The electron transfer (ET) dynamics of an unusually rigid
-stacked (porphinato)zinc(II)-spacer-quinone(PZn-Q) system, [5-[8'-(4' '-[8' ''-(2' '' ',5' '' '-benzoquinonyl)-1' ''-naphthyl]-1' '-phenyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (
2a-Zn), in which sub-van der Waals interplanar distances separate juxtaposed
porphyryl,aromatic bridge, and quinonyl components of this assembly, have been measured by ultrafast pump-probetransient absorption spectroscopy over a 80-320 K temperature range in 2-methyl tetrahydrofuran (2-MTHF)solvent. Analyses of the photoinduced charge-separation (CS) rate data are presented within the context ofseveral different theoretical frameworks. Experiments show that at higher temperatures the initially prepared
2a-Zn vibronically excited S
1 state relaxes on an ultrafast time scale, and ET is observed exclusively fromthe equilibrated lowest-energy S
1 state (CS
1). As the temperature decreases, production of the photoinducedcharge-separated state directly from the vibrationally unrelaxed S
1 state (CS
2) becomes competitive with thevibrational relaxation time scale. At the lowest experimentally interrogated temperature (~80 K), CS
2 definesthe dominant ET pathway. ET from the vibrationally unrelaxed S
1 state is temperature-independent andmanifests a subpicosecond time constant; in contrast, the CS
1 rate constant is temperature-dependent, exhibitingtime constants ranging from 4 × 10
10 s
-1 to 4 × 10
11 s
-1 and is correlated strongly with the temperature-dependent solvent dielectric relaxation time scale over a significant temperature domain. Respective electroniccoupling matrix elements for each of these photoinduced CS
1 and CS
2 pathways were determined to be ~50and ~100 cm
-1. This work not only documents a rare, if not unique, example of a system where temperature-dependent photoinduced charge-separation (CS) dynamics from vibrationally relaxed and unrelaxed S
1 statescan be differentiated, but also demonstrates a temperature-dependent mechanistic
transition of photoinducedCS from the nonadiabatic to the solvent-controlled adiabatic regime, followed by a second temperature-dependent mechanistic evolution where CS becomes decoupled from solvent dynamics and is determined bythe extent to which the vibrationally unrelaxed S
1 state is populated.