Photomodulation of the proton affinity and acid gated photochromism of a novel dimethylaminophenyl thiazole diarylethene

Article abstract of DOI:10.1016/j.dyepig.2011.05.006

The acid-gated photochromism and the photomodulation of fluorescence and proton affinity of a novel dimethylaminophenyl thiazole diarylethene in MeCN were investigated from a kinetic point of view. Photomodulation of the proton affinities has been estimat

Full text of DOI:10.1016/j.dyepig.2011.05.006

Dyes and Pigments 92 (2012) 838e846
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journal homepage: www.elsevier.com/locate/dyepig
Photomodulation of the proton affinity and acid gated photochromism
of a novel dimethylaminophenyl thiazole diarylethene
,
b
a
Y. Kutsunugia, C. Coudretb , J.C. Micheau , T. Kawai
*
a Graduate School of Materials Science, NAIST, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
b Université de Toulouse, UPS, IMRCP, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The acid-gated photochromism and the photomodulation of fluorescence and proton affinity of a novel
dimethylaminophenyl thiazole diarylethene in MeCN were investigated from a kinetic point of view.
Photomodulation of the proton affinities has been estimated from acid titrations and numerical modelling
of the acid induced T-photochromism. The basicities of the thiazole and dimethylamino protonation sites
are different by only one pK unit. Upon ring- closure, their relative proton affinities are reversed.
Our investigations underline the role of the thiazole protonation in the carbon-carbon bond weakening of
the closed form and validate the role of the proton as catalyst in the gated T-photochromism.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 3 February 2011
Received in revised form
4 May 2011
Accepted 6 May 2011
Available online 16 September 2011
Keywords:
Photomodulation
Gated-photochromism
Diarylethene
Thiazole
Kinetic modelling
Acid catalysis
1. Introduction
concerned with the photoregulation of catalytic activity has been
published by Ueno et al. [2] during the hydrolysis of p-nitrophenyl
The development of photoswitchable molecular devices is
encompassing very different fields, from the design of photo-
responsive systems able to regulate a chemical reaction to physical
applications in optical memory media with non-destructive readout
capability. These two issues are basically related to the following
questions: (i) how the photoisomerisation process affects its hosting
medium? (ii) How the medium affects the photochromic response?
From a more basic point of view, these two concepts are related to
the so-called photomodulation and gated-photochromism. To date,
most of the responsive dyes are constructed as bifunctional mole-
cules involving a photochromic unit connected to a stimuli sensitive
moiety. If the difference between the physico-chemical properties of
the two isomers of a photochromic molecule is sufficient to change
the macroscopic behaviour of a bulk medium, it is said that this
compound exhibits a performance of photomodulation. For instance,
Feringa et al. have shown that the fluorescence and chirality of
photoswitchable inherently dissymmetric cis and trans alkenes can
be reversibly photomodulated [1]. Moreover, since the first report
acetate catalysed by b-cyclodextrin in presence of a photo-
isomerisable cisetrans p-(phenylazo) benzoate, many other exam-
ples of the photoswitching of catalytic activity have been published
[3]. They are described in a review by Hecht et al. [4].
With the aim to lock the photochromic unit into a specific isomer
-especially towards light- a related phenomenon has been explored:
the gated-photochromism where the photoisomerisation reactivity
is made sensitive to some external stimuli such as reactive chem-
icals. One of the first attempts to observe gated-photochromismwas
reported by Irie et al. [5]. The effect is based on the inhibition of the
photochromic reactivity of a diarylethene when the molecule
adopts a parallel conformation. This conformation was stabilised by
the presence of suitably disposed interlocking arms. The molecule
regains photoactivity when the intramolecular lock is released by
the addition of hydrogen bond breaking or reducing agents. Another
example of gated photochromism is achieved by controlling either
the conformation or intra-molecular proton transfer of diary-
lethenes by using intra or inter molecular hydrogen bonds [6]. More
recently, it has been shown that the incorporation of acid-base
sensitive groups in diarylethenes might lead to multi-state molec-
ular switches i.e. to acid-gated photochromism [7].
* Corresponding author.
E-mail addresses: tkawai@ms.naist.jp (Y. Kutsunugi), coudret@chimie.ups-tlse.fr
(C. Coudret).
The relationships between photomodulation and gated photo-
chromism are illustrated on Scheme 1 in the particular case of a pH
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.006

Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
839
pH variations, recorded the UV/visible spectra, and build
a comprehensive model based on a square network as in Scheme
1. The specificity of our approach is that all species and all
processes are considered simultaneously under realistic condi-
tions. The details of the used numerical technique can be found in
a free access website [16]. Curve fitting allows the assessment of
several relatively inaccessible parameters such as quantum
yields, multiple-equilibria stability constants or kinetic rate
constants.
