JOHNH. MOORE, JR.
1130
An Investigation of the Low Energy Singlet-Triplet and Singlet-Singlet
Transitions in Ethylene Derivatives by Ion Impact1 by John H. Moore, Jr. Department of Chemistry, University of Maryland, College Park, Maryland ,90748
(Received November 28, 1071)
Publication costs assisted by the National Science Foundation
The lowest energy singlet-triplet and singlet-singlet transitions in ethylene and some of its simple derivatives have been investigated using the technique of ion-impact energy-loss spectroscopy. The energy of maximum transition intensity has been measured and the magnitude of the singlet-triplet splitting between the T and V states has been determined. In 1,3-butadiene a transition at 4.9 eV was observed in addition to the known transition to the lowest triplet state at 3.2 eV.
Introduction The ethylene molecule is the simplest olefin, and as a result this molecule and its derivatives are often studied in order to understand the properties of the carboncarbon double bond and the effects of nearby substit-' uents on the nature of the bond. Spectroscopic studies of ethylene and its derivatives are frequently hampered by the relative intensities of several of the most important transitions. The ?r* + ?r transition from the N) is ground state to the lowest triplet state (T particularly weak even for a singlet-triplet transition. This is apparently the result of a large difference between the ground state and excited state geometries which permits very little Franck-Condon overlap between the two configurations. By contrast both the spin-allowed ?r* +- ?r transition (V N) and the first Rydberg transition (R + N) are dipole-allowed and are intense features in the optical and electron-impact spectrum. However, the V N transition consists mostly of continuum and is usually overlapped by the R + N bands.2 Electronic selection rules for transitions induced by ion impact differ from optical selection rules, and for this reason a study has been made of a series of substituted ethylenes using the technique of ion-impact energy-loss spectroscopy. I n an ion-impact epectrometer a change in the internal energy of a sample molecule is detected by measuring the kinetic energy lost by an ion which has been inelastically scattered from the molecule. The spectrometer used in these experiments is similar in design and operation to a modern electron impact spectrometer. The instrument has been described in detail previouslya and will not be discussed here; however, it may be useful to consider some of the characteristics of ion-impact induced electronic transitions. For collision energies of up to several kiloelectronvolts, the duration of an ion-molecule collision is sufficiently long to permit electron exchange between the ion and the target molecule to occur with relatively large prob+ -
+ -
+ -
The Journal of Physical Chemistry, Vol. 76, No. 8 , 1078
ability. As a result singlet-triplet transitions are easily excited by ion impact. In addition, for the type of inelastic process being investigated here, it has been observed that scattering occurs only into a small solid angle in the forward direction. This is in contrast to electron impact wherein the scattering is more nearly isotropic. As a result it is possible to detect a larger fraction of the inelastically scattered current in an ion scattering experiment than is the case for electron scattering. Thus while the selection rules for ion-impact spectroscopy are expected to be similar to those which apply to low-energy electron-impact spectroscopy, weak transitions are often more easily detected in an ion spectrometer than in an electron spectrometer.
Results Electronic transitions induced by proton and He+ impact on ethylene and a variety of alkyl- and halogensubstituted ethylenes as well as vinyl methyl ether and l13-butadiene have been investigated. The collision energy was about 3.0 keV, and the energy resolution was about 0.35 eV. The sample gas pressures were in the vicinity of 10 mTorr. The reported spectral peak positions are the averages of measurements made on at least five different spectra. Energy-loss spectra of He+ inelastically scattered from the various ethylenes are presented in Figure 1. With the exception of 1,3-butadiene1 the most intense feature in each of these spectra is a peak at about 4 eV which corresponds to the transition to the lowest-lying triplet state, T N. Using his method of high pressure oxygen-induced optical absorption, Evans observed the absorption maximum of the T + N transition in ethylene at 4.6 eV.4 In the ion-impact energy-loss +-
(1) Work supported by grants from the National Science Foundation and the Research Corp. (2) For a recent review on the ethylene spectrum see: A. J. Merer and R. S. Mulliken, Chem. Rev., 69, 639 (1969). (3) J. H.Moore, Jr., J . Chem. Phys., 5 5 , 2760 (1971). (4) D. F. Evans, J . Chem. SOC., 1735 (1960).
1131
Low ENERGY TRANSITIONS IN ETHYLENE DERIVATIVES H+
E.03.0 koV
E.= 3.0 Lev
0
2
4 6 ENERGY
B I O LOSS (eVf
I
2
1
4
Figure 1. 3.0-keV He +-impact energy-loss spectra of ethylene, some simple substituted ethylenes, and 1,3-butadiene. The sample gas pressure was about 10 mTorr. These spectra were taken using a count-rate meter and the maximum scattered eignal strength was typically about 50 counts/sec.
