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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.

Herein, we show that the photoactive complexes [(Cp)Fe(arene)]+(Cp = cyclopentadienyl; arene = C6H6, C6H5Me) act as latent catalysts that allow for photochemical control over the onset of alkyd paint curing, without the need for antiskinning agents such as the volatile 2-butanone oxime normally used to prevent curing during paint storage. The highly soluble neutral complexes [(Cp)Fe(Ch)] and [(Cp)Fe(Ch′)] (Ch = cyclohexadienyl, Ch′ = methylcyclohexadienyl) readily convert to the photoactive complexes [(Cp)Fe(arene)]+upon oxidation in alkyd, allowing the latter to be dosed in a wide range of concentrations. Infrared and Raman studies show similar spectral changes of the alkyd paint matrix as have been observed in alkyd curing mediated by well-known, industrially applied cobalt- and manganese-based catalyst Co(neodecanoate)2and [(Me3TACN)2Mn2(μ-OOCR)3](OOCR). The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]+system performs equally well as these cobalt- and manganese-based catalysts in terms of drying time and outperform the manganese catalyst by showing a hardness development (increase) similar to that of the cobalt-based catalyst. Based on electron paramagnetic resonance and light-activity studies, we propose that photolysis of [(Cp)Fe(arene)]+generates short-lived active FeIIspecies, explaining the desired latency. The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]+alkyd curing systems presented herein are unique examples of intrinsically latent paint curing catalysts that (1) are based on an abundant and harmless transition metal (Fe), (2) do not require any antiskinning agents, and (3) show favorable performance in terms of drying times and hardness development.

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.

The iron(III) complex [Fe(tpena)]2+ (tpena = N,N,N′-tris(2-pyridylmethyl)ethylendiamine-N′-acetate) undergoes irreversible O2-dependent N-demethylcarboxylation to afford [FeII(SBPy3)(MeCN)]2+ (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO2 and formaldehyde, respectively. Time-resolved UV-vis absorption, Mössbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway. The first of these, proposed to be a low-spin iron(II)-radical ligand complex, reacts with O2 in the rate-determining step to produce a putative alkylperoxide complex. DFT calculations suggest that this evolves into an Fe(IV)-oxo species, which can abstract a hydrogen atom from a cis methylene group of the ligand to give the second spectroscopically identified intermediate, a high-spin iron(III)-hydroxide of the product oxidized ligand, [FeIII(OH)(SBPy3)]2+. Reduction and exchange of the cohydroxo/water ligand produces the crystallographically characterized products [FeII(SBPy3)(X)]2+/3+, X = MeCN, [Zn(tpena)]+.

A reactive high-valent dinuclear nickel(IV) oxido bridged complex is reported that can be formed at room temperature by reaction of [(L) 2 Ni(II) 2 (μ-X) 3 ]X (X = Cl or Br) with NaOCl in methanol or acetonitrile (where L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The unusual Ni(IV) oxido species is stabilized within a dinuclear tris-μ-oxido-bridged structure as [(L) 2 Ni(IV) 2 (μ-O) 3 ] 2+ . Its structure and its reactivity with organic substrates are demonstrated through a combination of UV-vis absorption, resonance Raman, 1 H NMR, EPR, and X-ray absorption (near-edge) spectroscopy, ESI mass spectrometry, and DFT methods. The identification of a Ni(IV)-O species opens opportunities to control the reactivity of NaOCl for selective oxidations The Ubbo Emmius fund of the University of Groningen, the European Research Council (StG, no. 279549, W.R.B.), NWO for a VIDI grant (723.014.010, D.J.M. and M.T.), The Netherlands Ministry of Education, Culture and Science (Gravity program 024.001.035, W.R.B.), MINECO (CTQ2014-59212-P and CTQ2015-70851-ERC, M.S.), Gen- Cat (2014SGR1202, M.S.), FEDER (UNGI10-4E-801, M.S.), and COST action CM1305 “ECOSTBio” (W.R.B., COSTSTSM- CM1305-29045) are acknowledged for financial support

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited-state structural evolution. The ultrafast Franck-Condon bright state relaxes to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman-active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light-driven half of the photocycle.

