Synthesis and photochemical properties of azidohemicyanine

Article abstract of DOI:10.1007/s11172-008-0185-6

Photochemical activity of azidohemicyanine (1-methyl-4-(4-azidostyryl) quinolinium iodide) was predicted by quantum chemical calculations and confirmed experimentally. The azidohemicyanine, which was synthesized, is characterized by a long-wavelength abso

Full text of DOI:10.1007/s11172-008-0185-6

Russian Chemical Bulletin, International Edition, Vol. 57, No. 7, pp. 1428—1434, July, 2008
1428
Synthesis and photochemical properties of azidohemicyanine
M. F. Budyka,a N. V. Biktimirova,a T. N. Gavrishova,a and V. I. Kozlovskiib
aInstitute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 514 3244. Eꢀmail: budyka@icp.ac.ru
bBranch of the Institute of Energy Problems for Chemical Physics, Russian Academy of Sciences,
142432 Chernogolovka, Moscow Region, Russian Federation
Photochemical activity of azidohemicyanine (1ꢀmethylꢀ4ꢀ(4ꢀazidostyryl)quinolinium iodide)
was predicted by quantum chemical calculations and confirmed experimentally. The azidohemicyꢀ
anine, which was synthesized, is characterized by a longꢀwavelength absorption band (LWAB) in
the spectral region 350—500 nm with a maximum at 417 nm; it decomposes with a quantum yield of
0.84 0.17 upon irradiation within the LWAB, the quantum yield being independent of the presence
of oxygen. The reaction products identified by ESI mass spectrometry include the corresponding
primary amine as well as azo, hydrazo, nitroso, and nitro compounds, some of them are unidentified.
The azidohemicyanine possesses the longestꢀwavelength visible light sensitivity among aromatic
azides known so far.
Key words: quantum chemical calculations, PM3 method, density functional theory, hemicyaꢀ
nine, aromatic azide, photodissociation, quantum yield, light sensitivity, ESI mass spectrometry.
Aromatic azides are widely used as photoaffinity laꢀ
dyes studied so far have a positively charged aromatic
nucleus, i.e., they are cations.
bels1—3 and agents for photodynamic therapy.4 Most
azides used at present are sensitive to hard UV radiation
(250—320 nm). At the same time, in the photoaffinity
modifications of biomacromolecules it is desirable to
expose the system under study to soft longꢀwavelength
UV radiation (350—400 nm) or to visible light (which is
better) in order to prevent its photodegradation. To this
end, the azide should have an absorption band in
the specified spectral region and decompose with high
quantum yield upon irradiation with light in this spectral
region.
Quantum chemical calculations of aromatic azides
with different structures and photochemical properties
showed10,11 that the azide photoactivity is governed by
the type of the molecular orbital (MO) that is filled in the
lowest singletꢀexcited state S1. If the σNN*ꢀMO (antiꢀ
bonding orbital with respect to the N—N(2) bond) is
filled, the azide is photoactive (quantum yield, ϕ, of its
photodissociation is higher than 0.1). In turn, filling of
the σNN*ꢀMO depends on the size and charge of the aroꢀ
matic πꢀsystem. If the πꢀsystem extends beyond a certain
threshold, the σNN*ꢀMO in the S1 state remains vacant
and the azide becomes photoinert (ϕ values decrease to
less than 0.01). Filling of differentꢀtype orbitals leads to
different structures of the azide group in the S1 state of
photoactive and photoinert azides, whereas in the ground
state (S0) both types of azides have structurally similar
azide groups. Calculations12 of a series of cataꢀcondensed
heteroaromatic azides revealed that the threshold numꢀ
ber of πꢀelectrons (photoactivity loss threshold) equals
22 for neutral and 18 for positively charged azides, reꢀ
spectively. As this number is exceeded, the azide photoꢀ
activity ceases.
