Photochemical Generation of Benzoazetinone by UV Excitation of Matrix-Isolated Precursors: Isatin or Isatoic Anhydride

Article abstract of DOI:10.1021/acs.jpca.0c02562

Benzoazetinone was photochemically generated by UV irradiation of isatin isolated in low-temperature Ar matrixes. Upon UV (λ = 278 nm) excitation of isatin, monomers of the compound underwent decarbonylation and the remaining part of the molecule adopted the benzoazetinone structure or the structure of its open-ring isomer α-iminoketene. The same products (benzoazetinone and α-iminoketene) were generated by UV (λ = 278 nm) induced decarboxylation of matrix-isolated monomers of isatoic anhydride. Photoproduced α-iminoketene appeared in the low-temperature matrixes as a mixture of syn and anti isomers. Photoproducts generated upon λ = 278 nm irradiation of matrix-isolated isatin were subsequently exposed to λ = 532 nm light. That irradiation resulted in the shift of the α-iminoketene-benzoazetinone population ratio in favor of the latter closed-ring structure. The next irradiation at 305 nm caused the shift of the α-iminoketene-benzoazetinone population ratio in the opposite direction, that is, in favor of the open-ring isomer. Neither benzoazetinone nor its α-iminoketene open-ring isomer was generated upon UV (λ = 278 nm) irradiation of phthalimide isolated in Ar matrixes. Instead, the UV-excited monomers of this compound underwent such phototransformations as oxo → hydroxy phototautomerism or degradation of the five-membered ring with release of HNCO and CO. The efficiency of these photoconversions was low.

Full text of DOI:10.1021/acs.jpca.0c02562

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Photochemical Generation of Benzoazetinone by UV-Excitation
of Matrix-Isolated Precursors: Isatin or Isatoic Anhydride
Hanna Rostkowska, Leszek Lapinski, and Maciej J. Nowak
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.0c02562 • Publication Date (Web): 22 Apr 2020
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Photochemical Generation of Benzoazetinone by UV-excitation of
Matrix-Isolated Precursors: Isatin or Isatoic Anhydride
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Hanna Rostkowska, Leszek Lapinski and Maciej J. Nowak*
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
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* Author to whom correspondence should be addressed: mjnowak@ifpan.edu.pl
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ABSTRACT:
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Benzoazetinone was photochemically generated by UV irradiation of isatin isolated in low-
temperature Ar matrixes. Upon UV ( = 278 nm) excitation of isatin, monomers of the
compound underwent decarbonylation and the remaining part of the molecule adopted
benzoazetinone structure or the structure of its open-ring isomer -iminoketene. The same
products (benzoazetinone and -iminoketene) were generated by UV ( = 278 nm) induced
decarboxylation of matrix-isolated monomers of isatoic anhydride. Photoproduced
-iminoketene appeared in the low-temperature matrixes as a mixture of syn and anti-
isomers. Photoproducts generated upon =278 nm irradiation of matrix-isolated isatin were
subsequently exposed to =532 nm light. That irradiation resulted in the shift of the
-iminoketene – benzoazetinone population ratio in favor of the latter closed-ring structure.
The next irradiation at 305 nm caused the shift of the -iminoketene - benzoazetinone
population ratio in the opposite direction, that is in favor of the open-ring isomer. Neither
benzoazetinone nor its -iminoketene open-ring isomer were generated upon UV ( = 278
nm) irradiation of phthalimide isolated in Ar matrixes. Instead, the UV-excited monomers of
this compound underwent such phototransformations as oxo → hydroxy phototautomerism or
degradation of the five-membered ring with release of HNCO and CO. The efficiency of these
photoconversions was low.
