J. Phys. Chem. 1988, 92, 1742-1746
1742
TABLE V: Calculated and Experimental Rotational and Centrifugal Distortion Constants
calcd"
exptlb
pyrrole calcdo exptlb
A', MHz E' C'
9453.6 9143.8 4648.1
9447.1 9246.7 4670.8
9102.3 8935.6 4509.1
9130.6 9001.4 4532 I
8052.6 5374.0 3223 1
AJ, kHz AJK
1.653 -0.186 1.657 0.661 1.297
1.749 -0.264 1.880 0.698 1.31 I
1.464 -0.310 1.625 0.589 1.076
1.532 -0.341 1.810 0.615 1.109
0.767 -0.286 2.214 0.286 0846
furan
AK 65
6,
thiophene calcd'
"This work. Wlodarczak et al. and therefore a very useful tool in the accurate experimental determination of harmonic and anharmonic force constants. An interesting feature of the calculated out-of-plane vibrational frequencies is also noted, namely, that these are lower than the
experimental fundamentals. Finally, as a further help to the deconvolution of the experimental spectra, the IR intensities of the absorption bands are calculated and found to be in agreement with the qualitative experimental results. Note Added in Proof. In a recent paper, Wlodarczak, Martinache, and Demaison ( J . Mol. Spectrosc. 1988, 127, 200) investigated the rotational spectra of furan and pyrrole and published experimental values for the rotational and centrifugal distortion constants of these molecules. They regretted that ab initio values are not often available. We have used the program SPECTRO (J. F. Gaw, University of Cambridge) to generate these quantities from our harmonic force fields; these are given in Table V in the A reduction, representation 1'.
Acknowledgment. E.D.S. is grateful to the EEC for financial support. Registry No. Furan, 110-00-9; pyrrole, 109-97-7; thiophene, 110-02-1.
Excited-State Propertles of Arylmethyl Radicals Containing Naphthyl, Phenanthryl, and Blphenyl Moieties' D. Weir: L. J. Johnston, and J. C. Scaiano* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario K l A OR6, Canada (Received: July 7 , 1987; In Final Form: October 6 , 1987)
The excited-stateproperties of radicals I-V have been examined in solution by using two-photon, two-laser excitation techniques. The radicals were normally generated by photolysis (308 nm) of the corresponding halomethyl precursor. The radicals were then excited by laser irradiation at 337 nm, and the fluorescence spectra, lifetimes, and, whenever possible, transient spectra (Le., I and IV) for the excited radicals were recorded. For example, for the 2-phenanthrylmethyl radical (I), the excited-state lifetime is 79 ns in toluene at room temperature, where it fluoresces with = 593 nm and its absorption spectrum shows A,, = 400 nm. The lifetime of excited I shows only a very minor temperature dependence ( E , = 0.6 kcal/mol in the 183-340 K range).
Introduction Recent reports by Meisel et al.334on the detection of emission and transient absorption from diphenylmethyl and related radicals in solution at room temperature have stimulated a number of studies of the excited-state properties of benzylic radicals. The excited state of the parent radical, benzyl, proved to be a rather elusive species at room temperature. In a recent study Meisel et alS5 have examined the temperature dependence of the fluorescence from this radical and have demonstrated that the expected fluorescence lifetime at room temperature would be around 800 ps. The short lifetime is due to the close proximity of the 12A2 and 22B2 levels, the latter providing an efficient deactivation pathway in solution at room temperature. Structural modifications (such as in diphenylmethy13,4,6.7or 1-naphthylcan lead to an increased separation between these two (1) Issued as NRCC No. 281 10. (2) Present address: Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556. (3) Bromberg, A.; Schmidt, K. H.; Meisel, D. J . A m . Chem. SOC.1984, 106, 3056. (4) Bromberg, A.; Schmidt, K. H.; Meisel, D. J . A m . Chem. SOC.1985, 107, 83. (5) Meisel, D.; Das, P. K.; Hug, G. L.; Bhattacharyya, K.; Fessenden, R. W. J . A m . Chem. SOC.1986, 108, 4706. (6) Scaiano, J. C.; Tanner, M.; Weir, D. J . A m . Chem. SOC.1985, 107, 4396. (7) Weir, D.; Scaiano, J. C. Chem. Phys. Let!. 1986, 128, 156.
