TPA-active D-π-D fluorophores with rigid, planar cores from phenylene to indenofluorene and indolocarbazole

Article abstract of DOI:10.1016/j.dyepig.2009.11.009

A series of D-π-D quadrupolar fluorophores with systematically varied π-conjugated cores were prepared using the Heck reaction and their single- and two-photon photophysical properties were examined. Introduction of extended π-conjugated cores such as indenofluorene/indolocarbazole was found to induce only small shifts in the absorption and emission maxima versus the phenylene linked fluorophore but greatly enhance their single- and two-photon absorption cross-sections. The effects of positional substitution on the properties of the indolocarbazole derivatives were also investigated.

Full text of DOI:10.1016/j.dyepig.2009.11.009

Dyes and Pigments 86 (2010) 63e67
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TPA-active De
p
eD fluorophores with rigid, planar cores from phenylene
to indenofluorene and indolocarbazole
,
b
a
a
Zhiqiang Liu a , Duxia Cao , Ying Chen , Qi Fang
*
a State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
b School of Material Science and Engineering, University of Jinan, Jinan 250022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of De
using the Heck reaction and their single- and two-photon photophysical properties were examined.
Introduction of extended -conjugated cores such as indenofluorene/indolocarbazole was found to induce
only small shifts in the absorption and emission maxima versus the phenylene linked fluorophore but
greatly enhance their single- and two-photon absorption cross-sections. The effects of positional substi-
tution on the properties of the indolocarbazole derivatives were also investigated.
Ó 2009 Elsevier Ltd. All rights reserved.
peD quadrupolar fluorophores with systematically varied p-conjugated cores were prepared
Received 19 October 2009
Received in revised form
29 November 2009
Accepted 30 November 2009
Available online 21 December 2009
p
Keywords:
Fluorescence
Two-photon absorption
Indenofluorene
Indolocarbazole
Quadrupolar systems
Heck reaction
1. Introduction
co-workers recently reported a systematic study on the structure-
property relationships of various quadrupolar fluorophores with
Since the pioneering work of Parthenopoulos and Rentzepis [1]
and Webb and co-workers [2] on three-dimensional optical data
storage and two-photon laser scanning fluorescence microscopy,
molecules with strong two-photon absorption (TPA) cross-sections
different types of p-conjugated cores. The results of this work
indicated that increasing the connector length between D or
A groups induces systematic and pronounced red-shifts and hyper-
chromic effects on the absorption bands [9].
(
d2) have attracted growing interest [3]. Many chromophores with
Most applications of TPA and two-photon excited fluorescence
(TPEF) require excitation in the near infrared region (700e900 nm),
meaning that a molecular engineering approach to enhance the TPA
cross section of the materials without inducing large shifts in
a large variety of structures have been synthesized and had their
TPA cross-sections measured, and fundamental efforts have been
made to elucidate structure-property relationships in these systems.
Among them, DepeD and AepeA one-dimensional quadrupolar
their absorption bands would be highly desirable. Although DepeD
systems have been successfully employed to enhance d2 values. The
quadrupolar intramolecular charge transfer taking place between
either the electron-donating (D) or electron-accepting (A) end-
quadrupolar compound 1 and its fluorene analogue 2 have been
exclusively examined as TPA-active compounds [10,11], little attention
has been paid to their spectral similarities, particularly the linear
absorption and single-photon excited fluorescence emission spectra in
dilute solutions, which span nearly the same regions despite their
molecular structural differences. Thus, in the current investigation, we
groups and the
p-conjugated cores is believed to be an important
factor in this enhancement [4]. Further efforts to introduce stronger
acceptors or donors as well as elongated cores to enhance this charge
transfer have confirmed this assumption [5]. Very large d2 values
have also been obtained with DeAeD, AeDeA, DeAeDeAeD, and
AeDeAeDeA systems [3,6e8]. However, this success is often at the
cost of reduced fluorescence quantum yield and/or pronounced red-
shift of the absorption and emission bands. Blanchard-Desce and
have examined the structure-function relationship of De
peD systems
with respect to lengthening of the -conjugated core, in particular
p
focusing on extended fluorene and carbazole-based systems.
