Controllable three-component luminescence from a 1,8-naphthalimide/Eu(iii) complex: White light emission from a single molecule

Article abstract of DOI:10.1039/c2cc17182a

A macrocycle-appended naphthalimide derivative and its Eu(iii) complex show triple luminescence from isolated naphthalimide (blue), aggregated naphthalimide excimers (green) and Eu centres (red) with the balance being sensitive to the degree of aggregation, allowing white light emission to be obtained from a single molecule. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17182a

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Chemical Communications
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Volume 48 | Number 22 | 14 March 2012 | Pages 2733–2824
ISSN 1359-7345
COMMUNICATION
M. D. Ward et al.
Controllable three-component luminescence from a 1,8-naphthalimide/Eu(III) complex: white light emission
from a single molecule

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Controllable three-component luminescence from a 1,8-naphthalimide/
Eu(III) complex: white light emission from a single moleculew
Alexander H. Shelton, Igor V. Sazanovich, Julia A. Weinstein and Michael D. Ward*
Received 18th November 2011, Accepted 22nd December 2011
DOI: 10.1039/c2cc17182a
A macrocycle-appended naphthalimide derivative and its Eu(III)
complex show triple luminescence from isolated naphthalimide
In this communication we report a new approach to this
problem that uses a combination of solvent-dependent aggrega-
tion of a functionalised aromatic (1,8-naphthalimide) chromo-
phore to generate tunable blue or blue/green luminescence, plus
partial naphthalimide - Eu(III) energy-transfer to generate some
additional sensitised red luminescence. The result is highly
aggregation-dependent luminescence that can be white in the
correct solvent composition and also in the solid state.
(blue), aggregated naphthalimide excimers (green) and Eu centres
(red) with the balance being sensitive to the degree of aggregation,
allowing white light emission to be obtained from a single molecule.
Emission of white light from a single molecule is an important
chemical target as it will considerably simplify the preparation
of luminescent display and lighting devices by removing the
need for two or three separate components (e.g. red/blue/green
luminophores) which may degrade at different rates or have
1
,8-Naphthalimide has several properties which make it
ideal for use in this context. It shows short-lived (o1 ns) blue
fluorescence from the singlet p–p* state, and efficient inter-
1
system crossing to the lowest triplet state (f
acts as energy-donor to Eu(III). The energy of this triplet state
E 0.95) which
different requirements for device fabrication. In molecular
ISC
9
species, white-light emission has been achieved in various
ways. De Cola et al. used a combination of blue-emitting Ir
and red-emitting Eu units to generate white light from a
ꢀ
1
18 500 cm ) is just high enough to sensitise the emissive D
5
(
0
ꢀ1 10
state of Eu(III) (17 500 cm ). In addition, aggregation of 1,8-
naphthalimide units can result in formation of excimers which
fluoresce at lower energy than the monomer fluorescence: this
is not normally seen in monomeric naphthalimides, but occurs
in covalently-linked bis-naphthalimides in which the two
2
dinuclear Ir(III)/Eu(III) complex, an approach which we have
3
also investigated. Williams and co-workers took a different
approach by using a combination of blue luminescence from
mononuclear Pt(II) complexes and the red luminescence resulting
from excimer formation when the complex aggregates in different
solvents, thereby achieving a solvent-dependent balance of blue and
1
1
fluorophores are held close together. Thus energy-transfer
from the naphthalimide triplet state to generate red emission
from Eu(III) is possible, and – if aggregation can be induced –
the fluorescence from the singlet state can be varied with a
balance of blue and green components.
4
red-emission bands from the same molecule. A balance between
monomer and excimer or exciplex emission has also been used by
5
other groups to generate white light emission, and a feature of this
The structural formula of the naphthalimide/macrocycle ligand
method is that it allows a degree of tunability by controlling the
degree of association between monomer units and hence the
3 3
H L is shown in Scheme 1. H L has a heptadentate binding
1
2
4,5
domain which will give kinetically inert Ln(III) complexes, and
we prepared from this EuL and GdL, the latter to use in control
experiments. Full details of all synthesis and characterisation,
including electronic absorption spectra (Fig. S1), are in Supporting
Information.
balance between high- and low-energy emissive components.
