SHMUEL MALKINAND ERNST FISCHER
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Vol. 66
TEMPERATURE DEPENDENCE OF PHOTOISOMERIZATION. PART 1I.l QUANTUM YIELDS OF cis e trans ISOMERIZATIONS IN AZO-COMPOUNDS BY SHMUELMALKINAND ERNST FISCHER Laboratory of Photochemistry and Spectroscopy, The W e i m n n Institute of Science, Rehovolh, Israel Received Mau .E41966
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Quantum yields of the cis p n s photoisomerization of azobenzene and related compounds were measured in the temperature range of +20 t o - 183 . For each compound the yields in both directions were measured with light at two wave lengths, one within each of the two main absorption bands of the spectrum in the visible and near-ultraviolet regions, respectively. The yields for the trans cis transformation were found to decrease sharply on cooling, whereas those for the Cis trans reaction change but little in this temperature range. These results corroborate conclusions from earlier result%’ and indicate the existence of energy barriers somewhere between the electronically-excited singlet state of one isomer (formed by light absorption) and the ground state of the second isomer. Preliminary results with stilbene are basically similar, but in this case the yield for the reaction cis .-,tram also is attenuated on cooling, though to a lesser extent.
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Introduction In Part I of this series’ it was shown that the cis e trans photoequilibria attained on irradiation of azobenzene and related compounds all show a pronounced shift toward the trans isomer, if photoequilibration is performed at low temperatures. In some cases irradiation at -180’ with light at any wave length in the visible and ultraviolet region results in complete transformation into the trans isomer. Since the photoequilibrium constant is determined by the relative quantum yields in both directions, &ikltrans/&anklcis, these results were interpreted as being due to the fact that &.t is little affected by cooling, whereas $t+c approaches zero a t sufKciently low temperatures. These conclusions were now checked by direct measurements of the quantum yields &,t and c#+c. a t temperatures down to -183O, in an attempt to throw some light on the mechanism of photoisomerization in these cases. Quantum yields of the reversible photoisomerization of azobenzene have been measured very accurately a t room temperature and a t various wave lengths by Zimmerman, et aL12who found that the is significantly smaller than unity, sum dWt &C, proving non-existence of a common excited initial state of the two isomers. An earlier paper by Birnbaum and Style38.3b describes results obtained with a series of derivatives of azobenzene. Earlier results are cited in the above two papers.
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Experimental Spectrophotometry and Irradiation.-The low-temperature technique described previously* was used, modified by using dewar vessels with three pairs of windows in experiments involving fluid solutions. This enabled irradiation a t right angles to the measuring beam. All experiments were carried out in a Cary Model 14 recording spectrophotometer. A small magnetic stirrer was introduced into the measuring cells to enaure mixing, a t least as long as the medium is not too viscous. At temperatures at which the medium forms a rigid glass, the system is essentially unmixed, since neither convectional nor diffusional mixing takes place. At intermediate temperatures the conditions are less defined and an effort usually was made to work under one of the two extreme conditions. Solvents.-The present report deals only with non-polar solvents. The lowest melting one used was methylcyclo(1) Part I: E. Fischer. J. A m . Chem. Soc., 82, 3249 (1960). (2, G. Zimmerman, L. Chow, and U. Paik, ibtcl., 80, 3528 (1958). (3) (8) P. P. Birnbaum and D. W. G. Style, Trans. Faraday Soc., 60, 1192 (1954); (b) P. P. Birnbaum, J. H. Linford, and D. W. G. Style, ibid., 49, 735 (1953). (4) Y. Hirshberg and E. Fiacher, Rm. Sci. Instr., SO, 197 (19591.
