Charge Separation Kinetics Research Articles

Enhanced charge separation and oxidation kinetics by loading Pt nanoparticles with hydrogenated TiO2 nanotubes

Charge separation and oxidation kinetics are both critical for solar energy conversion in photoelectrochemical cell, but it is difficult to enhance simultaneously by a semiconductor. Here, the hydrogenated TiO2 loaded by Pt nanoparticles with improved charge separation and oxidation kinetics photoanodes are developed, characterized, and tested. The TiO2 nanotubes are prepared by hydrothermal of Ti foils and used as substrates for hydrogenation and loaded by Pt through photodeposition. Ascribe to the synergetic of hydroxylated surface and Pt nanoparticles, the photocurrent of Pt/H:TiO2 is enhanced from 0.052 (TiO2) to 0.098 mA/cm2 at 1.0 V versus RHE for water oxidation and almost increased 1.3 times for sulfite oxidation. Further, the electrons’ transport time has been reduced 20%, and the incident photon-to-electron conversion efficiency for Pt/H:TiO2 has been increased by 2.5-fold for water oxidation under 350 nm illumination at 1.0 V versus RHE. The charge separation and oxidation kinetics efficiencies of Pt/H:TiO2 are improved to 62.2 and 39.9% compared with pristine TiO2 (54.2 and 27.6%) at 1.0 V versus RHE. The origin of the enhanced photoelectrochemical performances for Pt/H:TiO2 is due to the hydroxylated surface and the absorption of plasmon resonance by Pt nanoparticles.

A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in ...

Enhanced charge separation and transfer by Bi2MoO6@Bi2Mo2O9 compound using SILAR for photoelectrochemical water oxidation

The successive ionic layer absorption and reaction (SILAR) method has been successfully used for the thin film preparation of metal oxides. In this work, we firstly fabricate the hybrid Bi2MoO6@Bi2Mo2O9 photoanode by a one-step SILAR method. The optimized Bi2MoO6@Bi2Mo2O9 demonstrates a photocurrent about 0.160 mA cm−2 at 1.23 V vs. RHE, which is 3.6 and 8.0 times enhancement compared with pristine Bi2MoO6 and Bi2Mo2O9 for water oxidation. The enhanced efficiencies of charge separation and oxidation kinetics by Bi2MoO6@Bi2Mo2O9 are 20.5% and 54.6% at 1.23 V vs. RHE, which is higher than Bi2Mo2O9 (9.2% and 15.4%) and Bi2MoO6 (10.7% and 29.2%). Moreover, the electron transport time of Bi2MoO6@Bi2Mo2O9 is 0.038 ms at 300 W m−2 through the controlled intensity modulated photocurrent spectrometer analysis, which is lower than Bi2Mo2O9 (0.087 ms) and Bi2MoO6 (0.052 ms). The enhanced photoelectrochemical activities of Bi2MoO6@Bi2Mo2O9 can be attributed to the matched energy band between Bi2Mo2O9 and Bi2MoO6, which could not only improve the charge separation, but also accelerate the charge migration. Therefore, our research indicates an effective technique for fabricating Bi2MoO6@Bi2Mo2O9 by one step process, and thus may provide a new idea for the preparation of other hybrid ternary metal oxides.

Charge separation at an organic/inorganic nano-hybrid interface: atomistic simulations of a para-sexiphenyl ZnO system.

A prototypical organic/inorganic interface is considered which is formed by vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nano-cluster of 3903 atoms. Charge separation kinetics at the interface are investigated for their dependence on ultrafast optical excitation. In order to analyze the spatio-temporal evolution of the Frenkel exciton in the organic part and the formation of charge separated states a first principles parameterized Hamiltonian is introduced and the related time-dependent Schroedinger equation is solved. By determining the interface absorption spectrum the optically addressable states can be uncovered. The work continues our previous studies of J. Phys. Chem. Lett., 2018, 9, 209, but with a changed type of surface passivation. This prevents trapping of electrons close to the surface. Charge separated states are formed by direct optical excitation and also by exciton decay at the interface. Electron migration away from the interface into bulk regions becomes possible. The hole stays close to the interface for all excitation scenarios. Finally, it is demonstrated that energetic disorder is of minor influence.

