Photoinduced electron transfer in platinum(II) terpyridinyl acetylide complexes connected to a porphyrin unit

Article abstract of DOI:10.1021/ic048573q

A series of six new dyads consisting of a zinc or magnesium porphyrin appended to a platinum terpyridine acetylide complex via a para-phenylene bisacetylene spacer are described. Different substituents on the 4′ position of the terpyridinyl ligand were explored (OC₇H₁₅, PO₃Et₂, and H). The ground-state electronic properties of the dyads are studied by electronic absorption spectroscopy and electrochemistry, and they indicate some electronic interactions between the porphyrin subunit and the platinum complex. The photophysical properties of these dyads were investigated by steady-state, time-resolved, and femtosecond transient absorption spectroscopy in N,N-dimethylformamide solution. Excitation of the porphyrin unit leads to a very rapid electron transfer (2-20 ps) to the nearby platinum complex followed by an ultrafast charge recombination, thus preventing any observation of the charge separated state. The variation in the rate of the photoinduced electron transfer in the series of dyads is consistent with Marcus theory. The results underscore the potential of the para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance.

Full text of DOI:10.1021/ic048573q

Inorg. Chem. 2005, 44, 4806−4817
Photoinduced Electron Transfer in Platinum(II) Terpyridinyl Acetylide
Complexes Connected to a Porphyrin Unit
Cyrille Monnereau, Julio Gomez, Errol Blart, and Fabrice Odobel*
Laboratoire de Synthe`se Organique, UMR 6513 CNRS, FR CNRS 2465, Faculte´ des Sciences et
des Techniques de Nantes, BP 92208, 2 rue de la Houssinie`re, 44322 Nantes Cedex 03, France
Staffan Wallin, Anna Fallberg, and Leif Hammarstro1m*
Department of Physical Chemistry, Uppsala UniVersity, Box 579, SE-751 23 Uppsala, Sweden
Received October 12, 2004
A series of six new dyads consisting of a zinc or magnesium porphyrin appended to a platinum terpyridine acetylide
complex via a para-phenylene bisacetylene spacer are described. Different substituents on the 4 position of the
′
terpyridinyl ligand were explored (OC7H15, PO3Et2, and H). The ground-state electronic properties of the dyads are
studied by electronic absorption spectroscopy and electrochemistry, and they indicate some electronic interactions
between the porphyrin subunit and the platinum complex. The photophysical properties of these dyads were
investigated by steady-state, time-resolved, and femtosecond transient absorption spectroscopy in N,N-
dimethylformamide solution. Excitation of the porphyrin unit leads to a very rapid electron transfer (2−20 ps) to the
nearby platinum complex followed by an ultrafast charge recombination, thus preventing any observation of the
charge separated state. The variation in the rate of the photoinduced electron transfer in the series of dyads is
consistent with Marcus theory. The results underscore the potential of the para-phenylene bisacetylene bridge to
mediate a rapid electron transfer over a long donor−acceptor distance.
Introduction
nium polypyridine complexes8 because of their outstanding
photochemical properties due to their metal-to-ligand charge
transfer (MLCT) excited state. More recently, several teams
have shown the remarkable luminescent properties of acetyl-
ide platinum complexes that are potentially useful for the
development of electroluminescent materials.2,9,10,11 However,
there are many fewer studies on the use of platinum(II)
square-planar complexes for photoinduced electron or energy
transfers. Arakawa and co-workers showed that platinum
The utilization of coordination compounds for the devel-
opment of multicomponent photoactive systems is of great
interest for applications in the area of solar energy
conversion.1-5 In this area, a great deal of attention has
been particularly given to ruthenium,5,6 osmium,7 and rhe-
* Author to whom correspondence should be addressed. E-mail:
Fabrice.Odobel@chimie.univ-nantes.fr (F.O.).
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4806 Inorganic Chemistry, Vol. 44, No. 13, 2005
10.1021/ic048573q CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/21/2005

Electron Transfer in Pt(II) Terpyridinyl Acetylide Complexes
Scheme 1 . Structures of the Molecules Studied in This Work
diimine dithiolate complexes display promising properties
for titanium dioxide sensitization, thus making these com-
pounds suitable for dye-sensitized photovoltaic cells.12 More
recently, Eisenberg and co-workers reported several systems
containing a platinum phenanthroline bis(arylacetylide)
platinum complex connected to electron acceptors and donors
that undergo efficient photoinduced charge separations.13,14
Harriman and co-workers reported a fast intramolecular
energy transfer from the second singlet excited-state of a
zinc porphyrin to a ruthenium bipyridine fragment connected
through a σ ethynyl platinum bisphosphine core.15
As an ongoing research program, we are interested in the
development of electronically coupled multicomponent sys-
tems in order to perform light-induced functions such as a
long-range electron transfer in a single step.16,17 Platinum
acetylide complexes with polypyridine ligands were appeal-
ing molecular building blocks for such a goal, because the
carbon-platinum bond of the acetylide linkage may allow
large electronic coupling with the neighboring unit. Further-
more, upon the photoinduced electron transfer, the reduced
platinum complex could potentially be an active redox
catalyst for water reduction.18 More specifically, we were
interested in merging the properties of the platinum com-
plexes with those of a porphyrin chromophore. This latter
fluorescent dye can serve as a light input element because it
strongly absorbs in the visible spectrum,4 and the terpyridine
platinum acetylide complex could be the electron acceptor
element of the photosystem. Another reason for choosing
these molecular structures relies on the fact that, after the
electron transfer from the porphyrin excited state, the
negative charge is shifted further away from the oxidized
porphyrin because the electron is localized in the terpyridine
LUMO orbitals. As a result, the lifetime of the charge
separated state may be significant.
Scheme 1). By changing the substituent of the terpyridine
ligand and the metal complexed inside the porphyrin of these
dyads, it was possible to tune the redox potentials and the
excited-state properties of each module. As a result, the
quenching rate of the porphyrin singlet excited state in the
dyads can be varied.
Experimental Section
General Methods. 1H NMR spectra were recorded on a Bruker
ARX 400 MHz Bruker spectrometer. Chemical shifts for 1H NMR
spectra are referenced relative to the residual protium in the
deuterated solvent (CDCl3, δ ) 7.26 ppm). Mass spectra were
recorded on an EI-MS HP 5989A spectrometer or a JMS-700
(JEOL Ltd, Akishima, Tokyo, Japan) double-focusing mass spec-
trometer of reversed geometry equipped with an electrospray
ionization source. Thin-layer chromatography was performed on
aluminum sheets precoated with Merck 5735 Kieselgel 60F254
.
Column chromatography was carried out either with Merck 5735
Kieselgel 60F (0.040-0.063 mm mesh) or with SDS neutral
alumina (0.05-0.2 mm mesh). Air-sensitive reactions were carried
out under argon in dry solvents and glassware. Chemicals were
purchased from Aldrich and used as received. Compounds tetrakis-
(trisphenyl)phosphine palladium,19 [Pt(COD)Cl2],20 terpyridinePtCl
complex 521, 2,6-di-2-pyridyl-4(1H)-pyridone,22 2,2′:6′,0.2′′-terpy-
ridine, and 4′-[((trifluoromethyl)sulfonyl)oxy]-2,2′:6′.2′′-terpyri-
dine22 were prepared according to literature methods.
In this work, we report on the preparation and the
spectroscopic and electrochemical properties of a series of
new dyads composed of a zinc or magnesium porphyrin
linked to a platinum(II) acetylide terpyridine complex (cf.
(9) (a) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung,
K.-K.; Zhu, N. Chem.sEur. J. 2001, 7, 4180. (b) Whittle, C. E.;
Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg. Chem. 2001,
40, 4053. (c) Cummings, S. D.; Eisenberg, R. Inorg. Chem. 1995, 34,
2007. (d) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem.
2000, 39, 2708. (e) Yam, V. W.-W. Acc. Chem. Res. 2002, 35, 555.
(10) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476.
(11) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653.
(12) (a) Islam, A.; Sugihara, H.; Hara, K.; Pratap Singh, L.; Katoh, R.;
Yanagida, M.; Takahashi, Y.; Murata, S.; Arakawa, H. New J. Chem.
2000, 24, 343. (b) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.;
Katoh, R.; Yanagida, M.; Takahashi, Y.; Murata, S.; Arakawa, H.;
Fujihashi, G. Inorg. Chem. 2001, 40, 5371.
UV-visible absorption spectra were recorded on a UV-2401PC
Shimadzu spectrophotometer. Fourier transform infrared spectra
were recorded in pressed KBr pellets on a Bruker Vector 22
spectrometer.
The electrochemical measurements were performed with a
potentiostat-galvanostat MacLab model ML160 controlled by
resident software (Echem version 1.5.2 for Windows) using a
conventional single-compartment three-electrode cell. The working
electrode was a Pt wire 10 mm long, the auxiliary was a Pt wire,
and the reference electrode was the saturated potassium chloride
calomel electrode (SCE). The supported electrolyte was 0.1 N Bu4-
NPF6 in N,N-dimethylformamide (DMF), and the solutions were
purged with argon before the measurements. All potentials are
(13) McGarrah, J. E.; Kim, Y.-J.; Hissler, M.; Eisenberg, R. Inorg. Chem.
2001, 40, 4510.
(14) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355.
(15) Harriman, A.; Hissler, M.; Trompette, O.; Ziessel, R. J. Am. Chem.
Soc. 1999, 121, 2516.
(16) Odobel, F.; Suresh, S.; Blart, E.; Nicolas, Y.; Quintard, J.-P.; Janvier,
P.; Le Questel, J.-Y.; Illien, B.; Rondeau, D.; Richomme, P.; Haupl,
T.; Wallin, S.; Hammarstro¨m, L. Chem.sEur. J. 2002, 8, 3027.
(17) Blart, E.; Suzenet, F.; Quintard, J.-P.; Odobel, F. J. Porphyrins
Phthalocyanines 2003, 7, 207.
(18) Zhang, D.; Wu, L.-Z.; Li, Z.; Han, X.; Yang, Q.-Z.; Zhang, L.-P.;
Tung, C.-H. J. Am. Chem. Soc. 2004, 126, 3440.
(19) Schlosser, M. Organometallics in Synthesis: A Manual; Wiley and
Sons: New York, 1994.
(20) Peters, T. B.; Bohling, J. C.; Arif, A. M.; Gladysz, J. A. Organome-
tallics 1999, 18, 3261.
(21) Hobert, S. E.; Carney, J. T.; Cummings, S. D. Inorg. Chim. Acta 2001,
318, 89.
(22) Potts, K. T.; Konwar, D. J. Org. Chem. 1991, 56, 4815.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4807

