Biomedical subjects
Publications and source records attributed to Andrew J Goshe.
Supramolecular porphyrin prisms have been obtained via coordinative self-assembly and characterized by 1H NMR, PFG NMR, electronic absorption spectroscopy and synchrotron-based measurements of solution phase X-ray scattering and diffraction.
We report on two multi-chromophore building blocks that self-assemble in solution and on surfaces into supramolecular light-harvesting arrays. Each building block is based on perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophores. In one building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 3 and 6 carbons of pyromellitimide to form a cross-shaped molecule (PI-PDI(4)). In the second building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 1 and 7 carbons of a fifth PDI to produce a saddle-shaped molecule (PDI(5)). These molecules self-assemble into partially ordered dimeric structures (PI-PDI(4))(2) and (PDI(5))(2) in toluene and 2-methyltetrahydrofuran solutions with the PDI molecules approximately parallel to one another primarily due to pi-pi interactions between adjacent PDI chromophores. On hydrophobic surfaces, PDI(5) grows into rod-shaped nanostructures of average length 130 nm as revealed by atomic force microscopy. Photoexcitation of these supramolecular dimers in solution gives direct evidence of strong pi-pi interactions between the excited PDI chromophore and other PDI molecules nearby based on the observed formation of an excimer-like state in <130 fs with a lifetime of about 20 ns. Multiple photoexcitations of the supramolecular dimers lead to fast singlet-singlet annihilation of the excimer-like state, which occurs with exciton hopping times of about 5 ps, which are comparable to those observed in photosynthetic light-harvesting proteins from green plants.
A dicationic molecular receptor bearing two cofacially disposed terpyridyl-Pd-Cl units forms stable 1:1 host-guest complexes with planar, neutral platinum(II) complexes. When the guest is modified to incorporate a pyridine group, the now basic guest is protonated by trifluoroacetic acid in acetonitrile solutions. The basic yellow guest forms a stable, deep red 1:1 host-guest complex with the yellow palladium receptor. Addition of trifluoroacetic acid to this host-guest complex leads to the displacement of the guest from the receptor. It is proposed that the dissociation of the guest is caused by electrostatic repulsion between the dicationic receptor and the positively charged protonated guest. Addition of base restores the host-guest complex. This protonmotive translocation of the guest from the host to the solution is discussed in terms of the mechanisms that drive molecular motors, the power stroke and the Brownian ratchet. It is concluded that the system is best described as a molecular switch that operates by the same mechanism as one stroke of a molecular motor
Two supramolecular trigonal prisms, each bearing three molecular clefts are shown to form 1:6 and 1:7 host-guest complexes with 9-methylanthracene and one of the prisms forms a 1:2 host-guest complex with a tritopic tri-anthracene guest that registers with the recognition sites of the host.
A di-positively charged metal-based receptor is shown to form 1:1 or 2:1 association complexes with rigid, linear two-site guests depending on the site separation, suggesting that electrostatic repulsion controls the association nuclearity.
Molecular receptors, consisting of either two parallel cofacially disposed terpyridyl-Pd-Cl+ or terpyridyl-Pt-Cl+ units, are described. Concerted rotation of these units about the molecular spacer can alter their separation between 6.4 and 7.2 A to accommodate the dimensions of molecular guests. Neutral and anionic planar complexes of platinum(II) were investigated as guests to determine if metal-metal interaction between the host and guest metals could stabilize host-guest association. With a neutral guest, it was found that host-guest formation is signaled by a color change from light yellow to deep red. For one of the anionic guests, a visible absorption band appears upon host-guest formation with the platinum receptor that is ascribed to transitions associated with a Pt-Pt interaction. The association constants found for the neutral guest with the palladium and platinum receptors are large, suggesting that metal-metal interaction contributes to the molecular recognition. The structures of the host-(neutral)guest complexes in solution have been determined by 1H NOESY spectra. A crystal structure of the platinum host-(neutral)guest complex is the same as that found in solution and confirms the presence of a Pt-Pt interaction. Temperature-dependent (195)Pt NMR spectra in solution provide a quantitative estimate of the conformational interconversions of the free platinum receptor.
A molecular receptor consisting of a spacer bearing two cofacially disposed terpyridyl-palladium-ligand (terpy-Pd-L) units rigidly separated by about 7 A has been investigated for molecular recognition of planar aromatic molecules. It is found that although the receptor forms stable 1:2 host-guest association complexes with 9-methylanthracene (9-MA), the guest undergoes very rapid site exchange within the receptor and with external free 9-MA. A crystal structure of the 2:1 adduct shows one 9-MA in the molecular cleft defined by the two terpy-Pd-L units and the other resides on an outside face of one terpy-Pd-L unit. To establish the site residency time of the guests, a number of tethered molecules were prepared. These involve an anthracene molecule tethered to a pyridine ligand bound to the palladium atoms to form intramolecular host-guest adducts. Rotating-frame Overhauser effects were used to infer the site residency of the anthracene guests in the receptor. Variable-temperature (1)H NMR spectroscopy of the intramolecular host-guest complexes has revealed that the site residency time of the anthracene guests is 1.6 x 10(-5) sec at 20 degrees C and 1.3 sec at -90 degrees C in acetone solution. Whereas the guests are thermodynamically stable, they are kinetically very labile. A crystal structure of one of the tethered host-guest adducts reveals the expected structure which is the same as that determined in solution by (1)H rotating-frame Overhauser enhancement spectroscopy experiments.