Multinuclear PtII Complexes: Why Three is Better Than Two to Enhance Photophysical Properties

Article abstract of DOI:10.1002/chem.202001510

The self-assembly of platinum complexes is a well-documented process that leads to interesting changes of the photophysical and electrochemical behavior as well as to a change in reactivity of the complexes. However, it is still not clear how many metal u

Full text of DOI:10.1002/chem.202001510

Accepted Article
Accepted Article
Title: Multinuclear Pt(II) Complexes: Why Three is Better Than Two to
Enhance Photophysical Properties
Authors: Luisa De Cola, Sourav Chakraborty, and Alessandro Aliprandi
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10.1002/chem.202001510
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Multinuclear Pt(II) Complexes: Why Three is Better Than Two to
Enhance Photophysical Properties
Sourav Chakraborty,[a] Alessandro Aliprandi,[a] and Luisa De Cola*[a,b]
Dedication ((optional))
Abstract: Self-assembly of platinum complexes is a well documented
process that leads to interesting changes of the photophysical and
electrochemical behavior as well modify the reactivity of the
complexes. However how many metal units must interact in order to
formation of assembled species does not only change the
absorption and emission energy, but due to a change of the
nature of the lowest transition and the increasing rigidity of the
packed units often lead to an enhancement of the emission
achieve the same desired properties of large assembly is still not clear. quantum yields (PLQY) and an elongation of the excited state
In this paper we aim to clarify the role of the number of interacting
Pt(II) unit lead to an enhancement of the spectroscopic properties and
lifetime.[20] Our group has recently reported a phosphorescent
mononuclear amphiphilic Pt(II)-complex[20] (PLQY of 1% in
how to address inter- vs intramolecular processes. We have, therefore, aerated dioxane) capable of producing
strong orange
synthesized and characterized a series of neutral multinuclear Pt(II)
complexes and studied their photophysical properties at different
concentration. Going from the monomer to the dimers we observe the
growth of a new emission band, and the enhancement of the emission
properties. Upon increasing the platinum units up to three, the
monomeric blue emission cannot be detected anymore and a
concentration independent bright yellow/orange emission due to
establishment of intramolecular metallophilic interactions is observed.
phosphorescence with a PLQY up to 84% in water/dioxane (95:5)
solution. The very high PLQY in aerated solution suggest also a
protection of the packed Pt(II) centers from dioxygen. Indeed, due
to the triplet nature of the platinum complexes emissions, one
would expect strong quenching, by dioxygen, of the long lived
(microsecond) excited state. The outstanding photophysical
properties of self-assembled structures of Pt(II) complexes have
gained high popularity in the last decades due to their good
performances into several applications such as organic light-
emitting diodes (OLEDs),[18, 34-39] field effect transistors (FETs),[40-
Introduction
41]
organic light emitting FETs (OLEFETs[42] and, more recently,
as sensors[43-49] and as labels in electrochemiluminescent
systems.[20, 50] However the self-assembly of such compounds
requires not only a precise chemical design, but also a tight
control of several parameters such as solvent composition,
temperature and pH.
Square planar Pt(II)-complexes, containing conjugated
coordinated ligands, are particularly interesting building blocks for
the creation of supramolecular nanostructures since their flat
geometry makes them prone to stack through noncovalent
interaction such as π–π stacking.[1-10] Furthermore, when Pt(II)
complexes are close enough (distance below 3.5 Å)[11-14]
metallophilic interactions between Pt centers may be established
and stable aggregates[15-17] can be observed possessing
spectroscopic properties which can be dramatically different from
the monomeric metal complex. [1-4, 18-23] Indeed the establishment
of ground state intermolecular interactions between protruding dz2
orbitals result in the formation of lower-lying molecular orbitals
thus new optical transition appears ascribed to Metal-Metal-to-
ligand Charge Transfer (MMLCT).[22-28] The changes in the optical
properties are directly correlated with the distance between the Pt
centers.[6, 11, 29-31] Thus, the shortest is the distance, the strongest
is the electronic Pt···Pt electronic interaction, the more
bathochromically shifted is the MMLCT band.[32-33] But the
An interesting open question is however how many units are
needed to obtained a full change in the spectroscopic behavior as
that observed for large assembled structure. A strategy to anwer
such a fundamental problem is to design multinuclear Pt(II)
complexes where the establishment of Pt···Pt and/or π-π
stacking interactions is obtained by polytopic auxiliary ligands.[51-
53]
For example Che et al[54] reported a series of trinuclear
tridentate cyclometalated platinum(II) complexes, tethered by tris-
phosphine auxiliary linkers, and compared the photophysical
properties with their mono- and binuclear homologues. However
the behavior of the reported multinuclear Pt(II)-complexes in
solution has not been reported at differet concentration to unravel
the intermolecular vs intramolecular interactions.
