80 different push-pull type organic chromophores which possess Donor-Acceptor (D-A) and Donor-Thiophene-Acceptor-Thiophene (D-T-A-T) structures have been systematically investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/6-311G* level. The introduction of thiophene (T) in the chain has allowed us to monitor the effect of π-spacers. Benchmark studies on the methodology have been carried out to predict the HOMO and LUMO energies and optical band gaps of the D-A systems accurately. The HOMO and LUMO energies and transition dipoles are seen to converge for tetrameric oligomers, and the latter have been used as optimal chain length to evaluate various geometrical and optoelectronic properties such as bond length alternations, distortion energies, frontier molecular orbital energies, reorganization energies and excited-state vertical transition of the oligomers. Careful analysis of our findings has allowed us to propose potential donor-acceptor couples to be used in organic photovoltaic cells.

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.