Search

Molecular symmetry is vital to the selection rule of vibrationally resolved electronic transition, particularly when the nuclear dependence of electronic wave function is explicitly treated by including Franck-Condon (FC) factor, Franck-Condon/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Our present study investigated the light absorption spectra of highly symmetric tetracene, pentacene, and hexacene molecules of point-group D2h, as well as their monobrominated derivatives with a lower Cs symmetry. It was found that the symmetry-breaking monobromination allows more vibrational normal modes and their pairs to contribute to FC/HT interference and HT coupling, respectively. Through a projection of a molecule's vibrational normal modes to its irreducible representations, a linear relationship between the FC/HT intensity to the polyacene's size was deduced alongside a quadratic dependence of the HT intensity. Both theoretically derived correlations were well justified by our numerical simulations, which also demonstrated an approximately 20% improvement on the agreement with experimental line shape if the HT theory is adopted to replace the FC approximation. Moreover, for these low-symmetry monobrominated polyacenes, the FC intensity was even weaker than its FC/HT and HT counterparts at some excitation energies, making the HT theory imperative to decipher vibronic coupling, a fundamental driving force behind numerous chemical, biological, and photophysical processes.

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., Ring changes: Rooted in advances in organic synthesis and combined with specialized techniques for generation and study of highly reactive molecules, longer and longer acenes have became accessible for experimental investigations. Combined with sophisticated modern computational chemistry techniques, an encompassing picture of this unusual class of alternant polycyclic aromatic hydrocarbons emerges.

The accurate multireference (MR) calculation of a strongly correlated chemical system usually relies on a correct preselection of a small number of active orbitals from numerous molecular orbitals. Currently, the active orbitals are generally determined by using a trial-and-error method. Such a preselection by chemical intuition and personal experience may be tedious or unreliable, especially for large complicated systems, and accordingly, the construction of active space becomes a bottleneck for large-scale MR calculations. In this work, we propose to automatically select the active orbitals according to the natural orbital occupation numbers by performing black box generalized valence bond calculations. We demonstrate the accuracy of this method through testing calculations of the ground states in various systems, ranging from bond dissociation of diatomic molecules (N2, C2, Cr2) to conjugated molecules (pentacene, hexacene, and heptacene) as well as a binuclear transition-metal complex [Mn2O2(H2O)2(terpy)2]3+ (terpy = 2,2':6,2″-terpyridine) with active spaces up to (30e, 30o) and comparing with the complete active space self-consistent field (CASSCF), density matrix renormalization group (DMRG)-CASSCF references, and other recently proposed inexpensive strategies for constructing active spaces. The results indicate that our method is among the most successful ones within our comparison, providing high-quality initial active orbitals very close to the finally optimized (DMRG-)CASSCF orbitals. The method proposed here is expected to greatly benefit the practical implementation of large active space ground-state MR calculations, for example, large-scale DMRG calculations.

Singlet fission is believed to improve the efficiency of solar energy conversion by breaking up the Shockley-Queisser thermodynamic limit. Understanding of triplet excitons generated by singlet fission is essential for solar energy exploitation. Here we employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. For the geminate recombination, the different rates were attributed to the significant difference in their related energy change based on the Redfield quantum dissipation theory. The process is mainly governed by the electron-phonon interaction in hexacene. On the other hand, the non-geminate recombination is of bimolecular origin through energy transfer. In the triplet-triplet bimolecular process, the rates along the two different optical axes in the a-b crystalline plane differ by a factor of 4. This anisotropy in the triplet-triplet recombination rates was attributed to the interference in the coupling probability of dipole-dipole interactions in the different geometric configurations of hexacene single crystals. Our experimental findings provide new insight into future design of singlet fission materials with desirable triplet exciton exploitations.

