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Spin-selective charge transport pathways through p-oligophenylene-linked donor-bridge-acceptor molecules
Amy M Scott1, Tomoaki Miura, Annie Butler Ricks
1Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
Journal of the American Chemical Society
|November 13, 2009
Summary
Researchers studied charge transfer in donor-bridge-acceptor molecules, finding superexchange dominates electron transfer pathways. The study quantifies distance-dependent electron recombination and spin dynamics.
Area of Science:
- Photochemistry and Photophysics
- Molecular Electronics
- Organic Chemistry
Background:
- Donor-bridge-acceptor (D-B-A) systems are crucial for understanding charge transfer processes.
- The influence of bridging units on electron transfer dynamics and spin coherence is of significant interest.
- Investigating distance-dependent charge separation and recombination is key to designing molecular electronic devices.
Purpose of the Study:
- To synthesize and characterize a series of D-B-A triads with varying p-oligophenylene bridge lengths.
- To investigate the charge transfer properties, including charge separation (CS) and charge recombination (CR), in these triads.
- To elucidate the underlying mechanisms of electron transfer and spin dynamics using advanced spectroscopic techniques.
Main Methods:
- Synthesis of D-B-A triads with 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) donor and naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor linked by p-oligophenylene (Ph(n)) bridges (n=1-5).
- Transient absorption spectroscopy to monitor photoinduced electron transfer and radical ion pair formation.
- Magnetic field effects (MFEs) and time-resolved electron paramagnetic resonance (TREPR) spectroscopy to probe spin dynamics and electronic coupling.
Main Results:
- Photoexcitation quantitatively generated a spin-coherent radical ion pair, DMJ(+*)-An-Ph(n)-NI(-*).
- Both CS and CR reactions showed exponential distance dependencies with similar damping coefficients (β ≈ 0.35 Å⁻¹).
- Superexchange was identified as the dominant mechanism for both singlet and triplet charge recombination pathways, with distinct distance dependencies (β = 0.48 Å⁻¹ for singlet, β = 0.35 Å⁻¹ for triplet).
Conclusions:
- The charge transport mechanism in these D-B-A triads is governed by superexchange, without a transition to hopping.
- The spin-spin exchange interaction (2J) exhibits an exponential distance dependence (α = 0.36 Å⁻¹), consistent with triplet CR.
- Understanding these distance-dependent electronic coupling and spin dynamics is crucial for designing efficient molecular charge transport systems.
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