Localised to intraligand charge-transfer states in cyclometalated platinum complexes: An experimental and theoretical study into the influence of electron-rich pendants and modulation of excited states by ion binding

Article abstract of DOI:10.1039/b816375h

The neopentyl ester of 1,3-di(2-pyridyl)benzene-5-boronic acid (dpy-B) is a useful intermediate in the divergent synthesis of N^C^N-coordinating, 1,3-di(2-pyridyl)benzene ligands, HLⁿ, that carry aryl substituents at the 5-position of the centr

Full text of DOI:10.1039/b816375h

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Localised to intraligand charge-transfer states in cyclometalated platinum
complexes: an experimental and theoretical study into the influence of
electron-rich pendants and modulation of excited states by ion binding†
David L. Rochester,a Ste´phanie Develay,a Stanislav Za´lisˇ*b and J. A. Gareth Williams*a
Received 18th September 2008, Accepted 14th November 2008
First published as an Advance Article on the web 26th January 2009
DOI: 10.1039/b816375h
The neopentyl ester of 1,3-di(2-pyridyl)benzene-5-boronic acid (dpy-B) is a useful intermediate in the
divergent synthesis of NŸCŸN-coordinating, 1,3-di(2-pyridyl)benzene ligands, HLn, that carry aryl
substituents at the 5-position of the central ring. The platinum(II) complexes, PtLnCl, of several such
ligands have been prepared, incorporating pendant anisoles, arylamines, an oxacrown, and an
azacrown, all of which are strongly luminescent in solution at 298 K. The emission of the complexes is
partially quenched by oxygen, and all of the compounds are very efficient sensitisers of singlet oxygen.
The quantum yields of 1O2 formation have been measured on the basis of the intensity of the O2 1Dg
emission at 1270 nm, and are in the range 0.25–0.65. Density functional theory (DFT) calculations have
been carried out that include the effect of the solvent, on the unsubstituted complex PtL1Cl and on the
derivatives incorporating p-dimethylaminophenyl and phenyl-15-mono-N-azacrown-5 pendants
(PtL9Cl and PtL12Cl respectively). Absorption spectra have been simulated on the basis of the calculated
singlet excitations: they closely resemble the experimental spectra. In particular, the DFT successfully
accounts for the appearance of low-energy absorption bands that accompany the introduction of the
aryl pendants, indicating the participation of the aryl group in the HOMO but not significantly in the
LUMO. The calculated lowest energy triplet excitation of PtL1Cl is close to the observed 0–0 emission
maximum of this complex in solution. Taking together data for this series of complexes and related
compounds previously studied, the energies of the lowest-energy spin-allowed absorption bands are
shown to correlate approximately linearly with the oxidation peak potential. The emission energies
show a similar correlation in toluene, but in CH2Cl2 the value for PtL9Cl is anomalously low. The
differing emission properties of this complex in the two solvents suggest a switch to a TICT-like state in
CH2Cl2 (TICT = twisted intramolecular charge transfer), stabilised in the more polar environment.
Transient DC photoconductivity measurements confirm that the dipole moment of the triplet excited
state is larger in CH2Cl2 than in toluene. The azacrown PtL12Cl displays similar behaviour. Binding of
metal ions such as Ca2+ to the azacrown unit of this complex leads to a pronounced blue shift in the
emission, which can be readily understood in terms of the large increase in the TICT energy that will
accompany the binding of the metal ion to the lone pair of the azacrown nitrogen atom.
following absorption of light, such as energy and electron transfer,
Introduction
central to the field of light-to-chemical energy conversion.4 Long
Transition metal complexes that emit efficiently from triplet
lifetimes under ambient conditions can also be exploited in sensor
excited states are the subject of a great deal of current research
technology, not only offering a means of discriminating from
interest. Apart from the insight into fundamental excited state
short-lived background emission, but also allowing lifetimes to
processes that they offer, there are numerous applications of
be used as a reporter parameter, in contrast to the normally used
methods based on intensity or wavelength.5,6 The design of triplet
emitters that respond to their environment or to the presence of
analytes of interest through modulation of the lifetime and/or
emission profile is key to the development of such sensors.7
As far as platinum(II) complexes of polyimine ligands are
concerned, the number of compounds that emit efficiently in
solution at room temperature has increased greatly over the past
decade.8 The use of cyclometalating ligands, or co-ligands such as
acetylides, has proved to be a particularly successful strategy for
promoting luminescence: the strong ligand-field associated with
such ligands ensures that the severely distorted metal-centred
(d–d) states—population of which is responsible for non-radiative
such compounds. For example, the new generation of display
screen technology–organic light-emitting devices (OLEDs)–can
benefit from the incorporation of such materials to induce
emission from otherwise wasted triplet states.1–3 Meanwhile, triplet
excited states of transition metal complexes may be sufficiently
long-lived to allow time for useful chemical processes to occur
aDepartment of Chemistry, University of Durham, Durham, U.K. DH1 3LE.
E-mail: j.a.g.williams@durham.ac.uk
bJ. Heyrovsky Institute of Physical Chemistry, Dolejskova 3, 182 23 Prague
8, Czech Republic. E-mail: stanislav.zalis@jh-inst.cas.cz
† Electronic supplementary information (ESI) available: Additional syn-
thetic details; Fig. S1–S7; Tables S1 and S2. See DOI: 10.1039/b816375h
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decay of simple Pt complexes such as Pt(bpy)Cl2—are displaced to
high energies.8,9 A number of studies during the past decade have
reported on luminescent platinum-based sensory systems.10 Some
of these make use of binding moieties such as crown ethers and
related ligands, appended onto the core of emissive complexes.11
Binding of the analyte modulates the properties of the excited
states, its influence being manifest through a change in the emission
characteristics, typically the intensity.
We have been exploring the chemistry of platinum(II) com-
plexes with terdentate cyclometalating ligands based on 1,3-di(2-
pyridyl)benzene (HL1), which offer the metal ion an NŸCŸN
coordination environment (Chart 1).12,13 These compounds are
amongst the brightest Pt-based emitters in solution at room
temperature; e.g. for the parent complex, [PtL1Cl], Ulum = 0.60
and t = 7.2 ms in deoxygenated dichloromethane.12 The complexes
are thermally stable and sublimable, rendering them appropriate
for incorporation into OLEDs: high device efficiencies have been
obtained in this way,14,15 including near-IR and white light emitting
devices that exploit excimer or aggregate emission.16,17 In earlier
work, we found that the introduction of a range of simple aryl
substituents into the central 4-position of the NŸCŸN ligand (Chart
1) allowed the emission energy Eem to be tuned over a range of
~3500 cm-1.13 Eem decreased in the order R = H > mesityl >
2-pyridyl > 4-tolyl > 4-biphenylyl > 2-thienyl, in line with the
increasing electron richness (= decreasing oxidation potential)
of the complex. However, for the derivative incorporating a p-
dimethylaminophenyl substituent, PtL9Cl, the emission energy
was substantially lower than that anticipated on this basis. We
tentatively attributed this, and other differences from the other
complexes (vide infra), to the introduction of an emissive triplet
intraligand charge transfer (3ILCT) state.13 In this contribution,
we probe in more detail the nature of the excited state of PtL9Cl
through a combination of experimental and theoretical methods.
We report the synthesis and luminescence properties of a range
of new complexes incorporating other electron-rich substituents,
including phenols, amines, oxacrown and azacrown ethers (Chart
2), and explore the balance between charge-transfer and ligand-
localised emission in such systems.
Results and discussion
1. Synthesis of new aryl-appended, NŸCŸN-coordinating ligands
and their Pt(II) complexes
The new ligands HL10–15 were prepared by one or other of
two closely related methods, as shown in Scheme 1. Previously,
we found that Suzuki cross-coupling reactions of aryl boronic
acids with the key intermediate 1,3-di(2-pyridyl)-5-bromobenzene
(dpyb-Br) provide a convenient, divergent route to aryl-substituted
NŸCŸN ligands HLn. For example, ligands HL5 and HL6 were
prepared in this way from mesitylboronic acid and 4,4¢-biphenyl-
1-boronic acid respectively. In the current work, this method was
employed to prepare ligands HL9, HL13 and HL14.
A potentially more versatile strategy would be to make use
of the reverse coupling, in which the boronic acid unit is
incorporated into the dipyridylbenzene, thus generating a versatile
intermediate for coupling with a wider range of aryl halides.
The new intermediate dpyb-B, the neopentylglycolate ester of
1,3-di(2-pyridyl)benzene-5-boronic acid (Scheme 1), was prepared
readily by a Miyaura cross-coupling reaction18 between dpyb-
Br and bis(neopentyl)glycolato diboron (B2neo2), catalysed by
Pd(dppf)Cl2 in the presence of potassium acetate. The strategy
Chart 1 Structures of the complexes investigated in the previous study.12,13
Chart 2 Structures of the new complexes described here.
