The photochemistry of an aryl pentazole in liquid solutions: The anionic 4-oxidophenylpentazole (OPP)

Article abstract of DOI:10.1016/j.jphotochem.2013.12.008

This paper reports a study on the photolysis of an aryl pentazole in solution. 4-Oxidophenylpentazole (OPP) was irradiated in water and in acetonitrile at its first absorption band (320 and 370 nm, respectively) and at 193 nm. 4-Oxidophenylazide (OPA) was

Full text of DOI:10.1016/j.jphotochem.2013.12.008

Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The photochemistry of an aryl pentazole in liquid solutions:
The anionic 4-oxidophenylpentazole (OPP)
Uzi Geigera, Yehuda Haasa,∗, Dan Grinsteinb
a Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Jerusalem 91904, Israel
b IMOD, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper reports a study on the photolysis of an aryl pentazole in solution. 4-Oxidophenylpentazole
(OPP) was irradiated in water and in acetonitrile at its first absorption band (320 and 370 nm, respectively)
and at 193 nm. 4-Oxidophenylazide (OPA) was also investigated under similar conditions for comparison.
NMR and UV–vis absorption spectroscopies of the products show that the major stable products of OPP
photolysis in acetonitrile are OPA (which is also a thermal product), 4,4ꢀ-dihydroxyazobenzene, and 4-
aminophenol while in water the main products are OPA and indophenol. The final product distribution
obtained upon 193 nm irradiation is essentially the same as that found for the longer wavelength sources
used. In these experiments some of the products are formed upon cleavage of the CN bond connecting
the phenyl and the pentazole rings. These reactions are due to secondary collisional processes and not to
a primary photochemical event. A mechanism accounting for the results is proposed.
Received 20 October 2013
Received in revised form
22 November 2013
Accepted 10 December 2013
Available online 17 December 2013
Keywords:
Aryl pentazoles
Cyclo-pentazolate anion
Photochemistry
© 2014 Elsevier B.V. All rights reserved.
Para oxidophenylazide
Para oxidophenylpentazole
1. Introduction
of aryl pentazoles has been reported to date. In this paper we
report an initial photochemical study of aryl pentazoles, using
Aryl pentazoles were first synthesized by Lifschitz in 1915 [1]
and later extensively studied by Huisgen and Ugi in the late 1950s
and early 1960s [2–7]. Interest in aryl pentazoles has recently been
revived due to the reports that the cyclo-pentazolate anion (cyclo-
N5−) was formed in the gas phase from 4-hydroxyphenylpentazole
(HOPP) [8] and 4-dimethylaminophenylpentazole [9]. The all-
nitrogen cyclo-pentazolate anion was detected by mass spectrome-
try. Attempts to repeat this reaction thermally in the bulk have been
unsuccessful [5,10], although ceric ammonium nitrate (CAN) oxida-
tion of 4-methoxyphenyl pentazole did yield products believed to
indicate the prior formation and degradation of cyclo-N5− [11]. This
inference is in contrast with a previous paper [12] that concluded
that no direct observation of the anion in solution was demon-
strated. I−n any case, at the present time there is a consensus that
cyclo-N5 has not been isolated by reactions of aryl azides in the
bulk.
4-oxidophenylpentazole (OPP) as the reactant. Theoretical calcu-
lations indicate that the neutral N5 ring radical spontaneously
decomposes, whereas cyclo-N5− is calculated to be relatively stable
[14–17]. The choice of an anionic aryl-pentazole precursor, rather
than a neutral one was motivated by the notion that formation of
cyclo-N5− would be in this case a homolytic bond cleavage whereas
if a neutral precursor is chosen the cleavage is heterolytic and extra
energy is needed to separate the nascent cation and anion. For this
reason, and due to the high stability of OPP and HOPP relative to
other aryl pentazoles, we have chosen OPP for this work.
OPP and the corresponding phenol (HOPP) have been studied
by several groups [3,8,10]. Both species were found to be unsta-
ble at room temperature, releasing N2 to form 4-oxidophenylazide
(OPA) and 4-hydroxyphenylazide (HOPA), respectively. The activa-
tion energies measured for both processes are about 20 kcal/mol
[10]. OPP and OPA were characterized by proton, 13C, and 15N NMR
spectroscopies in solution [10].
