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J. Phys. Chem. A 2010, 114, 10388–10394
Interaction of Ketocyanine Dye with a Co2+ Ion: An Electronic Spectroscopic Study Sanjib Kr Sardar, Kambalapalli Srikanth, and Sanjib Bagchi* Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur-741252, Nadia, West Bengal, India ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: July 16, 2010
The interaction of a ketocyanine dye with a cobalt(II) ion has been studied in solution by monitoring the electronic absorption and emission spectral characteristics of the dye. A new absorption band at a longer wavelength appears in solutions containing cobalt(II) ions. An isosbestic point is observed for systems containing a fixed dye concentration and varying Co2+ ion concentration, pointing to the formation of a complex. The stiochiometry of the complex has been found to be 1:1. Equilibrium constant has been determined from the observed data. The nature of interaction between the dye (S0 state) and the Co(II) ion is mostly electrostatic. Spectroscopic results have been supported by DFT/TDDFT calculation. The fluorescence band is characterized by a small blue shift. In the concentration range of 10-3-10-4 M of the Co(II) ion, a quenching of the dye fluorescence is noticed. The Stern-Volmer plot points to the operation of both static and dynamic mechanisms of quenching. For a micromolar concentration of the Co(II) ion, however, an enhancement of fluorescence intensity with a slight blue shift has been observed, which has been explained in terms of formation of a different type of complex in the S1 state at this concentration level. The value of lifetime increases at the micromolar level of concentration of the Co(II) ion, where the intensity increases and then remains practically unchanged as more salt is added to the system. Values of the decay constant for the different photophysical processes have been calculated. Complexation in the S1 state is characterized by a slower decay of the excited dye by a nonradiative path. 1. Introduction
SCHEME 1: Ketocyanine Dye Used in the Present Works
N-substituted derivatives of 2,5-bis[propylene]cyclopentanone, commonly known as ketocyanine dyes, are characterized by solvent-sensitive absorption and fluorescence properties.1,2 Due to the presence of electron-donor (amino) and electronacceptor (carbonyl) groups in the molecule, the electronic transition is associated with intramolecular charge transfer (ICT), and as such, the electronic spectral characteristics of the molecule are very much sensitive to the microenvironment. Electronic spectroscopic and photophysical properties of these molecules have been extensively studied in recent years.3-10 These compounds are also used as laser dyes and have industrial application in polymer imaging systems.11-14 The electronic spectral properties of this class of compounds are modified due to their interaction with metal ions. The effects of alkali and alkaline earth metal ions on the ground (S0) and the first singlet excited state (S1) have been reported in the several communications from our laboratory.15-18 It has been shown that the hard metal ions, for example, Li+, M2+ (M ) Mg, Ca, Ba, Sr) bind weakly to the carbonyl center, and the nature of interaction is mostly electrostatic. Doroshenko et al. have studied the interaction of bis(azacrown)-substituted derivatives of a ketocyanine dye with Mg2+ and Ba2+ ions.8 The objective of the present work is to study the interaction of a transition-metal ion with a ketocyanine dye. To this end, we have studied the electronic absorption and steady-state and time-resolved fluorescence properties of a ketocyanine dye (Scheme 1) in the presence of Co2+ ions in a solvent. The solvents used are acetonitrile (ACN), acetone (AC), and methanol (MEOH). Quantum chemical * To whom correspondence should be addressed.
