Kinetic modelling of the photochromism and metal complexation of a spiropyran dye: Application to the Co(II) - Spiroindoline- diphenyloxazolebenzopyran system

Article abstract of DOI:10.1016/j.dyepig.2010.06.005

A spirobenzopyran containing 6-chloro and 8-diphenyloxazole substituents has been investigated in acetonitrile solution by nanosecond laser photolysis at room temperature. In degassed solution, a short-lived transient (5 μs) has been identified as the triplet state of the closed spiro form. The ratio between the singlet and triplet pathways of ring opening has been determined from oxygen quenching measurements. In the presence of Co(II), UV irradiation is needed to induce the complexation. The fast second order rate constant for the 1:1 complex formation has been determined. It is suggested that such kinetic parameter could be used to characterise the properties of metallochromic spiropyrans.

Full text of DOI:10.1016/j.dyepig.2010.06.005

Dyes and Pigments 89 (2011) 324e329
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Kinetic modelling of the photochromism and metal complexation
of a spiropyran dye: Application to the Co(II) e Spiroindoline-
diphenyloxazolebenzopyran system
,
,
b
b
b
M.I. Zakharova a c, C. Coudret a, V. Pimienta a, J.C. Micheau a , M. Sliwa , O. Poizat , G. Buntinx ,
*
S. Delbaere b, G. Vermeersch b, A.V. Metelitsa c, N. Voloshin d, V.I. Minkin c
a Université de Toulouse, IMRCP, UMR 5623, F-31062 Toulouse, Cedex, France
b Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Université des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq
Cedex and Faculté des Sciences Pharmaceutiques et Biologiques, 3 rue du Professeur Laguesse, F-59006 Lille, Cedex, France
c Institute Phys. Org. Chem., Southern Federal University, Rostov on Don, Russia
d Southern Scientific Center of Russian Academy of Science, Rostov on Don, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
A spirobenzopyran containing 6-chloro and 8-diphenyloxazole substituents has been investigated in
acetonitrile solution by nanosecond laser photolysis at room temperature. In degassed solution, a short-
lived transient (5 ms) has been identified as the triplet state of the closed spiro form. The ratio between the
singlet and triplet pathways of ring opening has been determined from oxygen quenching measurements.
In the presence of Co(II), UV irradiation is needed to induce the complexation. The fast second order rate
constant for the 1:1 complex formation has been determined. It is suggested that such kinetic parameter
could be used to characterise the properties of metallochromic spiropyrans.
Received 29 January 2010
Received in revised form
23 April 2010
Accepted 16 June 2010
Available online 26 June 2010
Keywords:
Spiropyran
Ó 2010 Elsevier Ltd. All rights reserved.
Photochromism
Metaleion complexation kinetics
1. Introduction
time-resolved studies in organic photochemistry. The main result of
femtosecond transient investigations [2] is that the ring opening
The design and synthesis of functional dye molecules that could
serve as molecular devices for sensors, photo-switching, and signal
transduction is an area of intense activity. Among the compounds that
could be used for such purpose are the photochromic spiropyrans [1]
(Scheme 1).
Spiropyrans belong to a group of photo-switchable organic
molecules which photochromism involves UV light induced cleavage
of the CeO bond of the neutral spirocyclic form SP producing
a strongly coloured merocyanine open form MC. The latter slowly
decaying through a thermal process back to the SP form, this allows
reversible switching between a colourless closed form SP and
conjugated zwitterionic form MC. Spiropyrans have been extensively
studied due to their potential applications in optical switching, high
density optical storage, image processing etc.
occurs in a sequential way involving at least three intermediate
states in the following stages. The first step is assigned to the
formation of an electronic upper excited state which evolves during
a radiationless transition towards the reactive excited state. The
lifetime of this transition is about 40e100 fs. Then, there is a vibra-
tional relaxation to an intermediate state where the CeO bond of the
pyranic cycle is broken. This cisoïd zwitterionic state which is often
called “X” evolves during 200e350 fs. Then, radiationless transition
from X leads to the formation of the colored merocyanine form MC
after appropriate isomerizing bond rotations. The duration of this
last stage lies between 1 and 10 ps depending on the structure of the
spiropyran under investigation and the reaction medium [3]. Since
the pioneering work of Fischer [4], all time-resolved experiments
point to the very fast formation of the relaxed merocyanine open
form under the transoïd-transoïd-cis configuration (TTC).
