C O M M U N I C A T I O N S
Figure 3. Electronic absorption spectra of 2 and 5 in dichloromethane.
(Inset) Spectra of 2 and 5 in dichloromethane and of 5 in hexane.
Figure 2. X-ray crystal structure of heptacene 5.
Acknowledgment. We are grateful to the Office of Naval
Research and the National Science Foundation (MRI No. 0319176)
for support of this work. We thank Prof. M. R. Wasielewski
(Northwestern University) for helpful discussion.
chromatography on silica gel yielded a solution of heptacene 5,
which upon concentration and cooling yielded tiny, yellow-green
flattened needlelike crystals (Figure 2). The crystals shattered if
cooled below 150 K, but were stable at 200 K. Despite their size,
diffraction was sufficiently sharp from one of the largest single
needles (crystal size 0.15 × 0.02 × 0.005 mm, mass ca. 15 ng) for
structure solution. The asymmetric unit contained one and two
halves molecules of 5 with extensive disorder in the TTMSS groups.
Nonetheless, the structure of heptacene 5 is unambiguous (Figure
2).16 As with hexacene 2, the aromatic core of heptacene 5 is
essentially planar.
Supporting Information Available: Procedures for the preparation
of 1-5, characterization data, crystallographic CIF files for 2 and 5,
UV-vis-NIR spectra of silylethynyl-substituted anthracene-hepata-
cene. This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Reese, C.; Roberts, M.; Ling, M.-M.; Bao, Z. Mater. Today 2004, 20.
(2) Campbell, R. B.; Robertson, J. M. Acta Crystallogr. 1962, 15, 289.
(3) (a) Clar, E. Chem. Ber. 1939, 72B, 2137. (b) Marschalk, C. Bull. Soc.
Chim. 1939, 6, 1112. (c) Angliker, H.; Rommel, E.; Wirz, J. Chem. Phys.
Lett. 1982, 87, 208.
While solutions of 5 decompose in a few hours when exposed
to air, the crystals are relatively stable, remaining unchanged for
up to a week exposed to air and laboratory lighting. Heptacene 5
(4) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104, 4891.
(5) Bailey, W. J.; Liao, C.-W. J. Am. Chem. Soc. 1955, 77, 992.
(6) Clar, E. Chem. Ber. 1942 75, 1283.
1
is very soluble, allowing characterization by H and 13C NMR, as
well as cyclic voltammetry. The oxidation (0.470 V vs SCE) and
reduction (-0.830 V vs SCE) potentials again show an electro-
chemical HOMO-LUMO gap that matches well with the gap
extracted from the absorption edge of the optical spectrum (1.30
eV redox, 1.36 eV (912 nm) optical).
(7) Miller, G. P.; Briggs, J. Org. Lett. 2003, 5, 4203.
(8) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang, Q.; Sonmez, G.; Yamada,
J.; Wudl, F. Org. Lett. 2003, 5, 4433 and references therein.
(9) (a) Schleyer, P. v. R.; Manoharan, M.; Jiao, H.; Stahl F. Org. Lett. 2001,
3, 3643. (b) Wiberg, K. J. Org. Chem. 1997, 62, 5720. (c) Houk, K. N.;
Lee, P. S.; Nendel, M. J. Org. Chem. 2001, 66, 5517 and references
therein.
While there has been speculation that unsubstituted heptacene
may possess an open-shell ground state, the sharp 1H NMR signals
obtained for 4 and 5 confirm that these functionalized heptacenes
are closed-shell species. Analysis by EPR spectroscopy further
supports the closed-shell nature of these compounds.
(10) Bendikov, M.; Duong, H. M.; Starkey, K.; Houk, K. N.; Carter, E. A.;
Wudl, F. J. Am. Chem. Soc. 2004, 126, 7416.
(11) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15. (b)
Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem.
Mater. 2004, 16, 4980.
(12) For example, Morgenroth, F.; Berresheim, A. J.; Wagner, M.; Mu¨llen, K.
Chem. Commun. 1998, 1138.
(13) Payne, M. M.; Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2004,
6, 3325.
The absorption spectra of 2 and 5 are presented in Figure 3.
Hexacene 2 has its longest-wavelength λmax at 738 nm, correspond-
ing to a 100-nm red-shift compared to that of a similarly substituted
pentacene. Unlike 2, heptacene 5 shows strong evidence for
aggregate formation in its absorption spectrum. The longest-
wavelength absorption in a good solvent (CH2Cl2) is at λmax ) 852
nm, with a pronounced shoulder at 825 nm. In a poor solvent
(hexanes) this absorption is blue-shifted to λmax ) 835, with a
second sharp peak at λmax ) 810 nm. Nevertheless, the fusion of
an additional aromatic ring leads to a further ∼100 nm red-shift in
absorption compared to hexacene. A plot of the absorption edge
of silylethynylated acenes anthracene-heptacene versus 1/(acene
length) yields a y-intercept of 0.16 eV, suggesting a small but
nonzero energy gap for the corresponding polymer, polyacene.
(14) X-ray data for 2; triclinic, space group P1h; a ) 8.5611(2) Å, b ) 15.5772-
(4) Å, c ) 18.1567(6) Å, R ) 75.3201(12)°, â ) 81.3490(11)°, γ )
80.6678(12)°, V ) 2296.09(11) Å3; Z ) 1; Dc ) 1.118 Mg‚m-3; T )
90.0(2) K. Nonius KappaCCD diffractometer, graphite-monochromated
sealed-tube Mo KR X-rays. Structure solved and refined using SHELX-
97 (Sheldrick, G. M. Programs for crystal structure solution and
refinement, Uni. Go¨ttingen, Germany). Final R(F) ) 5.47%, wR2(F2) )
13.92% for 540 parameters and 10465 data.
(15) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV. Mater.
2003, 15, 2009.
(16) X-ray data for 5; triclinic, space group P1h; a ) 9.2305(4) Å, b ) 24.6811-
(13) Å, c ) 25.4062(12) Å, R ) 94.319(3)°, â ) 95.069(3)°, γ ) 91.020-
(3)°, V ) 5747.3(5) Å3; Z ) 1+2(1/2); Dc ) 1.063 Mg‚m-3; T ) 200(2)
K. Bruker-Nonius ×8 Proteum diffractometer, multilayer-optic conditioned
rotating-anode Cu KR X-rays. Structure solved and refined as in ref 14.
Final R(F) ) 8.06%, wR2(F2) ) 13.92% for 1142 parameters and 14577
data.
JA051798V
9
J. AM. CHEM. SOC. VOL. 127, NO. 22, 2005 8029