Structure-property relationships and 1O2 photosensitisation in sterically encumbered diimine PtII acetylide complexes

Article abstract of DOI:10.1002/chem.201300759

A series of sterically encumbered [Pt(L)(σ-acetylide)₂] complexes were prepared in which L, a dendritic polyaromatic diimine ligand, was held constant (L=1-(2,2′-bipyrid-6-yl)-2,3,4,5-tetrakis(4-tert- butylphenyl)benzene) and the cis ethynyl co-ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron-withdrawing phenyl-CF₃ substituents (4) with electron-donating pyrenes (5) resulted in a red shift of both the lowest-energy absorption (ΔE=3300cm^-1, 61nm) and emission bands (ΔE=1930cm^-1, 64nm). The emission, assigned in each case as phosphorescence on the basis of the excited-state lifetimes, switched from being ³MMLL′CT-derived (mixed metal-ligand-to-ligand charge transfer) when phenyl/polyphenylene substituents (3, 4, 6) were present, to ligand-centred ³ππ* when the substituents were more conjugated aromatic platforms [pyrene (5) or hexa-peri-hexabenzocoronene (7)]. The novel Pt^II acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited-state lifetimes suggested they would be good candidates for use as singlet-oxygen photosensitisers. Determined by in situ photooxidation of 1,5-dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl-5 as sensitiser (k_obs=39.3×10^-3min^-1) was over half that of the known ¹O₂ sensitiser tetraphenylporphyrin (k_obs=78.6×10^-3min ^-1) under the same conditions. Measured ¹O₂ quantum yields of complexes 5 and 7 were half and one-third, respectively, of that of TPP, and thus reveal an efficient triplet-triplet energy-transfer process in both cases. A polyarylated bipyridine ligand was used to prepare a series of sterically encumbered [Pt(L)(σ-acetylide)₂] complexes (L=1-(2,2′-bipyrid-6-yl)-2,3,4,5-tetrakis(4-tert-butylphenyl)benzene), the optical properties of which were tuned effectively by changing the electronic character, extent of π-conjugation, and steric bulk of the cis ethynyl co-ligands. Singlet-oxygen photosensitising properties were also explored (see figure). Copyright

Full text of DOI:10.1002/chem.201300759

FULL PAPER
DOI: 10.1002/chem.201300759
1
Structure–Property Relationships and O2 Photosensitisation in Sterically
Encumbered Diimine PtII Acetylide Complexes
Deanne Nolan,[a] Belꢀn Gil,[a] Longsheng Wang,[b] Jianzhang Zhao,[c] and
Sylvia M. Draper*[a]
Abstract: A series of sterically encum-
bered [Pt(L)(s-acetylide)2] complexes
were prepared in which L, a dendritic
polyaromatic diimine ligand, was held
assigned in each case as phosphores-
cence on the basis of the excited-state
trum, which along with their long trip-
let excited-state lifetimes suggested
they would be good candidates for use
as singlet-oxygen photosensitisers. De-
termined by in situ photooxidation of
1,5-dihydroxynaphthalene (DHN), the
photooxidation rate with pyrenyl-5 as
sensitiser (kobs =39.3ꢀ10ꢀ3 minꢀ1) was
lifetimes,
switched
from
being
3MMLL’CT-derived (mixed metal–
ligand-to-ligand charge transfer) when
phenyl/polyphenylene substituents (3,
4, 6) were present, to ligand-centred
3pp* when the substituents were more
conjugated aromatic platforms [pyrene
(5) or hexa-peri-hexabenzocoronene
(7)]. The novel PtII acetylide complexes
5 and 7 absorb strongly in the visible
region of the electromagnetic spec-
constant
(L=1-(2,2’-bipyrid-6-yl)-
2,3,4,5-tetrakis(4-tert-butylphenyl)ben-
zene) and the cis ethynyl co-ligands
were varied. The optical properties of
the complexes were tuned by changing
the electronic character, extent of p
conjugation and steric bulk of the
ethynyl ligands. Replacing electron-
withdrawing phenyl-CF3 substituents
(4) with electron-donating pyrenes (5)
resulted in a red shift of both the
1
over half that of the known O2 sensi-
tiser tetraphenylporphyrin (kobs =78.6ꢀ
10ꢀ3 minꢀ1) under the same conditions.
1
Measured O2 quantum yields of com-
plexes 5 and 7 were half and one-third,
respectively, of that of TPP, and thus
reveal an efficient triplet–triplet
energy-transfer process in both cases.
Keywords: acetylide ligands · che-
lates · photochemistry · photophy-
sics · platinum
lowest-energy
absorption
(DE=
3300 cmꢀ1, 61 nm) and emission bands
(DE=1930 cmꢀ1, 64 nm). The emission,
Introduction
Polyphenylenes may be considered to be monodisperse,
polyaromatic dendrimers due to their stiff radial arms, strin-
AHCTUNGTRENNUNG
gent geometry and controllable pore size.[1] Hexaphenylben-
zene (HPB) derivatives are a subset of this group and have
been applied as host materials for the detection of volatile
organic compounds,[2] (e.g., via a pentaaryl PtII terpyridyl
complex),[3] in the generation of large-surface support
frameworks for multichromophoric arrays[4] and in artificial
photosynthesis.[5] The functionalisation of sterically demand-
ing polyphenylenes with bipyridyl units has given rise to a
variety of shape-persistent coordination complexes in which
the rigid polyaromatic dendrons essentially encapsulate the
metal centre.[6]
Numerous polyarylated organometallic compounds have
been prepared, including some with bulky substituents that
exhibit correlated rotation,[7] and HPB PdII pincer com-
plexes that behave as homogeneous Lewis acid catalysts.[8]
The HPBs are interesting synthetic precursors for graphene-
like hexa-peri-hexabenzocoronene (HBC) platforms,[9] and
p-conjugated transition metal compounds have been exten-
sively investigated as triplet emitters in optoelectronic devi-
ces.[10] PtII acetylides are particularly viable materials for
such applications.[11] The optical properties of mixed poly-
pyridyl metal–alkynyl systems can be tuned by functionalisa-
[a] Dr. D. Nolan, Dr. B. Gil, Prof. S. M. Draper
School of Chemistry, University of Dublin, Trinity College
College Green, Dublin 2 (Ireland)
Fax : (+353)1-6712826
E-mail: smdraper@tcd.ie
[b] Dr. L. Wang
School of Chemical and Chemical Engineering
Ministry of Light Industry, Hubei University of Technology
Wuhan City 430068 (P. R. China)
[c] Prof. J. Zhao
State (National) Key Laboratory of Fine Chemicals
School of Chemical Engineering, Dalian University of Technology
Room E-208 West Campus, Dalian 116024 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300759. It contains crystal
data and structure refinement details for 2 (Table S1) and 3
(Table S2); ORTEPs of 2 (Figure S1) and 3 (Figure S2); temperature-
dependent 1H NMR spectra of 4 and 5 (Figure S3a, S3b); concentra-
1
tion-dependent H NMR spectra of 3 and 4 (Figure S4a, S4b); con-
1
centration-dependent H NMR spectra of 5 (Figure S5); normalised
UV/Vis absorption spectra of L, 1 and 3 in CH2Cl2 (Figure S6); nor-
malised emission spectra of 3, 4 and 6 in CH2Cl2 solution at 298 K
[lexc <350 nm (Figure S7a), lexc >420 nm(Figure S7b)] and at 77 K
(Figure S8); and UV/Vis absorption spectra showing photooxidation
1
of DHN with complex 4 as O2 photosensitiser (Figure S9).
