Ultra-thin films of amphiphilic lanthanide complexes: multi-colour emission from molecular monolayers

Article abstract of DOI:10.1039/d1cc02628c

We report the synthesis and Langmuir-Blodgett deposition of 4 brightly emissive lanthanide amphiphiles that can be co-deposited to give multi-emissive ultra-thin films where two, three and four distinct lanthanide emission profiles are observed. To the be

Full text of DOI:10.1039/d1cc02628c

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Ultra-thin films of amphiphilic lanthanide
complexes: multi-colour emission from molecular
Cite this: DOI: 10.1039/d1cc02628c
monolayers†
Received 19th May 2021,
Accepted 12th July 2021
Alex T. O’Neil,
John A. Harrison
and Jonathan A. Kitchen
*
DOI: 10.1039/d1cc02628c
rsc.li/chemcomm
We report the synthesis and Langmuir–Blodgett deposition of 4
With this in mind we have designed an amphiphilic ligand
brightly emissive lanthanide amphiphiles that can be co-deposited capable of forming brightly emissive lanthanide complexes for
to give multi-emissive ultra-thin films where two, three and four the express purpose of forming multi-emissive films. Utilising
distinct lanthanide emission profiles are observed. To the best of our previously developed 1,2,3-triazole ‘‘click’’ synthetic strategy
3
1
our knowledge, this is the first report of a four-component emissive to functionalise 2,6-pyridyldiamides, we have been able to
Langmuir–Blodgett film.
introduce an alkyl chain for LB deposition into a 2,6-pyridyl-
dicarbonyl core (ligand 1, Fig. 1).
3
+
3+
3+
3+
The development of advanced multi-functional molecular materials
3
Complexes of Ln(1) (where Ln = Eu , Tb , Dy , Sm and
3
+
is a highly active research area in modern chemical and materials La ) were prepared by refluxing three equivalents of 1 with
1
–3
science.
Within this field, luminescent lanthanide-based one equivalent of lanthanide salt [either Ln(Cl) ÁxH O or
3
2
materials are attractive targets owing to their unique optical Ln(CF SO )ÁxH O] and three equivalents of triethylamine, in
3
3
2
properties, e.g. solid-state emission, line-like emission bands, DCM:MeOH (1 : 1) under microwave irradiation. The resulting
2
–7
easily tuned quantum yields and long-lived excited states.
clear yellow solutions were subjected to vapour diffusion of
These characteristics have led to these compounds being used diethyl ether which resulted in off-white precipitates. Decanting
7
7
8
8
as sensors, imaging agents, lasers, optical displays, and off the solvent and air drying the precipitates yielded off-white
4
,6,9
multi-emissive materials.
Multi-emissive materials (in this solids in good yields (51–77%). Complex formation was con-
3
+
context materials either made up of multiple Ln containing firmed by elemental analysis, IR and HRMS (see ESI† for
molecular species or molecular species containing two or more details). Lanthanide coordination within the NO2 pocket is
3
+
10–17
Ln ions)
in particular are high value targets owing to their suggested by the notable shift to lower energy for both the
À1
À1
propensity towards more advanced and complex applications CQO stretches from 1745 cm
including molecular barcodes, ratiometric sensors, molecular (amide) to a merged peak at 1636–3 cm , a shift commonly
logic gates and white emissive solids.
functional materials, it is desirable to immobilise Ln multi-
(carboxyl) and 1669 cm
12
13
À1
18
14,16,19
3+
32
To translate into associated with coordination to Ln ions.
3+
3
Photophysical properties of 1 and the Ln(1) complexes were
emissive species onto surfaces, however only limited examples measured using 0.01 mM solutions in either MeOH or MeCN.
5
,15,16,19–25
exist of surface immobilised multi-emissive species.
