Metal-ion complexation and photochromism of triphenylmethane dye derivatives incorporating monoaza-15-crown-5 moieties

Article abstract of DOI:10.1021/ja9943694

Three types of triphenylmethane leuconitrile derivatives incorporating one, two, and three monoaza-15-crown-5 moieties have been designed to attain photocontrol of metal-ion complexation primarily by intramolecular electrostatic repulsion. Photochromism o

Full text of DOI:10.1021/ja9943694

5448
J. Am. Chem. Soc. 2000, 122, 5448-5454
Metal-Ion Complexation and Photochromism of Triphenylmethane
Dye Derivatives Incorporating Monoaza-15-crown-5 Moieties
Keiichi Kimura,*,† Ryoko Mizutani,‡ Masaaki Yokoyama,‡ Ryuichi Arakawa,§ and
Yoshiaki Sakurai
Contribution from the Department of Applied Chemistry, Faculty of Systems Engineering,
Wakayama UniVersity, Sakae-dani 930, Wakayama, Wakayama 640-8510, Japan,
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
Yamada-oka 2-1, Suita, Osaka 565-0871, Japan, Department of Applied Chemistry,
Faculty of Engineering, Kansai UniVersity, Suita, Osaka 564-8680, Japan, and Technology Research
Institute of Osaka Prefecture, 7-1, Ayumino 2-Chome, Izumi, Osaka 594-1157, Japan
ReceiVed December 14, 1999. ReVised Manuscript ReceiVed April 18, 2000
Abstract: Three types of triphenylmethane leuconitrile derivatives incorporating one, two, and three monoaza-
15-crown-5 moieties have been designed to attain photocontrol of metal-ion complexation primarily by
intramolecular electrostatic repulsion. Photochromism of the crowned triphenylmethane leuconitriles is influenced
by the cation complexation of their crown ether moiety to a considerable extent. Molecular orbital calculations
allowed some prediction regarding the possibility of photochemical control of metal-ion complexation by the
photochromic crown ethers. High efficiency in the photocontrol has been evidenced by electrospray-ionization
mass spectrometry, NMR spectroscopy, and thermal reaction kinetics.
Triphenylmethane dyes, such as Malachite Green and Crystal
Scheme 1
Violet, often show photochromism. For instance, Malachite
Green leuconitrile in solution ionizes to a triphenylmethyl cation
(or quinoid cation) and cyanide anion on UV-light irradiation,
thus turning green.1 Crystal Violet derivatives seem to undergo
similar photoionization with a color change to blue.2 This type
of photochromism, bringing about remarkable environmental
changes from electrically neutral to ionic (cationic) states, can
be used for photocontrol of physical properties.3-5
We have already designed a Malachite Green leuconitrile
derivative carrying a benzo-15-crown-5 moiety, to control metal-
ion complexation of its crown ether moiety by the photoion-
ization of its photochromic moiety.6 The benzocrown ether
moiety of the electrically neutral Malachite Green derivative
can bind a metal ion under dark conditions. Photoinduced
ionization of the Malachite Green moiety to its corresponding
triphenylmethyl (quinoid) cation decreases the metal-ion com-
plexing ability of the crown ether moiety, because the resulting
organic cation is likely to repel the metal ion in the crown ether
ring due to the intramolecular electrostatic repulsion. However,
the photochemical changes in the metal-ion complexing ability
of the crowned Malachite Green was not very remarkable, since
the positive charge formed by photoionization of the Malachite
Green moiety is a little too far from the crown-ether-complexed
metal cation to repel the metal ion efficiently.
