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Delayed fluorescence from triplet-triplet annihilation in solution. Is the T2 state involved?
Volume 161, number4,5
CHEMICAL PHYSICSLEmERS
22September1989
DELAYED FLUORESCENCE FROM TRIPLET-TRIPLET ANNIHILATION IN SOLUTION. IS THE T2 STATE INVOLVED? * W.G. McGIMPSEY *I C. EVANS b, C. BOHNE a, S.R. KENNEDY a and J.C. SCAIANO a*b ’ Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6 b Ottawa-Carleton Chemistry Institute, Department of Chemistry. Universityof Ottawa, Ottawa, Ontario, Canada KIN 6NS
Received 15 March 1989;in final form 22June 1989
The possibility of T2 involvement in the delayed fluorescence of 9,10-dibromoanthracene ( DBA) has been investigated by studying the decrease of delayed fluorescence with the addition of 1,3_octadiene, an efficient triplet quencher with energy higher than the TI of DBA. It is concluded that the reduction of delayed emission intensity could be explained by the quenching of the T, state which mediates intersystem crossing, and that it is not necessary to invoke T2 generation via T-T annihilation.
1. Introduction Triplet-triplet annihilation (TTA) is a rare process in solution under conditions of cw lamp excitation, although it is responsible for product formation in the case of anthracene dimerization [ 11. On the other hand, TTA can be an important process under flash (e.g. pulsed laser) excitation; in fact for long-lived triplets ( zTr 10 ps) the lifetime can be almost exclusively controlled by TTA under pulsed excitation. The most common manifestations of TTA are second-order kinetics and delayed fluorescence. The latter is the result of TTA leading to ground (So) and excited (S, ) singlet states, i.e. T, +TI +S,+S,
.
(1)
Spin statistics dictate that in the triplet-triplet encounter complex nine spin states with equal probability can be produced; l/9 are singlets, l/3 triplets and 5/9 quintets [ 2 1. The quintet encounter complex is dissociative giving back two triplets, because the energy for the lowest quintet state is normally not accessible in the encounter of two triplet molecules 131. Thus, the formation of the fluorescent S, state has to occur through the singlet and/or triplet channels. * Issued as NRCC 29987.
342
Experimental rate constants for triplet-triplet annihilation are much closer to the diffusion-controlled limit than expected on the basis of spin statistics for the singlet channels [ 2,4-61. In some triplet encounters this must lead to formation of T, and So states, since the production of S, and T, would be endothermic. It is generally assumed that the triplets formed would be in the lowest, T, state, although T2 formation is also energetically viable in most cases. To explore the possibility of Tz formation, we decided to use 9,lOdibromoanthracene (DBA) as a test molecule; the high efficiency of T,+ reverse intersystem crossing (RISC) [7-121 makes this a convenient choice as delayed fluorescence can be used to probe for Tz formation. To test for T2 involvement, the quenching of the delayed fluorescence of DBA by 1,3-octadiene, a quencher with a triplet energy higher than the T1 energy of DBA, but lower than that of TZ, was studied.
2. Experimental 9,10-dibromoanthracene (DBA; 98%) from Aldrich and cyclohexane (spectroscopic grade) from BDH were used without purification, whereas 1,3octadiene from Wiley Organics was distilled. Solutions (2.5 ml) were prepared in 7x 7 mm2 quartz
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume 161,number 4,5
22 September 1989
CHEMICAL PHYSICS LETTERS
cells and were deaerated by nitrogen bubbling for 15 min. Steady-state fluorescence spectra were recorded
using a Perkin-Elmer IS-5 spectrofluorimeter equipped with a PE-3600 data station. UV-visible absorption spectra were obtained on a HP-845 1A diode array spectrometer. The laser flash photolysis system at NRC has been described earlier [ 13,141. DBA was excited by a Molectron W-24 nitrogen laser (337.1 nm, q 8 ns); time-resolved absorption measurements were detected with a system using a photomultiplier, whereas emission spectra were obtained with an EG&G gated and intensified optical multichannel analyser (OMA) [ 15 1.
