6078
J. Phys. Chem. 1991, 95, 6078-6081
-
ns/(240gu) ns = 3 X 10-3/gu. If only the shoulder observed at 630 nm in the absorption spectrum is assigned to the So SI transition, the radiative lifetime is calculated to be ca. 2.9gUbs and & is approximately 2 X 1O4/g,. The observed SIlifetime of 650 ps is shorter than found for planar aromatic hydrocarbons. If the quantum yield for triplet formation is near unity as found in ref 16, this implies a faster intersystem crossing rate for the three-dimensional aromatic C, molecule compared with two-dimensional planar aromatics. A somewhat faster intersystem crossing rate is expected for the nonplanar Cso structure since the excited states are not pure m* in the conventional sense of planar aromatics where the irreducible representations spanned by the u and ?r orbitals are mutually exclusive due to the plane of symmetry. Thus, the one-center spin-orbit coupling between singlet and triplet states (which vanishes for pure TA* states of planar aromatics24)exists in this
case. According to calculations,z the lowest u n ~ ~ ~ ~molecular pied orbital involved in the low-energy excitations of Cbocontains ca. 94% carbon 2p, character ( I is the local radial direction). Thus, the low-energy states may contain up to 5% 2px and 2py character. This situation allows for direct spin-orbit coupling between the low-energy mr*-like states through one-center matrix elements of the type (PzlLxIPy) and (PzlLylPx). Acknowledgment. We acknowledge Dr. Allen Smith of Drexel University for valuable discussions. This research was supported by NSF and NIH grants to R.M.H. and was partially funded by NSF grant DMR 88-19885. (24) McClure, D. S. J . Chem. Phys. 1952, 20, 682. McGlynn, S.P.; Azumi. T.; Kinoshita, M. Molecular Spectroscopy of rhe Triplet Srare; Prentice-Hall: Englewood Cliffs, NJ, 1969; Chapters 5, 6. (25) Satpathy, S.Chem. Phys. ,?.err. 1986, 130, 545.
Photochemistry of the o-Nltrobenzyl System In Solution: IdentHicatlon of the Biradlcai Intermediate in the Intramolecular Rearrangement R. W. Yip,* Y. X. Wen, Dzpartement de Chimie, UniversitC du QuPbec d Montreal, C.P. 8888, Succ. A, Montreal, Quebec, Canada H3C 3P8
D. Gravel,* R. Giawn,* Dzpartement de Chimie, UniversitE de Montreal, C.P. 6210, Montreal, QuPbec, Canada H3C 3Vl
and D. K. Sharma Canadian Picosecond Laser Flash Photolysis Centre, Concordia University, Montreal, Canada H3G 1 M8 (Received: June 3, 1991)
Transient absorption spectra from o-nitrobenzyl p-cyanophenyl ether (ONBCPE) in acetonitrile recorded at nanosecond times has been resolved into Cauchy-Gauss band components. The band at 460 nm has been assigned to the biradical formed from the intramolecular abstraction of the benzylic hydrogen by the triplet excited state. These results show that, in the case of ONBCPE, the +quinonoid intermediate is formed from both the singlet as well as the triplet excited state, with =40% of the total reaction originating from the singlet excited state.
