New stable colored complex of Al3+ with 1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (BIPS)

Article abstract of DOI:10.1002/poc.4012

Ultraviolet-visible (UV-vis) absorption, flash photolysis, and ¹H and ¹³C NMR spectroscopy were used to investigate the mechanism of formation and structure of complexes in the 1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (SP, also well known as BIPS) with Al³⁺ (inorganic salts) in ethanol (EtOH), methanol (MeOH), and in aqueous MeOH solutions. A labile (equilibrium constants ≤ 5000 M^?1) complex of SP and Al³⁺ with broad absorption band in the UV-vis with λ_max = 380 nm appeared promptly in the presence of an excess of Al³⁺. The slow formation of a stable complex (SC) between Al³⁺ and two merocyanine (MC) forms of SP with an intensive absorption band at λ_max = 430 nm is observed with a yield of 1.0 upon keeping the solutions of these two compounds at constant concentration ratio [Al³⁺] ≥[SP]/2 in the dark. The rate constants of such SC formation were close to the corresponding rate constant of the transformation of SP into MC in the dark (5.0×10^?5-1×10^-3 s^-1, depending upon the solvent). The photolysis of the SC with visible light (λ > 400 nm) results in the total conversion of the SC into SP. The SC forms promptly after the addition of Al³⁺ at concentrations of the same order as those of MC. This method allows detection of [Al³⁺] to concentrations as low as 50 nM. The kinetics and thermodynamic parameters of the SP and MC reactions and of their complex formation with Al³⁺ are discussed.

Full text of DOI:10.1002/poc.4012

Received: 26 June 2019
DOI: 10.1002/poc.4012
Revised: 7 August 2019
Accepted: 12 August 2019
R E S E A R C H A R T I C L E
3
+
New stable colored complex of Al with 1',3',3'‐
trimethylspiro[2H‐1‐benzopyran‐2,2'‐indoline] (BIPS)
1
,2
1
1
Peter P. Levin
| Nikolay E. Belikov
| Irina I. Levina
|
1
1
3
Alexander S. Tatikolov | Andrey A. Khodonov
| Igor V. Khudyakov
1
Emanuel Institute of Biochemical
Abstract
Physics, Russian Academy of Sciences,
1
13
Moscow, Russia
Ultraviolet‐visible (UV‐vis) absorption, flash photolysis, and H and C NMR
spectroscopy were used to investigate the mechanism of formation and struc-
ture of complexes in the 1',3',3'‐trimethylspiro[2H‐1‐benzopyran‐2,2'‐indoline]
2
Semenov Institute of Chemical Physics,
Russian Academy of Sciences, Moscow,
Russia
3+
(SP, also well known as BIPS) with Al (inorganic salts) in ethanol (EtOH),
3
Department of Chemistry, Columbia
methanol (MeOH), and in aqueous MeOH solutions. A labile (equilibrium con-
University, New York, NY
−
1
3+
stants ≤ 5000 M ) complex of SP and Al with broad absorption band in the
Correspondence
UV‐vis with λ
= 380 nm appeared promptly in the presence of an excess of
3+
max
Igor V. Khudyakov, Department of
Chemistry, Columbia University, New
York 10027, NY, USA.
3+
Al . The slow formation of a stable complex (SC) between Al and two
merocyanine (MC) forms of SP with an intensive absorption band at λmax
=
Email: startatj@aol.com
4
30 nm is observed with a yield of 1.0 upon keeping the solutions of these
3+
two compounds at constant concentration ratio [Al ] ≥[SP]/2 in the dark.
The rate constants of such SC formation were close to the corresponding rate
−
5
‐3 ‐1
constant of the transformation of SP into MC in the dark (5.0×10 ‐1×10 s ,
depending upon the solvent). The photolysis of the SC with visible light (λ >
4
00 nm) results in the total conversion of the SC into SP. The SC forms
3+
promptly after the addition of Al at concentrations of the same order as those
3+
of MC. This method allows detection of [Al ] to concentrations as low as 50
nM. The kinetics and thermodynamic parameters of the SP and MC reactions
3+
and of their complex formation with Al are discussed.
KEYWORDS
BISP, complex, inorganic salts of Al, merocyanine, negative photochromism
1
| INTRODUCTION
The sensitivity of the photochromic properties of
spirocompounds towards solvent and different additives
makes them valuable material for different applications,
such as bio‐imaging, sensors, logic gates, and molecular
Spirocompounds are well‐known valuable photochromic
[2]
systems (see for review Durr and Bouas‐Laurent ). Their
photolysis leads to the highly colored merocyanine (MC)
intermediates. A number of spirocompounds undergo
efficient thermal opening of a spiro cycle with the forma-
tion of a corresponding MC as well. Scheme 1 presents
transformation of a specific spirocompound, namely,
[
3–8]
switches.