Smart or functional surfaces that exhibit complex multimodal responsivity, e.g., to light, heat, pH, etc., although highly desirable, require a combination of distinct functional units to achieve each type of response and present a challenge in achieving combinations that can avoid cross-talk between the units, such as excited-state quenching. Compounds that exhibit multiple switching modalities help overcome this challenge and drastically reduce the synthetic cost and complexity. Here we show that a bis-spiropyran photochrome, which is formed through coupling at the indoline 5-position using redox chemistry, exhibits pH-gated photochromism, with opening of the spiro moiety by irradiation with UV light and the expected reversion by either heating or irradiation with visible light gated by protonation/deprotonation. Remarkably, when the photochrome is oxidized to its dicationic form, bis-spiropyran(2+), visible light can be used instead of UV light to switch between the spiro and merocyanine forms, with locking and unlocking of each state achieved by protonation/deprotonation. The formation of the bis-spiropyran unit by electrochemical coupling is exploited to generate "smart surfaces", i.e., polymer-modified electrodes, avoiding the need to introduce an ancillary functional group for polymerization and the concomitant potential for cross-talk. The approach taken means not only that the multiresponsive properties of the bis-spiropyran are retained upon immobilization but also that the effective switching rate can be enhanced dramatically.

The iron(III) complex [Fe(tpena)]

Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited state processes. Here the excited state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency.

Monolayers of fluorinated light-driven molecular motors were synthesized and immobilized on gold films in an altitudinal orientation via tripodal stators. In this design the fimctionalized molecular motors are not interfering and preserve their rotary function on gold. The wettability of the self-assembled monolayers can be modulated by UV irradiation.

Driving molecular rotary motors using visible light (530-550 nm) instead of UV light was achieved using palladium tetraphenylporphyrin as a triplet sensitizer. Visible light driven rotation was confirmed by UV/vis absorption, circular dichroism and H-1 NMR spectroscopy and the rotation was confirmed to be unidirectional and with similar photostationary states, despite proceeding via a triplet instead of a singlet excited state of the molecular motor. Energy transfer proceeds in both inter- and intramolecular fashion from the triplet state of the porphyrin to the motor. Stern Volmer plots show that the rate of intermolecular quenching of the porphyrin excited state by the molecular motor is diffusion-controlled.

Dynamic control over the formation of H- or J-type aggregates of chromophores is of fundamental importance for developing responsive organic optoelectronic materials. In this study, the first example of photoswitching between a nonstacked and an intramolecularly H-stacked arrangement of perylenebisimides (PBI) is demonstrated. The system is composed of a central switching unit (overcrowded alkene) tethered to two PBI chromophores. cis-trans isomerization of the switching unit, induced by alternate irradiation at 312 and 365 nm, can drive two PBI chromophores reversibly between an intramolecularly "aggregated" and "nonaggregated" state. Distinct changes in UV-vis absorption and fluorescence spectra are observed following photoisomerization. This approach allows for efficient control of intramolecular H-stack formation with no significant intermolecular interactions spanning over at least 4 orders of magnitude of concentration (from 10(-8) to 10(-4) M) and a range of solvents and temperatures.

The synthesis, characterization, and electrochemical, photophysical, and photochemical properties of the binuclear compounds [(Ru(H-8-bpy)(2))(2)((Metr)(2)Pz)](PF6)(2) (1) and [(Ru(D-8-bpy)(2))(2)((Metr)(2)Pz)](PF6)(2) (2), where bpy is 2,2'-bipyridine and H-2(Metr)(2)Pz is the planar ligand 2,5-bis(5'-methyl-4'H-[1,2,4]triaz-3'-yl)-pyrazine, are reported. Electrochemical and spectro-electrochemical investigations indicate that the ground-state interaction between each metal center is predominantly electrostatic and in the mixed-valence form only a low level of ground-state delocalization is present. Resonance Raman, transient, and time-resolved spectroscopies enable a detailed assignment to be made of the excited-state photophysical properties of the complexes. Deuteriation is employed to both facilitate spectroscopic characterization and investigate the nature of the lowest excited states.

Reversible noninvasive control over the generation of singlet oxygen is demonstrated in a bicomponent system comprising a diarylethene photochromic switch and a porphyrin photosensitizer by selective irradiation at distinct wavelengths. The efficient generation of singlet oxygen by the photosensitizer is observed when the diarylethene unit is in the colorless open form. Singlet oxygen generation is not observed when the diarylethene is converted to the closed form. Irradiation of the closed form with visible light (>470 nm) leads to full recovery of the singlet oxygen generating ability of the porphyrin sensitizer.

