Photochemistry of Aryl Pentazoles: Para -Methoxyphenylpentazole

Article abstract of DOI:10.1021/jp5108813

The photolysis of para-methoxyphenyl pentazole (MeOPP) in methylcyanide (MeCN), investigated in the far UV (FUV) and near UV (NUV) is compared with the photolysis of para-methoxyphenyl azide (MeOPA). The main photoproduct of MeOPP is MeOPA, which, due to

Full text of DOI:10.1021/jp5108813

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Article
Photochemistry of Aryl Pentazoles: para Methoxyphenylpentazole at 193 nm
Uzi Geiger, and Yehuda Haas
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5108813 • Publication Date (Web): 08 Feb 2015
Downloaded from http://pubs.acs.org on February 18, 2015
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Figure 1: UV-vis spectra of MeOPP (solid) and MeOPA (dashed), 14M in MeCN at -
27.5 °C. TDDFT calculated data of the first strong UV absorption bands optically are
shown as red lines, solid for MeOPP and dashed for MeOPA, see Section 3.5 and Table
S2 for calculation details.
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Figure 2: The UV-vis spectra of irradiated MeOPP (solid line) and MeOPA (dashed) in
MeCN (A) FUV experiment (B) NUV experiment. 28 µM solutions, 60 sec irradiation at -
27.5 °C.
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Figure 3: UV-vis spectra in MeCN ensuing from the analysis for the trans- (green) and
cis- (orange) azo isomers.
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Figure 4: (A) Kinetic traces following a FUV irradiation of MeOPP (17 µM total, 15.5 µm
MeOPP) in MeCN. (B) blowup of initial stages of irradiation without the MeOPP trace.
Analysis data are given as points and lines denote exponential (MeOPP) or
biexponential fits. The decay time of MeOPP (assuming exponential decay) is 41 sec.
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Figure 5: Normalized (maximum intensity set to 1) UV-vis spectra of U1 from the NUV
irradiation of MeOPP (solid line) and U2 from the irradiation of MeOPA (dashed).
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Figure 6: Kinetic traces from the NUV irradiation of 43 µM MeOPP. Values for U1 are
in concentration equivalent aryl molecules, as best fits the data if conservation of
mass is considered. MeOPP decay time = 21 sec
Figure 7: UV-vis spectrum of U3 formed upon FUV irradiation of an 18 µM MeOPP or
MeOPA solution in 10% TFE : 90% MeCN.
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Figure 8: The aromatic part of the proton NMR spectrum of 30 min FUV irradiated 1.85
mM MeOPP solution in CD3CN measured at -35 °C (irradiation done at -27.5 °C).
Compound abbreviations listed in Table S1.
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Figure 9: A schematic presentation of the energy level diagram of MeOPA along the N-
N2 bond distance (singlet states only). X is the ground state singlet and the labels A, B,
etc. denote singlet states in rising energy at the Franck Condon region.
H C
N
N
3
N
N
O
N
h

V
,
MeOPP
N
U
F
U
,

V
h
IVR
S0, v
S12
S1
IC
33%
H C
H C
3
H C
3
N
N
N
N
N
N
3
IC
50%
N
N
N
N
N
N
O
N
O
N
O
N
%
44
~
C
,
S
2N 2
I
-
-
T
N
+
H C
H C
N
3
3
O
N
U1
O
N
MeOPA
MeOPN
Scheme 1: A schematic presentation of the photochemical dissociation of MeOPP,
showing the competition between MeOPA production and a non-reactive route leading
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back to S0. S12 is the 12th singlet state; according to DFT calculations it is the optically
active state at ~193nm.
-
N
+
H C
3
N
O
N
h

V
,
N
U
MeOPA
F
U
,

V
h
IVR
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-
-
-
S12
S2
N
+
N
+
N
+
IC
40%
H C
N
H C
3
N
H C
3
N
3
O
N
O
N
O
N
IC
60%
ISC
-
S1
S
N
T
+
H C
3
N
H C
67%
-N2
H C
3
3
ISC
83%
O
N
O
N
O
N
MeOPN
MeOPN
H3C
N
17%
O
H C
N
N
O
3
U2
O
CH
3
t-Azo
Tar
Scheme 2: A schematic summary of MeOPA photochemistry, including the assumption
regarding the formation of tars from the singlet nitrene or didehydroazepine and
identification of U2 with these products.
