Chemistry Reference
In-Depth Information
The data reported by Kaur et al.[
62
] are briefly summarized: the LDI-TOF-MS
showed the mass required for the nonacene 75. The UV/vis-NIR spectra showed
weak absorptions at 1,033, 921, and 846 nm, along with strong ones at 655, 600, and
550 nm. A strong blood-red fluorescence with a maximum below 700 nm was
observed. The electronic ground state of nonacene derivatives 75 and 76 was also
investigated computationally [
63
]. As with the parent acenes larger than hexacene
[
55
], an instability of the spin-restricted Kohn-Sham description was detected.
The identity of the nonacene 75 was challenged by the Anthony group on the
basis of the very weak S
0
!
S
1
transition and the fluorescence in the visible
[
61
]. Purushothaman et al. [
61
] argued that an alternant hydrocarbon such as
nonacene 75 should emit from the S
1
state according to Kasha's rule. As the
S
0
!
S
1
transition of 75 is in the NIR, no fluorescence in the visible should be
observed. To support their argument, Purushothaman et al. measured the absorption
and fluorescence spectrum of the endoperoxide 71 derived from their nonacene 70c.
Although the substituents differ from Miller's nonacene, the endoperoxide shows
the typical vibrational fine structure of the
p
band of its pentacene chromophore
(roughly 650, 600, and 550 nm). The endoperoxide is strongly fluorescent and emits
just below 700 nm. The correspondence with the data reported by Kaur et al. [
62
]
led the Anthony group to conclude that the observations reported for 75
may arise
from a decomposition product that makes the bulk of the sample
[
61
]
.
5 Conclusions and Outlook
The chemistry of higher acenes dates back to the 1930s. Yet significant
improvements have only been made since the ground-breaking work of the
Anthony group who employed silylethynyl groups for kinetic stabilization of the
highly reactive
ˀ
systems of hexacene, heptacene, and nonacene. The rediscovery
of the Strating-Zwanenburg reaction for acene synthesis paved the way to the
photogeneration of the parent acene systems in PMMA or cryogenic matrices.
This approach made acenes up to nonacene available for spectroscopic study. It
remains to be seen whether yet larger acenes can be generated.
Acknowledgments We are grateful for continued support from the Deutsche Forschungsge-
meinschaft and the Fonds der Chemischen Industrie.
References
1. Clar E, John F (1930) Ber Dtsch Chem Ges 63B:2967
2. Philippi E (1929) Monatsh Chem 53(54):638
3. Clar E (1939) Chem Ber 72:2137
4. Wang C, Dong H, Hu W, Liu Y, Zhu D (2012) Chem Rev 112:2208
5. Pron A, Gawrys P, Zagorska M, Djurado D, Demadrille R (2010) Chem Soc Rev 39:2577
