Off Fluorescent Indicator of

5090

J. Phys. Chem. B 2007, 111, 5090-5100

Solvent-Driven Multiply Configurable On/Off Fluorescent Indicator of the pH Window: A Diethylenetriamine Bearing Two End Pyrene Fragments Yasuhiro Shiraishi,* Yasufumi Tokitoh, 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: December 28, 2006; In Final Form: February 20, 2007

Fluorescence behaviors of a simple-structured molecule (L), a diethylenetriamine bearing two end pyrene fragments, have been investigated in water. Effects of adding a less-polar organic solvent (acetonitrile: MeCN) on the emission behaviors have been studied by means of steady-state and time-resolved fluorescence measurements. L dissolved in water shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission shows an “on-off” intensity profile against the pH window (pH 2-12), whereas the excimer emission shows an “off-on” profile. Upon MeCN addition, the monomer emission maintains the “on-off” profile. In contrast, the “off-on” profile of the excimer emission is drastically changed: L shows two more types of profiles, “off-on-off-on” and “off-on-off”, along with the MeCN concentration increase, thus behaving as a multiply configurable fluorescent indicator of the pH window. The MeCN-driven excimer emission switching of L is triggered by (i) the decrease in stability of the intramolecular ground-state dimer (GSD) formed between the end pyrene fragments, which suppresses the direct photoexcitation of GSD (suppression of the “static” excimer formation), leading to a decrease in the excimer emission intensity at basic pH; and (ii) the decrease in polarity of solution, which allows formation of a “dynamic” excimer via a monomer-to-excimer transition, resulting in an enhancement of the excimer emission intensity at acidic-neutral pH.

1. Introduction Design of supramolecular systems performing as elementary electronic devices, such as sensors, switches, logic gates, and molecular-level machines, is an area of intense research activity and of tremendous significance to the development of miniaturized device components.1 Potential application of these components in optical and electronic molecular-scale computational devices has attracted considerable effort to this research area.2 Among the research, design of fluorescent signaling molecules has attracted a great deal of attention.3 So far, a wide variety of supramolecular systems whose emission properties can be modulated by external inputs, such as temperature,4 light,5 redox potential,6 and metal ions, have been proposed.7 In particular, molecular systems behaving as a fluorescent indicator of the pH window have attracted much attention, because the proton (H+) concentration is simply and easily controlled (high operability).8-11 However, most of the fluorescent pH indicators proposed before show a simple “on-off”8 or “off-on”9 fluorescence intensity profile against the pH window. For creation of more sophisticated device components, development of pH indicators capable of detecting even a small pH change is necessary. Recent interest is, therefore, focused on more integrated systems capable of showing “on-off-on”10 and “off-on-off”11 profiles. However, a system showing more detailed profiles had not been proposed. In addition, these systems basically show single pH-fluorescence intensity profiles; in other words, one molecular system shows only one profile type. A molecular system behaving as a “multiply configurable” fluorescent pH indicator, whose pH-fluorescence * To whom correspondence should be addressed. Phone: +81-6-68506271. Fax: +81-6-6850-6273. E-mail: [email protected].

intensity profile can be modulated easily by simple external stimuli, had not been developed. Earlier, we 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.12 The intensity of the monomer emission is strong at acidic pH, but decreases with a pH increase. This is triggered by a pH-induced deprotonation of the nitrogen atoms of the polyamine chain, leading to an electron transfer from the unprotonated nitrogen atoms to the photoexcited pyrene fragments. In contrast, the intensity of the excimer emission at acidic pH is very weak, but increases with a pH increase. 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 within L. The increase in the excimer emission intensity associated with a pH increase is due to the bending of the polyamine chain driven by a deprotonation of nitrogen atoms, leading to an increased GSD stability.

In the present work, effects of adding a less-polar organic solvent (acetonitrile, MeCN; 0-50%) to water on the emission behaviors of L have been investigated. We found that the L molecule behaves as the first multiply configurable fluorescent pH indicator: the shape of the pH-excimer emission intensity profile can be reconfigured easily by the MeCN concentration. The monomer emission shows an “on-off” intensity profile

