Spectroscopy and photoredox properties of soluble platinum(II) alkynyl complexes

Article abstract of DOI:10.1016/j.poly.2009.10.005

Six complexes of platinum(II) with a terdentate π-acceptor ligand, 2,6-(N-(n-hexyl)benzimidazol-2′-yl)pyridine), and different ethynylbenzene ligands were synthesized and investigated by means of optical absorption, luminescence, and time-resolved emission spectroscopy. These complexes display similar photophysical and electrochemical properties as previously investigated analogs with the 2,2′:6′,2″-terpyridine ligand. The energy of the luminescence band maximum is a function of the nature of the chemical substituents attached to the ethynylbenzene ligand, luminescence intensities and lifetimes correlate with the luminescence wavelength according to the energy-gap law. The emissive excited states of some of these complexes are quenched reductively with efficiencies near the diffusion-controlled limit, even for moderate electron donors such as phenothiazine or triphenylamine. A complex with a dimethylamine substituent attached to the ethynylbenzene ligand exhibits photophysical properties that are strongly dependent on the protonation state of the amine. A dimer complex with a diethynyl-substituted xanthene bridging ligand displays absorption and emission behavior that is essentially identical to that of some of the monomeric platinum complexes investigated in this work. Short Pt(II)-Pt(II) contacts are only observed in the crystal structure of a precursor complex. A key feature of the new complexes is their good solubility in common organic solvents, thanks to the presence of two hexyl chains that are attached to the terdentate ligands.

Full text of DOI:10.1016/j.poly.2009.10.005

Polyhedron 29 (2010) 857–863
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Spectroscopy and photoredox properties of soluble platinum(II) alkynyl complexes
Akram Hijazi a, Mathieu E. Walther a, Céline Besnard b, Oliver S. Wenger c,
*
a Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
b Laboratory of X-ray Crystallography, University of Geneva, 24 quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
c Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany
a r t i c l e i n f o
a b s t r a c t
p
-acceptor ligand, 2,6-(N-(n-hexyl)benzimidazol-20-
Article history:
Six complexes of platinum(II) with a terdentate
Received 22 June 2009
Accepted 2 October 2009
Available online 8 October 2009
yl)pyridine), and different ethynylbenzene ligands were synthesized and investigated by means of optical
absorption, luminescence, and time-resolved emission spectroscopy. These complexes display similar
photophysical and electrochemical properties as previously investigated analogs with the 2,20:60,200-ter-
pyridine ligand. The energy of the luminescence band maximum is a function of the nature of the chem-
ical substituents attached to the ethynylbenzene ligand, luminescence intensities and lifetimes correlate
with the luminescence wavelength according to the energy-gap law. The emissive excited states of some
of these complexes are quenched reductively with efficiencies near the diffusion-controlled limit, even
for moderate electron donors such as phenothiazine or triphenylamine. A complex with a dimethylamine
substituent attached to the ethynylbenzene ligand exhibits photophysical properties that are strongly
dependent on the protonation state of the amine. A dimer complex with a diethynyl-substituted xan-
thene bridging ligand displays absorption and emission behavior that is essentially identical to that of
some of the monomeric platinum complexes investigated in this work. Short Pt(II)–Pt(II) contacts are
only observed in the crystal structure of a precursor complex. A key feature of the new complexes is their
good solubility in common organic solvents, thanks to the presence of two hexyl chains that are attached
to the terdentate ligands.
Keywords:
Platinum
Absorption
Luminescence
Photoredox chemistry
Photosensitizer
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
the Ru(bpy)32+ complex and sacrificial electron donors [10], recent
work demonstrated that this is also possible with acetylide
Platinum(II) complexes have been of interest to inorganic
photochemists for many years. Particularly prominent examples
complexes of Pt(II) [11]. The MLCT excited-state structures of
2+
Ru(bpy)3 and related d6 metal diimines makes these complexes
include the tetracyanoplatinates [1], Pt(II) diimine complexes [2],
good photosensitizers for inter- and intramolecular electron trans-
fer reactions [12], and the same is true for Pt(II) diimine acetylides
[13]. However, the latter are comparatively little explored in this
respect. In the course of our ongoing research program on long-
range electron transfer [14], we became interested in Pt(II)-based
photosensitizers, mainly because literature studies suggested that
the excited-state properties of these compounds can be varied
rather drastically with relative ease [8,15]. However, previously
investigated Pt(II) acetylide complexes with bipyridine and terpyr-
idine ligands are rather poorly soluble, which can be a problem for
complicated molecular architectures such as covalent donor-
bridge-acceptor molecules [13b,15a,16]. We have therefore
decided to conduct a study on the photophysical and photoredox
properties of Pt(II) alkynyls with 2,6-bis(benzimidazol-20-yl)pyri-
dine (bzimpy) ligands (Scheme 1). The latter can be alkylated eas-
ily, which results in greatly improved solubility with respect to
terpyridine-based complexes, provided the alkyl chains are suffi-
ciently long [17]. Through variation of the substituent R at the eth-
ynylbenzene ligand, the electronic structure of the complex can be
4ꢀ
and the Pt2(pop)4 (pop =
l
-P2O5H2) compound [3]. Metal–metal
interactions in these and related materials are responsible for very
rich photophysics and photochemistry, including luminescence
phenomena [1a,2c,2d], vapochromism [4], thermochromism [5],
and unusual photoredox behavior [3,6]. More recently, plati-
num(II) diimine acetylide and cyclometalated platinum(II) com-
plexes have received increasing attention. The latter have very
favorable emission properties and are promising for applications
as triplet emitters in organic light-emitting diodes [7]. Pt(II) dii-
mine acetylide complexes were found to exhibit similar electronic
2+
structures to the Ru(bpy)3 (bpy = 2,20-bipyridine) complex [8],
i.e., relatively long-lived metal-to-ligand charge transfer (MLCT)
excited states that make these complexes good excited-state oxi-
dants and reductants [9]. Inspired by the early studies that re-
ported on the photochemical generation of H2 from water using
* Corresponding author. Fax: +49 551 39 3373.
