Photochromism of a spiropyran derivative of 1,3-calix[4]crown-5

Article abstract of DOI:10.1016/S0022-2860(02)00242-9

A new spiropyran derivative containing a calixcrown ring has been synthesised. Thermal conversion from merocyanine to the spiropyran form was studied in ethanol and in acetonitrile, and equilibrium and kinetic data were compared with those of the parent 6-nitrospiropyran. The kinetics of thermal decay of the merocyanine form of the calixcrown-spiropyran molecule shows a biexponential character in both solvents, while the decay is much slower in ethanol than in acetonitrile.

Full text of DOI:10.1016/S0022-2860(02)00242-9

Journal of Molecular Structure 614 (2002) 69–73
www.elsevier.com/locate/molstruc
Photochromism of a spiropyran derivative of 1,3-calix[4]crown-5
a,b,
a,b
c
c
c
´
*
´
´ ´
¨
, Peter Baranyai , Istvan Bitter , Alajos Gru¨n , Eva Koszegi ,
Andras Grofcsik
Miklos Kubinyi , Krisztina Pal , Tamas Vidoczy
a,b
a,b
a,b
´
´
´
´
aDepartment of Physical Chemistry, Budapest University of Technology and Economics, Budafoki ut 8, 1521 Budapest, Hungary
bChemical Research Center, Hungarian Academy of Sciences, P.O. Box 77, 1525 Budapest, Hungary
cDepartment of Organic Chemical Technology, Budapest University of Technology and Economics 1521 Budapest, Hungary
Received 20 November 2001; accepted 19 December 2001
Abstract
A new spiropyran derivative containing a calixcrown ring has been synthesised. Thermal conversion from merocyanine to
the spiropyran form was studied in ethanol and in acetonitrile, and equilibrium and kinetic data were compared with those of the
parent 6-nitrospiropyran. The kinetics of thermal decay of the merocyanine form of the calixcrown-spiropyran molecule shows
a biexponential character in both solvents, while the decay is much slower in ethanol than in acetonitrile. q 2002 Elsevier
Science B.V. All rights reserved.
Keywords: Photochromism; Spiropyran derivatives; Merocyanine; Calixarene
1. Introduction
most extensively studied groups of photochromic
compounds [3]. The reversible photochromic process
is shown in Scheme 1. Upon UV irradiation,
spirobenzopyran (1a) (10,30,30trimethyl-6-nitro-
spiro[2H-1-benzopyran-2,20-indoline or, in short, 6-
nitrospiropyran) is converted to its coloured mero-
cyanine form (1b) by cleavage of the C–O bond. The
process is reversed either thermally or by irradiation
with visible light. The stability of the merocyanine
form depends on the environment and on the
substituents (Scheme 1).
Photochromic materials have attracted much
attention recently because of their potential appli-
cations as optical filters, optical switches, optical
memories and other molecular devices. Photochro-
mism is defined as a reversible transformation of a
chemical species between two forms, with different
absorption spectra, shifted in one or both directions by
absorption of electromagnetic radiation [1].
The photochromic behaviour of spiropyrans was
discovered by Fisher and Hinshberg in 1952 [2]. In the
course of time spiropyrans have become one of the
A large number of spiropyran derivatives have
been synthesised in recent years. Incorporation of a
crown ether moiety to the spiropyran skeleton offers a
great variety of properties, including the control of the
photoisomerisation of the spiropyran ring by com-
plexing alkali cations [4–7]. Crowned spirobenzo-
pyrans were also attached to polymers and
membranes [8,9]. In contrast to a fairly large number
*
Corresponding author. Address: Department of Physical
Chemistry, Budapest University of Technology and Economics,
Budafoki ut 8, 1521 Budapest, Hungary. Tel.: þ36-1-463-1484;
fax: þ36-1-463-3939.
E-mail address: agrofcsik@mail.bme.hu (A. Grofcsik).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00242-9

70
A. Grofcsik et al. / Journal of Molecular Structure 614 (2002) 69–73
Scheme 1.
of papers published on crowned spiropyrans, no
publications have been found on the inclusion of
both the spiropyran and the calixcrown ring in the
same molecule.
was to combine the unique properties of the calix[4]-
arene ring, the crown ether ring and the spiropyran
skeleton. With the help of UV/Vis spectroscopy we
compared the spiropyran–merocyanine equilibrium of
1 and 4, and studied the thermal decays of their
merocyanine forms in ethanol and in acetonitrile.
Calixarenes are macrocyclic phenol–formal-
dehyde condensation products, which form versatile
building blocks for supramolecular host compounds
[10,11]. They are regarded as the third generation of
supramolecular receptors after cyclodextrins and
crown ethers [12].
2. Experimental
In this paper we wish to report on the synthesis and
spectroscopic studies of photochromic behaviour of
compound 4 (Scheme 2). The purpose of the synthesis
Compound 4 was prepared from the corresponding
1,3-calix[4]crown-5 (2), which was synthesised by the
method described in the literature [13].
Scheme 2.