These systems, compounds 1e6 in Fig. 1, contain highly rigid and
planar
nylamino moieties were chosen as the electron-donating end-
groups with styryl bridging units to the varying -conjugated cores.
The linking units employed in this study were: 1,4-phenylene (1);
p-conjugated cores, with solubilizing alkyl groups. Diphe-
p
* Corresponding author. Tel./fax: þ86 531 88362782.
E-mail address: zqliu@sdu.edu.cn (Z. Liu).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.11.009

64
Z. Liu et al. / Dyes and Pigments 86 (2010) 63e67
N
N
X
Ar
X
N
2
Ar
C8H17
N
C8H17
N
C4H9C4H9
Ar =
N
C5H11
N
C8H17
N
C8H17
C2H5 C2H5
C4H9C4H9
1
2
3
4
5
6
Fig. 1. Synthesis of quadrupolar fluorophores 1e6. Reagents and conditions: Pd(OAc)2, P(o-tol)3, DMF, NEt3, reflux, 24 h. X ¼ I, for 1, 2 and 4; X ¼ Br, for 3, 5 and 6.
9,9-diethyl-9H-fluoren-2,7-yl (2); 9-n-pentyl-9H-carbazole-2,7-yl
(3); 6,6,12,12-tetra-n-butyl-6,12-dihydroindeno[1,2-b]fluoren-2,8-yl
(4); 6,12-di-n-octyl-6,12-dihydro-6,12-diazaindeno[1,2-b]fluoren-
2,8-yl (5) and 6,12-di-n-octyl-6,12-dihydro-6,12-diazaindeno[1,2-b]
fluoren-3,9-yl (6). Indenofluorene and indolocarbazole moieties
have been widely employed in organic light emitting diode (OLED)
and organic transistor materials, although there are few reports
of their use in TPA chromophores [12, 13]. A compound similar to 4
with methyl rather than n-butyl substituents has recently been
described in the patent literature and shown to exhibit high electro-
luminescent efficiency [14]. Lin and co-workers also reported
recently the synthesis and two-photon properties of a multipolar
chromophore derived from triphenylamine with three diphenyla-
minoindenofluorenyl branches [15]. To understand the effects of
positional substitution, if any, isomers 5 and 6 were also prepared.
(m, 20H), 7.18e7.21 (m, 10H), 7.38 (m, 4H), 7.43 (d, J ¼ 8 Hz, 2H),
7.53 (s, 2H), 7.64 (d, J ¼ 8 Hz, 2H). 13C NMR (CDCl3, 100 MHz)
d:
13.85, 23.10, 26.04, 40.59, 54.60, 113.87, 119.58, 120.61, 122.99,
123.71, 124.50, 125.38, 127.18, 127.26, 127.91, 129.14, 129.28, 131.87,
136.23, 141.07, 147.20, 147.62, 150.45, 151.68; MS: m/z (%): 1016.6
(Mþ, 100); Anal. calcd for C76H76N2: C, 89.72; H, 7.53; N, 2.75;
found: C, 89.76; H, 7.55; N, 2.70.
5: Yellow powder, Yield 52%. m.p. 178e180 ꢀC; 1H NMR (CDCl3,
400 MHz)
d
: 0.79 (t, J ¼ 8 Hz, 6H), 1.20 (broad, 12H), 1.32 (m, 4H), 1.42
(m, 4H), 1.90 (m, 4H), 4.34 (t, J ¼ 7 Hz, 4H), 6.95e7.07 (m, 14H),
7.14e7.22 (m, 14H), 7.34e7.40 (m, 8H), 7.87 (s, 2H), 8.07 (d, J ¼ 8 Hz,
2H). 13C NMR (CDCl3, 100 MHz)
d: 14.07, 22.63, 27.43, 28.87, 29.25,
29.48, 31.88, 43.30, 98.53,106.28,116.73,120.27,122.44,122.78,122.97,
123.77, 124.46, 127.29, 127.41, 128.49, 129.28, 132.00, 135.46, 136.84,
142.17, 147.117, 147.64; MS: m/z (%) 1018.7 (Mþ, 100); Anal. calcd for
C74H74N4: C, 87.19; H, 7.32; N, 5.50; found: C, 87.31; H, 7.28; N, 5.48.
