Lewis acid and base triggered molecular switch

Article abstract of DOI:10.1039/b905884b

A new kind of trigger for molecular switch has been developed using a photochromic spirooxazine derivative as a template system. It is found that the ring-opening and ring-closing interconversion of spirooxazine derivative can be performed by Lewis acid a

Full text of DOI:10.1039/b905884b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Lewis acid and base triggered molecular switch
a
Kunpeng Guoab and Yi Chen
*
Received 24th March 2009, Accepted 22nd May 2009
First published as an Advance Article on the web 25th June 2009
DOI: 10.1039/b905884b
A new kind of trigger for molecular switch has been developed using a photochromic spirooxazine
derivative as a template system. It is found that the ring-opening and ring-closing interconversion of
spirooxazine derivative can be performed by Lewis acid and base trigger instead of photoinducement.
With Lewis acid trigger, the ring-closing species converts to ring-opening species via formation of
a complex, and the complexed ring-opening species is thermal stable and photoinactive. Meanwhile, the
complexed ring-opening species converts completely back to ring-closing species by decomplexation
with Lewis base trigger, and the interconversion between ring-opening and ring-closing species can be
cycled.
ring-closing interconversion. In this paper, we report a new kind
Introduction
of trigger for ring-opening and ring-closing interconversion of
spirooxazine derivative. It is found that the ring-opening and
ring-closing interconversion of spirooxazine derivative can be
performed by Lewis acid and base trigger via complexing and
decomplexing. Moreover, the inhibited interconversion between
the ring-opening and ring-closing species of spirooxazine deriv-
ative in some solvents by phototrigger can be accomplished with
Lewis acid and base trigger, which provides a promising way of
manipulating ring-opening and ring-closing interconversion for
spirooxazine derivatives. The principle of ring-opening and ring-
closing interconversion with Lewis acid (FeCl3) and base
(diethanolamine, DEA) trigger is outlined in Scheme 1.
Great interest is currently devoted to bistable molecules pre-
senting two forms whose interconversion can be modulated by an
external stimulus.1 The design of such molecular-level switching
devices is directly linked to the chemistry of signal generation,
transfer, conversion, storage and detection. Molecules with
switchable properties are of considerable practical and funda-
mental interest as the development of robust systems will open up
new avenues and possibilities for regulating cellular processes
and potential applications to drug delivery systems, optical
devices and sensors.2
Typical bistable molecules are the so-called photochromic
compounds, which is defined as a reversible change induced by
light radiation, between two states of a molecule having different
absorption spectra.3 Of all potential applications of photochromic
compounds, photoswitch is one of important and attractive
application fields.4 The basic requirements for photoswitch are
bistability and nondestructive detection.5 For most photochromic
spirooxazines, thermal fading after UV irradiation is one of their
intrinsic characteristics,6 and it is useless for switching purposes
since the information is spontaneously erased after a relatively
short time. Another shortage of photochromic compounds for
photoswitch is destructive detection because two states are
photochemically active. The approach to avoid destructive detec-
tion is usually to build dual-mode systems in which two reversible
processes can be addressed by means of two different stimuli.7
The ring-opening and ring-closing interconversion of spiro-
oxazines are usually triggered by photoinduction. Although
acidichromism of spirooxazines has been reported by using
strong acids, such as HCl, AcOH and CF3COOH, as triggers,8
the reversal of ring-opening isomers is usually inhibited because
addition of strong base NaOH results in the damage of
spirooxazines and limits the cycling of ring-opening and
Experimental
General
1H NMR spectra were recorded at 400 MHz with TMS as an
internal reference and CDCl3 as solvent. MS spectra were recorded
with a GC-TOF MS spectrometer. UV absorption spectra were
measuredwithanabsorptionspectrophotometer (HitachiU-3010).
All chemicals for synthesis were purchased from commercial
suppliers, and solvents were purified according to standard proce-
dures. Reactionswere monitoredby TLC silicagel plate(60F–254).
Column chromatography was performed on silica gel (Merck,
70–230 mesh). A low-pressure mercury lamp (30W) and a Xeon
light (500W), with different wavelength filters, were used as light
sources for photocoloration and photobleaching, respectively.
aLaboratory of Organic Optoelectronic Functional Materials and
Molecular Engineering, Technical Institute of Physics and Chemistry,
The Chinese Academy of Science, Beijing, 100190, PR China. E-mail:
yichencas@yahoo.com.cn; Fax: +86 10 6487 9375; Tel: +86 10 8254 3595
bGraduate School of Chinese Academy of Sciences, Beijing, 100049, PR
China
Scheme 1 The principle of ring-opening and ring-closing interconver-
sion of spirooxazine derivative 1c with Lewis acid and base trigger.