Singlet fission (SF) creates two triplet excitons following absorption of a photon by two electronically interacting chromophores. Quaterrylene-3,4:13,14-bis(dicarboximide) (QDI) is a strongly absorbing chromophore that readily fulfills the energetic requirements for SF, E(S1) > 2E(T1), and thus should undergo rapid and efficient SF. SF was studied in thin films of the QDI derivative N,N-bis(2,6-diisopropylphenyl)-QDI (ArQDI), which undergoes SF in

Hexacene (Hc) is highly promising for singlet fission (SF). However, the number of SFs in Hc is extremely limited. As far as Hc dimers in solution are concerned, there is no report on the observation of the dissociation process from a correlated triplet pair (TT) to an individual one. The emphasis in this study is on the first observation of the quantitative TT generation together with the orientation-dependent photophysical discussions for TT dissociation usingpara- andmeta-phenyl-bridged Hc dimers. Moreover, the activation enthalpies of Hc dimers in TT dissociation are smaller than those of pentacene (Pc) dimers, whereas the relative entropic contributions for Gibbs free energy of activation are much larger than the enthalpic ones in both Hc and Pc dimers. This implies that the vibrational motions are responsible for the intramolecular conformation changes associated with the TT dissociation. Consequently, “enthalpy-entropy compensation” has a large impact on the rate constants and quantum yields. acceptedVersion

Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large., We report the synthesis and photophysical characterization of highly exoergic singlet fission in a hexacene dimer revealing exciton dynamics that follow the energy gap law.

Larger acenes, such as hexacene, are increasingly considered as promising materials for applications in optoelectronic devices. We studied electronic interface properties and the molecular orientat...

Summary Singlet fission is known to improve solar energy utilization by circumventing the Shockley-Queisser limit. The two essential steps of singlet fission are the formation of a correlated triplet pair and its subsequent quantum decoherence. However, the mechanisms of the triplet pair formation and decoherence still remain elusive. Here we examined both essential steps in single crystalline hexacene and discovered remarkable anisotropy of the overall singlet fission rate along different crystal axes. Since the triplet pair formation emerges on the same timescale along both crystal axes, the quantum decoherence is likely responsible for the directional anisotropy. The distinct quantum decoherence rates are ascribed to the notable difference on their associated energy loss according to the Redfield quantum dissipation theory. Our hybrid experimental/theoretical framework will not only further our understanding of singlet fission, but also shed light on the systematic design of new materials for the third-generation solar cells., Graphical Abstract, Highlights • Remarkable anisotropy of the overall singlet fission along different crystal axes • The correlated triplet pair emerges on the same timescale along both crystal axes • The quantum decoherence is predominantly driven by electron-phonon coupling • The anisotropic decoherence is due to the directional difference of its energy loss, Spectroscopy; Theoretical Photophysics; Quantum Phenomena

The crystal structures and compressibility parameters of benzene and a number of polycyclic aromatic hydrocarbons (PAHs) were calculated by the methods of the first-principles density functional theory with the gradient approximation of exchange and correlation potentials in the PBE form accounting for van der Waals interactions (optPBE–vdWB) at pressures of 0–20 GPa. A comparison with experimental data for benzene, naphthalene, tetracene, and pentacene demonstrated the high accuracy of our calculations. All the compounds have similar compressibilities, bulk moduli (8–12 GPa), and their pressure derivatives (6.9–7.5). The similarity of the calculated parameters indicates the main role of a decrease in interatomic distances during PAH compression and weak deformation of the molecules and benzene rings. The compressibility is weakly dependent on the number of atoms (benzene rings) in the molecule or crystal structure type (most of the PAHs have the space group P21/a). Compounds with many benzene rings and with a denser structure of rings (cyclic pyrene and coronene) are less compressible than less dense PAHs (tetracene and hexacene). Some PAHs (benzene, phenanthrene, pyrene, coronene) have high-pressure modifications, but a correct description of their structures allowing calculation of their elastic moduli has not been made so far. The obtained data on PAH compressibility can be used to develop high-temperature equations of state and calculation of the equilibrium composition of liquid and solid components of the C–O–H system.

Although a number of synthetic methodologies have been developed to prepare stable polycyclic aromatic hydrocarbons (PAHs), much less research has been devoted to functionalizing the peripheries of molecules to tune the self-assembly ability or introduce functional groups without altering their photophysical properties. Herein, we report twisted "X"-shaped molecules prepared through annulation of hexacene with benzoanthracene on the zigzag edge, and an investigation of their photophysical properties and self-assembly properties. The shape-complementary "X"-shaped molecules prefer to dimerize, while the π-extension would lead to one-dimensional π-stacking. Our findings give some insights into the design of stable PAHs without disturbing the electronic structures.

