Fluorination Position: A Study of the Optoelectronic Properties of Two Regioisomers Using Spectroscopic and Computational Techniques

The influence of the fluorination positions on the optical and electronic properties of a pair of donor-acceptor (DA)-based regioisomers is explored. In the regioisomers, the fluorination was varied between the 2 and 3 positions on benzothiadiazole (BTD) acceptor units. Although the structural variation between the regioisomers is small, significant variations in the electronic properties of the two compounds were observed. These were observed most markedly in the excited-state properties with a 15 nm (280-390 cm-1) red shift of the emission between the regioisomers. The combination of resonance Raman spectroscopy (RRS) and density functional theory (DFT) calculations was used to probe the possible causes of the observed variations. The analysis suggested a F···S through-space interaction as being responsible for tuning both the electronic properties and rigidity of the compounds. Shifting the fluorine atoms shifted the location of the F···S interaction, changing which part of the molecule was locked down, and showed a variation in the overall rigidity of the molecule. In this series, the influence could be varied between the core and periphery. This study adds to a growing body of work demonstrating the effectiveness of selective fluorination in tailoring the properties of organic molecules.