From a more fundamental point of view, they are of considerable
interest since these molecules are key-compounds for fast
Depending on the donoreacceptor character of the substituents
and the polarities of the solvents, the triplet excited-state channel
can also play a role in the dynamics of MC formation [5]. Knowledge
of the relative importance of the triplet pathway is of interest since
* Corresponding author. Tel.: þ33 561556275; fax: þ33 561558155.
E-mail address: micheau@chimie.ups-tlse.fr (J.C. Micheau).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.06.005

M.I. Zakharova et al. / Dyes and Pigments 89 (2011) 324e329
325
3.5 104
H3C
Cl
CH3
H3C
CH3
hν
3 104
2.5 104
2 104
1.5 104
1 104
5000
0
N
O
N
Cl
N
O
Δ
O
N
O
SP
MC (TTC)
Scheme 1. Photochromism of the spiroindoline-diphenyloxazolebenzopyran.
diffusion-controlled quenching of the longer lived SP triplet state
could occur if no special precaution to eliminate oxygen from the
sample is taken.
Another interesting property of photochromic spiropyrans is the
possibility to form complexes with metal ions in solvents of medium
polarity such as acetonitrile [6]. The complexation is due to the
possibility of electrostatic interactions between the metal cation and
the phenate anion of the merocyanine form. Moreover, substituting
the original structure by a suitably placed appropriate heterocyclic
moiety has been used to increase the metal ion binding capabilities
of these systems [7]. Such features require further studies to eluci-
date the mechanism of complexation.
300
350
400
450
lambda (nm)
Fig. 1. UV spectrum of 1 in acetonitrile solution. Lack of absorbance in the visible range
(not shown) indicates the absence of traces of open merocyanine form.
2.2. Laser flash photolysis
Since many years, an intense effort has been devoted to the
synthesis and the structural determination of the various chelating
spiropyrans and their metal complexes [8]. However, only few
quantitative characterisations of their complexation behaviour have
been attempted. A possible explanation of this lack of quantitative
results could lie in the relatively cumbersome data collection and
analysis necessary for extracting association constants between the
various metal-ions and the spiropyrans under investigation [9,10].
Although these thermodynamic parameters could be useful to direct
the synthetic efforts towards the best adapted structures for a given
application, only very few experimental values can be found in
the literature. This is why we want to present a kinetic approach
that could be used to obtain quantitative information about both
reaction mechanism and structureeproperties relationships,
namely the analysis of fast Absorbance vs time traces recorded
during photochromism and metal-ion complexation processes.
The aim of this manuscript is to analyse the kinetics of the early
step of the Co(II) complexation of a newly synthesized spiroindo-
Nano-microsecond transient absorption experiments were per-
formed using a laser flash photolysis apparatus. Excitation pulses at
355 nm (7e8 ns,1 mJ) were provided by a 20-Hz Nd:YAG laser (DIVA
II, Thales laser). The probe light was provided by a Xe lamp (XBO
150 W/CR OFR, OSRAM). Acetonitrile solutions of 1 were contained
in a quartz cell (10 ꢀ 10 mm2 section). The total number of photons
was around 1.8 ꢀ 10þ5 photons/pulse. Within the laser beam,
the total irradiated volume was about 100
mL where the total number
of molecules was around 4.6
ꢀ
10þ5 for
a
concentration
of 7.7 ꢀ 10ꢁ5 mol Lꢁ1 giving an absorbance of 1 at 355 nm. Degassed
solutions were obtained by bubbling Argon. The transmitted light
was analyzed by a spectrometer equipped with a photomultiplier
(R1477-06, Hamamatsu) coupled to
(TDS 540, Tektronix).