Hexacene is a valuable aromatic compound with special optoelectronic properties. Here, a stable new precursor of hexacene, 5,6,15,16-tetrahydro-5,16-epoxyhexacene, was synthesized through a three-step route in a total yield of 31%. Then two methods for the synthesis of hexacene based on this new precursor were developed. One is realized by dehydration of 5,6,15,16-tetrahydro-5,16-epoxyhexacene by thermal activation on the Cu(110) surface under ultrahigh vacuum (UHV) conditions. The other is copper powder-catalyzed dehydrogenation of another precursor 6,15-dihydrohexacene, which provided pure hexacene by vacuum sublimation in the dark. Organic thin-film transistors (OFETs) were fabricated using hexacene through a vacuum deposition method. And a comprehensive study of the hole-transfer properties of OFETs was performed. The best film mobility of hexacene was observed at 0.123 cm2 V−1 s−1 with an on/off ratio of 1.16 × 104 and a threshold of 2.65 V.

Understanding the plasmonic cavity induced electric field enhancement in a hybrid nanosystem is of paramount importance in the development of new optical devices. In this work, using linear response time dependent density functional theory calculations (LR-TDDFT), we demonstrate interesting modulations in the optical absorption spectra and electric field enhancements of a hybrid nanosystem consisting of linear acenes of fused benzene rings (benzene up to hexacene, arranged both in the parallel and perpendicular geometrical orientations) placed inside a sodium nanodimer gap. We show that the striking effect of placing linear acenes in a plasmonic gap is to create “asymmetric-dip” structures in the broad optical spectrum of the localized plasmon. This result is in tune with a recent experimental study [H. Imada, K. Miwa, M. Imai-Imada, S. Kawahara, K. Kimura and Y. Kim, Phys. Rev. Lett., 2017, 119, 013901] predicting that the origin of the dips is due to the energy transfer between the plasmon and molecular excitons. We also demonstrate that variations in the geometrical orientations of linear acenes in the gap region affect the absorption profile in a very moderate way with small, but visible changes in the absorption amplitudes. In addition, a particular emphasis is given to the modulations in the optical absorption spectra and electric field enhancements as a result of incorporating an external electric field in the Hamiltonian. Our theoretical findings may help in the design of hybrid nanosystems for ultra-sensitive molecular sensing applications.

Bu çalışmada, π-konjugasyon yapısına sahip hekzasen molekülü, farklı halojen atomlar (Flor, Klor, Brom) ve molekülde farklı konumlandırılmış siyano grupları ile dizayn edilmiş, elektronik, optik ve yük transfer özellikleri zamana bağlı ve zamandan bağımsız Yoğunluk Fonksiyon Teorisi (YFT) ile incelenmiştir. Dizayn edilen beş hekzasen türevinin yapı ve özellik arasındaki ilişkiyi kurabilmek için, moleküler geometri, reorganizasyon enerji, HOMO-LUMO orbital enerjileri, iyonizasyon potansiyeli (IP), elektron afinitesi (EA), geometriden dolayı nötr ve yüklü durumlara bağlı olarak bağ uzunluğu değişimi (BUD) üzerine substitüe etkisi incelenmiş ve etkiyi daha iyi anlayabilmek için sonuçlar hekzasen ve antrasen moleküllerin sonuçlarıyla karşılaştırılmıştır. Moleküllerin, elektron enjeksiyon bariyerinin hekzasen molekülünden daha düşük olduğu ve iyi bir boşluk taşıyıcısı olarak bilinen antrasenle kıyaslanabilir derecede etkili boşluk yük transfer oranına sahip olduğu belirtilmiştir. Dizayn edilen hekzasen türevlerinin, optoelektronik alanda potansiyel uygulamalar için ilgi çekebileceğini ve malzeme biliminde umut vaad edeceğini umuyorum., In this study, the π-conjugated hexacene molecule was designed with different halogen atoms (Fluorine, Chlorine, Bromine), differently positioned cyano groups and its electronic, optical and charge transfer properties were investigated by time-dependent and time-independent Density Functional Theory (DFT). The substituent effect on the molecular geometry, reorganization energy, frontier orbitals, ionization potential (IP) and bond length alternation (BLA) depending on nötr and charged states, electronic affinity (EA) of the five molecules were investigated to establish the relationship between structures and properties .To gain a better understanding of the substituent effect on the charge transport property, the results of designed molecules were compared with hexacene and antracene molecules. The electron injection barrier of the new designed molecules is lower than hexacene molecule and have more effective hole charge transfer property which is comparable to that of antracene known as a good hole charge carrier. I belived that, new designed hexacene derivatives may be used as optoelectronic devices which could attract great attention in material sciences.

