Pentacene, Diels-Alder reactions

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

A -sulfinylacetamide 297 in greater than 90% yield when a catalytic amount of methyltrioxorhenium is employed. Futhermore, the hetero-Diels-Alder adduct is highly soluble in both chlorinated and ethereal solvents. A detailed investigation of the retro-Diels-Alder reaction of 298 by thermogravimetric analysis revealed an onset temperature of 120 °C and complete conversion of bicycle 298 to pentacene 296 at 160 °C, which are temperatures compatible with the polymer supports typically used in electronics applications. The electronic properties of these newly prepared OTFTs are similar to those prepared by traditional methods. Later improvements to this chemistry included the use of A -sulfinyl-/< r/-butylcarbamate 299 as the dienophile <2004JA12740>. The retro-Diels-Alder reaction of substrate 300 proceeds at much lower temperatures (130 °C, 5 min with FlTcatalyst 150 °C, Ih with no catalyst). 

Recently a novel approach for a high yield synthesis of another soluble pentacene precursor was demonstrated.[249,250] The synthesis involves a Lewis acid-catalyzed Diels-Alder reaction of pentacene and N-sulflnylacetamide.[251-253] OTFTs fabricated by spin-casting a chloroform solution of the precursor on substrate followed... 

The first example of a pentacene precursor was a tetrachlorocyclohexadiene adduct prepared by the Mullen group (Figure 5.3.9a) [47]. This derivative is soluble in dichloromethane and forms good films by spin-coating. After deposition, the pentacene film is formed by a thermally activated retro Diels-Alder reaction expelling tetrachlorobenzene as the by-product. The hole mobilities of OFETs prepared from these pentacene precursors depended greatly on the annealing temperature... 

Pascal has reported a synthetic protocol for the preparation of 9,11,20,22-tetrap henyltetrabenzo[o,c,l,n]pentacene (145), a polycyclic aromatic hydrocarbon, by double Diels-Alder reaction between bisbenzyne precursor 143 and phencyclone 144 (Equation 12.39) [79]. Unfortunately, however, the reaction was found to be extremely low-yielding, with compound 145 being obtained in only 1-2% yield after purification. This problem was most likely a result of the high temperature used to generate the bisbenzyne intermediate. 

The centrosymmetric dimer 24 formed by reaction between the two most reactive sites (C7 and C14) and in this sense resembles the photodimers of parent anthracene, tetracene, and pentacene [38]. The dimer 25 formed in the dark reaction resulted from reaction of the most reactive ring of one monomer with the next to terminal ring of the other molecule [33]. Another interesting dimer 26 was isolated as a by-product of the hexacene synthesis by the Anthony group [33]. It is formed by Diels-Alder reaction between the reactive ring containing the C7 and C14 atoms and the alkynyl group of another TIBS-hexacene. As one intact hexacene unit is... 

Higher molecrolar weight polynuclear aromatic hydrocarbons (PAHs) containing the anthracene nucleus have also been found to react with maleic anhydride. These ring systems, however, can differ widely in reaction rates. Typical examples of those systems that undergo the Diels-Alder reaction are 1,2,5,6-dibenzanthracene (a), 2,3,6,7-dibenzanthracene (pentacene) (b), and 9,10-diphenylanthracene (c). 

Attempts have been made to deposit TIPS-pentacene from solution as the functional layer in a pentacene/C60 bilayer photovoltaic device. Careful optimization of deposition conditions, optimal concentration of mobile ion dopants, thermal postfabrication annealing, and the addition of an exciton-blocking layer yielded a device with a moderate white-light PCE of 0.52% [41]. Since TIPS-pentacene derivatives rapidly undergo a Diels-Alder reaction with fiillerene, the assembly of potentially more efficient bulk-heterojunction photovoltaic devices from TIPS-pentacene and fiillerene derivatives were not possible [42]. The energy levels of the TIPS-pentacene-PCBM adduct (PCBM is [6,6]-phenyl C61-butyric acid methyl ester) ineffectively supports the photoinduced charge transfer. 

The Diels-Alder reaction ([4+2] cycloaddition) [8] provides one of the most powerful methods for the constmc-tion of PAHs because it affords one-step formation of the six-membered ring framework, the basic unit of these derivatives. This chapter, based on the results of our work, will highUght the synthesis of substituted oligoacenes including anthracene (1), tetracene (2), and pentacene (3) cores by Diels-Alder reactions using arynes and quinones as dienophile components and furans, thiophene-1,1-dioxides, and o-quinodimethane as diene components. The synthetic methodology will be accompanied by discussions on molecular and packing stmctures as weU as solid-state optical properties. 

First, synthetic procedures for pentacene (3) and its derivatives win be described in detail. Pentacene (3) was first synthesized by Clar and John [75] in 1929 using the Elbs reaction [76] for the preparation of the pentacyclic framework 96 from 4,6-dibenzoyl-m-xylene (95) followed by dehydrogenation of 96 (Scheme 14.31). In 1953, Bailey and Madoff also prepared 3 by a Diels-Alder reaction of 1,2-dimethylenecyclohexane (97) with p-benzoquinone (61),... 

Diels-Alder reaction of tetrakis (pentafluorophenyl)porphyrin with pentacene and naphthacene gave mono-adduct product within 1 min under microwave irradiation with 83% and 23% yields, respectively whereas only 22% and no reaction was observed in classical heating. Bis-adducts was also formed in microwave heating, which were not obtained by classical method (Silva et al., 2005). 

This trend is revealed, for example, by the rates of Diels-Alder addition reactions of anthracene, naphthacene, and pentacene, in which three, four, and five rings, respectively are linearly fused. The rate data are shown in Table 9.3. The same trend can be seen in the activation energy and the resonance energy gained when cycloreversion of the adducts 9-12 yields the aromatic compoimd, as shown in Scheme 9.3. 

An ingenious route to the solubilization of pentacene involves addition of substituent groups that can be removed thermally after film formation is complete. The reactive nature of the central aromatic ring in pentacene makes it a good diene for Diels-Alder type reactions, and the reversibility of this reaction makes this approach ideal for such a functionalization strategy. 

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