Excited State Dynamics in Unidirectional Photochemical Molecular Motors.

Unidirectional photochemically driven molecular motors (PMMs) convert the energy of absorbed light into continuous rotational motion. As such they are key components in the design of molecular machines. The prototypical and most widely employed class of PMMs is the overcrowded alkenes, where rotational motion is driven by successive photoisomerization and thermal helix inversion steps. The efficiency of such PMMs depends upon the speed of rotation, determined by the rate of ground state thermal helix inversion, and the quantum yield of photoisomerization, which is dependent on the excited state energy landscape. The former has been optimized by synthetic modification across three generations of overcrowded alkene PMMs. These improvements have often been at the expense of photoisomerization yield, where there remains room for improvement. In this perspective we review the application of ultrafast spectroscopy to characterize the excited state dynamics in PMMs. These measurements lead to a general mechanism for all generations of PMMs, involving subpicosecond decay of a Franck-Condon excited state to populate a dark excited state which decays within picoseconds via conical intersections with the electronic ground state. The model is discussed in the context of excited state dynamics calculations. Studies of PMM photochemical dynamics as a function of solvent suggest exploitation of intramolecular charge transfer and solvent polarity as a route to controlling photoisomerization yield. A test of these ideas for a first generation motor reveals a high degree of solvent control over isomerization yield. These results suggest a pathway to fine control over the performance of future PMMs.

Control of Photoconversion Yield in Unidirectional Photomolecular Motors by Push-Pull Substituents.

Molecular motors based on the overcrowded alkene motif convert light energy into unidirectional mechanical motion through an excited state isomerization reaction. The realization of experimental control over conversion efficiency in these molecular motors is an important goal. Here, we combine the synthesis of a novel "push-pull" overcrowded alkene motor with photophysical characterization by steady state and ultrafast time-resolved electronic spectroscopy. We show that tuning of the charge transfer character in the excited state has a dramatic effect on the photoisomerization yield, enhancing it to near unity in nonpolar solvents while largely suppressing it in polar solvents. This behavior is explained through reference to solvent- and substituent-dependent potential energy surfaces and their effect on conical intersections to the ground state. These observations offer new routes to the fine control of motor efficiency and introduce additional degrees of freedom in the synthesis and exploitation of light-driven molecular motors.

Selective Analysis of Redox Processes at the Electrode Interface with Time-Resolved Raman Spectroscopy.

Electrochemistry and electrochemical reactions are increasingly important in the transition to a sustainable chemical industry. The electron transfer that drives such reactions takes place within nanometers of the electrode surface, and follow-up chemical reactions take place within the diffusion layer. Hence, understanding electrochemical reactions requires time-, potential-, and spatially resolved analysis. The confocal nature of Raman spectroscopy provides high spatial resolution, in addition to detailed information on molecular structure. The intrinsic weakness of nonresonant Raman scattering, however, is not sensitive enough for relatively minor changes to the solution resulting from reactions at the electrode interface. Indeed, the limit of detection is typically well above the concentrations used in electrochemical studies. Here, we show that surface-enhanced Raman scattering (SERS) and resonance Raman (rR) spectroscopy allow for spatially and time-resolved analysis of solution composition at (<1-2 nm) and near (within 5 µm) the electrode surface, respectively, in a selective manner for species present at low (<1 mM) concentrations. We show changes in concentration of species at the electrode surface, without the need for labels, specific adsorption, or resonance enhancement, using a SERS-active gold electrode prepared readily by electrochemical surface roughening. A combination of smooth and roughened gold electrodes is used to distinguish between surface and resonance enhancement using the well-known redox couples ferrocene and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). We discuss the impact of specific adsorption on the spectral analysis with the ruthenium(II) polypyridyl complex, [Ru(bpy)3]2+. The dual function of the electrode (surface enhancement and electron transfer) in the analysis of solution processes is demonstrated with the reversible oxidation of TMA (4,N,N-trimethylaniline), where transient soluble species are identified in real time, with rapid spectral acquisition, making use of localized enhancement. We anticipate that this approach will find use in elucidating electro(catalytic) reactions at electrode interfaces.

Stereodivergent Chirality Transfer by Noncovalent Control of Disulfide Bonds.

