The multiradical character of one- and two-dimensional graphene nanoribbons

Multireference Averaged Quadratic Coupled Cluster (MR-AQCC) Study of the Geometries and Energies for ortho-, meta- and para-Benzyne

The diradical benzyne isomers are excellent prototypes for evaluating the ability of an electronic structure method to describe static and dynamic correlation. The benzyne isomers are also interesting molecules with which to study the fundamentals of through-space and through-bond diradical coupling that is important in so many electronic device applications. In the current study, we utilize the multireference methods MC-SCF, MR-CISD, MR-CISD+Q, and MR-AQCC with an (8,8) complete active space that includes the σ, σ*, π and π* orbitals, to characterize the electronic structure of ortho-, meta- and para-benzyne. We also determine the adiabatic and vertical singlet-triplet splittings for these isomers. MR-AQCC and MR-CISD+Q produced energy gaps in good agreement with previously obtained experimental values. Geometries, orbital energies and unpaired electron densities show significant through-space coupling in the o- and m-benzynes, while p-benzyne shows through-bond coupling, explaining the dramatically different singlet-triplet gaps between the three isomers.

On Entangled Singlet Pure Diradicals

This work addresses a class of conjugated hydrocarbons that are expected to be singlet diradicals according to the topological Hückel Hamiltonian while possibly satisfying full on-bond electron pairing. These systems possess two degenerate singly occupied molecular orbitals (SOMOs), but aromaticity brought by properly positioned six-membered rings does prevent Jahn-Teller distortions. Density functional theory (DFT) calculations performed on two emblematic examples confirm the strong bond-length alternation in the closed-shell solutions and the clear spatial symmetry in the open-shell spin-unrestricted determinants, the latter solution always being found to have significantly lower energy. Since the SOMOs are here of different symmetry, the wave function is free from ionic valence-bond component, and spin decontamination of the unrestricted DFT solutions and wave function calculations at the CASSCF-plus-second-order-perturbation level confirm the expected pure diradical character of such molecules. In contrast to disjoint diradicals, the SOMOs of present systems have large amplitudes on neighbor atoms, and we propose to name them entangled pure diradicals, further providing some prescription rules for their design. Additional calculations point out the qualitative contrast between these molecules and the related diradicaloids.

Two-Dimensional Nonbenzenoid Heteroacene Crystals Synthesized via In-Situ Embedding of Ladder Bipyrazinylenes on Au(111)

Precisely introducing topological defects is an important strategy in nanographene crystal engineering because defects can tune π-electronic structures and control molecular assemblies. The synergistic control of the synthesis and assembly of nanographenes by embedding the topological defects to afford two-dimensional (2D) crystals on surfaces is still a great challenge. By in-situ embedding ladder bipyrazinylene (LBPy) into acene, the narrowest nanographene with zigzag edges, we have achieved the precise preparation of 2D nonbenzenoid heteroacene crystals on Au(111). Through intramolecular electrocyclization of o-diisocyanides and Au adatom-directed [2+2] cycloaddition, the nonbenzenoid heteroacene products are produced with high chemoselectivity, and lead to the molecular 2D assembly via LBPy-derived interlocking hydrogen bonds. Using bond-resolved scanning tunneling microscopy, we determined the atomic structures of the nonbenzenoid heteroacene product and diverse organometallic intermediates. The tunneling spectroscopy measurements revealed the electronic structure of the nonbenzenoid heteroacene, which is supported by density functional theory (DFT) calculations. The observed distinct organometallic intermediates during progression annealing combined with DFT calculations demonstrated that LBPy formation proceeds via electrocyclization of o-diisocyanides, trapping of heteroarynes by Au adatoms, and stepwise elimination of Au adatoms.

