J. Phys. Chem. 1990, 94, 6731-6734
6731
Triplet State Z / E Isomerization of a p-Styrylstilbene, a Partly Adiabatic Process Mikael Sundahl,*>+Olof Wennerstrom,+ Kjell Sandros,* Departments of Organic and Physical Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden
Tatsuo Arai, and Katsumi Tokumaru Department of Chemistry, University of Tsukuba, Tsukuba. Ibaraki 305, Japan (Received: January 18, 1990: In Final Form: March 27, 1990)
The quantum yields for biacetyl-sensitizedZ / E photoisomerizationof the isomers of 3,3”,5,5”-tetra-terr-butyl-4’-styrylstilbene, Z Z , ZE, and EE, have been determined. Z Z and Z E showed increased isomerization quantum yields with increased concentration due to a quantum chain process with ’EE* as the chain-carrying species. The photostationary state consists of a mixture of Z E and EE with more of the latter at higher concentration. Addition of azulene to the photoisomerization reaction drastically increases the proportion of EE at the photostationary state from ca. 50% to 99%. The TI-T, absorption spectra for all three isomers are identical as is the triplet lifetime ( T = 3.5 ps at 298 K). From a temperature study of the triplet lifetime ( T = 120 ps at 193 K) we conclude that there are two important triplet-state conformations: one with a planar E,E structure and the other where one double bond is twisted close to 90’. The energy difference between the two conformations was determined to be 4.4 f 0.5 kcal mol-!, with the planar structure being the more stable. From quenching experiments the triplet energy of EE was determined to be 41 kcal mo1-I.
knowledge, the first example of an adiabatic singlet-state 2-fold isomerization. In this paper we return to this styrylstilbene, 22, Z E , and E E , and report on a mechanistic investigation of the triplet-state Z / E photoisomerization.
Introduction Delocalization of *-electrons over the entire r-system in oligo@-phenylenevinylenes) is not very pronounced in the ground state. However, on doping (oxidation or reduction), delocalizing of 7-electrons results in interesting new properties such as electrical conductivity.’ This is also evident for cyclic oligomers of pphenylenevinylene compounds for which large ring current effects have been observed only for anionic derivatives and not for the neutral (undoped) compounds.2 Apparently, conjugation or delocalization of *-electrons is more pronounced in ionic derivatives than in the neutral compounds. Delocalization of r-electrons might also be more important in electronically excited states. We have recently shown that in some macrocycles with r-perimeters built from phenylenevinylene units, multiple Z / E photoisomerizations can occur by an adiabatic mechanism, Le., the rearrangement proceeds concerted on the triplet excited energy surface, and the product is formed in its excited state.3 The mechanism is completely different from that of the photoisomerization of stilbene under the same conditions, which shows that the presence of a cyclic *-system might indeed cause the photoreaction to proceed by a route different from the normal one. Symmetry considerations have revealed the close analogy between cyclic and large linear *-systems built from the same repeating unit.4 The first example of adiabatic Z / E photoisomerizations in olefins with aromatic substituents became known through the work of Tokumaru and co-workers when they reported on ”one-way photoisomerization” in 2-( 3,3-dimethylb~tenyl)anthracene.~ The photoisomerization of this molecule proceeds from the 2 to the E isomer entirely on the triplet surface with no deactivation at the conformation with perpendicular r-systems (90° twist at the double bond), resulting in a photostationary state consisting of the E isomer only. Later Sandros and Becker showed that three different 9-styrylanthracenes isomerize adiabatically on both the excited singlet-state and triplet-state surfaces.6 It is of interst to test whether or not linear oligomers with p-phenylenevinylene units behave like their cyclic counterparts on irradiation. Somewhat surprisingly even a styrylstilbene shows interesting new Z / E photoisomerization behavior when compared to stilbene. In a recent paper we have shown that the Z / E photoisomerization on direct excitation of 3,3”,5,5”-tetra-tertbutyl-4’-styrylstilbene proceeds by a I - and 2-fold adiabatic mechanism on the singlet excited-state surface.’ This is, to our
