Fluorescence response of TICT-active aminostilbenes to copper(II) ions: Redox reaction vs ion recognition

Article abstract of DOI:10.1007/s11164-012-0627-5

Cu(II)-selective fluorescence enhancement (1, 2, and 4) or fluorescence quenching (3) was observed for aminostilbenes 1-4 in acetonitrile. The fluorescence responses result from efficient Cu(II)-mediated oxidation of 1-4 that forms new fluorescent species rather than from any specific noncovalent interactions. Evidence of redox reactions includes irreversible Cu(II) titration spectra, spectroscopic observation of the radical cations, and isolation of oxidized aminostilbene dimers. These results provide a new method for synthesis of tetrasubstituted tetrahydrofurans and suggest that aminostilbenes with twisted intramolecular charge-transfer activity are potential fluorescence-enhanced Cu(II) chemodosimeters. The role of Cu(II)-mediated redox reactions should be always taken into account in mechanisms for sensing of arylamine-based Cu(II)-selective fluoroionophores.

Full text of DOI:10.1007/s11164-012-0627-5

Res Chem Intermed (2013) 39:19–32
DOI 10.1007/s11164-012-0627-5
Fluorescence response of TICT-active aminostilbenes
to copper(II) ions: redox reaction vs ion recognition
•
Cheng-Kai Lin Jye-Shane Yang
Received: 28 September 2011 / Accepted: 31 October 2011 / Published online: 14 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Cu(II)-selective fluorescence enhancement (1, 2, and 4) or fluorescence
quenching (3) was observed for aminostilbenes 1–4 in acetonitrile. The fluorescence
responses result from efficient Cu(II)-mediated oxidation of 1–4 that forms new
fluorescent species rather than from any specific noncovalent interactions. Evidence
of redox reactions includes irreversible Cu(II) titration spectra, spectroscopic
observation of the radical cations, and isolation of oxidized aminostilbene dimers.
These results provide a new method for synthesis of tetrasubstituted tetrahydrofu-
rans and suggest that aminostilbenes with twisted intramolecular charge-transfer
activity are potential fluorescence-enhanced Cu(II) chemodosimeters. The role of
Cu(II)-mediated redox reactions should be always taken into account in mechanisms
for sensing of arylamine-based Cu(II)-selective fluoroionophores.
Keywords Aminostilbenes ꢀ Oxidation ꢀ
Twisted intramolecular charge transfer (TICT) ꢀ
Chemodosimeters ꢀ Fluoroionophores
Introduction
Because of the biological and environmental importance of Cu(II) [1, 2], much
effort has been devoted to development of Cu(II)-selective fluorescent probes or
chemosensors for real-time detection [3–10]. Among the different fluorescence
This paper is dedicated to Professor Kazuhiko Mizuno on the occasion of his retirement from Osaka
Prefecture University.
C.-K. Lin ꢀ J.-S. Yang (&)
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
e-mail: jsyang@ntu.edu.tw
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C.-K. Lin, J.-S. Yang
sensing modes, ion-induced fluorescence enhancement or ratiometric fluorescence
responses are more useful than fluorescence quenching in terms of practical
applications. However, Cu(II) is known to be an effective fluorescence quencher via
excited-state energy and/or electron transfer [11]. Thus, to obtain chemosensors
performing Cu(II)-induced fluorescence enhancement, the fluorescence-enhanced
signaling pathways must be more efficient than the concomitant fluorescence
quenching processes. Previous successful strategies include:
1
Cu(II)-induced inhibition of the photoinduced electron-transfer (PET) process
occurring in the free fluoroionophores (i.e., PET-based probes) [3, 4];
Cu(II)-promoted deprotonation of donor–acceptor (D–A) type fluoroionophores
that facilitates a fluorescence intramolecular charge transfer (ICT) process (i.e.,
ICT-based probes) [5, 6];
2
3
4
Cu(II)-induced excimer formation [7, 8]; and
Cu(II)-induced state inversion from a weakly fluorescent n, p* state to a
fluorescent p, p* state [9, 10].
