Effects of Metal Cation Coordination on Fluorescence Properties of a

8812

J. Phys. Chem. B 2007, 111, 8812-8822

Effects of Metal Cation Coordination on Fluorescence Properties of a Diethylenetriamine Bearing Two End Pyrene Fragments Yasuhiro Shiraishi,* Katsutake Ishizumi, Go Nishimura, and Takayuki Hirai Research Center for Solar Energy Chemistry and DiVision of Chemical Engineering, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka 560-8531, Japan ReceiVed: March 15, 2007; In Final Form: May 14, 2007

Fluorescence properties of a diethylenetriamine bearing two end pyrene fragments (L) have been studied in water, where effects of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the emission properties of L have been studied. Without metal cations, L shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (IM) is strong at acidic pH but decreases with a pH increase because of an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene fragment. The excimer emission is due to the static excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The excimer emission intensity (IE) is weak at acidic pH but increases with a pH increase because of the GSD stability increase associated with the deprotonation of the polyamine chain. Addition of metal cations leads to IM decrease, where chelation-driven IM enhancement does not occur even with diamagnetic Zn2+ and Cd2+ at any pH. This is because a pyrene-metal cation π-complex, formed via a donation of π-electron of the pyrene fragment to the adjacent metal center, suppresses the monomer photoexcitation. IE also decreases upon addition of metal cations because the pyrene-metal cation π-complex weakens π-stacking interaction of the end pyrene fragments, leading to GSD stability decrease. The emission properties of L-Zn2+ complexes were studied by means of time-resolved fluorescence decay measurements, and the effects of adding a less-polar organic solvent were also studied to clarify the detailed emission properties.

1. Introduction Design of fluorescent signaling devices is an area of intense research activity and of tremendous significance to the field of molecular sensor/device fabrication.1 So far, various supramolecular systems whose emission properties can be modulated by external stimuli, such as temperature,2 light,3 redox potential,4 and pH,5 have been proposed. Among these devices, metal cations are often used as the external stimulus, which promote emission enhancement or quenching, associated with the coordination with ligand groups. A vast variety of molecular systems driven by metal cations, which act in organic,6 water,7 or organic/water mixture media,8 have been proposed. Among them, pyrene-containing molecular systems have been studied extensively because they demonstrate a distinctive excimer emission.9 Various pyrene systems capable of showing the metal-induced excimer enhancement or quenching have been proposed; however, most of these act in organic10 or organic/ water mixture media.11 To the best of our knowledge, there are only two reports of pyrene systems acting in total water.12 Both systems, however, show metal-induced excimer enhancement; a system showing metal-induced excimer quenching in total water had not been proposed. Earlier, we have reported that a diethylenetriamine bearing two end pyrene fragments (L) dissolved in water shows a pHcontrolled dual-mode fluorescence consisting of monomer and excimer emissions.13 The intensity of the monomer emission (IM) is strong at acidic pH but decreases with a pH increase. This is due to the pH-induced deprotonation of the nitrogen * To whom correspondence should be addressed. E-mail: shiraish@ cheng.es.osaka-u.ac.jp. Fax: +81-6-6850-6273. Tel.: +81-6-6850-6271.

atoms of the polyamine chain, leading to an electron transfer (ET) from the unprotonated nitrogen atoms to the photoexcited pyrene fragment. In contrast, the intensity of the excimer emission (IE) at acidic pH is very weak but increases with a pH increase. The excimer emission of L is due to the “static” excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments within L. The IE increase associated with a pH increase is due to the bending of the polyamine chain driven by the deprotonation of the nitrogen atoms, leading to an increased GSD stability.

In this work, effects of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the emission properties of L have been studied in water. We found that addition of all of these metal cations leads to both IM and IE decreases. This is the first pyrenecontaining molecular system showing metal cation-induced excimer emission quenching in total water. We describe here that the IM and IE quenching is basically due to the formation of a pyrene-metal cation π-complex via a donation of π-electron of the pyrene fragment to the adjacent metal center. This suppresses the monomer photoexcitation, leading to IM quenching and weakens the π-stacking interaction of the two pyrene fragments (GSD stability decrease), leading to IE quenching. The emission properties of L-Zn2+ complexes were studied by means of time-resolved emission decay measure-

10.1021/jp072081s CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

A Diethylenetriamine with End Pyrene Fragments

J. Phys. Chem. B, Vol. 111, No. 30, 2007 8813

TABLE 1: Stepwise Protonation Constants of L Determined in Aqueous NaCl (0.15 M) Solution at 298 K

a

reacna

constant

H + L ) HL H + HL ) H2L H + H2L ) H3L log β

8.97 ( 0.05 6.02 ( 0.14 3.48 ( 0.38 18.47

Charges are omitted for clarity.

