The influence of terminal push-pull substitution on the electronic structure and optical properties of pentacenes |
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Authors: | Tönshoff Christina Bettinger Holger F |
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Affiliation: | 1. Institut für Organische Chemie, Universit?t Tübingen, Auf der Morgenstelle 18, 72076 Tübingen (Germany), Fax: (+49)?7071‐29‐5244;2. Lehrstuhl für Organische Chemie 2, Ruhr‐Universit?t Bochum, 44780 Bochum (Germany) |
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Abstract: | The synthesis of 2,3-R(2)-9,10-(OMe)(2)-substituted pentacenes (R=OMe, F, Br, CN; 1-4) from 2,3-R(2)-9,10-dimethoxy-6,13-dihydro-6,13-ethanopentacene-15,16-diones (α-diketone-bridged precursors) by photochemically induced bis-decarbonylation (Strating-Zwanenburg reaction) is described. Under matrix-isolation conditions (solid Ar, 10 K) the S(1) transitions of 1 and 2 undergo hypsochromic and those of 3 and 4 bathochromic shifts compared to parent pentacene. The S(1) transition wavelengths correlate well with the difference of substituent parameters σ(p). A computational analysis of the excited states at the CAM-B3LYP/6-311+G** level of theory provides an assignment of the electronic transitions. Photolysis in solution at room temperature yields red [R=OMe (1)], blue [R=Br (3), F (2)], and green [R=CN (4)] pentacenes. The compounds are oxygen-sensitive and have low solubility, but their formation can be monitored by UV/Vis and, in the case of R=CN, also by (1)H NMR spectroscopy. The S(1) transition in 4 does not show the typical pentacene fine structure in the electronic absorption spectrum. Photogeneration in the presence of oxygen leads to a number of photoproducts that could be identified by monitoring the reaction by (1)H NMR spectroscopy for R=OMe. |
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Keywords: | acenes density functional calculations donor–acceptor systems matrix isolation photochemistry |
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