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Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems
K Kilså1, J Kajanus, A N Macpherson
1Department of Physical Chemistry and Department of Organic Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden.
Journal of the American Chemical Society
|July 18, 2001
Summary
Photoinduced electron transfer was studied in donor-bridge-acceptor systems. Conjugated bridges facilitate electron transfer, while non-conjugated bridges enable energy transfer, with coupling dependent on the energy gap.
Area of Science:
- Photochemistry
- Molecular electronics
- Supramolecular chemistry
Background:
- Donor-bridge-acceptor (DBA) systems are crucial for artificial photosynthesis and molecular devices.
- Understanding electron transfer mechanisms in DBA systems is key to designing efficient light-harvesting and charge-transport materials.
Purpose of the Study:
- To investigate photoinduced electron transfer in DBA systems with varying bridge electronic structures.
- To elucidate the role of the bridging unit in dictating electron vs. energy transfer pathways.
- To quantify electronic coupling between donor and acceptor moieties.
Main Methods:
- Synthesis of DBA systems with zinc porphyrin donors, gold(III) porphyrin acceptors, and varied bridging chromophores.
- Picosecond transient absorption spectroscopy to probe excited-state dynamics.
- Analysis using Marcus and Rehm-Weller equations to determine electron transfer parameters.
Main Results:
- Systems with pi-conjugated bridges exhibited solvent-dependent quenching rates, indicative of Marcus-type electron transfer.
- Systems with a broken conjugation bridge showed solvent-independent quenching, consistent with Förster energy transfer.
- Picosecond spectroscopy confirmed zinc porphyrin radical cation formation only in pi-conjugated systems.
- Electronic coupling was estimated at 5-20 cm⁻¹ for conjugated systems, correlating with energy gaps.
Conclusions:
- The electronic structure of the bridging unit critically controls the photoinduced charge transfer pathway (electron vs. energy transfer).
- Electronic coupling is influenced by the energy gap between donor and bridge excited states, aligning with quantum mechanical predictions.
- DBA systems with conjugated bridges show efficient photoinduced electron transfer, paving the way for advanced molecular electronic applications.
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