Photochromism of Schiff base compounds derived from N,N'-bis(2-aminophenyl) isophthalamide: Structure and photosensitivity

Article abstract of DOI:10.1016/j.molstruc.2010.06.008

The photochromic behaviors of four Schiff bases derived from N,N'-bis(2-aminophenyl)isophthalamide were studied to reveal the substituent effect on the photosensitivity. On UV radiation, all of compounds 1-4 exhibit photochromic behavior in solution throu

Full text of DOI:10.1016/j.molstruc.2010.06.008

Journal of Molecular Structure 977 (2010) 274–278
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Photochromism of Schiff base compounds derived from
N,N0-bis(2-aminophenyl)isophthalamide: Structure and photosensitivity
b
a
a
a
a
Qinghua Wang a, , Lizhen Cai , Fei Gao , Qiurong Zhou , Fengping Zhan , Qingxiang Wang
*
a Department of Chemistry and Environmental Science, Zhangzhou Normal University, Zhangzhou 363000, PR China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China
a r t i c l e i n f o
a b s t r a c t
The photochromic behaviors of four Schiff bases derived from N,N0-bis(2-aminophenyl)isophthalamide
were studied to reveal the substituent effect on the photosensitivity. On UV radiation, all of compounds
1–4 exhibit photochromic behavior in solution through intramolecular hydrogen atom transfer, and the
tautomerism equilibrium moved toward the keto-form. The solution and solid state fluorescence were
also studied. After irradiated by 254 nm UV light, the solution fluorescence emission of compounds 1
and 4 went down, but the fluorescence emission intensified for compound 3. The solid state fluorescence
(ss-FL) emission of compound 1 went down after UV irradiation. However, the ss-FL emission of 3 and 4
did not show any observable difference before and after UV irradiation. The influences of solvent, wave-
length of irradiation and acidity on the adsorption spectra of the title compounds were also investigated.
Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
Article history:
Received 2 April 2010
Received in revised form 6 June 2010
Accepted 7 June 2010
Available online 12 June 2010
Keywords:
Photochromism
Schiff base
Vanillin
Fluorescence
Deprotonation
1. Introduction
2. Experimental method
Photochromic compounds, which can switch reversibly be-
tween two forms by light excitation at certain wavelength, have
great potential for optoelectronic applications such as optical
memory and switches [1–9]. During the past decades, many types
of photochromic compounds have been synthesized and character-
ized, such as diarylethenes, fulgides and azobenzene. Among these,
the Schiff bases are one of the most important categories. The typ-
ical photochromic behavior of Schiff bases is the photo-induced
alteration of molecular structure based on inherent enol-keto tau-
tomerism (Scheme 1) [10–14]. In general, the ortha-hydroxyl
imine unit in molecular structure is a prerequisite for the tautom-
erism of Schiff bases [15]. However, Herzfeld pointed out that the
enol-keto tautomerism may also exist in para-hydroxyl imine type
Schiff bases through solvent-conducted proton transfer [16]. Many
photochromic products of Schiff bases are unstable in solution and
only transient adsorption spectra can be detected by laser flash
photolysis technique [17]. To investigate the substituent effect on
the photosensitivity, four Schiff bases were synthesized and their
photochromic behaviors were studied. Schiff base compounds 1–
4 were prepared by the usual condensation of aldehyde with
N,N0-bis(2-aminophenyl)isophthalamide (Scheme 2).
2.1. Reagents and apparatus
Chemicals were purchased from ACROS and used without fur-
ther purification, except for chloroform was distilled before use.
The 1H NMR spectra were recorded on a 400 MHz Bruker Avance
III spectrometer, with DMSO-d6 as solvent. ESI-MS or APCI-MS
spectra were recorded on a ThermoFinnigan Decax-30000 ma-
chine. Elemental analyses were performed on a Elementar Vario-
III elemental analyzer. IR spectra were measured on Nicolet Avatar
360 spectrometer as KBr discs. The UV–vis adsorption spectra were
recorded on a XinMao UV-750PC spectrophotometer. Fluorescence
spectra were recorded on a Perkin-Elmer LS-55 luminescence
spectrophotometer.
2.2. Material and preparation
2.2.1. Synthesis of N,N0-bis(2-aminophenyl)isophthalamide (5)
Compound 5 has been reported in previous paper [18], here it
was synthesized by a different route. Isophthalic acid (0.33 g,
2 mmol) and o-phenylenediamine (1.73 g, 16 mmol) were dis-
solved in a mixed solvent of 10 ml of DMF and 10 ml of CH2Cl2.