Synthesis of Tridecacene by Multistep Single-Molecule Manipulation

Acenes represent a unique class of polycyclic aromatic hydrocarbons that have fascinated chemists and physicists due to their exceptional potential for use in organic electronics. While recent advances in on-surface synthesis have resulted in higher acenes up to dodecacene, a comprehensive understanding of their fundamental properties necessitates their expansion toward even longer homologues. Here, we demonstrate the on-surface synthesis of tridecacene via atom-manipulation-induced conformational preparation and dissociation of a trietheno-bridged precursor on a Au(111) surface. The generated tridecacene has been investigated by scanning tunneling microscopy and spectroscopy (STM/STS), combined with first-principles calculations. We observe that the STS transport gap (1.09 eV) shrinks again following the gap reopening of dodecacene (1.4 eV). Spin-polarized density functional theory calculations confirm an antiferromagnetic open-shell ground-state electronic configuration for tridecacene in the gas phase. Interestingly, tridecacene's open-shell character is significantly reduced upon interaction with the Au(111) surface despite being only physisorbed. The interaction with the surface leads to a lowering of the magnetization of tridecacene, a reduced gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), compared to the gas phase, and a reduced relative energy to the nonmagnetic state, making it nearly isoenergetic. These observations show qualitatively that the influence of the Au(111) substrate on the properties of long acenes is significant, which is important for interpreting the measured STS transport gaps. Our work contributes to a fundamental understanding of the electronic properties of long acenes, confirming a nonmonotonous length-dependent HOMO-LUMO gap, and to the development of multistep tip-assisted synthesis of elusive compounds.

Energy Decomposition Scheme for Rectangular Graphene Flakes

We show-to our own surprise-that total electronic energies for a family of m × n rectangular graphene flakes can be very accurately represented by a simple function of the structural parameters m and n with errors not exceeding 1 kcal/mol. The energies of these flakes, usually referred to as multiple zigzag chains Z(m,n), are computed for m, n < 21 at their optimized geometries using the DFTB3 methodology. We have discovered that the structural parameters m and n (and their simple algebraic functions) provide a much better basis for the energy decomposition scheme than the various topological invariants usually used in this context. Most terms appearing in our energy decomposition scheme seem to have simple chemical interpretations. Our observation goes against the well-established knowledge stating that many-body energies are complicated functions of molecular parameters. Our observations might have far-reaching consequences for building accurate machine learning models.

The effect of water on gold supported chiral graphene nanoribbons: rupture of conjugation by an alternating hydrogenation pattern

In the last few years we have observed a breakpoint in the development of graphene-derived technologies, such as liquid phase filtering and their application to electronics. In most of these cases, they imply exposure of the material to solvents and ambient moisture, either in the fabrication of the material or the final device. The present study demonstrates the sensitivity of graphene nanoribbon (GNR) zigzag edges to water, even in extremely low concentrations. We have addressed the unique reactivity of (3,1)-chiral GNR with moisture on Au(111). Water shows a reductive behaviour, hydrogenating the central carbon of the zigzag segments. By combining scanning tunnelling microscopy (STM) with simulations, we demonstrate how their reactivity reaches a thermodynamic limit when half of the unit cells are reduced, resulting in an alternating pattern of hydrogenated and pristine unit cells starting from the terminal segments. Once a quasi-perfect alternation is reached, the reaction stops regardless of the water concentration. The hydrogenated segments limit the electronic conjugation of the GNR, but the reduction can be reversed both by tip manipulation and annealing. Selective tip-induced dehydrogenation allowed the stabilization of radical states at the edges of the ribbons, while the annealing of the sample completely recovered the original, pristine GNR.

Pushing Up the Size Limit of Boron-doped peri-Acenes: Modular Synthesis and Characterizations

Heteroatom-doped peri-acenes (PAs) have recently attracted considerable attention considering their fascinating physical properties and chemical stability. However, the precise sole addition of boron atoms along the zigzag edges of PAs remains challenging, primarily due to the limited synthetic approach. Herein, we present a novel one-pot modular synthetic strategy toward unprecedented boron-doped PAs (B-PAs), including B-[4,2]PA (1 a-2), B-[4,3]PA (1 b-2) and B-[7,2]PA (1 c-3) derivatives, through efficient intramolecular electrophilic borylation. Their chemical structures are unequivocally confirmed with a combination of mass spectrometry, NMR, and single-crystal X-ray diffraction analysis. Notably, 1 b-2 exhibits an almost planar geometry, whereas 1 a-2 displays a distinctive bowl-like distortion. Furthermore, the optoelectronic properties of this series of B-PAs are thoroughly investigated by UV/Vis absorption and fluorescence spectroscopy combined with DFT calculation. Compared with their parent all-carbon analogs, the obtained B-PAs exhibit high stability, wide energy gaps, and high photoluminescence quantum yields of up to 84 %. This study reveals the exceptional ability of boron doping to finely tune the physicochemical properties of PAs, showcasing their potential applications in optoelectronics.

