J. Phys. Chem. 1994,98, 9291-9296
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Photochemistry of Squaraine Dyes. 8. Photophysical Properties of Crown Ether Squaraine Fluoroionophores and Their Metal Ion Complexes+ Suresh Das,***K. George Thomas,* K. J. Thomas,$ Prashant V. Kamat,**§ and M. V. George'v**§J Photochemistry Research Unit, Regional Research Laboratory, Trivandrum 695 019, India, and Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: March 29, 1994; In Final Form: May 31, 1994"
The photophysical properties of a series of red-sensitive, highly fluorescent bis [C(monoaza-crown ether)phenyllsquaraine fluoroionophores a r e reported. These dyes are soluble in both polar and nonpolar solvents with fluorescence quantum yield varying from 0.01 to 0.85. These dyes readily form aggregates in aqueous solutions. These aggregates dissociate in 8-cyclodextrin solutions as the dye molecules form 1:1 complex with P-cyclodextrin. This complexation process is accompanied by a significant enhancement in the fluorescence yields of these dyes. Complexation of the fluoroionophores with alkali-metal ions brings about drastic reductions in their fluorescence quantum yield as well a s significant changes in their redox properties.
Introduction
Experimental Section
Design of new fluoroionophores, which essentially consist of fluorophores linked to ionophores, is an active area of research due to their potential applications in the field of molecular information processingl-3 and in the sensing of low concentrations of biologically important cations such as Li+, Na+, K+, and Ca2+. Recently, there have been several reports on the design and study of such fluoroionophores, some of which contain crown ether moieties as the ionophoric unit.k15 However, most of the reported compounds absorb in the near UV to visible region. Watersoluble fluoroionophores with absorption and emission bands in the near-infrared region are more suited for intracellular applications.2 Squaraine dyes are known to have strong absorption and emission properties in the near-infrared region. Due to the intramolecular charge-transfer nature of these electronic transitions, which are highly sensitive to substitutional changes as well as the nature of the solvent medi~m,~"19these dyes would be ideally suited for the design of new fluoroionophores. An additional property that these dyes possess is their ability to undergo reversible redox processes. Ionophores covalently linked to redox-active residues are very important as potential sensitive probes for metal ions20-22 as well as for transport of metal ions across membranes.23 In our earlier preliminary communication, we highlighted the alkali-metal-ion recognition capability of squaraine dyes by fluorometric and electrochemical technique^.^^ Here we report on the synthesis, characterization, detailed photophysical studies, and the metal-binding properties of this new class of squaraine fluoroionophores 1-3.