E-mail address: oliver.wenger@chemie.uni-goettingen.de (O.S. Wenger).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.10.005

858
A. Hijazi et al. / Polyhedron 29 (2010) 857–863
room temperature, 0.57 ml (4.1 mmol) of 1-bromohexane in 8 ml
N,N-dimethylformamide was added dropwise. This reaction mix-
ture was heated to 120 °C for 48 h, then cooled to room tempera-
ture, and finally poured into an aqueous solution containing 10%
of NH4Cl. The aqueous phase was extracted with chloroform, and
the chloroform extract was washed first with brine and then with
de-ionized water. Subsequent solvent evaporation resulted in
326 mg (0.68 mmol) of pure 2,6-di(N-hexylbenzimidazol-20-
yl)pyridine (hex2bzimpy). 1H NMR (CDCl3, 400 MHz, 25 °C): 8.38
(d, J = 7.8 Hz, 2H), 8.10 (t, J = 7.8 Hz, 1H), 7.90 (m, 2H), 7.50 (m,
2H), 7.43–7.38 (m, 4H), 4.70 (t, J = 7.5 Hz, 4H), 1.74 (m, 4H), 1.06
(m, 12H), 0.65 (m, 6H).
For complexation of hex2bzimpy to platinum(II), step c in
Scheme 2, we followed a method developed previously for com-
plexation of a doubly methylated bzimpy ligand to the same metal
center [4b]. In this procedure, 333 mg (0.7 mmol) of hex2bzimpy
and 200 mg (0.5 mmol) of K2PtCl4 (Sigma–Aldrich) were refluxed
in 30 ml de-ionized water for 3 days. Then the water was evapo-
rated, and the solid residue was recrystallized from methanol
and diethyl ether. This yielded 300 mg (0.4 mmol) of red [Pt(hex2b-
zimpy)Cl]Cl. 1H NMR (CD3OD, 400 MHz, 25 °C): 8.45 (t, J = 7.6 Hz,
1H, Hbzimpy), 7.96 (m, 2H, Hbzimpy), 7.43 (d, J = 8.2 Hz, 2H, Hbzimpy),
7.32 (m, 4H, Hbzimpy), 6.96 (t, J = 7.6 Hz, 2H, Hbzimpy), 4.12 (m, 4H,
Hhexyl), 1.77 (m, 4H, Hhexyl), 1.38 (m, 12H, Hhexyl), 0.98 (m, 6H, Hhex-
yl). ESI-MS (m/z): 709.2385 (calc.) 709.2364 (obs.). For X-ray dif-
fraction, single crystals were grown by slow diffusion of heptane
into dichloromethane. The [Pt(hex2bzimpy)Cl]+ cation crystallized
Scheme 1. The platinum(II) complexes investigated in this work.
modified [8c,18]. As indicated in Scheme 1, we varied R from
strongly electron-withdrawing (NO2) to strongly electron-donating
(N(CH3)2).
Since dimeric transition metal complexes with face-to-face d8
metal centers including Rh(I)–Rh(I) [19], Ir(I)–Ir(I) [20] and
Pt(II)–Pt(II) dimers [3] are among the photophysically and photo-
chemically most interesting systems, we also explored a xan-
thene-bridged Pt(II) alkynyl dimer with the abovementioned
bzimpy ligand (Scheme 1, right).
2. Experimental
2ꢀ
most easily with the ZnCl4 counter-ion. Crystallographic details
for the obtained structure are as follows: red platelet,
The platinum(II) complexes from Scheme 1 were synthesized
following the sequence of reactions outlined in Scheme 2. The first
step (a) is the synthesis of the 2,6-(benzimidazol-20-yl)pyridine
(bzimpy) ligand following a previously published synthetic proto-
col [21]. Hexyl groups were attached to the resulting molecule
using a procedure describing the alkylation of bzimpy with longer
alkyl chains [22]. In a typical procedure (step b in Scheme 2), we
suspended 200 mg (8.3 mmol) NaH in 8 ml dry tetramethylurea
and added 350 mg (1 mmol) of bzimpy under nitrogen atmo-
sphere. After heating to 80 °C for 2.5 h and subsequent cooling to
0.30 ꢁ 0.25 ꢁ 0.08 mm.
