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Non-exponential phosphorescence decay of phenanthrene in biphenyl
Volume
123, number
CHEMICAL
5
NON-EXPONENTIAL Shoji TAEN
Received
PHOSPHORESCENCE
and Yasuhiko
Department of Chemrstry,
PHYSICS
24 January
LETTERS
DECAY OF PHENANTHRENE
1986
IN BIPHENYL
GONDO
Faculty of Saence, Kyushu Unrcersrty 33. Hakozakr. Htgashrku, Fukuoka 812. Japan
19 June 1985; in final form 1 November
1985
Non-exponential phosphorescence decays of phenanthrene in biphenyl polycrystals have been observed. It is found that excitation with short duration quickens the decay while the decay is slower after excitation with weaker intensity. The origin of the non-exponentiality is ascribed to the distance-dependent interactions between guest molecules in the lowest excited triplet state.
1. Introduction Energy transfer has been observed in a variety of pairs of organic molecules. In most of these organic molecules, energy transfer occurs from an excited donor to a ground-state acceptor. However, energy transfer from an excited donor to an excited acceptor can also occur. Azumi and McGlynn [ 1J demonstrated that two triplet-state molecules interact directly with each other and energy transfer occurs from one tripletstate molecule to the other. According to Kellogg [2 ] , the necessary conditions for this type of energy transfer to occur are satisfied in phenanthrene. As the direct interaction occurs between triplet-state molecules with short distance, high density of triplet-state distribution is desirable for the observation of its effect. Phenanthrened10 is suitable for this purpose because of its long lifetime. Nakashima et ‘al. [3] observed a non-exponential phosphorescence decay of phenanthrened10 in a rigid solvent by means of high-density excitation with a Q-switched ruby laser. They concluded by a special analysis that the nonexponentiality was due to the direct resonance interaction bewteen triplet-state phenanthrenedilO. We have reported the excitation duration and light intensity effects on the temperature-independent delayed fluorescence (TIDF) and concluded that the mechanism for TIDF is the direct resonance interaction between triplet-state molecules [4] . According to 0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
this mechanism, nonexponential phosphorescence decay of phenanthrene in biphenyl is expected to be observed when the guest concentration is very high. Here we report nonexponential phosphorescence decays of phenanthrenedn in biphenyl polycrystals. The effects of excitation duration and excitation light intensity are also reported. Accordingly, the mechanism for TIDF is reexamined.
2. Experimental Phenanthrene (Tokyo Kasei, EP) was subjected to the chemical treatment described previously [5]. Biphenyl (Wake, EP) was twice recrystallized from ethanol and was twice subjected to zone refming over 200 passes [4] . As a square-wave excitation source, mechanically chopped radiation from a 250 W highpressure mercury lamp (Ushio Electric, USH-250D) was used in combination with a 5 cm optical-path aqueous filter. The mechanical chopping was effected by means of an electromagnetic shutter (Vincent Associates, 26LOAOX5), operated by means of a home-made control system incorporating multivibrators (Toshiba, TC4528BP). The operation of the shutter was monitored on a digital memory device (Nicolet 1170 signal averager), displaying the output of a phototransistor illuminated by a He-Ne laser (NEC, GLGSOOO). The analyzing apparatus has been de441
Volume 123. number 5
CHEMICAL PHYSICS LETTERS
scribed previously [4] . In the present paper, t signifies the time elapsed after the cessation of the excitation. Concentration of phen~~~ne was chosen to be 3.2 X lo-* moljmol. All the measurements were made at 7’7IL Phosphorescence decay curves were observed at 525 nm, where the triplet-triplet absorption spectrum does not overlap the phosphorescence spectrum [6]. Delayed fluorescence decays were observed at 370 mu, where the delayed fluorescence intensity is the strongest.
