Electron Transfer Modelling of Electrical Dark- and Photoconductivity of Redoxactive Ion Pairs
Titel:
Electron Transfer Modelling of Electrical Dark- and Photoconductivity of Redoxactive Ion Pairs
Auteur:
Kisch, Horst
Verschenen in:
Comments on inorganic chemistry
Paginering:
Jaargang 16 (1994) nr. 3 pagina's 113-132
Jaar:
1994-07-01
Inhoud:
Ion pair charge-transfer (IPCT) complexes consisting of dianionic dithiolene metallates [ML2]2- M = Ni, Pd, Pt, Cu, Zn, and dicationic 4,4'- and 2,2'-bipyridinium derivatives (A2+) exhibit IPCT bands in the VIS-NIR region of the electronic absorption spectrum. When both components are planar, the solid state structure consists of mixed donor-acceptor stacks while that is not the case when one or both are non-planar. By proper selection of the two components the driving force of electron transfer (ΔG12) from the dianion to the dication is varied from 0.7 to -0.1 eV. It is shown that the Hush relation between optical and thermal electron transfer is fulfilled for a number of thirty-two complexes. The reorganization energy is in the range of 60 kJ·mol-1 for the d8 complexes while it is almost two times larger for the d10 zinc compounds. The extent of charge delocalization is typical for outer-sphere complexes as indicated by the parameter α2 which is in the range of 10-4. Correspondingly, the interaction between the two redox states is rather weak as suggested by the values of 220-360 cm-1 calculated for the exchange matrix element. Free activation enthalpies ΔG* of electron transfer, as calculated within the Hush-Marcus model, amount from 0.15 to 0.73 eV. It is found that the electrical dark conductivity of these composite solids can be quantitatively predicted from ΔG* in the range from 10-10 to 10-3 ω-1 cm-1 in the case of complexes consisting of planar components (Class I) while in the presence of a nonplanar acceptor or copper as the central metal (Class II) no similar relation is observed. The electrical photoconductivity is wavelength dependent and exhibits a maximum in the region of the IPCT band. Laser excitation of the solids produces a transient photovoltage (Dember Voltage) with a half-life of about 20 milliseconds. The rcsults suggest that charge generation occurs by electron transfer and charge migration by a hopping mechanism. When a photoisomerizable olefinic acceptor is employed, the conductivity is ten times larger for the cis-isomer.
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