Excited state photochemistry

A number of elegant transient absorption studies have shown that nickel porphyrins possess a rich and complicated excited state photochemistry (3-5). In these studies it was concluded that in basic solvents such as piperidine and pyridine the state of... 

In the preceding chapters it has been seen that the chemical reactions in thermal ( dark ) chemistry originate from the ground state of a molecule, whereas photochemical reactions originate from its excited states. Photochemistry is therefore different from thermal chemistry because electronically excited molecules are different from ground state molecules. 

The thermal generation of reactive excited states ("photochemistry without light") has been reported before (36). 

Because of the much greater driving forces potentially available in reactions between substrates and excited state molecules, difficult—but valuable—electron transfer reactions, such as the oxidation of water or chloride ion, may be accessed through excited state photochemistry. The question of how to separate hole-electron pairs generated in a quenching reaction, how to provide kinetic pathways to lead these two highly reactive species far apart from each another, and how to couple in some useful chemistry are currently of interest. 

Some of the more interesting and valuable redox processes are multielectron in nature, suggesting the utility of coupling a two- or many-electron event into an excited state process. The study of the excited state photochemistry and photophysics of binuclear and polynuclear (cluster) molecules is thus becoming of importance, and two-electron reactions are being identified. 

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)... 

The singlet excited state photochemistry of cyclopropenes is dominated by ring cleavage reactions (Section IV.B.2), but in the triplet manifold [2 -I- 2] cycloaddition is the major pathway. Because of inefficient intersystem crossing the first excited triplet state of a cyclopropene is best populated by sensitization. Despite the added energy involved with these species there is no effective pathway available for ring cleavage in the absence of added olefin stereospecific dimerization to a cis-transoid-cis-tricyclohexane, e.g. 145, occurs. The smallest of the C(3) substituents is endo in the product ". A stepwise... 

The eyeloaddition is, therefore, forbidden in the ground state (no thermally activated reaction) and allowed via an excited state (photochemistry). 

Figure 3.17 Excited-state photochemistry of free 2 and hypothesized primary pathway of the antibody—2 complexes (gray). Dashed lines nonradiative transition solid lines radiative transitions. FC Franck-Condon excited state [64].

Excited state photochemistry. (Reprinted from The American Ceramic Society, www.ceram-ic.org. With permission. Copyright 1990. All rights reserved.)... 

Keywords Computational chemistry Excited states Photochemistry Electronic spectroscopy Non-adiabatic chemistry Jahn-Teller theory... 

Hirao, K., Unno, S., Miura, H., and Yonemitsu, O., Singlet and triplet excited state photochemistry of tricyclo[5.2.2.0 > ]undeca-3,10-dien-9-one derivatives, Chem. Pharm. Bull, 25, 3354,1977. 

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