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Circular Dichroism Photoswitching with a Twist: Axially Chiral Hemiindigo Christian Petermayer, and Henry Dube J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07839 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Journal of the American Chemical Society

Circular Dichroism Photoswitching with a Twist: Axially Chiral Hemiindigo Christian Petermayer,† Henry Dube†* † Ludwig-Maximilians-Universität München, Department für Chemie and Munich Center for Integrated Protein Science CIPSM, D-81377 Munich, Germany.

Supporting Information Placeholder ABSTRACT: Chiroptical properties play a crucial role

not only for molecular structures and their functions but also for advanced applications such as molecular sensing, absolute asymmetric synthesis, or information processing and storage. Manipulating chiroptical characteristics in a predictable and reversible fashion by outside means is therefore a highly desirable option to enhance the functions and reporting abilities of a molecular system. Herein we present axially chiral hemiindigo photoswitches showing unusual chiroptical changes upon visible light irradiation. While absorption remains high throughout the spectrum the corresponding ECD signals can be reversibly erased and reestablished in an ON/OFF manner upon photoswitching. Taken together with exceptionally high thermal bistabilities, leading to half-lives of the metastable states up to 3400 years at ambient temperature, and high photoswitching quantum yields these chiral hemiindigos offer unique possibilities for e.g. smart molecule, photonic materials, or sensing applications. The development of chiral photoswitches1-2 is motivated by several intriguing application prospects such as molecular information storage with nondestructive readouts,3-4 absolute asymmetric synthesis,5-9 or high resolution sensing.10-12 Consequently many common photoswitches have been either transformed directly into chiral derivatives by introducing chiral information close to the core chromophore structure4, 13-21 or been used as additives in chiral amplifying environments such as gels,2224 peptides,25-33 liquid crystals,34-37 or polymers38-39. In virtually all cases electronic circular dichroism (ECD) changes upon photoswitching are closely tied to changes in the absorption. Either different ECD signals with (possibly) changing signs are obtained if absorption remains high after switching or ECD signals disappear when the absorption also disappears. In this work we show how such behavior can be disentangled using newly developed axially chiral hemiindigo (HI)40-43 photoswitches 1 to 3 (Figure 1). These unusual chiroptical photoswitches operate entirely in the visible region of light and particularly derivatives 2 and 3 allow reversible erasing and reestablishing of strong ECD signals after photoswitching

while absorption always remains high in the same wavelength region. In this way an ECD-only readout of the double bond configuration can be established throughout a broad spectral range, which could be of interest in advanced molecular photonic and information storage devices as well as absolute asymmetric synthesis applications. This intriguing behavior is explained by the molecular design of HIs 1 to 3, which possess a strongly defined twisted structure in the Z but a much less twisted one in the E isomeric state (Figure 1a and b). Additionally it could be shown that the chiral axis remains completely stable upon photoswitching in these molecules enabling many cycles of ECD switching without loss of function. Photoswitches 1 to 3 consist of a HI core structure bearing a strong electron donating aniline fragment (Figure 1). This particular molecular setup was shown earlier by our group to result in highly efficient visible-light responsive photoswitches.40 The indoxyl N atom is substituted with an ortho-tolyl or naphthyl group establishing a chiral axis along the connecting N-C single bond. An adjacent methyl group at the indoxyl fragment introduces additional steric constraints effectively raising the energy barrier of thermal atropisomerization (racemization process). HIs 1 to 3 were synthesized in three steps (for details see the Supporting Information). The ortho-tolyl or naphthyl substituent at the indoxyl N atom is installed early on in the synthesis as the previously reported late-stage arylation40 was not successful in this case, presumably because of the considerable sterical hindrance the system. The conformations of 1 to 3 in solution were determined by absorption, ECD, and NMR spectroscopy supported by quantum chemical calculations (for details see the Supporting Information). Separation of Z and E isomers as well as enantiomers was achieved using reverse phase and chiral HPLC at different temperatures, respectively. For all HIs 1 to 3 thermal double-bond isomerizations are exceptionally slow at ambient temperatures, leading to halflives of up to 3400 years for the metastable E isomers at 25 °C in DMSO. With such high thermal bistability these HIs are effectively rendered into P-type photoswitches. The conformations of HIs 1-3 are substantially twisted in the Z isomer while in the E isomer twisting is strongly reduced. This is seen in the theoretical description as well as experimentally from enlarged upfield shifts of Z isomer proton signals compared to E isomers (see Figure 2b and S10).

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Figure 1. Axially chiral HIs 1 to 3. Photoisomerization leads to interconversion between Z and E isomeric states having different colors or intensities in solution. Thermal double bond isomerizations do not occur even at elevated temperatures. Thermal atropisomerizations are faster for Z than for E isomers. a) Isomer interconversion of HI 1. Cuvettes show DMSO solutions of pure isomers. b) Isomer interconversion of HI 2. Cuvettes show heptane/EtOAc (93/7) solutions of pure isomers. c) Thermal atropisomer interconversion of HI Z-3 is very slow.

