ConspectusElectrons and protons are the simplest particles in chemistry, and their transfers are among the most fundamental chemical reactions. It is increasingly recognized that these two particles often transfer in the same elementary kinetic step, resulting in the most common type of proton-coupled electron transfer (PCET). PCET has evolved from a curiosity to a major research field that is central to a broad range of processes in chemistry, biology, and materials science.PCET evolved from electron transfer, in both its experimental and theoretical origins. One wonders how the field would be different if it had been called electron-coupled proton transfer. This equivalent terminology illustrates that the proton is on equal footing to the electron, making PCET perhaps the simplest case where the quantum properties of both an electron and a nucleus need to be considered.The fundamental understanding of PCET in solution builds on the remarkably impactful theory of electron transfer (ET) developed by R. A. Marcus and others. At a basic level, ET theory is marked by a quadratic dependence of the reaction barrier on the reaction free energy (ΔG⧧ on ΔG°), with normal and 'inverted' regions separated by a barrierless region (ΔG⧧ = 0), plus an electronic coupling that determines the electron tunneling probability. The theory for PCET includes additional essential elements: the quantum mechanical treatment of the transferring proton(s) as tunneling particles, multiple channels corresponding to reactant and product electron-proton vibronic states, vibronic coupling rather than electronic coupling, and a distribution of proton donor-acceptor distances.Our recent studies of ultrafast intramolecular PCET in molecular triads were the first to demonstrate the corresponding free-energy dependence for PCET, including the inverted region. Inverted behavior was previously thought to be difficult to observe experimentally for PCET because it connects vibronic states rather than electronic states. Due to the more closely spaced vibronic state energy levels compared to electronic state energy levels, there is usually a nearly barrierless pair of reactant and product vibronic states that obviates the inverted region. For these molecular triads, however, the vibronic coupling is very small for the barrierless pair, allowing observation of the hallmark inverted region.While looking for ultrafast PCET, we discovered a new elementary chemical reaction that we denoted proton-coupled energy transfer (PCEnT). In PCEnT, proton transfer (PT) is coupled to electronic excitation energy transfer. As with PCET, PT is required for the reaction to be thermodynamically accessible. In our molecular triads, PT occurs within the phenol-pyridine acceptor unit, concerted with electron transfer to a photoexcited anthracene (PCET) or electronic excitation energy transfer from a photoexcited anthracene (PCEnT). The dominant reaction depends on the molecular substituents and reaction conditions. A theory for PCEnT with some of the same essential elements as PCET theory, along with some fundamental differences, has been developed and applied to a triad system.

Photocatalysis offers a promising approach for renewable energy conversion and storage, but short lifetimes of charge-separated states in photocatalysts due to charge recombination limit its utility. Here we report an organic molecule with an acceptor–donor–acceptor configuration that can self assemble into highly crystalline nanoparticles. Transient absorption spectroscopy reveals that these crystalline assemblies can induce an ultra-long-lived charge-separated state of up to 1.2 s, attributed to initial symmetry-breaking charge separation, followed by charge hopping across closely packed molecules. These self-assembled nanoparticles have an impressive photocatalytic H2 evolution rate of 126 mmol g−1 h−1 with an external quantum efficiency of 12 Achieving charge separation with minimal energy loss remains a key challenge in photocatalysis, but traditional approaches often suffer from rapid charge recombination or inefficient energy utilization. Now it has been shown that symmetry-breaking charge separation within organic crystalline nanoparticles can generate long-lived charge-separated states, enabling efficient photocatalytic hydrogen production.

Protein aggregation into amyloid fibrils underlies numerous human diseases, yet the most widely used fluorescent probe, Thioflavin T (ThT), offers an incomplete picture of the process and fails to detect certain fibril structures. Here, we introduce and characterize the photophysical properties of DANIR-2b(2OH), a water-soluble push-pull dye that overcomes these limitations. It successfully binds early prefibrillar aggregates and small fibrils of the human Islet Amyloid Polypeptide that elude detection by ThT, which we confirm by time-resolved cryo-electron microscopy of aliquots taken during the kinetic assays. We further demonstrate that DANIR-2b(2OH) can also track the aggregation of other amyloid proteins, such as insulin and Aβ1-42. The protein-dye interaction was characterized via steady-state and time-resolved fluorescent spectroscopy. DANIR-2b(2OH) features environment-sensitive emission, high photostability, and a straightforward synthesis. Critically, it provides a substantially lower noise level in standard plate-reader assays, allowing the tracking of aggregation processes that are not visible in standard ThT measurements. This establishes DANIR-2b(2OH) as a highly sensitive and broadly applicable probe for real-time amyloid aggregation measurements and imaging.

