A new route to the synthesis of near-infrared absorbing pyrazinopyrazine bridged dyes with intramolecular charge transfer character

Article abstract of DOI:10.1039/c4cc01084a

A simple and versatile procedure was developed for the synthesis of pyrazino[2,3-b]thieno[3,4-e]pyrazine derivatives with intramolecular charge transfer character. The compounds obtained exhibited a narrower band gap compared to those with one pyrazine bridge. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01084a

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A new route to the synthesis of near-infrared
absorbing pyrazinopyrazine bridged dyes with
intramolecular charge transfer character†
Cite this: Chem. Commun., 2014,
50, 4245
Received 11th February 2014,
Accepted 4th March 2014
Liang Zhang, Kin Cheung Lo and Wai Kin Chan*
DOI: 10.1039/c4cc01084a
www.rsc.org/chemcomm
A simple and versatile procedure was developed for the synthesis of
The key intermediate in the synthesis of PTPs is a diamino
pyrazino[2,3-b]thieno[3,4-e]pyrazine derivatives with intramolecular substituted thienopyrazine (1a–b). The synthesis of a similar
charge transfer character. The compounds obtained exhibited a 2,3-diaminoquinoxaline analogue was reported in the literature,13 but
narrower band gap compared to those with one pyrazine bridge.
it involves a three-step process in which the diamine first reacts with
oxalic acid, which is followed by the reaction with SOCl2 and then
Quinoxaline and thieno[3,4-b]pyrazine derivatives with donor/ with ammonia gas under high pressure (60 atm). Another approach
acceptor units have attracted tremendous interest in recent involves a reaction between dimethyl oximidate and 1,2-diamino-
years due to their narrow bandgap and potential applications in benzene in the presence of an acid catalyst.14 However, the oximidate is
organic photovoltaic cells.1–4 These molecules exhibited intra- not stable in air and at high temperature, and the reaction yield is low.
molecular charge transfer characters, resulting in low electronic
We have developed a new bis(2,2,2-trifluoroethyl) oximidate 3
transition energy. The pyrazine moieties served as a bridge between by which a diaminopyrazine unit can be constructed from the
the electron donor and acceptor. In this work, we attempted to corresponding 1,2-diamine in one step. Compared with dimethyl
synthesize a new series of heterocyclic donor–acceptor type molecules oximidate, trifluoroethyl oximidate is more stable and can be
by further extending the pyrazine bridging unit between the donor stored in the refrigerator for months. In addition, it has higher
and acceptor systems. The target molecules are pyrazino[2,3-b] reactivity towards 1,2-diamines. 3 was synthesized by a modified
pyrazine derivatives, which exhibit absorption in the visible-NIR region. procedure15 (see ESI†) in which cyanogen gas was bubbled into a
According to the known literature procedure, the synthesis of this sodium trifluoroethoxide solution in trifluoroethanol at ꢀ50 1C.
type of molecules can be achieved by the condensation between
Reaction between 2a–b and 3 in refluxing dioxane (Scheme 1)
1,2-diketones and 5,6-diamino-2,3-dicyanopyrazine5,6 or quinoxaline- yielded 1a–b in moderate yield (65–84%). By the same method,
2,3-diamine, or by the coupling of 1,2-diaminobenzene with 2,3-diamino-quinoxaline could also be obtained from 1,2-diamino-
5,6-dichloro-2,3-dicyanopyrazine, which is then followed by oxidation benzene in one step. Compound 1b may undergo simple condensation
with DDQ or PbO2.7–10 It was also reported that tetraazapentacene and with various aromatic 1,2-diketones, yielding various pyrazinopyrazine
hexacene derivatives could be obtained by a palladium catalyzed derivatives (Scheme 2). Compounds 4a–b were obtained from
coupling reaction between an aromatic diamine and 1,2-dichloro- 2,20-bipyridine substituted diketone, and may be used as ligands
quinoxaline followed by oxidation with MnO2.11,12 To the best of our for various transition metal complexes. It has been demonstrated
knowledge, pyrazino[2,3-b]thieno[3,4-e]pyrazine (PTP) and its derivatives that the presence of transition metal complexes could strongly
have not been reported. We herein report a new and simplified perturb the ICT transition in some functional ligands, resulting
synthetic procedure for PTPs, which allows us to synthesize a variety
of new low bandgap molecules/polymers that contain pyrazino[2,3-b]
pyrazine as the bridging unit. The molecules obtained showed a further
red shift in optical absorption compared to the thienopyrazine
analogues, and they have promising potential in low bandgap
photosensitizers or new type of charge transport materials.