The interaction between two TEMPO spin centers connected to a photoswitchable overcrowded alkene changes from noncoupled (three-line EPR spectrum) in the trans state, where the two spin centers are separated by similar to 22 angstrom, to strongly coupled (five-line EPR spectrum) in the cis state, where the separation is similar to 7 angstrom, upon photoswitching. Importantly, the performance of the alkene switching unit is essentially unaffected by the spin centers.

Tetra-peri-(tert-butyl-benzo)-di-peri-(pyrimidino)-coronene 1, the parent compound of the nitrogen heterosuperbenzene family N-HSB, is employed as a novel monotopic ligand in the formation of [Pd(eta(3)-C3H5)(1)]PF6 2 and [Ru(bpy)(2)(1)](PF6)(2) (where bpy = 2,2'-bipyridine 3a and d(8)-2,2'-bipyridine 3b). These N-coordinated complexes are fully characterized by H-1 NMR and IR spectroscopy and ESI-MS. Metal coordination has a profound effect on both the absorption and the emission properties of 1. Pd(II) coordination causes a red-shift in the low-energy absorptions, a decrease in the intensity of the n-pi* absorptions, and a quenching of the emission. Ru(II) coordination causes absorption throughout the visible region and creates two new complexes that join an elite group of compounds known as "black" absorbers. 3a and 3b possess two discernible (MLCT)-M-1 bands. The one of exceptionally low energy (lambda(max) = 615 nm) has an associated (MLCT)-M-3 emission ( lambda(max) = 880 nm) due to the unprecedented electron delocalization and acceptor properties of the rigid aromatic N-HSB 1. Both Ru(II) complexes are near-IR emitters with unusually protracted emission lifetimes of 320 ns at 77 K. They are photochemically inert, and their electrochemical properties are consistent with the presence of a low-lying pi* orbital on 1. The first two reversible reductions (E-1/2 (CH3CN), -0.54 V, -1.01 V vs SCE) are due to the stepwise reduction of 1 and are anodically shifted as compared to [Ru(bpy)(3)](2+). Temperature- and concentration-dependent NMR studies on 2 and 3a suggest extensive aggregation is occurring in solution.

In this contribution the polymerization of terthiophene, to form an alkene bridged alpha,alpha-coupled sexithiophene polymer, is controlled by light; i.e. the electropolymerizability of the monomer 1F is switched off and on with UV and visible light, respectively. The system comprises of both bis-terthiophene and photochromic dithienylethene units. The presence of a light-switchable unit allows on-off switching of the electropolymerization of the monomer with light. Furthermore the incorporation of the dithienylethene in the polymer backbone increases dramatically the homogenity of the polymer formed (i.e., only sexithiophene units are formed). The derived films are robust and fully retain electrochromic behavior as has been demonstrated through cyclic voltammetry while spatial control (patterning) is readily achieved by applying simple optical masking techniques.

The fully reversible three-state blue/red/off emission from photo-/electrochromic substituted bis-thiaxanthylidenes is reported. The blue luminescence of the most stable (anti-folded) conformer of dimethyl- and dimethoxy-bis-thiaxanthylidene can be switched off by photochemical conversion to the meta-stable (syn-folded) conformer and switched on again by thermal reversion to the anti-folded state. The red luminescence of the bis-thiaxanthylium dication can be switched on by oxidation at approximately 1.0 and 1.2 V vs SCE of the syn- and anti-folded conformers respectively and switched off or to blue by reduction at approximately 0.35 V vs SCE.

The excited-state properties of the transition metal complexes tris(2,2'-bipyridine) ruthenium(II) and tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) are examined using picosecond time-resolved luminescence spectroscopy. For both complexes, direct observation of a short-lived high-energy emission with a lifetime of less than 4 ps is reported. Upon deuteriation of the complexes the lifetime of the high-energy emission shows a marked increase with a biexponential decay (20 and approximately 300 ps components). Examination by time-resolved excited-state resonance Raman shows that for the perprotio complexes features attributable to the 3MLCT excited state are formed within 4 ps, while for the perdeuterio a rise time of approximately 20 ps is observed in the 3MLCT features. The results indicate that the emission in both cases may be 1MLCT in origin and are discussed with respect to heterogeneous electron 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.