Luminescence and Light-Driven Energy and Electron Transfer from an Exceptionally Long-Lived Excited State of a Non-Innocent Chromium(III) Complex.

Photoactive metal complexes employing Earth-abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non-innocence to tune the luminescence and photochemistry of the excited state of the [CrN6 ] chromophore [Cr(tpe)2 ]3+ with close to octahedral symmetry (tpe=1,1,1-tris(pyrid-2-yl)ethane). [Cr(tpe)2 ]3+ exhibits the longest luminescence lifetime (τ=4500 µs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2 ]3+ are redox non-innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2 ]3+ surpass those of the classical photosensitizer [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n-butyl)amine).

Hybridization and Covalency in the Group 2 and Group 12 Metal Cation/Rare Gas Complexes.

We provide a consistent set of interaction energy curves for the group 2 (IIA) and group 12 (IIB) metal cation/rare gas complexes, M+-RG, where M+ = Be+-Ra+ and Zn+-Hg+ and RG = He-Rn. We report spectroscopic constants derived from these, compare them with available data, and discuss trends in the values. We gain insight into the interactions that occur using a range of approaches: reduced potential energy curves; charge and population analyses; molecular orbital diagrams and contour plots; and Birge-Sponer plots. Although sp hybridization occurs in the Be+-RG, Mg+-RG and group 12 M+-RG complexes, this appears to be minimal and covalency is the main aspect of the interaction. However, major sd hybridization occurs in the heavier group 2 M+-RG systems, which increases their interaction energies but there is minimal covalency. Examination of Birge-Sponer plots reveals significant curvature in many cases, which we ascribe to the changing amounts of hybridization or covalency as a function of internuclear separation. This suggests why the use of a simple electrostatics-based model potential to describe the interactions is inadequate.

Molecular Ruby under Pressure.

The intensely luminescent chromium(III) complexes [Cr(ddpd)2 ]3+ and [Cr(H2 tpda)2 ]3+ show surprising pressure-induced red shifts of up to -15 cm-1 kbar-1 for their sharp spin-flip emission bands (ddpd=N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine; H2 tpda=2,6-bis(2-pyridylamino)pyridine). These shifts surpass that of the established standard, ruby Al2 O3 :Cr3+ , by a factor of 20. Beyond the common application in the crystalline state, the very high quantum yield of [Cr(ddpd)2 ]3+ enables optical pressure sensing in aqueous and methanolic solution. These unique features of the molecular rubies [Cr(ddpd)2 ]3+ and [Cr(H2 tpda)2 ]3+ pave the way for highly sensitive optical pressure determination and unprecedented molecule-based pressure sensing with a single type of emitter.

Nonclassical crystallization in vivo et in vitro (II): Nanogranular features in biomimetic minerals disclose a general colloid-mediated crystal growth mechanism.

Recent research has shown that biominerals and their biomimetics (i) typically form via an amorphous precursor phase, and (ii) commonly display a nanogranular texture. Apparently, these two key features are closely related, underlining the fact that the formation of biominerals and their biomimetics does not necessarily follow classical crystallization routes, and leaves a characteristic nanotextural imprint which may help to disclose their origins and formation mechanisms. Here we present a general overview of the current theories and models of nonclassical crystallization and their applicability for the advance of our current understanding of biomineralization and biomimetic mineralization. We pay particular attention to the link between nonclassical crystallization routes and the resulting nanogranular textures of biomimetic CaCO3 mineral structures. After a general introductory section, we present an overview of classical nucleation and crystal growth theories and their limitations. Then, we introduce the Ostwald's step rule as a general framework to explain nonclassical crystallization. Subsequently, we describe nonclassical crystallization routes involving stable prenucleation clusters, dense liquid and solid amorphous precursor phases, as well as current nonclassical crystal growth models. The latter include oriented attachment, mesocrystallization and the new model based on the colloidal growth of crystals via attachment of amorphous nanoparticles. Biomimetic examples of nanostructured CaCO3 minerals formed via these nonclassical routes are presented which help us to show that colloid-mediated crystal growth can be regarded as a wide-spread growth mechanism. Implications of these observations for the advance in the current understanding on the formation of biomimetic materials and biominerals are finally outlined.

Nonclassical crystallization in vivo et in vitro (I): Process-structure-property relationships of nanogranular biominerals.

