Photophysical behavior of open-shell, first-row transition metal meso-tetraphenyltetrabenzoporphyrins: insights from experimental and theoretical studies.

The ground and excited state properties of two metallo-tetraphenyltetrabenzoporphyrins (MTPTBP) have been investigated by a combination of DFT/TDDFT and transient absorption spectrometry to draw a complete picture of the excited state deactivation. The Cu(II) and Co(II) complexes were chosen to investigate the impact of the half-filled d orbitals on the photophysical properties of the tetrapyrrole macrocycle. The first observed transient in CuTPTBP was assigned to the triplet state that equilibrated with a ligand-to-metal charge transfer (LMCT) state. Ground state repopulation, completed within 53 ps, occurred via a lower-lying LMCT state. The dependence of the observed lifetime on solvent polarity confirmed the participation of the LMCT state in the overall deactivation process. For CoTPTBP, the first observed transient, a π-localized triplet state, converted to a hot d,d state, wherein intramolecular cooling occurred and completed within 3 ps. The cooled d,d state decayed to the ground state in an exponential manner with a 17 ps lifetime.