Recently, a new strategy based on the oxidizing ability of Cu(II) has emerged:
namely, the fluorescent probes (chemodosimeters) are oxidized by Cu(II) and the
resulting products fluoresce more strongly [12–18]. Cu(II)-mediated redox reactions
are particularly effective for arylamines in organic solvents such as acetonitrile
(MeCN) [19–23]. Because many Cu(II)-selective fluoroionophores reported in the
literature contain arylamino moieties and have been tested in MeCN, their
fluorescence sensing mechanism should be re-evaluated by considering the potential
contribution of Cu(II)-mediated redox reactions.
Aminostilbenes are useful fluorophores for constructing fluorescent probes [24–
26]. We have shown that in the presence of strong amino donors (e.g., those in
aminostilbenes 1 and 2) an inherently non-fluorescent or weakly fluorescent twisted
intramolecular charge transfer (TICT) state is formed and its formation dominates
the excited decay in polar solvents [27]. For example, the fluorescence quantum
yield (Ufl) of 1 and 2 in hexane is high (Ufl [ 0.5), but it becomes extremely low
(Ufl & 0.006) in MeCN. The dramatic decrease in fluorescence quantum efficiency
for 1 and 2 on going from nonpolar to polar solvents is a consequence of the
formation of a TICT state, which is more favorable in more polar solvents [27–29].
Because TICT state formation is associated with the strong electron-donating
methoxy group, modification of its electronic character by interaction with metal
ions might switch off the TICT processes and thus turn on the fluorescence. Indeed,
we have applied this concept to the design of a bimodal fluoroionophore (i.e.,
fluorescence off–on and ratiometric dual fluorescence) in response to alkaline metal
ions (Mg(II), Ca(II), and Ba(II)) [25]. In an effort to further explore the Cu(II)-
sensing performance of these TICT fluorophores and to investigate the relative
importance of Cu(II)-mediated redox reactions and noncovalent Cu(II) recognition
in the observed fluorescence responses, we have investigated the fluorescence
behavior of aminostilbenes 1–3 and the acetylhydrazone derivatives 4. The results
reported herein show that 1–3 undergo efficient and selective redox reactions with
Cu(II) in MeCN and the resulting fluorescence of the oxidized stilbene dimers is
stronger for 1 and 2. The redox reaction also dominates the fluorescence response of
123

Fluorescence response of TICT-active aminostilbenes
21
4 to Cu(II), despite the presence of a potential Cu(II) acceptor (i.e., o-methoxy-
benzaldehyde acetylhydrazone) [3, 10, 30]. Preparative isolation of the stilbene
dimer was performed for aminostilbene 5.
O
OCH3
OCH3
N
NH
CH3
CH3
N
N
R
N
CH3
4
5
1
2
3
R = H
R = CH3
R = 4-C6H4OCH3
Results and discussion
The synthesis of aminostilbenes 1 and 2 has been reported [27]. The synthesis of 3
and 4 is shown in Scheme 1, in which intermediates 6–9 are all known compounds
[31, 32]. The key reaction in Scheme 1 is the palladium-catalyzed C–N coupling
reaction between 9 and 5-bromoanisaldehyde, which requires the phosphine ligand
10 and the base K3PO4 for success [33]. The resulting compound 11 was methylated
into 12, which was then reacted with acetic hydrazide to form the desired compound
4. It should be noted that the product is a mixture of both trans and cis isomers in
respect of the C=N bond.
