Quantities such as the PR are often of interest in the study of conjugated polymers where they may be used to define an effective length scale for an exciton, which need not be the same as the conjugation length r hole (or A) ) can be interpreted as two-dimensional plots with the same axes (r elec , r hole ). In that case, distance is measured in units of atoms or functional groups, depending on how the molecule is partitioned, Overall size of the exciton is limited by the size of the molecule as an upper bound, and L coh is then limited by PR adiag .