Nathan D. McClenaghan

Dendrimers based on ruthenium(II) and osmium(II) polypyridine complexes and the approach of using complexes as ligands and complexes as metals

The use of the “complexes as ligands and complexes as metals” synthetic strategy for the preparation of luminescent and redox-active Ru(II) and Os(II) dendrimers, the dominant synthetic approach for this novel class of compounds, is reviewed. A few comments on the photophysical and redox properties of the metal dendrimers are provided and an overview of alternative synthetic approaches is also presented.

BF2-Azadipyrromethenes: Probing the Excited-State Dynamics of a NIR Fluorophore and Photodynamic Therapy Agent

BF(2)-Azadipyrromethene dyes are a promising class of NIR emitter (nonhalogenated) and photosensitizer (halogenated). Spectroscopic studies on a benchmark example of each type, including absorption (one and two photon), time-resolved transient absorption (ps-ms) and fluorescence, are reported. Fast photodynamics reveal that intense nanosecond NIR fluorescence is quenched in a brominated analog, giving rise to a persistent (21 μs) transient absorption signature. Kinetics for these changes are determined and ascribed to the efficient population of a triplet state (72%), which can efficiently sensitize singlet oxygen formation (ca. 74%), directly observed by (1)Δ(g) luminescence. Photostability measurements reveal extremely high stability, notably for the nonhalogenated variant, which is at least 10(3)-times more stable (Φ(photodeg.) = < 10(-8)) than some representative BODIPY and fluorescein dyes.

Recent advances in luminescent polymetallic dendrimers containing the 2,3-bis(2′-pyridyl)pyrazine bridging ligand

Abstract Some new developments in the area of metal-based light-harvesting dendrimers based on the 2,3-bis(2′-pyridyl)pyrazine bridging ligand are presented, with particular regard to unidirectional energy transfer, enhanced light absorption, coupling dendritic structures with electron donors, and measurement of some of the ultra-fast processes occurring in this class of compounds.

Photocatalyzed sulfide oxygenation with water as the unique oxygen atom source.

In our research program aiming to develop new ruthenium-based polypyridine catalysts for oxidation we were interested in combining a photosensitizer and a catalytic fragment within the same complex to achieve catalytic light-driven oxidation. To respond to the lack of such conjugates, we report here a new catalytic system capable of using light to activate water molecules in order to perform selective sulfide oxygenation into sulfoxide via an oxygen atom transfer from H(2)O to the substrate with a TON of up to 197 ± 6. On the basis of electrochemical and photophysical studies, a proton-coupled electron-transfer process yielding to an oxidant Ru(IV)-oxo species was proposed. In particular, the synergistic effect between both partners in the dyad yielding a more efficient catalyst compared to the bimolecular system is highlighted.

Enantiopure dendritic polyoxometalates: chirality transfer from dendritic wedges to a POM cluster for asymmetric sulfide oxidation.

The quest for new catalytic and highly enantioselective processes is currently one of the most intensively studied areas in chemical synthesis. In this context, chiral polyoxometalates (POMs) have become a topic of recent interest owing to their potential application in medicine and asymmetric catalysis. Among numerous applications of POMs, catalysis is by far the most studied, owing to the enormous versatility that POMs offer in the clean synthesis of fine chemicals. However, few chiral POM compounds for asymmetric catalysis are known. Of those reported, none are based on dendritic structures, although dendritic counter cations are known to play a critical role in determining the properties of anionic POMs. Two main approaches have been developed for the synthesis of chiral POM-based frameworks. The first involves the use of a chiral species (organic moieties or metal complexes) as a chirality transfer agent. The second strategy is based on spontaneous resolution upon crystallization in the absence of any chiral auxiliary, which yields conglomerates. For example, enantiopure hafnium-substituted POMs, which are obtained by spontaneous resolution upon crystallization in the absence of any chiral source have been reported. Recently, two enantiomerically pure 3D chiral POM-based architectures were developed from achiral moieties without any chiral auxiliaACHTUNGTRENNUNGry.[6b–c] However, despite their potential applications in catalysis and separation, enantiopure POM frameworks are still under represented. The search for suitable enantiomerically pure POM-based hybrid materials remains a challenging issue in synthetic chemistry and materials science. To date, chiral dendritic POM systems, as well as the study of chiroptical activity of enantiopure POM-based frameworks in asymmetric catalysis have, to the best of our knowledge, never been reported. Herein, we report the synthesis and characterization of four enantiopure polyoxometalate-cored dendrimers, which contain n-propyl [(R)-(+)-6 and (S)-( )-6] and epoxy [(R)(+)-7 and (S)-( )-7] groups, based on electrostatic interactions between enantiopure dendritic quaternary ammonium ions and an achiral trianionic POM. To the best of our knowledge, these compounds represent the first examples of optically active dendritic POM systems. Importantly, the POM cluster displays chiroptical effects, indicating chirality transfer from the enantiopure dendritic ammonium ions. The optical and chiroptical properties of the n-propyl-terminated POM-cored dendrimers (R)-(+)-6 and (S)-( )-6 have been demonstrated in solution by UV/Vis spectroscopy, circular dichroism (CD) and vibrational circular dichroism (VCD) spectroscopy, as well as fluorimetry. In addition, the use of 6 as a catalyst in the asymmetric oxidation of thioanisole with aqueous H2O2 provides the corresponding optically active sulfoxide with up to 14% enantiomeric excess (ee) as a result of a chirality transfer process from the dendritic wedge to the catalytically active POM unit. Despite the modest enantioselectivity, this proof-of-principle catalytic experiment demonstrates and confirms the transfer of chiroptical properties from organic moieties to a catalytically active POM unit. Interestingly, the catalyst was recovered and reused in three successive cycles without discernable [a] C. Jahier, Dr. M. Cantuel, Dr. N. D. McClenaghan, Dr. T. Buffeteau, Dr. D. Cavagnat, Dr. S. Nlate ISM, UMR CNRS No. 5255, Universit Bordeaux I 351 Cours de la Lib ration, 33405 Talence Cedex (France) Fax: (+33)5-4000-6994 E-mail : s.nlate@ism.u-bordeaux1.fr [b] Dr. F. Agbossou UCCS UMR CNRS N8 8181, Universit de Lille 59652 Villeneuve d’Asq Cedex (France) [c] Dr. M. Carraro, Prof. M. Bonchio Department of Chemical Sciences and ITM-CNR University of Padova, Via Marzolo, 1 35131 Padova (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901512.

