A cation-specific, light-controlled transient chromoionophore based on a benzothiazolium styryl azacrown ether dye

Article abstract of DOI:10.1021/ja964154j

A benzothiazolium styryl azacrown ether dye (1) and its complexation with Ba²⁺ and Na⁺ cations in acetonitrile solution have been studied by UV-vis absorption and emission spectroscopy. trans-1 is found to act as a normal chromoionophore for Ba²⁺ and Na⁺ at high concentration (≤ 10^-2 M) by complexation with the azacrown and as a light-controlled, transient chromoionophore specific for Ba²⁺ at low concentration (10^-5-10^-3 M). A quantitative analysis of the complexation and thermal reactions following photolysis has enabled a detailed mechanism to be proposed: trans-1 photoisomerization produces cis-1, which is stabilized in the presence of Ba²⁺ by dual intramolecular complexation of the cation with both azacrown and sulfonate groups.

Full text of DOI:10.1021/ja964154j

3456
J. Am. Chem. Soc. 1997, 119, 3456-3461
A Cation-Specific, Light-Controlled Transient Chromoionophore
Based on a Benzothiazolium Styryl Azacrown Ether Dye
Igor K. Lednev, Ronald E. Hester, and John N. Moore*
Contribution from the Department of Chemistry, The UniVersity of York,
Heslington, York YO1 5DD, U.K.
ReceiVed December 2, 1996X
Abstract: A benzothiazolium styryl azacrown ether dye (1) and its complexation with Ba2+ and Na+ cations in
acetonitrile solution have been studied by UV-vis absorption and emission spectroscopy. trans-1 is found to act as
a normal chromoionophore for Ba2+ and Na+ at high concentration (g10-2 M) by complexation with the azacrown
and as a light-controlled, transient chromoionophore specific for Ba2+ at low concentration (10-5-10-3 M). A
quantitative analysis of the complexation and thermal reactions following photolysis has enabled a detailed mechanism
to be proposed: trans-1 photoisomerization produces cis-1, which is stabilized in the presence of Ba2+ by dual
intramolecular complexation of the cation with both azacrown and sulfonate groups.
Introduction
and this interaction may be either intermolecular22-24 or
intramolecular, as for chromo- or fluoroionophores.25-30 Many
chromo- and fluoroionophores which are sensitive to different
ions have been investigated, mainly in solution17,26,29-45 but also
Macrocyclic ionophores based on polyether derivatives have
attracted much attention because of their selective affinity for
cations1-6 and their application as functional units within
supramolecular systems.7-11 The receptor and carrier charac-
teristics of macrocyclic molecules enable metal or molecular
ions to be recognized and transported and may facilitate the
development of supramolecular ionic devices. Significant
progress has been made in the development of sensing devices,
with different types of response being available from ionophores
with different functional properties, including ion-selective
electrodes9,12-14 and optrodes.9,15,16 The majority of ion and
molecular sensors are based on specific interactions which result
in easily recognizable changes in the sensor molecule, com-
monly observed via absorption, luminescence, or redox potential
properties.6-9,17-21 A strong interaction between the ionophore
and a dye is necessary to produce a strong optical response,
(19) Inouye, M.; Noguchi, Y.; Isagawa, K. Angew. Chem., Int. Ed. Engl.
1994, 33, 1163.
(20) Dieing, R.; Morisson, V.; Moore, A. J.; Goldenberg, L. M.; Bryce,
M. R.; Raoul, J.-M.; Petty, M. C.; Gar´ın, J.; Saviro´n, M.; Lednev, I. K.;
Hester, R. E.; Moore, J. N. J. Chem. Soc., Perkin Trans. 2 1996, 1587.
(21) Jørgensen, T.; Hansen, T. K.; Becher, J. Chem. Soc. ReV. 1994, 23,
41.
(22) Wolfbeis, O. S.; Schaffar, B. P. H. Anal. Chim. Acta 1987, 198, 1.
(23) Shimomura, M.; Shinohara, E.; Kondo, S.; Tajima, N.; Nagata, Y.;
Koshiishi, K. Sens. Mater. 1992, 4, 29.
(24) Shimomura, M.; Honma, A.; Kondo, S.; Shinohara, E.; Tajima, N.;
Koshiishi, K. Sens. Actuators 1993, B13-14, 629.
(25) Supramolecular Chemistry I-Directed Synthesis and Molecular
Recognition; Weber, E., Ed.; Springer-Verlag: Berlin, 1993.
(26) Dix, J. P.; Vo¨gtle, F. Chem. Ber. 1980, 113, 457.
(27) Takagi, M. In Cation Binding by Macrocycles. Complexation of
Cationic Speacies by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel
Dekker, Inc.: New York, 1990; Chapter 11, pp 465-498.
(28) Zollinger, H. Color Chemistry: Synthesis, Properties and Applica-
tions of Organic Dyes and Pigments; VCH: Weinheim, 1987.
