Crown ether styryl dyes25. Application of molecular mechanics to the study of self-organization into dimeric complexes, regio-, and stereoselectivity of cation-dependent [2+2]-photocycloaddition in photochromic crown ethers

Regio- and stereoselectivity of [2+2]-photocycloaddition in complexes containing crown ether styryl dyes and alkaline-earth metal cations were studied using molecular mechanics. The main assumption is that the rate of the allowed concerted photocycloaddition correlates with the relative energies of the ground states of dimeric adducts, which are treated as pre-reaction complexes, and the resulting cyclobutanes. These energies are estimated by molecular mechanics within the MMX parameterization. The calculated characteristics of different dyes are studied as functions of the structure of the heterocyclic moiety, the structure and size of the N-substituent, the size of the crown ether cycle, and the nature of the metal cation. A comparison between the computational results and the experimental data showed that the observed quantum yield of the reaction is governed by the relative energies of the dimeric complexes and the resulting cyclobutanes as well as by the mutual arrangement of dye molecules in the dimeric complexes. Both factors are closely interrelated. The approach used in this work furnishes an explanation for the experimental data and provides a guideline for supramolecular control of regio- and stereochemistry of cation-dependent [2+2]-photocycloaddition.