6: Pale yellow powder. Yield 65%. m.p. 170e172 ꢀC; 1H NMR
2. Experimental section
(CDCl3, 400 MHz)
d
: 0.86 (t, J ¼ 6 Hz, 6H), 1.26 (broad, 12H), 1.38
(m, 4H), 1.46(m, 4H), 1.96 (m, 4H), 4.41 (t, J ¼ 7 Hz, 4H), 7.01e7.15
(m, 14H), 7.23e7.29 (m, 14H), 7.37 (d, J ¼ 7 Hz, 2H), 7.45 (d, J ¼ 7 Hz,
4H), 7.66 (d, J ¼ 7 Hz, 2H) 8.02 (s, 2H), 8.32 (s, 2H). 13C NMR (CDCl3,
2.1. Synthesis and characterization of compounds 1e6
All reactions were carried out under a nitrogen atmosphere using
standard Schlenk techniques. NMR spectra were obtained by using
Bruker Avance 400 spectrometers. All spectra were recorded in
CDCl3. Chemical shifts are reported relative to tetramethylsilane and
are referenced to residual proton or carbon resonances in CDCl3.
Mass spectra were recorded on Applied Biosystems Voyager DE STR
(MALDI-TOF) spectrometers. Elemental analyses were performed
on a PE 2400 autoanalyser. The melting points were measured on
a METTLER-TOLEDO DSC822e differential scanning calorimeter at
a heating rate of 20 ꢀC minꢁ1 under nitrogen atmosphere.
100 MHz) d: 14.06, 22.63, 27.45, 28.93, 29.23, 29.47, 31.86, 43.53,
99.08, 108.60, 118.23, 122.85, 122.99, 123.29, 124.09, 124.36, 124.57,
125.18, 127.00, 128.14, 128.44, 129.27, 132.70, 136.57, 141.45, 146.77,
147.79; MS: m/z (%) 1018.7 (Mþ, 100); Anal. calcd for C74H74N4: C,
87.19; H, 7.32; N, 5.50; found: C, 87.26; H, 7.26; N, 5.53.
2.2. Photophysical properties measurement
Linear absorption spectra with C ¼ 1.0 ꢂ 10ꢁ6 mol Lꢁ1 were
recorded on a Shimadzu UV 2550 spectrometer. Single-photon
excited fluorescence (SPEF) spectra with C ¼ 1.0 ꢂ 10ꢁ6 mol Lꢁ1 were
measured on an Edinburgh FLS920 fluorescence spectrometer. The
General procedure for Heck reaction. A mixture of dihalo-
substituted aromatic center (1.0 mmol), 4-N,N-diphenylaminos-
tyrene (0.81 g, 3.0 mmol), tri-o-tolylphosphine (0.15 g, 0.49 mmol),
palladium(II) acetate (0.050 g, 0.22 mmol), 10 mL of redistilled
triethylamine and 10 mL of redistilled DMF were heated to 80 ꢀC
under nitrogen with stirring. After reacting for more than 20 h,
the mixture were cooled to room temperature and poured into the
water and then extracted with CHCl3. The organic phase was dried
over MgSO4 and concentrated under reduced pressure and purified
by column chromatography on silica gel using chloroform/petro-
leum ether (1:3 v/v) as the eluent.
SPEF quantum yields
F for the compound were determined relative
to coumarin 307 [16] using a standard method [17]. TPEF experi-
ments with C ¼ 1.0 ꢂ 10ꢁ4 mol Lꢁ1 were performed with a femto-
second Ti: sapphire laser (80 MHz, 200 fs pulse width, Spectra
Physics) as pump source and a USB 2000 þ Micro Fiber Spectrometer
(Ocean optics) as recorder. More details on the experimental setup
and the method can be found in previous work [18]. The TPA cross-
sections of 1e6 in toluene and THF solutions were measured by
using TPEF measurement technique and coumarin 307 as reference
solution [19].
