2486
D. SCHULTE-FROHLINDE, H. BLUME, AND H. GUSTEN Together with K =
(4)
K then may be calculated from the slope of the (linear) plot of In (Xl - XI") against f, thus furnishing one equation for the two unknowns q51 and 42. A second equation is furnished by the requirement that a t photoequilibrium equation 1 equals zero and therefore
&f1
Yol. 60
+
q52e2
we finally arrive a t
In using eq. 4 f is obtained by graphical integration, feeding the integrand with the experimental D's (as a function of time). X1 and XZare determined spectrophotometrically, from the absorption spectra of the pure isomers A and B and that of their mixtures. In rigid media no mixing by diffusion or convection occurs. Equation 1 has to be modified, resulting in a rather complicated equation which can be solved only approximately. The first approximation in fact leads to eq. 4, V denoting the volume passed by the photoactive light.
PHOTOCHEMICAL c~s-~~u~s-ISOMERIZATION OF SUBSTITUTED STILBENES BY DIETRICH SCHULTE-FROHLINDE, HARTWIG BLUME, AND HANSGUSTEN Kemjorschungszentrum Karlsruhe, Strahlenchemisches Laboratorium, Karlsruhe, Germany Received May 10, 1966
The quantum yields of cis-trans-isomerization of stilbenes have been measured a4 functions of substitution, solvent, temperature, concentration, and wave length of the exciting light. Six independent criteria indicate a markedly reduced lifetime of the excited cis-singlebstate (Table V). This short lifetime is responsible for the differences in photochemical behavior of stereoisomeric stilbenes. The results are discussed in the context of the triplet-theory in photochemical cistrans-isomerization.
Introduction According to Lewis and co-workers' the c i s trans-isomerization of stilbenes does not occur in the excited singlet state. Otherwise cis-stilbene also should give measurable fluorescence. Thus, two possibilities for the mechanism of isomerization remain. Either the rearrangement occurs in the triplet state or the electronic excitation energy after internal conversion, and present as point heat in the molecule, results in thermal rearrangement in the So-ground state. Recent authors presume a transition in electronic excited Forsterb postulates a cis-trans-isomerization of the C=C double bond via the triplet state, both parts of the molecule being twisted by 90 degrees a t the double bond. Earlier Huckela and Mulliken' considered twisted states in cis-trans-isomerization. The results of this work are discussed in the context of the triplet theory. Experimental Materials.4-Nitro-4'-methoxytilbene, cj.' 4,4'-Dinitrostilbene: preparation according to Walden and Kernbauma; m.p. 298", Xmal 308 mp, log e 4.577 (dimethylformamide). cis-Isomer: lra~-4,4'-dinitrostilbene wa8 (1) G. N. Lewis, T. T. Magel, and D. Lipkin, J . A m . Chem. Soc., 62, 2973 (1940).
(2) P. P. Birnbaum and D. W. G. Style, J . Chum.Soc., 1192 (1955). (3) G. Zimmermann, L. Chow, and U. Paik, J . A m . C h m . Soc., 80, 3528 (1958). (4) D. Schulte-Frohlinde, Liebigs Ann. Chum., 612, 138 (1958). (6) Th. Ftlrster, Z . EZskfroohem., 66, 716 (1952). (6) E. Hackel, 2. Phyuile, 60, 423 (1930). (7) R. 8. Mulliken, Phys. Rm., 41, 751 (1932). (8) P. Walden and K. Kernbaum, Em., 28, 1968 (1890).