There are a few azido dyes (phenanthridine and acriꢀ
dine derivatives) sensitive to shortꢀwavelength visible
light. For instance, azidoethidium dyes decompose with
quantum yields from 0.5 to 1.0 on exposure to light with
λ = 436 nm.5 9ꢀAzidoacridinium has a longꢀwavelength
absorption band (LWAB) in the region 400—470 nm and
decomposes with a quantum yield of 0.7—1.0 upon irraꢀ
diation within the LWAB region.6 However, this comꢀ
pound is readily hydrolyzed with the formation of acriꢀ
done in the presence of trace amounts of water, which
makes practical application of 9ꢀazidoacridinium diffiꢀ
cult. 9ꢀ(4ꢀAzidophenyl)acridinium has an absorption
band in the same spectral region and is more stable to
hydrolysis, but the quantum yield of its photodissociaꢀ
tion is very low (<0.01).7,8 Other azido dyes that have
absorption bands in the longerꢀwavelength region are
also insensitive to light.9 It should be noted that all azido
The number of πꢀelectrons in azido derivatives of
cyanine and triphenylmethane dyes studied earlier9,11 is
larger than these threshold values; as a consequence, all
these compounds appeared to be photoinert. At the same
time it is the hemicyanine πꢀsystem that includes 18 elecꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1402—1408, July, 2008.
1066ꢀ5285/08/5707ꢀ1428 © 2008 Springer Science+Business Media, Inc.

Photochemistry of azidohemicyanine
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1429
Solvents were purified using conventional procedures.17 Ethaꢀ
nol ("distilled, highest purity" grade) was distilled prior to use.
Synthesis of (E)ꢀ4ꢀ(4ꢀazidostyryl)ꢀ1ꢀmethylquinolinium iodide
(1) (the reaction and purification were carried out under red light).
To a solution of (E)ꢀ4ꢀ(4ꢀazidostyryl)quinoline18 (60 mg) in anhyꢀ
drous MeCN (5 mL), MeI (0.5 mL) was added and the reaction
mixture was heated at 40 °C for 5 h and then kept at 20 °C for 12 h.
To the cooled reaction mixture, petroleum ether (5 mL) was added
and the precipitate was filtered off and recrystallized from
MeCN—petroleum ether. The yield was 80%, brownꢀyellow crysꢀ
tals, decomp. temp. 185 °С. ESI MS, found: m/z 287.110 [М]+.
C18H15N4+. Calculated: М = 287.130. IR, ν/cm–1: 2127 (N3);
1593, 1567 (СН=СН); 1424, 1369 (СН3), 837 (рꢀС6Н4).
1H NMR, δ: 9.4, 9.15 (both d, 1 Н, quinoline H, J = 9 Hz);
7.9—8.7 (m, 8 Н, quinoline H, С6Н4, СН=СН); 7.25 (d, 2 Н,
С6Н4, J = 8 Hz); 4.62 (s, 3 Н, Me).
trons; therefore, it was of interest to apply the photoactivꢀ
ity criterion to azidohemicyanine using a theoretical apꢀ
proach developed earlier.
To this end, in the present study we have carried out
semiempirical (РМ3) and density functional (B3LYP/6ꢀ
31G*) quantum chemical calculations of 4ꢀ(4ꢀazidostyrꢀ
yl)ꢀ1ꢀmethylquinolinium iodide (azidohemicyanine 1)
in the ground (S0) and lowestꢀlying singlet excited (S1)
states. The structural formula of 1 and the atomic numꢀ
bering scheme are given below. The S1 state of the aziꢀ
dohemicyanine molecule is characterized by filling of
the σNN*ꢀMO, which suggests that this compound is
photochemically active (see a preliminary communiꢀ
cation13).
A DRShꢀ500 mercury lamp was used as the source of UV light.
The 365ꢀnm spectral line was cut using UFSꢀ6 and BSꢀ7 filters. An
SDKꢀ470 ultrabright lightꢀemitting diode (emission band with
a maximum at λ = 463 nm and a halfꢀwidth of 20 nm) was used as
the source of visible light. A band with a maximum at λ = 479 nm
was cut using a ZhSꢀ16 light filter. From the light generated by
a KGMꢀ24ꢀ250 halogen lamp, a band with a maximum at λ = 485 nm
and a halfꢀwidth of 8 nm was cut using a ZhSꢀ12 glass filter and an
IFꢀ475 interference filter.
Photochemical studies were carried out at room temperature in
airꢀsaturated or degassed solutions (in the latter case, three freezꢀ
ing—evacuation (down to 0.01 Torr)—thawing cycles were used).