pentane (m.p. - 144’) which, like other pure non-hydroxylic solvents, has the advantage of low viscosity down to ita melting point. For lower temperatures mixtures of several solvents were used, such as methylcyclohexane with either methylcyclopentane, isohexane, or isooctane. The viscosity of all these mixtures increases on cooling, and rigid glasses are formed at appropriate temperatures. All solvents were purified by psaeing them through columna .filled with “Woelm” alumina,4 and the solvent was distllled from a K/Na alloy i n vacuo onto the solute in the measuring cell, which then was fused off .4 This assured complete absence of air and hydroxylic contaminants. Light Sources.-A Philips spectral lamp (125-watt) with a stabilized power supply was used throughout. The mercury emission lines were isolated either with Corning glass filter combinations (at 436 and 365 mp) or with a combination of such liltens and a solution of NiClr a t 313 mp (cf. P@ I where one of the filters was erroneously deslgnated 0620 instead of 0160). Irradiations were carried out either at right angles to the measuring beam (in mixed fluid media) or in the direction of the measuring beam (in rigid media). I n this case the dewar with the measuring cell was rotated through 90’ during irradiation. Light Intensity.-Hatchard and Parker’s ferrioxalate actinometric method was used in the form modified to low intensities.6 Three ml. of actinometric solution is put into a spectrophotometer cell similar to the one used in photoisomerization, and irradiated for a certain time under conditions exactly identical with those pertaining during photoisomerization. One-half ml. of phenanthroline solution then is added, and the optical density of the resulting solution at 510 mp is measured in situ. Calibrations were made according to Parker.6 At 436 mp the absorption of ferrioxalate is rather small, and the actinometric results at this wave length of irradiation light therefore were checked by measuring the relative light intensities (in quanta/time unit) at 436 and 313 or 365 mfi, usin Bowen’s proportional quantum counting method, as mojified by Weber and Teale.e A solution of Rhodamine B in ethylene glycol served as “fluorescent screen.” The intensity of the fluorescence excited in it with light a t any wave length which is totally absorbed is proportional to the uantal intensity of the exciting light. The intensities of t%e monochromatic light incident on the solutions (expressed as quanta/minute) were about 1.2 X 1017 a t 313 mp, 2.5 x 1017a t 365 mp, and 1.2 X 10’’ a t 436 mp. Compounds.-Azobenzene (I): A commercial product was purified chromatographically on alumina and recrystallized from hexane. Z,Z’-Azonaphthalene (11): this was synthesized according to Friedlander as described before’ and recrystallized from isooctane. 1-Phenylazonaphthalene (111): This was obtained by Martynoff’s methods and recrystallized from hexane. (5) C. G. Hatchard and C. A. Parker, Proc. Roy. Soc. (London), 8 2 3 6 , 518 (1956). (6) G. Weber and F. W. J. Teale, Trans. Faraday SOC.,63, 646 (1957). (7) M. Frankel, R. Wolovsky, and E. Fischsr. J. Chem. Soc., 3441 (1955). (8) M. Martynoff, Bull. aoc. chim. Francs, 18, 216 (1951).
Dec., 1962
QUANTUMYIELDSOF cistitruns ISOMERIZATIONS IN AZO-COMPOUNDS I
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I11 The cis isomers were prepared b ultraviolet irradiation of solutions in petrol ether, followedlby chromatography on alumina, elution, and evaporation to dryness. T o prevent thermal and photochemical transformation into the trans isomer, all the above manipulations were carried out in a room at Od", using only red light for illumination. Pure cis isomers were used only for the determination of their extinction coefficients. Procedure for Determination of Photoisomerization Kinetics.-The isomeric composition was determined spectrophotometrically as a function of time of irradiation a t each wave length of photoactive light. In each kinetic series the starting composition waa chosen to be aa different aa possible from the final composition, Le., trans -P cis isomerizations started with the pure trans isomer, and cis -t trans conversions with a mixture containing rn much cis isomer as is obtainable by irradiation a t a suitable wave length and temperature. No photokinetic experiments were made with the pure cis isomers beoause of the latter's thermal instability. During the kinetic experiments full spectral curves were taken after each irradiation. Experiments were discarded if isosbestic points were not obtained, or if rolonged irradiation did not cause an asymptotic approac to a final state. These two basic requirements were absent only in very few experiments, and could be blamed on technical reasons. Calculation of Absolute Quantum Yields.-Using Zimmerman's basic rate equation2 the experimental composition us. time curves were analyzed either graphically or by an integration method, as detailed in the Appendix. Thermal isomerization could be disregarded a t the temperatures of all kinetic experiments carried out.
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Results Absorption Spectra.-These have been analyzed in detail for azobensene by Jaffe, Yeh, and Gardner? who distinguished between three main absorption bands of which the weakest one, in the visible region, is ascribed to the lone pair of electrons on the nitrogen atom, and therefore lacks in stilbene.lo As shown in Part I, the vibrational fine structure of all three trans-asonaphthalenes (1,l'; 1,2' ; 2,2') is enhanced on cooling and is completely absent in the cis isomers. However, as seen in Fig. lb, the cis isomer of compound I11 possesses such fine structure in its spectrum. A comparison of the spectra of the two isomers (Fig. 1) shows in all cases that the main absorption bands differ only slightly in their position. It may be inferred that the electronic energy levels are spaced in a rather similar way in both isomers. Quantum Yields.-The results are summarized in Fig. 2 for all four compounds investigated. In Part I we showed that the pronounced shift of the photoequilibrium in favor of the trans isomer on cooling also occurs in the two other azonaphthalenes and also in polar solvents (ethanol-methanol, tetrahydrofuran). It is therefore reasonable to (9) H. H. Jaffe, Si-Jung Yeh, and R. W. Gardner, J . Mol. Splctry., 2, 120 (1958).
