A novel carbazole-based fluorescent probe: 3,6-Bis-[(N-ethylcarbazole-3-yl)-propene-1-keto]-N-ethylcarbazole

Article abstract of DOI:10.1016/j.cclet.2009.07.003

A novel carbazole-based compound 5, 3,6-bis[(N-ethylcarbazole-3-yl)-propene-1-keto]-N-ethylcarbazole has been designed, synthesized and characterized. The absorption and fluorescence spectra in solvents of different polarities prove that the compound has a distinct intramolecular charge transfer character. Compound 5 can be used as a new class of fluorescent probe or biosensor due to its sensitivity to the local microenvironment such as solvent polarity.

Full text of DOI:10.1016/j.cclet.2009.07.003

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Chinese Chemical Letters 21 (2010) 9–12
www.elsevier.com/locate/cclet
A novel carbazole-based fluorescent probe:
3,6-Bis-[(N-ethylcarbazole-3-yl)-propene-1-keto]-N-ethylcarbazole
Li Ling Hao, Jun Fen Li, Pei Hua Lin, Rui Feng Dong,
*
Du Xin Li, Shao Min Shuang, Chuan Dong
Research Center of Environmental Science and Engineering, School of Chemistry and Chemical Engineering,
Shanxi University, Taiyuan 030006, China
Received 24 March 2009
Abstract
A novel carbazole-based compound 5, 3,6-bis[(N-ethylcarbazole-3-yl)-propene-1-keto]-N-ethylcarbazole has been designed,
synthesized and characterized. The absorption and fluorescence spectra in solvents of different polarities prove that the compound
has a distinct intramolecular charge transfer character. Compound 5 can be used as a new class of fluorescent probe or biosensor due
to its sensitivity to the local microenvironment such as solvent polarity.
# 2009 Chuan Dong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Carbazole; Fluorescent probe; Intramolecular charge transfer; Synthesis
An intramolecular charge transfer (ICT) compound has an electron-donating group and an electron-accepting
group connected through a p-conjugated linker [1,2]. ICT is a fundamental process and has always been attracting
considerable attention as a topic of central importance in various photophysical, photochemical and biochemical
processes [3,4], e.g. fluorescent probe or biosensor for studying different microenvironment [5–7], organic light-
emitting diodes [8–10], organic material for non-linear optics [11,12] and two-photon absorption materials [13,14].
Carbazole and its derivatives are one of the most important fundamental structures in the field of organic electrical and
optical materials [15]. It is well known that carbazole and its derivatives exhibit large p-conjugation system, special
rigid structure and good hole transporting properties and can be easily modified at the 3- and 6-positions to tune the
optical properties [16–18]. In addition, carbazole and its derivatives can be used as a strong electron-donating
chromophore [19,20]. Up to now, the studies on synthesis and the photoluminescence properties of new carbazole-
containing compounds are yet a significant subject [21].
Herein, we design and synthesize a novel carbazole-based molecule: 3,6-bis[(N-ethylcarbazole-3-yl)-propene-1-
keto]-N-ethylcarbazole (compound 5, shown in Scheme 1). In this compound, the N-ethylcarbazole group is used as
the central core to connect with the other two N-ethylcarbazole units in one molecule. The absorption and fluorescence
spectra in solvents of different polarities prove that compound 5 has a distinct ICT character.
* Corresponding author.
E-mail address: dc@sxu.edu.cn (C. Dong).
1001-8417/$ – see front matter # 2009 Chuan Dong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.07.003

