The emission and absorption characteristics of a conjugated poly(phenylene bithiophene) and a monomeric model compd. were investigated as a function of [Li+], [Na+], [K+], and [Ca2+]. The calix[4]arene bithiophene receptor that is present in both compds. provides selectively 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, increasing [Na+] provides a shift of the emission that is consistent with an equil. mixt. of bound and unbound receptor. The polymer displays a larger shift in the emission in response to Na+ and due to multiple binding sites lacks an isoemissive point. The chain length of the polymer also has an effect on this behavior. This behavior may be due to energy migration to regions of the polymer which do not have bound Na+ and can relax to lower energy conformations. This description is also borne out by the redn. in the lifetimes of the excited states with increasing [Na+] for both the polymer and the model system. This mechanism may provide a route to systems which can function as digital indicators at crit. concns. of analytes. [on SciFinder(R)]