Synthesis and photoluminescent properties of two novel tripodal compounds containing coumarin moieties

Article abstract of DOI:10.1016/j.saa.2009.02.013

Two novel tripodal compounds, tris[2-(7-diethylamino-coumarin-3-carboxamide)ethyl]amine (Tren-C1) and tris[2-(benzo[5,6]coumarin-3-carboxamide)ethyl]amine (Tren-C2), were synthesized and characterized. The UV-vis and fluorescence properties of Tren-C1 and Tren-C2 in solutions were investigated. These two compounds exhibited strong blue emission under ultraviolet light excitation. The maximal fluorescence emission occurred at about the level of 10^-5 mol/L. The chromophore units in the tripodal compounds shown a little interaction in the ground state, while the interactions in the excited state was notable and which leads to a broad and bathochromic shift of the emission bands.

Full text of DOI:10.1016/j.saa.2009.02.013

Spectrochimica Acta Part A 73 (2009) 168–173
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and photoluminescent properties of two novel tripodal compounds
containing coumarin moieties
Tianzhi Yua,∗, Peng Zhanga,b, Yuling Zhaob, Hui Zhanga, Jing Menga,b, Duowang Fana
a Key Laboratory of Opto-Electronic Technology and Intelligent Control (Lanzhou Jiaotong University), Ministry of Education, 88 West Anning Road, Lanzhou 730070, China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel tripodal compounds, tris[2-(7-diethylamino-coumarin-3-carboxamide)ethyl]amine (Tren-C1)
and tris[2-(benzo[5,6]coumarin-3-carboxamide)ethyl]amine (Tren-C2), were synthesized and character-
ized. The UV–vis and fluorescence properties of Tren-C1 and Tren-C2 in solutions were investigated.
These two compounds exhibited strong blue emission under ultraviolet light excitation. The maximal
fluorescence emission occurred at about the level of 10−5 mol/L. The chromophore units in the tripodal
compounds shown a little interaction in the ground state, while the interactions in the excited state was
Received 1 November 2008
Received in revised form 5 February 2009
Accepted 6 February 2009
Keywords:
Synthesis
Photoluminescence
Tripodal compounds
Coumarin derivative
notable and which leads to a broad and bathochromic shift of the emission bands.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
trap-site formation and energy transfer among fluorophores, with
a possibility of trap-site migration which results in quenching
Coumain and their derivatives have great interests for many
years. They are an important class of organic compounds, which
exhibit not only excellent biological and medical activities [1–3],
but also have super thermal stability and outstanding optical
properties including an extended spectral response, high quan-
tum yields, superior photostability. Optical applications of these
compounds, such as laser dyes, nonlinear optical chromophores,
fluorescent whiteners, fluorescent probes, polymer science, optical
recording and solar energy collectors, have been widely investi-
gated [4–10]. More important, coumarin dyes were used as blue,
green and red dopants in organic light-emitting diodes (OLEDs)
[11–13]. However, coumarin dyes are easily self-quenched in high
concentration due to the intermolecular interactions and aggrega-
tions, so as the light-emitting materials they are always doped in
the host materials at appropriate concentration to fabricate OLEDs
with reasonable, luminant efficiency [14–16].
Strong fluorescence of coumarins is attributed to the charge
transfer from the styryl to the carbonyloxy group [17]. Their absorp-
tion and fluorescence characteristics are significantly altered by
appropriate substitutions in the coumarin ring [18]. However, the
coumarin derivatives are easily quenched in the high concentration,
the quenching processes always cause emission band broadening
as well as bathochromic shift [19]. The general physical descrip-
tion of the self-quenching processes involves a combination of
[20]. Therefore, incorporation of bulky steric spacer or nonplanar
molecular moieties may prevent the chromophores from closing
in and interacting. Cheng et al. [21] appended coumarin-6 onto
the poly(methyl methacrylate) (PMMA) backbone, with the spacer
of methylene units to minimize overlapping of chromophores.