A distinct nanogranular fine structure is shared by a wealth of biominerals from several species, classes and taxa. This nanoscopic organization affects the properties and behavior of the biogenic ceramic material and confers on them attributes that are essential to their function. We present a set of structure-relationship properties that are rooted in the nanogranular organization and we propose that they rest on a common pathway of formation, a colloid-driven and hence nonclassical mode of crystallization. With this common modus operandi, we reveal the most fundamental and wide spread process-structure-property relationship in biominerals. With the recent increase in our understanding of nonclassical crystallization in vitro and in vivo, this significant process-structure-property relationship will serve as a source for new design approaches of bio-inspired materials.

Assignment of the vibrations of the S0, S1, and D0 (+) states of perhydrogenated and perdeuterated isotopologues of chlorobenzene.

We report vibrationally resolved spectra of the S1←S0 transition of chlorobenzene using resonance-enhanced multiphoton ionization spectroscopy. We study chlorobenzene-h5 as well as its perdeuterated isotopologue, chlorobenzene-d5. Changes in the form of the vibrational modes between the isotopologues and also between the S0 and S1 electronic states are discussed for each species. Vibrational bands are assigned utilizing quantum chemical calculations, previous experimental results, and isotopic shifts, including those between the (35)Cl and (37)Cl isotopologues. Previous work and assignments of the S1 spectra are discussed. Additionally, the vibrations in the ground state cation, D0 (+), are considered, since these have also been used by previous workers in assigning the excited neutral state spectra.

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of bromobenzene and its perdeuterated isotopologue: Assignment of the vibrations of the S(0), S(1), and D(0)(+) states of bromobenzene and the S(0) and D(0)(+) states of iodobenzene.

We report vibrationally resolved spectra of the S1←S0 transition of bromobenzene using resonance-enhanced multiphoton ionization spectroscopy. We study bromobenzene-h5 as well as its perdeuterated isotopologue, bromobenzene-d5. The form of the vibrational modes between the isotopologues and also between the S0 and S1 electronic states is discussed for each species, allowing assignment of the bands to be achieved and the activity between states and isotopologues to be established. Vibrational bands are assigned utilizing quantum chemical calculations, previous experimental results, and isotopic shifts. Previous work and assignments of the S1 spectra are discussed. Additionally, the vibrations in the ground state cation, D0 (+), are considered, since these have also been used by previous workers in assigning the excited neutral state spectra. We also examine the vibrations of iodobenzene in the S0 and D0 (+) states and comment on the previous assignments of these. In summary, we have been able to assign the corresponding vibrations across the whole monohalobenzene series of molecules, in the S0, S1, and D0 (+) states, gaining insight into vibrational activity and vibrational couplings.

HM⁺-RG complexes (M = group 2 metal; RG = rare gas): Physical vs. chemical interactions.

Previous work on the HM(+)-He complexes (M = Be-Ra) has been extended to the cases of the heavier rare gas atoms, HM(+)-RG (RG = Ne-Rn). Optimized geometries and harmonic vibrational frequencies have been calculated using MP2 theory and quadruple-ζ quality basis sets. Dissociation energies for the loss of the rare gas atom have been calculated at these optimized geometries using coupled cluster with single and double excitations and perturbative triples, CCSD(T)theory, extrapolating interaction energies to the basis set limit. Comparisons are made between the present data and the previously obtained helium results, as well as to those of the bare HM(+) molecules; furthermore, comparisons are made to the related M(+)-RG and M(2+)-RG complexes. Partial atomic charge analyses have also been undertaken, and these used to test a simple charge-induced dipole model. Molecular orbital diagrams are presented together with contour plots of the natural orbitals from the quadratic configuration with single and double excitations (QCISD) density. The conclusion is that the majority of these complexes are physically bound, with very little sharing of electron density; however, for M = Be, and to a lesser extent M = Mg, some evidence for chemical effects is seen in HM(+)-RG complexes involving RG atoms with the higher atomic numbers.

HM⁺ and HM⁺­He (M = Group 2 metal): chemical or physical interactions?

We investigate the HM(+)­He complexes (M = Group 2 metal) using quantum chemistry. Equilibrium geometries are linear for M = Be and Mg, and bent for M = Ca-Ra; the explanation for this lies in the differing nature of the highest occupied molecular orbitals in the two sets of complexes. The difference primarily occurs as a result of the formation of the H-M(+) bond, and so the HM(+) diatomics are also studied as part of the present work. The position of the He atom in the complexes is largely determined by the form of the electron density. HM(+)…He binding energies are obtained and are surprisingly high for a helium complex. The HBe(+)…He value is almost 3000 cm(-1), which is high enough to suspect contributions from chemical bonding. This is explored by examining the natural orbital density and by population analyses.

