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Comparative study of the kinetic behavior of ground and excited state radical pairs in micellar solution
Volume 173, number 4
CHEMICAL PHYSICS LETTERS
12 October 1990
Comparative study of the kinetic behavior of ground and excited state radical pairs in micellar solution * J.C. Scaiano ’ and Ji-Liang Shi Division of Chemistry NationalResearch Council, Ottawa,Ontario. Canada KlA OR6
Received 20 June 1990
The kinetics of geminate processes of ground and excited diphenylmethyl radical pairs have been examined in micellar solution using laser photolysis techniques. The radicals do not escape from the micelles in the submicrosecond time scale in which geminate processes-which are controlled by spin evolution -take place. Excited radical lifetimes are 120-140 ns in micellized radical pairs, but become comparable to typical homogeneous values (circa 250 ns) in the presence of magnetic fields.
1. Introduction
Several studies over the last decade have examined the behavior of triplet-derived radical pairs (RP) and radical ion pairs in various organized media, particularly micelles [ l-61. It is now believed that the kinetic behavior of geminate RPs is determined by the interplay of spin evolution and diffusional characteristics. Benzylic radicals have played an important role in time-resolved studies of RPs in micelles, largely due to their ease of generation and detection by optical spectroscopy. In particular diphenylmethyl radicals, which are the subject of this study, have been examined by Turro et al. [ 7 1. As part of a systematic study of the dynamics and magnetic field effects on the behavior of RPs, we now report a comparative study of the ground and excited states of diphenylmethyl radicals generated according to the reaction Ph2CHCOCHPh2 2
-co
2Ph2CH.
(1)
ing to a RP where one of the radicals is electronically excited. Alternatively, the same result can be achieved using two-laser two-color techniques [ 8 1. In addition, we were interested in examining whether isoenergetic energy transfer between the partners in this RP would have any effect on the dynamics or spectroscopy of the radicals. Carbon centered RPs in micelles typically have geminate lifetimes between 0.1 and 1 ps [7,9-121. In order to examine the effect of electronic excitation on micellar geminate behavior it is convenient to select an excited radical with a lifetime in this range. In this sense the diphenylmethyl radical, with an excited state lifetime around 250 ns [ 13-151 is an ideal choice. In this study we have employed laser flash photolysis techniques to examine the behavior of the diphenylmethyl-diphenylmethyl RP in anionic and cationic micelles. We have also examined the photochemistry of l,ldiphenylacetone, where the rapid escape of CH&O’ effectively yields isolated diphenylmethyl radicals, as a reference compound.
This reaction only yields ground state radicals following the rapid decarbonylation of PhzCHCO’ [ 7 ]; however, some of the diphenylmethyl radicals are readily excited by the tail of the laser pulse thus lead-
2. Experimental
* Issuedas NRCC 31525.
1,1,3,3_tetraphenylacetone (TPA) was prepared by a modification of the method reported by Dean et al. [ 16] which was described in some detail in .an
’ To whom correspondence should be addressed.
0009-2614/90/S 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
271
Volume 173, number 4
CHEMICAL PHYSICS LETTERS
earlier publication [ 15] _ 1,I-diphenylacetone (DPA ) from Aldrich was purified by recrystallization from ethanol. Sodium dodecyl sulfate (SDS), cetyltrimethylammonium chloride (CTAC), bromide (CTAB) and other surfactants were from the same sources as indicated earlier [ 12,171. The preparation of DPA-containing micellar solutions was carried out by adding surfactant and water to enough ketone to yield a 5 x 1Oe3 M solution. The mixture was sonicated at ~45°C. After standing overnight the clear solution was used. For TPA ( 1x 10e3 M) the solutions were frequently turbid due to incomplete dissolution; in these cases they were filtered through a Millex-CG1, filter. Typical absorbances at 308 nm were 0.4-0.6 for DPA and 0.25-0.4 for TPA. The samples ( z 2 ml) were contained in cells constructed of 7x 7 mm2 Suprasil tubing. They were deaerated by bubbling with oxygen-free nitrogen. The magnetic fields were supplied by the same home-built magnet employed earlier [ 171. A Lumonics TE-8602 excimer laser operated with Xe/HCl mixtures (308 nm, x 5 ns, 640 mJ/pulse) was employed for excitation. Front face excitation at a circa 20” angle with respect to the analyzing beam was used for most experiments (to allow room for the magnet), although a few measurements were also carried out with a 90” geometry (for H=O). Further details have been reported elsewhere [ 15,181. A few fluorescence spectra were recorded with an EG&G gated-intensified optical multichannel analyzer.
