Synthesis and spectroscopic properties of a hexapyrenylbenzene derivative

Article abstract of DOI:10.1021/ol0618196

(Chemical Equation Presented) In this paper, we present a synthetic approach to the first hexapyrenylbenzene starting from 4,5,9,10- tetrahydropyrene. Absorption and fluorescence spectroscopic measurements show strong and red-shifted fluorescence both from locally excited pyrene states and from the excitonic manifold of the aggregate.

Full text of DOI:10.1021/ol0618196

ORGANIC
LETTERS
2006
Vol. 8, No. 22
5037-5040
Synthesis and Spectroscopic Properties
of a Hexapyrenylbenzene Derivative†
Dirk Rausch and Christoph Lambert*
Institut fu¨r Organische Chemie, Julius-Maximilians-UniVersita¨t, Am Hubland,
D-97074 Wu¨rzburg, Germany
lambert@chemie.uni-wuerzburg.de
Received July 24, 2006
ABSTRACT
In this paper, we present a synthetic approach to the first hexapyrenylbenzene starting from 4,5,9,10-tetrahydropyrene. Absorption and
fluorescence spectroscopic measurements show strong and red-shifted fluorescence both from locally excited pyrene states and from the
excitonic manifold of the aggregate.
Highly symmetrical, star-shaped molecules such as hexaaryl-
benzenes can be used to investigate basic aspects of energy
transfer, delocalization, or storage in chromophore aggregates
as they occur, e.g., in the natural light-harvesting systems
LH I and LH II.1-4 For this reason, we designed a model
compound 8 which is based on the pyrene chromophore: six
pyrenyl substituents are arranged in a regular manner and
held together by a central benzene core. This simple model
compound shall serve to explore the basic photophysical
aspects of such 6-fold molecular arrangements.
delayed fluorescence, and rather long fluorescence lifetimes)
and well understood.8
In addition, hexaarylbenzenes have attracted interest in
material science in recent years9-11 for applications as light-
emitting and charge-transport layers in OLEDs,12 precursors
(4) Bottari, G.; Torres, T. Chem. Commun. 2004, 2668-2669.
(5) Fo¨rster, T. Angew. Chem., Int. Ed. 1969, 8, 333.
(6) Lewis, F. D.; Kurth, T. L. Can. J. Chem. 2003, 81, 770-776.
(7) Wahl, P.; Krieger, C.; Schweitzer, D.; Staab, H. A. Chem. Ber. 1984,
117, 260-276.
(8) Bree, A.; Vilkos, V. V. B. Spectrochim. Acta, Part A 1971, 27, 2333-
2354.
(9) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001, 3, 2887-
2890.
Pyrenyl subchromophores were chosen because the pho-
tochemistry of pyrene is quite rich (excimer formation,5-7
† Dedicated to Prof. Dr. Manfred Christl on the occasion of his 65th
birthday.
(1) Hu, X.; Schulten, K. Phys. Today 1997, 50, 28-34.
(2) Lambert, C. Angew. Chem., Int. Ed. 2005, 44, 7337-7339.
(3) Piet, J. J.; Biemans, H. A. M.; Warman, J. M.; Meijer, E. W. Chem.
Phys. Lett. 1998, 289, 13-18.
(10) Rathore, R.; Burns, C. L.; Abdelwahed, S. A. Org. Lett. 2004, 6,
1689-1692.
(11) Sun, D. L.; Rosokha, S. V.; Kochi, J. K. Angew. Chem., Int. Ed.
2005, 44, 5133-5136.
(12) Jia, W. L.; Wang, R. Y.; Song, D. T.; Ball, S. J.; McLean, A. B.;
Wang, S. N. Chem.-Eur. J. 2005, 11, 832-842.
10.1021/ol0618196 CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/06/2006

