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Highly oriented films of an electron accepting polymer semiconductor, polyN, N'-bis (2-octyldodecyl) -1, 4, 5, 8-naphthalenedicarboximide-2, 6-diyl-alt-5, 5'- (2, 2'-bithiophene) (PNDI2OD-T2), are obtained by two different methods, namely directional epitaxial crystallization (DEC) on 1, 3, 5-trichlorobenzene (TCB) and epitaxy on friction transferred poly (tetrafluoroethylene) (PTFE) substrates. Two distinct polymorphs with unprecedented intrachain resolution are identified by high-resolution transmission electron microscopy (HR-TEM). Form I is obtained by DEC on TCB, whereas highly oriented films of form II are obtained on PTFE substrates after melting at T = 300 °C and cooling at 0. 5 K/min. In form I, both electron diffraction and HR-TEM indicate a segregated stacking of bithiophene (T2) and naphthalene diimide (NDI) units forming separate columns. In form II, a ∼c/2 shift between successive π-stacked chains leads to mixed π-overlaps of T2 and NDI. Form I can be transformed into form II by annealing at T > 250 °C. The different π-stacking of NDI and T2 in the two polymorphs have characteristic signatures in the UV-vis spectra, especially in the charge transfer band around 750 nm which is also observed in spin-coated films.
Brinkmann et al. (Wed,) studied this question.