Citation

Hongwei Wua, Bin Wu, Xiyuan Yu, Pei Zhao, Wenbo Chen, Liangliang Zhu. Self-twisting for macrochirality from an achiral asterisk molecule with fluorescence-phosphorescence dual emission[J]. Chin. Chem. Lett., 2017, 28(11): 2151-2154  

Hongwei Wua^a,b, Bin Wu^a, Xiyuan Yu^a, Pei Zhao^a, Wenbo Chen^c, Liangliang Zhu^a    

Chiral nano architectures have been attracting great interests owing to their beautiful and diverse structural features [1-3]. As compared with those single structures or building blocks, the assembled twisted or helical structures could possibly either amplify the molecular properties or develop some unique functions [4, 5]. Recently, the supramolecular chirality of selfassembled systems based on achiral building blocks has got some peculiar attentions [6-8], and it is very important to understand the symmetry breaking and chiral amplification [9] process of such kind of self-assembly, as well as to introduce or develop new functions. It has been reported that some achiral molecules can occasionally self-assemble into chiral nanostructures, and the formation of one predominant chiral nanostructure is also possible from the achiral component. For example, an achiral C₃-symmetric molecule (abbreviated as BTAC) was found to form chiral twists through gelation, whereby unequal amounts of left and righthanded twists were subsequently obtained and the gel showed a clear CD signal without any chiral additives [10]. Subsequently, many derivatives of such shaped compound [11-13] are also capable of ability to form chiral self-assembly. In addition, these materials also show exciting chiral luminescent properties and exhibit potential applications in chiral sensing [14], bioimaging [15], chiral organic light-emitting diodes (OLEDs) [16] and so on. It remains very interesting but challenging to develop more advanced achiral materials which can be assembled into chiral aggregates and show more diverse photophysical properties, e.g., multicolor luminescence, fluorescence and phosphorescence dual emission [17], circularly polarized light and so on.

Inspired by the above situations, herein, we harnessed a single asterisk fluorescence-phosphorescence dual-emission molecule [18] 1 (Fig. 1) to investigate its chiral self-assembly process from achiral nature. According to our previous report, six amide bonds in this compound can result in strong multiple intermolecular hydrogen bonding to behave diverse self-assemblies for a crystallization-driven self-assembly, which can be precisely controlled by the change of the solvents [18]. Similar to the molecule BTAC [13], we expect that multiple amide bonds of 1 would also be conducive to form distorted molecular conformation to facilitate the chiral self assembly. To our delight, the chiral self-assembly was observed in a high concentration solution of 1, and the chiral twists increased with prolonging the time. The assembly process is illustrated in Fig. 1. Moreover, such a self-progressing self-assembly can be in response to a tunable fluorescence-phosphorescence dual-emission behavior. This is the first study demonstrating that a C₆-symmetric and achiral low-molecularweight compound is capable of optical activity and controllable macroscopic chirality. These findings provide a new template for understanding the role of supramolecular chirality in the performance of material optoelectronics.

Figure 1

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Fig. 1. The structure and self-assembly diagram of BTAC and 1.

The chemical structure of compound 1 has been reported according to our previous literature [18]. The measurement tools can also be referenced by our previous report [18]. Prior to chracteriazation, 1 (20mg) were dissolved in DMF (10mL), then divided into 10 parts (each 1 mL), and added DMF and water to obtain different solutions of two solvents in different proportions.

TEM was firstly employed to study the variation of aggregated morphologies. Compared to a rod-like self assembly in mixed solution at low concentration of 1 [18], remarkable nanotwists were found with the increase of the concentration of the compound 1 in DMF with 70% or 80% water. There is no obvious morphological alteration of the self assembly of compound 1 in DMF with 60% water along with time from 0h to 48h (Figs. 2a-c). This solvent selectivity is similar to the literature [19], hence it is important to increase the content of water for further triggering the self-twisting. Interestingly, the self assembly under this condition is self-progressive with prolonging the time. Rod-like achiral aggregates with an average length of 1 μm dominate at 0h in DMF with 70% water, however, the number of nanotwists gradually increased during first 24h, featuring many rod aggregates were bent and intertwined with each other, and then turned into elbow-shaped twists. The twist morphology became quite remarkable in 48h (Figs. 2d-f). The similarphenomena can be seen inTEM of 1 prepared fromDMFwith 80%water, and thenanotwists became even more curved and entangled with self-assembly in 48h (Figs. 2g-i). This self-progressive process is similar to a onedimensional self-seeding technique, whereby molecules in the free state can be continuously gathered onto the aggregates [20], leading to larger or more complex assemblies. As it can be seen from our results, nanotwists of 1 from DMF with 80% water at 48h were significantly larger and more entangled than those at 0h. The self-assembly effect will not be further changed when the water fraction reached up to 90% and more [19].

Figure 2

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Fig. 2. The TEM morphological variations of 1 (1.87mmol/L) along with time: (a) 0h, (b) 24h, (c) 48h prepared from DMF with 60% water; (d) 0h, (e) 24h, (f) 48h prepared from DMF with 70% water; (g) 0h, (h) 24h, (i) 48h prepared from DMF with 80% water. Scale bar: 1 μm.

