Citation:
Wellauer, J.; Pfund, B.; Becker, I.; Meyer, F.; Prescimone, A.; Wenger, O.S. Iron(III) Complexes with Luminescence Lifetimes of up to 100 ns to Enhance Upconversion and Photocatalysis. J. Am. Chem. Soc. ASAP.
https://doi.org/10.1021/jacs.4c18603
Summary Figure:
Background:
A good photocatalyst has a number of useful properties:
1. Absorb light at a wavelength that is easily accessible and does not interfere with other molecules. Ideally, this is visible light because most organic molecules do not absorb visible light (which is why everything is a white powder) and LEDs in visible light colors are easy to acquire.
2. Once the molecule has absorbed the energy in light, the catalyst needs to not immediately release that energy. Most molecules will absorb light, kick an electron up to the excited state, and then immediately relax back down and either release a different photon or wiggle their way back and release kinetic (heat) energy and be unproductive. If the catalyst is going to do something useful to another molecule, it needs to hold onto that energy long enough that it can find the right molecule.
3. Not undergo significant chemical change upon oxidation or reduction. For a photocatalyst to be a catalyst, it needs to be able to turnover in a catalytic cycle, which means being both an oxidant and a reductant.
For more background on photochemistry, I recommend this review: https://pubs.acs.org/doi/10.1021/acs.joc.6b01449
The most common photocatalysts for photochemistry use precious metals, usually iridium or ruthenium, usually as coordinatively saturated complexes. These have the useful properties that enable reliable photocatalysis.
For example, Ir(ppy)3 absorbs blue light, has an extremely long lived excited state (1900 ns), and reliably serves as a one-electron oxidant or reductant (going from Ir(III) to Ir(II) or Ir(IV)) that can be reverted back to Ir(III) to close the cycle.
However, Iridium currently costs about $150/gram (for reference, gold is ~$90/gram). Finding other photocatalysts that don't cost a grad student's kidney would be a relevant broader impact, and other metals also might have interesting photochemical properties and therefore intellectual merit.
Previously, some Fe(III) photocatalysts had been made that are able to absorb light, but they discharged the energy far too quickly (excited state lifetime of at most 2 nanoseconds). Again, for reference, Ir(ppy)3 was 2000 ns. In this report, the Wenger lab designed an iron(III) photocatalyst that has a lifetime of 98 ns (so close to 100!).
How it works:
The idea behind this report is that if you give the iron center something to discharge the energy into, you can keep the energy on the complex for longer without losing it as a photon.
First, a photon excites the iron complex via a ligand-to-metal charge transfer (LMCT). For those unfamiliar, this means that an electron that was on the ligand (usually in the ligand-metal bond) becomes excited and moves to a higher energy orbital somewhere on the metal. This means that there is now an unpaired electron on the metal complex, making it a doublet. The electron is also in a higher energy state, which means it is excited, hence this state is the "doublet excited state". (Note that for most precious metals like iridium, there are actually 2 unpaired electrons, making it a triplet. The difference is because iridium and iron have different numbers of d electrons.)
Second, the energy of this excited state is transferred to another part of the molecule by exciting a different pair of electrons. The original triplet is relaxed back to the ground state, but another part of the molecule is now in a excited state. This is called a "triplet-triplet energy transfer" (TTET) or a "doublet-triplet energy transfer" (DTET), depending on whether the metal excited state is a doublet or triplet. If the two parts of the molecule have excited states with similar energy levels, then this energy transfer is reversible and can go in either direction. You lose a little bit of energy each time, but this can extend the lifetime of the excited state by keeping the energy in a "reservoir".
Based on the figure directly above, the goal is to keep bouncing back and forth with the Kdtet as long as possible before either the Kem (photon emission) or Knr (non radiative decay) processes release the energy unproductively.
What the Wenger lab did is take a known iron(III) complex, [Fe(ImPP)2]+, that has the ability to absorb light but a short-lived excited state, and attach a chromophore (anthracene derivatives) to the end of the ligand that can absorb the energy. The exact energy that the chromophore absorbs could be tuned by adding different substituents, improving the transfer.
Initial questions and key findings:
1. Can you attach a chromophore to a photocatalyst to extend the lifetime of the excited state?
A: Yes, the base complex [Fe(ImPP)2]+ has a lifetime of 0.27 ns. Attaching an anthracene at the end of each side of the ImPP ligand improves this to 1.4 ns, a 5-fold improvement. The wavelength of light absorbed remained the same (635 nm) as did the quantum yield (0.07%), so all that the chromophore seems to do is increase the lifetime of the excited state.
2. Can you modify the chromophore to improve the properties of the photocatalyst?
A: Yes, adding nitrile groups to the anthracene and adding more anthracene units onto the ligand scaffold increases the lifetime of the catalyst to 98 ns. The more anthracene units added onto the complex, the longer the lifetime.
3. Can you use these catalysts for photocatalysis and do reactions?
A: Yes, the authors tested both the short-lived parent complex and the complex with the longest lifetime in 3 different photocatalysis reactions. In each reaction, the long-lived complex provided higher yields of the desired product, showing that these are improved catalysts compared to the parent complex.
Takeaways:
This report serves as a wonderful proof of concept of the ability to attach organic chromophores to the ligand and extend the excited state lifetime of metal complexes, which had not previously been shown for iron. The authors also demonstrated the link between modifying the substituents on the chromophore and the energy gap, opening up an obvious starting point for future work.
Medicinal chemists across the world probably are not going to toss out their iridium bottles and start switching to iron anytime soon. These catalysts still have a long way to go before they start replacing the extremely reliable and well established precious metal catalysts. However, that process does need to happen eventually, and work like this helps push the field along one innovation at a time.
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