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Isotope and temperature effects on the lifetime of excited diphenyl ketyl radicals
Volume 129, number 2
CHEMICAL PHYSICS LETTERS
22 August 1986
ISOTOPE AND TEMPERATURE EFFECTS ON THE LIFETIME OF EXCITED DIPHENYL KETYL RADICALS * Linda J. JOHNSTON,
D.J. LOUGNOT
’ and J.C. SCAIANO
Dwisron of Chemistry, Nattonal Research Councrl of Canada, Ottawa, Ontario, Canada KIA OR6
Received 11 May 1986
The lifetime of excited diphenyl ketyl radicals is lengthened by deuterium substitution. The largest effect is obyrved by substitution at the hydroxyllc positioni for example, the lifetimes are 3.9, 4.2, 8.7 and 10.5 ns for (C6H5)2COH, (C&)#OH, (C6Hs)2COD and (C&)2COD, respectively, in toluene or toluene-da at room temperature. Deuterium substitution in the solvent has no effect other than providing a different atom for the hydroxylic position in the ketyl radical.
1. Introduction
The photophysics and photochemistry of excited free radicals have been the subject of considerable interest in the last few years [l-7 1. Unfortunately, direct, time-resolved studies of their reactivity are frequently limited by their short lifetimes. An exception to this behavior is the example of the excited diphenylmethyl radical, which at room temperature in solution has a lifetime of around 260 ns [ 1,3]. This has allowed extensive studies of its intermolecular reactivity [3]. In the case of the 1-naphthylmethyl radical the excited state lifetime is =35 ns, again, sufficiently long to allow studies of its intermolecular reactivity [4,5]. Other radicals have considerably shorter lifetimes at room temperature in solution; these short lifetimes frequently limit the type of work that can be carried out. An extreme case of this behavior is benzyl, which at room temperature has an excited state lifetime of less than 1 ns, in spite of the fact that at 77 K its lifetime exceeds 1 ~.ts[8 ] ; the reasons for this unusual temperature dependence have been recently studied [9]. The excited diphenyl ketyl radical has been the subject of several studies dealing, in particular, with * Issued as NRCC 25478. ’ Visiting Scientist from Laboratoire de Photocbimie G&&ale, UA-431/CNRS, 68093, Mulhouse Cedex, France.
0 009.2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
its fluorescent properties [6,7,10-141. Studies of its intermolecular reactions have been limited to a couple of recent measurements by Nagarajan and Fessenden [7] and competitive studies by Baumann et al. [lo]. We are presently engaged in a detailed study of the excited state behavior of short-lived reaction intermediates. We find that excited diphenyl ketyl radicals are representative of a large group of excited reaction intermediates, in particular, free radicals, which have solution lifetimes of l-5 ns, i.e. sufficiently long to allow their detection rather easily, but at the same time short enough to limit severely the possibilities of studying other properties (such as substituent effects, intermolecular reactivity and energy transfer) in any detail. While picosecond techniques would have ready access to this time scale, the pump-probe techniques normally used for short-lived intermediates are limited to short (
(1)
We have used two-laser excitation techniques where slow radical formation is not a problem, but that are limited to the time resolution of nanosecond techniques, in our system less than 2 ns. In this paper we report the details of a study of isotope and tempera205
Volume 129, number 2
CHEMICAL PHYSICS LETTERS
ture effects aimed at establishing the magnitude of these effects on the lifetimes of excited diphenyl ketyl radicals. This study was also undertaken in the hope that the results obtained would allow us to establish the best conditions for other studies of substituent effects and intermolecular reactivities and perhas also suggest some of the trends to be expected in other excited reaction intermediates.
