Effect of substituents on redox, spectroscopic and structural properties of conjugated diaryltetrazines - A combined experimental and theoretical study

Article abstract of DOI:10.1039/c0cp01553a

Two series of new soluble conjugated compounds containing tetrazine central ring have been synthesized. The three-ring compounds have been synthesized by the reaction of aryl cyanide (where aryl = thienyl, alkylthienyl, phenyl or pyridyl) with hydrazine followed by oxidation of the intermediate product with diethyl azodicarboxylate. The five-ring compounds have been prepared using two pathways: (i) reaction of 5-cyano-2,2′-bithiophene (or its alkyl derivative) with hydrazine; (ii) via Suzuki or Stille coupling of 3,6-bis(5-bromo-2-thienyl)-1,2,4,5-tetrazine with a stannyl or boronate derivative of alkylthiophene. UV-vis spectroscopic properties of the synthesized compounds are strongly dependent on the nature of the aryl group, the position of the solubilizing substituent and the length of the molecule, showing the highest bathochromic shift (λ_max > 440 nm) for five-ring compounds with alkyl groups attached to C_α carbon in the terminal thienyl ring. An excellent linear correlation has been found for spectroscopically determined and theoretically calculated (TD-B3LYP/6- 31G*) excitation energies. With the exception of dipyridyl derivative, the calculated lowest unoccupied molecular orbital (LUMO) level of the investigated molecules changes within a narrow range (from -2.63 to -2.41 eV), in line with the electrochemical data, which show a reversible reduction process with the redox potential varying from -1.23 V to -1.33 V (vs. Fc/Fc⁺). The electrochemically determined positions of the LUMO levels are consistently lower by 0.9 to 1.2 eV with respect to the calculated ones. All molecules readily crystallize. Single crystal studies of 3,6-bis(2,2′-bithien-5-yl)-1,2,4,5- tetrazine show that it crystallizes in a P2₁/c space group whose structural arrangement is not very favorable to the charge carriers flow within the crystal. Powder diffraction studies of other derivatives have shown that their structural organization is sensitive to the position of the solubilizing substituent. In particular, the presence of alkyl groups attached to C _α carbon in the terminal thienyl ring promotes the formation of a lamellar-type supramolecular organization.

Full text of DOI:10.1039/c0cp01553a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
PCCP
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 2690–2700
www.rsc.org/pccp
PAPER
Effect of substituents on redox, spectroscopic and structural properties
of conjugated diaryltetrazines—a combined experimental and theoretical
studyw
Ewa Kurach,a David Djurado,b Jan Rimarcik,c Aleksandra Kornet,a
a
c
d
a
´
Marek Wlostowski, Vladimir Lukes, Jacques Pecaut, Malgorzata Zagorska*
and Adam Pron*b
Received 19th August 2010, Accepted 11th November 2010
DOI: 10.1039/c0cp01553a
Two series of new soluble conjugated compounds containing tetrazine central ring have been
synthesized. The three-ring compounds have been synthesized by the reaction of aryl cyanide
(where aryl = thienyl, alkylthienyl, phenyl or pyridyl) with hydrazine followed by oxidation
of the intermediate product with diethyl azodicarboxylate. The five-ring compounds have been
prepared using two pathways: (i) reaction of 5-cyano-2,20-bithiophene (or its alkyl derivative) with
hydrazine; (ii) via Suzuki or Stille coupling of 3,6-bis(5-bromo-2-thienyl)-1,2,4,5-tetrazine with
a stannyl or boronate derivative of alkylthiophene. UV-vis spectroscopic properties of the
synthesized compounds are strongly dependent on the nature of the aryl group, the position
of the solubilizing substituent and the length of the molecule, showing the highest bathochromic
shift (lmax > 440 nm) for five-ring compounds with alkyl groups attached to Ca carbon in the
terminal thienyl ring. An excellent linear correlation has been found for spectroscopically
determined and theoretically calculated (TD-B3LYP/6-31G*) excitation energies. With the
exception of dipyridyl derivative, the calculated lowest unoccupied molecular orbital (LUMO)
level of the investigated molecules changes within a narrow range (from ꢀ2.63 to ꢀ2.41 eV),
in line with the electrochemical data, which show a reversible reduction process with the redox
potential varying from ꢀ1.23 V to ꢀ1.33 V (vs. Fc/Fc+). The electrochemically determined
positions of the LUMO levels are consistently lower by 0.9 to 1.2 eV with respect to the
calculated ones. All molecules readily crystallize. Single crystal studies of 3,6-bis(2,20-bithien-5-yl)-
1,2,4,5-tetrazine show that it crystallizes in a P21/c space group whose structural arrangement is
not very favorable to the charge carriers flow within the crystal. Powder diffraction studies of
other derivatives have shown that their structural organization is sensitive to the position of the
solubilizing substituent. In particular, the presence of alkyl groups attached to Ca carbon in the
terminal thienyl ring promotes the formation of a lamellar-type supramolecular organization.
a Faculty of Chemistry, Warsaw University of Technology,
Noakowskiego 3, 00 664 Warszawa, Poland.
Introduction
E-mail: zagorska@ch.pw.edu.pl
b INAC/SPrAM (UMR 5819 CEA-CNRS-Univ. J. Fourier-Grenoble 1),
Conjugated molecules and macromolecules play an impor-
tant role in modern materials science. They can be used as
semiconductors in organic electronics1,2 or optoelectronics3
components of electrochemical sensors4,5 advanced electrode
materials6 and other types of functional materials. All these
applications require designing molecules (macromolecules)
with precisely tuned physicochemical properties. Control of
their HOMO (Highest Occupied Molecular Orbital) and LUMO
(Lowest Unoccupied Molecular Orbital) level positions is
especially important in this respect since to a first approximation
they govern the electronic, spectroscopic and redox properties
of a given molecule. As a general rule, introduction of an electron
accepting group to a conjugated molecule (macromolecule)
´
Laboratoire d’Electronique Moleculaire Organique et Hybride,
17 Rue des Martyrs, 38054 Grenoble Cedex 9, France.
E-mail: adam.pron@cea.fr
c Institute of Physical Chemistry and Chemical Physics,
Slovak University of Technology in Bratislava,
SK-81 237 Bratislava, Slovakia
d INAC/SCIB CEA Grenoble, 17 Rue des Martyrs,
38054 Grenoble Cedex 9, France
w Electronic supplementary information (ESI) available: Synthesis of
all molecules. The depiction of bonds forming the main conjugation
chain. Schematic representation of the relative energy between experi-
mental and B3LYP HOMOs and LUMOs for studied molecules. The
calculated TD-B3LYP optical transitions for studied molecules.