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Scheme 1 Synthetic routes to the new ligands.
is directly analogous to that developed previously in our lab-
oratory for the boronic acid derivatives of terpyridine and 4¢-
phenylterpyridine, which serve as versatile intermediates in the
preparation of substituted terpyridines.19 The subsequent cross-
coupling reaction of dpyb-B with 4-bromo-N,N-diphenylaniline
proceeded readily under standard conditions to generate HL10.
The method was readily extrapolated to the p-methoxy-substituted
analogue, HL11, and to the azacrown- and oxacrown-appended lig-
ands HL12 and HL15. The requisite 4-bromophenyl-N-monoaza-
15-crown-5 for the synthesis of HL12 was prepared by standard
cyclisation methods (see ESI†).
The platinum(II) complexes of the ligands were obtained upon
reaction with K2PtCl4, using either acetic acid or a mixture of
acetonitrile and water as the solvent. Typical reaction times were
2–3 days at reflux; further details for individual complexes are
provided in the Experimental. Crystals of PtL10Cl and PtL12Cl suit-
able for X-ray diffraction analysis were obtained from solutions in
CH2Cl2. Unfortunately, disorder hampered a full refinement, but
the data was more than adequate to confirm the identity of the
complexes (see Fig. S6 and S7 in the ESI†).
band and the weak but distinct S0→T1 absorption band, we
previously assigned the emission from this complex to a state of
predominant p–p* character, in which some contribution from the
metal nevertheless promotes the triplet radiative decay constant.12
Such an interpretation is supported by the DFT calculations of
Sotoyama et al., who modelled the parent PtL1Cl and methyl
derivative PtL3Cl in the gas phase.20 In the present work, time-
dependent density functional theory (TD-DFT) calculations have
been carried out to explore the nature of the excited states, taking
into account the influence of the solvent. These procedures have
been applied to investigate the profound differences observed
between PtL1Cl and the amino-substituted complex PtL9Cl.
Calculations were performed using B3LYP or PBE0 functionals
together with CPCM solvent description with CH2Cl2 as the
solvent.
Initially, ground-state DFT geometry optimisation calculations
were attempted in order to assess the reliability of the method:
calculated results were compared to those obtained experimentally
by X-ray crystallography for two of the complexes for which
diffraction data is available (PtL3Cl and PtL6Cl).13 Table S1 in
the ESI† reveals the excellent agreement between calculated and
experimental data.
2. Density functional theory (DFT) calculations of PtL1Cl
TD-DFT was then applied to the parent unsubstituted complex
PtL1Cl at its energy-minimised geometry, in order to identify the
main orbital components of the singlet transitions, together with
Based upon the highly structured profile of the emission spectrum,
and the very small Stokes shift between the highest energy emission
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Table 1 TD-DFT B3LYP/CPCM (CH2Cl2) singlet excitation energies (eV) for [PtL1Cl] with oscillator strength larger than 0.001, and corresponding
experimental values
State Main components (%)
Transition energya/eV (nm) Oscillator Strength Expt. band max./nm Extinction coefficient e/M-1 cm-1
a3A 75 (HOMO → LUMO)
b3A 90 (HOMO → LUMO + 1)
a1A 92 (HOMO → LUMO)
b1A 92 (HOMO → LUMO + 1)
2.64 (471)
2.82 (440)
3.20 (387)
3.25 (382)
3.62 (343)
3.62 (342)
—
—
485
454
—
401
380
—
240
270
—
7010
8690
—
0.004
0.155
0.011
0.005
0.081
0.355
c1A
97 (HOMO - 2 → LUMO)
d1A 91 (HOMO - 1 → LUMO)
g1A 67 (HOMO - 1 → LUMO + 1) 3.96 (313)
332
290
6510
22200
i1A
78 (HOMO - 4 → LUMO)
4.19 (296)
(p→p*) ligand
a Corresponding wavelength (nm) in parenthesis.
their energies and oscillator strengths. The data are summarised
in Table 1 and pertinent frontier orbitals are illustrated in
Fig. 1(a). The results aid in the interpretation of a number
of experimental observations. The lowest energy spin-allowed
transitions in absorption, around 380–400 nm, can be attributed to
the HOMO → LUMO and HOMO → LUMO + 1 transitions. An
absorption spectrum has been simulated based on the excitation
energies and their oscillator strengths, which matches closely the
experimental spectrum (Fig. 2). Similarly, the lowest-energy triplet
state is seen to be comprised predominantly of HOMO → LUMO
character (Table 1), and the calculated energy is remarkably close
to that of the 0–0 band found in the experimental spectrum
(2.64 and 2.53 eV respectively). Some further comments and
caveats on the assignment of bands in the spectrum by DFT are
provided in the ESI†. For both singlet and triplet, the net charge
distribution that accompanies the formation of the excited state is
principally “outwards” from Pt/Cl/phenyl to the pyridine rings
Fig. 2 Calculated absorption spectrum of [PtL1Cl] (red solid line) in
CH2Cl2 at 298 K, simulated using the TD-DFT data of Tables 2 and 3;
also shown is the experimental spectrum under the same conditions (blue
dashed line).
(Fig. 1a), in such a way that there will be little change in the
dipole moment, as is clear from the calculated dipole differences
for the excitations listed in Table 2. This would account for the
relatively weak solvatochromism observed for this complex (e.g.
a difference of only 300 cm-1 between the emission maxima in
toluene and acetonitrile, representing the extremes of polarity for
solvents in which the complex is soluble).
3. DFT calculations of PtL9Cl: trends in absorption spectra
The 4-(dimethylamino)phenyl-substituted complex PtL9Cl is
bright orange as opposed to the yellow colour of the parent
complex PtL1Cl. This is due to a strong, broad absorption in
the visible region, lmax = 431 nm, e = 8880 in CH2Cl2 at 298 K
(Fig. 3). Previously we postulated that the emergence of this
band might be due to a significant participation of the pendant
aminophenyl group in the HOMO.13 We now demonstrate that
DFT calculations of PtL9Cl, modelled in the presence of CH2Cl2
as the solvent, unequivocally support this hypothesis. The frontier
orbitals of PtL9Cl, calculated by DFT using B3LYP/CPCM, are
shown in Fig. 1(b). The excitation energies of singlet transitions
with oscillator strengths greater than 0.001 are listed in Table
S2 in the ESI†. Table 2 compares the energies of the lowest-
lying transitions of PtL9Cl with those of PtL1Cl, together with
Fig. 1 Frontier orbitals of (a) PtL1Cl, and (b) PtL9Cl, calculated by DFT
and including CH2Cl2 as the solvent.
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Table 2 TD-DFT B3LYP/CPCM (CH2Cl2) lowest lying singlet and triplet excitation energies (eV) and the dipole moment differences between excited
and ground state, calculated for [PtL1L], [PtL9L] and [PtL12L]
State
Main components (%)
Transition energya/eV (nm)
Oscillator strength
Dm/D
[PtL1L]
[PtL9L]
[PtL12L]
a3A (T1)
b3A (T2)
a1A (S1)
b1A (S2)
a3A (T1)
b3A (T2)
a1A (S1)
b1A (S2)
a3A (T1)
b3A (T2)
a1A (S1)
b1A (S2)
75 (HOMO → LUMO)
90 (HOMO → LUMO + 1)
92 (HOMO → LUMO)
92 (HOMO → LUMO + 1)
77 (HOMO → LUMO)
93 (HOMO → LUMO + 1)
95 (HOMO → LUMO)
95 (HOMO → LUMO + 1)
90 (HOMO → LUMO)
78 (HOMO → LUMO + 1)
97 (HOMO → LUMO)
97 (HOMO → LUMO + 1)
2.64 (471)
2.82 (440)
3.20 (387)
3.25 (382)
2.36 (525)
2.39 (518)
2.65 (467)
2.65 (466)
2.33 (531)
2.35 (526)
2.58 (480)
2.60 (477)
—
—
0.004
0.155
—
0.94
2.35
1.10
3.99
13.80
15.95
26.89
22.61
16.23
16.09
23.21
27.52
—
0.041
0.042
—
—
0.040
0.039
a Corresponding wavelength (nm) in parenthesis.
Table 3 Ground state UV-visible absorption data of the new platinum
complexes in CH2Cl2 at 298 K, with data for PtL1Cl and PtL9Cl for
comparison
the dipole moment differences between excited and ground states.
The HOMO in PtL9Cl is localised primarily on the pendent 4-
dimethylaminophenyl group, rather than on the Pt-NŸCŸN unit
as it is in PtL1Cl. On the other hand, the closely lying LUMO
and LUMO + 1 remain localised on the Pt-NŸCŸN moiety. Thus,
the lowest-energy, HOMO → LUMO and HOMO → LUMO + 1
transitions have clear-cut intraligand charge transfer character.
