Ultrafast charge separation in a photoreactive rhenium-appended porphyrin assembly monitored by picosecond transient infrared spectroscopy

Article abstract of DOI:10.1021/ja0539802

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated a

Full text of DOI:10.1021/ja0539802

Published on Web 03/14/2006
Ultrafast Charge Separation in a Photoreactive
Rhenium-Appended Porphyrin Assembly Monitored by
Picosecond Transient Infrared Spectroscopy
Anders Gabrielsson,† Frantisˇek Hartl,‡ Hong Zhang,‡ John R. Lindsay Smith,†
Michael Towrie,§ Anton´ın Vlcˇek, Jr.,| and Robin N. Perutz*,†
Contribution from the Department of Chemistry, UniVersity of York, York, YO10 5DD, United
Kingdom, Van’t Hoff Institute for Molecular Sciences, Molecular Photonic Materials, UniVersity
of Amsterdam, Nieuwe Achtergracht 166, Amsterdam, NL-1018 WV, The Netherlands,
Rutherford Appleton Laboratory, Central Laser Facility, CCLRC, Didcot, Oxon, OX11 0QX,
United Kingdom, and Department of Chemistry, Queen Mary, UniVersity of London,
London, E1 4NS, United Kingdom
Received June 16, 2005; E-mail: rnp1@york.ac.uk
Abstract: Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmet-
alloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent
of the porphyrin. The resulting complexes, abbreviated as [Re(CO)3(Pic)Bpy-MgTPP][OTf] and [Re(CO)3-
(Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)3(bpy) units and the
metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent
quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the
porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption
(TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of
the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the
TRIR spectra by a low-frequency shift in the ν(CO) bands of the Re(CO)3 moiety similar to that observed
by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO)3(Pic)Bpy-MTPP][OTf] results
in characteristic TRVIS spectra of the S1 state of the porphyrin that decay with a time constant of 17 ps (M
) Mg) or 24 ps (M ) Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or
ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron
transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The
short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence
of triethylamine.
Introduction
through covalent, coordination, or hydrogen bonds as well as
mechanically interlocked systems.4,9-19 Photoinduced charge
transfer is typically studied by time-resolved spectroscopy.
Vibrational spectroscopic methods can provide an unequivocal
demonstration of changes in charge distribution and are central
to the current study. Therien’s demonstration by time-resolved
Research into donor-bridge-acceptor assemblies capable of
photoinduced electron transfer has proliferated in recent years.1-6
Particular interest has been devoted to porphyrin-containing
supramolecules because of their resemblance to nature’s choice
of chromophores, their tunability, and low reorganization energy
upon reduction or oxidation.7-9 There are now many different
porphyrins linked to organic electron donors and/or acceptors,
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† University of York.
‡ University of Amsterdam.
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§ Rutherford Appleton Laboratory.
| Queen Mary, University of London.
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J. AM. CHEM. SOC. 2006, 128, 4253-4266
4253

