Aggregation of N-Heteropolycyclic Aromatic Molecules: The Acridine Dimer and Trimer

Polycyclic aromatic hydrocarbons and their nitrogen-substituted analogues are of great interest for various applications in organic electronics. The performance of such devices is determined not only by the properties of the single molecules, but also by the structure of their aggregates, which often form via self-aggregation. Gaining insight into such aggregation processes is a challenging task, but crucial for a fine-tuning of the materials properties. In this work, an efficient approach for the generation and characterisation of aggregates is described, based on matrix-isolation experiments and quantum-chemical calculations. This approach is exemplified for aggregation of acridine. The acridine dimer and trimer are thoroughly analysed on the basis of experimental and calculated UV and IR absorption spectra, which agree well with each other. Thereby a novel structure of the acridine dimer is found, which disagrees with a previously reported one. The calculations also show the changes from excitonic coupling towards orbital interactions between two molecules with decreasing distance to each other. In addition, a structure of the trimer is determined. Finally, an outlook is given on how even higher aggregates can be made accessible through experiment.

Detection of diheptacendiyl diradical intermediate in the cycloreversion of diheptacene to heptacene

Cycloreversion of diheptacenes, the covalently bound dimers of heptacene, in the solid state produces heptacene. In addition, diheptacendiyl diradical can be detected by ESR spectroscopy. The diradical has a small singlet-triplet energy gap of -0.02 kJ mol-1 (-4.8 × 10-3 kcal mol-1) in favor of the singlet state and is persistent in solid heptacene.

Isolated Dimers Versus Solid-State Dimers of N-Heteropolycycles: Matrix-Isolation Spectroscopy in Concert with Quantum Chemistry

In this work, matrix-isolation spectroscopy and quantum-chemical calculations are used together to analyse the structure and properties of weakly bound dimers of the two isomers benzo[a]acridine and benzo[c]acridine. Our measured experimental electronic absorbance spectra agree with simulated spectra calculated for the equilibrium structures of the dimers in gas-phase, but in contrast, disagree with the simulated spectra calculated for the structures obtained by optimising the experimental solid-state structures. This highlights the sensitivity of the electronic excitations with respect to the dimer structures. The comparison between the solid-state and gas-phase dimers shows how far the intermolecular interactions could change the geometric and electronic structure in a disordered bulk material or at device interfaces, imposing consequences for exciton and charge mobility and other material properties.

Pyridine Dimers and Their Low-Temperature Isomerization: A High-Resolution Matrix-Isolation Spectroscopy Study

The bonding between two neutral aromatic compounds, especially small ones, has been controversially debated in the last decades, and terms like "π-stacking" had to be revised. Surprisingly, despite of many experimental and computational work, there is still no clear consensus about the structure of and the bonding in the pyridine dimer. In this work, for different isomeric forms of the pyridine dimer, the structures and bonding were elucidated by combining high-resolution matrix-isolation spectroscopic results with quantum-chemical calculations. High-resolution IR spectra of Ne matrices at 4 K containing pyridine were recorded for different concentrations and upon annealing to 10 and 12 K, relying on three isotopologues of pyridine. The spectra show the presence of hydrogen-bonded, T-shaped, and stacked forms of weakly-bound pyridine dimers. Among these, the hydrogen-bonded isomer is identified as the lowest-energy form. The results provide for the first time conclusive information about the interaction between two pyridine dimers.

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.

Benzo-Fused Perylene Oligomers with up to 13 Linearly Annulated Rings

The longer acenes with more than six linearly fused six-membered rings are still fascinating chemists and physicists because of their unique photophysical properties and their high potential for organic electronics applications. Unfortunately, with increasing size (seven and more rings) these compounds rapidly lose chemical stability. Besides kinetic and chemical stabilization approaches introducing either bulky or electron-withdrawing groups or both, such systems also have been stabilized by peri-annulation. Although strictly spoken, these peri-annulated compounds are no longer real acenes, they have fascinating properties as well. Herein, we describe the first synthesis of a new series of peri-annulated acenes with up to 13 linearly fused rings, which is unprecedented till date. Furthermore, this new series contains perylene units connected through benzene rings along their [b,k]edges, responsible for unique absorption and emission properties.

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.

Low-Energy Electronic Excitations of N-Substituted Heteroacene Molecules: Matrix Isolation Spectroscopy in Concert with Quantum-Chemical Calculations

N-Heteropolycycles are attractive as materials in organic electronic devices. However, a detailed understanding of the low-energy electronic excitation characteristics of these species is still lacking. In this work, the matrix isolation technique is applied to obtain high-resolution absorbance spectra for a series of tetracene and core-substituted N-analogues. The experimental electronic excitation spectra obtained for matrix-isolated molecules are then analysed with the help of quantum-chemical calculations. Additional lower energy excitation bands in the spectrum of the core-substituted N-derivatives of tetracene could be explained in terms of intensity borrowing from dipole-forbidden transitions due to Herzberg-Teller vibronic coupling. In the case of tetracene, evidence for the additional formation of London dimers (J aggregates) is found at higher tetracene concentrations in the matrix.

A Practical General Method for the Preparation of Long Acenes

The field of long acenes, the narrowest of the zig-zag graphene nanoribbons, has been an area of significant interest in the past decade because of its potential applications in organic electronics, spintronics and plasmonics. However the low solubility and high reactivity of these compounds has so far hindered their preparation on large scales. We report here a concise strategy for the synthesis of higher acenes through Diels-Alder condensation of arynes with a protected tetraene ketone. After deprotection by cleavage of the ketal, the obtained monoketone precursors cleanly yield the corresponding acenes through quantitative cheletropic thermal decarbonylation in the solid state, at moderate temperatures of 155 to 205 °C. This approach allows the preparation of heptacene, benzo[a]hexacene, cis- and trans-dibenzopentacene and offers a valuable new method for the synthesis of even larger acenes.

Photoconversion of 6,13-α-diketopentacene single crystals exhibiting light intensity-dependent morphological change

Photoconversion of diketopentacene single crystal.

On-surface synthesis of heptacene and its interaction with a metal surface

Heptacene was generated by surface-assisted didecarbonylation of an α-diketone precursor on a Ag(111) surface. Monitoring of the surface reaction and characterization of the adsorbed heptacene was performed with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The surface-assisted formation of heptacene occurs around 460 K. Both the heptacene and the precursor molecules are oriented along the high-symmetry directions of the (111) surface and their molecular π systems face towards the substrate. The interaction with the Ag(111) substrate is not laterally uniform, but appears to be strongest on the central part of the molecule, in line with the expectations from Clar's rule. In the STM images, heptacene shows a dumbbell shape, which may correspond to the substantial out-of-plane deformations of heptacene on Ag(111). As revealed by DFT, the center of the molecule is closer to the surface than the outer parts. In addition, the inner rings are most affected by charge redistribution between surface and molecule. Heptacene acts as an acceptor and receives a negative charge of -0.6e from the Ag(111) surface. Since vacuum-sublimable α-diketone precursors for even larger acenes are available, the approach is promising for the on-surface synthesis of higher acene homologues such as octacene and nonacene.

Strategies for the Synthesis of Higher Acenes

The outstanding performance of pentacene-based molecules in molecular electronics, as well as the predicted enhanced semiconducting properties of extended acenes, have stimulated the development of new synthetic methods and functionalization strategies for the preparation of stable and soluble acenes larger than tetracene with the aim of obtaining improved functional materials.

Facile Synthetic Approach to a Large Variety of Soluble Diarenoperylenes

Fused, extended π-systems such as larger acenes and heteroacenes are interesting compounds for organic thin-film transistors (TFTs). The larger the number of linearly cata-fused rings, the lower the stability of the acenes. By peri-fusion of additional rings, the stabilities can significantly be increased. Here we present a facile approach to use a diborylated dihydroanthracene as precursor to get diareno-fused perylenes in just two steps in high yields. The compounds show pronounced packing in the crystalline states by π-stacking. Promising candidates have been used to fabricate p-channel TFTs by vacuum sublimation showing field-effect mobilities up to 0.12 cm²  V^-1  s^-1 .

