GEORGEZIMMERMAN, LUE-YVNC CHOWAXD UR-JIN PAIK
3,525
VOl. so
[CONTRIBUTION FROM DEPARTMENT O F CHEMISTRY, BRYNA ~ ~ A WCOLLEGE] R
The Photochemical Isomerization of Azobenzenel BY GEORGEZIhfXERXAR,2 LUE-TUNG ClIO\V
.\KD ITN-JIN
PAIK
RECEIVED DECEMBER 23, 1957
T h e photochemical cis + trans and trans -+ cis isomerizations of azobenzene solutiolis in isooctane have been studied. Quantum yields for both processes have been determined with precisions of from 5 t o lOy0over wide ranges of wave length, concentration and light intensity. The quantum yield depends only on wave length and is approximately constant throughout a single absorption band. T h e average quantum yields are as follows: for the ultraviolet band, cis + trans 0.42 & 0.04. trans cis 0.11 i 0.01; for t h e Visible band, cis -+ trans 0.48 =k 0.05, trans cis 0.24 =!c 0.02. T h e results suggest t h a t the isomerization takes place as an ordinary thermal reaction of an electronic excited st:tte. -.f
-f
Introduction We have studied the photochemical cis-trans isomerization of azobenzene dissolved in isoiictane over a wide range of concentration, of light intensity and of energy per photon. Several other investig a t o r ~ " ~have made quantitative measurements of the photoisomerization of azobenzene, but only in one case4 have quantum yields for both directions been obtained, along with analogous quantum yields for a number of substituted derivatives. We feel that the present data for azobenzene are both more accurate and more extensive. In particular, we found that in order to obtain quantum yields of meaningful accuracy, it was important that the irradiating light contain less than 1% of stray light, i.e., light of wave lengths other than the mercury lines used as sources. If any mixture of the two isomers is irradiated with steady visible or ultraviolet light, the isotopic composition changes with time, approaching asymptotically the same photostationary composition regardless of initial composition or light intensity (over wide ranges), but depending strongly on wave length. In the stationary state the photochemical tvans-cis rate must be equal to the photochemical plus the thermal cis-trans rates. In general, for any irradiated solution a t any time all three rate processes are taking place simultaneously. We feel that any satisfactory analysis of experimental data ;houId account quantitatively for the entire bet avior in time of any solution. Experimental Reagents. ~-lin,is-Azobenzene.---Eastiiian Kodak Co., >,~liite label aziiimizene w-ns recrystallized three times from ctliaii~lli n red light, xitli previous refluxing in t h e dark. cis-Azobenzene. --The rnetliiid c a f prcparatinii of Hart1 I V X ~used Jvithout change; the !inal product was estiinatec contain less tliaii 0.02':,. o f trczns-isoirier (vide ;n,fia\. Isooctane and Benzene. --Eastman spcctro grade was retiistilled iii a 20 t.p. packed colunm. Analysis. -The isomeric ctimpositioii was tleteriiiiiied by careful detcrmiiiatioii o f cqitical densities with a Beckinail I ) U spcctrophiitonieter. Solutions were therniostated a t 25.0 =k 0.1' for all \\-ark. The alisnrption spectra of both i,onierh i n isoBctaiie (see Fig. 1) were determined carefully using :L iiuiiiticr of coiiceiitrations. For the cis isomer, be~
.
~~
(I)
.-
'l'hii
Xbcirk w a s s u p p u r t e d ti? a Frederick Gardner Cottrell
Research Corporation, which we gratefully a c k n o w l e d g e . I.I.T., Cambridge, Mass. piisch(i!ie;z, 36, 3.7 (1040), C. S a 2 i i i ; i w s r h . (-i) 1' 1'. Birnbaurn a n d 1). \V. ( > . Style, l ' r a ; ~ . ~Fd?-nday . S U C . 5, 0 , 1192 (lCltj-L) i c r , 11. Frankel a n d 15 \Volox-\ky, J . C ' h e i u I ' h y s , 2 3 ,
artley, .I. C i i e i ~S o c . , 633 (1'338). artley arid R , J . \V I,eFevre, i b i d , 531 (1939).
