The matrix-isolation IR spectrum of the o-quinonoid intermediate in the photolysis of 2-nitrobenzyl methyl ether

Article abstract of DOI:10.1016/S1386-1425(97)00186-8

Photolysis of 2-nitrobenzyl methyl ether in Ar and N₂ matrices at 12 K generated an intermediate with λ_max at 430 nm, and which was itself photolabile at 430-460 nm. Matrix IR spectra, as well as the UV-visible absorption, were obtai

Full text of DOI:10.1016/S1386-1425(97)00186-8

Spectrochimica Acta Part A 53 (1997) 2553–2557
The matrix-isolation IR spectrum of the o-quinonoid
intermediate in the photolysis of 2-nitrobenzyl methyl ether
a,
b
a
a
I.R. Dunkin *, J. G e˛ bicki , M. Kiszka , D. San ´ı n-Leira
a
Department of Pure and Applied Chemistry, Uni6ersity of Strathclyde, Thomas Graham Building, 295 Cathedral Street,
Glasgow G1 1XL, Scotland, UK
Institute of Applied Radiation Research, Technical Uni6ersity (Politechnika), Z
b
:
wirki 36, 90924 Ł o´ dz; , Poland
Received 13 May 1997; accepted 2 July 1997
Abstract
Photolysis of 2-nitrobenzyl methyl ether in Ar and N matrices at 12 K generated an intermediate with u
at 430
2
max
nm, and which was itself photolabile at 430–460 nm. Matrix IR spectra, as well as the UV-visible absorption, were
obtained for this species. An analogous intermediate had previously been observed in flash-photolysis studies of
2
-nitrobenzyl 4-cyanophenyl ether, and had been assigned an o-quinonoid structure on the basis of its kinetic
behaviour and the position of its UV-visible absorption. In the matrix studies with 2-nitrobenzyl methyl ether, the IR
spectra confirmed the o-quinonoid structure. © 1997 Elsevier Science B.V.
Keywords: Matrix IR spectra; Photolysis; UV-visible absorption
1
. Introduction
compounds and materials containing 2-nitroben-
zyl groups have been developed as photocleavable
polymers (for use as resists) [5], cryptands [6] and
DNA building blocks [7,8], and for the photo-
cleavage of oligonucleotides from solid-phase sup-
ports [9]. Other interesting applications can be
envisaged.
The mechanism of the photocleavage of 2-
nitrobenzyl ethers and some analogues has been
studied by flash photolysis [10,11], and reaction
pathways such as those shown in Scheme 1 are
understood to operate. Flash photolysis of 2-ni-
trobenzyl 4-cyanophenyl ether in acetonitrile led
to the observation of three transient species: the
The 2-nitrobenzyl group is well known to
provide photocleavable protection for hydroxy
compounds, such as alcohols, phenols and car-
boxylic acids [1,2], and has also been applied to
thiols [3] and aldehydes [4]. The majority of or-
ganic photocleavages eliminate a small molecule,
such as N or CO , as one of the fragments. The
photolysis of 2-nitrobenzyl derivatives is one of
the relatively rare photocleavage reactions in
which two substantial fragments are produced,
both capable of bearing functionality. As a result,
2
2
first, with a lifetime of about 50 ps and with umax
*
Corresponding author. Tel.: +44 41 5524400; fax: +44
4
1
1 5525664.
at 650 nm, was identified as the triplet excited
386-1425/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII S1386-1425(97)00186-8