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N
N
N
N
N
h
5
4
O
N
O
N
1
N
N
N
H3C
H3C
3
2

S0(V)
N
N
N
N
O
N
O
N
+
N
H3C
N+
N-
N
H3C
Scheme 3: A sketch of biradical mediated ultrafast relaxation and dissociation of
MeOPP
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The Photochemistry of Aryl Pentazoles: para Methoxyphenylpentazole.
U. Geiger and Y. Haas*1
Institute of Chemistry, the Hebrew University, Jerusalem, Israel
Abstract
The photolysis of para methoxyphenyl pentazole (MeOPP) in methylcyanide (MeCN),
investigated in the far UV (FUV) and near UV (NUV) is compared with the photolysis of para
methoxyphenyl azide (MeOPA). The main photo-product of MeOPP is MeOPA, which, due to
further photon absorption, yields mostly 4,4’ dimethoxyazobenzene as the final thermally
stable product. The overall product quantum yield of both MeOPP and MeOPA is about 0.5 due
to competition with internal conversion (IC) to S0 implying an ultrafast non-reactive processes.
Both pentazole and azide yield only 4,4’ dimethoxyazobenzene (trans and cis isomers, trans
first) upon FUV photolysis in dilute solutions, whereas in the NUV, though the azo isomers are
still the major products, other species are also formed. The yields of these products are 17%
and 10% for the azide and the pentazole, respectively. The difference in yield indicates
pentazole photo-reactions that do not proceed via the azide. The products observed upon NUV
photolysis may be the precursors of tars commonly produced in azide photochemistry. The
exclusive production of azo compounds in the FUV indicates a rapid route to the triplet nitrene,
generally considered to be the precursor of azo products.
Introduction
Aryl pentazoles (see Figure 1 for the structure of methoxyphenyl pentazole) have been known
since the 1950’s, when they were first synthesized by Ugi and Huisgen1,2,3,4,5,6. Cleavage of the
CN bond connecting the phenyl and the pentazole rings in these materials was considered as
possible route to prepare polynitrogen compounds such as pentazolic acid (HN5) or the cyclo
1Yehuda.haas@mail.huji.ac.il; Tel:+97226585067
1
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pentazolate anion (N5 ). Interest in aryl pentazoles increased after they were used by two
groups to experimentally prepare the pentazolate anion in the gas phase: Christe et al. using
high energy gas phase collisions on para oxido phenyl pentazole (OPP)7 and Östmark et al using
laser desorption ionization on para dimethyl amino phenyl pentazole (DMAPP)8. Theory
predicts that the five membered nitrogen ring is a possible building block of stable all-nitrogen
compounds9,10 making the pentazolate anion the key to unlocking pure nitrogen chemistry – if
synthesized in large quantities.
Attempts to produce the pentazolate anion in the bulk by means of thermal- and electro-
chemistry were largely unsuccessful11,12, with the possible exception of cerium ammonium
nitrate oxidation13,14 yielding products indicative of N5 production, but without directly
detecting the anion itself. The claim was refuted by a subsequent paper15. A theoretical study of
the system16 showed that N5 could be produced in the gas phase by electronic excitation and
-
-
therefore photochemistry might be used in the bulk. All aryl pentazole dissociation reactions
must compete with the thermodynamically favorable dinitrogen expulsion and production of
the corresponding aryl azide, implicating that photochemical reaction paths of these materials
might avoid the well-known photochemistry of the azide, characterized by the formation of a
nitrene17,18. The solution phase photochemistry of OPP was examined by us in a previous
paper19 and that of DMAPP by Portius et al.20 and by us21. In these cases no C—N bond cleavage
was observed and all photochemistry occurred via the azide-nitrene route.
Azides, and even more so, pentazoles, are high energy density materials (HEDMs). On
dissociation, they release large amount of energy, much of it in translational energy of the
products and are considered as potential propellants and explosives. The source of their
extreme exothermicity is the fact that the triple NN bond is very stable (EDIS=945 kJ//mol, 226
kcal/mol): nearly six times the single NN bond (EDIS=160 kJ, 38.4 kcal/mol) and about twice the
NN double bond (EDIS=456 kJ, 109 kcal/mol). Therefore, a polynitrogen system based on single
and double NN bonds is extremely metastable and tends to spontaneously release dinitrogen
(N2) accompanied by large energy release. The barrier to N2 expulsion of these compounds is
2
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relatively small, 30-50 kcal/mol for organic azides and about 20-25 kcal/mol for the most stable
pentazoles.
