Centennial Isomers: A Unique Fluorinated Azobenzene Macrocyclus with Dual Stability Over 120 Years

A macrocyclic azobenzene with unique thermal stability, demonstrating potential for use in optical data storage material is presented: The Z‐isomer of this novel photoswitch exhibits unparalleled thermal stability, with a thermal half‐life surpassing 120 years at 25 °C. This stability is attributed to the strategic fluorination at two ortho‐ and both para‐positions. Comparative analyses involving its non‐fluorinated counterpart, ortho‐only‐fluorinated variant, and open‐chain analog are performed. Employing NMR and UV–vis spectroscopy, X‐ray diffraction, alongside Arrhenius, Eyring, and DFT calculations, revealed insights into its extraordinary stability. Furthermore, when incorporated into poly(methylmethacrylate), this material showcase efficient switching with visible light in the solid state, emphasizing its potential for optical data storage applications.


Introduction
In the contemporary digital era, the significance of electronic data storage cannot be overstated.At the time of writing, the global digital data sphere has reached a staggering magnitude, estimated at over 44 to 59 zettabytes (ZB) in 2020, [1] with projections indicating a doubling every two years [2] -a phenomenon colloquially referred to as the "data deluge".This exponential growth, fueled by many sources, including the proliferation of internet-connected devices, cloud computing, and the advent of DOI: 10.1002/adfm.202313268artificial intelligence (AI), underscores the formidable challenge of data management and preservation.The administration of vast digital repositories transcends logistics, representing a pivotal issue resonating across sectors.Proficient data storage significantly impacts businesses, research, and socio-economic structures, underscoring the imperative of data integrity, accessibility, and sustainability. [3]raditional data storage modalities, such as magnetic hard drives [4] and solid-state drives (SSD), [5] have commendably served the purposes of data storage.Magnetic storage, with its long-established reliability and high capacity, coexists with the newer, nimble SSDs, which excel in speed and energy efficiency.However, neither is impervious to drawbacks.Magnetic storage may succumb to data degradation over time, and SSDs have finite lifespans defined by write-cycle limitations.
This confluence of data growth and the intrinsic limitations of existing storage technologies has rekindled interest in optical data storage, reviving its potential to address the data storage predicament effectively.Optical data storage (ODS), leveraging the unique properties of light, offers distinctive advantages over traditional methods.It exhibits inherent resilience to magnetic interference and holds the promise of ultra-high storage densities.For example, a study demonstrated up to 0.2 terabytes (TB) of storage capacity by employing up to 20 layers of embedded data, showcasing the potential for ultrahigh-density ODS. [6]Another perspective notes that the theoretical maximum storage density for an aberration-free objective with a high numerical aperture (NA) of 1.4 is on the order of TB per disc, indicating substantial storage potential. [7]An impressive improvement in this respect was reported using a thermally very stable dipentaerythritol pentaacrylate based monomer with up to 10 TB of data on a DVD sized disk with a record storage time of 250 years at 27 °C. [8]hese are important advances in the field, especially if one considers that most data stored is archived data that does not need erasing and re-writing.1a] However, with few exceptions [9] these are not rewritable.In order to achieve this, organic molecular switches have been used: [10] In the realm of data storage, the simplest unit of information, the "bit," operates on a binary principle, embodying two distinct values: 0 and 1.This binary foundation, which is ingrained in the digital landscape, also finds its echo in the molecular world, particularly within the context of molecular switches.Molecular switches are intricate compounds capable of transitioning between distinct structural states in response to external stimuli.Much like the binary 0 and 1 representation in classical computing, molecular switches toggle between two metastable states.
Several types of molecular switches, [11] such as spiropyrans, [12] stilbenes, [13] stiffstilbenes, [14] or diarylethenes, [15] are welldocumented in the literature.While spiropyrans offer a large spectral change upon switching, they suffer from photodegradation, which is the loss of chemical properties by extended or repeated exposure to light, and thermal instability.Therefore, they are insufficient as candidates for long-term optical storage.Much better suited are the diarylethenes: Due to the pericyclic nature of the switching mechanism, the isomers are thermally independent.Moreover, switching by visible light has become possible in some cases [16] and the first applications in the realm of data storage have been reported. [17]Another promising candidate is the "stiff-stilbene" with an astonishing half-life time of ≈1 000 years at room temperature. [14,18]18a] This is a problem for the optical read-out and also for the alignment of such molecular switches as they stay in motion and are not locked into position.Additionally, optical storage technologies at the present time are based on lasers with wavelengths of 405 nm (blue, "Blu-Ray") up to 780 nm (red, "CD"); but it also has to be said that the writeread-delete-rewrite technology may well evolve to accommodate these photoswitches as well.