ments. In addition, effects of adding a less-polar organic solvent (acetonitrile: MeCN) on the emission properties of L-Zn2+ complexes were also studied for detailed descriptions of the emission properties. 2. Experimental Section 2.1. Materials. All of the materials used were of the highest commercial quality, which were supplied from Wako and Tokyo Kasei and used without further purification. Water was purified by the Milli Q system. The L molecule was synthesized according to procedure described previously.13 2.2. Spectroscopic Measurements. Steady-state fluorescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer (λexc ) 360 nm). Absorption spectra were measured on an UV-visible photodiode-array spectrometer (Shimadzu; Multispec-1500).14 Nanosecond time-resolved fluorescence decay measurements were carried out on a PTI-3000 apparatus (Photon Technology International) using a Xe nanoflash lamp filled with N2 as the excitation light source (λexc ) 358 nm).13 These spectra were measured at 298 ( 0.1 K using a 10 mm path length quartz cell. All of the measurements were carried out with NaCl to maintain the ionic strength of the solution (I ) 0.15 M). The concentration of L in the solution was adjusted to 25 µM, where metal chlorides were used as the metal source. For reproduction of the data, all of the measurements were carried out in an aerated condition. Fluorescence quantum yields (Φf) of L were determined by comparison of the integrated corrected emission spectrum of standard quinine, which was excited at 366 nm in aqueous H2SO4 (1 M) solution (Φf ) 0.55, at 298 K).15 2.3. Potentiometric Measurements. Potentiometric pH titrations were carried out on a COMTITE-550 potentiometric automatic titrator (Hiranuma Co., Ltd.).16 Aqueous solutions (50 mL) containing L (8 µmol) with and without Zn2+ (8 µmol) were kept under dry argon with an ionic strength of I ) 0.15 M (NaCl). The titrations were done at 298 ( 0.1 K using an aqueous NaOH (0.35 mM) solution as a base, and at least two independent titrations were performed. The protonation constants of L and the stability constants for coordination of L with Zn2+ were determined by means of the nonlinear least-squares program HYPERQUAD, where the Kw () [H+][OH-]) value used was 10-13.73 (at 298 K).17 The stepwise protonation constants of L determined in water are summarized in Table 1.13

Figure 1. pH-dependent change in (A) fluorescence spectra (λexc ) 360 nm; 298 K) of L in aqueous NaCl (0.15 M) solution. (B) pHdependent change in the emission intensities of (closed symbol, IM) monomer emission monitored at 376 nm and (open symbol, IE) excimer emission monitored at 480 nm. The dotted lines in the figure denote the molar fraction distribution of the different L species.

3. Results and Discussion 3.1. Effect of pH. The fluorescence properties of L in water without metal cations were described previously,13 hence, only briefly described here. Figure 1A shows pH-dependent change in fluorescence spectra of L in water (λexc ) 360 nm). At acidic pH, L shows a distinctive fluorescence at 370-420 nm, assigned to a monomer emission from the locally excited pyrene fragment. Figure 1B (closed symbol) plots the monomer emission intensity monitored at 376 nm (IM) against pH, where dotted lines in the figure denote the mole fraction distribution of the different L species in solution, which is calculated from the protonation constants determined potentiometrically. IM decreases with a pH increase. This is due to the pH-induced deprotonation of the nitrogen atoms of the polyamine chain within L (Scheme 1). This triggers an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene monomer, resulting in IM quenching.18 The pH-IM profile (Figure 1B, closed symbols) and the deprotonation sequence established by means of 1H and 13C NMR spectra (Scheme 1) indicate that (i) the fully protonated form of L (H3L3+) exhibits the most intense IM, (ii) monodeprotonation of the polyamine chain that occurs on the central nitrogen atom (H2L2+ formation)

SCHEME 1: Protonation/Deprotonation Sequence of L with pH

8814 J. Phys. Chem. B, Vol. 111, No. 30, 2007

Shiraishi et al. SCHEME 2: Emission Mechanism of L in Water for (A) Fully or Partially Protonated Species (H3L3+, H2L2+, HL+) and (B) Fully Deprotonated L Speciesa,b

a

0 < R, β < 1. bτM . τE′ > τE.

nm. In contrast, as shown in Figure 2C, spectra collected at 480 nm (excimer emission) show a red-shifted band with maximum intensity at around 360 nm, whose spectra are consistent with the red-shifted GSD absorption (Figure 2A). These findings clearly suggest that the excimer emission of L is due to the static excimer formed via direct GSD excitation. Table 2 summarizes the decay times and preexponential factors of the emitting components for monomer (monitored at 396 nm) and excimer emissions (monitored at 480 nm) at the respective pH, determined by means of nanosecond time-resolved emission decay measurements (λexc ) 358 nm). The decay profiles of the excimer emission obtained at the respective pH are successfully fitted by the sum of two exponentials, where no negative preexponential, i.e., a rise-time, is detected. These data clearly support the direct GSD excitation mechanism for the excimer emission of L in water (Scheme 2B). The IE increase associated with a pH increase (Figure 1B) is ascribed to the pH-induced GSD stabilization, which is triggered by a decrease in electrostatic repulsion of the polyamine chain of L, associated with the deprotonation of the nitrogen atoms.20 The repulsion decrease leads to chain bending of the polyamine bringing two end pyrene fragments closer and, hence, stabilizes the GSD (Scheme 1); the red-shifted GSD absorption actually increases with a pH increase (Figure 2A). As shown in Table 2, lifetime of the excimer component increases with a pH increase (H2L2+ (4.6 ns) < HL+ (5.3 ns) < L (6.3 ns)), although ET from the unprotonated nitrogen atoms to the excimer species

Figure 2. pH-dependent change in (A) absorption spectra and excitation spectra (B) collected at 376 nm (monomer emission) and (C) collected at 480 nm (excimer emission) of L in aqueous NaCl (0.15 M) solution at 298 K.

leads to 40% IM decrease, and (iii) strongest IM quenching occurs upon removal of proton from HL+. As shown in Figure 1A, L also shows strong excimer emission at 420-600 nm. As plotted in Figure 1B (open symbols), the intensity of the excimer emission monitored at 480 nm (IE) increases with a pH increase. This emission is due to the “static” excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments within L.13 As shown in Figure 2A, L shows a redshifted absorption at >350 nm assigned to GSD.19 As shown in Figure 2B, excitation spectra of L collected at 376 nm (monomer emission) show maximum intensity at around 340

TABLE 2: Decay Times (τ) and Preexponential Factors (a) of Monomer (M) and Excimer (E) Components for Emissions of L in Aqueous NaCl (0.15 M) Solution, as a Function of pH (λexc ) 358 nm; 298 K) τ/ns (a/%) H3L

3+

H2L

pH

λem/nm

M

E

2.0

396 480 396 480 396 480 480

120.7 (100) 120.7 (2.1)

3.97 (-37.5)a 3.97 (97.9)

4.7 7.3 13.0 a

M

97.1 (74.8) 97.1 (-1.1)

HL+

2+

E

M

L E

M

E

χ2

6.28 (77.1), 30.8b (22.9)b

3.30 2.60 1.63 1.86 3.61 2.63 1.62

4.56 (25.2) 4.56 (100) 79.2 (61.5) 79.2 (3.4)

5.34 (38.5) 5.34 (96.6)

The negative preexponential factor is due to much shorter lifetime of the excimer component than the monomer component. b The long lifetime excimer component is assigned to the H2O-solvated L species (see text; ref 13).