2. Experimental Benzophenone (Aldrich) was recrystallized twice from ethanol/water mixtures. Toluene (BDH) was dried over molecular sieves (4A) before use. Toluened8 (99.8% D; Merck, Sharp and Dohme) was used as received. Methyl methacrylate (BDH) was washed with alkali and distilled under vacuum. Laser flash photolysis studies were carried out using the same system described elsewhere [3,14]. Ketyl radicals were generated by photoreduction of benzophenone (h,, , d10 or l3C), excited at 308 nm, by toluene (hg or d8) used as solvent. In the “all protic” system at 29 1 K the lifetime for this process (i.e. reaction (1)) is 230 ns (vide infra). The radicals were then excited by the pulses from a 337 nm laser, allowing for a sufficiently long delay to ensure that ketyl radical formation was at least 90% complete. For kinetic studies the 337 nm pulses were from a PBA model LN-1000 laser, providing pulses of =600 ps duration and approximately 1 mJ/pulse. Kinetic measurements were carried out by monitoring the ketyl radical fluorescence at 580 nm. Fluorescence spectra were recorded using a Molectron W-24 nitrogen laser for excitation (337 nm, =:8 ns, QlO mJ/pulse) and an EC&G gated intensified optical multichannel analyzer (OMA) to record the spectra. A 20 ns gate, timed to coincide with the 337 nm laser excitation was used. One-laser, nanosecond transient absorption measurements used to establish the formation of ketyl radicals and its time evolution were carried out with 308 nm excitation (Lumonics TE-860.2 excimer laser) and the same system described before. Excitation of the ketyl radicals at 337 nm leads to some unavoidable excitation of ground state benzophenone. This does not interfere with the measurements because it does not lead to ketyl fluorescence (an advantage of the slow radical 206
22 August 1986
formation here). That is, the 337 nm laser does not yield ketyl fluorescence unless it is preceded by the “synthesis” 308 nm pulse. Several experiments were carried out with the methyl ether of a-phenylbenzoin to generate the methoxydiphenylmethyl radical according to the reaction PhCOC(OCH3)Ph2 + PhtO + Ph,COCH, .
(2)
The precursor benzoin ether was synthesized from (Y,a’-diphenylacetophenone following a literature procedure [ 151.
3. Results and discussion Fig. 1 shows the fluorescence spectra of (C6Hs)2COH and Of (C6H5)&CH3 in toluene at room temperature. The spectrum of (C,H&OH agrees with, and is an improvement over similar spectra in solution already available in the literature [ 1l131. No vibrational structure can be identified in fii. 1, and no further resolution was observed upon deuterium substitution (vide infra). However, a small increase in resolution and some narrowing of the emission band were observed at low temperature (190 K in toluene). Fig. 2 illustrates two representative traces, one corresponding to the instrument function (using the PRA laser) and the other to the fluorescence decay for (C6H5)2CoD at 280 K. Lifetimes were recorded in this way for a series of benzophenone-toluene isotopic combinations; these have been summarized in table 1. In order to obtain further evidence for the origin of the luminescence shown in fig. 1, the fluorescence intensity was measured at several delays after the 308 nm generation of the ketyl,radicals, i.e. the delay between the two lasers was changed while keeping the OMA gate coincident with 337 nm excitation. The intensity of the ketyl radical fluorescence (I) was shown to be a function of the delay between the two lasers (see fg 3). The fluorescence intensity, which serves as a probe for ketyl radical concentration, reached a maximum 1, with a growth lifetime,of 215 f 15 ns which agrees well with the decay lifetime (23 1 f 5 ns) of triplet benzophenone after 308 nm excitation. These results provide further support for the assign-
Volume 129, number 2
400
CHEMICAL PHYSICS LETTERS
500
700
600 Wavelength,
800
nm
Fig. 1. Fluorescence spectra of radicals in toluene at 291 K; (+) (CeHs)aCOH and
ment of the 580 nm fluorescence to the ketyl radical. As can be seen from table 1, the general trend is towards lengthening of the excited ketyl radical lifetimes upon deuterium substitution; however, deuteration at the hydroxylic position has the largest effect. This is in fact quite fortunate, since this is the position
22 August 1986
(0) (C6H5)2~~H3.
Table 1 Lifetimes of excited ketyl radicals at room temperature in toluene Radical
Solvent
(CeH5)zCOH (CeHs)zCOH (CeHs)aCOD (CeDs 12’9 (CeDs)aCCH (C, Ds )sCDD (C~H~)Z’~COH (CeHs)sCCCHs
C6w33
CeDsCDs CeDsCDs C6Hs
CH3
CeDsCDs CeDsCDs CeHsCHs CeHsCHs
7*/n~ a) 3.9 4.0 b) 8.1 4.2 4.4b) 10.5 3.9 3.5
a) Typical errors are f 0.5 ns. b) In the presence of 5 X 10m2 hi of 1 P-cyclohexadiene; under these conditions, more than 80% of the ketyl radicals are formed through hydrogen abstraction from 1 P-cyclohexadiene [ 161.
