Synthesis of new substituted tetrazines: Electrochemical and spectroscopic properties

Article abstract of DOI:10.1039/b310737j

Original tetrazines substituted by heterocyclic rings have been prepared. Their syntheses, as well as their electrochemical and spectroscopic features, are described. Calculations have also been made on the cation radicals, anion radicals and neutral compounds and are in correct agreement with the experimental results. All compounds are electroactive, both in oxidation and reduction, and display two absorption bands in the UV and visible regions of the spectrum. Reduction potentials and maximum wavelengths are correlated with the electron-rich character of the heterocyclic substituent on the tetrazine ring. None of these compounds gives good quality polymers upon electro-oxidation, which was unexpected, especially for the bis(2-pyrrolyl) tetrazine. This latter result can be explained by the occurrence of a self-deprotonation equilibrium in the cation radical.

Full text of DOI:10.1039/b310737j

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P A P E R
Synthesis of new substituted tetrazines: electrochemical and
spectroscopic properties
a
a
a
a
a
Pierre Audebert,* Sa ¨ı d Sadki, Fabien Miomandre, Gilles Clavier, Marie Claude Verni e` res,
a
b
Merzaka Saoud and Philippe Hapiot
a
Laboratoire de Photophysique et Photochimie Supramol e´ culaires et Macromol e´ culaires
t
(
CNRS UMR 8531), Ecole Normale Sup e´ rieure de Cachan, 61 Avenue du P Wilson, 94235,
Cachan, France. E-mail: audebert@ppsm.ens-cachan.fr
b
Laboratoire d’Electrochimie, Synth e` se et Electrosynth e` se Organiques (CNRS UMR 6510),
Universit e´ de Rennes 1, Campus de Beaulieu, 35042, Rennes cedex, France
Received (in Montpellier, France) 3rd September 2003, Accepted 13th October 2003
First published as an Advance Article on the web 12th February 2004
Original tetrazines substituted by heterocyclic rings have been prepared. Their syntheses, as well as their
electrochemical and spectroscopic features, are described. Calculations have also been made on the cation
radicals, anion radicals and neutral compounds and are in correct agreement with the experimental results. All
compounds are electroactive, both in oxidation and reduction, and display two absorption bands in the UV and
visible regions of the spectrum. Reduction potentials and maximum wavelengths are correlated with the
electron-rich character of the heterocyclic substituent on the tetrazine ring. None of these compounds gives
good quality polymers upon electro-oxidation, which was unexpected, especially for the bis(2-pyrrolyl)
tetrazine. This latter result can be explained by the occurrence of a self-deprotonation equilibrium in the cation
radical.
electron-rich and electron-poor structures have become
desirable targets for the achievement of such properties.
Introduction
7
–11
The search for new optically and electroactive compounds is
still active, given their potential in fields such as sensors.
In this article, we report one of the first syntheses of pyrrole
and thiophene substituted tetrazines, in addition to a pyr-
1
–4
12
With regard to such aims, fluorescence and electroactivity
are particularly interesting properties, because fluorescent
and/or electroactive molecules may constitute the active com-
ponent of a device. In this respect, the tetrazine family appears
as a very promising and fascinating one. Tetrazines are highly
coloured and electroactive heterocycles, displaying the follow-
ing special properties. (1) They have a very high electron affi-
nity, which makes them easily reducible (actually they are
role-pyrazole substituted tetrazine (see Fig. 1). We also report
their electrochemical and absorption properties.
Experimental
Synthesis of the monomers
The synthetic approach followed in our work for tetrazines 2
and 3 is detailed in Scheme 1. The synthesis of bis(thien-2-yl)-
tetrazine (4) and bis(pyrrol-2-yl)tetrazine (5) was performed
following a separate route.
5
the electron poorest C–N heterocycles). (2) They have a low
lying p* orbital, resulting in a n-p* transition in the visible
light range.