This prompted us to incorporate monoazacrown ether moieties
to a Malachite Green skeleton at its dimethylamino groups and
synthesize a Malachite Green leuconitrile possessing a bis-
(monoaza-15-crown-5) structure 1. In general, the photoioniza-
tion of Malachite Green leuconitriles forms a positive-charge
delocalized triphenylmethyl cation, which is the origin of the
green color (Scheme 1). In the cationic form of 1, some positive
charge is located at the nitrogen atom of its dimethylamino
groups. Since the nitrogen atoms are included in the monoaza-
crown ether rings of 1, efficient electrostatic repulsion can be
expected on photoionization of its Malachite Green moiety
owing to the proximity of a crown-ether complexed metal ion
to the resulting positive charge. We recently communicated
highly efficient, all-or-none type switching in the photocontrol
of metal-ion complexation of 1.7 We have furthermore designed
triphenymethane dye derivatives 2 and 3, in which one and three
monoaza-15-crown-5 rings are incorporated into a triphenyl-
methane skeleton, respectively. Here we describe in detail the
syntheses of crowned triphenylmethane derivatives 1-3 and
their behavior in the metal-ion complexation, photochromism,
and complexation photocontrol.
† Wakayama University.
‡ Osaka University.
§ Kansai University.
Technology Research Institute of Osaka Prefecture.
(1) Bertelson, R. C. In Photochromism; Brown, G. H., Ed.; Wiley-
Interscience: New York, 1971; pp 45-431.
(2) Luck, W.; Sand, H. Angew. Chem., Int. Ed. Engl. 1964, 3, 570-
573.
(3) Irie, M. J. Am. Chem. Soc. 1983, 105, 2078-2079.
(4) Irie, M.; Schnabel, W. Macromolecules 1986, 19, 2846-2850.
(5) Willner, I.; Sussan, S.; Rubin, S. J. Chem. Soc., Chem. Commun,
1992, 100-101.
(6) Kimura, K.; Kaneshige, M.; Yokoyama, M. Chem. Mater. 1995, 7,
945-950.
(7) Kimura, K.; Mizutani, R.; Yokoyama, M.; Arakawa, R.; Matsuba-
yashi, G.; Okamoto, M.; Doe, H. J. Am. Chem. Soc. 1997, 119, 2062-
2063.
10.1021/ja9943694 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/27/2000

Monoaza-15-crown-5 Moieties
J. Am. Chem. Soc., Vol. 122, No. 23, 2000 5449
hydroxide and 7.5 × 10-5 mol dm-3 picric acid were introduced into
a stoppered vial and shaken vigorously by a reciprocating shaker under
dark conditions for 15 min.8 After phase separation, the aqueous phase
was subjected to absorption-spectral measurements. The percent of
extraction was calculated as 100 × (A0 - A)/A0, where A0 and A denote
absorbances of picrate ion (355 nm) for the aqueous phase before and
after the extraction, respectively.
Spectrophotometric Measurements. Acetonitrile solutions contain-
ing a Malachite Green derivative (1 × 10-5 mol dm-3) and an
appropriate concentration of an alkali metal perchlorate were prepared
and their absorption spectra were then taken under dark conditions.
Photoirradiation was made by using UV light (240-400 nm), which
was obtained by passing a light from a 500 W Xe lamp through a
color filter of Toshiba UV-D33S. UV-light-induced coloration rates
for triphenylmethane derivative acetonitrile solutions with and without
an alkali metal perchlorate were determined at room temperature, by
using a laboratory-made setup consisting of a 500-W Xe lamp, a 1-cm
quartz cell containing a measuring solution, a monochromator with 0.5-
mm slit (Shimadzu SPG-100ST), and a silicon photodiode (Hamamatsu
Photonics, S2387-33R). The wavelengths for detection were 625, 620,
and 590 nm for the systems of 1, 2, and 3, respectively. The first-
order rate constant of coloration (k) was calculated, which is defined
by the following equation: log(At - A∞)/(A0 - A∞) ) kt, where At, A0,
and A∞ are the absorbance at T(measured time) ) t, T ) 0, and T ) ∞,
respectively.