dissolved in benzene and irradiated using a high pressure quartz mercury arc lamp (Osram S 81). The irradiated solution was chromatographed on activated alumina (neutral) and the resulting cis-isomer ww crystallized from benzene; m.p. 187O, XmX 320 mp, log e 4.193 (dimethylformamide). 4-Nitro-3'-methoxystilbene: 3.4 g. of m-methoxybenzaldehyde, 4.5 g. of p-nitrophenylacetic acid, and 3 ml. of piperidine were heated for three hours to 180-190"; yellow crystals were recrystallized from ethanol; m.p. 87-88', yield, 5.0 g. cis-Isomer: 1.8 g. of trans-4-nitro-3'-methoxystilbene was dissolved in 60 ml. of benzene and irradiated using a high pressure quartz mercury arc lamp (Osram S 81) for thirty hours. When placed on an activated alumina (neutral) chromatographic column the cis-isomer moved more rapidly aa a zone of light yellow color. Repeatedly chromatographed, the cis-isomer was obtained as a pale yellow colored oil. After three days, the oil crystallized; m.p. 3334", XmX 320 mp, log e 3.984 (benzene). Anal. Calcd. for ClsH1803N(255.3): C, 70.58; H, 5.13; N, 5.49. Found: C, 71.38; H, 5.23; N, 5.39. trans-Isomer: elution from the column by means of acetone and recrystallization from acetone-alcohol; m.p. 878 8 O , Xmax 357 mp, lo c 4.403 (benzene). Anal. Calcd. for ClsHlrOIN (255.3): 70.58; H, 5.13; N , 5.49. Found: C,70.94; H, 5.22; N, 5.22. 4-Nitro-4'-aminostilbene: the method of preparation of Calvin and Bucklesg (trans-isomer), and the &transseparation according to Calvin and Alter,'o were used without alteration. PNitro-4'-dimethylaminostilbene-truns-isomer: preparation according to Pfeifferll; m.p. 250-251°, Xmax 432 mp, log e 4.476 (benzene). cis-Isomer: 1.5 g. of trans-4nitro-4'-dimethylaminostilbene in 750 ml. of benzene waa irradiated for five hours in a high pressure mercury immersion arc lamp (Heraeus Q 81) a t 40". A sodium nitrite solution filter absorbed all light of shorter wave length than 366 mp.
6,
(9) M. Calvin and R. E. Buckles, J . Am. Chem. Soc., 62, 3324 (1940). (IO) M. Calvin and H. W. Alter, J. Chum. Phys., 19, 765 (1951). (11) P. Pfeiffer. Be?., 48, 1796 (1915).
PHOTOCHEMICAL ISOMERIZATION OF SUBSTITUTED STILBENES
Dec., 1962
The solution was placed on an inactivated basic alumina chromatographic rolumn (2.5 X 100 em.) and the pale pink colored cis-isomer zone eluted with ether. After evaporation, the residue waa taken up in a few drops of benzene and c r y s t d i e d in the cold to give red scaly crystals; m.p. 104105’, yield 18 mg.; Xm., 418 mp, log c 3.944 (benzene). Anal. Calcd. for ClsHlsNnO~ (268.3): N, 10.44. Found: N, 10.98. trans-Stilbene-4-diazonium Fluoroborate: A suspension of 400 mg. of pure trans-4-amino-stilbene in 50 ml. of 50% HC1 waa diazotized under stirring with a solution of 190 mg. of NaN03 in 1 ml. of H20 a t -5”. After one hour of stirring the precipitate was filtered off. The diazonium fluoroborate waa precipitated from the filtrate with a few milliliters of HBFa. The orange precipitate waa drawn off, washed with a few milliliters of cold HBF, and an excess of ether, and crystallized aa orange crystals from acetic acid: XmX 396 mp, log e 4.468 (50y0 methanol-water). Anal. Calcd. for CIaHI1N2BFr (294.1): C, 57.17; H, 3.77; N, 9.53; B, 3.68. Found: C, 56.18; H, 4.14; N, 9.65; B, 3.80. cis-Stilbene-4-diazonium Fluoroborate: cis-4-aminostilbene hydrochloride waa prepared according to Schmidt1* by chromatogra ihic cis-trans-separation of irradiated 4nitrostilbene andsubsequent reduction of the isolated cis-4nitrostilbene. A suspension of 400 mg. of cis-4-aminostilbene hydrochloride waa diazotized under stirring with 0.75 ml. of a 1 M solution of NaNO2 in water a t 20”. The oil and orange colored precipitate formed were filtered off. The diazonium fluoroborate waa Erecipitated from the filtrate with 0.5 ml. of 40% HBFn. T e precipitate thus formed was trans-stilbene-4-diazonium salt. At f20” cis-stilbene-4diazonium salt remained dissolved in the atrate. At -20” the &-diazonium salt crystallized. After drawing off, the salt waa washed with ether. The bright yellow colored n’s-atilbene-4-diazonium fluoroborate dissolved easilv, thus making recrystallization or reprecipitation difficuli;. XmX 372 mp, log e 4.008 (50% methanol-water). Anal. Calcd. for C1dH11NzBF4(294.1): C, 57.17; H , 3.77; N, 9.53. Found: C, 57.14,: H, 3.90; N,9.88.