6
These properties make tetrazine based molecules and mate-
rials attractive targets for devices targeted for optical and/or
electrochemical applications. In addition, low band gap poly-
mers could be expected from structures alternating tetrazine
rings and any electron-rich moiety like an oligothiophene,
since molecules and supramolecules providing alternating
Synthesis of bis(N-pyrrolyl)tetrazine and (N-dimethylpyra-
zolyl-N-pyrrolyl)tetrazine. Bis(dimethylpyrazol-1-yl)tetrazine
(1; 2.57 g, 2 mmol), prepared according to ref. 13, was
dissolved in 50 cm of dry THF and 4 mmol of pyrrolyl lithium
(prepared by reaction of 2 mmol of pyrrole with 2 mmol of
3
Fig. 1 Formulae of the tetrazine compounds investigated.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 8 7 – 3 9 2
387

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Scheme 1 Synthesis of compounds 2 and 3.
3
ꢀ
butyllithium in 20 cm THF) was added at 0 C. After 15 min
stirring and warming to room temperature, the THF was eva-
porated, the mixture taken up in dichloromethane (DCM) and
chromatographed (eluant DCM–petroleum ether 2:1) The first
orange product was bis(N-pyrrolyl)tetrazine (2), yield 30–35%.
The second product was (N-dimethylpyrazolyl-N-pyrrolyl)-
Electrochemistry and spectroscopy measurements
The electrochemical studies were performed using an EG&G
PAR 273 potentiostat, interfaced to a PC computer.
For CV measurements, the reference electrode used was an
AgCl/Ag electrode. This reference electrode was checked vs.
ferrocene as recommended by IUPAC. Potential values are
thus given with respect to ferrocene. Triethyl phosphate (Fluka
puriss.) was used, assuming the same potential for ferrocene.
Tetrabutylammonium perchlorate was purchased from
Fluka (puriss). Acetonitrile (Aldrich, 99.8%), dichloromethane
(SDS, 99.9%) and toluene (Aldrich 99.5%) were used as
received. All solutions were deaerated by bubbling argon gas
for a few minutes prior to electrochemical measurements.
Transmission spectra were recorded on a dual beam spec-
trometer (Perkin Elmer Lambda 9). UV-vis spectroelectro-
tetrazine (3), yield 15–20%.
2
1
. H NMR (CDCl ): 7.86 (4H, t, J ¼ 2.6 Hz, a-pyrrolic-H);
3
1
3
6
1
3
.51 (4H, t, J ¼ 2.6 Hz, b-pyrrolic-H); C: 158.09 (tetrazine),
18.86, 114.33, (pyrrole). Anal. calcd: C 56.56, H 3.79, N
9.60; found: C 56.43, H 4.29, N 37.09.
1
3
. H NMR (CDCl ): 7.89 (2H, t, J ¼ 2.5 Hz, a-pyrrolic-H);
3
6
.51 (1H, t, J ¼ 2.5 Hz, b-pyrrolic-H); 6.20 (1H, s, pyrazole);
.71 (3H, s, methylpyrazole), 2.40 (3H, s, methylpyrazole);
C: 160. 67, 158.07 (tetrazine), 154.77, 143.99, 119.66,
15.17, 112.32 (pyrazole and pyrrole), 15.21, 14.56 (methylpyra-
2
1
3
1
chemistry was performed using
a homemade cell; the
zole). Anal. calcd: C 54.76, H 4.58, N 40.64; found: C 55.25, H
4
working electrode was made of ITO coated on glass. Platinum
and silver wires were used, respectively, as counter and
pseudo-reference electrodes and were located on the cell sides
in order not to disturb the beam. A double UVIKON 923
.64, N 38.80.
Synthesis of bis(thien-2-yl)tetrazine and bis(pyrrol-2-yl)tetra-
zine. The syntheses were performed via the dihydrotetrazine
route, but this intermediate, which is moderately stable in
the case of thiophene and quite unstable in the case of pyrrole,
was not isolated but immediately aromatised into tetrazine.
Preparation of the dihydrotetrazine. To 2.5 g of 2-cyanopyrrole
(Bio-Tek Kontron) spectrophotometer was used for the
spectral acquisition.