a digitalized oscilloscope
2.3. NMR and UVevisible measurements under irradiation
Compound 1 was dissolved in CD3CN (C ¼ 2.8.10ꢁ4 M). 1H NMR
spectra were recorded at 233 K on a 500 MHz NMR spectrometer
equipped with a TXI probe. UVevisible spectra were recorded using
a micro-volume optical fibre probe (optical path ¼ 2 mm) inserted
into the NMR tube (diameter ¼ 5 mm) and connected to a Cary 50
UVevisible spectrophotometer in order to measure simultaneously
the absorbance and the extent of photoisomerization.
line-diphenyloxazolebenzopyrane able to produce
a chelating
merocyanine isomer using a short-lived light pulse. The incorpora-
tion of a chelating substituent into a spiropyran molecule in the
vicinity of the phenate leads to a chelating bidentate ligand. The
nitrogen containing 4,5-diphenyl-1,3-oxazol-2-yl substituent has
been selected as it is well known that oxazole derivatives, especially
those containing aromatic rings in positions 2 and 5, are good
fluorophores [11,12]. Such substituent makes the compound more
versatile as metal-induced fluorescence modulation could be used
for analytical purposes.
Photoirradiation was carried out directly into the NMR tube
using a thermostated home-built apparatus with a 1000 W high-
pressure HgeXe lamp and a filter (Schott 11FG09: 259 <
with lmax ¼ 330 nm, T ¼ 79%).
l < 388 nm
2. Experimental
Firstly, the 1H NMR and the UVevisible spectra of initial solution
were recorded. Then, an irradiation for 3 min was applied leading
to the appearance of a low intensity wide band at z615 nm
characterizing the formation of a small amount of the expected
photomerocyanine. However, in NMR spectroscopy, the conversion
was not sufficient to allow us to observe the corresponding signals
of the merocyanine. Increasing the time of irradiation led to the
formation of new absorption bands which were attributed to some
photodegradation. Possible aggregation has been ruled-out since
the amount of produced merocyanine remained always very low.
2.1. Products
The synthesis of compound 1 (C36H31N2O2Cl, FW ¼ 558.5) has
been already published [13]. UV spectrum is displayed on Fig. 1.
Cobalt perchlorate hexahydrate, Co(ClO4)2, 6H2O; FW ¼ 366.74;
(99.999% pure) was purchased from Alfa Aesar and used without
further purification. Acetonitrile was of HPLC grade with T% >82 at
197 nm.

326
M.I. Zakharova et al. / Dyes and Pigments 89 (2011) 324e329
0.06
0.05
0.04
0.03
0.02
0.01
0
3.2. Analysis of kinetic traces
Fig. 3 shows two kinetic traces recorded on at 510 and 640 nm, i.e.,
on both sides of the isosbestic point, for degassed (black traces)
and aerated (blue traces) solutions. In both solutions, the transient
responses at 510 and 640 nm evolve simultaneously and show an
initial instantaneous rise and a subsequent slower decay (510 nm)/
rise (640 nm) component. In the degassed solution, a time constant
9.3µs
5µs
3µs
2µs
0.55µs
of about 5
of oxygen, this evolution occurs with a much shorter lifetime (around
0.25 s). In addition, whereas the amplitude of the fast rising
ms is measured for the slow component. In the presence
V
m
component at 640 nm is nearly similar to that observed in
the degassed solution (a), the amplitude of the slow rising compo-
nent is much weaker (c-a < b-a), which clearly indicates that the slow
reaction of production of the open form is quenched by oxygen. These
results demonstrate that the open merocyanine form of 1 is produced
via two different ways, a fast process, kinetically unresolved at the
time resolution of the experiment, and a slow oxygen-sensitive
process. The fast reaction is likely arising at the excited singlet state.
Within our time scale, this process appears as instantaneous as it is
well known that the singlet state reaction pathway leads to the open
form in few picoseconds or less [16]. The oxygen-sensitive process
is likely involving the triplet state T1, since triplet states are usually
strongly quenched by oxygen. Accordingly, we assign with confi-
dence the transient absorption band in the 450e550 nm domain
(Fig. 2) to the lowest excited triplet state of the closed form of 1.