The mixture was placed in an ice bath and then NMM (1.1 ml,
10 mmol), HOBt (0.65 g, 4.8 mmol) and EDC (0.92 g, 4.8 mmol)
were added successively. The mixture was stirred under ice bath
for one more hour, and then stirred under room temperature for
24 h. To the mixture 10 ml of CH2Cl2 was added, and then the
* Corresponding author. Tel./fax: +86 596 2591445.
E-mail address: wqh_1974@yahoo.com.cn (Q. Wang).
0022-2860/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.06.008

Q. Wang et al. / Journal of Molecular Structure 977 (2010) 274–278
275
AOCH3); APCI-MS (ꢀ): m/z = 613.4 ([MAH]ꢀ). Anal. Calcd.% for
O
C
36H30N4O6: C, 70.35; H, 4.92; N, 9.12. Found C, 70.21; H, 4.83;
O
N
O
N
N, 9.26.
Experimental data for 4: compound 4 was prepared from 5 and
H
H
H
hv1
hv2
or
N
2-hydroxy-1-naphthaldehyde in 63% yield. mp: 283–284 °C; 1H
NMR (400 MHz, DMSO-d6) d: 10.40 (s, 1H, AOH), 9.77 (s, 2H,
ANH), 9.56 (d, 1H, AOH), 8.76(s, 1H, CH@N), 8.59 (s, 1H, CH@N),
8.43–6.60 (m, 24H, ArH); ESI-MS (ꢀ): m/z = 653.7 ([MAH]ꢀ). Anal.
Calcd.% for C42H30N4O4: C, 77.05; H, 4.62; N, 8.56. Found C, 77.46;
H, 4.53; N, 8.43.
H
H
H
cis-keto form
enol-imine form
trans-keto form
Scheme 1.
solution was washed with water for three times. The organic layer
was collected and the CH2Cl2 was removed by vacuum. The residue
was washed by 5% NaHCO3, water and 30% ethanol successively to
give the pure 5. Yield 71%; mp: 201–202 °C. ESI-MS (ꢀ): m/
z = 345.1 ([MAH]ꢀ).
3. Results and discussion
3.1. Spectral studies of the compounds under light irradiation
The irradiation time-dependent adsorption spectra of com-
pounds 1–4 were shown in Fig. 1. The irradiation to the solution
of 1 with 254 nm UV light turned the colorless solution into yellow,
and a new adsorption band appeared near 400 nm and intensified
with irradiation time (Fig. 1a). This adsorption band was attributed
to the trans-keto-form. Concurrently, the adsorption band around
2.2.2. General procedure for the synthesis of compounds 1–4
Compound 5 (0.069 g, 0.2 mmol) and a corresponding aldehyde
(0.4 mmol) were dissolved in 15 mL of ethanol and the mixture
was refluxed for 6 h, the precipitate was filtered and washed by
ethanol.
275 nm aroused from the
p ?
pꢁ transition of imine unit of enol-
Experimental data for 1: compound 1 was prepared from 5 and
2-hydroxybenzaldehyde in 59% yield. mp: 213–215 °C; IR(KBr,
cmꢀ1): 3276, 1664, 1653, 1613, 1521, 1447, 756; 1H NMR
(400 MHz, DMSO-d6) d: 12.85 (s, 2H, AOH), 10.23 (s, 2H, ANH),
8.93 (s, 2H, ACH@N), 8.53 (s, 1H, ArH), 8.16 (d, J = 7.6 Hz, 2H,
ArH), 7.72–7.66 (m, 5H, ArH), 7.50–7.48 (m, 2H, ArH), 7.38–7.34
(m, 6H, ArH), 6.94 (t, J = 7.2 Hz, 2H, ArH), 6.85 (d, 2H, J = 8.4 Hz,
ArH); ESI-MS (+): m/z = 555.1 ([M + H]+). Anal. Calcd.% for
form went down with irradiation time [19]. The four clear isos-
bestic points at 254, 309, 359 and 368 nm, respectively indicated
the presence of a two-component equilibrium system in the solu-
tion. When irradiated by 365 nm UV light, the solution of com-
pound 1 turned into light-yellow in color, and then changed back
to colorless as soon as the light was off. The adsorption spectra
after irradiation did not have any changes. The 365 nm light may
have induced compound 1 to switch from enol to cis-keto-form,
which was unstable and readily switched back to enol-form
through intramolecular proton transfer. For compound 2, irradia-
tion with 254 nm light also generated a new adsorption peak at
384 nm. Yet, this adsorption peak intensified within 15 s of irradi-
ation time, and then diminished with time (Fig. 1b). This indicated
that the keto-form of compound 2 was unstable in the chloroform
solution. We thought this to be caused by the AOH presence on the
para-site of the imine unit. Therefore compound 2 was unable to
take the trans-keto-form (Scheme 2). Furthermore, the hydrogen
bond between the hydroxyl H and the methoxyl O in enol-form
of compound 2 may have made the tautomerism equilibrium
moved toward the enol-imine direction. There was no isosbestic
point for compound 2, indicating the existence of a multi-compo-
nents equilibrium system in the solution. The keto-form of both
1 and 2 would switch back to enol-form after reflux in chloroform.