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INTRODUCTION
Azetinone molecules are interesting because of the partially antiaromatic character of their
four-membered ring.1,2 The saturated azetidinone moiety is a common structural fragment of
natural semi-synthetic and synthetic -lactam anitibiotics.3,4 It has also been recognized as a
building block for the synthesis of many biologically important compounds, such as amino
acids, peptides, alkaloids, aminosugars and others.5
Azetinone itself is an extremely unstable compound, even at low temperature. Hence,
the attempts to obtain this species were not successful.1 The azetinone structure, with four-
membered ring, is by ca. 50 kJ mol-1 less stable than its open-ring -iminoketene isomer.2 In
contrast to that, the energy of the closed-ring structure of benzoazetinone is lower (by more
than 30 kJ mol-1) than that of its open-ring -iminoketene isomer. Although benzoazetinones
are highly reactive and difficult to obtain, there are several reports on successful preparation
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of benzoazetinone or its derivatives.6-9 Only some N-alkylbenzoazetinones with tertiary alkyl
groups can be stable at room temperature. Pyrolysis of isatoic anhydride, followed by trapping
of the products at 77 K, was reported to yield benzoazetinone itself.6 Similarly, trapping of the
products of 1-methyl-1,2,3-benzotriazin-4(1H)-one pyrolysis in a layer of solid argon at 15 K
led to stabilization of N-methylbenzoazetinone in an Ar matrix.7 UV-induced decarbonylation
of N-methoxyisatin isolated in an Ar matrix yielded N-methoxybenzoazetinone.8 Recently,
N-deuterated benzoazetinone and related deuterated open-ring -iminoketene were
photochemically generated from deuterated 2-formylphenylazide isolated in an Ar matrix.9
This latter photoconversion occurred via deuterated 2-formylphenylnitrene diradical, which
was the primary photoproduct directly obtained by N2 detachment from deuterated
2-formylphenylazide.
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In the current work, we attempted to generate benzoazetinone by in situ UV excitation
of matrix-isolated precursors: isatin, phthalimide or isatoic anhydride (Scheme 1). To convert
into benzoazetinone, such precursors need to undergo UV-induced decarbonylation (isatin,
phthalimide) or decarboxylation (isatoic anhydride).
In the literature, there are many examples of photodecarboxylation or
photodecarbonylation processes,8,10-12 which occur in molecules having one or more C=O
groups in the structure (see Scheme S1 in the Supporting Information). Hence, it seemed
possible that photodecarbonylation or photodecarboxylation of isatin, phthalimide or isatoic
anhydride might yield benzoazetinone. The experiments carried out within the current work
confirmed some of these expectations. We demonstrated that photodecarbonylation of isatin
as well as photodecarboxylation of isatoic anhydride yielded benzoazetinone and its open-ring
isomer -iminoketene. It is the first investigation on photochemical transformations of
isolated molecules of unsubstituted isatin. Phthalimide molecule, despite the structural
similarity to isatin (Scheme 1), did not undergo UV-induced conversion into benzoazetinone
and its open-ring -iminoketene form.
EXPERIMENTAL SECTION
The compounds used in the current study as photochemical precursors were commercial
products. Phthalimide (purity > 99%) was provided by Sigma-Aldrich, isatin ( > 98%) and
isatoic anhydride ( > 98%) were purchased from TCI Europe. For each matrix-isolation
experiment, a solid sample of the studied compound was placed in a miniature glass oven
located inside the vacuum shroud of the cryostat cooled by Sumitomo SRDK-408D2 closed-
cycle helium refrigerator. The cryostat was then evacuated and the compound was heated (to
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400 K for isatin, 430 K phthalimide, 420 K isatoic anhydride) by a resistive wire wrapped
around the microoven. Vapor of the compound was deposited, together with a large excess of
argon (Linde, 6.0 purity) onto a CsI substrate mounted on the cold (15K) tip of the cryostat.
The infrared (IR) absorption spectra were recorded with 0.5 cm−1 resolution in the 4000-400
cm−1 range, using a Thermo Nicolet iS50R FTIR spectrometer equipped with a KBr beam
splitter and a DTGS-KBr detector. The spectra in the 690-220 cm-1 range were recorded with
1 cm-1 resolution using the same spectrometer but equipped with a Solid Substrate beam
splitter and a DTGS-PE detector. Matrix-isolated monomers of isatin, phthalimide and isatoic
anhydride were irradiated with UV (max = 278 nm, FWHM = 15 nm) light or with UV
(max = 305 nm, FWHM = 15 nm) light emitted by 6060 LG Innotek diodes. The optical
power of UV light generated by these LEDs was 100 mW. In some experiments, high-
pressure mercury lamp HBO200, fitted with a water filter and cut-off WG295 or WG320
Schott filters, was used as a source of UV light. Matrixes were also irradiated with visible 532
nm light of a green laser pointer (30 mW).
