Oxygen- and sulfur-containing positively charged polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1021/ol902366y

[Chemical Equation Presented] Benzo[5,6]naphthaceno[1,12,11,10-jklmna] xanthylium (BNAX) and benzo[5,6]naphthaceno[1,12,11,10-jklmna]thioxanthylium (BNATX) salts were synthesized. Comparison of the UV-vis absorption and emission spectra of them and dibenz

Full text of DOI:10.1021/ol902366y

ORGANIC
LETTERS
2009
Vol. 11, No. 24
5686-5689
Oxygen- and Sulfur-Containing
Positively Charged Polycyclic Aromatic
Hydrocarbons
Dongqing Wu, Wojciech Pisula,† Monika C. Haberecht,§ Xinliang Feng,* and
Klaus Mu¨llen*
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany
feng@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de
Received October 19, 2009
ABSTRACT
Benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium (BNAX) and benzo[5,6]naphthaceno[1,12,11,10-jklmna]thioxanthylium (BNATX) salts were
synthesized. Comparison of the UV-vis absorption and emission spectra of them and dibenzo[jk,mn]naphtho[2,1,8-fgh]thebenidinium (DBNT)
salts suggests that the incorporation of different heteroatoms in the same PAH core changes their optoelectronic properties profoundly.
Ordered columnar liquid crystalline phases were observed for di- and tridodecyl-substituted BNAX salts. X-ray scattering and molecular modeling
indicate that three BNAX molecules self-assemble into a disklike structure, which further organizes into hexagonal columnar phases.
Large polycyclic aromatic hydrocarbons (PAHs), as subunits
of graphene, have been exploited in various organic elec-
tronic devices including field-effect transistors, injection
layers, and solar cells.1 Compared with their all-hydrocar-
bon analogues, PAHs containing heteroatoms such as
nitrogen in the aromatic framework, either neutral or
positively charged, exhibit unprecedented chemical and
physical properties.2
Very recently, we described the synthesis and self-
assembly of dibenzo[jk,mn]naphtho[2,1,8-fgh]thebenidinium
(DBNT, 1, Figure 1) salts,3 which were the largest nitrogen-
containing positively charged PAHs. Compared with the
well-studied DBNT salts,3,4 their analogues containing other
heteroatoms such as sulfur and oxygen have been relatively
unexplored. In this paper, we present the first examples of
oxygen- and sulfur-containing positively charged PAHs,
namely benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthy-
lium (BNAX, 2) and benzo[5,6]naphthaceno[1,12,11,10-
jklmna]thioxanthylium (BNATX, 3) salts (Figure 1). Fol-
lowing the order of nitrogen, oxygen, and sulfur, these three
types of positively charged PAHs show obvious bathochro-
mic shifts of their UV-vis absorption and emission bands,
† Present address: Evonik Degussa GmbH, Process Technology &
Engineering, Process Technology - New Processes, Rodenbacher Chaussee
4, D-63457, Hanau-Wolfgang, Germany.
§ Present address: BASF SE, Polymer Research, Carl-Bosch-Str. 38,
D-67056, Ludwigshafen, Germany.
(1) (a) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A.
Chem. ReV. 2005, 105, 1491. (b) Wu, J. S.; Pisula, W.; Mu¨llen, K. Chem.
ReV. 2007, 107, 718.
(2) (a) Pieterse, K.; van Hal, P. A.; Kleppinger, R.; Vekemans, J.;
Janssen, R. A. J.; Meijer, E. W. Chem. Mater. 2001, 13, 2675. (b) Draper,
S. M.; Gregg, D. J.; Madathil, R. J. Am. Chem. Soc. 2002, 124, 3486. (c)
Wu, D. Q.; Zhi, L. J.; Bodwell, G. J.; Cui, G. L.; Tsao, N.; Mu¨llen, K.