for photoconductive polycyclic aromatic hydrocarbons,13-16
and discotic liquid crystals.17
Scheme 1. Synthesis of
Molecular modeling of 8 without ester groups (see Figure
1) shows that owing to the pyrene H atoms in the 1, 3, 4,
2,7-Dibromo-4,5,9,10-tetrahydropyrene 2
Subsequently, 2,7-dibromotetrahydropyrene 2 was mono-
lithiated in the key step with n-BuLi at -90 °C and reaction
with octylchloroformate led to the ester 3 in one step in
moderate to good yields.24 The ester 3 was easily oxidized
with 2 equiv of DDQ to give 4 (Scheme 2).23
Figure 1. Molecular modeling of 8 without ester functionalities.
Scheme 2. Synthesis of 2
and 10 positions the pyrenyl substituents adopt a propeller-
like geometry in which the pyrenyl rings have a dihedral
angel of ca. 58° to the central benzene ring.
In this context, it is essential that pyrene is bound in the
2 position to the central benzene ring because this minimizes
direct orbital overlap with the benzene core because of the
pyrene HOMO and LUMO having nodes through the 2 and
7 positions.18
Although most pyrene-derived chromophores are based
on 1-substituted pyrene, substitution of pyrene in the 2
position represents a general problem because electrophilic
substitution is usually strongly favored in the 1 position
followed by substitutions in the 3, 6, or 8 position.19,20 Thus,
a direct access to 2,7-disubstituted pyrenes is almost impos-
sible. Therefore, the synthesis of 2,7-substituted pyrenes re-
quires an indirect route. The bridged biphenyl 1 can selec-
tively be substituted by electrophiles in the 2 and 7 positions
followed by back-oxidation to the corresponding pyrenes.21,22
In this communication, we present the synthesis of the
hexapyrenylbenzene 8, which carries long alkylester groups
in the 7 position for better solubility. 4,5,9,10-Tetrahydro-
pyrene 1 was synthesized from pyrene according to a
literature procedure.23,22 Compound 1 was converted to 2,7-
dibromotetrahydropyrene following a modified bromination
method of Harvey et al.,21 who used a solution of 4,5,9,10-
tetrahydropyrene in acetic acid (Scheme 1).
The reaction of 4 with trimethylsilylacetylene (TMSA) in
a standard Hagihara/Sonogoshira coupling yielded 5 in
60%.25 Deprotection of 5 with TBAF in THF gave 6 in
quantitative yields (Scheme 3).26
Scheme 3. Synthesis of 6
(13) Watson, M. D.; Fechtenkotter, A.; Mu¨llen, K. Chem. ReV. 2001,
101, 1267-1300.
(14) Keegstra, M. A.; DeFeyter, S.; DeSchryver, F. C.; Mu¨llen, K. Angew.
Chem., Int. Ed. 1996, 35, 774-776.
The resulting terminal acetylene 6 was then coupled with
4 in the same coupling procedure used before to obtain 7.25
Workup of this tolan compound is difficult, owing to high
insolubility, but washing of the crude product with cold
dichloromethane gave 7 that was pure enough for the last
conversion step (Scheme 4).
(15) Kobayashi, K.; Kobayashi, N.; Ikuta, M.; Therrien, B.; Sakamoto,
S.; Yamaguchi, K. J. Org. Chem. 2005, 70, 749-752.
(16) Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071-4074.
(17) Kobayashi, K.; Kobayashi, N. J. Org. Chem. 2004, 69, 2487-2497.
(18) For DFT calculations, see the Supporting Information.
(19) Vollmann, H.; Becker, H.; Corell, M.; Streeck, H.; Langbein, G.
Ann. 1937, 531, 1-159.
(20) Minabe, M.; Takeshige, S.; Soeda, Y.; Kimura, T.; Tsubota, M.
Bull. Chem. Soc. J. 1994, 67, 172-179.
(21) Lee, H.; Harvey, R. G. J. Org. Chem. 1986, 51, 2847-2848.
(22) Conner, M. D.; Scott, D. A.; Collard, M. D.; Liotta, L. C.; Schiraldi,
D. A. J. Org. Chem. 1999, 64, 6888-6890.
(23) Musa, A.; Sridharan, B.; Lee, H.; Mattern, L. M. J. Org. Chem.
1996, 61, 5481-5484.
(24) Ghera, E.; Bendavid, Y. J. Org. Chem. 1988, 53, 2972-2979.
5038
Org. Lett., Vol. 8, No. 22, 2006

