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

The synthesis of 2,3-R(2)-9,10-(OMe)(2)-substituted pentacenes (R=OMe, F, Br, CN; 1-4) from 2,3-R(2)-9,10-dimethoxy-6,13-dihydro-6,13-ethanopentacene-15,16-diones (α-diketone-bridged precursors) by photochemically induced bis-decarbonylation (Strating-Zwanenburg reaction) is described. Under matrix-isolation conditions (solid Ar, 10 K) the S(1) transitions of 1 and 2 undergo hypsochromic and those of 3 and 4 bathochromic shifts compared to parent pentacene. The S(1) transition wavelengths correlate well with the difference of substituent parameters σ(p). A computational analysis of the excited states at the CAM-B3LYP/6-311+G** level of theory provides an assignment of the electronic transitions. Photolysis in solution at room temperature yields red [R=OMe (1)], blue [R=Br (3), F (2)], and green [R=CN (4)] pentacenes. The compounds are oxygen-sensitive and have low solubility, but their formation can be monitored by UV/Vis and, in the case of R=CN, also by (1)H NMR spectroscopy. The S(1) transition in 4 does not show the typical pentacene fine structure in the electronic absorption spectrum. Photogeneration in the presence of oxygen leads to a number of photoproducts that could be identified by monitoring the reaction by (1)H NMR spectroscopy for R=OMe.     

Synthesis and cofacial pi-stacked packing arrangement of 6,13-bis(alkylthio)pentacene

[reaction: see text] 6,13-Bis(alkylthio)pentacenes directed toward organic field-effect transistors (OFETs) were synthesized by the ZnI(2)-mediated reaction of trans-6,13-dihydroxy-6,13-dihydropentacene with alkylthiols, followed by the dehydrogenative aromatization of the resulting trans-6,13-bis(alkylthio)-6,13-dihydropentacenes with p-chloranil. The X-ray crystallographic analysis of 6,13-bis(methylthio)pentacene reveals that this compound is arranged as a result of cofacial pi-stacking with S-S and S-pi interactions.     

Stabilizing Pentacene By Cyclopentannulation

A new class of stabilized pentacene derivatives with externally fused five‐membered rings are prepared by means of a key palladium‐catalyzed cyclopentannulation step. The target compounds are synthesized by chemical manipulation of a partially saturated 6,13‐dibromopentacene precursor that can be fully aromatized in a final step through a DDQ‐mediated dehydrogenation reaction (DDQ=2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone). The new 1,2,8,9‐tetraaryldicyclopenta[fg,qr]pentacene derivatives have narrow energy gaps of circa 1.2 eV and behave as strong electron acceptors with lowest unoccupied molecular orbital energies between ?3.81 and ?3.90 eV. Photodegradation studies reveal the new compounds are more photostable than 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene).     

Preparation of 6,13-bis(trimethylsilyl)pentacene and formation of second-ring Diels-Alder adduct of pentacene

6,13-Bis(trimethylsilyl)pentacene was synthesized by a coupling reaction of bicyclic dilithiobutadiene with diiodonaphthalene followed by aromatization. Diels-Alder reaction of 6,13-bis(trimethylsilyl)pentacene with dienophiles afforded the corresponding second-ring adducts. Elimination of two silyl groups gave the second-ring Diels-Alder adducts of parent pentacene.     

Photochemical synthesis of pentacene and its derivatives

A novel alpha-diketone precursor of pentacene, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was prepared and converted successfully to pentacene in 74 % yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange-pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74 % yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow, and only 6,13-endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n-pi* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state. Because the T-T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground-state pentacene to give the 6,13-endoperoxide. The alpha-diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8- and 2,9-dibromopentacene and 2,6-trianthrylene were also prepared and their photoconversion was performed.     