Fig. 2. Time-resolved emission from (CaHs)sCOD in tolueneds at 280 K (+) and instrument response with PRA Laser (continuous line) under identical conditions.
where the deuterium atom is supplied by the solvent. Deuterium substitution in the solvent appears to have no effect other than as an H/D source (table 1). The longer lifetime under these conditions substantially facilitates time-resolved studies; for example, it was straightforward to determine the rate constant for fluorescence quenching by methylmethacrylate, which led to k, = 3.6 X lo9 M-l s-l in toluene-dg. 207
22 August 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 2
o.o3[->I 0.02
ooo--qA B
250
D
560
7b
lob0 Y-
'
,A~"---
120
cl/" d / P
80
r'" 40
1
A AA
I 0
A A
250
500 750
AA
1000
I
I
I
I
I
3.5
4.0
4.5
5.0
5.5
*
1000/T Fig. 4. Arfhenius plots for fluorescence lifetimes from (a) (CeHs)sCOH in toluene and (b) (CeHs)aCOD in toluene-da; only the filled points were used to derive the Arrhenius parameters.
Time, ns
Fig. 3. (top) Decay of benaophenone triplet at 590 nm after excitation by the “synthesis” pulse (308 nm); (middle) fluorescence intensity of (CeHs)s COH as a function of the delay between the “synthesis” and excitation pulse (3 37 nm) ; (bottom) first-order treatment of the triplet decay of benzophenone )riplet (right) and the fluorescence growth from (CeHs)sCOH (left).
This may be compared with values of 1.7 X lOlo and 2.6 X 109 M-l s-r reported by Baumann et al. [6] in acetonitrile, based on competitive (Stern-Volmer) studies. The difference between these two literature values has not been explained. The lifetimes of excited diphenyl ketyl radicals in organic solvents are known to be controlled by their chemical decay, rather than by photophysical processes. For example, Nagarajan and Fessenden [7] have dem-
208
onstrated that 532 nm laser excitation of the benzophenone ketyl leads to extensive bleaching (monitored at 335 nm) that has been attributed to as yet unidentified chemical reactions leading to radical depletion. We thought it would be interesting to examine the temperature dependence of representative examples and to compare this behavior with that observed in benzyl and related radicals. Fig. 4 shows Arrhenius plots for (C6H,)#OH and (C6Hg)2&D in toluene andtoluene-dg, reSptXtiVdy.TheSeplOtSlead to: -log(r*) = (9.22 + 0.05) - l:‘3”;;
f-or (C,H,),h, -log(r*) = (9.41 * 0.1) - 17;$=
for(c,Hg)&oD, where the errors are quoted as k.2 u. Although the slopes for the two plots show only
Volume 129, number 2
CHEMICALPHYSICS LETTERS
minor differences, only the lifetime for (C6HShCOD levels off at low temperatures. Similar results have been observed for benzyl and benzyl-d, radicals, both of which show temperature-independent fluorescence lifetimes below =115 K, as well as for triphenylmethyl and a-methylbenzyl radicals [9]. For (C,H,)COH in fig. 4 the longest lifetime corresponds to 10.9 ns at 185 K which is very close to the melting point of the solvent. Several studies report lifetime values between 17 and 23 ns at 77 K for this radical [7,17]. If we assume that the lifetime will level off at approximately that value, then extrapolation of the Arrhenius data in fig. 4 indicates that the lifetime will become temperature independent at -150 K. For both (C6HS)&H and (CeHg)@D, the temperaturedependent lifetimes are assumed to be determined by chemical reactions. The nature of the reactions responsible for excited state deactivation cannot be established from the present data; however, judging from the isotope effects (H/D) of =2.2, it seems clear that transfer of the H atom from the hydroxylic position must play an important role in the transition state. Interaction with the solvent may play a role in aqueous solvents, where the lifetimes are considerably shorter, but is unlikely to be a dominant mode in nonaqueous solvents since lifetimes in toluene (table 1) and in acetonitrile with 1% cyclohexane [7] are similar. Two other possibilities which are currently being examined are the addition of the hydroxylic hydrogen to one of the aromatic rings, or the ejection of a hydrogen atom from the excited ketyl; the latter would lead to the highly unstable H atom. However, systems leading to other unstablelspecies (e;‘g.aryl radicals) are not unprecedented under conditions photon excitation [ 181.
