Significantly Boosted and Inversed Circularly Polarized Luminescence

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Significantly Boosted and Inversed Circularly Polarized Luminescence from Photo-Generated Radical Anions in Dipeptide Naphthalenediimide Assemblies yuan wang, Yuqian Jiang, Xuefeng Zhu, and Minghua Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02269 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry Letters

Significantly Boosted and Inversed Circularly Polarized Luminescence from Photo-generated Radical Anions in Dipeptide Naphthalenediimide Assemblies Yuan Wang†,‡,⊥, Yuqian Jiang§,⊥ Xuefeng Zhu† and Minghua Liu*,†,‡ †Beijing

National Laboratory for Molecular Science, CAS Key Laboratory of Colloid

Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡University

§ CAS

of Chinese Academy of Sciences, Beijing 100049, China.

Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem

and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing 100190, P. R. China. AUTHOR INFORMATION Corresponding Author:

E-mail: [email protected] (M.L.).

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ABSTRACT: Circularly polarized luminescence (CPL) reflects the excited state properties of the chiral system. However, compared to the singlet and triplet excited states, there are still many unknowns about the CPL from the double excited state. Here, using the self-assembly strategy of a dipeptide substituted naphthalenediimide (NDI-GE) and the photo-generated radical anions, we have explored the ground state (CD) and excited state (CPL) chiral characteristics of neutral NDI and NDI•- radical anions assembly. The neutral gelator assemblies showed CPL with the dissymmetry factor glum in an order of 10-3, the radical anion exhibited an inversed CPL signal with significantly enhanced glum of 10-1. TDDFT calculation revealed that upon formation of the radical anions, the direction of the dipole moment changed, thus leading to the inverse of CD and CPL. The present work opens a new platform to develop CPL materials based on doublet excited state.

TOC GRAPHICS

KEYWORDS: radical anions, circularly polarized luminescence, chiral amplification, chiral inversion, supramolecular assembly

ACS Paragon Plus Environment

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In recent years, circularly polarized luminescence (CPL) has been attracting widespread attentions. One interesting is in the understanding of the chirality at an excited state since CPL just reflects the excited state properties of the chiral system.1 In view of the emission from the excited states, singlet,2-5 doublet6 and triplet state are all possible to emit CPL.7 However, although CPL has been extensively investigated based on the emission from singlet and triplet excited states, it still remains much unknown about the CPL from the doublet excited state. In particular, CPL switching for singlet and doublet excited states has not been reported. Another interests in CPL is from the developing new chiroptical materials since CPL materials have potential applications in optical display,

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optical storage,

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chiroptical materials,12-13 and so

on.14-19In order to develop CPL materials, both chirality and luminescence should be combined within molecules and their assemblies. Thus, a series of derivatives of -conjugated molecules with the chiral substitutions like binaphtol, amino acids and so on were designed.20-21 In contrast to inorganic complexes, these small organic molecules have the advantages of easy modification, multifunctional and readily regulation.22-23 However, organic chiral molecules usually exhibit small dissymmetry factor of about 10-3-10-5.24 Luminescence dissymmetry factor (glum) is defined as the difference between left (IL) and right (IR) circularly polarized radiation and expressed as glum = 2 ×(IL －IR)/(IL ＋IR), where IL and IR refer to the intensity of left- and right-handed CPL, respectively. The value of glum represents the level of polarization and ranges from -2 to +2.1, 25 Although self-assembly or doping in liquid crystal can improve the glum, ignoring the influence of |m| has limitations on the development of CPL materials with high glum. Usually, high luminescence

dissymmetry

is

obtained

for

magnetic

dipole

allowed

and

electric

dipole-suppressed transitions.22 Therefore, unfavourable transitions in organic systems make the enhancement of glum a challenging task.


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Theoretically, glum is simply approximated by 4|m|cosθ / |μ|, where |m| and |μ| are the magnitudes of magnetic and electric transition dipole moments vectors, respectively.1,

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Therefore, if the chiral molecule is reasonably designed to have small |μ| and simultaneously a larger |m|, it will be an excellent strategy to obtain high glum CPL materials. However, many of the natural organic molecules have only negligible |m|. Accordingly, it comes to the need of fabricating the radical based CPL materials, which is supposed to have larger |m| due to unpaired electrons.27 Here, we have designed a naphthalenediimide (NDI) substituted by a chiral dipeptide and investigated their radical generation as well as the CPL activity.

