Molecular complexes transfer states

Thus, irradiation in the transfer band results in practically the whole molecular complex being found in the complete transfer state. The course of the curve corresponding to the increasing ESR signal (Fig. 20) is typical of a number of PCSs and is well described by the relationship AI = where a is constant. 

Optical charge transfer (CT) is commonly observed in un-symmetrical molecules or molecular complexes in which there are sites of distinctly different ionization energies and electron affinities. The origin and properties of optical charge transfer transitions provide the basis for this account. A convenient place to begin chemically is with mixed-valence compounds and two examples are shown below (1-3). In the first (eq 1), the sites of different oxidation states are held in close... 

Excited molecular complexes of the donor-acceptor type are called excimers if formed from identical molecules and exiplexes if originated from different molecules. From the theory, it is concluded that photochemical influence will more readily accelerate electron transfer in a weak donor-acceptor pair than in a strong pair (Juillard and Chanon 1983). An organic molecule in an electron-excited state is a more active oxidant or stronger reducer than the same molecule in a ground state. 

An electronic excited state of a metal complex is both a stronger reductant and oxidant than the ground state. Therefore, complexes with relatively long-lived excited states can participate in inter-molecular electron transfer reactions that are uphill for the corresponding ground state species. Such excited state electron transfer reactions often play key roles in multistep schemes for the conversion of light to chemical energy ( 1). 

Because charge-transfer transitions of lanthanides in complexes involve ligand electrons that participate in chemical bonding at molecular scale, the energy levels of charge-transfer states are more sensitive to environments and particle size than that of the 4f and 5d states. The intensity and energy of charge-transfer transitions vary in nanocrystals as a function of particle size. This subject is covered in a recent review by Tanner (Tanner, 2005). 

However, for n,ir triplet aromatic ketones and aldehydes, the increased spin delocalization onto the rings should reduce the zero-field splitting, and the failure of their ESR detection may be attributed mainly to their short lifetimes. Hayashi and coworkers (290) have examined the ESR fine-structure parameters of a number of molecular complexes in their phosphorescent state and have derived the dependence of the fine-structure values upon the degree of charge-transfer character in the lowest triplet state of the molecular complexes. Some of their results are collected in Table 15. 

EXCITED CHARGE-TRANSFER COMPLEXES - Complexes produced by excitation of ground state molecular complexes for which there is conclusive evidence for association in the ground state. 

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