In contrast, azobenzene and many azobenzene-based materials are resistant to photobleaching; in one example over 20 000 switching cycles were reported. [19]In azobenzenes, typically, the more thermally stable isomer is in the E (trans) configuration, whereas the less stable isomer is in the Z (cis) configuration. [20]These two species are typically sufficiently distinct that these molecular transitions can form the basis for encoding data, at the molecular scale.10a,b,21] Although most reports concern 2D or 3D ODS with azobenzenes, indeed, azobenzenes have been used in 4D ODS in combination with nanocrystal quantum rods in poly(methylmethacrylate). [22]Here, the writing and read-out process was based on polarization transfer from the two-photon absorbing nanocrystal quantum rod to the azodyes only in the correct orientation, but not another.Therefore, if a technology based on the reorientation of azobenzenes by repeated light absorption is required, longevity of the less stable isomer is not required per se; [23] however, as not all light that is absorbed will lead to isomerization and much energy is lost as heat, these materials warm up.The storage time in these cases is dependent on the molecular environment.21a] However, azobenzenes have never been considered as direct data storage molecules, because their thermal half-life time is prohibitively low. [24]r an optical molecular switch to serve directly as a viable candidate for data storage, i.e., without the detour of supramolecular ordering, it must fulfill a set of critical attributes: The paramount requirement, in this case, is an extended thermal half-life, ensuring the stability of the metastable isomer (most often the Zisomer) over extended periods of time. [25]This thermal stability is necessary to safeguard against data loss due to thermal relaxation-a fundamental concern in data storage.Additionally, an optical switch must exhibit well-separated UV-vis absorption spectra for its isomeric states.This spectral differentiation is instrumental in facilitating precise data readout and writing, reducing the risk of information crossover.Photostability, though not the primary focus, remains significant, especially when considering the longevity of data storage devices and their resilience to repeated read-write cycles.Furthermore, practical data storage demands compatibility with solid matrices and switchability in the solid state.Many researchers have sought to solve this issue by incorporating azobenzenes into polymers or dendrimers, which aids the controlled dilution of the dye and thus increases the penetration depth of light.The ability to function as an optical switch in a solid matrix is essential, ensuring seamless integration into storage devices while retaining its switching capability.
The issue of preparing azobenzenes with long thermal halflife times and separating the n-* absorption bands for the E and Z isomer, which should enable switching with visible light, has been addressed by a number of strategies.For example, the ethylene bridged azobenzene (diazocine) is a cyclic azobenzene, which can be switched with visible light. [26]The strain of the 8-membered ring, in which the azo group is incorporated, unusually results in a thermally stable Z-isomer.More importantly, the n-* absorption maxima of both isomers are well separated.However, the half-life times are low with 4.5 h at 28.5 °C and could not significantly increased in recent years. [27]24c] The half-life time remained low at 2.4 days in an aqueous solution.The first azobenzene type molecule achieving a longer half-life time as well as switching with visible light, was reported by Hecht and coworkers, who noted that a tetra-ortho-fluoro-azobenzene had a thermal half-life of 2 years in acetonitrile. [28] Other approaches included azobenzene derivatives, in which one of the benzene rings was replaced by pyrazoles. [30]For one compound, a halflife time of ca.2.7 years at 25 °C was reported with excellent photostability and selectivity for the respective photostationary states (PSS).The record for such molecules is currently an azopyrazole with a Z-isomer thermal half-life of ≈46 years in DMSO-d 6 . [31]e recently reported an azobenzene macrocycle (HOAM) with a half-life time of 36.4 h at 70 °C h in acetonitrile (Figure 1), [32] but early attempts to improve the thermal stability of the Z-isomer by substitution did not extend the half-life, but rather shortened it. [33]That this macrocycle cannot easily be adapted to a system with higher half-life times was also found by K. S. von Krbek and coworkers who replaced the oxygen atoms by methylene groups; this gave a half-life time of 19 h at 45 °C in DMF. [34]ased on studies demonstrating extended half-life times of Zazobenzenes with fluorine substituents, [28,35] our objective was to systematically introduce fluorine atoms into the HOAM ring.This approach aimed to combine the influence of the fluorine atoms with the inherent ring strain to develop molecular switches exhibiting prolonged half-lives.Through strategic fluorine substitutions at both the ortho and para positions of the HOAM macrocycle, can now report thermal half-lives of up to 120 years at 25 °C and ≈14 370 years at 0 °C.Such longevity positions these azobenzenes as pioneering candidates for direct long-term data storage.Furthermore, we have optimized the synthetic procedures to ensure high yield and efficacy.20d,36]

