Synthesis, crystal structure and solid state photoluminescence of [Pt(trpy)(C{triple bond, long}CPh)]SbF6 (trpy=2,2′:6′,2″-terpyridine)

Article abstract of DOI:10.1016/j.ica.2006.08.061

The synthesis and characterisation of an orange polymorph of [Pt(trpy)(C{triple bond, long}CPh)]SbF₆ is described where trpy = 2,2′:6′,2″-terpyridine. An X-ray crystal structure determination at 293 K reveals that the cations are planar and stacked head-to-tail with alternating Pt?Pt distances of 3.604(1) and 4.412(1) ?. The perpendicular distances between successive cation planes are constant along the stack at a value of 3.33 ?. Crystal structure determinations at 240 and 200 K show that reducing the temperature to 200 K has no significant effect on the cation arrangement. However, below 200 K there is a phase change that we have not been able to characterise, but which has an effect on the solid state photoluminescence exhibited by [Pt(trpy)(C{triple bond, long}CPh)]SbF₆. Thus, whereas at temperatures of ≥200 K, a broad peak with two components at ca. 566 and 597 nm is observed, below 200 K a longer wavelength peak develops that red-shifts as the temperature is lowered [λ(em)_max = 637 nm at 80 K]. We assign the ≥200 K emission as ³MLLCT in origin, since the X-ray data show that ligand-ligand (LL) and not metal-metal (MM) interactions are important at T ≥ 200 K. On the other hand, the long wavelength emission observed below 200 K is typical of ³MMLCT emission, suggesting that the phase change leads to d_z² (Pt) s(-) d_z² (Pt) orbital interactions. Of particular interest is that the cation exhibits ³MLCT emission in dichloromethane that maximizes at 619 nm, i.e., the high temperature solid state emission occurs at a shorter wavelength, an unexpected result since intermolecular interactions in the solid usually cause the emission to occur at longer wavelengths. A possible explanation for this unexpected result is given.

Full text of DOI:10.1016/j.ica.2006.08.061

Inorganica Chimica Acta 360 (2007) 1633–1638
www.elsevier.com/locate/ica
Synthesis, crystal structure and solid state photoluminescence
of [Pt(trpy)(C„CPh)]SbF6 (trpy=2,20:60,200-terpyridine)
a
a,
a
a
Riaan Buchner , John S. Field *, Raymond J. Haines , Lesibana P. Ledwaba ,
¨
b
b
a
Robert McGuire Jr. , David R. McMillin , Orde Q. Munro
a
School of Chemistry, University of KwaZulu-Natal, Pietermaritzburg 3209, South Africa
b
Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084, USA
Received 12 August 2006; accepted 30 August 2006
Available online 14 October 2006
Abstract
The synthesis and characterisation of an orange polymorph of [Pt(trpy)(C„CPh)]SbF6 is described where trpy = 2,20:60,200-terpyri-
dine. An X-ray crystal structure determination at 293 K reveals that the cations are planar and stacked head-to-tail with alternating
˚
Ptꢀ ꢀ ꢀPt distances of 3.604(1) and 4.412(1) A. The perpendicular distances between successive cation planes are constant along the stack
˚
at a value of 3.33 A. Crystal structure determinations at 240 and 200 K show that reducing the temperature to 200 K has no significant
effect on the cation arrangement. However, below 200 K there is a phase change that we have not been able to characterise, but which has
an effect on the solid state photoluminescence exhibited by [Pt(trpy)(C„CPh)]SbF6. Thus, whereas at temperatures of P200 K, a broad
peak with two components at ca. 566 and 597 nm is observed, below 200 K a longer wavelength peak develops that red-shifts as the
temperature is lowered [k(em)max = 637 nm at 80 K]. We assign the P200 K emission as 3MLLCT in origin, since the X-ray data show
that ligand–ligand (LL) and not metal–metal (MM) interactions are important at T P 200 K. On the other hand, the long wavelength
3
emission observed below 200 K is typical of MMLCT emission, suggesting that the phase change leads to dz2 ðPtÞ–dz2 ðPtÞ orbital inter-
