Porphyrazines with annulated diazepine rings. 4. Synthesis and properties of MgII tetradiazepinoporphyrazine carrying exocyclic styryl fragments

Article abstract of DOI:10.1142/S1088424612501052

A novel tetradiazepinoporphyrazine Mg^II complex bearing eight peripheral styryl substituents, [St₈TDzPAMg(H₂O)] (St = -CH=CHAr, where Ar = 4-methoxyphenyl) was prepared by template cyclotetramerization of the corresponding precursor - 5,7-distyryl substituted diazepino-2,3-dicarbonitrile in the presence of Mg^II butoxide in n-butanol. UV-visible and ¹H NMR spectral data indicate that the complex is strongly aggregated in non-coordinating solvents (dichloromethane, chloroform, benzene), it is dimeric in pyridine, whereas it is predominantly monomeric in dimethylsulfoxide and dimethylformamide. The fluorescence response is high for solutions in which the monomeric form is prevalent, but it is strongly quenched as the content of the dimer is increased. Evidence was obtained that dimerization occurs due to intermolecular hydrogen bonding between acidic CH₂ groups in the diazepine ring (6H form) of one molecule with meso- and/or diazepine N atoms of another molecule, dimerization being also contributed by the presence of chlatrated water. In the presence of fluoride anions the dimer is destroyed with formation of the monomeric species, which is changed to the 1H form upon heating, as indicated by ¹H NMR spectra. Copyright

Full text of DOI:10.1142/S1088424612501052

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 968–976
DOI: 10.1142/S1088424612501052
Published at http://www.Worldscinet.Com/jpp/
Porphyrazines with annulated diazepine rings. 4. Synthesis
and properties of MgII tetradiazepinoporphyrazine carrying
exocyclic styryl fragments
Pavel A. Stuzhin*a◊, Pavel Tarakanova, Svetlana Shiryaevaa, Anna Zimenkovaa,
Oscar I. Koifmana◊, Elisa Violab, Maria Pia Donzellob◊ and Claudio Ercolanib◊
a Department of Organic Chemistry and Technology, Ivanovo State University of Chemical Technology, Friedrich
Engels Pr-t, 7, Ivanovo 153000, Russia
b Dipartimento di Chimica, Università di Roma “La Sapienza”, p.le A. Moro 5, Roma 00185, Italy
Dedicated to Professor Tebello Nyokong on the occasion of her 60th birthday
Received 18 March 2012
Accepted 10 May 2012
ABSTRACT: A novel tetradiazepinoporphyrazine MgII complex bearing eight peripheral styryl
substituents, [St8TDzPAMg(H2O)] (St = -CH=CHAr, where Ar = 4-methoxyphenyl) was prepared by
template cyclotetramerization of the corresponding precursor — 5,7-distyryl substituted diazepino-
1
2,3-dicarbonitrile — in the presence of MgII butoxide in n-butanol. UV-visible and H NMR spectral
data indicate that the complex is strongly aggregated in non-coordinating solvents (dichloromethane,
chloroform, benzene), it is dimeric in pyridine, whereas it is predominantly monomeric in
dimethylsulfoxide and dimethylformamide. The fluorescence response is high for solutions in which
the monomeric form is prevalent, but it is strongly quenched as the content of the dimer is increased.
Evidence was obtained that dimerization occurs due to intermolecular hydrogen bonding between acidic
CH2 groups in the diazepine ring (6H form) of one molecule with meso- and/or diazepine N atoms
of another molecule, dimerization being also contributed by the presence of chlatrated water. In the
presence of fluoride anions the dimer is destroyed with formation of the monomeric species, which is
changed to the 1H form upon heating, as indicated by 1H NMR spectra.
KEYWORDS: porphyrinoids, stylbenoids, tetradiazepinoporphyrazines, dimerization, electronic
absorption and emission spectra.
the central p-chromophore followed by light emission
close to the near IR region. Tetrapyrazinoporphyrazines
carrying exocyclic styryl fragments were recently also
prepared and their spectral and electrochemical pro-
perties investigated [4–6]. In our studies of symmetrical
tetradiazepinoporphyrazines (TTDzPA) [7] and low-
symmetry monodiazepinotribenzoporphyrazines [8] it
was observed that the presence of non-planar quasi-
aromatic 1,4-diazepine ring(s) annulated to the central
pyrrole rings has remarkable effects on the physico-
chemical properties of the central porphyrazine macro-
cycle. Very recently, evidence was obtained [9] that
tetradiazepinoporphyrazines exhibit a tendency to form
stable dimers with strong exciton coupling between
INTRODUCTION
Peripheral substituents in phthalocyanines (Pc) and
related macrocyclic complexes, as well as solvation
effects have a strong influence on their spectral,
photochemical and photophysical properties [1].