calculations at the density functional theory (DFT) level have also been done to study the complexation of the dye with the Co2+ ion. 2. Experimental Section 2.1. Materials. The ketocyanine dye was prepared by the method described in the in the literature.1a,3a The purity of the compound was checked by infrared (IR), UV-vis absorption, and fluorescence spectral data. For example, the synthesized dye was characterized by IR bands in a KBr disk, 1620, 1580, 1485, and 1400 cm-1, and λmax(absorption) at 525 nm and λmax(fluorescence) at 622 nm in ethanol. Acetone [E. Merck], acetonitrile [E. Merck], and methnol [E. Merck] were dried according to the standard procedure.19,20 All of the solvents were distilled prior to the experiment using calcium hydride. Cobalt chloride hexahydrate (CoCl2, 6H2O) [Merck] was dried by controlled heating under vacuum so as to avoid the possibility of getting oxidized to cobalt oxide. Cobalt(II) perchlorate hexahydrate [Sigma-Aldrich] was also dried cautiously in vacuum. The concentration of the dye was in the range of 10-5-10-7 M in all of the spectroscopic measurements. The concentrations of cobalt chloride and cobalt perchlorate solution varied in the range of 10-4-10-6 M. 2.2. Steady-State Spectral Measurement. The UV-vis absorption spectral studies were performed on a U-4100 Hitachi
10.1021/jp102595z 2010 American Chemical Society Published on Web 09/09/2010
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Figure 1. Optimized structures in the ground state of the dye (a) and dye-Co2+ complex (b) with carbonyl oxygen; (c) HOMO and LUMO pictures of the dye.
spectrophotometer fitted with a temperature controller unit. Some experiments have also been performed on a Cary 300 BIO spectrophotometer fitted with a temperature controller unit. The steady-state fluorescence emissions were measured on Horiba Jobin Yvon FluoroMax-3 as well as on a Perkin Elmer LS 55 spectrofluorimeter. 2.3. Time-Resolved Fluorescence Measurement. Fluorescence decay in the picosecond domain was measured by timecorrelated single-photon counting (TCSPC) technique using a Horiba Jobin Yvon time-resolved fluorimeter. Decay curves were analyzed using IBH DAS-6 decay analysis software. Intensity decay curves were fitted with an exponential decay function as given by eq 1.21
F(t) )
∑ ai exp(-t/τi)
(1)
where ai are the relative contributions to the lifetime component τi. The decay parameters were recovered using a nonlinear leastsquares fitting procedure. Fittings with χ2 values around 1 were taken as acceptable. The fluorescence decay of the dye was measured as a function of the concentration of metal ions. In the present study, the fluorescence intensity as a function of time t for the dye in pure solvent as well as in metal ion solution could be fitted with a single exponential equation. 2.4. DFT Calculation. DFT calculations have been done on the ketocyanine dye and the dye-cobalt(II) ion complex. Calculations were carried out using the B3LYP22-25 functional. The SDD pseudopotential26 basis set was used on Co2+, and the 6-31G(d,p) basis set27,28 was used for all other atoms.
Geometry optimizations were followed by frequency calculations to ascertain that the optimized structures were true minima. The default criteria for geometry optimizations were used. All calculations were carried out using the Gaussian 03 program.29 It is known that cobalt(II) chloride in acetonitrile exists predominantly as a tetrahedral species (either [CoCl2(ACN)2] or [CoCl3(ACN)]-, where ACN ) acetonitrile) particularly in dilute solution.30,31 In the present work, a tetrahedral geometry of the Co2+ center has been taken for the dye-Co2+ complex. Two chlorine atoms and one acetonitrile molecule (bound to Co through the N atom) were taken as the three groups around the Co2+ ion. The whole unit thus formed was attached either to the carbonyl oxygen or the N center of the dye. 3. Result and Discussion 3.1. Optimized Structures in the Ground State. The optimized geometries of the dye and Co2+ ion-dye complexes as obtained by DFT calculation are shown in Figure 1a and b. The dye is planar in the region contained between two N centers (except the aliphatic H atoms). Substituents on the N center in both the wings, however, make a small angle with the plane. The molecule has a symmetry plane passing through the carbonyl group, and the dipole moment comes out as 5.12 D along the carbonyl axis. As the dye contains both oxygen and nitrogen centers, a Co(II) ion can bind on either of the centers. Calculations have been done on both systems. It appears that the Co(II) ion-ketocyanine dye system with the Co(II) ion binding on oxygen of the dye was found to be lower in energy than that with the Co(II) ion binding on the N by ∼20 kcal mol-1. Calculations on both the quartet state as well as the doublet state of the Co(II)-ketocyanine complex were done.