Efficient light harvesting and subsequent energy transfer (EnT) to trivalent lanthanide ions (LnIII) are critical for achieving high-performance luminescent LnIII complexes. However, because of the complexity of the underlying dynamic processes, in-depth investigations of their sensitization mechanisms are still limited. In this work, we systematically studied three neutral LnIII complexes (GdIIIL, TbIIIL, and EuIIIL), all featuring the same carbostyril ligand, using steady-state and transient absorption and emission spectroscopy. Our study revealed efficient intersystem crossing (ISC) from the ligand-centered lowest singlet excited state (S1) to the ligand-centered lowest triplet excited state (T1) in GdIIIL, as well as dual-channel EnT from both ligand-centered S1 and T1 to the metal-centered excited state (LnIII*) in TbIIIL and EuIIIL. More importantly, the competition between ISC and EnT from the ligand-centered S1 determines the predominant sensitization pathway, resulting in the main EnT from T1 in TbIIIL, while EnT is from S1 in EuIIIL. These findings provide deeper insights into the sensitization mechanisms of LnIII complexes and offer valuable guidance for the design of new LnIII-based luminescent materials.

We have developed two new star-shaped donor-acceptor oligomers, named TxBT and TxNT, with a truxene donor core and either 2,1,3-benzothiadiazole (BT) or a naphtho[1,2-c:5,6-c ']bis[1,2,5]thiadiazole (NT) unit, respectively. Femtosecond transient absorption spectroscopy suggested that both oligomer nanoparticles (NPs) generate long-lived triplet charge-transfer (CT) states following photoexcitation, which undergo reductive quenching by ascorbate. TxNT NPs generate a larger population of reduced species that accumulate and escape recombination compared to TxBT NPs, indicating more efficient charge separation. TxNT NPs show significantly higher hydrogen evolution rate (54 mmol h-1 g-1) compared to TxBT NPs, which is comparable to the performance of the most efficient heterojunction polymer NP systems. Additionally, morphological analysis revealed that Pt deposition was significantly lower on TxBT than on TxNT NPs. These findings highlight the critical role of triplet CT states, tuning molecular energy levels, optimizing excited-state dynamics, and engineering NP architecture to increase photocatalytic hydrogen evolution of organic photocatalysts. To our knowledge, this is the first report where triplet CT states can mediate photocatalytic hydrogen evolution in donor-acceptor oligomer NPs.

Multi-electron donation remains a challenge for CO2 photocatalytic conversion to multi-electron products due to the efficient Auger recombination or annihilation at multiple excitation conditions for conventional molecules or semiconductor photocatalysts. In this paper, we demonstrated possible multi-electron donation within a quantum dot (QD)/metal complex hybrid photocatalyst system when multiple metal complexes are attached to one QD. Structural characterization first confirmed the number of [Re(4,4 '-R-bpy)(CO)3Br] catalysts (bpy = 2,2 ' bipyridine) attached per QD. The time-dependent density functional theory (TD-DFT) calculation identified that photoexcited electrons directly reside on the ligand of the metal complexes. Combining the studies from transient visible and infrared spectroscopies, we reveal that the efficient multi-electron transfer from one excited QD can be achieved when two metal complexes are anchored to one QDs with an electron injection time shorter than one ps. The transferred electrons are localized at the Re-complex while the holes are delocalized in the QD with a long lifetime. This can guarantee efficient multi-electron donation during photocatalytic reactions. Consequently, such multiple catalysts attachment facilitates the CO2 photocatalytic reduction, where unconventional methane production involving the donation of eight electrons has been significantly enhanced with an enhanced CH4 evolution rate of 130 mu mol/g/h and apparently quantum yield of 1.7 % in acetonitrile medium with triethanolamine as sacrificial electron donor. This work establishes a strategy to control CO2 reduction products via tuning the multi-electron donation pathways through molecular engineering.

