Plasmon mediated coherent population oscillations in molecular aggregates.

The strong coherent coupling of quantum emitters to vacuum fluctuations of the light field offers opportunities for manipulating the optical and transport properties of nanomaterials, with potential applications ranging from ultrasensitive all-optical switching to creating polariton condensates. Often, ubiquitous decoherence processes at ambient conditions limit these couplings to such short time scales that the quantum dynamics of the interacting system remains elusive. Prominent examples are strongly coupled exciton-plasmon systems, which, so far, have mostly been investigated by linear optical spectroscopy. Here, we use ultrafast two-dimensional electronic spectroscopy to probe the quantum dynamics of J-aggregate excitons collectively coupled to the spatially structured plasmonic fields of a gold nanoslit array. We observe rich coherent Rabi oscillation dynamics reflecting a plasmon-driven coherent exciton population transfer over mesoscopic distances at room temperature. This opens up new opportunities to manipulate the coherent transport of matter excitations by coupling to vacuum fields.

X-ray-determined structure of the technetium complex [Tc2(µ-CO)2(NC5H5)2(CO)6] revisited: [Tc2(µ-OMe)2(NC5H5)2(CO)6] as the correct formulation.

Some of us reported previously the structure of di-µ-carbonyl-bis[tricarbonyl(pyridine)technetium], [Tc2(µ-CO)2(C5H5N)2(CO)6], as the main product of the reaction of [Tc2(CO)10] with pyridine at room temperature, using the reagent itself as solvent [Zuhayra et al. (2008). Inorg. Chem. 47, 10177-10182]. On the basis of an X-ray analysis of the product, a molecular structure was proposed with two bridging carbonyls displaying very unusual geometrical features, not explained at the time. Subsequent chemical considerations, coupled with density functional theory (DFT) calculations, prompted us to revise the original structure determination. Using the original raw diffraction data, we have now performed new refinements to show that the previously proposed `bridging carbonyls' actually correspond to bridging methoxide groups, and that the crystals analyzed at the time therefore would correspond to the complex di-µ-methoxido-bis[tricarbonyl(pyridine)technetium], syn-[Tc2(µ-OMe)2(NC5H5)2(CO)6]. This methoxide-bridged complex likely was a minor side product formed along with the main product in the above reaction, perhaps due to the presence of trace amounts of methanol and air in the reaction mixture.

Structural Disorder as the Origin of Optical Properties and Spectral Dynamics in Squaraine Nano-Aggregates.

In contrast to regular J- and H-aggregates, thin film squaraine aggregates usually have broad absorption spectra containing both J-and H-like features, which are favorable for organic photovoltaics. Despite being successfully applied in organic photovoltaics for years, a clear interpretation of these optical properties by relating them to specific excited states and an underlying aggregate structure has not been made. In this work, by static and transient absorption spectroscopy on aggregated n-butyl anilino squaraines, we provide evidence that both the red- and blue-shifted peaks can be explained by assuming an ensemble of aggregates with intermolecular dipole-dipole resonance interactions and structural disorder deriving from the four different nearest neighbor alignments─in sharp contrast to previous association of the peaks with intermolecular charge-transfer interactions. In our model, the next-nearest neighbor dipole-dipole interactions may be negative or positive, which leads to the occurrence of J- and H-like features in the absorption spectrum. Upon femtosecond pulse excitation of the aggregated sample, a transient absorption spectrum deviating from the absorbance spectrum emerges. The deviation finds its origin in the excitation of two-exciton states by the probe pulse. The lifetime of the exciton is confirmed by the band integral dynamics, featuring a single-exponential decay with a lifetime of 205 ps. Our results disclose the aggregated structure and the origin of red- and blue-shifted peaks and explain the absence of photoluminescence in squaraine thin films. Our findings underline the important role of structural disorder of molecular aggregates for photovoltaic applications.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes.

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang-Rhys factors below 0.01 for the high-frequency, 1500 cm-1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

Subcomponent self-assembly of circular helical Dy6(L)6 and bipyramid Dy12(L)8 architectures directed via second-order template effects.

In situ metal-templated (hydrazone) condensation also called subcomponent self-assembly of 4,6-dihydrazino-pyrimidine, o-vanillin and dysprosium ions resulted in the formation of discrete hexa- or dodecanuclear metallosupramolecular Dy6(L)6 or Dy12(L)8 aggregates resulting from second-order template effects of the base and the lanthanide counterions used in these processes. XRD analysis revealed unique circular helical or tetragonal bipyramid architectures in which the bis(hydrazone) ligand L adopts different conformations and shows remarkable differences in its mode of metal coordination. While a molecule of trimethylamine acts as a secondary template that fills the void of the Dy6(L)6 assembly, sodium ions take on this role for the formation of heterobimetallic Dy12(L)8 by occupying vacant coordination sites, thus demonstrating that these processes can be steered in different directions upon subtle changes of reaction conditions. Furthermore, Dy6(L)6 shows an interesting spin-relaxation energy barrier of 435 K, which is amongst the largest values within multinuclear lanthanide single-molecular magnets.

