4246
J. Phys. Chem. 1991, 95,4246-4249
Photophyslcal Properties of Tetracene Derivatives in Solution. 2. Halogenated Tetracene Derivatives C. Burgdorff, S. Ebrbardft and H A .Liibmannsr6ben* Institut f o r Physikalische und Theoretische Chemie, Technische Universitat Braunschweig, Hans-Sommer-Strasse I O , 0-3300 Braunschweig, FRG (Received: July 12, 1990)
Photophysical properties of the halogenated tetracene derivatives S-chlore (MCT), $1 ldichlore ( E T ) , 5,6,1 1,124etrachlore (TCT), 5-brome (MBT), and an isomeric mixture of 5,11- and 5J2dibromotetracene (DBT) were studied in toluene solution. The influences of the internal heavy-atom effect and of the close proximity of substituents in TCT were investigated. For the molecules investigated internal conversion (IC) with a quantum yield of QIc = 0.4-0.5 at room temperature was found to be an important pathway for the deactivation from the SIstate. From the temperature dependence of the quantum yields of fluorescence (QF)and intersystem crossing (Qw)the experimental activation energies E, for the rate constants kIF of (Sl-T2) ISC were obtained according to an Arrhenius-typerelationship. Approximate Arrhenius parameters were also detemuned for kc In a combined evaluation of QFand Qm with the measured triplet lifetime fT,the effect of the heavy-atom perturbation in the molecules on Qy and QF can be accounted for. A consistent description of the photophysical processes is presented that allows the determination of E, for the halotetracenes investigated from the results of room-temperature measurements of QIX, QF, and TT.
1. Introduction
Polycyclic aromatic hydrocarbons (PAH) are traditional model compounds for the investigation of primary photophysical processa in large organic molecules. Nonradiative transitions often play an important role in the intramolecular deactivation from the first excited singlet state (SI).In the absence of photochemical reactions the fluorescence quantum yield (QF) is given by Here kF, klsc. and kIc are the rate constants of fluorescence, intersystem crossing (ISC), and internal conversion (IC), and T~ and T~ denote the fluorescence and radiative lifetimes. Many earlier photophysical studies have focused, partly for the sake of simplicity, on PAH molecules for which klc is negligible and the sum of the quantum yields QF and Qtsc is unity. M-substituted anthracenes are the most prominent examples in this re~pect.l-~ Recently the investigation of IC has also attracted increasing attention. Some of the reasons for this interest are as follows: (1) In solution, where usually processes occur from the vibrationally relaxed SIstate, IC is ubiquitous in aromatic molecules with an SIstate energy E @ , ) I 20000 ~m-'.4*~(2) Increasing experimental evidence suggests that IC in large molecules is intrinsically related to the Occurrence of interesting intramolecular or dewar photochemical reactions such as cis-trans i~omerization~~ formation.&I0 (3) The excess energy dependences of k c and kw in nonrelaxed molecules, measured in bulk and molecular beam experiments,"-" provide insight into the dynamics in excited molecular states. (4) Experimental activation energies obtained from the temperature dependence of klsc or kIcof PAH molecules in solution can be used to explore transitions to other electronic states.l4-l6 If both IC and ISC are observed in one molecule, the distinction and separate characterization of the processes are necessary. The direct determination of k c or Qlc can be achieved with involved or experimental techniques such as photothermal ~pectroscopy'~ surface electron ejection measurements in molecular beam studies.'* Alternatively, kIc can indirectly be obtained from independent measurements of at least two of the more conveniently accessible parameters QF, QIsc. and T~ In our previous workI9 we have shown that nonradiative processes in perylene and tetracene and some of their derivatives are aptly described by Arrhenius-type relationships: 'Institut fnr Organische Chemic, Technische Universitit Braunschweig, Hagenring 30, D -3300 Braunschweig, FRG. To whom correspondence should be addressed.
