Correlation between Excited-State Intramolecular Proton-Transfer and

Oct 2, 2014 - Excited-state intramolecular proton-transfer (ESIPT) and singlet-oxygen (1O2) quenching activities of intramolecularly hydrogen-bonded ...
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Correlation between Excited-State Intramolecular Proton-Transfer and Singlet-Oxygen Quenching Activities in 1‑(Acylamino)anthraquinones Shin-ichi Nagaoka,* Hikaru Endo, and Keishi Ohara Department of Chemistry, Faculty of Science and Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan

Umpei Nagashima Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan S Supporting Information *

ABSTRACT: Excited-state intramolecular proton-transfer (ESIPT) and singlet-oxygen (1O2) quenching activities of intramolecularly hydrogenbonded 1-(acylamino)anthraquinones have been studied by means of static and laser spectroscopies. The ESIPT shows a substituent effect, which can be explained in terms of the nodal-plane model. The ESIPT activity positively and linearly correlates with their 1O2 quenching activity. The reason for this correlation can be understood by considering ESIPTinduced distortion of their ground-state potential surface and their encounter complex formation with 1O2. Intramolecularly hydrogenbonded hydroxyanthraquinones found in aloe also show a similar positive and linear correlation, which can be understood in the same way.

1. INTRODUCTION Much attention has been directed, from both of experimental and theoretical viewpoints, toward excited-state intramolecular proton transfer (ESIPT)1 and singlet-oxygen (1O2, 1Δg state) quenching.2 Both of them play key roles in many photochemical and photobiological processes, and are associated with significant applications in several fields. In previous papers,3−5 Nagaoka and co-workers studied the ESIPT and 1O2 quenching activities of intramolecularly hydrogen-bonded hydroxyanthraquinone derivatives (HAQs) found in aloe. ESIPT-active HAQs molecules were found to have a high 1O2-quenching function. The reason for this was interpreted by considering ESIPT-induced distortion of their ground-state potential surface and their encounter complex formation with 1O2, which is attached near to the proton donor and acceptor participating in the ESIPT. Furthermore, the ESIPT activity of the surveyed HAQs linearly correlated with their 1O2 quenching activity, but the reason for the linear relationship has not yet been elucidated well. In intramolecularly hydrogen-bonded molecules, the ESIPT has been studied more extensively than the 1O2 quenching. The ESIPT activity depends on susceptibility to a tautomerism between normal and proton-transferred forms (Scheme 1).6−8 In addition, the ESIPT activity is sometimes affected by another tautomerism through rotation of the proton donor and/or © XXXX American Chemical Society

Scheme 1. Tautomerism between Normal and ProtonTransferred Forms in 1-(Acylamino)anthraquinones6−8

acceptor participating in the ESIPT (Scheme 2).9 Such a rotamerism was first reported in methylsalicylate, whose proton donor and acceptor (OH and O=COCH3 groups, respectively) are neither bulky nor fixed.9 The ESIPT activity of HAQs found in aloe would also depend on the susceptibility to either the former or both of the two tautomerisms, and the tautomerism(s) would have an influence on the 1O2 quenching activity Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: August 19, 2014 Revised: September 30, 2014

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Scheme 2. An Example of Tautomerism through Rotation of Proton Donor and Acceptor Participating in ESIPT9

because the ESIPT activity correlates with the 1O2 quenching activity. In the present work, to clarify the reason for the abovementioned positive- and linear-relationship between the ESIPT and 1O2 quenching activities, we have studied those activities of intramolecularly hydrog en-bonded 1-(acylamino)anthraquinones (AYAAQs, Chart 1) by means of static and Chart 1. Molecular Structures of AYAAQs and Numbering System for Atoms

Figure 1. Schematic and nonadiabatic representation of potential curves of AYAAQs. (a) Potential curves of S0 and S1 states along ESIPT coordinate. Abs. denotes photoabsorption. SWE and LWE stand for short and long wavelength emissions, respectively. (1) AAAQ. The potential curves of the S0(T) and S1(T) states are respectively located much higher in energy than those of the S0(N) and S1(N) states, and are not drawn here. (2) TFAQ. (b) Potential curves of encounter complexes with 1O2 and 3O2 in which AYAAQs molecules are in S0 state. ΔE and ΔE0 denote activation energies of the 1 O2 quenching. (1) Encounter complexes between AAAQ and 1,3O2. (2) Encounter complexes between TFAQ and 1,3O2.

