Each of these is well tested experimentally and known to produce relatively long-lived excited states with enough energy to be transferred to other metal centers or the conduction band of TiO 2. In addition, nine experimentally studied catalysts Chart 2 with a variety of metal centers, catalyzed reactions, reaction rates, mechanisms, and spectral coverage were chosen naming and ChemDraw structures also in Table S1. The absorption spectra of most of the catalysts are such that the Ru-MLCT band of each complex could be individually excited.
However, exciting the MLCT of each light harvester would mostly likely also result in some excited state population for Re , Co1 , and Co4 , which have broad absorption at wavelengths longer than nm, possibly leading to different pathways in the excited state dynamics. For each light harvesting donor the fully symmetry unrestricted relaxed ground state singlet , and excited state triplets, MLCT and metal centered MC , as well as the doublet and quartet oxidized forms have been optimized Figure S1.
Each metal centered catalyst has been fully relaxed to its singlet and triplet, as well as reduced doublet and quartet states Figure S2. The optimizations are symmetry unrestricted to account for Jahn-Teller effects and allow the geometric changes to be mapped with the electronic changes across various symmetries. For example, a system singlet can be constructed from the light harvester BPY ground state GS singlet and the catalyst Co3 ground state both having energy of zero giving a total energy of zero, or from the TDDFT calculated excited state singlet of the BPY and the ground state of Co3 with an energy of 2.
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Other types of triplets and quintets can be formed through excited state energy transfer EET, red or electron transfer ET, orange or green, which is catalytically active. Combinations result in two triplets, formed by a singlet-triplet and doublet-doublet, and two quintets, formed by combinations of a quartet and a doublet.
The doublet-doublet triplet and one of the quintets contain the low energy 2 Co I catalytically active form. The active quintet is often very high in energy due to the high energy 4 Ru III. These constructed states are approximate states of a weakly bound system. The reorganization around the metal centers used here are at most 0.
The singlet S o , triplet T 1 , and quintet Q 1 [ BPY Co3 ] constructed bimetallic states, where the electron transfer states ET that form catalytically active catalysts are green, other ET states are orange, and excited energy transfer EET states are red. The energy of each state is the sum of the calculated energies of each individual state.
To demonstrate the competing electron or energy transfer pathways for each catalyst we begin by constructing bimetallic dimers with each of the catalyst acceptors and a single donor, e. In Ru light harvesting complexes, the initial singlet excitation is quickly converted in fs timescales to a 3 MLCT state. The lifetime of this 3 MLCT determines the timescale for subsequent charge or energy transfers. In order to preform catalysis with the second metal center, the catalytically active state needs to not be deactivated into a lower energy excited state or through back electron transfer to reform the initial complex.
Thus systems where that the electron transfer state is the most thermodynamically stable state are preferable, like [BPY Co1].
The N3 dye is one of the best performing Ru dyes for dye-sensitized solar cells since its energy alignment allows for fast efficient transfer of electrons from both the excited state singlet and triplet into the conduction band of TiO 2. In Figure 4 we compare the linear and bent ligand Pd catalysts Pd1 and Pd2 respectively.
The linear ligand Pd1 doublet has a delocalized spin across the whole ligand like the 3 Pd II state Figure S2 and fairly small geometric changes between the reduced doublet and the excited state triplet indicating possible interconversion pathways between the doublet and triplet. Therefore, even if the doublet is initially formed, the lowest energy EET catalytically inactive 3 Pd II state is the most likely thermodynamic product.
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The bent ligand Pd2 promotes a 2 Pd I metal and monodentate ligand centered state where there are larger geometric changes from the ground state, possibly geometrically isolating the catalyst in this minimum Figure S2. Thus even though the EET also has some driving force for formation in the bent system, from either the hot vertical singlet and triplet donor excitations or from the triplet MC state, the most likely product of photoexcitation is the thermodynamically stable ET catalytically active form, making this system a good candidate for photoinduced catalysis.
DQP has an impressive microsecond excited state lifetime and shows particular promise for dyad formation, due to its structural propensity to form linear arrays. Figure 5 shows two alternative cases for DQP systems.
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As we have observed previously, 12 the doublets of each complex undergo more geometric change from the ground state than the excited state triplet MLCTs, indicating that this ET state might be observable in experiments even though there are lower energy triplets. Thus for systems of particular interest, more extensive geometry vs. Which state will be the final observed state would have to be explored through further geometric-electronic structure construction of potential energy surfaces to map the competing energy and electron transfer surfaces and their crossings helping to identify suitable systems for emerging possibilities to perform more computationally expensive dynamic simulations of excited state processes.
The largest difference in the donors is the balance of the charge separated ET vs. Here we examined the competing energy and electron transfer states from dyads formed from nine experimental carbon dioxide reduction or oxygen or hydrogen production catalysts and three well-studied Ru II light harvesters. Stable low energy excited state energy transfer states, like those seen in Ru II -Co III model systems, 12 form the majority of the thermodynamically stable states for these systems.
Finally, a couple of promising catalytic systems can be identified where photoexcitiation is expected to result in thermodynamically stable charge transfer to bring about a catalytically active acceptor, e. In addition to providing a broad basis for comparative analysis and screening, these calculations highlight some particularly important aspects for the investigated type of photocatalytic donor-acceptor systems. Firstly, the calculations show that optimizing both the donor and acceptor sites separately allows for the formation of several combined D-A states of sufficiently low energy to be accessible following the initial donor photoexcitation, and they can be characterized and compared theoretically even if not all such combined states are easily accessible experimentally.
