Transition metal complexes, photoactive

Among what have been widely employed as model compounds for Chi, are porphyrins, phthalocyanines, and some photoactive transition metal complexes, which are more stable and easier to obtain than Chi. Interfacial layers of these insoluble compounds are generally prepared by means of vacuum sublimation or solvent evaporation. 

Dyads and triads based on the photoactive, multibridging [Ru(bpz)3] (bpz = bipyrazine) complex directly bound to transition metal complexes were obtained by following the procedures previously reported for the generation of symmetric heptanuclear supermolecules (67-69). Such systems contain a tris(bpz)ruthenium (II) ion [RuJ attached to bis(bpy)chlororuthenium(II)/(III) [Rup], or penta-cyanoferrate(II)/(III) complexes via a bpz bridging ligand, as shown for the... 

Herein, we wdl discuss several approaches that have led from molecular entities to supramolecular soft and hard systems. In particular, we will show how the molecular structure can be modified to induce the controlled self-assembly of transition metal complexes into sophisticated photoactive arrays with imusual properties derived from the structiu-e of the metal complexes and their intermolecular interactions in the ground and/or excited electronic states within the assemblies. We will start with a survey of the photophysical properties of selected transition metal complexes, followed by an overview of the aggregation mechanism they can undergo to. We will focus our attention on soft assemblies... 

A photoactive metal center is introduced in these systems. The ruthenium bipyridyl complexes are coordinated to the emeraldine base to form the corresponding polymer complexes as described above." The incorporation of the ruthenium centers to the pyridyl backbone has been also reported to give the ruthenium complexes Conjugated ruthenium bipyridine complexes thus obtained are evaluated to be photorefractive materials. Other transition metal complexes can be employed to form the corresponding polymer complexes. The pyridine unit is replaced by bithienyl, 1,4-diazabutadiene, ethylene, benzimidazole or thiazole. ... 

This chapter reviews the major advances in the field of photochemistry and photocatalysis by transition metal compounds published in 2013-2014. Particular attention has been given to (i) photocatalysis in synthesis, and in the conversion of sunlight energy into chemical energy (ii) photoreactivity (iii) biomedical applications of photoactive transition metal complexes, e.g. as photo-CORMs and PDT agents. 

Numerous examples of photoinduced electron transfer reactions can be found in the literature, making use of Ru(bipy)3 as photoactive component [6], the redox partner being an inorganic cation, a transition metal complex or an organic species. 

Recent work (20, 21) in our laboratory has focused upon the use of transition metal compounds to sensitize the energy-storing valence isomerization of norbornadiene, NBD, to quadricyclene, Q (Reaction 3). In particular we have found that a catalytic amount of CuCl functions as an effective and quite specific sensitizer for this transformation. Conversions of greater than 90% have been achieved since Cu(I) is ineffective as a catalyst for the energy-releasing reverse reaction. Spectral and photochemical evidence support a mechanism which features a 1 1 ClCu—NBD complex as the photoactive species. As illustrated in Figure 3, an obvious consequence of complexation is a shift of the absorption spectrum of the system into a region accessible to the 313-nm irradiation used. Possible pathways by which the photo-excited complex relaxes to Q have been discussed (12). 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

In this part, complex electrochemical interfaces and electrochemical reactions on surfaces with various molecules in solvents will be discussed. Examples are the oxidation and evolution of hydrogen on different transition metal surfaces, the reduction of oxygen on several surfaces as well as carbon monoxide reactions, and a complex photoactive reaction in a solar cell. 

Rhodium(III) systems are formally analogous to cobalt(III) in that both are d systems. The larger Dq values for second-row transition metals makes all the Rh(III) complexes more analogous to the cyano complexes of Co(III). Studies with Rh(III) have the advantage that the rate of decay of the photoexcited states can be measured. The area was reviewed by Mpnsted and Mdnsted and by Skibsted. The photoactive state appears to be the ( E for Rh(L)5X symmetry), and the rate constants, for phosphorescent decay from this state in the solids at 77 K are given in Table 7.3. A more recent and extensive compilation has been given in a review by Forster. Some values of ik in representative solvents are given in Table 7.4. 

The prospect of developing new materials of relevance to the emerging field of molecular electronics, modelling electron-transfer processes in biological systems and producing new electroactive and photoactive catalysts has led in recent years to considerable interest in transition metal polypyridyl complexes. Two recent examples of the application of the OTTLE spectroelectrochemical technique to the study of these fascinating systems are described here. 

A full report has been published on the resonance Raman spectra of M(CO)4 (di-imine) (M = Cr, Mo, or W), and their relevance to the photosubstitution reactions of the complexes has been discussed.It was observed that the complexes are only photoactive if (CO ) shows resonance enhancement of Raman intensity. This enhancement is a result of the CT transition causing delocalization of the negative charge over the cw-carbonyls and hence reduction of 7i-backbonding with the metal and weakening of the M—CO bond. 

The photochemical displacement of the arene is also observed for the complexes ( / -cp)(7 -arene)M (M = Fe, Ru). This reaction occurs from the photoactive a E ligand field excited state. A linear correlation exists between log (0/(l-0)) and o-p, the Hammett parameter for a series of complexes with chloro-and methyl-substituted arenes. The data indicate that a small amount of negative charge builds up at the arene in the transition state for the reaction that results in arene dissociation in these systems. The temperature dependence for the photodissociation of the arene ligand indicates that the metal-arene bond is almost broken in the a E excited state. When the analogous pentamethylcyclopentadienyl complexes (7 -cp )( / -arene)M (M = Fe, Ru) are photolyzed, the quantum yields for arene release are lower than those found for the unsubstituted compounds... 

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