Fluorescent quinolizinium ionic liquids (salts) with unexpectedly high quantum yields up to >99%

Article abstract of DOI:10.1039/c1jm11556a

Quinolizinium ionic liquids (salts) featuring an unbranched cation core have been prepared, characterized and found to show extremely high fluorescence quantum yields in solution (Φ_f > 99%, λ_em near 334 nm) and bright cyan fluorescence in molten state (λ_em near 465 nm).

Full text of DOI:10.1039/c1jm11556a

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Fluorescent quinolizinium ionic liquids (salts) with unexpectedly high quantum
yields up to >99%†
ab
a
a
a
a
a
Zhengjian Chen, Shiguo Zhang, Xiujuan Qi, Shimin Liu, Qinghua Zhang and Youquan Deng*
Received 12th April 2011, Accepted 17th May 2011
DOI: 10.1039/c1jm11556a
Quinolizinium ionic liquids (salts) featuring an unbranched cation
core have been prepared, characterized and found to show extremely
high fluorescence quantum yields in solution (F > 99%, l near
position. Unlike (iso)quinolinium, the quinolizinium ion is symmetric
11
and the p-electron densities are equally distributed over both rings.
The annelated quinolizinium derivatives have been used as important
f
em
1
2
DNA intercalators. It is of further interest to note that the quino-
lizinium ion provides a structural motif for designing a new type of
ionic liquid with an unbranched cation core.
3
4
34 nm) and bright cyan fluorescence in molten state (lem near
65 nm).
In this paper, two small-molecule quinolizinium ions were found
suitable for making ionic liquids, by incorporating weakly interacting
Recently, functional materials designed from ionic liquids, which are
a class of neoteric solvents composed entirely of ions, are receiving
increasing interest, since ionic liquids can be custom-tailored to satisfy
the functional requirements for building organic materials by
anions (NTf
the quinolizinium ionic liquids. First of all, [EO
prepared via a quaternization reaction between a methylpyridine and
2
, BF
4
and DCA). Scheme 1 shows the synthesis route of
2
EMPy]Br was
1
changing either the cation or anion species. Up to now, functional-
ꢀ ꢀ
2 2 5
BrCH COOC H in ethanol at 60 C for 5 h and 80 C for 15 h. The
ized ionic liquids have achieved great success in acting as luminescent
bromide salt precursors were then synthesized through a Westphal
reaction, modified from a previously reported solid-phase proce-
13
2–4
5
6
7
materials, magnetic fluids, liquid crystals, photochromic switches
and many more. The recently reported luminescent ionic liquids
could be classified into three structurally distinct types, i.e., proton-
dure. For example, the mixture of 3,4-hexanedione and 1 equiv. of
triethylamine was added dropwise into a THF solution of 1.2 equiv.
2
ated polyamidoamine dendrimers, polycyclic aromatic benzobis
of [EO EMPy]Br under reflux and mechanical stirring. After further
2
3
4
imidazolium), and transition metal-containing anions. The ben-
(
stirring for 3 h, a white solid of [EEQu]Br was obtained by filtration
and recrystallization from methanol by addition of diethyl ether.
zobis(imidazolium)-based ionic liquids characteristically exhibit
3
strong fluorescence with quantum yields up to 91% in methanol. In
Finally, an anion metathesis reaction from a bromide ion to NTf
LiNTf ), BF (NaBF4) or DCA (AgDCA) in water, afforded the
2
many fields, the required quantum yields of fluorescent materials
should be as high as possible. In neat ionic liquids, the associated
species that are energetically different show excitation wavelength-
(
2
4
desired ionic liquids or low melting salts. The structures and
compositions of the newly prepared compounds were confirmed by
8
dependent fluorescence. Our recent studies show that the fluores-
1
13
H-NMR, C-NMR, FT-IR, electrospray ionization mass spec-
trometry (ESI-MS, positive ion mode), Br ion-selective electrode and
moisture meter. In addition to measuring some basic physicochemical
properties such as melting point, thermal stability, density, viscosity
and conductivity of these quinolizinium salts, their unexpected and
rather interesting fluorescence characteristics were particularly
emphasized, i.e. high quantum yields up to >99% in solution with lem
cence of the neat ionic liquids can be dramatically enhanced when
9
confined into mesoporous silica gel.
Although a number of cation species of ionic liquids have been
developed and studied, all of them are limited to have a common
structure with at least one side chain (or proton) in their cation cores,
such as dialkylimidazolium. The cationic side chains can be removed
by nucleophilic substitution with anions under heating and lead to
10
low thermal stability. So, one possible way to develop novel ionic
liquids is to find a substitute for the branched cation cores. The
quinolizinium ion, structurally derived from naphthalene, is charac-
terized by a bridgehead quaternary nitrogen atom in the fused
a
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail:
ydeng@licp.cas.cn; Fax: +86-931-4968116
b
Graduate School of the Chinese Academy of Sciences, Beijing, 100039,
China
†
Electronic supplementary information (ESI) available: synthesis and
characterisation of quinolizinium salts and computational details. See
DOI: 10.1039/c1jm11556a
Scheme 1 Synthesis of quinolizinium ionic liquids (salts).