Controlling dynamic stereochemistry is an important challenge, as it is not only inherent to protein structure and function but often governs supramolecular systems and self-assembly. Typically, disulfide bonds exhibit stereodivergent behavior in proteins; however, how chiral information is transmitted to disulfide bonds remains unclear. Here, we report that hydrogen bonds are essential in the control of disulfide chirality and enable stereodivergent chirality transfer. The formation of S-S···H-N hydrogen bonds in solution can drive conformational adaption to allow intramolecular chirality transfer, while the formation of C=O···H-N hydrogen bonds results in supramolecular chirality transfer to form antiparallel helically self-assembled solid-state architectures. The dependence on the structural information encoded in the homochiral amino acid building blocks reveals the remarkable dynamic stereochemical space accessible through noncovalent chirality transmission.

Taming Tris(bipyridine)ruthenium(II) and Its Reactions in Water by Capture/Release with Shape-Switchable Symmetry-Matched Cyclophanes.

Electron/proton transfers in water proceeding from ground/excited states are the elementary reactions of chemistry. These reactions of an iconic class of molecules─polypyridineRu(II)─are now controlled by capturing or releasing three of them with hosts that are shape-switchable. Reversible erection or collapse of the host walls allows such switchability. Some reaction rates are suppressed by factors of up to 120 by inclusive binding of the metal complexes. This puts nanometric coordination chemistry in a box that can be open or shut as necessary. Such second-sphere complexation can allow considerable control to be exerted on photocatalysis, electrocatalysis, and luminescent sensing involving polypyridineRu(II) compounds. The capturing states of hosts are symmetry-matched to guests for selective binding and display submicromolar affinities. A perching complex, which is an intermediate state between capturing and releasing states, is also demonstrated.

Correction: The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome.

Correction for 'The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome' by Luuk Kortekaas et al., Chem. Soc. Rev., 2019, 48, 3406-3424, DOI: 10.1039/C9CS00203K.

Three-State Switching of an Anthracene Extended Bis-thiaxanthylidene with a Highly Stable Diradical State.

A multistable molecular switching system based on an anthracene-extended bis-thiaxanthylidene with three individually addressable states that can be interconverted by electrochemical, thermal, and photochemical reactions is reported. Besides reversible switching between an open-shell diradical- and a closed-shell electronic configuration, our findings include a third dicationic state and control by multiple actuators. This dicationic state with an orthogonal conformation can be switched electrochemically with the neutral open-shell triplet state with orthogonal conformation, which was characterized by EPR. The remarkably stable diradical shows kinetic stability as a result of a significant activation barrier for isomerization to a more stable neutral closed-shell folded geometry. We ascribe this activation barrier of ΔG⧧(293 K) = 25.7 kcal mol-1 to steric hindrance in the fjord region of the overcrowded alkene structure. The folded closed-shell state can be converted back to the diradical state by irradiation with 385 nm. The folded state can also be oxidized to the dicationic state. These types of molecules with multiple switchable states and in particular stable diradicals show great potential in the design of new functional materials such as memory devices, logic gates, and OFETs.

Filter paper based SERS substrate for the direct detection of analytes in complex matrices.

Surface-enhanced Raman spectroscopy (SERS) is an emerging analytical technique for chemical analysis, which is favourable due to its combination of short measurement time, high sensitivity and molecular specificity. However, the application of SERS is still limited, largely because in real samples the analyte is often present in a complex matrix that contains micro/macro particles that block the probe laser, as well as molecular contaminants that compete for the enhancing surface. Here, we show a simple and scalable spray-deposition technique to fabricate SERS-active paper substrates which combine sample filtration and enhancement in a single material. Unlike previous spray-deposition methods, in which simple colloidal nanoparticles were sprayed onto solid surfaces, here the colloidal nanoparticles are mixed with hydroxyethyl cellulose (HEC) polymer before application. This leads to significantly improved uniformity in the distribution of enhancing particles as the film dries on the substrate surface. Importantly, the polymer matrix also protects the enhancing particles from air-oxidation during storage but releases them to provide SERS enhancement when the film is rehydrated. These SERS-paper substrates are highly active and a model analyte, crystal violet, was detected down to 4 ng in 10 µL of sample with less than 20% point-by-point signal deviation. The filter paper and HEC effectively filter out both interfering micro/macro particles and molecular (protein) contaminants, allowing the SERS-paper substrates to be used for SERS detection of thiram in mud and melamine in the presence of protein down to nanogram levels without sample pre-treatment or purification.

Engineering the Oxidative Potency of Non-Heme Iron(IV) Oxo Complexes in Water for C-H Oxidation by a cis Donor and Variation of the Second Coordination Sphere.

