Photochemical formation of substituted pentacenes

Article abstract of DOI:10.1021/jo800728p

(Chemical Equation Presented) Substituted pentacenes (8a, 8b, 14a, and 14b) were prepared by Strating-Zwanenburg photodecarbonylation of diones (7a, 7b, 13a, and 13b). The compounds are red and stable in the solid state under inert atmosphere as well as i

Full text of DOI:10.1021/jo800728p

Photochemical Formation of Substituted Pentacenes
Yuewei Zhao, Rajib Mondal, and Douglas C. Neckers*
Center for Photochemical Sciences,1 Bowling Green State UniVersity, Bowling Green, Ohio 43403
neckers@photo.bgsu.edu
ReceiVed April 1, 2008
Substituted pentacenes (8a, 8b, 14a, and 14b) were prepared by Strating-Zwanenburg photodecarbo-
nylation of diones (7a, 7b, 13a, and 13b). The compounds are red and stable in the solid state under
inert atmosphere as well as in degassed solutions, but not in air. Each is soluble in common organic
solvents where, unless protected, they are oxygen sensitive.
Introduction
More recent syntheses of pentacenes have employed the
elimination of 6 and 13 functional groups from dihydro
precursors other than carbonyl groups13 though Ono’s group
did report using the Strating-Zwanenburg reaction to synthesize
pentacene- and bromine-substituted derivatives.9a,b
Pentacene emerged in the early 1990s2 as the candidate
among organic semiconductors to be used in “plastic” electron-
ics.3 It has the most nearly ideal thin film characteristics so far
observed among p-type semiconductors4 and the highest charge
carrier (hole) mobilities of any small molecule.5,6 Pentacene,
however, is sensitive to oxygen, difficult to purify, and rather
insoluble in many organic solvents.7 There are few reports of
modified pentacene derivatives substituted at positions other than
6 and 13 positions,8,9 because the classic preparative methods
for preparing polyacenes reported by Clar,10 and later Bailey,11
do not allow for routine structural modification. These methods
used Meerwein Pondorf Verley reduction of dione assemblies
that, as well, led to incorrect conclusions about heptacene.12
Given the immense possibilities of poly(acenes), we have
explored flexible photochemical routes to 2,3,9,10-substituted
pentacenes. Photochemical reactions14 can be carried out at
ambient conditions, are hands off, and provide the opportunity
(1) Contribution No. 672 from the Center for Photochemical Sciences.
(2) (a) Garnier, F.; Peng, F. Z.; Horowitz, G.; Fichou, D. AdV. Mater. 1990,
2, 592. (b) Peng, X.; Horowitz, G.; Fichou, D.; Garnier, F. Appl. Phys. Lett.
1990, 57, 2013. (c) Dodabalapur, A.; Katz, H. E.; Torsi, L.; Haddon, R. C. Science
1995, 269, 1560. (d) Torsi, L.; Dodabalapur, A.; Katz, H. E. J. Appl. Phys. 1995,
78, 1088. (e) Torsi, L.; Dodabalapur, A.; Rothberg, L. J.; Fung, A. W. P.; Katz,
H. E. Science 1996, 272, 1462. (f) For a summary reference see: Laquindanum,
J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Chem. Mater. 1996, 8,
2542. (g) See: Dimitrakopoulos, C. D.; Maelenfant, P. R. L. AdV. Mater. 2002,
14, 99.
(9) (a) Uno, H.; Yamashita, U.; Kikuchi, M.; Watanabe, H.; Yamada, H.;
Okujima, T.; Ogawa, T.; Ono, N. Tetrahedron Lett. 2005, 46, 1981. (b) Yamada,
H.; Yamashita, Y.; Kikuchi, M.; Watanabe, H.; Okujima, T.; Uno, H.; Ogawa,
T.; Ohara, K.; Ono, N. Chem. Eur. J. 2005, 11, 6212. (c) Chan, S. H.; Lee,
H. K.; Wang, Y. M.; Fu, N. Y.; Chen, X. M.; Cai, Z. W.; Wong, H. N. C.
Chem. Commun. 2005, 66. (d) Stone, M. T.; Dnderson, H. L. J. Org. Chem.
2007, 72, 9776. (e) Anthony, J. E.; Gierschner, J.; Landis, C. A.; Parkin, S. R.;