3
actions. Of particular interest is that the cation exhibits MLCT emission in dichloromethane that maximizes at 619 nm, i.e., the high
temperature solid state emission occurs at a shorter wavelength, an unexpected result since intermolecular interactions in the solid usually
cause the emission to occur at longer wavelengths. A possible explanation for this unexpected result is given.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Luminescence; Platinum complexes; Terpyridine complexes
1. Introduction
[20]. A recurring theme in the reports by Yam and co-work-
ers, is that the luminescence properties of mononuclear
Pt(trpy)(r-acetylide) derivatives are markedly dependent
on the extent to which the lumaphores aggregate in fluid
solution and in the solid state [15,18]. One example is
of direct relevance here.The purple [Pt(trpy)(C„CPh)]-
Terpyridyl (trpy, where trpy = 2,20:60,200-terpyridine)
ligand complexes of platinum(II) with a chloride ion as
the fourth ligand bonded to platinum have received consid-
erable attention insofar as their photoluminescence proper-
ties are concerned [1–14]. However, only five reports have
appeared on the photophysics of complexes where a r-acet-
ylide ligand is bound to a Pt(trpy) unit [15–19]; and one
report on the photophysics of cyanide ligand complexes
of the type [Pt{40(R)-trpy}(CN)]+ (R = H, CN, or NMe2)
3
PF6 complex exhibits monomeric MLCT emission in ace-
tonitrile but, due to dz2 ðPtÞ–dz2 ðPtÞ orbital interactions,
exhibits 3MMLCT (MMLCT = metal–metal bond-to-
ligand charge-transfer) emission at a much longer wave-
length of ca. 800 nm in the solid state [15]. This observation
has prompted this report on the crystal structure and solid
state photophysics of the hexafluoroantimonate salt of the
same cation, i.e., [Pt(trpy)(C„CPh)]SbF6, a salt that is both
*
Corresponding author. Tel.: +27 33 2605239; fax: +27 33 2605009.
E-mail address: fieldj@ukzn.ac.za (J.S. Field).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.08.061

1634
R. Bu¨chner et al. / Inorganica Chimica Acta 360 (2007) 1633–1638
00
00
different in colour (orange) and that exhibits very different
emission behaviour to that reported for the purple hexaflu-
orophosphate salt in the solid state [15].
(DMSO-d6): d 8.69 (2H, d, H6,6 ) 8.26 (2H, s, H3,3 ) 8.22
0
0
00
00
(2H, s, H3 ,5 ) 8.04 (2H, t, H4,4 ) 7.68 (2H, m, H5,5 ) 7.46–
7.24 (5H, m, C6H5) 2.50 (1H, s, H4 ). 13C NMR (DMSO-
0
0
0
00
00
d6): d 160.0–154.2 (4C, m, C2,2 ,6 ,2 ) 144.8 (2C, s, C4,4 )
0
00
0
00
2. Experimental
142.2 (1C, s, C4 ) 142.0 (1C, s, C2 ) 144.0 (2C, s, C6 ,6 )
00
129.8 (2C, s, C5,5 ) 128.9–126.8 (6C, m, C6H5C„C) 126.5
00
0
0
2.1. Materials
(2C, s, C3,3 ) 124.8 (2C, s, C3 ,5 ) 103.7 and 98.6 (2C, s,
C6H5C„C). UV–Vis (50 lM in MeCN): kmax/nm (e,
The organic solvents used in the syntheses were of
HPLC grade, and with the 2,20:60,200-terpyridine, phenyl-
acetylene, Bu3SnCl and butyllithium (1.6 M solution in
M
ꢁ1 cmꢁ1) 244 (2.5 · 104); 259 (6.1 · 103); 311 (8.3 · 103);
330 (9.0 · 103); 343 (9.2 · 103); 430 (2.5 · 103).
n
hexanes) came from Aldrich Chemicals and were used as
received. The [Pt(PhCN)2Cl2] and AgSbF6 were obtained
from Strem Chemicals and also used as received. The
[Pt(trpy)Cl]SbF6 was synthesized by the reaction in aceto-
nitrile of [Pt(PhCN)2Cl2] with an equimolar amount of
AgSbF6 followed by addition of 2,20:60,200-terpyridine, as
described in the literature [7]. The DMSO-d6 used for
recording the NMR spectra, as well as the acetonitrile
and dichloromethane used for the infrared, UV–Vis
absorption and solution emission measurements, were of
spectroscopic grade and obtained from Merck Chemicals.
For the UV–Vis absorption and solution emission mea-
surements the solvents were deoxygenated prior to use by
a stream of argon.
