Segregated versus Mixed Interchain Stacking in Highly Oriented Films of Naphthalene Diimide Bithiophene Copolymers

Highly oriented films of an electron accepting polymer semiconductor, poly {[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalene-dicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDI20D-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 (HRTEM). Form I is obtained DEC on TCB, whereas highly oriented films of form II are obtained on PTFE substrates after melting at T=300 degrees C and cooling at 05 k/min. In form I, both electron diffraction and HR-TEM Indicate a segregated sticking of bithiophene (T2) and naphthalene diimide (NDI) units forming separate columns. In form II, a similar to c/2 shift between successive pi-stacked chains leads to Mixed pi-overlaps of 12 and NDI. Form I can be transformed into form II by annealing at T>250 degrees C The different pi-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.