Photochemisty amd Spectrophotometry The samples in 2 X 1 cm. quartz cells were irradiated with
a stabilized hi h pressure mercury arc lamp (Osram HBO
200). Heat raiiation waa absorbed by a thermostated water filter (quartz windows, 7 cm. path length). The different mercury-lines were isolated by means of interference filters (Schott u. Gen., type IL). The purity of the radiation waa examined by measuring the transmission spectra of the filters. Every filter has a single band of transmission greater than one per cent. Thus the percentage of light of the neighboring Hg emisirion line, mr
313 365 405 579
Tranemismon of the filter > 1 %, mr
303-329 350-395 373429 575-608
mercury emission lines is less than 0.1%. The change of isomeric composition waa measured with a Zeiss-apectrophotometer PMQ I1 through the 2-cm. cell path length, and at 90 degrees to the irradiation over the 1-cm. path length. Thus, a decrease of the error introduced by the calculation formula,‘ especidly at higher optical densities, waa effected. The optical density normall did not exceed 0.2. The systematic error usually rangei from 5 to 10%. An error calibration curve waa used to eliminate this remaining error. The calculation of quantum yields from experimental data was carried out m published earlier.‘ The intensity of the light incident upon the solution waa measured with a calibrated thermopile (Pyro-Werk, Hannover). The thernio-current was measured in a compensating circuit. The absolute error in the determination of quantum ’ is yields is f10 %; a relative error of not more than f 5 % calculated from the reproducibilit of the measurements. All spectra were taken with a 8ary 14 recording spectrophotometer at room temperature. Spectra and quantum yields at high concentrations were measured in special cella. (12) P. Schmidt, U. 9. At. Energy Comm., UCRL-8883 (1959).
2487
Quartz plates of 5 mm. thicknees and, different metal folios were pressed together forming cella with 100 to 0.5 fi path length. The path length waa determined by means of infrared interference.
Results 1. Effect of Solvent and Substituent. (a) Quantum Yields of trans + cis-Isomerization.Different solvents do not affect the quantum yields of trans -t cis-isomerization of unsubstituted stilbene4 and 4,4’-dinitrostilbene (Table I). HowTABLE I QUANTUM YIELDSFOR PHOTOCHEMICAL cis e trans-IsoMEBIZATION OF SUBST~TUTED STILBENES IN DIFFERENT SOLVENTS (EXCITING WAVE LENGTH 366 mp) Equilibrium
% Ci8
Solvent
Qt-te
Qet
Q.
(a) 4,4’-Dinitrostilbene 0.34 0.34 0.34 0.33
0.61 0.63 0.63 0.64
(b) 4-Nitro-3’-methoxystilbene 0.61 0.25 89.0 Cyclohexane 0.38 0.40 85.5 Benzene 0.39 0.41 83.5 p-Xylene 0.28 0.40 Methanol 82.5 0.20 0.38 69.0 Dimethylformamide
0.86 0.78 0.80 0.68 0.58
Benzene Chloroform Methanol Dimethylformamide
(c)
Benzine Benzene Paraffin oil Chloroform Ethanol Methanol Dimethylforniamide
80 83 80.5 82
0.27 0.29 0.29 0.31
4-Nitro-4’-methoxystilbene4 91 73 74 56 60 29 17
0.67 0.40 0.34 0.17 0.13 0.07 0.035
0.24 0.43 0.26 0.37 0.40 0.48 0.42
0.91 0.83 0.60 0.54 0.53 0.55 0.45
(d) 4-Nitro-4’-aminostilbene 75 34 0
0.36 0.44 0.30 0.20
0.80 0.54 0.30 0.20
(e) 4-Ni tro4’dimethylaminostilbene Cyclohexane 52.5 0.20 0.40 0.016 0.40 Benzene 6.5 0 0 0.31 Ethanol Dimethylform0 0 0.15 amide
0.60 0.42 0.31 0.15
Cyclohexane Benzene Ethan01 Dimethylformamide
0
0.44 0.10 0 0
ever substitution of one of the nitro groups by an electron donating group results in the dependence of the trans -+ cis-quantum yield upon the polarity of the solvent. This effect grows as the electron donating properties of the second substituent increase (Table I). I n the 4,4’-position, the influence of the substituent on quantum yields is especially strong, while in the 4,3‘-position a marked decrease in influence occurs. Consequently we have to consider the non-specific chemical effect of the substituents, as well as of the ~ o l v e n t . ~