laser spectroscopies. A steric effect of every bulky acyl-group in AYAAQs (OCR in Chart 1) prevents a break of the intramolecular hydrogen bond and the subsequent rotation of the proton donor (N1−H1) around C1−N1 bond. Furthermore, in contrast to the case of methylsalicylate (Scheme 2),9 the proton acceptor (C9O9) is so fixed as to exclude the rotamerism in AYAAQs as well as in HAQs. Accordingly, the rotamerism shown in Scheme 2 is highly unlikely to be present in AYAAQs, and only the susceptibility to the proton-transfer tautomerism shown in Scheme 1 is thought to affect the ESIPT and 1O2 quenching activities. In fact, Smith and co-workers6−8 showed that some AYAAQs molecules have an asymmetric double-minimum potential-surface with a barrier to ESIPT in the lowest-excited 1(π,π*) state (S1 state). The ESIPT activity in AYAAQs depends on the relative energies of the normal S1state and the tautomeric proton-transferred S1-state (S1(N) and S1(T) states, respectively, in Figure 1a). AYAAQs as our sample have the advantage of being able to exclude any possibility of the rotamerism shown in Scheme 2. In ESIPT-active AYAAQs, the potential surface of the ground state (S0 state) is also distorted along the ESIPT coordinate. Such potential distortion in the S0 state (Figure 1a) would have an influence on the 1O2 quenching activity. In this paper, the ESIPT activity depending on the relative energies of the S1(T) and S1(N) states in AYAAQs is explained in terms of the nodal-plane model,10,11 which is a qualitative theoretical model but allows us to recognize important features of the ESIPT immediately. Many chemists have cited the nodalplane model, and recognized usefulness of the explanation12 even in quite recent years.1,13−17 Furthermore, the positive and linear relationship between the ESIPT and 1O2 quenching activities in AYAAQs is interpreted on the basis of ESIPTinduced distortion of their S0-state potential surface and their encounter complex formation with 1O2.

2. EXPERIMENTAL SECTION 1-(Acetylamino)anthraquinone (AAAQ), 1(chloroacetylamino)anthraquinone (CAAQ), 1(dichloroacetylamino)anthraquinone (DCAQ), and 1(trifluoroacetylamino)anthraquinone (TFAQ) (Chart 1) were prepared according to procedures similar to those reported previously.7 The electron-accepting property of the substituent in the acyl group (R in Chart 1) becomes stronger in the order of AAAQ, CAAQ, DCAQ, and TFAQ. Ethanol used for our spectral measurements was commercially available and used without further purification. All the measurements were performed at room temperature. The setup and experimental procedures were described in a previous paper.3 Briefly, the absorption spectra of AYAAQs were measured in ethanol with a Shimadzu UV mini-1240 spectrophotometer. The fluorescence spectra were measured in ethanol with a Shimadzu RF-5000 spectrofluorophotometer and analyzed by using curve-fitting.18 The fluorescence quantum yields (ϕ’s) were determined by comparing the fluorescence-emission spectra of AYAAQs with that of 9,10diphenylanthracene in ethanol (ϕ = 1.0)19 after they had been corrected for the spectral sensitivity of the detector. In this study, 1O2 was produced through triplet photosensitization of rose bengal in an air-saturated ethanol solution. The solubility of rose bengal is high in ethanol. When a quencher (Q) such as AYAAQs is added to the solution, 1O2 is quenched in the following way: 1

kQ

O2 + Q → 3O2 + quenching product

B

(1)

