New two-photon absorption organic chromophores containing imino and hydroxyl groups: Synthesis, ESIPT and chemosensors

Article abstract of DOI:10.1016/j.tet.2013.04.121

In this work, a variety of new conjugated chromophores containing imino and hydroxyl groups are presented. Excited state intramolecular proton transfer (ESIPT) of these chromophores under one- and two-photon irradiation was surveyed. One-photon absorption

Full text of DOI:10.1016/j.tet.2013.04.121

Tetrahedron 69 (2013) 5355e5366
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New two-photon absorption organic chromophores containing
imino and hydroxyl groups: synthesis, ESIPT and chemosensors
*
*
Fang Gao , Xinchao Wang, Hongru Li , Xiaojuan Ye
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a variety of new conjugated chromophores containing imino and hydroxyl groups are
presented. Excited state intramolecular proton transfer (ESIPT) of these chromophores under one- and
two-photon irradiation was surveyed. One-photon absorption spectra show the presence of internal
hydrogen bond in the organic dyes containing ortho-hydroxyl group, while the corresponding dyes
carrying para-hydroxyl group do not exhibit intramolecular H-bonding effect. In the most of aprotic and
protic solvents, the extended chromophores bearing ortho-hydroxyl group exhibit well-separated dual
emission bands with large Stokes shift (ca. 160 nm), as contrast, the molecules containing para-hydroxyl
group exhibit only single band with normal Stokes shift (ca. 50 nm). Two-photon absorption (TPA) in-
duced competitive ESIPT emission can also be observed by near-infrared (near-IR) femtosecond laser to
the molecules carrying ortho-hydroxyl group. The experiments show that the molecules carrying ortho-
hydroxyl group are able to undergo ESIPT under one- and two-photon excitation, while the molecules
containing para-hydroxyl groups do not exhibit such properties. 2-((30,40-Dimethoxyl-phenylethyleneyl-
phenyl-4-ylimino)methyl)phenol (C1) and 2-((30,40,50- trimethoxyl-phenylethyleneyl-phenyl-4-ylimino)
methyl)phenol (C3) exhibit regular and selective response to Zn2þ in DMF. The molecular modeling was
further performed to analyze ESIPT occurrence theoretically.
Received 11 February 2013
Received in revised form 16 April 2013
Accepted 26 April 2013
Available online 2 May 2013
Keywords:
Chromophore
Two-photon
ESIPT
Chemosensor
Zn2þ
Molecular modeling
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Photoinduced tautomerization of an ESIPT molecule suggests
that the four-level cycle process (E/E
*
/K
*
/K/E) is a real
/K coupled
Photoinduced excited state intramolecular proton transfer
(ESIPT) is well accepted as significantly important basic chemical
process in nature and life systems, and thus it has been drawing
considerable attention experimentally and theoretically.1e6 Photo-
induced enol (E) and keto (K) forms tautomerization process is the
most representative ESIPT phenomenon. Normally, such photo-
tautomerization process gives rise to an abnormally large Stokes
chemical reaction, and thus the energy barrier of E
*
*
with five or six or rarely seven-membered ring transient state
should be barrierless or negligible,13e16 which in turn allows such
process. It is generally considered that the small size molecules
normally have more reactivity than the similar large size com-
pounds. This interprets that the most reported ESIPT molecules
possess small chemical size, which could be favorable for barrier-
less or negligible energy barrier.
On the other hand, the large size of ESIPT molecules would be of
great benefit for the extensions of TPA application potentials under
near-IR femtosecond laser, which has various physical advantages
such as low energy, deep penetration, and negligible damage to the
normal biological tissues. However, the experimental and theo-
retical investigation on ESIPT of large molecules has been seldomly
reported because the chemical preorganization and huge compu-
tations of the large size ESIPT molecules are great challenges for
organic and physical chemists. Only a few enlarged molecules were
demonstrated to undergo ESIPT experimentally.17e20
shift without self-absorption because the absorption is from E/E
and the emission originates from K /K. It means that E form is
more stable in the ground state, while K form is more stable in the
excited state. On the other hand, normal emission (E /E) may be
*
*
*
observed to an ESIPT molecule due to its competitive nature in the
excited state. Consequently, an ESIPT molecule generally exhibits
dual emission bands, and the second emission band is character-
ized with large Stokes shift. These unique ESIPT properties have
been found in a variety of applications such as fluorescence sensors
for various metal or anion ions,7,8 electroluminescence materials
for optical devices,9,10 bio-imaging reagents for live cells.11,12
TPA properties of organic dyes make it possible to utilize near-IR
laser.21e25 Jen and co-workers reported new 2-(20-hydroxyphenyl)
benzothiazole derivatives with obvious TPA cross-sections to sur-
vey TPA ESIPT.20 The shoulder in the TPA emission bands is an
* Corresponding authors. Tel./fax: þ86 23 65102531; e-mail addresses: fanggao
1971@gmail.com (F. Gao), hrli@cqu.edu.cn (H. Li).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.04.121

5356
F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
indication of the occurrence of ESIPT of these benzothiazole de-
rivatives under near-IR laser.20 While unfortunately, no well-
separated dual emission bands have been observed for organic
chromophores under near-IR laser irradiation so far. It could be
ascribed to its normal emission decay competition under two-
photon process, and thus ESIPT emission could not be distin-
guished well from the normal emission.
Experimental section part). The first warm condensation reaction
could occur firstly between piperidine and 4-fluoro-benzaldehyde
to yield 4-piperidinyl-benzaldehyde so that the intermediate 4-
piperidinyl-benzaldehyde was produced, which could be isolated
and characterized by 1H NMR spectrum (Fig. S1, see Supplementary
data). And it was further confirmed that 4-piperidinyl-benzalde-
hyde could be efficiently and rapidly pꢀroduced between piperidine
and 4-fluoro-benzaldehyde at 80e90 C without organic solvents.
Further experiments suggest that 4-piperidinyl-benzaldehyde
could react with 4-nitro-phenylacetic acid to yield the precursor 4-
piperidinyl-40nitro-diphenylethylene through the second warm
condensation reaction using the carbanion produced from 4-nitro-
phenylacetic acid as the additive to formyl group in 4-piperidinyl-
benzaldehyde.30 The chemical structure of 4-piperidinyl-40-nitro-
diphenylethylene was confirmed by 1H NMR spectrum (Fig. S2, see
Supplementary data). A parallel experiment was also carried out
without organic solvents between phenylacetic acid and 4-fluoro-
benzaldehyde using piperidine as the reactant and the catalyst. 1H
NMR spectrum of 4-piperidinyl-diphenylethylene further confirms
the assumption (Fig. S3, see Supplementary data).
Inspired by the efforts, we propose that it could be efficient
approach to preorganize enlarged TPA ESIPT molecules if photo-
tautomerized enoleketo moiety is attached to TPA chromophores
via covalent bond. It was well demonstrated that salicylidene me-
thylamine has the ability to undergo ESIPT through an ideal bar-
rierless quasi-six-membered ring state process with reasonable
dual emission bands.26 This means that the adjacent hydroxyl and
imino groups are attractive segments to build enlarged TPA mole-
cules. It is quite possible to observe well-separated dual emission
bands in salicylidene methylamine-based chromophores under
near-IR femtosecond laser excitation.
