Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron transfer (DET) activation of alkyl halide initiators following absorption of light. Characterizing DET rate coefficients (<i>k_act</i>) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs where synthetic handles of core-substitution and <i>N</i>-aryl substitution enable tunability of the electronic and spin character of the catalyst excited state as well as DET reaction driving force ( ). Using Stern-Volmer quenching experiments through variation of diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of <i>k_act</i>are reported as a function of for four PCs: four triplet reactants and four singlets with <i>k_act</i> values ranging from 1.1´10⁸ M^-1s^-1 where DET itself controls the rate to 4.8´10⁹ M^-1s^-1 where diffusion is rate limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of adiabatic Marcus electron transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where <i>k_act</i> can be estimated if is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on driving force. Next, on the basis of measured <i>k_act</i> values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In <i>N</i>-naphthyl-containing PCs characterized by near-unity intersystem crossing, the T₁ is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S₁ is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in <i>N</i>-phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S₁ under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S₁ and T₁ reactant states produce the PC^·+ radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The sub-unity PC^·+ yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron transfer (DET) activation of alkyl halide initiators following absorption of light. Characterizing DET rate coefficients (<i>k_act</i>) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs where synthetic handles of core-substitution and <i>N</i>-aryl substitution enable tunability of the electronic and spin character of the catalyst excited state as well as DET reaction driving force ( ). Using Stern-Volmer quenching experiments through variation of diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of <i>k_act</i>are reported as a function of for four PCs: four triplet reactants and four singlets with <i>k_act</i> values ranging from 1.1´10⁸ M^-1s^-1 where DET itself controls the rate to 4.8´10⁹ M^-1s^-1 where diffusion is rate limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of adiabatic Marcus electron transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where <i>k_act</i> can be estimated if is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on driving force. Next, on the basis of measured <i>k_act</i> values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In <i>N</i>-naphthyl-containing PCs characterized by near-unity intersystem crossing, the T₁ is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S₁ is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in <i>N</i>-phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S₁ under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S₁ and T₁ reactant states produce the PC^·+ radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The sub-unity PC^·+ yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

Structure and chemistry of the solid electrolyte interphase (SEI) on a high capacity conversion-based anode: NiO

Electroanalytical techniques are specialized tools with high-sensitivity that when combined with electron transfer theory can shed light on the mechanisms of highly complex, heterogeneous, multi-step reactions – including SEI formation on LiB anodes.

Stacking Effects on Charge Transfer Dynamics in Fluctuating DNA

Base-stacked structure is an important feature of DNA molecules. But dynamics study on the influences of the stacking effects on charge transfer in DNA is yet rare.In this article, a general rule about the relationship of onsite energies of same bases in a stack is derived by H ̈uckel theory. It is found that the base in the middle position of the stack has lower onsite energy than the bases at the terminals due to squeezing effect, which is different from previous studied neighboring base effect. The former is along-range effect while the latter acts in a short range. Semiempirical MNDO calculations on (A:T) n (n=1∼10) systems verfied the H ̈uckel analysis. From this perspective,the so-called incoherent hopping mechanism is actually somewhat coherent due to the squeezing effect. To understand these stacking effects on charge transfer in DNA,a cross-scale method which combines classical MD simulations, quantum mechanism calculations, Marcus electron transfer theory and kinetic Monte Carlo simulations is developed and applied on hole dynamics in (A:T) n (G:C) (n=1∼10) systems. Although no superexchange mechanism is explicitly involved in the studied systems, a crossover from strong to weak distance-dependency of hole arrival rates, which is an experimentally observed property of hole dynamics in DNA and is thought an evidence of the conversion from superexchange to hopping mechanism, also appears. We attribute it to the stacking effects. Such a result provides a new idea on explaining the crossover of different distance-dependencies of charge transfer rates in DNA. In addition, the squeezing effect may be a new driving force for long-range charge transfer. At the same time, some technical methods developed in the dynamics, e.g. calculations of onsite energies and electronic couplings in a stack, and simulated hole dynamics, etc.,can be generalized to other complex molecular systems with charge transfer behaviors.<br><br>

