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Formation of an exciplex with mixed ligands in the mercury-photosensitized luminescence of alcohol–amine mixtures in the liquid phase
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Spectrochimica Acta Part A 70 (2008) 265–269
Formation of an exciplex with mixed ligands in the mercury-photosensitized luminescence of alcohol–amine mixtures in the liquid phase Shunzo Yamamoto ∗ , Michiyo Uno, Yoshimi Sueishi Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Received 28 June 2006; received in revised form 14 July 2007; accepted 25 July 2007
Abstract The liquid-phase mercury-photosensitized luminescence of tert-butyl alcohol (TL)–tert-butylamine (TM) mixtures has been investigated by a steady-state illumination method over a wide range of substrate concentrations. The emission bands from exciplexes (HgTL* and HgTM*) between an excited mercury atom and an alcohol or an amine molecule were observed at about 330 nm and 370 nm, respectively, in TL and TM solutions in cyclohexane. Two other bands appeared at 405 nm and 455 nm for TM at high concentrations. These bands were previously assigned to two types of 1:2 exciplexes (HgTM2 * and HgTM2 **). In TL–TM mixed solutions, a new band appeared at about 400 nm. The intensity of this band increased with increasing concentrations of TL and TM. This band was attributed to an exciplex with mixed ligands (HgTLTM*). This band was observed for the first time in this study. The energized intermediate, (HgTLTM*)= , formed between HgAL* and AM can be effectively stabilized by collisions with solvent molecules in solution, while it decomposes to HgAM* and AL in the gas phase. The results for TL–TM mixtures can be explained by the proposed reaction mechanism. © 2007 Elsevier B.V. All rights reserved. Keywords: Photosensitized luminescence; Mercury exciplex; Mixed ligands; Emission band
1. Introduction It is well known that excited triplet Cd [1] and Hg [2] atoms form charge-transfer complexes with some compounds containing an N or an O atom in the gas phase. The exciplexes for alcohols give smooth, structureless, symmetrical emission bands at about 300 nm (Hg) and 400 nm (Cd), and those for amines give emission bands at about 340 nm (Hg) and 450 nm (Cd). The mercury-photosensitized luminescence of amine (AM)–alcohol (AL) mixtures has been investigated in the gas phase [3–6]. It was found that for AM–AL mixtures, the intensity of emission band from an HgAL* complex decreased with increasing amine pressure, while the emission intensity from an HgAM* complex increased upon addition of AL. It was reported that the ligand-exchange reaction, HgAL* + AM → HgAM* + AL, effectively occurred in AM–AL mixtures. We have investigated the liquid-phase mercury-photosensitized luminescence of water, saturated alcohols, ethers and
∗
Corresponding author. Tel.: +81 86 251 7835; fax: +81 86 251 7853. E-mail address: [email protected] (S. Yamamoto).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.07.040
amines [7], and found that the emission bands from exciplexes (HgS*) between an excited mercury atom and solvent molecules shifted to the red as the solvent polarity increases. We also observed an emission band from an exciplex (HgAM*) between an excited mercury atom and amine molecule in some solutions of the amine in alkanes, alcohols and ethers. This band also shifts to the red with increasing solvent polarity. The dipole moment of HgAM* was estimated from the solvatochromic shift of the exciplex emission band. It was found that a new band appeared with some dialkylamines at high concentrations on the long-wavelength side of the band of HgAM*, while two new bands appeared for some monoalkylamines at high concentrations. The intensity ratios of the bands observed at high concentrations to that of HgAM* were found to be proportional to the concentrations of the amines. These bands were assigned to 1:2 exciplexes (HgAM2 * for dialkylamines and HgAM2 * and HgAM2 ** for alkylamines) [8]. Since the concentrations of substrates can be changed to high values in solution, we have investigated the mercuryphotosensitized luminescence of tert-butylamine (TM)–tertbutyl alcohol (TL) mixtures over a wide range of concentrations.
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Fig. 1. Normalized emission spectra for mercury-photosensitized luminescence of TM at 25 ◦ C at 0.0063 (1), 0.032 (2), 0.063 (3), 0.094 (4) and 0.160 mol dm−3 (5) in cyclohexane.
We have found emission spectra with three to five component bands which are attributed to different types of exciplexes. The present paper reports the details of mercury-photosensitized luminescence of TL–TM mixtures in the liquid phase. 2. Experimental 2.1. Materials tert-Butylamine (TM) was used after drying with potassium hydroxide and trap-to-trap distillation, and tert-butyl alcohol (TL) (G.R. grade) was used as supplied. Cyclohexane (G.R. grade) was used after drying and distillation. Naphthalene (G.R. grade) was used after recrystallization.