10.1021/jp0689823 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007

Multiply Configurable Fluorescent pH Indicator

J. Phys. Chem. B, Vol. 111, No. 19, 2007 5091

TABLE 1: Fluorescence Quantum Yield (Φf) of L Measured in Aqueous Solution with Different MeCN Concentrations at Selected pH (λexc ) 340 nm; 298 K) MeCN (%)

pH

Φf

0

2.0 12.0 2.0 12.0 2.0 4.3 2.0 4.3

0.15 0.068 0.15 0.039 0.12 0.15 0.064 0.074

5 10 50

against the pH window at all MeCN concentrations, while the excimer emission shows three types of pH-intensity profiles, such as “off-on”, “off-on-off-on”, and “off-on-off”, which are precisely changeable by the MeCN concentration. We describe here that this MeCN-driven excimer emission switching of L is triggered by (i) the decrease in GSD stability, which suppresses the direct excitation of GSD (suppression of the “static” excimer formation), leading to decrease in the excimer emission intensity at basic pH; and (ii) the decrease in polarity of solution, which allows the formation of a “dynamic” excimer via a monomer-to-excimer transition, resulting in an enhancement of the excimer emission intensity at acidic-neutral pH. Time-resolved fluorescence decay measurements were carried out to clarify the changes in emission properties with the MeCN concentration. Temperature dependence of the emissions was also studied for detailed descriptions of the emission properties. 2. Experimental Section General. All of the reagents 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 procedures described previously.12 Spectroscopic Measurements. Steady-state fluorescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer. Absorption spectra were measured on an UV-visible photodiode-array spectrometer (Shimadzu; Multispec-1500).13 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.8e,f,9,12 These spectra were measured at 298 ( 0.1 K (unless otherwise noted) with a 10 mm path length quartz cell under aerated conditions. The measurements were carried out in the presence of NaCl to maintain the ionic strength of the solution (I ) 0.15 M), where the L concentration in solution was adjusted to 25 µM. The solubility of L decreases with an increase in the MeCN concentration: addition of >60% MeCN suppresses the dissolution of a 25 µM L; therefore, measurements were done with 0-50% MeCN. Fluorescence quantum yields (Φf) of L were determined by a comparison of the integrated corrected emission spectrum of standard quinine, which was excited at 366 nm in an aqueous H2SO4 (1 M) solution (Φf ) 0.55, at 298 K).14 The Φf of L measured with different MeCN concentrations at selected pH are summarized in Table 1. Potentiometric Measurements. Potentiometric pH titrations were carried out on a COMTITE-550 potentiometric automatic titrator (Hiranuma Co., Ltd.).8e,f,9,11,15 Aqueous solutions (50 mL; I ) 0.15 M (NaCl)) containing L (25 µM) with a required amount of MeCN were kept under dry argon. The titrations were done at 298 ( 0.1 K using an aqueous NaOH (0.35 M) solution, and at least two independent titrations were performed. The

Figure 1. pH-dependent change in fluorescence spectra (λexc ) 340 nm; 298 K) of L measured in aqueous NaCl (0.15 M) solution (A) without MeCN, (B) with 5% MeCN, (C) with 10% MeCN, and (D) with 50% MeCN.