(29) Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552.
(30) Le´tard, J. F.; Lapouyade, R.; Rettig, W. Pure Appl. Chem. 1993,
65, 1705.
(31) Martin, M. M.; Plaza, P.; Meyer, Y. H.; Badaoui, F.; Bourson, J.;
Lefe`vre, J.-P.; Valeur, B. J. Phys. Chem. 1996, 100, 6879.
(32) Martin, M. M.; Plaza, P.; Dai Hung, N.; Meyer, Y. H.; Bourson, J.;
Valeur, B. Chem. Phys. Lett. 1993, 202, 425.
(33) Bourson, J.; Valeur, B. J. Phys. Chem. 1989, 93, 3871.
(34) Fery-Forgues, S.; Le Bris, M.-T.; Guette˘, J.-P.; Valeur, B. J. Phys.
Chem. 1988, 92, 6233.
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
(1) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem.
ReV. 1995, 95, 2529.
(2) Cram, D. J.; Cram, J. M. Container Molecules and Their Guests;
Royal Society of Chemistry: Cambridge, 1994.
(3) Transition Metals in Supramolecular Chemistry; Fabbrizzi, L.; Poggi,
A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994.
(4) Dietrich, B.; Viout, P.; Lehn, J.-M. Macrocyclic Chemistry; VCH:
Weinheim, 1993.
(5) AdVances in Supramolecular Chemistry; Gokel, G. W., Ed.; JAI Press
Inc.: Greenwich, CT, 1993; Vol. 3.
(6) Cation Binding by Macrocycles. Complexation of Cationic Speacies
by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker, Inc.: New
York and Basel, 1990.
(7) Lehn, J.-M. Supramolecular Chemistry. Concepts and PerspectiVes;
VCH: Weinheim, 1995.
(8) Fabbrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 197.
(9) Wright, J. D. Chem. Brit. 1995, 374.
(10) Lednev, I. K.; Petty, M. C. AdV. Mater. 1996, 8, 615.
(11) Crown Compounds: Toward Future Applications; Cooper, S. R.,
Ed.; VCH Publishers, Inc.: New York, 1992.
(35) Le´tard, J.-F.; Delmond, S.; Lapouyade, R.; Braun, D.; Rettig, W.;
Kreissler, M. Recl. TraV. Chim. Pays-Bas 1995, 114, 517.
(36) Dumon, P.; Jonusauskas, G.; Dupuy, F.; Pe´e, Ph.; Rullie`re, C.;
Le´tard, J.-F.; Lapouyade, R. J. Phys. Chem. 1994, 98, 10391.
(37) Mrozek, H.; Nikol, H.; Vogler, A.; Vo¨gtle, F. J. Photochem.
Photobiol. A: Chem. 1994, 84, 227.
(38) Mason, A.; Sheridan, A.; Sutherland, I. O.; Vincent, A. J. Chem.
Soc., Chem. Commun. 1994, 2627.
(12) Reinhoudt, D. N. Recl. TraV. Chim. Pays-Bas 1996, 115, 109.
(13) Pranitis, D. M.; Teltingdiaz, M.; Meyerhoff, M. E. Crit. ReV. Anal.
Chem. 1992, 23, 163.
(14) Kimura, K.; Shono, T. In Cation Binding by Macrocycles: Com-
plexation of Cationic Species by Crown Ethers; Inoue, Y., Gokel, G. W.,
Eds.; Marcel Dekker, Inc.: New York, 1990; Chapter 10, pp 429-463.
(15) Alava-Moreno, F.; Pereiro-Garcia, R.; Diaz-Garcia, M. E.; Sanz-
Medel, A. Sens. Actuators 1993, 13-14, 276.
(16) Lambeck, P. V. Sens. Actuators 1992, B8, 103.
(17) Valeur, B.; Bardez, E. Chem. Brit. 1995, 31, 216.
(18) Beer, P. D. In AdVances in Inorganic Chemistry; Sykes, A. G., Ed.;
Academic Press, Inc.: San Diego, 1992; Vol. 39, pp 79-157.
(39) Schmittel, M.; Ammon, H. J. Chem. Soc., Chem. Commun. 1995,
687.
(40) Alfimov, M. V.; Gromov, S. P.; Lednev, I. K. Chem. Phys. Lett.
1991, 185, 455.
(41) Lednev, I. K.; Gromov, S. P.; Ushakov, E. N.; Alfimov, M. V.;
Moore, J. N.; Hester, R. E. Spectrochim. Acta 1992, 48a, 799.
(42) Lednev, I. K.; Fomina, M. V.; Gromov, S. P.; Stanislavsky, O. B.;
Alfimov, M. V.; Moore, J. N.; Hester, R. E. Spectrochim. Acta 1992, 48a,
931.