On irradiation with UV light, the solution of 3 produced a new
adsorption band around 430 nm which may be attributed to the
C
34H26N4O4: C, 73.63; H, 4.73; N, 10.10. Found C, 73.51; H, 4.90;
N, 10.19.
Experimental data for 2: compound 2 was prepared from 5 and
Vanillin in 65% yield. mp: 215–216 °C; IR(KBr, cmꢀ1): 3373, 1666,
1590, 1516, 1448, 1290; 1H NMR (400 MHz, DMSO-d6) d: 10.07
(s, 2H, AOH), 9.83 (s, 2H, ANH), 8.65 (s, 2H, ACH@N), 8.58 (s, 1H,
ArH), 8.28–8.20 (m, 4H, ArH), 7.72–7.66 (m, 3H, ArH), 7.44–7.39
(m, 4H, ArH), 7.31 (t, J = 7.2 Hz, 2H, ArH), 7.23 (t, J = 7.2 Hz, 2H,
ArH), 6.88 (d, J = 8.0 Hz, 2H, ArH), 3.79 (s, 6H, AOCH3); APCI-MS
(+): m/z = 615.3 ([M + H]+). Anal. Calcd.% for C36H30N4O6: C,
70.35; H, 4.92; N, 9.12. Found C, 70.46; H, 4.81; N, 9.30.
Experimental data for 3: 3 was prepared from 5 and 2-hydroxy-
3-methoxybenzaldehyde in 70% yield. mp: 221–223 °C; 1H NMR
(400 MHz, CDCl3) d: 14.47 (s, 2H, AOH), 9.27 (s, 2H, ANH), 8.84
(s, 1H, ArH), 8.44 (d, 2H, CH@N), 8.25 (d, 2H, ArH), 7.92(s, 2H,
ArH), 7.74–7.70 (t, 1H, ArH), 7.44–7.40 (t, 2H, ArH), 7.27–7.19
(m, 4H, ArH), 6.76–6.72(t, 2H, ArH), 6.53 (d, 4H, ArH), 2.95 (s, 6H,
Scheme 2.

276
Q. Wang et al. / Journal of Molecular Structure 977 (2010) 274–278
2.0
1.8
1.6
1.4
1.2
(a)
(c)
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
0
1
2
3
4
5
0
1
3
4
2
1.0
5
0.8
0.6
0.4
5
0.2
0.0
0
240
290
340
390
440
490
250
300
350
400
450
500
Wavelength /nm
Wavelength /nm
(d)
2.000
1.800
1.600
1.400
1.200
1.000
0.800
0.600
0.400
0.200
0.000
2.500
2.000
1.500
1.000
0.500
0.000
0
0
1
2
3
4
5
6
7
1
2
3
4
5
6
7
0
7
7
0
245
295
345
395
445
495
245
345
445
545
Wavelength /nm
Wavelength /nm
Fig. 1. UV–vis adsorption spectral changes for compounds 1–4 in CHCl3 solution (5 ꢂ 10ꢀ5 mol/l) with 254 nm UV light irradiation. Curve 0, before irradiation. (a) Compound
1, irradiation time gap is 10 s; (b) compound 2, the time gap is 5 s; (c) compound 3, the time gap is 20 s; (d) compound 4, the time gap is 10 s;
n ? pꢁ transition of keto-form (Fig. 1c) [19], however the intensity
of the new generated band is much more weak than that of com-
pound 1. In compound 3 the hydroxy H atoms formed hydrogen
bonds with the adjacent methoxyl, so it is much more difficult
for the intramolecular proton transfer to take place in 3 than in
compound 1.
(a)1000
800
600
1a
1a'
400
200
0
1b'
1b
200
300
400
500
600
700
800
Wavelength (nm)
(b) 1000
(c)
600
1
2
3
800
600
400
200
0
3
400
200
0
4
2
1
400
450
500
550
600
650
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2. Solution fluorescence spectra with 254 nm UV light irradiation (5 ꢂ 10ꢀ5 mol/l in CHCl3) (a) compound 1. Left, excitation spectra; right, emission spectra; 1a and 1a0
are before UV irradiation; 1b and 1b0 are after UV irradiation for 1 min; (b) compound 3. kex = 346 nm. 1, before irradiation; 2, 3, after UV irradiation for 1 and 2 min,
respectively; (c) compound 4. kex = 484 nm. 1, before irradiation; 2, 3, 4, after the irradiation of UV light for 1, 2 and 5 min, respectively.

