Photoinduced Electron Transfer Reactions Research Articles

Advances in exploration of photoinduced electron transfer reactions involving small molecules probed by magnetic field effect

The review focuses on photoinduced electron transfer (PET) reactions between small molecules and various kinds of chemical and biological systems using a weak external magnetic field (MF). Laser flash photolysis is a competent tool to characterize the intermediates which are formed due to PET. A weak MF, very close to the hyperfine interaction of the system, has the potential to inhibit or enhance reaction channels for singlet and triplet states, which eventually effects the product distribution. At first, well-documented examples of PET involving small molecules like derivatives of phenazines, carbazoles and acridines with classical electron donors in varying homogeneous and heterogeneous media have been discussed and the influence of a weak MF on the dynamics of PET is highlighted. Secondly, utilization of magnetic field effect (MFE) to probe PET in protein pockets has been described. Thirdly, an extensive discussion on PET involving nucleobases, nucleosides, nucleotides and nucleic acids and subsequent MFE on such reactions has been reported. Next, MFE has been exploited to study PET involving nanomaterials. Finally, some very recent studies of MFE have been discussed. Thus, this review is an attempt to unravel various aspects of PET in a large number of systems of varying dimensions by means of several facets of MFE like B1/2 parameter, its capability to authenticate the initial spin state and distance dependence property.

The use of a tertiary solvent mixture to study solvent dynamic effects in the photoinduced electron transfer reaction of excited singlet pyrene and indole

The solvent dynamic effects in the PET reactions were studied in a series of tertiary solvent mixtures PA/BN/DEP, spanning a wide range of dielectric constants (5.90 < εs < 23.4) while maintaining refractive index and viscosity constant.

Photoinduced electron transfer reaction for synthesis of tetrahydroquinoline derivatives with conjugated structure using chlorophyll b‐modified magnetic titanium dioxide photocatalyst

Photocatalytic conversion of organic compounds has recently emerged as a cost‐effective, safe, and easy‐to‐operate procedure to synthesize value‐added materials. In this study, the magnetic titanium dioxide‐based (Fe3O4/SiO2/TiO2) photocatalyst was designed to synthesize conjugated derivatives of tetrahydroquinoline through a photoinduced electron transfer (PET) reaction. Chlorophyll b was immobilized on the surface of magnetic titanium dioxide, as a natural visible‐light‐sensitive compound using 3‐aminopropyltriethoxysilane (APTES) as a coupling agent containing terminal amine (Fe3O4/SiO2/TiO2‐NH2‐Chb) to improve light harvesting ability. Fourier transform infrared (FT‐IR), powder X‐ray diffraction (PXRD), thermogravimetric analysis (TGA), energy‐dispersive X‐ray spectrometer (EDS), and vibrating sample magnetometer (VSM) results confirmed the successful synthesis of Fe3O4/SiO2/TiO2‐NH2‐Chb. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images displayed the preserved spherical morphology of Fe3O4/SiO2/TiO2‐NH2‐Chb. Chlorophyll b‐modified magnetic titanium dioxide was highly active under visible‐light irradiation toward the cyclization of (E)‐3‐[4‐(dimethylamino)phenyl)]‐1‐arylprop‐2‐en‐1‐one with 1‐aryl‐1H‐pyrrole‐2,5‐dione to achieve new tetrahydroquinoline derivatives with conjugated structure in high yields at ambient temperature, in air. The incorporation of chlorophyll b in the photocatalyst plays an important role in the photocatalytic mechanism, facilitating photoinduced electron transfer to the conduction band of TiO2. Moreover, the magnetic property enabled easy recovery of the photocatalyst and improved its reusability up to three runs. The characteristics of tetrahydroquinoline derivatives were studied by FT‐IR, CHN, 1H NMR, and 13C NMR analyses.

