A Modular Synthetic Approach to Conjugated Pentacene Di‐, Tri‐, and Tetramers

Abstract

Pentacene and functionalized pentacenes are leading candidates for small-molecule semiconductor applications and have been widely explored over the past decade.[1] On the other hand, examples of pentacene polymers [2, 3a-c] and oligomers [3] remain scarce, even though they present obvious opportunities for discovery. In comparison to polymers, monodisperse oligomers provide the added benefit of structural homogeneity. Furthermore, oligomers allow for the systematic study of structure-property relationships, in which a property of interest (e.g., HOMO-LUMO gap (HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital)) is examined as a function of oligomer length. If oligomers of sufficient length are accessible, the effective conjugation length can be determined by observing saturation of a property; that is, when the oligomer starts to behave like the polymer. [4] To date, the synthesis of functionalized pentacenes has relied, to a large extent, on the addition of nucleophiles to acenequinones such as 6,13-pentacenequinone, followed by reduction to form the aromatic pentacene framework.[1] This methodology cannot, however, easily be adapted to the synthesis of conjugated oligomers.[3d] Such synthetic limitations are due, in part, to the reactive nature of the pentacene core and to the limits of current methodology for desymmetrizing the pentacene chromophore. It is unlikely that this issue can be effectively addressed by condensation [5a] and cycloaddition [5b-f] reactions to form acenes, since such methods often produce isomeric mixtures. Oxidative acetylenic homo-and cross-coupling reactions of terminal and/or haloacetylenes (e.g., Hay and Cadiot-Chodkiewicz reactions) are a well-established means of assembling carbon-rich materials.[6] These reactions, when paired with recently developed methods to form unsymmetrical pentacenes, [7] would serve as the foundation for the synthesis of a homologous series of pentacene oligomers.Pentacenes that bear a terminal acetylene functionality, such as 1 or 2 (Scheme 1), are quite reactive, [7a] and have been ineffective to date when isolated and used as precursors for synthetic elaboration.[8] To circumvent this problem, a protection method has been developed [9] so that the synthetic potential of terminal acetylenes can be accommodated and exploited. Herein, we report the synthesis of protected building blocks such as 3 and 4, and describe their application to metal-mediated coupling reactions used in the synthesis of a series of conjugated pentacene oligomers (5-8, Scheme 2). The properties of these oligomers are explored by crystallographic, spectroscopic, and electrochemical methods.The construction of pentacene oligomers 6-8 begins with desymmetrizing 6,13-pentacenequinone (9, Scheme 3). Addition of one equivalent of the appropriate lithium acetylide (10 a-d) to a suspension of 9 in THF afforded monoaddition products 11 a-d in good yields (80-96 %). Addition of an excess of acetylide 10 e (ca. 3 equiv) to ketones 11 a-d, followed ...

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

“…The dimer has good solubility in typical organic solvents (4 mg ml −1 ) such as THF, CH 2 Cl 2 , CHCl 3 and toluene, but NC slowly decomposes over several days in solutions under exposure to air and light (Supplementary Methods 1). The known pentacene monomer 6,13-bis(tri- iso butylsilylethynyl)pentacene (TIBS) served as a reference28.…”

Allenylidene Complexes Based on Pentacenequinone

“…We decided to commence from [RuCl 2 (PPh 3 ) 3 ] ( 3 )12 and the heteroscorpionate complex [Ru(bdmpza)Cl(PPh 3 ) 2 ] ( 5 ) [bdmpza = bis(3,5‐dimethylpyrazol‐1‐yl)acetate],13 which are known to form quite stable allenylidene complexes 14. As a starting point for the pentacenone component, the known 13‐hydroxy‐13‐[(triisopropylsilyl)ethynyl]pentacen‐6‐one ( 1 )15 was converted into the propargyl alcohol 2 by desilylation with TBAF in thf (Scheme ). Finally, following the procedure of Selegue, pentacenone 2 was combined with [RuCl 2 (PPh 3 ) 3 ] ( 3 ) in thf under reflux, which led to the formation of a deep‐blue solution 16.…”

“…X‐ray structure determinations were carried out with a Bruker‐Nonius Kappa‐CCD diffractometer. Hbdmpza,21 [RuCl 2 (PPh 3 ) 3 ],12 [Ru(bdmpza)Cl(PPh 3 )2],13 and [(triisopropylsilyl)ethynyl]pentacen‐6‐one15 were prepared according to the literature.…”

Evidence for Charge-Transfer Mediation in the Primary Events of Singlet Fission in a Weakly Coupled Pentacene Dimer

“…6,13-Bis(tri-isobutylsilylethynyl)pentacene (TIBS) was used as a reference (Figure 1). 59 In the absorption spectra of NC-p/TIBS, the (S 0 S 0 ) to (S 1 S 0 )/(S 0 ) to (S 1 ) transitions are characterized by vibrational fine structure (also known as Franck-Condon progression) in the range of 450-650 nm. However, the long-wavelength onset of the absorption undergoes a red shift on going from non-polar to polar solvents.…”