ArF and KrF lasers-induced gas-phase photolysis of selenophene and telluropllene: Extrusion of Te and Se and intramolecular 1,3-H shift competing with β-C-C cleavage in C4H4 residue

Article abstract of DOI:10.1021/jo991867m

ArF (193 nm) and KrF (248 nm) laser-induced photolysis of gaseous selenophene and tellurophene (C₄H₄M, M = Se and Te) has been examined. It is shown that, unlike thiophene and furan, selenophene and tellurophene cleave both M-C bonds and yield the elemental heteroatom (Se, Te), 1-buten-3- yne, and ethyne. The proposed mechanism involves an intermediate ·HC=CH- CH=CH· diradical that decomposes via two competitive pathways, namely, 1,3-H shift to 1-buten-3-yne and/β-cleavage to two molecules of ethyne. It is shown that the relative importance of the channels depends both on the energy of the photon and on the heteroatom. Specifically, the 1,3-H shift/β- cleavage ratios are 2.3 (193 nm, M = Se), 3.6 (248 nm, M = Se), 1.4 (193 nm, M = Te), and 10.5 (248 nm, M = Te). The inertness of the Te residuum and the high preference for the 1,3-H shift in KrF laser photolysis of tellurophene suggest that this photolysis can serve as a source of the C₄H₄ diradical for mechanistic studies.

Full text of DOI:10.1021/jo991867m

J . Org. Chem. 2000, 65, 2759-2762
2759
Ar F a n d Kr F La ser -In d u ced Ga s-P h a se P h otolysis of Selen op h en e
a n d Tellu r op h en e: Extr u sion of Te a n d Se a n d In tr a m olecu la r
1
4 4
,3-H Sh ift Com p etin g w ith â-C-C Clea va ge in C H Resid u e
,
†
‡
J osef Pola* and Akihiko Ouchi
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague,
Czech Republic, and National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba,
Ibaraki 305-8565, J apan
Received December 6, 1999
ArF (193 nm) and KrF (248 nm) laser-induced photolysis of gaseous selenophene and tellurophene
4 4
(C H M, M ) Se and Te) has been examined. It is shown that, unlike thiophene and furan,
selenophene and tellurophene cleave both M-C bonds and yield the elemental heteroatom (Se,
•
Te), 1-buten-3-yne, and ethyne. The proposed mechanism involves an intermediate HCdCH-CHd
•
CH diradical that decomposes via two competitive pathways, namely, 1,3-H shift to 1-buten-3-yne
and â-cleavage to two molecules of ethyne. It is shown that the relative importance of the channels
depends both on the energy of the photon and on the heteroatom. Specifically, the 1,3-H shift/â-
cleavage ratios are 2.3 (193 nm, M ) Se), 3.6 (248 nm, M ) Se), 1.4 (193 nm, M ) Te), and 10.5
(
248 nm, M ) Te). The inertness of the Te residuum and the high preference for the 1,3-H shift in
KrF laser photolysis of tellurophene suggest that this photolysis can serve as a source of the C
diradical for mechanistic studies.
4 4
H
In tr od u ction
Surprisingly, the photolysis of the congener rings con-
taining Se and Te remains essentially unknown. Thus,
the only reported data are the transient absorption spec-
The photolysis of furan1 and thiophene1e,2 has been
extensively studied in order to elucidate the mechanism
of these reactions. It has been demonstrated for both
3
tra in the flash photolysis of gaseous selenophene and
those from the UV laser photodeposition of Te from
compounds that the initial photochemical event is the
tellurophene in solution.4
formation of the Dewar isomers1
e,g,2e
and that the nature
In this work, we examined the photolysis of seleno-
phene and tellurophene in the gas phase using mono-
chromatic laser radiation at 193 and 248 nm and revealed
that these compounds photolytically cleave in a com-
pletely different way than do their O- and S-atom-con-
taining congeners. We report that selenophene and tel-
lurophene decompose via the same mechanism into the
heteroatom, ethyne, and 1-buten-3-yne and that the ratio
of these hydrocarbons is controlled by the energy of the
photon and by the heteroatom.
of the final products depends on the photolytic conditions.
