Facile synthesis of azobenzene‐embedded conjugated macrocycles for optically switchable single‐crystal transistors and tunable supramolecular assemblies

A series of new π‐conjugated macrocycles (AzoM‐n‐E, n = 1–3) incorporating azobenzene units have been synthesized by a facile strategy in one‐pot reaction. The resultant azobenzene‐embedded macrocycles feature intrinsic photoresponsive behaviors and intriguing supramolecular assembling properties. The smallest macrocycle AzoM‐1‐E with a rigid planar conjugated backbone structure is used to prepare the single crystal transistors, showing reversible optical tunability. The moderate size macrocycle AzoM‐2‐E assembles into a dimer in the form of interpenetration through π‐π stacking between azobenzene units. The largest macrocycle AzoM‐3‐E with enhanced flexibility can adaptively assemble with various types of electron‐deficient guests accompanied by distortion of azobenzene. Typically, AzoM‐3‐E assembles with the planar F4‐TCNQ to form a tetragonal geometry by C‐F···π and π‐π interactions, while the assembly with ellipsoidal C70 via π‐π interactions induces AzoM‐3‐E to form a boat‐shaped geometry. This work will shed new light on the development of functional conjugated macrocycles in organic electronics.

In organic optoelectronic systems, photo regulation of electronic structure properties of molecule is of great value for the development of functional organic electronic materials. [39,40][43] While the macrocyclic host molecules with inherent photo response are relatively limited.The incorporation of azobenzene into macrocyclic scaffolds is an effective strategy in constructing the photo-responsive host molecules for the light-controlled self-assembly in supramolecular chemistry. [44]Recently, a series of azobenzene macrocycles with different structures have been reported, and the encapsulation and release of optically controlled guest molecules have been successfully realized. [45]Yuan et. [46]designed H-bonded azobenzene (azo)-macrocycles that can precisely control the quantitative release and capture for bipyridinium guests.Huang et. [47]ynthesized a series of azobenzene-based macrocyclic arenes with light-controlled molecular encapsulation and release via a fragment-cyclization method.These studies have greatly enriched the application of azo-macrocycles in the controllable supramolecular assembly.However, there are still rare reports on exploring the application of azo-macrocycles in organic electronics.
To expand the application of azo-macrocycles in organic electronics, we incorporated azobenzene into the conjugated skeleton of macrocycles through a facile one-pot reaction to synthesize a series of macrocycles with gradually increasing size, AzoM-1-E, AzoM-2-E, and AzoM-3-E (Figure 1).The structure and spatial conformation of these macrocycles are confirmed by the X-ray crystallographic analysis.The planar rigid conformation of AzoM-1-E with small size makes it easy to form single crystals through strong intermolecular π-π stacking and facilitates the transport of charge carriers between molecules, which shows reversible photoresponsive properties when used as organic semiconductors in single crystal organic field-effect transistors (OFETs) devices.The drain-source current of single crystal device increases by 13.9 times and the charge mobility increases by three times after the irradiation with Ultraviolet-visible (UV) light for 1 min, which can be restored gradually when placed in dark.The reversible photo regulation cycles can be conducted four times without obvious degradation of performance.What's more, the other two larger homologues AzoM-2-E and AzoM-3-E adopt flexible twisted geometries and are suitable for supramolecular assemblies.In the solid, the two AzoM-2-E molecules assemble into a dimer in the form of interpenetration through π-π stacking between azobenzene units.The largest macrocycle AzoM-3-E with more flexibility can accommodate various types of electron-deficient guests adaptively.Typically, AzoM-3-E forms a tetragonal geometry by C-F⋅⋅⋅π and π-π interactions between F4-TCNQ and electron-rich phenanthrene units.When assembled with ellipsoidal C 70 via π-π interactions, AzoM-3-E converts to a boat-shaped geometry.The distortions of dihedral angle between the two benzene rings of azobenzene groups are contributed by these supramolecular assemblies, which shows adaptability of azobenzene in π-π interactions.

Synthesis
The macrocycles AzoM-1-E, AzoM-2-E, and AzoM-3-E were synthesized by a simple one-pot Suzuki cross-coupling reaction using Buchwald Pd-precatalyst as shown in Scheme S1.The reaction was carried out at 55 • C for 24 h, the residue was first purified by a short column chromatography.Then macrocycles AzoM-1-E, AzoM-2-E, and AzoM-3-E were further purified and obtained by preparative GPC using CHCl 3 at a rate of 16 mL/min with the yields of 21.6%, 8.2%, and 3.0%, respectively.The chemical structures of three macrocycles were confirmed by nuclear magnetic resonance (NMR) spectroscopy, high-resolution electrospray ionization mass spectrometry and single crystal X-ray diffraction analysis.