The temperature dependence of concerted proton-electron transfer (CPET) reactions of two anthracene-phenol-pyridine (An-PhOH-py) triads is investigated in toluene. Light excitation forms an anthracene local excited state (1*An), which undergoes CPET to form a charge separated state (CSS, An•--PhO•-pyH+), which in turn undergoes CPET charge recombination (CR). In toluene, compared with polar solvents, the CSS is energetically destabilized. First, this makes another reaction competitive with CPET, which we propose is proton-coupled energy transfer (PCEnT) from 1*An to form the short-lived excited state keto tautomer of the phenol-pyridine subunit (*[PhO═pyH]). Second, it puts CR deep into the Marcus inverted region, and CSS lifetimes therefore reach several nanoseconds at room temperature. The slow kinetics makes CR to the anthracene triplet state (3*An) competitive, as well as another reaction that is strongly activated and dominates CSS deactivation at T ≥ 240 K for one of the triads. The latter is proposed to be CR via initial formation of the same [*PhO═PyH] state as above by an unusual electron transfer (ET) from An•- to pyH+, instead of CR with the juxtaposed PhO•. The two different pathways to form *[PhO═pyH] lead to CSS yields and lifetimes that vary significantly with temperature, and in markedly different ways between the triads. This is rationalized by the differences in the energies of the states involved. The results broaden the scope and understanding of the still rare phenomena of inverted CPET and PCEnT and may aid toward their use in solar fuels and photoredox catalysis.

Exciton migration in organic polymer dots (Pdots) is crucial for optimizing photocatalytic reactions at the particle surface, such as hydrogen evolution and carbon dioxide reduction. Despite the use of Pdots in photocatalysis, there is still a need for better understanding of exciton diffusion within these systems. This study investigates the exciton diffusion in PFBT Pdots stabilized with different weight percentages of the co-polymer surfactant PS-PEG-COOH and doped with perylene red as an internal quencher. Time-resolved fluorescence quenching data yields a quenching volume that the excitons explore during their lifetime (Vq), which is comparable to the volume of the hydrophobic core of PFBT Pdots. This indicates that excitons can migrate to the particle surface with high probability and suggests that the intrinsic exciton diffusion length (LD approximate to 19 nm) for PFBT is significantly larger than previously reported in Pdot studies from the literature (5.3 and 8.6 nm). Additionally, a larger quenching rate constant (kq) and smaller volume (Vq) is observed for the higher PS-PEG-COOH weight ratio, which are attributed to their smaller core. The study provides insights into the exciton migration within Pdots, with important implications for photocatalysis.

A practical deconstructive arylation of aliphatic alcohols has been developed using a synergistic photoredox proton-coupled electron transfer (PCET) and nickel dual catalytic system. The method efficiently generates alkyl radicals via concerted PCET-mediated β-scission, enabling the formation of C(sp3)-C(sp2) bonds between alcohols and aryl halides. Optimization studies revealed a broad functional group tolerance and high chemoselectivity, with good yields even for challenging tertiary alcohol substrates. Mechanistic insights from transient absorption spectroscopy confirmed the dominance of a PCET pathway for radical generation. This strategy expands the utility of alcohols as alkyl radical precursors in cross-coupling reactions, offering a versatile tool for constructing complex molecular architectures.

Industrial dinitrogen (N-2) reduction to ammonia in the Haber-Bosch synthesis is essential for producing fertilizers and, consequently, food. Methods wherein the energy for nitrogen activation is supplied by light could provide more sustainable alternatives to existing ones. The combination of a photosensitizer and a lanthanide catalyst is reported for an effective >2e(-) reduction of N-2 in what is the first transition-metal-free molecular photocatalyst for ammonia synthesis. The lanthanide is Earth-abundant Sm. The reaction proceeds at ambient pressure and temperature, with high turnover numbers (up to 98), with visible light irradiation in aqueous solvent mixtures and even pure water, and it uses an environmentally benign non-metallic sacrificial reductant. Nitrite and nitrate were also efficiently reduced to ammonia. Thus, the first photocatalytic co-reduction of nitrite and bicarbonate to urea using an Sm-based photocatalyst was achieved.