Specifically, such compounds have been
studied as dark or light‐controlled chemosensors for
[
8–11]
3+
[8–15]
metal ions
and Al
in particular.
There is
always a demand for reliable and facile sensors for salts
[
16–20]
and compounds of Al.
J Phys Org Chem. 2019;e4012.
wileyonlinelibrary.com/journal/poc
© 2019 John Wiley & Sons, Ltd.
2 of 11
https://doi.org/10.1002/poc.4012

3
of 11
LEVIN ET AL.
used. The solvent used for NMR (CD OD) had 99.8 atom
3
%
D and 0.2 vol. % H O. Its composition was confirmed
2
by NMR.
2.3 | NMR
1
13
SCHEME
1
.
1',3',3'‐trimethylspiro[2H‐1‐benzopyran‐2,2'‐
H and C NMR spectra were recorded on a Bruker
Avance III‐500 spectrometer with the working frequency
of 500 and 126 MHz, respectively. Chemical shifts are
indoline] (SP, well known as BIPS), which was studied in the
present communication.
given relative to tetramethylsilane. CD OD was also used
3
as an internal standard (δ 3.35 and 49.3 ppm, for methyl
groups in H and C measurements, respectively). We
used COSY spectroscopy for attribution of NMR signals
The focus of our research was the kinetics and mecha-
1
13
3+
nism of complex formation of Al
with 1',3',3'‐
trimethylspiro[2H‐1‐benzopyran‐2,2'‐indoline] (SP)/MC
(
a COSY spectrum is not presented). The concentration
in ethanol (EtOH), methanol (MeOH), and MeOH/H O
2
of SP in CD OD in all NMR measurements was 6.0 ×
3
mixtures (up to 0.5 mole fraction). Previous studies of
‐
2
3+
10 M; the concentration of Al salts varied from 1.0 ×
0 M to 0.3 M.
spirocompounds as chemosensors for Al demonstrated
‐
2
1
the formation of relatively labile complexes (LCs) МС‐
3+.[12–15]
Al
The instability of these complexes limits the
application of such systems. In the present work, the sta-
3+
2.4 | Chromatography
ble complex (SC) of Al with two MC of SP is described.
Aluminum salts are known as Lewis acids. Al salts of
strong acids undergo hydrolysis in the water‐containing
alcoholic solutions leading to acidic media. It is known
that strong acids are able to protonate MC of SP and
Preparative adsorption column chromatography was per-
formed on Kieselgel 60 (0.040‐0.063 mm, Merck) or on
HPLC‐chromatograph Smart Line 1000 (Knauer).
[
21–23]
essentially stabilize it that way.
Therefore, it is of
3+
+
interest to compare the reactivity of Al and H towards
SP/MC.
Despite its apparent simplicity, Al produces multiple
hydro and aqua complexes in the presence of water and
2
.5 | UV‐vis absorption and fluorescence
spectroscopies
3+
Ultraviolet‐visible (UV‐Vis) absorption spectra were mea-
sured in quartz cells with optical paths length l= 10 mm
on a Shimadzu UV‐2140PC spectrophotometer. The
kinetics of the photocoloration and photodecoloration
processes were taken on a fiber‐optic set spectrophotom-
eter HR‐2000 and spectrofluorimeter USB4000‐FL (Ocean
Optics, USA) in a quartz cells (l=10 mm). The irradiation
was performed with the UV (270 ≤ λ ≤ 370 nm) or vis
III
[24,25]
polynuclear Al soluble compounds as well.
These
multiple species are in dynamic equilibrium, which
depends on pH, the concentrations of salt, and other fac-
tors. The irreversible process of complex formation of the
3+
sensor with Al is required for the quantitative measure-
ments of the concentration of a dissolved salt of Al.
(
λ ≥ 400 nm) light of an LC8 lamp (Hamamatsu, Japan,
2
2
| EXPERIMENTAL
.1 | SP
2
irradiance 180 mW/cm ). An appropriate light filters
were used. The samples were stirred during irradiation.
The fluorescence measurements were performed on a
Fluorat‐02 Panorama spectrofluorimeter (Lumex, Russia).
[26]
SP was prepared as previously described in.
Purity
(
≥98%) was confirmed by high‐performance liquid chro-
1
matography (HPLC). SP structure was verified by
and C NMR (see Section 3.5).
H
13
2.6 | Flash photolysis
A conventional flash photolysis setup with UV‐vis
[
27]
2.2 | Other reagents
absorption registration was used.
The transient
absorption spectra and the kinetics of formation and
decay of intermediates were measured in the time range
t ≥ 10 μs in the spectral region 400 ≤ λ ≤ 800 nm. The
photoexcitation of the SP solutions (5.0 × 10 M) was
performed through the appropriate light filter (270 ≤ λ
Chemicals Al (NO ) x 9 H O, AlCl x 6 H O, hydrochloric
3
3
2
3
2
acid (HCl) 37 wt.%, and solvents MeOH and EtOH were
sourced from Sigma‐Aldrich. They were used as received.