Theoretical modelling
ꢃ2
or 3 g of 2-cyanothiophene (ꢁ2.7 ꢂ 10 mol), dissolved in 8
The calculations were performed using the Gaussian 98 pack-
age for density functional and solvation calculations. Gas
mL of absolute ethanol, was added 0.42 g of powdered sulfur
(
14
ꢃ2
sulfur flower, 1.3 ꢂ 10 mol in sulfur) and 2 mL of fresh
phase geometries and electronic energies were calculated by
full optimisation without imposed symmetry of the conforma-
tions using the B3LYP density functional with the 6-31G*
ꢃ2
hydrazine monohydrate (4 ꢂ 10
mol). The mixture was
refluxed for 1.5 h and produced hydrogen sulfide (CAUTION!
use a well-ventilated hood). The mixture was cooled to room
temperature where the dihydrotetrazine crystallised and was
quickly filtered off, dried in vacuo and quickly used for the next
step (approx. yields 60–70%).
15,16
basis set,
starting from preliminary optimisations
performed with semi-empirical methods. The quality of the
obtained minima was checked by running frequency calcula-
tions. We also checked that the spin contamination remained
Syntheses of the tetrazines 4 and 5. The dihydrotetrazines (1 g)
were dissolved in ethanol (200 mL). In the case of 4, excess
sodium nitrite (about one mass equivalent relative to the sub-
strate) was added to the solution, which was stirred overnight.
2
low (s < 0.78) for all the open-shell B3LYP calculations
(
cation and anion radical). Solvation free energies were calcu-
lated on the gas-phase-optimised conformations according to
the SCRF (self consistent reaction field) method using the
ꢀ
In the case of 5, the solution was simply stirred at 50 C in the
15
IPCM method and the B3LYP density functional in aceto-
presence of air. In both cases the tetrazines slowly crystallised
upon cooling of the solution to room temperature and were
recovered in several successive crops (150–250 mg each).
nitrile (e ¼ 36.64). In this method, the solvent is treated as a
continuum of uniform dielectric constant in which the solute
is placed into a cavity defined as an isodensity surface of
Overall yield 60–70%.
4
the molecules. The value for the isodensity surface was chosen
ꢃ1 16
as used in previous published
1
. H NMR (CDCl
3
): 8.31 (2H, dd, b2-H); 7.73 (2H, dd,
as 0.001 electron bohr
calculations.
,
a-H); 7.31, (2H, dd, b3-H), Jab2 ¼ 1.29 Hz, Jab3 ¼ 5.06 Hz,
1
3
J
1
2
b2b3 ¼ 3.95 Hz;
C: 161.66 (tetrazine), 136.12, 132.59,
31.08, 129.13 (thiophene). Anal. calcd: C 48.72, H 2.44, N
2.74; found: C 48.82, H 2.42, N 22.67.
5
Results and discussion
1
. H NMR (DMSO-d6): 12.22 (1H, s, NH) 7.16 (2H, m,
Synthesis of the monomers
1
3
b-pyrrolic-H); 6.34 (1H, q, a-pyrrolic-H); C: 157.89 (tetra-
zine), 124.89, 124.72, 112.79, 110.74 (pyrrole). Anal. calcd: C
The first part of the synthetic scheme followed in our work is
based on the beautiful synthetic work of Hiskey et al.,
1
3,17–19
5
6.56, H 3.79, N 39.60; found: C 56.43, H 4.29, N 37.09.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
3
88
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 8 7 – 3 9 2

View Online
who devised a very efficient synthesis of our basic starting
compound: bis(dimethylpyrazolyl)tetrazine 1. It should, how-
ever, be pointed out that the reaction involved had been dis-
Electrochemical behaviour of the tetrazine derivatives
All tetrazines are reducible, either in acetonitrile or in dichloro-
methane, according to their solubility. Most of them give well-
defined cyclic voltammograms, as exemplified on Fig. 2. On a
few occasions, the voltammograms are not fully reversible,
showing that the anion radical undergoes chemical evolution.
The most likely process for this is probably the coupling of
two anion radicals, since it appears to be relatively general
and not limited to the compounds having an acidic hydrogen.
However, it is not possible to perform a mechanistic study,
because of the too-narrow scan rate range underwhich a
reaction occurs.
The potentials are given in Table 1 and correlate well with
the electron-rich or -poor character of the heterocycle substi-
tuted on the tetrazine, since the highest reduction potential is
displayed by the bis(dimethylpyrazolyl)tetrazine 1 and the
lowest by the bis(a-pyrrolyl)tetrazine 5.