Angew. Chem., Int. Ed. 2007, 46, 5417. (d) Takase, M.; Enkelmann, V.;
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46, 5524. (e) Bunz, U. H. F. Chem.sEur. J. 2009, 15, 6780. (f) Miao, S.;
Appleton, A. L.; Berger, N.; Barlow, S.; Marder, S. R.; Hardcastle, K. I.;
Bunz, U. H. F. Chem.sEur. J. 2009, 15, 4990.
(3) Wu, D. Q.; Feng, X. L.; Takase, M.; Haberecht, M. C.; Mu¨llen, K.
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10.1021/ol902366y  2009 American Chemical Society
Published on Web 11/17/2009

functionalized BNAX salts. In our work, the Kumada
coupling of 4b-d with Grignard reagents produces the
alkyl-substituted dibenzo[a,j]xanthenes (4e-g) in good
yields. Subsequent oxidization of 4 with bromine yields
the corresponding 14-phenyl-14-dibenzo[a,j]xanthenylium
bromides (5a-d). The photocyclization of 5 in acetic acid
under 300 nm UV light results in BNAX bromides 2 as
purple solids without the need for further purification. All
1
molecules were characterized by H/13C NMR spectros-
copy, FD/MALDI-TOF mass spectrometr,y as well as
elemental analysis (Supporting Information).
Figure 1
.
DBNT (1), BNAX (2), and BNATX (3) salts.
For the synthesis of the sulfur-containing BNATX salt,
a modified synthetic route was developed in this work
(Scheme 2): the acid-catalyzed ether-sulfide exchange
indicating that heteroatoms can effectively adjust the pho-
tophysical properties of PAHs without alteration of the
aromatic skeleton. The bulk organization of alkyl-chain-
substituted BNAX is also presented. Both di- (2c) and
tridodecyl-substituted (2d) BNAX salts can form columnar
liquid crystalline phases with large unit cells. This can be
attributed to the formation of disklike substructures by the
self-assembly of three BNAX molecules.
Scheme 2. Synthesis of BNTAX Salt
The synthesis of various BNAX bromides (2) proceeds
as follows (Scheme 1): a Claisen condensation of ap-
Scheme 1. Synthesis of BNAX Bromides
reaction6 between 2-methoxynaphthalene and naphthalene-
2-thiol gives dinaphthalen-2-ylsulfane (6), which can
further condense with benzaldehyde to provide 14-phenyl-
14H-dibenzo[a,j]thioxanthene (7).7 Then, 14-phenyldiben-
zo[a,j]thioxanthenium tetrafluoroborate (9) is obtained
based on a two-step procedure through the oxidization of
7 and subsequent dehydrogenation of the resulting 14-
phenyl-14H-dibenzo[a,j]thioxanthen-14-ol (8). Since com-
pound 7 cannot be directly oxidized with bromine as in
the case of 4, we utilize a two-step oxidization-dehydration
process. Irradiation of 9 with 300 nm UV light yields
benzo[5,6]naphthaceno[1,12,11,10-jklmna]thioxanthyli-
um tetrafluoroborate (BNATX BF4, 3) as purple needles.
propriate arylaldehydes and naphthols in acetic acid results
in 14-phenyl-14H-dibenzo[a,j]xanthenes 4a-d.5 Mono-
(4b), di- (4c), and tribromodibenzo[a,j]xanthene (4d)
constitute useful building blocks for the synthesis of
(6) Radhakrishnan, K.; Lin, C. H. Synlett 2005, 2179.
(5) (a) Claisen, L. Ber. Deutsch. Chem. Ges. 1887, 20, 665. (b) Claisen,
L.; Claparede, A. Ber. Deutsch. Chem. Ges. 1887, 14, 2460.
(7) (a) Dilthey, W.; Quint, F.; Heinen, J. J. Prakt. Chem. 1939, 152,
49. (b) Dilthey, W.; Quint, F.; Stephan, H. J. Prakt. Chem. 1939, 152, 99.
