T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered in
US.Patent Office
0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 17
AUGUST 24,1978
Search for Steric Hindrance to Quenching of Triplet State Alkylbenzene Vapors by cis-Piperylene and Biacetyl Merlyn D. Schuh Department of Chemistry, Davidson College, Da vidson, North Carolina 28036 (Received March 9, 1978) Publication costs assisted by the Petroleum Research Fund and Da vidson College
Rate constants, ranging between 3.9 and 21 X 1O'O M-l s-l, for quenching of triplet state alkylbenzenes by cis-piperyleneand biacetyl have'been measured in the vapor phase using a flash sensitization technique. Steric and energy transfer is most efficient hindrance to energy transfer is significant only for 1,4-di-tert-butylbenzene, for a coplanar orientation of alkylbenzene and biacetyl or cis-piperylene at a distance of 4.3-5.5 A.
Introduction Bimolecular electronic energy transfer occurs by at least three (1) electron-exchange interaction requiring close contact of donor and acceptor molecules; (2) resonance energy transfer over large distances up to 50 A, arising from dipole-dipole interactions; and (3) radiative energy transfer involving absorption by an acceptor of a photon emitted by a donor. Transfers of singlet energy have been found to occur by all three mechanisms, whereas spin conservation requirements permit triplet energy transfer only by an electron exchange mechanism. The requirement of a collision during electron-exchange energy transfer should make steric hindrance likely when bulky groups surround the chromophores of the donor and/or acceptor. Yet steric hindrance to energy transfer has been reported in only a few none of which involve transfer of triplet energy in the vapor phase, and little is known about orientation and distance requirements for efficient energy transfer. Furthermore, some studies may be questioned since they involved singlet energy transfer, which may involve contributions from two mechanisms, or they were made in condensed phase and may be complicated by solvent perturbations. For instance, in ref 5 the decrease in quenching constant for triplet energy transfer from sterically hindered alkylbenzenes to cis-piperylene is attributed to steric hindrance. However these studies were made in isopentane at 77 K where diffusional contact between donor and acceptor 0022-365417812082-1861$01 .OO/O
molecules is severely restricted and static quenching contributes significantly. In searching for steric effects it is important to use sterically hindered donors or acceptors which have similar triplet state energies. Moreover, the efficiency of electron-exchange energy transfer is proportional to the overlap of the donor TI So emission and So T1 acceptor absorption spectra. This changes with shifts in triplet energies, which may arise from increased substitution onto the donor and/or acceptor molecules, and can mask steric effects. We have measured rates of triplet energy transfer from alkylbenzenes t o biacetyl and cispiperylene in order to (1) compare the effect of steric hindrance on energy transfer to different chromophores, (2) demonstrate the masking of steric hindrance by variation of donor triplet energy, (3) find out the effect of the solid solvent in the experiments of ref 5, and (4)determine the orientation and distance requirements for efficient energy transfer.
-
-
Experimental Section Chemicals. Chromatoquality benzene and biacetyl with respective purities of 99.9 and 99.5% were obtained from Matheson Chemical Co. cis-Piperylene with a purity of greater than 97 % was obtained from Tridom-Fluka Chemical Co. All alkylbenzenes were obtained from Aldrich Chemical Co. and had purities ranging between 97 and 99.9%. Previous experiments have shown these 0 1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
purities to be adequate, and all samples were used directly after three to four freeze-pump-thaw cycles. All experiments were done at room temperature (25 "C) except for the studies of 1,4-di-tert-butylbenzene in which case samples were maintained a t 58.5 f 0.5 " C in a styrofoam-insulated chamber. The alkylbenzene pressure was between 1.0 and 20.0 Torr for all samples except 1,2,4,5-tetramethylbenzene (0.12 Torr), 1,2,3,5-tetramethylbenzene(0.46 Torr), and 1,3-diethylbenzene (0.84 Torr). In determining the rate constants for quenching by biacetyl the biacetyl pressure ranged between 0.001 and 0.035 Torr. In determining the cis-piperylene quenching constants the biacetyl pressure was 0.001-0.002 Torr, except for 1,4-di-tert-butylbenzene and tert-butylbenzene for which the biacetyl pressure was 0.040 and 0.035 Torr, respectively. cis-Piperylene pressures ranged between 5 X and 0.010 Torr. Apparatus. The method of Parmenter and Ring and apparatus have been described previouslya8 Kinetic Model. The following reaction scheme is consistent with the photochemical behavior of mixtures of alkylbenzenes, cis-piperylene, and biacetyl.
