J. Phys. Chem. 1994, 98, 11439- 11443
11439
Flash Photolysis of Nitrated Stilbenes in Acetonitrile Solutions: Triplet-State Lifetimes and Photoproduct Formation Rodney A. J. Borg Aeronautical and Maritime Research Laboratory (AMRL)-DSTO, P.O. Box 4331, Melbourne, Victoria 3001, Australia Received: April 1.5, 1994; In Final Form: July 28, 1994@
Triplet-state lifetimes for 2,4-dinitrostilbene and 2,2',4,4'-tetranitrostilbene of 50 and 25 ns respectively have been determined in acetonitrile solutions using laser flash photolysis. Five other highly nitrated stilbenes, possessing the common structural feature of three nitro groups in the 2, 4, and 6 positions of one benzene ring, have also been studied under the same conditions. For these molecules, rapid photoproduct formation precludes the determination of triplet-state lifetimes and the observed transient species has a lifetime of 34 ns .
Introduction Interest in the trans cis photoisomerization of nitrated stilbene derivatives has yielded a wealth of data on the photophysical properties of these The presence of nitro groups enhances the efficiency of intersystem cr0ssing,4%~ and this facilitates studies of the resultant triplet states. Previous studies have shown that the triplet-state lifetimes are sensitive to solvent polarity, viscosity, and temperature,'S2J0J More recently it was proposed that the decrease in triplet-state lifetime observed in the series of molecules 4,4'-dinitrostilbene(4,4'DNS), 2,2'-dinitrostilbene (22'-DNS), and 2,2',4,4',6,6'-hexanitrostilbene (HNS) was a result of increasing steric hindrance (Chart l).l2 The transient absorption observed at 440 nm for HNS in acetonitrile solution was attributed to a photoproduct with a decay lifetime of 21 ns,12 and it was inferred that the triplet-state lifetime of HNS must be less than the lifetime of this transient. Although triplet-state lifetimes for mononitrated2,l3 and dinitrated'~~.'~ stilbene derivatives in a variety of solvents have been reported, these values cannot be used to test the above hypothesis due to the effects of the solvent on triplet-state lifetimes. Since the hypothesis is presently based on the triplet-state lifetimes of only three molecules, a more detailed investigation is warranted. The aim of this study is to ascertain how the triplet-state lifetime varies for a wider range of nitrated stilbenes in acetonitrile solution. Flash photolysis experiments have been performed for dinitrated, trinitrated, tetranitrated, and pentanitrated stilbene derivatives (Chart 1) to determine if steric hindrance alone can explain the observed trend in triplet-state lifetimes of nitrated stilbenes. Twisting about the central double bond, caused by steric hindrance, can affect the relative positions of the So and TI states of the molecule^,^ and a reduction in the separation of the SO and T1 states is expected to decrease the triplet-state lifetime.3J4J5 In conjunction with the transient lifetime measurements, a series of calculations were performed to provide a measure of the separation of the SO and T1 states independent of a qualitative assessment of the steric factors. A comparison of the calculated singlet-triplet energy gap with the observed triplet-state lifetimes will establish if a correlation exists between these two parameters.
-
'
Experimental Section Materials. The preparation and purification of the trans isomers of 4,4'-DNS and HNS have been described previously.'* ~~
@
Abstract published in Advance ACS Abstracts, October 1, 1994.
