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potassium ferricyanide systems containing organic solvents
Chemical Physics Letters 474 (2009) 285–289
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Singlet oxygen-sensitized delayed emissions from hydrogen peroxide/gallic acid/potassium ferricyanide systems containing organic solvents Hiroshi Ishii a, Kazuo Tsukino a, Masahiko Sekine b, Munetaka Nakata b,* a b
Japan Applied Technology Inc., Yaho, Kunitachi, Tokyo 186-0011, Japan Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, Naka-Cho, Koganei, Tokyo 184-8588, Japan
a r t i c l e
i n f o
Article history: Received 4 March 2009 In final form 23 April 2009 Available online 3 May 2009
a b s t r a c t Fourier-transform chemiluminescence spectra of H2O2/gallic acid/K3[Fe(CN)6] systems containing organic solvents were measured. Emission bands with peaks around 530 and 700 nm were observed in systems containing solvents with a carbonyl group such as N,N-dimethylformamide, and those with a hydroxyl group such as methanol, respectively. The relative band intensities depended strongly on the concentration of these organic solvents. The emission species are attributed to gallic acid–ferricyanide complexes excited by energy transfer from singlet oxygen dimol, (1O2)2. The effects of organic solvents are interpreted in terms of intermolecular interactions of gallic acid–ferricyanide complexes, water molecules and organic solvents. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Khan and Kasha [1] proposed in 1966 that emission from any energetically favorable species could be observed as chemiluminescence by the direct energy transfer from excited oxygen molecular pairs. A number of papers have since been published on the chemiluminescence of this origin, so-called ‘singlet oxygen-sensitized delayed fluorescence (SOSDF)’ [2–8]. A schematic energy diagram shown in Fig. 1 summarizes the mechanism of the generation of singlet oxygen dimol, (1O2)2, followed by the excitation of organic compounds, in which the lowest excited electronic state, (1Dg)(1Dg), and/or the second excited state, P 1 1 1 ð1 þ g Þð Dg Þ, of ( O2)2 are involved [1]. The excited states of ( O2)2 can be generated by (i) energy transfer from sensitizers [3,4,7,8] (path a), (ii) laser radiation (1270 nm, path b) [9,10], (iii) decomposition of peroxide (paths c and d) [2,5], etc. Processes (iii) result in chemiluminescence instead of fluorescence from organic compounds, but it is included here as a part of the mechanisms of SOSDF. The sensitizers used in past studies to generate excited (1O2)2 species were acetonaphtone [3], C60 [4,8], and phenalene-1-one [7]. One should note that in SOSDF the organic compounds excited by the energy transfer from (1O2)2 usually show emission with a wavelength longer than that corresponds to the transition from P 3 P the first excited state (1Dg)(1Dg) to the ground state ð3 g Þð gÞ of (1O2)2. In a recent Letter [11], we reported on the new design of a Fourier-transform chemiluminescence (FT-CL) spectrometer to observe weak chemiluminescence. This spectrometer was used to * Corresponding author. Fax: +81 42 388 7909. E-mail address: [email protected] (M. Nakata). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.04.062
measure the chemiluminescence spectra in decomposition of hydrogen peroxide (H2O2) in the presence of gallic acid (3,4,5-trihydroxybenzoic acid) and potassium ferricyanide (K3[Fe(CN)6]). This is known as a typical ternary system that evolves singlet oxygen molecules, where hydrogen peroxide reacts as an oxygen source, gallic acid as an efficient receptor of oxygen from hydrogen peroxide [12], and K3[Fe(CN)6] is a model of peroxidase in an enzyme-catalyzed reaction [13]. The wavelength of the observed broad emission band, 580 nm, differed from the reported values of the transitions in (1O2)2 from the excited electronic state, (1Dg)v=0(1Dg)v=0, to the ground state, P P ð3 Þ , 633 nm, and to the vibrationally excited ground ð3 g Þv ¼0 P g v3¼0 P 3 state, ð g Þv ¼1 ð g Þv ¼0 , 703 nm [14]. Since the observed emission was suppressed by addition of a small amount of sodium azide used as a quencher, we inferred that singlet oxygen played an important role as a trigger for the observed emissions. Since we observed strong dependence of the intensity and the peak wavelength on the concentration of K3[Fe(CN)6], we attributed the spectral features to the solvent effect based on the strong chargeand/or K+, but transfer interaction between (1O2)2 and FeðCNÞ3 6 no further evidence was provided. In a similar system such as H2O2/gallic acid/acetaldehyde studied by Yoshiki et al. [13], a characteristic dependence of chemiluminescence intensity on the concentrations of H2O2 and gallic acid was reported. Though the authors had suspected oxidation of acetaldehyde by H2O2 as one of the simplest mechanisms of this chemiluminescence, they excluded it as hardly realistic [13]. In the present Letter, we have extended our study by investigating the effect of organic solvents added to the H2O2/gallic acid/ K3[Fe(CN)6] system on the chemiluminescence. Our interest is
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H. Ishii et al. / Chemical Physics Letters 474 (2009) 285–289
Sensitizers S1
(1Σg+)(1Δ g)
T1 (path a)
Excited states
( Δ g)( Δ g) 1
(path b)
Ground state
Organic Compounds
Oxygen Dimol
1
(path c)
(path d)
Ground state
SOSDF
Ground state Decomposition of Peroxide
Fig. 1. An energy diagram for singlet oxygen-sensitized delayed emission. Excited states of singlet oxygen dimol are generated by energy transfer from sensitizer (path a), by excitation of oxygen in the ground state (path b), by decomposition of peroxide (paths c and d) etc. Organic compounds are excited by energy transfer from excited states of singlet oxygen dimol to give emissions.
