Infrared Transition Moment Orientational Analysis on the Structural Organization of the Distinct Molecular Subunits in Thin Layers of a High Mobility n-Type Copolymer

The IR-based method of infrared transition moment orientational analysis (IR-TMOA) is employed to unravel molecular order in thin layers of the semiconducting polymer poly[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene) (P(NDI2OD-T2)). Structure-specific vibrational bands are analyzed in dependence on polarization and inclination of the sample with respect to the optical axis. By that the molecular order parameter tensor for the respective molecular moieties with regard to the sample coordinate system is deduced. Making use of the specificity of the IR spectral range, we are able to determine separately the orientation of atomistic planes defined through the naphthalenediimide (NDI) and bithiophene (T2) units relative to the substrate, and hence, relative to each other. A pronounced solvent effect is observed: While chlorobenzene causes the T2 planes to align preferentially parallel to the substrate at an angle of 29°, using a 1:1 chloronaphthalene:xylene mixture results in a reorientation of the T2 units from a face on into an edge on arrangement. In contrast the NDI unit remains unaffected. Additionally, for both solvents evidence is observed for the aggregation of chains in accord with recently published results obtained by UV-vis absorption spectroscopy.