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Transient absorption spectral evidence for phenyl nitrene as the chain propagator in the photo-initiated autocatalytic chain decomposition of phenyl azide and phenyl isocyanate
Volume
98, number
2
TRANSIENT
CHEMICAL
ABSORPIION
SPECTRAL
AS THE CHAlN PROPAGATOR DECOMPOSITION Natalie
PHYSICS
EVIDENCE
17June1983
LETTERS
FOR PHENYL NITRENE
IN THE PHOTO-INITIATED
AIJTOCATALYTIC
CHAIN
OF PHENYL AZIDE AND PHENYL ISOCYANATE
B. FEILCHENFELD
and Walter H. WADDELL
Department of C?temistr_vmrci Ceuter for the Joining of ~fateriais. Carrregie-lllellon University, Pirtsbltrgil. Penns_vlxania 152,713.US.4
Transxnt absorptton spectra nere recorded 1.5 ns to 6 ps foIlo\sing a 266 nm laser pulse for phenyl aide and for phenyl wxyanatc in aerated axtonttrilc and 3methylpentanc soluttons. Trnnsicnt spectra which are independent of concentraUon and of dsl;lv time. xc essentialI) tdenticti for pltenyl azide and for phenyl isocyanate, except at h&her energies where piten) 1 .tzidc .horbs, ad are assigned as th.tt of triplet phcnyl nitrene. Stnce there is no spectral evidence for a second speCWC_ pilcn~~~ nitrate IS !houpht the chn%n propzgxtoi in the autocntrtlytic ch;un decomposition that occurs for phenyl midc .uuld for phcnyl Isocynnatc.
1. Introduction
which decomposes trogen
The piloto-i~~itiated chain decomposition observed upon 151 nm irradiation of concentrated sofutions of phenyl uide (PhN3) was manifest as quantum yields of disappearance of phenyl azide (Q-PhN3) that greatly exceeded unit efficiency [I ] _For example, at [PhN,] = 3 X 10-I M. @-PhN3 = 300. The chain decomposition is thought tooccur by reaction of phenyl nitrene. the direct irradiation product [2--S] 111~ PllN: PhN + N2 .
Cl)
wrth phcnyi azide to form either nvo pheny1 nitrenes .utd molecular mt rogen PhN f PhN; -3PhN+N,, or 1,l-diphenyltetratiadrene
(3) [6.7]
PhN + PhN; +Ph-N=N-N=N-Ph( f Pit-y=N-Ph
“=? 190
1.4-~pll~nyltetraaza~ene) (1,3-diphenyltetraazadiene),
(3)
Ph-N=N-N=N-Pb
into five phenyl nitrenes
and ni-
+ 2lW-J + N, _
(4)
Reaction (3) may also afford 1,Zdiphenyitetraazadiene which immediately affords azobenzene [6] and xiitrogen Ph-R--N-Ph
i- Nz _
+Ph-N=N-Ph
i-l=; Alternatrvely. azobenzene, which is formed exclusively as the E-isomer [S] may be formed by direct dimerization of two phenyi nitrenes 2PhN -+ Ph-N=N-Ph
_
(6)
The reaction of aryl nitrene with aryi azide occurs in solution at a near diffusion controlled rate of IO9 M-‘s-l [9]_Th e reaction with phenyl azide occurs for triplet phenyl nitrene [IO], which is the groundstate species [ 11 --f3]_ Direct evidence for the existence of 1,4diphenylte~ra~a~ene is not available; however, reactions of organometallics with aryl azides afford stable complexes of 1,4diaryltetraazadiene with cobalt [ 141, nickel and platinum [ 1S] . In addi0 009-~614/83/0000-0000/S
03.00 0 1983 Non-Holl~d
Volume 98, number 2
CHEhIICAL PHYSICS LE?TERS
tion, mass spectral analysis of the vapor phase reaction of the anion radical of phenyl nitrene with phenyl azide suggests the formation of the anion radical of 1,4_diphenyltetraazadiene [ 161. Upon irradiation of concentrated, deaerated solutions of phenyl isocyanate (PhNCO) a chain decomposition occurs since Q-PhNCO values of greater than unit efficiency are also measured [ 17]_ hY PhNCOPhN + CO . (7) Since a 1,4-adduct analogous to 1,4-diphenyltetraazadiene is not thought feasible, the reaction of a phenyl nitrene with phenyl isocyanate is thought to afford hvo phenyl nitrenes and carbon monoxide PhN + PhNCO -+ 3PhN + CO .