All melting points are uncorrected and were determined either on a Buchi melting point apparatus or on a Aldrich meltemp apparatus. All new compounds were fully characterized on the basis of analytical results and spectral data. IR spectra were recorded on a Perkin-Elmer Model 882 IR spectrometer and the UV- visible spectra on a Shimadzu 2100 spectrometer. IH and 13CNMR spectra were recorded on a JEOL EX 90 spectrometer. 13C N M R (22.5 MHz) resonances were assigned using QUAT and DEPT programs to determine the number of hydrogen attachments. Mass spectra were recorded on a Finnigan MAT Model 8340 or JEOL J M S AX 505 H A mass spectrometer. Quantum yields of fluorescence were measured by the relative method using optically dilute solutions with bis[C(dimethylamino)-2-hydroxyphenyl]squaraine (& = 0.84)25 in dichloromethane as reference. Spectroscopy grade solvents were used for all the measurements. For metal-binding studies, anhydrous metal perchlorates and dried solvents were used. Synthesis of 1-3. N-Phenylmonoaza- 12-crown-4-ether,26 N-phenylmonoaza- 15-~rown-5-ether,2~ and N-phenylmonoaza18-crown-6 ether2' were synthesized, adopting methods similar to the reported procedures. Crown-bearing squaraine dyes have been synthesized by refluxing the corresponding N-phenylmonoaza-crown ether (0.3 mmol) and squaric acid (0.15 mmol) in a solvent mixture containing 3 mL of benzene and 5 mL of 1-butanol, accompanied by azeotropic distillation of water. Recrystallization from a mixture (3: 1) of chloroform and n-hexane gave a 44% yield of bis(4-(monoaza-l2-crown-4ether)phenyl)squaraine (1, mp 225 OC (dec)) and a 27% yield of bis(4(monoaza- 15-crown-5 ether)phenyl)squaraine (2, mp 21 5 OC (dec)), whereas recrystallization from a mixture (1:3) of chloroform and benzene gave a 36% yield of bis(4-(monoaza18-crown-6 ether)phenyl)squaraine (3, mp 176 "C). Analytical results and spectral data of fluoroionophores 1-3 are summarized below. 1(44%): mp 225 OC (decomp); IR v, (KBr) 1586 cm-'; UV A,, (CHCl3) 634 nm (e 300000 M-1 cm-I); lH NMR (CDC13) 6 3.6-4.0 (32 H, m, CH2),6.85 (4 H, d, aromatic), 8.4 (4 H, d, aromatic); I3C NMR (CDCl3) 6 189.00 (C), 183.32 (C), 154.57 (C), 133.08 (CH), 120.29 (C), 113.31 (CH), 71.41 (CH2), 69.87 (CH2), 69.21 (CH2), 52.94 (CH2); exact mol wt calcd for C32H4108N~(MH+),58 1.2863; found, 581.2862 (FAB high resolution mass spectroscopy). 2(27%): mp 215 OC (decomp); IR vmax (KBr) 1595 c m l ; UV A, (CHCl3) 632 nm (c 270 000 M-1 cm-l); 1H NMR (CDCl3) 6 3.6-4.0 (40 H, m, CH2) 6.8 (4 H, d, aromatic) 8.4 (4 H, d, aromatic); 13C NMR (CDCl3) 6 188.64 (C), 182.97 (C), 153.56
1,n = l 2, n = 2
3, n = 3 t Dedicated to Professor C. N . R. Rao on his 60th birthday.
t Regional Research Laboratory, Trivandrum, India. 8 University of Notre Dame. 1 Also at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 012, India. *Abstract published in Advance ACS Abstracts, August 15, 1994.
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TABLE 1: Absorption and Emission Characteristicsof Bis$4-monoaza-crown ether)phenylhuaraines 1-3 in Different Solvents’ 1
2
, ,X
3 , ,X
Amax
solvents abs em +I-b (&2%) abs em 4f.b (*2%) abs em +f.b (*2%) benzene 635 652 0.84 633 652 0.89 635 654 0.78 chloroform 634 655 0.74 632 652 0.83 634 656 0.76 dichloromethane 636 660 0.48 634 651 0.50 636 660 0.43 trifluoroethanol 634 661 0.06 631 657 0.07 629 655 0.07 terr-butyl alcohol 635 658 0.55 isopropyl alcohol 638 662 0.23 636 660 0.34 638 663 0.28 ethanol 640 664 0.14 638 662 0.18 640 666 0.17 methanol 638 66 1 methoxyethanol 646 672 0.15 643 670 0.20 646 672 0.19 acetonitrile 637 662 0.11 634 659 0.14 636 66 1 0.13 water 640 667 0.014 644 667 0.015 water 0-CD (2.7 mM) 645 664 0.20 647 663 0.14 a The concentration of the dye employed was 4-5 pM. Optically dilute samples were used for & (optical density at 590 nm = 0.1).