protonation constants of L were determined by means of the nonlinear least-squares program HYPERQUAD, where Kw () [H+][OH-]) value used was 10-13.73 (at 298 K).16 The stepwise protonation constants of L determined in water are: log K(HL/ H‚L) ) 8.97, log K(H2L/HL‚H) ) 6.02, and log K(H3L/H2L‚ H) ) 3.48 (log β ) 18.47).12 The protonation constants determined with 5-50% MeCN were almost the same as that obtained in water (error 350 nm assigned to GSD.18 As shown in Figure 3Aii, excitation spectra of L collected at 376 nm (monomer emission) show a maximum intensity at 342 nm. In contrast, as shown in Figure 3Aiii, excitation spectra collected at 480 nm (excimer emission) show a red-shifted band with a maximum intensity at around 360 nm, whose spectra are consistent with the red-shifted GSD absorption (Figure 3Ai). These indicate that the excimer emission of L (Figure 1A) is due to the excimer formed by direct GSD excitation. Figure 4A shows decay profiles of the monomer (monitored at 396 nm; open symbol) and excimer emissions (monitored at 480 nm; closed symbol) of L in water at the respective pH. As shown in Table 2, the decay profiles are successfully fitted by a single exponential or sums of two or three exponentials, where no negative pre-exponential, that is, a rise-time, is detected. These clearly support the direct GSD photoexcitation mechanism for the excimer emission of L in water (Scheme 2). The IE increase associated with a pH increase (Figure 2B) 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.8f,9,19 The repulsion decrease leads to a chain bending of L bringing two end pyrene fragments closer and, hence, stabilizes the GSD (Scheme 1); the red-shifted GSD absorption actually increases with pH increase (Figure 3Ai). As shown in Table 2, lifetime of the excimer component increases with pH increase (H2L2+, 5.7 ns; HL+, 6.0 ns; L, 6.2 ns), although the electron transfer from the unprotonated nitrogen atoms to the excimer is accelerated at higher pH. This is due to the pH-induced GSD stabilization. As shown in Figure 2B (closed circle), a drastic IE increase occurs at pH > 8, where the fully deprotonated L species exist. As described,12 this is due to the formation of highly stabilized GSD by complete deprotonation of the polyamine chain (strong interaction of the end pyrene fragments by the chain bending) and by solvation of the species by H2O molecules. As shown in Table 2, at pH 12.0, two excimer components with lifetimes of 6.2 and 30 ns are detected with positive pre-exponential factors. The latter long lifetime 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 < 4, where the fully protonated H3L3+ mainly exists, excimer emission scarcely appears (Scheme 2A). At 4 < pH < 9, where partially protonated species (H2L2+, HL+) exist, both monomer and excimer emissions appear simultaneously (Scheme 2B), where the excimer emission is due to the static excimer formed by direct GSD excitation. At pH > 8, GSD is highly stabilized by complete deprotonation of the polyamine chain and by H2O solvation. Photoexcitation of GSD leads to formations of monomer and two types of excimer emissions with a short and a long lifetime, where the latter long-lifetime emission is due to the excimer highly stabilized by H2O solvation (Scheme 2C). 3.2. Effects of 5% MeCN (“Off-On-Off-On” Excimer Emission Intensity Profile). Figure 1B shows a pH-dependent change in the fluorescence spectra of L in water with 5% MeCN. As shown in Figure 2A (open circle), the pH-IM profile is similar to that obtained without MeCN (“on-off” profile8). As shown in Table 3, the lifetime of the monomer component for monomer emission (measured at 396 nm) obtained with 5% MeCN is longer than that obtained without MeCN (Table 2) at any pH. This is because the aprotic MeCN suppresses the quenching of the excited monomer by H2O to some extent.20 In contrast, the pH-IE profile (Figure 2B, open circle) differs



Figure 3. pH-dependent change in (i) absorption spectra and excitation spectra (ii) collected at 376 nm (monomer emission), and (iii) collected at 480 nm (excimer emission) of L in aqueous NaCl (0.15 M) solution at 298 K (A) without MeCN, (B) with 5% MeCN, (C) with 10% MeCN, and (D) with 50% MeCN.

SCHEME 1: Protonation/Deprotonation Sequence of L with pH

from that obtained without MeCN (closed circle): IE increases at pH 2-4 and decreases at pH > 5 but then increases again at pH > 8, resulting in an “off-on-off-on” profile. The IE value is similar to that obtained without MeCN at pH < 5 but is much lower at pH > 5, where HL+ and the fully deprotonated L species exist. As described,12 the lower IE value at pH > 8 (where the L species exist) is because less-polar MeCN weakens the π-stacking interaction of the end pyrene fragments21 and suppresses the solvation of the species by H2O, leading to a decrease in the GSD stability. As shown in Figures 3Bi and 3Ai, absorbance of the red-shifted GSD band actually becomes weaker by the MeCN addition. The suppression of the H2O solvation by MeCN is supported by the fact that, as shown in Table 3, the pre-exponential factor of the long lifetime excimer component formed by direct excitation of the H2O-solvated GSD (11.9%; pH 12) is much smaller than that obtained without MeCN (19.8%; Table 2). These findings clearly suggest that the MeCN addition suppresses the interaction of the end pyrene fragments and the solvation by H2O, resulting in an IE decrease at pH > 8. The lower IE value at pH 6-8 (where HL+ exist), in comparison to that obtained without MeCN (Figure 2B), is also due to the GSD destabilization by MeCN;21 this is also supported by the GSD absorption decrease (Figure 3Bi).

