1717
stant with increase in ring size. The dominant mechanism must then be the direct radical-fluorine collision, with at most a minor contribution from the T systems. BDPA behaves in a manner similar to GALV except that all 19F enhancements with BDPA have larger scalar components. BDPA must also couple primarily with the 19F directly, the larger scalar coupling resulting from a sterically more available unpaired electron on the radical. The stronger scalar components in fluorine polarizations with DPPH cannot be readily explained solely on the basis of direct collision at the F site. Nevertheless, indirect scalar coupling via ring orbitals seems unlikely, since neither calculated nor observed ionization potentials for the two B systems can be correlated with the F polarizations.5 A possible explanation for the DPPH results is longer lived collisions due to N-N
attraction between radical and receptor molecules, but multifield DNP measurements would be required to assess this hypothesis. In summary, DNP offers no evidence for spin coupling via either B system in (PNF,), with the three radicals tested. As a corollary, the ring B orbitals show no effects on the direct polarization of orbitals localized near fluorine. To a large extent, the exocyclic F atoms in (PNF& behave toward radical probes as though they were not conjugated to the ring T orbitals. This situation is in marked contrast to that of F in fluorobenzenes or C1 in phosphonitrilic chlorides. Acknowledgment. One of us (J. A. P.)acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We are grateful for the high-field nmr spectra obtained by Dr. D. Denney.
Photochemistry of Benzaldehyde Michael Berger, Irwin L. Goldblatt, and Colin Steel*
Contribution from the Department of Chemistry, Brandeis University, Waltham, Massachusetts 02154. Receiced July 6 , I972 Abstract: The photochemistry of benzaldehyde has been studied by determining the quantum yields of benzene and carbon monoxide formation, benzaldehyde consumption and phosphorescence emission, and by following the phos-
phorescence lifetime over a range of pressure and excitation wavelengths. These data together with information from triplet transfer studies allow the constructionof a model for the primary photochemical-photophysical processes (Figure 9) and the determination of most of the primary rate constants (Table 11). Excitation with 276-nm light, S o S p , results in the population of two vibrationally excited triplet states. At low pressures dissociation, yielding benzene and CO, occurs from the high vibrational levels of these states. Collisional deactivation of these states feeds the lower vibrational levels of the lowest triplet; from here there are no chemical decay channels and phosphorescence can be observed. In contrast to So --* S a excitation, So SI excitation results in no benzene or carbon monoxide formation and the phosphorescence yields are insensitive to pressure. There is however a significant quantum yield of benzaldehyde consumption and polymer formation can be observed. -+
-+
s part of our work on the gas-phase photochemistry
reported that n + B* excitation in the gas phase of azoalkanes' we have used benzaldehyde as a results in polymer formation and that at elevated temtriplet energy donor.2 Benzaldehyde is one of the peratures the quantum yield of benzaldehyde consumption was as high as 40." It has also been reported few molecules which exhibits a strong phosphorescence in the gas p h a ~ e ,and ~ , ~we believed that this emission that at wavelengths below 270 nm benzaldehyde dewould provide a very convenient monitor. Other composed into benzene and carbon monoxide. lo The aim of this work was to obtain absolute quantum workers have also used benzaldehyde as a triplet energy donor in the gas phasesa and in s ~ l u t i o n . ~ ~ 'yields of the primary photophysical and photochemical Our use of benzaldehyde as an energy donor was processes and to study the effect of wavelength on the hampered by our lack of understanding of the photosystem. In this way we hoped to determine the chemistry of benzaldehyde itself. mechanism for the population of the emitting state There were rather preliminary studies into the direct and determine from which state(s) dissociation into photolysis of gaseous benzaldehyde.*It has been benzene and carbon monoxide occurred.
A
(1) W. Clark and C. Steel, J . Amer. Chem. Soc., 93, 6347 (1971). (2) A preliminary account of the work reported in this paper was
presented by I. L. Goldblatt and C. Steel, Abstracts, 157th National Meeting of the American Chemical Society, Minneapolis, Minn., 1969. (3) P. Longin, C. R . Acad. Sci., 255, 865 (1962). (4) M. Stockburger, Z . Phys. Chem. (Frankfurt am Main), 31, 350 (1962). ( 5 ) (a) G. R. D e Mare, P. Goldfinger, G. Huybrects, E. Jonas, and M. Toth, Ber. Bunsenges. Phys. Chem., 73, 867 (1969). (b) G. R. De Mare, M. C. Fontaine, and M. Termonia, Chem. Phys. Lett., 11, 617 (1971). (6) N. C. Yang, Photochem. Photobiol., 7, 767 (1968). (7) N. C. Yang, J. I. Cohen. and A. Shani, J . Amer. Chem. Soc., 90, 3264 (1968).