Solvents were of a spectral grade. Water‐bidistillate was
−
5

LEVIN ET AL.
4 of 11
≤
370 nm). The quartz cuvette with optical paths l= 20
cm was used in these experiments.
All measurements were performed at room
temperature.
(K)
(
J)
3
3
| RESULTS AND DISCUSSION
.1 | Photochromism and
(
I)
H)
G)
(
thermochromism of SP
(
Absorption spectra and photochromic properties of SP/MC
[21,22,26,28,29]
(
A)
in alcohol solutions have been reported.
long‐wavelength band in the absorption spectrum of SP
has λ = 295 nм with extinction coefficient ε = 6.0 ×
The
max
−1
295
3
−1
1
0 M cm (see Figure 1a). Additionally,
the MC absorption band with λmax = 550 nm
FIGURE
absorption spectrum of 1',3',3'‐trimethylspiro[2H‐1‐benzopyran‐
,2'‐indoline] (0.01 M) in MeOH/H O mixtures with mole fraction
of water: (a) 0, (b) 0.022, (c) 0.0435, (d) 0.083, (e) 0.12, (f) 0.15, (g)
.189, (h) 0.25, (i) 0.31, (j) 0.409, and (k) 0.47
2
Long‐wavelength part of ultraviolet‐visible
(Figure 2) is barely seen in nonpolar
2
2
solvents but can be observed in relatively concentrated
[21,22]
alcoholic solutions in the dark.
It is formed in the
0
reaction ((1)). The extinction coefficient of MC absorption
band with λmax
=
+
[21,22]
(
MCH ) (Figure 2).
The intensity of this band is neg-
ligible in ethanol (EtOH).
k1
k−1
SP ⇌
MC
(1)
The flash photoexcitation of SP results in the forma-
tion of an additional amount of MC followed by its ther-
mal relaxation to the equilibrium concentration
(Equation (1)). The study of decay kinetics of this MC
leads to the values of k−1, which allows, in turn, the
[
28]
5
50 nm was estimated as a six‐fold of that of SP ε295.
Based on this value the equilibrium constants, K =
k1/k−1 were calculated (see Table 1):
1
It is worthwhile mentioning that a relatively small
determination of k = K × k−1 (Table 1). The obtained
1 1
shoulder was found at λ=430 nm in UV‐vis spectra of
values of these constants are similar to those reported in
other polar solvents to within 1 order of magnitude.
[28,29]
SP in MeOH as well as in H O/MeOH mixtures and it
2
was assigned to a small amount of protonated MC
An increase of the polarity of alcohol under transition
from EtOH (dielectric constant ε = 24) to MeOH (ε =
33) leads to a shift of equilibrium ((1)) to the right mostly
due to the increase of k (Table 1).
1
The increase of water content in the MeOH/H O mix-
2
ture results in the increase of the relative intensity of MC
+
absorption at λmax = 550 nm as well as MCH at λ = 430
(
G)
nm demonstrating the shift of the equilibrium ((1)) to the
right side (Figure 2). Figure 3 presents plots of k−1 and k1
vs. water content.
An increase of concentration of highly polar water
leads to the increase of the dielectric constant of
(
F)
(
(
E)
D)
(
C)
(
B)
(
A)
MeOH/H O mixture that stabilizes zwitterion MC and
2
leads to the pronounced increase in the rate of MC forma-
tion (see k in Table 1 and Figure 3). On the other hand,
1
the simultaneous decrease of k−1 is relatively
insignificant.
FIGURE 1 Ultraviolet‐visible absorption spectrum of SP (0.1
mM) in MeOH (a). Absorption spectra of the same solution were
measured promptly after addition of different concentrations of Al
The dependence of dielectric constant ε of a mixture
[
30]
MeOH/H O on H O mole fraction is linear.
At the
same time, similar plots of the constants, K , k , and k
−1
2
2
(
NO
3
)
3
× 9H
2
O (in mM): 0.2 (b), 0.4 (c), 0.9 (d),1.9 (e), 3.8 (f), and
1
1
upon H O mole fraction, essentially deviate from the lin-
ear dependence and nicely fit to exponentials. That fact
7
.3 (g). OD here and below stands for optical density. The role of
2
3
+
Al is discussed in Section 3.2 below

5
of 11
LEVIN ET AL.