However, as shown in Table 3, the lifetime of the excimer HL+ component (7.1 ns) is longer than that obtained without MeCN (6.0 ns; Table 2). This may be because the MeCN addition suppresses the quenching of the excimer species by H2O,20 although the excimer formation is suppressed by the GSD destabilization. In contrast, as shown in Figure 2B (open circle), IE at pH 3-6 is comparable to that obtained without MeCN (closed circle). However, as shown in Figure 3Bi, GSD absorption at this pH range is much weaker than that obtained without MeCN (Figure 3Ai). In addition, as shown in Figure 3Biii, the excitation spectra collected at 480 nm (excimer emission) at this pH range is similar to the spectra collected at 376 nm (monomer emission; Figure 3Bii), where a red-shifted GSD excitation band is very weak. These data strongly suggest that, at this pH range with 5% MeCN, a “dynamic” excimer forms from the excited monomer via a monomer-to-excimer transition (Scheme 3B).22 Several reports described that emission intensity of a dynamic excimer increases by adding a less-polar solvent,23,24 which is in general associated with a decrease in viscosity23 or polarity24 of the solution. In the present system, the solution viscosity increases by 5% MeCN addition (water, η ) 0.890 × 10-3 Pa‚s; with 5% MeCN, η ) 0.980 × 10-3 Pa‚s), whereas

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Figure 4. Decay profiles (λexc ) 337 nm; 298 K) of monomer emission (open symbols; monitored at 396 nm) and excimer emission (closed symbols; monitored at 480 nm) of L in aqueous NaCl (0.15 M) solution (A) without MeCN, (B) with 5% MeCN, (C) with 10% MeCN, and (D) with 50% MeCN. The respective pH is: (i) 2.0, (ii) 4.7, (iii) 7.4, and (iv) 12.0.

TABLE 2: Decay Times (τi) and Pre-exponential Factors (ai) of Monomer (M) and Excimer (E) Components for Emissions of L Measured in Water as a Function of pH (λexc ) 337 nm; 298 K) τi (ns) (ai (%)) H3L3+ λem pH (nm)

M

2.0 396 114.1 (100) 4.7 396 480 7.3 396 480 12.0 396

480

E

HL+

H2L2+ M

E

M

L E

M

E

χ2 2.53

94.3 (98.2) 94.3 (15.1)

5.69 (1.8) 5.69 (84.9)

2.82 1.96 79.7 (81.1) 79.7 (3.2)

6.01 (18.9) 6.01 (96.8)

2.96 3.18 60.2 6.15 1.35 (23.3) (75.3) 29.9a (1.4)a 60.2 6.15 1.51 (0.7) (79.5) 29.9a (19.8)a

a Long lifetime excimer component is assigned to the H2O-solvated L species (ref 10; Scheme 2C).

the dielectric constant decreases (water, ) 78.54; with 5% MeCN, ) 74.66).25 As described,24a excimer species have a nonpolar character and, hence, is stabilized more in less-polar

media. These facts clearly suggest that the polarity decrease of the solution by 5% MeCN addition allows the dynamic excimer formation. Figure 4Bii shows decay profiles of the monomer and excimer emissions measured at pH 4.7. Although the excimer emission probably contains a dynamic excimer, the rising edge of the excimer emission is similar to that of the monomer emission. In addition, as shown in Table 3, the decay profile of the excimer emission is successfully fitted by a sum of two exponentials assigned to monomer and excimer components with positive pre-exponential factors. The absence of a rise time suggests that the “static” excimer formed by direct GSD excitation is still involved in the excimer emission and the contribution of the dynamic excimer to the total excimer emission is lower than that of the static excimer. The emission mechanism of L in water with 5% MeCN is summarized as Scheme 3. The excimer of the HL+ species and the fully deprotonated L species are populated directly by the GSD photoexcitation, as is also the case without MeCN (Scheme 2B,C). In contrast, the excimer of the H2L2+ species (1E*) is populated directly by the GSD excitation and also indirectly from the locally excited monomer (1M*) via the monomer-toexcimer (1M* f 1E*) transition (Scheme 3B). In this case, excimer-to-monomer (1E* f 1M*) transition scarcely occurs. This is confirmed by the fact that, as shown in Figure 3Bii, excitation spectra collected at 376 nm (monomer emission) do not show a red-shifted GSD excitation band observed in the spectra collected at 480 nm (excimer emission; Figure 3Biii). The lack of the 1E* f 1M* transition may be because of the


J. Phys. Chem. B, Vol. 111, No. 19, 2007 5095 SCHEME 3: Emission Mechanism of L in Aqueous Solution with 5 or 10% MeCN for (A) Fully Protonated Species (H3L3+), Partially Protonated (B) H2L2+ and (C) HL+ Species, and (D) Fully Deprotonated Species (L)a

SCHEME 2: Emission Mechanism of L in Water for (A) Fully Protonated Species (H3L3+), (B) Partially Protonated Species (H2L2+, HL+), and (C) Fully Deprotonated Species (L)a

a

0 e R, β e 1. τM . τE′ > τE.

