Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline state: Probing the role of a metastable dimer

Cis-trans isomerization is one of the most common and well-investigated photoreaction of olefins in solution. This occurs via 180° rotation of one of the substituents on CC bond requiring large space around the double bond. One would expect that in crystals the neighboring molecules would prohibit such a process. In spite of this in early 1960s Schmidt and co-workers reported occurrence of such a process in crystals via 2 + 2 'meta cyclobutane' formation. In this study by examining the photochemistry of four cis-stilbazolium salts we show that the photoconversion to the corresponding trans isomers does not occur via 2 + 2 addition. Large separation that ensures considerable space between neighboring olefins favors cis-trans isomerization in crystals. An aspect that is puzzling is that the product trans isomer phase separates and recrystallizes within the parent cis crystals and photodimerizes to the cyclobutane dimer. Details of this phenomenon are to be understood.