Sensitised near-infrared luminescence from lanthanide(III) centres using Re(I) and Pt(II) diimine complexes as energy donors in d-f dinuclear complexes based on 2,3-bis(2-pyridyl)pyrazine

Article abstract of DOI:10.1039/b616423d

The luminescent transition metal complexes [Re(CO)₃Cl(bppz)] and [Pt(CC-C₆H₄CF₃)₂(bppz)] [bppz = 2,3-bis(2-pyridyl)pyrazine], in which one of the diimine binding sites of the potentially bridging ligand bppz is vacant, have been used as 'complex ligands' to make heterodinuclear d-f complexes by attachment of a {Ln(dik)₃} fragment (dik = a 1,3-diketonate) at the vacant site. When Ln = Pr, Nd, Er or Yb the lanthanide centre has low-energy f-f excited states capable of accepting energy from the ³MLCT excited state of the Pt(ii) or Re(i) centre, quenching the ³MLCT luminescence and affording sensitised lanthanide(iii)-based luminescence in the near-IR region. UV/Vis and luminescence spectroscopic titrations allowed measurement of (i) the association constants for binding of the {Ln(dik)₃} fragment at the vacant diimine site of [Re(CO)₃Cl(bppz)] or [Pt(CC-C₆H ₄CF₃)₂(bppz)], and (ii) the degree of quenching of the ³MLCT luminescence according to the nature of the Ln(iii) centre. In all cases Nd(iii) was found to be the most effective of the series at quenching the ³MLCT luminescence of the d-block component because the high density of f-f excited states of the appropriate energy make it a particularly effective energy-acceptor. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b616423d

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Sensitised near-infrared luminescence from lanthanide(III) centres using Re(I)
and Pt(II) diimine complexes as energy donors in d–f dinuclear complexes
based on 2,3-bis(2-pyridyl)pyrazine†
Frazer Kennedy,a Nail M. Shavaleev,b Thelma Koullourou,c Zoe¨ R. Bell,b John C. Jeffery,b Stephen Faulkner*c
and Michael D. Ward*a
Received 9th November 2006, Accepted 9th February 2007
First published as an Advance Article on the web 28th February 2007
DOI: 10.1039/b616423d
The luminescent transition metal complexes [Re(CO)3Cl(bppz)] and [Pt(CC–C6H4CF3)2(bppz)] [bppz =
2,3-bis(2-pyridyl)pyrazine], in which one of the diimine binding sites of the potentially bridging ligand
bppz is vacant, have been used as ‘complex ligands’ to make heterodinuclear d–f complexes by
attachment of a {Ln(dik)3} fragment (dik = a 1,3-diketonate) at the vacant site. When Ln = Pr, Nd, Er
or Yb the lanthanide centre has low-energy f–f excited states capable of accepting energy from the
3MLCT excited state of the Pt(II) or Re(I) centre, quenching the 3MLCT luminescence and affording
sensitised lanthanide(III)-based luminescence in the near-IR region. UV/Vis and luminescence
spectroscopic titrations allowed measurement of (i) the association constants for binding of the
{Ln(dik)3} fragment at the vacant diimine site of [Re(CO)3Cl(bppz)] or [Pt(CC–C6H4CF3)2(bppz)], and
(ii) the degree of quenching of the 3MLCT luminescence according to the nature of the Ln(III) centre. In
all cases Nd(III) was found to be the most effective of the series at quenching the 3MLCT luminescence
of the d-block component because the high density of f–f excited states of the appropriate energy make
it a particularly effective energy-acceptor.
of the [Ru(bipy)(CN)4]2− units is quenched by the Ln(III) cen-
tre with concomitant appearance of sensitised near-infrared
Introduction
luminescence.2 Alternatively, d-block complexes bearing a pen-
dant peripheral diimine site can be used to form adducts with
Ln(diketonate)3 fragments in low-polarity solvents; again, exci-
tation of the d-block component in the visible region generates
sensitised Ln(III)-based near-infrared luminescence following d →
f energy-transfer.3
In the last few years there has been a lot of interest from
many groups on the preparation and study of mixed-metal
d–f assemblies in which a strongly light-absorbing transition-
metal chromophore acts as an antenna group for sensitisation of
luminescence from the lanthanide(III) fragment.1–3 The use of d-
block chromophores as antenna groups in this way overcomes the
inherently low light absorption characteristics of lanthanide(III)
ions, and a very large collection of d-block units is available
which combine the desirable characteristics of chemical and
photochemical stability, the availability of well-characterised long-
lived excited states which can act as effective energy-donors,
and strong absorption of light at more or less any desired
wavelength. Generally, given the excited-state energies of the
d-block chromophores used, near-infrared emissive lanthanides
[Pr(III), Nd(III), Er(III), Yb(III)] have been used as the energy
acceptors.
The d → f energy-transfer process can be followed in two ways.
Firstly, the appearance of sensitised Ln(III)-based luminescence
is obvious evidence that energy-transfer has occurred; if a rise-
time is detectable then the energy-transfer rate can be estimated.