TABLE 3: Decay Times (τi) and Pre-exponential Factors (ai) of Monomer (M) and Excimer (E) Components for Emissions of L in Aqueous Solution with 5% MeCN as a Function of pH (λexc ) 337 nm; 298 K) τi (ns) (ai (%)) H3L3+ λem pH (nm)

M

1.8 396 142.8 (100) 4.7 396 480 7.3 396 480 12.0 396

480

E

HL+

H2L2+ M

E

M

L E

M

E

χ2 2.53

135.5 (72.1) 135.5 (10.0)

14.6 (27.9) 14.6 (90.0)

1.69 1.35 122.1 (41.4) 122.1 (0.6)

7.1 (58.6) 7.1 (99.4)

1.63 1.81 112.7 3.7 1.31 (5.5) (81.9) 31.2 (12.6) 3.7 2.07 (88.1) 31.2a (11.9)a

a Long lifetime excimer component is assigned to the H O-solvated 2 L species (ref 10; Scheme 3D).

short lifetime of the excimer (14.6 ns, Table 3); the rapid deactivation of the eximer probably suppresses the 1E* f 1M* transition. 3.3. Effects of 10% MeCN (“Off-On-Off” Excimer Emission Intensity Profile). Fluorescence spectra of L obtained with 10% MeCN is shown in Figure 1C, and the changes in IM and IE with pH are summarized in Figure 2 (triangle). The pH-

a

0 e R, β e 1. τM . τE′ > τE.

IM profile is similar to that obtained without MeCN (closed circle) and with 5% MeCN (open circle). In contrast, the pHIE profile changes drastically: IE increases with a pH increase but decreases at pH > 6, showing an “off-on-off” profile.11 IE at pH 3-6 is much higher than that obtained with 5% MeCN but at pH > 8 is lower. The lower IE at pH > 8 (where the fully deprotonated L species exist) is due to the GSD destabilization by MeCN increase; as shown in Figure 3Ci, at pH > 8, the GSD absorption obtained with 10% MeCN is actually weaker than that obtained with 5% MeCN (Figure 3Bi). This is because higher MeCN concentration suppresses the interaction of the end pyrene fragments of L and the H2O solvation. As shown in Table 4 at pH 12.0, the pre-exponential factor assigned to the H2O-solvated GSD for excimer emission (5.0%) is lower than that obtained with 5% MeCN (11.9%, Table 3), indicating that GSD is indeed destabilized by 10% MeCN. As shown in Figure 2B (triangle), IE at pH 3-6 (where H2L2+ exist) is much higher than that obtained with 5% MeCN. As shown in Figure 3Ci, GSD absorption at this pH range is much weaker than that obtained with 5% MeCN (Figure 3Bi). In addition, at this pH range, excitation spectra collected at 480 nm (excimer emission, Figure 3Ciii) is similar to the spectra collected with 5% MeCN (Figure 3Biii) and the spectra collected with 10% MeCN at 376 nm (monomer emission, Figure 3Cii). These clearly indicate that dynamic excimer contributes to the excimer emission of the H2L2+ species, as is also the case with 5% MeCN. However, as shown in Figure 4Ciii, the rising edge of the excimer emission is similar to that of the monomer emission. In addition, as shown in Table 4, the decay profile of

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TABLE 4: Decay Times (τi) and Pre-exponential Factors (ai) of Monomer (M) and Excimer (E) Components for Emissions of L Measured in Aqueous Solution with 10% MeCN as a Function of pH (λexc ) 337 nm; 298 K)a

TABLE 5: Decay Times (τi) and Pre-exponential Factors (ai) of Monomer (M) and Excimer (E) Components for Emissions of L Measured in Aqueous Solution with 50% MeCN as a Function of pH (λexc ) 337 nm; 298 K)a

τi (ns) (ai (%)) L3+

H3 λem pH (nm)

M

1.8 396 131.1 (68.0) 480 131.1 (0.2) 4.7 396 480 7.3 396 480 12.0 396

480

E

M

E

τi (ns) (ai (%))

HL+

H2L2+ M

H3L3+

L E

25.7 (32.0) 25.7 (99.8)

