Acceptor or donor (diaryl B or N)substituted octupolar truxene: Synthesis, structure, and charge-transfer-enhanced fluorescence

Article abstract of DOI:10.1021/jo061210i

(Graph Presented) Two diaryl B- and N-substituted truxene charge-transfer compounds B3 and N3 have been synthesized. The fluorescence intensities of several nonfunctionalized truxene compounds are 1 order of magnitude weaker than that of B3 and N3. To reveal the structure-property correlations, the X-ray structures of B3 and N3 and their precursors 3 and 4 have been determined. The extended molecular dimension, the especially shortened B-C bond, and the improved planarity of B3 can serve as direct structural evidence for the charge transfer.

Full text of DOI:10.1021/jo061210i

SCHEME 1. Synthesis of B3 and N3a
Acceptor or Donor (Diaryl B or N) Substituted
Octupolar Truxene: Synthesis, Structure, and
Charge-Transfer-Enhanced Fluorescence
Mao-Sen Yuan,† Qi Fang,*,† Zhi-Qiang Liu,† Jian-Ping Guo,‡
Hong-Yu Chen,† Wen-Tao Yu,† Gang Xue,† and
Dian-Sheng Liu‡
State Key Laboratory of Crystal Materials, Shandong
UniVersity, 250100 Jinan, P.R. China, and Institute of Modern
Chemistry, Shanxi UniVersity, Taiyuan, P.R. China
fangqi@icm.sdu.edu.cn
ReceiVed June 13, 2006
a Key: (a) PPA, 160 °C, 3 h; (b) Bromoethane, n-BuLi, THF, 0 °C, 12
h; (c) bromine, CH2Cl2, 25 °C, 12 h; (d) HIO3, I2, CH3COOH-H2SO4-
H2O-CCl4, 80 °C, 4 h; (e) dimesitylboron fluoride, n-BuLi, THF,
-78 °C, 2 d; (f) diphenylamine, K2CO3, Cu, 18-crown-6-ether, 1,2-
dichlorobenzene, reflux, 8 h.
Two diaryl B- and N-substituted truxene charge-transfer
compounds B3 and N3 have been synthesized. The fluores-
cence intensities of several nonfunctionalized truxene com-
pounds are 1 order of magnitude weaker than that of B3
and N3. To reveal the structure-property correlations, the
X-ray structures of B3 and N3 and their precursors 3 and 4
have been determined. The extended molecular dimension,
the especially shortened B-C bond, and the improved
planarity of B3 can serve as direct structural evidence for
the charge transfer.
The chemistry of truxene at 5-, 10-, 15-positions has acquired
appreciable development, and some of the resultant compounds
might serve as precursors for fullerenes.4-7 Recently, a variety
of large π-delocalized dendritic truxene derivatives have been
synthesized as a result of a series reactions initiated at the less
crowded para position (2-, 7-, 12-positions).2,8,9 However, the
CT strategy in the materials chemistry of truxene seems to be
considered less often.
Three-coordinate organoboron, with a vacant pz orbit, is an
excellent electronic acceptor when connecting to a π-delocalized
system, and some B(III) compounds indeed exhibit unique
optoelectronic properties.3b,10-14 Similarly, three-valence nitro-
The optoelectronic properties of an organic molecule are
correlated to its two structural characters: the π-conjugation
and the charge transfer (CT) property. Besides, molecular
symmetry is also a factor influencing some nonlinear optical
(NLO) processes. At a glance of the rigid π-conjugated structure
of truxene (1), namely 10,15-dihydro-5H-diindeno[1,2-a;1′,2′-
c]fluorene (Scheme 1), one can understand why this kind of
heptacyclic polyarene has received considerable attention in
recent years and shows some potential applications, such as in
organic light-emitting diodes (OLEDs),1 two-photo absorption,2
and the NLO chromophore.3a
(4) (a) Frutos, OÄ . de; Go´mez-Lor, B.; Granier, T.; Monge, M. AÄ .;
Gutie´rrez-Puebla, E.; Echavarren, A. M. Angew. Chem., Int. Ed. 1999, 38,
204. (b) Frutos, OÄ . de; Granier, T.; Go´mez-Lor, B.; Jime´nez-Barbero, J.;
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* To whom correspondence should be addressed. Fax: 86-513-88362782.