k ~ s c A exp(-E,/RT)
(2)
klc = A' exp(-E,'/RT)
(3)
where A, A', and E,, and E,' are the frequency factors and experimental activation energies of ISC and IC, respectively. We obtained E, with good accuracy from measurements of the temperature-dependent triplet absorbance Ao,.(T) and of the integral of the fluorescence spectrum F(T)employing the relationship
In [AOT(T)/F(T)] = -E,/(RT)
+ constant
(4)
The main advantage of eq 4 is that relative Ao,.(T) and F(T) data can be used for the determination of E, instead of absolute Qrsc and QF values, which are necessary in order to obtain, e.g., A'and E,' from
In [(I - QF - Q a c ) / Q ~ l= -E,'/(RT) +
(A'/~F) (5)
In the current work we have extended our investigations to the (1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (2) Maciejewski. A.; Steer, R. P. J . Phorochem. 1986, 35, 59. ( 3 ) LbhmannsrBkn, H . 4 . In h e r Spcerroscopy;Heldt, J., Lawruszuk, R., Eds.; World Scientific Publishers: Singapore, 1988. (4) Siebrand, W.; Williams, D. F. J . Chem. Phys. 1968, 49, 1860. (5) Plotnikov, V. G.; Dolgikh, B. A. Opr. Spekrrosc. (USSR)1977, 43, 522.
(6) Akcsson, E.; Bergstram, H.; Sundstram, V.; Gillbro, T. Chem. Phys.
Lett. 1986, 126, 385.
(7) Flom, S . R.; Nagarajan, V.; Barbara, P. F. J . Phys. Chem. 1986,90, 2085.
(8) Jahn, B.; Dreeskamp, H. Ber. Bunsen-Ges. Phys. Chem. 1984,88,42. (9) Hirayama, S.;Shimono, Y. J . Chem. Soc., Faraday Trans. 1984,80, 941. (IO) Schoof, S.;Guten, H. Ber. Bunsen-Ges. Phys. Chem. 1989,93,864. (1 1) Huang, C.-S.; Hsieh, J. C.; Lim, E. C. Chem. Phys. Lerr. 1974,28, 130. (12) Amirav, E.; Even, U.; Jortner, J. J . Chem. Phys. 1981, 75, 3770. (13) Mhmannsraben, H.-G.; Luther, K.; Stuke, M. J . Phys. Chem. 1987, 91, 3499. (14) Kcarvell, A,; Wilkinson, F. Transitions Non Radiar. Mol., Reun. Soc. Chim. Phys. 20th, 1969 1970, 125. (15) Lampert, R. A,; Phillips, D. J . Chem. Soc.. Faraday Trans. 2 1985, 81, 383. (16) Pantke, E. R.; Labhart, H. Chem. Phys. Lerr. 1972, 16, 255. (17) Jabben, M.; Garcia, N. A.; Braslavsky, S.E.; Schaffner, K.Phorochem. Photobiol. 1986, 43, 127. (18) Sneh, 0.;Amirav, A.; Cheshnovsky, 0. J . Chem. Phys. 1989, 91, 3532. (19) Komfort. M.; Mhmannsrilben, H.-G.;Salthammer, T. J . Phorochem. Photobiol. A 1990, 51, 215.
0022-365419112095-4246$02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4247
Halogenated Tetracene Derivatives
TABLE I: Wotophysical Propertles of Tetrrceme nod Halogenated Tetncene Derivatives in T o l W TET MCT DCT TCT 1O-3cr,/M-icm-l (A,/nm) 10.7 (475) 8.0 (488.5) 11.0 (499.8) 8.5 (551) 9.926 9.5527 0.16' 0.14 (365 K) 4.8 31.8 (465) 0.62 0.62 (365 K) 6.3
QF rF/nS
10-3cT/M-1cm-' (X,,/nm) Qisc 104~~/s
0.05 0.017 (365 K) 1.8 37.6 (480) 0.5 1 0.42 (365 K) 0.55
MBT
11.028
8.311b
0.065 0.02 (365 K) 1.7 42.2 (482.5) 0.53 0.45 (365 K) 0.35
0.26 0.13 (338 K) 7.9 42.0 (460) 0.29 0.40 (338 K) 0.01
DBT
9.0 (489)
12.0 (502)
0.004
0.005 0.001 (345
39.4 (485) 0.50
41.6 (485) 0.59
0.036
0.02
K)
"If not specified otherwise, room-temperature values are given. For QF and Qlsc a value at higher temperature is included to denote the size of temperature effects.
gm; 10
11
12
1
8
7
6,. 5
CI
DCT
i
--lo
UI
e
Figure 1. Tetracene molecule.
