Computational Study of Photo-oxidative Degradation Mechanisms of Boron-Containing Oligothiophenes

We have modeled possible photo-oxidative degradation pathways for a set of boron-containing oligothiophenes, which have potential use in organic electronic devices. Photogenerated reactive oxygen species such as hydroxyl radical, hydroperoxyl radical, and singlet and triplet molecular oxygen are taken into account in three main pathways, namely, sulfoxide formation, sequential addition, and stepwise singlet molecular oxygen addition. Density functional theory at the B3LYP level is used to assess the reaction kinetics and thermodynamics. Our findings show that the influence of the number of thiophene rings and the presence of boron is in most cases minor in terms of degradation. The formation of sulfoxide on the thiophene ring is among the easiest degradation pathways if hydroxyl radical is present in the system. The hydroxyl radical attack on the C_β of thiophene ring of BMBE-1T (2,5-bis­(E-dimesitylborylethenyl)­thiophene) forms the BMBE-1T­(C)­OH radical adduct which is kinetically and thermodynamically more favorable than the hydroperoxyl radical attack. The stepwise triplet molecular oxygen addition on the BMBE-1T­(C)­OH radical adduct has a free energy barrier around 19 kcal·mol^–1, and it results in thermodynamically stable degradation product via ring cleavage. Stepwise reactions with singlet molecular oxygen have energy barriers of roughly 40 kcal·mol^–1. Singlet molecular oxygen attack on the α-carbon of the thiophene ring is kinetically much more favored than the attack on the beta carbon. Our results elucidate the preferred degradation mechanism of the thiophene backbone of the selected photoactive oligomers. Moreover, the findings of this theoretical study clarify the photostability, and hence the potential drawbacks, of the large-scale use of this class of polythiophenes.