(8)
Phenyl nitrene would thus act as the propagating species in the photoinitiated chain decomposition reaction of phenyl isocyanate. Flash photolytic studies of phenyl azide have been made in the gas phase and in solution [ 18,19]_ The vapor phase spectrum of phenyl nitrene has been observed upon photolysis of phenyl azide [ 181 and of phenyl isocyanate [20]. Azobenzene was the only other species observed [ 181. In solution, absorbance increases at 366 nm were observed upon irradiation of phenyl azide; however, spectra were not reported
[191Thus, to address the question
of 1 P-diphenyltetraazadiene’s participation in the chain decomposition of phenyl azide ,pulsed laser spectroscopic studies of phenyl azide and of phenyl isocyanate were made in 3-methylpentane and in acetonitrile, solvents in which chain decomposition is observed [ 1,17]_ The fourth harmonic of a Nd : YAG laser was used to generate a 266 nm light pulse (10 ns width) and an ultraviolet-visible continuum from a synchronous xenon flash lamp used to obtain absorption spectra of the transients.
2. Experimental Phenyl azide was prepared according to the procedure of Lindsay and Allen [2 I] _Phenyl isocyanate (puriss, p-a., Fluka) was used as received. A&o&rile (Spectrograde. Burdick and Jackson) was used as received while 3-methylpentane (99+%Phillips Petroleum)
17 June 1983
was distilled from Dri-Na (Baker) prior to use. For all spectral studies a Quanta-Ray DCR Nd : YAG pulsed laser (1064 nm) was the excitation source. Two KDP doubling crystals were used to generate and a Pellin-Brocca prism used to isolate the fourth harmonic (266 nm). An EG&G xenon flash lamp (I m width) provided a continuum probe source from = 250 t9 550 nm, focused into essentially identical beams into a 1 mm quartz cuvette (Precision Cells) held in a brass mount having pairs of inlet and exit holes. The transmitted light was focused through an American ISA spectrograph (0.1 m. 100 grooves/ mm grating) onto a silicon intensified vidicon with ultraviolet scintillation (PAR model 1254) having a 12.5 mm window, 5 12 channels gated by a highvoltage pulse generator (EC&G PAR model 1211). Spectra were digitized by a multichannel detector controller (EG&G PAR model 1216) and processed by a Digital MINC 11 minicomputer that also synchronized laser escitation, flash lamp probing and detection. The actual spectra of the transients were displayed as AA (change in absorbance between irradiated and non-irradiated portions of the sample, corrected for initial differences in the two portions’ absorbance) as a function of wavelength_ The direct subtraction of absorbances was only valid for small conversions of starting material upon irradiation_ This was indeed the case since repeated spectral measurements of the same portion of the sample yielded identical results. Each spectrum was obtained at a predetermined delay time (15 ns to 6 11s) with respect to excitation_ All experimentation was performed under red illumination.