H
N
Br
H3CO
OCH3
1. P(OEt)3, Δ
7
3
2. NaH, THF
Pd2(dba)3, NaOtBu
DPPF, toluene 95%
benzaldehyde
Br
85%
Br
O
OCH3
Br
6
K3PO4,
Br
Pd2(dba)3, dioxane
90oC 12 h
1. P(OEt)3, Δ
NH2NH2, Pd/C
45%
2. NaH, THF
dioxane
90%
benzaldehyde
NO2
P(tBu)2
N
85%
R
O
NH2
NO2
OCH3
9
8
11 R = H
10
H
N
CH3
H2N
NaH, MeI, DMF, 70%
O
4
12 R = CH3
EtOH, HCl
48%
Scheme 1 Synthesis of compounds 3 and 4
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22
C.-K. Lin, J.-S. Yang
Table 1 Maxima of UV–
visible absorption (kabs) and
fluorescence (kf), and
fluorescence quantum yields
(Uf) of 1–4 in hexane (Hex)
and acetonitrile (MeCN)
Compd
Solvent
kabs (nm)
kf (nm)
Uf
1a
Hex
349
356
350
356
370
367
349
356
389
502
406
577
421
538
447
525
0.60
MeCN
Hex
0.007
0.65
2a
3
MeCN
Hex
0.005
0.47
MeCN
Hex
0.11
4
0.13
MeCN
0.003
a
Data from Ref. [27]
Photophysical data for 1–4 in hexane and MeCN are shown in Table 1. The
absorption maxima (kabs) of aminostilbenes 1–4 vary only slightly on changing from
hexane to MeCN. This indicates a small difference in dipole moment for the Frank–
Condon (FC) excited-state vs the ground state. In contrast, both the fluorescence
spectra and quantum yields (Uf) are highly dependent on solvent polarity. The large
red shift in the fluorescence spectra reflects strong ICT character in their fluorescing
excited states. It is notable that in hexane the fluorescence quantum yield of 4 (0.13)
is significantly lower than that of 1–3 (0.47–0.65), indicating that the additional
hydrazone group may induce new nonradiative decay processes (e.g., C=N bond
isomerization). The substantial decrease in Uf for 1 and 2 on going from hexane to
MeCN (Uf-Hex/Uf-MeCN [ 86) is a typical phenomenon of TICT state formation, and
the larger the Uf-Hex/Uf-MeCN ratio and the lower the Uf-MeCN value, the more
favorable is TICT state formation in MeCN [27]. The corresponding solvent effect
on Uf is somewhat weaker in 4 (Uf-Hex/Uf-MeCN = 43) as a consequence of the lower
Uf-Hex value. In contrast, the Uf-Hex/Uf-MeCN ratio is much lower for 3 (Uf-Hex
/
Uf-MeCN = 4.3) because of the larger Uf-MeCN value. It seems that introduction of
the second N-anisolyl group, as in 3, reduces the tendency to form a TICT state.
The ability of Cu(II) to oxidize aminostilbenes 1–3 in MeCN is shown by the
formation of the corresponding radical cations. Figure 1 shows a parallel change in
the absorption spectra of 3 on addition of Cu(II) and electrochemical oxidation. The
new absorption bands (460, 533, and 733 nm) are characteristic of radical cations of
triarylamines [34, 35]. In particular, the 733 nm band is close to the reported value
of 737 nm for the radical cation of 4, 40-dimethoytriphenylamine [22]. For 1 and 2,
the absorption intensity of these peaks is rather weak, as shown in Fig. 2a for 2. The
cyclic voltammetry (CV) of 1–3 shows that the relative stability of their radical
cations is 1 \ 2 \ 3, because reversible anodic waves are observed for 2 and 3 only
at a scan rate of 100 mV/s and only that for 3 remains reversible at a scan rate as
low as 5 mV/s (Fig. 3). We might conclude that oxidation of 1–3 is accompanied by
chemical reactions with the radical cation intermediates. Indeed, addition of
ethylenediaminetetraacetate (EDTA), a strong Cu(II) chelator, cannot restores the
absorption bands for all cases, indicating the occurrence of an irreversible reaction
between Cu(II) and aminostilbenes.
The proposed redox reaction between aminostilbenes 1–3 and Cu(II) (Eq. 1) is
indeed thermodynamically favorable.
