Polymer–peptide templates for controlling electronic interactions of organic chromophores

Abstract

Alignment, orientation and distance between organic chromophores mediate structure–property relationships, and have therefore been widely studied in order to enhance the performance of organic and polymeric semiconductor materials. Biological molecules such as peptides and polypeptides have been used as templates for the study of intermolecular interactions by manipulating the intermolecular distances at the nano-scale level. In this work, we have employed PEGylated, helical peptide templates to effectively display an array of organic chromophores (oxadiazole-containing phenylenevinylene oligomers, Oxa-PPV) in a co-facial or anti-facial manner. Three Oxa-PPV chromophores were strategically positioned on each template, at distances ranging from 6 to 17 Å from each other, as dictated by the chemical and structural properties of the peptide. The Oxa-PPV modified PEGylated helical peptides (produced via Heck coupling strategies) were characterized by a variety of spectroscopic methods. Circular dichroic (CD) spectroscopy showed the helical nature of the scaffolds and the observation of exciton coupled-CD suggested side chain interaction depending on the distance and dihedral angle between chromophores. Chromophores placed in close proximity (6 Å) had reduced emission as compared to the excitonic emission of chromophores placed 17 Å apart. Electronic contributions from multiple pairs of chromophores on a scaffold were detectable; the number and relative positioning of the chromophores dictated the absorbance and emission maxima, thus confirming the utility of these polymer–peptide templates for complex presentation of organic chromophores.