Introduction
SCHEME I
Nitrobenzene and substituted nitrobenzenes display a remarkably wide variety of photochemical reactions.' These include intermolecular2and intramolecular benzylic? and homobenzylicib hydrogen abstraction, photoaddition to olefins,' photoredox,5 heterolytic photocleavage of the benzylic C-C bond6.' in aqueous solutions, and photosubstitution.* Among these, the well-established general photoreactivity of o-nitrobenzyl systems (ONB, Scheme I) to display intramolecular hydrogen abstraction has stimulated considerable interests toward the exploitation of this reaction in the development of photochromic compound^,^ pho( I ) (a) Morrison, H. A. In The Chemistry of rhe Nirro and Nirroso Groups; Feuer, H., Ed.; Interscience: New York, 1969; Part I, Chapter 4, p 164. (b) Wpp, D. Top. Curr. Chem. 1975,55,49. (2) Hurley, R.; Testa. A. C. J . Am. Chem. Soc. 1968, 90,1949. (3) de Mayo. P.; Reid, S.T. Q.Reu. 1961, 15, 393. (4) Charlton. J. L.; Liao, C. C.; de Mayo, P. J . Am. Chem. Soc. 1971,93, 2463. (5) Wan, P.; Yates, K. J . Org. Chem. 1983,18, 136. (6) Wan, P.; Muralidharan; S. J . Am. Chem. Soc. 1988, 110, 4336. (7) Craig, B. B.; Weiss, R. G.; Atherton, S.J. J . Phys. Chem. 1987, 91, 5906. and references cited therein. (8) Cornelisse, J.; Lodder, G.; Havinga, E. Rev. Chem. Inrermed. 1979, 2. 231, (9) Dcaaauer, R.; Paris, J. P. In Aduances in Phorochemisrry; Noyes, W . A., Jr., Hammond, G. S.,Pita, J. N., Eds.; Interscience: N ~ W Yo&, 1963; Vol. I , p 275.
0022-3654/91/2095-6078S02.50/0
d H NO2
ON B
I
lkl-
d:H ct:H AA-
i' =;'I
CP
I BR
tolabile protecting groups,I0 and photoresists." The photorearrangement reaction (Scheme I) can proceed from the short-lived 0 1991 American Chemical Society
The Journal of Physical Chemistry, VO~. 95, No. 16, 1991 6079
Letters excited singlet state SI*( T < 10 ps)L2J3 to an o-quinonoid intermediate o-Q, or from the n-* excited triplet state" TI* to biradical BR, which has been post~lated'~ as an intermediate in the formation of 0-Q. The biradial can also give the cyclization product CP directly as has been observed in the case of geometrically constrained o-nitrobenzyl systems.16 Detection of the biradical, a key intermediate in the triplet pathway for the rearrangement, would thus provide an important way in which the mechanism could be delineated since the triplet excited state may or may not lead to reaction product.I2 Previously in our picosecond laser absorption investigation of o-nitrobenzyl p-cyanophenyl ether (ONBCPE)12 in which the singlet excited state was shown to be responsible for the formation of the o-quinonoid (absorption at 4 2 5 nm) at picosecond times, we had also obtained indirect evidence consistent with biradical absorption. We observed that the absorption in the 425-nm wavelength region (due to overlapping absorption from o-Q initially formed from the SI*,and absorption from the shortwavelength band of TI*) did not decrease as expected during the time of decay of the TI* which can be monitored by its longwavelength band at -650 nm. This suggests that another species with a similar absorption coefficient is formed from the triplet state. In a further study, the o-nitrobenzyl system was fused to a bicyclo[2.2.1]heptane or bicyclo[2.2.2]octane system to prevent the rapid formation of o-Q from the singlet excited state in order to provide a clear optical window in the 40+500-nm wavelength region with which to detect the However, despite this and other recent picosecond laser absorption spectroscopic studthe expected biradical intermediate BR has thus far evaded detection. In the present Letter, we report the identification of the biradical absorption band at 460 nm from a series of transient absorption spectra from ONBCPE recorded from about 50 ps to 10 ns by resolving the spectra into their component bands. From the kinetics of the component bands componding to the triplet excited state, the biradical, and the o-quinonoid intermediate, we have been able to establish the relative contributions of the singlet and triplet excited-state pathways in the formation of the o-quinonoid intermediate.