CCDC reference number 774463. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0cp01553a
c
2690 Phys. Chem. Chem. Phys., 2011, 13, 2690–2700
This journal is the Owner Societies 2011

View Online
electrode of a surface area of 3 mm2, a platinum wire counter
lowers its LUMO level whereas the presence of an electron
donating unit raises the HOMO level. Using the building
block strategy several low band gap, donor–acceptor type
conjugated compounds were prepared, suitable for photovoltaic
and other applications.7 Similarly, appropriate functionaliza-
tion of conjugated molecules (macromolecules) with electron
accepting groups resulted in the preparation of several organic
semiconductors with low lying LUMO levels suitable for
fabrication of n-channel organic field effect transistors
(OFETs) operating in air.8 In this context bis-substituted
tetrazines, containing aryl substituents, can be considered
as donor–acceptor–donor (DAD) molecules in which the
central unit is strongly electron deficient. They can serve as
building blocks for new low band gap conjugated polymers9
or, in some cases, as macromonomers, undergoing electro-
chemical polymerization.10 Due to their low lying LUMO
level they undergo reversible electrochemical reduction at
much higher potentials as compared to the reduction of their
thiophene homooligomers.11 This makes them promising
candidates for the use in electrochromic devices and, since
some of them are fluorescent, also in electrofluorochromic
ones.12 Synthetic and physical chemistries of tetrazine derivatives
have recently been profoundly discussed in two exhaustive review
papers.13,14
electrode and an Ag/0.1 M AgNO3 acetonitrile reference
electrode. The potential of the reference vs. Fc/Fc+ redox
couple was checked at the end of each experiment. X-Ray
powder diffraction investigations were carried out on an
X-Pert Pro MPD Philips diffractometer (cobalt Ka1 radiation;
l = 1.789 A) using Bragg–Brentano (y/2y) reflection geometry.
The detector was moved by 2y steps of 0.021 and the counting
time was at least 10 s per step. The divergence slit was
automatically adjusted giving a 10 mm irradiated length. In
the case of single crystals study, the X-ray Crystallography
Diffraction data were taken at 150 K using an Oxford-
Diffraction XCallibur
S Kappa geometry diffractometer
(MoKa radiation, graphite monochromator, l = 0.71073 A).
The cell parameters were obtained with intensities detected on
three batches of 5 frames. For three settings of f and y,
277 narrow data were collected for 11 increments in o with a
20 s exposure time. Unique intensities detected on all frames
using the Oxford-diffraction Red program were used to refine
the values of cell parameters. The hydrogen atoms were all
fixed in ideal positions. 10 794 reflections comprising 3751
independent ones were collected. The obtained final R indices
were R1 = 0.037 and wR2 = 0.0555.
Unsubstituted DAD conjugated molecules containing
thiophene and tetrazine units are difficult to process due to
their limited solubility. It can be significantly improved by
synthesizing their derivatives containing long alkyl substituents,
similarly as in the case of oligo- and poly(thiophenes).15
However, this approach must be exploited with caution since
essentially all physico-chemical properties of this family of
organic semiconductors depend on their regiochemistry—two
regioisomers may exhibit strikingly different electronic and
optical properties.16 Moreover, the presence of long alkyl
groups may have a profound effect on the supramolecular
organization of these compounds, frequently facilitating their
self-assembly through interdigitation.17
Theoretical calculations
The electronic ground state geometries of studied systems were
optimized using Density Functional Theory (DFT)18 based on
the Becke’s three parameter hybrid functional using the Lee,
Yang and Parr correlation functional for Gaussian (B3LYP).19
Based on the optimized geometries, the vertical transition
energies and oscillator strengths between the initial and final
states were computed by Time dependent version (TD-) of
DFT method.20 The 6-31G* basis set has been used21,22
and the calculations were done using the Gaussian 03
program package.23 The energy cut-off was of 10ꢀ3 kcal molꢀ1
and the final root mean square energy gradient was under
0.01 kcal molꢀ1
A
ꢀ1. The obtained optimal structures were
The main goal of this paper was to synthesize new regio-
chemically well defined DAD-type electroactive molecules of
improved processability and tunable HOMO–LUMO levels,
which can be of potential use in organic electrochemical,
electronic and optoelectronic devices. The relationships
between their optical, electrochemical properties and molecular
structure have been studied at a Density Functional Theory
(DFT) level.
checked by normal mode analysis (no imaginary frequencies
for all optimal geometries). The numerical integration of the
used functional was performed using the fine integration
grid which parameters were set as default. Due to the extreme
computational requirements, our theoretical model was
restricted to the single molecule without taking any solvent
nor temperature effects into account as a standard simplifica-
tion used in quantum chemical studies.
Experimental
Results and discussion
Procedures for the preparation of compounds studied in
this research are described in detail in ESIw together with
their spectroscopic (NMR, FTIR) and elemental analysis
data.
Synthesis
Two series of compounds were synthesized, namely tetrazines
with aryl and biaryl substituents. Their chemical formulae are
depicted in Chart 1.
UV-vis spectra of the synthesized compounds dissolved in
methylene chloride were recorded on a Varian Cary 5000
spectrometer. Cyclic voltammetry investigations of the synthe-
sized diaryltetrazines were carried out using an Autolab
potentiostat (Eco Chemie). The studied compounds were
dissolved in 0.1 M methylene chloride solution of Bu4NBF4.