3: Yellow powder. Yield 57%. m.p. 212e213 ꢀC; 1H NMR (CDCl3,
400 MHz) d: 0.90 (t, 3H), 1.42 (broad, 4H), 1.90 (m, 2 H), 4.25 (t, 2 H),
7.02e7.46 (m, 36 H), 8.00 (d, J ¼ 8 Hz, 2H); 13C NMR (CDCl3,100 MHz)
d: 12.97, 21.51, 27.74, 28.43, 42.00, 105.55, 116.67, 119.33, 121.35,
3. Results and discussion
121.97, 122.70, 123.45, 126.28, 126.55, 127.24, 128.26, 130.86, 134.40,
140.53, 146.20, 146.59; MS: m/z (%) 676 (Mþ, 100); Anal. calcd for
C57H49N3: C, 88.22; H, 6.36; N, 5.41; found: C, 88.41; H, 6.27; N, 5.37.
4: Yellow powder. Yield 65 %. m.p. 188e190 ꢀC; 1H NMR (CDCl3,
3.1. Synthesis
As shown in Fig. 1, compounds 1e6 were synthesized via cross-
400 MHz)
d: 0.59 (m, 20H), 1.02 (m, 8H), 1.97 (m, 8H), 6.95e7.08
coupling of the appropriate dihalo-substituted arene with two

Z. Liu et al. / Dyes and Pigments 86 (2010) 63e67
65
equivalents of 4-N,N-diphenylaminostyrene under typical Heck
reaction conditions, catalyzed by Pd(OAc)2 and P(o-tol)3. 1,4-diio-
dobenzene is commercially available. Indenofluorene was prepared
starting from 1,4-dibromo-2,5-dimethylbenzene by
a similar
method to that reported by Wang and co-workers [20] with a little
revision. Normally, the direct halogenation of fluorene will take
place at 2,7 positions [21]. Similar halogenations with I2-HIO3 in
acid condition also occur for indenofluorene at para-positions (C2
and C8 positions) in our work. However, the direct halogenations
of carbazole always occur at the 3,6 positions. Therefore, the
method of Müllen and co-workers was used to prepare the
2,7-dibromocarbazole starting from commercially available 4,40-
dibromobiphenyl [22]. The 2,8-dibromo-indolo[1,2-b]-carbazole
and 3,9-dibromo-indolo[1,2-b]-carbazole was synthesized by the
method reported by Beng and co-workers, with bromine atom
being introduced since the starting materials of the first step
(4-bromophenylhydrazine hydrochloride and 3-bromophenylhy-
drazine hydrochloride, respectively) to avoid the difficulty of
seperation of isomers in the normal direct halogenations [23a]. To
improve the solubility of these compounds, alkyls groups of varying
length were introduced at the bridging C or N atoms. Details of the
syntheses and characterizations of compounds 3e6 are given in the
experimental section, and those for their dihalo-arene precursors
are provided in the supporting information.
3.2. Linear absorption and single-photon excited fluorescence
Fluorophores 1e6 all display solubility in common solvents of
intermediate polarity, such as toluene, THF and chloroform. Single-
photon absorption and fluorescence spectra recorded for 1e6 are
shown in Fig. 2, and the relevant photophysical data are summarized
in Table 1. To our surprise, they exhibit quite similar photophysical
properties despite their structural differences. Examination of the
absorption spectra of the related fluorene-based compounds, 2 and
4, shows that upon extension of the core of these chromophores,
a negligible red-shift in their absorption maxima, lmax(abs), of less
than 2 nm takes place. As shown in Fig. 2a, these compounds display
distinct, broad absorption bands with maxima of 411e416 nm, and
also longer-wavelength shoulders. It should also be noted that an
increase in the length of the conjugated cores results in enhance-
Fig. 2. The normalized linear absorption (a) and single-photon excited fluorescence
emission spectra (b) of compounds 1e6 in toluene (1.0 mM).
