Gütlich, P.
Garcia, Yann
[UCL]
Photoreactivity is a widespread phenomenon in organic and inorganic chemistry. A compound may change its structural and/or electronic properties under light irradiation. Light-induced charge separation or cis–trans isomerization processes under light are well-known examples. A phase transformation may occur, with or without hysteresis. If the light-induced new phase possesses sufficiently long lifetime and differs in optical and/or magnetic properties such that it can be easily detected by optical or magnetic means, the material bears the potential for possible technical applications in switching or display devices.
This article deals with light-induced changes of the magnetic and/or optical properties of transition metal compounds to long lived metastable states. Figure 1 sketches the general situation: A complex compound with ground state spin S and orbital momentum Ɣ converts under light of wavelength λ, to a more or less metastable state with different spin and orbital momentum S′ and Ɣ′. The metastable state lies higher in energy and usually has weaker metal–ligand (M–L) bonds and accordingly longer bond lengths r(M–L) than the ground state. This is the consequence of an increasing population of antibonding molecular orbitals and simultaneously decreasing population of weakly π-backbonding molecular orbitals. The energy barrier between the potential wells governs the lifetime of the metastable state, which in turn depends on the horizontal and vertical displacements relative to each other. Switching back from the metastable state to the ground state may be possible by using light of different wavelength λ′. The two-potential-well scheme of Fig. 1 refers to transition metal complexes with strong and intermediate field strength ligands. In weak-field complexes the potential with the longer M–L bond length would be the ground state, but light-induced switching is still possible.


Bibliographic reference |
Gütlich, P. ; Garcia, Yann. Photomagnetism of molecular systems. In: Buschow K. H. J., The Encyclopedia of Materials : Science and Technology, Elsevier Ltd. 2001, p. 6951-6955 |
Permanent URL |
http://hdl.handle.net/2078/92668 |