Organic microspheres and microcrystals made from pyridine-functionalized chromophores and a chiral phosphoric acid

Abstract

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 (g_lum) 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.

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Introduction

Materials with circularly polarized luminescence (CPL) properties have recently attracted considerable research attention due to their appealing application prospects in light-emitting devices, three-dimensional displays, catalysis, and optical information storage and encryption.^1–10 CPL can be generated from chiral single-component chromophores and assemblies or those consisting of multiple components. A great number of organic molecular materials have been reported to exhibit CPL activity by covalently linking chiral motifs with luminescent chromophores.^1–10 This method sometimes suffers from tedious synthesis. In comparison, self-assembly offers a simple and powerful strategy to obtain CPL via chirality transfer and amplification among constituent subunits.^11–20 Thanks to the noncovalent nature of chiral assembled systems, their CPL properties can be readily modulated by changing the assembly conditions or applying an external stimulus.^11–20

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 (g_lum) of 0.23 when dispersed in methanol.³⁴ The high g_lum 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 g_lum in the order of 10⁻² and high photoluminescence quantum yields (Φ_PL).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 catalysis^38,39 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 g_lum 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.

Results and discussion

Compounds 1–5 have a linear shape and consist of a phenylpyridine skeleton substituted with different aryl groups. They were prepared by the palladium-catalyzed Suzuki coupling or Heck reaction from 4-(p-bromophenyl)pyridine via reported procedures.³⁶ The attachments of various aryl groups are expected to expand the conjugation of these molecules, leading to the modulation of their assembly behaviours and emission properties.

Compounds 1–5 exhibit gradually red-shifted emissions in dichloromethane (CH₂Cl₂) solutions from 1 to 5, with the emission maximum wavelength λ_emi varying from 335 to 461 nm (Table 1 and Fig. 2). Compounds 2–5 are highly emissive with fluorescence quantum yields (Φ_FL) of over 45%. However, the shortest compound 1 is only weakly emissive with a Φ_FL of 0.80%. The chiral acid (R)-BPA displays absorption between 250 and 340 nm with the absorption maximum wavelength λ_abs at 303 nm and an intense ultraviolent emission band with a λ_emi of 352 nm and a Φ_FL of 29.7% (Fig. S1, ESI†). The fluorescence emission lifetimes (τ) of these compounds are in the range of 0.32–2.34 ns.

Table 1 Photophysics data in CH₂Cl₂ solution^a

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

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Fig. 2 (a, c, e, g, and i) Absorption and (b, d, f, h, and j) photoluminescence (PL) spectral changes of 1–5 upon the treatment of (R)-BPA (up to 2.0 equiv.) in CH₂Cl₂ (5 × 10⁻⁵ M). The absorption spectrum of (R)-BPA (blue curves) is included for the purpose of comparison. (k and l) Images of 1–5 in CH₂Cl₂ under UV illumination in the (k) absence and (l) presence of 2 equiv. of (R)-BPA (5 × 10⁻⁴ M). Excitation wavelength: 260 nm for 1; 280 nm for 2–4; and 300 nm for 5.

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 λ_emi 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 λ_emi 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 λ_emi 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 Φ_FL values of 19.8%, 64.3%, 31.8% and 9.6% for 1[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph, respectively.

Fig. 3 (a–d) Fluorescence microscopy images of (a) 1[(R)-BPA]-sph, (b) 2[(R)-BPA]-sph, (c) 3[(R)-BPA]-sph, and (d) 4[(R)-BPA]-sph. The insets show the enlarged images of different spherical samples. (e and f) SEM images of microspheres 4[(R)-BPA]-sph.

Table 2 CPL data of solid microspheres and microrods^a

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

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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 λ_emi values of 501 and 558 nm, respectively (Table 2). Notably, the emission of 4[(R/S)-BPA]-rod (λ_emi = 501 nm) is somewhat blue-shifted with respect to those of 4[(R/S)-BPA]-sph (λ_emi = 540 nm) and the solution mixture of 4 with (R/S)-BPA (λ_emi = 564 nm). In addition, the crystalline rod samples also exhibit higher Φ_FL values than spherical samples. The blue-shifted emissions of the crystalline samples are likely a result of the rigidification effect and their higher Φ_FL can be ascribed to the reduced non-radiative decays in crystals. λ_emi 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 Φ_FL 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 (g_abs) 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 g_lum 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 g_lum with values of −4.62 × 10⁻⁴ and +5.11 × 10⁻⁴, respectively. Though the g_lum 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†).