Figure 3. pH-dependent change in (A) IM and (B) IE of L (λexc ) 360 nm; 298 K) measured in aqueous NaCl (0.15 M) solution with 1 equiv of respective metal cations.

may be accelerated at higher pH. This is due to the increased GSD stability. As shown in Figure 1B (open symbol), a drastic IE increase takes place at pH >8, where the fully deprotonated L species exist. This is due to the formation of highly stabilized GSD by complete deprotonation of the polyamine chain (strong π-stacking interaction of the end pyrene fragments by the chain bending) and by solvation of the species by H2O molecules.13 As shown in Table 2, at pH 13.0, two excimer components with lifetimes of 6.3 and 31 ns are detected with positive preexponential factors. The latter long-lived excimer component is assigned to the highly stabilized excimer formed by direct excitation of the H2O-solvated GSD. The overall emission mechanism of L in water can be summarized as Scheme 2. At pH 8, where fully deprotonated L species exist, monomer emission scarcely appears, while GSD is highly stabilized by complete deprotonation of the polyamine chain and by solvation of the species by H2O. The photoexcitation of GSD leads to formations of two types of excimer emissions with short and long lifetimes, where the latter long-lifetime emission is due to the excimer highly stabilized by the H2O solvation (Scheme 2B). 3.2. Effect of Metal Cations. Figure 3 shows pH-dependent change in IM and IE of L measured in water with 1 equiv of respective metal cations (Cu2+, Ag+, Hg2+, Cd2+, Zn2+). Table 3 summarizes the fluorescence quantum yields (Φf) at selected pH. It is well-known that, for polyamine-conjugated aromatic molecules, addition of Cu2+, Ag+, and Hg2+ quenches the monomer emission, via the following mechanisms: (i) Coordinated Cu2+ leads to an energy transfer21 from the excited monomer to the low-lying empty d-orbital of Cu2+. (ii) Coordinated Ag+ and Hg2+ enhance the intersystem crossing from the excited monomer to its triplet (nonfluorescent) excited state via the heavy atom effect.22 Also in the present L system, as shown in Figure 3, IM decreases with the addition of these metal cations (Cu2+, Ag+, Hg2+). It is also well-known that,

Figure 4. (A) pH-dependent change in fluorescence spectra (λexc ) 360 nm; 298 K) of L in aqueous NaCl (0.15 M) solution with 1 equiv of Zn2+. (B) pH-dependent change in (closed symbol) IM and (open symbol) IE. The dotted line in (B) denotes the molar fraction distribution of the different L species. The bold lines in (B) denote the pH-IM and pH-IE profiles of L obtained without metal cations.

TABLE 3: Fluorescence Quantum Yield (Φf) of L Measured in Aqueous NaCl (0.15 M) Solution with and without Metal Cations (1 equiv) at Selected pH (λexc ) 360 nm; 298 K) metal cation none Zn2+ Cd2+

pH 2.0 11.0 2.0 11.0 2.0 11.0

Φf

metal cation

pH

Φf

0.14 0.066 0.12 0.026 0.13 0.026

Cu2+

2.0 11.0 2.0 11.0 2.0 11.0

0.10 0.009 0.14 0.018 0.04 0.015

Ag+ Hg2+

for polyamine-conjugated aromatic molecules,22 coordination of diamagnetic Zn2+ and Cd2+ leads to IM enhancement at neutral-basic pH. This is because the coordination of these cations suppresses ET from the unprotonated nitrogen atoms to the excited monomer.23 As reported,24 a triethylenetetramine or tetraethylenepentaamine bearing two end pyrene fragments also shows IM enhancement upon addition of these cations at pH 6-12. However, as shown in Figure 3A, the present L system shows IM decrease upon addition of Zn2+ and Cd2+. In addition, IE also decreases upon addition of all of the metal cations (Figure 3B). To the best of our knowledge, this is the first pyrene system showing excimer quenching by all of these metal cations in total water. The reasons for IM decrease upon addition of Zn2+ and Cd2+ and for IE decrease upon addition of all of the metal cations must be clarified. Effect of Zn2+ coordination on the emission properties of L was studied first. Figure 4A shows pH-dependent change in fluorescence spectra of L obtained with 1 equiv of Zn2+. Figure 4B plots IM and IE against pH, where dotted lines in the figure denote the mole fraction distribution of the species, which is calculated from the protonation (Table 1) and stability constants (Table 4) determined potentiometrically. Coordination of Zn2+ with L takes place at pH >6, where three kinds of


Shiraishi et al.

TABLE 4: Stability Constants for Interaction between L and 1 equiv of Zn2+ Determined in Aqueous NaCl (0.15 M) Solution at 298 K

a

reacna

constant

Zn + L ) LZn LZn + OH ) LZn(OH) LZn(OH) + OH ) LZn(OH)2 Zn + L + H2O ) LZn(OH) + H

5.99 ( 0.34 5.91 ( 0.47 4.78 ( 0.97 -1.88 ( 0.25

Charges are omitted for clarity.