photoswitching
h
ν
H+
-
-
+
H+
H+
+
H+
2. Experimental
h
ν
2.1. Products
2.1.1. 4-(Dimethylamino)benzothioamide
H+
H+
To a slurry of 70% sodium hydrosulfide hydrate (0.92 g ¼ 10 mmol)
and magnesium chloride hexahydrate (1.02 g ¼ 5 mmol) in DMF
(10 mL) was added 4-(dimethylamino)benzonitrile (0.73 g ¼ 5 mmol)
in one portion, and the mixture was stirred at room temperature
for 2 h. The resulting green slurry was poured into 20 mL of
water, stirred for 20 min in 1 N HCl, then filtered and washed with
water to give pure compound by yellow solid. Yield 80%, 1H NMR
Scheme 1.
sensitive photochromic molecule [8]. All the necessary processes
construct a square network. The two horizontal processes, related
to the photoswitching between open and closed forms, lead to the
gating of the photochromic property (coloration or discoloration)
as the photoswitching process is made sensitive to protonation. The
two vertical processes involve “dark processes” and compare the
two isomer’s affinity for proton. Thus, along this point of view, one
can change this constant with light, thus “photomodulating” the
protonation equilibrium.
Several families of photochromic molecules can be used to
develop gated-photochromism and photomodulation phenomena.
Among them, spiropyranes and spirooxazines bearing crown-
ethers have been synthesised for analytical chemistry purposes
[9]. However, to increase the photochromic performance and the
fatigue resistance, it appears that looking towards the diarylethene
(DTE) family [10] could be more profitable. Indeed, the open and
closed isomers of photochromic diarylethenes have not only
different absorption UV/visible spectra but also many different
physical and chemical properties. For instance, the switching of
luminescence [11] and the photomodulation of ionic interactions
and reactivity [12] are some properties closely related to the pho-
tomodulation or gated-photochromism that have been rather well
documented. However, in more details, it must be pointed-out that
the most notorious papers in this domain are focused on only one of
the two quantitative aspects, either on photomodulation or on
gated-photochromism. For instance, Odo et al. [13] have shown that
(300 MHz, Acetone-d6): d 8.48 (br, NH2) 8.01 (d, 2H), 7.48 (d, 2H), 3.03
(s, 6H).
2.1.2. 4,5-Bis(2-methylbenzo[b]thiophen-3-yl)-2-(4-
(trifluoromethyl)phenyl)thiazole (1o)
2-hydroxy-1,2-bis(20-methylbenzo[b]thiophen-30-yl)ethanone
(1.76 g ¼ 5 mmol) was added to a solution of 4-(dimethylamino)
benzothioamide (0.65 g, 3.2 mmol) in trifluoroacetic acid (3.0 ml)
and the mixture was kept for 24 h at room temperature, diluted
with diethyl ether, and neutralised with aqueous solution of
sodium hydroxide. The aqueous phase was extracted with diethyl
ether, the extracts were washed in succession with water, and 20%
solution of sodium hydroxide, and water again, dried over MgSO4,
and evaporated. Yield 1.0 g (40%). 1H NMR (300 MHz, DMSO-d6)
d 7.90e7.88(m, 4H), 7.69(br, 2H), 7.30e7.26(m, 4H), 6.83(d, 2H),
3.02(s, 6H), 2.02(s, 6H) ppm. HRMS(EI) calcd. for C29H24N2S3 (Mþ)
496.1102, found (Mþ) 496.1100.
2.1.3. Solvent and reactants
MeCN for spectroscopy, anhydrous trifluoroacetic acid and
aqueous solution (70% acid) of HClO4 were of the highest purity
available. They were used as purchased.
2.2. Data recording and analysis
a pKa switching can be induced by the change in a
p-conjugated
system based on photochromism, but the possible variations of the
quantum yields under the influence of pH variations have not been
explored. Another example can be found in the Yumoto et al. [14]
paper about the control of the photoreactivity of some diary-
lethene derivatives by quaternarization of a pyridylethynyl group,
but the probable changes of pK’s according to the isomeric state
open or closed have not been scrutinised. Because photo-
modulation and gated-photochromism are intimately related, it
must be borne in mind that the two processes are occurring
simultaneously. This detail could be of importance, especially when
aiming towards potential practical applications. To the best of our
knowledge this is only in the paper by Lehn et al. [15] that the two
concepts (“photochemical pKa-modulation and gated photochromic
properties”) are investigated simultaneously.
2.2.1. Photochromism
Absorption spectra of the MeCN solutions of compound 1
(8 ꢀ 10ꢁ6 mol Lꢁ1) were recorded on an HP 8451 diode array
spectrophotometer. The photochemical irradiation was derived
from a 200 W, IR filtered, high pressure mercury lamp equipped
with single-line interference filters and high transmission optical
fibre. The photochemical reactor was a 1 cm ꢀ 1 cm quartz cell
(1 cm optical path) containing 2.5 cm3 of solution. The optical fibre
was fitted tightly at the neck of the cell. The monochromatic light
intensity was determined directly in the stirred reactor using an
acidic aqueous solution of potassium ferrioxalate (I3013 ¼ (2 ꢂ 0.4) ꢀ
10ꢁ6; I3065 ¼ (4 ꢂ 0.9) ꢀ 10ꢁ6 mol Lꢁ1 sꢁ1). The thermostated solu-
tion (25 ꢃC) was stirred with a magnetic stirrer bar. Absorption
maximum and irradiation wavelengths were monitored simulta-
neously until a PSS was reached. The kinetic modelling technique of
the Abs vs time traces using the home made “sa” software has been
already described [17].