2.4. Instrumentation
Microanalysis for carbon, hydrogen and nitrogen was
performed by Galbraith Laboratories, Knoxville, TN,
USA. Infrared spectra were recorded using a Shimadzu
FT IR 4300 spectrometer. H and 13C NMR spectra were
1
recorded at 298 K using a Varian Unity Inova 500 MHz
spectrometer. UV–Vis spectra were recorded at 295 K
using a Shimadzu UV-2101PC scanning spectrophotome-
ter. Emission spectra were recorded on a SLM-Aminco
SPF-500 C spectrofluorometer. The cryostat was an
Oxford Instruments model DN1704 liquid-nitrogen-cooled
system complete with an Oxford Instruments model ITC4
temperature controller. The excitation wavelength was
350 nm, with the scattered light removed by a 400 nm
long-wave-pass filter.
n
2.2. Synthesis of Bu3SnC„CPh
Butyllithium (5.35 ml of a 1.6 M solution in hexanes,
8.56 mmol) was added dropwise over a period of 10 min
2.5. Crystal structure determinations
to
a
vigorously stirred solution of phenylacetylene
The crystal and refinement data for [Pt(trpy)-
(C„CPh)]SbF6 obtained at 293, 240 and 200 K are sum-
marized in the Table 1. X-ray intensity data from an
orange needle-shaped crystal were collected with CRYSALIS
CCD 170 [21] on an Oxford Diffraction Xcalibur 2CCD
four-circle diffractometer fitted with an Oxford Instruments
Cryojet and temperature controller. Absorption correc-
tions were applied using a numerical face-indexing method
and the data reduced with the program CRYSALIS RED 170
[21]. The structure was solved using the program SHELXS97
[22]. All non-hydrogen atoms were assigned anisotropic
displacement parameters and the hydrogen atoms (in
calculated positions) a single overall isotropic displacement
parameter. The plots were obtained using the ORTEP pro-
gram [23].
(0.75 ml, 6.85 mmol) in dry THF (20 ml) at ꢁ78 ꢁC. The
mixture was stirred for a further 10 min after which
nBu3SnCl (2.78 ml, 10.28 mmol) added dropwise over a
period of ca. 30 min. The reaction mixture was then
allowed to warm up to room temperature and its volume
reduced to ca. 5 ml. The remaining liquid was extracted
into n-hexane (2 · 20 ml) and the combined extracts filtered
under dinitrogen using a P4 frit. The volume of the filtrate
was reduced to a minimum (ca. 2 ml) in vacuo to afford the
product as a clear light-yellow liquid. Yield: 1.61 g (60%).
IR (CH2Cl2, cmꢁ1): m(C„CPh): 2132m.
2.3. Synthesis of [Pt(trpy)(C„CPh)]SbF6
n
Two equivalents of Bu3SnC„CPh (0.17 g, 0.42 mmol)
were added to a suspension of [Pt(trpy)Cl]SbF6 (0.15 g,
0.21 mmol) in acetonitrile (10 ml) and the mixture heated
under reflux for 6 h. After allowing the mixture to cool
to room temperature, the product precipitated out as an
orange microcrystalline solid that was isolated by filtration,
washed successively with n-hexane, cold acetonitrile and
diethyl ether and dried in vacuo. Yield: 0.12 g (70%). Anal.
Calc. for C23H16F6N3PtSb: C, 36.10; H, 2.11; N, 5.49.