The azide concentration in the solutions was (2—20)•10–5 mol L–1
,
the optical path length in quartz cells was l = 1 cm, and the light
intensity was (5—50)•10–10 Einstein cm–2 s–1 (measured using
a PPꢀ1 cavity or a ferrioxalate actinometer).
This conclusion was confirmed experimentally. Comꢀ
pound 1 was synthesized; it is characterized by a LWAB
in the region 350—500 nm with a maximum at 417 nm.
The quantum yield of photodissociation of 1 is 0.84 0.17
irrespective of the presence of oxygen and the irradiation
wavelength (365, 379, or 385 nm). The photolysis prodꢀ
ucts identified by ESI mass spectrometry include a conꢀ
ventional mixture typical of photochemistry of arylazides,
namely, the corresponding primary amine as well as azo,
hydrazo, nitroso, and nitro compounds.
Results and Discussion
Molecular and electronic structure of the azidohemicyaꢀ
nine. Selected geometric parameters and electronic charꢀ
acteristics of the S0 and S1 states of the azidohemicyanine
optimized by the B3LYP/6ꢀ31G* and РМ3 methods are
listed in Table 1. Similarly to azides studied earlier,10—12
the azido group in molecule 1 has a quasiꢀlinear geomeꢀ
try (N—N—N bond angle is nearly 170°) and the termiꢀ
nal nitrogen atoms bear large positive charges. Comꢀ
pared to B3LYP/6ꢀ31G*, the РМ3 method somewhat
overestimates the atomic charges and bond lengths.
Excitedꢀstate calculations revealed a bending geomeꢀ
try of the azido group and elongation of the N(1)—N(2)
bond by 0.122 Å accompanied by a decrease in the order
of this bond by a value of 0.39; in addition, the charge on
the terminal N2 group decreases (see Table 1). As menꢀ
tioned earlier,11 this pattern of changes in the parameters
on going from the S0 to S1 state are characteristic of
photoactive azides.
Experimental
Quantum chemical calculations were carried out by the semiemꢀ
pirical method РМ314 (MOPACꢀ2002 program package) and usꢀ
ing the density functional theory (B3LYP functional, 6ꢀ31G* basis
set, GAUSSIANꢀ03 program package15). The azide geometries in
the S0 and S1 states were calculated with full geometry optimization.
Excitedꢀstate calculations were carried out by the РМ3 method with
inclusion of configuration interaction (CI = 7, eight electrons in
seven orbitals, 1225 configurations).
Electronic absorption spectra were measured on a Specord
Мꢀ40 spectrophotometer and IR spectra were recorded on a Specꢀ
trum BXꢀ2 FTꢀIR spectrometer (КВr pellets). 1H NMR spectra
were recorded on a Bruker DPXꢀ200 spectrometer in DMSOꢀd6
with Me4Si as the internal standard.
Changes in the geometric parameters upon excitation
depend on the type of the orbitals participating in
the electron transfer process. The compositions of the
(i) frontier MOs, namely, the highest occupied MO
(HOMO) and the lowest unoccupied MO (LUMO) in
ESI mass spectra were obtained using an original timeꢀofꢀflight
mass spectrometer with orthogonal ion inlet16 (positive ion deꢀ
tection).

1430
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Budyka et al.
Table 1. N(1)—N(2) and C(4)—N(1) bond lengths (r), bond orders (overlap populations) (p), N(1)—N(2)—N(3) bond
angles (ωNNN), and the Mulliken effective charges on the terminal N2 group (ZN ) in the ground state (S0) and in the
2
lowestꢀlying electronꢀexcited (S1) state of azidohemicyanine 1 calculated using different methods (with no counterion)
State
Computational
method
r/Å
p
ω
/deg
ZN /е
NNN
2
N(1)—N(2)
C(4)—N(1) N(1)—N(2) C(4)—N(1)
S0
S0
S1
PM3
B3LYP/6ꢀ31G*
PM3
1.278
1.247
1.400
1.419
1.399
1.316
1.29
0.48
0.90
1.06
0.57
1.66
169.5
171.8
129.5
0.45
0.26
0.17
the S0 state and (ii) the lowest and highest singly occupied
MOs (LSOMO and HSOMO, respectively) in the S1 state
are shown in Fig. 1. In the S0 state both frontier MOs are
πꢀorbitals localized on the aromatic nuclei and on the
central double bond. The σNN*ꢀMO of interest is localꢀ
ized on the azido group, being antibonding in character
with respect to the N—N(2) bond. Its energy level lies
3.26 eV higher than that of the LUMO and the σNN*ꢀMO
itself is the LUMO+7 orbital (according to РМ3 calcuꢀ
lations).
