Abstract
This short critical review is devoted to the synthesis and functionalization of various types of azaacenes, organic semiconducting compounds which can be considered as promising materials for the fabrication of n-channel or ambipolar field effect transistors (FETs), components of active layers in light emitting diodes (LEDs), components of organic memory devices and others. Emphasis is put on the diversity of azaacenes preparation methods and the possibility of tuning their redox and spectroscopic properties by changing the C/N ratio, modifying the nitrogen atoms distribution mode, functionalization with electroaccepting or electrodonating groups and changing their molecular shape. Processability, structural features and degradation pathways of these compounds are also discussed. A unique feature of this review concerns the listed redox potentials of all discussed compounds which were normalized vs. Fc/Fc+. This required, in frequent cases, recalculation of the originally reported data in which these potentials were determined against different types of reference electrodes. The same applied to all reported electron affinities (EAs). EA values calculated using different methods were recalculated by applying the method of Sworakowski and co-workers (Org. Electron. 2016, 33, 300–310) to yield, for the first time, a set of normalized data, which could be directly compared.
Highlights
Synthesis of AzaacenesFirst syntheses of azaacenes were reported at the end of 19th century by Fischer and Hepp [38,39]
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Acenes cannot be used as semiconductors transporting electrons in any organic electronic devices, including n-channel organic field effect transistors (OFETs), because their LUMO level is too high, implying low electron affinity (EA) values and by consequence very difficult electron injection [7,8]
Summary
First syntheses of azaacenes were reported at the end of 19th century by Fischer and Hepp [38,39]. This may induce steric problems which together with lower reactivity of o-dihalides containing electron accepting groups, may in some cases result in lowering the reaction yield [54] In view of these briefly discussed shortcomings of Buchwald-Hartwig coupling in the synthesis of azaacenes containing strongly electron accepting substituents, alternative approaches were proposed. The first reduction potential of azaacenes can be significantly increased and the band gap narrowed, as a result of two main factors or a combination of them: (i) increased number of aromatic rings and (ii) functionalization with electron withdrawing substituents These effects were discussed in detail in reference [69]. Decreasing (Eox2–Eox1) value in the case of azaacenes with larger central units can be rationalized by the fact that larger conjugated cores of the central unit better promote delocalization of the positive charge imposed upon the removal of the first electron, facilitating in this manner the removal of an additional electron from the second triphenylamine substituent
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