Chemical approaches to nanometre-scale logic gates

External stimulus-induced changes in molecular read-out give rise to bistable nanoscopic switches. Beyond simple bistable species, multistate (multi-input/output) systems lead to the idea of molecular logic gates. Since the appearance of a prototypical molecular logic gate in 1993, different strategies have been described for developing these molecular information processors. Notably, various photophysical processes have been employed to effect the requisite changes in photonic output as a function of the input. Equally, different questions have been addressed with varying degrees of success concerning the connectivity/integration of logic gates and the superposition/multiplexing of optical logic gates. After introducing pertinent processes available in the chemists toolbox for designing functional nanometric devices, illustrated with a few simple examples, more sophisticated combinatorial logic arrays are described, including molecular systems capable of numeracy.

Metal complexes as components of luminescent signalling systems

Abstract Freshly developing lines of research in luminescent signalling with metal complexes over the last year are critically discussed. Metal-containing moieties are found to serve as remarkable lumophores with delayed emission and also as efficient receptors especially for anionic species. The photophysical concepts of metal-centred (MC) excited states, metal to ligand charge transfer (MLCT) excited states, metal–metal to ligand charge transfer (MMLCT) excited states, photoinduced electron transfer (PET) processes and electronic energy transfer (EET) processes are all being exploited during these signalling operations. Novel sensors and switches are emerging as a result.

The Anthracen-9-ylmethyloxy Unit: An Underperforming Motif Within the Fluorescent PET (Photoinduced Electron Transfer) Sensing Framework

Compound 2, which was designed to act as a fluorescent sensor for calcium according to the PET (Photoinduced Electron Transfer) principle, shows a relatively small Ca2+-induced fluorescence enhancement factor (FE) of 1.8 whereas its close relative 1 is known to display a far higher FE value of 16. Though designed as fluorescent PET sensors for solvent polarity, compounds 5 and 6 also show negligible fluorescence enhancement as their environments are made progressively less polar even though their relatives 3 and 4 show limiting FE values of 53 and 3, respectively. Indeed, 3 and 4 are useful since they are fluorescent sensors for solvent polarity without being affected by Bronsted acidity. The poor sensory performance of 2, 5, and 6 relative to their cousins is attributed to the presence of an oxygen proximal to the 9-position of an anthracene unit, which opens up a CT (charge transfer) channel. Normal PET sensing service is resumed when the offending oxygen is deleted.

Direct observation of reversible electronic energy transfer involving an iridium center.

A cyclometalated iridium complex is reported where the core complex comprises naphthylpyridine as the main ligand and the ancillary 2,2′-bipyridine ligand is attached to a pyrene unit by a short alkyl bridge. To obtain the complex with satisfactory purity, it was necessary to modify the standard synthesis (direct reaction of the ancillary ligand with the chloro-bridged iridium dimer) to a method harnessing an intermediate tetramethylheptanolate-based complex, which was subjected to acid-promoted removal of the ancillary ligand and subsequent complexation. The photophysical behavior of the bichromophoric complex and a model complex without the pendant pyrene were studied using steady-state and time-resolved spectroscopies. Reversible electronic energy transfer (REET) is demonstrated, uniquely with an emissive cyclometalated iridium center and an adjacent organic chromophore. After excited-state equilibration is established (5 ns) as a result of REET, extremely long luminescence lifetimes of up to 225 μs result, compared to 8.3 μs for the model complex, without diminishing the emission quantum yield. As a result, remarkably high oxygen sensitivity is observed in both solution and polymeric matrices.

Copper Catalyst Activation Driven by Photoinduced Electron Transfer: A Prototype Photolatent Click Catalyst

PET cat. While the copper(II) tren ketoprofenate precatalyst 1 (see picture) is inactive at room temperature in methanol, it is quantitatively and rapidly reduced to its cuprous state upon light irradiation to provide a highly reactive click catalyst. By simply introducing air into the reaction medium the catalysis can be switched off and then switched on again by bubbling argon followed by irradiation.