Figure 1. A schematic diagram of the formation of NDI•- radical anions that emits circularly polarized luminescence. The L-NDI-GE gelator form organogel by molecular self-assembly to emit left-handed CPL; The L-NDI-GE organogel produces NDI•- radical anions by UV irradiation, enhanced and reversed the CPL.


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NDI is a kind of redox-active chromophore, easy to self assembly and could form radicals under redox reaction or UV irradiaition.28-30 The dipeptide glycyl glutamic acid is a chiral unit and can slef-assemble. By combining the two units, we have desigened the enantiomeric molecules, names as L-NDI-GE or D-NDI-GE (Supporting Information scheme S1). It was found that the compounds could form gels in methanol or mixed DMF/water system. When the gel was irradiated with UV-irradiation, NDI•- radical anions were generated. Interestingly, both neutral and radical anion gels showed CD and CPL (Figure 1). While the neutral assemblies showed the glum in an order of 10-3, the radical anion showed enhanced glum by two orders of magnitude to 10-1. In addition, the radical anions could be quenched by oxygen and reproduced by UV-irradiation, which can further realize reversible CPL switch based on the singlet-doublet excited states. To the best of our knowledge, this is the first report on the CPL switching between singlet-doublet excited states. L-NDI-GE or D-NDI-GE gelator is capable of self-assembly in solvent due to hydrogen bonding and π-π stacking.

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When L-NDI-GE gelator dispersed in methanol was heated to

form solution and then cooled down to room temperature, transparent gel could be formed (Figure S1). The gels were characterized by UV-vis, FL spectra, X-ray diffraction (XRD) and atomic force microscopy (AFM) (Figure S2 and S3). It revealed that during the gel formation, L-NDI-GE molecule self-assembled into 2D nanosheet, which immobilized the methanol. The gels are sensitive to the UV-irradiation. Upon UV-irradiation, the gels changed from colorless to brownish red, as shown in Figure S1 and Figure 2f. Interestingly, the production of the brownish red was formed immediately after irradiation and could diffuse into the other places in the gel, as shown in Figure S4. By using a 360 nm laser, a lithographic pattern such as a grid was obtained. Such pattern can be expanded to all the gel after two hours (Figure S4). This suggested that


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radical anions were produced by the irradiation and can diffuse in the gel through electron transfer. That is, electrons can be transferred in NDI-NDI ordered assembly (Detailed description is shown in Figure S4).

Figure 2. Spectral characterizations of NDI•- radical anions generated by UV irradiation. Time-dependent UV-vis spectra (a) of L-NDI-GE gel generated NDI•- radical anions. FL spectra


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(b) of L-NDI-GE gel generated NDI•- radical anions. (c) 1HNMR spectra of (i) L-NDI-GE, (ii) NDI•- radical anions generated by UV-irradiation and (iii) oxidized neutral L-NDI-GE in the O2. (DMSO-d6, 298 K) (d) EPR spectra of L-NDI-GE gel generated NDI•- radical anions after UV-irradiation. Deoxygenation Irradiation: max= 360 nm, 1 min. Red line, experimental data. Dark line, simulated data. [aN =1.04G (2N), aHring=1.79G (2H), aHring=1.77G (2H), aHalkl=0.20G (2H), aHalkl=0.26G (2H)]. (e) The molecular structures of neutral L-NDI-GE. (f) UV-irradiation induces NDI•- radical anions that change the gel coloring from white to brownish red, and neutral L-NDI-GE gel was restored after 8 h by natural O2 diffusion. (Gel=10 mg/mL, Methanol solvent, Irradiation: max= 360nm, 20 s).