Designing high performance all-small-molecule solar cells with non-fullerene acceptors: comprehensive studies on photoexcitation dynamics and charge separation kinetics

High definition of all-small molecule materials for organic solar cells allows slow separation of localized charges.

Ultrathin Fe-NiO nanosheets as catalytic charge reservoirs for a planar Mo-doped BiVO4 photoanode.

The energy conversion efficiency of a photoelectrochemical system is intimately connected to a number of processes, including light absorption, charge excitation, separation and transfer processes. Of these processes, the charge transfer rate at the electrode|electrolyte interface is the slowest and, hence, the rate-limiting step causing charge accumulation. Such an understanding underpins efforts focused on applying highly active electrocatalysts, which may contribute to the overall performance by augmenting surface charge accumulation, prolonging charge lifetime or facilitating charge transfer. How the overall effect depends on these individual possible mechanisms has been difficult to study previously. Aiming at advancing knowledge about this important interface, we applied first-order serial reactions to elucidate the charge excitation, separation and recombination kinetics on the semiconductor|electrocatalyst interfaces in air. The study platform for the present work was prepared using a two-step Mo-doped BiVO4 film modified with an ultrathin Fe-doped NiO nanosheet, which was derived from an Fe-doped α-Ni(OH)2 nanosheet by a convenient precipitation and ion-exchange method. The simulation results of the transient surface photovoltage (TSPV) data showed that the surface charge accumulation was significantly enhanced, even at an extremely low coverage (0.12-120 ppm) using ultra-thin Fe-NiO nanosheets. Interestingly, no improvement in the charge separation rate constants or reduction of recombination rate constants was observed under our experimental conditions. Instead, the ultra-thin Fe-NiO nanosheets served as a charge storage layer to facilitate the catalytic process for enhanced performance.

Atomistic Simulations of Charge Separation at a Nanohybrid Interface: Relevance of Photoinduced Initial State Preparation.

Charge separation kinetics at a nanohybrid interface are investigated in their dependence on ultrafast optical excitation. A prototypical organic/inorganic interface is considered. It is formed by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocluster of 3783 atoms. A first principle parametrized Hamiltonian is employed, and the photoinduced subpicosecond evolution of Frenkel-excitons in the organic part is analyzed besides the formation of charge separated states across the interface. The interface absorption spectrum is calculated. Together, the data indicate that the charge separation is based on the direct excitation of the charge separated states but also on the migration of created Frenkel excitons to the interface with subsequent decay. Further, the photoinduced interface dynamics are compared with data resulting from direct set-ups of an initially excited state. Mostly such set-ups lead to substantially different charge separation processes.

Mo6+ Doped BiVO4 with improved Charge Separation and Oxidation Kinetics for Photoelectrochemical Water Splitting

Monoclinic bismuth vanadate (BiVO4) has been widely applied as a promising n-type semiconductor for photoelectrochemical (PEC) water splitting because of its low cost, environmentally friendly, and relatively narrow band gap of ∼2.4eV. Here, we developed a facile fabrication of Mo doped BiVO4 photoanode on the fluorine-doped tin oxide substrate by electrodeposition method and used these samples to better understand the doping effect for charge separation and charge oxidation kinetics. Compared with the undoped BiVO4 photoanode, the optimized Mo doped BiVO4 (3AMo:BV) produced a much higher photocurrent of 1.91mA/cm2 at 1.23V vs. RHE under AM 1.5G illumination for water oxidation. The results of the photoelectric conversion kinetics for various samples revealed that the charge separation and oxidation kinetics efficiencies for 3AMo:BV sample are 74.42% and 49.25% at 1.23V vs. RHE, while the values for undoped BiVO4 are 48.04% and 32.98%, respectively. The improved PEC performance for Mo doped BiVO4 is mainly ascribed to the crystal deformation caused by larger tetrahedral ionic VO4 and higher photovoltage generated by the interface of photoanode and electrolyte.