Influence of Different Substitution Patterns on the 2D Crystalline Aggregation of Small Molecules on HOPG Surfaces.

A series of benzoic and terephthalic acid derivatives and their corresponding ethyl esters have been synthesized. These are decorated with dendritic units carrying alkoxy chains that differ in the number of alkoxy groups and/or their substitution pattern. These molecules were deposited from solution on highly oriented pyrolytic graphite, and the resulting supramolecular surface patterns have been studied by scanning tunneling microscopy at the solid/liquid interface. Thus, we were able to perform a systematic study on the influence of orthogonal functional groups governing the supramolecular assembly, that is, the alkoxy chains of dendritic units and the carboxylic acid (ester) functions, on the intermolecular interactions, that is, van der Waals interactions and hydrogen bonding between the adsorbed species that drive the pattern formation.

Plasmon-Enhanced Exciton Delocalization in Squaraine-Type Molecular Aggregates.

Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Two-dimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton-plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule-gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.

A Family of Heterobimetallic Cubes Shows Spin-Crossover Behaviour Near Room Temperature.

Using 4-(4'-pyridyl)aniline as a simple organic building block in combination with three different aldehyde components together with metal(II) salts gave three different Fe8 Pt6 -cubes and their corresponding Zn8 Pt6 analogues by employing the subcomponent self-assembly approach. Whereas the use of zinc(II) salts gave rise to diamagnetic cages, iron(II) salts yielded metallosupramolecular cages that show spin-crossover behaviour in solution. The spin-transition temperature T1/2 depends on the incorporated aldehyde component, giving a construction kit for the deliberate synthesis of spin-crossover compounds with tailored transition properties. Incorporation of 4-thiazolecarbaldehyde or N-methyl-2-imidazole-carbaldehyde yielded cages that undergo spin-crossover around room temperature whereas the cage obtained using 1H-4-imidazolecarbaldehyde shows a spin-transition at low temperatures. Three new structures were characterized by synchrotron X-ray diffraction and all structures were characterized by mass spectrometry, NMR and UV/Vis spectroscopy.

Charge-State-Dependent Fragmentation of [2.2]Based Metallosupramolecular Cyclic Helicates in the Gas Phase.

A detailed mass-spectrometric study provides insight into the gas-phase fragmentation pathways of a cyclic helicate selectively built from four iron(II) centers and six [2.2]cyclophane-based ligands through the subcomponent self-assembly approach. The charge state of the precursor ion, i. e., the number of triflate anions accompanying the metallo-supramolecular core, has a strong influence on the observed fragmentations. The triply charged ion shows loss of a neutral ligand whereas ions of higher charge fragment by up to three different charge-separating pathways to minimize the charge density of the ions. Additional subsequent fragmentations of highly charged fragment ions include redox processes as well as splitting of the unusual paracyclophane backbone.

A heterobimetallic tetrahedron from a linear platinum(II)-bis(acetylide) metalloligand.

Employing 4-ethynylaniline as a simple organic ligand we were able to prepare the stable trans-bis(acetylide)platinum(II) complex [Pt(L1)2(PBu3)2] as a linear metalloligand. The reaction of this metalloligand with iron(II) cations and pyridine-2-carbaldehyde according to the subcomponent self-assembly approach yielded decanuclear heterobimetallic tetrahedron [Fe4Pt6(L2)12](OTf)8. Thus, combination of these two design concepts - the subcomponent self-assembly strategy and the complex-as-a-ligand approach - ensured a fast and easy synthesis of large heterobimetallic coordination cages of tetrahedral shape with a diameter of more than 3 nm as a mixture of all three possible T-, S 4- and C 3-symmetric diastereomers. The new complexes were characterized by NMR and UV-vis spectroscopy and ESI mass spectrometry. Using GFN2-xTB we generated energy-minimized models of the diastereomers of this cage that further corroborated the results from analytical findings.

Chiral Self-Sorting Effects in the Self-Assembly of Metallosupramolecular Aggregates Comprising Ligands Derived from Tröger's Base.