Y
/
ZZ
EE
Experimental Section Materials. For the preparation and the purification of the styrylstilbenes, ZZ, Z E , and E E , see ref 7. Biacetyl from Aldrich (>99%, GC) was used without further purification. Azulene was purified by sublimation and anthracene by zone melting. All measurements were performed with spectroscopic grade methylcyclohexane as solvent. The solutions were normally freed from oxygen by argon flushing. In the low-temperature experiments the solutions were degassed by five freeze-pumpthaw cycles. The lifetime of the studied triplet at low temperature is >IO0 ~ sand , thus oxygen is not sufficiently removed by argon flushing. ( I ) See, e.g.: Gagnon, D. R.; Capsitran, J. D.;Karasz, F. E.; Lenz, R. W.; Antoun, S. Polymer 1987, 28, 567, and references therein. (2) Miillen, K.; Unterberg, H.; Huber, W.; Wennerstrom, 0.;Norinder, U.;Tanner, D.;Thulin, B. J . Am. Chem. Soc. 1984, 106, 7514. (3) (a) Sundahl, M.; Wennerstrom, 0.;Sandros, K.; Arai, T.; Okamoto, H.; Tokumaru, K. Chem. Phys. Lett. 1990, 168, 395. (b) Sundahl, M.; Wennerstrom, 0.; Sandros, K.; Norinder, U. Tetrahedron Lett. 1986, 27, 1063. (c) Sundahl, M.; Wennerstrom, 0.; Raston, I.; Norinder, U.Acfa Chem. Scand. 1988, 842, 367. (d) Sundahl, M.; Wennerstrom, 0. Ibid. 1988, 842, 127. (4) Norinder, U.;Wennerstrom, 0.; Wennerstrom, H . Tetrahedron 1985, 41, 713. (5) Arai, T.; Karatsu, T.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1983, 24, 2873. (6) Sandros, K.; Becker, H.-D. J . Phofochem. 1987, 39, 301. (7) Sandros, K.; Sundahl, M.; Wennerstrom, 0.;Norinder, U. J . Am. Chem. Soc. 1990, 112, 3082.
‘Department of Organic Chemistry. *Department of Physical Chemistry.
0022-3654/90/2094-673 1 $02.50/0
ZE
g
b 1990 American Chemical Societv
(-
Sundahl et al.
6732 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 0
[EEl/([ZEl+[EEl) t -
09
,-
-
0 6
-
14
.
A
d
0 8 0 7
16
r?'
c
-
r/
#'
05 0 4
-0
7
0 0004
0 0008
0 0012 [ E I M
Figure 1 Concentration dependence of photostationary-state composition, obtained by biacetyl sensitization, fitted to a straight line Intercept 0 47, slope 457 M-'
0
0.02
0.01
0.03
0.04 [ Z E I W
Figure 3. Concentration dependence of isomerization quantum yield of Z E fitted to a straight line. Intercept 0.58; slope 423 M-'. On sensitized excitation of Z E the only new product is EE. A0D 0.5
,
Q 1 9 3
X
1 0.4 a
0.3
I' h
. '
\
0.2 4 09 07 0.5
./
: 1
0
,
,
0.005 0.01 0.015 0.02 0.025 0.03 [ZZl(M)
Figure 2. Concentration dependence of isomerization quantum yield of
Z Z fitted to a straight line. Intercept 0.92; slope 37 M-I. On sensitized excitation of 22 a mixture of Z E and € E is produced. lsomerizarion Measurements. Irradiations were performed in an optical bench arrangement from Applied Photophysics using a 150-W xenon arc lamp and a monochromator (436 f 8 nm). Quantum yields were determined by using ferrioxalate actinometry.8 The isomerizations were followed by analytical HPLC, with a SiCN column and hexane as eluent. Laser Flash Photolyses. Biacetyl was excited by using a dye laser (Lambda Physics fl-3002, stilbene-3) pumped by an excimer laser (Lambda Physics EMG 101, XeCI, IO-ns fwhm). Experimental details concerning laser photolysis are described el~ewhere.~
400
440
480
520
560
6 0 0 run
Figure 4. T,-T, absorption spectra of Z Z , ZE, and EE (0, X, and A, respectively) obtained by biacetyl sensitization. The spectra were run under similar conditions. The AOD values (not normalized) are those recorded =lo0 ns after the laser pulse. 32.7'
;
-
3ZE'
A
_f
I
3"
3&
!I
~~
,t
zz
.?E
EE
Figure 5. Reaction model for the biacetyl sensitized Z / E isomerizations. Dotted arrows indicate energy-transfer processes.
