Significant Enhancement of Circularly Polarized Luminescence

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Significant Enhancement of Circularly Polarized Luminescence Dissymmetry Factors in Quinoline Oligoamide Foldamers with Absolute Helicity Dan Zheng, Lu Zheng, Chengyuan Yu, Yulin Zhan, Ying Wang, and Hua Jiang* College of Chemistry, Beijing Normal University, Beijing 100875, China

Org. Lett. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 03/28/19. For personal use only.

S Supporting Information *

ABSTRACT: When S- or R- oxazolylaniline enantiomers were attached to achiral quinoline oligoamide foldamers (QOFs), a single diastereomerically pure P- or M-handed foldamer was observed and exhibits negative or positive circularly polarized luminescence with the emission dissymmetry factors |glum| up to 0.038, which is significantly larger than that of QOF with incomplete chiral induction. More importantly, the CPL dissymmetry factors, together with the absorption dissymmetry factors, are enhanced with increases in the lengths of QOFs.

H

induction at the C-terminus of QOF, chiral oxazolylanilines12 were rationally chosen, because they were expected to form a stable three-center hydrogen bonding network with QOF, which would secure the highly efficient transference of the chiral information from central chirality to helicity and consequently fulfill complete control of helicity in QOF (Figure 1a). Hence, we designed and synthesized enantiomerically pure chiral and racemic oxazolylanilines with methyl and phenyl substitutes and their QOF-based derivatives with the lengths up to 32 quinoline units (Figure 1 and synthetical details see the Supporting Information). Here, we report our explorations of the CD and CPL properties in QOFs with absolute helicity. The populations of one pair of diastereomeric M- and Phelices can be readily rated by 1H NMR if a foldamer is long enough to undergo slow interconversion on the NMR time scale. For a chiral foldamer bearing, for example, S-chiral center, the proportions of two sets of signals for two diastereomers S-P and S-M helices will be revealed. However, in the case of absolute biasing, a single set of signals for single diastereomeric S-M or S-P helix will be observed. 1H NMR spectra of S-CQ4-a and S-CQ4-b exhibit the presence of a single set of sharp signals between 11.0 and 13.5 ppm for the four carbox amides that were expected to form three-center hydrogen bonding networks (see Figures 2a and 2b, as well as Figures S3−S5 in the Supporting Information). Among them, the signals appearing at 13−13.5 ppm are assignable to the resonances of the protons in the amide between oxazolylaniline and quinoline judged by the previous observations in QOFs.11,15,16 The VT 1H NMR experiments revealed that the single set of sharp signals for S-CQ4-a, S-CQ4-b, and SCQ4-b does not decoalescence when cooling to 183 K in

elical structures are very important in biomacromolecules such as DNA and peptides. Molecules bearing helical conformation exhibit large anisotropy in circular dichroism (CD) and circularly polarized luminescence (CPL), whose investigations are crucial not only for the understanding of molecular chiroptical activity but also for the developing of chiroptical or electro-optical materials,1 particularly, of materials showing CPL because of their potential applications in display devices2 and optical storage devices.3 Recently, CPL-active organic helical systems1c have attracted growing attention, because of their advantage of readily tunable emission properties and easy modification.1b,5 Aromatic foldamers fold into helical conformation driven by noncovalent forces exhibit equilibrium that occurs between the left (M)- and right (P)-handed enantiomers. Biasing of such equilibrium by attaching chiral moieties to aromatic foldamers generates one-handed preference in helicity,6 leading to potentially practical applications in chiral recognition, chiral electro-optical devices, and asymmetric catalysis.7 Chiral aromatic foldamers appear as appealing candidates for developing chiroptical or electro-optical materials, because of the inherent helical chirality. However, their CPL properties attract much less attention, in comparison with their CD,8 presumably because of both weak luminescence properties and poor chiral induction.6,9 Recent studies showed that both dissymmetry factors gabs and glum of chiral foldamers are significantly dependent on various factors, such as the stability of chiral foldamers and the diastereomeric excess value.9a,10 In fact, in most cases, chiral induction of aromatic foldamers is incomplete,6 which leads to inseparable mixtures of diastereomers. Therefore, the absolute control of helicity in aromatic foldamers is desperately crucial for developing diastereomerically pure aromatic foldamers with CPL properties. We focused on QOF because of its robust folded helical conformation and easy synthesis.11 To achieve absolute chiral © XXXX American Chemical Society