Microscopic dynamics of charge separation at the aqueous electrochemical interface

We have used molecular simulation and methods of importance sampling to study the thermodynamics and kinetics of ionic charge separation at a liquid water-metal interface. We have considered this process using canonical examples of two different classes of ions: a simple alkali-halide pair, Na+I-, or classical ions, and the products of water autoionization, H3O+OH-, or water ions. We find that for both ion classes, the microscopic mechanism of charge separation, including water's collective role in the process, is conserved between the bulk liquid and the electrode interface. However, the thermodynamic and kinetic details of the process differ between these two environments in a way that depends on ion type. In the case of the classical ion pairs, a higher free-energy barrier to charge separation and a smaller flux over that barrier at the interface result in a rate of dissociation that is 40 times slower relative to the bulk. For water ions, a slightly higher free-energy barrier is offset by a higher flux over the barrier from longer lived hydrogen-bonding patterns at the interface, resulting in a rate of association that is similar both at and away from the interface. We find that these differences in rates and stabilities of charge separation are due to the altered ability of water to solvate and reorganize in the vicinity of the metal interface.

Enhanced charge transfer kinetics of Fe2O3/CdS composite nanorod arrays using cobalt-phosphate as cocatalyst

Photoelectrochemical (PEC) water splitting is one of the most efficient strategies to meet the challenges of the global energy crisis. Although hematite (α-Fe2O3), is a suitable material for PEC, its performance is limited by passive surface state and slow charge transfer kinetics. Here, a successful design and fabrication of novel ternary Fe2O3/CdS/Co-Pi nanorod arrays (NRAs) photoanode is reported aimed at boosting charge separation and transfer kinetics within the bulk and at the electrode/electrolyte interface. As expected, the results reveal that α-Fe2O3 sluggish reaction kinetics is greatly ameliorated upon depositing CdS and Co-Pi, and surface charge recombination decreases even more, resulting in substantially enhanced photocurrent density. Moreover, noticeable onset potential negative shift evidenced by the I–V curve is found to result from the deposition of CdS and Co-Pi, which allows the ternary composite to generate photocurrent at much lower applied potential. We elaborate the synergistic effect of CdS and Co-Pi in achieving faster charge transfer kinetics and lower onset potential for photocurrent. These results provide new insights into the surface reaction kinetics of Co-Pi/semiconductor photoanode toward PEC application.

Bioinspired Molecular Electrets: Bringing Proteomic Approaches to Charge-Transfer Systems

The ability to control charge transfer at molecular and nanometer scales represents the ultimate level of electronic mastery, and its impacts on electronic, energy and other applications cannot be overstated. As electrostatic analogues of magnets, electrets possess ordered electric dipoles that present key paradigms for directing transduction of electrons and holes. We undertake a bioinspired approach in the design of molecular electrets based on anthranilamides.[J. Phys. Chem. Lett. 2011, 2, 503–508] Similar to protein helices, the anthranilamides possess intrinsic dipoles originating from ordered amide and hydrogen bonds, i.e., they are composed of non-native anthranilic residues.[J. Org. Chem. 2013, 78, 1994-2004; Pure Appl. Chem. 2015, 87, 779-792] Unlike proteins, however, the bioinspired molecular electrets have extended pi-conjugation along their backbones providing a means for efficient charge transfer. The anthranilic molecular electrets are polypeptides of non-native amino acids. This feature offers unexplored routes for bringing principles of proteomics into the de novo designs of electronic molecular systems and materials. In analogy, less than a couple of dozen transcribable native alpha-amino acids, each with a different single side chain, are the building blocks of proteins with countless structural and functional characteristics yielding the amazing diversity of life on our planet. Therefore, a similar set of Aa residues with different electronic properties should prove most essential for the design of molecular electrets with a wide range of charge-transfer properties. Unlike the native amino acids, each of the anthranic residues has two side chains that we selectively alter. Electrochemical and spectroelectrochemical studies revealed that the electronic properties of an anthranilic residue depend not only on the type of a substituent, but also on its exact position [J. Phys. Chem. Lett. 2016, 7, 758-764]. This regio-dependence of properties offers a larger diversity in anthranilamide residues than what native-type amino acids can offer, demonstrating once again the clear advantage of bioinspired over biomimetic or biomediated approaches. The bioinspired molecular electrets, indeed, rectify the kinetics of charge separation, i.e., they act as molecular diodes.[J. Am. Chem. Soc. 2014, 136, 12966-12973] The rates of electron transfer along the electret dipoles are faster than the rates against the dipoles. For processes with small driving forces, the modulation of the energies of the charge-transfer states accounts for the observed rectification trends. Conversely, for processes with a large graving force, such as charge-recombination, it is the asymmetry in the electronic coupling between the electret residues and the axillary photosensitizer. These effects become more pronounced as the length of the anthranilic oligomers increases. Another feature that is affected by the electret length is the kinetic isotope effect on charge-transfer kinetics. Replacing all amide protons with a deuterium significantly slows down charge separation and charge recombination. While the amide protons are not on the charge-transfer pathways, most likely, their modes of vibration couple with charge-transfer transition-state modes affecting its kinetics. Currently, we have developed more than 15 non-native anthranilic residues. Controlling the sequence of these residues in the molecular electrets allows for introducing barriers, wells, and rectifying junctions on the charge-transfer pathways. These findings and developments provide unexplored paradigms for energy science and organic electronics.