3 _Results and discussion Fig. 1 shows the effects of excitation duration and light intensity on the decay of the delayed fluorescence of phenanthrene in biphenyl. According to Misra [7], the intensity of the delayed fluorescence of phenanthrene in biphenyl is independent of temperature in the range from 6 to 80 IL Therefore, the delayed fluorescence observed at 77 K is TIDF alone. As excitation with short duration makes the TIDF decay steep and excitation with weak intensity makes the TIDF decay slow, we proposed before [4] that the mecha~sm for TIDF was the direct resonance interaction between guest molecules in the lowest excited triplet state. In this mechanism the energies that are
Fig. 1. Comparison between the excitation duration and excitation light intensity effects on the delayed fluorescence decay of a 3.2 X 10s2 mol/mol mixed crystal of phenantlnene ln biphenyl at 77 K. Decay curves are normalized at the initial lnten&y. Excitation durations sre (a) 8.63 II,(b) 14 ms, and (c) 8.65 s. Relative excitation light intensities are (a) I, (b) 1, and (c) 0.02. AR the decay curves are averages of five runs.
442
24 January 1986
stored separately in two triplet-state molecules are con~en~ated in one molecule by energy transfer and then part of the resultant highly excited state is degraded to the lowest excited singlet state. It is from here that a transition downward by emission of delayed fluorescence takes place. As the energy emitted as a photon of delayed fluorescence originates in the triplet-state molecules, the effect of the interaction may appear on the phosphorescence decay. If we observe the effect of the interaction on the phosphorescence decay, the dependence of the phosphorescence decay on the excitation duration and light intensity must qu~tatively be the same as that of the TIDF decay. Fig. 2 shows the observed phosphorescence decay curves under various excitation durations. It can easily be seen that shortening the excitation duration makes not only the phosphorescence intensity weak, but also the decay steep. The phosphorescence decay curves after excitations longer than 4 s are essentially the same as that after an excitation of 4 s. Fig. 3 shows the phosphorescence decay curves under various excitation light intensities. It can be seen that excitation with weak intensity makes the decay slow. When the excitation duration or excitation light intensity is
Fig. 2. Excitation duration effect on the phosphorescence decay of a 3.2 X 10v2 mol/mol mixed crystal of phenanthrene in biphenyl at 77 K. Excitation durations are (a) 8.635 s, (b) 0.501 s,(c) 0.202 s,(d) 0.100 s,(e) 0.029 s, and (f) 0.014 5.
Volume 12 3, number 5
CHEMICAL PHYSICS LETTERS
(a) (b)
0
2
4
tlS
Fig. 3. Excitation light intensity effect on the phosphorescence decay of a 3.2 X lo-* mol/mol mixed crystal of phenanthrene in biphenyl at 77 K. Relative excitation light intensities are (a) 1, (b) 0.6, (c) 0.26, (d) 0.13, (e) 0.05, and (f) 0.02.
altered, the resultant phosphorescence intensities are greatly different. In consequence, it is desirable to normalize the phosphorescence decay curves at the initial time for the convenience of comparison. Fig. 4 shows the normalized phosphorescence decay curves. Curves
(cl (a)
2 tls Fig. 4. Comparison between the excitation duration and excitation light intensity effects on the phosphorescence decay of a 3.2 X lo-* mol/mol mixed crystal of phenanthrene in biphenyl at 77 K. Excitation durations are (a) 8.65 s, (b) 14 ms, and (c) 8.65 s. Decay curves are normalized at the initial intensity. Relative excitation light intensities are (a) 1, (b) 1, and (c) 0.02. All the decay curves are averages of fwe runs.
24 January 1986
(b) and (c) show opposite tendencies when (a) is chosen as a reference. The observed effects of excitation duration and excitation light intensity are compatible with the mechanism of distance-dependent interaction between guest molecules in the lowest excited triplet state. One or two triplet-state molecules may be degraded to the ground state after the interaction. Therefore, an additional decay rate of the triplet state is present when interactions between the guest triplet-state molecules are operative. As the guest molecules may be distributed randomly in space, distances between the trlpletstate molecules after excitation are various. Consequently, the observed total decay curve is non-exponential because it is a sum of the decays with various decay rates. The distribution of the triplet-state molecules may be random after delta-pulse excitation. If the excitation duration is long, annihilation occurs simultaneously during the excitation. As pairs with shorter distances are annihilated with higher probabilities, pairs with longer distances survive with relatively higher probabilities. Therefore, the relative numbers of pairs with short distances decrease as the excitation is prolonged. In other words, the total decay after short-pulse excitation is steeper than the decay after prolonged excitation. When the excitation light intensity is weak, the total number of triplet-state molecules is small and the distances between the tripletstate molecules are generally large. Therefore, the decay after excitation with weak intensity is slower than the decay after excitation with stronger intensity. Though we stated before [4] that the distance-dependent interaction was the direct resonance interaction, we cannot rule out other interactions when the guest concentration is as high as that adopted here. The candidates for the distance-dependent interaction may be the direct resonance interaction [l-3 ] , the direct exchange interaction [8] , and the interaction through the host [8] . The major interaction which causes the phosphorescence decay to be nonexponential will be elucidated by a further study of the effects of the solvent, concentration, and magnetic field [9] .
