Unusually High Fluorescence Enhancement of Some 1,8

11824

J. Phys. Chem. B 2000, 104, 11824-11832

Unusually High Fluorescence Enhancement of Some 1,8-Naphthalimide Derivatives Induced by Transition Metal Salts B. Ramachandram, G. Saroja, N. B. Sankaran, and A. Samanta* School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India ReceiVed: January 27, 2000; In Final Form: August 29, 2000

Three-component systems, 1a-c and 2a,b, comprising 1,8-naphthalimide and 4-methoxy-1,8-naphthalimide as fluorophore, a dimethylamino moiety as guest binding site and a polymethylene group as spacer, have been synthesized and the fluorescence behavior of these systems has been studied in the absence and in the presence of the salts of several transition metal ions. The systems are found to be very weakly fluorescent compared to their constituent fluorophores (3 and 4) and this observation has been ascribed to photoinduced intramolecular electron transfer (PIET) between the electron rich amino moiety (donor) and relatively electron deficient fluorophore component (acceptor). Spectral and electrochemical data indicate the thermodynamic feasibility of PIET (exergonic free energy changes) in these multicomponent systems and PIET is found to be most efficient in systems where the fluorophore and the amino moiety are separated by two methylene groups. Fluorescence decay behavior of the systems suggest that PIET occurs by a through-space mechanism. In the presence of the transition metal ions, well-known for their fluorescence quenching abilities, the present systems exhibit significant fluorescence enhancement (FE). Moreover, it has been observed that guest-induced FE can even be severalfold higher than that expected from consideration of PIET in the system. It is suggested that a system can exhibit unusually high FE when the guest is capable of inducing FE by more than one means. In the present case, it is shown that preferential solvation of the fluorophore by the water molecules of the hydrated metal salts could be partially responsible for the high FE values.

1. Introduction

SCHEME 1

Systems capable of sensing guest molecules or ions are considered to be quite useful in a variety of applications and there is a great deal of current interest in the development of fluorosensors for species such as metal ions, protons, anions, etc.1,2 Photoinduced intramolecular electron transfer (PIET) is the most commonly exploited mechanism for the design of the fluorosensors, which are essentially multicomponent systems comprising a signaling moiety (fluorophore) and a guest binding site (commonly referred to as receptor1); the two are often separated by a spacer. The components are chosen such that PIET between the fluorophore and the receptor (usually a group containing one or more amino nitrogen atoms) quenches the fluorescence of the system. However, in the presence of a guest, which binds to the receptor engaging its lone pair of electrons, PIET communication between the receptor and the fluorophore gets cutoff and the fluorescence of the system is recovered. In other words, the presence of a guest is signaled by fluorescence enhancement (FE)3 of the system. This principle has been illustrated in Scheme 1. Over the past few years we have been studying the fluorescence behavior of some simple multicomponent systems toward metal ions and protons,4-7 primarily to explore the potential of these systems in sensing applications and while doing so, we have focused ourselves mainly on the transition metal ions (as the species to be sensed). This is because, unlike other metal ions such as those of alkaline or alkaline earth metals, most of these ions are known as notorious quenchers of fluorescence and FE resulting from the suppression of PIET is most often * Corresponding author. Email: [email protected].

nullified by the inherent fluorescence quenching ability of these ions.8,9 We have shown earlier4 that the quenching interaction between the metal ions and the fluorophore, known to be predominantly redox in nature,8,9 could be lowered significantly when the chosen fluorophore is electron deficient. It is to be noted in this context that an electron deficient fluorophore not only lowers the undesired fluorophore-metal ion quenching interaction, but also enhances the desirable PIET communication between the fluorophore and the receptor in the guest-free condition. We could observe fairly good FE of very simple fluorophore-spacer-receptor systems in the presence of efficient quenchers such as Fe3+ and Cr3+.4 In continuing our search for efficient systems involving

10.1021/jp000333i CCC: $19.00 © 2000 American Chemical Society Published on Web 11/14/2000

