Abstract
The research efforts on oligoacene systems are still relatively limited mainly due to the synthetic challenge and the extreme instability of longer acenes. Herein, these two issues have been overcome through elaborative modification and the stable pentacene species has been successfully synthesized. Additionally, a series of bis(diarylamino) compounds linked by variable-length oligoacene bridges ranging from one to five fused rings (benzene (1a), naphthalene (1b), anthracene (1c), tetracene (1d) and pentacene (1e)) have been prepared to probe the effect of the extent of π-conjugation on the electron transfer properties. Compound 1c exhibits a high planarity between the anthracyl bridge and the two nitrogen cores and the molecular packing shows a two-dimensional herringbone characteristic. Combined studies based on electrochemistry and spectroelectrochemistry demonstrate that (i) the electronic coupling across the oligoacene linkers between two diarylamine termini exponentially decrease with a moderate attenuation constant (β) of 0.14 Å−1 in these length-modulated systems and (ii) the associated radical cations [1a]+–[1e]+ are classified as the class II Robin–Day mixed-valence systems. Furthermore, density functional theory (DFT) calculations have been conducted to gain insight into the nature of electron transfer processes in these oligoacene systems.
Highlights
Oligoacenes and their derivatives have recently received an increasing interest because of their potential applications in a variety of optoelectronic devices such as light-emitting diodes (LEDs), photovoltaic cells, organic thin-film field-effect transistors (OFETs) and liquid crystals[11,12,13,14,15]
The target compounds 1a–1e were prepared via a Buchwald−Hartwig coupling of their corresponding oligoacene bromides with an excess of the appropriate aniline
The stability of 1e in the solution state was kept for several weeks when stored in air under dark conditions. 1e will degrade slowly when exposed to UV irradiation at 254 nm with the characteristic 1H NMR signals associated with the pentacene ring disappearing over time (See Fig. S1 in the Supporting Information)
Summary
Oligoacenes and their derivatives have recently received an increasing interest because of their potential applications in a variety of optoelectronic devices such as light-emitting diodes (LEDs), photovoltaic cells, organic thin-film field-effect transistors (OFETs) and liquid crystals[11,12,13,14,15]. Triarylamine has been identified as an almost ideal redox center to study the electron-transfer processes due to its reversibly oxidizable superiority[31,32,33,34,35,36,37,38,39,40,41]. As an ideal redox center, the triphenylamine moiety has been expected to act as the desirable probe for our evaluation of the electron transport in the oligoacene series. Our goal in the present study is to elaborate how a change in the extension of the fused aromatic ring will affect the redox, spectroscopic and electron transfer properties of the investigated series of compounds. We have employed a joint experimental and theoretical method involving electrochemistry, UV-vis-NIR spectroelectrochemistry, and density functional theory calculations to make an in-depth discussion on a series of diamine compounds featuring variable-length oligoacene linkers (benzene, naphthalene, anthracene, tetracene and pentacene)
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