Chem. Eur. J. 2013, 19, 15615 – 15626
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15615

tion of the polypyridyl ligand or by structural modification
of the acetylide. The strong spin–orbit coupling of the Pt
centre allows for efficient singlet–triplet mixing, which facili-
tates phosphorescence from the appended chromophore.
The strong-field acetylides ensure that non-radiative deacti-
vation of the emissive excited state is less likely.[12]
complexes were established by single-crystal X-ray crystal-
lography and temperature- and concentration-dependent
1H NMR experiments. Complexation results in encapsula-
tion of the PtII centre in a rigid, sterically encumbered poly-
phenylene environment that excludes the possibility of
metal–metal bonding interactions.
Since the first preparation of a diimine PtII bis-acetylide
by Che et al.,[13] compounds of this class have attracted con-
siderable interest. In 2000, a series of highly emissive Pt bi-
pyridyl bis(aryl acetylide) complexes were reported by Ei-
senberg et al., in which synthetic variation of the aryl acety-
lide ligand facilitated a comprehensive investigation into the
nature of the excited state.[14] Concomitant results described
by Schanze et al. and separately by Che et al. detailed the
influence of changing the electronic demand of the diimine
ligand (by varying substituents) on the emission energy.[15]
In both cases, the photophysics of these materials was
shown to be dominated by metal-to-ligand charge transfer,
Results and Discussion
Synthesis: The novel ligand 1-(2,2’-bipyrid-6-yl)-2,3,4,5-tetra-
kis(4-tert-butylphenyl)benzene (L) was prepared by [2+4]
Diels–Alder cycloaddition between 2,3,4,5-tetrakis(4-tert-bu-
tylphenyl)cyclopenta-2,4-dienone and 6-ethynyl-2,2’-bipyri-
dine (Scheme 1).[9b,26] Ligand L has the potential to coordi-
nate to a metal centre such as Pt in two different modes
(Scheme 1), namely, bidentate bipyridyl coordination to
yield [Pt
by pseudo-terpyridine coordination to yield [Pt-
(L:N^N^C)Cl] (2). The synthesis of 1 was achieved by heat-
ing L and cis-[PtCl2A(DMSO)2] to reflux in chloroform. In
ACHTUNGTRNE(NUNG L:N^N)Cl2] (1) or, once deprotonated and anionic,
3MLCT [Pt(5d)!p*
ACHTUNGTRENN(UNG N^N)]. In contrast, Castellano et al.
showed that incorporation of an alkynyl pyrene sensitises
ACHTUNGTRENNUNG
room-temperature intra
G
N
CHTUNGTRENNUNG
the pyrenyl core in fluid solution as a consequence of the
heavy-metal effect of the PtII centre.[16] Photoinduced intra-
molecular electron-transfer quenching has also been studied
for a series of chromophore–dyad complexes incorporating
diimine PtII acetylide complexes as the chromophoric
unit.[17] Alkynyl PtII polypyridyl complexes (bipyridyl or ter-
pyridyl) have found application in areas spanning photocata-
lytic hydrogen production,[18] vapoluminescent materials,[19]
chromophores in dye-sensitised solar cells[20] and lumines-
cent labels for bioconjugates,[21] and some exhibit lamellar
liquid-crystalline behaviour and thermoreversible metallo-
comparison, cyclometallated 2 proved more difficult to pre-
pare in reasonable yield despite attempting multiple synthet-
ic routes. These included using a variety of solvents,[27] alter-
native PtII salts[28] and preparing a dichloro-bridged PtII
dimer followed by water-induced CH activation.[29] Complex
2 was eventually isolated in poor yield from a CHCl3/EtOH
solution of 1 on standing. The conversion of diimine-coordi-
nated 1 to cyclometalated 2 was confirmed by 1H NMR
spectroscopy (in particular the absence of a singlet for an H
atom on the central phenyl ring of 2) and single-crystal X-
ray diffraction (see below).
gelation.[22]
The high yield of long-lived triplet excited states (e.g.,
The [Pt(L)ACTHUNGTERNNU(G C CR)2] complexes 3–7 were prepared by a
ꢁ
series of CuI-catalysed, chloride-to-alkyne exchange reac-
tions. The polyphenylene alkyne 1-(4-ethynylphenyl)-
2,3,4,5,6-penta-(4-tert-butylphenyl)benzene and fully cyclised
2-ethynyl-5,8,11,14,17-penta-tert-butylhexa-peri-hexabenzo-
coronene (HBC) were prepared by published procedures.[30]
The steric bulk of these alkynes meant that an excess of
alkyne co-ligand and extended reaction times were required.
Complexes 3–6 were fully characterised by NMR spectro-
scopy (1H and 13C{1H}, as well as 19F for 4), IR spectroscopy,
mass spectrometry and elemental analysis. As expected, sev-
eral of the proton signals of the bipyridyl ligand are affected
significantly by the nature of the acetylide and underwent
marked shifts upon complexation. Proton H6’, which is im-
mediately adjacent to the coordinated nitrogen atom
(Scheme 1), has the most-downfield of all the proton signals
in each complex; it shifts from d=8.60 ppm in L to d=
9.53 ppm for 1 and to d=10.26 ppm in 7.
