2,2’:6’,2’’-Terpyridine derivative with tetrazole motif and its analogues with 2-pyrazinyl or 2-thiazolyl substituents – Experimental and theoretical investigations

Article abstract of DOI:10.1016/j.molstruc.2019.127669

The synthesis of 4’-[4-(2-decyl-1,3,4-tetrazol-5-yl)phenyl]-2,2’:6’,2’’-terpyridine and its analogues which contain substituted at the central pyridine ring (positions 2 and 6) 2-pyrazinyl or 2-thiazolyl groups is reported. Photophysical, optical studies

Full text of DOI:10.1016/j.molstruc.2019.127669

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2,2':6′,2″-Terpyridine derivative with tetrazole motif and its analogues with 2-pyrazinyl
or 2-thiazolyl substituents – Experimental and theoretical investigations
Dawid Zych, Aneta Slodek, Jan Grzegorz Małecki
PII:
S0022-2860(19)31778-8
DOI:
https://doi.org/10.1016/j.molstruc.2019.127669
MOLSTR 127669
Reference:
To appear in: Journal of Molecular Structure
Received Date: 27 November 2019
Revised Date: 20 December 2019
Accepted Date: 28 December 2019
Please cite this article as: D. Zych, A. Slodek, J.G. Małecki, 2,2':6′,2″-Terpyridine derivative with
tetrazole motif and its analogues with 2-pyrazinyl or 2-thiazolyl substituents – Experimental and
theoretical investigations, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2019.127669.
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2,2':6',2''-Terpyridine derivative with tetrazole motif and its
analogues with 2-pyrazinyl or 2-thiazolyl substituents –
experimental and theoretical investigations
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Dawid Zych[a]*, Aneta Slodek[b], Jan Grzegorz Małecki[b]
[a] Institut für Silizium-Photovoltaik, Helmholtz-Zentrum Berlin für Materialien und Energie
GmbH, Kekuléstraße 5, 12489 Berlin, Germany
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[b] Institute of Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland
*Corresponding Author: dawidzych92@gmail.com (Dr. Dawid Zych)
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Graphical abstract
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Abstract
The synthesis of 4'-[4-(2-decyl-1,3,4-tetrazol-5-yl)phenyl]-2,2':6',2''-terpyridine and its
analogues which contain substituted at the central pyridine ring (positions 2 and 6) 2-
pyrazinyl or 2-thiazolyl groups is reported. Photophysical, optical studies and theoretical
calculations analysis with particular emphasis on the difference caused by the change of
substituents is presented.
Keywords: pyridine derivatives; tetrazole motif; condensation; cycloaddition; photophysical
properties; DFT studies
Introduction
The tetrazole is a well-known and commonly used motif in compounds dedicated to
various aspects of science and life [1]. The interest of this five-membered aromatic
heterocycle is visible in an increasing number of researches during the last years due to,
among others, its bioisosterism to carboxyl and amide group [2,3]. The most significant area

