Intramolecular excimers based on rigidly-linked platinum(II) complexes: Intense deep-red triplet luminescence in solution

Article abstract of DOI:10.1039/b807668e

By linking together two cyclometallated, N∧C∧N-coordinated platinum(ii) complexes through a xanthene core, intense luminescence in the red region of the spectrum is achieved in solution at room temperature (λ_max = 690 nm, Φ_lum = 0.20, τ = 1.7 μs in CH₂Cl₂) due to the efficient formation of an intramolecular excimer.

Full text of DOI:10.1039/b807668e

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Intramolecular excimers based on rigidly-linked platinum(II) complexes:
intense deep-red triplet luminescence in solution†
Ste´phanie Develay and J. A. Gareth Williams*
Received 8th May 2008, Accepted 18th June 2008
First published as an Advance Article on the web 7th July 2008
DOI: 10.1039/b807668e
By linking together two cyclometallated, N∧C∧N-
coordinated platinum(II) complexes through a xanthene
core, intense luminescence in the red region of the spectrum
is achieved in solution at room temperature (kmax = 690 nm,
Ulum = 0.20, s = 1.7 ls in CH2Cl2) due to the efficient
formation of an intramolecular excimer.
The formally forbidden radiative decay of triplet states of
conjugated organic ligands can be promoted by coordination to
metal ions with high spin–orbit coupling constants. A large range
of luminescent complexes of 3rd row metals, particularly Os(II),
Ir(III) and Pt(II), have been reported over the past few years.4 We
have been exploring the chemistry and applications of a family
of brightly emissive cyclometallated platinum(II) compounds
incorporating N∧C∧N-coordinating tridentate ligands based on
1,3-di(2-pyridyl)benzene (dpybH). These complexes emit from
triplet states with high luminescence quantum yields {e.g. for
Pt(dpyb)Cl, Scheme 1, Ulum = 0.6},5 and function efficiently in
OLEDs.6 At elevated concentrations in solution, they form highly
emissive excimers, giving rise to an additional band in the emission
spectrum centred at around 700 nm (Ulum ∼ 0.3). The effect is also
observed in polymer matrices and in neat thin films, offering a
strategy for creating single-dopant, white-light emitting devices
and near-IR OLEDs respectively.7
We reasoned that a molecular structure in which two
Pt(dpyb)Cl units were locked rigidly together in a face-to-face
manner might offer uniquely excimeric emission, providing a
novel route to high efficiency, deep-red triplet emitters in solution.
Tanaka et al. have shown that a 4,5-disubstituted xanthene skele-
ton can be used to bring together two metal–terpyridyl complexes
in such a way (e.g. mixed Ru, Pt derivatives were prepared),
with interplanar separations of the terpyridyl units that could
be appropriate for the present objective.8 The compound L(PtCl)2
(Scheme 1) was therefore targeted. 1,3-Di(2-pyridyl)benzene-5-
boronic acid (dpybH-B), obtained from the corresponding bromo
derivative, was subjected to a palladium-catalysed cross-coupling
reaction with 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene
(Br-xnth-Br), in the molar ratio 2 : 1.† The cross-coupling of the
The luminescence quantum yields of compounds that emit from
triplet states in the deep red and near infra-red region of the spec-
trum are generally low. The lower the energy, E, of an electronic
excited state, the more rapidly it typically undergoes non-radiative
decay through energy transfer to vibrational motions. Meanwhile,
the coefficient of spontaneous emission varies with E3, such that
radiative rate constants tend to fall off at long wavelengths.
Nevertheless, compounds that emit efficiently in the region 700–
900 nm are particularly attractive in biosensing and bioimaging
applications, owing to the relative transparency of tissue to such
wavelengths.1 In the case of triplet emitters, a further attraction is
that their longer luminescence lifetimes (often > 100 ns) may allow
time-resolved detection procedures to be employed to discriminate
from scattering and background fluorescence (< 10 ns).2 Red
triplet emitters are also under intensive investigation as phosphors
for organic light emitting devices (OLEDs), to trap and promote
light emission from the triplet excitons formed upon charge
recombination.3
Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
E-mail: j.a.g.williams@durham.ac.uk; Fax: +44 191 384 4737
† Electronic supplementary information (ESI) available: details of syn-
thetic procedures and characterisation. See DOI: 10.1039/b807668e
Scheme 1 The structures of the complexes investigated and the synthetic route employed.
4562 | Dalton Trans., 2008, 4562–4564 This journal is The Royal Society of Chemistry 2008
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first equivalent of dpybH-B took place readily under standard
conditions, but hydrodebromination of the xanthene proved to
be competitive with the introduction of the second dpyb unit,
such that the major product was H-xnth-dpybH. The use of
an excess of dpybH-B (4 equiv.) allowed the desired product,
xnth-(dpybH)2 = H2L, to be obtained. The mono- and dinuclear
platinum(II) complexes of the ligand were isolated upon treatment
with K2PtCl4 in acetonitrile–water (3 : 1 v/v). The introduction of
the second platinum(II) ion proved to be much slower than the
first: thus HLPtCl was isolated in 80% yield after 3 d, whereas
conversion to L(PtCl)2 required a total of 11 d at reflux.