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The second-order rate-constant (kQ) of the 1O2 quenching by Q was determined by measuring the phosphorescence-decay rate-constant (k) of 1O2 in the solution. The k values were measured with a Hamamatsu C7990-01 near-infrared fluorescence-lifetime measurement system in which the second harmonic of a Nd:YAG laser (532 nm) was used as the excitation light. According to ref 20, the k value is given by k = k 0 + k Q [Q]

normal form is stable, and in the S1 state ESIPT takes place and yields the proton-transferred form. The proton-transferred form shows LWE at a lower energy (Figure 1a-2), and a significant amount of the absorbed photon energy is dissipated as heat during the S0(N) → S1(N) → S1(T) → S0(T) → S0(N) cycle. The greatly Stokes-shifted LWE is thus an indication of ESIPT, which causes distortion and displacement of the potential surfaces of the S0 and S1 states. As shown in Figure 1a-2, ESIPT reduces the potential surface curvature of the S0 state owing to strong intramolecular hydrogen bonding. The spectra of AAAQ (Figure 2a), however, differ from those of TFAQ (Figure 2d). The Stokes-shift of AAAQ is much less than that of TFAQ, and the fluorescence emission of AAAQ in a short wavelength region originates from the S1(N) → S0(N) transition (SWE in Figure 1a-1). AAAQ in the S1 state thus does not seem to be susceptible to ESIPT, even though the N− H group at the 1-position is intramolecularly hydrogen-bonded to the carbonyl oxygen at the 9-position. In contrast to the case of TFAQ, distortion and displacement in S0- and S1-potential surfaces due to ESIPT as well as decrease in S0-potential curvature are absent in AAAQ. DCAQ shows dual fluorescence-emission in the short and long wavelength regions (Figure 2c). LWE here observed indicates ESIPT along the intramolecular hydrogen-bond as well as in TFAQ, and SWE originates from the S1(N) → S0(N) transition as well as in AAAQ. The seeming intensities of LWE and SWE of DCAQ are close to each other in Figure 2c. On the other hand, LWE dominates over SWE in TFAQ (Figure 2d), while SWE dominates over LWE in CAAQ and AAAQ, but the relative intensity of the fluorescence tail in the LWE region of CAAQ is stronger than that of AAAQ (Figure 2a,b). Accordingly, the experimental results can be stated in a qualitative way as follows: as the electron-accepting property of the substituent in the acyl group (R in Chart 1) becomes stronger in AYAAQs, LWE and SWE are enhanced and weakened, respectively. A similar result was previously reported in cyclohexane.6−8 To quantitatively examine the above-mentioned substituent effect on LWE and SWE in AYAAQs, the quantum yields of SWE and LWE (ϕSWE and ϕLWE, respectively) are necessary to be determined. To the best of our knowledge, the ϕSWE and ϕLWE values of AYAAQs have not yet been reported in contrast to the total fluorescence quantum yields.7 Accordingly, we have determined each of the ϕSWE and ϕLWE values by an integration of the area under the curve of the corresponding emission in ethanol. The ϕSWE and ϕLWE values thus obtained are given in Table 1, together with the ratio ϕLWE/ϕSWE. The greater the ϕLWE/ϕSWE value is, the more favorable the molecule is for

(2)

where k0 and [Q] respectively denote the natural-decay rateconstant of 1O2 and the molar concentration of Q in the solution. We can determine the kQ value from the slope of the k versus [Q] plot. Since intramolecularly hydrogen-bonded anthraquinones are likely to make self-associations,3,4 the experiments were made under [Q] < 10−4 M.

3. RESULTS AND DISCUSSION 3.1. ESIPT. Figure 2 shows the absorption and fluorescence spectra of AYAAQs in ethanol. As described in Section 1,

Figure 2. Absorption and fluorescence-emission spectra (dashed and solid lines, respectively) of (a) AAAQ, (b) CAAQ, (c) DCAQ, and (d) TFAQ in ethanol at room temperature. ε denotes the molar absorption coefficient. SWE and LWE stand for short and long wavelength emissions, respectively. The fluorescence-emission spectra were obtained by photoexcitation at 380 nm, and have not been corrected for the spectral sensitivity of the detector.