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Sardar et al. TABLE 1: Values of the Energy Maximum of Absorption, EC, of the Dye-Metal Ion Complex in Acetonitrilea metal ions EC/kcal mol
-1
Co2+
Mg2+ b
Ca2+ b
Sr2+ b
Ba2+ b
48.9 (585)
49.3 (580)
50.2 (570)
51.1 (559)
52.2 (548)
a
Values within brackets indicate the wavelength (nm) of maximum absorption. b Reference 18.
Figure 2. Absorption spectrum of the dye (concentration: 10-5 M) after correction due to Co2+ ion absorption in acetone solvent with varying concentrations of the Co2+ ion, (1) 0.0, (2) 2.3 × 10-4, (3) 4.6 × 10-4, (4) 6.9 × 10-4, (5) 9.2 × 10-3, and (6) 1.15 × 10-3 M. The variation of absorbance of the complex at 600 nm as a function of Co2+ ion concentration is shown in the inset.
The quartet state was found to be lower in energy (∼20 kcal mol-1). TDDFT calculation has also been done for the dye and the dye-Co(II) system. The HOMO and LUMO of the dye are shown in Figure 1c. It appears that due to the HOMO f LUMO transition, the electron density on the carbonyl group increases for the dye, pointing to ICT from N to O atom of the dye. Similar observation has been made for the complex dye. The HOMO-LUMO gap for the dye is 3.0406 eV, and those for the complexed dye are 2.6694 (R-spin) and 2.5788 eV (β-spin). Thus, the HOMO-LUMO gap for the dye decreases upon complexation. The calculated value of S0 f S1 absorption for the dye is 444 nm. The value can be compared with the value of the absorption maxima (467 nm) of the dye in a nonpolar solvent, for example, hexane. The agreement is good. For the dye-Co(II) system with Co2+ binding on the oxygen atom, the value of the maximum absorption for the S0 f S1 transition is 496 nm. Thus, the red shift of the absorption maximum is predicted from theoretical calculations. At this point, it may be mentioned that Andrzej Eilmes has recently done DFT and TDDFT calculations on the Li+ and Mg2+ complexes of a ketocyanine dye and arrived at similar conclusions.32 3.2. Absorption Studies. In a pure aprotic polar solvent, the dye forms a loose solvated species, and the absorption maximum of the dye is dependent on solvent polarity.3a For acetonitrile and acetone, the band maxima appear respectively at 503 and 495 nm. Upon addition of CoCl2 to the system, the spectrum shows absorption due to the dye and Co(II) ion. After correction for Co(II) absorption, it appears that a new band at longer wavelength grows in the absorption spectrum as the concentration of the salt increases. For a fixed dye concentration, an isosbestic point appears in the absoption spectrum in acetone and acetonitrile solvent containing varying amounts of the salt. Similar observation is made when Co(ClO4)2 is used in place of CoCl2. Thus, the spectral change is induced by a cation, that is, the Co2+ ion. Figure 2 shows representative spectra of the dye in acetone containing varying amounts of CoCl2. These results point clearly to the existence of two species, namely, the solvated dye and the dye-Co2+ ion complex in equilibrium. A plot of absorbance at higher wavelength versus the concentration of the Co(II) ion is show in the inset of Figure 2. The absorbance value saturates at higher concentration of the salt, indicating that a new species is formed. Thus, the absorption
maximum at around 500 nm corresponds to the solvated dye, and that at the longer wavelength is due to a dye-Co2+ ion complex. The results obtained earlier indicate that for alkali and alkaline earth metal ions, the new absorption band upon complexation appears at a longer wavelength with respect to the free dye absorption. For these metal ions, the nature of the interaction is mainly electrostatic, as evidenced by the systematic variation of the absorption maximum of the complex with the ionic potential value of the cations.18 In the case of the dye-Co2+ ion complex, also the new absorption band at a longer wavelength can similarly be explained as due to an interaction of Co2+ with the carbonyl group of the dye (S0 state). Results obtained from DFT calculation also indicate that interaction of the Co2+ ion with the oxygen center of the dye comes with lower energy relative to that for interaction at the N center, and the