BinaphthylâBipyridyl Cyclic Dyads as a Chiroptical Switch - Journal of

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Binaphthyl-Bipyridyl Cyclic Dyads as a Chiroptical Switch Kazuto Takaishi, Makoto Yasui, and Tadashi Ema J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01860 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Journal of the American Chemical Society

Binaphthyl-Bipyridyl Cyclic Dyads as a Chiroptical Switch Kazuto Takaishi,* Makoto Yasui, and Tadashi Ema* Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Okayama 700-8530, Japan

Supporting Information Placeholder ABSTRACT: A series of chiral cyclic dyads, axially chiral binaphthyls linked to a 3,3’-bipyridyl, was synthesized. The dyad 2 bearing methoxy groups exhibited ON/OFF properties in circularly polarized luminescence (CPL), yielding a |glum| of 1.6 × 10–3 or 0 without any change in fluorescence (FL). This type of CPL switch is unprecedented. Regioisomer 4 exhibited a dextro/levo rotation switching ability in []D. X-ray structures as well as experimental and theoretical analyses suggested that the switching properties depended on conformational changes.

defined as 2(IL–IR)/(IL+IR)) and optical rotation ([]D = –660), unlike typical binaphthyl compounds.7 The configurations of binaphthyl and bipyridyl were labeled as Rnp/Snp and Rpy/Spy, respectively. The configurations of 2–5 were determined based on X-ray crystallography and the DFT-calculated energy profiles, as described below.

Artificial molecular switches have been actively investigated for their potential utility in drug delivery systems, artificial muscles, photonic devices, and catalysts.1 Because nitrogencontaining aromatic rings, such as pyridine, pyrimidine, and triazole, can be protonated and deprotonated easily, these rings are often used as a trigger, for example, in rotaxanes2 and foldamers3. Only a handful of molecules have been identified that provide chiroptical signal switching, especially circularly polarized luminescence (CPL)4 switching.5 For example, Maeda and cowork-

ers reported anion-responsive helicenoids,5a and Akagi and coworkers reported thermotropic nematic liquid crystals. 5b In a previous study of the axially chiral binaphthyl-2,2’-bipyridyl cyclic dyad 1, we found that the (R)-binaphthyl produced (R)bipyridyl via intramolecular chiral transfer (Figure 1).6 It should be noted that a metastable (S)-bipyridyl conformer might be present in low quantities under equilibrium. We postulated that the dyad skeleton could be used to develop a new type of protonation-driven chiroptical switch, including CPL switch. In the present work, we designed the 3,3’-bipyridyl analogs 2 and 4 possessing OMe groups at the 3,3’-positions of the binaphthyl unit (Figure 1) under the following assumptions: (1) (R)-binaphthyl would strongly favor the formation of (R)bipyridyl due to the steric effects of substituents. (2) The protonated bipyridyl would form hydrogen bonds with the OMe oxygens, inducing an inversion from the (R)-bipyridyl to the (S)-bipyridyl. (3) The conformational change must lead to a change in the chiroptical properties. Herein, we report that the dyads 2 and 4 and non-substituted 3 and 5 act as protonation/deprotonation-activated molecular switches that display distinct conformational and chiroptical properties. Among these compounds, dyad 2 acts as an excellent CPL switch, whereas dyad 4 acts as a dextro/levo rotation switch. The dioxepin-fused binaphthyl (R)-6 was selected as a reference compound due to its intense CPL (glum = 1.4 × 10–3; glum was

Figure 1. Binaphthyl-bipyridyl cyclic dyads (Rnp)-1–5 and chiroptical reference binaphthyl (Rnp)-6.

The synthetic routes to (Rnp)-2–5 are shown in Schemes 1 and S1. (Rnp)-8, which was derived from (Rnp)-78, and 99 were used for the synthesis of (Rnp)-2. The coupling reaction was attempted, but the target was obtained in only 7% yield. Therefore, we selected a different method, the SNAr involving (Rnp)-8 and 3-bromo-2fluoropyridine, which gave the coupling precursor (Rnp)-10 in 87% yield. Then the intramolecular homocoupling gave (Rnp)-2

in 65% yield. (Rnp)-4 was obtained in moderate yield through the Williamson ether coupling of equimolar amounts of (Rnp)1110 and 126 with Cs2CO3 (31%). (Rnp)-3 and (Rnp)-5 were synthesized in a similar manner. 1 H NMR titrations indicated that these compounds were almost fully protonated at the two N atoms in the pyridine rings upon the addition of 2.1 equivalents of trifluoroacetic acid (TFA) (Figures S16–S18). They were then deprotonated upon the addition of 2.2 equivalents of triethylamine (Et3N). We investigated the conformational properties of (Rnp)-2–5. The DFT calculations were used to predict the stable structures of