123

Fluorescence response of TICT-active aminostilbenes
23
Fig. 1 Absorption spectra of 3
in MeCN upon (a) addition of 0,
0.5, 1.0, 1.5, and 2.0 equiv
Cu(ClO4)2ꢀ6H2O and
(b) electrochemical oxidation on
a Pt electrode with Bu4NPF6
(0.1 M) as the supporting
electrolyte at an applied
potential of 0.6 V versus Ag
wire for 0, 1, 3, 5, 10, 20, 35, 55,
75 and 120 min.
(c) Fluorescence spectra of 3
(1 9 10-5 M, excitation at
405 nm) in MeCN upon addition
of 0, 0.4, 0.8, 1.2, 1.6 and 2.0
equiv Cu(ClO4)2ꢀ6H2O
Fig. 2 Absorption (a) and fluorescence (b) spectra of 2 (1 9 10-5 M, excitation at 303 nm) in MeCN
upon addition of 0, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 equiv Cu(ClO4)2ꢀ6H2O
123

24
C.-K. Lin, J.-S. Yang
Fig. 3 Cyclic voltammograms of 1–3 (1 mM) in MeCN containing 0.1 M Bu4NPF6 with a scan rate of
100 mV/s (a, c, e) or 5 mV/s (b, d, f)
Aminostilbene þ CuðIIÞ ! Aminostilbene ꢀþ þ CuðIÞ
ð1Þ
The free energy change DG for this electron-transfer reaction can be estimated
from Eq. 2 [36–38]:
DG ¼ Eox ꢁ Ered ꢁ e2=de
ð2Þ
where Eox is the oxidation potential of the donor (aminostilbenes in this case) and
red is the reduction potential of the acceptor (Cu(II) in this case), d is the center-to-
E
center distance between donor and acceptor, and e is the dielectric constant of the
solvent. In polar solvents, for example MeCN, the Coulombic term (e2/de) can be
neglected [38]. On the basis of the Ered value of 0.96 V (vs SCE) for the Cu(II)/
Cu(I) couple [39] and Eox values of 0.68, 0.64, and 0.66 V (vs SCE) for 1–3
(Fig. 3), respectively, the DG values for the Cu(II)-mediated oxidation of 1–3 are
-0.28, -0.32, and -0.30 eV, respectively.
The nature of the chemical reactions associated with the radical cations of 1–3 in
MeCN was also investigated. Various products were generated and the ESI-TOF
mass spectra of 1 9 10-3 M MeCN solutions of 2 (m2 = 315) and 3 (m3 = 407)
123

Fluorescence response of TICT-active aminostilbenes
25
Fig. 4 Absorption (a) and fluorescence (b) spectra of 5 (1 9 10-5 M, excitation at 295 nm) in MeCN
on addition of 0, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 equiv Cu(ClO4)2ꢀ6H2O
containing 2 9 10-3 M Cu(II) suggests the presence of a dimeric form of 2
(m/z = 647) and 3 (m/z = 831). This is similar to the oxidative dimerization
reactions of stilbene derivatives that form tetrasubstituted tetrahydrofuran species
[40–42]. The oxidants could be metal oxides (e.g., VOF3 and FeCl3ꢀ6H2O) [40],
stilbene oxidases or laccase [41], or nitrite ion [42]. The occurrence of the same
reaction for 2 and 3 is, indeed, in agreement with the observed peaks in the mass
spectra (i.e., 2m2 ? 16 (O) ? 1 (H?) = 647 and 2m3 ? 16 (O) ? 1 (H?) = 831).