3. Results The electronic absorption spectrum of 2.14 X lOA M phenyl azide (PhN3) in acetonitrile (CH,CN) has a low energy onset at =Z292 nm, shoulders at 284 and 277 nm and an absorption maximum at 250 nm; ema, = 1.0 X lo4 M-l cm-l _The absorption spectrum of the transient(s) present 142 ns following laser excitation of 5.70 X lOA M PhN, in aerated CH,CN is shown in fig_ 1, which is the accumulation of 1000 scans obtained by flowing the solution at = 03 rnllmln. A low energy absorption onset at 560 nm and peaks at 487, 435,410,375 and 295 nm are notable features of 191
VoIu~nc 96, number [I
I
2
CHEMICAL I
1
I
PHYSICS
I
17 June
LElTERS
Table 1 Correlation coefficients for transient absorption spectra a) \Iolecule
Solvent
Spectra b)
.c)
ivd)
PhN3 _PhN3
CH&N
DS, cs, DS, CL,
0.975 0.988 0.905 0.968
6 6 4
PhNCO. PhNCO
I’h;\‘,,
this absorption. The low-energ absorption onset of the tr.msicnt(s) correlates with ihe decrease in intensity of the spectral output of the xenon flash lamp (750-550 nm). The transient absorption spectrum ~ecordcd 5.45 I_csfollowing lsser excitdtion IS essenrefill> identical. d correlation coefficient of 0.975 is obt~n~d. Tr.msient absorption spectra of d 1.14 X IO-? hi PllN 3 solution in aerated CH3CN (99 scans) obt.ked 142 ns and 5.55 JB after laser excitation are ko esscntlally identical to those obtained for the dllutc solution, fig. 1 ,ekcept for intemlty differences. Table 1 shoots examples of correlation coefficients of thcsc kinetic Jbsorption spectra.The transient absorption obtained 15 ns following laser excitation is *also \cry similar. S&uration of the 1.14 X lo-? M PhN3 m CH,CN solution with argon gx or with oxygen gas hxd no effrct upon the shape or the intensity of the Iranslcnt ribsorptions. Smular transient absorption spectra were also obramed 112 ns and 5.45 /JS following laser excitation of 2.71 X 10-l hiPhNj in aeratt d 3-methylpentane (3-MP; 99 scans)_ The transient absorption spectra of the stable products Ihat can be obtained upon Irradiation of phenyl azldr in CHJCN, namely E-azobenzene, nitrobenzene, =osybenzene or nitrobenzene [S], do not correspond with those sllo~n in fig. 1, which thus represent spec192
1983
PhNCO
DL CL CL cs c)
2
3-!aP
CL. cs
0.938
6
CH&N
us, DL cs, CL lx. CL
0.920 0.977
0.969
3 9 9
3-VP
CL. cs
0.948
6
CH3CN
cs. CL. CL. DS, DL. DS, DS.
cs CL cs DS DL DL cs
0.980 0.951 0.964 0.956 0.942 0.932 0.918
Z 9 3 3 1 6 1
3-JIP
es, cs CL, CL CL. cs
0.924 0.963 0.935
4 4 4
‘) Spectra correldtcd between 285 and 540 nm for PhN-, and 376 and 540 nm for PhNCCl for aerated solutions. b) D = dilute act v.dues. ewitatton. c, Least-squues d) Number of e) Correlation tions.
and C = conccntrared solution, see text for esS = 142 ns and L = 5 45 ps delay folloainp lllser fit. spectral sets. between Ar- (CS) .md &-saturated
(CL) solu-
of the intermediate(s) of the photochemical reaction. The electronic absorption spectrum of 2.56 X lo4 M phenyi isocy anate (PhNCO) in CH3CN has a low ener,v onset at = 280 nm, peaks at 277,269, 263 and 255 nm and an absorption maximum at 227 277=5X lo* M-l cm-l_ nm, En,, =l.lXl@and~ The absorption spectrum of the transient(s) present 142 ns following laser excitation of 6.15 X 1O-4 M PhNCO in aerated CH3CN (1000 scans flowing the solution at 1 ml/min) is essentially identical to those absorption spectra obtained(i) for 6.15 X lo4 M PhNCO at 5.45 ps following laser excitation, (ii) for l-23 X 10-lM PhNCO (99 scans) at 142 ns and 5 -45 tra
CHEMICAL
Volume 98, number 2
17 June 1983
PHYSICS LETTERS
absorption is assigned as that of triplet phenyl nitrene. The absorption spectra of the transient obtained upon pulsed laser excitation of phenyl isocyanate did not change with time from 142 ns to 5.45 ps, is similar to that spectrum obtained upon irradiation of phenyl isocyanate at 77 K in a 3-MP mat& [ 171, and is essentially identical to the transient absorption spectra obtained for phenyl azide. Hence. triplet phenyl nitrene is again formed. 01
Since the transient absorption spectrum of phenyl azide (i) does not vary with concentration x 10-2
00
WA’ELENCTH.
