Theoretical studies on triplet formations in nitrobenzoxadiazole (NBD) derivatives: The impact of donor group and heteroatom substitution

Abstract

Heavy-atom-free triplet photosensitizers (TPs) have attracted much interest due to the applications in triplet–triplet annihilation (TTA) and photodynamic therapy. Recently, several nitrobenzoxadiazole (NBD) derivatives have been reported to generate singlet oxygens. However, the mechanistic understanding of the intersystem crossing (ISC) process has not been studied, resulting in a lack of rational design strategies for creating effective TPs. Based on DFT and TD-DFT calculations, we found that the replacement of donor group (methylamino → methoxyl) and the heteroatom (oxadiazole → triazole) could induce a significant change in the nature of triplet excited states in comparison to that of the single excited states, and thus enable substantial spin-orbital couplings for efficient triplet conversions. We expect that modifying the electronic properties of the triplet excited states through donor/heteroatom substitution is a promising method for creating heavy-atom-free TPs in many chemical families of fluorophores.

Keywords

• Nitrobenzoxadiazole
• TD-DFT
• Spin-orbital coupling
• Molecular engineering