Phthalocyanines bearing eight peripheral stylbenoid
p-chromophores were synthesized [2] and are actively
investigated as artificial light-harvesting systems [3] in
which the excitation of eight peripheral stylbene moieties
in the near UV region can be effectively transferred to
SPP full member in good standing
*Correspondence to: Pavel A. Stuzhin, email: stuzhin@isuct.ru
COPYRIGHT
© 2012 WORLD SCIENTIFIC PUBLISHING

PORPHYRAZINES WITH ANNULATED DIAZEPINE RINGS
969
OMe
MeO
Me
Me
H2N
H2N
CN
CN
O
O
+
MeO
OMe
MeO
N
N
N
N
N
Mg
N
PhMe, t°
N
N
H2O
N
N
O
H
CN
CN
Mg(OBu)2
BuOH
N
N
MeO
N
N
Me
N
N
N
N
1
CN
CN
PhMe/pip
2
N
N
Me
MeO
OMe
MeO
OMe
MeO
3
[St8TDzPAMg(H2O)]
Scheme 1. Synthesis of the styryl substituted tetradiazepinoporphyrazine MgII complex [St8TDzPAMg(H2O)]
two porphyrazine p-chromophores, comparable with
that observed in sandwich or dimeric phthalocyanine
complexes [10]. Styryl substituted diazepines exhibit
fluorescence with large Stokes shift and can be used
as fluorescent dyes [11] or red dopants in organic
electroluminescence devices [12]. It was thought
that connection of the central p-chromophoric
system of a porphyrazine macrocycle with peripheral
styryl substituted diazepine fragments might lead to
peculiar optical properties. Following a preliminary
communication [13], it is reported here in detail on the
synthesis and physicochemical characterization of the
octastyryl substituted MgII-tetradiazepinoporphyrazine
[St8TDzPAMg(H2O)] (St = 4-MeOPhCH=CH-, see
Scheme 1). The synthesis of low-symmetry and
symmetrical MgII-porphyrazines containing styryl
substituted diazepine ring(s) has been also quite
recently reported [14] and variations observed in the
electronic absorption spectra in different solvents were
connected with solvatochromic effects. Based on our
electronic absorption and emission and 1H NMR spectral
measurements, the present work provides enough
information that the spectral properties of the styryl
stubstituted MgII-tetradiazepinoporphyrazines clearly
depend on their tendency to dimerization, a monomer/
dimer equilibrium which is very sensitive to the presence
of water and other ligands in the solvents.
Synthetic procedures
5,7-Di[(E)-2-(4-methoxyphenyl)ethenyl]-6H-
1,4-diazepine-2,3-dicarbonitrile (2). A mixture of
5,7-dimethyl-6H-1,4-diazepine-2,3-dicarbonitrile
(1)
(1.7 g, 10 mmol), 4-methoxybenzaldehyde (2.8 g,
20 mmol), toluene (40 mL) and piperidine (5 drops)
was refluxed with stirring for 6 h in a round-bottomed
flask equipped with a Dean-Stark attachment. During
the reaction, the color of the mixture slowly turned to
orange-red. After cooling and filtration of the mixture,
the bright-orange solid was washed with chloroform and
combined with the residue obtained from the chloroform
solution after chromatographic purification on silica (2;
total amount: 3.5 g; yield 85%, mp = 267 °C). Anal.
calcd. for C25H20N4O2 (408.45): C, 73.51; H, 4.94; N,
13.72%. Found: C. 73.75; H, 5.04; N, 13.66%. 1H NMR
(500 MHz; DMSO-d6; Me4Si): dH, ppm 8.042 (2H, d,
3J = 16.5 Hz, -CH=CH-), 7.746 (4H, d, 3J = 8.5 Hz, ArH),
7.002 (4H, d, 3J = 8.5 Hz, ArH), 6.994 (2H, d, 3J = 16.5
Hz, -CH=CH-), 5.34 (1H, br, eq-CH2), 3.793 (6H, s,
OCH3), 2.02 (1H, br, ax-CH2). IR (KBr): n, cm-1 2222w,
1599s, 1516s, 1304w, 1257s, 1173s, 978w, 827w, 771s.