0022-3654/88/2092-1742$01.50/0
Published
levels and, as a result, a longer lifetime and more efficient fluorescence. In our earlier study of 1-naphthylmethyl radical6 we made the intriguing observation that the absorption spectrum of the excited radical was very similar to that of triplet naphthalene. It would be interesting to establish if this characteristic is shared by other systems; however, our current knowledge of the properties of excited radicals is too limited to decide whether this is the case. In this paper we report the results of a series of studies undertaken to broaden our knowledge on free-radical kxcited states and to address the question raised above. Our experiments have led to the characterization of several radicals which had not been observed before. Our studies deal with the fluorescence spectra and lifetimes of all the systems examined and the transient absorption properties of those where such measurements proved feasible.
Experimental Section Materials and General Techniques. 2-(Bromomethy1)phenanthrene and 4-(bromomethy1)biphenyl were prepared by bromination (N-bromosuccinimide) of 2-methylphenanthrene and 4-methylbiphenyl, respectively.' I 2-(Bromomethyl)naphthalene (8) Johnston, L. J.; Scaiano, J. C. J . A m . Chem. SOC.1985, 107, 6368. (9) Scaiano, J. C.; Johnston, L. J. Pure Appl. Chem. 1986, 58, 1273. (10) Hilinski, E. F.; Huppert, D.; Kelly, D. F.; Milton, S.V.; Rentzepis, P. M. J . A m . Chem. SOC.1984, 106, 1951. 1988
by the American Chemical Society
Excited-State Properties of Arylmethyl Radicals
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1743
CHART I
I Oo5-
.0.06
Br
m
IV 0 0002 4
V
,L , - , ,
II
I
IT
VI 300
450
5 00
WAVELENGTH, nm
I
VI I
4 00
350
iH2
VIII
(Aldrich), 2-(chloromethy1)naphthalene (ICN), l-bromo-2(bromomethy1)naphthalene (Sigma), and 4,4’-dimethylbiphenyl (Aldrich) were recrystallized before use. Triethylamine was distilled from sodium hydroxide. Solvents were all spectrograde or Aldrich Gold Label and were used as received. Gas chromatography analyses were done on a Perkin-Elmer 8320 gas chromatograph with a 12-m BP1 on vitreous silica capillary column. Fluorescence spectra were recorded on a Perkin-Elmer LS-5 fluorescence spectrometer. Fluorescence lifetimes were measured by using a PRA single photon counting system with a hydrogen lamp. Laser Photolysis Experiments. These experiments were generally carried out using a flow system in order to avoid the accumulation of fluorescent photoproducts. Unless otherwise indicated, the samples were deaerated by nitrogen bubbling in a container connected to the reaction cell using Teflon lines. The irradiation cell was constructed of 7 X 7 mm2 square Suprasil tubing. Flow rates sufficient to ensure that each pair of laser shots excited a fresh portion of solution were maintained during the experiments. The details of our experimental setup and of the modifications required for two-laser experiments have been reported elsewhere.6J2 Briefly, a Lumonics TE860-2 excimer laser (308 nm, -5 ns, 1 2 0 mJ/pulse) was used as a synthesis laser, in order to generate the free radical of interest. After a suitable delay (typically 0.5-3 ps), the pulses from a Molectron UV-24 nitrogen laser (337.1 nm, -8 ns, 4 or 9 mJ/pulse) or a Candela UV-5OOM dye laser were used to excite the free radical under study. While the various radicals show absorption maxima at different wavelengths, they are all sufficiently absorbing at 337 nm to make nitrogen lasers adequate excitation sources. Our system also includes a PRA-1000 nitrogen laser (337.1 nm, -600 ps, -1 mJ/pulse); given the low energy per pulse delivered by this laser, it was only utilized when the excited radicals were very short lived and required the use of a short excitation pulse. The risetime of the detection system was 1.6 ns. In most cases the radicals were generated from the corresponding bromo precursor, with the exception of the radical from 4,4’-dimethylbiphenyl, which was generated by reaction of tertbutoxyl radicals with this substrate. tert-Butoxyl radicals were in turn generated by 308-nm excitation of di-tert-butyl peroxide. Kinetic measurements were carried out on traces initially acquired by a Tektronix R7912 transient digitizer interfaced to a PDP 11/23 computer which also controls the experiment. Spectral data were acquired by use of an EG&G gated intensified optical (1 1) Bullpitt, M.; Kitching, W.; Doddrell, D.; Adcock, W. J. Org. Chem. 1976, 41, 760.