With furan, the rearranged products, 2-cyclopropene-
carboxaldehyde and 2,3-butadienal (trapped as furan
Diels-Alder adducts), are produced in the liquid phase,1f
whereas the ring-contracted products, carbon monoxide
1
a-d
and cyclopropene (Hg-sensitized photolysis
), along
with other unsaturated hydrocarbons (direct photolysis1 ),
g,h
are the principal products in the gas phase.
With thiophene, the Dewar isomer (trapped as furan
Diels-Alder adducts) is produced in a solution of
thiophene in furan, whereas unsaturated hydrocarbons
Resu lts a n d Discu ssion
•
and carbon disulfide, arising via the postulated CHCH-
•
2a
The UV absorption spectrum of gaseous selenophene
shows an absorption band at 190 nm and a continuous
CHCHS and C
2
b-d
H
2
S intermediates or the detected
HCCS radical,2
2a
are produced in both direct and Hg-
5
absorption between 210 and 260 nm, and that of gaseous
sensitized photolysis in the gas phase.
tellurophene exhibits three major bands at 196, 220, and
These photolyses involve the extensive formation of
polymers, and their final products reveal the cleavage of
only one of the two C-X bonds.
2
75 nm, the maxima being shifted compared to those for
6
the n-hexane solution. Both compounds show sufficient
absorption in the region of ArF and KrF laser emission
(Figure 1) and can be photolyzed by laser radiation at
193 and 248 nm.
†
Academy of Sciences of the Czech Republic.
National Institute of Materials and Chemical Research.
‡
(1) (a) Srinivasan, R. J . Am. Chem. Soc. 1967, 89, 1758. (b)
Laser irradiation of gaseous selenophene and tel-
lurophene at these wavelengths results in the formation
of three hydrocarbons as the exclusive volatile products,
namely, 1-buten-3-yne, ethyne, and butadiyne, and in the
deposition of thin selenium and tellurium films.
Srinivasan, R. J . Am. Chem. Soc. 1967, 89, 4812. (c) Srinivasan, R.
Pure Appl. Chem. 1968, 16, 65. (d) Poquet, E.; Dargelos, A.; Chaillet,
M. Tetrahedron 1976, 32, 1729. (e) Rendal, W. A.; Clement, A.; Torres,
M.; Strausz, O. P. J . Am. Chem. Soc. 1986, 108, 1691. (f) Rendall, W.
A.; Torres, M.; Strausz, O. P. J . Org. Chem. 1985, 50, 3034. (g) Price,
D.; Ratajczak, E.; Sztuba, B.; Sarzynski, D. J . Photochem. 1987, 37,
2
73. (h) Hiraoka, H.; Srinivasan, R. J . Chem. Phys. 1968, 48, 2185.
2) (a) Wiebe, H. A.; Heicklen, J . Can. J . Chem. 1969, 47, 2965. (b)
Krishnamachari, S. L. N. G.; Ramsay, D. A. Discuss. Faraday Soc.
981, 71, 205. (c) Venkatasubramanian, R.; Krishnamachari, S. L. N.
(
(3) Krishnamachari, S. L. N. G.; Veinkitachalam, T. V. Chem. Phys.
Lett. 1979, 67, 69.
(4) Ouchi, A.; Yamamoto, K.; Koga, Y.; Pola, J . J . Mater. Chem. 1999,
1
G. Indian J . Pure Appl. Phys. 1991, 29, 697. (d) Cooper, D. L. Chem.
Phys. Lett. 1981, 81, 479. (e) Rendall, W. A.; Torres, M.; Strausz, O. P.
J . Am. Chem. Soc. 1985, 107, 723. (f) Krishnamachari, S. L. N. G.;
Veinkitachalam, T. V. Chem. Phys. Lett. 1978, 55, 116.
9, 563.
(5) Trombetti, A.; Zauli, C. J . Chem. Soc. A 1967, 1106.
(6) Fringuelli, F.; Taticchi, A. J . Chem. Soc., Perkin Trans. 1 1972,
199.