AYAAQs can exist in the two tautomeric forms: the normal and proton-transferred forms. In each of the S0 and S1 states, some AYAAQ molecules have an asymmetric double-minimum potential on which the two forms are represented as the minima (Figure 1a-2).6−8,21−23 TFAQ shows the fluorescence-emission in a long wavelength region (LWE) with an unusually great red-shift (Stokes-shift) from the absorption peak (Figure 2d). This observation can be explained in terms of ESIPT:6−8 photoexcitation of the normal form (stable S0-state species, S0(N) in Figure 1a-2) produces the S1(N) state, in which ESIPT along the intramolecular hydrogen-bond to produce the S1(T) state takes place rapidly, stabilizing the S1 state. The S1(T) state decays to the S0(T) state through the fluorescence-emission or nonradiative transition.24,25 Then the reverse proton transfer takes place in the S0(T) state, and the S0(N) species is regenerated. The relative stability between the normal and proton-transferred forms depends on the electronic state: in the S0 state, the

Table 1. ϕSWE, ϕLWE, ϕLWE/ϕSWE, kQ, and SOAC Values in Ethanol AYAAQs

ϕSWE

AAAQ CAAQ DCAQ TFAQ α-tocopherol (vitamin E)

3.6 × 10−3 1.9 × 10−3 5.4 × 10−4 10 -

6.4 × 107 7.3 × 107 9.7 × 107 1.2 × 108 1.2 × 108b

0.53 0.61 0.81 1.00 1.00

Singlet oxygen absorption capacity defined as kQsample/kQα‑tocopherol. See ref 28. Original SOAC values were determined through a competitive reaction. bReference 3. a

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ESIPT. The ϕLWE/ϕSWE value increases in the order of AAAQ, CAAQ, DCAQ, and TFAQ. Like this, as the electron-accepting property of the substituent R becomes stronger in AYAAQs, the ϕLWE/ϕSWE value and ESIPT activity increase. 3.2. Nodal-Plane Model. The above-mentioned substituent effect on ESIPT in AYAAQs can be explained in terms of the nodal-plane model.10,11,25,26 The presence of the phenyl ring composed of C1−C4, C4a, and C9a atoms has a great influence on the ESIPT (see Supporting Information). Figure 3

result, the S0(N) state becomes markedly lower in energy than the S0(T) state (Figure 1a). The nodal-plane model is consistent with the substituent effect on ESIPT in AYAAQs. In TFAQ with an electronaccepting substituent (R in Chart 1 = CF3) in the acyl group, the lone π electron on C1 is significantly delocalized in the S1 state (the curved arrow in Figure 3a). Owing to the delocalization of the lone π electron on C1, C1N1 and N1− H1 bonds are formed and dissociated, respectively. Then, since the lone π electron on C9a can also be delocalized, C9C9a and O9−H1 bonds are formed and C9O9 double bond is reduced to C9−O9 single bond. As a result, the S1(T) state is stabilized in comparison with the S1(N) state, and ESIPT is activated. Accordingly, LWE dominates over SWE, and the ϕLWE/ϕSWE value is great in TFAQ. The situation for the S1 state of AAAQ with an electrondonating substituent (R = CH3) is the reverse of that for TFAQ. In AAAQ, the delocalization of the lone π electron on C1 in the S1 state is hindered by the electron-donating property of CH3 (the curved arrow in Figure 3b). As a result, the S1(T) state is not stabilized in comparison with the S1(N) state, and ESIPT is not activated. Accordingly, LWE is not enhanced, and the ϕLWE/ϕSWE value is very small in AAAQ. The situation for the S1 state of DCAQ showing the dual fluorescence emission (SWE and LWE seemingly comparable to each other, see Figure 2c) is roughly intermediate between those of TFAQ and AAAQ. CAAQ is similar in situation to AAAQ, but slightly draws to DCAQ. Like this, the nodal-plane model is consistent with the experimental results that the ϕLWE/ϕSWE value and ESIPT activity increase as the electronaccepting property of the substituent R becomes stronger in AYAAQs. The substituent effect observed here can be explained in terms of our nodal-plane model. 3.3. 1O2 Quenching. Figure 4a shows the phosphorescence-emission spectrum of 1O2 produced through photo-

Figure 3. Nodal planes of wave functions in S0 and S1 states of AYAAQs. (a) TFAQ. (b) AAAQ.