Photoinduced Processes in Molecular and Supramolecular Arrays Based on Porphyrins and Acridinium Units

Multichromophoric arrays based on porphyrins are of high interest for their mimicry of naturally occurring photoinduced energy and electron transfer processes leading to energy collection and conversion. [1] Moreover, the rich photophysics and electrochemistry of porphyrins make them suitable components of molecular wires, switches or chemical sensors. [2] In this perspective, it is interesting to pursue the combination of porphyrins with units that can act as multiresponsive components by modifying their chemical and electronic structure upon chemical or redox stimuli. N-acridinium ions have been recently explored as a new class of switches that respond to various orthogonal stimulations (pH or electrochemical/photochemical inputs) that trigger their reversible conversion into the non-aromatic acridane form. [3] Few examples reporting the association of acridinium units and porphyrins indicate that these systems are characterized by fast photoinduced electron transfer reactions between the porphyrin donor and the acridinium acceptor. [4,5]We recently investigated the photophysical properties of molecular and supramolecular arrays based on porphyrins and acridinium units, where the type of porphyrin (free-base or metallated) and/or the form of the switchable component (acridinium or acridane) has been varied (Figure 1). We found that the association of porphyrins and acridinium ions always leads to ultrafast electron transfer processes, and that these reactions can be altered by changing the temperature or by converting the acridinium electron acceptor into an acridane energy donor. Moreover, the combination of two Zn-porphyrins with two acridinium/acridane units results in switchable molecular receptors able to coordinate suitable hosts. The photophysics of the arrays and of the complexes, analyzed by means of steady-state and time-resolved luminescence techniques and ultrafast transient absorption spectroscopy, will be herein presented and discussed.The work is supported by the H2020-MSCA-ITN-2017-765297 project “NOAH” and by the H2020-LC-SC3-2020-RES-RIA-101006839 project “CONDOR”.[1] M. Beyler, L. Flamigni, V. Heitz, J.-P. Sauvage, B. Ventura, Photochem. Photobiol. 2014, 90, 275-286.[2] Beletskaya I, Tyurin VS, Tsivadze AY, Guilard R, Stern C. Chem. Rev. 2009, 109, 1659-1713.[3] H.-P. J. de Rouville, J. Hu, V. Heitz, ChemPlusChem, 2021, 86, 110-129.[4] A. Edo-Osagie, D. Sánchez-Resa, D. Serillon, E. Bandini, C. Gourlaouen, H.-P. Jacquot de Rouville, B. Ventura, V. Heitz, C. R. Chim. 2021, 24, 1-9.[5] H. Kotani, K. Ohkubo, M. J. Crossley, S. Fukuzumi, J. Am. Chem. Soc., 2011, 133, 11092-11095.Figure 1. Structures of the studied arrays and schemes of the occurring photoinduced processes: a) bisacridinium-Zn(II) porphyrin conjugate, b) bisacridinium-diphenylporphyrin conjugate, c) bis(acridinium/acridane)-Zn(II) porphyrin receptors. Figure 1

Photoactivable alizarin and eugenol-based materials for antibacterial applications

We report herein the design of a new visible-light epoxy-based alizarin photosensitizer (AE) of iodonium salt (Iod) for the cationic polymerization (CP) of epoxidized eugenol monomers under visible-light irradiation. The combination of AE and Iod demonstrates highly efficient photoinitiating properties under air. As highlighted by fluorescence, laser flash photolysis (LFP) and electron paramagnetic resonance spin-trapping (ESR ST) experiments, AE acts as an electron donating molecule, and reacts with Iod under light exposure through its excited singlet and triplet states via a photoinduced electron transfer reaction, producing thus protonic acids, H+, able to initiate CP. Also, we demonstrate by EPR ST that the addition of diepoxy eugenol monomer may lead to the formation of reactive radical species which may be oxidized by Iod to form cationic species. Thus, new bio-based and photoactivable materials were designed by cationic photopolymerization with the use of alizarin-based photosensitizer and eugenol derived monomers. By tuning the weight proportion of eugenol derived epoxy monomers (mono- (EE) and di-epoxidized (EdE) eugenol), we can modulate the thermal and mechanical properties of the resulting materials. The new AE-based materials have demonstrated two interesting properties: first, the presence of EE in the polymer network has considerably decreased the number of colony forming units (CFUs) at the surface of materials whatever the bacterial strains used, and these samples demonstrate photoactivable properties as they produce, under visible-light irradiation, singlet oxygen, a biocide agent against bacteria. Interestingly, the irradiation of the AE/Iod/EE(50)/EdE(50) materials lead to a tremendous decrease of the number of E. coli and S. aureus CFUs at their surface. A reduction by 100% of the bacterial adhesion is observed upon visible-light exposure.

