Analysis of the optical absorption spectra of diradical oligomers in diacetylene crystals

We report on electronic and vibrational properties of the diradical short-chain reaction intermediates of the low-temperature polymerization reaction in diacetylene single crystals. The positions, shapes and intensities of the zero-phonon lines and of vibronic lines of the polarized optical absorption series are analyzed. Theoretical model calculations concerning the vibrational and excitonic states in the molecular aggregates are performed and applied to the experiments.