photophysical properties of the following five halogenated tetracene derivatives (cf. Figure 1): 5-chlorotetracene (MCT); $1 l-dichlorotetracene (DCT); 5,6,11,12-tetrachlorotetracene(TCT); 5-bromotetracene (MBT); and an isomeric mixture of 5,11- and 5,12-dibromotetacene (DBT). Stationary fluorescence measurements and transient absorption measurements after laser flash photolysis were carried out. Central points of our interest were as follows: The temperature dependence of and the influence of the internal heavy-atom effect (internal HAE) on the nonradiative transitions, and the proximity effects in multiply substituted molecules. For all molecules investigated IC is an important deactivation process even at room temperature, a feature that has hitherto not been reported for tetracene derivatives. The results obtained here allow a comparison with the properties of phenyl-substituted tetracenes reported in part 1 of this work.20
1 E
. ! i Y
i ...........-..
LOO
L50
500 -1inm
550
*-.......'......._,, -00
600
650
Figure 2. Singlet absorption (-), fluorescence (- -), and triplet-triplet absorption (--) spectra of DCT in toluene at room temperature.
2. Experimental Section
The preparation and spectroscopic characterization of the substances MCT, DCT, TCT, MBT, and DBT were carried out according to literature procedures.21 Tetracene (TET), rubrene (RUB), and anthracene (ANT) are commercially available and were purified by vacuum sublimation. Toluene of spectroscopic grade was used as solvent. For all measurements dilute solutions (c = l P - l @ M) were deoxygenized and sealed in 1 X 1 cm cells. The samples had to be prepared in the dark to avoid photooxidation. The UV-vis absorption spectra were recorded with a doublebeam spectrophotometer (Shimadzu UV-240). Fluorescence spectra were recorded with a quantum-correctedsmofluorimeter (Perkin-Elmer MPF-ME). For temperature control the samples were placed in special cuvette holders in Dewar vessels and cooled by circulation of gaesous N2 or heated by circulating water or glycol from a constant-temperature bath. Quantum yields of fluorescence at room temperature were determined relative to literature values of RUB = 0.9flZ0)and TET (QF = 0.16l). Fluorescence lifetimes were determined with a time-correlated single-photon-counting apparatus described in ref 22. The transient absorption spectroscopy was camed out with a laser flash apparatus, for which experimental details can be found in previous publication^.^^.^^ The apparatus allows laser excitation of the
(eF
(20) Burgdorff, C.; Kircher, T.; L6hmannsrbben. H.G. Spectrahim. Acra A 1988,44. 1137. (21) (a) Tanimoto, I.; Kushioka, K.; Kitagawa, T.; Maruyama, K. Bull. Chem. Sa.Jpn. 1979,52,3586. (b) Balodii, K. A.; Livdane. A. D.; Medne, R. S.;Neiland, 0. Ya. J. Org. Chrm. USSR 1979. IS, 343. (22) Dreeskamp, H.;Salthammer, T.; Liufcr, A. G. E. J . Lumin. 1989, 44, 161. (23) Lewitzka, F.;Mhmannsrbben, H.-G. Z . Phys. Chem. (Munich) 1986, 150. 69.
b 0LOO
;; e.'
L50
500 -1lnm
I
550
......
600
650
-...*.... 00 700
Figure 3. Singlet absorption (-), fluorescence -), and triplet-triplet absorption (--) spectra of TCT in toluene at room temperature. (.e
samples at 308 nm or with an excimer-pumped dye laser (350450 nm) and the detection of triplet molecules on a submicrosecond time scale. From the triplet decay the initial triplet absorbance AOAT) and the triplet lifetime TT were obtained.= Absolute values of triplet extinction coefficients (9)and Qlsc were determined by the energy-transfer and reference excitation methods."' The = 0.62, triplet parameters of the reference substances TET ~ ~ ( 4 nm) 6 5 = 31 800 M-'cm-') and ANT (QlX = 0.71, q(430 nm) = 45 500 M-' cm-I) were taken from refs 1 and 25.