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COMPUTATIONAL SECTION
The geometries of all structures considered in this work were fully optimized using the
density functional method DFT(B3LYP) with the Becke’s three-parameter exchange
functional13 and the Lee, Yang, Parr correlation functional.14 The 6-311++G(2d,p) basis set
was applied in these calculations. At the optimized geometries, the harmonic vibrational
frequencies and IR intensities were calculated at the same DFT(B3LYP)/6-311++G(2d,p)
level. The computed harmonic vibrational wavenumbers were scaled down by a factor of 0.95
for wavenumbers higher than 3000 cm−1, and a factor of 0.98 for wavenumbers lower than
3000 cm−1. All quantum-mechanical computations were carried out with the Gaussian 03
program.15
RESULTS AND DISCUSSION
4.1 Most Stable Isomeric Forms of Monomeric Isatin, Phthalimide and Isatoic
Anhydride. X-ray studies demonstrated that in the condensed phases the molecules of
isatin,16,17 phthalimide18 and isatoic anhydride19 adopt only their dioxo forms. However, in the
gas phase, where intermolecular interactions are absent, the relative energies and relative
populations of isomeric forms of these compounds may be different.
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For monomers of isatin, phthalimide and isatoic anhydride, the relative energies of
different isomeric structures (the dioxo tautomer and the conformers of the oxo-hydroxy
tautomer) were calculated at the DFT(B3LYP)/6-311++G(2d,p) level. In the case of isatoic
anhydride, the lowest energy has been obtained for the dioxo tautomer, while the calculated
energies of the oxo-hydroxy isomeric forms were higher by more than 45 kJ/mol (Table S1 in
the Supporting Information). For isatin and for phthalimide, the dioxo tautomers have also
been predicted to be the most stable. For these two latter compounds, the computed difference
between the energy of the most-stable dioxo tautomers and other oxo-hydroxy structures is
even bigger than it was for isatoic anhydride (see Tables S2 and S3 in the Supporting
Information). Big energy gap, substantially larger than the usual inaccuracy of the theoretical
DFT(B3LYP) prediction, means that in the gas phase only the molecules in the dioxo form
should be expected. Consequently, only the molecules of isatin, phthalimide and isatoic
anhydride in the dioxo tautomeric form should be trapped from the gas phase into the low-
temperature argon matrixes. Hence, in the infrared spectra of isatin, phthalimide and isatoic
anhydride isolated in Ar matrixes, only the IR bands due to vibrations of the dioxo structures
of these compounds are to be expected.
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4.2 Infrared Spectra of Monomeric Isatin, Phthalimide and Isatoic Anhydride
Isolated in Argon Matrixes. The IR spectrum of isatin monomers trapped in a low-
temperature argon matrix matches well the spectrum simulated theoretically for the dioxo
isomer (Figure 1 and Table S4 in the Supporting Information). The infrared spectra of
monomeric isatoic anhydride and monomeric phthalimide isolated in Ar matrixes (Figures 2,
3 and Tables S5, S6) are also accurately reproduced by the spectra theoretically simulated for
the dioxo tautomers of these compounds. The most characteristic features in the mid-IR
spectra of the three studied compounds are the bands observed at ca. 3450 cm-1, which are
assigned to the stretching vibrations of the N-H group. Other characteristic bands appearing in
these spectra near 1800 cm-1 are attributed to the coupled stretching vibrations of the C=O
groups. Although formally phthalimide and isatin are isomers, the general shape of their IR
spectra is different. The spectrum of matrix-isolated phthalimide is simpler than that of isatin.
This is due to different symmetry of the two molecules in question. The dioxo tautomer of
phthalimide is of higher C2v symmetry, whereas the dioxo tautomer of isatin is of lower Cs
symmetry (see Scheme 1).
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4.3 Effects of UV ( = 278 nm) Irradiation of Isatin and Isatoic Anhydride Isolated
in Low-Temperature Argon Matrixes. Isatin molecules isolated in an Ar matrix were
irradiated with UV light. The first UV ( > 320 nm) irradiation did not result in any changes
in the infrared spectrum of the compound. Next exposure to UV ( > 295 nm) light caused
only extremely small changes in the IR spectrum of isatin. This indicated occurrence of some
photochemical processes consuming the isatin precursor, but the yield of these
phototransformations was very low. Upon subsequent UV ( = 278 nm) irradiation, the
infrared spectrum of the isatin underwent substantial changes (Figure 4 a). After 4 hours of
UV ( = 278 nm) irradiation, the IR spectrum of the initial dioxo tautomer of isatin
disappeared nearly totally, reflecting consumption of this form in a photochemical reaction.
Concomitantly, a series of new bands due to photogenerated products appeared in the IR
spectrum. Also irradiation of isatoic anhydride isolated in an Ar matrix with UV ( = 278 nm)
light resulted in a conversion of the initial dioxo form of the compound into photoproducts.
Upon such irradiation, intensities of IR bands in the initial spectrum due to the dioxo tautomer
substantially decreased and a set of new bands appeared (Figure 4 b). It is easy to notice that
the IR spectrum of the photoproduct(s) generated from isatoic anhydride matches very well
the spectrum of the species photogenerated from isatin (compare red traces in both panels of
Figure 4). This suggests that the main photoproduct obtained by UV ( = 278 nm) irradiation
of isatin or isatoic anhydride is the same.