With the focus on tuning the charge transport and optoelectronic properties, various new derivatives were designed by substituting the oligocene moieties at end cores of 2,5-di(pyrimidin-5-yl)thieno[3,2-b]thiophene (Comp.1). The end core substitution effect of benzene, naphthalene, anthracene, tetracene, pentacene and hexacene on Comp.1 was comprehensively studied on the structure-property relationship, electro-optical properties, ionization potential (IP), electron affinity (EA) and hole/electron reorganization energies (λ(h)/λ(e)). The injection barrier was reduced by introducing the oligocene units at end cores. The substitution of anthracene, tetracene, pentacene and hexacene at end cores of Comp.1 significantly minimized the polarization resulting smaller λ(h) (Comp.1c – Comp.1f) than the referenced compound, i.e., pentacene. The smaller λ(h) values are illuminating that newly designed derivatives might be superior/analogous to pentacene, i.e., frequently used hole transport material. The fluorescence spectra, dipole moment, IP, EA and λ(h) values showed that targeted molecules would be not only good for organic light emitters but also efficient for organic semiconductors and photovoltaic devices. Keywords: Semiconductors, Oligocene, Density functional theory, Optoelectronic properties, Reorganization energy, Charge transport properties

Although hexacene was first synthesized in 1939, the thin film properties, which are interesting for future applications and fundamental research, have never been investigated. Therefore, we synthe...

On-surface synthesis provides a powerful method for the generation of long acene molecules, making possible the detailed investigation of the electronic properties of single higher acenes on a surface. By means of scanning tunneling microscopy and spectroscopy combined with theoretical considerations, we discuss the polyradical character of the ground state of higher acenes as a function of the number of linearly fused benzene rings. We present energy and spatial mapping of the tunneling resonances of hexacene, heptacene, and decacene, and discuss the role of molecular orbitals in the observed tunneling conductance maps. We show that the energy gap between the first electronic tunneling resonances below and above the Fermi energy stabilizes to a finite value, determined by a first diradical electronic perturbative contribution to the polyacene electronic ground state. Up to decacene, the main contributor to the ground state of acenes remains the lowest-energy closed-shell electronic configuration.

The addition of phenyllithium to a polycyclic quinone, 9,11,12,21,22,24-hexaphenyltetrabenzo[a,c,n,p]hexacene-10,23-dione (10), followed by SnCl2 -mediated reduction of the diol intermediate, yielded 9,10,11,12,21,22,23,24-octaphenyltetrabenzo-[a,c,n,p]hexacene (4). Crystallographic analysis of hexacene 4 showed it to possess a longitudinal twist of 184°, which was in good agreement with AM1 calculations. In addition to being the most twisted acene synthesized to this point, compound 4 contains within its substructure the most twisted naphthalene, anthracene, tetracene, and pentacene moieties described.

A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were synthesized. The precursor was synthesized through 7 steps in a total yield of 2.5%. The structure of precursor and thermally converted 2-bromohexacene were characterized by solid state NMR, IR, and absorption spectra, as well as by DFT computation analysis. It exhibited high stability in the solid state over 3 months, therefore can be utilized in the fabrication of opto-electronic devices. The organic thin-film transistors (OFETs) were fabricated by using 2-bromohexacene and parent hexacene through vaccum deposition method. The best film mobility of 2-bromohexacene was observed at 0.83 cm2 V−1 s−1 with an on/off ratio of 5.0 × 104 and a threshold of −52 V, while the best film mobility of hexacene was observed at 0.076 cm2 V−1 s−1 with an on/off ratio of 2.4 × 102 and a threshold of −21 V. AFM measurement of 2-bromohexacene showed smooth film formation. The averaged mobility of 2-bromohexacene is 8 fold higher than the non-substituted hexacene.

