Photophysics of Soret-excited Metallated Tetrapyrroles in Solution: Experimental and Theoretical Studies

Abstract

The photophysics of highly electronically excited states of a set of d0 and d10 metallated tetrapyrroles, which have different peripheral substituents and central metal atoms, macrocycle substitution patterns and macrocycle conformations, have been investigated both theoretically and experimentally. Theoretically, the energies of ground state molecular orbitals and the energies and rank in energy of the excited states have been calculated using density functional theory and time-dependent density functional theory methods. Experimentally, the steady-state absorption and fluorescence spectra have been measured. Temporal fluorescence profiles have been measured using a time correlated single photon counting system for the S1 state and a fluorescence upconversion system for the S2 state.

The decay mechanisms of highly electronically excited states are governed by the nonradiative S2 – S1 internal conversion. The possible existence of any excited state (such as S2' or dark state) lying close to the S2 state and its participation in the nonradiative decay processes of the S2 state has been discussed. The ultrafast nonradiative decay rates of the S2 – S1 internal conversion were interpreted on the basis of the energy gap law of radiationless transition theory. For magnesium tetraphenylporphyrin (MgTPP), the radiationless rates (knr)of its S2 state follows the prediction of the energy gap law for the weak coupling statistical limit case. However, the S2 – S1 electronic coupling energies of the other metalloporphyrins investigated fall within the inermediate to strong coupling range. The difference of knr relative to the weak coupling limit can be rationalized by the different magnitudes of electronic coupling energies. The magnitude of electronic coupling energies is the major factor in determining the radiationless depopulation rate constants of the S2 states in metallated tetrapyrroles which have S2 – S1 electronic coupling energies exceeding the weak coupling limit. In some cases, such as ZnOEP, the magnitude of Frack-Condon factor has only minor effect.

The photophysics of Soret-excited metallated corroles have also been investigated in this study. Primary work has shown that two metallated corroles examined have similar S2 – S1 interstate electronic coupling energies to that of CdTPP and thus the radiationless decay rates of Soret-excited S2 state are also determined by the magnitude of electronic coupling energies.