Magnetic field effect on ion pair dynamics upon bimolecular photoinduced electron transfer in solution

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. , We show here that thanks to of fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Bimolecular photoinduced symmetry-breaking charge separation of perylene in solution

Photoinduced symmetry-breaking charge separation (SB-CS) results in the generation of charge carriers through electron transfer between two identical molecules, after photoexcitation of one of them. It is usually studied in systems where the two reacting moieties are covalently linked. Examples of photoinduced bimolecular SB-CS with organic molecules yielding free ions remain scarce due to solubility or aggregation issues at the high concentrations needed to study this diffusion-assisted process. Here we investigate the excited-state dynamics of perylene (Pe) at high concentrations in solvents of varying polarity. Transient absorption spectroscopy on the subnanosecond to microsecond timescales reveal that self-quenching of Pe in the lowest singlet excited state leads to excimer formation in all solvents used. Additionally, bimolecular SB-CS, resulting in the generation of free ions, occurs concurrently to excimer formation in polar media, with a relative efficiency that increases with the polarity of the solvent. Moreover, we show that SB-CS is most efficient in room-temperature ionic liquids due to a charge-shielding effect leading to a larger escape of ions and due to the high viscosity that disfavours excimer formation.

Magnetically Boosted Generation of Intracellular Reactive Oxygen Species toward Magneto-Photodynamic Therapy

The generation of reactive oxygen species (ROS) in photodynamic therapy (PDT) involves excited-state intermediates with both singlet and triplet spin configurations, which provides possibilities to modulate the ROS production in PDT under an external magnetic field. Here, we present that magnetically modulated ROS production can promote PDT efficacy and develop a magnetic-field-assisted PDT (magneto-PDT) method for effectively and selectively killing cancer cells. The photosensitization reaction between excited-state riboflavin and oxygen molecules is influenced by the applied field, and the overall magnetic field effect (MFE) shows a moderate increase at a low field (<1000 G) and then a boost up to the saturation ∼100% at a high field (>1000 G). It is found that the spin precession occurring in radical ion pairs (electron transfer from riboflavin to oxygen) facilitates the O₂^•- generation at the low field. In comparison, the spin splitting in an encounter complex (energy transfer from riboflavin to oxygen) benefits the production of ¹O₂ species at the high field. The field modulation on the two types of ROS in PDT, i.e., O₂^•- and ¹O₂, is also demonstrated in living cells. The magneto-PDT strategy shows the capability to inhibit the proliferation of cancer cells (e.g., HeLa, RBL-2H3, and MCF-7) effectively and selectively, which reveals the potential of using the MFE on chemical reactions in biological applications.

Solvent and Temperature Effects on Photoinduced Proton-Coupled Electron Transfer in the Marcus Inverted Region

Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated (Science 2019, 364, 471-475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene-phenol-pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile:n-butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (-ΔG°) and reorganization energy (λ), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n-butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (ΔG*CR ∼ 140 meV). However, the solvent continuum model fails to accurately describe the changes in ΔG° and λ with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [J. Phys. Chem. B 1999, 103, 9130-9140], which predicts that λ decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both ΔG°CR and λ, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.

Magnetically modulated photochemical reaction pathways in anthraquinone molecules and aggregates

The chemical reactions involving excited-state radical pairs (RPs) of parallel/anti-parallel spin configurations are sensitive to magnetic field, leading to the possibilities of magnetically controlled synthesis of chemical compounds. Here we show that the reaction of anthraquinone (AQ) in sodium dodecyl sulfate (SDS) micellar solution under UV excitation is significantly influenced by applying external field. The steady state and time-resolved spectroscopies reveal that the reaction intermediate (pairs of AQH-SDS radicals) can undergo two distinct pathways depending on whether it is spin singlet or triplet, and the field is beneficial to the conversion between spin configurations of RPs. The applied field not only affects the reaction rate constant but also changes the final products. Besides, the aggregation of AQ molecules would change the population of singlets and triplets and thus enhance magnetic field effect. This work represents a promising way of controlling chemical reaction and improving reaction selectivity via magnetic field methods.

Magnetic-Field-Induced Modulation of Charge-Recombination Dynamics in a Rosarin-Fullerene Complex

Charge-recombination processes are critical for photovoltaic applications and should be suppressed for efficient charge transport. Here, we report that an applied magnetic field (0-1 T) can be used control the charge-recombination dynamics in an expanded rosarin-C₆₀ complex. In the low magnetic field regime (<100 mT), the charge-recombination rate slows down due to hyperfine coupling, as inferred from transient absorption spectroscopic analyses. In contrast, in the high field regime, i.e., over 500 mT, the charge-recombination rate recovers and increases because the Δg mechanism facilitates spin conversion to a triplet charge-separated state (S to T₀ ) that undergoes rapid charge-recombination to a localized rosarin triplet state. Therefore, we highlight the charge-recombination rate and the localized triplet state population can be modulated by the magnetic field in charge donor/acceptor non-covalent complexes.

Ion-Pair Dynamics upon Photoinduced Electron Transfer Monitored by Pump-Pump-Probe Spectroscopy

The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump-pump-probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe^•-). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S₁) + donor state were identified: one occurring in a few picoseconds from Pe^•-*(D₁) and one taking place within 100-200 fs from Pe^•-*(D _n>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.