3. Results and discussion A key result in this work is that the delayed fluorescence of DBA is more efficiently quenched by 1,3-octadiene than its prompt fluorescence. This delayed emission results from triplet-triplet annihilation (P-type), since emission resulting from thermal population of S, from T, (E-type delayed fluorescence) is improbable in polynuclear aromatics with large S-T gaps ( ;I 30 kcal/mol for DBA). Dienes are known to be efficient triplet quenchers when the process is exothermic [ 161. The T, state of DBA, monitored by the decay of the triplet-triplet absorption at 420 nm, is not quenched by 1,3_octadiene (k,(T,) < 10” M-’ s-l). This result suggests the participation of a state different from S1and T,; this state probably being TI. The possible steps involved in the triplet-triplet annihilation process are
Tz + Q + quenching ,
(10)
S, t Q + minor quenching.
(11)
The following factors can contribute to some extent to the decrease of the delayed fluorescence intensity after a selected delay time ( ta) following laser excitation in the presence of 1,3-octadiene: (i) quenching of the St state formed in the interaction of two triplets; (ii) decrease of the T, concentration at td, due to the quenching of S1 and/or T2, both species that mediate the formation of T, [ 8,171, and (iii) possible quenching of TS, if formed in the trip let-triplet annihilation process. The quenching of S, by 1,foctadiene was measured by steady-state fluorimetry and a typical SternVolmer plot is presented in fig. 1A. The fluorescence and absorption spectra of DBA are shifted 2 nm to the red in the presence of 1.0 M 1,3-octadiene. The fluorescence quenching is relatively inefficient and 1.4
A
‘0
hu so-s, &
9 -7.2,
(2)
(3)
S, -tT,,
(4)
s1 + srJ+Izv, ,
(5)
Tz +S,, 7’2
-‘T,,
(-3) (6)
T, ST, +S, (orSo)+&,,
(71
TI +T, +TI+%,
(8)
T, ST, -T,+S,,
(9)
0
0.3
0.6
0.9
1.2
[I ,3-octadlene]
Fig. 1. (A) Stem-Vohner plot for the quenching ofDBA (50 @Q tluorescence by l$octadiene. 1,=400 nm; emission spxtra were integrated between 435 and 500 nm. (9) Stem-Volmer plot for thequenchingofDBA (0.2 mM) T, formation by 1,3-octadiene. The initial triplet absorbance was measured at 420 nm. The symbols indicate experiments performed with different DBA and 1,3octadiene stock solutions.
343
Volume I6 1,number4,5
CHEMICAL PHYSICS LETTERS
some scatter in the Stem-Volmer constant values are observed for different experiments. The mean value (J&(S), six experiments) is 0.25f0.03 M-l. DBA has a Tz state slightly above ( w2 kcal/mol) the SI state and the intersystem crossing to T, has been reported to be exclusively mediated by Tz (eqs. (3) and (6)) [8,17]. As RISC (T&,, eq. (-3)) is quite efficient ( $RISC=0.17 [ 121)) T2*S1 interconversion may play a role in determining the value of K,,(S), For this reason the true Stem-Volmer slope for S,-only quenching would be smalle; than the measured one. The initial concentration of low lying triplets ( T1 ) formed using laser excitation was measured by the triplet-triplet absorption (Top OD) at 420 nm immediately following the laser pulse. The Stem-Volmer slope (&v (Tf) ) for 1,3_octadiene quenching of the T, formation was obtained from the slope of the plot of (TOP OD),/(Top OD) versus [1,3-octadiene], where (Top OD), and (Top OD) are the initial triplet absorbance values in the absence and presence of quencher, respectively, For each quencher concentration a fresh DBA solution was used, as this compound is destroyed with extensive laser irradiation; however, no substantial destruction was observed up to 500 laser shots of < 10 mJ per pulse. Fig. 1B shows the Stem-Volmer plot based on the yield for triplet formation, which presents results from three independent experiments performed with different DBA and 1,Eoctadiene stock solutions. The &(T,) value is 0.55+0.04 M-‘. If intersystem crossing would proceed directly from SI to TI the Stem-Volmey constants measured in the fluorescence experiment and for the triplet formation should have the same value. Clearly this is not the case. The much higher value obtained for T, formation confirms the participation of Tz in the intersystem crossing. It is interesting to note that the value of Ksv (T,) of 0.55 M-l is much lower than- the value for quenching by 1,3-octadiene ( 1.8 M-l ) obtained in a two-laser experiment where T2 was generated by photoexcitation of T, [ 121. Both values incorporate S I quenching and should therefore be approximately equal. The substantial difference observed cannot be ascribed to the participation of a direct +T, EC, Lim et al. [ 171 discarded this mechanism in their study of the temperature dependence on the fluo344
22 September 1989