TABLE 2
Singlet Excited-State Characteristics 1
solvent
1
2
3
3
2 ~
Amax(&-SAa nm ~
(ns)
(108 s-1)
kk (108 s-I)
~
7sb
(ns)
kP (lo8 s-l)
kP (lo8s-1)
7s’
(ns)
kk (108 s-1)
kk (108 s-1)
benzene 495 495 495 2.36 3.56 0.73 2.13 4.17 0.52 2.50 3.12 0.88 chloroform 480 480 480 1.25 5.92 2.1 1.20 6.91 1.42 1.45 5.2 1.70 water 465 468 0.07 2.0 140.8 0.11 1.36 89.5 water + 2.7 mM P-CDC 480 480 0.90 2.2 8.9 0.94 1.48 9.16 Difference absorption maximum of the excited singlet state. Singlet excited lifetime, T ~was , measured from the decay of the transient absorption 2.7 mM. The rate constants for the radiative (kr) and nonradiative (knr) decay were evaluated from the Qf and T~ measurements and at the A., using the expressions q4r = kr(kr + kn,) and T~ = l/(kr + knr). Intersystem crossing efficiency was assumed to be very small. (C), 133.00 (CH), 119.90 (C), 112.59 (CH), 71.00 (CH2), 70.23 (CH2), 69.81 (CH2), 68.08 (CH2) 53.13 (CH2); exact mol wt calcd for C ~ ~ H ~ ~ (MH+) O I O 669.3387; N ~ found 669.3372 (FAB high-resolution mass spectroscopy). 3 (36%): mp 176 OC; I R vmax (KBr) 1615 cm’; UV A, (CHC13) 634 nm (e 280 000 M-I c m l ) ; IH N M R (CDC13) 6 3.6-3.9 (48 H , m, CH2), 6.8 (4 H , d, aromatic), 8.35 (4 H, d, aromatic); l3C N M R (CDC13) 6 188.58 (C) 183.27 (C), 153.89 (C), 133.18 (CH), 128.05 (C), 112.59 (CH), 70.88 (CHz), 70.79 (CH2), 70.65 (CH2), 68.44 (CH2), 51.70 (CH2); exact mol wt calcd for C ~ H 5 7 0 1 2 N(MH+), ~ 757.3912; found 757.3916 (FAB high-resolution mass spectroscopy). Picosecond Laser Flash Photolysis. Picosecond laser flash photolysis experiments were performed with 532 nm laser pulses from a mode-locked, Q-switched Quantel YG-501 DP Nd:YAG laser system (output 2-3 mJ/pulse, pulse width 18 ps). The white continuum picosecond probe pulse was generated by passing the fundamental output through a D20/Hz0 solution. The excitation and the probe pulse were incident on the sample cell at right angles. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced with an IBM-AT computer. The details of the experimental setup and its operation are described elsewhere.28 Time zero in these experiments corresponds to the end of the excitation pulse. All the lifetimes, rate constants, and equilibrium constants reported in this study carry an experimental error of i5%.
-
Results and Discussion 1JC NMR Spectra. The high solubility of the squaraine fluoroionophores in organic solvents (e.g., solubility of 2 is greater than 1.5 M in CDCl3), facilitated the recording of their 1% N M R spectra (see Experimental Section for details). The I3C N M R spectrum of 2 in CDCl3, for example, showed two tertiary carbon signals, four quaternary carbon signals, and five secondary carbon signals. The two tertiary carbon peaks observed at 133.00 and 112.59 ppm have been assigned to the two sets of identical carbon
atoms at C4, CS,C4/, Cs’, and Cs, C7, C5/, C{ positions (see 1-3). The quarternary carbon signals a t 153.56 and 119.90 ppm are attributed to the two sets of identical carbon atoms situated at the c3, ci and cg, Cg’ positions, and the quarternary carbon signals at 188.64 and 182.97 ppm can be attributed to the two sets of identical carbon atoms at the C I , CI’ and Cl, CZ’positions of the central cyclobutane ring. In the case of 2 and 3, the secondary carbons of the crown moiety appear as five peaks, whereas for 1 they appear as four peaks. All the other spectral features of 1 and 3 are similar to those of 2. The spectral features of 1-3 indicate that the two halves of the molecules are identical, indicating the highly symmetric donor-acceptor-donor nature of these molecules. Excited State Characteristics. The photophysical properties of 1-3 have been studied in solvents