(ersc
3. Results and Discussion 3.1. Spectroscopic and Kinetic Properties. A summary of the photophysical properties of the molecules investigated here is given in Table I. For comparison, the data for tetracene have alsc been (24) Carmichael, 1.; Hug, G. L. J . Phys. Chem. Ref.Dara 1986, IS, 1. (25) Bcnsasson, R.; Land, E.J. J. Chrm. Sa.,Faraday Trans. 1971,67, 1904.
4248 The Journal of Physical Chemistry, Vol. 95, NO. 1 I, 1991
included.20 The spectra of singlet (So-SI) absorption (extinction coefficient es). fluorescence, and triplet (TI-T,) absorption (*) of DCT and TCT are shown in Figures 2 and 3. The spectra of MCT and the brominated compounds show vibrational structures similar to those of DCT. The singlet absorption spectra of MCT, DCT, and TCT exhibit increasing red spectral shifts with increasing chlorine substitution. The red spectral shifts are approximately additive for the first two chlorine atoms, 580 cm-I for MCT and 525 cm-l for DCT per chlorine atom. The red spectral shift is greatly enhanced to 725 cm-' per chlorine atom in TCT with halogen atoms in pen positions 5,6, and 11, 12. This is interpreted as a result of the so-called peri effect of the subs t i t u e n t ~and ~ ~ ,has ~ ~been related to sterical hindrance (overcrowding) and conjugative effects. The peri effect, which is also evident from the less distinct vibrational structures in the TCT spectra, has been discussed in detail for PAH derivative^.^*-^^ It is well-known that the rate constants for spin-forbidden, nonradiative processes are affected by the internal HAE and by the zero-vibrational energy difference of the electronic states involved in the transitions. Usually heavy-atom substitution in organic molecules leads to the enhancement of ISC and thus to a reduction of QF relative to the parent molecule. Upon heavyatom substitution a decrease of the SIstate energies is also often observed, whereas the triplet-state energies either remain unchanged or are lowered only to a small extent. If thermally activated (Sl-T,) ISC to a triplet state higher in energy than the SI state is competing with fluorescence decay, the red spectral shift of the SIstate can increase the (SI-T,) energy gap and consequently reduce the rate constant of the ISC transition. In such a case heavy-atom substitution of a molecule can have the unusual effect of increasing the fluorescence intensity. For the molecules investigated here, such an "inverse" heavy-atom effect ("inverse" HAE) is most evident in the high value of QF = 0.26 for TCT, which is 5 times larger than QF of MCT. Moreover, QF of TCT is 65 times larger than QF of MBT, whereas it is estimated with the atomic spin-orbit coupling parameter for C1 and Br3I that the heavy-atom perturbation in MBT is only 4 times that in TCT. A more quantitative description of the "inverse" HAE is given in section 3.2. With the measured values of QF and TF (cf. Table I) rate constants of fluorescence were found to be kF = (3-4) X lo7 s-l, which is in good agreement with experimental data of other tetracene derivativesZoand values calculated according to the Strickler-Berg formula.' From the Qlsc values obtained (cf. Table I) it is evident that for all halotetracenes investigated IC with a quantum yield of QIc = 0.4-0.5 is a major deactivation process at room temperature. It is notable that IC, which was found for the parent molecule TET (Qlc = 0.18 f 0.0332),is significantly enhanced by halogen atom substitution. On the contrary, only a minor contribution of IC was detectable at room temperature for phenyl-substituted tetracene derivatives investigated so far (e.g., Qlc 5 0.05 for RUBZo). 3.2. Temperature-Dependent Photophysical Properties. The temperature dependences of Qf and Qlsc of some of the investigated halotetracenes are shown in Figure 4. Only for TCT an increase of Qlsc with increasing temperature is observed. The temperature-dependent results were evaluated according to eqs 4 and 5 for the chlorinated compounds only. QF(T) of the brominated compounds was also determined, but due to the low fluorescence intensities the data have large uncertainties. From the linear plots of the combined evaluation of F(T) and AoT(T), according to eq 4, in Figure 5 the following experimental activation (26) Berlmam, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 197 1. (27) Maulding, D. R.;Roberts, B. G. J. Org. Chem. 1969, 34, 1734. (28) Clar, E.;Marshalk, Ch. Bull. Soc. Chim. Fr. (5) 1950, 433. (29) Balasubramaniyan, V. Chem. Reu. 1966, 66, 567. (30) JafE, H. H.; Chalvet, 0.J . Am. Chem. Soc. 1%3,85, 1561. (31) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Trlplet State; Prentice-Hall: E n g l e w d Cliffs: NY, 1969. (32) Wilkinson, F.In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley-Interscience: London, 1975; Vol. 2.