+
TABLE 1 Rate constants k
1
, k−1, kSC, kMCH , and the equilibrium constant K
1
, K
2
, K
3
, and K
5
4
4
+
4
×10‐2,
×10‐2,
K
5
×10‐5,
M
K
10
1
4
×
k
s
−1
−1a
,
k
s
1
×10 ,
k
s
SC ×10 ,
k
s
MCH ×10 ,
K
M
2
−1a
K
M
3
a
−1 b
−1a
−1a
−1 a
−1a
Solvent
EtOH
1.1
5.1
0.48
0.45
0.29
0.19
0.53
2.3
0.66
2.5
‐
7.1
49
3.1
0.8
‐
‐
MeOH
1.4
2.9
7.1
21
1.8
MeOH/H
2
2
O (mole fractions 0.80:0.20) 11
O (mole fractions 0.58:0.42) 44
3.3
3.9
1.6
0.4
0.24
0.10
MeOH/H
8.4
8.9
1
Note. The K ‐value was obtained from the band intensities of MC at λmax = 550 nm and the known concentration of SP assuming the corresponding extinction
4
−1
−1.[28]
coefficient to be 3.6 × 10 M cm
a
Determination error 10%.
b
Determination error 20%.
the ET increase. The dependence is not a straight line
Figure 4). A “forceful” fit into the linear law gives a slope
(
of −1.0. We conclude that E quite satisfactory describes
T
the observed dependence of K upon solvent composition.
1
However, the longest wavelength of MC in solvents
does not correlate with ET despite the fact that both
Dimroth‐Reichard's betaine molecule and MC are both
conjugated zwitterions. We observed the maximum
hypsochromic shift in MC band as 5 nm, under transition
from MeOH to MeOH/H O; mole fraction of H O is 0.5.
2
2
Such change of the solvents is characterized by E change
T
from 55.8 to 57.3 (kcal/mol), which is a noticeable differ-
ence. At the same time, such shift in MC band from 555
to 550 nm is a change in the light energy from 51.40 to
51.85 kcal/mol. The latter energy change is very small.
1
FIGURE 3 Plots of the rate constant k−1 and k vs. mole fraction
of water in MeOH. The solid lines are a fit to an exponential. All
experiments were done at [SP] = 10 mM. The figure also presents
rate constants ksc (squares) and kMCH+ (triangles) vs. water content.
‐5
The latter data were obtained under [SP] = 5.0 × 10 M and [Al
The longest wavelength of betaine is a charge‐transfer
5
(
NO
3
)
3
× 9H
2
O] = 2.5 × 10‐ M or [HCl] = 1 mM. The triangles and
[34]
(
CT) transition.
However, the longest waveband of MC
squares correspond to the right ordinate axis. Section 3.3 below
seems to be of some other nature. In particular, MC has
insufficient conjugation for CT transition compared with
a conjugated system of betaine (see Scheme 1).
explains the meaning of kSC and kMCH+
implies the linear dependence of the corresponding equi-
librium (1) free energy ΔG ° and activation free energies
1
#
#
1
ΔG , ΔG−
on the mole fraction of MeOH/H O
2
1
mixture.
It was found that the spectral and kinetic properties of
MC forms of some other spiro compounds are affected by
solvent polarity, specific solvation ability (hydrogen
[31]
bonding), and by other properties.
However, in the
case of SP/MC in MeOH/H O mixture, the linear experi-
2
#
#
mental plots of the G °, ΔG , and ΔG− on the mole
1
1
1
fraction indicate the absence of essential preferential
solvation effect by MeOH or H O (see Supporting Infor-
2
mation S2 also).
In this work, we alternatively decided to look for corre-
lations between K and the known Dimroth‐Reichard
1
[1,32,33]
parameter E of a solvent.
We got a dependence
T
of logK or ΔG ° in MeOH/H O mixture, see Figure 4:
1
1
2
FIGURE 4 Dependence of Δ G ° in MeOH/H O mixture upon
1
2
[
1]
(
G ° were calculated using van't Hoff equation. ) One
E parameter of the mixture (see the text). E values are borrowed
1
T
T
[32]
can see a systematic decrease of ΔG ° (growth of K ) with
from Dawbler et al
1
1

LEVIN ET AL.
6 of 11
3
+
3+
3
.2 | LC of SP and Al
of LC at λ max = 380 nm, l‐ optical path; [Al ]
and
o
[
SP] are the initial molar concentrations. The values of
K2 obtained from the fits of plots of OD380 vs. [Al
o
3+
The formation of a LC of the closed form of SP and Al
was observed by UV‐Vis absorption in alcoholic solutions
(NO3)3 × 9H O] using Equation I are presented in
2
as well as in MeOH/H O mixtures promptly after the
mixing of solutions of Al and SP. LC is characterized
Table 1. The increase of organic solvent polarity stabilizes
2
3+
LC. The value of K for MeOH is higher than that for
2
by the broad absorption band at λmax = 380 nm (see
EtOH (Table 1). However, K essentially decreases upon
2
Figure 1). The plots of UV‐vis absorption at λ
nm (OD380) vs. [Al ] (Figure 5) are characteristic for
the equilibrium (2):
= 380
the addition of water (see Table 1).
max
3+
A similar absorption with λmax = 380 nm appears upon
addition of an excess of HCl to SP solutions (Figure 5c).
