UV-induced photolysis of diethyl selenium and diethyl tellurium: extrusion of selenium and tellurium via molecular elimination of ethene

Article abstract of DOI:10.1016/S0022-328X(01)00818-X

KrF laser-induced photolysis of gaseous (C2H5)2M (M = Se, Te) compounds is shown to be a one-photon process controlled by the cleavage of both C-M bonds. It is dominated by the molecular elimination of ethene and yields elemental selenium and tellurium.

Full text of DOI:10.1016/S0022-328X(01)00818-X

Journal of Organometallic Chemistry 629 (2001) 93–96
www.elsevier.com/locate/jorganchem
UV laser-induced photolysis of diethyl selenium and diethyl
tellurium: extrusion of selenium and tellurium via molecular
elimination of ethene
Josef Pola a,*, Akihiko Ouchi b
a Laser Chemistry Group, Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Roz6ojo6a Str. 135,
16502 Prague 6, Czech Republic
b National Institute of Materials and Chemical Research, AIST, MITI, Tsukuba, Ibaraki 305, Japan
Received 18 February 2001; accepted 29 March 2001
Abstract
KrF laser-induced photolysis of gaseous (C2H5)2M (M=Se, Te) compounds is shown to be a one-photon process controlled
by the cleavage of both CꢀM bonds. It is dominated by the molecular elimination of ethene and yields elemental selenium and
tellurium. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Diethyl selenium; Diethyl tellurium; Laser photolysis; Molecular mechanism
Less attention has been paid to the photolysis of the
congener (C2H5)2M molecules with M=Se, Te. Thus,
1. Introduction
the photolysis of diethyl selenium has not been studied
The gas-phase photolysis of diethyl ether [1] and
at all and that of diethyl tellurium, although examined,
diethyl sulfide [2] has been examined to elucidate the
is poorly understood as to the mechanism. It was
mechanism of these reactions. It has been shown that
shown that laser photodissociation of (C2H5)2Te
the primary process is the fission of the CꢀO (CꢀS)
through a one-photon absorption at 193 and 248 nm [3]
’
’
’
bond to yield C2H5 and C2H5O (C2H5S ) radicals, and
that final photolytic products are accounted for mostly
by radical and not molecular steps. The C1–C4 hydro-
carbons, ethanol, acetaldehyde and ethoxybutanes were
produced from diethyl ether, and C2, C4 hydrocarbons,
ethanethiol, 4-methyl-3-thiahexane and methyl-3,5-
dithiaheptane were obtained from diethyl sulfide. The
O- and S-containing products are compatible with the
perseverance of the ethoxyl and ethylthiyl radicals and
confirm the cleavage of only one of the carbon–het-
eroatom bonds.
and through a multiphoton absorption at 358–395 nm
[4] yields ground-state Te atoms; these were presumed
to originate via the homolysis of the TeꢀC bond (Eq.
(1)).
3
’
(C2H5)2Te“Te( P2)+2C2H5
(1)
A more complicated photochemical scheme involving
C2H5Te transient was surmised for mercury–xenon
lamp-induced photodeposition of Te [5]. No Te-con-
taining short-lived species were, however, detected by
dye-laser mass spectroscopy [6], despite that a very high
sensitivity of this technique was proved with other alkyl
metals [6,7]. The formation of the observed Te photo-
product can be, in principle, rationalised in terms of
two different mechanisms: (a) the assumed TeꢀC ho-
molysis (Eq. (1)) and (b) a sequence of a two-fold
b-elimination of ethene [8] and elimination of H2 from
unstable hydrogen telluride (Scheme 1). Until now,
neither transient nor final stable hydrocarbon products
Scheme 1.
* Corresponding author. Tel.: +420-2-20390308; fax: +420-2-
20920661.
E-mail address: pola@icpf.cas.cz (J. Pola).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00818-X