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L.L. Hao et al. / Chinese Chemical Letters 21 (2010) 9–12
Scheme 1. Synthetic route of compound 5.
1. Experimental
1
Melting points were determined on x-5 melting point detector. IR spectra were recorded with an FTIR 1730. H
NMR spectra and 13C NMR spectra were obtained on a Bruker 300 MHz instrument. The absorption spectra were
recorded on a TU-1901 dual-beam UV–vis spectrophotometer and fluorescence spectra were obtained on a Carry
Eclipse fluorescence spectrofluorometer (Varian, USA) equipped with a 150 W xenon lamp source.
Compounds 2 and 3 were synthesized according to literature [2]. A mixture of AlCl3 (3.2 g), CH3COCl (3.2 g,
40.76 mmol) and CHCl3 (150.0 mL) were added in a 250 mL 3-necked flask equipped with a stirrer. Then the solution
which was obtained from compound 2 (2.0 g, 10.26 mmol) and CHCl3 (20.0 mL) was added in drop-by-drop. After
stirring for 16 h at room temperature, the reaction mixture was poured into 200 mL of ice water under vigorous stirring
and was kept stirring for 5 h. Yellow needle-like crystals of compound 4 were obtained after being recrystallized from
ethanol (1.83 g, 64.0% yield). The melting point was 183–185 8C. The synthesis method was improved based on
literature [22]. IR (KBr) cmÀ1: 3417, 3126, nC–H (carbazole and –COCH3); 3051, 2970, nC–H (–CH2CH3); 1662, 1625,
1
(carbazole); 1392, 1348, nC–N (carbazole); 802, dC–H (carbazole). H NMR
nC
(–COCH3); 1487, 1427, nC
O
C
(CDCl3): d 1.44–1.19 (3H, –CH3), 2.70 (6H, –COCH3), 4.38–4.31(2H, –CH2–), 8.71–7.21 (6H, carbazole).
Compound 3 (1.0 g, 4.48 mmol), 4 (0.5 g, 1.79 mmol) and ethanol (100.0 mL) were added in a 250 mL round-
bottomed flask, and the solution was obtained by stirring at room temperature. Then 10% NaOH aqueous solution
(3.0 mL) was dropped in. The reaction solution was stirred for 36 h at room temperature. Yellow powder of compound
5 was obtained (0.64 g, 51.8% yield). The melting point was 210–213 8C. IR (KBr) cmÀ1: 3413, 3126, nC–H
(carbazole); 3056, 2970, nC–H (–CH2CH3); 1674, 1647, nC
(–COCH CH–); 1624, 1593, nC (–COCH CH–);
O
C
1492, 1475, nC C (carbazole); 1390, 1348, nC–N (carbazole); 1018, nC–H (–COCH CH–); 804, 744, dC–H (carbazole).
1H NMR (CDCl3): d 1.46 (9H, –CH3), 4.39 (6H, –CH2–), 2.74 (4H, –CH CH–CO–), 8.91–7.22 (20H, carbazole). 13
C
NMR (CDCl3, d ppm): 18.26 (–CH3), 42.54 (–CH2–), 31.12 (–CH CH–), 113.26–150.11 (carbazole), 201.99 (–CO–).
In general, the typical d 13C value of –CH CH– in the region of from 80 to 160, but the region is wider with a
substituent group of O- or N-atom [23].
2. Results and discussion
The fluorescence quantum yield (wf) of compound 5 in different solvents are: carbon tetrachloride, 0.0045; diethyl
ether, 0.0138; tetrahydrofuran, 0.0333; chloroform, 0.135; dimethyl sulfoxide, 0.428; ethanol, 0.516, water, 0.0446. It
is obvious that compound 5 shows good luminescence in ethanol.
Typical absorption spectra of compound 5 in solvents of different polarities are shown in Fig. 1. The spectra exhibit
two main bands, whose maxima are located in the between 220 and 350 nm, and larger than 350 nm. The short
wavelength bands between 220 and 350 nm are attributed to carbazole moiety whereas the long wavelength bands

L.L. Hao et al. / Chinese Chemical Letters 21 (2010) 9–12
11
Fig. 1. Normalized absorption spectra of compound 5 in various solvents: (1) diethyl ether, (2) tetrahydrofuran, (3) chloroform, (4) ethanol.
larger than 350 nm are assigned to ICT transitions from carbazole moiety to carbonyl group moiety. The absorption
spectra of larger than 350 nm show minimal dependence on the solvent polarity with red shift of about 17 nm from
378 nm in diethyl ether to 395 nm in ethanol. So, the solvatochromism displayed by the absorption spectra is
negligible.
Typical fluorescence emission spectra of compound 5 are shown in Fig. 2. It is noted that with the increasing of the
solvent polarity, the fluorescence emission spectra dramatically shift from 430 nm in non-polar cyclohexane to 542 nm
in a strongly polar solvent, methanol. The shift of fluorescence spectra is larger than that of the absorption spectra,
which indicates that intensively photoinduced ICT takes place (refer to Scheme 2) within the molecule in the singlet
excited state. The distinctly solvatochromic effect can be attributed to the decrease of the energy in the excited singlet
state with the increase of the solvent polarity.
In summary, carbazole-based compound 5 with ICT character is successfully synthesized and characterized. The
solvatochromism exhibited by the compound is also discussed. Compound 5 can be used as a new class of fluorescent
probe or biosensor due to its sensitivity to the local microenvironment such as solvent polarity.
Fig. 2. Normalized emission spectra of compound 5 in various solvents: (1) cyclohexane, (2) chloroform, (3) acetone, (4) N,N-dimethylformamide,
(5) acetonitrile, (6) ethanol, (7) methanol. The arrow direction indicates increasing solvent polarity.

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Scheme 2. ICT process of compound 5.
Acknowledgments
This work was supported by the National Science Foundation of China (No. 20875059) and the Youth Science
Foundation of Shanxi Province (No. 20051005).
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