of two-
Experiments with (C6H5)2CCCH3 (see fig. 1) were carried out in the hope of observing longer lifetimes than those for (C,H,),COH. However, to our surprise, replacement of H by CH3 at the oxygen center leads, if anything, to a shortening of the lifetime (table 1). A study of the temperature dependence of the fluorescence lifetime for the etherderived radical leads to -log(r*) = (9.50 + 0.08) -
22 August 1986
structural differences between this radical and the ketyl radicals, a change in their chemical decay paths could account for these different behaviors. Product studies are planned in order to establish if this hypothesis is correct.
4. Conclusion Deuterium substitution in (CeH&!OH leads to increased excited radical lifetimes in all cases; however, the effect is largely due to substitution at the hydroxylic site. Thus, the lifetimes for (C6Hs),COH and (C6HS)2C0D in toluene and toluene.dg at 291 K are 3.9 and 8.7 ns, respectively. &rbstitution by methyl at the hydroxylic position does not lead to any lengthening of the lifetime, a rather surprising observation which is tentatively attributed to chemical deactivation paths different from those available to the ketyl radical itself. A study of the temperature dependence of the excited-state lifetimes led to linear Arrhenius plots in the 185-290 K temperature range for (C6Hg)2COH, but a curved Arrhenius plot was obtained for (C6HS)2CCD. The latter result is similar to results for several benzyl and substituted benzyl radicals [9 1. For (C6H5)2COH a comparison of the Arrhenius data and the reported lifetimes at 77 K suggests that a temperature-independent lifetime should be observable below 150K. From the point of view of future studies, temperature effects may be of only limited value as ways of increasing the lifetime of excited radicals; while they may facilitate the study of intramolecular effects (i.e. substituent effects), the situation may be quite different for intermolecular reactions, since in general these processes can be expected to have higher activation energies than those derived from fig. 4. A detailed examination of substituent effects on the spectroscopic properties and reactivity of excited ketyl radicals as well as product studies for several systems are in progress.
1560 f 80 2.3 RT
for (C6H5)20CH3.
Again the lifetime became temperature independent in the region below 180 K. In spite of the small
Acknowledgement Thanks are due to Dr. D. Meisel and Dr. R.W. Fessenden for the communication of unpublished 209
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CHEMICAL PHYSICS LETTERS
material and to Mr. S.E. Sugamori for technical assistance. DJL thanks the CNRS/NRC cooperative program for support and gratefully acknowledges the hospitality of the National Research Council of Canada.
References
VI A. Bromberg, K.H. Schmidt and D. Meisel, J. Am. Chem. Sot. 107 (1985) 83.
PI A. Bromberg and D.‘Meisel, J. Phys. Chem. 89 (1985) 2507. [31 J.C. Sceiano, M. Tanner and D. Weir, J. Am. Chem. Sot. 107 (1985) 4396. 141 L.J. Johnston and J.C. Scaiano, J. Am. Chem. Sot. 107 (1985) 6368. [51 E.F. Hilinski, D. Huppert, D.F. KeIley, S.V. Milton and P.M. Rentzepis, J. Am. Chem. Sot. 106 (1984) 1951; K. Tokumura, hi. Vdagawa and M. Itoh, J. Phys. Chem. 89 (1985) 5147. [61 H. Baumann, C. Merckel, H.-J. Timpe, A. Graness, J. Kleinschmidt, I.R. Gould and NJ. Turro, Chem. Phys. Letters 103 (1984) 497.