Fluorescence Enhancement of 1,8-Naphthalimide CHART 1

electropositive fluorophore component, we have embarked on this study in which 1,8-naphthalimide and 4-methoxy-1,8naphthalimide have been used as the fluorescing moiety. 1,8Naphthalimide and its derivatives have been the subject of several investigations dealing with their photophysical and photobiological behavior. 1,8-Naphthalimide and bisnaphthalimide derivatives are promising anticancer agents,10 the sulfonated derivatives are good antiviral agents with selective in vitro activity against the human immuno deficiency virus, HIV1.11 Brominated mono and bisnaphthalimide derivatives are photochemotherapeutic inhibitors for enveloped viruses in blood and blood products.12 The photoactivity and DNA-intercalation properties of 1,8-naphthalimide derivatives have also prompted studies on sequence-specific DNA- photonucleases.13 The photophysical behavior and excited-state reactivity of 1,8naphthalimide derivatives has been studied by Kossanyi and co-workers,14 Pardo et al.,15 Brown and co-workers,16 and others including ourselves.17-19 Even though the fluorophore-spacer-receptor systems based on the fluorophore, 5 do not show any significant FE in the presence of the transition metal ions,4 systems such as 1 and 2 (see Chart 1) are expected to be superior as these involve relatively electron deficient fluorophores (3 and 4). Surprisingly, as shown below, the FE observed with the present systems is much higher than what is expected from the consideration of PIET in the system. 2. Experimental Section 2.1. Materials. 1,8-Naphthalimide, 1,8-naphthalic anhydride, 4-amino-1,8-naphthalic anhydride, N,N-dimethylethylenediamine, and N,N-dimethylpropylenediamine from Aldrich and 4-chloronaphthalic anhydride from Acros Organics were used without any further purification. 4-amino-1,8-naphthalimide was recrystallized from ethanol prior to use in spectral measurements. Dibromobutane, ethyl bromide, sodiumhydride, N,N-dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (ACN), and dimethoxypropane were procured from Merck (India). The following metal salts were used in the investigation: Zn(H2O)6(ClO4)2, Cu(H2O)3(NO3)2, Ni(H2O)6(ClO4)2, Co(H2O)6(NO3)2, Fe(H2O)6(ClO4)3, Fe(H2O)6(ClO4)2, Mn(H2O)6(ClO4)2, Cr(H2O)6Cl2, Co(H2O)6Cl2. The metal salts used in this study were locally procured and were used without any purification. However, for some specific measurements, purification procedure such as recrystallization was adopted. Anhydrous CoCl2 was prepared from Co(H2O)6Cl2 following a standard procedure20 according to which the hydrated salt (once recrystallized) was refluxed

J. Phys. Chem. B, Vol. 104, No. 49, 2000 11825 with excess of dimethoxypropane for 6 h and subsequently the solvent was removed in a vacuum. The solvents, THF and ACN, used in the spectral studies, were rigorously purified by standard procedures. The purified solvents were found to be free from impurities and were transparent in the spectral region of interest. Further, it was confirmed by the measurement of ET(30) values21 of the solvents that ACN and THF employed in this study for spectroscopic measurements were almost free from moisture. 2.2. Synthesis of the Systems. For the syntheses of 1a and 1b, a common procedure was employed. Compound 1a was prepared by refluxing an ethanolic solution of 1,8-naphthalic anhydride (2.5 mmol, 3 mL) with N,N-dimethylaminoethylenediamine (3.04 mmol) for 4 h. After completion of the reaction, the solvent was removed and the residue was purified by column chromatography (silica gel, 20:80 mixture of hexane:ethyl acetate). Compounds 2a and 2b were obtained in two steps. In the first step, an ethanolic suspension of 4-chloro-1,8-naphthalic anhydride (0.5 g, 2.15 mmol in 5 mL ethanol) was refluxed with N,N-dimethylethylenediamine (0.29 mL, 2.6 mmol) for 6 h. The reaction mixture was cooled to room temperature and the solid product was recrystallized out on cooling. In the second step, the solid product (4-chloro analogue of 1a) obtained in the previous step (0.1 g, 0.35 mmol) was treated with sodium methoxide (0.1 M solution in 5 mL methanol) at room temperature for 6 h. The solvent was removed under vacuum and the product was purified by column chromatography (silica gel, 85:15 mixture of hexane:ethyl acetate). Compound 1c was obtained by in two steps: the first step consisted of preparation of N-(4-bromobutyl)-1,8-naphthalimide and the second step involved treatment of N-(4-bromobutyl)-1,8-naphthalimide with the dimethylamine hydrochloride salt. The reaction conditions were as follows. 1,8-Naphthalimide (0.5 g, 2.5 mmol) was allowed to react with previously washed sodium hydride (0.25 g, 10 mmol) in dry DMF (3 mL) for 1 h at room temperature. To this was added 1,4-dibromobutane (0.33 mL, 2.8 mmol), and the reaction was held at 80° with constant stirring for 24 h. The product was obtained on work up with water and extraction with ethyl acetate. Column purified N-(4-bromobutyl)-1,8-naphthalimide (0.1 g, 0.3 mmol) was dissolved in acetonitrile (5 mL) and to it added N,N-dimethylamine hydrochloride (0.03 g, 0.36 mmol). After addition of potassium carbonate (0.08 g, 0.6 mmol) and catalytic amount of the potassium iodide, the reaction mixture was allowed to stir continuously for 24 h at room temperature. After completion of the reaction, the solvent was evaporated and the resulted residue was purified by column chromatography. The details of the preparation, purification procedures, and the analytical data of the multicomponent systems and the fluorophores have been described elsewhere.22 2.3. Apparatus and Methods. The NMR spectra of the compounds (in CDCl3) at 25 °C were recorded on Bruker ACF200 spectrometer. Infrared (IR) spectra were recorded on the JASCO FT-IR/5300 spectrometer. The absorption and fluorescence spectra were recorded on JASCO UV-vis spectrophotometer (model 7800) and JASCO spectrofluorimeter (model FP-777), respectively. The fluorescence decay curves were recorded using IBH single photon counting spectrofluorimeter (model 5000). The instrument was operated using a thyratrongated flash lamp filled with hydrogen at a pressure of 0.5 atm. The lamp was operated at a frequency of 40 kHz and the pulsewidth of the lamp under the operating condition was ∼1.2 ns. The lifetimes were estimated from the measured fluorescence