(12) Scaiano, J. C. J . Am. Chem. Sac. 1980, 102, 7747.
Figure 1. Transient absorption spectra produced by 308-nmexcitation of 2-(bromomethy1)phenanthrene (I) and 4-(bromomethy1)biphenyl(IV) in benzene and 4,4’-dimethylbiphenyl in 5% di-tert-butyl peroxidebenzene (V). The spectra were recorded 3, 10, and 5 p after excitation for I, IV, and V, respectively.
o,‘2;
d
0
n
o
a
0.04
0.02
0
300
I
400
5 00
600
WAVELENGTH, nm
Figure 2. Transient absorption spectra measured 1.5 and 5.5 ws after 308-nm excitation of 2-(chloromethy1)naphthalene (11) and l-bromo2-(bromomethyl)naphthalene, (111) respectively, in cyclohexane.
multichannel analyzer (OMA) with an EG&G Model 1420 detector head or by using the point-by-point approach which is well-established in laser photolysis techniques. The latter approach was preferred for the measurements of the weak transient absorptions due to excited intermediates. On the other hand, OMA measurements were the technique of choice for all emission measurements.
Results (a) Generation and Characterization of Ground-State Radicals. Chart I shows the structures of the radicals examined in this work, along with those for VI, VII, and VIII, which have been the subject of various studies in our, or other, l a b o r a t ~ r i e s . ~ ~ *The - ’ ~ *radicals ~~ were usually generated from the corresponding chloro or bromo precursors, by 308-nm laser excitation which produces transient concentrations in the 10-40 gM range. The measurements reported in this section are all results from one-laser experiments. The mechanism of the photocleavage of the carbon-halogen bond in these systems has been the subject of several studies and on occasion rather controversial.’0*’4-’6This aspect of the problem (13) Tokumura, K.; Mizukami, N.; Udagawa, M.; Itoh, M. J . Phys. Chem. 1986, 90,3873. (14) Huppert, D.; Rand, S. D.; Reynolds, A. H.; Rentzepis, P. M. J . Chem. Phys. 1982, 77, 1214. (15) Slocum, G . H.; Kaufmann, K.; Schuster, G. B. J . Am. Chem. Sac. 1981, 103, 4625.
1744
Weir et al.
The .Iournal of Physical Chemistry, Vol. 92, No. 7, 1988
TABLE I: Fluorescence Spectra and Lifetimes for Various Benzylic Radicals in Solution at Room Temperature
'........ = I ....... ...... -hl ~- , ... : mi . i ~. .' ............ IW: I - ( ...... , ........(...... z r--I -- .,...-" . .I
I
........
,:
I
C-.
I
I....
.
L.
r.
.
1
5
I
;
:,
500
?
=I
...........
600 700 WAVELENGTH , nm
radical solvent I toluene 11 cyclohexane 11 benzene I1 methanol 11 triethylamine Ill cyclohexane Ill benzene 111 methanol IV benzene V benzene VI 2-methyltetrahydrofuran VI1 methanol VI11 hexane
Ann, nm
593 605
609 607 605 624 630 626
576 582 462" 590 545
7,ns 79 f 5 27 f 2 26 f 2 27 f 2 27 f 2 24 f 2 20 f 2 22 f 2 14 i~ 3 10 f 3 -0.8' 35 21
ref this work this work this work this work this work this work this work this work this work this work 5 8 13
"In ethanol at 77 K.'6 'Extrapolated from low-temperature data.5
Figure 3. Fluorescence spectra for radicals 1, 11, Ill, IV, and V in benzenc. Radicals were produced from the same precursors as in Figures 1 and 2 and were excited with a nitrogen laser.