The relative yield of ring opening from the singlet and triplet path-
ways can then be estimated from the amplitude ratio of the fast and
slow rising contributions in the 640 nm kinetics on Fig. 3 (see below).
V
400
450
500
550
600
650
700
wavelength (nm)
Fig. 2. Transient absorption spectra of compound
acetonitrile solution at 0.55, 2, 3, 5 and 9.3
1
in deaerated (argon-purged)
ms after the 355 nm pump excitation pulse.
3. Results and discussion
3.1. Irradiation of the free compound 1 in aerated
and degassed solution
The purpose of this experiment was to check if triplet quenching
by ambient oxygen could influence the dynamics of ring opening
[14,15]. Time-resolved experiments have been performed using
3.3. Kinetic modelling of the singlet vs triplet
ring opening pathways
a 10 ms time window.
Fig. 2 shows some typical transient absorption spectra recorded
after 355 nm excitation of a degassed solution of compound 1 in
acetonitrile. The spectrum recorded immediately after the laser pulse
shows a weak and unresolved absorption around 510 nm and a strong
band peaking at 640 nm. This last one can be attributed to the relaxed
open merocyanine form. Then, during a 10 ms time scale, the strong
640 nm band increases significantly whereas the weak absorbance at
510 nm decreases. An isosbestic point appears at 550 nm.
In order to check the validity of these assumptions, and to describe
semi-quantitatively the singlet vs triplet ring opening processes,
a global model [17] has been proposed. This model involves only four
species: the closed form SP, its singlet S1 and triplet T1 states, and the
open merocyanine form MC. For the sake of simplicity, all the inter-
mediate short-lived transient species such as open forms of higher
energies whatever their multiplicities, have been short-cut since they
are too fast to be seen within the ms time-scale kinetics. The processes
under consideration are based on classical photochemistry laws
such as deactivation or internal conversion, intersystem crossing and
ring opening. The kinetic model is then described by a system of
simple differential equations which can be integrated algebraically.
The assumed kinetic model is listed on Table 1.
The laser pulse is assumed to be sufficiently short vs the dura-
tion of the observed phenomena. At the end of the laser pulse
the produced singlet state S1 evolves within a picosecond time
scale according to three possible ways: (i) radiative or non radiative
(internal conversion) deactivation to the ground state of SP (ksd),
(ii) ring opening pathway leading to the open form MC (ksr), and
(iii) intersystem crossing to the triplet state (kisc). Therefore, the
concentrations of the free merocyanine [MC]p and triplet state [T]p
arising from S1 are respectively:
ꢀ
ꢁ
Tp ¼ kisc½S1ꢂ0=ðksd þ ksr þ kiscÞ
(1)
and
ꢀ
ꢁ
MCp ¼ ksr½S1ꢂ0=ðksd þ ksr þ kiscÞ
(2)
where [S1]0 is the total amount of S1 produced by the pulse. The
subscript “p” indicates that the concentrations of T and MC are to be
considered just after the end of the laser pulse. The singlet state
lifetime is then defined by sS ¼ 1/(ksd þ ksr þ kisc). Then, during the
Fig. 3. Optical density changes recorded at 640 and 510 nm after the 355 nm laser
pulse for a degassed solution (black traces) and an aerated solution (blue traces).
The red lines are the best fits to the oxygen free data.

M.I. Zakharova et al. / Dyes and Pigments 89 (2011) 324e329
327
Table 1
However, when the evolution is monitored through the absor-
List of processes used in the kinetic modeling of the singlet vs triplet ring opening of
the spiropyran 1.
bance, Beer’s law applies:
ꢂ
ꢃ
Absl
¼
¼
3lMC½MCꢂ þ 3lT½Tꢂ[
(8)
Processes
Rate constant Processes
h
ꢂ
ꢃ
SP / S1
S1 / SP
S1 / MC
S1 / T1
laser pulse
excitation process
Absl
3lMCðktrkiscsT þ ksrÞ þ kisc 3lT
ꢁ
3lMCktrsT
ksd
ksr
kisc
ktr
singlet state deactivation (radiative or not)
singlet state ring opening
intersystem crossing
i
expðꢁt=sTÞ [½S1ꢂ0sS
ð9Þ
T
1 / MC
triplet state ring opening
T1 þ O2 / SP þ O2 kq[O2]
triplet state quenching by dissolved oxygen
triplet state deactivation (radiative or not)
where 3lMC and 3lT are respectively the molar absorption coefficients
T
1 / SP ktd
of open merocyanine MC and triplet state T at wavelength l.