Fig. 3. Normalized emission spectra for mercury-photosensitized luminescence of TM–TL mixtures in cyclohexane at 25 ◦ C. [TM] = 0.0090 mol dm−3 , [TL] = 0 (1), 0.035 (2), 0.14 (3), 0.34 (4) and 0.66 mol dm−3 (5).
irradiation at 254 nm by means of a Shimadzu spectrofluorophotometer, model RF-1500 at 25 ◦ C. The solutions of mercury prepared by this method were quite stable and no microscopic droplets of mercury were observed in the solution. The quantum yield of mercury-photosensitized luminescence of TL was estimated using the fluorescence of naphthalene as a standard (φ = 0.23 ± 0.02 in cyclohexane [9]). 3. Results
For emission spectra measurements, solutions of mercury in cyclohexane including TL and TM were prepared by distilling cyclohexane with a drop of mercury under a nitrogen atmosphere, and by adding known amounts of TL and TM with a microsyringe. The emission spectra were measured upon
Absorption spectra of a solution of mercury in cyclohexane showed an absorption band with a doublet structure peaking at around 260 nm. The absorption spectrum is consistent with that reported previously [10]. This band was attributed to the 1 S → 3 P transition of atomic mercury, and emission from 0 1 Hg(3 P1 ) atoms was not observed in cyclohexane, but sensitized luminescence was observed on addition of amines and alcohols, indicating that relaxation from the 3 P1 state to the 3 P0 state by collision with solvent molecules must be fast. Hg(3 P0 ) atoms, but not Hg(3 P1 ) atoms were present in cyclohexane under irradiation at 254 nm.
Fig. 2. Concentration dependences of the intensity of band A for TL () and total intensity for TM (). [Solid lines show the values calculated by Eqs. (1) and (2). See text.]
Fig. 4. Emission spectrum for mercury-photosensitized luminescence of TM–TL mixture ([TM] = 0.0090 and [TL] = 0.14 mol dm−3 ) and its decomposition into three Gaussian curves.
2.2. Measurement
S. Yamamoto et al. / Spectrochimica Acta Part A 70 (2008) 265–269
Fig. 5. Concentration dependences of the intensities of bands A (), B () and C () for mercury-photosensitized luminescence of TM–TL mixtures in cyclohexane at 25 ◦ C. [TM] = 0.0090 mol dm−3 .
The emission spectra were obtained from the mercury solution in cyclohexane containing TL and TM. For TL, the shape of the emission band (band A) did not depend on the alcohol concentration and the band showed a slight red-shift with increasing alcohol concentration. This shift must be due to the increase in solvent polarity upon addition of TL. The emission intensity increased and reached a constant value with increasing alcohol concentration. From the constant value, the quantum yield of luminescence was obtained (φ = 0.061). Fig. 1 shows emission spectra obtained for the mercuryphotosensitized reaction of different concentrations of TM in cyclohexane. As mentioned previously [8], the emission spectra of alkylamines such as propylamine, butylamine and TM obtained at high concentrations of amines could be decomposed into three component bands (shown previously for propylamine). These bands were assigned to a 1:1 (band B) and two 1:2 (bands D and E) mercury exciplexes with amine. The emission spectrum of TM at 0.0063 mol dm−3 consists almost exclusively of band B. Fig. 2 shows the plots of the emission intensities versus [TL] or [TM] in mercury–TL and mercury–TM systems.
267
Fig. 7. Emission spectrum for mercury-photosensitized luminescence of TM–TL mixture ([TM] = 0.10 and [TL] = 0.34 mol dm−3 ) and its decomposition into five Gaussian curves.
Fig. 3 shows the emission spectra obtained for the solution containing TM at 0.0090 mol dm−3 and TL at various concentrations. With increasing TL concentration, the intensity of the band for TM (band B) decreases and the band for TL (band A) appears and its intensity increases. Fig. 3 also shows that the intensity of the long-wavelength edge increases. Fig. 4 shows a decomposition of the emission spectrum of TM–TL mixtures obtained at [TM] = 0.0090 mol dm−3 and [TL] = 0.34 mol dm−3 into three Gaussian curves. The spectra obtained at other concentration of TL shown in Fig. 3 could also be separated into three component bands (bands A, B and C for shortest-, middle- and longest-wavelength bands). Their shapes and positions are independent of the TL concentration, but their relative intensities change with concentration. These findings indicate that the three bands are ascribed to three different emitters, and their existence ratios change with TL concentration. Since the shape and position of bands A and B are consistent with those of the emission band of TL and the TM monomer band, these bands can be assigned to the HgTL* and HgTM* exciplexes, respectively. The shape and position of band C are inconsistent with those of 1:2 exciplexes observed for TM. Band C was observed in amine–alcohol mixtures for the first time. This band may be attributed to the exciplex with mixed ligands (HgTLTM*). Fig. 5 shows the concentration dependences of the intensities of bands A–C. Fig. 6 shows the emission spectra obtained for the solutions containing TM at 0.10 mol dm−3 and TL at various concentrations. With increasing TL concentration, the intensities of the bands for TM decrease and the intensity of the band for TL increases. The emission spectrum obtained at [TM] = 0.10 mol dm−3 and [TL] = 0.34 mol dm−3 consists of five bands (Fig. 7). 4. Discussion
Fig. 6. Normalized emission spectra for mercury-photosensitized luminescence of TM–TL mixtures in cyclohexane at 25 ◦ C. [TM] = 0.10 mol dm−3 , [TL] = 0 (1), 0.070 (2), 0.14 (3), 0.21 (4) and 0.34 mol dm−3 (5).
