Interplay Between Electronic and Steric Effects in Multiphotochromic Diarylethenes

The structures and electronic features of five complex multiphotochromic molecules incorporating two or three diarylethene units are investigated with quantum mechanical approaches. Four out of the five systems only display partial photochromism, and it is shown that the interplay between steric and electronic effects might explain this outcome. For a spiro-bonded system (I), the doubly closed isomer is reachable because the two photochromes are essentially independent and undergo no specific geometric stress. For a tetrathiafilvalene-bridged derivative (II), there is no steric hindrance, but promoting the electron toward the reactive orbital is not possible. In dimers sharing a central thiophene ring (III and IV), the absence of the closed–closed derivative can be understood by either the compactness of the molecule or by a combination of conformational and electronic (lack of photochromic orbital) factors. Eventually, the reactivity of the trimer, V, is related to the variations of the distances between reactive carbon atoms. This contribution therefore paves the way toward an atomic-scale description of elaborated coupled switches and gives hints for the design of more efficient multiaddressable structures, by proposing a new architecture.