Long-term shift towards shady and nutrient-rich habitats in Central European temperate forests.

Biodiversity world-wide has been under increasing anthropogenic pressure in the past century. The long-term response of biotic communities has been tackled primarily by focusing on species richness, community composition and functionality. Equally important are shifts between entire communities and habitat types, which remain an unexplored level of biodiversity change. We have resurveyed > 2000 vegetation plots in temperate forests in central Europe to capture changes over an average of five decades. The plots were assigned to eight broad forest habitat types using an algorithmic classification system. We analysed transitions between the habitat types and interpreted the trend in terms of changes in environmental conditions. We identified a directional shift along the combined gradients of canopy openness and soil nutrients. Nutrient-poor open-canopy forest habitats have declined strongly in favour of fertile closed-canopy habitats. However, the shift was not uniform across the whole gradients. We conclude that the shifts in habitat types represent a century-long successional trend with significant consequences for forest biodiversity. Open forest habitats should be urgently targeted for plant diversity restoration through the implementation of active management. The approach presented here can be applied to other habitat types and at different spatio-temporal scales.

Tailoring Magnetism of Graphene Nanoflakes via Tip-Controlled Dehydrogenation.

Atomically precise graphene nanoflakes called nanographenes have emerged as a promising platform to realize carbon magnetism. Their ground state spin configuration can be anticipated by Ovchinnikov-Lieb rules based on the mismatch of π electrons from two sublattices. While rational geometrical design achieves specific spin configurations, further direct control over the π electrons offers a desirable extension for efficient spin manipulations and potential quantum device operations. To this end, we apply a site-specific dehydrogenation using a scanning tunneling microscope tip to nanographenes deposited on a Au(111) substrate, which shows the capability of precisely tailoring the underlying π-electron system and therefore efficiently manipulating their magnetism. Through first-principles calculations and tight-binding mean-field-Hubbard modeling, we demonstrate that the dehydrogenation-induced Au-C bond formation along with the resulting hybridization between frontier π orbitals and Au substrate states effectively eliminate the unpaired π electron. Our results establish an efficient technique for controlling the magnetism of nanographenes.

π-Radical Cascade to a Chiral Saddle-Shaped Peropyrene.

Reactions of open-shell molecular graphene fragments are typically thought of as undesired decomposition processes because they lead to the loss of desired features like π-magnetism. Oxidative dimerization of phenalenyl to peropyrene shows, however, that these transformations hold promise as a synthetic tool for making complex structures via formation of multiple bonds and rings in a single step. Here, we explore the feasibility of using this "undesired" reaction of phenalenyl to build up strain and provide access to non-planar polycyclic aromatic hydrocarbons. To this end, we designed and synthesized a biradical system with two phenalenyl units linked via a biphenylene backbone. The design facilitates an intramolecular cascade reaction to a helically twisted saddle-shaped product, where the key transformations-ring-closure and ring-fusion-occur within one reaction. The negative curvature of the final peropyrene product, induced by the formed eight-membered ring, was confirmed by single-crystal X-ray diffraction analysis and the helical twist was validated via resolution of the product's enantiomers that display circularly polarized luminescence and high configurational stability.

Boosting quantum yields and circularly polarized luminescence of penta- and hexahelicenes by doping with two BN-groups.

The incorporation of boron-nitrogen (BN) units into polycyclic aromatic hydrocarbons (PAHs) as an isoelectronic replacement of two carbon atoms can significantly improve their optical properties, while the geometries are mostly retained. We report the first non-π-extended penta- and hexahelicenes comprising two aromatic 1,2-azaborinine rings. Comparing them with their all-carbon analogs regarding structural, spectral and (chir)optical properties allowed us to quantify the impact of the heteroatoms. In particular, BN-hexahelicene BN[6] exhibited a crystal structure congruent with its analog CC[6], but displayed a fivefold higher fluorescence quantum yield (φfl = 0.17) and an outstanding luminescence dissymmetry factor (|glum| = 1.33 × 10-2). Such an unusual magnification of both properties at the same time makes BN-helicenes suitable candidates as circularly polarized luminescence emitters for applications in materials science.

Observation of the Magnetic Ground State of the Two Smallest Triangular Nanographenes.

