Acene Series Research Articles

Gas Phase Ionization Energy of Heptacene.

The ionization energy is a fundamental property that is relevant to charge transport in organic semiconductors. We report adiabatic ionization energies (AIEs) of heptacene at 6.21 and 7.20 eV for the X̃+B2g and Ã+Au states, respectively, as the next larger member of the acene series using mass- and isomer-selective double imaging photoelectron photoion coincidence spectroscopy. The X̃+ state energy decreases monotonically with an increase in size within the homologous series of acenes and approaches an asymptotic limit [AIE(polyacene) = 5.94 ± 0.06 eV] based on a fit with an exponential decay function. As byproducts of heptacene formation from cycloreversion of diheptacenes, 5,18-, 7,16-, and 6,17-dihydroheptacene can be detected, and their AIE is similar to that of their largest acene subunit (anthracene and tetracene, respectively), in very good agreement with computational treatments.

Synthesis of polyacene by using a metal–organic framework

The acene series, an important class of linearly polycyclic aromatic hydrocarbons, are of interest owing to their unique physicochemical features. With an increase in the number of fused benzene rings, acenes display an evolution of electronic structure and properties. Thus, efforts have been devoted to the synthesis of longer acenes, with dodecacene being the longest acene (12 fused benzene rings) reported to date. However, the formation of polymeric acenes with numerous benzene rings, namely polyacene, has yet to be realized. Herein, we present a methodology for the synthesis of polyacene mediated by a metal–organic framework. Nanoconfined synthesis of precursor polymers in the channels of the metal–organic framework and the subsequent dehydro-aromatization reaction produced polyacene that was overwhelmingly longer than the previously reported acenes. The scalable synthesis of polyacene allowed us to unveil the stability and electronic properties of polyacene, paving the way for their widespread applications in optoelectronic and magnetic devices.

Pyridone-Doped Acenes with Improved Stability.

Besides the peripheral modification, the introduction of heteroatoms to modulate the property of longer acenes with improved chemical stability has been extensively studied for their potential applications in organic electronics. However, the utilization of 4-pyridone, a common unit in the air- and photo-stable acridone and quinacridone, to decorate higher acenes with increased stability has not been realized yet. Here we present the synthesis of a series of monopyridone-doped acenes up to heptacene via the palladium-catalyzed Buchwald-Hartwig amination of aniline and dibromo-ketone. The effect of pyridone on the property of doped acenes were investigated experimentally and computationally. With the π-extension of doped acenes, the pyridone ring shows the weakened conjugation and gradual loss of aromaticity. The doped acenes demonstrate enhanced stability in solution and maintain the electron communication between the acene planes.

Structural stability and the low-lying singlet and triplet states of BN-n-acenes, n=1-7.

The chemical stability and the low-lying singlet and triplet excited states of BN-n-acenes (n=1-7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet-triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon-only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet-triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN-n-acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low-lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron.

The role of crystal packing on the optical response of trialkyltetrelethynyl acenes.

The electronic and optical responses of an organic semiconductor (OSC) are dictated by the chemistries of the molecular or polymer building blocks and how these chromophores pack in the solid state. Understanding the physicochemical nature of these responses is not only critical for determining the OSC performance for a particular application, but the UV/visible optical response may also be of potential use to determine aspects of the molecular-scale solid-state packing for crystal polymorphs or thin-film morphologies that are difficult to determine otherwise. To probe these relationships, we report the quantum-chemical investigation of a series of trialkyltetrelethynyl acenes (tetrel = silicon or germanium) that adopt the brickwork, slip-stack, or herringbone (HB) packing configurations; the π-conjugated backbones considered here are pentacene and anthradithiophene. For comparison, HB-packed (unsubstituted) pentacene is also included. Density functional theory and G0W0 (single-shot Green's function G and/or screened Coulomb function W) electronic band structures, G0W0-Bethe-Salpeter equation-derived optical spectra, polarized ϵ2 spectra, and distributions of both singlet and triplet exciton wave functions are reported. Configurational disorder is also considered. Furthermore, we evaluate the probability of singlet fission in these materials through energy conservation relationships.