In human body, Zn2þ plays important role in ubiquitous cellular
physiological metabolism activities such as structural and catalytic
cofactors, regulators of gene expression, and apoptosis.27 Zinc de-
ficiency may cause very serious health problems such as genetic
diseases to the children. On the other hand, the other metal ions
2.2. ESIPT under two-photon excitation
such as Hg2þ display greatly similar chemical properties with Zn2þ
,
2.2.1. ESIPT under one-photon excitation. In order to reveal the
possibility of ESIPT under two-photon excitation to the molecules
C1eC6, we firstly surveyed one-photon ESIPT in various aprotic and
protic solvents. In aprotic solvents, the maximal absorption wave-
lengths of the molecules containing ortho-hydroxyl group (C1, C3,
C5) are bathochromically shifted with respect to those of the
molecules carrying para-hydroxyl group (C2, C4, C6) correspond-
ingly (w10 nm), which is clearly shown in Fig. 1 and Table 1. Fur-
thermore, the absorption maxima of the all molecules are
insensitive to the polarity of the aprotic solvents. The absorption
maxima of the partner C5/C6 show the red-shift as comparison
with those of the partners C1/C2 and C3/C4, respectively (>10 nm,
typically shown in Fig. S4, see Supplementary data), which could be
caused by stronger electron-donating ability of piperidinyl group.
The results suggest that there is internal charge transfer in the
ground states of these molecules.31
In protic solvents, the maximal absorption wavelengths are
blue-shifted as compared with those in aprotic solvents to the all
molecules, which suggests the influence of intermolecular H-
bonding between the solute and protic solvent on the absorption
spectra. However, as observed in aprotic solvents, the red-shift of
the absorption maxima of C1, C3, and C5 with respect to those of
corresponding contrasting dyes (C2, C4, and C6) can be seen from
Table 1 as well. Further experiments (typically shown in Figs. S5
and S6, see Supplementary data) show that the maximal absorp-
tion wavelengths of C1, C3, and C5 are hypsochromically shifted by
added NaOH aqueous solution. And, the absorption maxima of C1,
C3, and C5 are similar to those of C2, C4, and C6, respectively, if
excess NaOH is added. The changed absorption maxima of C1, C3,
and C5 can be recovered by further added HCl solution. The max-
imal absorption wavelengths of C2, C4, and C6 display only slight
variation as titrated NaOH aqueous solution. The results demon-
strate that internal H-bonding effect could account for the red-shift
of the absorption maxima to C1, C3, and C5 as comparisons with
those of C2, C4, and C6 correspondingly.
while they have quite contrasting physiological effects. Uptake of
Hg ions could induce renal dysfunction, calcium metabolism dis-
orders, and prostate cancer. It would be of great importance to
selectively recognize Zn2þ with fluorescence sensors.28,29
In recent years, our group has been focusing on the development
of new molecules with ESIPT emission under near-IR femtosecond
laser. To search for key experimental evidences of TPA ESIPT, a range
of new target conjugated molecules containing ortho-hydroxyl and
imino groups were synthesized in this work (Scheme 1). For com-
parisons, the corresponding contrast molecules containing hy-
droxyl group at the para-position were also synthesized (Scheme 1).
In this study, we demonstrate that the new salicylidene
methylamine-based chromophores with enlarged conjugation do
have ability to show competitive ESIPT emission under near-IR
femtosecond laser excitation. The possibility of these molecules as
chemosensors to Zn2þ was further evaluated. Molecular modeling
was also performed to interpret the experimental results.
H3CO
H3CO
H3CO
H3CO
HO
N
N
OH
C2
C1
H3CO
H3CO
H3CO
H3CO
H3CO
H3CO
HO
N
N
OH
C4
C3
N
HO
N
N
N
OH
C5
C6
Scheme 1. Chemical structures of the molecules studied in this study.
In the most of aprotic solvents, C1, C3, and C5 display well-
separated one-photon dual emission bands, while C2, C4, and C6
exhibit only one-photon single emission band, which are clearly
presented in Fig. 2 and Table 1 (also typically shown in Figs. S7 and
S8, see Supplementary data). The first maximal emission peaks of
C1, C3, and C5 exhibit normal Stoke shifts (ca. 50 nm), while the
second maximal fluorescence peaks show large Stokes-shifts (ca.
160 nm). As contrast, the maximal fluorescence peaks of C2, C4, and
2. Results and discussion
2.1. Synthesis: piperidinyl group introduction
The precursor 4-piperidinyl-40-nitro-diphenylethylene was
produced by twice typical heat condensation cascade reaction (see

F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
5357
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.1
364nm
376.4nm
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
THF
C1
C2
THF
C1
C2
364nm
376.4nm
275 300 325 350 375 400 425 450 475 500 525
275 300 325 350 375 400 425 450 475 500 525
Wavelength(nm)
Wavelength (nm)
0.5
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
390.1nm
390.1nm
377.6nm
377.6nm
THF
0.4
0.3
0.2
0.1
0.0
THF
C5
C6
C5
C6
275
300
325
350
375
400
425
450
475
275
300
325
350
375
400
425
450
475
Wavelength(nm)
(a)
Wavelength(nm)
(b)
Fig. 1. Representative actual (1a) and normalized (1b) UV/visible absorption spectra of C1, C2, C5, and C6 in THF (c¼1ꢂ10ꢃ5 mol/L).
Table 1
Table 1 (continued )
Representative absorption and emission parameters of the molecules in various
solvents
Solvents
DMSO
C1
C2
C3
C4
C5
C6
la,max 373
361
0.741
433
369
0.714
444
362
0.98
440
392
0.699
444
378
Solvents
Benzene la,max 377
0.379
lf,max 432, 533 410
0.0334 0.0282 0.0337
la,max 376.4
0.357
lf,max 420, 529 423
0.0137 0.00702 0.0124
Dioxane la,max 375.2
0.433
lf,max 410, 530 394
0.0150 0.0082 0.0131
Acetone la,max 374 363 370
0.411 0.459 0.418
lf,max 418, 523 414 425, 520 412
0.0340 0.0195 0.0262 0.0201 0.0187
CH3CN la,max 372 362 366 358 388
0.408 0.455 0.350 0.516 0.346
lf,max 423, 532 421 420, 528 416 433
0.0085 0.0056 0.0082 0.0039 0.0164
CH3OH la,max 369
0.408
lf,max 434, 530 427
0.0127 0.040
C2H5OH la,max 371 357
0.336 0.445
lf,max 433, 529 423
0.0145 0.050
la,max 378 367
0.433
427
C1
C2
C3
C4
C5
C6
3
0.75
lf,max 435
0.0031
0.828
442
0.0030
365
0.438
374
0.338
362
0.403
392
0.231
427, 539 426
0.115
390.1
0.385
379
0.310
3
F
0.0023 0.0056
0.0037 0.010
426, 528 428
0.022
362.4
0.483
la,max
,
lf,max: nm,
3
: molꢃ1.cmꢃ2
.
F
0.047
377.6
0.358
THF
364.0
0.486
373.6
0.415
3
C6 display only normal Stoke shifts (ca. 50 nm). This suggests that
the second emission bands of C1, C3, and C5 are caused by ESIPT
occurred via a reasonable quasi-six-membered ring transit states in
the excited states (Scheme 2), These also suggest that the first
emission bands of C1, C3, and C5 could originate from the similar
ground state precursors as those of C2, C4, and C6 correspondingly,
namely S0/S1 absorption spectral feature.32
ESIPT emission of C1, C3, and C5 is varied in protic solvents. In
methanol, ESIPT emission is diminished to C1 and C3, and it is even
disappeared to C5 (typically shown in Fig. S9, Supplementary data).