2
0
covered before, while 1 itself had been prepared for the
2
1
first time even earlier, although in low yields. The pyrazolyl
groups may act as soft leaving groups, allowing a nice range
of heterocyclic substituents to be introduced on the tetrazine
ring, provided that the basic character of the entering nucleo-
phile is not too strong and its hardness of medium strength.
This has been done in particular with hydrazine and ammo-
1
3,17–19
nia,
and we show here that the pyrrolyl anion is also
able to enter the tetrazine ring (Scheme 1), to give mono-
and disubstitution products. However, we noticed that this is
not the case for the much more reactive thiophene based
anions, where only tars are obtained.
Therefore, in order to prepare the more potentially interest-
ing bis(a-heterocyclic)tetrazines another strategy, based on
previously published work, has been employed. It had been
1
2
All the tetrazines bearing electron-rich heterocycles can also
be oxidised, although they do not give polymers except for 2,
which gives a very poor quality polymer. The potential values
are given in Table 2 and will be discussed in the next part, in
the light of the molecular modelling results.
shown in the past that sometimes hydrazine, alone or prefer-
2
2
ably in the presence of sulfur, could lead to the formation of
dihydrotetrazine from aromatic nitriles. Nevertheless, the reac-
tion requires highly concentrated conditions, as well as a neat
excess of hydrazine in many cases. No mechanism was sug-
gested by the previous authors; however, we have observed
that: (1) the colourless solution turns immediately light orange
as soon as the sulfur is added and (2) vigorous hydrogen
sulfide emission takes place throughout the reaction.
Molecular modelling on tetrazine compounds and their derived
charged species
The geometries of the prepared tetrazines were calculated.
Except for compound 1, which is slightly twisted in the neutral
form, all the investigated tetrazines are completely planar,
indicating that there is electron delocalisation and therefore
charge transfer between the tetrazine ring and the electron-rich
heterocycle, which enhances the interest of using tetrazine as
an attracting ring in designing NLO-active molecules. Simi-
larly, we found that the bonds connecting the heterocycle (pyr-
role or thiophene) to the tetrazine ring are shorter than the
values corresponding to the similar single bond: for example,
On the basis of these observations, we propose that the
active nucleophile in the solution be the fast addition product
of hydrazine to sulfur, namely H N–NH–SH, with the follow-
2
ing mechanism as shown in Scheme 2.
The regiochemistry, as well as the preference (relative to the
other nucleophiles still present like excess hydrazine) of both
nucleophilic attacks are consistent with the ‘‘alpha effect’’
exerted by both the sulfur and the second nitrogen atom linked
to the nitrogen responsible for the nucleophilic attack. If this
mechanism were to be right, only 1/2 equiv. of sulfur would
be required (relative to the starting material) to form again
one half of the first intermediate, which can react further with
the remaining nitrile. The previous authors used an undefined
stoichiometry (they described a general procedure with only
masses indicated, with the stoichiometry falling roughly
between 1 and 1/2) but we noticed that indeed the required
sulfur amount was 1/2 equiv.
˚
in 2 the N–C bond length is equal to 1.38 A and in 5 to 1.44
˚
A, again showing a good conjugation between the heterocycle
and the tetrazine. This confirms that the electronic connection
is rather strong in all compounds, and correlates well with the
results of the electrochemical studies showing the potential
dependence on the substituents.
Calculations were also performed on the charged species
anion and cation radicals) to predict where the unpaired elec-
tron will be localised after electron transfer. Two examples
are shown in Fig. 3 for the spin densities in the pyrrolyl
(
2
3
Fig. 2 Cyclicvoltammograms of bis-substituted tetrazinesin dichloro-
methane (þ0.1 M TBAP) on a platinum electrode: bis(N-pyrrolyl)-
tetrazine 2 (solid line) and bis(thienyl)tetrazine 4 (dashed line).