In UV-Vis absorption spectra of L-NDI-GE gels, strong absorption peaks appear at 381 nm and 360 nm, which are characteristic of the NDI π-π* transition polarized along the long axis of the chromophore,31 and the broad shoulder peaks at 400 nm are characterized by π-π stacking of NDI group due to the self-assembly in compared to the solution (Figure S2a). Upon UV-irradiation, new peaks exhibited at 488 nm, 531 nm, 614 nm and 753 nm, which are characteristic of NDI•- radical anions (Figure 2a).30, 33-34 Further, the absorption properties of the radical anions were simulated by theoretical calculations, and the results were matched with the experimental data (Supporting Information). These peaks increased with the irradiation and saturated about 20 s in the gel. Previously, NDI was reported to form radical anions in the presence of reductant.35 Here, NDI•- radical anions were generated only by UV-irradiation and were not doped with reducing agent. This may be attributed to the intermolecular photo-induced electron transfer. NDI moiety is known to be redox-active and can generate radicals upon light irradiation.36 During such process, a positively charged radical cations should be formed in the assembly, but we did not observe it in UV-vis spectra.37 This perhaps because of its high reactivity, leading to the rapid consumption of radical cations from the reactional media.38 In order to further confirm the formation of the radical anions in the gel, both Electron Paramagnetic Resonance (EPR) and 1H-NMR measurements were applied. The EPR confirmed


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the formation of paramagnetic NDI•- radical anions (Figure 2d). The experimental data (red line) displays hyperfine splitting identical to simulated NDI•- spectrum (black line), which is simulated according 2 N and 8 H, confirming a paramagnetic NDI•- radical anions formation from diamagnetic L-NDI-GE.35, 39-41 Figure 2c, e showed that when NDI•- radical anions were formed by UV irradiation, NDI core 4Ha protons (= 8.73 ppm) of NDI-GE and 4Hb (= 4.75 ppm) on adjacent naphthalimides disappeared.39 The results showed that eight hydrogens participated in the delocalization during the formation of NDI•- radical anions, which is consistent with EPR measurements (Figure 2d). In the presence of oxygen, the proton of 4Ha on benzene ring and 4Hb on naphthalimide reappeared at the same position as neutral L-NDI-GE,34-35, 42 and no other new peaks appeared, indicating that NDI•- radical anions produced by PET process is reversible.33, 37 The methanol gels of the neutral species showed the emissions at 425 nm and 455 nm, which are characteristic of NDI chromophore.43 After UV-irradiation, a new broad emission peak appeared at 560 to 700 nm (Figure 2b), which is attributable to the emission of the NDI•- radical anions. To further explain the luminescent species. We explored through experimental phenomena and theoretical computational simulations. It was observed by the Figure S4 experimental that the uneven brownish red grid gel (NDI•--NDI•-) became uniform assembly contains neutral species and NDI•- radical species (NDI-NDI•-) during the electron transfer process, but the fluorescence still showed weak red grid shape. In addition, to understand the excitation property in dimer (NDI-NDI•-), computational analyses were conducted using Gaussian 09. The calculation results show that due to the mismatch between the two energies, neutral NDI and NDI•- radical anions cannot form new absorption and emission (see Supporting Information for details). It is indicated that the red emission should be NDI•-- NDI•-, not the excimer of neutral species and NDI•- radical species (NDI-NDI•-).


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The fluorescence average lifetime of neutral L-NDI-GE gel and NDI•- radical anions gel measured as =1.07 ns and =7.19 ns, respectively (Figure S5). This is because the nonradiative decay rate of intersystem crossing and internal conversion may become faster for S1 → S0 transitionthan than for D1 → D0 transition.44 Therefore, excited state NDI gel have a shorter fluorescent average lifetime than excited state NDI•- gel. The radiacl anions can be converted into orginal neutral species upon exposure to oxygen or air. This can be visulized form Figure 2f, where the bronwrish color disappered gradually. In addition, such reversible change was confirmed through the 1H-NMR (Figure 2c). Since the NDI-GE gelator contains the chiral centers, we have further investigated their supramolecular chirality using CD and CPL spectra.1, 45 In the methanol gel, L-NDI-GE showed a negative exciton type Cotton split peak with a valley and a peak at 406 nm and 392 nm, and the crossover at 400 nm, respectively (Figure 3a). The crossover of the CD spectra occurred in the same position as the shoulder peak of the gel of UV-vis spectra (Figure S2a), this indicated that the chromophores have a strong intermolecular coupling and the molecular chirality localized at the peptide transferred to the assemblies.31, 46 For L-NDI•- radical anions gel, the positive Cotton effect appeared at 488 nm, 531 nm and 613 nm, which matched the absorption peak of the NDI•radical anions (Figure 3b). This indicated that the chiral information was also transferred to the radical anions gel as well. It should be noted that upon formation of the NDI•-, the neural species also partially existed and their CD spectrum was much weaked while keeping the shape (Figure S6). On the other hand, the D-enantiomer showed just the mirror-imaged CD spectra for both the neutral and radical anions gels. The intensity of CD signal increased depends on the time of UV irradiation, as seen in Figure S7a-c for details. Interestingly, the supramolecular chirality of