Similar to C-3 symmetrical compound, our compound is spatially distorted which was confirmed by theoretical simulation [18]. Such a distorted structure is conducive to the occurrence of symmetrical break and the formation of chiral self-assembly, and the concentration of the solution will have a huge influence on the formation of the chiral self-assembly. Namely, the compound will form small aggregates since there are not enough molecules in solution with low concentration, which is not conducive to the formation of twisted structures. However, the concentrated molecules are enough to form large aggregates in solution with high concentration, whereby some molecules will continue growing onto the aggregates after an initial assembly, bending and twisting, to undergo a chiral self-assembly finally.

To confirm such a self-assembly is a chiral process, the solutions were analyzed by CD spectroscopy. There is no CD signal of the compound 1 in DMF with 60% water with time from 0h to 48h (Fig. 3a), whereas it showed obvious signals in DMF with 70% water. Under that condition, 1 exhibited positive Cotton effects at 325nm and negative Cotton effects at 375nm (Fig. 3b), and then the signal intensity was greatly enhanced (about 3-fold at 48h as compared with that at 0h). Similarly, positive-negative Cotton effect pair of 1 and the enhancement process was also observed in DMFwith 80% water(Fig. 3c), wherethe self-assemblyat the status of 48h showed a very strong CD signal. These results are in good agreement with the morphological results observed from TEM, deeply proving that compound 1 underwent a chiral self-assembly by self-twisting process. Although a twist morphology assembled from achiral building blocks normally shows racemic nature and does not produce any CD signals, symmetry breaking can be implemented by better controlled self-assembly conditions [21-24]. In our case, self-twisting process served as a chiral amplification factoronce the symmetry breakingoccurs, leading to a superior optical activity when the system reaches a selfprogressive equilibrium.

Figure 3

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Fig. 3. CD spectra of 1 (1.87 mmol/L) in DMF/H2O with different water fractions: (a) DMF with 60% water, (b) DMF with 70% water, (c) DMF with 80% water.

In terms of our previous study, the photophysical properties of the molecule is very sensitive to the self-assembly patterns. In this work, therefore, we turn to investigate the dual-emission properties of compound 1 during the self-progressing assembly process. The long-wavelength emission band (~550nm) dominates the dual-band spectral curve of 1 in DMF with 60% water and no obvious emission changewas observedwith the time prolonged (Fig. 4a). However, the short-wavelength emission band (~425nm) of 1 boosted gradually in DMF with 70% and 80% water with prolonging the self-assembly time (Figs. 4b and c), while the long-wavelength emission band (~550 nm) remained unchanged. This spectral behavior hence caused the luminescent color conversion of 1 from yellow (CIE coordinate, 0.34, 0.42) to white (0.28, 0.33), and finally to blue (0.25, 0.28) under a UV light source (see the photoimages and the CIE coordinate diagram in Figs. 4e and f). At the same time, we also measured the emission lifetime of the compound in both of the emission bands (Figs. 4d1 and d2). The short-wavelength emission band (425 nm) shows a fluorescence lifetime (1.17 ns), however, the long-wavelength one (550 nm) exhibit a phosphorescence lifetime (20.32 μs). Such a measurement condition is similar to our previous literature [18], profoundly proving that a fluorescence-phosphorescence dual emission property of 1 is tunable along with the self-twisting process. These results indicate that our system showed three luminescent color in the same solution with time-resolved, and this phenomenon is rarely seen in single-molecule organic lightemitting systems. Therefore, we can conclude that chiral selfassembly here is more biased towards the fluorescence generation. This phenomenon can be deduced from those similar mechanism in our previous report [18, 25, 26], namely, the further chiral aggregations are favorable for emitting fluorescence as compared to the achiral building blocks.

Figure 4

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Fig. 4. The emission spectra of 1 (1.87 mmol/L) in DMF/H 2 O with different water fractions : (a) DMF with 60% water, (b) DMF with 70% water, (c) DMF with 80% water at the different time. (d) The emission lifetimes of 1 at different wavelengths (d1 and d2). (e) P hotographs of 1 in DMF with 80% water under a UV light (365 nm) at different time (0 h for yellow luminescence, 48 h for white luminescence and 120 h for blue luminescence). (f) CIE 1931 chromaticity diagram signifying the luminescent color coordinates for the corresponding states of (e).

In conclusion, a self-progressing chiral self-assembly from an achiral and C₆-symmetric molecule was demonstrated. The chiral amplification was found in the self-twisting process with prolonging the time. Moreover, such an establishment of supramolecular chirality can be used for tuning the photophysical behavior of the material. Our system shows three distinct luminescent colors with the change of time in the same solution system, on the basis of a proportional regulation of the fluorescence-phosphorescence dual emission. We emphasize that it is the first example signifying a C₆-symmetric and achiral lowmolecular-weight compound that is capable of optical activity and controllable macroscopic chirality. These findings can be valuable for bridging supramolecular chiral science and advanced optoelectronic materials [27, 28].

This work was supported by 2017 Natural Science Foundation of Shanghai (No. 17ZR1402400) and National Program for Thousand Young Talents of China. W. Chen thanks the Shanghai Pujiang Program (No. 15PJ1402600), the Natural Science Foundation of Shanghai (No. 17ZR1447100), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.