Influence of Donor–Acceptor Distance Variation on Photoinduced Electron and Proton Transfer in Rhenium(I)–Phenol Dyads

Theoretical Analysis of Selectivity Differences in Ketoreductases toward Aldehyde and Ketone Carbonyl Groups

Lactobacillus kefir alcohol dehydrogenase (LkADH) and ketoreductase from Chryseobacterium sp. CA49 (ChKRED12) exhibit different chemoselectivity and stereoselectivity toward a substrate with both keto and aldehyde carbonyl groups. LkADH selectively reduces the keto carbonyl group while retaining the aldehyde carbonyl group, producing optically pure R-alcohols. In contrast, ChKRED12 selectively reduces the aldehyde group and exhibits low reactivity toward ketone carbonyls. This study investigated the structural basis for these differences and the role of specific residues in the active site. Molecular dynamics (MD) simulations and quantum chemical calculations were used to investigate the interactions between the substrate and the enzymes and the essential cause of this phenomenon. The present study has revealed that LkADH and ChKRED12 exhibit significant differences in the structure of their respective active pockets, which is a crucial determinant of their distinct chemoselectivity toward the same substrate. Moreover, residues N89, N113, and E144 within LkADH as well as Q151 and D190 within ChKRED12 have been identified as key contributors to substrate stabilization within the active pocket through electrostatic interactions and van der Waals forces, followed by hydride transfer utilizing the coenzyme NADPH. Furthermore, the enantioselectivity mechanism of LkADH has been elucidated using quantum chemical methods. Overall, these findings not only provide fundamental insights into the underlying reasons for the observed differences in selectivity but also offer a detailed mechanistic understanding of the catalytic reaction.

Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in SnIV (l-Tryptophanato)2 Porphyrin Conjugates

Aromatic amino acids such as l‐tyrosine and l‐tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton‐coupled electron‐transfer, PCET), both ET‐only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a Sn^IV tetraphenylporphyrin and Sn^IV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l‐tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X‐ray analysis. The photophysical investigation of 1 and 2 in CH₂Cl₂ in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible‐light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal‐mediated analogues featuring Sn^IV porphyrin chromophores and l‐tyrosine as the redox active amino acid and well illustrate the peculiar role of l‐tryptophan with respect to PCET.

Optically Controlled Electron Transfer in a ReI Complex

Ultrafast optical control of intramolecular charge flow was demonstrated, which paves the way for photocurrent modulation and switching with a highly wavelength-selective ON/OFF ratio. The system that was explored is a fac-[Re(CO)₃ (TTF-DPPZ)Cl] complex, where TTF-DPPZ=4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine. DFT calculations and AC-Stark spectroscopy confirmed the presence of two distinct optically active charge-transfer processes, namely a metal-to-ligand charge transfer (MLCT) and an intra-ligand charge transfer (ILCT). Ultrafast transient absorption measurements showed that the ILCT state decays in the ps regime. Upon excitation to the MLCT state, only a long-lived ³ MLCT state was observed after 80 ps. Remarkably, however, the bleaching of the ILCT absorption band remained as a result of the effective inhibition of the HOMO-LUMO transition.

Dynamics of Photoinduced Energy Transfer in Fully and Partially Conjugated Polymers Bearing π-Extended Donor and Acceptor Monomers

The photophysical properties of donor (D)-acceptor (A) polymers were studied by designing two types of polymers, (D-σ-A)_n and (D-π-A)_n, with non-conjugated alkyl (sp³) and π-conjugated (sp²) linkers using π-extended donor and acceptor monomers that exhibit planar A-D-A structures. The non-conjugated alkyl linker provides structural flexibility to the (D-σ-A)_n polymers, while the π-conjugated linker retains the rigid structure of the (D-π-A)_n polymers. Photoinduced energy transfer occurs from the large donor to acceptor units in both polymers. However, the photoinduced energy transfer dynamics are found to be dependent on the conformation of the polymers, where the difference is dictated by the types of linkers between the donor and acceptor units. In solution, intramolecular energy transfer is relatively favorable for the (D-σ-A)_n polymers with flexible linkers that allow the donor and acceptor units to be proximally located in the polymers. On the other hand, intermolecular (or interchain) energy transfer is dominant in the two polymer films because the π-extended donor and acceptor units in polymers are closely packed. The structural flexibility of the linkers between the donor and acceptor repeating units in the polymers affects the efficiency of energy transfer between the donor and acceptor units and the overall photophysical properties of the polymers.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts

Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)₃(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na₂CO₃ and NaOH gave neutral fac-[Re(CO)₃(pz*H)(pypz)] and anionic Na{fac-[Re(CO)₃(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)₃(pz*H)(pypz)], and fac-[Re(CO)₃(pypz)₂Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently ³MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to Re^I → Re^II occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO₂ saturated media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_cat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)₃Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Long-Lived Charge-Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long‐lived triplet charge‐transfer (³CT) state, based on the electron spin control using spin‐orbit charge transfer intersystem crossing (SOCT‐ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT‐ISC takes 8 ns to produce the ³NI* state. Then the slow secondary CS (125 ns) gives the long‐lived ³CT state (0.94 μs in deaerated n‐hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as ¹NI*→¹CT→³NI*→³CT. With time‐resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron‐spin polarization pattern was observed for the naphthalimide‐localized triplet state. Our spiro compact dyad structure and the electron spin‐control approach is different to previous methods for which invoking transition‐metal coordination or chromophores with intrinsic ISC ability is mandatory.

Towards better understanding of photophysical properties of rhenium(I) tricarbonyl complexes with terpy-like ligands

Series of Re(I) carbonyls complexes were designed and synthesized to explore the impact of the triimine skeleton and number of methoxy groups attached to aryl substituents on their optoelectronic and thermal properties. The chemical structures of the prepared complexes were confirmed by ¹H and ¹³C NMR spectroscopy, HR-MS, elemental anlsysis, and X-ray measurements. DSC measuremtns showed that they melted in the range of 198-325 °C. Some of them form stable molecular glasses with high glass transition temperatures (158-173 °C). Experimentally obtained optical properties were supported by DFT calculations. The UV-Vis spectra display a series of overlapping absorption bands in the range 200-350 nm, and much weaker broad band in the visible spectral region, due to intraligand and charge transfer transitions, respectively. All synthesized complexes were emissive in solution and in solid state as powder. Moreover, when applied in diodes, some of them exhibited ability for emission of light under external voltage with maximum of electroluminescence band located at 591-630 nm.

Photophysical and biological investigation of phenol substituted rhenium tetrazolato complexes

The synthesis, structural and photophysical characterisation of four tricarbonyl rhenium(i) complexes bound to 1,10-phenanthroline and a tetrazolato ancillary ligand are reported. The complexes are differentiated by the nature (hydroxy or methoxy) and position (meta or para) of the substituent attached to the phenyl ring in conjugation to the tetrazole ring. The complexes exhibit phosphorescence emission from triplet charge transfer excited states, with the maxima around 600 nm, excited state lifetime decays in the 200-300 ns range, and quantum yield values of 4-6% in degassed acetonitrile solutions. The nature and position of the substituent does not significantly affect the photophysical properties, which remain unchanged even after deprotonation of the hydroxide group on the phenol ring. The interpretation of the photophysical data was further validated by resonance Raman spectroscopy and time-dependent density functional theory calculations. All the complexes are internalised within cells, albeit to variable degrees. As highlighted by a combination of flow cytometry and confocal microscopy, the species display diffuse cytoplasmic localisation except for the complex with the hydroxy functional group at the para position, which reveals lower accumulation in cells and more pronounced punctate staining. Overall, the complexes displayed low levels of cytotoxicity.

