Acene Series Research Articles

A DFT‐Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700-1100 K) are calculated by using B3LYP/6-31+G(d,p) geometries and BMK/6-311+G(3df,2p) single-point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity-enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.

The radical character of the acenes: A density matrix renormalization group study

We present a detailed investigation of the acene series using high-level wave function theory. Our ab initio density matrix renormalization group algorithm has enabled us to carry out complete active space calculations on the acenes from napthalene to dodecacene correlating the full pi-valence space. While we find that the ground state is a singlet for all chain lengths, examination of several measures of radical character, including the natural orbitals, effective number of unpaired electrons, and various correlation functions, suggests that the longer acene ground states are polyradical in nature.

Improving the TDDFT calculation of low‐lying excited states for polycyclic aromatic hydrocarbons using the Tamm–Dancoff approximation

AbstractFull TDDFT combined with the commonly used hybrid functional B3LYP has been known to greatly underestimate the 1La excitation energies of large linear acenes. This has cast doubt on the TDDFT results for the similar 1La‐type state in other conjugated π‐systems. Although increasing the amount of Hartree–Fock (HF) exchange energy in the employed functional could improve the excitation energy estimation of the 1La state, it would worsen the results for another lowest‐lying excited state,1Lb. Calculations of absorption and emitting energies relative to the 1La states for a series of linear acenes showed that a TDDFT scheme incorporating the Tamm–Dancoff approximation (TDDFT/TDA) could decrease the estimation errors by a factor of about 50%, but keep the levels of 1Lb states almost unchanged. Thus, the TDDFT/TDA scheme gives an overall description for the low‐lying excited states of linear acenes significantly better than the full TDDFT does. Furthermore, 16 nonlinear polycyclic aromatic hydrocarbons (PAHs) with various structures were examined to confirm the superiority of the TDDFT/TDA over the full TDDFT in its ability to describe the 1La states for conjugated π‐systems of large size. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Synthesis and Characterization of Anthra[2,3-b]thiophene and Tetraceno[2,3-b]thiophenes for Organic Field-Effect Transistor Applications

A series of thiophene-fused acenes, anthra[2,3-b]thiophene (1), tetraceno[2,3-b]thiophene (2), and 2-n-decyl-tetraceno[2,3-b]thiophene (3), were synthesized. The crystal structures of 1 and 2 were determined. A weak dipole results from the addition of the thiophene ring, and the crystal packing changes from the triclinic structure of acenes to orthorhombic, with the molecules maintaining the herringbone arrangement with the molecular long axis antiparallel to the neighbors. Theoretical calculation showed that the molecules have higher oxidation potential and comparable hole mobility as acene analogues with the same number of rings. Thin film transistor devices were fabricated from these materials. With compound 1, a field-effect mobility of 0.134 cm2/V s and on/off ratio of 108 can be achieved when deposited at a substrate temperature of 25 °C, whereas a mobility of 0.245 cm2/V s and on/off ratio of 106 were observed for compound 2 deposited at 80 °C.

Multiscale Modeling of Self-Assembled Monolayers of Thiophenes on Electronic Material Surfaces

Computer simulation programs, spanning different time and length scales, are used to describe the fundamentals of thin film growth morphology in organic self-assembled monolayers using thiophenes on gold as representative systems. Ab initio calculations created a catalog of the energetics between two N-[4-(thien-2ylethynyl)phenyl] hydroxyl ("1P" molecules) in vacuum and interactions in three orthogonal orientations (parallel, perpendicular, and gamma-phase) to a Au (111) surface. This energetic dataset was supplied as the input for kinetic Monte Carlo simulations of dimer and trimer representations of small organic molecules to describe both sub-monolayer and multilayer growth on a series of hypothetical model substrates. On strongly binding metallic-like substrates, sub-monolayers of the model organic molecules formed ordered phases in the x and y directions at high temperatures and a disordered polycrystalline structure at low temperatures with the molecules lying down. Only at high temperatures was a "phase inversion" observed from a completely flat to an upright structure, suggesting the upright phase to be kinetically limited. Results for multilayer deposition of 1P molecules on three substrates which differ in their binding energy to the molecule (from non-interacting to strongly binding substrates) provided a rich view of the polymorphism that can result from differing choices of temperature and flux conditions. Irrespective of the binding energy of the molecule to the substrate, on highly corrugated surfaces we always observed 3D-island growth of multiple layers of the thiophenes, in contrast to Stranski-Krastanov or Frank-van der Merwe growth on more uniform substrates. The qualitative picture we obtained agrees with the growth habits of other small organic molecule systems like the acene series. Finally, molecular dynamics studies were used to understand the packing structures of stable polymorphs of thiophene SAMs. Different deposition conditions and substrate-molecule binding captured different regimes of growth morphology, some of which have already been observed experimentally.