However, it is the σNN*ꢀMO rather than the aromatic
πꢀMO (formerly LUMO) that is filled in the relaxed S1
state (see Fig. 1), which is a prerequisite for photodissoꢀ
ciation of the azido group. This suggests that the azidoheꢀ
micyanine will decompose on exposure to light within
the LWAB with a quantum yield ϕ > 0.1, i.e., be a photoꢀ
active azide.
The kinetics of photolysis. The spectral changes upon
irradiation of a degassed solution of azide 1 in EtOH at
485 nm are shown in Fig. 2 as an example. The absorpꢀ
tion spectrum of the initial compound (spectrum 1) exꢀ
hibits an intense band with a maximum at λ = 417 nm
a
b
(molar extinction coefficient ε = 28900 L mol–1 cm–1
)
At
ln[(A0 –A∞)/(At – A∞)]
2.0
1.5
1.0
1´
2´
2
1
A
300
600
t/s
1
2
2.0
1.5
1.0
0.5
3
4
c
d
5
6
7
300
400
500
600 λ/nm
Fig. 2. Spectral changes in the course of photolysis of degassed
solution of azidohemicyanine 1 in EtOH (7.4•10–5 mol L–1) at
λ = 485 nm; light intensity I0 = 1.08•10–9 einstein cm–2 s–1
,
photolysis duration: 0 (1), 60 (2), 120 (3), 240 (4), 480 (5),
1380 (6), and 2900 s (7). The inset shows the kinetics of changes in
the optical density at 417 nm (1´) and its semilogarithmic anamorꢀ
phose (2´).
Fig. 1. Frontier MO compositions for azidohemicyanine 1: HOMO
(a) and LUMO (b) in the S0 state; LSOMO (c) and HSOMO (d) in
the S1 state.

Photochemistry of azidohemicyanine
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1431
and
in the visible region, and two weaker bands at 332
(ε 6100 L mol–1 cm–1) and 249 nm (ε 19600 L mol–1 cm–1
in the UV region (see Fig. 2, spectrum 1). Upon irradiaꢀ
tion, the shortꢀwavelength band is shifted to 240 nm, the
longꢀwavelength band disappears, and two new bands
appear near 280 and 530 nm (spectra 2—7).
The isosbestic points at 246.9, 354.1, and 471.7 nm
(see Fig. 2) retained their positions in the initial stage of
photolysis and gradually shifted on further irradiation.
Some kinetic curves showed extrema, e.g., the optical
density near 280 nm initially increased, passed through
a maximum, and then decreased. In addition, the optical
density of the reaction product near 530 nm continued to
increase after the intensity of the azide band at 417 nm no
longer decreased. These data suggest that the products of
azide photolysis undergo subsequent reactions.
a
longꢀwavelength tail spanning to 500 nm
lation of the ratio of the component concentrations at this
instant. This corresponds to the "switching on" a new
reaction or "switching off" a certain concurrent reaction.
Considering the azidohemicyanine structure, it is logꢀ
ical to assume that here we deal with the second case
where consumption of the azide is followed by "switching
off" the photodissociation of the azido group. but photoꢀ
isomerization of the central double bond of the styryl
fragment in the azide photolysis products still does ocꢀ
cur. Since cyanine dyes are prone to formation of various
complexes and aggregates,20 this process can also conꢀ
tribute to the mismatch of the kinetic curves measured at
different wavelengths.
For comparison, in Fig. 3 we present the results of
analysis of the reaction mixture using the method of prinꢀ
cipal components taking photolysis of an airꢀsaturated
solution of azide 1 (dotted curve) as an example. In this
case, we also observed the initial linear region followed
by gradual deviation of points from the straight line on
further irradiation, i.e., secondary reactions occurred.
The mismatch of the initial points in the curves (see Fig. 3)
can be explained by small changes in the azide absorption
spectrum on degassing and the mismatch of the curves on
the whole points to formation of different products in the
presence and in the absence of oxygen (see below).