Five ligands with either nitrile or isonitrile metal binding motifs have been synthesized based on the 2,8- or 3,9-disubstituted Tröger's base scaffold, respectively. These ligands self-assemble into dinuclear cyclic metallosupramolecular aggregates upon coordination to [(dppp)Pd(OTf)2 ] in a highly diastereoselective manner, by heterochiral self-sorting in a chiral self-discriminating manner as shown by ESI mass spectrometry, NMR spectroscopy, and single crystal XRD analysis. This observation is in contrast to earlier studies with ligands derived from Tröger's base that have larger metal binding motifs and bis(nitrile) and bis(isonitrile) ligands based on other rigid dissymmetric cores such as [2.2]paracyclophanes. Thus, the combination of these slim metal binding motifs with the rigid v-shaped 2,8- or 3,9-disubstituted Tröger's base scaffolds seems to be especially well preorganized to ensure high-fidelity social self-sorting behavior.

Better Together: Functional Heterobimetallic Macrocyclic and Cage-like Assemblies.

Metallosupramolecular chemistry has attracted the interest of generations of researches due to the versatile properties and functionalities of oligonuclear coordination complexes. Quite a number of different discrete cages were investigated, mostly consisting of only one type of ligand and one type of metal cation. Looking for ever more complex structures, heterobimetallic complexes became more and more attractive, as they give access to new structural motifs and functions. In the last years substantial success has been made in the design and synthesis of cages consisting of more than one type of metal cations, and a rapidly growing number of functional materials has appeared in the literature. This Minireview describes recent developments in the field of discrete heterometallic macrocycles and cages focusing on functional materials that have been used as host-systems or as magnetic, photo-active, redox-active, and even catalytically active materials.

Polymorphic chiral squaraine crystallites in textured thin films.

An enantiomerically pure (R)-2-methylpyrrolidine-based anilino squaraine crystallizes in two chiral polymorphs adopting a monoclinic C2 and an orthorhombic P21 21 21 structure, respectively. By various thin-film preparation techniques, a control of the polymorph formation is targeted. The local texture of the resulting textured thin films is connected to the corresponding optical properties. Special attention is paid to an unusual Davydov splitting, the anisotropic chiroptical response arising from preferred out-of-plane orientation of the crystallites, and the impact of the polymorph specific excitonic coupling.

Surface-Controlled Crystal Alignment of Naphthyl End-Capped Oligothiophene on Graphene: Thin-Film Growth Studied by in Situ X-ray Diffraction.

We report on the microstructure, morphology, and growth of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2) thin films deposited on graphene, characterized by grazing incidence X-ray diffraction (GIXRD) and complemented by atomic force microscopy (AFM) measurements. NaT2 is deposited on two types of graphene surfaces: custom-made samples where chemical vapor deposition (CVD)-grown graphene layers are transferred onto a Si/SiO2 substrate by us and common commercially transferred CVD graphene on Si/SiO2. Pristine Si/SiO2 substrates are used as a reference. The NaT2 crystal structure and orientation depend strongly on the underlying surface, with the molecules predominantly lying down on the graphene surface (face-on orientation) and standing nearly out-of-plane (edge-on orientation) on the Si/SiO2 reference surface. Post growth GIXRD and AFM measurements reveal that the crystalline structure and grain morphology differ depending on whether there is polymer residue left on the graphene surface. In situ GIXRD measurements show that the thickness dependence of the intensity of the (111) reflection from the crystalline edge-on phase does not intersect zero at the beginning of the deposition process, suggesting that an initial wetting layer, corresponding to 1-2 molecular layers, is formed at the surface-film interface. By contrast, the (111) reflection intensity from the crystalline face-on phase grows at a constant rate as a function of film thickness during the entire deposition.

Influencing the Self-Sorting Behavior of [2.2]Paracyclophane-Based Ligands by Introducing Isostructural Binding Motifs.

Two isostructural ligands with either nitrile (Lnit ) or isonitrile (Liso ) moieties directly connected to a [2.2]paracyclophane backbone with pseudo-meta substitution pattern have been synthesized. The ligand itself (Lnit ) or its precursors (Liso ) were resolved by HPLC on a chiral stationary phase and the absolute configuration of the isolated enantiomers was assigned by XRD analysis and/or by comparison of quantum-chemical simulated and experimental electronic circular dichroism (ECD) spectra. Surprisingly, the resulting metallosupramolecular aggregates formed in solution upon coordination of [(dppp)Pd(OTf)2 ] differ in their composition: whereas Lnit forms dinuclear complexes, Liso exclusively forms trinuclear ones. Furthermore, they also differ in their chiral self-sorting behavior as (rac)-Liso undergoes exclusive social self-sorting leading to a heterochiral assembly, whereas (rac)-Liso shows a twofold preference for the formation of homochiral complexes in a narcissistic self-sorting manner as proven by ESI mass spectrometry and NMR spectroscopy. Interestingly, upon crystallization, these discrete aggregates undergo structural transformation to coordination polymers, as evidenced by single-crystal X-ray diffraction.

