1,721,019 research outputs found
Relaxation Dynamics and Transient Behavior of Small Arenethiol Passivated Gold Nanoparticles
No full textNovel gold nanoparticles, passivated by monolayers of benzenethiol, biphenylthiol, and similar derivatives, have been synthesized and characterized using UV/vis, NMR, and Fourier transform infrared (FTIR) spectroscopies. The nanoparticle sizes have been evaluated using transmission electron microscopy and UV/vis spectroscopy; they show diameters between 2.1 and 4.7 nm, depending on the method of synthesis and the monolayer protecting group. Femtosecond transient absorption measurements show that the nanoparticles possess optical properties on the boundary between molecular and nanoparticle behavior. The smaller systems based on benzenethiol exhibit long-lived excited states with lifetimes on the order of a few nanoseconds, resembling those of small gold molecular type clusters. The larger nanoparticles protected with biphenylthiol and benzylthiol groups relax much more rapidly on a picosecond time scale, similarly to related citrate stabilized systems reported in the literature
Site-specific electronic couplings in dyads with MLCT excited states. Intramolecular energy transfer in isomeric Ru(II)-Ru(II) cyclometalated complexes
No full textThe rod-like binuclear complexes [(ttpy)Ru(tpy-ph2-phbpy)Ru(ttpy)]4+ and
[(ttpy)Ru(tpy-ph2-tpy)Ru(phtbpy)]4+ (for abbreviations, see text) have been synthesized and
characterized. In both complexes, the polypyridine Ru(II) centers have (N∧N∧N)Ru(N∧N∧N) and
(N∧N∧N)Ru(C∧N∧N) coordination environment. The two isomeric species differ in whether the
cyclometalating carbon resides on the bridging or on the terminal ligand. The two complexes have
virtually identical energy levels, but MLCT excited states of different (bridging or terminal) ligand
localization. They are thus ideally suited to investigate possible effects of excited-state localization on
intramolecular energy transfer kinetics. In fact, ultrafast spectroscopic measurements yield different
energy transfer time constants for the two isomers, with the bridge-cyclometalated complex (2.7 ps)
being faster than the terminal-cyclometalated one (8.0 ps). This difference can be explained in terms of
different electronic factors for Dexter energy transfer. The study highlights the peculiar intricacies of
intramolecular energy transfer in inorganic dyads involving MLCT excited states
Electronic Energy Transfer in a Multiporphyrin-based Molecular Box
No full textThe molecular box 1 comprises of two zinc-porphyrin metallacycles connected bytwo free-base 4’-trans-dipyridylporphyrins, axiallycoordinated to the zinc centers. The photophysics of 1 were studied in chloroform byemission and ultrafast absorption spectroscopy. In the molecular box, fast singlet energy transfer (main component, t=32 ps) is observed to occur from the zinc-porphyrin metallacycles to the free-base chromophores. From wavelength- dependent spectrofluorimetric data, the efficiencyof the energy-transfer (ET) process is estimated as 0.5. The lower-thanunityvalue is tentativelyattributed to the possibilityof a competing electron-transfer quenching pathway. Molecular box 1 can be considered to be a simple, self-assembling, six-chromophore antenna system. It has an inner cavity, 11.4 < wide, that could be used, in principle, to host a varietyof guest molecules and obtain higher-order assemblies
Photophysical Properties of Metal-Mediated Assemblies of Porphyrins
No full textThe metal-driven construction of multi-porphyrin assemblies, which exploits the formation of coordination bonds between peripheral basic site(s) on the porphyrins and metal centers, has recently allowed the design and preparation of sophisticated supramolecular architectures whose complexity and function begin to approach the properties of naturally occurring systems. Within this framework, meso-pyridyl/phenyl porphyrins (PyPs), or strictly related chromophores, can provide geometrically well-defined connections to as many as four metal centers by coordination of the pyridyl peripheral groups. Several discrete assemblies of various nuclearities, in which the pyridylporphyrins are linkers binding metalloporphyrins and/or coordination compounds, have been constructed in recent years. In this review, we summarize recent work from our laboratories on metal-mediate
The role of oxygen in the mechanism of the intramolecular photoredox reaction of Fe(III) protoporphyrinIX in alkaline aqueous ethanol.
