Published on Web 01/05/2006
Helical Conformational Dynamics and Photoisomerism of
Alternating Pyridinedicarboxamide/m-(Phenylazo)azobenzene
Oligomers
Chenyang Tie, Judith C. Gallucci, and Jon R. Parquette*
Contribution from the Department of Chemistry, The Ohio State UniVersity,
Columbus, Ohio 43210
Received July 14, 2005; Revised Manuscript Received November 18, 2005; E-mail: parquett@chemistry.ohio-state.edu
Abstract: Alternating sequences of pyridine-2,6-dicarboxamides and meta-(phenylazo)azobenzenes have
been assembled into oligomers composed of four (8) and eight (9) azobenzene linkages. X-ray
crystallography confirmed that oligomer 8 adopts a two-turn helical conformation with a helical pitch of
approximately 3.4 Å in the solid state. The presence of a two- and four-turn helical conformation of 8 and
9, respectively, in polar and nonpolar solvents was elucidated by the anisotropic upfield shifting of protons
located within the helices, NOE enhancements between protons oriented toward the helix interior, and the
diastereotopicity of the terminal benzyloxycarbonyl (CBz) methylene protons. 1H NMR line shape analysis
of the CBz methylene hydrogens at the chain ends revealed a dynamic equilibria interconverting M and P
helical conformations with energetic barriers (∆Gq) of 11.1 (∆Sq ) -19.4 ( 1.6 cal mol-1 K-1; ∆Hq ) 6.5
( 0.4 kcal/mol) for 8 and 13.8 kcal/mol (∆Sq ) -6.6 ( 6.2 cal mol-1 K-1; ∆Hq ) 11.8 ( 1.8 kcal/mol) for
9. Irradiation of the oligomers with 350 nm light induces an E f Z isomerization of the azo linkages that
decreases in efficacy at longer helix lengths. The suppression of E f Z isomerization is a consequence of
the contrasting behavior of the azo linkages located at the helix termini, which afford Z/E ratios similar to
those of model compound 7d, and the internal azo groups, which undergo significantly lower Z/E conversion
ratios compared with 7e.
following a structural perturbation.6 However, the polydisperse
nature of polymeric systems complicates analysis of the
Introduction
The interdependence of structure and function in biological
systems1 provides tremendous impetus for the design of
unnatural oligomers that adopt compact, folded conformations
resembling natural secondary structure.2 In these natural systems,
highly dynamic conformational processes serve to correlate local
and global structural motions.3 Understanding the nature of this
conformational interplay in synthetic systems will be important
for progress toward functional systems to be fully realized.4
Therefore, the development of folded abiotic systems that
respond to external stimuli represents a particularly important
objective in the search for function.5 Accordingly, intrinsically
helical polymers and oligomers have been shown to experience
M-P helical and helical-nonhelical conformational transitions
structural source of the conformational changes. The design of
discrete folded oligomers composed of isomerizable repeat units
should facilitate studies directed at understanding the delicate
interplay between local and global conformational dynamics and
structure. Recent studies in folded oligomers have shown that
conformational perturbations induced by protonation,7 metal
coordination,8 changes in solvent polarity,9,10 and anion11 or
cation12 binding effect corresponding changes in structure.
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J. AM. CHEM. SOC. 2006, 128, 1162-1171
10.1021/ja0547228 CCC: $33.50 © 2006 American Chemical Society