Effect of Irradiation Timing and Wavelength in the Time-Delayed, Two-Color Photolysis of 1, 2-Bis[(phenylseleno)methyl]benzene: Transient Targeting of o-Quinodimethane in Room-Temperature Solutions

Full text of DOI:10.1021/jo981360w

6
780
J . Org. Chem. 1998, 63, 6780-6781
Effect of Ir r a d ia tion Tim in g a n d Wa velen gth
in th e Tim e-Dela yed , Tw o-Color P h otolysis of
Sch em e 1
1
,2-Bis[(p h en ylselen o)m eth yl]ben zen e:
Tr a n sien t Ta r getin g of o-Qu in od im eth a n e in
Room -Tem p er a tu r e Solu tion s
Akihiko Ouchi* and Yoshinori Koga
National Institute of Materials and Chemical Research,
Tsukuba, Ibaraki 305-8565, J apan
Received J uly 14, 1998
mainly oligomers, polymers, and spiro dimer 4.8 To the best
of our knowledge, the photochemical reaction of 3 in room-
temperature solutions has not been reported so far. This
The utilization of lasers in organic photochemistry has
enabled many reactions that could not be conducted by the
use of conventional light, and some special techniques have
1
9
been developed and used for the photolysis, such as laser
jet and laser drop3 photolysis techniques. A main theme
in organic laser chemistry is the photolysis of photochemi-
cally generated short-lived intermediates. Multicolor laser
photolysis has been used in order to achieve maximum
efficiency in the photolyses by matching the wavelength to
both the starting materials and the intermediates. When
pulsed lasers are used in such multicolor photolysis,4 it
enables us to have precise control not only of the wave-
lengths but also of the timing of the irradiation. It means
that we can target a specific intermediate at a particular
time in the course of the reaction. This method, time-
delayed, multicolor photolysis, provides an additional means
for the study of the time profile of intermediates by product
analyses, and this is especially useful in the cases where
spectroscopic techniques are not applicable due to absorption
overlapping of the reaction intermediates.
is probably due to its high thermal reactivity, by which
thermal products are formed before its photochemical reac-
tion can proceed. In our laser photolysis experiments at
room temperature, we have observed the formation of
benzocyclobutene 5 (photochemical product of 3) together
with o-quinodimethane spiro dimer 4 (thermal product of
3) in both photolyses. The time profile of 3 was obtained by
delay-time dependence of 4 and 5 in the time-delayed, two-
color laser photolyses.
2
The strategy used in our experiments for the photolysis
of 3 was the fast generation of 3 and its photolysis before
the occurrence of its thermal reaction. The fast generation
of 3 was accomplished by the KrF excimer laser photolysis
of 1,2-bis[(phenylseleno)methyl]benzene 1 in 10- M aceto-
4
1
0
nitrile solution under a nitrogen gas atmosphere, and
successive photolyses of 3 were also conducted by lasers
(Scheme 1). The time-delayed, two-color photolysis was
conducted by one pair of laser pulses; the pair of laser pulses
consisted of one pulse of the KrF excimer laser [248 nm, 1.25
We report here the photolysis of o-quinodimethane5 [3,
5
,6-bis(methylene)cyclohexa-1,3-diene], a thermally unstable
2
1
-2
-1
short-lived intermediate, in room-temperature solutions by
using one-color and time-delayed, two-color4 excimer laser
photolysis techniques. o-Quinodimethane 3 has been ex-
× 10 photons‚m ‚pulse ] and one subsequent pulse of a
a,b
21 -2 -1
XeCl [308 nm, 1.88 × 10 photons‚m ‚pulse ] or a XeF
2
1
-2
-1
[351 nm, 1.88 × 10 photons‚m ‚pulse ] excimer laser,
which was flashed after various delay times ranging from 0
ns to 3 s. In comparison, the one-color photolyses were
performed by using a pulse of the KrF, XeCl, and XeF
excimer lasers with the same laser fluences as those in the
two-color photolysis. The yields of 4 and 5 and the con-
sumption of 1 were determined by HPLC analysis in
comparison with the authentic samples.
