TEMPERATURE DEPENDENCE OF PHOTOISOMERIZATION
R'Iethyl siibstitution of both mela positions of I causcs a red shift, A,,, 380 mp,32which is the only carbonium ion shift in the same direction as might be expected for the carbanion. Interestingly, the spectral shifts found
1153
for benzyl carbonium ions on methyl substitution are larger than the corresponding shifts in the spectrum of benzyl carbanions, whereas the shifts in the allyl systems arc of tho same magnitude.
Temperature Dependence of Photoisomerization.
111.'
Direct and Sensitized Photoisomerization of Stilbenes
by Shmuel Malkin and Ernst Fischer Photochemical Laboratory, The Weizmunn Institute of Science, Rehoaoth, Israel
(Received December 9 , 1963)
A quantitative and comparative study was made of the direct and sensitized photoisomerization and of the fluorescence of stilbene and several derivatives, in a wide temperature range. Quantum yields for all three processes were evaluated and their interdependence was studied. The yields of the direct trans cis photoconversion and of the fluorescence were found to change sharply on cooling, probably owing to a potential barrier of close to 2 kcal./mole in the photoconversion. No such temperature dependence was observed in the photoisomerization sensitized by benzophenone, down to - 140'. This indicates that the observed barrier is situated somewhere between the first excited singlet level and the active intermediate responsible for the actual isomerization. A comparison between stilbene and its p-bromo derivative makes it plausible that this barrier is due to the necessity, In stilbene, to pass a second, somewhat higher excited singlet level, unattainable directly, and by-passed in bromostilbene. The nature of this level is discussed. -+
Introduction froin tlhis laboratory, it was I n farlier showii that the quantum yield of the trans cis photoisonierization of aromatic azo compounds drops sharply with decreasing temperatures. These results were taken to indicate the existence of potential barriers somewhere along the path from the excited singlet trans niolecule to the ground singlet cis molecule. Similar preliminary results were obtained with stilbene' and confirmed by Dyck and J I c C l ~ r ethus , ~ indicating the existence of potential barriers also in this case. The photoisomerization of stilbene has been invcstigated by several authors. all of whom conclude that the first excited singlet state, reached by light absorption, is not directly responsible for the isomerization, which -+
probably occurs in a lower excited state reached subsequently. Evidence that this intermediate state is a triplet was advanced by Schulte-Fr~hlinde,~ Stegemeyer,5 and Dyck and M c C l ~ r c ,the ~ latter authors paying particular attention to the role of excited states as iritcrniediates in the course of energy degradation. Corninentirig on the potential barriers observed by the present authors, Stegenieyer5 suggests that these (1) Part 11: S. Malkin and E. Fischer, Symposium on Reversible Photorhernical Processes, Durham, N. C., April. 1962; J . Phys.
Chem., 66, 2482 (1962). (2) E. Fischer, J . A m . Chern. Sac., 82, 3249 (1960). (3) H . H. Dyck and D. S. McClure, J . Chem. Phys.. 36, 2326 (1962). (4) D. Schulte-Frohlinde, H. Blurne, and H. Gusten, J . Phys. Chem., 66,2486 (1962). ( 5 ) H . Stegemeyer, ibid., 66,2555 (1962)
Volume 68, Number 6
May, 1964
1154
SHMUEL MALKIN AND ERNST FISCHER
o
Stilbene
+ p-CL-Stilbene n
p-Br-Stilbene
t 16~ +I40
i
I?