focused on the question as to whether the emission bands originate from an excited state of the gallic acid and ferricyanide complex produced by the energy transfer from (1O2)2 generated in the dark decomposition of peroxide. To our knowledge, this is the first report on the chemiluminescence spectra observed in the whole region of visible radiation, which is caused by energy P 1 transfer from a state of higher energy, such as ð1 þ g Þð Dg Þ, than 1 1 1 the lowest exited electronic state, ( Dg)( Dg) of ( O2)2. The observed characteristic changes in the wavelength and intensity of chemiluminescence emissions from a variety of solutions containing organic solvents are discussed in terms of the intermolecular interaction involving the gallic acid–ferricyanide complex, water molecules and organic solvents, and the relaxation of the excited electronic states of (1O2)2. 2. Experimental method Chemiluminescence spectra were measured with an FT-CL spectrometer (MS-8310) developed by Japan Applied Technology Inc. [11]. The use of an optical system composed of a Savart plate and a polarizer and a data-acquisition system made of a charge coupled device enabled collection of weak emissions from twodimensional widespread area for an extended period. Water-miscible organic solvents, i.e., methanol, ethanol, acetone, N,N-dimethylformamide (DMF), 3-pentanone, and 2-propanol were used as co-solvents. All the chemicals with the highest attainable purity were purchased from Wako Pure Chemicals Ltd. and used without further purification. An aqueous solution of K3[Fe(CN)6] prepared freshly was poured to saturated gallic acid solution in a sample dish of 50 mm diameter. After addition of an organic solvent to the mixture, hydrogen peroxide (30%) was added and stirred vigorously, and then data-acquisition was started. The concentration of K3[Fe(CN)6] was either 200 or 1000 mM. The sampling time was set to 5 min so as to obtain a reasonable signal-to-noise ratio for a chemiluminescence spectrum. The temperature of the sample stage was kept at 300 K. 3. Results and discussion 3.1. Measurements of chemiluminescence spectra A chemiluminescence spectrum of the H2O2/gallic acid/ K3[Fe(CN)6] system, without organic solvents and with 200 mM of K3[Fe(CN)6], is displayed in Fig. 2a. This spectrum, where a broad emission band appears around 580 nm, is nearly identical to that reported in Ref. [11]. When an organic solvent of 2 ml was added to a solution containing H2O2, gallic acid, and K3[Fe(CN)6] of 1 ml
Fig. 2. Chemiluminescence spectra of the H2O2/gallic acid/K3[Fe(CN)6] system containing organic solvents: (a) no organic solvent, (b) DMF, (c) acetone, (d) 3pentanone, (e) methanol, (f) ethanol, and (g) 2-propanol. Solvents containing a carbonyl group are represented by closed circles, while those containing a hydroxyl group by closed triangles. The ratio of H2O2:gallic acid:K3[Fe(CN)6]:organic solvent was 1 ml:1 ml:1 ml:2 ml. The concentration of K3[Fe(CN)6] was 200 mM. Accumulation time for data collection was 5 min.
each, a remarkable spectral change was observed, as shown in Fig. 2b–g. By addition of DMF, for example, the peak wavelength of the chemiluminescence shifted from 580 to 530 nm with a significant enhancement of intensity. In contrast, a band with a peak at 700 nm appeared when 2-propanol was added; similar bands were observed by addition of methanol and ethanol. In summary, a broad emission around 530 nm was dominant for DMF (marked b), acetone (c), and 3-pentanone (d), whereas the band around 700 nm was dominant for methanol (e), ethanol (f), and 2-propanol (g). These two groups of organic solvents may be characterized by their functional groups; the former, which exhibited the 530 nm emission, contains a carbonyl group, whereas the latter that exhibited the 700 nm emission contains a hydroxyl group. A remarkable feature is that the 700 nm emission in the latter group is enhanced by extension of the carbon chain, in the order of methanol < ethanol < 2-propanol. We note that this spectral trend has first been detected in this study by the use of a spectrometer of the FT-CL type, unlike a normal experimental setup for measurements of chemiluminescence composed of a series of interference filters and a photomultiplier, with which no reliable calibration of relative intensities would be expected. To investigate the effect of organic solvents on the emission band intensity in more detail, chemiluminescence spectra were measured by varying the amounts of organic solvents relative to water. Typical spectral changes observed with the samples containing different concentrations of methanol are compared in Fig. 3. The spectrum of the most dilute sample a, with methanol:water of 1:100, showed an emission band at 530 nm with a shoulder band at 700 nm, being similar to that of pure water marked a in Fig. 2, as expected. In contrast, a spectrum of the sample with the highest concentration, 72:100 (d), showed a strong broad band at 700 nm but with a barely detectable 530 nm band. Similar experiments were carried out by adding various amounts of DMF to the H2O2/gallic acid/K3[Fe(CN)6] system. In contrast to the results for methanol, the observed chemiluminescence spectra exhibited less significant changes, as shown in Fig. 3; the band at 530 nm is dominant for samples with any amounts of DMF, while that around 700 nm is a shoulder of the 530 nm band. We note that the peak intensity depends on the concentration of DMF, but there is no apparent shift in the peak position in the molar ratio of DMF:water from 0.5:100 to 38:100.
H. Ishii et al. / Chemical Physics Letters 474 (2009) 285–289
Fig. 3. Chemiluminescence spectra of the H2O2/gallic acid/K3[Fe(CN)6] system containing methanol or DMF. The mol ratio of methanol to total water was 1:100 (a), 12:100 (b), 33:100 (c), 72:100 (d), while that of DMF to total water was 0.5:100 (a), 6:100 (b), 17:100 (c), 38:100 (d). The concentration of K3[Fe(CN)6] was 1000 mM. Accumulation time for data collection was 5 min.