CHART 1 R
NO2 2,4-DNS: R = H 2,2',4,4'-TNS: R = NO2
NO2
2,4,6-TriNS: R2 = H, R4 = H, Re = H 2,2',4,6-TNS: Rp = N02, R4 = H, R6 = H 2,4,4',6-TNS: R2 = H, R4 = NO2, Re = H PNS: R2 = NOn, R4 = NO2, Re = H HNS: R2 = NO2, R4 = NOn, Re = NO2
2,4-Dinitrostilbene (2,4-DNS) was synthesised using an established literature procedure16and recrystallized twice from glacial acetic acid to yield yellow crystals, mp 145-146 "C (lit.17 trans 143-145 "C, cis 127 "C). 2,4,6-Trinitrostilbene (2,4,6-TriNS) was prepared as reported in the 1iterature.l8 The product was recrystallised twice from ethanol to yield yellow needles, mp 158-160 "C (lit." 158 "C); 'H NMR (60 MHz, DMSO-d6), 6 9.2 (s, 2H) 7.5 (m, 6H) 6.8 (d, J = 16 Hz, 1H). 2,2',4,6Tetranitrostilbene (2,2',4,6-TNS) was prepared as described previously18 and recrystallized twice from ethanol to yield yellow needles, mp 181-182 "C (lit.17 181 "C); 'H NMR (300 MHz, DMSO-d6) 6 9.14 (s, 2H) 8.09 (d, 1H) 7.86 (d, 1H) 7.67 (m, 2H) 7.55 (d, J = 16.3 Hz, 1H) 7.11 (d, J = 16.3 Hz, 1H). Anal. Calcd for C14H8N408: C, 46.68; H, 2.24; N, 15.55; 0, 35.53. Found: C, 46.75; H, 2.14; N, 15.27; 0, 35.71. 2,4,4',6Tetranitrostilbene (2,4,4',6-TNS) was synthesised using a method described by Ullmann and Gschwind.19 The crude product was recrystallized twice from ethanol to yield yellow crystals, mp 197-199 "C (lit.17 196 "C); 'H NMR (300 MHz, DMSO-d6) 6 9.15 ( s , 2H) 8.29 (d, 2H) 7.88 (d, 2H) 7.78 (d, J = 16.7 Hz, 1H) 6.95 (d, J = 16.7 Hz, 1H). Anal. Calcd for C14H8N408: c , 46.68; H, 2.24; N, 15.55; 0, 35.53. Found: C, 46.89; H, 2.18; N, 15.35; 0, 35.57. 2,2',4,4'-Tetranitrostilbene (2,2',4,4'-TNS) was prepared using a modification of a literature procedure20 for synthesising 2,2',6,6'-tetranitrostilbene. A crude product of 2,4-dinitrobenzylbromide (70%) was prepared using a method described by Kuffner et aLZ1This crude product (5 g) was dissolved in ethanol (50 mL) to which a solution of KOH (1.21 g) in ethanol (15 mL) was added dropwise with stirring. A powder blue product precipitated and was filtered off and dried at the pump. Three recrystallizations from acetic acid gave fine pale yellow needles, mp 268-269 "C (lit.17 260 "C); 'H NMR (300 MHz, DMSO-d6) 6 8.81 (d,
0022-3654/94/2098-11439$04.50/0 Published 1994 by the American Chemical Society
Borg
11440 J. Phys. Chem., Vol. 98, No. 44, I994 2H) 8.62 (m, 2H) 8.21 (m, 2H) 7.79 (s, 2H). Anal. Calcd for C l d ~ N 4 0 8 :C, 46.68; H, 2.24; N, 15.55; 0, 35.53. Found: C, 46.61; H, 2.23; N, 15.16; 0, 35.84. 2,2’,4,4’,6-Pentanitrostilbene (PNS) was prepared by the nitration of 2,4,6-TriNSZ2 The crude product was recrystallized from acetone/petroleum spirit and then from benzenelacetone to give fine light-green needles, mp 199 “C (lit.22 198-199 “C); ‘H NMR (300 MHz, DMSO-&) 6 9.15 (s, 2H) 8.79 (d, 1H) 8.66 (d Of d, 1H) 8.11 (d, 1H) 7.78 (d, J = 16.5 Hz, 1H) 7.16 (d, J = 16.5 Hz, 1H). Anal. Calcd for C1a7N5010: C, 41.50; H, 1.74; N, 17.28; 0, 39.48. Found: C, 41.70; H, 1.67; N, 17.12; 0,39.44. Stilbene derivatives where the aryl rings have different substitution patterns will exhibit splitting of the nonequivalent vinylic protons in the lH NMR spectrum. A coupling constant of approximately 16 Hz is