For HIs 2 and 3 the increased sterical hindrance raises energy barriers for thermal atropisomerizations sufficiently to allow isolation of atropisomers in both Z and E isomeric states. The corresponding energy barriers for thermal atropisomerizations in heptane/ethyl acetate mixtures are 19.9/23.4 kcal/mol for Z-1/E-1, 23.1/26.1 kcal/mol for Z-2/E2, and 24.8/27.6 kcal/mol for Z-3/E-3, respectively. ECD spectra were recorded for pure enantiomers of both switching states (except for Z-1) showing a vast difference in the intensity of ECD signals between Z and E isomers, especially for 2 (Figure 3c). While Z-2 possesses strong signals throughout the visible spectral range reaching Δε values of 8.0 mol-1cm-1 the ECD signals of E-2 have very low intensities with Δε values below 0.17 mol-1cm-1. At the same time the absorption remains high over the visible range of the spectrum (Figure 3b). Z/E-3 exhibits similar behavior except for somewhat higher ECD responses in the E form. The corresponding g-factors mostly resemble the ones of 2 in intensity. Interestingly the ECD signals of E-2/3 are especially low at wavelengths of maximum absorption close to 500 nm. This behavior can qualitatively be understood by significantly reduced twisting in the molecular structures of E-2/3 strongly affecting the ECD spectrum (see Figure 2b and the Supporting Information for theoretically obtained ECD spectra). After establishing thermal stabilities for the different isomeric states of 1 to 3 their photoswitching properties and associated spectral and geometry changes could be investigated. HIs 1 to 3 display considerable photochromism, which enables following changes in isomer composition by the naked eye especially for 1 (Figure 1). Photoswitching efficiencies are very good for all derivatives leading to high isomer content at different wavelengths of irradiation. At 470 nm irradiation 93% of E-1 and at 625 nm 97% Z-1 are obtained in DMSO solution. The corresponding values for HIs 2/3 are lower in DMSO (see Table 1) but reach similar high values in less polar solvents such as mixtures of heptane/ethyl acetate, i.e. 84/83% E2/3 in the photostationary state (pss) at 435 nm and 98% Z2/3 in the pss at 530 nm. The corresponding photoquantum yields are also quite high with e.g. 49%/12% for the ZE/EZ photoisomerization at 450/520 nm for HI 1 or

≈30%/10% at 450/520 nm for HIs 2 and 3 in heptane/ethyl acetate mixtures. All quantified properties of HIs 1 to 3 are summarized in Table 1. In the following the geometry changes after photoisomerization of HI 2 are discussed in detail whereas analysis of HI 1 and 3 is given in the Supporting Information. Irradiation of a 93/7 heptane/ethyl acetate solution containing pure Z-2 with (Sa) configured chiral axis (see the Supporting Information for establishment of the absolute stereo-configuration) with 450 nm light at -20 °C resulted in the formation of exclusively E-2 with again (Sa) configured chiral axis as photoproduct.

Figure 2. Conformational analysis of HI 2. a) Molecular structures of (Ra)-Z-2 and (Ra)-E-2 optimized at the DFT B3LYP/6-311+G(d,p) level of theory. b) Aliphatic region of the 1H NMR spectrum (400 MHz, CD2Cl2, 27 °C) of Z-2 (red) and E-2 (blue). The corresponding aromatic signals are shown in Figure S5. Indicative signals of the methyl groups are upfield shifted in Z-2 compared to E-2.


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Table 1. Overview of photophysical properties of HIs 1 – 3. Pss Z/E = Z/E composition in the pss in % at wavelength in nm. QY ZE/EZ = quantum yield of Z to E or E to Z photoisomerization in % at wavelength in nm. G* DB equil. ZE/EZ = G* for thermal double bond isomerizations in kcal/mol. τ1/2 DB equil. ZE/EZ = extrapolated half-lives of respective Z or E isomers at 25 °C. G* Atrop. Z/E = G* for thermal atropisomerization of the respective Z/E atropisomers in kcal/mol. τ1/2 Atrop. Z/E = extrapolated half-lives of atropisomers at 25 °C.