Comparison of the interactions in the rare gas hydride and Group 2 metal hydride anions.

We study both the rare gas hydride anions, RG-H(-) (RG = He-Rn) and Group 2 (Group IIa) metal hydride anions, MIIaH(-) (MIIa = Be-Ra), calculating potential energy curves at the CCSD(T) level with augmented quadruple and quintuple basis sets, and extrapolating the results to the basis set limit. We report spectroscopic parameters obtained from these curves; additionally, we study the Be-He complex. While the RG-H(-) and Be-He species are weakly bound, we show that, as with the previously studied BeH(-) and MgH(-) species, the other MIIaH(-) species are strongly bound, despite the interactions nominally also being between two closed shell species: M(ns(2)) and H(-)(1s(2)). We gain insight into the interactions using contour plots of the electron density changes and population analyses. For both series, the calculated dissociation energy is significantly less than the ion/induced-dipole attraction term, confirming that electron repulsion is important in these species; this effect is more dramatic for the MIIaH(-) species than for RG-H(-). Our analyses lead us to conclude that the stronger interaction in the case of the MIIaH(-) species arises from sp and spd hybridization, which allows electron density on the MIIa atom to move away from the incoming H(-).

Vibrations of the S1 state of fluorobenzene-h5 and fluorobenzene-d5 via resonance-enhanced multiphoton ionization (REMPI) spectroscopy.

We report resonance-enhanced multiphoton ionization spectra of the isotopologues fluorobenzene-h5 and fluorobenzene-d5. By making use of quantum chemical calculations, the changes in the wavenumber of the vibrational modes upon deuteration are examined. Additionally, the mixing of vibrational modes both between isotopologues and also between the two electronic states is discussed. The isotopic shifts lead to dramatic changes in the appearance of the spectrum as vibrations shift in and out of Fermi resonance. Assignments of the majority of the fluorobenzene-d5 observed bands are provided, aided by previous results on fluorobenzene-h5.

Interaction of the NO 3pπ Rydberg state with Ar: potential energy surfaces and spectroscopy.

We present the experimental and simulated (2+1) REMPI spectrum of the C(2)Π state of the NO-Ar complex, in the vicinity of the 3p Rydberg state of NO. Two Rydberg states of NO are expected in this energy region: the C(2)Π (3pπ) and D(2)Σ(+) (3pσ) states, and we concentrate on the former here. When the C(2)Π (3pπ) state interacts with Ar at nonlinear orientations, the symmetry is lowered to C(s), splitting the degeneracy of the (2)Π state to yield C((2)A") and C((2)A') states. For these two states of NO-Ar, we calculate potential energy surfaces using second order Møller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction to describe the excited states, the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectrum, which is in excellent agreement with experiment, providing assignments for the observed spectral lines from the calculated rovibrational wavefunctions.

Interactions in the B(+)-RG complexes and comparison with Be(+)-RG (RG = He-Rn): evidence for chemical bonding.

Potential energy curves for the interaction of B(+) ((1)S) with RG ((1)S), RG = He-Rn, have been calculated at the CCSD(T) level of theory employing quadruple-ζ and quintuple-ζ quality basis sets. The interaction energies from these curves were subsequently point-by-point extrapolated to the basis set limit. Rovibrational energy levels have been calculated for each extrapolated curve, from which spectroscopic parameters are determined. These are compared to previously determined experimental and theoretical values. The potentials have also been employed to calculate the transport coefficients for B(+) traveling through a bath of RG atoms. We also investigate the interactions between B(+) and the rare gases via contour plots, natural population analysis (NPA), and molecular orbital diagrams. In addition, we consider the atoms-in-molecules (AIM) parameters. The interactions here are compared and contrasted with those for Li(+)-He and Be(+)-RG; it is concluded that there is significant and increasing dative covalent bonding for the Be(+)-RG and B(+)-RG complexes for RG = Ar-Rn, while the other species are predominantly physically bound.

Spectroscopy of the à state of NO-alkane complexes (alkane = methane, ethane, propane, and n-butane).

