Photoresponsive Ligands Research Articles

Synthesis and Coordination Chemistry of a Photoswitchable Bis(phosphine) Ligand

A 1,2-dithienylethene compound bearing bis(phosphine) groups (1o) represents a new class of photoresponsive ligands where there are steric and electronic differences between two photogenerated isomers. The coordination chemistry of this ligand class is demonstrated by preparing a gold(I) complex (2o) and a phosphine selenide (3o).

Photomodulation of conformational states. IV. Integrin‐binding RGD‐peptides with (4‐aminomethyl)phenylazobenzoic acid as backbone constituent

In previous studies we have investigated octapeptides backbone-cyclized by (4-amino)phenyl azobenzoic acid (APB) or (4-aminomethyl)phenylazobenzoic acid (AMPB) and containing the active-site sequence Cys-Ala-Thr-Cys-Asp from the thioredoxin reductase. The conformational and redox properties of these peptides were strongly dependent on the isomeric state of the azobenzene chromophore. Using the same approach we were successful in constructing photoresponsive ligands for alphavbeta3 integrin containing the Arg-Gly-Asp (RGD) sequence as binding motif. For achieving maximal conformational restriction of the peptide a reduced ring size compared to our previous azobenzene peptides was employed in the cyclic peptide c[Asp-D-Phe-Val-AMPB-Lys-Ala-Arg-Gly-]. Conformational properties of the trans and cis isomers of this peptide in solution were investigated by CD and NMR and were found to differ markedly from the thioredoxin derived azobenzene peptides. In a second peptide, c[Asp-D-Phe-Val-Lys-AMPB-Ala-Arg-Gly-], shifting the position of the chromophore lead to a marked decrease in affinity. With the availability of the x-ray structure of a cyclic RGD-pentapeptide bound to alphavbeta3 integrin (PDB entry 1L5G) modeling of possible bound conformations for trans and cis isomers of both azobenzene peptides was possible. Notably, both peptides in either isomeric form share the same overall conformation in the bound state according to our molecular dynamics simulations.

Synthesis and Solution Structure Analysis of a Bispyrenyl Bishydroxamate Calix[4]arene-Based Receptor, a Fluorescent Chemosensor for Cu2+ and Ni2+ Metal Ions

A bispyrenyl calix[4]arene-based receptor, 1, incorporating two hydroxamic acid functionalities has been designed for the optical detection of Cu2+ and Ni2+ metal ions in solution. The synthetic route is based on the utilization of pyrene-labeled, O-protected hydroxylamines that represent versatile building blocks for the generation of fluorescent siderophore-based chelators. Compound 1 is shown to exist in CDCl3 solution at room temperature as a mixture of mainly two conformers, namely, the cone and partial-cone species (partial-cone/cone ratio ca. 57/43). The full assignment of the proton and carbon resonances of the calix[4]arene core for both conformers is described. Ligand 1 exhibited in solution a dual fluorescence emission spectrum composed of monomer and excimer contributions, the latter resulting from intramolecular interactions between pyrene nuclei in the excited state. In methanol/water (80/20 v/v), the fluorescence intensity was shown to be sensitive to proton concentration and to the presence of transition-metal ions. Addition of Cu2+ and Ni2+ metal cations induced a dramatic quenching of fluorescence of 1 depending on the value of −log[H+]. The excimer-forming photoresponsive ligand 1 belongs to a new class of sensitive chemosensors for the selective detection of transition-metal species in aqueous media.

Transport and fluorescence emission properties of photoresponsive ligands

New photoresponsive azobenzene systems L1–L10 have been synthesised and characterised. On irradiation at ∼330 nm, they undergo conversion fromE toZ form to a varying extent, depending on the nature and position of substitution on the azobenzene rings. They revert toE form in the dark. Photochemical equilibria have been studied in acetonitrile,o-dichlorobenzene and DMF.E forms are stabilized more ino-dichlorobenzene than in acetonitrile. Two of the molecules L1 and L5 show enhanced transport of Cu2+ ions across liquid membrane on irradiation. Compounds L2 and L3 undergoE→Z photoisomerisation on irradiation at ∼330 nm accompanied by fluorescence enhancement.Z→E isomerisation in the dark is accompanied by decrease in the fluorescence intensity. These are the first reported examples of fluorescence enhancement onE→Z isomerisation across N=N bond. Protonation of azo groups in L2 and L3 is accompanied by fluorescence enhancement. The origin of the fluorescence enhancement in thecis isomer is explained on the basis of the inhibition of photoinduced electron transfer due to the nonplanar geometry of this isomer which reduces the effective conjugation of the nitrogen lone pair electrons with theπ electrons of the fluorophore.

Heterospin Systems Consisting of Organic Free Radicals and Magnetic Metal Ions by Self-Assembling Strategy. Diazodi(4-Pyridyl)Methane as Photo-Responsive Ligands for Metal-Carbene-Based Heterospin Magnets

Diazodi(4-pyridyl)methane and Mn(hfac)2 formed the orange-colored 1:1 complex having a spiral 1-D chain structure. While all the Mn(II) ions were found to be magnetically isolated before irradiation, a 1-D ferrimagnetic chain was established in the complex by an antiferromagnetic interaction of the 3d spins of Mn(II) with the 2p spins of photochemically generated carbenes (S = 2/2). Dark greenish bricks of the similar 1:1 complex from the diazo compound and Cu(hfac)2 exhibited after photolysis a χmolT-T profile characteristic of ferromagnetic interaction between the carbenes and the Cu(II) ions. The carbene centers were found to be stable up to ca. 230 K in both the complexes. The photochemically generated heterospin system will serve as a prototype of molecular photomagnetic recording devices.

Diazodi(4‐pyridyl)methane and Diazophenyl‐(4‐pyridyl)methane as Photoresponsive Ligands for Metal–Carbene Hetero‐Spin Systems

Diazodi(4‐pyridyl)methane and Diazophenyl‐(4‐pyridyl)methane as Photoresponsive Ligands for Metal–Carbene Hetero‐Spin Systems