cause of the thermal cis-trcins reaction, it was found necessary t o measure the times between dissolving t h e pure crystalline cis isomer arid making a reading on the spectrophotometer. Knowing the first-nrder rate constant (vide infru) for the thermal reaction, t h e optical density a t zero time could be calculated. A11 points on t h e cis absorption curve were obtained b y this surt of extrapolation. To check the purity of the crystalline cis isomer, this procedure was repeated for six successive recrystallizations a t t h e most sensitive wave length (340 1n.u). T h e residual variations in t h e extrapdated density indicated the purity mentioned above. The r a t e constant, k = 1.37 i 0.01 X min.-' for the thermal reaction x i s obtained from a least squares slope of t h e straight line obtained by plotting In (D, - D ) vs. 1. ( D = optical density, t = time.) Photochemistry.-Solutions of total azobetizeiie concentration, cg, from t o 1 0 - 5 wz were irradiated in a fused cyliiidrical quartz cell, tiieriiiostated at 25.0 i 0.1'. The solutions had a volume of 6.204 cc. and were stirred with a smill magnetic stirrpr lying just below the almost parallel beam of light. The btani filled about 807" of the cell wiiidows aiitl was prI863 for 254 inp. T h e purity of t h e r:idiatioii was tested iJi each case photographically with a sinall quartz Hilger spectrograph, and the relative intensities of the several coniporients of t h e compound lines were estimated tnicrophotonictrically. These relative intensities were needed t o calculate the proper average absorption for L: coml)ound line. The cell holder i r a s arranged so t h a t it could be alternately irradiated for a measured time interval and examined in the Beckinan spectrophotometer t o determine t h e isomer conipositioii. T h e Beckman wave length setting was fixed f i r a given total concentration, CO, and was chosen t o give the aiaxiniuiii precision for the analysis; e.g., 410 nip for cg = m , 315 rnp for G O = 2.5 X ? ? I . The intensity of the light iiicideiit upon the sclution, 10,was determined I]>- urztipl oxalate actiiicirnctry for all except the green and J.cilvn lines. Relative intensity measurements with ail Epplcy thermopile, useii in coujunc!ion with a Listoil-tmplifier ant1 an EsterliiicXngus recording ere u s d to check the actinometry arid to ol)tain values for the green and yellow lines. During a phiitochemical run the relative light intensity was monitored liy reflecting a small pcrtion rif the light on t o a n'eston PhotrCJiiic cell which, in turii, W:LS checked periodically against a n iiicniidescetit lauip iiiaintained a t a constaut voltage.
Results The experimental results are best expressed as fraction of cis isomer, >#, 21s a' function of time for each 10,ca and wave length, as shown in Figs. 2 and 3 . For most wave lengths the stationary state was approached from both sides~-for example, in Fig. 3 a solution, initially pure trans, was irradiated with wave length 313 1 n ~ and , after reaching the si-eady composition, y.. = 0 3 0 , was then irradiated with wave length t546 (or 336) giving the descending c i i r w and reaching a new stationary state. The %I
(;
%iniruerin.in, J . C h e m i ' i i y s . , 23, 82.7 (1955).
PHOTOCHEMICAL ISOMERIZATION OF AZOBENZENE
July 20, 1958
stationary state compositions which are given in Table I (y,) depended only on wave length and not on Ioor co for the ranges studied. I t was found that the infrared radiation present in the light sources and the presence of dissolved 0 2 (from about to 2 X lo-’ m) in the solutions had no observable effect on the photoisomerization. In all cases the isomerization was completely reversible, in agreement with previous work4 where fairly drastic tests failed to show the presence of any side reactions. In no case (including gaseous trans isomer a t low pressures) was luminescence of any sort observed. Analysis of Data.-The differential equation for the rate processes occurring assuming monochromatic light, complete stirring, Beer’s law and that the quantum yields in both directions are independent of Io and concentrations, can be written
3529
-1 1
i
w
I:/
0
I
08i
0 4 02 01
I
= path length through s o h . (cm.) dx = quantum yield for trans-cis; subscripts x and c refer,
respectively, t o trans and cis e ’ = molar absorption coeff, defined by the relation D ’ = optical density lne(Io/I) = D’ = ~ o l [ e ’ ~ ( c ’ ~ - ~ ’ ~ ) y ] V = volume in soln. in liters.