It has been suggested some time ago that cleavage of the C
N
bond connecting the pentazole and the aromatic rings might be
accomplished using photochemical methods [13]. However, to
the best of our knowledge, no experimental photochemical study
Formation of an azide by dinitrogen extrusion is the energeti-
cally preferred thermal reaction, and is expected to be important
also in photochemical reactions. It might be assumed that the azides
are the primary products which subsequently lead to other prod-
ucts, by thermal or photochemical secondary reactions. Whereas
the photochemistry and thermochemistry of many aryl azides
were extensively investigated [18–22], the photochemistry of para-
phenol azides and the corresponding anions is only poorly studied
∗
Corresponding author. Tel.: +972 26585067; fax: +972 25860622.
E-mail addresses: yehuda.haas@mail.huji.ac.il, yehuda@chem.ch.huji.ac.il
(Y. Haas), Dan grinstein@mod.gov.il (D. Grinstein).
1010-6030/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.12.008

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U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
Chart 1. Main compounds discussed in this article with their abbreviations (in parentheses) as neutral, mono anionic and di-anionic forms when applicable. BQI in this paper
is the short-hand description of 4-benzoquinone monoimine (para BQI). The ortho form (2-benzoquinone monoimine) is marked as such where appropriate.
[23,24]. We have therefore examined also the photochemistry
of OPA, and compared it with that of the OPP. The names and
structures of the compounds discussed in this paper are listed in
Chart 1.
conditions). 6 ml of water, 1 ml of HCl (12 mequiv.) and 1.302 g
(8.9 mmol) of p-aminophenol HCl were introduced to the reac-
tion vessel under continuous stirring. Upon drop-wise addition of
0.703 g (10.5 mmol) of NaNO2 dissolved in 4 ml water, the solu-
tion, stirred for 1 h, became dark blue due to the formation of
the diazonium salt. 20 ml of methanol were added and the ves-
sel was allowed to stabilize at −15 ◦C for 1.5 h. 0.813 g (12.5 mmol)
of sodium azide dissolved in 8 ml water were added slowly. During
the addition of the azide strong bubbling of the solution occurred
due to generation of gaseous nitrogen and a white precipitate was
formed. After allowing the solution to react for 1 h, the precipitate
was filtered through a cooled, jacketed filter funnel and washed
with ca. 150 ml freezer-cold 1:1 methanol:water solution to yield
the crude product. The crude product was re-dissolved in 13 ml
cold methanol and filtered into a dry-ice cooled filter bottle, into
which 10 ml of pure water were added and the precipitated mate-
rial was again filtered through a cold filter and washed with the
methanol:water mixture. The product HOPP was transferred to a
weighted vial and dried under vacuum at ca. −30 ◦C. Yield: 16%
(0.233 g, 1.43 mmol).
2. Experimental
2.1. Materials and equipment
P-aminophenol HCl (Fluka, purum, ≥99.0%), sodium nitrite
(Aldrich, 97.0%), sodium azide (Aldrich, purum, ≥99.0%), methanol
(BioLab, HPLC supra gradient), deuterium oxide (99.9 atom% D,
Aldrich), acetonitrile-D3 (D, 99.8%, Cambridge Isotope Laborato-
ries, inc.), hydrochloric acid (Sigma–Aldrich, 37% fuming), and
nitrogen-15 labeled sodium nitrite (98% + 15N, Cambridge Isotope
Laboratories, Inc.) were used as received. Tetramethyl ammonium
hydroxide (Sigma–Aldrich, 25 wt.% in methanol, TMAOH) was used
as received or diluted 10 times in dry MeCN or MeOH. Acetonitrile
(Fluka, for UV-spectroscopy, ≥99.8%) was dried over 4 A molecular
sieves, passed over a short column of molecular sieves and filtered
just prior to use. The water used was ultra-purified with a Branstead
Nanopure type I water system (Thermo Scientific).