Despite several reported dinuclear complexes,[25, 55-58] to the best
of our knowledge, not a clear answer to the above question has
been given. Indeed all the reported cases show a change in the
emission color upon a Pt-Pt interaction is established, but how the
enhancement of the emission properties is correlated to the
number of unit invoved in the assemblies is still under debate.
Herein we aim to answer such fundamental questions by
comparing the spectroscopic properties of a series of neutral
mono-, bis-, and tris-Pt(II) complexes at different concentrations
in order to assess intra vs intermolecular processes. The
luminescent compounds consist of a tridentate dianionic 2,6-
bis(1H-1,2,4-triazol-5-yl)pyridine[59] chelates and ancillary
[a]
[b]
Dr. S. Chakraborty, Dr. A. Aliprandi, Prof. L. De Cola
Institut de Science et d’Ingénierie Supramoléculaires,
CNRS, UMR 7006, Université de Strasbourg
8 rue Gaspard Monge, 67000 Strasbourg, France.
E-mail: decola@unistra.fr
Prof. L. De Cola
Institute for Nanotechnology (INT), Karlsruhe Institute of Technology
Hermann-von-Helmholtz-Platz 1
76344 Eggenstein-Leopoldshafen (Germany)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.202001510
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triethylene glycol-substituted pyridine ligands (Scheme 1). We
have prepared two dimers where the phenyl ring is substituted in
the 1,2 and 1,3 positions to evaluate the effects of inter vs
intramolecular interactions on the photophysical properties.
The rationalization of the photophysical behavior observed for the
different species at different concentration in solution allow the
design of
molecularly defined systems possessing high
phosphorescence quantum yields and long excited state lifetimes
even in aereated solvents.
Results and Discussion
Synthesis and Characterization
Scheme1. Chemical formulas of monoPt(II) Pt1, 1,3-bisPt(II), Pt2a, 1,2- bisPt(II), Pt2b, and 1,2,3-trisPt(II), Pt3 complexes.
The strategy used to prepare the multinuclear complexes is to
attach the pyridines substituted in 4 position with tetraethylene
glycol (TEG) chain to di- and tri-hydroxy benzene and then
reacting the pyridines with a platinum precursor. In particular, the
mono-pyridine linker L1 (see SI, Scheme S1) was functionalized
by refluxing a stirred THF solution of tetraethylene glycol
monomethyl ether with 4-(chloromethyl)-pyridine hydrochloride in
presence of NaH under nitrogen.[60] Whereas, the synthesis of bis-
pyridine (L2a, and L2b) and tris-pyridine (L3) linkers were
achieved in two steps. First the condensation of the mono-
tosylated tetraethylene glycol with appropriate di- and tri-hydroxy
benzene results in the formation of the corresponding di- and tri-
TEG precursors (see SI, Scheme S2-S4). These di- and tri-TEG
obtain the corresponding monoPt(II), Pt1, 1,3-bisPt(II) Pt2a, 1,2-
bisPt(II), Pt2b, and 1,2,3-trisPt(II), Pt3, complexes (Scheme 1).