indicative of a trans isomer; thus for 2,4,6-TriNS, 2,2’,4,6-TNS, 2,4,4‘,6-TNS, and PNS the NMR spectra indicate that the respective syntheses produced the trans isomers. HPLC grade acetonitrile (UNICHROM) and water subjected to reverse osmolysis were used. Apparatus. The experimental setup has been described previously,12 but in this work the Ndsilicate glass laser was replaced with a Nd:YAG laser (Continuum NY61-10) and the oscilloscope used was a Tektronix TDS 520 digitizing oscilloscope (500 MHz, 500 MSds) connected to an IBM-compatible PC. Oxygen was removed from solution by successive freezepump-thaw cycles to constant pressure (5 mTorr). Typically four or five cycles were required to achieve this level of vacuum. The decay curves were averaged over 10 laser shots and analyzed by performing a least-squares fit, incorporating convolution with the system response function, to a singleexponential function. The system response function was obtained by measuring the scattered light from a colloidal solution in a quartz cuvette placed in the same position as the cell for the transient absorption measurements. The temporal profile of the measured system response yields a full width at half-maximum of 10 ns. Oxygen quenching rate constants are calculated using LO21 = 1.68 x M1 and [02] = 0.265 x MZ3 for air-saturated acetonitrile and water solutions, respectively. Ultraviolethisible absorption spectra were recorded using a Varian Cary 3 spectrophotometer. ‘H NMR were recorded on a Varian EM360L (60 MHz spectra) or a Bruker AM 300 (300 MHz spectra). n e program HyperChem (AutoDesk, Windows Release 2) running on a PC (486) was used to obtain optimized geometries (minimum energies, minimal atomic forces) and energies for the singlet and triplet states of the nitrated stilbenes. The AM1 method was employed with the restricted Hartree-Fock Hamiltonian (RHF) for the singlet states and the unrestricted Hartree-Fock Hamiltonian (UHF) for the triplet states.
Results and Discussion Transient absorption for degassed and air-saturated acetonitrile solutions of 4,4‘-DNS, 2,4-DNS, and 2,2‘,4,4’-TNS excited at 355 nm are shown in Figure 1. Table 1 summarizes the lifetimes of the transient species extracted from the exponential fits for both the degassed and air-saturated solutions and includes values from the literature for comparison. The measured triplet-state lifetimes of 4,4‘-DNS and 2,4-DNS in acetonitrile compare favorably with previously reported values; however the earlier 2,4-DNS result was obtained in benzene (z = 50 ns),* a solvent expected to give a reduction in the triplet-state lifetime.’ The transient absorption of excited 4,4’-DNS at 500 nm, 2,4-DNS at 530 nm, and 2,2’-DNS at 440 nm have been assigned to T-T absorption of the triplet state.1,2J2 Given the similarity in the oxygen quenching rate constant of 2,2’,4,4’-TNS to the other
a.
0.4I
I
8 C
4 2
n .=f
0.6
,,,
C
0.41jk 0.2 0.0
0
50 100 Time (ns)
150
Figure 1. Transient absomtion of degassed hmuer trace’, and aerated acetonitrile solutions of (ai 4,4’-DNSWat500 nm, (b) 2,4-DNS at 530 nm, and (c) 2,2‘,4,4’-TNS at 460 nm. Dots represent the experimental data, and the solid line represents the curve of best fit.
