The fluorescence yield, eta approximate to 0.30, and Lifetime, tau approximate to 52 ps, of recently discovered 'red' polydiacetylene isolated chains [R. Lecuiller, J. Berrehar, C. Lapersonne-Meyer, M. Schott, Phys. Rev. Lett. 80 (1998) 4068] have been measured at 15 K. These values show that the relaxation scheme of the 'red-chain' exciton is very different from that of the usual 'blue chains', and that excited states of A, symmetry are not present in the optical gap of red chains. Moreover, the radiative lifetime is very short at tau(rad) approximate to 170 ps. (C) 1999 Elsevier Science B.V. All rights reserved.