Para methoxyphenyl pentazole (MeOPP) is a relatively stable phenyl pentazole that can be
stored in dry ice temperature for months and manipulated at moderate temperatures (< -20 °C)
for many hours with no noticeable thermal degradation. This pentazole derivative was studied
less often than DMAPP or OPP due to its poorer thermal stability and the difficulty of separating
it from the azide after synthesis (both DMAPP and OPP can be purified by washing or
recrystallizing with methanol-water mixtures while MeOPP cannot). MeOPP was, however, the
-
aryl pentazole investigated in the first claims for the thermal production of N5 in solution.13,14 It
is also expected to have a much lower contribution from its quinoidic structure and so a weaker
C—N bond with a bond order closer to 1, compared to DMAPP and OPP.
The photochemistry of the few aryl pentazoles studied thus far is reminiscent of the
photochemistry of the corresponding azides due to either the conversion of the pentazoles to
the azides as a first step or the direct formation of phenyl nitrenes from the pentazoles19,20. The
fate of the nitrenes produced from the pentazoles or the azide depends on the environment,
substrate concentration, and oxygen availability. The common stable end products of aryl azide
photochemistry are substituted azobenzene derivatives (nicknamed azo compounds in this
paper), anilines and azepines18. Any difference between the product distributions of pentazole
vs. azide having the same aryl component is an indication of a photochemical path available to
pentazoles and not to azides.
This article presents a preliminary investigation of the photochemical end products of para-
methoxypentazole (MeOPP) and its corresponding azide (MeOPA) in acetonitrile. As C—N bond
cleavage is predicted to proceed via a conical intersection starting from a highly excited
electronic state16, the effects of short excitation wavelength are compared with those of longer
wavelengths. Similar experiments with OPP19 did not discern any qualitative excitation
wavelength effects. The choice of solvent was due to the wide spectral window of acetonitrile,
and its small absorption at 193 nm.
1. Experimental
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2.1 Materials
P-anisidine (Alfa Aesar, 99%), HCl (Sigma, 37%), NaNO2 (Aldrich, 97%), NaN3 (Aldrich, purum),
MeOH (BioLab, HPLC supra gradient), hexanes (Sigma-Aldrich, 98.5%+),Acetonitrile (Bio Lab
inc., HPLC ultra gradient grade), trifluoroethanol (Alfa Aeasar, >99.5%) and Acetonitrile-d3
(Cambridge Isotope Laboratories, %D > 99.8%) were used as received. N-butyl nitrite was
synthesized according to Noyes22 and analyzed using proton NMR (CD3OD, shift (ppm): 4.72
(2H, t, wide), 1.71 (2H, p), 1.43 (2H, sextet), 0.97 (3H, q)).
2.2 Synthesis of MeOPP and MeOPA
MeOPP was synthesized according to Butler et al14 as advised by Östmark et al23 with slight
modifications:
A 100 ml beaker was immersed in a circulator cooled acetone bath. The circulator chiller was
set to -5°C to achieve 0°C in the reaction vessel. 0.40 gr (3.2 mmol) para-anisidine was dissolved
in 2.05 ml methanol and 0.53 ml (6.2 mmol) 37% HCl was added drop wise while stirring. 0.42
ml (3.6 mmol) n-butyl nitrite was added drop wise and the reaction was allowed 20 min to
complete. The color first changes to deep purple and later to red. The temperature was
reduced to below -20 °C and the solution diluted with 5.6 ml of a 1:1 water:methanol mixture.
45 ml chilled hexanes were added to form a two phase system and 0.228 gr (3.5 mmol) sodium
azide dissolved in 1.8 ml 3:2 water:methanol mixture was slowly injected into the lower phase.
The reaction was allowed to run for 30 min and the resulting solid product was filtered in a
cooled sintered glass filter and washed with 50 ml 3:2 water:methanol mixture cooled to dry ice
temperature. This solid was dried in vacuum while cooling to below -20 °C. NMR indicated that
the product was MeOPP contaminated with about 10% MeOPA.
Yield: (70±20) mg ((11±3)% based on MeOPP only) with (7.9±0.8)% MeOPA impurity, no other
contaminants were detected with proton NMR. Proton NMR (MEOPP): 8.09 (doublet, 2H), 7.22
(doublet, 2H), and 3.91 (singlet, 3H) ppm.