Free-Energy-Gap Law for Ultrafast Charge Recombination of Ion Pairs Formed by Intramolecular Photoinduced Electron Transfer.

In this article, regularities of ultrafast charge recombination (CR) kinetics in photoinduced intramolecular electron transfer in polar solvents are studied. The kinetics of charge separation and ensuing ultrafast CR are simulated within the framework of the multichannel stochastic model. This model accounts for the reorganization of both the solvent and a number of intramolecular high-frequency vibrational modes. The solvent relaxation is described in terms of two relaxation modes. For ultrafast CR, the free-energy-gap law strongly depends on the parameters: the electronic coupling, reorganization energy of intramolecular high-frequency vibrational modes, and the vibrational and solvent relaxation times. The semilog dependence of the CR rate constant on the free-energy gap varies from a parabolic shape to a nearly linear one with increasing the electronic coupling and decreasing the vibrational relaxation time. The dynamic solvent effect in CR is predicted to be large in the area of strong exergonicity and small in the area of weak exergonicity. This regularity is opposite to that observed for the thermal reactions.

Enhanced charge separation and oxidation kinetics of BiVO4 photoanode by double layer structure

Abstract Monoclinic bismuth vanadate (BiVO4) is a promising semiconductor for photoelectrochemical water splitting. Here, we developed a facile fabrication of BiVO4 double layer photoanode on the fluorine-doped tin oxide substrate by electrodeposition. The BiVO4 double layer photoanode is composed by a dense BiVO4 film as the inner layer and a nanoporous BiVO4 film as the outer layer. Compared to the BiVO4 single layer photoanode, the optimized BiVO4 double layer photoanode produced a much higher photocurrent of 1.15 mA/cm2 at 0.6 V vs. Ag/AgCl under AM 1.5G (100 mW/cm2) illumination. The results of the photoelectric conversion kinetics for different samples revealed that the charge separation and oxidation kinetics efficiencies for the BiVO4 double layer are 47.2% and 51.6% at 0.6 V vs. Ag/AgCl, while the values for BiVO4 single layer are 32.3% and 35.8%, respectively. The improved photoelectrochemical performance for BiVO4 double layer is mainly ascribed to the decrease of defect state at the interface after inserting a dense BiVO4 as an inner layer to prevent the recombination of photogenerated electron-hole pairs.

Photo-assisted synthesis of zinc-iron layered double hydroxides/TiO2 nanoarrays toward highly-efficient photoelectrochemical water splitting

A highly-matched semiconductor/cocatalyst is crucial to enhance the bulk charge separation and surface reaction kinetics of the photoelectrode in the solar water splitting system. In this work, well-aligned, hierarchical zinc-iron layered double hydroxide (LDH) is in situ synthesized on the surface of TiO2 by a facile and effective photo-assisted electrodeposition (PED) method. An experimental-computational combination study reveals that the photogenerated holes of TiO2 tend to travel to ZnFe-LDH which enhances the bulk charge separation; while ZnFe-LDH acts as a cocatalyst which accelerates the surface water oxidation reaction. The resulting TiO2/ZnFe-LDH-PE photoanode exhibits a largely enhanced PEC performance: the photocurrent density at 1.0V vs. RHE is 2.29 and 1.31 times higher than that of the pristine TiO2 and TiO2/ZnFe-LDH-E (prepared by a conventional electrosynthesis method) photoanode, with 150mV and 50mV of negative shift for onset potential. This can be ascribed to the enhanced interface interaction and highly-matched band structure between ZnFe-LDH and TiO2. It is expected that this strategy can be extended to other heterostructures for advanced performance in the fields of energy conversion and storage.

