5453
J. Phys. Chem. 1989, 93, 5453-5451 A rough interpretation of the phase space scaling factors shows that b73 = 0.007b59. This trend could be expected since both masses originate from direct cleavage reactions and the future loss of C H 3 enables rotations, which is not the case for H loss. The fragments with masses 41, 31, and 29 are all expected to be two-step fragmentation products that have the ion with mass 59 as intermediate. Therefore the scalings factors of these ions are comparable and are scaled relative to b31 in Table 111. The ion with mass 41 is formed by loss of HzO, 31 by loss of CzH4, and can be explained since 29 by loss of CHzO. The ratio b29 = loss of CzH4 is a rearrangement reaction while loss of C H 2 0 is interpreted as a direct cleavage. To interpret the ratio b41 = 0.03 b 3 ! ,two facts have to be considered. In the first place H 2 0 has only one and CzH4has two heavy atoms, and thus this ratio will be slightly influenced by the rotation of the fragments. This effect is expected to be smaller than the ratio of bS7and bT1for n-hexane since both reactions need a rearrangement of the atoms. In the second place the observation is made that the rearrangements necessary to lose a water molecule are more complicated because the oxygen atom is originally bound, on both sides, with two carbon atoms, while the loss of ethene only needs a proton transfer. ConcI usion
Temperature-dependent photoionization mass spectra can be reconstructed for several photon energies simultaneously by using
a simplified approximation of the quasi-equilibrium theory. In this model rates of fragmentation can be calculated by using activation energies and space phase scaling factors. The differences between the scaling factors can be understood mainly by the number of fragments, the presence of rearrangement, and the rotation of the fragmentation products. Temperature-variation PIMS measurements can be used as a calibration for the determination of the unknown internal energy of a set of molecules. The results in this paper show that based on such a calibration measurement a quite detailed interpretation of the observed fragmentation reactions is also possible. For a test of the statistical behavior of the molecular ions, one needs the best information available; therefore, simultaneous detection of the energy of the electron is necessary. Acknowledgment. We acknowledge Dr. P. G. Kistemaker for helpful discussions. This work is part of the research program of the stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter, FOM) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver- Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research, ZWO). Registry No. Hexane, 110-54-3; cyclohexane, 110-82-7; diethyl ether, 60-29-7.
Charge Transfer and Cis-Trans Photoisomerization’ T. W. Ebbesen,*,+,$K. Tokumaru,t M. Sumitani,O and K. Yoshiharag Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, Department of Chemistry, University of Tsukuba, Sakura-mura, Ibaraki 305, Japan, and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: May 2, 1988; In Final Form: December 19, 1988)
The photophysical and photochemical properties of cis- and trans-1,2-bis(1-methyl-4-pyridinio)ethylene dication in acetonitrile are examined. These properties are significantly altered by the charge-transfer complexation in the ground state. The free olefins undergo cis-trans photoisomerization with a quantum yield of ca. 0.3. The cis isomer does not fluoresce unlike the trans isomer (aF= 0.01, T F = 5 X lo-” s). The fluorescence of the trans isomer is quenched by complexation with iodide ions in the ground state. In addition, static quenching occurs within a 16-A radius. The constants of the CT complexation with the iodide ion (in the ground state) are 1.6 M-’ for cis and 4.6 M-’ for trans. Upon excitation in the charge-transfer absorption band, the trans iodide complex gives the trans radical with a quantum yield of 0.06. The cis iodide complex leads not only to the formation of the cis radical isomer with a quantum yield of 0.02 but also to the trans isomer dication (quantum yield 0.05). The latter reaction can be interpreted either as a geminate pair catalyzed event or as isomerization in the excited charge-transfer state from the results of the nanosecond and picosecond laser flash photolysis experiments.
Introduction
In order to study the effect of charge transfer in cis-trans isomerization, olefins substituted with quaternarized N-heteroaromatic groups are particularly well suited. Their behavior in the ground and excited states can be (selectively) modified by charge-transfer complexation with electron-donating counteranions. This enables comparison between the photochemistry induced from the charge-transfer state and that from the other excited states. Comparisons can also be made between the cis and trans isomers for various electron donors. Another advantage is that the reduced olefins are stable and have large absorption coefficient so that they can easily be identified by transient spectroscopy. *To whom correspondence should be sent at the following new address: Fundamental Research Laboratory, NEC Corporation, 4-1-1 Miyazaki, Mi amae-ku, Kawasaki 213, Japan. YUniversity of Notre Dame. *University of Tsukuba. 8 Institute for Molecular Science. Document No. NDRL-2943 from the Notre Dame Radiation Laboratory.