Fq. 2. Transient absorptron spectra (C) in aerated CHaCN recorded 142 laser excitnrion (lower curve) and if (C) in aerated CHsCN recorded 142 laser ewiration.
nm
of 1.14 X lo-* hl PhNs ns (S) folloxring pulsed 1.23 X IO-’ M PhNCO ns (S) following pulsed
~.rsfollowing laser excitation (fig. 3) and (iii) for the dilute and concentrated PhN, solutions obtained at 142 ns and 5.45 ps following laser excitation in CH,CN. table 1. One notable difference is the higher
to 5.70
x 10-4
M in aerated
from
‘IHsCN,
l_ 14
yet the
quantum yield of disappearance of phenyl azide (QPhN,) is greatly affected by decreasing concentration over this range [ 1] whereby +PhNs decreases from 10 to 2 to 0.7 at [PhN,] = 3 X 10e2, 3 X 10e3 and 3 X 1 OA M, respectively, and (ii) is essentially identical to that obtained upon irradiation of phenyl isocyanate, phenyl nitrene is assigned as the chain propagator in the photo-initiated chain decomposition reaction of phenyl azide. There is no spectral evidence for the intermediacy of 1,4_diphenyltetraazadiene within the limitations of our current experiments, namely absorptions between 250 and 550 nm for up to “6~ following laser excitation-
energy absorptions visible in the spectra obtained upon pulsed excitation ofPhNC0, compared to PhN3 _ Transient absorption spectra were also recorded for 1.79 X 10-l M PhNCO in aerated 3-MP (99 scans) 142 ns and 5.45 m following laser excitation and are essentially identical to corresponding spectra obtained for PhN,
in 3-MP, table 1.
4. Discussion The absorption spectra of the transient(s) obtained upon pulsed laser excitation of phenyl azide did not change with time from 15 ns to 5-45 us, hence only one species is thought present. Our preliminary measurements of transient lifetimes also indicate the presence of only one absorbing species over the %330-500 nm spectral range. Since the transient absorbs at energies similar to those reported by Reiser and co-workers [24], Smimov and Brichkin [5] and Waddell and Feilchenfeld [ 171 at 77 K in organic matrices and by Reiser et al. [9] at room temperature in solution and in polymeric matrices, the transient
Acknowledgement We are deeply indebted to Professor Robin M. Hochstrasser and the staff at the Regional Laser and Biotechnology Laboratory, University of Pennsylvania for use of the Nanosecond Absorption Spectrometer_ Particular hanksare due to Drs.PaulComelius,GaryM. Holtum and hlark Paczkowski for their expert technit: . assistance. We also wish to thank Drs. Celia H. . =e Go and Helen W. Richter of Carnegie-hlellon Ur._Jersity for their technicai assistance. This research was supported by a grant (DMR76-81561) from the National Science Foundation to the Center for the Joining of Materials at Carnegie-Mellon_
References
[l] W.H. WaddeU and CL. Go, J. Am. Chem. Sot. 104 (1982) 5804. [2] A. Reiser and V. Fraser, Nature 208 (1965)
682.
193
\‘olume 98. number 2
CHEMICAL
131 .k Rtiser, G. Boues and RJ_ Home, Trans. Faraday Sot. 62 (1966) 3162. 141 A. Rerser. H.M. Wagner, R. Marley and G. Bo\\es, Trans. Faraday Sot. 63 (1967) 2403_ 151 V.A. Smirnov and S.B. Brichkin, Chem. Phys. Letters 87 (1982) 518. 1‘51 L. Hornsr. A. Christmum and A. Gross, Chem. Ber. 96 (1963) 399T 171 P.4.S. Sm~rh. in. Nltrcncs, ed. L. Lwowski (\\‘ileyInrcrsrrrilcc. Nw York. 1970) p. 99. 181 C.L. Go and \I.H. \Vaddcll, submitted for publication. F.W. Wl1et.s. G.G. Terry. V. Williams nnd R. 191 A. R&x. \l.ulcy. Tr.ms. fxaddy Sot. 64 (1968) 3265. Il@l L Lcyshon .md A. Rciscr. Trims Farada) Sot. 68 (1972) 1918. 1111 C. Smohnsl.? . E. ~~.wxrm.m .md \\‘.A. Yager, J. Am. C-hem. Sot. 95 (1962) 3220. Smohnsky and \V.‘.A.Yager, J. Am. 11’1 C W.wxrnun,G. Chcm. Sot 86 (1964) 3166.