Manganese bipyridine tricarbonyl complexes show high efficiency and selectivity in electrochemical CO2 reduction (e-CO2RR) to CO. Efforts to shift selectivity toward HCOOH have been made by introducing second-sphere hydroxyl or amine functional groups and using amines or proton-coupled electron transfer (PCET) mediators. However, the direct spectroscopic evidence for the bifurcation pathways leading to CO and HCOOH remained elusive. Using stopped-flow mixing with decamethyl cobaltocene reductant and time-resolved infrared (TRIR) spectroscopy, we identified, for the first time, the key intermediates in this bifurcation pathway for an Mn complex with second-sphere hydroxyl groups in real time under catalytic conditions. The measured rate constants align with reported TOF values from electrochemical studies, validating the relevance of the results to e-CO2RR conditions. Our findings reveal that HCOOH production involves proton transfer from hydroxyl groups to the doubly reduced Mn center, forming the Mn-hydride intermediate, followed by CO2 insertion, leading to the Mn-formate intermediate. However, the inability of the resulting phenolate to rebind protons from weak acids like water leads to rapid catalyst degradation, limiting sustained catalysis. This work provides mechanistic insights and paves the way for designing molecular catalysts with enhanced selectivity and stability for HCOOH production during e-CO2RR.

Supramolecular surfactants provide a versatile platform to construct systems for solar fuel synthesis, for example via the self-assembly of amphiphilic photosensitizers and catalysts into diverse supramolecular structures. However, the utilization of amphiphilic photosensitizers in solar fuel production has predominantly focused on yielding gaseous products, such as molecular hydrogen (H2), carbon monoxide (CO), and methane (CH4) with turnover numbers (TONs) of synthetic catalysts typically in the range of hundreds to thousands. Inspired by biological lipid-protein interactions, we present herein a bio-hybrid assembly strategy that utilizes photosensitizers as surfactants to form micellar scaffolds that interface with enzymes, namely hydrogenases and formate dehydrogenases, for semi-artificial photosynthesis. Specifically, surfactants with a tris(2,2'-bipyridine)ruthenium(II) head group provide high photocatalytic activity upon association with the enzymes as their positively charged [Ru(bpy)3]2+ complex electrostatically interacts with the enzymes favorably to enable direct electron transfer at the micelle-enzyme interface. Time-resolved absorption and emission spectroscopy support the beneficial charge carrier dynamics of the reductively quenched [Ru(bpy)3]+ species when the enzymes are introduced in the micellar solution. Thus, a biohybrid concept is introduced for solar fuel synthesis using a biomimetic enzyme-micellar system, providing also a platform for other photocatalytic transformations using enzymes in the future.

Supramolecular surfactants provide a versatile platform to construct systems for solar fuel synthesis, for example via the self‐assembly of amphiphilic photosensitizers and catalysts into diverse supramolecular structures. However, the utilization of amphiphilic photosensitizers in solar fuel production has predominantly focused on yielding gaseous products, such as molecular hydrogen (H2), carbon monoxide (CO), and methane (CH4) with turnover number (TON) of synthetic catalysts typically in the range of hundreds to thousands. Inspired by biological lipid–protein interactions, we present herein a novel bio‐hybrid assembly strategy that utilizes photosensitizers as surfactants to form micellar scaffolds that interface with enzymes, namely hydrogenases and formate dehydrogenases, for semi‐artificial photosynthesis. Specifically, surfactants with a [ruthenium tris(2,2’‐bipyridine)]2+ head group provide high photocatalytic activity upon association with the enzymes as their positively charged [Ru]2+ center electrostatically interacts with the enzymes favorably to enable direct electron transfer at the micelle‐enzyme interface. Time‐resolved absorption and emission spectroscopy support the beneficial charge carrier dynamics of the reductively quenched [Ru]+ species when the enzymes are introduced in the micellar solution. Thus, a new concept is introduced for solar fuel synthesis using a biomimetic enzyme–micellar system, providing also a platform for other photocatalytic transformations using enzymes in the future.

A new elementary reaction, denoted proton-coupled energy transfer (PCEnT), has been recently reported in a series of donor-acceptor molecules. In this reaction, excited state energy transfer is made possible by a simultaneous transfer of a proton on the energy acceptor. This type of elementary reaction could have, by analogy to proton-coupled electron transfer, an important role in photochemistry and energy transportation of biological systems. In the previously reported case, the reaction is shown to occur intramolecularly in a covalently linked system in a 77 K glass. In this work, we identify a suitable bimolecular system for PCEnT and provide direct evidence for PCEnT in a room temperature solution using fluorescence spectroscopy. Based on these results, we discuss some simple design principles for PCEnT, including some of the current obstacles in designing a successful system.