nitrated stilbenes shown in Table 1, the transient absorption of this molecule is also considered to be due to the T-T absorption of the triplet state. The results shown in Table 1 indicate that the triplet-state lifetime increases for the molecules in the following order: 2,2’,4,4’-TNS < 2,4-DNS < 2,2’-DNS < 4,4’-DNS. It has been proposed recently that the triplet-state lifetime of nitrated stilbenes is correlated with the amount of twisting about the central double bond.12 Briefly, the triplet-state lifetime decreases as the separation of the SOand TI states is r e d u ~ e d . ~ JSteric ~J~ hindrance, leading to loss of molecular planarity, can result in a closer approach of the SOand T1 states. Consideration of steric factors would predict 2,4-DNS to have a longer lifetime than 2,2’-DNS yet the measurements show that 2,4-DNS has a slightly shorter lifetime. The steric effects for 2,2’,4,4’-TNS and 22-DNS will be similar since the nitro groups in the 4,4’ positions should not contribute to steric interactions. In addition, the presence of the 4,4’-nitro groups may be expected to enhance resonance in 2,2‘,4,4‘-TNS, thereby increasing the triplet-state lifetime.12 However, the triplet-state lifetime of 2,2‘,4,4’-TNS is less than half of the lifetime of 2,2’-DNS. Thus the trend in the triplet-state lifetime of nitrated stilbenes cannot be explained solely from a consideration of the steric factors. Since the influence of steric effects is ultimately based on the separation of the SO and T1 states, the measured triplet-state lifetimes were compared with estimates of the &-TI energy gap. Furthermore, it has been shown that the nonradiative lifetime of the triplet state of several cyclic aromatics increases with an increasing energy gap.14 Geometry optimization of the SO and T I electronic states was performed using AM1 calculations and the optimized energies calculated were used to obtain the So-Tl energy separation for selected nitrated stilbenes shown in Table 1. An additional single-point energy
Flash Photolysis of Nitrated Stilbenes
J. Phys. Chem., Vol. 98, No. 44, 1994 11441
TABLE 1. Triplet-State Lifetimes (z) and Oxygen Quenching Rate Constants ( k ~of ) Nitrated Stilbene Derivatives in Acetonitrile Solutionf 5 (ns) AE(SO-TI) (kcal/mol) molecule 1 (nm) degassed aerated k~ ( x 109 M-’ s-1 ) SO optimized SO at TI geometry 2,2’-DNS“ 440 55 f 3 27 f 2 11 f 2 42.3 11.3 55 f 2 6.0 f 0.5 45.4 31.8 4,4’-DNS” 500 125 f 5 4,4’-DNSb 4,4’-DNS‘ 2.4-DNSd 2,4-DNSc 2,2’4,4‘-TNS‘
170 f 17 125 f 3 50 f 5‘ 45 f 2 25.0 f 0.3
500 530 530 460
60f 1
5.4 5.2 f 0.3
26.7 f 0.5 19.9 f 0.1
9 f l 6.1 f 0.4
45.4 45.4 43.7 43.7 44.0
31.8 31.8 31.8 31.8 19.9
Results taken from reference 12. Result taken from reference 1. This work. Result taken from reference 2. e Benzene was the solvent for this lifetime measurement. f1is the transient absorption wavelength. The computed AM1 So-Tl energy gaps for these molecules is also shown. 0.3
I
0
I
50
I
100
I
1
150
Time (ns) Figure 2. Transient absorption of 2,4,6-T1iNS in a degassed acetonitrile solution. Transient absorption at 440 nm, 500 nm, and the difference are represented by curves a, b, and c respectively.
