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Detection of Trimethylenemethane by IR Spectroscopy The Result of an Unexpected Photoisomerization of Methylenecyclopropane in a Halogen-Doped Xe Matrix.

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Detection of Trimethylenemethane by
IR Spectroscopy: The Result of an Unexpected
Photoisomerization of Methylenecyclopropane
in a Halogen-Doped Xe Matrix**
By Giinther Maier,* Hans Peter Reisenauer, Klaus Lanz,
Raw Tross, Dorollike Jiirgen, B. Andes Hess, Jr., and
Lawrence J. Schrrad
Dedicated to Professor Rudolf Hoppe
on the occasion oj'l~is70th birthday
Cyclobutadiene (1) and trimethylenemethane (2) are the
parent molecules of two fundamentally different families of
conjugated hydrocarbons. Cyclobutadiene is the first member of the series of cyclic conjugated hydrocarbons (Kekule
compounds); trimethylenemethane, of the non-Kekule hydrocarbons.["
able of existence at temperatures up to - 150 oC.[5a1
We report here for the first time the IR spectrum of 2.
at isolating 2 in an argon matrix
Our previous
gave the following results: After irradiation (313 nm) of 3 in
argon at 10K a well-resolved ESR spectrum of 2 could be
recorded, but in the IR spectrum only the bands of
methylenecyclopropane 7 were recognizable.[*' When 3 was
subjected to flash pyrolysis (400-600 "C) and the fragments
were then isolated in argon at IOK, 2 could not be detected
even by ESR spectroscopy. The IR spectrum again showed
only the bands of 7. Diiodide 5["] was inert to irradiation
(Ar, 10K, 254 nm), and the combination of flash pyrolysis
(500-800 "C)/matrix isolation"'] again provided exclusively
Despite these disappointing results we recently reopened
our investigations, and our efforts have been rewarded :
When ketone 4 was irradiated (Ar, 10K, 300 nm) the ESR
spectrum of 2 was observed. In the IR spectrum besides the
bands of 7, two miniscule absorptions are seen at 758 and
500 ern-'[''] with an intensity ratio of roughly 4: 1 , These
correspond to the two most intense bands in the predicted IR
spectrum of 2"', I 3 ] (Fig. 1).
Following simple Hiickel theory, in 1 and 2 four 7~ electrons are distributed over four C atoms, and one electron is
placed in each of the two degenerate nonbonding molecular
orbitals. Accordingly, both the square form of 1 (D4,,)
and 2
(D3h)should have triplet ground states. This is indeed true
for 2 but not for 1. The distinct arrangements of the individual centers account for this difference.[21
During our studies of c y ~ l o b u t a d i e n ewe
[ ~ wondered
if matrix-isolation spectroscopy would be a
good tool for the study of trimethylenemethane. Compound
2 was first prepared in 1966 by D ~ w d ~by
~ "irradiation
4-methylenedihydropyrazole 3 in an organic matrix at
- 185 "C,and its existence was proven by ESR spectroscopy.
Despite great effortst6]our current understanding of 2 is still
quite limited. The only certainty is that the irradiation of
suitable precursors, for example 3,4, o r 7 (y radiation), gives
rise to triplet 2 in such low concentrations that it can be
detected only with ESR spectroscopy.[5.7 1 Species 2 is cap-
3115 5
3099.6 3031.2
0 . 0 - 9
1 -
0 4
313 nm
Prof. Dr. G. Maier. Dr. H. P. Reisenauer. Dr. K. Lanz,
D~pl.-Chem.R. Tross. DipLChem. D. Jiirgen
Institut fur Organische Chemie der Universitlt
Heinrich-Buff-Ring 58, D-W-6300 Giessen (FRG)
Prof. B. A. Hess. Jr.. Prof. L. J. Schaad
Department of Chemistry, Vanderhilt Univerity
Nashville. T N 37235 (USA)
Small Rings. Part 75. This research was supported by the Fonds der
Chemischen Industrie. - Part 74: G. Maier. A. Schick. I. Bauer, R. Boese.