(10) CJ'.,however, the recent paper by M. B. Robin and W. T. Birnpson, published [ J . Chem. Phys., 86, 580 (19SZ)l after the present communication was sent in.
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Fig. 1.-(a) Spectrum of azobenzene in isohexane, 0.7 X 10-4 M , at room temperature: 1, trans; 2, cis. (b) Spectrum of 1-phenylazonaphthalene in isohexane-methylcyclohexane ( l : l ) , 0.75 X lO-'M: 1, trans a t 5 ' ; 2, trans a t -160'; 3, cis at 5". (c) The progressive change of the spectrum of stilbene (in isohexane, 0.4 X lo-' M ) by irred+ tion with light at 313 mp. Numbers refer to time of irrahation (in minutes) at 25". (d) Spectrum of. a solution of 2,2'azonaphthalene, 0.32 X 10-4 M , in isohexane-methylcyclohexane (1: 1) at 10': 1, trans, 2,eis.
expect that the temperature dependence of the quantum yields described in Fig. 2 characterizes a t least all unsubstituted azo-compounds and does not depend on the solvent used. Furthermore the preliminary results with stilbene (Fig. 2c) indicate that this behavior may be of more general importance in cis trans photoisomerisations. The results may be summarized thus: (a) In accordance with Zimmerman's results2 at room temperature, the yields with ultraviolet light are lower than with visible light. (b) The yield of the process trans --+ cis in all cases decreases sharp1.v on cooling, but in a way peculiar to each compound and wave length of photoactive light. (c) The yield of the process cis + trans changes much less with the temperature, and does so in a rather irregular way.
Discussion The following ideas have been forwarded hitherto to explain the mechanism of photoisomerization. (1) According to Lewis, et a2.,11photoisomerination involves transformation of an electronically excited molecule into one a t a high vibrational level of the ground electronic state. In the latter suffcient energy is available for those rotations which can bring about isomerization. Eventually (1 1) G . N. Lewis, T. T. Magel, and D Lipkin, J . Am. Chem. Soc., 62, 2973 (1940).
lowing reason.I6 Both in stilbene and iii azocompounds the absorption bands of the cis isomer are either similar to that of the trans isomer or even shifted toward higher frequencies. This shows that the distance between the cis and trans levels in the excited states is at least as big as that between the cis and frans levels in the ground state, which is about 10 kcal./ mole. Accordingly the potential barrier for tip" + trans* should be larger than 10 kcal./mole. For a reaction occurring within less than see. such a high activation energy seems improbable. and also would imply a much sharper temperature dependence than even the one observed here. Furthermore, in stilbene only the trans isomer is fluorescent," showing that there is no cis* + trans* conversion. In azo-compounds neither isomer is fluorescent, and the same argument can be applied only by analogy. One therefore has to conclude that one or more intermediate excited states X of similar or lower energy than cis" and trans* are involved, and that hv
the sequences are schematically cis hu +
(+XI)
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X X' (+X)
cis*
+ trans, and trans trans* -+ cis. The general reaction scheme thus would be
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En, SY
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-140 -220t20 -60 TEMPERATURE ("C).
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Fig. 2.-Quantum yields, +o and &, for the cis + trans and trans + cis photoisomerization, respectively, as a function of temperature (in non-polar solvents): (a) azobenzene; (b) 1-phenylazonaphthalene; (c) stilbene; ( d ) 2,Z'-azonaphthalene. (The results for stilbene are only preliminary and less accurate than the others. They are brought here only for the sake of comparison.)
the surplus energy is lost t'o the medium, and the molecule stays eit'her in t'he cis or t,he trans form. ( 2 ) According to Birnbaum and Style3and other photoisomerization involves an intermediate st'ate common to both cis and trans isomers. In this state there is no energy barrier between the two isomers.l5 (3) According to Zimmerman2 t'he above two hypotheses contradict the experimental results, and it t,herefore has to be assumed that "the isomerization takes place as an ordinary thermal reaction of an electronic excited state," i.e., thermal intercoriversion of electronic excited cis* and trans* molecules across a (hypothetical) energy barrier. The latter must be low in order t'o enable such interconversion to occur within the short lifetime of the excited states (
