S. Mubeena, G. Annapareddy, M. N et al.
Journal of Molecular Structure 1243 (2021) 130876
cal optical excitation conditions [23]. Though the title compounds
1 and 2 contain anthracene in their structure, the estimated flu-
orescence quantum yields of these compounds using quinine sul-
fate as the reference were found to be very low (quantum yield
< 1%). The weak fluorescence of the title compounds compared to
anthracene is presumably due to the quenching of the fluorescnce
by fast intersystem crossing promoted by the heavy atom effect of
tungsten.
1.0
1.0
0.5
0.0
Em (decyl-PF )
6
0.5
Em 1 (methyl)
Abs 1 (methyl)
4. Conclusion
Em 2 (decyl)
In summary, two hexatungstate cluster-based compounds con-
taining 9,10-bis-alkyl imidazolium-anthracene as the cations were
synthesized and characterized. Both the solids maintain 1:1 sto-
ichiometry between the cation and the anion and the cationic
imidazolium fragment is projected toward the POM cluster by
coulombic interaction. The endothermicity of the structural incom-
patibility between anthracene and polyoxometalate cluster anion
is offset by the ionic interaction and further by the supramolecu-
lar association between them. The alkyl chain length is found to
influence the crystal structure of the compounds where the decyl
derivative (2) has a less crystal density than the methyl derivative
(1). This is obviously due to the modification of the packing pat-
tern by the length of the alkyl chain. The absorption and emission
spectra of the compounds revealed similar behavior to anthracene
except for the fluorescence quantum yield which was quenched in
the title compounds perhaps due to the heavy atom effect of tung-
sten.
Abs 2 (decyl)
0.0
300
350
400
450
500
550
Wavelength (nm)
Fig. 3. Electronic absorption and fluorescence spectra of compounds 1 and 2 in
DMSO.
modynamic reasons, we attempted to develop crystals based on
hexamolybdate clusters of the same dications. Unfortunately, the
crystals were too poor to structurally characterize. However, from
our previous experience, we found the hexamolybdate and hex-
atungstate compounds were isomorphic.
In the crystal structure of compound 1, there is a dimer for-
mation between two translation equivalent organic counterparts
along the x-axis. The C10 atom of the imidazole ring undergoes
a C–H•••π stacking interaction with anthracene ring of a trans-
5. Author credit statement
˚
lation equivalent (dC•••centroid ~ 3.6 A, <C–H•••centroid ~ 160 ). The
º
Shaik Mubeena synthesized the compounds and performed the
background research for the work. She was also directly involved
in preparation of the manuscript.
anthracene rings are one unit length away and thus the dimer is
not a π•••π stacked dimer, as observed in the previous report for
the mono-cationic anthracene-imidazolium compounds [24]. These
dimeric structures then meet the cluster ions through C–H•••π in-
Gayatri Annapareddy and Meghana N performed the photo-
physical studies, analyzed data and were involved in the prepara-
tion of the manuscript at various stages.
˚
teractions between C5 and O2 (dC•••O ~ 3.3 A, <C–H•••O7 ~ 160 );
º
˚
and, C9 and O7 (dC•••O ~ 3.2 A, <C–H•••O7 ~ 160 ). The relevant
º
Monima Sarma designed the project, supervised the students,
prepared the manuscript and made necessary corrections during
the production of the article.
packing modes in the crystal structure of compound 1 are shown
The crystal packing pattern of compound 2 is quite differ-
ent from that of compound 1. In this case, no dimer formation
as in compound 1 was observed. The organic counterpart under-
goes supramolecular C–H•••O interaction with the inorganic cluster
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
˚
(dC•••O ~ 3.1 A, <C–H•••O7 ~ 166 ). Repetition of this interaction by
º
inversion and translational symmetry results in a one-dimensional
chain-like packing pattern (Fig. 2b).
Acknowledgments
3.3. Absorption and emission spectroscopy
The author thanks Science and Engineering Research Board
(SERB), DST, India (Project No. SRG/2019/001365) for funding. MS
also acknowledges Prof. Samar K. Das, the University of Hyderabad
for valuable support.
The title compounds contain anthracene in their structure that
is a well-known fluorescent probe. Anthracene exhibits a blue flu-
orescence (λmax = 400–450 nm) when excited by ultraviolet light.
Both the absorption and emission spectra of anthracene show dis-
tinct vibrational structure due to the quantized vibrational levels
in both the ground and excited states [23]. The electronic spectro-
scopic behavior of the title compounds was examined in dimethyl
sulfoxide. The relevant spectra are shown in Fig. 3. The absorption
and fluorescence spectra of compounds 1 and 2 are similar and ex-
hibit highly structured bands due to the absorption and emission
of anthracene. For comparison, the fluorescence spectrum of the
PF6 salt of bis-decylimidazolyl anthracene is shown which over-
laps well with the spectra of the hexatungstate salts. The fluores-
cent quantum yield is the ratio of the number of emitted photons
to the number of absorbed photons. The common way to estimate
this parameter is to compare the fluorescence of the probe solution
with that of a reference solution (e.g. quinine sulfate) under identi-
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