The spectroscopic properties of the chains of the polydiacetylene (PDA) poly-4BCMU, diluted in their single-crystal monomer matrix are studied and discussed in terms of the single polymer chain electronic properties, the monomer structural changes, and their mutual influence. The polymer exciton transition energy shifts considerably (by 0.3 eV) as temperature decreases, down to a value that may be the lowest transition energy yet observed in a PDA. Two first-order structural transitions of the monomer crystal are observed near 320 and 220 K. They show up in the polymer spectroscopic properties-absorption and resonance Raman scattering-as discontinuities of both wave numbers and linewidths. So, the polymer chains behave as probes of the monomer crystal structural changes. A new electronic transition is observed below the main exciton one, possibly corresponding to a state of g symmetry. A weak fluorescence is observed at low temperature. Its origin coincides with the exciton absorption. Therefore, there is no configurational relaxation in the exciton state during the fluorescence lifetime.