N-Rich Fused Heterocyclic Systems: Synthesis, Structure, Optical and Electrochemical Characterization

Article abstract of DOI:10.1002/ejoc.201501283

A new class of N-rich fused heterocyclic compounds containing the triazolo-triazine moiety was synthesized and studied by cyclic voltammetry, UV/Vis spectroscopy, X-ray diffraction, and first principle computations. All the compounds show reversible or quasi-reversible reduction processes, with reduction potentials easily tunable within the class. LUMO energies as low as -3.95 eV have been measured. The UV/Vis spectra show highly structured absorptions, indicative of rigid molecular skeletons. The solid-state packing is dominated by π-π stacking interactions, which are promoted by weak C_Ar-H···N interactions, whereas face-to-edge contacts (T contacts), typical of many fused hydrocarbons, are largely absent.

Full text of DOI:10.1002/ejoc.201501283

DOI: 10.1002/ejoc.201501283
Full Paper
Crystal Packing
N-Rich Fused Heterocyclic Systems: Synthesis, Structure, Optical
and Electrochemical Characterization
Sandra Fusco,*[a,c] Cira Maglione,[a] Amalia Velardo,[b] Vincenzo Piccialli,[a] Rosalba Liguori,[c]
Andrea Peluso,[b] Alfredo Rubino,[c] and Roberto Centore*[a]
Abstract: A new class of N-rich fused heterocyclic compounds gies as low as –3.95 eV have been measured. The UV/Vis spectra
containing the triazolo-triazine moiety was synthesized and show highly structured absorptions, indicative of rigid molec-
studied by cyclic voltammetry, UV/Vis spectroscopy, X-ray dif- ular skeletons. The solid-state packing is dominated by π–π
fraction, and first principle computations. All the compounds stacking interactions, which are promoted by weak CAr–H···N
show reversible or quasi-reversible reduction processes, with re- interactions, whereas face-to-edge contacts (T contacts), typical
duction potentials easily tunable within the class. LUMO ener- of many fused hydrocarbons, are largely absent.
Introduction
aromatic hydrocarbons are endowed with weak H-bond donors
(aromatic C–H) and weak acceptors (π acceptor). Given the di-
rectionality of the π acceptor[12] it is not surprising that the
basic packing motif of fused aromatic hydrocarbons is the T
contact (face-to-edge) rather than the π stack (parallel shifted
face-to-face, FF contact); thus, truly planar graphite-like layers
are not observed in the packing of planar fused aromatic hydro-
carbons, even of large size. For this reason, the synthesis of
infinite π-stacked structures presents a challenging problem in
material chemistry. The realization of infinite planar structures
can be achieved by in-plane covalent bonding (e.g., graphite,
h-boron nitride) or by in-plane noncovalent bonding, but in this
case molecules must be endowed with suitable groups, prop-
erly positioned to form those planes by secondary interactions.
With respect to these points, replacement of CAr–H groups by
nitrogen atoms in aromatic hydrocarbons[13] can provide some
improvements.[14] First, pyridine-like N atoms in aromatic com-
pounds are strong in-plane H-bond acceptors, favoring the for-
mation of infinite planar layers of H-bonded molecules, through
in-plane CAr-H···N interactions. Second, the shorter van der
Waals radius of nitrogen can result in shorter stacking distances
In the last years significant efforts have been made to synthe-
size new organic compounds for applications in optoelectron-
ics, photonics, and organic electronics.[1] In all these fields, aro-
matic heterocyclic and polyheterocyclic systems have received
attention because of their improved performance compared
with aromatic hydrocarbons both in low molar mass[2,3] and in
polymer based materials.[4,5]
Some linearly fused aromatic hydrocarbons, such as pent-
acene, have shown good charge transport characteristics[6] and,
together with compounds containing donor–acceptor (D–A)
groups, are among the most studied classes of compounds for
organic semiconductors.[6,7] Nevertheless, higher acenes, such
as hexa- or heptacene and related compounds, suffer from im-
portant drawbacks, such as environmental instability, especially
under light conditions, which is determined by their high en-
ergy HOMOs.[8,9] The introduction of heteroatoms, for example
sulfur and nitrogen, proved very effective for overcoming that
difficulty.[7c,9,10]
The conductive properties of fused aromatic hydrocarbons
are not only related to their redox properties but also to solid-
state packing. The general rules on the modes of packing of
aromatic hydrocarbons were derived some years ago:[11] fused
[a] Department of Chemical Sciences, University of Naples “Federico II”,
Via Cinthia, 80126 Naples, Italy
E-mail: sandra.fusco@unina.it
roberto.centore@unina.it
www.unina.it
[b] Department of Chemistry and Biology, University of Salerno,
Via Giovanni Paolo II, 13284084 Fisciano, Salerno, Italy
[c] Department of Industrial Engineering (DIIN), University of Salerno,
Via Giovanni Paolo II, 132, 84084 Fisciano, Salerno, Italy
www.unisa.it
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201501283.