In order to afford a new example where photomodulation and
gated-photochromism are investigated simultaneously we have
submitted a novel pH-sensitive diarylethene to irradiation and

840
Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
2.2.2. Protonation
compounds Xi (in absorbance units), εxi(l) is the UV/visible spec-
0.08 mol Lꢁ1 MeCN solutions of TFA or HClO4 were added step-
by-step to a 8 ꢀ 10ꢁ6 mol Lꢁ1 solution of 1 in a 1 cm ꢀ 1 cm quartz
cell using a microliter syringe. Ratio of concentrations at 104
insured that the volume increase after 1000 equivalents of acid
added was only þ10%.
trum of the compound Xi in L molꢁ1 cmꢁ1 units and [X]i the
concentration of compound Xi in mol Lꢁ1 units. The concentrations
of the species [X]i are given by the model. εxi(l) are the solutions of
a system of linear equations.
2.2.5. Thermal bleaching
2.2.3. Establishment of a model
Colored solutions were obtained by irradiating a 3.5 ꢀ10ꢁ5 M
MeCN solution of 1 until a photostationary state (PSS) was reached
(ODmax ca 0.37). Then, the appropriate amount of HClO4 was added.
UVevis. kinetics traces of the absorbance at 552 nm (lmax of the
protonated closed form) were then acquired at 25 ꢃC. The whole
model including processes 1e17 has been used for kinetic data
analysis. Previously determined first and second protonation
equilibrium constants of the open form have been fixed while
mono- and di-protonation affinities and thermal kinetic rate of
ring-opening of the closed form have been adjusted to fit the
thermal bleaching kinetics.
Table 1 displays the list of processes occurring in the assumed
model, where 1o represents the neutral open form, 1oHþ the
mono-protonated open form, 1oH22þ the di-protonated open form,
1c the neutral closed form, 1cHþ the mono-protonated closed form
and 1cH22þ the unstable di-protonated closed form. This model is
the simplest possible taking into account all the qualitative
experimental observations. It is based on the same ideas than the
square network displayed in Scheme 1. Moreover, to increase the
chemical realism, according to Kolthoff et al. [18], the formation of
hydrated protons and water homoconjugates in MeCN has been
taken into account when using the partially hydrated strong
perchloric acid solution (molar ratio H2O:HClO4 ¼1:2.4). The cor-
responding formation constants of the water homoconjugates have
been gathered from the literature. The others have been adjusted
by the model from the fitting of various Absorbance vs acid added
and Absorbance vs time experimental curves.
3. Results and discussion
Recently, thiazole containing DTE-like molecules, such as
angular terarylenes, have been developed for fast thermal bleaching
rate purposes [19]. The presence of the heterocyclic nitrogen atom
makes the molecule sensitive to Lewis acidic cations. Thus, the
stability of the closed form of a triphenylangular terthiazole was
found to be considerably affected by the presence of protonation
sites [20]. In the present paper, we investigate the photochromism
in neutral and acidified MeCN of a new dimethylaminophenyl
thiazole diarylethene [21]. The presence of two protonation sites on
the dimethylamino group and on the thiazole ring makes the
compound 1 a strong candidate for proton gated-photochromism
and pKa photomodulation experiments. The key step of the
synthesis is the construction of the central thiazole ring via the
condensation of a symmetrical acyloin with a thioamide. For
comparisonpurpose a derived ammonium salt was also prepared by
room temperature methylation of the dimethylamino group of
compound 1 (Fig. 1).
With trifluoroacetic acid (TFA), only processes 9 and 10 have
been considered replacing Hþ by TFA and 1oHþ species by a loose
[1o.TFA] ion-pair. From the simultaneous fitting of the variations of
the absorbance at 360 and 428 nm as a function of the number of
equivalents of acid added, k9 has been estimated at 425 ꢂ 20
M
ꢁ1 sꢁ1 using k10 ¼ 1 sꢁ1
.
With perchloric acid (HClO4) which has been assumed to be
fully dissociated, the second protonation occurs and k9 and k11 have
been estimated at respectively 4 ꢀ104 and 4 ꢀ103 Mꢁ1 sꢁ1 using
k10 ¼ k12 ¼1.
2.2.4. Spectral recalculations
They are based on the Beer’s law: Abs(
l) ¼ Siεxi(l)*[X]i, where
Abs( ) is the recorded UV/visible spectrum of the mixture of
l
Table 1
Processes 1 to 8 are devoted to the description of the formation of water homo-
conjugates. They allow a correction of the acid concentrations. Processes 9 to 17
depict the first and second protonation of the open and closed form and the acid
catalysed ring- opening. Reverse rate constants have been put arbitrarily at 1 in
order to insure that the establishment of the various equilibria are sufficiently rapid.
In these conditions, the equilibrium constant is expressed by the same value than
the direct rate constant of the corresponding forward process.
