Direct Measurement of Photoinduced Charge Separation Distances in Donor−Acceptor Systems for Artificial Photosynthesis Using OOP-ESEEM

Abstract

The distance over which two photogenerated charges are separated in electron donor-acceptor systems for artificial photosynthesis depends on the structure of the system, while the lifetime of the charge separation and, ultimately, its ability to carry out useful redox chemistry depend on the electronic coupling between the oxidized donor and reduced acceptor. The radical ions produced by charge separation are frequently delocalized over the pi systems of the final oxidized donor and reduced acceptor, so that there is often significant uncertainty as to the average distance between the separated charges, especially in low dielectric constant media, where the Coulomb attraction of the ions may be significant and the charge distribution of the ions may be distorted, so that the average distance between them may be shorter than that implied by their chemical structures. The charge separation distances between photogenerated radical ions in three donor-acceptor molecules having different donor-acceptor distances were measured directly from their dipolar spin-spin interactions using out-of-phase electron spin echo envelope modulation (OOP-ESEEM). The measured distances in toluene at 85 K compare favorably to the calculated distances between the centroids of the spin distributions of the radical ions within the radical ion pairs. These results show that despite the intrinsically nonpolar nature of medium, the spin (and charge) distributions of the RPs are not significantly distorted by Coulomb attraction over these long distances. This study shows that OOP-ESEEM is well-suited for probing the detailed structural features of charge-separated intermediates that are essential to understanding how to design molecular structures that prolong and control charge separation for artificial photosynthesis.

Quantifying the exchange coupling in linear copper porphyrin oligomers

“…8 In contrast, EPR can probe exchange interactions of the order of 10 À3 cm À1 and lower. 2,3,9,10 In this study, we investigate electronic communication in linear copper porphyrin oligomers designed as molecular wires [11][12][13] by EPR spectroscopy via measurement of the exchange coupling between the copper centers. The copper oligomers chosen for this case study represent very large spin systems with Hamiltonian dimensions exceeding the capabilities of commonly used simulation approaches.…”

Determination of Radical–Radical Distances in Light‐Active Proteins and Their Implication for Biological Magnetoreception

“…+ n k )/6. [23] We analyzed the echo decays and frequency spectra based on published formalisms [21] and extracted dipolar and exchange couplings directly from least-squares fittings of simulations to experimental time traces.T he distances between the individual radicals in the RPs of the two proteins were calculated using the point-dipole approximation, [19] D(r) = À2785 mT 3 r À3 . As is already evident from am ere inspection of the experimental data, the simulations confirmed that D is larger in EcPL than in DmCry (see Figure 1), thus indicating as horter separation between the radicals in the pair of spins in the former protein.…”

“…[12d, 15] Herein, we present data from pulsed out-of-phase electron-spin-echo envelope modulation (OOP-ESEEM) EPR [16] spectroscopic studies that were performed to directly measure the strength of the exchange and dipolar interactions in the short-lived RP states of the Drosophila melanogaster cryptochrome (DmCry) and Escherichia coli DNAp hotolyase (EcPL), for which ET along the conserved Tr pt riad has been extensively studied by transient optical absorption spectroscopy. [11b,17] OOP-ESEEM has previously been applied to examine spin-correlated RPs in photosynthetic reaction centers, [18] donor-acceptor systems for artificial photosynthesis, [19] and, more recently,c omposite materials designed for organic photovoltaic devices. [20] In brief,t his EPR experiment comprises ac anonical two-microwave-pulse echo sequence [hn-t DAF -(x 1 ) x -t-(x 2 ) x -t-echo] applied after as hort laser pulse that initiates RP formation.…”

Feedback control optimisation of ESR experiments

“…OOP-ESEEM experiments used a sample of BDXANINI (FIG. 4), with the expected donor-acceptor distance of 26 Å, similar to Sample 1 in [58], but with naphthalimide (NI) rather than phthalimide (PI) derivative as the acceptor, and 2,5ditertbutylphenyl as the end group. BDXANINI was dissolved in 4-cyano-4 -pentylbiphenyl (5CB) liquid crystal at a concentration of 0.2 mM and degassed in a 4 mm OD quartz tube using the freeze-pump-thaw method prior to flame sealing under vacuum.…”

“…The method has the benefit of a smaller number of experimental Figure 4: The sample used in OOP-ESEEM experiments on a photo-generated radical pair. The group that proposed this system [58] used BDX tag for the biphenyl dioxolane derivative on the left, ANI tag for the 4-aminonaphthalene-1,8-imide (middle), and NI tag for the naphthalene-1,8-imide-4,5-imide (right), hence the overall BDXANINI tag for the construct. evaluations per iteration compared to other gradient-free techniques, such as genetic algorithms [29,47] or simulated annealing [48].…”