To isolate this product we used the structurally simpler compound, N,N-dimeth-
ylaminostilbene (5), because compound 5 (m = 224) has similar behavior to 2 in
terms of Cu(II) titration absorption spectra (i.e., reduction of ICT band intensity and
a noticeable increase in the baseline in the range 400–500 nm) (Fig. 4) and Cu(II)-
mediated dimerization (m/z = 463). Compared with 2, compound 5 is easier to
synthesize and NMR analysis of its dimer product is more straightforward. The
preparative oxidation reaction of 5 was performed at a concentration of
5 9 10-5 M in the presence of 2.0 equiv Cu(ClO4)2ꢀ6H2O. The reactant was
consumed after 12 h and the major product 13 (Scheme 2) was isolated by
preparative HPLC with a yield of 72 %. Although there are four stereogenic centers
in 13, the NMR spectrum shows only one set of signals (Fig. 5a, b) and its
regiochemistry can be determined on the basis of 2D NMR spectra (Fig. 5c, d). An
attempt to grow single crystals of 13 was not successful, and thus the
stereochemistry of the dimerization reaction could not be addressed. Scheme 2
shows our tentatively proposed mechanism for the Cu(II)-mediated dimerization
reaction. The head-to-head dimerization with the anilino rather than phenyl groups
located at the 2 and 5-positions of the tetrahydrofuran ring of 13 is consistent with
the expectation of a more stable carbocation at the a-position of the aminostilbenes.
This is the first example of Cu(II)-mediated dimerization and tetrahydrofuran
formation of aminostilbenes in MeCN.
The corresponding fluorescence responses of 1–3 to Cu(II) are also intriguing. As
represented by 2 as example, addition of Cu(II) results in enhanced and blue-shifted
fluorescence (Fig. 2b). The fluorescence intensity reaches a maximum by a factor of
20 (Uf = 0.1) at ca 1.2 equiv Cu(II), and further addition of Cu(II) leads to
fluorescence quenching. The fluorescence peak shifts from 577 to 408 nm at 1.2
123

26
C.-K. Lin, J.-S. Yang
Cu(II)
Cu(I)
N
N
N
H3C
CH3
H3C
CH3
H3C
CH3
5+.
5
H3C
CH3
N
CH3
H3C
CH3
N
N
N
O
H3C
H3C
CH3
x 2
H2O
-2H+
13
Scheme 2 Proposed mechanism for the Cu(II)-mediated dimerization reaction of compound 5
equiv Cu(II). The large blue shift of fluorescence (Dkf & 170 nm) indicates that the
new fluorescent species are of shorter conjugation length and/or weaker charge-
transfer character than their precursors 1–3. The new fluorescent species should
have larger fluorescence quantum yield than 1 and 2 to account for the phenomenon
of fluorescence enhancement. The observation of fluorescence quenching at higher
concentrations of Cu(II) could be understood in terms of the fluorescence quenching
nature of Cu(II) [11]. Because radical cations of arylamines are usually nonfluo-
rescent in solutions at ambient temperature [43, 44] and the observed fluorescence is
located at shorter wavelengths than the absorption of the radical cations, the new
fluorescent species cannot be attributed to the radical cations of 1–3. Consequently,
a potential candidate is the tetrahydrofuran product. For example, kf for N,N-
dimethylaniline (DMA) in MeCN is 348 nm, which is close to the 355 nm
fluorescence observed for 13 (Fig. 6). The somewhat longer fluorescence for 13
versus DMA could be attributed to the presence of a tetrahydrofuranyl group in the
former. Likewise, the N-methyl-N-phenyl-p-anisidine group (kf = 381 nm, Fig. 6)
in the dimerized product of 2 might be responsible for the 408 nm fluorescence
induced by Cu(II) (Fig. 2b). Thus, the fluorescence peak maxima between 577 and
408 nm (e.g., 470 nm at 0.2 equiv Cu(II)) could be simply attributed to overlapping
of fluorescence from intact 2 and the oxidized product. The immediate fluorescence
response of 2 to Cu(II), even at a concentration as low as 10-5 M, indicates that
formation of radical cation p-dimers is thermodynamically favorable, which drives
the dimerization reactions proposed above. Similar absorption titration spectra were
also observed for aminostilbenes 1 and 3 (Figs. 1a and 7a), although the
corresponding fluorescence response is off–on and on–off (Figs. 1c and 7b),
respectively. The different fluorescence response of 1–3 to Cu(II) reflects the
different TICT character of their excited states (vide supra). No fluorescence
123

Fluorescence response of TICT-active aminostilbenes
27
Fig. 5
a
1H, b 13C, c COSY, and d HMBC NMR spectra of 13 in CDCl3
Fig. 6 Normalized absorption
spectra of 13 (a) and
fluorescence spectra of 13 (b),
N,N-dimethylaniline (c), and
N-methyl-N-phenyl-p-anisidine
(d) in MeCN
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C.-K. Lin, J.-S. Yang
Fig. 7 a Absorption and b fluorescence spectra of 1 (1 9 10-5 M, excitation at 303 nm) in MeCN on
addition of 0, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 equiv Cu(ClO4)2ꢀ6H2O
response is detectable for aminostilbenes 1–3 with other metal ions, including
Mg(II), Ca(II), Ba(II), Ag(I), Co(II), and Ni(II).
