Synthesis and Photoresponse of Helically Folded Poly(phenyleneethynylene)s Bearing Azobenzene Moieties in the Main Chains

Abstract

Novel optically active poly­(phenyleneethynylene)­s bearing azobenzene moieties in the main chains [poly­(<b>1</b>–<b>2m)</b>, poly­(<b>1</b>–<b>2p</b>)] were synthesized by the Sonogashira–Hagihara coupling polymerization of 3′,5′-diiodo-4′-hydroxy-<i>N</i>-α-<i>tert</i>-butoxycarbonyl-d-phenylglycine hexylamide (<b>1</b>) with 3,3′-diethynylazobenzene (<b>2m</b>) and 4,4′-diethynylazobenzene (<b>2p</b>). The corresponding polymers [poly­(<b>1</b>–<b>2m</b>), poly­(<b>1</b>–<b>2p</b>)], with number-average molecular weights of 10700 and 9400, were obtained in 70% and 86% yields, respectively. CD and UV–vis spectroscopic analyses revealed that poly­(<b>1</b>–<b>2m</b>) and poly­(<b>1</b>–<b>2p</b>) formed predominantly one-handed helically folded structures in CHCl₃/THF mixtures. Poly­(<b>1</b>–<b>2m</b>) underwent a reversible conformational change between folded and unfolded structures upon UV and visible irradiation, as a result of <i>trans</i>–<i>cis</i> isomerization of the azobenzene moieties. On the other hand, poly­(<b>1</b>–<b>2p</b>) showed very little conformational transformation or azobenzene isomerization. The formation of helical structures was supported by conformational analysis based on the molecular mechanics (MM), semiempirical molecular orbital (MO), and density functional theory (DFT) methods