Surface-Promoted Evolution of Ru-bda Coordination Oligomers Boosts the Efficiency of Water Oxidation Molecular Anodes.

A new Ru oligomer of formula {[RuII(bda-κ-N2O2)(4,4'-bpy)]10(4,4'-bpy)}, 10 (bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine), was synthesized and thoroughly characterized with spectroscopic, X-ray, and electrochemical techniques. This oligomer exhibits strong affinity for graphitic materials through CH-π interactions and thus easily anchors on multiwalled carbon nanotubes (CNT), generating the molecular hybrid material 10@CNT. The latter acts as a water oxidation catalyst and converts to a new species, 10'(H2O)2@CNT, during the electrochemical oxygen evolution process involving solvation and ligand reorganization facilitated by the interactions of molecular Ru catalyst and the surface. This heterogeneous system has been shown to be a powerful and robust molecular hybrid anode for electrocatalytic water oxidation into molecular oxygen, achieving current densities in the range of 200 mA/cm2 at pH 7 under an applied potential of 1.45 V vs NHE. The remarkable long-term stability of this hybrid material during turnover is rationalized based on the supramolecular interaction of the catalyst with the graphitic surface.

Stepwise Folding and Self-Assembly of a Merocyanine Folda-Pentamer.

Here we report a synthetic protocol toward a merocyanine (MC) pentamer 1 which represents the first merocyanine oligomer longer than a dimer. By continuously decreasing the solvent polarity, we demonstrate the stepwise folding from partially folded monomeric and dimeric MC subunits (in chloroform) up to the full pentamer π-stack (in 75% methylcyclohexane/25% chloroform) and a subsequent self-assembly of pentamer 1 into larger aggregates (in 80% methylcyclohexane/20% chloroform). This hierarchical structure formation process became possible due to the predominant dipole-dipole interactions among MC dyes that allowed for a precise modulation of the energy landscape by the solvent polarity. This unprecedented stepwise control of dye assembly via hierarchical dipole-dipole interactions opens the door for a more precise control of dye-dye interactions in artificial multichromophoric ensembles.

Consecutive Charging of a Perylene Bisimide Dye by Multistep Low-Energy Solar-Light-Induced Electron Transfer Towards H2 Evolution.

A photocatalytic system containing a perylene bisimide (PBI) dye as a photosensitizer anchored to titanium dioxide (TiO2 ) nanoparticles through carboxyl groups was constructed. Under solar-light irradiation in the presence of sacrificial triethanolamine (TEOA) in neutral and basic conditions (pH 8.5), a reaction cascade is initiated in which the PBI molecule first absorbs green light, giving the formation of a stable radical anion (PBI.- ), which in a second step absorbs near-infrared light, forming a stable PBI dianion (PBI2- ). Finally, the dianion absorbs red light and injects an electron into the TiO2 nanoparticle that is coated with platinum co-catalyst for hydrogen evolution. The hydrogen evolution rates (HERs) are as high as 1216 and 1022 µmol h-1 g-1 with simulated sunlight irradiation in neutral and basic conditions, respectively.

Metadynamics for automatic sampling of quantum property manifolds: exploration of molecular biradicality landscapes.

We present a general extension of the metadynamics allowing for an automatic sampling of quantum property manifolds (ASQPM) giving rise to functional landscapes that are analogous to the potential energy surfaces in the frame of the Born-Oppenheimer approximation. For this purpose, we employ generalized electronic collective variables to carry out biased molecular dynamics simulations in the framework of quantum chemical methods that explore the desired property manifold. We illustrate our method on the example of the "biradicality landscapes", which we explore by introducing the natural orbital occupation numbers (NOONs) as the electronic collective variable driving the dynamics. We demonstrate the applicability of the method on the simulation of p-xylylene and [8]annulene allowing to automatically extract the biradical geometries. In the case of [8]annulene the ASQPM metadynamics leads to the prediction of biradical scaffolds that can be stabilized by a suitable chemical substitution, leading to the design of novel functional molecules exhibiting biradical functionality.