The absorption spectra of 1 showed an intense peak at 222 nm
Langmuir–Blodgett (LB) deposition is promising technique for and a broad three peak signal (265, 271 and 280 nm), with the
2
6,27
preparing multi-emissive lanthanide-based thin-films,
allows for multiple discrete amphiphilic species to be self- central pyridyl unit as seen in previous studies.
assembled into layers – either through sequential building up of UV-absorption spectra are observed for Ln(1) however, they
as it latter assigned to the n - p* and p - p* transitions from the
3
3,34
Similar
3
layers of different species (multi-layered approach), or through show a slight red shift in the broad peak at B270 nm and a
mixing different amphiphiles together prior to deposition (mixed significant increase in the absorbance.
2
2,28
amphiphile approach).
Additionally, LB also gives reproducible
The solid lanthanide complexes (excluding non-emissive
27,29,30
3+
deposition, and control over film thickness and shape.
La ) showed classical lanthanide emission under short-wave
UV irradiation (Fig. 1b). Solution based photoluminescence
studies were carried out on all emissive complexes with an
Chemistry, School of Natural and Computational Sciences, Massey University,
Auckland, New Zealand. E-mail: J.Kitchen@massey.ac.nz
3
excitation wavelength of 275 nm. Eu(1) showed the expected
†
Electronic supplementary information (ESI) available: Full synthesis and character-
3
+
5
7
0 J
red emission from the Eu with D - F (J = 0–4) transitions.
isation details and crystallographic information. CCDC 2046714. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/d1cc02628c
Tb(1)
3
, Sm(1)
3
and Dy(1)
3
also exhibited the typical emissive
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Fig. 1 – (Left) Reaction scheme for generating Ln(1)
3
complexes from 1. (Right) (a) Lanthanide complexes Ln(1)
, Tb(1) and Eu(1)
3
without excitation source and (b) under
shortwave UV irradiation: left to right – Sm(1)
3
, Dy(1)
3
3
3
.
5
7
transitions from D4 depopulation to corresponding FJ
4
6
4
(
J = 6–3), G depopulation to H (J = 5/2 À 11/2) and F
5
/2
J
9/2
6
depopulation to H
Dy(1) , the overall emission appears close to white with CIE
coordinates of x = 0.3346 and y =0.3829 (pure white emission
J
(J = 15/2 À 9/2) respectively. Notably for
3
1
4
being x,y = 0.33), owing to the combination of its emissive
4
6
4
6
blue ( F9/2 - H15/2), yellow ( F9/2 - H15/2) and weaker red
4
6
35
(
F
9/2 - H11/2 and 9/2) transitions (Figs. S65, S81 ESI†). Life-
time measurements for Eu(1) and Tb(1) , gave values of 1.960
3
3
and 0.984 ms respectively which fit to single exponential decay
functions indicative of single species emission. Excitation plots
3
+
3+
3+
of the major emitting peaks Eu (616 nm), Tb (545 nm), Dy
(
3
+
480 and 573 nm) and Sm (600 nm), showed similar spectral
features (200 nm to 300 nm) to their electronic absorption Fig. 2 – Surface pressure vs. area isotherms, of Ln(1) complexes (where
3
3
+
3+
3+
3+
spectra with two major excitation points around 230 nm and Ln = Eu , Tb , Dy and Sm ). Insert: Stability measurement of com-
À1
plexes – held at 30 mN m for 460 min.
275 nm, indicating the population of lanthanide excited states
through indirect excitation. Overall quantum yields of the
emissive lanthanide complexes were calculated by the relative
3
6
method in MeOH by comparing to Cs
3
[Eu(dpa)
3
]Á8H
2
O and (i.e. absorption at 220 nm and broad absorption between 260–
gave 290 nm). Importantly, on transfer to the surface, the complexes
gave a retained their excellent emissive properties (Fig. 4). Photophy-
sical properties (emission and excitation) remained unchanged
3
7
Cs
3
[Tb(dpa)
3
]Á8H
2
O standards. Eu(1)
3
, Tb(1) and Dy(1)
3
3
quantum yields of 22.4%, 11.9% and 2.9%, whilst Sm(1)
lower yield of 0.32%.