Burgdorff et al.
'
1 - 1 00 O
250
280
3h
3io
3io -TIK-
Figure 4. Temperature dependences of QF and Qlsc of halogenated tetracene derivatives in toluene. -1L.W
i
k
-17.N
'O
-
I
i -20.00
C 7
0.29
oio
-
a32 103ri'K-' l
-
0.33
41 I 0.35
Figure 5. Evaluation of temperature-dependent measurements of F(T) and AOAT) according to eq 4 for MCT (m), DCT (+), and TCT (*) and of QF(T) and QIsc(T)according to eq 5 for MCT ( 0 ) .
energies were obtained (estimated uncertainty f 150 cm-I): E, = 870 cm-' for MCT, 1110 cm-l for DCT, and 1580 cm-' for TCT. Also exemplary shown in Figure 5 is the evaluation of the QdT) and Qlsc(T) data of MCT according to eq 5. Since absolute values of QF(T) and QIx(T) are necessary for the determination of E,,' A, and A', only ranges of E,' = 1100-1600 cm-' and of 1010-1013s-l for the frequency factors A and A'for MCT, DCT, and TCT can be reported. The problems involved in the interpretation of experimental activation energies obtained from Arrhenius-type relationships of temperature-dependent properties of large molecules in solution J ~crude . ~ ~ , ~ ~ have repeatedly been pointed ~ u ~ . ~ ~ AsJ a~ very approximation E, can be equated with the energy difference between the ground vibrational levels of the SIand T2 electronic states. With E(Sl) from the average wavenumbers of the first absorption and emission maxima in toluene and the E, values given above, the T2 levels are then located at approximately 21 270 cm-' for MCT, 21 070 cm-' for DCT,and 19440 cm-' for TCT. Within experimental accuracy the T2state energies are thus approximately the same for MCT and DCT, while a significant decrease for E(T2) of TCT was found. This is in accordance with earlier observations that in PAH molecules simple substituents or solvent effects have a much larger influence on the SIstate energies than on triplet state energies. As for the SIstate, the larger red spectral shift found for E(T,) of TCT is attributed to the peri effect. Because of the limited experimental accuracy a detailed interpretation of the values found for the frequency factors and for E,' is not useful, but some comments and comparisons with the results for phenyltetra~enes'~ can be made. The values for A'are on the order of what is expected for spin-allowed radiationless
Halogenated Tetracene Derivatives transitions. This supports our assumption of the participation of IC in the deactivation from the SIstate of the molecules investigated. Since for the phenyltetracenesA = 109-1010s-I,l9 it can be assumed that the higher values of A = 1010-1013 s-l determined in the present work result from the enhancement of ISC by the internal HAE in the halotetracenes. For the phenyltetracenes approximate values of E,’ are in the range 2200-4500 cm-I,I9 which seems to be significantly larger than the values of E,’ = 1100-1600 cm-l for the chlorotetracenes. The reason for these lower experimental activation energies, which make IC an important process for the deactivation from the SIstate of the chlorotetracenes, is not clear. It is tempting to speculate about a correlation between the low value of E,’ and the presence of the C-CI group, which possibly provides promoting vibrational modes for IC. The vibrational spectroscopy of some phenyltetracenes has been studied in a molecular beam,33but the influence of vibrational modes on the dynamics of excited states of tetracene derivatives is presently not known. Finally, an assessment of the evaluation of the temperaturedependent data seems of interest. It is obvious that the often applied approach of extracting experimental activation energies from Arrhenius-type plots of In [(QIsc)-l - 11 versus inverse temperature is not appropriate since it neglects IC and leads, e.g., to negative E, values for MCT and DCT. For a further consistency test of the results obtained, a simplified description of the Ynverse” HAE (cf. section 3.1) in the halotetracenes can be introduced that is based on the following assumptions: (1) The measured triplet lifetimes are assumed to correspond to unimolecular nonradiative rate constants that are proportional to the square of the effective spin-orbit coupling factor $, for the overall heavy-atom perturbation in the molecule: kT = (TT)-I = constant X tz.(2)The rate constants for (S1-T2) ISC given by eq 2 are assumed to depend quadratically on the same coupling factor, Le., A = constant X t2. Since kF can assumed to be constant for the investigated molecules, eq 6 can be derived.
