Palladium-Assisted Reaction of 2,2-Dialkylbenzimidazole and Its Implication on Organic Solar Cell Performances

A 2,2-dimethyl-2H-benzimidazole (22MBI) pulling
unit has been synthesized as a potential high-solubility substitute
for benzothiadiazole and incorporated into a push–pull-type
copolymer used for decent-efficiency (∼3%) organic photovoltaic
devices. We herein replace the two methyl side groups of 22MBI by
longer alkyl (ethyl, butyl, and hexyl) side chains to further improve
the solubility. However, the copolymers replaced by the new pulling
units, 2,2-diethyl/dibutyl/dihexl-2H-benzimidazole
(22EBI/22BBI/22HBI), lose favorable optical characteristics and exhibit
negligible (<0.5%) power conversion efficiency. Intrigued by this
anomalous side-group effect of 2,2-dialkyl-2H-benzimidazole
(22BI), we carry out time-dependent density functional theory calculations
on a series of 22BI-based copolymers with various lengths of 2,2-dialkyl
side chains (methyl, ethyl, butyl, and hexyl), but no discernible
difference in equilibrium structure nor in electronic structure is
found between them. We hence formulate a hypothesis that 22BI may
either isomerize into 1,2-dialkylbenzimidazole (12BI) or lose one
of its alkyl (≥ethyl) chains as olefin to become 2-dialkylbenzimidazole
(2BI) because these aromatic products, unlike the quinoid-type 22BI, would exhibit unfavorable electronic
structure for organic photovoltaic (OPV) applications. Indeed, the
absorption spectra measured for the 22BI-based copolymers with long
dialkyl side chains are best reproduced by the calculations on 2BI-
and 12BI-based copolymers, and the two side products are calculated
to be more stable than 22BI, indicating the spontaneity of the proposed
reactions. The activation barriers are prohibitively high (>29
kcal/mol)
but could be reduced down to 8 kcal/mol in the presence of palladium-based
polymerization catalysts. Indeed, the presence of the predicted 12BI-containing
side products is confirmed by NMR spectra. A temperature-dependent
polymerization experiment shows that 22MBI is in fact subject to the
same type of isomerization when the temperature is raised to 150 °C
above the original polymerization temperature (90–110 °C),
further supporting the hypothesis from our calculations and explaining
the observed anomalous side-group effect.