Extensive spectral tuning of the proton transfer emission from green to red via a rational derivatization of salicylideneaniline

Article abstract of DOI:10.1016/j.cclet.2012.09.009

A series of salicylideneaniline derivatives 1a-1f were synthesized under mild condition in high yields, and characterized by ¹H NMR, HRMS, UV-vis and emission spectra. In solid and aprotic solvents 1a-1f exist mainly as E conformers that posses

Full text of DOI:10.1016/j.cclet.2012.09.009

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1279–1282
www.elsevier.com/locate/cclet
Extensive spectral tuning of the proton transfer emission from green
to red via a rational derivatization of salicylideneaniline
*
Ming Hui Luo, Hsing Yang Tsai, Hong Yi Lin, Sin Kai Fang, Kew Yu Chen
Department of Chemical Engineering, Feng Chia University, 40724 Taichung, Taiwan
Received 9 July 2012
Available online 23 October 2012
Abstract
A series of salicylideneaniline derivatives 1a–1f were synthesized under mild condition in high yields, and characterized by 1H
NMR, HRMS, UV–vis and emission spectra. In solid and aprotic solvents 1a–1f exist mainly as E conformers that possess a six-
membered-ring hydrogen bond and undergo excited-state intramolecular proton transfer (ESIPT) reactions, resulting in a proton-
transfer tautomer emission. Depending on the electronic donor or acceptor strength of the substituent in either the HOMO or LUMO
site, a broad tuning range of the emission from green (1c) to red (1a) has been achieved.
# 2012 Kew Yu Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: ESIPT; Schiff bases; Salicylideneaniline derivatives; DFT calculations
Schiff bases are aldehyde- or ketone-like compounds in which the carbonyl group is replaced by an imine or
azomethine group [1–6]. They are widely used as pigments and dyes [7–11], catalysts [12–14], liquid crystals [15–17],
intermediates in organic synthesis [18–22] and also exhibit a broad range of biological activities [23,24]. For example,
salicylideneaniline (1d, Scheme 1) derivatives are effective against Mycobacterium tuberculosis H37Rv, exhibiting an
MIC value of 8 mg/mL [25]. On the other hand, the excited-state intramolecular proton transfer (ESIPT) reaction of
salicylideneaniline derivatives has been investigated for past years [26,27], which incorporates transfer of a hydroxy
proton to the imine nitrogen through an intramolecular six-membered-ring hydrogen-bonding system. The resulting
proton-transfer tautomer possesses significant differences in structure and electronic configuration from its
corresponding normal species. Accordingly, a large Stokes shifted S01 ! S00 fluorescence (the prime sign denotes the
proton-transfer tautomer) was observed. This unusual photophysical property has found many important applications.
Prototypical examples are probes for solvation dynamics [28,29] and biological environments [30,31], fluorescence
microscopy imaging [32], near-infrared fluorescent dyes [33], photochromic materials [34], chemosensors [35–37]
and recent application in the field of organic light emitting diodes [38]. We now report the synthesis, characterization,
spectroscopic properties and complementary density functional theory (DFT) calculations of X-salicylidene-Y-aniline
compounds (1a–1c) with X = NO2 as an electron acceptor substituent and Y = OMe as an electron donor substituent.
* Corresponding author.
E-mail address: kyuchen@fcu.edu.tw (K.Y. Chen).
1001-8417/$ – see front matter # 2012 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.2012.09.009

