Metal states, photochemistry

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

J3, 4). Low-lying electronic excited states of metal-metal bonded complexes often involve significant changes in the electron density associated with the metal-metal bond, compared to the ground electronic state. Accordingly, study of the photochemistry of metal-metal bonded complexes not only provides potential new reaction chemistry but also provides insight into, and confirmation of, the electronic structure. This symposium volume affords us an opportunity to record the state of the field of metal-metal bond photochemistry. The aim of this article is to summarize recent research results from this laboratory and to place them in perspective in relation to results from other laboratories. 

Of significant interest are the Ru(bipy)3"+ and Cr(bipy)3 + complexes. The former, along with derivatives, can be used as sensitizers in photolytic systems such as the photolysis of water, and the Ru /Ru couple can be tuned by varying the nature of the bipy ligand. The Cr(bipy)3 + cation is one of the standard substances for probing excited state photochemistry see Photochemistry of Transition Metal Complexes)... 

Hasselbrink E 1994 State-resolved probes of moleoular desorption dynamios induoed by short-lived eleotronio exoitations Laser Spectroscopy and Photochemistry at Metal Surfaces ed E H L Dai and W Ho (Hong Kong World Soientifio) p 685... 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

Even when the d-d state is at much higher energy than the emitting level, it can still be of paramount importance in the photophysics and photochemistry of the system. Indeed, a major contributor to the temperature-dependent loss of emission intensity in luminescent metal complex based sensor materials is nonradiative decay via high-energy d-d excited states.(15) The model for this is shown in Figure 4.4A. The excited state lifetime is given by... 

While the area of the photochemistry of metal complexes with DNA is still at an early stage of development, substantial progress has been made in the last ten years. It is apparent from the different chapters of this review that a knowledge of the photophysimetal complex s excited state can be used to predict the type of reaction caused to the DNA. 

Oxygenated Bronzes , P. Hagenmuller, Progr. Solid State Chem., 1971, 5, 71. The Photochemistry of Transition-metal Co-ordination Compounds , W. L. 

Over the past several years we have been interested in determining to what extent the photochemistry of complex ions of various transition metal ions resemble thermal reaction chemistry as to products, and to what extent the behavior varies with the wave length or type of excited state produced. 

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