To explain the experimental results obtained for TL, TM and TL–TM mixtures in this study, the following set of reactions is
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considered under low TM concentration conditions: Hg(1 S0 ) + hν → Hg(3 P1 )∗
I0
Hg(3 P1 )∗ + S → Hg(3 P0 )∗ + S
k0
Hg(3 P0 )∗ + S → Hg(1 S0 ) + S
k1
Hg(3 P0 )∗ + TL → HgTL∗
k2
Hg(3 P0 )∗ + TM → HgTM∗
k3
HgTL∗ → Hg(1 S0 ) + TL + hνA HgTL∗ → Hg(1 S0 ) + TL
αk4
HgTM∗ → Hg(1 S0 ) + TM + hνB ∗
HgTM → Hg( S0 ) + TM HgTL + TM → HgTLTM
βk5
∗
HgTL∗ + TM → HgTM∗ + TL
IA = I0 α
γk6 (1 − γ)k6
HgTLTM∗ → Hg(1 S0 ) + TL + TM + hνC ∗
HgTLTM → Hg( S0 ) + TL + TM 1
∗
IB = I0 β δk7
k4 k2 [TL] k1 + k2 [TL] + k3 [TM] k4 + k6 [TM]
(3)
k3 [TM] k1 + k2 [TL] + k3 [TM]
+I0 β
(1 − δ)k7
HgTLTM + TL → Hg( S0 ) + 2TL + TM 1
Applying the steady-state approximation to TL–TM mixtures, the following relations are obtained:
(1 − β)k5
1
∗
Fig. 8. Effect of TL on the emission intensity of band A for TM–TL mixtures in cyclohexane at 25 ◦ C (concentrations of TM and TL are the same as those for Fig. 3). [Solid line shows the values calculated by Eq. (3). See text.]
(1 − α)k4
k2 [TL] (1 − γ)k6 [TM] k1 + k2 [TL] + k3 [TM] k4 + k6 [TM]
(4)
and
k8
Here, S denotes solvent, HgTL* and HgTM* are mercury exciplexes with TL and TM, and HgTLTM* denotes a mercury exciplex with mixed ligands. HgTL*, HgTM* and HgTLTM* fluoresce producing bands A, B, and C, respectively. Since it was observed that the intensity of band A decreased on the addition of TM, while that of band B did not decrease on the addition of TL, the reaction, HgAM* + AL → HgTLTM*, is not included in the above reaction mechanism. As mentioned above, the emission from Hg(3 P1 ) atoms cannot be observed in the liquid phase, this indicates that relaxation from the 3 P1 state to the 3 P0 state by collision with solvent molecules is fast. Applying the steady-state approximation, the following relations are obtained for mercury–TL and mercury–TM systems: IA = I0
αk2 [TL] k1 + k2 [TL]
(1)
IB = I0
βk3 [TM] k1 + k3 [TM]
(2)
where α and β denote the fractions of emission processes in the deactivations of HgTL* and HgTM*. The solid lines in Fig. 2 show the values calculated by Eqs. (1) and (2) using the following constants and ratios of the rate constants: αI0 = 1.16, βI0 = 1.09, k2 /k1 = 6.70 mol−1 dm3 and k3 /k1 = 13.0 mol−1 dm3 .
k1 + k3 [TM] IB = 0 k1 + k2 [TL] + k3 [TM] IB +
k2 [TL] k1 + k3 [TM] (1 − γ)k6 [TM] k3 [TM] k1 + k2 [TL] + k3 [TM] k4 + k6 [TM]
(5)
where IB0 is the emission intensity of band B in the absence of TL and is given by Eq. (2). Figs. 8 and 9 show plots of IA and IB /IB0 versus [TL]. The solid lines in Figs. 8 and 9 show the values calculated by Eqs. (3) and (5) using the ratio of the rate constants
Fig. 9. Effect of TL on the emission intensity ratio of band B (IB /IB0 ) for TM–TL mixtures in cyclohexane at 25 ◦ C (concentrations of TM and TL are the same as those for Fig. 3). [Solid line shows the values calculated by Eq. (4). See text.]