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decay curves and the lamp profiles using a nonlinear leastsquares iterative fitting procedure.23 The quality of the fit was assessed by the plot of the standard deviation and the chi-square values. The cyclic voltammetric measurements were carried out with Cypress model CS-1090/CS-1087 computer controlled electroanalytical system. Ag/AgCl was used as the reference electrode, a glassy carbon as the working electrode and a Pt wire as the auxiliary electrode. The redox potentials were measured in N2 bubbled acetonitrile using 0.1 M TBAP as the supporting electrolyte. The scanning speed was maintained at 100 mV/s. The fluorescence quantum yields of 1 were measured using naphthalimide as the reference compounds (φf ) 5.0 × 10-2 in acetonitrile),17 while the quantum yields of 2 were measured using 4-aminophthalimide as the reference compound (φf ) 0.63 in acetonitrile).24 A solution of 1 and 2 in tetrahydrofuran or acetonitrile was prepared with absorbance (OD ) ∼ 0.2) same as that of the reference compound at the exciting wavelength (λexc was 320 nm for 1 and 350 nm for 2). The quantum yields of the fluorophores in aqueous solution were measured using solutions with an absorbance of 0.05 (due to poor solubility). The fluorescence spectra were then measured under the same operating conditions and settings. The quantum yields were determined by comparing the areas underneath the fluorescence spectra. The fluorescence measurements were carried out by using ∼10-5 M solutions of the compounds. The effect of the metal ions on the fluorescence intensity was examined by adding a few µL of a stock solution of the metal ions to a known volume of the solution of 1 or 2 (2 mL). The addition was limited to 100 µL such that the volume change was insignificant. 3. Results 3.1. Redox Behavior. The redox behavior of 3 and 4 has been studied first with a view to have an idea on how electron deficient these fluorophore components are compared to 5 or others used earlier.4-7 It is to be noted that while oxidation was observed at 1.27,4 1.5,4 and 1.6 V5 for 5, 4-aminophthalimide and 4-amino-7-nitrobenz-2-oxa-1,3-diazole respectively, no oxidation peak could be observed for 3 and 4 in the range of 0-2 V. This observation clearly indicates that 3 and 4 are comparatively electron deficient and hence, the multicomponent systems, 1 and 2, involving these fluorophores are expected to be superior to those based on 5 and others state above. While no oxidation peak could be observed for 3 and 4, reduction was observed at -1.00 and -1.11 V for 3 and 4, respectively. These values are consistent with the nature of the substituent in the ring. 3.2. Photophysical Behavior. 3.2.1. Absorption Spectra. The UV-vis absorption characteristics of 1 and 2 (Figure 1) are found to be very similar to those of the respective fluorophores in both THF and ACN. In any given solvent, a red-shift of the spectral maxima along with an increase in the spectral width could be observed as one passes from the parent system, 1 to the methoxy derivative, 2. This behavior can be understood taking into consideration of the difference in the electron donating ability of the substituent at 4-position and the nature of the lowest excited state of the systems. In the case of 3, the lowest excited state is known to be π,π* in nature.17a However, it appears from the spectral nature (broadness and the location) of 2 that the lowest excited state of the fluorescing moiety is charge transfer (CT) in nature. The charge transfer is made possible due to the electron donating nature of the methoxy group at 4-position. It may be noted that 5, where the charge

Figure 1. Absorption spectra of 1b and 2b in tetrahydrofuran (a) and acetonitrile (b).