To simplify the procedure for processing of the kinetꢀ
ic curves, photolysis of azide 1 was carried out following
two different lightꢀabsorption patterns. When the sample
was photolyzed at 365 nm, conditions for almost 100%
light absorption (optical density at the irradiation
wavelength Aex > 1) were met in the very beginning
of the process. In this case, the quantum yield of
azide decomposition was calculated from the slope of
the initial linear region of the kinetic curve using
the equation
)
The occurrence of several reactions in the system
clearly manifested itself in analysis of the spectral changꢀ
es by the method of principal components (spectral data
processing program is described in Ref. 19). The score
plot representing the experimental spectra shown in Fig. 2
in the basis set of the first three singular vectors is preꢀ
sented in Fig. 3. Each point in Fig. 3 corresponds to
a particular spectrum of the reaction mixture shown in
Fig. 2; the analysis was performed for the spectral region
from 40000 to 16480 cm–1 with an increment of 80 cm–1
,
i.e., the initial matrix included 295 optical density values
for each of the seven spectra. The initial linear region of
the plot (see Fig. 3, solid line) means that only one
reaction resulting in a single compound or several conꢀ
current reactions at constant relative proportion of prodꢀ
uctsꢀcomponents (only their concentrations vary) occur
in this stage of azide photolysis. Deviation of the next
pointꢀspectrum from the initial linear region implies vioꢀ
At = A0 – ελϕI0t(A0 – A∞)/A0,
(1)
3
where At, A0, and A∞ are the optical densities of the reacꢀ
tion mixture at the observation wavelength λ at the instant
t, at t = 0, and after completion of the process (A∞ value
was estimated at long photolysis times); ελ is the extincꢀ
tion coefficient of azidostyrylquinoline (L mol–1 cm–1) at
the observation wavelength; and I0 is the intensity of
incident light (Einstein cm–2 s–1). The maximum of the
azide LWAB was used as the observation wavelength.
Photolysis with visible light at the edge of the longꢀ
wavelength tail of the absorption band corresponded to
a reaction in a thin optical layer (optical density at
the irradiation wavelength Aex < 0.1). The quantum yield
ϕ was calculated from the equation
5´
6´
4´
7´
3´
6
2´
1
1
5
4
3
2
7
1´
2
Fig. 3. Analysis of reaction mixture (see Fig. 2) by the method of
principal components (solid curve); experimental spectra are preꢀ
sented in the basis set of the first three singular vectors; points 1—7
correspond to the spectra 1—7 in Fig. 2. Analysis of the reaction
mixture in the case of photolysis of airꢀsaturated solution of azidoheꢀ
micyanine 1 by the method of principal components using the same
coordinates; points 1´—7´ correspond to the spectra recorded at
a photolysis duration of 0, 60, 120, 240, 540, 840, and 1500 s,
respectively (see text).
ln[(A0 − A∞)/(At − A∞)] = 2.3ε ϕI0t,
(2)
ex
where εex is the extinction coefficient of azidostyrylꢀ
quinoline (L mol–1 cm–1) at the irradiation wavelength.

1432
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Budyka et al.
The inset (see Fig. 2, curve 2´) shows an example of proꢀ
cessing of experimental data in the coordinates of Eq. (2).
The quantum yields (ϕ) of photodissociation of aziꢀ
dohemicyanine 1 in EtOH on exposure to light with difꢀ
ferent wavelengths and under different photolysis condiꢀ
tions calculated from the kinetic curves are listed below
(error is 20%). As can be seen, azide 1 exposed to light
with the wavelength within the LWAB decomposes with
a high quantum yield irrespective of the presence of oxyꢀ
gen and irradiation wavelength; the average ϕ value
is 0.84 0.17.
20—25%), the reaction mixture contained two major
components, the initial azide characterized by the most
abundant peak in the mass spectrum (m/z 287, 100%)
and the corresponding primary amine (m/z 261, 59%);
peaks of other compounds were more than tenfold weakꢀ
er (see Table 2). Therefore, in this stage the amine can be
considered as the only reaction product; this is consistent
with the isosbestic points in the absorption spectra at
a given instant (see Fig. 2, spectra 1—4). This amine,
(E)ꢀ4ꢀ(4ꢀaminostyryl)ꢀ1ꢀmethylquinolinium iodide, is
the simplest hemicyanine dye characterized by a LWAB
near 530 nm in the absorption spectra of photolysis
products.