Usually this is not possible, because efficient d → f energy-
transfer occurs on a timescale of nanoseconds and the detectors
commonly used for measuring NIR luminescence lifetimes cannot
resolve such short rise-times because of their long response
time. We have observed rise-times for sensitised Ln(III)-based
luminescence only when the energy-transfer process is relatively
slow and a rise time of the order of 100 ns could be detected by
deconvolution techniques.2a Alternatively, if the d-block energy-
donor is luminescent, the degree of luminescence quenching—
manifested in a reduction of the luminescence lifetime—provides
quantitative information on energy-transfer.2b,3d This is technically
much easier due to the availability of very fast, sensitive detectors
in the visible region which allow residual luminescence lifetimes
on the nanosecond scale to be measured easily.
We have been following two quite distinct strategies for
preparation of such d–f arrays. Crystallisation of luminescent
cyanometallate complex anions such as [Ru(bipy)(CN)4]2− with
lanthanide cations affords a wide range of cyanide-bridged
Ru(II)/Ln(III) coordination networks in which the luminescence
aDepartment of Chemistry, University of Sheffield, Sheffield, UK S3 7HF.
E-mail: m.d.ward@Sheffield.ac.uk
bSchool of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS
In a recent paper we showed how luminescence from the
complexes [Re(bpym)(CO)3Cl] and [Pt(bpym)(CC–C6H4CF3)2]
(bpym = 2,2ꢀ-bipyrimidine) was completely quenched on binding
of various Ln(diketonate)3 fragments (Ln = Nd, Er, Yb) to the
vacant site of the bipyrimidine bridging ligand.3c The d → f
cDepartment of Chemistry, University of Manchester, Oxford Road, Manch-
ester, UK M13 9PL. E-mail: stephen.faulkner@manchester.ac.uk
† Electronic supplementary information (ESI) available: Tables S1 and S2
and Fig. S1. See DOI: 10.1039/b616423d
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◦
˚
energy-transfer in these systems was clearly fast compared to the
limitations of the equipment, i.e. > 109 s−1. In order to slow down
the energy-transfer so that residual luminescence lifetimes can be
determined, which will allow us to see which d/f combinations
in an isostructural series are the best for achieving sensitised
near-infrared luminescence, we have prepared a related series
of complexes based on the longer bridging ligand 2,3-bis(2-
pyridyl)pyrazine (bppz) (see Scheme 1). In this paper we describe
the syntheses, structural and photophysical properties of this set of
d-block complexes and their d/f adducts with a range of different
Ln(diketonate)3 fragments.
Table 1 Selected bond distances (A) and angles ( ) for the structure of
[Re(CO)3Cl(bppz)]·CHCl3
Re(1)–C(51)
Re(1)–C(61)
Re(1)–C(41)
1.920(6)
1.932(6)
2.006(6)
Re(1)–N(21)
Re(1)–N(11)
Re(1)–Cl(1)
2.161(4)
2.171(5)
2.4592(16)
C(51)–Re(1)–C(61)
C(51)–Re(1)–C(41)
C(61)–Re(1)–C(41)
C(51)–Re(1)–N(21)
C(61)–Re(1)–N(21)
C(41)–Re(1)–N(21)
C(51)–Re(1)–N(11)
C(61)–Re(1)–N(11)
90.8(2)
88.7(2)
92.4(2)
C(41)–Re(1)–N(11)
N(21)–Re(1)–N(11)
C(51)–Re(1)–Cl(1)
C(61)–Re(1)–Cl(1)
C(41)–Re(1)–Cl(1)
N(21)–Re(1)–Cl(1)
N(11)–Re(1)–Cl(1)
92.0(2)
74.38(17)
92.83(19)
90.93(18)
176.33(15)
82.75(13)
86.11(13)
98.5(2)
169.0(2)
93.72(19)
172.9(2)
96.2(2)
The complex [Pt(CC–C6H4CF3)2(bppz)] is a new member of the
series of Pt(II)–diimine-diacetylide complexes which have attracted
considerable attention recently for their luminescence properties,5
the trifluoromethyl substituents on the phenylacetylide ligands
enhance the 3MLCT energy and lifetime.3c,5c It was prepared in 64%
yield from the known complex [Pt(bppz)Cl2]6 by cross-coupling
with F3C–C6H4-CCH in the usual way.5 We could not obtain X-ray
quality crystals of it, but we did get crystals of the non-fluorinated
analogue [Pt(CC–C6H4CH3)2(bppz)] which was prepared using the
same method. The structure and metric parameters are in Fig. 2
and Table 2. The Pt(II) centre has the usual nearly square-planar
geometry, with the mono-coordinated bppz ligand having a similar
conformation to that observed for [Re(CO)3Cl(bppz)]; thus the
pyridyl ring containing N(11) twisted away from the plane of the
adjacent pyrazine ring by 14◦, whereas the pendant pyridyl ring
Scheme 1 (i) Ln(dik)3(H2O)2/CH2Cl2.
Results and discussion
Syntheses and crystallography
The complex [Re(CO)3Cl(bppz)] has been prepared before,4 we
obtained crystals of the chloroform solvate from an NMR sample.
The structure and geometry of the complex molecule (Fig. 1,
Table 1) contain no surprises, with the metric parameters of the
pseudo-octahedral Re(I) centre being typical for complexes of
this type. The coordinated diimine unit is not planar, with the
coordinated pyridyl ring containing N(11) being twisted from
the mean plane of the pyrazine ring by 18◦, in order to avoid
steric interference with the pendant pyridyl ring [containing N(31)]
which is twisted from the mean plane of the pyrazine ring by 44◦.