An improved synthesis of pentacene: rapid access to a benchmark organic semiconductor

Pentacene is an organic semiconductor used in a variety of thin-film organic electronic devices. Although at least six separate syntheses of pentacene are known (two from dihydropentacenes, two from 6,13-pentacenedione and two from 6,13-dihydro-6,13-dihydroxypentacene), none is ideal and several utilize elevated temperatures that may facilitate the oxidation of pentacene as it is produced. Here, we present a fast (-2 min of reaction time), simple, high-yielding (≥ 90%), low temperature synthesis of pentacene from readily available 6,13-dihydro-6,13-dihydroxypentacene. Further, we discuss the mechanism of this highly efficient reaction. With this improved synthesis, researchers gain rapid, affordable access to high purity pentacene in excellent yield and without the need for a time consuming sublimation.     

Pentacene disproportionation during sublimation for field-effect transistors

At moderate temperatures in flowing gas, pentacene undergoes a disproportionation reaction to produce 6,13-dihydropentacene (DHP) and a series of polycondensed aromatic hydrocarbons, including the previously unknown peripentacene (PP). The process requires activation by heating to 320 degrees C and is possibly catalyzed by impurities such as DHP, 6,13-pentacenequinone (PQ), Al, or Fe found in the starting materials. These impurities also result in a decrease in the intrinsic field-effect mobility (FEM) of pentacene crystals. Subsequent purifications remove such impurities, thus inhibiting the formation of the disproportionation products and increasing the FEM of pentacene (2.2 cm(2)/Vs). These results clarify the importance of purification of semiconductive materials for measurements of intrinsic mobility and optimal device performance.     

Synthesis and Characterization of 6,13‐Diamino‐Substituted Pentacenes

A series of 6,13‐diamino‐substituted pentacenes 1 a – d has been prepared and characterized as a new class of pentacene derivatives with strong donor ability and enhanced solubility in common organic solvents. The spectroelectrochemical and DFT studies revealed that the two‐electron oxidation process was accompanied by the substantial structural change into a butterfly‐like conformation of the pentacene moiety. More importantly, the extent of deformation from the planar pentacene moiety in the dications of 6,13‐diaminopentacene is tunable by varying the N‐substituents.     

Synthesis, characterization, and reactions of 6,13-disubstituted 2,3,9,10-tetrakis(trimethylsilyl)pentacene derivatives

Pentacene has been actively studied as relevant materials in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs). However, the low solubility and low stability of pentacene in common organic solvents have hindered its applications. When exposed to light or at high concentration, pentacene is found to dimerize easily. Many research groups are currently working on the design and synthesis of novel substituted pentacenes, but few of them systematically reported physical properties such as molecular spectroscopy and electronic properties, which might elucidate the influence of substituents on HOMO-LUMO gaps. Furthermore, the reactive nature of the central ring in pentacenes makes pentacenes good dienes for Diels-Alder reactions. In this paper, a series of soluble 6,13-disubstituted 2,3,9,10-tetrakis(trimethylsilyl)pentacenes were synthesized and characterized. Their reactions, structures, and physical properties were also studied. In addition, bulky o-carboranyl substituted pentacene derivative 15 and 6-chloro-2,3,9,10-tetrakis(trimethylsilyl)pentacene (16) were synthesized for the first time. Compound 16 possesses the largest dihedral angle (7.7° with two adjacent benzene rings) and shows a wave structure. Diels-Alder reactions with acceptable efficiency were carried out between 16 and various dienophiles.     

Synthesis and morphology of asymmetric,alkyne-functionalised pentacene and 2-fluoroanthradithiophene

The performance of 6,13-(bis-triisopropylsilylethynyl)pentacene (TIPS-pentacene) and 1,1′-difluoro-5,11-(bis-triethylsilyl)acetylene-anthra[2,3-b:6,7-b′]dithiophene (TES-FADT) is highly dependent on the morphology enforced by their alkyl groups, with only triisopropylsilylethynyl and triethylsilylethynyl producing viable transistor devices, respectively. Asymmetric triisopropylsilylacetylene- and triethylsilylacetylene-functionalised pentacene and 2-fluoroanthradithiophene were synthesised to study the effects a small change to the solubilising groups has on the thin film morphology.     

Synthesis of a Soluble Pentacene Derivative: 6,13-bis(m-trifluoromethyl phenyl ethenyl) Pentacene †

We report the synthesis and characterization of a new pentacene derivative, 6,13- bis(m-trifluoromethyl phenylethynyl)pentacene. This pentacene derivative is soluble in common organic solvents and found to have the HOMO energy level at ?5.45 eV in solution.     

