Full Paper
given in Figure 3A and Figure S8 in the Supporting Informa-
tion. It was found that the absorptions resemble those record-
ed in bulk solution. The lack of obvious broadening and/or
red-shift of the peak indicate that the arrangements of the
naphthalene units in the core are rather loose and no strong
[
25,26]
association occurs.
The fluorescence spectra of 2a–d in bulk solutions have
been investigated in detail. Upon excitation, concentration-de-
pendent emission has been detected (Figure S11 in the Sup-
porting Information). Besides, the fluorescence emission of
2
a–d can be also heavily influenced by the polarity of the sol-
vents. It was found the intensity of the fluorescence emission
À1
at the same concentration (for example, 100 mmolL ) deceas-
es significantly from ethanol to CH Cl and toluene (Figure 3B,
2
2
Figure S12) accompanied a hypsochromic effect. Meanwhile,
a significant decrease of the fluorescence quantum yield (Ff)
was noticed. For example, F at an excitation of 285 nm was
Figure 4. A) Photos of 2a without (left) and with (right) 365 nm UV irradia-
tion. B) Photos of an ethanol solution of 2b (0.1 molL ) without (left) and
f
À1
recorded to be 0.11–0.15 for 2a–d in ethanol, which decreases
with 254 nm (top right) or 365 nm (bottom right) UV irradiation. C) Photos
of letters written by a brush pen using the ethanol solution shown in B on
tinfoil taken without (left) and with 254 nm (middle) or 365 nm (right) UV ir-
radiation. D) The same letters written on peanut leaf taken without (left) and
with 254 nm (top right) or 365 nm (bottom right) UV irradiation.
to approximately 0.05 in CH Cl (Table S4 in the Supporting In-
2
2
formation). The polarity-dependent fluorescence of 2a–d can
be further clarified by measurements carried out in ethanol/
H O mixtures. When the water content was gradually in-
2
creased, the emission wavelength was found to gradually shift
to longer wavelengths (Figure 3C, Figure S13 in the Support-
ing Information). The luminescence intensity first increases,
passes through a maximum and then decreases. For 2d, the
emission becomes quite broad in pure water (Figure S13C in
the Supporting Information), presumably due to its lowest
water solubility. To get more details, the fluorescence decay
profiles of 2a–d were recorded in three different environments
photoluminescence were detected for 2a–d (Figure 4A, Fig-
ure S15 and S16 in the Supporting Information). This indicates
that no strong p–p interaction exists among naphthalene units
although they might form loosely-packed cores, which is con-
sistent with the results from UV/Vis, XRD and SAXS measure-
ments. We have also used the solution of 2b in ethanol
À1
(
i.e., in ethanol, CH Cl and solvent-free state) and significant
(0.1 molL , Figure 4B) as the ink with which letters were writ-
2
2
differences have been noticed. Considering 2b as an example,
monoexponontial decay was observed in ethanol with a fluo-
rescence lifetime (t) of 9.83 ns (Figure 3D, Table S4 in the Sup-
porting Information). In CH Cl , biexponontial decay was de-
ten by a brush pen on various substrates including tinfoil, tita-
nium sheet, filter paper, plastic film, silica gel plate, peanut leaf
and bracket plant leaf (Figure 4C and D, Figure S17 in the Sup-
porting Information). It can be seen that on all the substrates
the letters are luminescent even after they are totally dry. This
is in contrast to other reported photoluminescent inks which
show substrate-sensitive luminescent properties and/or
2
2
tected with t1 and t2 to be 3.20 ns (21.75%) and 7.52 ns
78.25%), respectively. In solvent-free state, biexponontial
decay was also observed with t and t to be 1.50 ns (81.81%)
(
1
2
[
47–49]
and 8.35 ns (18.19%), respectively. Similar results were ob-
tained for 2a, 2c and 2d (Figure S14 and Table S4 in the Sup-
porting Information). The solvent polarity-dependent fluores-
cence should be ascribed to the change of the content of non-
radiative energy transfer triggered by the variation of the mi-
croenvironment surrounding the naphthalene units.
become optically silent upon drying.
To the best of our
knowledge, this is the first example of RTILs which can be uti-
lized as photoluminescent inks. Compared to previously re-
[
25,26]
ported neutral photoluminescent inks,
The inks demon-
strated here owns the advantage of easily property optimiza-
tion through simple ion-exchange, by which complicated and
time-consuming organic synthesis can be avoided. Carrying an
ionic moiety, the RTILs inks may also have better adhesion to
some oppositely-charged substrates. In addition, since RTILs
have negligible vapor pressure and are nonflammable, the let-
ters on the substrates can be preserved longer and the ink is
safer to be used in practical applications. Efforts to fully dem-
onstrate these advantages are currently underway and will be
reported in near future.
Application as photoluminescent inks
For organic compounds or nanosized materials which are pho-
toluminescent in bulk solutions, self-quenching usually occurs
in solvent-free state due to the aggregation of the chromo-
phores. If the aggregation is suppressed by effectively disturb-
ing the interaction among p-units, however, photoluminescent
[25,26,45,46]
properties can preserve.
It is quite interesting to check
whether 2a–d, which were functionalized with the branched
alkyl chains only at one end of the naphthalene unit, can also
preserve photoluminescent properties in solvent-free states.
Upon illumination by a portable UV lamp at 365 nm, strong
Conclusions
In summary, we have introduced a new strategy to construct
p-conjugated, photoluminescent RTILs by using branched alkyl
Chem. Eur. J. 2016, 22, 6286 – 6293
6291
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