When compared with the similar band observed in the
K2
3+
3þ
3þ
presence of Al , only insignificant differences can be
SP þ AI
⇌ SP −AI
(2)
LC
identified. These minor differences are the extinction
coefficients and the form of a shoulder near λ = 340
nm. This band apparently belongs to a protonated SP
A kinetic treatment of Scheme 2 leads to Equation I
where ε380 is the extinction coefficient
+
[21,22]
(
SPH ) formed in the acid‐base equilibrium ((3)).
The K values (Equation (3)) obtained in MeOH and in
Â
Ã
þ
3
2
½LCꢀ ¼ ð2 OD380=ε380lÞ ¼ Al3
þ ½SPꢀ þ 1=K2 −
0
0
ꢁ
ꢀ
ꢁ
Â
Ã
Â
Ã
MeOH/H
2
O mixtures are smaller than those
3þ
3þ
0:5
ꢁ
Al
þ ½SPꢀ þ 1=K2 2 − 4 Al 0½SPꢀ0
;
0
0
K3
SP þ Hþ ⇌ SPH
þ
(3)
(
I)
3
+
of K for Al (see Table 1). This finding can be attrib-
2
3
+
uted to the three times larger positive charge on Al .
3+
+
(
C)
B)
Al and H can associate with the unshared pair of
electrons of either O or N atoms of SP. Resonance Raman
(
[22]
studies performed elsewhere
led to the conclusion of
(
A)
protonation at the Natom of SP at relatively low concentra-
tion of HCl in MeOH. The water molecules form hydrogen
bonds with N atom of SP, thus making this atom with
3+
+
unshared pairs of electrons screened off for Al and H .
The addition of concentrated HCl is accompanied by the
addition of a minor amount of water in solution. However,
at the highest used [HCl], the added amount of water is
smaller than 0.02 mole fraction. Such low amounts do not
affect the observed spectral and kinetic properties of the
studied systems. The same holds true for Al salts, which
contain coordinated water (crystallohydrates).
3 3 2
FIGURE 5 Plots of OD380 vs. [Al (NO ) x9H O] in EtOH (a) and
MeOH (b). (c) Plot of OD380 vs. [HCl] for equilibrium ((3)), see
below. [SP]= 0.1 mM in all solutions. The solid lines are the fits to
equation (I)
SCHEME 2 Chemical shifts (ppm) in
NMR spectra of SP (top) and SC (bottom)
1
in CD
3
3
OD of H NMR (two left structures)
1
and C NMR (two right structures). Here
n=1/2

7
of 11
LEVIN ET AL.
FIGURE 6 Ultraviolet‐visible absorption spectra of a solution SP (3.0 × 10‐ M) and Al (NO (1.5 × 10‐5 M) in EtOH. The spectra were
5
3 3
)
taken with every 10 min for the first hour after solution preparation, then every 30 min for the next 4 h, then every hour for the next 3 h. One
final spectrum was taken at 24 h. One can see an isosbestic point testifying to quantitative conversion of SP into SC
3
.3 | Slow formation of a SC of two MC
3
+
with Al and the protonated form of МС
+
(C)
(
МСH )
(
B)
(
A)
3+
The formation of a SC of two MC and Al was observed
by UV‐vis absorption spectroscopy in alcohols and
MeOH/H O mixture. In this section, the solutions with
2
‐5
‐4
low [SP] = 1.0 × 10 ‐1.0 × 10 M and with comparable
3
+
to [SP] concentration of Al will be considered. The
absorption spectra of promptly prepared solutions change
with time. The spectrum demonstrates growth of absorp-
tion at λmax = 430 nm and the concurrent decrease of
absorption at λmax = 295 nm, see Figure 6:
3+
FIGURE 7 Plots of OD430 vs. time for a solution SP/Al in
MeOH (a); MeOH/H O (0.2 mole fraction) (b); MeOH/H O (0.42
mole fraction) (c). We used [SP] = 5.0 × 10 , [Al3+] = 2.5 × 10‐5 M.
2
2
‐5
The plots of the absorption at λmax = 430 nm (OD430
)
vs. time nicely fit with exponential function (Figure 7).
The solid lines are a fit to the exponential functions
The amount of SC formed and the rate of SC (OD430
)
growth (kSC in Table 1) are
3+
3+
independent of [Al ] in the range [Al ] ≥ [SP]/2.