C31H37N5ClPtꢂC31H37N5ClPtꢂCl4ZnꢂCH4O,
Mr = 1653.60, monoclinic, space group P21/c, a = 13.690(5) Å,
b = 24.557(7) Å, c = 19.249(6) Å, b = 102.990(10)°, V = 6306(4) Å3,
Z = 4, Mo K
Stoe IPDS 2 diffractometer, 234 074 reflections measured, 22 422
independent reflections, 11 604 reflections with |Fo| > 2 (Fo),
Rint = 0.063. Final values R[F2 > 2 (F2)] = 0.038, R(F2) = 0.079 and
S = 1.05 (H-atom parameters restrained).
a radiation (k = 0.7103 Å), l
= 5.11 mmꢀ1, T = 110 K,
r
r
x
Scheme 2. Synthesis of the [Pt(hex2bzimpy)Cl]+ precursor complex (upper half) and synthesis of the diethynyl-substituted xanthene ligand (lower half) used for the
synthesis of the platinum dimer Pt2.

A. Hijazi et al. / Polyhedron 29 (2010) 857–863
859
Numerous synthetic protocols for the attachment of alkynyl li-
gands to complexes of platinum(II) have been reported in recent
years [4b,8,15b,18,23]. In our work, we have used the following
general procedure for the synthesis of the various complexes
shown in Scheme 1. [Pt(hex2bzimpy)Cl]Cl was reacted with 2
equivalents of commercial alkynyl ligand and 0.1 equivalents of
copper(I) iodide in a 2:1 (v:v) mixture of methanol and diisopro-
pylamine. After stirring at 65 °C overnight, the solvent and the vol-
atile amine base were evaporated. The solid residue was washed
with methanol, acetone, and diethyl ether. Physical characteriza-
tion data for the five Pt–R complexes from Scheme 1 are as follows:
Pt–NO2: C39H41N6O2ClPt. Synthesized in 61% yield. 1H NMR
(CD2Cl2, 400 MHz, 25 °C): 8.80–8.75 (m, 3H, Hbzimpy), 8.56 (m,
J = 8.8 Hz, 2H, Halkynyl), 7.70 (d, J = 8.8 Hz, 2H, Halkynyl), 7.60 (m,
6H, Hbzimpy), 7.29 (m, 4H, Hbzimpy), 4.90 (t, J = 7.5 Hz, 4H, Hhexyl),
2.01 (m, 4H, Hhexyl), 1.50 (m, 4H, Hhexyl), 1.33 (m, 8H, Hhexyl), 0.89
(m, 6H, Hhexyl). ESI-MS (m/z): 820.2938 (calc.) 820.2943 (obs.).
Pt–F: C39H41N5FClPt. Synthesized in 83% yield. 1H NMR (CD2Cl2,
400 MHz, 25 °C): 8.58 (t, J = 7.8 Hz, 1H, Hbzimpy), 8.48 (d, J = 8.2 Hz,
2H, Halkynyl), 8.38 (d, J = 8.2 Hz, 2H, Halkynyl), 7.64 (t, J = 7.6 Hz, 2H,
Hbzimpy), 7.49 (m, 4H, Hbzimpy), 7.29 (m, 4H, Hbzimpy), 4.72 (t,
J = 7.4 Hz, 4H, Hhexyl), 1.92 (m, 4H, Hhexyl), 1.31 (m, 12H, Hhexyl),
0.91 (m, 6H, Hhexyl). ESI-MS (m/z): 793.2993 (calc.) 793.3000 (obs.).
Pt–H: C39H42N5ClPt. Synthesized in 63% yield. 1H NMR (CD2Cl2,
400 MHz, 25 °C): 8.35 (m, 1H, Hbzimpy), 8.15 (d, J = 8.0 Hz, 2H, Halky-
nyl), 8.07 (d, J = 7.5 Hz, 2H, Hbzimpy), 7.53 (m, 4H, Hbzimpy), 7.36 (m,
1H, Halkynyl) 7.31 (m, 4H, Hbzimpy), 7.05 (d, J = 8.0 Hz, 2H, Halkynyl),
4.52 (m, 4H, Hhexyl), 1.77 (m, 4H, Hhexyl), 1.23 (m, 12H, Hhexyl),
0.83 (t, J = 7.0 Hz, 6H, Hhexyl). ESI-MS (m/z): 774.3004 (calc.)
774.2994 (obs.)
ꢀ20 °C, and 0.95 g (3.75 mmol) of iodine in 8 ml dry diethyl ether
was added slowly. This mixture was stirred at ꢀ20 °C during 1 h
and then at room temperature over night. Workup consisted of a
washing with aqueous Na2S2O3 solution, subsequent extraction
of the aqueous phase with diethyl ether, and evaporation of the
solvents in the combined organic phases. Pure 2,7-di(t.-butyl)-
4,5-diiodo-9,9-dimethylxanthene was obtained by recrystalliza-
tion from diethyl ether and hexane. The yield was 70%. 1H NMR
(CDCl3, 400 MHz, 25 °C): 7.72 (m, 2H), 7.38 (m, 2H), 1.63 (s, 6H),
1.33 (s, 18H). This bromo/iodo exchange turned out to be necessary
for the following reaction.