rescence quantum yield and triplet-triplet absorption spectra of 9- and 9,10-substituted anthracenes (DBA included). The 1.8 M- ’ value obtained in the two laser experiments does not incorporate the quenching of higher triplet states (n>, 3), as this Stem-Volmer slope is independent of the triplet excitation wavelength [ 121. A possible explanation for the lower value obtained in the present study is that, following the T2 quenching, the T, state of DBA may be populated by energy transfer from the triplet diene generated in reaction (10). Considering the acyclic diene triplet lifetimes (usually c 50 ns [ l&l9 ] ) and DBA ground-state concentration (0.2 mM) the energy transfer process cannot occur randomly in solution. The formation of T, by the triplet diene would have to occur in the geminate solvent cage and should be dependent on the solvent viscosity. The delayed fluorescence from the TTA process was measured with OMA detection. With this equipment data can be acquired with a short gate at a predetermined delay after the laser pulse, so that the prompt fluorescence is excluded. The luminescence spectra were obtained 1.Ow after the lasei pulse with a 120 ns gate starting at the end of the delay period. As already indicated, 1,3-octadiene does not quench the T, state of DBA. TTA is only a minor process under the conditions used and the decay of DBA triplets (T , ) follows approximately first-order kinetics. Thus, the decrease in the intensity of delayed fluorescence in the presence of 1,3-octadiene cannot be the result of a change in the decay kinetics of triplet DBA. For each diene concentration fresh DBA solutions were used and the emission spectra were integrated between 420 and 480 nm; within the 5 nm resolution employed, the addition of diene did not induce any changes in the spectral distribution of the delayed fluorescence. For each quencher concentration the lifetime of DBA was checked by its absorption at 420 nm to ensure that spurious quenchers, such as oxygen, were not inadvertently added. The dependence of the intensity ratio in the absence and presence of quencher (IO/I) with the 1,3-octadiene concentration is shown in fig. 2A. The delayed fluorescence intensity decrease in the presence of 1,3octadiene must be corrected for S1 fluorescence quenching and for the lower T, concentration due to quenching of S, and/or Tz during the intersystem crossing process. As TTA is a bimolecular process
CHEMICAL PHYSICS LETTERS
Volume 161, number 4,5
OT.2 [l ,t-octediene]
Fig. 2. Stern-Volmer plot for the quenching of DBA (0.2 mM) delayed fluorescence by 1,3_octadiene. (A) total quenching and (B) intensity decrease corrected for T, formation and singlet quenching (see text ).
the decrease of T, concentration will affect the delayed fluorescence intensity as a quadratic factor. The corrected lo/Z value is given by
0 10
r
JO
1
= r l-t&v(S) CDCT
tQ1
22 September 1989
It should be noted that given the yield of RISC (reaction ( - 3) ) of 0.17, the maximum corrected lo/l would be = 1.6, if the yield of Tz was three times the yield of S, via TTA. We estimate that a value of (I,,/&,, of 1.15 would be required before a compelling argument for reaction (9) could be made. Although the participation of TZ in the TTA process cannot be inferred from the experiments presented, it is important to mention that the corrected DBA emission values are within experimental error 1.0, but consistently higher than this value. In the case of anthracene (data not shown), where no contribution of the T2 route would be expected due to a much lower RISC yield, the values for the corrected delayed fluorescence intensity were scattered around 1.0. In any case, it is clear that if T2 is formed in the TTA process its contribution to the delayed fluorescence is small. Finally, a potentially interesting point for further investigation is that the differences in Stem-Volmer constants for fluorescence quenching and triplet formation could be explored to obtain information about the T2 state, i.e. lifetimes, of other anthracene derivatives, where the RISC quantum yield is much lower than the one observed for DBA.
(12) Acknowledgement The fact that &(T,) could incorporate complications, like back energy transfer from the triplet diene, does not invalidate the approach used to correct the decrease in delayed fluorescence. The last term in eq. (12) takes into account the change in triplet concentration; experimentally, Kw (T,) is the factor which takes into account the experimental decrease in T, concentration. Similarly, &v(S) represents the way in which the fluorescence from S, responds to the addition of diene, which may incorporate some T2 quenching as a result of S,-Tz interconversion. If the formation of T2 and excitation of S1 through RISC was operating in the delayed fluorescence process the (lo/Z),,, should be larger than 1.0 and the slope of the (&/I),,, versus 1,J-octadiene concentration should give Ksv ( T2 ). The fact that the corrected values are within the experimental error of unity (fig. 23 ) suggests that it is not necessary to invoke reaction (9) to explain the delayed emission.