UV-vis (CHCl3): lmax, nm (log e) 357 (4.77), 436 (4.40).
Tetrakis-2,3-{5,7-di[(E)-2-(4-methoxyphenyl)-
ethenyl]-6H-1,4-diazepino}porphyrazinato(aquo)-
magnesium(II),
[St8TDzPAMg(H2O)].4H2O
(3).
Metallic magnesium (100 mg, 4 mmol) was refluxed with
stirring in n-butanol (50 mL) in the presence of a few
crystals of iodine until complete formation of Mg(OBu)2.
After addition of dicarbonitrile 2 (0.5 g, 1.2 mmol), the
reaction mixture was kept boiling for further 18 h. After
cooling, the solvent was evaporated and the residue was
washed with water acidified with acetic acid, then with
water, methanol and diethyl ether and brought to constant
weight under vacuum (yield 0.44 g; 87%). Anal. calcd.
for C100H82MgN16O9.4H2O: C, 68.70; H, 5.19; N, 12.82%.
Found: C, 68.55; H, 5.25; N, 12.80%. MS (MALDI-TOF):
m/z 1659 (100%, [M + H]+), 1681 (16%, [M + Na]+),
1697 (8%, [M + K]+), 3318 (5%, [M2 + 2H]+), 3340 (5%,
EXPERIMENTAL
Solvents and reagents
Solvents and chemicals were used as purchased.
Pyridine was made anhydrous by distillation over
KOH. 5,7-dimethyl-6H-1,4-diazepine-2,3-dicarboni-
trile (1) was prepared by condensation of diamino-
maleodinitrile and acetylacetone following the reported
procedure [15].
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P.A. STUZHIN ET AL.
[M2 + H + Na]+). 1H NMR (500 MHz, DMSO-d6, 393 K,
Me4Si): dH, ppm 8.038 (8H, d, 3J = 16.5 Hz, -CH=CH-),
to afford the MgII complex of the corresponding
styryl substituted tetradiazepinoporphyrazine 3. This
complex shows in MALDI-TOF mass-spectrum an
intense molecular ion peak [M + H]+ at m/z = 1659
(Fig. 1). Remarkably, an additional lower intensity
peak, corresponding to the molecular ion of the dimeric
species, is also seen. According to elemental analyses,
complex 3 was isolated as a hydrated compound,
formulated as [St8TDzPAMg(H2O)].4H2O, in which
one water molecule is assumed to be directly bound to
MgII. The presence of the MgII(H2O) moiety is typical
for MgII complexes of porphyrazines as was established
by single-crystal X-ray work on MgII phthalocyanine
[16] and observed also for other MgII porphyrazines
[5,17]. The presence of additional water molecules,
variable from batch to batch, is also typical for other
tetradiazepinoporphyrazines [7, 9] and can be related
to the hydrogen bonding abilities of diazepine and/or
meso-nitrogen atoms.
3
3
7.671 (16H, d, J = 7.9 Hz, ArH), 6.993 (16H, d, J =
3
7.9 Hz, ArH), 7.545 (8H, d, J = 16.5 Hz, -CH=CH-),
2
2
5.692 (4H, d, J = 12.2 Hz, eq-CH2), 4.916 (4H, d, J =
12.2 Hz, eq-CH2), 3.898 (24H, s, OCH3). IR (KBr): n,
cm-1 1602s, 1513s, 1462w, 1252s, 1174s, 1114w, 1031m,
967w, 824w. UV-vis (CH2Cl2 + 2.5% pyridine): lmax, nm
(log e) 379 (4.85), 415 (4.85), 670 (4.46), 702 (4.44).
Spectral measurements
Infrared spectra were taken on the IR-spectrometer
AVATAR 360 FT-IR using KBr pellets. UV-vis spectra
were recorded using a Hitachi U-2000 instrument and
1H NMR spectra were obtained on a Bruker Avance 500
MHz spectrometer. Elemental analyses were performed
on a CHN analyser Flash EA 1112. Mass-spectrometric
measurements were carried out on a MALDI-TOF Bruker
Ultraflex spectrometer. The fluorescence emission and
excitation spectra were recorded on a Fluorescence
Spectrophotometer (Cary Eclipse, Varian). For the
emission measurements, the excitation wavelength was
set at 400 nm or 630 nm, and fluorescence was recorded
up to 900 nm. For the excitation spectra, the emission
was observed at 740 nm. Both in emission (lexc = 400 nm)
and in fluorescence excitation measurements, an internal
emission filter which cut off radiations below 430 nm
was used.