3IL) in PtII and IrIII complexes, a consequence of an efficient
intersystem crossing process, has seen them employed as ef-
ficient triplet photosensitisers. Alkynyl PtII polypyridyl com-
plexes have shown application in areas such as photocataly-
sis[18b,23] and triplet–triplet energy-transfer (TTET) process-
es, for example, triplet–triplet annihilation-based upconver-
sion (TTA) and singlet-oxygen sensitisation.[24] The ability of
heavy-metal complexes to sensitise the conversion of triplet
oxygen to highly reactive singlet oxygen is under intense
scrutiny for several applications, particularly in photody-
namic therapy.[25]
This paper reports a new series of diimine PtII bis-acety-
lides in which the diimine ligand is held constant while the
electronic character and steric demands of the appended al-
kynyl chromophores are varied. The series offers an unpre-
cedented increase in the degree of steric encumbrance on
the PtII centre and an opportunity to determine, from an un-
usual perspective, the optical properties as a function of
structure. The bulky diimine 1-(2,2’-bipyrid-6-yl)-2,3,4,5-tet-
rakis(4-tert-butylphenyl)benzene is a polyphenylene bipyri-
dine comprising seven aromatic rings. Its unusual asymmet-
ric design combines functionalized polyaryl substituents with
a bidentate diimine moiety. The structural features of the
The poor solubility of 7 precluded the measurement of its
13C {1H} NMR spectrum, but for the other complexes 3–6
two acetylenic 13C signals were observed for each acetylide
ligand. The13C–195Pt coupling constants of Ca and Cb were
identified by a HMBC experiment on account of the three-
ꢁ
bond coupling of HA (the CH signal closest to the C C
moiety) to Cb on each acetylide (Scheme 1).
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Chem. Eur. J. 2013, 19, 15615 – 15626

Diimine PtII Acetylide Complexes
FULL PAPER
Scheme 1. Synthesis of bipyridyl ligand L, PtII chloride precursor 2 and alkynyl complexes 3–7. i) Benzophenone, 150–2008C, 6 h, 84%; ii) cis-[PtCl2-
(DMSO)2] (1 equiv), CHCl3, 558C, 48 h, 77%; iii) C2H5OH/CHCl3, 7%; iv) Aryl acetylene (2 equiv), CH2Cl2, iPr2NH, CuI (cat.); 3 62%, 4 55%, 5 31%,
6 29%, 7 77%.
ACHTUNGTRENNUNG
ꢁ
For each member of the family of compounds [Pt(L)
CR)2] (3–7), the asymmetric diimine ligand results in chemi-
(C
persists in complexes 3–6, as evidenced by variable-tempera-
ture 1H NMR experiments. Interestingly, the additional
steric bulk of pyrenyl-bearing 5 appears to give rise to even
more restricted internal rotation than in 3, 4 and 6. By 908C
(Supporting Information Figure S3b), several of the proton
ring signals have coalesced to very broad featureless signals
between d=5.9 and 6.8 ppm but have not yet sharpened;
more energy (i.e., higher temperature) is required for the
molecule to cross the rotational barrier imposed by steric
congestion.
To investigate the effect of intermolecular aggregation,
concentration-dependent NMR studies were carried out
over the range 0.1–10 mm (for compounds 1 and 3–6, poor
solubility of 7 precluded its measurement, except at very
low concentrations). For compounds 1, 3, 4 and 6, only very
slight changes under these conditions were observed [i.e., up
to a maximum of 0.05 ppm downfield shift from 0.1 to
10 mm; Supporting Information Figures S4a (3) and Fig-
ure S4b (4)]. Pyrenyl-containing complex 5, studied over the
range of 0.1 to 10 mm, exhibited different behaviour (Sup-
porting Information Figure S5). Between d=7.8 and
10.3 ppm (Supporting Information Figure S5b), a gradual
downfield shift of the aromatic proton signals occurred (e.g.,
the doublet centred at d=10.15 ppm). It appears that the
cally inequivalent acetylide co-ligands giving rise to two n-
(C C) infrared stretching bands with values in good agree-
ꢁ
A
ment with those obtained for analogous complexes.[14,31]
Variable-temperature/concentration 1H NMR spectroscopy:
The 1H NMR spectrum of 1 is complex due to both the
asymmetric nature of L and the steric constraints within the
molecule. Free rotation is sufficiently slowed on the NMR
timescale to broaden the proton signals, and some of the
phenyl ring protons, which should yield two linked doublets
in an AB substitution pattern, appear as four separate
phenyl proton resonances. This effect is more apparent in al-
kynyl complexes 3–7 and was investigated by temperature-
dependent 1H NMR experiments (20–1008C, [D6]DMSO).
On increasing the temperature, the signals due to the tert-
butyl-substituted phenyl ring protons sharpen to give rise to
fewer and more-resolved doublets, which are shifted down-
field (e.g., see spectra for CF3-containing complex 4 in Sup-
porting Information Figure S3a). This is indicative of in-
creased rotational motion, as geometrically equivalent pro-
1
tons become magnetically equivalent on the H NMR time-
scale. This high degree of intramolecular steric constraint
Chem. Eur. J. 2013, 19, 15615 – 15626
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15617

S. M. Draper et al.
1H NMR spectroscopic properties of 5 are influenced both
by intramolecular steric encumbrance and, in contrast to 1,
3, 4 and 6, some intermolecular aggregation. This feature
was investigated further in a subsequent photophysical in-
vestigation.
In combination with the crystal structure of 3 (see below)
we concluded that the interactions causing the restricted
movements in 1, 3, 4 and 6 are predominantly intramolecu-
lar. In these cases, the influence of intermolecular interac-
tions on the observed NMR spectra and consequent optical
properties is minimal.
X-ray crystallography: [PtACTHNUTRGENUGN(L:N^N^C)Cl] (2) was obtained
as fine orange needles from a solution of 1 in chloroform/
ethanol on standing. It crystallised in monoclinic space
group C2/c with one molecule of 2 and one water molecule
in the asymmetric unit (Supporting Information Figure S1).
The geometry about the Pt centre is distorted square-planar
(C1-Pt1-Cl1 105.6(9)8; N1-Pt1-C1 76.7(1)8) and deviates
slightly from those typically determined for N^N^C/PtII sys-
tems (C-Pt-Cl ca. 1008, N-Pt-C ca. 828)[32] due to the steric
demands of the [N^N^C] pseudo-terpyridine and its extend-
ed substituents (e.g., ring G in Figure 1a). The [N^N^C]
ligand moiety is almost planar, with tilt angles of 10.8 and
11.98 for rings B and C, with respect to the plane of the cen-
tral phenyl ring A (Table 1). The four propeller-like phenyl
Table 1. Tilt angles made between the central ring A and the peripheral
aromatic rings in 2 and 3. Rings are identified by a letter according to
Figures 1 and 2.