94
J. Pola, A. Ouchi / Journal of Organometallic Chemistry 629 (2001) 93–96
by a Gentec ED-500 joulmeter connected to a Tek-
tronix T912 10 MHz storage oscilloscope.
The progress of the photolysis was monitored by
FTIR spectroscopy (a Shimadzu FTIR 4000 spectrome-
ter) using absorption bands at 1233 cm−1 ((C2H5)2Se)
and 1198 cm−1 ((C2H5)2Te), and by gas chromatogra-
phy on a Gasukuro Kogyo 370 chromatograph (pro-
grammed temperature 30–150°C), a 60 m long capillary
(Neutra Bond-1) and 2 m long SUS Unipak S columns.
Both chromatographs were equipped with flame-ionisa-
tion detectors and connected with a Shimadzu CR 5A
Chromatopac data processor. The gaseous samples
were introduced by a gas-tight syringe (Dynatech Preci-
sion Sampling). The photolytic products were identified
by the gas chromatography using the comparison of
retention times with those of authentic samples.
For the examination of the dependence of (C2H5)2Se
and (C2H5)2Te depletion on the laser fluence, the en-
trance reactor window was cleaned before each experi-
ment. This helped to circumvent the data
irreproducibility caused by a gradual decay of laser
power within the reactor, which was due to some
formation of Se and Te films on the reactor window.
The quantum yields of both processes were deter-
mined on the basis of initial depletion of (C2H5)2M (as
measured by IR spectroscopy) and the amount of laser
energy absorbed in (C2H5)2M (as measured by the
Gentec ED-500 joulmeter).
Fig. 1. UV absorption spectrum of (a) (C2H5)2Se and (b) (C2H5)2Te.
Conditions: 1 Torr, pathlength 9 cm.
of the gas-phase (C2H5)2Te photolysis have been iden-
tified, despite that the differentiation between these two
pathways can be made on the basis of the determined
yields of hydrocarbon products and consideration of
relative rate constants for combination and dispropor-
’
tionation of the C2H5 radical [9].
Diethyl selenium [14] and diethyl tellurium [15] were
prepared using the reported procedures and distilled as
a fraction with b.p. 106°C and 82°C/100 Torr, respec-
tively. Their purity (better than 98%) was checked by
In conjunction with our previous studies on the
gas-phase UV laser photolysis of ketones [10],
organosilicon [11], organogermanium [12] compounds
and selenophene and tellurophene [13], we explored in
this work KrF laser photolysis of diethyl selenium and
diethyl tellurium. We reveal that these compounds pho-
tolytically cleave to elemental selenium and tellurium
and that the heteroatom extrusion occurs mostly by a
molecular elimination of ethene.
1
chromatography and H-NMR spectroscopy.
3. Results and discussion
The absorption spectrum of diethyl selenium consists
of absorption bands at 204, 210 and 230 nm and that of
diethyl tellurium (almost coincident with that reported
[3,16]) shows absorption bands at 201, 217 and 246 nm
(Fig. 1). The absorptivity at 248 nm of diethylselenium
(3×10−3 Torr−1 cm−1) is significantly lower than that
of diethyl tellurium (1.5×10−1 Torr−1 cm−1).
The KrF laser photolysis of diethyl selenium results
in an instant formation of a white fog and that of
diethyl tellurium affords a dark fog, both of which
descend onto the reactor walls when the irradiation is
ceased. The white fog creates films that are initially
white in colour and later pink, whereas the dark fog
grows into a dark powder. XPS measurements proved
that these materials correspond to elemental selenium
and tellurium. Apart from these solid products, various
C1–C4 hydrocarbons are produced. The presence of
these hydrocarbons and that of elemental selenium and
tellurium, but not of any Se- and Te-containing organic
2. Experimental
Laser photolysis experiments were performed on
gaseous samples of (C2H5)2M (M=Se, Te) by using an
LPX 210i (KrF) laser operating at 248 nm with a
repetition frequency 10 Hz. The samples of (C2H5)2Se
(11 Torr) or (C2H5)2Te (8 Torr) in helium (total pres-
sure 790 Torr) were irradiated in a reactor (140 ml in
volume) which was equipped with a sleeve with a
rubber septum and PTFE valve, and consisted of two
orthogonally positioned Pyrex tubes (both 3 cm in
diameter), one (9 cm long) fitted with two quartz
windows and the other (13 cm long) furnished with two
NaCl windows. The laser beam of different fluences
(full-width at half-maximum typically 23 ns) effective
on the area of 2.6 cm2 was monitored for energy output