3
3
We next investigated the ability of the Ln(1) complexes to on deposition when compared to the solution-based measure-
self-assemble at an air–water interface and form Langmuir ments (Fig. S58–S66, in the ESI†). The monolayer films
films. 20 mL aliquots of complex (using CHCl :MeOH (20 : 1) remained highly emissive over extended periods of time with
3
as the spreading solvent) were applied onto the surface of a films still emissive after 6 months post deposition (Fig. S47, in
pure water sub-phase at room temperature. All complexes the ESI†). The Dy(1) monolayer film also retains its close to
3
showed near identical surface-pressure vs. area isotherms in white single-component emission, with CIE coordinates of x =
which an exponential increase in surface pressure was observed 0.3416 and y = 0.3876 (Fig. 3). Multi-layering LB deposition
upon decrease of the trough area (Fig. 2). The films collapsed at studies were next carried out to both gain insight into the nature
À1
2
5
3–59 mN m with molecular areas of 78–81 Å . The stability of the deposition process and to increase emission intensity. All
of the Langmuir films were also evaluated by keeping the complexes showed film deposition on emersion and partial
monolayers at the liquid-condensed phase for prolonged periods deposition on immersion of the substrate which is indicative
3
8
of time (460 min) while monitoring the surface pressure. All of a Y-type multi-layered film. Emission intensity for the Eu(1)3
complexes displayed excellent stability profiles (Fig. 2), thus giving complex was observed to increase as 3, 5, and 7 successive layers
the ideal situation for film deposition.
Langmuir–Blodgett deposition was carried out on either a an 8 layered Eu(1)
were deposited (Fig. S83 – see ESI† for details). FT-IR spectra of
3
film on a CaF
2
slide showed distinct bands
À1
square quartz slide (30 mm  30 mm, 1 mm thick) for photo- associated with the CQO stretch at 1638 cm , notably in the
physical or a circular CaF
FT-IR studies. Monolayer deposition was observed on emersion further indicating that the complex remains intact on deposition
up-stroke) of the slides from the water sub-phase with transfer (Fig. S57 and Table S3, ESI†).
ratios (tr) of ca. 1 indicating near complete coverage of the slides With the near identical Langmuir film properties (i.e. iso-
Fig. S45 and S46 and Table S1, in the ESI†). Successful transfer therm and stability profiles) of these complexes, attempts to
2
window (ø = 20 mm, 1 mm thick) for region associated with the complex and not the base ligand,
(
(
3
+
of the molecular units was confirmed by UV/vis-absorption and sequentially layer different Ln complexes on each other were
FT-IR (see below). UV/vis-absorption of the monolayer films carried out. By multi-layering different lanthanide complexes,
3 3
retained the same spectral features as observed in solution specifically Eu(1) with a second layer of Tb(1) , we were able to
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Fig. 5 – Steady-state emission from monolayered mixed Eu(1)
3 3
: Tb(1)
(
1 : 1) film. Images are of (left) Eu(1) : Tb(1) (1 : 1) in solution (middle) multi-
3
3
layered mixed film and (right) monolayer mixed amphiphile film under
shortwave UV irradiation.
Fig. 3 – (a) 1931 CIE chromaticity diagram with calculated CIE coordinated
of monolayer LB films of Eu(1)
Eu(1) : Dy(1) (1: 10), Eu(1) : Tb(1)
Dy(1) : Sm(1) (1: 1: 10 :50) see ESI† for coordinates.
3
, Tb(1)
3
, Sm(1)
3
Dy(1)
3
, Eu(1)
3
: Tb(1)
: Tb(1)
3
(1 : 1), Langmuir film formed from this mixed amphiphile system were,
3
3
3
3
:Dy(1)
3
(1 :1 :10) and Eu(1)
3
3
:
as expected, near identical to the films of the mono-component
3
3
systems (Fig. S68 and S69, in the ESI†). Upon deposition of the
3
+
3+
mixed Tb /Eu film onto a quartz slide, yellow/orange emission,
similar to that seen in solution, was observed (Fig. 5). This mixed
amphiphile approach appeared to achieve the same emission
3+
3+
tuning as the multi-layered Tb /Eu film, however the film has
superior emission uniformity across the slide (Fig. 5, inset). For
this reason, further deposition studies were carried out using the
mixed amphiphile approach.