= -E,/(RT) + constant (6) The main advantage of eq 6 is that the effective heavy-atom perturbation, which in large molecules is usually not very wellknown,”J’ is tentatively accounted for by the use of experimental triplet lifetimes. Thus it is possible to obtain the experimental activation energies E, from room-temperature measurements of Q I ~QF, , and TT. To eliminate the unspecified constant in eq 6, we have calculated with the data from Table I the following E, valua relative to E, = 870 cm-I of MCT: E, = 1010,2150,920, and 1050 cm-l for DCT, TCT, MBT, and DBT, respectively. It is seen that this simple model does not only reproduce quantitatively essential results of the measurements of the temperaturedependent parameters but also gives meaningful data for the bromotetracenes for which no temperature-dependent measurements were performed: In accordance with the direct experimental determination, a slight increase in the calculated E, values relative
In
(33) Mhmannsrdbcn, H.-G.; Bahatt, D.; Even, U.J. Chem. Phys. 1990,
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4249
to the monosubstituted halotetracenes is obtained for the disub stituted compounds and a strong increase as a result of the peri effect for TCT. It is notable that the calculated E, value for TCT is too large, probably because here the T1 state energy is lower than in the other halotetracenes and thus the measured triplet lifetime is decreased not only by the internal HAE but also due to the dependence of kT on the (TI-So) energy gap. Thus our results suggest that relative to the other halotetracenes investigated, the spectral properties of TCT are determined by large red spectral shifts of the SI,T2, and TI states. This is very similar to the relation found between RUB and other pheny1tetra~enes.I~ 4. Conclusions
The results of the present study indicate that eqs 2-6 provide a simple description of the temperature-dependent photophysical properties of the halotetracenes investigated: It was shown that with the evaluation of data according to eqs 4 and 5, the investigation of the temperature dependences of QF and Qlsc allows the distinction of ISC and IC as competing processes in the deactivation from the SIstate. With the assumption of Arrhenius-type relationships for both klsc and klc, the experimental activation energy E, for ISC can accurately be determined, whereas only approximate values were obtained for the frequency factors and the experimental activation energy for IC. Because of the low solubility of the compounds, investigations could be performed only in a limited temperature range. Therefore the possibility of a temperature-independent contribution to kIc in eq 3 cannot be excluded with certaint~.)~Work is in progress to measure QF(T) of the halotetracenes over a wide temperature range, e.g, in PMMA matrices. (2) The photophysical properties of the halotetracenes investigated are strongly affected by the internal HAE and by the energy gap for (Sl-Tz) ISC. For all compounds investigated, IC with a quantum yield of QIc = 0.4-0.5 at room temperature is an important pathway for the deactivation from the SIstates. This was unexpected, since in the parent molecule TET Qlc at room temperature is only 0.18 and is even significantly lower in pheny1tetra~enes.I~ Due to the high values of Qlc, no highly fluorescent halotetracene was found and, as a result of the “inverse” HAE, the largest value of QF = 0.26 was observed for TCT. (3) With the combined evaluation of the experimental data obtained for Qlsc, QF, and TT according to eq 6 it is possible to tentatively take into account the influence of the overall heavyatom perturbation in the molecules on Qlsc and Qp This is of importance since the effective spin-orbit coupling parameter is difficult to obtain for large molecules. Moreover, with eq 6 it is possible to obtain the experimental activation energy E, for ISC from room-temperature measurements of QF, Qlsc, and TT only. Acknowledgment. It is a pleasure to thank Prof. H. Dreeskamp for his support and A. Muller for her technical assistance. This work was financially supported by the Deutsche Forschungsgemeinschaft.
94, 4025.
(34) Miller, J. C.; Meek, J. 99, 8175.
S.;Stricklet, S.J. J . Am. Chem. Soc. 1977.
(35) Von E w i s Of Menar, M.; Bendig, J.; Siegmund, M. J. Prcrkt. Chem. 1983, 325, 75.