3.2. Assignment of the chemiluminescence spectra As shown in Figs. 2 and 3, two distinct emission bands were observed at 530 and 700 nm for all the organic solvents studied and with different ratios to water, but their relative intensities were varied. One might assign these emission bands to (1O2)2, which is known to emit red lights around 633 nm from the excited electronic state, (1Dg)v=0(1Dg)v=0, to the ground state, P P 3 703 nm to the vibrationally exð3 g Þv ¼0 ð g Þv ¼0 , and that around P P 3 cited ground electronic state, ð3 g Þv ¼1 ð g Þv ¼0 [14]. However, the wavelength of 530 nm observed in our study deviates far from that of the 633 nm emission. The 700 nm emission is much closer to the reported 703 nm emission of (1O2)2, but one can hardly assign this emission to the vibrationally excited ground electronic state, 703 nm, because it has exceedingly higher emission intensity than that to the ground vibronic state, 633 nm. Therefore, it is hard to attribute the bands observed in the present study to the direct emission from (1O2)2. To solve the question on whether the two emission bands arise from (1O2)2, we have examined the effect of a singlet oxygen quencher, sodium azide, on the chemiluminescence. Fig. 4 shows the spectra with and without sodium azide in solutions containing methanol or DMF. The emission bands of 700 nm are dominant for methanol, whereas those of 530 nm are dominant for DMF, as reported in 3.1. When 10 mM of sodium azide was added to the solutions, both emission bands at 700 and 530 nm were clearly suppressed; this led us to the conclusion that the two emis-
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Fig. 4. Influence of sodium azide, a scavenger for singlet oxygen, on the chemiluminescence spectra of H2O2/gallic acid/K3[Fe(CN)6] systems containing methanol or DMF. The amounts of H2O2, gallic acid, K3[Fe(CN)6], and organic solvent were 1, 1, 0.5, and 2 ml, respectively. An amount of 0.5 ml of water (solid line) or 10 mM sodium azide solution (dashed line) was added to the solution 2 min after the reaction started. The concentration of K3[Fe(CN)6] was 600 mM in common, and the accumulation time for data collection was 5 min.
sion bands originate only indirectly from the (1O2)2 generated in decomposition of hydrogen peroxide in the present system. Our additional finding is that the suppression in the methanol case is apparently less significant than that in the DMF case. We attribute this observation to the dynamic behavior of the electronic states of P singlet oxygen, 1D and 1 þ g ; see 3.3. Gallic acid solution is known to have no absorption in the visible-light region. However, it seems reasonable to choose a gallic acid-FeðCNÞ3 6 complex as a candidate to account for the two emission bands, because gallic acid forms complexes with Fe3+ ion [15] in aqueous solutions with Fe3+ ion:gallic acid ratios of 1:1, 1:2 and 1:3; water molecule plays an additional ligand in the 1:2 complex. It is also reported that a ternary complex of gallic acid/Fe3+ ion/glycine (1:1:1) showed an absorption spectrum with a peak at 530 nm [16], which is similar to one of the emissions of our observation. One can thus assume that the energy of (1O2)2 generated in the decomposition of peroxide is transferred to gallic acid-FeðCNÞ3 6 complexes like the energy transfer from singlet oxygen to its acceptors, phthalocyanin [2–4], dibenzanthrone [5], rubrene [6], and Nile Blue A perchlorate [7], as discussed in Introduction. One can also assume that organic molecules excited by energy transfer from (1O2)2 emit radiation with wavelength longer than 633 nm due to the transition from the lowest excited electronic state, P P 3 (1Dg)v =0(1Dg)v =0, to the ground state ð3 g Þv ¼0 ð g Þv ¼0 . This holds in cases of phthalocyanin [2–4], Nile Blue A perchlorate [7], and dibenzanthrone [5], which show emission bands around 710,
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(a) Weak Interaction between Water and Solvent Energy Transfer Solvent H2O2
Gallic Acid
Solvent
Water
Gallic Acid (O2) 2*
H2O2 Fe(CN) 3+ 6
Fe(CN)6
Water
3+
~ 530 nm emission
(b) Strong Interaction between Water and Solvent Energy Transfer Solvent H2O2 H2O2
Gallic Acid
Solvent
Water
Fe(CN)63+
(O2) 2*
Gallic Acid
Water
~ 700 nm emission
Fe(CN)63+
Fig. 5. A schematic model to interpret two distinct emissions appearing around 530 and 700 nm: (a) Weak interaction between water and solvents such as DMF, and gallic Pþ 1 1 1 nm emission. (b) Strong interaction between water and solvents such acid–FeðCNÞ3þ 6 complex excited by energy transfer from the excited state ð g Þð Dg Þ of ( O2)2 gives 530 Pþ 3þ as methanol, and gallic acid–FeðCNÞ6 complex excited by energy transfer from the excited state ð1 g Þð1 Dg Þ and/or (1Dg)(1Dg) of (1O2)2 gives 700 nm emission.