The simulated spectrum based on these data shown in Fig. 3
qualitatively describes the experimental spectrum: the marked
difference between the spectra of PtL9Cl and PtL1Cl corresponds
closely to what is observed in practice (Fig. 3 versus Fig. 2). The
new HOMO → LUMO transition gives rise to a low-energy band
that has no counterpart in PtL1Cl, with a predicted wavelength of
467 nm (experimental value 431 nm).
Complex and substituent Absorbancea lmax/nm (e/M-1cm-1)
PtL1Clb H–
380 (8690), 401 (7010), 454 (270), 485 (240)
380 (6410), 431 (8880)
380 sh (7200), 422 (7160), 495 (110)
423 (7400), 498 (100)
PtL9Clc Me2N-C6H4–
PtL10Cl Ph2N-C6H4–
PtL11Cl
(p-MeO-C6H4)2N-C6H4–
PtL12Cl azacrown
PtL13Cl MeO-C6H4–
PtL14Cl (MeO)2-C6H3–
PtL15Cl oxacrown
360 (7260), 379 (6100), 432 (8010)
364 (4070), 381 (6300), 420 (7200), 497 (120)
362 (3670), 381 (6200), 422 (6970), 497 (100)
363 (3380), 381 (5930), 420 (6400), 498 (120)
a For bands > 350 nm. b Data from reference 12. c Data from reference 13.
ox
(Eabs) and the oxidation potential Ep for the collective set of
complexes (Fig. 4). That the energy depends on the oxidation
potential alone, the reduction potential seemingly irrelevant, can
be readily rationalised from the DFT results. Inspection of the
frontier orbitals of PtL1Cl in Fig. 1a reveals that the 4-position
of the phenyl ring makes a major contribution to the HOMO but
only a minor contribution to the LUMO and is a node in LUMO +
1. Thus, the HOMO (and hence oxidation) should be significantly
affected by the substituent, whereas the LUMO (hence reduction
potential) and LUMO + 1 will be much less so. The observed red
shifts with increasing electron-donating ability of substituent thus
reflect an increase in the HOMO level and a LUMO that remains
essentially unchanged.
4. Luminescence
Fig. 3 Calculated absorption spectrum of [PtL9Cl] (red solid line) in
CH2Cl2 at 298 K, simulated using the TD-DFT data of Tables 3 and S1†;
also shown is the experimental spectrum under the same conditions (blue
dashed line).
The new complexes prepared in this study PtL10–15Cl are all
brightly luminescent, emitting in the green–yellow region of the
spectrum. Luminescence data for the new complexes, together
with values for PtL1Cl and PtL9Cl, are summarised in Table 4.
For the oxo-substituted series PtL13–15Cl (Fig. S2 in the ESI†),
the emission maxima are shifted a little towards the red, in line
with the donating character of the phenolic oxygen atoms. Their
emission properties are similar to PtL1–8Cl, in that (i) the spectra
are quite narrow, with some evidence of vibrational structure,
(ii) they undergo self-quenching at elevated concentrations (kQ =
2–5 ¥ 109 M-1 s-1) which is accompanied by the appearance of
The new aryl-substituted complexes prepared in this study,
together with those from the previous work, have absorption
maxima between the values of PtL1Cl and PtL9Cl (Table 3),
reflecting the progressively increasing contribution of the aryl
pendant to the HOMO as the pendant becomes more electron
rich. A remarkably linear correlation is observed between the
energy of the lowest-energy, spin-allowed intense absorption band
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Table 4 Luminescence data for the platinum complexes, in CH2Cl2 solution at 298 K
a
1
d,f
Complex and substituent
Emission lmax/nm
Ulum degassed
t0/msb degassed
kQSQb/109 M-1s-1
kQO2c/108 M-1s-1
U O2
PtL1Cle H–
491, 524, 562
588
557
572
580
528, 555 sh
533, 560 sh
529, 560 sh
0.60
0.46
0.29
0.30
0.28
0.44
0.49
0.37
7.2
12
9.0
11
9.5
9.6
11
5.3
0.12
0.29
0.53
< 0.10
2.7
9.1
24
21
37
23
13
16
14
0.65
0.34
0.24
0.26
0.30
—
PtL9Cle Me2N-C6H4–
PtL10Cl Ph2N-C6H4–
PtL11Cl (p-MeO-C6H4)2N-C6H4–
PtL12Cl azacrown
PtL13Cl MeO-C6H4–
PtL14Cl (MeO)2-C6H3–
PtL15Cl oxacrown
4.5
2.5
0.55
—
10
b
SQ
a Luminescence quantum yield determined using [Ru(bpy)3]Cl2 as the standard. t0 is the lifetime at infinite dilution and kQ the self-quenching rate
constant, determined from the intercept and slope, respectively, of a plot of the measured decay rate constant against concentration. c Bimolecular rate
constant for quenching by molecular oxygen, assuming that [O2] = 2.2 ¥ 10-3 M in CH2Cl2 at 1 atm air. d Quantum yield of singlet oxygen formation
measured using perinaphthenone as the standard. e Data for PtL1Cl and PtL9Cl from references 12 and 13 respectively, except for U 1O2 (this work).
1
f Quantum yields of O2 formation have also been measured during this work for the previously synthesised series of complexes: PtL2Cl, 0.55; PtL3Cl,
0.78; PtL5Cl, 0.60; PtL6Cl, 0.56; PtL7Cl, 0.61; PtL8Cl, 0.43.
Fig. 4 Plot of the energy of the lowest-energy spin-allowed absorption
ox
maximum Eabs (in CH2Cl2) against the oxidation peak potential Ep (in
90% CH3CN–10% CH2Cl2) for the aryl-substituted complexes at 298 K,
highlighting the remarkably linear correlation. The straight line represents
the best fit through the data, gradient = 2900 cm-1/V.
a brightly emissive excimer band centred around 700 nm; and
(iii) they are weakly negatively solvatochromic. These features in
common with the previously investigated complexes suggest that
the emission emanates from essentially the same type of excited
state, and that the shift to the red can be interpreted in terms of
the electron-donating nature of the substituents raising the energy
of the HOMO, as for the singlet excitations in absorption. Indeed,
when the emission energy Eem of the collective series of complexes
in dichloromethane is plotted against Epox, a remarkably linear
correlation is observed (Fig. 5a), mirroring the trend found in
absorption.
Fig. 5 Plot of the emission energy of the complexes in (a) dichloro-
methane, and (b) toluene, against the oxidation peak potential (in 90%
The anomalous behaviour of PtL9Cl: solvent-induced switching
of excited states. Despite the largely good correlation between
Eem and Epox, the emission energy of PtL9Cl is anomalously low
(Fig. 5a), lying well below the extrapolated line based on the other
complexes. In contrast, we find that, for the corresponding plot
of emission energies in toluene, PtL9Cl appears at the ‘expected’
CH3CN–10% CH2Cl2 for both plots) at 298 K. The solid lines represent
the best fit to the data, excluding (a), and including (b), the data point for
PtL9Cl; the respective gradients are 4900 and 4500 cm-1/V.
This suggests that there may be some additional stabilisation of
the excited state of this complex in CH2Cl2, that does not apply
ox
position based on its Ep value, as it did in absorption (Fig. 5b).
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to the other complexes, or a switch to a different type of excited
state.
Previously, we had noted that the emission of PtL9Cl appeared
to be anomalous in some other respects too:13 (i) it displays
strong positive solvatochromism, (varying by almost 4000 cm-1
between CCl4 and MeCN), as opposed to the weak negative
solvatochromism of the other complexes; (ii) the emission band
is broad and structureless in CH2Cl2 and MeCN, but has the
characteristic, partially structured profile in CCl4 and toluene,
similar to that of the other complexes; (iii) shows much less
propensity to self-quenching or excimer formation than the other
complexes, at elevated concentrations in CH2Cl2.
We have sought to probe in more detail whether the nature
of the emissive excited state of PtL9Cl does indeed change on
going from CH2Cl2 to toluene. The limited solubility of the
complex in toluene limits the range of concentrations available
in this solvent. However, it proves to be informative to compare
the emission properties in CH2Cl2 with those in 95% toluene–
5% CH2Cl2. Comparison of the emission spectrum in dilute and
concentrated solution in CH2Cl2 reveals only a little difference
between the profiles (Fig. 6a, 40-fold difference in concentration):
there is a slightly increased intensity in the red-end tail of the
spectrum, which could be due to an excimer, but clearly only a
small proportion relative to the monomeric species. Similarly, the
lifetime of emission is reduced by only a relatively small factor of
~2 over this large concentration range. In contrast, in 95% toluene–
5% CH2Cl2, the more concentrated solution displays a broad, low-
energy band centred around 700 nm resembling that displayed
by the other complexes and attributable to an excimer (Fig. 6b).