+ hv
A 'A
-+
'A
A + hvf
(2)
3A
(3)
-+ -
+
'A
(1)
'A A 'A + B --c A 'BII
lA + CPs (AcP)*
+
A,
(4)
(5)
+ cP,
3A
3A
A (?) B - A + 3B
-
3A + CP
-
A
3A + A
3P + B
3P
3B
+ 3P 2A
CPor tP
-
d(3A)/dt = k3('A) d(3B)/dt
(8) (9)
7A-l
(10)
+ 3B
(11)
B + hv,
(13)
B
(14)
B
d(lA)/dt = E(t)- ' ~ f l ( l A )
(7)
(12)
--
singlet state biacetyl molecules are formed by reaction 5. These molecules appear to decay photochemically and have no effect on the reaction ~ c h e m e .Reaction ~ 6 is also unimportant a t the low cis-piperylene pressure. The triplet state energy of cis-piperylene is estimated to be above that of biacetyl by less than 1.3 kcal/mol,1° and we have found h15 to be less than 1/1000 collisional. Process 11 is also possible but inefficient due to the low biacetyl pressure and very short triplet lifetime of cispiperylene resulting from cis-trans isomerization. On the other hand, both processes 11and 15 have recently been reported in mixtures of 2,3-pentanedione and cyclopentadiene.ll Process 11 evidently occurs because cyclopentadiene is restricted to the cis configuration, and its triplet lifetime is longer than that of cis-piperylene. The formation and decay of singlet state alkylbenzenes closely parallel the flash intensity vs. time profile and lead to maximum triplet state concentration shortly after the flash has expired. The shape of the phosphorescence vs. time profile arises from competition between biacetyl triplet formation, which dominates at short times and leads to increasing intensity, and biacetyl triplet decay, which dominates a t long times and occurs exponentially with lifetime 7,. T , and t,,,, the time when phosphorescence reaches its maximum, are the two experimental parameters. The flash profile, E(,,, can be fitted by the sum of two exponentials, and an analytical solution for the timedependent behavior of the triplet biacetyl concentration, (3B),can be obtained by solving differential eq 16-18. Tf,
(6)
CPor t P
3B 3B + CP
Merlyn D. Schuh
+ 3P
(15)
A, B, cP, and t P refer to alkylbenzene, biacetyl, cis-piperylene, and trans-piperylene, respectively. Superscripts denote spin multiplicity, the v subscript refers to vibrational excitation, and the I1 subscript on B signifies the second excited singlet state. The formation of an exciplex is stated explicitly only in reaction 6 since the existence of an exciplex is essential to an explanation of quenching without electronic energy transfer. Vibronically excited alkylbenzene molecules are formed by the excitation flash. At the pressure of our experiments not all of these molecules relax to a Boltzmann distribution, but this has little effect on the triplet alkylbenzenes. The low flash energies used in these experiments have been shown to preclude triplet-triplet annihilation of alkylbenzenes, and kinetic analysis has revealed that in the absence of cis-piperylene reactions 7,8, 10, 13, and 14 are the most important decay channels of 3A and 3B,8b which are vibrationally relaxed. The relatively weak absorbance of biacetyl at the excitation wavelengths and the large alkylbenzene/biacetyl pressure ratio ensure that nearly all of the few excited
= k7 T
~
-
(16)
'TA-'(~A)
(17)
h8(3A)(B)- T,,-'(~B)
(18)
+ hl&A) + k&B) + h,(cP) = h13 + k14 + kl&CP) -
(19)
(20)
~