Thanks are due to the Natural Sciences and Engineering Research Council (Canada) for partial support of this work and to Mr. SE. Sugamori for technical assistance. CB thanks the Conselho Nacional de Desenvolvimento Cientifico e Technol6gico (Brazil) for a post doctoral fellowship.
References [ I ] J.L. Charlton, R. Dabestani and J. Saltiel, J. Am. Chem. sot. 10s (1983) 3473.
[ 21 I. Saltiel and B.W. Atwater, Advan. Photochem. 14 ( 1988) [ 31 k. Dick and B. Nickel, Chem. Phys. 78 (1983) 1. [ 41 A. Yekta and N.J. Turro, Mol. Photochem. 3 (1972) 307. (51 I. Saltiel, G.R. Marchand, W.K. Smothers, S.A. Stout and J.L. Charlton, J.Am. Chem. Sot. 103 (1981) 7159. [6] J. Saltiel, S. Ganapathy and B.W. Atwater, J. Am. Chem. Sot. 109 (1987) 1209.
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[7]T. WilsonandA.M.Halpem,J.Am.Chem.Soc. 102’(1980) 7212. [S] K.-C. Wu and W.R. Ware, J. Am. Chem. Sot. 101 (1979) 5906. [ 9 1S. Kobayashi, K. Kikuchi and H. Kokubun, Chem. Phys. 27 (1978) 399. [lOI L.H. CatalaniandT. Wilson, J. Am. Chem. Sot. LO9(1987) 7458. [ 111H. Fukumura, R. Kikuchi, K. Koike and H.J. Kokubun, Photochem. Photobiol. 42A ( 1988) 283. [ 121W.G. McGimpsey and J.C. Scaiano, J. Am. Chem. Sot. 111 (1989) 335. [ 131J.C. Scaiano, J. Am. Chem. Sot. 102 ( 1980) 7747.
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[ 141 J.C. Scaiano, M. Tanner and D. Weir, J. Am. Chem. SOC. 107 (1985) 4396. [ 151 C. Bohne and J.C. Scaiano, J. Am. Chem. Sot. 111 ( 1989) 2409. [ 161 P.J. Wagner and I. Kochevar, J. Am. Cham. Sot. 90 ( 1968) 2232. [17] E.C. Lim, J.D. Laposa and J.M.H. Ytt, J. Mol. Spectry. 19 (1966) 412. [ 181 R.A. Caldwell and M. Sin& J. Am. Chem. Sot. 104 ( 1982) 6121. [ 191 T. Mi, R.A. Caldwell and L.A. Melton, J. Am. Chem. Sot. 111(1989)457.
CHEMICAL PHYSICSLEmERS
22September1989
DELAYED FLUORESCENCE FROM TRIPLET-TRIPLET ANNIHILATION IN SOLUTION. IS THE T2 STATE INVOLVED? * W.G. McGIMPSEY *I C. EVANS b, C. BOHNE a, S.R. KENNEDY a and J.C. SCAIANO a*b ’ Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6 b Ottawa-Carleton Chemistry Institute, Department of Chemistry. Universityof Ottawa, Ottawa, Ontario, Canada KIN 6NS
Received 15 March 1989;in final form 22June 1989
The possibility of T2 involvement in the delayed fluorescence of 9,10-dibromoanthracene ( DBA) has been investigated by studying the decrease of delayed fluorescence with the addition of 1,3_octadiene, an efficient triplet quencher with energy higher than the TI of DBA. It is concluded that the reduction of delayed emission intensity could be explained by the quenching of the T, state which mediates intersystem crossing, and that it is not necessary to invoke T2 generation via T-T annihilation.