650, and 660 nm, respectively. Conversely, the energy of the 530 nm emission observed in the present study is higher than that of the 633 nm emission. It is possible that energy transfer P 1 from a higher state of (1O2)2, for example ð1 þ g Þv ¼0 ð Dg Þv ¼0 , occurs in the decomposition of peroxide in the present system; a similar mechanism of energy transfer has been proposed for the thermal decomposition of photo-oxidized 1,4-dimethoxy-9,10-dipyridylanthracene [17] and for a reaction of pyrogallol with IO 4 in KOH/ K2CO3 solution [18], but only the emission intensities measured with a series of interference filters were reported. 3.3. Interpretation of the solvent effects No chemiluminescence spectra were observed with our spectrometer when one of the chemical species in the present system, i.e., H2O2, gallic acid, or K3[Fe(CN)6], was omitted. This finding implies that all the three species interact with one another and play important roles in the emission mechanism. Though the structure of gallic acid-FeðCNÞ3 6 complex is still unknown, one needs to consider at least two types, because two emission bands have been observed at 530 and 700 nm. We can assume from our observations that the relative abundance of these complexes depends strongly on the amount of organic solvents. Our interpretation of this solvent effects is schematically presented in Fig. 5: Hydrogen peroxide is decomposed to produce electronically excited (1O2)2 by a catalyst of gallic acid-FeðCNÞ3þ 6 complex. This complex, by interaction with water molecules and by energy transfer from (1O2)2, is led to emissions. If an organic solvent is added to the solution, water molecules may interact with the organic solvent as well as gallic acid–FeðCNÞ3þ 6 complex. If the intermolecular interaction between water and the organic solvent is weak, as in the case of DMF shown in Fig. 5a, an emission band due to the complex appears around 530 nm. If the interaction between water molecules and the organic solvent is much stronger, as in the case of methanol with hydrogen bonding, the interaction between the complex and water molecules is relatively weakened, as shown in Fig. 5b, so that the emission from the complex appears around 700 nm. In other words, the emission of gallic acid-FeðCNÞ3 6 complex depends on whether water molecules interact with the complex or with the organic solvent. This interpretation is supported by the fact that a hydroxyl group can work both as a hydrogen donor and an acceptor, whereas a carbonyl group works only as an acceptor. There is yet another possibility that relaxation from the second P 1 1 1 excited state ð1 þ g Þð Dg Þ to the lowest excited state ( Dg)( Dg) is accelerated by an organic solvent. We assume that gallic
complexes can be excited by energy transfer from acid-FeðCNÞ3 6 P 1 the two excited states of (1O2)2: 488 nm for ð1 þ g Þð Dg Þ and 1 1 633 nm ( Dg)( Dg), as shown in Fig. 1. If the relaxation from P 1 1 1 ð1 þ g Þð Dg Þ to ( Dg)( Dg) is not accelerated, the emission around 530 nm is dominant as in the case of DMF. On the other hand, if an organic solvent containing a hydroxyl group such as methanol accelerates the relaxation more effectively than those containing a carbonyl group such as DMF, the emission around 700 nm must be dominant. Then the intensity of the 700 nm emission band should increase with the concentration of methanol, although it is non-linear, as shown on the upper side of Fig. 3. The relaxation may be accelerated effectively even in the solution of DMF with the highest concentration, as shown on the lower side of Fig. 3, where the intensity of the 700 nm emission increases relatively to that of the 530 nm emission. If the relaxation is accelerated more effectively as the number of carbon chains in alcohols increases, then we can understand the fact that the intensity of 700 nm emission is enhanced in the order of methanol < ethanol < 2-propanol, as shown in Fig. 2. However, we still have no conclusive evidence on this interpretation, so that further experimental results are awaited to confirm this mechanism.
4. Summary Chemiluminescence spectra of H2O2/gallic acid/K3[Fe(CN)6] system containing organic solvents were measured with an FT-CL spectrometer. Two emission bands were observed around 530 and 700 nm. The 530 nm band was dominant in organic solvents with a carbonyl group such as N,N-dimethylformamide, while the 700 nm band was dominant in organic solvents with a hydroxyl group such as methanol. Relative intensities of the two emission bands strongly depended on the amounts of organic solvents. Since the wavelengths of these emissions were far from those of (1O2)2, the emitting species are attributed to the gallic transfer from the two acid-FeðCNÞ3 6 complexes excited by energy P 1 Þð Dg Þ and (1Dg)(1Dg). excited electronic states of ð1 O2 Þ2 ; ð1 þ g The observed solvent effects on the chemiluminescence spectra are interpreted in terms of intramolecular interactions between complex and organic solwater molecules, gallic acid-FeðCNÞ3 6 vents; the 700 nm emission band is dominant when the interaction between water molecules and organic solvents is strong like hydrogen bonding. Another possibility has been proposed to explain the solvent effects that organic solvents with a hydroxyl P 1 group such as methanol accelerate relaxation from ð1 þ g Þð Dg Þ to (1Dg)(1Dg) of (1O2)2.
H. Ishii et al. / Chemical Physics Letters 474 (2009) 285–289
Acknowledgement The authors thank Professor Kozo Kuchitsu (BASE, Tokyo University of A & T) for his helpful discussions. References [1] [2] [3] [4] [5] [6] [7]
A.U. Khan, M. Kasha, J. Am. Chem. Soc. 88 (1966) 1574. Y. Fu, A.A. Krasnovsky Jr., C.S. Foote, J. Am. Chem. Soc. 115 (1993) 10282. A.A. Gorman, I. Hamblett, J. Hill, J. Am. Chem. Soc. 117 (1995) 10751. S.T. Murphy, K. Kondo, C.S. Foote, J. Am. Chem. Soc. 121 (1999) 3751. S.R. Abbott, S. Ness, D.M. Hercules, J. Am. Chem. Soc. 92 (1970) 1128. T. Willson, J. Am. Chem. Soc. 91 (1969) 2387. P.T. Chou, Y.C. Chen, C.Y. Wei, M.Z. Lee, J. Am. Chem. Soc. 120 (1998) 4883.