The final spectrum does not change during at least one
week. The ksc values are very close to the rate constant
k1 (Equation (1)) of the opening of SP into MC (see
Table 1 and Figure 3). The limiting stage of SC produc-
tion in the dark is the formation of MC. A similar conclu-
Thus, SC is a stable compound with stoichiometry of
two MC and one Al , which is formed overall in reaction
4) (see Sections 3.4 and 3.5 below). In these experiments,
the extinction
3+
(
3
þ
2
MC þ Al →SC
(4)
[12]
sion was made in Minkovska et al
complexes of metal cations and of Al in particular with
on the formation of
3+
coefficient of MC, which is a constituent of the SC, was
3
+
4
−1
−1
other spirocompounds. A decrease of a ratio [Al ]/[SP]
to less than ½ leads to a corresponding decrease of the
amount of the formed [SC].
estimated as ε430 = 3.0×10 M cm .
NMR experiments confirm the structure of SC as
3+
2MC:1Al on the molar basis and demonstrate that

LEVIN ET AL.
8 of 11
3
+
3+
Al forms the SC by an attachment of Al to the pheno-
late moiety of MC (see Section 3.5 below).
in the presence of divalent and trivalent metal ions has
demonstrated that the presence of water in the concentra-
tion more than 0.2 mole fraction prevents complex forma-
The addition of water accelerates both the formation of
MC from SP and that of SC to the same extent (see values
k1 and ksc in Table 1 and Figure 3) despite the fact that
the equilibrium concentration of LC is significantly
reduced by the presence of water (see Table 1 and Section
.2). Apparently, LC is not necessarily involved as a pre-
cursor in the formation of the SC. Water has no effect
on the final amount of SC formed even under molar ratio
SP: 1Al . The latter fact means that reaction (4) is irre-
versible at concentrations of water up to 0.5 mole
fraction.
The formation of similar absorption with λmax = 430
nm is observed in solutions of SP containing HCl in the
dark.
[
35,36]
tion.
One may expect that in the case of SP, the
presence of water would prevent quantitative analysis of
3+
other metal ions except for Al .
Photoillumination of the colored MC stabilized with
3+
+
3
Al or with Н by visible light completely regenerates
the starting SP, LC, or SPH+. Storage of the illuminated
solutions in the dark leads to the regeneration of SP as
well. This cycle can be repeated multiple times without
noticeable consumption of reagents. SС is a stable com-
pound with negative photochromism. Systems demon-
strating negative photochromism are of interest for
light‐controlled devices, eg, molecular switches controlled
3+
2
[21,22]
+
[8]
However, in the case of H , the equilibrium
by visible light.
(5) is observed
K5
MC þ Hþ ⇌ MCH
þ
(5)
3
.4 | Fast formation of a SC of two MC
3+
+
with Al and of protonated МС (МСH )
[21,22]
.
It is necessary to have an excess of HCl in order
to reach the maximum possible OD430 and the complete
In this section, the experiments were performed with
+
conversion of SP into SPH . The value of the extinction
solutions of the relatively large [SP] (1‐10 mM) and small
+
3+
coefficient of MCH obtained for the samples with com-
[Al ] (0.05‐50 μM). Under such [SP], the registration of
+
plete conversion of MC into MCH coincides with that
the MC absorption at λmax = 550 nм (see Section 3.1) is
already possible and the prompt formation of the SC
absorption with λmax = 430 nm is observed after the intro-
of SC and with the value estimated from the spectrum
+
[21]
of MCH presented in Yoda et al.
The rate constants
+
3+
of MCH formation (kMCH+ in Table 1 and Figure 3)
obtained from the curves similar to those presented in
Figure 6 are slightly smaller than k and k . The exis-
duction of the small amounts of Al to SP solutions. At
the same time, the MC band with λmax = 550 nm is pre-
served (see Figure 8):
1
SC
tence of equilibrium (5) results in an apparent decrease in
the rate of MCH formation.
A gradual decrease of [HCl] leads to a decrease of the
maximum obtainable OD430. It is assumed that under
such conditions [HCl] is still larger than [SP]. The value
The direct measurements attest to the fast reaction of
the complex formation between metal ions and MC forms
+
[
35–37]
of spirocompounds in solutions.
The onset time of
of K obtained from the plots of limit OD
values vs.
430
5
[
HCl] using equation similar to Equation I are presented
in Table 1.
K5 strongly depends upon the amount of added water.
K5 decreases 10 times upon the addition of 0.2 mole frac-
tion of water and 20 times upon the addition of 0.42 mole
+
fraction of water (see Table 1). Apparently, H with its
3+
thrice lower charge than Al forms a LC. Both com-
plexes demonstrate similar absorption bands and close
values of extinction coefficients due to the fact that they
have the same chromophore MC. Small differences are
observed only in the intensities of absorption shoulders
at λ = 360 and λ = 330 nm due to the insignificant differ-
ences in the vibration structure.