Experimental Section
Synthesis. Bis[4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-
phenyl]phenylmethanenitrile or Biscrowned Malachite Green Leu-
conitrile 1. N-(4-Bromophenyl)-monoaza-15-crown-5 (4.3 mmol),
which was prepared by the reaction of N-phenyl-monoaza-15-crown-5
and N-bromosuccinimide (NBS) in refluxing CCl4 for 3 h, was dissolved
in anhydrous tetrahydrofuran (THF) (8 cm3) and the solution was kept
at -78 °C in a liquid nitrogen bath under an argon atmosphere. A
hexane solution of butyllithium (BuLi) (4.9 mmol) was injected
gradually into the THF solution with stirring. To the mixture was added
dropwise a THF (3 cm3) solution of methyl benzoate (1.9 mmol). The
reaction mixture was allowed to warm slowly to room temperature and
then stirred for an additional hour. After the reaction, the THF was
evaporated off under vacuum and water (about 25 cm3) was added to
the residue. The aqueous phase was then neutralized by 0.1 mol dm-3
hydrochloric acid. Extraction with dichloromethane, followed by
vacuum evaporation of the solvent, afforded a dark-green oily product
of biscrowned Malachite Green hydroxide, which was used for the
subsequent cyanization without further purification. The biscrowned
Malachite Green leucohydroxide (2.3 mmol) was dissolved in dimethyl
sulfoxide (DMSO) (about 5 cm3) and heated at 60 °C in a hood.
Hydrochloric acid (3.4 mmol) and then KCN (12.6 mmol) were added
to the solution and the mixture was stirred for 10 min. For complete
dissolution of the KCN, an appropriate amount (up to 20 cm3) of water
was added. The reaction mixture turned light yellow and then a crude
product of biscrowned Malachite Green leuconitrile 1 precipitated.
Recrystallization of the filtered sticky precipitate from hexane yielded
a pale-green solid of 1 (30%): mp 112 °C; 1H NMR (270 MHz, CDCl3)
δ 3.5-3.8 (m, 40H, CH2O), 6.5-6.58 (d, J ) 9 Hz, 4H, o-H of NPh),
7.01 (d, J ) 9 Hz, 4H, m-H of NPh), 7.2-7.4 (m, 5H, PhC); MS, m/e
678 (55%), 703 (M+, 100%). Anal. Calcd for C40H53N3O8: C, 68.25;
H, 7.59; N, 5.96. Found: C, 68.27; H, 7.74; N, 5.82.
Similarly, thermal reverse reaction rates of 1 were measured at 30,
40,and 50 °C after UV-light irradiation for 5 min. Arrhenius plots of
the reverse reaction rates afforded straight lines, from the slopes of
which the values of activation energy for the reverse reaction were
calculated. The concentrations of 1 and solvent were the same as those
for the photoinduced coloration reaction.
Electrospray Ionization Mass Spectrometry (ESI-MS). A sector-
type mass spectrometer (JEOL-D300) connected with a laboratory-
made ESI interface was used to obtain ESI-MS spectra. A sample
solution was sprayed with a flow rate of 0.15 cm3 h-1 at the tip of a
needle applied by a voltage 3.5 kV higher than that of a counter
electrode. A heated nitrogen gas flowing between the needle and counter
electrode was used to enhance desolvation of charged droplets sprayed.
The resulting ions were then introduced into the vacuum system through
the first and the second skimmers of a mass analyzer. The pressures of
differential pumping stages were about 1 and 1 × 10-3 Torr,
respectively. A rotary pump and a mechanical booster pump in their
region were floating electrically to suppress a discharge. The voltage
of the first skimmer was the same as that of the counter electrode and
was 50 V higher than that of the second skimmer in the present case.
The solutions for mass spectrometry contained an equimolar amount
(1 × 10-4 mol dm-3) of a triphenylmethane derivative and a metal
salt, but no additive for promoting the ionization. Photoirradiation was
made by using UV light (240-400 nm), which was obtained by passing
a light from a 500 W Xe lamp equipped with a quartz waveguide
through a color filter of Toshiba UV-D33S.
