Electron Transfer Modelling of Electrical Dark- and Photoconductivity of Redoxactive Ion Pairs
- 1 July 1994
- journal article
- research article
- Published by Informa UK Limited in Comments on Inorganic Chemistry
- Vol. 16 (3), 113-132
- https://doi.org/10.1080/02603599408035855
Abstract
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 (ΔG 12) 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.Keywords
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