Boosting the circularly polarized luminescence of small organic molecules via multi-dimensional morphology control

By regulating the composition of solvents, the assembled nanostructures of chiral molecules transformed from 0D nanospheres to 3D nanoflakes, which showed significantly amplified circularly polarized luminescence.


Introduction
Chirality is a basic characteristic of nature which can be incisively and vividly exemplied by molecular, macromolecular or supramolecular levels of bioorganic systems, such as amino acids, enzymes, proteins, sugars, RNA and DNA. [1][2][3][4][5][6][7][8] In recent years, excited state chirality, namely circularly polarized luminescence (CPL), has aroused much attention due to its potential for application in many elds. 9-20 CPL originates from giving priority to one handed circularly polarized emission and the level of polarization is usually characterized by the luminescence dissymmetry factor (g lum ). The value of g lum ranges from À2 to +2 and the maximum value of |g lum | represents completely le or right-handed circularly polarized light. [21][22][23] To broaden the application of CPL-active materials, it is necessary to pursue large g lum of chiral luminescent systems. To date, the largest g lum value with about 1.3 was obtained in lanthanide complexes, that is cesium tetrakis(3-heptauoro-butylryl-(+)-camphorato) Eu(III) complexes. 24 On the other hand, in contrast to lanthanide complexes, research on simple organic molecules with CPL activity has become popular due to the inherent advantages of organic luminescent molecules. 25,26 Compared with lanthanide complexes, the emission of pure organic CPL-active materials could be tuned more exibly by regulating the electronic levels of the excited state. For instance, many strategies have been realized to modulate the electronic state of organic molecules, such as by chemical transformations of substituents, self-assembly and by changing the environment through external stimuli, e.g. temperature, pH, concentration, light, magnetism and solvents. [27][28][29][30][31][32][33][34][35][36][37][38] Unfortunately, the g lum value of organic systems is relatively low and generally falls in the range of 10 À5 to 10 À3 , which precludes them from being the best candidates for CPL research. To amplify the luminescence dissymmetry factors, different approaches have been adopted, which include the formation of receptor-ion complexes, congurational changes upon binding with guest ions or molecules and self-assembly of chiral molecules. [39][40][41][42][43][44][45][46] For example, Tang and co-workers, by introducing the concept of aggregation-induced emission, realized the amplication of g lum , which could be regarded as an excellent approach for fabricating efficient CPL-active organic solid materials. 25,47,48 Very recently, we have demonstrated that energy transfer, including Förster resonance energy transfer and the photon upconversion process, could remarkably amplify the g lum value. [49][50][51] Kawai and co-workers have found that self-assembly could be used as an approach for amplifying the g lum value. 52 In addition, they have demonstrated that one-dimensional selfassembled nanostructures exhibited larger g lum values than zero-dimensional aggregates. However, detailed demonstrations of morphology-dependent circularly polarized emission with regulated large g lum values in pure organic molecule-based systems have rarely been reported.
Herein, we report an interesting CPL-active organic system, which shows morphologically dependent CPL activity with controllable g lum values, as shown in Scheme 1. This category of molecules was originally synthesized as a privilege catalyst for asymmetric reactions. 53,54 Different morphological features of the nanostructures from the same chiral small molecule could be exibly tuned by changing the mixing ratio of THF and water. Upon increasing the volume fraction of water, the nanostructures changed from 0D nanospheres to 2D and 3D nano-akes. By trapping the chiral molecule association into different supramolecular architectures, the g lum value of the specic nanostructures showed a remarkable amplication from 10 À4 to 10 À2 . Particularly, compared with the monomeric state of the chiral emissive molecule, the g lum value showed an amplication factor of two orders of magnitude. Thus, avoiding tedious organic synthesis, by simply regulating the composition of solvents, there still is great potential for enhancing the CPL activity.

