Topochemistry. Part XVIII. The solid-state photochemistry of some heterocyclic analogues of trans-cinnamic acid

Abstract

β-2-Furylacrylic acid irradiated with λ > 290 mµ gives an oligomer of D.P. ∼30 as well as dimer identified chemically and by its n.m.r. spectrum as the furyl analogue of β-truxinic acid. β-2-Thienylacrylic acid is dimorphic: its (thermodynamically) stable modification polymerises while the metastable form gives the β-truxinic dimer and small amounts of oligomer. 5-Methyl-2-thienylacrylic acid yields oligomer. β-3-Pyridylacrylic acid dimerises to the β-truxinic analogue; β-3-indolylacrylic acid is light-stable. According to the u.v. spectra all oligomers contain partially reduced ring systems.

The photo-behaviour of this series of compounds is interpreted in terms of the packing arrangement of the monomers and of the reactivity of the ring systems: the formation of β-truxinic dimers follows from the parallelism of monomers related by a 4 Å translation axis; polymerisation is initiated by the attack of photo-excited exocyclic [graphic omitted] groups on sufficiently close double bonds of reactive (furan, thiophen) ring systems.