The 1H-NMR spectra of Pt1, Pt2a and Pt2b complexes (Figures
S4, S14, S23) show a large downfield shift of pyridine protons
adjacent to the nitrogen donor atom, confirming the metal-ligand
coordination. The other pyridine protons, as well as the protons
from auxiliary tridentate linkers, also shifts downfield compared to
their non-complexed analogs. The appearance of a sharp singlet
in 19F-NMR of complex Pt1, Pt2a and Pt2b at δ = -64.2, -64.5 and
-64.4 ppm respectively, suggest overall symmetry of these
complexes (Figure S6, S15, S24). But in case of complex Pt3 two
peaks (-64.09 and -64.10) were observed in 19F-NMR spectrum
(Figure S32), suggesting the presence of two different chemical
1
precursors
were
subsequently
treated
with
4-
environments for the three platinum complex units. In H-NMR
(chloromethyl)pyridine hydrochloride in presence of sodium
hydride under nitrogen to generate the expected TEG
functionalized 1,3-bispyridine (L2a), 1,2-bispyridine (L2b), and
1,2,3-trispyridine (L3) linkers in good to moderate yields (See SI,
(Figure S33) the appearance of two sets of aromatic peaks, for
the Pt3, in 1:2 ratio further confirms that we have a trimetallic
system in which the central platinum feels a different chemical
environment than the external platinum units. The chemical
characterization was completed by high resolution mass
spectrometry analysis (See SI). All the mass peaks are
isotopically resolved and found in well agreement with the
theoretically predicted isotopic distribution, further confirming the
formation of desired complexes.
1
Scheme S2-S4). The H NMR spectra of the linkers (L1-L3)
shows a distinctive sharp singlet, at δ = 4.61, 4.51, 4.58 and 4.62
ppm respectively, due to the benzylic –OCH2 linkage, two
doublets for the pyridine protons in aromatic region and the
characteristic signals for glycol chains in between 3.5 to 4.3 ppm
suggest the formation of the anticipated pyridine linkers which
was further supported by the ESI-MS data (see SI for detailed
characterization data).
Photophysical Characterization
In order to synthesize the desired Pt(II) complexes, Pyridine
linkers L1-L4 were treated with PtCl2(DMSO)2 as the platinum
The complexes Pt1-Pt3 were fully characterized by using
electronic absorption, steady-state and time-resolved emission
spectroscopy at different concentration from 1 to 100 µM in
CH2Cl2 (DCM). As shown in Figure 1a, the absorption spectra of
precursor,
2,6-bis[3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl]
pyridine[59] as the tridentate auxiliary ligand (py-CF3-trzH2) in
CHCl3, and in the presence of Hünig base (iPr2EtN) at 50 °C to
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Pt1 display intense bands in the UV region (λ = 254 nm, 300 nm,
337 nm). These transitions are mainly attributed to intra-
Both complexes Pt2a and Pt2b show the aggregate intense
emission band at around 580 nm along with the peaks in blue
region, which shows slight shifts featuring emission maxima at
466 and 494 mm for Pt2a and 468 and 497 mm for complex Pt2b
(PLQY = 2.9% and 2.7% for Pt2a and Pt2b, respectively at 5x10-
6
M). The absorption spectra suggest that the aggregation is
already in the ground state as clearly seen (figure 2a and b)
compared with the reference compound Pt1 in which the band in
the visible region is much less pronounced than for Pt2a and Pt2b.
This is true even though working at the same molar concentration
means that for the dimer we have twice the number of platinum
units than for the monomer and therefore a double absorbance,
in the absence of other factors. Such data are confirmed by the
excitation spectra of both the complexes at λem = 580 nm, clearly
displaying a lower energy excitation band that trace up to 500 nm
(see SI, figures S37 and S40) which is ascribed to 3MMLCT
transitions due to the establishment of Pt···Pt metallophilic
interactions.
Figure 1. (a) UV-Vis absorption spectra and (b) emission spectra, λexc = 375
nm, for complex Pt1 at variable concentration (1 µM to 100 µM) in DCM.
ligand (1IL) and metal-perturbed inter-ligand charge transfer
(1ILCT) states. At lower energy, between 360-450 nm, broad and
featureless weak bands with molar extinction coefficients, ε,
between 0.99 and 4.3×103 M-1 cm-1 (calculated at λabs = 402 nm)
for all the complexes were assigned to transitions involving the
spin-allowed singlet-manifold metal-to-ligand charge transfer,
1MLCT, and the forbidden singlet to triplet, 3MLCT, partially
allowed because the presence of the heavy atom. These
excitation processes have been previously reported for similar
cyclometalated platinum(II) complexes.[37] Increasing the
concentration from 1 µM to 100 µM does not lead to a change in
the features of the absortion spectra suggesting that Pt1 exist in
the monomeric form in such range of concentrations. The
absence of concentration dependent aggregation is also evident
in the emission spectra that are all characterized by a moderate
structured emission (PLQY = 1.5%) in the blue region of the
visible spectrum with maximum at 463 nm, and vibrational bands
at 491, 525, and a shoulder at 570 nm (figure 1b). Also the excited
state lifetimes seems to be unaffected by the concentration, with
an average value of 188 ns in aerated DCM solvent (see Table 1
for details). The steady state luminescence as well as the excited-
state lifetime of the complex are typical of the monomeric form of
such class of compounds.[19-20, 29, 50, 60-61]
Figure 2. (a, b) UV-Vis absorption spectra and (c, d) Normalized (at 467 nm)
Emission spectra (upon exc = 375 nm) for complex Pt2a (top) and Pt2b (bottom)
at variable concentration (100 µM to 1 µM) in DCM.