calculation was performed for each nitrated stilbene for the ground singlet state at the optimized geometry of the TI state. If the triplet-state lifetime is solely dependent on the &-TI energy gap and increases with increasing AE(So-T1), then from the lifetime measurements we would expect the calculated AE(So-Tl) to increase for the series 2,2’,4,4’-TNS < 2,4-DNS < 2,2’-DNS .c 4,4’-DNS. The AM1 calculations do not reflect this expectation. The absence of a direct correlation between the measured triplet-state lifetime of nitrated stilbenes and the AM1 computed &-TI energy gap may be due to the neglect of solvent effects in the calculations or other factors important in determining triplet-state lifetimes such as the degree of overlap of the appropriate wave functions24in the nonradiative intersystem crossing from TI to SO. The relatively small tripletstate lifetime for 2,2’,4,4’-TNS may be due in part to an enhancement of this intersystem crossing efficiency because of the increased number of nitro groups for this molecule. Photoproduct formation is not apparent following laser irradiation of acetonitrile solutions of 4.4’-DNS, 2,4-DNS, and 2,2’,4,4‘-TNS. Laser excitation of the degassed or air saturated solutions did not alter the visible absorption spectrum. Previous work indicates that degassed acetonitrile solutions of 2,2’-DNS do not form photoproducts upon laser irradiation at 355 nm although there is some evidence that air-saturated solutions form photoproducts.12 In addition to the lack of a color change upon irradiation, none of the transient absorption measurements for these nitrated stilbenes exhibit a growth component on the nanosecond time scale investigated. In Figure 2 the result for the flash photolysis of 2,4,6-TriNS in a degassed acetonitrile solution is shown. In this figure there is a transient absorption measured at 440 nm (curve a) which clearly shows a decay superimposed on a growth component. At 500 nm the transient absorption consists solely of the growth component, also shown in Figure 2 (curve b). To analyze the decay component, the growth component is subtracted from the
0.15
d
0
50
100
150
Time (ns)
Figure 3. Growth component of the transient absorption of degassed (upper trace) and aerated acetonitrile solutions of 2,4,6-TriNS. growth and decay trace to produce the difference shown in Figure 2 (curve c). The visible absorption of the solution also changed as a consequence of the laser excitation. These observations indicate that photoproduct formation has occurred. Analogous behavior is observed for acetonitrile solutions of 2,4,4’,6-TNS, 2,2’,4,6-TNS, PNS, and HNS. Attempts to fit the growth component to an exponential rise proved unsuccessful even when the system response function was incorporated. However, the decay component is exponential, and a decay lifetime can be extracted. In Figure 3 the decay component and the exponential fit incorporating the system response function is presented for 2,4,6-TriNS. Measurements at other absorption wavelengths indicate that the decay component is exponential over a 40 nm range. Similar decay components are obtained for each of the molecules listed above, except HNS (Figure 4) where a biexponential curve was required to obtain an adequate fit to the data. The decay lifetime for degassed and aerated solutions is shown in Table 2. In the case of HNS two lifetimes are given, one for the faster component and one for the longer lived component. It is immediately apparent from these results that the lifetime of the decaying transient species is almost identical for each of the molecules in this table. From the low oxygen quenching rate shown in Table 2 it is likely that the species responsible for the decay component is not a triplet state. It also appears that the type of species formed from each of the nitrated stilbenes shown in Table 2 is the same because of the similarity in the decay lifetime. A common structural feature of the nitrated stilbenes susceptible to photoproduct formation is the presence of three nitro groups on one ring in the 2, 4, and 6 positions. Thus it is proposed that the unidentified species is related to this structural feature. Although various photoproducts resulting from the photodecomposition of solid HNS have been identified,25the formation of these products involves multistep processes from the parent molecule. It is difficult to envisage that any of these