This conformation of a mono-coordinated bppz ligand is typical.4b
Fig. 2 Molecular structure of [Pt(CC–C6H4CH3)2(bppz)].
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for the structure of
[Pt(CC–C6H4CH3)2 (bppz)]
Pt(1)–C(51)
Pt(1)–C(61)
Pt(1)–N(11)
1.948(7)
1.959(7)
2.053(5)
Pt(1)–N(21)
C(51)–C(52)
C(61)–C(62)
2.061(6)
1.204(9)
1.170(10)
C(51)–Pt(1)–C(61)
C(51)–Pt(1)–N(11)
C(61)–Pt(1)–N(11)
89.3(3)
174.5(3)
95.8(2)
C(51)–Pt(1)–N(21)
C(61)–Pt(1)–N(21)
N(11)–Pt(1)–N(21)
96.9(2)
173.1(2)
78.1(2)
Fig. 1 Molecular structure of the metal complex unit of [Re(CO)3-
Cl(bppz)]·CHCl3.
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containing N(31) is twisted out of the plane of the pyrazine ring
by 60◦. The molecules are stacked such that the planar central
Pt(diimine) portions of the structures overlap, but they are offset
such that there are no short Pt · · · Pt contacts.
˚
˚
7.40 A, cf. separations of ca. 6.3 A in related complexes with bpym
as bridging ligand.3c Adducts of [Pt(CC–C6H4CF3)2(bppz)] with
Ln(diketonate)3 units also clearly formed in CH2Cl2 solution, on
the basis the colour change in the Pt(II) chromophore associated
with Ln(III) binding at the other site of bppz (see Fig. 4), but none
afforded X-ray quality crystals.
Adducts of these complexes [Cl(CO)3Re(l–bppz)Ln(hfac)3]
(hereafter, Re-Ln) or [Cl(CO)3Re(l–bppz)Ln(tta)3] (hereafter, Re-
Lnꢀ) were simply prepared by mixing a 1 : 1 molar ratio of
[Re(CO)3Cl(bppz)] and the appropriate Ln(diketonate)3(H2O)2 in
a CH2Cl2/hexane mixture and allowing the solution to evaporate.
For the series Re-Lnꢀ, containing the diketonate ligand tta (anion
of thenoyl-trifluoro-acetylacetonate) X-ray quality crystals were
obtained for Ln = Gd (the analogue with Ln = Nd was described in
an earlier communication).7 The structure and metric parameters
of Re-Gdꢀ are in Fig. 3 and Table 3. The fact that both diimine sites
are now coordinated to metal ions results in a more symmetrical
arrangement of the bridging ligand, with the pyridyl rings
coordinated to Re(1) and Gd(1) being twisted out of the plane
of the central pyrazine ring by 22◦ and 37◦, respectively. Gd(1) is
in an N2O6 8-coordinate environment, from the three diketonate
ligands and one diimine donor, the coordination geometry is ap-
proximately square antiprismatic, with O(51)/O(54)/O(61)/O(64)
forming one square plane and N(11)/N(21)/O(41)/O(44) forming
the other. The Re · · · Gd separation across the bridging ligand is
Fig. 4 Normalised luminescence spectra of [Pt(CC–C6H4CF3)2(bppz)],
(a) at 77 K in a 2-Me-thf glass (solid line) and (b) at room temperature in
CH2Cl2 solution.
We also prepared the known related complex [Re(CO)3Cl(dpq)]
[dpq = 2,3-bis(2-pyridyl)quinoxaline]8 to see if Ln(diketonate)3
adducts could be formed, however this was less successful, with the
only crystalline product obtained, [Re(CO)3Cl(dpq)][Gd(hfac)3-
(H2O)2]·C6H6, showing that the two components [Re(CO)3Cl-
(dpq)] and [Gd(hfac)3(H2O)2] are not associated in the solid
state but have co-crystallised. Although it is not what we
had hoped for it is not without interest and the details are
included as ESI.† In view of the relatively weak association
of Ln(diketonate)3 fragments to the vacant binding site of
[Re(CO)3Cl(dpq)] we did not investigate this system further, but
confined our solution luminescence studies to the [Cl(CO)3Re(l-
bppz)Ln(hfac)3] (Re-Ln), [(CF3C6H4CC)2Pt(l-bppz)Ln(hfac)3]
(Pt-Ln) and [(CF3C6H4CC)2Pt(l-bppz)Ln(tta)3] (Pt-Lnꢀ) series.
Solution photophysical studies
(i) Studies of Re-based and Pt-based luminescence quenching:
d → f energy-transfer rates. The 1MLCT absorption maximum
of [Re(CO)3Cl(bppz)] is at ca. 400 nm (depending on the solvent)
and its luminescence emission maximum was reported to be at
670 nm in MeCN.4a,4c We found the luminescence in aerated
CH2Cl2 solution to be centred at 665 nm with s = 13 ns, and we also
found that the unstructured luminescence maximum at 77 K (2-
Me-thf glass) is at 575 nm, giving a 3MLCT energy of 17 400 cm−1.