of wide-ranging polarities, and these results are summarized in Tables 1 and 2. The S,& transitions of similar squaraine dyes have been proposed to arise from charge-transfer transitions involving the 0-atoms and the central cyclobutane ring with only minor contributions from the amino moieties.29 The absorption spectra of the crown ether squaraines 1-3 are slightly red shifted compared to that of bis((dimethy1amino)phenyl)squaraine (Amm = 628 nm). An increase in thelength of the alkyl group substituentson thenitrogen brings about a similar shift in the absorption spectra.16 The absorption and emission spectra of these dyes are fairly independent of the nature of the solvent with respect to aprotic solvents, whereas in protic solvents they shift, depending upon the hydrogen-bonding ability of the solvent. In trifluoroethanol, a strongly hydrogen-bonding solvent, for example, the absorption maximum is 629 nm whereas in a weakly hydrogen-bonding solvent such as methoxyethanol the absorption maximum is considerably red shifted to 646 nm. The plot of absorption maxima of these dyes in protic solvents of varying polarity versus the solvent Taft parameter, R * , showed a linear dependence. The observed hypsochromic shift in absorption maxima can be attributed to the formation of specific hydrogen-bonded solutesolvent complexes. Similar solvent hydrogen-bonding effects were earlier observed for bis(benzothiazolylidene)squaraines.‘g
Photochemistry of Squaraine Dyes
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1 .o
0.6
a 2m
0.5
0
)
V
-
e 8 2 -3 0.0 0.2
-
-0.5
400
450
500
550
600
500
Wavelength, nm Figure 1. Time-resolvedtransient absorption spectra recorded following 532-nm laser pulse (pulse width 18 ps) excitation of 3 in benzene. Inset shows the decay profile of the singlet excited state as monitored from its absorbance at 495 nm. (Time zero in these experiments corresponds to the end of the excitation pulse.)
600
7 00
Wavelength, nm Figure 2. Effect of addition of acetonitrile on the absorption spectra of an aqueous solution of 3 (1.7 pM). Acetonitrile v/v (a) O%, (b) 5%, (c) 20%, and (d) 40%. I v)
c The fluorescence quantum yield (&) and the singlet excited .C ) these dyes are extremely sensitive to the state lifetime ( T ~ of 3 solvent polarity (Tables 1 and 2). The fluorescence quantum 0 yield of 2 in a relatively nonpolar solvent such as benzene (T* = 0.59) is very high (& = 0.89). However, in a polar solvent such * c .as acetonitrile (T* = 0.75) it is very low (& = 0.14). The singlet v) C excited state absorption spectra and lifetimes of these dyes were 0 c C studied by picosecond laser flash photolysis using 532 nm laser H pulses as the excitation source. The time-resolved transient c absorption spectra of singlet excited state of 3in benzene is shown .-0v) in Figure 1. The excited singlet exhibits absorption maximum .-In E at 495 nm. The decay of this transient absorption closely matches w with the recovery of the bleaching a t 635 nm. The lifetime of 600 650 700 7 D the singlet excited state was determined by fitting the transient Wavelength, nm absorption decay to the first order kinetics. The absorption Figure 3. Influence of 8-cyclodextrin [&CD] on the emission spectrum maximum and lifetime of singlet excited dyes are summarized of 2 (5.1 pM) in aqueous solutions. [b-CD] (a) 0.06, (b) 0.6, (c) 2.2, and in Table 2. The singlet lifetimes of these dyes varied from 70 ps (d) 2.7 mM. Inset shows the plot of quantum yield of fluorescence (&) in water to 2.5 ns in benzene. The absence of long-lived transients of 2 versus [o-CD] in aqueous solutions. which could be attributed to triplet excited states indicates that the intersystem crossing efficiency is very low in all three dyes. to form linear J-type aggregates in dry acetonitrile solution^.'