the same T,-T, absorption (A- = 505 nm) with the same lifetime (77. = 3.5 ps at 298 K). The decays are monoexponential, and the spectra are time independent, which means that the Z / E photoisomerization must occur in less than 100 ns (the time resolution in the experiment). The transients are quenched by azulene (ET = 39.8 kcal mol-I, k , = 4.6 X IO9 M-' s-I 1, by anthracene (ET = 42.5 kcal mol-', k , = 3.0 X lo8 M-ls-'), a nd by oxygen ( k , = 5.0 X lo9 M-l s-l).Io Equation 1 gives an estimate of the triplet energy of E E (41 kcal mol-'):
Results and Discussion Isomerizations. The isomerizations were performed by using biacetyl ((3-4) X IO-* M) as a sensitizer ( ET = 55 kcal mol-I). On irradiating any one of the three isomers a photostationary-state mixture, containing Z E and EE only, is approached. The relative proportion of E E increases with the concentration (Figure I ) . Addition of azulene, a triplet quencher, also afffects the composition at the photostationary state. With a total olefin concentration of 0.9 mM and an azulene concentration of 1.8 mM, the resulting mixture after prolonged irradiation contained 98.6% E E and 1.4% Z E . Starting with Z Z under similar experimental conditions, we observed, for conversions L 15%, the same isomer ratio ( E E I Z E ) as at the photostationary state. For small conversions the amount of Z E could not be accurately determined. The isomer ratio of the photostationary state mixture also depends on the temperature. In one experiment with a total olefin concentration of 0.14 m M the composition changed from 50% E E at 298 K to 60% E E at 193 K . The quantum yields for isomerization of both Z Z and Z E increase with the concentration (Figures 2 and 3). In contrast the quantum yield for isomerization of E E is constant (4 = 0.50, at 298 K). The isomerization behavior will be explained in terms of a quantum chain process. Laser Photolyses. The Tl-T, absorption spectra of the three isomers (3 X M) observed on biacetyl sensitization, excitation at 425 nm, are presented in Figure 4. All three isomers show
Reaction Model. Irradiation of any isomer of the title compound in the presence of biacetyl results in a mixture of Z E and E E in proportions that vary with the concentration, temperature, and addition of quencher. All experimental observations are consistent with the following simple reaction model (Figure 5 ) . The 3EE* and 3E,p* are in rapid dynamic equilibrium. The jE,p* state on the triplet surface corresponds to a structure with one planar E double bond and the other twisted close to 90°, Le., the transition-state geometry for thermal isomerization in the ground state. As a result of the small energy difference between the ground and triplet state for this geometry, the unimolecular decay from )E,p* is rapid. As no Z Z is present in the photostationary-state mixtures, unimolecular decay from the corresponding structure with one Z double bond, 3Z,p*, is of little
(8) Hatchard, C. G.;Parker, C. A. Proc. R. Soc. London 1956, ,4235, 518. (9) Arai, T.; Karatsu, T.; Tsuchiya, M.; Sakuragi, H.; Tokumaru, K. Chem. Phyc. L e u . 1988. 149, 161.