is not really relevant to the work described here. In our experiments photocleavage of the C-X bond provides a convenient source of radicals I-IV, regardless of the details of the mechanism. When the solvent used is benzene, we can readily detect the corresponding r-complexes (Le., C6H6 Cl and C6H6 Br) with absorption maxima at 490 and -540 nm, in addition to the spectra of the corresponding arylmethyl radical. Figures I and 2 show the absorption spectra of radicals I-V in hydrocarbon solvents. The time delay between excitation and the recording of the spectrum was adjusted so as to ensure the complete decay of any n-complexes (if appropriate) or triplet states; the latter are readily detectable at short delay times (e.g., A,, = 480 nm for 2-(bromomethyl)phenanthrene), but in all cases were shorter lived than the free radicals and therefore could be spectrally separated by careful choice of the delay time (see figure captions). Radical half-lives under our experimental conditions were a few microseconds (5-10 p s being a typical range), and the corresponding decays were dominated by secopd-order processes, as expected. For example, in the case of IV, which gave remarkably good second-order fits, we obtain 2kJc = 4.7 X lo6 cm/s at 350 nm. The A,,, values for I1 and IV agreed well those reported in the litrrature.20~21 ( b ) Fluorescence Spectra and Lifetimes. Radical emission spectra were recorded following 337-nm excitation of the radical after a delay comparable with those utilized to record the transient absorption spectra of Figures 1 and 2. The spectra, which generally showed some vibrational structure, are illustrated in Figure 3; the general features of these spectra parallel those of other well-characterized benzylic radical^.^-^^"^^^^*-^* We note that the state responsible for emission is not the same one responsible for absorption. The transition associated with emission is too weak to be readily detectable in the absorption mode. We emphasize that these spectra are quite different from the fluorescence from is 593 nm, the corresponding precursors. For example, for I, while for 2-phenanthrylbromomethane A,, is 368 nm. Table 1 summarizes the fluorescence data and includes results illustrating the rather small solvent dependence observed. A rough
-
-
(16) Slocum, G. H.; Schuster, G. 8.J . Org. Chem. 1984, 49, 2177. (17) Biihler, R. E.; Ebert, M. Nature (London) 1967. 214, 1220. (18) Buhler, R. E. Helv. Chim. Acta 1968, 51, 1558. (19) Louwrier, P. F. W.; Hamill, W. H. J. Phys. Chem. 1969, 7 3 , 1707. (20) Kigawa, H.; Takamuku, S.; Toki, S.; Toki, S.; Kimura, N.; Takeda, S . , Tsumori, K.; Sakurai, H. J. A m . Chem. Soc. 1981, 103, 5176. (21) Porter. G.; Strachan, E. Trans. Furadaji Soc. 1958, 54, 1595. (22) Johnson, P. M.; Albrecht, A. C. J . Chem. Phys. 1968, 48, 8 5 1 ( 2 3 ) Branciaid-Larcher, C.; Migirdicyan, E. Chem. Phys. 1973, 2, 95. (24) Okamura, T.; Tanaka, 1. J . Phys. Chem. 1975, 79, 2728. (25) Sinirnov. V . '4.; Brichkin, S. 8.;Efimov, S. P. Opt. Spektrosk. 1983, 3 5 . 463. ( 2 6 ) Okainura, T.; Obi, K.;Tanaka, I. Chem. Phys. Lett. 1973, 20, 90. ( 2 7 ) Pellois, A.; Ripoche, J. Specrrochim. Acta, Part A 1970, 26A, 1051. ( 9 8 ) Ripoche. J C. R . Hebd. Seances Acod. Sci. 1966, 262, 30.
n
LLL 300
I
I
500 WAVELENGTH, nm
400
600
Figure 4. Transient absorption spectra for the doublet-doublet transition for excited radicals I ( 0 , toluene) and IV (A,benzene). Insert: Decay traces for the absorption (points, monitored at 400 nm) and the fluorescence (solid line, monitored a t 600 nm) of 1 in toluene.
