Photophysics and self-assembly of symmetrical and unsymmetrical cationic oligophenylene ethynylenes

A study of the synthesis, photophysical behavior and self-assembly of a series of symmetrical cationic oligophenylene ethynylenes is reported. Aqueous solutions of all these compounds exhibit strong and structured absorption in the ultraviolet (maxima near 305 nm and 355 nm) with a broad fluorescence in the range 370–600 nm. While all of these compounds show strong fluorescence in methanol, the fluorescence yields in water are variable and substituent dependent. Transient absorption, presumably attributable to a triplet is found for all of the oligomers studied thus far. All of the compounds having a net positive charge exhibit strong complex formation when they are added to aqueous solutions containing the anionic biopolymer carboxymethylcellulose. The complexes are characterized by a pronounced red shift in the oligomer absorption spectrum and a red shifted and generally intense fluorescence. We attribute the spectral shifts due to the formation of “J-dimers” in each case. The results of this preliminary study suggest that these compounds may be useful in sensing due to their strong tendency to associate with anionic biomacromolecules.