Sherman, J. B.; Bakus, R. C. Chem. Commun. 2007, 4746. (f) Wang, Y. M.; Fu,
N. Y.; Chan, S. H.; Lee, H. K.; Wong, H. N. C. Tetrahedron 2007, 63, 8586.
(g) Takahashi, T.; Li, S.; Huang, W. Y.; Kong, F. Z.; Nakajima, K.; Shen, B. J.;
Ohe, T.; Kanno, K. I. J. Org. Chem. 2006, 71, 7967. (h) Perepichka, D. F.;
Bendikov, M.; Meng, H.; Wudl, F. J. Am. Chem. Soc. 2003, 125, 10190. (i)
Okamato, T.; Senatore, M. L.; Ling, M. M.; Mallik, A. B.; Tang, M. L.; Bao,
Z. N. AdV. Mater. 2007, 19, 3381. (j) Schuster, I. I.; Craciun, L.; Ho, D. M.;
Pascal, R. A., Jr Tetrahedron 2002, 58, 8875. (k) Benard, C. P.; Geng, Z.; Heuft,
M. A.; VanCrey, K.; Fallis, A. G. J. Org. Chem. 2007, 72, 7229.
(3) Dimitrakopoulos, C. D.; Pursushothaman, S.; Kymissis, J.; Callegari, A.;
Shaw, J. M. Science 1999, 283, 822.
(4) Leising, G.; Stadlober, B.; Haas, U.; Haase, A.; Palfinger, C.; Gold, H.;
Jakopic, G. Microelectron. Eng. 2006, 83, 831.
(5) Kraft, A. ChemPhysChem 2001, 2, 163.
(6) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104, 4891.
(7) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Secrist, T. Chem.
Mater. 2004, 16, 4960.
(10) (a) Clar, E.; John, F. Chem. Ber. 1929, 62, 3027. (b) Clar, E.; John, F.
Chem. Ber. 1930, 63, 2987. (c) Clar, E.; John, F. Chem. Ber. 1931, 64, 2194.
(11) Bailey, W. J.; Madoff, M. J. Am. Chem. Soc. 1953, 75, 5603.
(8) In that pentacenes have such good charge carrier properties, subsequent
preparation of substituted pentacenes, e.g., 6,13-bis(triisopropylsilylethynyl)-
pentacene, which are stable and have acceptable field mobilities was reported.
(12) (a) Clar, E. Ber. Dtsch. Chem. Ges. 1942, 75B, 1330. (b) Boggiano, B.;
Clar, E. J. Chem. Soc. 1957, 2681.
5506 J. Org. Chem. 2008, 73, 5506–5513
10.1021/jo800728p CCC: $40.75 2008 American Chemical Society
Published on Web 06/14/2008

Photochemical Formation of Substituted Pentacenes
SCHEME 1. Pentacenes: Unsymmetrical 2,3-Disubstitutiona
a Reagents and conditions: (i) n-BuLi, -50 to -60 °C; (ii) Suzuki coupling reaction, Cs2CO3, Pd(PPh3)4 (catalyst); (iii) n-BuLi, 1,2-dibromobenzene,
-50 to -60 °C; (iv) Chloranil, refluxing in dry toluene; (v) OsO4, MMO, water/acetone, Na2S2O4; (vi) Swern oxidation, trifluoroacetic anhydride, -70 °C;
(vii) 395 nm light or light g450 nm.
SCHEME 2. Pentacenes: 2,3,9,10 Multiple Substitutionsa
a Reagents and conditions: (i) n-BuLi, toluene, -50 to -60 °C; (ii) refluxing in dry toluene, 2 h; (iii) Suzuki coupling reaction, Cs2CO3, Pd(PPh3)4
(catalyst); (iv) OsO4, MMO, Na2S2O4; (v) Swern oxidation, trifluoroacetic anhydride, -70 °C; (vi) 395 nm light or light g450 nm.
to produce images and, in the current case, to prepare the
poly(acene) in situ. The conditions used in the case of
heptacene15–17 point to just a few of the advantages of the
Strating-Zwanenburg photoprocess.
affect appropriate substitutions, chloranil aromatization, and
osmium tetraoxide followed by modified Swern oxidation to
produce the appropriate bridged diones. The routes are distin-
guished by the critical intermediate dibromide 2 and tetrabro-
mide 9. The specific experimental processes, of which the
“benzyne” step proved most problematic (low yields because
of water-sensitivity of BuLi and the poor solubility of the
intermediate compound), are detailed in the Experimental
Section or the Supporting Information. Each dione was found
to be exceedingly photosensitive requiring protection from light
both during and after synthesis.
Results and Discussion
Scheme 1 was used for the syntheses of 2,3-disubstituted
pentacenes which we called “one-sided”, and Scheme 2 was
used for 2,3,9,10-tetrasubstituted pentacenes, i.e. “two-sided”.
Both use the critical pentaene 1 with subsequent “naphthyne”
cycloadditions to build the pentacene ring, Suzuki coupling to
(14) (a) Turro, N. J. Angew. Chem., Int. Ed. 1972, 11, 331. (b) Eaton, P. E.;
Cole, T. W. J. Am. Chem. Soc. 1964, 86, 3157.
