Excited-state charge coupled proton transfer reaction via the dipolar functionality of salicylideneaniline

Article abstract of DOI:10.1016/j.cclet.2013.01.011

Based on design and synthesis of salicylideneaniline derivatives (1a-1d), we demonstrate a prototypical system to investigate the excited-state intramolecular charge transfer (ESICT) coupled excited-state intramolecular proton transfer (ESIPT) reaction vi

Full text of DOI:10.1016/j.cclet.2013.01.011

Chinese Chemical Letters 24 (2013) 145–148
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Chinese Chemical Letters
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Original article
Excited-state charge coupled proton transfer reaction via the dipolar functionality
of salicylideneaniline
*
Tzu-Chien Fang, Hsing-Yang Tsai, Ming-Hui Luo, Che-Wei Chang, Kew-Yu Chen
Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history:
Based on design and synthesis of salicylideneaniline derivatives (1a–1d), we demonstrate a prototypical
system to investigate the excited-state intramolecular charge transfer (ESICT) coupled excited-state
intramolecular proton transfer (ESIPT) reaction via the dipolar functionality of the molecular framework.
In solid and aprotic solvents 1a–1d exist mainly as E conformers that possess an intramolecular six-
membered-ring hydrogen bond. Compounds 1a–1c exhibit a unique proton-transfer tautomer emission,
while compound 1d exhibits remarkable dual emission due to the different solvent-polarity
environment between ESICT and ESIPT states. Time-dependent density functional theory (TDDFT)
calculations are reported on these Schiff bases in order to rationalize their electronic structure and
absorption spectra.
Received 27 September 2012
Received in revised form 5 December 2012
Accepted 14 December 2012
Available online 4 February 2013
Keywords:
ESIPT
ESICT
Dipolar functionality
Salicylideneaniline derivatives
ß 2013 Kew-Yu Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
1. Introduction
carried out with femtosecond techniques to investigate the
dynamics of the ESIPT reactions. Based on the chemical design,
Schiff bases are some of the most widely used organic
compounds [1–6]. They are used as dyes and pigments [7–11],
catalysts [12–14], liquid crystals [15–17], and intermediates in
organic synthesis [18–22]. Moreover, several Schiff bases exhibit a
broad range of biological activities [23]. For example, salicylide-
neaniline derivatives are effective against Mycobacterium tubercu-
losis H37Rv [24]. The excited-state intramolecular proton transfer
(ESIPT) reaction of salicylideneaniline derivatives [25,26], which
incorporates transfer of a hydroxy proton to the imine nitrogen
through an intramolecular six-membered-ring hydrogen-bonding
system, has been investigated in the past. The proton transfer dyes
have found many important applications. Typical examples
include probes for solvation dynamics [27] and biological
environments [28], photochromic materials [29], chemosensors
[30–32], and organic light-emitting diodes [33]. Furthermore,
several relevant examples have recently been explored by
anchoring the ESIPT molecules with a strong electron-donating
substituent such as dialkylamines, so that upon Franck–Condon
excitation, excited-state intramolecular charge transfer (ESICT)
may take place. In an effort to probe this fundamental issue, studies
of the ESIPT reaction in 2-hydroxy-4-(di-p-tolylamino)benzalde-
hyde and N,N-dialkyllamino-3-hydroxyflavone [34] have been
we herein demonstrate a practical approach to investigate the
ESICT/ESIPT coupled reaction via the dipolar functionality of X-
salicylidene-Y-aniline compounds with X = N(CH3)2 as an electron
donor substituent and Y = CN as an electron acceptor substituent.
2. Experiment
The general procedure for the synthesis of Schiff bases (1a–1d):
Nonsubstituted/substituted aniline (5.4 mmol) and formic acid
(0.1 mL) were added to a mixture of nonsubstituted/substituted 2-
˚
hydroxybenzaldehyde (4.7 mmol) and molecular sieves 4 A (0.5 g)
in ethanol (25 mL) at room temperature. The mixture was refluxed
for 12 h. After cooling, the mixture was poured into the cold water
and extracted with CH2Cl2 and dried with anhydrous MgSO4. After
solvent was removed, the crude product was purified by silica gel
column chromatography with eluent CH2Cl2 to produce the title
compounds (1a–1d) (Scheme 1) in 90% yield. Characterization
data: 1a: 1H NMR (400 MHz, CDCl3):
d 13.26 (br, 1H), 8.62 (s, 1H),
7.36–7.44 (m, 4H), 7.30–7.25 (m, 3H), 7.04 (d, 1H, J = 8.5 Hz), 6.96
(t, 1H, J = 7.5 Hz); MS (FAB) m/z (relative intensity) 198 (M+H+,
100); HRMS calcd. for C13H12NO 198.0919, found 198.0916. 1b: 1H
NMR (400 MHz, CDCl3):
d 12.60 (br, 1H), 8.59 (s, 1H), 7.71 (d, 2H,
J = 8.0 Hz), 7.44 (m, 2H), 7.33 (d, 2H, J = 8.0 Hz), 7.03 (d, 1H,
J = 8.5 Hz), 6.98 (t, 1H, J = 8.0 Hz); MS (FAB) m/z (relative intensity)
223 (M+H+, 100); HRMS calcd. for C14H11N2O 223.0871, found
* Corresponding author.
223.0875. 1c: 1H NMR (400 MHz, CDCl3):
d 13.85 (br, 1H), 8.42
E-mail address: kyuchen@fcu.edu.tw (K.-Y. Chen).
1001-8417/$ – see front matter ß 2013 Kew-Yu Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.01.011