L-Zn2+ complexes, such as LZn2+, LZn(OH)+, and LZn(OH)2, appear with a pH increase. Within these complexes, all of the nitrogen atoms of L coordinate with Zn2+, indicating that ET to the excited pyrene monomer would be much suppressed. However, as shown by closed symbols in Figure 4B, IM is much lower than that obtained without metal cations (bold line). As reported for ligand-conjugated aromatic molecules,25 coordination of transition metal cations with the ligand leads to a formation of metal-aromatic π complex via a donation of π-electron of the aromatic moiety to the adjacent cations at room temperature. As reported,26 some polyamine-conjugated anthracene molecules show strong IM decrease upon coordination with Zn2+ because of suppression of the monomer excitation by Zn2+-anthracene π-complex formation.27 The IM decrease of L at pH >6 upon coordination with Zn2+ (Figure 4B) may probably be due to the formation of Zn2+-pyrene π-complex.28 Figure 4B (open symbol) shows change in IE with pH. At 6 < pH < 9, where LZn2+ and LZn(OH)+ species form mainly, IE is higher than that obtained at lower pH; however, the value is much lower than that obtained without metal cations (bold line). In contrast, at pH >9, where LZn(OH)2 exist, IE increases more, although the value is ca. 40% lower than that obtained without metal cations. These suggest that all three L-Zn2+ complexes show excimer emission and IE increases with the OH- coordination to the Zn center (LZn2+ < LZn(OH)+ < LZn(OH)2). Figure 5A,C shows pH-dependent changes in absorption and excitation (collected at 480 nm; excimer emission) spectra measured with 1 equiv of Zn2+. As is also the case without metal cations (Figure 2A,C), the red-shifted GSD absorption and excitation bands increase with a pH increase. These suggest that L-Zn2+ complexes still form GSD via the π-stacking interaction of the two end pyrene fragments, and the excimer emission is due to the “static” excimer formed via direct GSD photoexcitation. Comparison of the absorption spectra obtained without metal cations (Figure 2A), however, reveals that the GSD absorption obtained with Zn2+ (Figure 5A) is much weaker. This may be due to the Zn2+-pyrene π-complex formation; the donation of π-electron of the pyrene fragments to the adjacent Zn2+ leads to a decrease in π-stacking interaction of the two pyrene fragments. This results in formation of a weak GSD, leading to weak IE (Figure 4B). IE increases with the OH- coordination to Zn2+ center associated with a pH increase (LZn2+ < LZn(OH)+ < LZn(OH)2). As shown in Figure 5A, >370 nm absorbance of L rises with a pH increase at pH 9, where LZn(OH)2 form, this absorption decreases, while the GSD absorption (at around 360 nm) increases. This risen absorption at >370 nm is assigned to the Zn2+-pyrene π-complex. Similar absorption rise is observed in some ligandconjugated anthracene systems.29 The decrease of >370 nm absorption at pH >9 is due to the OH- coordination to the Zn2+ center. This leads to a decrease in electron negativity of Zn2+ center30 and, hence, suppresses the Zn2+-pyrene π-complex formation.26a As a result of this, GSD stability increases with the OH- coordination. The relatively strong IE enhancement at

Figure 5. pH-dependent change in (A) absorption and excitation spectra (B) collected at 376 nm (monomer emission) and (C) collected at 480 nm (excimer emission) of L in aqueous NaCl (0.15 M) solution with 1 equiv of Zn2+.

pH >9 (Figure 4B) is therefore due to the increased GSD stability by the coordination of two OH- to Zn2+. As shown in Figure 3B, IE of L is also quenched by other metal cations (Cu2+, Ag+, Hg2+, Cd2+). The strong IE quenching upon Cd2+ and Ag+ addition is explained by the strong metalpyrene π-complex formation.26,31 As shown in Figure 6A,B, the GSD absorption (at around 360 nm) obtained with Cd2+ and Ag+ is much weaker than that obtained with Zn2+ (Figure 5A), while the rise in >370 nm absorption is quite strong. In addition, the risen absorption still appears even at basic pH, although these metal centers should be coordinated with OH-, as is also the case for Zn2+. These indicate that the Cd2+- and Ag+pyrene π-complexes are much stronger than that for Zn2+, as is also observed for related polyamine-conjugated aromatic molecules.26,29 These strong π-complexes suppress the GSD formation, resulting in almost zero IE at any pH (Figure 3B). Upon Hg2+ addition (Figure 6C), GSD absorption is also weaker than that obtained without metal cations due to the Hg2+-pyrene π-complex formation.32 The GSD absorbance is, however, similar to that obtained with Zn2+ (Figure 5A), indicating that stability of GSD of the L-Hg2+ complexes is similar to that of L-Zn2+ complexes. However, as shown in Figure 3B, IE obtained with Hg2+ at basic pH is weaker than that obtained with Zn2+. This may probably be due to the acceleration of the intersystem crossing from the excimer to their triplet (nonfluorescent) excited state via the heavy atom effect by Hg2+.22 Upon Cu2+ addition (Figure 6D), the GSD absorption is much weaker than that obtained with Hg2+ (Figure 6C), although the >370



Figure 7. Change in IE of L (λexc ) 360 nm) with the stepwise addition of Zn2+ to an aqueous NaCl (0.15 M) solution of (A) pH 8 and (B) pH 11.5 at 298 K. Inset: Change in fluorescence spectra of the solutions.