(13) (a) Brown, A. R.; Pomp, A.; de Leeuw, D. M.; Klaassen, D. B. M.;
Havinga, E. E.; Herwig, P.; Mullen, K. J. Appl. Phys. 1996, 79, 2136. (b) Herwig,
P. T.; Mullen, K. AdV. Mater. 1999, 11, 480. (c) Afzali, A.; Dimitrakopoulos,
C. D.; Breen, T. L. J. Am. Chem. Soc. 2002, 124, 8812. (d) Vets, N.; Smet, M.;
Dehaen, W. Tetrahedron Lett. 2004, 45, 7287. (e) Weidkamp, K. P.; Afzali, A.;
Tromp, R. M.; Hamers, R. J. J. Am. Chem. Soc. 2004, 126, 12740. (f) Afzali,
A.; Dimitrakopoulos, C. D.; Graham, T. O. AdV. Mater. 2003, 15, 2066.
(15) (a) Mondal, R.; Shah, B.; Neckers, D. C J. Am. Chem. Soc. 2006, 128,
9612. (b) Bettinger, H. F.; Mondal, R.; Neckers, D. C. Chem. Commun. 2007,
48, 5209.
(16) Mondal, R.; Okhrimenko, A.; Shah, B.; Neckers, D. C. J. Phys. Chem.
B. 2008, 112, 11.
(17) Strating, J.; Zwanenburg, B.; Wagenaar, A.; Udding, A. C. Tetrahedron
Lett. 1969, 3, 125.
J. Org. Chem. Vol. 73, No. 14, 2008 5507

Zhao et al.
FIGURE 1. Absorption (a) and emission (b) spectra of diones in toluene (λEx ) 450 nm); [7a] ) 2 × 10-4 M; [13a] ) 2.5 × 10-4 M; [7b] ) 1
× 10-4 M; [13b] ) 1 × 10-4 M.
FIGURE 2. Emission spectra of 7b at different excitation wavelengths in toluene (5 × 10-5 M).
TABLE 1. The Properties of Diones
compds
ꢀ/M-1 cm-1 (λn-π*/nm)a
λ
Em/nma
500
FT-IR/ν (cm-1
)
7a
1165 (465)
2922.4 (br), 1734.6, 1580.2, 1471.7, 1398.3, 1191.9, 1099.9, 904.2,
733.1, 614.1
7b
1083 (466)
845 (466)
508
500
2960.4, 2866.7, 1735.0, 1512.6, 1459.4, 1395.2, 1362.4, 1265.0, 1194.7,
1109.5, 1015.9, 954.8, 912.3, 886.5, 830.9, 747.6, 689.7
3015.5, 2920.9, 1734.4, 1475.5, 1452.2, 1377.7, 1263.4, 1188.7, 1111.6,
1024.3, 951.2, 913.4, 758.7, 728.8
13a
13b
1337 (468)
508
2958.8, 2867.5, 1735.7, 1513.7, 1477.2, 1394.4, 1362.2, 1267.8, 1195.5,
1111.3, 1014.8, 993.6, 955.4, 911.0, 872.4, 830.5, 748.9
a Absorption and emission in toluene.
Properties: Diones 7a, 7b, 13a, and 13b. Diones 7a, 7b,
13a, and 13b (13c is unstable and decomposed during separation
on a column) absorb between 420 and 510 nm in toluene
showing characteristically broad n-π* bands. Absorption
maxima appear at 465 (ꢀ ) 1165), 466 (ꢀ )1083), 466 (ꢀ )
845), and 468 nm (ꢀ ) 1337), respectively (Figure 1a). A
fluorescence emission at 500 nm was observed in toluene for
2-methylphenyl-substituted diones, 7a and 13a (Figure 1b),
while 4-tert-butylphenyl substituted diones, 7b and 13b, also
showed two small peaks around 600 and 650 nm assigned to
the emission of photolysis products, the pentacenes, besides the
maximum at 508 nm.
The diones are exceedingly photosensitive, so the longer the
excitation wavelength, the more obvious the emission of the
pentacene (Figure 2). Almost all the emission originates from
the pentacene with 500 nm excitation light in the case of 7b
with some 8b (the photolysis product of 7b) produced during
measurement of the fluorescence.
The molar extinction coefficient (ꢀ, in toluene) of the diones
and infrared data are given in Table 1. Each diketone has a
characteristic CdO peak around 1735 cm-1
.
Photodecarbonylation Reactions. This Strating-Zwanenburg
decarbonylation was originally attractive as a potential
approach to the now predicted to be unstable OdCdCdO.15
Consequently in the Okhrimenko-Mondal report, decarbo-
nylation was shown to be from the singlet state of the dione
with no evidence in transient UV experiments as a first
approximation for any reaction intermediate precedent to
poly(acene). As these authors point out, the matter of biradical
intermediates in the Strating-Zwanenburg reaction is not
spoken to by transient UV spectroscopic methods, and must
be addressed by TRIR. Under steady state conditions, both
395 nm light from an LED or continuous wavelength light
(g450 nm) worked well for the Strating-Zwanenburg
photodecarbonylation, the results of which can be recorded
by either NMR or UV-vis spectrometers. For example, if a
5508 J. Org. Chem. Vol. 73, No. 14, 2008

Photochemical Formation of Substituted Pentacenes
FIGURE 3. Transformation of 7a to 8a in sealed and degassed systems: (a) colors of 7a CDCl3 solution before and at the end of irradiation; (b)
1
the evolution of H NMR signals of 7a with the irradiation.