Org. Lett., Vol. 11, No. 24, 2009
5687

Further characterization of the product by FD/MALDI-
phenomena have been recognized in the study of small
heterocations such as pyridinium, pyrylium, and thiopyrylium
salts.8 Our results, therefore, demonstrate the possibility of
fine-tuning the optoelectronic properties of PAHs through
the incorporation of different heteroatoms without changing
the aromatic framework.
1
TOF mass spectrometry, H/13C NMR spectroscopy, and
elemental analysis is in accordance with expectations
(Supporting Information).
In order to understand the effect of different heteroatoms
on the optoelectronic properties of charged PAHs with the
same aromatic framework, the UV-vis absorption spectra
of 14-phenyl-DBNT tetrafluoroborate 1a, BNAX bromide
2a, and BNATX tetrafluoroborate 3 in dichloromethane
solutions are compared in Figure 2a. Remarkably, the three
The supramolecular chemistry of alkyl-substituted BNAX
salts was also studied in this work. Scanning electron
microscopy images of compound 2b (7-dodecylbenzo-
[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium bromide)
drop-cast from a methanol solution exhibit fibrous ag-
gregates (Figure S3, Supporting Information) close to the
nanofibers formed from 14-hexyl-DBNT tetrafluoroborate
1b.3 This suggests a similar self-assembly mechanism in
methanol. On the other hand, the supramolecular organi-
zation of positively charged PAHs substituted with more
than one alkyl chain in the bulk is still unknown. Thus,
the liquid crystalline properties of the di- (2c, 4,10-dido-
decylbenzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium
bromide)- and tridodecyl (2d, 4,7,10-tridodecylbenzo[5,6]-
naphthaceno[1,12,11,10-jklmna]xanthylium bromide)-substi-
tuted BNAX bromides were investigated by two-dimensional
wide-angle X-ray scattering (2D-WAXS). For this purpose,
extruded filaments with macroscopically aligned columnar
structures were prepared from both compounds at 120 °C.
Neither compound reveals any phase transition in differential
scanning calorimetry (DSC) scans within a temperature range
of -100 to +300 °C. The 2D patterns shown in Figure 3 are
Figure 3
at 30 °C.
. 2D-WAXS patterns of (a) 2d and (b) 2c both recorded
Figure 2. (a) UV-vis absorption spectra of 1a (black), 2a (red),
and 3 (blue) in CH2Cl2, measured at 20 °C. (b) Fluorescence spectra
of 1a (black), 2a (red), and 3 (blue) in dichloromethane (20 °C,
excited at 350 nm).
similar for both compounds indicating identical supramolecular
structures. The distribution of the reflections in the pattern is
characteristic for a discotic liquid crystalline phase and can be
separated into an equatorial and meridional plane. The equatorial
small angle reflections are assigned to typical columnar stacks
formed from the discotic molecules. Identical to most discotics
the columns are well-aligned in the orientation of the extruded
fiber direction. The positions of the reflections are thereby
related to a unit cell characteristic for the intercolumnar
arrangement. In this case, a hexagonal cell with a parameter of
compounds show very different absorption spectra, especially
in the visible region. The maximum absorptions for 1a, 2a,
and 3 are located at 512 (log ε ) 4.51), 573 (log ε ) 4.71),
and 600 nm (log ε ) 4.31), respectively, thus reflecting an
apparent red shift by 61 (N to O) and 27 nm (O to S). The
emission spectra are recorded by using the same excitation
wavelength of 350 nm for 1a, 2a and 3; the results are
demonstrated in Figure 2b. The three spectra show struc-
tureless peaks which are centered at 531, 593, and 643 nm,
respectively. Obviously, this red-shift trend following the
sequence of nitrogen, oxygen, and sulfur is consistent with
their maximum absorptions in the visible region. Similar
(8) (a) Saeva, F. D.; Olin, G. R. J. Am. Chem. Soc. 1980, 102, 299. (b)
Novikov, V. N.; Tymyanskii, Y. R.; Feigelman, V. M.; Knyazhanskii, M. I.
