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Concentration quenching of triplet phenanthrene in propan-1,2-diol
Volume 49, number 2
15 July 1977
CHEMICAL PHYSICS LETTERS
CONCENTRATION QUENCHING OF TRIPLET PHENANTHRENE IN PROPAN-1,2-DIOL Michael J. PILLING and Margaret A. RUSSELL Physical Chemistry Laboratory,
Oxford, OXI SQZ, UK
Received 9 December 1976 Revised manuscript received 25 April 1977
The lifetime of triplet phenanthrene in propan-l&iiol has been studied over a range of temperatures and ground state concentrations. The results demonstrate that the pseudo-first-order decay constant is linear in scavenger concentration, and that the resulting second order rate constant is too large to be explained by impurity quenching. Ground state quenching is implicated; its temperature dependence is much weaker than that of a diffusion-controlled reaction at high temperatures (220-300 K), but it tends towards diffusion-controlled behaviour at lower temperatures (<200 K).
1. Introduction The evidence for triplet excimers in aromatic hydrocarbons is contradictory [l] _ Several emission features have been attributed to such a species [2], but impurity emission has, in each case, been a potential source of ambiguity [ 11. Concentration quenching of aromatic triplets has been postulated to proceed via a triplet excimer [3,4]. The case of phenanthrene is particularly interesting, since it has no singlet excimer; Birks [I] has attributed the results of Langelaar et al. [4] to the presence of an anthracene impurity at the 100 ppm level. Recently Aiiawa et al. [S] observed an emission spectrum at 525 nm for phenanthrene in iso octane which they ascribed to the triplet excimer; the time dependence of the emission supports this conclusion. In this paper we provide further kinetic evidence for concentration quenching of triplet phenanthrene in propan-1,2-diol.
agent) was purified by distillation at 1 torr (boiling point = 54°C) and stored under nitrogen. Phenanthrene (Oxford Organic Chemicals, purified by CrO, oxidation in acetic acid, separated by chromatography on silica gel and recrystallised three times from ethanol) was shown by fluorescence spectroscopy to contain 5 10 ppm anthracene. t-stilbene (BDH Ltd.) was used without further purification.
3. Results Fig. 1 shows a plot of the first-order rate constant for triplet phenanthrene decay versus concentration lo/
a6-
2. Experimental Triplet phenanthrene was studied by laser flash photolysis. The experimental arrangement and method of analysing the results have been described previously [6]_ The photolysis cehs were of 5 mm or 1 mm path length. Propan-1,Zdiol (BDH Ltd., Laboratory Re-
Fig. 1. Plot of the first order rate constant, kl, for decay of triplet phenanthrene versus ground state concentration, T = 250 K.
343
Volume
49, number
CHEMICAL
2
PHYSICS
15 July 1977
LETTERS
ground state concentration. At 250 K, the decay is entire!y first order and the results unambiguous. At higher temperatures there is also a second-order component arising from triplet-triplet annihilation and the analysis becomes more complex [6j. The optical
density at the laser frequency
IOkTfKJ
Fi” =_ 2. Arrhenius plot for the second order rate constants, lihi, for quenching of triplet phenanthrene by (a) phenan-
threne
(e) and (b) t-stilbene (0).