With a combination of blue and green emitting organic chromo-
phores and a red-emitting Eu(III) unit, three-colour R/G/B emission
from a single molecule has been achieved in some cases generating
6
white light emission overall. A variety of other approaches has
The effect of solvent-induced aggregation on luminescence is
shown by the free ligand H L. In aqueous solution the
also been studied including preparation of a molecule containing
complementary blue- and orange-emitting units between which
energy-transfer is blocked because of the requirement for excited-
state proton transfer, leading to concentration-independent white
3
fluorescence spectrum shows the typical intense band centred
at 396 nm, extending down into the blue region of the visible
7
3
spectrum. An isoabsorbing solution of H L in MeCN shows a
luminescence; and a deceptively simple mononuclear Ir(III)
complex which shows broad, panchromatic (white) luminescence
8
from a mixed excited state has been reported.
Department of Chemistry, University of Sheffield, Sheffield S3 7HF,
UK. E-mail: m.d.ward@sheffield.ac.uk
w Electronic supplementary information (ESI) available: Experimental
information and additional figures. See DOI: 10.1039/c2cc17182a
Scheme 1 Structural formulae of the compounds in this paper.
Chem. Commun., 2012, 48, 2749–2751 2749
This journal is c The Royal Society of Chemistry 2012

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compared to the luminescence spectrum in water; Fig. 2, black
trace, and Fig. 3) a reduction in intensity and change in shape
cf. Fig. 1) of the higher energy fluorescence band and the
(
appearance of the lower-energy excimer emission component
centred at about 460 nm which extends down into the green
region of the spectrum. Given that hydrogen-bonding between
3
molecules of EuL is not possible in the way that it is for H L due
to the absence of acidic protons to act as donors, this aggregation
to give excimer emission most likely arises from p-stacking. The
balance between these two higher-energy emission components
in MeCN is somewhat concentration-dependent, but at a concen-
tration of 8 mM we see three emission components from a single
molecule in the blue, green and red regions that combine to give
light with CIE coordinates of 0.27, 0.25 (Fig. 3) which is at the
blue end of the region generally considered ‘‘white’’ (CCT =
Fig. 1 Luminescence spectra of H
circles) based on isoabsorbing solutions at the excitation wavelength.
F = 0.022 in 1 : 1 MeCN/water.
3
L in water (squares) and MeCN
(
reduced intensity and slight changes in the relative intensities
of the vibronic components for this fluorescence band and an
additional broad low-energy feature centred at 475 nm which
1
9500 K).z
To investigate the kinetics of the naphthalimide - Eu
1
1
energy-transfer process we used transient absorption spectro-
scopy to examine its triplet state. Excitation of EuL at 355 nm
in degassed MeCN resulted in appearance of an excited-state
absorption spectrum whose main feature is at 470 nm, with
weaker bands at 440 and 370 nm, exactly characteristic of the
is characteristic of an excimer (Fig. 1).
Given the nature of H L it is possible that both p–p stacking
between naphthalimide units, and hydrogen bonding involving
3
2
H-bond donors (–CO H groups) and H-bond acceptors (the
naphthalimide N and O atoms) on different molecules, contri-
1
bute to the aggregation. The H NMR spectrum of H L in
3
9
,10a
naphthalimide p–p* triplet excited state (Fig S3);
the
decay of the transient absorption spectrum allowed determi-
nation of the lifetime of this triplet excited state to be 11 ms.
Time-resolved measurements on the 615 nm sensitised Eu-based
emission band in MeCN revealed – in addition to the expected
MeCN is broad and poorly-defined, characteristic of aggrega-
tion, but the spectrum becomes sharp as soon as a few drops of
water are added to the sample (Fig. S2). This implies that
H-bonding plays an important role in aggregation of H L in
3
12
slow decay (t = 1.1 ms, F = 0.12y ) – a grow-in with a time
constant of 14 ms (Fig. S4, ESIw). This matches well the 11 ms
decay of the donor (naphthalimide triplet state), confirming the
occurrence of naphthalimide - Eu(III) energy-transfer in EuL.
solution, even though the excimer emission component must
arise from p-stacking between the aromatic naphthalimide units.
