Stimuli-Responsive Polymers. VII. Photomodulated Chiroptical Switches: Periodic Copolyaramides Containing Azobenzene, Phenylene, and Chiral Binaphthylene Main-Chain Linkages* GARY D. JAYCOX Experimental Station, DuPont Central Research and Development, Wilmington, Delaware 19880-0328 Received 27 January 2003; accepted 9 June 2003 ABSTRACT: Conformationally restricted copolyaramides containing a combination of 4,4'-azobenzene, 1,4-phenylene, and chiral 2,2'-binaphthylene main-chain segments exhibit photoresponsive chiroptical behavior stemming from multiple trans– cis-isomer- ization reactions triggered within their polymer backbones. In contrast to their more randomly constructed counterparts, copolymer variants endowed with periodic back- bone structures undergo reversible, wavelength-dependent inversions in their optical rotations in response to multiple ultraviolet-light/visible-light illumination cycles. Sim- ilar behavior is also observed for a smaller oligomer fitted with a periodic arrangement of its monomer units. In their present forms, the periodic constructs constitute a new class of solution-based, photomodulated chiroptical switches that may be suitable for applications in a number of emerging technological areas. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 566 –577, 2004 Keywords: trans–cis-azobenzene isomerization; azobenzene stimuliphores; helical polymers; axial dissymmetry; exciton coupling; circular dichroism; optical rotation; stimuli-sensitive polymers; photophysics; azo polymers INTRODUCTION Molecules that adopt single-handed helical motifs exhibit optical rotations and circular dichroism (CD) spectra that are strongly tied to their three- dimensional architectures. 1 The reversible dis- ruption of quasistable helical geometries by light, heat, or other stimuli provides a basis for the rational design of new materials with stimuli- responsive chiroptical properties. Polymers and oligomers falling within this category are poten- tially well suited for optical recording media, mo- lecular switching devices, and other photonics- based applications. Efforts within this laboratory remain focused on the development of helical condensation poly- mers and a related series of low-molecular-weight constructs that undergo well-defined structural perturbations in response to externally applied stimuli. 2–6 Endowed with conformationally re- stricted backbone geometries, our polymers are minimally comprised of two essential, main-chain components. These include a chiral helical direct- ing group and a stimuli-responsive chromophore or stimuliphore. Axially dissymmetric 2,2'- binaphthylene linkages serve as effective helical directors when positioned along a polymer back- bone at regular intervals. Chiral helical confor- mations result when only the R- or S-optical iso- mer is used to construct the polymer chain. The *This report is DuPont Contribution 8353. Correspondence to: G. D. Jaycox (E-mail: gary.d.jaycox@ usa.dupont.com) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 566 –577 (2004) © 2003 Wiley Periodicals, Inc. 566