We report on the first coherent control experiments on a purely electronic exciton state in an extended quasi-perfect organic quantum wire, a polydiacetylene chain isolated in the crystalline matrix of its own monomer. The time-integrated luminescence of a single wire is measured as the relative phase between two exciting sub-picosecond laser pulses is varied. From visibility functions the exciton dephasing time is extracted and its temperature dependence studied. Our work points the predominant role of thermalization upon the phase relaxation dynamics. By means of microscopic imaging spectroscopy we also show that despite local excitation coherent control is achieved on states delocalized over the chain at the micrometric scale.