Intramolecular Photoinduced Energy and Electron Transfer Reactions in Phenanthroimidazole-Boron Dipyromethane Donor-Acceptor Dyads.

Donor-acceptor systems in which a donor phenanthroimidazole (PhI) is directly connected to a BODIPY acceptor (Dyad1) and separated by an ethynyl bridge between PhI and BODIPY (Dyad2) have been designed, synthesized, and characterized by various spectroscopic and electrochemical techniques. Optical absorption and 1H NMR characteristics of both dyads with those of constituent individuals suggest that there exists a minimum π-π interaction between phenanthroimidazole and BODIPY. Quenched emission of both the dyads was observed when excited either at phenthaoimidazole absorption maxima or at BODIPY absorption maxima in all three investigated solvents. The detailed spectral analysis provided evidence for an intramolecular photoinduced excitation energy transfer (PEnT) from the singlet excited state of phenanthroimidazole to BODIPY and photoinduced electron transfer (PET) from the ground state of phenanthroimidazole to BODIPY. Transient absorption studies suggest that charge-separated species (PhI•+ - BODIPY•-) are generated at a rate constant of (1.16 ± 0.01) × 108 s-1 for the dyads Dyad1 and (5.15 ± 0.03) × 108 s-1 and for Dyad2 whereas energy transfer rate constants were much higher and were on the order of (1.1 ± 0.02) × 1010 s-1 and (1.6 ± 0.02) × 1010 s-1 for Dyad1 and Dyad2, respectively, signifying their usefulness in light energy harvesting applications.

Poly‐p‐phenylene terephthamide modified poly(1,3,4‐oxadiazole) fibers with superior ultraviolet resistance

AbstractPoly‐p‐phenylene‐(1,3,4‐oxadiazole) (POD) fibers have attracted tremendous attention due to the excellent thermal stability, however, their poor ultraviolet (UV) resistance seriously restricted the application. Some efforts have been devoted to improving the UV stability of POD, while the methods are generally too complicated or costly. This work applied a simple but effective approach to obtain POD/PPTA composite fibers by blending poly‐p‐phenylene terephthamide (PPTA) with POD. The results showed that the obtained POD/PPTA composite fibers exhibited enhanced tensile strength and heat resistance compared to the pure POD fibers. The POD/10%PPTA composite fibers retained 68% strength and 43% elongation even after 24 h UV accelerated aging. The improvement of UV resistance was attributed to the photoinduced electron transfer reaction in POD/PPTA composite fibers, which has been demonstrated by both experimental and simulation studies.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions.

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)4Cl(L)]+ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH+, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(μ-pz)Ru(py)4Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

Photosensitized activation of diazonium derivatives for C–B bond formation

Aryl diazonium salts are ubiquitous building blocks in chemistry, as they are useful radical precursors in organic synthesis as well as for the functionalization of solid materials. They can be reduced electrochemically or through a photo-induced electron transfer reaction. Here we provide a detailed picture of the ground and excited-state reactivity of a series of 9 rare and earth abundant photosensitizers with 13 aryl diazonium salts, which also included 3 macrocyclic calix[4]arene tetradiazonium salts. Nanosecond transient absorption spectroscopy confirmed the occurrence of excited-state electron transfer and was used to quantify cage-escape yields, i.e. the efficiency with which the formed radicals separate and escape the solvent cage. Cage-escape yields were large; increased when the driving force for photo-induced electron transfer increased and also tracked with the C-N2 + bond cleavage propensity, amongst others. A photo-induced borylation reaction was then investigated with all the photosensitizers and proceeded with yields between 9 and 74%.

Full theoretical protocol for the design of metal-free organic electron donor-spacer-acceptor systems.

A user-friendly (time-dependent) density functional theory based algorithm is proposed to design new donor-spacer-acceptor systems for electron transfer reactions. This algorithm is focused on metal-free organic compounds, most of which contain aromatic or alkene moieties. The oxidation and reduction potentials are calculated, together with the excited-state energy difference including the zero-point energy and the structural properties required to calculate an electron transfer Gibbs free energy change. The proposed algorithm has been tested on well-known systems, while two new compounds are suggested for photoinduced intramolecular electron transfer reactions using this scheme. The methodology here presented is intended to be a tool for synthetic physical-chemists, allowing them to evaluate the properties of hypothetical systems before the synthesis, enabling the study of limitless combinations of donor-spacer-acceptor arrangements.