A dual emissive monolayer was also observed via mixed
3 3
solutions of Eu(1) and Dy(1) . At a 1 : 1 ratio the emission from
3
+
3+
Dy is significantly weaker than the Eu emission and is barely
observable, however by increasing the ratio to 1 : 10 (Eu: Dy) we
were able to achieve similar emission intensity for steady-state
emission (Fig. S70, ESI†). Attempts at colour tuning different
combinations of Dy(1) :Eu(1) and Dy(1) :Sm(1) were carried
Fig. 4 – (a) Monolayer LB of Eu(1)
under shortwave UV irradiation, and (b) without shortwave UV irradiation.
3 3 3 3
, Sm(1) , Dy(1) , Tb(1) and blank slide
3
3
3
3
out in solution in an attempt to improve overall white emission
generate a dual emissive surface (which gave an overall yellow/ but were found ineffective (see ESI† for details).
orange emission). Whilst multi-layering was found to increase
With successful dual emissive films readily prepared, we
overall emission intensity and give multiple emission bands, next investigated the possibility of developing triply emissive,
unfortunately the films were routinely non-uniform, as seen mixed amphiphile films by incorporating three emissive com-
3 3 3
with the green, red and orange emissions observed (Fig. 5). In plexes. Indeed, a solution of Eu(1) , Tb(1) and Dy(1) in a
order to circumvent the issues observed when using a multi- 1 : 1 : 10 (Eu : Tb : Dy) ratio gave a triply emissive film where distinct
3+
layered approach, we moved to a mixed amphiphile approach. emission from all three Ln ions was obtained (Fig. S72, ESI†).
We believed that with the different complexes having near The final aspect of our study was to incorporate four visibly
identical Langmuir film isotherms, by mixing different amphi- emitting lanthanide complexes into a quadruply emitting film.
philes together in solution, deposition should allow multi-emissive We extended the technique used to make dual and triply emissive
3+
monolayers to be achieved.
Chloroform solutions for LB deposition were prepared contain- the optimum ratio of 1 : 1 : 10 : 50 [Eu(1)
ing different ratios of the emissive complexes. This approach giving near equal emission intensities for each Ln (Fig. 6).
allowed us to achieve relatively similar intensities from each lantha- X-ray photoelectron spectroscopy (XPS) was performed on
films by incorporating Sm as the fourth emitting complex with
3
:Tb(1) :Dy(1) :Sm(1) ]
3
3
3
3+
3+
nide ion (Ln ) and tune the emission profile in solution before films to confirm the atomic composition. XPS revealed the
deposition. Initially we employed a dual emissive system consisting presence of O, N, and C on all films indicating the presence of 1
3 3
of a 1 : 1 ratio of Eu(1) and Tb(1) – the solution state emission on the surface. Importantly, for all non-mixed monolayers, the
3+
3+
profile showed both Tb and Eu emission and gave a yellow/ distinct peaks associated with the 3d3/2 and 3d5/2 binding
orange emission with CIE coordinates of x = 0.4530 and y =0.4828 energy bands for lanthanide ions (see Fig S99–S102, ESI†)
3 3 3
(Fig. S78 in the ESI†). The properties (isotherm and stability) of the were seen. In the case of Eu(1) , Tb(1) and Sm(1) monolayer
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2
3
4
5
6
7
8
9
J. A. Kitchen, Coord. Chem. Rev., 2017, 340, 232–246.
S. V. Eliseeva and J.-C. G. B u¨ nzli, Chem. Soc. Rev., 2010, 39, 189–227.
J.-C. G. B u¨ nzli, Eur. J. Inorg. Chem., 2017, 5058–5063.
M. Hasegawa and A. Ishii, Coord. Chem. Rev., 2020, 421, 213458.
J.-C. G. B u¨ nzli, Coord. Chem. Rev., 2015, 293-294, 19–47.
