Spectroscopy study on the photochromism of Schiff bases N,N'-bis(salicylidene)-1,2-diaminoethane and N,N'-bis(salicylidene)-1,6-hexanediamine.

Article abstract of DOI:10.1016/S1386-1425(00)00353-X

The photochromism of Schiff bases N,N'-bis(salicylidene)-1,2-diaminoethane (BSE) and N,N'-bis(salicylidene)-1,6-cyclohexanediamine (BSH) was studied by steady-state and time-dependent fluorescence, UV Vis absorption spectroscopy and theoretical chemistry calculations. The experimental results show that BSH can perform the photochromism easier than BSE, may be due to the molecular topology difference.

Full text of DOI:10.1016/S1386-1425(00)00353-X

Spectrochimica Acta Part A 57 (2001) 149–154
www.elsevier.nl/locate/saa
Spectroscopy study on the photochromism of Schiff Bases
N,N%-bis(salicylidene)-1,2-diaminoethane and
N,N%-bis(salicylidene)-1,6-hexanediamine
Jianzhang Zhao a,b, Bing Zhao a,*, Juzheng Liu c, Weiqing Xu a,
Zongmu Wang d
a Key Laboratory of Supramolecular Structure and Spectroscopy, Jilin Uni6ersity, Changchun 130023, PR China
b Department of Chemistry, Jilin Uni6ersity, Changchun 130023, PR China
c Department of Chemistry and Chemical Engineering, Southeast Uni6ersity, Nanjing 210096, PR China
d College of Life Science, Jilin Uni6ersity, Changchun 130023, PR China
Received 11 May 2000; received in revised form 12 July 2000; accepted 12 July 2000
Abstract
The photochromism of Schiff bases N,N%-bis(salicylidene)-1,2-diaminoethane (BSE) and N,N%-bis(salicylidene)-1,6-
cyclohexanediamine (BSH) was studied by steady-state and time-dependent fluorescence, UV–Vis absorption
spectroscopy and theoretical chemistry calculations. The experimental results show that BSH can perform the
photochromism easier than BSE, may be due to the molecular topology difference. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Time-dependent spectroscopy; Photochromism; Salicylidineaniles; Molecular topology
of high-density information storage system, elec-
tro- or light-driven information display device,
1. Introduction
optical calculation and other non-linear optical
The 21st century will be a phonic century,
devices. Salicylidineaniles are one of the choices
during which the information will be transferred
owing to their photochromism caused by in-
and processed by a new kind of media: light. So,
tramolecular hydrogen transfer [1–3] (Scheme 1).
a whole family of functional compounds or mate-
We believe that the topology of the molecules
rial have to developed to meet the need in the filed
would affect their photochromism behavior exten-
sively, so we studied the photochromism of N,N%-
* Corresponding author. Present address: Key Laboratory
bis(salicylidene)-1,2-diaminoethane (BSE) and
of Supramolecular Structure and Spectroscopy, Jilin Univer-
N,N%-bis(salicylidene)-1,6-cyclo-hexanediamine
sity, Changchun 130023, People’s Republic of China. Fax:
(BSH), and the experimental results are consistent
with this hypothesis.
+86-431-8923907.
E-mail address: wqxu@mail.jiu.edu.cn (B. Zhao).
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00353-X

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J. Zhao et al. / Spectrochimica Acta Part A 57 (2001) 149–154
Scheme 1. Photochromism of salicylidineaniles.
2. Experimental
to elucidate the aggregation state of BSE and BSH
in solution and solid state, the UV–Vis spectra and
fluorescence spectra were determined first.
The Schiff bases BSE and BSH were synthesized
as shown in Scheme 2. All the reagents were
analytically pure and were purified before use.
UV–Vis absorption spectra were recorded with a
Shimadzu UV-3100 UV–VIS–NIR recording
spectrophotometer. The fluorescence spectra were
recorded with a Shimadzu RF-5301 PC spec-
trofluorophotometer. All the solutions were stored
in the dark for more than 72 h before use to
prevent photochromism caused by room light. A
light source of a 500 W Xenon lamp with grating
monochromator was used in the photochromism
study. The fluorescence quantum yield of BSE and
BSH in CHCl3 solution was determined with
quinine sulfate as reference [4].
The UV–Vis data of BSE and BSH (Table 1)
show that the absorption of casting films move
more bathochromically than that of solution. The
absorption peaks in chloroform, ethanol and ace-
tonitrile are almost the same, but the absorption in
tetrachloride carbon moves more bathochromi-
cally than that in the three solvents already men-
tioned and is almost the same as that of the casting
film. We found that the maxima absorption posi-
tions do not change accordingly when the concen-
tration of the chloroform solution was changed in
the range of 1×10−5 to 1×10−4 M. These results
infer that the molecules of BSE or BSH in dilute
chloroform, ethanol and acetonitrile solution are
independent, and there is no extensive interaction
between solute molecules.
3. Results and discussion
3.2. Time-dependent fluorescence spectra [7–10]:
3.1. Steady-state UV–Vis absorption and
photochromism of BSE and BSH
fluorescence spectra of BSE and BSH
The solution emission spectra of BSE and BSH
would change with irradiation; this is one manifes-
tation of their photochromic ability.
It is known that the aggregation state of the
compound would affect its character [5,6]. In order
Scheme 2. Synthesis of BSE and BSH.