2
554
I.R. Dunkin et al. / Spectrochimica Acta Part A 53 (1997) 2553–2557
state of the starting material, the second, with a
lifetime of about 1 ns and umax at 460 nm, was
identified as a triplet biradical (see 3), while the
third, which persisted beyond 10 ns, had umax at
about 420 nm and was assigned an o-quinonoid
structure (see 2) [11].
Despite the interest in this reaction and its
potential importance for novel ‘photochemical ef-
fect’ materials, there appears to have been no
previous study of the reactive intermediates by
IR spectroscopy. In this paper we report matrix-
isolation studies of the photolysis of 2-nitroben-
zyl methyl ether 1 and the IR spectrum of the
o-quinonoid intermediate 2.
3. Results and discussion
Fig. 1 shows UV-visible spectra of 1 in an Ar
matrix, before and after photolysis. A weak broad
absorption umax=430 nm was observed to arise
during the initial photolysis at 254 nm, and this
was bleached by subsequent photolysis at 460 nm.
Similar results were obtained with N matrices.
2
Allowing for differences in temperature, medium
and starting material, this observation agrees
closely with the absorption (umax=421 nm) of the
relatively long lived intermediate observed after
flash photolysis of the 4-cyanophenyl analogue of
1
[11]. This intermediate was previously assigned
an o-quinonoid structure (see 2). Not surprisingly,
in our matrix experiments we saw no absorption
at longer wavelengths that could be attributed to
the triplet biradical 3.
2
. Experimental
-Nitrobenzyl methyl ether 1 was prepared by
Table 1 lists the observed IR absorptions of 1
2
isolated in Ar and N matrices. There is good
2
treatment of 2-nitrobenzyl alcohol (Aldrich) with
dimethyl sulphate under phase transfer catalysis
agreement between the spectra for the two differ-
ent matrix hosts, small differences in band posi-
tions and intensities being attributed to matrix site
effects. Assignments have been made by reference
to standard tabulations of data [15]. All these
bands diminished in intensity when the matrices
were irradiated with 254 nm light, while a large
number of new bands arose. Amongst the latter
were absorptions which could be attributed to the
[
12]. At room temperature in acetonitrile 1 had
3
−1
−1
umax=261 nm (‡=5400 dm mol
search grade argon (min. 99.9994%) and nitrogen
cm ). Re-
(
min. 99.995%) were obtained from BOC.
The matrix-isolation cold cell and its method
of use have been described in detail previously
13,14]. Ar and N matrices containing 1 were
[
2
prepared by evaporation of 1 onto a spectro-
scopic window cooled to 12 K, with simulta-
neous deposition of an excess of the host gas.
For UV-visible spectra, the cell was fitted with
quartz external windows and matrices were de-
posited on a CaF cold window, while for IR
2
spectra, a KBr cold window and KBr external
windows were used.
UV-visible spectra were recorded on a Shi-
madzu UV-250 instrument, with
a
double
monochromator. IR spectra were recorded on a
Bomem MB-100 FTIR spectrometer. Photolyses
at 254 nm were carried out with unfiltered low
pressure Hg arcs (Vilber Lourmat, 2×4 W or
2
×8 W), and at 430910 and 460910 nm with
a high pressure Hg arc (Oriel, 200 W) via a high
radiance grating monochromator (Applied Pho-
tophysics, f/3.4).
Scheme 1.