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong, China. E-mail: waichan@hku.hk; Fax: +852 2857 1586; Tel: +852 2859 8943
† Electronic supplementary information (ESI) available: TDDFT calculation
results, solvatochromism data, cyclic voltammetry results, and experimental
Scheme 1 (i) ꢀ50 1C/trifluoroethanol. (ii) 3/dioxane/reflux, 5 h.
procedures. See DOI: 10.1039/c4cc01084a
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Table 1 Solvatochromic properties of 5 and 6 in different solvents
lmax (nm)
Solvent
5
6
Toluene
DCM
THF
400, 893
398, 913
391, 846
357, 594
365, 593
354, 587
Ethyl acetate
Acetone
Acetonitrile
DMF
390, 868
388, 850
387, 857
392, 841
397 (0.980; 36 090)
870 (0.183; 2360)
398 (0.991; 36 990)
859 (0.179; 2330)
353, 580
352, 581
350, 579
355, 588
345 (0.649; 37 810)
584 (0.363; 5360)
343 (0.690; 34 860)
578 (0.358; 5450)
Toluenea
DMFa
a
Results obtained from TDDFT calculations. The first number in the
parenthesis is the oscillator strength f, and the second number is the
measured extinction coefficient e (in L molꢀ1 cmꢀ1).
Scheme 2 Synthesis of PTP derivatives. (iii) Ethanol/p-toluenesulfonic
acid (0.1 equiv.)/reflux, 12 h.
much more significant than that in 6, suggesting that the electron
density redistribution upon excitation is stronger in 5 than that in 6.
To gain more insight into the electronic transitions of the molecules,
the molecular orbitals of 5 and 6 were computed by TDDFT calcula-
tions using the CAM-B3LYP Coulomb-attenuated functional18 and the
6-311G(d,p) basis set. The spatial plots of the HOMOs and LUMOs of 5
and 6 are shown in Fig. 2. The HOMOs of both molecules are largely
localized at the terthiophene unit, while the LUMOs are mainly located
at the pyrazine (for 6) and pyrazinopyrazine bridge (for 5). In 6, although
the electronic transition is mainly from the terthiophene to the pyrazine
ring, the electron density spreads throughout the whole molecule in
both the HOMO and the LUMO, resulting in no significant change in
electron density on the phenylene rings. On the other hand, in 5, all
parts of the molecule participate in the electron redistribution in a
larger scale. In the HOMO, most of the electron density is located on
the terthiophene ring, with almost no contribution from the phenylene
rings. However, in the LUMO, most of the electron density is located on
the pyrazinopyrazine ring, and some even reaches the phenylene rings.
Compared to 6, the larger electron redistribution of 5 causes the excited
state to be very different from the ground state, resulting in a much
stronger solvatochromism.
in a further red-shift in the absorption band.16,17 Compound 5
was used as a model compound for the investigation of detailed
optical and electrochemical properties. The molecules obtained
may form the skeleton of low bandgap molecules/complexes
with potential applications in optoelectronics.