M. Nussbdumer. Ckcni. Brr. 1992, 125. 2111 2117.
'(.> VCH ~ ~ r . l u ~ . s g e s e l l .iirhH,
~ ~ ~ u W-6940
Weinlwiiii,I YY3
2000 1500
F [cm-'~
Fig. 1. IR spectrum of 2: a) calculated (ah initio UMP2/6-31G*), b) measured
(Xe matrix, 12K. difference of the spectra after irradiation of 7/Br, with light
with a wavelength of 254 nm followed by irradiation with light with a wavelength of 313 nm). The bands with negative values of A arise from compound 7.
&---254 nm
Even more importantly, we found that these same bands
(756, 499 cm- ') arose when 5 was subjected to flash pyrolysis at 600°C with xenon, the product mixture condensed at
IOK, and the matrix finally irradiated (254 nm). This observation was initially confusing, since 5 and methylenecyclopropane 7 formed in the thermolysis are photostable in
xenon when irradiated separately. We conducted a long list
of experiments to solve this mystery. The results: a) The
source of 2 in these reactions is not 5 but 7. b) The photoconversion 7 -+2 takes place in a xenon or a xenon/argon matrix
(limiting ratio 1 :4), but not in a pure argon matrix. c) Halo-
S 10.00f .25/0
A n g m . Chem. Int. Ed. Engi. 1993. 32. No. 1
gen atoms (I., Br', or CI') must be present in the irradiation.
d) The same IR spectrum is recorded regardless of which
halogen is used.
Based on these results we devised two methods for the
production of 2. The halogen, that is, bromine o r iodine, is
pyrolyzed with xenon at 750°C. The halogen atoms that
arise are frozen onto a window cooled to 1 0 K along with a
gaseous mixture of 7/xenon, which is introduced separately.
A Hg low-pressure lamp (254 nm) is then used for the irradiation. The formation of 2 by this procedure is faster and
more effective than by the direct irradiation of halogen molecules (C12, Br,, or I,) in a xenon matrix in the presence
Of 7.
The identification of trimethylenemethane (2) formed in
the xenon matrix is based primarily on the comparison of the
measured with the calculated (ab initio UMP2/6-31 G* ;
Table 1) I R spectrum (Fig. 1). Because of the D,, symmetry
atoms X' form exciplexes with xenon when irradiated, which
then immediately relax to form the triatomic charge-transfer
complexes 9.r151
The exciplexes can be detected by UV absorption spectroscopy (Xe/I.: A,, = 280 nm, Xe/Br.:
A,,, = 310 nm, Xe/CI': I,. = 354 nm), 9 with emission
spectroscopy (9 -+ Xe Xe X'
hv).[''' We presume
that the energy stored in 9 is transferred to 7 across a long
distance (ratio 7:Xe and X':Xe < 1 :1000). Exactly how the
energy is relayed and how the ring cleavage of 7 is induced
remains open. In any case the energy stored in 9 (50-
X' + hv
+ 7
2 + Xe + Xe
Table I . Calculated vibrational spectrum of 2; symmetry of the vibrations,
wavenumbers i n cm-',
intensity in kmmol-I.
A I'
of 2, only 14 of the 24 fundamental vibrations are IR active
and should result in 8 IR bands (6E', 2A;; calculated spectrum Fig. 1 top). The five most intense bands correlate well
with the observed bands. The two strongest absorptions are
assigned to the out-of-plane deformations ( A ; ) of the hydrogen atoms (756cm-') and C atoms of the framework
(499 cm- I ) according to the calculations. The asymmetric
C-C stretching frequency ( E ' )calculated a t 1550 cm- may
correlate to the band observed at 1418 cm-'. The IR band
registered at 1456 cm- does not have an equivalent in the
calculated spectrum and probably results from a combination vibration. Two weak "doublets" are observed in the
region typical for C-H stretching frequencies; these correlate
well with the two calculated E' type IR absorptions at 3345
and 3237 cm- Matrix effects probably cause the splitting
of these bands.
