Tunable emission in lanthanide coordination polymer gels based on a rationally designed blue emissive gelator

Article abstract of DOI:10.1039/c5cc02709h

Rational design and synthesis of a new low molecular weight gelator (LMWG) having 9,10-diphenylanthracene core and terminal terpyridine is reported. Tb^III and Eu^III ion coordination to a LMWG results in green and pink emissive coordination polymer gels, respectively, with coiled nanofiber morphology. Further, control over stoichiometry of LMWG:Tb^III:Eu^III leads to yellow and white light emitting bimetallic gels.

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Tunable emission in lanthanide coordination polymer
gels based on a rationally designed blue emissive
gelator
Papri Sutar, Venkata M. Suresh and Tapas Kumar Maji*
superior compared to the monochromatic (that emits entire visible
regions) and dichromatic (that emit in blue and yellow regions) due
to the finer colour rendering properties and high quantum
efficiencies.8 To date a handful of trichromatic white light emitting
materials, suitable for solid state lighting applications have been
reported. However, realization of white-light-emission based on
CPG is yet to be accounted.
Rational design and synthesis of a new low molecular
weight gelator (LMWG) having 9,10-diphenylanthracene
core and terminal terpyridine is reported. TbIII and EuIII
ion coordination to LMWG results in green and pink
emissive coordination polymer gels respectively with coiled
nanofiber morphology. Further, control over stoichiometry
of LMWG:TbIII:EuIII leads to yellow and white light
emitting bimetallic gels.
In this work, we report rational design of a new LMWG, 9,10-
(4-carboxyphenyl)anthracenedi-{[3-([2,2’;6’,2’’]terpyridin-4’ylami
Over past few decades, enormous research interest has been directed
towards the development of new functional supramolecular gels
from low molecular weight gelators (LMWGs).1 Easy solution
processability and dynamic nature of such soft materials hold great
promise for applications in diverse fields, including biomedicine,
nanoelectronics and catalysis.2 Recently, the concept of coordination
polymer gels (CPGs) has been emerged where coordination of metal
ions to a suitably designed LMWG results in supramolecular self-
assembled networks.3 Such soft organic-inorganic hybrids are highly
promising since the incorporation of metal ions renders an
no)-propyl]-amide (L) having
a blue emitting 9,10-diphenyl
anthracene core and terminal terpyridine groups and its gelation
properties. Coordination of L through terpyridine with TbIII and EuIII
result in CPGs (TbL and EuL) which show green and pink emission
respectively. Moreover, coordination of both TbIII and EuIII to L
yields bimetallic CPGs (TbEu1, TbEu2) in which precise
stoichiometric control over L:EuIII:TbIII ratio leads to tunable yellow
and white emissions (Scheme 1).
opportunity to emulate its unique redox, optical, electronic and
4
magnetic properties in supramolecular gels.3a,
Particularly,
metallogel of lanthanide ions (LnIII) are of specific interest as they
show narrow band emissions (red: EuIII, PrIII, SmIII ; green: TbIII, ErIII
; blue: TmIII, CeIII) with high quantum yields when bind to suitable
π-chromophoric linker.5 Such unique emission properties of LnIII
ions, stemming from sensitized f-f electronic transition has been
investigated in bulk crystalline coordination polymers and exploited
for solid state lighting and sensing applications.6 However, low
solution processability greatly limits their application in device
fabrication and biological sciences. In this regard, lanthanide
coordination polymer gels (CGPs) are superior as reversible sol-gel
behaviour enhances their processability and would facilitate easy
fabrication of large-area and flexible displays, portable sensing kits.
Recently, white-light-emitting materials have merited
immense research interest due to their potential application in solid
state lighting.7 White-light-emitting materials can be trichromatic in
which blue, green and red light emitting components are introduced
to cover the entire visible spectral window. Such materials are
Scheme1 Schematic showing the self-assembly of
L though H-bonding and π-π
stacking interactions and its coordination to LnIII forming luminescent CPGs.