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NDI•- gel inversed compared to the neutral NDI-GE gel with the same chiral center. Meanwhile the CD switch of radical anions assemblies can be obtained by alternative UV-irradiation and oxygen diffusion (Figure S7d).

Figure 3. Chiroptical properties of neutral NDI-GE gel (transparent) and NDI•- radical anions gel (brownish red). (a) CD spectra of L-NDI-GE gel (blue line) and D-NDI-GE gel (red line). (b) CD spectra of NDI•- (L-NDI-GE) radical anions gel (blue line) and NDI•- (D-NDI-GE) radical anions gel (red line). (c) CPL spectra of L-NDI-GE (blue line) and D-NDI-GE (red line) gel formed in methanol (excited at 360 nm). (d) Luminescence dissymmetry factor (glum) for NDI•radical anions (excited at 360 nm). Gel=10 mg/mL, methanol solvent.

Circularly polarized luminescence (CPL) represents the chiral characteristic of the excited state.

Thus, we further measured the CPL from both the neutral and the NDI•- radical anions


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gels. The CPL spectrum of NDI-GE was observed at 460 nm, corresponding to the fluorescence emission of NDI chromophore (Figure S8a). The calculated dissymmetry factor |glum| of the CPL signal for the neutral NDI-GE gel was |0.0047| (Figure 3c). For the NDI•- radical anions gel, the CPL moved to around 610 nm and the signal reversed in comparison with the CPL of the neutral species (Figure S8). This inversion of CPL is consistent with the CD spectrum. Remarkably, the NDI•- radical anions gels showed high |glum| factor, which is in the range of |0.24-0.18| (Figure 3d), in different measuring batches. Obviously, the glum was enhanced by two orders of magnitude from 10-3 to 10-1 from the neutral to the NDI•- radical anions assembly. Usually, such a high glum is rare in organic molecules. Mori et al have proposed that there are some correlations between glum and gabs in molecular state.47 Here, it seems that the assembly system is not the same case. This may be due to the weak correlation between the chirality of the ground state (gabs) and the chirality of the excited state (glum) in supramolecular systems.46, 48-49 In addition, we could not detect the CPL in corresponding solution (Figure S9). This indicated that the chiral signals were only detected in the assemblies while molecules could transfer the chirality to the assemblies and be amplified.50 As mentioned above, NDI•- radical anions can alternatively appear in the presence of oxygen and UV-irradiation. Thus, we observed a CPL switch by alternative UV-irradiation and natural O2 diffusion, which can be repeated several times (Figure S8c). Thus, the reversible switching of CPL from singlet-doublet excited states is realized. The NDI•- radical anions formation is not limited to the methanol. We have also prepared gels in DMF/H2O (v/v=1:2) mixed solvents (Figure S10). The assembled structure was characterized by multi-level helical nanofibers structure by AFM and XRD methods (Figure S11). Simultaneously obtained many similar results such as UV-vis, FL, EPR, Fluorescence lifetime and photogerated radical anions to those of the methanol gel (Figures S2, S12-14).