[Pt(CC–C6H4CF3)2(bppz)] is much more strongly luminescent; in
aerated CH2Cl2 solution its 1MLCT absorption band is at 420 nm,
and its luminescence maximum is at 600 nm with s = 250 ns. At
77 K (2-Me-thf glass) a structured luminescence profile (Fig. 4) is
seen with the lowest-energy maximum at 520 nm, giving a 3MLCT
energy of 19 200 cm−1.
Fig. 3 Molecular structure of [Cl(CO)3Re(l-bppz)Gd(tta)3] (F atoms of
the tta ligands omitted for clarity, one of the tta ligands is shown with
hollow bonds for clarity).
◦
˚
Table 3 Selected bond distances (A) and angles ( ) for the structure of
[Cl(CO)3Re(l–bppz)Gd(tta)3]
Gd(1)–O(61)
Gd(1)–O(44)
Gd(1)–O(54)
Gd(1)–O(41)
Gd(1)–O(64)
Gd(1)–O(51)
Gd(1)–N(11)
Gd(1)–N(21)
2.306(3)
2.345(3)
2.347(3)
2.357(3)
2.377(3)
2.386(3)
2.564(4)
2.674(3)
Re(1)–C(91)
Re(1)–C(71)
Re(1)–C(81)
Re(1)–N(31)
Re(1)–N(24)
Re(1)–Cl(1)
1.916(5)
1.920(5)
1.932(5)
2.162(4)
2.171(3)
2.4620(15)
C(91)–Re(1)–C(71)
C(91)–Re(1)–C(81)
C(71)–Re(1)–C(81)
C(91)–Re(1)–N(31)
C(71)–Re(1)–N(31)
C(81)–Re(1)–N(31)
C(91)–Re(1)–N(24)
C(71)–Re(1)–N(24)
87.6(2)
91.5(2)
91.5(2)
172.62(17)
99.09(17)
91.54(17)
98.43(17)
170.25(17)
C(81)–Re(1)–N(24)
N(31)–Re(1)–N(24)
C(91)–Re(1)–Cl(1)
C(71)–Re(1)–Cl(1)
C(81)–Re(1)–Cl(1)
N(31)–Re(1)–Cl(1)
N(24)–Re(1)–Cl(1)
96.04(16)
74.55(13)
93.08(17)
89.24(15)
175.41(15)
83.88(10)
82.79(10)
We could observe the effect of a Ln(diketonate)3 fragment
binding to the vacant site of each of these by carrying out
UV/Vis and luminescence titrations, adding small portions of
Ln(hfac)3(H2O)2 to solutions of the d-block complex in CH2Cl2.
The results obtained with [Pt(CC–C6H4CF3)2(bppz)] are in Fig. 5.
1
As the {Ln(hfac)3} fragment associates, the MLCT transition
of the Pt(II) complex is red-shifted from 425 nm to 450 nm
1494 | Dalton Trans., 2007, 1492–1499
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[Gd(III), whose lowest energy excited state is at ≈ 32 000 cm−1] or
has no f–f excited states at all [La(III), 4f0; Lu(III), 4f14].
Rather surprisingly, titration of [Pt(CC–C6H4CF3)2(bppz)] with
Gd(hfac)3(H2O)2 resulted in a very substantial degree of quenching
of the Pt(II)–based luminescence intensity [cf. Fig. 5(b)] with
the lifetimes being reduced from 250 ns to 18 ( 1) ns. This
has occurred despite there being no significant red-shift in the
emission maximum of the Pt–Gd adduct compared to free
[Pt(CC–C6H4CF3)2(bppz)]; thus, although the 1MLCT absorption
maximum is red-shifted by ca. 1300 cm−1 the corresponding
3MLCT emission maximum is scarcely affected so the effects of the
energy-gap law will not be significant in reducing the luminescence.
This quenching must therefore be ascribed to an alteration in the
vibrational behaviour of the complex when the Ln(dik)3 fragment
binds which facilitates non-radiative deactivation. To confirm that
this is a mechanical effect and not related to the specific electron
configuration or paramagnetism of Gd(III) (4f7), we performed
the same experiment using diamagnetic Lu(hfac)3(H2O)2 and
achieved an essentially identical result, with >90% quenching
of luminescence intensity and reduction in the residual Pt-based
luminescence lifetime to 17 ( 1) ns. Fitting the luminescence
intensities during these titrations to a 1 : 1 binding isotherm allows
determination of the association constant of the equilibrium in eqn
(1) as 4.6 × 104 M−1 for Gd(III) and 9.6 × 104 M−1 for Lu(III), with
the smaller (and hence more Lewis-acidic) Lu(III) ion giving the
higher association constant.
Comparable titrations with other Ln(hfac)3(H2O)2 species [Ln =
Pr, Nd, Er, Yb, see Fig. 5], which have low-energy f–f levels
capable of quenching the Pt(II)-based 3MLCT state, result in
similar changes in the UV/Vis and luminescence data, with
the exception that the luminescence quenching goes nearer to
completion because of the additional effects of d → f energy-
transfer. In each case we could measure the residual lifetime of
the Pt(II)-based luminescence at the end of the titration, and
also determine an association constant (cf. eqn (1)). The data are
summarised in Table 4. Looking first at the association constants,
there is a general increase by approximately a factor of two
from the least to the most Lewis-acidic Ln(III) ion, although
the variation is not smooth. Secondly, the degree of quenching
of the Pt(II)-based luminescence, as shown by the luminescence
lifetime of the Pt-Ln adduct at the end of the titration, varies
between different lanthanides. The shortest residual lifetime for
Pt(II)-based luminescence occurs in Pt-Nd (s ≈ 1 ns); on the basis
of eqn (2), where sq is the ‘quenched’ lifetime (i.e. 1 ns) and su is the
Fig.