The greater extent of intermolecular interaction in toluene is also
manifest in the larger influence of concentration on the lifetime of
the monomer band, falling from 11 to 1.4 ms over the 40-fold range
displayed in Fig. 6b. Meanwhile, the decay kinetics registered at
700 nm reveal a clear-cut grow-in of the emission not observed for
the same wavelength in CH2Cl2 (Fig. 6c), in line with bimolecular
excimer formation in the former solvent.
Thus, the observations collectively point to a change in the
nature of the emissive excited state of PtL9Cl in the more
polar solvents (CH2Cl2 and CH3CN), to one in which the
intermolecular interactions that lead to excimers are inhibited.
A change to a state with a more complete transfer of charge
from the amino pendant to the core of the complex seems
likely, perhaps reminiscent of the Twisted Intramolecular Charge
Transfer (TICT) states of structurally related organic molecules
like p-dimethylaminobenzonitrile, which are sufficiently stabilised
in more polar solvents to become the lowest-energy excited states.21
Although the exact nature of such states remains the subject of
much fascinating experimental and theoretical study,22 the general
consensus is in favour of a twisted model, in which the initially
formed locally excited state undergoes a radiationless reorganisa-
tion in polar solvents to a state in which the dimethylamino group
is perpendicular to the ring, and hence electronically decoupled,
accompanied by transfer of charge to the acceptor terminus.
Thus, whilst the emission of such molecules is usually strongly
solvatochromic, the absorption may be much less so, as is indeed
the case for PtL9Cl.
Fig. 6 (a) Emission spectra of PtL9Cl in CH2Cl2 at concentrations of 1 ¥
10-5 M (dotted line) and 4 ¥ 10-4 M (solid line). (b) The corresponding
spectra at the same concentrations in toluene, revealing the formation
of a strongly emissive excimer. (c) Decay kinetics of emission of PtL9Cl
registered at 700 nm in the concentrated solutions (4 ¥ 10-4 M) in
dichloromethane (blue) and toluene (red), highlighting the grow-in of the
excimer in toluene only.
Meanwhile, McMillin and co-workers have noted that the
introduction of ILCT character in 4-amino-substituted platinum
terpyridyl complexes substantially reduces their tendency to
1734 | Dalton Trans., 2009, 1728–1741
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undergo quenching reactions with donating solvents such as
acetonitrile, probably because of the increased electron density
at the metal.8a,23 A similar effect in the present instance might
reasonably account for the lower propensity to excimer fomation
in CH2Cl2.
Diarylamine complexes. Intrigued whether this behaviour
would also be seen for complexes with slightly less electron-rich,
diarylamine pendants (as opposed to the dialkylamino unit of
PtL9Cl), we investigated the complexes PtL10Cl and PtL11Cl. In
dichloromethane solution, the emission energies lie in the order
PtL10Cl > PtL11Cl > PtL9Cl (Fig. 7), in line with the expected
order of increasing electron-donating ability of the amine, but the
emission spectral profiles, lifetimes and self-quenching constants
of the arylamine complexes are all similar to those of PtL9Cl,
suggesting that they too have more ILCT character in this solvent.
Careful inspection of the spectra at high concentration compared
to dilute solution does, however, reveal a little more marked
concentration dependence than PtL9Cl, with somewhat enhanced
low-energy emission, whilst the self-quenching rate constants are
actually a little larger for the arylamine complexes, despite the
larger expected steric bulk, suggesting that the switch to TICT
behaviour is somewhat less clear-cut for these complexes.
If the above interpretation is correct, then the dipole moment
of the emissive triplet state of the compound should be larger
in CH2Cl2 than in toluene, owing to a greater degree of charge
separation. The change in dipole moment of a molecule upon
formation of an excited state can frequently be probed by Stark
electroabsorption spectroscopy—in which the influence of a strong
electric field on the absorption transitions is probed—but the
technique is limited to excited states that are formed directly
from the ground state upon excitation, viz. normally the singlet
states.24 In the present case, the emissive triplet states are formed
subsequent to light absorption, by intersystem crossing from the
singlet. The related technique of transient DC photoconductivity
(TDCP) can allow such states to be probed.25,26 Briefly, the change
in capacitance of a cell containing a solution of the compound,
across which is applied a strong electric field, is monitored during,
and immediately following, pulsed laser excitation. Any difference
between the dipole moment in the excited compared to the
ground state will result in the molecules realigning within the
electric field, equivalent to transient flow of charge. The TDCP
signal scales with mex2–mgd2. TDCP data obtained for PtL9Cl
2
in toluene could be fitted satisfactorily on the basis of (mex
–
mgd2) = 150 ( 20) Debye2 and tex = 12 ms, the latter value
being fully consistent with the observed lifetime (a representative
TDCP photoresponse and fit is shown in the ESI†, Fig. S4).
Attempts to measure the response in CH2Cl2 were unsuccessful,
owing to breakdown of the solvent under the applied electric
field. However, data could be obtained in 1,2-dichloroethane,
a similar solvent both in terms of chemical composition and
polarity, giving a value of (mex2–mgd2) = 250 ( 20) Debye.2 The
significantly larger value in the latter solvent is consistent with the
notion of a greater extent of charge transfer under more polar
conditions.
Fig. 7 Absorption and emission spectra of the diarylamino complexes
PtL10Cl (blue) and PtL11Cl (green) in CH2Cl2 at 298 K, together with that
of PtL9Cl (red) for comparison.
Triplet state inversion according to solvent has recently been
described by Castellano et al. in a very different class of
5. Chemically-induced switching of excited states: an
azacrown-substituted complex PtL12Cl
platinum(II) complexes.27 They noted that in Pt(dbbpy)(–C C–
∫
3
∫
∫
f–C C–f–C C–Ph)2, emission from a CT state was observed
only in the least polar solvents (e.g. n-hexane) whereas emission
from a 3IL state localised on the acetylides dominates under more
In our previous study, we noted that the addition of acid to a
dichloromethane solution of PtL9Cl leads to a substantial shift in
both the emission band and the lowest-energy absorption band to
higher energy: lmaxabs shifts from 431 to 410 nm; lmaxem from 588 to
482 nm.13 The effect is reversed upon addition of an organic base
such as Me4N+OH-. These changes are clearly due to the fact that
protonation of the pendant amine nitrogen atom will attenuate or
even essentially eliminate its participation in the molecular orbital
previously identified as HOMO in Fig. 1(b). The HOMO will
now return to being centred primarily on the NŸCŸN unit, as in
PtL1Cl, restoring the localised p–p* state as the emissive state;
indeed, the emission characteristics of [PtL9HCl]+ are similar to
those of PtL1Cl.13
3
polar conditions. In their case, the CT increases in energy with
increasing solvent polarity, and the effect is more readily probed
on the basis of the variation in the decay kinetics as a function of
solvent polarity, because of the large difference in magnitudes of
the lifetimes of the two types of excited state.
Although the DFT calculations on PtL9Cl do not reveal
evidence for such a twisted state becoming lowest in energy in
dichloromethane (the amine group being essentially coplanar with
the phenyl ring in the energy-minimised geometry), it should be
noted that the difference in energy involved is small, ~1000 cm-1
(0.12 eV) on the basis of the data trend in Fig. 5a, and clearly
very sensitive to the solvent. Moreover, states with an extensive
degree of charge separation—of which the proposed TICT-like
state would be one—are generally poorly modelled by B3LYP,
especially when the occupied and virtual orbitals involved in the
excitation do not overlap.28
We sought to investigate whether this modulation of the excited
states could be extended to other systems, triggered by species
other than the proton. Reversible switching between excited states
with significantly different emission properties, induced by target
analytes, can be exploited in ratiometric sensing applications, for
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example, as in many of the calcium sensors used in physiological
studies.29 As a proof of principle, we chose to prepare the
azacrown-substituted complex PtL12Cl. The azacrown unit is well-
known to bind several divalent metal ions, Mn+, a process that
involves the use of the lone pair on the azacrown nitrogen atom.30
The positioning of the latter at the position corresponding to the
N atom in PtL9Cl should lead to metal-ion triggered modulation
of emission.
In the absence of added metal ions, the excited state properties
of PtL12Cl are indeed very similar to those of PtL9Cl (Tables 3
and 4). TD-DFT calculations confirm that the azacrown has a
directly analogous effect to the –NMe2 unit, as shown by the
assignments and calculated energies of the lowest-lying singlet
and triplet transitions listed in Table 2, which are very similar for
the two complexes.