Pss Z/E [%] ([nm])

QY ZE/EZ [%] ([nm])

G* DB equil. ZE/EZ [kcal/mol]

τ1/2 DB equil. ZE/EZ at 25 °C

1/DMSO

97(625)/93(470)

33(467)/9 (600)

30.6/32.1

99 a/1140 a

1/Hept/EA1)

96(600)/96(435)

49(450)/12(520)

26.4/27.1

30 d/104 d

2/DMSO

96(617)/56(450)

12(450)/12(520)

30.5/31.7

78 a/614 a

2/Hept/EA2)

98(530)/84(435)

34(450)/10(520)

3/DMSO

98(595)/43(450)

5(450)/8(520)

3/Hept/EA3)

98(530)/83(435)

27(450)/9(520)

HI/ Solvent

31.8/32.7

G* Atrop. Z/E [kcal/mol]

τ1/2 Atrop. Z/E at 25 °C

19.9/23.4

43 s/286 m

23.1/26.1

164 m/19.3 d

24.8/27.6

2.01 d/0.622 a

750 a/3427 a

Heptane/ethyl acetate mixtures: 1) 83/17; 2) 93/7; 3) 87/13. of (Sa)-3 at 19 °C (alternating 435 nm and 505 nm irradiation to the respective pss), kinetics of the thermal racemization of (Sa)-Z-3 at 19 °C (violet), and irreversible photodegradation (green).

Figure 3. ECD photoswitching of enantiomerically pure HI 2 and 3 in heptane/ethyl acetate (93/7 and 83/17, respectively) solutions. a) Photoswitching of 2 and 3 with blue and green light leads to loss and reestablishment of the ECD spectrum while absorption remains high. b) Molar absorption of pure (Sa)-Z-2 (red) and (Sa)-E-2 (blue). c) Molar ECD spectra of pure (Sa)-Z-2 (red) and pure (Sa)-E-2 (blue). d) ECD spectra of (Ra)-2 (-20 °C, solid) and (Sa)-3 (19 °C, dashed) in the pss at different irradiation wavelengths (98% enriched in (Ra)-Z-2 at 520 nm, 88% (Sa)-Z-3 at 505 nm, red; 82% enriched in (Ra)-E-2 at 450 nm, 83% (Sa)-E-3, blue). e) ECD changes observed at 420 nm (grey bar in subfigure d) upon repetitive photoswitching of (Ra)-2 at -20 °C (alternating 450 and 520 nm irradiation to the respective pss, left) and of (Sa)-2 at 24 °C (435 and 505 nm irradiation to the pss, right), kinetics of the thermal racemization of (Ra)-Z-2 at 24 °C (violet), and irreversible photodegradation (green). f) ECD changes observed at 435 nm (grey bar in subfigure d) upon repetitive photoswitching

Likewise, irradiation of pure (Sa)-E-2 with 520 nm light results in exclusive formation of (Sa)-Z-2 as photoproduct (Figure 3d and e). This result shows that upon photoswitching no coupled motion between the aniline fragment and the rotatable N-o-tolyl moiety occurs and only the central double bond is photoisomerized. Therefore, the absolute stereoconfiguration of the chiral axis is retained upon photoswitching and no racemization occurs. As the ECD signals are vastly different between Z-2 and E-2 isomers HI 2 can be used for reversible photoinduced ECD eradication and restoration in the visible region of the spectrum while absorption remains high throughout this process. This behavior was evidenced by repetitive photoswitching of the ECD signals of 2 in heptane/ethyl acetate 93/7 solution at -20 °C and at 24 °C without racemization or significant photofatigue over up to 36 cycles (Figure 2e). The apparent ECD signal decay occurring during the cycling experiments at 24 °C could be attributed to thermal racemization in the Z isomeric state over the prolonged course of the experiment. HI 3 overcomes this shortfall by possessing increased thermalstability of its atropisomers resulting in convenient ECD photoswitching and atropisomer characterization at 19.2 °C (Figure 3d and f). In summary, we have developed axially chiral hemiindigo photoswitches providing highly unusual and reversible photocontrol over their spectral properties. Upon photoswitching the absolute stereo-configuration of their chiral axes remains stable giving direct experimental evidence for the absence of photoinduced racemizations and thus coupled gearing motions. The different molecular twists in the Z and E isomeric switching states lead to strong modulation of the entire ECD spectrum. Especially the ECD spectrum of E-2 possesses very low intensity compared to



Z-2 allowing efficient and completely reversible photoswitching of the ECD spectrum in an ON/OFF manner while maintaining strong absorption throughout the spectral range. This uncoupling of absorption changes from ECD modulations could prove very useful in future lightcontrolled molecular information processing or sensing applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures for 1-3, characterization including mp. 1H NMR, 13C NMR, IR, (HR)MS, conformational analysis, photophysical data including molar absorption coefficients, isomer compositions in the pss at different wavelengths, quantum yields, kinetic analyses of the interconversion of isomers at different temperatures, and computational details.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the Deutsche Forschungsgemeinschaft (SFB 749/3, A12) for financial support. We further thank the Deutsche Forschungsgemeinschaft (DFG) for an EmmyNoether fellowship (DU 1414/1-1), and the Cluster of Excellence ’Center for Integrated Protein Science Munich’ (CIPSM) for financial support.

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