  The excitonic luminescence of an isolated polydiacetylene polymer chain in
its monomer matrix is studied by micro-photoluminescence. These chains behave
as perfect 1D semiconducting wires with the expected 1/$\sqrt{E}$ density of
states between 5 and 50 K. The temperature dependence of the homogeneous width
is explained by interaction with longitudinal acoustic phonons of the crystal
in the range of temperature explored. The optical phonons of the chain which
are involved in the vibronic transitions are found to have coherence times
ranging from 300 to 600 fs.