Synthesis
All three cyclic azobenzenes HOAM, [32] FOAM, and 2FOAM were obtained in three synthetic steps based on the respective 2-nitrophenol derivative (Scheme 1) after an initial Williamson ether synthesis [37] to introduce the ortho-dibenzylic ether bridge, the nitro groups were subsequently reduced with elemental iron particles under mild conditions (Bechámp reduction). [38]The resulting anilines were subjected to oxidative coupling on the surface of MnO 2 to form the azobenzene compounds under oxygen-free conditions. [39]The overall yields were 37% (HOAM), 82% (FOAM), and 84% (2FOAM).2-FOMe, [40] which was prepared as a reference compound to assess the impact of the macrocyclic architecture, was synthesized starting from 2-fluoro-4-methoxyaniline through oxidative coupling with MnO 2 under oxygen-free conditions with a yield of 72% (For further details see the Supporting Information).

Crystal Structures
The synthesized compounds could be crystallized by solvent evaporation (Et 2 O or DCM / hexane) and characterized by X-ray single crystal diffraction (XRD).To obtain the Z-isomers, the solution was irradiated with 530 nm for 1 h before crystallization (for the photochemistry of these compounds see below).
When examining the crystal structures (Table 1), it becomes clear that the ring structure in the macrocyclic molecules forces the two fluorine atoms in ortho-position to the azobridge onto the same side, irrespective of electronic or steric repulsion.This is not the case for the acyclic reference molecule FOMe: here, the fluorine atoms can avoid each other's proximity both in the Eand the Z-isomers by positioning themselves on opposite sides of the azobridge.Furthermore, the cyclic ortho-dibenzylether bridge in the E-isomers shows different inclinations toward the azobenzene.In E-HOAM, the plane of the aromatic ring in the bridge is oriented perpendicular to the plane of the azobenzene, [41] but the E-FOAM shows a tilt, whereas the E-2FOAM bridge exhibits ) The atom numbering for this molecule differs from that of the cycles as it is symmetrical in the E-form.However, it is unsymmetrical for the Z-form and to avoid confusion when comparing the bond lengths and angles, the numbers have been adjusted for the molecule; Therefore, the numbers here differ from those in the CIF-file.In general, the crystal structure of the macrocyclic Z-isomers HOAM, FOAM, and 2FOAM appear more similar to one another.However, the dihedral angle "C 01 -N 01 = N 02 -C 20 " follows the trend of 2.1°(Z-HOAM) < 6.9°(Z-FOAM) < 7.0°(Z-2FOAM) < 7.2°(Z-FOMe).This can be interpreted as increased ring strain for the Z-isomers with an additional fluorine atom in ortho-position next to the azo-bridge.This observation holds apparently irrespective of whether the switching unit is incorporated into a macrocycle or not.However, the relative change of the dihedral angles upon switching is very similar for all compounds (6.6°HOAM, 8.0°F OAM, 8.1°2FOAM, 7.2°FOMe).