S. Yamamoto et al. / Spectrochimica Acta Part A 70 (2008) 265–269
269
The energized intermediate, (HgTLTM*)= , formed between HgTL* and TM can be stabilized by collisions with solvent molecules in solution, but it may decompose effectively into HgTM* + TL in the gas phase (the ligand exchange reaction, HgTL* + TM → HgTM* + TL, is exothermic). As mentioned above, the emission spectrum obtained for a TM–TL mixture at [TM] = 0.10 mol dm−3 and [TL] = 0.34 mol dm−3 could be decomposed into five component bands (Fig. 7). The five emission bands are those from HgTL* (band A), HgTM* (band B), HgTLTM* (band C), HgTM2 * (band D) and HgTM2 ** (band E). Therefore, in this case we could observe all of the possible emission bands. Fig. 10. Effect of TL on the intensity ratio of band C to band A for TM–TL mixtures in cyclohexane at 25 ◦ C (concentrations of TM and TL are the same as those for Fig. 3). [Solid line shows the values calculated by Eq. (5). See text.]
(k6 /k4 = 140 mol−1 dm3 ), γ = 0.93 and the values mentioned above for the ratios of rate constants k2 /k1 and k3 /k1 . As shown in Figs. 8 and 9, the agreements between the observed and the calculated values are good. The following equation for the intensity ratio of band C to band A is also obtained by applying the steady-state approximation to the TM–TL mixtures: IC k7 δ γk6 [TM] = (6) α k4 k7 + k8 [TL] IA Fig. 10 shows plots of IC /IA against [TL]. Since the values of γ and k6 /k4 have already been obtained, the value of IC /IA can be calculated by Eq. (5), assuming the values of γ/α and k8 /k7 . The solid line in Fig. 10 shows the values calculated using γ/α = 0.66 and k8 /k7 = 1.76 mol−1 dm3 . The agreement between the observed and the calculated values for IC /IA is good. The emission band from a new type of mercury exciplex (HgTLTM*) was observed for the first time in mixed solutions of TL, TM and mercury in cyclohexane. The concentration dependences of the intensities of bands A–C could be explained well by the proposed reaction mechanism. As shown in previous papers, the following ligand-exchange reaction in an exciplex occurs effectively in the gas phase: HgTL∗ + TM → HgTM∗ + TL However, in solution this reaction is only minor, and the formation of the exciplex with mixed ligands is a major process. This difference between the major reactions in solution and in the gas phase can be explained by the following reactions: HgTL∗ + TM → (HgTLTM∗ )= (HgTLTM∗ )= + S → HgTLTM∗ (HgTLTM∗ )= → HgTM∗ + TL
5. Conclusion The emission bands from HgTL* and HgTM* exciplexes appeared at about 330 nm and 370 nm, respectively, in TL and TM solution in cyclohexane. Two other bands were observed at 405 nm and 455 nm for TM at high concentrations. These bands were previously assigned to two types of 1:2 exciplexes. The emission band from an exciplex with mixed ligands (HgTLTM*) was observed in TL–TM mixed solution for the first time. As shown previously, the following ligandexchange reaction in an exciplex occurs effectively in the gas phase: HgTL∗ + TM → HgTM∗ + TL In the liquid phase, however, the formation of the exciplex with mixed ligands was found to be a major process. The energized intermediate, (HgTLTM*)= , formed between HgTL* and TM can be stabilized by collisions with solvent molecules in solution, but it may decompose effectively into HgTM* + TL in the gas phase. References [1] L.F. Phillips, Acc. Chem. Res. 7 (1974) 135. [2] A.B. Callear, Chem. Rev. 87 (1987) 355. [3] S. Yamamoto, T. Nagaoka, Y. Sueishi, N. Nishimura, Chem. Lett. (1992) 621. [4] S. Yamamoto, T. Nagaoka, Y. Sueishi, N. Nishimura, J. Chem. Soc., Faraday Trans. 90 (1994) 2021. [5] S. Yamamoto, T. Ohba, J. Chem. Soc., Faraday Trans. 92 (1996) 729. [6] S. Yamamoto, M. Ikeda, Y. Sueishi, Phys. Chem. Chem. Phys. 2 (2000) 335. [7] S. Yamamoto, M. Doi, J. Chem. Soc., Faraday Trans. 93 (1997) 729. [8] S. Yamamoto, M. Doi, N. Ban, J. Chem. Soc., Faraday Trans. 94 (1998) 2361. [9] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, London, 1965. [10] M.K. Phibbs, B.de.B. Darwent, J. Chem. Phys. 18 (1950) 679.