Then; Abslðt/NÞ ¼ 3Ml CðktrkiscsT þ ksrÞ[½S1ꢂ sS
and
(10)
0
slow evolution occurring after the jump within the triplet time
scale, [T] and [MC] obey respectively to the following differential
equations:
h
ꢂ
ꢃi
Abslðt ¼ 0Þ ¼ 3Ml CðktrkiscsT þ ksrÞ þ kisc 3Tl
[½S1ꢂ0sS
ꢁ
3lMCktrsT
d½Tꢂ=dt ¼ ꢁðktr þ ktdÞ½Tꢂ
and
(3)
(4)
(5)
ð11Þ
which gives rise to the % of MC arising from triplet ¼
d½MCꢂ=dt ¼ ktr½Tꢂ
giving rise to mono-exponential kinetics:
ꢂ
ꢃ.
Abslðt/NÞ ꢁ Abslðt ¼ 0Þ Abslðt/NÞ
ꢂ
ꢃ.h
i
¼ kisc 3lMCktr
s
T ꢁ 3lT
3lMCðktrkiscsT þ ksrÞ
(12)
½Tꢂ ¼ ½Tꢂpexpðꢁt=sT
Þ
At a wavelength where 3Tl
ꢃ
3lMC the previous expression from
and
eq. (7): ktrkisc
From the ratio of the triplet state lifetime in the degassed
(0.25 s) and aerated (5 s) solutions, it appears that (5e0.25)/
s
T/(ktrkiscsT þ ksr) is recovered.
½MCꢂ ¼ ½MCꢂpþktr½TꢂpsTð1 ꢁ expðꢁt=sTÞÞ
where
(6)
m
m
s
T is the degassed triplet lifetime, sT ¼ 1/(ktr þ ktd)
5 ¼ 95% of the triplet state is quenched in the presence of oxygen
and thus the yield of the open form arising from the triplet state is
expected to be reduced by a factor of 20. According to an absor-
bance change ¼ 0.021 (amplitude of the red curve at 640 nm ¼ b-a)
in the degassed solution (Fig. 3), it is thus expected that this
amplitude was only 0.021/20 ¼ 0.001 in the aerated solution.
When the experiment is performed in aerated solution,
s
the triplet lifetime is reduced to:
¼ 1=ðktr þ ktd þ kq½O2ꢂÞ
TðO2Þ
z0:25
m
s, therefore kq½O2ꢂ ¼ 1=
s
ꢁ 1=sT ¼ ðð1=0:25Þ ꢁ 1=5Þ
TðO2Þ
msꢁ1 ¼ 3:8 ꢀ 106
s
ꢁ1. Then, taking into account that in air satu-
rated acetonitrile [18], the concentration of dissolved oxygen is
about [O2]diss ¼ 1.9 ꢀ 10ꢁ3 mol Lꢁ1, a value of kq ¼ 2 ꢀ 109 Mꢁ1 sꢁ1 is
Such a value is close to the experimental value of DOD z 0.001 (c-
obtained, in agreement with the literature (kq ¼ 1.9 ꢀ 109 Mꢁ1 sꢁ1
)
a). It is found that a/b ¼ 62% of the ring opening originates from the
singlet state S1 and (b-a)/b ¼ 38% from the triplet T1. These results
confirm that the mechanism shown in Table 1 is roughly sufficient
to interpret the singlet vs triplet ring opening in spiropyran 1.
From these considerations, the following relationships between
[19].
Within the framework of the model displayed on Table 1, the
relative proportion of the merocyanine MC formed from singlet and
triplet states can be calculated as:
the various rate constants can be derived: ksr/(ktrkisc) ¼ 8
ms;
ktr þ ktd ¼ 2 ꢀ 10þ5 sꢁ1 and kq ¼ 2 ꢀ 109
M .
