Azobenzene Ligands Research Articles

A Highly Efficient Near-Infrared-Emissive Copolymer with a N=N Double-Bond π-Conjugated System Based on a Fused Azobenzene-Boron Complex.

Fused azobenzene-boron complexes (BAzs) show highly efficient near-infrared (NIR) emission from the nitrogen-nitrogen double bond (N=N) containing π-conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron-nitrogen (B-N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor-acceptor (D-A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λPL =751 nm, ΦPL =0.25) and in the film (λPL =821 nm, ΦPL =0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation-induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π-conjugated materials.

A Highly Efficient Near‐Infrared‐Emissive Copolymer with a N=N Double‐Bond π‐Conjugated System Based on a Fused Azobenzene–Boron Complex

AbstractFused azobenzene–boron complexes (BAzs) show highly efficient near‐infrared (NIR) emission from the nitrogen–nitrogen double bond (N=N) containing π‐conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron–nitrogen (B−N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor–acceptor (D–A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λPL=751 nm, ΦPL=0.25) and in the film (λPL=821 nm, ΦPL=0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation‐induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π‐conjugated materials.

Impact of the Alkyne Substitution Pattern and Metalation on the Photoisomerization of Azobenzene-Based Platinum(II) Diynes and Polyynes.

Trimethylsilyl-protected dialkynes incorporating azobenzene linker groups, Me3SiC≡CRC≡CSiMe3 (R = azobenzene-3,3'-diyl, azobenzene-4,4'-diyl, 2,5-dioctylazobenzene-4,4'-diyl), and the corresponding terminal dialkynes, HC≡CRC≡CH, have been synthesized and characterized. The CuI-catalyzed dehydrohalogenation reaction between trans-[Ph(Et3P)2PtCl] and the deprotected dialkynes in a 2:1 ratio in iPr2NH/CH2Cl2 gives the platinum(II) diynes trans-[Ph(Et3P)2PtC≡CRC≡CPt(PEt3)2Ph], while the dehydrohalogenation polycondensation reaction between trans-[(nBu3P)2PtCl2] and the dialkynes in a 1:1 molar ratio under similar reaction conditions affords the platinum(II) polyynes, [-Pt(PnBu3)2-C≡CRC≡C-]n. The materials have been characterized spectroscopically, with the diynes also studied using single-crystal X-ray diffraction. The platinum(II) diynes and polyynes are all soluble in common organic solvents. Optical-absorption measurements show that the compounds incorporating the para-alkynylazobenzene spacers have a higher degree of electronic delocalisation than their meta-alkynylazobenzene counterparts. Reversible photoisomerization in solution was observed spectroscopically for the alkynyl-functionalized azobenzene ligands and, to a lesser extent, for the platinum(II) complexes. Complementary quantum-chemical modeling was also used to analyze the optical properties and isomerization energetics.

Photoswitchable NIR-Emitting Gold Nanoparticles.

Photo-switching of the NIR emission of gold nanoparticles (GNP) upon photo-isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo-isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo-isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo-isomerization process was followed in real-time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo-isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo-isomerization, rather than competing with it. These results pave the road to the development of new, NIR-emitting, stimuli-responsive nanomaterials for theranostics.

Photoswitchable NIR‐Emitting Gold Nanoparticles

AbstractPhoto‐switching of the NIR emission of gold nanoparticles (GNP) upon photo‐isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo‐isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo‐isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo‐isomerization process was followed in real‐time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo‐isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo‐isomerization, rather than competing with it. These results pave the road to the development of new, NIR‐emitting, stimuli‐responsive nanomaterials for theranostics.

Photoswitchable arene ruthenium and pentamethylcyclopentadienyl rhodium complexes containing o-sulfonamide azobenzene ligands: Synthesis, characterization and cytotoxicity

A new series of arene chlorido ruthenium and pentamethylcyclopentadienyl chlorido rhodium complexes containing o-sulfonamide azobenzene ligands with an exocyclic NN bond coordination pattern have been synthesized. These complexes undergo readily E→Z photoisomerization followed by thermal Z→E isomerization (upon resting in the dark). The ruthenium complexes showed low-micromole-ranged cytotoxicity towards a panel of cancer cells. Western blotting and flow cytometric analyses suggest that 1) they are potent apoptotic inducers for cancer cells, probably through a caspase-3 dependent apoptotic pathway, and that 2) they have a much stronger ability to induce HeLa cancer cell apoptosis than cisplatin, a commercial anticancer drug.

