Photoisomerization and Thermal Isomerization of Shuttlecock- and Bowl-Equipped (Phenylazo)pyridines

Photoswitchable catalysis using organometallic complexes

Photoswitchable catalysis using organometallic complexes: a ligand design perspective.

Photoswitchable and pH responsive organoplatinum(ii) complexes with azopyridine ligands

Several platinum(ii) complexes with ligands containing azo groups have been prepared and structurally characterised, and their photoswitching between trans and cis azo group isomers has been studied. The azo groups in the cationic complexes [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄X)][PF₆], X = H, OH or NMe₂, and in the dicationic complex [Pt(bipy)(4-H₂NC₆H₄-N[double bond, length as m-dash]N-C₆H₅)₂][OTf]₂ undergo trans to cis photoswitching on irradiation at 365 nm. The complex [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄NMe)₂][PF₆] also exhibits a reversible halochromic effect on protonation to give the dicationic complex [PtMe(bipy)(4-NC₅H₄-NH[double bond, length as m-dash]N-4-C₆H₄NMe₂]²⁺. The nature of the frontier orbitals in the platinum(ii) complexes depends on the charge on the complex and on the degree of metal-ligand π-bonding.

Excited-State Decay in the Photoisomerisation of Azobenzene: A New Balance between Mechanisms

The mechanism of the photoisomerisation of azobenzene has been studied by means of multiconfigurational ab initio calculations. Our results show that it is necessary to account for the dynamic electron correlation in the location of the critical points (CASPT2 optimizations) to obtain a correct description of the topography of the potential energy surfaces of the low energy singlet excited states. By using this methodology, we have found that the state populated by the initial excitation is the S₂ (ππ*) state, which decays very efficiently to the S₁ (nπ*) state at a pedal-like non-rotated geometry. In the S₁ state, relaxation leads to a rotated geometry where the system decays to the ground state, in which further relaxation can lead to either the trans or cis geometries. However, the S₁ /S₀ conical intersection seam also extends to planar geometries, so this reaction path is also accessible for rotation-constrained systems. Our results explain the experimental observations satisfactorily.

Design and Synthesis of Photodissociable Ligands Based on Azoimidazoles for Light-Driven Coordination-Induced Spin State Switching in Homogeneous Solution

Light-switchable azoimidazoles were rationally designed and synthesized, and their performance was investigated as photodissociable ligands (PDL) and for spin state switching of Ni porphyrins. The rationally designed ligands exhibit a high photochemical conversion rate (trans → cis > 98%) and no measurable fatigue over a large number of switching cycles at room temperature under air. As compared to the known phenylazopyridines, the phenylazoimidazoles exhibit a much stronger affinity as axial ligands to Ni porphyrin in the binding trans configuration and a low affinity in their cis form. This affinity switching was used to control the coordination number of Ni(2+). Concomitant with the change in coordination number is the change of the spin state from triplet (high spin) to singlet state (low spin). We report on phenylazoimidazole-based PDLs that switch the paramagnetic ratio of the investigated nickel species by up to 70%. Consequently, azoimidazoles exhibit considerably higher switching efficiencies than previously described pyridine-based PDLs.

A mild CuBr–NMO oxidative system for the coupling of anilines leading to aromatic azo compounds

An efficient methodology was developed for the synthesis of aromatic azo derivatives under mild reaction conditions utilizing CuBr with N-methylmorpholine N-oxide as an oxidant.

Using coaxial electrospinning to fabricate core/shell-structured polyacrylonitrile–polybenzoxazine fibers as nonfouling membranes

We used coaxial electrospinning to produce core/shell polyacrylonitrile (PAN)–benzoxazine (BA) fibers. The PAN–PBA core/shell fibers that we obtained after curing exhibited low surface energies and excellent biononfouling properties.

Photoswitchable organoplatinum complexes containing an azobenzene derivative of 3,6-di(2-pyridyl)pyridazine

The new, unsymmetrical azobenzene-tagged ligand 4-(4-azobenzene)-3,6-di(2-pyridyl)pyridazine, adpp, forms complexes with platinum(II) and platinum(IV), which exist as a mixture of geometrical isomers. The complexes are characterized primarily by their NMR spectra, while the structures of the ligand and its complexes [PtMe2(adpp)], [PtBrMe2(CH2C6H4-4-t-Bu)(adpp)], and [PtBrMe2(CH2C6H3-3,5-t-Bu2)(adpp)] have been structurally characterized. In solution, the compounds undergo easy photochemical trans−cis switching of the azobenzene group, with subsequent slow thermal isomerization back to the more stable trans-azobenzene isomer.

Light-induced spin change by photodissociable external ligands: a new principle for magnetic switching of molecules

Magnetic bistability in spin-crossover materials generally is a collective phenomenon that arises from the cooperative interaction of a large number of microscopic magnetic moments within the crystal lattice in the solid state. We now report on individual molecules in homogeneous solution that are switched between the diamagnetic and paramagnetic states at room temperature by light-driven coordination-induced spin-state switching (LD-CISSS). Switching of the coordination number (and concurrently of the spin state) was achieved by using Ni-porphyrin as a square-planar platform and azopyridines as photodissociable axial ligands. The square-planar Ni-porphyrin is diamagnetic (low-spin, S = 0), and all complexes with axial ligands are paramagnetic (high-spin, S = 1). Association constants were determined for all conceivable 1:1 and 1:2 porphyrin/azopyridine complexes. The binding constants of the trans azopyridines are larger than those of the corresponding cis isomers. Thus, upon irradiation with UV light (365 nm, trans → cis) and visible light (455 nm, cis → trans), switching of the magnetic properties was achieved. Upon substitution of the azopyridines at the 4- and 4'-positions with larger substituents, the difference in trans and cis association constants, and thus the switching efficiency, was increased. A photoinduced, reversible switching between 20 and 68% paramagnetic Ni species in solution was achieved with isopropyl substituents at room temperature.