Photoswitching, Photostability, and Thermal Stability of the Z-Isomers
The photoswitching properties were characterized by UV-vis spectroscopy and NMR spectroscopy.DMSO was chosen as a solvent for all measurements to enable thermal relaxation experi-ments at elevated temperatures and ensure the comparability between all measurements.All the molecules in this study exhibited photoswitching behavior upon exposure to visible light.In particular, upon exposure to green light (530 nm, 100 mW cm −2 , 5 min), the respective Z-isomers were generated, (Figure 2A, Table 1) yielding PSS 530nm values ranging from 91% (Z-FOMe) to 97% (Z-2FOAM), with the open chain reference molecule giving 83%.Reverting the switches to the E-isomers was achieved by irradiating them with blue light (415 nm, 80 mW cm −2 , 5 min), resulting in the conversion of the respective E-isomers with PSS 415nm values ranging from 74% (E-HOAM) to 93% (E-2FOAM).This results in a remarkably high switching efficiency of ≈90% for 2FOAM (77% for tetra-ortho-fluoro-azobenzene (F 4 )). [28]The UV-vis spectra for HOAM and FOMe have been discussed elsewhere; [32,40] FOAM and 2FOAM are very similar and therefore, only 2FOAM is discussed here in details (see the Supporting Information for a collection of all spectra).The UV-vis spectrum (Figure 2B) for E-2FOAM exhibited the characteristic azobenzene -* absorption band [42] at 325 nm (E-HOAM: 290 nm, E-FOAM: 300 nm, E-FOMe: 300 nm, F 4 : [29] 305 nm) with an emission coefficient of  325nm = 8 871 M -1 cm -1 (E-FOAM  300nm = 10 604 M -1 cm -1 ), which is significantly lower in comparison to F 4 ( 305nm = 18 000 M -1 cm -1 ). [29]Furthermore, a secondary absorption band, corresponding to the n-* transition was observed at ≈415 nm for Z-2FOAM and 465 nm for E2FOAM, characteristic of ortho-substituted azobenzenes (Z-F 4 : 414 nm, E-F 4 : 456 nm). [29]These n-* bands are very well separated from each other, with a difference of their maxima of Δ50 nm (F 4 : Δ = 42 nm). [29]This feature not only enables photoswitching using visible light through the n-* absorption band, but it also makes the two isomers discernible with the naked eye (Figure 1B).Because the photostability is a vital process parameter for potential ODS materials, the photostability was tested with over 35 successive irradiation cycles between both isomers (irradiation wavelengths of 530 and 415 nm).With a concentration of 0.0936 μm mL −1 and using LEDs with the power of 50 mW cm −2 (530 nm) and 30 mW cm -2 (415 nm), only minor photodegradation was observed with a relative change of 0.5% for green irradiation and 1% for blue irradiation (Figure 3B).