Photophysical Study of DPPTT‐T/PC70BM Blends and Solar Devices as a Function of Fullerene Loading: An Insight into EQE Limitations of DPP‐Based Polymers

Diketopyrrolopyrrole (DPP)‐based polymers have been consistently used for the fabrication of solar cell devices and transistors due to the existence of intermolecular short contacts, resulting in high electron and hole mobilities. However, they also often show limited external quantum efficiencies (EQEs). In this contribution, the authors analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation, and recombination kinetics in thin films and solar devices of a DPP‐based donor polymer, DPPTT‐T (thieno[3,2‐b]thiophene‐diketopyrrolopyrrole copolymer) blended with varying weight fractions of the fullerene acceptor PC70BM. From the correlations between photoluminescence quenching, transient absorption studies, and EQE measurements, it is concluded that the main limitation of photon‐to‐charge conversion in DPPTT‐T/PC70BM devices is poor exciton dissociation. This exciton quenching limit is related not only to the low affinity/miscibility of the materials, as confirmed by wide angle X‐ray diffraction diffraction and transmission electron microscopy data, but also to the relatively short DPPTT‐T singlet exciton lifetime, possibly associated with high nonradiative losses. A further strategy to improve EQE in this class of polymers without sacrificing the good extraction properties in optimized blends is therefore to limit those nonradiative decay processes.

Programming Interfacial Energetic Offsets and Charge Transfer in β-Pb0.33V2O5/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States

Semiconductor heterostructures for solar energy conversion interface light-harvesting semiconductor nanoparticles with wide-band-gap semiconductors that serve as charge acceptors. In such heterostructures, the kinetics of charge separation depend on the thermodynamic driving force, which is dictated by energetic offsets across the interface. A recently developed promising platform interfaces semiconductor quantum dots (QDs) with ternary vanadium oxides that have characteristic midgap states situated between the valence and conduction bands. In this work, we have prepared CdS/β-Pb0.33V2O5 heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/β-Pb0.33V2O5 heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/β-Pb0.33V2O5 heterostructures relative to CdSe/β-Pb0.33V2O5 heterostructures suggest a positive shift in the VB edge potential and, therefore, an increased driving force...

Singlet Oxygen Generation and Photoinduced Charge Separation of Tetra Polyethyleneglycol Functionalized Zinc Phthalocyanine‐Fullerene Dyad

AbstractUtilization of polyethyleneglycol (PEG) functionalized zinc phthalocyanine, (PEG)4ZnPc in (i) singlet oxygen generation, and (ii) in building energy harvesting donor‐acceptor systems using fullerene, C60 as an acceptor via the well‐known metal‐ligand axial coordination approach is reported. The (PEG)4ZnPc was found to be capable of producing singlet oxygen with a quantum yield, ΦΔ of 0.77 in toluene, a value higher than that obtained for pristine (t‐bu)4ZnPc (ΦΔ=0.54) carrying no PEG groups, revealing its usefulness in photodynamic therapy applications. Spectroscopic studies revealed efficient binding of phenylimidazole functionalized fullerene, C60Im with 1:1 stoichiometry to (PEG)4ZnPc. Binding constant K for the formation of (PEG)4ZnPc:ImC60 dyad was found to be 6×103 M−1 revealing moderate stability. Geometric and electronic studies of the dyad was arrived by B3LYP/3‐21G(∗︁) method. The HOMO level was found to be on zinc phthalocyanine entity while the LUMO level was found to be on the C60 entity suggesting formation of (PEG)4ZnPc·+:ImC60· − charge separated state during the process of electron transfer reaction. Redox studies on the (PEG)4ZnPc:ImC60 dyad enabled accurate determination of the oxidation and reduction potentials of the donor‐acceptor system, and to evaluate free‐energy changes associated for the charge separation process. Kinetics of photoinduced charge separation and recombination in the (PEG)4ZnPc:ImC60 dyad was investigated using femtosecond transient absorption studies. Relatively long‐lived charge separated states were confirmed for the dyad suggesting their potential usefulness in energy harvesting applications.