A q-pod 2e temperature controlled cuvette holder system
equipped with a magnetic stirrer was used for irradiation (Quan-
tum North West). An USB-4000 fiber optic spectrometer was used
with a DH-2000-BAL or DT-mini-2-GS light source for UV–vis
absorption measurements, all from Ocean Optics. Two concen-
tration ranges were used, as the NMR measurements required
much higher concentration than practical for UV absorption spec-
trometry. The concentration required for NMR (∼10−2 M) was too
high for UV experiments, for which a ∼10−4 M concentration was
suitable to avoid saturation. The spectrometer and light source
were either attached to the q-pod perpendicular to the irradi-
ation direction for the low concentration experiments or to an
uncooled cuvette holder nearby for the high concentration experi-
ments (measurements were taken as quickly as possible). A Bruker
Avance II 500 MHz NMR spectrometer was used for all NMR mea-
surements. The photolysis light sources were either a pulsed ArF
excimer laser with variable firing rate (Neweks Ltd., PSX-100) or
a 150 W xenon lamp fitted with a water filter and the appropri-
ate low-pass and high-pass filters for the experiment (Hamamatzu
E7536 with L2175).
HOPA was prepared from HOPP by allowing the material to
warm to 25 ◦C.
OPP was freshly prepared for each experiment by adding excess
base (usually TMAOH) to a HOPP solution. OPA was likewise pre-
pared from HOPA, or by warming OPP to 25 ◦C.
2.3. Procedure of photochemical experiments
OPP and OPA were irradiated in water at +5 ◦C or MeCN at
−27.5 ◦C using either a pulsed ArF excimer laser at 193 nm or
the xenon lamp equipped with the appropriate filters. Low con-
centration experiments were performed as follows: 2 ml of the
appropriate solvent were loaded into a 10 mm × 10 mm quartz
cuvette equipped with a magnetic stirrer; the cuvette was placed
in the q-pod holder and allowed to equilibrate to the set tem-
perature. A solution of ca. 1 mg HOPP in 1 ml MeOH (for water)
or MeCN was prepared in a 5 ml vial and kept in dry ice. From
this mother solution about 5 ␮l were added to the cuvette and,
if OPA was needed, the solution was heated to 25 ◦C for 30 min
quantitatively yielding HOPA. Ca. 10 ␮l of 10× diluted TMAOH was
added to convert HOPP or HOPA to OPP or OPA, respectively. The
irradiation of the low concentration solution lasted no more than
5 min and was analyzed by UV–vis spectroscopy only.
2.2. Synthesis of OPP and OPA
High concentration experiments were performed as follows:
1.5 ml of the solvent, usually deuterated, were loaded into a
10 mm × 10 mm cuvette with a magnetic stirrer. The cuvette was
placed into the q-pod holder and allowed to equilibrate to the set
temperature. Roughly 5 mg of HOPP were weighed and added to
the cuvette. If OPA was needed in MeCN the system was heated
to 25 ◦C for 30 min to quantitatively yield HOPA; in water the base
HOPP was synthesized by a method similar to that used by Ugi
and Huisgen [2] and previously by us [25]. In a typical synthesis
a 100 ml glass flask (the reaction vessel) was immersed in ace-
tone in a jacketed reactor, through which ethanol was circulated
at −7.5 ◦C (no freezing occurred in the reaction vessel under these

U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
55
bonding, which might strongly affect the shift even at low water
concentrations. To clarify this point UV–vis absorption spectra of
OPP in MeCN:water solutions of different ratios were measured
(Fig. 1). In addition, the absorption spectrum of OPP in MeOH (capa-
ble of making hydrogen bonds, dielectric constant similar to MeCN)
was taken for comparison and is shown in the Supplementary Data
(SD1).
A 1% water impurity in MeCN accounts for 40% of the shift
and 5% account for 60% of the shift. These data, together with the
large MeOH shift (ꢀmax = 321 nm) compared to MeCN, indicate the
importance of hydrogen bonding.
Fig. 1. Normalized UV–vis absorption spectra of solutions of OPP in: dry MeCN
(A, ꢀmax = 368 nm); 1% water in MeCN (B, ꢀmax = 345 nm); 5% water in MeCN (C,
ꢀmax = 336 nm); and water (D, ꢀmax = 317 nm).
3.2. Irradiation in MeCN
was added first to allow good solubility. 15 ␮l of TMAOH was used
to convert the HOPP or HOPA to OPP or OPA, respectively. The irra-
diation of the high concentration solution lasted up to 4 h and was
analyzed by NMR spectroscopy, or, after appropriate dilution, by
UV–vis spectrometry. 1D proton and nitrogen and 2D COSY, HSQC
and HMBC spectra were used to identify the constituents of the
product mixture.
For validating the identification of the products it was often ben-
eficial to prepare the acid forms of the ions, as NMR data are more
abundant for the neutral acid. 5 ␮l of glacial acetic acid was added
directly to the NMR tube containing ca. 1 ml solution.