ꢁ1 sꢁ1
% of MC arising from triplet ¼ slow evolution=
ðinstantaneous þ slow evolutionÞ
3.4. Metal complexation
¼ ðamplitude at t/N ꢁ amplitude at t ¼ 0Þ=
amplitude at t/N ¼ ktr½TꢂpsT=ð½MCꢂpþktr½TꢂpsT
¼ ktrkiscsT=ðktrkiscsT þ ksrÞ
Þ
Metal complexation occurs from the negatively charged phenate
substituent in the merocyanine MC open form. In the case of
ð7Þ
Fig. 4. Variation of the absorbance at 630 and 530 nm for compound 1 in the presence of Co(II) ions in non-degassed acetonitrile at room temperature (left: [Co] ¼ 10ꢁ4 mol Lꢁ1
right: [Co] ¼ 6.3 ꢀ 10ꢁ4 mol Lꢁ1) after 355 nm nanosecond laser pulse.
;

328
M.I. Zakharova et al. / Dyes and Pigments 89 (2011) 324e329
Table 3
spiropyrans, in which an SP4MC thermochromic equilibrium is
present, there is always a small quantity of MC available for the
complexation [20]. In these conditions, although it is assumed to be
rapid, the rate of the metal complexation reaction (MC þ M / MCM)
is limited by the SP / MC ring opening process [21].
Effect of the structure of the spiropyran on the second order rate constant of Co(II)
complexation. a, b: results from ref. 22b in non-degassed acetonitrile; 1: our result.
On the contrary, in the specific case of compound 1, the ther-
mochromic equilibrium is not operative as there is no perceptible
trace of merocyanine open form in the UV spectrum recorded in
acetonitrile solution. UV irradiation is needed to provide the amount
of merocyanine MC required to trigger the metal complexation.
The fast relaxation of the MC þ M / MCM (k2) complexation
reaction has been recorded in the merocyanine band at 630 and also
at 530 nm after 355 nm nanosecond laser pulse. The 530 nm band is
assigned to the 1:1 metal complex because within ionophoric
spiropyrans, the recognition of a metal cation is accompanied by
a strong hypsochromic shift of the absorption band of the mer-
ocyanine form. Since the formation of the relaxed open form occurs
R1
R6
R8
k2 (L molꢁ1 sꢁ1
)
CH3
C6H5
C3H7
NO2
NO2
Cl
OCH3
OCH3
DPO
4 ꢀ 10þ8
5 ꢀ 10þ8
1.5 ꢀ 10þ7
a
b
1
in the 10
m
s time domain, it is expected that the complexation,
the phenate anion. From all these considerations, it appears that
kinetic rate constants of metal complexation are very sensitive to
the stereo-electronic effect of the various substituents. Therefore,
in combination with complexation equilibrium constants they
could profitably be used to characterize newly synthesized photo-
chromic and metallochromic spiropyrans.
whose relaxation time ranges on more than hundreds of
on the already relaxed open form. This assumption is confirmed by
ms, arises
the absence of fast phase during the evolution of the complexation.
During the MC þ M / MCM (k2) fast process we have: Absl
¼
3lMC½MCꢂ þ 3Ml CM½MCMꢂÞ[ with
3
>
3
and 35B30
<
3
. Hence,
630
MC
630
530
ð
MCM
MCM
on Fig. 4 the decrease at 630 nm corresponds mainly to the
consumption of the merocyanine while the increase at 530 nm
characterizes the complex build-up. Numerical fitting of the
relaxation kinetics recorded just after the laser flash shows a mono-
exponential behaviour in accordance with the assumed process of
formation of the 1:1 metal complex from the relaxed merocyanine
[22]. The observed pseudo first-order rate constant is expressed as
kobs ¼ k2 ꢀ [M] where [M] is the added metal concentration.