we can assume that the intramolecular charge transfer during the
excitation are also symmetrical, which means the charge transfer
only involving nearly the same domains from the electron-
donating triphenylamino to the phenyl of central core. What should
be pointed out, although the fluorene and indenofluorene keep the
bridge with high rigidity and planarity, the phenyls in these cores
seems not conjugated very well to each other. For example, we had
previously reported crystal structure of indenofluorene [24]. The
five-membered rings in indenofluorene do not fit the Hückel
aromaticity rule. The bridging CeC bond lengths in indenofluorene
are both 1.468 Å, which is significantly longer than those of the
phenyl rings (1.391 and 1.404 Å). These less conjugation may also
contribute to the spectral similarity.
ment of the extinction coefficients (3). For example, the value of 3 for
compound 4 is nearly twice that of 1.
The carbazole-based compounds 3, 5 and 6 show quite different
trends. Although the absorption spectrum of 3 is nearly identical to
that of 1, that of 5 displays a lmax value of 416 nm with a secondary
peak at 440 nm, both of which are significantly red-shifted relative
to that of 3 (5 nm for lmax and 20 nm for the longer-wavelength
shoulders). In addition, changing the position of the electron-
donating substituents on the cores substantially alters the spectral
properties. Compared with spectrum of 5, the main absorption band
of 6 is blue-shifted by 27 nm, and the value of 3 is also decreased by
nearly 20%.
Table 1
The photophysical properties of the compounds 1e6 in toluene.a
In addition to the similarity of the absorption spectra, the fluo-
rescence emission properties of these compounds are also quite
similar despite the differences in their molecular structures. As
shown in Fig. 2b, compounds 1e4 emit light in the blue-green spectral
region with emission maxima, lmax (em), of 450e455 nm. Compound
5 exhibits a maximum at 473 nm, a red-shift of ca. 20 nm. Although
linear absorption of 6 is greatly blue-shifted relative to those of others,
its emission maximum is similar to those of compounds 1e4 (around
453 nm). All of their emission spectra contain secondary peaks at
longer wavelengths. Their fluorescence quantum yields, V, are also
sizeable (0.60e0.84).
lmax(abs)/nmb
log
3
lmax(em)/nmc
F
d2/GMd
1
2
3
4
5
6
412
412
411
413
416
389
4.96
5.13
5.10
5.21
5.36
5.10
455
451
451
450
473
453
0.82
0.84
0.81
0.66
0.62
0.60
696
870
910
1280
1770
880
a
b
c
All single-photon-physical properties were measured as 1.0
Only the longest-wavelength absorption peaks are shown.
Emission measurements excited at the absorption maximum.
mM solutions.
d
TPA cross-sections were measured by the comparative TPEF method [19] on
1.0 ꢂ 10ꢁ4 M solutions excited by a fs laser at 730 nm with Coumarin 307 as
The spectral similarity of these compounds is interesting.
Considering the molecular symmetry of compounds 1e5, maybe
reference.

66
Z. Liu et al. / Dyes and Pigments 86 (2010) 63e67
Fig. 3. Crystal structures of indenofluorene [24] (left) and indolocarbazole [23c] (right). (All the H atoms and most of the C atoms of the solubilizing alkyl groups were omitted for
clarity.)
However, the differences between those of compounds 4 and 5
could be related to the differences in the bond lengths of their cores.
Differing from that of indenofluorene, the five-membered rings in
indolocarbazole fit the Hückel aromaticity rule. As mentioned above,
for indenofluorene, the bridging CeC bond lengths are significantly
longer those of the phenyl rings. However, in indolo[3,2-b]carbazole
[23c], the bridging CeC bonds lengths are 1.443 Å (see Fig. 3), which
is closer to those of the analogous phenyls (1.410 and 1.417 Å).