J. Mater. Chem., 2011, 21, 8979–8982 | 8979
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around 334 nm and bright cyan fluorescence in molten state with lem
the quinolizinium salts, with decomposition temperature (T ) ranging
d
ꢀ
around 465 nm.
from 265 to 408 C. Interestingly, the quinolizinium bromides exhibit
ꢀ
Prior to property testing, each salt would be dried at 80 C and
ꢁ3
comparable thermal stability to their BF -based counterparts
4
ꢁ2
ꢀ
1
0 –10 mbar for 20 h, and the water content in these salts could be
(ꢃ350 C), nevertheless, in traditional ionic liquids, the nucleophilic
reduced to 35–302 ppm (see Table S1 in ESI†). The water content was
determined by means of Karl-Fischer titration on a Metrohm
4
Br anion is much less thermally stable than BF , e.g. [EMIm]Br
ꢀ
ꢀ
10
(311 C) vs. [EMIm]BF (450 C). This finding should be attributed
4
8
31 KF Coulometer. The bromide ion concentration in Br-free salts
to the unbranched cation core in the quinolizinium ion, offering no
side chain to react with anions. However, not all anions can take
advantage of the quinolizinium ion’s resistance to heating. For
was estimated to be less than 0.15 wt%, by using a Br ion-selective
electrode (DX280-Br, Mettler-Toledo) on a Mettler-Toledo Sev-
1 13
enMulti meter. H-NMR/ C-NMR spectra (ppm, d) were recorded
ꢀ
example, [EEQu]DCA is thermally stable only up to 281 C, in spite
ꢀ
on a Varian Mercury Plus spectrometer (400 MHz). FT-IR spectra
of 408 C for [EEQu]NTf . Hence, the thermal decomposition of the
2
ꢁ1
(
cm ) were obtained in KBr discs on a Thermo Nicolet 5700 FT-IR
quinolizinium salts depends on the nature of both the cation and
spectrophotometer. ESI-MS spectra (positive ion mode, m/z) were
determined in methanol solution on a Bruker Daltonics APEX II 47e
FTMS. The melting point was determined on a differential scanning
anion. The two NTf -based ionic liquids were determined to show
2
moderate density, viscosity and conductivity, like [MMQu]NTf2:
ꢁ1
ꢁ1
ꢀ
1.49 g mL , 37.2 cP and 8.42 mS cm at 60 C.
e
calorimeter model DSC822 (Mettler-Toledo) with a rate of
ꢁ1
Intense photoluminescence is the most attractive feature of the
quinolizinium compounds. A preliminary study was carried out to
investigate their UV-Vis absorption and fluorescence emission char-
acteristics in solution. As shown in Fig. 1a, the absorption and
ꢀ
1
0 C min under an N
2
atmosphere. The thermal decomposition
temperature was measured on a Pyris Diamond Perkin-Elmer
ꢁ1
ꢀ
2
TG/DTA at a rate of 10 C min under an N atmosphere. The
density was determined using a 10 mL volumetric flask calibrated
with water by mass method. The viscosity was determined on a Sta-
binger Viscosimeter SVM 3000/GR. The conductivity was measured
on a Mettler-Toledo SevenMulti meter. The temperature for deter-
2
emission spectra of [EEQu]NTf in ethanol each consist of two
vibrational bands, and are roughly in a mirror-image relation to each
other. The fluorescence peak located at 334 nm (3.713 eV) is close to
the corresponding absorption peak at 329 nm (3.769 eV), resulting in
ꢁ1
mining density, viscosity and conductivity was maintained to 60 ꢂ
a small Stokes shift (Dn) of 445 cm (0.056 eV). The small Stokes
shift indicates that there is only small geometry modification between
ꢀ
0
.1 C by means of an external temperature controller. The absorp-
14
tion spectra were determined using an Agilent 8453 diode array
spectrophotometer with a deuterium lamp as the light source. The
emission spectra were recorded on a Hitachi F-7000 FL spectro-
photometer equipped with a Xe lamp. The integrating sphere
0 1
the ground state (S ) and the first excited state (S ).