146
T.-C. Fang et al. / Chinese Chemical Letters 24 (2013) 145–148
Scheme 1. The synthetic route of 1 and the structures of 1a–1d.
(s, 1H), 7.15–7.40 (m, 6H), 6.26 (dd, 1H, J1 = 8.5 Hz, J2 = 2.0 Hz), 6.19
(s, 1H), 3.42 (q, 4H, J1 = 7.0 Hz), 1.23 (t, 6H, J = 7.0 Hz); 13C NMR
solely a long wavelength emission (>540 nm) in cyclohexane.
Fig. 1 clearly shows a large separation of the energy gap between
the 0–0 onset of the absorption and emission. The Stokes shift of
the emission, defined by peak (absorption)-to-peak (emission) gap
in terms of frequency, is calculated to be >8500 cmꢀ1 for 1a–1c.
Accordingly, the assignment of 540–575 nm emission for 1a–1c in
cyclohexane to a proton-transfer tautomer emission is unambigu-
ous, and ESIPT takes place from the phenolic proton O–H to the
imine nitrogen, forming the keto-amine tautomeric species.
Fig. 2 shows the absorption and emission spectra of 1d in
various solvents. It is apparent that the absorption spectrum of 1d
is a close resemblance to that of 1c. Despite the similarity in
absorption spectra, in which the S0 ! S1 peak wavelengths are
both located at 370–390 nm for 1c and 1d, the corresponding
emission reveals remarkable differences. In contrast to a unique
proton-transfer emission in 1c, dual emission maximized at 445
and 573 nm was observed for 1d in cyclohexane. The 573 nm band
can clearly be assigned to the keto-amine emission resulting from
ESIPT, while the 445 nm band is absent in 1c and is accordingly
ascribed to the fluorescence of the enol-imine species.
As shown in Fig. 2, similar dual emission was observed for 1d in
other solvents. However, as the solvent polarity increases, the
intensity ratio for the keto-amine versus enol-imine intensity
decreases, approaching zero in acetonitrile. While the peak
wavelength of the keto-amine emission remains unchanged, the
enol-imine emission of 1d reveals appreciable solvent-polarity
dependence, being shifted from 445 nm in cyclohexane to 489 nm
in acetonitrile. The fluorescence spectra shift significantly to the
red upon increasing solvent polarity, indicating an intramolecular
charge transfer characteristic for the excited state of 1d. In view of
the high electron affinity of the cyano group and the low ionization
potential of the diethylamino group, the occurrence of ESICT in 1d
is expected.
(100 MHz, CDCl3):
d 164.27, 160.41, 151.80, 148.77, 133.70,
129.21, 125.42, 120.73, 109.08, 103.72, 97.78, 44.55, 12.67; MS
(FAB) m/z (relative intensity) 269 (M+H+, 100); HRMS calcd. for
C
17H21N2O 269.1654, found 269.1650. Selected data for 1d: 1H-
NMR (400 MHz, CDCl3):
d 13.28 (br, 1H), 8.41 (s, 1H), 7.66 (d, 2H,
J = 8.5 Hz), 7.28 (d, 2H, J = 8.5 Hz), 7.18 (d, 1H, J = 9.5 Hz), 6.29 (dd,
1H, J1 = 9.5 Hz, J2 = 2.5 Hz), 6.18 (d, 1H, J = 2.5 Hz), 3.44 (q, 4H,
J = 7.5 Hz), 1.24 (t, 6H, J = 7.5 Hz); 13C NMR (100 MHz, CDCl3):
d
164.07, 162.13, 153.06, 152.54, 134.40, 133.38, 121.62, 119.12,
108.89, 108.21, 104.31, 97.52, 44.67, 12.65; MS (FAB) m/z (relative
intensity) 294 (M+H+, 100); HRMS calcd. for C18H20N3O 294.1606,
found 294.1603.
3. Results and discussion
Scheme 1 shows the synthetic route of 1 and the structures of
compounds 1a–1d. These Schiff bases were prepared through
condensation reactions between substituted salicylic aldehydes
and substituted anilines [35]. The dominance of a E isomer for 1a–
1d, namely intramolecular hydrogen-bond formation between O–
H and N, is firmly confirmed by single-crystal X-ray analyses [36–
39]. In good agreement with these observations, the 1H NMR
spectra (in CDCl3) revealed significantly downfield signals at d > 12
for all compounds 1a–1d, giving a clear indication of the strong
hydrogen-bond formation.
Fig. 1 shows the absorption and emission spectra of 1a–1c in
cyclohexane. Compound 1a exhibits a S0 ! S1 electronic transition
at 340 nm. The longest wavelength absorption band of 1b (1c) is
slightly red-shifted relative to that of 1a; this can be explained by
the fact that the addition of an electron-withdrawing group
(electron-donating group) at the benzene (phenol) ring decreases
(increases) the LUMO (HOMO) energy level and hence decreases
the energy gap. As for the steady-state emission, 1a–1c all exhibit
To gain more insight into the electronic properties of
1a–1d, quantum mechanical calculations were performed using
Fig. 1. Normalized absorption (left) and emission (right) spectra of 1a, 1b and 1c in cyclohexane.