shows the nodal patterns of the wave functions in the S0 and S1 states of AYAAQs. As in the case of a particle in a twodimensional rectangular potential box,27 the wave function has no nodal plane in the S0 state, while that has one nodal plane in the S1 state. In the S0 state of AYAAQs, π electrons are distributed mainly over the anthraquinone nucleus, and are used to form the double bonds. In the S1 state (1(π,π*) state), whose wave function has one nodal plane perpendicular to the plane of the anthraquinone nucleus, the π electrons are distributed except on the nodal plane. Accordingly, one can write at least two double bonds along the nodal plane in the S1 state, and one double bond is across the nodal plane from the other, as in benzene (see Supporting Information). As shown in Figure 3, when the two double bonds are C2C3 and C4C4a, lone π electrons are seemingly localized at C1 and C9a atoms. If ESIPT yielding the proton-transferred form takes place in the S1 state, the two lone π electrons can be significantly delocalized owing to formation of C1N1 and C9C9a bonds. In some AYAAQs molecules favorable to the delocalization, the lone π electrons facilitate the rearrangement of the bonds to produce the proton-transferred form. The S1(T) state then becomes markedly lower in energy than the S1(N) state owing to the delocalization. The proton-transferred form is preferred in the S1 state of such AYAAQs molecules because of the more favorable nodal pattern. In contrast, the wave function has no nodal plane in the S0 state of AYAAQs. Accordingly, the proton-transferred form is not preferred in the S0 state because C1N1 and C9C9a double bonds cannot be formed: if these double bonds were formed, C1 and C9a atoms would become pentavalent. As a

Figure 4. (a) Phosphorescence-emission spectrum from 1O2 produced through photosensitization from rose bengal in ethanol at room temperature. (b) Phosphorescence decay curves of 1O2 in absence (solid curve) and presence (dots) of DCAQ (7.53 × 10−5 M) in ethanol.

sensitization from rose bengal in ethanol, and Figure 4b shows the rise and decay curves obtained by monitoring the phosphorescence-emission at 1273 nm in the presence and absence of DCAQ. The phosphorescence decay after subtraction of background counts due to dark current is wellcharacterized by a single exponential decay. The phosphorescence-decay rate-constant k increases when DCAQ is added to the solution (Figure 4b), showing that DCAQ can quench D

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O2 (reaction 1). 1O2 has an electron-accepting property,2 and electron-transfer from DCAQ would deactivate 1O2.3 The dependence of the rate constant k on the concentration of TFAQ is shown in Figure 5. By using eq 2, the second-order 1

Figure 6. kQ versus ϕLWE/ϕSWE plot for AYAAQs (closed circles). An arrow at the right vertical axis indicates the kQ value of TFAQ. The plot gives a linear fit with a slope of 5.6 × 106 M−1 s−1 and an intercept of 7.0 × 107 M−1 s−1 (solid line). The plot for HAQs (open circles) in ethanol4 is also shown together. AS1, AS2, 1-HAQ, 1,5-DHAQ, and 1,8-DHAQ denote aloesaponarin I, aloesaponarin I 3-O-methyl ether, 1-hydroxyanthraquinone, 1,5-dihydroxyanthraquinone, and 1,8-dihydroxyanthraquinone, respectively. Arrows at the right vertical axis indicate the kQ values of AS1 and AS2. The plot gives a linear fit with a slope of 3.0 × 107 M−1 s−1 and an intercept of 5.3 × 105 M−1 s−1 (broken line).

Figure 5. Dependence of k on [TFAQ]. The k values were obtained by monitoring the phosphorescence decay of 1O2 at 1273 nm in ethanol at room temperature. [TFAQ] denotes the molar concentration of TFAQ.

