A R T I C L E S
Miao et al.
2
3,24
25
program)
and the GIAO-CMO method are computed at
aromaticity as a local ring-by-ring rather than a global molecular
property, 6 and 8 can be viewed as overall aromatic, yet the
central dihydropyrazine ring in both is strongly (locally)
antiaromatic, with an additional global component. However,
upon dehydrogenation the strong local antiaromaticity disap-
pears. On the other hand, part of the relative stability of 6 and
8 comes from the increased HOMO-LUMO gap (discussed
below), another experimental and calculational observation that
is at odds with both simple H u¨ ckel theory and the expectations
fuelled by antiaromatic compounds such as cyclobutadiene.
While 8 is easily isolated and characterized, 6a and 6b are less
persistent, although 6b is sufficiently stable to be characterized.
Moreover, the HOMO-LUMO gap of aromatic 5a,b is
smaller than that of the formally antiaromatic 6a,b. As shown
in Figure 9 for 5′ and 6′, the computed LUMO energy difference
(-3.0 eV for 5′ vs -1.5 eV for 6′) is greater than that of the
HOMO (-5.2 eV for 5′ to -4.6 eV for 6′). Hence, the band
gap increases from 2.3 eV for 5′ to 3.1 eV in 6′. The
experimental optical band gaps, obtained from the intersection
of absorption and emission traces of 5a,b and 6a (2.1 and 2.6
eV, respectively) agree reasonably well. For comparison, the
solution electrochemical HOMO-LUMO gap is 2.16 eV.
PW91/IGLO-III//B3LYP/6-311+G** for uniformity.
As expected, all the individual local NICS(0)πzz values of 7 and
the overall ΣNICS(0)πzz sums are highly diatropic and are similar
to those of tetracene (Figure 10). Both their total LMO and total
CMO NICS(0)πzz data are almost identical. The LMO details are
instructive. Note that the “local” and “total” LMO NICS(0)πzz data
for both 7 and tetracene are very similar. Evidently, the large
negative NICS(0)πzz values of the individual rings are due to their
23
“
local” diatropicity (i.e., the π contributions involving each ring
itself). The “remote” π contributions (from the other rings) are
rather small. In line with Clar’s rule, which states that structures
with a maximum number of localized sextet rings are advantageous
energetically, the diatropic contributions of the aromatic subunits
of both 7 and tetracene are localized. The inner rings of 7 and also
2
6
more diatropic than the outer rings.
The behavior of 8 is different. Both LMO and CMO
ΣNICS(0)πzz(-28 ( 2 ppm) reveal 8 to be weakly aromatic
despite its 4n π perimeter (Figure 10). Note that the paratropic
contributions of the dihydropyrazine ring of 8 are rather
delocalized (see Figure 10). Unlike 7, the “remote” LMO
contributions of the individual aromatic rings of 8 (shown in
green) are distinctly paratropic. For this reason, the ΣNICS(0)πzz
of both tetracene and 7 (approximately -145 ppm) are much
greater in magnitude than that of 8. Note that the benzenoid
rings of 8 have significantly less diatropic individual “total”
LMO NICS(0)πzz values compared to their corresponding rings
in 7, but their “local” LMO NICS(0)πzz values are similar (see
Figure 10). The “local” aromaticites of the bezenoid rings of 8
are preserved, despite the presence of the antiaromatic dihydro
ring.
Conclusion
In conclusion, we have prepared a redox interconvertible pair
of heterocyclic tetracene derivatives, the aromatic 5a,b and 7
and their dihydro derivatives 6 and 8. These compounds are
potentially useful as active layers in thin-film transistors. The
formation of the requisite films for 5-8 is challenging and under
current investigation. While 5a,b are persistent, 6a,b are readily
oxidized to 5a,b in air. The 4n+2 π species, 5a,b and 7, have
small band gaps and are red-fluorescent. In contrast, their 4n π
dihydro derivatives, 6a,b and 8, are green-fluorescent and have
significantly larger band gaps. The 4n π compounds, 6a,b and
The magnetic criteria suggest that 8 and its derivatives 6a,b
are aromatic, even though less so than 7 and 5a,b, at least when
aromaticity is viewed as a global property. If one considers
8
5
, display significantly reduced aromaticity when compared to
a,b and 7. The paratropicity of the antiaromatic (4n π)
(
(
21) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H. J.; Hommes,
N. J. R. V J. Am. Chem. Soc. 1996, 118, 6317–6318.
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Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. ReV. 2005,
dihydropyrazine subunits is delocalized throughout the system.
Thus, although they are easily oxidized, large 4n π compounds
like 6a,b, 6′, and 8 are not appreciably destabilized relative to
their 4n+2 π congeners, 5a,b, 5′, and 7.
1
05, 3842–3888. (c) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.;
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8
(
(
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Acknowledgment. We thank the National Science Foundation
(STC Program, DMR-0120968 (S.B., S.R.M.) and Grants CHE
0
716718 (P.S., J.W.) and CHE 0548423 (U.B., S.B.M., S.M.B.)
for financial support and Dr. J. Anthony and A. Appleton for helpful
discussions.
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(
(
1
Supporting Information Available: Synthetic details and
NMR spectra as well as the CIF files for 5-8. This material is
available free of charge via the Internet at http://pubs.acs.org.
Org. Lett. 2003, 5, 23–26. (c) Heine, T.; Schleyer, P. v. R.;
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344 J. AM. CHEM. SOC. 9 VOL. 130, NO. 23, 2008