Fig. 5 (a) UV-vis absorption and (b) photoluminescence (PL) spectra of microspheres 1[(R)-BPA]-sph–4[(R)-BPA]-sph. (c) CD, (d) CPL, and (e) g_lum spectra of microspheres 1[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph. Excitation wavelength: 345 nm for 1[(R/S)-BPA]-sph and 400 nm for 2[(R/S)-BPA]-sph–4[(R/S)-BPA]-sph, respectively.

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 g_lum 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 g_lum 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 g_lum 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.

Fig. 6 (a) UV-vis absorption and (b) PL spectra of microrods 4[(R)-BPA]-rod and 5[(R)-BPA]-rod. (c) CD, (d) CPL, and (e) g_lum spectra of microrods 4[(R/S)-BPA]-sph and 5[(R/S)-BPA]-sph. Excited at 400 nm for 4[(R/S)-BPA]-rod and 430 nm for 5[(R/S)-BPA]-rod, respectively.

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.

Fig. 8 PXRD patterns of (a) microspheres of 1[(R)-BPA]-sph–4[(R)-BPA]-sph and (b) microrods of 4[(R)-BPA]-rod and 5[(R)-BPA]-rod. The data of the bare quartz substrate is included in panel (a) as a control sample. The simulated patterns of 4[(R)-BPA] and 5[(R)-BPA] are included in panel (b).

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 P=O 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 P=O 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

Fig. 9 FTIR spectra of (R)-BPA, microspheres of 1[(R)-BPA]-sph–4[(R)-BPA]-sph and microrods of 4[(R)-BPA]-rod and 5[(R)-BPA]-rod as KBr pellets. The wavenumbers in cm⁻¹ of N–H, P=O, and P–O vibrations are indicated in the figure.

Conclusions

In summary, a convenient method for fabricating CPL-active multicolor microspheres and microcrystals is presented based on the reaction and co-assembly of pyridine-containing compounds with a chiral binaphthyl phosphoric acid. Four pairs of chiral microspheres with blue to yellow emissions have been obtained with g_lum in the order of 10⁻⁴. Two pairs of microcrystals with green and yellow CPLs have been prepared with a higher g_lum in the order of 10⁻³. The origins of CPLs are related to the chirality transfer from the chiral phosphoric framework to the conjugated pyridinium chromophores. The improved g_lum of microcrystals versus amorphous microspheres suggests that the ordered molecular packing in the crystalline samples plays a role in boosting the CPL performance. The relatively low g_lum values may be partially caused by the long distance between the chiral binaphthyl framework and the pyridinium moieties. In the future, we intend to use chiral binaphthyl phosphoric acids with steric bulky substituents to prepare microstructures,^38,39 which may be helpful in improving the CPL properties. In addition, these CPL-active microspheres and microcrystals with well-defined shapes are believed to have potential for chiroptical applications as optical resonators and building blocks for photonic devices.^21–25