transfer is expected to be even higher, exhibits further Stokesshifted broad absorption and emission CT bands.16 The spectral data of 1 and 2 along with those for 3 and 4 in THF and ACN have been collected in Table 1. 3.2.2. Fluorescence Properties. The fluorescence spectral data of 1 and 2 in THF and ACN have been collected in Table 1, and some representing spectra are shown in Figure 2. While 1a-c display fairly structured fluorescence (having mirror image relationship with the absorption) with the maximum appearing at around 380 nm, 2a and 2b exhibit distinctly different unstructured fluorescence band in both polar and nonpolar media. Further, as evident from the data presented in Table 1, the fluorescence band of 2 displays significant solvatochromism, as is expected of a charge transfer band. It may be noted in this context that 5 (or its derivative), which contains 4-amino group as the electron donor, also exhibits exclusively the chargetransfer emission.16 The fluorescence quantum yields of the multicomponent systems and the constituting fluorophores are shown in Table 2. The presented data can be summarized as follows: As far as the fluorescence yields of the bare fluorophores are concerned,

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J. Phys. Chem. B, Vol. 104, No. 49, 2000 11827

TABLE 1: Absorption and Fluorescence Spectral Data of the Multicomponent Systems and the Constituent Fluorophores in THF and ACN compound solvent

1a

1b

1c

3

2a

2b

4

320 (s) 330 342

320 (s) 330 340

315 (s) 330 343

340 (s) 359 375 (s)

345 (s) 359 375 (s)

345 (s) 354 375 (s)

320 (s) 330 340 357 380 398 (s)

320 (s) 330 342 357 380 398 (s)

320 (s) 330 340 356 380 398 (s)

315 (s) 329 342 360 377 396 (s)

366

368

436

430

427

319 (s) 330 341 320 (s) 330 340 362 384 400 (s)

319 (s) 330 341 320 (s) 330 341 363 385 400 (s)

320 (s) 330 340 320 (s) 330 340 363 380 400 (s)

315 (s) 330 343 315 (s) 329 341 362 379 400 (s)

345 (s) 360 375 (s) 367

345 (s) 360 375 (s) 368

340 (s) 357 380 (s) 368

439

438

438

320 330 341

λmax(abs) (nm)

(s)a

THF λmax(ex)b (nm) λmax(flu)c (nm)

λmax(abs) (nm) ACN λmax(ex)b (nm) λmax (flu)c (nm)

368

a (s) represents a shoulder. b Obtained by monitoring the respective fluorescence bands. c Fluorescence spectra were measured on excitation at 320 nm for 1 and 3 and 350 nm for 2 and 4.

TABLE 2: Fluorescence Quantum Yielda of 1 and 2 and the Constituent Fluorophores (3 and 4) in Tetrahydrofuran and Acetonitrileb compound

tetrahydrofuran

fc

acetonitrile

fc

1a 1b 1c 3 2a 2b 4

3.0 × 10-4 5.6 × 10-4 2.9 × 10-3 1.1 × 10-2 6.5 × 10-2 0.15 0.76

37 20 4

6.9 × 10-3 9.6 × 10-3 9.1 × 10-3 4.0 × 10-2 3.9 × 10-2 4.9 × 10-2 0.78

6 4 4

12 5

20 16

a (10% for values of 10-2 or higher and ( 15% for values less than 10-2; b Also shown is the factor (f) representing the ratio of the fluorescence yield of the constituent fluorophore to that of the sensor system. c This represents the upper limiting value of FE for the system when the PIET communication between the fluorophore and the amino moiety is cutoff completely.

4 fluoresces much more strongly compared to 3. The fluorescence yield of 3 in ACN is higher than that in THF. However, the fluorescence quantum yield of 4 is very similar in the two solvents. As far as the multicomponent systems are concerned, the fluorescence yields of 1a-c or 2a,b are significantly lower than those of the respective constituent fluorophores in any given solvent. The factor by which the fluorescence yield of a given multicomponent system is lower than that of its constituent fluorophore is also shown in Table 2 for all the systems. It should also be noted that among the different sensor systems of any given series, the fluorescence yield is the lowest for the systems with two methylene spacer units and it increases as the number of methylene unit is increased. The fluorescence decay behavior of the systems was studied using a ns time-resolved instrument. However, the fluorescence lifetimes of 3 were found to be lower than or very close to 1 ns (the instrumental resolution). This observation is consistent with the literature reports.17,18 Since the fluorescence lifetimes of PIET quenched systems (1a-c) are expected to be even lower, no time-resolved fluorescence measurements could be carried out on 1a-c. However, as seen from Figure 3, 4 displays a single exponential fluorescence decay with a lifetime of 7.1 ns