Optoacoustic (photoacoustic) imaging enables high-resolution optical imaging at depths well beyond optical microscopy, revolutionizing optical interrogation of tissues. Operation in the near-infrared (NIR) is nevertheless necessary to capitalize on the technology potential and reach depths of several centimeters. Using Flash NanoPrecipitation for highly-scalable single-step encapsulation of hydrophobic hexacene at self-quenching concentrations, we propose quenched fluorescence-dye nanoparticles as a potent alternative to NIR metal nanoparticles for strong optoacoustic signal generation. Comprehensive hexacene-based nanoparticle characterization was based on a 5-step approach that examined the physicochemical features (Step 1), optoacoustic signal generation (Step 2), stability (Step 3), biocompatibility (Step 4) and spectral sensitivity (Step 5). Using this characterization framework we showcase the discovery of two nanoparticle formulations, QH2-50 nm and QH2-100 nm that attain superior stability characteristics and optimal optoacoustic properties compared to gold standards commonly employed for near-infrared optoacoustics. We discuss encapsulation and self-quenching (ESQ) of organic dyes as a promising strategy to generate optimal optoacoustic particles.

Motiviert durch die erfolgreiche Anwendung von Pentacen als organisches Halbleitermaterial beschäftigt sich diese Arbeit mit den Oberflächeneigenschaften der beiden nächstgrößeren Acenen, Hexacen und Heptacen. Im Anschluss an eine mehrstufige Molekülsynthese wurden im Ultrahochvakuum definierte Schichten auf Au(110) und Cu(110)-O(2x1) hergestellt. Die elektronischen Eigenschaften sowie die molekulare Orientierung von Monolagen bis mehreren Nanometern wurden mittels Photoelektronenspektroskopie (XPS und UPS) sowie Röntgenabsorptionsspektroskopie (XAS) untersucht. Hexacen auf Au(110) zeigte insgesamt eine starke Ähnlichkeit zu Pentacen auf Au-Substraten. Die gemessenen Zustandsdichten im Valenzband von nanometerdicken Molekülfilmen stimmten im Bereich niedriger Bindungsenergien mit DFT berechneten Werten sehr gut überein und es ließen sich die drei am höchsten besetzten Molekülorbitale HOMO, HOMO-1 und HOMO-2 zuordnen. Gut aufgelöste Resonanzstrukturen in XAS Messungen konnten ebenfalls als Übergänge in die beiden am niedrigsten unbesetzten Molekülorbitale LUMO und LUMO+1 interpretiert werden. Ultradünne Schichten aus wenigen Monolagen zeigten eine Wechselwirkung zwischen Hexacen und Au(110) an der Grenzfläche. Zusätzlich wurde bei Hexacen auf Cu(110)-p(2x1)O ein hochgeordnetes Schichtwachstum beobachtet mit Ausrichtung der Moleküle parallel zu den Sauerstoffreihen des Substrats. Deutliche Differenzen zu Hexacen und Pentacen ergaben hingegen die Oberflächenexperimente von Heptacen auf Au(110). Insbesondere bei Multilagenfilmen mit einer Schichtdicke von mehreren Nanometern zeigten sich in den PES und XAS Spektren Unterschiede im Vergleich zu Hexacen und Pentacen. Zudem wurden in IR-Spektren der Heptacenfilme auf Au(110) neue intensive Signale detektiert, die auf eine mögliche Reaktion der Moleküle im Festkörper hindeuten.

Diese Arbeit setzt sich auseinander mit der Synthese, Charakterisierung und vor allem mit der Reaktivität der (großen) Acene in Lösung, in organischen Gläsern, in kryogenen Edelgasmatrices und unter sauer-oxidierenden Bedingungen. Ein besonderes Augenmerk wurde hierbei auf das Verhalten von Heptacen gelegt, über welches bislang nur sehr wenig bekannt war. Die erhaltenen Ergebnisse von Heptacen jeweils mit denen der bisher bekannten kleineren Homologen wie Pentacen und Hexacen verglichen.Das Wissen um dessen Reaktivität trägt unter Anderem zum Verständnis der Reaktivität kleinerer Homologen, sowie des Konzeptes der sogenannten aromatischen Stabilität, bei. Außerdem kann es für zukünftige Generationen bei der Entwicklung von organischen Halbleitern auf Basis von Acenen hilfreich sein. Weiterhin konnte Diheptacen sowie das Dikation von Heptacen - ein überraschen persistenter Antiaromat - in rauschender Schwefelsäure vollständig charakterisiert werden.