Khim. Geterotsikl. Soedin. 1988, 1320. (c) Beddoes, R.; Heyes, D.; Menon,
R. S.; Watt, C. I. F. J. Chem. Soc., Perkin Trans. 2. 1996, 307.
5688
Org. Lett., Vol. 11, No. 24, 2009

3.97 nm for 2d is derived, while for 2c a slightly smaller
parameter of 3.49 nm is determined.
model for the columnar liquid crystal of 2d is illustrated
in Figure 4. In this model, three BNAX salts self-assemble
The wide angle meridional reflections in both patterns of
2d and 2c are attributed to a π-stacking distance of 0.34 nm
between aromatic cores within the columnar structures. The
molecules are packed in a nontilted manner with their planes
perpendicular to the columnar axis. The structural analysis
suggests that 2d forms a columnar liquid crystalline phase
due to the typical hexagonal unit cell, nontilted stacked disks,
and finally the appearance of an amorphous halo, which is
attributed to the disordered alkyl chains (Figure S4, Sup-
porting Information). Interestingly, the organization and the
corresponding dimensions of the liquid crystalline phase do
not change at higher temperatures (Figure S5, Supporting
Information, 150 °C) suggesting a pronounced stability of
this thermal state.
The main difference between 2c and 2d is obvious from
the meridional reflections owing to π-stacking correlations.
Since 2d is more waxy, the compound is therefore better
macroscopically aligned, leading to reflections with a smaller
azimuthal distribution. On the other hand, the sharp but rather
isotropic scattering intensities for 2c can be attributed to
poorer orientation of the powder-like sample.
It is interesting that the parameters of the unit cell for
both compounds are almost two times larger than the
simple molecular size (the fully stretched lengths of 2c
and 2d are 2.0 and 2.2 nm, respectively.9), which is very
similar to what we have observed in other discotic liquid
crystal materials consisting of positively charged PAHs.10
It is reasonable to assume that a kind of oligomer structure
is first formed in the liquid crystal phase of 2c and 2d
due to noncovalent forces between the molecules, includ-
ing dipole-dipole, ionic, and amphiphilic interactions.
These oligomeric discs then form discotic columnar stacks
by the local phase separation between their soft and rigid
parts.11 According to the parameters obtained by WAXS,
the effective number of molecules Z per repeat distance
(or slice) for 2c and 2d is determined as 2.97 and 2.90 at
30 °C (Supporting Information).12 Therefore, a molecular
Figure 4. Scheme illustration of the unit cell formed by 2d.
into a disklike trimer. These trimers then form columnar
superstructures which further arrange into the hexagonal
columnar mesophases. The dimension of the disklike
trimer is in good agreement with the dimension of 2d in
this model. The molecular packing of 2c in the bulk is
similar.
In this work, the synthesis of a class of unprecedented
large oxygen- and sulfur-containing positively charged
BNAX and BNATX salts was developed. These molecules
exhibited interesting optoelectronic properties which strongly
depend on the heteroatoms in their aromatic framework.
Columnar liquid crystals were formed by the trimers of alkyl-
chains-substituted BNAX salts due to noncovalent intermo-
lecular forces. Combined with our previous results,3 extend-
ing the synthetic methodology to even larger positively
charged PAHs with different heteroatoms is currently
underway in our laboratory. Physical studies of these
compounds as well as our exploration of their applications
are also in progress.
Acknowledgment. This work was financially supported
by the Max Planck Society through the program ENERCHEM,
the German Science Foundation (Korean-German IRTG),
DFG Priority Program SPP 1355, and DFG Grant No. AN
680/1-1.
(9) The MM2 force field was used to calculate the minimum energy
conformation during computer simulations.
Supporting Information Available: Experimental details
and characterization data of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(10) Wu, D. Q.; Pisula, W.; Enkelmann, V.; Feng, X. L.; Mu¨llen, K.
J. Am. Chem. Soc. 2009, 131, 9620.
(11) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. ReV. 2007, 36,
1902.
(12) Lava, K.; Binnemans, K.; Cardinaels, T. J. Phys. Chem. B 2009,
113, 9506.
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