Photochemical reactivity distortions

Correlations between Excited State Distortions and Photochemical Reactivity... 

It is, thus, important that the ruthenium(II) complexes that are to be used as building blocks of the future machines contain sterically hindering chelates so as to force the coordination sphere of the metal to be distorted from the perfect octahedral geometry. We will discuss the photochemical reactivity of rotaxanes and catenanes of this family as well as non-interlocking systems like scorpionates since the lability of bulky monodentate ligands could also lead to useful photosubstitution reactions. 

Second we determine the distortions of molecules by using the combination of electronic and pre-resonance Raman spectra when the electronic spectra are not well enough resolved to be used alone. Third, we show in detail how to determine excited state distortions by using Raman spectroscopy even when the electronic spectra do not show vibronic structure. Once the distortions are determined, we discuss aspects of the meanings of the distortions. For example, we relate the distortions of W(CO)spyridine to the photochemical reactivity of the molecule. We show how the pattern of distortions in a molecule can be used to provide insight useful in the orbital assignment of the transition giving rise to that excited state. Finally, we discuss some of the unusual spectroscopic features which are caused by the distortions and which can be quantitatively interpreted once the distortions are known. 

I. W(CO)s(pyridine) 3. The electronic spectra of this molecule show vibronic structure, but only one progression is apparently observed. (In fact, the progression does not arise from one normal mode its assignment led to the discovery of the MIME which is discussed in Section V.) Because the spectrum cannot be further resolved under any of the conditions which have been tried and because insufficient detail is available from the electronic spectra, Raman information must be used in order to calculate the distortions. This example provides an excellent illustration of how the combination of pre-resonance Raman and electronic spectroscopy are used in conjunction to obtain the desired information. This molecule is also an excellent example for discussion because the distortions provide a detailed basis for interpreting the orbitals involved in the electronic transition and because the relationship between distortions and the molecule s photochemical reactivity can be compared. 

Therefore, it is modulated by the intermolecular displacement occurring in a phonon motion. Photochemical aggregation reactions, such as dimerization or polymerization reactions, can be assisted by the occurrence of strong electron-phonon interaction in the reactive electronic state. This strong electron-phonon interaction creates a local lattice-deformation in the reactive (excited) electronic state. The deformation traps the electronic excitation and, at the same time, it may provide a local preformation of the product lattice if the distortion is along the reaction co-ordinate. Both these features assist a photochemical aggregation reaction. 

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