^ In Similar very low intersystem crossing efficiencies have also been the present study however the exact nature of the aggregate could noted for several other squaraine dyes.25J0931 The decrease in not be ascertained. The intermolecular charge-transfer interacsinglet excited-state lifetime and fluorescence quantum yield with tions between the donor (amino) and acceptor (cyclobutane ring) increasing solvent polarity could be attributed to complex as proposed earlier for formation of aggregates of bis(alky1amiformation between the solute and solvent in polar ~ o l v e n t s . ~ ~ J ~nopheny1)squaraines in DMSO/water solutions32 and LangmuirThe values of radiative and nonradiative decay rate constants Blodgett could lead to the formation of the aggregates of listed in Table 2 show the influence of such solute-solvent the crown ether squaraines. complexation on the deactivation pathways of the excited singlet Interaction with @-Cyclodextrin.Addition of low concentrastate. tions of 0-cyclodextrin (53 mM) to aqueous solutions of 2 and Aggregation Phenomena. The chromoionophores 2 and 3 are 3 bring about a significant enhancement in the intensity of their water soluble, but their solubility is relatively low ( 1 5 pM). absorption bands, indicating that complexation of the dyes with However when solutions of thesedyes are prepared in water from B-cyclodextrin (8-CD) also leads to a breaking up of the stock solutions in acetonitrile a rapid decrease in the absorbance aggregates. The changes in absorption spectra are accompanied of the dyes is observed till they reach stable values. This process by an enhancement of fluorescence quantum yield (Figure 3) is accompanied by a slight broadening of the absorption band. upon addition of 0-CD to an aqueous solution of 2. (Corrections The changes in the absorption spectra with time is attributed to have been made in fluorescence yield measurements for the the formation of dye aggregates. The process of dye aggregation changes in theabsorbance at the excitation wavelength that occur is reversible. Addition of acetonitrile to the aqueous solutions due to p-CD complexation.) The enhancement in fluorescence leads to a recovery of the original monomeric absorption band yields cannot be attributed to a simple breaking up of the of these dyes. The changes in the absorption spectra recorded aggregate, since addition of acetonitrile, which also leads to a upon addition of acetonitrile to aqueous solution of 3 is shown break up of the aggregate, does not lead to any significant change in Figure 2. in the emission properties. The fluorescence quantum yield, &, is 0.02 in water and 0.03 in 40% acetonitrile-water mixture and The photoconducting properties of squaraine dyes arise out of this very minor effect is attributed to the change in the solvent intermolecular interactions. To understand these processes, there polarity brought about by the addition of acetonitrile. The have been several studies on the formation of squaraine aggregates enhancement of fluorescence yield in the presence of 0-CD can in solutions.32-j4 Both bis(2,4-dihydroxyphenyl)squaraine and therefore be attributed to specific complex formation between the bis(2,4,6-trihydroxyphenyl)squaraine dyes have been shown
e
Y
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I\
-10
a -0.1
L
-301
-40
-
-o'2 -0.3
400
500
450
550
600
650
Wavelength, nm
1
0
2
I
1
I
4
6
8
I/[&co],
Figure 5. Transient absorption spectra of excited singlet of 3 in aqueous solution containing (a) no j3-CD and (b) 3 mM &CD (excitation 532 nm, A? = 0 ps). Inset shows the decay of the transient absorption at 465 nm.