A series of iron(IV) oxo complexes, which differ in the donor (CH2py or CH2COO-) cis to the oxo group, three with hemilabile pendant donor/second coordination sphere base/acid arms (pyH/py or ROH), have been prepared in water at pH 2 and 7. The νFe═O values of 832 ± 2 cm-1 indicate similar FeIV═O bond strengths; however, different reactivities toward C-H substrates in water are observed. HAT occurs at rates that differ by 1 order of magnitude with nonclassical KIEs (kH/kD = 30-66) consistent with hydrogen atom tunneling. Higher KIEs correlate with faster reaction rates as well as a greater thermodynamic stability of the iron(III) resting states. A doubling in rate from pH 7 to pH 2 for substrate C-H oxidation by the most potent complex, that with a cis-carboxylate donor, [FeIVO(Htpena)]2+, is observed. Supramolecular assistance by the first and second coordination spheres in activating the substrate is proposed. The lifetime of this complex in the absence of a C-H substrate is the shortest (at pH 2, 3 h vs up to 1.3 days for the most stable complex), implying that slow water oxidation is a competing background reaction. The iron(IV)═O complex bearing an alcohol moiety in the second coordination sphere displays significantly shorter lifetimes due to a competing selective intramolecular oxidation of the ligand.

Photophysics of First-Generation Photomolecular Motors: Resolving Roles of Temperature, Friction, and Medium Polarity.

Light-driven unidirectional molecular rotary motors have the potential to power molecular machines. Consequently, optimizing their speed and efficiency is an important objective. Here, we investigate factors controlling the photochemical yield of the prototypical unidirectional rotary motor, a sterically overcrowded alkene, through detailed investigation of its excited-state dynamics. An isoviscosity analysis of the ultrafast fluorescence decay data resolves friction from barrier effects and reveals a 3.4 ± 0.5 kJ mol-1 barrier to excited-state decay in nonpolar media. Extension of this analysis to polar solvents shows that this barrier height is a strong function of medium polarity and that the decay pathway becomes near barrierless in more polar media. Thus, the properties of the medium can be used as a route for controlling the motor's excited-state dynamics. The connection between these dynamics and the quantum yield of photochemical isomerization is probed. The photochemical quantum yield is shown to be a much weaker function of solvent polarity, and the most efficient excited-state decay pathway does not lead to a strongly enhanced quantum yield for isomerization. These results are discussed in terms of the solvent dependence of the complex multidimensional excited-state reaction coordinate.

Electrochemical Ring-Opening and -Closing of a Spiropyran.

The bistability of molecular switches is an essential characteristic in their use as functional components in molecular-based devices and machines. For photoswitches, light-driven switching between two stable states proceeds via short-lived changes of the bond order in electronically excited states. Here, bistable switching of a ditertbutyl-substituted spiropyran photoswitch is instead demonstrated by oxidation and subsequent reduction in an overall four-state cycle. The spiropyran structure chosen has reduced sensitivity to the effect of secondary electrochemical processes such as H+ production and provides transient access to a decreased thermal Z-E isomerization barrier in the one electron oxidized state, akin to that achieved in the corresponding photochemical path. Thus, we show that the energy needed for switching spiropyrans to the merocyanine form on demand, typically delivered by a photon, can instead be provided electrochemically. This opens up further opportunities for the utilization of spiropyrans in electrically controlled applications and devices.

Correction: The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome.

Correction for 'The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome' by Luuk Kortekaas et al., Chem. Soc. Rev., 2019, 48, 3406-3424, DOI: .

Visible-Light-Driven Rotation of Molecular Motors in a Dual-Function Metal-Organic Framework Enabled by Energy Transfer.

The visible-light-driven rotation of an overcrowded alkene-based molecular motor strut in a dual-function metal-organic framework (MOF) is reported. Two types of functional linkers, a palladium-porphyrin photosensitizer and a bispyridine-derived molecular motor, were used to construct the framework capable of harvesting low-energy green light to power the rotary motion. The molecular motor was introduced in the framework using the postsynthetic solvent-assisted linker exchange (SALE) method, and the structure of the material was confirmed by powder (PXRD) and single-crystal X-ray (SC-XRD) diffraction. The large decrease in the phosphorescence lifetime and intensity of the porphyrin in the MOFs upon introduction of the molecular motor pillars confirms efficient triplet-to-triplet energy transfer between the porphyrin linkers and the molecular motor. Near-infrared Raman spectroscopy revealed that the visible light-driven rotation of the molecular motor proceeds in the solid state at rates similar to those observed in solution.