For step e in Scheme 2, 540 mg (0.94 mmol) of the diiodo-xan-
thene and 1.80 g (18 mmol) trimethylsilylethynyl were placed in a
100 ml reaction flask along with 108 mg (0.09 mmol) of Pd(PPh3)4
catalyst and 18 mg (0.09 mmol) of copper(I) iodide. After addition
of 20 ml tetrahydrofuran and 20 ml diisopropylamine, the reaction
mixture was deoxygenated thoroughly by bubbling nitrogen gas
during 30 min. Then, the coupling reaction was carried out at
50 °C for 48 h. After cooling to room temperature, 50 ml of diethyl
ether were added, and the organic solution was washed with aque-
ous NH3, brine, and water. The organic phase was dried over anhy-
drous MgSO4 prior to solvent evaporation. The desired coupling
product was purified by crystallization from hexane, affording a
pale yellow solid. The yield was 35%. 1H NMR (CDCl3, 400 MHz,
t
25 °C): 7.38 (m, 4H), 1.63 (s, 6H, CH3), 1.34 (s, 18H, Bu), 0.33 (s,
18H, TMS).
Removal of the trimethylsilyl protecting groups (step f in
Scheme 2) occurred as follows: 0.10 g (0.2 mmol) of the coupling
product from above were dissolved in 5 ml tetrahydrofuran. Then,
a volume of 1.2 ml of a 1 M solution of tetrabutylammonium fluo-
ride (1.2 mmol) in THF was added slowly at room temperature.
After stirring for 5 h at room temperature and subsequent solvent
evaporation, the solid residues were taken up in diethyl ether and
water. The organic phase was dried over MgSO4 and evaporated to
dryness. Column chromatography on silica gel with pentane eluent
gave the doubly deprotected product in essentially quantitative
yield. 1H NMR (CDCl3, 400 MHz, 25 °C): 7.41 (m, 4H), 3.37 (s, 2H,
Pt–CH3: C40H44N5ClPt. Synthesized in 98% yield. 1H NMR
(CD2Cl2, 400 MHz, 25 °C): 8.18 (m, 2H, Halkynyl), 7.71 (m, 1H, Hbzim-
py), 7.46–7.15 (m, 10H, Hbzimpy + Halkynyl), 6.98 (d, J = 7.5 Hz, 2H,
Hbzimpy), 2.35 (s, 3H, CH3), 4.34 (t, 4H, Hhexyl), 1.79 (m, 4H, Hhexyl),
1.29 (m, 12H, Hhexyl), 0.90 (m, 6H, Hhexyl). ESI-MS (m/z): 788.3160
(calc.) 788.3165 (obs.).
Pt–N(CH3)2: C41H46N6ClPt. Synthesized in 85% yield. 1H NMR
(CD2Cl2, 400 MHz, 25 °C): 8.34 (t, J = 8.6 Hz, 1H, Hbzimpy), 8.13 (d,
J = 8.1 Hz, 2H, Halkynyl), 8.05 (d, J = 8.1 Hz, 2H, Halkynyl), 7.49 (m,
2H, Hbzimpy), 7.30 (m, 2H, Hbzimpy), 7.18 (d, J = 8.6 Hz, 2H, Hbzimpy),
7.00 (d, J = 8.3 Hz, 2H, Hbzimpy), 6.92 (d, J = 8.6 Hz, 2H, Hbzimpy),
4.52 (t, J = 7.4 Hz, 4H, Hhexyl), 1.79 (m, 4H, Hhexyl), 1.38–1.27 (m,
12H, Hhexyl), 0.90 (m, 6H, Hhexyl). MS (ES, m/z): 817.3426 (calc.)
817.3452 (obs.).
We note that reproducible results from elemental analysis
could not be obtained for any of the six new complexes reported
herein. Anion exchange does not help solve this problem. It is plau-
sible that this is due to the fact that these complexes retain organic
solvent molecules with tenacity. This problem has been encoun-
tered previously with platinum(II) complexes of methylated bzim-
py ligands [4]. With hexyl-substituted ligands the situation can be
expected to become even worse. However, all 1H NMR spectra can
be fully interpreted and sample integrity can be verified on this ba-
sis. Moreover, we point out that elemental analysis is not espe-
cially useful in determining the integrity of a complex vis-à-vis
luminescence spectroscopy [24]. Samples which yield acceptable
results from elemental analysis frequently turn out to be impure
by luminescence spectroscopic criteria.
t
ethynyl-H), 1.63 (s, 6H, CH3), 1.33 (s, 18H, Bu).
For the preparation of platinum dimer Pt2 from Scheme 1,
30 mg (0.04 mmol) of [Pt(hex2bzimpy)Cl]Cl were reacted with
6 mg (0.05 mmol) of 2,7-di(t.-butyl)-4,5-diethynyl-9,9-dimethylx-
anthene in presence of 2 mg (0.01 mmol) CuI in a mixture com-
prised of 5 ml methanol and 2.5 ml diisopropylamine. This
mixture was stirred at 80 °C overnight. After evaporation of the
solvent and the amine base, the solid residue was subjected to col-
umn chromatography on silica gel. The eluent was a mixture of
acetone/water/saturated aqueous KNO3 solution in 100:10:1
(v:v:v) proportion. Pure Pt2 was obtained as a dark red solid in
75% yield after precipitation with aqueous KPF6. 1H NMR (CD2Cl2,
400 MHz, 25 °C): 8.88 (m, 2H, Hbzimpy), 8.55 (m, 8H, Hbzimpy), 7.47
(d, J = 2.3 Hz, 2H, Hxanthene), 7.39 (m, 8H, Hbzimpy), 7.28 (d,
J = 2.3 Hz, 2H, Hxanthene), 7.10 (m, 4H, Hbzimpy), 4.88 (m, 4H, Hhexyl),
4.36 (m, 4H, Hhexyl), 1.87 (s, 6H, Hxanthene), 1.74 (m, 8H, Hhexyl), 1.41
(s, 18H, Hxanthene), 1.23 (m, 24H, Hhexyl), 0.82 (t, J = 6.7 Hz, 12H,
Hhexyl). ESI-MS (m/z): 820.2938 (calc.) 820.2943 (obs.).