FIGURE 4. UV-vis spectra changes of 7a during the irradiation in toluene (3 × 10-5 M).
CDCl3 solution of 7a in a sealed NMR tube was completely
degassed by freeze-pump-thaw cycles before photolysis,
the solution turned red from yellow during the photolysis
while recorded changes in the NMR spectra reflect the
transformation from diketone to pentacene, Figure 3.
ated in an NMR tube, 8a and 14a were obtained as brown solids
and 8b and 14b as red solids.
If a solution of a pentacene is transferred to an open cell it
turns colorless (or pale yellow) resulting, as we show, from
endoperoxide formation. Irradiating a degassed toluene solution
of 7a in a cuvette produced new, structured absorption bands
from 385 to 625 nm with maxima at 408, 505, 542, and 587
nm (Figure 4). The latter three peaks, assigned to the π-π*
transition of 8a,9a,k increased for 40 min of photolysis, then
decreased gradually, disappearing completely after 75 min. The
peak around 400 nm is assigned to the endoperoxide (confirmed
by MALDI-TOF analysis), which became the sole product after
80 min of irradiation in the cuvette, which cannot be firmly
sealed like the special NMR tube.
Pentacenes without modification at the 6 and 13 positions
are reported either to dimerize ([4 + 4] cycloaddition) or be
oxidized to the 6,13-endoperoxide9a–c,k in solution with the
appearance of new 1H NMR peaks (6 and 13 positions) at ∼5.06
ppm for the dimer and ∼6.29 ppm for the endoperoxide. In our
experiments, no new peaks were observed in the 5.0-7.0 ppm
region, though peaks resulting from the 6 and 13 protons of
the diketones gradually decreased as the irradiation progressed
to completion in 2 h ((). Subsequently, the spectrum remained
unchanged even following an additional 1 h of irradiation. The
pentacenes thus formed also remain for almost one month in
the sealed tube, indicating that they do not react via the reactive
central ring by dimerization. The molecular weight of each
pentacene was observed by DIP-MASS (see the Supporting
Information) and the elemental composition confirmed by
HRMS. After evaporating the solvent from the samples irradi-
To confirm that the peak around 400 nm belonged to the
endoperoxide, a solution of 7b saturated with oxygen was
irradiated and the results recorded by UV-vis spectroscopy
(Figure 5a). After irradiation, MALDI-TOF analysis confirmed
the peaks around 400 nm originate from endoperoxide and
during the irradiation no peaks for 8b were observed, indicating
8b from the photolysis immediately reacts with oxygen.
J. Org. Chem. Vol. 73, No. 14, 2008 5509

Zhao et al.
FIGURE 5. (a) Absorption spectra of 7b (3 × 10-4 M) at different irradiation (395 nm) times in oxygen-saturated toluene; (b) absorption spectra
of 7b (3 × 10-4 M) in degassed toluene solution [before (line 1) and after (line 2) the fluorescence measurement].
FIGURE 6. NMR spectra of the endoperoxide of 8a in oxygen-saturated CDCl3 solution.
However, in the solution containing 7b, the absorbance persists
almost an hour, meaning the step from 7b to 8b is the slow but
predominant process. Transformation to pentacene can be
affected even by the weak light of a fluorometer during
measurement (Figure 5b).
Pentacenes. The absorption and fluorescence spectra of
pentacenes 8a, 8b, 14a, and 14b are shown in Figure 8. Their
photophysical parameters are listed in Table 2. The emissions
of 8a and 14a are nearly the same, whereas there is an
obvious red shift (11 nm for the first peak and 24 nm for the
second peak) observed for 8b and 14b.
Excitation spectra of 8a, 8b, 14a, and 14b (Figure 9) are
similar to their absorption spectra, meaning emission mainly
occurs from their singlet excited states (S1) and that there are
no spurious products in the system.
The formation of endoperoxide can also be detected from
the NMR spectra (Figure 6). A CDCl3 solution of 7a in an
NMR tube was saturated with oxygen for 20 min following
which the tube was irradiated. Before the irradiation, the
protons of the bridge carbon atoms appear at 5.35 ppm in
1
the H NMR spectrum. As a result of the irradiation, the
diketone NMR signal decreases and a new peak ascribed to
the protons of the carbon atoms attached to the oxygen bridge
at 6,13 positions in the endoperoxide appears at about 6.32
ppm. MALDI-TOF analysis also supports formation of
endoperoxide in the oxygen-purged solution.