in THF. On the other hand, the decay behavior of 2 (Figure 3b), which contains the same fluorophore, is clearly biexponential and can be best fitted to the function, I(t) ) B1 exp(t/τ1) + B2 exp(-t/τ2) in both THF and ACN. The decay parameters for 2a,b, and 4 in THF and ACN are collected in Table 3. 3.3. Effect of the Metal Ions. Typical change in the fluorescence behavior of the multicomponent systems induced by the transition metal salt has been illustrated in Figure 4. Initial addition of the metal ions to a solution of 1 or 2 leads to an increase in the fluorescence intensity of the system. Further addition of the salt increases the fluorescence intensity of the system. However, at a certain concentration of the metal ion, the fluorescence intensity reaches a maximum and beyond this concentration, quenching of fluorescence could be observed. No noticeable spectral shift, similar to what has been seen in some cases,4,6 could be observed for the present systems. The maximum FE values obtained for different derivatives of 1 and 2 in the presence of the metal salts have been collected in Tables 4 and 5, respectively. As can be seen, the observed FE values are extraordinarily high in some cases. The highest FE could be observed in the case of 1a in the presence of Zn2+, known to be an inefficient quencher. It is worth noting that even efficient quenchers such as Fe3+ and Cr3+ give rise to excellent FE. The metal ion induced changes in the fluorescence decay behavior of 2a and 2b have also been studied. With the addition of the metal salts, the fluorescence decay profile changes from biexponential to a single exponential one with gradual disappearance of the short-lived component. The fluorescence decay curves become single exponential for concentrations (of the metal ions) corresponding to the maximum FE. The effect of the metal ions on the fluorescence decay behavior of these multicomponent systems is clearly evident from a comparison of the decay profiles shown in Figures 3b and 5. 4. Discussion While the absence of any oxidative peak between 0-2 V for the constituent fluorophores indicates just the electron deficient

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Figure 2. Fluorescence spectra of 1b and 2b in tetrahydrofuran (a) and acetonitrile (b). The excitation wavelength for 1b was 320 nm, while that for 2b was 350 nm. The fluorescence intensities of the systems have been normalized at the peak for clarity.

nature of 3 and 4, the measured reduction potentials and the spectral data allow a quantitative estimation of the thermodynamic driving force for the PIET process in 1 and 2 in terms of the free energy changes (∆G). The ∆G values have been calculated using ∆G ) 23.06[Eox(recep) - Ered(fluor)] - E0,0, where Eox(recep) represents the oxidation potential for the amino moiety, Ered(fluor) represents the reduction potential of the fluorophore and E0,0 denotes the energy of the fluorescent state. The E0,0 values used in the calculation of ∆G have been estimated from the location of the first peak position in the fluorescence spectrum. The oxidation potential of triethylamine (0.49 V) has been used as Eox(recep).25 The measured ∆G values have been shown in Table 6. As can be seen, PIET is thermodynamically feasible for the two sets of compounds. It is pertinent to note here that a similar calculation on multicomponent system involving the fluorophore, 5 yielded a ∆G value of only - 6.6 kcal/mol.4 Even though PIET is thermodynamically feasible in all the systems, the actual extent of PIET in any system depends on

Figure 3. Fluorescence decay profiles of 4 (a) and 2b (b) in tetrahydrofuran. The excitation wavelength was 350 nm. Solid lines indicate the best fit to the measured decay profiles. The decay curves for 2b were found to be best represented by a biexponential decay function while those for 4 were found to be single exponential. The fitting parameters have been discussed in the text. The exciting lamp profiles are also shown in the figure. Fluorescence was monitored at 435 nm.

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TABLE 3: Fluorescence Lifetime (τ) of 2a, 2b, and 4 and Relative Weightage of the Lifetime Components (in Brackets) in Tetrahydrofuran and Acetonitrilea tetrahydrofuran

acetonitrile

compound

τ1 (ns)

τ2 (ns)

τ1 (ns)

τ2 (ns)

2a

0.4 (74%) 0.4 (89%) 7.1

7.2 (26%) 7.0 (11%)

0.1 (97%) 0.4 (98.4%) 8.1

8.2 (3%) 8.1 (1.6%)

2b 4 a

Excitation wavelength was 350 nm and fluorescence decays were monitored at 520 nm.

Figure 4. Fluorescence spectra of 1a (10-5 M) in tetrahydrofuran in the presence of Ni(H2O)6(ClO4)2. Ni2+ concentrations, in increasing order of the fluorescence intensity, are 3.8 × 10-5, 1.5 × 10-4, 1.9 × 10-4, 2.6 × 10-4, 3.8 × 10-4, 4.9 × 10-4, and 9.7 × 10-4 M. λex ) 320 nm.