Synthesis of 10-Phenanthrenols via Photosensitized Triplet Energy Transfer, Photoinduced Electron Transfer, and Cobalt Catalysis.

Due to the inert redox activity and high triplet energy, radical chemistry of 1,3-dicarbonyl compounds usually requires prefunctionalization substrates, external oxidant, and high-energy UV light. Here, we report a visible-light-driven photocatalyst/cobaloxime system composed of a photosensitized energy transfer reaction (PEnT) and photoinduced electron transfer reaction (PET) and with an interrupted 6π-photocyclization/dehydrogenative aromatization in one pot to synthesize 10-phenanthrenols. Preliminary mechanistic studies revealed that fac-Ir(ppy)3 plays the dual roles of energy transfer catalysis for photocycloaddition via 1,2-biradical intermediates of 1,3-dicarbonyl compounds and photoredox/cobaloxime catalysis dehydrogenative aromatization of 1,4-biradical rather than the intermediates via 6π photocyclization in the tandem reaction. In contrast to previous well-established radical chemistry of 1,3-dicarbonyl compounds, we provide a new strategy for the activation of 1,3-dicarbonyl compounds under visible light catalysis, affording a novel cyclization strategy with extremely high atom economy for the synthesis of 10-phenanthrenols.

Redfield Propagation of Photoinduced Electron Transfer Reactions in Vacuum and Solution.

Dynamical simulations of ultrafast electron transfer reactions are of utmost interest. To allow for energy dissipation directly into an external surrounding environment, a solvent coupling model has been deduced, implemented, and utilized to describe the photoinduced electron transfer dynamics within a model triad system herein. The model is based on Redfield theory, and the environment is represented by harmonic oscillators filled with bosonic quanta. To imitate real solvents, the oscillators have been equipped with frequencies and polarization lifetimes characteristic of the corresponding solvent. The population was found to transfer through the energetically lowest electron transfer route regardless of the medium. The condensed population transfer dynamics were observed to be highly dependent on the solvent parameters. In particular, an increase in the solvent coupling entailed a detainment in the population transfer from the initially prepared diabatic state and a promotion in the population transfer through the other electron transfer route. Two explanations based on the diagonal and off-diagonal matrix elements of the Kohn-Sham Fock matrix, respectively, have been provided. The lifetime of the populated partially charge-separated state was prolonged with increasing solvent polarity, and it was explained in terms of attractive interactions between the solvent's dipole moments and the fragments' charges. The high-frequency vibrational fine-structure in the correlation function was demonstrated to be important for the transfer dynamics, and the importance of dephasing effects in polar solvents was verified and precised to concern the optical polarization of the solvents.

(Invited) Ultrafast Formation of Long-Lived Charge-Separated State at High Energy Level By Two-Photon Ionization

