NA+ SPECIFIC EMISSION CHANGES IN AN IONAPHORIC CONJUGATED POLYMER

The emission and absorption characteristics of a conjugated poly(pheny lene bithiophene), 2, and a monomeric model compound, 3, were investig ated as a function of [Li+], [Na+], [K+], and [Ca2+]. The calix-[4]are ne bithiophene receptor that is present in both 2 and 3 provides selec tively for Na+ and the absorption and emission characteristics are not affected by Li+, K+, or Ca2+. Both systems display absorption spectra which are relatively insensitive to Na+; however, the Stokes shift of the emission is reduced by added Na+ For the model system 3, increasi ng [Naf] provides a shift of the emission that is consistent with an e quilibrium mixture of bound and unbound receptor. The polymer 2 displa ys a larger shift in the emission in response to Na+ and due to multip le binding sites lacks an isoemissive point. The chain length of the p olymer also has an effect on this behavior. This behavior may be due t o energy migration to regions of the polymer which do not have bound N a+ and can relax to lower energy conformations. This description is al so borne out by the reduction in the lifetimes of the excited states w ith increasing [Na+] for both the polymer 2 and the model system 3. Th is mechanism may provide a route to systems which can function as digi tal indicators at critical concentrations of analytes.