Photophysical data for HLPtCl and L(PtCl)2 are compiled in
Table 1, together with corresponding values for the parent complex
Pt(dpyb)Cl for comparison. The absorption spectrum of L(PtCl)2
(Fig. 1) is similar to that of Pt(dpyb)Cl in the near-UV/visible
region (350–450 nm), with bands in this region attributed to
charge-transfer transitions involving the metal and halide co-
ligand, whilst the absorbance at shorter wavelengths is increased
due to the superposition of p–p* transitions associated with the
xanthene moiety. A very weak absorption band centred at 496 nm
is attributed to the spin-forbidden S0→T1 transition, enhanced by
the large spin–orbit coupling constant of the Pt(II) ion.
resembles that displayed by the parent complex Pt(dpyb)Cl at
elevated concentrations, which arises from the diffusion-controlled
formation of intermolecular excimers. In that case, however, the
excimer band is always accompanied by structured emission from
isolated molecules (kmax = 490 nm) and, at lower concentrations,
the monomeric emission is exclusively observed (Fig. 1). Thus, the
emission from L(PtCl)2 is attributed to very rapid formation of
intramolecular excimers following absorption of light, at a rate that
precludes any significant emission from the monomeric excited
state. The luminescence of L(PtCl)2 decays mono-exponentially
with a long lifetime of 1.7 ls, indicative of an emitting state of
triplet character. Notably, the grow-in of the emission was too
fast to be resolved on the nanosecond timescale following pulsed
laser diode excitation (using equipment with a response function
∼ 0.5 ns). This contrasts with the inter-molecular excimer emission
displayed by Pt(dpyb)Cl, whose growth is characterised by a much
slower time-constant of the order of 1 ls.5 The difference is readily
interpreted in terms of the diffusion-controlled nature of excimer
formation in the intermolecular case, whereas in L(PtCl)2, the
two units are held in close proximity.
Such a description inevitably raises the question whether there is
already, in the ground state of L(PtCl)2, some interaction between
the two Pt(N∧C∧N) units. Indeed, many examples of platinum(II)
complexes are known that display Pt–Pt interactions in the ground
2
state, through overlap of cofacial dz orbitals. Such interactions
may lead to low-energy emission spectra, as in the many examples
of excited states designated MMLCT {metal–metal-bond-to-
ligand charge transfer, or dr*(Pt2) → p*(L)}.9–11 Two pieces of
experimental data suggest to us that there is no significant ground-
state interaction in L(PtCl)2, and that an excimeric description is
the more appropriate. Firstly, for aggregate species involving a Pt–
Pt dr* HOMO, a low-energy band in the absorption/excitation
spectrum would be anticipated. No such band is detectable in the
spectra of L(PtCl)2: the lowest-energy absorption band at 495 nm
corresponds to the S0→T1 transition at 485 nm in the mononuclear
Pt(dpyb)Cl {indeed, a more appropriate model is arguably the 4-
methyl-substituted analogue of Pt(dpyb)Cl, for which the value of
495 nm is identical to that of L(PtCl)2}. Secondly, ground-state Pt–
Pt interactions are stabilised with decrease in temperature, leading
to well-documented red-shifts in the corresponding absorption
and emission bands.12 In the case of L(PtCl)2, no such shift is seen
as the temperature is lowered in fluid solution. On the contrary,
in a rigid glass at 77 K, exclusively monomer-like emission is
observed (Fig. 2). This striking observation strongly suggests
Fig. 1 Absorption (blue line) and emission spectrum (red, kex = 420 nm)
of L(PtCl)2, at a concentration of 10−6 M in degassed CH2Cl2 at 298 K. The
emission spectra of Pt(dpyb)Cl at 10−6 M and 2 × 10−4 M (green solid and
dashed lines respectively) are also shown.