Monnereau et al.
quoted relative to SCE. In all of the experiments, the scan rate was
100 mV/s for cyclic voltammetry and 15 Hz for pulse voltammetry.
Fluorescence spectra were recorded on a SPEX Fluorolog II
Systems fluorimeter and were corrected for the wavelength-
dependent response of the detector system. Fluorescence lifetimes
were determined using a time-correlated single-photon counting
setup. The samples were excited at 400 nm with 200 kHz, 150 fs
laser pulses by frequency-doubling the output from an amplified
Ti:sapphire laser system. The emission was detected at a right angle
using magic angle polarizers and 470 nm cutoff filters to remove
scattered excitation light. The full width at half maximum (fwhm)
of the response function measured from a scattering sample was
about 70 ps.
Femtosecond transient absorption measurements were made using
an amplified 1 kHz Ti:sapphire laser system. An optical parametric
amplifier (TOPAS) was used to produce 630 nm, 120 fs excitation
pulses. The pump passed a chopper in which every other pulse
was blocked before it was focused in the sample cell (1 mm quartz
cuvette). The sample cell was mounted on a holder that moved up
and down with a frequency of about 1 Hz. The probe beam was
led through a delay line and focused on a CaF2 plate where a white
light continuum (WL) was generated. A beam splitter was used to
produce a WL reference beam. The probe and reference were
focused through the slit of a monochromator and detected by two
512 pixel diode arrays. The transient absorption signal was
calculated for each pair of shots (pump-probe vs probe only) and
then averaged. Transient absorption spectra are the average of 10
scans with 1000 shots at each time step. The absorbance of the
sample was about 0.3 at the excitation wavelength. The response
function measured as the fwhm of the signal from pure solvent is
150-200 fs. The transient spectra were corrected for the spectral
chirp (200-300 fs over the detected spectral window) by visually
determining the zero time location in the 3D data file.
Pt Complex with 4′-Diethylphosphonate-2,2′:6′,2′′-terpyridine
(4). Terpyridine diethylphosphonate, 1 (0.098 g, 0.266 mmol), was
added to a suspension of [Pt(COD)Cl2] (0.100 g, 0.266 mmol) in
water (20 mL). The stirred suspension was heated to 60 °C for 1
h, and the resulting orange solution was filtered over Celite. To
the filtrate was added an aqueous solution of NaBF4 (0.2 g of NaBF4
in 5 mL of water). The solution was cooled in an ice bath, and the
orange precipitate was filtered and repeatedly washed with water,
then with diethyl ether. Upon drying under a vacuum, an orange
1
solid was isolated, 4 (0.176 g, yield 93%). H NMR (400 MHz,
d6-DMSO) δ: 8.98 (d, 2H, 3J ) 5.6 Hz), 8.96 (d, 2H, 3J ) 8 Hz),
8.83 (d, 2H, 3J ) 13.2 Hz), 8.53 (dd, 2H, 3J ) 8 Hz, 3J ) 7.1 Hz),
3
3
8.00 (dd, 2H, J ) 7.1 Hz, J ) 5.6 Hz), 4.25 (m, 4H), 1.33 (m,
6H). IR (KBr disk, ν/cm-1): 3048 (m), 1606 (s), 1413 (s), 1143
(s), 1034 (m), 785 (s), 596 (s). MS-FAB (m/z) Calcd for C19H20-
ClN3O3PPt ) 599.1. Found ) 599.9 (M)+.
Pt Complex with 4′-Heptoxy-2,2′:6′,2′′-terpyridine (6). To a
suspension of [Pt(COD)Cl2] (0.162 g, 0.432 mmol) in water (30
mL) was added 0.150 g (0.432 mmol) of 3, and the suspension
was heated to 60 °C for 1 h. The resulting yellow solution was
filtered over Celite. To the filtrate was added a solution of 0.2 g of
NaBF4 in water (5 mL), giving rise to a red-orange precipitate.
The suspension was cooled in an ice bath. The solid was filtered
and repeatedly washed with water and diethyl ether. Upon drying
under a vacuum, a red-orange solid was isolated, 6 (0.265 g, yield
1
3
92%). H NMR (400 MHz, d6-DMSO) δ: 8.86 (d, 2H, J ) 5.2
3
3
3
Hz), 8.64 (d, 2H, J ) 8.0 Hz), 8.50 (dd, 2H, J ) 8.0 Hz, J )
3
3
7.2 Hz), 8.27 (s, 2H), 7.92 (dd, 2H, J ) 7.2 Hz, J ) 5.2 Hz),
4.36 (m, 2H), 1.86 (m, 2H), 0.80-1.50 (m, 11H). IR (KBr disk,
ν/cm-1): 2926 (m), 1617 (s), 1608 (s), 1480 (s), 1434 (s), 1225
(s), 1084 (m), 784 (s). MS-FAB (m/z) Calcd for C22H25ClN3OPPt
) 577.1. Found ) 578.0 (M)+.
General Procedure for the Preparation of Platinum Terpy-
ridyl Acetylide Complexes (12-14). 1-Ethynyl-4-triisopropylsi-
lylethynylbenzene, 7 (0.050 g, 0.1775 mmol); CuI (5 mg); and
iPr2NH (1 mL) were added to a suspension of [(R1-trpy)PtCl](BF4)
(0.16 mmol) in distilled CH2Cl2 (20 mL) under argon. The mixture
was stirred overnight at room temperature and resulted in a reddish
colored suspension. The solid was filtered and washed with CH2-
Cl2, and then tetrahydrofuran (THF). Upon drying under a vacuum,
the awaited product was isolated.
4′-Diethylphosphonate-2,2′:6′,2′′-terpyridine (1). Diethyl phos-
phite (2 mL, 15.7 mmol), Et3N (1.46 mL, 10.5 mmol), PPh3 (6.9
g, 26.3 mmol), and Pd(Ph3)4 (0.67 g, 0.6 mmol) were added to a
suspension of 4′-triflate-2,2′:6′,2′′-terpyridine (2 g, 5.3 mmol) in
distilled toluene (80 mL). The reaction mixture was set under argon
and heated to reflux. After 12 h, the mixture was evaporated to
dryness. The resulting crude reaction mixture was purified by
column chromatography on silica (eluted with a gradient of AcOEt
in petroleum ether, from 10 to 70%). A white solid was isolated,
1 (1.2 g, yield 92%). 1H NMR (200 MHz, CDCl3) δ: 8.85 (d, 2H,
[Pt(trpyPO3Et2)(CtCC6H4CtCTIPS)]BF4 (12). Complex 12
1
was isolated as a deep red solid, yield 51%. H NMR (400 MHz,
3
3
4
3
3Jp ) 14 Hz), 8.72 (m, 2H), 8.60 (dt, 2H, J ) 1.1 Hz, J ) 7.9
d6-DMSO) δ: 9.22 (d, 2H, J ) 5.3 Hz), 8.95 (d, 2H, J ) 7.9
3
3
3
4
3
3
Hz), 8.87 (d, 2H, J ) 13.2 Hz), 8.53 (dd, 2H, J ) 7.9 Hz, J )
Hz), 7.87 (ddd, 2H, J ) 1.8 Hz, J ) 7.6 Hz, J ) 7.9 Hz), 7.36
6.5 Hz), 7.99 (dd, 2H, 3J ) 6.5 Hz), 7.53 (d, 2H, 3J ) 8 Hz), 7.44
4
3
3
(ddd, 2H, J ) 1.1 Hz, J ) 4.9 Hz, J ) 7.6 Hz), 4.27 (m, 4H),
1.35 (t, 6H, 3J ) 7.2 Hz). MS-EI (m/z) Calcd for C19H20N3O3P )
369.1. Found ) 369.1 (M)+.
3
3
(d, 2H, J ) 8 Hz), 4.24 (m, 4H), 1.35 (t, 6H, J ) 6.6 Hz), 1.13
(s, 21H). IR (KBr disk, ν/cm-1): 2942 (m), 2864 (s), 2150 (s),
2118 (s), 1605 (s), 1409 (s), 1098 (m), 781 (s), 474 (s). MS-FAB
(m/z) Calcd for C38H45N3O3PPtSi ) 845.2. Found ) 845.9 (M)+.
[Pt(trpy)(CtCC6H4CtCTIPS)]BF4 (13). Complex 13 was
isolated as a garnet solid, yield 45%. 1H NMR (400 MHz,
4′-Heptoxy-2,2′:6′,2′′-terpyridine (3). 1-Bromoheptane (1.3 mL)
and KOH (1.5 g) were added to a suspension of dipyridilpyridone
(0.300 g, 1.2 mmol) in acetone (21 mL) and DMF (8 mL). This
stirred suspension was heated to reflux for 5 h. The mixture was
evaporated to dryness. A total of 100 mL of water was added, and
the solution was extracted with CH2Cl2 (3 × 100 mL). The
combined organic layers were dried on MgSO4 and filtered. The
resulting pale yellow solution was evaporated to dryness, and the
crude product was purified by column chromatography on neutral
alumina (petroleum ether/AcOEt, 90:10). A white solid was isolated,
3
d6-DMSO) δ: 9.02 (d, 2H, J ) 6.2 Hz), 8.6-8.5 (m, 5H), 8.46
(dd, 2H, 3J ) 8.0 Hz, 3J ) 7.1 Hz), 7.88 (dd, 2H, 3J ) 6.2 Hz, 3J
3
) 7.1 Hz), 7.44 (2d, 4H, J ) 8.4 Hz), 1.13 (s, 21H). IR (KBr
disk, ν/cm-1): 2944 (m), 2865 (s), 2152 (s), 2120 (s), 1711(s),
1604 (s), 1455 (s), 1109 (m), 839 (m), 771 (s), 559 (s). MS-FAB
(m/z) Calcd for C34H36N3PtSi ) 709.2. Found ) 709.3 (M)+.
[Pt(C7H15O-trpy)(CtCC6H4CtCTIPS)]BF4 (14). Complex 14
was isolated as a red solid, yield 92%.1H NMR (400 MHz, d6-
1
3 (0.342 g, yield 82%). H NMR (200 MHz, CDCl3) δ: 8.10 (d,
2H, 3J ) 4.2 Hz), 8.04 (d, 2H, 3J ) 8.0 Hz), 7.43 (d, 2H, 3J ) 7.9
3
3
3
3
3
Hz), 7.27 (dd, J ) 7.1 Hz, J ) 7.1 Hz), 6.74 (dd, 2H, J ) 7.1
acetone) δ: 9.32 (d, 2H, J ) 5.5 Hz), 8.68 (d, 2H, J ) 8 Hz),
3
3
3
3
Hz, J ) 5.4 Hz), 3.64 (t, 2H, J ) 6.2 Hz), 0.5-1.6 (m, 11H).
8.53 (dd, 2H, J ) 8.0 Hz, J ) 7.0 Hz), 8.28 (s, 2H), 7.96 (dd,
MS-EI (m/z) Calcd for C22H25N3O ) 347.2. Found ) 347.2 (M)+.
2H, 3J ) 7 Hz, 3J ) 5.5 Hz), 7.47 (d, 4H,3J ) 8 Hz), 4.47 (t, 2H,
4808 Inorganic Chemistry, Vol. 44, No. 13, 2005