The measurements were performed using a platinum working
For simplicity, in the subsequent text compounds 1–5 will
be termed ‘‘three-rings’’ and compounds 6–9—‘‘five-rings’’.
4, 5, 7, 8, 8a, 9 have not been reported in the literature,
the remaining compounds were synthesized and studied for
comparative reasons.
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 2690–2700 2691

View Online
Scheme 3 Preparation of five-ring compounds via Stille or Suzuki
coupling of dibromo-derivative of 3.
the synthetic route but also results in better yields as checked
in the synthesis of 6 and 8 (see ESIw).
Unsubstituted as well as substituted three-rings (1–5) readily
dissolve in chlorinated solvents. An unsubstituted five-ring
compound (6) is less soluble, however, the introduction of long
alkyl chains improves its solubility, most significantly in the
case of 9.
Chart
research.
1
Tetrazine derivatives synthesized and studied in this
Scheme 1 Pinner’s type synthesis of diaryltetrazines.
Scheme 2 Synthetic route to alkyl substituted five-ring compounds.
Three-rings were prepared in high yields using a modifi-
cation of Pinner’s procedure, in its simplest version shown in
Scheme 1.
The synthesis pathway involves the reaction of aryl
cyanide and hydrazine to yield 1,4-dihydrotetrazine—an inter-
mediate product—which is then converted to the desired
diaryltetrazine using a mild oxidant as for example isoamyl
nitrite as recommended in ref. 24. In our syntheses we have
however used a different oxidizing agent, namely diethyl
azodicarboxylate which assures clean oxidation and high yield
of the reaction.25
The same synthetic strategy can be applied to five-ring
compounds already exploited by Audebert et al.10 It was tested
by us in the preparation of 6 and 8 as one of the applied
methods. This reaction pathway is however inconvenient for
the derivatives containing alkyl groups because it involves a
synthesis of cyano- and alkyl-substituted bithiophene which
requires several steps, as shown in Scheme 2.
One may however ascertain that dibromo derivative of
3 can serve as a building block for the synthesis of a whole
series of five-ring compounds via Stille or Suzuki coupling
(see Scheme 3). This strategy not only significantly facilitates
Fig. 1 Schematic structure of studied molecules, their optimal
B3LYP/6-31G* bond lengths (in A) and dihedral angles between the
aromatic rings (in degrees).
c
2692 Phys. Chem. Chem. Phys., 2011, 13, 2690–2700
This journal is the Owner Societies 2011

View Online
Theoretical optimal structure
ring is the shortest (1.31 A) whereas C–C bond linking the
tetrazine central ring with the substituent is the longest. The
value of 1.48 A is found for 1 and 2 (phenyl and pyridine
substituent), for 3 (thienyl substituent) this distance is 1.45 A.
The presence of alkyl groups in various positions of the thienyl
substituent has different influence on the neighboring bonds.
In general, shortening of the double bond is compensated by
the elongation of the single bonds. The changes are within
10ꢀ2 A.
The presence of single bonds connecting the tetrazine unit with
thiophene groups in the studied molecules leads to the formation
of a large number of conformations. Based on the previous
work of Liu et al.26 we have restricted our calculations only
to the symmetric all-trans conformations which exhibit the
lowest total energies with respect to the steric conditions for
oligothiophenes and related compounds.
The optimal B3LYP geometries for the non-substituted
smallest molecules (three-ring compounds) (1, 2, 3) are totally
planar. As presented in Fig. 1, the alkyl substitution in the
inner Cb position is responsible for a small torsion of 31
calculated for 4. The presence of two neighboring thiophene
rings in the five-ring series leads to a certain torsion between
them. The torsional angle in the non-substituted five-ring, 6, is
1661. The effect of the thiophene ring connection with the
tetrazine moiety on the geometry of the five-ring molecule can
be estimated from the comparison of this dihedral angle with
the corresponding angle in simple bithiophene. The calculated
B3LYP/6-31G* torsion angle for bithiophene is 1571.27
Subsequent DFT or ab initio MP2 quantum-chemical studies
for this molecule28,29 report the energy minima for the
anti-conformers at the torsion angles between 1421 and 1521. It
seems, therefore, that the presence of the central tetrazine ring is
responsible for the lowering of the torsion angle between the
neighboring thiophene rings. The theoretical calculations of
other five-rings reveal that the planarity slightly increases upon
substitution with alkyl groups. The torsion angle of 1711 is found
for 7 where the alkyl substituents in the terminal thiophene rings
are located in the outer Cb position. The electron–donor effect of
the alkyl substitution is thus reflected in the observed small
dihedral angle changes (from 2 to 51).
The aromaticity of studied molecules and its relation to their
molecular structure can be described by the bond length
alternation (BLA) parameter.27,30 This parameter might be
defined as the averaged sum of the absolute values of the
differences between the bond length (di) and the averaged bond
ꢀ
length (d):
M
P
ꢀ
jdi ꢀ dj
i¼1
BLA ¼
ð1Þ
M
In this definition, M stands for the number of aromatic bonds
(M = 20 for 1 and 2 and M = 18 for 3 to 5 and M = 30 for 6
to 9). With respect to this definition, a very small BLA value
indicates effective aromatic structure, e.g. BLA of benzene is
zero. The smallest BLA values of 0.038 and 0.030 were found
for the optimal electronic ground state geometries of 1 and 2,
respectively (see Table 1). Three-ring compounds containing
thienyl substituents show larger BLA values (from 0.125 to
0.128 A). Elongation of the molecule by adding two thienyl
rings (five-rings) leads to an increase of BLA by ca. 0.013 A.