The sunlight-driven reduction of CO2 into fuels and platform chemicals is a promising approach to enable a circular economy. However, established optimization approaches are poorly suited to multivariable multimetric photocatalytic systems because they aim to optimize one performance metric while sacrificing the others and thereby limit overall system performance. Herein, we address this multimetric challenge by defining a metric for holistic system performance that takes multiple figures of merit into account, and employ a machine learning algorithm to efficiently guide our experiments through the large parameter matrix to make holistic optimization accessible for human experimentalists. As a test platform, we employ a five-component system that self-assembles into photocatalytic micelles for CO2-to-CO reduction, which we experimentally optimized to simultaneously improve yield, quantum yield, turnover number, and frequency while maintaining high selectivity. Leveraging the data set with machine learning algorithms allows quantification of each parameter's effect on overall system performance. The buffer concentration is unexpectedly revealed as the dominating parameter for optimal photocatalytic activity, and is nearly four times more important than the catalyst concentration. The expanded use and standardization of this methodology to define and optimize holistic performance will accelerate progress in different areas of catalysis by providing unprecedented insights into performance bottlenecks, enhancing comparability, and taking results beyond comparison of subjective figures of merit.

AbstractDye-sensitized photoelectrodes consisting of photosensitizers and molecular catalysts with tunable structures and adjustable energy levels are attractive for low-cost and eco-friendly solar-assisted synthesis of energy rich products. Despite these advantages, dye-sensitized NiO photocathodes suffer from severe electron-hole recombination and facile molecule detachment, limiting photocurrent and stability in photoelectrochemical water-splitting devices. In this work, we develop an efficient and robust biohybrid dye-sensitized NiO photocathode, in which the intermolecular charge transfer is enhanced by a redox polymer. Owing to efficient assisted electron transfer from the dye to the catalyst, the biohybrid NiO photocathode showed a satisfactory photocurrent of 141±17 μA·cm−2 at neutral pH at 0 V versus reversible hydrogen electrode and a stable continuous output within 5 h. This photocathode is capable of driving overall water splitting in combination with a bismuth vanadate photoanode, showing distinguished solar-to-hydrogen efficiency among all reported water-splitting devices based on dye-sensitized photocathodes. These findings demonstrate the opportunity of building green biohybrid systems for artificial synthesis of solar fuels.

We show here that soap films-typically expected to host symmetric molecular arrangements-can be constructed with differing opposite surfaces, breaking their symmetry, and making them reminiscent of functional biological motifs found in nature. Using fluorescent molecular probes as dopants on different sides of the film, resonance energy transfer could be employed to confirm the lack of symmetry, which was found to persist on timescales of several minutes. Further, a theoretical analysis of the main transport phenomena involved yielded good agreement with the experimental observations.

A series of 2,1,3-benzothiadiazole-Au(i)-L complexes have been synthesised, structurally characterised and investigated for their photophysical properties. These are the first organometallic Au(i) complexes containing a C-Au bond on the highly electron-deficient benzothiadiazole unit. The complexes exhibit solution-phase phosphorescence at room temperature, assigned to the intrinsic triplet state of the benzothiadiazole unit that is efficently populated through its attachment to gold. Comparison with routinely reported Au(i) complexes, which include intervening alkenyl linkers, suggests that previous assignments of their phosphorescence as 1 pi -> pi*(C 00000000000000000 00000000000000000 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 00000000000000000 00000000000000000 CR) might be incomplete. Our observations affirm that, in addition to the heavy atom effect, breaking symmetry in the involved aryl motif may be of importance in controlling the luminescence properties. A series of 2,1,3-benzothiadiazole-Au(i)-L complexes have been synthesised, structurally characterised and investigated for their photophysical properties.

Direct excitation of aromatic compounds grants access to high‐energy intermediates that can be utilised in organic synthesis. Understanding and predicting the substituent effects at the excited state for aromatic molecules remains challenging for the synthetic photochemist. In this work, we present an experimental and computational investigation of the excited state of the isomeric chloroanilines, which promptly react by losing the chloride when the amino group is in para position, but are non‐reactive and non‐emissive in the meta and ortho isomers. XMS‐CASPT2//CASSCF computations explain this apparent contradiction of the meta‐ortho selectivity rule of Zimmerman, which originates from the substituent effects lowering to a different extent the barrier to populate the prefulvenic conical intersection that deactivates non‐radiatively the singlet excited state of the chloroanilines.

Proton-coupled electron transfer reactions of tyrosine are instrumental to many redox reactions in nature. By balancing the driving forces for electron and proton transfer, the mechanism can be changed from step-wise to concerted transfer (CEPT).