[4-(Dimethylamino)phenyl][4-(1,4,7,10-tetraoxa-13-azacyclopen-
tadec-13-yl)phenyl]phenylmethanenitrile or Monocrowned Mala-
chite Green Leuconitrile 2. A similar reaction of the crowned
phenyllithium with 4-(N,N-dimethylamino)benzophenone (3.7 mmol)
afforded monocrowned Malachite Green leucohydroxide, which was
reacted with KCN to yield a crude product of the corresponding
leuconitrile. Recrystallization from hexane afford a pale-green solid
1
of 2 (20%): mp 125 °C; H NMR (270 MHz, CDCl3) δ 2.98 (s, 6H,
NCH3), 3.5-3.8 (m, 20H, CH2O), 6.5-6.8 (m, 4H, o-H of NPh), 6.9-
7.1 (m, 4H, m-H of NPh), 7.2-7.4 (m, 5H, PhC); MS, m/e 289 (42%),
530 ((M + 1)+, 100%). Anal. Calcd for C32H39N3O4: C, 72.60; H,
7.37; N, 7.93. Found: C, 72.34; H, 7.53; N,7.70.
Tris[4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)phenyl]-
methanenitrile or Triscrowned Crystal Violet Leuconitrile 3.
4-Carboxylphenyl-monoaza-15-crown-5 was obtained by bubbling dry
CO2 gas into the crowned phenyllithium THF solution over 1 h,
followed by neutralization by HCl. Dimethyl sulfate (2.8 mmol) and
4-carboxylphenyl-monoaza-15-crown-5 (2.5 mmol) were added to a
mixture of dry acetone (90 cm3) and K2CO3 (2.8 mmol). After the
reaction mixture was refluxed for 3 h, the acetone was evaporated off
under vacuum. The residue was extracted with water and diethyl ether
and the solvent evaporation of the organic phase afforded 4-methoxy-
carbonylphenyl-monoaza-15-crown-5. The reaction of crowned phen-
yllithium with 4-methoxycarbonylphenyl-monoaza-15-crown-5 (1.9
mmol) afforded triscrowned Crystal Violet leucohydroxide, which was
reacted with KCN to yield a crude product of the corresponding
leuconitrile. Recrystallization from benzene/hexane afforded a pale-
Molecular Orbital Calculation. The calculation by the MNDO-
PM3 method9 was performed with the MOPAC program (version 6)
using a Macintosh computer. The structural output was recorded by
using the CAChe program by Sony Tektronix.
Results and Discussion
Design of Triphenylmethane Leuconitriles Carrying Crown
Ether Moieties. The syntheses of crowned triphenylmethane
leuconitriles 1-3 are based on the reaction of a benzophenone
derivative with a phenyllithium derivative, followed by cyan-
ization with KCN, as outlined in Scheme 2. A phenyllithium
derivative carrying a monoaza-15-crown-5 moiety can be
obtained by treating its corresponding phenyl bromide derivative
with butyllithium. The nucleophilic addition of the crowned
phenyl anion to an appropriate benzoate or benzophenone
derivative in THF can afford the corresponding crowned
1
green solid of 3 (30%): mp 279 °C; H NMR (270 MHz, CDCl3) δ
3.5-3.8(m, 60H, CH2O), 6.57 (d, J ) 9 Hz, 2H, o-H of NPh), 7.03 (d,
J ) 9 Hz, 2H, m-H of NPh); MS, m/e 626 (25%), 921 ((M + 1)+,
100%). Anal. Calcd for C50H72N4O12: C, 65.20; H, 7.83; N, 6.09.
Found: C, 64.73; H, 7.70; N, 5.98.
Other Materials. Alkali metal salts were of analytical grade. The
solvents for the measurements were spectro-grade from Dojindo. Water
was deionized.
Metal-Ion Extraction. Equal volumes (3 cm3) of 2.1 × 10-3 mol
dm-3 crowned triphenylmethane 1,2-dichloroethane solution and an
aqueous solution containing a mixture of 0.1 mol dm-3 alkali metal
(8) Pedersen, C. J. Fed. Proc. 1968, 27, 1305-1309.
(9) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209-220.

5450 J. Am. Chem. Soc., Vol. 122, No. 23, 2000
Kimura et al.