12 October 1990
decay process. This is not surprising, and is an indication of essentially quantitative separation of the RP. We attribute this to rapid exit of the acetyl radicals from the micelle; under these conditions the decay of the radicals results from random encounters and the kinetic behavior is expected to parallel that in homogeneous solution. In the case of TPA the behavior is quite different, For example the traces in SDS reveal a fast decay with a lifetime around 2 10 ns leading to variable amounts of residual absorption (vide infra). The decay kinetics are in agreement with the values reported by Turro et al. [ 7,111 under similar (but not identical) conditions. Fig. 1 (top) shows a typical trace obtained at full laser power, leading to about 40% residual. These residuals have usually been attributed to radical exit from the micelle and the kinetics for geminate and exit processes are interpreted using a kinetic model proposed earlier [ 17,191. However, in this case we were surprised by the amount of residual, which would require rates of radical exit in excess of 1O6s- ’ for diphenylmethyl; comparison with other radicals and molecules [ 19,201 suggests that this value would be too high for a hydrophobic radical of tbis molecular size. Further, the amount of
3. Results and discussion 3. I. Ground state radical decay 308 nm laser excitation of either DPA or TPA in any of the micelles under study yields the intense and characteristic absorption spectrum from diphenylmethyl with ,I,,,,,= 332 nm [ 15 1. The absorptions due to acetyl radicals formed from DPA are expected to be too weak to be detectable in the presence of diphenylmethyl; no signals from CH$O’ were anticipated and none were observed. Photolysis of DPA in miceilar solutions led to very long-lived signals, with half lives in excess of 20 ps and where the decay was dominated by second-order contributions to the
272
0
0.4
0.8
1.2
1.6
Time p Fig. 1. Transient decay traces for ground state diphenylmethyl (from TPA) in SDS 0.2 M monitored at 332 run for full (top) laser dose and 4.7% dose (bottom).
12 October1990
CHEMICAL PHYSICS LETTERS
Volume 173, number 4
residual observed is circa twice that reported by Turro et al. [7]; it has been our experience that these values are considerably more reproducible that this. Studies of the power dependence of the micellar photochemistry of TPA show that the residual signals have a different origin in this case; thus, when the laser energy is reduced to z 5% of the available dose the residual is absent (fig. 1, bottom). We attribute the residual to a two-photon process induced at high laser powers. The residual absorption (such as that in fig. I, top) was a quadratic function of the total concentration of diphenylmethyl generated. It is interesting to note that Turro [ 71 has established and we have confirmed that the spectrum of the residual agrees well with that for diphenylmethyl. This is consistent with the idea of a two-photon process that removes one diphenylmethyl radical from the RP thus leaving the other one as a long-lived detectable transient. Such removal can involve the photochemistry of Ph&H- of Ph,CHCO’, presumably to yield a species which exits rapidly from the micelle or is unreactive towards Ph&H‘. Thus, our low laser dose data show that there is no need to invoke exit of diphenylmethyl radicals from the micelle. This means that the rates of decay measured can be completely attributed to geminate processes. Application of magnetic fields led to a dramatic decrease of the rate of radical decay; fig. 2 shows such a decay in SDS micelles recorded at 3 kG. Note in the expansion of this figure the growth observed following laser excitation; this is due to ground state repopulation concurrent with the decay of the excited radical (vide infra). This is not observed in the absence of a magnetic field because it is masked by