Acknowledgement The present work was partially supported by Grantin-Aid for Scientific Research No. 474221 from the Ministry of Education, Science and Culture. 443
Volume 123, number 5
CHEMICAL PHYSICS LETTERS
24 January 1986
References
161 J.S. Brinen and W.G. Hodgson, J. Chem. Phys. 47 (1967)
[II T. Azumi and S.P. McGlynn, J. Chem. Phys. 38 (1963) 2773; 39 (1963) 1186. PI R.E. Kellogg, J. Chem. Phys. 41(1964) 3046. I31 N. Nakashima, Y. Kume and N. Mataga, J. Phys. Chem. 79 (1975) 1788. [41 S. Taen and Y. Gondo, Bull. Chem. Sot. Japan 58 (1985) 41. [51 S. Yahiro, H. Sekiya, S. Taen and Y. Gondo, Mem. Fat. Sci. Kyushu Univ. Ser. Cl3 (1982) 227.
[71 T.N. Misa, J. Chem. Phys. 58 (1973) 1235.
2946.
PI H. Sternlicht, G.C. Nieman and G.W. Robinson, J. Chem. Phys. 38 (1963) 1326; 39 (1963) 1610; J. Jortner, S.A. Rice, J.L. Katz and S. Choi, J. Chem. Phys. 42 (1965) 309. [91 R.E. Merrifield, J. Chem. Phys. 48 (1968) 4318; R.C. Johnson and R.E. Merrifield, Phys. Rev. Bl (1970) 896.
123, number
CHEMICAL
5
NON-EXPONENTIAL Shoji TAEN
Received
PHOSPHORESCENCE
and Yasuhiko
Department of Chemrstry,
PHYSICS
24 January
LETTERS
DECAY OF PHENANTHRENE
1986
IN BIPHENYL
GONDO
Faculty of Saence, Kyushu Unrcersrty 33. Hakozakr. Htgashrku, Fukuoka 812. Japan
19 June 1985; in final form 1 November
1985
Non-exponential phosphorescence decays of phenanthrene in biphenyl polycrystals have been observed. It is found that excitation with short duration quickens the decay while the decay is slower after excitation with weaker intensity. The origin of the non-exponentiality is ascribed to the distance-dependent interactions between guest molecules in the lowest excited triplet state.
1. Introduction Energy transfer has been observed in a variety of pairs of organic molecules. In most of these organic molecules, energy transfer occurs from an excited donor to a ground-state acceptor. However, energy transfer from an excited donor to an excited acceptor can also occur. Azumi and McGlynn [ 1J demonstrated that two triplet-state molecules interact directly with each other and energy transfer occurs from one tripletstate molecule to the other. According to Kellogg [2 ] , the necessary conditions for this type of energy transfer to occur are satisfied in phenanthrene. As the direct interaction occurs between triplet-state molecules with short distance, high density of triplet-state distribution is desirable for the observation of its effect. Phenanthrened10 is suitable for this purpose because of its long lifetime. Nakashima et ‘al. [3] observed a non-exponential phosphorescence decay of phenanthrened10 in a rigid solvent by means of high-density excitation with a Q-switched ruby laser. They concluded by a special analysis that the nonexponentiality was due to the direct resonance interaction bewteen triplet-state phenanthrenedilO. We have reported the excitation duration and light intensity effects on the temperature-independent delayed fluorescence (TIDF) and concluded that the mechanism for TIDF is the direct resonance interaction between triplet-state molecules [4] . According to 0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
this mechanism, nonexponential phosphorescence decay of phenanthrene in biphenyl is expected to be observed when the guest concentration is very high. Here we report nonexponential phosphorescence decays of phenanthrenedn in biphenyl polycrystals. The effects of excitation duration and excitation light intensity are also reported. Accordingly, the mechanism for TIDF is reexamined.