0022-365418912093-5453$01SO10
With these considerations in mind, the olefin 1,2-bis( l-methyl4-pyridinio)ethylene (BPE2+) was chosen for the purpose of this study, in line with our previous work on bipyridinium ~ a l t s l -and ~ other 01efins.~-~ Although the photochemistry of N-heteroaromatic substituted olefins has been studied by various g r o ~ p s , ~ that - I ~ of olefins (1) Ebbesen, T. W.; Levey, G.; Patterson, L. K. Nafure (London) 1982, 298, 545. ( 2 ) Ebbesen, T. W.; Ferraudi, G. J . Phys. Chem. 1983, 87, 3717. (3) Ebbesen, T. W.; Manring, L. E.; Peters, K. S . J. Am. Chem. Soc. 1984, 106. 7400.
(4) Arai, T.; Karatsu, 1983, 24, 2873. ( 5 ) Karatsu, T.; Arai,
T.; Sakuragi, H.; Tokumaru, K. TefrahedronL e f f . T.; Sakuragi, H.; Tokumaru, K. Chem. Phys. L e f f .
1985. - - ,115. 9. ~
(6) Hamaguchi, H.; Tasumi, M.; Karatsu, T.; Arai, T.; Tokumaru, K. J . A m . Chem. SOC.1986, 108, 1698. (7) Bong, P.-H.;Shim, S . C.; Chae, K. H.; Nakashima, N.; Yoshihara, K. J . Photochem. 1985, 31, 223. (8) Bong, P.-H.; Kim, H. J.; Chae, K. H.; Shim, S . C.; Nakashima, N.; Yoshihara, K. J . Am. Chem. SOC.1986, 108, 1006. (9) Whitten, D. G.; McCall, M. T. J . A m . Chem. SOC.1969, 91, 5097.
0 1989 American Chemical Society
5454
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
containing two quaternarized rings is not well understood due to the lack of information on the cis isomers and the role of the counteranions. This was shown in our preliminary account of the photochemistry of ?rans-BPE2+." Furthermore, little is known about the difference in charge-transfer behavior of cis and trans isomers. Such a quantitative study and comparison requires stable cis isomers in the ground state in the presence of charge-transfer complexing agents, which is apparently not always the case,I5 and identification and characterization of cis and trans radical isomers. Recently, we described the radical properties of cis and trans isomers of BPE2+and found that these radicals undergo one-way photoisomerization from cis to trans only.18 We have also shown that the cis-BPE2+ is stable at room temperature if the ionic strength is sufficiently high even in the presence of electron-donating agents.17 Thus it is possible to present here a complete comparison of the properties of cis- and trans-BPE2+ in the presence and absence of charge-transfer states.
Ebbesen et al.
w
0 Z
/"IS
U
m [L
a=q,
0 v)
m
'+
U
Experimental Section
The nanosecond flash photolysis experiments were carried out with a Quanta Ray Nd:YAG laser using both the 354- and 266-nm excitation wavelengths. The excitation and the analysis beams had a right-angle geometry, with the excitation beam hitting the ground surface of the side of the sample cell for even distribution of photons. Anthracene and naphthalene in cyclohexane were used as actinometer^.'^ Slit widths of 2 nm were used in the actinometry measurements to obtain accurate peak absorbance values. A Quantel Nd:YAG laser was the excitation source in the picosecond flash photolysis experiments, the pulse width being ca. 18 ps. The picosecond absorbance spectra were measured between 400 and 760 nm, with a new apparatus described elsewhere.20 The fluorescence lifetime was measured with a Hamamatsu C-979 steak camera with an S-20 cathode.21x22 The ground state spectra were measured with a Cary 219 spectrophotometer. The syntheses of trans- and cis-1,2-bis( l-methyl-5pyridinio)ethylene have been described e1sewhere.l' One of the authors developed a severe skin rash on the hands during the preparation. Since the exact cause is not known, we recommend precaution in preparing and handling these compound. It was found to be very difficult to obtain the trans-free cis isomer after quaternization with CHJ. This cis isomer isomerized to trans in the dark at low ionic strength, further complicating the preparation. However, the fraction of residual trans in the cis preparations used in this study was always less than 0.1 as determined by relative fluorescence intensity of the trans isomer. Acetonitrile was the only solvent used for analyzing the photochemistry of BPE2+ because other solvents, such as water and alcohols, are known to react with the solute upon excitation." The ionic strength was maintained relatively constant by addition of LiC104 (0.5 M). The error margin of the results is