UV-vis spectra
The UV-vis spectra in different solvents of the MgII
complex 3 and the related tert-butylphenyl substituted
complex [(tBuPh)8TDzPAMg(H2O)] (4) [9] are shown
for comparison in Fig. 2. As exemplified in Fig. 2a, the
spectrum of 3 in CH2Cl2 (dashed line), and other non-
coordinating solvents (CHCl3, benzene), shows the
presence of a low-intensity broad featureless absorption
in the Q-band region (600–800 nm) which is indicative of
the occurrence of strong aggregation. As can be seen, the
spectrum in CH2Cl2 containing pyridine (ca. 2%) or in
100% pyridine (Figs. 2a and 2b, respectively; solid lines)
is better resolved and shows two peaks in the same region
with maxima at ca. 670 and 702–705 nm. In going from
pyridine to DMSO (Fig. 2c, solid line) the absorption at
670 nm is decreased and practically disappears in DMF
(Fig. 2d, solid line), in which the spectrum shows the
RESULTS AND DISCUSSION
Synthesis and characterization
The precursors for the template synthesis of styryl
substituted diazepinoporphyrazines, i.e. 1,4-diazepine-
2,3-dicarbonitriles carrying styryl groups at the
5,7-positions, can be conveniently pre-
pared from 5,7-dimethyl-1,4-6H-diazepine-
2,3-dicarbonitrile (1) [11], in which the
CH-acidity of methyl groups is high enough
to allow the easy condensation with aryl-
aldehydes having electron-donating sub-
stituents. Previously [11b], 4-metho-
xystyryl substituted 1,4-6H-diazepine-2,3-
dicarbonitrile (2) was prepared with 10%
yield by condensation of 1 with 4-metho-
xybenzaldehyde in benzene in the presence
of catalytic amounts of piperidine. The use
of toluene instead of benzene (Scheme 1)
allowed us to conduct the reaction at higher
temperature and the yield of 2 was increased
to 85%. The bis-styryl substituted dinitrile
2, similarly to procedures with other
diazepine-2,3-dicarbonitriles [7a, 9], can be
Fig. 1. MALDI-TOF mass-spectrum of styryl substituted tetradiazepinopor-
phyrazine 3
cyclotetramerised under Linstead conditions
(magnesium butoxide in refluxing n-butanol)
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PORPHYRAZINES WITH ANNULATED DIAZEPINE RINGS
971
The splitting of the Q-band originates from the exciton
coupling between two neighboring p-chromophores
constituting the dimeric unit, similarly to sandwich and
dimeric phthalocyanine and porphyrazine complexes
[10]. This encourages the same assignment for the double
peaked spectrum of 3.
It should be noted that water-soluble tetrasulfonated
phthalocyanines [18] and quaternized octapyridyl
substituted tetrapyrazinoporphyrazines [19] show a
similar double-peaked Q-band absorption due to essential
dimerization in water solution. However, dimerization of
these species is easily suppressed upon dilution of water
solutions, and they exist exclusively in the monomeric
form with a single Q-band not only in DMSO, but also
in pyridine solution. In contrast to that, the dimeric form
of 3 is more stable; it exhibits a double Q-band and obey
the Lambert–Bouger–Baehr law in CH2Cl2 containing
pyridine (> 2.5%) and in pure pyridine when explored in
the 0.005–0.05 mM concentration range.
As can be seen in Fig. 2, the Q-band maxima for
complex 3 are shifted bathochromically by 20–25 nm
as compared to those of complex 4. This is indicative
that styryl substituents in 3 exhibit stronger electron-
t
donor effect on the diazepine rings than the BuPh
groups in 4. The minimal residual molecular aggregation
can not explain the observed broadness of the Q-band
absorptions for 3. Such a broadening, however, might
be contributed by the presence in the macrocycle 3 of
conformational isomers having different orientation of
the styryl fragments in respect to the diazepine rings (e.g.
with cisoid and transoid –N=C…CH=CH- fragments).
Indeed, the presence of two conformations of styryl
groups was observed in the X-ray crystal structure of
bis(4-diethylaminostyryl)substituted 1,4-diazepine-2,3-
dicarbonitrile [11b]. We have also observed that the
profile of the spectra of 3 taken in pyridine, DMSO and
DMF may partly depend on the amount of water present.