Thermochemistry and Kinetics of Hydrogen Abstraction by Methyl Radical from Polycyclic Aromatic Hydrocarbons

Thermodynamic and kinetic properties relating to hydrogen abstraction by methyl radical from various sites in polycyclic aromatic hydrocarbons (PAHs) have been investigated. The reaction enthalpies (298 K), barriers (0 K), and activation energies and pre-exponential factors (700-1100 K), have been calculated by means of density functional theory, specifically with B3-LYP/6-311G(d,p) geometries, followed by BMK/6-311+G(3df,2p) single-point energy calculations. For uncongested sites in the PAHs, a reasonable correlation is obtained between reactivities (as characterized by the reaction barriers) and reaction enthalpies. This is reflected in a Bell-Evans-Polanyi (BEP) relationship. However, for congested sites, abstraction is accompanied both by lower reaction enthalpies (due to relief of steric strain) and also by reduced reactivities (due to significantly increased steric hindrance effects in the transition structures), so that the BEP relationship does not hold. In addition, the reaction enthalpies and kinetic parameters for the series of linear acenes indicate that abstraction is more difficult from the central rings.

Roles of polycyclic compounds in increasing breakdown strength of LDPE film

Polycyclic compounds of acene series without radicals were added to the material of LDPE (low density polyethylene) film to investigate the roles of aromatic compounds in an increase in the breakdown strength (BDS) of the film. The additives used were naphthalene, anthracene, tetracene and pentacene (group A). Dc and impulse BDSs of the film with anthracene were the highest among the films with the group A additives. It was estimated from our experiments that dc breakdown of the film in this study is mainly due to the electron avalanche mechanism. Furthermore, the dependence of BDS upon the type of radical connected to anthracene (An) was measured using three kinds of additive (group B), which are 9,10-dibromoanthracene (Br-An), 9-nitroanthracene (NO-An) and 1-aminoanthracene (NH-An). DcBDS of the films with radicals are arranged in the following order: (PE/NO-An)ges(PE/Br-An)>(PE/An)>(PE/NH-An). DcBDS of (PE/NO-An) was approximately 1.65 times higher than that of the film without an additive. The dependence of dcBDS upon the type of radical connected to An was discussed using the experimental results with UV absorbance, DSC and TSC. The dependence of dcBDS on the kind of group B additive corresponds to the dependence of trap depth for electron carriers on the type of connected radical: the electron-accepting or electron-donating, or halogen type. The excitation of the additive at the collision with electron carriers is considered to increase dcBDS for all the films with the additives of group B and An. The modification of the morphology is not observed significantly in the films studied

Electronic and Crystal Engineering of Acenes for Solution‐Processible Self‐Assembling Organic Semiconductors

High charge-carrier mobility in bis- silylethynylated acenes: This article highlights new organic semiconductor materials (see figure) with pronounced p- and n-type charge-carrier mobilities. Particular emphasis is given to the evolution of the electronic properties with increasing number of annulated rings in the ascending bis-silylethynylated acene series as well as the rational supramolecular crystal engineering.

Calculation and analysis of the excited states and fine structure of vibronic spectra of anthracene, anthracene-d 10, and tetracene

Structural dynamic models of the excited states of some molecules of the acene series (anthracene, anthracene-d10, and tetracene) have been developed and the vibronic structure of the fluorescence and absorption spectra of these molecules has been calculated by the parametric method of the theory of vibronic spectra. Good agreement is obtained between the calculated and experimental spectra. It is shown that the system of parameters determined for these acenes in the second-order approximation of the method is highly transferable in the acene series. The models proposed adequately describe the real structure of the excited molecules and make it possible to predict quantitatively the fine vibrational structure of the spectra, including relatively weak effects (weak bands, the spectral variations in the series of these molecules, and the variations upon deuterium substitution). The specific features of the structural variations in the investigated acene molecules upon their excitation and the dynamics of these variations in the series of these molecules are studied.

Superconductivity in acenes

The vibronic coupling between the lowest unoccupied molecular orbital (LUMO) and molecular vibrations is calculated and analyzed with respect to a series of acenes. The Raman active CC stretching modes of 1400–1600 cm −1 have large coupling constants in small acenes, while both low Raman active modes of 200–300 cm −1 and CC stretching modes of 1400–1600 cm −1 play an important role in the electron–phonon coupling in large acenes. The possible superconducting transition temperatures of the monoanions of benzene (C 6H 6) and acenes are roughly estimated, on the basis of the hypothesis that vibronic interactions between the LUMO and intramolecular vibrations would play an essential role in the occurrence of superconductivity in these molecules.

