Isomerization of C=N Double Bonds

UV (14:86)

S S

MeO S

S

MeO

26M,E 26P,Z

UV (89:11)

S S

MeO S

S

MeO

∆T

∆T

26P,E 26M,Z

Scheme 28: Molecular motor 26 based on alternating isomerizations of a C=C double bond and thermal steps to achieve unidirectional rotation.^34,35

Leigh´s group used the second widely applied photochromic system in the context of molecular machines: The reversible interconversion of fumaramide to maleamide changes the binding constant of a station in a catenane rotary motor.³⁶

2.3.5.2 Isomerization of C=N Double Bonds

In contrast to stilbene, which has a high activation barrier for the thermal Z to E isomerization of 40 kcal/mol and azobenzene of which the barrier is still 23 kcal/mol, N-benzylideneaniline has a small barrier of 16-17 kcal/mol. Thus, irradiation experiments must be conducted at low temperature or on a short time scale. Irradiation of N-benzylideneaniline 27E (λ_max = 368 nm) at −140 °C results in the formation of 27Z (λmax = 306 nm, Scheme 29).⁹⁷ Changing the benzylidene ring for a pyrrole, increases the thermal barrier considerably to almost 20 kcal/mol, which results in switches that can be operated at room temperature with thermal half-lives of several seconds.⁹⁸

UV

∆T N

N

27^E 27^Z Scheme 29: Photochromism of N-benzylideneaniline 27.⁹⁷

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Photochromism does not necessarily require an aryl substituent on the nitrogen atom of the C=N double bond, as oximes and oxime ethers are known to switch as well, although there are only scattered reports in the literature. When the oxime ether 28E is irradiated with UV-light, the photoproduct 28Z forms (Scheme 30). Although no thermal half-life was given by the authors, a certain stability of Z oximes and oxime ethers is evident, as both isomers can be separated preparatively.⁹⁹

UV

UV N OMe

N OMe

28E 28Z

Scheme 30: Photochromism of oxime ether 28. The Z form is surprisingly stable.⁹⁹

In the negative photochromic 2,3-diazabutadienes (azines) both C=N double bonds can undergo an E to Z isomerization,¹⁰⁰ although it is also possible to access the EZ form photochemically, since the EE form absorbs further red then the EZ and the ZZ forms. Both Z double bonds can revert thermally, although the second isomerization is much slower, allowing to access the EZ isomer also via the thermal pathway. Azine derivative 29 yields 56% ZZ, 38% EZ, and 6% EE upon irradiation with 436 nm, although an even higher ZZ content would be expected for irradiation at around 450 nm (ΦEE to EZ = 0.02, ΦEZ to EE = 0.01, ΦEZ to ZZ = 0.01, and ΦZZ to EZ = 0.2). Utilization of 480 nm results in the exclusive formation of 29EZ. Whereas the thermal isomerization from ZZ to EZ occurs at room temperature and has a ∆G^‡ = 23.3 kcal/mol, the second reaction from EZ to EE requires temperatures

> 50 °C due to an activation barrier of ∆_G^‡ = 26.4 kcal/mol (Scheme 31).¹⁰¹

Scheme 31: Photochromism of azine 29, where the EZ isomer is directly accessible via selective irradiation of EE or sufficiently low temperature for the thermal reverse reaction from ZZ.¹⁰¹

Although there is a lot of literature about the biological activity and the metal complex formation of formazanes, their photochromic properties have also been studied. There has been a long debate about the structures of red and yellow formazane, the red ones being stable in non-protic solvents, such as benzene, and the yellow ones being stable in protic solvents like ethanol.¹⁰² Crystal structures proved the red one as a six membered ring, which is formed via a hydrogen bond.¹⁰³ Going to nonpolar solvents, the C=N bond of 30EZ (λmax = 500 nm) isomerizes under irradiation first to form the yellow all-trans-formazane 30EE (λmax = 405 nm, Scheme 32).¹⁰⁴ This one could undergo further E/Z isomerization involving four different species over all, which makes the elucidation of the photoreaction rather complex, although usually no starting material is left. The thermal back reaction follows pseudo first order kinetics and has a half-life of around 4 h at 0 °C, although it highly depends on the purity of the solvent.¹⁰⁵

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Scheme 32: Isomerization between the red (EZ) and yellow (EE) form of formazane 30.¹⁰⁴

Shifting the irradiation wavelength further to the red has been accomplished by variation of the aryl units. The blue formazane 31EZ undergoes the Z to E isomerization under 578 nm irradiation to the yellow 31EE, while the reverse reaction can be conducted either thermally or with 436 nm (Scheme 33).

The thermal back reaction proceeds faster compared to the parent 30EE and interestingly there are only two species present, suggesting that the other isomers revert even faster.¹⁰²

vis

Scheme 33: Formazane derivative 31 can be switched with 578 nm from Z to E and with 436 nm or heat from E to Z.¹⁰²

Replacing the azo group of a formazane by a carbonyl group, which can also form a hydrogen bond has a dramatic effect on the switching properties, especially as it can increase the thermal half-life (Scheme 34). The hydrazone 32Z (λmax = 398 nm) is converted to the corresponding E isomer 32E (λmax = 373 nm)

Scheme 34: Negative photochromic hydrazone based switch 32 with a thermal half-life of 2700 years at 25 °C.¹⁰⁶

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Acylhydrazones can be easily synthesized from a hydrazide and an aldehyde or ketone, allowing for the simple introduction of various substituents. Although many derivatives were found to be photochromic, including electron donating or withdrawing substituents, heterocycles, and polycyclic aromatic hydrocarbons, most of them require UV-light for both isomerization directions. The biggest effects are observed when changing the ketone/aldehyde half of the switch. In the pyridine derivative 33E (λmax = 292 nm) the photochemically produced Z form is considerably stabilized due to hydrogen bonding (Scheme 35). In the case where 2-pyrenyl is used instead of 2-pyridyl the absorption maximum of the E isomer shifts to the visible region (λmax = 403 nm) and renders the system negative

Scheme 35: P-type photochromism of an acylhydrazone 33, where the Z isomer is stabilized via a hydrogen bond.¹⁰⁷