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Fig. 15 Mid-IR spectra (Ar, 10 K) of the photochemical decomposition of 7 and formation of
heptacene. Bottom ( red ): IR difference spectrum after 2.5 h of irradiation (385
450 nm);
bands pointing downward decrease during irradiation and those pointing upward increase. Middle
( blue ): IR spectrum of 37 obtained after deposition for 75 min at 195 C. Top ( black ): IR spectrum
computed for 7 at the RB3LYP/6-31G* level of theory scaled by 0.985. Reprinted with permission
from [ 22 ]. Copyright 2009 American Chemical Society
< λ <
Fig. 16 The SOMO-1 ! SOMO transitions of the radical ions of acenes measured in solid argon
at 10 K. Reprinted with permission from [ 22 ]. Copyright 2009 American Chemical Society
a yellow substance was washed from the substrate. This could be investigated by
LDI-TOF-MS and showed the mass of the heptacene dimer among other signals at
higher masses (presumably oxygenated products) [ 22 ]. As the concentration of
oxygen is low in the matrix isolation system, oxidation occurs presumably after
opening of the system to air and during sample preparation for MS measurements.
Note that the colorless pentacene photodimer 38 (from a [4+4] cycloaddition,
Fig. 17 ) is reported to be easily oxidized under ambient conditions, although it
only consists of naphthalene subunits [ 38 ]. Hence, facile oxidation of heptacene
dimers is also likely.
Anthony and co-workers reported that their silylethynyl hexacene dimers,
although well defined in the solid state, only gave the mass of the monomer in
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