Swanson et al. [22] replaced diethylamine by nonplanar molec-
ular moieties (diphenylamine) in 7-position of coumarin ring to
obtain C-6S and C-7S. Lee et al. [15] synthesized a new fluores-
cent dopant coumarin-545P having incorporated five strategically
placed “methyl” steric spacers on julolidyl ring system and used
it as a dopant in the Alq3-hosted OLED device, the result showed
notable improvement in luminance efficiency and more resistance
to concentration quenching than the analogue coumarin-545T.
In this paper, we report the synthesis and characterization of
two new compounds. One is tris[2-(7-diethylamino-coumarin-3-
carboxamide)ethyl]amino (Tren-C1), another is tris[2-(benzo[5,6]
coumarin-3-carboxamide)ethyl]amine (Tren-C2) (Scheme 1). The
strategies for designing the tripodal compounds are: firstly, attach-
ing three coumarin fluorophores onto the terminal amido groups
of tren is expected to separate chromophores each other, keeping
the optical properties of the molecules identical to the pristine
coumarin fluorophores; secondly, the flexibility of the tripodal
compounds can increase the spatial extent of the molecules and
weaken three chromophores aligned in parallel, which may effec-
tively relieve the interaction between the terminal groups and
prevent combination of trap-site formation. The UV–vis and pho-
toluminescent spectra of Tren-C1 and Tren-C2 were investigated in
details.
∗
Corresponding author. Tel.: +86 931 4956935; fax: +86 931 4938756.
E-mail address: ytz823@hotmail.com (T. Yu).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.02.013

T. Yu et al. / Spectrochimica Acta Part A 73 (2009) 168–173
169
Scheme 1. Synthetic routines to the compounds Tren-C1, Tren-C2, model-C1 and model-C2.
2. Experimental
7-(diethylamino)-coumarin-3-carboxyl acid (2), were synthesized
according to the Ref. [23].
2.1. Materials and methods
7-(Diethylamino)-coumarin-3-carboxylic acid (6 g, 0.023 mol)
was dissolved in 50 mL of dichloromethane, and then thionyl chlo-
ride (10 mL) was dropped stepwise under ice bath. The mixture
was refluxed for 10 h. The resulting solution was evaporated to dry-
ness under reduced pressure and the residue was dispersed in dry
toluene (20 mL). After elimination of the solvent under reduced
pressure, and the process was repeated twice. The residue was
dried under vacuum to obtain the 7-(diethylamino)-coumarin-3-
carboxnyl chloride (3).
4-(N,Nꢀ-diethylamino) salicylaldehyde from Zhejiang Huadee
Dyestuff Chemical Co. Ltd. (China) was recrystallized from
ethanol. 2-Hydroxyl-1-(1-naphthaldehyde) was obtained from
Acros Organics. Tris(2-aminoetheyl)amine (tren) was purchased
from Fluka. Thionyl chloride was freshly distilled prior to use. All
other chemicals were analytical grade reagent.
IR spectra (400–4000 cm−1) were measured on a Shimadzu
IRPrestige-21 FT-IR spectrophotometer. 1H NMR spectra were
obtained on Unity Varian-500MHz. C, H, and N analyses were
obtained using an Elemental Vario-EL automatic elemental anal-
ysis instrument. Melting points were measured by using a X-4
microscopic melting point apparatus made in Beijing Taike Instru-
ment Limited Company, and the thermometer was uncorrected.
UV–vis absorption and photoluminescent spectra were recorded
on a Shimadzu UV-2550 spectrometer and on a Perkin Elmer LS-55
spectrometer, respectively.
7-(Diethylamino)-coumarin-3-carbonyl chloride (1.4 g, 5 mmol)
was dissolved in anhydrous pyridine (10 mL) and added with stir-
ring to a solution of tris(2-aminoethyl)amine (0.2 g, 1.36 mmol) in
anhydrous pyridine (10 mL). After 5 h at room temperature, the
resulting mixture was poured into 100 mL water and extracted
with chloroform (3× 50 mL). The organic phase was washed with
water (2× 50 mL) and dried over anhydrous MgSO4. After filter-
ing, the filtrate was evaporated to dryness under reduced pressure.