The UV–vis absorption spectra measured after Xe lamp irradi-
ation of OPP and OPA at low (Ca. 10−4 M) and high (Ca. 10−2 M)
concentrations, together with the absorption spectrum of OPP itself
and OPA, are shown in Fig. 2; changing the excitation source to
193 nm led to practically identical product spectra in the high con-
centration solution, but different spectra are observed in the low
concentration solution; in this case broad absorption bands charac-
teristic of tar particles, similar to other azides [18,20] were observed
(see SD2, SD3).
The most prominent, narrow absorption bands appearing
immediately upon irradiation of low OPP concentration at 242 and
251 nm (shown in SD3) are attributed to the anion of BQI. Their
intensity diminishes in less than a minute and two broader bands
centered at 500 and 615 nm are observed (Fig. 2); they also appear
upon OPA irradiation, (Fig. 3, below). These bands are assigned to
polymers such as polyacetonitrile and poly aminophenol.
At higher concentrations (Fig. 2C.) a major new absorption peak
appears at 475 nm as well as another single peak at about 250 nm.
These bands are tentatively assigned to DHAB− and PAP−, respec-
tively.
3. Results
3.1. OPP spectra in water-acetonitrile mixtures
The first absorption band of OPP is blue shifted upon chang-
ing the solvent from MeCN to water by a relatively large shift
(0.542 eV), from 368 to 317 nm (Fig. 1). This large difference may
be due either to polarity change, in which case a low water
impurity in MeCN would have a negligible effect, or to hydrogen
Fig. 2. UV–vis absorption spectra in MeCN at −27.5 ◦C of: (A) OPP (40 ␮M, full line) and OPA (40 ␮M, dashed line); (B) a 45.5 ␮M OPP solution irradiated at 350–390 nm for
60 s (full line) and a 50 ␮M OPA solution irradiated at 280–390 nm for 10 s (dashed line); (C) a 28,000 ␮M OPP solution irradiated at 350–390 nm for 150 min (full line) and
a 41,000 ␮M OPA solution irradiated at 280–390 nm for 90 min (dashed line). C was measured following 50 times dilution for the OPP solution and 100 times for the OPA
solution in a 2 mm cuvette for a total of 250× and 500× optical dilution for OPP and OPA, respectively.

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U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
in the same product absorption spectra as upon using carefully
dried MeCN.
The tentative assignments of the products at low and high
OPP concentrations suggested by UV–vis spectra were confirmed
and improved by NMR measurements carried out using high
concentrations of OPP. Fig. 4 shows representative proton NMR
spectra of irradiated solutions of OPP and OPA in MeCN; the same
spectra are found following irradiation of OPP or OPA at 193 nm. It is
noted that in general the product distribution of the major non-OPA
products of OPP irradiation (Table 1) are similar in MeCN to those
recorded upon OPA irradiation (except for the long wavelength irra-
diation of OPP). All other NMR spectra, including the proton NMR
spectrum of an OPA solution after irradiation and acidification with
acetic acid, are shown in the Supplementary Data (SD4).
Fig. 3. UV–vis absorption spectra at −27.5 ◦
C of OPA solutions (48 ␮M) after
280–390 nm irradiation for 10 s in wet (0.25% water) MeCN (full line) and in dry
MeCN (dashed line). The spectrum of the dry solution was recorded 1 min after irra-
diation, the spectrum of the wet solution 10 min after irradiation and is identical to
the one taken immediately after irradiation.
3.3. Photochemistry in water
Being anions, OPP and OPA readily dissolve in water, in con-
trast with other aryl pentazoles and organic azides. Fig. 5 shows
the UV–vis spectra of OPP and OPA solutions in water at 5 ◦C and
those obtained after irradiating OPP at 5 ◦C at high and low con-
centrations. The most prominent new absorption bands in the low
concentration samples are at 255 and 262 nm, slightly shifted from
the product found for low concentration in wet MeCN (Fig. 2) and
are assigned BQI in water. In the high concentration case the first
main band appears at 632 nm (IP−) and another one appears after
further irradiation at 450 (probably LIP−) nm, with no qualitative
difference between OPP and OPA or between the two excitation
sources (other spectra are shown in the Supplementary Data (SD5)).