Spectrochimica Acta Part A 70 (2008) 265–269
Formation of an exciplex with mixed ligands in the mercury-photosensitized luminescence of alcohol–amine mixtures in the liquid phase Shunzo Yamamoto ∗ , Michiyo Uno, Yoshimi Sueishi Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Received 28 June 2006; received in revised form 14 July 2007; accepted 25 July 2007
Abstract The liquid-phase mercury-photosensitized luminescence of tert-butyl alcohol (TL)–tert-butylamine (TM) mixtures has been investigated by a steady-state illumination method over a wide range of substrate concentrations. The emission bands from exciplexes (HgTL* and HgTM*) between an excited mercury atom and an alcohol or an amine molecule were observed at about 330 nm and 370 nm, respectively, in TL and TM solutions in cyclohexane. Two other bands appeared at 405 nm and 455 nm for TM at high concentrations. These bands were previously assigned to two types of 1:2 exciplexes (HgTM2 * and HgTM2 **). In TL–TM mixed solutions, a new band appeared at about 400 nm. The intensity of this band increased with increasing concentrations of TL and TM. This band was attributed to an exciplex with mixed ligands (HgTLTM*). This band was observed for the first time in this study. The energized intermediate, (HgTLTM*)= , formed between HgAL* and AM can be effectively stabilized by collisions with solvent molecules in solution, while it decomposes to HgAM* and AL in the gas phase. The results for TL–TM mixtures can be explained by the proposed reaction mechanism. © 2007 Elsevier B.V. All rights reserved. Keywords: Photosensitized luminescence; Mercury exciplex; Mixed ligands; Emission band
1. Introduction It is well known that excited triplet Cd [1] and Hg [2] atoms form charge-transfer complexes with some compounds containing an N or an O atom in the gas phase. The exciplexes for alcohols give smooth, structureless, symmetrical emission bands at about 300 nm (Hg) and 400 nm (Cd), and those for amines give emission bands at about 340 nm (Hg) and 450 nm (Cd). The mercury-photosensitized luminescence of amine (AM)–alcohol (AL) mixtures has been investigated in the gas phase [3–6]. It was found that for AM–AL mixtures, the intensity of emission band from an HgAL* complex decreased with increasing amine pressure, while the emission intensity from an HgAM* complex increased upon addition of AL. It was reported that the ligand-exchange reaction, HgAL* + AM → HgAM* + AL, effectively occurred in AM–AL mixtures. We have investigated the liquid-phase mercury-photosensitized luminescence of water, saturated alcohols, ethers and
∗
Corresponding author. Tel.: +81 86 251 7835; fax: +81 86 251 7853. E-mail address: [email protected] (S. Yamamoto).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.07.040
amines [7], and found that the emission bands from exciplexes (HgS*) between an excited mercury atom and solvent molecules shifted to the red as the solvent polarity increases. We also observed an emission band from an exciplex (HgAM*) between an excited mercury atom and amine molecule in some solutions of the amine in alkanes, alcohols and ethers. This band also shifts to the red with increasing solvent polarity. The dipole moment of HgAM* was estimated from the solvatochromic shift of the exciplex emission band. It was found that a new band appeared with some dialkylamines at high concentrations on the long-wavelength side of the band of HgAM*, while two new bands appeared for some monoalkylamines at high concentrations. The intensity ratios of the bands observed at high concentrations to that of HgAM* were found to be proportional to the concentrations of the amines. These bands were assigned to 1:2 exciplexes (HgAM2 * for dialkylamines and HgAM2 * and HgAM2 ** for alkylamines) [8]. Since the concentrations of substrates can be changed to high values in solution, we have investigated the mercuryphotosensitized luminescence of tert-butylamine (TM)–tertbutyl alcohol (TL) mixtures over a wide range of concentrations.
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Fig. 1. Normalized emission spectra for mercury-photosensitized luminescence of TM at 25 ◦ C at 0.0063 (1), 0.032 (2), 0.063 (3), 0.094 (4) and 0.160 mol dm−3 (5) in cyclohexane.
We have found emission spectra with three to five component bands which are attributed to different types of exciplexes. The present paper reports the details of mercury-photosensitized luminescence of TL–TM mixtures in the liquid phase. 2. Experimental 2.1. Materials tert-Butylamine (TM) was used after drying with potassium hydroxide and trap-to-trap distillation, and tert-butyl alcohol (TL) (G.R. grade) was used as supplied. Cyclohexane (G.R. grade) was used after drying and distillation. Naphthalene (G.R. grade) was used after recrystallization.