Figure 6. pH-dependent change in absorption spectra of L in aqueous NaCl (0.15 M) solution measured with 1 equiv of (A) Cd2+, (B) Ag+, (C) Hg2+, and (D) Cu2+ at 298 K.

nm absorption rise is weaker than that obtained with Cd2+ and Ag+ (Figure 6A,B). This may be due to the Cu2+-pyrene π-complex formation.33 This suggests that the stability of GSD of the L-Cu2+ complexes is greater than that of the L-Cd2+ and L-Ag+ complexes but is less than that of the L-Zn2+ and L-Hg2+ complexes. However, IE is almost zero at any pH (Figure 3B). The strong IE quenching by Cu2+ (Figure 3B) may therefore be due to the low GSD stability and also an energy transfer from the excimer to the low-lying empty d-orbital of Cu2+.21 3.3. Emission Properties of L-Zn2+ Complexes. Emission properties of the L-Zn2+ complexes were studied further. Figure 7A,B shows changes in IE of L with the stepwise addition of Zn2+ to aqueous solutions of pH 8 and 11.5. As shown in Figure 4B, at pH 8, LZn2+ and LZn(OH)+ exist at the same mole fractions, whereas, at pH 11.5, LZn(OH)2 exist predominantly. At both pH, IE decreases with the Zn2+ addition, but the decreases stop upon addition of 1 equiv of Zn2+. These indicate that the excimer emission of L demonstrates a 1:1 response to Zn2+ at the entire pH range.

Time-resolved fluorescence decay measurements were carried out to clarify the photophysical properties of the L-Zn2+ complexes. Figure 8 shows decay profiles of the monomer (monitored at 396 nm) and excimer (monitored at 480 nm) emissions measured at the respective pH (λexc ) 358 nm). Table 5 summarizes the decay times and the preexponential factors for the respective emitting components determined from the mole fraction distribution of the species (Figure 4B) and the decay data obtained without metal cations (Table 2). The decay profiles for both monomer and excimer emissions obtained at pH 7.2 and 9.0 are fitted by the sums of more than two exponentials, as expected from the complicated distribution of the species (Figure 4B), whereas the excimer emission profile obtained at pH 11.5 (where LZn(OH)2 exists) is successfully explained by a single exponential. At pH 7.2, HL+ (49%) and LZn2+ (38%) exist (Figure 4B). The decay profile of the monomer emission obtained at pH 7.2 is almost explained by the sum of two exponentials assigned to the excimer HL+ and monomer LZn2+ components, where contribution of the monomer HL+ component to the total emission is negligibly small. It is notable that lifetime of the monomer LZn2+ component (104 ns) is longer than that of the monomer HL+ (79 ns; Table 5), although IM of LZn2+ is weaker than that of HL+ (Figures 1B and 4B). This means that the excited monomer LZn2+ is stabilized more than the excited monomer HL+, although photoexcitation of the monomer LZn2+ is suppressed by the Zn2+-pyrene π-complex formation. The stabilization of the excited monomer LZn2+ may be due to the suppression of ET from the nitrogen atoms of L to the excited pyrene fragments by Zn2+ coordination.23,24 In contrast, the decay profile of the excimer emission is almost explained by the sums of two exponentials assigned to the excimer HL+ and LZn2+ components. The lifetime of the excimer LZn2+ component (2.1 ns) is less than half that of the excimer HL+ (5.3 ns), indicating that stability of the excimer LZn2+ is lower than that of HL+. At pH 9.0, LZn(OH)+ (46%) and LZn(OH)2 (47%) exist predominantly, with a small amount of LZn2+ (3%) (Figure 4B). As shown in Table 5, the decay profile of the monomer emission


Figure 8. Fluorescence decays of L in aqueous NaCl (0.15 M) solution with 1 equiv of Zn2+ at pH (A) 7.2, (B) 9.0, and (C) 11.5 (λexc ) 358 nm) monitored at 396 nm (monomer emission, closed symbol) and at 480 nm (excimer emission, open symbol). For judging the quality of the fit, autocorrelation functions (AC) and weighted residuals (WR) are also shown in the figure.

obtained at this pH is fitted by three exponentials assigned to monomer LZn2+, excimer LZn(OH)+, and excimer LZn(OH)2 components. At this pH, mole fraction of LZn(OH)+ is quite high (46%; Figure 4B), but contribution of the monomer LZn(OH)+ to the total emission is negligibly small. In contrast, contribution of the monomer LZn2+ is much higher, although the mole fraction of LZn2+ is very small at this pH (Figure 4B). This indicates that the monomer emission of LZn(OH)+ is much weaker than that of LZn2+. This may be due to the OH- coordination to the Zn center: the coordinated OHtransfers their electrons to the adjacent excited pyrene fragments and, hence, quenches the excited monomer. Similar IM quenching by OH- coordination is observed in several systems.34 In contrast, decay profile of the excimer emission is almost explained by two exponentials assigned to the excimer LZn(OH)+ and LZn(OH)2 components (Table 5). The lifetime of the respective components are determined to be LZn(OH)+ (3.0 ns) and LZn(OH)2 (4.6 ns), both which are longer than that of LZn2+ (2.1 ns). The lengthened lifetime of these excimer components is because the GSD of the L-Zn2+ complexes is stabilized by OH- coordination due to suppression of the Zn2+pyrene π-complex formation. The above findings indicate that

