Hemiindigo: Highly Bistable Photoswitching at the Biooptical Window

Article pubs.acs.org/JACS

Cite This: J. Am. Chem. Soc. 2017, 139, 15060-15067

Hemiindigo: Highly Bistable Photoswitching at the Biooptical Window Christian Petermayer, Stefan Thumser, Florian Kink, Peter Mayer, and Henry Dube* Ludwig-Maximilians-Universität München, Department für Chemie and Munich Center for Integrated Protein Science CIPSM, D-81377 Munich, Germany S Supporting Information *

ABSTRACT: Hemiindigo is a long known chromophore that absorbs in the blue part of the spectrum but has almost completely been ignored as potential photoswitch. Herein we show how the absorption of hemiindigo is shifted to the red part of the visible spectrum and how nearly perfect photoswitching can be achieved using blue or green and red light. Five derivatives were investigated giving very high isomeric yields in both switching directions, i.e. >90% E isomer after irradiation with 470 to 530 nm light and 99% Z isomer with 590 up to 680 nm light. At the same time the thermal bistability is extraordinarily high leading to half-lives of the pure isomeric states of up to 83 years at 25 °C. The herein developed photoswitches show photochromism in the visible enabling the two isomeric states to be distinguished by the naked eye. Substituted hemiindigos therefore constitute extremely promising new photoswitches with excellent properties for applications in biology, chemistry, or material sciences.

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INTRODUCTION Shifting the wavelengths for photoswitching into the red part of the spectrum while maintaining high thermal stability of the induced metastable states is a topic of very high current interest in many chemistry related fields.1−16 This combination of properties opens up new opportunities for multiresponsive molecular systems17,18 or toward orthogonal photoswitching,18−22 eliminates side reactions of harmful UV irradiation, and offers deep signal penetration into tissue2,5,7 for biological and photomedical applications.23−27 Even without complete visible light responsiveness the establishment of highly bistable photoswitching remains a challenge at the current state of the art.28 On this topic we have recently reported a hemithioindigo photoswitch that can be operated using green and red light with a thermal half-life of the metastable E isomer of 30 days at 25 °C.29 Hemiindigo is structurally related to hemithioindigo30−45 and consists of an indigo and a stilbene fragment sharing a common central double bond (Figure 1a). It is known as a chromophore for more than 100 years and was first described by Adolf von Baeyer in 1883 naming the compounds obtained from condensation of indoxylic acid and aldehydes “Indogenide”.46 Despite its facile synthesis,47,48 established spectroscopic properties,48−50 and relation to hemithioindigo30−43 and aurone,51−53 the photochemistry of hemiindigo is essentially unexplored especially with regard to applications as a photoswitch. As the absorption profile of unsubstituted hemiindigo resides partially in the visible part of the electromagnetic spectrum, the possibility of reversible double bond photoisomerization using exclusively visible light is highly tempting. However, the very few earlier investigations in this direction show a rather unfavorable photochemistry of © 2017 American Chemical Society

hemiindigo where unwanted behavior such as light induced [2 + 2] cycloadditions50,54 or triplet generation54 seem to hamper stable and efficient photoswitching. To the best of our knowledge only one such example was reported so far using a pyrole-derivative and UV/vis light for excitation.55−59 In this work we show how donor substitution of hemiindigo leads to formidable photoswitches with nearly perfect reversible switching behavior in the red part of the visible spectrum. We prepared five derivatives of hemiindigo with different amine substituents at the stilbene fragment as well as different substituents at the nitrogen atom of the indigo fragment (Figure 1) and analyzed their photophysical properties in detail. Amine substituents were used to establish a strong push (electron-donating stilbene)−pull (electron-accepting carbonyl group) character across the central double bond, which leads to a significant red shift of the absorption. These derivatives offer precise photocontrol of their geometry at wavelengths up to 565 nm in the Z to E switching direction and up to 680 nm in the E to Z direction, consistent switching behavior in different solvents, and an extraordinarily high thermal stability of the individual isomeric Z and E states. Combined with their easy access via a simple two-step synthesis, substituted hemiindigos therefore constitute a highly promising new photoswitching system for broad applications in all chemistry-related fields.

