Structural Influence on Exciton Migration and Singlet Oxygen Photosensitization in Porphyrinic Metal–Organic Coordination Networks

Abstract: A hexagonal three-dimensional (3D) metal−organic coordination network (MOCN) [(ZnTPyP)·0.75 DMSO]n (3D Helix) and a one-dimensional (1D) coordination polymer [(ZnTPyP)·DMF]n (1D Ladder) (ZnTPyP = 5,10,15,20-tetrakis(4-pyridyl)porphyrinatozinc(II)) are prepared and their photo- physical properties are investigated to assess their ability to promote efficient exciton energy migration through singlet−singlet annihilation processes and to photosensitize singlet oxygen (1O2(g)). The presence and absence of annihilation in 3D Helix and 1D Ladder, respectively, are indicative of their ability to efficiently promote exciton migration. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to demonstrate the presence of interporphyrin couplings in the 3D Helix structure. Using Forster’s theory of energy transfer, the relative efficiencies of exciton migration across the bulk were correlated with the structural parameter κ2/ r6, indicative of the relative orientations and distance between the donor and acceptor. Concurrently, based on the magnitude of 1O2(g) phosphorescence (1280 nm), it was noted that 3D Helix photosensitizes 1O2(g) more efficiently than 1D Ladder (by roughly 1 order of magnitude). During this study, a new two-dimensional (2D)-MOCN was prepared, 2D Grid [(ZnTPyP)·4CHCl3]n, but weak Zn···N interactions and evaporation of CHCl3 transformed 2D Grid into a multiphasic mixture (ZnTPyP Morph) containing both 3D Helix and other 1D ladder-like species.