L is synthesized in one-step via amide coupling (see ESI for
details) between 9, 10-(4-carboxyphenyl)anthracene and 2,2′;6′,2″
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terpyridin-4′-yl-propane-1,3-diamine and characterized using 1H- neighbouring L and forms fibrils. These fibrils on aggregation form
NMR (Fig. S1, S2 and S3).9 Methanolic solution of L (1 × 10-4 M) fibers which further entangle to form typical fibrous gel architecture.
shows a broad absorption in the range 280-310 nm which could be The presence of H-bonding and π-π stacking in gel are supported by
assigned to the π-π* transition of 9,10-diphenyl anthracene (ant) powder X-ray diffraction (PXRD) and Fourier transform infrared
core and terminal terpyridine (tpy) groups (Fig. S4, S5). Excitation spectra (FTIR) analysis. PXRD of the xerogel shows broad peak at
at 280 nm results in strong blue emission with maxima at 428 nm 2θ =21.8o, corresponding to the d-spacing value of 4.0 Å which
which can be considered as combined emission from ant and tpy supports the presence of π-π stacking in gel (Fig. S9). Moreover,
(Fig. S6, S7). Interaction of L with TbIII ions is studied in methanol FTIR of L xerogel on comparison with L exhibits decrease in -N-H
solution by gradually adding TbIII (10-4 M) to solution of L (10-4 M) and -C=O stretching frequency from 3436 cm-1 to 3424 cm-1 and
(Fig. 1a). With incremental addition of TbIII, absorption spectrum from 1645 cm-1 to 1638 cm-1 respectively, indicating the presence of
shows a significant bathochromic shift at 280 nm with appearance of intermolecular H-bonding in this assembly (Fig. S10). The
a broad shoulder at 327 nm, suggesting complexation of TbIII to the absorption spectrum of L xerogel shows maxima around 350 nm.
tpy groups of L.10 The corresponding emission spectra of the Upon exciting at 350 nm it shows intense blue emission with
solution (λex= 280 nm) shows a decrease in intensity at 428 nm with maxima at 435 nm, originating from both ant core and tpy (Fig.
concomitant appearance of sharp peaks at 490 nm, 546 nm, 587 nm S11). Emission spectra of gel film prepared on quartz substrate is
5
and 623 nm which can be attributed to D4-7FJ (J= 6-3) transitions of also recorded and it shows similar emission behaviour with a
TbIII (Fig. 1b). This sensitized emission further confirms the binding maximum at 435 nm (Fig. S12).
of TbIII to the tpy of L.11 Interestingly, emission of L at 428 nm is
not supressed completely even after addition of 2 eq of TbIII ions.
This residual emission is attributed to the central ant core which
probably does not involve in sensitization of TbIII due to large spatial
distance between ant and metal ion. Similar changes in absorption
spectra are also observed in case of EuIII ion (Fig. 1c). However, the
emission spectra exhibits weak bands at 592 nm and 618 nm,
5
5
assigned to the D0-7F1 and D0-7F2 transition of EuIII and a feeble
decrease in emission at 428 nm (Fig. 1d). This result indicates weak
energy transfer from tpy to EuIII.
Fig. 2 (a) Photographs of
TEM image of xeogel. (d) Photographs of TbL gel under day and UV light, (e)
and (f) TEM images of TbL xerogel. (g) Comparison of PXRD of (black), TbL(blue)
L gel under day and UV light, (b) FESEM image and (c)
L
L
and EuL(red) xerogel (low angle peak in TbL and EuL xerogel is highlighted in
box). (h) Probable coordination environment around TbIII in TbL which leads to
the formation of coiled-nanofiber morphology.
Fig. 1 Spectral changes in (a), (c) absorption and (b), (d) emission of
L on
incremental addition of TbIII and EuIII in methanol respectively. Arrows indicate
observed changes.