Electron Transfer in Pt(II) Terpyridinyl Acetylide Complexes
3J ) 8 Hz), 1.36 (s, 36H), 1.3-1.18 (m, 12H), 0.89 (3H, t). IR
(KBr disk, ν/cm-1): 2960 (m), 2865 (s), 2151 (s), 2118 (s), 1616
(s), 1608 (s),1463 (s), 1099 (m), 801 (m), 473 (s). MS-FAB (m/z)
Calcd for C41H50N3OPtSi ) 823.3. Found ) 823.3 (M)+.
terpyridine platinum complex [(R1-trpy)Pt Cl](PF6) (0.035 mmol),
CuI (4 mg), and Pr2NH (1 mL). The suspension was then stirred
i
overnight at room temperature. The resulting mixture was filtered
on Celite, and the Celite was washed with CH2Cl2. The filtrate was
evaporated to dryness. The residue was solubilized in CH2Cl2 (4
mL), precipitated with petroleum ether (20 mL), and filtered. The
green solid was then washed with a mixture of CH2Cl2/petroleum
ether (1:5). Finally, the complex was solubilized in THF, precipi-
tated with an aqueous solution of KPF6, and filtered, and the
precipitate was washed with water.
5-((4′-Trisisopropylsilylethynylphenyl)ethynyl)-10,15-bis-(3′,5′-
di-tert-butylphenyl)-porphyrin (8). 1-Ethynyl-4-triisopropylsilyl-
ethynylbenzene, 7 (0.250 g, 0.887 mmol), and distilled Et3N (7
mL) were added to a solution of 10-bromo-5,15-bis-(3,5-di-tert-
butyl-phenyl)-porphyrin (0.390 g, 0.506 mol) in distilled toluene
(33 mL). This solution was degassed by 15 min of N2 bubbling
under sonication. AsPh3 (38 mg, 0.124 mmol) and Pd2(dba)3-
CHCl3 (39 mg, 0.038 mmol) were then added. The dark red solution
was once again degassed. The stirred reaction mixture was then
heated at 60 °C for 20 h. The solvents were removed by rotary
evaporation, and the crude product was purified by flash column
chromatography on silica gel, eluted with a mixture of CH2Cl2/
petroleum ether (20:80), leading to a brown solid, 8 (408 mg, yield
81%). 1H NMR (200 MHz, CDCl3) δ: 10.17 (s, 1H), 9.82 (d, 2H,
3J ) 4.5 Hz), 9.29 (d, 2H, 3J ) 4.5 Hz), 9.03 (d, 2H, 3J ) 5.6 Hz),
9.00 (d, 2H, 3J ) 5.6 Hz), 8.12 (s, 4H), 7.98 (d, 2H, 3J ) 8.1 Hz),
[(Et2O3P-trpy)Pt(CtCC6H4CtC-ZnP)]PF6 (15). Yield 61%.
Rf ) 0.21 on neutral alumina eluted with CH2Cl2/CH3OH (98:2).
3
1H NMR (400 MHz, d6-DMSO) δ: 10.29 (s, 1H), 9.85 (d, 2H, J
3
3
) 5.2 Hz), 9.45 (d, 2H, J ) 5.2 Hz), 9.32 (d, 2H, J ) 4.7 Hz),
8.80-9.05 (m, 8H), 8.56 (m, 4H), 8.15 (2H, d, 3J ) 7.8 Hz), 8.07
(s, 4H), 7.88 (s, 2H), 7.8 (d, 2H, 3J ) 7.8 Hz), 4.53 (m, 4H), 1.54
(s, 36H), 0.95 (m, 6H). IR (KBr disk, ν/cm-1): 2961 (m), 2182
(s), 2115 (s), 1591 (s), 1061 (m), 792 (m). MS-FAB (m/z) Calcd
for C77H75N7O3PPtZn ) 1435.5. Found ) 1437.9 (M)+.
[(trpy)Pt(CtCC6H4CtC-ZnP)]PF6 (16). Yield 72%. Rf )
3
1
7.84 (s, 2H), 7.68 (d, 2H, J ) 8.1 Hz), 1.57 (s, 36H), 1.20 (s,
0.36 on neutral alumina eluted with CH2Cl2/CH3OH (98:2). H
21H), -2.46 (s, 2H). IR (KBr disk, ν/cm-1): 2963 (m), 2864 (s),
2150 (s), 1462 (s), 1261 (s), 1097 (m), 799 (m). MS-FAB (m/z)
Calcd for C67H78N4Si ) 966.6. Found ) 967.8 (M + H)+.
5-((4′-Ethynylphenyl)ethynyl)-10,15-bis-(3′,5′-di-tert-butylphe-
nyl)-porphyrin (9). Bu4NF (1M in THF, 0.57 mL) was added to
a solution of the porphyrin 8 (0.140 g, 0.142 mmol) in distilled
THF (150 mL). The reddish solution was stirred at room temper-
ature under argon for 1 h and turned to dark green. A total of 100
mL of brine was added to the reaction mixture, and the solution
was extracted with CH2Cl2 (3 × 100 mL). The combined organic
layers were dried on MgSO4 and filtered, and the solvent was
removed by rotary evaporation. The crude product was purified by
flash column chromatography on silica gel (eluted with a mixture
of CH2Cl2/petroleum ether/Et3N, 19:80:1). A blue-green solid was
3
NMR (400 MHz, d6-DMSO) δ: 10.30 (s, 1H), 9.83 (d, 2H, J )
5.5 Hz), 9.45 (d, 2H, 3J ) 5.5 Hz), 9.30 (d, 2H, 3J ) 3.2 Hz), 8.95
(d, 2H, 3J ) 5.0 Hz), 8.89 (d, 2H, 3J ) 5.0 Hz), 8.5-8.75 (m, 5H),
8.14 (d, 2H, 3J ) 7.8 Hz), 8.07 (s, 4H), 8.01 (m, 2H), 7.88 (s, 2H),
3
7.78 (d, 2H, J ) 8 Hz), 1.53 (s, 36H). IR (KBr disk, ν/cm-1):
2960 (m), 2868 (s), 2184 (s), 2113 (s), 1591 (s), 1475 (s), 1453
(s), 1247 (s), 1208 (s), 844 (m). MS-FAB (m/z) Calcd for
C73H66N7PtZn ) 1299.4. Found ) 1300.0 (M)+.
[(C7H15Otrpy)Pt(CtCC6H4CtC-ZnP)]PF6 (17). Yield 45%.
Rf ) 0.20 on neutral alumina eluted with CH2Cl2/CH3OH (98:2).
1H NMR (400 MHz, d6-DMSO) δ:10.23 (s, 1H), 9.82 (d, 2H, 3J )
6 Hz), 9.43 (d, 2H, 3J ) 6 Hz), 9.15 (m, 2H), 8.94 (d, 2H, 3J ) 4.8
3
Hz), 8.88 (d, 2H, J ) 4.8 Hz), 8.64 (m, 2H), 8.50 (m, 2H), 8.23
3
(m, 2H), 8.11 (d, 2H, J ) 8.1 Hz), 8.07 (s, 4H), 7.91 (m, 2H),
1
3
isolated, 9 (110 mg, yield 93%). H NMR (400 MHz, CDCl3) δ:
7.88 (s, 2H), 7.73 (d, 2H, J ) 8.1 Hz), 4.27 (m, 2H), 1.56 (s,
3
3
36H), 0.8-1.4 (m, 13H). IR (KBr disk, ν/cm-1): 2960 (m), 2870
(m), 2184 (s), 2113 (s), 1607 (s), 1209 (s), 1084 (m), 793 (s). MS-
FAB (m/z) Calcd for C80H80N7OPtZn ) 1413.5. Found ) 1414.0
(M)+.
10.19 (s, 1H), 9.85 (d, 2H, J ) 4.3 Hz), 9.36 (d, 2H, J ) 4.3
Hz), 9.08 (d, 2H, 3J ) 4.3 Hz), 9.01 (d, 2H, 3J ) 4.3 Hz), 8.12 (s,
4H), 7.98 (d, 2H, 3J ) 9.8 Hz), 7.83 (s, 2H), 7.70 (d, 2H, 3J ) 9.8
Hz), 3.22 (s, 1H), 1.55 (s, 36H), -2.91 (s, 2H). IR (KBr disk,
ν/cm-1): 2961 (m), 2183 (s), 2110 (s), 1465 (m), 1368 (s), 1075
(m), 765 (m). MS-FAB (m/z) Calcd for C58H58N4 ) 810.5. Found
) 811.7 (M + H)+.
[5-((4′-Ethynylphenyl)ethynyl)-10,15-bis-(3′,5′-di-tert-butylphe-
nyl)-porphyrinato]magnesium(II) (11). To a solution of the
porphyrin 9 (110 mg, 0.137 mmol) in distilled CH2Cl2 (9 mL) were
i
[5-((4′-Ethynylphenyl)ethynyl)-10,15-bis-(3′,5′-di-tert-butylphe-
nyl)-porphyrinato] Zinc(II) (10). A solution of Zn(OAc)2‚2H2O
(0.454 g, 2.07 mmol) in 25 mL of MeOH was added to a solution
of 9 (0.100 g, 0.1034 mmol) in CH2Cl2 (50 mL). The reddish
reaction mixture was then stirred at room temperature for 1 h, and
the solvents were rotary evaporated. The green solid was solubilized
in dichloromethane, and the solution was washed with 100 mL of
water. The organic layer was dried with MgSO4 and filtered, and
the solvent was removed by rotary evaporation. A green solid was
added distilled Pr2NH (0.5 mL) and magnesium iodide (380 mg,
1.37 mmol). The green solution was stirred at room temperature
under argon for 30 min. A total of 20 mL of water was then added,
and the solution was extracted with CH2Cl2 (3 × 20 mL). The
combined organic layers were dried on MgSO4 and filtered, and
the solvent was removed by rotary evaporation. The crude product
was purified by column chromatography on alumina gel (eluted
with a mixture of CH3CN/Et3N, 99:1). A bright green product was
1
isolated, 11 (88 mg, yield 78%). H NMR (400 MHz, CDCl3) δ:
1
3
3
isolated, 10 (0.105 mg, yield 99%). H NMR (200 MHz, CDCl3)
10.12 (s, 1H), 9.86 (d, 2H, J ) 4.3 Hz), 9.28 (d, 2H, J ) 4.3
Hz), 9.04 (d, 2H, 3J ) 4.3 Hz), 8.99 (d, 2H, 3J ) 4.3 Hz), 8.11 (s,
4H), 7.98 (d, 2H, 3J ) 8.4 Hz), 7.81 (s, 2H), 7.72 (d, 2H, 3J ) 8.4
Hz), 3.22 (s, 1H), 1.57 (s, 36H), -2.90 (s, 2H). IR (KBr disk,
ν/cm-1): 2960 (m), 2184 (s), 2108 (s), 1590 (s), 1460 (m), 1375
(s), 1260 (s), 1060 (m), 792 (s). MS-ES (m/z) Calcd for C59H60-
MgON4 ) 864.46. Found ) 864.46 (M + CH3OH)+.
3
3
δ: 10.15 (s, 1H), 9.82 (d, 2H, J ) 4.3 Hz), 9.30 (d, 2H, J ) 3.6
Hz), 9.09 (d, 2H, 3J ) 4.0 Hz), 9.03 (d, 2H, 3J ) 4.0 Hz), 8.11 (s,
4H), 7.95 (2H, d, 3J ) 7.7 Hz), 7.84 (s, 2H), 7.66 (d, 2H, 3J ) 7.7
Hz), 1.58 (s, 36H). IR (KBr disk, ν/cm-1): 2961 (m), 2868 (s),
2186 (s), 2107 (s), 1592 (s), 1463 (m), 1384 (s), 1363 (s), 1259
(s), 1060 (m), 793 (s). MS-FAB (m/z) Calcd for C58H56N4Zn )
872.4. Found ) 874.5 (M)+.
General Procedure for the Preparation of the Dyads Con-
General Procedure for the Preparation of Dyads Containing
Zinc Porphyrin (15-17). To a solution of porphyrin 10 (38.3 mg,
0.0437 mmol) in distilled CH2Cl2 (40 mL) were added the
taining a Magnesium Porphyrin (18-20). To a solution of the
porphyrin 11 (25 mg, 0.030 mmol) were added Pr2NH (0.5 mL),
CuI (1 mg), and the platinum terpyridine complex (0.025 mmol)
i
Inorganic Chemistry, Vol. 44, No. 13, 2005 4809