2.3 Photolysis of MeOPP and MeOPA
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All irradiations were performed in a q-pod 2e temperature controlled cell holder with magnetic
stirring (Quantum North West) at -27.5 °C and 750 RPM, in a 10X10 mm quartz cell. The
excitation sources were either a 150W xenon lamp fitted with a water filter, an appropriate
optical filter, and a concentrator lens (Hamamatsu E7536 with L2175) or a pulsed ArF excimer
laser (Neweks Ltd., PSX-100) with a diverging lens before the sample. The lens to sample
distance in the laser setup was set to allow 2.5 mW of laser power to illuminate ~1 cm2 of the
sample. The ArF laser and the Xe lamp, when used with a 235-400 nm filter will be denoted as
FUV (Far UV) and NUV (Near UV), respectively. A Bruker Avance II 500 MHz NMR spectrometer
was used for all NMR measurements.
For low concentrations experiments (LC, ~10-5 M), 1-5 mg MeOPP were dissolved in 1 ml MeCN
cooled to -40 °C. 5-20 μl of this liquor diluted in 2 ml MeCN were used for irradiation. Products
of LC experiments were analyzed using UV-vis absorption only: a DH-2000 light source and a
USB4000 spectrometer (Ocean Optics) were connected via fiber optics at a perpendicular angle
to the excitation orientation. Measurements were done a few seconds after irradiation to allow
equilibration and mixing.
For medium concentrations (MC, ~10-3 M), ~1 mg MeOPP was dissolved in cooled CD3CN (0.75
ml, -40 °C) in a small vial. 240 µl of this solution were added to .75 ml CD3CN in the cuvette, at
the working temperature. 40μl of the irradiated solution were diluted in 160 μl MeCN in a 2
mm optical path cell to affect a X25 optical dilution. A spectrum was collected using a different
light source (DH-2000-BAL, Ocean Optics) that allows recording spectra down to 225 nm only.
No cooling was needed, as the measurement was fast enough to permit neglect of thermal
reactions.
In high concentrations experiments (HC, ~10-2 M), 3-10 mg MeOPP were dissolved in MeCN or
CD3CN (1.5 ml) directly in the cuvette at the working temperature. For UV-vis absorption
measurements a sample (5μl) of the irradiated solution was taken and diluted in 245 or 495 μl
MeCN in a 2 mm optical path cell to allow an X250 or X500 optical dilution. A spectrum was
collected without cooling but quickly enough to avoid significant thermal react. UV-vis
absorption was performed using equipment as for the medium concentration experiments.
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MeOPA was produced by heating MeOPP to 30 °C for 30 min and then cooling to -27.5 °C prior
to excitation. Complete conversion was obtained as the first order reaction half-life of this
reaction was measured at 25 °C and found to be 2.4 min.
The proton NMR shift of CD3CN was measured relative to TMS at -35 °C and found to be 1.97
ppm. This was later used to reference all reported spectra. A frequency ratio  of 25.145020%
was used to reference the carbon shifts.
2.4 Analysis of UV-vis spectra
UV-vis spectra were analyzed under the assumption that at any time the recorded spectrum is a
∑
linear combination of a set of several component spectra, i.e.
where At,λ is
the absorbance at wavelength λ and time t assuming an optical path of 1 cm, m is the number
of materials, εi,λ is the extinction coefficient of material i at wavelength λ and Ci,t is the
concentration of material i at time t. This assumption allows us to extract the spectra of
unidentified substances as long as they exhibit distinct kinetic behavior. The full details of the
analysis method used are given in the supporting information (S1).
A shortcoming of this method is that the spectra produced by it are not necessarily those of
pure materials if different products follow similar kinetics.
2.5 Error analysis
A major contribution to the errors in estimating yields is due the unavoidable thermal
degradation of MeOPP while manipulating the samples. Small variations in temperature change
the MeOPA content of the solutions. In addition, spectral contributions by species with closely
related kinetic behavior lead to errors in the estimated concentrations of the products due to
spectral contributions by species having closely related kinetic behavior. However, these errors
are largely canceled when the ratios of concentrations are considered. Therefore,
concentration ratios should be regarded as much more precise than absolute values of the
concentrations themselves. The latter turn up to have rather large error bars, as found upon
repetitive runs.