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RESULTS AND DISCUSSION Synthesis. The synthesis of hemiindigos 1 to 5 is shown in Scheme 1 and commences with a condensation reaction between commercially available indoxyl acetate and a

Received: July 19, 2017 Published: September 25, 2017 15060

DOI: 10.1021/jacs.7b07531 J. Am. Chem. Soc. 2017, 139, 15060−15067

Article

Journal of the American Chemical Society

electrophile delivers the N-propylated hemiindigos 3 and 4 in very good yields of up to 92%.61 A Buchwald−Hartwig palladium-catalyzed cross-coupling62 allows for facile installation of an aromatic substituent at the indigo-amine function and delivers hemiindigo 5 in 50% yield. Purification of the derivatives 1 to 3 was conveniently achieved by crystallization and of 4 and 5 by reversed-phase HPLC delivering very pure material. Conformations of Substituted Hemiindigos. Crystal structure analysis is provided for hemiindigos Z-1 and Z-2 as well as for E-3 and E-4 (see Figure 1b). Derivatives Z-1, E-3, and E-4 do not accumulate sterical hindrance between the indigo- and the stilbene fragment and are therefore planar structures. Consequently small torsion angles around the rotatable single bond adjacent to the double bond are found for Z-1 (8.9°), E-3 (7.5°), and E-4 (6.1°). Only Z-2 shows some more pronounced twisting of the stilbene fragment (torsion angle of 14.5°). Introducing substituents at the indoxyl-N atom larger than hydrogen should lead to strongly increased steric repulsion of the stilbene fragment in the Z isomeric state. Therefore, the Z isomers of hemiindigos Z-3 to Z-5 are not expected to be completely planar anymore. Apparently the situation is different for their E isomers, where a planar structure can be expected and is indeed found in the crystalline state of E-3 and E-4. As we have incomplete record of the detailed molecular structures of all derivatives in the Z and E isomeric state, we optimized both isomers of all five hemiindigos on the B3LYPGD3BJ/6-311G(d,p) level of theory. Additionally the polarizable continuum model (PCM) for implementation of DMSO solvent effects was employed for the optimization. For both Z and E isomers of 3 and 4 an initial conformational search has been conducted on the force field MM3* level of theory with a Monte Carlo Multiple Minimum (MCMM) search algorithm and an energy threshold of 1.91 kcal/mol in the gas phase to account for the conformationally flexible n-propyl chains. The global minima geometries were then first optimized on the MPW1K/6-311G(d,p) and subsequently reoptimized on the B3LYP-GD3BJ/6-311G(d,p) PCM(DMSO) level of theory. Frequency analysis confirmed all structures to be minimum structures since no imaginary frequencies have been found. We also provide the corresponding calculated absorption spectra

Figure 1. Hemiindigos 1 to 5 investigated in this work. (a) Molecular structures of 1 to 5. (b) Crystal structures of hemiindigos Z-1, Z-2, E3, and E-4. (c) Crystals of Z-2 showing strong red−green dichroism.

benzaldehyde.60 The protonated hemiindigo 1 and 2 are obtained in good yields of 82% and 69%, respectively. Deprotonation and substitution with n-propyl iodide as an

Scheme 1. Synthesis of Hemiindigos 1 to 5 via Condensation of Indoxyl Acetate and Different Substituted Benzaldehydesa

a

The NH-proton can easily be replaced by alkyl or aromatic substituents. 15061


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Figure 2. Extinction coefficients of both Z (bold lines) and E isomers (dotted lines) of hemiindigos 1 to 5 measured in solvents of different polarity (toluene in blue, THF in black, and DMSO in red): (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. (f) Individual isomers can be distinguished in solution by the naked eye: 1 = Z-1 (in toluene), 2 = E-1 (in toluene), 3 = Z-4 (in DMSO), and 4 = E-4 (in DMSO).