(43) Gromov, S. P.; Fyedorova, O. A.; Ushakov, E. N.; Stanislavsky,
O. B.; Lednev, I. K.; Alfimov, M. V. Dokl. Akad. Nauk SSSR 1991, 317,
1134.
S0002-7863(96)04154-6 CCC: $14.00 © 1997 American Chemical Society

Cation-Specific Transient
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3457
in films.10,46-49 In chromoionophores, the color change usually
occurs because of charge redistribution within the dye unit upon
ion complexation,9,27 and the most dramatic and specific color
changes have been found to occur when the ionophore and dye
units share common atoms; for example, one or more hetero-
atoms of a crown ether ionophore may also form part of the
π-conjugated system of a dye. The most promising examples
of such compounds include monoazacrown ether derivatives in
which the nitrogen heteroatom is within the π-conjugated system
of a chromophore or fluorophore,26,28,37,39 and absorption and/
or emission color changes have been studied for several styryl
and azobenzene monoazacrown derivatives.26,29,30,33,34 In gen-
eral, cation-specific color changes are observed on complexation
in solvents of moderate polarity, but the sensitivity is not high.
For example, the stability constant for complexation with alkali
and alkaline-earth metal cations in acetonitrile is typically 2 or
3 orders of magnitude smaller for monoaza-15-crown-5 ether
styryl dyes30,33,34 than for monoaza-15-crown-5 ether itself,50
arising from the electron-withdrawing effect of the dye and the
partial positive charge on the azacrown nitrogen atom which
results.51
Here we report studies of a benzothiazolium styryl dye (1),
containing both an aza-15-crown-5 ether and an alkylsulfonate
substituent, and its complexation with barium and sodium
cations in acetonitrile solution. These studies demonstrate,
remarkably, that the same chromoionophore may act as a sensor
for barium ions in two different concentration ranges. While
the dye acts as a normal chromoionophore for Ba2+ and Na+ at
high concentrations, it acts as a transient, cation-specific, light-
controlled chromoionphore for Ba2+ at low concentrations. A
detailed analysis of the kinetics of the cis-trans thermal
isomerization reaction enables the stability constant for cation
complexation with the cis-isomer formed on photolysis to be
determined, and a five-state, quantitative reaction scheme is
presented which summarizes the observations. These studies
are relevant to the molecular design and development of
sensitive cation-specific optical sensors and of photocontrolled
ion-release systems.
Macrocyclic molecules can be used as ion-selective carriers,
and the development of triggered ion-capture, -release, and/or
-transport devices is an important goal. For photoresponsive
macrocyclic chromoionophores, it is possible to control the ion-
binding and transport processes with light,9,52,53 forming the basis
for photocontrolled ion-switching devices.7,54 The photocontrol
of cation binding can be achieved by several different mecha-
nisms, such as photoinduced structural change within the crown
ether group or photocontrolled intramolecular coordination
between a crowned cation and a pendant ionic group.52 The
majority of such systems are based on the photoisomerization
of chromophores, such as azobenzenes, thioindigos, and spiro-
pyrans, or on the photodimerization of anthracene.19,52,55-58 In
all of these cases, photolysis results in the formation of a new
species with an affinity for cations which is different from that
of the starting compound. One example is a benzothiazolium
styryl dye containing a 15-crown-5 ether, which has been shown
to have a higher stability constant for complex formation with
alkaline-earth metal cations in the cis-isomer form.40 This arises
because the stabilizing effect of intramolecular coordination
between the crowned cation and the negatively charged alkyl-
sulfonate substituent of the benzothiazolium dye is possible only
in the cis-isomer form.40
Experimental Section
The synthesis of the trans-isomer of betaine 2-{2-[4-(4,7,10,13-
tetraoxa-1- azacyclopentadecyl)phenyl]ethenyl}-3-(3-sulfopropyl)ben-
zothiazolium (1) has been described previously.59 2-[4-(Dimethylamino)-
styryl]-3-ethylbenzothiazolium iodide (2), 1-(3-sulfopropyl)pyridinium
hydroxide, sodium 9,10-anthraquinone-2-sulfonate, acetonitrile (anhy-
drous), potassium 4-sulfo-1,8-naphthalic anhydride (SNA), 1,8-naph-
thalic anhydride (NA), NaClO4, and Ba(ClO4)2 (all from Aldrich) were
used as received.
A Hitachi U-3000 spectrophotometer was used to measure absorption
spectra, using cells of 1, 5, or 10 cm pathlength. A 250 W Xe lamp
(XENOPHOT HLX 64655) fitted with a 450 nm long-pass filter was
used for sample photolysis. For the kinetic studies, the sample was
photolyzed (ca. 1 min) and transferred immediately to the spectrometer,
the lid was closed, and the absorbance at a fixed wavelength was
measured as a function of time. A Shimadzu RF-150X spectrofluo-
rimeter, with perpendicular sampling geometry, was used to measure
the emission spectra; the sample was contained in a 1 cm pathlength
cell, and the concentration chosen to give an absorbance of ca. 0.04 at
the excitation wavelength. All measurements were made at room
temperature (ca. 18 °C). The data were fitted using the SPSS statistical
package, which allowed nonlinear regression analysis to be performed;
fitted values are reported with uncertainties of (2σ.