Shiraishi et al. OH- coordination to the L-Zn2+ complex leads to stability decrease of the excited monomer (shortening lifetime), while leading to stability increase of the excimer (lengthening lifetime). 3.4. Effects of Less-Polar Solvent. To further clarify the excimer emission properties of the L-Zn2+ complexes, effects of adding a less-polar organic solvent (acetonitrile: MeCN) to water were studied in comparison to the case without metal cations. Figure 9A shows change in fluorescence spectra of L measured at pH 13.0 without metal cations, as a function of MeCN concentration. IE decreases with the MeCN concentration increase,13 along with a blue-shift of the emission (solvatochromic shift)35 owing to decrease in polarity of the solution. This IE decrease associated with the MeCN concentration increase is due to the GSD destabilization and the suppression of the H2O solvation to GSD.13 As shown in Figure 9B, the GSD absorption band blue-shifts with the MeCN concentration increase, along with the absorbance decrease. This is because the MeCN addition weakens π-stacking interaction of the two pyrene fragments and, hence, destabilizes GSD. As shown in Figure 9D, the GSD excitation band (λem ) 480 nm) decreases in parallel, indicating that the GSD destabilization by MeCN leads to IE decrease (Figure 9A). Figure 10A shows decay profiles of the excimer emission measured at pH 13.0 with different MeCN concentrations, and the decay times and preexponential factors obtained are summarized in Table 6. The preexponential factor of the long-lived excimer emission, assigned to the H2O-solvated GSD, decreases with the MeCN concentration increase. This suggest that the MeCN addition actually suppresses the solvation of GSD by H2O molecules.13 Figure 11A shows change in fluorescence spectra of L measured with 1 equiv of Zn2+ at pH 11.5 (where LZn(OH)2 exist), as a function of MeCN concentration. The MeCN concentration increase leads to a solvatochromic shift of the excimer emission, as is also the case without metal cations (Figure 9A), but does not lead to IE decrease. As shown in Figure 11B, the GSD absorption decreases with the MeCN concentration increase, but the blue-shift of the absorption is not observed. This means that GSD of the LZn(OH)2 complex is more stable than that of the fully deprotonated L species against the MeCN addition. This high GSD stability of the LZn(OH)2 is probably because the Zn2+ coordination with the polyamine ligand of L keeps the two end pyrene fragments close even with MeCN, thus resulting in lower IE decrease against the MeCN concentration increase (Figure 11A). As shown in Figure 11A, 10% MeCN addition leads to IE increase. Figure 11D shows excitation spectra of L collected at 480 nm (excimer emission) with 1 equiv of Zn2+ at different MeCN concentrations. The GSD excitation band decreases with the MeCN concentration increase, as is also the case without Zn2+ (Figure 9D). However, with >5% MeCN, a new excitation band appears at a lower wavelength region (λmax ) 343 nm), which is similar to the excitation band for monomer emission (Figure 11C). This implies that the excimer emission of the LZn(OH)2 complex is formed via a monomer-to-excimer transition (“dynamic” process)36 as well as the direct photoexcitation of GSD (static process). Figure 11C (inset) shows change in fluorescence spectra when excited at 340 nm. IM increases with the MeCN concentration increase, associated with IE increase. This IM increase is because the addition of aprotic MeCN suppresses the quenching of the excited monomer by H2O.37 As reported, the monomer-to-excimer transition is enhanced with a decrease in polarity of the solution. 35b,38 The IE increase (Figure 11C, inset) is therefore due to the acceleration of the monomer-to-excimer transition by the MeCN addition. These



TABLE 5: Decay Times (τ) and Preexponential Factors (a) of Monomer (M) and Excimer (E) Components for Emissions of L in Aqueous NaCl (0.15 M) Solution with 1 equiv of Zn2+, as a Function of pH (λexc ) 358 nm; 298 K) τ/ns (a/%) HL+ pH 7.2 9.0 11.5

λem/nm 396 480 396 480 480

M 79.2 (-12.8)

a

LZn(OH)+

LZn2+ E

M

E

M

5.34 (41.3) 5.34 (39.2)

104.4 (58.7) 104.4 (1.7) 104.4 (44.7)

2.13 (59.1) 60.7 (-14.1)a 60.7 (1.9)

LZn(OH)2 E

3.00 (20.3) 3.00 (86.4)

E

χ2

4.60 (35.0) 4.60 (11.7) 4.60 (100)

1.99 1.23 1.96 1.08 2.91

a The negative preexponential factors are due to the shorter lifetime of the monomer HL+ and the monomer LZn(OH)+ components than that of the monomer LZn2+ component.

Figure 10. Decay profiles of excimer emission (monitored at 480 nm) of L in aqueous NaCl (0.15 M) solution (A) without metal cations (pH 13) and (B) with 1 equiv of Zn2+ (λexc ) 358 nm), (a) without MeCN, (b) with 1% MeCN, (c) with 5% MeCN, and (d) with 10% MeCN. For judging the quality of the fit, autocorrelation functions (AC) and weighted residuals (WR) are also shown in the figure.

Figure 9. Change in (A) fluorescence (λexc ) 360 nm), (B) absorption, and excitation spectra (C) collected at 376 nm (monomer emission) and (D) collected at 480 nm (excimer emission) of L in aqueous NaCl (0.15 M) solution of pH 13.0 (298 K) (a) without MeCN, (b) with 1% MeCN, (c) with 5% MeCN, and (d) with 10% MeCN. Inset: Change in fluorescence spectra (λexc ) 340 nm).

findings suggest that the excimer emission of LZn(OH)2 also contains a dynamic excimer. Figure 9C (inset) shows fluorescence spectra when excited at 340 nm without metal cation. IE decreases with the MeCN concentration increase, and almost no IM change is observed. This is because the excited monomer of the fully deprotonated L species is strongly deactivated by

TABLE 6: Decay Times (τ) and Preexponential Factors (a) for Short- and Long-Lived Excimer Components for Excimer Emission of the Fully Deprotonated L Species Measured at pH 13.0 with Different MeCN Concentrations (λexc ) 358 nm, λem ) 480 nm)a MeCN/%

τshort/ns (ashort/%)

τlong/ns (along/%)

χ2

0 1 10

6.28 (77.1) 6.11 (81.8) 5.37 (95.6)

30.8 (22.9) 30.7 (18.2) 30.1 (4.4)

1.86 1.01 1.49

a The long lifetime excimer component is assigned to the H2Osolvated L species.13

ET from the unprotonated nitrogen atoms. In contrast, with Zn2+, coordination of Zn2+ with all three nitrogen atoms of L suppresses ET, thus accelerating the monomer excitation and the monomer-to-excimer transition.