Scheme 2 Synthesis of compounds 4 and 5.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 8 7 – 3 9 2
389

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Table 1 Standard potentials (in V vs. ferrocene) for the tetrazine
reduction in acetonitrile (AN) and dichloromethane (DCM)
interest. The case of 5 is particularly surprising, because in
addition its oxidation occurs at a very low potential (Table
). However, this potential rises noticeably when one adds a
2
ꢀ
ꢀ
E (AN)/V vs. Fc
E (DCM)/V vs. Fc
moderate acid such as acetic acid to the solution (Fig. 4). This
behaviour would be in accordance with the existence of a self
deprotonation equilibrium, in the cation radical species,
1
2
3
4
5
ꢃ0.96
ꢃ1.07
ꢃ0.97
ꢃ1.22
ꢃ1.22
Not fully rev.
ꢃ1.01
25
according to the following equation:
ꢃ1.02
ꢃ1.21
ꢃ1.25
Table 2 Experimental and B3LYP/6-31G* estimated oxidation
potentials of the various tetrazine compounds
It is known that deprotonated pyrrole cation radicals in
general do not polymerise (some work has shown that pyrrole
polymerisation is not possible in the presence of non-nucleo-
a
þ
p
E /V vs. Ag/Ag
Calcd
2
6,27
philic strong bases
). The cation radical of 5, due to the
1
2
3
4
5
1.02
0.73
0.80
0.62
0.0
> 1.8 (ill-defined peak)
1.27
smaller charge on the ring associated with the delocalised char-
acter, should not be able to polymerise. This would also
account for the unusually low peak potential, which would
be shifted towards lower values because the internal protona-
tion-deprotonation equilibrium would stabilise the oxidised
form.
1.45
> 1.8 (ill-defined peak)
b
0.60 (resp. 0.90 )
a
In volt. Bis(2-pyrrolyl)tetrazine (compound 5) is taken as an internal
b
reference in the B3LYP calculations. In the presence of acetic acid.
Spectroscopic features of the tetrazine derivatives
compounds. In the radical anion, the unpaired electron is loca-
lised in the centre of the molecule (namely on the tetrazine
ring) contrarywise to the radical cation where the unpaired
electron is mainly present on the 2-positions of the pyrrole
moieties [this is especially true in the case of bis(1-pyrrolyl) tet-
razine, 2]. This localisation should in principle help the oxida-
tive polymerisation, indicating that the difficulties encountered
in the polymer formation are mainly due to other factors than
the electronic properties of the radical cation.
The UV-visible spectra of all the compounds have been
obtained in dichloromethane and are displayed on Fig. 5; the
corresponding maxima are listed in Table 3. They all display
a very strong band in the UV region, corresponding to the
p-p* allowed transition, and a much less intense band in the
visible region, corresponding to the n-p* transition, which is
responsible for the colour of the compounds. The wavelength
of the UV band correlates clearly with the donor-acceptor bal-
ance of the substituted heterocycle, as it could be expected,
while this is less obvious in the case of the visible band. This
shows clearly that the donor character of an a-pyrrole is con-
siderably higher than that of a N-pyrrole, due to the complete
conjugation in the former case, as previously suggested by the
theoretical calculations.
While there is little influence of the heterocycle on the visible
band, one should, however, notice that the case of the a-pyr-
role compound 5 lies apart, since it displays a strong batho-
chromic absorption shift relative to the other four
compounds. This may also be in relation with the fact that
the proton born by the pyrrole rings may also be linked to
the nearest nitrogen of the neighbour tetrazine ring.
We have attempted to perform spectroelectrochemistry on
ITO at the first reduction potential of tetrazine 4 (ꢃ1.4 V);
unfortunately, as evidenced by the CV in Fig. 6(a), the anion
radical is no longer stable when generated on an ITO
electrode. The spectra recorded at this potential show the
emergence of two new bands [see Fig. 6(b)]: one at shorter
2
8
B3LYP calculations generally allow good predictions of the
variation trends for a family of molecules. We performed such
calculations using the IPCM methods to estimate the solvation
2
4
free energies. These estimations are gathered in Table 2: the
values of the oxidation peak potentials display a good agree-
ment between the expected and experimental data, especially
in the case where well-defined peaks could be obtained,
because the relative differences are quasi-constant (of course,
the absolute values from the calculations do not correlate,
because they depend on an arbitrary choice of internal refer-
ence). The trend in the variation of Eox is correctly predicted
by the calculations, especially for the pyrrole family, while
there is some disagreement for the thiophene derivative 4.