Fusion of three benzene rings in a triangular fashion gives rise to the smallest open-shell graphene fragment, the phenalenyl radical, whose π-extension leads to an entire family of non-Kekulé triangular nanographenes with high-spin ground states. Here, we report the first synthesis of unsubstituted phenalenyl on a Au(111) surface, which is achieved by combining in-solution synthesis of the hydro-precursor and on-surface activation by atomic manipulation, using the tip of a scanning tunneling microscope. Single-molecule structural and electronic characterizations confirm its open-shell S = 1/2 ground state that gives rise to Kondo screening on the Au(111) surface. In addition, we compare the phenalenyl's electronic properties with those of triangulene, the second homologue in the series, whose S = 1 ground state induces an underscreened Kondo effect. Our results set a new lower size limit in the on-surface synthesis of magnetic nanographenes that can serve as building blocks for the realization of new exotic quantum phases of matter.

Dimethylnonacethrene - en route to a magnetic switch.

Dimethylnonacethrene is the first derivative of the cethrene family that is energetically more stable than the product of its electrocyclic ring closure. Compared to the shorter homologue dimethylcethrene, the new system is EPR-active, because of a significantly lowered singlet-triplet gap, and displays remarkable stability. Our results suggest that adjustment of the steric bulk in the fjord region can enable realisation of diradicaloid-based magnetic photoswitches.

The taming of Clar's hydrocarbon.

Triangulene is the smallest non-Kekulé graphene fragment known as Clar's hydrocarbon. Due to its open-shell electronic structure, triangulene is a promising molecular building block of carbon-based organic materials for spintronics and quantum molecular science. It comprises six benzenoid rings arranged in a triangular shape with two unpaired electrons delocalized over the entire conjugated core, making this molecule highly reactive. A triplet ground state is predicted for this hydrocarbon by Ovchinnikov's rule, or Lieb's theorem, in accord with Hund's rule. The pioneering work on triangulene was performed almost 70 years ago by Erich Clar, who attempted to prepare the pristine compound. Since then, several synthetic approaches to prepare this molecule have been exploited. The extreme reactivity of triangulene can be circumvented using on-surface techniques or by installation of sterically demanding substituents, which kinetically stabilize the diradical core against oligomerization in solution. The first two examples of a persistent derivative of triangulene were simultaneously and independently developed last year. This article presents a historical development in the synthesis of triangulene and its derivatives and outlines possible future applications in ferromagnetic materials, electrically conductive polymers or quantum computing.

Subphthalocyanine-triangulene dyads: Property tuning for light-harvesting device applications.

Organic photovoltaics relies on the development of stable chromophores and redox-active organic molecules with tailor-made HOMO/LUMO energies. Here, we present the synthesis and properties of novel dyads composed of boron subphthalocyanine (SubPc) and triangulene units, connected either at the peripheral position of the subphthalocyanine or at the axial boron. The connectivity has strong implications for the absorption and fluorescence properties of the dyads, as well as their redox properties. While the SubPc unit has a bowl shape, triangulene is a planar structural unit that allows dyads to dimerize in the solid state on account of π-stacking interactions as shown by X-ray crystallography of one of the dyads. The electronic properties were also studied computationally by density functional theory methods. Excellent agreement between experimental and computed data were obtained, showing that our computational method is a strong tool in the rational design of optimum molecules to ultimately obtain finely tuned molecules for device applications.

Nonacethrene Unchained: A Cascade to Chiral Contorted Conjugated Hydrocarbon with Two sp3-Defects.

We demonstrate that structurally complex carbon nanostructures can be achieved via a synthetic approach that capitalizes on a π-radical reaction cascade. The cascade is triggered by oxidation of a dihydro precursor of helical diradicaloid nonacethrene to give a chiral contorted polycyclic aromatic hydrocarbon named hypercethrene. In this ten-electron oxidation process, four σ-bonds, one π-bond, and three six-membered rings are formed in a sequence of up to nine steps to yield a 72-carbon-atom warped framework, comprising two configurationally locked [7]helicene units, a fluorescent peropyrene unit, and two precisely installed sp3-defects. The key intermediate in this cascade is a closed nonacethrene derivative with one quaternary sp3-center, presumably formed via an electrocyclic ring closure of nonacethrene, which, when activated by oxidation, undergoes a reaction cascade analogous to the oxidative dimerization of phenalenyl to peropyrene. By controlling the amount of oxidant used, two intermediates and one side product could be isolated and fully characterized, including single-crystal X-ray diffraction analysis, and two intermediates were detected by electron paramagnetic resonance spectroscopy. In concert with density functional theory calculations, these intermediates support the proposed reaction mechanism. Compared to peropyrene, the absorption and emission of hypercethrene are slightly red-shifted on account of extended π-conjugation and the fluorescence quantum yield of 0.45 is decreased by a factor of ∼2. Enantiomerically enriched hypercethrene displays circularly polarized luminescence with a brightness value of 8.3 M-1 cm-1. Our results show that reactions of graphene-based π-radicals-typically considered an "undefined decomposition" of non-zero-spin materials-can be well-defined and selective, and have potential to be transformed into a step-economic synthetic method toward complex carbon nanostructures.