(44) Gromov, S. P.; Fomina, M. V.; Ushakov, E. N.; Lednev, I. K.;
Alfimov, M. V. Dokl. Akad. Nauk SSSR 1990, 314, 1135.
(45) Alfimov, M. V.; Fedorov, Y. V.; Fedorova, O. A.; Gromov, S. P.;
Hester, R. E.; Lednev, I. K.; Moore, J. N.; Oleshko, V. P.; Vedernikov, A.
I. Spectrochim. Acta, A, in press.
(46) Lednev, I. K.; Petty, M. C. J. Phys. Chem. 1995, 99, 4176.
(47) Lednev, I. K.; Petty, M. C. J. Phys. Chem. 1994, 98, 9601.
(48) Lednev, I. K.; Petty, M. C. Langmuir 1994, 10, 4185.
(49) Lednev, I. K.; Petty, M. C. Thin Solid Films 1996, 285, 683.
(50) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem.
ReV. 1991, 91, 1721.
(51) Lednev, I. K.; Hester, R. E.; Moore, J. N. J. Chem. Soc., Faraday
Trans., in press.
(52) Shinkai, S. In Cation Binding by Macrocycles. Complexation of
Cationic Speacies by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel
Dekker, Inc.: New York and Basel, 1990; Chapter 9, pp 397-428 and
references therein.
(53) De Silva, A. P.; McCoy, C. P. Chem. Ind. 1994, 992.
(54) Kimura, K. Coord. Chem. ReV. 1996, 148, 41.
(55) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Phys. Chem. 1992,
96, 5614.
(56) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Chem. Soc., Perkin
Trans. 2 1992, 613.
Results and Analysis
Cation Complexation with the Azacrown of trans-1.
Acetonitrile solutions of trans-1 and the model dye, trans-2,
exhibited closely similar UV-vis absorption spectra, indicating
that their electronic structures and spectral properties are
determined principally by the dye group. The absorption
spectrum of trans-1 (ca. 4 × 10-6 M) in acetonitrile solution
(λmax ) 522 nm) was found to change on addition of Ba(ClO4)2
at >10-3 M, as illustrated in Figure 1a,b, and a color change
from dark pink to ochre was observed in forming this complex
(λmax ) 438 nm). Such changes are typical of metal ion
(57) Kimura, K.; Yamashita, T.; Yokoyama, M. Chem. Lett. 1991, 965.
(58) Inouye, M.; Ueno, M.; Kitao, T.; Tsuchiya, K. J. Am. Chem. Soc.
1990, 112, 8977.
(59) Lednev, I. K.; Fyedorova, O. A.; Gromov, S. P.; Alfimov, M. V.;
Moore, J. N.; Hester, R. E. Spectrochim. Acta 1993, 49a, 1055.

3458 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
LedneV et al.
Figure 3. UV-vis absorption spectrum of 1 (ca. 2 × 10-5 M) in
Figure 1. UV-vis absorption spectra of 1 (ca. 4 × 10-6 M) in
acetonitrile solution: (a) in the absence of added barium salt, (b) in
the presence of Ba(ClO4)2 (ca. 0.3 M), (c) after >450 nm photolysis in
the presence of Ba(ClO4)2 (ca. 1 × 10-4 M). UV-vis emission spectra
of 1 (ca. 1 × 10-6 M) on 520 nm excitation: (d) in the absence of
added barium salt, (e) in the presence of Ba(ClO4)2 (ca. 1 × 10-4 M)
and with nonattenuated fluorimeter lamp excitation.
acetonitrile solution, in the presence of NaClO4 at (a) 0, (b) 5.5 × 10-3
,
(c) 2.6 × 10-2, (d) 7.2 × 10-2, and (e) 0.25 M.
than that on complexation with Ba2+ (∆λ ) 84 nm); the relative
magnitude of these shifts is similar to those reported for metal
cation complexation with aza-15-crown-5 ether groups within
other styryl chromoionophores.9,26,29,33,51
Cation Association with the Sulfonate Group. The sul-
fonate group within 1 provides a second possible site for cation
association and this process must be considered in the quantita-
tive study of such dyes. The possibility of Ba2+ and Na+
association with the sulfonate group in acetonitrile solution has
been investigated here by NMR, IR, and UV-vis absorption
studies of 1 and other sulfonates. These studies are nontrivial
because of the need to study the reaction at a sulfonate
concentration lower than the cation concentration at which
effective association occurs.