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Fragments of the experimental IR spectra of photoproducts, extracted from the spectra
recorded after exposure of matrix-isolated isatin and isatoic anhydride to UV ( = 278 nm)
light, are presented in Figure 5 (a, b). The most characteristic features of these spectra are the
strong absorptions at 1843 or 1846/1838 cm-1. Intense bands appearing in the 1860 - 1820
cm-1 wavenumber range are typical of the stretching (C=O) vibrations of the carbonyl group
attached to a four-membered ring.7-9,20-22 Hence, benzoazetinone may be treated as one of the
obvious candidates for the structure of the main photoproduct generated by
photodecarbonylation of isatin or by photodecarboxylation of isatoic anhydride (see Scheme
2).
In the IR spectra of matrix-isolated N-methylbenzoazetinone7 and N-deuterated
benzoazetinone9 (compounds having the structures similar to that of benzoazetinone), the
bands due to the C=O vibrations were found at 1843 cm-1 and 1836 cm-1, respectively. The
spectral positions of these bands are very close to the positions of the bands observed in the
current study in the spectra recorded after UV ( = 278 nm) irradiation of isatin and isatoic
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anhydride. This supports the assumption that the photoproduced species has the
benzoazetinone structure.
Geometry of benzoazetinone has been theoretically optimized at the
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DFT(B3LYP)/6-311++G(2d,p) level. The optimized structure of the molecule is not planar,
with the angle between the N-H vector and the plane defined by the C5, N and C7 atoms as
large as 42° (see Table S7 for atom numbering). At the optimized geometry, the infrared
spectrum of benzoazetinone has been calculated at the same DFT(B3LYP)/6-311++G(2d,p)
level. This spectrum (Figure 5 c) reproduces well the experimental spectra (Figure 5 a, b) of
photoproducts obtained upon UV ( = 278 nm) irradiation of isatin and isatoic anhydride. The
agreement between the theoretical and experimental spectra shown in Figure 5 provides a
convincing evidence that benzoazetinone was photoproduced from isatin as well as from
isatoic anhydride isolated in low-temperature Ar matrixes (Table 1 and Table S7 in the
Supporting Information).
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Carefull analysis of the IR spectra recorded after UV ( = 278 nm) irradiation of Ar
matrixes containing monomers of isatin or isatoic anhydride revealed the presence of the
bands which could not be attributed to the spectrum of benzoazetinone. The most
characteristic of these bands are those observed in the 2400 – 2000 cm-1 region (Figure 6).
The bands indicate that benzoazetinone is not the only product photogenerated from both
reactants. In the spectrum recorded after UV ( = 278 nm) irradiation of the dioxo form of
matrix-isolated isatin, alongside the bands due to benzoazetinone (with the most prominent
absorption feature at 1843 cm-1, Figure 5 a), a group of bands was observed in the 2200 –
2100 cm-1 range (Figure 6 a). Within this group, the medium-strong absorptions at 2147 and
2144 cm-1 can be assigned to photogenerated CO, released from UV-excited molecule of
isatin.23,24 Two other medium-strong bands appeared at 2126 and 2111 cm-1. These bands
have their close counterparts observed at 2129 and 2114 cm-1 in the spectrum of UV-
irradiated isatoic anhydride (Figure 6 b). The two bands in question appeared in the spectral
region, where the infrared absorptions due to "antisymmetric" C=C=O stretching vibrations of
various open-ring ketenes were previously observed.7-9 Therefore, these bands can be treated
as the spectral indications of two anti and syn isomers (iminoketene-1 and iminoketene-2, see
Scheme 2) of an open-ring isomer (6-imino-2,4-cyclohexadien-1-ketene) of benzoazetinone.
This interpretation is strongly supported by a close match between the spectral positions of
the lower-wavenumbers bands (2111 and 2114 cm-1) and the wavenumber (2110 cm-1) at
which a band of iminoketene-2 was previously observed.25 Small differences between the
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wavenumbers given above are easily understandable, taking into account the presence of
different partners (CO, CO2 and N2) in the same matrix cage with the in-situ-generated
iminoketene product.