6
tensively studied from both synthetic and physical stand-
points.7 The thermal reactions of 3 at room temperature or
below have been well established, and 3 is reported to form
(1) Reviews: (a) Scaiano, J . C.; J ohnston, L. J . In Organic Photochemistry;
Padwa, A., Ed.; Marcel Dekker: New York, 1989; Vol. 10, pp 309-355. (b)
Wilson, R. M.; Schnapp, K. A.; Hannemann, K.; Ho, D. M.; Memarian, H.
R.; Azadnia, A.; Pinhas, A. R.; Figley, T. M. Spectrochim. Acta, Part A 1990,
4
1
2
6, 551-558. (c) Wilson, R. M.; Adam, W.; Schulte Oestrich, R. Spectrum
In the case of one-color photolyses, the yields of 4 and 5
and the consumption of 1 by a pulse of the excimer laser,
991, 4, 8-17. (d) Wilson, R. M.; Schnapp, K. A. Chem. Rev. 1993, 93, 223-
49.
were 7.0%, 6.3%, and 91% with the KrF laser and 2.2%, 0%,
(2) E.g.: (a) Refs 1b-d. (b) Wilson, R. M.; Schnapp, K. A.; Glos, M.; Bohne,
and 44% with the XeCl laser.1
1,12b
No reaction proceeded
C.; Dixon, A. C. Chem. Commun. 1997, 149-150. (c) Adam, W.; Schneider,
K.; Stapper, M.; Steenken, S. J . Am. Chem. Soc. 1997, 119, 3280-3287.
with the XeF laser, which was due to the lack of the
absorption of 1 at 351 nm. Despite the larger number of
photons, lower consumption of 1 in the XeCl laser photolysis
than that of the KrF laser can be explained by the difference
(3) E.g.: (a) Banks, J . T.; Scaiano, J . C. J . Am. Chem. Soc. 1993, 115,
6
409-6413. (b) Miranda, M. A.; P e´ rez-Prieto, J .; Font-Sanchis, E.; K o´ nya,
K.; Scaiano, J . C. J . Org. Chem. 1997, 62, 5713-5719.
4) E.g.: (a) Bendig, J .; Mitzner, R. Ber. Bunsen-Ges. Phys. Chem. 1994,
(
9
9
8, 1004-1008. (b) Ouchi, A.; Koga, Y. Tetrahedron Lett. 1995, 36, 8999-
002. (c) J im e´ nez, M. C.; Miranda, M. A.; Scaiano, J . C.; Tormos, R. Chem.
-
1
-1
in molar absorptivity of 1 (16 800 M ‚cm at 248 nm and
-
1
-1
Commun. 1997, 1487-1488.
5) Reviews: (a) McCullough, J . J . Acc. Chem. Res. 1980, 13, 270-276.
b) Charlton, J . L.; Alauddin, M. M. Tetrahedron 1987, 43, 2873-2889. (c)
Scaiano, J . C.; Wintgens, V.; Netto-Ferreira, J . C. Pure Appl. Chem. 1990,
2, 1557-1564. (d) Martin, N.; Seoane, C.; Hanack, M. Org. Prep. Proced.
Int. 1991, 23, 237-272.
6) Review: Kametani; T.; Nemoto, H. Tetrahedron 1981, 37, 3-16 and
references cited therein.
7) (a) Flynn, C. R.; Michl, J . J . Am. Chem. Soc. 1973, 95, 5802-5803.
1800 M ‚cm at 308 nm) and also by the participation of
13b
(
different excited states
at each laser wavelength. The
(
formation of 514 is rationalized by further photolysis of 3
within the same laser pulse (Scheme 1, laser 1 ) laser 2 )
6
(
(9) Spectroscopic studies on the photolysis of 3 have been reported at
-196 °C in organic matrixes7 and at 10-15 K in argon matrixes.
a,b
7f,h
(
(
b) Flynn, C. R.; Michl, J . J . Am. Chem. Soc. 1974, 96, 3280-3288. (c) Tseng,
(10) Thus generated 3 was trapped with maleic anhydride in a the yield
of 43%.1
3.
