From molecules through clusters to crystals

Photochemical behaviour of coordination compounds described in previous chapters results mainly from electronic interactions between the central metal atom or ion and ligands in the hrst coordination sphere. An increased size of molecular systems to clusters and nanosized crystals expands the possibility of photoinduced electron transfer between the discrete electronic states to excitation within bands. Furthermore, interactions of nanoparticles with molecules yield unique materials, combining structural versatility of molecular species with collective properties of solids. 

Bioinorganic Photochemistry Grazyna Stochel, Malgorzata Brindell, Wojciech Macyk, Zofia Stasicka, Konrad Szacilowski 2009 Grazyna Stochel, Malgorzata Brindell, Wojciech Macyk, Zofia Stasicka, Konrad Szacilowski. ISBN 978-1 -05-16172-5 

Excitation of the crystalline material, leading to the electron transfer from the VB to the CB by UV or visible light, corresponds to energy of photons in the range of approximately 1-4 eV. The materials characterized by similar bandgap energy can be classified as semiconductors or wide bandgap semiconductors. 

The dependence of bandgap energy (Eg) on crystal size is given by [1]  

A significant difference between electronic properties of molecular and crystalline structures reflects in other spectral features and photochemical behaviour -these topics are described later. 

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