Monnereau et al.
Scheme 2 . General Route for the Preparation of the Platinum Complexes Studied in This Work
in distilled CH2Cl2 (25 mL). The mixture was stirred overnight at
room temperature under argon. The mixture turned to very dark
green. For 18, a precipitate formed during the reaction, and it was
directly filtered. This precipitate was then washed with petroleum
ether (30 mL) and then with CH2Cl2 (60 mL).
The procedure was slightly different for 19 and 20, where no
precipitate appeared. A total of 30 mL of water was added to the
reaction mixture, and the solution was extracted with CH2Cl2 (3 ×
20 mL). The combined organic layers were dried on MgSO4 and
filtered, and the solvent was removed by rotary evaporation. The
dark green residue was solubilized in THF, precipitated with an
aqueous solution of KPF6, and filtered. Finally, the filtrate was
washed with water and then with diethyl ether.
(d, 2H, 3J ) 4.6 Hz), 8.83 (d, 2H, 3J ) 4.6 Hz), 8.62 (m, 2H), 8.46
3
(m, 2H), 8.21 (m, 2H), 8.09 (d, 2H, J ) 7.9 Hz), 8.05 (s, 4H),
3
7.87 (m, 2H), 7.85 (s, 2H), 7.70 (d, 2H, J ) 7.9 Hz), 4.23 (m,
2H), 1.54 (s, 36H), 0.8-1.55 (m, 13H). IR (KBr disk, ν/cm-1):
2961 (m), 2867 (m), 2180 (s), 2110 (s), 1600 (s), 1210 (s), 1068
(m), 778 (s). MS-FAB (m/z) Calcd for C80H80MgN7OPt ) 1375.6.
Found ) 1375.6 (M)+.
Results
Preparations of the Compounds. The general synthetic
route for the preparation of the new molecular systems is
summarized in Scheme 2.
[(Et2O3P-trpy)Pt(CtCC6H4CtC-MgP)]PF6 (18). Yield 60%.
Rf ) 0.21 on neutral alumina eluted with CH2Cl2/CH3OH (98:2).
The synthesis of the dyads 15-20 required the initial
preparation of the two key building blocks, namely, a
metalloporphyrin (modules 10 and 11) and a platinum chloro
terpyridine complex (modules 4-6). The porphyrin module
8 was easily available from a Sonogashira cross-coupling
reaction using Lindsey conditions23 between the correspond-
ing 15-bromo-5,10-diaryl porphyrin24 and 1-ethynyl-4-tri-
isopropylacetylene-benzene 725 followed by desilylation with
fluoride anion. The terpyridine ligands 1 and 3 were prepared
by modification of the literature procedures. The 4′-dieth-
ylphosphonate-2,2′:6′,2′′-terpyridine 1 was previously de-
scribed by Gra¨tzel and co-workers26 from a palladium
3
1H NMR (400 MHz, d6-DMSO) δ: 10.28 (s, 1H), 9.82 (d, 2H, J
3
3
) 5.5 Hz), 9.40 (d, 2H, J ) 5.5 Hz), 9.35 (d, 2H, J ) 5.5 Hz),
3
3
8.80-9.05 (m, 8H), 8.58 (t, 2H, J ) 7.8 Hz), 8.54 (t, 2H, J )
3
7.8 Hz), 8.14 (d, 2H, J ) 8 Hz), 8.10 (s, 4H), 7.88 (s, 2H), 7.78
(d, 2H, 3J ) 8 Hz), 4.53 (m, 4H), 1.54 (s, 36H), 0.95 (m, 6H). IR
(KBr disk, ν/cm-1): 2960 (m), 2182 (s), 2117 (s), 1590 (s), 1045
(m), 791 (m). MS-FAB (m/z) Calcd for C77H75MgN7O3PPt )
1396.5. Found ) 1396.3 (M)+.
[(trpy)Pt(CtCC6H4CtC-MgP)]PF6 (19). Yield 83%. Rf )
1
0.36 on neutral alumina eluted with CH2Cl2/CH3OH (98:2). H
3
NMR (400 MHz, d6-DMSO) δ: 10.27 (s, 1H), 9.81 (d, 2H, J )
3
3
5.5 Hz), 9.39 (d, 2H, J ) 5.5 Hz), 9.26 (m, 2H), 8.91 (d, 2H, J
) 5.2 Hz), 8.84 (d, 2H, 3J ) 5.2 Hz), 8.65 (m, 5H), 8.54 (m, 2H),
8.13 (d, 4H, 3J ) 7.8 Hz), 8.08 (s, 4H), 8.00 (m, 2H), 7.87 (s, 2H),
(23) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266.
(24) (a) Uyeda, H. T.; Zhao, Y.; Wostyn, K.; Asselberghs, I.; Clays, K.;
Persoons, A.; Therien, M. J. J. Am. Chem. Soc. 2002, 124, 13806. (b)
Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C.; Hupp, J.
T.; Angiolillo, P. J.; Therien, M. J. J. Am. Chem. Soc. 2000, 122,
7017.
7.78 (d, 2H, J ) 7.8 Hz), 1.56 (s, 36H). IR (KBr disk, ν/cm-1):
3
2960 (m), 2180 (s), 2114 (s), 1590 (s), 1210 (s), 838 (m). MS-
FAB (m/z) Calcd for C73H66MgN7Pt ) 1260.5. Found ) 1260.3
(M)+.
(25) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir
1999, 15, 1121.
(26) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Pechy, P.; Rotzinger, F. P.;
Humphry-Baker, R.; Kalyanasundaram, K.; Gra¨tzel, M.; Shklover, V.;
Haibach, T. Inorg. Chem. 1997, 36, 5937.
[(C7H15O-trpy-PO3Et2Pt(CtCC6H4CtC-MgP)]PF6 (20).
Yield 90%. Rf ) 0.20 on neutral alumina eluted with CH2Cl2/CH3-
OH (98:2). 1H NMR (400 MHz, d6-DMSO) δ: 10.26 (s, 1H), 9.77
(d, 2H, 3J ) 4.8 Hz), 9.40 (d, 2H, 3J ) 4.8 Hz), 9.07 (m, 2H), 8.89
4810 Inorganic Chemistry, Vol. 44, No. 13, 2005