K. L.; Michl, J . J . Am. Chem. Soc. 1977, 99, 4840-4842. (d) Roth, W. R.;
Biermann, M.; Dekker: H.; J ochems, R.; Mosselman, C.; Hermann, H.
Chem. Ber. 1978, 111, 3892-3903. (e) Roth, W. R.; Scholz, B. P. Chem. Ber.
(11) TLC analyses of the laser reaction products showed the existence
of a significant amount of compounds at the origin, which were expected to
be oligomers and polymers.8.
(12) The results were obtained from the average of (a) two and (b) three
independent runs.
(13) (a) Ouchi, A.; Koga, Y. Chem. Commun. 1996, 2075-2076. (b) Ouchi,
A.; Koga, Y. J . Org. Chem. 1997, 62, 7376-7383.
1
981, 114, 3741-3750. (f) Chapman, O. L.; McMahon, R. J .; West, P. R. J .
Am. Chem. Soc. 1984, 106, 7973-7974. (g) Trahanovsky, W. S.; Macias, J .
R. J . Am. Chem. Soc. 1986, 108, 6820-6821. (h) Chapman, O. L.; J ohnson,
J . W.; McMahon, R. J .; West, P. R. J . Am. Chem. Soc. 1988, 110, 501-509.
(8) E.g.: Errede, L. A. J . Am. Chem. Soc. 1961, 83, 949-954.
S0022-3263(98)01360-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/05/1998

Communications
J . Org. Chem., Vol. 63, No. 20, 1998 6781
F igu r e 1. Yields of spiro dimer 4 and benzocyclobutene 5 as a function of the delay time of (a) KrF and XeCl lasers and (b) KrF and XeF
1
2a
lasers and (c) increased yield of 5 as a function of the delay time of the KrF and XeF lasers with second-order kinetic fitting: 4, open
-
4
symbols; 5, closed symbols. Combinations of lasers: KrF-XeCl (O, b); KrF-XeF (0, 9). Concentration: 10 M 1 in acetonitrile. Optical
2
1
21
-2
-1
path: 1 mm. Laser fluence: 1.25 × 10 (KrF) and 1.88 × 10 (XeCl and XeF) photons‚m ‚pulse
.
laser 3). This result is in accord with the formation of 5 by
conventional light photolysis of 3 in low-temperature
matrixes.7
a,b,f,h
Figure 1a,b shows the yields of spiro dimer 4 and
benzocyclobutene 5 as a function of the delay time of the
two laser pulses in the time-delayed, two-color photolyses.
The consumption of 1 in the two-color photolyses, 91-94%,
was independent of the delay times and almost the same as
that in the one-color photolysis by KrF laser, 91%. As seen
in Figure 1a,b, the additional formation of 5 strongly
depended on the delay time. At short delay times, the yields
of 4 and 5 were almost the same as those of the one-color
photolysis with the KrF excimer laser. However, a maxi-
mum on the yield of 5 and a minimum on the yield of 4 were
observed at the delay time of ca. 20 ms.
F igu r e 2. Laser fluence dependence on the increased yield of
benzocyclobutene 5 as a function of the XeCl excimer laser
fluence.1 5: b. Concentration: 10 M 1 in acetonitrile. Optical
2a
-4
2
1
path: 1 mm. Delay time: 2 ms. KrF laser fluence: 1.25 × 10
-
2
-1
-2
-1
photons‚m ‚pulse (100 mJ ‚cm ‚pulse ).
The increase of the yield of 5 in the two-color photolyses
is explained by the photolysis of 3 by the XeCl/XeF laser
pulse, i.e., the photolysis of two-photon intermediate 3 to
form 5 (Scheme 1: laser 1 ) laser 2 ) KrF laser, laser 3 )
XeCl/XeF laser). However, another possibility is the pho-
tolysis of one-photon intermediate 2a /b through a two-
photon process (Scheme 1: laser 1 ) KrF laser, laser 2 )
laser 3 ) XeCl/XeF laser) because the presence of intermedi-
conducted with different precursors;5c,7d,g the decay is ex-
plained by the thermal dimerization.