This could be caused by intermolecular H-bonding effect between
the solutes and methanol solvent. The active hydrogen atom could
be restrained by intermolecular H-bonding effect, and the internal
proton transfer in the excited state is prohibited.33 This is the major
factor that leads to quench ESIPT emission in stronger protic sol-
vents such as acetic acid to C1, C3, and C5, respectively (typically
shown in Fig. S10, see Supplementary data). While in ethanol,
propanol or butanol, the dual emission bands with excellent sep-
aration can be detected to C1, C3, and C5 (typically shown in
429, 529 421
426, 546 425
0.0090
377.3
0.493
F
0.00553 0.0633
362.4
0.429
364.0
0.453
371.2
0.369
389.2
0.382
3
411, 534 393
0.00513 0.0490
361 389
0.424 0.493
433, 548 425
0.0048
376
0.486
433
0.0511
365
0.382
431
0.0244
371
0.288
432
0.0258
384
0.359
436
0.0011
429, 536 427
0.0128
377
0.363
F
3
F
3
F
354
0.407
366
0.404
347
0.365
383
0.359
433
0.0532
384
0.316
435
3
429, 532 429
0.0094
371
F
0.014
359
0.443
3
0.395
436, 524 425
F
0.0070
374
0.432
435
0.0178 0.0634
DMF
365
0.392
429
386
0.365
434
ꢁ
ꢁ
ꢁ
ꢁ
Fig. S11, see Supplementary data). Furthermore, IE =IN (IE =IN
represents the intensity ratio of the first emission band to the
second emission band, wherein IE is the representative of the
3
0.398
lf,max 432
ꢁ
F
0.0021
0.0011 0.0034
0.0024 0.0087

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F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
225
200
175
150
125
100
75
1.2
Normal emission
Normal emission
1.0
0.8
0.6
0.4
0.2
0.0
THF
C1
C1
C2
C5
C6
C2
C5
C6
ESIPT emission
50
ESIPT emission
25
0
400
450
500
550
600
650
700
400
450
500
550
Wavelength (nm)
(b)
600
650
Wavelength (nm)
(a)
Fig. 2. Representative actual (2a) and normalized (2b) linear emission spectra of C1, C2, C5, and C6 in THF excited by 350 nm (c¼1ꢂ10ꢃ5 mol/L).
is promoted by the suitable pulling effect caused by intermolecular
visible ligth or near-IR laser
H-bonding between the solute and ethanol or butanol.34
Further experiments show that the second emission bands of C1
and C3 in alcohols are lowered by added NaOH aqueous solution
(typically shown in Fig. S12, Supplementary data), and the dual
emission bands tend to become single emission band. The lost
ESIPT emission is back by further added HCl solution. This dem-
onstrates further that the second emission band is from ESIPT ex-
cited by one-photon absorption.
R2
R2
*
H
R1
O
H
O
R1
N
N
R3
R3
R2
R2
R3
*
R1
R3
H O
R1
H O
2.2.2. ESIPT under two-photon excitation. TPA optical properties of
the molecules were further surveyed by near-IR Ti:sapphire fem-
tosecond laser system in various aprotic and protic solvents. C5
shows dual TPA emission bands, while C6 displays single TPA
emission band in THF under 730 nm femtosecond laser excitation,
which is typically shown in Fig. 3. The emission maximum of the
first TPA emission band of C5 is almost identical to the maximal TPA
emission wavelength of C6 (ca. 470 nm). And, the maximal emis-
sion wavelength of the second emission band of C5 is almost
identical to that of linear ESIPT emission (ca. 550 nm). Hence, the
emission maximum of the second TPA emission band of C5 is much
longer than that of the first TPA emission band. The partners C1/C2
and C3/C4 also exhibit such contrasting TPA optical properties
(typically shown in Fig. S13, see Supplementary data). The results
suggest that the first emission band could be assigned to TPA nor-
mal emission, and the second emission band is from TPA ESIPT
N
N
Scheme 2. Typical four-level cyclic process of ESIPT for the resolved molecules.
ꢁ
intensity of ESIPT emission intensity, IN refers to the normal
emission intensity) ratios of C1, C3, and C5 in ethanol are much
larger than those in methanol (such as in ethanol, C1: 2.780, C3:
2.964, C5: 0.227, in methanol, C1: 0.121, C3: 0.192), and they are
greater than those in aprotic solvents as well (C1: 0.288 in benzene,
0.520 in THF; C3: 0.241 in benzene, 0.431 in THF; C5: 0.200 in
benzene, 0.097 in THF). IE =IN ratios even become much larger in
butanol (C1: 2.992, C3: 3.044, C5: 0.378). The results show that
ESIPT emission becomes the competitive pathway to deactivate the
excited singlet state in ethanol or butanol. This indicates that ESIPT
ꢁ
ꢁ
40
35
140
ESIPT emission
ESIPT emission
C5
C6
30
25
20
15
10
5
C1
C2
120
100
80
60
40
20
0
Normal emission
Normal emission
0
350
400
450
500
550
600
650
700
300
350
400
450
500
550
600
650
700
Wavelength(nm)
Wavelength (nm)
(a)
(b)
Fig. 3. Representative TPA emission of C1, C2, C5, and C6 in THF under 730 nm femtosecond laser irradiation.

F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
5359
emission for C1, C3, and C5. The results suggest that C1, C3, and C5
are able to undergo ESIPT under TPA process, while C2, C4, and C6
do not show such properties.
intensities of C1 and C3 in DMF are gradually increased reaches
approximate constants with small red-shift as the added of Zn2þ
aqueous solution. The sensing response shows that the equilibrium
of zinc complex with C1 and C3 is formed, respectively. As contrast,
owing to the stronger internal H-bonding effect of C5, it is hard to
lose the active hydrogen in OH group of C5 to form the complex
with Zn2þ. Consequently, C5 does not show nice sensing response
ꢁ
ꢁ
IE =IN ratios in TPA emission spectra of C1, C3, and C5 are much
larger than those in one-photon emission spectra under 730 nm
femtosecond laser irradiation (such as in THF, to C1: 1.321, C3: 2.167,
C5: 2.932). In benzene, the molecules exhibit even only ESIPT
emission excited by 730 nm femtosecond laser (typically shown in
Fig. S14, see Supplementary data). Even in strong protic methanol
or basic solvents DMF, near-IR femtosecond laser induced ESIPT
emission also becomes the major deactivation decay in the excited
states of C1, C3, and C5 (typically shown in Fig. S15, see
Supplementary data). ESIPT emission is also strong competitive in
two-photon process excited by the other near-IR femtosecond laser
wavelengths (typically shown in Fig. S16, see Supplementary data).
It was demonstrated that ESIPT of salicylidene methylamine was
completed through femtosecond-level process.26 Therefore, it is
reasonable to assume ESIPT in the excited states of C1, C3, and C5
could occur in femtosecond time range. This means that near-IR
femtosecond pulsed laser could well match ESIPT process of the
molecules. As a consequence, ESIPT becomes predominant path-
way to deactivate the excited states of C1, C3, and C5 during two-
photon process induced by femtosecond laser excitation. It is no-
to Zn2þ
.