The absorption and emission spectra of PtL12Cl were monitored
as a function of added group I (Li+, Na+ and K+) and group II
(Mg2+, Ca2+ and Ba2+) metal ions and Zn2+ in air-equilibrated
MeCN at room temperature. This solvent was selected owing to
the greater solublity of metal triflate salts therein compared to
dichloromethane. Little effect was observed for the group I metal
ions, but the divalent metal ions led to a marked change in the
emission spectrum, which shifted substantially to higher energy
and increased in intensity. The strongest response was observed
for calcium; the emission spectra prior to and after addition of an
excess of Ca2+ are shown in Fig. 8. The observed change is evidently
directly analogous to the effect of protonation on PtL9Cl, and can
be interpreted in terms of the inversion of the relative energies of
the NŸCŸN-localised and TICT excited states. Thus, upon binding
of Ca2+ to the pendent amine, the latter is increased in energy such
that the former becomes the emissive state (Scheme 2).
Fig. 8 Absorption and emission spectra of PtL12Cl (3.6 ¥ 10-5 M) in
air-equilibrated MeCN (containing Bu4NClO4, 0.1 M, as inert electrolyte)
(red lines), and corresponding spectra in the presence of calcium triflate
(1 ¥ 10-3 M) (blue lines); lex = 374 nm (isosbestic point), T = 298 K.
∫
[Pt(phbpy)(–C C–Ar)] where Ar is a unit such as benzo-18-crown-
5 or 15-monoazacrown-5. In these cases, the emission of the
unbound form is very low, either because it involves an ILCT state
that is of low-energy and subject to efficient non-radiative decay, or
because of quenching of the excited state by photoinduced electron
transfer from the amine lone pair. The compounds thus function
as “turn-on” sensors. The present system differs and, indeed, is
unusual amongst azacrown-appended lumophores in general, in
that both forms are highly emissive, and the metal binding leads
to a clear-cut shift in the spectrum, and not just to an intensity
change.
In contrast to the profound effect on the triplet state lumi-
nescence, the change in the absorption spectrum in the presence
of an excess of Ca2+ is less pronounced, with the lowest-energy
band shifting only from 426 to 414 nm (Fig. 8). This is consistent
with the observation described earlier that the absorption of the
amine complex PtL9Cl lies in line with the absorbance maxima of
6. Benzo-18-crown-6-substituted complex, PtL15Cl
The oxacrown-substituted complex PtL15Cl displays similar lumi-
nescence properties to the dimethoxyphenyl-substituted complex
PtL14Cl, as expected. The modulation of the absorption and
emission spectra of this complex in response to added divalent
metal cations M2+ was investigated. Experiments were carried out
in MeCN solution, using the same metal triflate salts as those
screened in the study of PtL12Cl. Again, little effect was observed
for the group I metal ions. The divalent metal ions Mg2+, Ca2+
and Zn2+ led to a blue-shift in the lowest-energy spin-allowed
ox
the other complexes on the basis of Ep (Fig. 3): it is only for the
emissive excited state where the additional stabilisation is observed
for this complex (Fig. 5a).
Somewhat related examples of how the excited states of
azacrown-substituted platinum complexes can be modulated by
ion binding have been reported by Yam and by Wu.11 They
+
∫
have studied complexes of the type [Pt(tpy)(–C C–Ar)] or
Scheme 2 Schematic illustration of the proposed effect of binding of H+ or metal ions such as Ca2+ upon the relative energies of the ligand-centred and
intramolecular charge-transfer excited states in complexes PtL9Cl and PtL12Cl.
1736 | Dalton Trans., 2009, 1728–1741
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absorption band, which was most significant for Zn2+(Fig. 9). This
effect can again be interpreted in terms of the influence of metal
binding on the phenolic oxygen atoms, which will render them less
electron-donating and hence lower the HOMO energy, shifting
the excitations towards the higher energies observed for PtL1Cl.
On the other hand, the emission band—though showing some
decrease in intensity upon addition of metal ions—displays no
shift: the profile is unchanged (Fig. 9). This might suggest that the
affinity of the excited state for the metal ions is lower than that of
the ground state, a plausible possibility given the redistribution of
electron density away from the crown and towards the platinum
centre upon excitation.
are known to quench 1O2 once generated,32 the fact that this
effect is observed only for these two systems amongst the family
of complexes suggests some chemical reactivity associated with
the basic dialkylamino units, possibly N-oxidation. In line with
this result, we also note an empirical observation that these two
complexes display evidence of long-term instability: over a period
of days in aerated solution, a new high-energy band appears in
the emission spectrum centred around 510 nm, at the expense of
the original emission band, which would be consistent with amine
oxidation or cleavage.
Thus, whilst it might be tempting to suggest that complexes
such as PtL12Cl could be viable luminescent sensors for metal
ions, amenable to time-resolved detection procedures owing to
their long lifetimes, the development of such systems to practicable
applications would face tough challenges. In particular, the oxygen
sensitivity, singlet oxygen generation, and questionable long-term
stability would all compromise the potential performance.
Summary and conclusions
The new compound 1,3-di(2-pyridyl)-5-boronic acid neopentyl-
glycolate, dpyb-B, has been shown to be a versatile intermediate for
the preparation of 5-aryl-appended NŸCŸN-binding ligands. The
new ligands prepared in this study—which incorporate a variety
of electron rich aromatics with amino and alkoxy substituents—
all lead to highly luminescent platinum(II) complexes. They have
quantum yields in solution in the range 0.3–0.5 and lifetimes of
the order of 10 ms at room temperature. Density functional theory
calculations which include the effect of the solvent demonstrate
remarkable success in simulating observed absorption spectra, and
confirm that the trend to lower excited-state energies upon aryl
substitution is associated with the participation of the pendant
group in the HOMO. In dichloromethane solution, the lower-
than-anticipated emission energy of the amino complexes is
probably due to the stabilisation of a TICT-like state under the
more polar conditions. By incorporating the amine atom into
an azacrown, the effect can be reversed through the binding of
divalent metal ions such as Ca2+, leading to the profound changes
in emission profile observed. In contrast, there is no change in
emission profile upon binding of metal ions to the oxocrown
analogues, despite a shift of the absorption to higher energies. All
the complexes are shown to be very efficient sensitisers of the 1Dg
state of oxygen in solution. Overall, the results demonstrate the
rich potential of the family of NŸCŸN-coordinated platinum(II)
complexes, and the ability to switch between different excited
states—each highly luminescent—with distinct properties through
simple stuctural modification.
Fig. 9 Absorption and emission spectra of the benzo-18-crown-6 ap-
pended complex, PtL15Cl (1.7 ¥ 10-5 M) in MeCN solution (containing
0.1 M Bu4NClO4 as an inert electrolyte) (red lines), and the corresponding
spectra in the presence of zinc triflate (5 ¥ 10-4 M) (blue lines); lex = 365 nm
(isosbestic point), T = 298 K.
7. Oxygen sensitivity and 1O2 formation
The luminescence of all of the complexes in solution is partially
quenched by dissolved molecular oxygen, with bimolecular rate
contants of the order of 109 M-1s-1 in CH2Cl2 at 298 K (Ta-
ble 4). The values are largest for the most electron-rich, amino-
substituted complexes. The triplet states of many transition metal
complexes can act as sensitisers of singlet oxygen, O2 1Dg, by energy
ꢀ
3
-
transfer to the
ground state of molecular oxygen. For the
g
present set of complexes, the quenching of the visible luminescence
by oxygen was accompanied by the appearance of emission
in the near-IR, attributable to the formation and subsequent
1
radiative decay of the Dg state, which emits at ~1270 nm. The
quantum yields of singlet oxygen formation by the complexes
in CH2Cl2 under irradiation at 355 nm were determined using
perinaphthenone as a standard.31 With the exception of PtL9Cl
and PtL12Cl, no significant quenching of the generated singlet
oxygen by the complexes was observed: the rate of decay of
the 1Dg emission in the near-IR was 1.2 ¥ 104 s-1, essentially
identical to that measured following production via the standard.
In the case of the Me2N, and azacrown–substituted complexes,
however, the rate of decay of the 1O2 was increased by a factor of
around 2, corresponding to a bimolecular quenching rate constant
of ~5 ¥ 108 M-1s-1. Although some transition metal complexes
Experimental
Proton and 13C NMR spectra, including NOESY and COSY,
were recorded on Varian 300, 400 or 500 MHz instruments.
Chemical shifts (d) are in ppm, referenced to residual protio-
solvent resonances, and coupling constants are in Hertz. Electron
ionisation (EI) mass spectra were recorded with a Micromass
AutoSpec spectrometer. Electrospray ionisation (ES) mass spectra
were acquired on a time-of-flight Micromass LCT spectrometer
with methanol or acetonitrile as the carrier solvent. All solvents
used in preparative work were at least Analar grade, and water
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R
was purified using the Puriteꢀ system. Solvents used for optical
3J 5.0, 4J 1.5, H6), 8.57 (1H, t, 4J 1.5, H2¢), 8.29 (2H, 4J 1.5, H4¢),
7.93 (2H, d, 3J 8.0, H3), 7.82 (2H, td, 3J 7.5, 4J 1.5, H4), 7.66 (2H,
d, 3J 8.5, Hb), 7.29 (6H, m, Ha¢, H5), 7.18 (4H, d, 3J 8.5, Hb¢), 7.15
spectroscopy were HPLC grade.
