Theoretical investigation of the energy transmission mechanism in phthalocyanine-benzimidazole photosensitizers used for singlet oxygen generation

The present work is focused on investigating the spatial factors regarding the energy transmission mechanism occurring in four second-generation phthalocyanine-benzimidazole photosensitizers proposed for singlet oxygen production (H₂Pc, ZnPc, GaClPc and InClPc) through analyses from the Quantum Theory Atoms in Molecules (QTAIM). Hence, electron and spin densities obtained by means of Density Functional Theory (DFT) are considered here. The QTAIM quantities calculated for these phthalocyanines allowed us to investigate the atoms presenting the most relevant variations in the electronic structure moving from the singlet ground state to the first triplet excited state, $S_0\to T_1$, indicating the regions of the photosensitizers where the transmission of energy from these molecules to oxygen (O₂) could take place. This analysis was corroborated by spin densities that pointed out the positions where the highest rates of singlet oxygen formation would take place. Hence, the nitrogen and mainly carbon atoms from the inner phthalocyanine ring are pointed as the docking sites for energy transmission aiming singlet oxygen generation from phthalocyanine-benzimidazole photosensitizers. Finally, while the spatial features seen in metallophthalocyanines remain similar for all metals in the oxidation states considered here [Zn(II), Ga(III) and In(III)], the H₂Pc compound shows small differences with the respect.