Chemically accurate singlet-triplet gaps of organic chromophores and linear acenes by the random phase approximation and σ-functionals

Predicting the energy differences between different spin-states is challenging for many widely used ab initio electronic structure methods. We here assess the ability of the direct random phase approximation (dRPA), dRPA plus two different screened second-order exchange (SOX) corrections, and σ-functionals to predict adiabatic singlet-triplet gaps. With mean absolute deviations of below 0.1 eV to experimental reference values, independent of the Kohn-Sham starting point, dRPA and σ-functionals accurately predict singlet-triplet gaps of 18 organic chromophores. The addition of SOX corrections to dRPA considerably worsens agreement with experiment, adding to the mounting evidence that dRPA+SOX methods are not generally applicable beyond-RPA methods. Also for a series of linear acene chains with up to ten fused rings, dRPA, and σ-functionals are in excellent agreement with coupled-cluster single double triple reference data. In agreement with advanced multi-reference methods, dRPA@PBE and σ-functional@PBE predict a singlet ground state for all chain lengths, while dRPA@PBE0 and σ-functional@PBE0 predict a triplet ground state for longer acenes. Our work shows dRPA and σ-functionals to be reliable methods for calculating singlet-triplet gaps in aromatic molecules.

Hexakis-TIPS-Alkynylated Nonacenes: Persistent and Processible

Four substituted nonacenes were prepared and characterized by UV-vis and EPR spectroscopy and X-ray crystallography. The compounds are the most stable and soluble nonacenes to date - due to six strategically placed triisopropylsilyl(TIPS)-ethynyl groups. They are stable for several weeks in the solid state. In dilute solution their half-life is 5-9 h. Crystal structure analyses of two nonacenes prove their structures. A nonacene derivative was tested in a solution-processed transistor and exhibits ambipolar charge transport (μe =0.007 cm2 /Vs; μh =0.023 cm2 /Vs).

Polyradical character assessment using multireference calculations and comparison with density-functional derived fractional occupation number weighted density analysis

The biradicaloid character of different types of polycyclic aromatic hydrocarbons (PAHs) based on small band gaps is an important descriptor to assess their opto-electronic properties. In this work, the unpaired electron densities and numbers of unpaired electrons (NU values) calculated at the high-level multireference averaged quadratic coupled-cluster (MR-AQCC) method are used to develop a test set to assess the capabilities of different biradical descriptors based on density functional theory. A benchmark collection of 29 different compounds has been selected. The DFT descriptors contain primarily the fractional occupation number weighted electron density (FOD) based on simplified thermally-assisted-occupation density functional theory (TAO-DFT) calculations, but the singlet-triplet energy difference and other descriptors denoted as y0 and nLUNO have been considered as well. After adjustment of the literature-recommended finite temperatures, a very good, detailed agreement between unpaired density and FOD analysis is observed which is also manifested in excellent statistical correlations. The other two descriptors also show good correlations even though the absolute scaling is not satisfactory. A new linear fit of FOD data to the MR-AQCC reference values leads to an improved regression relation for determining the recommended finite temperature value in dependence of the Hartree-Fock exchange. This provides the basis for fast and reliable assessment of the biradical character of many classes of PAHs without the need for performing computationally extended MR calculations.

Electronic Structure Modulation of Nanographenes for Second Order Nonlinear Optical Molecular Materials

Nanographenes (NGs) have drawn extensive attention as promising candidates for next-generation optoelectronic and nonlinear optical (NLO) materials, owing to its unique optoelectronic properties and high thermal stability. However, the weak polarity or even non-polarity of NGs (resulting in weak even order NLO properties) and the high chemical reactivity of zigzag edged NGs hinder their further applications in nonlinear optics, thus stabilization (lowering the chemical reactivity) and polarizing the charge distribution in NGs are necessary for such applications of NGs. The fusion of heptagon and pentagon endows the azulene with the character of donor-acceptor, and the B=N unit is isoelectronic to C=C unit. The introduction of polar azulene and BN are idea to polarize and stabilize the electronic structure of NGs for NLO applications. In the present review, a survey on the functionalization and applications of NGs in nonlinear optics is conducted. The engineering of the electronic structure of NGs by topological defects, doping and edge modulation is summarized. Finally, a summary of challenges and perspectives for carbon-based NLO nanomaterials is presented.

Structural stability and the low-lying singlet and triplet states of BN-n-acenes, n = 1-7

The chemical stability and the low-lying singlet and triplet excited states of BN-n-acenes (n = 1-7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet-triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon-only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet-triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN-n-acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low-lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron.