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(Rnp,Spy)-3 (14%), reflecting small energy differences in G° (1.1 kcal/mol) and G‡ (17.6 kcal/mol). The abundance ratio agreed well with the Boltzmann distribution, 87:13, based on the 1.1 kcal/mol energy difference predicted by DFT calculations. The VT 1H NMR spectra of 3 indicated that (Rnp,Rpy)-3 and (Rnp,Spy)-3 interconverted more rapidly as the temperature increased (Figure S21). A trend similar to that observed in (Rnp)-3 was predicted for (Rnp)-3･2H+, but the (Rnp,Spy)-form was not detected experimentally due to a relatively large energy difference (2.5 kcal/mol).

Scheme 1. Synthesis of (Rnp)-2 and (Rnp)-4

(Rnp)-2–5 and (Rnp)-2–5 ･ 2H+ and the transition state (TS) structures between the (Rnp,Rpy)-form and the (Rnp,Spy)-form (Figures 2 and S3–S10 and Table 1).11 For (Rnp)-2, two stable conformations were found: the (Rnp,Rpy)-form was the most stable structure, and the (Rnp,Spy)-form was the second most stable structure, less stable by 8.0 kcal/mol. The instability of (Rnp,Spy)-2 might have been due to steric repulsion between the methoxy groups and the pyridine rings and electronic repulsion between the N and O atoms. The rotational barrier, G‡, was 18.4 kcal/mol, indicating that the interconversion of the two conformers occured easily at room temperature. By contrast, for (Rnp)-2･2H+, the (Rnp,Spy)-form was more stable than the (Rnp,Rpy)-form by a huge energy difference, 11.7 kcal/mol. The distances between the hydrogen atoms of NH and the oxygen atoms of OMe in the (Rnp,Spy)-form were 1.72 Å, indicating the presence of hydrogen bonds. The two intramolecular hydrogen bonds stabilized the conformation and facilitated the drastic conformational change from (Rnp)-2. The G‡ of (Rnp)-2･2H+ was low (13.1 kcal/mol), and the TS was slightly stabilized, although the hydrogen bond distances were relatively long (2.52 Å). The 1H NMR spectra of (Rnp)-2 and (Rnp)2･2H+ showed that only most stable conformers could be distinguished due to the huge energy difference from metastable conformers, and no changes were detected, even at –60 °C (Figure S20). Once (Rnp)-2 was protonated, the signals of bipyridyl ( of 0.13–0.15 ppm), binaphthyl ( of 0.02–0.07 ppm) and methylene protons ( of 0.06–0.11 ppm) shifted as a result of a conformational change of the bipyridyl moiety. Moreover, a NH signal appeared at 9.35 ppm in CDCl3 and at 8.21 ppm in CD2Cl2 for (Rnp)-2･2H+ and at 4.72 ppm in CDCl3 and at 6.57 ppm in CD2Cl2 for (Rnp)-3･2H+ indicating the possible presence of intramolecular hydrogen bonds in (Rnp)-2･ 2H+ (Figures S16 and S17).12 The CD spectra also changed due to protonation, reflecting the changes in the biaryl dihedral angles (Figure S15).13 The (Rnp,Rpy)-form of (Rnp)-3, which bore no OMe groups, was slightly stable (1.1 kcal/mol). 1H NMR and EXSY experiments in CDCl3 at room temperature revealed that (Rnp)-3 was present as an equilibrium mixture of (Rnp,Rpy)-3 (86%) and

Figure 2. DFT-optimized structures of (Rnp)-2, (Rnp)-2･2H+, and their transition states during epimerization of the bipyridyl axis, calculated at the B3LYP/6-31+G(d,p) level.

