Photo-induced dimer-diradical formation in diacetylene crystals at low temperatures

Optical absorption changes induced by 308-nm laser pulses and by electron p ulses at low temperatures were studied for crystalline 5,7-dodecadiyne-1,12 -diol bis[phenylcarbarnate] (TCDU; R-C=C-C=C-R, R=(CH2)(4)O(CO)C6H5). The d imer diradical which shows an absorption band at 3.05 eV is not formed by t he electron-pulse irradiation, but is formed efficiently by irradiation wit h the 308-nm laser pulses. By exciting crystals with an electron pulse and a laser pulse sequentially, it is shown that the collision of excited speci es with the lowest triplet state is a crucial step in generating the dimer diradical. The mechanism of the formation of the dimer diradical is discuss ed.