Crystal structure and photoresponsive semiconductor performance of AzoM-1-E
Single crystals of AzoM-1-E were easily obtained by slow diffusion from chloroform to methanol/acetonitrile in bottles within 2 days.X-ray crystallographic analysis reveals that whole skeleton of AzoM-1-E adopts a plane conformation with the azobenzene units from the side view (Figure 2A), which makes it easy to obtain single crystals.The planar skeleton of AzoM-1-E is stacked in parallel along the same direction and arranged in the cross arrangement.The interplanar distance between adjacent parallel molecules is 3.43 Å (Figure 2B), which indicates that the parallel molecules are stacked through π-π interactions.Moreover, the close interplanar distance with 3.43 Å based on π-π stacking is conducive to intermolecular carrier transport in single crystal OFETs. [48]Besides, the C-H⋅⋅⋅π interactions with the distances of 2.46 Å and 2.70 Å between the ends of cross arranged azobenzene units and adjacent molecular planes were observed (Figure 2B).The intermolecular π-π stacking The photoresponsive behavior of AzoM-1-E in solution was investigated by UV-Vis absorption spectra and 1 H NMR spectra.The absorption intensity of AzoM-1-E at 330 nm decreased by 30% and slightly increased slightly at 440 nm after UV irradiation (365 nm, 120 mW/cm 2 ) for 30 s, and it almost completely recovered to the original absorption intensity with further visible light irradiation (420 nm, 130 mW/cm 2 ) for 30 s (Figure S3a).The absorption variation indicates the reversible conversion of azobenzene groups between trans-and cis-form.In addition, the photoisomerization of the azobenzene group led to the shift of the 1 H NMR signal peak in the aromatic region after AzoM-1-E was exposed to UV light for 1 min, and the photoisomerization efficiency of azobenzene was 77% according to integral ratio of signal peak.Further, the restoration ratio was 61% after exposure to 420 nm visible light for 2 min.While the signal peak could be completely recovered after being placed in the dark for 1 day (Figure S7).Similarly, the UV-Vis spectra of AzoM-1-E crystal also shows variations after alternating treatment with UV irradiation and the dark environment (Figure 3A).The absorption intensity at 343 nm decreased by 15% and slightly increased at 267 nm after UV irradiation (365 nm, 120 mW/cm 2 ) for 1.0 min, and continuous irradiation caused no significant change in the absorption intensity, indicating that the photostationary appeared.Further, the absorption intensity could be completely restored by being kept in the dark for 4 h.These variations in absorption spectra represent the configuration transformation of azobenzene group between trans-form and cis-form.These reversible spectral changes could be repeated for several cycles with almost no significant changes in absorption intensity, which are shown in Figure 3B.While visible light irradiation (420 and 470 nm) caused no obvious recovery of absorption intensity, which was attributed to the compact molecular packing, resulting in almost no significant difference in weak absorption (in the range 400-550 nm) caused by the n-π* transition bands of trans-and cis-azobenzene.To further investigate the conversion rate of crystals under UV irradiation, AzoM-1-E crystals irradiated by UV light were dissolved in CDCl 3 , the 1 H NMR spectra was compared with sample before irradiation.As shown in Figure S6, the light conversion efficiency of crystal is only 18%, which is lower than the efficiency of 77% in solution.
The single crystals of AzoM-1-E were grown on octyltrichlorosilane-modified SiO 2 /Si substrates by drop-casting methods as organic semiconductor for fabrication of optically tunable OFETs.The polarized optical microscopy images of AzoM-1-E single crystals were shown in Figure 4B.The bottom gate/top contact OFETs were fabricated according to "gold strips" methods (Figure 4A).The single crystals of AzoM-1-E exhibit p-type charge transporting behavior with a highest hole mobility of 0.013 cm 2 /V s (Figure S13).The optically tunable behaviors for FETs were explored by the change of transfer curves on UV irradiation or placed in the dark for certain periods.As shown in Figure 4D, the transfer curves had changed significantly on UV irradiation for 50s, and the threshold voltage (V TH ) moved in a positive direction.Specifically, the device current I DS (V G = V DS = −70 V) was increased by 13.9 times (from 3.88 × 10 −9 to 5.4 × 10 −8 A) after exposure to UV light for 50 s.And, the transfer curve was restored by placing in the dark for 4 h, and I DS reverted to 4.1 × 10 −9 A. Such reversible variation of transfer curves and I DS can be repeated for four cycles without fatigue upon the successive UV light irradiation and placed in the dark.In addition, the charge carrier mobility of AzoM-1-E single crystal was increased by three times (from 1.2 × 10 −3 to 3.6 × 10 −3 cm 2 /V s) after UV light irradiation for 50 s, and it reverted by placed in the dark (Figure 4C).Such reversible variation of charge carrier mobility was repeated for four cycles upon the successive UV light irradiation and placed in the dark.Due to the damage of continuous UV light and current to single crystals, the performance of the device began to decline after about four cycles.