Scheme 4. Synthesis of the Hexaarylbenzene 8
Figure 3. UV/vis spectrum (red) and fluorescence spectrum (black)
of 8 in dichloromethane. Deconvolution of the fluorescence
spectrum is in blue and green.
played. Table 1 shows the absorption and emission maxima
and the corresponding extinction coefficients of 8.
The absorption spectrum shows the typical allowed La
band at 28 800 cm-1 and the much weaker forbidden Lb band
at 25 100 cm-1. Both bands display vibronic structure and
are distinctly shifted to lower energy compared to ethyl
pyrene-2-carboxylate (La ) 30 000 cm-1, ꢀ ) 4.5 × 104 M-1
cm-1, Lb ) 25 500 cm-1, ꢀ ) 2 × 103 M-1 cm-1 in
cyclohexane).27 Furthermore, the spectrum is dominated by
an intense unresolved band at 35 700 cm-1. This band is at
an energy similar to that for the Bb and Ba bands of ethyl
pyrene-2-carboxylate (Bb ) 35 500 cm-1, ꢀ ) 2.5 × 104
M-1 cm-1, Ba ) 38 000 cm-1, ꢀ ) 7 × 104 M-1 cm-1). All
bands are much more intense than those of ethyl pyrene-2-
carboxylate, but they do not quite reach the 6-fold intensity
as expected. In addition, there is a broad and unresolved band
at ca. 21 500 cm-1 which is absent in ethyl pyrene-2-
carboxylate and which we tentatively assign to an excitation
in the excitonic manifold of the pyrene hexaaggregate. In
this way, 8 is unlike pyrene which shows a concentration-
dependent excimer fluorescence but no corresponding ab-
sorption band. The emission spectrum of 8, peaked at 24 100
cm-1, also shows a vibronic fine structure but is much less
resolved than that of the pyrene parent compound. At the
low-energy side, there is a broad and unresolved shoulder
Finally, a conventional alkyne trimerization reaction of 7
with Co2(CO)8 provided the desired compound 8 in 25%
yield. The low yield is mainly caused by two precipitation
steps which were needed for purification (Scheme 4). Figure
2 shows the MALDI-ToF mass spectrum of 8.
Table 1. Absorption and Emission Maxima and Extinction
Coefficients of 8 in Dichloromethane
λmax (nm)
ν˜max (cm-1
)
ꢀ (M-1 cm-1
)
Figure 2. MALDI-ToF mass spectrum of 8.
absorption
emission
280
347
398
465
35700
28800
25100
21500
348000
178000
8400
In Figure 3, the UV/vis spectrum together with the corre-
sponding emission spectrum in dichloromethane are dis-
1200
415
483
24100
20700
(25) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org.
Lett. 2000, 2, 1729-1731.
Org. Lett., Vol. 8, No. 22, 2006
5039

which is more intense in polar solvents (e.g., acetonitrile)
than in moderately or apolar solvents. The emission spectrum
is independent of the concentration such that excimer
formation between two molecules of 8 could be excluded.
This broad emission band (given in blue in Figure 3) of 8 is
more obvious after deconvolution of the emission band into
four equally broad (900 cm-1) Gaussian bands (given in
green in Figure 3) according to the vibrational structure
which then yields the fifth band as the difference to the
experimental spectrum. This fifth band is lower in energy
than the lowest-energy absorption band and might be due to
the emission from the excitonic manifold (or two adjacent
pyrene moieties), whereas the vibrationally resolved band
is due to the emission from locally excited pyrene chro-
mophores. In contrast, the model monomer 7-phenyl-pyrene-
2-carbonic acid octylester does not show any concentration-
dependent excimer fluorescence. Thus, the red-shifted emission
appears to be a unique property of the hexameric aggregate.
Further spectroscopic characterizations (i.e., time-resolved
measurements), particularly to prove the origin of the blue
band, are in progress and will be disclosed elsewhere.
In conclusion, we described a synthetic approach for the
synthesis of hexapyrenylbenzene chromophores which shows
strong and red-shifted fluorescence both from locally excited
pyrene states and from the excitonic states of the aggregate.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (La 991/4-3 and Gra-
duiertenkolleg 1221) as well as by the Heraeus GmbH and
Wacker AG.
Supporting Information Available: Synthetic procedures
and characterization of products. This material is available
free of charge via the Internet at http://pubs.acs.org.
(26) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic
Synthesis, 2nd ed.; John Wiley & Sons: New York, 1991.
(27) Vasak, M.; Whipple, M. R.; Berg, A.; Michl, J. J. Am. Chem. Soc.
1978, 100, 6872-6877.
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