1,3-Di(2-pyridyl)benzene (HL1) and 1,3-di(2-pyridyl)-5-
bromobenzene (dpyb-Br) were prepared by palladium-catalysed
reaction of 2-(tributylstannyl)-pyridine with 1,3-dibromobenzene
and 1,3,5-tribromobenzene respectively, as described pre-
viously.33,13 4-Bromo-N,N-diphenylaniline was prepared by a
modified Ullmann coupling from diphenylamine and 4-bromo-
iodobenzene;34 the dianisyl analogue was prepared in the same
way. 1,3-Di(2-pyridyl)-5-(p-dimethylaminophenyl)benzene, HL9,
was prepared by Suzuki cross-coupling of dpyb-Br with p-
(dimethylamino)phenylboronic acid; data were identical to those
from the different method of preparation used in our previous
work, in which the Stille and Suzuki reactions were carried out
in the opposite sequence.13 The synthesis and characterisation
of 4-bromophenyl-N-monoaza-15-crown-5 are described in the
ESI†.
3
(2H, m, Ha), 7.04 (2H, t, J 7.5, Hc¢). MS (ES+): m/z 476 [M +
H]+. Anal. calcd. for C36H29N3O2: C, 80.7; H, 5.5; N, 7.8%. Found
C, 80.3; H, 5.7; N, 7.5%.
5-[4-N,N-Di(4-methoxyphenyl)aminophenyl]-1,3-di(2-
pyridyl)benzene HL11
Compound HL11 was prepared used the same procedure as for
HL10, using dpyb-B (290 mg, 0.84 mmol) and 4-bromo-N,N-
di(anisyl)aniline (301 mg, 0.78 mmol). Work-up and purification
by chromatography on silica using the same eluant as for HL10
1
gave the product as a colourless solid (111 mg, 25%). H NMR
3
(400 MHz, CDCl3): d 8.76 (2H, d, J 4.5, H6), 8.59 (1H, s, H2¢),
8.31 (2H, s, H4¢), 7.98 (2H, d, 3J 8.0, H3), 7.88 (2H, t, 3J 7.5, H4),
7.63 (2H, d, 3J 8.5, Hb), 7.35 (2H, m, H5), 7.12 (4H, d, 3J 8.5, Hb¢),
7.05 (2H, d, Ha), 6.89 (4H, d, 3J 8.5, Ha¢), 3.81 (CH3). MS (ES+):
m/z 536 [M + H]+. Anal. Calcd. for C34H25N3: C, 85.9; H, 5.3; N,
8.8%. Found C, 86.0; H, 5.4; N, 8.2%.
1,3-Di(2-pyridyl)benzene-5-boronic acid neopentylglycol
ester (dpyb-B)
A mixture of dpyb-Br (110 mg 0.34 mmol), potassium acetate
(118 mg, 1.20 mmol), bis(neopentyl glycolato)-diboron (87 mg,
0.39 mmol) and 1,1¢-[bis(diphenylphosphino)]ferrocene palla-
dium(II) dichloride (20 mg, 0.027 mmol) in anhydrous DMSO
(5 mL) was degassed by three freeze–pump–thaw cycles and back-
filled with nitrogen. The mixture was heated at 80 ◦C for 6 h under
nitrogen. After cooling to ambient temperature, toluene (15 mL)
was added, and the solution washed with water (5 ¥ 20 mL).
The organic layer was dried over anhydrous potassium carbonate,
filtered and the solvent removed under reduced pressure to give a
pale brown solid, which was recrystallised from dichloromethane–
5-[4-(N-Monoaza-15-crown-5)phenyl]-1,3-
di(2-pyridyl)benzene HL12
This compound was prepared in the same way as HL10 using dpyb-
B (154 mg, 0.45 mmol), 4-bromophenyl-N-monoaza-15-crown-
5 (93 mg, 0.25 mmol), sodium carbonate (37 mg, 0.35 mmol)
and Pd(PPh3)4 (34 mg, 0.029 mmol) in a mixture of toluene :
ethanol : water (3 : 3 : 1 mL respectively). The mixture was heated
at 80 ◦C for 3 d before work up as for HL10. The crude product was
purified by chromatography on aluminium oxide, gradient elution
from hexane to 70 : 30 hexane : ethyl acetate (Rf = 0.8 in ethyl
1
methanol solution to give the product (114 mg, 90%). H NMR
(500 MHz, CDCl3): d 8.74 (1H, t, 4J 1.5, H2¢), 8.72 (2H, dd, 3J 5.0,
4J 1.5, H6), 8.48 (2H, d, 4J 1.5, H4¢), 7.89 (2H, dt, 3J 8.0, 4J 1.5, H3),
7.76 (2H, td, 3J 8.0, 4J 2.0, H4), 7.23 (2H, ddd, 3J 7.5, 3J 5.0, 4J 1.5,
H5), 3.82 (4H, s, –CH2–), 1.05 (6H, s, CH3). 13C NMR (126 MHz,
CDCl3): d 157.7, 149.8 (C6), 139.3, 136.8 (C4), 133.1 (C2¢), 128.1
(C4¢), 122.2 (C5), 121.0 (C3), 72.49 (CH2), 32.1 (C-alkyl quat), 22.1
(CH3). HRMS (ES+): m/z 345.17702 [M + H]+; [C21H22N2O211B]
requires 345.17689.
acetate) yielding a clear oil (122 mg, 88%). H NMR (300 MHz,
1
3
4
CDCl3): d 8.73 (2H, d, J 4.5, H6), 8.48 (1H, t, J 1.5, H2¢), 8.23
4
3
3
(2H, d, J 1.5, H4¢), 7.88 (2H, d, J 8.0, H3), 7.78 (2H, td, J
8.0, J 1.7, H4), 7.65 (2H, d, J 8.5, Hb), 7.26 (2H, dd, J 5.0, J
4
3
3
4
1.0, H5), 6.76 (2H, d, J 8.5, Ha), 3.80 (4H, t, J 6.0) and 3.66
(16H, m), alicyclic protons. MS (ES+): m/z 547 [M + Na]+, 526
[M + H]+.
3
3
5-(4-Methoxyphenyl)-1,3-di(2-pyridyl)benzene HL13
5-(4-N,N-Diphenylaminophenyl)-1,3-di(2-pyridyl)benzene HL10
This compound was prepared from dpyb-Br (106 mg, 0.34 mmol),
4-methoxyphenylboronic acid (58 mg, 0.38 mmol), sodium car-
bonate (149 mg, 1.41 mmol) and Pd(PPh3)4 (40 mg, 0.035 mmol),
suspended in a mixture of toluene : ethanol : water (4 : 4 : 1 mL
respectively). The mixture was heated at 80 ◦C for 60 h. Purification
was performed by chromatography on silica gel, elution 50 : 50
hexane : diethyl ether (Rf = 0.7 in diethyl ether) followed by
recrystalisation from hexane (121 mg, 94%). 1H NMR (500 MHz,
A mixture of dpyb-B (178 mg, 0.52 mmol), 4-bromo-N,N-
diphenylaniline (169 mg, 0.52 mmol), and sodium carbonate
(66 mg, 0.62 mmol, in 0.2 mL water), in dimethoxyethane
(10 mL) was degassed and back-filled with nitrogen gas.
Tetrakis(triphenylphosphine)palladium(0) (48 mg, 0.042 mmol)
was added under a stream of nitrogen, and the mixure was heated
◦
at 80 C for 60 h. The solvent was then removed under reduced
3
4
4
pressure, and the residue taken into dichloromethane (40 mL) and
washed with water (4 ¥ 30 mL). After separation of the organic
layer and drying over anhydrous potassium carbonate, the solvent
was evaporated and the residue purified by chromatography on
silica gel, gradient elution from hexane to 80 : 20 hexane : diethyl
ether (Rf = 0.4 in 75 : 25 hexane : diethyl ether), followed by
recrystallisation from hexane : diethyl ether yielding a colourless
solid (76 mg, 31%). 1H NMR (400 MHz, CDCl3): d 8.75 (2H, dd,
CDCl3): d 8.74 (2H, dd, J 5.0, J 1.5, H6), 8.54 (1H, t, J 1.5,
4 3
H2¢), 8.26 (2H, d, J 1.5, H4¢), 7.89 (2H, d, J 7.5, H3), 7.79 (2H,
3
4
3
3
td, J 7.5, J 1.5, H4), 7.71 (2H, d, J 8.5, Hb), 7.27 (2H, dd, J
6.0, J 4.5, H5), 7.01 (2H, d, J 8.5, Ha), 3.87 (3H, s, CH3). 13C
NMR (126 MHz, CDCl3): d 159.5, 157.4, 149.8 (C6), 142.0, 140.5,
136.9 (C4), 133.5, 128.6 (Ca), 126.1 (C4¢), 124.1 (C2¢), 122.5 (C5),
121.0 (C3), 114.3 (Cb), 55.5 (CH3). HRMS (ES+): m/z 339.14903
[M + H]+, [C23H19N2O] requires 339.14919.