An epitaxial graphene platform for zero-energy edge state nanoelectronics

Graphene's original promise to succeed silicon faltered due to pervasive edge disorder in lithographically patterned deposited graphene and the lack of a new electronics paradigm. Here we demonstrate that the annealed edges in conventionally patterned graphene epitaxially grown on a silicon carbide substrate (epigraphene) are stabilized by the substrate and support a protected edge state. The edge state has a mean free path that is greater than 50 microns, 5000 times greater than the bulk states and involves a theoretically unexpected Majorana-like zero-energy non-degenerate quasiparticle that does not produce a Hall voltage. In seamless integrated structures, the edge state forms a zero-energy one-dimensional ballistic network with essentially dissipationless nodes at ribbon-ribbon junctions. Seamless device structures offer a variety of switching possibilities including quantum coherent devices at low temperatures. This makes epigraphene a technologically viable graphene nanoelectronics platform that has the potential to succeed silicon nanoelectronics.

Polycyclic Hydrocarbons from [4n]Annulenes: Correlation versus Hybridization Forces in the Formation of Diradicaloids

The conceptual connections between [4n] Hückel antiaromaticity, disjoint orbitals, correlation energy, pro-aromaticity and diradical character for a variety of extended π-conjugated systems, including some salient recent examples of nanographenes and polycyclic aromatic radicals, are provided based on their [4n]annulene peripheries. The realization of such structure-property relationships has led to a beneficial pedagogic exercise establishing design guidelines for diradicaloids. The antiaromatic fingerprint of the [4n]annulene peripheries upon orbital interactions due to internal covalent connectors gives insights into the diradicaloid property of a diversity of π-conjugated molecules that have fascinated chemists recently.

Carbon-based nanostructures as a versatile platform for tunableπ-magnetism

Emergence ofπ-magnetism in open-shell nanographenes has been theoretically predicted decades ago but their experimental characterization was elusive due to the strong chemical reactivity that makes their synthesis and stabilization difficult. In recent years, on-surface synthesis under vacuum conditions has provided unprecedented opportunities for atomically precise engineering of nanographenes, which in combination with scanning probe techniques have led to a substantial progress in our capabilities to realize localized electron spin states and to control electron spin interactions at the atomic scale. Here we review the essential concepts and the remarkable advances in the last few years, and outline the versatility of carbon-basedπ-magnetic materials as an interesting platform for applications in spintronics and quantum technologies.

TAO-DFT fictitious temperature made simple

Over the past few years, thermally-assisted-occupation density functional theory (TAO-DFT) [J.-D. Chai, J. Chem. Phys., 2012, 136, 154104] has been proved to be an efficient electronic structure method for investigating the ground-state properties of large electronic systems with strong static correlation effects. In TAO-DFT, the strength of static correlation in an electronic system at zero temperature is closely related to the so-called fictitious temperature (i.e., the temperature of the corresponding noninteracting reference system). In this work, we propose a simple model to define the optimal system-independent fictitious temperature of a given energy functional in TAO-DFT. Besides, we employ this model to determine the optimal system-independent fictitious temperature of a global hybrid functional in TAO-DFT as a function of the fraction of exact exchange. In addition, we adopt TAO-DFT with various global hybrid functionals and system-independent fictitious temperatures to explore the ground-state properties of several electronic systems with strong static correlation effects, such as the linear acenes and cyclic carbon chains. Furthermore, we discuss the role of exact exchange and an optimal system-independent fictitious temperature in TAO-DFT. Owing to the much reduced self-interaction error, TAO-DFT with exact exchange and an optimal system-independent fictitious temperature can accurately predict the radical character and bond length alternation of cyclic carbon chains (with even number of carbon atoms), which are challenging problems for traditional electronic structure methods.

Optimal system-independent fictitious temperature θ of TAO-GH as a function of the fraction of exact exchange a_x.

One-step functionalization of mildly and strongly reduced graphene oxide with maleimide: an experimental and theoretical investigation of the Diels-Alder [4+2] cycloaddition reaction