Scheme 1. Chemical structures of the synthesized compounds. Only one iso-
mer is shown for 5.
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Scheme 2. Scheme of synthesis of compounds 1–5.
between the layers compared with those of all-carbon-contain- pounds similar to 1 and 2 are present in the literature[18] and
ing compounds.[14]
they were prepared according to a different multi-step synthetic
route that requires the preliminary preparation of 3-hydrazino-
aceno-1,2,4-triazine derivatives and their subsequent condensa-
tion with a carboxylic acid.
Herein, with the aim of providing a contribution to the study
of structure-property relationships in fused-ring heteroaromatic
systems,[15] we report the synthesis, the structures, and chemi-
cophysical properties of some new compounds obtained by
condensation of 3,4-diamino-1,2,4-triazoles[16] with aromatic
1,2-diketones. Their chemical structures are shown in Scheme 1.
Electrochemical Potentials
The oxidation and reduction potentials of 1–5, measured by
cyclic voltammetry at room temperature on thin films drop-cast
from dichloromethane on a glassy carbon disk electrode, are
reported in Table 1. The current/voltage curves are reported in
the Supporting Information.
Results and Discussion
Synthesis
The general synthetic route for compounds 1–5 is shown in
Scheme 2. It is based on the condensation of 3,4-diamino-5-
heptyl-1,2,4-triazole with different fused diketones. The conden-
sation was carried out in acetic acid at reflux, overnight.
The diamino-triazole starting reagent was obtained by one-
step reaction of octanoic acid with diaminoguanidine mono-
hydrochloride in polyphosphoric acid.[16a] Diketones 9,10-
phenanthrenedione, 1,2-acenaphthenequinone, and 1,10-phen-
anthroline-5,6-dione are commercially available. Pyrene-1,2-di-
one and pyrene-4,5,9,10-tetraone were prepared by oxidation
of pyrene with the catalytic system RuCl3·xH2O/sodium perio-
date, by following a reported procedure.[17]
Table 1. Redox potentials (vs. Fc+/Fc, Volt), HOMO and LUMO energies [eV]
and calculated dipole moments (μ, Debye) of 1–5.[a]
Eox
Ered
exp.
HOMO
exp.
LUMO
exp.
μ
calcd.
calcd.
1
2
3
4
5
1.22
0.99
–
0.95
1.01
–1.51
–1.39
–1.15
–1.33
–1.15
–1.98
–1.72
–1.54
–1.72
–1.55
–6.32
–6.10
–
–6.05
–6.11
–3.59
–3.71
–3. 95
–3.77
–3.95
–3.12
–3.38
–3.56
–3.38
–3.55
11.2
10.1
5.8
10.2
9.5[b]
[a] The calculated entries are referred to compounds analogous to those of
Scheme 1, with R = CH3. [b] For the noncentrosymmetric isomer.
HOMO and LUMO energies have been calculated by using
the value 5.1 V for the reduction potential of the redox couple
Fc+/Fc.[19]
The overall yields of the synthetic route shown in Scheme 2
were satisfactory, ranging between 65 and 94 % for compounds
1–4. In the case of 5, the formation of two regioisomers is ex-
pected from the synthesis, and they were indeed observed (see
All the reduction processes are reversible and the reported
the Experimental Section). However, from the chromatographic Ered in Table 1 was taken as a numerical average between the
analysis, only the less polar isomer was isolated in pure form, anodic and cathodic peaks. For oxidation processes, which are
and the yield in this case was 25 %. Few examples of com- all irreversible, the onset of current signal was considered.
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Compound 1 is composed of a triazolo-triazine unit con- show that the additional electron of the anionic species is
densed with two fused phenyl rings, whereas 2 has two phenyl mainly localized on the triazolo-triazine moiety, with compara-
groups attached by a covalent bond. This creates a new planar tively smaller contributions of the hydrocarbon fragment. The
system with a couple of additional π electrons in comparison LUMO of 5 is predicted to be completely delocalized over the
with 1, which causes a decrease in the oxidation potential and two triazolo-triazine moieties, and indeed the cyclic voltamme-
an increase in the reduction potential. In 3, which is isoelec- try of 5, reported in the Supporting Information, exhibits two
tronic with 2, the substitution of two CArH units with nitrogen peaks, one at ca. –1.1 V vs. Fc+/Fc, and the second at ca. –1.3 V,
atoms further increases the reduction potential and lowers the which could be attributed to reduction in two different elec-
LUMO energy by 0.24 eV. This behavior is in line with other tronic states, obtained by the superposition of the two states
results showing that the introduction of nitrogen atoms in the in which the additional electron is wholly localized on one of
molecular systems lowers the LUMO energies.[15c] Notably, com- the two triazole-triazine units.[21b–21e] The resonance effect in-
pound 4, with one more ring and a more extended conjugation creases significantly the reduction potential of 5, which is about
path, has lower oxidation and higher reduction potentials com- 200 mV higher than that of its homologue 2.