Singlet fission has the great potential to overcome the Shockley–Queisser thermodynamic limit and thus promotes solar power conversion efficiency. However, the current limited understandings of detailed singlet fission mechanisms hinder a further improved design of versatile singlet fission materials. In the present study, we combined ultrafast transient infrared spectroscopy with ab initio calculations to elucidate the roles played by the vibrational normal modes in the process of singlet fission for hexacene. Our transient infrared experiments revealed three groups of vibrational modes that are prominent in vibronic coupling upon photoexcitation. Through our computational study, those normal modes with notable Franck-Condon shifts have been classified as ring-twisting modes near 1300.0 cm−1, ring-stretching modes near 1600.0 cm−1, and ring-scissoring modes near 1700.0 cm−1. Experimentally, a ring-stretching mode near 1620.0 cm−1 exhibits a significant blue-shift of 4.0 cm−1 during singlet fission, which reaction rate turns out to be 0.59 ± 0.07 ps. More interestingly, the blue-shifted mode was also identified by our functional mode singlet fission theory as the primary driving mode for singlet fission, suggesting the importance of vibronic coupling when a correlated triplet pair of hexacene is directly converted from its first excited state singlet exciton. Our findings indicate that the ultrafast transient infrared spectroscopy, in conjunction with the nonadiabatic transition theory, is a powerful tool to probe the vibronic fingerprint of singlet fission.

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.

The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1 may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 1(TT) state. Here, we probe the 1(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive 1(TT) state, which decays to 2×T1 with a time constant of 270 fs. However, the decay of S1 and the formation of 1(TT) occur on different timescales of 180 fs and

Noncovalent interactions determine in large part the thermodynamic aspects of molecular packing in organic crystals. Using a combination of symmetry-adapted perturbation theory (SAPT) and classical multipole electrostatics, we describe the interaction potential energy surfaces for dimers of the oligoacene family, from benzene to hexacene, including up to 5000 configurations for each system. An analysis of these surfaces and a thorough assessment of dimers extracted from the reported crystal structures underline that high-order interactions (i.e., three-body nonadditive interactions) must be considered in order to rationalize the details of the crystal structures. A comparison of the SAPT electrostatic energy with the multipole interaction energy demonstrates the importance of the contribution of charge penetration, which is shown to account for up to 50% of the total interaction energy in dimers extracted from the experimental single crystals; in the case of the most stable cofacial model dimers, this con...

We have investigated optoelectronic properties of higher acenes: pentacene, hexacene, heptacene, octacene, nonacene, decacene and their boron-nitride (BN) analogues, within the framework of density functional theory (DFT). We have also investigated the optoelectronic properties of acenes modified by BN substitution. Calculated optoelectronic properties encompasses: oxidation and reduction potentials, electron and hole reorganization energies and energy difference between excited first singlet and triplet states ΔE(S1−T1). Oxidation and reduction potentials indicate significantly better stability of BN analogues, comparing with their all-carbon relatives. Although higher acenes possess lower electron and hole reorganization energies, with both best values much lower than 0.1 eV, their BN analogues also have competitive values of reorganization energies, especially for holes for which reorganization energy is also lower than 0.1 eV. On the other hand ΔE(S1−T1) is much better for BN analogues, having values that indicate that BN analogues are possible applicable for thermally activated delayed fluorescence.

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.

We describe the synthesis and electronic properties of the novel interrupted hexacene 5,16-diphenylcyclobuta[1,2-b:3,4-b′]dianthracene (1). The synthesis is rooted in a sequential twin Sonogashira coupling of 2,3-dibromo-9,10-dihydro-9,10-epoxyanthracene followed by a Vollhardt 2+2+2 cyclotrimerization to give o-bromoaryl(trimethylsilyl) 8, after selective bromination. A Diels-Alder cycloaddition of the aryne from 8 and 1,3-diphenylisobenzofuran gave 1 after deoxygenation. Photophysical studies suggest that the fused cyclobutane ring been the two anthracene rings allows through-conjugation, but does not allow 1 to behave as a conjugated acene. Attempts to synthesize the parent analogue, cyclobuta[1,2-b:3,4-b′]dianthracene (2) were not successful, but detailed calculations are reported for both 1 and 2.

