Excited state competition in fac-[Re-I(dppz)(CO)(3)(py-PTZ)](+)

Following Re-I --> dppz (dppz is dipyrido[3,2-a:2',3'-c]phenazine) metal-to -ligand charge transfer (MLCT) excitation of fac-[Re(dppz)(CO)(3)(py-PTZ)]( +) (py-PTZ is 10-(4-picolyl)phenothiazine), there is a dynamic competition between intramolecular electron and energy transfer. Laser flash photolysis at 420 nm in 1,2-dichloroethane produces the MLCT excited state fac-[Re-II (dppz(.-))(CO)(3)(py-PTZ)](+)*. It undergoes intramolecular energy transfer to give the dppz-localized pi pi* excited state [Re-I((3)dppz)*(CO)(3)(py- PTZ)](+) (3 pi pi*) in competition with electron transfer to give the redox -separated state (RS) [Re-I(dppz(.-))(CO)(3)(py-PTZ(.+))](+). The pi pi* st ate decays to the ground state with k = 1.7 x 10(4) s(-1) (tau = 60 +/- 15 mu s) and the RS state decays with k = 9.1 x 10(6) s(-1) (tau = 110 +/- 10 ns). The competition ratio for formation of the two is [RS]/[(3)pi pi*] sim ilar to 2.4. Subsequent (3)pi pi* --> RS interconversion is slow oven thoug h the process is favored by 0.43 eV.