Found: C, 36.10; H, 2.25; N, 5.71%. IR (KBr, cmꢁ1):
m(C„CPh): 2128w; m(trpy): 1605s, 14ꢁ87m, 1478m, 1454s,
1439s, 1400s, 1035m, 772s; mðSbFÞ6 658vs. 1H NMR
3. Results and discussion
The r-acetylide complex [Pt(trpy)(C„CPh)]SbF6 was
synthesised in a good yield by means of a metathesis reac-
tion in which the chloro-analogue, [Pt(trpy)Cl]SbF6, was
reacted in acetonitrile under reflux with two equivalents
of nBu3SnC„CPh. The compound was isolated as an
orange microcrystalline solid by rapidly cooling the reac-
tion mixture to room temperature. Single crystals suitable
for the purposes of an X-ray diffraction study were grown
by dissolving the compound in acetonitrile at room temper-

R. Bu¨chner et al. / Inorganica Chimica Acta 360 (2007) 1633–1638
1635
Table 1
a
Crystal and refinement data for [Pt(trpy)(C„CPh)]SbF6
Temperature (K)
a (A)
293
240
200
˚
7.521(4)
14.691(4)
20.755(6)
91.35(3)
2293(1)
2.217
7.521(1)
14.640(2)
20.683(3)
90.74(1)
2277(1)
2.232
7.501(1)
14.604(2)
20.632(2)
90.57(1)
2260(1)
2.249
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Dc (g cmꢁ3
)
l (mmꢁ1
)
7.34
7.39
7.44
Total number of reflections measured
Independent reflections
Rint
19385
7165
0.102
19471
7111
0.067
19259
7053
0.064
Reflections observed [I > 2r(I)]
3403
0.066
0.146, 0.231
308, 0
0.000
4076
0.060
0.115, 0.176
308, 0
0.000
4341
0.058
0.106, 0.177
308, 0
0.000
4.13(Pt), ꢁ2.20(Pt)
b
R1 [I > 2r(I)]
c
R1b, wR2 (all data)
Number of refined parameters,d restraints
(shift/esd)max
ꢁ3
˚
Maximum, minimum Dq (e A
)
2.63(Pt), ꢁ2.29(Pt)
3.49(Pt), ꢁ2.03(Pt)
a
Crystal data common to all temperatures: formula (fw), C23H16F6N3PtSb (765.23 S); crystal system, monoclinic; space group, P21/n (no. 14); Z, 4;
F(000), 1432.
P
P
b
R1 = iFoj ꢁ jFci/ jFoj.
P
P
2
2
1=2
2
c
wR2 ¼ ½ ½wðF o2 ꢁ F 2c Þ ꢂ= ½wðF o2Þ ꢂꢂ where w ¼ 1=½r2ðF o2Þ þ ð0:1PÞ ꢂ and P ¼ ðF o2 þ 2F 2c Þ=3.
d
Refinement on F2.
ature and allowing the solvent to evaporate slowly over a
period of days. Note that when the orange compound is
dissolved in refluxing acetonitrile and the solution allowed
to cool slowly to room temperature, i.e., over a period of
days, a purple crystalline material forms at the bottom of
the flask along with a few orange crystals. However, despite
repeated attempts including hand selection of crystals, it
was not possible to separate an analytically pure purple
compound; nor could single crystals suitable for an X-ray
diffraction study be isolated. Nevertheless, an infrared
spectrum of the purple compound recorded as a KBr disc
showed a peak at ca. 2130 cmꢁ1 that is clearly assigned
to the stretching frequency of the C„C triple bond of
the phenylacetylide ligand. Thus, we suspect (but cannot
prove) that the purple material is a second polymorph of
[Pt(trpy)(C„CPh)]SbF6.
stacked head-to-tail with the Ptꢀ ꢀ ꢀPt distances alternating
˚
between 3.363 and 3.375 A in a zig-zag chain [15]. As noted
by the authors, these distances are short enough to support
extended dz2 ðPtÞ–dz2 ðPtÞ orbital interactions along the plat-
inum atom chain [15]. Fig. 2 gives a side-on view of the
cation stack in [Pt(trpy)(C„CPh)]SbF6. Here the cations
are also stacked head-to-tail, but with alternating Ptꢀ ꢀ ꢀPt
C(8)
C(7)
C(9)
C(10)
C(6)
C(5)
C(12)
C(4)
N(2)
C(11)
C(13)
C(14)
N(1)
Pt
C(3)
3.1. Crystal structure of orange [Pt(trpy)(C„CPh)]SbF6
at 293 K
N(3)
C(16)
C(1)
C(15)
C(2)
A perspective view of the cation in [Pt(trpy)(C„CPh)]-
SbF6 is given in Fig. 1 with a selection of intramolecular
bond lengths and angles being listed in the caption. None
of these shows any unusual features and are similar to
those measured for other terpyridyl ligand complexes of
platinum(II) [2–4,7,14,24]. The phenyl ring of the acetylide
ligand is almost co-planar with the Pt(trpy) moiety as evi-
denced by a dihedral angle between these two units of only
1.1ꢁ. In fact the cation as a whole is essentially planar, with
a maximum deviation of any one atom from a mean plane
C(17)
C(18)
C(23)
C(22)
C(19)
C(20)
C(21)
˚
Fig. 1. Perspective view of the cation in [Pt(trpy)(C„CPh)]SbF6. Selected
interatomic distances (A) and angles (ꢁ): Pt–N(1) 2.04(1), Pt–N(2) 1.95(1),
Pt–N(3) 2.06(1), Pt–C(16) 1.97(1); N(1)–Pt–N(2) 80.5(2), N(1)–Pt–N(3)
160.7(4), N(1)–Pt–C(16) 99.1(4), N(2)–Pt–N(3) 80.3(4), N(2)–Pt–C(16)