As we have previously demonstrated with both hexacene and
heptacene, PMMA matrices stabilize reactive poly(acenes)
substantially. The absorption and emission spectra of 7a in a
PMMA matrix were similar to those in solution though the
absorption became broader and an approximate 20 nm red shift
occurred in the emission peak (Figure 7a). Photolysis of 7a in
PMMA matrix also produces 8a (Figure 7b). Thus 8a is very
stable in PMMA matrix and the spectra remain unchanged after
3 h of irradiation.
Conclusion
We report the facile photodecarbonylation of pentacene diones
7a, 7b, 13a, and 13b to pentacenes 8a, 8b, 14a, and 14b. The
pentacenes are stable to dimerization and as isolated solids
(under inert atmosphere), but they form endoperoxides with
oxygen in solution. The facile synthetic approach will be useful
on surfaces as the drive toward plastic electronics goes forward.
(18) (a) Wolak, M. A.; Jang, B. B.; Palilis, L. C.; Kafafi, Z. J. Phys. Chem.
2004, 108, 5492. (b) Seybold, P. G.; Gouterman, M.; Callis, J. Photochem.
Photobiol. 1969, 9, 229.
(19) Rigaudy, J.; Scribe, P.; Breliere, C. Tetrahedron 1981, 37, 2585.
5510 J. Org. Chem. Vol. 73, No. 14, 2008

Photochemical Formation of Substituted Pentacenes
FIGURE 7. Absorption and emission (λEx ) 470 nm) spectra (a) of 7a and its absorption changes (b) with irradiation time (light g450 nm) in
PMMA matrix.
FIGURE 8. Normalized absorption and emission spectra of 8a, 8b, 14a, and 14b in toluene (λEx ) 550, 550, 545, and 555 nm, respectively).
TABLE 2. The Photophysical Properties of Pentacenes Obtained
b
compds
ꢀ/M-1 cm-1 (λ/nm)a
λ
Em/nma
Øfl
8a
8b
14330 (505), 28220 (542), 34790 (587) 605, 660 0.027
12260 (507), 2287 (544), 26660 (590) 602, 646 0.088
14a
10150 (511), 19680 (550), 20920 (596) 605, 660 0.049
14b
10060 (516), 17960 (555), 17080 (603) 613, 668 0.067
a Absorption and emission in toluene. b Measured in toluene with
Rose Bengal in basic ethanol as the ref 18.
Experimental Section
Bicyclo[2.2.2]oct-2,3,5,6,7-pentaene (1). 1 was prepared according
to a literature procedure.19
2,3-Dibromo-5,6,9,10-tetrahydro-6,9-etheno-7,8-dimethylenean-
thracene (2). n-BuLi (1.6 M in hexane) (0.88 mL, 1.41 mmol) was
added dropwise under argon to 60 mL of a dry toluene solution
containing 1 (200 mg, 1.28 mmol) and 1,2,4,5-tetrabromobenzene
(505 mg, 1.28 mmol) at -50 to -60 °C. After being stirred for
6 h at this temperature, the reaction mixture was warmed slowly
to rt followed by another 1 h of stirring. Methanol (5 mol) was
then added to quench the excess of n-BuLi, solvents were removed
on a rotary evaporator, and the resulting solid mixture was applied
to a silica gel column for purification. Elution was started with
hexanes and ended with 10% (vol) of dichloromethane (DCM) in
hexanes. The collected fractions were evaporated to isolate the
desired compound (250 mg, yield 50%). Mp 142-143 °C; 1H NMR
(300 MHz, CDCl3) δ 3.462 (s, 4H), 3.95 (m, 2H), 4.872 (s, 2H),
5.092 (s, 2H), 6.482 (m, 2H), 7.375 (s, 2H); 13C NMR (300 MHz,
CDCl3) δ 30.9, 52.2, 101.9, 121.6, 133.4, 133.6, 135.2, 143.6; mass
spectrum (DIP-MS) m/z M+ 390 (50), 375 (20), 310 (30), 230 (55),
104 (100); HRMS (EI) m/z calcd for C18H14Br2 387.9463 (M+),
found 387.9467.
FIGURE 9. Excitation spectra or 8a, 8b, 14a, and 14b in toluene (λEm
) 643, 660, 650, and 670, respectively).
2,3-Di(2-methylphenyl)-5,6,9,10-tetrahydro-6,9-etheno-7,8-
dimethyleneanthracene (3a). Into a 100-mL 2-necked round-
bottomed flask was placed 2 (200 mg, 0.51 mmol), o-tolylboronic
acid (210 mg, 1.53 mmol), Cs2CO3 (602 mg, 1.8 mmol), 40 mL of
toluene, and 20 mL of water. After this the reaction mixture was
degassed for 30 min with argon with strong stirring, then Pd(PPh3)4
(20 mg, 0.017 mmol) was introduced; this was followed by 48 h
of reflux. The reaction mixture was cooled to rt, the solvent was
evaporated on a rotary evaporator, and the residue was applied to
a silica gel column for purification. The column was washed with
a gradient eluent (starting with hexane and ending with 20% DCM
(vol) in hexane). Condensation of the fraction provided the pure
compound 3a (170 mg, 81%). 1H NMR (300 MHz, CDCl3) δ 2.039
(s, 6H), 3.598 (s, 4H), 3.964 (m, 2H), 4.877 (s, 2H), 5.097 (s, 2H),
6.501 (m, 2H), 6.960-7.078 (m, 10H); 13C NMR (300 MHz,
CDCl3) δ 20.7, 34.7, 57.4, 101.5, 124.8, 126.7, 129.7, 130.7, 132.5,
J. Org. Chem. Vol. 73, No. 14, 2008 5511

Zhao et al.