Table 2 summarizes the observed labs and lem values, which suffer
no significant impact from altering the alkyl chain length (methyl and
2 4
ethyl), anion species (Br, NTf , BF and DCA) and solvent type
(
150 mm diameter, BaSO
4
coating, Edinburgh Instruments)
(ethanol, CH CN and THF). In addition, the fluorescence decay of
3
measurements of quantum yields (absolute error < 2.5%) were con-
ducted on an Edinburgh FSL920 spectrofluorimeter equipped with
a Xe lamp (Xe900, 230ꢃ2000 nm), at an excitation wavelength of
315 nm. The emission lifetime was also determined on the FSL920
with a nanosecond hydrogen flash lamp (nF900, 110–850 nm) as an
excitation source, and the lifetime measurements at the peak emission
wavelength of (ꢃ334 nm) were performed at a pulse duration of 1.0
ns and a pulse repetition rate of 40 kHz.
Table 1 shows some basic properties of the quinolizinium salts.
Differential scanning calorimetry (DSC) studies reveal that, as
2 4
expected, replacing the Br anion with NTf , BF or DCA leads to
a low melting point (T ). In particular, three bromide-free salts melt
m
Fig. 1 The absorption and emission spectra (a) and fluorescence decay
(b) of [EEQu]NTf2 in 5 ꢄ 10ꢁ6 mol Lꢁ1 ethanol solution at ambient
abs
ꢀ
ꢀ
ꢀ
below 100 C, such as [MMQu]NTf : 60.5 C, [EEQu]NTf
2
2
: 50.4 C
ꢀ
ꢁ1
7
and [EEQu]DCA: 67.9 C, and thus fall into the ionic liquid class.
Thermogravimetric analysis (TGA) shows good thermal stability of
conditions. Dn (cm ) ¼ 10 ꢄ (1/l ꢁ 1/lem) is the Stokes shift, s the
2
lifetime, and c the linear fitting parameter.
Table 1 Some basic properties of quinolizinium salts
a
ꢀ
b
ꢀ
c
r (g mL
ꢁ1
d
h (cP)
e
k (mS cm
ꢁ1
Salt
T
m
( C)
T
d
( C)
)
)
[
[
[
[
[
[
[
[
MMQu]Br
MMQu]NTf
MMQu]BF
MMQu]DCA
EEQu]Br
237.1
60.5
186.4
137.5
221.6
50.4
346
408
347
265
347
405
353
281
2
1.49
37.2
49.3
8.42
4.70
4
EEQu]NTf
EEQu]BF
EEQu]DCA
2
1.44
4
115.5
67.9
a
b
Melting point. Decomposition temperature at 5% weight loss. Density at 60 C. Viscosity at 60 C. Conductivity at 60 C.
c
ꢀ
d
ꢀ
e
ꢀ
8
980 | J. Mater. Chem., 2011, 21, 8979–8982
This journal is ª The Royal Society of Chemistry 2011

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Table 2 Absorption and emission data of quinolizinium salts
a
a
a
,
a b
Salt
Solvent
l
abs (nm)
l
em (nm)
s (ns)
F
f
(%)
[
[
[
[
[
[
[
[
[
[
EEQu]NTf
2
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
329
329
329
328
328
328
329
328
328
330
334
334
334
334
334
334
334
334
333
336
5.33
5.38
—
—
—
—
—
—
>99
>99
>99
95
94
96
89
82
95
86
EEQu]BF
4
EEQu]DCA
MMQu]NTf
MMQu]BF
MMQu]DCA
EEQu]Br
MMQu]Br
2
4
EEQu]NTf
EEQu]NTf
2
2
CH CN
THF
3
5.40
4.87
a
Measured in 5 ꢄ 10ꢁ6 mol L solutions at room conditions. The absolute quantum yield measured by integrating sphere method.
ꢁ1
b
[
EEQu]NTf
2
in ethanol follows a single exponential function, with
Computer simulations were conducted to better understand the
unprecedented high fluorescence quantum yields of the only bicyclic
quinolizinium fluorophore. As shown in Fig. 2a, the optimized
ground-state geometry of the [MMQu] cation adopts a planar
conformation, and the three C–N bonds are fully delocalized to be
a typical fluorescence lifetime of 5.33 ns, as shown in Fig. 1b.
Moreover, the fluorescence intensity of the quinolizinium
compounds is several times stronger than that of naphthalene
1
5
(
F
f
¼ 20%) at the same condition (see Fig. S2 in ESI†). This finding
prompted us to determine their absolute fluorescence quantum yields
F ) using integrating sphere method. To our surprise, the F values of
ꢀ
almost of the same length 1.384–1.389 A, close to these C–C bond
ꢀ
lengths (1.364–1.438 A). When excited to the S state (see Fig. S5 in
(
f
f
1
ꢀ
the only bicyclic naphthalene-like quinolizinium fluorophore range
up to 82–99%, as listed in the last column in Table 2. In particular,
three Br-free [EEQu] compounds in ethanol are all capable of giving
ESI†), only a modest change in the bond lengths (ꢁ0.024–+0.029 A)
of [MMQu] occurs, without perceivable distortion of the aromatic
plane, answering for the above-mentioned small Stokes shift value.