rate constants (kQ values) of the 1O2 quenching by AYAAQs have been determined from the slopes of the k versus [Q] plots, and are listed in Table 1, together with SOAC values (measures of 1O2 quenching).28 The kQ value of TFAQ is close to that of vitamin E (α-tocopherol, 1.2 × 108 M−1 s−1 in ethanol),3 which is well-known as an efficient 1O2 quencher.2,29 The kQ value of TFAQ is great (about twice as great as that of AAAQ in Table 1). Furthermore, as explained in Section 3.1, the ϕLWE/ϕSWE value is also great in ESIPT-active TFAQ, in which LWE dominates over SWE. These situations for TFAQ are close to those for aloesaponarin I (methyl 3,8-dihydroxy-1methyl-9,10-dioxo-9,10-dihydroanthracene-2-carboxylate, abbreviated as AS1).3 Accordingly, as in AS1, TFAQ is thought to form stable encounter complexes with 1O2, which is attached near to the proton donor and acceptor participating in the ESIPT. Then, the encounter complexes are favorable for the 1 O2 quenching, and show a great kQ value as a whole.3 Thus, the formation of the encounter complexes related to ESIPT is an accelerator of the 1O2 quenching. In contrast, the kQ value of AAAQ is small (about half of kQ of TFAQ in Table 1). Furthermore, LWE of AAAQ is negligibly weak, the ϕLWE/ϕSWE value is very small, and AAAQ is not susceptible to ESIPT (Section 3.1). Accordingly, the 1O2quenching accelerator present in TFAQ is absent in AAAQ. AAAQ would form no stable encounter complex with 1O2, or even if such a stable encounter complex is formed, it would not be favorable for the 1O2 quenching. DCAQ showing the dual fluorescence-emission (SWE and LWE seemingly comparable to each other, see Figure 2c) is, also in 1O2 quenching, roughly intermediate between TFAQ and AAAQ (Table 1). CAAQ is similar to AAAQ, but slightly draws to DCAQ. Like this, the kQ value increases in the order of AAAQ, CAAQ, DCAQ, and TFAQ, as the electronaccepting property of the substituent R becomes stronger in AYAAQs. 3.4. Correlation between ϕLWE/ϕSWE and kQ Values. Figure 6 shows the kQ versus ϕLWE/ϕSWE plot for AYAAQs, together with that for HAQs in ethanol.4 ESIPT-active TFAQ and -inactive AAAQ have high and low 1O2-quenching

functions, respectively. In AYAAQs, the ESIPT activity (ϕLWE/ϕSWE) linearly correlates with the 1O2 quenching activity (kQ), as in HAQs (Figure 6). The reason for this correlation in AYAAQs can be qualitatively explained in the following way as in HAQs.4,5 Schematic potential curves of the S0 and S1 states along the ESIPT coordinate in AYAAQs are summarized in Figure 1, together with potential curves of the encounter complexes with 1 O2 and 3O2 in which the AYAAQs molecules are in the S0 state. As mentioned in Section 3.3, TFAQ in the S0 state would form the stable encounter complexes with 1O2 ({TFAQ···1O2}), and the N1−H1...O9C9 moiety participating in ESIPT would play an important role also in the 1O2 quenching through the complex formation. So it is expected that the quenching-reaction coordinate from {TFAQ···1O2} to {TFAQ···3O2} has a component parallel to the reaction coordinate of ESIPT. If the two coordinates were perpendicular to each other, the correlation between kQ and ϕLWE/ϕSWE would not be evident. Accordingly, the schematic potential curves of {TFAQ···1O2} and {TFAQ···3O2} projected on the 1 O2-quenching-reaction coordinate would be given by Figure 1b-2. Here, TFAQ is in the S0 state, {TFAQ···3O2} is lower in energy than {TFAQ···1O2}, and the molecular structure of TFAQ at the crossing of the two potential curves would be similar to that of the proton-transferred form. The potential crossing puts a barrier between {TFAQ··· 1 O 2 } and {TFAQ···3O2}; that is, an activation energy is necessary to the {TFAQ···1O2} → {TFAQ···3O2} reaction (ΔE in Figure 1b2). The activation energy of the TFAQ complex is small, because ESIPT in TFAQ reduces the S0-potential curvature along the ESIPT coordinate as explained in Section 3.1. In contrast, since ESIPT is absent in AAAQ, the potential surfaces of the S0 and S1 states are basically similar in shape to each other, and the normal form is represented as the common minimum (Figure 1a-1). As described in Section 3.1, in contrast to the case in TFAQ, a reduction in S0-potential curvature is absent in AAAQ, and so the activation energy of the 1O2 → 3O2 E