179.5(4), N(3)–Pt–C(16) 100.1(5). Atom positions are plotted using the
293 K data and the program ORTEP with 50% displacement ellipsoids.
through all the non-hydrogen atoms of 0.10 A. Insofar as
˚
the packing of the cations in crystals of [Pt(trpy)-
(C„CPh)]SbF6 is concerned, it is very different to that
reported for the purple hexafluorophosphate salt, [Pt(trpy)-
(C„CPh)]PF6 [15]. In the latter salt, the cations are

1636
R. Bu¨chner et al. / Inorganica Chimica Acta 360 (2007) 1633–1638
a
4.412(1) Å
Pt
3.604(1) Å
Pt
b
Fig. 2. View of a cation stack in [Pt(trpy)(C„CPh)]SbF6. Successive
cations are related by centres of inversion on the [a]-axis of the unit cell.
The Ptꢀ ꢀ ꢀPt distances shown are those obtained from the 293 K data.
Pt
distances that are not only much longer than those
observed for [Pt(trpy)(C„CPh)]PF6, but also very different
˚
Pt
in length viz 3.604(1) and 4.412(1) A. The upper Ptꢀ ꢀ ꢀPt
distance limit for significant dz2 ðPtÞ–dz2 ðPtÞ orbital interac-
˚
tions is about 3.5 A [25]. As a result, the Ptꢀ ꢀ ꢀPt distances
in crystals of [Pt(trpy)(C„CPh)]SbF6 are too long to allow
for finite dz2 ðPtÞ–dz2 ðPtÞ orbital overlap, with the result that
orange [Pt(trpy)(C„CPh)]SbF6 and purple [Pt(trpy)-
(C„CPh)]PF6 exhibit very different emission behaviour
in the solid state (see below). The perpendicular distances
(or interplanar spacings) between successive mean planes
through the non-hydrogen atoms of the cation in
Fig. 3. View perpendicular to the plane defined by the three nitrogen
atoms of the two types of cation interaction in [Pt(trpy)(C„CPh)]SbF6.
(a) Ptꢀ ꢀ ꢀPt = 3.604(1) A (at 293 K), (b) 4.412(1) A (at 293 K).
˚
˚
the solid emission by the compound at 240 and 200 K.
However, as at 293 K, p–p interactions are expected to
influence the photoluminescence by [Pt(trpy)(C„CPh)]-
SbF6 in the solid state (see below).
[Pt(trpy)(C„CPh)]SbF6 are constant at
a value of
˚
3.33 A. Thus, the difference between the two interaction
Ptꢀ ꢀ ꢀPt distances observed for [Pt(trpy)(C„CPh)]SbF6
can be attributed to the different extents to which adjacent
platinum atoms are slipped with respect to a line drawn
perpendicular to the cation planes; the greater the slippage
the longer the Ptꢀ ꢀ ꢀPt distance. This is illustrated in Fig. 3.
Attempts were made to collect intensity data sets at tem-
peratures below 200 K, specifically at 160, 120 and 80 K,
but without success. It became apparent that the crystal
underwent a phase change below 200 K, as evidenced by
the fact that the unit cell on which the data were indexed
at 293, 240 and 200 K, did not appear as an option for
the data obtained at the lower temperatures. In fact, it
was not possible to discern using the CRYSALIS CCD 170 soft-
ware, a unit cell on which a full sphere of reflections could
be indexed. Interestingly, warming the crystal from temper-
atures below 200 K back up to 200, 240 and 293 K and re-
collecting the intensities, gave data sets that solved for the
same structures as summarized in Table 1.
˚
Note that an interplanar spacing of 3.33 A is well within
˚
the upper distance limit of about 3.8 A for p–p interactions
in organic species [26].
3.2. Crystal structures of orange [Pt(trpy)(C„CPh)]SbF6
at lower temperatures
As the data in Table 1 show, there is no phase change on
cooling crystals of [Pt(trpy)(C„CPh)]SbF6 to 240 and
200 K but there is the expected systematic decrease in the
unit cell volume. Insofar as the structural parameters rele-
vant to the discussion of the solid emission behaviour of
[Pt(trpy)(C„CPh)]SbF6 are concerned these are as follows.