1H NMR spectra were acquired on a Bruker Avance 400 MHz
spectrometer. All chemical shifts are reported relative to the tetra-
methylsilane signal. Deuterated solvents were bought from the
Cambridge Isotope Laboratories. Electrospray ionization mass
spectrometry was performed on a Finnigan MAT SSQ 7000 instru-
ment. Methanol (HPLC grade, VWR) was used to solubilise the
compounds. Optical absorption data were measured on a Cary
5000 UV–Vis–NIR spectrometer from Varian. Luminescence spec-
tra were obtained using a Fluorolog-3 instrument from Horiba.
Luminescence decays were measured on a home-built setup
comprised of a Quantel Brilliant Nd:YAG excitation source with
Preparation of the diethynyl-substituted xanthene followed the
reaction sequence summarized in the lower half of Scheme 2 [25].
Commercial 2,7-di(t.-butyl)-4,5-dibromo-9,9-dimethylxanthene
(0.60 g, 1.24 mmol) was dissolved in 50 ml anhydrous diethyl
ether and cooled to ꢀ5 °C. After dropwise addition of a mixture
comprised of 1.7 ml 1.6 M n-butyllithium in hexane and
N,N,N0,N0-tetramethylethylene diamine, the reaction mixture was
stirred at ꢀ5 °C for 20 min. Then the solution was cooled to

860
A. Hijazi et al. / Polyhedron 29 (2010) 857–863
an integrated Magic Prism OPO, and a detection system consisting
of a Spex 270 M monochromator, a Hamamatsu photomultiplier,
and a Tektronix TDS 540B oscilloscope. All optical spectroscopic
measurements were performed with dichloromethane of spectro-
photometric grade. Cyclic voltammetry was performed using a
Versastat3-100 potentiostat equipped with the K0264 micro cell
kit from Princeton Applied Research. A silver wire served as a
quasi-reference electrode. The supporting electrolyte was tetrabu-
tylammonium hexafluorophosphate. The solution was deoxygen-
ated prior to voltammetry sweeps by bubbling nitrogen gas. The
scan rate was 200 mV/s.
dation can occur. However, as mentioned above, the lowest-energy
excited state is not of pure MLCT-type but contains also varying
amounts of LLCT character for the different Pt–R complexes [8c].
The Pt–N(CH3)2 sample represents an extreme case with a particu-
larly important LLCT contribution [28]. These mixed MLCT/LLCT
emissions give rise to poorly structured emission bands. The
appearance of stronger vibronic structure in the Pt–NO2 spectrum
suggests that in this complex the emissive excited state contains
significant character of yet another type of electronic excitation,
namely a so-called intraligand (IL) transition. Nitro-substituted
compounds often have a low-lying LUMO on the NO2-group, hence
it is plausible that an IL state on the ethynylbenzene ligand is sig-
nificant in this case. In general, the more electron-withdrawing the
ethynylbenzene-substituent, the more important the IL contribu-
tion becomes as a result of the shift of the MLCT levels to higher
energies.
The luminescence lifetimes in de-aerated dichloromethane
solution vary considerably among the five Pt–R complexes
(Fig. 2, left panel). As seen from the right panel of Fig. 2, the emis-
sion decay rate constant (sꢀ1) is very nearly an exponential func-
tion of the emission energy (Eem). Here, Eem is defined as the
energy of the luminescence band maximum. Notably, in the Pt–
N(CH3)2 complex the emissive excited-state decays almost two or-
ders of magnitude faster than in the Pt–NO2 compound. The en-
ergy-gap law predicts an exponential dependence of the rate
constant for nonradiative excited-state decay on the energy of the
electronic origin of the emission [26]. In Fig. 2 we plot the total ex-
cited-state decay rate constant (that is, radiative and nonradiative
parts) versus the energy of the emission band maximum, yet the
exponential function is clearly discernible. On the one hand, this
must be due to similar radiative decay rate constants for all five
complexes, which, considering the similar oscillator strengths
associated with the MLCT absorptions in Fig. 1, is not surprising.
On the other hand, the data suggests that the energetic difference
between electronic origin and luminescence band maximum is rel-
atively constant among the five Pt–R complexes from Scheme 1. At
any rate, the key point here is that there is an exponential correla-
tion between the excited-state lifetime and the emission energy
(Eem) similar to that observed for d6 metal diimines or other Pt(II)
alkynyl complexes [8b,27].