The NMR and IR techniques are relatively insensitive and
require sulfonate concentrations of >10-4 M, at which studies
of trans-1 are complicated by aggregate formation. NMR
studies of trans-1 (4 × 10-4 M), and IR studies of the sulfonate
band of 1-(3-sulfopropyl)pyridinium hydroxide (3 × 10-4 M),
both indicated a relatively high stability constant for Ba2+
-
-
SO3 association in acetonitrile of K ′ > 3 × 103 M-1. UV-
vis spectroscopy may be used at lower concentrations, but in
this case the association is monitored indirectly; cation associa-
tion with alkylsulfonates such as 1 does not result in changes
in the UV-vis spectrum, and consequently, we have studied
arylsulfonates which do show observable changes. A UV-vis
study of sodium 9,10-anthraquinone-2-sulfonate (3 × 10-5 M)
gave an approximate value of K ′ ) (3 ( 2) × 105 M-1 for
Ba2+ association in acetonitrile. After testing several other
compounds, we have found the most useful arylsulfonate to be
the potassium salt of 4-sulfo-1,8-naphthalic anhydride (SNA),
and we have studied it along with its nonsulfonated analogue,
1,8-naphthalic anhydride (NA), for comparison.
Figure 2. Ba(ClO4)2 concentration dependence of (a) the absorbance
of 1 at 520 nm, fitted to eq 1, and (b) kobsd, fitted to eq 8. NaClO4
concentration dependence of (c) the absorbance of 1 at 520 nm, fitted
to eq 1, and (d) kobsd, fitted to eq 8.
complexation, and the concentration dependence of the absorb-
ance, A, at a fixed wavelength fits well to eq 1, where Ao and
A∞ are the absorbances at zero and infinite metal ion concentra-
tions ([M]), respectively, indicating that 1:1 ligand-cation
complexes are formed.29 The stability constant for complex
A ) (Ao + A∞K[M])/(1 + K[M])
(1)
The UV-vis absorption spectra of SNA and NA are different,
as shown in Figure 4a,b. The addition of Ba(ClO4)2 at >10-5
M to an acetonitrile solution of SNA (ca. 3 × 10-7 M) resulted
in changes in the spectrum, and increasing the Ba(ClO4)2
concentration to 10-3 M resulted in a spectrum (Figure 4c)
similar to that of NA. By comparison, the addition of Ba(ClO4)2
at e10-2 M did not affect the absorption spectrum of NA in
acetonitrile; the spectrum showed very small changes only when
the concentration of Ba(ClO4)2 was increased further to >0.05
M. These results indicate that the sulfonate group of SNA
complexes with Ba2+ at a concentration of 10-5-10-4 M.
Isosbestic points in the spectra would be expected for 1:1
complex formation, but these points were poorly defined; the
changes in the spectrum were small and subject to error, but it
is also possible that several different complexes form, including
those involving perchlorate ions or additional SNA molecules.
The dependence of the absorbance on Ba2+ concentration fits
well to eq 1, which applies for the formation of only one
formation, K, was estimated to be 70 ( 15 M-1 (Figure 2a)
and was found to be independent of the concentration of 1 within
the range from 1 × 10-5 to 1 × 10-7 M. The large spectral
changes indicate that the electronic structure of 1 is altered
significantly on cation complexation. This can be attributed to
coordination of Ba2+ to the azacrown ether and to the significant
charge redistribution within the PhN+dCCdCPhN chro-
mophore which results. The spectrum of 2 did not change on
addition of Ba2+, indicating that complexation does not occur
in the absence of the azacrown ether.
The absorption spectrum of trans-1 in acetonitrile was studied
as a function of NaClO4 concentration for comparison (Figure
3). The formation of a complex (λmax ) 498 nm) between Na+
and the azacrown of trans-1 is evident from these spectra, and
eq 1 was used to obtain K ) 35 ( 10 M-1 (Figure 2c). The
long-wavelength absorption band of trans-1 exhibited a smaller
hypsochromic shift on complexation with Na+ (∆λ ) 24 nm)

Cation-Specific Transient
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3459
At ) A∞ + (A0 - A∞)exp(-kobsdt) (2)
The addition of Ba(ClO4)2 was found to increase the lifetime
of cis-1 substantially. The absorption spectra of 1 in acetonitrile
solution containing Ba(ClO4)2 at 1 × 10-4 M were measured
before (similar to Figure 1a) and ca. 1 min after irradiation
(Figure 1c); the large change in the absorbance on irradiation
resulted in a distinct color change from dark pink to light pink,
attributable to a slower recovery time and a higher concentration
of the cis-isomer in the photostationary state. The initial
spectrum was found to recover fully in the dark, and the recovery
kinetics were studied for 1 (5 × 10-7 M) in the presence of
Ba(ClO4)2 at concentrations in the range of 1 × 10-6-2 ×
10-2 M; all of the kinetic data fit well to eq 2 to give the
observed rate constant, kobsd, as a function of Ba2+ concentration
(Figure 2b).