I
+IO0
IIAII-..-i__l_ I I I +60 +20 -20 -60 -100 -140 -180 +20 -20 -60 -100
Tern pera t u re ,OC
-
Irans) and & (trans Figure 1 . Temperature dependence of the quanturn yields in direct photoisomeriaation, & (cis Solutions in rnethyIcyclohexanc--isohexttne, at, temperatures rip to 25", and in decalin at higher temperahres.
exist between excited (probably triplet) trans and cis states of the same multiplicity, while the other authors3s4 believe that the harriers exist between the excited singlet and the triplet state of one isomer. Howcver, the cvidcnce in all cases is not sufficiently accurate and complete to provide conclusive proof of either hypothcsis. The results to be reported here include a dctailed and accurate investigation of the quantum yicld of the cis % lrans photoisonierization of stilbene and several derivatives over a wide range of tcniperatures, a parallel investigation of the fluorescence yicld of the trans isomers, and, finally, a study of the quantuni yield of photoisonierization of stilbene sensitized hy benzophenone. l.6
Results 1. Direct I'holoisomerization. Quanturn yields were calculatcd' from thc kinctics of the isomerization in both directions and the composition of the photostationary state attained aftcr prolonged irradiation. Evaluation of the latter is solnewhat coniplicatcd in stilbene, as comparcxd with aroiiiatic azo compounds, by the irreversible photocyclization and deliydrogcnahv : cis-stilbenc + (dihydrophetion of the cis isoliier7~* nanthrrne) -+ phciianthrcnc '213. Fortunatcly, thc yield of this rcaction is much lower than that of the isomerization and practically vanishes a t tcnipcra-
+
The Journal of l'hysieal Chemistry
-180
-140
-
cis).
tures below -50'. JIorcover, the quantity of phenanthrene formed can be estimated casily from i t s sharp absorption peak a t 2,50 rnp, and the true spectrum of the photostationary mixture in the absence of cyclization evaluated. The same holds also for p-chloro- and pbrornostilbenc. With the naphthyl analog, 1,2-di(a-naphthyl)cthylene, irreversible photoreactions predominate over photoisonierizatioii, and no quantitatativc nieasurenients could be made, though the qualitative behavior of this compound is similar to that of the three stilbenes investigated, shown in Fig. I. Substitution of the naphthyl groups in the ppositions would probably prevent such cyclization. The spectral changes involved in isonierii,at'ion are exeniplified in Fig. 2 . The terriperature dependence of the quantum yields is characteristic of the compounds and varies only little with t,he viscosity and the chemical nature of the medium, as shown by observations on solutions in hydrocarbons, paraffin oil, alcohols, cthers, and plastic films (polystyrene). In cis all cases + t , the quantum yield for the trans isonicrization, increases from very low values (about 0.002) a t -180' to higher values a t higher tempera-
-
(6) S. M:tlkin. Null. Res. Council Israel, A l l , 208 (1962). (7) I T . Stogerneyer. %. .\'aturforsch., 17b 153 (1902); 13. Stegemeyer and 14.-11. Perkampus, Z . p h y s i k . Chem.~(I'mnkfurt),39, 125 ( 1 9 G 3 ) .
(8) 11. V . Surgent and C. J. Tirnmon, .I. Am. Chem. Soc., 85, 2186 (1%:3),
where previous literature is listed.
TEMPERATURE DEPENDENCE OF PHOTOISOMERIZATIOX
/ '
I
I
a
O+
1155
curves reflects competition among several deactivation pathways of the molecule, following primary excitation. One of these paths brings about isomerization and depends strongly on temperature, while the others do not cause isomerization and their rates do not change significantly with the temperature. This oversiniplified schenie is shown in Fig. 3 and sugces to explain SI
I I
I I
I I
hv
hv
\
\ \ \
\
I
.
.
cis
SO
'0
trans
OC
c
L..L---L& 250
Figure 3. Scheme of energy levels involved in photoisomerization.
yx)
Figure 2. Spectral changes resulting from irradiation of solutions in methylcyclohexane-isohexane a t 313 mp, for the periods indicated: parts a and b, a t room temperature; c and d, at - 180'; a and c, p-chlorostilbene (2.4 X 10-6 M);b and d, 1,2-di(a-naphthyl)ethylene(4 X M).