The component in the Q-band area at ca. 700 nm shows
higher intensity in carefully dried solvents, whereas
small additions of water decrease the intensity of this
band with a concomitant increase of the short-wave
component at ca. 670 nm. This is exemplified in Fig.
2d (dashed line), which shows the spectrum of 3 in
DMF containing 2% of water which is characterized
by the prevalent presence of the double peaked profile
in the concomitant disappearance of the absorption
at ca. 700 nm. In line with the above findings for 3, it
was observed for 4, that a single Q-band spectrum of
the monomeric species detected in diluted DMSO and
DMF solutions with low water content (dotted lines
in Figs 2c and 2d) is changed to that of the dimeric
species by addition of water or pyridine [9]. It should
be noted that UV-vis spectra of 3,4,5-trimethoxystyryl
substituted MgII-tetradiazepinoporphyrazines in various
solvents were recently reported [14b]. Similarly to our
4-methoxystyryl derivative 3, they contain a broad
Q-band absorption typical of strongly associated species
Fig. 2. UV-vis spectra of MgII complex of styryl substituted
tetradiazepinoporphyrazine 3: (a) aggregated form in CH2Cl2
(dashedline)andspectralchangeafteradditionofca2%pyridine
(solid line), (b) in pyridine, (c) in DMSO, (d) in DMF (solid
line), after addition of 2% water (dashed line). Spectra of tert-
butylphenyl substituted complex [(tBuPh)8TDzPAMg(H2O)]
in corresponding solvents are shown by dotted lines. Adapted
from Ref. [9]
single Q-band at 702 nm accompanied by vibrational
shoulders on its higher energy side. Dotted lines in
Figs. 2b, 2c, 2d indicate a parallel solvent dependence
observed recently [9] for the tert-butylphenyl substituted
complex 4 in that the spectrum goes from the presence of
two well-separated absorptions with maxima at 644 and
684 nm in pyridine to a single narrow intense absorption
at 677–679 nm in DMSO and DMF.
The spectra with a single Q-band observed in
DMF are typical for monomeric metal complexes
of symmetrical porphyrazines and can be definitely
assigned as belonging to the usual monomeric form of
complexes 3 and 4. The spectra with a double Q-band
were first observed for complexes of phenyl substituted
tetradiazepinoporphyrazine and suggestion was made
that the less-intense long-wave component might arise
from n-p* transition involving lone pairs of the diazepine
nitrogens [7]. However, in our recent study [9] clear
evidence was obtained that the parent spectrum seen in
pyridine solution for 4 (Fig. 2b, dotted line), displayed
identical also in CH2Cl2, benzene, THF, or acetone,
belongs to a stable dimeric form of the complex 4.
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P.A. STUZHIN ET AL.
in CH2Cl2 and tetrahydrofurane and a double-peaked
Q-band characteristic for the dimeric form in a number
of other solvents. Taking into account our results on
the dependence of the UV-vis spectra of 3 and 4 from
residual water in DMSO and DMF, it cannot be excluded
that the split Q-band which was observed in [14b] for the
3,4,5-trimethoxystyryl derivative in these solvents might
also be correlated to the level of water content of the
used solutions. We have established that the fluorescence
emission spectra are even more sensitive to the factors
influencing complex 3 in its monomer/dimer interchange.
480–500 and ca. 700 nm (lexc = 400 nm) [5]. For the
corresponding styryl substituted dinitrile precursors
a broad fluorescence emission band is observed in
the region 400–600 nm. Noteworthy, the related
pyrazinoporphyrazine
MgII
macrocycle
bearing
peripheral methyl groups instead of styryl groups, i.e.
[Me8TPyzPAMg(H2O)], shows, as expected, only a single
emission band in the near-IR region with maximum at
643 nm (lexc = 400 nm) due to the porphyrazine core [5].
The fluorescence excitation spectra registered in the
three solvents (pyridine, DMSO and DMF) all showing
the fluorescence emission at lem = 740 nm, are given
in Fig. 3 (right side, curves 3). Such type of spectra is
typical of symmetrical porphyrazines in their monomeric
form. In the region 600–800 nm the spectra show only
one narrow intense band at 702–708 nm having mirror
image symmetry with the emission spectrum in the
same region. Comparing the excitation spectra with
the corresponding absorption spectra (Fig. 3, curves 3
and 4), it is noticed that the absorption band at ca.