1
0.1021/jo991867m CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/07/2000

2
760 J . Org. Chem., Vol. 65, No. 9, 2000
Pola and Ouchi
depletion at each laser irradiation wavelength reveals
that these two photolyses are also one-photon processes,
as the slope of the dependence for a double-logarithmic
plot is close to unity in each case (Figure 3). The initial
quantum yield of both photolyses was estimated as ca.
0
.6.
The relative amounts of the hydrocarbon products
essentially do not change with the photolysis progress,
they depend slightly on the laser fluence, and they
depend significantly on the laser wavelength. Thus, the
hydrocarbon distribution (in relative mole %) in the ArF
-
2
laser photolysis at fluences of 7-40 mJ cm s1-buten-
-yne, 36-42; ethyne, 54-60; and butadiyne, ∼4sdiffers
significantly from that observed for the KrF laser pho-
3
-
2
tolysis at fluences of 8-60 mJ cm s1-buten-3-yne, 81-
8
4; ethyne, 15-17; and butadiyne, 1-2.
As with the photolysis of selenophene, these relative
F igu r e 1. UV absorption spectrum of (a) gaseous tellurophene
amounts are also not subject to changes when the
photolyses are conducted in 13-26 kPa of hydrogen or
nitrogen.
XPS measurements of the deposited films are in accord
with the elemental tellurium having a Te 3d5/2 electron
binding energy of 573.2 eV.
P h otolysis Mech a n ism . The observed products of the
4 4
laser photolysis of C H M (M ) Se and Te) compounds
can be rationalized by a mechanism for a 2-fold cleavage
(0.5 kPa) and (b) gaseous selenophene (0.1 kPa).
The measured amounts of the three hydrocarbon
products account for all of the depletion of both sele-
nophene and tellurophene. This shows that no hydrocar-
bon undergoes photolysis after its formation. (Ethyne and
1
-buten-3-yne do not absorb at 248 nm, and their
independent photolysis with 193-nm radiation revealed
that irradiation times at least 2 orders of magnitude
longer than those used for the photolysis of tellurophene
and selenophene were required for a noticeable decay in
either of them.)
of the C-M bonds that involves transient formation of a
•
•
HCdCH-CHdCH diradical undergoing both 1,3-H shift
(
into 1-buten-3-yne) and â-C-C cleavage (into two mol-
ecules of ethyne). The intermediacy of the seemingly
elusive C diradical gains support from the thermo-
Selen op h en e. The 193- and 248-nm photolyses of
selenophene yield 1-buten-3-yne and ethyne as major
products and butadiyne as a minor product. Concomi-
tantly, a pink film is deposited from the gas phase onto
the reactor surface. An examination of the laser-fluence-
dependent depletion of selenophene at each laser irradia-
tion wavelength shows that the slope of this dependence
for a double-logarithmic plot is close to unity (Figure 2).
This confirms that both photolyses are one-photon pro-
cesses. The initial quantum yield of both processes was
assessed as 0.5-0.6.
4
H
4
8
chemical data, from the identification of the cis-butadi-
enediyl diradical as a stable product on the singlet
potential energy surface for the interconversions of C H
4 4
species (MINDO/3 calculations, ref 9), and from ab initio
10
investigations of ethyne reactions. A plausible route for
the minor formation of butadiyne involves cleavage of the
ethyne C-H bond and subsequent recombination of the
•
two HC
2
radicals. The steps for the formation of the
major products are given in Scheme 1.
The relative importance of the 1,3-H shift and the
The hydrocarbon product distribution (in relative mole
â-C-C cleavage in the C
1
0
4
H
4
diradical is reflected by the
-buten-3-yne/(0.5 × ethyne) ratios. This ratio is 2.3 (
.3 for the 193-nm photolysis of selenophene, 3.6 ( 0.3
%
) in the ArF laser photolysis (fluence, 6.6-55 mJ
-2
cm )s1-buten-3-yne, 47-53; ethyne, 41-47; and butadi-
yne, 5-6sis essentially independent of the laser fluence
and the photolysis progress. The product distribution in
the KrF laser photolysis is also independent of the
photolysis progress, but somewhat different for fluences
(
low-fluence range) and 2.0 ( 0.2 (high-fluence range) for
the 248-nm photolysis of selenophene, 1.4 ( 0.2 for the
1
2
93-nm photolysis of tellurophene, and 10.5 ( 0.5 for the
48-nm photolysis of tellurophene. These values show
-
2
lower than 55 mJ cm (1-buten-3-yne, 58-66; ethyne,
that the 1,3-H shift is, in all instances, more feasible than
the â-C-C cleavage, that the 1,3-H shift is enhanced with
the 248-nm photons, and that this enhancement is more
pronounced with tellurophene than with selenophene.