Received: February 2, 2019

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DOI: 10.1021/acs.orglett.9b00450 Org. Lett. XXXX, XXX, XXX−XXX

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Figure 3. Crystal X-ray structure of racemic tetramer: (a) (±)S/RCQ4-a (side view), (b) the three-center hydrogen bonding network between oxazolylaniline moiety and quinoline group (the rest of the parts were deleted for the sake of clarity).

metric space group P1̅. As expected, each unit cell thus contains two pairs of M and P helices. More importantly, in the crystal, the S-asymmetric center always accompanies the Phanded helicity and the R-asymmetric center always accompanies the M-handed helicity, consistent with the previous observations by Huc et al.6b In contrast with the silent CD spectra of the racemate (±)S/ R-CQ4-a, the spectra of chiral tetramers S-CQ4-a and S-CQ4-b display bands with the same positive of negative sign for any given wavelength in the absorption region of the quinoline rings between 250 nm and 450 nm (Figure 4a), demonstrating

Figure 1. (a) Illustration of absolute control of helicity of foldamers. (b) The structures of QOFs containing oxazolylaniline motifs S/RCQn-a and S/R-CQn-b, S/R-PCQ2n-b, (±)S/R-CQ4-a and model compound S-CQ8-m.

Figure 2. Parts of the 1H NMR spectra of (a) S-CQ4-a, (b) S-CQ4-b, (c) S-CQ8-b, (d) S-CQ16-b, (e) S-PCQ8-b, (f) S-PCQ16-b, and (g) SPCQ32-b in CDCl3 at 298 K.

CD 2 Cl 2 (see Figures S1 and S2 in the Supporting Information), indicating that the chiral inductions are quantitative in S-CQ4-a and S-CQ4-b, and the diastereomeric excess (de) is >99%. Moreover, the 1H NMR spectra of SCQ4-b and R-CQ4-b are identical to each other (see Figures S3 and S4). The absolute chiral inductions are attributed to the stable hydrogen bonding network between chiral oxazolylanilino groups and quinoline oligoamides. To confirm the formation of a three-center hydrogen bond network between oxazolylanilino moieties and quinoline oligoamides, racemic tetramer (±)S/R-CQ4-a, which bears an (±) oxazolylanilino group, was used to grow crystals. X-ray diffraction (XRD) reveal that one pair of enantiomers cocrystallize as true racemates and the existence of a three-center hydrogen bond network between the amide proton and both adjacent quinoline nitrogen and oxazoline nitrogen in each enantiomer (see Figure 3 and the Supporting Information). The structure of (±)S/R-CQ4-a belongs to the centrosym-

Figure 4. (a) CD and CPL spectra of S-CQ4-a, S-CQ4-b, and R-CQ4b in DCM (c = 1.0 × 10−5 M). (b) Ultraviolet-visible light (UV-vis) and fluorescence spectra (excited at 310 nm) of S-CQ4-a, S-CQ4-b, and R-CQ4-b in DCM (c = 5.0 × 10−6 M).

that the helical senses are the same. The positive signs of the peaks between 350 and 450 nm indicate a P helical sense. The gabs values of S-CQ4-a and S-CQ4-b and S-CQ4-b are +0.020 and +0.019, respectively (see Table S2 in the Supporting Information). The CD spectra of R- and S-CQ4-b are mirror images, showing a M helical sense for R-CQ4-b. These almostinvariable values of |gabs| clearly display that the helical bias of these tetramers is completely governed by the chiral nature of the S- or R-oxazolylanilino center but is independent of its substituents. By combination with 1H NMR and XRD data, B

DOI: 10.1021/acs.orglett.9b00450 Org. Lett. XXXX, XXX, XXX−XXX

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Figure 5. (a) CD and CPL spectra of S/R-CQ4,8,16-b in DCM (c = 1.0 × 10−5 M). b) CD and CPL spectra of S/R-PCQ8,16,32-b in DCM (c = 1.0 × 10−5 M). (c) UV-vis and fluorescence spectra (λex = 310 nm) of S/R-CQ4,8,16-b in DCM (c = 5.0 × 10−6 M). (d) UV-vis and fluorescence spectra (λex = 310 nm) of S/R-PCQ8,16,32-b in DCM (c = 5.0 × 10−6 M).