210
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[7] V. Nagarajan and R.W. Fessenden, Chem. Phys. Letters 112 (1984) 207. [ 81 J.D. Laposa and V. Morrison, Chem. Phys. Letters 28 (1974) 270; T. Okamura, K. Obi and I. Tanaka, Chem. Phys. Letters 26 (1974) 218; T. Okamura and I. Tanaka, J. Chem. Phys. 79 (1975) 2728. [9] D. Meisel, P.K. Das, G.L. Hug, K. Bhattacharyya and R.W. Fessenden, J. Am. Chem. Sot., submitted for publication. [lo] H. Baumamt, K.P. Schumacher, H.-J. Timpe and V. Rehak, Chem. Phys. Letters 89 (1982) 315. [ll] M.R. Topp,Chem. Phys. Letters 39 (1976) 423. [ 12 ] 0. Brede, W. Hehnstreit and R. Mehnert, Z. Physik. Chem. (Leipzig) 256 (1975) 5 13. [ 13 ] K. Razi Naqvi and UP. Wild, Chem. Phys. Letters 41 (1976) 570. [14] J.C. Scaiano, J. Am. Chem. Sot. 102 (1980) 7747. [15] A. Werner, Chem. Berich. 39 (1906) 1298. [16] M.V. Encinasand J.C. Scaiano, J. Am. Chem. Sot. 103 (1981) 6393. [17] K. Obiand H. Yamaguchi, Chem. Phys. Letters 54 (1978) 448. [18] J.C. Scaiano and P.J. Wagner, J. Am. Chem. Sot. 106 (1984) 4626.
CHEMICAL PHYSICS LETTERS
22 August 1986
ISOTOPE AND TEMPERATURE EFFECTS ON THE LIFETIME OF EXCITED DIPHENYL KETYL RADICALS * Linda J. JOHNSTON,
D.J. LOUGNOT
’ and J.C. SCAIANO
Dwisron of Chemistry, Nattonal Research Councrl of Canada, Ottawa, Ontario, Canada KIA OR6
Received 11 May 1986
The lifetime of excited diphenyl ketyl radicals is lengthened by deuterium substitution. The largest effect is obyrved by substitution at the hydroxyllc positioni for example, the lifetimes are 3.9, 4.2, 8.7 and 10.5 ns for (C6H5)2COH, (C&)#OH, (C6Hs)2COD and (C&)2COD, respectively, in toluene or toluene-da at room temperature. Deuterium substitution in the solvent has no effect other than providing a different atom for the hydroxylic position in the ketyl radical.
1. Introduction
The photophysics and photochemistry of excited free radicals have been the subject of considerable interest in the last few years [l-7 1. Unfortunately, direct, time-resolved studies of their reactivity are frequently limited by their short lifetimes. An exception to this behavior is the example of the excited diphenylmethyl radical, which at room temperature in solution has a lifetime of around 260 ns [ 1,3]. This has allowed extensive studies of its intermolecular reactivity [3]. In the case of the 1-naphthylmethyl radical the excited state lifetime is =35 ns, again, sufficiently long to allow studies of its intermolecular reactivity [4,5]. Other radicals have considerably shorter lifetimes at room temperature in solution; these short lifetimes frequently limit the type of work that can be carried out. An extreme case of this behavior is benzyl, which at room temperature has an excited state lifetime of less than 1 ns, in spite of the fact that at 77 K its lifetime exceeds 1 ~.ts[8 ] ; the reasons for this unusual temperature dependence have been recently studied [9]. The excited diphenyl ketyl radical has been the subject of several studies dealing, in particular, with * Issued as NRCC 25478. ’ Visiting Scientist from Laboratoire de Photocbimie G&&ale, UA-431/CNRS, 68093, Mulhouse Cedex, France.