0.2
428 nm
0.15
nꢃ
List of processes
Rate of process
Rate constant
of process
þ þ H2O / H3Oþ
k1[H][H2O]
k2[H3Oþ]
k1 ¼ 1.4 ꢀ 102
0.1
1
2
3
4
5
6
7
8
H
H3Oþ / Hþ þ H2O
H3Oþ þ H2O / H5Oþ2
H5Oþ2 / H3Oþ þ H2O
H5Oþ2 þ H2O / H7Oþ3
H7Oþ3 / H5Oþ2 þ H2O
H7Oþ3 þ H2O / H9Oþ4
H9Oþ4 / H7Oþ3 þ H2O
1o þ Hþ / 1oHþ
k2 ¼ 1
k3[H3Oþ][H2O]
k4[H5Oþ2 ]
k3 ¼ 2.7 ꢀ 103
k4 ¼ 1
k5[H5Oþ2 ][H2O]
k6[H7Oþ3 ]
k5 ¼ 3.3 ꢀ 104
k6 ¼ 1
0.05
k7[H7Oþ3 ][H2O]
k8[H9Oþ4 ]
k7 ¼ 1.2 ꢀ 104
k8 ¼ 1
360 nm
9
k9[1o][ Hþ]
k10[1oHþ]
k9 ¼ see text
k10 ¼ 1
1oHþ / 1o þ Hþ
0
10
11
12
13
14
15
16
17
750
250
500
1000
1oHþ þ Hþ / 1oH22þ
1oH22þ / 1oHþ þ Hþ
1c þ Hþ / 1cHþ
k11[1oHþ][Hþ]
k11 ¼ 4ex103 Mꢁ1 sꢁ1
k12 ¼ 1
k12[1oH22þ
]
number of equivalents of TFA added
k13[1c][Hþ]
k14[1cHþ]
k13 ¼ 103 Mꢁ1 sꢁ1
k14 ¼ 1
1cHþ / 1c þ Hþ
1cHþ þ Hþ / 1cH22þ
1cH22þ / 1cHþ þ Hþ
1cH22þ / 1oH22þ
k15[1cHþ][Hþ]
k15 ¼ 104 Mꢁ1 sꢁ1
k16 ¼ 1
Fig. 1. Variations of the absorbance monitored at two selected wavelengths during the
addition of TFA in a MeCN solution of 1o; ([1o]0 ¼ 7.7 ꢀ 10ꢁ6 M). Two independent
experiments are displayed (filled circles and open circles). Dots are the experimental
data points, solid lines are the simulations by the model.
k16[1cH22þ
k17[1cH22þ
]
]
k17 ¼ (1.5 ꢂ 0.1) ꢀ 10ꢁ1 sꢁ1

Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
841
1.4
N
N
(irr.) = 365 nm; (obs.) = 365 nm
1.2
1
365 nm
N
S
Light
N
S
(irr.) = 405 nm; (obs.) = 405 nm
0.8
0.6
0.4
0.2
0
405 nm
S
S
S
S
1o
1c
(irr.) = 365 nm; (obs.) = 530 nm
Scheme 2.
3.1. Photochromism
(irr.) = 405 nm; (obs.) = 530 nm
In neutral MeCN, compound 1 exhibits the expected P-photo-
chromism of the diarylethene (DAE) series, i.e. photocoloration
under UV irradiation and photobleaching under visible light
(Scheme 2).
0
20
40
60
80
100
120
140
160
time (s)
Fig. 3. Determination of the quantum yields of photocoloration
(Foc ¼ 0.52) and
In order to determine the quantum yields of photocoloration
and photobleaching together with the UV/visible absorption
spectrum of the closed form, a photokinetic analysis of the absor-
bance vs time kinetic curves recorded under continuous mono-
chromatic irradiation has been carried-out. Under 365 nm UV
irradiation, the system slowly evolves until a photosteady state
(PSS) at which the rate of photocoloration is exactly counter-
balanced by the rate of photobleaching (Fig. 2). The presence of
a clear-cut isosbestic point is a strong indication that only two
isomers are present and that there is no photodegradation.
By combining photocoloration and photobleaching records, it is
photobleaching (Fco ¼ 0.27) from the kinetic modelling of the Absorbance vs time
curves recorded under continuous monochromatic irradiation for photocoloration at
365 nm (black dots) and at 405 nm for photobleaching (open squares). ([1o] þ [1c]) ¼
4.8 ꢀ 10ꢁ5 mol Lꢁ1 in neutral MeCN solution. At PSS(365), [1c]/[1o] ¼ 3.1, at PSS(405),
[1c]/[1o] ¼ 0.05.
The kinetic analysis also delivers the PSS conversion rate
(z76% of closed form under 365 nm irradiation) and consequently
the molar absorption coefficient of the closed isomer 1c at
the observation wavelength (530 nm) can be estimated at
13,200 L molꢁ1 cmꢁ1
.
possible using a simple P-photochromism model (1o) / 1c h
n;
h
n
) to find out the two quantum yields of photocoloration (Foc) and
3.2. Fluorescence
photobleaching (Fco) without the need of a time- and product-
consuming chemical isolation of the closed isomer (Fig. 3). The
modelling also delivers the conversion rate ([1c]/[1o]) which has
been achieved at the PSS. Therefore the whole UV/visible spectrum
of the closed isomer 1c can be estimated (see below).