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[8] A.A. Krasnovsky Jr., C.S. Foote, J. Am. Chem. Soc. 115 (1993) 6013. [9] A.A. Krasnovsky Jr., Y.V. Roumbal, A.V. Ivanov, R.V. Ambartzumian, Chem. Phys. Lett. 430 (2006) 260. [10] A.A. Krasnovsky Jr., Y.V. Roumbal, A.A. Strizhakov, Chem. Phys. Lett. 458 (2008) 195. [11] K. Tsukino, T. Satoh, H. Ishii, M. Nakata, Chem. Phys. Lett. 457 (2008) 444. [12] T. Miyazawa, K. Nakagawa, Biosci. Biotechnol. Biochem. 62 (1998) 829. [13] Y. Yoshiki, H. Yuan, T. Iida, M. Kawane, K. Okubo, T. Ishizawa, S. Kawabata, Photochem. Photobiol. 68 (1998) 802. [14] W. Adam, D.V. Kazakov, V.P. Kazakov, Chem. Rev. 105 (2005) 3371, and references cited therein. [15] S.A. Kazmi, M.S. Qureshi, Z. Maqsood, Inorg. Chim. Acta 137 (1987) 151. [16] A.E. Fazary, M. Taha, Y.H. Ju, J. Chem. Eng. Data 54 (2009) 35. [17] R. Chapelon, C. Balny, Mol. Photochem. 3 (1971) 255. [18] J.-M. Lin, M. Yamada, Anal. Chem. 71 (1999) 1760.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Singlet oxygen-sensitized delayed emissions from hydrogen peroxide/gallic acid/potassium ferricyanide systems containing organic solvents Hiroshi Ishii a, Kazuo Tsukino a, Masahiko Sekine b, Munetaka Nakata b,* a b
Japan Applied Technology Inc., Yaho, Kunitachi, Tokyo 186-0011, Japan Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, Naka-Cho, Koganei, Tokyo 184-8588, Japan
a r t i c l e
i n f o
Article history: Received 4 March 2009 In final form 23 April 2009 Available online 3 May 2009
a b s t r a c t Fourier-transform chemiluminescence spectra of H2O2/gallic acid/K3[Fe(CN)6] systems containing organic solvents were measured. Emission bands with peaks around 530 and 700 nm were observed in systems containing solvents with a carbonyl group such as N,N-dimethylformamide, and those with a hydroxyl group such as methanol, respectively. The relative band intensities depended strongly on the concentration of these organic solvents. The emission species are attributed to gallic acid–ferricyanide complexes excited by energy transfer from singlet oxygen dimol, (1O2)2. The effects of organic solvents are interpreted in terms of intermolecular interactions of gallic acid–ferricyanide complexes, water molecules and organic solvents. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Khan and Kasha [1] proposed in 1966 that emission from any energetically favorable species could be observed as chemiluminescence by the direct energy transfer from excited oxygen molecular pairs. A number of papers have since been published on the chemiluminescence of this origin, so-called ‘singlet oxygen-sensitized delayed fluorescence (SOSDF)’ [2–8]. A schematic energy diagram shown in Fig. 1 summarizes the mechanism of the generation of singlet oxygen dimol, (1O2)2, followed by the excitation of organic compounds, in which the lowest excited electronic state, (1Dg)(1Dg), and/or the second excited state, P 1 1 1 ð1 þ g Þð Dg Þ, of ( O2)2 are involved [1]. The excited states of ( O2)2 can be generated by (i) energy transfer from sensitizers [3,4,7,8] (path a), (ii) laser radiation (1270 nm, path b) [9,10], (iii) decomposition of peroxide (paths c and d) [2,5], etc. Processes (iii) result in chemiluminescence instead of fluorescence from organic compounds, but it is included here as a part of the mechanisms of SOSDF. The sensitizers used in past studies to generate excited (1O2)2 species were acetonaphtone [3], C60 [4,8], and phenalene-1-one [7]. One should note that in SOSDF the organic compounds excited by the energy transfer from (1O2)2 usually show emission with a wavelength longer than that corresponds to the transition from P 3 P the first excited state (1Dg)(1Dg) to the ground state ð3 g Þð gÞ of (1O2)2. In a recent Letter [11], we reported on the new design of a Fourier-transform chemiluminescence (FT-CL) spectrometer to observe weak chemiluminescence. This spectrometer was used to * Corresponding author. Fax: +81 42 388 7909. E-mail address: [email protected] (M. Nakata). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.04.062
measure the chemiluminescence spectra in decomposition of hydrogen peroxide (H2O2) in the presence of gallic acid (3,4,5-trihydroxybenzoic acid) and potassium ferricyanide (K3[Fe(CN)6]). This is known as a typical ternary system that evolves singlet oxygen molecules, where hydrogen peroxide reacts as an oxygen source, gallic acid as an efficient receptor of oxygen from hydrogen peroxide [12], and K3[Fe(CN)6] is a model of peroxidase in an enzyme-catalyzed reaction [13]. The wavelength of the observed broad emission band, 580 nm, differed from the reported values of the transitions in (1O2)2 from the excited electronic state, (1Dg)v=0(1Dg)v=0, to the ground state, P P ð3 Þ , 633 nm, and to the vibrationally excited ground ð3 g Þv ¼0 P g v3¼0 P 3 state, ð g Þv ¼1 ð g Þv ¼0 , 703 nm [14]. Since the observed emission was suppressed by addition of a small amount of sodium azide used as a quencher, we inferred that singlet oxygen played an important role as a trigger for the observed emissions. Since we observed strong dependence of the intensity and the peak wavelength on the concentration of K3[Fe(CN)6], we attributed the spectral features to the solvent effect based on the strong chargeand/or K+, but transfer interaction between (1O2)2 and FeðCNÞ3 6 no further evidence was provided. In a similar system such as H2O2/gallic acid/acetaldehyde studied by Yoshiki et al. [13], a characteristic dependence of chemiluminescence intensity on the concentrations of H2O2 and gallic acid was reported. Though the authors had suspected oxidation of acetaldehyde by H2O2 as one of the simplest mechanisms of this chemiluminescence, they excluded it as hardly realistic [13]. In the present Letter, we have extended our study by investigating the effect of organic solvents added to the H2O2/gallic acid/ K3[Fe(CN)6] system on the chemiluminescence. Our interest is
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H. Ishii et al. / Chemical Physics Letters 474 (2009) 285–289
Sensitizers S1
(1Σg+)(1Δ g)
T1 (path a)
Excited states
( Δ g)( Δ g) 1
(path b)
Ground state
Organic Compounds
Oxygen Dimol
1
(path c)
(path d)
Ground state
SOSDF
Ground state Decomposition of Peroxide
Fig. 1. An energy diagram for singlet oxygen-sensitized delayed emission. Excited states of singlet oxygen dimol are generated by energy transfer from sensitizer (path a), by excitation of oxygen in the ground state (path b), by decomposition of peroxide (paths c and d) etc. Organic compounds are excited by energy transfer from excited states of singlet oxygen dimol to give emissions.