Tel: 86-513-88362782.
(8) Kanibolotsky, A. L.; Berridge, R.; Skabara, P. J.; Perepichka, I. F.;
Bradley, D. D. C.; Koeberg, M. J. Am. Chem. Soc. 2004, 126, 13695.
(9) (a) Pei, J.; Wang, J. L.; Cao, X. Y.; Zhou, X. H.; Zhang, W. B. J.
Am. Chem. Soc. 2003, 125, 9944. (b) Cao, X. Y.; Zhang, W. B.; Wang, J.
L.; Zhou, X. H.; Lu. H.; Pei, J. J. Am. Chem. Soc. 2003, 125, 12430. (c)
Duan, X. F.; Wang J. L.; Pei, J. Org. Lett. 2005, 7, 4071.
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† Shandong University.
‡ Shanxi University.
(1) Kimura, M.; Kuwano, S.; Sawaki, Y.; Fujikawa, H.; Noda, K.; Taga,
Y.; Takagi, K. J. Mater. Chem. 2005, 15, 2393.
(2) Zheng, Q. D.; He, G. S.; Prasad, P. N. Chem. Mater. 2005, 17, 6004.
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C. Chem. Eur. J. 1998, 4, 2129. (b) Stahl, R.; Lambert, C.; Kaiser, C.;
Wortmann, R.; Jakober, R. Chem. Eur. J. 2006, 12, 2358.
10.1021/jo061210i CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/29/2006
7858
J. Org. Chem. 2006, 71, 7858-7861

atmosphere. Catalyzed by Cu powder, N3 was synthesized from
4 and diphenylamine.
The crystals of B3, N3, 3, and 4 have been determined to be
with the space groups of I4h3d(B3), P63 (N3), and Pa3h(3 and
4), and the molecular point groups are C3 (for B3) and C3h (for
3, 4). There are two kinds of N3 molecules in the crystal: one
belongs to C3 point group with its truxene moiety being shallow-
bowl-shaped, and the other slightly deviates from C3 symmetry.
The π-conjugation of all four of these structures is fairly good.
The truxene moiety of B3, 3, and 4 has a perfect planar octupolar
C3h symmetry with the mean deviations from their individual
least-squares plane being 0.017(4), 0.060(4), and 0.060(5) Å
respectively. The significance of C3h point group is its pure
octupolar symmetry. The idea of octupoles was introduced to
NLO field in early 1990s and acquired development since
then.15,16
As shown in Figure 1, three para-C atoms (2-, 7-, 12-) form
an equilateral triangle with the edge length of 9.768(6) Å for
B3, which is longer than those of 3 (9.646(6) Å) and 4 (9.644-
(5) Å). Considering the electron-withdrawing nature of -B(Mes)2,
we believe that this two-dimensional extension is the result of
CT. This kind of octupolar CT from a π-body to three acceptors
(or conversely from three donors to a central π-body), such as
in B3, may be put as π-A3 (or π-D3), as an extension of the
well-established A-π-A or D-π-D model for quadrupolar
CT and D-π-A model for dipolar CT.17
The bond length of B-C3 (1.558(7) Å) is shorter than that
of B-C14 (1.565(8) Å) and B-C23 (1.588(8) Å), implying
certain p-π conjugation between the boron and the truxene
moiety. The calculated HOMO orbital of a model compound
of B3 also shows a bonding nature for the B-C3 bonds (see
Figure 6 in the Supporting Information). The B atom and its
three-bonded C atoms are perfectly coplanar, while the N atom
in N3 is 0.15(1) Å above the plane of its three bonded C atoms.
