Through-bond excited energy transfer mediated by an amidinium-carboxylate salt bridge in Zn-porphyrin free-base porphyrin dyads

Excited energy-transfer processes were investigated for a supramolecular Zn-porphyrin free-base porphyrin dyad, ZnPA-2 x FbPC-2, in which beta-octaalkylated meso-diarylporphyrins are connected through an amidinium-carboxylate salt bridge. The rate of energy transfer in the dyad (1.3 x 10(9) s(-1)) is substantially slower than that in the previously reported dyad, ZnPA-1FbPC-1 (4.0 x 10(9) s(-1)), in which meso-tetraarylporphyrins are connected through the same amidinium-carboxylate salt bridge. The F?rster-type mechanism can explain only minor parts of these rates (3.3 x 10(8) and 5.1 x 10(8) s(-1), respectively). Thus, Dexter-type through-bond energy transfer may be invoked. Indeed, bridge-mediated electronic processes would be favored in ZnPA-1 x FbPC-1 over ZnPA-2 x FbPC-2 on the basis of steric and electronic factors. Sterically, the phenyl groups in ZnPA-2 and FbPC-2 are more closely perpendicular to the porphyrin planes than those in ZnPA-1 and FbPC-1. Electronically, the energy and symmetry of the occupied frontier orbitals should favor ZnPA-1 x FbPC-1 over ZnPA-2 x FbPC-2 in terms of electronic interactions through the bridge. Therefore, the observed trend (ZnPA-1 x FbPC-1>ZnPA-2 x FbPC-2), consistent with these considerations, lends further support to the through-bond mechanism. Thus, the amidinium-carboxylate salt bridge is effective in mediating through-bond energy transfer even though the bond is noncovalent.     

Switchable antenna: a star-shaped ruthenium/osmium tetranuclear complex with azobis(bipyridine) bridging ligands

A star-shaped Ru/Os tetranuclear complex, in which a central Os unit is linked to three peripheral Ru units by 4,4'-azobis(2,2'-bipyridine) (azobpy) bridging ligands, was prepared to examine the unique photodynamics regulated by its redox state. The Ru/Os tetranuclear complex exhibits Ru-based luminescence at 77 K, whereas the three-electron reduction (one for each azobpy) of the Ru/Os complex results in luminescence from the Os unit. The photoexcited state of the Ru/Os complex rapidly decays into low energy metal-to-ligand charge-transfer states, in which the excited electron is localized in the azobpy ligand in the form of azobpy(.-). Upon the one-electron reduction of the azobpy ligands, the above-mentioned low-energy states become unavailable to the photoexcited complex. As a result, an energy transfer from the Ru-based excited state to the Os-based excited state becomes possible. Ultrafast transient absorption measurements revealed that the energy transfer process consists of two steps; intramolecular electron transfer from the terminal bipyridine ligand (bpy(.-)) to form azobpy(2-) followed by a metal-to-metal electron transfer. Thus, the Ru/Os tetranuclear complex collects light energy into the central Os unit depending on the redox state of the bridging ligands, qualifying as a switchable antenna.     

Nagelamide J, a novel dimeric bromopyrrole alkaloid from a sponge Agelas species

A novel dimeric bromopyrrole alkaloid, nagelamide J (1), with antimicrobial activity has been isolated from an Okinawan marine sponge Agelas species, and the structure and stereochemistry were elucidated from spectroscopic data. Nagelamide J (1) is the first bromopyrrole alkaloid possessing a cyclopentane ring fused to an amino imidazole ring.     

Tuning of fluorescence properties of aminoterpyridine fluorophores by N-substitution

Several N-alkyl and N-phenyl derivatives of 6-amino- () and 6,6'-diamino-2,2':6',2'-terpyridine () were synthesized, and their fluorescence properties were studied. A successive red-shift was observed as the number of the N-substituted groups increased. It was also shown that the susceptivity of the fluorophores to a solvent varied considerably according to the mode of the N-substitution. While the monoamino-tpys (tpy: 2,2':6',2'-terpyridine) suffered almost complete quenching of their fluorescence in ethanol, the fully N-alkylated diamino-tpys and retained their fluorescence. The results show that N-substitution is a useful way to tune both the radiation energy and solvent susceptivity of the fluorescence of the amino-tpys.     

Iterative two-step strategy for C2-C4′ linked poly-oxazole synthesis using Suzuki-Miyaura cross-coupling reaction

An iterative method for the synthesis of C2-C4′ linked poly-oxazoles has been developed. This efficient two-step repetitive process includes TBS-iodine exchange reaction and Suzuki-Miyaura cross-coupling reaction with oxazolylboronate 8, which allows appending a bis-oxazole moiety per each iteration. The synthesis of bis-, tris-, tetrakis-, pentakis-, and hexakis-oxazoles (10, 14, 22, 18, and 24) was achieved starting from the common intermediate 7 in 1-5 steps.     

