On the Homoconjugation of Two Acceptor Groups

Homoconjugation and Intramolecular Charge Transfer in Extended Aromatic Triptycenes with Different π-Planes

“…Oxidation of the third arm of the extended triptycenes was again performed in a mixture of acetic acid and dichloromethane at room temperature. Here, the concentration of nitric acid needed to be increased to 2.5 mol/L in comparison to former oxidations, giving already a hint that the two electron-poorer aromatic paddles are influencing the reactivity of the remaining veratrole unit via transannular or through-space interactions. ,− , By applying this higher concentration of the oxidant, triptycenes 5 ( F 2 Q 1 ) and 13 ( F 1 P 1 Q 1 ) were isolated in 73 and 89% yields, respectively. Under the same conditions, a full oxidation of P 2 V 1 to P 2 Q 1 ( 10 ) required 3 h of reaction time to give the desired product in 89%.…”

“…Because triptycene possesses three aromatic wings, it offers the opportunity to place up to three different extended aromatic units in close and exact proximity to each other in various ratios to study homoconjugation or charge-transfer effects in more detail by a combination of UV/vis spectroscopy, cyclovoltammetry, and DFT calculations. This idea was first time already discussed in 1963 by de Groot and J. H. van der Waals and investagetd by using electron resonance spectroscopy. , To our surprise, there are only three examples in the literature where three different aromatic units are ortho -connected via the two methine bridges. − De Wit and Wynberg reported on “ interannular π–π interactions between the aromatic rings in these systems ”, which have been identified by a combination of UV/vis and CD spectroscopy, comparing two triptycene isomers with a benzene, a naphthalene, and a 2,3- or 3,4-thienyl unit. − …”

“…Indeed, hydrogen abstraction did not occur under conditions that, e.g., lead to triphenylmethyl cations for analogue systems . Later on, the following question arose: if the π-electrons of the three phenylene rings are somehow in conjugation through space (which is often called homoconjugation) and influence each other electronically or if these react similarly as three independent benzene or xylene rings. − Bartlett already reported in one of his seminal contributions the different oxidation potential of triptycene quinone in comparison to ortho -xyloquinone and benzoquinone as well as the observation that the maximum of the absorption spectrum of triptycene is shifted by about 6–7 nm in comparison to triphenylmethane, suggesting that there is some kind of conjugation . Thereafter, it was long debated whether such a homoconjugation effect exists or not. − In 1983, Martin et al unequivocally proved the existence of homoconjugation in triptycenes by photoelectron spectroscopy in combination with theoretical calculations .…”

1,4,5,8-Tetramethoxyanthracene

“…There has been considerable ongoing research involving the electrochemistry of compounds incorporating multiple quinones. Some of these compounds are anthracene derivatives (Joze®ak et al, 1989), while others are based on triptycene architecture (Doerner et al, 1998;Joze®ak et al, 1989;Quast & Fuchsbauer, 1986;Russell & Suleman, 1981). 1,4,5,8-Tetramethoxyanthracene (1,4,5,8-TMA), (I), has been used as an intermediate in the syntheses of some of these compounds.…”

Synthesis of a novel building block for the preparation of multi-chromophoric sensitizers for panchromatic dye-sensitized solar cells

“…This approach provides a convenient pathway for 3 synthesizing various hetero-aromatic constructs that feature extended domains of aromaticity and specific peripheral functionality. [7][8][9][10][11][12][13][14][15][16][17][18][19][20] This condensation reaction can be used to construct complex motifs in which chromophores and anchoring groups are present on a fused heteroaromatic scaffold suitable for mediating electron transfer. [6,20] Although the synthesis reported previously for P 2 -PZ was successful, the final step employed harsh acidic conditions to generate the carboxylic acid functionality from the di-nitrile precursor (Scheme 1), and this in turn required a subsequent Williamson ether synthesis to repair the damaged ether functionalities.…”