Delayed photoacidity produced through the triplet-triplet annihilation of a neutral pyranine derivative

A novel pyranine derivative, EtHPTA-OH, was synthesized via the substitution of the anionic sulfonate groups with neutral diethylsulfonamide groups. The photophysical and photochemical properties of EtHPTA-OH were studied using photoluminescence quenching and transient absorption spectroscopy. The singlet state of EtHPTA-OH was found to be highly photoacidic (pKa* = 8.74 in acetonitrile). A series of aniline and pyridine bases were used to investigate excited-state proton transfer (ESPT) from singlet EtHPTA-OH, and rate constants for singlet quenching via ESPT were determined (kq = 5.18 × 109 to 1.05 × 1010 M-1 s-1). EtHPTA-OH was also found to exhibit a long-lived triplet state which reacts through a triplet-triplet annihilation (TTA) process to reform singlet EtHPTA-OH on timescales of up to 80 μs. Detection of ESPT photoproducts on timescales comparable to that of TTA singlet regeneration provides strong evidence for photoacidic behavior stemming from the regenerated singlet EtHPTA-OH.

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

Photoinduced PCET meets catalysis, and the accumulation of multiple redox equivalents is of key importance.

Recent advances in bioinspired proton-coupled electron transfer

Fundamental aspects of PCET continue to attract attention. Understanding this reaction type is desirable for small-molecule activation and solar energy conversion.

Activationless Multiple-Site Concerted Proton-Electron Tunneling

The transfer of protons and electrons is key to energy conversion and storage, from photosynthesis to fuel cells. Increased understanding and control of these processes are needed. A new anthracene-phenol-pyridine molecular triad was designed to undergo fast photoinduced multiple-site concerted proton-electron transfer (MS-CPET), with the phenol moiety transferring an electron to the photoexcited anthracene and a proton to the pyridine. Fluorescence quenching and transient absorption experiments in solutions and glasses show rapid MS-CPET (3.2 × 1010 s-1 at 298 K). From 5.5 to 90 K, the reaction rate and kinetic isotope effect (KIE) are independent of temperature, with zero Arrhenius activation energy. From 145 to 350 K, there are only slight changes with temperature. This MS-CPET reaction thus occurs by tunneling of both the proton and electron, in different directions. Since the reaction proceeds without significant thermal activation energy, the rate constant indicates the magnitude of the electron/proton double tunneling probability.

Affinity-Enhanced Luminescent Re(I)- and Ru(II)-Based Inhibitors of the Cysteine Protease Cathepsin L

Two new Re(I)- and Ru(II)-based inhibitors were synthesized with the formulas [Re(phen)(CO)₃(1)](OTf) (7; phen = 1,10-phenanthroline, OTf = trifluoromethanesulfonate) and [Ru(bpy)₂(2)](Cl)₂ (8; bpy = 2,2'-bipyridine), where 1 and 2 are the analogues of CLIK-148, an epoxysuccinyl-based cysteine cathepsin L inhibitor (CTSL). Compounds 7 and 8 were characterized using various spectroscopic techniques and elemental analysis; 7 and 8 both show exceptionally long excited state lifetimes. Re(I)-based complex 7 inhibits CTSL in the low nanomolar range, affording a greater than 16-fold enhancement of potency relative to the free inhibitor 1 with a second-order rate constant of 211000 ± 42000 M^-1 s^-1. Irreversible ligation of 7 to papain, a model of CTSL, was analyzed with mass spectroscopy, and the major peak shown at 24283 au corresponds to that of papain-1-Re(CO)₃(phen). Compound 7 was well tolerated by DU-145 prostate cancer cells, with toxicity evident only at high concentrations. Treatment of DU-145 cells with 7 followed by imaging via confocal microscopy showed substantial intracellular fluorescence that can be blocked by the known CTSL inhibitor CLIK-148, consistent with the ability of 7 to label CTSL in living cells. Overall this study reveals that a Re(I) complex can be attached to an enzyme inhibitor and enhance potency and selectivity for a medicinally important target, while at the same time allowing new avenues for tracking and quantification due to long excited state lifetimes and non-native element composition.

