Novel 2-alkoxy- and 2-alkylthio-substituted pyrimidines containing 2-(1-methyl-1H-pyrrol-2-yl)vinyl moieties: optical and electrochemical properties

Article abstract of DOI:10.1016/j.mencom.2019.01.014

A series of novel Y-shaped dyes bearing the D–π–A–π–D fragment comprising central 2-alkoxy-or 2-alkylthio-substituted pyrimidine core and terminal 2-(1-methyl-1H-pyrrol-2-yl)vinyl moieties have been synthesized and studied as potential materials for the organic electronics. The obtained compounds demonstrated an efficient absorption at ～410 nm with the high molar absorption coefficient (29790–40620 dm³ mol^?1 cm^?1) and fluorescence emission at ～480 nm (Φ_F ～ 7.5–8.6%). Thin films of 2-ethyloxy- or 2-ethylthiopyrimidines exhibited the red-shifted photoluminescence (～600 nm).

Full text of DOI:10.1016/j.mencom.2019.01.014

Mendeleev
Communications
Mendeleev Commun., 2019, 29, 47–49
Novel 2-alkoxy- and 2-alkylthio-substituted pyrimidines
containing 2-(1-methyl-1H-pyrrol-2-yl)vinyl moieties:
optical and electrochemical properties
Ekaterina A. Komissarova,a,b Maksim V. Dmitriev,b Ivan G. Mokrushin,b Alexander N. Vasyanin,b
Igor V. Lunegov,b Elena V. Shklyaevab and Georgii G. Abashev*a,b
a Institute of Technical Chemistry, Ural Branch of the Russian Academy of Sciences, 614013 Perm,
Russian Federation. E-mail: gabashev@psu.ru
b Perm State University, 614990 Perm, Russian Federation
DOI: 10.1016/j.mencom.2019.01.014
C(18)
C(14)
A series of novel Y-shaped dyes bearing the D–p–A–p–D frag-
ment comprising central 2-alkoxy- or 2-alkylthio-substituted
pyrimidine core and terminal 2-(1-methyl-1H-pyrrol-2-yl)vinyl
moieties have been synthesized and studied as potential
materials for the organic electronics. The obtained compounds
demonstrated an efficient absorption at ~410 nm with the high
molar absorption coefficient (29790–40620 dm3 mol–1 cm–1)
and fluorescence emission at ~480 nm (FF ~ 7.5–8.6%). Thin
films of 2-ethyloxy- or 2-ethylthiopyrimidines exhibited the
red-shifted photoluminescence (~600 nm).
C(9)
N(1)
C(10)
N(4)
C(8)
C(7)
C(17)
C(16)
C(5)
C(3)
C(13)
C(12)
C(11)
C(15)
C(6)
C(4)
N(3) C(2)
N(2)
C(1)
N
N
S(1)
C(19)
N
N
Me
Me
C(20)
C(22)
XAlk
X = O, S
Alk = Et, n-hexyl
C(21)
C(24)
C(23)
Pyrimidine as a highly electron-deficient p-system is often incor-
porated into the structure of small push-pull chromophores,
oligomers, and polymers as their electron-withdrawing heterocyclic
fragment. Its integration with the p-electron-excessive moieties
through a p-conjugated bridge provides an efficient intramole-
cular charge transfer (ICT) upon excitation, which can in its turn
induce luminescence and nonlinear properties. Thus, pyrimidine-
containing chromophores of D–p–A–p–D type have found practical
applications as fluorescent probes for detection of Hg2+ and Cu2+
cations,1 as materials for pH sensors,2–4 OLEDs5 and OFET6
devices, and solar cells.7 Fluorescent pyrimidine derivatives are
potential probes for the two-photon microscopy.8 Photoluminescent
properties of pyrimidines are summarized in the comprehensive
review.8
In this work, target 2-alkoxy-4,6-bis[(E)-2-(1-methyl-1H-pyrrol-
2-yl)vinyl]pyrimidines 7, 8 and 2-alkylthio-4,6-bis[(E)-2-(1-methyl
-
1H-pyrrol-2-yl)vinyl]pyrimidines 9, 10 were prepared via an
easy-to-perform high yielding two-step process (Scheme 1). The
initial 2-hydroxy- and 2-mercapto-4,6-dimethylpyrimidine hydro-
chlorides 1 and 2 were synthesized via cyclization of acetylacetone
with urea or thiourea, respectively, in ethanol in the presence
of conc. HCl.11–13 Their consequent alkylation in the presence of
14
K2CO3 or NaOH15 (see Scheme 1) gave pyrimidines 3–6. The
key step of synthesis involved an aldol condensation between
2-substituted 4,6-dimethylpyrimidines 3–6 and N-methylpyrrole-
2-carboxaldehyde in boiling aqueous 5 m NaOH solution in the
presence of Aliquat 336 as the phase-transfer catalyst,16 which
resulted in the target perfectly stable D–p–A–p–D pyrimidines
7–10 soluble in common organic solvents. Their molecular
To continue our previous studies,9 here we report on the
synthesis and characterization of Y-shaped push-pull chromo-
phores containing the central electron-withdrawing pyrimidine
core and incorporating 2-ethoxy, 2-ethylthio, 2-hexyloxy or 2-hexyl-
thio groups and two 4,6-positioned bis(1-methyl-1H-pyrrol-2-yl)-
vinyl moieties. N-Methylpyrrole fragments have been embedded
into the structure of target chromophores as the heterocycles
possessing the powerful electron-donating ability similar to that
of primary organic functionalities such as amino, dialkylamino
and hydroxy groups.10
1
structures were characterized by H, 13C NMR spectroscopy
2 equiv.
CHO
N
Me
Me
Me
Me
i or ii
Me
iii
N
N
N
N
· HCl
XH
XAlk
1 X = O
3–6
2 X = S
2-Hydroxy- and 2-mercapto-4,6-dimethylpyrimidines hydro-
chlorides are generally considered as useful building blocks since
the two methyl groups are favorable as methylene components of
the Knoevenagel condensation providing an opportunity to prepare
symmetrical and asymmetrical chromophores of D–p–A–p-D
or D–p–A–p–D' types with the elongated p-conjugated chain.
On the other hand, the hydroxy and mercapto groups can be
readily alkylated by alkyl halides of various structure thereby
increasing the solubility of resulting chromophores in common
organic solvents.9(b)–(d)
N
N
N
N
Me
Me
3, 7 X = O, Alk = Et
4, 8 X = O, Alk = n-hexyl
5, 9 X = S, Alk = Et
XAlk
7–10
6, 10 X = S, Alk = n-hexyl
Scheme 1 Reagents and conditions: i, AlkBr, K2CO3, DMF, reflux, 5 h;
ii, AlkBr, NaOH, DMSO, stirring, 48 h; iii, Aliquat 336, NaOH (5 m),
reflux, 6 h.
© 2019 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 47 –

