Synthesis and blue-violet two-photon excited fluorescence of a new organoboron compound

Article abstract of DOI:10.1016/j.molstruc.2007.03.036

A new A-π-D-π-A type organoboron compound, 3,6-bis(dimesitylboryl)-N-butyl-carbazole (abbreviated as BBC), with trivalent boron as electron acceptor and carbazole as electron donor and π-conjugated core, has been synthesized and its single and two-photon

Full text of DOI:10.1016/j.molstruc.2007.03.036

Available online at www.sciencedirect.com
Journal of Molecular Structure 874 (2008) 46–50
www.elsevier.com/locate/molstruc
Synthesis and blue-violet two-photon excited fluorescence
of a new organoboron compound
a,
b
a
c
a
Duxia Cao *, Zhiqiang Liu , Guozhong Li , Guoqun Liu , Guohui Zhang
a
School of Materials Science and Engineering, University of Jinan, Jinan, 250022 Shandong, China
State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100 Shandong, China
School of Materials and Chemical Engineering, Zhongyuan University of Technology, Zhengzhou, 450007 Henan, China
b
c
Received 28 August 2006; received in revised form 13 March 2007; accepted 19 March 2007
Available online 27 March 2007
Abstract
A new A-p-D-p-A type organoboron compound, 3,6-bis(dimesitylboryl)-N-butyl-carbazole (abbreviated as BBC), with trivalent
boron as electron acceptor and carbazole as electron donor and p-conjugated core, has been synthesized and its single and two-photon
related photophysical properties experimentally investigated. Pumped by 720 nm laser pulses in femtosecond regime, it showed strong
two-photon excited blue-violet fluorescence at 393 nm in toluene and 403 nm in THF. The measured two-photon absorption cross-sec-
tion by two-photon fluorescence method in toluene and THF is 34 GM and 38 GM, respectively.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Two-photon excited fluorescence; Organoboron compound; Blue-violet fluorescence; Carbazole
1. Introduction
[5], and can serve as emissive and/or electron-transport lay-
ers in OLEDs [6]. In the past years, we firstly presented the
application of trivalent organoboron compounds for TPEF
[7–9]. Experiments demonstrated the trivalent organoboron
compounds synthesized by our group possess strong TPEF
properties and large two-photon absorption (TPA) cross-
section. Recently, we designed and synthesized a new
A-p-D-p-A type compound basing on trivalent boron atom
electron acceptor and aromatic heterocycle conjugated
bridge. The introduction of the aromatic heterocycle carba-
zole as a p-conjugated bridge are based on this aromatic
system is much richer in electronic density with a pair of
alone electrons on nitrogen atom compared to a benzene
and fluorene and has perfect coplanar conformation. In this
paper, we report single and two-photon related photophys-
ical properties of this A-p-D-p-A type compound.
In recent years, a considerable interest has been devoted
to research into organic molecules with strong two-photon
excited fluorescence (TPEF) due to their potential use in
advanced photonic applications such as three-dimensional
optical data storage [1], up-converted lasing [2] and two-
photon fluorescence excitation microscopy [3,4]. The fur-
ther development of these applications greatly benefit from
the synthesis of new organic molecules exhibiting strong
TPEF in desirable wavelength regions. But most of the
molecules reported recently mainly emit TPEF in yellow-
to-red light region and the molecules emitting TPEF in
blue-violet light region are fairly lacking.
Boron atom, especially three-coordinated boron, with its
vacant p-orbital, is a useful p electron acceptor in conju-
gated organic molecular materials. These materials exhibit
interesting fluorescence emission, first- and second order
nonlinear optical properties, and fluoride ion sensing ability
2. Experimental
2.1. Chemicals and instruments
*
Corresponding author. Tel.: +86 531 8276 5974; fax: +86 531 8797
Nuclear magnetic resonance spectra were recorded on a
MercuryPlus-400 spectrometer. Mass spectrum was
4453.
