INVESTIGATION OF THE LOWEST ELECTRONIC STATES OF OSMIUM(II) TETRATOLYLPORPHYRINS - PHOTOPHYSICS OF METALLOPORPHYRIN (D,PI-ASTERISK) CHARGE-TRANSFER STATES

The nature of the lowest excited states of a series of osmium(II) porp hyrins has been investigated using static and time-resolved emission s pectroscopy and ultrafast transient absorption measurements. It is fou nd that a metal-to-ring 3(d,pi) charge transfer excited state is the l owest excited state of the Os(II)P(CO)(L) and Os(II)P(L)2 complexes, w here the porphyrin macrocycle (P) is either tetratolyl- or octaethylpo rphyrin and the axial ligand L is a sigma-donor such as pyridine. Prev ious studies on OsTTP(CO)(py) had led to the assignment of the lowest excited state as the 3(pi,pi) state of the porphyrin ring. The result s on the Os(II) porphyrins can be contrasted with those found previous ly for the analogous Ru(II) porphyrins, in which the lowest excited st ate was found to switch from 3(pi,pi) in the RuP(CO)(L) complexes to 3(d,pi) in the RuP(L)2 compounds. The studies further establish the e lectronic origin of the red-region features in the absorption spectra of metalloporphyrin 3(pi,pi) and 3(d,pi*) excited states. The combine d results demonstrate the critical role played by pi-accepting and cr- donating axial ligands in controlling the nature of the lowest excited states, the electronic properties, and excited state dynamics of the biologically-relevant d6 metalloporphyrins.