133.7, 134.0, 135.7, 138.7, 144.1; mass spectrum (DIP-MS) m/z
M+ 412 (100), 397 (40), 325 (40); HRMS (EI) m/z calcd for C32H28
412.2191 (M+), found 412.2188.
CDCl3) δ 31.3, 34.4, 51.3, 68.4, 123.4, 124.4, 125.0, 125.9, 127.7,
128.8, 129.6, 131.9, 132.8, 136.2, 137.4, 138.6, 139.5, 149.3; mass
spectrum (DIP-MS) m/z (M+ - C2H4O2) 543 (100), 557 (20);
MALDI-TOF 626 (M+ + Na); HRMS (ESI) m/z calcd for C44H42O2
625.3083 (M+ + Na), found 625.3079.
2,3-Di(2-methylphenyl)-5,6,7,12,13,14-hexahydro-6,13-etheno-
pentacene (4a). n-BuLi (1.6 M in hexane) (0.69 mL, 1.1 mmol)
was added dropwise under argon to 50 mL of a dry toluene solution
containing 3a (200 mg, 0.49 mmol) and 1,2-dibromobenzene (236
mg, 1 mmol) at -50 to -60 °C. After 3 h of stirring, the reaction
mixture was warmed slowly to rt. Then 3 mol of methanol was
added to quench the reaction. Following removal of the solvents
the crude residue was applied to a silica gel column for purification.
Elution (hexane to 25% (vol) DCM in hexane) was used to wash
the column. White pure 4a (149 mg) was obtained (yield 62%)
Preparation of 2,3-Di(2-methylphenyl)-6,13-dihydro-6,13-etha-
nopentacene-15,16-dione (7a) and 2,3-Di(4-tert-butylphenyl)-6,13-
dihydro-6,13-ethanopentacene-15,16-dione (7b). Under argon at-
mosphere trifluoroacetic anhydride (3 mL) was added dropwise to
a stirred solution of dry dimethyl sulfoxide (DMSO, 1.2 mL) in 30
mL of DCM kept at -50 to -60 °C. After 15 min, a solution (1.2
mL of dry DMSO in 15 mL of DCM) of 6a (150 mg, 0.29 mmol)
or 6b (200 mg, 0.33 mmol) was added very slowly. The resulting
mixture was stirred for 2 h after which 2 mL of diisopropylethyl-
amine was introduced dropwise and this reaction mixture was stirred
for another 2 h. The mixture was maintained at reduced temperature
under argon for the entire time after which the resulting bright
yellow solution was warmed to rt and extracted with DCM followed
by washing the organic layer with water. Evaporation of solvent
provided crude product, 7a, which was purified by recrystallization
with DCM and hexane (pale yellow solid, 73 mg, yield 49%). Pure
yellow 7b (105 mg, yield 53%) was obtained following separation
1
from the fraction. Mp 178-180 °C; H NMR (300 MHz, CDCl3)
δ 2.028 (s, 6H), 3.612 (s, 4H), 4.329 (m, 2H), 6.873-7.104 (m, 16
H); 13C NMR (300 MHz, CDCl3) δ 32.9, 34.5, 54.4, 124.8, 125.9,
126.7, 128.9, 129.9, 130.9, 133.0, 134.6, 135.8, 138.5, 139.4, 140.5;
mass spectrum (DIP-MS) m/z M+ 488 (100), 398 (25), 347 (25),
180 (70); HRMS (EI) m/z calcd for C38H32 488.2503 (M+), found
488.2519.
2,3-Di(2-methylphenyl)-6,13-dihydro-6,13-ethenopentacene (5a).
A dry toluene solution containing 244 mg of 4a (0.5 mmol) and
chloranil (246 mg, 1 mmol) was refluxed in a 100-mL round-
bottomed flask for 2 h. The reaction mixture was then cooled to rt
and solvent was evaporated on a rotary evaporator. The reaction
residue was dissolved in 50 mL of DCM and washed with 2 N
NaOH solution (2 × 10 mL) and ultimately with brine (1 × 10
mL). The organic layer was dried (anhydrous MgSO4), filtered, and
evaporated. The crude product was further purified by column
chromatography on silica gel to give 5a (white solid, 230 mg, 95%).