3
3
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5-(3,4-Dimethoxyphenyl)-1,3-di(2-pyridyl)benzene HL14
3J(195Pt) 39, H6), 7.96 (2H, td, 3J 7.5, 4J 1.5, H4), 7.74 (2H, dd, 3J
8.0, 4J 1.5, H3), 7.61 (2H, s, H3¢), 7.50 (2H, d, 3J 8.5, Hb), 7.29 (6H,
m, Ha¢, H5), 7.16 (6H, m, Ha, Hb¢), 7.05 (2H, tt, 3J 7.0, 4J 1.0, Hc¢).
13C NMR (101 MHz, CDCl3): d 167.3, 152.5, 147.7, 147.3, 141.4,
139.3 (C4), 136.6, 135.8, 129.5, 127.7 (Cb), 124.5, 124.2, 123.5,
123.2 (Cc¢), 123.1 (C3¢), 119.4 (C3). Anal. Calcd. for C34H24N3PtCl:
C, 57.9; H, 3.4; N, 6.0%. Found C, 57.1; H, 3.4; N, 5.9%.
PtL11Cl From HL11 (81 mg, 0.15 mmol) giving a deep yellow
solid (25 mg, 22%). 1H NMR (400 MHz, CDCl3): d 9.38 (2H, d,
3J 6.0, J(195Pt) 40, H6), 7.96 (2H, td, 3J 7.5, 4J 1.5, H4), 7.75 (2H,
d, 3J 8.0, H3), 7.60 (2H, s, H3¢), 7.44 (2H, d, 3J 8.5, Hb), 7.29 (2H,
m, H5), 7.12 (4H, d, 3J 7.0, Hb¢), 7.03 (2H, d, Ha), 6.87 (4H, d, 3J
7.0, Ha¢). Anal. Calcd. for C34H24N3PtCl: C, 57.9; H, 3.4; N, 6.0%.
Found C, 57.1; H, 3.4; N, 5.9%.
Ligand HL14 was prepared as described for HL13, using 3,4-
dimethoxyphenylboronic acid (103 mg, 0.57 mmol) in place of
methoxyphenylboronic acid. Purification by chromatography on
silica gel, elution gradient from hexane to diethyl ether (Rf = 0.5 in
diethyl ether), yielded a colourless solid, which was recrystallised
from diethyl ether (64 mg, 31%). 1H NMR (500 MHz, CDCl3): d
3
4
4
8.75 (2H, d, J 4.5, H6), 8.55 (1H, t, J 1.5, H2¢), 8.27 (2H, d, J
1.5, H4¢), 7.92 (2H, d, 3J 8.0, H3), 7.82 (2H, td, 3J 7.5, 4J 1.5, H4),
7.31 (4H, m, H5, Hd, Hb), 6.98 (1H, d, J 8.0, Ha), 4.00 (3H, s,
3
m-CH3), 3.94 (3H, s, p-CH3). 13C NMR (126 MHz, CDCl3): d
157.3, 149.7 (C6), 149.2, 148.9, 142.3, 140.4, 137.0, 134.0, 126.2
(C4¢), 124.2 (C2¢), 122.5 (C5), 121.0 (C3), 119.9, 111.5 (Ca), 110.7,
56.2 (m-CH3), 56.1 (p-CH3). MS (ES+): m/z 369.3 [M + H]+.
PtL12Cl From HL12 (112 mg, 0.21 mmol) giving an orange solid
1
3
(100 mg, 68%). H NMR (500 MHz, CDCl3): d 9.28 (2H, d, J
5.6, J(195Pt) 36.5, H6), 7.91 (2H, t, 3J 7.5, H4), 7.69 (2H, d, 3J 7.5,
5-(3,4-Benzo-18-crown-6)-1,3-di(2-pyridyl)benzene HL15
H3), 7.51 (2H, s, H3¢), 7.46 (2H, d, J 8.5, Hb), 7.23 (2H, t, J
3
3
HL15 was prepared using the procedure used for HL10–12, starting
from dpyb-B (284 mg, 0.83 mmol), 4¢-bromobenzo-18-crown-6
(337 mg, 0.86 mmol), sodium carbonate (119 mg, 1.12 mmol),
and Pd(PPh3)4 (38 mg, 0.033 mmol) in a mixture of toluen◦e :
ethanol : water (6 : 6 : 1 mL). The mixture was heated at 80 C
for 80 h. Purification of the crude material by chromatography
on aluminium oxide, gradient elution from hexane to ethyl acetate
(Rf = 0.3 in ethyl acetate), yielded the product as a clear, colourless
oil (192 mg, 43%). 1H NMR (500 MHz, CDCl3): d 8.74 (2H, d, 3J
4.5, H6), 8.53 (1H, t, 4J 1.5, H2¢), 8.23 (2H, d, 4J 1.5, H4¢), 7.89 (2H,
6.5, H5), 6.74 (2H, d, J 8.5, Ha), 3.81 (4H, t, J 6.0, Hb), 3.71
(8H, m, Hg,e), 3.67 (8H, m, Ha,d). 13C NMR (126 MHz, CDCl3):
d 167.5, 159.7, 152.3, 147.0, 141.2, 139.1 (C4), 137.0, 129.1, 127.8
(Cb), 123.2 (C5), 122.6 (C3¢), 119.4 (C3), 111.9 (Ca), 71.5, 70.4, 70.3,
68.7 (Cb), 52.7 (Ca). MS (ES+): m/z 778.2 [M + Na+], 719.4 [M–
Cl]+. HRMS (ES+): 777.17832 [M + Na]+; [C32H34N3O4PtCl23Na]
requires 777.17778.
3
3
PtL13Cl From HL13 (48 mg, 0.14 mmol) giving a bright yellow
solid (55 mg, 72%). 1H NMR (500 MHz, CDCl3): d 9.33 (2H, d,
3J 5.5, J(195Pt) 40, H6), 7.94 (2H, t, 3J 8.0, H4), 7.73 (2H, d, 3J 7.5,
3
3
4
d, J 7.5, H3), 7.79 (2H, td, J 7.5, J 1.5, H4), 7.28 (4H, m, Hb,
Hd, H5), 6.98 (1H, d, 3J 8.0, Ha), 4.28 (2H, t, 3J 4.5, Ha¢), 4.22 (2H,
t, 3J 4.5, Ha), 3.96 (4H, t, 3J 4.5, Hb, Hb¢), 3.80 (4H, m, Hg , Hg¢),
3.74 (4H, m, Hd, Hd¢), 3.70 (4H, s, He, He¢). 13C NMR (126 MHz,
CDCl3): d 157.4, 149.8, 149.3, 149.0, 142.2, 140.5, 137.0, 134.6,
126.3 (C4¢), 124.2 (C2¢), 122.5 (C5), 121.1 (C3), 120.6, 114.5 (Cb),
113.9 (Cd), 71.0, 71.0, 70.9, 69.9, 69.8, 69.6, 69.4. HRMS (ES+):
m/z 543.24895 [M + H]+, [C32H35N2O6] requires 543.24896; m/z
565.23048 [M + Na]+, [C32H34O6N223Na] requires 565.23090.
H3), 7.56 (2H, s, H3¢), 7.55 (2H, m, Hb), 7.27 (2H, t, J 6.0, H5),
3
7.07 (2H, d, 3J 8.0, Ha), 3.88 (3H, s, CH3). 13C NMR (126 MHz,
CDCl3): d 167.4, 160.2, 159.2, 152.5 (C6), 141.3, 139.2 (C4), 136.7,
134.5, 128.1 (Cb), 123.4 (C5), 123.2 (C3¢), 119.5 (C3), 114.5 (Ca),
55.6 (CH3). Anal. Calcd. for C23H17N2OPtCl.1/4CH2Cl2: C 47.4,
H 3.0, N 4.8%. Found: C 47.9, H 2.9, N 4.8%.