The sensing response of C1 and C3 to other metal ions was also
evaluated. The fluorescence sensing response and the photo-
luminescence of C1 and C3 with added metal ions at the equilib-
rium states are shown in Fig. 5. It is clear that the titration of Zn2þ
causes remarkable increasing of fluorescence emission in DMF,
while slight or negligible changes occur as added other metal ions
(also see Supplementary data, Fig. S19). It means that the sensing
effect of Zn2þ can be well distinguished from the other metal ions.
The results demonstrate that C1 and C3 are effective chemosensors
for Zn2þ.36 The linear characteristics of 1:1 complexation action is
produced from the curve fitting of the fluorescence intensity to the
reciprocal of the Zn2þ concentration, and the equilibrium constants
are determined from the BenesieHildebrand plots to 0.69ꢂ104 L/
mol (C1) and 1.80ꢂ104 L/mol (C3), respectively (Fig. 6). It is obvious
that C3 is more sensitive than C1, suggesting again the roles of
substituent groups.
The sensing response in other hydrophilic solvents such as THF
to other metal ions was also surveyed for C1 and C3 (typically
shown in Fig. S20, see Supplementary data). It is not regular or
insensitive response of the absorption and fluorescence spectra to
various metal ions. However, the active hydrogen in C1 and C3 can
be loosed by unshared pair electrons in N atom of DMF, thus they
exhibit excellent sensing response to Zn2þ in DMF. Some sensing
response to Zn2þ was also observed in methanol for C1 and C3,
respectively. It is also interesting that ESIPT emission bands of C1
and C3 are disappeared as added Zn2þ in methyl alcohol (typically
shown in Fig. S21, see Supplementary data). We also found that that
TPA ESIPT emission bands of C1 and C3 in methanol or DMF could
be quenched by added Zn2þ under near-IR femtosecond laser ex-
citation (typically shown in Fig. S22, see Supplementary data). The
results suggest in turn that ESIPT does be presence in C1 and C3
during one- and two-photon excitation process, and the adjacent O
and N atoms are key factors for the formation of the Zn complex.
ꢁ
ꢁ
ticed that IE =IN ratios of C5 are larger than those of C1 and C3 in
THF during two-photon process. At present, we do not understand
deeply such experimental phenomena. We assume that it could be
caused by more efficient TPA of C5 under near-IR femtosecond laser
irradiation.
It was further determined TPA cross-sections of the molecules
based on TPA fluorescent methods. Typical TPA cross-sections of
the molecules C1eC6 excited by 730 nm femtosecond laser are
shown in Table 2. The data show that the partner C5/C6 has larger
TPA cross-sections than the partners C1/C2 and C3/C4. This could be
caused by the larger dipole moment changes between the excited
state and the ground state of the partner C5/C6 than those of the
partners C1/C2 and C3/C4 due to stronger electron-donating ability
of piperidinyl group35 (E to E
, such as to C1: 0.675 D, C3: 0.892 D,
*
C5: 1.656 D, see Molecular modeling part).
Table 2
TPA cross-sections of the molecules under 730 nm femtosecond lasera
Solvents
TPA cross-sections (GM)
2.4. Molecular modeling
C1
C2
C3
C4
C5
76
101
C6
2.4.1. Geometry parameters. Molecular modeling was performed to
analyze these experimental phenomena. The calculation shows
that the transition states from Keto to Enol are presence in the
ground and excited states to C1, C3 and C5 (typically shown in
Fig. S23, see Supplementary data). While as contrast, no transition
states exist to C2, C4 and C6. Hence, the theoretical calculation
suggests the impossibility of internal proton transfer in the ground
and excited states of C2, C4, and C6, while C1, C3, and C5 have large
possibility to undergo ESIPT.
Benzene
THF
17
26
7
12
21
34
9
14
25
57
a
1 GM¼10ꢃ50 cmꢃ1 s photonꢃ1, the errors do not exceed 15%.
2.3. The possibility as fluorescence sensors to Zn2D
The possibility as fluorescence chemosensors for metal ions was
evaluated to the molecules. C2, C4, and C6 do not exhibit regular or
negligible fluorescence response to metal ions in various hydro-
philic solvents such as DMF (typically shown in Fig. S17, see
Supplementary data). This could be ascribed to the absence of the
adjacent hydroxyl and imino groups. Although there is the pres-
ence of the adjacent C]N and OH in C5, it does not exhibit regular
sensing response to metal ion in various hydrophilic solvents such
as DMF (typically shown in Fig. S18, Supplementary data). As
contrast, C1 and C3 display remarkably regular sensing response to
Zn2þ in DMF.
The distances of HeO and HeN (in salicylidene methylamine
part) in the transition states of C1, C3, and C5 are between those of E
and K forms in S0 and S1 states (typically shown in Table S1, see
Supplementary data). On the other hand, the distance of OeN in the
transition states is much smaller than those of E and K forms in
both S0 and S1 states to C1, C3, and C5. The shorter OeN means that
the transition state is the state with the strongest internal hydrogen
bond strength in S0 and S1 of C1, C3, and C5, respectively, which
meets the perquisite condition for the occurrence of ESIPT.
The calculation shows that in S0 and S1 of E forms of C2, C4, and
C6, the distance HeN is much longer as comparisons with that of
As shown in Fig. 4a, the absorption spectra of C1 and C3 in DMF
are gradually red-shifted and broadened by the continuous titra-
tion of Zn2þ aqueous solution. At the end, a new broadened ab-
sorption band is appeared. Fig. 4b shows that the emission
ꢀ
C1, C3, and C5 correspondingly, even up to about 7.0 A (Table S1).
Hence, internal H-bonding interaction between H and N could be

5360
F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
0.35
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-4
-5
-4
-5
C3
5.00
×
10
0.30
0.25
0.20
0.15
0.10
0.05
0.00
5.00
5.00
×
×
10
10
C1
5.00×
10
-5
-5
2.15
1.00
5.00
×10
2.15
1.00
5.00
×10
-5
-6
-5
-6
×
10
10
×
×
10
10
×
-6
-6
0.00×10
0.00
2+]
Zn
×10
2+]
[
Zn
[
280 300 320 340 360 380 400 420
280
310
340
370
400
430
460
Wavelength (nm)
Wavelength (nm)
(a)
300
250
200
150
100
50
-3
400
350
300
250
200
150
100
50
1.50×10
-4
-4
-4
-5
-5
-5
-6
C3
5.00×10
C1
-3
1.50×10
2.15×10
1.00×10
5.00×10
2.15×10
1.00×10
-4
2.15
5.00
1.00
5.00
2.15
1.00
×
10
10
10
10
10
10
-4
-4
-5
-5
-5
×
×
×
×
×
0.00×10
2+
-6
0.00
×10
Zn
2+
Zn
0
0
375 400 425 450 475 500 525 550 575
370
410
450
490
530
570
Wavelength (nm)
Wavelength (nm)
(b)
Fig. 4. Representative UV/visible spectra (a) and fluorescence spectra (b) of C1 and C3 (c¼1ꢂ10ꢃ5 mol/L) in DMF at room temperature as the function of added Zn(OAc)2 various
aqueous stock solutions (c¼5ꢂ10ꢃ3 to 1.0 mol/L).
negligible or inexistence in S0 and S1 of C2, C4, and C6, respectively.