J. Zhao et al. / Spectrochimica Acta Part A 57 (2001) 149–154
151
Table 1
UV–Vis absorption and fluorescence spectra data of BSE and BSHa
UV–Vis absorption spectra data
Fluorescence spectra data
CHCl3 C2H5OH CCl4
CHCl3
C2H5OH CH3CN CCl4
Casting film
Casting film
Solid
BSE
BSH
259
320
255
316
256
318
254
315
255
317
253
314
252
325
265
330
uex 276
uem 370
uex 283
uem 370
357
434
357
435
282
472
280
466
331
494
451
498
472
509
296
506
323
319
a Concentration of the solution: 5×10−5 M; substrate of the casting film: CaF2. uex and uem are optimal excitation and emission
wavelengths (nm).
The original emission peak at 370 nm for BSE
in chloroform solution will decrease with irradia-
tion and a new red-shifted peak centered at 460
nm appeared and intensified (Fig. 1). This is
similar for BSH (Fig. 2). The original emission
peak at 370 nm decreased slightly and the peak at
460 nm increased about ten times. The data in
Figs. 1 and 2 mean that the photochromic ability
of BSE and BSH in chloroform solution is differ-
ent; it is easier for BSH to perform the photo-con-
version, based upon the irradiation time needed in
the photochromism.
More attention should be paid to the pho-
tochromism of solid BSE and BSH if some poten-
tial practical use were considered. Therefore, we
also studied the photochromism of solid BSE and
BSH. The solid emission spectra of BSE would
change slightly when its crystal was irradiated
with light of 472 or 300 nm. However, the solid
emission spectra of BSH change extensively with
irradiation, and the process is so rapid that it is
difficult to monitor the spectral changes by the
usual time-dependent fluorescence spectroscopy.
We therefore use the ‘ time course’ technique to
study the change of the fluorescence intensity at a
fixed wavelength (Fig. 3). The data in Fig. 3 show
that the fluorescence intensity at 506 nm for BSH
decreases sharply with irradiation and becomes
stable after about 10 s of irradiation. But, for
BSE, no substantial change was observed.
decay kinetics is drastically different from that of
BSE. The decay kinetics of irradiated BSH is
nonlinear and is rapid (Fig. 5).
These results are consistent with that deduced
from Figs. 1 and 2, namely the fluorescent ability
of BSH is stronger than that of BSE, and the BSH
can perform the photochromism more quickly
than BSE (for the cause, vide infra)
3.3. Time-dependent UV–Vis spectra
In order to prove the presented change further,
we conducted the UV–Vis absorption spectra
study of the irradiated solution. The experimental
results show that the UV–Vis spectra of BSE or
BSH would change accordingly when it was irra-
diated (Figs. 6 and 7).
For BSE (Fig. 6), the original absorption peaks
centered at 259 and 320 nm decreased with irradi-
ation, and a new red-shifted absorption peak at
The decay kinetics of the irradiated BSE and
BSH are also studied by time-dependent fluores-
cence spectroscopy [6–8]. The results show that
decay kinetics of BSE is nearly linear and the
process is rather slow (Fig. 4). But, for BSH, the
Fig. 1. Solution emission spectral changes for BSE with irradi-
ation (in chloroform, 5×10−5 M; uex=276 nm). From a to l,
each irradiation time=100 s; total, 1100 s.