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The spectral positions of the bands due to "antisymmetric" stretching vibrations of the
iminoketene-1 and iminoketene-2 structures have been theoretically calculated at the
DFT(B3LYP)/6-311++G(2d,p) level. Although the theoretically computed wavenumbers
2147 and 2132 cm-1 differ slightly from the experimentally observed spectral position (the
shift between theory and experiment is a mere result of the applied scale factor), the predicted
spectral distance (=15 cm-1) between the band due to iminoketene-1 and the band due to
iminoketene-2 is exactly the same as the spectral distances within the pairs of the
experimentally observed bands (2126 and 2111 cm-1 , Fig. 6 a) or (2129 and 2114 cm-1, Fig.
6 b). All this spectral evidence speaks in favor of the intepretation of the bands observed in
the 2130 - 2110 cm-1 spectral range in terms of two iminoketene-1 and iminiketene-2 open-
ring isomers of benzoazetinone. Moreover, this assignment has been further supported by the
results of the experiments on ring-opening/ring-closure photoinduced transformations, which
are described in section 4.5.
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The most intense band, present in the spectrum recorded after UV ( = 278 nm)
irradiation of matrix-isolated isatoic anhydride, is the very strong IR absorption at ca. 2343
cm-1 (Figure 6 b). This band is split into several components. It appears only in the spectrum
of UV-irradiated isatoic anhydride; no analogous strong IR absorption appears in the
spectrum of UV-irradiated isatin. The very characteristic frequency26,27 and huge intensity
strongly suggest that this band must be due to the antisymmetric stretching vibration of CO2
molecule, which has been released from the UV excited isatoic anhydride. All the
experimental observations and their interpretations described above combine into a consistent
picture (Scheme 2) of photoreactions occuring for isatin and isatoic anhydride monomers
trapped in Ar matrixes and irradiated with UV ( = 278 nm) light.
4.4 Effects of UV ( = 278 nm) Irradiation of Phthalimide Isolated in Low-
Temperature Argon Matrixes. The photochemical behavior of phthalimide, the formal
isomer of isatin, appeared to be quite different. Phototransformations of phthalimide
monomers isolated in an Ar matrix and irradiated with UV ( = 278 nm) light were slow but
noticeable. After 5.5 hours of UV irradiation at 278 nm, the bands initially present in the IR
spectrum of the dioxo form of phthalimide diminished by ca. 20%, while a set of new IR
bands emerged.
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Extracted IR spectrum of the photogenerated product(s) is presented in Figure 7. In
this spectrum, one of the new IR absorption bands appeared at 3545 cm-1, where IR bands due
to O-H stretching vibrations are expected. Presence of this band suggests that one of the
photogenerated products has an O-H group in its structure. One of the most probable
candidates for such a photoproduced species is the oxo-hydroxy tautomer of phthalimide. The
oxo-hydroxy tautomer may be formed by UV-induced migration of hydrogen atom from
nitrogen to one of the vicinal oxygen atoms of the dioxo form of phthalimide. The comparison
of the experimental spectrum of the photoproducts generated upon UV ( = 278 nm)
irradiation of matrix-isolated phthalimide with the spectrum theoretically simulated for the
oxo-hydroxy form of the compound is presented in Figure 7 and Table S8 in the Supporting
Information. The theoretical IR bands predicted for the hydroxy tautomer of phthalimide have
their counterparts in the experimental spectrum of the photogenerated species. Good
agreement between these two sets of bands suggests that the oxo-hydroxy form of phthalimide
is indeed photoproduced (Scheme S2 in the Supporting Information). Analogous
oxo→hydroxy phototautomerizations in systems without intra-molecular hydrogen-bonds are
well-known. The oxo→hydroxy phototransformation was first observed for
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4(3H)-pyrimidinone28,29 and later for a number of other matrix-isolated compounds containing
lactam groups.30-34 Therefore, occurrence of this type of phototautomerization may be
expected also for phthalimide.
Other characteristic bands, appearing upon UV ( = 278 nm) irradiation in the
spectrum of matrix-isolated phthalimide, are those found in the 2300 – 2100 cm-1 region, see
Figure 7. The split absorption at 2265 -2225 cm-1 can be attributed to the stretching vibrations
of HNCO,35-37 whereas absorption bands at 2147 cm-1 and 2140 cm-1 are due to the CO
stretching vibration of carbon monoxide.23,24 The presence of these bands in the spectra of
phthalimide isolated in an Ar matrix and exposed to UV ( = 278 nm) light could indicate the
decomposition of the five-membered (maleimide) ring of the compound into HNCO and CO
fragments. The remaining fragment of the molecule, with a six-membered ring, might adopt
the o-benzyne structure. But no experimental IR absorptions, appearing in the spectrum of
photoproducts generated from phthalimide, were assigned to o-benzyne. This may be related
to very low intensities theoretically predicted for IR bands of this species.38 Hence,
photogeneration of o-benzyne from UV-excited phthalimide was not spectroscopically
supported, see Scheme S2 in the Supporting Information.