Trimesityltriangulene: a persistent derivative of Clar's hydrocarbon.

Triangulene, known as Clar's hydrocarbon, is a prototypical non-Kekulé diradical comprised of six benzenoid rings fused in a triangular shape. We synthesized and characterized its trimesityl derivative, illustrating that three bulky substituents installed in the centers of the zigzag edges suffice to protect all reactive positions. This work brings prospects to use triangulene and its open-shell analogs in spintronic materials via solution-phase synthesis.

Cooperative Weak Dispersive Interactions Actuate Catalysis in a Shape-Selective Abiological Racemase.

A simple abiological host-guest system demonstrates racemase activity with catalytic rate enhancements of 104 without employing traditional functional groups. Cooperative weak interactions enhanced through shape-complementarity between the catalyst active site and the reaction transition state drive this activity, as proposed by Pauling for enzymes. In analogy to the Jencks' concept of catalytic antibodies, it is shown that a hapten resembling the planar transition state of the bowl inversion acts as a potent inhibitor of this catalytic process. In contrast, no substrate/product inhibition is detected, and a relatively weak binding of the substrate is observed (Ka ≈ 102 M-1 at 293 K). This simple box-and-bowl system demonstrates that shape selectivity arising from cooperative dispersive forces suffices for the emergence of a catalytic system with an enzyme-like thermodynamic profile.

Fullerene Wires Assembled Inside Carbon Nanohoops.

Carbon-nanohoop structures featuring one or more round-shaped cavities represent ideal supramolecular hosts for spherical fullerenes, with potential to form host-guest complexes that perform as organic semiconductors in the solid state. Due to the tight complexation between the shape-complementary hosts and guests, carbon nanohoops have the potential to shield fullerenes from water and oxygen, known to perturb the electron-transport process. Many nanohoop receptors have been found to form host-guest complexes with fullerenes. However, there is only a little or no control over the long-range order of encapsulated fullerenes in the solid state. Consequently, the potential of these complexes to perform as organic semiconductors is rarely evaluated. Herein, we present a survey of all known nanohoop-fullerene complexes, for which the solid-state structures were obtained. We discuss and propose instances where the inclusion fullerene guests form discrete supramolecular wires, which might open up possibilities for their use in electronic devices.

Cycloparaphenylene Double Nanohoop: Structure, Lamellar Packing, and Encapsulation of C60 in the Solid State.

A new member of the cycloparaphenylene double-nanohoop family was synthesized. Its π-framework features two oval cavities that display different shapes depending on the crystallization conditions. Incorporation of the peropyrene bridge within the nanoring cycles via bay-regions alleviates steric effects and thus allows 1:1 complexation with C60 in the solid state. This nanocarbon adopts a lamellar packing motif, and our results suggest that the structural adjustment of this double nanohoop could enable its use in supramolecular and semiconductive materials.

Rules of Nucleophilic Additions to Zigzag Nanographene Diones*.

Nucleophilic addition of carbon-centered nucleophiles to nanographene ketones represents a valuable late-stage method for the functionalization of zigzag nanographenes, but its use is rare in the chemical literature. Using two model systems, non-Kekulé triangulene-4,8-dione and Kekulé anthanthrone, we identify unexpected regioselectivities and uncover the rules that govern these reactions. Considering the large number of nanographene ketones that have been reported since the pioneering work of Eric Clar, this method enables synthesis and exploration of hitherto unknown functionalized nanographenes.

Cycloparaphenylene-Phenalenyl Radical and Its Dimeric Double Nanohoop*.

The first example of a neutral spin-delocalized carbon-nanoring radical was achieved by integration of the open-shell phenalenyl unit into cycloparaphenylene (CPP). Spin distribution in this hydrocarbon is localized primarily on the phenalenyl segment and partially on the CPP segment as a consequence of steric and electronic effects. The resulting geometry is reminiscent of a diamond ring, with pseudo-perpendicular arrangement of the radial and the planar π-surface. The phenylene rings attached directly to the phenalenyl unit give rise to a steric effect that governs a highly selective dimerization pathway, yielding a giant double nanohoop. Its π-framework made of 158 sp2 -carbon atoms was elucidated by single-crystal X-ray diffraction, which revealed a three-segment CPP-peropyrene-CPP structure. This nanocarbon shows a fluorescence profile characteristic of peropyrene, regardless of which segment gets excited. These results in conjunction with DFT suggest that adjusting the size of the CPP segments in this double nanohoop could deliver donor-acceptor systems.

