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

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 DFT and TDDFT calculations show that the ground state geometries are not significantly altered by substitutions on the porphyrin macrocycle. However, they do affect the orbital energies, and the energy and the rank in energy of the excited states accessible in the near uv-region of the spectrum of the metalloporphyrins studied. The TDDFT calculations of the excited states predict that a number of singlet and triplet excited states lie in the vicinity of the 21Eu state. One or more of them may contribute to the fast relaxation of the S2 state of the metalloporphyrins.;The S2 – S1 energy gap, ΔE(S2 – S1), is controlled by both solvent and porphyrin polarizabilities for a given metalloporphyrin. A good linear correlation between ΔE(S 2 – S1) and the Lorenz-Lorentz solvent polarizability function, f1, is obtained for ZnTPP, MgTPP and ZnTBP metalloporphyrins in the set of solvents employed. Extrapolation of such plots to f1 = 0 gives the ΔE(S2 – S1) of the bare molecules observed in a supersonic expansion. In addition, the slope of such plot for ZnTBP is different from those of ZnTPP and MgTPP because of different porphyrin polarizabilities.;The S2 depopulation rates have been calculated based on the S2 fluorescence lifetimes obtained. For a given metalloporphyrin in several fluid solvents, the logarithms of the radiationless decay rates of the S2 states decrease linearly with the increase of ΔE(S 2 – S1). However, the slopes of such energy gap law plots of ZnTPP and ZnTBP are different from that of MgTPP. The internal conversion efficiency is close to 1.0, except for CdTPP, where it is only 0.69. The fact and the equality of the S2 decay and S1 rise times indicate that the only important relaxation pathway of the S2 state is S 2 – S1 internal conversion. Further analysis indicates that there is no evidence of significant contributions from heavy-atom induced S2 – Tn (n > 2) intersystem crossing or of the existence of a dark or S2' state and its participation in the nonradiative decay processes of the S2 state.;The decay mechanisms of highly electronically excited states are governed by the nonradiative S2 – S1 internal conversion. The ultrafast S2 – S1 internal conversion rates are determined by two factors; The magnitude of the Franck-Condon factor and the S2 – S1 interstate electronic coupling energy, b2el , according to the Fermi Golden Rule. For MgTPP, the change in k nr with ΔE is determined exclusively by the magnitude of F, which varies inverse exponentially with ΔE, and thus strictly follows the predictions of the energy gap law for the weak coupling, statistical limit case. However, the other metalloporphyrins investigated have S2 – S1 electronic coupling energies falling within the intermediate to strong coupling range. The difference of knr relative to the weak coupling limit can be rationalized by the different magnitudes of β el. Thus βel is the major factor in determining the radiationless depopulation rate constants of the S2 states in metallated tetrapyrroles which have S2 – S1 interstate electronic coupling energies exceeding the weak coupling limit In some cases, such as ZnOEP, the magnitude of F 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 βel.