Figure 1. a) Perspective view of 2 with selected atom numbering and ring
labelling. b) Packing arrangement of 2 viewed down the c axis. Distances
are measured from N1 and the centroid of ring C on an adjacent mole-
cule. Solvent molecules have been removed for clarity. Selected bond
Tilt angle with respect to ring A [8]
2
3
B: 10.8; C: 11.9; D: 73.8; E: 85.8; F: 79.4; G: 75.3
B: 54; C: 49; D: 62; E: 66; F: 62; G: 55
ꢀ
ꢀ
ꢀ
lengths [ꢂ] and angles [8]: Pt1 C1 2.008(2), Pt1 N2 2.112(2), Pt1 N1
ꢀ
2.044(3), Pt1 Cl1 2.319(10); C1-Pt1-N1 76.7(11), N1-Pt1-N2 84.9(10), C1-
Pt1-Cl1 105.6(9), N2-Pt1-Cl1 92.9(8).
substituents twist at angles between 73.8 and 85.88 from the
plane of ring A.[30b,33] Pyridine ring C is involved in an off-
set, face-to-face p-stacking interaction with pyridine ring B
on an adjacent molecule and this gives rise to head-to-tail
dimeric pairs in the crystal lattice (Figure 1b).
causes the ethynyl moieties, particularly Pt1-C9-C10, to de-
viate quite significantly from linearity (1698), and the phenyl
rings of the acetylide ligands are similarly forced to tilt out
of the PtII coordination plane.
Yellow needles of 3 suitable for single crystal X-ray crys-
tallography were grown from a CH2Cl2 solution layered with
methanol. Complex 3 crystallises in the tetragonal space
group I41/a, and the asymmetric unit contains one molecule
of 3 (Figure 2), one molecule of methanol and three water
molecules. Unfortunately, poor crystal quality, despite exten-
sive consequent refinement, precludes very detailed discus-
sion of the structure. As for 2, the geometry of the PtII
centre can be described as distorted square-planar, but in
this case the steric bulk of the substituents combines to push
the acetylide ligands closer together. As a result, the bite
angles of the ligands about the metal centre deviate from
those of similar systems in the literature: the C9-Pt1-C7
bond angle of 858 is slightly smaller and the N2-Pt1-C9
angle of 1008 slightly larger than expected (standard ranges:
89–94 and 94–978 respectively).[19a,31,34] The bulky diimine
A significant degree of steric strain is evident in the solid-
state structure of 3. For example, the distances between
phenyl acetylide C76 and C36 on ring F and between phenyl
acetylide C72 and C26 on ring E are both only slightly
larger than the sum of the van der Waals radii of two
carbon atoms (3.40 ꢂ). Such short contact distances would
suggest reduced rotational freedom in these phenyl rings
and are consistent with the temperature dependence and un-
1
usual features in the H NMR spectrum.
UV/Vis absorption spectra: The absorption spectra of ligand
L and complexes 1 and 3–7 were recorded (Table 2). Di-
AHCTUNGTREGiNNNU mine ligand L shows the broad and featureless absorption
spectrum typical of a non-planar polyphenylene and is com-
posed of high-energy (l=200–320 nm) ligand-centred p–p*
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Diimine PtII Acetylide Complexes
FULL PAPER
AHCTUNGTRENNUNG
and are attributable to metal-to-ligand charge transfer
(1MLCT) [d(Pt)!p*
ACHTUNGTRNE(NUNG N^N)] with some contribution from
3MLCT. The additional weak shoulder at 481 nm (e=
0.02mꢀ1 cmꢀ1) originates from a metal-centred (MC) transi-
tion and is a common feature of the absorption spectra of
similar [PtACHTNUGTRENUNG(bpy)Cl2] (bpy=2,2’-bipyridine) systems (Sup-
porting Information Figure S6).[12,36]
The photophysical properties of the acetylide complexes
are best considered in two groups: 5 and 7, and 3, 4 and 6.
Complexes 5 (ethynyl-pyrene) and 7 (ethynyl-HBC), which
contain extended alkynyl p-conjugated platforms, have well-
resolved, highly structured absorption bands arising from
the p–p* transitions on the appended planar chromophores
(Figure 3). The shape of these bands is typical of their re-
Figure 3. Normalised UV/Vis absorption spectra of complexes 3–7 in
CH2Cl2 (10ꢀ5 m). Inset: lowest-energy absorption band.
Figure 2. Perspective view of 3 and diagram showing selected atomic
numbering and aromatic-ring labelling). Solvent molecules have been re-
moved for clarity.
spective chromophore, although they are slightly red-shifted,
a consequence of s donation from the acetylide onto the PtII
centre. In the case of 7, the additional low energy band at
414 nm probably has some Pt-perturbed intraACTHUNTGRNENUGliACHUTNTGERNNUGgand charac-
ter and some charge-transfer contribution. This band was
also observed recently in the absorption spectra of both a
and n–p* transitions (Supporting Information Figure S6).[35]
Coordination to the PtII centre in the form of [Pt-
Table 2. Room-temperature UV/Vis spectral data (CH2Cl2, ꢂ10ꢀ5 m) and electrochemical data[a] for diimine ligand L, [Pt
ACHTUNGTERN(NUNG L:N^N)Cl2] (1) and related
ꢁ
acetylide complexes [Pt
G
E
ox
pa
(PtII–PtIII
[V][b]
)
Eox
(N^N/C CR)
Ered
(DEp [mV])[d]
1=2ACHTUNGTRENN(UNG N^N/C CR)[V]
ꢁ
ꢁ
lmax [nm]
E
4
(10ꢀ eꢀ
G
[V][b]
ACHTUNGTRENNUNG
234 (5.3), 250 (5.1), 279 (3.8), 304 sh (2.2)
250 (5.6), 278 (4.6), 337 (1.6), 380 (0.8), 418 sh (0.3), 481 (0.02)
261 (8.9), 292 sh (5.8), 356 (1.6), 411 (0.7)
257 (7.3), 267 (7.4), 283 (7.0), 302 sh (5.4), 357 (1.4), 401 (0.8)
232 (14.4), 248 sh (11.3), 280 (8.1), 291 (9.6), 347 sh (4.7), 364 (7.4),
384 (6.9), 397 (5.2), 462 (0.8)
–
+1.07
+1.01
+1.01
–
ꢀ1.96 (273)[c]
ꢀ1.74 (169)
ꢀ1.90 (101)
ꢀ1.85 (191)
ꢀ1.86 (85)
1
+0.70
+0.56
+0.87
+0.37
3 (tBu)
4 (CF3)
5 (Pyr)
+0.83
6 (HPB)
7 (HBC)
251 (15.7), 279 (10.9), 313 (6.1), 445 (0.6)
231 (34.8), 241 (31.0), 260 (14.3), 317 (6.1), 332 (8.3), 348 (16.5),
364 (31.0), 395 (11.7), 414 (5.0), 445 (1.4)
+0.58
+0.45
–
ꢀ1.92 (104), ꢀ2.20 (43)
ꢀ1.90 (81)
+0.80
[a] In deoxygenated dichloromethane (0.1m, nBu4NPF6) at 298 K, scan rate=100 mVsꢀ1, versus Fc/Fc+. [b] Irreversible/quasi-reversible oxidation proc-
ox
pa
ess, E (anodic peak potential) quoted. [c] Return oxidation wave associated with this reduction is poorly defined. [d] DEp =EpaꢀEpc, peak potential sep-
aration.