At 240 K, the Ptꢀ ꢀ ꢀPt distances alternate between 3.607(1)
3.3. Photophysical properties of [Pt(trpy)(C„CPh)]SbF6
The UV–Vis absorption spectrum of [Pt(trpy)(C„ CPh)]-
SbF6 measured in acetonitrile exhibits an intense peak at
244 nm with a shoulder at 259 nm, a vibrationally struc-
tured band between 300 and 350 nm, and a broad band at
430 nm. As is the case for other terpyridyl ligand complexes
of platinum(II), the high energy peaks are assigned to
1(p–p*) transitions of the terpyridyl ligand [2,3,15,20].
Two possibilities exist for the assignment of the broad low
˚
˚
and 4.443(1) A while the interplanar spacing is 3.32 A. At
200 K, the Ptꢀ ꢀ ꢀPt distances alternate between 3.603(1)
˚
˚
and 4.454(1) A while the interplanar spacing is 3.31 A.
The Ptꢀ ꢀ ꢀPt distances are not significantly different to those
obtained at 293 K and, therefore, dz2 ðPtÞ–dz2 ðPtÞ orbital
interactions are not expected to play a role in determining

R. Bu¨chner et al. / Inorganica Chimica Acta 360 (2007) 1633–1638
1637
energy band, a dp(Pt) ! p*(trpy) metal-to-ligand charge-
transfer (1MLCT) transition or an acetylide-to-terpyridine
ligand-to-ligand charge-transfer (1LLCT) transition. We
1
tentatively make the MLCT assignment, as indeed have
other workers for the broad band that is observed in the
400–500 nm region of the absorption spectra of mononu-
clear terpyridyl ligand complexes of platinum(II) with a
r-acetylide as the co-ligand [15–17].
The emission spectra of [Pt(trpy)(C„CPh)]SbF6 mea-
sured in acetonitrile and dichloromethane at room temper-
ature are shown in Fig. 4. The emission is broad,
asymmetric and unstructured in both solvents but there
are differences in the peak intensities and maxima. Specifi-
cally, the signal in acetonitrile is weaker and maximizes at
608 nm, whereas the more intense peak recorded in dichlo-
romethane maximizes at 619 nm. These observations are
consistent with a 3MLCT assignment to the emission in fluid
solution, since (1) the emissions from planar complexes of
platinum(II) tend to be quenched in a coordinating solvent
(like acetonitrile), and (2) the wavelength of the emission
maximum for 3MLCT emission is dependent on the polarity
of the solvent [27]. In fact, a similar assignment has been
offered for the emission recorded in acetonitrile from
[Pt(trpy)(C„CPh)]PF6 [15] as well as [Pt(trpy)(C„CPh)]-
ClO4 [16].
Solid state emission spectra of a microcrystalline sample
of orange [Pt(trpy)(C„CPh)]SbF6were recorded at 40 K
intervals over the temperature range 80–280 K and are
shown in Fig. 5. At 280 K the solid emission spectrum
comprises a broad peak with two barely discernable com-
ponents at ca. 566 and 597 nm of near equal intensity
and separated in energy by ca. 900 cmꢁ1. The emission
intensity increases on lowering the temperature to 200 K,
and the resolution improves with little or no shift in the
band maxima. However, below 200 K, a longer wavelength
component develops as shown in Fig. 5. This component is
different in that the maximum shifts to longer wavelength
as the temperature decreases. In assigning the solid emis-
sion at temperatures of P200 K, we first note that it is clear
from the X-ray structural data that neither component can
Fig. 5. Solid state emission spectra recorded on the orange polymorph of
[Pt(trpy)(C„CPh)]SbF6 at 40 K intervals over the temperature range 80 K
(a) to 280 K (f).
3
be assigned to emission from a MMLCT state since the
˚
Ptꢀ ꢀ ꢀPt distances of >3.6 A are too long to support finite
dz2 ðPtÞ–dz2 ðPtÞ orbital overlap [25]. On the other hand,
˚
the interplanar spacings of ca. 3.3 A indicate that there
are extended p–p interactions between adjacent terpyridyl
moieties at all temperatures investigated. To the extent that
intermolecular forces influence the solid-state emission, the
emission has 3MLLCT character because ligand–ligand
(LL) interactions, not metal–metal (MM) interactions,
influence the transition energies. We note that the
900 cmꢁ1 spacing in the solid-state emission spectrum of
[Pt(trpy)(C„CPh)]SbF6 is smaller than the vibrational
progressions of 1300–1500 cmꢁ1 normally associated with
the m(CC) and m(CN) stretching modes of a terpyridyl
ligand [2,3,15,19]. This may mean that the two components
observed at 200, 240 and 280 K have different origins.