E-mail address: duxiacao@ujn.edu.cn (D. Cao).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.03.036

D. Cao et al. / Journal of Molecular Structure 874 (2008) 46–50
47
obtained on an Agilent 5973N MSD spectrometer. Ele-
mental analyse was carried out on a PE 2400 autoanalyzer.
n-Butyllithium in hexane solution was obtained from Acros
Ltd. Dimesitylboron fluoride (BF(Mes)2, Mes = 2,4,6-
trimethylphenyl) was purchased from Aldrich Ltd. Other
reagents, such as carbazole and 1-bromobutane were pur-
chased from Shanghai Reagents and were used as received
directly without further purification. All of the solvents
were freshly distilled before using.
2.2.2. 3,6-Bis(dimesitylboryl)-N-butyl-carbazole (BBC)
Under the cover of N2, 1.6 g (4.2 mmol) S-2 in 50 mL
freshly distilled THF was injected into a three-necked flask.
Then the flask was cooled to ꢀ78 ꢁC for about 30 min,
until 14 mmol n-butyllithium in hexane solution was
dropped. The flask was kept at ꢀ78 ꢁC by dry ice-acetone
bath for about one hour then warmed naturally to room
temperature to get the lithium salt. The mixture was cooled
to ꢀ78 ꢁC again, and 2.5 g (9.3 mmol) dimesitylboron fluo-
ride, which had been resolved in 10 mL freshly distilled
THF, was injected quickly via a dried injector. The mixture
was continuously stirred overnight and then was poured
into 200 mL distilled water and extracted with dichloro-
methane. After a rotary evaporator removed the organic
solvent, the product was purified through column chroma-
tography on silica gel using chloroform–petroleum ether (1:
9) as eluent. White powders with yield 35% were obtained.
1H NMR (CDCl3, 400 MHz), dH (ppm): 0.90 (t,
J = 7.3 Hz, 3H), 1.09–1.42 (m, 4H), 1.94 (s, 24H), 2.24 (s,
12H), 4.23 (t, J = 7.8 Hz, 2H), 6.75 (s, 8H), 7.23 (d,
J = 9.3 Hz, 2H), 7.55 (d, J = 8.8 Hz, 2H), 8.23 (s, 2H).
Anal. calcd for C52H59B2N: C, 86.79; H, 8.26; N, 1.95.
Found: C, 86.55; H, 8.18; N, 1.97. MS: m/z (%) 719 (M+,
100).
2.2. Synthesis
The synthetic strategy is outlined in Scheme 1. The title
compound was prepared in three steps via butylation, bro-
mination and boronation reaction. Substitution of 9-
hydrogen atom on carbazole using butyl group afforded
N-butyl-carbazole (S-1) [10]. 3,6-dibromo-N-butyl-carba-
zole (S-2) can be prepared from S-1 and two equivalent
Br2 in acetic acid. Finally, the title compound BBC was
obtained by substitution of two Br atoms on the 3,6-posi-
tion of S-2 using dimesitylboryl group.
2.2.1. 3,6-Dibromo-N-butyl-carbazole (S-2)
7.4 g (0.033 mol) N-butyl-carbazole was dissolved in
50 mL acetic acid, and then 3.4 mL (0.066 mol) Br2 in
50 mL acetic acid was added dropwise during 1 h. The mix-
ture was continuously stirred at room temperature for fur-
ther 2 h. A great deal of white precipitate formed and the
separated precipitate was repeatedly washed with ethanol
to give white needle-like crystals with a yield 70%. 1H
NMR (CDCl3, 400 MHz), dH (ppm): 0.89 (t, J = 7.3 Hz,
3H), 1.14–1.46 (m, 2H), 1.54–1.92 (m, 2H), 4.56 (t,
J = 7.6 Hz, 2H), 7.21 (d, J = 9.0 Hz, 2H), 7.54 (d,
J = 8.8 Hz, 2H), 7.99 (s, 2H).