The elution started with hexane and ended with 30% (vol) DCM
1
on a silica gel column eluted with DCM. Data for 7a: H NMR
(300 MHz, CDCl3) δ 2.019 and 2.085 (6H), 5.350 (s, 2H), 6.999
and 7.091 (8H), 7.526 (m, 2H), 7.790, 7.843 and 7.853 (8H); 13C
NMR (300 MHz, CDCl3) δ 20.3, 60.7, 124.9, 125.1, 125.4, 127.1,
127.2, 127.9, 129.3, 129.8, 131.4, 131.9, 132.3, 132.6, 133.7, 135.7,
140.8, 185.1; mass spectrum (DIP-MS) m/z (M+ - C2O2) 458 (100);
HRMS (ES) m/z calcd for C38H26O2 515.2011(MH+), found
1
515.1993. Data for 7b: H NMR (300 MHz, CDCl3) δ 1.298 (s,
18), 5.327 (s, 2H), 7.088-7.320 (m, 8H), 7.522 9M, 2H),
7.856-7.965 (m, 8H); 13C NMR (300 MHz, CDCl3) δ 17.7, 20.6,
46.9, 110.8, 111.2, 111.6, 113.2, 114.3, 115.2, 115.7, 118.1, 118.8,
119.7, 124.1, 126.6, 135.8, 171.4; mass spectrum (DIP-MS) m/z
(M+ - C2O2) 543 (100), 410 (60); HRMS (EI) m/z calcd for
C44H38O2 598.28718 (M+), found 598.21775.
1
in hexane. Mp 263-265 °C dec; H NMR (300 MHz, CDCl3) )
2.065 and 2.131 (d, 6H), 5.407 (m, 2H), 7.054-7.129 (b, 10 H),
7.445 (m, 2H), 7.719-7.819 (m, 8H); 13C NMR (300 MHz, CDCl3)
δ 34.7, 50.3, 121.0, 121.4, 124.1, 125.7, 127.0, 127.5, 129.1, 130.6,
131.9, 136.0, 138.3, 142.1, 142.7; mass spectrum (DIP-MS) m/z
484 (100); HRMS (EI) m/z calcd for C38H28 484.2191 (M+), found
484.2190.
2,3,9,10-Tetrabromo-5,6,7,12,13,14-hexahydro-6,13-ethenopen-
tacene (9). To a solution containing 1 (200 mg, 1.28 mmol) and
1,2,4,5-tetrabromobenzene (1.06 g, 2.69 mmol) in 60 mL of dry
toluene was added 1.85 mL of n-BuLi in hexane (1.6 M) dropwise
at -50 to -60 °C. The procedure was similar to that described for
the preparation of 2. Elution was begun with hexane and ended
with 20% DCM in hexane. The purified compound was obtained
2,3-Di(2-methylphenyl)-6,13-dihydro-6,13-ethanopentacene-15,16-
diol (6a) and 2,3-di(4-tert-butylphenyl)-6,13-dihydro-6,13-ethano-
pentacene-15,16-diol (6b). Osmium tetroxide [1 mL solution of
2.5% (w)] in tert-butyl alcohol was added to 100 mL of a mixed
solution of acetone and water (1:5 in volume) containing 250 mg
of 4-methylmorpholine N-oxide (2.14 mmol) in a 500-mL round-
bottomed flask. After 10 min of stirring, an acetone solution
containing 0.5 mmol of 5a or 5b (242 mg or 284 mg) was
introduced and a suitable amount of acetone added to make the
solution transparent. The reaction mixture was then stirred at rt for
48 h. Sodium dithionite (400 mg) was added to this reaction
mixture, then it was stirred for another 20 min to yield a
heterogeneous solution. The suspension in the mixture was removed
by filtering it through a pad of cerite and washing the pad with
acetone. The solvent from the filtered solution was evaporated on
a rotary evaporator, and the residue was applied to a silica gel
column and washed with gradient eluent. For 6a (6b), the eluent
started from hexane and ended with 25% (30%) ethyl acetate in
hexane. The collected solvent was evaporated to provide the pure
compound (white solid, yield 90% for 6a 92% for 6b). Data for
6a: Mp 193-195 °C dec; 1H NMR (300 MHz, CDCl3) δ
1.987-2.097 (m, 8H), 4.224 (s, 2H), 4.653 (s, 2H), 6.990-7.072
(d, 8H), 7.428 (m, 2H), 7.720-7.858 (m, 8H); 13C NMR (300 MHz,
CDCl3) δ 14.2, 51.3, 68.5, 123.1, 123.4, 124.8, 125.4, 126.0, 127.0,
127.7, 129.8, 131.7, 132.8, 135.9, 137.4, 137.8, 140.4, 144.3; mass
spectrum (DIP-MS) m/z (M+ - C2H4O2) 458 (100); MALDI-TOF
541 (M+ + Na); HRMS (ESI) m/z calcd for C38H30O2 541.2144
(M+ + Na), found 541.2139. Data for 6b: Mp 188-190 °C dec;
1H NMR (300 MHz, CDCl3) δ 1.290 (s, 18), 2.336 (br, 2H), 4.074
(br, 2H), 4.608 (s, 2H), 7.015-7.045 (m, 4H), 7.181-7.248 (m,
4H), 7.429 (m, 2H), 7.767-7.843 (m, 8H); 13C NMR (300 MHz,
1
in a yield of 350 mg (44%). Mp 295-296 °C dec; H NMR (300
MHz, CDCl3) δ 3.506 (s, 8H), 4.276 (m, 2H), 6.840 (m, 2H), 7.365
(s, 2H); 13C NMR (300 MHz, CDCl3) δ 32.4, 53.8, 121.6, 133.4,
135.4, 139.2, 139.8; mass spectrum (DIP-MS) m/z M+ 624 (40),
544 (10), 464 (10), 384 (8), 151 (100); HRMS (EI) m/z calcd for
C24H16Br4 625.79413 (M+), found 625.79255.