Owing to no internal pulling effect of N for H, the distance of HeO in
hydroxyl group of E forms of C2, C4, and C6 becomes shorter than
C1 and C5, respectively. Fig. 7 shows that the energy barrier is
presence in intramolecular proton transfer in S0 and S1, and the
energy barrier is much larger in S0. Furthermore, E form is more
stable in the ground state. These results indicate that the occur-
rence of ground state internal proton transfer (E/K) is difficult,
while the reverse proton transfer (K/E) may take place, which is
favorable for four-level cycle phototautomerization process.3a,15
We further analyzed the relative energy parameters in S0 and S1
states of all molecules (typically shown in Table S2, see
Supplementary data). The energy difference between E and K,
ꢀ
that of C1, C3, and C5 correspondingly (such as to C1: 0.955 A, C2:
ꢀ
0.943 A). The calculation also shows that that in the K forms in S0
and S1 of C2, C4, and C6, the distance of HeN in Schiff moiety is
ꢀ
ꢀ
shorter (such as to C1: 1.012 A, C2: 0.995 A) than that in C1, C3, and
C5 correspondingly, while the distance of HeO in K forms of C2, C4,
ꢀ
ꢀ
and C6 are much longer (such as to C1: 1.806 A, C2: 6.184 A). This
further indicates that C1, C3, and C5 have much larger possibility to
undergo ESIPT than C2, C4, and C6 correspondingly.
represented as DE, is much lower in the excited state than that in the
The important dihedral angle :C1NC2C3 associated with ESIPT
process was also calculated (Table S1). A large torsion in :C1NC2C3
in S0 of the tautomers to all molecules (such as in E of C1: 42.537ꢀ)
is presence, while :C1NC2C3 tends to be reduced in S1 (such as in
ground state. Meanwhile, the data suggest that K form has a higher
energy than E form in S0 state to all molecules, indicating that it is
difficult to undergo internal proton transfer in S0 (E/K). Direct
reaction energy barrier (E/K), shown as Ed#, shows further that
D
E
*
of C1: ꢃ0.186ꢀ). Such reduction in C1, C3, and C5 is more obvious
an energy barrier exist between the phototautomers E and K in both
S0 and S1 states to C1, C3, and C5, respectively (such as to C1: S0,
21.364 kJ/mol, S1, 7.414 kJ/mol). As contrast, the reverse reaction
than that in C2, C4, and C6 correspondingly (such as in E of C2:
43.570ꢀ to 11.375ꢀ), and :C1NC2C3 in E
*
of C2, C4, and C6 is still
ꢀ
over 10 C. As contrast, the dihedral angle :C1NC2C3 is almost in
the same plane in the excited states of all phototautomers (E, TS,
and K forms) of C1, C3, and C5. In particular, :C1NC2C3 is negli-
energy barrier (K/E, shown as Er#) in the ground state is much
D
lower than that in the excited states to C1, C3, and C5, respectively
(such as to C1: So, ꢃ1.23 kJ/mol, S1, 3.468 kJ/mol). This further
demonstrates that the occurrence of ground state intramolecular
proton transfer (GSIPT, E/K/E) is much harder than that of ESIPT
gible in TS
*
of C1, C3, and C5. Consequently, the negligibly twisted
configuration in the excited states allows the extension of the
conjugated
p
-electron in the entire molecular framework of C1, C3,
(E/E
*
/K /K/E). On the other hand, the small Er# in ground
D
*
and C5, respectively,37 and the energy barrier in the ESIPT process
caused by the bond twisting is reduced. In a word, the dihedral
angles suggest further that it is possible to undergo ESIPT to C1, C3,
and C5, while it is quite difficult to C2, C4, and C6.
state implies that reasonability to undergo reverse internal proton
transfer (namely from K/E) to C1, C3, and C5, respectively.
D
E in S0 of C2, C4, and C6 is much larger than those of C1, C3, and
C5 correspondingly (such as to C1 in S0, 22.594 kJ/mol, C2 in S0
36.656 kJ/mol), and it becomes more remarkable in S1 (such as to
C1 in S1, 3.946 kJ/mol, C2 in S1 27.535 kJ/mol). This further suggests
that internal proton transfer is much harder in both the ground and
excited states of C2, C4, and C6, particularly in the excited states.
2.4.2. Potential energy curves of phototautomerization in S0 and
S1. Representative plots of the potential curves of intramolecular
proton transfer as a function of OeH distance in S0 and S1 states of

F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
5361
Fig. 5. (a) Fluorescence sensing response of C1 and C3 (c¼1ꢂ10ꢃ5 mol/L in DMF) to various metal ions, concentration of Zn2þ is 5ꢂ10ꢃ5 mol/L in DMF, and the concentration of the
other metal ions are 5ꢂ10ꢃ4 mol/L in DMF; (b) and (c) are the photoluminescence of C1 and C3 in DMF with added various metal ions excited by 365 nm light (from left to right:
blank, Kþ, Naþ, Liþ, Mg2þ, Ca2þ, Zn2þ, Fe3þ, Ni2þ, Cd2þ, Hg2þ). All the experiments were detected at the equilibrium states.
Supplementary data). Obvious electron transfer between electron-
12
donating part and salicylidene methylamine part during the exci-
tation from HOMO to LUMO is shown. The larger electron density
distribution in the phenolic ring in LUMO is the driving force for
10
C1
C3
internal proton transfer in the excited state. The active hydrogen
atom in the transition state exhibits the larger positive charge than
8
that in E and K forms in S0 and S1 (typically shown in Table S3,
Supplementary data). As E is excited to E , the part of negative
*
6
4
2
charge is transferred from the hydroxyl to the imino group. As
a result, the negative charge in proton donor (O atom) is reduced
(less basicity, more acidity, such for C1: in S0, ꢃ0.549e, in S1,
ꢃ0.547e), and the negative charge in proton receptor (N atom) is
increased (less acidity, more basicity, such as for C1: S0, ꢃ0.562e, S1,
ꢃ0.606e), which is favorable for ESIPT in the excited state.38 As
contrast, such changes are smaller or negligible for C2, C4, and C6,
respectively (typically shown in Table S4, Supplementary data).
If internal proton transfer occurs, C2, C4, and C6 have to go
through a much larger geometry change than C1, C3, and C5 cor-
respondingly. As a consequence, the dipole moment of tautomers of
C2, C4, and C6 displays much larger changes in both the ground and
excited states than those of C1, C3, and C5 correspondingly (such as
in S0, C1: 2.135 D, C2: 8.002 D; in S1, C1, 1.579 D, C2, 8.949 D), which
in turn demonstrates that internal proton transfer could be very
hard in S0 and S1 to C2, C4, and C6.
0
0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 0.0040
2+
Zn
[
]
/ mol/L
Fig. 6. BenesieHildebrand plots of the fluorescence intensities of C1 and C3 against
the reciprocal of Zn2þ in DMF.
2.4.3. Frontier orbital, atomic charge, and dipole moment. HOMO
and LUMO orbitals of the molecules are characterized with
type symmetric nature (typically shown in Fig. S24, see
pep

5362
F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
120
110
100
90
300
290
280
270
260
250
Excited state
80
70
60
50
K
*
K
*
E*
E*
40
30
20
60
Ground state
Excited state
240
40
50
40
30
20
10
0
30
20
10
0
Ground state
K
K
E
E
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.8
1.0
1.2
1.4
1.6
1.8
2.0
R
O-H
(angstrom)
R
(angstrom)
O-H
(b)
(a)
Fig. 7. Proton transfer reaction potential energy curves relaxed along the OeH distance at ground and the first single excited state for C1 (7a) and C5 (7b), respectively.