(see below in the “Theoretical Description” section and the Supporting Information for further details on the computations). Two cases can be distinguished: Close to planar geometry for both Z and E isomers (hemiindigos 1 and 2) and twisted Z but planar E isomer geometry (hemiindigos 3 to 5). For 1 and 2 the calculations show that there is a slight twist in the Z isomers around the single bond adjacent to the central double bond (14.6° for Z-1 and 14.0° for Z-2) but full planarity is achieved in the E isomeric form. The twist in Z-1 therefore appears to be a bit more pronounced than suggested by the crystal structure of (8.9°). This can be explained by the apparently non-negligible steric interactions between the NH proton and the ortho-protons of the stilbene fragment, which are overcome in part by crystal packing forces. The planarity in the E isomers is likely a result of favorable C−H···lonepair interactions between the ortho-protons of the stilbene fragment and the carbonyl moiety favoring a short distance. For the Z isomers of 3 to 5 the stilbene fragment is more severely twisted (34° for Z-3, 37° for Z-4, and 26° for Z-5). This twist however is not extremely large and should therefore

not strongly impede the conjugation across the chromophore. The corresponding E isomers of 3 to 5 are consistently found to possess a planar hemiindigo core-chromophore structure. In Z-5 the toluene residue at the indoxyl-N atom is also twisted but only with a 40° angle. In the corresponding E isomer the toluene residue at the indigo fragment shows an increased twist angle of 69°. The lesser 40° twist angle of this substituent in Z5 together with its apparent close proximity to an orthohydrogen atom of the stilbene fragment (2.70 Å) suggests favorable dispersive and CH−π interactions, which reduce the twist angle in this case. These interactions could therefore also explain the comparatively small 26° twist angle of the stilbene fragment. The theoretically obtained structures of Z-1, Z-2, E-3, and E-4 correspond very well with the experimentally determined structures in the crystalline state. In solution the Z and E isomers can be assigned clearly by NOE cross signals between protons of the indoxyl-N substituents and the double-bond proton as well as close by protons of the stilbene fragment. For the full analysis see the Supporting Information. Less information is available concerning twisting of the stilbene fragment, but several solution 15062


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Journal of the American Chemical Society Table 1. Physical and Photophysical Properties of Hemiindigo (HI) Photoswitches 1 to 5a ϕZ/E (%) (at nm)

ϕE/Z (%) (at nm)

24 ± 2 (467)

9±2 (600)

−

−

DMSO

19 ± 2 (467)

11 ± 2 (600)

toluene

−

−

THF

−

−

DMSO

16 ± 2 (467)

10 ± 2 (600)

toluene

−

−

THF

−

−

DMSO

23 ± 2 (467)

9±2 (600)

toluene

−

−

THF

−

−

DMSO

22 ± 2 (467)

7±2 (600)

toluene

−

−

THF

−

−

22 ± 2 (467)

2±1 (625)

HI

solvent

1

toluene THF

2

3

4

5

DMSO a

isomer yield in the pss ΔG* (therm. Z/E (%) (nominal LED nm) equil.) (kcal mol−1) 99% 87% 94% 89% 98% 89% 99% 83% 99% 90% 95% 81% 99% 93% 99% 93% 98% 95% 99% 95% 96% 90% 99% 98% 99% 95% 98% 86% 93% 97%

Z (617 E (470 Z (617 E (435 Z (617 E (470 Z (617 E (470 Z (617 E (470 Z (617 E (505 Z (617 E (470 Z (625 E (470 Z (617 E (470 Z (617 E (470 Z (617 E (470 Z (680 E (505 Z (617 E (470 Z (617 E (470 Z (625 E (470

nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm) nm)