Triphenylphosphine Photorelease and Induction of Catalytic Activity from Ruthenium-Arene Complexes Bearing a Photoswitchable o-Tosylamide Azobenzene Ligand

The reactivity of cationic arene ruthenium complexes bearing a photoswitchable o-tosylamide azobenzene ligand toward various phosphorus nucleophiles was investigated. The resulting phosphine-ruthenium complexes containing an azobenzene ligand were isolated as the Z isomer. Under appropriate reaction conditions, quantitative triphenylphosphine photorelease from the complex was achieved through Z → E isomerization of the ligand. This process was applied to the photoinitiation of the catalytic aza-Morita–Baylis–Hillman reaction.

Photoswitchable Arene Ruthenium Complexes Containing o-Sulfonamide Azobenzene Ligands

A series of arene ruthenium complexes containing o-sulfonamide azobenzene ligands were synthesized and found to exhibit uncommon coordination pattern with an exocyclic N═N bond. Upon irradiation, these complexes cleanly undergo E → Z photoisomerization followed by thermal Z → E isomerization (upon resting in the dark) whose rate is dependent on the solvent, the nature of the arene group, the sulfonamide moiety, and azobenzene substitution, as revealed by structure–property studies.

Synthesis and Properties of New Dimeric η2-Diyne Complexes of Cobalt Linked through an Azobenzene Ligand

The reaction between 4,4′-diiodoazobenzene (0) with excess trimethylsilylacetylene or dec-1-yne in the presence of catalytic amounts of PdCl2(PPh3)2 and CuI, Sonogashira coupling conditions, gave rise to the formation of 4,4′-bis(trimethylsilylethynyl)azobenzene (1) and 4,4′-bis(dec-1-ynyl)azobenzene (2), in good yield, in addition to icosa-9,11-diyne (3) as byproduct in the last case. The analogous reactions from 0 and the alkynyl-cobalt complexes Co2(CO)4(μ-L-L)(μ2-η2-SiMe3C2)](C≡CH) (L-L = dppm, dmpm) afforded 12–14. Complexes 4–7 and 15 have been obtained by direct reaction between Co2(CO)8 and the organic ligands 1–3. In order to increase the stabilization of Co–Co bonds, diphenylphosphinomethane-substituted alkyne-carbonyl complexes 8–11 were prepared by substitution reaction of carbonyl ligands in the presence of Me3NO at the Co2(CO)6 units from 4–7. All products were characterized by analytical and spectroscopic data (IR, 1H, 13C, and 31P NMR, and MS). Crystals of 9 suitable for single-crystal X-ray diffraction were grown, and the molecular structure of this compound is discussed. In this paper we report a comparative electrochemical study of these complexes by means of cyclic and square-wave voltammetry techniques, the trans–cis photoisomerization study of azobenzene compounds, and the thermotropic liquid-crystalline behavior.

Light-responsive three-dimensional microstructures composed of azobenzene-based palladium complexes

We describe not only fleeting assembly of photoisomerizable azobenzene-based palladium complexes into microstructured crystalline architectures but also their light-responsive functions. A transformation in the crystalline morphology from two-dimensional (2D) parallelogram-like sheets to three-dimensional (3D) cuboid- or rhombus-like structures was achieved by changing the solvent from tetrahydrofuran (THF) to acetone and N,N-dimethylformamide (DMF). The sizes of the structures, ranging from a few hundred nanometers to several hundred micrometers, were also modified by varying the complex concentration. In stark contrast to the very stable 2D sheets in the THF-H2O suspensions, exposure of 3D structures in polar DMF-H2O suspensions to ultraviolet (UV) light led to fast disassembly of the structures into isolated metal complexes and further dissociation of free azobenzene ligands from the complexes. In acetone-H2O suspensions, interestingly, disassembly of 3D cuboid-like structures into isolated complex components occurred upon exposure to UV light without further dissociation of azobenzene ligands from the palladium complexes. Considering the photoisomerization ability of the azobenzene-based palladium complex in common organic solvents, the π-stacking interactions that support 3D structures are likely to be sufficiently weak that they might be broken by the UV-induced trans-to-cis isomerization in more polar solvent mixtures. As a consequence, disassembly proceeded under UV light irradiation. Moreover, the effect of solvent polarity on the UV-assisted dissociation (in DMF-H2O) may be associated with the coordination ability of solvent molecules with the metal center.