J. Pola, A. Ouchi / Journal of Organometallic Chemistry 629 (2001) 93–96
95
compounds, reveals that photolysis of (C2H5)2M (M=
Se, Te) can be rationalised by cleavage of both CꢀM
bonds.
propene (0–1.0), methane (1–5) and ethyne (0–1) —
does not noticeably change within the ca. 10–100%
photolysis progress in the range of fluence, 30–210
mJ cm−2
.
3.1. Photolysis of diethyl selenium
3.2. Photolysis of diethyl tellurium
The initial quantum yield of the laser photolysis (the
number of (C2H5)2Se molecules decomposed by one
absorbed photon) was estimated as 0.08–0.10. The
slope of the initial (C2H5)2Se depletion versus laser
fluence in a double-logarithmic plot is very close to 1
(Fig. 2), which indicates that the photolysis is a one-
photon process. Apart from the white films deposited
on the reactor walls, the photolysis yields hydrocarbons
whose distribution (in relative mol%) — ethene (80–
86), n-butane (8–13), ethane (4–6), propane (0.5–2.0),
The initial quantum yield of the laser photolysis (the
number of (C2H5)2Te molecules decomposed by one
absorbed photon) was estimated as 0.4–0.5. The slope
of the initial (C2H5)2Te depletion versus laser fluence in
a double-logarithmic plot is very close to 1 (Fig. 2).
This reveals that the photolysis is a one-photon process
as assessed earlier for KrF laser photolysis of
(C2H5)2Te in the absence and excess of hydrogen [3,4].
Apart from the dark dust collected on the reactor
bottom, the photolysis yields hydrocarbons whose dis-
tribution (in relative mol%) — ethene (62–68), n-bu-
tane (27–20), ethane (8–10), propane (0.5–2.0),
propene (0–1.0) and methane (0–2) — does not no-
ticeably change within the ca. 10–100% photolysis pro-
gress in the range of fluence, 55–160 mJ cm−2
.
3.3. Photolysis mechanism
We assume that the observed high amounts of
ethene, n-butane and ethane (and the insignificant
amounts of other hydrocarbons) are consistent with the
one-photon absorption process in which the energy
delivered by the photons at 248 nm (corresponding to
ca. 480 kJ Einstein−1) is barely enough to remove both
ethyl groups from Se and Te. The average bond energy
for the SeꢀC [17] and TeꢀC [5,18] bonds in (C2H5)2M
has been speculated as being ca. 250 kJ mol−1. It is
thus conceivable that the formed hydrocarbon frag-
ments may possess only very small excess energy and
that they are deactivated by excess helium gas. The
observed small amounts of propene, propane and meth-
Fig. 2. Dependence of initial rate of (a) (C2H5)2Se and (b) (C2H5)2Te
photolysis on laser fluence.
’
ane reflect an unimportant role of cleavage of the C2H5
’
radical and of reactions of the CH3 radical (addition to
C2H4, cross-disproportionation and combination with
’
Fig. 3. Hydrocarbon product yield (in relative mole percent) at
different stages of (a) (C2H5)2Se photolysis with fluence 127 mJ cm−2
and (b) of (C2H5)2Te photolysis with fluence 100 mJ cm−2. n-C4H10
(ꢀ); C2H6 (ꢁ); C2H4 (ꢂ).
the C2H5 radical).
The distribution of the most significant hydrocarbon
products — ethene, ethane and n-butane — and
their invariant ratios in different stages of the photoly-
sis (Fig. 3) clearly show that the homolysis of (C2H5)2M
into M atoms and two ethyl radicals (Eq. (1)) cannot be
regarded as a major photolytic pathway. In accordance
’
with the known disproportionation (2C2H5 “C2H4+
’
C2H6)/combination (2C2H5 “n-C4H10) rate ratio, kd/
kc, for the ethyl radical (ꢀ0.13, [9]), the clean MꢀC
homolysis would have yielded n-butane in a large ex-
cess over equal amounts of ethane and ethene.
The observed dominance of ethene over both n-bu-
tane and ethane thus reveals that more than 90% of
ethene in both photolyses must be produced via a
Scheme 2.