Mendeleev Commun., 2019, 29, 47–49
Table 1 Calculated absorption maxima of S0 ® S1 excitations of pyrimidines
C(9)
C(18)
N(1)
7–10 obtained from the TDDFT//PBE0-D3/def2-TZVPD calculations in gas
C(8)
C(7)
phase.
C(17)
C(16)
C(10)
C(14)
Oscillator
strength
N(4)
C(5)
Compound
l
max/nm
E/eV Major contribution
C(3)
C(13)
C(12)
C(11)
C(6)
C(15)
C(4)
N(3)
7
8
406.4
406.1
411.1
410.7
1.2405
1.2312
1.1308
1.1182
3.051 HOMO ® LUMO (99%)
3.053 HOMO ® LUMO (99%)
3.016 HOMO ® LUMO (97%)
3.019 HOMO ® LUMO (97%)
C(2)
N(2)
9
C(1)
S(1)
10
C(19)
C(20)
C(21)
be about 7.5–8.6%. The optical band gap (Egopt) averaged 2.65 eV
(see Table S1).
C(22)
C(24)
It was interesting to compare the optical characteristics
of newly synthesized pyrimidines 7–10 and those of reported
4,6-bis[(E)-2-(1-methyl-1H-pyrrol-2-yl)vinyl]pyrimidine.2 The
main difference between these compounds is the absence of a
2-positioned substituent in the pyrimidine ring of the latter.
The data on absorption and emission of THF solutions of these
compounds are very close (see Table S1). The data on absorption
and emission of 4,6-bis[(E)-2-(1-methyl-1H-pyrrol-2-yl)vinyl]-
pyrimidine in CH2Cl2 solutions (see Table S1) are rather close to
the values of optical characteristics acquired for the compounds
obtained in the present work. Some decrease in the fluorescence
quantum yields is the result of strong electron donating effect of
alkyloxy and alkylmercapto groups, which partially diminishes
the electron deficiency of a pyrimidine core and, as a result,
reduces the efficiency of ICT process. At the same time, the
presence of aliphatic tails at C2 position allows these dyes to form
aggregates, and, consequently, the properties of an excited state
become different.
Intense long wavelength absorption maxima in the UV spectra
of films obtained from chlorobenzene solutions of pyrimidines
7–10 appear in the region of 395–416 nm, thus undergoing a
very small hypsochromic shift (~6 nm) towards those of the cor-
responding pyrimidine solutions in THF. On the contrary, the
values of absorption edges of these thin films are definitely red-
shifted (~35–60 nm). The investigation of fluorescence spectra of
the films has revealed that the pyrimidines containing hexyloxy or
hexylthio groups (compounds 8 and 10) do not fluoresce, while
the pyrimidines bearing ethyloxy or ethylthio moieties (7 and 9)
have demonstrated emission in the 600 nm range. The absence
of fluorescence in the case of pyrimidines 8 and 10 containing
the long-chain substituents is an example of aggregation-caused
quenching of fluorescence (ACQ effect).17 The formation of
aggregates in solid state was also confirmed by single crystal
X-ray analysis of compound 10.
The electrochemical behavior of the synthesized chromophores
containing an electroactive N-methylpyrrole units in their structure
is of special interest. The presence of free C5 and C5' positions
in these heterocyclic moieties allows them to form substituted
bispyrrole structures as a result of electrochemical oxidation (its
mechanism has been described in detail18). The cyclic volt-
ammetry method (CV) was used to obtain the electrochemical
characteristics of the prepared compounds and to estimate the
values of their HOMO/LUMO energy levels and electrochemical
band gaps (Egel). The obtained data and experimental details are
given in Online Supplementary Materials and also summarized
in Table 2.
The CV curves of pyrimidines 7–10 (Figures S12 and S13,
see Online Supplementary Materials) comprise two reversible
oxidation waves derived from the redox processes in pyrrole
fragments of these compounds. The CV curve shapes as well as
the values of redox potentials appeared to be very similar within
this group of pyrimidines (7–10). At the same time, it has been