Results and discussion
Initially, the primary photophysical investigations of monomeric and aggregated R-or S-SPAn were carried out by employing a series of optical testing methods. As shown in Fig. S1a, † upon increasing the volume fraction of water, the emission colour varied from deep blue to cyan under a UV lamp. The emission spectra also showed an obvious red shi. The maximum emission peak of R-SPAn in pure THF solution was located at 422 nm, which was identical to the one in the aggregate state in the mixed solvent (THF/H 2 O ¼ 1/1) (Fig. 1a). Upon increasing the volume fraction of water (f w ) to 85%, the emission spectrum showed a bathochromic shi to 432 nm with the emergence of a shoulder peak at 460 nm. When the water fraction reached 90%, the vibronic peaks tended to disappear and the maximum emission peak was located at 460 nm. Interestingly, the emission quantum yield (F em ) showed an increasing trend from 0.44 (pure THF solution) to 0.6 (f w 50%). However, aer increasing the water fraction to 85%, the emission dramatically quenched (F em ¼ 0.04), which should be due to the aggregation-caused quenching of luminescence. Finally, upon increasing the water fraction to 90%, the quantum yield reached 0.13 ( Fig. S1b and S2a †). As shown in Fig. S2b, † upon addition of water, the absorption spectra of R-SPAn showed a slight bathochromic shi and broadening, which indicated the formation of aggregates. The nature of different emitters of aggregates was investigated by emission lifetime measurement (Table S1 †). The emission lifetime for f w 0% and 50% exhibited a monoexponential decay with a lifetime of 8 ns for emission monitored at 422 nm, which could be attributed to the luminescence of monomeric molecules. Upon increasing the water fraction to 85%, the aggregates exhibited a fast biexponential decay and triple-exponential decay lifetime monitored at 432 nm and 460 nm, respectively. The emergence of new emissive species with a lifetime of about 2.3 ns and a longer lifetime of about 6.8 ns may be attributed to the luminescence of the aggregates and excimer, respectively. The lifetime for f w 90% monitored at 460 nm exhibited a triple-exponential decay and the emergence of a long-lived lifetime of about 10 ns could further conrm the formation of an excimer. [55][56][57] It could be concluded that the formation of the aggregates and excimer was responsible for the characteristic spectral features observed at higher water fractions. 58 To test the chirality of the monomeric and self-assembled states, circular dichroism (CD) and CPL measurements were carried out under different conditions. In pure THF solution, the CD spectra of the monomeric molecule showed an obvious Cotton effect corresponding to the absorption bands (Fig. 1b). The CD signals from 200 to 300 nm could be assigned to the electronic transitions of the spiral chromophore with axial chirality, 59 while the signals in the range of 350-400 nm were assigned to the anthracene units, respectively. Mirror-image CD spectra were observed for the R and S-enantiomers. Since the anthracene chromophores were achiral, the CD signals located at 350 nm to 400 nm could be assigned to the intramolecular chirality transfer from spiral chromophore to anthracene. The CD spectra of R-or S-SPAn under various conditions are shown in Fig. S3. † The CD intensity corresponding to the anthracene chromophore exhibited amplication accompanying the increasing water fraction. We used the absorption dissymmetry factor (g CD ) to evaluate the change of chirality. The absorption of an asymmetric factor (g CD ) is dened as g CD ¼ D3/3 ¼ 2(3 L À could signicantly amplify the g CD by controlling the solvents. Thus, by controlling the morphology of nanostructures aggregated at various water fractions, amplied supramolecular chirality was obtained. The amplied g CD laid the foundation for enhanced CPL as discussed below. The CPL spectra could provide more direct evidence for chiral dissymmetry in the uorescence of the monomer and aggregates ( Fig. 1c and S5 †). Upon increasing the volume fraction of water, the intensity of the CPL was obviously enhanced. The extent of chiral dissymmetry in uorescence is quantied using the anisotropy factor, g lum , of CPL, which is given by the equation g lum ¼ 2(I L À I R )/(I L + I R ), where I L and I R are the intensities of the le-and right-handed circularly polarized emissions, respectively. 