Table 1. Photophysical Properties, Absorption, and Luminescence Dissymmetry Factors of Oligomers S-CQ4,8,16-b and SPCQ8,16,32-b in Dichloromethane at 298 K compound

gabs

glum

ΦFa (%)

τ (ns)

krb (× 107 s−1)

knr (× 108 s−1)

S-CQ4-b S-CQ8-b S-CQ16-b S-PCQ8-b S-PCQ16-b S-PCQ32-b

0.019 0.031 0.036 0.028 0.036 0.044

0.015 0.025 0.029 0.021 0.031 0.038

1.61 8.87 10.20 5.30 7.04 10.10

0.70 1.18 1.63 1.07 1.15 1.53

2.3 7.5 6.3 5.0 6.1 6.6

14.1 7.7 5.5 8.9 8.1 5.9

ΦF measured using a calibrated integrating sphere. bThe radiative rate constant kr and nonradiative rate constant knr were obtained from the equations: kr = ΦF/τ and knr = (1 − ΦF)/τ, respectively. a

lino groups can also be assigned as single S-P or R-M diastereomeric helices, respectively, according to the 1H NMR (recall Figure 2, as well as Figures S3−S5 in the Supporting Information) and CD spectra (Figures 5a and 5b). The CD spectra of S- and R-CQ4,8,16-b exhibit mirror image Cotton effects in the absorption region of the quinoline rings between 250 nm and 450 nm but with different intensities. The |Δε| at 388 nm increased linearly with the increase in the number of quinoline units (see Table S2 in the Supporting Information, Figure 5a, and Figure S7 in the Supporting Information). Interestingly, their |gabs| values also increased from ca. 0.019 to 0.036, displaying a similar trend to |Δε|. The phenomenon is completely different from the paracyclophane oligomers where the molar ellipticity [θ] increase with the increase in the number of the stacked π-electron systems but their gabs values are almost identical, because of its unfolded conformation.13 The linear increase in |Δε| and |gabs| of S- and R-CQ4,8,16-b indicate that there is no loss of chiral transference from chiral oxazolylanilino motif to QOFs as the chains of QOFs lengthened.10 Similarly, the values |gabs| of longer chiral S- and R-PCQ8,16,32-b also increase from 0.028 to 0.044 (Table 1 and Figure S2).

one can conclude that S-CQ4-a and S-CQ4-b are diastereomerically pure P helices while R-CQ4-b is a diastereomerically pure M helix. S-CQ4-a and S-CQ4-b exhibit positive CPL with the maximum emission at 450 nm and negligible spectral difference, because of their similar CD properties (Figure 4a). The CPL spectra of S- and R-CQ4-b are mirror images of each other (Figure 4a). The magnitudes of CPL of these QOFs were evaluated by glum. The |glum| values for S-CQ4-a, S-CQ4-b, and R-CQ4-b are almost same as 0.015 (see Table S2 in the Supporting Information), regardless of the different substituents in the chiral centers as the observations in gabs. Moreover, the UV-vis absorption and fluorescence emission spectra of SCQ4-a, S-CQ4-b, and R-CQ4-b are almost identical (Figure 4b). The 1H NMR spectra of S- and R-CQ8-b also feature a single set of sharp signals (see Figure 2, as well as Figure S3 in the Supporting Information), indicating a complete helicalhanded-ness bias because they are longer than S-CQ4-a and S-CQ4-b. The CD spectra of S- and R-CQ8-b are mirror images (Figure 5a). Therefore, S- and R-CQ8-b can be assigned as single S-P and R-M diastereomeric helices, respectively. Analogously, other longer QOFs bearing S- or R-oxazolylaniC