Water appears to strongly affect the outcome of the irradia-
tion, in both OPP and OPA as can be seen from Fig. 3: even a small
amount of water changes the absorption of the products to 250 and
260 nm. These absorption bands are due to species that are more
stable than those obtained in dry solutions (absorption peaks at
242 and 251 nm) and are compatible with the formation of BQI in
MeCN, possibly hydrogen-bonded to water. In the high concentra-
tion experiments no special measures for solvent dehydration were
needed and the use of commercial CD3CN (not anhydrous) resulted
Fig. 4. Proton NMR spectra and major peak assignments of: (A) OPP irradiated at 350–390 nm for 150 min; (B) OPA irradiated at 280–390 nm for 90 min. Both irradiations
were carried out in CD3CN at −27.5 ◦C and both spectra were taken at −35 ◦C. The full assignments are discussed in a subsequent Section 3.4.

U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
57
Table 1
The photochemical product distribution obtained upon irradiation of OPP and OPA solutions in MeCN using different light sources. The main product from OPP irradiation is
always OPA, (not included in the table). The unknown product shows splitting characteristic of a 1,2,4 substituted aromatic ring (AX, AXY, and AY splitting for the 7.14, 6.47,
and 5.77 peaks, respectively). Percentages derived from proton NMR measurements.
Experimental conditions
Product (aromatic proton shift)
DHAB− (7.46, 6.42)
PAP− (6.36, 6.27)
Unknown (7.14, 6.47, 5.77)
OPP 193 nm irradiation
OPA 193 nm irradiation
OPP 350–390 nm irradiation
OPA 250–390 nm irradiation
50%
47%
28%
52%
36%
34%
59%
32%
5.5%
4.2%
13%
3.2%
Abbreviations according to Chart 1
Fig. 6. UV–vis absorption spectra of an OPP solution (23 mM) in water at 5 ◦C after
260 min irradiation at 325–390 nm: (A) as is and (B) acidified with acetic acid. Both
spectra diluted 50× in water and using a 2 mm cuvette for a total 250× optical
dilution.
Table 2
Major products obtained upon OPA irradiation in MeCN and in water at different
concentrations. Products of OPP irradiation are identical apart from OPA being the
dominant product.
Solvent
Low concentration
High concentration
Polyacetonitrile
Polyaminophenol
PAP−
DHAB−
PAP−
MeCN
Fig. 5. UV–vis absorption spectra in water at 5 ◦C of: (A) OPP (24 ␮M), (B) OPA
(24 ␮M), (C) a 24 ␮M OPP solution irradiated by a Xe lamp (270–380 nm filter) for
90 s, and (D) a 23,000 ␮M OPP solution irradiated by a Xe lamp (325–390 nm filter)
for 4 h and 20 min. D was measured following 50 times dilution in a 2 mm cuvette
for a total of 250× optical dilution.
BQI
IP−
Water
LIP−
(the dominant product). As seen in the table different end products
are found for the two solvents.
After addition of excess acetic acid the UV–vis changes to the
spectrum shown in Fig. 6, with the 632 nm absorption band shifted
to 496 nm, agreeing well with the protonation of IP− to IP.
A representative H NMR spectrum of an OPP solution irradiated
in D2O is shown in Fig. 7. Two main products appear in addition
to OPA after the irradiation: one with broad peaks and the other
with sharp ones, both are in the aromatic region. Their proposed
assignments are shown in the figure and are compared with liter-
ature in Section 3.4.
3.4. Product identification
The products were identified mainly on the basis of NMR data,
and are also compatible with the UV–vis absorption spectra. Table 3
summarizes the data collected for the identified products and lit-
erature references (if available). The NMR data consisted of both
proton and carbon data via the HSQC and HMBC methods. Iden-
tification was helped by acidification of the solution as literature
data on the acid form is more extensive than on most mono and
di-anionic molecules of interest. The identification of the anionic
forms matching the neutral ones, especially for molecules with
more than one such form, is based on integration of proton NMR
signals.
Fig. 8 presents NMR of OPP, labeled at the 2/5 positions by using
15N isotopically enriched sodium nitrite in the synthesis, after Xe
lamp irradiation in water. The signals shown are those of OPP, OPA,
N2 and the centrally labeled azide ion only. The same signals, apart
from the azide ions, were also detected after OPP and OPA irradi-
ation in MeCN and in a solution of OPP in MeCN allowed to react
thermally for a while (shown in SD6).