The crude was purified by chromatography on silica gel using
chloroform/methanol (1:24, v/v) as the eluent to give tris[2-
(7-diethylamino-coumarin-3-carboxamide)ethyl]amino (Tren-C1)
(0.52 g, 42.7%). m.p. 116–118 ◦C. IR: 3348 (N–H), 2970, 2930, 2870,
2816 (C–H, aliphatic), 1701 (C O, lactone), 1635 (C O, amide), 1620,
2.2. Synthesis and characterization of Tren-C1 and the model
compound C1
1585, 1512 cm−1
.
1H NMR (CDCl3, TMS): 8.97 (s, 3H, 4-H), 8.62 (s,
The synthetic route was shown in Scheme 1. The interme-
diates, ethyl 7-(diethylamino)-coumarin-3-carboxylate (1) and
3H, NH), 7.38 (d, J = 9.2 Hz, 3H, 5-H), 6.60 (d, J = 9.2 Hz, 3H, 6-H), 6.42

170
T. Yu et al. / Spectrochimica Acta Part A 73 (2009) 168–173
ride (6) were synthesized according to the produce described in the
literature [24].
Tris[2-(benzo[5,6]coumarin-3-carboxamide)ethyl]amine (Tren
-C2) and N-butyl-benzo[5,6]coumarin-3-carboxamide (model-
C2) were synthesized following the procedures similar to that
described for Tren-C1 and the model-C1, respectively.
Tris[2-(benzo[5,6]coumarin-3-carboxamide)ethyl]amine (Tren
-C2): yield: 70.2%. m.p. 133–135 ◦C. IR (KBr pellet cm−1): 3350
(N–H), 3059, 3010 (Ar-H), 2949, 2882, 2821 (C–H, aliphatic), 1709
(C O, lactone), 1650 (C O, amide), 1603 (C C), 1570, 1533, 1511,
1217, 1174. 1H NMR (500 MHz, CDCl3, ı, ppm): 9.44 (s, 3H, 4-H), 9.05
(s, 3H, NH), 8.28–6.99 (m, 18H, aryl-H), 3.68 (m, 6H, CONH–CH2),
2.93 (t, J = 5.9 Hz, 6H, N–CH2). Anal. Calc. for C48H36O9N4 (%): C,
70.93; H, 4.46; N, 6.89. Found: C, 70.76; H, 4.61; N, 6.83.
N-butyl-benzo[5,6]coumarin-3-carboxamide
(model-C2):
yield: 60.2%. m.p. 82–84 ◦C. IR (KBr pellet cm−1): 3332 (N–H), 3061
(Ar-H), 2954, 2932, 2868 (alkyl-CH), 1705 (C O, lactone), 1652
(C O, amide), 1603 (C C), 1567, 1537, 1516, 1374, 1212, 1152. 1H
NMR (500 MHz, CDCl3, ı, ppm): 9.70 (s, 1H, 4-H), 8.89 (s, 1H, NH),
8.47–7.50 (m, 6H, aryl-H), 3.52 (m, 2H, CONH–CH2), 1.65 (m, 2H,
CH2), 1.44 (m, 2H, CH2), 0.98 (t, J = 6.4 Hz, 3H, CH3). Anal. Calc. for
Fig. 1. UV–vis absorption spectra of the tripodal compounds and the model
compounds in dichloromethane at room temperature (Tren-C1, 1 × 10−5 mol/L;
model-C1, 3 × 10−5 mol/L; Tren-C2, 2.4 × 10−5 mol/L; model-C1, 7.2 × 10−5 mol/L).