Table 1. Photophysical data of Platinum complexes Pt1-Pt3 in DCM (5×10–
5 M) at room temperature.
λabs nm
λex
nm
λem,max
nm
Lifetime (τ)
As expected at high concentration both Pt2a and Pt2b (figure 2c
and d) show a great tendency to form intermolecular interactions
rather than only intramolecular ones. However, a careful
examination of the emission spectra upon excitation at 375 nm, in
DCM solutions reveals that the two compounds behave differently
at low concentration. Indeed the relative emission intensities of
the monomeric band (467 nm) versus the aggregate band (580
nm) is not the same for the two systems, as one would predict
from their chemical structure. For Pt2a the distance between the
two complexes, despite the flexibility of the glycol chains, is
definitely larger than for Pt2b and indeed at 1x10-6 M and at 5x10-
Complex
(ε [103 M-1
ФPL
ns
cm-1])
460,
485,
525,
254 (33.2)
300 (22.2)
337 (3.9)
τ1 = 188.2 (88%)
τ2 = 3.8 (12%)
375
0.015
Pt1
402 (0.99)
254 (66.4)
300 (38)
337 (8.3)
402 (2.4)
254 (56)
300 (32)
337 (7.2)
402 (2.1)
256 (69.6)
300 (36.6)
337 (9.8)
402(4.3)
τ1 = 1013.6 (88%)
τ2 = 14.4 (12%)
τ1 = 258.1 (88%)
τ2 = 11.7 (12%)
580
460
580
460
Pt2a
Pt2b
Pt3
375
375
400
0.029
0.027
0.38
τ1 = 1029.0 (66%)
τ2 = 21.8 (34%)
τ1 = 235.8 (38%)
τ2 = 14.5 (62%)
6
M solutions the ratio between the high and low energy bands
(467 vs 580 nm) is much higher for complex Pt2a than for Pt2b
(figure 2d). Such behavior can be rationalized assuming that at
such concentration the predominant interactions are
intramolecular and therefore the capability to establish
590
τ = 1093.1
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intramolecular metallophilic interactions is higher for Pt2b rather
than Pt2a. Increasing the concentration leads to a large increase
of the emission intensity at 580 nm compared to the band at 467
nm suggesting the establishment of intermolecular interactions.
Not surprisingly in such range of concentrations both dimers
behave similarly (figure 2c). A possible mechanism is depicted in
scheme 2 where while Pt1 is more stable in the solvated
monomeric form, the dimers are more prone to intercalation with
other molecules. The corresponding excited state lifetimes of the
bands at 460 nm and 580 nm of Pt2a and Pt2b at the different
concentrations are shown in table S2 and S3 of the supporting
info respectively. The longer lifetime component, attributed to
3MMLCT decay, increases its relative weight at higher
concentration (See SI table S2 and S3). Noticeably, for both
complexes on decreasing the concentration from 100 to 1 µM the
lifetime of the long component attributed to the 3MMLCT transition
decreases
Pt3 complex displays a more intense low-energy absorption band
centered at ca. 461 nm and stretching towards 500 nm (Figure 3a
inset). We ca definitely state that the bands at lowest energy are
metal-metal-to-ligand charge transfer (1MMLCT and 3MMLCT)
transitions[12, 62] associated to the formation of Pt∙∙∙Pt and π-π
stacking interactions. In the case of Pt3 the MMLCT bands are
independent from the concentration and are present even in very
dilute conditions suggesting strong intramolecular electronic
interactions. Upon excitation at 400 nm, Pt3 shows only one very
intense emission band centered at 590 nm (Figure 3) with a PLQY
value up to 38% in air-equilibrated DCM solutions. The excited
state decays with a mono-exponential kinetic of around 1 µs (see
table S4), which is ascribed to a 3MMLCT transition resulting from
intramolecular metallophilic interactions. The ground state
interaction between platinum units is also confirmed by the
appearance of the typical MMLCT bands stretching up to 500 nm
in the excitation spectra of complex Pt3, monitored at the
emission maximum. The concentration dependency of the
emission spectra in range of 100 to 1 µM shows no effect on the
emission profile of complex (figure 3b) as well as the lifetime
decay of the excited state component remains unchanged (see
table 2). In addition, for such complex the elimination of the
dioxygen from the solution does not have any pronounced effect
on the exited state life time, which showed only a marginal
elongation (scheme 2).