λ /nm
365
365
479
485
485
ex
ϕ
0.80
0.86*
0.89
0.85
0.80*
In addition to the amine, the degassed solution conꢀ
tained the following final reaction products: the correꢀ
sponding azo compound (m/z 259 [M]2+, doubly charged
* Degassed solution.
Reaction products. Usually, photolysis of aryl azides
results in the corresponding primary and secondary
amines, as well as azo, hydrazo, azoxy, nitroso, and nitro
compounds, etc.21 The percentage of these compounds in
the reaction mixture depends on the initial azide concenꢀ
tration22 and therefore the results of preparative photolyꢀ
sis of concentrated solutions cannot directly be applied to
dilute solutions used in spectral studies.
ion, M = 518) and a hydrazo compound (m/z 260 [M]2+
,
doubly charged ion, M = 520). The mass spectrum also
exhibits weaker peaks of unidentified products with m/z
218.097, 229.593 ([M]2+, doubly charged ion, M = 459)
and 295.080 (see Table 2), which can be attributed to
secondary reaction products.
When photolysis was carried out in the presence of
oxygen, the amine remained the major reaction product,
the percentages of the amine, azo, and hydrazo comꢀ
pounds being almost unchanged (100, 62, and 8%, reꢀ
spectively). In addition, oxygenꢀcontaining products
formed including the corresponding nitroso (m/z 275),
Using ESI mass spectrometry, we have analyzed diꢀ
lute reaction mixtures (concentrations ~10–5 mol L–1
)
used in the kinetic studies when measuring the quantum
yields, determined the compositions of the reaction mixꢀ
tures at different durations of irradiation of the solutions
of azide 1, and compared them with spectrophotometric
data. The results obtained are presented in Table 2.
In the initial stage of photolysis corresponding to the
absorption spectrum 3 in Fig. 2 (azide conversion
nitro (m/z 291), and azoxy compounds (m/z 267, [M]2+
,
doubly charged ion, M = 534), as well as the same unidenꢀ
tified products as in the degassed solution (see Table 2).
It is noteworthy that the percentages of the nitroso and
nitro compounds in the photolysis of azidohemicyanine
Table 2. Relative peak intensities (I/Imax (%)) in ESI mass spectra of the photolysis products of azidohemicyanine solutions
irradiated at λ = 365 nm
Producta
Structurea
m/z
I/Imax (%)
Airꢀsaturated
solution
Degassedb
solution
+
C18H17N2
QuSt—NH2
QuSt—NO
QuSt—NHOH
QuSt—NO2
QuSt—N=N—StQu
QuSt—NH—NH—StQu
QuSt—N=N(O)—StQu
261.116
275.098
277.090
291.099
259.099
260.106
267.097
218.097
229.593
295.080
100
9
6
100 (59)
—
5 (—)
—
62 (60)
8c (—)
—
15 (2)
14 (—)
15 (5)
C18H15N2O+
C18H17N2O+
+
C18H17N2O2
4
2+
C36H30N42+
63
8c
6
17
5
C36H32N4
C36dH30N4O2+
—
—
—
—
d
—
d
—
14
a Shown are the molecular formulas with no anions; for the structures, see Scheme 1.
b The results of analysis of the reaction mixture in the initial stage of photolysis (azide peak with m/z 287.105 is the strongest in the
mass spectrum) are given in parentheses.
c Peak intensity corrected for the isotope peak [М + 2] of the azo compound ion.
d Unidentified product.

