Holger F. Bettinger

Synthesis, stability, and photochemistry of pentacene, hexacene, and heptacene: a matrix isolation study.

The photochemical bisdecarbonylation of bridged alpha-diketones (Strating-Zwanenburg reaction) to give the oligoacenes pentacene (2), hexacene (3), and heptacene (4) is investigated in solid inert gas matrices at cryogenic temperatures. The photodecomposition using visible light irradiation cleanly produces the corresponding oligoacene without formation of observable intermediates. This synthetic approach to the higher acenes allows a comprehensive comparative study of their electronic absorption and infrared spectral properties under identical conditions for the first time. In addition, the route makes it possible to investigate the thermal and photochemical stability of these higher acenes and addresses the problem of heptacene stability which dates back almost 70 years. This largest known member of the acene series is found to be unstable at room temperature. Furthermore, all oligoacenes 2-4 undergo a photoredox reaction upon 185 nm excitation, resulting in the concurrent formation of radical cations and anions in the noble gas matrix. These polaron states of the oligoacenes are stable under the conditions of their generation but collapse to the uncharged acenes upon visible light irradiation.

Electronic structure of higher acenes and polyacene: The perspective developed by theoretical analyses

The hypothetical polymer obtained by linear annelation of benzene units, polyacene (PAC) (C4H2)n, has received considerable attention over the last 50 years. This interest is due to the unusual electronic structure that is assumed to result in usual physical properties. The review summarizes the theoretical investigations of PAC research. The most recent computational analyses available in the literature are based on density functional theory (DFT) for PAC and on the complete active space self-consistent field (CASSCF) method for oligoacenes and suggest an undistorted symmetrical structure with an antiferromagnetic (AFM) coupling of electrons.

Metal‐Free Conversion of Methane and Cycloalkanes to Amines and Amides by Employing a Borylnitrene

The selective transformation of methane into more reactive molecules, considered to be one of the “holy grails” of chemistry, lies at the heart of our understanding of chemical reactivity and has potentially far-reaching practical implications. The challenge in achieving this transformation arises from the low reactivity of methane. An economically feasible process for C H activation is not available, but is highly desirable owing to the abundance of methane as the major constituent of natural gas. Although natural gas is the most abundant, low-cost, carbon-based feedstock, most basic chemicals are produced today indirectly from petroleum in energy-extensive processes. While superacids, free radicals and radical cations, and enzymatic systems can be used to functionalize simple hydrocarbons, much success has been achieved in the field of transition-metal chemistry. A typical theme of transitionmetal-mediated C H bond activation is the oxidative addition of an alkane to a coordinatively unsaturated metal center [LnM ] [Eq. (1)], which is usually generated in situ by thermal or photochemical decomposition of a suitable precursor. The alkane RH acts then as a nucleophile towards the electrophilic metal center [LnM ] [Eq. (1)].

Construction of an internally B3N3-doped nanographene molecule

The synthesis of a hexa-peri-hexabenzocoronene (HBC) with a central borazine core is described. The solid-state structure of this BN-doped HBC (BN-HBC) is isotypic with that of the parent HBC. Scanning tunneling microscopy shows that BN-HBC lies flat on Au(111) in a two-dimensional pattern.

Reaction of the ethynyl radical, C2H, with methylacetylene, CH3CCH, under single collision conditions: Implications for astrochemistry

The reaction between the ethynyl radical, C2H (X 2Σ+), and methylacetylene (X 1A1′), which yields ethynylallene, pentadiyne, and butadiyne, has been studied at the density functional (B3LYP/6-311+G**) and coupled cluster (coupled-cluster single double perturbative triple/cc-pVTZ) levels of theory. These results agree with data from crossed molecular beam experiments where ethynylallene (10) and pentadiyne (13) have been observed. The C2H(1) radical initially attacks the π system of methylacetylene (2) without an entrance barrier to form Z-1-ethynylpropen-2-yl (3) or Z-2-ethynylpropen-1-yl (4) in highly exothermic reactions. Geometric considerations as well as the computed enthalpies suggest Z-1-ethynylpropen-2-yl (3) to be the dominant initial intermediate. Assuming single collision conditions as found in cold molecular clouds in the interstellar medium and distinct planetary atmospheres, numerous rearrangements may ensue the initial reaction step before ejection of a hydrogen atom or a methyl group relea...

B3N3 Borazine Substitution in Hexa-peri-Hexabenzocoronene: Computational Analysis and Scholl Reaction of Hexaphenylborazine

The doping of graphene molecules by borazine (B(3)N(3)) units may modify the electronic properties favorably. Therefore, the influence of the substitution of the central benzene ring of hexa-peri-hexabenzocoronene (HBC, C(42)H(18)) by an isoelectronic B(3)N(3) ring resulting in C(36)B(3)N(3)H(18) (B3N3HBC) is investigated by computational methods. For comparison, the isoelectronic and isosteric all-B/N molecule B(21)N(21)H(18) (termed BN) and its carbon derivative C(6)B(18)N(18)H(18) (C6BN), obtained by substitution of a central B(3)N(3) by a C(6) ring, are also studied. The substitution of C(6) in the HBC molecule by a B(3)N(3) unit results in a significant change of the computed IR vibrational spectrum between 1400 and 1600 cm(-1) due to the polarity of the borazine core. The properties of the BN molecule resemble those of hexagonal boron nitride, and substitution of the central B(3)N(3) ring by C(6) changes the computed IR vibrational spectrum only slightly. The allowed transitions to excited states associated with large oscillator strengths shift to higher energy upon going from HBC to B3N3HBC, but to lower energy upon going from BN to C6BN. The possibility of synthesis of B3N3HBC from hexaphenylborazine (HPB) using the Scholl reaction (CuCl(2)/AlCl(3) in CS(2)) is investigated. Rather than the desired B3N3HBC an insoluble and X-ray amorphous polymer P is obtained. Its analysis by IR and (11)B magic angle spinning NMR spectroscopy reveals the presence of borazine units. The changes in the (11)B quadrupolar coupling constant C(Q), asymmetry parameter η, and isotropic chemical shift δ(iso)((11)B) with respect to HPB are in agreement with a structural model that includes B3N3HBC-derived monomeric units in polymer P. This indicates that both intra- and intermolecular cyclodehydrogenation reactions take place during the Scholl reaction of HPB.

The Longest Acenes

For a long time, the largest known member of the acene series was hexacene, consisting of six linearly fused benzene rings. The next higher member, heptacene, is so highly reactive that either stabilization using substituents is required or matrix isolation techniques need to be employed for the detection of the parent hydrocarbon. This Record Review summarizes recent research that culminated in the synthesis of substituted and parent nonacene.

On-Surface Synthesis of BN-Substituted Heteroaromatic Networks.

We report on the bottom-up fabrication of BN-substituted heteroaromatic networks achieved by surface-assisted polymerization and subsequent cyclodehydrogenation of specifically designed BN-substituted precursor monomers based on a borazine core structural element. To get insight into the cyclodehydrogenation pathway and the influence of molecular flexibility on network quality, two closely related precursor monomers with different degrees of internal cyclodehydrogenation have been employed. Scanning tunneling microscopy shows that, for both monomers, surface-assisted cyclodehydrogenation allows for complete monomer cyclization and the formation of covalently interlinked BN-substituted polyaromatic hydrocarbon networks on the Ag(111) surface. In agreement with experimental observations, density functional theory calculations reveal a significantly lower energy barrier for the cyclodehydrogenation of the conformationally more rigid precursor monomer, which is also reflected in a higher degree of long-range order of the obtained heteroaromatic network. Our proof-of-concept study will allow for the fabrication of atomically precise substitution patterns within BNC heterostructures.