the fast geminate decay. At low powers the decay is very slow, but ev-entually complete (i.e. no residual). Table 1 summarizes the data in various micellar systems. 3.2. Excited radical decay The kinetics for excited diphenylmethyl radical decay can be monitored either by transient absorption at around 355 nm or by its fluorescence decay in the 500-600 nm range. Both signals could be detected in these systems and led to the same lifetimes. The signal-to-noise was significantly better for the fluorescence and all the values reported here are based on this method. These measurements were performed at 530 nm. Photolysis of DPA in micellar solutions led to excellent decay traces with lifetimes in the 250-265 ns range, i.e. virtually identical to the values in homogeneous inert solvents [ 13-151. Fig. 3 shows a representative decay trace in CTAC micelles. No magnetic field effect (up to 5 kG) was observed in any of the micelles examined. Again, this is the expected result if complete separation of the RP takes place when the mobile CHsCO’ radical is involved. Emission spectra recorded for diphenylmethyl were identical whether they were based on DPA, TPA, or two-laser experiments where the radicals were reexcited by a 337 nm pulse at different delays after their generation. Thus, the interaction with the second radical and the possibility of energy hopping appears to have no effect on the spectroscopic properties of Ph&H ’ . The excited lifetimes for the radicals produced from TPA were shorter than those recorded with DPA
Table 1 Kinetic data for the decay of ground and excited state radical pairs Substrate
Surfactant (M)
DPA DPA DPA
SDS (0.2) CTAC (0.05) CTAB (0.05)
TPA TPA TPA
SDS (0.2) CTAC
(0.05)
CTAB (0.05)
Ground state r(w) >20 >20 215 0.21 0.84 0.84
Excited state r(ns)(H=O)
T(ns)(H=3 kG)
259 254 194
265 250 200
120 140 127
245 235 190
&rn (s-l)
4.4x 106 3.2x 10’ 2.7x lo6
273
CHEMICAL PHYSICS LETTERS
Volume 173, number 4
0.02 t
Fig. 2. Transient trace monitored at 330 nm for TPA in SDS micelles in the presence of a 3 kG field. Insert: Expansion of the region indicated showing a detail of the groundstate repopulation.
as a precursor. Typical lifetimes for SDS, CTAC and CTAB have been included in table 1. Even shorter lifetimes (around SO- 100 ns) were recorded in so-
I2 October 1990
dium octyl sulfate and sodium decyl sulfate, although the signal-to-noise in these systems was rather poor. We attribute these short lifetimes to geminate processes in the excited RP. We believe the excited RP must retain its triplet character, and the observed decay must be due to the sum of the rate constants for fluorescent decay and for spin evolution to a singlet excited RP which decays rapidly either by chemical means or by deactivation to a ground state RP. Application of a magnetic field leads to an increase in the fluorescence lifetime which approaches the value for isolated radicals as determined from the DPA experiments. As usual, we interpret these observations in terms of a slow-down of triplet-singlet interconversion resulting from Zeeman splitting of the triplet sublevels. The high-field values have been included in table 1. A few observations are worth noting. In the case of CTAB the high field lifetimes, as well as the DPAderived value are around 200 ns, i.e. shorter than the typical ( s 250 ns) values obtained in other systems. Most likely the presence of bromide as a counterion reduces the excited state lifetime; consistently with
260 240 220 8 6
200
. +*
t+
+
+
‘t
1
l I
180
+.
+
i+ t*r t
160
f
0.4
0.8
1.2
1.6
Time, w
2
3
4
5
Field, kG Fig. 3. Magnetic field dependence of the lifetimes for excited diphenylmethyl radicals in CTAC micelles (0.05 M) using DPA (0 ) and TPA(m) as precursors. Insert: DPA (left) and TPA (right) derived fluorescence decay traces at zero field.