Stacking Effects on Charge Transfer Dynamics in Fluctuating DNA

Base-stacked structure is an important feature of DNA molecules. But dynamics study on the influences of the stacking effects on charge transfer in DNA is yet rare.In this article, a general rule about the relationship of onsite energies of same bases in a stack is derived by H ̈uckel theory. It is found that the base in the middle position of the stack has lower onsite energy than the bases at the terminals due to squeezing effect, which is different from previous studied neighboring base effect. The former is along-range effect while the latter acts in a short range. Semiempirical MNDO calculations on (A:T) n (n=1∼10) systems verfied the H ̈uckel analysis. From this perspective,the so-called incoherent hopping mechanism is actually somewhat coherent due to the squeezing effect. To understand these stacking effects on charge transfer in DNA,a cross-scale method which combines classical MD simulations, quantum mechanism calculations, Marcus electron transfer theory and kinetic Monte Carlo simulations is developed and applied on hole dynamics in (A:T) n (G:C) (n=1∼10) systems. Although no superexchange mechanism is explicitly involved in the studied systems, a crossover from strong to weak distance-dependency of hole arrival rates, which is an experimentally observed property of hole dynamics in DNA and is thought an evidence of the conversion from superexchange to hopping mechanism, also appears. We attribute it to the stacking effects. Such a result provides a new idea on explaining the crossover of different distance-dependencies of charge transfer rates in DNA. In addition, the squeezing effect may be a new driving force for long-range charge transfer. At the same time, some technical methods developed in the dynamics, e.g. calculations of onsite energies and electronic couplings in a stack, and simulated hole dynamics, etc.,can be generalized to other complex molecular systems with charge transfer behaviors.<br><br>

Consecutive Marcus Electron and Proton Transfer in Heme Peroxidase Compound II-Catalysed Oxidation Revealed by Arrhenius Plots

Electron and proton transfer reactions in enzymes are enigmatic and have attracted a great deal of theoretical, experimental, and practical attention. The oxidoreductases provide model systems for testing theoretical predictions, applying experimental techniques to gain insight into catalytic mechanisms, and creating industrially important bio(electro)conversion processes. Most previous and ongoing research on enzymatic electron transfer has exploited a theoretically and practically sound but limited approach that uses a series of structurally similar (“homologous”) substrates, measures reaction rate constants and Gibbs free energies of reactions, and analyses trends predicted by electron transfer theory. This approach, proposed half a century ago, is based on a hitherto unproved hypothesis that pre-exponential factors of rate constants are similar for homologous substrates. Here, we propose a novel approach to investigating electron and proton transfer catalysed by oxidoreductases. We demonstrate the validity of this new approach for elucidating the kinetics of oxidation of “non-homologous” substrates catalysed by compound II of Coprinopsis cinerea and Armoracia rusticana peroxidases. This study – using the Marcus theory – demonstrates that reactions are not only limited by electron transfer, but a proton is transferred after the electron transfer event and thus both events control the reaction rate of peroxidase-catalysed oxidation of substrates.

Theoretical study of charge-transport and optical properties of organic crystals: 4,5,9,10-pyrenediimides

This work presents a systematic study of the conducting and optical properties of a family of aromatic diimides reported recently and discusses the influences of side-chain substitution on the reorganization energies, crystal packing, electronic couplings and charge injection barrier of 4,5,9,10-pyrenediimide (PyDI). Quantum-chemical calculations combined with the Marcus–Hush electron transfer theory revealed that the introduction of a side chain into 4,5,9,10-pyrenediimide increases intermolecular steric interactions and hinders close intermolecular π–π stacking, which results in weak electronic couplings and finally causes lower intrinsic hole and electron mobility in t-C5-PyDI (μh = 0.004 cm2 V−1 s−1 and μe = 0.00003 cm2 V−1 s−1) than in the C5-PyDI crystal (μh = 0.16 cm2 V−1 s−1 and μe = 0.08 cm2 V−1 s−1). Furthermore, electronic spectra of C5-PyDI were simulated and time-dependent density functional theory calculation results showed that the predicted fluorescence maximum of t-C5-PyDI, corresponding to an S 1→S 0 transition process, is located at 485 nm, which is close to the experimental value (480 nm).

Theoretical study of charge-transport and optical properties of indeno[1,2-b]fluorene-6,12-dione-based semiconducting materials

The conducting and optical properties of a series of indeno[1,2-b]fluorene-6,12-dione (IFD)-based molecules have been systematically studied and the influences of butyl, butylthio and dibutylamino substituents on the reorganization energies, intermolecular electronic couplings and charge-injection barriers of IFD have been discussed. The quantum-chemical calculations combined with electron-transfer theory reveal that the incorporation of sulfur-linked side chains decreases reorganization energy associated with hole transfer and optimizes intermolecular π–π stacking, which results in excellent ambipolar charge-transport properties (μh = 1.15 cm2 V−1 s−1 and μe = 0.08 cm2 V−1 s−1); in comparison, addition of dibutylamino side chains increases intermolecular steric interactions and hinders perfect intermolecular π–π stacking, which results in the weak electronic couplings and finally causes the low intrinsic hole mobility (μh = 0.01 cm2 V−1 s−1). Furthermore, electronic spectra of butyl-IFD, butylthio-IFD and dibutylamino-IFD were simulated and compared with the reported experimental data. Calculations demonstrate that IFD-based molecules possess potential for developing novel infrared and near-infrared probe materials via suitable chemical modifications.