*b,
Kun
Tang
b,
Jiang-Yang
Shao
b,
Zili
Chen
*a and
Yu-Wu
Zhong
*bc
aDepartment of Chemistry, Renmin University of China, 59# Zhongguancun Street, Haidian District, Beijing 100872, China. E-mail: zilichen@ruc.edu.cn
bBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 2 Bei Yi Jie, Zhong Guan Cun, Haidian District, Beijing 100190, China. E-mail: lizhongqiu@iccas.ac.cn; zhongyuwu@iccas.ac.cn
cSchool of Chemical Sciences, University of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Shijingshan District, Beijing 100049, China
First published on 7th December 2022
Well-defined luminescent micro/nanostructures are important building blocks for miniaturized photonic devices. In this work, a simple method is employed to prepare organic microspheres and crystalline microrods with circularly polarized luminescence (CPL) properties. The in situ protonation and co-assembly of a series of pyridine-functionalized chromophores and a chiral binaphthol phosphoric acid give rise to blue-to-yellow emissive amorphous microspheres and green- and yellow-emissive crystalline microrods with good fluorescence quantum yields (9.6–64.6%). The luminescence dissymmetry factors (glum) of microrods (∼10−3) are found to be one order of magnitude larger than those of microspheres. These microstructures and related materials are further characterized by fluorescence microscopy, scanning electron microscopy, single-crystal and powder X-ray diffraction, and Fourier transform-infrared spectroscopy analyses. The well-defined morphologies and promising luminescence properties of these microstructures make them potentially useful for chiral photonic applications.
Organic nanophotonics focus on the photonic studies of organic nanostructures or microstructures with well-defined morphologies, including the preparation of photofunctional structures with different dimensionalities and their applications in the fabrication of various miniaturized photonic devices such as lasers, waveguides, and logic gate circuits.21–25 In this regard, organic nano/microstructures with CPL activity represent an advanced type of material allowing us to manipulate polarized light in photonic devices.26–30 Among them, CPL-active structures with a one-dimensional (1D) or a two-dimensional (2D) shape can be relatively readily obtained.26–30 In particular, this is true for those formed as a result of ordered or helical molecular packing, leading to efficient chirality transfer and/or chirality amplification. In comparison, zero-dimensional (0D) spherical micro/nanostructures with CPL activity are less known.31,32 Micro/nanoparticles or spheres are typically amorphous and they are very likely CPL-inactive owing to the unordered molecular arrangement and irregular spatial organization of luminophores, though chiral molecular components are present in these structures.33 Polymeric microspheres recently reported by Yamamoto and co-workers are one striking example, and they show a particularly high luminescence factor (glum) of 0.23 when dispersed in methanol.34 The high glum value is related to the formation of a liquid crystalline mesosphere in these microspheres.
In chiral assemblies, noncovalent interactions, such as hydrogen bonding interactions, π–π stacking, and metal–metal interactions, play significant roles in determining their photophysical properties.11–20 We recently employed the in situ reaction and crystallization of pyridine-functionalized chromophores with the chiral camphor sulfonic acid to prepare crystalline microplates with full-color CPL with glum in the order of 10−2 and high photoluminescence quantum yields (ΦPL).28 When treated with a strong protonic acid, these pyridine compounds are transformed into protonated pyridinium derivatives, to which the chiral sulfonic anions are bound by hydrogen bonding interactions in the solid state.35–37 Pyridinium derivatives emit intense photoluminescence with a π/π* localized or intramolecular charge transfer (ICT) character, accompanied by distinct CPL due to the effective chirality transfer from chiral anions to pyridinium chromophores. Considering the widespread use of C2-symmetric chiral binaphthyl phosphoric acids in organic catalysis38,39 and the high utilities of binaphthyl frameworks in preparing CPL-active materials,40–45 we are interested in examining the possibility of preparing CPL-active micro/nanostructures by the reaction of pyridine derivatives with the chiral binaphthyl phosphoric acid as a strong protonic acid. For this purpose, the reactions of (R/S)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphate ((R/S)-BPA) with achiral pyridine-containing chromophores 1–5 have been attempted (Fig. 1; only (R)-BPA is displayed). By varying the preparation conditions, microspheres and crystalline microrods with multicolor CPLs have been obtained from this binary assembly, with glum in the order of 10−4 and 10−3, respectively.