Experimental Section Materials. All reagents were the best grade commercially The preparations of azoisopropane (Alp), 2,3-diazabicyclo[2.2.l]hept-2-ene (DBH), a n d 2,3-diazabicyclo[2.2.2]oct-2-
available.
(8) F. Almasy, J . Chem. Phrs., 30, 528,634,713 (1933). (9) G. R. De Mare, M. C. Fontaine, and P. Goldfinger, J . Org. Chem., 33, 2528 (1968). (10) (a) M. de Hemptinne, J . Phj,s. Radium, 9, 357 (1929); (b) C. R. Acad. Sci., 186, 1295 (1928). ( I 1) F. E. Blacet and D. Vanselow, Abstracts, 13 1th National Meeting of the American Chemical Society, Miami Beach, Florida, 1957.
Berger, Goldblatt, Steel
Photochemistry of Benzaldehyde
1718 -----
I
20001
I 3
0
1
300
310
"" 1 7-
7
120
330
WAVELENGTH 0.5 f n m : 350
,TO
360
,w
Figure 2. Solution-phase absorption spectrum of benzaldehyde in then 4 T* region.
Table I. Quantum Yields in the Photolysis of Benzaldehyde
I
265
2 70
275
280
WAVELENGTH
285
290
(nm)
Figure 1. Gas-phase absorption spectrum of benzaldehyde in the 265-290-nm region.
ene (DBO) have been described previ~usly.~Benzaldehyde was
JAIP
DBH
0.021 0.047 0.051 0.065 0.075 0.091 0.100 0.154 0.154 0.172 0.201 0.207 0.307 0.359 0.622
276 276 726 276 276 276 276 276 2 76 276 276 276 276 276 276 276
0.025 0.043 0.065 0.092 0.151 0.154 0.207 0.307 0.409 0.622
284 284 284 284 284 284 284 284 284 284
0.34
0.87 0.42 0.29 0.26 0.20 0.21 0.18 0.20 0.20 0.17
0.046 0.183 0.567
328 328 328
0.39 0.41 0.41
0.006 0.002 0.002
@ ~ ~ C ' ~ ~ ( CAs G Hwe ~ )have . pointed out in the Experimental Section absolute quantum yields for X e x c l t 284 nm were particularly hard to obtain and it could be
-
+
Journal of the American Chemical Society
2
Figure 12. Ratio of emission yields for X e x c i t 284 nm and h,,,it 276 nm as a function of added ethane: (0)0.2 Torr of benzaldehyde, ( 8 )0.2 Torr of benzaldehyde C Z Has~ added gas.
+
that the latter inequality is due to a systematic error. If the difference between benzaldehyde consumption and benzene formation does monitor SI reactivity, then the results could indicate that for Xenclt 276 nm intersystem crossing is sufficiently rapid to compete favorably with internal conversion, but is somewhat slower in the case of 284-nm photolysis. Theoretical calculations have shown30 that nonradiative decay rates can be very sensitive functions of energy. Moreover the possibility that there are several triplet states lying close to S2I7 could also influence the coupling rates. (7) General Discussion. Although in the main the experimental results are rather pleasingly encompassed by the model given in Figure 9 and by eq I-VI, there are certain problems which remain. We have indicated in section 4c that the experimental evidence points to the existence of a state rvwith a lifetime -lo-* sec, which can ultimately be deactivated to the phosphorescing triplet TI0. The state r v cannot rapidly interconvert to TIVbecause if this were so there would effectively only be one excited state involved and we should expect no sharp break in the l/@dee276(C6H6) us. pressure curve (Figure 3); it would also be hard to explain the increase of @'el,,27b in the 1-10Torr region (Figure 6) or the continued formation of benzene in the presence of excess DBH (Table 111). We have also presented evidence to show that rvcannot have a large amount of So or SI character. But if it is a triplet what state can it be? It is generally assumed that internal conversion between states occurs much more rapidly than lo8 sec-1.22 Thus we have a problem in associating rv with T2", say. If rv were in fact a TI state, but with a distribution of the excess vibrational energy differing from that of TIv, we should also expect rapid inter31 Self-consiscommunication between Tiv and I'v, tent-field MO calculations indicate that there are pos) of A' symmetry lying besibly four 3 ~ , 7 r * ( ~ L ,states tween S2('Lb) and T1(3U) which is of A" symmetry." The symbols in parentheses are often to describe the diflerent states in aromatic carbonyl compounds. If we consider only the T and n electrons of benzaldehyde, we can represent the electronic structure of T? by ( ~ 1 ) ? ( ~ ? ) * ( ~ 3 ) z ( ( n ~ ) 1 ( ~ ~and * ) 1 (that n ) ? of TI by ( T , ) ' ( T ~ ) ~ ( T ~ ) ~ ( T * )'(n)'. ~ ( T ~ *It) can therefore be seen that the T, -+TI process involves essentially the transfer of an n electron t o the 7r4 orbital. Such orbitals are spatially orthogonal and could result in a very low value for the electronic part of the matrix element (30) A . Nitzan, J. Jortner, and P. M. Rentzepis, Chem. P h y s . Lett., 8, 445 (1971). (31) D. Bunker, J . Chem. P ~ J J s40, . , 1946 (1964).