C
18 H17 O3N (%): C, 73.20; H, 5.80; N, 4.74. Found: C, 73.36; H, 5.68;
(s, 3H, 8-H), 3.58 (m, 6H, CONH–CH2), 3.44–3.39 (m, 12H, 7-N–CH2),
2.88 (t, J = 6.1 Hz, 6H, N–CH2), 1.23 (t, J = 7.2 Hz, 18H, CH3). Anal. Calc.
for C48H57O9N7 (%): C, 65.81; H, 6.56; N, 11.19. Found: C, 65.76; H,
6.61; N, 11.23.
N, 4.83.
3. Results and discussion
The model compound C1, N-butyl-7-(diethylamino)-coumarin-
3-carboxamide, was obtained from 7-(diethylamino)-coumarin-3-
carbonyl chloride and butylamine following a procedure similar
to that described for Tren-C1. The crude was purified by chro-
matography on silica gel using ethyl acetate/petroleum (1:5, v/v),
m.p. 107–109 ◦C. IR (KBr pellet, cm−1): 3336 (N–H), 3043 (aryl-
CH), 2969, 2929, 2866 (alkyl-CH), 1697 (C O, lactone), 1644 (C O,
amide), 1617, 1582, 1525, 1509, 1377, 1133. 1H NMR (CDCl3, ı, ppm):
8.77 (s, 1H, NH), 8.71 (s, 1H, 4-H), 7.42 (d, J = 9.2 Hz, 1H, 5-H),
6.65 (d, J = 9.2 Hz, 1H, 6-H), 6.51 (s, 1H, 8-H), 3.48–3.41 (m, 2H,
CONH–CH2), 3.48–3.41 (m, 4H, 7-N–CH2), 1.60 (m, 2H, CH2), 1.42
(m, 2H, CH2), 1.24 (m, 6H, CH3), 0.95 (t, J = 6.4 Hz, 3H, CH3). Anal.
Calc. for C18 H24O3N2 (%): C, 68.33; H, 7.65; N, 8.85. Found: C, 68.40;
H, 7.61; N, 8.79.
3.1. Synthesis
The synthetic routes and the structures of the investigated com-
pounds were shown in Scheme 1. The key step of the synthetic
procedures is Knoevenagel condensation of the corresponding aro-
matic aldehydes with diethyl malonate. The hydrolysis of ethyl
coumarin-3-carboxylate was easily realized by alkaline hydrolysis
followed by acidification to afford the derivatives of the coumarin-
3-carboxylic acids. Subsequent treatment with thionyl chloride
led to the corresponding coumarin-3-carbonyl chlorides that then
reacted with tris(2-aminoethyl)amine (tren) or/and butylamine to
afford the tripodal compounds and the model compounds, respec-
tively.
The center N atom of tris(2-aminoethyl)amine (tren) is approxi-
mately sp3 hybridized, and the three alkyl groups occupy corners of
a tetrahedron. When the coumarin fluorophores were incorporated
into the tris(2-aminoethyl)amine, the tripodal compounds were
obtained which show three-dimensional spaces. Three coumarin
fluorophores were attached to an apical N atom through the isolat-
ing chains and formed a cage cavity. The molecular design concept
2.3. Synthesis and characterization of Tren-C2 and the model
compound C2
Ethyl benzo[5,6]coumarin-3-carboxylate (4), benzo[5,6]coum-
arin-3-carboxylacid (5)and benzo[5,6]coumarin-3-carboxnyl chlo-
Fig. 2. The PL spectral features of Tren-C1 (a, ꢀex = 420 nm) and Tren-C2 (b, ꢀex = 400 nm) in dichloromethane at room temperature.

T. Yu et al. / Spectrochimica Acta Part A 73 (2009) 168–173
171
376 nm [28], the longer wavelength band of Tren-C1 is red-shifted
to 416 nm due to the fluorophores bearing an electron-acceptance
group in 3-position, but the absorption spectral shapes of them
are markedly alike in dichloromethane solution, which displayed a
rather weak band at shorter wavelength followed by a intense band
at longer wavelength. Compared with Tren-C2, there is a remark-
able red shift in the main absorption band of Tren-C1, which was
interpreted to the extension of the chromophore from the electron
donating group (i.e. 7-diethylamino) to the electron withdrawing
group (i.e. 3-carbonyl) through the benzopyranone ring, and it turns
out that energy required for an electronic transition decreases as the
extent of conjugation increase and the absorption band occurs at
longer wavelength.