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of tetrazole investigation can be found in a broadly defined medicine [4–6] followed by
coordination chemistry [7,8] and materials chemistry [9].
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In the area of 2,2':6',2''-terpyridines with a substituent at the 4' position, there is a lot of the
investigations what already has been covered extensively by scientific literature [10–12].
During last years, scientists have become interested in the isomeric or analogues of 2,6-bis(2-
pyridyl)pyridine (2,2':6',2''-terpyridine) which contain substituted other heteroaryls at
positions 2 and 6 of pyridine [13–17]. They are known to exhibit attractive optical and
electronic properties, as well as, their complexes exhibit antitumor activity [18–20].
The systems terpyridine-tetrazole composed of a tetrazole functional group conjugated to
a terpyridine moiety by phenyl linker have been described first time by G.-C. Guo and co-
workers in 2016, where the tetrazole motif is unsubstituted by any additional groups [21]. The
authors reported an azole-based metal-organic framework toward direct white-light emissions,
where as metal cadmium was used. The crystal structure of ligand was described and also
photophysical properties were studied, 4-(tetrazol-5-yl)phenyl-2,2':6',2''-terpyridine reveals
intense absorption in the solid-state in the range of 200-400 nm and weak absorption in the
range of 400-750 nm, the absorption in the methanol solution shows the intense band at
around 300 nm. In the case of emission in the methanol solution, the maximum is observed at
415 nm while the emission in the solid-state is shifted up to 567 nm.
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In 2017, M. Massi et al. published four terpyridine functionalized by protonated or
methylated tetrazoles spaced by phenyl or pyridyl ring [22]. Authors noticed significant
decreasing quantum yields in the case where the tetrazole moiety is methylated in comparison
to protonated. Reported molecules were studied as sensors for selected cations. Moreover,
they were investigated as sensitizers for lanthanide luminescence. They proofed that
terpyridine-functionalized tetrazole systems are worthy of investigation due to the useful
luminescence properties.
Y.-Q. Wang successfully used 4-(tetrazol-5-yl)phenyl-2,2':6',2''-terpyridine analog i.e. 4-
(tetrazol-5-yl)phenyl-4,2':6',4''-terpyridine in Zn(II)-MOF synthesis which was tested as
multi-responsive luminescent sensor for the quantitative detection of antifungal agrochemical
2,6-dichloro-4-nitroaniline in methanol, and also ions such as Fe3+, CrO42- or Cr2O72- in water
[23].
Taking into account above described consideration, we present the synthesis and study of
the photophysical properties of 4'-[4-(2-decyl-1,3,4-tetrazol-5-yl)phenyl]-2,2':6',2''-terpyridine
and its analogues which contain substituted at the central pyridine ring at position 2 and 6

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heteroaryls 2-pyrazinyl or 2-thiazolyl with particular emphasis on the difference caused by the
change of substituent.
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Results and discussion
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The synthetic route for obtaining the target molecules 3A-3C is presented in Scheme 1. As
a starting material 4-cyanobenzaldehyde was used in aldol condensation with appropriate
acetyl derivatives resulting in 1A-3C with satisfactory yields (40-45%). Obtained 4-(4-
cyanophenyl)pyridine derivatives were used in the cycloaddition reaction [3+2] with sodium
azide followed by the alkylation with decyl bromide based on the previously reported method
what resulted in the target compounds 3A-3C with moderate yields (28-34%) [24,25]. The
structures of compounds were confirmed by 1H and 13C NMR, mass spectrometry HRMS, and
also in the case of 3C XRC analysis (Figure 1, Table S1). The detailed synthesis procedures
with confirmation of structures are presented in Supplementary Information (Figure S1-S14).
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C
H
10 21
H
N
N
N
N
A
N
N
N
N
N
CN
CN
N
O
c
b
B
C
a
R =
+
N
S
N
R
CH
3
O
R
N
2
R
R
N
R
R
N
R
1 (Y=40-45%)
3 (Y=28-34%)
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Scheme 1. Synthetic route to obtain target molecules. Reagents and reaction conditions:
a) KOH, EtOH, NH3(aq), room temp., 24 h; b) NaN3, NH4Cl, DMF, 120 °C, 12 h; c) decyl
bromide, K2CO3, DMF, 70 °C, 16 h.
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Figure 1. Perspective views of the molecular structure of 3C with the atom numbering.
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Displacement ellipsoids are drawn at the 50% probability level.