Figure 4. UV-vis absorption spectra of (a) NA (3 × 10-7 M) in
acetonitrile (offset), (b) SNA (3 × 10-7 M) in acetonitrile in the absence,
and (c) in the presence of Ba(ClO4)2 (6.9 × 10-4 M). Inset: Ba(ClO4)2
concentration-dependence of the absorbance of SNA at 354 nm, fitted
to eq 1.
The lifetime of cis-1 was measured as a function of the
concentration of 1 in the range of 5 × 10-8-5 × 10-6 M, at
two fixed Ba(ClO4)2 concentrations. The lifetime of cis-1 was
found to be independent of the chromoionophore concentration
and to be 92 ( 4 and 258 ( 3 s for [Ba2+] ) 1 × 10-5 and 5
× 10-4 M, respectively.
complex, giving K ′ ) (2.2 ( 0.5) × 104 M-1 at 354 nm (Figure
4 inset) and K ′ ) (3 ( 1) × 104 M-1 at 337 nm.
The addition of NaClO4 at e10-2 M resulted in very small
changes in the UV-vis absorption spectrum of SNA in aceto-
nitrile and significant changes occurred only at NaClO4
concentrations of g0.1 M. The absorption spectrum of SNA
at [NaClO4] ) 1.3 M was found to be similar to that at
[Ba(ClO4)2] ) 7 × 10-4 M, but the interpretation of the data is
complicated by the observation that the spectrum of NA also
showed changes on addition of NaClO4 at >0.1 M; this may
result from the association of Na+ cations with the oxygen atoms
of the anhydride group. It may be concluded that the association
constant for the interaction of the sulfonate group of SNA with
The lifetime of cis-2 remained almost unchanged on addition
of Ba(ClO4)2 up to a concentration of 0.16 M, indicating that
the transient color change does not occur in the absence of
azacrown and sulfonate groups. The lifetime of cis-1 in
acetonitrile was not affected by the addition of NaClO4 at
concentrations below 5 × 10-3 M, indicating that the effect is
cation-specific, and the lifetime increased only at Na+ concen-
trations of g10-2 M (Figure 2d).
Fluorescence Measurements. The fluorescence spectra of
1 (10-6 M) in acetonitrile solution in the absence and presence
of Ba(ClO4)2 (10-4 M) were measured in two different ways
using a fluorimeter. In one case, the excitation beam (520 nm)
was attenuated by ca. ×2000 with a neutral-density filter and
an iris; a small decrease of ca. 10-15% in the fluorescence
intensity was found on addition of the barium salt. In the second
case, the excitation beam was not attenuated and a large decrease
of ca. 85% in the fluorescence intensity was observed on
addition of the barium salt (Figure 1d,e). This effect was
observed in <1 min after exposure of the sample to the
fluorimeter excitation beam. The profiles of the fluorescence
spectra (λmax ) 583 nm) were similar in each case.
Na+ in acetonitrile is e102 M-1
.
These studies of sulfonate-cation association indicate that
any quantitative study of sulfonated dyes in acetonitrile requires
this effect to be considered. This association occurs readily at
low Ba2+ concentrations and, consequently, we have been able
only to estimate lower limits for the K′ values for alkylsul-
fonates, which would be the best models for 1. The values we
have obtained for arylsulfonates may be influenced by charge
redistribution arising from the aromatic ring to which the
sulfonate is directly attached, but nevertheless they serve as
accessible models. Thus, for 1, we conclude that the Ba2+
-
-
SO3 association constant is likely to be in the range of K ′ )
Discussion
-
104-106 M-1, while that for Na+-SO3 association is likely
to be K ′ e 102 M-1
.
All of the observations can be interpreted by a five-state
scheme including cis- and trans-isomers and metal ion com-
plexation with sulfonate and azacrown groups (Figure 5). The
general scheme for the thermal back-reaction of cis-trans
isomerization in the presence of metal ions in solution is given
by eqs 3-6
Isomerization of 1. Styryl dyes photoisomerize readily,
giving a distinct change in the UV-vis absorption spectrum, but
this change often cannot be seen by eye because the cis-isomers
usually are unstable at ambient temperature and rapidly revert
to the trans-isomers in the dark.59 The photoisomerization
reactions of trans-1 and -2 in acetonitrile were studied by visible
irradiation (λ > 450 nm) into the long-wavelength absorption
band. Photoisomerization was observed, and the kinetics of the
reverse cis-trans thermal isomerization were studied by
measuring the time-dependence of the absorbance at a fixed
wavelength (520 nm) after irradiation ceased. Color changes
were not observed visually because of the rapid recovery times
and the low concentration of the cis-isomer in the photo-
stationary state. The kinetic data fit well to eq 2, where At is
the absorbance at time t after irradiation ceased, A0 and A∞ are
the initial and final absorbances, and kobsd is the observed rate
constant obtained from the fit. This analysis gave lifetimes (1/
kf
cis-L
9
8 trans-L
(3)
(4)
(5)
(6)
kc
cis-LM 98 trans-LM
cis-L + M {K } cis-LM
cis
trans-L + M {K } trans-LM
trans
where L is the ligand and M the metal cation, kf and kc are the
rate constants of thermal cis-trans isomerization of 1 in free
and cation-complexed forms, respectively, and Kcis and Ktrans
are the stability constants for complexation of cis- and trans-1,
k
obsd) of ca. 26 and 10 s for cis-1 and cis-2, respectively, in
acetonitrile at room temperature.

