Theoretical Models for Photochromic Systems

Photo induced spectral changes of some molecules containing electronic π systems are investigated and the effect of electrocyclic reactions (ring closure etc) on their electronic structure is considered. Two types of molecules are studied 1) Molecules which exhibit two stable states characterized by different π electron configurations1) such as 2) electron transfer systems containing a "π electron bridge" linking donor and acceptor which can be modified by a photo induced reaction for example Static electronic π systems are described by a nearest neighbor Huckel Theory, i.e. by a Hamiltonian of the type where ai+, ai are the usual Fermion creation and destruction operators, ε the p electron energy and Γij the coupling between nearest neighbor p orbitals. Due to rotation of molecular fragments Γij depends on the orientation of the relevant p orbitals.