In this context it is interesting to comment the partial BLA*
value for the hypothetical main conjugated backbone of
the molecules studied. In this model, only the C–C, C–N
and N–N bonds are accounted (see Fig. S1 in ESIw, M = 11
for 1 to 5 and M = 19 for 6 to 9). The calculation results are
also presented in Table 1 and they indicate the minimization of
the bond length differences between the selected alternating
Although the molecules of the three-ring series contain
substituents of different chemical nature (phenyl, pyridyl or
thienyl), the mutual comparison of selected bonds is still quite
interesting. As presented in Fig. 1, the N–N bond in the central
Table 1 The BLA values and the calculated B3LYP/6-31G* excitation energies (E). The second presented excitation energy was selected
according to the dominant oscillator strength value (f > 0.6). The order of the excitation energy is indicated by the values written in k-column.
Values written in italic and with asterisk symbol (*) stand for the BLA* of selected bonds (see Fig. S1 in ESIw)
Compound
BLA/A
k
E/eV
2.15
Transitiona
k
E/eV
4.23
f
Transitiona
1
0.038
0.043*
0.030
0.033*
0.126
0.034*
0.125
0.036*
0.128
0.033*
0.141
0.031*
0.140
0.034*
0.141
0.030*
0.141
0.032*
1
H - L
(86%)
H - L
(86%)
12
0.9861
H ꢀ 3 - L + 1
(84%)
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
2.24
2.23
2.18
2.23
2.24
2.24
2.24
2.24
7
4
4
4
3
3
3
3
3.97
3.51
3.41
3.34
2.68
2.65
2.57
2.54
1.0456
0.8096
0.6091
1.1925
1.6334
1.7206
1.8308
1.9948
H ꢀ 1 - L + 1
(76%)
H - L + 1
(76%)
H ꢀ 1 - L
(86%)
H ꢀ 1 - L
(85%)
H - L + 1
(65%)
H - L + 1
(80%)
H - L + 1
(76%)
H - L + 1
(81%)
H - L + 1
(83%)
H - L + 1
(82%)
H ꢀ 1 - L
(86%)
H ꢀ 2 - L
(80%)
H ꢀ 2 - L
(84%)
H ꢀ 2 - L
(86%)
H ꢀ 2 - L
(84%)
a
H stands for HOMO and L stands for LUMO. Values in parentheses represent the percentages of the excitation contributions to individual
transitions.
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 2690–2700 2693

View Online
alkylthienyl groups (4 and 5) as compared to the case of
unsubstituted three-ring (3). Two factors inducing this
bathochromic shift can be postulated: slightly increased
electron donating effect of the alkylthienyl substituents as
compared to non-substituted thienyl group and/or alkyl group
induced planarization of the molecule. These suggestions are
confirmed by the results of quantum chemical calculations
(vide infra).
By comparison of the three-ring and the five-ring series it
can be evoked that the conjugation length has the most
pronounced effect on the position of the second band, which
is bathochromically shifted in five-ring compounds by
ca. 80–90 nm as compared to the corresponding three-ring
ones (see spectra of 5 and 8 presented in Fig. 2). Again, as in
the case of three-ring compounds, five-rings containing alkyl
substituents (7, 8, 9) absorb at higher wavelengths than their
unsubstituted analogue (6).
Fig. 2 UV-vis spectra of 5 and 8 recorded in CH2Cl2.
The first transition band, in the vicinity of 540 nm, clearly
seen in compounds 1–5, in 6 and 7 is present as a very weak
shoulder. In 8, 8a and 9 it is not detectable. The position of this
band is less dependent on the molecule length and the type of
substituent, as typically found for this type of transition.32
Based on the optimal B3LYP electronic ground state
geometries, the fifteen TD-B3LYP vertical excitation energies
were calculated (see Table S1, ESIw). For all studied molecules
the first excitation energies has very low oscillator strength.
The values collected in Table 1 show that the energies for
three-ring molecules (1 to 5) are ranged from 2.15 to 2.24 eV.
In the case of five-ring molecules (6 to 9), the position of the
first excitation energy is 2.24 eV. The obtained results are in
agreement with the experiment where, for the majority of
cases, very weak and broad band or shoulders are detected
in this energy region.
single and double bonds with the increase of molecular size.
Our calculations also show that not only the extension of the
molecule length but also the addition of an alkyl substituents
have an effect on the BLA* values calculated for the molecules
studied.
Spectroscopic properties
The positions of UV-vis absorption bands of diaryltetrazines
depend on the nature of the aryl rings, adjacent to the tetrazine
ring and more particularly on their electron withdrawing/
electron donating properties.24 Representative spectra of the
three-ring and five-ring series are shown in Fig. 2 whereas
the UV-vis spectroscopic data for all compounds studied are
collected in Table 2.
In all three-ring compounds the second band dominates the
spectrum. In 1 and 2 this band is hypsochromically shifted
with respect to the corresponding band in unsubstituted
tetrazine, where it is located at 320 nm.31 This shift clearly
manifests the electron withdrawing properties of the pyridyl
substituent and, to a lesser extent, the phenyl one. In compounds
with thienyl groups lmax of the second band is located at
higher wavelengths than in unsubstituted tetrazine due to
electron donating properties of these substituents. In addition
this band is strongly influenced by the presence of the alkyl
substituent and its position in the thienyl ring. lmax of the
second band is bathochromically shifted for tetrazines with
Comparing with the experiment, the optical transitions
showing the dominant oscillator strength larger than 0.6 are
connected with excitation energies of different order. It seems
that the relevant highest excitation energies are connected with
higher orders (see k values in Table 1). The energy of 4.23 eV
obtained for 1 represents the twelfth transition. For the
five-ring series (6 to 9) these energies are the third transitions.