Figure 2. 3.0-keV proton-impact energy-loss spectra of ethylene, some simple substituted ethylenes, and 1,3-butadiene. The sample gas pressure was about 10 mTorr. The maximum scattered signal strength was typically about 150 counts/sec.
spectrum this peak occurs at 4.3 eV. The energy of the maximum singlet-triplet excitation produced by ion impact on the alkyl-substituted ethylenes is very
little changed from the maximum in the et’hylenespectrum. For the chloro-substitut,ed ethylenes the T + N peak is at somewhat lower energies than in ethylene The Journd of Physical Chemistry, Vol. 76,No. 8,1972
JOHNH. MOORE, JR.
1132 -~
Table I : Energy-Loss Peak Positions in the Ion-Impact Spectra of Simple Ethylene Derivativeso T+N max, eV
T+N other measurements
V+N max, eV
V+N other measurements
Ethylene
4.3
4.6 (4) 4.4 (9)
7.8
Butene-1 cis-Butene-2
4.3 4.2
7 . 6 (8) 7.7 (7, 9, 14) 7 . 1 (10, 15) 7 . 1 (10, 15)
trans-Butene-2 Vinyl chloride 1,l-Dichloroethylene Tetrachloroethylene 1,l-Difluoroethylene Vinyl methyl ether 1,3-Butadiene
4.2 4.0 3.9 4.2 4.6 4.2 3.2
Molecule
(16) 4.3 (7)
7.3 7.2-7.5 7.5 6.9 6.9
3.2 (4) 3.3 (7)
7.6 6.7 6.1
7.0 (10, 15) 6.75 (8) 6.45 (8) 6.3? (8) 7.5 (17) 5.9 (15) 5.8-6.1 (7)
AE,
eV
3.5 3.0 3.0-3.3 3.3 2.9 3.0 3.0 2.5 2.9
a Other measurements are listed in the form: optical results/electron-impact results. References are given in parentheses. AE represents the singlet-triplet splitting between the T and V states as measured peak-to-peek.
and in 1,l-difluoroethylene the peak occurs at higher energy. In butadiene the lowest triplet state moves to lower energy and the peak of the T + N transition is a t 3.2 eV. I n addition a peak appears at 4.9 eV which corresponds to a transition t o a state which lies below the lowest energy singlet state. Evans has reported singlet-triplet absorptions peaking at 3.2 and 3.9 eV.4 He suggests that the triplet states are 3B, and 3A,, respectively. However, these two states are theoretically predicted to lie at 3.9 and 4.6 eV by Pariser and Parrs and at 3.4 and 4.4 eV by Sidman.6 In the low-energy electron-impact spectrum obtained by Oosterhoff, et there are peaks at 3.3 and 3.8 eV and a very weak transition at about 4.8 eV. Proton-impact -spectra are presented in Figure 2. Electron-exchange scattering cannot occur in a protonmolecule collision since the proton does not have an electron and as a result only singlet-singlet transitions occur in these spectra. There are two intense transitions in the 6-9-eV region of both the optical and electron-impact spectrum of ethylene-the first Rydberg transition, R + N and the n* + n transition to the lBlu state, V + N. The intensity maximum of the V + N transition is reported to be a t 7.6 eV in the optical spectrum and 7.7 eV in the electron-impact s p e c t r ~ m . ~The . ~ first peak in the ionimpact spectrum of ethylene is at 7.8 eV. The disparity between these measurements is probably caused by variations in the intensity of the R + N and other underlying transitions. I n contrast to the ethylene case, the R + N and V + N transitions in transbutene-2 produce two well-separated peaks a t 6.2 and 7.5 eV, respectively. I n 1,3-butadiene the V + N transition at 6.2 eV falls well below the Rydberg transitions. The Journal of Ph@ical Chemistry, Vol. Y6, No. 8,1972
Discussion The peak energies of the transitions to the lowest triplet state and the corresponding singlet state of each of the sample molecules are presented in Table I. Peak energy measurements from optical and low-energy electron-impact spectra are included for comparison. Since the T state and the V state of ethylene and its simple derivatives have the same electronic configuration (excepting the difference in electron spin multiplicity), the energy difference between these two states is a quantity of interest. As indicated in Table I, the singlet-triplet splitting as measured peak-to-peak is 3.5 eV in ethylene and decreases to about 3.0 eV in most of the substituted ethylenes. The V + N maxima in the ion-impact spectra is higher in energy than in the corresponding optical and electron-impact spectra. It is of interest to note that this trend holds for ethylene in which the R + N and V + N bands are superimposed, as well as for cis- and trans-butene2 in which these two bands are reasonably well separated.1° Anomolous intensity behavior in the 7-8-oV regicjn of the ethylene spectrum has been observed by other investigators. Ross and Lassettrell have suggested that an electronic quadrupole transition at 7.45 eV may ( 5 ) R. Pariser and R. G. Parr, J . Chem. Phys., 21, 767 (1953). (6) J. W. Sidman, ibid., 27, 429 (1957). (7) H. H.Brongersma, J, A. v.d. Hart, and L. J. Oosterhoff, “Fast
Reactions and Primary Processes in Chemical Kinetics,” S. Claesson, Ed., Interscience, New York, N. Y.,1967, p 211. (8) G. Heraberg, “Electronic Spectra of Polyatomic Molecules,” Van Nostrand, Princeton, N. J., 1966. (9) J. P. Doering and A. J. Williams 111, J . Chem. Phvs., 47, 4180 (1967). (10) J. T. Gary and L. W. Pickett, ibid., 22, 599 (1954). (11) K.J. Ross and E. N. Lassettre, ibid., 44, 4633 (1966).