Besides the IR spectrum discussed, the typical ESR spectrum of 2 should also be measureable. This was achieved
only with difficulty, as only very broad signals can be obtained in xenon matrices. A series of experiments with ketone
4 as the precursor and various argon/xenon ratios showed
that the ESR bands were less resolved with an increasing
proportion of xenon in the matrix.
A chemical structural proof of trimethylenemethane was
found in the isomerization of 2 back to 7 upon irradiation
(31 3 nm). This photoreaction is evident in the difference
spectrum (Fig. 1 bottom). The bands of the product 7 appear
as negative peaks, those of the starting material 2 as positive
peaks. Light with a wavelength of 254 nm transforms 2 into
s-trans-butadiene (6). Because of this photolability, a certain
stationary concentration of 2 cannot be exceeded (maximum
How does one explain the photoexcitation of 7, a compound that does not absorb in the UV range? We assume
that the photochemically or thermally generated halogen
Angew. ('hiwi. Inr. Ed. EngI. 1993. 32, No. 1
d" VCH
75 kcalmol-') is enough for the conversion of 7 into the
excited singlet state of 2,[16,' I which then undergoes intersystem crossing t o the triplet ground state. At 10 K the kinetic energy is too low to allow the back reaction to 7, once 2 is
If this description is true, irradiation in the presence of
halogen atoms may be a fundamentally new method for the
generation of highly reactive species in a xenon matrix. This
could be applied to cases for which the previous methods
(matrix photolysis of absorbing starting materials or the
combination flash pyrolysis/matrix isolation) cannot be applied.
Received: September 19, 1992 [Z5582IE]
German version: Angew. Chem. 1993, f05, 119
[I] M. J. S. Dewar. The Molecular Orhitul The0r.v oforgunic Molecules, McGraw-Hill, New York, 1969, p. 232 -236.
121 W. T. Borden, E. R. Davidson, J. Am. Chem. Soc. 1977, 99, 4587-4594.
[3] Review: G. Maier, Angew. Chem. 1988, 100,317-341; Angew. Chem. Int.
Ed. EngI. 1988, 27. 309-332. and references therein
141 K . Lanz, Diplomurbeit, Universitit Giessen. 1981. See also footnote 14 in
[S] a) P. Dowd. J. An?. Chem. Soc. 1966, 88. 2587-2589; b) P. Dowd, K .
Sachdev, ihid. 1967,89. 715-716; c) P. Dowd. A. Gold, K . Sachdev. ihid.
1968,90,2715- 2716; d) R. J. Baseman, D. W. Pratt. M. Chow. P. Dowd,
ihid 1976, Y8, 5726-5727; e) P. Dowd, M. Chow, ihid. 1977. Y9. 64386440; f) P. Dowd, M. Chow, Tetrahedron 1982,38, 79% 807.
[6] Reviews: a) P. Dowd, Arc. Chem. Res. 1972,5,242 -248; b) J. A. Berson.
rhrd. 1978, f1, 446-453; c ) W. T. Borden, E. R. Davidson, ihid. 1981, 14.
69- 76; d) J. A. Berson, Cupturuhle Dirudiculs o/ 1/11> Trimerh~li,nemrthun~
Series in Diradiculs (Ed.: W. T. Borden). John Wiley, New York. 1982,
pp. 151-194; e)D. A. Dougherty, Acc. Chem. Rex 1991.24. 88-94.
171 a) K. Takeda. H. Yoshida, K . Hayashi, S. Okamura, Bull. Inst. C'hrm. Re.\.
Kyoto Univ. 1967.45,5562;b) T. Yamaguchi, M. Irie. H. Yoshida. Chem.
Lert. 1973, 975-978; c) H. Yoshida, 0 . Edlund, Clirm. Phys. Lcrt. 1976,
42, I07 110; d) 0.Claesson, A. Lund, T. Gillbro, T. Ichikawa, 0. Edlund.
H. Yoshida. J. Clirm. Ph.v.9. 1980, 72, 1463-1470.
[Sl Cf. the Same result from the matrix irradiation of 4: A. KrantL. cited in
footnote 14 in [SeI.