When excited at 360 nm in MeCN, the open form 1o exhibits
a strong fluorescence peaking at ca. 440 nm (Fig. 4). The large
Stokes shift (z80 nm) suggests the presence of a twisted inter-
molecular charge transfer (TICT) excited state.
7 105
0.2
0.15
0.1
100% open form
6 105
5 105
365 nm
irradiation
4 105
3 105
V
2 105
0.05
24% open form
1 105
0
0
250
300
350
400
450
500
550
600
650
400
450
500
550
600
lambda (nm)
wavelength (nm)
Fig. 2. Photocoloration of compound 1o ([1o]0 ¼ 7.7 ꢀ 10ꢁ6 M) under 365 nm irradi-
ation in MeCN. Note the formation of the closed form absorbing at 410 and 530 nm
(wide band) with an isosbestic point at 380 nm. The concentrations of the closed form
at the PSS is: [1c]PSS ¼ 5.8 ꢀ 10ꢁ6 M.
Fig. 4. Decay of the fluorescence of the open form during the photocoloration process
1o / 1c under 365 nm irradiation. The residual fluorescence at the 365 nm PSS is 24%
of the initial signal.

842
Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
intramolecular non luminescent quencher of the DMAPT’s excited
states (either FC or TICT).
b
a
N
N
N
3.3. Acidochromism
DMAPT
A very nice bathochromic spectral shift is observed upon acid
addition in a MeCN solution of the open form 1o. It is interesting to
remark that trifluoroacetic (TFA) and perchloric (HClO4) acid give
rise to the same phenomenon. However, this occurs not for the
same amount of equivalents. Typically, it needs about 1000
equivalents of TFA to reach a maximum, while only less than 5 are
sufficient for HClO4.
N
S
N
S
N
S
S
S
TICT
FC
DBTE
1o
DMAPT*
A determination of the association constants between 1o and
acid has been performed from the numerical modelling of the
variation of the absorbance during the incremental addition of acid
equivalents using a simple A þ B ) / C model:
Scheme 3.
It is indeed clear from the structure of compound 1o that the
central moiety, the 2-(p-dimethylaminophenyl)thiazole DMAPT
shows the classic push-pull system, with the N,N-p-dimethylami-
nophenyl as the donor group and the thiazol-2-yl as the acceptor
one (Scheme 3a). This chromophore is very close to the one studied
by Dey et al. [22], namely the 2-(p-dimethylaminophenyl)benzo-
thiazole DMAPBT. The main low energy band in the DMAPBT
absorption spectrum was attributed to the charge transfer transi-
tion between the electron donating dimethylamino group and the
electron withdrawing benzothiazolyl one: it is indeed sol-
vatochromic with a lmax in MeCN of ca 355 nm that equates nicely
the main transition of the open form 1o. Thus, 1o is a bichromo-
phoric system, with a DMAPT unit fused to a dibenzothienylethene
(DBTE) photochromic one (Scheme 3a). The DMAPT-localised
excited state would then adopt two extreme conformations,
a planar “Franck-Condon” (FC) one, with the geometry of the
ground state, and a relaxed twisted one (TICT) in which the charges
are fully separated, and responsible of the long wavelength emis-
sion (Scheme 3b).
Upon continuous irradiation, the fluorescence signal decays in
a parallel way to the photoisomerisation process. These results
confirm that the conversion rate at the 365 nm PSS is z76% of the
closed form, a value identical to that obtained from the kinetic
modelling displayed on Fig. 3. These data also indicate that the
closed isomer 1c is not contributing to the total fluorescence
spectrum. A very rough energetic scheme can be proposed
(Scheme 4) for both situations: in the open state, the molecule’s
Franck Condon planar excited state can relax either by the photo-
cyclization of the DBTE part or by luminescence via a TICT inter-
mediate. In the closed state the DBTEc unit is acting as an
For TFA addition : 1o þ TFA)/ ½1o:TFAꢄ K1oTFA
[1o.TFA] is a loose ion-pair where the proton is closer to 1o than to
TFA.
h
i
For HClO4 addition : 1o þ Hþ)/ 1:Hþ
K1oH
HClO4 is assumed to be fully dissociated.
This model allows the determination of the association
constants of the open form 1o with TFA, K1oTFA at around
425 ꢂ 25 Mꢁ1 and with Hþ, K1oH z 4 ꢀ10þ4 Mꢁ1. The result with
TFA deserves to be compared with a recent NMR determination of
the TFA association constant with a terthiazole photochromic
derivative [23] (z27 Mꢁ1) where the association is occurring at the
thiazole rings.