3460 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
LedneV et al.
Figure 5. Proposed scheme for the photochemical (hν) and thermal (∆) reactions of 1 in acetonitrile, in the presence of Ba2+, along with stability
constants and rate constants.
respectively, with the metal cation. The establishment of
equilibria eqs 5 and 6 will occur more rapidly than the thermal
isomerization reactions in eqs 3 and 4, and therefore the system
is at equilibrium, from the viewpoint of complexation, at all
times during the dark reactions in eqs 3 and 4. For this
condition, and assuming that only a small fraction of the metal
cations are complexed (i.e., that the concentration of free cations
is essentially equal to the total concentration of the salt), the
kinetic equation for the dark reaction is
105 M-1, kf ) (3.9 ( 0.4) × 10-2 s-1, and kc ) (3.1 ( 2.0) ×
10-3 s-1 were obtained for Ba2+. The fitted value of kf is in
agreement with the measured lifetime in the absence of Ba2+
,
and kc is an order of magnitude lower than kf, indicating that
the cis-isomer is stabilized by the presence of Ba2+
. This
stabilization occurs at Ba2+ concentrations of 10-5-10-3 M,
below those at which Ba2+ complexation with the azacrown of
-
trans-1 occurs but in the range where Ba2+-SO3 association
occurs. The fitted stability constant for cis-1-Ba2+ formation,
Kcis ) 2.7 × 105 M-1, is within the range of K ′ ) 104-106
[cis-L]Σ ) [cis-L]Σ0 exp(-kobsdt)
(7)
-
M-1 estimated for Ba2+-SO3 association, and this suggests
that cation association with the sulfonate plays an important
role in the stabilization mechanism. Barium-sulfonate ion
association alone is most unlikely to stabilize the cis-isomer,
and the most likely mechanism is the formation a “closed” cis-
isomer form in which a “tail-biting” intramolecular Ba2+ cation
bridge connects the alkylsulfonate and azacrown ether groups
of 1 (Figure 5). This proposal is substantiated by earlier studies
of benzothiazolium styryl dyes containing a 15-crown-5 ether
group and a propylsulfonate chain.40,43 In this case, tail-biting
intramolecular coordination was proposed to occur for the cis-
isomer in the presence of alkaline-earth metal cations at low
concentration. A strong hypsochromic shift of the dye absorp-
tion band was observed at low cation concentrations, and this
where kobsd is the observed rate constant, t is the time after
photolysis ceased, [cis-L]Σ ) [cis-L] + [cis-LM], [cis-L]Σ
[cis-L]Σ at time t ) 0, and
)
0
kobsd ) kf/(1 + Kcis[M]) + kc/(1 + 1/Kcis[M])
(8)
Equation 7 indicates that the kinetics of thermal cis-trans
isomerization are monoexponential at any concentration of metal
cation; it may be rewritten as eq 2 to give the time-dependence
of the experimentally observed absorbance. Equation 8 gives
the observed rate constant as a function of cation concentration.
The data fit well to eq 8, as shown in Figure 2b,d for Ba2+
and Na+, respectively. Fitted values of Kcis ) (2.7 ( 1.0) ×

Cation-Specific Transient
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3461
shift decreased substantially as the metal cation concentration
was increased to give a spectrum which resembled that of the
cis-isomer in the absence of metal cations: this change was
attributed to the sulfonate associating preferentially with free
metal cations in solution at high cation concentrations. The
effects ascribed to this tail-biting interaction were not observed
for an analogue with an ethyl rather than a propylsulfonate group
pendant to the benzothiazolium dye.
of 1. Thus, 1 may be used with a fluorimeter to detect Ba2+ at
low concentrations by comparison of the emission intensity with
that of a sample of 1 in the absence of Ba2+; this technique,
although giving a less distinct effect, is complementary to the
probes of UV-vis absorption or color change.
It can be expected that the ion selectivity of this effect will
be determined both by the association efficiency of different
cations with the sulfonate group and by the strength of the
additional complexation with the azacrown ether ring. The
contrasting effectiveness of Ba2+ and Na+ cations in complexing
cis- and trans-1 reported here confirms this; while 1 acts as a
normal chromoionophore for Na+, the weak association of this
cation with the sulfonate group in acetonitrile renders it
ineffective as a transient chromoionophore for sensing low
concentrations of Na+. Preliminary studies of the stabilization
of cis-1 in the presence of different alkali and alkaline-earth
metal cations in acetonitrile show that the selectivity rules are
different from those predicted solely on the basis of the crown
ether ring size/ion size effect; these results will be reported in
full elsewhere.