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S. M. Draper et al.
cis and a trans PtII phosphine complex containing two ethyn-
yl-HBC ligands.[30b]
tum yields, of compounds L, 1 and 3–7 in the solid state and
in CH2Cl2 solution (298 and 77 K) are summarised in
Table 3.
Upon excitation at high energy (lexc =315 nm), the emis-
sion of L is dominated by an intense, high-energy band cen-
tred at 365 nm (solid state)/ 375 nm (solution), with excited-
state lifetimes in the nanosecond region (Table 3, Figure 4).
The most significant feature in the spectra of complexes 3,
4 and 6 is a low-energy absorption band that is red-shifted
with increasing electron-donating character of the acety-
lide.[14,15,37] This low-energy band, which is absent in L and
PtII precursor 1 and buried for the most part under the PAH
absorption bands in 5 and 7, appears at highest energy in 4
(l=401 nm), at 411 nm in 3 and at 445 nm for complex 6.
The band is particularly sensitive to the polarity of the sol-
vent and exhibits a significant degree of negative solvato-
chromism suggesting considerable involvement of charge
transfer (e.g., in 6 the band occurs at 472 nm in toluene and
421 nm in acetonitrile).[12] The close electronic coupling of
the acetylide and 5d Pt metal orbitals in 3, 4 and 6 means
that the HOMO is most likely composed of a mixture of
ꢁ
both Pt 5d and C CR p orbitals. The HOMO–LUMO tran-
sition is probably best described as metal-perturbed (mixed
metal–ligand)-to-ligand charge transfer MMLL’CT [Pt(d)/p-
T
ACHTUNGTRENNUNG
(N^N)].[28,38] A series of PtII diimine dithiolate
ꢁ
complexes in which the electronic demand of both diimine
and dithiolate have been varied independently showed simi-
lar behaviour.[39]
Figure 4. Normalised emission spectra of L and [Pt(L)Cl2] (1) and excita-
tion spectrum of L in CH2Cl2 solution at 298 and 77 K [lexc/nm].
Electrochemical properties: Cyclic voltammetric studies
were carried out on 1 mm solutions of L and complexes 1
and 3–7 in CH2Cl2 containing 0.1m nBu4PF6 as electrolyte
(Table 2). In all six PtII complexes the first oxidation process
is a quasi-reversible/irreversible oxidation wave between
+0.37 and +0.87 V (Table 2), typical of the oxidation of PtII
to PtIII.[40] The metal-centred oxidation was most difficult in
4 (CF3) and easiest in 5, and this corresponds to the change
in the electron density on the metal as a result of the elec-
tron-donating ability of the acetylide. For complexes 3, 4
and 6, the trend observed in the UV/Vis spectrum holds: the
PtII centres in complexes 3 and 6 are significantly easier to
oxidise than that of 4.
The small Stokes shifts between the lowest-energy excitation
band and onset emission and the short lifetimes allow this
emission to be assigned as fluorescence from the bipyridine
chromophore (1pp*). An additional lower-energy emission
band is also evident in the spectrum (507 nm in solution),
which has a similar lifetime. These bands feature in the
emission spectra of all PtII complexes of L in this work and
are characteristic of polyphenylene/bipyridine derivatives in
general.[30b,41]
As is typical of [PtACTHNUTRGNE(UNG bpy)Cl2]-derived systems, complex 1 is
essentially non-emissive as a result of facile non-radiative
decay via low-lying d–d excited states and exhibits extreme-
ly weak ligand-centred fluorescence (l=353 nm). However,
at 77 K in frozen CH2Cl2, an intense, broad, structureless
emission band is observed at lmax =566 nm, significantly red-
shifted from bipyridine-related luminescence bands
(Figure 4). This is tentatively assigned to a mixture of
In 7, the PtII/PtIII oxidation process is coincident with the
irreversible first oxidation on the HBC platform, and thus a
broadened and unstructured reverse wave arises.[30b] In gen-
eral the second quasi-reversible/irreversible oxidation ob-
served in each complex corresponds to the oxidation of L or
the alkyne and occurs between +0.80 and +1.07 V. One re-
duction process occurs within the solvent window; it is local-
ized on the diimine ligand and appears at less negative po-
tentials in the complexes than in the free ligand (Table 2).
The dependence of the PtII oxidation on the alkynyl co-
ligand substituent agrees with the red shift of the lowest-
energy band in the UV/Vis absorption spectra on going
from electron-withdrawing to electron-rich substituents. It is
3
3MLCT and d–d.[12,36]
The photoluminescence data measured for complexes 3–7
are detailed in Table 3 and, as for the UV absorption spec-
tral data, are divided into two groups of compounds accord-
ing to their analogous photophysical behaviour, that is, com-
plexes 3, 4 and 6 are considered separately to 5 and 7.
ꢁ
also further evidence of the participation of both the C CR
Complexes 3, 4 and 6 (3MMLL’CT): The solid-state emis-
sion spectra of 3, 4 and 6 at 298 K (Figure 5, top) are domi-
nated by a broad, structureless emission, which shows red
shifts in the order 6>3>4. Complex 4 containing an elec-
p and Pt 5d orbitals in the HOMO of compounds 3, 4 and 6
and the assignment of the lowest-energy transition in these
cases as MMLL’CT.
tron-withdrawing CF3-substituted acetylide has the most
em
max
Emission spectroscopy: The results of the photophysical
blue shifted emission (l =524 nm) and 3 and 6 show red
measurements, including excited-state lifetimes and quan-
shifts as the acetylide gains more electron-donating charac-
15620
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15615 – 15626

Diimine PtII Acetylide Complexes
FULL PAPER
Table 3. Emission data for L, [Pt(L)Cl2] (1) and complexes 3–7 in the solid state and in solution (CH2Cl2) at 298 and 77 K.
