Spacer dependence of intersystem crossing (ISC) affected by a remote metal unpaired electron was studied in a series of copper(II) porphyrin-free base porphyrin dimers. In these dimers, fluorescence of the free base moiety is remarkably quenched by the copper counterpart which is linked via a phenanthrene, a naphthalene, or a benzene. Fluorescence lifetimes and quantum yields of the free base moiety in the heterodimers are 1/10-1/35 of the free base monomer depending upon the spacer, whereas no change was observed in the free base homodimers. The changes in the copper(II)-free base dimers are attributed to an enhancement of ISC in the free base half, due to interaction with an unpaired electron in Cu(II) ion of the other half. Estimated ISC rates from the experimental results exhibit a strong correlation with the number of bonds of the linkage but not with the center-to-center distance of the two halves. An expression for the ISC rate in the presence of an unpaired electron is derived. Exchange interaction between the copper unpaired electron and free base pi-electrons gives rise to mixing between the excited states of the dimer when the free base moiety is in singlet and triplet excited states, and enhances ISC in the free base part. The observed spacer dependence leads to the suggestion that the ISC rate is governed by through-bond exchange interaction.