Crystal Structures of β-Methylchalcogenated Tetrathienoacenes: From One-Dimensional π-Stacking to Sandwich Pitched π-Stacking Structure

The control and prediction of crystal structures of molecular semiconductors is critical to optimize such solid-state properties as carrier mobility, although it still presents a challenge owing to the complicated interplay of various intermolecular interactions in the solid state. We recently reported that methylthio and methoxy groups can act as a “controller” of crystal structure from the herringbone to the rubrene-like pitched π-stacking in a series of acenedithiophenes and simple acenes, such as naphthalene and anthracene. In this study, we focused on tetrathienoacene (4TA), which affords a one-dimensional π-stacking structure, and investigated the effect of methylchalcogenation on its crystal structure. The crystal structures of newly synthesized 4TA derivatives were altered significantly; the methoxy and methylthio derivatives crystallized into a “sandwiched pitched π-stacking” structure, whereas the methylseleno 4TA crystallized into a “pseudo π-stacking” structure. These structural changes were explained by the analysis of intermolecular interactions estimated by the symmetry-adapted perturbation theory method. The interplay between the CH-π interactions induced by the methylchalcogenation and the inherent nature of the π-π interactions of the 4TA cores determined the crystal structures of the 4TA derivatives. We conclude that the mutual effects of the parent π-conjugated system and the methylchalcogeno groups are important to define the packing structure.

Impact of exciton delocalization on exciton-vibration interactions in organic semiconductors

Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Green's function based methods for the electronic structure and non-perturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that the degree of exciton delocalization controls these interactions, with thermally activated crystal phonons predominantly coupling to delocalized states, and molecular quantum fluctuations predominantly coupling to localized states. Based on this picture, we quantitatively predict and interpret the temperature and pressure dependence of excitonic peaks in the acene series of organic semiconductors, which we confirm experimentally, and we develop a simple experimental protocol for probing exciton delocalization. Overall, we provide a unified picture of exciton delocalization and vibrational effects in organic semiconductors, reconciling the complementary views of finite molecular clusters and periodic molecular solids.

Effects of site-specific substitution to hexacene and its effect towards singlet fission

The singlet fission (SF) process, which is involved in organic solar cell, is a spin allowed and extremely fast internal conversion process by which a photo-excited singlet exciton produces two triplets. To accelerate this fission and to increase the efficiency of solar cell, designing of new materials/molecules is an interesting area of research and our current interest. Several alternate hydrocarbons of the acene series and their derivatives with structural variety are desirable for this purpose. Therefore, we have theoretically modeled and investigated different substituted hexacenes in detail. Different electron donating groups (EDGs) as well as electron withdrawing groups (EWGs) and few groups of other varieties are chosen for site-specific monosubstitution to the hexacene ring for manipulation of their excited singlet-triplet energy levels and thereby to match the SF driving force (Δ) and triplet-triplet annihilation (TTA) deactivation force (Ω). The geometries, electronic structures, frontier molecular orbital (FMO) energies, optimization of excited state and calculation of Δ and Ω of the substituted hexacenes are investigated with Time Dependent Density functional theory (TDDFT) method using B3LYP/6-31G∗ basis set. After a detail theoretical analysis on sixty five (65) hexacene derivatives, the δ-triisopropylsilylethynyl (-TIPS) hexacene and β-NO2 hexacene are predicted to exhibit good SF characteristics.

Dodecacene Generated on Surface: Reopening of the Energy Gap.

The acene series represents a model system to investigate the intriguing electronic properties of extended π-electron structures in the one-dimensional limit, which are important for applications in electronics and spintronics and for the fundamental understanding of electronic transport. Here, we present the on-surface generation of the longest acene obtained so far: dodecacene. Scanning tunneling spectroscopy gives access to the energy position and spatial distribution of its electronic states on the Au(111) surface. We observe that, after a progressive closing of the gap and a stabilization to about 1 eV at the length of decacene and undecacene, the energy gap of dodecacene unexpectedly increases to 1.4 eV. Considering the acene series as an exemplary general case, we discuss the evolution with length of the single tunneling resonances in comparison with ionization energy, electronic affinity, and optical gap.

From Palladium to Gold Catalysis for the Synthesis of Crushed Fullerenes and Acenes.