As shown in Figure 2, the B3 crystal has large channels along
[111] direction of the cubic cell, which may allow some
molecules in and off, and therefore may find some application
as a chemosensor. The N3 crystal has a 63 screw symmetry. As
expected, this polar noncentrosymmitric crystal possesses
2-order NLO property with a measured powder SHG intensity
(1064 f 532 nm) as strong as that of the powder KDP sample.
As shown in Figure 3 and Table 1, the absorption peaks of
N3 (λmax ) 364 nm) and B3 (λmax ) 362 nm) have a ∼60 nm
FIGURE 1. ORTEP drawing of (a) B3, (b) 3, and (c) N3. H-atoms
are omitted for clarity. The structure of 4 is very similar to that of 3.
gen is a typical donor center, and some triarylamines have
already been used as certain photofunctional materials.
In this paper, we introduce a diarylboryl acceptor or diaryl-
amino donor to the truxene moiety and investigate the structures
and optical properties of the resultant compounds.
The approach to compounds B3 and N3 is outlined in Scheme
1. 3-Phenylpropionic acid was converted to truxene after
acylation and condensation by step-heating to 160 °C in
phosphoric acid and in N2 atmosphere. To remove active
hydrogen atoms and to enhance the solubility, full alkylation
was carried out at the 5-, 10-, and 15-positions, obtaining
compound 2. Bromoethane was used because ethyl has an
appropriate size, matching the thickness of the B3 molecule to
facilitate crystallization (see Figure 1). Bromination or iodination
of 2 afforded 3 or 4, respectively. Schlenk techniques were used
to obtain B3 by reacting 3 with dimesitylboron (mesityl ) 2,4,6-
trimethylphenyl) fluoride in the presence of n-BuLi and in N2
red-shift relative to that of 2. The extinction coefficient (ꢀmax
)
is increased on a scale of 0.90 (2):0.98 (N3):1.21 (B3). N3 and
B3 emit strong blue-violet fluorescence at λmax ) 390 and 386
nm, respectively. The fluorescence quantum yields (Φ) are 0.27
for N3 and 0.34 for B3, which are about 4-5 times that (0.07)
of 2 and rank in the largest Φ values in the small truxene
molecules. It is noteworthy that the measured fluorescence
intensities (Figure 3) or the scaled intensities in terms of Φꢀmax
(Table 1) of N3 and B3 are 1 order of magnitude that of 2. The
emissive rate of N3 and B3, scaled by values of Φ/τ, are also
1 order of magnitude larger than that of 2 (Table 1). We believe
(15) (a) Zyss, J. Nonlinear Optics 1991, 1, 3. (b) Vance, F. W.; Hupp,
J. T. J. Am. Chem. Soc. 1999, 121, 4047.
(11) (a) Noda, T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11, 283.
(b) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714.
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(14) (a) Liu, Z.-Q.; Fang, Q.; Wang, D.; Cao, D.-X.; Xue, G.; Yu, W.-
T.; Lei, H. Chem. Eur. J. 2003, 9, 5074. (b) Liu, Z.-Q.; Fang, Q.; Cao,
D.-X.; Wang, D.; Xu G.-B. Org. Lett. 2004, 6, 2933.
(16) Cho, B. R.; Lee, S. J.; Lee, S. H.; Son, K. H.; Kim, Y. H.; Doo,
J.-Y.; Lee, G. J.; Kang, T. I.; Lee, Y. K.; Cho, M.; Jeon, S.-J. Chem. Mater.
2001, 13, 1438.
(17) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J. E.; Fu, J.-Y.;
Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
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W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653.