the fluorescence lifetime, and k3 are estimated from literature values (which need not be known very accurately since they are very large relative to k7-hl5),and process 11 is disregarded. Arbitrary values of hg, 'TA, and ' T are ~ substituted into the integrated equation for (3B),which is plotted vs. time, and t,, values are taken directly from these plots and used to form a family of curves of t, vs. T*-' for different values of ' T ~Experimental values of 7 and the average of five t,, values per run are then used with this family of curves to evaluate TA as a function of biacetyl and cis-piperylene pressures. hg and hg are determined from the slopes of plots of TA-' vs. (B) with (cP) and (A) held constant and ' T ~ -vs. ~ (cP)with (B) and (A) held constant, respectively (see eq 19). , Values of k 8 and hg were obtained this way for all alkylbenzenes except tert-butylbenzene and 1,4-di-tertbutylbenzene. For these molecules the sensitized biacetyl emission was weak, and scattered light prevented ,t from being readily distinguished for some pressures. So a steady state method was used. The relative quantum yield of phosphorescence equals the area under the oscilloscope intensity vs. time trace and is given by eq 21 if reactions 5,6, and 11 are
+,
'TP =
d'p
--[
1
akl3d'ST
k7
+ hg(B) + k,(cP) + hl,(A)
]
-
(21)
unimportant. @ST is the quantum yield of S1 T1 intersystem crossing and a is the proportionality constant
The Journal of Physical Chemisfry, Vol. 82, No. 17, 1978
Steric Hindrance in Triplet State Quenching
0
I
4
8
12
16
I
I
I
I
1863
]
lV
in x104
1-1
1.0
2
0
I
I
1 2 3 pressure in microns
0
4
Figure 1. Top curve is a plot of vs. biacetyl pressure for 2.5 Torr of 1,3,5-trimethyIbenzene. The ordinates and absicissae are on the right and top of the graph, respectively. Bottom curve is a plot of vs. cis-piperylene pressure for 1.9 Torr of methylbenzene. The ordinates and abscissae are on the left and bottom of the graph, respectively. The slopes of the top and bottom curves are 13 f 1.5 X 10" and 18 f 2 X 10" M-' s-l,respectively.
I
'
I
I
4
8
8
(clr-plperylene)
I
10
In microns
Figure 3. Plots of T ~ :/T:$ $ vs. cis-piperylene pressure for 1.9 Torr of fert-butylbenzene (top curve) and 1.O Torr of 1,4di-fert-butylbenzene (bottom curve). The slopes of the top and bottom curves are 77 and 55 Torr-', respectively. The biacetyl pressures corresponding to the top and bottom curves are 0.035 and 0.040 Torr, respectively.
14 -
12
I
I
I
I
I
I
1
-
I
-
-
2-
I .026 0
.04
.08
l / (biacetyl)
.I2
.I6
.20
1
I
Figure 2. Plot of l/$p vs. the reciprocal biacetyl pressure for 1.0 Torr of 1,4-di-fert-butylbenzene. The slope/intercept ratio equals 0.0054
Torr.
between 4p and the absolute quantum yield of phosphorescence. This relation shows that in the absence of cis-piperylene a plot of l/$pvs. l/(B) has a slope/intercept ratio given by slope - k7 + klo(A) (22) intercept k8 From eq 21 it follows that a plot of ~ ~ 4 p O / ~ where ~ p $ ~T ,~ O and 4pO are determined in the absence of cis-piperylene, vs. (cP)has a slope given by slope = kg/[k7 + ha@) + k1o(A)1 (23) For tert-butylbenzene h, and k7 + hlo(A) were determined using the time-dependent method discussed earlier and kg was determined using eq 23. For a mixture of 1,4-di-tert-butylbenzene and 2 X lo" Torr of biacetyl t,, was not dominated by scattered light and was used to estimate k7 + klo(A) as less than 8500 s-l. Application of this value to eq 22 and 23.yielded k, and k9. Results Typical plots of vs. (B) and vs. (cP) are shown in Figure 1. Figures 2 and 3 show plots of l/$pvs. l/(B)
,034
I/ (1Pt42-ET)
I n microns''
Plot of k , vs. 1/(IP
Figure 4.
I
I
I
,030
2
I
I
,038 in ev-'
+ 0.42 - ET)* in (electron volts)-*.