The results presented here also are relevant to the develop-
ment of photoionic molecular devices and may be described in
this context.60 Compound 1 may be considered as a device for
two categories of photoionic signal processing. First, as a one-
input/one-output functionality; the presence of Ba2+ at g10-2
M (ionic input) results in a substantial photonic output change,
observed as an increase in the visible transmittance. Second,
as a two-input/one-output functionality; the presence of both
Ba2+ at 10-5-10-3 M (ionic input) and visible irradiation
(photonic input) are required to give a substantial photonic
output change, observed again by an increase in the visible
transmittance or by a decrease in the fluorescence intensity.
The observation that kobsd is independent of dye concentration
is consistent with the occurrence of the reactions in eqs 3-6,
and in accordance with eq 8, this indicates that Kcis also is
independent of dye concentration. This suggests strongly that
1:1 1-Ba2+ complexes are formed at these Ba2+ concentrations
(ca. 10-5-10-3 M), for which Ba2+ complexation with the
azacrown of trans-1 is not significant. This is consistent with
the proposed structure of cis-1-Ba2+, and the observed
dependence of kobsd on Ba2+ concentration may be attributed to
-
the concentration of associated Ba2+-SO3 groups present in
solution prior to irradiation. This association effectively captures
the Ba2+ cation in the trans-isomer form, for which intramo-
lecular tail-biting is not possible, and facilitates the formation
of the “closed” cis-isomer on photoisomerization.
Fitted values of Kcis ) 13 ( 10 M-1, kf ) (3.4 ( 0.4) ×
10-2 s-1, and kc ) (6.6 ( 5.0) × 10-3 s-1 were obtained for
Na+. The fitted value of kf again is in agreement with the
measured lifetime in the absence of added salt, and again kc is
lower than kf, indicating that the cis-isomer is stabilized by the
presence of Na+. In this case, the cis-isomer is stabilized only
at Na+ concentrations of >10-2 M, above those at which Na+
complexation with the azacrown of trans-1 occurs, and the fitted
stability constant is also within the limit of K ′ e 102 M-1
-
estimated for Na+-SO3 association. Thus, the mechanism
by which cis-1 is stabilized by Na+ is not clear, although it is
Conclusions
evidently quite different from that proposed for stabilization by
Ba2+
.
This study demonstrates that 1 may act as a light-controlled,
transient chromoionophore which is specific and sensitive to
metal cations at low concentration, in addition to acting as a
normal chromoionophore for metal cations at high concentration.
A detailed analysis of the complexation and kinetic data has
enabled a quantitative understanding of the mechanism to be
developed which will facilitate comparison with other molecules
which may be designed to exhibit similar but different com-
plexation and photochemical properties. Derivatives of 1 with
different azacrown, dye, or functionalized alkyl chains may offer
good opportunities for the design of molecules with different
properties, and the approach described here for their quantitative
study may be used to inform strategies for molecular design.
These complexation properties indicate that 1 acts as a normal
chromoionophore for sodium and barium cations at [M] g 10-2
M, through complexation with the azacrown ether. Compound
1 is insensitive to Na+ at lower concentrations. For Ba2+ at
<10-3 M, 1 is insensitive to the presence of barium ions alone
but the cis-isomer formed on visible photolysis is stabilized by
dual intramolecular complexation of Ba2+ with both sulfonate
and azacrown units, providing a color change which indicates
the presence of Ba2+ cations in solution at 10-5-10-3 M.
Species 1 is insensitive to visible photolysis alone because of
the fast cis-trans thermal isomerization. Thus, 1 acts as a
cation-specific, light-controlled transient chromoionophore,
providing a transient visual sensor for Ba2+ cations only in the
presence of both Ba2+ and light. The sensor returns to the
starting state over ca. 10 min in the dark.
The fluorescence emission of 1 offers another probe of ion
complexation. The decrease in the fluorescence intensity of 1,
in the presence of Ba2+ cations at 10-5-10-3 M and on
irradiation by the full output of a fluorimeter lamp at 520 nm,
may be attributed to the relatively high concentration of the
stabilized cis-1-Ba2+ species formed in the photostationary
state. These data are consistent with the occurrence of
fluorescence from only the trans-isomer, and not the cis-isomer,
Acknowledgment. We thank Professor M. V. Alfimov and
Dr O. A. Federova of the Photochemistry Department, Institute
of Chemical Physics, the Russian Academy of Sciences,
Moscow, for providing the sample of 1. We acknowledge the
support of DuPont and of EPSRC, the latter through research
grant and Advanced Fellowship (J.N.M.) awards.
JA964154J
(60) De Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. Nature 1993,
364, 42.