In the case of compound 1o, it is not easy to say where the first
protonation takes place. Dey et al. [22] have studied the influence of
protonation of the absorption spectrum of the DMAPBT chromo-
phore. This investigation is interesting, because of the presence of
basic nitrogen atoms on both electron donating and withdrawing
groups. Thus, if the protonation occurs on the thiazole moiety
enhancing its electron withdrawing ability, one expects a lowering
of the charge transfer energy. On the other hand, protonation of the
dimethylamino group should destroy its electron donating ability
thus moving the transition to higher energy. A similar result is also
assumed for the twice protonated species. This leads us to assign
the protonation sites of compound 1o from the spectral changes of
the DMAPT chromophore’s main transition. The bathochromic
spectral shift observed on Fig. 5 would be indicative that the first
protonation of the open form 1o is occurring on the thiazole
DMAPT*-DBTEo
DMAPT*-DBTEc
FC
FC
DMAPT-DBTEo*
TICT
TICT
DMAPT-DBTEc*
abs
abs
photoisom.
fluo
photoisom.
Open compound
Closed compound
Scheme 4.

Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
843
0.25
0.2
0.15
0.1
0.05
0
should imply that the ion pair between the trifluoroacetate and the
1oHþ cation should be loose or more or less dissociated.
The spectral variations in presence of an excess of HClO4 have
been analysed by considering the second protonation. This was
done by taking into account a second equilibrium:
a
b
c
h
i
h
i
k
1o:Hþ þ Hþ)/ 1o:H22þ K1oH2
allowing the estimation of
a second association constant
K
1oH2 z 4 ꢀ10þ3 Mꢁ1 i.e. one order of magnitude lower than for the
first protonation. This result confirms the presence of two
protonation sites.
It is not simple to compare the values of these association
constants with some pKa values found in the literature. The first
reason stays in the structural differences between compound 1 and
the closest parent molecule, namely the DMAPBT (dimethyl ami-
nophenyl benzothiazole). The second is related to the solvent used,
pure water vs MeCN/water mixture. Nevertheless, if we consider
the values published by Dey et al. [22] for the monocation and the
dication of DMAPBT at respectively 2.9 and 0.3 in water, it is
possible to estimate their corresponding values in MeCN by adding
ca 3pK units [25]. Finally, the recalculation of the proton affinity
constants from the corrected pKa lead to two results not too far
from the expected order of magnitude, namely 8 ꢀ 105 Mꢁ1 for the
mono- protonation and to 500 Mꢁ1 for the di-cation formation.
The UV spectra of the mono- and di-protonated species have
also been recalculated from the species distribution delivered by
the model (Fig. 7).
c
b
a
k
250
300
350
400
450
lambda (nm)
Fig. 5. Spectral variation during the addition of TFA on 1o in anhydrous MeCN.
a: [1o] ¼ 8 ꢀ 10ꢁ6 M; [TFA] ¼ 0; b: [TFA] ¼ 8 ꢀ 10ꢁ4 M (100equivalents); c ¼ 200eq.;
k: 1000 equivalents, [TFA] ¼ 8 ꢀ 10ꢁ3 M. Note the bathochromic shift from 360 nm to
428 nm. Isobestic point at 390 nm indicates that 1o and [1o.TFA] are the only
absorbing species.
heterocycle. Moreover, as displayed on Fig. 6, the step-by-step
addition of only some equivalents of HClO4 is sufficient to
observe a gradual bleach of the 428 nm band and the appearance of
a new blue shifted absorption at 290 nm. This observation is
consistent with what should be expected if the second protonation
of 1o occurs on the dimethylamino group [24]. The spectral shift is
completed after the addition of less than 300 equivalents of HClO4
and the process is fully reversible. The 428 nm band is recovered
then converted into the initial 360 nm band during the neutrali-
sation of the HClO4 in excess by the addition of the corresponding
amount of triethylamine.
Fig. 8 displays the recalculated UV/visible spectra of the neutral
closed form (b). Spectrum c shows the shifted spectrum of the
closed form in presence of aqueous perchloric acid.
As for the open form, the spectra of both closed forms (b and c)
are dominated by a sharp absorption in the blueeUV region that is
likely to come from the central p-dimethylaminophenyl thio-
imidate moiety (note that the thiazole aromatic ring does not exist
in the ring-closed form). Again, applying the same analysis as Dey
et al. can help in the protonation site assignment. Indeed, the main
transition in 1c is hypsochromically shifted in presence of free
protons. Consequently it is likely that the mono-protonation of 1c
occurs on the dimethylamino group. The basicity locus inversion of
It is remarkable that whatever the acid used (TFA or HClO4)
the intermediate “monoprotonated” spectrum is preserved. This
0.2
3 104
2.5 104
2 104
1.5 104
1 104
5000
0
a
c
b
0.15
a
0.1
b
c
0.05
n
0
250
300
350
400
450
250
300
350
400
450
500
lambda (nm)
lambda (nm)
Fig. 6. Gradual decrease of the 428 nm band during the step-by-step addition of an
excess of HClO4 in a 8 ꢀ 10ꢁ6 M solution of 1o. a: 20; b: 40; c: 60; n: 280 equivalents
added. Note the increase of the absorption at 292 nm.
Fig. 7. UV/visible spectrum of the free open form (a); recalculated spectrum of the
mono-protonated open form (b) and of the di-protonated open form (c). The spectrum
of the ion-pair with TFA is identical to (b).