Experimental

Instrumental information for physical measurements

The UV-vis absorption spectra were recorded at rt using the TU-1810DSPC spectrometer from Beijing Purkinje General Instrument Co., Ltd. The photoluminescence spectra were recorded at rt using an F-380 spectrofluorimeter from Tianjin Gangdong Sci. & Tech. Development Co., Ltd. The absolute fluorescence quantum yields were determined using a Hamamatsu Absolute PL Quantum Yield Spectrometer C11347 with the aid of an integrating sphere. The Hamamatsu C11367-11 instrument with single photon counting was employed to obtain the excited-state lifetimes. The morphologies of microstructures were investigated by SEM using a Hitachi SU8010 instrument at 10 kV. A Rigaku Saturn 724 diffractometer was used to perform the single-crystal X-ray analysis at 173 K in a rotating anode (Cu Kα; λ = 1.54184 Å). The structures were solved with SHELXS-97 and refined on Olex2. The crystallographic data are summarized in Table S1 (ESI†). The powder X-ray diffraction (PXRD) was carried out using a Rigaku D/max-2500 instrument (Cu Kα; λ = 1.54 Å). The fluorescence microscopy characterization was run using an Olympus IX83 Inverted fluorescence microscope by UV irradiation at 325–375 nm. A JASCO J-1700 spectrometer was used to obtain the CD spectra. The JASCO CPL-300 spectrometer was used to record the CPL spectra. The Bruker VERTEX 70v spectrometer was used to record the FTIR spectra.

Methods for TDDFT calculations

The electronic structures of [1-H]⁺–[5-H]⁺ were first optimized using the Gaussian 09 program package by density functional theory (DFT) calculations with B3LYP/6-31G** and the consideration of the solvent (CH₂Cl₂) effect. TDDFT calculations were carried out on the optimized structures with a long-range correlated CAM-B3LYP functional.⁴⁹

Synthesis and characterization

General information. Compounds 1–3 and 5 are known and prepared according to literature studies.³⁶ Compound 4 was prepared using a similar procedure as described below. Because of the overlap of signals, the numbers of the observed ¹³C NMR peaks are smaller than expected.

Synthesis of 4-((pyren-1-yl)phen-4-yl)pyridine (4). To a solution of 4-(bromophen-4-yl)pyridine (500 mg, 2.14 mmol) and 1-pyrenylboronic acid (1.053 g, 4.28 mmol) in 20 mL of THF, tetrakis(triphenylphosphine)palladium (70 mg, 0.060 mmol) was added. A solution of K₂CO₃ (1.48 g, 10.7 mmol) in 10 mL of water was then added. The reaction mixture was bubbled with nitrogen for 10 min, followed by stirring at 95 °C for 12 h. After cooling to room temperature, THF was evaporated. The crude product was extracted with copious CH₂Cl₂. The organic phase was then washed with brine and dried over MgSO₄. The product was purified by column chromatography on silica gel using petroleum ether/ethyl acetate 5/1 as the eluent and was obtained as a faint yellow solid (483.2 mg) in 63.5% yield. ¹H NMR (400 MHz, CDCl₃): δ 8.77–8.69 (d, J = 5.3 Hz, 2H), 8.28–8.17 (m, 4H), 8.13 (s, 2H), 8.10–7.98 (m, 3H), 7.82 (dd, J = 12.7 8.7 Hz, 4H), 7.65 (d, J = 5.6 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 150.42, 147.94, 142.17, 136.96, 136.70, 131.49, 131.37, 130.95, 130.87, 128.49, 127.76, 127.67, 127.48, 127.41, 127.01, 126.14, 125.32, 125.00, 124.89, 124.73, 121.60. HRMS (MALDI) calcd for [M]⁺ C₂₇H₁₇N: 355.1356. Found: 355.1356.

Synthesis of microspheres 1[(R/S)-BPA]-sph. To a solution of 1 (500 μL, 6.44 mM, 0.0032 mmol) in CH₂Cl₂, (R)-BPA (190 μL, 16.8 mM, 0.0032 mmol) was added in THF. The mixed solution was injected into 1.0 mL of ethyl acetate. The solution was transferred to a quartz plate by direct dropping. The solvent volatilized naturally to give 1[(R)-BPA]-sph as white microspheres on the plate, which were directly used for various measurements. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.47 (d, J = 5.9 Hz, 2H), 7.92 (dd, J = 14.2, 8.5 Hz, 4H), 7.70 (d, J = 5.7 Hz, 2H), 7.61 (d, 2H), 7.58–7.48 (m, 5H), 7.43 (t, J = 7.2 Hz, 2H), 7.35 (d, J = 8.5 Hz, 2H), 7.26 (t, J = 8.0 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 156.02, 148.67, 148.58, 141.83, 134.50, 132.42, 131.47, 131.28, 130.59, 129.53, 128.31, 127.53, 127.02, 126.13, 124.93, 123.23, 122.02, 122.00, 121.72, 121.70. ESI-HRMS (P) calcd for C₁₁H₁₀N [M – BPA + H]⁺: 156.0808; found: 156.0808. Microspheres 1[(S)-BPA]-sph were prepared using the same method as 1 and (S)-BPA.