2.3. Measurements
Linear absorption spectra with C = 1.0 · 10ꢀ5 mol Lꢀ1
were recorded on a PE Lambda 35 UV/VIS spectrometer.
Single-photon excited fluorescence (SPEF) spectra with
C = 1.0 · 10ꢀ5 mol Lꢀ1 were measured on an Edinburgh
FLS920 fluorescence spectrometer. The SPEF quantum
yields U for the compound were determined relative to cou-
marin 307 [11] using a standard method [12]. TPEF exper-
H
N
C4H9
N
KI, KOH
CH3(CH2)2CH2Br
+
DMSO
S-1
C4H9
N
CH3COOH
S-1
S-2
2
Br2
+
Br
Br
B
S-2
C4H9
N
n-BuLi
THF
2
FB(Mes)2
+
B
BBC
Scheme 1. Synthetic strategy of the title compound.

48
D. Cao et al. / Journal of Molecular Structure 874 (2008) 46–50
iments with C = 1.0 · 10ꢀ3 mol Lꢀ1 were performed with a
femtosecond Ti: sapphire laser (76 MHz, 200 fs pulse
width, Coherent Mira 900) as pump source and a streak
camera (Hamamatsu, model: C5680) in conjunction with
an imaging spectrograph (Hamamatsu, model: C5094) as
recorder. The TPA and TPEF cross-sections of BBC in tol-
uene and THF solutions were measured by using TPEF
measurement technique and coumarin 307 as standard
solution [13].
2.4. Theory calculation
We performed DFT (density functional theory) quan-
tum chemical calculations on the compound by using
Gaussian 03 [14]. The functional used includes Becke’s
three parameter hybride functional [15] in conjunction with
the Lee–Yang–Parr correlation functional [16], which is
abbreviated as B3LYP. The basis set is taken as 6-31G.
HOMO and LUMO of the compound studied are obtained
using the visualization package Gauss View [17].
Fig. 1. Linear absorption spectra of BBC in various solvents.
3. Results and discussion
3.1. Linear absorption and single-photon excited fluorescence
properties
The data of linear absorption and SPEF properties of
BBC in four solvents with different polarity are summa-
rized in Table 1. From Table 1, Figs. 1 and 2, one can
see that absorption peak position and absorbance show lit-
tle dependence on solvent polarity, but SPEF properties
are obviously influenced by solvent polarity. The peak
positions of SPEF are red-shifted and the fluorescence
quantum yields increased. The bathochromic phenomena
indicate that the polarity of the compound in excited state
is much larger than that in ground state. The difference in
dipole moment between excited state and ground state
(Dlge) can be estimated by Lippert–Mataga equation [18]:
Fig. 2. Single-photon excited fluorescence spectra of BBC in various
solvents.
respectively. From Fig. 3, Stokes’ shifts are basically pro-
portional to the orientational polarizability Df. The slope
m of the fitted line is 3.11 · 103 cmꢀ1. The theory geometry
optimization procedure gives the total volume of a mole-
Dm ¼ mabs ꢀ mSPEF ¼ 2Dl2geDf =hca3 þ const
ð1Þ
˚
cule and its equivalent a value (8.0 A) was obtained by
with Df = [(e ꢀ 1)/(2e + 1)] ꢀ [(n2 ꢀ 1)/(2n2 + 1)].
assuming the molecule to be a sphere having this volume.