Another possibility is that the 900 cmꢁ1 spacing corre-
sponds to a ‘beat frequency’ associated with two higher fre-
quency components [28]. Either way, it is interesting that
the solid-state emission occurs at a shorter wavelength than
that recorded in fluid solutions. Usually, intermolecular
interactions in the solid or a concentrated glass cause the
emission to occur at a longer wavelength [3,7,29–31]. One
possible explanation is as follows. Recall that the cations
are planar and that any one cation is sandwiched between
two others oriented in the opposite direction (see Fig. 3). In
view of the dipole moments, this arrangement is stabilizing
for the ground state, but it may be destabilizing for the
excited state which has a different charge distribution.
We now turn to the changes in the solid emission that
manifest at temperatures below 200 K. As already noted,
a long wavelength emission appears and the maximum
shifts from 602 nm at 160 K to 637 nm at 80 K. Accompa-
nying this red shift is a narrowing of the bandwidth such
that, at 80 K, the full-width-at-half-maximum (fwhm)
value is ca. 1500 cmꢁ1. This kind of behaviour is associated
with 3MMLCT emission, i.e., where there is a finite
dz2 ðPtÞ–dz2 ðPtÞ orbital interaction that increases as the
temperature decreases due to further shortening of the
Ptꢀ ꢀ ꢀPt distance [2,3,9,15]. Unfortunately, we do not have
Fig. 4. Room temperature emission spectra of [Pt(trpy)(C„CPh)]SbF6 in
degassed CH2Cl2 (a) and CH3CN (b).

1638
R. Bu¨chner et al. / Inorganica Chimica Acta 360 (2007) 1633–1638
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¨
the requisite X-ray crystal structure determinations at 80,
120 and 160 K to support this conclusion, but it is likely
˚
that the Ptꢀ ꢀ ꢀPt distances shrink to less than 3.5 A as a
McMillin, G.C. Summerton, J. Chem. Soc., Dalton Trans. (1999)
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4. Conclusions
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This work has demonstrated that changing the counter-
ion from the hexafluorophosphate to the larger hexafluoro-
antimonate anion, causes a dramatic change in the colour
and solid state emission of their salts with the
[Pt(trpy)(C„CPh)]+cation. Thus, whereas the former is
purple the latter is orange and, whereas the solid emission
exhibited by the hexafluorophosphate salt is characterized
by a long wavelength band at 800–830 nm [15], there is
no evidence for such a band in the emission spectra
recorded for [Pt(trpy)(C„CPh)]SbF6 (see Fig. 5). How-
ever, this change is not clear-cut since, as described above,
[Pt(trpy)(C„CPh)]SbF6 can be induced to crystallize in a
purple form by slow cooling of a solution of the compound
in hot acetonitrile. Though the purple sample thus
obtained is not analytically pure, we have been able to
record emission spectra of the purple sample as a function
of temperature that can be regarded as genuinely represen-
tative of the purple polymorph. These spectra are domi-
nated by a broad band that increases in intensity as the
temperature is lowered and which maximizes at ca.
780 nm at 80 K. As such, its solid state photoluminescence
is similar to that recorded for the purple hexafluorophos-
phate salt [Pt(trpy)(C„CPh)]PF6 [15]. We have also
recorded the excitation spectrum of the purple polymorph
and note that is dominated by absorption bands that
appear towards the red end of the spectrum, i.e., at about
600 nm. This accounts for the purple colour.
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Acknowledgements
We thank the University of KwaZulu-Natal and the
South African National Research Foundation for financial
support. The Purdue authors thank the Petroleum Re-
search Foundation for Grant No. ACS-PRF 4036-AC3.
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1
Based on previous studies in our laboratories [9], we speculate that on
cooling the crystal below 200 K the platinum atoms shift their positions
within the cation plane, i.e., the interplanar spacings remain essentially
unchanged. In so doing the platinum atoms of successive cations adopt
new positions that are more eclipsed with respect to a line drawn
perpendicular to the cation stack with the result that the Ptꢀ ꢀ ꢀPt distances
shorten.