Electron Transfer in Pt(II) Terpyridinyl Acetylide Complexes
Table 1. Electrochemical Data for Compounds 10-18a
potential range -0.6 to -0.9 V, the first reversible one-
electron reduction couple can be assigned to the platinum
terpyridine complex. With reference to the other studies on
platinum polypyridine complexes,10,14,29 this reduction is most
probably localized on the terpyridine ligand, but some
contribution of the platinum metal may also exist. It is
interesting to observe that the position of this redox couple
is strongly affected by the substituent on the terpyridine. An
electron withdrawing group, such as phosphonate, stabilizes
the reduced terpyridine and makes the reduction more easily
accessible at a lower potential, whereas an electron donating
substituent, such as ether, increases the energy of the π*
LUMO orbital, thereby increasing the reduction potential of
the complex. This expected trend is clearly observed in the
reference platinum complexes 12-14 as well in the dyads
15-20. The second reduction wave located in the range of
-1.1 to -1.3 V is assigned to the porphyrin reduction
because waves at comparable potentials were found in the
reference porphyrins. These results, coupled with the lumi-
nescence study (see below), give the energetics for the
photoinduced electron transfer processes from the porphyrin
excited singlet state (cf. Table 1).
porphyrin
Pt complex
reduction oxidation reduction oxidation
∆GCS (eV)
E
1/2 (V)
E1/2 (V)
E1/2 (V)
E1/2 (V)
from porphyrin
10
11
12
13
14
15
16
17
18
19
20
-1.19
-1.24
0.84
0.71
-0.61
-0.79
-0.90
-0.59
-0.80
-0.94
-0.58
-0.79
-0.93
1.23
1.05
0.94
-1.17
-1.20
-1.21
-1.14
-1.29
-1.28
0.79
0.83
0.78
0.70
0.70
0.70
-0.59
-0.34
-0.26
-0.66
-0.45
-0.31
a Recorded in dimethylformamide with 0.1 M Bu4NPF6 as the supporting
electrolyte; scan rate: 100mV/s; all potentials referenced vs SCE. E1/2
(Epa - Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials,
respectively. ∆GCS is calculated from Weller eq 1.
)
catalyzed cross-coupling reaction between the 4′-bromo
terpyridine and diethyl phosphite. We showed that the
replacement of the 4′-bromo terpyridine by 4′-triflate terpy-
ridine was beneficial, because it avoids the preparation of
4′-bromo terpyridine that requires the costly lachrymator
phosphorus oxybromide reagent and it afforded 4′-phospho-
nate terpyridine 1 with a much higher yield (92%). Terpy-
ridine substituted by the ether substituent was obtained by
the nucleophilic substitution of the corresponding pyridone
with bromoheptane in an 80% yield. The chelation of
terpyridines 1-3 with platinum involved heating the ligand
with Pt(COD)Cl220 (COD ) cycloocta-1,5-diene) in water.21
An anion methathesis reaction with a solution of sodium
tetrafluoroborate led to colored precipitates 4-6 with high
yields (>90%). The chloro platinum complexes 4-6 were
subsequently reacted with alkyne 7, 9, or 11 in the presence
of a catalytic amount of copper iodide and triethylamine and
led to the formation of an acetylide platinum bond with good
yields.3,11
Electrochemical Study. Cyclic voltammetry has been
performed in DMF to determine the redox potentials of the
dyads 15-20. The electrochemical data are collected in Table
1. Half-wave potentials (E1/2) were confirmed by square-
wave voltammetry, and they are in excellent agreement with
the values found by cyclic voltammetry.
The attributions of the redox processes have been made
by comparison with the reference compounds and are in
complete agreement with those reported by other groups for
similar electroactive units.4,10,27 For dyads 15-20, in the
anodic region, one reversible wave was observed at around
0.8 V, which can be confidently assigned to the oxidation
of the porphyrin macrocycle because platinum oxidation
potentials on reference complexes 12-14 are higher than
0.9 V and the oxidation potentials of the reference zinc and
magnesium porphyrin 10 and 11 occur around 0.8 V.28
In the cathodic region, both the porphyrin and platinum
complexes display several electrochemical processes. In the
Electronic Absorption Spectra. The electronic absorption
spectra of the molecules described in this study have been
recorded in dimethyl formamide (DMF) where all of the
compounds are soluble. The UV-vis absorption data are
summarized in Table 2.
The absorption characteristics of the platinum(II) terpyridyl
acetylide complexes 12-14 are in good agreement with those
reported by other groups for similar structures (Figure
1a,b).10,11 They display intense absorption bands in the UV
region (ꢀ ≈ 5 × 104 M-1 × cm-1) that are assigned to π-π*
transitions localized on terpyridine (cf. Figure 1a). The lower-
energy broad absorption band located around 420 nm
is generally assigned to a MLCT transition.10,11 The most
recent molecular orbital calculations made on this type of
complex have shown that the highest lying occupied orbitals
(HOMOs) contain an important contribution of both platinum
and an alkyne ligand.29,31 Therefore, the broad absorption
around 420 nm is probably not a pure MLCT transition but
rather (what is designed as) a MMLL′CT transition (mixed-
metal-ligand-to-ligand charge transfer) already described by
Eisenberg for platinum diimine dithiolate complexes.2 In this
type of transition, the electron density originates from an
orbital of a mixed metal and acetylide composition and shifts
to an unoccupied π* orbital localized on the terpyridine
ligand. The position of this band is very sensitive to the
substituent born on the terpyridine ligand. This is consistent
with the well-known participation of the π* LUMO of the
terpyridine into the MMLL′CT electronic transition. An
electron withdrawing substituent such as diethyl ester phos-
phonate (molecules 12, 15, and 18) decreases the energy of
(29) McInnes, E. J. L.; Farley, R. D.; Rowlands, C. C.; Welch, A. J.;
Rovatti, L.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 1999, 4203.
(30) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.;
Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620.
(31) (a) van Slageren, J.; Klein, A.; Zalis, S. Coord. Chem. ReV. 2002,
230, 193. (b) Fernandez, S.; Fornies, J.; Gil, B.; Gomez, J.; Lalinde,
E. J. Chem. Soc., Dalton Trans. 2003, 822.
(27) Redmore, N. P.; Rubtsov, I. V.; Therien, M. J. J. Am. Chem. Soc.
2003, 125, 8769.
(28) O’Shea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J. S. Inorg. Chem.
1996, 35, 7325.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4811