Then the difference in dipole moment between the ground
and excited states in solution Dlge is 12.5 D. The depen-
dence of fluorescence quantum yield on solvent polarity
is unusual because a general trend for the D–A systems is
In this equation, h is Planck’s constant, c is the speed of
light, a is the cavity radius of the molecule, Df is orientation
polarizability reflecting the polarity of solvent, e and n is
the dielectric constant and the refractive index of solvent,
Table 1
Photophysical properties of BBC
emax (104 cmꢀ1 Mꢀ1
)
k
(nm)
Dm (cmꢀ1
)
Ua
k
(nm)
reb(GM)c
rb(GM)c
max
abs
max
SPEF
max
TPEF
Solvents
k
(nm)
Hexane
Toluene
THF
CH3CN
a
364
366
368
366
3.9
3.6
3.5
3.4
374, 392
385
1294
1348
1793
2322
0.52
0.68
0.73
0.74
393
403
23
28
34
38
394
400
Fluorescence quantum yields determined using coumarin 307 as standard.
b
c
TPEF cross-section (re) and TPA cross-section (r) at kex = 720 nm.
1GM = 10ꢀ50 cm4 s photonꢀ1 moleculeꢀ1
.

D. Cao et al. / Journal of Molecular Structure 874 (2008) 46–50
49
Fig. 3. Stokes shift Dm vs orientational polarizability Df of the solvents.
(The scatter dots are the experimental data and the line is the linearly fitted
result.)
that fluorescence quantum yield decreases with increasing
solvent polarity. An origin of this unexpected behavior
may be a decrease of nonradiative deactivation rate [19].
We also investigated the charge transfer mode of the
compound. Fig. 4 illustrates the electron density distribu-
tion of the frontier molecular orbitals (HOMO and
LUMO). In the HOMO, the electrons are mainly concen-
trated on the nitrogen atom and carbazole ring, while in
the LUMO, the electrons are spread to the boron atoms.
The theory calculation indicates that charge transfer from
N-butyl-carbazole to boron atoms upon excitation from
HOMO to LUMO.
Fig. 4. The electron density distribution of the frontier molecular orbitals
of BBC.
3.2. Two-photon excited fluorescence properties
to that of SPEF spectra. Just like solvent effect on SPEF,
the peak position of TPEF is red-shifted with the increase
of solvent polarity from toluene to THF. kmax(TPEF) is
TPEF was collected at a direction perpendicular to the
pump beam. During TPEF measurement, volume re-
absorption effect occurred because concentrated solutions
(1.0 · 10ꢀ3 mol Lꢀ1) were used. To minimize the re-absorp-
tion effect, the excitation beam was focused as closely as
possible to the quartz cell wall, which faced the slit of imag-
ing spectrograph. From linear absorption spectra (Fig. 1),
one can see that the compound is linear optical penetrable
in the range from 400 to 1000 nm. Therefore, any emission
induced by excitation at this wavelength range should be
derived from multiphoton absorption process.
When a Ti: sapphire femtosecond laser output 720 nm
laser is used as the pump source, the compound shows
strong blue-violet TPEF in toluene (393 nm) and THF
(403 nm) solution. TPEF peak positions of the compound
in both solvents are obvious red-shifted relative to that of
their corresponding SPEF spectra, which may result from
re-absorption effect in concentrated solution [20]. In
Fig. 5, one can see that the short wavelength side of the
SPEF and the long wavelength side of the linear absorption
band have a certain extent overlapping. The spectral pro-
files of the TPEF spectra of the compound are very similar
Fig. 5. The UV absorption, single-photon-excited fluorescence (SPEF)
spectra with C = 1.0 · 10ꢀ5 mol Lꢀ1 and two-photon-excited fluorescence
(TPEF) spectrum with C = 1.0 · 10ꢀ3 mol Lꢀ1 of BBC in THF.

50
D. Cao et al. / Journal of Molecular Structure 874 (2008) 46–50
shifted from 393 nm in toluene to 403 nm in THF. All the
spectral characters discussed above seems to indicate that
either single or two-photon excited initial state may relax
to the same fluorescence emission state.
References
[1] D.A. Parthenopoulos, P.M. Rentzepis, Science 245 (1989) 843.
[2] G.S. He, C.F. Zhao, J.D. Bhawalkar, P.N. Prasad, Appl. Phys. Lett.