The observed preference for the 1,3-H shift over the
2
1
7-35; and butadiyne, 5-7) and for fluences higher than
-
2
00 mJ cm (1-buten-3-yne, 40-50; ethyne, 40-50; and
butadiyne, 8-10).
These relative amounts are not affected by the addition
of an inert gas (13-26 kPa of hydrogen or nitrogen). XPS
measurements of the deposited films confirmed the
presence of elemental selenium (binding energy of the
4 4
â-C-C cleavage in the postulated C H diradical under
photolytic conditions differs from the relative importance
of these pathways theoretically predicted for a thermally
3
d electrons, 55.5 eV), and the UV spectrum of the films
9
equilibrated system, for which the (one-step) conversion
showed a broad absorption band with the maximum at
of the cis-butadienediyl diradical to ethyne is energeti-
cally more feasible than the (multistep) conversion to
1
90 nm typical for amorphous selenium.7
Tellu r op h en e. The 193- and 248-nm photolyses of
tellurophene yield the same three hydrocarbons as those
of selenophene (1-buten-3-yne, ethyne, and butadiyne)
and produce black films on the reactor surface. An
examination of the laser-fluence-dependent tellurophene
(
8) Zhang, M.-Y.; Wesdemiotis, C.; Marchetti, M.; Danis, P. O.; Ray,
J . C.; Carpenter, B. K.; McLafferty, F. W. J . Am. Chem. Soc. 1989,
111, 8341 and references therein.
(
9) Kollmar, H.; Carrion, F.; Dewar, M. J . S.; Bingham, R. C. J . Am.
Chem. Soc. 1981, 103, 5292.
10) Gey, E.; Wiegratz, S.; K u¨ hnl, W.; Ondruschka, B.; Saffert, W.;
Zimmermann, G. J . Mol. Struct. (THEOCHEM) 1989, 184, 69.
(
(7) Vasko, A. J . Non-Cryst. Solids 1990, 3, 225.

Photolysis of Selenophene and Tellurophene
J . Org. Chem., Vol. 65, No. 9, 2000 2761
F igu r e 2. Depletion of selenophene in (a) ArF and (b) KrF laser photolysis as a function of laser fluence.
F igu r e 3. Depletion of tellurophene in (a) ArF and (b) KrF laser photolysis as a function of laser fluence.
Sch em e 1
The observed occurrences of the 1,3-H shift and the
â-C-C cleavage are related to the photon energy and the
mass of the M atom. The energy delivered by the 193-
and 248-nm photons corresponds, respectively, to ca. 620
-
1
and 480 kJ einstein , while the dissociation energy of
both the Se-C13 and the Te-C bonds can be speculated
14
-
1
to be ca. 250 kJ mol . It is thus conceivable that
absorption of the 248-nm photons is barely sufficient to
break both M-C bonds, whereas absorption of the 193-
nm photons delivers ca. 100 kJ mol more energy than
needed for this cleavage. We assume that the remarkably
greater enhancement of the 1,3-H shift when using the
193-nm photons instead of the 248-nm photons with
tellurophene compared to that with selenophene may
-
1
1
-buten-3-yne, which takes place via several stable
intermediates (cyclobutadiene and bicyclo[1.1.0]butenes).