2. Experimental Phenanthrene (Tokyo Kasei, EP) was subjected to the chemical treatment described previously [5]. Biphenyl (Wake, EP) was twice recrystallized from ethanol and was twice subjected to zone refming over 200 passes [4] . As a square-wave excitation source, mechanically chopped radiation from a 250 W highpressure mercury lamp (Ushio Electric, USH-250D) was used in combination with a 5 cm optical-path aqueous filter. The mechanical chopping was effected by means of an electromagnetic shutter (Vincent Associates, 26LOAOX5), operated by means of a home-made control system incorporating multivibrators (Toshiba, TC4528BP). The operation of the shutter was monitored on a digital memory device (Nicolet 1170 signal averager), displaying the output of a phototransistor illuminated by a He-Ne laser (NEC, GLGSOOO). The analyzing apparatus has been de441
Volume 123. number 5
CHEMICAL PHYSICS LETTERS
scribed previously [4] . In the present paper, t signifies the time elapsed after the cessation of the excitation. Concentration of phen~~~ne was chosen to be 3.2 X lo-* moljmol. All the measurements were made at 7’7IL Phosphorescence decay curves were observed at 525 nm, where the triplet-triplet absorption spectrum does not overlap the phosphorescence spectrum [6]. Delayed fluorescence decays were observed at 370 mu, where the delayed fluorescence intensity is the strongest.
3 _Results and discussion Fig. 1 shows the effects of excitation duration and light intensity on the decay of the delayed fluorescence of phenanthrene in biphenyl. According to Misra [7], the intensity of the delayed fluorescence of phenanthrene in biphenyl is independent of temperature in the range from 6 to 80 IL Therefore, the delayed fluorescence observed at 77 K is TIDF alone. As excitation with short duration makes the TIDF decay steep and excitation with weak intensity makes the TIDF decay slow, we proposed before [4] that the mecha~sm for TIDF was the direct resonance interaction between guest molecules in the lowest excited triplet state. In this mechanism the energies that are
Fig. 1. Comparison between the excitation duration and excitation light intensity effects on the delayed fluorescence decay of a 3.2 X 10s2 mol/mol mixed crystal of phenantlnene ln biphenyl at 77 K. Decay curves are normalized at the initial lnten&y. Excitation durations sre (a) 8.63 II,(b) 14 ms, and (c) 8.65 s. Relative excitation light intensities are (a) I, (b) 1, and (c) 0.02. AR the decay curves are averages of five runs.
442
24 January 1986
stored separately in two triplet-state molecules are con~en~ated in one molecule by energy transfer and then part of the resultant highly excited state is degraded to the lowest excited singlet state. It is from here that a transition downward by emission of delayed fluorescence takes place. As the energy emitted as a photon of delayed fluorescence originates in the triplet-state molecules, the effect of the interaction may appear on the phosphorescence decay. If we observe the effect of the interaction on the phosphorescence decay, the dependence of the phosphorescence decay on the excitation duration and light intensity must qu~tatively be the same as that of the TIDF decay. Fig. 2 shows the observed phosphorescence decay curves under various excitation durations. It can easily be seen that shortening the excitation duration makes not only the phosphorescence intensity weak, but also the decay steep. The phosphorescence decay curves after excitations longer than 4 s are essentially the same as that after an excitation of 4 s. Fig. 3 shows the phosphorescence decay curves under various excitation light intensities. It can be seen that excitation with weak intensity makes the decay slow. When the excitation duration or excitation light intensity is
Fig. 2. Excitation duration effect on the phosphorescence decay of a 3.2 X 10v2 mol/mol mixed crystal of phenanthrene in biphenyl at 77 K. Excitation durations are (a) 8.635 s, (b) 0.501 s,(c) 0.202 s,(d) 0.100 s,(e) 0.029 s, and (f) 0.014 5.