67 (1995) 3703.
TPA cross-section r and TPEF cross-section re are
important parameters in evaluating materials’ TPA and
TPEF properties [13,21]. There are many methods to get
them such as Z-scan, TPEF measurement technique and
so on. In this paper, the latter are used to get r and re of
the compound and coumarin 307 is selected as standard
solution. The TPEF cross-sections (re) were obtained refer-
ring to the following equation [21]:
[3] W. Denk, J.H. Strickler, W.W. Webb, Science 248 (1990) 73.
[4] L. Ventelon, S. Charier, L. Moreaux, J. Mertz, M. Blanchard-Desce,
Angew. Chem. Int. Ed. 40 (2001) 2098.
[5] Z.Q. Liu, M. Shi, F.Y. Li, Q. Fang, Z.H. Chen, T. Yi, C.H. Huang,
Org. Lett. 7 (2005) 5481.
[6] C.D. Entwistle, T.B. Marder, Chem. Mater. 16 (2004) 4574.
[7] Z.Q. Liu, Q. Fang, D. Wang, D.X. Cao, G. Xue, W.T. Yu, H. Lei,
Chem. Eur. J. 9 (2003) 5074.
[8] Z.Q. Liu, Q. Fang, D.X. Cao, D. Wang, G.B. Xu, Org. Lett. 6 (2004)
2933.
Cr nr F
C n F r
[9] D.X. Cao, Z.Q. Liu, Q. Fang, G.B. Xu, G. Xue, G.Q. Liu, W.T. Yu,
J. Organomet. Chem. 689 (2004) 2201.
[10] W. Wang, Q. Fang, Z.Q. Liu, D.X. Cao, M.Z. Deng, Acta Chim.
Sinica 63 (2005) 1323.
re ¼ re;r
ð2Þ
In this equation, r stands for the reference compound, n for
the refractive index, C for the concentration and F for the
integral fluorescence intensity.
[11] G.A. Reynolds, K.H. Drexhage, Opt. Commun. 13 (1975) 222.
[12] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991.
[13] M.A. Albota, C. Xu, W.W. Webb, Appl. Opt. 37 (1998) 7352.
[14] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C.
Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B.
Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J.
Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin,
R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma,
G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D.
Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G.
Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T.
Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challa-
combe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C.
Gonzalez , J.A. Pople, Gaussian 03, Revision A.1, Gaussian, Inc.,
Pittsburgh, PA, 2003.
r ¼ re=U0
ð3Þ
U0 is TPEF quantum yield. Up to now, it is difficult to
measure U0 strictly. Here we assumed that U0 are constant
over the entire spectral range and U0 = U in our experi-
ment. TPA cross-section at 720 nm in toluene and THF
is measured 34 GM and 38 GM in this way, respectively.
4. Conclusion
A new A-p-D-p-A type organoboron compound has
been synthesized. The single and two-photon related
photophysical properties are reported. The compound
exhibits strong two-photon up-converted blue-violet fluo-
rescence with 720 nm laser excitation. DFT quantum
chemical calculation indicates that upon excitation to the
excited state, charge transfer from N-butyl-carbazole to
boron atoms takes place.
[15] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[16] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[17] Gaussian Inc., GaussView 3.0, 2003.
[18] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum
Press, New York, 1983, p. 190.
[19] R. Stahl, C. Lambert, C. Kaiser, R. Wortmann, R. Jakober, Chem.
Eur. J. 12 (2006) 2358.
[20] D.X. Cao, Q. Fang, D. Wang, Z.Q. Liu, G. Xue, G.B. Xu, W.T. Yu,
Eur. J. Org. Chem. (2003) 3628.
[21] C. Xu, W.W. Webb, J. Opt. Soc. Am. B 13 (1996) 481.
Acknowledgements
This work was supported by the Shandong Encouraging
Fund for Excellent Scientists (2006BS04016) and the
Doctoral Fund of University of Jinan (B0415).