(Oligo-)Thiophene Functionalized Tetraazaperopyrenes: Donor–Acceptor Dyes and Ambipolar Organic Semiconductors

Peri-Decoration of a Tetraazaperylene with Urea Units: Chiral Octaazaperopyrenedioxides (OAPPDOs) and Their Optical and Chiroptical Properties

Octaazaperopyrenedioxides (OAPPDOs) are a new class of fluorescent polycyclic aromatic hydrocarbons based on a tetraazaperylene core that is formally condensed with N-substituted urea units in the two opposite peri positions. Here, we report the synthesis of series of substituted OAPPDO derivatives with different N-substitution patterns (H, alkyl, benzyl) in the peri positions, including bay-chlorinated OAPPDOs. Starting from the latter, a series of bay-arylated OAPPDOs was synthesized by Suzuki cross coupling, which resulted in the formation of helically chiral OAPPDO derivatives. The electrochemical and photophysical properties were investigated by UV/Vis and fluorescence spectroscopy as well as cyclic voltammetry. The P and M enantiomers of a phenylated OAPPDO were separated by semipreparative HPLC and further analyzed by CD spectroscopy. The frontier orbital energies, the mechanism of the isomerization, the electronic excitation and the CD spectrum (TD-DFT) were computed and compared to the experimental data. The reversible 1e^- oxidation of the OAPPDOs generates the corresponding radical cations, one of which was characterized by EPR spectroscopy. The reversible oxidation process was also systematically investigated by spectro-electrochemistry.

Small-molecule ambipolar transistors

Ambipolar transistor properties have been observed in various small-molecule materials. Since a small energy gap is necessary, many types of molecular designs including extended π-skeletons as well as the incorporation of donor and acceptor units have been attempted. In addition to the energy levels, an inert passivation layer is important to observe ambipolar transistor properties. Ambipolar transport has been observed in extraordinary π-electron systems such as antiaromatic compounds, biradicals, radicals, metal complexes, and hydrogen-bonded materials. Several donor/acceptor cocrystals show ambipolar transport as well.

Band Formation at Interfaces Between N-Heteropolycycles and Gold Electrodes

Efficient charge injection at organic semiconductor/metal interfaces is crucial for the performance of organic field effect transistors. Interfacial hybrid band formation between electronic states of the organic compound and the metal electrode facilitates effective charge injection. Here, we show that a long-range ordered monolayer of a flat-lying N-heteropolycyclic aromatic compound on Au(111) leads to dispersing occupied and unoccupied interfacial hybrid bands. Using angle-resolved two-photon photoemission we determine their energy level alignment and dispersion relations. We suggest that band formation proceeds via hybridization of a localized occupied molecular state with the d-bands of the Au substrate, where the large effective mass of the d-bands is significantly reduced in the hybrid band. Hybridization of an unoccupied molecular state with the Au sp-band leads to a band with an even smaller effective mass.

An unconventional route to an ambipolar azaheterocycle and its in situ generated radical anion

Herein we report the synthesis, characterization and application of an azaheterocycle 4 obtained via an unprecedented C-N coupling. The neutral azaheterocycle undergoes one-electron reduction to form an air-stable radical anion in situ, which provides added benefit towards operational stability of the device during n-type charge transport. The unusual stability of this radical anion is due to the fact that the fused cyclopentane ring upon reduction forms aromatic cyclopentadienyl anion, and the negative charge delocalizes over the nearly planar azaheterocycle core. The present azaheterocycle can be considered as a mimic of a fullerene fragment, which shows balanced ambipolar charge transport in space charge limited current (SCLC) devices with moderate hole (μh) and electron (μe) mobilities (μh = 2.96 × 10-3 cm2 V-1 s-1 and μe = 1.11 × 10-4 cm2 V-1 s-1). Theoretical studies such as nucleus independent chemical shifts (NICS) calculations, anisotropy of the induced current density (ACID) plots, spin density mapping and anisotropic mobility calculations were performed to corroborate the experimental findings.

Perhalogenated Tetraazaperopyrenes and Their Corresponding Mono- and Dianions

Chlorination and bromination of 2,9-perfluoropropyl-substituted tetraazaperopyrenes (TAPPs) under forcing conditions resulted in fully core-halogenated TAPP derivatives, devoid of hydrogen atoms at the polycyclic aromatic core. The octahalogenation stabilized the reduced mono- and dianionic compounds sufficiently to allow for their characterization. The additional ortho-chlorination led to an improvement of the electron mobility compared to the bay-substituted tetrachloro-TAPP when employed as an n-channel semiconductor in thin-film transistors.

Novel highly substituted thiophene-based n-type organic semiconductor: structural study, optical anisotropy and molecular control

Oligothiophenes and their functionalized derivatives have been shown to be a viable option for high-performance organic electronic devices.

Twisting the TAPPs: Bay-Substituted Non-planar Tetraazapero-pyrenes and their Reduced Anions

A new synthesis of tetraazaperopyrenes (TAPPs) starting from a halogenated perylene derivative 3,4,9,10- tetrabromo-1,6,7,12-tetrachloroperylene (1) gave access to bay-substituted TAPPs for the first time. Selective lithiation of the bromine-positions and subsequent addition of tosyl azide led to the formation of the tetraazidotetrachloroperylene (2), which was subsequently reduced by addition of sodium borohydride to the corresponding tetraaminotetrachloroperylene (3). Oxidation to its semiquinoidal form 4 and subsequent cyclization with acid chlorides gave rise to a series of bay-chlorinated TAPPs. Whereas the aromatic core of the previously studied ortho-substituted TAPPs was found to be planar, the steric pressure of the two chlorine substituents on each side leads to the twist of the peropyrene core of approximately 30 degrees, a structural feature also observed in other bay-substituted perylene derivatives. An experimental and computational analysis reveals that introducing chloride substituents at these positions leads to slightly increased electron affinities (EA) enabling the selective generation and characterization of the reduced mono-anionic radicals and closed shell di-anionic species. These anions were isolated and characterized by UV/Vis spectroscopy and EPR or NMR, respectively. Processing of the bay-chlorinated TAPPs in n-channel organic TFTs revealed electron mobilities of 0.001 to 0.003 cm2  V-1  s-1 . These reduced electron mobilities compared to the ortho-halogenated TAPPs are thought to be rooted in the less densely packed solid-state structures.