Scheme 2
supported by the fact that the metal-ion extractability for
monocrowned Malachite Green 2 is in the order Na+ > K+
>
Li+. Therefore, the K+ selectivity over Na+ for biscrowned
Malachite Green 1 suggests that K+ can be sandwiched
intramolecularly by two adjacent crown ether rings, as was the
case with previously reported bis(crown ether) derivatives.11,12
Thus, the intramolecular formation of sandwich-type 2:1 (crown
ether ring/metal ion) complexes resulted in high complexing
ability and extractability of 1 for K+. In the triscrowned Crystal
Violet leuconitrile 3, a selectivity for the larger alkali metal
ions such as Cs+ might be expected by cooperative metal-ion
complexation with the three crown ether rings. The extractability
of 3 with respect to alkali metal ions is in the order Cs+ > K+
> Li+ > Na+, as expected.
Mass spectrometry also gives some information about the
selectivities of crowned triphenylmethane leuconitriles 1-3 on
cation complexation. ESI-MS was taken with acetonitrile
solutions containing a crowned triphenylmethane and a mixture
of Li+, Na+, and K+ (and Cs+). In ESI-MS for the system of
biscrowned Malachite Green 1 (Figure 2), strong peaks assigned
to 1:1 (1/metal ion) complexes of K+ and Na+ were observed
as well as a tiny peak for the corresponding Li+ complex,
suggesting that the metal-ion complexing ability of 1 is in the
order K+ > Na+ . Li+. In the mass spectrum for the 2 system
(Figure 3), the peak intensity for 1:1 (2/metal ion) complexes
is in the order Na+ > K+ . Li+. In the monocrowned Malachite
Green, some formation of sandwich-type 2:1 (2/metal ion)
complexes was also found. The mass spectrum for the system
containing triscrowned Crystal Violet 3 and a mixture of Li+,
Na+, K+, and Cs+ showed that the peak intensity for 1:1 (3/
metal ion) complexes is in the order Na+ ∼ K+ > Cs+ (Figure
4). Peaks assigned to 1:2 (3/metal ion) complexes with the Na+
and K+, especially with the former metal ion, were also seen
in the mass spectrum, indicating that triscrowned Crystal Violet
Figure 1. Alkali metal-ion extraction with crowned triphenylmethane
leuconitrile derivatives under dark conditions: (0) 1, (O) 2, and (4)
3.
triphenylmethane leucohydroxides, which can be converted into
their leuconitrile derivatives 1- 3 by treatment with KCN.
Grignard reactions using a phenylmagnesium bromide possess-
ing the monoazacrown ether moiety instead of the crowned
phenyllithium were also attempted, but they failed probably due
to the Mg2+ complexation of the crown ether moiety.
Complex Formation with Alkali Metal Ions. For rapid
screening of metal-ion complexing ability of crowned triphen-
ylmethane leuconitriles, 1-3, under dark conditions, extraction
was carried out with a 1,2-dichloroethane solution of a crown
ether derivative from an aqueous solution containing an alkali
metal picrate. The results for metal-ion extraction were sum-
marized in Figure 1. Biscrowned Malachite Green 1 extracts
alkali metal ions in the order K+ > Na+ . Li+, preferring K+
to Na+. Monoaza-15-crown-5 itself can complex Na+ more
strongly than K+ on the basis of the size-fit concept.10 This is
(11) Kimura, K.; Maeda, T.; Shono, T. Talanta 1979, 26, 945-949.
(12) Kimura, K.; Maeda, T.; Tamura, H.; Shono, T. J. Electroanal. Chem.
Interfacial Electrochem. 1979, 95, 91-101.
(10) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S. Chem. ReV. 1991, 91,
1721-2085.

Monoaza-15-crown-5 Moieties
J. Am. Chem. Soc., Vol. 122, No. 23, 2000 5451
Figure 2. ESI-MS for acetonitrile solution of biscrowned Malachite Green leuconitrile 1 and alkali metal (Li+, Na+, and K+) perchlorates under
dark (a) and photoirradiated (b) conditions. [1] and [MClO4]: 2 × 10-4 mol dm-3 each. UV light: 240-400 nm for 30 s.