Table 2. Excited state lifetimes (τ) of Pt2a, Pt2b, and Pt3 at different conc.
with and without oxygen recorded at em = 580 nm, upon exc = 375 nm.
Concentration
(M)
Pt2a
Pt2b
Pt3
τ, ns
τ, ns
τ, ns
Scheme 2. Mechanistic illustration of the intra and intermolecular aggregation
of mono-, bis- and tris-nuclear Pt(II)-complexes in effect of concentration. The
monomeric form of the complexes (at dilution) are shown at bottom and the
formation of aggregated species (at higher concentration) on top.
τ1 = 1058.1 (94%)
τ2 = 15.4 (6%)
τ1 = 1013.6 (88%)
τ2 = 14.6 (12%)
τ1 = 851.6 (75%)
τ2 = 13.5 (25%)
τ1 = 782.7 (71%)
τ2 = 13.2 (28%)
τ1 = 702.9 (62%)
τ2 = 12.8 ns (38%)
τ1 = 1107.1 (66%)
τ2 = 12.6 (34%)
τ1 = 1072.4 (76%)
τ2 = 22.9 (24%)
τ1 = 1029.0 (66%)
τ2 = 21.8 (34%)
τ1 = 855.4 (52%)
τ2 = 23.9 (48%)
τ1 = 796.2 (33%)
τ2 = 20.7 (67%)
τ1 = 708.4 (44%)
τ2 = 22.0 (56%)
τ1 = 902.1 (53%)
τ2 = 24.3 (47%)
1×10-4
5×10-5
1×10-5
5×10-6
1×10-6
1084.7
1093.1
1122.6
1126.7
1222.5
1480.4
gradually from 1 to 0.7 µs suggesting an effective dioxygen
quenching when the intramolecular interactions are reduced.
Indeed the excited state lifetimes recorded in deaerated 1 µM
solutions for both complex Pt2a and Pt2b are close to the one
recorded for aerated high concentrated (100 M) solutions as
shown in table 2.
1×10-6
Deaerated
Interestingly, on increasing the number of Pt(II) units from two to
three the scenario changes completely as shown in figure 3.
We can conclude therefore that for Pt3 the 3MMLCT emitting state
is tightly packed, as shown by the high emission quantum yield
and by the lack of diffusion of the dioxygen that should quench
such long lived emitters.
Conclusions
To rationalize the properties emerging from the self-assembly of
luminescent platinum complexes, and in particular the role played
by the number of units to reach and enhancement of the
photophysical properties, we have investigated a series of neutral
mono-, bis- and tris-Platinum(II) complexes. The compounds
based on platinum metal ions, a tridentate N^N^N chromophoric
Figure 3. (a) UV-Vis absorption spectra, and (b) Emission spectra (upon exc
=
400 nm) for complex Pt3 at variable concentration (0.1 mM to 1.0 µM) in DCM.
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Keywords: Aggregation-Induced Emission • Metallophilic
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10.1002/chem.202001510
Chemistry - A European Journal
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Multinuclear Pt(II) complexes provide
valuable insight on the formation of
closed shell Pt···Pt metallophilic
interactions and on the corresponding
MMLCT excited state. Persistent
aggregation induced emission, which
is independent from the media, is
observed for the trinuclear species
demonstrating that three is better than
two.
Sourav Chakraborty, Alessandro
Aliprandi, Luisa De Cola*
Page No. – Page No.
Multinuclear Pt(II) Complexes: Why
Three is Better Than Two to Enhance
Photophysical Properties
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