1. Introduction Triplet-triplet annihilation (TTA) is a rare process in solution under conditions of cw lamp excitation, although it is responsible for product formation in the case of anthracene dimerization [ 11. On the other hand, TTA can be an important process under flash (e.g. pulsed laser) excitation; in fact for long-lived triplets ( zTr 10 ps) the lifetime can be almost exclusively controlled by TTA under pulsed excitation. The most common manifestations of TTA are second-order kinetics and delayed fluorescence. The latter is the result of TTA leading to ground (So) and excited (S, ) singlet states, i.e. T, +TI +S,+S,
.
(1)
Spin statistics dictate that in the triplet-triplet encounter complex nine spin states with equal probability can be produced; l/9 are singlets, l/3 triplets and 5/9 quintets [ 2 1. The quintet encounter complex is dissociative giving back two triplets, because the energy for the lowest quintet state is normally not accessible in the encounter of two triplet molecules 131. Thus, the formation of the fluorescent S, state has to occur through the singlet and/or triplet channels. * Issued as NRCC 29987.
342
Experimental rate constants for triplet-triplet annihilation are much closer to the diffusion-controlled limit than expected on the basis of spin statistics for the singlet channels [ 2,4-61. In some triplet encounters this must lead to formation of T, and So states, since the production of S, and T, would be endothermic. It is generally assumed that the triplets formed would be in the lowest, T, state, although T2 formation is also energetically viable in most cases. To explore the possibility of Tz formation, we decided to use 9,lOdibromoanthracene (DBA) as a test molecule; the high efficiency of T,+ reverse intersystem crossing (RISC) [7-121 makes this a convenient choice as delayed fluorescence can be used to probe for Tz formation. To test for T2 involvement, the quenching of the delayed fluorescence of DBA by 1,3-octadiene, a quencher with a triplet energy higher than the T1 energy of DBA, but lower than that of TZ, was studied.
2. Experimental 9,10-dibromoanthracene (DBA; 98%) from Aldrich and cyclohexane (spectroscopic grade) from BDH were used without purification, whereas 1,3octadiene from Wiley Organics was distilled. Solutions (2.5 ml) were prepared in 7x 7 mm2 quartz
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume 161,number 4,5
22 September 1989
CHEMICAL PHYSICS LETTERS
cells and were deaerated by nitrogen bubbling for 15 min. Steady-state fluorescence spectra were recorded
using a Perkin-Elmer IS-5 spectrofluorimeter equipped with a PE-3600 data station. UV-visible absorption spectra were obtained on a HP-845 1A diode array spectrometer. The laser flash photolysis system at NRC has been described earlier [ 13,141. DBA was excited by a Molectron W-24 nitrogen laser (337.1 nm, q 8 ns); time-resolved absorption measurements were detected with a system using a photomultiplier, whereas emission spectra were obtained with an EG&G gated and intensified optical multichannel analyser (OMA) [ 15 1.
3. Results and discussion A key result in this work is that the delayed fluorescence of DBA is more efficiently quenched by 1,3-octadiene than its prompt fluorescence. This delayed emission results from triplet-triplet annihilation (P-type), since emission resulting from thermal population of S, from T, (E-type delayed fluorescence) is improbable in polynuclear aromatics with large S-T gaps ( ;I 30 kcal/mol for DBA). Dienes are known to be efficient triplet quenchers when the process is exothermic [ 161. The T, state of DBA, monitored by the decay of the triplet-triplet absorption at 420 nm, is not quenched by 1,3_octadiene (k,(T,) < 10” M-’ s-l). This result suggests the participation of a state different from S1and T,; this state probably being TI. The possible steps involved in the triplet-triplet annihilation process are
Tz + Q + quenching ,
(10)
S, t Q + minor quenching.
(11)
The following factors can contribute to some extent to the decrease of the delayed fluorescence intensity after a selected delay time ( ta) following laser excitation in the presence of 1,3-octadiene: (i) quenching of the St state formed in the interaction of two triplets; (ii) decrease of the T, concentration at td, due to the quenching of S1 and/or T2, both species that mediate the formation of T, [ 8,171, and (iii) possible quenching of TS, if formed in the trip let-triplet annihilation process. The quenching of S, by 1,foctadiene was measured by steady-state fluorimetry and a typical SternVolmer plot is presented in fig. 1A. The fluorescence and absorption spectra of DBA are shifted 2 nm to the red in the presence of 1.0 M 1,3-octadiene. The fluorescence quenching is relatively inefficient and 1.4
A
‘0
hu so-s, &
9 -7.2,
(2)
(3)
S, -tT,,
(4)
s1 + srJ+Izv, ,
(5)
Tz +S,, 7’2
-‘T,,
(-3) (6)
T, ST, +S, (orSo)+&,,
(71
TI +T, +TI+%,
(8)
T, ST, -T,+S,,
(9)
0
0.3
0.6
0.9
1.2
[I ,3-octadlene]
Fig. 1. (A) Stem-Vohner plot for the quenching ofDBA (50 @Q tluorescence by l$octadiene. 1,=400 nm; emission spxtra were integrated between 435 and 500 nm. (9) Stem-Volmer plot for thequenchingofDBA (0.2 mM) T, formation by 1,3-octadiene. The initial triplet absorbance was measured at 420 nm. The symbols indicate experiments performed with different DBA and 1,3octadiene stock solutions.