tures and eventually approaches a limiting value. The temperature a t which this limiting value is attained varies greatly from compound to compound, e.g., - 160' for p-bromostilbene and -30' for stilbene. This should be compared with azo compounds,' where a limiting value is not reached even a t room temperature, and with azomethines and azoxybenzene, where +t does not change with temperatureQdown to -BO', suggesting that the observed value is the limiting one attained even a t this low temperature. The peculiar shape of the yield us. temperature
the facts mentioned hitherto. If the rates of those steps which do not cause isomerization, k F (fluorescence) and kz (nonradiative deactivation), do not change with the temperature, while those of the steps causing isomerization, i.e., kl and k3, increase on raising the temperature, then a t higher temperatures both & and +c will reach limiting values because then k l >> /CF, kl >> kz, k3 >> kq. This means that an equilibrium between the excited forms of the two isomers will then be established rapidly, and the sum & & should approach unity. This is indeed the case for stilbene and pchlorostilbene, but not for pbroniostilbene. The latter differs also in the shape of its dt us. temperature curve, with & staying constant down to -160' and then dropping sharply on further cooling (Fig. 1).
+
(9)
S. Mal!& and E. Fischer, to be published. Volume 68.IVumber 6 M a g , 1964
1156
SHMUEL MALKIN A N D EHNST FISCHER
Dyck and RScClure, whose observations are qualitatively similar to the above, suggest that the primarily reached excited state, S*, can pass into the excited state responsible for isonierization in two ways. One of these predominates in stilbene and the other in p bromostilhene, probably because of the enhancement of singlet-triplet transitions by the heavy Br atom. This fact suggests that kl, rather than k3, in the above scheme changes with the temperature. 2. Fluorescence 01 trans-Stilbene. In order to test the above hypothesis, the temperature dependence of the fluorescence intensity was investigated. As seen in Fig. 3, the sharp decrease of Gt on cooling should cause a concurrent rise in the yield of fluorescence. This was proved by Dyck and illcClure3 to be qualitatively true. The following results prove the quantitative interdependence of isomerization and fluorescence in stilbene. The luminescence spectra of stilbene and p-chloro- and p-bromostilbene were measured over a wide range of temperature. The areas of the spectral curves, some of which are shown in Fig. 4, were assumed to be proportional to the quantum yields a t each temperature. Table I sun~marizesthe observations, with the quantum yields of fluorescence, #F, expressed as fractions of the highest quantum yield #F-, reached a t -183' with stilbene. Table I: Temperature Dependence of +t and @ F / + F - in Solutions of Stilbene (0.8 x 10-6 mole/l.) and pChlorostilbene" (0.85 X 10-6 mole/l.) in MethylcyclohexaneIsohexane
a
T.
--Stilbene-
OC.
mt
@F/@Frn
+25 -40 - 65 - 80 - 90 - 100 - 105 - 123 - 140 - 174 - 183
0.50 0.46 0.31
0.08 0.18 0.50
0.18
0.71
0.12 0.07 0.04
0.74 0.90 0.97
0.006
1.00
0.60 0.45
0.11 0.39
0.29
0.62
0.20
0.68
0.13 01
0.70 0.70
;.o,
According to the scheme in Fig. 3, #tand expressed by akl,/(kl
n
I1
p-CL-Stilbene (trans)
Stilbene (trans)
I1
1I
I 1
I1
A
I!
n I\
cis- Stilbene
(mp) Figure 4. Emission spectra of stilbene and derivatives, excited with light at 313 mp: full lines, at 25'; dashed lines, a t - 180". The intensity scales a t both temperatures are arbitrary. The ernission of cis-stilbene was measured at - 180' and corrected for the emission of traces of transstilbene accompanying the cis isomer.
where k F and k2 denote the rate constants of radiative and radiationless transitions from the excited to the ground singlet states, respectively, and LY is a teinperature-independent constant giving the probability of the excited intermediate (triplet?) to be transformed to the cis isomer. If +' denotes the yield of the radiationless transition, given by k 2 / ( k l ICz k F ) , the following expression will relate the various yields
QF/9Frn
mt
For both compounds the fluorescence yields refer t o
=
350
-gChloroatilbene---
+F-
+ +
#F
of
stilbene.