A. Grofcsik et al. / Journal of Molecular Structure 614 (2002) 69–73
71
and was used without further purification. The
solvents were of HPLC grade.
For kinetic and equilibrium measurements of the
spiropyran–merocyanine system a Cintra 10e UV/Vis
spectrophotometer was used. Before the determi-
nation of thermal equilibrium the samples were kept
in dark overnight. For production of the merocyanine
form in sufficient concentration, the quartz cell
containing the sample was illuminated by the
365 nm line of a 200 W high-pressure mercury arc
lamp. The visible part of the light was cut out by
aqueous CuSO4 solution and an UG1 filter. In order to
study the kinetics of the thermal ring closure of the
merocyanines, we determined the absorbance of the
visible band as a function of time.
Fig. 1. UV/visible absorption spectrum of 5 £ 1025 mol dm23
ethanolic solution of 4 taken after overnight storage in the dark (a),
2 min (b), 15 min (c) and 1 h (d) after illumination with UV light for
1 min. The spectra were taken at 25 8C.
3. Results and discussion
Compound 2 was (i) first monoalkylated with ethyl
bromoacetate in the presence of barium oxide as base.
This process was followed by (ii) hydrolysis with
ethanolic KOH and (iii) subsequent chlorination with
oxalyl chloride to afford the calixarene acid chloride,
which was allowed to react (iv) without isolation with
the respective N-hydroxyethyl-indolo-spiropirane (3)
[14] which finally furnished the target molecule.
Details of the synthesis will be described elsewhere.
The structure was verified by 1H, 13C NMR, and
elemental analysis.
Fig. 1 shows the UV/visible spectra of the
5 £ 1025 molar ethanolic solution of 4. Spectrum a
was taken with the sample kept in dark, spectra b, c
and d were obtained 2, 15 and 60 min after a short
illumination with UV light. It is clearly seen that the
merocyanine form, produced by UV light, slowly
decays to its original spiropyran form. The maximum
of the visible band in ethanol appears at 540 nm both
for 4b (the merocyanine form of 4) and for 1b. The
band maxima in acetonitrile are at 561 and at 557 nm
for 4b and 1b, respectively.
6-Nitrospiropyran (1) was purchased from Aldrich
In 2 £ 1024 mol dm23 solution of 4 in ethanol an
absorbance of 0.079 was measured. This is about 50%
higher than the absorbance of 1 (0.053) under the
same conditions. With the molar absorption coeffi-
cient 1max ¼ 3.54 £ 104 dm23 mol21 cm21 of the
merocyanine form found in the literature [15],
(7.6 ^ 1.5) £ 1023 equilibrium constant of the spir-
opyran–merocyanine equilibrium has been obtained
for 1 in ethanol in the dark at 25 8C, which is close to
the value of (8.4 ^ 2.7) £ 1023 determined by
Flannery [16]. Absorption coefficients of different
spiropyran derivatives published so far were found to
be between 3.5 £ 104 and 6 £ 104 dm23 mol21 cm21
[15]. It is mainly solvent polarity and not the quality
of the substituent that influences the absorption
coefficient. Lower values were obtained in polar
solvents, higher ones in apolar solvents. Assuming the
same absorption coefficient for 4b and 1b, we
Fig. 2. Thermal decay curves for 2 £ 1025 mol dm23 solutions of
4b in ethanol (full line) and in acetonitrile (dashed line) at 25 8C.
The decay curves were recorded by monitoring the absorbance of
the visible band of the merocyanine form. Initial absorbances are
normalised to 1.

72
A. Grofcsik et al. / Journal of Molecular Structure 614 (2002) 69–73
calculated
the
equilibrium
constant
(1.1 ^ 0.3) £ 1022 for the 4a and 4b system in
ethanol at 25 8C. Similar estimates for the equilibrium
constants in acetonitrile at 25 8C produced
(3.8 ^ 1.5) £ 1023 and (3.4 ^ 1.5) £ 1023 for 1 and
4, respectively (The published equilibrium constant
for 1 in acetonitrile at 45 8C was 8.2 £ 1023 [17].)
Typical decay curves of the merocyanine form of 4
in ethanol and acetonitrile are shown in Fig. 2. For the
sake of comparison the initial absorbances are
normalised to 1. As can be seen in the figure, the
rate of ring closure is much slower in ethanol than in
acetonitrile, which points to the stabilizing effect of
ethanol on the merocyanine form.
Analysis of the thermal decay curves of 4b
revealed that a single exponential curve was not
suitable for the description of the ring closure process.
Figs. 3(a) and (b) show the decay curves in
acetonitrile together with the residuals for both single
and double exponential fits, respectively. One thou-
sand and five hundred data points were used in the
fitting procedures. For the sake of clarity only 31
experimental data points are shown in each diagram
(one point in every 50 s). The much better fit of Fig.
3(b) suggests that the ring closure kinetics of the
merocyanine form of 4 shows a biexponential
character. (Rate constants are given in Table 1.) In
acetonitrile the weights of the two exponentials are
nearly the same. Double exponential decay kinetics
has also been determined for the ethanolic solution of
4. Comparison of the decay curves in the two solvents
reveals two differences. One is that the slower decay
can be characterised with a rate constant, which is
Fig. 3. Single exponential (a) and double exponential (b) fits to the
experimental decay curve of 2 £ 1025 mol dm23 solution of 4b in
acetonitrile. The fitted and the experimental absorbances are
indicated by full lines and squares, respectively. The residuals are
also shown in each diagram.
Table 1
Observed rate constants of thermal ring closure of 1b and 4b at 25 8C in s21
Compound
Solvent
Ethanol
k1
Acetonitrile
Reference
k2
k1
k2
1b
1b
1b
1b
1b
4b
–
7.57 £ 1024
3.76 £ 1024
4.39 £ 1024
6.9 £ 1024
–
–
2.89 £ 1023
2.04 £ 1023
This work
[16]
–
1.72 £ 1022
[18]a
[19]
–
2.99 £ 1023
[20]
1.70 £ 1022
4.50 £ 1024
1.33 £ 1022
3.55 £ 1023
This work
a
Calculated rate constants from the Arrhenius parameters given in Tables 1 and 2 of Ref. [18].

A. Grofcsik et al. / Journal of Molecular Structure 614 (2002) 69–73
73
about one order of magnitude lower in ethanol than in
References
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