The Increased Longevity of Z-OAMs
Based on the long thermal half-life times (t 1/2 ) of the Z-HOAM, we hypothesized that for the fluorinated macrocycles, the half-life times for the Z-isomers might be even longer.In order to ensure complete comparability between the thermal half-life times of Z-FOMe, Z-HOAM, Z-FOAM, and Z-2FOAM, all measurements were performed in DMSO as the solvent, with five measurements of the t 1/2 at elevated temperatures between 90 °C and 110 °C (Figure 4).
The temperature of 25 °C ("room temperature) is particularly relevant in terms of the material´s applicability.Ideally, an ODS device should be able to operate at this temperature.In addition, half-life times of thermally long-lived switches are provided at many different temperatures in the literature, but this temperature is often the point of reference.Therefore, the half-life time for this temperature was extrapolated with the aid of the Arrhenius Equation (1)(Figure 4A).
Where k is the rate constant, A the Arrhenius factor, E a the activation energy, R the universal gas constant and T the Temperature.
For instance, the macrocyclic compound Z-HOAM had a thermal half-life t 1/2(25°C) = 3.8 years.In contrast, introducing fluorine atoms in the ortho-position (Z-FOAM) increased the value to t 1/2(25°C) = 106 years.By introducing two additional fluorine atoms in the para-position, the thermal half-life time further increased to t 1/2(25°C) = 120 years for Z-2FOAM -a value unprecedented in the literature.The confinement of the azobenzene moiety in a macrocycle appeared to be the most relevant structural feature: Without it, the half-life decreased dramatically by a factor of ca.  2) and the plots derived from it contain additional and mechanistically relevant information (Table 2).
Where  is the transmission coefficient, k B the Boltzmann constant, T the temperature, h the Planck´s constant, ΔG ‡ the Gibbs energy of activation.The Eyring equation allows to assess the contribution of the enthalpy, entropy, and Gibbs free energy of the transition states.In the case of azobenzenes, these are indicative of the isomerization mechanism: Azobenzene can undergo thermal-and photoisomerization through various mechanisms.Commonly proposed mechanisms for thermal isomerization include rotational, inversion, inversion-assisted rotation, and concerted inversion (Figure 5). [29,40,43]Inversion mechanisms have a characteristic angle inversion angle (C─N═N, )  1 ≈120°and  2 ≈180°and a dihedral angle (C─N═N─C, ф) of ф ≈180°(in plane), while inversion-assisted rotation has additional rotation around the dihedral angle ф ≈90°(out of plane).24b] In contrast, thermal relaxation does not involve excitation and takes place in the ground state S 0 PES.This process requires overcoming a transition state (TS).Currently, for the parent azobenzene the rotational pathway is considered to be the preferred one. [44]However, in macrocyclic compounds, this pathway might be sterically less accessible, and the inversion pathway is favored as described by Rau et al. [45] To better understand the prolonged half-live observed in FOAM and 2FOAM, we conducted a DFT calculation using B3LYP/cc-pVDZ level of theory (Table 3). [46]We simulated the solvent DMSO (consistent with the experiment) using the solvation model based on density (SMD) in a self-consistent reaction field (SCRF) and applied Grimme D3-correction [47] to include dispersion effects.For the ground state, only positive frequencies were obtained, and for the TS, only one imaginary frequency.For the cyclic compounds, a TS was found, which lies in between the inversion-assisted rotation (IAR) mechanism and the inversion mechanism.For TS-2FOAM the C─N═N angle was  1 = 178°,  2 = 120°, while the dihedral angle (C─N═N─C) was ф = 123°.This dihedral angle lies between the values for a clear inversion mechanism (ф = 180°), and a clear inversion-assisted rotation (ф = 90°).This observation can be made for all OAM´s.The imaginary frequency of each TS was in the range of −500 to −250 cm −1, which is similar to the inversion mechanism as observed by Lopez et al.Based on these calculations and together with the observation that the relative change in the dihedral angles from Z to E is almost the same for all four compounds, we hypothesize that a change in ring strain does not play a major role for the different half-life times.Rather, a high dihedral angle in the Z-isomer in the macrocycles is blocking the rotational movement, which in turn leads to higher energy TSs and longer half-life times.At the same time, the energies of the Z-ground states of the FOAMs are relatively decreased (compared to Z-HOAM).
In summary, the calculations support our hypothesis regarding the increased stability of the Z-FOAM´s, emphasizing the importance of the fluorination in ortho-position and the macrocyclic structure for the long-lived Z-isomer(Table 4).