Fig. 1 Schematic diagram showing the reaction of (R)-BPA with 1–5 to give CPL-active microspheres and crystalline microrods. |
Compounds 1–5 exhibit gradually red-shifted emissions in dichloromethane (CH2Cl2) solutions from 1 to 5, with the emission maximum wavelength λemi varying from 335 to 461 nm (Table 1 and Fig. 2). Compounds 2–5 are highly emissive with fluorescence quantum yields (ΦFL) of over 45%. However, the shortest compound 1 is only weakly emissive with a ΦFL of 0.80%. The chiral acid (R)-BPA displays absorption between 250 and 340 nm with the absorption maximum wavelength λabs at 303 nm and an intense ultraviolent emission band with a λemi of 352 nm and a ΦFL of 29.7% (Fig. S1, ESI†). The fluorescence emission lifetimes (τ) of these compounds are in the range of 0.32–2.34 ns.
Compound | λ abs/nm | λ emi/nm | Φ FL /% | τ/ns |
---|---|---|---|---|
a Excitation wavelength: 260 nm for (R)-BPA and 1; 280 nm for 2–4; 300 nm for 5. ND = not determined. The absolute ΦFL values of the emissions of 1–5 with the addition of (R)-BPA have not been determined because the degrees of protonation and thus the emission intensities are dependent on the solution concentration. b Absolute fluorescence quantum yields (1 × 10−6 M). | ||||
(R)-BPA | 303, 324 | 352 | 29.7 | 2.20 |
1 | 252 | 335 | 0.80 | 0.99 |
1 + (R)-BPA | 289, 327 | 354 | ND | ND |
2 | 298 | 389 | 72.4 | 1.30 |
2 + (R)-BPA | 303, 349 | 478 | ND | ND |
3 | 341 | 427 | 45.7 | 0.32 |
3 + (R)-BPA | 280, 376 | 534 | ND | ND |
4 | 280, 346 | 427 | 50.7 | 2.34 |
4 + (R)-BPA | 302, 360 | 564 | ND | ND |
5 | 312, 379 | 461 | 60.6 | 2.06 |
5 + (R)-BPA | 302, 397 | 583 | ND | ND |
When treated with (R)-BPA, distinct absorption spectral red-shifts were observed for the solutions of 2–5 as a result of the protonation of the pyridine unit. However, such a spectral shift is less obvious for 1 due to the overlap between the absorption spectra of 1 and (R)-BPA. In the presence of (R)-BPA, the original emissions of 1–5 were diminished and a new emission band with a longer λemi appeared. The new emissions of the longer molecules 2–5 are attributed to the ICT process targeted at the pyridinium motif,35–37 which is supported by the time-dependent density functional theory (TDDFT) calculations of the protonated cationic moieties (Fig. S2, ESI†). The emission of (R)-BPA is barely visible in the solutions of 2–5 in the presence of (R)-BPA, reflecting the efficient energy transfer between them. The emission spectral changes caused by the addition of acid are in agreement with the emission color changes of these compounds (Fig. 2k and 2l). The observed emission band of 1 in the presence of (R)-BPA is more likely associated with (R)-BPA based on the consistency of their emission wavelengths and the weak emission of 1.
When treated with (R)-BPA or (S)-BPA, compounds 1–5 show mirror-image circular dichroism (CD) signals in CH2Cl2 (Fig. S3, ESI†). These signals are largely related to the chirality information of (R)-BPA or (S)-BPA in the wavelength range of 240–340 nm. The very weak CD signals in the longer wavelength region between 340 and 450 nm (except 1) are associated with the ICT absorptions of the pyridinium compounds. However, the solutions of these protonated pyridinium molecules display no detectable CPL signals, suggesting that the excited-state chirality transfer is negligible in the solution state (Fig. S4, ESI†).