T.-C. Fang et al. / Chinese Chemical Letters 24 (2013) 145–148
147
Fig. 2. Normalized absorption (left) and emission (right) spectra of 1d in cyclohexane, diethyl ether, dichloromethane and acetonitrile.
Fig. 3. Computed frontier orbitals of 1a–1d. The upper graphs are the LUMOs and the lower ones are the HOMOs.
Table 1
emission, while compound 1d exhibits remarkable dual emission
due to the different solvent-polarity environments between ESICT
and ESIPT states. The results make further rational design of the
ESICT/ESIPT coupled systems feasible simply by modifying the
molecular structures.
Calculated (TDDFT/B3LYP) and experimental parameters for 1a–1d.
HOMOa
LUMOa
Band gapa
fa
Eg
b
Compound
1a
1b
1c
1d
ꢀ6.02
ꢀ6.11
ꢀ5.25
ꢀ5.29
ꢀ2.39
ꢀ2.63
ꢀ1.81
ꢀ1.99
3.63
3.48
3.44
3.30
0.3537
0.3548
1.0334
1.1569
3.66
3.57
3.40
3.18
Acknowledgment
f is the oscillator strength for the lowest energy transition.
a
TDDFT/B3LYP calculated values.
We thank the National Science Council (No. NSC 101-2113-M-
035-001-MY2) for the financial support.
b
At absorption maxima (Eg = 1240/lmax, in eV).
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