Addition of water leads to the classic electrolytic disso-
FIGURE 8 Long‐wavelength part of the ultraviolet‐visible
+
+
+
ciation of a salt‐like MCH . Therefore, H (H O ) is not
3
spectrum of SP in MeOH in the presence of Al (NO ) × 9H O in
3
3
2
3+
3
+
able to compete with Al in complex formation with
a small concentration; [Al ], μM: (a) 0.0, (b) 0.5, (c) 1.0, (d) 1.5, (e)
MC. The studies of dynamics of MC of substituted BIPS
2.0, (f) 2.5, (g) 3.0, (h) 3.5, and (i) 4.0. [SP] =0.01 M in all solutions

9
of 11
LEVIN ET AL.
3+
of Al by using SP solutions. The detection limit as low
as 50 nM can be estimated (see the Supporting Informa-
tion). Similar results were obtained with AlCl3.
(A)
3.5 | NMR study of SP, LC, and SC
(
B)
1
13
H‐, C‐, and COSY‐NMR (Section 2.3) study was per-
formed in order to figure out the binding sites of SP in
3+
LC and two MC with Al in SC. Scheme 2 presents
1
13
chemical shifts in H and C NMR spectra of SP and
FIGURE 9 Plots of OD430 vs. the concentration of Al (NO
O (a) and of HCI (b) in MeOH solutions of SP (5 mM). The lines
are the fits of experimental data to linear functions
3
)
3
× 9
SC in CD OD obtained. Our NMR data are consistent
3
H
2
with
[
38–40]
published data for SP in other solvents.
Minor
deviations observed are due to the specific solvation of
1
the equilibrium ((1)) being equal to 1/(k + k ) is rather
SP by MeOH. H NMR spectra of MC in SC are in accor-
1
−1
3+
+.[41] 13
short (approximately ≤5 s). That means that all Al is
able to be consumed promptly (less than in 5 s) in the
irreversible reaction (4) by the quickly appearing MC.
dance with the reported data for МСН
spectra of MC as a component of SP are obtained for
the first time.
C NMR
The intensity of the promptly formed band at λ
30 nm is directly proportional to [Al ] (see Figures 8
and 9a). The slope of this plot (Figure 9a)
=
NMR spectra of aliphatic and aromatic regions of SP
obtained prior to addition and promptly after addition
of an excess Al (NO ) × 9 H O (LC spectrum) are sim-
max
3+
4
3
3
2
gives an apparent extinction coefficient of SC, which is
times larger than that obtained in Section 3.2 for one
ilar to each other. That means that the formed LC com-
plex (see Section 3.2) is a complex of SP as was
suggested, but not a complex of MC. We refer the
reader to spectra a and b in Figure S1. The difference
between spectra is due to essential broadening of lines
2
MC in the complex. This fact testifies to the structure of
3+
SC as consisting of two MC with one Al .
+
In the case of H , we observed equilibrium (5). An
apparent extinction coefficient obtained by a similar
in LC spectrum due to equilibrium (2), see above. We
3+
1
manner like that for Al from the slope of a linear plot
observed also a signal of H O (δ 4.85 ppm in H NMR
2
+
for H (Figure 9b) was 2 times smaller than a similar
spectrum) and of a certain acidic proton (a broad line
1
value obtained in Section 3.3. The dependence of
at δ ~10 ppm in H NMR spectrum). The latter signal
+
[
MCH ] vs initial concentration of HCl ([HCl] ) can be
(0.04 M at the concentration of salt being equal to 0.2
o
+
described by a simple equation (II) since the [MC] is prac-
tically constant in these experiments. The knowledge of
genuine ε430 obtained in the
M) is probably due to H of HNO formed by hydrolysis
3
of Al (NO ) .
3
3
1
Further examination of H NMR spectra of LC demon-
strates additional signals of individual weakly acidic pro-
tons at δ 7.3 (1H), 7.6 (2H), and 7.75 (1H) ppm in the LC
spectrum, which are absent in the spectrum of SP. One
can suggest that these signals are due to H of water mol-
þ
½
MCH ꢀ ¼ OD430=ðε430lÞ ¼ ½HClꢀ =ð1 þ 1=ðK ½MCꢀÞÞ
o
5
(
II)
3+
previous section allowed for calculation of K ‐values,
ecules in the coordination sphere of Al in LC. These
signals disappear in the spectrum of SC: compare spectra
b and d (Figure S1).
5
which turn out to be rather close to the values obtained
in Section 3.3 (see Table 1).
3+
The abrupt further increase of [Al ] or [HCl] leads
The broadened signals of olefinic protons of LC at C3‐
C4‐position are observed at δ 5.77 and 6.96 ppm with the
coupling constant of J=9.5 Hz. The starting SP had cou-
pling J=10.3 Hz. A decrease of J under formation of LC
means a weakening of a double bond order in the
complex.
eventually to a prompt disappearance of the band at λmax
=
550 nm, and only the band at λmax = 430 nm is
observed. This means that the kinetics of the reaction
becomes the same as described in Section 3.3 when the
formation of MC according to equilibrium ((1)) is the
limiting stage of the process.