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In order to gain a deep insight into the structure of obtained compounds, the theoretical
calculations based on DFT method with various exchange-correlation potentials (B3LYP,
CAM-B3LYP, M06, wb97XD) with 6-31G(d,p) basis set were conducted (Table S2). The
highest value of the calculated energy gap was achieved by molecule 3A and the lowest for
3C. Comparing the theoretical energy gaps with experimental values (Table 3), the results
obtained with B3LYP exchange-correlation potential differ about 0.98-1.26 eV whereas in the
case of wb97XD the values are even doubled. The B3LYP/6-31G(d,p) optimized structures
with contours of orbitals (HOMO-1, HOMO, LUMO, LUMO+1) are depicted in Figure 2 and
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Table S3. The contribution of
a
particular part of molecules 3A-3C
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(tetrazole/Ph/pyridine/substituent) in the creation of selected orbitals is presented in Figure 3.
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Figure 2. B3LYP/6-31G(d,p) optimized structures with contours of selected orbitals.
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Figure 3. Contribution of tetrazole/C6H4/pyridine/substituent in creation of selected orbitals.

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The meaningful differences between compounds 3A, 3B vs. 3C were observed. In the
case of the first subgroup (3A and 3B), the contribution of 2-pyrazinyl in the creation of
unoccupied molecular orbitals (LUMO (15 vs. 34 %) and LUMO+1 (44 vs. 58 %)) is
significantly higher in comparison to 2-pyridyl. The highest molecular orbitals are built in 3%
from 2-pyridyl and 1% from 2-pyrazinyl which proves the lower electron-donating character
of pyrazine compared to pyridine. The contribution of the rest parts of molecules 3A and 3B
takes part in the creation of HOMO at a similar rate, only 2% higher for tetrazole, phenyl and
pyridine in the case of 3B. Comparison of 3A and 3B with molecules 3C containing 2-
thiazolyl substituent revealed the definitely stronger electron-donating character of thiazol,
HOMO is created in 66% by substituents, whereas percentage value of their participation in
unoccupied orbitals is between above-mentioned values in the case of 3A-3B (LUMO (24 %)
and LUMO+1 (49 %)). The participation of tetrazole motif for molecules 3A and 3B is the
highest in HOMO, i.e. 26 and 28%, respectively, whereas for 3C in HOMO-1 (28%).
Moreover, due to the absence of crystallographically determined structures of 3A and 3B,
the theoretically calculated angles between the planes: pyridine – phenyl (Θ1) and pyridine –
substituent (Θ2) were determined (Table 1). It be noticed that the angles between plane of
pyridine and plane of phenyl (Θ1) are similar (≈35°) 3A>3B>3C, whereas the angles between
planes of pyridine and substituent (Θ2) differ significantly, the highest value was observed for
3A (35.14°) followed by 3B (33.95°) whereas the same angle in the case of 2-thiazolyl
substituent equals 17.66° what is lower about 17° in comparison to 3A-3B. Based on the
crystallographic data the Θ1 and Θ2 angles at 3C are 21.18° (8.16°) and 1.66° (1.38°), 4.16°
(9.08°), respectively (in parenthesis are given values for the second molecule in asymmetric
unit). The results of theoretical calculations showed significant differences between target
compounds what will have a direct influence on the photophysical properties.
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Table 1. DFT calculated (B3LYP/6-31G(d,p)) angles Θ1 and Θ2.
Θ2
R
Θ1
H
C
21 10
N
N
N
N
N
R
Θ1 [°]
35.20
35.09
34.85
Θ2 [°]
35.14
33.95
17.66
3A
3B
3C
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As the first, the thermal properties, as the parameter which is almost unpredictable, of
obtained compounds 3A-3C have been examined by using the thermogravimetric analysis
(TGA) in the range of temperature to 900 °C under a nitrogen atmosphere. Obtained results
showed that within the studied group of compounds, the change of the pyridine substituents
does not have a significant influence on the thermal stability of molecules (Table 2, Figure
S15). The temperatures of 5% decomposition are very similar (T5%=282-296 °C). In the case
of T10%, the highest value was observed for 3B (348 °C), the lowest for 3C (297 °C). The char
residue at 900 °C was the highest also for 3B (29%) and the lowest for 3C (12%). It can be
concluded that among the studied group of compounds, the molecule 3B is the most thermally
stable.
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Table 2. Thermal properties of target molecules 3A-3C.
TGA
Char residue
at 900°C [%]
T5% [°C]
T10% [°C]
Tmax [°C]
287
296
282
300
348
297
295, 503,
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3A
3B
3C
295, 384, 489, 706
300, 378, 492
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(a)
(b)
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Figure 4. (a) Absorption and (b) emission spectra recorded in the chloroform solution
(c = 10–5 mol/L) of 3A-3C.
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Table 3. Photophysical data for 3A-3C recorded in the chloroform solution.
Eg opt Eg opt
PL λem [nm] /
Φ
χ2
[eV]
em
[eV]