TABLE I alkylbenzene benzene methylethyln-propyltert-butyl1,2-dimethyl1,3-dimethyll,4-dimethyl1,3-diethyIl,4-diethyl1,4-di-tert-butyl1,2,4-trimethyl1,3,5-trimethyl1,2,3,44etramethyl1,2,3,5-tetramethyl1,2,4,5-tetramethyl-
k , (10"
k , (10"
M-1 s - l )
M-1 s - l )
3 . 9 i 0.5 6.6 i 0.8 6.9 i 0.8
20i 2 18 i 2
6 . 8 i 0.8 6.7 ?: 0.8 7.7 t 0.9
19. 2 18 i 2
9.3 i 1.0 8.8 i 1.0 1oi 1 11 i 1 ~ 3 . i1 0.4 1 2 i 1.5 1 3 2 1.5 1 2 i 1.5 15+ 2 14r 2
16 i 2
18i 2 1 5 +2 ~ 7 . 0 t 0.9 17r 2
21 i 2.5 21 r 2.5
and rP$pO/r OdP vs. (cP) for tert-butylbenzene and 1,4di-tert-butyrbenzene. Table I lists values of h8 and kg obtained in this work. In conjunction with a charge-transfer explanation of the biacetyl quenching (vide infra) a plot of ha vs. 1/(IP + 0.42 - ET)2,where IP and ET are the ionization potentials and triplet state energies of alkylbenzenes (reported in ref 12),
1864
The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
respectively, appears in Figure 4.
Discussion The linearity of all plots in Figures 1-3 is consistent with the proposed mechanism and reveals the absence of process 11which would lead to negative deviation in Figure 3. Additional support for our data comes from the good agreement between the kg value for benzene in Table I and that reported in ref 13, namely, 1.7 X 10” M-’ s-l, which was obtained using cis-trans isomerization of butene-2 as the triplet probe. The efficiency of triplet energy transfer may be influenced by factors which must be estimated before steric hindrance can be distinguished. For instance, the distance of approach required for efficient energy transfer may depend upon factors such as the lifetime of the complex. Since the ionization potentials and triplet energies of the monoalkylbenzenes are similar, differences in k8 and kg for these compounds should be due to the substituents. Moreover, the different sizes of the alkyl groups should produce different interaction potentials with the acceptor molecules14and should lead to different collision complex lifetimes. However, the constancy of k8 and kg for these alkylbenzenes shows that the lifetimes of the complexes have little effect on the quenching efficiencies. Mechanisms. With the exception of 1,4-di-tert-butylbenzene ha increases with increasing number of substituents on the benzene ring, opposite the expected trend if steric effects were dominant. Dexter15has shown that the probability of energy transfer by an exchange mechanism is given by
where u is the frequency in wavenumbers, fDcu, is the spectral distribution of donor (D*) phosphorescence normalized on a frequency scale, FA(v)is the molar decadic extinction coefficient of the acceptor (A), K2 is a constant with units of energy squared, R is the mutual distance between D* and A, and L is a constant called the “effective Bohr radius”. L and R are not amenable to experimental determination, however, the probability should be proportional to the overlap of donor phosphorescence and acceptor absorption spectra. Vapor phase alkylbenzene phosphorescence spectra have not been reported, but they are likely similar to solid phase spectra, which range between 20 500 and 29 000 cm-’. In general the 0,O energy gap decreases and the emission intensity becomes more heavily weighted on the red end of the spectrum as the benzene ring becomes more substituted.l22l6 If the So TI biacetyl absorption spectrum is approximated by the mirror image of the phosphorescence, it extends from about 20000 to 24000 cm-’, and the overlap between alkylbenzene emission and biacetyl absorption spectra increases with decreasing alkylbenzene triplet energy. Thus the trend in k8 is consistent with a simple exchange mechanism. A charge-transfer mechanism is also consistent with the k8 values. For a charge transfer mechanism the quenching constant is given by
-
where p is the density of states, F is the Franck-Condon term, EcT is the energy of the charge-transfer state, and ET is the triplet state energy. Since alkylbenzene-biacetyl charge-transfer states have not been observed, EcT must
Merlyn D. Schuh
be determined from the formula ECT = IP - EA + C (26) where IP is the ionization potential of the alkylbenzene, EA is the electron affinity of biacetyl, and C is the Coulomb stabilization energy of the complex. The electron affinity for biacetyl, which makes a constant contribution to E C T , is not available but can be estimated from its half-wave reduction potential of -0.42 eV in 15% ethan01.l~ Coulomb stabilization energies are unknown for alkylbenzene-biacetyl complexes. However C is generally small relative to IP for organic complexes,18and for structurally analogous complexes C is expected to change little. Thus a plot of k8 vs. 1/(IP + 0.42 - ET)2should increase linearly if p F and the matrix elements coupling triplet, ground, and charge-transfer states are constant. Although complete donation of an electron probably does not occur, the linearity of Figure 4 indicates the possible importance of coupling to a state with charge-transfer character. The constancy of kg and increase in k8 with increasing substitution make it seem that biacetyl and cis-piperylene quench alkylbenzenes by different mechanisms. However the constancy of the kg values is misleading because they are nearly equal to the hard sphere collisional constant for spheres of 5 A radius and cannot be much larger. Comparison of k8 and kg values permits a