The separate optimizations provide evidence for a competition between the formation of low-energy electron and excitation energy transfer states. The presence of EET as the final lowest energy activated state might at first sight appear surprising considering that most of the catalytically active acceptor sites only absorb light at significantly higher energies compared to the light-harvesting donor sites Figure 1.
The calculations, however, show the importance of structural relaxations to form relaxed excited acceptor states i. Given the lack of acceptor absorption, the competitive formation of such EET states over the formation of the desired ET states that form the catalytically active reduced acceptor site might therefore be easily overlooked without the presented computational characterization of the full set of combined D-A states.
Thus, quantum chemistry can be used to identify the few cases that are potentially interesting for real photocatalytic applications. Once promising donor-acceptor pairs have been identified optimizations of bound supramolecular systems could be used to investigate the interactions between units. Secondly, the calculations demonstrate the importance of combining suitable donor and acceptor properties to form photocatalytic D-A complexes that promote the photo-induced formation of the desired catalytically active state in a non-trivial manner. In particular, photocatalysis has more competing pathways compared to using the catalytic acceptor parts by themselves in an electrocatalytic fashion that only involves electrochemical reduction of the acceptor but no electronic excitations.
On one hand, the energy balance between the different reduced acceptor states is effectively governed by the formation of a catalytically active ET state alone similar to electrocatalysis. However, the desired energy and spin state for the investigated D-A systems using typical ruthenium dyes, such as the ones investigated here, have a clear energetic preference for a low-spin oxidized final state due to the energetics of the dye. This could, however, conceivably be modified for D-A systems featuring more complicated donor photophysics with alternative low-energy spin states for the oxidized donor.
On the other hand, these calculations show that the important balance between ET and EET that governs which will be the lowest energy activated D-A state can be influenced not only by the choice of acceptor e. Bound photoinduced catalytic complexes are expensive to make and study experimentally. However, weakly bound, donor-acceptor systems of Ru-light harvesters and experimentally studied catalysts can be constructed from the fully relaxed excited states of the individual sites, saving computational cost and allowing systems to be considered in a mix-and-match fashion.
Here we presented the thermodynamics of the competing electron and energy transfer states that might result from initial photoexcitation of a donor. We are able to characterize all of the possible energy and electron transfer states in weakly interacting photosensitizer-catalyst dimer systems by optimizing the donor and acceptor separately.
This provides identification of the most interesting combinations of photosensitizers and catalysts in a computationally efficient way, and thus also provides guidance to single out the D-A combinations where it will be interesting to proceed in future work with more elaborate computational investigations of explicitly bound supramolecular potential energy surfaces and the dynamics after the initial excited state to provide a more detailed picture of the excited state decay pathways.
Three key factors to which state, energy or electron transfer, wins out in a system emerge. First, metal centers that have low energy high spin forms like Co provide many low energy competing states, including excited state energy transfer with low energy triplets and thermodynamically stable high spin quartet electron transfer states, which are useful for applications like luminescent solar concentrators. Second, ideal light harvesters would have maximum spectral coverage N3 , structural aspects like readily forming vectoral arrays DQP , and driving force to preform energy or electron transfer.
While current Ru II harvesters have some of these characteristics, the methods used to extend lifetimes of tridentate complexes have often decreased the 3 MLCT excited state energy, reducing the driving force for charge separation via electron transfer. This can be seen in the limited number of dyads whose ET green state is below the 3 MLCT excited state energies in Figure 1 , making some of the most common light-harvesters less than ideal.
And lastly, acceptors that have less spectral coverage below nm provide the opportunity to selectively excite the light harvesting site, directing the initial excited state evolution toward photoinduced processes. This method of constructing donor-acceptor systems allows theoretical chemists to characterize the balance of electron and energy transfer states of photoexcited donor-acceptor pairs, pin-pointing promising pairs for photoinduced electron and energy transfer in weakly bound systems.
Analyzing the possible dyads formed from these known catalysts, the [D Pd2 ] and [D Co1 ] systems are good candidates for photoinduced oxygen or hydrogen production as the most thermodynamically stable form is the designed charge separated catalytically active form. Supporting Information. Additional computational details including the spin densities, electronic structure diagrams, and the complete list of TD-DFT calculated transitions.
J Chem Phys. Author manuscript; available in PMC Sep PMID: Lisa A. Copyright notice. The publisher's final edited version of this article is available at J Chem Phys. Abstract The rapidly growing interest in photocatalytic systems for direct solar fuel production, such as hydrogen generation from water splitting is grounded in the unique opportunity to achieve charge separation in molecular systems provided by electron transfer processes. Keywords: quantum chemistry, density functional theory, donor-acceptor, excited state evolution, supramolecular systems, photochemistry, charge separation, electron transfer, excited state energy transfer.
Introduction There is rapidly growing interest in photocatalytic systems for direct solar fuel production, such as carbon dioxide reduction and oxygen and hydrogen generation from water splitting. Open in a separate window. Chart 1. Chart 2. Figure 1. Figure 2. Figure 3. The book will focus on the dynamic aspects of the electron injection, exciton and carrier relaxation processes, as well as coherence effects, which continue to provide the impetus and the greatest challenge for the development of new methodologies.
D in Physical Chemistry from the University of Chicago in He then went on to hold An Associate position in He has been a visiting fellow at Tokyo Metropolitan University and visiting scientist in the protein engineering department at Genentech Inc. He has been an active member of many scientific committees and international conferences including the International Symposium of the U. His main research interests are the development of ultrafast microscopy methods applied to electron and excitation transfer at interfaces and in inhomogeneous systems, time-resolved laser spectroscopy of reactive intermediates, interactions between host-guest systems and redox proteins.
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