New ferrocenyl derivative with controllable aggregation-induced emission (AIE) characteristics

Article abstract of DOI:10.1007/s11458-010-0110-z

A novel molecular switch, 7-(N,N-diethylamino)-2-oxo-2H-chromen-4-yl ferrocene carboxylate (FCC), was synthesized and fully characterized by ¹H NMR, ¹³C NMR, and HRMS. Taking advantage of the properties of ferrocene as an electron do

Full text of DOI:10.1007/s11458-010-0110-z

Front. Chem. China 2010, 5(2): 241–246
DOI 10.1007/s11458-010-0110-z
RESEARCH ARTICLE
switch, PET could quench the fluorescence. Ferrocene has a
sandwich-type molecular structure and a high degree of
electron-rich aromatic system, and hence can be used as a
donor in the PET system [11]. Its emergence is a milestone in
the development of modern chemistry since it can be
experienced reversible one-electron oxidation in most com-
mon solvents. In addition, coumarins are interesting fluor-
ophores with regard to their photochemical and photophysical
properties. Herein, we designed a new fluorescence molecular
switch, FCC, based on electroactive ferrocene linking with a
coumarin as a fluorescent unit (Figure 1).
New ferrocenyl derivative
with controllable
aggregation-induced
emission (AIE)
characteristics
Dong ZHANG, Liangliang ZHU, Hong LI and
The fluorescence of many chromophoric organics becomes
weak when forming aggregation. Through aggregating, the
molecules would form less emissive species, for example,
excimers, and would lead to a reduction in the luminescence
efficiency. This is a thorny problem in the development of
photoluminescent materials [12]. Meanwhile, the “concentra-
tion quenching” in the fluorescence signal is the important
challenge that restricts the application of multifunctional
photochromic molecules to high-density optical memory
systems [13]. The best approach is to develop new
luminophoric materials that have an “aggregation-induced
emission (AIE)” property [14]. This technology can be
applied in areas of biologic probes [15], chemical sensors
[16], photochromic materials [17], and organic light-emitting
diodes (OLED) [18]. In addition, some positive attempts of
AIE materials have been made in the area of fluorescence
molecular switches [19]. In the same way, the target molecule
(FCC) we reported here presents AIE property in mixed
acetonitrile-water media. Furthermore, because b-cyclodex-
trin (b-CD) can form stable 1∶1 complex with ferrocene or
ferrocenyl derivatives [20], we use randomly methylated b-
CD (Me-b-CD) to form inclusion complex with the ferrocene
(Fc) unit of FCC, which can influence the aggregation of FCC
and can further bring great impact to the AIE property.
Jianhua SU (✉)
A novel molecular switch, 7-(N,N-diethylamino)-2-oxo-
2H-chromen-4-yl ferrocene carboxylate (FCC), was
1
synthesized and fully characterized by H NMR, 13C
NMR, and HRMS. Taking advantage of the properties
of ferrocene as an electron donor active unit and the
coumarin as a fluorescent unit, the dyad FCC shows a
fast and reversible redox-switchable fluorescence emis-
sion. In sharp contrast to most photoluminescent
chromophores, FCC has a unique enhanced emission
through aggregation. The change of electrochemical
signals (CV and DPV) indicated that the ferrocene (Fc)
unit of FCC could form inclusion complex with Me-b-
cyclodextrin (CD). This inclusion complex could further
weaken the aggregation-induced emission (AIE) effect
remarkably. This advance paves the way to introduce
AIE property into molecular devices applications.
Keywords ferrocene, cyclodextrin, molecular switch,
fluorescence, aggregation-induced emission (AIE)
1 Introduction
2 Experimental
Molecular devices have developed at a good pace in the past
decades with ever-increasing interest for molecular computa-
tion and molecular electronics [1–8]. As a fundamental and
important class of molecular devices, fluorescence molecular
switches [9,10] can realize fluorescent “on-off ” function by
controlling the chemical changes, such as coordination, redox,
energy transfer, protonation, and isomerism.
2.1 Chemicals and instruments
Oxalyl chloride and DMF were dried by 4A molecular sieve
and distilled before use. Anhydrous CH2Cl2 was distilled
from calcium hydride. Anhydrous THF was distilled from
sodium wire. Commercially available reagents were used
without further purification unless otherwise stated. Com-
pounds were visualized with UV light at 254 and 365 nm.
Melting points were determined on a hot-plate melting point
apparatus and are corrected. H NMR and 13C NMR spectra
were obtained by using a Bruker AV 400 spectrometer. High-
resolution mass spectra were performed with an HP5989 mass
One general mechanism in fluorescence molecular switches
is photo-induced electron transfer (PET). In the off state of the
Received January 22, 2010; accepted February 14, 2010
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, 130 Meilong Road,
Shanghai 200237, China
1
E-mail: bbsjh@ecust.edu.cn
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2010