11442 J. Phys. Chem., Vol. 98, No. 44, I994
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TABLE 2. Transient Lifetimes (r)and Oxygen Quenching Rate Constants ( k ~for ) Photoexcited Nitrated Stilbene Derivatives in Acetonitrile Solutionu molecule
growth
2.4.6-TriNS 2;2',4,6-TNS 2,4,4',6-TNS PNS HNS (fast) HNS (slow)
500 500 500 500 520 520
I (nm) growth + decay 440 420 420 420 440 440
t (ns)
degassed 32.6 f 0.4 33 f 1 34.1 f 0.5 32.9 f 0.3 4.5 f 0.3 34 f 2
aerated
( x 109 M-'
31.2 f 0.4 29.3 f 0.4 27.9 f 0.2 29 f 1 5.1 f 0.5 35 f 3
s-1
)
0.9 f 0.5 2.3 f 0.8 3.9 f 0.4 2.4 f 0.9
1 is the transient absorption wavelength. 0.10
I
I
1
1
0
50
100
150
0.10
I ,
1
1
I
I
I
I
al
0
C
m 0.05
e
SI
9
0.00
Time (ns)
Figure 4. Growth component of the transient absorption of degassed (upper trace) and aerated acetonitrile solutions of HNS. products would be formed on the time scale involved in the flash photolysis experiment. Thus, the observed transient species with a 34 ns lifetime is likely to be a precursor to the photoproducts observed in the photodecomposition of solid HNS. A previous study on HNS obtained the equivalent of a singleexponential fit to the decay.12 As a result of the faster temporal response of the apparatus used in this current study, two components have been resolved; a fast component with a lifetime of approximately 5 ns and a slow component with a lifetime (34 ns) similar to that observed following laser excitation of the other 2,4,6-trinitrostilbene derivatives. The fast component represents another species that very likely decomposes to form the longer lived species. Since the intersystem crossing in HNS is expected to be highly efficient,'* the possibility remains that the fast component is the triplet state of HNS. In an attempt to increase the HNS excited triplet-state lifetime, studies were performed in more polar solvents. In water, single exponential decays with longer lifetimes in degassed (83 & 2 ns) and aerated (76 f 2 ns) aqueous solutions were obtained (Figure 5 ) . The oxygen quenching rate constant is (4 & 2) x lo9 M-' s-'. The photolysis of HNS in water produces much less photoproduct than in acetonitrile. This is further supported by the observation that photoexcitation of acetonitrile solutions of HNS results in a substantial change in the UVhisible absorption spectrum, whereas the change for the water solution is less pronounced. It is also apparent that the rapidly decaying component is not as prominent relative to the longer lived component (see Figure 5). The best fit to the data is obtained with a single-exponential curve with a small nondecaying component representing about 15% of the total absorption at the decay maximum. Although the lifetime is longer in aqueous solutions than in acetonitrile solutions, it seems likely that the species observed in both cases is the same photoproduct. Although triplet-state lifetimes of nitrated stilbenes can be increased by increasing the solvent polarity, the triplet state of
-I
100
200 300 400 Time (ns) Figure 5. Transient absorption at 440 nm of degassed (upper trace) and aerated aqueous solutions of HNS. 0