3. Results and discussion
The left panel of Fig. 1 shows the optical absorption spectra of
the five Pt–R complexes from Scheme 1 in dichloromethane solu-
tion at room temperature. All of them exhibit intense absorptions
above 25 000 cmꢀ1 and weaker bands in the visible spectral range.
Based on prior work on structurally related platinum(II) complexes
with 40-tolylterpyridyl ligands [8c], the intense UV bands can be
attributed to electronic transitions occurring on the bzimpy and
ethynylbenzene ligands, whereas the lower-energy absorptions
are a mixture of metal-to-ligand charge transfer (MLCT) transitions
and ethynylbenzene-to-bzimpy ligand-to-ligand charge transfer
(LLCT) transitions. From Fig. 1 it is seen that the lowest-energy
absorption bands are susceptible to the chemical substituents at-
tached to the ethynylbenzene ligand. A simplistic but useful view
is that ethynylbenzene ligands with electron-donating para-sub-
stituents increase the electron density at the metal center, thereby
making them easier to oxidize and facilitating the MLCT transition,
whereas electron-withdrawing para-substituents have the oppo-
site effect. Since the MLCT energy shifts as a function of the bzimpy
substituent, the relative MLCT and LLCT contributions to the low-
est-lying excited state vary over the series of complexes with
R = NO2, F, H, CH3, N(CH3)2.
This can explain the variations observed in the luminescence
spectra of the five Pt–R complexes in the right panel of Fig. 1. All
of them were measured on ꢃ10ꢀ5 M dichloromethane solutions
at room temperature, and they have been normalized to the same
maximum intensity. This representation allows direct observation
of two trends: (i) the more electron-donating the acetylide para-
substituent, the further the emission shifts to lower energies; (ii)
the greater this red-shift, the more the band shape becomes
unstructured and begins to approach that of a Gaussian. The trend
in emission band energies reflects the ease with which metal oxi-
The dimethylamino-substituted complex stands out from the
five Pt–R complexes in that it exhibits strongly pH-dependent
absorption and emission properties (Fig. 3). In the unprotonated
form, even dilute solutions of the complex are deep purple due
Fig. 2. Left panel: Luminescence decays (in semilogarithmic representation) of the
Fig. 1. Optical absorption (left panel) and luminescence spectra (right panel) of the
Pt–R complexes from Scheme 1
in freeze–pump–thaw deoxygenated 10ꢀ5 M
five Pt–R complexes from Scheme
1
in room temperature dichloromethane
dichloromethane solutions. Excitation occurred at 450 nm with laser pulses of
solutions. For emission spectroscopy, excitation occurred at 450 nm. Emission
intensities are not to scale; all luminescence bands were normalized to a common
maximum intensity.
ꢃ10 ns duration. Right panel: Semilogarithmic plot of the luminescence decay rate
constant (sꢀ1) as a function of the energy of the luminescence band maximum (Eem
)
for the five Pt–R complexes.

A. Hijazi et al. / Polyhedron 29 (2010) 857–863
861
to the abovementioned low-energetic LLCT transition, but upon
addition of trifluoroacetic acid the solution turns pale yellow. In
the series of optical absorption spectra from the left panel of
Fig. 3, isosbestic points are discernible at 468, 350 and 280 nm,
indicating the existence of just two distinct chemical species.
Luminescence spectroscopy reveals a strong blue-shift of the emis-
sion (right panel of Fig. 3) accompanied by an increase in lumines-
(TPA). For these experiments, Pt–F was excited at 450 nm and its
emission intensity at 580 nm was measured as a function of PTZ
or TPA concentration. Both of these two tertiary amines induce
strong luminescence quenching with nearly diffusion-controlled
rate constants kQ. This is particularly noteworthy in the case of
TPA which is a rather poor donor (E1/2 = 1.06 V versus SCE). For
2+
comparison, TPA quenches the Ru(bpy)3 excited state only with
cence intensity and lifetime (
s).
kQ ꢄ 106 Mꢀ1 sꢀ1, that is roughly three orders of magnitude less
efficient than 10-methylphenothiazine [9]. Thus, the large quench-
ing constants observed here suggest that the excited state of Pt–F is
substantially more oxidizing than that of Ru(bpy)32+. Indeed, the
results obtained from electrochemical measurements (Table 1)
support this hypothesis.
These observations can be explained by a lowering of the ethyn-
ylbenzene-based HOMO upon protonation of the dimethylamino-
group, leading to a changeover in the lowest-energetic (and emis-
sive) excited state from mainly LLCT to mainly MLCT/IL [28].
Dimethylamino-protonation has a minor influence on the bzimpy-
and Pt(II)-based orbitals, hence the MLCT and bzimpy-IL states re-
main nearly unaffected and only the LLCT state is raised in energy.