194
PHYSICS LETTERS [ i3]
17 June 1983
R.M. Moriarity, R. R&man and G-l. King, J. Am. Chem. Sot. 88 (1966) 842. [ 141 S. Otsuka and A. Nalwnura, Inorg. Chem. 7 (1968) 2.542. [ 1.51 P. Overbosch, G. van Koten and 0. Overbeck, J. Am. Chem. Sot. 102 (1980) 2091. [ 161 R.N. McDonald and AK. Chowdhury, J. Am. Chem. Sot. 102 (1980) 5118. [ 17) W.H. Waddell and N.B. Feilchenfeld, submitted for publication. [ 181 P.A. Lehman and R.S. Berry, J. Am. Chem. Sot. 95 (1973) 8614. [ 191 B.A. DeCraff, D.W. Gillespie and R.J. Sundberg, J. Am. Chem. Sot. 96 (1974) 7491. [20] G. Porter, R.S.F. Ward and B. Ward, Proc. Roy. Sot. A303 (1968) 139. 1211 R.O. Lindsay nnd C.F.H. .411en, Orsnnic Synthesis 3 (1955) 710.
98, number
2
TRANSIENT
CHEMICAL
ABSORPIION
SPECTRAL
AS THE CHAlN PROPAGATOR DECOMPOSITION Natalie
PHYSICS
EVIDENCE
17June1983
LETTERS
FOR PHENYL NITRENE
IN THE PHOTO-INITIATED
AIJTOCATALYTIC
CHAIN
OF PHENYL AZIDE AND PHENYL ISOCYANATE
B. FEILCHENFELD
and Walter H. WADDELL
Department of C?temistr_vmrci Ceuter for the Joining of ~fateriais. Carrregie-lllellon University, Pirtsbltrgil. Penns_vlxania 152,713.US.4
Transxnt absorptton spectra nere recorded 1.5 ns to 6 ps foIlo\sing a 266 nm laser pulse for phenyl aide and for phenyl wxyanatc in aerated axtonttrilc and 3methylpentanc soluttons. Trnnsicnt spectra which are independent of concentraUon and of dsl;lv time. xc essentialI) tdenticti for pltenyl azide and for phenyl isocyanate, except at h&her energies where piten) 1 .tzidc .horbs, ad are assigned as th.tt of triplet phcnyl nitrene. Stnce there is no spectral evidence for a second speCWC_ pilcn~~~ nitrate IS !houpht the chn%n propzgxtoi in the autocntrtlytic ch;un decomposition that occurs for phenyl midc .uuld for phcnyl Isocynnatc.
1. Introduction
which decomposes trogen
The piloto-i~~itiated chain decomposition observed upon 151 nm irradiation of concentrated sofutions of phenyl uide (PhN3) was manifest as quantum yields of disappearance of phenyl azide (Q-PhN3) that greatly exceeded unit efficiency [I ] _For example, at [PhN,] = 3 X 10-I M. @-PhN3 = 300. The chain decomposition is thought tooccur by reaction of phenyl nitrene. the direct irradiation product [2--S] 111~ PllN: PhN + N2 .