DOI: 10.1021/acs.orglett.9b00450 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters In the CPL spectra, S-CQ4,8,16-b and S-PCQ8,16,32-b exhibit substantial signals due to both increases in length. The CPL spectra of R-M and S-P helices of corresponding QOFs are mirror images each other (Figures 5a and 5b). The glum values obviously became larger with increases in the lengths of chiral QOFs (Table 1). As expected, the |glum| values of R-M and S- P helices are almost same (see Table 1, as well as Table S2). The significant increase in dissymmetry factors would be ascribed, in part, to the increased helical aromatic stacks with the increase in the lengths of the chiral QOFs. For the first time, the length-dependent glum was observed in foldamers and completely different from the previous report in which the glum values are almost the same or even slightly less with the increase in the helical overlap of bridged helicenes.1a,4a,14 These values are on the order of 10−2 and significantly larger than those for helically chiral molecules reported to date,1a,9a,b,11b,14,15 for instance, functionalized [7]helicene-like molecule exhibits a |glum| value of 0.021 for the dispersed state.16 In addition, the ratios of dissymmetry factors (glum/ gabs) are in the range of 0.77 to 0.87 (Table S2), suggesting that the dissymmetry of these chiral QOFs is mainly kept in the excited state.15 The UV−vis and fluorescence spectra of each pair of S- and R-CQ4,8,16-b, and of S- and R-PCQ8,16,32-b are approximately the same (see Figures 5c and 5d). The quantum yields and lifetimes of these chiral QOFs exhibit the increased trend with the increase in the lengths of QOFs (Table 1). The nonradiative decay has a substantial contribution in the shortest chiral tetramer S-CQ4-b. Interestingly, the nonradiative decay constants decrease with the increase in the lengths of S-CQ4,8,16-b (Table 1). The similar variation was also observed for S-PCQ8,16,32-b (Table 1). The ratios of knr/kr are 61−9 for S-CQ4,8,16-b and 17−9 for S-PCQ8,16,32-b (Table S1), respectively, which suggests that the increase in the lengths of chiral QOFs is obviously favorable for the excitedstate decays via radiative channel, presumably because of the more-rigid structure in the excited state that originates from strong aromatic stackings in longer QOFs. For further insight into the effect of the absolute chiral induction on absorption and luminescence dissymmetry factors of chiral QOFs, we synthesized the model compound S-CQ8m bearing S-phenethylamino group that slightly biases the equilibrium between P- and M-handed helix (Figure 1).6a,b 1H NMR spectrum revealed there are two sets of signals in the ratio of 10:1 corresponding the de of 82%, suggesting incomplete chiral induction (Figure S6 in the Supporting Information). The CD and CPL spectra of S-CQ8-m feature similar patterns to those of S-CQ8-b but with much smaller intensities (Figure S9 in the Supporting Information). The values of gabs and glum are 0.021 and 0.016, respectively, which are significantly smaller than those of S-CQ8-b with absolute helicity (Table 1). These data clearly demonstrate that the extent of chiral induction has a significant effect on both absorption and luminescence dissymmetry factors. In summary, we have synthesized a series of QOFs with chiral oxazolylanilino motifs that exhibit absolute chiral inductions due to the stable three-center hydrogen bonding network between chiral groups and achiral framework of QOFs, leading to the formation of single diastereomerically pure M or P helix without further resolution, as required for nonracemic helicenes.7 These QOFs with absolute helicity display enhanced CD signals, together with a significant increase in CPL. Importantly, both absorption and lumines-

cence dissymmetry factors enhance with the increase in the lengths of aromatic foldamers. The control of absolute helicity would provide many chances for developing diastereomerically pure aromatic foldamers with unprecedented chiroptical features.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00450. Experimental procedures, characterization of synthesized oligomers, and additional data (PDF) Accession Codes

CCDC 1886419 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ying Wang: 0000-0002-7015-7228 Hua Jiang: 0000-0002-9917-2683 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21472015 and 21672026). We also thank Applied Photophysics for their generous assistance on CPL measurements.

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REFERENCES

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DOI: 10.1021/acs.orglett.9b00450 Org. Lett. XXXX, XXX, XXX−XXX