103~"
Figure 4. Plot of l / ( & - bf) versus reciprocal of &cyclodextrin concentration for the fluorescenceyield enhancement of 2 upon addition
of 8-cyclodextrin. the dye and 8-CD. Similar interaction with 8-CD has also been observed for other squaraine dyes.18 The formation of dye-8CD complexes which would lead to the break-up of the aggregate could be described by eq 1, -M
M,
+M
2M
+&CD F? -8-CD
Z[M---&CD]
(1)
where Mz = aggregate and M = monomer. The fluorescence enhancement of 2 and 3 could be analyzed by the Benesi-Hildebrand equation for a 1:l complex formation between monomer and 8-cyclodextrin (eq 2),
-=-1
l
4; - 4f 4; - 4:
+
1
WJ;- 4:) [B-CDI
Wavelength, nm
Figure 6. Effect of LiC104 concentrations on the absorption spectrum
of 2 (7.4pM) in 30% (v/v) acetonitrile-toluene mixture. [LiC104] (a) 0, (b) 0.25, (c) 0.5,and (d) 2.5 mM.
(2)
where K is the equilibrium constant, 4: is the fluorescence quantum yield of the monomer, #I{ is the quantum yield of fluorescence of the dye-8-CD complex, and 4fis the observed quantum yield of fluorescence. The dependence of l / ( & - 4f) on the reciprocal concentration of 0-CD, was found to be linear, indicating a 1:l complex formation between the dye and 0-CD (Figure 4). The values for the equilibrium constant ( K ) and fluorescence quantum yield of the dye-P-CD complex (4{) determined from the slope and intercept of these plots were 1100 M-1 and 0.24 for 2 and 1750 M-1 and 0.16 for 3, respectively. Thus, the crown ether derivatives of squaraine dyes which are relatively nonfluorescent in aqueous media can substantially enhance their fluorescence quantum yield through hydrophobic interactions. Picosecond laser flash photolysis studies on the dye-8-CD complexes (of 2 and 3) showed that the absorption spectra of the excited singlet states werevery similar to those of the uncomplexed dyes (Table 2). However, the excited singlet-state lifetime were much longer for the complexes than for the free dyes in aqueous solution. The absorption spectra of singlet excited dye, 3, recorded in the absence and in the presence of 8-CD are shown in Figure 5. The decay traces in the inset of Figure 5 highlights the longer lifetime of the excited state in 8-CD solution. The nearly 10-fold enhancement in singlet excited-state lifetime observed in the solutions containing &CD, parallels the increase in fluorescence yield observed for these dyes. Complexation with 8-CD would lead to a suppression of the nonradiative internal conversion modes of the excited singlet state. Indeed the nonradiative decay rate constants determined from the dfand T $values (Table 2) support this argument to be true. For the dye 2 the k,, decreased from
600
650
700
750
Wavelength (nm) Figure 7. Influence of LiC104 on the emission spectrum of 2 (7.4pM) in 30% (v/v) acetonitrile-toluene mixture.[LiC104] (a) 0, (b) 0.375,(c) 1.0, and (d) 1.87 mM.
1.4 X 1010 to 8.9 X 108 s-1, and for 3 it decreased from 8.95 X 109 to 9.16 X 108 s-1 in 8-CD solutions. Cation-Binding Properties. Complexation properties of redsensitive fluoroionophores, 1and 2, with lithium and sodium ions have been demonstrated using fluorescence spectroscopy. Significant changes in the absorption and emission spectra were observed in the presence of alkali-metal ions. Fluoroionophore 1 has an absorption maximum at 639 nm and an emission maximum at 661 nm (& = 0.51) in 30% (v/v) acetonitriletoluene mixture. A gradual red shift in absorption maximum was observed upon addition of lithium perchlorate. Similar changes in the absorption spectrum was also noted for 2. The absorption and emission spectra of 2 recorded upon addition of lithium perchlorate (0-2 mM) to the dye solution in 30% (v/v) acetonitrile-toluene mixture are shown in Figures 6 and 7, respectively. A well-defined isosbestic point a t 625 nm indicates the existence in equilibrium of the uncomplexed and complexed
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9295
Photochemistry of Squaraine Dyes
0.6
0.4 8
1 /[M+], M - I
b 0
o'2
F 0
1
2
3
[M+l, 0.0
I
0
I
I
I
I
10
20
30
40
[Mfl, mM Figure 8. Plot of quantum yield of fluorescence (&) of 1 (3.0pM) versus metal ion concentration in 30% (v/v acetonitrile-toluene mixture. (a) NaC104; (b) LiC104. Inset shows the plot of 1 /(&$-&) versus reciprocal concentration of lithium ion concentration.
