Zhou and co-workers synthesized three A-DA'D-A (acceptor-donor-acceptor'donor-acceptor) type molecules, namely Y6, Y6-C4, and Y6-C5, to illustrate how side-chain engineering can manipulate molecular stacking in films (Figure ). The central backbone of Y6 possesses a twisted DA'D framework, owing to the branched alkyl chain 2-ethylhexyl with the opposite manner attaching to the two sp -hybridized nitrogen atoms in the pyrrole motif. All three acceptors had similar absorption spectra, with the maximum absorption peak located at 731 nm in solution, indicating that the absorption properties of Y6, Y6-C4, and Y6-C5 were barely affected by different alkyl chains in dilute solution. However, the changes from solution to film exhibited large differences in molecular packing. Y6 displayed an obvious red shift of about 100 nm, while Y6-C4 and Y6-C5 showed a red shift of only ~50 nm from solution to film absorption. Grazing-incidence wide-angle Xray scattering results indicated that the Y6 film adopted the preferential face-on orientation. By moving the branching point of the pyrrolic alkyl chain from second to fourth, the Y6-C4 film exhibited favorable edge-on orientation to the substrate.