The theoretically calculated energy values are systematically
lower than the experimental ones (the lowest shift of 0.05 eV
was found for 5, the highest of 0.26 eV for 6). Nevertheless
a simple linear relationship can be found between these
Table 2 Maxima of absorption bands (lmax) measured in solution spectra (CH2Cl2 solvent) of diaryltetrazines studied in this research
Compound
First band/nm (eV)
Second band/nm (eV)
Third band/nm (eV)
1
2
3
4
545 vwa (2.28)
547 vwa (2.27)
531 vwa (2.34)
546 vwa (2.27)
274 (4.43)
298 (4.16)
347 (3.57)
353 (3.51)
—
—
278 (4.46)
266 (4.66)
283 (4.38)
283 (4.38)
274 (4.52)
279 (4.44)
276 (4.49)
278 (4.46)
286 (4.34)
5
6
7
8
8a
9
534 vwa (2.32)
366 (3.39)
421 (2.94)
433 (2.86)
445 (2.79)
443 (2.80)
455 (2.72)
vwsb
vwsb
—
—
—
a
b
Very weak. Very weak shoulder.
c
This journal is the Owner Societies 2011
2694 Phys. Chem. Chem. Phys., 2011, 13, 2690–2700

View Online
delocalized not only over the central tetrazine part, but
electron clouds are shifted to the adjacent aryls, too. Their
lobes are perpendicularly oriented to the aromatic chain. Both
LUMO and (HOMO ꢀ 1) orbitals are of p-type. In the case of
the optical transitions with the first dominant oscillator
strength, the transitions to the (LUMO + 1) orbitals are
important for all investigated molecules. As it can be seen in
Fig. 4, this orbital is delocalised only over the tetrazine unit
and the adjacent molecular units have minimal influence on
this electron distribution. This orbital has an antibonding p
type character. With respect to this analysis, the first optical
transition (first band) for compounds 1 and 2 belongs to the
n–p* band. In the case of other molecules studied (3 to 9),
the first optical transition has a clear p–p* character. An
interesting situation appears for optical transitions (second
band) with the first dominant oscillator strength (f > 0.6). The
contribution from HOMO-to-(LUMO + 1) is significant and
therefore the n–p* character of this transition is expected for
thienyl molecules.
Fig. 3 Linear dependence between experimental and theoretical
(TD-B3LYP/6-31G*) excitation energies with the first dominant oscillator
strengths (second band).
results (see Fig. 3 and eqn (2)). The correlation coefficient of
Electrochemical properties
experimental energies (Eexp) vs. theoretical lowest excitation
energies (ETD–B3LYP
) with the first dominant oscillator
Tetrazines substituted with thienyl or bithienyl substituents
are electrochemically active showing a reversible redox couple
at negative potentials and an irreversible oxidation peak at
positive potentials with no signs of electropolymerization
upon consecutive scans. The only exceptions are 6, already
described in the literature10 and 7 which undergo oxidative
electrochemical polymerization upon potential scanning. A
representative voltammogram for the series of tetrazine
derivatives is shown in Fig. 5 whereas the electrochemical
data determined for all compounds studied are collected in
Table 3.
In a recent paper24 redox behavior of various tetrazine
derivatives were discussed with special emphasis on the
correlation between the electron withdrawing/donating properties
of the substituents and the standard potential of the reversible
redox couple. According to the literature the standard
potential of the reversible redox couple in unsubstituted
tetrazine, E01, is ꢀ1.16 V vs. Fc/Fc+.33 Among all compounds
studied only in 1 a shift of E01 towards higher potentials is
observed (E01 = ꢀ1.12 V) which can be treated as a manifestation
of electron accepting properties of pyridyl substituents.
The values of this potential, recorded for phenyl, thienyl
and bithienyl derivatives of tetrazine, are consistently lower,
indicating that they have a weakly electron donating effect on
the central tetrazine ring. One can however notice a measurable
effect of the presence the alkyl substituents on E01 in the three-
ring series. The unsubstituted compound 3 shows the highest
value of E01, whereas in the case of 4, in which the alkyl group
is attached in the 4 position (inner b position) of the thienyl
ring, this potential is lowered by 100 mV. An intermediate
value of E01 is found for 5 in which the alkyl substituent is
located in the a position of the thienyl ring. The reduction
process is little dependent on the conjugation length since the
values of E01 for 3 (unsubstituted three-ring compound) and 6
(unsubstituted five-ring compound) are very close. In the five-
ring series the effect of the alkyl substituents is very weak and
the values of E01 are very close for all derivatives studied. This
strength reached 0.991.
Eexp = 0.158 + 0.995 ꢁ ETD–B3LYP (eV)
(2)
Based on the presented calculations and their correlation
with the experimental results, the general effect of the alkyl
group substitution on the absorption spectra of the molecules
studied can be analyzed. First, the transition energies for the
unsubstituted molecules are higher than those for the alkyl
substituted ones. This is connected with a better planarity of
the alkyl derivatives (see Fig. 1) as well as a slight increase of
the electron donating properties of the thienyl groups caused
by the presence of alkyl substituents. The strongest donating
effect is predicted for 9, where the excitation energy of 2.54 eV
with largest oscillator strength was obtained (see Table 1).
To understand the physical background of the observed
spectroscopic properties as well as to elucidate the electro-
chemical processes occurring in the synthesized molecules, it is
useful to examine the shapes of the most important molecular
orbitals. Since the shapes of these orbitals are only minimally
affected by the substitution with the alkyl groups, the sub-
sequent analysis will be presented only for the unsubstituted
molecules. As shown in Table 1, the transition from the
Highest Occupied to the Lowest Unoccupied Molecular
Orbitals (HOMO-to-LUMO) contributes significantly to the
first excitation energy only for compounds 1 and 2. These
contributions are practically constant (ca. 85%). The addition
of thiophene rings to the tetrazine molecule is connected with
the transition of (HOMO ꢀ 1)-to-LUMO for 3 to 5 and with
(HOMO ꢀ 2)-to-LUMO for 6 to 9. The HOMO orbitals for
all studied molecules are uniformly delocalized only over the
nitrogens of central tetrazine (see Fig. 4) and they are
connected with the free electron pairs of these atoms. These
orbitals have a typical non-bonding n character. The LUMO
orbitals show the inter-ring bonding character and their lobes
are uniformly spread along the tetrazine central ring and the
neighbouring aryl groups. The (HOMO ꢀ 1) orbitals are
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 2690–2700 2695

View Online
Fig. 4 Plots of B3LYP orbitals for unsubstituted molecules contributing to the selected excitation energies (see in Table 1).