700 nm is the one responsible for the emission band
with maximum at 722–727 nm, while the absorption
band at 660–670 nm seen in DMSO and pyridine does
not contribute to the fluorescence response at all. These
findings, in agreement with indications from the above
UV-vis spectra, further support the assignment of the
absorption band at ca. 700 nm as due to the monomeric
Fluorescence spectra
The fluorescence measurements for the MgII complex
3 were made for aerated solutions in pyridine, DMSO
and DMF. Curves 1 and 2 in Fig. 3 (left side) are the
fluorescence spectra obtained under excitation with
lexc = 400 and 630 nm, respectively. The fluorescence
emission band at 722–727 nm observed using both
types of excitation is typical of the porphyrazine core,
whereas the broad emission band at ca. 480–490 nm is
assigned to the presence of the styryl fragments. These
data fit well with the fluorescence results obtained by
us for the octastyryl substituted MgII complex of the
tetrapyrazinoporphyrazine analog [St8TPyzPAMg(H2O)]
which also shows two fluorescence emission bands at
Fig. 3. Fluorescence emission (1) - lexc = 400 nm, (2) - lexc = 630 nm, fluorescence excitation (3), (lreg = 740 nm) and absorption (4)
spectra of MgII complex 3 in DMF, DMSO and pyridine
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PORPHYRAZINES WITH ANNULATED DIAZEPINE RINGS
973
Fig. 4. Changes in absorption (a) and fluorescence emission (b) (lex = 630 nm) spectra of the MgII complex 3 in DMSO (1) after
addition of 10% water (2) and tbaF (3)
fluorescent macrocycle and of the absorption at 660–670
nm to the non-fluorescent dimeric species.
[St8TPyzPAMg(H2O)] (13 nm). This suggests that the
relaxed excited-state geometry is more different from the
ground-state geometry for the porphyrazine macrocycle
bearing diazepine rings than for the tetrapyrazino analog.
In this fact may reside the reason for the low effectiveness
of the excitation transfer from the periphery to the central
p-chromophore. The larger conformational changes in
the excited state for 3 evidently also negatively affect the
fluorescence quantum yield (FF). Its values measured in
DMF for lexc = 630 nm (FF = 0.097) and for lexc = 400
nm (FF = 0.022 and 0.061 for emission of stylbenoid and
porphyrazine p-chromophores, respectively) are lower
than that observed for the tetrapyrazino analog (FF =
0.23–0.26 in DMSO [5]). It should be also noted that the
intensity of the stylbenoid fluorescence band is increased
relative to that of the porphyrazine fluorescence when the
content of the monomeric species is decreased in going
from DMF to DMSO and pyridine solutions (compare
profiles of curves 1 in Fig. 3, right side). This indicates
that dimerization quenches the fluorescence of the
porphyrazine chomophore, but has a lower effect on the
emission of the stylbenoid fragments.
Figure 4 shows the spectral variations of both the
absorption and emission spectra of 3 in DMSO caused by
addition of H2O and tetrabutylammonium fluoride (tbaF).
The observed changes seen by addition of H2O (Fig. 4a,
spectra 1 and 2, black → green) indicate a decrease of the
amount of the monomeric species (maximum at 704 nm)
and a concomitant increase of the dimeric species (664
nm); a reverse spectral change (black → red) favoring
the monomer takes place upon addition of the fluoride
ion (Fig. 4a, spectrum 3). In keeping with these findings,
the fluorescence is decreased in the presence of water and
increased upon addition of fluoride (Fig. 4b) indicating
coherent changes of the ratio monomer/dimer. While
water stabilizes the dimeric form of the MgII complex 3,
the addition of fluoride anions destroys it with formation
of the monomeric form. The effect of fluoride might
be connected with its ability to substitute coordinated
water and/or rearrange hydrogen bonds responsible for
dimerization.
