ature, and the reaction mixture was stirred overnight. Water and
CH2Cl2 were added to the reaction mixture in sequence. The CH2-
Cl2 extract was dried over anhydrous Na2SO4 and concentrated to
dryness. Chromatography of the residue with CH2Cl2 afforded 5
(13.73 g, 95%). Mp: 185 °C (dec.). 1H NMR (300 MHz, CDCl3):
δ 10.414 (2H, s), 8.373 (2H, s). 13C NMR (75 MHz, THF-d8): δ
191.1, 141.9, 137.2, 116.2. IR (neat): 1692. HREI: calcd for
(C8H4O2I2) 385.830 083, found 385.829 281.
4,5-Bis(trimethylsilylethynyl)benzene-1,2-dicarbaldehyde (6a).
5 (0.772 g, 2 mmol), trimethylsilylacetylene (0.45 mL, 3.2 mmol),
CuI (0.03 g, 0.16 mmol), and Pd(PPh3)2Cl2 (0.056 g, 0.08 mmol)
were put together and purged with argon for 10 min. THF (20 mL)
and NEt3 (8 mL) were added in sequence. The reaction mixture
was stirred at room temperature under argon overnight. After all
solvents were removed, CH2Cl2 and water were added to the
residue. The CH2Cl2 layer was separated and dried with anhydrous
Na2SO4. Removal of CH2Cl2 resulted in a solid residue. Chroma-
tography of the residue with CH2Cl2 gave a yellow solid (6a, 0.52
g, 79%). Mp: 135-137 °C. 1H NMR (300 MHz, CDCl3): δ 10.457
(2H, s), 8.025 (2H, s), 0.299 (18H, s). 13C NMR (75 MHz,
CDCl3): 190.9, 134.9, 134.7, 130.9, 104.88, 101.3, -0.25. IR
(neat): 1694. HREI: calcd for (C18H22O2Si2) 326.115 837, found
326.115 193.
2,3,9,10-Tetrakis(trimethylsilylethynyl)-6,13-pentacenequino-
ne (7a). 6a (0.163 g, 0.5 mmol) and 1,4-cyclohexanedione (0.031
g, 0.276 mmol) were dissolved in C2H5OH (9 mL) at room
temperature. KOH (1 M aqueous, 0.1 mL) was added to it. The
reaction mixture was stirred at 55 °C for 5 min. The red-brown
precipitate was collected with vacuum filtration, washed with water,
and then dried at 60 °C in a vacuum oven overnight to give a yellow
solid 6a (0.102 g, 60%). Mp: >320 °C. 1H NMR (300 MHz, THF-
d8): δ 8.833 (4H, s), 8.369 (4H, s), 0.329 (4H, s). 13C NMR (75
MHz, THF-d8): δ 182.0, 135.1, 135.0, 132.8, 129.5, 126.38, 103.6,
101.0, 0.1. IR (neat): 1675. HREI: calcd for (C42H44O2Si4)
692.241 845, found 692.242 263.
FIGURE 3. Thin-film absorption spectra of 9a-c.
unchanged from each at -1.30 eV (vs ferrocene), whereas 9c
differs in the HOMO energy level from 9a and 9b. The
difference in the HOMO energy level may be attributed to the
weakly π-donating property of 3,5-di-tert-butylphenyl moieties
and the weakly π-accepting property of the silyl groups. We
anticipate that attaching different electron-donating or -accepting
moieties to the terminal rings of pentacene may tune or even
alter the electronic properties of pentacene derivatives. The
attachment of substituents on the terminal ring can be realized
by the reliable palladium-based coupling reaction with 4,5-
diiodobenzene-1,2-dicarbaldehyde. The HOMO-LUMO gaps
estimated from the oxidation and reduction potentials are 1.735,
1.760, and 1.693 eV for 9a, 9b, and 9c, respectively. Those
values are among the lowest HOMO-LUMO gaps reported in
the literature for pentacene derivatives.12-15
Compared with their absorptions in solution, compounds
9a-c all demonstrated a significant red shift (or displacement)
of the absorption spectrum in thin films grown from their CHCl3
solutions (Figure 3). The red shifts, characteristic for conjugated
organic compounds, indicate strong electronic interaction be-
tween molecules in the films.23,24 A displacement of 68, 37,
and 35 nm with respect to the absorptions in CH2Cl2 solutions
was obtained in films for compounds 9a, 9b, and 9c, respec-
tively. In addition, peak broadening was also observed in their
thin-film absorptions, indicating strong intermolecular interac-
tion. These thin films were quite stable. When exposed to the
light and air in the lab, no significant change was observed in
their absorptions for two weeks.