PtL14Cl From HL14 (64 mg, 0.17 mmol) giving a yellow solid
(64 mg, 65%). 1H NMR (500 MHz, CDCl3): d 9.28 (2H, d, 3J 5.5,
3
4
3
J(195Pt) 40, H6), 7.92 (2H, td, J 7.5, J 1.0, H4), 7.72 (2H, d, J
8.0, H3), 7.52 (2H, s, H3¢), 7.23 (2H, ddd, 3J 7.0, 3J 6.0, 4J 1.0, H5),
7.16 (2H, m, Hb, Hd), 6.97 (1H, d, 3J 8.7, Ha), 4.01 (3H, s, m-CH3),
3.96 (3H, s, p-CH3). 13C NMR (126 MHz, CDCl3): d 167.2, 160.4,
152.4 (C6), 149.5, 148.7, 141.2, 139.1 (C4), 136.8, 135.0, 123.3 (C5),
123.3 (C3¢), 119.5 (C3), 119.4 (Cb), 111.7 (Ca), 110.5 (Cd), 56.4 (m-
CH3), 56.2 (p-CH3). Anal. calcd for C24H19N2O2PtCl.1/4CH2Cl2:
C, 45.3; H, 3.1; N, 4.3. Found: C, 45.1; H, 3.3; N, 4.2%.
Platinum complexes
The platinum(II) complexes of the ligands were prepared by
reaction with potassium tetrachloroplatinate(II) using one of two
methods. For Pt10–11Cl and Pt13–14Cl, a mixture of the ligand (50–
100 mg) and K2PtCl4 (1 equiv.) in acetic acid (typically 3 mL) was
degassed by three freeze–pump–thaw cycles and back-filled with
nitrogen gas. The mixture was refluxed for 72 h. The resulting
yellow or orange precipitate was separated by centrifugation and
washed successively with water, ethanol and diethyl ether (3 ¥
5 mL of each). The solid was then extracted into dichloromethane,
filtered to remove small amounts of insoluble residue, and the
solvent then evaporated to give the product. The complexes PtL9Cl
and Pt12Cl, which feature basic nitrogen atoms in the pendants, as
well as Pt15Cl, were prepared by mixing a solution of the ligand in
acetonitrile (typically 50 mg in 5 mL) with a solution of K2PtCl4
in water (1 equiv. in 2.5 mL), both of which had been previously
purged with nitrogen gas, and refluxing the mixture under nitrogen
for 72 h. Work-up was similar to that used for the preparations in
acetic acid.
PtL15Cl From HL15 (84 mg, 0.16 mmol) giving a yellow solid
(33 mg, 28%). 1H NMR (400 MHz, CDCl3): d 9.31 (2H, d, 3J 5.7,
3
4
3
J(195Pt) 40, H6), 7.94 (2H, td, J 7.5, J 1.5, H4), 7.72 (2H, d, J
8.0, H3), 7.53 (2H, s, H3¢), 7.26 (2H, m, H5), 7.16 (1H, s, Hd), 7.15
(1H, m, Hb), 6.97 (1H, d, 3J 8.5, Ha), 4.29 (1H, t, 3J 4.5, Ha¢), 4.23
(1H, t, J 4.5, Ha), 3.98 (2H, m, Hb, Hb¢), 3.81 (2H, m, Hg , Hg¢),
3
3.75 (2H, m, Hd, Hd¢), 3.71 (2H, s, He, He¢). MS (ES+): m/z 794.2
[M + Na]+, m/z 768.3 [M–Cl + MeOH]+.
Electrochemical measurements
Cyclic voltammetry was carried out using a mAutolab Type III
potentiostat with computer control and data storage via GPES
Manager software. Solutions of concentration 1 mM in 90%
CH3CN/10% CH2Cl2 were used, containing [Bu4N][BF4] as the
PtL10Cl From HL10 (76 mg, 0.16 mmol) giving a yellow solid
(97 mg, 88%). 1H NMR (400 MHz, CDCl3): d 9.35 (2H, d, 3J 5.5,
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1728–1741 | 1739
©

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supporting inert electrolyte. A three-electrode assembly was em-
ployed, consisting of a platinum working electrode, platinum wire
counter electrode and platinum flag reference electrode. Solutions
were purged for 5 minutes with solvent-saturated nitrogen gas
with stirring, prior to measurements being taken without stirring.
The voltammograms were referenced to a ferrocene-ferrocenium
couple as the standard (E0 = 0.40 vs. SCE).
1 were used at the excitation wavelength, and the absorption
spectrum of the sample was verified after the measurements to
ensure that no electrochemical degradation had occurred, induced
by the field. Data was analysed by a global fitting procedure
described previously,38 based on that developed by Smirnov et al.25
Theory and calculations
The electronic structures were calculated by density functional
theory (DFT) methods using the Gaussian 0339 program package.
Low-lying singlet and triplet excitation energies were calculated
at optimized geometries by time-dependent DFT (TD-DFT).
DFT calculations employed hybrid functionals; either B3LYP40
or Perdew, Burke, Ernzerhof41 (PBE0). The solvent was de-
scribed by the polarizable conductor calculation model (CPCM).42
Optimized excited-state geometry was calculated for the lowest
triplet state by unrestricted Kohn–Sham (UKS) procedure. For
H, C, N, O and Cl atoms, either polarized 6–31g* double – z
basis sets43 for geometry optimization and vibrational analysis,
or cc-pvdz correlation consistent polarized valence double – z
basis sets44 (TD-DFT) were used, together with quasirelativistic
effective core pseudopotentials and corresponding optimized set
of basis functions for Pt.45 The MO plots were drawn using
the GaussView software. The spectral simulation was performed
using the GaussSum software.46 All calculated transitions are
included. Gaussian shapes of the absorption bands are assumed.
The fwhm value of 0.4 eV used corresponds to typical experimental
bandwidths of MLCT bands.47
Photophysical measurements
Absorption spectra were measured on a Biotek Instruments
XS spectrometer, using quartz cuvettes of 1 cm path length.
Steady-state luminescence spectra were measured using a Jobin
Yvon FluoroMax-2 spectrofluorimeter, fitted with a red-sensitive
Hamamatsu R928 photomultiplier tube; the spectra shown are
corrected for the wavelength dependence of the detector, and
the quoted emission maxima refer to the values after correction.
Samples for emission measurements were contained within quartz
cuvettes of 1 cm path length modified with appropriate glassware
to allow connection to a high-vacuum line. Degassing was achieved
via a minimum of three freeze-pump-thaw cycles whilst connected
to the vacuum manifold; final vapour pressure at 77 K was <
5 ¥ 10-2 mbar, as monitored using a Pirani gauge. Luminescence
quantum yields were determined using quinine sulfate in 1M
H2SO4 (f = 0.54635) and [Ru(bpy)3]Cl2 in air-equilibrated aqueous
solution (f = 0.02836) as standards; estimated uncertainty in f is
20% or better.
The luminescence lifetimes of the complexes were measured
by time-correlated single-photon counting, following excitation
at 374.0 nm with an EPL-375 pulsed-diode laser. The emitted
light was detected at 90◦ using a Peltier-cooled R928 PMT after
passage through a monochromator. The estimated uncertainty in
the quoted lifetimes is 10% or better. Bimolecular rate constants
for quenching by molecular oxygen, kQ, were determined from
the lifetimes in degassed and air-equilibrated solution, taking
the concentration of oxygen in CH2Cl2 at 0.21 atm O2 to be
2.2 mmol dm-3.37
Acknowledgements
We thank EPSRC for support (grant ref. EP/D500265/1) and for a
DTA studentship to D. L. R.; Frontier Scientific for supplying key
starting materials; E.U. COST D35; and the Grant Agency of the
Academy of Sciences of the Czech Republic (KAN100400702) and
Ministry of Education of the Czech Republic (OC 139) for support
to S. Z. We thank Prof. J. T. Hupp and Dr J. McGarrah for access
to and assistance with instrumentation for TDCP. The authors are
indebted to Drs S. Navaratnam and R. Edge for their assistance
in measuring 1O2 quantum yields, carried out with support from
CCLRC Daresbury Laboratory, and to Dr J. A. Weinstein for
discussions.
The quantum yields of singlet oxygen production were obtained
by measuring the intensity of the Dg near-IR luminescence in
1
an air-saturated solution in CH2Cl2, and comparing it with that
measured for a solution of perinaphthenone {1-H-phenalen-1-
one, U(1O2) = 0.9531}. Solutions with optical density between 0.1
and 0.2 at 355 nm were used in each case. The samples were excited
at 355 nm using the third harmonic of a Nd : YAG laser, and the
emission in the near-IR was detected using a liquid-N2-cooled
germanium photodiode detector. A filter was used to eliminate
light of < 1100 nm. Measurements at a range of incident light
powers were made, and the quantum yields determined from
the ratio of the gradients of intensity vs. power for the sample
and standard, after correcting for any small differences between
the absorbance of the sample and standard solutions at 355 nm.
Representative plots are shown in the ESI†.
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