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4.5 Photoreversible Conversions between Closed-ring Benzoazetinone and Open-
ring -Iminoketene. Monomers of isatin isolated in an Ar matrix and irradiated with UV
( = 278 nm) light were subsequently irradiated with green (=532 nm) light (Figure 8). Upon
this irradiation the IR bands at 2126 and 2111 cm-1 (due to two isomers of -iminoketene)
decreased in intensity (Figure 8 b), whereas the intensity of the band at 1843 cm-1 (due to
closed-ring benzoazetinone) increased. The observed intensity changes are the spectral
signatures of the photoinduced conversion transforming the open-ring -iminoketene into
benzoazetinone, where the four-membered ring is closed. Analogous ring-closure
conversions, occurring upon irradiation of matrix-isolated -iminoketene with visible (=500
nm) light, were observed for compounds structurally similar to benzoazetinone:
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N-methylbenzoazetinone7 and N-deuterated benzoazetinone.9 In the current study, both
iminoketene-1 and iminoketene-2 isomers are consumed in the ring-closure photoprocess.
That implies that excitation with green (=532 nm) light also induces the syn-anti
photoisomerization converting one -iminoketene isomer into the other.
The argon matrix containing isatin isomers subjected to irradiations at =278 nm and
then at =532 nm was subsequently irradiated with =305 nm light generated by an UV LED.
Upon this latter irradiation, the intensity of the band at 1843 cm-1 (due to closed-ring
benzoazetinone) decreased, whereas the intensities of the bands at 2126 and 2111 cm-1 (due to
two isomers of -iminoketene) increased (Figure 8 c). This is a clear indication of the ring-
opening photoconversion that occurs upon =305 nm irradiation and transforms the closed-
ring benzoazetinone into two isomers of -iminoketene. The effects of irradiation at 532 nm
and 305 nm allowed separation of the IR spectra of benzoazetinone and its open-ring
-iminoketene isomers (Figure 9). The experimental IR bands assigned to the -iminoketene
product are listed in Table S9 in the Supplementary Information.
The intensity changes of the IR bands at 2126 / 2111 cm-1 (due to -iminoketene
isomers) and at 1843 cm-1 (due to benzoazetinone), reflecting ring-closure transformation
occurring upon irradiation at 532 nm, were quantitatively measured. Upon this irradiation, the
intensity of the bands at 2126 / 2111 cm-1 diminished by ca. 40%, whereas the intensity of the
band at 1843 cm-1 increased by 5%, with respect to the intensities of these bands in the
spectrum recorded after the initial irradiation at 278 nm. (Figure 8 a, b) The intensity changes,
reflecting the ring-opening transformation occurring upon the subsequent irradiation at 305
nm (the third irradiation, following exposure of the matrix to =278 nm and =532 nm light),
were also quantitatively measured. Upon this latter irradiation at 305 nm, the intensity of the
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bands at 2126 / 2111 cm-1 increased by 50%, while the intensity of the band at 1843 cm-1
decreased by 7%, with respect to the intensities of these bands in the spectrum recorded after
the initial irradiation at 278 nm. (Figure 8 a, c). These data show that after a prolonged
UV( = 278 nm) irradiation (consuming nearly all of the isatin precursor), the amount of
benzoazetinone is ca. 7.5 times larger than the combined amount of both isomers of open-ring
-iminoketene, produced upon the same irradiation at =278 nm.
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Similar ratio of closed-ring benzoazetinone and open-ring -iminoketene was generated
(Figure 6 a, b) upon a prolonged UV ( = 278 nm) irradiation that consumed nearly all of the
isatoic anhydride precursor. In spite of low population, IR spectral signature of
photoproduced -iminoketene was clearly seen in the spectra recorded after =278 nm
irradiation. This was possible mainly because of the very high ( > 1000 km mol-1) absolute
intensity of the IR band due to the "antisymmetric" stretching vibration of the C=C=O
fragment.
CONCLUSIONS
In the current work, photochemical behavior of isolated molecules of unsubstituted isatin has
been studied for the first time. The study demonstrated that monomers of the compound,
isolated in Ar matrixes and excited with UV ( = 278 nm) light, undergo
photodecarbonylation. The main molecular fragment remaining after release of CO, adopts
either the closed-ring benzoazetinone structure or the open-ring structure of -iminoketene.