The luminescence spectrum of EuL in water likewise is
dominated by the naphthalimide fluorescence peak centred in
this case at 385 nm. In addition we can see the series of line-like
emission bands at lower energy that is characteristic of Eu(III)
We also compared the Eu emission lifetime for EuL in H
O from which a hydration number of 2 was found for the
Eu(III) centre, as expected given the 7-coordinate environment
2
O and
D
2
5
7
0 2
with the most intense component, as usual, being the D - F
13
provided by the ligand.
line at 615 nm (Fig. 2, red line). This combination of intense blue/
violet and weaker red emission components generates overall
purple emission with coordinates of 0.23, 0.10 in the standard
CIE colour coordinate system. In contrast, in MeCN some
aggregation occurs and we see (for an isoabsorbing solution
Given the different CIE colour coordinates of EuL in water
and MeCN, we investigated the luminescence in mixed
MeCN/water systems of different proportions but at a fixed
concentration. We observed a smooth variation in lumines-
cence colour from purple (in water) to white (in pure MeCN)
as shown in Fig. 2, with the arrow in the inset showing the
direction of change of the colour as the proportion of water in
Fig. 2 Luminescence spectra of EuL in different solvents; all solutions
were isoabsorbing at the excitation wavelength. Solvents were
water/MeCN mixtures with the following water : MeCN proportions:
red, 100 : 0; blue, 75 : 25; green, 50 : 50: cyan, 25 : 75; black, 0 : 100. The
inset shows how the CIE coordinates vary: the coordinates based on
the blue, green and cyan spectra overlap closely. The arrow shows the
direction of change as the MeCN proportion increases; ‘‘X’’ denotes the
pure white point (0.33, 0.33).
Fig. 3 Expansion of the luminescence spectrum of EuL in MeCN
(black trace in Fig. 2) showing clearly the three separate luminescence
components (A, isolated naphthalimide; B, naphthalimide excimer; C,
europium), with (inset) a picture of the luminescence from a sample of
EuL under these conditions. * = Raman scattering artefact.
2
750 Chem. Commun., 2012, 48, 2749–2751
This journal is c The Royal Society of Chemistry 2012

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component from a Eu(III) centre in the same molecule, allows
tunable three-component emission from a single molecule which
appears white under some conditions. In solution EuL shows white
emission in MeCN solution from a balance of purple/blue
(naphthalimide fluorescence), blue/green (naphthalimide excimer)
and red (Eu) components. In the solid state H L gives white
3
emission on its own due to aggregation effects broadening the
luminescence spectrum such that its emission is panchromatic.
We thank the Leverhulme Trust for financial support.
Notes and references
z CCT = Correlated Colour Temperature.
Fig. 4 Luminescence spectra of H
3
L (green), EuL (red) and GdL
y Quantum yield for Eu-based emission estimated from F = t/t
n
where t = 1.1 ms and, t
ref. 12).
n
= the ‘‘natural’’ radiative lifetime of 9.5 ms
(blue) as isoabsorbing thin films. The insets show (a) white emission
(
3
from solid H L; (b) bright cyan emission from EuL; and (c); how the
z Time-resolved measurements show that this broad luminescence has
three decay components with lifetimes of 20 ns, 250 ns and 4.5 ms; the
first two of these can reasonably be ascribed to excimer-type fluores-
cence from naphthalimide units in different aggregation states, and the
4.5 ms second (which was a very small contributor) could be a trace of
phosphorescence from the triplet state.
CIE coordinates for the three compounds vary.
the solvent mixture decreases. Part of this is due to the known
reduction in fluorescence quantum yield (blue component) as
9
the solvent polarity is decreased, but as water disappears from
1
(a) G. M. Farinola and R. Ragni, Chem. Soc. Rev., 2011, 40, 3467;
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the solvent mixture the lower-energy excimer feature starts to
become significant as aggregation occurs and the blue/green
balance changes. Thus the overall emission colour can be
tuned according to solvent composition which alters the
balance between the three emission components.