3. Conclusions
analysis. Melting points of the samples were detected by Beijing
Fukai melting point apparatus.
This work has successfully established efficient chemical ap-
proach to develop new enlarged organic chromophores with TPA
ESIPT. It is demonstrated that internal H-bonding plays key roles
in ESIPT of the organic chromophores. The substituent groups
and the solvents also exhibit significant influence on ESIPT under
one- and two-photon process. It is further demonstrated that the
organic dyes containing ortho-hydroxyl and imino groups display
competitive TPA ESIPT emission under near-IR femtosecond
laser wavelengths. C5 exhibits the larger TPA cross-sections
than C1 and C3 caused by the stronger electron-donating nature
of piperidinyl group. C1 and C3 show excellent sensing response
to Zn2þ in DMF due to the metal coordination effect. The mo-
lecular modeling is agreement well with the experimental
observation.
4.3. One- and two-photon spectral determination
Cintra spectrophotometer was used for the measurement of the
UV/visible absorption spectra, while the emission spectra were
detected with Shimadzu RF-531PC spectrofluorophotonmeter.
Rodamin 6G in ethanol (
F, 0.94) and quinine sulfate in 0.5 M H2SO4
(F
, 0.546)40 were used as references to measure the fluorescence
quantum yields of the molecules herein. The low concentration of
the molecules was used to measure the fluorescence quantum yields
in order to avoid self-quenching of fluorescence emission. Herein,
the optical density at the excited wavelength for the determine
fluorescence quantum yields is below 0.1. The fluorescence quantum
yields were calculated according to the following equation:41
Z
ꢀ
ꢁ
n20A0 If lf
d
lf
4. Experiment section
Ff
¼
Ff0
(1)
Z
ꢀ
ꢁ
n2A If0 lf
d
lf
4.1. Reagents and materials
wherein n0 and n are the refractive indices of the solvents, A0 and A
show the optical densities at excitation wavelength, Ff and F0f are
the quantum yields, and the integrals are the area of the fluores-
cence bands, to the reference and sample, respectively.
Organic chemicals and reagents were got from Aldrich
unless otherwise specified. The organic solvents were further
processed using well-known standard laboratory techniques
before use.39
Two-photon excited fluorescence spectra were produced by
pumped Ti:sapphire femtosecond laser (Spectra-Physics Ltd., Tsu-
nami mode-locked, 80 MHz, <130 fs, average power ꢄ700 mW)
tuned at step of 10 nm in the range of 700e880 nm, and Ocean
Optics USB2000 CCD camera with the detecting range of
Zn(OOCCH3)2$2H2O, Mg(OOCCH3)2$4H2O, KOOCCH3, NaOOC
CH3$3H2O, LiCl, Ca(OOCCH3)2$H2O, Fe(NO3)3$9H2O, Cu(OOCCH3)2$
H2O, NiCl2$6H2O, Hg(OOCCH3)2, and Cd(OOCCH3)2$2H2O were
used as received from Shanghai Chemical Reagent Corporation.
The new target chromophores (C1, C3, C5) and the contrasting
molecules (C2, C4, C6) were shown in Scheme 1, including
2-((30,40-dimethoxyl-phenylethyleneyl-phenyl-4-ylimino)methyl)
phenol (C1), 4-((30,40-dimethoxyl-phenylethyleneyl-phenyl-4-yli
mino) methyl)phenol (C2), 2-((30,40,50-trimethoxyl-phenylethyl
eneyl-phenyl-4-ylimino)methyl)phenol (C3), 4-((30,40,50-trimetho
xyl-phenylethyleneyl-phenyl-4-ylimino) methyl)phenol (C4), 2-
((40-piperidinyl-phenylethyleneyl-phenyl-4-ylimino)methyl)phe-
nol (C5), and 4-((40-piperidinyl-phenylethyleneyl-phenyl-4-ylimi
no)methyl)phenol (C6), which were firstly reported herein.
180e880 nm was used to record the spectra. TPA cross-section (s)
was obtained by up-conversion fluorescence method using
5ꢂ10ꢃ4 mol/L fluorescein in 0.1 mol/L solution of NaOH as reference
sample.42 All determined samples were bubbled with N2 for 15 min
to eliminate O2 before the detection. TPA cross-sections of the
molecules were calculated by the following equations:43
sTPE
s
¼
(2)
FF
TPEccal ncal
S
sTPE
¼
scal
(3)
4.2. Chemical structural characterization
c
n Scal
1H and 13C nuclear magnetic resonance (NMR) spectra were
determined at room temperature with a Bruker 500 MHz appara-
tus. Tetramethylsilane (TMS) was used as an internal standard to
determine the chemical shift. CE440 elemental analysis meter ob-
tained from Exeter Analytical Inc. was performed in the elemental
wherein
s
represents as TPA cross-section, sTPE denotes two-
photon excited crossing section, c is the concentration of refer-
ence and sample molecules, n shows the refractive index of the
solvent, and S is two-photon up-conversion fluorescence intensity,
cal is shown as the reference.

F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
5363
4.4. Determination of the coordination constants
7.45 (m, 3H, AreH), 7.66 (m, 3H, AreH), 9.03 (s, 1H, AreCH]NeAr),
13.19 (s, 1H, AreOH). IR (KBr pellet, cmꢃ1
¼3440 (s), 3002 (m),
2930 (m), 2834 (w), 1618 (m), 1602 (w), 1505 (s), 1259 (m), 1220 (s),
1021 (m) cm
ꢃ1 13C NMR (DMSO-d6, 125 MHz):
111.785, 116.596, 119.169, 119.395, 120.076, 121.867, 125.531, 127.191,
128.324, 128.691, 129.973, 132.535, 133.222, 136.428, 146.562,
148.841, 148.961, 160.332, 162.584. Elemental analysis: C23H21NO3,
C, 76.76 (76.91), H, 5.89 (5.73), N, 3.90 (3.74).
)
n
BenesieHildebrand equation was employed to analyze the
binding constant K of the complex formed by the metal ion and the
sample through the measurement of the fluorescence spectra
changes.44 The equation is described as following for 1:1 complex:
.
d¼55.508, 109.214,
ꢀ
ꢁ
ꢀ
ꢁ
F0=ðF0 ꢃ FÞ ¼ F0= F0 ꢃ Fcomplex þ F0= F0 ꢃ Fcomplex ꢂ 1=K
ꢂ 1=½Mꢅ
4.6.3. 4-((30,40-Dimethoxyl-phenylethyleneyl-phenyl-4-ylimino)
methyl)phenol (C2). C2 was synthesized as C1. Light yellow solid,
yield: 56.3%, mp: 241e243 ꢀC. 1H NMR (DMSO-d6, 500 MHz, TMS):
(4)
wherein K denotes the binding constant, F0 represents as the in-
tegrated fluorescence intensity of free ligand, F is the observed
integrated fluorescence intensity, Fcomplex shows the emission of
the ligandemetal complex, and [M] is the concentration of added
metal ion. The binding constant is calculated by the ratio intercept/
slope from the plot of F0/F0ꢃF versus 1/[M].
d
¼3.77 (s, 3H, AreOCH3), 3.83 (s, 3H, AreOCH3), 6.88 (d, J¼8.5 Hz, 2H,
AreH), 6.95 (d, J¼8.5 Hz, 1H, AreCH]CHeAr), 7.10 (d, J¼8.5 Hz, 1H,
AreCH]CHeAr), 7.16 (s, 2H, AreH), 7.23 (d, J¼8.0 Hz, 3H, AreH),
7.59 (d, J¼8.5 Hz, 2H, AreH), 7.78 (d, J¼8.5 Hz, 2H, AreH), 8.52 (s,1H,
AreCH]NeAr), 10.15 (s, 1H, AreOH). IR (KBr pellet, cmꢃ1
)
n¼3407
(s), 3018 (m), 2930 (m), 2850 (w), 1625 (m), 1609 (w), 1505 (s), 1442
(m) 1164 (m),1268 (s),1012 (m) cm
.