absorption maximum due to the S0 f S1 transition of the dye in the complex form appears at longer wavelength. Table 1 lists the experimentally observed values of the energy of maximum absorption (EC) of the dye complexed with different metal ions. As discussed in our earlier work, with an increase in interaction of the dye with alkaline earth metal ions, the absorption band of the dye metal ion complex (EC) moves to lower energy (longer wavelength). The value of EC as obtained in the present work indicates that the extent of interaction in a particular solvent is greater for the Co2+ ion than that for the alkaline earth metal ions. Note that Co2+ has an ionic radius value close to that of Mg2+.33 Thus, the two ions have similar values of ionic potential. The closeness of the EC values for the two metal ions, as is evident from Table 1, indicates that the interaction of the dye with two metal ions is also similar. Thus, the nature of binding of the dye with the Co2+ ion is mostly electrostatic, as found in the case of the Mg2+ ion. A slightly stronger interaction for the Co2+ ion is presumably due to involvement of a partial covalent nature of binding. The interaction of dye with the Co2+ ion in solution can be represented by the following equilibrium
D · · · S + nCo2+ ) D · · · (Co2+)n + S K ) CC /[CS(CCo2+)n]
(2)
Here, D · · · S and D · · · (Co2+)n represent the solvated dye and complexed dye, respectively, n is the minimum number of Co2+ ions participating in the equilibrium, and CS, CC, and CCo2+ represent the molar concentrations of the solvated dye, metalcomplexed dye, and free metal ion, respectively, in the solution. In the present case, the wavelengths corresponding to the solvated dye and complexed dye (λS and λC, respectively) are far apart, and it can be assumed that at λS and λC, the absorbing species are the solvated dye and complex dye, respectively. Denoting the absorbance at a wavelength λi by Ai, we have the following equation
log(AC /AS) ) n log CCo2+ + log((CK)/S)
(3)
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Figure 3. Plot of log(AC/AS) versus log(C0) for the dye-Co2+ system in acetone solvent.
TABLE 2: Values of the Equilibrium Constant (K) for the Co2+ Ion Dye (S0 State) Interaction at Various λ Values in Different Solvents at 298 K solvent
λ/nm
K
R2
average K
ACN
495 500 504 510 515 485 490 495 500 505
255 ( 10 250 ( 10 257 ( 10 252 ( 10 254 ( 10 523 ( 20 550 ( 20 579 ( 20 547 ( 20 498 ( 20
0.97 0.98 0.98 0.96 0.96 0.99 0.99 0.99 0.99 0.99
254 ( 10
AC
539 ( 20
where C and S is the molar absorbance of the complexed and the solvated dye at λC and λS, respectively. The CCo2+ term in eq 3 can be written as CCo2+ ) C0 - nCC, where C0 is the concentration of the total Co2+ added. In our experiment, we kept the concentration of the total dye fixed (10-5 M) and varied the concentration of the Co2+ ion (10-4-10-3 M), that is, C0 is 10-20 times higher than the total dye concentration (CT). As CC e CT, under the present experimental conditions, we have C0 . CC and log(CCo2+) can be approximated by log(C0). The value of n can thus be determined from the slope of the plot of log(AC/AS) versus log(C0). Figure 3 shows a representative plot. The value of n is in the range of 0.93-0.95 for all of the systems. This points to the formation of a 1:1 complex. The equilibrium constant for 1:1 complexation (n ) 1) can also be found from an analysis of the spectral data as follows. Equation 2 can be rearranged (with n ) 1) as
1 + KCCo2+ ) (CC + CS)/CS ) CT /CS
(4)
If AT and AS represent the absorbance value at λS for the total dye (solvated + complexed) and the solvated dye, respectively, we have the following equation
AT /AS ) 1 + KCCo2+
(5)
The value of AT is the absorbance of the solution when C0 ) 0. We have determined the value of K at several wavelengths in the absorption band of the dye using eq 5. Table 2 lists the K values in different solvents. Note that for acetonitrile, the K values are almost independent of wavelength; for acetone, the
Figure 4. Fluorescence spectrum of the dye in acetonitrile solvent with varying concentrations of CoCl2, (1) 0.0, (2) 2.3 × 10-4, (3) 4.6 × 10-4, (4) 6.9 × 10-4, (5) 9.2 × 10-4, and (6) 1.15 × 10-3 M.