Crystal structures of AzoM-2-E and AzoM-3-E
The single crystals of AzoM-2-E were easily grown by slow diffusion from chloroform to methanol/acetonitrile in bottles within 3 days.As shown in Figure 5A, the moderate size macrocycle AzoM-2-E is distorted to a certain extent for the crowded spatial structure and assembles into a dimer in the form of interpenetration through π-π stacking between azobenzene units.And, the π-π stacking between interpenetrating azobenzene is shown in the inset.This self-assembly behavior also causes a large distortion of azobenzene group from the original plane conformation, in which the dihedral angles between the two benzene rings of interpenetrating azobenzene group are 33.88 • .And the dihedral angles of the other two azobenzene are 8.72 • and 10.41 • respectively.These distortions indicate that the conformation of azobenzene has certain adaptabilities according to the self-assembly.The crystal packing of supramolecular dimers is also shown in Figure 5B.
Due to the more distorted and flexible conformations compared with AzoM-2-E, the single crystals of AzoM-3-E were not obtained after repeated attempts under various conditions.While the flexible cavity of AzoM-3-E can accommodate various types of electron-deficient guests, and two representative co-crystals of AzoM-3-E with two electron-deficient guests, planar F4-TCNQ and ellipsoidal C 70 , were easily obtained by slowly diffusing from carbon disulfide solution to n-hexane.As shown in Figure 5C, in the co-crystals of AzoM-3-E and F4-TCNQ, the macrocycle presents a symmetrical tetragonal geometry.The planar guest F4-TCNQ is fixed in the middle of two phenanthrene groups in the opposite position of AzoM-3-E and a "sandwich" structure is formed.The plane of F4-TCNQ is approximately parallel to the plane of phenanthrenes, and the distances from F atoms to the plane of two phenanthrenes are 3.01 Å and 2.97 Å respectively, which indicates a strong C-F⋅⋅⋅π interactions.It is the interwoven noncovalent interactions that limits the distortion of AzoM-3-E in this direction.What's more, the other two phenanthrene groups in the opposite position of AzoM-3-E also approach each other through folding, forming a similar conformation in the other direction.Through the intermolecular interaction between the two phenanthrene groups and other molecules, the F4-TCNQ⊂AzoM-3-E forms a stable and orderly molecular arrangement structure with a plane spacing of 3.75 Å (Figure 5D).In addition to F4-TCNQ, fullerene C 70 is also a common guest with unique ellipsoid geometry.
The existence of many π electrons on the surface of C 70 makes it exhibit a strong π-π interactions with the aro-