Fig. 3. Normalized emission spectra for mercury-photosensitized luminescence of TM–TL mixtures in cyclohexane at 25 ◦ C. [TM] = 0.0090 mol dm−3 , [TL] = 0 (1), 0.035 (2), 0.14 (3), 0.34 (4) and 0.66 mol dm−3 (5).
irradiation at 254 nm by means of a Shimadzu spectrofluorophotometer, model RF-1500 at 25 ◦ C. The solutions of mercury prepared by this method were quite stable and no microscopic droplets of mercury were observed in the solution. The quantum yield of mercury-photosensitized luminescence of TL was estimated using the fluorescence of naphthalene as a standard (φ = 0.23 ± 0.02 in cyclohexane [9]). 3. Results
For emission spectra measurements, solutions of mercury in cyclohexane including TL and TM were prepared by distilling cyclohexane with a drop of mercury under a nitrogen atmosphere, and by adding known amounts of TL and TM with a microsyringe. The emission spectra were measured upon
Absorption spectra of a solution of mercury in cyclohexane showed an absorption band with a doublet structure peaking at around 260 nm. The absorption spectrum is consistent with that reported previously [10]. This band was attributed to the 1 S → 3 P transition of atomic mercury, and emission from 0 1 Hg(3 P1 ) atoms was not observed in cyclohexane, but sensitized luminescence was observed on addition of amines and alcohols, indicating that relaxation from the 3 P1 state to the 3 P0 state by collision with solvent molecules must be fast. Hg(3 P0 ) atoms, but not Hg(3 P1 ) atoms were present in cyclohexane under irradiation at 254 nm.
Fig. 2. Concentration dependences of the intensity of band A for TL () and total intensity for TM (). [Solid lines show the values calculated by Eqs. (1) and (2). See text.]
Fig. 4. Emission spectrum for mercury-photosensitized luminescence of TM–TL mixture ([TM] = 0.0090 and [TL] = 0.14 mol dm−3 ) and its decomposition into three Gaussian curves.
2.2. Measurement
S. Yamamoto et al. / Spectrochimica Acta Part A 70 (2008) 265–269
Fig. 5. Concentration dependences of the intensities of bands A (), B () and C () for mercury-photosensitized luminescence of TM–TL mixtures in cyclohexane at 25 ◦ C. [TM] = 0.0090 mol dm−3 .
The emission spectra were obtained from the mercury solution in cyclohexane containing TL and TM. For TL, the shape of the emission band (band A) did not depend on the alcohol concentration and the band showed a slight red-shift with increasing alcohol concentration. This shift must be due to the increase in solvent polarity upon addition of TL. The emission intensity increased and reached a constant value with increasing alcohol concentration. From the constant value, the quantum yield of luminescence was obtained (φ = 0.061). Fig. 1 shows emission spectra obtained for the mercuryphotosensitized reaction of different concentrations of TM in cyclohexane. As mentioned previously [8], the emission spectra of alkylamines such as propylamine, butylamine and TM obtained at high concentrations of amines could be decomposed into three component bands (shown previously for propylamine). These bands were assigned to a 1:1 (band B) and two 1:2 (bands D and E) mercury exciplexes with amine. The emission spectrum of TM at 0.0063 mol dm−3 consists almost exclusively of band B. Fig. 2 shows the plots of the emission intensities versus [TL] or [TM] in mercury–TL and mercury–TM systems.
267
Fig. 7. Emission spectrum for mercury-photosensitized luminescence of TM–TL mixture ([TM] = 0.10 and [TL] = 0.34 mol dm−3 ) and its decomposition into five Gaussian curves.
Fig. 3 shows the emission spectra obtained for the solution containing TM at 0.0090 mol dm−3 and TL at various concentrations. With increasing TL concentration, the intensity of the band for TM (band B) decreases and the band for TL (band A) appears and its intensity increases. Fig. 3 also shows that the intensity of the long-wavelength edge increases. Fig. 4 shows a decomposition of the emission spectrum of TM–TL mixtures obtained at [TM] = 0.0090 mol dm−3 and [TL] = 0.34 mol dm−3 into three Gaussian curves. The spectra obtained at other concentration of TL shown in Fig. 3 could also be separated into three component bands (bands A, B and C for shortest-, middle- and longest-wavelength bands). Their shapes and positions are independent of the TL concentration, but their relative intensities change with concentration. These findings indicate that the three bands are ascribed to three different emitters, and their existence ratios change with TL concentration. Since the shape and position of bands A and B are consistent with those of the emission band of TL and the TM monomer band, these bands can be assigned to the HgTL* and HgTM* exciplexes, respectively. The shape and position of band C are inconsistent with those of 1:2 exciplexes observed for TM. Band C was observed in amine–alcohol mixtures for the first time. This band may be attributed to the exciplex with mixed ligands (HgTLTM*). Fig. 5 shows the concentration dependences of the intensities of bands A–C. Fig. 6 shows the emission spectra obtained for the solutions containing TM at 0.10 mol dm−3 and TL at various concentrations. With increasing TL concentration, the intensities of the bands for TM decrease and the intensity of the band for TL increases. The emission spectrum obtained at [TM] = 0.10 mol dm−3 and [TL] = 0.34 mol dm−3 consists of five bands (Fig. 7). 4. Discussion
Fig. 6. Normalized emission spectra for mercury-photosensitized luminescence of TM–TL mixtures in cyclohexane at 25 ◦ C. [TM] = 0.10 mol dm−3 , [TL] = 0 (1), 0.070 (2), 0.14 (3), 0.21 (4) and 0.34 mol dm−3 (5).