We have recorded (1+1) resonance-enhanced multiphoton ionization spectra of complexes formed between NO and the alkanes: CH(4), C(2)H(6), C(3)H(8), and n-C(4)H(10). The spectra correspond to the à ← X̃ transition, which is a NO-localized 3s ← 2pπ* transition. In line with previous work, the spectrum for NO-CH(4) has well-defined structure, but this is only partially resolved for the other complexes. The spectra recorded in the NO(+)-alkane mass channels all show a slowly rising onset, followed by a sharp offset, which is associated with dissociation of NO-alkane, from which binding energies in the X̃ and à states are deduced. Beyond this sharp offset, there is a further rise in signal, which is attributed to fragmentation of higher complexes, NO-(alkane)(n). Analysis of these features allows binding energies for (NO-alkane)···alkane to be estimated, and these suggest that in the NO-(alkane)(2) complexes, the second alkane molecule is bound to the first, rather than to NO. Calculated structures for the 1:1 complexes are reported, as well as binding energies.

Bioinspired Materials: From Living Systems to New Concepts in Materials Chemistry.

Nature successfully employs inorganic solid-state materials (i.e., biominerals) and hierarchical composites as sensing elements, weapons, tools, and shelters. Optimized over hundreds of millions of years under evolutionary pressure, these materials are exceptionally well adapted to the specifications of the functions that they perform. As such, they serve today as an extensive library of engineering solutions. Key to their design is the interplay between components across length scales. This hierarchical design-a hallmark of biogenic materials-creates emergent functionality not present in the individual constituents and, moreover, confers a distinctly increased functional density, i.e., less material is needed to provide the same performance. The latter aspect is of special importance today, as climate change drives the need for the sustainable and energy-efficient production of materials. Made from mundane materials, these bioceramics act as blueprints for new concepts in the synthesis and morphosynthesis of multifunctional hierarchical materials under mild conditions. In this review, which also may serve as an introductory guide for those entering this field, we demonstrate how the pursuit of studying biomineralization transforms and enlarges our view on solid-state material design and synthesis, and how bioinspiration may allow us to overcome both conceptual and technical boundaries.

Universal structure motifs in biominerals: a lesson from nature for the efficient design of bioinspired functional materials.

Biominerals are typically indispensable structures for their host organism in which they serve varying functions, such as mechanical support and protection, mineral storage, detoxification site, or as a sensor or optical guide. In this perspective article, we highlight the occurrence of both structural diversity and uniformity within these biogenic ceramics. For the first time, we demonstrate that the universality-diversity paradigm, which was initially introduced for proteins by Buehler et al. (Cranford & Buehler 2012 Biomateriomics; Cranford et al. 2013 Adv. Mater.25, 802-824 (doi:10.1002/adma.201202553); Ackbarow & Buehler 2008 J. Comput. Theor. Nanosci.5, 1193-1204 (doi:10.1166/jctn.2008.001); Buehler & Yung 2009 Nat. Mater.8, 175-188 (doi:10.1038/nmat2387)), is also valid in the realm of biomineralization. A nanogranular composite structure is shared by most biominerals which rests on a common, non-classical crystal growth mechanism. The nanogranular composite structure affects various properties of the macroscale biogenic ceramic, a phenomenon we attribute to emergence. Emergence, in turn, is typical for hierarchically organized materials. This is a clear call to renew comparative studies of even distantly related biomineralizing organisms to identify further universal design motifs and their associated emergent properties. Such universal motifs with emergent macro-scale properties may represent an unparalleled toolbox for the efficient design of bioinspired functional materials.

ß-Amyloid impairs axonal BDNF retrograde trafficking.

The neurotrophin, brain-derived neurotrophic factor (BDNF), is essential for synaptic function, plasticity and neuronal survival. At the axon terminal, when BDNF binds to its receptor, tropomyosin-related kinase B (TrkB), the signal is propagated along the axon to the cell body, via retrograde transport, regulating gene expression and neuronal function. Alzheimer disease (AD) is characterized by early impairments in synaptic function that may result in part from neurotrophin signaling deficits. Growing evidence suggests that soluble ß-amyloid (Aß) assemblies cause synaptic dysfunction by disrupting both neurotransmitter and neurotrophin signaling. Utilizing a novel microfluidic culture chamber, we demonstrate a BDNF retrograde signaling deficit in AD transgenic mouse neurons (Tg2576) that can be reversed by γ-secretase inhibitors. Using BDNF-GFP, we show that BDNF-mediated TrkB retrograde trafficking is impaired in Tg2576 axons. Furthermore, Aß oligomers alone impair BDNF retrograde transport. Thus, Aß reduces BDNF signaling by impairing axonal transport and this may underlie the synaptic dysfunction observed in AD.