191 P. S. Skell, R. G. Doerr, J. Am. Chem. Soc. 1967, 89, 4688-4692.
[lo] I n analogy to the generation of ally1 radical from ally1 iodide: G. Maier.
H. P. Reisenauer, 8 . Rohde, K. Dehnicke, Chem. 5er. 1983. ff6.732-740.
1111 Spectrometer: Bruker F T I R IFS-85; these bands could not be found with
the grating instrument we had used previously[4].
1121 757 and 511 c m - ' (DZP/SCF;corrected): C. P. Blahous 111. Y. Xie, H. E
Schaefer 111, J Chem. Pliys. 1990, 92, 1174 -1 179; these authors also refer
to the importance of the matrix IR spectrum of 2.
[13] Our first calculation (November 1987; UHF/6-31G*) gave values of 731
and 523 cm I . The most recent data (UMP2/6-31Gt) are listed in Table 1
We also calculated the spectra anticipated for the isotopomers of 2. Current efforts are directed toward the experimental determination of these
[14] The yield given is based on the calculated band intensities of 2 (UMP2/631G*) and 7 (MP2/6-31G*).
Verlugsgrsrllschufr nihH. W-6940 Weinheim. 1993
$ 10.00f .B/O
[I51 a) M. E. Fajardo, V. A. Apkarian, J, Chem. Phjs. 1986, 85. 5660-5681;
ihrd. 1988, 89, 4102-4123; rbicl. 1988, 89. 4124-4136. The absorption
spectra we measured correspond to the reported excitation spectra of the
initially formed exciplexes 8. The same is true of the emission spectra ofthe
triatomic species 9; b) I. Last, T. F. George, J. Chem. P h u . 1987, 86.37873794; I. Last, T. F. George, M. E. Fayardo, V. A. Apkarian, ihid. 1987.87.
5917 - 5927.
[I61 Triplet 2 is 27.5 kcalmol-’ higher in energy than 7. singlet 2 is
15.3 kcalmol-’ higher in energy than triplet 2: R. Janoschek, Chrm. Unscrer Zeit 1991, 25. 59-66, R. Janoschek, University of Graz. private
[17] A single-electron transfer (SET) mechanism for the ring opening of 7
cannot be assumed. The energy for the irradiation IS not enough to ionize
7 (IP = 9.57 eV; K. B. Wiberg. G. B. Ellison. J. J. Wendoloski. C . R. Brundle. N . A. Kuebler. J. A m . Chrn?.So<. 1976. 98. 7179-7187).
Didodecylsexithiophene-A Model Compound
for the Formation and Characterization of Charge
Carriers in Conjugated Chains**
redox states of 1; a dimerization of the radical cation 1‘+ to
(1);+ was also observed.
Controlled electrochemical and chemical oxidation (with
iron trichloride), and reduction (with potassium) led to the
different redox states of 1, which were characterized by cyclic
voltammetry. and absorption (Table 1) and ESR spectroscopy. The cyclic voltammogram exhibits two reversible
waves in the oxidation part (EY = 0.34V and E ; =
0.54 V[81) which correspond to a one-electron transfer in
each step leading to the radical cation 1“ and the dication
l ” , respectively (Fig. 1). The first oxidation potential of 1 is
close to that ofpolythiophene (E” = 0.30 Vr9]),which shows
only one extremely broad redox wave because of the chainlength distribution in the polymer. Since the alkyl side chains
improve the solubility, the reduction of 1 could also be examined. Two further reversible one-electron transfer steps were
evident in the reduction cycle of the cyclic voltammogram
( E : = - 2.27 V and E: = - 2.40 V) providing the radical
anion 1‘- and the dianion 1‘- (Fig. 1).