Cl)
wrth phcnyi azide to form either nvo pheny1 nitrenes .utd molecular mt rogen PhN f PhN; -3PhN+N,, or 1,l-diphenyltetratiadrene
(3) [6.7]
PhN + PhN; +Ph-N=N-N=N-Ph( f Pit-y=N-Ph
“=? 190
1.4-~pll~nyltetraaza~ene) (1,3-diphenyltetraazadiene),
(3)
Ph-N=N-N=N-Pb
into five phenyl nitrenes
and ni-
+ 2lW-J + N, _
(4)
Reaction (3) may also afford 1,Zdiphenyitetraazadiene which immediately affords azobenzene [6] and xiitrogen Ph-R--N-Ph
i- Nz _
+Ph-N=N-Ph
i-l=; Alternatrvely. azobenzene, which is formed exclusively as the E-isomer [S] may be formed by direct dimerization of two phenyi nitrenes 2PhN -+ Ph-N=N-Ph
_
(6)
The reaction of aryl nitrene with aryi azide occurs in solution at a near diffusion controlled rate of IO9 M-‘s-l [9]_Th e reaction with phenyl azide occurs for triplet phenyl nitrene [IO], which is the groundstate species [ 11 --f3]_ Direct evidence for the existence of 1,4diphenylte~ra~a~ene is not available; however, reactions of organometallics with aryl azides afford stable complexes of 1,4diaryltetraazadiene with cobalt [ 141, nickel and platinum [ 1S] . In addi0 009-~614/83/0000-0000/S
03.00 0 1983 Non-Holl~d
Volume 98, number 2
CHEhIICAL PHYSICS LE?TERS
tion, mass spectral analysis of the vapor phase reaction of the anion radical of phenyl nitrene with phenyl azide suggests the formation of the anion radical of 1,4_diphenyltetraazadiene [ 161. Upon irradiation of concentrated, deaerated solutions of phenyl isocyanate (PhNCO) a chain decomposition occurs since Q-PhNCO values of greater than unit efficiency are also measured [ 17]_ hY PhNCOPhN + CO . (7) Since a 1,4-adduct analogous to 1,4-diphenyltetraazadiene is not thought feasible, the reaction of a phenyl nitrene with phenyl isocyanate is thought to afford hvo phenyl nitrenes and carbon monoxide PhN + PhNCO -+ 3PhN + CO .
(8)
Phenyl nitrene would thus act as the propagating species in the photoinitiated chain decomposition reaction of phenyl isocyanate. Flash photolytic studies of phenyl azide have been made in the gas phase and in solution [ 18,19]_ The vapor phase spectrum of phenyl nitrene has been observed upon photolysis of phenyl azide [ 181 and of phenyl isocyanate [20]. Azobenzene was the only other species observed [ 181. In solution, absorbance increases at 366 nm were observed upon irradiation of phenyl azide; however, spectra were not reported
[191Thus, to address the question
of 1 P-diphenyltetraazadiene’s participation in the chain decomposition of phenyl azide ,pulsed laser spectroscopic studies of phenyl azide and of phenyl isocyanate were made in 3-methylpentane and in acetonitrile, solvents in which chain decomposition is observed [ 1,17]_ The fourth harmonic of a Nd : YAG laser was used to generate a 266 nm light pulse (10 ns width) and an ultraviolet-visible continuum from a synchronous xenon flash lamp used to obtain absorption spectra of the transients.
2. Experimental Phenyl azide was prepared according to the procedure of Lindsay and Allen [2 I] _Phenyl isocyanate (puriss, p-a., Fluka) was used as received. A&o&rile (Spectrograde. Burdick and Jackson) was used as received while 3-methylpentane (99+%Phillips Petroleum)
17 June 1983
was distilled from Dri-Na (Baker) prior to use. For all spectral studies a Quanta-Ray DCR Nd : YAG pulsed laser (1064 nm) was the excitation source. Two KDP doubling crystals were used to generate and a Pellin-Brocca prism used to isolate the fourth harmonic (266 nm). An EG&G xenon flash lamp (I m width) provided a continuum probe source from = 250 t9 550 nm, focused into essentially identical beams into a 1 mm quartz cuvette (Precision Cells) held in a brass mount having pairs of inlet and exit holes. The transmitted light was focused through an American ISA spectrograph (0.1 m. 100 grooves/ mm grating) onto a silicon intensified vidicon with ultraviolet scintillation (PAR model 1254) having a 12.5 mm window, 5 12 channels gated by a highvoltage pulse generator (EC&G PAR model 1211). Spectra were digitized by a multichannel detector controller (EG&G PAR model 1216) and processed by a Digital MINC 11 minicomputer that also synchronized laser escitation, flash lamp probing and detection. The actual spectra of the transients were displayed as AA (change in absorbance between irradiated and non-irradiated portions of the sample, corrected for initial differences in the two portions’ absorbance) as a function of wavelength_ The direct subtraction of absorbances was only valid for small conversions of starting material upon irradiation_ This was indeed the case since repeated spectral measurements of the same portion of the sample yielded identical results. Each spectrum was obtained at a predetermined delay time (15 ns to 6 11s) with respect to excitation_ All experimentation was performed under red illumination.
