Picosecond Studies of Ruthenium(II) and Ruthenium(III) Porphyrin Photophysics
Tait, C.D., Holten, D., Barley, M., David Dolphin, and James, B.R.
J. Am. Chem. Soc.,
The results are reported of picosecond and slower time scale transient absorption and emission measurements on a series of Ru(II) and Ru(III) octaethylporphyrins (Ru(OEP)(L1)(L2)). The Ru(II) porphyrins are of 3 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 approximately 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 2 new transient absorption peaks (log ε ~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 photophysics behavior is observed for RuII(OEP)(P-n-Bu3)2. The lowest excited state of this complex is assigned as a (dπ,eg(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 RuII(OEP)(NO)(OMe) in toluene with a 35-ps flash decay in 2 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 dπ + NO(π*)] CT states predicted to be in the proper energy range from previous theoretical work. Finally, excited RuIII(OEP)(P-n-Bu3)(Br) decays completely in ?35 ps, possibly via a (π,dπ) ring-to-metal CT excited state. The 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 Os(II) porphyrins.