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Regioregular Polymer Analogous Thionation of Naphthalene Diimide–Bithiophene Copolymers
Polymer
analogous thionation of the n-type conjugated polymer PNDIT2
is investigated using Lawesson’s reagent (LR). Detailed high-temperature
NMR spectroscopic investigations show that due to the copolymer structure,
two out of the four available carbonyl groups present in the naphthalene
diimide (NDI) comonomer are sterically less hindered and react preferentially.
This leads to regioselective thionation in the <i>trans</i>-configuration even for a large excess of LR. For high degrees of
O/S conversion, signals of minor intensity show up in addition pointing
to undesired side reactions. These signals could not be eliminated
despite further optimized reaction conditions including different
aromatic solvents and reaction temperatures. Compared to PNDIT2, the
resulting 2S-<i>trans</i>-PNDIT2 features strong aggregation,
lower solubility, an 80 nm bathochromic shift of the charge-transfer
band, a by 0.22 eV lower LUMO energy level, a lower thermal stability,
and higher melting temperatures (<i>T</i><sub>m</sub>).
As the combination of the lower thermal stability and higher melting
points renders the characterization of thermal transitions challenging,
fast scanning calorimetry (flash-DSC) is successfully used to determine <i>T</i><sub>m</sub>. With increasing O/S conversion, <i>T</i><sub>m</sub> first increases but then decreases, which is ascribed
to a combined effect of stronger main chain interactions and increasing
chemical defects. Microstructural order and field-effect electron
mobilities decrease with increasing O/S conversion compared to PNDIT2