The existence of the maximum for the yield of 5 in Figure
1
a,b means that the highest concentration of 3 was reached
at ca. 20 ms delay time. This result indicates the existence
of a slow process for the generation of 3. On the other hand,
the formation of a considerable amount of 5 by the one-color
photolysis of the KrF laser implies the existence of another
very fast process, which enables 3 to absorb the third photon
within the same laser pulse (<30 ns). The origin of the two
processes is not clear at present because the rise of a
transient species has been generally ignored in conventional
flash photolysis studies.
1
5
ate 2a /b was also observed by flash photolysis experiments.
To clarify the mechanism of the additional formation of 5,
an experiment on the XeCl laser fluence dependence was
conducted in the time-delayed, two-color photolysis. Figure
2
shows the increased yield of 5 as a function of the XeCl
laser fluence at a fixed delay time of 2 ms. The increased
yield is defined as the difference in the yield of 5 between
the time-delayed, two-color photolysis and the one-color KrF
laser photolysis. As seen in Figure 2, the increased yield of
The wavelength dependence in photochemical conversion
3
f 5 is best shown as the difference in the increased yield
of 5 at the maximum of Figure 1a,b. The increased yield of
was 2.8-fold higher for the XeCl laser (308 nm) than the
5
5
was proportional to the first power of the XeCl laser
XeF laser (351 nm). This result cannot be explained by the
fluence. This result indicates that the formation of 5 by the
XeCl laser irradiation proceeded through a one-photon
process; i.e., 3 f 5 but not 2a /b f 3 f 5.
difference in molar absorptivity of 3 at each wavelength
16
7b
because the reported molar absorptivity is 2.5- to 3.8-fold
larger at 351 nm than at 308 nm. This tendency is opposite
Figure 1c shows the increased yield of 5 as a function of
the delay time of the KrF and XeF lasers. This time profile
corresponds to the decay curve of 3 in conventional flash
photolysis experiments15 because the increased yield of 5
reflects the concentration of 3 at each delay time. As seen
in Figure 1c, 3 decreased with second-order kinetics. This
is consistent with the results of spectroscopic experiments
to that of the increased yield of 5. The reported λmax of the
first absorption band of 3 is 370, and 308 nm is located just
5c,7b,16
at the minimum of its shorter wavelength side.
There-
fore, the involvement of higher excited state, S2, is expected
in the photochemical conversion 3 f 5. Indeed, participation
8b,c
of the S2 state for the conversion 3 f 5 has been postulated;
however, it is believed that the S0 f S2 transition should be
buried under the more intense S0 f S1 band. We think that
our result on the wavelength dependence on the increased
yield of 5 is consistent with this prediction on the involve-
ment of the S0 f S2 transition for the conversion 3 f 5,
which is still not explicitly observed by spectroscopic means.
(14) It is important to note that 5 is not formed by the thermal reaction
of 3 below room temperature. Cf. Cava; M. P.; Deana, A. A. J . Am. Chem.
Soc. 1959, 81, 4266-4268.
(15) Transient absorption was observed at 320 nm by the photolysis of 1
by using a pulse of a 266 nm laser, and the absorption is in good accord
with that of the ortho-substituted benzyl radical:16 unpublished result.
However, the decay of 3 was difficult to observe because of the overlapping
of several species such as phenyl seleno radicals, diphenyl diselenide,
intermediate 2a /b, and other unidentified species.
Su p p or tin g In for m a tion Ava ila ble: Experimental details
and preparation of compounds 1, 4, and 5 (4 pages).
(16) Fujiwara, M.; Mishima, K.; Tamai, K.; Tanimoto, Y.; Mizuno, K.;
Ishii, Y. J . Phys. Chem. A 1997, 101, 4912-4915.
J O981360W