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Scheme 2 Synthesis of spirooxazine derivative 1c.
Material
Spirooxazine 1c was synthesized according to the synthetic route
presented in Scheme 2, and the detailed procedures and spectra
data were as follows: a mixture of 1-nitroso-4-N,N-diethy-
lamino-2-phenol (3.88 g, 20 mmol) and 1,3,3-trimethyl-2-meth-
yleneindoline (3.5 ml, 20 mmol) in EtOH (50 ml) was refluxed.
After no starting material was detected by TLC plate, the
mixture was cooled. The resulting solution was concentrated and
purified by flash column chromatography with petroleum/
acetone (3:1) as eluent to afford target compound 1c (orange
Fig. 1 Absorption changes of 1c (20 mM, DCM) with 365 nm light
irradiation (periods: 0, 10, 20 s).
It seemed that photochromism is inhibited in those solvents
although the mechanism is not known.
1
solid) in 18% yield. M.p. ¼ 143–145 ꢀC. H NMR (CDCl3):
Some interesting results were obtained when Lewis acid was
added to the solution of 1c. It is found that addition of FeCl3
(0.01M, DCM) to the solution of 1c promoted a color change of
solution from colorless to blue immediately, as shown in Fig. 2,
the absorption band at 628 nm increased with increase of amount
of FeCl3 till 5.0 equiv. of FeCl3 was added. Further investigation
found that the blue solution was attributed to the absorption of
7.36 (s, 1H), 7.21 ꢁ 7.18 (m, 2H), 7.07 (d, J ¼ 7.2 Hz, 1H), 6.86
(t, J1 ¼ 7.4 Hz, J2 ¼ 7.4 Hz, 1H), 6.57 (d, J ¼ 7.8 Hz, 1H), 6.24
(dd, J1 ¼ 2.7 Hz, J2 ¼ 2.7 Hz, 1H), 6.05 (d, J ¼ 2.7 Hz, 1H), 3.32
ꢁ 3.27 (q, 4H), 2.77 (s, 3H), 1.36 (s, 3H), 1.29 (s, 3H), 1.30 (t, 6H).
13C NMR (CDCl3): 149.4, 148.7, 148.1, 147.4, 136.4, 129.1,
128.0, 121.6, 120.6, 119.7, 107.2, 104.2, 98.9, 97.2, 51.7, 44.6,
29.8, 25.7, 20.8, 12.8. HRMS (TOF-MS EI, m/z) [M+] calcd. for
C21H27N3O: 337.0443, found: 337.0440.
Results and discussion
Spectral photochromic properties of spirooxazine 1 were studied
in dichloromethane (DCM) solution. 1 showed ring-opening and
ring-closing photoisomerization with UV/Vis light irradiation.
The photoisomerization of 1 was described in Scheme 3, and
absorption spectra changes were presented in Fig. 1. Before
irradiation, the absorption maximum of 1c at 343 nm (3 ¼ 1.6 ꢂ
104). Upon irradiation with UV light, a new band at 607 nm,
which corresponded to the ring-opening isomer 1o, appeared,
and absorption intensity increased with increasing the irradiation
time till photostationary state was reached. The process was
accompanied by color change of the solution from colorless to
blue. The new band at 607 nm decreased and disappeared when
the sample was irradiated with visible light (lmax $ 450 nm) or
kept in the dark, and accompanying this process the color change
of solution from blue to colorless. This coloration and decolor-
ation can be cycled with UV/Vis light irradiation. It was worth
noting that no color change was observed when spirooxazine 1c
was dissolved in some solvents such as acetonitrile, tetrahydro-
furan, benzene and cyclohexane with UV light irradiation.
Fig. 2 Absorption changes of 1c (20 mM, DCM) with addition of FeCl3
(0, 13, 40, 100 mM, DCM).
Fig. 3 Absorption changes of 1o (dot) with addition of FeCl3 (solid) (13,
25, 50, 100 mM, DCM).
Scheme 3 Ring-opening and ring-closing photoisomerization of 1c with
UV/Vis light irradiation.
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complex 1o$FeCl3. As presented in Fig. 3, addition of FeCl3
(0.01M, DCM) to the solution of 1o produced an absorption
band of 1o at 607 nm bathochromic shifted to 628 nm.