Compounds (Rnp)-4, (Rnp)-5, and their protonated species had large G‡ values (32.6–34.6 kcal/mol) due to steric repulsion between the facing hydrogen atoms of two methylenes adjacent to the bipyridyl axis. These G‡ values indicated that the conformers did not interconvert. Indeed, each species (Rnp,Rpy)-5 and (Rnp,Spy)-5 could be isolated. (Rnp,Spy)-4･2H+ was stabilized by hydrogen bonds (1.70 Å) similar to those observed in (Rnp,Spy)-2･2H+. Compound (Rnp,Spy)-5 may feature repulsion between the NH protons and the naphthyl protons, leading to a 5.0 kcal/mol instability. The linkers mainly affected the rotational barrier height.


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Journal of the American Chemical Society Table 1. Energy Differences and Rotational Barriers of (Rnp)-2–5a compd 2 2･2H+ 3 3･2H+ 4 4･2H+ 5 5･2H+

G (kcal/mol)

rotational barrierb

(Rnp,Rpy)

(Rnp,Spy)

(kcal/mol)

stable 11.7 stable stable stable 4.8 stable stable

8.0 stable 1.1 2.5 9.2 stable 0.2 5.0

18.4 13.1 17.6 17.7 32.6 34.6 33.3 33.1

a

Calculated via DFT at the B3LYP/6-31+G(d,p) level. bEnergy difference between the (Rnp,Rpy)-form and the transition state.

The X-ray structures of (Rnp,Rpy)-2, (Rnp,Spy)-3, (Rnp,Rpy)-4, (Rnp,Rpy)-5, and (Rnp,Spy)-5 were determined to be similar to the computationally optimized structures (Figure 3). A slightly unstable (Rnp,Spy)-form of 3 was obtained from an equilibrium mixture of conformers, possibly because this form was more readily crystallized than the (Rnp,Rpy)-form.

no significant differences between the conformations of the ground and excited states. Surprisingly, (Rnp)-2･2H+ did not display CPL, although its FL intensity and wavelength were indistinguishable from those of (Rnp)-2. The CD signal at the longest wavelength Cotton effect also became very small (|gabs| ≤ 1.0 × 10–4), which was not inconsistent with the CPL results. After the addition of Et3N to a solution of (Rnp)-2･2H+, CPL was recovered. This is the first example yet reported of ON/OFF CPL switching without changing the FL,5 suggesting the dyes’ utility in anti-counterfeiting applications.4f The possibility that TFA induced a simple solvent effect was eliminated by analyzing the CPL of (Rnp)-6 in a solution containing 100 eq. TFA (Figure S14). The CPL and FL of (Rnp)-6 remained unchanged in the presence/absence of TFA. This result indicated that the CPL ON/OFF change was specific to (Rnp)-2 and arose from the excited state conformations of the binaphthyl unit, which differed in (Rnp)-2 and (Rnp)-2･2H+. Imai, Cheng, and coworkers independently reported that the strengths and signs of the CPL of binaphthyl compounds were correlated with their dihedral angles.15 Indeed, the dihedral angles of the TD DFT-optimized excited state structures were 91° for (Rnp,Rpy)-2, 119° for (Rnp,Spy)-2, 86° for (Rnp,Rpy)-2･ 2H+, and 100° for (Rnp,Spy)-2･2H+ (note that the equilibrium abundance ratios of these species could not be determined) (Figure S9). The dihedral angle of the excited binaphthyl moiety appeared to contribute significantly to the CPL switching properties, although other factors may also be relevant. We performed a TD DFT analyses of compound 2 to obtain the rotatory strength (R) of the S1 transition. The R value of (Rnp,Rpy)-2 (22.1× 10–40 erg esu cm/G) was 11 times the value obtained from (Rnp,Spy)-2･2H+ (2.1× 10–40 erg esu cm/G). This result further supported the relationship between CPL/CD and the conformation.

Figure 3. X-ray structures of (a) (Rnp,Rpy)-2, (b) (Rnp,Spy)-3, (c) (Rnp,Rpy)-4, (d) (Rnp,Rpy)-5, and (e) (Rnp,Spy)-5 (thermal ellipsoids at 50% probability).