of ground state phenanthrene. At this temperature the natural decay constant of triplet phenanthrene (=0.03 s-l) is, by comparison, negligible- The plot shows good linearity and a very small zero-concentration intercept. Fig. 2 shows an Arrhenius plot for the second-order rate constant for the process 3M*+
1M?!!+z
lM
,
(1)
where 3M* is the triplet state and ‘M the ground state of phenanthrene. On the basis of fig. I, the first order decay, after correction for natural decay, was attributed entirely to reaction (1). The concentration was varied by a factor of 20; the small experimental scatter and the lack of any correlation of the scatter with [‘Ml suggest that this assumption is justified. Fig. 2 also shows an Arrhenius plot for the quenching of phenanthrene by t-stilbene. This reaction is diffusioncontrolled: the non-linearity arises mainly from the non-Arrhenius temperature dependence of the solvent viscosity [7] _
was kept below 0.15 to
minimize any concentration gradient in the cell and both the triplet-triplet annihilation rate constant and kbi showed no apparent concentration dependence. An analysis of the apparent kinetics for a system with first- and second-order decay Channels, with the level of inhomogeneity existing in the experimental system, showed that the distortion of the kinetics is unimportant. Fig. 2 demonstrates that the decay cannot be attributed to impurity quenching. At room temperature, the ratio kbi to the rate constant for quenching by t-stilbene is 3 X 10w3, whilst the impurity level is < 10-s_ As the temperature falls, the ratio becomes larger and, at temperatures below 200 K, kbi approaches the diffusion-controlled value. t-stilbene and anthracene are both ff at aromatic molecules with almost identical van der Waals radii [8], and they shouId show very similar diffusion characteristics. t-stilbene thus provides a good model for anthracene, which could not be studied directly because of its large extinction coefficient at the laser frequency. Langelaar et al. [4] showed that concentration quenching in ethanoi has a different temperature dependence from that of the solvent fluidity. Their results were, however, not unambiguous for phenanthrene. In addition, diffusion-controlled reactions themselves can show quite marked deviations from the behaviour expected from the Stokes-Einstein equation [9]_ Fig. 2, by virtue of the contrast it affords between a diffusion-controlled reaction and one attributed to concentration quenching, provides cogent evidence in favour of reaction (1). A possible mechanism for the reaction is described by the scheme: 3M*
+
!+--{3M* IMM.
+ 1M)+
3D*k,lM
-I- ‘M
,
e
4. Discussion Fig. 1 -provides convincing evidence that, in the millimolar concentration range, the first-order decay constant for triplet phenanthrene varies linearly with
344
where {3M* + lM)is the encounter pair and 3D* the triplet excimer. kd is the diffusion rate constant. Ap-
plying the steady-state and 3D* ,
approximation
to c3M* + lM)
CHEMICAL PHYSICS LETTERS
Volume 49, number 2
Pe = k,l(k_,
+ kc)
(3)
-
Eq. (2) can be simplified the non-diffusion-controlled (a)
k-,
for two limiting cases in regime (k, < k_d):
; kbi = kekdlk-d
the excimer
is limited by the rate of formation of from the encounter pair. This could arise acceptance
cone for formation
[9] have recently analysed the kinetics of processes limited in this way; their analysis, requires a cone of acceptance of only 10” in the present case, which seems unrealistically small. The results also require that relative rotational diffusion inside the solvent cage shows a weaker temperature dependence than does translational diffusion. of the excimer.
@)
kc e k-e The reaction
Schurr
Langelaar et al. [4] considered d scheme similar to that given above, except that they omitted the first step and equated the rate constant to pkd, where p is the temperature-dependent probability of forming 3D*; it is equal to the bracketed term in eq. (2). At low temperatures, because of the weak temperature dependence of k, or k, , kbi + kd .
*
The reaction
if there were a limited
1.5 July 1977
References [l] [2]
and Schmitz
; kbi = kckekdIk_ek_d
-
is limited by the rate of crossing from the triplet excimer to the repulsive surface of the ground state dimer. The results require that this has a weak temperature dependence.
[3] [4] [S]
J.B. Birks, Rept. Progr. Phys. 38 (1975) 903. J. Langelaar. R.P.H. Rettschnick, A.M.P. hmbooy
and
GJ. Hoytink, Chem. Phys. Letters 1 (1968) 609. R.B. Cundall and A.J.R. Voss, Chem. Commun. (1969) 116. J. Langelaar, G. Jansen, R.P.H. Rcttschnick and G.J. Hoytink, Chcm. Phys. Letters 12 (1971) 86. M. Aikawa, T. Takemura and H. Baba, Bull. Chem. Sot. Japan 49 (1976)
437.
[6] E.J. Marshall, N.A. Philipson and M.J. Pilling, J. Chem. Sot. [7 [S] [9]
Faraday
II 72 (1976)
830.
] F.S. Ddinton, M.S. Henry, M.J. Pdling and P-C. Spencer, J. Chem. Sot. Faraday I, to be published. J-T. Edward, J. Chem. Educ. 47 (1970) 261_ J.M. Schurr and K.S. Schmitz, J. Phys. Chcm.