The quest for organic materials with improved optoelectronic properties has stimulated the development of new strategies for the preparation of polycyclic aromatic hydrocarbons. Within this context, transition metal catalysis offers unparalleled opportunities for the assembly of complex molecular architectures. The palladium-catalyzed direct C-H arylation provides straight access to biaryls without the need of prefunctionalization at the nucleophilic site, which is attractive from the perspective of the synthesis of polyarenes. Mechanistically, this reaction was found to be different from an electrophilic aromatic substitution, involving the abstraction of a proton by an external base in the key metalation step. Using readily available C27 truxene as the starting material, a concise synthetic route consisting of a threefold benzylation and subsequent palladium-catalyzed arylation led to C60 polyarenes, also referred to as "crushed fullerenes", which could be converted into C60 fullerene by laser-induced cyclodehydrogenation in the gas phase or by thermal cyclodehydrogenation on a platinum surface. A conceptually related strategy based on the use of a highly electrophilic gold(I) complex as the catalyst for the threefold intramolecular hydroarylation of truxene derivatives was applied for the synthesis of decacyclenes. Using gold(I) catalysis, we have also developed a variety of synthetically useful protocols for the cycloisomerization of readily available 1,n-enynes as well as for the addition for nucleophiles to these unsaturated substrates. In one of these transformations, 1,7-enynes bearing aryl-substituted alkynes undergo formal [4 + 2] cycloaddition reactions via gold(I)-catalyzed 6-exo-dig cyclization and intramolecular Friedel-Crafts-type reaction to form tricyclic compounds bearing a dihydronaphthalene core. A related transformation led to a general synthesis of hydroacenes, which are known to be stabilized precursors of the corresponding conjugated acenes with enhanced solubility. A wide variety of dihydrotetracenes featuring electron-donating and electron-withdrawing groups, as well as dihydropentacene and dihydrohexacene could be easily obtained. A simple variation of our synthetic route led to tetrahydro-derivatives of higher acenes with up to 11 linearly fused six-membered rings. The dehydrogenation of tetrahydroacenes on a metallic substrate using the tip of a scanning tunneling microscopy instrument or by thermal annealing enabled the preparation of the whole series of higher acenes from heptacene up to previously unknown undecacene, whose structure was confirmed by noncontact atomic force microscopy. This work provided a unique opportunity for the analysis of the evolution of the transport gap in the acene series on Au(111). Furthermore, heptacene was also generated by on-surface dehydrogenation on Ag(001) from tetrahydroheptacene and a dibrominated derivative.

Acridinium Based Organic Molecular Single Electron Transistor for High Performance Switching Applications

The conductance and switching behaviour of acene series derivative `Acridinium' based organic molecular single electron transistor have been assessed in the vicinity of Density Functional Theory (DFT) and Non-equilibrium Green's Function (NEGF) formalisms based ab-initio approach. Computationally, acridinium has been obtained by nitrogen substitution of its parent molecule anthracene. Our analysis of structural and chemical properties predicts relatively high electrical conductivity for acridinium than anthracene owing to the high degree of delocalization associated with the former. The performance of acridinium as a quantum dot of SET has been studied by calculating charging energies and charge stability diagram. It has been observed that acridinium SET is power efficient and possesses better switching behaviour than anthracene and other acene derivative SETs reported so far. Moreover, the negative gate bias operation of acridinium SET is observed to be energetically more favourable than the positive bias operation.

FT-IR spectroscopy of carbonized acenes: a possible key for the UIBs/AIBs origins

The complete acene series from naphthalene to anthracene, tetracene and pentacene were carbonized directly in a differential scanning calorimeter (DSC) special crucible heating up to 700 °C under autogenous pressure. In these conditions the carbonization of all the acenes was detected as an endothermal process occurring above 550 °C. The resulting carbonized acenes were studied by FT-IR spectroscopy showing a band pattern matching the spectrum of a mature coal and the unidentified infrared bands (UIBs) as well as the aromatic infrared bands (AIBs) detected in the interstellar and in certain circumstellar media. It is speculated that carbonized organic matter could be a valid model to explain the carriers of the UIBs/AIBs bands in space. Carbonized PAHs can be conceived as carbon dust grains formed under the action of shock waves, which are quite common in certain circumstellar and interstellar media.

Evolution of the Optical Gap in the Acene Series: Undecacene.

Undecacene, the largest member of the acene family, was photogenerated in a polymer matrix under cryogenic conditions from a precursor with two α-diketone bridges. The electronic absorption of undecacene extends into the NIR region, but the spectrum is dominated by two strong absorptions in the UV/Vis range. The HOMO-LUMO transition is continuously shifted to lower energies as the number of rings of the acene increases (E vs. 1/N, N=number of rings), which is in line with the strong electron correlation calculated by DFT/MRCI.