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M.H. Luo et al. / Chinese Chemical Letters 23 (2012) 1279–1282
Scheme 1. The synthetic route of 1 and the structures of 1a–1f.
1. Results and discussion
Scheme 1 shows the synthetic route of 1 and the structures of the salicylideneaniline derivatives 1a–1f. These Schiff
bases were prepared through condensation reactions between substituted salicylic aldehydes and substituted anilines.
The structures of the products were characterized by 1H NMR spectroscopy and high resolution mass spectrometry
(HRMS) [39]. In the 1H NMR studies, the existence of a strong intramolecular hydrogen bond between O–H and N is
evidenced by the observation of a large downfield shift of the proton peak at d > 12 ppm for all compounds 1a–1f, the
values of which are in the order 1a (17.19 ppm) > 1b (17.10 ppm) > 1c (16.93 ppm) > 1d (13.26 ppm) > 1e
(12.60 ppm) > 1f (12.44 ppm) in dry CDCl3. The dominance of a E isomer for 1a–1f is strongly supported by DFT
geometry optimization (Fig. 1). These results are consistent with those of previous studies on other salicylideneaniline
derivatives [40,41].
Fig. 2 shows the absorption and emission spectra of 1a–1e in chloroform. For clarity, the absorption and emission
spectra of 1f are omitted in Fig. 2 because the difference between 1e and 1f is small. The absorption spectra of 1a–1f
Fig. 1. DFT (B3LYP/6-31G**) geometry-optimized structures and computed frontier orbitals of 1a–1e. The upper graphs are the LUMOs and the
lower ones are the HOMOs.

M.H. Luo et al. / Chinese Chemical Letters 23 (2012) 1279–1282
1281
Fig. 2. Normalized absorption (left) and emission (right) spectra of 1a (red line), 1b (blue line), 1c (cyan line), 1d (black line) and 1e (green line) in
chloroform solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Table 1
Calculated (DFT/B3LYP) parameters for salicylideneaniline derivatives 1a–1f.
Compounds
HOMO (eV)
LUMO (eV)
Eg (eV)
Compounds
HOMO (eV)
LUMO (eV)
Eg (eV)
1a
1b
1c
ꢁ5.88
ꢁ6.49
ꢁ6.87
ꢁ2.25
ꢁ2.51
ꢁ2.80
3.63
3.98
4.07
1d
1e
1f
ꢁ6.06
ꢁ6.15
ꢁ6.19
ꢁ2.11
ꢁ2.44
ꢁ2.37
3.95
3.71
3.82
are dominated by characteristic p–p* transitions at ꢀ350 nm (normal form). In addition to the higher energy
absorption at 350 nm, compounds 1a–1c exhibit a lower energy electronic transition at ꢀ460 nm (tautomer form). The
enol-imine (normal form) $ keto-amine (tautomer form) tautomeric equilibrium in the ground electronic state of
salicylideneaniline derivatives has also been reported recently [42]. As for the steady-state emission, the emission peak
greatly shifts from 510 nm in 1c to 615 nm in 1a in chloroform. The occurrence of ESIPT in 1d–1f is supported by the
anomalously large Stokes shifted emission with respect to the absorption peak wavelength. The tendency of the
spectral shift can be explained by the fact that the addition of electron-withdrawing groups (–NO2) at the phenol ring
(1b and 1c) decreases the HOMO energy level and hence increases the keto-tautomer energy gap, while the addition of
an electron-withdrawing group (–CN or –CF3) at the benzene ring (1e or 1f) decreases the LUMO energy level and
hence decreases the energy gap. Note that the substitution of three of the hydrogen atoms in 1c by three methoxy
groups at the benzene ring, forming 1a, increases the electron density on the benzene ring, so the HOMO (LUMO)
energy level of 1a is delocalized mainly on the benzene (salicylidene) moiety (Fig. 1). As a result, 1a has a higher
HOMO and a lower LUMO and hence a smaller energy gap relative to 1d (Table 1).
2. Conclusion
1
A series of salicylideneaniline derivatives 1a–1f have been synthesized and characterized by H NMR, HRMS,
UV–vis and emission spectra. Via a systematic derivatization of the excited-state intramolecular proton-transfer
system, salicylideneaniline, the proton-transfer emission can be extensively tuned from green (1c) to red (1a),
generating a new family of proton transfer fluorescent dyes.
Acknowledgment
The project was supported by the National Science Council (No. NSC 101-2113-M-035-001-MY2).
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