Q. Wang et al. / Journal of Molecular Structure 977 (2010) 274–278
277
For compound 4, there are four adsorption bands around 319,
371, 442 and 476 nm. The adsorptions at 319 and 476 nm may
be assigned to the enol-form, while the adsorptions at 371 and
442 nm may be assigned to the keto-form. The absorption bands
of the keto-form can be detected before UV irradiation, which
may be due to the large dissociation constant of the naphthol hy-
droxyl. When irradiated by UV light, the enol-adsorption bands
around 319 and 476 nm diminished, while the keto-adsorption
bands around 371 and 436 nm intensified with irradiation time
(Fig. 1d). There are three clear isosbestic points at 294, 350 and
470 nm. The UV light caused the enol-keto tautomerism equilib-
rium moved toward the keto-form.
were unstable in ethanol and the tautomerism equilibrium readily
moved toward the enol-imine form.
The solution and solid state fluorescence spectra of the title com-
pounds were also taken and studied (Figs. 2 and 3). Compound 1 was
firstly excited at 362 nm and it had an intense emission at 534 nm.
Then the solution was irradiated by a 254 nm UV lamp (20 W) for
one minute and the fluorescence spectrum was taken again. The
emission at 534 nm went down and blue-shifted to 530 nm. At the
same time, the excitation spectra split into two peaks (Fig. 2a). We
thought the OAHꢃꢃꢃN hydrogen bond would no longer be retained
in the trans-keto-form (Scheme 1) and the rigidity of the molecular
conjugated plane decreased. Therefore the emission intensity de-
creased. Similar to compound 1, the fluorescence emission of com-
pound 4 in solution also went down after being irradiated with UV
light (Fig. 2c). However, for compound 3, the solution fluorescence
emission intensity increased upon UV irradiation, at the same time
the maximum emission wavelength blue-shifted to 540 nm
(Fig. 2b). In compound 3, the electron pushing methoxyl adjacent
to the hydroxyl might stabilize the excited state of the keto-form
and resulted in the increase of fluorescence emission.
Compared with 1, 3 and 4, the fluorescence emission of com-
pound 2 was very weak (not show here). We believed that the pre-
requisite for strong fluorescence emission was that the hydroxyl on
the ortho-site of the imine unit, which might help in retaining the
rigidity of the molecular structure through intramolecular OAHꢃꢃꢃN
hydrogen bond.
The solid state fluorescence emission of compound 1 went
down after being irradiated with UV light (Fig. 3). However, the so-
lid state fluorescence spectra of 3 and 4 did not show any observa-
ble change upon UV irradiation. We believed that in 3, the
methoxyl involved in intermolecular hydrogen bonding, while in
The UV–vis spectral studies of the title compounds under UV
light irradiation were also conducted in ethanol. The results indi-
cated that all of the compounds were non-photochromic in etha-
nol. We supposed that the keto-form of the title Schiff bases
800.0
700
1a'
600
500
1b'
400
300
200
100
0.0
450.0
500
600
700.0
nm
Fig. 3. Solid state fluorescence emission spectra of compound 1 (kex = 412 nm). 1a0
is before irradiation and 1b0 is after UV irradiation for 3 min.
(a)
(b)
2.500
2.500
2.000
1.500
2.000
2
1.500
1.000
0.500
0.000
2
1.000
0.500
0.000
1
3
1
3
200
300
400
Wavelength /nm
500
600
200
300
400
Wavelength /nm
500
600
(c)
(d)
2.500
2.000
1.500
1.000
0.500
0.000
3.000
2.500
2.000
1.500
1.000
0.500
0.000
2
1
2
3
1
3
200
300
400
500
600
200
300
400
500
600
Wavelength /nm
Wavelength /nm
Fig. 4. UV–vis adsorption spectral changes with titration by base or acid (a) compound 1; (b) compound 2; (c) compound 3; (d) compound 4; 1, Schiff base; 2, 1 + NaOH; 3,
1 + HCl.