Although polycyclic conjugated hydrocarbons (PCHs) and their analogues have gained great progress in the fields of organic photoelectronic materials, the in‐depth study on present PCHs is still limited to hexacene or below because longer PCHs are insoluble, unstable, and tediously synthesized. Very recently, various strategies including on‐surface synthesis are developed to address these issues and many higher novel PCHs are constructed. Therefore, it is necessary to review these advances. Here, the recent synthetic approach, basic physicochemical properties, single‐crystal packing behaviors, and potential applications of the linearly fused PCHs (higher than hexacene), including acenes or π‐extended acenes with fused six‐membered benzenoid rings and other four‐membered, five‐membered or even seven‐membered and eight‐membered fused compounds, are summarized. Ministry of Education (MOE) Published version G.Z. acknowledges financial support from the National Natural Science Foundation of China (Nos. 51561135014 and U1501244), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), Science and Technology Program of Guangzhou (No. 2019050001), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No. 2017B030301007), MOE International Laboratory for Optical Information Technologies, the 111 Project. Q.Z. acknowledges AcRF Tier 1 (Grant Nos. RG 111/17, RG 2/17, RG 114/16, and RG 113/18) and Tier 2 (Grant Nos. MOE 2017‐T2‐1‐021 and MOE 2018‐T2‐1‐070), Singapore. W.C. acknowledges financial support from the National Natural Science Foundation of China (No. 21905100).

Exciton condensation, a Bose-Einstein-like condensation of excitons, was recently reported in an electronic double layer (EDL) of graphene. We show that a universal quantum signature for exciton condensation can be used to both identity and quantify exciton condensation in molecular systems from direct calculations of the two-electron reduced density matrix. Computed large eigenvalues in the particle-hole reduced-density matrices of pentacene and hexacene EDLs reveal the beginnings of condensation, suggesting the potential for exciton condensation in smaller scale molecular EDLs.

On-surface synthesis represents a successful strategy to obtain designed molecular structures on an ultra-clean metal substrate. While metal surfaces are known to favor adsorption, diffusion, and chemical bonding between molecular groups, on-surface synthesis on non-metallic substrates would allow the electrical decoupling of the resulting molecule from the surface, favoring application to electronics and spintronics. Here, we demonstrate the on-surface generation of hexacene by surface-assisted reduction on a H-passivated Si(001) surface. The reaction, observed by scanning tunneling microscopy and spectroscopy, is probably driven by the formation of Si-O complexes at dangling bond defects. Supported by density functional theory calculations, we investigate the interaction of hexacene with the passivated silicon surface, and with single silicon dangling bonds.

Polyacenes have attracted considerable attention due to their various applications in organic optoelectronic materials. This study focuses on linear polyacenes and their electron affinity (EA) and ionization potential (IP) properties. We have employed our recent implementation of EA/IP equation of motion coupled cluster singles and doubles (EA/IP-EOMCCSD) methods which are accurate, computationally efficient and are capable of treating large systems employing reasonable basis sets size. The EA/IP results obtained for naphthalene, anthracene, tetracene, pentacene, hexacene and heptacene are in a good agreement with experiment. Comparison between quality of excitation energies obtained from IP-EOMCCSD and EE-EOMCCSD formalisms were also studied.

Non-covalent functionalization via physisorption of organic molecules provides a scalable approach for modifying the electronic structure of graphene while preserving its excellent carrier mobilities. Here we investigated the physisorption of long-chain acenes, namely, hexacene and its fluorinated derivative perfluorohexacene, on bilayer graphene for tunable graphene devices using first principles methods. We find that the adsorption of these molecules leads to the formation of localized states in the electronic structure of graphene close to its Fermi level, which could be readily tuned by an external electric field. The electric field not only creates a variable band gap as large as 250 meV in bilayer graphene, but also strongly influences the charge redistribution within the molecule-graphene system. This charge redistribution is found to be weak enough not to induce strong surface doping, but strong enough to help preserve the electronic states near the Dirac point of graphene., 17 pages, 7 figures, supporting information

The [4+2] cycloaddition of hexacene and C60 yielded 1:1 and 1:2 adducts. The highest yield of 1:1 adduct was 90% by heating hexacene and C60 in toluene. The formation of 2 was achieved by reacting 1 with an excess amount of C60 in the presence of silica gel 60 with a yield up to 69%. The function of silica gel was rationalized by the effect of concentration enhancement inside the pore of silica gel. The reaction was examined systematically by changing the parameters of solvent, temperature, reaction time, and the pore size of silica gel. Both purified compounds 1 and 2 can be used for the fabrication of bulk heterojunction solar cells.