1
F =-
+
D‘
1 - e-D’ 1 in m i n . ; Ioin Einsteins/min.
I
“1L A ! _ L .
where
~
‘20
~2%
250
300
365
1313
350
Y
400
8
450
500
A(..)
Fig. 1.-Absorption spectra of a s - and trans-azobenzene, at 25”, dissolved in isooctane (in this fig only, E is defined using base 10 logs instead of natural logs as in the text) oc-
-
_ I _
- _- _-- .-
-1
9c-
The first two terms on the right are the photochemical rates in the two directions and the third term is the thermal cis-trans rate. Setting dyldt = 0 for t = , solving for (bC, and substituting into (1) gives dy
dt
=
A (ym
- Y) - k y ( F - / F
-
1)
(2)
where A = 10/+~e’~/L‘y,. For all experimental conditions used, a careful numerical comparison shows that the second term in ( 2 ) can be neglected compared with dyldt, giving the equation dz/dt =
- Az F
(3)
where z = (y, - y). If we let I(z) = J F dz,/a then we can write I ( z ) = -At Constant, and a plot of I ( z ) against t gives the constant, A , and hence l o $ x . In order to find I(z), the function F can be represented as the power seriesg F(D’) = 1 D’/2 + D’=/12 - Dt4/72O ...., .. (D’< 5 ) For D’ > 5 it is a good enough approximation to write F(D’) = D’. For various ranges of D’values one obtains the following valid approximations for
+
+
+
m
Range of D’ D’ F,
p log
(9) B. 0. Peirce, “ A Short Table of Integrals,” 3rd Ed., Ginn and Co.. New York. N. Y., 1929, No. 764, p . 90.
Fig. 2.-Change of isomeric composition with time for solutions irradiated with wave length 313 mp: A, B, C and D are for the same light intensity, IO,and for co = 10“. 2 . 5 X loF6, and respectively; D’ is the same a s D except using the lower time scale. E and F are for co = and IO/^ and I0/10, respectively. 2.5 X
plots of I(z) against t using the appropriate approximation. I n all cases good straight lines were obtained, omitting the last 2 or 3 points close to z m where the uncertainty becomes very large. Table I gives the values of obtained in this way. The values of +c were calculated. from the steady state equation; here the term containing the thermal reaction constant k cannot be neglected. Alternatively and more generally i t was found possible to use the differential equation directly by fitting an empirical function to the experimental data and then differentiating. This procedure was much more lengthy and gave the same results
GEORGE ZIMMER~IXN, LUE-YUXG CHOWAND UX-JIN P'IIK
3530
Vol. YU
TABLE I QUAXTUM YIELDSFOR ISOMERIZATION OF AZOBENZENE IN ISOOCTANE AT 25' IO
A
k
i /
-6
rnY1.s
L ._ . >V.' .
ya
0.075 U.796 ,794
,805
YX
+ 9c
0.57 0.52 .54 .51 .51 .54 .50 ,52 .GO .60
,803 .i93 .40 ,810 . 40 ,812 I48 ,388 .48 ,390 .51 ,133 .i2 2.i4 .Z .55 ,125 .i8 .27 .51 2.81 ,147 .82 3.73 .2Y .65 ,143 .83 10 - 3 ~ 5 0 .2@ .4e5 . 17db .i2 j4r.j 17.9 .%I .40 .192 ,64 2 . 5 x 10-6 1 8 . 1 . "-4_ .40 . 188 ,G4 578 2 . 5 X 10-5 2 3 . 1 .23 .44 ,045 .67 D.c. mercury arc used; in all other cases, 60 cycle a.c. Eenzene as solvent; taken from M. A . thesis of L. Y . Chow, Bryn Mawr College.