We next evaluated the FL and CPL characteristics of 2–5. Dyads (Rnp)-2 and (Rnp)-3 displayed blue fluorescence, whereas the other dyads did not. The FL of (Rnp)-2 (Figure 4b) and (Rnp)-3 (Figure S13) originated from the binaphthyl unit, and the absolute FL quantum yields (FL) were 0.15, 0.11, respectively. These FL values were typical of binaphthyl compounds,7,14 suggesting that FL quenching was not caused by the presence of the bipyridyl moiety. Fortunately, (Rnp)-2 showed a clear (–)-CPL (glum = –1.6 × 10–3) (Figure 4a). This glum value is as large as or larger than the values reported for simple biaryl compounds.4e (Rnp)-2 and (Snp)-2 were almost mirror images, indicating that the effects of the linearly polarized emission were negligible. The sign of the Cotton effect gabs value for (Rnp)-2 (/–7.7 × 10–4) at the longest absorption wavelength (340 nm) was equal to that of glum, indicating

Figure 4. (a) CPL and (b) FL spectra of (Rnp)-2 (solid line) and (Snp)-2 (dot line). Conditions: CH2Cl2, 1.0 × 10–5 M, 20 °C, light path length = 1 cm. ex = 300 nm.

Finally, we anticipated that protonation/deprotonation of 2– 5 would lead to a change in the specific optical rotation, []D (Table 2). A large []D, 24–176, was observed upon protonation, and the addition of Et3N restored the []D values to the



native values. By contrast, []D of (Rnp)-6 (15) was smaller than the corresponding values obtained from dyads 2–5, although the absolute value was large (660). These results indicated that a large []D required proton acceptor moieties. Among (Rnp)-2–5, (Rnp)-4 produced the largest change ([]D of 176), intriguingly, in a dextro/levo rotation inversion. Dextro/levo rotational switching in 4 was also observed in different solvents (CH2Cl2, or CH3CN) in the presence of acid (TsOH) or base (DIPEA). It is not clear why (Rnp)-4 exhibited dextro/levo switching with a large []D. However, the dihedral angles of binaphthyl ( = 12.8°) and bipyridyl ( = 49.1°) in (Rnp,Rpy)-4 and (Rnp,Rpy)-4･2H+ differed significantly (Figure S5 and S6). The change in the CD spectra of 4 upon protonation probably reflected the changes in the dihedral angles, and the CD signal at the longer wavelength, which could affect the rotation of the sodium D-line, was altered. (Figure S15). (Rnp)-4 is an extremely rare example of a protonation-activated dextro/levo rotation switch.16

Table 2. []D of (Rnp)-2–6 after Addition of TFA and Et3N. compd (Rnp,Rpy)-2 (Rnp,Rpy)-3a (Rnp,Rpy)-4 (Rnp,Rpy)-5 (Rnp,Spy)-5 (Rnp)-6

[]D after addition of TFA (2.5 eq)

[]D after addition of Et3N (2.5 eq)

[]D

+214 +420 –23 +104 –297 –660

+238 +472 +153 +211 –435 –675

24 52 176 107 138 15

Conditions: CHCl3, c 0.10, 25 °C, light path length = 5.0 cm a Equilibrium mixture of diastereomers.

In summary, we have developed a new class of axially chiral binaphthyl-bipyridyl cyclic dyads with unique chiroptical switching and structural properties. Protonation/deprotonation of (Rnp)-2 triggered unprecedented CPL-ON/OFF switching without an FL change, and (Rnp)-4 displayed very rare dextro/levo rotation switching. Experimental and theoretical analyses suggested that the switching properties resulted from an inversion of the bipyridyl axis mediated by intramolecular hydrogen bonds or a change in the biaryl dihedral angles. Dyads 2 and 4 will open up new doors in the fields of chiroptical materials, photoluminescence, and electroluminescence.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Synthesis, spectra, and computational details (PDF) X-ray crystallographic data for (Rnp,Rpy)-2, (Rnp,Spy)-3, (Rnp,Rpy)-4, (Rnp,Rpy)-5, and (Rnp,Spy)-5 (CIF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (KT) *E-mail: [email protected] (TE)

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ORCID Kazuto Takaishi: 0000-0003-4979-7375 Tadashi Ema: 0000-0002-2160-6840

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by JSPS KAKENHI Grant Nos. JP15K17826 and JP17K05786 and the Foundation for The Promotion of Ion Engineering. We thank Dr. Hiromi Ota of Okayama University for X-ray analyses, the N-BARD, Hiroshima University and JASCO Corporation for CPL measurements, and Otsuka Electronics Co., Ltd for measurements of the absolute fluorescence quantum yields.

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