factors such as the fluorophore-receptor distance and the relative orientation of the two components.26 Experimentally, the extent of PIET in a multicomponent system could be best obtained by comparing its fluorescence quantum yield with that of its constituent fluorophore. It can be seen from Table 2, among all the systems studied, the factor by which the fluorescence yield of a given system is lower (relative to that of its constituent fluorophore) as a result of PIET is the highest in the case 1a. It can also be seen that for a given set of compounds, say 1, the extent of PIET is the highest for the system in which the fluorophore and the receptor are separated by two methylene spacer units. With an increase in the number of the methylene groups, the fluorescence yield increases indicating a decrease of PIET in the system. The fluorescence decay behavior of the systems allows us to determine whether PIET in these multicomponent systems takes place by a through-space or a through-bond mechanism. While for the first set of compounds, the decay profiles could not be measured (for reasons stated earlier), the fluorescence decay profiles of 2 are clearly biexponential with a short-lived component (major) and a long-lived minor component. This biexponential nature of the decay and the lifetimes values of the components can be understood considering that PIET in the multicomponent systems takes place by a through-space mechanism. The short-lived component is clearly the result of PIET quenching. Since the minor long-lived component has a lifetime

very similar to that of the constituent fluorophore (Table 3), it must be arising from a small fraction of the molecules in which the relative orientation of the receptor and the fluorophore and the distance between the two do not allow a through-space overlap of the electron donor and the acceptor orbitals that is required for PIET. Similar through-space PIET has been observed in many systems where the electron donor and the acceptor moieties are linked by a flexible aliphatic chain.26 One may wonder, how does the transition metal ions, wellknown for their quenching abilities,8,9,27 induce, in the first place, FE of the multicomponent systems. We have answered to this question taking into consideration of the individual interactions operative in the presence and in absence of the guest.6 While the interaction between the fluorophore and the metal ion leads to fluorescence quenching, that between the receptor and the metal ion results in FE. Therefore, whether the net result of these two opposing interactions (quenching or FE) is determined by the extent of quenching due to the former interaction and the enhancement due to the latter. The extent of quenching can be quantitatively estimated using Stern-Volmer equation,27 I0/I ) 1 + kqτ0[Q], where I0 is the original fluorescence intensity, I is the intensity of the quenched fluorescence, kq is the quenching constant, τ0 is the lifetime of the unquenched fluorophore, and [Q] is the concentration of the quencher (metal ions in the present case). It is known that the metal ions such as Fe3+ and Cr3+ can quench the fluorescence very efficiently with the rate constants typically in the diffusion-controlled range (1010 M-1 s-1 or more).6,8,9 According to the above equation, 1 mM solution of a metal ion with kq value of 2 × 1010 M-1 s-1 quenches the fluorescence intensity of 4 (the fluorophore component for which the lifetime is available, τ0 ) 8.1 ns in ACN) by 14%. However, it is interesting to note that this quenching interaction becomes insignificant in the case of 2a or 2b, which contains the same fluorophore with considerably shorter lifetime. According to Stern-Volmer equation, the extent of fluorescence quenching predicted for 2b (with a lifetime of 0.4 ns) is only 0.8% (assuming an identical kq value and 1 mM solution of the metal ion). Therefore, a metal ion, which is otherwise a strong quencher, behaves as a poor quencher toward a system, whose fluorescence has already been quenched by PIET. When the quenching interaction of a metal ion becomes unimportant, FE in its presence does not surprise one. However, what is quite unusual is the fact that the FE values are unusually high for 1a-c. The upper limiting value of FE of a fluorophore-spacer-receptor system in the presence of a guest is essentially the factor by which the fluorescence yield of the system is lower (due to PIET) relative to that of its constituent fluorophore. Hence, assuming complete recovery, one expects only 37-fold FE for 1a and 20-fold FE for 1b in the presence of the metal ions that do not have any quenching ability (Table 2). Since Zn2+ ion has the least quenching ability among the d-block metal ions studied, one expects 1a to exhibit not more than 37-fold FE in its presence. However, one can see from Table 4, the FE value observed for 1a is as high as 2150 with nonquenching Zn2+ and 1800 with Ni2+ in THF. With Fe3+ and Cr3+, the most efficient quenchers among the first row of transition metal ions,8,9 the observed FE values are 430 and 350, respectively. Even Mn2+ that contains five unpaired electrons in the high spin state, displays 450-fold enhancement. For 1b, even though the FE values are substantially lower in the range of 50-120, they are nevertheless much higher than the expected 20-fold FE. Both the FE values of such large magnitude and FE values higher than the PIET limiting values are contrary to our expectation. Interestingly, for 2a and 2b,

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TABLE 4: Fluorescence output of 1a, 1b, and 1c as a Function of Different Metal Ion Input in Tetrahydrofuran and Acetonitrilea 1a

1b

tetrahydrofuran metal ion 3+

Cr Mn2+ Fe3+ Co2+ Ni2+ Cu2+ Zn2+

[M]b

(M)

8.0 × 10 1.6 × 10-4 4.7 × 10-5 2.0 × 10-4 9.7 × 10-4 5.9 × 10-4 1.8 × 10-4 -4

FEc,d 350 450 430 450 1800 300 2150

acetonitrile [M]b

FEc,d

(M)