343
Volume I6 1,number4,5
CHEMICAL PHYSICS LETTERS
some scatter in the Stem-Volmer constant values are observed for different experiments. The mean value (J&(S), six experiments) is 0.25f0.03 M-l. DBA has a Tz state slightly above ( w2 kcal/mol) the SI state and the intersystem crossing to T, has been reported to be exclusively mediated by Tz (eqs. (3) and (6)) [8,17]. As RISC (T&,, eq. (-3)) is quite efficient ( $RISC=0.17 [ 121)) T2*S1 interconversion may play a role in determining the value of K,,(S), For this reason the true Stem-Volmer slope for S,-only quenching would be smalle; than the measured one. The initial concentration of low lying triplets ( T1 ) formed using laser excitation was measured by the triplet-triplet absorption (Top OD) at 420 nm immediately following the laser pulse. The Stem-Volmer slope (&v (Tf) ) for 1,3_octadiene quenching of the T, formation was obtained from the slope of the plot of (TOP OD),/(Top OD) versus [1,3-octadiene], where (Top OD), and (Top OD) are the initial triplet absorbance values in the absence and presence of quencher, respectively, For each quencher concentration a fresh DBA solution was used, as this compound is destroyed with extensive laser irradiation; however, no substantial destruction was observed up to 500 laser shots of < 10 mJ per pulse. Fig. 1B shows the Stem-Volmer plot based on the yield for triplet formation, which presents results from three independent experiments performed with different DBA and 1,Eoctadiene stock solutions. The &(T,) value is 0.55+0.04 M-‘. If intersystem crossing would proceed directly from SI to TI the Stem-Volmey constants measured in the fluorescence experiment and for the triplet formation should have the same value. Clearly this is not the case. The much higher value obtained for T, formation confirms the participation of Tz in the intersystem crossing. It is interesting to note that the value of Ksv (T,) of 0.55 M-l is much lower than- the value for quenching by 1,3-octadiene ( 1.8 M-l ) obtained in a two-laser experiment where T2 was generated by photoexcitation of T, [ 121. Both values incorporate S I quenching and should therefore be approximately equal. The substantial difference observed cannot be ascribed to the participation of a direct +T, EC, Lim et al. [ 171 discarded this mechanism in their study of the temperature dependence on the fluo344
22 September 1989
rescence quantum yield and triplet-triplet absorption spectra of 9- and 9,10-substituted anthracenes (DBA included). The 1.8 M- ’ value obtained in the two laser experiments does not incorporate the quenching of higher triplet states (n>, 3), as this Stem-Volmer slope is independent of the triplet excitation wavelength [ 121. A possible explanation for the lower value obtained in the present study is that, following the T2 quenching, the T, state of DBA may be populated by energy transfer from the triplet diene generated in reaction (10). Considering the acyclic diene triplet lifetimes (usually c 50 ns [ l&l9 ] ) and DBA ground-state concentration (0.2 mM) the energy transfer process cannot occur randomly in solution. The formation of T, by the triplet diene would have to occur in the geminate solvent cage and should be dependent on the solvent viscosity. The delayed fluorescence from the TTA process was measured with OMA detection. With this equipment data can be acquired with a short gate at a predetermined delay after the laser pulse, so that the prompt fluorescence is excluded. The luminescence spectra were obtained 1.Ow after the lasei pulse with a 120 ns gate starting at the end of the delay period. As already indicated, 1,3-octadiene does not quench the T, state of DBA. TTA is only a minor process under the conditions used and the decay of DBA triplets (T , ) follows approximately first-order kinetics. Thus, the decrease in the intensity of delayed fluorescence in the presence of 1,3-octadiene cannot be the result of a change in the decay kinetics of triplet DBA. For each diene concentration fresh DBA solutions were used and the emission spectra were integrated between 420 and 480 nm; within the 5 nm resolution employed, the addition of diene did not induce any changes in the spectral distribution of the delayed fluorescence. For each quencher concentration the lifetime of DBA was checked by its absorption at 420 nm to ensure that spurious quenchers, such as oxygen, were not inadvertently added. The dependence of the intensity ratio in the absence and presence of quencher (IO/I) with the 1,3-octadiene concentration is shown in fig. 2A. The delayed fluorescence intensity decrease in the presence of 1,3octadiene must be corrected for S1 fluorescence quenching and for the lower T, concentration due to quenching of S, and/or Tz during the intersystem crossing process. As TTA is a bimolecular process
CHEMICAL PHYSICS LETTERS
Volume 161, number 4,5
OT.2 [l ,t-octediene]
Fig. 2. Stern-Volmer plot for the quenching of DBA (0.2 mM) delayed fluorescence by 1,3_octadiene. (A) total quenching and (B) intensity decrease corrected for T, formation and singlet quenching (see text ).
