  Optical excitation of a chain in a polymer film may result in formation of an
excimer, a superposition of on-chain excitons and charge-transfer excitons on
the originally excited chain and a neighboring chain. The excimer emission is
red-shifted compared to that of an on-chain exciton by an amount depending on
the interchain coupling $t_\perp$. Setting up the excimer wavefunction and
calculating the red shift, we determine average $t_\perp$ values, referred to a
monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy $p$-phenylene
cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1, 4
p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the
effect of interchain distance on the emission.