M

E

χ2 2.15 2.27

128.9 (68.2) 128.9 (8.8)

18.4 (31.8) 18.4 (91.2)

2.16

116.0 (51.4) 116.4 (2.6)

12.5 (48.6) 12.5 (97.4)

λem pH (nm)

1.8 396 25.2 (100) 480 25.2 (100) 4.7 396

3.59

480

1.74

7.3 396

3.62

480

95.4 6.10 1.18 (8.9) (90.3) 30.2a (0.8)a 95.4 6.10 1.87 (0.5) (94.5) 30.2a (5.0)a

12.0 396

a Long lifetime excimer component is assigned to the H O-solvated 2 L species (ref 10; Scheme 3D).

the excimer emission is fitted by a sum of two exponentials assigned to monomer and excimer components with positive pre-exponential factors. These suggest that static excimer formed via direct GSD excitation is still involved in the excimer emission for the H2L2+ species. As shown in Table 4, lifetime of the monomer component at pH 4.7 (128.9 ns) is relatively shorter than that obtained with 5% MeCN (135.5 ns), although in general the lifetime of the monomer emission increases with an addition of aprotic MeCN to water.20 This is because the monomer-to-excimer transition is accelerated by 10% MeCN addition; this results in rapid decay of the monomer species. The overall emission mechanism of L with 10% MeCN can be summarized as Scheme 3, as is also the case with 5% MeCN. As shown in Figure 3Cii, the excitation spectra collected at 376 nm do not show a red-shifted GSD excitation band, indicating that the 1E* f 1M* transition scarcely occurs, as is also the case with 5% MeCN. 3.4. Effects of 50% MeCN (“Off-On-Off” Excimer Emission Intensity Profile). Addition of 50% MeCN leads to significant changes in pH-fluorescence intensity profiles. As shown in Figure 2A (square), IM decreases drastically at pH > 3 with the formation of H2L2+, while 10, 5, and 0% MeCN additions still show strong IM. The most notable change is the pH-IE profile showing an “off-on-off” profile (Figure 2B, square). IE increases drastically with the H2L2+ formation; the value is highest among all MeCN concentrations. IE decreases with HL+ formation, but the value is higher than that obtained with other MeCN concentrations. A further IE decrease occurs at pH > 8 (formation of the fully deprotonated L species); however, the value is comparable to that obtained without MeCN, whereas 5 and 10% MeCN leads to a significant IE decrease. As shown in Figure 3Di, upon 50% MeCN addition, red-shifted GSD absorption does not appear at any pH, indicating that GSD is strongly destabilized. In addition, as shown in Figure 3Diii, excitation spectra collected at 480 nm (excimer emission) are almost the same as that collected at 376 nm (monomer emission; Figure 3Dii), where a red-shifted GSD

M

480

E

HL+

L2+

H2 M

E

M

L E

M

E

χ2 4.43 2.36

3.42 (89.1) 3.42 (-12.4)

27.0 (10.9) 27.0 (100)

1.98 2.07 2.91 (89.4) 2.91 (-33.6)

26.7 (10.6) 26.7 (100)

2.38 2.11 2.04 (98.2) 2.04 (-13.3)

25.5 1.51 (1.8) 25.5 1.12 (100)

SCHEME 4: Emission Mechanism of L in Aqueous Solution with 50% MeCN for (A) Fully Protonated Species (H3L3+) and (B) Partially and Fully Deprotonated Species (H2L2+, HL+, L)a

a

τE > τM.

excitation band scarcely appears. These facts clearly suggest that the dynamic excimer contributes predominantly to the excimer emission at any pH. Figure 4D shows decay profiles of the monomer and excimer emissions of L obtained with 50% MeCN at the respective pH value, and Table 5 summarizes the decay times and the preexponential factors of the respective emitting components. It is notable that (i) the rising edge of the excimer emissions monitored at pH 4.7, 7.3, and 12.0 is obviously delayed versus that of the monomer emissions; and (ii) these excimer emission decay profiles are explained by the sum of two exponentials assigned to excimer (with a positive pre-exponential factor) and monomer components (with negative pre-exponential factor). These findings again indicate that, with 50% MeCN, the dynamic excimer contributes predominantly to the excimer emissions. As shown in Table 5, the lifetime of the monomer components (95 ns). This is due to acceleration of the monomer-to-excimer transition by the polarity decrease of the solution, leading to rapid quenching of the excited monomer. As shown in Table 5, lifetimes of the excimer components for H2L2+, HL+, and L species are similar (25-27 ns); however, IE decreases in the order of H2L2+ > HL+ > L (Figure 2B).