242
Dong ZHANG, Liangliang ZHU, Hong LI and Jianhua SU
Figure 1 The synthetic route of FCC.
spectrometer and UV-Vis spectra on a Varian Cary 500 2H, CH2), 7.17 (d, 4H, J = 8.4 Hz, Ph–H), 7.28 (t, 2H, J =
spectrophotometer (1-cm quartz cell used). Fluorescent 8.8 Hz, Ph–H), 7.42 (t, 4H, J = 7.2 Hz, Ph–H).
spectra were recorded on a Varian Cary Eclipse fluorescence
spectrophotometer.
2.3 7-N,N-diethylamino-4-hydroxycoumarin (2) [22]
Cyclic voltammetry (CV) and differential pulse voltam-
metry (DPV) experiments were performed with a CHI 660C
electrochemical workstation using a normal three-electrode
cell with a Pt working electrode, a Pt wire auxiliary electrode,
and Ag/AgCl reference electrode in saturated KCl solution.
All potentials are quoted with respect to Fcþ /Fc under a
nitrogen atmosphere at 25°C. The experiments were carried
out in solutions containing 0.1 mol$L–1 of [(nBu)4N]PF6 as
supporting electrolyte. Deoxygenation of the solutions was
achieved by bubbling nitrogen for at least 10 min, and the
working electrode was cleaned after each run. CV curves were
recorded at a scan rate of 50 mV$s–1, and DPV curves were
recorded at a scan rate of 20 mV$s–1.
A mixture of diphenyl malonate (1) (41.7 g, 163 mmol), 3-
(diethylamino)phenol (26.88 g, 163 mmol) and toluene
(250 mL) was protected by argon. The mixture was refluxed
for 9 h. Upon cooling to room temperature, compound 2 was
precipitated and filtered off and washed with toluene, giving
yellow powder. Yield: 13.4 g; 35%. 1H NMR (400MHz,
DMSO-d6, 25°C, TMS): d (ppm) 1.09 (t, 6H, J = 7.2 Hz,
CH3), 3.39 (d-d, 4H, J = 7.2 Hz, CH3), 3.43 (s, 2H, CH2), 6.42
(s, 1H, Ar–H), 6.64 (d, 1H, J = 8.8 Hz, Ar–H), 7.53 (d, 1H, J =
8.8 Hz, Ar–H).
2.4 Ferrocene carbonyl chloride (3)
A mixture of ferrocene carboxylic acid (2.32 g, 10 mmol), dry
methylene chloride (50 mL), freshly distilled oxalyl chloride
(6.4 g, 50 mmol), and DMF (7 drops) was stirred in the dark
for 12 h at room temperature. Then, the contents evaporated to
dryness under reduced pressure. The residue was dissolved in
anhydrous THF (30 mL) for the next step reaction without
further purification.
2.2 Diphenyl malonate (1) [21]
A mixture of malonate (22.0 g, 212 mmol) and phenol (39.9 g,
424 mmol) was protected by argon. Through the addition
funnel, phosphorus oxychloride (23 mL, 246 mmol) was
slowly added dropwise at 0°C–5°C over a period of 50 min.
The reaction was immediate as evidenced by the evolution of
hydrogen chloride. The reaction mixture was stirred 1.5 h at
115°C, and then, hydrogen chloride evolution ceased. When
the mixture was cooled to 80°C, the upper clear liquid was
poured into water (100 mL) and extracted with ethyl ether
(75 mL ꢀ 3). The combined organic phase was washed with
brine and dried over anhydrous MgSO4. The filtrate was
evaporated under reduced pressure to afford 1 (41.7 g, 77%).
1H NMR (400MHz, CDCl3, 25°C, TMS): d (ppm) 3.87 (s,
2.5 7-(N,N-Diethylamino)-2-oxo-2H-chromen-4-yl
ferrocenecarboxylate (FCC)
A mixture of 7-N,N-diethylamino-4-hydroxycoumarin (2)
(2.56 g, 11 mmol), anhydrous THF (50 mL), and triethyla-
mine (2.1 mL) was protected by argon. A solution of ferrocene
carbonyl chloride (10 mmol) in anhydrous THF (30 mL) was
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