observed K values shows slight wavelength dependence. This may arise due to slight overlap of the absorption spectra of the solvated and complex dyes. Values of K can also be determined by a similar analysis by studying absorbance at λC. The results are the same within the experimental inaccuracy. Values of K are higher than those for the alkali/alkaline earth metal-dye complex.18 This is also consistent with the fact that the dye-Co2+ ion interaction is stronger than the dye-alkali/ alkaline earth metal interaction, as discussed earlier. Note that K in acetonitrile solution is lower than the value of K in acetone. This is intelligible in view of eq 2, where Co2+ replaces one solvent molecule from the solvated dye to form a complex. Acetonitrile being more polar than acetone, a stronger ACN-dye interaction relative to a AC-dye interaction is expected. Thus, the process represented by eq 2 is more favorable for acetone. No extra absorption band can be detected when methanol is used as a solvent. This can be explained as follows. It is known that the dye forms a stronger hydrogen-bonded complex with alcohols. The dye-solvent interaction is stronger than the dye-cobalt ion interaction, and no complexation takes place. Similar observation has been made with earlier studies involving alkali/alkaline earth metal ions.16-18 As discussed earlier, the binding in the newly formed species is mostly electrostatic. We have studied the dye-Co2+ interaction by adding inert salt, for example, tetra-n-butyl ammonium bromide, in the reaction mixture. Addition of inert salt does not affect the absorption spectrum of the dye in pure solvent. However, the absorption due to the complex decreases with a simultaneous increase in the absorption due to free Co2+ ions upon addition of salt. K values have been determined in the presence of inert salt by the method discussed above. The observed K value decrease as the ionic strength of the medium increases. Thus, the values of K are 100 and 15 for 10-3 and 10-2 M, respectively, inert salt concentrations. The value of K is practically independent of the use of CoCl2 and Co(NO3)2 as the source of the Co2+ ion. These results support the electrostatic nature of the binding. 3.3. Fluorescence Studies. The fluorescence band of the dye appears as broad and structureless with a maximum at around 560 nm in ACN and 543 nm in AC. Upon addition of the Co2+ ion to the solution of the dye, there is a blue shift of the band maximum, as can be seen from Figure 4. The extent of the blue shift is small (∼5-10 nm) in the range of salt concentrations studied. The blue shift is relatively greater when Co(II) chloride is added in place of Co(II) perchlorate.