A design strategy for stable light-sensitive palladium complexes

We describe the important determinants of stabilizing/destabilizing azobenzene-based palladium complexes capable of undergoing repeated light-triggered conformation changes. Inspired by highly warped palladium complexes with distorted azobenzene ligands, three different types of azobenzene ligands (1–3) were chosen to modify both the molecular structure (from a ubiquitous planar to a twisted trans-azobenzene) and the solubility of the azobenzenes in organic solvents. Single-crystal X-ray structure analyses were carried out for ortho-alkylated azobenzene (ortho-Az: 1b), meta-alkylated azobenzene (meta-Az: 2b) and PdCl2(ortho-Az)2 (1-Pd). Whereas both 2b and 3 lacking the meta and ortho substituents had planar structures, ortho-alkylated 1b was highly distorted by 68.06° from planarity. In stark contrast to PdCl2(meta-Az)2 (2-Pd) and PdCl2(Az)2 (3-Pd), which quickly dissociated in organic solvents, 1-Pd was very stable, especially in nonpolar solvents, so that it was possible to successfully purify and characterize them. The results suggest that unusually distorted trans-azobenzene is hardly influenced by the complexation reaction which requires considerable distortion of the azobenzene unit, thus stabilizing mononuclear palladium complexes. In nonpolar solvents, 1-Pd underwent repeated conformation changes under alternating UV and visible light irradiation. However, in polar solvents, the UV-triggered conformation change was accompanied by facile light-assisted breaking of the N:→Pd bond. Even dark incubation in polar organic solvents caused the dissociation of azobenzene ligands from the complexes. The breaking rate of the N:→Pd bond increased in the order of benzene ≈ dichloromethane < acetone < DMF, with more polar solvents inducing faster dissociation. The solvent polarity effect on the stability of azobenzene-based complexes can be interpreted in terms of the degree of polarization of the metal–ligand bond formed as a consequence of interactions between the palladium ion as a soft acid and azobenzene nitrogen as a hard base.

Photoresponsive smart template for reversible cell micropatterning.

A novel micropatterned smart template based on transparency photolithography allows the spatial control of cell micropatterning. By utilizing the photoinduced reaction of azobenzene ligands and cyclodextrin-terminated alkanesilane via host-guest recognitions, cells can be easily controlled to adhere reversibly in well defined areas.

A palladacyclic azobenzene derivative as a catalyst for carbon-carbon bond formation reactions

A bis-chelated palladacycle in which the Pd atom is [(C, N)(C, N)]-tetracoordinated to two azobenzene ligands proved to be a moderately active catalyst for the Suzuki and Heck reactions. A voltammetric study revealed that the two-electron oxidation of this complex is accounted for by the irreversible PdII/PdIV transition.

Photoresponsive Release from Azobenzene-Modified Single Cubic Crystal NaCl/Silica Particles

Azobenzene ligands were uniformly anchored to the pore surfaces of nanoporous silica particles with single crystal NaCl using 4-(3-triethoxysilylpropylureido)azobenzene (TSUA). The functionalization delayed the release of NaCl significantly. The modified particles demonstrated a photocontrolled release by trans/cis isomerization of azobenzene moieties. The addition of amphiphilic solvents, propylene glycol (PG), propylene glycol propyl ether (PGPE), and dipropylene glycol propyl ether (DPGPE) delayed the release in water, although the wetting behavior was improved and the delay is the most for the block molecules with the longest carbon chain. The speedup by UV irradiation suggests a strong dependence of diffusion on the switchable pore size. TGA, XRD, FTIR, and NMR techniques were used to characterize the structures.

ChemInform Abstract: Dinuclear Cyclopalladated Azobenzene Complexes: Crystal Structure Analysis of Homologous Series

Abstract Organometallic palladium(II) complexes obtained by cyclometallation reactions of azobenzene ligands or Schiff bases are of considerable interest in the context of practical applications such as lumunescent or liquid-crystalline materials. The shape and the dimensions of the molecules can play an important role in the physical properties of these new molecular materials. Various dinuclear cyclopalladated azobenzene complexes structurally characterised are reviewed with the intention to provide insights into the design of new dimeric metallomesogens with a suitable molecular shape.

Unusual azobenzene/bipyridine palladacycles: structural, dynamical, photophysical and theoretical studies