Table 2 summarizes the key results and compares the major
products of OPA photolysis in MeCN with those found in water, the
assignments are fully discussed in the next subsection. OPP irradi-
ation leads to the same products in addition to production of OPA
The identification of LIP is based only on H NMR data and com-
parison to calculated references and is therefore less reliable.
Other products were obtained in low yields, not sufficient
for 13C NMR analysis (analyzed by proton NMR integration). The

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U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
Fig. 7. Proton NMR spectrum of a 19 mM OPP solution in D2O at 5 ◦C after a 325–390 nm irradiation for 100 min.
Fig. 8. 15N NMR spectrum of a 36.5 mM OPP solution in water at 5 ◦C after 4 h irradiation at 305–390 nm.
Table 3
Identified and suspected products with their experimental data and literature references. Each reference is for both NMR and UV–vis data, where applicable.
Compound
Experimental data
Literature data
Notes and references
In DMSO-d6 [26]
DHAB (CD3CN)
1H NMR: 7.72, 6.98
1H NMR: 7.71, 6.94
13C NMR: 161.0, 145.1, 124.1, 115.9
1H NMR: 7.46, 6.42
13C NMR: 160.8, 146.1, 125.0, 116.6
UV–vis: 465–470 nm
DHAB− (CD3CN)
In water [27]
13C NMR: 171.9, 140.6, 123.6, 118.9
UV–vis: 475 nm
DHAB−2 (CD3CN)
PAP (CD3CN)
1H NMR: 6.78, 6.24
13C NMR: 169.6, 140.3
1H NMR: 6.58, 6.50
1H NMR: 6.485, 6.426
13C NMR: 148.25, 140.46, 115.51,
115.26
In DMSO-d6 [28]
13C NMR: 149.2, 139.9, 115.8, 115.3
PAP− (CD3CN)
1H NMR: 6.36, 6.27
UV–vis: 256, 320 nm
From an authentic material sample
13C NMR: 160.4, 133.5, 118.8, 116.8
UV–vis: 250, 330 nm
BQI (water)
IP (water)
UV–vis: 255, 262 nm
UV–vis: 255, 263 nm
1H NMR: 7.32, 6.96
UV–vis: 496 nm
In water[29]
In water [30]
1H NMR: 7.22, 6.86
UV–vis: 496 nm
IP− (water)
LIP (water)
1H NMR: 7.34, 6.72
1H NMR: 7.18, 6.8
UV–vis: 637 nm
In water [30]
[31]
UV–vis: 632 nm
1H NMR: 7.28, 7.00 (obtained after
acidification, poor quality due to
aggregation)
1H NMR: 7.1, 6.89 (Predicted
spectrum, as experimental data not
available)
LIP− (water)
1H NMR: 6.70, 6.54
Not available
13C NMR: 159.1, 133.3, 118.6, 118.6
UV–vis data for these compounds are also largely unknown due,
probably to overlap with the absorption of other substances in
the mixture and/or poor signal to noise ratio. The data on these
compounds are available in the Supplementary Data (SD7 and
SD8).
4. Discussion
4.1. Main products
The photochemical reactions of many aryl azides are known to
proceed via an aryl nitrene intermediate [20–22]. Our data show

U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
59
Scheme 1. Main routes proposed for the photochemistry of OPP in the studied media.
Scheme 2. An outline of the reaction of BQI with OPA in water, forming indophenolate and the azide ion. The possible photoreaction of IP− with water to give LIP− is also
depicted.
that irradiation of OPP in MeCN produces mainly OPA, and by
analogy with the rich literature on aryl azide photochemistry [20],
irradiation of OPA leads to the final products by reactions of the
nitrene intermediate (Scheme 1). There is no evidence that another
mechanism is operative in the case of high energy irradiation of OPP
in MeCN or water (at 193 nm). It is noted that in this case a good
separation between the excitation of OPP and OPA could not be
achieved.