80 (1976)
1934.
345
15 July 1977
CHEMICAL PHYSICS LETTERS
CONCENTRATION QUENCHING OF TRIPLET PHENANTHRENE IN PROPAN-1,2-DIOL Michael J. PILLING and Margaret A. RUSSELL Physical Chemistry Laboratory,
Oxford, OXI SQZ, UK
Received 9 December 1976 Revised manuscript received 25 April 1977
The lifetime of triplet phenanthrene in propan-l&iiol has been studied over a range of temperatures and ground state concentrations. The results demonstrate that the pseudo-first-order decay constant is linear in scavenger concentration, and that the resulting second order rate constant is too large to be explained by impurity quenching. Ground state quenching is implicated; its temperature dependence is much weaker than that of a diffusion-controlled reaction at high temperatures (220-300 K), but it tends towards diffusion-controlled behaviour at lower temperatures (<200 K).
1. Introduction The evidence for triplet excimers in aromatic hydrocarbons is contradictory [l] _ Several emission features have been attributed to such a species [2], but impurity emission has, in each case, been a potential source of ambiguity [ 11. Concentration quenching of aromatic triplets has been postulated to proceed via a triplet excimer [3,4]. The case of phenanthrene is particularly interesting, since it has no singlet excimer; Birks [I] has attributed the results of Langelaar et al. [4] to the presence of an anthracene impurity at the 100 ppm level. Recently Aiiawa et al. [S] observed an emission spectrum at 525 nm for phenanthrene in iso octane which they ascribed to the triplet excimer; the time dependence of the emission supports this conclusion. In this paper we provide further kinetic evidence for concentration quenching of triplet phenanthrene in propan-1,2-diol.
agent) was purified by distillation at 1 torr (boiling point = 54°C) and stored under nitrogen. Phenanthrene (Oxford Organic Chemicals, purified by CrO, oxidation in acetic acid, separated by chromatography on silica gel and recrystallised three times from ethanol) was shown by fluorescence spectroscopy to contain 5 10 ppm anthracene. t-stilbene (BDH Ltd.) was used without further purification.
3. Results Fig. 1 shows a plot of the first-order rate constant for triplet phenanthrene decay versus concentration lo/
a6-
2. Experimental Triplet phenanthrene was studied by laser flash photolysis. The experimental arrangement and method of analysing the results have been described previously [6]_ The photolysis cehs were of 5 mm or 1 mm path length. Propan-1,Zdiol (BDH Ltd., Laboratory Re-
Fig. 1. Plot of the first order rate constant, kl, for decay of triplet phenanthrene versus ground state concentration, T = 250 K.
343
Volume
49, number
CHEMICAL
2
PHYSICS
15 July 1977
LETTERS
ground state concentration. At 250 K, the decay is entire!y first order and the results unambiguous. At higher temperatures there is also a second-order component arising from triplet-triplet annihilation and the analysis becomes more complex [6j. The optical
density at the laser frequency
IOkTfKJ
Fi” =_ 2. Arrhenius plot for the second order rate constants, lihi, for quenching of triplet phenanthrene by (a) phenan-
threne
(e) and (b) t-stilbene (0).