Figure 5. Temperature-dependent change in (i) fluorescence (λexc ) 340 nm) and (ii) absorption spectra of L in aqueous NaCl (0.15 M) solution without MeCN at pH (A) 2.0, (B) 4.7, (C) 7.3, and (D) 12.0. The respective temperature is: 278, 283, 298, 313, 323, and 333 K.

Figure 6. Arrhenius plots for the ratio IE/IM obtained in an aqueous NaCl (0.15 M) solution (A) without MeCN and (B) with 50% MeCN at pH (square) 2.0, (triangle) 4.7, (open circle) 7.3, and (closed circle) 12.0, respectively.

As shown in Table 5, the lifetime of the monomer components decreases with the deprotonation degree of the species (H2L2+ > HL+ > L), because of acceleration of the electron transfer from the unprotonated nitrogen atoms to the excited monomer. The rapid monomer quenching at higher pH suppresses the monomer-to-excimer transition, thus, probably leading to a lower IE value (Figure 2B). The excimer species must also be quenched more at higher pH by the electron transfer. The excimer lifetime of the highly deprotonated species similar to that of the species of lower deprotonation degree (Table 5) may be due to the shortened distance between the two pyrene fragments within the highly deprotonated species by the deprotonation of the polyamine chain (Scheme 1). This accelerates the monomerto-excimer transition, thus, showing a lifetime similar to that of the species of lower deprotonation degree. The overall emission mechanism of L with 50% MeCN can be summarized as Scheme 4. The partially and fully deprotonated species (H2L2+,HL+, L) produce a dynamic excimer. As shown in Table 5, the lifetimes of the excimer components for all of these species are longer than that of the monomer components. This indicates that the excimer-to-monomer transition (1E* f 1M*) is favored with 50% MeCN (Scheme 4), although additions of

0, 5, and 10% MeCN do not allow the 1E* f 1M* transition because of the shorter 1E* lifetime (Schemes 2 and 3). 3.5. Properties of Monomer and Excimer Emissions. The above results reveal that different MeCN concentrations lead to the formation of different types of excimer species; 0% MeCN produces a static excimer, whereas 50% MeCN produces a dynamic excimer. To further clarify the properties of the monomer and excimer emissions of L with different MeCN concentrations, temperature dependences of the emissions were studied. Figures 5i and 5ii show a temperature-dependent change in fluorescence and absorption spectra of L in water without MeCN. At any pH (Figure 5i), the entire spectrum becomes weaker as the temperature rises. As also observed for related aromatic molecules,26 these heat-induced IM and IE decreases are due to the gradual predominance of the nonradiative decay process of the excited species relative to the radiative decay process. As shown in Figure 5ii, GSD absorption also decreases with a rise in temperature at the entire pH range. This means that GSD is destabilized by the temperature increase; this may also affect the heat-induced IE decrease (Figure 5i). Figure 6A shows a change in the ratio of the monomer and excimer emission intensity (IE/IM) obtained without MeCN, with the reciprocal of the temperature (1/T). At pH 2.0 (square), where H3L3+ exists, the ratio increases with the rise in temperature. This is because, at this pH, IE is very weak at the entire temperature range (Figure 5Ai) and, hence, the heat-induced IM decrease affects strongly on the ratio increase. At pH 4.7 (triangle), where H2L2+ exist, the ratio decreases with the temperature increase, meaning that a temperature increase affects the IE decrease more strongly than the IM decrease. This is due to the GSD destabilization with a rise in temperature (Figure 5Bii). At this pH, IM increases with a rise in temperature at 5, in comparison to that obtained without MeCN, is due to the decreased GSD stability. In contrast, IE at pH 2-4 is similar to that obtained without MeCN. This is because the addition of less-polar MeCN allows the formation of a dynamic excimer. With 10% MeCN, a decrease in the GSD stability at pH > 5 becomes more apparent, while the formation of the dynamic excimer at pH 2-4 is further accelerated, resulting in an expression of “off-on-off” IE profile. (3) With 50% MeCN, L still shows an “on-off” IM profile and an “off-on-off” IE profile. The entire IE intensity is much

