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 ...

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 ...

Give me five! Pentacene‐based polycyclic aromatic hydrocarbon (PAH) dyads are synthesized via an unsymmetrical pentacene building block. These molecules exhibit cofacial solid‐state π‐stacking as a result of the large aromatic chromophores. The choice of the PAH attached to the pentacene (see picture) influences the electronic properties as determined by UV/Vis absorption/emission spectroscopy and cyclic voltammetry.

Graphyne allotropes of carbon are fascinating materials, and their electronic properties are predicted to rival those of the “wonder material” graphene. One allotrope of graphyne, having rectangular symmetry rather than hexagonal, stands out as particularly attractive, namely 6,6,12-graphyne. It is currently an insurmountable challenge, however, to design and execute a synthesis of this material. Herein, we present synthesis and electronic properties of molecules that serve as model compounds. These oligomers, so-called radiaannulenes, are prepared by iterative acetylenic coupling reactions. Systematic optical and redox studies indicate the effective conjugation length of the radiaannulene oligomers is nearly met by the length of the trimer. The HOMO-LUMO gap suggested by the series of oligomers is still, however, higher than that expected for 6,6,12-graphyne from theory, which predicts two nonequivalent distorted Dirac cones (no band gap). Thus, the radiaannulene oligomers present a suitable length in one dimension of a sheet, but should be expanded in the second dimension to provide a unique representation of 6,6,12-graphyne.

Versatile, iterative synthetic protocols to form expanded [n]radialenes have been developed (n=3 and 4), which allow for a variety of groups to be placed around the periphery of the macrocyclic framework. The successful use of the Sonogashira cross-coupling reaction to complete the final ring closure demonstrates the ability of this reaction to tolerate significant ring strain while producing moderate to excellent product yields. The resulting radialenes show good stability under normal laboratory conditions in spite of their strained, cyclic structures. The physical and electronic characteristics of the macrocycles have been documented by UV-visible spectroscopy, electrochemical methods, and X-ray crystallography (four derivatives), and these studies provide insight into the properties of these compounds as a function of pendent substitution in terms of conjugation and donor/acceptor functionalization.