Temperature-Dependent Mechanistic Transition for Photoinduced Electron Transfer Modulated by Excited-State Vibrational Relaxation Dynamics
详细信息 查看全文
- 作者：Youn K. Kang ; Timothy V. Duncan ; Michael J. Therien
- 刊名：Journal of Physical Chemistry B
- 出版年：2007
- 出版时间：June 21, 2007
- 年：2007
- 卷：111
- 期：24
- 页码：6829 - 6838
- 全文大小：304K
- 年卷期：v.111,no.24(June 21, 2007)
- ISSN：1520-5207
文摘
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 prepared2a-Zn vibronically excited S1 state relaxes on an ultrafast time scale, and ET is observed exclusively fromthe equilibrated lowest-energy S1 state (CS1). As the temperature decreases, production of the photoinducedcharge-separated state directly from the vibrationally unrelaxed S1 state (CS2) becomes competitive with thevibrational relaxation time scale. At the lowest experimentally interrogated temperature (~80 K), CS2 definesthe dominant ET pathway. ET from the vibrationally unrelaxed S1 state is temperature-independent andmanifests a subpicosecond time constant; in contrast, the CS1 rate constant is temperature-dependent, exhibitingtime constants ranging from 4 × 1010 s-1 to 4 × 1011 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 CS1 and CS2 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 S1 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 S1 state is populated.
-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 prepared2a-Zn vibronically excited S1 state relaxes on an ultrafast time scale, and ET is observed exclusively fromthe equilibrated lowest-energy S1 state (CS1). As the temperature decreases, production of the photoinducedcharge-separated state directly from the vibrationally unrelaxed S1 state (CS2) becomes competitive with thevibrational relaxation time scale. At the lowest experimentally interrogated temperature (~80 K), CS2 definesthe dominant ET pathway. ET from the vibrationally unrelaxed S1 state is temperature-independent andmanifests a subpicosecond time constant; in contrast, the CS1 rate constant is temperature-dependent, exhibitingtime constants ranging from 4 × 1010 s-1 to 4 × 1011 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 CS1 and CS2 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 S1 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 S1 state is populated.