Substituent effects on the electronic characteristics of pentacene derivatives for organic electronic devices: dioxolane-substituted pentacene derivatives with triisopropylsilylethynyl functional groups

The intramolecular electronic structures and intermolecular electronic interactions of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-pentacene (TP-5 pentacene), and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5 pentacene) have been investigated by the combination of gas-phase and solid-phase photoelectron spectroscopy measurements. Further insight has been provided by electrochemical measurements in solution, and the principles that emerge are supported by electronic structure calculations. The measurements show that the energies of electron transfer such as the reorganization energies, ionization energies, charge-injection barriers, polarization energies, and HOMO-LUMO energy gaps are strongly dependent on the particular functionalization of the pentacene core. The ionization energy trends as a function of the substitution observed for molecules in the gas phase are not reproduced in measurements of the molecules in the condensed phase due to polarization effects in the solid. The electronic behavior of these materials is impacted less by the direct substituent electronic effects on the individual molecules than by the indirect consequences of substituent effects on the intermolecular interactions. The ionization energies as a function of film thickness give information on the relative electrical conductivity of the films, and all three molecules show different material behavior. The stronger intermolecular interactions in TP-5 pentacene films lead to better charge transfer properties versus those in TIPS pentacene films, and EtTP-5 pentacene films have very weak intermolecular interactions and the poorest charge transfer properties of these molecules.     

Hetero Diels–Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron-Rich Pentacene

Herein, the first hetero Diels–Alder (DA) reactions with a stable, dicationic urea azine derived azo dienophile, synthesized by two-electron oxidation of a neutral urea azine are reported. Several charged DA products were synthesized in good yield and fully characterized. The DA adduct of anthracene is in thermal equilibrium with the reactants at room temperature, and the reaction enthalpy and entropy were determined from the temperature-dependent equilibrium constant. Furthermore, base addition to solutions of the pentacene DA product led to deprotonation, cleavage of the N−N bond, and formation of an electron-rich 6,13-bisguanidinyl-substituted pentacene. The redox and optical properties of this new pentacene derivative were studied. Furthermore, the dication resulting from its two-electron oxidation was synthesized and fully characterized. The results disclose a new elegant route to electron-rich pentacene derivatives.     

On the mechanism of peripentacene formation from pentacene: computational studies of a prototype for graphene formation from smaller acenes

The formation of peripentacene during the high-temperature vacuum sublimation of pentacene (P) in the presence of trace amounts of 6,13-dihydropentacene (DHP) has been studied computationally with density functional theory. Computational and kinetic analyses indicate that competing mechanisms involving a series of H atom transfers initiated by hydrogen transfer from DHP to P can account for the formation of peripentacene. The overall reaction is predicted to proceed with a free energy barrier of 36.1 kcal/mol and to be autocatalytic. Kinetic modeling supports the proposed mechanism.     

6,13-dibromopentacene [2,3:9,10]-bis(dicarboximide): a versatile building block for stable pentacene derivatives

6,13-Dibromopentacene [2,3:9,10]-bis(dicarboximide) (1) was synthesized for the first time by using in situ generated benzo[1,2-c:4,5-c']difuran as a key intermediate. Compound 1 exhibits good photostability in comparison to other pentacene derivatives and it can be further functionalized by Pd-catalyzed coupling reactions to give a series of soluble and stable functional pentacenes.     