It can be only reconciled with another, much less feasible
9
(higher-energy), pathway to 2 C
2
H
2
, as predicted from
the vibrationally excited (trans-butadienediyl) C
4
4
H dirad-
ical. The latter reaction can conceivably take place with
hot molecules produced in the intense laser beam, but
an explanation of the observed relative occurrences of
both laser-photolytic pathways is not straightforward
because of the possible involvement of both nonequilib-
rium laser-induced conditions and energy-randomized
thermal processes. The accomplished electronic transi-
tions (π-π* excitation of the diene system or a system
4 4
reflect a different energy partitioning between the C H
radical and the two different M atoms, as it can be
assumed that the greater amount of energy being carried
away by the heavier Te atom leaves the C H diradical
4 4
less energetic and hence less prone to the higher-energy
â-C-C cleavage.
The absence of an effect of excess nitrogen and hydro-
gen gases on the relative importance of the 1,3-H shift
4 4
and â-C-C cleavage indicates that the C H diradical is
5
,6,11
involving the M atom,
possible interference of nearby
1
2
Rydberg states ) and also post-excitation events can
differ with each M and wavelength.
(
12) Nyulaszi, L.; Veszpremi, T. Acta Chim. Hung. 1993, 130, 811
and references therein.
13) Batt, L. In The Chemistry of Organic Selenium and Tellurium
(
(
11) (a) Sch a¨ fer, W.; Schweig, A.; Gronowitz, S.; Taticchi, A.;
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K.,
1986; Vol. 1.
(14) (a) Mullin, J . B.; Irvine, S. J . C. J . Vac. Sci. Technol., A 1986,
4, 700. (b) McAllister, T. J . Cryst. Growth 1989, 96, 552.
Fringuelli, F. J . Chem. Soc., Chem. Commun. 1973, 541. (b) Kuder, J .
E. In Organic Selenium Compounds: their chemistry and biology;
Klayman, D. L., G u¨ nther, W. H. H., Eds.; Wiley: New York, 1973.

2
762 J . Org. Chem., Vol. 65, No. 9, 2000
Pola and Ouchi
too short-lived to undergo stabilizing collisions with N
and H . This view is in accord with the fact that the
photolyses in the presence of hydrogen do not lead to any
2
had two sidearms, one fitted with a rubber septum and the
other connecting to a standard vacuum manifold. The reactor
accommodated metal, quartz, and KBr substrates, which,
covered with solid material deposited in the course of photoly-
sis experiments, were transferred for measurements of their
properties by UV and photoelectron spectroscopy. The laser
beam of different fluences was passed through a slit, and its
output energy was measured by a joulemeter coupled with a
10-MHz storage oscilloscope.
The progress of the photolysis was monitored directly in the
reactor by infrared spectroscopy and, after expansion of helium
into the reactor, also by gas chromatography [chromatographs
equipped with a 60 m × 0.25 mm i.d. Neutra Bond-1 capillary
column and with a packed 2 m × 3 mm i.d. Unipak S SUS
column, programmed temperature (30-150 °C), helium carrier
gas]. The sampling was conducted using a gastight syringe.
Both chromatographs were equipped with flame-ionization
detectors (FID) and coupled with a chromatograph data
processor. The depletion of selenophene and tellurophene was
followed by infrared spectroscopy using diagnostic bands at
2
detectable amounts of hydrocarbons (e.g., n-butane,
1
-butene, propene, and ethene) that might have been
formed through reactions (e.g., H-abstraction, radical
disproportionation, recombination, and cleavage) of the
C
4
H
4
diradical with H
The results reported here on the preference of the 1,3-H
shift over â-C-C-cleavage in the C diradical produced
2
.
4
H
4
from photolytically activated selenophene and tellurophene
reveal interesting aspects of the laser photolysis of heavy-
atom-containing organic compounds. The results also
suggest that the mechanism of the 1,3-H shift after
electronic excitation may be different than the multistep
9
channel operating in the ground electronic state. The
observed dependence of the competition between the two
photochemical paths on the photon energy and the mass
of the heavy atom can be used for various synthetic
applications in which some photochemical event can be
enhanced or diminished by proper choice of the hetero-
atom and photolysis wavelength. The system itself is
-
1
-2
-1
-1
6
6
85 cm (absorption coefficient, 3.3 × 10 kPa cm ) and
-1 -2 -1
(absorption coefficient, 3.4 × 10 kPa-1 cm ),
88 cm
respectively. The photolytic products, i.e., ethyne, 1-buten-3-
yne, and buta-1,3-diyne, were monitored by a combination of
FTIR spectral and GC methods. The formation of ethyne and
1-buten-3-yne was followed by FTIR spectroscopy using diag-
nostic absorptions at 731 and 619 cm . Absorption coefficients
determined for authentic samples of ethyne and 1-buten-3-
•
suited for photochemical generation of the HCdCH-
-1
•
CHdCH diradical and for studies of reactions of this
diradical with specifically designed reactive scavengers.
