A Way to Stable, Highly Emissive Fluorubine Dyes: Tuning the Electronic Properties of Azaderivatives of Pentacene by Introducing Substituted Pyrazines

Abstract

Pentacene and its derivatives are among the most important examples of π-electron-rich molecules used in organic field effect transistors. The replacement of CH groups by nitrogen atoms opens an elegant way to generate highly electron-deficient molecules, known as oligoazaacenes. We describe the synthesis and spectroscopic properties of two novel derivatives of this family, namely the zwitterionic and quinoidal conjugated forms of dihydro-5,6,7,12,13,14-hexaazapentacene (fluorubine). We outline a powerful strategy to tune the electronic properties of these redox-active azaacenes by the selective introduction of substituted pyrazines. Their acidochromic and solvatochromic behaviour is investigated experimentally and interpreted with the help of theoretical calculations. The simple "exchange" of substituents or protonation is shown to significantly alter the spectroscopic and electronic properties of these remarkably stable π-systems. Their exceptional optical properties, such as high fluorescence quantum yields combined with a redox-active behaviour, make them promising candidates for sensor materials. Additional marked features in the solid state, such as herringbone packing in combination with short π-π distances, will open access to electronic materials.

Stable and Easily Accessible Functional Dyes: Dihydrotetraazaanthracenes as Versatile Precursors for Higher Acenes

“…Furthermore, we could not detect NH signals in the 1 H NMR spectrum. Additionally, the pentacene obtained shows similar spectroscopic properties to that of the mesoionic hexaazapentacene developed by Fleischhauer et al.,33 particularly with respect to the broad‐structured absorption behaviour in the orange to red region of the visible spectrum. However, the absorption is, perhaps due to the electron‐withdrawing cyano residues, slightly redshifted by approximately 21 nm compared with the hexaazapentacene described by Fleischhauer.…”

WITHDRAWN: Hydrogen-bonding versus π–π stacking in the design of organic semiconductors: From dyes to oligomers

“…The crystalline structure of the N-heteroacenes has yet to be elucidated [82]; however, it is assumed [83] that similar to the acenes, a herringbone orientation of the neighboring molecules is characteristic for the majority of the non-substituted N-heteroacenes (Fig. 19).…”

Fluorescent mesomorphic pyrazinacenes

“…), could find use as novel thermoresponsive detectors or switches with appropriate modification using hydrophilic ''dendritic wedges'' as described here. We have also observed (as have others in similar compounds 29 ) the variation of electronic absorption and emission wavelengths depending on local acidity in these compounds and we are currently studying this feature with a few to develop pH indicators for use in non-polar solvents. 49 These compounds are worthy targets for use in fluorescence lifetime imaging applications 36 and we will report our efforts in that direction later.…”

“…Fleischhauer and co-workers have prepared a series of highly fluorescent pyrazinacenes based on the fluorubine core which showed solvatochromic and halochromic properties. 29 Here, we present the synthesis and characterisation of a series of compounds based on a similar fluorubine (5,6,7,12,13,14-hexaaza-6,13-dihydropentacene) core. Simple N-alkylation can give a range of highly photoluminescent compounds exhibiting a variety of liquid-crystalline phases, and which can form fluorescent gels in various solvents.…”

“…30 Fleischhauer and co-workers utilised condensation of substituted phenylenediamines with tetrachloropyrazine followed by oxidation to give substituted fluorubines. 29 Their method allows a degree of control over the position of substitutions when preparing N-substituted pyrazinacenes. Our approach to the synthesis of fluorubine derivatives and other pyrazinacene compounds involves a double nucleophilic attack at the 2,3…”