6.0 × 10 2.0 × 10-4 3.4 × 10-5 1.2 × 10-4 1.8 × 10-4 2.7 × 10-5 9.3 × 10-5 -5

1c

tetrahydrofuran 44 65 34 53 57 48 50

[M]b

(M)

4.6 × 10 4.1 × 10-4 3.2 × 10-5 1.0 × 10-3 3.1 × 10-4 2.8 × 10-4 1.2 × 10-4 -4

acetonitrile

FEc,d 50 75 70 60 120 45 120

[M]b

(M)

2.0 × 10 8.0 × 10-5 1.7 × 10-5 4.9 × 10-5 8.8 × 10-5 2.0 × 10-5 5.5 × 10-5 -5

FEc,d 27 110 26 35 36 34 35

tetrahydrofuran [M]b

(M)

3.0 × 10 5.5 × 10-4 1.2 × 10-4 1.1 × 10-3 2.2 × 10-4 4.6 × 10-4 6.8 × 10-4 -4

acetonitrile

FEc,d

[M]b (M)

FEc,d

1.0 9.0 7.9 6.1 8.8 4.4 8.5

2.5 × 10 2.0 × 10-4 1.1 × 10-4 3.1 × 10-4 2.0 × 10-4 2.3 × 10-4 4.4 × 10-4

7.9 8.9 9.0 8.6 9.1 1.3 9.4

-4

a Experimental condition: ∼ 2 × 10-5 M solution of the compounds in tetrahydrofuran (THF) and/or acetonitrile was used at 298 K, λ exc ) 320 nm, excitation and emission bandwidths were 1.5 and 5 nm, respectively. b represents the concentration of the metal ion for which maximum FE was observed; any further increase in concentration led to fluorescence quenching. c With reference to the fluorescence intensity of the respective compound in the absence of metal ion, 103φf for 1a, 1b, and 1c are respectively 0.3, 0.56, 2.9 in THF and 6.9, 9.6, and 9.1 in ACN. d (15%.

TABLE 5: Fluorescence Response of 2a and 2b as a Function of Different Metal Ion input in Tetrahydrofuran and Acetonitrilea 2a tetrahydofuran

2b acetonitrile

tetrahydrofuran

acetonitrile

metal ion

[M]b (M)

FEc,d

[M]b (M)

FEc,d

[M]b (M)

FEc,d

[M]b (M)

FEcd

Cr3+ Mn2+ Fe2+ Co2+ Ni2+ Cu2+ Zn2+

4.6 × 10-4 1.8 × 10-4 6.3 × 10-5 8.0 × 10-4 2.2 × 10-4 3.5 × 10-4 3.4 × 10-4

8.2 12.1 12.0 9.6 12.2 8.6 12.7

5.0 × 10-4 6.0 × 10-4 3.9 × 10-4 4.7 × 10-4 7.1 × 10-4 3.5 × 10-4 3.3 × 10-4

25.5 21.2 15.0 26.4 26.6 23.3 28.3

3.0 × 10-4 2.3 × 10-4 3.1 × 10-5 2.0 × 10-4 1.1 × 10-4 2.9 × 10-4 1.7 × 10-4

4.8 6.1 6.0 5.0 6.2 5.3 6.1

3.7 × 10-4 4.0 × 10-4 3.3 × 10-4 3.3 × 10-4 5.1 × 10-4 3.5 × 10-4 4.4 × 10-4

16.0 13.4 10.0 16.4 17.2 14.5 17.4

a Experimental condition: ∼2 × 10-5 M solution of the compounds in tetrahydrofuran and acetonitrile were used at 298 K, λ exc ) 350 nm, excitation and emission bandwidths were 1.5 and 3 nm, respectively. b Represents the concentration of the metal ion for which maximum FE was observed, any further increase in concentration led to fluorescence quenching. c With reference to the fluorescence intensity of the respective compound in the absence of metal ion; φf for 2a and 2b are respectively 0.065 and 0.15 in THF and 0.039 and 0.049 in ACN. d (5%.

the maximum FE values observed (Table 5) are in close agreement with the expected values. To find out an explanation for the contrasting behavior of the two set of systems we looked for any fundamental difference in the fluorescence properties of the two fluorophores. The fluorescence response of 1,8-naphthalimide is known to be dependent on the solvent polarity.17,18 With increase in the polarity of the medium, the fluorescence efficiency of 1,8naphthalimide increases due to a change in the spacing and ordering of two close-lying π-π* and n-π* states.17 The fluorescence yield of 3 and 4 has been measured in different solvents and the values are shown in Table 7. It can be seen that while the fluorescence yield of 3 is considerably higher in polar media, that for 4 is not that sensitive to the solvent. Keeping in mind that the transition metal salts are usually hydrated,20,28 one can expect an increase in the polarity of the immediate surroundings of the fluorophore on the addition of the metal salts due to preferential solvation of the fluorophore by the water molecules. The chosen fluorophore component (3) is such that its fluorescence efficiency depends significantly on the polarity of the media. If it is assumed that the water molecules from the hydrated salts used in the study preferentially solvates the fluorophores, then according to the data presented in Table 7, one can expect an FE of the systems (1a-c) by a factor of ∼22 (THF solution) or 6 (ACN solution) just due to a change in the microenvironment of the fluorophore. In such a situation, the observed FE, which is due to both metal ion binding and changes in the microenvironment of the fluorophore, can be as large as ∼ 814 (22 × 37) in THF, a value that is still lower than the maximum FE observed with 1a in THF in the