This relationship allows the determination of [M] from the
4. Conclusion
In this paper, we have analysed the photochromism and metal-
lochromism of a DPO substituted spirobenzopyran. In this compound,
as there is no thermal ring opening, UV irradiation in presence of
the metal ion is necessary to trigger the complexation. In deaerated
acetonitrile, about 38% of the ring opening proceeds from a triplet
state exhibiting a lifetime around 5
m
s. In presence of oxygen, the
s indicating a rapid quenching by
ꢁ1). In the 10
s time scale open mer-
measurement of kobs
.
triplet lifetime is reduced to 0.25
m
The results of the kinetic traces analysis are gathered on Table 2.
Careful examination of the data shows that the kinetic traces
amplitudes are independent of the metal concentration. The differ-
ences jAbs0 ꢁ AbsNj are of the same order of magnitude: 1.09 ꢀ 10ꢁ2
vs 0.99 ꢀ 10ꢁ2 at 530 nm and 2.93 ꢀ 10ꢁ2 vs 3.10 ꢀ 10ꢁ2 at 630 nm for
the 1.0 ꢀ 10ꢁ4 and 6.3 ꢀ 10ꢁ4 mol Lꢁ1 Co(II) concentrations, respec-
tively. Since the metal concentration is in excess in both cases, the
amplitudes are only determined by the amount of merocyanine
produced by the laser pulse. At the end of the evolution, all the mer-
ocyanine is complexed. The second-order complexation rate constant
oxygen (kq ¼ 2 ꢀ 10þ9 Mꢁ1
s
m
ocyanine accumulates. In presence of Co(II) ions, a fast complexation
occurs leading to a hypsochromic shift. The second order rate constant
of complexation has been determined at 1.5 ꢀ 10þ7 Mꢁ1
s
ꢁ1. We
propose to use the complexation kinetic rate constants for the quan-
titative characterisation of these dye molecules. Although, it necessi-
tates time resolved absorbance measurements in the ms time scale,
the main advantage of this approach is the possibility to consider
new types of spiropyrans metal sensors [24] in which the metal
complexation occurs only after photoisomerization.
k2 is around 1.5 ꢄ 0.9 ꢀ 10þ7 Mꢁ1
s
ꢁ1. It is not easy to compare
this value with literature data, because with the exception of the two
pioneering papers by Görner and Chibisov [23] there are no more
published studies. However, looking at Table 3, it is obvious that our
value is more than one order of magnitude lower than those already
published for Co(II) complexation of similar spiropyrans in acetonitrile
solutions.
Acknowledgements
MIZ thanks the France-Russia network of the “Ministère de
l’Education Nationale, de l’Enseignement Supérieur et de la
Recherche » for a grant. CC, JCM and MS gratefully acknowledge the
French CNRS for financial support in the framework of the Inter-
national Research Group nꢅ 93 « PHENICS ».
This difference can be interpreted by considering some struc-
tural features (see structure on Table 3): the presence of a chloro
substituent instead of a nitro, the size of the nitrogen substituent of
the indoline and the bulky DPO heterocycle limiting the access to
References
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Table 2
Initial and final amplitudes and observed apparent rate constants of the mono-
exponential relaxation kinetic curves recorded in the merocyanine and metallic
complex absorption bands.
Curve [M] (mol Lꢁ1
)
Abs0
AbsN
kobs (sꢁ1
)
k2 (L molꢁ1 sꢁ1
)
530
630
530
630
10ꢁ4
1.29 ꢀ 10ꢁ2 2.35 ꢀ 10ꢁ2 1.9 ꢀ 103 1.9 ꢀ 10þ7
5.03 ꢀ 10ꢁ2 2.1 ꢀ 10ꢁ2 2.6 ꢀ 103 2.6 ꢀ 10þ7
1.25 ꢀ 10ꢁ2 2.24 ꢀ 10ꢁ2 6.0 ꢀ 103 9.5 ꢀ 10þ6
3.42 ꢀ 10ꢁ2 3.18 ꢀ 10ꢁ3 4.8 ꢀ 103 7.6 ꢀ 10þ6
10ꢁ4
6.3 ꢀ 10ꢁ4
6.3 ꢀ 10ꢁ4

M.I. Zakharova et al. / Dyes and Pigments 89 (2011) 324e329
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