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A similar positive and linear relationship between the ϕLWE/ ϕSWE and kQ values is seen in HAQs (Figure 6). Accordingly, eq 7 would also hold true in HAQs, and the positive and linear relationship would be able to be explained in terms of the ESIPT-induced reduction in S0-potential curvature and the encounter complex formation between 1O2 and HAQs. In fact, a similar asymmetric double-minimum potential-surface with a barrier to ESIPT in the S1 state is expected in HAQs together with coexistence of S1(N) and S1(T) species at equilibrium at room temperature.21 The intramolecular hydrogen bonds of HAQs would be strong and less likely to be broken by solvent molecules.3 In Figure 6, the slope of the plot for AYAAQs is less than that for HAQs. The proton donors participating in ESIPT of AYAAQs and HAQs are NH and OH, which have one and two nonbonding orbitals, respectively. The first step of the 1O2 quenching through the encounter complex formation would involve electron transfer from AYAAQs or HAQs to 1O2,2 some of which would be attached to the spaces of the nonbonding orbitals with negative electrostatic potential.3,30 Since the number of the nonbonding orbital of NH is half of that of OH, the 1O2 attachment to AYAAQs would be less favorable than that to HAQs. In addition, since HAQs with OH show greater spin−orbit interaction than AYAAQs with NH,31 the spin−orbit interaction would also accelerate the 1O2 → 3O2 quenching containing spin inversion in HAQs. Owing to these effects, the frequency factor of the 1O2 quenching (A in eq 7) for AYAAQs would be less than that for HAQs. As a result, the slope for AYAAQs is thought to be less than that for HAQs in Figure 6. Like these, our explanation is again consistent with the experimental results shown in Figure 6. Next we will examine whether the b value in eq 7 is near to unity as suggested above. Equation 7 is transformed into

quenching is great in AAAQ (ΔE0 in Figure 1b-1). Therefore, ΔE for ESIPT-active TFAQ (Figure 1b-2) is less than ΔE0 for ESIPT-inactive AAAQ (Figure 1b-1) or than the activation energy in the case that AAAQ forms no stable encounter complex with 1O2. As a result, ESIPT-active TFAQ is considered to show the higher 1O2-quenching activity than AAAQ (Figure 6). CAAQ and DCAQ are intermediate between AAAQ and TFAQ. However, the reason for the linear relationship between the ϕLWE/ϕSWE and kQ values (Figure 6) has not yet been elucidated well. A possible interpretation will be given below. Smith and co-workers suggested that the S1(N) and S1(T) species of AYAAQs coexist at equilibrium at room temperature.7 Then, the ϕLWE/ϕSWE value, which depends on the abundance ratio of the two excited species is given by ϕLWE /ϕSWE = a ·exp(E N *−T */RT )

(3)

where R and T denote the gas constant and temperature, respectively. EN*−T* is obtained by subtracting the S1(T) energy from the S1(N) one. The Franck−Condon factors for the S1(N) → S0(N) and S1(T) → S0(T) transitions do not need to be the same as each other, especially if the S1 and S0 geometries are different from each other. Accordingly, the constant a is not necessarily equal to unity. As the ESIPT activity increases, the potential curvature of the S0 state along the ESIPT coordinate decreases (Figure 1a). As the S0-potential curvature along the ESIPT coordinate decreases, the ΔE value decreases (Figure 1b) and ΔE0 − ΔE increases. On the other hand, as the ESIPT activity increases, the EN*−T* value increases (Figure 1a). Accordingly, since ΔE0 − ΔE increases with EN*−T*, ΔE0 − ΔE is assumed to be given by ΔE0 − ΔE = b·E N *−T * + c

(4) ln(k Q − k Q 0) = ln(A /ab) + (c − ΔE0)/RT + b·ln(ϕLWE /ϕSWE)

where b and c denote constants, and b > 0. The c value is equal to ΔE0 − ΔE for the case that the S1(N) and S1(T) energies are equal to each other. The details of parameter b will be described below. When Figure 1b-2 represents the reaction potential curve for the 1O2 quenching as in TFAQ, the kQ value is given by k Q = A ·exp( −ΔE /RT ) + k Q 0

(8) 0

Here, kQ is equal to the intercept of the plot given in Figure 6, according to eq 7. Based on eq 8, the ln(kQ − kQ0) versus ln(ϕLWE/ϕSWE) plot for AYAAQs is drawn in Figure 7, together with the corresponding plot for HAQs. In Figure 7, the ln(kQ − kQ0) value increases linearly with ln(ϕLWE/ϕSWE) with a slope close to unity (1.00 and 0.99 for AYAAQs and HAQs,

