TY  - GEN
AV  - public
ID  - heidok33641
Y1  - 2023///
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/33641/
A1  - Zong, Wansheng
CY  - Heidelberg
TI  - Synthesis and Characterization of Large Azaacenes and Stable Azaacene Radical Anions
N2  - The objective of this thesis was the synthesis and characterization of larger (aza)acenes (> pentacenes) and their radical anions. The thesis focuses on the stabilization of azaacenes by positioning the silylethynyl groups adjacent to the central pyrazine / pyrazinopyrazine rings, which renders azaacenes immune to Diels-Alder reactions, butterfly dimerization and endoperoxide formation (Figure 1). 
Six (aza)heptacenes containing 0, 2, 4 or 6 nitrogen atoms and four triisopropylsilyl (TIPS) ethynyl substituents were synthesized. The position of the four TIPS ethynyl substituents determines the (aza)heptacenes? stability. The symmetric 7,16-diaza-6,8,15,17-tetrakis (triisopropylsilylethynyl)heptacene (DAH1), in which the two nitrogen atoms occupy the central ring and the four TIPS-ethynyl substituents fill the four positions in the two directly adjacent rings, is much more stable than any hitherto reported (aza)heptacenes and survives several weeks in solution on the benchtop according to UV-Vis studies. Thin film transistors with ?e = 0.042 cm2V-1 s-1 were produced for DAH1. The tetraazaheptacene (TAH) is also reasonably stable (> 7 d), but hexaazaheptacene (HAH) is such a strong oxidant that undergoes spontaneous reduction (? 5 d) according to UV-Vis studies.
A series of 5,8,13,16-tetraethynyl-6,7,14,15-tetraazahexacenes were synthesized ? all were persistent in solution. Upon crystallization, the derivatives with TIPS ethynyl substituents equilibrate into their didehydrotetrazecines (cyclic biscarbodiimide) isomers in the solid state. This solid-state ring opening is enforced by the bulk of the substituents, crystal packing energy and aromaticity given in terms of Clar sextets ? with increasing steric bulk the crystal lattice itself enforces ring opening. This process is fully reversible, upon dissolution, the steric force disappears and the bis-carbodiimides revert into the azaacene form. The reversible rearrangement process was also observed in the pentacene and heptacene congeners.
ER  -