(IO) Back energy transfer has been taken into account when calculating the quenching rate constants (cf. ref 1 I ) . ( I I ) Sandros, K . .4cru Chem. Scand. 1964, 18, 2355.
k, = 6
X
109/{1
+ e-AE/(Rn]
(1)
Z/E Isomerization of a p-Styrylstilbene
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6733
importance. Probably the 3Z,p* has considerably higher energy than 3E,p*. The following reactions are of relevance for this system: excitation:
D
-
ID*
I, excitation rate (D = biacetyl)
ID*
4
3D*
4isc
=z
1
energy transfer:
3D*+ ZZ- D + 3ZZ* 3D*+ ZE D + 3ZE* 3D*+ E E D + 3EE* 3EE*
+ ZE
'EE*
+ ZZ
-
-
+ 3ZE* E E + 3ZZ* EE
\\
0.003 0.0035 0.004 0.0045 0.005 0.0055
(W)
Figure 6. Temperature dependence of equilibrium constant, K . Straight line fit of In K versus TI,for three different pairs of values of k,, and kEp: 0,kEE= 5 X IO3 s-l and kEp= 2 X IO6 s-I; 0 , k,, = 3 X IO3 s-' and k,, = 2 X lo7 s-l; 0 ,k, = 0 s-I and kEp= 2 X IO8 s-I.
kelZE kelzz
kcaljmol
isomerizations: 3ZZ*
4
3ZE* -+ 3E,3*
3EE* * )E,p*
4
3EE*
K (equilibrium constant) 50
unimolecular decay: 3E,p* ---* a Z E 3EE*
+ (1 - a)EE
-
EE
30
kEp
10
kEE 10
The olefin concentrations in the isomerization measurements are selected to give complete energy transfer from biacetyl to the olefins. The energy-transfer rate constant from biacetyl is assumed to be equal for all three isomers. Consistent with the isomerization behavior, energy transfer from 3EE* but not from 3ZZ* or 3ZE* is assumed. The unimolecular decay from jEE* is too slow to be of importance at room temperature (kE# >> kEE). In this model the important decay from the triplet surface is from the 3E,p* state at low olefin concentration, whereas on increased concentration or on addition of a substance with low triplet energy a competing bimolecular decay, through quenching of 3EE*, also becomes important. For sensitized photoisomerization from ZE, a steady-state treatment of 3E,p* gives d[jE,p*]/dt = I - k ~ , [ ~ E , p *=] 0
(2)
The rate of isomerization to EE is d[EE]/dt = (1 - a)k,p[3E,p*]
+ kelZE[3EE*][ZE]
(3)
(2) and (3) give 4jso
= 1-a
+ k,tz~[ZE]/(Kk,p)
(4)
where dis0is the initial quantum yield for isomerization. The concentration dependence of the photostationary state composition is [EEl/([ZEl
+ [EEI)
= 1 -a
+ ketZ€[ZEl/(KkEp) (5)
For sensitized photoisomerization from ZZ the equilibrated triplet is reached from another source, but otherwise the reaction scheme is identical. The steady-state treatment of 3E,p* again gives eq 2. The rate of isomerization of ZZ is
+ ketZz[3EE*][ZZ]
(6)
= 1 + ket~~[ZZl/(Kk~p)
(7)
-d[ZZ]/dt = I (2) and (6) give diso
From the obtained concentration dependence of the isomerization quantum yield for ZE it follows (eq 4) that kelZE/(KkEp) = 420 M-I. As kE& = ps-I (which is equal to kobs,at room temperature) we obtain ketZE= 0.12 X IO9 M-' s-l, close to the value obtained from the concentration dependence of the photo-
1
zz ZE EE Figure 7. Energy diagram for ground and excited states of the p styrylstilbene.