Folding and fluorescence enhancement with strong odd-even effect for a series of merocyanine dye oligomers.

A series of merocyanine (MC) oligomers with a varying number of chromophores from two to six has been synthesized via a peptide synthesis strategy. Solvent-dependent UV/vis spectroscopic studies reveal folding processes for the MC oligomers driven by strong dipole-dipole interactions resulting in well-defined π-stacks with antiparallel orientation of the dyes. Whilst even-numbered tetramer 4 and hexamer 6 only show partial folding into dimeric units, odd-numbered trimer 3 and pentamer 5 fold into π-stacks of three and five MC units upon decreasing solvent polarity. In-depth 2D NMR studies provided insight into the supramolecular structure. For trimer 3, an NMR structure could be generated revealing the presence of a well-defined triple π-stack in the folded state. Concomitant with folding, the fluorescence quantum yield is increased for all MC oligomers in comparison to the single chromophore. Based on radiative and non-radiative decay rates, this fluorescence enhancement can be attributed to the rigidification of the chromophores within the π-stacks that affords a pronounced decrease of the non-radiative decay rates. Theoretical investigations for the double and triple dye stacks based on time-dependent density functional theory (TD-DFT) calculations indicate for trimer 3 a pronounced mixing of Frenkel and charge transfer (CT) states. This leads to significant deviations from the predictions obtained by the molecular exciton theory which only accounts for the Coulomb interaction between the transition dipole moments of the chromophores.

Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation.

Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly, this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead, detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time, macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity, which is reminiscent of catalytic activities of enzymes at active sites.

metaFALCON: A Program Package for Automatic Sampling of Conical Intersection Seams Using Multistate Metadynamics.

The multistate metadynamics for automatic exploration of conical intersection seams and systematic location of minimum energy crossing points in molecular systems and its implementation into the software package metaFALCON is presented. Based on a locally modified energy gap between two Born-Oppenheimer electronic states as a collective variable, multistate metadynamics trajectories are driven toward an intersection point starting from an arbitrary ground state geometry and are subsequently forced to explore the conical intersection seam landscape. For this purpose, an additional collective variable capable of distinguishing structures within the seam needs to be defined and an additional bias is introduced into the off-diagonal elements of an extended (multistate) electronic Hamiltonian. We demonstrate the performance of the algorithm on the examples of the 1,3-butadiene, benzene, and 9H-adenine molecules, where multiple minimum energy crossing points could be systematically located using the Wiener number or Cremer-Pople parameters as collective variables. Finally, with the example of 9H-adenine, we show that the multistate metadynamics potential can be used to obtain a global picture of a conical intersection seam. Our method can be straightforwardly connected with any ab initio or semiempirical electronic structure theory that provides energies and gradients of the respective electronic states and can serve for systematic elucidation of the role of conical intersections in the photophysics and photochemistry of complex molecular systems, thus complementing nonadiabatic dynamics simulations.

Ultrafast Photodynamics of Glucose.

We have investigated the photodynamics of ß-d-glucose employing our field-induced surface-hopping (FISH) method, which allows us to simulate the coupled electron-nuclear dynamics, explicitly including nonadiabatic effects and light-induced excitation. Our results reveal that from the initially populated S1 and S2 states, glucose returns nonradiatively to the ground state within about 200 fs. This takes place mainly via conical intersections (CIs), whose geometries in most cases involve the elongation of a single O-H bond, whereas in some instances, ring-opening due to dissociation of a C-O bond is observed. Experimentally, excitation to a distinct excited electronic state is improbable due to the presence of a dense manifold of states bearing similar oscillator strengths. Our FISH simulations, explicitly including a UV laser pulse of 6.43 eV photon energy, reveal that after initial excitation, the population is almost equally spread over several close-lying electronic states. This is followed by a fast nonradiative decay on the time scale of 100-200 fs, with the final return to the ground state proceeding via the S1 state through the same types of CIs as observed in the field-free simulations.