abs
λmax [nm]
τ [ns] (weight %)
Stokes shift [cm-1] [%]
351/3357,
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240, 314
2.99
0.984 3.43
3.60
3A
361/4147
378/3485,
392/4430
2.05 [0.22 (46.16)
3.61 (53.84)]
243, 311, 334
239, 310, 332
1
1.136 3.17
0.997 3.22
3.30
3.33
3B
3C
385/4146
15
1.66
Eg opt= 1241/λem
.
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The novel compounds 3A-3C demonstrate comparable absorption behaviours with three
bands in the range of 220 - 350 nm (Figure 4a). The transition bands at 240 and 310 nm can
be associated with the π–π* transition within the conjugated system whereas the peaks at
343 nm for compounds 3B and 3C can be ascribed to intramolecular charge transfer (ICT)
from the donor to acceptor units confirmed by the distribution of electron density of their
HOMOs and LUMOs. In sharp contrast to the compound 3A in which the pyridyl substituents
participate in neither in HOMO nor LUMO, the substituents in 3B (pyrazinyl and phenyl
rings) and 3C (thiazolyl rings) take an essential part in the stabilization the HOMO and
LUMO (Figure 3). It can be concluded that 3A gives the π–π* transition along with the charge
transfer, whereas compounds 3B and in particular 3C have the π–π* transition along with the
intramolecular charge-transfer character. It is worth paying attention to the analysis of the
dipole moment and the planarity of molecules which are also connected with the charge
transfer in molecules. The theoretical results demonstrate that the dipole moment in the
ground state calculated in chloroform medium among the studied compounds is the highest
for compound 3C (6.64 D) suggesting significant charge separation and thus effective ITC
transition (Tables S4). In addition, the most planar structure of 3C (Table 1) verifies the more
effective charge separation in 3C in relation to the other compounds. Comparison of the
maximum absorption of 3A-C compounds with the described 4'-(1-substituted-2,3-triazol-4-
yl)phenyl-2,2':6',2''-terpyridine derivatives shows that the novel compounds possess maxima
absorption shifted around 20 nm to longer wavelengths with broader absorption spectra [11].
To gain a deeper insight into electronic properties TD-DFT calculations using 4 various
functionals: B3LYP, CAM-B3LYP, wB97XD, M06 (Table S5) were performed. Among
employed functionals, both B3LYP and M06 estimated the calculated absorption maxima of
3A-3C according to experimental data. The obtained absorption maxima with the other
functionals CAM-B3LYP and wB97XD significantly deviated about 50 nm from the
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experimental data. A blue shifting in calculated absorption maxima (using both B3LYP and
M06) of 3A-3C compared to these in solution is observed and these shifts are about 20 nm.
The highly energetic absorption bands of 3A-3C possess much lower oscillator strength than
bands in low energy region (Table S7) however the intensity of highly energetic absorption
bands of 3A-3C follows the same trends as that obtained in solution increasing in the order
3C→3B→3A.
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The electronic character of substituents in compounds 3A-3C reflects strongly on their
fluorescent behaviour contrary to absorption spectra (Figure 4b). Concerning the emission
spectra, the emission maxima of 3B with the most electron-deficient (pyrazinyl) and 3C with
the most electron-rich substituents (thiazolyl) among discussed compounds are red-shifted by
31 and 24 nm, respectively, compared to the emission maximum of 3A bearing the pyridine
units. The emission peaks of 3A-3C are considerably red shifted in the following order
3A→3C→3B, analogous spectral behaviours were found for the chromophores with the same
substituents [17]. The emission spectra of 3A-3C simulated by the TD-DFT calculation
exhibit consistency with experimental data when functional B3LYP and M06 were applied
similarly to simulated absorption spectra (Figure S6). The emission maxima of 3A-3C
demonstrate the same trend in shifting to longer wavelengths, however, these shifts are not as
significant as compared to experimental data. The fluorescence decays of 3A-3C (τ = 1.66–
2.99 ns) were satisfactorily fitted with mono (3A and 3C) pointing out the emission from a
single excited state and bi-exponential (3B) functions. Furthermore, the fluorescence quantum
yield varies from 1% (3B) through 11% (3A) to 15% (3C). The presence of electron-donating
2-thiazolyl substituents in 3C, as well as its planarity, causes pronounced red-shifted and
structureless emission band with relatively high quantum yield. It is worth noting that the
obtained values of fluorescence quantum yield of 3A-3C are much higher than those shown
for 2,2':6',2'' terpyridine, 2,6 di(thiazol 2 yl)pyridine, and 2,6 di(pyrazin 2 yl)pyridine
derivatives (Φ= 0.14-0.93%) [17].
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Furthermore, the druglikeness of all obtained compounds was predicted based on the
computational calculations of ADME parameters (absorption, distribution, metabolism, and
excretion), pharmacokinetic properties, druglike nature and medicinal chemistry friendliness.
Obtained data are presented in Supporting Information in Table S9.
Conclusions