ΔG* (therm. E/Z equil.) (kcal mol−1)

equilibration half-life of pure Z isomer at 25 °C

equilibration half-life of pure E isomer at 25 °C

−

24.1

−

15 h

−

24.5

−

1.2 d

−

26.9

−

70 d

−

23.8

−

8.9 h

−

22.4

−

50 min

−

25.8

−

11 d

30.3

30.5

60 a

83 a

28.0

27.8

1.2 a

0.9 a

28.2

28.7

1.7 a

4.0 a

24.5

24.6

1.2 d

1.4 d

28.3

28.6

2.0 a

3.4 a

27.7

27.8

0.7 a

0.9 a

30.5

30.5

83 a

83 a

29.5

29.5

15 a

15 a

25.4

25.7

6d

9d

Polarity (ET(30)64 /kcal mol−1) of the solvents: toluene = 33.9; THF = 37.4; DMSO = 45.1.

properties hint at a rather small torsion angle, which does not strongly impede conjugation in the chromophore. For all derivatives 1 to 5 the extinction is rather high for both isomers, which excludes a severe distortion.63 For hemiindigos 1, 2, and 5, the extinction of the S0−S1 transition is considerably higher for the Z isomers compared to their corresponding E isomers. The trend is reversed for hemiindigos 3 and 4 where the extinction of this band in the E isomers is equally high or higher compared to the Z isomers. The latter behavior hints at some conjugation impediment stemming from distortion in the Z isomeric state of 3 and 4, which is released in the E isomer. However, these are not very large effects and overall the conformations of the core-chromophore structure in solution can be regarded as not severely twisted and well conjugated, resembling those found in the crystal structures and in the theoretical descriptions. Physical and Photophysical Properties. For evaluation of the photophysical properties we measured the extinction coefficients for Z and E isomers of all hemiindigo derivatives (Figure 2). The extinctions of derivatives 2, 4, and 5 are of special interest as they are located at the longest wavelengths, but all derivatives show considerable photochromism (e.g., 55 nm for 4 in THF). The solvatochromism is comparatively small for all derivatives. The largest solvatochromic shift is observed for derivative Z-2 where the absorption maximum shifts 30 nm toward longer wavelengths when going from apolar toluene to highly polar DMSO (positive solvatochromism).

The photoisomerization was scrutinized using irradiation wavelengths between 365 and 680 nm in different solvents (see the Supporting Information for the full list of conditions tested). Irradiation was continued until the photostationary state (pss) was reached and no further changes were observed in the isomer composition. The pss with highest isomeric ratios are reported in Table 1 for hemiindigos 1 to 5 together with the respective wavelengths of irradiation. For selected wavelengths we also report quantum yields for the Z to E (ϕZ/E) and Z to E (ϕE/Z) photoisomerizations in selected solvents. As can be seen the photoisomerization of 1 to 5 proceeds efficiently and gives very high isomeric ratios in both switching directions. The photoswitching behavior is rather independent of the solvent for all derivatives, which is again in considerable contrast to many members of the related hemithioindigo photoswitch family.39−41 The photoisomerization performance of derivative 4 is particularly impressive: switching with 470 to 505 nm light gives >95% yield of the E isomer while irradiation with wavelengths up to 680 nm delivers back the Z isomer virtually quantitatively. Similar good performance but at somewhat shorter wavelengths is achieved for all other derivatives as shown in Table 1. This behavior does not change even if the nature of the solvent is changed drastically (from toluene to THF to DMSO). Additionally, we have examined the photoisomerization behavior in mixtures of H2O and small amounts of DMSO, DMF, or THF (see Supporting Information for all measured spectra). In some cases tiny 15063