pared with those of 2. According to this trend, the reduction
potential of 5, which contains a linear sequence of six fused
rings, is higher than those of 4 and 2 and the LUMO energy is
lower. Actually, the features of the frontier orbitals of 5 seem
optimized (i.e., HOMO energy is higher and LUMO energy is
lower) compared with those of recently reported heteroacenes
of comparable length.[9b]
The computed ground-state dipole moments of 1–5 are re-
ported in Table 1. It is widely considered that high dipole mo-
ments are detrimental for charge transport, because strong
charge–dipole interactions could limit charge diffusion.[22] Com-
pound 3 exhibits a comparatively low dipole moment (5.8 D),
whereas 5 possesses two isomers, a centrosymmetric one, with
a dipole moment that is clearly zero, and a noncentrosymmetric
All these results suggest that the presence of heteroatoms isomer, the predicted dipole moment of which is 9.5 D. Thus,
affects the LUMOs more than the extension of the conjugation both 3 and the centrosymmetrical isomer of 5 should be well
path. Additionally, it is notable that the LUMOs of 2–5 all lie suited for applications in nanoelectronics. We will return to this
below –3.7 eV, which is a critical value to avoid H2O reduc- point when we consider structural changes upon injection of
tion.[20] Moreover, the reduction potentials of 3 and 5 are com- an additional electron (see below).
parable with those of PCBM60 and N,N′-dioctyl-perylendi-
immide, which, measured by us under the same conditions and
with the same setup, are –1.0 V. This result suggests that these UV/Vis Absorption and Emission
compounds are potential n-type active molecules for organic
The absorption spectra of compounds 1–5 recorded in CH2Cl2
transistors and organic photovoltaics.
solution (10–5
M
) are reported in Figure 2. Compounds 1–4
Full optimizations of the neutral and anionic structures of 1–
5, carried out at density functional theory level, using the B3LYP
functional and the 6-311+G(d,p) basis set, in dichloromethane,
leads to LUMO energies slightly higher than the experimental
values. The discrepancy depends both on the know difficulty of
reaching high accuracy in computing reduction free energies
and on the fact that experimental values have been measured
on thin films and thus could include resonance effects due to
stacking interactions, which lower the LUMO energies.[21] Nota-
bly, the observed changes of the LUMO energies within the
class of investigated compounds is well reproduced by calcula-
tions. The computed isodensity surfaces, reported in Figure 1,
show highly structured spectra with vibronic progressions both
in the shorter and longer wavelength regions, as a consequence
of their rigid skeleton. The absorption onset is redshifted on
going from 1 to 4, as a consequence of the more extended π
conjugation. The absorption onset of 5 is located between
those of 3 and 4.
Figure 2. UV spectra of compounds 1–5 in dichloromethane.
Compound 4 exhibits a highly structured absorption at
Figure 1. Computed isodensity surfaces of HOMO of anionic species of 1–5.
longer wavelength, in which three vibronic peaks are visible at
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433, 459, and 489 nm, which, on the basis of the predicted low oscillator strength, which increases for 4 and 5, in agree-
absorption wavelengths (see below), can be assigned to ment with the observed spectra.
vibronic progression of
a vibrational mode with ω ≈
The computed electronic density variation upon excitation
to the first excited singlet of 1–5 are reported in Figure 3. In-
spection of Figure 3 shows that for all compounds, the transi-
tion to the lowest excited state involve MOs that are delocalized
on almost all molecular rings. Notwithstanding this, a signifi-
cant fraction of electron density is transferred from the hydro-
carbon and triazole rings to the triazine, as occurs in analogous
compounds.[23]
In the case of 5, electronic excitation involves MOs delocal-
ized over the whole molecule, evidencing the presence of exci-
tonic couplings between two quasi-degenerate transitions. In-
deed, TDDFT computations predict two transitions falling at 438
and 397 nm, for the centrosymmetric isomer, and at 424 and
380 nm, for the non-centrosymmetric isomer. The presence of
two transitions destroys the fine vibrational structure, so that
the absorption bands appear broad and unstructured.