Results and Discussion Transient spectra were obtained by using the pump-probe technique which has been described in detail.I6 The sample solution (2 mm path cell) were excited with single 30-50-p355-nm third harmonic pulses (1-2 mJ) obtained with a Nd:YAG mode-locked laser system. At different optical delays, the absorption of the solution was interrogated with a continuum probe pulse (ca. 410-750 nm) generated by 1.06-rm excitation of a solution containing 30% H3P04in HzO. The spectra were recorded in sets. Each spectrum of the set corresponded to a different delay time and was obtained by typically averaging nine shots which represented a reasonable compromise between improved signal to noise ratio and marginal improvement upon ~
~~
(IO) (a) Htbert,J.; Gravel, D. Can. J . Chem. 1974,52, 187. (b) Gravel, D.; Htbert, J.; Thoraval, D. Can. J . Chrm. 1983,61,400. (c) Gravel, D.; Murrav. S.: Ladouceur. G. Chem. Commun. 1985. 1828. For excellent teviews &e: Amit, B.; Zehavi, U.; Patchornik, A. Isr: J. Chem. 1974, 12, 103. Pillai, W. N. R. Synthesis 1980, 1. (1 1) Reichmanis, E.; Gooden, R.; Wilkins, Jr., C. W.; Schonhorn, H.J . Polvm. Sci.. Polvm. Chem. Ed. 1983. 21. 1075. (12) Yip, R. W.; Sharma, D. K.; G i a k n , R.; Gravel, D. J. Phys. Chem.
1985. 89. 5328. (13) Yip, R. W.; Sharma, D. K. Reo. Chem. Inrrrmrd. 1989, 11, 109. (14) Yip, R. W.; Sharma, D. K.; Giasson, R.; Gravel, D. J . Phys. Chem. 1984,88, 5770. (15) de Mayo, P.; Reid, S. T.Q.Rcv. 1961, 15, 393. (16) Gravel, D.; G b n , R.; Blanchet, D.; Yip, R. W.; S h a m , D. K. Can.
J . Chem., in press. (17) (a) Craig, B. B.; Atherton, S. J. Proc. SHE.Inr. Soc. Opr. Eng. 1984, 182, 95. (b) Craig, B. 8.;Atherton, S.J.; Chem. Phys. b i t . 1986, 127, 7. (18) (a) Schupp. H.;Wong, W. K.; Schnabel. W. 1.Photochem. 1987,36, 85. (b) Wong, W. K.; Schupp, H.;Schnabel. W. Macromolecules 1989,22, 2 176.
- sops - Ins - - - - -- -
400
5ns lonS
500
600
Wavelength, nm
Figure 1. Transient absorption spectra of o-nitrobenzyl p-cyanophenyl ether (ONBCPE) in acetonitrile; 2 m m path length cell.
further averaging due to low-frequency drift by the OMA system. The absorption spectra were fitted to absorption bands described by Cauchy-Gauss product functions of the type F(x - xo) = A[ 1/( 1
+ EZ(x- x ~ ) ~[exp(-C(x )]
- xO)')] (1)
where x is the wavelength of the band, in nm; xo is the wavelength of the peak of the band, in nm; E is the Cauchy factor; C is the Gauss factor; and A is the band amplitude, by using a program (pci 16) developed by Pitha and JonesI9 which employed the Marquardt optimization method. We have previously identified two intermediates from transient absorption spectroscopy of o-nitrobenzyl phenyl ether and onitrobenzyl p-cyanophenyl ether in THF.I2 They are the oquinonoid intermediate,which displays a single band at -425 nm, and the triplet excited state," which possessestwo absorption bands (400-425 and -600-650 nm) of similar intensities ( T = 300-400 ps for ONBCPE). The o-quinonoid absorption which appeared at times comparable with the excitation pulse (-35 ps) and persisted beyond our time resolution (=lo ns) was attributed to intramolecular H-abstraction reaction from the excited singlet state. Thus the principal features of the transient absorption spectra are the following: (1) at picosecond times, two absorption bands: one at =425 nm due to superimposed 0-Q and TI* a b sorption, and the other at -650 nm due to TI* absorption; (2) at times >-lo ns, a single absorption band at -425 nm due to 0-Q.