fluoroionophores. Similar changes in absorption spectra were also observed upon addition of sodium perchlorate. The fluorescence quenching of 1 upon addition of 20 mM lithium perchlorate as well as sodium perchlorate are shown in Figure 8. A significant quenching of fluorescence (38%) was observed upon addition of 20 mM lithium perchlorate. In the case of sodium perchlorate, the absorption and emission maxima remain unchanged, while 4% quenching of fluorescence was observed upon addition of 1 mM of sodium perchlorate and 18% quenching of fluorescence was observed upon addition of 20 mM of sodium perchlorate (Table 3). Even at low concentrations of lithium perchlorate (1 mM), a noticeable quenching of fluorescence (8%) occurred. The dyeto-metal ion stoichiometry and stability constant, K,, of 1 for complexation with metal ion have been investigated by expression 3,
(3) where K , is the stability constant of the complex, &the quantum yield of fluorescence of uncomplexed dye, &' the quantum yield of fluorescence of complexed dye, $f the observed quantum yield of fluorescence, and [MI the concentration of metal perchlorate. Plot of I/(@ - &) against the reciprocal of lithium ion concentration showed a linear dependence in the concentration range 0-10 mM, indicating a 1:1 metal-to-ligand complexation for 1 in this concentration range (Figure 8, inset). Stability constant (K,) of 2.71 X lo2 M-I was obtained from the ratio of intercept and slope. The deviation from linearity found above 10 mM is attributed to the formation of 2: 1 metal toligand complexes.
5
4
6
mM
Figure 9. Plot of quantum yield of fluorescence (bf)of 2 (7.5pM) versus metal ion concentration in 30% (v/v) acetonitrile-toluene mixture (a) @f) versus reciprocal NaC104; (b) LiC104. Inset shows the plot of l/($$of lithium ion concentration.
The emission maximum of 2 remains unchanged upon addition of LE104 as well as NaC104. In the presence of 1 mM of lithium perchlorate, 26% quenching of fluorescence was observed and for 1 mM of sodium perchlorate, 20% quenching of fluorescence was observed (Figure 9, Table 3). Plot of l/(& - &) versus reciprocal of metal ion concentration showed a linear dependence for lithium perchlorate as well as sodium perchlorate in the concentration range 0-2 mM, indicating a 1:l metal ion to dye complexation. A typical example observed in the case of lithium perchlorate is shown in the inset of Figure 9. Stability constants, K,, were calculated as 1560 M-' for lithium perchlorate and 1300 M I for sodium perchlorate. (All the K, values reported here are within the experimental error of *5%.) Blank experiments were done with a similar squaraine dye, bis(4-dioctadecylpheny1)squaraine (4),35 which does not possess the crown appendages. Only slight reduction in quantum yield of fluorescence (1%) was observed upon addition of 1 mM of LiC104 to a solution of 4 in acetonitrile-toluene mixture (30/70, v/v), whereas in the case of NaC104, absorption and emission properties remain unchanged. At higher concentrations (5 mM) of LiC104, absorption as well as emission maxima shift to a longer wavelength by 1 nm and the quenching of fluorescence is limited to about 6%. In the case of NaC104 (5 mM), the absorption as well as emission maxima remain unchanged, and a slight quenching of fluorescence (1%) was observed. These results clearly indicate that the complexation of the metal ions is predominantly by the crown ether moiety. The absorption and emission spectra of the squaraine dyes have been suggested to arise from charge