95.6 1 March 21, 1973
1725
coupling the states.** In this respect it is interesting crossing from SI to TI, reaction 13 Figure 9, actually to note that there are several references in the literature occurs via T2. to the involvement of upper triplet states in photoHarrison and L o ~ s i n gcarriad ~ ~ out the Hg-sensitized chemistry. This could indicate that the upper triplets decomposition of benzaldehyde using light intensities in these cases do not relax very rapidly to the lowest which, judging by the published were about triplet. One group of compounds where two triplets one million times greater than ours. Under their conare apparently involved are lactones and it has been ditions the major product was a yellowish polymer but suggested that one triplet has an n,x* configuration 18% of the yield was C6H6and CO. They found that and the other a K,T* c o n f i g u r a t i ~ n . ~Also ~ M i ~ h l ~67~ of the CaHa was formed by a molecular route and has studied a hydrocarbon in whose photochemistry 33% by a radical route which they suggested was an upper triplet is involved. It turns out that the C~HF,CHO* + CsH5. CHO. followed by CeH5. molecule has essentially two spatially separate chromoCHO. -t CsH6 CO. However in a study of the phoric groups, the lowest triplet being associated with direct photolysis of benzaldehyde Majer, et a1.,,16conone and the next triplet with the other. Another poscluded that all the benzene was formed molecularly. sibility which should not be neglected is that r is some Our results, which show that the quantum yield of benstate in which the nuclear geometry is sufficiently diszene formation tends toward unity at low pressure, torted so that it could be regarded as an isomer of benzalimply that radical reactions cannot be of major imdehyde. portance since it is unlikely that the fate of every For simplicity in Figure 9 we have shown the interphenyl radical would be benzene. Moreover we have system crossing for X e x c i t 276 nm occurring directly observed that when 0.2 Torr of 0, was added to 0.7 from S,. Operationally this description predicts the Torr of benzaldehyde the quantum yield of benzene observed kinetics; the important fact is that there are formation was 0.33 which is close to the value obtained two states with very different lifetimes both triplet in in the absence of oxygen (Table I). Oxygen would be character from which benzene is formed. Of course expected to scavenge any radical intermediates very we cannot distinguish between this operational model effectively. I n the work by Harrison and L o ~ s i n git~ ~ and other models which have processes too fast for our is possible that the increased importance of radical reexperiments to detect. Such a process would be inactions in their system was in part due to the process ternal conversion from Sz to SIv before intersystem Hg(3P1) C 6 H 5 C H 0 + Hg-H C6HjCO. comcrossing occurs. However the observations that polyC6H5CH0 + peting with energy transfer Hg(3P1) meric build-up seemed to be more severe for n + K* CeiH,CHO(T) Hg('So). photolysis than for x 3 K* photolysis and that the For the S o -P S , band we have discussed the photopolymer apparently originated from S , reaction could chemistry resulting from Xexcit 276 and 284 nm. In support the supposition that intersystem crossing compreliminary studies we have found even stronger prespetes favorably with internal conversion. Very rapid sure and wavelength effects for X e x c i t