Comparing the absorption bands in Figs 2 and 3 found that both
the profile of band and absorption wavelength are the same,
which indicated that the absorption at 628 nm corresponded to
complex 1o$FeCl3. The investigation of decoloration found that
the complex 1o$FeCl3 is photoinactive upon irradiation with
visible light, the blue solution of 1o$FeCl3 could not be bleached
to colorless solution, and no absorption change was detected in
absorption spectrum. Moreover, complex 1o$FeCl3 also showed
thermal stability and the blue solution was not faded when the
Similar results were obtained when other Lewis acids were
employed as triggers. It is found that using AlCl3, SnCl4, BF3
Et2O, and ZnCl2 as triggers, the colorless solution of 1c could
also be converted to blue solution of complex 1o$X. Like
1o$FeCl3, the blue solution of 1o$X could be bleached to
a colorless solution of 1c with addition of DEA, and coloration
and decoloration could be cycled. Fig. 6 showed the absorption
changes of 1c with addition of AlCl3 solution, both absorption
profile and absorption wavelength of 1o$AlCl3 are similar to
those of 1o$FeCl3.
Different solvents were employed in the investigation of ring-
opening and ring-closing interconversion of spirooxazine 1 with
Lewis acid and base trigger. No color change was observed when
1c dissolved in acetonitrile (CH3CN) with UV light irradiation,
and it seemed that the ring-opening and ring-closing photo-
conversion of 1 was inhibited in CH3CN. By using Lewis acid and
base trigger instead of phototrigger, the ring-opening and ring-
closing interconversion of spirooxazine 1 was, however, accom-
plished in CH3CN. As presented in Fig. 7, the absorption band at
613 nm, which corresponded to complex 1o$BF3 Et2O, appeared
when BF3 Et2O (0.01M, CH3CN) was added to the solution of 1c
in CH3CN and the absorption increased with increase of amount
of BF3 Et2O till about 10 equiv. BF3 Et2O was added. The addi-
tion of DEA could also bleach the blue solution of 1o$BF3 Et2O
back to the colorless solution of 1c, and the color switching could
ꢀ
temperature is over 60 C.
The bleaching of the complex 1o$FeCl3 is explored by using
Lewis base as trigger It is found that the blue solution of
1o$FeCl3 was bleached quickly when diethanolamine (DEA) was
added and the decoloration of 1o$FeCl3 could be performed
completely. As presented in Fig. 4, the absorption band at
628 nm was disappeared completely with addition of DEA, and
the absorption band at 344 nm, which corresponding to 1c,
reappeared. These indicated that the addition of DEA trans-
ferred 1o$FeCl3 to 1c. The mechanism of bleaching probably
combined two processes in which FeCl3 bound with DEA and
deviated from 1o$FeCl3. Switching properties exhibited that
coloration and decoloration between 1c and 1o$FeCl3 could be
cycled with FeCl3 and DEA trigger although the optical density
of 1o$FeCl3 was decreased significantly after 5 cycles (Fig. 5).
Fig. 6 Absorption changes of 1c (20 mM, DCM) with addition of AlCl3
(0, 40, 80, 160, 240 mM, DCM).
Fig. 4 Absorption changes of 1o$FeCl3 with addition of DEA (0, 66,
100, 166 m M, DCM).
Fig. 5 Cycle number of coloration and decoloration with FeCl3 and
Fig. 7 Absorption changes of 1c (20 mM, CH3CN) with addition of BF3
Et2O (0, 66, 106, 160, 213 mM, CH3CN).
DEA trigger in solution.
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derivative as a model compound. It has demonstrated that
the interconversion between ring-opening and ring-closing
species can be performed and cycled via complexing and
decomplexing. The colored complex species shows thermal
stability and photoinactivity, and can be bleached back to
colorless species. This provides a promising way of manipulating
ring-opening and ring-closing interconversion for spirooxazine
derivatives, especially when photoinduced interconversion is
inhibited.
Scheme 4 Compounds employed in control experiments.
Acknowledgements
We are grateful to the National Nature Science Foundation of
China (No. 60337020) and Beijing Natural Science Foundation
(No. 4082033) for financial support.
Scheme 5 A probable mechanism of ring-opening conversion with
Lewis acid trigger and the structure of the resulting complex.
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Conclusion
In summary, a Lewis acid and base triggered molecular
switch has been developed by employing
a spirooxazine
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