With the above results in mind, it is interesting to investigate the fluorescence
behavior of aminostilbenes containing a potential Cu(II) acceptor to address the
relative importance of Cu(II)-mediated oxidation vs noncovalent Cu(II) binding. In
this context, we targeted compound 4, because the o-methoxybenzaldehyde
acetylhydrazone moiety has been suggested to be a Cu(II)-selective acceptor [3,
10, 30]. Figure 8 shows the absorption and fluorescence titration spectra of 4 in
MeCN with Cu(II). In conjunction with the observation of a mass peak at m/z = 815
for 4 (m4 = 399), the great similarity in the responses of both absorption and
fluorescence spectra to those of 2 suggests that the redox reaction discussed above
for 1–3 is also responsible for the observed spectral changes in 4. In other words,
complexation of Cu(II) with 4, if it occurs, does not inhibit oxidation of 4 by Cu(II).
It should be noted that there is no change in the absorption spectra of 4 in the
presence of 10 equiv Cu(II) in tris(hydroxymethyl)aminomethane)ꢀHCl (10 mM,
pH = 7.0) buffer containing 50 % (v/v) water–MeCN, which indicated that the
redox reaction became negligible in aqueous solutions [19].
Fig. 8 a Absorption and b fluorescence spectra of 4 (1 9 10-5 M, excitation at 303 nm) in MeCN on
addition of 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 equiv Cu(ClO4)2ꢀ6H2O
123

Fluorescence response of TICT-active aminostilbenes
29
Conclusion
The spectroscopic behavior of a series of aminostilbenes (1–5) in response to Cu(II)
has been reported. Cu(II)-mediated oxidation of 1–5 followed by dimerization and
nucleophilic addition of water molecules account for changes in the electronic
absorption and emission spectra, which is not affected by the presence of the
potential Cu(II) receptor o-methoxybenzaldehyde acetylhydrazone. Thus, possible
involvement of redox reactions should be taken into account in the signal-
transduction mechanism of any Cu(II)-responsive and Cu(II)-oxidizable fluoroion-
ophores in nonaqueous solutions. The dimerization reactions observed also provide
a means of synthesis of substituted tetraarylfurans.
Experimental
General methods
Electronic spectra were recorded at room temperature (23 1 °C). UV–visible
spectra were measured on a Cary300 double beam spectrophotometer. Fluorescence
spectra were recorded on a PTI QuantaMaster C-60 spectrometer and corrected for
instrumental nonlinearity. The optical density (OD) of all solutions was approx-
imately 0.1 at the wavelength of excitation. A N2-outgassed solution of anthracene
(Uf = 0.27 in hexane) [45] was used as standard for determinations of fluorescence
quantum yield and the solvent refractive index correction was applied. The
excitation wavelength was 355 nm. An error of 10 % is estimated for the
fluorescence quantum yields.
Materials
All commercially available materials were used as received. Solvents for
photochemical and electrochemical measurements were HPLC-grade. CH2Cl2 was
dried over calcium hydride and distilled before use. The synthesis of aminostilbenes
1 and 2 has been reported [27]. Synthetic procedures and characterization data for
new compounds are shown below.