ꢃ1 13C NMR (DMSO-d6,125 MHz):
4.5. Fluorescence sensing to metal ions
d
¼55.866, 60.075, 103.806, 113.310, 115.664, 121.336, 121.450,
127.168, 127.371, 127.478, 127.576, 127.828, 130.685, 132.971, 134.515,
137.238, 150.940, 153.054, 159.461, 160.655. Elemental analysis:
C23H21NO3, C, 76.76 (76.96), H, 5.89 (5.78), N, 3.90 (3.79).
Little volume of metal ions stock solution (5ꢂ10ꢃ3 to 1 mol/L)
was added into
3 mL the corresponding sample solution
(1ꢂ10ꢃ5 mol/L) in various hydrophilic solvents to check the spectral
4.6.4. 3,4,5-Trimethoxyl-40-amine-diphenylethylene. The synthesis
of 3,4,5-trimethoxyl-40-nitro-diphenylethylene is similar to that of
3,4-dimethoxyl-40-nitro-diphenylethylene (isolated yield: 68.2%).
3,4,5-Trimethoxyl-40-amine-diphenylethylene is prepared as that
of 3,4-dimethoxyl-40-amine-diphenylethylene, yield, 47.4%. 1H
sensing response. Total added volume is guaranteed to be less than
10 mL so that the concentration of the sample is ensured to display
negligible change. Before the titration, the blank experiments
without metal ions using the same procedure were performed to
check neglected effect of added water on the spectral properties.
The solution was stirred intensively or sonicated for 30 min to reach
equilibrium for every titration, and the UV/visible spectra as well as
the emission spectra were subsequently measured. No obvious ef-
fect on both UV/visible spectra and emission spectra was observed
as the titration of limited water volume.
NMR (DMSO-d6, 500 MHz, TMS):
d
¼3.86 (s, 3H, AreOCH3), 3.91 (s,
6H, Ar-OCH3), 6.67 (d, J¼8.5 Hz, 2H, AreH), 6.70 (s, 2H, AreH), 6.84
(d, J¼16.5 Hz, 1H, AreCH]CHeAr), 6.92 (d, J¼16.5 Hz, 1H, AreCH]
CHeAr), 7.32 (d, J¼8.0 Hz, 2H, AreH).
4.6.5. 2-((30,40,50-Trimethoxyl-phenylethyleneyl-phenyl-4-ylimino)
methyl)phenol (C3). The preparation of C3 was carried out as that of
C1, golden yellow solid, yield: 54.2%, mp 121e122 ꢀC. 1H NMR
4.6. Synthesis
The synthesis route was depicted in Scheme 3.
(DMSO-d6, 500 MHz, TMS):
d
¼3.68 (s, 3H, AreOCH3), 3.85 (s, 6H,
AreOCH3), 6.96 (d, J¼6.0 Hz, 2H, AreH), 6.99 (m, 2H; AreCH]
CHeAr), 7.26 (d, J¼8.0 Hz, 2H, AreH), 7.42 (m, 1H, AreH), 7.47 (d,
J¼9.0 Hz, 2H, AreH), 7.67 (m, 3H, AreH), 9.03 (s, 1H, AreCH]
NeAr), 13.16 (s, 1H, Ar-OH). 13C NMR (DMSO-d6, 125 MHz):
4.6.1. 3,4-Dimethoxyl-40-amine-diphenylethylene. 3,4-Dimethoxyl-
40-nitro-diphenylethylene was prepared by the condensation of 3,4-
dimethoxyl-benzaldehyde and 4-nitro-phenylacetic acid based on
routine experimental condition (the isolated yield: 72.4%).45 Stan-
nous chloride dehydrate was added slowly in ethanol solution con-
taining 3,4-dimethoxyl-40-nitro-diphenylethylene. The mixture was
kept refluxing for 24 h at argon atmosphere. The yielded solid was
dissolved in dilute NaOH solution and its pH was adjusted to 8e9
with dilute hydrochloric acid. The solid was obtained by the filtra-
tion, which was further purified with column chromatography using
the mixture of methylene chloride and petroleum ether (v/v¼1:1) as
eluent. Brown-yellow solid, yield: 46%, mp 153e154 ꢀC. 1H NMR
d
¼56.371, 60.567, 104.443, 117.073, 119.757, 122.383, 127.527,
127.844, 129.276, 133.117, 133.726, 136.599, 137.894, 147.340,
153.548, 160.813, 163.214. IR (KBr pellet, cmꢃ1
)
n
¼3438 (s), 3026
(m), 3065 (w), 2938 (m), 2834 (w), 1618 (m), 1577 (w), 1506 (s),
1450 (m), 1347 (m), 1236 (m), 1132 (s), 996 (m). Elemental analysis:
C27H23NO4, C, 76.22 (76.35), H, 5.45 (5.29), N, 3.29 (3.16).
4.6.6. 4-((30,40,50-Trimethoxyl-phenylethyleneyl-phenyl-4-ylimino)
methyl)phenol (C4). The synthesis of C4 was carried out as the that
of C2, light yellow solid, yield: 52.5%, mp: 236e237 ꢀC. 1H NMR
(CDCl3, 500 MHz, TMS):
d
¼3.75 (s, 3H, AreOCH3), 3.80 (s, 3H,
AreOCH3), 5.23 (s, 2H, AreNH2), 6.55 (d, J¼8.5 Hz, 2H, AreH), 6.82 (d,
J¼16.5 Hz,1H, AreH), 6.91 (t, J¼8.5 Hz, 2H, AreCH]CHeAr), 6.98 (t,
J¼10.0 Hz,1H, AreH), 7.14 (s,1H, AreH), 7.24 (d, J¼8.5 Hz, 2H, AreH).
