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 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.     

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.     

Photo precursor for pentacene

6,13-Dihydro-6,13-ethanopentacene-15,16-dione gave pentacene efficiently both in solid and in solution by irradiation of light.     

High-performance,solution-processed organic thin film transistors from a novel pentacene precursor

The Lewis acid-catalyzed Diels-Alder reaction of the organic semiconductor pentacene with N-sulfinylacetamide yields a soluble adduct. Spin-coated thin films of this adduct undergo solid-phase conversion to form thin films of pentacene at moderate temperatures. Organic thin film transistors fabricated by spin-coating this adduct, followed by thermal conversion to pentacene, exhibit the highest mobility reported to date for a solution-processed organic semiconductor.     

Photooxidation and reproduction of pentacene derivatives substituted by aromatic groups

Pentacene derivatives substituted by aromatic groups at the 6,13-positions were prepared and investigated for their electronic properties and the photoaddition reaction with oxygen. The pentacene derivatives substituted by 2-thienyl and phenyl groups reacted with oxygen in solution under light and afforded their endoperoxides. These first-order kinetic constants were evaluated to be 1.5×10⁻³ s⁻¹ and 2.7×10⁻³ s⁻¹. The pentacene derivative with pentafluorophenyl groups was relatively stable in solution. The thermolysis and photolysis of the endoperoxide with 2-thienyl groups in solution afforded the pentacene derivative with yields of 30 and 44%, respectively. In addition, UV irradiation (254 nm) of the thin film of the endoperoxide was studied, which indicated the reproduction of the pentacene derivative.     

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.     

Intramolecular Diels-Alder Reaction of Thioaldehydes

Intramolecular Diels-Alder reaction of thioaldehydes which were generated from bis(trimethylsilyl) sulfide and dienals gave the corresponding bicyclic adducts.     

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.     

Synthesis and thermolysis of a Diels-Alder adduct of pentacene and thiophosgene

Pentacene has excellent semi-conducting properties but its practical use in organic thin film transistors (OTFTs) gives rise to a lot of problems caused by its sensitivity to oxygen and its very low solubility. In order to solve the problems involved in the use of pentacene, we have synthesized a Diels-Alder adduct of pentacene with thiophosgene.     

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.     

A soluble pentacene: synthesis, EPR and electrochemical studies of 2,3,9,10-tetrakis(trimethylsilyl)pentacene

A soluble 2,3,9,10-tetrakis(trimethylsilyl)pentacene (1) was synthesized; the discovery of the radical cationic character of in solution through EPR measurement has provided insights into the sensitivity of acenes towards light and oxygen.     

Completely regioselective, highly stereoselective syntheses of cis-Bisfullerene

cis-Bisfullerene[60] adducts of 6,13-disubstituted pentacenes (R = Ph, 4'-hydroxymethylphenyl) are synthesized in 75% to 85% isolated yields under kinetically controlled Diels-Alder conditions. The cycloadditions are completely regioselective and highly stereoselective, with only traces of the diastereomeric trans-bisfullerene[60] adducts forming.     

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).     

Lewis Acid promoted tandem intermolecular Diels-Alder/intramolecular allylation reactions of silyl-substituted 1,3-butadienes leading to multisubstituted 7-norbornenones and related polycyclic compounds

7-Norbornenones of exo,exo-disubstituted patterns were formed highly selectively in good yields from Lewis acid-promoted tandem intermolecular Diels-Alder/intramolecular allylation reactions. The intermolecular Diels-Alder reaction between 1,4-bis(trimethylsilyl)-1,3-butadienes or 1-trimethylsilyl-1,3-butadienes with maleic anhydride in the presence of newly sublimed AlCl3 afforded their corresponding cycloaddition adducts, which underwent AlCl3-mediated intramolecular allylation of the carbonyl group by the in situ generated allylsilane moiety affording 7-norbornenones of exo,exo-disubstituted patterns.     

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.     

Chemistry of 1,16-Dihydroxytetraphenylene and 2,3,9,10-Tetrakis(trimethylsilyl) pentacene

Recently, four tetraphenylenols, as well as several non-substituted and substituted 2,3,9,10-tetrakis(trimethylsilyl)pentacenes were realized in our laboratories. In this presentation, the synthesis, hydrogen bonding properties, and chiral recognition studies of (S)- 1,16-dihydroxytetraphenylene (1) and its (R)-enantiomer will be delineated.1,16-Bis(diphenylphosphino)tetraphenylene (2) was also realized from 1 and was employed in the formation of a stable platinum complex. The synthesis of soluble 2,3,9,10-tetrakis(trimethylsilyl)pentacene (3) was achieved and fully characterized by physical and spectroscopic methods.     

The Shapiro reaction of barrelene derivatives: the influence of annelation on acene formation

The possible formation of pentacene from a tosylhydrazone of 6,13-dihydro-6,13-ethenopentacene under the conditions of the Shapiro reaction is explored, as previous work demonstrated that the tosylhydrazone of barrelene (bicyclo[2.2.2]octatriene) yields benzene under these conditions [C. Weitemeyer, T. Preuss, and A. de Meijere, Chem. Ber., 1985, 118, 3993]. The computational analyses based on homodesmotic equations involving the anions, and monomeric (including the dimethyl ether solvate) and dimeric organolithium compounds reveals that benzene formation is exothermic, but pentacene formation is endothermic due to the increased stability of the lithium derivative and the decreased stability of pentacene. The computational predictions are confirmed by experimental investigations.     

Excitation energy transfer in tris(8-hydroxyquinolinato)aluminum doped with a pentacene derivative

We investigate the excitation energy transfer in a guest-host molecular system consisting of a pentacene derivative, namely 6,13-bis(2,6-dimethylphenyl)pentacene (DMPP), doped into tris(8-hydroxyquinolinato)aluminum (Alq(3)) using steady-state and time-resolved photoluminescence (PL) spectroscopy. The concentration dependent energy transfer rate and efficiency are calculated and analyzed in terms of the F?rster resonance energy transfer model. A relatively long excitation transfer time ( approximately 0.6-3.4 ns depending on the DMPP concentration) and a large transfer radius (31-36 A) are obtained. The F?rster radius calculated directly from the Alq(3) PL-DMPP absorption spectral overlap (26 A) is smaller than the transfer radii obtained from the PL studies, which suggests that excitation energy migration within Alq(3) plays an important role in the energy transfer process, effectively elongating the transfer radius and increasing the transfer rate and efficiency.