We note that compounds 4 and 5, both connected in the
2-positions, seem much easier to oxidise, indicating a greater
stabilisation of the electrogenerated radical cations.
The question why 2 and, more surprisingly, 5 do not poly-
merise is therefore still open to discussion and of particular
Fig. 3 Spin density (isodensity at 0.004 a.u.) from B3LYP calculations for radical anions (a and c) and radical cations (b and d) of 2 (top) and 5
(bottom).
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
3
90
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 8 7 – 3 9 2

View Online
ꢃ1
Fig. 4 Cyclic voltammograms (v ¼ 100 mV s ) of 0.01 M bis(a-pyr-
role)-s-tetrazine 5 in triethylphosphate–0.1 M tetrabutylammonium
perchlorate. Reference Ag/Ag (CH CN): (curve a) without acetic
þ
3
acid and (curve b) in the presence of acetic acid (25% relative to the
same electrolyte solvent).
Fig. 6 CV and spectroelectrochemistry of 4 in acetonitrile (þTBAP)
ꢃ1
on an ITO electrode. (a) CV (full line) at 100 mV s . The dotted line
shows the background current in the absence of substrate on the same
electrode. Potentials are given with respect to a bare Ag wire quasi-
reference. (b) Spectra recorded at ꢃ1.4 V after the following waiting
times: (curve a) a few s, (curve b) 1.5 min, (curve c) 3 min, (curve d)
5
.5 min, (curve e) 7 min, (curve f) 9 min, (curve g) 11 min.
Concluding remarks
In this article, we have described the properties of a few new
heterocycle substituted tetrazines, which display high to very
high reduction potentials. Although they do not polymerise,
they constitute an intriguing family of molecules that may find
applications in various fields of physical chemistry such as the
previously mentioned NLO materials. The third-order NLO
properties are currently under investigation for this family
and beyond.
.
Fig. 5 UV-vis absorption spectra of compounds 1 (–), 2 (---), 3 (– – ),
(ꢄ ꢄ ꢄꢄ ꢄ ꢄ) and 5 (—).
4
a
Table 3 Spectroscopic features of the tetrazine derivatives
ꢃ1
ꢃ1
lmax/nm
e/L mol cm
1
2
3
4
5
278
295
295
347
360
49 800
47 750
48 300
52 230
49 530
References
1
2
3
4
5
F. B e´ dioui, J. Devynck and C. Bied-Charreton, J. Mol. Catal.
A: Chem., 1996, 92, 1411.
G. Bidan, B. Divisia-Blohorn, M. Lapkowski, J. M. Kern and
J. P. Sauvage, J. Am. Chem. Soc., 1992, 114, 598.
R. P. Kingsborough and T. M. Swager, Prog. Inorg. Chem., 1999,
48, 123.
a
All spectra recorded in dichloromethane except for
spectrum is recorded in acetonitrile
5 whose
J. P. Lang, H. Kawaguchi and K. Tatsumi, Inorg. Chem., 1997,
6, 6447.
3
A. R. Katritzky, Handbook of Heterocyclic Chemistry, Pergamon
Press, Oxford, 1986.
6
7
8
W. Kaim, Coord. Chem. Rev., 2002, 230, 127.
J. Roncali, Chem. Rev., 1997, 97, 173.
L. Groenendaal, M. J. Bruining, E. H. J. Hendrickx, A. Persoons,
J. A. J. M. Vekemans, E. E. Havinga and E. W. Meijer, Chem.
Mater., 1998, 10, 226.
wavelengths that can probably be ascribed to dihydrotetrazine
this yellow compound has an absorption maximum in the UV
with a bandtail up to 480 nm), and another one at longer wave-
lengths, which is probably due to oligomers resulting from
coupling reactions.
(
9
J. K u¨ rsti, P. R. Surjan, M. Kertesz and G. Frapper, Synth. Met.,
1
993, 55–57, 4338.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 8 7 – 3 9 2
391

View Online
19 M. D. Coburn, G. A. Buntain, B. W. Harris, M. A. Hiskey, K. Y.
Lee and D. G. Ott, J. Heterocycl. Chem., 1991, 28, 2049.
