A DFT and TD-DFT study on emodin and purpurin and their functionalized molecules to produce promising organic semiconductor materials

Density functional theory (DFT) computations were done to explore the optical and electronic properties of two conjugated molecules, emodin and purpurin, as potential organic semi-conductors. The molecules were functionalized to explore the impact of functionalization on the electronic and optical properties. The properties calculated include reorganization energy (λh and λe), adiabatic ionization potential (IP), adiabatic electron affinity (EA), chemical hardness (n), HOMO and LUMO energies, and HOMO-LUMO energy gap (Eg) via B3LYP/6–3++G (d, p) method. In addition, the maximum absorption (λmax) and oscillator strength (f) at the excited states in vacuum and solvent (Ethanol) were investigated using time-dependent density functional theory (TD-DFT). The introduction of functional groups to emodin was considered to convert the molecule from a p-type into an n-type material, while purpurin is considered as an n-type material, its functionalization with NO2 and 2F resulted in a slight increase in λe values, which is considered detrimental for the process of charge-transport. However, the functionalized molecules have shown an increase in EA and a decrease in LUMO energy level, indicating their potential use as n-type materials. Furthermore, to have an understanding of the intermolecular interactions in emodin and purpurin molecules, Hirshfeld surface analysis and energy framework were studied.