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CHEMICAL PHYSICSLETTERS
this, no effect is observed in CTAC. The observation of long lifetimes in SDS and CTAC raises some questions regarding triplet sublevel interconversion; in principle, Zeeman splitting can only slow down intersystem crossing from T, and T_, but not T,,. While our decay traces frequently show a few fast points preceding the main decay (fig. 3, right insert), it is not clear whether this reflects T,, behavior, or minor impurity emission; we note, however, that a similar component is not observed in PDA traces such as the one in the left insert in fig. 3. Interestingly, it is possible to separate the factors responsible for excited state decay in TPA-derived RPs. Thus, the lifetime at H=O is related to &.,, according to f-‘(H=O)=kgcm+k~~~),
(2)
where kgem refers to the processes that lead to access of the singlet surface via spin evolution and the spontaneous decay refers to the emissive and radiationless processes involved in the decay of diphenylmethyl; it can be estimated as either the reciprocal of the high field lifetime or the DPA-derived lifetime. We have preferred the latter approach to avoid any uncertainty relating to the To behavior (vide supra). We were surprised to find that in the case of SDS the value of ksem is essentially identical to its value for the ground state RP (i.e. the reciprocal of 0.21 ps, see table 1). However, we feel that this is probably just a coincidence, since in the cases of CTAC and CTAB the values are different, k_,, being (2.7-3.2) X lo6 s-’ for the excited state and 1.2~ lo6 s-’ for the ground state. The fact that the differences are larger in the larger micelles suggests that k&,, tends to differ under conditions where its value is largely controlled by hypefine interactions, while other mechanisms (spin-orbit coupling, spin-rotational coupling [21,22] ) may differ less for the two RPs. Further studies would clearly be needed before the generality of these ideas can be established.
4. Conclusion Our studies show that micellar geminate processes in triplet-derived RPs are not dramatically different for ground and excited states; only in the case of large micelles, where we presume that hypefine interac-
I2 October1990
tions are dominant, are significant differences observed. We note that while free radicals are known to be excellent quenchers of excited radicals, such mechanisms would not be expected to be operative for a spin correlated (triplet) RP. Finally, our studies show that the exit of diphenylmethyl radicals from the micelles (SDS, CTAC or CTAB) is too slow to compete with submicrosecond geminate processes. The residual absorptions which frequently detected in the decay traces should be attributed to two-photon processes induced at high laser doses.
Acknowledgement Thanks are due to SE. Sugamori and G. Charette for technical assistance.
References [I] N.J.Turro,PureAppl.Chem. 58 (1986) 1219, [2] J.C. Scaiano,Trans. Roy. Sot. Can. 21 (1983) 133. [3] Y. Tanimoto and M. Itoh, in: Physical organic chemistry 1986, Vol. 31, ed. M. Kobayashi (Elsevier, Amsterdam, 1987) p. 257. [4]N.J. Two, and B. Kraeutler, Accounts Chem. Res. 13 ( 1980) 360. [5] S. Nagakura, and H. Hayashi, Intern. J. Quantum Chem. 18 (1984) 571. [6] U.E. Steiner, and T. Ulrich, Chem. Rev. 89 (1989) 51. [ 71 I.R. Gould, M.B. Zimmt, N.J. Two, B.H. Baretz and G.F. Lehr, J. Am. Chem. Sot. 107 ( 1985) 4607. [ 8 ] J.C. Scaiano, L.J. Johnston, W.G. McGimpscy and D. Weir, Accounts Chem. Res. 2 1 ( 1988) 22. [9] J.C.Scaiano, Accounts Chem. Res. 15 (1982) 252. [ 101N.J.Two, M.B. Zimmt, X.G. Lei, 1-R.Gould, KS. Nitsche and Y. Cha, J. Phys. Chem. 91 (1987) 4544. [ 111NJ. Two, M.B. Zimmt and I.R. Gould, J. Phys. Chem. 92 (1988) 433. [ 121J.C. Scaiano and D.J. Lougnot, J. Phys. Chem. 88 (1984) 3379. [ 131A. Bromberg, K.H. Schmidt and D. Meisel, J. Am. Chem. Sot. 106 (1984) 3056. [ 141 A. Bromberg, K.H. Schmidt and D. Meisel, J. Am. Chem. Sot. 107 (1985) 83. [ 15] J.C. Scaiano, M. Tanner and D. Weir, J. Am. Chem. Sot. 107 (1985) 4396. [ 161 D.O. Dean, W.B. Dickinson, O.R. Quayle and CT. Lester, J. Am. Chem. Sot. 72 (1950) 1740. [ 17 ] C.H. Evans and J.C. Scaiano, J. Am. Chem. Sot. 112( 1990.) ~2694.