J. Org. Chem, Vol. 71, No. 20, 2006 7859

TABLE 1. Spectroscopic Data of 1-4, N3, and B3 in THF
a
a
λabs ꢀmax (×104,
λF
(nm)
τc
Φꢀmax (×103, Φ/τ
(nm) M-1 cm-1
)
Φb (ns)
M-1 cm-1
)
(ns-1
)
1
2
3
4
273
305
312
316
7.0
9.0
9.4
9.4
9.8
361
0.07 12.5
0.07 12.6
4.9
6.3
0.94
0.94
26.5
41.1
0.006
0.006
0.03
0.02
0.13
0.08
368
376
368
0.01
0.01
0.3
0.5
2.1
4.4
N3 364
B3 362
390, 405 0.27
386, 403 0.34
12.1
a Linear absorption and fluorescence maxima with c ) 1.0 × 10-5 mol/
L. b Fluorescence quantum yields determined using coumarin 307 with c
) 1.0 × 10-5 mol/L as standard. c Fluorescence lifetime.
In conclusion, we have carried out a detailed synthesis-
structure-property study on a series of truxene derivatives. The
X-ray diffraction data have revealed the structural evidence of
CT of B3 relative to its parent compounds at the ground state.
The spectroscopic data and theoretical calculations suggest the
photoinduced CT of B3 and N3 at the excited state. The
acceptor- or donor-functionalized octupolar compounds B3 and
N3 emit greatly enhanced blue fluorescence in comparison with
that of the nonfunctionalized compounds. The main subject
compounds in this work as well as their derivatives in future
may find some application, such as the blue-light-emitting and
two-photon materials, the hole-transport (N3) and electron-
transport (B3) materials in OLEDs, the 2-order NLO materials
(N3), and the chemosensors (B3 crystal).
Experimental Section
2,7,12-Tri(dimesitylboryl)-5,5′,10,10′,15,15′-hexaethyltrux-
ene (B3). n-BuLi (2.8 M solution in n-hexane, 4.50 mL, 12.6 mmol)
was added to a stirred solution of compound 3 (1.10 g, 1.48 mmol)
in THF (30 mL) under nitrogen at -78 °C over 20 min, followed
by natural warming to room temperature. After reacting for a further
4 h, the reactants were cooled to -78 °C again, and dimesitylboron
fluoride (2.20 g, 8.2 mmol) in THF (15 mL) was injected over 30
min. The temperature was allowed to naturally rise to room
temperature and the mixture continuously stirred for 2 days. The
reactants were then diluted with ethyl acetate, washed with water,
and dried over magnesium sulfate. After solvents were removed,
the crude product was obtained. After purified by column chro-
matography on silica gel, eluting with dichloromethane-petroleum
ether (1: 20), compound B3 was obtained (0.45 g, 24%) as a white
powder. The single crystal was successfully obtained by slow
evaporation of their THF solution in air: mp > 400 °C; 1H NMR
(CDCl3, 300 MHz, ppm) δ 0.17 (s, 18H), 2.06 (s, 42H), 2.34 (s,
18H), 2.92 (s, 6H) 6.86 (s, 12H) 7.49-8.28 (m, 9H); 13C NMR
(CDCl3, 75 MHz, ppm) δ 151.91, 146.71, 144.25, 144.06, 142.05,
140.95, 138.83, 138.49, 135.10, 129.84, 128.19, 124.20, 56.90,
29.17, 23.58, 21.28, 8.46; MALDI-TOF m/z 1254.0 [M+], 1225.1
[M - 29]+. Anal. Calcd for C93H105B3: C, 88.99; H, 8.43. Found:
C, 88.63; H, 8.46. Crystal data for B3 (C93H105B3): Mr ) 1255.20,
cubic, space group I4h3d, a ) 36.383(4) Å, V ) 48160(8) Å3, Z )
16, Fcalcd ) 0.692 g/cm3, T ) 293(2) K, crystal dimensions 0.40 ×
0.40 × 0.30 mm3, R1 ) 0.0693 (wR2 ) 0.1625) [I > 2σ(I)].