The UV-vis absorption spectrum of 5 measured in CH2Cl2 is
shown in Fig. 1. An intense absorption peak centered at 398 nm
(3.1 eV) and a very broad absorption peak at 913 nm (1.4 eV) are
observed, and the absorption tail extends beyond 1 mm. The absorp-
tion spectrum of model compound 6 with one pyrazine bridge is also
shown for comparison. It can be seen that both absorption peaks of 5
show significant red shift compared to those of 6 (lmax at 365 and
593 nm, corresponding to a shift of 0.3 and 0.7 eV for the high and
low energy transitions, respectively). The absorption spectra of 5 were
measured in different solvents (see Fig. S2, ESI†), and the results are
summarized in Table 1. A negative solvatochromism was observed,
and the effect of changing the polarity of solvents was more
significant for the lower energy transition. This is assigned to
be intramolecular charge transfer (ICT) transition. The hypso-
chromic shifts indicate that the excited states are less polar
than the ground states for both molecules, and such shift in 5 is
To further assess the solvatochromism, the polarizable continuum
model19 was applied using toluene and DMF as the solvent models.
The calculated absorption peaks, the corresponding oscillator strengths
( f ) and measured molar extinction coefficients (e) are shown in
Table 1. The details of the TDDFT results are shown in Tables S1–S6
(ESI†). In Table S1 (ESI†), for compound 5, the TDDFT results only
qualitatively match the experimental results and the solvatochromic
effect of the two absorptions bands was underestimated. On the other
hand, for compound 6, the TDDFT results agree very well with the
experimental data, with a difference of less than 0.02 eV. Nevertheless,
for both compounds 5 and 6, both TDDFT and experimental results
clearly indicate a stronger solvatochromic effect in the low energy
absorption band. Regarding the intensity of the absorption bands, the
low energy absorption bands are less intense than the high energy ones
for both 5 and 6. As shown in Fig. S1 and Tables S2 and S3 (ESI†), for
6, the absorption band at 357 nm corresponds to two electronic
transitions, in which the major component is the ICT transition
from the terthiophene ring to the pyrazine/phenylene rings, whereas
Fig. 1 UV-vis absorption spectra of 5 and 6 (1.5 ꢁ 10ꢀ5 M) in CH2Cl2.
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which is common in both molecules. On the other hand, although
both compounds exhibit a reversible reduction process, the reduction
potentials are quite different (E1/2 = ꢀ0.87 V for 5 and ꢀ0.95 V for 6).
These are assigned to the reduction of pyrazinopyrazine (for 5) and
pyrazine (for 6) moieties. In addition, 5 exhibits another irreversible
reduction process at a slightly higher potential (ca. ꢀ1.1 V), but such a
process is observed at ca. ꢀ1.5 V in 6. This may be due to the further
reduction of the N-heterocyclic bridging units. From these results, it
can be concluded that the lowering of the bandgap in 5 compared to
that in 6 is due to the pyrazinopyrazine bridge.
In conclusion, we have demonstrated a new and versatile synthetic
methodology for the synthesis of aromatic polycyclic molecules with
interesting optical and electronic properties. By this approach, mole-
cules with donor–acceptor units bridged by pyrazinopyrazine can be
synthesized easily by the reaction between derivatives of 1b and
various substituted 1,2-diketones. It can also be further extended to
the synthesis of functional polymeric materials.
The work reported in this communication was supported by the
Research Grants Council of the Hong Kong Special Administrative
Region, China (HKU700311P, 700613P and T23-713/11). Partial
financial support from the Hong Kong/Macao Scholar Collaborative
Research Scheme, NSFC China (project no. 51328302) is also
acknowledged. The computational work was conducted in part
using the HKU Information Technology Services computing facilities
that are supported in part by the Hong Kong UGC Special
Equipment Grant (SEG HKU09).
Fig. 2 Spatial plots (isovalue = 0.02) of the HOMO and LUMO of (a) 5 and
(b) 6 in toluene from TDDFT calculations.
Table 2 HOMO and LUMO levels measured by cyclic voltammetry and
estimated from DFT calculations for 5 and 6
a
HOMO
(eV)
LUMO
(eV)
Eg
HOMOcalc
(eV)
LUMOcalc
(eV)
Compound
(eV)
Notes and references
5
6
ꢀ5.13
ꢀ5.08
ꢀ4.03
ꢀ3.47
1.10
1.61
ꢀ5.12
ꢀ5.20
ꢀ3.66
ꢀ3.02
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This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 4245--4247 | 4247