c
2696 Phys. Chem. Chem. Phys., 2011, 13, 2690–2700
This journal is the Owner Societies 2011

View Online
Table 4 Calculated HOMO and LUMO levels and electrochemically
determined LUMO levels, in eV
B3LYP/6-31G*
HOMO
Experimentala
LUMO
LUMO
Compound
1
2
3
4
5
6
7
8
9
ꢀ3.76
ꢀ3.54
ꢀ3.62
ꢀ3.53
ꢀ3.58
ꢀ3.65
ꢀ3.65
ꢀ3.63
ꢀ3.62
ꢀ6.75
ꢀ6.17
ꢀ6.12
ꢀ5.94
ꢀ5.77
ꢀ5.46
ꢀ5.39
ꢀ5.25
ꢀ5.17
ꢀ3.19
ꢀ2.63
ꢀ2.65
ꢀ2.59
ꢀ2.45
ꢀ2.57
ꢀ2.52
ꢀ2.45
ꢀ2.41
a
Electrochemically determined.
molecule length since 2 (containing phenyl substituent) and 3
(containing thienyl substituent) start to oxidize at very similar
potentials. The same applies to 3 and 6 which differ in their
length. From the onsets of the first reduction peak, considering
the absolute potential scale34 the so called ‘‘electrochemical
LUMO level’’ can be calculated according to eqn (3).35
ELUMO = ꢀ(Ered onset + 4.8) eV
(3)
where Ered onset denotes the onset of the first reduction peak
(with respect to the Fc/Fc+ couple, see column 5 in Table 3).
In Table 4 the electrochemically determined values of
LUMO energies are compared with the calculated ones. It is
instructive to compare the theoretically calculated LUMO
levels with the electrochemically determined ones (see Table 4).
First, we notice that with the exception of 1, theoretical LUMO
levels change very little within the whole series of the
compounds studied. Similarly weak dependence of the nature
the substituent and the length of the molecule on its LUMO
level is observed in the electrochemical experiments. It should
also be noted that the calculated LUMO levels are consistently
higher by 0.9 to 1.2 eV with respect to the electrochemically
determined ones. Finally, the experimental electrochemical
results, together with the analysis of the frontier orbital shapes
(see Fig. 4), indicate that the reduction process affects the
whole molecule in the compounds of the three-ring series since
the LUMO orbitals show the inter-ring bonding character and
their lobes are uniformly spread along the tetrazine central
ring and the neighbouring aryl rings. In the five-ring series the
reduction process involves the increase of the electron density
in the three core-rings whereas the two terminal ones are little
affected. This explains the experimental observation of the
weak dependence of the E01 potential on the molecule length.
To the contrary, the lobes of the HOMO orbitals are
localized on the tetrazine central ring (see Fig. 4), indicating
that the oxidation process is initiated by abstraction of
an electron from the tetrazine moiety. This observation is
consistent with a weak dependence of the electrochemical
oxidation peak onset on the nature of the aryl substituents.
Fig. 5 Cyclic voltammogram of 8; concentration 5 ꢁ 10ꢀ4 M;
electrolyte 0.1 M Bu4BF4 in CH2Cl2; reference electrode Ag/Ag+
scan rate 50 mV sꢀ1, (a) reduction part and (b) oxidation part.
;
Table 3 Oxidation and reduction potentials of diaryltetrazines
determined from cyclic voltammetry investigations. Potential values
are given with respect to the ferrocene couple (Fc/Fc+), in V
Peak maximum
Peak onset
E1red
E1ox
E1 red
E2 ox
Compound
E10
1
2
3
4
5
6
7
8
8a
9
ꢀ1.17
ꢀ1.39
ꢀ1.30
ꢀ1.44
ꢀ1.43
ꢀ1.28
ꢀ1.28
ꢀ1.32
ꢀ1.39
ꢀ1.34
ꢀ1.06
ꢀ1.27
ꢀ1.20
ꢀ1.25
ꢀ1.19
ꢀ1.17
ꢀ1.17
ꢀ1.17
ꢀ1.20
ꢀ1.17
ꢀ1.12
ꢀ1.33
ꢀ1.25
ꢀ1.35
ꢀ1.31
ꢀ1.23
ꢀ1.23
ꢀ1.25
ꢀ1.30
ꢀ1.26
ꢀ1.04
ꢀ1.26
ꢀ1.18
ꢀ1.27
ꢀ1.22
ꢀ1.15
ꢀ1.15
ꢀ1.17
ꢀ1.22
ꢀ1.18
—
0.70
0.75
0.70
0.74
0.83
0.81
0.80
0.79
0.74
is due to the presence of an unsubstituted spacer between the
substituted thienyl ring and the tetrazine central unit. All these
data seem to indicate that the reduction process essentially
takes place in the central tetrazine unit. This is fully consistent
with the DFT calculations.
Structural organization in the solid state
As already stated oxidation of all compounds, occurring in
the range of 0.7–0.8 V vs. Fc/Fc+, is irreversible and, with the
exception of 6 and 7, does not lead to electropolymerization. It
is very little dependent on the nature of the substituent and the
The preparation and the electroactive properties of 6 have
already been reported,10 however its structure in the solid state
has not yet been determined. We succeeded in preparing single
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 2690–2700 2697

View Online
overlapping molecular orbitals which is believed to promote a
high electronic delocalization. In Fig. 7 details of the stacking
of three molecule packets constituting the second column are
shown with a view along the stacking axis. The tetrazine rings
are shifted and the deviation from a regular face-to-face
stacking is measured to be equal to 571. Moreover the terminal
thienyl rings do not overlap at all in this stacking sequence.
