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A handle on charge reorganization - Nature Chemistry
Photoredox catalysts offer a promising approach to performing reactions with high energetic requirements, however, the influence of solvent and counter ions is not fully understood. Now, a microwave-based technique is shown to give direct insight into their effects on charge reorganization during catalysis.
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The rearrangement of charge is a central aspect in many light-driven processes in biology, chemistry and materials science with the absorption of light often triggering a plethora of processes during which electrons — and often nuclei — reorganize in molecules. Clear examples are found in natural photosynthesis, in which a key step is the formation of a charge-separated state, and in organic solar cells, where the interface between an electron-donating and an accepting material induces the formation of isolated charges. The time-resolved detection of such charge rearrangement is usually indirect, for instance, exploiting changes in optical absorption that characterize the different species that are formed, but the assignment of optical signatures to specific species can be difficult to do in an unambiguous way.
Now, writing in Nature Chemistry , Rumbles, Reid and co-workers describe 1 a more direct technique for probing the rearrangement of charge following the absorption of light and use it to characterize the ion-pair reorganization that occurs in photoredox catalysts after photoexcitation. The technique, time-resolved dielectric loss spectroscopy (TRDL, and also called time-resolved microwave conductivity) has its origin in radiation chemistry in the 1970s where it was used for the time-resolved detection of ionic species made by pulsed irradiation 2 . Among the pioneers of this technique was John Warman at Delft University of Technology who used TRDL in several forms to study the excited-state properties of molecules in solution and charge transport in dielectric liquids and organic materials 2 , 3 , 4 .
In the TRDL approach, microwaves are used to probe changes in the dielectric properties of a sample after irradiation with a short pulse from a laser or an electron accelerator 5 . This dielectric response is a complex quantity, consisting of a real and an imaginary part. The real part corresponds to the transient changes in the dielectric constant (or the ability of an electric field to polarize the charge distribution), and the imaginary part describes the dielectric loss. This latter loss component is interesting as it corresponds to the absorption of microwaves and can be interpreted as a conductivity. As a result, TRDL has been used extensively by a limited number of groups to perform time-resolved conductivity measurements, for example, photoconductivity measurements materials for solar cells or determinations of charge carrier mobilities in organic semiconductors 5 , 6 .
Although less common, TRDL can also be used to measure excited-state dipole moments. Dipolar species can absorb microwave power through rotational motion, the same phenomenon used to heat food in a microwave oven, which works through the rotational motion of dipolar water molecules. If the dipole moment of a molecule in solution changes on photoexcitation it can be measured via losses in the transient dielectric signal. Such experiments played a key role in proving that full charge separation can take place over large distances in donor–bridge–acceptor systems 3 .
In all of the examples above, it is the dielectric loss (or the imaginary part of the dielectric constant) that determines the response. Changes in the real dielectric constant are related to the ease with which a charge distribution can be polarized, for instance, when measuring excited-state polarizabilities of conjugated molecules to gain insight into the nature and degree of delocalization of excited states. Change in the real dielectric constant are also observed in the case of a very rapid intramolecular rearrangement of the dipole moment.
Microwave-based dielectric loss spectroscopy methods therefore offer a direct handle on charge distribution in molecular systems in solution. Rumbles and co-workers use the TRDL approach to unravel some intriguing aspects of photoredox catalysts 1 . Such catalysts can drive reactions with high kinetic or thermodynamic barriers using the absorption of photons. Common photoredox catalysts consist of a d 6 -metal centre — like the iridium complex interrogated by Rumbles, Reid and colleagues (Fig. 1a ) — surrounded by multiple ligands. When they are photoexcited, metal-to-ligand charge-transfer states are generated that exhibit substantial changes in charge distribution, and hence a substantial change in dipole moment compared to the ground state. The team’s TRDL data show that this charge rearrangement is closely coupled to the surroundings of the complex, and counter ions that form a closely bound ion pair with the metal complex are shown to substantially affect the charge distribution in the excited state. The size and polarity of the counter ion were also seen to strongly affect the reactivity of the photocatalyst, pointing to the effects of ion pairing.
Fig. 1: Structure of photoredox catalysts. adapted from ref. 1 ., Springer Nature Ltd.
a , Structure of the [Ir[dF(CF 3 )ppy] 2 (dtbpy)]X compound, where X – represents the counter ions BAr F 4 ? and PF 6 ? . b , The proximity of the counter ion strongly affects the overall charge distribution and the catalytic activity.
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It is almost impossible to quantify such ion-pairing effects using optical measurements, but Rumbles, Reid and colleagues have used a revitalized form of TRDL to gain direct insight into the charge distribution in the ground and excited states of Ir complexes with different counter ions. Their ground-state measurements showed that small negative counterions (PF 6 ? in this case) form tightly bound ion pairs that behave as a permanent dipole moment, generating a dielectric loss signal. The team were also able to derive the overall dipole moment using a new quantitative modelling approach. When they replaced the small counter ion by a much larger one (BAr F 4 ? ) they saw the Ir complexes behave as individual ions in solution, rather than forming tightly bound ion pairs, and hence the dipole moment of the ground-state complex itself determined the dielectric loss signal (Fig. 1b ).
In the excited state the results become even more intriguing. From measuring the changes in the real and imaginary dielectric constant, a detailed picture emerges where the ion pairing not only affects the magnitude of the dipole moment, but also directs the nature of the excited state itself through the electric field that it exerts on the Ir complex. The effects of the counter ions on the nature of the excited state and the specific polarization that their presence induce have direct consequences for the catalytic activity, resulting in up to a fourfold increase in reaction rate when ion-pairing is removed. These results give important new insights into the effects of pairing with counter ions and solvent polarity on the catalytic activity of this whole class of photoredox catalysts.
The work of Rumbles, Reid and colleagues may extend further than just the specifics of the photoredox systems studied here. They show that reviving a somewhat eccentric form of dielectric loss spectroscopy and combining it with modern numerical modelling approaches can result in a unique method by which to shine a direct light on excited-state charge distribution in molecular systems. This will certainly lead to many more insights into other photoinduced processes that are almost impossible to obtain by other techniques.
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Department of Chemical Engineering, Delft University of Technology, Delft, The Netherlands
Ferdinand C. Grozema
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Grozema, F.C. A handle on charge reorganization. Nat. Chem. (2022). https://doi.org/10.1038/s41557-022-00987-0
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Published : 01 July 2022
DOI : https://doi.org/10.1038/s41557-022-00987-0
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