60,61 As shown in Fig. 1d, in pure THF solution and f w 50%, the g lum values were about 2.1 Â 10 À4 and 2.4 Â 10 À4 , respectively. Upon increasing the volume fraction of water, the g lum value increased by an order of magnitude. The maximum g lum values for f w 85% and 90% were 7.2 Â 10 À3 and 2.9 Â 10 À2 , respectively. The  Scanning electron microscopy (SEM) measurement was employed to investigate the morphological transformation of aggregates under different conditions. As shown in Fig. S7a, † the R-SPAn molecule formed amorphous structures upon evaporation of THF. When the water fraction reached 50%, two kinds of nanoparticles with a rough surface and with a hole could be observed ( Fig. 2a and b). Upon increasing the fraction of water to 85%, two-dimensional nanoakes were observed ( Fig. 2c and d). Finally, when the water fraction reached 90%, three-dimensionally stacked akes, which exhibit the largest g lum value ( Fig. 2e and f), could be obtained. The morphological transformation of S-SPAn nanostructures at various water fractions is shown in Fig. S6. † Thus, by varying the composition of the solvent, a dramatically changed CPL dependent on the morphological transformation could be obtained. We also tried higher volume fractions of water, such as 96% and 98%. Unlike the sample with a water fraction of 90%, the one with f w 96% exhibited the structure of nanobelts, while hollow microspheres were observed in f w 98% (Fig. S7b and c †). However, it is hard to get a plausible CPL signal under these conditions. This was due to the weak luminescence at a higher volume fraction of water. 62 The ripening process of R-SPAn in the water fraction of 90% was monitored by CPL measurement, as shown in Fig. 3a. The time-dependent CPL investigations in the water fraction of 90% showed that the CPL intensity dramatically increased. The timedependent emission was also investigated, and it showed a similar tendency to CPL, as shown in Fig. S8. † The g lum value at different ripening times is shown in Fig. 3b. In the rst two minutes, the g lum value of the obtained sample was 2.2 Â 10 À4 . Aer 4 hour ripening, the g lum value reached 2.6 Â 10 À2 , and remained at an almost constant value, conrmed by testing the sample aer 24 hours (2.7 Â 10 À2 ). SEM was carried out to carefully investigate the morphological transformation at different ripening times. In the rst 10 minutes, mono-disperse nanoparticles with a size distribution of about 140 nm were obtained, as shown in Fig. 3c. When the ripening time reached 30 minutes, the nanoparticles stacked together and fused to form a laminated ake structure with rough edges (Fig. 3d). Aer 4 hours, stacked 3D nanoakes with a smooth surface were observed (Fig. 3e). Accompanying the morphology transformation from 0D nanospheres to 3D nano-akes, the CPL activity also showed amplication. The g lum value was amplied by two orders of magnitude to 0.027. The enhanced CPL activity of time-dependent morphology transformation in f w 90% suggested that the CPL activity exhibited morphological dependence.
To further clarify the formation of nanostructures with gradually enhanced CPL, X-ray diffraction (XRD) was carried out, as  shown in Fig. 4a. The diffraction pattern of the R-SPAn cast lm exhibited only one broad peak which indicated the formation of amorphous structures. For the f w 50% sample, three diffraction peaks were observed at 2q values of 7.01 , 7.37 and 21.94 with d spacings of 1.26 nm, 1.19 nm, and 0.41 nm, respectively. Clearly, upon increasing the volume fraction of water to 85% and 90%, three new diffraction peaks were observed at 2q values of 10.46 , 11.71 and 16.55 . The whole diffraction peaks of f w 90% gave d spacings of 1.20, 0.85, 0.75 and 0.54 nm with a d spacing ratio of about 1 : , indicating a body-centred cubic packing of the molecules. 63 The rst order diffraction peak of 3D nanoakes was located at a 2q value of 7.34 while the peak of 2D nanoakes was located at a 2q value of 7.24 , which indicated a closer molecular packing of the 3D nanoakes than the 2D ones. In addition, the selected area electron diffraction (SAED) of the nanostructures obtained at f w 85% and 90% showed ordered diffraction patterns (Fig. S9 and Table S2 †). The obtained interlattice spacing was about 0.53-0.54 nm, which could be estimated from the result of XRD. These results suggested that the obtained 2D and 3D nanoakes had ordered molecular packing and a crystalline nature to some extent.
