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Jesus Nieto-Pescador1, Baxter Abraham2, Jingjing Li2

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|May 9, 2017
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Summary

Investigating photoexcited Zn-porphyrin dyes on TiO2, this study reveals ultrafast heterogeneous electron transfer (HET) occurs in 80 fs. This process is independent of linker-induced energy level alignment changes, suggesting unique transition states.

Keywords:
PorphyrinoidsTiO2heterogeneous electron transferultrafast transient absorption

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Area of Science:

  • Photochemistry
  • Materials Science
  • Physical Chemistry

Background:

  • Heterogeneous electron transfer (HET) is crucial for photocatalysis and solar energy conversion.
  • Understanding the factors influencing HET rates, such as electronic coupling and energy level alignment, is essential.
  • Zn-porphyrin dyes offer tunable electronic properties for studying interfacial electron transfer.

Purpose of the Study:

  • To investigate the kinetics of HET between photoexcited Zn-porphyrin chromophores and colloidal TiO2.
  • To explore the influence of linker-induced dipole reorientation and level alignment on HET rates.
  • To elucidate the underlying mechanisms of HET at the molecular level.

Main Methods:

  • Femtosecond transient absorption spectroscopy was employed to study dye molecules in solution and adsorbed on TiO2 films.
  • A series of Zn-porphyrin chromophores with varying linker orientations were synthesized.
  • Quantum-mechanical modeling incorporating electronic-vibronic coupling and finite acceptor bandwidth was used for comparison.

Main Results:

  • Intramolecular energy transfer was observed as a competing relaxation pathway in solution.
  • Ultrafast heterogeneous electron transfer (HET) occurred on the TiO2 film in 80 fs, faster than intramolecular processes.
  • Identical HET times were measured despite a 200 meV difference in excited-state level alignment, indicating insensitivity to energy level shifts.

Conclusions:

  • HET at the dye-TiO2 interface is extremely rapid, occurring within 80 fs.
  • The electron transfer rate is largely independent of the excited-state energy level alignment within the studied range.
  • HET likely proceeds through a distribution of transition states, rather than conventional surface or bridge-mediated states.