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CHEMICAL PHYSICS LETTERS
[18] J.C. Scaiano, J. Am. Chem. Sot. 102 (1980) 7747. [ 19 ] J.C. Scaiano, E.B. Abuin and L.C. Stewart, J. Am. Chem. Sot. 104 (1982) 5673. [20] J.C.Se1wynandJ.C. Scaiano,Can. J.Chem. 59 (1981) 663.
276
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[21]I.D.R. Stevens, M.T.H. Liu, N. Soundarajan and N. Paike, Tetrahedron Letters 30 ( 1989) 48 1. [ 221 T. Uhich, U.E. Steiner and W. Schlenker, Tetrahedron 42 (1986) 6131.
CHEMICAL PHYSICS LETTERS
12 October 1990
Comparative study of the kinetic behavior of ground and excited state radical pairs in micellar solution * J.C. Scaiano ’ and Ji-Liang Shi Division of Chemistry NationalResearch Council, Ottawa,Ontario. Canada KlA OR6
Received 20 June 1990
The kinetics of geminate processes of ground and excited diphenylmethyl radical pairs have been examined in micellar solution using laser photolysis techniques. The radicals do not escape from the micelles in the submicrosecond time scale in which geminate processes-which are controlled by spin evolution -take place. Excited radical lifetimes are 120-140 ns in micellized radical pairs, but become comparable to typical homogeneous values (circa 250 ns) in the presence of magnetic fields.
1. Introduction
Several studies over the last decade have examined the behavior of triplet-derived radical pairs (RP) and radical ion pairs in various organized media, particularly micelles [ l-61. It is now believed that the kinetic behavior of geminate RPs is determined by the interplay of spin evolution and diffusional characteristics. Benzylic radicals have played an important role in time-resolved studies of RPs in micelles, largely due to their ease of generation and detection by optical spectroscopy. In particular diphenylmethyl radicals, which are the subject of this study, have been examined by Turro et al. [ 7 1. As part of a systematic study of the dynamics and magnetic field effects on the behavior of RPs, we now report a comparative study of the ground and excited states of diphenylmethyl radicals generated according to the reaction Ph2CHCOCHPh2 2
-co
2Ph2CH.
(1)
ing to a RP where one of the radicals is electronically excited. Alternatively, the same result can be achieved using two-laser two-color techniques [ 8 1. In addition, we were interested in examining whether isoenergetic energy transfer between the partners in this RP would have any effect on the dynamics or spectroscopy of the radicals. Carbon centered RPs in micelles typically have geminate lifetimes between 0.1 and 1 ps [7,9-121. In order to examine the effect of electronic excitation on micellar geminate behavior it is convenient to select an excited radical with a lifetime in this range. In this sense the diphenylmethyl radical, with an excited state lifetime around 250 ns [ 13-151 is an ideal choice. In this study we have employed laser flash photolysis techniques to examine the behavior of the diphenylmethyl-diphenylmethyl RP in anionic and cationic micelles. We have also examined the photochemistry of l,ldiphenylacetone, where the rapid escape of CH&O’ effectively yields isolated diphenylmethyl radicals, as a reference compound.