844
Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
3.5 104
3 104
2.5 104
2 104
1.5 104
1 104
5000
0
dimethylamino site. Moreover, in strong acidic conditions, this
spectrum is no longer stable and undergoes a slow thermal
bleaching (Fig. 10). The second protonation of the closed form
concerns the heterocyclic imino nitrogen of the former thiazole
ring. Since no UVevis spectrum can be extracted for this twice
protonated species, the latter must evolve quickly into the open
form. This mechanistic scheme is analogous to what was observed
on a parent terthiazole photochromic compound [19,21].
The rate of ring opening depends on the amount of acid added.
For low HClO4 concentrations, the rate of bleaching is roughly
proportional to the amount of acid added. On the contrary, when
the acid concentration is sufficiently high, a saturation phenom-
enon (the rate no longer increases) occurs and the kinetic traces are
superposed (Fig. 11).
a
b
c
b
c
The following kinetic scheme with kinetic equations numbering
according to Table 1 has been used to describe the acid catalysed
thermal bleaching of 1.
250
300
350
400
450
500
550
600
650
h
i
lambda (nm)
1c þ Hþ)/ 1cHþ
R ꢁ 13 þ R ꢁ 14
Fig. 8. UV/visible spectra of the neutral closed form b. c: shifted spectrum of the closed
form in strongly acidic solution. For the sake of comparison, the spectrum of the
neutral open form is labelled: a.
h
i
h
i
1cHþ þ Hþ)/ 1cHþ2 þ
R ꢁ 15 þ R ꢁ 16
h
i
1o þ Hþ)/ 1oHþ
R ꢁ 9 þ R ꢁ 10
the first protonation which takes place on the thiazole in the open
form and on the dimethylamino in the closed form has to be related
to the photochromic reaction that affects the thiazole ring. Another
spectral change concerns the long wavelength transition at ca
500 nm. It is then likely that the slight tailing in the red and the
simplification of the band’s envelope could be due to a charge effect
coming from the protonation [26].
h
h
i
i
h
i
1oHþ þ Hþ)/ 1oHþ2 þ
R ꢁ 11 þ R ꢁ 12
h
i
1cHþ2 þ / 1oHþ2 þ
R ꢁ 17
The model considers the simultaneous mono- and di-protonation
of both the closed and open form. It is assumed that the set of
rapid equilibria are continuously displaced under the influence of
the irreversible ring opening process. This situation is similar to the
hydration of the flavylium ion as described by Mc Clelland et al. [27]
where multiple equilibria are displaced by a coupling with an
irreversible reaction. With such a model fitted with the appropriate
parameters i.e. the already estimated proton affinities of the open
form 1o, it is possible to approximately reproduce a set of four
3.4. Acid catalysed T-photochromism
Under aqueous HClO4 addition, there is a rapid variation of the
spectrum of the closed form as depicted on Fig. 9.
After few seconds, the closed form 1c exhibits the same UV/
visible spectrum as with TFA. This result is an indication that
a protonation (like the loose ion-pair) is taking place at the
0.2
0.5
0.4
0
0.15
2
0.3
V
0.1
4
6
0.2
0.1
0
0.05
8
0
250
300
350
400
450
500
550
600
300
350
400
450
500
550
600
650
lambda (nm)
wavelength (nm)
Fig. 9. Rapid variation (from 0 to 8 s) of the UV/visible spectrum of the closed form
after HClO4 addition in MeCN and vigourous shaking. [1c]0 ¼ 5 ꢀ10ꢁ6 M;
[HClO4]0 ¼ 2.5 ꢀ10ꢁ3 M. Note the red-shift of the closed form spectrum with solvent
change. Spectrum 8 is no longer stable and undergoes a slow bleaching.
Fig. 10. Acid catalysed thermal bleaching of the closed form, [1c]0 ¼ 2 ꢀ 10ꢁ5 M.
Changes in
D
t are indicated by bold solid line spectra
D
t ¼ 600 s, then 1200, then 1800,
2400 and 3000 s. [HClO4] added ¼ 4 ꢀ10ꢁ3 M.

Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
845
Table 3
0.4
0.35
0.3
Photomodulation of the proton affinity constants.
Compound 1
Mono- protonation
Di-protonation
Open 1o
Closed 1c
4 ꢀ 10þ4 Mꢁ1
4 ꢀ 10þ3 Mꢁ1
10þ3 Mꢁ1
10þ4 Mꢁ1
a
b
0.25
0.2
the mono- and di-cation, their ratio depend on the open 1o or
closed 1c state of the isomer under consideration.
Open and closed isomers behave in a reverse way. For the open
isomer, the dication formation is less easy than the mono. On
the contrary, for the closed form, there is a cooperative effect, the di-
protonation is easier. For the open form 1o, the mono-protonation
site is then assumed to be at the thiazole ring with the higher
stability constant and the second at the dimethylamino site with the
lower constant. The situation for the closed form 1c is different, di-
protonated species are kinetically unstable, therefore the only
protonated species is the mono-protonated one where protonation
is occurring at the dimethyl-amino site. Consequently, the attack of
the second proton is taking place on the thiazole moiety. This
second protonation is expected to give rise to a weakening of the
ring-closing bond as it was already suggested in the acid triggered
terthiazole T-photochromism [19]. It is possible that this affinity
change could be related to a proton swap between the two basic
sites during the photoswitch of the monoprotonated species.