To explain the experimental results obtained for TL, TM and TL–TM mixtures in this study, the following set of reactions is
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considered under low TM concentration conditions: Hg(1 S0 ) + hν → Hg(3 P1 )∗
I0
Hg(3 P1 )∗ + S → Hg(3 P0 )∗ + S
k0
Hg(3 P0 )∗ + S → Hg(1 S0 ) + S
k1
Hg(3 P0 )∗ + TL → HgTL∗
k2
Hg(3 P0 )∗ + TM → HgTM∗
k3
HgTL∗ → Hg(1 S0 ) + TL + hνA HgTL∗ → Hg(1 S0 ) + TL
αk4
HgTM∗ → Hg(1 S0 ) + TM + hνB ∗
HgTM → Hg( S0 ) + TM HgTL + TM → HgTLTM
βk5
∗
HgTL∗ + TM → HgTM∗ + TL
IA = I0 α
γk6 (1 − γ)k6
HgTLTM∗ → Hg(1 S0 ) + TL + TM + hνC ∗
HgTLTM → Hg( S0 ) + TL + TM 1
∗
IB = I0 β δk7
k4 k2 [TL] k1 + k2 [TL] + k3 [TM] k4 + k6 [TM]
(3)
k3 [TM] k1 + k2 [TL] + k3 [TM]
+I0 β
(1 − δ)k7
HgTLTM + TL → Hg( S0 ) + 2TL + TM 1
Applying the steady-state approximation to TL–TM mixtures, the following relations are obtained:
(1 − β)k5
1
∗
Fig. 8. Effect of TL on the emission intensity of band A for TM–TL mixtures in cyclohexane at 25 ◦ C (concentrations of TM and TL are the same as those for Fig. 3). [Solid line shows the values calculated by Eq. (3). See text.]
(1 − α)k4
k2 [TL] (1 − γ)k6 [TM] k1 + k2 [TL] + k3 [TM] k4 + k6 [TM]
(4)
and
k8
Here, S denotes solvent, HgTL* and HgTM* are mercury exciplexes with TL and TM, and HgTLTM* denotes a mercury exciplex with mixed ligands. HgTL*, HgTM* and HgTLTM* fluoresce producing bands A, B, and C, respectively. Since it was observed that the intensity of band A decreased on the addition of TM, while that of band B did not decrease on the addition of TL, the reaction, HgAM* + AL → HgTLTM*, is not included in the above reaction mechanism. As mentioned above, the emission from Hg(3 P1 ) atoms cannot be observed in the liquid phase, this indicates that relaxation from the 3 P1 state to the 3 P0 state by collision with solvent molecules is fast. Applying the steady-state approximation, the following relations are obtained for mercury–TL and mercury–TM systems: IA = I0
αk2 [TL] k1 + k2 [TL]
(1)
IB = I0
βk3 [TM] k1 + k3 [TM]
(2)
where α and β denote the fractions of emission processes in the deactivations of HgTL* and HgTM*. The solid lines in Fig. 2 show the values calculated by Eqs. (1) and (2) using the following constants and ratios of the rate constants: αI0 = 1.16, βI0 = 1.09, k2 /k1 = 6.70 mol−1 dm3 and k3 /k1 = 13.0 mol−1 dm3 .
k1 + k3 [TM] IB = 0 k1 + k2 [TL] + k3 [TM] IB +
k2 [TL] k1 + k3 [TM] (1 − γ)k6 [TM] k3 [TM] k1 + k2 [TL] + k3 [TM] k4 + k6 [TM]
(5)
where IB0 is the emission intensity of band B in the absence of TL and is given by Eq. (2). Figs. 8 and 9 show plots of IA and IB /IB0 versus [TL]. The solid lines in Figs. 8 and 9 show the values calculated by Eqs. (3) and (5) using the ratio of the rate constants
Fig. 9. Effect of TL on the emission intensity ratio of band B (IB /IB0 ) for TM–TL mixtures in cyclohexane at 25 ◦ C (concentrations of TM and TL are the same as those for Fig. 3). [Solid line shows the values calculated by Eq. (4). See text.]