This reaction only yields ground state radicals following the rapid decarbonylation of PhzCHCO’ [ 7 ]; however, some of the diphenylmethyl radicals are readily excited by the tail of the laser pulse thus lead-
2. Experimental
* Issuedas NRCC 31525.
1,1,3,3_tetraphenylacetone (TPA) was prepared by a modification of the method reported by Dean et al. [ 16] which was described in some detail in .an
’ To whom correspondence should be addressed.
0009-2614/90/S 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
271
Volume 173, number 4
CHEMICAL PHYSICS LETTERS
earlier publication [ 15] _ 1,I-diphenylacetone (DPA ) from Aldrich was purified by recrystallization from ethanol. Sodium dodecyl sulfate (SDS), cetyltrimethylammonium chloride (CTAC), bromide (CTAB) and other surfactants were from the same sources as indicated earlier [ 12,171. The preparation of DPA-containing micellar solutions was carried out by adding surfactant and water to enough ketone to yield a 5 x 1Oe3 M solution. The mixture was sonicated at ~45°C. After standing overnight the clear solution was used. For TPA ( 1x 10e3 M) the solutions were frequently turbid due to incomplete dissolution; in these cases they were filtered through a Millex-CG1, filter. Typical absorbances at 308 nm were 0.4-0.6 for DPA and 0.25-0.4 for TPA. The samples ( z 2 ml) were contained in cells constructed of 7x 7 mm2 Suprasil tubing. They were deaerated by bubbling with oxygen-free nitrogen. The magnetic fields were supplied by the same home-built magnet employed earlier [ 171. A Lumonics TE-8602 excimer laser operated with Xe/HCl mixtures (308 nm, x 5 ns, 640 mJ/pulse) was employed for excitation. Front face excitation at a circa 20” angle with respect to the analyzing beam was used for most experiments (to allow room for the magnet), although a few measurements were also carried out with a 90” geometry (for H=O). Further details have been reported elsewhere [ 15,181. A few fluorescence spectra were recorded with an EG&G gated-intensified optical multichannel analyzer.
12 October 1990
decay process. This is not surprising, and is an indication of essentially quantitative separation of the RP. We attribute this to rapid exit of the acetyl radicals from the micelle; under these conditions the decay of the radicals results from random encounters and the kinetic behavior is expected to parallel that in homogeneous solution. In the case of TPA the behavior is quite different, For example the traces in SDS reveal a fast decay with a lifetime around 2 10 ns leading to variable amounts of residual absorption (vide infra). The decay kinetics are in agreement with the values reported by Turro et al. [ 7,111 under similar (but not identical) conditions. Fig. 1 (top) shows a typical trace obtained at full laser power, leading to about 40% residual. These residuals have usually been attributed to radical exit from the micelle and the kinetics for geminate and exit processes are interpreted using a kinetic model proposed earlier [ 17,191. However, in this case we were surprised by the amount of residual, which would require rates of radical exit in excess of 1O6s- ’ for diphenylmethyl; comparison with other radicals and molecules [ 19,201 suggests that this value would be too high for a hydrophobic radical of tbis molecular size. Further, the amount of
3. Results and discussion 3. I. Ground state radical decay 308 nm laser excitation of either DPA or TPA in any of the micelles under study yields the intense and characteristic absorption spectrum from diphenylmethyl with ,I,,,,,= 332 nm [ 15 1. The absorptions due to acetyl radicals formed from DPA are expected to be too weak to be detectable in the presence of diphenylmethyl; no signals from CH$O’ were anticipated and none were observed. Photolysis of DPA in miceilar solutions led to very long-lived signals, with half lives in excess of 20 ps and where the decay was dominated by second-order contributions to the
272
0
0.4
0.8
1.2
1.6
Time p Fig. 1. Transient decay traces for ground state diphenylmethyl (from TPA) in SDS 0.2 M monitored at 332 run for full (top) laser dose and 4.7% dose (bottom).