Heating the mixture of
L and Tb(NO3)3.6H2O (molar
ratio=1:1) in the aforementioned CHCl3/THF (1:1) solvent mixture
followed by cooling results in a stable coordination polymer gel
(CPG) (TbL) (Fig. 2d). Likewise, EuL is also prepared by using 1:1
molar ratio of L and Eu(NO3)3.6H2O. The presence of coordinated
TbIII and EuIII in respective TbL and EuL xerogels is confirmed by
energy dispersive X-ray spectroscopy (EDXS) analysis (Fig. S13,
S14). CHN analysis of TbL and EuL xerogels indicate the probable
formula as [Tb(L)(NO3)3.2THF] and [Eu(L)(NO3)3.2THF]
respectively. To get insight into the mode of packing in CPG, TbL
xerogel is analysed by TEM and FESEM images. The TbL xerogel
exhibits uniformly distributed coiled-nanofiber morphology with
diameter of 80-150 nm (Fig. 2e, 2f, 2h and Fig. S15) which is higher
than that of L nanofibers (40-50 nm). It is expected that the 1D self-
assembled primary structure of LMWG with pendent tpy groups on
Heating solution of L (10-3M) in CHCl3/THF (2:1), followed by
cooling at room temperature results in an opaque gel that remained
stable upon inversion (Fig. 2a). Field emission scanning electron
microscopy (FESEM)) images of xerogel reveal the fibrous
morphology. Nanofibers are several micrometer long with
approximate diameter of 40-50 nm (Fig. 2b, 2c and Fig. S8).
Interestingly, presence of nanorings is also observed which are
formed via coiling of nanofibers.12 Transmission electron
microscopy (TEM) images also support the existence of nanofibers
and nano-rings. Formation of nanofibers can be explained based on
the self-assembly of L as shown in the Scheme 1. In binary mixture
of solvents like CHCl3/THF, L assembles through the H-bonding
between amide groups and π-π stacking between ant cores of
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the outer surface would form higher ordered structures through ratios are simultaneously added into L solution during gelation. The
coordination-driven interaction with LnIII ions. This phenomenon has bimetallic CPG TbEu1 with TbIII:EuIII = 1:1.2 is prepared which
also been affirmed by PXRD analysis. L xerogel and metallogels shows strong greenish yellow emission under UV light (Fig. S18).
The corresponding emission spectrum displays bands characteristic
of L, TbIII and EuIII in which emission from TbIII at 545nm (green)
predominant over EuIII peaks at 590 nm and 615 nm (red) (Fig. 3c).
The CIE coordinate of TbEu1 is calculated to be (0.24, 0.38) (Fig.
3e). We envisioned that by further increasing EuIII concentration, the
emission feature of bimetallic CPG could be further tuned towards
white light. Finally, by changing the (TbIII:EuIII) ratio to 1:2, we are
able to prepare CPG (TbEu2) which show near white light emission
(Fig. 3d).The CIE coordinate of TbEu2 is calculated to be
(0.28,0.34) (Fig. 3e).13 Upon excitation at 340 nm the absolute
quantum yield (QY) of TbEu2 is found to be 5.0%. The presence of
both TbIII and EuIII in TbEu2 xerogel is confirmed by EDAX (Fig.
S19). Excellent processability and colour tunability of these “soft
hybrids” prompted us to exploit them for coating on various
substrates. As seen in inset, Fig. 3a-d quartz substrates coated with
corresponding gels show strong red, green, yellow and white light
emission. The white light emitting gel has also been used to fabricate
white-light-emitting-diode (WLED) by simply coating commercial
blue LED with TbEu2. Upon turning on the LED, it displays bright
white light (Fig. 3f).
