The Effect of Single-Atom Substitution on Structure and Band Gap in Organic Semiconductors

Abstract

We systematically altered the molecular structure of 5-methoxynaphtho[1,2-d]thiazole (NTH) by replacing the terminal hydrogen atom with halogens (Cl, Br, I) to study the effect of single-atom substitution in modulating the crystal packing and optical band gap. The parent compound (NTH) and Br and I derivatives of NTH crystallized in the same space group, wherein only Br- and I-substituted molecular crystals displayed isostructural interaction topologies. The crystal-packing similarities in structurally equivalent motifs were established using the numerical descriptors isostructurality (Is) and cell similarity (π) indices. An energy framework analysis was implemented to obtain a qualitative picture of the 3D topology displaying the predominant interactions in supramolecular architectures of the NTH derivatives. A decrease in optical band gap from 3.48 to 3.07 eV was observed with an increasing atomic number of halogens in NTH derivatives, signifying the direct role of halogen atoms in the electronic properties of organic crystals. The reduction of the optical band gap in 5-methoxynaphtho[1,2-d]thiazole derivatives was visualized from the band structure and projected density of states obtained by employing DFT calculations. The outcome suggests the potential of halogenation in tailoring the optoelectronic properties of organic functional materials.