Monnereau et al.
Table 2. Absorption and Luminescence Data for Compounds 8-20a
absorption characteristics
luminescence
E0-0 (eV)
b
λ
max (ꢀ) nm (dm3 mol-1 cm-1
)
λ
em (nm)
τ
em (ns)
Φem
10
11
443 (257 000), 572 (8200), 623 (16 100)
226 (17 200), 277 (17 700), 314 (18 100),
442 (290 000), 539 (3300),
630, 688
640, 652
1.97
1.94
3.6
7.6
0.15
0.31
578 (11 500), 629 (29 000)
294 (53 500), 308 (44 400), 356 (11 500),
468 (6400)
292 (54 000), 309 (42 200), 348 (14 600),
446 (8200)
292 (45 100), 305 (38 000), 377 (12 700),
435 (6700)
286 (55 300), 318 (36 400), 337 (27 400),
360 (26 500), 446 (222 900),
574 (13 700), 628 (30 900)
12
13
14
15
∼670
∼2.1c
∼2.2c
∼2.3c
1.97
20
0.003
0.006
0.01
∼640
40
605
60
635, 690
<0.04d
0.012
16
17
445 (209 000), 574 (11 200), 626 (25 300)
388 (25 300), 309 (19 100), 321 (13 000),
337 (17 900), 446 (135 000),
574 (7000), 626 (17 500)
265 (44 000), 288 (43 000), 307 (33 600),
322 (33 300), 337 (31 100),
632, 686
634, 696
1.97
1.97
<0.04d
<0.04d
0.024
0.018
18
652, 702
1.94
<0.04d
0.034
446 (300 000), 536 (7400),
580 (13 800), 633 (39 000)
266 (32 300), 286 (32 700), 316 (24 700),
446 (219 000), 534 (4900),
580 (10 100), 632 (29 700)
265 (67 100), 286 (43 100), 308 (33 400),
322 (33 200), 337 (31 000),
19
20
652, 703
651, 702
1.94
1.94
<0.04d
<0.04d
0.009
0.021
446 (356 300), 533 (6200),
580 (13 700), 632 (39 600)
a Recorded in dimethylformamide at room temperature. b Measured with ZnTTP (Φ ) 0.040)4 or Ru(bpy)3(PF6)2 (Φ ) 0.062)30 (for Pt emission) in
CH3CN as standards. c From emission maxima at 77 K. d Additional components corresponding to the unquenched porphyrin were attributed to impurities.
Figure 1. Electronic absorption spectra recorded at room temperature in DMF: (a) reference platinum(II) terpyridine arylacetylide complexes 12-14 and
(b) the magnesium porphyrin dyads 18-20 along with the reference magnesium porphyrin 11.
the LUMO pyridine orbital and, hence, leads to a lower
energy MMLL′CT absorption. Conversely, an electron
donating substituent such as ether (molecules 14, 17, and
20) raises the π*(terpy) energy, thus increasing the energy
of the MMLL′CT transition, which results in a blue shift of
the absorption band (cf. Table 2). These substituent effects
were apparent from the position of the emission wavelength
maxima that reflect the energy level order of the lowest
MMLL′CT excited state (see below). It is important to
observe that the MMLL′CT transition of the platinum
complexes has a low absorption coefficient (ꢀ ≈ 5 × 103
M-1 cm-1) compared to that of the porphyrin transition in
this region (cf. Table 2).
Porphyrin Modules. The porphyrin chromophores feature
intense absorptions in the visible region. The introduction
of a π-conjugated spacer that electronically interacts with
the porphyrin units leads to a decrease of the energy of the
porphyrin electronic transitions and an inversion of the
relative Q-band intensities.16 Changes in the relative transition
strengths for Q(0,0) and Q(1,0) bands have been explained
by Gouterman: Abs[Q(0,0)]/Abs[Q(1,0)] is proportional to
the square of the energy difference E(a2u - eg) - E(a1u
-
eg), where the latter corresponds to the energies of the singlet
transitions.32 Thus, smaller Q(1,0) bands indicate that the
energy difference between singlet transitions is relatively
large in these porphyrins, as compared to those of tet-
4812 Inorganic Chemistry, Vol. 44, No. 13, 2005