+
40
35
Y,
30
crn.-l. 26
20
2. Effect of Irradiating Wave Length on Quantum Yields.-The quantum yields of the cistrunsisomerization of 4-nitro-4'-dimethylaminostilbene are independent of irradiating wave length, even if irradiated in different absorption bands (Fig. 1, Table 11). This is another argument against cis @ trans-isomerization in the excited singlet state. Normally S T-transitions occur from the &-state only. If this is valid for stilbenes and if the isomerization occurs via the triplet state, there should be no dependence of quantum yield upon the wave length of the exciting light. On the other hand, diazonium salts decompose in the excited singlet state. Quantum yields of this decomposition depend upon the energy of exciting light (Table 111)
i -
-
TABLE I1 O F CZS-trUnS-ISOMERIZATION O F &NITRO4'-DIMETHYLAMINOSTlLBENE AS A FUNCTION OF EXCITING
QUANTUM YIELDS
WAVELENGTH Equilibrium
300
400 500 (mfi). Fig. 1.-Ultraviolet absorption spectra of tran,s- and cis4-nitro-4'-dimethylaminostilbene in cyclohexane: the monochromatic wave lengths used in irradiation are indicated by arrows. The spectrum was taken with a Cary 14 recording spectrophotometer a t room temperature; c = 7.22 X mole/l. (- * - photochemical equilibrium for exciting light of 405 mF).
The influence of substitution and solvents on the quantum yields of trans +- cis-isomerization might be explained by solvation in the case of the excited singlet state. Interpreting the shift of fluorescence wave length caused by different solvents, Lippert13.14 has already postulated solvation of excited states by orientation polarization for the same molecules. The trans + cis-isomerization probably is hindered by solvation. (b) Quantum Yields of cis -+ trans-Isomerization.-The quantum yields of cis -, trans-isomerization are in a first approximation independent of substitut,ion and of solvents. Lewis, Magel, and Lipkiril concluded that the excited cis-S,-state was extremely short lived for t'wo reasons: cisst,ilbenes do not give measurable fluorescence and absorption spectra have no fine structure. If we assume that the S1 + T-transition in cis-isomers occurs faster than the arrangement of orientation polarizat'ion of t'he solvent, no more influence,upon the cis trans-quantum yield should be expected. Relaxation time in the solvents used is about s e ~ o n d . ' ~Therefore the excitpd cis-S1-state has a lifetime of less than lo-" second. I n those cases in which the quantum yield of the c i s -+ transisomerization also is affect,ed by t,he solvent, it may be t,hat. the lifetime of the excited cis-S1state is of the same order as the relaxation time of the solvent,s. --t
(13) E. Lippert, %. C i r k l r o r h c m . , 61, 083 (1957); Z. Safurforsch., loa, 541 (1955). (14) E. Lippert w i t h W. Ljidpr. F. Moll, W. Nhgele, H. Boos, H. I ' r i g ~ niiil ~ , J. 9 ~ i l ~ o l r l - ~ ~ l ; i n k ~ nAngrrr.. a t ~ i n , Chrm., 73, 695 (1901).
w
% cis
313 366 405 436
(a) Solvent: Cyclohexane 19.0 0.14 0.45 52.5 0.20 0.40 60.5 0.16 0.42 64.0 0.16 0.37
313 366 405 43G
(b) Solvent: Benzene 3.0 0.015 0.42 6.5 0.016 0.39 11.0 0.015 0.40 13.5 0.013 0.40
Qt-c
Qs
Qc-t
0.59 0.60 0.58 0.53 0.44 0.41 0.42
0.41
TABLE 111 QUANTUM YIELDS OF THE PHOTODECOMPOSITION OF 2-METHYL-6- [N,N-DIMETHYLAMINO] -PHENAZINEDIAZONIUM FLUOROBORATE-(3) AS FUNCTION O F EXCITIKG WAVE LENGTH,SOLVENT 0.1 N HCl15 Wave length of 313 366 405 579 exciting light, mfi
Bands Quantum yield
I
I1
I1
I11
0.25
0.17
0.16