3. Results The electronic absorption spectrum of 2.14 X lOA M phenyl azide (PhN3) in acetonitrile (CH,CN) has a low energy onset at =Z292 nm, shoulders at 284 and 277 nm and an absorption maximum at 250 nm; ema, = 1.0 X lo4 M-l cm-l _The absorption spectrum of the transient(s) present 142 ns following laser excitation of 5.70 X lOA M PhN, in aerated CH,CN is shown in fig_ 1, which is the accumulation of 1000 scans obtained by flowing the solution at = 03 rnllmln. A low energy absorption onset at 560 nm and peaks at 487, 435,410,375 and 295 nm are notable features of 191
VoIu~nc 96, number [I
I
2
CHEMICAL I
1
I
PHYSICS
I
17 June
LElTERS
Table 1 Correlation coefficients for transient absorption spectra a) \Iolecule
Solvent
Spectra b)
.c)
ivd)
PhN3 _PhN3
CH&N
DS, cs, DS, CL,
0.975 0.988 0.905 0.968
6 6 4
PhNCO. PhNCO
I’h;\‘,,
this absorption. The low-energ absorption onset of the tr.msicnt(s) correlates with ihe decrease in intensity of the spectral output of the xenon flash lamp (750-550 nm). The transient absorption spectrum ~ecordcd 5.45 I_csfollowing lsser excitdtion IS essenrefill> identical. d correlation coefficient of 0.975 is obt~n~d. Tr.msient absorption spectra of d 1.14 X IO-? hi PllN 3 solution in aerated CH3CN (99 scans) obt.ked 142 ns and 5.55 JB after laser excitation are ko esscntlally identical to those obtained for the dllutc solution, fig. 1 ,ekcept for intemlty differences. Table 1 shoots examples of correlation coefficients of thcsc kinetic Jbsorption spectra.The transient absorption obtained 15 ns following laser excitation is *also \cry similar. S&uration of the 1.14 X lo-? M PhN3 m CH,CN solution with argon gx or with oxygen gas hxd no effrct upon the shape or the intensity of the Iranslcnt ribsorptions. Smular transient absorption spectra were also obramed 112 ns and 5.45 /JS following laser excitation of 2.71 X 10-l hiPhNj in aeratt d 3-methylpentane (3-MP; 99 scans)_ The transient absorption spectra of the stable products Ihat can be obtained upon Irradiation of phenyl azldr in CHJCN, namely E-azobenzene, nitrobenzene, =osybenzene or nitrobenzene [S], do not correspond with those sllo~n in fig. 1, which thus represent spec192
1983
PhNCO
DL CL CL cs c)
2
3-!aP
CL. cs
0.938
6
CH&N
us, DL cs, CL lx. CL
0.920 0.977
0.969
3 9 9
3-VP
CL. cs
0.948
6
CH3CN
cs. CL. CL. DS, DL. DS, DS.
cs CL cs DS DL DL cs
0.980 0.951 0.964 0.956 0.942 0.932 0.918
Z 9 3 3 1 6 1
3-JIP
es, cs CL, CL CL. cs
0.924 0.963 0.935
4 4 4
‘) Spectra correldtcd between 285 and 540 nm for PhN-, and 376 and 540 nm for PhNCCl for aerated solutions. b) D = dilute act v.dues. ewitatton. c, Least-squues d) Number of e) Correlation tions.
