RSC Advances
Paper
system of the tosylate anion. Hence, these data demonstrate
that the crystal-state AIE effect can be achieved using chloride,
bromide and tosylate as the counter-ion for pyridinium.
Importantly, nature of counter-ion inuences the solid-state
emission maximum of AIEgens. Thus, change of the counter-
ion in AIEgen 2 from tosylate (2b) to chloride (2a) resulted in
a noticeable bathochromic shi of more than 70 nm (Fig. 3B.).
The dependence of the solid-state emission maximum on the
nature of the counter-ion simplies the design of AIEgens with
the desired emission wavelength.
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Conclusions
All synthesized pyridinium salts 1–5 demonstrate AIE properties
with up to 187 times emission increase in the crystal-state as
compared to solution. Lack of emission for 2b in solution and
frozen DMSO matrix (solid amorphous state) speaks against the
RIR effect as the origin of the AIE properties for pyridinium salt
2b. Single crystal X-ray analyses provide clear evidence for the
presence of non-covalent intermolecular pyridinium–pyr-
idinium and pyridinium–p interactions in the crystal-state of
AIEgens 2–5. Consequently, crystal-state AIE effect can be
attributed to the intermolecular p+–p+ and p+–p interactions
involving pyridinium cations. Hence, non-covalent interactions
between two neighboring pyridinium subunits in a crystal-state
are an efficient means to accomplish AIE. The nature of counter-
ion in pyridinium salts 2–5 does not affect emission efficiency
(PLQY) of the crystal-state AIEgens, however, the counter-ion
does inuence the crystal-state emission maximum of AIE-
gens. The use of the non-covalent pyridinium–pyridinium or
pyridinium–p interactions in the design of AIEgens is alterna-
tive and complementary approach to routinely used means to
avoid the ACQ effect.3,4 Further studies on the design of the
crystal-state AIEgens based on interactions between cationic p
systems will be reported in due course.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was funded by Taiwan–Lithuania–Latvia Tripartite
Cooperation Fund (Project “New materials and technologies for
very-high color rendering and high sunlight spectrum resem-
blance OLED lighting sources”). We thank Dr A. Mishnev and
Dr D. Stepanovs for X-ray crystallographic analysis and P.
Dimitrijevs for DLS measurements.
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