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We presented novel terpyridine and its analogues containing a 2-decyl-1,3,4-tetrazol-5-yl
substituent. Performed photophysical and optical studies which were analysed based on the
results of DFT/TD-DFT calculations. Obtained results showed that within the studied group
of target compounds, the change of the pyridine substituents does not have a significant
influence on the thermal stability of molecules. What is more, the variation of substituents at
the pyridine ring can effectively change the bandgap, 3A (2-pyrydinyl substituents) gives the
π–π* transition along with the charge transfer, whereas compounds 3B (2-pyrazinyl
substituents) and in particular 3C (2-thiazolyl substituents) have the π–π* transition along
with the intramolecular charge-transfer character. The emission maxima of 3B with the most
electron-deficient (pyrazinyl) and 3C with the most electron-rich substituents (thiazolyl) are
red-shifted by 31 and 24 nm, respectively, compared to the emission maximum of 3A bearing
the pyridine units. The presence of electron-donating 2-thiazolyl substituents in 3C, as well as
its planarity, causes pronounced red-shifted and structureless emission band with relatively
high quantum yield. Moreover, obtained results of theoretical investigation of druglikeness
may indicate that obtained trident ligands can exhibit interesting biological properties.
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Acknowledgement
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This work was supported by the Ministry of Science and Higher Education, Poland
Diamentowy Grant 0215/DIA/2015/44. D. Zych was financed by the National Science Centre
of Poland ETIUDA6 2018/28/T/ST5/00005. Calculations have been carried out using
resources provided by Wroclaw Centre for Networking and Supercomputing (http://wcss.pl),
grant No.18. CCDC 1570874 contains the supplementary crystallographic data for compound
3C. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk
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Highlights
•
Novel terpyridine and its analogues containing a 2-decyl-1,3,4-tetrazol-5-yl
substituent
•
Performed photophysical and optical studies analysed based on the results
of DFT/TD-DFT calculations
•
•
Variation of substituents at the pyridine ring effectively change the bandgap
Significant changes in the photophysical parameters in reference to substituent
at the pyridine ring

Author Contribution Statement
Dawid Zych: Conceptualization; Data collection; Funding acquisition; Investigation;
Methodology; Writing - original draft, review & editing.
Aneta Slodek: Data collection; Investigation; Methodology; Writing - original draft, review
& editing.
Jan Grzegorz Malecki: Data collection; Investigation; Methodology; Writing - original draft,
review & editing.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: