Linear C2-symmetric polycyclic benzodithiophene: efficient, highly diversified approaches and the optical properties

Two facile approaches to two new series of the seven-rings fused benzodithiophene-based polycyclic aromatics are developed in good yields.     

Aromatization-driven Grob-fragmentation approach toward polyazaacenes. Synthesis and amination of multi-functionalized diazapentacenes

A general approach toward the synthesis of multi-functionalized diazapentacene derivatives 1, using 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene (TDCp, 2), a substituted benzene-1,2-diamine (ADA, 6), and a naphthalene-1,4-dione (BQ, 3) as the building units, is described. The synthesis basically entails three operations: (i) oxidation of the dichloroetheno-bridge in the Diels-Alder cycloadduct 4 of TDCp and 3, (ii) condensation of the 1,2-diketone 5 thus generated with an ADA to give quinoxaline-fused polycyclic compounds 7, followed by (iii) an one-pot, three-reaction process keyed upon the base- or acid-catalyzed aromatization-driven Grob-type fragmentation to produce quinoxaline ring-embedded diazapentaceneesters 1. The diazapentacene derivative 1a underwent the nucleophilic aromatic ipso-amination with primary and secondary amines to afford the amino-substituted derivatives 12, which tend to self-assemble in solid state driven by the cofacial π-stacking interactions, demonstrated by the crystal packing structures of 12a and 12f.     

N-Doped Nonalternant Nanoribbons with Excellent Nonlinear Optical Performance

Two novel N-doped nonalternant nanoribbons ( NNNR-1 and NNNR-2 ) featuring multiple fused N-heterocycles and bulky solubilizing groups were prepared via bottom-up solution synthesis. NNNR-2 achieves a total molecular length of 33.8 Å, which represents the longest soluble N-doped nonalternant nanoribbon reported to date. The pentagon subunits and doping of N atoms in NNNR-1 and NNNR-2 have successfully regulated their electronic properties, achieving high electron affinity and good chemical stability enabled by the nonalternant conjugation and electronic effects. When applied a laser pulse of 532 nm, the 13-rings nanoribbon NNNR-2 shows outstanding nonlinear optical (NLO) responses, with the nonlinear extinction coefficient of 374 cm GW⁻¹, much higher than those of NNNR-1 (96 cm GW⁻¹) and the well-known NLO material C₆₀ (153 cm GW⁻¹). Our findings indicate that the N-doping of nonalternant nanoribbons is an effective strategy to access another type of excellent material system for high-performance NLO applications, which can be extended to construct numerous heteroatom-doped nonalternant nanoribbons with fine-tunable electronic properties.     

Conformational reactions of D2-symmetric twisted acenes

Computational analyses of the longitudinally twisted polycyclic aromatic hydrocarbons 9,18-diphenyltetrabenz[a,c,h,j]anthracene (1), 9,10,19,20-tetraphenyltetrabenzo[a,c,j,l]naphthacene (2), and 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (3) were performed at the B3LYP/6-31G(d) level of theory. Ground state and transition state structures were located for two classes of conformational reactions in these molecules: twist inversions (enantiomerization or racemization reactions) and phenyl rotations, and the free energies of activation for these processes were calculated. Where possible, the computational results were compared with both new and existing experimental data for the rates of these conformational reactions. Multiple pathways were identified for some processes, but the low-energy transition states were often quite different from those that common chemical intuition might suggest.     

Diazadiboraacenes: Synthesis,Spectroscopy and Computations

The incorporation of heteroatoms into hydrocarbon compounds greatly expands the chemical space of molecular materials. In this context, B−N doping takes a center stage due to its isosterism with a C=C-bond. Herein, we present a new and modular synthetic concept to access novel diazadiborabenzo[b]triphenylenes 7 a–h using the B−N doped biradical 16 as intermediate. Characterization of the photophysical properties revealed the emission spectra of the diazadibora benzo[b]triphenylenes 7 a–h can conveniently be tuned by small changes of the substitution on the boron-atom. All of the diazadibora compounds show a short life-time phosphorescence. Additionally, we were able to rationalize the excited-state relaxation of the diazadiboraacene 7 a via intersystem crossing by quantum chemical calculations. The new synthetic strategy provides an elegant route to various novel B−N doped acenes with great potential for applications in molecular materials.     

On-Surface Synthesis and Determination of the Open-Shell Singlet Ground State of Tridecacene**

The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open-shell character has so far been missing. Here, we present the on-surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many-body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet-triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.     

Methodologies for the synthesis of pentacene and its derivatives

In recent years, due to its high hole-mobility, high on/off current ratio and low threshold voltage, pentacene and its derivatives have found increasing application in the fabrication of light-emitting diodes, field-effect transistors and photovoltaic cells. It has also emerged as an alternative to silicon due to its similar performance to inorganic semiconductors. Pentacene cannot be isolated from the petroleum fractions like other acenes such as anthracene or tetracene, and therefore it needs to be chemically synthesized. The first successful synthesis of pentacene was reported in early 19th century where pentacene was obtained via dehydrogenation of 6,14-dihydropentacene. Since then a number of methods have been reported for the synthesis of pentacene. This review describes various strategies used for the synthesis of pentacene and its derivatives reported since 2005.     

Time-dependent density functional study of the electronic spectra of oligoacenes in the charge states −1, 0, +1, and +2

We present a systematic theoretical study of the five smallest oligoacenes (naphthalene, anthracene, tetracene, pentacene, and hexacene) in their anionic, neutral, cationic, and dicationic charge states. We used density functional theory (DFT) to obtain the ground-state optimised geometries, and time-dependent DFT (TD-DFT) to evaluate the electronic absorption spectra. Total-energy differences enabled us to evaluate the electron affinities and first and second ionisation energies, the quasiparticle correction to the HOMO–LUMO energy gap and an estimate of the excitonic effects in the neutral molecules. Electronic absorption spectra have been computed by combining two different implementations of TD-DFT: the frequency–space method to study general trends as a function of charge-state and molecular size for the lowest-lying in-plane long-polarised and short-polarised π → π^* electronic transitions, and the real-time propagation scheme to obtain the whole photo-absorption cross-section up to the far-UV. Doubly ionised PAHs are found to display strong electronic transitions of π → π^* character in the near-IR, visible, and near-UV spectral ranges, like their singly charged counterparts. While, as expected, the broad plasmon-like structure with its maximum at about 17–18 eV is relatively insensitive to the charge-state of the molecule, a systematic decrease with increasing positive charge of the absorption cross-section between 6 and 12 eV is observed for each member of the class.