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J. Zhao et al. / Spectrochimica Acta Part A 57 (2001) 149–154
Fig. 2. Solution emission spectral changes for BSH with
irradiation (in chloroform, 5×10−5 M; uex=283 nm). From
a to i, each irradiation time=40 s; total, 320 s.
Fig. 4. Decay kinetics of the irradiated solid BSE (irradiation
time, 150 s; reading wavelength, 472 nm; monitor at 509 nm).
upon the irradiation time needed to perform the
photo-conversion.
291 and 369 nm appeared and increased. BSH
show similar change with irradiation (Fig. 7), the
original peaks at 255 and 316 nm decreased and
two new red-shifted peaks centered at 281 and 362
nm appeared and increased with irradiation. The
new generated absorption peaks at 291 nm (Fig.
6, BSE) and 281 nm (Fig. 7, BSH) may be at-
tributed to the keto form (extended a,b-unsatu-
rated ketone structure) [11]. The isobestic points
in Figs. 6 and 7 mean that an equilibrium system
presents in the solution; no other molecules are
involved in the photochromism process. This is
consistent with our knowledge of the pho-
tochromism of salicylidineaniles. It should be
pointed out again that it is easier for BSH to
conduct the photochromism than BSE, based
3.4. Molecular structure analysis: explanation for
the experimental results
Our experiment show that the spectroscopic
difference of BSE and BSH lie in two points.
First, the fluorescence intensity of BSH is stronger
than that of BSE. For example, the quantum yield
of BSE and BSH are 0.203 and 0.291, respec-
tively. It should be kept in mind that the chro-
mophore of BSE and BSH is almost the same.
Second, it is easier for BSH to perform the pho-
tochromism than BSE. These problems can be
solved by topology analysis of BSE and BSH
molecules. We put forward a temporary hypothe-
sis that the structure of enol form or the enol–
keto form (one proton transferred, intermediate
Fig. 3. Time course curves of solid BSE (a) and BSH (b): (a)
irradiation wavelength, 472 nm; monitored wavelength, 509
nm; (b) irradiation wavelength, 296 nm; monitored wave-
length, 506 nm.
Fig. 5. Decay kinetics of the irradiated solid BSH (irradiation
time, 150 s; reading wavelength, 296 nm; monitor at 506 nm).

J. Zhao et al. / Spectrochimica Acta Part A 57 (2001) 149–154
153
Scheme 3. Molecular structure analysis of the photochromism
of BSE and BSH.
BSH, however, a stronger repulsion presents be-
tween them. As a result, the first proton of
enol-BSH will be transferred easier than enol-
BSE. As soon as one proton was transferred
(driven by irradiation), the optimized enol–keto-
BSE show that the two chromophores became
closer to each other and interaction between them
is increased. Unfortunately, this strong attraction
is unfavorable to the photochromism because the
remaining proton is difficult to be transferred. For
BSH, this is the opposite of the analysis men-
tioned for BSE, namely only weaker attraction
presents in the one proton transferred form, the
enol–keto form, in which the two chromophores
become far away from each other. This structure
information may be used to explain our experi-
mental results.
Fig. 6. UV–Vis absorption spectral changes for BSE with
irradiation (in chloroform, 5×10−5 M; uex=276 nm). From
a to u, each irradiation time=200 s; total, 4000 s.
of the photochromism) is different from each
other for BSE and BSH (Scheme 3).
The structures pictured in Scheme 3 are optimal
ones with minimal energy (optimized with the
AM1 method). We will find several key points in
these structures (Table 2). First, the two chro-
mophores in enol-BSE are far away from each
other and there is no strong interaction between
them. For the two chromophores in the enol-
The difference of fluorescence intensity of solid
BSE and BSH may be caused by the rigidity of
the molecular skeleton. The enol form of BSH is
Table 2
Distant of the chromophores (shown by the distance of OꢀO)
,
for the structures in Scheme 3 (A)
Compounds
Enol form
Enol–keto form
Fig. 7. UV–Vis absorption spectral changes for BSH with
irradiation (in chloroform solution, 5×10−5 M; uex=283
nm). From a to y, each irradiation time=100 s; total, 2500 s.
BSE
BSH
8.639
3.882
4.654
5.048

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J. Zhao et al. / Spectrochimica Acta Part A 57 (2001) 149–154
Acknowledgements
a more rigid structure than BSE for the interac-
tion of the two chromophores, so its fluorescence
intensity is stronger than BSE, in which there is
no interaction between the two chromophores.
After one proton had been transferred, the ꢀOH
group remaining in the enol–keto form of BSE
became closer to the newly generated carbonyl
group, so it is difficult for the remaining proton to
transfer, due to the static charge pulling force.
But, for BSH, no such hydrogen bond exists in its
enol–keto form, because the two chromophores
became further away from each other than in the
enol form, and the remaining proton can be trans-
ferred freely. So, it is easier for BSH to perform
the photochromism completely than BSE. This
analysis is consistent with our experimental
results.
This work was supported by the National
Nature Science Foundation of China (29633010).
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bases BSE and BSH were studied by steady-state
and time-dependent fluorescence spectra and
UV–Vis absorption spectra. The experimental re-
sults show that BSH can perform the pho-
tochromism easier than BSE, perhaps due to the
molecular topology difference.
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