(5)

where A stands for the frequency factor of the 1O2 quenching in which the encounter complexes between 1O2 and AYAAQs are formed, and kQ0 denotes the second-order rate-constant of the 1 O2 quenching without the encounter complex formation. By using eq 4, eq 5 is rewritten as k Q = A ·exp{(c − ΔE0)/RT }· exp(b·E N *−T */RT ) + k Q 0 (6)

According to eq 3, eq 6 is transformed into k Q = (A /ab) ·exp{(c − ΔE0)/RT } ·(ϕLWE /ϕSWE)b + k Q 0 (7)

In eq 7, the kQ value is nearly proportional to ϕLWE/ϕSWE, when the b value is near to unity and/or the range of ϕLWE/ϕSWE is not wide. Therefore, eq 7 would be consistent with the experimental results of AYAAQs (Figure 6), and the linear relationship between the ϕLWE/ϕSWE and kQ values would be able to be explained in terms of the ESIPT-induced reduction in S0-potential curvature and the encounter complex formation between 1O2 and AYAAQs.

Figure 7. ln(kQ − kQ0) versus ln(ϕLWE/ϕSWE) plot for AYAAQs (closed circles) and the corresponding plot for HAQs (open circles). Arrows at the right vertical axis indicate the ln(kQ − kQ0) values of TFAQ, AS1, and AS2. The plot for AYAAQs gives a linear fit with a slope of 1.00 and an intercept of 15.7 (solid line). The plot for HAQs gives a linear fit with a slope of 0.99 and an intercept of 17.2 (broken line). F

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respectively). Since the slope of the plot drawn in Figure 7 is equal to the b value according to eq 8, the b value would be close to unity. Therefore, eq 7 would be rewritten as

(9)

Like this, the kQ value would be proportional to ϕLWE/ϕSWE, and the proportional relationship is consistent with the plots shown in Figure 6.

4. CONCLUSIONS The ESIPT and 1O2 quenching activities of AYAAQs have been studied by means of the static and laser spectroscopies. The ESIPT shows the substituent effect that can be explained in terms of the nodal-plane model. The ESIPT activity positively and linearly correlates with the 1O2 quenching activity. The reason for this correlation can be understood by considering the ESIPT-induced distortion of the S0-state potential surface and the encounter complex formation with 1O2. HAQs molecules found in aloe also show the positive and linear correlation, which can be understood in the same way. ASSOCIATED CONTENT

S Supporting Information *

Explanations of S0-potential stabilization due to resonance of phenyl ring in TFAQ (Figure S1) and of nodal plane, lone π electrons, and double bonds in S1 state of TFAQ (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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k Q = (A /a) ·exp{(c − ΔE0)/RT } ·(ϕLWE /ϕSWE) + k Q 0

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Article

AUTHOR INFORMATION

Corresponding Author

* Phone: 81-89-927-9592; fax: 81-89-927-9590; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Professor Dan Huppert of Tel Aviv University on the occasion of his 70th birthday. S.N. owes special thanks to the late Professor Paul F. Barbara of the University of Texas at Austin, who enabled us by calling S.N.’s attention to ESIPT of AYAAQs to undertake the present study. S.N.’s thanks are also due to Professor Hiroyuki Teramae of Josai University for his valuable discussion on the nodal-plane model. The work reported in this paper was presented at the Symposium “Photoinduced Proton Transfer in Chemistry and Biology” as a part of the Physical Chemistry Division session at the 248th ACS National Meeting & Exposition, August 10−14, 2014, San Francisco, California. S.N. expresses his sincere thanks to Professor Kyril Solntsev of Georgia Institute of Technology and Professor Pi-Tai Chou of National Taiwan University for their kind invitation to the symposium. S.N. thanks the Research Center for Computational Science at the Okazaki Research Facilities of the National Institutes of Natural Sciences for the use of the computers and the Library Program Gaussian 09 in drawing of Figure S1. This work was partly supported by a Grant-in-Aid for Challenging Exploratory Research (No. 24658123) from the Japan Society for the Promotion of Science. G

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