tained these given values, ours from Table I being listed in parentheses for comparison: ox a t 436 b" mp, 0.36 (0.27); q5c a t 436 m p , 0.45 (0.55); +x a t 365 mp, 0.20 (0.12). The two sets of values for 313 m p are not comparable because of the polychromatic character of the light used by Birnbaum and Style. These differences may be real and due I to the different solvents used; in fact, the above t t I change of & a t 436 mp is the same as the change in Fig. 4.-Piots of 1 ( z ) 1's. time: the iiitercepts for zero time Table I between isooctane and benzene as solvents. have been chosen arbitrarily for convenience. Curves A4n We feel, however, that this change is just barely B and C correspond to curves B, E and F in Fig 2. Curves significant if a t all and that the above differences of D and F each correspond to t h e two curves for 365 and 436 our results from those of Eirnbaum and Style are not serious or possibly not even real. mp respectively in Fig 3. Curve E is for 578 mp and cg = 4
? .
-I-
-
+,
m : e, 9 represent y increasing with time; 2.3 X 0, 0 represent y decreasing with time.
as integration and hence was used only as a check on a few runs. I t is interesting to note that, below a certain value of CO, y as a function of time becomes independent of CO, e.g., for wave lengths 546 and 578 mp for both co = and the photochemical conversion takes place a t the same rate. Azobenzene solutions (or perhaps azonaphthalene solutions,i which absorb much more strongly in yellow and red light) could be used for actinometry extending almost to red light; the solutions are chemically stable (to OZ), have slowly varying and fairly large quantum yields and seem to be relatively insensitive to temperature variations. The results of interest are summarized in Table I Each entry and 2.5 x 10-6 is an average of for co = several individual runs. Figure 5 shows the quantum yields as a function of wave length, the other variables investigated being without substantial effect. Birnbaum and Style4 using ethanol as the solvent and somewhat greater light intensities, ob-
Discussion I n the absence of appreciable fluorescence it must be assumed that the excited molecules eventually return to the ground state by internal conversion followed by loss of the excess vibrational energy through collisions with the solvent molecules. It seems unlikely that in a condensed phase the vibrationally excited molecules in the process of "cooling off" isomerize appreciably compared with the observed quantum yields. We can apply the same reasoning here as in the case of hInO4- decomposition previously discussed.8 If E represents the excess vibrational energy immediately after internal conversion and e the height of the potential energy barrier separating the two isomers in the ground state. then the probability (hence the quantum yield) of isomerization (by this sort of mechanism) is given approximately by the expression (1 - e/E)I2 where n is the number of "classically excited" modes of vibration. This model predicted a strong monotonic decreasing quantum yield with increasing wave length of excitation, which is certainly not the present case. Further, taking e = 23 kcal., the average activation energy
PHOTOCHEMICAL ISOMERIZATION OF AZOBENZENE
July 20, 1958
found for the thermal reaction,’JO and n > 15,11 the quantum yields for all wave lengths considered are less than 0.01. This point of view is not consistent with the model of Lewis, et UZ.,’~ for the stilbenes, nor with Olson’s model of cis-trans isomerization.la Prom the data can be seen a t once that there cannot be a single excited state, indistinguishable regardless of which isomer was excited by the radiation, since such a common state would imply that cpx dC = 1, which is not the case (Table I, last column) .14 These considerations suggest that isomerization takes place by a normal thermal rate process in an electronic excited state in the potential energy surface of which there is a barrier (probably small-say about 5 kcal.) separating two configurations corresponding approximately to the cis and trans configurations of the ground state. The simplest kinetic mechanism consistent with this model can be represented by the scheme I, ka kr
3531
+
cI-c*-’x*--x ki
kr
Is
where the asterisk denotes the excited state, the k’s first-order rate constants, and the I’s the rate of light absorption by each isomer. One would expect the configuration after absorption of a photon to correspond to that of the original absorbing molecule, because of the Franck-Condon principle. The constants, ka and k4, represent thermal isomerization and k1 and kz represent quenching to the ground state with no appreciable loss of configuration. For this case the quantum yields are kl/kJ and a symgiven by l/& = 1 kl/k3 (1 metrical expression for &. Note that if ka