EXCITED CHARGE-TRANSFER SYSTEMS be contributing to the apparent intensity of the V + N peak. A recent theoretical work by Buenker, et uZ.,12 discusses the V + N bands of ethylene in terms of the electronic and geometrical structure of excited states in 7-8-eV region. They conclude that verticle transitions are forbidden for the V N excitation. If the V + N peak corresponds to a nonvertical transition, it would not be surprising to learn that the position of the intensity maximum is a function of the excitation mechanism. One of the most interesting features of the spectra observed in this work is the intense transition at 4.9 eV in l,&butadiene. It seems probable that this is a singlet-triplet transition; however, the appearance of this feature in the He+-impact spectrum and its absence from the proton-impact spectrum does not prove that the upper state of this transition is a triplet. Robin,
1133 et uZ,,l* postulate the presence of a CH* + R , “mystery band” in the butadiene spectrum at about 5 eV. This assignment seems unlikely for the transition observed here since no feature in the other ethylene spectra can be readily identified with the olefin mystery band. Also since the mystery band is a spin-allowed transition, it would be expected to appear in the proton-impact ~pectrum.l*-~’ (12) R.J. Buenker, S. D. Peyerimhoff, and H. L. Hsu, Chem. Phys. Lett., 11, 65 (1971). (13) M.B. Robin, R. R. Hart, and N. A. Kuebler, J . Chem. Phys., 44, 1803 (1966). (14) J. A. Simpson and S. R. Mielczarek, ibid., 39, 1606 (1963). (15) L. C. Jones and L. W. Taylor, Anal. Chem., 27, 228 (1955). (16) G. P. Semeluk and R. D. Stevens, Can. J . Chem., 49, 2452 (1971). (17) G.BBlanger and C. Sandorfy, J. Chem. Phys., 55, 2055 (1971).
Primary Processes in Excited Charge-Transfer Systems by N. Orbach, R. Potashnik, and M. Ottolenghi* Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel
(Received September 7, 1971)
Publication costs assisted bv the U.S. National Bureau of Standards
The relative yields of triplet state, fluorescent exciplex, and radical ions, generated by the interaction of an excited acceptor (pyrene or anthracene) with N,N-diethylaniline (DEA) as donor, are followed using fast laser photolysis techniques. The effects of temperature and solvent polarity on the relative yields of the products are examined. In nonpolar solvents the data are found to be inconsistent with.a mechanism in which intersystem crossing takes place from the relaxed exciplex, in competition with fluorescence. In polar systems, part of the observed pyrene triplet (3P*)is generated by the relatively slow recombination: PDEA+ + 3P* DEA. However, most of 3P*is already present at the end of the laser pulse, prior to any substantial decay of P- and DEB+. It is suggested that the principal path of intersystem crossing in both polar and nonpolar solvents is a fast process competing with vibrational or solvent relaxation of the excited system.
+
+
Introduction The primary process of intersystem crossing (ISC) from the singlet to the triplet manifold is of major importance in determining the physical and chemical consequences of light absorption by molecules. However, apart from the familiar effects of enhanced ISC due to interactions of the excited molecule with heavy atom or paramagnetic neighbors, very little is known concerning other environmental effects on the rate of crossing to the triplet state. Recently,’ the importance of charge-transfer interactions in enhancing ISC have been shown in relation to the quenching of the fluorescence of aromatic molecules by electron donors or acceptors. Pulsed laser photolysis experiments were carried out: (a) in non-
polar solvents, where the excited state donor (D)acceptor (.4) interaction is associated with the generation of a fluorescent exciplex (A-D +) *,2 e.g.
‘A*
+ D +‘(A-D+)* +A + D + hv
(1)
(b) in polar liquids, where deactivation is associated with the formation of solvated radical ion^^,^ (1) C.R. Goldschmidt, R. Potashnik, and M. Ottolenghi, J . Phys. Chem., 75, 1025 (1971). (2) H. Leonhardt and A. Weller, Ber. Bunsenges. Phvs. Chem., 67, 791 (1963). (3) (a) H.Knibbe, D. Rehm, and A. Weller, ibid., 72,257 (1968): (b) M.Koizumi and H. Yamashita, 2.Phys. Chem. (Frankfurt am Main), 57, 103 (1968); (c) K . Kawai, N. Yamamoto, and H. Tsubomura, Bull. Chem. SOC.Jap., 42, 369 (1969); (d) K. H. Grellmann, A. R. Watkins, and A. Weller, J . Lumin., 1, 2, 678 (1970). The Journal of Physical Chemistry, Vol. 76,No. 8,1974