The absorption spectra of the multiple chromophore assem-
blies Tren-C1 and Tren-C2 show peak shapes and absorbances in
dichloromethane solution that represent linear additions of the
absorption spectra of their component chromophores. It was clearly
indicated that three fluorophores in the tripodal compounds are
independent units and little interactions each other in ground state.
Fig. 3. Fluorescence emission intensity as a function of concentration for Tren-C1
and Tren-C2 in dichloromethane at room temperature.
3.3. Fluorescence properties
of this work was to realize the absence of significant interactions
between the three coumarin fluorophore groups in the tripod com-
pounds and compel them to emit simultaneously.
The fluorescence behaviors of Tren-C1 and Tren-C2 versus the
concentrations in dichloromethane were shown in Fig. 2. The emis-
sion peaks of Tren-C1 and Tren-C2 located at around 458 and
445 nm, respectively. The emission peak of Tren-C1 was red-shifted
by 13 nm with respect to that of Tren-C2. This is due to the electron
repelling groups (diethylamino) in the 7-position of the coumarin
rings cause a shift of fluorescence band to longer wavelength [29].
In addition, it was found that the concentration required for the
maximum emission intensity is different in Tren-C1 and Tren-C2,
which implies that concentration plays an important role in the dif-
ferent dye molecules. There is the strongest blue emission at about
concentration of 1.0 × 10−5 mol/L for Tren-C1 and 4.0 × 10−5 mol/L
for Tren-C2, respectively.
Fig. 3 shows the fluorescence emission intensity of the tripo-
dal compounds versus various concentrations in dichloromethane
solutions. The emission intensity is noticed to increases gradually
with increase in the concentration and reaches a maximum for a
certain value of dye concentration (1.0 × 10−5 mol/L for Tren-C2,
4 × 10−5 mol/L for Tren-C2). A further increase in the dye concen-
tration decreased the fluorescence emission due to self-absorption
or self-quenching of the dye molecules (concentration quench-
ing). By reason of short distance between monofluorophores at
3.2. UV–vis absorption spectra
Fig. 1 shows UV–vis absorption spectra of Tren-C1, Tren-C2,
model-C1 and model-C2 in the solution of dichloromethane. Obvi-
ously, the spectrum of Tren-C1 presents two intense absorption
bands at 258 and 416 nm, while Tren-C2 has two intense absorp-
tion bands at 261 and 331–387 nm. These absorption bands of the
two compounds should be attributed to the ethylenic and ben-
zenoid transition [25], respectively. Generally, coumarins have been
reported to have two band groups of UV absorptions at 270–280
and 310–350 nm, arising from ␲–␲* transitions [26]. Two benzo-
coumarins had been also reported to show two band groups at
274–287 and 322–347 nm regions and the band at 322–347 nm
was expected to be the result of overlap of ␲–␲* band with n–␲*
band [27]. Compared with benzocoumarins [27], the second band
of Tren-C2 was bathochromically shifted due to the presence of the
carbonyl group substituted in 3-position which conjugates with
the pyranone ring. Contrasted with the absorption spectrum of
7-diethylamino-coumarin which exhibits two peaks at 256 and
Fig. 4. The normalized fluorescence spectra of the tripodal compounds and their corresponding model compounds in dichloromethane at room temperature. (a) Tren-C1
(1 × 10−5 mol/L) and model-C1 (3 × 10−5 mol/L); (b) Tren-C2 (8× 10−6 mol/L) and model-C2 (2.4 × 10−5 mol/L).

