Picosecond Studies of Ruthenium(II) and Ruthenium(III) Porphyrin Photophysics

We report the results of picosecond and slower time scale transient absorption and emission measurements on a series of ruthenium(II) and ruthenium(III) octaethylporphyrins (Ru(oep)(l1)(l₂)). The ruthemum(II) porphyrins are of three types. First are the carbonyl complexes in which L₁ = CO and L₂ = EtOH, pyridine (py), or 1-methylimidazole (1-MeIm), which all exhibit similar spectral and kinetic behavior. The lowest excited states of these molecules are assigned as the ring ³T(π_{, π}*) in agreement with previous work on similar complexes. The ³T(π, π*)state has a lifetime of-75 μs at room temperature in degassed solution, as measured by decay of phosphorescence emission at 657 nm or relaxation of absorption changes induced by a 10-ns laser flash. Absorption difference spectra obtained upon excitation with a 35-ps flash do not decay over a 6-ns time scale, in agreement with the slower measurements. The spectra exhibit bleaching in the ground-state bands and the appearance of two new transient absorption peaks (log ∊-3.7) near 720 and 815 nm; these features are consistent with the assignment of the transient as a metalloporphyrin (π, π*) triplet. The ¹Q(π, π*) lifetimes in these carbonyl complexes appear to be <35 PS. Different photophysical behavior is observed for Ru^II(OEP)(P-n-Bu₃)₂. We assign the lowest excited state of this complex as a (d_π, e_g(π*)) metal-to-ring charge transfer (CT) state. The lifetime of this transient is 12 ± 3 ns, as measured by decay of the absorption changes in toluene following picosecond excitation. The absorption difference spectrum contains a distinct new absorption peak near 710 nm, which is expected for a (d, π*) CT excited state on the basis of resemblances with ground-state spectra of metalloporphyrin π-anion radicals. The absorption changes observed upon excitation of Ruⁿ(OEP)(NO)(OMe) in toluene with a 35-ps flash decay in two steps, with lifetimes of ≤50 ps and >5 ns. These components are tentatively assigned to relaxation of the ring ¹Q(π, π*) and ³T(π, π*) excited states. The triplet decay may proceed via ring (π) →[metal d_π+ No(π*)]CT states predicted to be in the proper energy range from previous theoretical work. Finally, excited Ru¹¹¹-(OEP)(P-n-Bu₃)(Br) decays completely in ≤35 ps, possibly via a (π, d_π) ring-to-metal CT excited state. Our results are discussed in terms of recent absorption and emission measurements and calculations on d⁶ metalloporphyrins and are compared to previous picosecond measurements on analogous osmium(II) porphyrins.