focused on the question as to whether the emission bands originate from an excited state of the gallic acid and ferricyanide complex produced by the energy transfer from (1O2)2 generated in the dark decomposition of peroxide. To our knowledge, this is the first report on the chemiluminescence spectra observed in the whole region of visible radiation, which is caused by energy P 1 transfer from a state of higher energy, such as ð1 þ g Þð Dg Þ, than 1 1 1 the lowest exited electronic state, ( Dg)( Dg) of ( O2)2. The observed characteristic changes in the wavelength and intensity of chemiluminescence emissions from a variety of solutions containing organic solvents are discussed in terms of the intermolecular interaction involving the gallic acid–ferricyanide complex, water molecules and organic solvents, and the relaxation of the excited electronic states of (1O2)2. 2. Experimental method Chemiluminescence spectra were measured with an FT-CL spectrometer (MS-8310) developed by Japan Applied Technology Inc. [11]. The use of an optical system composed of a Savart plate and a polarizer and a data-acquisition system made of a charge coupled device enabled collection of weak emissions from twodimensional widespread area for an extended period. Water-miscible organic solvents, i.e., methanol, ethanol, acetone, N,N-dimethylformamide (DMF), 3-pentanone, and 2-propanol were used as co-solvents. All the chemicals with the highest attainable purity were purchased from Wako Pure Chemicals Ltd. and used without further purification. An aqueous solution of K3[Fe(CN)6] prepared freshly was poured to saturated gallic acid solution in a sample dish of 50 mm diameter. After addition of an organic solvent to the mixture, hydrogen peroxide (30%) was added and stirred vigorously, and then data-acquisition was started. The concentration of K3[Fe(CN)6] was either 200 or 1000 mM. The sampling time was set to 5 min so as to obtain a reasonable signal-to-noise ratio for a chemiluminescence spectrum. The temperature of the sample stage was kept at 300 K. 3. Results and discussion 3.1. Measurements of chemiluminescence spectra A chemiluminescence spectrum of the H2O2/gallic acid/ K3[Fe(CN)6] system, without organic solvents and with 200 mM of K3[Fe(CN)6], is displayed in Fig. 2a. This spectrum, where a broad emission band appears around 580 nm, is nearly identical to that reported in Ref. [11]. When an organic solvent of 2 ml was added to a solution containing H2O2, gallic acid, and K3[Fe(CN)6] of 1 ml
Fig. 2. Chemiluminescence spectra of the H2O2/gallic acid/K3[Fe(CN)6] system containing organic solvents: (a) no organic solvent, (b) DMF, (c) acetone, (d) 3pentanone, (e) methanol, (f) ethanol, and (g) 2-propanol. Solvents containing a carbonyl group are represented by closed circles, while those containing a hydroxyl group by closed triangles. The ratio of H2O2:gallic acid:K3[Fe(CN)6]:organic solvent was 1 ml:1 ml:1 ml:2 ml. The concentration of K3[Fe(CN)6] was 200 mM. Accumulation time for data collection was 5 min.
each, a remarkable spectral change was observed, as shown in Fig. 2b–g. By addition of DMF, for example, the peak wavelength of the chemiluminescence shifted from 580 to 530 nm with a significant enhancement of intensity. In contrast, a band with a peak at 700 nm appeared when 2-propanol was added; similar bands were observed by addition of methanol and ethanol. In summary, a broad emission around 530 nm was dominant for DMF (marked b), acetone (c), and 3-pentanone (d), whereas the band around 700 nm was dominant for methanol (e), ethanol (f), and 2-propanol (g). These two groups of organic solvents may be characterized by their functional groups; the former, which exhibited the 530 nm emission, contains a carbonyl group, whereas the latter that exhibited the 700 nm emission contains a hydroxyl group. A remarkable feature is that the 700 nm emission in the latter group is enhanced by extension of the carbon chain, in the order of methanol < ethanol < 2-propanol. We note that this spectral trend has first been detected in this study by the use of a spectrometer of the FT-CL type, unlike a normal experimental setup for measurements of chemiluminescence composed of a series of interference filters and a photomultiplier, with which no reliable calibration of relative intensities would be expected. To investigate the effect of organic solvents on the emission band intensity in more detail, chemiluminescence spectra were measured by varying the amounts of organic solvents relative to water. Typical spectral changes observed with the samples containing different concentrations of methanol are compared in Fig. 3. The spectrum of the most dilute sample a, with methanol:water of 1:100, showed an emission band at 530 nm with a shoulder band at 700 nm, being similar to that of pure water marked a in Fig. 2, as expected. In contrast, a spectrum of the sample with the highest concentration, 72:100 (d), showed a strong broad band at 700 nm but with a barely detectable 530 nm band. Similar experiments were carried out by adding various amounts of DMF to the H2O2/gallic acid/K3[Fe(CN)6] system. In contrast to the results for methanol, the observed chemiluminescence spectra exhibited less significant changes, as shown in Fig. 3; the band at 530 nm is dominant for samples with any amounts of DMF, while that around 700 nm is a shoulder of the 530 nm band. We note that the peak intensity depends on the concentration of DMF, but there is no apparent shift in the peak position in the molar ratio of DMF:water from 0.5:100 to 38:100.