The emission spectra of the compounds, which are all fluo-
rescent, are reported in Figure 4.
1310 cm–1.[23b,24] Compound 5 shows a broad absorption ex-
tending from 350 to 500 nm, without any vibronic activity,
probably because of excitonic couplings between two quasi-
degenerate transitions (see below). Vibronic activities are also
shown by compounds 2 and 4 in the range 280–350 nm, attrib-
utable to the aromatic hydrocarbon skeleton. Less intense vi-
bronic activities are also observed for compounds 1–3 in the
same wavelength region.
Vertical absorption wavelengths predicted by time-depend-
ent density functional theory (TDDFT) are reported in Table 2,
together with the experimental absorption maxima and com-
puted oscillator strengths.
Table 2. Experimental and predicted absorption wavelengths [nm], experi-
mental extinction coefficients (ε/103·dm3 mol–1 cm–1) of 1–5 in dichloro-
methane, predicted oscillator strengths (f) and optical band gap [eV].
(opt)
λ [exp.] λ [calcd.]
ε
f
Eg
1
2
3
4
5
ca. 420 410
1.6
16
32
2.9
4.5
18
2.3
2.9
24
4.1
11
12
0.94
1.2
4.4
0.045
0.13
0.38
0.050
0.13
0.37
0.066
0.095
0.061
0.26
0.14
0.11
0.24
0.02
0.29
2.77
336
298
364
333
ca. 442 449
400 400
ca. 304 292
ca. 445 434
2.55
2.51
2.41
2.48
373
296
489
344
327
450
398
312
365
329
483
329
323
438[a]
414
397
449[b]
424
380
0.06
0.31
0.11
[a] Centrosymmetrical isomer. [b] Non-centrosymmetrical isomer.
The theoretical absorption wavelengths are in very good
agreement with experimental values. The lowest energy transi-
tions of compounds 1–3 are characterized by a comparatively
Figure 4. Emission spectra of compounds 1–5 in dichloromethane; excitation
wavelengths: 298, 230, 296, 340, and 312 nm for 1–5, respectively.
Figure 3. Computed electronic density variation upon excitation to the first excited singlet of 1–5. The violet and blue lobes represent electron density
increase and depletion upon excitation, respectively.
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The emission spectra were recorded in CH2Cl2 solution (10–
for compounds 1, 2, 3, 5 and 10–7
M
for 4). Compound 1
6
M
exhibits a maximum emission at about 460 nm, with the onset
at 410 nm, which is consistent with theoretical calculations that
assign the very broad absorption of 1, extending from 280 to
450 nm, to the existence of three different electronic states (cf.
Table 2). The emission bandwidth of 1 extends up to ca. 650 nm
and suggests the presence of several Franck–Condon-active vi-
brational modes. This behavior is possibly due to the fact that
in 1, at variance with 2–5, the triazolo-triazine and the naphth-
alene rings are connected by a five- rather than a six-membered
cycle, leading to larger equilibrium geometry displacements
upon transitions. For all the other molecules, the emission spec-
tra are very similar, exhibiting modest Stokes shifts, as expected
for extensively delocalized π-electron systems.
The Simulated Photoelectron Spectrum of 3–
Figure 5. The predicted photoelectron spectrum of the anion of 3 as a func-
tion of the vibrational energy excess. Inset: the Franck–Condon weighted
density of states for electron transfer between an anionic and a neutral mol-
ecule of 3, as a function of the energy difference for the electron transfer
reaction, at T = 298 K.
As discussed above, compounds 3 and 5 are candidates for
applications in organic electronics as components for electron
transport materials. In such materials, charge transport occurs
by a series of successive electron-transfer reactions between
neutral and localized radical anions, which are promoted by the
presence of π stacking interactions in the solid phase. Apart
from geometrical arrangements in the solid phase, for a mol-
ecule to be an effective charge carrier it is necessary that the
transition from the neutral to the charged state takes place with
small reorganization energies; that is, the equilibrium positions
of the two states has to coincide as much as possible. Informa-
tion about changes in the equilibrium geometries upon transfer
of an electron or a hole can be inferred from the band shapes
of photoelectron spectra of neutral or anionic species: narrow
bands, such as those observed in the vibrationally resolved
photoelectron spectrum of C60, pyrene, and perylene anions,[25]
are characteristic of molecules that undergo small geometrical
changes, being therefore well suited as charge carriers in or-
ganic materials. We have thus simulated the photoelectron
spectrum of the anion of 3 at 298 K, by using the computed
equilibrium geometries and normal modes of vibration of 3 in
its neutral and anionic state and the Kubo generating function
approach to reconstruct the band shape.[26] The simulated pho-
toelectron spectrum of anion 3 is reported in Figure 5, as a
function of the vibrational energy excess. The peak at ω = 0,
corresponds to the 0–0 transition, and, as in the case of pyrene,
perylene, and fullerene anions, is the most intense. The
whole spectrum is characterized by a total bandwidth of ca.