For large-scale graphene applications, such as the production of polymer-graphene nanocomposites, exfoliated graphene oxide (GO) and its reduced form (rGO) are presently considered to be very suitable starting materials, showing enhanced chemical reactivity with respect to pristine graphene, in addition to suitable electronic properties (i.e., tunable band gap). Among other chemical processes, a suitable way to obtain surface decoration of graphene is through a direct one-step Diels-Alder (DA) reaction, e.g. through the use of dienophile or diene moieties. However, the feasibility and extent of decoration largely depends on the specific graphene microstructure that in the case of rGO sheets is not easy to control and generally presents a high degree of inhomogeneity owing to various on-plane functionalization (e.g., epoxide and hydroxyl groups) or in-plane lattice defects. In an effort to gain some insights into the covalent functionalization of variably reduced GO samples, we present a combined experimental and theoretical study on the DA cycloaddition reaction of maleimide, a dienophile functional unit well-suited for chemical conjugation of polymers and macromolecules. In particular, we considered both mildly and strongly reduced GOs. Using thermogravimetry, Raman and X-Ray photoelectron spectroscopy, and elemental analysis we show evidence of variable chemical reactivity of rGO as a function of the residual oxygen content. Moreover, from quantum mechanical calculations carried out at the DFT level on different graphene reaction sites, we provide a more detailed molecular view to interpret experimental findings and to assess the reactivity series of different graphene modifications.

Preparative-scale synthesis of nonacene

During the last years we have witnessed progressive evolution of preparation of acenes with length up to dodecacene by on-surface synthesis in ultra-high vacuum or generation of acenes up to decacene in solid matrices at low temperatures. While these protocols with very specific conditions produce the acenes in amount of few molecules, the strategies leading to the acenes in large quantities dawdle behind. Only recently and after 70 years of synthetic attempts, heptacene has been prepared in bulk phase. However, the preparative scale synthesis of higher homologues still remains a formidable challenge. Here we report the preparation and characterisation of nonacene and show its excellent thermal and in-time stability.

Acenes, or linearly fused benzene rings, have both fundamental scientific interest and potential for electronic and material utility, but synthesis of acenes with more than six rings are difficult due to dimerization and degradation. Here the authors prepare nonacene and demonstrate that it is stable in inert conditions.

Variational quantum eigensolver simulations with the multireference unitary coupled cluster ansatz: a case study of the C2v quasi-reaction pathway of beryllium insertion to H2 molecule

Variational quantum eigensolver (VQE)-based quantum chemical calculations have been extensively studied as a computational model using noisy intermediate-scale quantum devices. VQE uses a parametrized quantum circuit defined through an “ansatz”...

Computational predictions of adaptive aromaticity for the design of singlet fission materials

The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.

Electrocatalytic Isoxazoline-Nanocarbon Metal Complexes

We report the synthesis of new carbon-nanomaterial-based metal chelates that enable effective electronic coupling to electrocatalytic transition metals. In particular, multiwalled carbon nanotubes (MWCNTs) and few-layered graphene (FLG) were covalently functionalized by a microwave-assisted cycloaddition with nitrile oxides to form metal-binding isoxazoline functional groups with high densities. The covalent attachment was evidenced by Raman spectroscopy, and the chemical identity of the surface functional groups was confirmed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The functional carbon nanomaterials effectively chelate precious metals Ir(III), Pt(II), and Ru(III), as well as earth-abundant metals such as Ni(II), to afford materials with metal contents as high as 3.0 atom %. The molecularly dispersed nature of the catalysts was confirmed by X-ray absorption spectroscopy (XAS) and energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping. The interplay between the chelate structure on the graphene surface and its metal binding ability has also been investigated by a combination of experimental and computational studies. The defined ligands on the graphene surfaces enable the formation of structurally precise heterogeneous molecular catalysts. The direct attachment of the isoxazoline functional group on the graphene surfaces provides strong electronic coupling between the chelated metal species and the conductive carbon nanomaterial support. We demonstrate that the metal-chelated carbon nanomaterials are effective heterogeneous catalysts in the oxygen evolution reaction with low overpotentials and tunable catalytic activity.

Shedding Light on the Chemistry and the Properties of Münchnone Functionalized Graphene

Münchnones are mesoionic oxazolium 5-oxides with azomethine ylide characteristics that provide pyrrole derivatives by a 1,3-dipolar cycloaddition (1,3-DC) reaction with acetylenic dipolarophiles. Their reactivity was widely exploited for the synthesis of small molecules, but it was not yet investigated for the functionalization of graphene-based materials. Herein, we report our results on the preparation of münchnone functionalized graphene via cycloaddition reactions, followed by the spontaneous loss of carbon dioxide and its further chemical modification to silver/nisin nanocomposites to confer biological properties. A direct functionalization of graphite flakes into few-layers graphene decorated with pyrrole rings on the layer edge was achieved. The success of functionalization was confirmed by micro-Raman and X-ray photoelectron spectroscopies, scanning transmission electron microscopy, and thermogravimetric analysis. The 1,3-DC reactions of münchnone dipole with graphene have been investigated using density functional theory to model graphene. Finally, we explored the reactivity and the processability of münchnone functionalized graphene to produce enriched nano biomaterials endowed with antimicrobial properties.

Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAH) are a prominent substance class with a variety of applications in molecular materials science. Their electronic properties crucially depend on the bond topology in ways that are often highly non-intuitive. Here, we study, using density functional theory, the triplet states of four biphenylene-derived PAHs finding dramatically different triplet excitation energies for closely related isomeric structures. These differences are rationalised using a qualitative description of Clar sextets and Baird quartets, quantified in terms of nucleus independent chemical shifts, and represented graphically through a recently developed method for visualising chemical shielding tensors (VIST). The results are further interpreted in terms of a 2D rigid rotor model of aromaticity and through an analysis of the natural transition orbitals involved in the triplet excited states showing good consistency between the different viewpoints. We believe that this work constitutes an important step in consolidating these varying viewpoints of electronically excited states.

Tuning the edge states in X-type carbon based molecules for applications in nonlinear optics

Novel carbon based "X-type" graphene nanoribbons (GNRs) with azulenes were designed for applications in nonlinear optics in the present work, and the second order nonlinear optical (NLO) properties of those X-type GNRs were predicted using the sum-over-states (SOS) model. The GNRs with edge states are feasibly polarized. The effects of zigzag edges on the NLO properties of GNRs are scrutinized by passivation, and the electronic structures of GNRs are modulated with heteroatoms at the zigzag edges for improved stability and NLO properties. Those nanomaterials were further functionalized with electron-donating and electron-withdrawing groups (NH₂/NO₂) to enhance the NLO responses, and the connection of those functional groups at the azulene ends play a determinant role in the enhancement of the NLO properties of those X-type nanoribbons, e.g., the static first hyperpolarizability (〈β₀〉) changes from -783.23 × 10^-30 esu to -1421.98 × 10^-30 esu. The mechanism of such an enhancement has been investigated. Through two-dimensional second order NLO spectra simulations, particularly besides the strong electro-optical Pockels effect and optical rectification responses, strong electronic sum frequency generations and difference frequency generations are observed in those GNRs. The strong second order NLO responses of those GNRs in the visible light region bring about potential applications of these carbon nanomaterials in nonlinear nanophotonic devices and biological nonlinear optics.

Energetics of Formation of Cyclacenes from 2,3-Didehydroacenes and Implications for Astrochemistry

The carriers of the diffuse interstellar bands (DIBs) are still largely unknown although polycyclic aromatic hydrocarbons, carbon chains, and fullerenes are likely candidates. A recent analysis of the properties of n-acenes of general formula C4n+2 H2n+4 suggested that these could be potential carriers of some DIBs. Dehydrogenation reactions of n-acenes after absorption of an interstellar UV photon may result in dehydroacenes. Here the reaction energies and barriers for formation of n-cyclacenes from 2,3-didehydroacenes (n-DDA) by intramolecular Diels-Alder reaction to dihydro-etheno-cyclacenes (n-DEC) followed by ejection of ethyne by retro-Diels-Alder reactions are analyzed using thermally assisted occupation density functional theory (TAO-DFT) for n=10-20. It is found that the barriers for each of the steps depend on the ring strain of the underlying n-cyclacene, and that the ring strain of n-DEC is about 75 % of that of the corresponding n-cyclacene. In each case, ethyne extrusion is the step with the highest energy barrier, but these barriers are smaller than CH bond dissociation energies, suggesting that formation of cyclacenes is an energetically conceivable fate of n-acenes after multiple absorption of UV photons.

Pushing the Limits of Acene Chemistry: The Recent Surge of Large Acenes

Acenes, consisting of linearly fused benzene rings, are an important fundamental class of organic compounds with various applications. Hexacene is the largest acene that was synthesized and isolated in the 20th century. The next largest member of the acene family, heptacene, was observed in 2007 and since then significant progress in preparing acenes has been reported. Significantly larger acenes, up to undecacene, could be studied by means of low-temperature matrix isolation spectroscopy with in situ photolytic generation, and up to dodecacene by means of on-surface synthesis employing innovative precursors and highly defined crystalline metal surfaces under ultrahigh vacuum conditions. The review summarizes recent experimental and theoretical advances in the area of acenes that give a significantly deeper insight into the fundamental properties and nature of the electronic structure of this fascinating class of organic compounds.

