Various types of topochemical [2+2] photocycloaddition behaviors are described focusing on the recent results of a series of alkyl pyridylethenylcinnamate derivatives. Highly stereo regular polymerization and “absolute” asymmetric synthesis were achieved by using crystal-lattice controlled [2+2] photocyclo-additions. Product control by employing co-crystallization of solvent molecules and a low temperature photo irradiation technique is demonstrated in the topochemical photoreactions of diolefin molecules. A methodology for the synthesis of highly strained compounds, [2.2]paracyclophane derivatives, in the crystalline state is also described.