M2: Molecular Triads and Charge-Transfer Assemblies

Single molecules with linked photosensitizer, electron donor, and acceptor for one-particle solar fuels.

Idea

Create single molecules or complexes that perform the series of reactions: a light absorber connected to an electron donor (which gets oxidized) and an acceptor (which gets reduced). Upon illumination, charge separation occurs within this molecule (electron goes one way, the “hole” the opposite way). This mimics the natural reaction center.

Rationale

If successfully designed, a molecular triad could directly use light to split water within one molecular architecture – e.g. a photosensitizer (P) linked to an oxidation catalyst (D donor) and a reduction catalyst (A acceptor). Such assemblies have been studied (e.g., dye molecules with attached catalysts) and can achieve initial charge separation, but stability and completing the full cycle are challenges. The advantage is no need for external wiring or membranes – it’s a “one-particle” solution like a homogeneous system.

Prerequisite Themes

Electron transfer theory (Marcus theory, etc.); proton-coupled electron transfer (PCET) mechanisms; coordination chemistry to link multi-functional units.

Dependencies

Often requires a sacrificial reagent in early tests. Ultimately, needs to be coupled with regeneration cycles or anchored to surfaces to remove products. Can complement L1 or L3 (as the light-absorbing part of the triad) and uses catalysts from H1–H3.

Signs of Progress

Successful water-splitting by a single molecular assembly (a few have achieved two-step charge separation, but not full fuel so far); improved longevity through self-repair or reversible charge separation; incorporation of PCET relays (like internal bases or proton relays) that make charge separation more efficient – similar to tyrosine-histidine in natural PSII.

Base Camp M2.1: Electron Transfer Theory and Kinetics

Scope

To design molecular triads, you must control electron transfer rates. This base-camp covers Marcus theory basics (normal vs inverted region), the role of driving force (ΔG) and reorganization energy, and distance dependence of tunneling.

Stepping-stones

For a given triad, identify all possible electron/hole transfer pathways. Solve a few simplified rate equations or use Marcus theory equations qualitatively to see how changing donor-acceptor distance or driving force affects rate.

Resources

• Marcus, Rudolph – Electron transfer reactions in chemistry: Theory and experiment (Nobel Lecture), Angew Chem Int Ed 32.8 (1993): 1111–1121. Why: Marcus’s Nobel lecture provides a relatively accessible summary of the electron transfer theory.
• Meyer, Thomas – Chemical approaches to artificial photosynthesis, Acc Chem Res 22.5 (1989): 163–170. Why: Discusses early molecular assemblies for photoinduced electron transfer and the importance of kinetic competition.
• Closs, Gerhard & Miller, John – Tunneling and Protein Electron Transfer: Experimental Studies, Science 240.4851 (1988): 440–447. Why: Provides empirical evidence on how electron transfer rate decays with distance (β factor), key for triad design.

Base Camp M2.2: Synthetic Chemistry of Multifunctional Molecules

Scope

Learn how to actually make a molecule that has three parts (or more). This includes understanding of coordination chemistry if attaching metal catalysts, or organic synthesis for linking chromophores and catalysts.

Stepping-stones

Look at examples of triads: how were they synthesized step-by-step? Identify the linker units in the molecular structure. Consider issues like solubility and purification.

Resources

• Nieuwland, M. & Reek, J. – Molecular Photosystems for Solar Fuel Production, Catalysis Journal 3.1 (2019): 213–236. Why: A review focusing on molecular systems for solar fuels including sections on how various groups have constructed their assemblies.
• Gust, Devens et al. – Artificial photosynthetic triad, J Am Chem Soc 109.24 (1987): 846–847. Why: The famous carotene-porphyrin-quinone triad paper. Exemplifies a successful construction of a triad.
• Lehn, Jean-Marie – Supramolecular Chemistry: Concepts and Perspectives (Wiley-VCH, 1995). Why: Broader perspective on how to design and assemble multi-component molecules.

Base Camp M2.3: Photophysical Characterization of Triads

Scope

Once a triad is made, determining if it actually does what you want (photoinduced charge separation) is non-trivial. You need techniques like transient absorption spectroscopy, EPR, or time-resolved IR to see charges.

Stepping-stones

Understand what a charge-separated state’s spectral signature might be. Look at a kinetic trace from a pump-probe experiment for a known triad and interpret where the different exponential decay components come from.

Resources

• Wasielewski, Michael – Photoinduced electron transfer in supramolecular systems for artificial photosynthesis, Chem Rev 92.3 (1992): 435–461. Why: A comprehensive review on photophysics of donor-acceptor systems, covering how to measure and analyze electron transfer.
• Harriman, Anthony – Luminescence of multi-component molecular assemblies: an elementary introduction to the role of kinetic considerations, J Photochem Photobiol A 82.1-3 (1994): 3–17. Why: Didactic piece on interpreting luminescence quenching as sign of electron transfer.
• Kovalenko, Sergey et al. – Ultrafast Photochemistry of Multichromophoric Systems in Solution, Chemical Physics 259.2-3 (2000): 209–227. Why: Introduces idea that conformation affects electron transfer – relevant for flexible triads.

Base Camp M2.4: PCET in Molecular Systems

Scope

Focus on understanding PCET at a molecular level: when a molecule like a phenol transfers an electron, a proton hops too. Learn about designing intramolecular PCET – e.g. incorporating a proton relay in a catalyst so that when it gets reduced it also picks up a proton.

Stepping-stones

Familiarize with model systems, such as ruthenium polypyridyl complexes with a pendant acid/base. Understand how the rate of PCET can differ from purely electron transfer or purely proton transfer.

Resources

• Hammes-Schiffer, Sharon & Stuchebrukhov, Alexei – Theory of coupled electron and proton transfer reactions, Chem Rev 110.12 (2010): 6939–6960. Why: A thorough review of PCET theory.
• Weinberg, David et al. – Proton-Coupled Electron Transfer, Accounts of Chemical Research 50.4 (2017): 986–993. Why: Approachable account discussing PCET in the context of solar fuels.
• Costentin, Cyrille et al. – Concerted proton-electron transfers: electrochemical and spectroscopic evidence, J Am Chem Soc 131.44 (2009): 15938–15939. Why: Experimental work showing PCET in action through electrochemistry and spectroscopy.