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Ultrafast Processes in Bimetallic Dyads with Extended Aromatic Bridges. Energy and Electron Transfer Pathways in Tetrapyridophenazine-bridged Complexes
No full textThe energy and electron transfer processes taking place in binuclear polypyridine complexes of-ruthenium and osmium based on the tetrapyrido[3,2-a:2¢,3¢-c:3¢¢,2¢¢-h:2¢¢¢-3¢¢¢-j]phenazine bridging ligand-(tpphz) have been investigated by ultrafast absorption spectroscopy. In the binuclear complexes, each-chromophore is characterized by two spectrally distinguishable metal-to-ligand charge transfer (MLCT)-excited states: MLCT1 (with promoted electron mainly localized on the bpy-like portion of tpphz, higher-energy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lower-energy). In the homodinuclear complexes Ru(II)-Ru(II) and Os(II)-Os(II), MLCT1 f MLCT0 relaxation-(intraligand electron transfer) is observed, with strongly solvent-dependent kinetics (ca. 10-10 s in CH2Cl2,-ca. 10-12 s in CH3CN). In the heterodinuclear Ru(II)-Os(II) complex, *Ru(II)-Os(II) f Ru(II)-*Os(II) energy-transfer takes place by two different sequences of time-resolved processes, depending on the solvent: (a)-in CH2Cl2, ruthenium-to-osmium energy transfer at the MLCT1 level followed by MLCT1 f MLCT0 relaxation-in the osmium chromophore, (b) in CH3CN, MLCT1 f MLCT0 relaxation in the ruthenium chromophore-followed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)-Os(III)-species, the *Ru(II)-Os(III) f Ru(III)-Os(II) electron transfer quenching is found to proceed by two-consecutive steps in CH3CN: intraligand electron transfer followed by ligand-to-metal electron transfer.-On a longer time scale, charge recombination leads back to the ground state. Altogether, the results show-that the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transfer-processes with its electronic levels.
Photochemistry and Photophysics of Coordination Compounds: Rhodium
No full textRhodium(III) polypyridine complexes and their cyclometalated analogues display photophysical properties of considerable interest, both from a fundamental viewpoint and in terms of the possible applications. In mononuclear polypyridine complexes, the photophysics and photochemistry are determined by the interplay between LC and MC excited states, with relative energies depending critically on the metal coordination environment. In cyclometalated complexes, the covalent character of the C – Rh bonds makes the lowest excited state classification less clear cut, with strong mixing of LC, MLCT, and LLCT character being usually observed. In redox reactions, Rh(III) polypyridine units can behave as good electron acceptors and strong photo-oxidants. These properties are exploited in polynuclear complexes and supramolecular systems containing these units. In particular, Ru(II)-Rh(III) dyads have been actively investigated for the study of photoinduced electron transfer, with specific interest in driving force, distance, and bridging ligand effects. Among systems of higher nuclearity undergoing photoinduced electron transfer, of particular interest are polynuclear complexes where rhodium dihalo polypyridine units, thanks to their Rh(III)/Rh(I) redox behavior, can act as twoelectron storage components. A large amount of work has been devoted to the use of Rh(III) polypyridine complexes as intercalators for DNA. In this role, they have proven to be very versatile, being used for direct strand photocleavage marking the site of intercalation, to induce long-distance photochemical damage or dimer repair, or to act as electron acceptors in long-range electron transfer processes
Aluminium pyridyl porphyrins. Versatile platforms for self assembling of electron transfer photosystems
No full textTowards the design of artificial photosystems, non-covalent self-assembling strategies exhibit several attractive features: mild conditions, combinatorial flexibility, repair capability. Their
application to intrinsically non-symmetric systems is not obvious, however. We have recently described the spontaneous self-assembly of a triad for photoinduced charge separation based
on a bifunctional Aluminium pyridylporphyrin platform [1]. There, the Lewis acid and basic functions (metal center and meso-pyridyl group) were used to bind, respectively, electron
acceptor and donor units. We are now working to extend such type of strategy for the assembling of systems forphotocatalytic hydrogen evolution. For instance, on the same platform, complex units with hydrogen evolving catalytic activity could be coordinated via the meso-pyridyl group. On the other hand, axial coordination at the Al center can be exploited to reversibly bind a variety of
sacrificial electron donors containing carboxylic functions
Atmospheric reactivity of 1,3-butadiene - nitrogen monoxide and acrolein - nitrogen monoxide systems.
No full textThe simulated photooxidation of 1,3-butadiene and acrolein in the presence of nitrogen oxides has been investigated by irradiation of NO- 1,3-butadiene -air and NO- acrolein - air mixtures in a 760 L smog chamber. Data on the consumption rate of the organic species and on the conversion rate from NO to NO2 were obtained
Photophysics of Metal-mediated Assemblies of Porphyrins and Perylene Bisimides
No full textInteresting supramolecular architectures can be obtained exploiting the exocyclic coordination ability of pyridylporphyrins. Thus, coordination to metal centers can be used to build metal-bridged porphyrin assemblies (e.g., 1). On the other hand, axial coordination to metalloporphyrins can be used to produce a variety of side-to-face porphyrin assemblies (e.g., 2). The two structural motifs can be combined in higher order assemblies (e.g., 3). Other interesting supramolecular systems can be designed along the same lines using pyridyl-functionalized perylene bisimides (e.g., 4). The photophysics of these multichromophoric assemblies is characterized by the occurrence of fast intercomponent energy and electron transfer processes. Recent examples will be discusse
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