HNS is not observed because of the very fast reactive pathway leading to the observed photoproduct. Conclusion Acetonitrile solutions of nitrated stilbene derivatives excited by laser pulses at 355 nm exhibit transient absorption in the visible range from about 400 nm up to 620 nm. There appears to be two distinct types of transient absorption depending on the nitrated stilbene being excited. For some of the nitrated stilbenes the transient absorption appears as an exponential decay with no readily apparent underlying absorption. In this case the transient absorption can be attributed to the T-T absorption of the triplet state. The other type of behavior shows distinct absorption growing underneath a decay component. For the second type of behavior it is possible to observe growth absorption alone by changing the monitor wavelength. The growth component represents absorption due to a photoproduct resulting from the laser excitation. The trend in the triplet-state lifetimes of 4,4'-DNS (125 ns), 2,2'-DNS (55 ns'*), 2,4-DNS (50 ns), and 2,2',4,4'-TNS (25 ns) in degassed acetonitrile solutions cannot be explained solely on the basis of loss of molecular planarity due to steric hindrance. Furthermore, the observed trend does not correlate with calculated values of the S0-T' energy gap. Other factors, such as the role of multiple nitro groups on the rate of intersystem crossing and the use of S0-T' energy gaps calculated for gas phase rather than solvated molecules, need to be considered. Triplet-state lifetimes of nitrated stilbenes with three nitro groups on one benzene ring cannot be determined due to rapid photoproduct formation. For 2,4,6-T1iNS, 2,4,4',6-TNS, 2,2',4,6TNS, PNS, and HNS a transient with a lifetime of 34 ns is obtained. In the case of HNS a much faster component is also observed in photoexcited acetonitrile solution with a lifetime of 5 5 ns. Attempts to confirm this as the triplet state by repeating the experiment in more polar solvents were unsuccessful. Photoexcitation of HNS in degassed water yielded a
Flash Photolysis of Nitrated Stilbenes transient absorption with a decay lifetime of 83 ns and the identity of the absorbing species is likely to be the same as the species observed in acetonitrile solution with a lifetime of 34 ns. It seems apparent that if the triplet state of HNS is formed, it decays very rapidly due to a reactive pathway. Acknowledgment. I am indebted to Dr. I. J. Dagley for numerous useful discussions and for preparing PNS. I appreciate the efforts of Dr. M. Kony for obtaining the NMR spectra and Dr. K. J. Smit for stimulating discussions. Thanks are also due to Dr. K. P. Ghiggino for access to facilities within the School of Chemistry, University of Melbourne, and to Andrew Clayton for assistance. References and Notes (1) Gomer, H.; Schulte-Frohlinde, D. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 1208.
(2) Schulte-Frohlinde,D.; Gomer, H. Pure Appi. Chem. 1979,51,279. (3) Schulte-Frohlinde,D.; Blume, H.; Gusten, H. J. Phys. Chem. 1962, 66, 2486. (4) Gegiou, D.; Muszkat, K. A,; Fischer, E. J. Am. Chem. SOC.1968, 90, 3907. (5) Bent, D. V.; Schulte-Frohlinde, D. J. Phys. Chem. 1974, 78,451.
J. Phys. Chem., Vol. 98, No. 44, 1994 11443 (6) Gomer, H. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 1199. (7) Gomer, H.; Schulte-Frohlinde, D. J . Phys. Chem. 1981,85, 1835. (8) Gruen, H.; Gomer, H. J. Phys. Chem. 1989, 93, 7144. (9) Sun, L.; Gomer, H. J. Phys. Chem. 1993, 97, 11186. (10) Gomer, H.;Schulte-Frohlinde, D. J . Phys. Chem. 1978, 82, 2653. (11) Pisanias, M. N.; Schulte-Frohlinde, D: Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 662. (12) Smit, K. J. J . Phys. Chem. 1992, 96, 6555. (13) Bent, D. V.; Schulte-Frohlinde, D. J. Phys. Chem. 1974, 78, 446. (14) Calvert, J. G.; Pitts, Jr., J. N. Photochemistry; Wiley: New York, 1967; p 301. (15) Gorman, A. A.; Beddoes, R. L.; Hamblett, I.; McNeeney, S. P.; Rescott, A. L.; Unett, D. J. J. Chem. SOC., Chem. Commun. 1991, 963. (16) Hargreaves, K. R.; McGookin, A. J. SOC.Chem. Id.(London)1950. 69, 186. (17) Dictionary of Organic Compounds, 5th ed.; Buckingham, J.; Donaghy, S. M., Eds.; Chapman and Hall: New York, 1982. (18) Bishop, G.; Brady, 0. L. J . Chem. SOC. 1922, 121, 2364. (19) UUmann, F. M.; Gschwind, M. Berichte 1908, 41, 2291. (20) Reich, S. Berichte 1913, 45, 804. (21) Kuffner, F.; Lenneis, G.; Bauer, H. Monatsh. Chem. 1%0,91, 1152. (22) Challenger, F.; Clapham, P. H. J . Chem. SOC. 1948, 1612. (23) Murov, S. L. Handbook of Photochemistry; Dekker: New York, 1973. (24) El-Sayed, M. A. J . Chem. Phys. 1963, 38, 2834. (25) Kayser, E. G. NSWC Report-TR-90-60; Naval Surface Warfare Center, MD, 1989.