estimate of the fluorescence intensity may be obtained from the fact that the total integrated fluorescence intensity from VI1 is approximately 5% of that from the diphenylketyl radical. For the latter a fluorescence quantum yield of 0.03 has been estimated.29 Fluorescence lifetimes were determined from the time evolution of the emission at or near the corresponding maximum (see Table I). The lifetimes are generally insensitive to the polarity and hydrogen donor ability of the solvent. It should be noted that, given the short excited radical lifetimes and the low transient concentrations, quenching of the excited radicals by halogen atoms or their complexes is unlikely. Quenching by the precursor arylmethyl halide is also improbable given the moderate reactivity of excited diphenylmethyl radical toward carbon tetrachloride (k, = 1.6 X IOs M-' s-I ). The reactivity of other radicals examined is lower than that of diphenylmethyl (see section e below).6 ( c ) Transient Absorption Spectra of Excited Radicals. Experiments of this type are substantially more difficult than the rest of the measurements included in this study. In fact, only a few spectra of this type have been reported in the l i t e r a t ~ r e . ~ ~ ~ . ~ ~ ~ ~ Our experiments were quite successful for I, for which we were able to obtain a detailed absorption spectrum for the excited free radical. Figure 4 shows the corresponding doublet-double absorption spectrum, showing ,A, = 400 nm. This spectrum was recorded in very dilute solution (typically -2 X lo4 M) in order to achieve selective excitation of the radical at 337 nm. Lifetime measurements at 400 nm led to a lifetime of 75 ns, in good agreement with the value determined from the fluorescence measurements. The insert in Figure 4 shows an overlap of the transient absorption and fluorescence decays for I. The residual absorption in the absorption trace is due to production of a small (29) Johnston, L. J.; Lougnot, D. J.; Wintgens, V.; Scaiano, J. C. J . Am. Chem. SOC.,accepted for publication. (30) Nagarajan, V.; Fessenden, R. W. Chem. Phys. L e f t . 1984, 112, 207.
The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988
Excited-State Properties of Arylmethyl Radicals
1745
TABLE II: Temperature Dependence of Excited Radical Decay log
radical
I I11
solvent
toluene toluene IV toluene VI 2-methyltetrahydrofuran VI1 methanol Ph2CH 2-methyltetrahydrofuran
E,'
('4ls-I)
0.6 b -0.07 3.83
7.56 b 7.79 11.9
0.3 0.8
7.7 7.3
Trange,
K
ref
183-340 213-293 180-293 115-180
this work this work this work 5
211-297 345 nm), which is outside the experimental error of the -80-ns lifetime determined for excited I. The agreement between the fluorescence and absorption lifetimes for excited I (vide supra) provides additional confirmation for our assignment. Further, the position of the radical fluorescence band (b,o = 593 nm) is at too long a wavelength to correspond to fluorescence emission from phenanthrene or one of its derivatives. The agreement between fluorescence and absorption lifetimes was also confirmed at other temperatures (vide infra). We have also been able to characterize the transient absorption from excited radical IV*. The corresponding spectrum has been included in Figure 4 and was recorded in experiments similar to those already described for I*, although measurements were considerably more difficult in the case of IV as a result of its short excited-state lifetime; the value of 14 ns obtained from transient absorption measurements is fully consistent with the value derived from fluorescence measurements (see Table I). No attempt was made to characterize the excited state of V using absorption techniques, since its absorption can be anticipated to be similar to IV yet even more difficult to obtain as a result of its very short lifetime. Attempts to detect excited I1 in the absorption mode were unsuccessful. In some experiments we observed a very weak absorption at ca. 430 nm, but it was not possible to identify and assign this absorption conclusively. Attempts to characterize transient absorption from 111 were unsuccessful, although in this particular system transient phenomena involving C-Br cleavage may mask other processes leading to weak signals. This is the only system among those described in this report in which we have observed photochemical degradation of the radical upon photoexcitation in inert (e.g., benzene) solvents. This aspect of the photochemistry of 111, leading to the cleavage of the C-Br bond, is discussed in a separate section. (d) Temperature Dependence of the Lifetimes. These measurements were carried out for a few representative radicals and are based on the decay of the fluorescence emission. Whenever possible, the values at the extremes of the temperature range were confirmed by using transient absorption measurements. Table I1 shows the results of our measurements, along with literature values for VI and VII. We note that the use of toluene as a solvent is not a subject of concern, because any benzyl radicals generated via hydrogen abstraction would not be significantly fluorescent through most of the temperature range studied, and in fact (31) Selwyn, J. C.; Scaiano, J . C. Can. J . Chem. 1981, 59, 663
1 4
8
12
16
TIME , p ~ Figure 5. Decay at 360 nm of transient produced by 308-nm excitation (A)and 308- 430-nm excitation (0) of l-bromo-2-(bromomethyl)naphthalene in benzene.