found that the HOMO levels of pyrimidines 7 and 9 (Alk = Et)
C(23)
Figure 1 Molecular structure of compound 10.
and elemental analysis.† The structure of compound 10 was
additionally confirmed by X-ray analysis (Figures 1, S11, see
Online Supplementary Materials).‡
Optical properties of pyrimidines 7–10 were investigated
by UV-VIS absorption and photoluminescence spectroscopy
(Figures S18–S20, Table S1). THF was chosen as the solvent in
order to avoid the influence of acid traces, which are sometimes
present in chloroform usually used for spectroscopic measure-
ments. All compounds appeared to be photostable and did not
undergo cis–trans-isomerization under the conditions of spectral
analysis. Each of the prepared compounds demonstrated three
absorption bands: two weak ones in the region of 243–316 nm
(e = 6540–11890 dm3 mol–1 cm–1) and an intensive one about
410–415 nm (e = 29790–40620 dm3 mol–1 cm–1). The first band
centered at 243 nm is ascribed to the aromatic p–p* electronic
transitions in pyrimidines, the second, less intense, band centered
at 285 nm corresponds to the n–p* electronic transitions involving
lone pair of a nitrogen atom. The long-wavelength absorption
bands (~410 nm) are the result of the intramolecular charge-
transfer process occurring in the photo excited compounds; the
large values of their molar absorption coefficients confirm the
high efficiency ICT from the electron excessive N-methylpyrrol
moiety to the electron-deficient pyrimidine cycle.
The obtained data have confirmed the fact that the meta-
arrangement of substituents prevents an effective delocalization
of the electron density along the conjugated chain. Consequently,
the compounds absorb light in the UV and visible region. TDDFT
calculations performed to examine the vertical singlet excitations
at PBE0-D3/def2-TZVPD level in gas phase revealed that the
strongest band in the absorption spectra of pyrimidines 7–10
correspond to S1 state with the main contribution of HOMO to
LUMO (Figure S23). The calculated absorption maxima are in
a good agreement with the experimental values obtained for
THF solutions (Table 1). Absorption maxima of 2-alkylthio-
substituted pyrimidines 9, 10 are slightly red-shifted vs. those
of their alkoxy analogues 7, 8. The same effect was observed in
the emission spectra of these compounds: the emission maxima
of alkylthio derivatives 9, 10 are slightly red-shifted as well. The
fluorescence quantum yields (FF) were estimated and found to
†
For NMR and elemental analysis data, see Online Supplementary Materials.
Crystal data for 10: C24H30N4S, M = 406.58, orthorhombic, space group
‡
Pca21, a = 15.847(5), b = 16.217(5) and c = 8.899(3) Å, V = 2286.9(12) Å3,
T = 295(2) K, Z = 4, m(MoKa) = 0.158 mm–1. The final refinement
parameters: R1 = 0.0593, wR2 = 0.1487 [for observed 2498 reflections
with I > 2s(I)]; R1 = 0.0907, wR2 = 0.1711 (for all independent 3801
reflections), S = 0.979. Largest diff. peak/hole: 0.190/–0.225 eÅ–3.
CCDC 1844962 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk.
– 48 –

Mendeleev Commun., 2019, 29, 47–49
Table 2 Comparison of empirical and calculated values of EHOMO/ELUMO
and Egel for pyrimidines 7–10.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2019.01.014.
EHOMO
/
ELUMO
/
Egel/
EHOMO
/
ELUMO
/
Eg/
Compound
eVa
eVa
eVb
eVc
eVc
eVc
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–2.00
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–1.5
–1.93
–1.94
–1.99
–2.00
LUMO (calc)
LUMO (exp)
–2.5
–3.5
–4.5
–5.5
–3.54
–3.54
–3.56
–3.59
–5.35
–5.55
–5.36
–5.51
–5.37
–5.51
HOMO (exp)
HOMO (calc)
–5.40
–5.56
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10
Figure 2 Frontier molecular orbital energy level diagram of pyrimidines
7–10.
Received: 12th July 2018; Com. 18/5641
– 49 –