With the above solution-state spectral changes in mind, we embarked on the preparation of nanostructures or microstructures from the in situ reactions of 1–5 with (R/S)-BPA. To our delight, by natural solvent evaporation of a mixture of 1–4 with equal equiv. of (R/S)-BPA in mixed solvents of CH2Cl2/tetrahydrofuran (THF)/ethyl acetate (EA) (see details in the Experimental section), microspheres 1[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph with a diameter of 1.0–2.0 μm were obtained (Fig. 3 and Fig. S5 (ESI†); sph stands for microspheres). In this mixed solvent system, CH2Cl2 and THF behave as the good solvents; while EA is a poor solvent for the in situ formed pyridinium salt. When the solvents evaporate gradually, microspheres are generated on the quartz plate. The binary chemical compositions of these microspheres are supported by NMR and mass spectral analyses. The similar procedure for the reactions of 5 with (R/S)-BPA however only gave irregular solids. This difference may be caused by the relatively lower solubility of 5 and 5[(R/S)-BPA] in these solvents. Microspheres 1[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph exhibit blue, green, yellow and green-yellow emissions, respectively. The λemi of 1[(R/S)-BPA]-sph (470 nm) is distinctly red-shifted with respect to that of the mixed solution (354 nm) of 1 with (R)-BPA, probably caused by the molecular aggregation of BPA in the solid state. However, 2[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph (496, 552, and 540 nm, respectively) exhibit similar λemi with respect to corresponding solution mixtures (478, 534, and 564 nm, respectively), suggesting that similar protonated products are obtained in two states. Microspheres 1[(R/S)-BPA]-sph have a τ of around 35 ns; however, 2[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph have a shorter τ of 3.93–4.54 ns (Table 2). This again reflects the different emission natures of these samples. All the samples exhibit moderate to good emission efficiencies, with ΦFL values of 19.8%, 64.3%, 31.8% and 9.6% for 1[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph, respectively.
Compound | λ emi/nm | Φ FL /% | τ/ns | g lum (× 10−4) |
---|---|---|---|---|
a Excitation wavelength: 345 nm for 1[(R/S)-BPA]-sph; 400 nm for 2[(R/S)-BPA]sph–4[(R/S)-BPA]-sph and 400 nm for 4[(R/S)-BPA]-rod, 430 nm for 5[(R/S)-BPA]-rod. b Absolute fluorescence quantum yields. c g lum = [2 × (IL − IR)]/(IL + IR), where IL and IR stand for the emission intensities of left- and right-hand CPL, respectively. The values are determined by glum = [ellipticity/(32980/ln10)]/total fluorescence intensity at a specific emission wavelength. | ||||
1[(R)-BPA]-sph | 470 | 19.8 | 35.0 | −2.99 |
1[(S)-BPA]-sph | 470 | 20.7 | 35.1 | +1.45 |
2[(R)-BPA]-sph | 496 | 64.3 | 5.59 | −2.18 |
2[(S)-BPA]-sph | 496 | 64.4 | 5.58 | +2.26 |
3[(R)-BPA]-sph | 552 | 31.8 | 4.00 | −4.62 |
3[(S)-BPA]-sph | 552 | 32.4 | 3.93 | +5.11 |
4[(R)-BPA]-sph | 540 | 9.60 | 7.11 | +2.92 |
4[(S)-BPA]-sph | 540 | 10.5 | 7.54 | −1.14 |
4[(R)-BPA]-rod | 501 | 16.3 | 4.89 | −18.4 |
4[(S)-BPA]-rod | 501 | 17.2 | 4.64 | +20.3 |
5[(R)-BPA]-rod | 558 | 32.6 | 2.39 | −17.7 |
5[(S)-BPA]-rod | 558 | 31.7 | 2.31 | +11.2 |
The above microsphere samples are essentially non-crystalline (see Discussion later). We are also interested in preparing well-defined organic crystals from the in situ reactions of 1–5 with (R/S)-BPA, in the hope of their potential applications in photonics.21–25 In a previous study, we found that the reactions of these pyridine-functionalized chromophores in THF with perchloric acid (HClO4) readily yielded plate-shaped microcrystals with appealing polarized emission properties.36 However, when similar nanoprecipitation conditions were attempted with 1–5 and (R/S)-BPA, no crystals were formed due to the better solubilities of the resulting salts with respect to those with ClO4− counteranions. In this regard, other reaction conditions were attempted. Gratifyingly, the natural evaporation of the solution of 4 with (R/S)-BPA in mixed CH2Cl2 and EA and the nanoprecipitation of 5 with (R/S)-BPA in mixed