Two types of Pd(ii) azobenzene/bipyridine complexes with unusual coordination mode of azobenzenes, PdCl{(mu-Cl)(mu-R(1)C(6)H(3)N=NC(6)H(3)R(2))}Pd(bpy) 1a-4a and [(bpy)PdCl(mu-NH(2)C(6)H(3)N=NC(6)H(4))Pd(bpy)]Cl 3b were formed by the reaction of dicyclopalladated azobenzenes (DMF)PdCl(mu-R(1)C(6)H(3)N=NC(6)H(3)R(2))PdCl(DMF) with excess of bpy, where bpy=2,2'-bipyridine; R(2)=H and R(1)=H (1), CH(3) (2), NH(2) (3) or R(1)=N(CH(3))(2) and R(2)=NO(2) (4). Neutral species 1a-4a were obtained in acetone, while in DMSO or MeOH the ionic complex 3b was produced. When dissolved, 3b decomposes to 3a and free bpy; however in DMSO upon addition of bpy 3b crystallizes again. X-ray structures of all complexes confirmed breaking of one Pd-N bond in the initial precursors, thus allowing rotation of one phenyl ring and positioning of both Pd atoms on the same side of the azobenzene ligand. Two Pd atoms are connected by the azobenzene ligand and in neutral complexes additionally by the Cl-bridge. In all complexes in the solid-state azobenzenes act simultaneously as monodentate C- and bidentate C,N-donors while bpy acts as bidentate donor. Variable-temperature (1)H NMR experiments established that structures of 1a-4a in DMF and DMSO at ambient temperature are not consistent with solid-state structures due to the fast exchange of one of the bpy nitrogen atoms bound to the Pd atom with solvent molecules. Theoretical studies confirmed the experimental structures as the most stable isomers. Photoabsorption and photoemission properties of the new complexes have been measured and photoabsorption is rationalized by time dependent DFT calculations. The presence of bpy significantly increases the intensity of fluorescence either in the solution (4a) or in the solid state (3a, 4a, 3b) at ambient temperature.

A reversibly photoswitchable mononuclear palladium(ii) complex with ortho-diethylated azobenzene ligands

A mononuclear palladium(II) complex (PdCl2(2Et-Azo)2), with sterically bulky ortho-diethylated azobenzenes (2Et-Azo) in the long-lived cis form, was designed to achieve reversible photoswitching originating from a reversible change in its molecular structure. An X-ray crystallographic study indicated that the metal center in the complex has a trans square-planar structure, with two 2Et-Azo ligands coordinated to a palladium ion in a monodentate manner. Alternating UV and visible light irradiation of PdCl2(2Et-Azo)2 gave rise to reversible changes in the molecular structure between the trans and cis forms, confirmed by UV-vis absorption and NMR spectroscopic measurements. Neither significant decomposition nor fast dissociation of PdCl2(2Et-Azo)2 was observed by UV and visible light irradiation. Furthermore, slow thermal back-isomerization occurred over the course of 2 days at ambient temperature, which may be closely related to the stability of the cis form of the ortho-alkylated azobenzene ligand.

Optical tuning of molecule‐induced magnetism at non‐magnetic semiconductor nanoparticles

AbstractWe have demonstrated that it is possible to reversibly photo‐control the room temperature magnetism in a diluted magnetic semiconductor based on zinc oxide nanoparticles coated with azobenzene‐derivatized thiols (Azo‐ZnO NPs). These photochromic ligands provide a photo‐function and at the same time act as a trigger for the generation of ferromagnetism at ZnO–S interface. UV irradiation increases the saturated magnetization of the Azo‐ZnO NPs due to trans‐ to cis‐isomerization in azobenzene‐derivatized ligands, and following irradiation with visible light the magnetization is decreased due to inverse isomerization. X‐ray photoelectron spectroscopy reveal that the photo‐induced magnetization changes are strongly related to photo‐induced changes in the chemical state of zinc atom in the ZnO. These photo‐magnetic effects can be attributed to photo‐induced changes in surface states of the ZnO due to photoisomerization in azobenzene ligands, which is accompanied by alterations in the lowest unoccupied molecular orbital level of the ligand, since magnetic properties of ZnO depend on carrier concentration, which correlated with surface states of the ZnO.magnified image

Regulating Copper‐Binding Affinity with Photoisomerizable Azobenzene Ligand by Construction of a Self‐Assembled Monolayer

Einer bleibt, einer geht: Selbstorganisierte Monoschichten (SAMs) aus trans- oder cis-Azobenzolliganden können CuII-Ionen komplexieren. Die an die trans-SAM gebundenen Ionen werden in cyclovoltammetrischen Redoxzyklen (CV) freigesetzt, während die cis-Komplexe stabil bleiben (siehe Bild). Bestrahlung der cis-SAM mit sichtbarem Licht führt zu einer cis-trans-Photoisomerisierung, wodurch ebenfalls eine Freisetzung der Cu-Ionen resultiert.

Regulating Copper‐Binding Affinity with Photoisomerizable Azobenzene Ligand by Construction of a Self‐Assembled Monolayer

Catch and release! Self-assembled monolayers (SAMs) of both trans- and cis-azobenzene ligands can bind CuII ions. Ions bound to the trans ligand are released by cyclic voltammetry (CV) redox scans whereas those bound to the cis ligand are not (see picture). Irradiation of the cis-SAM with visible light leads to removal of the bound Cu ions by cis-to-trans photoisomerization.