20 R. N. Butler, F. L. Scott and R. D. Scott, J. Chem. Soc. C, 1970,
2510.
21 C.-H. Lin, E. Lieber and J. P. Horwitz, J. Am. Chem. Soc., 1954,
76, 427.
22 M. O. Abdel-Rahman, M. A. Kira and M. N. Tolba, Tetrahedron
Lett., 1968, 3871.
1
0
G. Zotti, S. Zecchin, G. Schiavon, A. Berlin, G. Pagani,
M. Borgonovo and R. Lazzaroni, Chem. Mater., 1997, 9,
2
876.
J. Cao, J. W. Kampf and M. D. Curtis, Chem. Mater., 2003,
5, 404.
1
1
2
1
1
(a) (1-Bithienyl)-s-tetrazine was claimed to have been prepared in
a recent paper, but characterisation of the molecule was not
provided: F. Bentiss, M. Lagren e´ e, M. Traisnel, B. Mernari
and H. El Attari, J. Heterocycl. Chem., 1999, 36, 149; (b) While
this work was being reviewed, a very recent article describing
the synthesis of pyrrole and thiophene substituted tetrazines was
published: J. Soloducho, J. Doskocz, J. Cabaj and S. Roszak,
Tetrahedron, 2003, 59, 4761.
23 Hybrid density functional methods with the 6-31G* base have
been used previously for calculations on neutral and radical
cations of hetorocycles containing sulfur atoms and found to give
a
good qualitative description of their properties.(a) V.
Hern a´ ndez, H. Muguruma, S. Hotta, J. Casado and J. T. L o´ pez
Navarrete, J. Phys. Chem. A, 2000, 104, 735; (b) T. Keszthelyi,
M. M.-L. Grage, J. F. Offersgaard, R. Wilbrandt, C. Svendsen,
O. S. Mortensen, J. K. Pedersen and H. J. A. Jensen, J. Phys.
Chem. A, 2000, 104, 2808.
1
3
4
D. E. Chavez, R. D. Gilardi and M. A. Hiskey, Angew. Chem.,
Int. Ed., 2000, 39, 1791.
1
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery,
Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi,
V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala,
Q. Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J.
Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson,
W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-
Gordon, E. S. Replogle and J. A. Pople, GAUSSIAN 98 (Revision
A.11), Gaussian, Inc., Pittsburgh, PA, 2001.
24 (a) J. B. Foresman, T. A. Keith, K. B. Wiberg, J. Snoonian and
M. J. Frisch, J. Phys. Chem., 1996, 100, 16 098; (b) For a general
review about solvation methods, see: J. Cramer and D. G.
Truhlar, Chem. Rev., 1999, 99, 216.
25 There is no pK data for the s-tetrazines, besides the known fact
that a strong acid is required to protonate the relatively elec-
tron-deficient nitrogens in polyazines where the number of nitro-
5
gens is greater than 3. Similarly, there are no data on the pyrrole
cation radical, which could be, however, slightly above 1, since
pyrrole polymerises with some difficulty in pure water, but not
at all as soon as an even very weak base is added. When the
pyrrole bears an attracting moiety (like the tetrazine in the present
case), it should be even lower, which renders the aforementioned
proposition on the protonation equilibrium acceptable. (See ref. 5,
p. 153).
1
1
5
6
A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
P. C. Hariharan and J. A. Pople, Chem. Phys. Lett., 1972,
1
6, 217.
D. E. Chavez and M. A. Hiskey, J. Energet. Mater., 1999,
7, 357.
D. E. Chavez and M. A. Hiskey, J. Heterocycl. Chem., 1998, 35,
329.
26 M. Zhou and J. Heinze, J. Phys. Chem., 1999, 103, 8451.
27 L. Guyard, P. Hapiot, M. Jouini, J. C. Lacroix, C. Lagrost and
P. Neta, J. Phys. Chem., 1999, 103, 4009.
28 J. Waluk, J. Spanget-Larsen and E. W. Thulstrup, Chem. Phys.,
1995, 200, 201.
1
1
7
8
1
1
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
3
92
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 8 7 – 3 9 2