Supramolecular assembly of AzoM-2-E and AzoM-3-E
To further investigate the supramolecular assembly behaviors of AzoM-2-E and AzoM-3-E, several electron-deficient guests with similar conformation (ellipsoidal molecules C 70 and C 60 , planar molecules F4-TCNQ, TCNQ and 4F-TTN) are selected to compare the binding constants by 1 H NMR titration.As shown in Figure 6A, by gradually adding F4-TCNQ to the AzoM-3-E solution, the aromatic protons moved significantly.With the addition of F4-TCNQ, the peak of proton a on the inner side of the ring exhibited an obvious upfield shift, and the peak gradually widened within 1.0 eq.When continuously added 2.0 eq., the peak continued to shift upfield, while the peak shape gradually recovered.The aromatic protons b, c, and e showed similar changes.Meanwhile, the peaks of some aromatic protons d, f, g, h, were also shifted downfield.These variations indicate supramolecular interactions between AzoM-3-E and F4-TCNQ.By fitting the 1 H NMR titration data, the association constant K a was calculated to be 3.1 × 10 3 M −1 (323 K) (Figure 6B), with a 1:1 binding model (Figure S26).Similarly, 1 H NMR titration experiments were performed to study supramolecular interactions between AzoM-3-E and C 70 (Figure S32).Evidently, the binding ability of C 70 with AzoM-3-E is slightly weaker compared with F4-TCNQ, and the association constant K a was calculated to be 7.89 × 10 2 M −1 (323 K) (Figure S33).Through 1 H NMR titration experiments, it is found that the binding constants of TCNQ (Figure S27, S28) and 4F-TTN (Figure S34, S35) with AzoM-3-E, which have the similar configuration to F4-TCNQ, decrease successively, indicating that the binding constant is related to the relative electrostatic attraction strength between the electron-deficient guest and host (Table 1).On the other hand, C 60 has a slightly stronger supramolecular interaction with AzoM-3-E than C 70 with a similar conformation.For comparison, we also studied the binding ability of AzoM-2-E to these guest molecules by 1 H NMR titration and the association constants are shown in Table 1.According to these data, the binding constants of AzoM-2-E and these electron-deficient guests are slightly smaller than AzoM-3-E, indicating that AzoM-3-E with larger flexible cavity has stronger binding ability to these guests.These results reflect the size-dependent properties of macrocycle AzoM-2-E and AzoM-3-E on the host-guest assemblies.the intermolecular dislocation stacking mode (Figure S2a) from the perspective of electrical distribution.In contrast, the macrocycle inner skeletons and cavity of AzoM-2-E and AzoM-3-E tend to show negative and neutral charge.
Obviously, photoisomerization of AzoM-2-E leads to smaller cavities and dense negative charge distribution (Figure 7C), which indicates the application potential of AzoM-2-E in reversible binding and release through light regulation.While the cavity size and electrical distribution of AzoM-3-E change slightly after photoisomerization (Figure 7D).However, AzoM-3-E shows the better supramolecular assembly performance because of its larger cavity and adaptive flexible configuration.
The thermodynamic stability calculations were used to further analyze the supramolecular assembly ability of AzoM-3-E with C 70 and F4-TCNQ, respectively.The lower binding energy represents more stable molecular configuration and stronger binding ability.As shown in Figure 7A, the energy (ΔE), enthalpy (ΔH), and Gibbs free energy (ΔG) of assembled complex C 70 ⊂AzoM-3-E compared with the monomer AzoM-3-E are −0.124eV, −0.124 eV, −0.053 eV, respectively, which indicates the strong binding ability between AzoM-3-E and C 70 .By comparation, the ΔE, ΔH, and ΔG of assembled complex F4-TCNQ⊂AzoM-3-E are −0.522eV, −0.522 eV, −0.327 eV, respectively.The lower binding energy indicates that F4-TCNQ is easier to assemble and form stable complexes with AzoM-3-E than that of C 70 .These results are completely consistent with the experimental trends of the binding constant obtained by 1 H NMR titration, and the bind constant of AzoM-3-E and F4-TCNQ (3.1 × 10 3 ) is much greater than that of AzoM-3-E and C 70 (7.9× 10 2 ).

CONCLUSION
In summary, we synthesized a series of π-conjugated azomacrocycles AzoM-1-E, AzoM-2-E, and AzoM-3-E with gradually increasing sizes through a facile strategy in one-pot reaction using Buchwald Pd-precatalyst.These macrocycles featuring intrinsic photoresponsive behaviors were explored by UV-Vis absorption spectra, NMR, DFT calculations.According to the X-ray single crystal analysis, the smallest macrocycle AzoM-1-E has a completely planar conformation with rigid backbone, which is profited to form intermolecular π-π stacking and C-H⋅⋅⋅π interaction.Benefiting from its ordered π-π stacking and intrinsic photoresponsive properties, the single crystal field effect transistor devices based on AzoM-1-E show reversible optical switchable property.The drain-source current increases by 13.9 times and charge mobility increases by three times after irradiation with UV light for 1 min, which can be gradually restored by placing the device in dark.The single crystal analysis reveals that the moderate size AzoM-2-E with flexible twisted geometries forms a dimer by interpenetrating assembly of azobenzene groups based on π-π interactions.Due to the enlarged ring size, the largest macrocycle AzoM-3-E is more flexible, which can adaptively assemble with various types of electron-deficient guests.The cocrystal analysis shows that the planar electron-deficient F4-TCNQ assembles with AzoM-3-E to form a tetragonal geometry by C-F⋅⋅⋅π and π-π interactions between F4-TCNQ and electron-rich phenanthrene units, while the ellipsoidal C 70 assembles with AzoM-3-E to form a boat-shaped geometry via π-π interactions.In addition, the angles of azobenzene groups are distorted with the changes of the AzoM-2-E and AzoM-3-E conformations caused by supramolecular assembly, which reflect the flexible adaptability of azobenzene groups.To further explore the supramolecular assembly behaviors of AzoM-2-E and AzoM-3-E, 1 H NMR titrimetric experiments were conducted to compare the binding constants with several electron-deficient guests with similar conformation.This work provides significant opportunities for the development of novel functional macrocycles based on conformation design for the application in organic electronics and supramolecular assemblies.
[CCDC contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.