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TABLE 3: Spectroscopic and Thermodynamic Parameters for the Co2+ Ion Dye (S1 State) Interaction in Different Solvents at 298 K solvent
ES1/kcal mol-1
EC1/kcal mol-1
K1
ACN AC
53.36 ( 0.5 53.18 ( 0.5
50.59 ( 0.5 52.02 ( 0.5
358 ( 6 689 ( 8
A shift of the emission maximum indicates that the Co2+ ion interacts with the dye in the S1 state. Our previous studies with alkali/alkaline earth metal salts indicate that a new band in the red appeared upon addition of a metal ion. This was rationalized in view of increased charge density on the carbonyl oxygen in the S1 state, leading to a stronger dye-metal ion interaction in the S1 state relative to that in the S0 state. A blue shift, as observed in the present case, indicates that the nature of interaction is different than that in the case of alkali/alkaline earth metal ions. In the present case, the dye molecule has a donor (carbonyl O) and an acceptor (N) center. The nature of the transition is π f π* with some ICT character.2 This is reflected in the strong solvatochromism exhibited by the dyes. In the ground state, the Co2+ ion binds to the oxygen of the carbonyl group, and the all-trans configuration of the double bonds gives an almost planar structure of the dye. Upon excitation, different configurations are possible, and the N atom may come into the proximity of the Co2+ ion and take part in complexation. Thus, the emitting state is modified. In the event of complexation involving also the N center, the possibility of ICT in the excited state is reduced. The modified emitting state presumably has higher energy than the normal ICT emitting state which exists in absence of the Co(II) ion. Thus, the observed blue shift is intelligible in terms of a modification of the emitting state of the dye. The energy of the fluorescence maximum for the dye (S1 state) bound to the Co2+ ion and the constant representing the interaction have been determined by a method described earlier.18 The observed fluorescence energy, E(F), can be considered as a mole fraction average of the solvated and the complexed dye, ES1 and EC1, respectively. Thus, one gets
E(F) ) (CSES1 + CCEC1)/(CS + CC)
(6)
Assuming equilibrium between two species in solution, one gets
E(F) ) ES1 + EC1K1CCo2+ - K1E(F)CCo2+
(7)
F0 /F ) 0.99 + 590Q + (3 × 104)Q2
where F0 and F are the fluorescence intensities in the absence and presence, respectively, of the quencher, that is, the Co2+ ion, and Q is the concentration of the Co2+ ion. Results thus point to the simultaneous existence of static and dynamic quenching. It is interesting to note that the sum of the constants (K and K1) representing ground- and excited-state complexation of the dye equals ∼620, which close to the coefficient of Q in eq 9. The change of intensity of fluorescence at the µM concentration level of the Co2+ ion is interesting. A small (∼20%) but significant enhancement of fluorescence intensity has been observed at this level of concentration of Co2+, despite the fact that the transition-metal ions are very good quenchers. Table 4 shows the variation of fluorescence intensity as a function of the concentration of the Co2+ ion in acetone and acetonitrile. Note that initial addition of Co2+ ion to a solution of the dye leads to an increase in the fluorescence intensity. At a certain concentration of the metal ion, the intensity reaches a maximum, and beyond this, a quenching could be observed. Enhancement of the fluorescence intensity attended with a blue shift of the emission maximum of a fluorophore in the presence of transition-metal ions has also been reported by other workers for fluorophore-spacer-receptor system.34-36 The phenomenon was explained by postulating a binding of a transition-metal ion to the receptor site, causing a decrease in photoinduced electron transfer (PET). Results obtained in the present case can be explained by an interaction of the Co2+ ion with the dye molecule in the excited state. The observed value of the equilibrium constant (K) for the ground-state complexation or the excited-state complexation (K1) indicates that the extent of ground-state complexation is insignificant at the µM level of Co2+ ion concentration. Thus, the characteristic fluorescence feature at this concentration range is indicative of a different complex species. Presumably an encounter complex, (D · · · Co2+)*, is formed upon collision of a Co2+ ion with the dye molecule in the S1 state (D*). In the mM concentration range, the ground-state complexation, however, predominates, leading to a quenching. Enhancement of the fluorescence intensity at the µM concentration level indicates that the resulting encounter complex is also fluorescent. Thus, the state of affairs at this level of concentration can be expressed by the following kinetic scheme k1