of ground state phenanthrene. At this temperature the natural decay constant of triplet phenanthrene (=0.03 s-l) is, by comparison, negligible- The plot shows good linearity and a very small zero-concentration intercept. Fig. 2 shows an Arrhenius plot for the second-order rate constant for the process 3M*+
1M?!!+z
lM
,
(1)
where 3M* is the triplet state and ‘M the ground state of phenanthrene. On the basis of fig. I, the first order decay, after correction for natural decay, was attributed entirely to reaction (1). The concentration was varied by a factor of 20; the small experimental scatter and the lack of any correlation of the scatter with [‘Ml suggest that this assumption is justified. Fig. 2 also shows an Arrhenius plot for the quenching of phenanthrene by t-stilbene. This reaction is diffusioncontrolled: the non-linearity arises mainly from the non-Arrhenius temperature dependence of the solvent viscosity [7] _
was kept below 0.15 to
minimize any concentration gradient in the cell and both the triplet-triplet annihilation rate constant and kbi showed no apparent concentration dependence. An analysis of the apparent kinetics for a system with first- and second-order decay Channels, with the level of inhomogeneity existing in the experimental system, showed that the distortion of the kinetics is unimportant. Fig. 2 demonstrates that the decay cannot be attributed to impurity quenching. At room temperature, the ratio kbi to the rate constant for quenching by t-stilbene is 3 X 10w3, whilst the impurity level is < 10-s_ As the temperature falls, the ratio becomes larger and, at temperatures below 200 K, kbi approaches the diffusion-controlled value. t-stilbene and anthracene are both ff at aromatic molecules with almost identical van der Waals radii [8], and they shouId show very similar diffusion characteristics. t-stilbene thus provides a good model for anthracene, which could not be studied directly because of its large extinction coefficient at the laser frequency. Langelaar et al. [4] showed that concentration quenching in ethanoi has a different temperature dependence from that of the solvent fluidity. Their results were, however, not unambiguous for phenanthrene. In addition, diffusion-controlled reactions themselves can show quite marked deviations from the behaviour expected from the Stokes-Einstein equation [9]_ Fig. 2, by virtue of the contrast it affords between a diffusion-controlled reaction and one attributed to concentration quenching, provides cogent evidence in favour of reaction (1). A possible mechanism for the reaction is described by the scheme: 3M*
+
!+--{3M* IMM.
+ 1M)+
3D*k,lM
-I- ‘M
,
e
4. Discussion Fig. 1 -provides convincing evidence that, in the millimolar concentration range, the first-order decay constant for triplet phenanthrene varies linearly with
344
where {3M* + lM)is the encounter pair and 3D* the triplet excimer. kd is the diffusion rate constant. Ap-
plying the steady-state and 3D* ,
approximation
to c3M* + lM)
CHEMICAL PHYSICS LETTERS
Volume 49, number 2
Pe = k,l(k_,
+ kc)
(3)
-
Eq. (2) can be simplified the non-diffusion-controlled (a)
k-,
for two limiting cases in regime (k, < k_d):
; kbi = kekdlk-d
the excimer
is limited by the rate of formation of from the encounter pair. This could arise acceptance
cone for formation
[9] have recently analysed the kinetics of processes limited in this way; their analysis, requires a cone of acceptance of only 10” in the present case, which seems unrealistically small. The results also require that relative rotational diffusion inside the solvent cage shows a weaker temperature dependence than does translational diffusion. of the excimer.
@)
kc e k-e The reaction
Schurr
Langelaar et al. [4] considered d scheme similar to that given above, except that they omitted the first step and equated the rate constant to pkd, where p is the temperature-dependent probability of forming 3D*; it is equal to the bracketed term in eq. (2). At low temperatures, because of the weak temperature dependence of k, or k, , kbi + kd .
*
The reaction
if there were a limited
1.5 July 1977
References [l] [2]
and Schmitz
; kbi = kckekdIk_ek_d
-
is limited by the rate of crossing from the triplet excimer to the repulsive surface of the ground state dimer. The results require that this has a weak temperature dependence.
[3] [4] [S]
J.B. Birks, Rept. Progr. Phys. 38 (1975) 903. J. Langelaar. R.P.H. Rettschnick, A.M.P. hmbooy
and
GJ. Hoytink, Chem. Phys. Letters 1 (1968) 609. R.B. Cundall and A.J.R. Voss, Chem. Commun. (1969) 116. J. Langelaar, G. Jansen, R.P.H. Rcttschnick and G.J. Hoytink, Chcm. Phys. Letters 12 (1971) 86. M. Aikawa, T. Takemura and H. Baba, Bull. Chem. Sot. Japan 49 (1976)
437.
[6] E.J. Marshall, N.A. Philipson and M.J. Pilling, J. Chem. Sot. [7 [S] [9]
Faraday
II 72 (1976)
830.
] F.S. Ddinton, M.S. Henry, M.J. Pdling and P-C. Spencer, J. Chem. Sot. Faraday I, to be published. J-T. Edward, J. Chem. Educ. 47 (1970) 261_ J.M. Schurr and K.S. Schmitz, J. Phys. Chcm.
80 (1976)
1934.
345