Nickel porphyrin-C₆₀ dyads: Significance of low-lying porphyrin orbitals to suppress excited charge and energy transfer

Two nickel porphyrin-C 60 dyads, viz., meso-(tri(toly)porphyrinato)nickel-C 60 (NiTTP-C 60 ), and meso-(tri(triphenylamine)porphyrinato)nickel-C 60 (NiTPA-C 60 ) were newly synthesized and studied by various techniques including optical absorption, electrochemistry, DFT and time dependent-DFT (TD-DFT) and femtosecond transient absorption, to establish the photochemical events. While electrochemical studies examined the redox properties of the dyads, the spectrum of the radical ion species was obtained from the chemical oxidation method. Although energetically photoinduced energy and/or electron transfer from excited nickel porphyrin to C 60 was possible, femtosecond transient absorption studies were not conclusive of the occurrence of either of these events. The low-lying orbitals of the d 8 -configured nickel porphyrin were attributed to this phenomenon. It appears that nonradiative decay of the excited nickel porphyrin via the low-lying orbitals dominated the relaxation path instead of either energy or electron transfer involving covalently linked C 60 , including NiTPA-C 60 having an easily oxidizable porphyrin. TD-DFT studies further supported this mechanism, revealing that the excited charge transfer was only possible in these dyads from the significantly higher electronic excited states, viz., the 35 th and 39 th excited states in the cases of NiTTP-C 60 and NiTPA-C 60 . The present study highlights the significance of low-lying orbitals in metalloporphyrins to suppress energy and electron transfer events in covalently linked donor-acceptor systems.