and C = conccntrared solution, see text for esS = 142 ns and L = 5 45 ps delay folloainp lllser fit. spectral sets. between Ar- (CS) .md &-saturated
(CL) solu-
of the intermediate(s) of the photochemical reaction. The electronic absorption spectrum of 2.56 X lo4 M phenyi isocy anate (PhNCO) in CH3CN has a low ener,v onset at = 280 nm, peaks at 277,269, 263 and 255 nm and an absorption maximum at 227 277=5X lo* M-l cm-l_ nm, En,, =l.lXl@and~ The absorption spectrum of the transient(s) present 142 ns following laser excitation of 6.15 X 1O-4 M PhNCO in aerated CH3CN (1000 scans flowing the solution at 1 ml/min) is essentially identical to those absorption spectra obtained(i) for 6.15 X lo4 M PhNCO at 5.45 ps following laser excitation, (ii) for l-23 X 10-lM PhNCO (99 scans) at 142 ns and 5 -45 tra
CHEMICAL
Volume 98, number 2
17 June 1983
PHYSICS LETTERS
absorption is assigned as that of triplet phenyl nitrene. The absorption spectra of the transient obtained upon pulsed laser excitation of phenyl isocyanate did not change with time from 142 ns to 5.45 ps, is similar to that spectrum obtained upon irradiation of phenyl isocyanate at 77 K in a 3-MP mat& [ 171, and is essentially identical to the transient absorption spectra obtained for phenyl azide. Hence. triplet phenyl nitrene is again formed. 01
Since the transient absorption spectrum of phenyl azide (i) does not vary with concentration x 10-2
00
WA’ELENCTH.
Fq. 2. Transient absorptron spectra (C) in aerated CHaCN recorded 142 laser excitnrion (lower curve) and if (C) in aerated CHsCN recorded 142 laser ewiration.
nm
of 1.14 X lo-* hl PhNs ns (S) folloxring pulsed 1.23 X IO-’ M PhNCO ns (S) following pulsed
~.rsfollowing laser excitation (fig. 3) and (iii) for the dilute and concentrated PhN, solutions obtained at 142 ns and 5.45 ps following laser excitation in CH,CN. table 1. One notable difference is the higher
to 5.70
x 10-4
M in aerated
from
‘IHsCN,
l_ 14
yet the
quantum yield of disappearance of phenyl azide (QPhN,) is greatly affected by decreasing concentration over this range [ 1] whereby +PhNs decreases from 10 to 2 to 0.7 at [PhN,] = 3 X 10e2, 3 X 10e3 and 3 X 1 OA M, respectively, and (ii) is essentially identical to that obtained upon irradiation of phenyl isocyanate, phenyl nitrene is assigned as the chain propagator in the photo-initiated chain decomposition reaction of phenyl azide. There is no spectral evidence for the intermediacy of 1,4_diphenyltetraazadiene within the limitations of our current experiments, namely absorptions between 250 and 550 nm for up to “6~ following laser excitation-
energy absorptions visible in the spectra obtained upon pulsed excitation ofPhNC0, compared to PhN3 _ Transient absorption spectra were also recorded for 1.79 X 10-l M PhNCO in aerated 3-MP (99 scans) 142 ns and 5.45 m following laser excitation and are essentially identical to corresponding spectra obtained for PhN,
in 3-MP, table 1.
4. Discussion The absorption spectra of the transient(s) obtained upon pulsed laser excitation of phenyl azide did not change with time from 15 ns to 5-45 us, hence only one species is thought present. Our preliminary measurements of transient lifetimes also indicate the presence of only one absorbing species over the %330-500 nm spectral range. Since the transient absorbs at energies similar to those reported by Reiser and co-workers [24], Smimov and Brichkin [5] and Waddell and Feilchenfeld [ 171 at 77 K in organic matrices and by Reiser et al. [9] at room temperature in solution and in polymeric matrices, the transient
Acknowledgement We are deeply indebted to Professor Robin M. Hochstrasser and the staff at the Regional Laser and Biotechnology Laboratory, University of Pennsylvania for use of the Nanosecond Absorption Spectrometer_ Particular hanksare due to Drs.PaulComelius,GaryM. Holtum and hlark Paczkowski for their expert technit: . assistance. We also wish to thank Drs. Celia H. . =e Go and Helen W. Richter of Carnegie-hlellon Ur._Jersity for their technicai assistance. This research was supported by a grant (DMR76-81561) from the National Science Foundation to the Center for the Joining of Materials at Carnegie-Mellon_
References
[l] W.H. WaddeU and CL. Go, J. Am. Chem. Sot. 104 (1982) 5804. [2] A. Reiser and V. Fraser, Nature 208 (1965)
682.
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\‘olume 98. number 2
CHEMICAL
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17 June 1983
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