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2.6 TD-DFT Calculations
All calculations were performed at the B3LYP/CC-pVDZ level using the Gaussian software
package24.
2. Results
Figure 11 displays the UV-vis absorption spectra of MeOPP and MeOPA. The UV-vis spectrum of
the MeOPP synthesis product without subtraction of the 8% MeOPA spectrum is shown in
Figure S1. The proton NMR spectrum of both compounds can be seen in Figure S2, the 8%
MeOPA contamination was determined using these NMR data.
Table S1 lists all the compounds mentioned in the paper with the abbreviations used. Literature
UV-vis absorption spectra and NMR data (ppm) are presented when available.
Figure 1: UV-vis spectra of MeOPP (solid) and MeOPA (dashed), 14M in MeCN at -27.5 °C.
TDDFT calculated data of the first strong UV absorption bands optically are shown as red
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lines, solid for MeOPP and dashed for MeOPA, see Section 3.5 and Table S2 for calculation
details.
3.1 LC photolysis results
The UV-vis spectra of LC MeOPP and MeOPA solutions in MeCN irradiated in the far and near
UV are shown in Figure 2. In both cases any residual reactant peak is overlapped by other
absorption bands and a few new absorption bands appear: 357 and 250 nm for the FUV
irradiation; and 449, 308, and 251 nm in the case of NUV irradiation.
Figure 2: The UV-vis spectra of irradiated MeOPP (solid line) and MeOPA (dashed) in MeCN
(A) FUV experiment (B) NUV experiment. 28 µM solutions, 60 sec irradiation at -27.5 °C.
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The spectrum of FUV irradiated MeOPP can be satisfactorily reproduced by adding two new
absorbing compounds, in addition to MeOPP and MeOPA. The spectra of the added compounds
fit well the literature spectra of the trans and cis isomers of 4,4′-Dimethoxyazobenzene25, as
shown in Figure 3. To better distinguish between the two azo spectra (i.e. to attain a better
kinetic difference between the two) the FUV irradiated MeOPA solution was further irradiated
at 360 nm and then at 280 nm using the Xe lamp equipped with appropriate interference filters.
This procedure is known to cause transcis and then cistrans isomerization26 and therefore
allows better spectral separation (see Figure S3 for the resulting spectra). The vanishing
absorption of the cis-azo spectrum at ~380 nm – is an artifact due to the imperfect kinetic
separation between it and the trans isomer spectrum and some linear mixing between the two
spectra. The contributions of these factors are estimated to be smaller than 5%, based on many
repetitions.
The spectral data were converted to concentrations using the measured extinction coefficients
of MeOPP (12,000 M-1cm-1 at 275 nm) and MeOPA (13,000 M-1cm-1 at 256 nm). The conversion
to concentrations is based on assuming that no other products except MeOPA and the two azo
compounds contribute (as indicated by the good fit to the spectral data), on conservation of
mass, and on the fact that production of one azo molecule consumes two pentazole ones (the
aryl equivalence of azo is twice its concentration). The maximum deviation from the
conservation of mass, as measured by the sum of aryl-equivalent concentrations at any given
time, is lower than 1 µM (6% of the 17 µM solution used); this figure is used as a lower limit to
the error of the analysis method. The analysis resulted in spectra (Figure 3) and kinetic traces
(Figure 4) in which the cis isomer constituted (50±7)% of the azo isomers after 60 sec. The
calculated extinction coefficient of the trans isomer is very high (34,000 M-1cm-1) relative to
that reported in toluene by ref 25 (25,000 M-1cm-1). A strong solvent dependence of extinction
coefficients was previously found27 for other azobenzene derivatives, wherein molar extinction
coefficients higher than 30,000 M-1cm-1 were reported.
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Figure 3: UV-vis spectra in MeCN ensuing from the analysis for the trans- (green) and cis-
(orange) azo isomers.
The kinetic traces from a FUV irradiation of MeOPP (Figure 4A) show that initially only MeOPA
appears (first second) followed by the production of only the trans-azo isomer (next 4 seconds)
and then followed by the appearance of the cis isomer. On the time scale used, all compounds
are assumed to be formed in any irradiation period, but at the beginning stages the cis azo
concentrations are apparently below the noise level as can be more clearly seen in Figure 4B.