5
Effects on (a) absorption and (b) luminescence spec-
10−5 M) on titration with
tra of [Pt(CC–C6H4CH3)2(bppz)] (2
×
[Yb(hfac)3(H2O)2] in CH2Cl2 solution. Part (c) shows the extent of
quenching of the luminescence intensity of [Pt(CC–C6H4CH3)2(bppz)] vs.
the amount of added [Ln(hfac)3(H2O)2] for a range of Ln, these data were
used to calculate the 1 : 1 association constants associated with eqn (1)
(see main text and Table 5).
(which causes a deepening of the colour), and the luminescence is
quenched to an extent depending on the identity of the Ln centre.
The luminescence quenching is manifested in both a reduction
in luminescence intensity (Fig. 5), and also the appearance of
a short-lived component in the time-resolved measurements. As
more of the Ln(hfac)3(H2O)2 is added to a fixed amount of [Pt(CC–
C6H4CF3)2(bppz)] during the titration, the luminescence decay
shows both a long-lived component characteristic of residual free
[Pt(CC–C6H4CF3)2(bppz)] and a short-lived component charac-
teristic of the adduct Pt-Ln. The long lived component becomes
less significant and the short-lived component dominates as the
titration proceeds, because of the equilibrium shown in eqn (1)
(where ‘M–NN’ denotes a metal complex ligand with a vacant
diimine site, and ‘dik’ denotes a diketonate).
Table 4 Summary of stability constant data and Pt(II)-based 3MLCT
luminescence data for the series Pt-Ln, Pt-Lnꢀ and Re-Ln
Pt–Ln series
Pt–Lnꢀ series
Re–Ln series
s/nsa
K/M−1
s/nsa
K/M−1
s/nsa
K/M−1
M–NN + Ln(dik)3(H2O)2 = M–NN–Ln(dik)3 + 2H2O
(1)
The red-shift of the absorption maximum during the titration
is a consequence of the bppz LUMO being reduced in energy
on coordination of an electropositive Ln(III) centre. It might be
expected that this would also result in a reduction in the 3MLCT
energy and luminescence lifetime, following the energy-gap law.9
The effects of this can be seen by using Ln(III) centres which
cannot quench the excited state by energy-transfer, either because
the Ln(III) concerned has only very high-energy f–f excited states
Pr
3
1
18
2
2
17
4.8 × 104
5.6 × 104
4.6 × 104
8.6 × 104
1.9 × 105
9.6 × 104
7
1
70
4
6
—
9.0 × 103
1.5 × 104
2.1 × 104
2.2 × 104
7.0 × 103
2
2
3
2
2
2
1.3 × 105
4.1 × 104
2.5 × 104
2.8 × 105
2.4 × 105
2.3 × 104
Nd
Gd
Er
Yb
Lu
b
b
—
a Detector response function, 0.5 ns, values are therefore quoted to the
nearest nanosecond. b Not measured.
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‘unquenched’ lifetime taken from either the Pt-Gd adducts (i.e. 18
ns) or the Pt-Lu adduct (i.e. 17 ns) in the absence of energy-transfer
effects, the Pt → Nd energy-transfer rate kEnT is ≈ 109 s−1.
block component, i.e. without this gradient the d → f energy-
transfer will be inefficient at room temperature.10 Nd(III) has a
particularly high density of f–f excited states of appropriate energy
(between ca. 11 000 and 16 000 cm−1) which overlap well with the
luminescence from [Pt(CC–C6H4CF3)2(bppz)], and both we3d and
others1 have observed that Nd(III) is for this reason a particularly
effective quencher of luminescence for d-block chromophores
which luminesce in the 600–700 nm region in fluid solution. Pr(III),
Yb(III) and Er(III) all have fewer f–f states in the relevant region
and energy-transfer to them is accordingly slower. In the Pt-Lnꢀ
series the least effective energy-acceptors are Pr(III) and Yb(III),
which is a consequence of the energy-accepting f–f level being
kEnT = 1/sq–1/su
(2)
For the other lanthanides the residual Pt(II)-based lifetime is a
little longer, between 2 and 3 ns, implying a slightly slower value
for kEnT with the smallest value being ≈ 3 × 108 s−1 for Pt →
Pr energy-transfer. All of these residual lifetimes sq are close to
the limit of what can be measured accurately with our instrument
(whose instrument response function is ca. 0.5 ns) but the factor of
3 difference between the shortest and the longest—1 ns, for Pt-Nd,
vs. 3 ns, for Pt-Pr—is significant and implies that Nd(III) is the
most effective quencher of the Pt(II) chromphore. The reasons for
this are discussed later.