stronger than that obtained with other MeCN concentrations (0, 5, and 10%). In this case, GSD does not form at any pH. Timeresolved fluorescence decay measurements reveal that, at this MeCN concentration, dynamic excimer contributes predominantly to the excimer emission. In this condition, monodeprotonated H2L2+ species show the highest IE, and IE decreases with the deprotonation degree of the species. (4) Without MeCN, a temperature increase leads to decreases in IM, IE, and GSD absorption. The thermal stability of GSD for highly deprotonated species is higher and, hence, the heatinduced IE decrease is smaller at higher pH. With 50% MeCN, a temperature increase also leads to IM and IE decreases. The ∆H value for the excimer formation of the HL+ species is lowest among the species. This may be due to the relatively short distance between the two pyrene fragments and to the relatively low deprotonation degree, leading to an efficient monomer-toexcimer transition. Acknowledgment. This work was partly supported by the Grant-in-Aid for Scientific Research (15360430) and that on Priority Areas (417) “Fundamental Science and Technology of Photofunctional Interfaces” (17029037) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We are grateful to the Division of Chemical Engineering for the Lend-Lease Laboratory System. G.N. thanks the Japan Society of Promotion of Science (JSPS) Research Fellowships for Young Scientist. References and Notes (1) (a) de Silva, A. P.; McClenaghan, N. D.; McCoy, C. P. MolecularLeVel Electronics, Imaging and Information, Energy and EnVironment, in Electron Transfer in Chemistry, Vol. 5; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001. (b) Balzani, V. Molecular DeVices and Machines: A Journey into the Nano World; Wiley-VCH: Weinheim, Germany, 2003. (c) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, Germany, 2001. (d) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348. (e) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (2) (a) Irie, M., Ed. Special Issue: Photochromism: Memories and Switches. Chem. ReV. 2000, 100, 1683. (b) Amendola, V.; Fabbrizzi, L.; Foti, F.; Licchelli, M.; Mangano, C.; Pallavicini, P.; Poggi, A.; Sacchi, D.; Taglietti, A. Coord. Chem. ReV. 2006, 250, 273. (c) Raymo, F. M.; Tomasulo, M. Chem. Eur. J. 2006, 12, 3186. (d) Jiang, G.; Wang, S.; Yuan, W.; Jiang, L.; Song, Y.; Tian, H.; Zhu, D. Chem. Mater. 2006, 18, 235. (e) Trieflinger, C.; Ro¨hr, H.; Rurack, K.; Daub, J. Angew. Chem., Int. Ed. 2005, 44, 6943. (f) Rudzinski, C. M.; Nocera, D. G. In Optical Sensors and Switches; Schanze, K. S., Ed.; Marcel Dekker: New York, 2001, pp 1-99. (3) (a) de Silva, A. P.; McClenaghan, N. D. Chem. Eur. J. 2004, 10, 574. (b) Raymo, F. M. AdV. Mater. 2002, 14, 401. (c) Brown, G. J.; de Silva, A. P.; Pagliari, S. Chem. Commun. 2002, 2461. (d) 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. (e) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P. Acc. Chem. Res. 1999, 32, 846-853. (f) de Silva, A. P.; Fox, D. B.; Moody, T. S.; Weir, S. M. Trends Biotechnol. 2001, 19, 29-34. (g) Czarnik, A. W. Fluorescent Chemosensors for Ion and Molecular Recognition; American Chemical Society: Washington, DC, 1992. (4) Uchiyama, S.; Kawai, N.; de Silva, A. P.; Iwai, K. J. Am. Chem. Soc. 2004, 126, 3032. (5) (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. 2001, 3, 1833. (6) (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. Eur. J. 1996, 2, 1243. (c) Zhang, G.; Zhang, D.; Guo, X.; Zhu, D. Org. Lett. 2004, 6, 1209. (7) (a) de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. J. Am. Chem. Soc. 1997, 119, 7891. (b) de Silva, S. A.; Amorelli, B.; Isidor, D. C.; Loo, K. C.; Crooker, K. E.; Pena, Y. E. Chem. Commun. 2002, 1360. (c) Yang, R.-H.; Chan, W.-H.: Lee, A. W. M.; Xia, P.-F.; Zhang, H.-K.; Li, K. J. Am. Chem. Soc. 2003, 125, 2884. (d) Bodenant, B.; Fages, F.; Delville, M.-M. J. Am. Chem. Soc. 1998, 120, 7511.

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Off Fluorescent Indicator of

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