Ultrafast Excited State Dynamics in a First Generation Photomolecular Motor.

Efficient photomolecular motors will be critical elements in the design and development of molecular machines. Optimisation of the quantum yield for photoisomerisation requires a detailed understanding of molecular dynamics in the excited electronic state. Here we probe the primary photophysical processes in the archetypal first generation photomolecular motor, with sub-50 fs time resolved fluorescence spectroscopy. A bimodal relaxation is observed with a 100 fs relaxation of the Franck-Condon state to populate a red-shifted state with a reduced transition moment, which then undergoes multi-exponential decay on a picosecond timescale. Oscillations due to the excitation of vibrational coherences in the S1 state are seen to survive the ultrafast structural relaxation. The picosecond relaxation reveals a strong solvent friction effect which is thus ascribed to torsion about the C-C axle. This behaviour is contrasted with second generation photomolecular motors; the principal differences are explained by the existence of a barrier on the excited state surface in the case of the first-generation motors which is absent in the second generation. These results will help to provide a basis for designing more efficient molecular motors in the future.

O2 Activation by Non-Heme Thiolate-Based Dinuclear Fe Complexes.

Iron centers featuring thiolates in their metal coordination sphere (as ligands or substrates) are well-known to activate dioxygen. Both heme and non-heme centers that contain iron-thiolate bonds are found in nature. Investigating the ability of iron-thiolate model complexes to activate O2 is expected to improve the understanding of the key factors that direct reactivity to either iron or sulfur. We report here the structural and redox properties of a thiolate-based dinuclear Fe complex, [FeII2(LS)2] (LS2- = 2,2'-(2,2'-bipyridine-6,6'-iyl)bis(1,1-diphenylethanethiolate)), and its reactivity with dioxygen, in comparison with its previously reported protonated counterpart, [FeII2(LS)(LSH)]+. When reaction with O2 occurs in the absence of protons or in the presence of 1 equiv of proton (i.e., from [FeII2(LS)(LSH)]+), unsupported µ-oxo or µ-hydroxo FeIII dinuclear complexes ([FeIII2(LS)2O] and [FeIII2(LS)2(OH)]+, respectively) are generated. [FeIII2(LS)2O], reported previously but isolated here for the first time from O2 activation, is characterized by single crystal X-ray diffraction and Mössbauer, resonance Raman, and NMR spectroscopies. The addition of protons leads to the release of water and the generation of a mixture of two Fe-based "oxygen-free" species. Density functional theory calculations provide insight into the formation of the µ-oxo or µ-hydroxo FeIII dimers, suggesting that a dinuclear µ-peroxo FeIII intermediate is key to reactivity, and the structure of which changes as a function of protonation state. Compared to previously reported Mn-thiolate analogues, the evolution of the peroxo intermediates to the final products is different and involves a comproportionation vs a dismutation process for the Mn and Fe derivate, respectively.

Impact of binding to the multidrug resistance regulator protein LmrR on the photo-physics and -chemistry of photosensitizers.

Light activated photosensitizers generate reactive oxygen species (ROS) that interfere with cellular components and can induce cell death, e.g., in photodynamic therapy (PDT). The effect of cellular components and especially proteins on the photochemistry and photophysics of the sensitizers is a key aspect in drug design and the correlating cellular response with the generation of specific ROS species. Here, we show the complex range of effects of binding of photosensitizer to a multidrug resistance protein, produced by bacteria, on the formers reactivity. We show that recruitment of drug like molecules by LmrR (Lactococcal multidrug resistance Regulator) modifies their photophysical properties and their capacity to induce oxidative stress especially in 1O2 generation, including rose bengal (RB), protoporphyrin IX (PpIX), bodipy, eosin Y (EY), riboflavin (RBF), and rhodamine 6G (Rh6G). The range of neutral and charged dyes with different exited redox potentials, are broadly representative of the dyes used in PDT.

Noncommutative Switching of Double Spiropyrans.

