The absorption and emission properties of excitons are investigated in a pure 'red' phase of luminescent polydiacetylene wires. The values of the binding energy (E-b = 0.556 eV) and Bohr radius (r similar to 14 angstrom) make the 'red' exciton similar to the one of the extensively studied 'blue' phase, so the conformational modifications which lead to a reordering of excited states and luminescent emission do not strongly affect the exciton structure. The opening of a thermally activated non radiative channel leads to an efficient quenching of the intense luminescence observed at low temperature. (C) 2007 Elsevier B.V. All rights reserved.