This finding is consistent with a mechanism in which MeOPP’s initial reaction upon FUV
irradiation is the formation of MeOPA by nitrogen extrusion. Absorption of another photon
detaches a second N2 molecule forming p-methoxyphenyl nitrene (MeOPN), which then forms
trans-azo via a bimolecular reaction with either MeOPP, MeOPA, or with another nitrene
molecule18.
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Figure 4: (A) Kinetic traces following a FUV irradiation of MeOPP (17 µM total, 15.5 µm
MeOPP) in MeCN. (B) blowup of initial stages of irradiation without the MeOPP trace.
Analysis data are given as points and lines denote exponential (MeOPP) or biexponential fits.
The decay time of MeOPP (assuming exponential decay) is 41 sec.
When operated at 2.5 mW, the 193 nm laser delivers 4.0 nmols of photons per second. the
initial absorbance of the solution at 193 nm was 0.294, 91% of which may be attributed to
MeOPP (the extinction coefficients of MeOPP and MeOPA at 193 nm are 17,000 and 23,000 M-
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1cm-1, respectively, and the MeOPA contamination was ~8%), which means that 1.9 nmol of
photons were absorbed by MeOPP during the first second of irradiation. At the same time 0.95
nmol of MeOPP were decomposed (the total volume of the solution was 2 ml and the
concentration decreased from 15.25 to 14.78 µM) so that the quantum yield of the reaction
was 0.50±0.08. As no other products were detected it is assumed that all unreacted excited
molecules relax to the ground state without producing the azide. A similar analysis of the
irradiation of MeOPA leads to a QY of 0.60±0.09.
The spectrum obtained upon NUV irradiation of MeOPP is not the same as that obtained upon
FUV excitation. In addition to MeOPP, MeOPA, and the two azo isomers, the spectrum of at
least one more component is needed for a good fit of the data. The spectrum, due to yet
unidentified compound(s), (U1, Figure 5) shows two sharp peaks at 204 and 232 nm, a broad
and flat absorption band at 280-312 nm, and another broad absorption at ~445 nm with a
shoulder towards the blue. It may represent more than one product, having the same time-
dependent characteristics. U1 is obtained by averaging the spectra during the first 60 seconds
of irradiation. As the reaction advances MeOPP concentration is reduced, becoming eventually
smaller than MeOPA concentration (Figure 6). If the pentazole and the azide yield different
products, the structure of U1 should be a function of time. Attempts to obtain the spectrum
after a much shorter time (<10 sec) were unsuccessful due to unacceptable low S/N ratio.
A similar result holds for NUV irradiated MeOPA: one more spectrum is necessary in order to fit
the data. This spectrum denoted U2 in Figure 5, is less noisy than U1 and contains many of the
bands observed in U1: a 232 nm peak, a strong 307 nm peak and a broad absorption at 445 nm
with a shoulder towards the blue. These similarities between the two spectra indicate that after
prolonged irradiation irradiation of MeOPP U1 should be similar to U2.
These spectra have not been reported yet. Their relatively large yield calls for further study of
their thermal reactions, which may lead to formation of the tars observed by thermal and
photochemical aryl azide reactions18.
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Figure 5: Normalized (maximum intensity set to 1) UV-vis spectra of U1 from the NUV
irradiation of MeOPP (solid line) and U2 from the irradiation of MeOPA (dashed).
The kinetic analysis of the NUV irradiation of MeOPP (Figure 7A) shows that the percentage of
the trans azo ((50±7)% upon FUV irradiation) changes to a much larger cis isomer contribution
after NUV irradiation ((90±3)% cis isomer after 60 seconds of irradiation). The species giving rise
to the U1 spectrum constitute (10±4)% of the initial concentration after 60 seconds. The azide
appears to react more slowly (MeOPP and MeOPA decay times are 21 and 53 sec, respectively)
and accumulates to concentrations higher than MeOPP, so that the final stages of irradiation
are largely the irradiation of MeOPA with some residual MeOPP. While there may be a kinetic
difference between the pentazole derived products and the azide ones, in practice this
difference was too small to detect. An attempt to analyze the initial 5 seconds of irradiation,
when the pentazole is in large excess relative to the azide, failed due to the large noise at these
shorter times.