I.R. Dunkin et al. / Spectrochimica Acta Part A 53 (1997) 2553–2557
2555
bands assigned to this intermediate in both Ar
and N2 matrices. There is excellent agreement
between the two matrix hosts. In most experi-
ments, irradiation at 460 nm (rather than 430 nm)
was chosen so as to minimize further photolysis of
the starting material. Even so, a very small
amount of photolysis of 1 occurred, as shown by
the appearance of the strongest two bands of 1 as
weak bands in the difference spectrum (Fig. 2),
but this caused no ambiguity in assigning bands
to the intermediate.
Apart from an OH group, the functionality in 2
would be expected to give rise to IR bands only in
the skeletal region of the spectrum, where assign-
ments are difficult to make with certainty. For
this reason, the observed IR spectra of the inter-
Table 1
−
1
IR absorptions (cm ) of 2-nitrobenzyl methyl ether 1 isolated
in Ar and N2 matrices at 12 K
a
Ar matrices
N2 matrices
Assignment
3
2
2
2
2
2
2
1
1
093 vwb
998
935
911
890
3093 vw
3002 Â
2938 Ã
2915 Ì
2896 Ã
2830 Å
2090 vw
1874 vw
1617
w(C–H) (aromatic)
Fig. 1. UV-visible spectra of 1 in an Ar matrix at 12 K: (a)
before photolysis; (b) after 6.5 min photolysis at 254 nm; (c)
after a further 30 min photolysis at 460910 nm. The spectra
all have the background subtracted.
w(C–H) (CH , CH )
3
2
830
090 vw
870 vw
618
Combination band
Combination band
w(CꢀC) (aromatic)
expected final products, 2-nitrosobenzaldehyde 5
1
598
(
(
w(CꢀO) at 1724 (Ar) or 1719 (N ), w(NO) at 1527
2
1
1
1
581 vw
535 vs
469
1581 vw
1536 vs
1481
1454
1382
1351 s
1303 vw
1204 w
1116 s
1079
981 m
932 vw
859
w(CꢀC) (aromatic)
w (NO )
−1
Ar) or 1528 cm
(N )) [16] and methanol
2
a
2
(
cm
w(OH) 3340 and 3230 (Ar), or 3362 and 3222
w(CꢀC) (aromatic)
−
1
(N )). The overall reaction thus followed
1452
2
1
1
1
1
378 m
the normal course, but clearly more than one
product species was present. Amongst the IR
bands which arose on initial photolysis at 254 nm,
were a group which diminished in intensity on
subsequent long wavelength photolysis (430910
or 460910 nm). These latter IR bands behaved
in exactly the same way as the broad electronic
absorption at 430 nm, and can therefore be as-
signed, reasonably confidently, to the same inter-
mediate species. Fig. 2 shows a portion of an IR
difference spectrum showing the IR bands which
diminish on 460 nm photolysis and which are
assigned to the intermediate with the 430 nm
electronic absorption. Table 2 lists all the IR
344 s
309
203 m
w (NO )
s
2
1120 s
1078
9
9
8
7
wa(C–O–C)
83 m
33 vw
60
92
796
734 m
684
728 m
84
k(C–H) (aromatic)
6
a
IR spectra were recorded on a Bomem MB-100 FT instru-
ment with MCT detector.
b
Bands were weak unless denoted: vs, very strong; s, strong;
m, medium; vw, very weak.

2
556
I.R. Dunkin et al. / Spectrochimica Acta Part A 53 (1997) 2553–2557
−
1
Fig. 2. An IR difference spectrum in the region 1800–500 cm
K, followed by 3 h photolysis at 460910 nm. The spectrum shows bands which diminished (downwards) and arose (upwards) on
60 nm photolysis. The former are assigned to the o-quinonoid species 2. The two strongest bands of 1 (marked with an asterisk)
appear weakly in the spectrum, indicating a very small amount of further photolysis of 1 at 460 nm.
obtained after 1.5 h photolysis of 1 with 254 nm light in Ar at 12
4
mediate do not allow its unequivocal identifica-
tion as the o-quinonoid species 2. Nevertheless,
the observed spectrum accords with structure 2
very well. Firstly, the spectrum is dominated by a
small number of strong bands, especially that at
expected on stereoelectronic grounds. The possi-
bility that more than one stereoisomer of 2 was
present cannot be ruled out, but the IR spectra
show few splittings that would suggest that this
was the case.
−
1
1
244 cm
(Ar matrix), which are much stronger
Additional support for the identification of the
intermediate as 2 comes from the observation that
on photolysis it gives a new species (possibly more
than one) with prominent IR bands at 1637, 1091
than those arising from photolysis of the interme-
diate (positive-going bands in Fig. 2), and this
suggests a highly polar structure. Secondly, the
spectrum contains the expected w(OH) absorp-
tion. Thirdly, the CꢀN(O)OH moiety is very simi-
lar to a nitrone, differing from the latter by
replacement of N-alkyl or -aryl by N–OH; and
the observed very strong band at 1244 and strong
−
1
and 756 cm , but no carbonyl band in the
−
1
region of 1700 cm . This is consistent with the
formation of the bicyclic intermediate 4, in which
the aromatic ring is regained. Significant further
photolysis of 4 would not be expected to occur at
460 nm.
−
1
band at 1556 cm
(Ar matrix) agree well with
+
−
the expected w(N –O ) and w(CꢀN) absorp-
Finally, it should be noted that evidence for the
o-quinonoid ketene 6 was previously obtained in
Ar-matrix studies of the photolysis of 2-nitroben-
zaldehyde [19]. This species had umax at 435 nm,
very close to that of 2, but proved to be very
photosensitive; so, despite careful selection of
photolysis wavelengths, only ketene bands at 2118
tions, respectively [15,17,18]. Finally, the strong
−
1
bands at 1124 and 950 cm
(Ar matrix) can
reasonably be attributed to w (C–O–C) and w(N–
a
OH) (by comparison with oximes), respectively
[15]. Thus all the expected features for 2 are
present in the matrix IR spectra. The stereochem-
istry shown for 2 in Scheme 1 cannot, of course,
be deduced from the spectroscopic data, but is
−
1
and 2107 cm
could be observed in the IR, and
these only weakly.