A. J. Amoroso and S. J. A. Pope, Chem. Soc. Rev., 2015, 44, 4723–4742.
J.-C. G. B u¨ nzli, Trends Chem., 2019, 1, 751–762.
J.-C. G. B u¨ nzli, Chem. Rev., 2010, 110, 2729–2755.
1
1
1
0 Z. Zhang, Y. Chen, H. Chang, Y. Wang, X. Li and X. Zhu, J. Mater.
Chem. C, 2020, 8, 2205–2210.
1 O. Kotova, S. J. Bradberry, A. J. Savyasachi and T. Gunnlaugsson,
Dalton Trans., 2018, 47, 16377–16387.
2 J. Wang, Y. Suffren, C. Daiguebonne, S. Freslon, K. Bernot, G. Calvez,
L. Le Poll `e s, C. Roiland and O. Guillou, Inorg. Chem., 2019, 58,
2
659–2668.
1
1
3 T. Sun, Y. Gao, Y. Du, L. Zhou and X. Chen, Front. Chem., 2021, 8.
4 S. SeethaLekshmi, A. R. Ramya, M. L. P. Reddy and S. Varughese,
J. Photochem. Photobiol., C, 2017, 33, 109–131.
Fig. 6 – Quadruple steady-state emission from mixed monolayer LB film 15 H. Brunckova, E. Mudra, L. Rocha, E. Nassar, W. Nascimento, H. Kolev,
of Eu(1)
3
: Tb(1)
3
: Dy(1)
3
: Sm(1) (1 : 1 : 10 : 50). Insert: Mixed multi-layered
A. Kovalcikova, Z. Molcanova, M. Podobova and L. Medvecky, Appl. Surf.
Sci., 2021, 542, 148731.
6 T. Wang, P. Li and H. Li, ACS Appl. Mater. Interfaces, 2014, 6,
film under shortwave UV irradiation.
1
1
12915–12921.
7 M. Mart ´ı nez-Calvo, O. Kotova, M. E. M ¨o bius, A. P. Bell, T. McCabe,
J. J. Boland and T. Gunnlaugsson, J. Am. Chem. Soc., 2015, 137,
1983–1992.
18 M. A. Hern ´a ndez-Rodr ´ı guez, C. D. S. Brites, G. Antorrena, R. Pi n˜ ol,
R. Cases, L. P ´e rez-Garc ´ı a, M. Rodrigues, J. A. Plaza, N. Torras, I. D ´ı ez,
films, by comparing the relative signal area of the 3d5/2 peak to
the nitrogen 1s peak (and accounting for differences in photo-
electron escape depths), a ratio of approx. 1: 15 (Ln :N) was
obtained, indicating the presence of the (1: 3) M :L species on the
3
+
A. Millan and L. D. Carlos, Adv. Opt. Mater., 2020, 8, 2000312.
´
surface (Fig. S107–S109, ESI†). XPS was also carried out on the 19 H. Liu, T. Chu, Z. Rao, S. Wang, Y. Yang and W.-T. Wong, Adv. Opt.
Mater., 2015, 3, 1545–1550.
0 N. Marets, S. Kanno, S. Ogata, A. Ishii, S. Kawaguchi and M. Hasegawa,
ACS Omega, 2019, 4, 15512–15520.
dual (1 : 1) and quadruple (1 : 1 : 1 : 1) mixed films. Results con-
firmed the presence of multiple Ln ions (Fig. S103–S106, ESI†).
2
3+
To conclude, we have shown that using the amphiphilic 21 J. Ma and B. Yan, Dyes Pigm., 2018, 153, 266–274.