The interlayer interaction energy and its components of the bilayer graphene were obtained by extrapolation of the interlayer interaction energies and their components of the dimers formed by graphene with benzene, naphthalene, anthracene, phenanthrene, pyrene, tetracene, perylene, pentacene and hexacene. The resulting interaction energy of the bilayer graphene is −1.82 kcal/mol (or −79 meV) per carbon atom. The dispersion energy represents 70% of the total attractive energy between two graphene layers. In contrast, the electrostatic component is responsible for 25% of the total attractive interaction and the induction term contributes 5% to the stability of two graphene layers.

Singlet fission is an important candidate to increase energy conversion efficiency in organic photovoltaics by providing a pathway to increase the quantum yield of excitons per photon absorbed in select materials. We investigate the dependence of exciton quantum yield for acenes in the strong light-matter interaction (polariton) regime, where the materials are embedded in optical microcavities. Starting from an open-quantum-systems approach, we build a kinetic model for time-evolution of species of interest in the presence of quenchers and show that polaritons can decrease or increase exciton quantum yields compared to the cavity-free case. In particular, we find that hexacene, a typically poor singlet-fission candidate, can feature a higher yield than cavity-free pentacene when assisted by polaritonic effects. Similarly, we show that pentacene yield can be increased when assisted by polariton states. Finally, we address how various relaxation processes between bright and dark states in lossy microcavities affect polariton photochemistry. Our results also provide insights on how to choose microcavities to enhance similarly related chemical processes., 12 pages, 4 figures

Seven longitudinally twisted acenes (an anthracene, two tetracenes, three pentacenes, and a hexacene) have been synthesized by the addition of aryllithium reagents to the appropriate quinone precursors, followed by SnCl2 -mediated reduction of their diol intermediates, and several of these acenes have been crystallographically characterized. The new syntheses of the three previously reported twisted acenes, decaphenylanthracene (1), 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (2), and 9,10,11,12,13,14,15,16-octaphenyldibenzo[a,c]tetracene (14), resulted in a reduction of the number of synthetic steps. As a consequence their overall yields were increased by factors of 50-, 24-, and 66-fold, respectively. All of the twisted acene syntheses reported here are suitable for the synthesis of at least gram quantities of these remarkable hydrocarbon materials.

Triphenylene units stabilize hexacenes as dibenzo¬hex¬acenes. The hexacenes formed by a statistical Yamamoto coupling of dibromobenzene with dibromo-TIPS-pentacene or, alternatively, Stille coupling with dimethyldibenzostannole. UV-vis spectroscopy and quantum chemical calculations show, that despite the double benzannulation, the species have hexacene-type character, as evidenced by their red-shifted absorption characteristics. Charge carrier mobility of dibenzohexacene 1a was determined up to 0.2 cm2/(Vs).

Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particle–particle random-phase approximation calculation. The 1 A g ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state 3 B 2u is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state 1 B 2u is a zwitterionic state to the short axis. The excited 1 A g state gradually switches from a double-excitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the 1 B 2u and excited 1 A g states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved.

cited By 7; International audience; Surface-assisted reduction of specially designed air-stable precursors allows us to study single hexacene molecules on Au(111) by scanning tunneling microscopy and spectroscopy, mapping with intramolecular resolution their extended electronic eigenstates.

For a long time, the largest known member of the acene series was hexacene, consisting of six linearly fused benzene rings. The next higher member, heptacene, is so highly reactive that either stabilization using substituents is required or matrix isolation techniques need to be employed for the detection of the parent hydrocarbon. This Record Review summarizes recent research that culminated in the synthesis of substituted and parent nonacene.

Pi-extended thienoacenes that comprise alternatively arranged anthracene and thieno[3,2-b]thiophene moieties and have 8 or 13 aromatic rings were synthesized. the delocalization of their homo and lumo orbitals is over the entire molecules, and low-lying homo levels endow them good stability. (c) 2014 elsevier ltd. all rights reserved.

Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10-100 ps) but significantly slower than pentacene (80-110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family.