3000 cm–1, typical of molecules that undergo small displace-
ments from the equilibrium positions upon electron transfer
from an anionic to a neutral molecule, comparable to that of
C60 and perylene anions,[25a,25c] and significantly narrower than
that of the pyrene anion.
where Vel is the electronic coupling element and F(ΔE,T) is the
Franck–Condon weighted density of states.
k = (2π/h) |Vel|2 F(ΔE,T)
(1)
F(ΔE,T) for the electron hopping reaction shown in Equa-
tion (2) has been obtained from the convolution of the simu-
lated photoelectron spectrum of 3– (Figure 5), with the simu-
lated electron attachment spectrum of the neutral species (not
shown).[25,26] The resulting F(ΔE,T) is shown in the inset of Fig-
ure 5, as a function of the energy difference of reaction (2).
3– + 3 → 3 + 3–
(2)
Assuming Vel = 0.05 eV, a lower limit value for stacked mol-
ecules, and taking the computed value of F(ΔE,T) at ΔE ≈ 0,
corresponding to charge transfer occurring in resonance condi-
tions, charge hopping is predicted to occur on femtosecond
timescales, showing that 3 is potentially suited for technological
applications.
Solid-State Structure
The X-ray molecular structure of 1 is reported in Figure 6. The
molecule has a flat shape with the alkyl chain in the most ex-
tended conformation.
The packing is shown in Figure 7. Molecules are arranged in
infinite planar layers through weak H-bond interactions be-
tween in-plane CAr–H donor and in-plane N acceptors [C15–
From photoelectron spectra it is also possible to obtain a first H···N1i 0.929, 2.534, 3.445(6) Å, 167°, i: x, –1 + y, –1 + z; C20–
estimate of the rates for charge hopping between two adjacent H···N5ii 0.930, 2.541, 3.401(7) Å, 154°, ii: 1 –x, 1 –y, –z]. As may
molecules. Indeed, according to Fermi's golden rule, the rate be inferred from Figure 7 (b), the molecular layers are parallel
constant of a nonradiative transition is given by Equation (1) to the lattice planes (2 1 –1), and this is consistent with 2 1 –1
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Figure 6. X-ray molecular structure of 1. Thermal ellipsoids are drawn at 30 %
probability level.
Figure 8. X-ray structure of the two crystallographically independent mol-
ecules of 2. Thermal ellipsoids are drawn at 30 % probability level.
being the most intense reflection of the diffraction pattern. As
a result, the stacking distance between the molecular layers is
the Bragg spacing of the corresponding lattice planes, d =
3.53 Å. Closest contacts between C atoms of adjacent layers are
C11···C15i 3.397(5) Å and C21···C14i 3.394(5) Å (i: 1 –x, –y, –z).
Figure 9. (a) Face view of ribbons of molecules of 2. (b) Edge view of the
same ribbons of (a).
Figure 7. (a) An infinite planar layer of molecules of 1. (b) Edge view of the
same layer of (a).
Hirshfeld fingerprint plots[27] of 1 and 2 are reported in Fig-
ures 10 and 11. The fingerprint plot is a graphical two-dimen-
sional map that indicates the distribution of the interactions for
a single molecule in the crystal.[28] In the plot, for each point of
the Hirshfeld surface enveloping the molecule in the crystal,
the distance di to the nearest atom inside the surface and the
distance de to the nearest atom outside the surface are re-
ported. The color of each point in the plot is related to the
abundance of that interaction, from blue (low) to green (high)
to red (very high).
The two crystallographically independent molecules of 2 are
shown in Figure 8. They differ for the conformation of the alkyl
tail that is trans-planar in A whereas it shows a gauche-type
torsion angle in B. The average planes of the fused-ring molec-
ular portion are not coplanar in the two independent mol-
ecules, making a dihedral angle of 15.73(2)°. This, of course,
prevents the formation of truly planar parallel stacked layers as
in the case of 1. The packing is shown in Figure 9. Ribbons of
A are formed by weak H-bond interactions between CAr–H do-
nor and N acceptors [C16A–H···N1Ai 0.949, 2.494, 3.356(5) Å,
151°, i: –0.5 + x, 0.5 – y, z]. In a similar way, ribbons of B are
formed [C16B–H···N1Bi 0.951, 2.433, 3.372(5) Å, 169.4°; C19B–
H···N1Bi 0.950, 2.638, 3.588(5) Å, 178.2°, i: –0.5 + x, 1.5 – y, z]. The
ribbons are generated, crystallographically, by the glide plane
perpendicular to b, and are therefore parallel to a. The two
ribbons are linked in an open structure, again by weak H-bond-
ing between CAr–H donor and N acceptors of crystallographi-
cally different molecules [C20A–H···N5B 0.950, 2.730, 3.485(4) Å,
137°; C21A–H···N2B 0.951, 2.683, 3.557(4) Å, 153°]. In this way,
a double layer of undulated planes is formed. The double layers
are piled along c.
