Stimuli-responsive polymers. 9. Photo-regulation of optical rotations in chiral polyesters: Altering responsive outputs with conformationally flexible backbone elements Gary D. Jaycox * DuPont Central Research and Development, Biochemical Science and Engineering, E328-205B Experimental Station, Wilmington, DE 19880-0328, USA Received 21 August 2006; received in revised form 2 November 2006; accepted 3 November 2006 Available online 1 December 2006 Abstract Conformationally restricted polyesters comprising alternating azobenzene and chiral binaphthylene main-chain units undergo photo-induced oscillations in optical rotatory power when stimulated with sequential intervals of ultraviolet and visible light exposure. The incorporation of flexible (oligo)ethylene oxide backbone segments into these constructs has resulted in a new family of materials having vastly altered photo-responsive chiroptical properties. Light-regulated outputs were found to be strongly dependent upon the length of the flexible backbone segments employed and on their specific locations of placement within the polyester chain. Most notably, the insertion of flexible oligo(ethylene oxide) spacer elements between neighboring binaphthylene and azobenzene main-chain units had a pronounced damping effect on the photo-dynamic behavior exhibited by these chiral systems. Similar results were noted for a related series of oligomeric model compounds that were specifically designed to mimic local monomer sequences present within the larger polyester structures. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Azo polymers; Optical rotation; Light-regulated chiroptical response 1. Introduction Polymers and oligomers that contain atropisomeric R- or S-2,2 0 -binaphthylene segments exhibit novel chiroptical prop- erties that stem from their conformationally dynamic, three- dimensional architectures [1]. The inclusion of main-chain azobenzene stimuliphores into macromolecules of this kind allows for the controlled disruption of local and more global backbone geometries via a series of light-modulated transe cis-isomerization events. As reported earlier [2e11], these reversible structural changes often lead to dramatic fluctua- tions in optical rotatory power and circular dichroism spectra that are strongly correlated to the nature of the solvent medium and other environmental influences. Properly designed, these photo-responsive materials are potentially well suited for the fabrication of sensors and optical switching and recording elements that have relevance to a number of emerging technologies. Previous studies carried out in this laboratory have demon- strated that photo-regulated chiroptical changes tend to be most pronounced for macromolecular constructs that are endowed with conformationally restricted main-chain geome- tries [2e8]. In these systems, steric and electronic perturba- tions afforded by local transecis-azobenzene isomerization reactions are efficiently transmitted throughout the larger polymer structure. As shown in Fig. 1, levorotatory specific rotations recorded for polyester 1R dissolved in THF oscillate over a span of several hundred deg dm 1 g 1 cm 3 when this system is stimulated by multiple UV lightevisible light illumi- nation cycles. This chiroptical output can be conveniently monitored at the sodium D-line (589 nm) and is thus well removed from the dual ‘‘input’’ light frequencies that serve to drive the polymer’s response. Similar results have been * Tel.: þ1 302 695 7138; fax: þ1 302 695 9799. E-mail address: gary.d.jaycox@usa.dupont.com 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.11.006 Polymer 48 (2007) 82e90 www.elsevier.com/locate/polymer