The comparison between the absorption and the
excitation spectra obtained for the complex 3 leads to
some additional considerations. As shown in Fig. 3
(curves 3 and 4), absorption of the peripheral stylbenoid
p-chromophores in the region 400–600 nm contributes
strongly to the broad and intense Soret band in the
absorption spectra. The lower intensity of this band in the
excitation spectra seems to indicate that the excitation is
not effectively transferred from the stylbenoid moieties
to the central porphyrazine p-chromophore. Noticeably,
the Stokes shift observed for complex 3 in the near-IR
region is ca. 20 nm, which is larger than that found
for the styryl substituted tetrapyrazinoporphyrazine
1H NMR spectra
1H NMR spectra of the MgII complex 3 taken at room
temperature in different deuterated solvents contain
broad poorly resolved lines due to strong aggregation
effects at the concentrations used for measurements
(0.1–1 mM). The quality of the spectra improves at
elevated temperatures. The spectra of 3 recorded in
DMSO-d6 at 383 K are presented in Fig. 5 in comparison
with the spectra of the corresponding dinitrile precursor
2 in DMSO-d6. For both 3 and 2 the singlets of methoxy
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974
P.A. STUZHIN ET AL.
Fig. 6. UV-vis spectra of concentrated (~2 mM) solution of
MgII complex 3 in DMSO-d6 (a) and its change after addition
1
of TBAF (b) and successive heating in the course of H NMR
measuremts (c)
indicative of a rigid framework and consequent inhibited
inversion of the 6H-1,4-diazepine rings in the MgII
complex 3. This can be in connection with the existence
of 3 in a stable dimeric form, in a fashion similar to the
case previously discussed for the related tert-butylphenyl
derivative [9]. These results indicate that, differently
from the findings for the diluted DMSO solutions used
for UV-vis and fluorescence measurements, in which the
MgII complex 3 exists predominantly in its monomeric
1
Fig. 5. H NMR spectra of dicarbonitrile 2 ((a), (b)) and MgII
complex 3 ((c), (d)) in DMSO-d6. Spectrum D is recorded after
addition of tbaF
groups are observed at 3.8–3.9 ppm and the characteristic
doublets of aromatic o- and m-protons are at ca. 7.7 and
7.0 ppm. Two doublets of the CH=CH group with typical
trans-splitting constant (3J = 16.5 Hz) are seen at 8.04
and 7.55 ppm for complex 3 (383 K, Fig. 5c) and at
ca. 8.0 and 7.0 ppm for the dinitrile 2 (343 K, Fig. 5b).
The doublet in the higher field, which is shifted downfield
by ca. 0.5 ppm for the MgII complex 3, belongs to the
CH protons neighboring the diazepine ring. It should
be noted that in the spectrum of 3 recorded at 383 K
additional broad lines of aggregates are still seen in the
aromatic region along with an additional signal of the
methoxy group at ca 3.8 ppm (Fig. 5c).
Two broad signals of diastereotopic CH2 protons of
the diazepine ring are detected for the dinitrile 2 at 5.3
and 2.0 ppm at room temperature (see Fig. 5a), but at
elevated temperature they disappear (see Fig. 5b) due to
rapid inversion of the diazepine ring. Such behaviour is
typical also for other simple 1,4-diazepines existing in
the 6H tautomeric form [7a, 9,20]. In contrast to that,
for the MgII complex 3 signals of diastereotopic CH2
protons are retained and even sharpened with the raise
of temperature and in DMSO-d6 at 383 K (Fig. 5c) they
appear at 5.70 and 4.92 ppm. The resonance signal at
5.70 ppm appears little broadened, while that at 4.92
ppm is seen as a well-resolved doublet with geminal
1
form, at the concentrations used for the H NMR study
the dimeric form is prevalent. The UV-vis spectrum of
this concentrated solution taken in a thin cuvette contains
a double peaked absorption (Q-band) expected for the
dimer (Fig. 6a).
Interestingly, the addition of tbaF to this concentrated
solution of 3 leads to dissociation of the dimeric species
with formation of the monomer with a single Q-band
(Fig. 6b), similarly as was observed for a diluted solution
(Fig. 4). The 1H NMR spectrum of this DMSO-d6
solution containing TBAF at 383 K is shown in Fig. 5d.
The signals of aromatic (7.09 and 7.90 ppm) and olefinic
protons (7.60 and 8.12 ppm) are shifted downfield in
comparison with the corresponding positions in the
spectrum taken in the same solvent in the absence of tbaF
(Fig. 5c). The most prominent feature of the spectrum in
the presence of F- ions is the disappearance of the signals
of diastereotopic CH2 protons, which are characteristic
for the 1,4-diazepine ring in the 6H tautomeric form,
and the appearance of the new signals at 8.61 and 5.37
ppm. These new signals can be assigned to the NH and
CH protons of the diazepine ring in the 1H form. The
corresponding NH and CH resonances were found at ca.