Finally the obtained intermolecular distances in this structural
arrangement range from 3.52 A for the shortest to 4.23 A for
the longest one, revealing some deviations from the full planar
geometry of the molecules as can clearly be seen in Fig. 6. In
the molecular crystallographic unit, deviations from the
perfect planarity range in between 3.51 to 201 for thienyl and
thienylene rings, respectively, to the perfect plane defined
by the tetrazine ring. Concerning the torsional angles of
thiophene rings, they range from 176.51 to 179.81 for the
two extremes revealing that the molecular stacking in the
solid state minimizes these values. This is in contrast with
Fig. 6 Perspective view of the monoclinic unit cell found after
refining the single crystal structure of 6.
crystals of 6 suitable for refining its crystallographic structure.
6 crystallizes in a P21/c space group, Z = 4 (one repeating
unit is constituted of one molecular and a half entity),
a = 5.8286(2) A, b = 22.7537(9) A, c = 19.7775(7) A and
b = 94.418(4)1 giving a density of d = 1.564 g cmꢀ3.w A
perspective view of the unit cell is shown in Fig. 6. The
molecular packing in the crystals of 6 is relatively complex.
The molecules are paired and the formed pairs are stacked
along two different types of columns. One column is oriented
along a direction close to the c axis, while the stacking direc-
tion of the second one is close to a parallel of the (b,c)
crystallographic plane. As a consequence, the molecular packets
constituting these columns stay relatively apart from each
other within the crystal structure. This structural arrangement
does not favor the formation of an extended lattice of p–p
Fig. 8 X-Ray powder profiles of: (a) 6 calculated from the single
crystal data, (b) 7 and (c) 9.
Fig. 7 Top view along the stacking axis of the molecular packets
in columns showing details of the stacking mode of neighbouring
molecules in which the shift from a face to face configuration can be
clearly seen.
Fig. 9 X-Ray powder profiles of: (a) 8 and (b) 8a. The inset is a zoom
of the 5–501 2y range in which the Bragg peaks, possibly belonging to
the same family as the most intensive one, are indexed accordingly.
c
2698 Phys. Chem. Chem. Phys., 2011, 13, 2690–2700
This journal is the Owner Societies 2011

View Online
the calculated values for the isolated molecules in which a
1661 torsional angle is found for the terminal thienyl ring.
(see 6 in Table 1).
thienyl ring induce the formation of a lamellar-type supra-
molecular organization.
The obtained experimental results have been correlated
with those derived from quantum chemical calculations. In
particular, the B3LYP calculations have shown that the
presence of the tetrazine central unit is responsible for the
lowering of the torsion angle between the adjacent aromatic
rings which leads to planarisation of the whole molecule. This
point is strongly confirmed by the analysis of X-ray data
obtained for single crystals of 6. The studied molecules also
exhibit different bond length alternation (BLA) parameters.
Although the largest molecules (6 to 9) have the highest BLA
values, the minimization of bond length differences between
the selected single and double bonds within the main conjuga-
tion chain has been found. The calculated optical transitions
with the dominant oscillator strengths show very good
linear relationship with the relevant experimental values. The
presented orbital analysis has also revealed that the contri-
bution from HOMO-to-(LUMO + 1) is for thienyl tetrazines
significant and the n–p* for character of this transition is
expected. The investigation of the shapes of frontier orbitals
showed that the oxidation of tetrazine derivatives is connected
with the electron abstraction from the tetrazine moieties.
To the contrary, the electrochemical reduction leads to the
increase of electron density in bonds linking tetrazine with
adjacent rings. The results obtained from quantum chemical
calculations are fully consistent with experimental findings.
For derivatives 7 to 9 only powder X-ray diffractograms
could be measured. In Fig. 8 powder X-ray profiles of 6, 7 and
9 are compared whereas in Fig. 9 the same profiles are shown
for 8 and 8a. It is clear that all powders studied are highly
crystalline. Thus, the presence of alkyl terminal groups of
different lengths and at different positions does not induce
noticeable structural disorder since no increase of the diffuse
background is observed in the recorded diffractograms. At
this stage, it seems interesting to more clearly elucidate the
structural arrangement of 7–9 by comparing their powder
diffractograms with that of 6, calculated from the single
crystal data.
First, we can notice that the X-ray profiles of 7 and 9, which
contain octyl substituents in positions 4 and 4, 5, respectively,
exhibit some similarities, suggesting probable structural
relationships with 6 (see Fig. 8). For these compounds, the
size of the unit cell does not seem larger than that determined
for 6. This indicates that, in spite of the presence of alkyls
chains, the resulting molecular packing is probably denser
than in 6.
As expected, X-ray profiles of 8 and 8a also show striking
similarity (Fig. 9). A low angle Bragg peak, corresponding to a
long distances of 24.7 A for 8 and 24.3 A for 8a, shows
the highest intensity in both cases. This indicates that the
symmetry of the unit cells of these compounds, which differ
only in the length of the alkyl substituent is certainly different
from the symmetry of the unit cells of 6, 7 and 9. Moreover,
this intensive low angle Bragg peak seems to be the first order
of a family of Bragg reflections; higher order reflections of this
family are indicated in the inset of Fig. 9. Since this distance
corresponds to the length of the molecule, it can therefore
be postulated that 8 and 8a may exhibit a lamellar like
structure, in which the molecules would stand upright in the
lamellae. This structural arrangement is probably facilitated
by the presence of terminal alkyl substituents in position 5
(outer Ca carbon).
Acknowledgements
Financial support from Polish Ministry of Science and Higher
Education through research project no. NN205105735 in years
2008–2011 is acknowledged.
References
1 A. R. Murphy and J. M. J. Frechet, Chem. Rev., 2007, 107, 1066.
2 S. Allard, M. Forster, B. Souharce, H. Thiem and U. Scherf,
Angew. Chem., Int. Ed., 2008, 47, 4070.
To summarize this part of the research, it is clear from the
X-ray diffraction studies that the supramolecular organization
in the five-ring series is very dependent on the position of the
substituent. In particular, the presence of the alkyl groups
attached to Ca carbon in the terminal thienyl ring promotes
the formation of a lamellar-type supramolecular organization.