Figure 10. Hirshfeld fingerprint plot of 1.
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Experimental Section
Materials and Instrumentation: All reagents, catalysts, and sol-
vents were purchased from Sigma–Aldrich and used as received,
without any further purification. Chromatographic separations were
performed by using standard column methods with silica gel (230–
400 mesh). Melting points were determined by temperature-con-
trolled optical microscopy (Zeiss Axioskop polarizing microscope
equipped with a Mettler FP90 heating stage). The thermal behavior
of the compounds (see the Supporting Information) was studied by
DSC (Perkin–Elmer Pyris instrument, scanning rate 10 °C/min, nitro-
gen flow). Elemental analyses were obtained with a Thermo Elec-
tron Flash 1112 apparatus. NMR spectra were obtained in CD2Cl2,
CDCl3, or 1,1,2,2-[D2]tetrachloroethane (C2D2Cl4) with a Varian 200
(200 MHz), Varian 500 (500 MHz) or Bruker Avance 400 (400 MHz)
spectrometer and referenced to the solvent signal. Chemical shifts
are reported in ppm. UV/Vis spectra were measured with a dual-
beam scanning spectrophotometer (Perkin–Elmer Lambda 900) at
25 °C, using samples prepared as dilute solutions in 1 cm quartz
cuvettes. MS (ESI, +) analyses were performed with a Micromass
Bio-Q triple quadrupole mass spectrometer equipped with electro-
spray ion source, using methanol as solvent.
Cyclic Voltammetry: Electrochemical measurements were per-
formed with a Metrohm 757 VA Computrace combined with an
AUTOLAB potentiostat-galvanostat. Voltammetric experiments were
performed at 25 °C, in a single-compartment three-electrode cell.
The medium was purged with a high-purity nitrogen stream. A
setup made up of glassy carbon electrode (GC) as working elec-
trode, a platinum wire as pseudo-reference electrode and a plati-
num wire as counter electrode was used. The compounds were
Figure 11. Hirshfeld fingerprint plots of the two crystallographically inde-
pendent molecules of 2.
The plots show, as a common feature, a green area centered
at (di, de) = (1.8, 1.8), corresponding to the π–π stacking con-
tacts.[27a] These are the most abundant intermolecular interac-
tions in both compounds. The “wings“ at the upper left (di, de) =
(1.2, 1.9) and lower right (di, de) = (1.9, 1.2) corresponding to
the C–H···π interactions typical of the herringbone packing (T
contacts)[27a] are largely absent. In this sense, the plots of Fig-
ures 10 and 11 are more similar to the plot of benzodicoronene
rather than to naphthalene, anthracene, phenanthrene, and
pyrene, in which T contacts are relevant.[27a] As compared with
the plots of pure fused hydrocarbons, an additional distinctive
feature is represented by the two spikes at di + de = 2.4 Å,
pointing to the lower left of the plots and symmetrically dis-
posed with respect to the diagonal. They correspond to the
CAr–H···N interactions. Therefore, the green area centered at di =
de ≅1.8 Å and the two spikes can be considered as distinct
features of the fingerprint plots of N-rich fused aromatic hydro-
carbons.
filmed on the working electrode from a 10–5
triazolotriazine derivative in CH2Cl2 and a solution of 100 m
M
solution of each
of
M
tetrabutylammonium-hexafluorophosphate (nBu4NPF6) in CH3CN
was employed as inert electrolyte. The measurements were per-
formed in the range –2.2 to +2.2 V at a speed of 100 mV/sec. Ferro-
cene/Ferrocinium (Fc/Fc+) redox couple was used as internal refer-
ence.
X-ray Analysis: All data for crystal structure determinations were
measured with
a
Bruker–Nonius KappaCCD diffractometer
equipped with Oxford Cryostream 700 apparatus, using graphite-
monochromated Mo-Kα radiation (0.71073 Å). Reduction of data
and semiempirical absorption correction were performed with the
SADABS program.[29] The structures were solved by direct methods
(SIR97 program[30]) and refined by the full-matrix least-squares
method on F2 using the SHELXL-97 program[31] with the aid of the
program WinGX.[32] H atoms were generated stereochemically and
refined by the riding model. The analysis of the crystal packing
was performed by using the program Mercury;[33] Hirshfeld surface
analysis was performed using the program CrystalExplorer.[34] Crys-
tal and refinement data are summarized in Table 3.