Quantum Corrections to the Polarization Energy in Linear Acene Series

Quantum Corrections to the Polarization Energy in Linear Acene Series

Bridged Annulenes

AbstractThe synthesis and aromaticity of 1,6‐methano[10]annulene and its analogues containing a heteroatom bridge suggested the possibility that the homologous series of bridged [4n + 2]annulenes 1, 2, 3, etc., formally derived from the acene series, could be developed. In the pursuit of this concept a set of systematically bent bridged [14]annulenes which permitted a scrutinization of the steric prerequisite of the Hückel rule — the requirement of a planar ring skeleton — were built up. Moreover, a new synthetic strategy based on the synthon cycloheptatriene‐1,6‐dicarboxaldehyde has provided access to the two next higher homologues (syn‐series) of 1,6‐methano[10]annulene, i.e., syn‐1,6:8,13‐bismethano[14]annulene and syn, syn‐1,6:8,17:10,15‐trismethano[18]annulene, respectively.

Towards a unified theoretical treatment of the transition state in the reactions of unsaturated molecules—III : A study of two photochemical reactions

The experimental results concerning the photodeuteration of various benzene derivatives and the photooxidation of a series of acenes are found to be explicable by means of the delocalized transition state model.

Low resistivity polyacenequinones

AbstractA group of polyacenequinone radical polymers prepared from a homologous series of heterocyclic acenes and two aromatic anhydrides including mellitic trianhydride were studied in an attempt to produce materials with enhanced conduction. The specific resistivities, thermal activation energies, and energy intervals of these materials are reported. It has been shown that the conductivity of these polymers can be enhanced by about three orders of magnitude by the substitution of heterocyclic acenes.

Annellation effects in the pyrene series and the classification of absorption spectra

The classification of absorption bands in the acene series (α, p, β-bands) was first based on the annellation effect and later on the polarization of the bands with regard to the molecular axes (L b,L a,B b,B a-bands). Both give the same result since annelation axes and molecular axes coincide. This is not so in the pyrene series. Annellation axes and molecular axes are different and it is shown that the classification of bands can be based only on the annellation effects. Moreover, the electronic asymmetry of the pyrene system is demonstrated by a striking asymmetric annellation effect. A number of higher annellated pyrenes have been synthesized: 1·-benzonaphtho-(2″·3″ :6·7)-pyrene, dinaphto-(2'·3 :1·2)-(2″·3″:6·7)-pyrene, 1·14,4·5-dibenzo-pentacene, 5·,15·16-dibenzohexacene, 6·7,16·17-dibenzoheptacene, 1·16,4·5-dibenzohexacene and 7·8,17·18-dibenzo-octacene.

Asymmetric annellation effects—VI

Strong constant spectral shifts are recorded as stated in the preceding chapters when benzene rings are fused together to form the acene or phene series. These shifts are constant and from them an order number K or K p for each member in the annellation series may be derived. In striking contrast to this, very different spectral shifts are produced by the successive connection of two diphenyl systems to benzene (I) forming triphenylene (II) and tetrabenzanthracene (III), which is formally a symmetric annellation. The first shift is much bigger than the second one. The nature of this electronic asymmetry becomes obvious if the second shift is deducted from the β-band (in √A) of triphenylene (II). This means the elimination of the influence of the angular rings. The result is a β-band at 2237 A, which is almost identical with the β-band found in the absorption spectrum of naphthalene which is at 2210 A in alcoholic solution. It is obvious that one of the two annellations produces an induced benzenoid ring, marked “Bz”, and extends the central aromatic system of the benzene ring to a naphthalene system, whilst the second annellation does not extend it to an anthracene system but forms an empty ring, marked “E”. It must be concluded therefore that each inherent benzenoid ring (as indicated by the circles) can transfer two π-electrons only to the induced benzenoid ring (Bz), a conclusion which was reached in the preceding chapters for different reasons. It appears that the formation of an induced π-electron sextet (Bz) has preference over a symmetric distribution of π-electrons. If this interpretation is correct then the same principle should apply to the central acene system in a higher tetrabenzacene, without regard to the number of rings. There is only one sextet in an acene independent of its length and accordingly only two transferable π-electrons able to migrate from one ring to the other.

Equilibrium Between Methyl-Acences and Methylenedihydroacenes

TAUTOMERISM beween methyl-acenes (I, II, III, IV, V) and methylene-dihydroacenes (la, Ila, IIIa, IVa, Va, respectively) has often been postulated to explain side -chain reactivity of aromatic compounds but hitherto no methylene-dihydroacene has beprt isolated. This weakness in the hypothesis has rfow been removed by the synthesis of pure 6-methylene-6 : 13-dihydropentacene (Va). The 'anellation principle'1 requires a gradual transition in properties in the acene series, and it may be inferred, therefore, that (IV), (III), (II) and (I) will also contain the corresponding methylene forms, but in decreasing proportions.