Excited-State Dynamics Leading either to Triplet Formation or Coordinative Expansion following Photolysis of Cu(II)-5,10,15,20-<em>meso-tetrakis</em>(phenyl)porphyrin or Cu(II)-5,10,15,20-<em>meso-tetrakis</em>(N-methylpyridium-4-yl)porphyrin: A DFT, TD-DFT, Luminescence and Femtosecond Time-Resolved Absorbance Study

• DOI: 10.20944/preprints202307.1515.v1
• Publication Date: 2023-07-25T02:42:36Z
• Authors: Ross McGarry; Lazaros Varvarezos; Mary T. Pryce; Conor Long

The photophysical properties of Cu(II) complexes with 5,10,15,20-meso-tetrakis(phenyl)porphyrin and 5,10,15,20-meso-tetrakis(N-methylpyridium-4-yl)porphyrin were examined by luminescence and femtosecond time-resolved absorbance methods respectively. These studies were supported by DFT and TD-DFT calculations which highlight the important role played by a ligand-to-metal charge-transfer states in directing the system towards either intersystem crossing to the triplet hypersurface or coordinative expansion to a five-coordinate quasi-stable intermediate. The latter processes occurs when the porphyrin is photolyzed in the presence of suitably located Lewis bases. Femtosecond time resolved absorbance measurements on Cu(II)-5,10,15,20-meso-tetrakis(N-methylpyridium-4-yl)porphyrin confirm that the coordinative expansion in water occurs in approximately 700 fs while crossing to the triplet hypersurface takes approximately 140 fs in the same solvent. These processes are mutually exclusive, but both can occur simultaneously depending on the environment of the porphyrin. The ratio of the two processes depends on the relative orientation of the Lewis base with respect to the copper atom at the time of excitation. As a consequence, copper porphyrins such as these, are excellent probes to the environment of the porphyrin, and can be used to identify the location of the porphyrin when interacting with DNA fragments.