New ferrocenyl derivative with controllable aggregation-induced emission (AIE) characteristics
243
added dropwise at 0°C–5°C over a period of 50 min. The resulted in little fluorescence. However, the emission intensity
reaction mixture was stirred for 10 h at room temperature. of FCC was enhanced dramatically upon the addition of Fe
Then, the solution was filtered free of triethylamine hydro- (ClO4)3 to the solution (red line). The emission spectrum of
chloride, and the solvent evaporated under reduced pressure. oxidized FCC in THF displays typical emission band at
The crude product was further purified by column chromato- 390 nm (lex = 340 nm), which is indeed the mirror images of
graphy on silica gel using ethyl acetate/petroleum ether (1/3) the UV-Vis absorption spectrum, implying that the fluores-
to afford orange solid. The product was recrystallized from cence of FCC is due to the coumarin unit. Because in the
methanol to give pure FCC as orange needle-like crystals. initial state, the photo-induced electron transfer (PET) from
Yield: 3.80 g, 85%; m.p. 112°C–114°C. 1H NMR (400 MHz, the Fc unit to the excited electron acceptor coumarin unit
CDCl3, 25°C, TMS): d (ppm) 1.23 (t, 6H, J = 7.2 Hz, CH3), would result in fluorescence quenching, with FCC showing
3.44 (d-d, 4H, J = 7.2 Hz, N–CH2), 4.33 (s, 5H, Cp–H), 4.60 no fluorescence basically. However, after oxidation by Fe
(t, 2H, J = 2 Hz, Cp–H), 5.01 (t, 2H, J = 2 Hz, Cp–H), 6.22 (s, (ClO4)3, the electron-donating abilities of the Fc unit are
1H, Ar–H), 6.55 (d, 2H J = 2.4 Hz, Ar–H), 6.62 (d, 2H J = reduced, and the PET function would be restrained, leading to
8.8 Hz, Ar–H), 7.50 (d, 2H J = 8.8 Hz, Ar–H). 13C NMR fluorescence enhancement. Importantly, the electron donating
(400 MHz, CDCl3, 25°C, TMS): d(ppm) 12.44 (2C, CH3), abilities of the Fc unit can be reversibly modulated by the
44.86 (2C, N–CH2), 68.70 (1C), 70.20 (5C, CpC), 70.79 (2C, chemical or electrochemical redox reaction [11]. This result
CpC), 72.66 (2C, CpC), 97.58 (1C), 98.42 (1C), 104.17 (1C), demonstrates the possibility of establishing a new redox-
108.66 (1C), 123.67 (1C), 151.31 (1C), 156.42 (1C), 159.76 fluorescence switch based on FCC.
(1C), 163.15 (1C), 168.10 (1C). HRMS (EI): m/z [FCC] calc.
445.2909, found 445.0972.
3 Results and discussion
3.1 Synthesis
Figure 1 outlines the synthesis of the target molecule, 7-(N,N-
Diethylamino)-2-oxo-2H-chromen-4-yl ferrocene carboxylate
(FCC). Esterification of phenol followed by reaction with
phosphorus oxychloride led to diphenyl malonate (1) in good
yield of 77%. 7-N,N-Diethylamino-4-hydroxycoumarin (2)
was synthesized easily in toluene, which was precipitated
during the reaction and separated by simple filtration. The
treatment of ferrocene carboxylic acid with an excessive
Figure 2 Normalized absorption (black) and fluorescence (blue,
amount of freshly distilled oxalyl chloride and a catalytic
l
ex = 340 nm) spectra of the THF solution (1.0 ꢀ 10–5 mol$L–1) of
amount of DMF afforded ferrocene carbonyl chloride (3),
which was dissolved in anhydrous THF for the final reaction
without further purification. The final esterification in
anhydrous THF at temperature around 0°C–5°C for 50 min
and then at room temperature for 10 h afforded FCC in high
yield of 85%. Compounds 1, 2, and FCC have been
FCC, at 298 K, and change in fluorescence spectra (red, lex
340 nm) toward oxidation (in the presence of Fe(ClO4)3).
=
3.3 AIE property
FCC shows a clear AIE property in mixed acetonitrile-water
media (Figure 3) with different water/acetonitrile ratios. The
emission from the acetonitrile solution of FCC is quite weak.
However, with the increase in the proportion of water, the
fluorescence gradually increased. When the volume fraction
of water was increased to 60%, the fluorescence intensity
1
characterized by H NMR spectroscopy, 13C NMR spectro-
scopy, and HRMS (EI) spectrometry.
3.2 Fluorescence molecular switch
The optical properties of FCC were measured by UV-Vis and increased to the strongest. However, the fluorescence started
fluorescence spectroscopy in THF solution, as shown in to decrease swiftly upon further increase in the proportion of
Figure 2. FCC shows an intense absorption at 350 nm (lmax
,
water. Under UV light (lex = 365 nm), this AIE phenomenon
black line), and the molar extinction coefficient at lmax is could be clearly observed by photograph (Figure 4). Since
determined to be 2.26 ꢀ 104 L$mol–1$cm–1. The fluorescence water is an antisolvent for FCC, the molecules must have
spectrum of FCC was excited at 340 nm (blue line), which aggregated in the acetonitrile-water and exhibited strong
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