Deciphering the incognito role of water in a light driven proton coupled electron transfer process

Light induced multisite electron proton transfer in two different phenol (simple and phenol carrying an intramolecularly hydrogen bonded base) pendants on acridinedione dye (ADD) and an NADH analogue was studied by following fluorescence quenching dynamics in an ultrafast timescale. In a simple phenol derivative (ADDOH), photo-excited acridinedione acquires an electron from phenol intramolecularly, coupled with the transfer of a proton to solvent water. But in a phenol carrying hydrogen bonded base (ADDDP), both electron and proton transfer occur completely intramolecularly. The sequence of this electron and proton transfer process was validated by discerning the pH dependency of the reaction kinetics. Since photo-excited ADDs are stronger oxidants, the sequential electron first proton transfer mechanism (ETPT) was observed in ADDOH and hence there is no change in the PCET reaction kinetics kETPT ∼ 6.57 × 109 s-1 in the entire pH range (pH 2-12). But the phenol carrying hydrogen bonded base (ADDDP) unleashes concerted electron proton transfer where the PCET reaction rate decreases upon decreasing the pH below its pKa. Noticeably, the concerted EPT process in ADDDP mimics the donor side of photosystem II and it occurs by two distinct pathways: (i) through direct intramolecular hydrogen bonding between the phenol and amine, kDEPT ∼ 12.5 × 1010 s-1 and (ii) through the bidirectional hydrogen bond extended by the water molecule trapped in between the proton donor and acceptor, which mediates the proton transfer and serves as a proton wire, kWMEPT ∼ 2.85 × 1010 s-1. These results unravel the incognito role played by water in mediating the proton transfer process when the structural elements do not favor direct hydrogen bonding between the proton donor and acceptor in a concerted PCET reaction.

Formation of a long-lived radical pair in a Sn(iv) porphyrin–di(l-tyrosinato) conjugate driven by proton-coupled electron-transfer

A surprisingly long-lived radical pair state is achieved in a tin-porphyrin/l-tyrosine conjugate by exploiting a photochemical PCET quenching mechanism.

Influence of Proton Acceptors on the Proton-Coupled Electron Transfer Reaction Kinetics of a Ruthenium-Tyrosine Complex

A polypyridyl ruthenium complex with fluorinated bipyridine ligands and a covalently bound tyrosine moiety was synthesized, and its photo-induced proton-coupled electron transfer (PCET) reactivity in acetonitrile was investigated with transient absorption spectroscopy. Using flash-quench methodology with methyl viologen as an oxidative quencher, a Ru³⁺ species is generated that is capable of initiating the intramolecular PCET oxidation of the tyrosine moiety. Using a series of substituted pyridine bases, the reaction kinetics were found to vary as a function of proton acceptor concentration and identity, with no significant H/D kinetic isotope effect. Through analysis of the kinetics traces and comparison to a control complex without the tyrosine moiety, PCET reactivity was found to proceed through an equilibrium electron transfer followed by proton transfer (ET-PT) pathway in which irreversible deprotonation of the tyrosine radical cation shifts the ET equilibrium, conferring a base dependence on the reaction. Comprehensive kinetics modeling allowed for deconvolution of complex kinetics and determination of rate constants for each elementary step. Across the five pyridine bases explored, spanning a range of 4.2 pK_a units, a linear free-energy relationship was found for the proton transfer rate constant with a slope of 0.32. These findings highlight the influence that proton transfer driving force exerts on PCET reaction kinetics.

Photoinduced Electron Transfer Coupled to Donor Deprotonation and Acceptor Protonation in a Molecular Triad Mimicking Photosystem II

The first artificial donor-sensitizer-acceptor compound in which photoinduced long-range electron transfer is coupled to donor deprotonation and acceptor protonation is reported. The long-lived photoproduct stores energy in the form of a radical pair state in which the charges of the donor and the acceptor remain unchanged, much in contrast to previously investigated systems that exhibit charge-separated states comprised of electron-hole pairs. This finding is relevant for light-driven accumulation of redox equivalents, because it exemplifies how the buildup of charge can be avoided yet light energy can be stored. Proton-coupled electron transfer (PCET) reactions at a phenol donor and a monoquat acceptor triggered by excitation of a Ru(II) sensitizer enable this form of photochemical energy storage. Our triad emulates photosystem II more closely than previously investigated systems, because tyrosine Z is oxidized and deprotonated, whereas plastoquinone B is reduced and protonated.