278
Q. Wang et al. / Journal of Molecular Structure 977 (2010) 274–278
4 the naphthyl group involved in the intermolecular
p
–
p
stacking
controlled the shift of the enol-keto tautomerism equilibrium. The
results indicated that when the naphthol hydroxyl was presented
in the molecular structure of 4, keto-form could be detected even
before UV irradiation; however when methoxyl was presented in
compound 3, the enol-form was much more favored. Furthermore,
the methoxyl present in meta-position will enhance the fluores-
cence emission of keto-form. The influences of acidity on the
adsorption spectra were also studied. The results indicated that
when titrated by base, the anionic forms of the Schiff bases gener-
ated new adsorption bands at longer wavelength, which are quite
similar to that of keto-forms induced by UV irradiation.
interaction. Thus, the strong intermolecular interaction in 3 and 4
prohibited the molecular conformation change upon UV irradia-
tion. So the solid state fluorescence spectra did not change after
UV irradiation.
3.2. Deprotonation–protonation
The phenolic group and imine unit are quite sensitive to base
and acid. So, the influences of acidity on the UV–vis adsorption
spectra of the title Schiff bases were also studied (Fig. 4). When ti-
trated with NaOH, all of the four compounds generated a new
adsorption band at longer wavelength (1 at 405 nm, 2 at 391 nm,
3 at 413 nm and 4 at 430 nm), and these new generated adsorption
bands are assigned to the deprotonated forms of the title Schiff
bases. The maximum wavelength adsorption bands of the deproto-
nated forms are quite similar to that of keto-forms induced by UV
irradiation (also see Fig. 1). On the other hand, when titrated with
HCl, for all the compounds the adsorption bands moved to shorter
wavelength with the absorbance diminished. The striking adsorp-
tion spectral changes may be caused by the protonation of the
imine unit (ACH@NH+A).
Acknowledgements
This work was supported by Natural Science Foundation of Fuj-
ian, PR China (2008J0237).
References
[1] T. Fujiwara, J. Harada, K. Ogawa, J. Phys. Chem. B 108 (2004) 4035.
[2] A. Nakano, T. Yamazaki, A. Osuka, Chem. Eur. J. 6 (2000) 3254.
[3] D. Gust, T.A. Moore, A.L. Moore, Acc. Chem. Res. 34 (2001) 40.
[4] M. Engeser, L. Fabbrizzi, M. Vlicchelli, D. Vsacchi, Chem. Commun. (1999) 1191.
[5] K. Ogawa, J. Harada, J. Mol. Struct. 647 (2003) 211.
[6] H. Tian, S. Yang, Chem. Soc. Rev. 33 (2004) 85.
4. Conclusions
[7] F. Lo, J. Kuo, N. Teeng, Jpn. J. Appl. Phys. Part . 43 (2004) 4852.
[8] X. Zhou, F. Zhang, Z. Tung, Chem. Lett. 33 (2004) 1006.
[9] V. Yam, C. Ko, N. Zhu, J. Am. Chem. Soc. 126 (2004) 12734.
[10] K. Amimoto, T.J. Kawato, Photochem. Photobiol. C 6 (2005) 207.
[11] G.R. Desiraju, Angew. Chem. Int. Ed. Engl. 34 (1995) 2311.
[12] A. Elmali, M. Kabak, E. Kavlakoglu, Y. Elerman, T.N. Durlu, J. Mol. Struct. 510
(1999) 207.
[13] L. Zhao, Q. Hou, D. Sui, Y. Wang, S. Jiang, Spectr. Acta Part A 67 (2007) 1120.
[14] H. Fukuda, K. Amimoto, H. Koyama, T. Kawato, Tetrahedron Lett. 50 (2009)
5376.
[15] L. Antonov, S. Kawauchi, M. Satoh, Dyes Pigm. 38 (1998) 157.
[16] R. Herzfeld, P. Nagy, Curr. Org. Chem. 5 (2001) 373.
[17] A. Ohsima, A. Monotake, T. Arai, J. Photochem. Photobiol. A 162 (2004) 473.
[18] X. Bao, J. Yu, Y. Zhou, Sens. Actuators, B 140 (2009) 467.
[19] Y. Hong, J. Xiao, S. Chen, W. Wang, K. Huang, Chin. J. Org. Chem. 28 (2008)
1404.
In this paper four Schiff bases have been synthesized and their
photochromic behaviors were studied to reveal the substituent ef-
fect on the photosensitivity. The influences of the wavelength of
UV irradiation, solvent and acidity on the photochromic properties
were also investigated. The results indicated that all of the com-
pounds were photochromic in chloroform on irradiation with
254 nm UV light. However, upon 365 nm UV irradiation, none of
the compounds displayed photochromic property. The solvent also
played a pivotal role on the photochromic behavior; all the title
compounds were photochromic in chloroform but non-photochro-
mic in ethanol. The substituent group closed to the imine unit also