To gain further insight into the non-covalent interactions, Fourier transform infrared (FT-IR) spectra of the nanostructures were obtained and are shown in Fig. 4b. The peaks located at 885 and 846 cm À1 could be attributed to the P-O stretching vibration (aromatic ring). The spectra with sharp peaks at around 1214 cm À1 could be ascribed to the P]O stretching vibration. In addition, the peak appearing at 1442 cm À1 could be ascribed to the stretching vibration of -CH 2 . The band appearing at 3052 cm À1 could be ascribed to the stretching vibration of unsaturated C-H single bonds. 64 Upon increasing the fraction of water, the stretching vibration of the hydroxyl group of the phosphate moiety shied toward lower wavenumbers, from 3427 cm À1 (f w 0%) to 3382 cm À1 (f w 90%). This was indicative of the existence of hydrogen bonding between intermolecular hydroxyl groups. Simultaneously, the peak became broader, which suggested the existence of an enhanced hydrogen bonding. These results indicated that the enhanced intermolecular hydrogen bond might be the main reason for enhancing the molecular stacking of 2D and 3D nanoakes with relatively higher g lum values. 65 To gain a deep insight into the CPL amplication upon increasing the fraction of water, molecular packing analyses were carried out. We applied Materials Studio as a simulation tool for theoretically predicting the growth morphology of the S-SPAn single crystal. The simulation was performed using MS's morphology component based on the attachment energies. Firstly, the single-crystal of S-SPAn was incubated with THF/ water mixed solvent. As shown in Fig. 5a, the crystal morphology of SPAn was plate-like, which was in agreement with the SEM results. The detailed crystallographic data are shown in Table S3. † Based on the XRD patterns, the growth in the thickness of the microplates was along the [001] direction, which could be attributed to the molecular packing based on pp interaction of anthracene groups and hydrogen bonding between intermolecular hydroxyl groups as shown in Fig. 5b. With all of this in mind, we could speculate on a plausible mechanism for the amplication of CPL dissymmetry factor (g lum ). When the water fraction was relatively low, the SPAn molecules aggregated in the form of microspheres, which was an amorphous packing mode and showed a relatively low g lum ($10 À4 ). Upon increasing the water content, SPAn molecules could aggregate in the form of microplates in crystalline packing mode, and the organized packing could amplify the CPL dissymmetry factor (g lum $ 10 À3 ). Furthermore, when the water fraction was elevated to a higher level, more crystal units packed in the thickness direction, i.e., more ordered structures, were involved in the CPL generation process. Thus, g lum could be further amplied (g lum $ 10 À2 ).

Conclusions
In summary, by tailoring the composition of the solvent (THF/ H 2 O), the same chiral emitter R-or S-SPAn could be constructed into various nanostructures, including 0D nanospheres, 2D and 3D nanoakes. Accompanying the morphological transformation, an amplied circularly polarized emission is observed. The maximum g lum value could reach 0.029, which is a relatively large value in pure organic systems. The strong intermolecular interaction resulted in the formation of a more orderly and compact arrangement of nanostructures. The good packing of nanostructures induced a strong excimer emission, which could contribute a large g lum value. The morphological dependence of emissive nanostructures on CPL activity can lead to controlled modulation of chiroptical properties, offering great potential for fabricating chiroptical organic nanomaterials.

Preparation
S-SPAn and R-SPAn were dissolved in tetrahydrofuran (THF) with a volume of 1 mL, 500 mL, 150 mL and 100 mL, respectively. And then 500 mL, 850 mL and 900 mL of water were added to the latter three. Thus, the nanostructures with various morphologies could be obtained. The nal concentration of R-or S-SPAn of all samples was 1.5 mM.
Characterization UV-vis spectra, uorescence spectra and CD spectra were obtained using a Hitachi UV-3900, Zolix Omin-l500i monochromator with a photomultiplier tube PMTH-R 928 and JASCO J-810 spectrometers, respectively. CPL measurements were performed with a JASCO CPL-200 spectrometer. XRD analysis was performed on a Rigaku D/Max-2500X-ray diffractometer (Japan) with CuKa radiation (l ¼ 1.5406Å), operated at a voltage of 40 kV and a current of 200 mA. FTIR studies were performed with a JASCO FTIR-660 spectrometer. SEM was performed on a Hitachi S-4800FE-SEM with an accelerating voltage of 10 kV. TEM and selected area electron diffraction were performed on a transmission electron microscope, JEM1011. The uorescence lifetime measurements were recorded on an Edinburg FLS-980 uorescence spectrometer using time-correlated single photon counting.

Conflicts of interest
There are no conicts to declare.