H. Ishii et al. / Chemical Physics Letters 474 (2009) 285–289
Fig. 3. Chemiluminescence spectra of the H2O2/gallic acid/K3[Fe(CN)6] system containing methanol or DMF. The mol ratio of methanol to total water was 1:100 (a), 12:100 (b), 33:100 (c), 72:100 (d), while that of DMF to total water was 0.5:100 (a), 6:100 (b), 17:100 (c), 38:100 (d). The concentration of K3[Fe(CN)6] was 1000 mM. Accumulation time for data collection was 5 min.
3.2. Assignment of the chemiluminescence spectra As shown in Figs. 2 and 3, two distinct emission bands were observed at 530 and 700 nm for all the organic solvents studied and with different ratios to water, but their relative intensities were varied. One might assign these emission bands to (1O2)2, which is known to emit red lights around 633 nm from the excited electronic state, (1Dg)v=0(1Dg)v=0, to the ground state, P P 3 703 nm to the vibrationally exð3 g Þv ¼0 ð g Þv ¼0 , and that around P P 3 cited ground electronic state, ð3 g Þv ¼1 ð g Þv ¼0 [14]. However, the wavelength of 530 nm observed in our study deviates far from that of the 633 nm emission. The 700 nm emission is much closer to the reported 703 nm emission of (1O2)2, but one can hardly assign this emission to the vibrationally excited ground electronic state, 703 nm, because it has exceedingly higher emission intensity than that to the ground vibronic state, 633 nm. Therefore, it is hard to attribute the bands observed in the present study to the direct emission from (1O2)2. To solve the question on whether the two emission bands arise from (1O2)2, we have examined the effect of a singlet oxygen quencher, sodium azide, on the chemiluminescence. Fig. 4 shows the spectra with and without sodium azide in solutions containing methanol or DMF. The emission bands of 700 nm are dominant for methanol, whereas those of 530 nm are dominant for DMF, as reported in 3.1. When 10 mM of sodium azide was added to the solutions, both emission bands at 700 and 530 nm were clearly suppressed; this led us to the conclusion that the two emis-
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Fig. 4. Influence of sodium azide, a scavenger for singlet oxygen, on the chemiluminescence spectra of H2O2/gallic acid/K3[Fe(CN)6] systems containing methanol or DMF. The amounts of H2O2, gallic acid, K3[Fe(CN)6], and organic solvent were 1, 1, 0.5, and 2 ml, respectively. An amount of 0.5 ml of water (solid line) or 10 mM sodium azide solution (dashed line) was added to the solution 2 min after the reaction started. The concentration of K3[Fe(CN)6] was 600 mM in common, and the accumulation time for data collection was 5 min.
sion bands originate only indirectly from the (1O2)2 generated in decomposition of hydrogen peroxide in the present system. Our additional finding is that the suppression in the methanol case is apparently less significant than that in the DMF case. We attribute this observation to the dynamic behavior of the electronic states of P singlet oxygen, 1D and 1 þ g ; see 3.3. Gallic acid solution is known to have no absorption in the visible-light region. However, it seems reasonable to choose a gallic acid-FeðCNÞ3 6 complex as a candidate to account for the two emission bands, because gallic acid forms complexes with Fe3+ ion [15] in aqueous solutions with Fe3+ ion:gallic acid ratios of 1:1, 1:2 and 1:3; water molecule plays an additional ligand in the 1:2 complex. It is also reported that a ternary complex of gallic acid/Fe3+ ion/glycine (1:1:1) showed an absorption spectrum with a peak at 530 nm [16], which is similar to one of the emissions of our observation. One can thus assume that the energy of (1O2)2 generated in the decomposition of peroxide is transferred to gallic acid-FeðCNÞ3 6 complexes like the energy transfer from singlet oxygen to its acceptors, phthalocyanin [2–4], dibenzanthrone [5], rubrene [6], and Nile Blue A perchlorate [7], as discussed in Introduction. One can also assume that organic molecules excited by energy transfer from (1O2)2 emit radiation with wavelength longer than 633 nm due to the transition from the lowest excited electronic state, P P 3 (1Dg)v =0(1Dg)v =0, to the ground state ð3 g Þv ¼0 ð g Þv ¼0 . This holds in cases of phthalocyanin [2–4], Nile Blue A perchlorate [7], and dibenzanthrone [5], which show emission bands around 710,
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(a) Weak Interaction between Water and Solvent Energy Transfer Solvent H2O2
Gallic Acid
Solvent
Water
Gallic Acid (O2) 2*
H2O2 Fe(CN) 3+ 6
Fe(CN)6
Water
3+
~ 530 nm emission
(b) Strong Interaction between Water and Solvent Energy Transfer Solvent H2O2 H2O2
Gallic Acid
Solvent
Water
Fe(CN)63+
(O2) 2*
Gallic Acid
Water
~ 700 nm emission
Fe(CN)63+
Fig. 5. A schematic model to interpret two distinct emissions appearing around 530 and 700 nm: (a) Weak interaction between water and solvents such as DMF, and gallic Pþ 1 1 1 nm emission. (b) Strong interaction between water and solvents such acid–FeðCNÞ3þ 6 complex excited by energy transfer from the excited state ð g Þð Dg Þ of ( O2)2 gives 530 Pþ 3þ as methanol, and gallic acid–FeðCNÞ6 complex excited by energy transfer from the excited state ð1 g Þð1 Dg Þ and/or (1Dg)(1Dg) of (1O2)2 gives 700 nm emission.