Excitonic and charge transfer interactions in tetracene stacked and T-shaped dimers

Extended quantum chemical calculations were performed for the tetracene dimer to provide benchmark results, analyze the excimer survival process, and explore the possibility of using long-range-corrected (LC) time-dependent second-order density functional tight-biding (DFTB2) for this system. Ground- and first-excited-state optimized geometries, vertical excitations at relevant minima, and intermonomer displacement potential energy curves (PECs) were calculated for these purposes. Ground-state geometries were optimized with the scaled-opposite-spin (SOS) second-order Møller-Plesset perturbation (MP2) theory and LC-DFT (density functional theory) and LC-DFTB2 levels. Excited-state geometries were optimized with SOS-ADC(2) (algebraic diagrammatic construction to second-order) and the time-dependent approaches for the latter two methods. Vertical excitations and PECs were compared to multireference configuration interaction DFT (DFT/MRCI). All methods predict the lowest-energy S₀ conformer to have monomers parallel and rotated relative to each other and the lowest S₁ conformer to be of a displaced-stacked type. LC-DFTB2, however, presents some relevant differences regarding other conformers for S₀. Despite some state-order inversions, overall good agreement between methods was observed in the spectral shape, state character, and PECs. Nevertheless, DFT/MRCI predicts that the S₁ state should acquire a doubly excited-state character relevant to the excimer survival process and, therefore, cannot be completely described by the single reference methods used in this work. PECs also revealed an interesting relation between dissociation energies and the intermonomer charge-transfer interactions for some states.

On-surface synthesis of graphene nanostructures with π-magnetism

Graphene nanostructures (GNs) including graphene nanoribbons and nanoflakes have attracted tremendous interest in the field of chemistry and materials science due to their fascinating electronic, optical and magnetic properties. Among them, zigzag-edged GNs (ZGNs) with precisely-tunable π-magnetism hold great potential for applications in spintronics and quantum devices. To improve the stability and processability of ZGNs, substitutional groups are often introduced to protect the reactive edges in organic synthesis, which renders the study of their intrinsic properties difficult. In contrast to the conventional wet-chemistry method, on-surface bottom-up synthesis presents a promising approach for the fabrication of both unsubstituted ZGNs and functionalized ZGNs with atomic precision via surface-catalyzed transformation of rationally-designed precursors. The structural and spin-polarized electronic properties of these ZGNs can then be characterized with sub-molecular resolution by means of scanning probe microscopy techniques. This review aims to highlight recent advances in the on-surface synthesis and characterization of a diversity of ZGNs with π-magnetism. We also discuss the important role of precursor design and reaction stimuli in the on-surface synthesis of ZGNs and their π-magnetism origin. Finally, we will highlight the existing challenges and future perspective surrounding the synthesis of novel open-shell ZGNs towards next-generation quantum technology.

Synthesis, crystal structure and charge transport characteristics of stable peri-tetracene analogues

peri-Acenes have shown great potential for use as functional materials because of their open-shell singlet biradical character. However, only a limited number of peri-acene derivatives larger than peri-tetracene have been synthesized to date, presumably owing to the low stability of the target compounds in addition to the complicated synthesis scheme. Here, a very simple synthesis route for the tetrabenzo[a,f,j,o]perylene (TBP) structure enables the development of highly stable peri-tetracene analogues. Despite a high degree of singlet biradical character, the compounds with four substituents at the zigzag edge show a remarkable stability in solution under ambient conditions, which is better than that of acene derivatives with a closed-shell electronic configuration. The crystal structures of the TBP derivatives were obtained for the first time; these are valuable to understand the relationship between the structure and biradical character of peri-acenes. The application of peri-acenes in electronic devices should also be investigated. Therefore, the semiconducting properties of the TBP derivative were investigated by fabricating the field-effect transistors.

Highly stable peri-tetracene analogues with a high degree of singlet biradical character were synthesized in a very simple route, and their crystal structures and semiconducting properties were investigated.