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However, the NDI•- radical anions in the DMF/Water solvents is much stable in the air, as shown in Figure S15a. It took 7 days to return to the original state under natural diffusion of oxygen and dark conditions (Figure S15b). Free radicals are generally unstable in the room temperature and oxygen atmosphere. However, the in situ generated radical in the gel matrix are relatively stable.29 It has been reported that the gel matrix can block the oxygen to some extent.51-52 However, the difference in stability exhibited by NDI•- radical anions in different gels. On the one hand, it is speculated that methanol solvents have poor oxygen shielding ability compared with DMF/H2O solvents. On the other hand, may be attributed to different molecular assembly patterns in the gels (Figure S3, S11).37 Further studies on gel chirality showed that the chiral signals (CD and CPL) of the gel formed by mixed DMF/H2O (V/V=1:2) were contrary to that formed in methanol (Figure S16, S17). This may be caused by different molecular packing.53-55 Similarly, the CPL signal of the NDI•- radical anions gel (DMF/H2O (v/v=1:2) was also detected, which the luminescence asymmetry factor glum was changed from |0.0023| to |0.26-0.20| (Figure S17), enhanced by two orders of magnitude from 10-3 to 10-1 from the neutral to the radical anions assembly. In addition, it was also observed that the CD and CPL of the NDI•- radical anions gel were reversed compared to the chirality of the neutral NDI-GE gel.


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Figure 4. Theoretical modeling of L-NDI-GE. The first excited state transition electric dipole moments (μ) and the molecular orbitals between which the transition mainly occurs for L-NDI-GE in neutral state (a, c) and negatively charged state (b, d) respectively.

Chiral reversal is a phenomenon that could not be ignored. The supramolecular assemblies formed in any solvent (methanol or mixed DMF/H2O (v/v,1:2)), the NDI•- radical anions assembly compared with the neutral assembly, showed chiral reversal of CD or CPL. In order to better understand this phenomenon, theoretical calculation is carried out through TDDFT. As shown in Figure 4, L-NDI-GE molecular geometry had been reorganized after absorbing one electron, and the electron probability densities of molecular orbitals (MO) attending to the first excited state transition had also been changed as well. Considering the transition dipole moment


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(μ) describes the electron transition between the ground state and excited state, the direction of transition electric dipole moment changes when neutral L-NDI-GE is reduced to L-NDI•--GE, further leading to chiral (CD and CPL) reversal in the supramolecular system.56-57 As mentioned above, we observed the CPL from radical anions gel. In comparison with the neutral species, the glum of the radical anion is two order of magnitude higher. On the one hand, for organic molecules, the magnetic dipole transition moment (m) is typically smaller than the electric dipole term.24 However, for systems containing a large number of unpaired electrons (NDI•- radical anions), the contribution of magnetic dipole transition moment (m) effect cannot be neglected.27,

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This unpaired electrons (NDI•- radical anions) realized the state that

conventonal organic molecuels could not. On the other hand, the formation of NDI•- radical anions is usually a process of electric dipole-forbidden transitions, which helps to increase the glum. Overall, according to the formula 4|m|cosθ / |μ|, the large |m|, small || would contribute together to produce a high glum in the NDI•- radical anions gel system. By UV-irradiation and natural diffusion of oxygen, NDI•- radical anions and neutral NDI alternately appear, which in turn regulates the strength and position of CPL.Therefore, for obtaining high glum value CPL-active materials, the introduction of unpaired electrons may be a promising strategy. In summary, we have designed enantiomeric dipeptide substituted naphthalenediimide gelators and investigated their self-assembly and the chiroptical properties. The L-NDI-GE or D-NDI-GE gelators are capable self-assemble to form transparent gel in either methanol or DMF/H2O solvent. Upon UV-irradiation, a large amount of unpaired electrons are generated in the gels (radical anions). Due to the self-assembly the localized chirality at the peptide was transferred to the whole assemblies and thus both the neutral and radical anion gels showed supramolecular chirality as well as CPL. However, the chirality of the neutral assembly reverses


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compared to the chirality of the radical anions assembly. Remarkably, the radical anions gel showed a higher dissymmetric factor glum of 10-1, which is two order of magnitude than that of the neutral gel. In addition, the chiroptical properties of NDI•- radical anions methanol gel and the neutral species is reversibly switchable via the alternative UV-irradiation and oxygen diffusion. The present work opened a new platform for generating CPL-active materials with high glum by taking advantage of the doublet excited state. ASSOCIATED CONTENT Supporting Information. The synthesis of gelator, the preparation process of gel, the experimental steps of radical anion generation, the method and process of theoretical simulation calculation, and detailed characterization data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.L.). ORCID Minghua Liu: 0000-0002-6603-1251 Author Contributions ⊥These authors contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (21890734), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12020200).


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Significantly Boosted and Inversed Circularly Polarized Luminescence

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