Compound 3
A mixture of 6 (0.52 g, 2.0 mmol), 7 (0.50 g, 2.2 mmol), NaOtBu (0.27 g,
2.8 mmol), DPPF (0.044 g, 0.040 mmol), and Pd2(dba)3 (0.018 g, 0.020 mmol) in
10 mL of anhydrous toluene under N2(g) was heated at 90 °C for 12 h. The solution
was cooled and the solvent was removed under reduced pressure to afford the crude
product. Further purification was performed by column chromatography to provide
1
the final product. Yellow solid; yield 95 %; mp 117–118 °C; H NMR (400 MHz,
CDCl3): 3.79 (s, 6H), 6.82 (dd, J = 3.5 and 12 Hz, 4H), 6.89–7.00 (m, 4H), 7.06
(d, J = 9 Hz, 4H), 7.20 (t, J = 7.3 Hz, 1H), 7.32 (t, J = 7.3 Hz, 4H), 7.46
(d, J = 7.5 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3): 55.5, 114.7, 120.5, 126.1,
123

30
C.-K. Lin, J.-S. Yang
126.2, 126.6, 127.0, 127.2, 128.4, 128.6, 129.6, 137.9, 140.8, 148.3, 156.0 ppm; IR
(KBr): 978, 1506, 1599 cm-1; ESI-HRMS calcd for C28H25NO2: 407.1885, found
407.1884.
Compound 4
A mixture of 12 (0.34 g, 1.0 mmol) and acethydrazide (0.15 g, 2.0 mmol) in
absolute ethanol was stirred at RT for 12 h. Solvent was removed and the residue
was purified by column chromatography (CH2Cl2–acetone = 20:1) to provide the
final product. Pale yellow solid; yield 48 %; mp 210–212 °C; 1H NMR (400 MHz,
DMSO-d6): 2.12/1.91 (s, 3H), 3.25 (s, 3H), 3.86 (s, 3H), 6.75 (m, 2H), 6.99
(d, J = 16.0 Hz, 1H), 7.11–7.22 (m, 4H), 7.33 (d, J = 8.8 Hz, 2H), 7.53
(d, J = 8.0 Hz, 2H), 7.59 (s, 1H), 8.47/8.32 (s, 1H), 11.40/11.25 (s, 1H) ppm;
13C NMR (100 MHz, DMSO-d6): 20.7, 22.0, 56.4, 56.5, 113.7, 115.6, 115.9, 121.9,
123.7, 123.8, 125.1, 125.2, 126.4, 127.3, 127.6, 127.7, 127.88, 127.91, 128.4, 128.6,
128.8, 129.1, 138.1, 138.2, 141.2, 141.7, 141.9, 149.2, 155.0, 165.9, 172.3 ppm; IR
(KBr): 959, 1590, 1671 cm-1; ESI-HRMS calcd for C25H25N3O2: 399.1947, found
399.1943; Anal. Calcd for C25H25N3O2: C, 75.16, H, 6.31, N, 10.52. Found: C,
75.02, H, 6.29, N, 10.51.
Compound 11
A mixture of 9 (0.98 g, 5.0 mmol), 5-bromoanisaldehyde (1.2 g, 5.5 mmol), K3PO4
(1.5 g, 7.0 mmol), 10 (0.025 g, 0.075 mmol), and Pd2(dba)3 (0.045 g, 0.050 mmol)
in 20 mL anhydrous dioxane under N2(g) was heated at 90 °C for 12 h. The solution
was cooled and solvent was removed under reduced pressure to afford the crude
product. Further purification was performed by column chromatography to provide
1
the final product. Orange solid; yield 45 %; mp 190–192 °C; H NMR (400 MHz,
DMSO-d6): 3.89 (s, 3H), 6.97–7.05 (m, 3H), 7.13–7.23 (m, 3H), 7.35 (t,
J = 7.6 Hz, 2H), 7.42–7.48 (m, 4H), 7.54 (d, J = 8.0 Hz, 2H), 8.31 (s, 1H),
10.34 (s, 1H) ppm; 13C NMR (100 MHz, DMSO-d6): 56.6, 114.5, 115.9, 116.5,
124.9, 125.3, 126.5, 127.3, 127.4, 128.2, 128.6, 128.9, 129.1, 136.7, 138.1, 144.2,
156.7, 198.4 ppm; IR (KBr): 3311, 2863, 1674, 1590, 967 cm-1; ESI-HRMS calcd
for C22H19NO2: 329.1416, found 329.1412; Anal. Calcd for C22H19NO2: C, 80.22,
H, 5.81, N, 4.25. Found: C, 80.02, H, 5.69, N, 4.21.