8.0 and 6.1–6.2 ppm for the pyrimidine fused 1H-1,4-
2
splitting constant J = 12.2 Hz. For a solution of 3 in
diazepines
(6,8-diarylpyrimido[4.5-b]-1.4-diazepine-
pyridine-d5 the resonances of the olefinic CH=CH and
diazepine CH2 protons are shifted to the lower field and
at 383 K they are observed at 8.49, 8.12 and 6.73, 5.98
ppm, respectively. The presence of two CH2 resonances
observed even at elevated temperatures is clearly
4-ols) [21]. The UV-vis spectrum of the solution taken
1
after the H NMR measurement at elevated temperature
differs from the initial spectrum obtained after addition of
tbaF (Fig. 6b) and contains a broad and featurless
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PORPHYRAZINES WITH ANNULATED DIAZEPINE RINGS
975
absorptioninthe600–900nmregion(Fig. 6c).Theoretical
calculations of the excited states of the MgII complex
of monodiazepine annulated tribenzoporphyrazine
[DzBz3PAMg] [8b] predict that such broadening of
the Q-band could be expected for the porphyrazines
with fused diazepine rings in the 1H form. The visible
spectral region for the 6H species [6HDzBz3PAMg],
typically for porphyrazine complexes, contains only
two p–p* transitions of the macrocyclic p-chromophore
(Q-band), while transitions involving p- and n-orbitals of
the diazepine ring were found in the UV region. For the
1H tautomer [1HDzBz3PAMg] the p-orbital localized on
the diazepine ring is located very closely to the highest
occupied p-MO of porphyrazine macrocycle. This leads
to a number of additional intense transitions involving
charge transfer from the diazepine ring to the central
porphyrazine core which are strongly mixed with the
p–p* transition localised on the macrocycle.
by a UV-vis spectrum showing a double peaked Q-band
area with strong exciton coupling, whereas the complex
appears to be largely in its monomeric form in diluted
DMSO and especially in DMF solution. Results of
emission spectroscopy measurements indicate that only
the monomeric form is fluorescent, whereas the
fluorescence of the porphyrazine p-chromophore is
1
quenched in the dimeric form. H NMR spectra provide
evidence that dimerization occurs due to intermolecular
hydrogen bonding between acidic CH2 groups in the
diazepine ring of one macrocycle with the meso- and/
or diazepine N atoms of the adjacent macrocycle,
clathrated water also contributing to dimerization.
Addition of fluoride anions to solutions of the dimer
favors monomerization and the monomer is stabilized
with the diazepine rings in the 1H form. Further studies
are being conducted on the structure, dimerization
processes and tautomerism of the diazepine rings in
tetradiazepinoporphyrazines.
The available UV-vis and 1H NMR spectral data let us
to assume that conversion of the dimeric form of the MgII
complex 3 to the monomeric 1H form upon addition of the
fluoride anions occurs in two steps. The diazepine rings
in the dimer exist in the 6H form, as established by the
presence of two characteristic signals of diastereotopic
(axial and equatorial) CH2 protons. One can suppose that
dimerization occurs due to hydrogen bonding interaction
involving the CH2 group in one molecule and meso- or/
and diazepine N atoms of the other closely lying molecule
within the dimer, locally inserted clathrated water within
the two molecules presumably also contributing to
dimerization. The occurrence of H-bonding leads to the
observed strong downfield shift of the resonance signal
belonging to the axial CH2 proton participating in this
interaction from its “normal” position (ca. 2 ppm in the
precursor 2) and very likely determines its broadening.
Fluoride anions upon their addition substitute water
molecules participating in hydrogen bonding interaction
and/or coordinated to the magnesium ion. This leads to
dissociation of the dimers and formation of the monomer
with the diazepine rings in the 6H form characterised by
a single Q-band (Fig. 6b). Upon heating the 6H tautomer
is transformed to the 1H tautomer which exhibits a
broad absorption in the visible region (Fig. 6c) and is
evidently stabilized by the hydrogen bonding of fluoride
anions with the NH groups of the enamine 1H form
(F-…HN(diazepine)).
Acknowledgements
The financial support from Russian Foundation of
Basic Research (grant RFFI 10-03-01069-a) and Russian
Ministry for Science and Education (the Federal Target
Program “Nanotechnology and Nanomaterials,” GK
14740.11.0268) is gratefully acknowledged.
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