The emission spectrum of the protonated Pt–N(CH3)2 complex is
similar to that of Pt–NO2, indicating a mixed MLCT/IL emissive ex-
cited state with an important IL contribution. This assignment is
Based on the (ground-state) electrochemical potentials for one-
electron reduction of the Pt–R complexes (E(Pt/Ptꢀ)) and their ex-
cited-state energies (Eem), it is possible to estimate the redox
potentials for one-electron reduction in the excited state (E(Pt*/
Ptꢀ)). It turns out that the Pt–F complex, along with Pt–H and
Pt–NO2, is a potent oxidant with E(Pt*/Ptꢀ) = 1.12–1.15 V versus
SCE in dichloromethane. This explains the comparatively strong
reductive quenching of the Pt–F luminescence even by the poor do-
nor TPA. On average, the Pt–R excited-state potentials for one-elec-
tron reduction in Table 1 are about 200 mV more positive than
those reported previously for analogous Pt(II) complexes with ter-
pyridine instead of bzimpy [16a]. This renders the new complexes
suitable for photosensitization of (long-range) electron transfers
even with weak to moderate donors. This is an attractive feature
in view of driving-force dependence studies because of the large
driving-force range that can potentially be probed.
consistent with the observed excited-state lifetime of 1.4
de-aerated dichloromethane solution.
ls in
Fig. 4 shows a Stern–Volmer plot based on time-integrated
luminescence intensities measured for dichloromethane solutions
of the Pt–F complex in the presence of two reductive quenchers,
namely 10-(p-xylyl)-phenothiazine (PTZ) and triphenylamine
Up to concentrations of 5 ꢁ 10ꢀ4 M no significant self-quench-
ing of the luminescence was observed for the Pt–R complexes in
dichloromethane solution.
Given the unusual photoredox chemistry of many d8d8 metal
dimers [3,19,20], the photophysical and electrochemical properties
of the xanthene-bridged dimer (Pt2) were investigated with partic-
ular focus on spectroscopic evidence for metal–metal interactions.
However, the absorption and luminescence data in Fig. 5 fail to
provide such evidence. Up to 35 000 cmꢀ1 the absorption spectrum
of Pt2 in dichloromethane resembles very closely that of the Pt–R
complexes with R = NO2, F, H, CH3. Only at higher energies there
is additional extinction in Pt2 due to the bridging xanthene back-
bone, but for the emission behavior this is irrelevant. The lumines-
cence spectrum of Pt2 is nearly identical to that of Pt–NO2,
indicating that the emission originates from an excited state with
Fig. 3. Left: Absorption spectral changes associated with protonation (with
trifluoroacetic acid) of the tertiary amine in the Pt–N(CH3)2 complex in CH2Cl2.
Right: Comparison of the luminescence spectra, relative luminescence intensities,
and luminescence lifetimes in unprotonated and protonated forms of Pt–N(CH3)2 in
de-aerated CH2Cl2 solution. The excitation wavelength was 468 nm, which corre-
sponds to an isosbestic point in absorption.
mixed MLCT/IL character. The luminescence lifetime of 1.5 ls in
de-aerated dichloromethane (Table 1) is in line with this assign-
ment. Interacting Pt(II)–Pt(II) centers usually exhibit metal–me-
tal-to-ligand charge transfer (MMLCT) emission that is
Table 1
Emission band maxima (Eem), ground- and excited-state electrochemical potentials
for one-electron reduction (E(Pt/Ptꢀ) and E(Pt*/Ptꢀ)), and luminescence lifetimes (
s)
of the complexes from Scheme 1.
Compound
Eem (eV)a
E(Pt/Ptꢀ)b
E(Pt*/Ptꢀ)c
s
(ns)d
(V vs. SCE)
(V vs. SCE)
Pt–NO2
Pt–F
Pt–H
Pt–CH3
Pt–N(CH3)2
Pt2
2.18
2.16
2.15
2.00
1.77
2.15
ꢀ0.83
ꢀ0.82
ꢀ0.83
ꢀ0.83
ꢀ0.87
ꢀ0.87
1.15
1.14
1.12
0.97
0.70
1.08
1054
561
479
88
20
1502
a
Emission band maxima (Eem) in dichloromethane.
In dichloromethane solution with tetrabutylammonium hexafluorophosphate
b
Fig. 4. Stern–Volmer plot based on time-integrated luminescence intensities
observed for dichloromethane solutions of Pt–F as a function of 10-(p-xylyl)-
phenothiazine (PTZ) or triphenylamine (TPA) concentration. For both types of
reductive quenchers, excitation of Pt–F occurred at 450 nm, and detection was at
580 nm. The straight lines represent linear regression fits to the experimental data.
as electrolyte.
c
Calculated based on the assumption that E00 ꢄ Eem + 0.2 eV for all our
compounds.
d
Measured in freeze–pump–thaw deoxygenated dichloromethane solution.

862
A. Hijazi et al. / Polyhedron 29 (2010) 857–863
The only compound in this study for which there is direct
experimental evidence for Pt(II)–Pt(II) interactions is the
[Pt(hex2bzimpy)Cl]+ precursor complex used for the synthesis of
2ꢀ
all alkynyl complexes reported herein. With the ZnCl4 counter-
ion this particular complex gives crystals suitable for X-ray analy-
sis. As seen from Fig. 6, in the resulting structure there are Pt(II)–
Pt(II) contacts as short as 3.38 Å between metal complexes ar-
ranged in a slightly twisted head-to-tail fashion. This is the range
in which significant overlap between the 5dz2 orbitals from neigh-
boring complexes is expected to occur [5,29c], and this is in line
with the orange-red (rather than yellow) color of the respective
crystals [2e,5,29c].