Benzo[cd]triangulene: A Spin 1/2 Graphene Fragment.

How does edge modification affect spin distribution in open-shell graphene fragments? We investigated this effect by analyzing spin-delocalization in benzo[cd]-triangulene, a spin 1/2 graphene fragment composed of seven benzenoid rings fused in a hybrid zigzag/armchair fashion. Six rings of this system form the core of Clar's hydrocarbon triangulene, to which an additional ring is annulated in the zigzag region. The singly occupied molecular orbital (SOMO) of this hydrocarbon radical resembles both SOMOs of triangulene, but the spin density is distributed over the core in a nonuniform fashion. The uneven spin distribution is reflected in the reactivity-reaction with oxygen occurs selectively at a position with the highest spin density-and correlates nicely with relative stabilities of the corresponding Clar resonance structures. The spin distribution is different from that of a topologically similar compound composed of the same number of sp2 carbon atoms but featuring six rings only, illustrating the impact of subtle structural changes on spin-density distribution. This compound was characterized by means of UV-vis and electron paramagnetic resonance spectroscopy, cyclic voltammetry, mass spectrometry, and X-ray crystallography. The experimental results are supported by density functional theory calculations.

Gram-Scale Synthesis and Supramolecular Complex of Precursors of Clar's Hydrocarbon Triangulene.

We present to date the most efficient gram-scale synthesis of triangulene-4,8-dione and 12-hydroxytriangulene-4,8-dione, the precursors of Clar's hydrocarbon, in overall yields >50%. The direct dihydroprecursors of triangulene, obtained upon reduction of triangulene-4,8-dione, were stabilized in a supramolecular complex with a tetracationic cyclophane ExBox4+ and characterized by single-crystal X-ray crystallography. This result represents the first step in an endeavor to stabilize the fragile core of triangulene in an inclusion complex in solution and solid state.

Molecular Magnetic Switches.

The design and synthesis of molecular switches is of growing importance considering the steep increase in the production of consumer electronics in the recent years. The function of these devices is based on binary electronic circuits and can be achieved by use of bistable magnetic materials. This article reviews four types of molecular systems, which can be switched between two spin states in response to an external stimulus, illustrating working principles that can motivate the design of systems for practical applications. As an outlook, organic diradicaloid molecules are introduced as potential molecular magnetic switches that do not rely on the use of metals.

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch.

We describe the synthesis and properties of 13,14-dimethylcethrene, a prototype of a chiral diradicaloid photochemical switch that can be transformed reversibly via conrotatory electrocyclization to its more stable closed form by light (630 nm) or heat and back to its open form by light (365 nm). This system illustrates how the chemical reactivity of a diradicaloid molecule can be translated into a switching function, which alters substantially all electronic parameters, namely, the HOMO-LUMO and the singlet-triplet (ST) energy gaps, and the degree of helical twist. As a result, distinct changes in the optical and chiroptical properties of this system were observed, which allowed us to monitor the switching process by a variety of spectroscopic techniques, including NMR, UV-vis, and CD. In comparison to the previously reported parent molecule cethrene, this system benefits from two methyl substituents installed in the fjord region, which account for the stability of the closed form against oxidation and racemization. The methyl substituents increase the ST energy gap of 13,14-dimethylcethrene by ∼4 kcal mol-1 in comparison to cethrene. Our DFT calculations reveal that the larger ST gap is a result of electronic and geometric effects of the methyl substituents and show the potential of related systems to act as magnetic switches at room temperature.

The Three C's of Cethrene.

Molecules that contain one or more unpaired electrons delocalized within a π-conjugated backbone are promising candidates for applications in spin electronics or simply 'spintronics'. Our group develops functional organic materials based on π-conjugated hydrocarbon molecules, where the electrons are unpaired either in the ground state or in the excited state that is low in energy and can be populated thermally. We aim to learn how to introduce and control a multitude of properties, namely, optical, chiroptical, magnetic, and conductive, in a bulk material made of these molecules, by manipulating spin interactions between the unpaired electrons. The first model system that was developed in our group is a hydrocarbon named cethrene, which has a diradicaloid singlet ground state and a low-lying triplet excited state. In this article, the structural parameters and their impact on the properties and reactivity of cethrene are discussed within the realm of the three C's that symbolize cethrene's C-shape, chirality, and chameleonic reactivity.