Conclusions
CCDC 1423276 (for 1), and 1423277 (for 2) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
N-Rich fused-ring heterocyclic compounds containing the tri-
azolo-triazine moiety show remarkable properties both at the
molecular level and in the solid state. As compared with pure
fused hydrocarbons, extensive replacement of CAr–H by nitro-
gen atoms reduces the LUMO energies to a level comparable
with well-known organic n-type semiconductors. In particular,
the lowest energy LUMO (–3.95 eV) is reached by the com-
pounds of the series featuring the highest ratio of nitrogen to
carbon atoms in the fused-ring system (N/C = 0.5 in 3 and 5).
The solid-state packing is dominated by π-stacking interactions
between truly planar or undulated molecular sheets, whereas T
contacts (face-to-edge) are absent.
Typical Procedure. Synthesis of 3-Heptylacenaphtho[2′,1′-h]-
[1,2,4]triazolo[4,3-b][1,2,4]triazine (1): 3,4-Diamino-5-heptyl-
1,2,4-triazole (1.03 g, 4.04 mmol) and 1,2-acenaphtenequinone
(1.47 g, 8.09 mmol) were suspended in glacial acetic acid (13 mL)
and the reaction mixture was heated to reflux overnight to give an
orange solution. The reaction mixture was then cooled to room
temperature and poured into cold water (100 mL). A bright-yellow
solid formed, which was collected by vacuum filtration and dried in
an oven at 100 °C. Purification was carried out by column chroma-
tography (silica gel; ethyl acetate/chloroform, 1:2), yield 94 %; m.p.
Eur. J. Org. Chem. 2016, 1772–1780
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Pyreno[5′,4′,9′,10′]bis-3-heptyl-[1,2,4]triazolo[4,3-b][1,2,4]tri-
azine (5): In this case, the formation of two different geometric
isomers is expected in the reaction (see Schemes 1 and 2). Purifica-
tion by column chromatography indeed gave two fractions: a less
polar one containing only one isomer and the second containing
both. All measurements were performed on the fraction containing
only one isomer. Yield for the isolated isomer: 25 %; m.p. 350 °C
Table 3. Crystallographic data for 1 and 2.
1
2
Formula
M
C21H21N5
343.43
C23H23N5
369.46
Pna21
22.088(5)
15.826(4)
10.826(3)
90
Space group
a [Å]
P1
¯
10.287(4)
10.435(8)
10.661(5)
110.85(3)
97.86(3)
116.35(3)
897.0(8)
2
b [Å]
c [Å]
α [°]
ꢀ [°]
1
(dec.); H NMR (CD2Cl2, 500 MHz, 25 °C): δ = 9.42 (d, J = 9.42 Hz, 2
H), 9.23 (d, J = 9.22 Hz, 2 H), 8.09–8.04 (m, 2 H), 3.49 (t, 2 H), 2.14
(m, 2 H), 1.63–1.38 (m, 8 H), 0.98 (t, 3 H) ppm. 13C NMR (100 MHz,
C2D2Cl4): δ = 153.7, 149.2, 147.0, 143.7, 139.5, 130.7, 129.8, 128.8,
128.1, 126.2, 31.5, 29.1, 28.8, 26.3, 24.3, 22.5, 14.1 ppm. MS (ESI, +):
m/z calcd. for C34H36N10 585.0; found 586.2 [M+·H], 608.1 [M+·Na+],
624.0 [M+·K+]. C34H36N10 (584.73): calcd. C 69.84, H 6.21, N 23.95;
found C 69.72, H 6.18, N 23.93.