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Dong ZHANG, Liangliang ZHU, Hong LI and Jianhua SU
Figure 3 Fluorescence spectra (lex = 340 nm) of FCC (4.0 ꢀ
10–5 mol$L–1, at 298 K) in acetonitrile with different amount of
H2O.
Figure 5 SEM image of FCC nanoparticles obtained from its
suspension containing 60% volume fractions of water in
acetonitrile.
fluorescence. This phenomenon indicates that FCC molecules
aggregated markedly when the volume fraction of water
reached 60%. The aggregation should be nanodimensional
[23,24]. Scanning electron microscopy (SEM) shows the
nanoparticle size distributions of the sample, which was
prepared by a reprecipitation method from acetonitrile
solution with distilled water (Figure 5).
oxidized FCC (FF = 0.523) is much higher than FCC in the
solutions (free, FF = 0.002) as well as aggregates (AIE, FF =
0.014). Therefore, whether oxidized FCC has AIE property, it
is hard to detect.
3.4 Adduct formed with cyclodextrins
The restriction of intramolecular rotation (RIR) in the
aggregates is the main cause of the AIE effect [25]. The
intramolecular rotation is active in the solutions, which is
restricted in the aggregates due to the physical constraint. As
any molecular motions consume energy, the aggregation
changes the nonradiative path to the radiative decay, resulting
in the enhancement of fluorescence. In addition, the aggrega-
tion could weaken the energy loss from the ruleless
circumcolumnar movement of ferrocene unit [26]. However,
it will form aggregates with larger dimensions when the water
content is very high ( > 70%). Moreover, these amorphous
aggregates are only weakly fluorescent (Figure 4), which is
similar to the fluorescence of solid of FCC. Therefore, AIE is
a unique phenomenon only in the particular size range.
Unfortunately, we could not evaluate whether oxidized FCC
has AIE property. Using anthracene as the standard (FF =
0.27) [27], we observed that the fluorescence quantum yield of
Cyclodextrins (CDs) has internal cavities, which can
accommodate a wide range of guest molecules, ranging
from polar compounds to apolar compounds. The slightly
apolar cyclodextrin cavity tends to be occupied by appropriate
“guest molecules”, which are less polar than solvent
molecules [28]. The ability of CDs to include a part of an
organometallic compound has been demonstrated by numer-
ous researchers [29]. This kind of inclusion generally modifies
the chemical, electrochemical, and photochemical properties
of organometallics. Ferrocenyl derivatives have been most
studied in these organometallics.
Moreover, it has been demonstrated that the orientation of
ferrocene in b-CD is nearly parallel to the molecular axis of b-
CD. The electrochemical parameters of redox active ferrocene
unit are often strongly affected by the presence of the host, b-
CD, affording a valuable tool to investigate the complexation
Figure 4 Photograph of the emission of FCC (4.0 ꢀ 10–5 mol$L–1, lex = 365 nm, at 298 K) in acetonitrile with different amount of H2O
(from left to right: 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%).
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