Multi-triphenylamine-substituted porphyrin-fullerene conjugates as charge stabilizing "antenna-reaction center" mimics

A new concept of charge stabilization via delocalization of the pi-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin pi-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the delocalization of the pi-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates.     

Enantioselective total synthesis of (+)-ottelione A, (-)-ottelione B, (+)-3-epi-ottelione A and preliminary evaluation of their antitumor activity

Enantioselective total synthesis of (+)-ottelione A (1) and (-)-ottelione B (2), novel and potent antitumor agents from a freshwater plant, and (+)-3-epi-ottelione A (3), the earlier proposed stereostructure of 1, was efficiently achieved starting from the known tricyclic compound 10. The synthesis involved the following key steps: i) coupling reactions of aldehydes 8 and 9 with the aromatic portion 7 (8+7-->15 and 9+7-->27), ii) base-induced hemiacetal-opening/epimerization reactions of the cyclic hemiacetals 6 and 27 (6-->17 and 27 a-->26 a), and iii) Corey-Winter's reductive olefination of the cyclic thiocarbonates 21 and 36 (21-->22 and 36-->37). The present total synthesis fully established the absolute configuration of these natural products. The cell growth inhibition profile, COMPARE analysis, and tubulin inhibitory assay of (+)-3-epi-ottelione A (3) and its O-acetyl derivative 24 demonstrated that these unnatural substances could be prominent lead compounds for the development of anticancer agents with a novel mode of action.     

Synthesis, structures, and properties of meso-phosphorylporphyrins: self-organization through P-oxo-zinc coordination

The synthesis, structures, and optical and electrochemical properties of meso-phosphorylporphyrins are described. The copper-catalyzed carbon-phosphorus cross-coupling reaction of a meso-iodoporphyrin with di-n-butyl phosphite and diphenylphosphane oxide has proved to be an efficient and general method for the synthesis of meso-phosphorylporphyrins. Zinc phosphorylporphyrins thus obtained readily undergo self-organization through P-oxo-Zn coordination to form noncovalently linked, cofacial porphyrin dimers or linear oligomers, which have been characterized by spectroscopic methods and X-ray crystallographic analyses. In toluene, CH(2)Cl(2), and CHCl(3), the zinc phosphorylporphyrins exist mostly as dimers or monomers, depending on their concentrations, the temperature, and the presence of additives. The self-association constants for dimerization in toluene have been determined by UV/Vis absorption titration measurements. The meso-diphenylphosphorylporphyrin dimer displays excitonic coupling of the Soret band with a splitting energy of 940 cm(-1). Fluorescence lifetimes of the zinc phosphorylporphyrins have been found to be affected only slightly by the concentration of the solution, and by the addition of triphenylphosphane oxide, suggesting that the effect of dimerization on their photodynamics in the S(1) state is negligible. On the other hand, the effect of dimerization is clearly reflected in their electrochemical oxidation processes, as the initially produced radical cations are efficiently delocalized over the two porphyrin rings. These findings demonstrate the potential utility of meso-phosphorylporphyrins as new models for the special pair in photosynthesis and as new building blocks for porphyrin-based supramolecular materials.     

Supramolecular triad and pentad composed of zinc-porphyrin(s), oxoporphyrinogen, and fullerene(s): design and electron-transfer studies

By adopting a "covalent-coordinate" bonding approach, novel supramolecular pentad and triad molecules composed of zinc-porphyrin(s), fullerene(s), and oxoporphyrinogen redox-/photoactive entities have been constructed, and also characterized by means of spectral and electrochemical techniques. The geometry and electronic structures of the pentad and the triad were deduced by means of DFT calculations. Free-energy calculations suggested that the photoinduced electron/energy transfer from the zinc-porphyrin (ZnP) singlet-excited state to the imidazole modified fullerene (ImC(60)) acceptor and oxoporphyrinogen (OxP) entities is feasible for both the triad and the pentad. The charge-separation rates (k(CS)) determined from picosecond time-resolved emission studies were higher for pentad (C(60)Im:ZnP)(2)-OxP than for the corresponding triad, C(60)Im:ZnP-OxP. A comparison of the k(CS) values previously reported for the covalently linked bis(zinc-porphyrin)-oxoporphyrinogen triad suggests that employing a fullerene acceptor improves the electron-transfer rates. Nanosecond transient absorption studies provide evidence for the occurrence of electron-transfer processes. Lifetimes of the radical ion pairs (tau(RIP)) are in the range of hundreds of nanoseconds, which indicates that there is charge stabilization in the supramolecular systems.