S. Yamamoto et al. / Spectrochimica Acta Part A 70 (2008) 265–269
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The energized intermediate, (HgTLTM*)= , formed between HgTL* and TM can be stabilized by collisions with solvent molecules in solution, but it may decompose effectively into HgTM* + TL in the gas phase (the ligand exchange reaction, HgTL* + TM → HgTM* + TL, is exothermic). As mentioned above, the emission spectrum obtained for a TM–TL mixture at [TM] = 0.10 mol dm−3 and [TL] = 0.34 mol dm−3 could be decomposed into five component bands (Fig. 7). The five emission bands are those from HgTL* (band A), HgTM* (band B), HgTLTM* (band C), HgTM2 * (band D) and HgTM2 ** (band E). Therefore, in this case we could observe all of the possible emission bands. Fig. 10. Effect of TL on the intensity ratio of band C to band A for TM–TL mixtures in cyclohexane at 25 ◦ C (concentrations of TM and TL are the same as those for Fig. 3). [Solid line shows the values calculated by Eq. (5). See text.]
(k6 /k4 = 140 mol−1 dm3 ), γ = 0.93 and the values mentioned above for the ratios of rate constants k2 /k1 and k3 /k1 . As shown in Figs. 8 and 9, the agreements between the observed and the calculated values are good. The following equation for the intensity ratio of band C to band A is also obtained by applying the steady-state approximation to the TM–TL mixtures: IC k7 δ γk6 [TM] = (6) α k4 k7 + k8 [TL] IA Fig. 10 shows plots of IC /IA against [TL]. Since the values of γ and k6 /k4 have already been obtained, the value of IC /IA can be calculated by Eq. (5), assuming the values of γ/α and k8 /k7 . The solid line in Fig. 10 shows the values calculated using γ/α = 0.66 and k8 /k7 = 1.76 mol−1 dm3 . The agreement between the observed and the calculated values for IC /IA is good. The emission band from a new type of mercury exciplex (HgTLTM*) was observed for the first time in mixed solutions of TL, TM and mercury in cyclohexane. The concentration dependences of the intensities of bands A–C could be explained well by the proposed reaction mechanism. As shown in previous papers, the following ligand-exchange reaction in an exciplex occurs effectively in the gas phase: HgTL∗ + TM → HgTM∗ + TL However, in solution this reaction is only minor, and the formation of the exciplex with mixed ligands is a major process. This difference between the major reactions in solution and in the gas phase can be explained by the following reactions: HgTL∗ + TM → (HgTLTM∗ )= (HgTLTM∗ )= + S → HgTLTM∗ (HgTLTM∗ )= → HgTM∗ + TL
5. Conclusion The emission bands from HgTL* and HgTM* exciplexes appeared at about 330 nm and 370 nm, respectively, in TL and TM solution in cyclohexane. Two other bands were observed at 405 nm and 455 nm for TM at high concentrations. These bands were previously assigned to two types of 1:2 exciplexes. The emission band from an exciplex with mixed ligands (HgTLTM*) was observed in TL–TM mixed solution for the first time. As shown previously, the following ligandexchange reaction in an exciplex occurs effectively in the gas phase: HgTL∗ + TM → HgTM∗ + TL In the liquid phase, however, the formation of the exciplex with mixed ligands was found to be a major process. The energized intermediate, (HgTLTM*)= , formed between HgTL* and TM can be stabilized by collisions with solvent molecules in solution, but it may decompose effectively into HgTM* + TL in the gas phase. References [1] L.F. Phillips, Acc. Chem. Res. 7 (1974) 135. [2] A.B. Callear, Chem. Rev. 87 (1987) 355. [3] S. Yamamoto, T. Nagaoka, Y. Sueishi, N. Nishimura, Chem. Lett. (1992) 621. [4] S. Yamamoto, T. Nagaoka, Y. Sueishi, N. Nishimura, J. Chem. Soc., Faraday Trans. 90 (1994) 2021. [5] S. Yamamoto, T. Ohba, J. Chem. Soc., Faraday Trans. 92 (1996) 729. [6] S. Yamamoto, M. Ikeda, Y. Sueishi, Phys. Chem. Chem. Phys. 2 (2000) 335. [7] S. Yamamoto, M. Doi, J. Chem. Soc., Faraday Trans. 93 (1997) 729. [8] S. Yamamoto, M. Doi, N. Ban, J. Chem. Soc., Faraday Trans. 94 (1998) 2361. [9] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, London, 1965. [10] M.K. Phibbs, B.de.B. Darwent, J. Chem. Phys. 18 (1950) 679.