The spiropyran family of photochromes are key components in molecular-based responsive materials and devices, e.g., as multiphotochromes, covalently coupled dyads, triads, etc. This attention is in no small part due to the change in properties that accompany the switch between spiropyran and merocyanine forms. Although the spiropyran is a single structural isomer, the merocyanine form represents a family of isomers (TTT, TTC, CCT, etc.) and protonation states. Combining two spiropyrans into one compound increases the number of possible structures dramatically and the interaction between the units determines, which are impeded due to intramolecular quenching of excited states. Here, we show that the coupling of two spiropyran photochromes through their phenol units yields favorable interactions (crosstalk) between the components that provides access to species inaccessible with the component monospiropyran alone. Specifically, the ring opening of one spiropyran unit, which is thermally stable at -30 °C, prevents ring opening of the second spiropyran unit. Furthermore, whereas protonated E- and Z-monomerocyanines were previously shown to undergo thermal- and photo-equilibration, the corresponding protonated E- and Z- bimerocyanines are thermally stable and show one-way photoisomerization from the Z,Z- to an emissive E,E-bimerocyanine form. Subsequent deprotonation at room temperature resets the system to the bispiro ring-closed form, but deprotonation at -30 °C yields the otherwise inaccessible bimerocyanine form. This form is photochemically inert but undergoes a two-step thermal relaxation via the merocyanine-spiropyran form, showing that the connection at the phenol units provides sufficient intramolecular interaction to fine-tune the complex isomerization pathways of spiropyrans and demonstrating noncommutability in photo- and pH-regulated multistep isomerization pathways.

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome.

Spiropyrans have played a pivotal role in the emergence of the field of chromism following their discovery in the early 20th century, with almost ubiquitous use in materials applications especially since their photochromism was discovered in 1952. Their versatility continues to lend them to application in increasingly diverse fields not least due to recent discoveries of properties that have expanded their utility extensively. This review provides an overview of their rich history and highlights the contemporary relevance of the spiropyrans.

cis Donor Influence on O-O Bond Lability in Iron(III) Hydroperoxo Complexes: Oxidation Catalysis and Ligand Transformation.

The FeIII/FeII redox potentials for [Fe(tpen)]2+/3+, [Fe(tpena)]+/2+, and [Fe(tpenO)]+/2+ (N-R-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine, where R = CH2C6H4N, CH2COO-, CH2CH2O-, respectively) span 470 mV with the oxidation potentials following the order [FeII(tpenO)]+ (MeOH) < [FeII(tpena)]+ (MeCN) < [FeII(tpen)]2+ (MeCN). In their +3 oxidation states the complexes react with 1 equiv of H2O2 to give the purple [FeIII(OOH)(HL)]n+ (n = 2 for L = tpena, tpenO; n = 3 for L = tpen). A pyridine arm is decoordinated in these complexes, furnishing a second coordination sphere base which is protonated at ambient pH. The lifetimes of these transient species depend on how readily the substrate (sometimes the solvent) is oxidized and reflect the trend in both the O-O bond lability and oxidizing potency of the putative iron-based oxidant derived from the iron(III) peroxides. In methanol solution, [FeIII(tpenO)]2+ and [FeIII(tpena)]2+ exist in their Fe(III) states and hence the formation of [FeIII(OOH)(Htpena)]2+ and [FeIII(OOH)(HtpenO)]2+ is instantaneous. This is in contrast to the short lag time that occurs before adduct formation between [FeII(tpen)]2+ and H2O2 due to the requisite prior oxidation of the solution-state iron(II) complex to its iron(III) state. Stabilization of the +3 iron oxidation state in the resting state catalysts affords complexes that activate H2O2 more readily with the consequence of higher yields in the oxidation of the C-H bonds using H2O2 as terminal oxidant. The presence of a cis monodentate carboxylato donor increases the rate of oxidation by hydrogen atom transfer in comparison to the systems with an alkoxo or pyridine in this position. Competing with substrate oxidation is the oxidative modification of the alkoxido group in [FeIII(tpenO)]2+, converting it to a carboxylato group in the presence of H2O2: in effect, transforming tpenO to tpena.

Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation.

Lewis acid (LA) activation by coordination to metal oxido species has emerged as a new strategy in catalytic oxidations. Despite the many reports of enhancement of performance in oxidation catalysis, direct evidence for LA-catalyst interactions under catalytically relevant conditions is lacking. Here, we show, using the oxidation of alkenes with H2O2 and the catalyst [Mn2(µ-O)3(tmtacn)2](PF6)2 (1), that Lewis acids commonly used to enhance catalytic activity, e.g., Sc(OTf)3, in fact undergo hydrolysis with adventitious water to release a strong Brønsted acid. The formation of Brønsted acids in situ is demonstrated using a combination of resonance Raman, UV/vis absorption spectroscopy, cyclic voltammetry, isotope labeling, and DFT calculations. The involvement of Brønsted acids in LA enhanced systems shown here holds implications for the conclusions reached in regard to the relevance of direct LA-metal oxido interactions under catalytic conditions.