The end products of OPP and OPA irradiation in MeCN are
assumed to be derived from triplet nitrene by analogy with other
aryl azides [20]; the currently accepted literature mechanisms
for their production are included in Scheme 1. The low concen-
tration experiments in dry MeCN show an overlap of two main
absorptions: one is identified as the polymer of PAP which has an
absorption band at 500 nm [32] and the other, whose absorption
peaks at 615 nm, may be due to poly-acetonitrile, though regret-
tably literature references for the latter are not yet available. OPA,
PAP and DHAB are the main stable products of OPP irradiation at
concentrations high enough to use NMR spectroscopy. Another, as
yet unidentified, product in MeCN has the characteristic H NMR
peak splitting of a 1,2,4 substituted aromatic ring. This product
appears to undergo further photoreaction when more energetic
photons are used, but upon irradiation at 350–390 nm it remains
as one of the stable products (OPP irradiation). It should be noted
that whereas the relative amounts of the end product change for
different excitation sources, no new products are found in any
of these cases, indicating a quantitative rather than a qualitative
difference.
In water, the main products observed are not known to be
derived from a nitrene, and so is para-BQI (4-BQI) produced in
wet MeCN and in water under low concentration conditions. A
related compound (ortho-BQI, 2-BQI) was found upon irradia-
tion of 2-hydroxyphenylazide in various solvents, mostly in protic
solvents and specifically in water [23]. The unstable anion of BQI
may be viewed as a resonance form of singlet nitrene, which may be
further stabilized by addition of a proton to form neutral BQI. On the
other hand BQI is known to produce leuco indophenols with phe-
nols (when a para hydrogen is present) by electrophilic aromatic
substitution and these are easily oxidized by atmospheric oxy-
gen or by another BQI molecule to indophenols [30,33]. If BQI (an
electrophile) attacks OPA (a nucleophile) releasing an azide anion
rather than a proton as is the case for phenols, indophenol should
be directly formed, as indeed is the case at higher concentrations
of OPP or OPA (Scheme 2).

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U. Geiger et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 53–61
Scheme 3. Two possible reaction routes of nitrogen-15 labeled OPP at the 2/5 position with BQI: thermal or photochemical nitrogen extrusion to form OPA, which proceeds
to react with BQI to form IP and centrally labeled azide ion, or a direct reaction of OPP with BQI to form IP and cyclo-N5−; the latter might decompose thermally to centrally,
terminally and non labeled azide ions.
4.2. C N bond cleavage
is not absorbed by it. Otherwise, several more products are found,
presumably due to reactions of a nitrene formed by extrusion of
another N2 molecule. In general, the products of OPP and OPA pho-
tolysis were the same, as shown by independently studying OPA
photolysis. Under the experimental conditions employed here no
direct photochemical C N bond cleavage was observed. None the
less, C N bond cleavage occurred, presumably by reaction with
benzoquinone imine (BQI). This is, to the best of our knowledge,
the first observation of this reaction. Likewise, the photochemical
production of BQI from OPA was realized for the first time.
In MeCN the photochemistry of OPA is similar to that of other
aryl azides leading mainly to the formation of para aminophenol
(PAP) and 4,4ꢀ-dihydroxyazobenzene (DHAB). In water the photo-
chemistry leads to the formation of BQI which reacts with another
OPA molecule to form indophenol (IP).
The reaction between BQI and OPP or OPA to produce indophe-
nols (Scheme 2) involves the cleavage of the C N bond. OPP labeled
with 15N at the 2/5 positions of the pentazole ring was used to inves-
tigate this reaction (Fig. 8). While labeled OPA derived from the
labeled OPP produced the centrally labeled azide ion as expected
(Scheme 3), OPP also produced only this extra signal, indicating
that the only significant reaction is of BQI and thermally or pho-
tochemically produced OPA. If the C N bond of OPP would have
−
been cleaved and cyclo-N5 produced, two more signals should
have been observed – that of cyclo-N5 itself and that of its other
−
thermal dissociation product, the terminally labeled azide ion.
It is of interest to compare our results with previous work that
reported formation of cyclo-N5− from HOPP. The scission of the CN
bond was reported by Vij et al. [8] in the gas phase using mass spec-
trometry detection. The reaction probably proceeds by first forming
OPP (which is in equilibrium with HOPP, and whose concentration
can be increased using pyridine) followed by colliding the anion
with high energy argon or nitrogen. The result is somewhat sur-
prising, as extrusion of nitrogen from OPP is energetically preferred.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2013.12.008.
−
However, intact cyclo-N5 was only observed using high collision
energies (75 eV), whereas at low collision energies the azide was
formed. In solution, collision energies are limited by the solvent,
and thus our results are compatible with those of Ref. [8].
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