As seen from the electron density surface of [MMQu] in Fig. 2b, the
positive charge is highly delocalized into the aromatic system, leaving
only +0.208 e charge located at the quaternary nitrogen atom
(see Table S2 in ESI†). Likewise, two frontier orbitals (HOMO and
LUMO, Fig. 2c, d) are also substantially delocalized over the entire
ring. The HOMO–LUMO gap was overrated to be 4.15 eV, relative
to the experimental value of absorption energy: 3.78 eV (328 nm). In
fact, the quaternary nitrogen atom is an electron-withdrawing group,
which helps to transfer about ꢁ1.08 e from the hydrogen atoms and
methyl groups into the [MMQu] ring (see Table S2†), resulting in an
electron-rich aromatic system. To sum up, the high quantum yields of
the quinolizinium fluorophore should be related to the following
the highest F
100%). Three Br-free [MMQu] compounds also maintain a high
level of F (94–96%). As for the two bromides, their F values are
f
value of >99%, close to the theoretical maximum
(
f
f
relatively low, 82% for [MMQu]Br and 89% for [EEQu]Br respec-
tively, on account of heavy atom (Br)-induced fluorescence quench-
f
ing. In other solvents, high F s could also be obtained from [EEQu]
NTf , i.e. CH CN (95%) and THF (86%). In theory, high fluores-
2
3
cence yields require polycyclic aromatic systems. To our knowledge,
the quinolizinium ring should be the first small aromatic system
(bicyclic) that gives a F value near 100%.
f
structural reasons: 1) no significant structural difference between S
and S states; 2) the well-delocalized positive charge and frontier
orbitals; and 3) the electron-rich system.
0
1
Fluorescence emission was also observed in the condensed qui-
nolizinium salts. For example, the emission spectrum from the solid
Fig. 2 Optimized ground-state geometry of the [MMQu] cation (a, bond
ꢀ
length A), color-coded electron density surface (b) and frontier molecular
Fig. 3 Excitation wavelength dependent emission spectra in the molten
ꢀ
2
a 365-nm cold light source (136 mW cm ).
orbitals: HOMO (c) and LUMO (d). The calculations were performed in
the gas phase at B3LYP/6-31 + G(d,p) level of theory.
[EEQu]NTf2 at 60 C and cyan fluorescence under irradiation by
ꢁ
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[
EEQu]NTf2 is composed of two overlapping bands peaked at
Notes and references
3
50 nm (see Fig. S3 in ESI†), in a shape similar to the case in solution.
1
T. Torimoto, T. Tsuda, K. Okazaki and S. Kuwabata, Adv. Mater.,
010, 22, 1196.
However, the molten [EEQu]NTf exhibits structureless emission
2
2
bands, as shown in Fig. 3. Unlike the cases in solution and the solid
state, the emission peak in the liquid state increases with excitation
wavelength, in agreement with the previous studies on imidazolium
2 J. Huang, H. Luo, C. Liang, I. Sun, G. A. Baker and S. Dai, J. Am.
Chem. Soc., 2005, 127, 12784.
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A. J. Boydston, C. S. Pecinovsky, S. T. Chao and C. W. Bielawski, J.
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8
ionic liquids. In detail, the emission peak in Fig. 3 would vary from
453 to 486 nm, when the excitation wavelength was shifted from
365 to 410 nm. The emission wavelength of the molten salt falls
4
5
S. f. Tang, A. Babai and A. V. Mudring, Angew. Chem., Int. Ed., 2008,
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within the visible range, and a bright cyan fluorescence could be seen
under irradiation by an ultraviolet cold light source (365 nm). A
plausible explanation for the cyan fluorescence should be the presence
of a large number of associated species (such as ion clusters), which
are energetically different and naturally occurring in ionic liquids via
4
8,16
hydrogen-bonding, p–p stacking and/or electrostatic interactions.
Similar results were also obtained from [MMQu]NTf (see Fig. S3
and S4 in ESI†).
2
In conclusion, with [MMQu]NTf and [EEQu]NTf etc. we
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present a new type of quinolizinium ionic liquid featuring an
unbranched cation core. The key structural feature makes the small
bicyclic quinolizinium fluorophore exhibit very strong fluorescence
9
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(
ꢃ334 nm) with quantum yields up to >99% in solution, which raises
the possibility of designing and fabricating high-efficiency fluorescent
compounds with small molecular weights. The emission properties of
these quinolizinium salts can be dramatically tuned by altering the
phase state, and bright cyan fluorescence (ꢃ465 nm) in the molten
state was observed.
1
1
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Acknowledgements
This work was supported by the National Natural Science Foun-
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8
982 | J. Mater. Chem., 2011, 21, 8979–8982
This journal is ª The Royal Society of Chemistry 2011