At the end of this section, we will mention that we did
not observe the fluorescence of a pure MC. SC demon-
strates weak fluorescence at λ = 550 nm. Spectrophotom-
etry is a convenient and reliable method of determination
3+
NMR spectra of solutions of Al , SP, and LC undergo
essential changes during their storage in the dark (see
Figure S2). These changes reflect the conversion of SP
into MC. The rates of the slow formation of SP in the dark
measured by NMR were the same as those obtained in

LEVIN ET AL.
10 of 11
UV‐vis experiments. The most important transformations
of NMR spectra are the following: (a) two non‐equivalent
methyl groups (3'; Scheme 2) in SP or LC (two distinct
singlets at δ 1.18 and 1.31 ppm) became magnetically
equivalent (the singlet for six equivalent H at δ 1.87
corresponding nitro MC zwitterions for complexing with
a metal ion.
In this work, we presented data related to aluminum
nitrate. Experiments with AlCl led to the same spectro-
3
scopic, kinetic and thermodynamic observations.
[34,41–43]
ppm) due to quasi‐planar structure of MC in SC,
b) the signal of N‐CH observed for SP and LC at 2.73
(
3
4
| CONCLUSIONS
ppm is downfield shifted to 4.16 ppm for SC due to the
[
42]
appearance of a positive charge on N‐atom in MC,
The main achievement of this work is the discovery of a
and (c) the two doublets of the olefinic protons at
C3‐C4‐position of SP or LC (δ 5.77 and 6.96 ppm with
the coupling constant of 10.2 and 9.5 Hz, respectively)
were downfield shifted in SC (doublets at δ 8.72 and
3 +
colored SC of Al
with two MC forms of the simple
spirocompound BIPS. The alcoholic solutions of BIPS
3+
can be used as a sensitive chemosensor of Al . It was
3+
demonstrated that [Al ] can be measured by “simple”
7
.77 ppm with the coupling constant of J=16.4 Hz) indi-
spectophotometry. The method has a high sensitivity
due to the large extinction coefficient of the complex
[34,41–43]
cating the trans‐MC form.
The essential unshielding of protons 5‐8 (Scheme 2)
can be attributed to the binding of phenolate anion with
a metal ion. The same conclusion one can make consider-
with two MC ligands, and the limit of a detectable
3+
Al
concentration was estimated as 50 nM (see
Supporting Information S3 also). It was shown that
13
ing chemical shifts in the C NMR spectrum (Scheme 2).
The signal of atom C bound to O is essentially unshielded
3+
strong acids cannot compete with Al , and that SP col-
3+
lects all Al from alcohols or from aqueous alcohols.
1
in MC compared with that in SP. H‐NMR spectra of SC
3+
The study of selectivity of a new chemosensor for Al
obtained at different ratios of components demonstrate
is in progress.
that SP has an established composition two MC and one
3+
A new Al complex is a stable reversible photoswitch
3+
Al . See the spectra in Figure S3.
with a negative photochromism, which may find an
application as UV and visible light‐activated light switch.
The comprehensive set of equilibria and rate con-
stants of the studied processes was obtained. In addi-
tion, we found that the studied SC possesses negative
photochromism.
Based on the NMR and other data presented above, we
conclude that a new complex has the following structure
(Scheme 3):
NMR spectra of SC, which had been kept for a long
enough time in the dark, exhibited narrow signals. This
observation testifies to the fact that the exchange pro-
cesses disappear with time. SC is the stable individual
chemical substance. LCs of MC obtained for other
AUTHORS AGREEMENT
3+
spirocompounds and metal ions and in particular Al
demonstrated broad NMR signals of MC fragment in a
The authors believe that they equally contributed to the
manuscript (doing experiments, discussing data, prepar-
ing the manuscript, etc.) The work was not supported
by a specific grant.
[14,34]
complex due to the exchange.
The presence of
electron‐withdrawing nitro‐group in widely studied nitro
spirocompounds reduced the capability of the
ACKNOWLEDGEMENTS
The authors are grateful to Dr. Alexey V. Laptev, Ms.
Irina A. Melnikova, and Dr. Alexey Yu. Lukin for their
help with the synthesis of BIPS. We thank Professor Olga
V. Demina for her assistance with chemicals.
ORCID
Peter P. Levin https://orcid.org/0000-0002-7197-5775
Nikolay E. Belikov https://orcid.org/0000-0001-9566-7227
Irina I. Levina https://orcid.org/0000-0001-5791-3343
Andrey A. Khodonov
011
Igor V. Khudyakov https://orcid.org/0000-0002-0217-1167
https://orcid.org/0000-0001-9123-
8
SCHEME 3 Structure of a complex

11 of 11
LEVIN ET AL.
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