solvents of CH2Cl2 and THF gave rod-like microcrystals 4[(R/S)-BPA]-rod and 5[(R/S)-BPA]-rod, respectively (Fig. 4 and Fig. S6, ESI†). These microrods have lengths of 50–100 μm and diameters of 0.5–2 μm. They exhibit green and yellow emissions, with λemi values of 501 and 558 nm, respectively (Table 2). Notably, the emission of 4[(R/S)-BPA]-rod (λemi = 501 nm) is somewhat blue-shifted with respect to those of 4[(R/S)-BPA]-sph (λemi = 540 nm) and the solution mixture of 4 with (R/S)-BPA (λemi = 564 nm). In addition, the crystalline rod samples also exhibit higher ΦFL values than spherical samples. The blue-shifted emissions of the crystalline samples are likely a result of the rigidification effect and their higher ΦFL can be ascribed to the reduced non-radiative decays in crystals. λemi of 5[(R/S)-BPA]-rod (558 nm) is also blue-shifted than that of the solution mixture of 5 with (R/S)-BPA (583 nm). Microcrystals 4[(R)-BPA]-rod and 5[(R)-BPA]-rod have τ of a few of ns and ΦFL values of 16.3% and 32.6%, respectively (Table 2).
Fig. 4 (a and d) Fluorescence microscopy and (b, c, e, and f) SEM images of microrods (a–c) 4[(R)-BPA]-rod and (d–f) 5[(R)-BPA]-rod. |
The chiral optical activities of microspheres 1[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph were examined by CD and CPL spectral analyses (Fig. 5). The pseudo-mirror-image CD spectra were recorded for these materials with (R)- and (S)-BPA. In addition, some weak CD signals beyond 350 nm are present for 2[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph. Because these signals are too weak, their absorption dissymmetry factors (gabs) have not been determined. Depending on the molecular chirality of BPA, multicolor CPLs from 470 to 560 nm were recorded from these microspheres. Microspheres 1[(R)-BPA]-sph–3[(R)-BPA]-sph show negative CPLs; while positive CPLs are observed for 1[(S)-BPA]-sph–3[(S)-BPA]-sph. However, 4[(R/S)-BPA]-sph microspheres show an opposite trend. The reason for this difference is not clear at this stage. The molecular aggregation in the solid state may result in different excited states. The luminescence factors glum of these CPLs are in the order of 10−4 at the maximum emission wavelength. Among them, 3[(R/S)-BPA]-sph exhibit the highest glum with values of −4.62 × 10−4 and +5.11 × 10−4, respectively. Though the glum values of these microspheres are low, these results reflect that the excited-state chirality transfer from BPA is more effective in the solid state than that in the solution state. In the latter case, no CPLs could be observed at all (Fig. S4, ESI†).
In comparison to microspheres, microcrystals 4[(R/S)-BPA]-rod and 5[(R/S)-BPA]-rod are characterized with better CPL properties (Fig. 6). The distinct mirror image CD and CPL spectra have been obtained for crystals containing (R)- and (S)-BPA. The glum values are in the order of 10−3 at the maximum CPL wavelength. Microcrystals 4[(R/S)-BPA]-rod show values of −1.85 × 10−3 and +2.04 × 10−3; while 5[(R/S)-BPA]-rod possesses glum values of −1.77 × 10−3 and +1.12 × 10−3, respectively. These values are one order of magnitude larger than those of the above-discussed microspheres, suggesting the beneficial role of crystalline structures in promoting the chirality transfer. Considering that these samples are crystals, the CPL measurements may suffer from the influence of potential linearly polarized luminescence. In order to clarify this issue, the CPL spectra were subjected to repeated measurements by rotating the substrate to a certain degree (from 0° to 270°). Indeed, the CPL intensities and shapes only show a slight degree of variation during these measurements (Fig. S7, ESI†), supporting the reliabilities of the CPL and the glum data of these crystalline samples. In addition, in order to reduce the influence of the scattering effect during the measurement, the solid samples are placed in front of the detector as close as possible. The observations of the distinct mirror-image CD and CPL spectra of the rod samples (Fig. 6 and Fig. S7, ESI†) suggest that the influence of the scattering effect, in any, is insignificant.