#t
4cc
- T - T - r ~ - - ~ -7l T - ~
+ + kF); 122
4~ = The .Journal of Phyaical Chemistry
kF/(ki
+ k$ +
#F
kF)
may be
(I)
+ 4' +
=
1
(11)
Dividing eq. I1 by 4 F m in order to be able t o use the relative fluorescence yields + F / + F ~ , we get
This equation leads to a linear relationship be tween the relative fluorescence yield and #t if dt experimental $F's a t 25
+
+ + + +
and -115' we havc 0.065 = k ~ / ( k ~ /is k1) and 0.13 = /rF/(lrF ICs). Defining Zli = kl? ks kl and rearranging we have Zk ' ( k ~ kb) = 0.13/0.063, and therefore a t 25' kl = k F lis, or Zk = 2kl. 1)efining the yields of steps 1 and 5 a t 25' by cpl = IC,/ Z k ; $6 = k s / Z k , we have $1 = k1/2kl = 0.5, and thcrefore $6 = 1 - $F - $1 = 0.4%. I n other words, a t room temperature the chances of step 1 and 5 are almost equal in p-bromostilbenc. The scheme of Fig. 9 is of course speculative, but suffices to explain the results obtained. The current investigation, as well as others in this field, may serve to emphasize how photoisornerization and related phenomena cvokc more general theoretical problems, such as singlet-triplet transitions, evaluation of energy levels and potential curves, and transfer of excitation energy between niolecules. A more thorough evaluation of current theories will bc published scparatcly, together with a comparison of the results in stilbene with those obtained in compounds containing lone electron pairs (aromatic azo and azoxy compounds, azorncthines). At this point only a few gcncral conclusions will be indicated. Dyck and McClure3 havc proved conclusively that the order of the central ethylenic bond in the lowest excited state of stilbenc is only slightly lower than in the ground state. This rules out isomerization in thc lowest excited singlet state, aiid also indicates that earlier IJCAO-HAIO calculations, leading to a much lower bond order in this state, are inaccurate. This leaves little choic but to assunie that isonicrization involves one or more of the triplet states possible in stilbene. As to the nature of the intcrniediate singlet level S2, one possible approach is using the concept of and levels introduced by I'ariser" and others. I'ariser ass~~riies the operation of selection rules allowing only H - nonradiative singlet-triplet transitions. If S1 aiid T have the same sign and Sz a different one, the transition SI T would be forbidden and S2 T allowed, necessitating the sequence SI SZ---c T, in which S, + S L would involve passing a potential barrier. A reversal of the positions of SI and Sz would then eliininatc the need to pass such a barrier. This might well be the case in thc cz's isomer, although too sniall a harrier would escape detection in any case. In azo compounds the n - ~ *transition might be involved. I t is remarkable that the tcrnpcrature dependence of $t was observed only in those conipounds whose central part is synimetrical, ie., stilbenes
+
+
+
+
+
The Journal
-
of
PhZ/sical Chemistry
- -
and azo compounds, but not azoniethines (-CH=X-) and azoxy conipounds (--X=N-) .9
I1
0 Experimental Spectrophotometry, Irradiation, Actinometry, Analysis, and Calculation o j Qimntum Yields. All these were essentially as described in part 11.' For the sake of better spectral definition, the 313-nip group of mercury emission lines was isolated by a combination of a Corning filter KO. 98633, 5 mm. of a solution of potassium chrornate in water (300 nlg./l.), and 10 nun. of a solution of potassiun~biphthalatel5 in water (25 g,/l.). Most expcrimcnts were carried out with solutions in carefully dried methylcyclohexane -isohexanc (1 : 1) mixtures. S o difference in photoisornerixation bchavior was observed between air-saturated solutions and solutions in which the solvent was distilled under high vacuum from K-Sa alloy onto the solute in the nieasuring cell, the latter then being fused off. A tiny magnetic stirrer served to ensure mixing of the solutions being irradiated, whenever their viscosity was not too high. Sensitized Photm'somerization. Experiments a t high concentrations were carried out in the above solvent mixture or in methylcyclohexane, in cells having a light path of 10 or 25 mm. Samples of 0.1 ml. were withdrawn after appropriate irradiation periods and