Energy versus Electron Transfer: Controlling the Excitation Transfer in Molecular Triads

The photochemistry of Ru^II coordination compounds is generally discussed to originate from the lowest lying triplet metal-to-ligand charge-transfer state (³ MLCT). However, when heteroleptic complexes are considered, for example, in the design of molecular triads for efficient photoinduced charge separation, a complex structure of ¹ MLCT states, which can be populated in a rather narrow spectral window (typically around 450 nm) is to be considered. In this contribution we show that the localization of MLCT excited states on different ligands can affect the following ps to ns decay pathways to an extent that by tuning the excitation wavelength, intermolecular energy transfer from a Ru^II -terpyridine unit to a fullerene acceptor can be favored over electron transfer within the molecular triad. These results might have important implications for the design of molecular dyads, triads, pentads and so forth with respect to a specifically targeted response of these complexes to photoexcitation.

Mono- and di-nuclear Re(i) complexes and the role of protonable nitrogen atoms in quenching emission by hydroquinone

Using the simplest type of supramolecular architecture as an easy approach to understand the quenching mechanism of rhenium(i) compounds.

Unusual distance dependences of electron transfer rates

There are regimes in which electron transfer rates increase with increasing donor–acceptor distance, leading to rate maxima at large donor–acceptor separations.

Fluorescence probes for prokaryotic and eukaryotic cells using Re(CO)3+complexes with an electron withdrawing ancillary ligand

Re(CO)₃⁺complexes with an ancillary ligand present an electron withdrawing effect suitable for cell imaging.

Conformational Change with Steric Interactions Affects the Inner Sphere Component of Concerted Proton-Electron Transfer in a Pyridyl-Appended Radical Cation System

Proton-coupled electron transfer (PCET) model systems combine one-electron oxidants and bases to generate net hydrogen atom acceptors. We have generated two persistent pyridyl-appended radical cations: 10-(pyrid-2-yl)-10H-phenothiazinium (PPT•+) and 3-(pyrid-2-yl)-10-methyl-10H-phenothiazinium (MPTP•+). EPR spectra and corresponding calculations indicate phenothiazinium radical cations with minimal spin on the pyridine nitrogen. Addition of hindered phenols causes the radical cations to decay, and protonated products and the corresponding phenoxyl radicals to form. The ΔG° values for the formation of intermediates (determined through cyclic voltammetry and pKa measurements) rule out a stepwise mechanism, and kinetic isotope effects support concerted proton–electron transfer (CPET) as the mechanism. Calculations indicate that the reaction of PPT•+ + tBu3PhOH undergoes a significant conformational change with steric interactions on the diabatic surface while maintaining the hydrogen bond; in contrast, MPTP•+ + tBu3PhOH maintains its conformation throughout the reaction. This difference is reflected in both experiment and calculations with ΔG(⧧)MPTP•+ < ΔG(⧧)PPT•+ despite ΔG°MPTP•+ > ΔG°PPT•+. Experimental results with 2,6-di-tert-butyl-4-methoxyphenol are similar. Hence, despite the structural similarity between the compounds, differences in the inner sphere component for CPET affect the kinetics.

Moving protons and electrons in biomimetic systems

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H(+)) and one electron (e(-)), which is equivalent to transfer of a hydrogen atom (H(•)). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.

New insight into photo-induced electron transfer with a simple ubiquinone-based triphenylamine model

A ubiquinone-based triphenylamine system was designed as a simple model to study the photo-induced electron transfer (PET).