F
I G U R E 2 (A) Crystal structures (top and side views) of AzoM-1-E.(B) Crystal packing structures of AzoM-1-E.Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawing at 50% probability for ellipsoids.Alkyl chains are omitted for clarity.

F
I G U R E 3 (A) Absorption spectra of AzoM-1-E crystal under alternating irradiation with UV (365 nm, 120 mW/cm 2 ) for 1 min and standing in dark for 4 h.(B) The changes in absorption of AzoM-1-E crystal at 343 nm under alternating irradiation with UV and standing in dark.and C-H⋅⋅⋅π interactions result in the ordered arrangement of AzoM-1-E.

F
I G U R E 4 (A) Schematic of AzoM-1-E single crystal organic field-effect transistor (OFET) devices fabrication and UV irradiation process with "gold strips" method.(B) Polarized optical microscopy images of the AzoM-1-E single crystal with rotations of 0 • , 45 • , 90 • .(C) The reversible modulation of I DS (V G = V DS = −70 V) (red) and charge mobility (blue) and (d) I DS -V G transfer curves for four cycles after alternating UV irradiation (1 min) and dark placing (4 h).

F
I G U R E 5 (A) X-ray crystal structure of twisted macrocycle AzoM-2-E and dimer formed by mutual interpenetrating assembly of azobenzene on AzoM-2-E.(B) Crystal packing of AzoM-2-E.(C) Top view of X-ray crystal structure of the complex F4-TCNQ⊂AzoM-3-E.(D) Two adjacent complex structure.(E) Crystal packing.(F) Top view and (G) side view of X-ray crystal structure of the complex C 70 ⊂AzoM-3-E, (H) Crystal packing.ORTEP drawing at 50% probability for ellipsoids.The hydrogen atoms are omitted for clarity.
matic system.According to the cocrystal C 70 ⊂AzoM-3-E, C 70 (blue) molecules is assembled into the cavity of AzoM-3-E, while the other two C 70 (purple) are filled among the molecules.As shown in Figure 5F, the distances between the F I G U R E 6 (A) 1 H NMR spectra (toluene-D 8 ) at 323K for the titration of AzoM-3-E with gradually increase of F4-TCNQ.(B) The chemical shift changes of H a on AzoM-3-E on addition of F4-TCNQ.surface of ellipsoidal C 70 located in the cavity (blue) and the plane of benzene inside azobenzene in the opposite position of the macrocycle are both 3.25 Å, presenting the symmetrical π-π interactions.The C 70 filled outside the macrocycles is close to the two phenanthrenes in the opposite position and azobenzene units through π-π interactions, and the distances between the surface of C 70 and the plane of phenanthrene are both 3.21 Å (Figure 5G).The intermolecular π-π interactions of C 70 and AzoM-3-E result in a stable and orderly crystal packing (Figure 5H).Interestingly, observed from the cocrystals of C 70 ⊂AzoM-3-E and F4-TCNQ⊂AzoM-3-E, two guest molecules with different conformations are assembled with the macrocycle host through different noncovalent interactions, resulting in the change of AzoM-3-E skeleton.As shown in Figure S15, the planar electron-deficient guest F4-TCNQ can induce two phenanthrene groups on the AzoM-3-E to approach each other, forming a tetragonal geometry like "sandwich" through C-F⋅⋅⋅π interactions.While the ellipsoidal C 70 can open the phenanthrene unit of both sides of AzoM-3-E to convert to a boat-shaped configuration.In addition, due to the assembly of AzoM-3-E with C 70 , the azobenzene units are distorted greatly, and the dihedral angles between the two benzene rings of azobenzene units on AzoM-3-E are 60.78 • , 38.60 • , 60.78 • , and 38.60 • , respectively.When assembled with F4-TCNQ, the dihedral angles between the two benzene rings of azobenzene units on AzoM-3-E are 41.59 • , 42.18 • , 41.59 • , and 42.18 • , respectively.
cam.ac.uk/data_request/cif.] AzoM-1-E: 2224095 AzoM-2-E: 2224096 F4-TCNQ⊂AzoM-3-E: 2224097 C 70 ⊂AzoM-3-E: 2224098 A C K N O W L E D G M E N T S This work was financially supported by the National Key R&D Program of China (grant number: 2018YFA0703200), the National Natural Science Foundation of China (grant numbers: 61890940 and 52073063), the Program for Professor of Special Appoint-ment (Eastern Scholar) at the Shanghai Institutions of Higher Learning, and the Natural Science Foundation of Shanghai (grant numbers: 22ZR1405800 and 23ZR1405100).