0.15
0.1
c & d
0.05
0
0
2000
4000
6000
8000
1 104
time (s)
Fig. 11. Kinetic records of the thermal bleaching of the closed form 1c (3 ꢀ 10ꢁ5 mol Lꢁ1
)
M
under strong acidic conditions: [HClO4] ¼ 4.2 ꢀ 10ꢁ3 M (a); 7 ꢀ 10ꢁ3 M (b); 1.05 ꢀ10ꢁ2
(c); 1.4 ꢀ 10ꢁ2 M (d). Dots are experimental data points; solid lines: simulation by the
model. Note that the model reproduces quite well the superposition of the two kinetic
curves (c and d) recorded at the higher acid concentrations. The slight departures seen on
curves a and b could be attributed to some model imperfections or experimental bias.
kinetic traces recorded at variable initial acid concentrations. The
ability of the model to reproduce the traces superposition (satu-
ration effect) at the higher acid concentration must be pointed-out.
Such a feature is an argument for the global validation of the model
although it cannot be excluded that some missing processes could
occur. Within the framework of these hypothesis, the model allows
4. Conclusion
Compound 1 exhibits the required features to display some
gated-photochromism and photomodulation properties, namely
the presence of a photochromic DAE-like kernel associated with
two basic sites of different proton affinities.
the estimation of the mono (z10þ3 Mꢁ1) and di- (z10þ4 Mꢁ1
)
From the point of view of gated-photochromism, several effects
deserve to be pointed-out. There are clear-cut spectral shifts of both
the open and closed form under protonation. Quantum yields of
both photocoloration and photobleaching are sensitive to the
presence of TFA. When compound 1 is mono-protonated, its
quantum yield of photocoloration is decreased by about 40%, while
its quantum yield of photobleaching increases by about the same
amount. It is liable that this effect originates from a specific reac-
tivity of the excited states either at the level of the deactivations or
avoided crossing paths. The last point to be mentioned is the
transformation of a typical DAE-like P-photochromism into a vari-
able T-photochromism under the influence of HClO4 addition.
A mechanism including the lack of stability of the di-protonated
closed form has been developed and it has been shown that it is
able to reproduce rather accurately the effect of HClO4 initial
concentration.
Our approach was based on numerical modelling and multi-
experiment curve fitting. This specific technique was applied to
handle either equilibrium (titration) or time dependent (kinetic
traces) data. We have been able to estimate the photomodulation of
the proton affinity. Ring-closure increases the proton affinity of the
heterocyclic nitrogen. This last point is remarkable because
a similar phenomenon of basicity increase in the photochromic
kernel has already been observed in a previous P- to T-photo-
chromism switch with a terthiazole compound.
protonation constants of the closed form and the first-order
thermal ring opening rate constant (kopening ¼ 1.5 ꢂ 0.5 ꢀ10ꢁ1 sꢁ1).
3.5. Analysis of the gated-photochromism and photomodulation
Results gathered in Table 2 show that compound 1 displays
a gated-photochromism effect under mild and strong acid condi-
tions. On one hand, spectral shifts around 70e100 nm are observed
on the absorption spectra of the open and closed isomers. On the
other hand, the photocoloration/photobleaching quantum yields
ratio is decreased by a factor of more than 2 in mild acidic medium.
It is likely that protonation could change the excited state deacti-
vation and evolution paths. Moreover, strong acid triggers the ring-
opening of the otherwise stable closed form 1c. Fig. 4 also illus-
trates the photomodulation of the fluorescence of compound 1o.
The open form 1o is fluorescent while the closed form 1c is not.
Moreover, thanks to the numerical modelling of the acid
induced T-photochromism, photomodulation of the proton affini-
ties has also been observed (Table 3). Although there is only one
order of magnitude difference between the stability constants of
Table 2
Gated-photochromism of 1. The quantum yields of photocoloration and photo-
bleaching in mild acidic conditions have been determined in the presence of 1000
equivalents of TFA. Strong acidic conditions are attained with HClO4.
From this analysis, it appears that many photochromic mole-
cules bearing ion-affinity sites or reactive fragments are relevant to
the gated-photochromism or photomodulation concept. Their
quantitative studies are not easy because all these effects are
coupled and cannot be isolated without difficulties. However, by
combining careful spectroscopic titration or kinetic monitoring
with numerical data analysis it could be possible to reach some
lmax (open) εmax
lmax (closed) εmax
FAB FBA
(nm)
(open) (nm) (closed)
Neutral
Acid (mild)
Acid (strong) 292
360
428
27,300 530; 405
27,200 552; 305
20,000 Not stable
13,200
10,700
Not stable
0.52 0.27
0.32 0.39
e
e

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Y. Kutsunugi et al. / Dyes and Pigments 92 (2012) 838e846
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