Switching in the Solid State
The new azobenzene macrocycles have very promising properties for ODS.20d,36] This was showcased by dispersing the molecule 2FOAM, as the most promising candidate into PMMA (polymethyl methacrylate) (Figure 7).To achieve this, PMMA (500 mg) and 2FOAM (50 mg) were dissolved in toluene, filtered through a syringe filter, and then cast into a petri dish, allowing the solvent to slowly evaporate under ambient conditions.The resultant PMMA-2FOAM composite, now tinged orange due to the presence of 2FOAM, remained optically clear (Figure 6).The thickness of the film was measured using an electronic caliper and was 100 ± 15 μm.This polymer film was subjected to multiple cycles of irradiation with blue (415 nm, 20 mW cm -2 ) and green light (530 nm, 25 mW cm −2 ), enabling the writing and erasing of "data" on the polymer.As an illustrative example, we utilized light to create the well-known motif of the "Bremer Stadtmusikanten" (the "town musicians of Bremen") [49] in the polymer film by using a mask when irradiatingthe film.To demonstrate the efficacy of the approach, samples with lower concentrations 2FOAM (2 mg and 5 mg) in a PMMA matrix (500 mg) were also prepared, highlighting the pronounced switchability and thermal relaxation characteristics of 2FOAM within the solid PMMA matrix, as demonstrated by UV-vis spectroscopy (Figure 7).The thermal half-life of 2FOAM in the PMMA composition was determined at 90 °C by the lowest concentrated sample (2 mg in 500 mg PMMA) after irradiation with green light.The polymer was glued inside a cuvette with glycerin (non-solvent) and heated to 90 °C (below the glass transition temperature (T g ) of PMMA) for 4 days.The thermal half-life was determined to be t 1/2, PMMA, 90°C = 69 h (Figure S15, Supporting Information), which is faster than in the solvent DMSO t 1/2, PMMA, 90°C = 91 h and contrary to the belief that the half-life time in a polymer matrix below the T g is prolonged. [50]However, covalently bonded 2FOAM to the PMMA matrix, higher concentrations, and different polymer blends need to be further studied to fully understand the behavior of 2FOAM in polymer blends.
Table 3. Calculated results of HOAM, FOAM, 2FOAM, and FOMe for the rate constant (k), the Arrhenius factor (A), the activation energy (E a ), Enthalphy (ΔH ‡ ), Entropy (ΔS ‡ ), Gibbs-Energy (ΔG ‡ ) and the half-life times for the isomerization of the respective Z-isomers at room temperature (t 1/2 (25 °C)).Table 4. Results of the DFT calculations with B3LYP/cc-pVTZ level of theory and Grimme D3-dispersion correction in the gas phase and in solution (DMSO).For the solvent simulation, a self-consistent reaction field (SCRF) with the SMD model for the solvent DMSO, the B3LYP/cc-pVDZ level of theory, and Grimme D3-dispersion correction were used. [48]The calculated TS shows an IAR mechanism.The given energy values are the difference to the respective Z-isomer.36e,53]

Conclusion
Optical data storage that can be written, read, erased and re-written is likely to impact heavily on data storage technologies.One approach is using molecular switches, but for the development of a direct writing -erasing -re-writing system, light should be the only stimulus.Azobenzenes, although very photostable, suffer from too short of thermal half-life times.Here, the synthesis of an extremely thermally stable macrocyclic azobenzene compound is reported: The extrapolated half-life values span the range of a human life-time and even exceed it when cooled (Z-2FOAM: t 1/2 , 0 °C ≈14 370 years).This is unprecedented in the literature.Through comprehensive analysis encompassing NMR, UV-vis, and XRD studies on four distinct azobenzenes in both isomeric forms, we have established that the combined influence of the macrocyclic structure and ortho-fluoro-substitution is crucial for achieving such exceptional stability.Further fluorination in the para-position leads to even higher half-life times.A key mechanistic feature is the difficulty in accessing the TS for a thermal relaxation due to geometric constraints.This prolonged half-life time, coupled with efficient and safe photoconversion using visible light, underscores the effectiveness of our molecule as a highly viable switch.This bi-stability holds potential for versatile I/O switching applications in data storage, but it may also become useful biological, physical, and materials science as a form of information storage.

Experimental Section
Only the key equipment, methods, and experimental procedures are listed.For more details see the Supporting Information.
Equipment for the Irradiation Experiments: A UPLED power supply powered the high-power LED modules from Thorlabs (US, NJ): M365L3 (365 nm), M415L4 (415 nm), M530L4 (530 nm), M590L4 (590 nm).A collimator was used to focus the light beam.The light intensity was measured using an ILT2400 hand-held light meter manufactured by International Light Technologies, Inc., (Peabody, USA).Additionally, a 420 nm (1.0 W) and a 525 nm LED (1.0 W) from EPILED were built in laboratories.
UV-vis spectra were recorded with a resolution of 0.5-0.1 nm on a UV-2700 spectrometer from Shimadzu (Shimadzu, Kyoto, Japan) with a double monochromator.In all cases, DMSO (spectroscopy grade) was used as Figure 6.Thermal relaxation of Z−2FOAM.The computational method was B3LYP/cc-pVTZ level of theory including the Grimme D3 [47] correction for the gas phase.B3LYP/cc-pVTDZ level of theory and SCRF with SMD correction for the solvent DMSO and Grimme D3 [47] correction was used for the solvent calculation.The data is referenced on the Z-isomer.Shown is the IAR pathway for Z-2FOAM toward E-2FOAM via the TS.In the TS, the vibronic imaginary frequency is shown shaded.For the sake of brevity, only the synthesis of 2FOAM is presented here.For all other compounds see the Supporting Information.