0 009.2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
its fluorescent properties [6,7,10-141. Studies of its intermolecular reactions have been limited to a couple of recent measurements by Nagarajan and Fessenden [7] and competitive studies by Baumann et al. [lo]. We are presently engaged in a detailed study of the excited state behavior of short-lived reaction intermediates. We find that excited diphenyl ketyl radicals are representative of a large group of excited reaction intermediates, in particular, free radicals, which have solution lifetimes of l-5 ns, i.e. sufficiently long to allow their detection rather easily, but at the same time short enough to limit severely the possibilities of studying other properties (such as substituent effects, intermolecular reactivity and energy transfer) in any detail. While picosecond techniques would have ready access to this time scale, the pump-probe techniques normally used for short-lived intermediates are limited to short (
(1)
We have used two-laser excitation techniques where slow radical formation is not a problem, but that are limited to the time resolution of nanosecond techniques, in our system less than 2 ns. In this paper we report the details of a study of isotope and tempera205
Volume 129, number 2
CHEMICAL PHYSICS LETTERS
ture effects aimed at establishing the magnitude of these effects on the lifetimes of excited diphenyl ketyl radicals. This study was also undertaken in the hope that the results obtained would allow us to establish the best conditions for other studies of substituent effects and intermolecular reactivities and perhas also suggest some of the trends to be expected in other excited reaction intermediates.
2. Experimental Benzophenone (Aldrich) was recrystallized twice from ethanol/water mixtures. Toluene (BDH) was dried over molecular sieves (4A) before use. Toluened8 (99.8% D; Merck, Sharp and Dohme) was used as received. Methyl methacrylate (BDH) was washed with alkali and distilled under vacuum. Laser flash photolysis studies were carried out using the same system described elsewhere [3,14]. Ketyl radicals were generated by photoreduction of benzophenone (h,, , d10 or l3C), excited at 308 nm, by toluene (hg or d8) used as solvent. In the “all protic” system at 29 1 K the lifetime for this process (i.e. reaction (1)) is 230 ns (vide infra). The radicals were then excited by the pulses from a 337 nm laser, allowing for a sufficiently long delay to ensure that ketyl radical formation was at least 90% complete. For kinetic studies the 337 nm pulses were from a PBA model LN-1000 laser, providing pulses of =600 ps duration and approximately 1 mJ/pulse. Kinetic measurements were carried out by monitoring the ketyl radical fluorescence at 580 nm. Fluorescence spectra were recorded using a Molectron W-24 nitrogen laser for excitation (337 nm, =:8 ns, QlO mJ/pulse) and an EC&G gated intensified optical multichannel analyzer (OMA) to record the spectra. A 20 ns gate, timed to coincide with the 337 nm laser excitation was used. One-laser, nanosecond transient absorption measurements used to establish the formation of ketyl radicals and its time evolution were carried out with 308 nm excitation (Lumonics TE-860.2 excimer laser) and the same system described before. Excitation of the ketyl radicals at 337 nm leads to some unavoidable excitation of ground state benzophenone. This does not interfere with the measurements because it does not lead to ketyl fluorescence (an advantage of the slow radical 206
22 August 1986
formation here). That is, the 337 nm laser does not yield ketyl fluorescence unless it is preceded by the “synthesis” 308 nm pulse. Several experiments were carried out with the methyl ether of a-phenylbenzoin to generate the methoxydiphenylmethyl radical according to the reaction PhCOC(OCH3)Ph2 + PhtO + Ph,COCH, .
(2)
The precursor benzoin ether was synthesized from (Y,a’-diphenylacetophenone following a literature procedure [ 151.
3. Results and discussion Fig. 1 shows the fluorescence spectra of (C6Hs)2COH and Of (C6H5)&CH3 in toluene at room temperature. The spectrum of (C,H&OH agrees with, and is an improvement over similar spectra in solution already available in the literature [ 1l131. No vibrational structure can be identified in fii. 1, and no further resolution was observed upon deuterium substitution (vide infra). However, a small increase in resolution and some narrowing of the emission band were observed at low temperature (190 K in toluene). Fig. 2 illustrates two representative traces, one corresponding to the instrument function (using the PRA laser) and the other to the fluorescence decay for (C6H5)2CoD at 280 K. Lifetimes were recorded in this way for a series of benzophenone-toluene isotopic combinations; these have been summarized in table 1. In order to obtain further evidence for the origin of the luminescence shown in fig. 1, the fluorescence intensity was measured at several delays after the 308 nm generation of the ketyl,radicals, i.e. the delay between the two lasers was changed while keeping the OMA gate coincident with 337 nm excitation. The intensity of the ketyl radical fluorescence (I) was shown to be a function of the delay between the two lasers (see fg 3). The fluorescence intensity, which serves as a probe for ketyl radical concentration, reached a maximum 1, with a growth lifetime,of 215 f 15 ns which agrees well with the decay lifetime (23 1 f 5 ns) of triplet benzophenone after 308 nm excitation. These results provide further support for the assign-
Volume 129, number 2
400
CHEMICAL PHYSICS LETTERS
500
700
600 Wavelength,
800
nm
Fig. 1. Fluorescence spectra of radicals in toluene at 291 K; (+) (CeHs)aCOH and
ment of the 580 nm fluorescence to the ketyl radical. As can be seen from table 1, the general trend is towards lengthening of the excited ketyl radical lifetimes upon deuterium substitution; however, deuteration at the hydroxylic position has the largest effect. This is in fact quite fortunate, since this is the position
22 August 1986
(0) (C6H5)2~~H3.