+
ground-state benzyl has a very sharp absorption spectrum and is essentially transparent at 337 nm.5 Clearly, benzylic radicals derived from PAHs show virtually no temperature dependence when compared with the parent radical in the series (Le., benzyl, VI). ( e ) Intermolecular Quenching by Carbon Tetrachloride. Several intermolecular reactions of excited diphenylmethyl and 1-naphthylmethyl radicals have been examined in earlier studies from our Among these, the reaction with carbon tetrachloride leads to chlorine atom abstraction and is one of the ones showing more selectivity. The lifetime of excited I was measured as a function of carbon tetrachloride concentration in benzene, resulting in a value of 1 X lo6 M-l s-l for k,, the bimolecular rate constant. For excited radical 11essentially the same lifetimes were obtained in either cyclohexane or 1:l cyclohexane-carbon tetrachloride. This allows us to estimate an upper limit of 1 X lo6 M-' s-I for k,. Similar results were obtained for 111. For comparison excited diphenylmethyl and 1naphthylmethyl radicals are quenched by carbon tetrachloride with k, values of 1.6 X lo8 and 4 X lo6 M-' s-l, respectively.6*8 U, Photofragmentation of 1-Bromo-2-Naphthylmethyl Radicals. Laser irradiation of I11 at 337 nm in benzene or cyclohexane leads to irreversible bleaching (monitored at 360 nm). However, the precursor (1-brome2-(bromomethyl)naphthalene)also absorbs at the laser wavelength and produces additional absorption due to 111 which partially compensates for the bleaching. This complication was readily circumvented by irradiating the radical with the pulses from a dye laser (Amu = 430 nm). Figure 5 shows the effect of one- and two-laser irradiation in benzene. The bleaching at 360 nm is also accompanied by the appearance at 540 nm of a new absorption which can be readily characterized as the bromine atom-benzene charge-transfer complex.'* The fact that this signal is not observed upon irradiation of I11 in cyclohexane is consistent with this assignment. Elimination of the bromine atom from 111 would produce a 1,3biradical which would be expected either to yield 2-methylnaphthalene via hydrogen abstraction or to undergo ring closure to give a tricyclic hydrocarbon. Comparison of the products formed from 308 nm alone and 308- plus 430-nm irradiation of 1-bromo-2-(bromomethyl)naphthalenein cyclohexane indicates that irradiation of I11 does not yield an appreciable amount of 2-methylnaphthalene. The two-laser irradiation gives a more complex mixture of products which were not all identified.32
-
Discussion The results herein provide a number of additional examples of the fluorescence and absorption properties of excited free radicals in solution at room temperature. In general, the fluorescence (32) The major product from either lamp (300 nm) or 308-nm laser photolysis is 1-bromo-2-methylnaphthalene along with small amounts of products resulting from oxygen trapping of the 1-bromo-2-naphthylmethyl radical.