Compound 12
Sodium hydride (0.47 g, 12.0 mmol) was added slowly at 0 °C to a solution of 11
(0.99 g, 3.0 mmol) in 10 mL DMF and the mixture was stirred for 10 min.
Iodomethane (0.60 mL, 9.0 mmol) was added, and stirring at RT was continued for
12 h. The reaction mixture was extracted three times with CH2Cl2. The combined
organic layers were dried over MgSO4 and the solvent was removed under reduced
pressure. Further purification was performed by column chromatography to provide
the final product. Compound 12: yield 70 %; mp 132–134 °C; 1H NMR (400 MHz
CDCl3): 3.94 (s, 3H), 6.83 (d, J = 8.8 Hz, 2H), 6.93–6.98 (m, 2H), 7.06
123

Fluorescence response of TICT-active aminostilbenes
31
(d, J = 16.0 Hz, 1H), 7.22 (t, J = 7.3 Hz, 1H), 7.32–7.40 (m, 5H), 7.49 (d,
J = 7.4 Hz, 2H), 7.63 (s, 1H), 10.64 (s, 1H) ppm; 13C NMR (100 MHz, CDCl3):
40.4, 56.1, 113.0, 117.1, 123.0, 125.9, 126.2, 127.1, 127.5, 128.4, 128.6, 128.9,
131.9, 137.8, 142.1, 148.6, 158.3, 189.4 ppm; IR (KBr): 2859, 1685, 1593,
968 cm-1; ESI-HRMS calcd for C23H21NO2:343.1572, found 343.1572; Anal.
Calcd for C23H21NO2: C, 80.44, H, 6.16, N, 4.08. Found: C, 80.44, H, 5.99, N, 4.10.
Compound 13
A solution of Cu(ClO4)2ꢀ6H2O (0.0185 g, 0.05 mmol) in 5 mL MeCN was added
dropwise to a solution of 5 (0.0558 g, 0.025 mmol) in 500 mL MeCN at RT, and
mixture was stirred for 12 h. Solid K2CO3 (0.01 g, 0.075 mmol) was added, and the
mixture was stirred for 0.5 h. MeCN was removed and the residue was extracted
three times with CH2Cl2. The combined organic layers were dried over MgSO4
and the solvent was removed under reduced pressure. Further purification was
performed by preparative HPLC (hexane–THF = 85:15) to provide the final
product. Pale yellow solid; yield 72 %; mp 148–150 °C; 1H NMR (400 MHz,
CDCl3): 2.91 (s, 12H), 3.67 (dd, J = 5.1 and 2.2 Hz, 2H), 5.33 (dd, J = 5.1 and
2.2 Hz, 2H), 6.68 (d, J = 7.0 Hz, 4H), 7.10–7.14 (m, 6H), 7.18 (d, J = 6.0 Hz,
4H), 7.21 (d, J = 7.0 Hz, 4H) ppm; 13C NMR (100 MHz, CDCl3): 40.6, 62.6, 87.3,
112.5, 126.6, 127.0, 128.0, 128.4, 129.6, 138.6, 150.1 ppm. ESI-HRMS calcd for
C32H34N2O:462.2671, found 462.2677.
Acknowledgments Financial support for this research was provided by the National Science Council of
Taiwan, ROC. The authors also thank Dr Kang-Ling Liau for providing compound 3.
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