2,7,12-Tri(N,N-diphenylamino)-5,5′,10,10′,15,15′-hexaethyl-
truxene (N3). A mixture of compound 4 (1.20 g, 1.35 mmol), K2-
CO3 (2.80 g, 20.3 mmol) powder, fresh Cu (0.30 g, 4.69 mmol)
powder, 18-crown-6 ether (0.20 g, 0.83 mmol), diphenylamine (1.20
g, 7.09 mmol), and 1,2-dichlorobenzene (20 mL) was heated to
reflux with stirring under nitrogen atmosphere. After being reacted
for a further 8 h, the mixture were cooled to room temperature and
filtered off under suction. Then the filtrate was condensed and
purified by column chromatography on silica gel, eluting with
dichloromethane-petroleum ether (1:10), and compound N3 was
obtained (0.71 g, 52%) as a white powder. The single crystal was
FIGURE 2. Top: [111] view of the cubic B3 crystal showing large
channels with a diameter about 5 Å. That is why its density (0.692
g/cm3) is very small. Bottom: c-axis view of the hexagonal N3 crystal.
The blue molecules have a C3 point group, and the green molecules
slightly deviate from C3. The narrow phenyl channels are located at
the 63 and 21 sites. The white phenyls are in one layer, and the yellow
phenyls are in the neighboring layer.
FIGURE 3. Absorption and fluorescence spectra of B3, N3, and 2 in
THF with c ) 1.0 × 10-5 mol/L.
that the fluorescence enhancement of N3 and B3 is associated
with photoinduced CT. This octupolar kind of CT in exciting
process does not require a dipole moment change and therefore
may not followed by a large Stokes loss.
A calculation for a model compound of B3 by using the time-
dependent density functional theory (TDDFT) shows that the
negative charge on any B atom in LUMO and LUMO+1 is 1
order of magnitude larger than that in HOMO (see the
Supporting Information for details).
7860 J. Org. Chem., Vol. 71, No. 20, 2006

successfully obtained by slow evaporation of the CH2Cl2 solution
in air: mp 310-312 °C; 1H NMR (CDCl3, 400 MHz, ppm) δ 0.17
(t, 18H, J ) 7.03 Hz), 1.88-1.93 (m, 6H), 2.82-2.87(s, 6H), 6.99-
7.30 (m, 36H) 8.08 (d, 3H, J ) 8.58 Hz); 13C NMR (CDCl3, 75
MHz, ppm) δ 154.19, 147.94, 146.27, 142.36, 138.33, 135.59,
129.20, 125.30, 124.13, 122.61, 121.98, 117.70, 56.51, 29.18, 8.66;
MALDI-TOF m/z 1011.0 [M+], 981.7 [M - 29]+. Anal. Calcd for
C75H69N3: C, 88.98; H, 6.87; N, 4.15. Found: C, 88.68; H, 6.89;
N, 4.01. Crystal data for N3 (C75H69N3): Mr ) 1012.33, hexagonal,
space group P63, a ) 32.199(7) Å, c ) 12.957(4) Å, V ) 11634-
(5) Å3, Z ) 8, Fcalcd ) 1.156 g/cm3, T ) 298(2) K, crystal
dimensions 0.54 × 0.12 × 0.09 mm3, R1 ) 0.1069 (wR2 ) 0.2709)
[I > 2σ (I)].
Acknowledgment. We thank the National Natural Science
Foundation of China (No. 20472044), the Ph.D. Foundation of
the Ministry of Education of China, and the Foundations of the
Key Laboratory of Organofluorine Chemistry of the Chinese
Academy of Sciences for financial support for this work.
Supporting Information Available: Syntheses and character-
izations of compounds 1-4; CIF files for 3, 4, B3, and N3;
computational work and results for B3. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO061210I
J. Org. Chem, Vol. 71, No. 20, 2006 7861