Dispersion-Driven Conformational Isomerism in σ-Bonded Dimers of Larger Acenes†

known to be a problem in those applications, and it has been investigated in some detail. As the larger acenes have become experimentally accessible, studying their properties seems worthwhile. From a theoretical point of view, the butterfly structure of the dimers with four intramolecularly pstacked acene sub-systems makes them very interesting and challenging molecules. It was recently reported that a tert-butyl substituted hexaphenylethane derivative displays a rare case of bond length isomerism, which is mainly caused by attractive London dispersion interactions between the bulky substituents. Herein, we investigate possible extensions of this concept of dispersion-driven isomerism. It is shown that covalently bonded dimers of larger acenes can display an unusual conformational isomerism that is driven by strong intramolecular dispersion interactions between the p-stacked acene subunits. The two postulated structures are the common open form and a bent conformer with a p-stacked alignment of bent acene subunits (Figure 1). The stacked form was already found by Zade et al. in their theoretical investigation of the dimerization reaction of heptacene, and was described as an intermediate. The acene dimers considered here are certainly not the ideal molecules for making functional systems (such as switches, parts of molecular motors) but should merely serve as model compounds for the basic process of dispersion-driven isomerism. It can be expected that similar double-minimum-shaped potential energy curves (PECs) to those reported here are inherent in other (supramolecular) p-systems. With increasing number of annulated rings, the mostly additive dispersion interaction of the acene subunits should at some point overcome the ring bending and Pauli repulsion energy, and the stacked conformer should become more stable. This is related to the folding of long n-alkane chains, where the folded structure is energetically favored after a distinct turning point, as recently shown by L ttschwager et al. Herein, we present PECs for the symmetric opening of the stacked to the open conformer for the heptacene and nonacene dimers. Opposed to the dimerization reaction itself, this conformational process does not involve any orbital crossing and can thus be investigated using single-reference quantum chemical methods. For the heptacene dimer, accurate wavefunction-based calculations were carried out, employing the efficient LPNO-CEPA implementation by Neese and co-workers. The coupled electron pair approach (CEPA, version “1”) was already shown to yield accurate results for general thermochemistry and non-covalent interactions. Combined with the localized pair natural orbitals (LPNO) approximation, significant computational speedups can be achieved without much loss of accuracy. Application of somewhat simplified computational methods is mandatory as the heptacene dimer comprises 96 atoms, which is rather challenging size for a correlated wavefunction treatment. Concerning the here relevant non-covalent interactions, the LPNO-CEPA reference method was already applied to protein–ligand interaction energies with good success. For comparison, two conceptually different flavors of dispersion corrected density functional theory (DFT) are employed, the well-established atom-pairwise DFT-D3 and a non-local Scheme 1. Prototypical dimerization reaction of larger acenes with an odd number of benzene rings. Figure 1. Stacked (left) and open (right) forms that were used for the construction of the potential energy curve of the heptacene dimer and analogously for the nonacene dimer. The atoms marked in black were kept fixed while relaxing all remaining degrees of freedom. The arrow indicates the distance variable (reaction coordinate) used throughout.

Borazine and Benzene Homo- and Heterodimers

The homodimers of benzene and borazine as well as a heterodimer consisting of one benzene (bz) and one borazine (bor) molecule are investigated using MP2, SCS-MP2, and CCSD(T) theories in conjunction with basis sets of up to quadruple-zeta quality. Dimer geometries were completely optimized using the resolution of the identity approximation of MP2 with a QZVPP basis set and characterized by computation of harmonic vibrational frequencies using triple-zeta basis sets. While significant higher order correlation effects beyond MP2 are important for the benzene dimer, these are very small for the borazine dimer and intermediate for the heterodimer. The spin-component scaling (SCS) correction of MP2 produces binding energies for the borazine dimer that are too low but yields very good agreement with CCSD(T) for the heterodimer. The decrease in the intermolecular distance in the sandwich (S) configurations from bz(2) via bz-bor to bor(2) is accompanied by an increased binding energy and a change from second-order stationary points to a minimum for bor(2). The T isomer is less stable than the S configuration for bor(2), but it is preferred over the S and a parallel-displaced (PD) arrangement in the heterodimer. The following order of stability is obtained for the minima at the extrapolated CCSD(T) level: T(bz-bor) > S(bor(2)) > PD(bz-bor) > PD(bor(2)) > T(bor(2)) > PD(bz(2)). The most stable isomer at all levels of theory, T(bz-bor), features a NH...pi interaction.