TABLE 6: Reduction Potential (Ered) and Energy of the Fluorescence State (E0,0) of 3 and 4 and the Free Energy Changes Associated with PIET (∆G) in 1 and 2 compound Ered (V)a E0,0 (kcal/mol)b 3 4

-1.00 -1.11

79.0 65.3

compound ∆G (kcal/mol) 1 2

-44.6 -28.4

a Details of the measurement conditions are described in the Experimental Section; cyclic voltammetric traces for the reduction of all the systems were found to be reversible. b E0,0 values have been estmated from the location of the first fluorescence maximum of the fluorophores.

presence of nonquenching Zn2+. In acetonitrile, while the expected maximum FE value for 1a is 36 (6 × 6), the observed value is 65. The fact that a change in the microscopic polarity around the fluorophore, induced by the water molecules of the hydrated salts, is indeed one possible reason for the high FE values exhibited by 1a-c is evident from the following observation. Since the fluorescence quantum yield of 4 is not significantly dependent on the solvent polarity (in fact, unlike the other two systems, the yield is even lower in aqueous medium), one expects the multicomponent systems 2a and 2b, which are made up of the fluorophore component 4, not to exhibit an FE higher than that predicted by PIET. In fact, the maximum FE values (28 and 17 respectively in ACN and 13 and 6 in THF) are in fairly good agreement with those predicted by PIET alone. Since the hydrated metal salts are usually contaminated with protons (generated from partial hydrolysis of the salts), it is often argued that the enhancement actually results from the protonation of the receptor moiety. In this context, it is to be noted that even if the protons are assumed to contribute to the

Fluorescence Enhancement of 1,8-Naphthalimide

J. Phys. Chem. B, Vol. 104, No. 49, 2000 11831 by more than one means. Preferential solvation of the fluorophore component of the multicomponent system by the water molecules of the hydrated salts of the metals has been identified as one possible origin of the unusually high FE shown by some of the systems. Acknowledgment. Financial support for this research was obtained from Council of Scientific and Industrial Research (CSIR). Fellowship received by B.R. and G.S. from CSIR is gratefully acknowledged. References and Notes

Figure 5. Fluorescence decay profile of 2b (10-5 M) in tetrahydrofuran in the presence of 2.7 × 10-4 M of Co(H2O)6(NO3)2. Shown also in the figure are the exciting lamp profile and the single-exponential fit (τ ) 8.1 ns) to the fluorescence decay. The excitation wavelength was 350 nm and fluorescence was monitored at 435 nm.

TABLE 7: Fluorescence Quantum Yielda of 3 and 4 as a Function of the Solvent compound 1,4-dioxane tetrahdrofuran acetonitrile 3 4

3.1 × 10-3 0.76

1.1 × 10-2 0.76

methanol

waterb

4.0 × 10-2 4.4 × 10-2 0.24 0.78 0.85 0.46b

a Unless otherwise mentioned, ( 10% for values more than 10-2 and (15% for values less than 10-2. b(15%, because of poor solubility of the system in aqueous solution.

observed FE of the systems, the maximum value of FE is not expected to be more than that predicted by PIET. The fact that the fluorescence enhancement observed here is largely due to the metal ions and not due to the contaminated protons is evident from the observation that the FE values exhibited by a given multicomponent system comprising a solvent insensitive fluorophore is almost the same with a hydrated salt of cobalt, [Co(H2O)6]Cl2, or with its anhydrous salt (prepared following a standard procedure).25 Hence, it is quite obvious that a severalfold higher FE values observed for the multicomponent systems, 1a and 2a, which are comprised of the solvent sensitive fluorophores, in the presence of a hydrated salt compared with those observed in the presence of the corresponding anhydrous salt has to be attributed to a change in the microscopic environment of the fluorophore. 5. Conclusion Fluorescence behavior of some structurally simple multicomponent systems has been studied in the presence and in absence of the transition metal salts. The metal salts are found to induce unusually high FE, much higher than that expected from consideration of PIET in the systems. The results can be accounted for assuming a guest to be capable of inducing FE

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