650, and 660 nm, respectively. Conversely, the energy of the 530 nm emission observed in the present study is higher than that of the 633 nm emission. It is possible that energy transfer P 1 from a higher state of (1O2)2, for example ð1 þ g Þv ¼0 ð Dg Þv ¼0 , occurs in the decomposition of peroxide in the present system; a similar mechanism of energy transfer has been proposed for the thermal decomposition of photo-oxidized 1,4-dimethoxy-9,10-dipyridylanthracene [17] and for a reaction of pyrogallol with IO 4 in KOH/ K2CO3 solution [18], but only the emission intensities measured with a series of interference filters were reported. 3.3. Interpretation of the solvent effects No chemiluminescence spectra were observed with our spectrometer when one of the chemical species in the present system, i.e., H2O2, gallic acid, or K3[Fe(CN)6], was omitted. This finding implies that all the three species interact with one another and play important roles in the emission mechanism. Though the structure of gallic acid-FeðCNÞ3 6 complex is still unknown, one needs to consider at least two types, because two emission bands have been observed at 530 and 700 nm. We can assume from our observations that the relative abundance of these complexes depends strongly on the amount of organic solvents. Our interpretation of this solvent effects is schematically presented in Fig. 5: Hydrogen peroxide is decomposed to produce electronically excited (1O2)2 by a catalyst of gallic acid-FeðCNÞ3þ 6 complex. This complex, by interaction with water molecules and by energy transfer from (1O2)2, is led to emissions. If an organic solvent is added to the solution, water molecules may interact with the organic solvent as well as gallic acid–FeðCNÞ3þ 6 complex. If the intermolecular interaction between water and the organic solvent is weak, as in the case of DMF shown in Fig. 5a, an emission band due to the complex appears around 530 nm. If the interaction between water molecules and the organic solvent is much stronger, as in the case of methanol with hydrogen bonding, the interaction between the complex and water molecules is relatively weakened, as shown in Fig. 5b, so that the emission from the complex appears around 700 nm. In other words, the emission of gallic acid-FeðCNÞ3 6 complex depends on whether water molecules interact with the complex or with the organic solvent. This interpretation is supported by the fact that a hydroxyl group can work both as a hydrogen donor and an acceptor, whereas a carbonyl group works only as an acceptor. There is yet another possibility that relaxation from the second P 1 1 1 excited state ð1 þ g Þð Dg Þ to the lowest excited state ( Dg)( Dg) is accelerated by an organic solvent. We assume that gallic
complexes can be excited by energy transfer from acid-FeðCNÞ3 6 P 1 the two excited states of (1O2)2: 488 nm for ð1 þ g Þð Dg Þ and 1 1 633 nm ( Dg)( Dg), as shown in Fig. 1. If the relaxation from P 1 1 1 ð1 þ g Þð Dg Þ to ( Dg)( Dg) is not accelerated, the emission around 530 nm is dominant as in the case of DMF. On the other hand, if an organic solvent containing a hydroxyl group such as methanol accelerates the relaxation more effectively than those containing a carbonyl group such as DMF, the emission around 700 nm must be dominant. Then the intensity of the 700 nm emission band should increase with the concentration of methanol, although it is non-linear, as shown on the upper side of Fig. 3. The relaxation may be accelerated effectively even in the solution of DMF with the highest concentration, as shown on the lower side of Fig. 3, where the intensity of the 700 nm emission increases relatively to that of the 530 nm emission. If the relaxation is accelerated more effectively as the number of carbon chains in alcohols increases, then we can understand the fact that the intensity of 700 nm emission is enhanced in the order of methanol < ethanol < 2-propanol, as shown in Fig. 2. However, we still have no conclusive evidence on this interpretation, so that further experimental results are awaited to confirm this mechanism.
4. Summary Chemiluminescence spectra of H2O2/gallic acid/K3[Fe(CN)6] system containing organic solvents were measured with an FT-CL spectrometer. Two emission bands were observed around 530 and 700 nm. The 530 nm band was dominant in organic solvents with a carbonyl group such as N,N-dimethylformamide, while the 700 nm band was dominant in organic solvents with a hydroxyl group such as methanol. Relative intensities of the two emission bands strongly depended on the amounts of organic solvents. Since the wavelengths of these emissions were far from those of (1O2)2, the emitting species are attributed to the gallic transfer from the two acid-FeðCNÞ3 6 complexes excited by energy P 1 Þð Dg Þ and (1Dg)(1Dg). excited electronic states of ð1 O2 Þ2 ; ð1 þ g The observed solvent effects on the chemiluminescence spectra are interpreted in terms of intramolecular interactions between complex and organic solwater molecules, gallic acid-FeðCNÞ3 6 vents; the 700 nm emission band is dominant when the interaction between water molecules and organic solvents is strong like hydrogen bonding. Another possibility has been proposed to explain the solvent effects that organic solvents with a hydroxyl P 1 group such as methanol accelerate relaxation from ð1 þ g Þð Dg Þ to (1Dg)(1Dg) of (1O2)2.
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Acknowledgement The authors thank Professor Kozo Kuchitsu (BASE, Tokyo University of A & T) for his helpful discussions. References [1] [2] [3] [4] [5] [6] [7]
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