Figure 3. ESI-MS for acetonitrile solution of monocrowned Malachite Green leuconitrile 2 and alkali metal (Li+, Na+, and K+) perchlorates under
dark (a) and photoirradiated (b) conditions. [2] and [MClO4]: 2 × 10-4 mol dm-3 each. UV light: 240-400 nm for 30 s.
3 quite easily form multinuclear metal-ion complexes with the
metal ions. However, the metal-ion order in the MS peak
intensity for 3 is different from that in the extractability. This
is probably because Cs+ forming 1:1 (3/metal ion) complexes
is easier to extract to an organic phase than Na+ and K+ capable
of forming multinuclear metal-ion complexes. Despite the
relatively large cation size, Cs+ can be complexed by the tris-
(crown ether) derivative, but there is no significant peak for
the multinuclear complexes with Cs+. This implies some
cooperative effect of three adjacent crown ether rings on the
Cs+ complexation, although the effect is not so remarkable as
that for the biscrowned Malachite Green 1.
Photoionization in the Presence of Metal Ions. UV-light
irradiation on acetonitrile solutions of crowned triphenylmethane
leuconitriles 1-3 changed their absorption spectra remarkably
(Figure 5). Under dark conditions, the solutions of crowned

5452 J. Am. Chem. Soc., Vol. 122, No. 23, 2000
Kimura et al.
Figure 4. ESI-MS for acetonitrile solution of triscrowned Crystal Violet leuconitrile 3 and alkali metal (Li+, Na+, K+, and Cs+) perchlorates under
dark (a) and photoirradiated (b) conditions. [3] and [MClO4]: 2 × 10-4 mol dm-3 each. UV light: 240-400 nm for 30 s.
Figure 6. Metal-ion effects on UV-light-induced coloration (ionization)
of crowned triphenymethane leuconitriles 1 (a), 2 (b), and 3 (c) in
acetonitrile. Salt: NaClO4 (9); KClO4 (b); and CsClO4 (2). [crowned
triphenylmethane]: 1 × 10-5 mol dm-3. UV light: light from a 500-W
Xe lamp.
To elucidate the photochromism of crowned triphenyl-
methanes 1-3 in the presence of crown-ether-complexing metal
ions, the coloration reaction was followed spectrophotometri-
cally while irradiating with UV light. In general, addition of a
metal salt to an acetonitrile solution of a crowned triphenyl-
methane leuconitrile increases the polarity of the medium. This
Figure 5. Absorption spectra of acetonitrile solutions of crowned
triphenylmethane derivatives with and without photoirradiation: (a) 1
system, (b) 2 system, and (c) 3 system. Broken line: without
photoirradiation. Full line: after UV-light irradiation for 5 min.
[crowned triphenylmethane]: 1 × 10-5 mol dm-3. UV light: 240-
400 nm.
accelerates the photoinduced ionization of its triphenylmethane
leuconitrile moiety to a conjugated triphenylmethyl cation,
unless its crown ether moiety binds metal ions. However, if
the crown ether moiety is very likely to bind metal ions
powerfully, especially in the case of high stability of the crown
ether complexes with a given metal ion, the photoisomerization
must be suppressed on account of the intramolecular electrostatic
repulsion. Actually, the addition of 1 × 10-5 mol dm-3 NaClO4
to an acetonitrile solution of biscrowned Malachite Green 1
enhanced the photoinduced coloration, but the increasing Na+
concentrations again suppressed it (Figure 6a). In the K+-1
system, on the other hand, the addition of only 1 × 10-5 mol
dm-3 K+ decreased the coloration rate of 1. This difference
between the two metal ions in the addition effect is reflected in
the metal-ion complexing ability of 1, i.e., the higher complexing
triphenylmethane leuconitriles are essentially colorless and
transparent. On photoirradiation, the solutions of Malachite
Green derivatives 1 and 2 and Crystal Violet derivative 3 turned
green and blue, respectively. The photoinduced coloration of
crowned triphenylmethane leuconitriles indicates that the tri-
phenylmethyl cations resulting from the heterolytic C-C bond
scission are in highly conjugated forms. For instance, the
Malachite Green derivatives produce quinoid-like cations.