3 T. Rauch, M. Boberl, S. F. Tedde, J. Furst, M. V. Kovalenko,
G. Hesser, U. Lemmer, W. Heiss and O. Hayden, Nat. Photonics,
2009, 3, 332.
4 U. Lange, N. V. Roznyatovskaya and V. M. Mirsky, Anal. Chim.
Acta, 2008, 614, 1.
5 M.-J. Spijkman, J. J. Brondijk, T. C. T. Geuns, E. C. P. Smits,
T. Cramer, F. Zerbetto, P. Stoliar, F. Biscarini, P. W. M. Blom and
D. M. de Leeuw, Adv. Funct. Mater., 2010, 20, 898.
6 A. Malinauskas, J. Malinauskiene and A. Ramanavicius,
Nanotechnology, 2005, 16, R51.
¨
¨
7 Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109,
5868.
Conclusions
8 A. Pron, P. Gawrys, M. Zagorska, D. Djurado and R. Demadrille,
Chem. Soc. Rev., 2010, 9, 2577.
9 J. Soloducho, J. Doskocz, J. Cabaj and S. Roszak, Tetrahedron,
2003, 59, 4761.
10 P. Audebert, S. Sadki, F. Miomandre and G. Clavier, Electrochem.
Commun., 2004, 6, 144.
11 G. Barbarella, P. Ostoja, P. Maccagnani, O. Pudova, L. Antolini,
D. Casarini and A. Bongini, Chem. Mater., 1998, 10, 3683.
12 Y. Kim, J. Do, E. Kim, G. Clavier, L. Galmiche and P. Audebert,
J. Electroanal. Chem., 2009, 632, 201.
To conclude, we have synthesized a series of solution processable
diaryl tetrazine derivatives consisting of either three or five
aromatic rings, the majority of them have never been reported.
The synthesized compounds show tunable spectroscopic
properties and interesting electrochemical behavior associated
with a reversible reduction at relatively high potentials and
an irreversible oxidation. The solid state structure of the
synthesized five-ring compounds is very sensitive to the
position of the solubilizing alkyl substituents. It has been
shown that substituents attached to Ca carbon in the terminal
13 N. Saracoglu, Tetrahedron, 2007, 63, 4199.
14 G. Clavier and P. Audebert, Chem. Rev., 2010, 110, 3299.
15 I. Osaka and R. D. McCullough, Acc. Chem. Res., 2008, 41, 1202.
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 2690–2700 2699

View Online
16 It is instructive to compare the strikingly different optical and redox
properties of regioregular ht–ht coupled poly(alkylthiophene)s, first
synthesized by (a) T. A. Chen and R. D. Rieke, J. Am. Chem. Soc.,
1992, 114, 10087; (b) R. R. McCullough and R. D. Lowe, J. Chem.
Soc., Chem. Commun., 1992, 70 with hh–tt or tt–hh coupled poly-
(alkylthiophene)s synthesized by; (c) M. Zagorska and B. Krische,
Polymer, 1990, 31, 1379; (d) R. M. Souto-Maior, K. Hinkelmann,
H. Eckert and F. Wudl, Macromolecules, 1990, 23, 1268.
17 T. J. Prosa, M. J. Winokur, J. Moulton, P. Smith and A. J. Heeger,
Macromolecules, 1992, 25, 4364.
J. V. Foresman, Q. Ortiz, A. G. Cui, S. Baboul, J. Clifford,
B. B. Cioslowski, K. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,
GAUSSIAN 03, Revision A.1, Gaussian, Inc., Pittsburgh, PA,
2003.
24 Y.-H. Gong, F. Miomandre, R. Meallet-Renault, S. Badre,
´
L. Galmiche, J. Tang, P. Audebert and G. Clavier, Eur. J. Org.
Chem., 2009, 6121.
18 R. G. Parr and W. Yang, Density-Functional Theory of Atoms and
Molecules in Chemistry, Springer-Verlag, New York, 1991.
19 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
25 M. Wlostowski and A. Olszewski, in preparation.
26 F. Liu, P. Zuo, L. Meng and S. J. Zheng, THEOCHEM, 2005, 726,
161.
20 F. Furche and R. Ahlrichs, J. Chem. Phys., 2002, 117, 7433.
21 G. A. Petersson and M. A. Al-Laham, J. Chem. Phys., 1991, 94,
6081.
27 V. Lukes, R. Solc, J. Rimarck, S. Guillerez and B. Pe
THEOCHEM, 2009, 910, 104.
´
pin-Donat,
28 S. M. Bouzzine, S. Bouzakraoui, M. Bouachrine and M. Hamidi,
THEOCHEM, 2005, 726, 271.
22 V. Rassolov, J. Comput. Chem., 2001, 22, 976.
23 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. Hratchian, 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, J. B. Raghavachari,
29 W. J. D. Beenken, Chem. Phys., 2008, 349, 250.
30 D. Jacquemin, E. A. Perpete, H. Chermette, I. Ciofini and
C. Adamo, Chem. Phys., 2007, 332, 79.
31 A. R. Katritzky, Handbook of Heterocyclic Chemistry, Pergamon
Press, Oxford, 1986.
32 J. Waluk, J. Spanget-Larsen and E. W. Thulstrup, Chem. Phys.,
1995, 200, 201.
33 H. Fischer, T. Muller, I. Umminger, F. A. Neugebauer, H. Chandra
¨
and M. C. R. Symons, J. Chem. Soc., Perkin Trans. 2, 1988,
413.
34 S. Trasatti, Pure Appl. Chem., 1986, 58, 955.
35 T. Johansson, W. Mammo, M. Svensson, M. R. Andersson and
O. Inganas, J. Mater. Chem., 2003, 13, 1316.
c
2700 Phys. Chem. Chem. Phys., 2011, 13, 2690–2700
This journal is the Owner Societies 2011