90
90
γ [°]
V [Å3]
3784.4(17)
8
1.297
173(2)
0.080
Z
Density [g/cm3]
1.271
T [K]
296(2)
0.079
μ [mm–1
]
Computational Details: Computations were carried out at the den-
sity functional theory level, using the B3LYP functional and the 6-
311+G(d,p) basis set. Reduction potentials were computed from the
optimum geometries of the neutral and anionic species, including
solvent effects by means of the Polarizable Continuum Model
(PCM). The unrestricted formalism was used for the anionic species;
alkyl substituents were replaced by methyl groups. Solvent (di-
chloromethane) was included by using the Polarizable Continuum
Model (PCM). Excited-state energies were computed by using the
time dependent formalism (TDDFT) at the same level of computa-
tions using ground-state optimized geometries. All computations
were performed with the G09 package. Photoelectron spectra were
simulated by using the Kubo generating function approach, which
allows all normal modes of vibrations to be included in the compu-
tation of spectral band shapes, taking properly into account both
effects due to Duschinsky's mixing and the changes of vibrational
frequencies upon transition. All computations were carried out by
a development version of MolFC package;[35] full details can be
found in ref.[26]
Reflections collected
8128
3125 (0.0939)
0.0677
18456
7239 (0.0625)
0.0637
0.1317
Unique refl. (Rint
R1 [I > 2σ(I)]
)
wR2 (all data)
0.1514
187 °C (see the Supporting Information); 1H NMR (CD2Cl2, 200 MHz,
25 °C): δ = 8.51 (d, J = 8.49 Hz, 1 H), 8.39 (d, J = 8.38 Hz, 1 H), 8.31–
8.26 (m, 2 H), 8.01–7.92 (m, 2 H), 3.34 (t, 2 H), 2.17 (m, 2 H), 1.65–
1.37 (m, 8 H), 0.95 (m, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
154.9, 149.7, 149.4, 147.7, 138.8, 131.0, 130.3, 129.9, 129.1, 128.8
(shoulder), 128.7, 126.6, 123.3, 122.0, 31.6, 29.2, 28.8, 26.5, 24.3, 22.6,
14.0 ppm. MS (ESI, +): m/z calcd. for C21H21N5 343.4; found 366.2
[M+·Na+], 382.1 [M+·K+]. C21H21N5 (343.43): calcd. C 73.44, H 6.16, N
20.39; found C 73.29, H 6.01, N 20.32.
3-Heptylphenanthro[10′,9′-h][1,2,4]triazolo[4,3-b][1,2,4]triazine
(2): Yield 83 %; m.p. 144 °C (see the Supporting Information); 1H
NMR (CD2Cl2, 200 MHz, 25 °C): δ = 9.09 (d, J = 9.08 Hz, 1 H), 9.06
(d, J = 8.86 Hz, 1 H), 8.87 (d, J = 8.37 Hz, 2 H), 7.83–7.64 (m, 4 H),
3.43 (t, 2 H), 2.09 (m, 2 H), 1.71–1.39 (m, 8 H), 0.95 (m, 3 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 148.2, 146.9, 143.5, 138.8, 132.5,
132.3, 131.6, 131.5, 128.4, 128.1, 127.4, 126.7, 125.4, 124.5, 122.8,
122.6, 31.6, 29.2, 28.8, 26.2, 24.3, 22.5, 14.0 ppm. MS (ESI, +): m/z
calcd. for C23H23N5 369.4; found 370.3 [M + ·H+], 392.2 [M + ·Na+],
408.3 [M + ·K+]. C23H23N5 (369.47): calcd. C 74.77, H 6.27, N 18.96;
found C 74.59, H 6.12, N 18.89.
Acknowledgments
Thanks are due to the University of Naples “Federico II” and the
University of Salerno for RX, NMR, UV/Vis, ESI-MS and Elemental
Analysis facilities. The authors also thank the Ministero dell'Uni-
versità e della Ricerca (MIUR) (PON02_00556_3306937, RELIGHT
project) for financial support in the framework of the National
Operational Programme for Research and Competitiveness
2007–2013. R. C. thanks the COST Association for support and
critical discussions within the COST Action CM-1402-Crystallize.
3-Heptyl-1,10-phenanthrolino[5′,6′-h][1,2,4]triazolo[4,3-b]-
[1,2,4]triazine (3): Yield 85 %; m.p. 259 °C; 1H NMR (CD2Cl2,
200 MHz, 25 °C): δ = 9.35 (d, J = 9.34 Hz, 1 H), 9.32–9.16 (m, 5 H),
7.79–7.73 (m, 2 H), 3.65 (t, 2 H), 2.12 (m, 2 H), 1.39–1.36 (m, 8 H),
0.96 (m, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 154.6, 154.2,
149.6, 149.1, 149.0, 147.0, 142.8, 138.8, 134.5, 132.7, 126.0, 125.0,
124.7, 124.0, 31.6, 29.2, 28.8, 26.4, 24.4, 22.5, 14.0 ppm. MS (ESI, +):
m/z calcd. for C21H21N7 371.4; found 372.4 [M + ·H]. C21H21N7
(371.44): calcd. C 67.90, H 5.70, N 26.40; found C 67.72, H 5.55, N
26.33.
Keywords: Fused-ring systems · Nitrogen heterocycles ·
Crystal engineering · Cyclic voltammetry
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Received: October 5, 2015
Published Online: February 24, 2016
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