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

CASSCF with Extremely Large Active Spaces Using the Adaptive Sampling Configuration Interaction Method

The complete active space self-consistent field (CASSCF) method is the principal approach employed for studying strongly correlated systems. However, exact CASSCF can only be performed on small active spaces of ∼20 electrons in ∼20 orbitals due to exponential growth in the computational cost. We show that employing the Adaptive Sampling Configuration Interaction (ASCI) method as an approximate Full CI solver in the active space allows CASSCF-like calculations within chemical accuracy (<1 kcal/mol for relative energies) in active spaces with more than ∼50 active electrons in ∼50 active orbitals, significantly increasing the sizes of systems amenable to accurate multiconfigurational treatment. The main challenge with using any selected CI-based approximate CASSCF is the orbital optimization problem; they tend to exhibit large numbers of local minima in orbital space due to their lack of invariance to active-active rotations (in addition to the local minima that exist in exact CASSCF). We highlight methods that can avoid spurious local extrema as a practical solution to the orbital optimization problem. We employ ASCI-SCF to demonstrate a lack of polyradical character in moderately sized periacenes with up to 52 correlated electrons and compare against heat-bath CI on an iron porphyrin system with more than 40 correlated electrons.

Two Cycling Centers in One Molecule: Communication by Through-Bond Interactions and Entanglement of the Unpaired Electrons

Many applications in quantum information science (QIS) rely on the ability to laser-cool molecules. The scope of applications can be expanded if laser-coolable molecules possess two or more cycling centers, i.e., moieties capable of scattering photons via multiple absorption-emission events. Here we employ the equation-of-motion coupled-cluster method for double electron attachment (EOM-DEA-CCSD) to study the electronic structure of hypermetallic molecules with two alkaline-earth metals connected by an acetylene linker. The electronic structure of the molecules is similar to that of two separated alkali metals; however, the interaction between the two electrons is weak and largely dominated by through-bond interactions. The communication between the two cycling centers is quantified by the extent of the entanglement of the two unpaired electrons associated with the two cycling centers. This contribution highlights the rich electronic structure of hypermetallic molecules that may advance various applications in QIS and beyond.

Mapping the optoelectronic property space of small aromatic molecules

Small aromatic molecules and their quinone derivatives find use in organic transistors, solar-cells, thermoelectrics, batteries and photocatalysts. These applications exploit the optoelectronic properties of these molecules and the ease by which such properties can be tuned by the introduction of heteroatoms and/or the addition of functional groups. We perform a high-throughput virtual screening using the xTB family of density functional tight-binding methods to map the optoelectronic property space of ~250,000 molecules. The large volume of data generated allows for a broad understanding of how the presence of heteroatoms and functional groups affect the ionisation potential, electron affinity and optical gap values of these molecular semiconductors, and how the structural features - on their own or in combination with one another - allow access to particular regions of the optoelectronic property space. Finally, we identify the apparent boundaries of the optoelectronic property space for these molecules: regions of property space that appear off limits for any small aromatic molecule.

Toward an understanding of electronic excitation energies beyond the molecular orbital picture

Can we gain an intuitive understanding of excitation energies beyond the molecular picture?

Excited States and Optical Properties of Hydrogen-Passivated Rectangular Graphenes: A Computational Study

In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) π-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configurationinteraction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects in accurate description of the excited states. In addition to the optical gap, spin gap of each molecule was also computed using the same methodology. Calculated spin gaps exhibit a decreasing trend with the increasing sizes of the molecules, suggesting that the infinite graphene has a vanishing spin gap.

Benzo-Fused Periacenes or Double Helicenes? Different Cyclodehydrogenation Pathways on Surface and in Solution

Controlling the regioselectivity of C-H activation in unimolecular reactions is of great significance for the rational synthesis of functional graphene nanostructures, which are called nanographenes. Here, we demonstrate that the adsorption of tetranaphthyl- p-terphenyl precursors on metal surfaces can completely change the cyclodehydrogenation route and lead to obtaining planar benzo-fused perihexacenes rather than double [7]helicenes during solution synthesis. The course of the on-surface planarization reactions is monitored using scanning probe microscopy, which unambiguously reveals the formation of dibenzoperihexacenes and the structures of reaction intermediates. The regioselective planarization can be attributed to the flattened adsorption geometries and the reduced flexibility of the precursors on the surfaces, in addition to the different mechanism of the on-surface cyclodehydrogenation from that of the solution counterpart. We have further achieved the on-surface synthesis of dibenzoperioctacene by employing a tetra-anthryl- p-terphenyl precursor. The energy gaps of the new nanographenes are measured to be approximately 2.1 eV (dibenzoperihexacene) and 1.3 eV (dibenzoperioctacene) on a Au(111) surface. Our findings shed new light on the regioselectivity in cyclodehydrogenation reactions, which will be important for exploring the synthesis of unprecedented nanographenes.