Table 1 Lifetimes of excited ketyl radicals at room temperature in toluene Radical
Solvent
(CeH5)zCOH (CeHs)zCOH (CeHs)aCOD (CeDs 12’9 (CeDs)aCCH (C, Ds )sCDD (C~H~)Z’~COH (CeHs)sCCCHs
C6w33
CeDsCDs CeDsCDs C6Hs
CH3
CeDsCDs CeDsCDs CeHsCHs CeHsCHs
7*/n~ a) 3.9 4.0 b) 8.1 4.2 4.4b) 10.5 3.9 3.5
a) Typical errors are f 0.5 ns. b) In the presence of 5 X 10m2 hi of 1 P-cyclohexadiene; under these conditions, more than 80% of the ketyl radicals are formed through hydrogen abstraction from 1 P-cyclohexadiene [ 161.
Fig. 2. Time-resolved emission from (CaHs)sCOD in tolueneds at 280 K (+) and instrument response with PRA Laser (continuous line) under identical conditions.
where the deuterium atom is supplied by the solvent. Deuterium substitution in the solvent appears to have no effect other than as an H/D source (table 1). The longer lifetime under these conditions substantially facilitates time-resolved studies; for example, it was straightforward to determine the rate constant for fluorescence quenching by methylmethacrylate, which led to k, = 3.6 X lo9 M-l s-l in toluene-dg. 207
22 August 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 2
o.o3[->I 0.02
ooo--qA B
250
D
560
7b
lob0 Y-
'
,A~"---
120
cl/" d / P
80
r'" 40
1
A AA
I 0
A A
250
500 750
AA
1000
I
I
I
I
I
3.5
4.0
4.5
5.0
5.5
*
1000/T Fig. 4. Arfhenius plots for fluorescence lifetimes from (a) (CeHs)sCOH in toluene and (b) (CeHs)aCOD in toluene-da; only the filled points were used to derive the Arrhenius parameters.
Time, ns
Fig. 3. (top) Decay of benaophenone triplet at 590 nm after excitation by the “synthesis” pulse (308 nm); (middle) fluorescence intensity of (CeHs)s COH as a function of the delay between the “synthesis” and excitation pulse (3 37 nm) ; (bottom) first-order treatment of the triplet decay of benzophenone )riplet (right) and the fluorescence growth from (CeHs)sCOH (left).