(DMSO-d6, 500 MHz, TMS):
d
¼3.67 (s, 3H, AreOCH3), 3.84 (s, 6H,
AreOCH3), 6.89 (d, J¼8.0 Hz, 2H, AreH), 6.93 (s, 2H, AreH), 7.16 (s,
1H, AreCH]CHeAr), 7.19 (s,1H, AreCH]CHeAr), 7.24 (d, J¼8.5 Hz,
2H, AreH), 7.61 (d, J¼8.0 Hz, 2H, AreH), 7.78 (d, J¼8.5 Hz, 2H,
AreH), 8.52 (s, 1H, AreCH]NeAr), 10.15 (s, 1H, AreOH). IR (KBr
4.6.2. 2-((30,40-Dimethoxyl-phenylethyleneyl-phenyl-4-ylimino)
methyl)phenol (C1). At argon atmosphere, 2-hydroxy-benzalde-
hyde was slowly added into ethanol solution containing 3,4-
dimethoxyl-40-nitro-diphenylethylene. The mixture was kept stir-
red at refluxing for a few hours. After cooling, the solid was ob-
tained by filtration. The purified C1 was obtained with twice
recrystallization in ethanol. Light yellow solid, yield: 56.5%, mp
pellet)
1602 (s), 1505 (s), 1466 (m), 1250 (s), 1005 (m) cmꢃ1
(DMSO-d6, 125 MHz):
n
¼3431 (s), 3271 (s), 3026 (w), 2938 (m), 2843 (w), 1632 (m),
.
13C NMR
d
¼50.106, 114.994, 116.460, 116.504, 123.921,
126.150, 126.527, 127.247, 127.652, 128.775, 129.010, 132.878,
148.201, 150.955, 160.256, 163.798. Elemental analysis: C27H23NO4,
C, 76.22 (76.37), H, 5.45 (5.32), N, 3.29 (3.13).
145e146 ꢀC. 1H NMR (DMSO-d6, 500 MHz, TMS):
AreOCH3), 3.83 (s, 3H, AreOCH3), 6.98 (m, 3H, AreH), 7.12 (m, 1H,
AreH), 7.21 (d, J¼11.5 Hz, 2H, AreCH]CHeAr), 7.27 (m, 1H, AreH),
d¼3.78 (s, 3H,
4.6.7. 4-Piperidinyl-40-amine-diphenylethylene. 4-Piperidinyl-40-
nitro-diphenylethylene was prepared using 4-fluorobenzaldehyde

5364
F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
R2
R2
piperidine
COOH
O2N
R1
CHO
O2N
+
R1
100-140 0C
R3
R3
C2H5OH
SnCl2(H2O)2
C1: R1, R2: OCH3, R4: OH, R3, R5: H
C2: R1, R2: OCH3, R3, R4: H, R5: OH
R2
H2N
R1
C3: R1, R2, R3: OCH3, R4: OH, R5: H
C4: R1, R2, R3: OCH3, R4: H, R5: OH
R3
OHC
R2
R5
C2H5OH
R4
R1
R4
N
R3
R5
(a)
piperidine
100-140 0C
COOH
O2N
O2N
F
CHO
+
N
SnCl2(H2O)2 C2H5OH
H2N
N
C5: R4: OH, R5: H
C6: R4: H, R5: OH
OHC
R5
C2H5OH
R4
R5
R4
N
N
(b)
Scheme 3. Synthesis routes of the molecules C1eC6.
and 4-nitro-phenylacetic acid as reactants (yield: 76.3%). 4-
Piperidinyl-40-amine-diphenylethylene was synthesized as 3,4-
dimethoxyl-40-amine-diphenyl-ethylene. Yellow solid, yield: 45.1%,
(m), 1243 (s), 1180 (m) cmꢃ1 13C NMR (DMSO-d6, 125 MHz):
.
d
¼25.061, 54.567, 115.542, 116.622, 117.251, 119.204, 119.541. Ele-
mental analysis: C20H22N2O, C, 78.40 (78.56), H, 7.24 (7.13), N, 9.14
(9.02).
mp: 140e141 ꢀC; 1H NMR (DMSO-d6, 500 MHz, TMS):
d
¼1.53 (m,
2H, AreNC5H10), 1.60 (m, 4H, AreNC5H10), 3.14 (t, J¼5.25 Hz, 4H,
AreNC5H10), 5.18 (s, 2H, AreNH2), 6.53 (d, J¼8.5 Hz, 2H, AreH), 6.79
(t, J¼14.5 Hz, 2H, AreH), 6.87 (d, J¼8.5 Hz, 2H, AreCH]CHeAr),
7.20 (d, J¼8.5 Hz, 2H, AreH), 7.32 (d, J¼8.5 Hz, 2H, AreH).
4.6.9. 4-((40-Piperidinyl-phenylethyleneyl-phenyl-4-ylimino)methyl)
phenol (C6). C6 was prepared as the that of C2, yellow solid, yield:
51.3%, mp, 259e260 ꢀC; 1H NMR (DMSO-d6, 500 MHz, TMS):
d
¼1.56
(m, 2H, AreNC5H10), 1.62 (t, 4H, J¼8.0 Hz, AreNC5H10), 3.19 (t, 4H,
J¼5.25 Hz, AreNC5H10), 6.91 (m, 4H, AreH), 7.02 (d, 1H, J¼16.5 Hz,
AreCH]CHeAr), 7.12 (d, 1H, J¼16.0 Hz, AreCH]CHeAr), 7.21 (d,
J¼8.5 Hz, 2H, AreH), 7.43 (d, J¼9.0 Hz, 2H, AreH), 7.55 (d, 2H,
J¼8.0 Hz, AreH), 7.78 (d, 2H, J¼8.5 Hz, AreH), 8.51 (s, 1H, AreCH]
4.6.8. 2-((40-Piperidinyl-phenylethyleneyl-phenyl-4-ylimino)methyl)
phenol (C5). C5 was synthesized as the that of C1, yellow solid,
yield: 52.5%, mp: 223e224 ꢀC. 1H NMR (DMSO-d6, 500 MHz, TMS):
d
¼1.56 (d, J¼5.0 Hz, 2H, AreNC5H10), 1.61 (d, J¼4.5 Hz, 4H,
AreNC5H10), 3.20 (t, J¼5.25 Hz, 4H, AreNC5H10), 6.93 (d, J¼8.5 Hz,
2H, AreH), 6.98 (m, 2H, AreCH]CHeAr), 7.06 (d, J¼16.5 Hz, 1H,
AreH), 7.19 (d, J¼16.5 Hz, 1H, AreH), 7.43 (m, 5H, AreH), 7.65 (m,
3H, AreH), 9.02 (s, 1H, AreCH]NeAr), 13.21 (s, 1H, AreOH). IR
NeAr), 10.11 (s, 1H, AreOH). IR (KBr pellet)
n
¼3431 (s), 2938 (m),
2803 (w), 1602 (s), 1500 (s), 1521 (s), 1243 (s), 1171 (m) cmꢃ1
NMR (DMSO-d6, 125 MHz):
.
13C
d
¼25.478, 55.478, 109.161, 109.333,
111.281, 111.772,115.663,119.847,121.416,124.051, 125.856,126.981,
127.605, 127.704, 127.844, 136.136, 130.664, 134.808, 148.666,
(KBr pellet)
n
¼3860 (s), 2930 (m), 2850 (w), 1609 (s), 1514 (s), 1449

F. Gao et al. / Tetrahedron 69 (2013) 5355e5366
5365
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Acknowledgements
We thank financial support from CSTC2012jjB50007 and
CSTC2010BB0216. F.G. thanks the warm encouragements from the
Ministry of Education, China (NCET-10-0876). We appreciate Fun-
damental Research Funds for the Central Universities (CDJZR
10220006). We always thank the support from National Science
Foundation of China and Key Laboratory of Photochemical Con-
version and Optoelectronic Materials, TIPC, Chinese Academy of
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