Investigating the effect of mixed halogens in the properties of copper cysteamine by density functional theory

A new type of copper cysteamine photosensitizer, Cu-Cy: Cu₃Cl (SR)₂ (R = CH₂CH₂NH₂), was reported and later, there was another experimental report of Cu-Cy synthesis with other halogen elements (Cu-Cy-X, X = F, Br, and I), these later group of cysteamines contained Cl as an impurity. To realize the full potential applications of these cysteamines as photosensitizers, it is imperative to fundamentally understand the effect of Cl on the stability and optical properties of Cu-Cy-F/Br/I. In this paper, the stability of Cl-doped Cu-Cy-X (X = F, Br, and I) and their electronic and optical properties pertaining to their application as photosensitizers were investigated by using density functional theory (DFT). It was found that mixing Cu-Cy-Br and Cu-Cy-I with Cl increased their thermodynamic stability as well as made the materials mechanically harder. In addition, mixing halogens in Cu-Cy rendered a significant impact on their optical properties, namely the mixing affected Cu-Cy's deexcitation to their ground states during the photosensitizing process. In addition, it was found that after the initial photoexcitation within these Cu-cysteamines, the intersystem crossing can take place due to spin–orbit coupling from the Cu atoms.