2,3-Dimethylphenyl(4-tert-butylphenyl)pentacene 8a (8b) and
2,3,9,10-Tetramethylphenyl(4-tert-butylphenyl)pentacene 14a (14b).
Transparent diketone (7a, 7b, 13a, 13b) solutions (5 mg in about
0.7 mL of CDCl3) in Norell Young valve NMR tubes were
degassed by using freeze-pump-thaw cycles in the dark.
Subsequently the sealed, oxygen-free diketone solutions were
irradiated with an UV-LED lamp (395 ( 25 nm) or high-pressure
mercury lamp (a filter was used to cut off the light below 450
nm) as the light source. During the irradiation, the 1H NMR
signal (around 5.3 ppm) of the 6 and 13 positioned protons of
the diketone were monitored to follow the transformation from
diketone to pentacene. Reaction completion was indicated by
the disappearance of 6 and 13 proton signals of the diketone.
The pentacenes were obtained as follows: Following the complete
disappearance of the diketone signals, the tube was transferred
to a dry box and solvent was removed in vacuum. There were
no apparent side products (NMR) so we estimate 100% yield.
The dried pentacenes were redissolved in either C6D6 or CDCl3
and the NMR spectra recorded. Data for 8a: 1H NMR (300 MHz,
CDCl3) δ 2.187 (s, 6 H), 7.051-7.109 (m, 8H), 7.314-7.353
5512 J. Org. Chem. Vol. 73, No. 14, 2008

Photochemical Formation of Substituted Pentacenes
(m, 2H), 7.881-7.959 (m, 4H), 8.701 (d, 4H), 9.004 (s, 2H);
mass spectrum (DIP-MS) m/z M+ 458 (100); HRMS (EI) m/z
calcd for C36H26 458.20345 (M+), found 458.20348. Data for
8b: 1H NMR (300 MHz, CDCl3) δ 1.326 (s, 18 H), 7.030-7.216
(m, 8H), 7.890-8.075 (m, 6H), 8.675 (d, 4H), 8.974 (s, 2H);
mass spectrum (DIP-MS) m/z M+ 543 (100); HRMS (EI) m/z
calcd for C42H38 542.29735 (M+), found 542.29733. Data for
Department of Development (03-054). The MALDI-TOF was
purchased with the assistance of the National Science Founda-
tion (NSF 02344796).
Supporting Information Available: General experimental
procedures, synthesis, and characterization of 3b, 4b, 5b, 10,
1
1
11a, 11b, 11c, 12a, 12b, 12c, 13a, 13b, and 13c, H NMR
14a: H NMR (300 MHz, C6D6) δ 2.193 (s, 6H), 2.218 (s, 6H),
7.058-7.136 (w, 16H), 7.915 (s, 4H), 8.737 (s, 4H), 9.046 (s,
2H); mass spectrum (DIP-MS) m/z M+ 638 (100); HRMS (EI)
m/z calcd for C50H38 638.29735 (M+), found 638.29757. Data
for 14b: 1H NMR (300 MHz, C6D6) δ 1.351 (s, 36H),
7.115-7.292 (m, 16H), 8.017 (s, 4H), 8.706 (s, 4H), 8.997 (s,
2H); mass spectrum (DIP-MS) m/z M+ 807 (100), 675 (40);
HRMS (EI) m/z calcd for C62H62 807.48515 (M+), found
807.48525.
spectra evolutions reflecting the production of pentacene (8b,
14a, 14b) endoperoxides, emission spectra of 7a, 13a, and 13b
under different excitation lights, 1H, 13C NMR spectra for
synthesized compounds, MALDI-TOF spectra for diols and
pentacene endoperoxides, FT-IR for diones, and DIP-MS for
pentacenes. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the Office
of Naval Research (ONR-N00014-06-1-0948) and the Ohio
JO800728P
J. Org. Chem. Vol. 73, No. 14, 2008 5513