In the present transient absorption study of ONBCPE in acetonitrile ((0.8-1.0) X 1W2 M), the time-resolved spectra were similar to those which we observed for the two o-nitrobenzyl phenyl ethers in THF-two bands at picosecond times, and at longer delay times, a single band at -425 nm due to 0-Q (Figure 1). In addition, a biradical route to e Q via TI* (Scheme I) should result in an increase absorption of the o-Q at e425 nm at a rate identical with that of the decay of its biradical precursor. Experimentally, in the 400-500-nm region, we find what appears to be a single absorption band which gains in intensity from 1 to 10 ns (see Figure 1) which is consistent with a triplet pathway in which o-Q is formed from the biradical. A similar observation has been reported for o-nitrobenzyl esters by Schnabel and co-workers.I8 The apparent shift in the baseline of the spectrum at 10 ns is an experimental artifact due to thermal lensing effect which appears a t 8-10 n s after excitation. This was verified by flash excitation studies of the solution, and of different solvents by themselves at different intensities. To further analyze the band structure of the absorption in the 425-nm region we began by fitting the spectra at 10 ns to the function described by eq 1 since there does not appear to be significant time-dependent changes between 8 and 10 ns. The (19) Pitha, J.; Jones, R. N. NRCC Bull. 1968, No. 12.
6080 The Journal of Physical Chemistry, Vol. 95, No. 16, 1991
Letters 1.57
TABLE 1: Optidled Fitting Panmeters for Transient Absorption S w e t n Bands from ONBCPE in A c e t d t d k o Gauss factor Cauchy factor delay peak C B time position, nm 0.008 f 0.004 0.005 f 0.004 50 ps 649 f 7 (4) I ns 0.014 f 0.005 0.0005 f 0.005 460 i 17 (7) 10 ns 421 f 3 (6) 0.022 f 0.002 0.012 f 0.005 OValues in parentheses are the number of samples. The band shape Cauchy and Gauss factors are defined in eq 1. 0.4 1
0.oJ 0
5-2
2
'
'
4
'
'
6
'
'
0
'
1
10
Delay Time, ns
-d
Figure 3. Kinetics of the 649-, 460-, and the 421-nm bands due to the triplet excited state, the biradical, and the o-quinonoid intermediate, respectively. Equations 2a, 2b, and 2c were used to calculate the three respective kinetic curves, with kl = 3.0 X IO9 s-l and k2 = 0.13 X IO9 s-'. (0)649-nm T* band; (A) 460-nm biradical band; (0)421-nm o-quinonoid band.
. Ci
0.1
nn 400 Y."
from the triplet excited state. The populations of the three species can be expressed by the following equations (cf. ref 20): TI*(?) = a. exp(-k,t) 500
600
Wavelength, nm
Figure 2. Curve fit of the 421-nm (heavy dash) and 460-nm (light dash) transient absorption bands from ONBCPE in acetonitrile at 1 ns. fit should describe a single species due to 0-Q. We obtained optimization of the band at 421 f 3 nm, and Cauchy and Gauss factors of B = 0.012 & 0.005 and C = 0.022 f 002, respectively (six spectra). Spectra corresponding to a delay time of 1 ns were next selected for optimization with the 421-nm band position and band factors B and C fixed to the previously optimized values. These spectra correspond to absorption due to o-Qand possibly the biradical. Contribution from the triplet excited state is not expected to be significant because of its short lifetime (-300 ps, vide infra). Optimizations were carried out for different number of bands but significant contribution were found only for bands at 421 nm and a new band at 460 f 17 nm. In the presence of 1 M 5methyl-5-(hydroxymethy1)-1,3-cyclohexadiene,a diene which is known to quench the triplet lifetime of ONBCPE,I2 the amount compared with of the 460-nm band at 1 ns was reduced by =WO the band area from ONBCPE without diene. At times < I ns, we were unable to resolve a third band in the 400-450-nm wavelength region due to the triplet excited state. The weakness of the absorption band at 460 nm and that of the TI* in comparison to the strong overlapping absorption of 0-Q formed from the singlet excited state is a probable explanation for the negative result. The long-wavelength band due to TI* was, however, optimized at 649 f 7 nm. The optimization parameters are summarized in Table I and the fit for the 421- and 460-nm bands for the transient absorption spectrum of ONBCPE at 1 ns is shown in