This may be compared with values of 1.7 X lOlo and 2.6 X 109 M-l s-r reported by Baumann et al. [6] in acetonitrile, based on competitive (Stern-Volmer) studies. The difference between these two literature values has not been explained. The lifetimes of excited diphenyl ketyl radicals in organic solvents are known to be controlled by their chemical decay, rather than by photophysical processes. For example, Nagarajan and Fessenden [7] have dem-
208
onstrated that 532 nm laser excitation of the benzophenone ketyl leads to extensive bleaching (monitored at 335 nm) that has been attributed to as yet unidentified chemical reactions leading to radical depletion. We thought it would be interesting to examine the temperature dependence of representative examples and to compare this behavior with that observed in benzyl and related radicals. Fig. 4 shows Arrhenius plots for (C6H,)#OH and (C6Hg)2&D in toluene andtoluene-dg, reSptXtiVdy.TheSeplOtSlead to: -log(r*) = (9.22 + 0.05) - l:‘3”;;
f-or (C,H,),h, -log(r*) = (9.41 * 0.1) - 17;$=
for(c,Hg)&oD, where the errors are quoted as k.2 u. Although the slopes for the two plots show only
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minor differences, only the lifetime for (C6HShCOD levels off at low temperatures. Similar results have been observed for benzyl and benzyl-d, radicals, both of which show temperature-independent fluorescence lifetimes below =115 K, as well as for triphenylmethyl and a-methylbenzyl radicals [9]. For (C,H,)COH in fig. 4 the longest lifetime corresponds to 10.9 ns at 185 K which is very close to the melting point of the solvent. Several studies report lifetime values between 17 and 23 ns at 77 K for this radical [7,17]. If we assume that the lifetime will level off at approximately that value, then extrapolation of the Arrhenius data in fig. 4 indicates that the lifetime will become temperature independent at -150 K. For both (C6HS)&H and (CeHg)@D, the temperaturedependent lifetimes are assumed to be determined by chemical reactions. The nature of the reactions responsible for excited state deactivation cannot be established from the present data; however, judging from the isotope effects (H/D) of =2.2, it seems clear that transfer of the H atom from the hydroxylic position must play an important role in the transition state. Interaction with the solvent may play a role in aqueous solvents, where the lifetimes are considerably shorter, but is unlikely to be a dominant mode in nonaqueous solvents since lifetimes in toluene (table 1) and in acetonitrile with 1% cyclohexane [7] are similar. Two other possibilities which are currently being examined are the addition of the hydroxylic hydrogen to one of the aromatic rings, or the ejection of a hydrogen atom from the excited ketyl; the latter would lead to the highly unstable H atom. However, systems leading to other unstablelspecies (e;‘g.aryl radicals) are not unprecedented under conditions photon excitation [ 181.
of two-
Experiments with (C6H5)2CCCH3 (see fig. 1) were carried out in the hope of observing longer lifetimes than those for (C,H,),COH. However, to our surprise, replacement of H by CH3 at the oxygen center leads, if anything, to a shortening of the lifetime (table 1). A study of the temperature dependence of the fluorescence lifetime for the etherderived radical leads to -log(r*) = (9.50 + 0.08) -
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structural differences between this radical and the ketyl radicals, a change in their chemical decay paths could account for these different behaviors. Product studies are planned in order to establish if this hypothesis is correct.
4. Conclusion Deuterium substitution in (CeH&!OH leads to increased excited radical lifetimes in all cases; however, the effect is largely due to substitution at the hydroxylic site. Thus, the lifetimes for (C6Hs),COH and (C6HS)2C0D in toluene and toluene.dg at 291 K are 3.9 and 8.7 ns, respectively. &rbstitution by methyl at the hydroxylic position does not lead to any lengthening of the lifetime, a rather surprising observation which is tentatively attributed to chemical deactivation paths different from those available to the ketyl radical itself. A study of the temperature dependence of the excited-state lifetimes led to linear Arrhenius plots in the 185-290 K temperature range for (C6Hg)2COH, but a curved Arrhenius plot was obtained for (C6HS)2CCD. The latter result is similar to results for several benzyl and substituted benzyl radicals [9 1. For (C6H5)2COH a comparison of the Arrhenius data and the reported lifetimes at 77 K suggests that a temperature-independent lifetime should be observable below 150K. From the point of view of future studies, temperature effects may be of only limited value as ways of increasing the lifetime of excited radicals; while they may facilitate the study of intramolecular effects (i.e. substituent effects), the situation may be quite different for intermolecular reactions, since in general these processes can be expected to have higher activation energies than those derived from fig. 4. A detailed examination of substituent effects on the spectroscopic properties and reactivity of excited ketyl radicals as well as product studies for several systems are in progress.
1560 f 80 2.3 RT
for (C6H5)20CH3.
Again the lifetime became temperature independent in the region below 180 K. In spite of the small
Acknowledgement Thanks are due to Dr. D. Meisel and Dr. R.W. Fessenden for the communication of unpublished 209
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material and to Mr. S.E. Sugamori for technical assistance. DJL thanks the CNRS/NRC cooperative program for support and gratefully acknowledges the hospitality of the National Research Council of Canada.
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