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

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.

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.

High-Resolution Electronic Excitation and Emission Spectra of Pentacene and 6,13-Diazapentacene Monomers and Weakly Bound Dimers by Matrix-Isolation Spectroscopy

N-Heteropolycycles are among the most promising candidates for applications in organic devices. For this purpose, a profound understanding of the low-energy electronic absorbance and emission characteristics is of crucial importance. Herein, we report high-resolution absorbance and fluorescence spectra of pentacene (PEN) and 6,13-diazapentacene (DAP) in solid neon obtained using the matrix-isolation technique. Accompanying DFT calculations allow the assignment of specific vibrationally resolved signals to corresponding modes. Furthermore, we present for the first time evidence for the formation of van der Waals dimers of both substances. These dimers exhibit significantly different optical characteristics resulting from the change of electronic properties evoked by the incorporation of sp² nitrogen into the molecular backbone.

Modulating the Electronic and Solid-State Structure of Organic Semiconductors by Site-Specific Substitution: The Case of Tetrafluoropentacenes

The properties as well as solid-state structures, singlet fission, and organic field-effect transistor (OFET) performance of three tetrafluoropentacenes (1,4,8,11: 10, 1,4,9,10: 11, 2,3,9,10: 12) are compared herein. The novel compounds 10 and 11 were synthesized in high purity from the corresponding 6,13-etheno-bridged precursors by reaction with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate at elevated temperatures. Although most of the molecular properties of the compounds are similar, their chemical reactivity and crystal structures differ considerably. Isomer 10 undergoes the orbital symmetry forbidden thermal [4+4] dimerization, whereas 11 and 12 are much less reactive. The isomers 11 and 12 crystallize in a herringbone motif, but 10 prefers π-π stacking. Although the energy of the first electric dipole-allowed optical transition varies only within 370 cm-1 (0.05 eV) for the neutral compounds, this amounts to roughly 1600 cm-1 (0.20 eV) for radical cations and 1300 cm-1 (0.16 eV) for dications. Transient spectroscopy of films of 11 and 12 reveals singlet-fission time constants (91±11, 73±3 fs, respectively) that are shorter than for pentacene (112±9 fs). OFET devices constructed from 11 and 12 show close to ideal thin-film transistor (TFT) characteristics with electron mobilities of 2×10-3 and 6×10-2  cm2  V-1  s-1 , respectively.

Synthesis of Unsymmetrical Derivatives of Pentacene for Materials Applications

Pentacene shows unique electronic properties that have long been appreciated and exploited. Over the past 20 years, new synthetic schemes have been developed to address some of the problems encountered with pristine pentacene (e.g., stability and solubility), and pentacene derivatives have become a mainstay in the realm of organic semiconductors in applications such as organic light-emitting diodes, organic field-effect transistors (OFETs), and organic photovoltaics. At the onset of our work, the vast majority of known pentacene derivatives featured a symmetrical structure, often as the result of synthetic protocols that rely on nucleophilic additions to 6,13-pentacenequinone (PQ). The assembly of pentacenes featuring an unsymmetrical framework held great appeal, but the stepwise formation of derivatives, in which a specific function might be incorporated through each individual addition step, did not exist. This Account presents contributions from our lab and others to the synthesis and study of unsymmetrical pentacene derivatives. PQ offers an ideal platform for desymmetrization through the sequential addition of nucleophiles to each of the two ketone groups. Addition can be completed in a one-pot protocol, or through individual steps in which the product of the first addition is isolated and used as a precursor in the divergent synthesis of a series of structurally related molecules. This general approach has been used to assemble pentacene derivatives appended with alkynyl/aryl/alkyl groups, polarized frameworks via substitution with donor and/or acceptor groups, and conjugated oligomers linked by butadiynyl moieties. Stepwise substitution also provides derivatives with remarkable functionality, including pentacene-porphyrin dyads, pendent TEMPO free radicals, cyanoacrylic acid anchor groups (for incorporation into dye-sensitized solar cells), and derivatives with ambipolar behavior for OFET devices. The study of intramolecular singlet fission (iSF) has emerged as one of the most fruitful applications of unsymmetrical pentacene derivatives. SF involves the spontaneous splitting of a photoexcited singlet state (S₁) in one chromophore into a pair of triplets (T₁) shared with a neighboring chromophore. Pentacene derivatives are particularly well suited for this since E(S₁) ≥ 2E(T₁) satisfies the thermodynamic requirements for SF, and they have the additional feature that two chromophores can be tethered together by a "spacer" that allows spectroscopic studies of iSF to be done in dilute solution. From a synthetic perspective, the major advantage of the dimeric structure is the ability to modify the spacer, which allows for control over the distance, geometric relationship, and electronic coupling between the two pentacene groups. Dimeric pentacenes are central to providing an in-depth understanding of the molecular mechanism of SF, often providing advances not possible from measurements in the solid state.