Benzoazetinone and its open-ring isomer -iminoketene were also obtained as the products of
UV ( = 278 nm) induced photodecarboxylation of isatoic anhydride. Thus, in the present
paper, we provide the first report on photogeneration of unsubstituted and non-deuterated
benzoazetinone and on the IR spectrum of this compound (see Table 1 and Table S7 in the
Supporting Information). Our identification of the products of the investigated
phototransformations was based on the analysis of the IR spectra recorded after UV ( = 278
nm) irradiation of matrix-isolated isatin and isatoic anhydride. The assignment of
benzoazetinone and -iminoketene structures to the photogenerated products was further
supported by the observation of the photoinduced ring-closure and ring-opening conversions
transforming the open-ring (-iminoketene) and closed-ring (benzoazetinone) products into
each other.
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Interestingly, phthalimide molecules isolated in Ar matrixes and excited with UV ( =
278 nm) light did not phototransform to benzoazetinone and its open-ring -iminoketene
isomer. Instead, matrix-isolated molecules of phthalimide underwent a photoinduced
dioxo→oxo-hydroxy conversion, leading to generation of the oxo-hydroxy tautomer of the
compound, see Scheme S2 in the Supporting Information. HNCO and CO were also detected
as minor products of the UV-induced transformations of phthalimide.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at
https://pubs.acs.org/doi …………..
Calculated relative energies of different isomers of isatin, isatoic anhydride and
phthalimide; Tabulated infrared spectra of isatin, isatoic anhydride, phthalimide,
benzoazetinone and -iminoketene; Effects of UV irradiation of phthalimide; Schemes
of photochemical reactions.
AUTHOR INFORMATION
Corresponding Author
Maciej J. Nowak – Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-
668 Warsaw, Poland; orcid.org/0000-0002-0693-4109; Email: mjnowak@ifpan.edu.pl.
Authors
Hanna Rostkowska – Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46,
02-668 Warsaw, Poland; orcid.org/0000-0002-09186-0705.
Leszek Lapinski – Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-
668 Warsaw, Poland; orcid.org/0000-0003-2896-4007.
Author Contributions
The manuscript was written through contributions of all authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was financed by the Institute of Physcics, Polish Academy of Sciences.
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References
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Table 1 Approximate assignment of the most pronounced absorption bands, observed in the
IR spectrum of photoproducts generated upon UV (=278 nm) irradiation of isatin and isatoic
anhydride isolated in Ar matrixes, to the normal modes calculated for benzoazetinone at the
DFT(B3LYP)/6-311++G(2d,p) level of theory.
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Ar matrix
Isatoic anhydride
Calculation
Isatin
photoproduct
DFT(B3LYP)/6-311++G(2d,p)
photoproduct
a
I
I
Ath
km mol-1
Approximate
description
cm-1
cm-1
cm-1
rel.
rel.
3415, 3409, 3407
124 3424, 3408
570 1846, 1838
238 1606
85
3370
1844
1604
1445
1361
1295
1181
956
26  NH
570  C=O
190  CC
28  CH
1843
570
262
55
16
56
25
22
26
55
25
1606
1439, 1437
1370
54 1440, 1437
16 1370
17  CC
1293
55 1294
33  CH
1191 broad
968
23 1192 broad
46 971
27  CN,  CC
27  R,  CN,  CH
21  CC
830
31 831
818
740
68 741
737
69  CH
497, 492
31 493
472
108  NH
a The theoretical wavenumbers higher than 3000 cm−1 were scaled by a factor of 0.95,
whereas those lower than 3000 cm−1 were scaled by 0.98.
Abbreviations: rel. relative;  stretching;  bending;  wagging.
Wavenumbers of the strongest bands in a multiplet are underlined.
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Figures and captions
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O
O
O
O
N H
O
N
N
H
O
O
H
isatin
phthalimide
isatoic anhydride
Scheme 1. Chemical structures of the most stable, dioxo forms of isatin, phthalimide and
isatoic anhydride.
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Scheme 2. Photochemical reactions induced by UV (=278 nm) irradiation of matrix-isolated
isatin and isatoic anhydride.
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Figure 1. Experimental IR spectrum of isatin monomers isolated in a low-temperature argon
matrix at 15 K (a), compared with harmonic wavenumbers and IR intensities calculated for
the dioxo tautomer of the compound at the DFT(B3LYP)/6-311++G(2d,p) level (b). The
theoretical wavenumbers higher than 3000 cm−1 were scaled by a factor of 0.95, whereas
those lower than 3000 cm−1 were scaled by 0.98.