The differences in the optical properties of compounds 5 and 6
could be related to the differences in the positions of their electron-
donating groups relative to the indolocarbazole cores. Similar spectral
differences were also observed in other indolocarbazole derivatives
reported by Leclerc and co-workers [13,23c]. In compound 5, the
electron-donating groups are located at the 2- and 8-positions of the
core, so electron-delocalization from terminal amines to the center
will be permitted. However, in compound 6, they are at the 3- and 9-
positions, para- to the bridging N atoms, so electron-delocalization
from terminal amines to the center will be forbidden, but electron-
delocalization from terminal amine to bridging N atoms should be
permitted, meaning that the conjugation length of 6 is shorter than
that of 5.
different in many cases. For example, a compound similar to our
compound 2 had also been studied with the TPEF technique by Fitilis
et al. [11]. However, the TPA value reported in that paper is smaller
than both other researcher's [10] and ours. So the comparison
between the results from different group should be very careful.
However, the result under the same condition should be reliable
to investigate the structure-dependence. In this work, the TPA cross-
sections of these compounds were measured by the comparative
TPEF measurement technique, using femtosecond laser pulses as the
light source, as described previously. As with most of the reported
quadrupolar TPA fluorophores at low concentration, in all cases the
TPEF spectra are quite similar to the single-photon fluorescence
spectra, except of lower intensity and with more noise (see Fig. 4).
The pronounced vibronic structures in the single-photon data
are not as apparent in the two-photon data. This maybe due to the
spectrometer slits in the two-photon experiment are normally
opened wider to get enough signal level. However, the two-photon
excitation spectra contain maxima which are significantly blue-
shifted with respect to those of single-photon absorption (see Fig. 5).
This is likely a consequence of the effect of molecular symmetry as
manifested in the parity-derived selection rules [3]. It well known,
that the spectral selection rule of TPA (g / g) is different from that of
single-photon absorption (g / u) for the centrosymmetric struc-
tures. But both SPEF and TPEF emission processes, with only one
photon involved, should abide by the same rule (u / g).
3.3. Two-photon absorption and two-photon
excited fluorescence emission
Being limited by the wavelength range of our laser system, we are
unlikely to have found the maximum TPA cross-sections. However,
based on the d2 values obtained at 730 nm, as given in Table 1, it
seems that elongation of the central core significantly enhances the
TPA. The TPA cross-section of compound 5 is as large as 1770 GM at
730 nm, which is comparable to those of other reported efficient TPA
systems. However, the TPA cross-section of compound 6 is only 880
Up to now, the accurate measurement of TPA cross-section value
is still difficult to do. The TPA cross section value had been found to
depend on the experimental condition, such as the laser, concen-
tration, solvents and the references for TPEF method [3a,3b]. So, even
the same compound, the data reported by different group maybe also
Fig. 4. Two-photon excited fluorescence emission spectra of 1e6 in toluene
(1.0 ꢂ 10ꢁ4 mol Lꢁ1).
Fig. 5. Two-photon excitation spectra of 1e6 in toluene (1.0 ꢂ 10ꢁ4 mol Lꢁ1).

Z. Liu et al. / Dyes and Pigments 86 (2010) 63e67
67
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4. Conclusion
In conclusion, we have synthesized a series of DepeD systems
with different cores, ranging from benzene to indenofluorene
and indolocarbazole. Systematic comparison of their single- and
two photophysical properties reveals that the systems with longer
cores, such as indenofluorene and indolocarbazole have signifi-
cantly enhanced one- and two-photon absorptions, but only small
shifts in their absorption or emission bands.
Acknowledgement
Financial support from the National Natural Science Foundation
of China (20802026, 50803033) Natural Science Foundation of
Shandong Province (Q2008F02) Shandong Encouraging Fund for
Excellent Scientists (BS2009CL013) is gratefully acknowledged. The
authors thank Dr. Jonathan C. Collings of Durham University for his
valuable suggestions.
[18] Liu ZQ, Fang Q, Wang D, Cao DX, Xue G, Yu WT, et al. Chem Eur
2003;9:5074e84.
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