12 October1990
CHEMICAL PHYSICS LETTERS
Volume 173, number 4
residual observed is circa twice that reported by Turro et al. [7]; it has been our experience that these values are considerably more reproducible that this. Studies of the power dependence of the micellar photochemistry of TPA show that the residual signals have a different origin in this case; thus, when the laser energy is reduced to z 5% of the available dose the residual is absent (fig. 1, bottom). We attribute the residual to a two-photon process induced at high laser powers. The residual absorption (such as that in fig. I, top) was a quadratic function of the total concentration of diphenylmethyl generated. It is interesting to note that Turro [ 71 has established and we have confirmed that the spectrum of the residual agrees well with that for diphenylmethyl. This is consistent with the idea of a two-photon process that removes one diphenylmethyl radical from the RP thus leaving the other one as a long-lived detectable transient. Such removal can involve the photochemistry of Ph&H- of Ph,CHCO’, presumably to yield a species which exits rapidly from the micelle or is unreactive towards Ph&H‘. Thus, our low laser dose data show that there is no need to invoke exit of diphenylmethyl radicals from the micelle. This means that the rates of decay measured can be completely attributed to geminate processes. Application of magnetic fields led to a dramatic decrease of the rate of radical decay; fig. 2 shows such a decay in SDS micelles recorded at 3 kG. Note in the expansion of this figure the growth observed following laser excitation; this is due to ground state repopulation concurrent with the decay of the excited radical (vide infra). This is not observed in the absence of a magnetic field because it is masked by
the fast geminate decay. At low powers the decay is very slow, but ev-entually complete (i.e. no residual). Table 1 summarizes the data in various micellar systems. 3.2. Excited radical decay The kinetics for excited diphenylmethyl radical decay can be monitored either by transient absorption at around 355 nm or by its fluorescence decay in the 500-600 nm range. Both signals could be detected in these systems and led to the same lifetimes. The signal-to-noise was significantly better for the fluorescence and all the values reported here are based on this method. These measurements were performed at 530 nm. Photolysis of DPA in micellar solutions led to excellent decay traces with lifetimes in the 250-265 ns range, i.e. virtually identical to the values in homogeneous inert solvents [ 13-151. Fig. 3 shows a representative decay trace in CTAC micelles. No magnetic field effect (up to 5 kG) was observed in any of the micelles examined. Again, this is the expected result if complete separation of the RP takes place when the mobile CHsCO’ radical is involved. Emission spectra recorded for diphenylmethyl were identical whether they were based on DPA, TPA, or two-laser experiments where the radicals were reexcited by a 337 nm pulse at different delays after their generation. Thus, the interaction with the second radical and the possibility of energy hopping appears to have no effect on the spectroscopic properties of Ph&H ’ . The excited lifetimes for the radicals produced from TPA were shorter than those recorded with DPA
Table 1 Kinetic data for the decay of ground and excited state radical pairs Substrate
Surfactant (M)
DPA DPA DPA
SDS (0.2) CTAC (0.05) CTAB (0.05)
TPA TPA TPA
SDS (0.2) CTAC
(0.05)
CTAB (0.05)
Ground state r(w) >20 >20 215 0.21 0.84 0.84
Excited state r(ns)(H=O)
T(ns)(H=3 kG)
259 254 194
265 250 200
120 140 127
245 235 190
&rn (s-l)
4.4x 106 3.2x 10’ 2.7x lo6
273
CHEMICAL PHYSICS LETTERS
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0.02 t
Fig. 2. Transient trace monitored at 330 nm for TPA in SDS micelles in the presence of a 3 kG field. Insert: Expansion of the region indicated showing a detail of the groundstate repopulation.