By Peter Bauerle,* Uwe Segelbacher, Kai-Uwe Caudl,
Dieter Huttenlocher, and Michael Mehring
Oligothiophenes are among the best investigated model
compounds for electrically conducting polymers.’’] The excellent properties, which in some respects surpass those of
the related polymers,[’] are attained by stepwise chemical
assembly leading to compounds with well-defined structures
and controlled chain and conjugation length. The stability of
the oligothiophenes in both their neutral and oxidized forms
allows the precise characterization of the electronic structure
and the charge carrier responsible for the conductivity along
the conjugated chains.13] The characterization of oligothiophenes in solution, in which the cooperative interactions
characteristic of the solid state are eliminated, is more diffcult because of their inherent reactivity in the oxidized state
(12 I
5 ) and their poor solubility with increasing chain length
(n2 6). However, when the reactive terminal positions141are
blocked or solubilizing alkyl groups are introduced to the
o l i g ~ m e r , the
[ ~ ~precise characterization of the oligomers in
solution is possible.
In this context we have synthesized 3””-4‘-didodecyI2,2’: 5‘,2”: 5”,2”’: 5“‘,2““:5””,2”“’-sexithiophene (I), whose
alkyl substituents in contrast to the compounds in previous
studies are fixed at defined positions, and whose structure
is established unambiguously by NMR spectroscopy and
X-ray structure analysis.“. I’ Sexithiophene 1 is the first compound of this type that can be oxidized to form a stable
dication and also reduced to form a stable dianion. We report here on the preparation and characterization of the
Dr. P. Biuerle, Dr. K.-U. Gaud1
lnstitut fur Organische’Chemie und Isotopenforschung der Universitit
Pfaffenwaldring 55. D-W-7000 Stuttgart 80 (FRG)
Dipl.-Phys. U. Segelbacher. DiplLPhys. D. Huttenlocher.
Prof. M. Mehring
2. Physikalisches Institut der Universitit
Pfaffenwaldring 57. D-W-7000 Stuttgart 80 (FRG)
Thiopfienes. Part 9. This research was supported by the Deutsche
Forschungsgemeinschaft (SFB 329) and the Bundesminister fur
Forschung und Technologie (TK 0325). We thank Dr. A. Grupp and Dr.
S. Sariciftci for helpful discussions. - Part 8 [h].
9‘; VCH
m h H , W-6940 Wemheim, 1993
Fig. 1. Cyclic voltammogram of 1 recorded with a scan rate of 100 mVs- The
oxidation was measured in dichloromethdne, the reduction in T H E The potentials are referenced to ferrocene (Fc)/Fc+
The stability of all the redox states allowed their characterization by absorption spectroscopy. In comparison to the
unsubstituted sexithiophene 2 (i,,,
432 nm, E =
2.87 eV),[31the neutral dialkylsexithiophene 1 absorbs as expected at a somewhat shorter wavelength (&, = 43 6 nm,
E = 2.97 eV), due to the steric interaction of the alkyl chains
with the conjugated i~ system. This interaction is also reflected in the crystal structure of 1 by the twisting of the relevant
thiophene rings by 10.8°.[71Two bands are observed in the
absorption spectrum of cation radical I * + , each with shoulders [sh] at higher energies ( E = 0.87, 1.14[sh], 1.60,
1.81[sh] eV) (Fig. 2 top); the structure and position of these
bands are almost identical to those of 2+ 13] (Table I), indicating that the conjugated system becomes more planar during the transition from 1 to 1”. Variable-temperature measurements show that the absorption spectrum changes with
decreasing temperature; only two of the transitions
( E = 1.60 and 0.87 eV) can be attributed to 1.’. These tran-
Table 1. Physical properties of the various redox states of I in comparison to 2 (values
in parentheses from ref. 131).
I”] la1
E , [eV] [b] 2.97 (2.87) 1.60 (1.59)
E2 Lev1 [bl
0.87 (0.X4)
1.81 (1.81 [c])
1.12 (0.98 [c])
1.47 (1.36)
1.31 (1.24)
[a] Redox potentials determined by cyclic voltammetry measured vs. Fc;Fc+.
[b] Transition energies determined from absorptions spectra. [c] These transitions were
previously assigned [3] to the monocation 2’+.
S ll).OO+.2SlO
Angrn. Chem. Ini. Ed. Ennl. 1993. 32. N o . I
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methylenecyclopropane, spectroscopy, matrix, halogen, detection, unexpected, results, photoisomerization, doped, trimethylenemethane
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