Figure 2. The absorption at the 460-nm band overlaps with that expected for the short-wavelength band of the triplet excited state. Therefore, if the 460-nm band were due to the biradical derived from triplet and the two bands were similar in intensities, little or no absorption changes would result during decay of the triplet. However, as the o-quinonoid is formed from the biradical, decay of the 460-nm band is expected together with increase of the 421 -nm band of the o-quinonoid. Kinetically, intramolecular hydrogen abstraction from the triplet excited state leading to the o-quinonoid intermediate via the biradical can be represented by the series of two first-order reactions with rate constants k l and k2 (see Scheme I). Other competing reactions by T I * and BR, will reduce the yields of BR and 0-Q (FER and r,, respectively)
BR(t) = flER[exp(-klt)
o-Q(?)= o-Q(O) + O,QU
- exp(-k2t)l
(2a) (2b)
+ l / ( h - k2)[k2 exp(-ht) - kl exp(-kzt)l)
(2c) where a,, = Tl*(0) is the yield of TI* at t = 0;f l ~ , = rBRa,,[kl/(k2 - kl)];o-Q(0)is initial concentration of the o-quinonoid from the singlet excited state; and &Q = reQa,).In the case where the rate of decay of the triplet state is much greater than that for the formation of 0-Q, (k,>> k2), decay of the biradical could be described by a single-exponential function BR(t) = rBRao eXp(-kzl) and eq 2c reduces to
(2W
o-Q(t) = o-Q(O) + r,Qao[l - exp(-k2t)] (2d) The area of the band component for the o-quinonoid (A, = 421 nm) at different delay time was fitted to eq 2c' by using a modified Simplex optimization algorithm.21 In all, 35 spectra from 7 sets were used for the global fit which gave a Ilk2 rise time of 7.6 f 1.8 ns. The areas of the TI* and the BR band components were substantially smaller than those of the o-Q. To reduce the random error for these weaker signals, the results for a given delay time from several sets were averaged. The fit of the results from 23 spectra for TI* to eq 2a gave a I l k l decay time of 330 f 115 ps. The kinetics of the BR is expected to rise and decay according to eq 2b with the same kl and k2 as those obtained from the kinetic treatment of the 0-Q and the TI*. The calculated curve from eq 2b and the experimental points (derived from 23 spectra) for the 460-nm band component attributed to the BR, together with the results for the 0-Q and the TI*, are shown on Figure 3. These results support the assignment for the BR. To eliminate the possibility that the 460-nm band was a consequence of nonlinear processes, the excitation intensity f,, was varied to determine, if there was a threshold intensity, or a quadratic (Ie:), or higher power, intensity dependence for the 460-nm band. We obtained the following relative band areas, 0.33 (0.71), 0.77 (1.6), 1.4 (3.4), for the different relative f,, (in parentheses). The signal appears to be linear with I,, except at the highest intensities where a sublinear regime is indicated. The (20) Frost. A. A.; Pearson, R. G. Kinetics and Mechanism, 2nd ai.; Wiley: New York, 1961. (21) (a) Nelder, J . A.; Mead, R. Compur. J . 1965, 7, 308. (b) Cooper, J. W. Introduction to Pascal for Scientists; Wiley: New York, 1981; pp 185-197.
J . Phys. Chem. 1991,95,6081-6086 latter may originate from saturation, a diminution of the triplet lifetime due to triplet-triplet annihilation, or other effects; and in any event it is incompatible with a IC: or higher dependence. The yield of the o-quinonoid formed from singlet excited state (o-Q(O)) as a fraction of the total o-quinonoid ( 0 - Q ( - ) ) formed can also be obtained from the kinetics. The value for o-Q(O) can be obtained by band fitting at 1 ns, where the contribution due to absorption from TI* is negligible, and the value of o-Q(-) can be obtained from the plot to eq 2c'. We obtained a value of o-Q(O)/o-Q(-) = 0.43 f 0.05-that is, approximately 40% of the o-quinonoid is formed from the singlet pathway. From the relative area obtained from the decay of the biradical and the rise of the o-quinonoid, we obtained a limiting value for the relative oscillator strengths f (f= 4.139 X lo4 .f c ( v ) dv) of the two species, fN/fBR 1 4.6 f 1.1. The inequality corresponds to a yield of the o-quinonoid from the biradical of