New ferrocenyl derivative with controllable aggregation-induced emission (AIE) characteristics
245
processes [30]. The apolar FCC is hard to dissolve in water; 3.5 Influence of AIE with cyclodextrins
also the solubility of b-CD in nonaqueous solvents have
FCC possesses the AIE property and can form an adduct with
Me-b-CD besides. Could Me-b-CD affect its AIE property?
We examined the dependence of fluorescence quantum yield
(FF) on the solvent composition of the acetonitrile-water
mixture using anthracene as the standard (FF = 0.27) [27]. As
seen in Figure 7, in the presence of Me-b-CD, the FF of FCC
has a similar change with AIE property as mentioned above.
However, the magnitude of this FF change is so weak that it is
hard to be detected.
similar difficulty. However, Me-b-CD has a good solubility in
a variety of organic solvents, which were introduced to
determine the inclusion complex. Figure 6 shows the CV and
DPV data in CH3CN solution containing 0.1 mol$L–1 [(n-
Bu)4N]PF6 as supporting electrolyte, contrasting the effect of
Me-b-CD on the electrochemical behavior of FCC. The
comparison of CV (Figure 6 (a)) demonstrates that the
complexation of Fc unit of FCC with Me-b-CD is detected
experimentally by a shift in the half-wave potential (E1/2) to
more positive values (DE1/2 = + 48 mV) and by a decrease in
the peak currents (Ipa). The anodic potential shift reveals that
the reduced form (Fc) of the guest is more stabilized by
complexation than the oxidized form (Fcþ). The compar-
ison of the DPV (Figure 6 (b)) about the inclusion complex
exhibits similar electrochemical signal change.
Figure 7 Relative quantum yields of FCC (4.0 ꢀ 10–5 mol$L–1
,
at 298 K) versus solvent composition of the water/acetone mixture,
in the absence (black) of and presence of Me-b-CD (10 equiv.).
The adduct formed with Me-b-CD has better solubility than
free FCC in the mixed solvent with high proportion of water.
Thus, the aggregation is difficult to form, and SEM shows no
nanoparticle. Therefore, Me-b-CD could indirectly affect the
AIE property though inclusion complexation.
Figure 6 Evolution of the (a) CV and (b) DPV of FCC (1 ꢀ
10–3 mol$L–1) in CH3CN with [(n-Bu)4N]PF6 (0.1 mol$L–1) as
supporting electrolyte, scanned at 20 mV$s–1, from 0.4 V to
1.0 V when Me-b-CD is added: from 0 (black line) to 5 equiv. (red
line).
4 Conclusion
In summary, we have designed and synthesized a novel ferro-
cenyl derivative containing a coumarin fluorophore—FCC.
The synthetic strategy is straightforward with high yield. FCC
is a redox-controlled fluorescence molecular switch due to
PET mechanism, which is the first example with AIE property
based on ferrocene at the same time. Though forming adducts
with Me-b-CDs, the AIE property could be affected greatly.
We introduced this unique AIE performance into the field of
molecular devices for the first time.
While these voltammetric data are qualitative observations
of inclusion complex, the best and more general methodology
to obtain the corresponding equilibrium constants relies on the
experimental voltammograms. However, this approach may
show limitations as the mechanism of the complexation
reactions becomes more complicated [20]. The association
constants (Kass) between b-CD and Fc unit [29] is greater than
Kass between b-CD and coumarin unit [31]. Moreover, from
such large changes of electrochemical signals (Figure 6), we
can determine that the Me-b-CD formed an inclusion complex
with the Fc unit of FCC, rather than the coumarin unit.
Acknowledgements The work was supported by the National
Natural Science Foundation of China, the State Key Development
Program for Basic Research of China (Grant No. 2006CB806200)
and Scientific Committee of Shanghai. The authors thank Prof. H.
Tian (ECUST) for his assistance in experiments and discussion.
Frontiers of Chemistry in China Vol. 5, No. 2, 2010

246
Dong ZHANG, Liangliang ZHU, Hong LI and Jianhua SU
13. Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y.,
Jianhua SU was born in 1965 and com-
pleted his B.S. studies in Fine Chemicals
(1987) at East China University of Science
and Technology (ECUST, Shanghai). He
received his Ph.D. degree in 1997 from
ECUST. His current research interests
include the chemicals for daily use and the
development of interdisciplinary materials
science that determines the electronic and
optical properties of materials.
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Z.; et al, Chem. Commun. 2006, 1, 70–72
15. Huang, J.; Wang, M.; Zhou, Y.; Weng, X.; Shuai, L.; Zhou, X.;
Zhang, D., Bioorg. Med. Chem. 2009, 17, 7743–7748
16. Zheng, Y. S.; Hu, Y. J., J. Org. Chem. 2009, 74, 5660–5663
17. Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y.,
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18. Liu, Y.; Tao, X.; Wang, F.; Dang, X.; Zou, D.; Ren, Y.; Jiang, M.,
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19. Yang, Z.; Chi, Z.; Yu, T.; Zhang, X.; Chen, M.; Xu, B.; Liu, S.;
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