(Invited) Si/NiCoSex Nanorods Photocathode for Enhanced Photoelectrochemical Hydrogen Production

Hydrogen is considered as the ideally sustainable alternative to fossil fuels. Photoelectrochemical (PEC) water splitting is attractive as it can store solar energy in the form of hydrogen. Silicon as an earth-abundant and efficient visible light absorption material has been widely used in PEC for hydrogen generation. However, achieving a high photocurrent density still faces a challenge when utilizing silicon as a photocathode owing to its sluggish hydrogen evolution reaction (HER) kinetics and low light harvesting capability. Here, we design a novel one-dimensional photocathodes - Si/NiCoSex nanorods using a facile photo-assistant electrodeposition method and demonstrate their enhanced PEC hydrogen production performance with high photocurrent density. Si nanorods were covered with NiCoSex electrocatalysts, where electrons efficiently diffuse across the electrode/electrolyte interface. The synergy effects of the high light harvesting of Si nanorod morphology and NiCoSex electrocatalyst enhanced the production and utilization of photogenerated electrons. The high quality junctions between one-dimensional semiconductor and electrocatalyst for charge collection and utilization is significant to enhance the efficiency of water splitting for hydrogen generation. The improved charge separation and hydrogen evolution reaction kinetics on the electrode-electrolyte interface were confirmed. Furthermore, we expect that inducting the buried junction of Si or optimizing the electrode/electrolyte interface might further boost the PEC-HER performance of the Si/NiCoSex nanorods, making it a more competitive candidate for PEC-HER photocathode. The reported new Si/NiCoSex nanorods photocathode offers a new strategy for preparing efficient solar energy conversion devices.

Effect of Spacer Connecting the Secondary Electron Donor Phenothiazine in Subphthalocyanine-Fullerene Conjugates in Promoting Electron Transfer Followed by Hole Shift Process.

Sequential electron/hole transfer between energetically well-positioned entities of photosynthetic reaction center models is one of the commonly employed mechanisms to generate long-lived charge-separated states. A wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies. In the present study, we report on the effect of spacer (direct or via phenoxy linkage) connecting the hole shifting agent, phenothiazine (PTZ), on photoinduced charge stabilization in subphthalocyanine-fullerene donor-acceptor conjugates. In these conjugates, the subphthalocyanine (SubPc) and fullerene (C60 ) served as primary electron donor and acceptor, respectively, while the phenothiazine entities act as hole shifting agents. The newly synthesized compounds were characterized by optical absorption and emission, computational, and electrochemical methods. The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes. Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents. In the conjugate where the phenothiazine entities are directly linked to SubPc, evidence for sequential electron transfer followed by hole shift leading to long-lived charge separated state was weak, primarily due to the delocalization of HOMO on both SubPc and PTZ entities. However, in case of the conjugate where the PTZ and SubPc are linked via phenoxy spacers, sequential electron transfer/hole shift was observed leading to the formation of long-lived charge-separated states. The present study brings out the importance of the spacer group connecting the hole shifting agent in model donor-acceptor conjugates to generate long-lived charge-separated states.

The appearance of reorganization of intramolecular low-frequency modes in the kinetics of electron transfer from the second excited state in zinc-porphyrin derivatives

An analysis of theoretical modeling results of ultrafast kinetics of photoinduced intramolecular charge separation from the second excited singlet state in the dyad Zn-tetraphenylporphyrin-aminonaphthalenediimide (Zn-TPP-ANDI) in a solution of toluene is presented. The calculations are performed within the framework of the stochastic multi-channel model, which includes four electron states (the ground, first and second excited singlet states, the state with charge separation), as well as their vibration sublevels corresponding to the excitation of highfrequency intramolecular vibration modes. A bimodal kinetic curve of population of the state with charge separation observed in experiments is quantitatively reproduced. The absolute yield values of the state with charge separation are determined. The results of the modeling show that intramolecular modes make a significant contribution to the reorganization of low-frequency modes. Quantum chemical calculations were performed, determining the degrees of freedom related to the intramolecular slow motion of nuclei of high amplitude in the dyad Zn-TPPANDI on going from the ground state to the state with charge separation.