The leaving Te and Se atoms are inert residues and will
not interfere with a deliberately added reactant that will
-
2
-2
yne at these wavenumbers were 2.5 × 10 and 2.2 × 10
-1
-1
kPa cm , respectively. Relative amounts of ethyne, 1-buten-
-yne, and buta-1,3-diyne were also determined by gas chro-
3
•
•
react exclusively with the HCdCH-CHdCH diradical
matography (Unipak S SUS column), considering that the FID
response factors of all three compounds are virtually the
same.15 The identification of all three products was performed
by GC-MS analysis using the Unipak S SUS column.
For the examination of the dependence of selenophene and
tellurophene depletion on the laser fluence, the entrance
reactor window was cleaned before each experiment. This
helped to circumvent the data irreproducibility caused by a
gradual decay of laser power within the reactor, which was
due to the Se and Te film buildup on the reactor window.
The UV spectra of gaseous selenophene, tellurophene,
ethyne, 1-buten-3-yne, and buta-1,3-diyne were recorded as
they were admitted to the reactor. UV spectra were also
recorded for the solid films as they were deposited on the
quartz substrates housed in the reactor prior to irradiation.
Properties of the photodeposited solids were also analyzed
by X-ray photoelectron spectroscopy.
and will not be exposed to other intervening steps.
Con clu sion
ArF and KrF laser-induced photolyses of selenophene
and tellurophene are revealed to occur via the extrusion
of the Se and Te atoms and via the two different
fragmentation channels (1,3-H shift and â-C-C cleavage)
•
•
of the postulated HCdCH-CHdCH diradical. The rela-
tive importance of these channels is dependent on the
excitation energy of the photon (193 or 248 nm) and on
the heteroatom.
The decomposition mode of tellurophene and sele-
nophene is different from that of furan and thiophene,
which cleave just one of the two available O-C or S-C
bonds.
Selenophene (Aldrich, 97% purity) was a commercial sample.
Tellurophene (>96% purity) was synthesized by a reported
The inertness of the selenium and particularly tel-
lurium residuum suggests that the photolysis of sele-
nophene and tellurophene can serve as an efficient
synthesis using bis(trimethylsilyl)-1,3-butadiyne and in situ
generated sodium telluride.16 1-Buten-3-yne (98% purity) was
obtained as previously reported.17 Ethyne (>99.9% purity) was
•
•
method for the generation of the HCdCH-CHdCH
purchased from Toho Asechiren K. K.
diradical in mechanistic studies.
Ack n ow led gm en t. The authors are grateful to Dr.
Z. Bastl for the XPS analyses. The work was supported
by the Ministry of Education, Youth and Sports of the
Czech Republic (Grant ME 191) and by the Science and
Technology Agency (J apan).
Exp er im en ta l Section
Laser photolysis experiments were carried out on gaseous
samples of selenophene (2.7 kPa) and tellurophene (0.5 kPa)
using an excimer laser (ArF and KrF radiation) operating with
a repetition frequency of 10 Hz at 193 and 248 nm, respec-
tively. The samples were irradiated in a reactor consisting of
two orthogonally positioned tubes (both 3 cm in diameter), one
J O991867M
(
(
(
15) Dietz, W. A. J . Gas Chromatogr. 1967, 5, 68.
16) Lohner, W.; Praefcke, K. Chem. Ber. 1978, 111, 3745.
17) Torneng, E.; Nielsen, C. J .; Klaeboe, P.; Hopf, H.; Priebe, H.
(
(
13 cm in length) furnished with KBr windows and the other
9 cm in length) equipped with quartz windows. The reactor
Spectrochim. Acta, A 1980, 36, 975.