Photochemical oligomerization pathways in 2,5-diiodothiophene film

The photochemical oligomerization mechanisms of 2,5-diiodothiopene in a multilayer film was studied with matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). The photo-dissociation of 2,5-diiodothiophene and subsequent reactions of the dissociated radical with surrounding molecules and other radicals can lead to the growth of oligothiophene species. The molecular weight distribution of the oligothiophene products indicated that the average number of iodine atoms in the oligothiophene molecule increases with the number of thiophenyl units in the molecule. All possible oligomerization mechanisms could be categorized in two pathways-the oligomer growth by insertion of a thiophenyl unit versus the growth by insertion of an iodothiophenyl unit. Using a modified Flory's model for a linear step polymerization process, we found that the probability of thiophenyl and iodothiophenyl insertion in each C{single bond}C bond formation is ∼0.63 and ∼0.37, respectively. The model also predicted the most probable oligomer length to be about 3-4, which was consistent with the photoluminescence and FTIR analysis.