Volume 12 3, number 5
CHEMICAL PHYSICS LETTERS
(a) (b)
0
2
4
tlS
Fig. 3. Excitation light intensity effect on the phosphorescence decay of a 3.2 X lo-* mol/mol mixed crystal of phenanthrene in biphenyl at 77 K. Relative excitation light intensities are (a) 1, (b) 0.6, (c) 0.26, (d) 0.13, (e) 0.05, and (f) 0.02.
altered, the resultant phosphorescence intensities are greatly different. In consequence, it is desirable to normalize the phosphorescence decay curves at the initial time for the convenience of comparison. Fig. 4 shows the normalized phosphorescence decay curves. Curves
(cl (a)
2 tls Fig. 4. Comparison between the excitation duration and excitation light intensity effects on the phosphorescence decay of a 3.2 X lo-* mol/mol mixed crystal of phenanthrene in biphenyl at 77 K. Excitation durations are (a) 8.65 s, (b) 14 ms, and (c) 8.65 s. Decay curves are normalized at the initial intensity. Relative excitation light intensities are (a) 1, (b) 1, and (c) 0.02. All the decay curves are averages of fwe runs.
24 January 1986
(b) and (c) show opposite tendencies when (a) is chosen as a reference. The observed effects of excitation duration and excitation light intensity are compatible with the mechanism of distance-dependent interaction between guest molecules in the lowest excited triplet state. One or two triplet-state molecules may be degraded to the ground state after the interaction. Therefore, an additional decay rate of the triplet state is present when interactions between the guest triplet-state molecules are operative. As the guest molecules may be distributed randomly in space, distances between the trlpletstate molecules after excitation are various. Consequently, the observed total decay curve is non-exponential because it is a sum of the decays with various decay rates. The distribution of the triplet-state molecules may be random after delta-pulse excitation. If the excitation duration is long, annihilation occurs simultaneously during the excitation. As pairs with shorter distances are annihilated with higher probabilities, pairs with longer distances survive with relatively higher probabilities. Therefore, the relative numbers of pairs with short distances decrease as the excitation is prolonged. In other words, the total decay after short-pulse excitation is steeper than the decay after prolonged excitation. When the excitation light intensity is weak, the total number of triplet-state molecules is small and the distances between the tripletstate molecules are generally large. Therefore, the decay after excitation with weak intensity is slower than the decay after excitation with stronger intensity. Though we stated before [4] that the distance-dependent interaction was the direct resonance interaction, we cannot rule out other interactions when the guest concentration is as high as that adopted here. The candidates for the distance-dependent interaction may be the direct resonance interaction [l-3 ] , the direct exchange interaction [8] , and the interaction through the host [8] . The major interaction which causes the phosphorescence decay to be nonexponential will be elucidated by a further study of the effects of the solvent, concentration, and magnetic field [9] .
Acknowledgement The present work was partially supported by Grantin-Aid for Scientific Research No. 474221 from the Ministry of Education, Science and Culture. 443
Volume 123, number 5
CHEMICAL PHYSICS LETTERS
24 January 1986
References
161 J.S. Brinen and W.G. Hodgson, J. Chem. Phys. 47 (1967)
[II T. Azumi and S.P. McGlynn, J. Chem. Phys. 38 (1963) 2773; 39 (1963) 1186. PI R.E. Kellogg, J. Chem. Phys. 41(1964) 3046. I31 N. Nakashima, Y. Kume and N. Mataga, J. Phys. Chem. 79 (1975) 1788. [41 S. Taen and Y. Gondo, Bull. Chem. Sot. Japan 58 (1985) 41. [51 S. Yahiro, H. Sekiya, S. Taen and Y. Gondo, Mem. Fat. Sci. Kyushu Univ. Ser. Cl3 (1982) 227.
[71 T.N. Misa, J. Chem. Phys. 58 (1973) 1235.
2946.
PI H. Sternlicht, G.C. Nieman and G.W. Robinson, J. Chem. Phys. 38 (1963) 1326; 39 (1963) 1610; J. Jortner, S.A. Rice, J.L. Katz and S. Choi, J. Chem. Phys. 42 (1965) 309. [91 R.E. Merrifield, J. Chem. Phys. 48 (1968) 4318; R.C. Johnson and R.E. Merrifield, Phys. Rev. Bl (1970) 896.