Temperature-dependent mechanistic transition for photoinduced electron transfer modulated by excited-state vibrational relaxation dynamics

The electron transfer (ET) dynamics of an unusually rigid pi-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-probe transient 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 of several 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 from the equilibrated lowest-energy S-1 state (CS1). As the temperature decreases, production of the photoinduced charge-separated state directly from the vibrationally unrelaxed S-1 state (CS2) becomes competitive with the vibrational relaxation time scale. At the lowest experimentally interrogated temperature (similar to 80 K), CS2 defines the dominant ET pathway. ET from the vibrationally unrelaxed S-1 state is temperature-independent and manifests a subpicosecond time constant; in contrast, the CS1 rate constant is temperature-dependent, exhibiting time constants ranging from 4 x 10(10) s(-1) to 4 x 10(11) s(-1) and is correlated strongly with the temperature-dependent solvent dielectric relaxation time scale over a significant temperature domain. Respective electronic coupling matrix elements for each of these photoinduced CS1 and CS2 pathways were determined to be similar to 50 and similar to 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 states can be differentiated, but also demonstrates a temperature-dependent mechanistic transition of photoinduced CS 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 by the extent to which the vibrationally unrelaxed S-1 state is populated.