Evolution of the Optical Gap in the Acene Series: Undecacene

AbstractUndecacene, the largest member of the acene family, was photogenerated in a polymer matrix under cryogenic conditions from a precursor with two α‐diketone bridges. The electronic absorption of undecacene extends into the NIR region, but the spectrum is dominated by two strong absorptions in the UV/Vis range. The HOMO–LUMO transition is continuously shifted to lower energies as the number of rings of the acene increases (E vs. 1/N, N=number of rings), which is in line with the strong electron correlation calculated by DFT/MRCI.

Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene.

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

Higher Acenes by On‐Surface Dehydrogenation: From Heptacene to Undecacene

AbstractA unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

Lateral Extension of a Benzodithiophene System: Construction of Heteroacenes Containing Various Chalcogens.

A series of linear acenes with five fused rings, which contain thiophene, selenophene, and tellurophene as the outmost rings, have been synthesized from well-known benzodithiophene (BDT). It was found that the optical, electrochemical properties and crystal packing motifs could be modulated by changing heteroatoms in the outmost rings.

An assessment of low-lying excitation energies and triplet instabilities of organic molecules with an ab initio Bethe-Salpeter equation approach and the Tamm-Dancoff approximation.

The accurate prediction of singlet and triplet excitation energies is an area of intense research of significant fundamental interest and critical for many applications. Most calculations of singlet and triplet energies use time-dependent density functional theory (TDDFT) in conjunction with an approximate exchange-correlation functional. In this work, we examine and critically assess an alternative method for predicting low-lying neutral excitations with similar computational cost, the ab initio Bethe-Salpeter equation (BSE) approach, and compare results against high-accuracy wavefunction-based methods. We consider singlet and triplet excitations of 27 prototypical organic molecules, including members of Thiel's set, the acene series, and several aromatic hydrocarbons exhibiting charge-transfer-like excitations. Analogous to its impact in TDDFT, we find that the Tamm-Dancoff approximation (TDA) overcomes triplet instabilities in the BSE approach, improving both triplet and singlet energetics relative to higher level theories. Finally, we find that BSE-TDA calculations built on effective DFT starting points, such as those utilizing optimally tuned range-separated hybrid functionals, can yield accurate singlet and triplet excitation energies for gas-phase organic molecules.

Heptacene: Characterization in Solution, in the Solid State, and in Films.

Acenes comprise an important class of organic semiconducting materials. As graphene nanoribbons of ultimate width, they are valuable atom-precise model systems for studying the properties of this form of nanoscale carbon materials. Heptacene is the smallest member of the acene series that could only be studied under matrix isolation conditions. Its existence in bulk had never been positively confirmed, despite efforts dating back more than 70 years. We report that the reduction of 7,16-heptacenequinone produces a mixture of two diheptacene molecules. The diheptacenes undergo thermal cleavage to heptacene at high temperatures in the solid state. Monitoring this cycloreversion by solid state 13C cross-polarized magic angle spinning NMR reveals that solid heptacene has a half-life time of several weeks at room temperature. The diheptacenes are valuable precursors for generating films of heptacene by vapor phase deposition that can be studied below or at room temperature.

Herringbone Pattern and CH-π Bonding in the Crystal Architecture of Linear Polycyclic Aromatic Hydrocarbons.

The herringbone pattern is a pervasive structural motive found in most molecular crystals involving aromatic compounds. A plot of the experimental sublimation enthalpies of members of increasing size of the acene, phenacene and p-phenyl families versus the number of carbons uncovers a linear relationship between the two magnitudes, suggesting a major role of CH-π bonding. In this work we undertake the task of evaluating the relevance of the edge-to-face interaction (or CH-π bond) in the overall reticular energy of the crystal, to quantitatively assess the importance of this structural element. Following a heuristic approach, we considered the series of acenes, phenacenes and p-phenyls and analyzed the edge-to-face interaction between the molecules as they occur in the experimental crystal network. Isolation of the relevant molecular dimers allows to incorporate some of the most sophisticated tools of quantum chemistry and get a reliable picture of the isolated bond. When compared to the experimental sublimation energy, our results are conclusive: this sole interaction is the largest contribution to the lattice energy, and definitively dictates the crystal architecture in all the studied cases. Elusive enough, the edge-to-face interaction is mainly dominated by correlation interactions, specifically in the form of dispersion and, to a less extent, of charge-transfer terms. A suggestive picture of the bond has been obtained by displaying the differences in local electron densities calculated by either correlated or non-correlated methods.