(
2 P. Coppo, M. Duati, V. N. Kozhevnikov, J. W. Hofstraat and
L. De Cola, Angew. Chem., Int. Ed., 2005, 44, 1806.
3
4
D. Sykes, I. S. Tidmarsh, A. Barbieri, I. V. Sazanovich,
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Finally, given the effect of aggregation on the luminescence
of these compounds, we examined their luminescence in thin
films (prepared by slow evaporation of MeOH solutions on a
J. A. G. Williams, Adv. Mater., 2007, 19, 4000; (b) W. Mroz,
´
C. Botta, U. Giovanella, E. Rossi, A. Colombo, C. Dragonetti,
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Chem., 2011, 21, 8653.
quartz plate). Fig. 4 shows luminescence spectra of H
3
L, GdL
and EuL measured using isoabsorbing thin films. Solid H
3
L
5 J. Karpiuk, E. Karolak and J. Nowacki, Phys. Chem. Chem. Phys.,
2010, 12, 8804.
showed none of the UV/blue emission characteristic of isolated
naphthalimide molecules: instead a broad emission band span-
ning the visible spectrum, but centred at 480 nm, was observed
6
(a) G.-L. Law, K.-L. Wong, H.-L. Tam, K.-W. Cheah and
W.-T. Wong, Inorg. Chem., 2009, 48, 10492; (b) G.-J. He,
D. Guo, C. He, X.-L. Zhang, X.-W. Zhao and C.-Y. Duan, Angew.
Chem., Int. Ed., 2009, 48, 6132.
(Fig. 4) because of aggregation effects.z Surprisingly, the colour
7
S. Park, J. E. Kwon, S. H. Kim, J. Seo, K. Chung, S.-Y. Park,
D.-J. Jang, B. M. Medina, J. Gierschner and S. Y. Park, J. Am.
Chem. Soc., 2009, 131, 14043.
of this broad luminescence of H L alone is in the white region
3
(
CIE coordinates 0.28, 0.35; CCT = 7800 K)z; however it is
relatively weak, probably because luminescence is quenched by
electron-transfer from the tertiary amine of the macrocycle.
Binding of Gd(III) into the macrocycle prevents this quenching
and the emission of GdL is much stronger (Fig. 4) but it appears
blue (CIE coordinates 0.19, 0.31; CCT = 21900 K)z, because the
red tail of the emission spectrum of GdL is reduced compared to
8 H. J. Bolink, F. De. Angelis, E. Baranoff, C. Klein, S. Fantacci,
E. Coronado, M. Sessolo, K. Kalyanasundaram, M. Gratzel and
M. K. Nazeeruddin, Chem. Commun., 2009, 4672.
V. Wintgens, P. Valat, J. Kossanyi, L. Biczok, A. Demeter and
T. Berces, J. Chem. Soc., Faraday Trans., 1994, 90, 411.
¨
9
´
10 (a) M. de Sousa, M. Kluciar, S. Abad, M. A. Miranda, B. de
Castro and U. Pischel, Photochem. Photobiol. Sci., 2004, 3, 639;
´
(b) J. P. Cross, M. Lauz, P. D. Badger and S. Petoud, J. Am. Chem.
Soc., 2004, 126, 16278.
3
H L (see Fig. S5, ESI, for comparison of normalised spectral
profiles), which must arise from differences in solid-state aggre-
gation because GdL cannot hydrogen-bond to itself. In EuL
however, addition of the sensitised Eu-based luminescence restores
some of the red component that is missing from the luminescence
of solid GdL, affording bright cyan luminescence from solid EuL
1
1 (a) Z. Xu, J. Yoon and D. R. Spring, Chem. Commun., 2010,
46, 2563; (b) T. C. Barros, P. B. Filho, V. G. Toscano and
M. J. Politi, J. Photochem. Photobiol., A, 1995, 89, 141;
(c) D. W. Cho, M. Fujitsuka, A. Sugimoto and T. Majima,
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12 M. H. V. Werts, R. T. F. Jukes and J. W. Verhoeven, Phys. Chem.
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(CIE coordinates 0.21, 0.31; CCT = 17600 K;z Fig. 4).
1
3 (a) D. Parker, Coord. Chem. Rev., 2000, 205, 109; (b) E. Toth,
´
In conclusion, the combination of tunable emission in the
blue/green region from the naphthalimide chromophore according
to its degree of aggregation, and a sensitised red emission
L. Helm and A. E. Merbach, in Comprehensive Coordination
Chemistry, ed. M. D. Ward, Elsevier, Oxford, 2nd edn, 2004,
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2749–2751 2751