qualitative choice between the exchange and charge-transfer mechanisms. Table I shows that kg exceeds k8 for all alkylbenzenes. In one respect this is consistent with an exchange mechanism since E T for cis-piperylene is greater than for biacetyl, and therefore the spectral overlap occurs over a greater wavelength range for cis-piperylene. Whether or not the actual spectral overlap is larger is not known since normalized absorption spectra for cis-piperylene and biacetyl are not available for comparison. It is more difficult to compare k8 and kg on the basis of a charge-transfer mechanism since the electron affinities of biacetyl and cis-piperylene are not known. However atomic oxygen is more electronegative than atomic carbon, and since biacetyl and cis-piperylene are structurally similar with two double bonds per molecule, it is expected that biacetyl has a larger electron affinity. However, this would make k8 larger than hg,in disagreement with Table I. On the basis of this qualitative discussion an exchange mechanism for quenching by both biacetyl and cis-piperylene best explains the difference between k8 and kg. Steric Hindrance. Table I shows that steric hindrance is absent or masked by the increasingly favorable electron-exchange interaction between donor and biacetyl as alkyl substitution increases, and only when two tert-butyl groups are present does steric hindrance abruptly reduce k8 by a factor of 3.6. Quenching by cis-piperylene shows little steric hindrance also. kg decreases slightly for 1,4dimethyl and 1,4-diethyl derivatives, but not until two tert-butyl groups are present does kg decrease abruptly by a factor of 2.3. Thus it appears that the optimum separation between alkylbenzene and acceptor is about the same for biacetyl and cis-piperylene. This is not surprising since both acceptors are similar in size and both possess two double bonds. The abrupt onset of steric interference allows an estimation of optimum separation between and orientation of acceptor and donor in the collision complex. Consideration of molecular models reveals that end-on approach of biacetyl and cis-piperylene perpendicular to the benzene ring should be sterically unhindered in all alkylbenzenes in Table I, but the decrease in k 8 and kg in the case of 1,4-di-tert-butylbenzenereveals that this is not the optimum orientation for energy transfer. On the other hand,
Steric Hindrance in Triplet State Quenching
coplanar approach of donor and acceptor allows maximum overlap of T orbitals and can be sterically hindered. The van der Waals radius for a methyl group is 2.0 A19 and twice this value represents the minimum interplanar distance between polymethylbenzenes and biacetyl or cis-piperylene since methyl groups on both of these acceptors fall within the plane of influence of the benzene methyl groups. Energy transfer is apparently quite efficient a t this distance. On the other hand, H atoms on tert-butyl groups extend about 3.5 A above the benzene plane,5 and the sum of 3.5 and 2.0 A gives the interplanar distance a t which energy transfer abruptly becomes less efficient. Thus the optimum interplanar separation for efficient energy transfer is 4.0-5.5 A. Since steric hindrance is very small if not absent for 1,4-diethylbenzene, this separation is even larger. Rotation of ethyl groups about the C-C bonds coplanar with the benzene ring makes the extension of ethyl H atoms above the plane vary between 2.0 and 3.5 A. So there is no static steric hindrance effect for 1,4-diethylbenzene, but the time averaged extension above the plane of the ethyl H atoms is at least 2.3 A, which increases the optimum interplanar distance to 4.3-5.5 A. This estimate is about the same as that reported by Froehlich and Morrison for quenching of singlet state alkylbenzenes by cis-piperylene in s ~ l u t i o nwhich , ~ occurs through vibrational energy dissipation and not electronic energy transfer. However several differences between their results and those in Table I exist. These workers reported a more gradual decrease in the quenching constant with increasing size of the mono-substituted alkyl group and number of methyl substituents. This may be due to the absence of favorable electron exchange or charge transfer interactions which would prevent the quenching constant from decreasing. However it could also arise from solvent interactions which lead to differences in the lifetimes of collision complexes. Froehlich and Morrison also reported that the quenching constants of a limited number of triplet alkylbenzenes in isopentane at 77 K decrease gradually with increasing size and number of alkyl group^.^ The difference between solid and vapor phase results may also be due to different collision complex lifetimes in the solid. However the small magnitude of the rate constants (range of 122-269 M-l s-l) indicates very limited diffusion of donor and acceptor molecules, and the difference in results is more likely due to the random fixed orientations of donor and acceptor molecules in a solid matrix which prevents formation of coplanar states for optimal energy transfer. In this case the efficiency of static energy transfer would be governed principally by the distance between and not orientation of the 7r orbitals of donor and acceptor molecules. The average distance would increase with increasing substitution and lead to the observed gradual decrease in quenching constant.