Monoaza-15-crown-5 Moieties
J. Am. Chem. Soc., Vol. 122, No. 23, 2000 5453
Figure 8. Photoinduced 1H NMR spectral changes of biscrowned
Malachite Green 1 in the presence and absence of Na+ or K+. [1] and
[metal ion]: 1 × 10-4 mol dm-3 each in CD3OD, UV-light irradiation
for 5 min. The arrows indicate new peaks based on the metal-ion
complexation.
Figure 7. MO calculation for leuconitrile and cationic forms of
crowned triphenylmethane derivatives 1-3. The numbers in the vicinity
of heteroatoms stand for their point net charges.
ability toward K+ than Na+, which is due to the bis(crown ether)
effect of 1. The Na+ system of monocrowned Malachite Green
2 exhibited a similar tendency to that of the Na+-1 system in
the addition effect on the photoinduced coloration, showing a
maximal rate constant at a Na+ concentration between 1 × 10-5
and 1 × 10-4 mol dm-3 (Figure 6b). In the K+-2 system, the
maximum in the coloration rate was found at the higher metal-
ion concentration. This is derived from the fact that Na+ can
fit into the monoaza-15-crown-5 ring better than K+ on the
formation of 1:1 (2/metal ion) complexes. In the photoinduced
coloration for the system of triscrowned Crystal Violet 3, the
suppression effect on addition of an alkali metal salt was
decreased in the order Na+ > K+ > Cs+ (Figure 6c). The tris-
(crown ether) derivative, 3, can form mulitinuclear complexes,
especially with Na+, as demonstrated in the mass spectra (Figure
4), and the resulting plural metal cations in a 3 molecule may
be more effective in the electrostatic repulsion with the conjugate
triphenylmethyl cation of 3 than a single metal cation. This
probably accounts for the metal-ion order in the addition effect
on the coloration rate of 3.
Photoinduced Switching of Metal-Ion Complexation. As
mentioned above, the UV-light-induced ionization rate of
crowned triphenylmethane leuconitriles 1-3, accompanied by
coloration, is influenced by the metal-ion complexation of their
crown ether moieties to a great extent. It is expected in the
opposite sense that the metal-ion complexing ability of the crown
ether derivative can be controlled by photoionization of their
triphenylmethane leuconitrile moieties, primarily based on the
electrostatic repulsion between crown-ether-complexed metal
ion(s) and a conjugated triphenylmethyl cation.
(MO) calculations were carried out by using the corresponding
dimethoxyamino derivatives of the crowned triphenylmethane
leuconitriles for simplicity (Figure 7). The photoionization of
crowned triphenylmethanes causes their appreciable configu-
ration changes. For instance, the triphenylmethyl carbon atom
in the leuconitrile form of biscrowned Malachite Green 1 is in
sp3 hybridization. It is therefore likely that two crown ether rings
can cooperate in forming intramolecular sandwich-type com-
plexes with a metal ion, especially K+. On the contrary, in the
quinoid-like cation of 1 formed by UV-light irradiation, the two
crown ether rings are more distant from each other than in the
leuconitrile form, so the cooperation is somewhat difficult for
the sandwich-type complex formation. The MO calculation data
for triscrowned Crystal Violet 3 suggest the formation of a
propeller-shaped conjugated triphenylmethyl cation, which
makes it more difficult for the three crown ether rings to
cooperate on complexing large metal ions such as Cs+ than its
corresponding leuconitrile form. Also, there is considerable
difference in the point net charges of the crown ether ring
heteroatoms (nitrogen and oxygen atoms) between the leuco-
nitrile and triphenylmethyl cationic forms of crowned tri-
phenylmethanes. In any of the crowned triphenylmethane
leuconitriles 1-3, the point net charges of the crown ether ring
heteroatoms are increased in the triphenylmethyl cationic form.
Taking account of the difference in the point net charges as
well as of the configuration, one can anticipate diminished
metal-ion complexing ability of the crown ether moiety of 1-3
in the corresponding conjugated triphenylmethyl cationic form.
