Measuring the size of excitons on isolated phenylene-vinylene chains: From dimers to polymers

We have determined the excess polarizability of the relaxed first excited-singlet-state $S_1$ on isolated chains of phenylene-vinylene oligomers and polymers by measuring the transient change in the microwave dielectric constant that occurs on flash photolysis of dilute solutions. The isotropic value of the excess polarizability volume, $\Delta {\alpha }^{\prime }{(S}_1),$ increases from 250 ${\mathrm{\AA }}^3$ for a dimer to 960 ${\mathrm{\AA }}^3$ for an octamer, and 1600 ${\mathrm{\AA }}^3$ for high molecular weight polymers. The value for the isolated polymer is close to that found for the unrelaxed $S_1$ state in electroabsorption (Stark effect) measurements on solid samples of poly(phenylene-vinylene), indicating that exciton delocalization is not strongly influenced by either post-excitation relaxation of the polymer backbone geometry, or by interchain interactions.