Self-assembled pyrazinacene nanotubes

Nanotubes of a pentacene derivative, 6,13-bis(1-n-dodecyl)-[a,c,l,n]-tetrabenzo-5,6,7,12,13,14-hexaazapentacene, have been prepared by a hierarchical self-assembly mechanism. The oligoazaacenes 1-3, referred to as pyrazinacenes due to their structures of linearly fused pyrazine heterocycles, can also be considered as two azatriphenylenes fused through a reduced pyrazine ring. Dissolution of 6,13-bis(1-n-dodecyl)-[a,c,l,n]-tetrabenzo-5,6,7,12,13,14-hexaaza pentacene in nearly boiling toluene followed by standing of the solution at room temperature yields self-assembled (sa) pyrazinacene (Pa) nanotubes (NT's), or sa-PaNTs. Self-assembled-PaNTs are formed after initial aggregation of the pyrazinacene giving a 130-nm-wide 2-dimensional tape followed by helical twisting of this tape into a hollow cylindrical form of 150-200 nm diameter which can exceed 10 μm in length. The morphologies of the tape and nanotube structures were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and electron absorption spectroscopy (UV/Vis). The latter indicates that the tubes may be formed by chromophore J-aggregation. Also, high resolution TEM of the tubes reveals that they can be composed of several tapes while powder X-ray diffraction revealed the lamellar structure of the tapes composing the tubes.     

Transition-metal-catalyzed formation of trans alkenes via coupling of aldehydes

[reaction: see text] Rh(2)(OAc)(4) catalyzed the formation of exclusively trans fluorinated alkenes from aldehydes and pentafluorobenzaldehyde tosylhydrazone salts, which were readily prepared from pentafluorobenzaldehyde using the Bamford-Stevens reaction. A series of pentafluorophenyl-containing alkenes were synthesized from aldehydes in moderate to good yields under mild reaction conditions in a one-pot reaction. It is the first report of coupling two different aldehydes to form exclusively trans alkenes.     

High‐Yield Excited Triplet States in Pentacene Self‐Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission

One of the major drawbacks of organic‐dye‐modified self‐assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13‐bis(triisopropylsilylethynyl)pentacene–alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time‐resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface.     

The addition of benzocyclobutenylidene to benzene : The facile rearrangement of spiro[benzocyclobutene-1,7'-cyclohepta-1',3',5'-triene] to 9a,10-dihydrobenz[α]azulene

Thermal decomposition of the sodium salts of benzocyclobutenone tosylhydrazone and 2-methylbenzocyclobutenone tosylhydrazone in benzene affords 9a,10-dihydrobenz[α]azulene 4 and trans-10-methyl-9a, 10-dihydrobenz[α]azulene 3, respectively. A mechanism involving initially the addition of the carbene benzocyclobutenylidene, or its 2-Me derivative, to the benzene ring is postulated. A proposed intermediate in the reaction, spiro [benzocyclobutene 1,7' cyclohepta-1',3',5'-triene] 12 has been synthesised, and shown to give rise to 4 under the reaction conditions. The rate of rearrangement of 12 → 4 has been measured, and the activation energy determined: E_a = 125.9 ± O.8 KJmol^?1 and A = 1.38 × lO¹⁴sec^?1. The mechanism for the rearrangement must involve ring opening of the benzocyclobutene moiety of 12 to give an o- xylylene intermediate which is postulated to possess considerable diradical character. At 71.8 °, this ring opening is 2.7 × 10⁶ times faster than the ring opening of the parent benzocyclobutene molecule. The decomposition of the sodium salt of 2-(7' -cyclohepta-1',3',5' trienyl)benzaldehyde tosylhydrazone has also been investigated and is shown to yield 4a,10-dihydrobenz[α]azulene, 9,10-dihydrobenz[α]azulene and 8,9-benzotricyclo [5.3.0.0^2.10]deca-3,5,8-triene. A mechanism involving intramolecular 1,3-dipolar addition of a diazo grouping to a cycloheptatriene Π-bond, followed by decomposition of the resulting pyrazoline intermediate, is proposed.     

Oxidative dinuclear addition of a PdI-PdI moiety to arenes: generation of μ-η3:η3-arene-Pd(II)2 species

We report the oxidative dinuclear addition of a Pd(I)-Pd(I) bond to arenes. The oxidative dinuclear addition products, which have a bi-π-allyl-type arene dipalladium(II) structure, were obtained from [2.2]paracyclophane, anthracene, tetracene, and pentacene. A systematic study of the reaction of [Pd(2)(CH(3)CN)(6)][BF(4)](2) with benzene and polyacenes showed that the larger polyacenes, tetracene and pentacene, afforded the oxidative dinuclear addition products, while benzene, naphthalene, and anthracene gave the π-sandwich Pd(I)-Pd(I) complexes.