I.R. Dunkin et al. / Spectrochimica Acta Part A 53 (1997) 2553–2557
2557
Table 2
[2] D. D o¨ pp, in: W.M. Horspool, P.-S. Song (Eds.), CRC
Handbook of Organic Photochemistry and Photobiology,
Ch. 81, CRC Press, Boca Raton, FL, 1995, pp. 1019–
1062.
−
1
IR bands (cm ) arising after 254 nm photolysis of 1 and
diminishing upon subsequent long wavelength photolysis
(460910 nm) in Ar and N2 matrices at 12 K
[
3] V.N. Rajasekharan Pillai, in: W.M. Horspool, P.-S. Song
(Eds.), CRC Handbook of Organic Photochemistry and
Photobiology, Ch. 62, CRC Press, Boca Raton, FL, 1995,
pp. 766–774.
Ar matrices
N2 matrices
Assignment
3
2
2
533a
943
115
3519
2946
2115
w(OH)
w(CH)
[4] M.J. Aurell, C. Boix, M.L. Ceita, C. Llopis, A. Torta-
jada, R. Mestres, J. Chem. Res. (S) (1995) 452; (M) (1995)
2569.
1587
1556 s
1384 m
1267 m
1244 vs
1205 m
1147 s
1124 s
1101
1552 s
1384 m
1270 s
1243 vs
1203 m
1149 s
1124 s
1098 m
991
w(CꢀN)
[
[
[
5] A. Reiser, Photoreactive Polymers, Wiley, New York,
989.
6] R. Warmuth, E. Grell, J.-M. Lehn, J.W. Bats, G.
Quinkert, Helv. Chim. Acta 74 (1991) 671.
7] P.A. Wender, C.K. Zercher, S. Beckham, E.-M. Haubold,
J. Org. Chem. 58 (1993) 5867.
1
+
−
w(N –O
)
wa(C–O–C)
[8] P. Ordoukhanian, J.-S. Taylor, J. Am. Chem. Soc. 117
1995) 9570.
9] M.M. Greenberg, J.L. Gilmore, J. Org. Chem. 59 (1994)
46.
(
9
9
8
7
93
50 s
42
[
956 s
844
746
w(N–OH)
7
[
[
[
10] D. Gravel, R. Giasson, D. Blanchet, R.W. Yip, D.K.
Sharma, Can. J. Chem. 69 (1991) 1193.
11] R.W. Yip, Y.X. Wen, D. Gravel, R. Giasson, D.K.
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Daum, H. D o¨ rr, N.P. DuPreez, V. Ehrig, W. Langer, C.
N u¨ ssler, H.-A. Oei, M. Schmidt, Helv. Chim. Acta 60
40
a
The relative intensities of bands were weak unless denoted:
vs, very strong; s, strong; m, medium.
Acknowledgements
(
1977) 301.
[
13] I.R. Dunkin, J.G. MacDonald, J. Chem. Soc. Perkin
Trans. 2 (1984) 2079.
We thank the Universities of Strathclyde and
Ł o´ dz
;
for an exchange scholarship for MK, and
[14] R. Withnall, I.R. Dunkin, R. Snaith, J. Chem. Soc.
Perkin Trans. 2 (1994) 1973.
the EPSRC and the Royal Academy of Engineer-
ing for a Fellowship for IRD.
[
15] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli,
The Handbook of Infrared and Raman Characteristic
Frequencies of Organic Molecules, Academic Press,
Boston, 1991.
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