Upon excitation at 420 nm, L(PtCl)2 displays intense lumines-
cence centred at 690 nm (Fig. 1), with a luminescence quantum
yield, Ulum, of 0.20 ( 0.04). The emission spectrum has a broad,
unstructured profile which is independent of the concentration (the
range 2 × 10−7–5 × 10−4 M was investigated). The band closely
Table 1 Photophysical data for the new complexes L(PtCl)2 and HLPtCl, and for the parent complex Pt(dpyb)Cl for comparisona
abs
max
em
max
em
max
f
Complex
k
b/nm (e/M−1 cm−1
)
k
/nm
Ulum
0.60h 7.2 (0.50) 5.3
510, 542, 588sh 0.40h 8.8 (0.55) 1.1
s0c,d/ls
kSQc/109 M−1 s−1 kQO2 e/108 M−1 s−1
k
f/nm (77 K) s0 /ls (77 K)
Pt(dpyb)Clg 332 (6510), 380 (8690),
401 (7010), 485 (240)
491, 524, 562
8.5
7.7
487, 521, 558
505, 543, 583
504, 540, 580
6.1
9.0
7.6
HLPtCl
386 (7950), 416 (5300),
495 (200)
386 (14650), 412 (9940), 690
495sh (350)
L(PtCl)2
0.20i 1.7 (0.25) 0.16
16
a In deoxygenated CH2Cl2 at 298 K except where stated otherwise. b For bands > 300 nm. c s0 is the lifetime at infinite dilution and kSQ the bimolecular
self-quenching rate constant, determined from the y-intercept and gradient, respectively, of a plot of s−1 versus concentration. d Values in air-equilibrated
solution are given in parentheses. e Bimolecular rate constant of quenching by O2, estimated from the lifetimes in degassed and air-equilibrated solutions.
f In diethyl ether–isopentane–ethanol, 2 : 2 : 1 by volume. g Data from ref. 5. h Measured using fluorescein in 0.1 M NaOH as the standard (U = 0.90).
i Measured using Ru(bpy)3Cl2 in air-equilibrated H2O as the standard (U = 0.028). Estimated uncertainty in Ulum is 0.04.
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that the formation of the emissive state responsible for the red
band at 690 nm is a thermally activated process, requiring at
least some motion of the two Pt(N∧C∧N)Cl units relative to
one another. Although the emitting state might involve Pt–Pt
interaction—in which case it could be formulated as an MMLCT
state that exists only in the excited state, this is not necessarily
the case, since p–p* or d–p* interactions could also account
for the stabilisation relative to the monomeric excited state.13
In a recent study of structurally related cationic anthraquinone-
bridged platinum terpyridyl complexes, Pt · · · Pt distances in the
most rigid systems (acetylide linkers) are estimated to be up to
red-emitting Pt(II) complexes,16 quantum yields remain much
lower for diimine complexes, whilst the benchmark platinum
porphyrins have Ulum values in solution of around 0.05.17 The use of
intramolecular excimer formation could open up a powerful new
approach to obtaining more efficient red emitters. Recent work
has shown that the excimer energies in mononuclear derivatives
of Pt(dpyb)Cl can be varied by at least 2000 cm−1 through simple
structural modification.18 Thus the present strategy could prove
versatile in the design of bright emitters with tuneable wavelengths
across the red and into the NIR.
˚
Acknowledgements
11 A, much longer than that necessary to support MMLCT
interactions.14 In the present instance, the charge neutrality of
the complexes eliminates the effect of electrostatic repulsion that
is expected in the cationic systems, such that closer separations
We thank EPSRC for finanical support of this work (grant ref.
EP/D500265/1).
˚
would be anticipated, but distances are still expected to be > 4.5 A
Notes and references
based on the corresponding ligands. In contrast, it should be noted
that adoption of angles significantly less than 90◦ between the
xanthene and N∧C∧N planes will favour close d–p* or p–p* (but
not Pt · · · Pt) interactions.
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Fig. 2 Emission spectra of L(PtCl)2 in diethyl ether–isopentane–ethanol
(2 : 2 : 1) at 77 K (blue) and in CH2Cl2 at 298 K (red). The spectrum of
HLPtCl in CH2Cl2 at 298 K is in green.
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Red emission is also observed from the compound in the solid
state, although the spectra show some variation according to
sample preparation (kmax in the range 670–710 nm, according
to the solvent from which the sample is evaporated and the
rate of evaporation). This probably reflects subtle differences in
intermolecular packing of the molecules, akin to those seen in
simple monometallic derivatives of Pt(dpyb)Cl.15
The singly complexed derivative HLPtCl displays an emission
spectrum that resembles the monomer emission of Pt(dpyb)Cl
(Fig. 2). At elevated concentrations in solution, only a weak
intermolecular excimer band is observed, much less intense than
that displayed by Pt(dpyb)Cl at comparable concentrations. This
lower propensity of HLPtCl to intermolecular excimer formation
can be rationalised in terms of the neighbouring uncomplexed
dipyridylbenzene unit offering the Pt(N∧C∧N) moiety protection
from intermolecular interactions, at least from one face, an effect
which is also reflected in the smaller self-quenching rate constant
kSQ (Table 1).
17 Handbook of Photochemistry, ed. M. Montalti, A. Credi, L. Prodi and
M. T. Gandolfi, CRC Press, Boca Raton, FL, 3rd edn, 2006.
18 S. Develay, L. Murphy, P. L. Brothwood and J. A. G. Williams, Inorg.
Chem., manuscript in preparation.
In conclusion, the high luminescence efficiency of L(PtCl)2 is
particularly remarkable given the low energy of the emission.
Although progress has been made recently in the design of
4564 | Dalton Trans., 2008, 4562–4564
This journal is
The Royal Society of Chemistry 2008
©