A similar analysis for U2 produced by the NUV irradiation of MeOPA shows it represents
(17±2)% of all products. The overall conversion quantum yield of both compounds (MeOPP and
MeOPA) under these conditions is 0.6±0.2 as calculated by taking into account the light
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intensity across the spectrum, the absorption of MeOPP and MeOPA, the experimental setup,
the percent of MeOPA contamination in the MeOPP sample, and the conversion upon the first
second of irradiation. The ratio of the two quantum yields (1.0±0.1) is more precise than the
absolute values of the QYs themselves (see section 2.5), as this number does not depend on the
experimental setup as long as the light intensity is kept constant (as indeed it was). The
observation that only 10% of all MeOPP products are not azo compounds, as opposed to 17%
for MeOPA, means that at most (60±20) % of the MeOPP molecules decompose initially to
MeOPA and that the majority of the end products derived from MeOPP are the azo compounds
(presumably via the formation of MeOPN as a first stage). The cis-trans isomer ratio following
FUV irradiation changes upon exposure to NUV light, but U1 or U2 are not formed. This is
compatible with the notion that these spectra are indeed the direct products of MeOPP and
MeOPA and not the decomposition products of the azo compounds (Figure S5). 30 minutes
heating to +30°C of a NUV irradiated MeOPA solution resulted in very little change in the
spectrum, indicating that U2 is thermally stable (Figure S6). The measured yield of U2 is
comparable to the 18% yield of unidentified tars reported in ref. 28.
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Figure 6: Kinetic traces from the NUV irradiation of 43 µM MeOPP. Values for U1 are in
concentration equivalent aryl molecules, as best fits the data if conservation of mass is
considered. MeOPP decay time = 21 sec
MeOPP and MeOPA were also irradiated by FUV light in a 1:9 trifluoroethanol (TFE): MeCN
mixture to check the effect of hydroxylic hydrogen atoms; a new spectrum (U3) was observed
(Figure 7), in addition to the bands of the two azo compounds. This new spectrum exhibits a
major peak at 224 nm and a minor one at 357 nm. U3 is probably due to the end product of the
protonated MeOPN as proposed by Kwok et al29.
Figure 7: UV-vis spectrum of U3 formed upon FUV irradiation of an 18 µM MeOPP or MeOPA
solution in 10% TFE : 90% MeCN.
3.2 MC and HC irradiation results
The NMR spectrum recorded after 30 min FUV irradiating 1.85 mM MeOPP solution in CD3CN is
presented in Figure 8. In addition to MeOPP, MeOPA, and the azo isomers 6 more major proton
peaks are detected: two were assigned to para-nitroanisole (PNA), produced by the addition of
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oxygen during sample extraction, and four show identical integration to each other and shift
values beyond the regular range of aromatic protons (U4). Using the NMR data a UV-vis
absorption spectrum of the compound can be extracted by subtracting a linear combination of
the spectra of the known compounds with the appropriate coefficients (Figure S8, see Figure S7
for the originally measured spectrum). This UV-vis spectrum is unlike any spectrum recorded in
the LC experiments and must therefore be due to different compound(s). It is also noted that
the prevalence of the trans azo isomer is much larger than under similar conditions at lower
concentrations.
Figure 8: The aromatic part of the proton NMR spectrum of 30 min FUV irradiated 1.85 mM
MeOPP solution in CD3CN measured at -35 °C (irradiation done at -27.5 °C). Compound
abbreviations listed in Table S1.
A similar concentration dependent behavior of azo compounds, including the shifting of the
photo-stationary equilibrium of the two isomers and a formation of a new compound, is known
in the literature30 and was explained by the production of an azo dimer that preferentially
decomposes to the trans isomer; the formation of U4 is compatible with mechanism. We found
reference for this hypothesis in this single article only, yet our findings seem to fit it quite well.
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The same pattern is seen upon FUV irradiation of MeOPA at similar concentrations (proton
NMR spectrum, Figure S9).
MeOPP and MeOPA were irradiated at even higher concentrations (~10-2 M) by both light
sources for 1 hour. A few new weak H-NMR peaks were detected upon FUV irradiation whereas
the NUV experiment resulted in a multitude of product peaks (Figure S10). The most prominent
new peak that appears after FUV irradiation at such concentrations is an unsplit peak at 6.84
ppm, similar to that of 2,7 dimethoxyphenazine, a known product of MeOPA upon pyrolysis31.
Comparing the FUV to the NUV results at least 4 new para substituted aryl rings are seen in the
NUV irradiated samples (multiple rings may belong to a single compound). No qualitative
difference between the results of MeOPP and MeOPA (Figure S11) irradiation were detected in
the HC experiments.