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T. Yu et al. / Spectrochimica Acta Part A 73 (2009) 168–173
high concentration, the intermolecular charge transfer may occur
in the excited state. The characteristic of fluorescence, which are
affected by any excited state process involving interactions of the
excited molecule with its close environment [30] and close molec-
ular packing in two-dimensional systems, may lead to significant
fluorescence concentration quenching [31]. It is found that the
quenching tendency of Tren-C1 was larger than that of Tren-C2.
To compare the structure of Tren-C2 and Tren-C2, there is a strong
electron donating substituents (diethylamino-) in the 7-position
of the fluorophore units in Tren-C1. This nonrigid derivative have
been found to have reduced fluorescence yield in environments of
high polarity, which is attributed to the nonradiative deactivation
of the derivative due to the formation of the twisted intramolecular
charge transfer (TICT) state from the singlet excited state. The for-
mation of the TICT state depends on the electron donor–acceptor
capabilities of the involved partners and on the solvent polarity
which stabilizes the highly polar TICT state [32]. The intramolecular
twist induces a large charge separation along the molecular skele-
ton between the donor group N(CH2CH3) and the acceptor moiety
and leads to an increase of the polarity. It was speculated that the
collision between dye and solvent molecules results in vibrational
coupling and favouring efficient internal conversion. However, in
the case of Tren-C2 (rigid fluorophores), the possibility of TICT for-
mation is very small because the rigid structure inhibits rotation
about the C–N bond, thereby preventing TICT state formation.
Fig. 4 shows the normalized fluorescent spectra of the tripo-
dal compounds and their corresponding model compounds in
dichloromethane. It is clearly shown that the emission band of Tren-
C1 became a little boarder than that of model-C1 (Fig. 4(a)). In the
case of Tren-C2, the emission band was not only broadening but also
bathochromically shifted by 18 nm compared with the emission
band of the model-C2 (Fig. 4(b)). It may be attributed to interac-
tion of fluorophore units of the tripodal compounds with their close
environment in the excited state. In the case of Tren-C1, the diethy-
lamino group rotated around the C–N bond and twisted out of the
plane of the aromatic ring. It may effectively weaken fluorophore
units align in parallel and reduce the interaction of fluorophore
units. However, the fluorophore units of Tren-C2, a rigid planar
conjugation system, may lead to an aggregation of the molecules
in parallel.
Fig. 6. The PL spectra of Tren-C1, model-C1, Tren-C2 and model-C2 in thin films at
the room temperature.
absorption and photoluminescent spectroscopy were fabricated by
spin-coating the acetone solutions onto clean quartz substrates.
Compared with UV–vis absorption and PL spectra of these com-
pounds in the diluted solution. Figs. 5 and 6 show the same results,
however, the PL bands of the materials in the thin films get broader
than that in the diluted solutions, which corresponding to the
excimers formed by collision between an excited fluorophore and
an unexcited. It is suggested that the compounds in the thin films
have more intermolecular interaction than that in solution situa-
tions due to the compact piling of the molecules in the thin films.
4. Conclusions
Two novel tripodal compounds containing coumarin moieties
and their corresponding model compounds were synthesized and
characterized. All of them exhibited strong blue emissions in dilute
solutions. The concentration dependence of the photolumines-
cence in the tripodal compounds were investigated and discussed.
The photoluminescence quench was due to the concentration
quenching phenomena. Our results indicated that these two new
compounds are potential useful as the dopants to make organic
light-emitting diodes.
The UV–vis absorption and photoluminescent behaviors of the
compounds doped in poly(methyl methacrylate) were also investi-
gated as shown in Figs. 5 and 6. Mixed systems of the compounds
with PMMA were prepared by dissolving these components with
a certain weight ratios into acetone. The samples for UV–vis
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Grant 60776006), the Program for Changjiang
Scholars and Innovative Research Team in University (IRT0629) and
also supported by ‘Qing Lan’ talent engineering funds (QL-05-23A)
by Lanzhou Jiaotong University.
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