the decrease of T, concentration will affect the delayed fluorescence intensity as a quadratic factor. The corrected lo/Z value is given by
0 10
r
JO
1
= r l-t&v(S) CDCT
tQ1
22 September 1989
It should be noted that given the yield of RISC (reaction ( - 3) ) of 0.17, the maximum corrected lo/l would be = 1.6, if the yield of Tz was three times the yield of S, via TTA. We estimate that a value of (I,,/&,, of 1.15 would be required before a compelling argument for reaction (9) could be made. Although the participation of TZ in the TTA process cannot be inferred from the experiments presented, it is important to mention that the corrected DBA emission values are within experimental error 1.0, but consistently higher than this value. In the case of anthracene (data not shown), where no contribution of the T2 route would be expected due to a much lower RISC yield, the values for the corrected delayed fluorescence intensity were scattered around 1.0. In any case, it is clear that if T2 is formed in the TTA process its contribution to the delayed fluorescence is small. Finally, a potentially interesting point for further investigation is that the differences in Stem-Volmer constants for fluorescence quenching and triplet formation could be explored to obtain information about the T2 state, i.e. lifetimes, of other anthracene derivatives, where the RISC quantum yield is much lower than the one observed for DBA.
(12) Acknowledgement The fact that &(T,) could incorporate complications, like back energy transfer from the triplet diene, does not invalidate the approach used to correct the decrease in delayed fluorescence. The last term in eq. (12) takes into account the change in triplet concentration; experimentally, Kw (T,) is the factor which takes into account the experimental decrease in T, concentration. Similarly, &v(S) represents the way in which the fluorescence from S, responds to the addition of diene, which may incorporate some T2 quenching as a result of S,-Tz interconversion. If the formation of T2 and excitation of S1 through RISC was operating in the delayed fluorescence process the (lo/Z),,, should be larger than 1.0 and the slope of the (&/I),,, versus 1,J-octadiene concentration should give Ksv ( T2 ). The fact that the corrected values are within the experimental error of unity (fig. 23 ) suggests that it is not necessary to invoke reaction (9) to explain the delayed emission.
Thanks are due to the Natural Sciences and Engineering Research Council (Canada) for partial support of this work and to Mr. SE. Sugamori for technical assistance. CB thanks the Conselho Nacional de Desenvolvimento Cientifico e Technol6gico (Brazil) for a post doctoral fellowship.
References [ I ] J.L. Charlton, R. Dabestani and J. Saltiel, J. Am. Chem. sot. 10s (1983) 3473.
[ 21 I. Saltiel and B.W. Atwater, Advan. Photochem. 14 ( 1988) [ 31 k. Dick and B. Nickel, Chem. Phys. 78 (1983) 1. [ 41 A. Yekta and N.J. Turro, Mol. Photochem. 3 (1972) 307. (51 I. Saltiel, G.R. Marchand, W.K. Smothers, S.A. Stout and J.L. Charlton, J.Am. Chem. Sot. 103 (1981) 7159. [6] J. Saltiel, S. Ganapathy and B.W. Atwater, J. Am. Chem. Sot. 109 (1987) 1209.
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