The fundamental crystalline excitation of 5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (TCDU) single crystals results in emission of two luminescence bands peaked at 4.28 eV and 2.57 eV, both of which show wide widths and significant degree of natural polarization at low temperatures. Measurements of the decay characteristics indicate that the initial state of the 4.28-eV luminescence is singlet, while that of the 2.57-eV band is triplet in nature. Analysis of the temperature dependence of Urbach tail shows that exciton-phonon coupling is strong enough to form self-trapped state of excitons. Therefore, we identify that the 4.28-eV and 2.57-eV luminescence are due to radiative recombination of the self-trapped excitons (STEs) in TCDU crystals. Excitation of the crystal with intense 4.66-eV laser light results in the formation of dimer diradicals, with expense of the concentration of only the triplet STE. The result shows that the triplet STEs is the metastable precursor responsible for the dimer-diradical formation, which initiates the polymerization of this diacetylene crystal. (C) 2003 American Institute of Physics.