2
low in energy [1G4 for Pr(III) and F5/2 for Yb(III), both at ≈
10 000 cm−1] with concomitantly poor spectroscopic overlap with
the emission spectrum of the Pt(II) donor unit. The relatively slow
energy-transfer to Pr(III)—the slowest in the series—implies that
the next highest energy level of Pr(III), 1D2 at ≈ 17 000 cm−1, is just
too high in energy to be efficiently populated at room temperature
Use of the complexes Ln(tta)3(H2O)2 to generate the series of
adducts Pt-Lnꢀ (with Ln = Pr, Nd, Gd, Er, Yb) gave compa-
rable results with some important similarities and some minor
differences. As before, addition of the {Ln(tta)3} fragment to the
secondary binding site of [Pt(CC–C6H4CF3)2(bppz)] results in a
red-shift of the Pt(II)-based 1MLCT absorption and quenching of
3
by the MLCT state of [Pt(CC–C6H4CF3)2(bppz)] following the
arguments given earlier.
Measurements of UV/Vis and luminescence spectra during the
titration of [Re(CO)3Cl(bppz)] with Ln(hfac)3(H2O)2 to generate
the Re-Ln series in CH2Cl2 solution (Ln = Pr, Nd, Gd, Er, Yb)
showed that, as with the Pt-Ln series, the association constants
for adduct formation are ca. 105 M−1. During the titration the
stabilisation of the bppz LUMO results in a red-shift of the Re(I)-
centred MLCT absorption from 414 nm to 450 nm, and also in
progressive quenching of the luminescence band at 670 nm. In
this series the Re-Gd adduct (in which there is no d → f energy-
transfer) is nearly quenched to start with (sq = 3 ns), presumably
because of the introduction of additional vibrational deactivation
pathways; the additional effects of energy-transfer with the other
{Ln(hfac)3} fragments make relatively little difference, giving sq ≈
2 ns in every case. The values of sq for Ln = Pr, Nd, Er, Yb are
sufficiently similar that no analysis of the relative Re → Ln energy-
transfer rates in this series is possible. However, as with the two
series Pt-Ln and Pt-Lnꢀ, the occurrence of d → f energy-transfer
is apparent from the appearance of sensitised luminescence from
the Ln(III) centre in each case.
3
the MLCT luminescence, and from the luminescence intensity
data recorded during the titration the 1 : 1 association constants
could be determined. These are significantly smaller than for the
Pt-Ln series, by on average one order of magnitude. This may be
ascribed to either (i) the greater bulk of the tta ligands compared to
hfac, which slightly hinders formation of the 8-coordinate adducts,
and/or (ii) the higher electron-withdrawing effect of the hfac
ligands (each with six F atom substituents) which will increase
slightly the partial positive charge on the Ln(III) centre and thereby
result in slightly stronger electrostatic bonding for the hfac series
Pt-Ln compared to the tta series Pt-Lnꢀ. Also we found that the
variation in binding constants throughout the Pt-Lnꢀ series no
longer correlates with the size of the Ln(III) ion, in fact Yb(III)
gives the weakest binding to [Pt(CC–C6H4CF3)2(bppz)], possibly
because the combination of bulkier tta ligands and the smaller
ionic radius results in a higher degree of steric crowding in the
8-coordinate adduct which overcomes the expected slight increase
in electrostatic metal/ligand interactions.
The luminescence lifetimes in this series, however, clearly show
the same general behaviour as observed in the Pt-Ln series. The
adduct Pt-Gdꢀ shows the lowest degree of quenching of the Pt(II)
3MLCT luminescence because of the absence of any energy-
transfer component, with the residual lifetime su of 70 ns. The
other energy-accepting lanthanides have a much greater quenching
effect, with (again) Nd(III) causing the highest degree of quenching
(sq ≈ 1 ns) and Pr(III) causing the least (sq = 7 ns). Thus, on the
basis of eqn (2), the Pt → Ln energy-transfer rates in this series
vary from ≈ 109 s−1 (for Ln = Nd) to 1.4 × 108 s−1 (for Ln = Pr).
