Effect of positional isomerism and alkyl substituent length on 2D and 3D supramolecular organizations of alkylthienyl-benzothiadiazole-alkylthienyl DAD semiconductors

• Systematic study of self-organization of alkylthienyl-substituted benzothiadiazoles. • Rational correlation between their 2D (STM) and 3D (XRD) supramolecular structures. • Effect of positional isomerism and substituent length on intermolecular interactions. • Role of weak chalcogen interactions in the formation of 3D and 2D ordered structures. • Factors inducing differences in 2D and 3D supramolecular structures (polymorphism). Donor-acceptor–donor semiconductors consisting of central benzothiadiazole unit disubstituted with alkylthienyl groups of different lengths (alkyl = hexyl or octyl) were investigated. Isomerism had little effect on their spectroscopic and electrochemical properties. In nonpolar solvents (toluene), they showed photoluminescence quantum yields (PLQY) exceeding 90 %. They could be electrochemically reversibly reduced and irreversibly oxidized, exhibiting electrochemical band gaps, E g el , in the range 2.62–2.65 eV. The experimental data were supported by DFT calculations. Complementary structural studies, involving scanning tunneling microscopy and X-ray diffraction, revealed, however, the effect of isomerism and alkyl chain length on their 3D and 2D organizations. 2D organizations of monolayers deposited on HOPG showed structural similarity to the flat layers found in 3D single crystals. However, a more complex dependence of the alkyl substituents length on 2D organizations was found compared to 3D ones. Structural dimerization of molecules within a layer was another feature, revealing the important role of chalcogen interactions between the benzothiadiazole groups of neighboring molecules, which forced their opposite orientation. Quantitative differences between the unit cell parameters of 2D and 3D order had two origins: i) interactions of C–H∙∙∙π type between π-stacked layers in the single crystal, and ii) an effect of the substrate (HOPG).