Photoinduced electron transfer reactions play an important role in various photo-energy conversion processes. A key transient species enabling these processes is the charge-separated (CS) state produced by electron transfer, and its large formation rate constant, high energy level and long lifetime are required for its efficient utilization to the subsequent energy conversion and photoredox reactions. However, the formation rate constant is mutually correlated with the energy gap of electron transfer reactions, and it is difficult to optimize these parameters simultaneously, as shown by the Marcus theory and experimental investigations accumulated to date. In the present study, to create the CS state with the large formation rate, high energy level and long lifetime, we experimentally demonstrated an electron transfer reaction driven by two-photon ionization using a model system of pyrene (Py) as an electron donor and biphenyl (BP) as an acceptor. Photoionization enables ultrafast electron ejection from the donor to the acceptor which is 3-4 nm away from the donor. This long interionic distance effectively inhibits the geminate recombination, and the high energy level of the produced CS state is realized thanks to total energy of two photonsWe first investigated two-photon ionization of pyrene in the absence of the electron acceptor. Photoinduced dynamics of pyrene in acetonitrile solution was investigated under various excitation intensities using transient absorption spectroscopy. Upon photoexcitation at 355 nm, the absorption band at 445 nm, which is ascribable to the radical cation of pyrene, was observed with the S1 state absorption under higher excitation power, while only the S1 absorption appears at low intensities. This result shows that pyrene is ionized by the multiphoton process. Furthermore, the excitation power dependence of this ionization was simulated by a numerical simulation to quantitatively analyze the ionization quantum yield in higher excited state. The absorption signal intensities of the S1 state and radical cation were reproduced by the simulation, and the ionization yield was estimated to be 0.20.The excitation wavelength effect on the ionization was also investigated for optimization of the condition to produce the radical cation. The ionization dynamics was systematically measured by photoexcitation at 340, 355 and 370 nm. The absorption peak due to the radical cation is scarcely seen under 340 nm excitation condition. Under the 370 nm excitation, on the other hand, the quantity of the produced radical cation was maximized among the three conditions. This difference can be interpreted by distinct resonance of the excitation wavelength with the S1-S0 absorption. That is, under 370 and 355 nm excitation where the molar absorption coefficients were respectively 180 and 340 M-1cm-1, the S1 state produced by the first photon absorption can further absorbs the second photon in the excitation pulse. However, under 340 nm excitation at which the absorption coefficient is 4600 M-1cm-1, the second photon absorption by the S1 state is inhibited by the large absorption of the major remaining ground state molecules, which behaves as an intrinsic filter to hide the Sn-S1 absorption. Thus, the stepwise two-photon excitation does not effectively proceed under this condition. Taken together, the photoexcitation at the wavelength region, where the ground state absorption is not prominent, is crucial for efficient ionization driven by stepwise two-photon excitation.We finally performed the proof-of-concept experiments on two-photon ionization-driven electron transfer of the pyrene-biphenyl system. Under the intense excitation condition optimized above, pyrene was excited in a two-photon manner, which can be confirmed by the instantaneous appearance of the radical cation. Other absorption bands show up around 400 and 640 nm in a few tens of picoseconds. These bands are attributable to the radical anion of biphenyl. A detailed global analysis revealed that the anion radical is produced with a time constant of 200 fs, which is followed by vibrational cooling of the resultant ion radicals and the S1 state with a time constant of 16 ps. This observation demonstrates that an electron ejected from pyrene is directly captured by biphenyl on an ultrafast timescale. The energy level of the CS state thus formed was estimated to be higher than that of the S1 state of pyrene by 0.53 eV, which is also confirmed by non-quenching behavior of pyrene fluorescence by biphenyl. In addition, the subsequent ionic dissociation without a remarkable geminate recombination in the sub-nanosecond to nanosecond time region effectively avoids the quantity loss of the CS state. The results in the present work show that two-photon excitation method enables ultrafast formation of the long-lived CS state at a high energy beyond the traditional framework of electron transfer reactions. Figure 1

Photoswitching Behavior of Flavin-Inhibitor Complex in a Nonphotocatalytic Flavoenzyme.

An unprecedented photoswitching phenomenon of flavin-inhibitor complexes in a flavoenzyme was revealed by femtosecond transient absorption spectroscopy. The vast majority of flavoenzymes, including monomeric sarcosine oxidase (MSOX), perform non-light-driven physiological functions. Yet, the participation of flavin cofactors in photoinduced electron transfer reactions is widespread. MSOX catalyzes the oxidative demethylation of sarcosine; methylthioacetate (MTA) is a substrate analog inhibitor that forms a complex with MSOX exhibiting intense absorption bands over the whole visible range due to flavin-MTA charge transfer (CT) interactions. Here, we demonstrate that upon excitation, these CT interactions vanish during a barrierless high quantum yield reaction in ∼300 fs. The initial complex subsequently geminately re-forms in a few nanoseconds near room temperature in a thermally activated way with an activation energy of 28 kJ/mol. We attribute this hitherto undocumented process to a well-defined photoinduced isomerization of MTA in the active site, as corroborated by experiments with the heavier ligand methylselenoacetate. Photoisomerization phenomena involving CT transitions may be further explored in photocatalytic and photoswitching applications of flavoenzymes.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Deep‐Red Luminescent Molybdenum(0) Complexes with Bi‐ and Tridentate Isocyanide Chelate Ligands