Synthesis of microspheres 2[(R/S)-BPA]-sph. To a solution of 2 (500 μL, 7.65 mM, 0.0038 mmol) in CH₂Cl₂, (R)-BPA (110 μL, 34.5 mM, 0.0038 mmol) in THF was added. The mixture was then injected into 3.0 mL of ethyl acetate. The solvent volatilized naturally to afford 2[(R)-BPA]-sph as green microspheres on the plate. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.46 (d, J = 5.9 Hz, 2H), 7.92 (dd, J = 16.0, 8.0 Hz, 4H), 7.78–7.65 (m, 6H), 7.61 (d, J = 8.8 Hz, 2H), 7.57 (d, J = 8.8 Hz, 2H), 7.43 (t, J = 7.2 Hz, 2H), 7.35 (d, J = 8.5 Hz, 2H), 7.25 (t, J = 7.0 Hz, 2H), 7.02 (d, J = 8.8 Hz, 2H), 3.86 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 160.00, 153.58, 149.26, 149.17, 144.04, 143.55, 133.43, 132.58, 131.94, 131.19, 130.40, 128.26, 128.22, 127.81, 127.59, 127.06, 126.05, 124.78, 122.43, 122.29, 122.27, 121.99, 121.97, 114.54, 55.42. ESI-HRMS (P) calcd for C₁₈H₁₆NO [M – BPA + H]⁺: 262.1226; found: 262.1225. Microspheres 2[(S)-BPA]-sph were prepared using the same method as 2 and (S)-BPA.

Synthesis of microspheres 3[(R/S)-BPA]-sph. To a solution of 3 (500 μL, 6.96 mM, 0.0035 mmol) in CH₂Cl₂ was added (R)-BPA (102 μL, 34.5 mM, 0.0036 mmol) in THF. The mixture was then injected into 4.0 mL of ethyl acetate. The solvent volatilized naturally to give 3[(R)-BPA]-sph as yellow microspheres on the plate. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.43 (d, J = 5.9 Hz, 2H), 7.92 (dd, J = 14.1, 8.5 Hz, 4H), 7.71 (d, J = 5.7 Hz, 2H), 7.67–7.47 (m, 8H), 7.43 (t, J = 7.5 Hz, 2H), 7.36 (d, J = 8.6 Hz, 2H), 7.29 – 7.20 (m, 3H), 7.03 (d, J = 16.3 Hz, 1H), 6.94 (d, J = 8.3 Hz, 2H), 3.84 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ 160.02, 155.00, 149.03, 148.93, 142.23, 141.27, 132.76, 132.54, 131.25, 131.20, 130.48, 129.40, 128.29, 128.19, 127.84, 127.25, 127.07, 126.09, 124.85, 124.79, 122.55, 122.19, 122.17, 121.88, 121.86, 114.35, 55.39. ESI-HRMS (P) calcd for C₂₀H₁₈NO [M – BPA + H]⁺: 288.1383; found: 288.1381. Microspheres 3[(S)-BPA]-sph were prepared using the same method from 3 and (S)-BPA.