2,3,6,9,10,13-Hexa(trimethylsilylethylnyl)-6,13-dihydropenta-
cene-6,13-diol (8a). Isopropylmagnesium chloride (2 M in THF,
2.2 mL, 4.4 mmol) and trimethylsilylacetylene (0.62 mL, 4.35
mmol) were dissolved in THF (5 mL) and heated for 2 h at 60 °C.
7a (0.22 g, 0.32 mmol) was added. The reaction mixture was stirred
at 60 °C overnight and quenched by adding 6% aqueous NH4Cl.
CH2Cl2 was added. The CH2Cl2 extract was washed with saline
twice and then with water and dried with anhydrous Na2SO4.
Removal of CH2Cl2 resulted in a solid residue, chromatography of
which with CH2Cl2/hexane (2:1) produced a yellow solid 8a (0.192
1
We have shown that the synthetic scheme designed is viable
for the preparation of symmetrical silylethynylated pentacenes.
The attachment of ethynyl moieties on terminal rings offers more
opportunities to tune the electronic properties of the molecules.
Prompted by the results that they are soluble and stable and by
their strong aggregation in thin films, investigation of their
potential charge-carrier mobility is currently underway.
g, 67%). Mp: 250 °C (dec.). H NMR (300 MHz, THF-d8): δ
8.505 (4H, s), 8.110 (4H, s), 5.960 (2H, s), 0.307 (36H, s), 0.210
(18H, s). 13C NMR (75 MHz, THF-d8): δ 140.3, 133.3, 132.8,
126.1, 123.5, 109.1, 104.4, 98.3, 91.2, 69.4, 0.4, 0.2. IR(neat): 2143,
839. Anal. Calcd: C, 70.21; H, 7.25. Found: C, 70.16; H, 7.31.
2,3,6,9,10,13-Hexa(trimethylsilylethynyl)pentacene (9a). 8a
(0.136 g, 0.15 mmol) was dissolved in THF (5 mL). A solution of
SnCl2 (0.172 g) in 3 M HCl (0.86 mL) was added with a syringe
under argon. The reaction mixture was sonicated for 2.5 h. Filtration
under argon gave a deep blue solid 9a (0.124 g, 95%). Mp: 285
Experimental Section
1
4,5-Diiodobenzene-1,2-dicarbaldehyde (5). Oxalyl chloride (10
mL, 0.116 mol) was dissolved in CH2Cl2 (187 mL) at -78 °C. A
solution of DMSO (18.75 mL, 0.264 mol) in CH2Cl2 (37.5 mL)
was added in 2.5 h. The solution was stirred at -78 °C for another
10 min. A solution of 4 (14.6 g, 0.037 mol) in DMSO (7.5 mL)
and THF (75 mL) was added slowly in 1.5 h at -78 °C. After the
reaction mixture was stirred at -78 °C for 5 h, NEt3 (53 mL) was
slowly added. The temperature was raised slowly to room temper-
°C (dec.). H NMR (300 MHz, CDCl3): δ 9.047 (4H, s), 8.198
(4H, s), 0.557 (18H, s), 0.361 (36H, s). 13C NMR (75 MHz,
CDCl3): δ 133.7, 131.0, 130.6, 126.4, 121.7, 118.9, 111.3, 103.5,
102.0, 99.2, 0.3, 0.1. IR(neat): 2144, 837. HRFAB: calcd for
(C52H62Si6 + H ) 855.354 548, found 855.355 000.
Acknowledgment. The authors thank Dr. Felix N. Castellano
for allowing us to use his group’s instruments. We sincerely
thank Dr. Brigitte Wex for many insightful discussions. We
thank Dr. Bipin Shah for his help in preparing the manuscript.
The authors appreciate help from Radiy Islangulov and Solen
Kinayyigit in measuring fluorescence lifetimes and the elec-
trochemical data. B.R.K. thanks the University Research Board
(23) Ostroverkhova, O.; Shcherbyna, S.; Cooke, D. G.; Egerton, R. F.;
Hegmann, F. A.; Parkin, S. R.; Anthony, J. E. J. Appl. Phys. 2005, 98,
033701.
(24) Pope, M.; Swenberg, C. E. Electronic Processes in Organic crystals
and Polymers, 2nd ed.; Oxford University Press: New York, 1999.
J. Org. Chem, Vol. 71, No. 5, 2006 2157