AbstractIn octahedral complexes, molybdenum(0) has the same 4d6 valence electron configuration as ruthenium(II), which is beneficial for establishing energetically low‐lying metal‐to‐ligand charge transfer (MLCT) excited states. Those MLCT states often show luminescence, and they can furthermore undergo photoinduced electron and energy transfer reactions that are of interest in the context of solar energy conversion, sensing, or photocatalysis. Molybdenum is roughly 100 times more abundant than ruthenium, and it seems desirable to increase our fundamental understanding of the photophysical properties of complexes made from non‐precious metals. We report here on the luminescence behavior of two new homoleptic molybdenum(0) isocyanide complexes, one with three bidentate, the other with two tridentate chelate ligands. The key novelty is the incorporation of thiophene units into the ligand backbones, causing strongly red‐shifted photoluminescence with respect to comparable molybdenum(0) isocyanides with phenylene units in the ligand backbones. Combined experimental and computational studies provide detailed insight into the photophysical properties of this compound class. This work is relevant for the development of new luminescent compounds with possible applications in lighting and sensing, and it complements current research efforts on photoactive complexes with other abundant transition metal and main group elements.

The initial charge separation step in oxygenic photosynthesis

Photosystem II is crucial for life on Earth as it provides oxygen as a result of photoinduced electron transfer and water splitting reactions. The excited state dynamics of the photosystem II-reaction center (PSII-RC) has been a matter of vivid debate because the absorption spectra of the embedded chromophores significantly overlap and hence it is extremely difficult to distinguish transients. Here, we report the two-dimensional electronic-vibrational spectroscopic study of the PSII-RC. The simultaneous resolution along both the visible excitation and infrared detection axis is crucial in allowing for the character of the excitonic states and interplay between them to be clearly distinguished. In particular, this work demonstrates that the mixed exciton-charge transfer state, previously proposed to be responsible for the far-red light operation of photosynthesis, is characterized by the ChlD1+Phe radical pair and can be directly prepared upon photoexcitation. Further, we find that the initial electron acceptor in the PSII-RC is Phe, rather than PD1, regardless of excitation wavelength.

Elucidating the Mechanism of Bimolecular Photoinduced Electron Transfer Reactions.

The current developments in photoredox chemistry are stimulating a renewed interest for bimolecular photoinduced electron transfer reactions. Their investigation, initiated in the 1960s using conventional photochemical tools, resulted in a relatively simple reaction scheme. More recent studies, using not only spectroscopic techniques with better time resolution and extended spectral/temporal windows but also molecular dynamics simulations, reveal a more complex picture. This Perspective focuses on the results of these latest studies with neutral organic reactants, highlighting the time dependence of the quenching rate, the effect of mutual orientation of the reactants on the electronic coupling, and their consequence on the nature of the reaction product. Remaining questions, such as the occurrence of distant electron transfer in nonviscous liquids are also addressed, and possible directions toward their answer are proposed.

Coherent Two-Dimensional and Broadband Electronic Spectroscopies.

Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

Microwave Boosting of Interfacial Tunneling Electron Transfer in a Quantum Dot-Sensitized Photoelectrode

Abstract Microwaves (MWs) with a frequency of sub–gigahertz or multi–gigahertz can promote a rotation of polar molecules and a vibration of electric charges through an interaction with the alternating electric fields which comprise the MWs. The MWs can also promote the rotation of dipoles formed at the interface of Schottky junctions and semiconductor heterojunctions (e.g. p–n junction), leading us to expect that photoinduced electron transfer reaction at the interface can be perturbed by the dipole rotation induced by MWs. In this paper, we successfully demonstrated that 2.45 GHz MWs can boost the photocurrents of FTO/TiO2 and FTO/TiO2/CdS electrodes, which are typically used electrode structures in quantum dot- or dye-sensitized solar cells. From photoelectrochemical analysis using precisely controlled MW power and illuminant wavelength, a model is proposed in which the tunneling electron transfer reactions occurring in photoelectrodes are accelerated by MW-induced perturbations of the interfacial dipoles at the tunneling interfaces. The model proposed here would open the way for novel applications of MWs in the fields of photo-, electro-, and photoelectrocatalysis with the aim of solar energy conversion.