Medium (T [K])
lem [nm] (lexc [nm])
t [ns] (lexc/lem [nm])[b]
Fem
L
solid (298)
solid (77)
365max, 512 (315)
39 (295/365), 1010 (370/512)
360max, 508, 554sh (300)
375max, 502 (315), 477, 487, 497, 507max
531sh (368)
CH2Cl2 (298)[a]
,
11 (295/375), 18 (370/504)
<10ꢀ3[f]
CH2Cl2 (77)
358max, 490 (315)
1
3
solid (298)[c]
solid (77)[c]
non-emissive
362wk (310)
353wk (290)
360wk, 566max, 627sh (309)
non-emissive[c]
CH2Cl2 (298)[a,c]
CH2Cl2 (77)
poorly emissive[c]
<10ꢀ3[g]
solid (298)
solid (77)
358wk, 543max (316)
537max, 574 (360)
357, 375, 386sh, 502, 604max (325), 476, 487,
496, 508, 604max, 668sh (425)
1600 (370/543)
16 (295/373)
134 (13%), 20 (87%) (370/500) 647 (370/600)
0.016[g]
CH2Cl2 (298)[a]
CH2Cl2 (77)
360wk, 517max, 542 sh, 572sh (290–390)
4
6
5
solid (298)
solid (77)
362wk, 524max, 561sh (315)
488, 524, 559sh (360)
355, 374, 387sh, 476sh, 533max (360), 477, 487,
508, 545max, 607sh (425)
363wk (295), 480max, 514, 545, 592sh (400)
2100 (370/524)
CH2Cl2 (298)[a]
19 (370/476) 363 (24%), 19 (76%) (370/506),
702 (89%), 39 (11%) (370/550)
0.017[g]
0.076[g]
0.042[g]
CH2Cl2 (77)
solid (298)
solid (77)
361 (305), 599max (470)
1800 (370/599)
360wk (295), 600max, 647 (482)
395max, 413, 472, 502, 609 (345) 477, 496sh,
506, 609max, 661sh (434)
CH2Cl2 (298)[a]
18 (370/473)
19 (84%), 241 (16%) (370/502) 484 (370/609)
CH2Cl2 (77)
365wk, 541max (300), 539max, 578sh, 613sh (440)
solid (298)[d]
360 (310)
355wk (310)
395, 415, 449max, 478, 495sh, 644sh, 663,
679sh (360)
662max, 719, 738sh (545)
484, 501, 540sh, 660, 674sh, 715, 734 (448)
poorly emissive[d]
solid (77)[d]
poorly emissive[d]
CH2Cl2 (298)[a] 10ꢀ5
m
m
9 (370/448), 23 (370/477), 4650 (370/661)
CH2Cl2 (298)[a] 10ꢀ3
CH2Cl2 (77)
730 (370/662)
7
solid (298)[d]
solid (77)[d]
372 (312), 472, 493, 593 (360)
367 (309), 558, 583max, 638 (390-505)
452sh, 473max, 481sh, 491, 501, 525, 536, 545sh,
560[e] (360)
poorly emissive[d]
poorly emissive[d]
CH2Cl2 (298)[a]
18 (370/475), 20 (370/504)
0.077[g]
CH2Cl2 (77)
477, 509, 524sh, 551sh, 572max, 618, 630sh,
667sh (402)
[a] Argon-degassed solution, 10ꢀ5 m. [b] Estimated error less than tꢃ10%; [c] Very weakly emissive in solid state and in solution (298 K). [d] Poorly
emissive in solid state. [e] Tail of emission band extends to 700 nm. [f] Measured with quinine sulfate as a standard, based on three repeat measure-
ments.[45] [g] Measured with 4’,6-diamidino-2-phenylindole dihydrochloride (DAPI) as a standard, based on three repeat measurements.[46]
em
max
em
max
ter (3 (tBu): l =543 nm; 6 (HPB): l =599 nm). The ex-
lifetimes, Table 3) derived from the hexaphenylbenzene core
dominates the spectrum (upon excitation at low energy).
The most significant and lowest-energy emission band shifts
to lower energy, from l=545 nm (4) to l=604 nm (3) and
to l 609 nm (6). The long excited-state lifetimes (e.g., 3: t=
647 ns) and large Stokes shifts of this emission suggests it is
triplet in origin. The lifetime for 4 at l=550 nm fits two
components [t=702 ns (89%), t=39 ns (11%)], and this
suggests the presence of two energetically close emissive
states, most likely due to partial overlap of the lower-energy
bipyridine emission and the metal-influenced phosphores-
cent band. The emission maxima undergo large rigidochro-
mic shifts and profile sharpening on lowering the tempera-
ture to 77 K; for example, in 3 a shift of DEs =2675 cmꢀ1
cited-state lifetimes of all three complexes are similar (1.0–
2.1 ms, solid state, 298 K). At low temperature, the emission
profiles become asymmetric and sharper than at room tem-
perature (Figure 5, bottom), and each has a degree of vi-
bronic fine structure that reveals the magnitude of involve-
ment of the diimine ligand (N=C, C=C) in the emissive ex-
~
cited state (e.g., 4: n(C
N) =1298 cmꢀ1).
= =
C/C
In optically dilute, degassed CH2Cl2 solution (298 K), sev-
eral emissive states are evident for complexes 3, 4 and 6
(Supporting Information Figure S7a and S7b). In addition to
the lower-energy bipyridine-based emission, high-energy IL
fluorescence is seen at 360 nm. In the case of polyphenylene
1
complex 6, intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand fluorescence (with short nanosecond
Chem. Eur. J. 2013, 19, 15615 – 15626
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15621

S. M. Draper et al.
Figure 6. Normalised emission spectra at 298 and 77 K of 5 and 7 in
CH2Cl2 solution (10ꢀ5 m) (lexc [nm]).
1
be assigned as pp* based on the hexa-peri-hexabenzocoro-
nene chromophore.[43] There is no evidence of the narrow-
bandwidth phosphorescence (3pp*) at 578 nm that has been
shown to be a feature of PtII acetylide HBC complex-
es.[30b,43,44] However, in the case of pyrenyl complex 5, the
lowest-energy band in the emission spectrum (663 nm) has a
lifetime of 4.65 ms. In combination with the profile of emis-
sion from this complex, this transition can be assigned as
phosphorescence originating from a triplet intra
li
E
3
[16]
ꢁ
on the pyrene chromophore [ pp*(C C-Pyrene)].
highest quantum yields of this family of complexes are re-
corded for 5, 6 and 7 (Fem =4.2, 7.6 and 7.7%, respectively).
This is a result of the intense polyaromatic-platform-centred
fluorescence and phosphorescence observed for these com-
plexes.
Figure 5. Normalised solid state emission spectra of 3, 4 and 6. Top:
298 K (lexc: 3, 315 nm; 4, 316 nm; 6, 380 nm). Bottom: 77 K (lexc: 3,
360 nm; 4, 360 nm; 6, 547 nm).
was measured (Figure S8). These large thermally induced
Stokes shifts are characteristic of charge transfer emis-
sion.[42]
The decrease in emission energy with increasing electron-
donating power of the acetylide co-ligand is a consistent fea-
ture of the spectra obtained in the solid state, in solution,
and at room and low temperature. This fact, along with the
UV/Vis data and electrochemical measurements, reaffirms
the assignment of the lowest-energy transition as MMLL’CT
At 77 K in frozen CH2Cl2, intraACHTNUGRTENNUlG iACHTUNGTRNEgNUNG and phosphorescence,
based on the p-conjugated aromatic co-ligands, dominates
the emission spectra of 5 and 7 (Figure 6). Complex 7 dis-
plays a narrow-bandwidth emission centred at 577 nm and a
shoulder at 618 nm. This corresponds to a triplet excited
3
ꢁ
state localised on the HBC platform, pp*
G
3
ilarly, pp* phosphorescence from the pyrene chromophore
occurs at 660 nm. The extremely small thermally induced
Stokes shift (5: DEs =69 cmꢀ1) confirms that emission from
5 and 7 is not derived from a charge-transfer excited state
ꢁ
[Pt(d)/p
G
ACHTUNGTRNE(NUNG N^N)].