The variation in Pt → Ln energy-transfer rates in both series
may be accounted for by considering the availability of f–f excited
states on the Ln(III) acceptor which are capable of acting as
(ii) Studies on sensitised near-infrared luminescence from
Nd(III), Er(III) and Yb(III). We could demonstrate in these series
of adducts that d → f energy-transfer is occurring, not just from
the quenching of the d-block component (previous section) but
from the appearance of sensitised luminescence from the Ln(III)
centre in many cases. The adducts were prepared in situ by
combining [Re(CO)3Cl(bppz)] or [Pt(CC–C6H4CF3)2(bppz)] with
a five-fold excess of the appropriate Ln(diketonate)3(H2O)2 species
at a concentration sufficiently high to ensure a substantial degree
of association based on the known association constants from
Table 4. Although association may not be complete (cf. eqn (1)),
by selective excitation of the d-block chromophore at >400 nm
(where the Ln-diketonates do not absorb) and monitoring only
sensitised Ln(III)-based luminescence in the near-IR region, only
the associated d–f adducts were being interrogated. In this way
3
energy acceptors. For energy-transfer to occur from the MLCT
state of [Pt(CC–C6H4CF3)2(bppz)] to the Ln(III) centre there
must be f–f excited levels available which are at least 2000 cm−1
3
below the MLCT level, but not so far below that there is no
we could see the characteristic luminescence of Yb(III) (2F5/2
→
3
4
overlap with the MLCT emission spectrum. The gradient of >
2F7/2 at 980 nm), Nd(III) (4F3/2 → I11/2 at 1060 nm), Pr(III) (1D2 →
2000 cm−1 for d → f energy-transfer is necessary to prevent
3F4 at 1030 nm) and Er(III) (4I13/2 → I15/2 at 1530 nm) in the
4
thermally-activated back energy-transfer from Ln(III) to the d-
series Re-Ln and Pt-Ln (Table 5). The Er(III)–based luminescence
1496 | Dalton Trans., 2007, 1492–1499
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[Re(CO)3Cl(dpq)]11 and Ln(dik)3·2H2O12 were prepared according
to the literature methods. Mass spectra (EI and FAB) were
recorded on a VG-Autospec instrument. UV/Vis absorption
spectra were recorded on a Cary 50 spectrometer using CH2Cl2
solutions. Steady-state luminescence spectra were recorded in
aerated CH2Cl2 solutions, diluted to give an absorbance of 0.2
or less at the excitation wavelength, on a Perkin-Elmer LS-50
luminescence spectrometer. Luminescence lifetimes of the d-block
complexes were acquired on an Edinburgh Instruments ‘Mini-s’
instrument operating under single photon counting conditions,
equipped with an EPL-405 picosecond pulsed diode laser (410
nm) as excitation source and a Hamamatsu R-928 photomultiplier
detector. Wavelength selection for lifetime measurements was via
the use of a band-pass filter. Instrumentation used to measure
time-resolved emission spectra from lanthanides in the near-
IR region has been described in a previous paper;13 selective
Table 5 Lifetimes of sensitised Ln(III)-based luminescence measured in
CH2Cl2 solution using excitation at 430 nm into the Re(I)-based or Pt(II)-
based MLCT absorptions
Complex
s/ls
Re-Nd
Pt-Nd
Re-Yb
Pt-Yb
Pt-Pr
0.2a
0.2a
9.0b
7.4b
0.2c
a Measured at 1060 nm, estimated error 0.05 ls. b Measured at 980 nm,
estimated error 0.2 ls. c Measured at 1030 nm, estimated error 0.05 ls.
was very weak and reliable lifetime values could not be obtained,
however the characteristic luminescence, which could only arise
from d → f energy-transfer, was present.
For the Yb(III) and Nd(III) adducts in each series, however,
time-resolved measurements were possible. For Re-Yb and Pt-Yb
the Yb(III)–based luminescence lifetimes from the Yb(hfac)3(NN)
centres were 9 and 7.4 ls, respectively; for Re-Nd and Pt-Nd
the sensitised Nd(III)–based luminescence lifetimes were likewise
similar to one another at about 0.2 ls each. These are fairly
typical values for such Ln(diketonate)3(NN) luminophores in
aprotic, non-coordinating solvents, with the much lower values
for Nd(III) arising from the presence of low-energy f–f states which
are particularly easily quenched by overtones of C–H oscillators
in either the solvent or the ligand set.3 For Pt-Pr we could observe
weak sensitised Pr(III)-based luminescence with a lifetime of ca.
0.2 ls; however, for Re-Pr the Pr(III)-based luminescence was
almost non-existent, probably because the much shorter lifetime of
the Re(I)-based 3MLCT state compared to the Pt(II)-based 3MLCT
state means that Re → Pr energy-transfer is not fast enough to
compete effectively with other deactivation pathways.
1
excitation at 430 nm of the Re(I)-based or Pt(II)-based MLCT
absorptions was achieved using a dye laser which was in turn
pumped by an N2 laser, as described previously.13 Calculations
of association constants from UV/Vis spectroscopic titration
data were performed with the programs NMRTit-HG (for 1 : 1
complexes) kindly provided by Prof. C. A. Hunter of the University
of Sheffield.14
Syntheses
[Pt(CC-C6H4CF3)2(bppz)]: A mixture of Pt(bppz)Cl2 (1.97 g,
3.9 mmol), iPr2NH (5 cm3, 35.4 mmol) and anhydrous CuI
(40 mg, catalyst) in freshly distilled CH2Cl2 (70 cm3) under N2
was stirred for 10 min, followed by the addition of 4-ethynyl-
a,a,a-trifluorotoluene (2 cm3, 12.3 mmol). The suspension was
stirred under N2 at room temperature for 24 h. After this time,
the resultant dark red suspension was evaporated to dryness to
remove traces of iPr2NH and purified by column chromatography
on silica eluting with MeOH–CH2Cl2 (1 : 100 to 5 : 95). The orange
coloured fraction containing the product was reduced in volume
and hexane added to precipitate the product. Yield: 64%. Anal.
Calc. for C32H18F6N4Pt: C, 50.1; H, 2.4; N, 7.3. Found: C, 49.9; H,
2.2; N, 7.1%. ESMS: m/z 768 (M+). UV VIS (CH2Cl2): nm (e) 423
(7100), 287 (53 000).
Conclusions
The luminescent complexes [Re(CO)3Cl(bppz)] and [Pt(CC–
C6H4CF3)2(bppz)], which both have a vacant diimine binding site,
form d–f dinuclear adducts in which {Ln(diketonate)3} fragments
bind at the secondary diimine site; association constants are in
the range 104 M−1–105 M−1 in CH2Cl2 solution. Compared to the
analogous bipyrimidine-bridged d–f dinuclear complexes that we
described recently3c the use of bppz as bridging ligand provides
two advantages: (i) it is a better base, so association constants
for {Ln(diketonate)3} binding are higher; (ii) the greater d–f
separation slows down d → f energy-transfer to the extent that
it can be quantified for different Ln(III) species by measuring the
differential degrees of quenching of the d-block luminescence.