D* + Co2+ y\z (D · · · Co2+)*
where K1 is the binding constant for the following process
(9)
(10)
k2
Co2+ + S · · · D (S1 state) ) Co2+ · · · D (S1 state) + S
(8) Values of ES1, EC1, and K1 that fit eq 7 can thus be determined by a linear regression analysis. Values of the parameters have been listed in Table 3. It appears that the value of K1 is greater than that of K, indicating a relatively stronger complex in the S1 state. This is intelligible in view of the increased electron density on the carbonyl oxygen in the LUMO of the dye (Figure 1c). Note that a quenching of fluorescence is observed (Figure 4) in the concentration range of 10-3-10-4 M of Co2+ ions. We have analyzed the results on quenching by the Stern-Volmer (SV) plot. The S-V plot deviates from linearity. The result can be best fitted by the following equation for acetonitrile solvent
where k1 is the bimolecular rate constant for the formation of the complex and k2 is the rate constant for its decomposition. In the absence of any quenching, the observed fluorescence intensity (F) can be written according to the following scheme
F ) (F0 + FCKbCCo2+)/(1 + KbCCo2+)
(11)
where F0 is the fluorescence intensity of uncomplexed dye (when the concentration of the Co2+ ion is 0) and FC is the fluorescence intensity when the emitting species is solely the complex. Kb is the binding constant given by Kb ) k1/k2. It appears from eq 11 that a plot of F versus log CCo2+ (CCo2+ is the equilibrium concentration) will be sigmoid. For the condition where the dye concentration is much less than the
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TABLE 4: Steady-State Fluorescence Intensity (F) of the Dye with Increasing Concentration of Co2+ Ions in Acetone and Acetonitrile Solventa Acetone 6
2+
6
2+
10 [Co ]/M F (at 543 nm)
0.00 569.6
1.54 571.0
1.74 592.3
1.98 614.2
2.30 632.3
3.05 640.9
3.80 631.9
4.54 618.9
5.28 605.3
7.45 582.9
11.0 551.1
14.4 525.5
13.4 338.9
19.7 329.5
31.7 314.2
42.0 300.7
Acetonitrile 10 [Co ]/M F (at 560 nm) a
0.00 314.2
0.48 326.9
1.39 337.2
1.60 347.9
2.77 352.8
4.14 353.2
6.84 347.9
9.50 344.0
The concentration of the dye is ∼10-7 M.
Figure 5. Plot of fluorescence intensity (F) of the dye as a function of log [Co2+] in acetonitrile.
TABLE 5: τ Value of the Dye in Acetonitrile Solvent As a Function of Co2+ Ion Concentration at 298 K [Co2+]/(M)
τ/ps
Φ
10-8kr/s-1
0 1.54 × 10-6 2.75 × 10-6 4.56 × 10-6 9.19 × 10-6 1.48 × 10-5 2.85 × 10-5
614 623 626 629 630 630 631
0.46 0.47 0.48 0.49 0.49 0.49 0.49
7.5 7.5 7.7 7.8 7.8 7.8 7.8
average kr/s-1
10-8knr/s-1
7.7 × 108
8.6 8.4 8.3 8.2 8.1 8.1 8.1
cobalt ion concentration, CCo2+ can be replaced by the analytical concentration [Co2+]. Figure 5 shows a representative plot. The value of Kb can be obtained from a sigmoid plot. Thus, the Kb value is 6.9629 × 105 for acetonitrile and 5.6715 × 105 for acetone. The value of FC as obtained from the plot is close to the maximum intensity value. The fluorescence decay profile of the dye both in the presence and in the absence of the Co2+ ion has been studied. Decay curves in both cases are best represented by a single exponential fit. The results have been given in Table 5. Note that the lifetime, τ, increases when the concentration of the metal is very low. After that, the value of τ remains practically unchanged. The range of concentration where the value of τ shows increasing trend (0-5 × 10-6 M) is very similar to the concentration range where the enhancement of fluorescence takes place (Table 4). This suggests that the increase of the τ value and the fluorescence enhancement have the same origin. According to the proposed kinetic scheme (eq 10), the two emitting species are present in solution at the µM concentration level of Co2+. The τ value at zero Co2+ ion concentration corresponds to the D* state, and the limiting value of τ at higher Co2+ ion concentration is characteristics of the (D · · · Co2+)* state. Values of the quantum yield of fluorescence, as measured using rhodamine as a standard, are listed in Table 5. Note that
Figure 6. Plot of 1/τ of the dye as a function of log[Co2+] in acetonitrile.