Synthesis of microspheres 4[(R/S)-BPA]-sph. A solution of 4 (500 μL, 5.63 mM, 0.0028 mmol) in CH₂Cl₂ was added (R)-BPA (250 μL, 11.48 mM, 0.0028 mmol) in THF. The mixture was then injected into 2.0 mL of ethyl acetate. The solvent volatilized naturally to afford 4[(R)-BPA]-sph as green-yellow microspheres on the plate. ¹H NMR (400 MHz, DMSO-d₆): δ 8.81 (d, J = 6.6 Hz, 2H), 8.41 (d, J = 7.9 Hz, 1H), 8.35 (d, J = 7.6 Hz, 1H), 8.31 (d, J = 6.7 Hz, 1H), 8.26 (s, 2H), 8.24 – 8.18 (m, 3H), 8.18–8.06 (m, 7H), 8.03 (d, J = 8.1 Hz, 2H), 7.85 (d, J = 8.3 Hz, 2H), 7.52–7.44 (m, 4H), 7.35–7.30 (m, 2H), 7.22 (d, J = 8.5 Hz, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ 170.78, 152.96, 149.41, 149.32, 144.86, 143.47, 136.29, 134.47, 132.26, 131.82, 131.43, 131.10, 131.00, 130.82, 128.95, 128.52, 128.37, 128.23, 128.09, 128.03, 127.83, 127.00, 126.90, 126.51, 126.07, 125.66, 125.50, 125.39, 124.74, 124.63, 124.46, 123.35, 122.46, 122.43, 121.92, 121.90. ESI-HRMS (P) calcd for C₂₇H₁₈N [M – BPA + H]⁺: 356.1434; found: 356.1438. Microspheres 4[(S)-BPA]-sph were prepared using the same method from 4 and (S)-BPA.

Synthesis of microcrystals 4[(R/S)-BPA]-rod. A solution of 4 (5.0 mg, 0.0141 mmol) and (R)-BPA (5.0 mg, 0.0143 mmol) in 2 mL of CH₂Cl₂ was injected into 0.5 mL of ethyl acetate. The solvent volatilized naturally to provide 4[(R)-BPA]-rod as green microcrystals on the plate. The characterization data are the same as those of the above-prepared microspheres 4[(R)-BPA]-sph. Microcrystals 4[(S)-BPA]-rod were prepared using the same method from 4 and (S)-BPA.

Synthesis of microcrystals 5[(R/S)-BPA]-rod. To a solution of 5 (3.0 mL, 1.75 mM, 0.0052 mmol) in CH₂Cl₂ was added (R)-BPA (100 μL, 34.5 mM, 0.0052 mmol) in THF under ultrasonication. The ultrasonication was continued for 2 min. After aging for 30 min at rt, 5[(R)-BPA]-rod appeared as yellow microcrystals on the bottom of the vial. ¹H NMR (400 MHz, DMSO-d₆): δ 8.90–8.69 (m, 3H), 8.65–8.53 (m, 2H), 8.38–8.26 (m, 4H), 8.20 (s, 2H), 8.17–8.02 (m, 11H), 7.67 (d, J = 16.1 Hz, 1H), 7.50–7.45 (m, 4H), 7.33 (t, J = 7.7 Hz, 2H), 7.22 (d, J = 8.6 Hz, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ 149.59, 149.50, 146.28, 140.30, 134.87, 132.30, 131.65, 131.52, 131.12, 131.08, 131.06, 130.95, 130.67, 128.93, 128.57, 128.40, 128.30, 128.18, 127.94, 127.90, 127.27, 126.89, 126.81, 126.51, 125.98, 125.82, 125.71, 125.29, 124.80, 124.55, 124.04, 123.84, 122.65, 122.54, 122.52, 121.93, 121.91. ESI-HRMS (P) calcd for C₂₉H₂₀N [M – BPA + H]⁺: 382.1589; found: 382.1590. Microcrystals 5[(S)-BPA]-rod were prepared using the same method from 5 and (S)-BPA.

Preparation of single crystals. Single crystals of 4[(R)-BPA] were obtained by natural solvent evaporation of the solution of 4 (0.51 mM) containing 1.0 equiv. of (R)-BPA in mixed CH₂Cl₂/THF/EA (2 mL/1 mL/8 mL). Single crystals of 5[(R)-BPA] were obtained by natural solvent evaporation of the solution of 5 (0.82 mM) containing 1.0 equiv. of (R)-BPA in mixed THF/CH₂Cl₂ (1 mL/6 mL).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial supports from the National Natural Science Foundation of China (grants 21925112, 22090021, 21872154, 21871294, and 21975264), the Youth Innovation Promotion Association CAS, and the Beijing Natural Science Foundation (grant 2191003) are acknowledged.

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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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