In the Pt/Ln series in particular the rates of Pt → Ln energy-
transfer vary over nearly an order of magnitude with Pr(III) and
Yb(III) being less effective energy-acceptors than Nd(III). The d →
f energy-transfer generates sensitised near-infrared luminescence
from the Ln(III) ions.
[Pt(CC-C6H4CH3)2(bppz)] was prepared and purified in an
exactly similar way. Yield: 67%. Anal. Calc. for C32H24N4Pt: C,
58.3; H, 3.7; N, 8.5. Found: C, 58.3; H, 3.7; N, 8.6%. FAB MS:
+
m/z 682 (16%, {M + Na} ), 660 (60%, M+). UV VIS (CH2Cl2):
nm (e) 438 (7000), 271 (48 000). X-Ray quality crystals were grown
by evaporation of a CH2Cl2 solution of the compound.
The adduct [Cl(CO)3Re(l-bppz)Ln(tta)3] (Re-Gdꢀ) was prepared
for X-ray structural determination by slow partial evaporation of
mixed CH2Cl2–hexane solution (1 : 1 v/v, initial volume approx.
25 ml) containing Re(bppz)(CO)3Cl and Gd(TTA)3·2H2O in a
1 : 1 molar ratio. In 3–5 days orange needle-like crystals of the com-
plexes were formed. Anal. Calcd. for GdReC41O9ClN4H22S3F9:
C, 36.2; H, 1.6; N, 4.1. Found: C, 36.4; H, 1.4; N, 4.2%.
IR (CH2Cl2), cm−1: 2028, 1932, 1908. The remaining adducts
were generated directly in solution for spectroscopic and lu-
minescence studies by combining [Pt(CC–C6H4CF3)2(bppz)] or
[Re(CO)3Cl(bppz)] with the appropriate Ln(dik)3·2H2O in dry
CH2Cl2 (see main text).
Experimental
General details
Organic reagents and metal salts were purchased from Sigma-
Aldrich and used as received. [Re(CO)3Cl(bppz)],4 [Pt(bppz)Cl2],6
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Table 6 Crystallographic data for the crystal structuresa
Complex
[Re(CO)3Cl(bppz)]·CHCl3
[Pt(CC–C6H4CH3)2 (bppz)]
[Cl(CO)3Re(l-bppz) Gd(tta)3]
Formula
Formula weight
T/K
C18H11Cl4N4O3Re
659.31
173(2)
Monoclinic, P2(1)/c
6.4657(16)
19.437(5)
17.053(4)
90
94.969(5)
90
2135.0(9)
4
2.051
6.220
C32H24N4Pt
659.64
173(2)
Monoclinic, P2(1)/c
9.3819(16)
19.480(4)
14.6718(16)
90
90.031(19)
90
2681.4(8)
4
1.634
5.260
C41H22ClF9GdN4O9ReS3
1360.71
100(2)
Monoclinic, P2(1)/c
Crystal system, space group
˚
a/A
8.849(4)
˚
b/A
38.843(11)
13.404(5)
90
101.26(4)
90
4519(3)
4
2.000
4.426
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
D
calc/Mg m−3
l/mm−1
Reflections (total, independent, Rint
Data/restraints/parameters
Final R1, wR2 indicesb
)
11953, 4571, 0.0376
4571, 0, 271
0.0334, 0.0949
16986, 6118, 0.0640
6118, 0, 337
0.0394, 0.0702
28948, 10353, 0.0428
10353, 0, 622
0.0342, 0.0782
a All measurements were made on a Bruker SMART diffractometer using Mo-Ka radiation. b R1 value is based on selected data [with I > 2r(I)]; wR2
value is based on all data.
The compound [Re(CO)3Cl(dpq)][Gd(hfac)3(H2O)2]·C6H6 was
prepared in exactly the same way as Re-Gdꢀ, by slow evaporation
of a 1 : 1 mixture of [Re(CO)3Cl(dpq)] and [Gd(hfac)3(H2O)2]
in CH2Cl2/benzene solution. Orange crystals were isolated in
which it was shown that the {Gd(hfac)3} unit was not bound to
the [Re(CO)3Cl(dpq)] fragment (see main text). Anal. Calcd. for:
C45H28N4ClF18GdO11Re: C, 35.5; H, 1.9; N, 3.7. Found: C, 35.5; H,
1.6; N, 4.1% (NB: This assumes 1.5 C6H6 of crystallisation. Only
one C6H6 molecule was explicitly located in the crystal structure
but an area of badly disordered solvent was also present which
could not be modelled).
restraints. An area of diffuse, disordered electron density that
could not be satisfactorily modelled was eliminated using the
SQUEEZE command in PLATON. There is a large residual
−3
˚
˚
electron density peak of ≈ 5 e A located close (0.8 A) to the
Gd atom.
CCDC reference numbers 626876–626879.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b616423d
Acknowledgements
We thank the EPSRC for financial support.
X-Ray crystallography
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