Synthesis of 2,2′-((1,2-Phenylenebis(methylene))bis(oxy))dianiline (S6):
Iron powder (4.41 g, 78.9 mmol, 10.0 equiv.) was added portion-wise to a solution of S5 (3.57g, 7.89 mmol, 1.00 equiv.) in MeOH (100 mL).Subsequently, HCl (1 m, 5 mL) was slowly added to the reaction.The reaction mixture was heated to 90 °C for 70 min.After cooling down the mixture, it was filtered.The solid residue was washed with ethyl acetate (100 mL).The combined organic layers were washed with water (3 × 200 mL) and brine (1 × 100 mL).The organic layer was dried over MgSO 4 and concentrated under reduced pressure.The crude product was purified by column chromatography (ethyl acetate : n-hexane, 2 : 1) to obtain the product as a white solid, which was not stable under ambient conditions and should be used immediately or stored under nitrogen in the dark (3.08 g, 7.85 mmol, 99%).(734 mg, 1.87 mmol, 1.00 equiv.) was dissolved in toluene (30 mL).A stream of nitrogen was passed through the solution for 1 h.MnO 2 (1.30 g, 15.0 mmol, 8.00 equiv.) was added and the suspension was heated to 110 °C for 48 h.The hot reaction mixture was filtered through celite and the celite rinsed with chloroform (2 x 30 mL).The filtrate was evaporated to dryness under reduced pressure, and the residue was purified by column chromatography (DCM/n-hexane, 2:1) to obtain final compound (625 mg, 1.61 mmol, 86%) as a yellow solid.

Figure 1 .
Figure 1.Overview of the synthesized cyclic azobenzene compounds.FOMe serves as an open-chain analogue, to gauge the influence of ring strain on the system.

a
more planar orientation with respect to the azobenzene.The inclination is evident in the dihedral angle labeled "C 01 -N 01 = N 02 -C 20 " (Ψ) and the twist angle of the azobenzene aromatic rings.The twist is measured by the angle between the two layers, which is determined by the positions of the atoms in the azobenzene's phenyl ring.(Graphical visualization see Supporting Information) E-2FOAM exhibits an arrangement of the azobenzene motif that is the closest to the planar parent molecule azobenzene (180°).The measured dihedral angles C 01 -N 01 = N 02 -C 20 (Ψ) are 175.5°(E-HOAM)< 178.9 (E-FOAM) = 178.9°(E-2FOAM)< 180°( E-FOMe) and the twist angles of the azobenzene aromatic rings are 24.9°(E-HOAM)> 5.3°(E-FOAM) > 4.1°(E-2FOAM).

Figure 5 .
Figure 5. Graphical visualization of the possible mechanisms for the thermal relaxation with remarkedly changes in angles during the back-isomerization for the inversion, inversion assisted rotation, rotation, and concerted inversion mechanism.

Figure 7 .
Figure 7. UV-vis spectra of 2FOAM immersed in PMMA (conc.5 mg in 500 mg PMMA) after irradiation with blue light (415 nm), green light (530 nm), and thermal treatment of the Z-isomer at 90 °C for 1 day and 4 days.An image of the "Bremer Stadtmusikanten" was produced by irradiation on a 2FOAM/PMMA polymer mixture (conc.50 mg in 500 mg PMMA) with green light using a mask.The picture can be erased by blue light and the composite irradiates with green light again to program a new picture.

Table 2 .
Summary of 1 H-NMR spectroscopic studies of the irradiation of HOAM, FOAM, 2FOAM, and FOMe, after irradiation with 530 nm and 415 nm in the solvent DMSO-d 6 .The % values for the E-and Z-isomer were obtained by the integration of the CH 2 -proton signals of the ether bridge (conc = 7.55 μmol mL -1 ).