and instead falls within the range expected for a pp* emis-
Complexes 5 and 7 (3pp*): In contrast to complexes 3, 4 and
6, complexes 5 and 7 are essentially non-emissive in the
solid state at 298 K and at 77 K. This can be attributed to
quenching of the luminescence due to aggregation (e.g., p–p
stacking) of the extended alkynyl p-conjugated platforms. In
argon-degassed, optically dilute (10ꢀ5 m) CH2Cl2 solution at
room temperature, the spectra of 5 and 7 are dominated by
emission from their respective highly conjugated aromatic
co-ligands. In each case these exhibit an extended emission
band with a complex vibronic structure (Figure 6). The emis-
sion spectrum of 7 is very similar to that recorded for the
free ethynyl-HBC ligand.[30b] The onset of luminescence
occurs at 473 nm and extends to 700 nm. With excited-state
lifetimes in the nanosecond region (Table 3), this band can
sive state.[16,37b]
The self-quenching of luminescence from complexes 5
and 7 in the solid state prompted us to perform measure-
ments on a solution of 5 in CH2Cl2 at higher concentration
(solubility issues precluded a similar study on 7). In 10ꢀ3
m
solution, all of the fluorescence from the pyrene-acetylide
chromophore has been quenched (Table 3), leaving solely
3pp* phosphorescence from the pyrenyl chromophore. A
noticeable decrease in phosphorescence lifetime is also evi-
dent (0.73 ms), but no concentration effects are seen in the
corresponding UV/Vis spectrum. It can be concluded that,
at higher concentrations, pyrenyl-bearing complex 5 self-
quenches in solution, most likely by stacking of pyrene chro-
mophores, similar to that observed in concentration-depend-
15622
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15615 – 15626

Diimine PtII Acetylide Complexes
FULL PAPER
1
ent H NMR spectroscopy. This aggregation has little effect
mixture irradiated and UV spectra were recorded at 2–
on the electronic structure of the triplet excited state, but it
does result in a shortening of the excited-state lifetime, simi-
lar to that observed elsewhere for PtII HBC acetylide com-
plexes.[43]
The photoluminescence of complexes 5 (Pyr) and 7
(HBC) is independent of the coordinated diimine ligand.
The observed phosphorescence from both complexes is
almost identical to that of analogous phosphine-containing
10 min intervals over the course of 60 min. The known com-
ꢁ
pounds [Pt
(bpy)
E
G
ployed as reference materials. Each of the compounds meas-
ured proved to be photostable under the reaction conditions
employed, and this precludes absorption spectral changes
due to decomposition or photochemical reaction of the com-
plex.
Relatively efficient photooxidation of DHN is evident
from the absorption spectra of 5 and 7 (Figure 7a, b). In
ꢁ
ꢁ
systems such as cis-[Pt
HBC)2ACHTUNGTRENNUNG
G
2ACHUTGTNRENN(GU dppe)], trans-[PtACHTUTGNREN(NUGN C C-
ꢁ
(C C-Pyr)
U
phosphorescence is evidently
derived from a 3pp* excited
state located on the extended
aromatic chromophore. The
effect of extending the p conju-
gation of the acetylide is to in-
troduce an energetically acces-
sible 3pp* excited state, local-
ised on the alkynyl moiety, that
is lower in energy than
3MMLL’CT [Pt(d)/p
p*(N^N)].
N
ꢁ
ACHTUNGTRENNUNG
PtII complexes as O2 sensitisers
for DHN photooxidation: The
ability of PtII complexes 5 (Pyr)
and 7 (HBC) to sensitise 1O2
production was examined in a
series of photooxidation experi-
ments. The long-lived, low-lying
triplet excited states (3IL)
formed by photoexcitation of 5
and 7 sensitise the conversion
1
Figure 7. Photooxidation of DHN (1ꢀ10ꢀ4 m) with PtII complexes 5 and 7, [Pt
(bpy)
E
(bpy)3]2+
ꢁ
and TPP as 1O2 photosensitisers (5ꢀ10ꢀ6 m) in CH2Cl2/CH3OH (9/1 v/v), irradiated with a halogen bulb lamp
(power output: 3 mWcmꢀ2). a) UV/Vis absorption spectral change of DHN with time with 5 as sensitizer.
b) Complex 7 as sensitizer. c) Plots of lnACHTNUTRGNEUNG(Ct/C0) versus irradiation time. d) Plots of chemical yield of juglone
3
1
of O2 to the highly reactive O2
species. The model reaction in-
vestigated was the oxidation of
1,5-dihydroxynaphthalene
versus irradiation time.
(DHN) to juglone by 1O2 generated in situ (Scheme 2),
which can be monitored by the reduction in absorption of
DHN as it is consumed over the course of the reaction (e=
7664mꢀ1 cmꢀ1 at l=301 nm) and the resulting increase in ab-
sorption at l 427 nm due to the formation of juglone (e=
3811mꢀ1 cmꢀ1).[24c,47]
contrast, complex 4 (CF3) behaves quite poorly, as is evident
from the marginal changes in absorption with irradiation
time (Figure S9, Supporting Information). This may be ex-
plained by its poor absorption of visible light and relatively
short triplet excited-state lifetime. A quantitative compari-
son may be achieved by plotting lnACTHNUTRGNE(UNG Ct/C0) again irradiation
For each experiment, an excess of DHN was maintained
(5 mol% of photosensitiser relative to DHN), the reaction
time (Ct =DHN concentration at time t, C0 =initial DHN
concentration), the slope of which can be used to derive the
pseudo-first-order oxidation rate constants (see Experimen-
tal Section). The photooxidation rate with pyrenyl-5 as sen-
sitiser (kobs =39.3ꢀ10ꢀ3 minꢀ1) is four times higher than that
ꢁ
(C CPh)2] (kobs =9.2ꢀ10ꢀ3 minꢀ1) and is more
of [PtACTHNURGTENN(UG bpy)ACHUTNGTRENNGUN
1
than half that of the known O2 sensitiser tetraphenylpor-
phyrin (TPP, kobs =78.6ꢀ10ꢀ3 minꢀ1) under these conditions.