4. Summary and conclusions
Fig. 5. Left: Absorption spectrum (solid trace) of the platinum(II) dimer (Pt2) from
Scheme 1 in dichloromethane at room temperature. Right: Luminescence spectrum
of Pt2 in CH2Cl2 measured after excitation at 450 nm. The excitation spectrum of
this emission (detected at 580 nm) is shown in the left panel (dashed trace). The
inset in the right panel shows the temporal evolution of the luminescence signal at
580 nm after excitation at 450 nm with laser pulses of ꢃ10 ns duration (semilog-
arithmic representation).
There are strong analogies between the previously investigated
platinum(II) 2,20:600,200-terpyridine (tpy) alkynyls and the plati-
num(II) 2,6-(N-(n-hexyl)benzimidazol-20-yl)pyridine (hex2bzimpy)
alkynyl complexes from this work. Substitution of ethynylbenzene
ligands with either electron-withdrawing or electron-donating
groups leads to the same trends in photophysical and electrochem-
ical properties in both classes of complexes. Intraligand emission
originating from the terdentate ligand is important for the complex
with the nitro-substituted ethynylbenzene ligand, whereas for all
other substituents investigated, the emission originates from an
excited state that has mixed MLCT/LLCT character with their rela-
tive amounts varying over the series of complexes. Luminescence
lifetimes correlate with the emission wavelength according to
the energy-gap law. There are two key differences between the
tpy and hex2bzimpy complexes: (i) the former are poorly soluble
in many solvents and must frequently be investigated in dimethyl-
formamide solution, whereas the hex2bzimpy complexes are solu-
ble in most common organic solvents; (ii) the hex2bzimpy
complexes of platinum(II) are about 200 mV easier to reduce than
those formed with tpy. This latter feature makes the hex2bzimpy
complexes even better photooxidants than the tpy complexes.
Combined with their favorable solubility properties, this makes
them suitable as photosensitizers in more complicated molecular
architectures where solubility is often an important issue, for
example as initiators of intramolecular long-range electron trans-
fer reactions in donor-bridge-acceptor dyads and triads. We note
that solubility issues could also be addressed by incorporating hex-
yl chains into tpy ligands, but this appears synthetically less
straightforward.
characterized by a significant red-shift with respect to the MLCT
emission bands of the monomeric analogs [2,6]. This is because
the overlap of 5dz2 orbitals between two Pt(II) centers leads to
the formation of bonding (d
orbitals. With both 5dz2 orbitals fully occupied, the d
comes the HOMO in such a Pt(II) dimer, and in complexes with
conjugated ligands the LUMO is usually a * orbital located on the
r) and antibonding (d
r*) molecular
r
* orbital be-
p
-
p
ligand [2,6]. As a result, the HOMO–LUMO transition is an MMLCT
transition. The red-shift of the MMLCT emission in the dimer com-
pared to the MLCT luminescence band of the respective monomer
reflects the destabilization of the dr* orbital with respect to the
non-interacting 5dz2 orbitals. In the case of Pt2 no such red-shift
is observed, hence the Pt(II)–Pt(II) interaction must be small.
We note that metal–metal interactions have been found in plat-
inum complexes with Pt(II)–Pt(II) distances shorter than 4.5 Å
[2e,5,29]. In a structurally related Pt(II) terpyridine dimer complex
with a rigid 1,8-di(ethynyl)-substituted anthraquinone bridging li-
gand, the metal–metal distance is roughly 11 Å [30], and the lack of
any noticeable metal–metal interaction between the two cationic
platinum complexes was explained by the long distance between
them. A recent study reports on weak metal–metal interactions
in xanthene-bridged cyclometalated Pt(II) dimer complexes in
the excited state [31]. However, a study on a charge-neutral plati-
num dimer with a 4,5-di(isocyanide) substituted xanthene bridg-
ing ligand makes no explicit mention of noticeable Pt(II)–Pt(II)
interactions [32].
There is no spectroscopic evidence for any significant electronic
interaction between the two metal centers in our platinum(II) di-
mer. A too long Pt(II)–Pt(II) distance caused by the rigid xanthene
backbone and electrostatic repulsion between the two cationic
platinum entities are responsible for this. Short Pt(II)–Pt(II) con-
tacts are only observed in the crystal structure of the [Pt(hex2bzim-
py)Cl]+ precursor complex.
Supplementary data
CCDC 736725 contains the supplementary crystallographic data
for [Pt(hex2bzimpy)Cl]+. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Fig. 6. Crystal packing of the cationic [Pt(hex2bzimpy)Cl]+ complexes showing the
dimeric structure in zig-zag fashion with alternating short (3.38 Å) and long
(5.09 Å) Pt–Pt distances. Hydrogen atoms, counter-ions, and solvent molecules are
omitted for clarity. One of the hexyl chains is disordered and only the hexyl-C-
atoms attached directly to the N-atoms are shown.
Acknowledgments
This work was financed by the Swiss National Science Founda-
tion (Grants Nos. PP002-110611 and 200021-117578), and the

A. Hijazi et al. / Polyhedron 29 (2010) 857–863
863
(c) C. Monnereau, J. Gomez, E. Blart, F. Odobel, S. Wallin, A. Fallberg, L.
Hammarström, Inorg. Chem. 44 (2005) 4806.
Swiss State Secretariat for Education and Research (Grant No.
C07.0063) in the framework of COST action D35.
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