τ values parallel the Φ values. The radiative decay constant kr as calculated by kr ) Φ/τ islisted in Table 5. It appears that the kr value is practically unaffected by complexation. The knr values calculated using the relation knr ) (1 - Φ)/τ are also listed in Table 5. Note that the values of knr decrease as the concentration of Co2+ increases and reaches a limiting value. While the knr values for zero Co2+ ion concentration (knr0) represent the nonradiative decay constant for the D* state, the limiting value of knr ()knrc), calculated using the limiting τ value, corresponds to the complex (D · · · Co2+)*. Note that the nonradiative decay of the (D · · · Co2+)* state is slower than that of the D* state. At any cobalt ion concentration, the observed value of the constant kr + knr ) 1/τ would be given by the mole fraction average of 1/τ values of the two species. Thus, we get
1/τ ) kr + x0knr0 + xcknrc
(12)
where x0 and xc represent the mole fraction of D* and (D · · · Co2+)*, respectively, in solution. Considering the kinetic scheme as given by eq 10, one gets
1/τ ) (1/τ0 + (knr0 - knrc)KbCCo2+)/(1 + KbCCo2+)
(13) Thus, the plot of 1/τ versus log[Co2+] would be sigmoid in nature. Figure 6 shows a representative plot. The value of the binding constant for the complex, as calculated from such sigmoid plot, compares well the value of Kb obtained from intensity variation. 4. Conclusion Interaction of the Co2+ ion with a ketocyanine dye in the S0 and S1 states has been studied by an electronic spectral study.
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A new red-shifted absorption band appears upon complexation. An isosbestic point is observed for systems containing a fixed dye concentration and varying Co2+ ion concentration, pointing to the formation of a dye (S0 state)-Co2+ ion complex. The stoichiometry of the complex and equilibrium constant has been determined from the observed data. The nature of binding in the complex is mostly electrostatic. Fluorescence of the dye in the presence of Co2+ is characterized by a slight blue shift. In the concentration range of 10-3-10-4 M of Co2+ ions, a quenching by both ground- and excited-state complexation has been observed. For a micromolar concentration of the Co2+ ion, an enhancement of fluorescence intensity accompanied by a small blue shift of the fluorescence band has been observed. The value of lifetime increases for a low concentration of Co2+ ion and then remains practically unchanged as more salt is added to the system. The range of concentrations where the value of τ increases is very similar to the concentration range where the enhancement of fluorescence takes place, indicating that the two phenomena have a common origin. Results have been explained in terms of formation of a different type of complex (encounter complex) between the Co2+ ion and the dye molecule in the excited state at this concentration level. The binding constant for the Co2+-dye (S1 state) complex has been determined. Acknowledgment. S.K.S. acknowledges financial support from UGC, India. The authors thank the reviewer for his constructive suggestions. References and Notes (1) (a) Kessler, M. A.; Wolfbeis, O. S. Spectrochim. Acta, Part A 1991, 47, 187. (b) Das, P. K.; Pramanik, R.; Banerjee, D.; Bagchi, S. Spectrochim. Acta, Part A 2000, 56, 2763. (c) Shannigrahi, M.; Pramanik, R.; Bagchi, S. Spectrochim. Acta, Part A 2003, 59, 2921. (2) Reichardt, C. Chem. ReV. 1994, 94, 2319. (3) (a) Banerjee, D.; Laha, A. K.; Bagchi, S. J. Photochem. Photobiol., A 1995, 85, 153. (b) Banerjee, D.; Mondal, S.; Ghosh, S.; Bagchi, S. J. Photochem. Photobiol., A 1995, 90, 171. (c) Banerjee, D.; Bagchi, S. J. Photochem. Photobiol., A 1996, 101, 57. (d) Shannigrahi, M.; Bagchi, S. J. Photochem. Photobiol., A 2004, 168, 133. (e) Shannigrahi, M.; Bagchi, S. J. Phys. Chem. B 2004, 108, 17703. (4) Marcotte, N.; Fery-Forgues, S. J. Photochem. Photobiol., A 2000, 130, 13. (5) Doroshenko, A. O.; Pivovarenko, V. G. J. Photochem. Photobiol., A 2003, 156, 5. (6) Doroshenko, A. O.; Bilokin, M. D.; Pivovarenko, V. G. J. Photochem. Photobiol., A 2004, 163, 95.
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