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Figure 3. Photoswitching and thermal isomerization behavior of substituted hemiindigos 3 and 4 in solution. Photoisomerization between (a) Z-3 and E-3 or (b) Z-4 and E-4 in DMSO proceeds almost quantitatively in both directions using green and red light, respectively. Clear isosbestic points are observed during the photoconversion. (c) 1H NMR spectra of a 3.0 mM solution of 3 in THF-d8 at 27 °C measured after the pss is reached at 470 nm irradiation wavelength (1:93% E-3 is obtained) and 590 nm (2:99% Z-3 is obtained). (d) The thermal stability of the individual isomers of 3 and 4 remains high in solvents of vastly different polarity. Energy barriers for the thermal Z to E (solid bars) and E to Z (hollow bars) isomerizations of hemiindigo 3 (black) and 4 (gray) in different solvents (toluene-d8, THF-d8, and DMSO-d6).

both the Z and E isomers are populated thermally, the firstorder kinetics of thermal Z to E and the reverse E to Z equilibration are slightly different and can be determined separately. We found that significantly higher bistabilities are observed when substituents are present at the indoxyl-N atom (3−5) as opposed to the protonated derivatives 1 and 2. Especially high barriers were found for the thermal isomerizations of hemiindigo 3, where equilibration half-lives of up to 83 years are determined for individual isomers in solution. Changing significantly the polarity of the solvent does not change this high bistability by much, again in strong contrast to most hemithioindigo photoswitches. Hemiindigos 4 and 5 exhibit somewhat reduced barriers for their thermal isomerizations, but the obtained stabilities are still very high warranting equilibration half-lives of individual states up to several years at 25 °C (Table 1). According to these results the substitution pattern of hemiindigos 3 and 4 provide paramount property profiles (Figure 3), as they combine highly efficient photoswitching at wavelengths up to 680 nm (nominal LED nm) with very high thermal bistability of the individual isomeric states (half-lives up to 83 years at 25 °C) in solvents of vastly different polarity. Theoretical Description of the Absorption. Based on the previously optimized structures (see the Conformational Analysis section) time-dependent B3LYP-GD3BJ/6-311+G(d,p) calculations were performed to obtain UV/vis spectra for both isomers of 1 to 5. The results for hemiindigos 3 and 4 are discussed in more detail below; for all other derivatives see the Supporting Information. According to the theoretical description the Franck−Condon S1 state mainly consists of a HOMO−LUMO excitation, which is a π−π* excitation for all calculated hemiindigo photoswitches

amounts of triethylamine have to be added to increase the switching performance. In these solvent mixtures the photoisomerization is still efficient, especially for derivatives 1, 3, and 4. At the same time the absorptions appear red-shifted even further and the time needed to reach the pss is considerably shorter hinting at improved switching efficiencies. In general the isomer yield obtained under irradiation at a certain wavelength is determined by the quantum yield ϕ and the difference of the extinction coefficients of both isomers of a photoswitch. The favorable pss compositions obtained for hemiindigos 1 to 5 are the result of significant ϕZ/E and ϕE/Z in combination with good photochromism, i.e. strongly different extinctions of the two isomers in the 450−500 nm range and beyond 570 nm. This pronounced photochromism is likely to be enhanced by the conformational differences of the Z and E isomers (see the conformation discussion above), where full planarity and conjugation of the chromophore can be achieved only in the E isomeric forms. The thermal isomerization properties of hemiindigos were tested for derivatives 1 to 5. Upon prolonged heating we observed full conversion to the Z isomers for 1 and 2. The barriers for thermal conversion of E to Z isomers were found to be in the range of 22.2 to 25.5 kcal/mol for 1 and 2, which is high enough to consider these derivatives as bistable for many applications. However, an equilibrium state with both Z and E isomers being present is observed when heating solutions of 3 to 5. The latter behavior is in stark contrast to hemithioindigo for which the Z isomer is in almost all cases the thermodynamically most stable state, allowing full thermal conversion of the corresponding E isomer.40,43 For the substituted hemiindigos 3 to 5, both isomers have very similar energies and none is clearly preferred thermodynamically. As 15064


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Figure 4. Theoretical description of the electronic transitions of (a) Z-4 and (b) E-4. The main transition is a HOMO−LUMO excitation for both Z and E isomers, which can essentially be described as π−π* excitation. Orbitals and associated energies were obtained at the TD-B3LYP-GD3BJ/6311+G(d,p) PCM(DMSO) level of theory based on structures optimized at the B3LYP-GD3BJ/6-311 G(d,p) PCM(DMSO) level.