3.3 Summary of experimental results
Table 1 summarizes the results of LC photolysis. A smaller number of MC and HC experiments
were carried out, but they yielded acceptable NMR spectra and thus helped in the assignment.
A strong concentration effect manifests itself by different UV-vis spectra as compared to the LC
runs; For instance, U1 and U2 were hardly detected under HC conditions. It is likely that the
formation of U1 and/or U2 in high concentrations is either retarded, or they are consumed by
secondary reactions. The discussion will focus on the LC data.
Table 1: A summary of LC irradiation of MeOPP and MeOPA
Reactant
MeOPP
FUV
NUV
Total conversion QY=0.50±0.08
Total conversion QY=0.6±0.2
End productsa: 100% azob ((50±7)% cis)
End products: (90±4)% azo
((90±3)% cis), (10±4)% U1
MeOPA
Total conversion QY=0.60±0.09
Total conversion QY=0.6±0.2
End products: (83±2)% azo
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End Products: 100% azo ((50±7)% cis)
((90±3)% cis), (17±2)% U2
a. Product distribution after 60 sec of irradiation at low concentration
b. The trans:cis ratio after 60 sec irradiation is given in parentheses
3.4 TD-DFTcCalculation results
TD-DFT calculations performed for MeOPP and MeOPA, summarized in Table S2, show good
agreement with the experimental spectral data (Figure 1): band maxima of 275 and 256 nm
were found experimentally for MeOPP and MeOPA, respectively, and calculated values of 279
and 256 nm were calculated for the lowest states with strong optical activity. MeOPA
calculation also reveals a lower lying singlet state of weaker optical intensity at 275 nm, which
is experimentally detected as a shoulder of the main peak. The energy of the different singlet
states were calculated along the N—N dissociation coordinate of MeOPA to achieve some
insight into the creation of MeOPN (Figure S12). The calculated energies of the dissociated
system are in good agreement with calculations on a similar system32 and to the dissociation
PES from other calculations33.
4.
Discussion
4.1
Preliminary comments
The main goal of this work was to further characterize the photochemistry of aryl pentazoles,
represented by MeOPP, in the near and far UV. Due to the large energy gain obtained upon N2
release, it was expected that MeOPA will play an important role in this process, similar to
thermally activated reactions of pentazoles, and to the few systems already studied
photochemically19,21. Thus acquaintance with azide photochemistry is called for. The NUV
photochemistry of aryl azides has been thoroughly characterized but the FUV analogue was
hardly investigated and was taken up in this work. The data indicate that a large percentage of
the excited molecules return to the ground state without reacting, a process that must be
ultrafast33. A full analysis requires the consideration of ultrafast events in the singlet manifolds
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of the azides and pentazoles (internal conversion) and possibly also intersystem crossing to the
triplet. Such processes often entail the transition through conical intersections, which are also
qualitatively considered.
The analysis is helped by quantum chemical calculations, performed by the TDDFT method. DFT
methods are very useful in predicting ground state and excited state properties; in the present
context it reproduced reasonably well the geometry of the pentazole, the UV absorption
spectra and the Raman spectra34. The DFT method is limited by being a single configuration
method, whereas understanding photochemical systems often requires multi-configuration
ones. Nonetheless DFT often provides a qualitative insight and points out points that require
higher level analysis. In this preliminary analysis it is found that TDDFT calculations on the
systems in the Franck-Condon region, as well as along the N- N2 stretch coordinate yield results
that help in proposing reaction mechanisms.
Two main themes were explored in this work. The first sketches the photochemistry of MeOPP
and compares it with that of the azide MeOPA. The second examines the photochemistry of
these two compounds in the near UV with that in the far UV, all in MecN solutions.
4.2 Comparison of MeOPP photolysis to that of MeOPA
Error! Reference source not found. outlines the main processes relevant to MeOPP
photochemistry in the near and far UV as revealed in this work. The experimental results show
that the photochemistry of MeOPP is very similar to that of the azide. Many of our
observations are consistent with the formation of MeOPA from MeOPP as a first photochemical
step (see Figure 1), and subsequent photolysis of MeOPA. There are some minor differences;
the main one is the appearance of the product U1 in the pentazole. U2, found upon photolysis
of MeOPA, is a different compound according to NMR and UV-vis spectroscopies. It is therefore
advantageous to begin the discussion with the more comprehensively studied aryl azide
photochemistry in the NUV.
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