ChemInform Abstract: PICOSECOND STUDIES OF RUTHENIUM(II) AND RUTHENIUM(III) PORPHYRIN PHOTOPHYSICS

Abstract

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 ruthenium(II) porphyrins are of three types. First are the carbonyl complexes in which Ll = CO and L2 = EtOH, pyridine (py), or I-methylimidazole (1-\1elm), which all exhibit similar spectral and kinetic behavior. The lowest excited states of these molecules are assigned as the ring 3T(7l',7l'*) in agreement with previous work on similar complexes. The 3T(7l',1I'*) state has a lifetime of ~75 J.lS at room temperature in degassed solution, as measured by decay of phosphorescence emission at 657 nm or relaxation of absorption changes induced by a lOons 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 E ~3.7) near 720 and 815 nm; these features are consistent with the assignment of the transient as a metalloporphyrin (11'.11'*) triplet. The lQ(7l',lI'*) lifetimes in these carbonyl complexes appear to be 50 ps and >5 ns. These components are tentatively assigned to relaxation of the ring IQ(7l',lI'*) and 3T(1I',1I'*) excited states. The triplet decay may proceed via ring (7l') -- (metal d .. + NO(1!'*)) CT states predicted to be in the proper energy range from previous theoretical work. Finally, excited Ru lll - (OEP)(P-n-Bu3)(Br) decays completely in !>35 ps, possibly via a (7l',d~) 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.