stationary-state composition, kelZE= 0.13 X lo9 M-'s-l. From the relationship (eq 1) for energy transfer we calculate a difference between the two triplet energies, ET(ZE) - ET(EE), of 2.3 kcal mol-'. From the concentration dependence of the quantum yield for isomerization of ZZ the rate constant for energy transfer to ZZ, kdz, is estimated at 0.01 1 X lo9 M-' s-I. Thus, the difference between the triplet energies of ZZ and EE is estimated at 3.8 kcal mol-'. Temperature Dependence of the Triplet Lifetime. Further information on the triplet energy surface was obtained from a temperature study of the triplet lifetime, starting from either ZE or EE in a biacetyl-sensitized experiment. To calculate the energy difference between 3EE* and 3E,p*, it is necessary to determine kEpand kEE, which is not possible from the experimental data. However, kEpand kEE can be estimated by comparison with data from similar compounds. As a first approximation kE is given the value for stilbene (2 X lo7 s-I).12 From the smarl change of the photostationary-state composition when the temperature is decreased from room temperature to 193 K, we conclude that the dominant decay from the triplet state, at 193 K, is still via jE,p*. Thus, the value for kEEis selected to be smaller than kob = 8 X lo3 s-I at 193 K. A plot of In K versus T I is shown in Figure 6 with three different pairs of values for kEpand kEE. The equilibrium constant K was determined from eq 8, where kobsis (8) K = (kobs - ~ E E ) / ( ~-EkPo d the observed rate constant for decay from the triplet state. The linear fit is good for all three pairs of values for kEpand kEE. AH for the reaction is determined to be 4.4 f 0.5 kcal mol-', and the at room temperature. assumption k, = 2 X lo7 s-l gives K = For stilbene a similar equilibrium, between 3E* and 3p*, is established, but here the perpendicular conformation is lowest in energy (K = 2.4-10).13 Several other system also show such an equilibrium, e.g., 1-styrylpyrene (K = 10-3)'4 and 2-styryl(12) Saltiel, J.; Rousseau, A. D.; Thomas, B. J . Am. Chem. Soc. 1983, 105, 763 1. (13) Gorner, H.; Schulte-Frohlinde, D. J . Phys. Chem. 1981, 85, 1835. (14) Arai, T.; Okamoto, H.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett. 1989, 1 5 7 , 46.
J . Phys. Chem. 1990, 94, 6734-6737
6734
naphthalene ( K = 0.7-1 .9).15 To ensure that the observed T,-T, absorption spectra have the same origin at the different temperatures, the complete spectrum was recorded at 203 K. The only observed low-temperature change of the spectrum was a slight red shift (8 nm). A summary of the thermodynamics is shown in Figure 7 (for ground-state and singlet-state energies see ref 7). Comparison with Diphenylbutadiene Isomerization.I6 The results prcsented in this paper are generally in accordance with those given by Yee et al. for the sensitized Z / E photoisomerization of I ,4-diphenylbutadiene. However, in their model bimolecular quenching of the excited triplet was suggested to give both E,E and (to a small extent) Z , E isomer, which is not necessary for our system. Although irradiation of Z Z with azulene present gives some Z E together with EE, the only product that is formed in quenched decay is E E . The unimolecular decay from the perpendicular triplet, 3E,p*, is fast enough to compete with bimolecular quenching, and thus some Z E must be formed. The expected ratio between EE and Z E should be given by eq 9. From this equation, with the experimentally obtained values of, k Az, k,,K, and a , the ratio is calculated as 59:l at [Az] = 1.8 m h , which is in fair agreement with that experimentally observed, 68:1. If quenched decay occurred from 3 Z E* , a concentration dependence for the isomerization of EE would be expected. This is not observed. It is remarkable to note that the excited triplet-state energy surfaces for diphenylbutadiene and for the investigated p styrylstilbene are very similar, while the excited singlet-state energy surfaces are different. The singlet-state isomerization mechanism for diphenylbutadiene resembles an ordinary diabatic mechanism, (15) Gorner, H.; Eaker, D.W.; Saltiel, J. J . Am. Chem. SOC.1981, 103, 7 164. (16) Yee. W. A,; Hug.S. J.; Kliger, D.S.J . Am. Chem. SOC.1988, 110, 2164.