Shiraishi et al. of Zn2+ at various MeCN concentrations (pH 11.5). With 0% and 1% MeCN, the decay profiles are successfully fitted by a single exponential assigned to the static excimer formed via the direct GSD excitation. In contrast, the profiles obtained with 5% and 10% MeCN are fitted by the sums of two exponentials with short- and long-lifetime components. The latter longlifetime component is assigned to the dynamic excimer formed via the monomer-to-excimer transition. This result clearly suggests that the excimer emission of LZn(OH)2 contains a dynamic excimer as well as a static excimer. As shown in Table 7, lifetime of the static excimer emission decreases slightly with the MeCN concentration (0% (4.60 ns) > 1% (3.44 ns) > 5% (3.33 ns) > 10% (3.31 ns)), due to the destabilization of GSD of the LZn(OH)2 complex by MeCN. In contrast, lifetime of the dynamic excimer emission increases with the MeCN concentration (5% (38 ns) < 10% (56 ns)). This is because the monomer-to-excimer transition is accelerated with the polarity decrease of the solution. The IE increase by 10% MeCN addition (Figure 11A) is therefore due to the contribution of the dynamic excimer. The above results indicate that GSD of the LZn(OH)2 complex is weaker than that of the fully deprotonated L species. Both GSD’s are destabilized by the addition of a less-polar MeCN, but the stability of GSD of the LZn(OH)2 complex is much higher than that of the fully deprotonated L species. This is because the Zn2+ coordination with the nitrogen atoms of L keeps the two pyrene fragments close even with MeCN. The IE decrease of LZn(OH)2 by the MeCN concentration increase is, therefore, very small, whereas the fully deprotonated L species show strong IE decrease. In addition, appearance of the dynamic excimer is observed for LZn(OH)2 with the MeCN concentration increase, due to the acceleration of the monomer-to-excimer transition by the polarity decrease of the solution. 4. Conclusion

Figure 11. Change in (A) fluorescence (λexc ) 360 nm), (B) absorption, and excitation spectra collected (C) at 376 nm (monomer emission) and (D) at 480 nm (excimer emission) of L in aqueous NaCl (0.15 M) solution of pH 11.5 (298 K) with 1 equiv of Zn2+, (a) without MeCN, (b) with 1% MeCN, (c) with 5% MeCN, and (d) with 10% MeCN. Inset: Change in fluorescence spectra (λexc ) 340 nm).

TABLE 7: Decay Times (τ) and Preexponential Factors (a) for Static and Dynamic Excimer Components for Excimer Emission of L Measured with 1 equiv of Zn2+ at pH 11.5 (Where LZn(OH)2 Species Exist) with Different MeCN Concentrations (λexc ) 358 nm, λem ) 480 nm) MeCN/%

τstatic/ns (astatic)%

0 1 5 10

4.60 (100) 3.44 (100) 3.33 (97.8) 3.31 (97.0)

τdynamic/ns (adynamic)%

χ2

38.3 (2.2) 56.2 (3.0)

2.91 3.88 1.66 2.22

The dynamic excimer formation within the LZn(OH)2 complex by the MeCN addition is confirmed by time-resolved fluorescence decay measurements. Figure 10B shows change in decay profile of the excimer emission with the MeCN concentration. The data clearly show that the decay rate of the excimer emission at >80 ns becomes slower with the MeCN concentration increase. Table 7 summarizes the decay times and preexponential factors for excimer emission of L with 1 equiv

Effect of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the fluorescence properties of a diethylenetriamine bearing two end pyrene fragments, L, has been studied in water. We found that L behaves as the first molecular switch capable of quenching pyrene excimer emission by the addition of all of these metal cations in water. The results obtained are summarized as follows. (1) Without metal cations, L shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (IM) is strong at acidic pH but decreases with a pH increase. In contrast, the excimer emission intensity (IE) is weak at acidic pH but increases with a pH increase. The excimer emission is due to the static excimer formed via a direct excitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The GSD stability increases with a pH increase due to the bending of the polyamine chain by deprotonation, resulting in pH-induced IE increase. (2) With metal cations, IM enhancement is not observed at any pH. The IM decrease with Zn2+ is because the coordination of Zn2+ with L forms a Zn2+-pyrene π complex via a donation of π-electron of the pyrene fragments to the Zn2+ center, leading to suppression of the monomer excitation. IE also decreases with the addition of metal cations at any pH. The IE decrease by the Zn2+ addition is because the metal-pyrene π complex formation weakens the π-stacking interaction of two end pyrene fragments, leading to decrease in the GSD stability. (3) IE of the L-Zn2+ complex increases with a pH increase (LZn2+ < LZn(OH)+ < LZn(OH)2). This is because the OHcoordination to the Zn2+ center leads to decrease in the Zn2+-