Dynamic Complex-to-Complex Transformations of Heterobimetallic Systems Influence the Cage Structure or Spin State of Iron(II) Ions.

Two new heterobimetallic cages, a trigonal-bipyramidal and a cubic one, were assembled from the same mononuclear metalloligand by adopting the molecular library approach, using iron(II) and palladium(II) building blocks. The ligand system was designed to readily assemble through subcomponent self-assembly. It allowed the introduction of steric strain at the iron(II) centres, which stabilizes its paramagnetic high-spin state. This steric strain was utilized to drive dynamic complex-to-complex transformations with both the metalloligand and heterobimetallic cages. Addition of sterically less crowded subcomponents as a chemical stimulus transformed all complexes to their previously reported low-spin analogues. The metalloligand and bipyramid incorporated the new building block more readily than the cubic cage, probably because the geometric structure of the sterically crowded metalloligand favours the cube formation. Furthermore it was possible to provoke structural transformations upon addition of more favourable chelating ligands, converting the cubic structures into bipyramidal ones.

Palladium(II)-Mediated Assembly of a M2L2 Macrocycle and M3L6 Cage from a Cyclopeptide-Derived Ligand.

A cyclic tetrapeptide L comprising two l-proline and two 3-amino-5-(pyridin-4-yl)benzoic acid subunits assembles into a dimetallic metallamacrocycle Pd2L2 and a trimetallic coordination cage Pd3L6 in the presence of suitable palladium(II) precursors. Pd2L2 recognizes organic anions in aqueous media, forming a particularly stable complex with sodium 2,6-naphthalenedisulfonate, in which two dianionic guest molecules reside in the cavity surrounded by the cyclopeptide ligands.

Subcomponent Self-Assembly of a Cyclic Tetranuclear FeII Helicate in a Highly Diastereoselective Self-Sorting Manner.

An enantiomerically pure diamine based on the 4,15-difunctionalized [2.2]paracyclophane scaffold and 2-formylpyridine self-assemble into an optically pure cyclic metallosupramolecular Fe4 L6 helicate upon mixing with iron(II) ions in a diastereoselective subcomponent self-assembly process. The cyclic assembly results from steric strain that prevents the formation of a smaller linear dinuclear triple-stranded helicate, and hence, leads to the larger strain-free assembly that fulfils the maximum occupancy rule. Interestingly, use of the racemic diamine also leads to a racemic mixture of the homochiral cyclic helicates as the major product in a highly diastereoselective narcissistic chiral self-sorting manner given the fact that the assembly contains ten stereogenic elements, which can in principle give rise to 149 different diastereomers. The metallosupramolecular aggregates could be characterized by NMR, UV/Vis and CD spectroscopy, mass spectrometry, and X-ray crystallography.

Efficient resolution of racemic crown-shaped cyclotriveratrylene derivatives and isolation and characterization of the intermediate saddle isomer.

The preparative resolution of a trifunctionalized C 3-symmetrical chiral cyclotriveratrylene derivative was achieved via high-performance liquid chromatography (HPLC) on a chiral stationary phase. This approach is a promising alternative to the previously reported resolution through formation of diastereomeric esters because it involves fewer synthetic steps and is less prone to thermal (re)racemization. During these studies an intermediate saddle conformer could also be isolated and characterized by 1H and 13C NMR spectroscopy. The HPLC separation method was further developed in order to allow investigations on the racemization behavior of the cyclotriveratrylene derivative.

Thermodynamically driven self-assembly of pyridinearene to hexameric capsules.

Pyridinearene macrocycles have previously shown unique host-guest properties in their capsular dimers including endo complexation of neutral molecules and exo complexation of anions. Here, we demonstrate for the first time the formation of hydrogen bonded hexamer of tetraisobutyl-octahydroxypyridinearene in all three states of matter - gas phase, solution and solid-state. Cationic tris(bipyridine)ruthenium(ii) template was found to stabilize the hexamer in gas phase, whereas solvent molecules do this in condensed phases. In solution, the capsular hexamer was found to be the thermodynamically favoured self-assembly product and transition from dimer to hexamer occurred in course of time. The crystal structure of hexamer revealed 24 N-HO direct intermolecular hydrogen bonds between the six pyridinearene macrocycles without any bridging solvent molecules. Hydrogen bond patterns correlate well with DFT computed structures. Thus, all structural chemistry methods (IM-MS, DOSY NMR, DFT, X-ray crystallography) support the same structure of the hexameric capsule that has a diameter of ca. 3 nm and volume of 1160 Å3.