Template-Assisted Benzannulation Route to Pentacene and Tetracene Derivatives and its Application to Construct Amphiphilic Acenes That Self-Assemble into Helical Wires

Pentacene is one of the most versatile organic semiconductors. New synthetic strategies to construct the pentacene skeleton are imperative to produce pentacene derivatives with appropriate solubility, stability, and optoelectronic properties for various applications. This paper describes a template-directed approach to pentacene derivatives. In the retrosynthesis, the acene skeleton is viewed as a laddered double strand polyene instead of the more intuitive linearly fused hexagons. Based on this vision, the template strand of polyene is constructed with Wittig olefination, whereas the second strand is accomplished with Knoevenagel condensation to produce pentacene and tetracene derivatives. The synthetic scheme is flexible enough to generate an array of acene derivatives with substitution patterns that were hitherto difficult to access. Amphiphilic pentacene and tetracene derivatives were also synthesized by the template strategy. One pentacene based amphiphilic rod-coil molecule undergoes self-assembly to form helical wire structures that were visualized with TEM.

Synthesis, physical properties, and chemistry of donor–acceptor-substituted pentacenes

Pentacenes bearing electron-donating and (or) -withdrawing groups, namely methoxy-, dialkylamino-, and nitroaryl moieties, are synthesized to afford polarized pentacenes. The optical, electrochemical, and chemical properties of these derivatives are explored. The cycloaddition reaction of selected derivatives with tetracyanoethylene (TCNE) is explored, and the experimental results are rationalized on the basis calculations using density functional theory (DFT). X-ray crystallographic data provides insight into the molecular structure and intermolecular interactions present in the solid state.

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.

Face-to-Face Packing of 2,3,9,10-Tetrasubstituted Pentacene Derivatives Revealed through a Solid State [4 + 4] Thermal Cycloaddition and Molecular Dynamic Simulation

2,3,9,10-Substituted pentacene tetraesters and pentacene diester-dinitriles were synthesized. These pentacene derivatives underwent an unusual solid state [4 + 4] thermal dimerization with good efficiency and complete stereoselectivity. This observation indicates this series of pentacene derivatives adopt π-π stacking geometry with large mutual overlap in solid state. This notion was confirmed by molecualr dynamic simulation.

Influence of spiral framework on nonlinear optical materials

A series of spiral donor-π-acceptor frameworks (i.e. 2-2, 3-3, 4-4, and 5-5) based on 4-nitrophenyldiphenylamine with π-conjugated linear acenes (naphthalenes, anthracenes, tetracenes, and pentacenes) serving as the electron donor and nitro (NO2 ) groups serving as the electron acceptor were designed to investigate the relationships between the nonlinear optical (NLO) responses and the spirality in the frameworks. A parameter denoted as D was defined to describe the extent of the spiral framework. The D value reached its maximum if the number of NO2 groups was equal to the number of fused benzene rings contained in the linear acene. A longer 4-nitrophenyldiphenylamine chain led to a larger D value and, further, to a larger first hyperpolarizability. Different from traditional NLO materials with charge transfer occurring in the one-dimensional direction, charge transfer in 2-2, 3-3, 4-4, and 5-5 occur in three-dimensional directions due to the attractive spiral frameworks, and this is of great importance in the design of NLO materials. The origin of such an enhancement in the NLO properties of these spiral frameworks was explained with the aid of molecular orbital analysis.

Thermal generation of pentacenes from soluble 6,13-dihydro-6,13-ethenopentacene precursors by a Diels-Alder-retro-Diels-Alder sequence with 3,6-disubstituted tetrazines

3,6-Substituted tetrazines 2 (a: R(2) = 2-pyridyl or b: CO(2)Me) react with 2,3,9,10-(R(1))(4)-dihydro-6,13-ethenopentacene 3 in solution at elevated temperature to the corresponding pentacene 1 (a: R(1) = H, b: OBn, c: F).