A Diethylenetriamine with End Pyrene Fragments pyrene π complex formation, leading to GSD stabilization. The excimer emission lifetime of the L-Zn2+ complex also increases with the OH- coordination. In contrast, the monomer emission lifetime of the L-Zn2+ complexes decreases with the OHcoordination. This is due to the electron transfer from the OHto the excited monomer. (4) Without metal cations at basic pH (where fully deprotonated L species exist), addition of less-polar acetonitrile (MeCN) leads to strong IE decrease due to the GSD destabilization. In contrast, with Zn2+ (where LZn(OH)2 exist), IE decrease is scarcely observed. This is because the Zn2+ coordination with L keeps the two pyrene fragments close and, hence, stabilizes the GSD. In addition, appearance of a dynamic excimer is observed for the LZn(OH)2 complex with the MeCN concentration increase. This is due to the acceleration of the monomerto-excimer transition by the polarity decrease of the solution. Acknowledgment. This work was partly supported by the Grant-in-Aid for Scientific Research (No. 19760530) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). We thank Mr. Yasufumi Tokitoh for his experimental assistance. We are also grateful to the Division of Chemical Engineering for the Lend-Lease Laboratory System. References and Notes (1) (a) Balzani, V. Molecular DeVices and Machines: A Journey into the Nano World; Wiley-VCH: Weinheim, Germany, 2003. (b) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515. (c) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348. (d) de Silva, A. P.; McClenaghan, N. D.; McCoy, C. P. Molecular Switches; Wiley-VCH: New York, 2000. (e) Czarnik, A. W. Fluorescent Chemosensors for Ion and Molecular Recognition; American Chemical Society: Washington, DC, 1992. (2) Uchiyama, S.; Kawai, N.; de Silva, A. P.; Iwai, K. J. Am. Chem. Soc. 2004, 126, 3032. (3) (a) Gobbi, L.; Seiler, P.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 674. (b) Beyeler, A.; Belser, P.; De Cola, L. Angew. Chem., Int. Ed. Engl. 1997, 36, 2779. (c) Raymo, F. M.; Tomasulo, M. J. Phys. Chem. A 2005, 109, 7343. (d) Raymo, F. M.; Giordani, S. Org. Lett. 2001, 3, 1833. (4) (a) Fabbrizzi, L.; Licchelli, M.; Mascheroni, S.; Poggi, A.; Sacchi, D.; Zema, M. Inorg. Chem. 2002, 41, 6129. (b) De Santis, G.; Fabbrizzi, L.; Licchelli, M.; Sardone, N.; Velders, A. H. Chem.sEur. J. 1996, 2, 1243. (c) Zhang, G.; Zhang, D.; Guo, X.; Zhu, D. Org. Lett. 2004, 6, 1209. (5) (a) Callan, J. F.; de Silva, A. P.; McClenaghan, N. D. Chem. Commun. 2004, 2048. (b) de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. Chem. Commun. 1996, 2399. (c) Cao, Y.-D.; Zheng, Q.-Y.; Chen, C.F.; Huang, Z.-T. Tetrahedron Lett. 2003, 44, 4751. (6) (a) Bergamini, G.; Ceroni, P.; Balzani, V.; Cornelissen, L.; van Heyst, J.; Lee, S.-K.; Vo¨gtle, F. J. Mater. Chem. 2005, 15, 2959. (b) Pina, F.; Passaniti, P.; Maestri, M.; Balzani, V.; Vo¨gtle, F.; Gorka, M.; Lee, S.K.; van Heyst, J.; Fakhrnabavi, H. Chem. Phys. Chem. 2004, 5, 473. (c) Licchelli, M.; Biroli, A. O.; Poggi, A.; Sacchi, D.; Sangermani, C.; Zema, M. Dalton Trans. 2003, 4537. (d) Amendola, V.; Fernandez, Y. D.; Mangano, C.; Montalti, M.; Pallavicini, P.; Prodi, L.; Zaccheroni, N.; Zema, M. Dalton. Trans. 2003, 4340. (e) Saudan, C.; Balzani, V.; Gorka, M.; Lee, S.-K.; Maestri, M.; Vicinelli, V.; Vo¨gtle, F. J. Am. Chem. Soc. 2003, 125, 4424. (f) Licchelli, M.; Linati, L.; Biroli, A. O.; Perani, E.; Poggi, A.; Sacchi, D. Chem.sEur. J. 2002, 8, 5161. (g) McSkimming, G.; Tucker, J. H. R.; Bouas-Laurent, H.; Desvergne, J.-P.; Coles, S. J.; Hursthouse, M. B.; Light, M. E. Chem.sEur. J. 2002, 8, 3331. (7) (a) Pina, J.; Seixas de Melo, J.; Pina, F.; Lodeiro, C.; Lima, J. C.; Parola, A. J.; Soriano, C.; Clares, M. P.; Albelda, M. T.; Aucejo, R.; GarcıáEspanã, E. Inorg. Chem. 2005, 44, 7449. (b) Alarcón, J.; Aucejo, R.; Albelda, M. T.; Alves, S.; Clares, M. P.; Garcıá-Espanã, E.; Lodeiro, C.; Marchin, K. L.; Parola, A. J.; Pina, F.; Seixas, de Melo, J.; Soriano, C. Supramol. Chem. 2004, 16, 573. (c) Clares, M. P.; Aguilar, J.; Aucejo, R.; Lodeiro, C.; Albelda, M. T.; Pina, F.; Lima, J. C.; Parola, A. J.; Pina, J.; Seixas de Melo, J.; Soriano, C.; Garcıá-Espanã, E. Inorg. Chem. 2004, 43, 6114. (d) Sclafani, J. A.; Maranto, M. T.; Sisk, T. M.; Van, Arman, S. A. Tetrahedron Lett. 1996, 37, 2193. (e) Ander, P.; Mahmoudhagh, M. K. Macromolecules 1982, 15, 213. (8) Bencini, A.; Berni, E.; Bianchi, A.; Fornasari, P.; Giorgi, C.; Lima, J. C.; Lodeiro, C.; Melo, M. J.; Seixas de Melo, J.; Parola, A. J.; Pina, F.; Pina, J.; Valtancoli, B. Dalton Trans. 2004, 2180.

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