diluted 100-fold. Their absorption spectra were then measurcd in 2 - m n ~cells in order to determine their isomeric coinposition (Fig. 6). At low Concentrations the solutions wcrc investigated directly in 1-mm. cells. At all teniperaturcs the spectrum of the mixture of benzophenone and stilbene was found to be identical with the sum of the spectra of the component solutions, indicating that probably no complex formation between the two solutes in their ground state takes place. Lumiiiescence Spectra. These were measured a t right anglcs to the exciting light. The latter was in all cases a t 313 nip, isolated from a Philips spectral lamp mercury are by means of an interference filter transmitting 2.5y0 a t 313 mp and less than 0.05% a t 302 m p . The resulting low intensities of exciting light caused only little isomerization during the recording of the luminescence spectra. The latter were eorrcctcd accordingly. The light emitted by the solutions entered a Bausch and Lomb 500-mni. monochromator driven by a synchronous motor. A Type 6256Q E.lI.1. photoniultiplier was placed opposite the (17) It. Pariser, J. Cksm. Phys.. 24, 250 (1956). (18) H . E. Hunt and W. Davis, Jr., J . Am. Chem. S o c , 69, 1416 (1947).
COMPARISON OF LIQUIDTHEORIES WITH SIGNIFICANT STRUCTURE THEORY
exit slit and its anode current recorded directly by a Kipp & Zonen micrograph recorder having a rnaxinial sensitivity of 0.1 pa. full scale. The spectral sensitivity of the photoniultiplicr over the range 302-406 nip was found to be constant within lo%, and the experimental curves therefore were not corrected accordingly. The cell contairiiiig the luiriinescent solution was placed inside a copper block cooled by liquid or gaseous air arid irisulatcd by a quartz dewar. The luminescence intensitiw in a range of temperatures were assuriied to be proportional to the areas beneath the recorded spectral curves. The relative luminescence yields were taken as proportional to the above intensities, after correcting them for the temperature dependence of the absorbance a t 313 m p . The latter was always smaller than the limit up to which the luminesecnce intensity was found proportional to the absorbance. The yields measured in this way were rcproducible to within 10%.
1163
Materials. trans- and cis-stilbene were commercial materials supplied by Light & Co. and further purified by crystallization from heptane (trans) or by chromatography on aluniina (cis). p-Bromo and pchlorostilbcne were prepared according to A n s c h u t ~ ~ ~ and 1,2-di(a-naphthyl)ethylene according to Wislicenus.*O Acknowledgments. Thc authors wish to thank Professors G. Stein and E. Iippert for most helpful discussions and 11r. 31. Kaganowitch for synthesizing the compounds investigated. They also gratefully acknowledge a research grant from the U. S. Xational Bureau of Standards, which served to finance part of the work described here.
(19) It. Anschiitz, Ber., 60, 1320 (1927). (20) W. Wislicenus and H. Wren, ibid.. 38, 502 (1905).
Comparison of Various Liquid Theories with the Significant Structure Theory
by Teresa S. Ree, Taikyue Ree, and Henry Eyring Departmnt of Chemistry, liniversity of Utah, Snlt Lake City, Utah (Receiwd Dr*.em.lJer 1.2, 1963)
The significant structure theory of liquids is applied to calculate the compressibility factor of argon gas and the reduced excess entropy, energy, and heat capacity, the compressibility, and the thermal expansion coefficient of the liquids of inert gases and nitrogen in equilibrium with their vapor. The calculated thermodynamic data are compared with experiment and with the values calculated from other theories. It is found that the significant structure theory agrees best with experiment. The quantum effect in liquids of the elements of low atomic weight is discussed. It is pointed out that the significant structure theory accounts for this effect very well, whereas the cell model explains the effect only when some correction factors are int~oducedinto the partition function.
I. Introduction The free-volu~netheory of 1,cnnardJones and Devonshirc1s2 is the most widely used theory of liquids.
thcir cclls. Moreover, they artificially introduced the extra factor e N into the partition function. Subse-