Thus, photoinduced changes of metal-ion complexing ability
in the crowned triphenylmethanes can be expected from the
viewpoint not only of the intramolecular electrostatic repulsion
To obtain important information about the configurations and
point net charges of each atom in the electrically neutral
leuconitrile and triphenylmethyl cationic forms, molecular orbital

5454 J. Am. Chem. Soc., Vol. 122, No. 23, 2000
Kimura et al.
but also of the difference in the configuration and point net
charges between the two forms.
peaks for the metal-ion complexes. This NMR observation is
in accord with the dramatic photoinduced disappearance of the
ESI-MS peaks assigned to the metal complexes of 1 and its
quinoid form (Figure 2b). Thus, the photoionization of their
triphenylmethane leuconitrile moiety allowed very efficient
release of metal ions in their crown ether moiety.
Photochemical switching of metal-ion complexation in the
three crowned triphenylmethane derivatives can be detected by
ESI-MS, as shown in Figures 2b-4b. Mass spectra for aceto-
nitrile solutions containing a crowned triphenylmethane leuco-
nitrile and a mixture of Li+, Na+, and K+ (and Cs+) after UV-
light irradiation (30 s) show only a single significant peak, which
can be assigned to their corresponding conjugated triphenyl-
methyl cations, in any systems of the crowned triphenylmethane
derivatives. The peaks for the metal complexes of the crowned
triphenylmethane leuconitriles almost disappeared on the photo-
irradiation. These mass-spectral changes have clearly verified
the photochemical switching of metal-ion complexation of
crowned triphenylmethane leuconitriles 1-3.
To confirm the all-or-none type photochemical switching of
metal-ion complexation by the biscrowned Malachite Green,
1H NMR spectra of 1-CD3OD solutions were measured in the
presence and absence of metal ions before and after UV-light
irradiation. The peaks assigned to crown ether ring protons of
1 as depicted in Figure 8 show significant differences in the
presence and absence of a metal ion and before and after
photoirradiation. The addition of an equimolar amount of K+
or Na+ caused the appearance of new peaks (highlighted by
arrows) that can be attributed to the metal-ion complexation by
the crown ether moiety. The complex formation constants were
determined to be 1.7× 104 and 8 × 103 dm3 mol-1 for the K+
and Na+ complexes, respectively, assuming the 1:1 stoichiom-
etry of 1 and metal ion. Any the peaks assigned to the metal-
ion complexes almost disappeared with UV-light irradiation.
The NMR spectra for photoirradiation in the presence of a metal
ion are very similar to that in the absence of it. The formation
constants for the metal complexes of conjugated quinoid cation
of 1 could not be determined due to the disappearance of the
Heating after turning off UV light brings about the thermal
reverse reaction from the conjugate triphenylmethyl cations to
the corresponding initial triphenylmethane leuconitriles. For
instance, such a thermal reverse reaction as shown in Scheme
1 proceeds gradually in the 1-metal ion systems. The thermal
reverse reaction for triphenylmethane lueuconitriles not carrying
any crown ether moiety is generally suppressed by adding any
metal ion due to the polarity increase, which in turn stabilizes
their corresponding triphenylmethyl cation form. To the
contrary, the metal-ion complex formation of triphenylmethane
leuconitrile 1 may function in a direction to enhance the thermal
reverse reaction. Very interestingly, the activation energy for
the thermal reverse reaction of 1-acetonitrile solution (1 × 10-5
mol dm-3) was decreased from 60.0 kJ mol-1 successively to
38.8, 25.7, and 23.7 kJ mol-1 by K+ addition at 1 × 10-5, 1 ×
10-4, and 1 × 10-3 mol dm-3. Obviously, the enhancing effect
on the thermal reverse reaction by the complex formation
surpasses the suppressing effect, indicating that the difference
in the metal-ion complexing ability is very remarkable, all-or-
none type, between the triphenylmethane leuconitrile and
triphenylmethyl cation forms.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research from the Ministry of Education,
Science, Culture, and Sports. We also acknowledge Yamada
Science Foundation for financial support.
JA9943694