The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
1885
Conclusions This work, to our knowledge, is the first report of steric hindrance to triplet energy transfer in the vapor phase and shows that the optimal energy transfer between triplet alkylbenzenes and biacetyl or cis-piperylene occurs for a coplanar arrangement in which the interplanar distance is 4.3-5.5 A. It appears that in the vapor phase energy transfer is not strongly dependent upon the collisional lifetime, which may not be true in condensed phase, and probably involves a simple electron exchange mechanism. Steric hindrance is masked by the increasingly favorable electron exchange interaction as the triplet energy of the donor decreases. Comparison of quenching by cis-piperylene in vapor and solid phases shows that in the latter case differences in quenching efficiency are probably due to the distance between donor and acceptor and is less dependent upon orientation of collision partners in close contact. Acknowledgment. Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We also acknowledge the Research Corporation for partial support of this work.
References and Notes F. Wilkinson in "Advances in Photochemistry", Vol. 3, W. A. Noyes, Jr., G. S. Hammond, and J. N. Pitts, Ed., Interscience, New York, N.Y., 1964, p 241 ff. A. A. Lamola in "Energy Transfer and Organic Photochemistry", P. A. Leermakers, A. Weissberger, A. A. Lamola, and N. J. Turro, Ed., Interscience, New York, N.Y., 1969, p 17 f f . W. G. Herkstroeter, L. B. Jones, and G. S. Hammond, J. Am. Chem. Soc., 88, 4777 (1966). K. Janda and F. S. Wettack, J. Am. Chem. Soc., 94, 305 (1972). P. M. Froehlich and H. A. Morrison, J . Am. Chem. Soc., 96, 332 (1974). C. C. Wamser, R. T. Medary, I.E. Kochevar, N. J. Turro, and P. L. Chang, J. Am. Chem. Soc., 97, 4864 (1975). (a) G. L. Loper and E. K. C. Lee, J . Chem. Phys., 63, 264, 3779 (1975). (a) C. S.Parmenter and B. L. Ring, J. Chem. Phys., 46, 1998 (1967); (b) J. M. Burke, M. D. Schuh, and H. M. Sporborg, ibid., 63, 3597 ( 1975). H. Ishikawa and W. A. Noyes, Jr., J . Chem. Phys., 37, 583 (1962). R. E. Kellogg and W. T. Stimpson, J . Am. Chem. Soc., 87, 4230 (1985). A. W. Jackson and A. J. Yarwood, Mol. Photochem., 8, 255 (1977). Values of ETand IP were taken from Tables 6.3 and 9.5, respectively, of J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, N.Y., 1970. M. W. Schmidt and E. K. C. Lee, J. Am. Chem. Soc., 92,3579 (1970). See D. L. Bunker, "Theory of Elementary Gas Reactions", Pergamon Press, New York, N.Y., 1966, Chapter 4. D. L. Dexter, J . Chem. Phys., 21, 836 (1953). R. V. Naumann, Ph.D. Dissertation, University of California, Berkeley, Calif., 1947. L. Metes, "Polarographic Techniques", Wiley-Interscience,New York, N.Y., 1965, p 686. A. Weller in "The Exciplex", M. S. Gordon and W. R. Ware, Ed., Academic Press, New York, N.Y., 1975, p 23. J. M. Robertson in "Determination of Organic Structures by Physical Methods", E. A. Braude and F. C. Nachod, Ed., Academic Press, New York, N.Y., 1955, p 494.