Electron Transfer in Pt(II) Terpyridinyl Acetylide Complexes
raarylporphyrins. This results from the attachment of a
conjugated bridge at the 5 meso position, which electronically
communicates with the porphyrin core, introducing an
asymmetry that splits the degenerate x- and y-polarized
transitions. Similar effects have been reported previously for
meso-ethynyl-susbtituted porphyrins.16,27 In addition, a broad-
ening of the Soret band was seen, which was attributed to
the splitting of the x and y components. In the UV region of
the dyads 15-20, both the porphyrin and Pt moieties show
moderate to strong absorption.
Steady-State Luminescence Spectra. The luminescence
spectra were recorded in a deaerated DMF solution for all
molecules. The emission data are listed in Table 2. The
porphyrins display emission spectra that are typical for this
class of fluorescent dyes.4,27 The energy of the lowest
porphyrin singlet excited state (E0-0) was calculated from
the average of the 0-0 Q-band maximum in the room-
temperature absorption and fluorescence spectra. For the
triplet MMLL′CT state of the platinum complex, E0-0 was
estimated from the first emission band maximum in a solvent
glass at 77 K where spectral distortion is minimized. Still,
the degree of spectral distortion that suggests the emission
Porphyrin-Platinum Complex Dyads. The absorption
spectra of dyads 18-20 are shown in Figure 1b. The spectra
of dyads 15-17 containing the zinc porphyrin are very
similar in terms of shape and position of the absorption
maxima (cf. Table 2). The visible part of the spectrum is
dominated by the intense absorption bands of the porphyrin
unit that give rise to intense Soret bands associated with
transitions to the second excited singlet state.4 The two Q
bands around 630 and 580 nm are ascribed to 0,0 and the
first vibronic transition to the first excited singlet state,
respectively.4 In the dyads, the Soret transition of the por-
phyrin, located around 420 nm, overlaps with the MMLL′CT
transition of the platinum complex. The MMLL′CT transition
is, therefore, not discernible because of its much lower molar
absorptivity. As a result, selective excitation of the platinum
complex is impossible, whereas that of the porphyrin unit is
easily feasible by light irradiation in the Q bands of the
porphyrin. Absorption of the platinum complex in the UV
region is more intense than that of the porphyrin unit; thus,
the π-π* transition of the terpyridine fragment can be clearly
distinguished (around 290, 320, and 340 nm), although they
overlap with the porphyrin transitions. It can be concluded
that in dyads 15-20, the absorption spectra are almost a
superposition of the spectra for the individual chromophores,
but with some significant differences. The porphyrin absorp-
tion bands are slightly red-shifted (3-5 nm), and the Soret
band is broader and has a less intense maximum than the
corresponding reference porphyrins 10 or 11 (cf. Figure 1b),
although the latter already contain the arylacetylide bridge.
Moreover, the intensity of the Q(1,0) absorption band is
lower relative to that of Q(0,0) band than it is in the refer-
ences. These changes indicate a significant electronic interac-
tion between the porphyrin and the platinum complex that
further splits the energies of the x- and y-polarized transitions.
Also, the electrochemical results suggest a weak but
noticeable interaction between the terpyridine and porphyrin
units, seen as a less negative potential for porphyrin reduc-
tion when the terpyridine has an electron withdrawing
phosphonato substituent (15 and 18). It is interesting that
the effect of the substituent in 15 and 18 is mediated over
such a long distance, over the platinum metal ion and the
para-diethynyl-phenyl bridge. Similar results were reported
in ref 27 where a significant interaction over ethylnyl-phenyl
spacers in porphyrin-diimide dyads has been observed as a
change in the porphyrin redox potentials depending on the
identity of the diimide.
maximum somewhat underestimates E0-0
.
The platinum complex emission is attributed to phospho-
rescence from the MMLL′CT excited state, and the spectra
are very similar to those reported for related platinum
terpyridyl arylacetylide complexes.10 The emission maxima
show that the energy of the MMLL′CT excited state
decreases steadily across the series of complexes 14 > 13
> 12 as the electron withdrawing strength of the substituents
increases. This is consistent with the MMLL′CT nature of
the emissive state, which involves the terpyridine π* LUMO
orbital of the complex. The room-temperature emission
lifetime was 60, 40, and 20 ns for 14, 13, and 12,
respectively. The emission quantum yield was ca. 1 × 10-2
for 14 and decreased in parallel to the lifetime in the order
14 > 13 > 12.
Fluorescence spectra of the reference porphyrins ZnP 10
and MgP 11 exhibit the typical Q(0,0) and Q(0,1) emission
bands around 630 and 690 nm, respectively.4 The fluores-
cence emission spectra clearly show that ZnP and MgP have
very similar emission energies (around 1.9 eV), although the
ZnP excited state is slightly higher than that of MgP, which
is consistent with what is generally observed for tetraarylpor-
phyrins.4
The luminescence spectra of both ZnP and MgP, contain-
ing dyads 15-20, were measured by excitation at the
maximum of the first Q band of the porphyrin where the
photons are exclusively absorbed by the porphyrin unit. The
fluorescence intensity of the porphyrin in dyads 15-20 is
almost quantitatively quenched by the nearby platinum
complex compared to the reference porphyrin 10 or 11 (cf.
Table 2).
The time-resolved fluorescence after excitation with 150
fs laser pulses at 400 nm showed a single-exponential decay
for the reference porphyrins 10 and 11 with lifetimes of 3.6
and 7.6 ns, respectively. The dyads instead showed a very
rapid decay component (τ < 40 ps) followed by a slower
component with a lifetime of a few nanoseconds, very similar
to that for the corresponding reference porphyrin. This
suggests that the porphyrin singlet excited state (S1) in the
dyads is quenched on a time scale shorter than what could
be resolved in these experiments (τ < 40 ps). This was
supported by the femtosecond transient absorption experi-
ments described below. The unquenched emission decay
component must correspond to some porphyrin impurity that
is not attached to an active Pt quencher. Because the short
lifetime component could not be resolved, the relative
(32) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Inorg.
Chem. 1980, 19, 386.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4813

Monnereau et al.
Figure 2. Transient absorption spectra of 10 after excitation with a 150 ps laser pulse at 630 nm. The spectra shown are at 1 ps (solid), 100 ps (dot-
dashed), and 4 ns (dashed) after excitation.
amplitude of the unquenched component could not be
determined in these experiments. The fluorescence yields and
the transient absorption results given below suggest that this
component corresponds to ca. 10% of the dyad samples and,
hence, that it dominates the fluorescence spectra.
Single-exponential fits to the transient absorption decay at
400-600 nm gave a S1 lifetime of 2-20 ps for the different
dyads (Table 3), leaving a small and long-lived (τ > 10 ns)
residual absorption with a spectral shape suggesting a T1
state. The magnitude of this residual T1 absorption relative
to the reference 10 or 11 agreed with the relative fluorescence
yield, suggesting that it arose from a fraction of unquenched
porphyrins rather than from a reaction in the intact dyads.
No other species could be resolved in the transient spectra.
Importantly, the recovery of the Soret- and Q-band bleach
followed the same kinetics as the S1 transient absorption
decay, both in terms of time constant and relative amplitude,
showing that the ground state was repopulated with the same
kinetics as the S1 decay.
Transient Absorption. The reference porphyrins 10 and
11 as well as the dyads were studied by transient absorption
spectroscopy following excitation with 150 fs laser pulses
at 630 nm. Immediately after the pulse, the transient spectra
of 10 and 11 are typical for the lowest excited state (S1) of
similar porphyrins, for example, ZnTPP,4 with a maximum
signal at 470 nm (cf. Figure 2). Some spectral dynamics are
seen on the time scale of ca. 10 ps (not shown), particularly
on the red side of the Soret band (430-450 nm). This is
indicative of some relaxation in the S1 state, possibly due to
rotation of the phenyl group of the bridge.27 On the much
longer time scale of some nanoseconds, this develops into a
spectrum that can be attributed to the lowest excited triplet
state (T1), with a transient absorption maximum at ca. 460
nm. The signal decay at 470 nm, where no picosecond
dynamics are seen, was used to follow the development
from the S1 to the T1 spectrum. The decay can be fitted to
a single exponential with a lifetime of ca. 2.5 ns that, given
the limited length of the optical delay line, is in good
agreement with the S1 lifetime determined from the fluo-
rescence decay above. It is clear from the recovery of both
the Soret- and Q-band bleach that the S1 state, to a large
extent, decays to the ground state, suggesting a triplet yield
of approximately 0.5, which is somewhat lower than, for
example, ZnTPP.4
Discussion
In this paper, we will focus the discussion only on the
potential deactivation processes from the porphyrin excited
singlet state because selective light excitation of the platinum
fragment is impossible, and both the emission and transient
absorption changes of the platinum unit are very weak
compared to those for the porphyrin.
In all dyads 15-20, the porphyrin fluorescence is quenched
compared to that of the reference porphyrin (10 or 11). The
transient absorption shows that the lifetime of the first
porphyrin singlet excited state (S1) is much shorter in the
dyads (τ ) 2-20 ps) than in the model porphyrins 10 (τ )
3.6 ns) and 11 (τ ) 7.6 ns). This shows that the covalent
attachment of a platinum complex to the porphyrin imparts
additional deactivation pathways compared to the model
porphyrin.
In all of the dyads 15-20, the initial spectrum after the
excitation pulse (cf. Figure 3) was in good agreement with
the S1 spectrum of the corresponding reference porphyrin.
The lifetime of the S1 state, however, was strongly reduced.
One major deactivation pathway in the reference porphy-
rins is intersystem crossing to the triplet (T1) state. An
4814 Inorganic Chemistry, Vol. 44, No. 13, 2005