96
J. Pola, A. Ouchi / Journal of Organometallic Chemistry 629 (2001) 93–96
References
different channel, i.e. via the b-elimination (Scheme 2)
taking place via a four-centre transition state (I).
Although the presumed intermediary species were not
observed, this molecular mechanism is in keeping with
the elusive nature of the RMH species [6] and with UV
photolysis of the H2M (M=Se [19], Te [20]) com-
pounds yielding selenium and tellurium.
The distribution of the final products thus reveals
that the b-elimination is a dominating route and that
intrusion of radical processes is small. The ascertained
mechanism of the laser photolysis of the (C2H5)2M
compounds cannot be compared to the mechanism of
homogeneous thermal decomposition of these com-
pounds, since the latter has been only studied under
conditions not circumventing heterogeneous catalysis.
In this process, a computer kinetics simulation revealed
that the b-elimination from (C2H5)2Te is insufficient to
describe experimental observations [18]. It was also
reported that the thermolysis of diethyl tellurium
catalysed by heated substrate or by deposited Te yields
slightly higher amounts of ethene relative to ethane and
takes place as a blend of almost equally important
b-elimination of ethene from (C2H5)2Te and expulsion
of ethane from intermediary C2H5TeH [21].
[1] (a) J.C. Calvert, J.N. Pitts, Photochemistry, Wiley, New York,
1966 (p. 448);
(b) C.A.F. Johnson, W.M.C. Lawson, J. Chem. Soc. Perkin
Trans. 2 (1974) 353;
(c) H. Mikuni, M. Takahasi, J. Photochem. 16 (1981) 179.
[2] (a) C.S. Smith, A.R. Knight, Can. J. Chem. 51 (1973) 780;
(b) C.S. Smith, A.R. Knight, Can. J. Chem. 54 (1976) 1290;
(c) Z.-X. Ma, C.-L. Liao, H.-M. Yin, C.Y. Ng, S.-W. Chin, N.L.
Ma, W.-K. Li, Chem. Phys. Lett. 213 (1993) 250.
[3] (a) J.J. Zink, P.D. Brewer, J.E. Jensen, G.L. Olson, L.W. Tutt,
Appl. Phys. Lett. 52 (1988) 1434;
(b) J.E. Jensen, P.D. Brewer, G.L. Olson, L.W. Tutt, J.J. Zink,
J. Vac. Sci. Technol. A 6 (1988) 2805;
(c) Y. Fujita, S. Fujii, T. Iuchi, J. Vac. Sci. Technol. A 7 (1989)
276.
[4] P.D. Brewer, Chem. Phys. Lett. 141 (1987) 301.
[5] J.B. Mullin, S.J.C. Irvine, J. Vac. Sci. Technol. A 4 (1986) 700.
[6] Y. Zhang, M. Stuke, Mater. Res. Soc. Symp. 131 (1989) 375.
[7] Y. Zhang, M. Stuke, Chem. Phys. Lett. 149 (1988) 310.
[8] C. Eaborn, M.M.S. Mahmoud, R. Taylor, J. Chem. Soc. Perkin
Trans. 2 (1982) 1313.
[9] M.J. Gibian, R.C. Corley, Chem. Rev. 73 (1973) 441.
[10] (a) J. Pola, R. Taylor, Tetrahedron Lett. 17 (1994) 2799;
(b) J. Pola, Y. Koga, A. Ouchi, Tetrahedron 53 (1997) 3757;
(c) J. Pola, A. Ouchi, K. Saito, K. Ishikawa, Y. Koga, Chem.
Phys. Lett. 262 (1996) 279.
[11] (a) J. Pola, J. Parsons, R. Taylor, J. Organomet. Chem. 489
(1995) C9;
In conclusion, the gaseous diethyl selenium and di-
ethyl tellurium photolytically cleave in a completely
different way than do their O- and S-atom containing
congeners. The photolysis of (C2H5)2M (M=Se, Te)
occurs via a two-fold four-centre molecular expulsion
of ethene, the paths not assumed in the photolysis of
diethyl tellurium earlier [3,4]. The reaction is promising
in the chemical vapour deposition of pure selenium and
tellurium films and nanoparticles and can find applica-
tion in microelectronics and photothermal optical
recording.
(b) J. Pola, A. Ouchi, M. Urbanova, Y. Koga, Z. Bastl, J. Subrt,
J. Organomet. Chem. 575 (1999) 246;
(c) J. Pola, R. Taylor, J. Organomet. Chem. 446 (1993) 13.
[12] (a) J. Pola, J. Parsons, R. Taylor, J. Chem. Soc. Faraday Trans.
88 (1992) 1637;
(b) J. Pola, R. Taylor, J. Organomet. Chem. 437 (1992) 271.
[13] J. Pola, A. Ouchi, J. Org. Chem. 65 (2000) 2759.
[14] M.L. Bird, F. Challenger, J. Chem. Soc. (1942) 570.
[15] M.P. Balfe, C.A. Chaplin, H. Phillips, J. Chem. Soc. (1938) 341.
[16] S.J.C. Irvine, J.B. Mullin, D.J. Robbins, J.L. Glasper, J. Elec-
trochem. Soc. 132 (1985) 968.
[17] L. Batt, in: S. Patai, Z. Rappoport (Eds.), The Chemistry of
Organic Selenium and Tellurium Compounds, vol. 1, Wiley,
Chichester, 1986.
[18] T. McAllister, J. Cryst. Growth 96 (1989) 552 (and references
therein).
Acknowledgements
[19] D.C. Dobson, F.C. James, I. Safarik, H.E. Gunning, O.P.
Strausz, J. Phys. Chem. 79 (1975) 771.
[20] D.J. Little, R.J. Donovan, R.J. Butcher, J. Photochem. 2 (1973/
74) 451.
[21] H. Dumont, A. Marbeuf, J.-E. Bouree, O. Gorochov, J. Mater.
Chem. 3 (1993) 1075.
The work was supported by the GAAVCR (Czech
Republic, grant no. A4072107) and by the STA
(Japan).
.