Quantitatively mapping cellular viscosity with detailed organelle information via a designed PET fluorescent probe

Viscosity is a fundamental physical parameter that influences diffusion in biological processes. The distribution of intracellular viscosity is highly heterogeneous, and it is challenging to obtain a full map of cellular viscosity with detailed organelle information. In this work, we report 1 as the first fluorescent viscosity probe which is able to quantitatively map cellular viscosity with detailed organelle information based on the PET mechanism. This probe exhibited a significant ratiometric fluorescence intensity enhancement as solvent viscosity increases. The emission intensity increase was attributed to combined effects of the inhibition of PET due to restricted conformational access (favorable for FRET, but not for PET), and the decreased PET efficiency caused by viscosity-dependent twisted intramolecular charge transfer (TICT). A full map of subcellular viscosity was successfully constructed via fluorescent ratiometric detection and fluorescence lifetime imaging; it was found that lysosomal regions in a cell possess the highest viscosity, followed by mitochondrial regions.

Proton-coupled electron transfer with photoexcited metal complexes

Proton-coupled electron transfer (PCET) plays a crucial role in many enzymatic reactions and is relevant for a variety of processes including water oxidation, nitrogen fixation, and carbon dioxide reduction. Much of the research on PCET has focused on transfers between molecules in their electronic ground states, but increasingly researchers are investigating PCET between photoexcited reactants. This Account describes recent studies of excited-state PCET with d(6) metal complexes emphasizing work performed in my laboratory. Upon photoexcitation, some complexes release an electron and a proton to benzoquinone reaction partners. Others act as combined electron-proton acceptors in the presence of phenols. As a result, we can investigate photoinduced PCET involving electron and proton transfer in a given direction, a process that resembles hydrogen-atom transfer (HAT). In other studies, the photoexcited metal complexes merely serve as electron donors or electron acceptors because the proton donating and accepting sites are located on other parts of the molecular PCET ensemble. We and others have used this multisite design to explore so-called bidirectional PCET which occurs in many enzymes. A central question in all of these studies is whether concerted proton-electron transfer (CPET) can compete kinetically with sequential electron and proton transfer steps. Short laser pulses can trigger excited-state PCET, making it possible to investigate rapid reactions. Luminescence spectroscopy is a convenient tool for monitoring PCET, but unambiguous identification of reaction products can require a combination of luminescence spectroscopy and transient absorption spectroscopy. Nevertheless, in some cases, distinguishing between PCET photoproducts and reaction products formed by simple photoinduced electron transfer (ET) (reactions that don't include proton transfer) is tricky. Some of the studies presented here deal directly with this important problem. In one case study we employed a cyclometalated iridium(III) complex. Our other studies with ruthenium(II) complexes and phenols focused on systematic variations of the reaction free energies for the CPET, ET, and proton transfer (PT) steps to explore their influence on the overall PCET reaction. Still other work with rhenium(I) complexes concentrated on the question of how the electronic structure of the metal-to-ligand charge transfer (MLCT) excited states affects PCET. We used covalent rhenium(I)-phenol dyads to explore the influence of the electron donor-electron acceptor distance on bidirectional PCET. In covalent triarylamine-Ru(bpy)₃²⁺/Os(bpy)₃²⁺-anthraquinone triads (bpy = 2,2'-bipyridine), hydrogen-bond donating solvents significantly lengthened the lifetimes of photogenerated electron/hole pairs because of hydrogen-bonding to the quinone radical anion. Until now, comparatively few researchers have investigated this variation of PCET: the strengthening of H-bonds upon photoreduction.

Photoinduced electron tunneling between randomly dispersed donors and acceptors in frozen glasses and other rigid matrices

In fluid solution un-tethered donors and acceptors can diffuse freely, and consequently the donor-acceptor distance is usually not fixed on the timescale of an electron transfer event. When attempting to investigate the influence of driving-force changes or donor-acceptor distance variations on electron transfer rates this can be a problem. In rigid matrices diffusion is suppressed, and it becomes possible to investigate fixed-distance electron transfer. This method represents an attractive alternative to investigate rigid rod-like donor-bridge-acceptor molecules which have to be made in elaborate syntheses. This perspective focuses specifically on the distance dependence of photoinduced electron transfer which occurs via tunneling of charge carriers through rigid matrices over distances between 1 and 33 Å. Some key aspects of the theoretical models commonly used for analyzing kinetic data of electron tunneling through rigid matrices are recapitulated. New findings from this rather mature field of research are emphasized.