In order to further probe the structures of the pyridine-functionalized compounds with BPA, the single crystals of 4[(R)-BPA] and 5[(R)-BPA] have been obtained from the natural solvent evaporation of the solutions containing 4 with 1.0 equiv. of (R)-BPA in mixed CH2Cl2/THF/EA (2/1/8, v/v/v) and 5 with 1.0 equiv. of (R)-BPA in mixed CH2Cl2/THF (1/6, v/v), respectively. These crystals possess the chiral C2 and P21 space groups, respectively, and the corresponding crystallographic data are summarized in Table S1 (ESI†). In both cases, the pyridine groups are protonated by chiral BPA, and the pyridinium hydrogen atoms are linked to the oxygen atoms of BPA by a strong hydrogen bond with a NH⋯O distance of 1.75 Å (Fig. 7). Beside from these strong NH⋯O hydrogen bonds, some weak CH⋯O bonds are evident from the single-crystal X-ray analysis between some hydrogen atoms from aromatic rings and the phosphate oxygen atoms. These weak hydrogen bonds have longer H⋯O distances in the range of 2.40–2.63 Å. In addition, hydrogen bonding and some π–π interactions are observed in the molecular packing of these crystals, e.g. between the pyridinium groups and pyrene planes of 4[(R)-BPA] (3.80 Å) and 5[(R)-BPA] (3.53 Å) and among different pyridinium groups of 4[(R)-BPA] (3.74 Å). When viewed from the b-axis of 4[(R)-BPA], an alternating aromatic fragment and a phosphate lamellar structure are observed. In the case of 5[(R)-BPA], head-to-tail dimeric stacking is observed among the long aromatic fragments and layered structures are evident when it is viewed from the a-axis. The hydrogen bonding interactions between the aromatic chromophores and the chiral BPA anions and the ordered molecular packings in the crystal structures are believed to play an important role in the chirality transfer.
Fig. 7 Thermal ellipsoid plots at 30% probability of the single-crystal structures and crystal packing of (a and b) 4[(R)-BPA] and (c and d) 5[(R)-BPA]. |
The powder X-ray diffraction (PXRD) spectra of the microsphere samples 1[(R)-BPA]-sph–4[(R)-BPA]-sph suggest that they are non-crystalline (Fig. 8; the broad peaks between 2θ of 15–30° are from the substrate background). In contrast, the high crystallinity of microcrystals is supported by the sharp diffraction peaks of 4[(R)-BPA] at the (001) plane and 5[(R)-BPA] at the (101) plane. This suggests that the ordered molecular arrangements of the crystalline samples are beneficial in enhancing the chirality transfer.
Fourier transform-infrared (FTIR) spectroscopy was further used to gain insight into the hydrogen bonding interactions in the microsphere and microcrystal samples. All these samples display an infrared (IR) peak at 3230–3250 cm−1, which can be assigned to the pyridinium N–H stretching vibration (Fig. 9).46 In contrast, compounds 1–5 are IR silent in this region (Fig. S8, ESI†). In addition, the PO and P–O peaks of free BPA are located at 1228 cm−1 and 954 cm−1, respectively.47 In contrast, the P–O peaks of microspheres and microcrystals exhibit a slight shift to the higher wavenumber region (956–960 cm−1) and their dominating PO stretching vibrations appear between 1240 and 1249 cm−1. These changes are in consistent with the presence of hydrogen bonding between pyridinium hydrogen and BPA oxygen atoms in both the microsphere and microcrystal samples.48
Footnote |
† Electronic supplementary information (ESI) available: TDDTF results, single crystal X-ray diffraction data, and other supplementary data. CCDC 2210345 and 2210218. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2tc04818c |
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