Photophysical Processes of Porphyrin and Corrin Complexes with Nickel and Palladium

Nickel(II) and palladium(II) ions are capable of forming complexes with macrocyclic terapyrrole structures such as the porphyrin or corrin ring. Many different derivatives of these complexes are synthesized and studied because these compounds have potential numerous applications, including catalysis, various light-driven chemical reactions and processes related to intramolecular and intermolecular energy redistribution. Nickel porphyrins exhibit neither fluorescence nor phosphorescence when excited with light; however, palladium porphyrins, when excited to the singlet state, very quickly transform into the triplet state, and unlike nickel porphyrins, deactivation of the excited states occurs by phosphorescence. Palladium corrin has dual luminescent properties and exhibits both a weak fluorescence and strong phosphorescence. These photophysical differences are based on the complex energetic redistribution of singlet and triplet excited states interacting with each other in the intersystem crossing process. Based on the results of calculations at the DFT/TDDFT and CASSCF/NEVPT2 levels of theory, the structure of electronic excited states of model nickel(II) and palladium(II) complexes with corrin and porphyrin macro-rings was characterized and potential paths of photophysical processes leading to the occupancy of low-lying triplet states were described. In nickel complexes, very low-energy triplet states are the main cause of the rapid radiationless deactivation of excited states via triplet photophysical pathways.