photoswitching behavior in the red region of the visible spectrum. Synthetically easily accessible hemiindigos 1 to 5 show a highly beneficiary property profile with almost quantitative Z to E and E to Z photoisomerization using blue or green and yellow to red light, respectively. They show pronounced photochromism that can be discerned by the naked eye, possess very high thermal stability of the individual isomeric states, and give sizable quantum yields for their photoreactions. This behavior is scarcely influenced by solvent polarity, which renders substituted hemiindigos into a very reliable photoswitching system. We believe that these new photoswitches are highly promising tools for very broad applications in all chemistry related fields especially in cases where damaging UV light needs to be avoided and exclusive photocontrol should be maintained without thermal deterioration of the switching states. With 680 nm responsiveness of its E isomer hemiindigo 4 reaches the biooptical window and should therefore be of special high interest to pharmaceutical and biomedicinal applications.

and isomers. Additionally, a small contribution from a HOMO−1 to LUMO excitation is also observed except for Z-4 (Figure 4). When the shape and size of the HOMOs for the E and Z isomer pairs of each hemiindigo are compared, no strong differences are found. However, the energies of the HOMOs are significantly different with a larger HOMO− LUMO gap being consistently observed for the Z isomers. By comparing the two different hemiindigos 3 and 4, a slight shift of HOMO localization toward the stilbene fragment in hemiindigo 4 can be recognized. The LUMO shapes are also very similar for Z-3, Z-4, and E‑4. They are characterized by a significant loss of electron density at the indoxyl-N atom as well as the stilbene fragment. Only E-3 differs from that LUMO distribution, as it retains a sizable contribution of the indoxyl-N atom. This LUMO is also more delocalized over the whole molecule compared to the LUMOs of Z-3, Z-4, and E-4. Energetically the LUMOs are rather similar for all isomers. Theoretical spectra were obtained from the time-dependent DFT calculations using 10 states for the description. The calculated spectra are in good agreement with the experimentally obtained ones regarding both the position of maxima and photochromicity between Z (hypsochromic absorption) and E isomers (bathochromic absorption). The observed photochromicity can essentially be traced back to a larger HOMO−LUMO gap for the Z isomers compared to the corresponding E isomers, regardless of the degree of planarization of the molecule in the different isomeric states (see details in the Supporting Information). The reasons for the different HOMO−LUMO gaps in both isomers are (1) a large stabilization of the HOMO in the Z isomers compared to the E isomers and (2) to a lesser degree a destabilization of the LUMO of the Z isomer compared to the E isomer.

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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/jacs.7b07531. Synthetic procedures for 1−5 as well as a full set of characterization data including melting points, 1H NMR, 13 C NMR, IR, (HR)MS, and elemental analyses, conformational analyses in solution and in the crystalline state, photophysical data including extinction coefficients, isomer compositions in the pss at different wavelengths, quantum yields, kinetic analyses of the interconversion of isomers at different temperatures, and computational details (PDF) X-ray crystal data for Z-1, Z-2, E-3, and E-4 (CIF)

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CONCLUSIONS In this work we have developed the photochemistry of the virtually unexplored hemiindigo chromophore and present a substitution pattern that gives access to nearly perfect 15065


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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Henry Dube: 0000-0002-5055-9924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.D. and C.P. thank the Deutsche Forschungsgemeinschaft (SFB 749, A12) for financial support. We further thank the “Fonds der Chemischen Industrie” for a Liebig fellowship, the Deutsche Forschungsgemeinschaft (DFG) for an EmmyNoether fellowship, and the Cluster of Excellence “Center for Integrated Protein Science Munich” (CIPSM) for financial support. We dedicate this work to Adolf von Baeyer (*31.10.1835 − †20.08.1917).

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