while for the p-styrylstilbene the mechanism is adiabatic (compare ref 7). Conclusions and Outlook
We have previously shown that the singlet-state Z / E isomerization mechanism of pstyrylstilbene is different from the accepted diabatic one for stilbene Z / E isomerization. Here we have shown that also the triplet-state mechanism differs. Several possibilities of changing the commonly encountered diabatic mechanism to an adiabatic mechanism exist. As previously shown by Tokumaru and co-workers5 and later by Sandros and Becker? exchanging one of the benzene rings in stilbene for an anthryl group changes the Z / E photoisomerization mechanism from diabatic to adiabatic. The selection between a triplet-state diabatic and adiabatic mechanism for styryl arenes is stated to depend on the triplet energy of the arene.” Therefore, we will investigate the effect of exchanging the central I,.l-phenylene, in the title molecule, for a 1,4-naphthylene, a 9,10-anthrylene, and a 4,4’-biphenylene, to see if this selection rule, between two-way and one-way isomerization, can be extended to bisstyryl aromatics or more precisely to see if any of the Z , Z isomers of these molecules show only a 2-fold but no single isomerization. The effect of extended conjugation on the mechanism, which seems to be important already in styrylstilbenes, will be pursued further simply by extending the conjugated chain from styrylstilbenes to bisstyrylstilbenes and so on. Acknowledgment. Financial support from the Swedish Natural Science Research Council and the Swedish Board of Technical Development is gratefully acknowledged. M.S. thanks his friends at the Laboratory of Prof. Tokumaru for generous help and valuable discussions during his stay at the University of Tsukuba, Department of Chemistry. (17) Arai, T.; Karatsu, T.; Misawa, H.; Kuriyama, Y.; Okamoto, H.; Hiresaki, T.; Furuuchi, H.; Zeng, H.; Sakuragi, H.; Tokumaru, K. Pure Appl. Chem. 1988, 60, 989. See also refs 5 and 6.
Further Evidence for Radlcal-Controlled Oscillations in the Belousov-Zhabotinskii Reaction: Large Effects of Ultraviolet Light and Silver Ions Linda Stuk, Jay Roberts, William D. McCormick,* and Zoltan Noszticziust Center f o r Nonlinear Dynamics and Department of Physics, University of Texas at Austin, Austin, Texas 78712 (Received: March 6 , 1990; I n Final Form: April 25, 1990)
We have performed experiments on the Belousov-Zhabotinskii reaction perturbed by ultraviolet light and by silver ions, and over a range of sulfuric acid concentrations, the reaction shows a large sensitivity to these perturbations. Our results support Forsterling and Noszticzius’ theory of malonyl radicals as a second control intermediate. Results of perturbation by both ultraviolet light and silver ions at the same time, however, are not adequately explained by either malonyl radical control or bromide control (or both) and suggest a third control intermediate such as bromomalonyl radicals.
Introduction
The possibility of non-bromide-controlled oscillations in the Belousov-Zhabotinskii reaction has been controversial for more than 10 years.] The original FKN mechanism2 explained the oscillations in terms of a delayed negative feedback loop which inhibits the autocatalytic production of bromous acid, HBr02. The heart of the negative feedback loop is the reaction Br- + HBrOz H+ 2HOBr (1)
+
-
*To whom correspondence should be addressed. Permanent address: Institute of Physics, Technical University of Budapest, H-I521 Budapest, Hungary. +
0022-3654/90/2094-6734$02.50/0
Since oscillations continue in the presence of silver ions,3 it is necessary to assume either that the precipitation of silver bromide does not occur fast enough to interfere with reaction 1 or that a second negative feedback loop exists. Forsterling and Noszticzius4 recently proposed a second control loop (1) Noyes, R. M.;Field, R. J.; Forsterling, H. D.;KorBs, E.; Ruoff, P. J . Phys. Chem. 1989, 93, 270. (2) Field, R. J.; Koros, E.; Noyes, R. M. J . Am. Chem. SOC.1972, 94, 8649. (3) Noszticzius, Z. J. Am. Chem. SOC.1979, 101, 3660. (4) Forsterling, H . D.; Noszticzius, Z. J . Phys. Chem. 1989, 93, 2740.
0 1990 American Chemical Society