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Figure 2. Experimental IR spectrum of isatoic anhydride monomers isolated in a low-
temperature argon matrix at 15 K (a), compared with harmonic wavenumbers and IR
intensities calculated for the dioxo tautomer of the compound at the
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DFT(B3LYP)/6-311++G(2d,p) level (b). The theoretical wavenumbers higher than 3000 cm−1
were scaled by a factor of 0.95, whereas those lower than 3000 cm−1 were scaled by 0.98.
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Figure 3. Experimental IR spectrum of phthalimide monomers isolated in a low-temperature
argon matrix at 15 K (a), compared with harmonic wavenumbers and IR intensities calculated
for the dioxo tautomer of the compound at the DFT(B3LYP)/6-311++G(2d,p) level (b). The
theoretical wavenumbers higher than 3000 cm−1 were scaled by a factor of 0.95, whereas
those lower than 3000 cm−1 were scaled by 0.98.
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Figure 4. The effects of UV (=278 nm) irradiation of isatin (a) and isatoic anhydride (b)
isolated in Ar matrixes at 15 K. Black traces represent the spectra after deposition of the
matrix, red traces illustrate the spectra after exposure to the UV (=278 nm) light. Total
duration of UV irradiation was 4 h for isatin and 95 min for isatoic anhydride.
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Figure 5. Fragments of the IR spectra of the photoproducts obtained after UV (=278 nm)
irradiation of isatin (a) and isatoic anhydride (b) isolated in low-temperature Ar matrixes,
compared with harmonic wavenumbers and IR intensities calculated for benzoazetinone at the
DFT(B3LYP)/6-311++G(2d,p) level (c). The theoretical wavenumbers higher than 3000 cm−1
were scaled by a factor of 0.95, whereas those lower than 3000 cm−1 were scaled by 0.98.
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Figure 6. The 2430 -1680 cm-1 range of the IR spectra of photoproducts generated upon UV
(=278 nm) irradiation of monomeric isatin (a) and monomeric isatoic anhydride (b) isolated
in low-temperature Ar matrixes; compared with harmonic wavenumbers calculated at the
DFT(B3LYP)/6-311++G(2d,p) level (c) for: benzoazetinone (red), isocyanic acid (turquoise
blue) and two isomers of iminoketene (green and violet), see Scheme 2. The theoretical
wavenumbers were scaled by a factor of 0.98. Arrows at the lower panel indicate spectral
positions of the experimental bands due to the stretching vibrations of CO2 (magenta), HNCO
(turquoise blue) and CO (blue).
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Figure 7. Fragments of the IR spectra of the photoproducts obtained upon UV (=278 nm)
irradiation of phthalimide isolated in low-temperature Ar matrixes (a), compared with
harmonic wavenumbers and IR intensities calculated for the oxo-hydroxy tautomer of
phthalimide at the DFT(B3LYP)/6-311++G(2d,p) level (b). The theoretical wavenumbers
higher than 3000 cm−1 were scaled by a factor of 0.95, whereas those lower than 3000 cm−1
were scaled by 0.98.
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Figure 8. The experimental spectra of photoproducts generated from isatin isolated in an Ar
matrix and subjected to a series of irradiations: the spectrum recorded after the first irradiation
with UV (=278 nm) light (a); the spectrum recorded after subsequent irradiation of the
matrix with green (=532 nm) light minus the spectrum recorded before this irradiation (b);
the spectrum recorded after three consecutive UV (=278, 532 and 305 nm) irradiations
minus the spectrum recorded before the last irradiation at 305 nm (c). The experimental
spectra are compared with harmonic wavenumbers and IR intensities calculated at the
DFT(B3LYP)/6-311++G(2d,p) level (d) for benzoazetinone (red), two isomers of
iminoketene (green and violet), see Scheme 2. The theoretical wavenumbers were scaled by a
factor of 0.98.
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Figure 9. Fragments of experimental difference spectrum obtained by subtraction of the
spectrum recorded after irradiation of matrix-isolated isatin with UV (=278 nm) light and
subsequently with green (=532 nm) light, from the spectrum recorded after three consecutive
UV (=278, 532 and 305 nm) irradiations (a). The experimental spectrum is compared with
harmonic wavenumbers and IR intensities calculated at the DFT(B3LYP)/6-311++G(2d,p)
level (b) for benzoazetinone (red), two isomers of iminoketene (green and violet), Scheme 2.
The theoretical wavenumbers higher than 3000 cm−1 were scaled by a factor of 0.95, whereas
those lower than 3000 cm−1 were scaled by 0.98.
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