1746 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
30 0
400 500 WAVELENGTH ,nm
600
Figure 6. Transient absorption spectra of excited I in toluene ( A ) and triplet phenanthrene in ethanol (0).
spectra show increasing red shifts with increasing degrees of ring delocalization. 9-Anthrylmethyl (VIII), recently reported by Itoh et al.,I3 appears to be the exception to this rule, with its 0,O band being blue-shifted with respect to other radicals such as IV. The reason for this behavior is unclear at this point. The question that we set out to answer when we initially undertook this study is now clearly resolved. The absorption spectrum of excited I is quite different from those of its triplet-state precursor and of triplet phenanthrene. Figure 6 shows an overlap of the absorption spectra for excited I and triplet phenanthrene and clearly illustrates this point. The spectrum of triplet 2(bromomethy1)phenanthrene is similar to that of phenanthrene, although more difficult to obtain due to large amounts of radical signal. Thus, the similarity observed previously between excited 1-naphthylmethyl radicals and naphthalene triplets cannot be generalized to other PAH-benzylic radicals. Interestingly, the absorption spectrum of excited IV does present considerable similarities with triplet biphenyls. In any event, the case of I provides an excellent example showing that those similarities cannot be generalized. The information available on the absorption spectra from radicals of this type is in fact so limited that there is little point in trying to compare or speculate on the origin of these differences. Further work on this area is clearly needed to resolve these questions. The parent benzyl radical (VI) shows nearly 4 kcal/mol activation energy for its excited-state decay.5 This has been at-
Weir et al. tributed to the need to populate the 22B2state that provides a pathway for radiationless decay. In a first approximation the activation energy can be interpreted as the Boltzmann factor associated with the population of the state responsible for decay. The activation energies obtained for I, 11, IV, and VI1 are clearly too small to allow a similar interpretation. If these activation energies reflected the Boltzmann factor required to populate a state responsible for radiationless decay, one would expect virtually no fluorescence at room temperature. Further, the calculations for 1-naphthylmethyl reported by Carsky and Z ~ ~ h r a d nsuggest ik~~ that the energy gap would be much larger for VI1 than for VI. Thus, we assign the near temperature independence of the excited-state lifetimes for I, 11, IV, and VI1 to the shut-off of the mechanism responsible for decay in the case of benzyl. The lifetimes for excited radicals I1 and 111 are 26 and 20 ns, respectively. In terms of differences in the rate constant for decay this corresponds to 1.2 X lo7 s-l, which would be the rate of bromine atom elimination from III* if the effect of the bromine atom on the photophysics of I11 (as compared with 11) can be neglected. In general, our results serve to emphasize that the excited states of free radicals can be sufficiently long lived to undergo intermolecular reactions quite readily; thus, in the case of I*, its reactions with carbon tetrachloride and with oxygen34occur with rate constants of 1 X lo6 and 5.7 X lo9 M-I s-', respectively. Excited benzylic radicals have also been shown to be excellent electron donors, but only modest to poor acceptors. For example, we were surprised to find that excited I1 was not quenched by triethylamine, to the point (see Table I) that the amine could be used as an inert solvent. Registry No. I, 19003-84-0; 11, 7419-61-6; 111, 113160-79-5: IV, 4939-76-8; V, 113180-34-0: VI, 2154-56-5; VII, 7419-60-5; VIII, 16407-06-0; CCl,, 56-23-5; C12, 7782-50-5; Z-BUO', 3141-58-0; tBuOO-1-Bu, 110-05-4; 2-(bromomethyl)phenanthrene, 241 7-66-5; 4(bromomethyl)biphenyl, 2567-29-5; 2-(bromomethyl)naphthalene,93926-4; 2-(chloromethyl)naphthalene, 2506-41-4; l-bromo-2-(bromomethyl)naphthalene, 37763-43-2; 4,4'-dimethylbiphenyI, 613-33-2; 2methylphenanthrene, 253 1-84-2; 4-methylbiphenyl, 644-08-6. (33) Carsky, P.: Zahradnik, R. J . Phys. Chem. 1970, 74, 1249. (34) This rate constant was determined in benzene at room temperature by using the same technique described in the case of excited diphenylmethyl.6 The reaction of I* with oxygen may involve singlet oxygen formation as has been suggested for excited diphenylmethyl radicaL6