transfer transitions. Even though the charge transfer is primarily confined to the central C402 unit (from the oxygen atom to the four-membered ring), there is a minor contribution from the anilino moiety also. Complexation of the cation by the crown ether moiety induces a reduction in the electron donating ability of the nitrogen atom and thereby hindering its ability to contribute to the intramolecular charge-transfer process. The fluoroionophore 1 is more readily complexed with lithium ion. This can be explained by comparing the cavity size of the
TABLE 3: Effect of LiC104 and NaC104 on the Photophysical Properties of Squaraine Chromoionophores 1, 2, and Model Compound 4 in 30% (v/v) Acetonitrile-Toluene Mixtures 1
2
4
0 mM LiC104 (1 mM) NaC104 (1 mM) 0 mM LiC104 (1 mM) NaC104 (1 mM) 0 mM LiC104 (1 mM) NaC104 (1 mM) 639 640 639 637 637 637 644 644 644 Adem) 661 661 661 657 657 657 662 662 662 +f 0.51 0.47 0.49 0.51 0.36 0.41 0.67 0.662 0.67 % quenching 8 4 29 20 1 0
Xmax(abs)
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N-aza-12-crown-4 ether (1.2-1.5 and the ionic diameter of the metal cations (ionicdiameter of Li+ = 1.36 8, and Na+ = 1.96 A).*! Ionic diameter of Li+ ion matches well with thecavity size of N-aza-12-crown-4 ether moiety. Fluoroionophore 2 is also complexed readily with Li+ ion even though the ionic diameter of Na+ ion matches with the cavity size of N-aza-15-crown-5 ether moiety (1.7-2.2 8,).11 The charge density of lithium ion is higher than sodium ion, and hence it leads to a more stable complex. Such effects were previously reported by Valeur and co-workers.l2 The complexation of these dyes with metal ions has also been found to alter the redox properties of these dyes. For example, it has been observed that the oxidation potential of 1 (0.705 V vs Ag/AgCl) shifts to less positive potentials (0.676 V in the presence of Li+ and 0.644 V in the presence of Na+) when metal ions (2 mM) are present in the acetonitrile solutions.24 Thus, these red-sensitive squaraine dyes are useful probes in recognizing metal ions by both fluorimetric and electrochemical analysis.
Conclusions Three new crown-ether-linked squaraine fluoroionophores 1-3 have been synthesized. The I3C N M R spectra of these dyes are indicative of their highly symmetric structure. The fluoroionophores 2 and 3 form aggregates in aqueous solution. Addition of 0-cyclodextrin breaks up this aggregate leading to the formation of a fluorescent dye-b-CD complex. The fluoroionophore bearing N-aza-12-crown-4 ether (1) as well as N-aza-15-crown-5 ether (2) complex with alkali-metal cations such as Li+ and Na+ and thus can be used as ion-sensitive near-infrared probes.
Acknowledgment. The authors thank the Council of Scientific and Industrial Research, Government of India (S.D., K.G.T., K.J.T., and M.V.G.) and the Office of Basic Energy Sciences of the Department of Energy (P.V.K. and M.V.G. (in part)). This is contribution No. RRLT-PRU-41 from RRL, Trivandrum and No. NDRL-3696 from the Notre Dame Radiation Laboratory. References and Notes (1) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (2) Bissell, R. A.; desilva, A. P.;Gunaratne, H. Q.N.; Lynch, P. L. M.; Maguire, G. E. M.; Sandanayake, K. R. A. S. Chem. SOC.Rev. 1992, 187. (3) de Silva, A. P.; Gunaratne, H. Q.N.; McRoy, C. P. Nature 1993, 364, 42. (4) Czarnik, A. W. In Frontiers in Supramolecular Organic Chemistry and Photochemistry; Schneider, H.-J., Durr, H., Eds.; VCH: New York, 1991; pp 109-122.
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