Furthermore, the chemical yield of juglone with increasing
irradiation time was calculated for each singlet-oxygen pho-
tosensitiser (Figure 7d). After 60 min of irradiation time,
complex 5 (78%) followed by HBC-7 (45%) achieved the
Scheme 2. Photooxidation of 1,5-dihydroxynaphthalene (DHN) to ju-
glone.[45]
Chem. Eur. J. 2013, 19, 15615 – 15626
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15623

S. M. Draper et al.
Table 4. Photooxidation-related parameters of PtII complexes 4, 5 and 7
7. The single-crystal X-ray structure of 3 strongly points to
an intramolecular nature of these interactions.
ꢁ
and reference compounds [Pt
(CH2Cl2/CH3OH 9/1 v/v, 298 K).
R
G
A
The opto-electronic character of the complexes was ex-
plored by UV/Vis absorption spectroscopy, cyclic voltamme-
try and photoluminescence studies. The lowest-energy ab-
sorption and emission maxima were red-shifted with increas-
ing electronic donicity of the acetylide ligands. Complexes 3,
4 and 6 display broad and increasingly red-shifted emission
profiles with lifetimes in the microsecond region (298 K, in
the solid state and in solution). This, and the dependence of
the PtII oxidation potential on the nature of the acetylide,
[a]
[c]
Complex
103 kobs
[minꢀ1
107 ui[b]
[mminꢀ1
]
FD
Yield
[%][d]
N
]
G
4 (CF3)
5 (Pyr)
7 (HBC)
0.29
0.29
0.16
0.35
0.23
0.57
0.20
0.62[c]
13.0
78.0
45.0
29.8
20.5
94.3
39.3
15.3
9.2
7.3
78.6
39.3
15.3
9.2
7.3
78.6
ꢁ
[Pt
[Ru
TPP
N
ACHTUNGTRENNUNG( Ph)2]
U
[a] Pseudo-first-order rate constant, ln
(Ct/C0)=ꢀkobst. [b] Initial rate of
DHN consumption, ui =kobsACHTUNTGRENNNUG
[DHN]. [c] Quantum yield of 1O2 generation
ꢁ
suggested that both the C CR p and the Pt 5d orbitals par-
with TPP (FD =0.62) as reference.[49,50] [d] Yield of juglone after 60 min
ticipate in the HOMO of these complexes. The emissive ex-
of reaction time.
cited states were assigned as 3MMLL’CT [Pt(5d)/p
(C
ꢁ
CR)!p*
N
highest conversion of DHN to juglone of the studied sys-
tems (Table 4).
As a final comparison, the singlet-oxygen quantum yields
of the PtII complexes were calculated by using the known
singlet-oxygen photosensitiser TPP as reference and 1,3-di-
and 7, which have p-conjugated platforms on their acetylide
co-ligands, was assigned as [3pp*
C
ꢁ
had extended excited-state lifetimes and were characteristic
of their appended polyaromatic chromophores. At 298 K,
complexes 5 and 7 exhibit high-energy fluorescence due to
their respective PAH segments, but at 77 K complex 5
1
phenylisobenzofuran (DPBF) as O2 scavenger (see Experi-
1
3
mental Section).[48] Complexes 5 and 7 showed O2 quantum
shows only pp* emission, which reveals efficient intersys-
yields which were half and one-third, respectively, of that of
TPP, and reveal an efficient triplet–triplet energy-transfer
process in both cases and correspond to the DHN photooxi-
dation results. Interestingly, the PtII reference complex es-
sentially matched the singlet-oxygen quantum efficiency of
TPP itself, and this reveals a highly efficient photosensitisa-
tem crossing from singlet to triplet emissive excited states.
Complex 5 also shows a notable concentration dependence
attributed to aggregation of the pyrenyl PAH fragments.
The properties of 5 and 7 are consistent with an energetical-
3
ly accessible pp* state that is localized on the acetylide and
lower in energy than the 3MMLL’CT state.
1
tion process when DPBF is employed as O2 scavenger. In
The triplet excited states of complexes 5 and 7 were
shown to be sufficiently long lived to effect the sensitisation
of 1O2, an effect shown by subsequent photooxidation of
DHN present in situ. In contrast, the excited state of 4 is in-
sufficiently long lived to afford an efficient conversion proc-
ess. The singlet-oxygen quantum yield of pyrenyl complex 5
and, even more effectively, the Pt complex employed as a
ꢁ
contrast, photooxidation of DHN is only reasonably effi-
cient. As the conversion of DHN to juglone is proposed to
1
involve initial trapping of O2 by DHN to form an adduct
(assumed to be the rate-determining step) followed by dehy-
dration to form juglone, it would appear that formation of
the adduct inhibits further DHN photooxidation.[47] This
suggests the involvement of one or more additional process-
reference, namely, [PtACHTNUTRGENNG(U bpy)CAHTUNGTRNE(NUGN C CPh)2], are of a similar order
1
es unrelated to O2, which makes the kinetic analysis signifi-
of magnitude to that of the known and readily utilised refer-
ence photosensitiser tetraphenylporphyrin.
cantly more complicated. DPBF appears to be a more suita-
1
ꢁ
(
ble substrate for O2 photoreaction sensitised by [Pt
G
This series of complexes allowed us to investigate the
photophysical effects of varying the steric bulk and electron-
ic demand of acetylide co-ligands in PtII complexes with the
same diimine structural motif. The novelty of these com-
plexes is derived from their inherent asymmetry, whereby a
polyarylated bipyridine with several potential coordination
modes partially envelops a photoactive metal centre. The
detailed investigation of such structure–property relation-
ships and efforts to exploit triplet excited states for real-
world purposes are fundamental to the quest for optimised
novel systems for opto-electronic and photosensitisation ap-
plications.
1
Ph)2] and shows an impressive O2 quantum efficiency.
Conclusions
ꢁ
A series of novel [Pt
(N^N)
N
a polyarylated bipyridine have been synthesised. The elec-
tronic character, steric bulk and p-electronic delocalization
of substituents on the acetylide co-ligands were systematical-
ly varied (R=tert-butyl, trifluoromethyl, pyrene, hexaphe-
nylbenzene, HBC). The complexes were characterised by
variable-temperature NMR and infrared spectroscopy, mass
spectrometry, elemental analysis and single-crystal X-ray
crystallography. The variable-temperature NMR experi-
ments supported the conclusion that the presence of a poly-
arylated bipyridine imposes considerable steric strain within
the molecules and affects markedly the rotational freedom
of one or more of the tert-butylphenyl rings in complexes 3–
Experimental Section
Electronic Supplementary Information (ESI) available: For experimental
details, including synthesis, characterisation and X-Ray crystallography,
see the Supporting Information.
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Chem. Eur. J. 2013, 19, 15615 – 15626

Diimine PtII Acetylide Complexes
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Received: February 26, 2013
Revised: July 2, 2013
Published online: September 24, 2013
15626
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Chem. Eur. J. 2013, 19, 15615 – 15626