´
. L o´ pez-Mu n˜ oz, D. Repetto, T. Ito,
3
+
22 M. Clemente-Le o´ n, E. Coronado, A
ligand 1, we are able to generate bright, visibly emissive Ln
complexes that are suitable for LB film deposition, thus generating
T. Konya, T. Yamase, E. C. Constable, C. E. Housecroft, K. Doyle and
S. Graber, Langmuir, 2010, 26, 1316–1324.
surface immobilised luminescent materials. Furthermore, the near 23 Q.-M. Fu, H. Fu, L. Hu, L. Liu, S.-Z. Liu, Z.-L. Du and W.-Y. Wong,
J. Inorg. Organomet. Polym. Mater., 2012, 22, 97–104.
4 J. Ni, Q.-M. Fu, L. Liu, Z.-R. Gu, Z. Zhou, F.-B. Li, S.-X. Zhang, S.-
Z. Liu and Z.-L. Du, Thin Solid Films, 2013, 537, 247–251.
identical nature of the films has allowed us to generate rare
examples of multi-emissive ultra-thin-films by taking a mixed
2
amphiphile approach to LB film formation. In doing so we have 25 L. Liu, M. Chen, J. Yang, S.-Z. Liu, Z.-L. Du and W.-Y. Wong, J. Polym.
Sci., Part A: Polym. Chem., 2010, 48, 879–888.
6 D. E. Barry, J. A. Kitchen, L. Mercs, R. D. Peacock, M. Albrecht and
T. Gunnlaugsson, Dalton Trans., 2019, 48, 11317–11325.
observed monolayers with emission from two, three and four
lanthanide ions. To the best of our knowledge this study
2
presents the first examples of triple and quadruple emission 27 D. E. Barry, J. A. Kitchen, M. Albrecht, S. Faulkner and T. Gunnlaugsson,
Langmuir, 2013, 29, 11506–11515.
8 S. Sundaresan, J. A. Kitchen and S. Brooker, Inorg. Chem. Front.,
from LB monolayers. Such mixed amphiphile, multi-emissive,
monolayers have potential to generate new materials with a host
2
2020, 7, 2050–2059.
of advanced properties. For example, by incorporating different 29 D. J. Wales and J. A. Kitchen, Chem. Cent. J., 2016, 10, 72.
3
0 A. Galanti, O. Kotova, S. Blasco, C. J. Johnson, R. D. Peacock,
S. Mills, J. J. Boland, M. Albrecht and T. Gunnlaugsson, Chem. –
Eur. J., 2016, 22, 9709–9723.
analyte specific receptors on different amphiphilic complexes,
systems capable of detecting four distinct analytes are possible.
This study represents rare examples of multi-emissive surfaces, 31 A. T. O’Neil, N. Zhang, J. A. Harrison, S. M. Goldup and J. A. Kitchen,
Supramol. Chem., 2021, DOI: 10.1080/10610278.2021.1955120.
an area we are actively developing to generate more advanced
surface immobilised supramolecular materials.
3
3
3
3
3
2 R. Jagannathan and S. Soundararajan, J. Coord. Chem., 1979, 9,
1–35.
3
3 T. Le Borgne, J.-M. B ´e nech, S. Floquet, G. Bernardinelli, C. Aliprandini,
P. Bettens and C. Piguet, Dalton Trans., 2003, 3856–3868.
4 F. Renaud, C. Piguet, G. Bernardinelli, J.-C. G. B u¨ nzli and
G. Hopfgartner, Chem. – Eur. J., 1997, 3, 1646–1659.
5 M. Vijayakumar, K. Mahesvaran, D. K. Patel, S. Arunkumar and
K. Marimuthu, Opt. Mater., 2014, 37, 695–705.
6 G. A. Crosby and J. N. Demas, J. Phys. Chem. A, 1971, 75, 991–1024.
Conflicts of interest
There are no conflicts to declare.
Notes and references
37 A. S. Chauvin, F. Gumy, D. Imbert and J. C. G. B u¨ nzli, Spectrosc. Lett.,
004, 37, 517–532.
D. B. Amabilino, D. K. Smith and J. W. Steed, Chem. Soc. Rev., 2017, 38 M. C. Petty, In Langmuir-Blodgett Films: An Introduction, Cambridge
6, 2404–2420. University Press: Cambridge, 1996, pp. 39–64.
2
1
4
Chem. Commun.
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