We present a fully reversible and highly efficient on-off photoswitching of magnetic resonance imaging (MRI) contrast with green (500 nm) and violet-blue (435 nm) light. The contrast change is based ...on intramolecular light-driven coordination-induced spin state switch (LD-CISSS), performed with azopyridine-substituted Ni-porphyrins. The relaxation time of the solvent protons in 3 mM solutions of the azoporphyrins in DMSO was switched between 3.5 and 1.7 s. The relaxivity of the contrast agent changes by a factor of 6.7. No fatigue or side reaction was observed, even after >100,000 switching cycles in air at room temperature. Electron-donating substituents at the pyridine improve the LD-CISSS in two ways: better photostationary states are achieved, and intramolecular binding is enhanced.
Two stable core‐modified rubyrins bearing one and two dithienylethene (DTE) units (1 and 2) have been synthesized. With one “closed‐form” DTE unit, 1 shows aromaticity associated with its conjugated ...circuit of 26 π‐electrons. In contrast, rubyrin 2 containing one “open‐form” DTE unit has nonaromatic properties.
Tuning aromaticity: Two stable core‐modified rubyrins bearing one (1) and two (2) dithienylethene (DTE) units have been synthesized. Compound 1, with a “closed‐form” DTE unit, has a cyclic conjugated system with 26 π‐electrons. In contrast, macrocycle 2 containing one “open‐form” DTE unit has nonaromatic properties.
The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin‐state switching to isolated molecules at room temperature ...(light‐driven coordination‐induced spin‐state switching, or LD‐CISSS). Whereas bistability and hysteresis in conventional spin‐crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD‐CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel–porphyrin as the transition‐metal complex and 3‐phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni–porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72 % paramagnetic Ni–porphyrin after irradiation at 365 nm, 32 % paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD‐CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light‐controlled magnetic levitation are conceivable applications.
Turn the spin: Nickel–porphyrins with appropriately designed axial photochromic ligands change their coordination number and consequently their spin state reversibly upon irradiation. Rational design led to a substituted 3‐phenylazopyridine as a photodissociable ligand with a switching efficiency of 40 % (see figure). Thus, the magnetic susceptibility of a homogeneous solution was switched by a factor of more than two at room temperature.
Trioctyl-functionalized triazatriangulenium (trioctyl-TATA) deposited on Au(111) and Ag(111) surfaces by electrospray ionization was investigated using low-temperature scanning tunneling microscopy. ...The molecule surprisingly adsorbs with gauche rather than anti conformations of the octyl groups. We observed chiral amplification in the islands. Only one of the eight possible configurations of the octyl groups was found in homochiral hexagonal networks. Quantum-chemical calculations confirmed and explained the preference for the gauche conformations of adsorbed trioctyl-TATA.
The controlled attachment of chromophores to metal or semiconducting surfaces is a prerequisite for the construction of photonic devices and artificial surface-based light-harvesting systems. We ...present an approach to mount porphyrins in ordered monolayers on Au(111) by self-assembly and verify it, employing STM, absorption spectroscopy, and quantum chemical calculations. The usual adsorption geometry of planar chromophores, flat on the surface or densely packed edge-on, is prevented by mounting the porphyrins upright on a molecular platform. An ethynyl unit as spacer and pivot joint provides almost free azimuthal rotation of the unsubstituted porphin. However, rotation of the larger triphenylporphyrin unit is sterically restricted: because the diameter of the substituted porphyrin is larger than the distance to its next neighbors, the phenyl substituents of neigboring molecules interact by dispersion force, which leads to an alignment of the azimuthal rotators.
The conductances of molecular model junctions comprising a triazatriangulenium platform with or without an ethynyl spacer and an upright Zn-porphyrin are probed with a low-temperature scanning probe ...microscope. The platform alone is found to be highly conductive. The ethynyl-linked Zn-porphyrin moiety reduces the conductance by three orders of magnitude and leads to an unexpected, non-monotonous variation of the force that was measured simultaneously at the tip of the microscope. Density functional theory calculations show that this variation results from an induced tilting of the porphyrin.
Molecules with Möbius topology have drawn increasing attention from scientists in a variety of fields, such as organic chemistry, inorganic chemistry, and material science. However, synthetic ...difficulties and the lack of functionality impede their fundamental understanding and practical applications. Here, we report the facile synthesis of an aggregation‐induced‐emission (AIE)‐active macrocycle (TPE‐ET) and investigate its analogous triply and singly twisted Möbius topologies. Because of the twisted and flexible nature of the tetraphenylethene units, the macrocycle adjusts its conformations so as to accommodate different guest molecules in its crystals. Moreover, theoretical studies including topological and electronic calculations reveal the energetically favorable interconversion process between triply and singly twisted topologies.
Twisted sister: A novel aggregation‐induced‐emission‐active macrocycle, TPE‐ET, was synthesized by a simple one‐step modified Sonogashira cyclization. TPE‐ET adjusts its conformation by virtue of different guest molecules, resulting in intriguing triply and singly twisted Möbius topologies. DFT calculations reveal that the interconversion of these conformations is energetically favorable.
Femtosecond time-resolved absorption measurements of a magnetically bi-stable azopyridine Ni-porphyrin in solution at room temperature show that the photo-induced dynamics are dominated by transient ...low-spin ⇄ high-spin interconversion involving Ni (d
2
) and (d, d) states.
Bacteriophage Diversity in the North Sea WICHELS, A; BIEL, S. S; GELDERBLOM, H. R ...
Applied and Environmental Microbiology,
11/1998, Letnik:
64, Številka:
11
Journal Article
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