Electron Transfer in Pt(II) Terpyridinyl Acetylide Complexes
Figure 3. Transient absorption spectra of dyad 16 after excitation with a 150 ps laser pulse at 630 nm. The spectra shown are at 1 ps (solid), 10 ps
(dot-dashed), and 4 ns (dashed) after excitation.
Table 3. Quenching Time Constants for the Porphyrin S1 State in
Dyads 15-20 Attributed to an Electron Transfer (See Text)
show no sign of aggregation, however, and all of the
quenching kinetics were single exponential. Moreover, the
dyad
τq (ps)
dyad
τq (ps)
degree of quenching (ca. 90% of the amplitude) was the same
in the fluorescence yield and the transient absorption
experiments, despite the large difference in concentration,
<0.01 mM versus 0.2 mM. We, thus, believe that the fraction
of aggregates is small and that it is not an important
contribution to the observed quenching.
15
16
17
2
6
20
18
19
20
2
5
10
enhancement of this process has been reported in Zn(II)
porphyrin-acceptor dyads with paramagnetic metals in the
acceptor moiety,33 presumably because of spin-spin cou-
pling. An enhanced intersystem crossing could occur also
with heavy metals such as platinum, as the result of an
increased spin-orbit coupling. This should lead to the
efficient formation of the triplet state, however. Instead, we
see a regeneration of the ground state with the same kinetics
as the S1 decay. Thus, an enhanced intersystem crossing
cannot be a significant contribution to the rapid S1 deactiva-
tion in the dyads (the very low yield of triplet observed
corresponds very well to the fraction of unquenched por-
phyrin and is, thus, not the result of reactions in the reactive
dyads).
It has recently been shown by Yam and co-workers that
platinum terpyridine acetylide complexes can form ag-
gregates both in the solid state and in solution.34 Porphyrins,
which are hydrophobic π-aromatic systems, are also prone
to aggregation in polar solvents. Therefore, it can be
anticipated that dyads 15-20 could form aggregates in
solution. These aggregates could also be responsible for
fluorescence quenching by a self-quenching process that has
been encountered many times with aggregated dyes. The
ground state and transient absorption spectra of the dyads
Other potential deactivation processes of the porphyrin
singlet excited state are the photoinduced energy or electron
transfer. The energy transfer from the excited singlet state
of the porphyrin to the platinum complex can be ruled out
on the basis of the free energy of this process, as determined
from the absorption and emission spectra of the components.
Singlet-to-singlet energy transfer is extremely unlikely
1
because the MMLL′CT state lies far above the S1 state of
the porphyrin (1.9 eV). The energy of the lowest singlet
excited state (1MMLL′CT) of the platinum complex can be
estimated from the red edge of the absorption band of the
reference complexes 12-14. This is found to be in the range
of 510-470 nm, corresponding to an excited-state energy
of 2.4-2.6 eV (cf. Table 2). A rapid (2-20 ps) S1-to-
3MMLL′CT energy transfer from the porphyrin is also
unlikely because the latter state is at 2.1-2.3 eV (cf. Table
2) in the series of dyads, that is, at 0.2-0.4 eV above the
porphyrin S1 state. Furthermore, no products of an energy
transfer process are seen, so that, if it occurred, the decay of
the platinum MMLL′CT state must be much faster than the
rate of its formation, that is, much faster than 2 ps for 15
and 18. Finally, an energy transfer to the singlet states of
the bridge can be excluded, because the lowest singlet excited
states of related phenylethynyl compounds are located at an
energy of >3 eV.35,36
(33) (a) Asano-Someda, M.; Kaizu, Y. Inorg. Chem. 1999, 38, 2303. (b)
Pettersson, K.; Kils, K.; Mrtensson, J.; Albinsson, B. J. Am. Chem.
Soc. 2004, 126, 6710.
(34) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002,
124, 6506.
(35) Biswas, M.; Nguyen, P.; Marder, T. B.; Khundkar, L. R. J. Phys. Chem.
A 1997, 101, 1689.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4815

Monnereau et al.
Figure 4. Transient absorption traces at 500 nm for 10 (stars), 15 (circles), and 16 (crosses) in the same experiments as those shown in Figures 2-3. The
lines are single- (10) or double-exponential (15 and 16) fits to the data (see text).
The reaction free energies for the photoinduced electron
transfers are gathered in Table 1. The photoinduced electron
transfer process is clearly exergonic, and it can be anticipated
that this process gives an important contribution in the
excited-state deactivation of the porphyrin. The calculations
of the photoinduced electron transfer driving force are based
on the Rehm-Weller equation37 (eq 1), where E0-0(P) is the
zero-zero excitation energy of the porphyrin and E0Ox(P+/
P) and E0Red(Pt+/Pt0) are the reduction potentials for the
porphyrin donor and the platinum complex acceptor couples,
respectively.
is a fit to the data according to eq 2, with the preexponential
factor (A) as the only floating parameter and λ set to 0.65
eV, which is a reasonable guess for this type of system.38
With few data points, which span only a part of the putative
Marcus curve, the result cannot, on its own, be taken as
evidence for an electron transfer, but it shows clearly that
the results are in agreement with a rapid electron transfer
from the excited porphyrin unit. The lack of a detectable
charge transfer intermediate implies that the subsequent
recombination is faster than the initial electron transfer. This
was previously observed for Zn(II) porphyrin-diimide dyads
linked via the same type of bridging molecules,27 and the
time scale of the initial electron transfer was very similar to
that of the present case.
∆G° ) E0Red(Pt+/Pt0) - E0Ox(P+/P) - E0-0(P)
(1)
The potentials were determined in the same solvent (DMF)
as that used for the photochemical experiments. Also, the
Coulombic work term is omitted because the electron transfer
leads only to a charge shift, with one net neutral species in
both the reactant and the product states. The exergonicity of
the electron transfer process varies in a large range in the
series of dyads, from fairly favorable in dyad 17 (∆G° )
-0.26 eV) to very favorable in dyad 18 (∆G° ) -0.66 eV).
The relative rate constant of the S1 decay in the series of
dyads varies with the driving force for an electron transfer,
in agreement with the Marcus expression36 (cf. Figure 4)
An electron transfer via arylethynyl bridging units has been
discussed in terms of a significant delocalization of the donor
or acceptor state over the bridge.27,35 This effect may be
entirely different for the charge separation and the subsequent
recombination reaction, so that the bridge may favor one of
the reactions over the other. For a series of phenylethylnyl-
bridged zinc(II) porphyrin-diimide dyads, Therien and co-
workers27 suggested that the porphyrin cation of the charge
separated state was delocalized over the bridge, whereas the
S1 excited state was not, on the basis of fits of the solvent
dependence of the rate constants. When the bridge of one
of the dyads was elongated with another phenylethynyl unit,
however, the rate of charge separation was only 3 times
slower, whereas the subsequent recombination was reduced
as much as 500 times. The much weaker distance dependence
for charge separation in that case suggests that this type of
bridge instead selectively mediates charge separations over
long distances. Possibly, this occurs by delocalization of the
(∆G0 + λ)2
kET ) A exp -
(2)
[
]
4λRT
where ∆G0 is the driving force according to eq 1 and λ is
the reorganization energy. In Figure 5, the parabolic curve
(36) (a) Hirata, Y.; Okada, T.; Mataga, N.; Nomoto, T. J. Phys. Chem.
1992, 96, 6559. (b) Hirata, Y.; Okada, T.; Nomoto, T. J. Phys. Chem.
1993, 97, 9677.
(37) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(38) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
4816 Inorganic Chemistry, Vol. 44, No. 13, 2005

Electron Transfer in Pt(II) Terpyridinyl Acetylide Complexes
Figure 5. Quenching rate constant as a function of electron transfer driving force for dyads 15-20. The line is a fit to the Marcus free energy dependence
of the electron transfer rate constant (eq 2).
porphyrin excited state over the bridge, which is enhanced
as the length of the π-conjugated bridge increases. It is, thus,
of interest to examine the effect of an elongation of the bridge
in the present porphyrin-Pt systems on the charge separation
and recombination rates, to elucidate whether one reaction
shows a weaker distance dependence than the other and
whether this effect can be used to selectively favor charge
separation.
distances. The development of new systems with longer
bridges, such as oligophenyl acetylene modules, is under way
in our laboratory in order to try to slow the rate of the
recombination process, while hopefully maintaining a rapid
charge separation.
Acknowledgment. L.H. acknowledges financial support
from the Knut and Alice Wallenberg Foundation and the
Swedish Foundation for Strategic Research and a Research
Fellow position from the Royal Swedish Academy of
Sciences. Financial support by La Rioja University (Spain)
for the salary of J.G. is gratefully acknowledged. This study
was partially subsidized by the French Ministry of Research
by the ACI “jeunes chercheurs” 4057.
In conclusion, an electron transfer from the porphyrin to
the platinum unit seems to be the only reasonable explanation
for the very rapid deactivation (τ ) 2-20 ps) of the lowest
excited singlet state of the porphyrin in the dyads 15-20.
The deactivation rate constant shows a Marcus-type variation
with a driving force for an electron transfer. The subsequent
recombination of the charge transfer state is even more rapid,
so that the regeneration of the ground state reactants follows
the same kinetics as the decay of the porphyrin excited state.
The results underscore the potential of phenyl-ethenyl
bridges to promote very rapid electron transfers over long
Supporting Information Available: Phosphorescence emis-
sion spectra of the reference complexes 12-14 recorded at 77
K. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC048573Q
Inorganic Chemistry, Vol. 44, No. 13, 2005 4817