Photochemistry of azidohemicyanine
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1433
I/Imax (%)
1 (9 and 4%, respectively) are almost an order of magnituꢀ
de lower than in the photolysis of neutral (E)ꢀ4ꢀ(4ꢀaziꢀ
dostyryl)quinoline, a precursor of azide 1 (60 and 40%,
respectively23). This can be due to the decrease in the
nitrene reactivity towards oxygen as a result of partial
localization of positive charge on the nitrene nitrogen
atom. At the same time, the positive charge does not
prevent nitrene dimerization to the azo compound beꢀ
cause the doubly charged ion peak of this product with
m/z 259 in the mass spectra in all cases is the next in
intensity (see Table 2).
a
80
60
40
20
The mass spectrum of the solution also exhibits a peak
with m/z 259 (see Table 2) in the initial stage of photolyꢀ
sis; however, here this is the peak of nitrene (product of
azide decomposition in the ionizing field). The m/z value
of the nitrene peak is the same as that of the azo comꢀ
pound peak (259), but the former originates from the
singly while the latter originates from the doubly charged
ion; therefore, one can distinguish between these peaks
with ease using the parameters of isotope peaks, namely,
the m/z value and peak intensity. The nitrene isotope
peak [М + 1] has m/z 260.127 and a relative intensity of
19.5%, whereas the azo compound isotope peak has m/z
259.625 and a doubled intensity (38.8%).
The regions in the mass spectra for the mass range
under study for the two reaction mixtures in the initial
and final stages of photolysis are shown in Fig. 4. The lack
of peaks with halfꢀinteger mass in the former case (see
Fig. 4, a) and the presence of these peaks in the latter case
(see Fig. 4, b), as well as their relative intensities allow
the peak with m/z 259 to be unambiguously assigned to
nitrene in the former and to the azo compound in the
latter case.
258
260
262
m/z
I/Imax (%)
b
80
60
40
20
258
260
262
m/z
Fig. 4. Mass spectra of reaction mixtures in photolysis of azidohemiꢀ
cyanine 1 in EtOH: a — initial stage of photolysis, the peak with
m/z 259 corresponds to nitrene (azide peak with m/z 287 is
the strongest in the mass spectrum); b — final stage of photolysis,
the peak with m/z 259 corresponds to the azo compound (no azide
peak in the spectrum). The peaks with m/z 261 in both spectra
correspond to amine.
The ion peak with m/z 277 in the mass spectra correꢀ
sponds to both the hydroxyamino derivative and substiꢀ
Scheme 1

1434
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Budyka et al.
Skopenko, Ukr. Khim. Zh. [Ukr. Chem. J.], 1984, 50, 296
(in Russian).
10. M. F. Budyka, T. S. Zyubina, J. Mol. Struct. (Theochem.), 1997,
419, 191.
11. M. F. Budyka, T. S. Zyubina, M. M. Kantor, Zh. Fiz.
Khim., 2000, 74, 1115 [Russ. J. Phys. Chem., 2000, 74, 995
(Engl. Transl.)].
12. M. F. Budyka, I. V. Oshkin, J. Mol. Struct. (Theochem.), 2006,
759, 137.
tuted aminophenol. It is noteworthy that the formation of
this compound is almost independent of the presence of
oxygen (see Table 2), namely, different compounds with
the same m/z value can probably form under different
conditions.
On the whole, the formation of all identified products
in the photolysis of azidohemicyanine 1 can be explained
by the reactions of the triplet nitrene21 (Scheme 1).
Thus, our studies confirmed the predicted photoꢀ
chemical activity of azides with positively charged conjuꢀ
gated πꢀsystem including at most eighteen electrons. Aziꢀ
dohemicyanine (4ꢀ(4ꢀazidostyryl)ꢀ1ꢀmethylquinolinium
iodide) was synthesized for the first time. The compound
has a LWAB in the region 350—500 nm and decomposes
with a quantum yield of 0.84 0.17 irrespective of the
presence of oxygen on exposure to light within the LWAB.
Among the known aromatic azides, the azidohemicyaꢀ
nine possesses the longestꢀwavelength visible light sensiꢀ
tivity and is characterized by the high quantum yield of
photodissociation of the azido group.
13. M. F. Budyka, N. V. Biktimirova, T. N. Gavrishova, V. I.
Kozlovskii, Mendeleev Commun., 2007, 17, 3.
14. J. J. P. Stewart, J. Comput. Chem., 1989, 10, 208, 221.
15. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchꢀ
ian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador,
J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G.
Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. AlꢀLaham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Revision B.02,
Gaussian Inc., Pittsburgh, PA, 2003.
The authors are grateful to A. G. Ryabenko for proꢀ
viding the program for analysis of spectral data by the
method of principal components.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 03ꢀ03ꢀ32116).
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Received May 31, 2007;
in revised form June 4, 2007