London dispersion in molecular chemistry--reconsidering steric effects

London dispersion, which constitutes the attractive part of the famous van der Waals potential, has long been underappreciated in molecular chemistry as an important element of structural stability, and thus affects chemical reactivity and catalysis. This negligence is due to the common notion that dispersion is weak, which is only true for one pair of interacting atoms. For increasingly larger structures, the overall dispersion contribution grows rapidly and can amount to tens of kcal mol(-1) . This Review collects and emphasizes the importance of inter- and intramolecular dispersion for molecules consisting mostly of first row atoms. The synergy of experiment and theory has now reached a stage where dispersion effects can be examined in fine detail. This forces us to reconsider our perception of steric hindrance and stereoelectronic effects. The quantitation of dispersion energy donors will improve our ability to design sophisticated molecular structures and much better catalysts.

Infrared vibrational and electronic transitions in the dibenzopolyacene family

We report experimental spectra in the mid-infrared (IR) and near-IR for a series of dibenzoacenes isolated in Ar matrices. The experiments are supported by Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations with both vibrational and electronic transitions studied. For the neutrals, we find good agreement between the experimental and B3LYP and BP86 results for all species studied. The band at about 1440 cm(-1) carries more intensity than in typical PAHs and increases in intensity with the size of the dibenzoacene molecule. For the ions the B3LYP approach fails to yield reasonable IR spectra for most systems and the BP86 approach is used. Electronic transitions dominate the vibrational bands in the mid-IR region for the large dibenzoacene ions. In spite of the very strong electronic transitions, there is still reasonable agreement between theory and experiment for the vibrational band positions. The experimental and theoretical results for the dibenzoacenes are also compared with those for the polyacenes.

Beyond pentacenes: synthesis and properties of higher acenes

Acenes consist of linearly annulated benzene rings. Their reactivity increases quickly with increasing chain length. Therefore acenes longer than pentacene are very sensitive towards oxygen in the presence of light and thus these molecules have not been well studied or have remained elusive in spite of synthetic efforts dating back to the 1930s. This review gives an historical account of the development of the chemistry of acenes larger than pentacene and summarizes the recent progress in the field including strategies for stabilization of higher acenes up to nonacene.

The influence of terminal push-pull substitution on the electronic structure and optical properties of pentacenes

The synthesis of 2,3-R(2)-9,10-(OMe)(2)-substituted pentacenes (R=OMe, F, Br, CN; 1-4) from 2,3-R(2)-9,10-dimethoxy-6,13-dihydro-6,13-ethanopentacene-15,16-diones (α-diketone-bridged precursors) by photochemically induced bis-decarbonylation (Strating-Zwanenburg reaction) is described. Under matrix-isolation conditions (solid Ar, 10 K) the S(1) transitions of 1 and 2 undergo hypsochromic and those of 3 and 4 bathochromic shifts compared to parent pentacene. The S(1) transition wavelengths correlate well with the difference of substituent parameters σ(p). A computational analysis of the excited states at the CAM-B3LYP/6-311+G** level of theory provides an assignment of the electronic transitions. Photolysis in solution at room temperature yields red [R=OMe (1)], blue [R=Br (3), F (2)], and green [R=CN (4)] pentacenes. The compounds are oxygen-sensitive and have low solubility, but their formation can be monitored by UV/Vis and, in the case of R=CN, also by (1)H NMR spectroscopy. The S(1) transition in 4 does not show the typical pentacene fine structure in the electronic absorption spectrum. Photogeneration in the presence of oxygen leads to a number of photoproducts that could be identified by monitoring the reaction by (1)H NMR spectroscopy for R=OMe.

Higher Order Acenes and Fused Acenes with Near-infrared Absorption and Emission

Higher order acenes and fused acenes represent two outstanding classes of building blocks to achieve near-infrared absorbing and emitting materials. Appropriate chemical modification and functionalization will lead to significant improvements in solubility and stability, thus making their applications practically possible. This short review summarizes the recent developments of the two types of polycyclic aromatic compounds mentioned above based on their physical and optical properties including absorption, emission, quantum yield, solubility, and stability.