(TbL and EuL) shows similar PXRD pattern with peaks at 2 =
14.1, 16.9, 18.6 and 21.8, corresponding to d-spacing values of
6.2 Å, 5.2 Å, 4.7 Å, and 4.0 Å respectively (Fig. 2g). The similarities
in PXRD pattern suggest that the primary assembly of L is retained
in CPGs. However, TbL and EuL show an additional low angle
peak at 4.3, (d-spacing value of 20.4 Å) which indicates formation
of higher-ordered self-assembled structures in CPGs on metal
coordination to tpy moieties.
In conclusion, the preceding results demonstrate a rational
design of a LMWG having blue luminescent core ant and a
metal chelator tpy which can also acts as a sensitizer for
lanthanide ions. Coordination driven self-assembly of LMWG
with LnIII (TbIII/EuIII) resulted in bright green/pink luminescent
coordination polymer gels. While, self-assembly of
L with
mixed metal ions with controlled stoichiometry of
L
:TbIII:EuIII
yielded bimetallic coordination polymer gels with tunable
emission including yellow and near white light. These soft
coordination polymer nanostructures are easy processable and
gentle heating of gels forms viscous liquids which can be easily
transferred to solid substrate without any loss in emission
intensity. Based on this approach a bright white light LED has
been fabricated. We believe that our design strategy of
trichromatic approach in coordination polymer gels would
provide new perspective for developing robust and easy
processable light emitting materials for potential applications in
optoelectronics and sensing
.
Fig. 3 Emission spectra of (a) TbL, (b) EuL, (c) TbEu1 and (d) TbEu2 (λex =280 nm).
(e) CIE chromatography of TbL(circle), EuL(triangle), TbEu1(square) and
TbEu2(solid circle). (f) Photograph of an illuminating ultraviolet LED (i), same LED
coated with TbEu2 gel under day light (ii) and LED showing bright white light
after switching on the LED(iii).
Acknowledgments
TKM is grateful to the Department of Science and Technology,
Government of India (GOI) and JNCASR, Bangalore for financial
support. PS and VMS are thankful to CSIR, GOI for fellowship.
Authors are thankful to Dr. M. Hariharan, IISER,
Thiruvananthapuram for quantum yield measurements.
Emission spectrum of TbL xerogel displays characteristic
peaks for TbIII (λex= 280 nm, Fig. S16) at 486 nm, 540 nm, 580 nm,
618 nm and L centred peak at 435 nm (Fig. 3a). Nevertheless,
emission spectrum of EuL xerogel (λex= 280 nm, Fig. S17) shows
well preserved L centred peak at 435 nm along with sharp peaks for
EuIII at 590 nm, 615 nm (Fig. 3b). TbL and EuL gels show intense
green and pink emissions respectively, under UV light (Fig. S18).
Respective CIE (Commission International de L’Eclairage)
coordinates of TbL and EuL emission are calculated to be (0.25,
Notes and references
Molecular Materials Laboratory, Chemistry and Physics of Materials
Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur,
Bangalore 560 064, India. *E-mail: tmaji@jncasr.ac.in, Tel:
0.43) and (0.26, 0.23) (Fig. 3e). The absolute quantum yield (QY) +918022082826; FAX: +91 8022082766.
upon excitation at 340 nm is 4.2% for TbL and 12.4% for EuL and
these values are comparable with other reported values of TbIII/EuIII
based matallogel and coordination polymers.5c, 8b, 8cAs seen in Fig.
3a, emission spectrum of TbL extends from 400 nm to 600 nm in
which blue (from L) and green (from TbIII) parts are major
components. Therefore, by further introducing a red component
(from EuIII) into TbL, the entire visible spectrum (400-800 nm)
could be covered via RGB approach. To validate this hypothesis,
bimetallic CPGs are prepared. TbIII and EuIII metal ions of different
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Table of Contents (TOC)
A new blue emissive gelator has been synthesized and its self-
assembly with TbIII and EuIII results in coordination polymer gels,
which show tunable emission based on stoichiometric control
over LMWG:TbIII:EuIII.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5