as a precursor. Typical lifetimes for SDS, CTAC and CTAB have been included in table 1. Even shorter lifetimes (around SO- 100 ns) were recorded in so-
I2 October 1990
dium octyl sulfate and sodium decyl sulfate, although the signal-to-noise in these systems was rather poor. We attribute these short lifetimes to geminate processes in the excited RP. We believe the excited RP must retain its triplet character, and the observed decay must be due to the sum of the rate constants for fluorescent decay and for spin evolution to a singlet excited RP which decays rapidly either by chemical means or by deactivation to a ground state RP. Application of a magnetic field leads to an increase in the fluorescence lifetime which approaches the value for isolated radicals as determined from the DPA experiments. As usual, we interpret these observations in terms of a slow-down of triplet-singlet interconversion resulting from Zeeman splitting of the triplet sublevels. The high-field values have been included in table 1. A few observations are worth noting. In the case of CTAB the high field lifetimes, as well as the DPAderived value are around 200 ns, i.e. shorter than the typical ( s 250 ns) values obtained in other systems. Most likely the presence of bromide as a counterion reduces the excited state lifetime; consistently with
260 240 220 8 6
200
. +*
t+
+
+
‘t
1
l I
180
+.
+
i+ t*r t
160
f
0.4
0.8
1.2
1.6
Time, w
2
3
4
5
Field, kG Fig. 3. Magnetic field dependence of the lifetimes for excited diphenylmethyl radicals in CTAC micelles (0.05 M) using DPA (0 ) and TPA(m) as precursors. Insert: DPA (left) and TPA (right) derived fluorescence decay traces at zero field.
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this, no effect is observed in CTAC. The observation of long lifetimes in SDS and CTAC raises some questions regarding triplet sublevel interconversion; in principle, Zeeman splitting can only slow down intersystem crossing from T, and T_, but not T,,. While our decay traces frequently show a few fast points preceding the main decay (fig. 3, right insert), it is not clear whether this reflects T,, behavior, or minor impurity emission; we note, however, that a similar component is not observed in PDA traces such as the one in the left insert in fig. 3. Interestingly, it is possible to separate the factors responsible for excited state decay in TPA-derived RPs. Thus, the lifetime at H=O is related to &.,, according to f-‘(H=O)=kgcm+k~~~),
(2)
where kgem refers to the processes that lead to access of the singlet surface via spin evolution and the spontaneous decay refers to the emissive and radiationless processes involved in the decay of diphenylmethyl; it can be estimated as either the reciprocal of the high field lifetime or the DPA-derived lifetime. We have preferred the latter approach to avoid any uncertainty relating to the To behavior (vide supra). We were surprised to find that in the case of SDS the value of ksem is essentially identical to its value for the ground state RP (i.e. the reciprocal of 0.21 ps, see table 1). However, we feel that this is probably just a coincidence, since in the cases of CTAC and CTAB the values are different, k_,, being (2.7-3.2) X lo6 s-’ for the excited state and 1.2~ lo6 s-’ for the ground state. The fact that the differences are larger in the larger micelles suggests that k&,, tends to differ under conditions where its value is largely controlled by hypefine interactions, while other mechanisms (spin-orbit coupling, spin-rotational coupling [21,22] ) may differ less for the two RPs. Further studies would clearly be needed before the generality of these ideas can be established.
4. Conclusion Our studies show that micellar geminate processes in triplet-derived RPs are not dramatically different for ground and excited states; only in the case of large micelles, where we presume that hypefine interac-
I2 October1990
tions are dominant, are significant differences observed. We note that while free radicals are known to be excellent quenchers of excited radicals, such mechanisms would not be expected to be operative for a spin correlated (triplet) RP. Finally, our studies show that the exit of diphenylmethyl radicals from the micelles (SDS, CTAC or CTAB) is too slow to compete with submicrosecond geminate processes. The residual absorptions which frequently detected in the decay traces should be attributed to two-photon processes induced at high laser doses.
Acknowledgement Thanks are due to SE. Sugamori and G. Charette for technical assistance.
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