A R T I C L E S
Gabrielsson et al.
infrared (TRIR) spectroscopy that porphyrins appended with
quinones and diimides can undergo subpicosecond photoinduced
charge separation is therefore of particular note.20-22
tential electron donors may be linked to the fac-[Re(CO)3(py)-
(bpy)]+ complex via either the bipyridyl unit or the pyridine
substituent.
Time-resolved IR spectroscopy has provided an exceptionally
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One of the aims of many studies of photoinduced electron
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charge-separated state as a mimic of photosynthesis. In this
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Charge Separation in Rhenium-Appended Porphyrin
A R T I C L E S
Chart 1
We show how this short lifetime can be reconciled with
reactivity at the remote site.
Experimental Section
General Procedures. Chemicals were obtained from the following
suppliers: benzophenone (Acros Organics); Re2(CO)10, Celite, activated
alumina, diisopropylethylamine, ferrocene, 3-picoline, [Bu4N][PF6], and
anhydrous magnesium bromide (Aldrich); zinc acetate, magnesium
sulfate, sodium carbonate, and thionyl chloride (Fisons); silver trifluo-
romethanesulfonate (Avocado); silica gel 60 and TLC plates F254
(Merck).
Solvents for general use were used as obtained from Fisher. Solvents
for Schlenk-line work were dried by refluxing over sodium/benzophe-
none (benzene, toluene, pentane, THF), over P2O5 (acetonitrile,
butyronitrile), or over CaCl2 and then P2O5 (dichloromethane). After
refluxing they were distilled and stored under argon. Triethylamine
(BDH) was dried over CaH2 and stored over 4 Å molecular sieves.
CDCl3 and CD2Cl2 were either used as obtained from Fluorochem
or Goss or dried over P2O5. Pyridine-d5 (Aldrich) was dried over 4 Å
molecular sieves. Dried NMR solvents were transferred into an NMR
tube fitted with a ptfe stopcock (Young) on a high-vacuum line and
sealed under argon.
Syntheses. 5-[4-(4-Methyl-2,2′-bipyridine-4′-carboxyamidyl)phe-
nyl]-10,15,20-triphenylporphyrin: Bpy-H2TPP. Abbreviations are
shown in Chart 1 and Scheme 1. 4-Methyl-2,2′-bipyridine-4′-carboxylic
acid (0.21 g, 0.98 mmol), synthesized according to a literature
procedure,77a was heated at reflux in thionyl chloride (6 cm3) under an
inert atmosphere overnight. Excess thionyl chloride was removed in
vacuo; the resulting yellow solid was dried for a further 30 min, and
triethylamine (20 cm3, dry) was added. NH2-H2TPP was synthesized
We have previously described zinc porphyrin systems linked
via an amide spacer to tungsten pentacarbonyl pyridine and
rhenium tricarbonyl 2,2′-bipyridine bromide.75,76 This design
allows the porphyrin moiety to be monitored by absorption and
emission spectroscopy, while the transition metal unit is revealed
by its structure-sensitive CO-stretching modes. The rhenium
tricarbonyl bromide complex [Re(CO)3BrBpy-ZnTPP] (Chart
1), however, has two disadvantages: first, it does not undergo
photoinduced reaction with intermolecular electron donors such
as triethylamine,23 and second, its solubility is very low in
common solvents. In the second generation complexes, we
aimed to increase solubility and increase the free energy of
electron transfer from metalloporphyrin to rhenium bipyridine
by replacing the bromine substituent with the 2-electron donor,
3-picoline.23 We showed that irradiation of the resulting [Re-
(CO)3(Pic)Bpy-ZnTPP][OTf] (Chart 1) into the long-wave-
length porphyrin absorption bands caused reaction at the remote
rhenium picoline site in the presence of triethylamine. We
showed that this excitation of the porphyrin resulted in electron
transfer from the porphyrin to the rhenium 2,2′-bipyridine center
that initiated reaction. This assembly may be considered as a
dyad without triethylamine or a triad with it. It exhibits
photoinduced reaction at a site remote from the chromophore,
one of the categories of photoactive supramolecule recognized
by Balzani and Scandola.1
by nitration of tetraphenylporphyrin,77b followed by reduction.77c
A
solution of NH2-H2TPP (0.77 g, 1.22 mmol) in dichloromethane (50
cm3, dry) was added by cannula, and the reaction mixture was stirred
for 1 h at room temperature and then heated for 2.5 h at reflux. The
solvent was removed, and the purple solid was purified by column
chromatography (Si-60, dichloromethane, dichloromethane with 4%
methanol). The second fraction was collected, the solvent was removed,
and the purple solid was recrystallized by slow evaporation of an ethanol
dichloromethane mixture (1:1 v/v) to yield the desired compound Bpy-
H2TPP (0.5 g, 0.61 mmol, 62%). The 1H NMR spectrum was in good
agreement with that reported earlier.78
1H NMR (500 MHz, THF-d8): δ -2.78 (2 H, br, s, inner NH); 2.32
(3 H, s, Bpy CH3); 7.10 (1 H, d, J 4.9 Hz, Bpy); 7.60-7.66 (9 H, m,
p-, m-phenyl); 7.83 (1 H, dd, J 4.9 Hz, J 0.6 Hz Bpy); 8.18 (2 H, d, J
8.1 Hz, m-bridging phenyl); 8.07-8.12 (8 H, m, 6 o-phenyl and 2
bridging phenyl); 8.32 (1 H, s, Bpy); 8.43 (1 H, d, J 4.77 Hz, 1 Bpy);
8.69-8.73 (7 H, m, 6 â- pyrrolic H and 1 Bpy); 8.80 (2 H, br,
â-pyrrolic); 9.00 (1 H, s, Bpy); 10.41 (1 H, s, br, CONH).
UV/vis (THF): λmax/nm (ꢀ/dm3 mol-1 cm-1) ) 373 (31 000), 416
(31 000), 514 (19 800), 549 (11 100), 592 (6900), 647 (5900).
ESI-MS: m/z ) 826 (100% MH+).
In this paper, we report the photophysics of [Re(CO)3(Pic)-
Bpy-ZnTPP][OTf] and [Re(CO)3(Pic)Bpy-MgTPP][OTf] (Chart
1) and provide the first detailed time-resolved transient absorp-
tion and infrared study of a porphyrin appended with a transition
metal carbonyl moiety. We demonstrate that photoinduced
charge separation from the excited porphyrin to Re(bpy) can
occur on a picosecond time scale and that the fast charge-
recombination results in a vibrationally excited ground state.
High-resolution FAB-MS: For [C56H39N7O]+, observed mass
825.3221, calculated mass 825.3216, difference 0.5 mD. In this and
other syntheses, high-resolution mass spectrometry was used in
preference to elemental analysis because of unreliable results with
elemental analysis of metalloporphyrins.
5-{4-[Rhenium(I)tricarbonyl(3-picoline)-4-methyl-2,2′-bipyridine-
4′-carboxyamidyl]phenyl}-10,15,20-triphenylporphyrinatozinc(II)
Trifluoromethanesulfonate; [Re(CO)3(Pic)Bpy-ZnTPP][OTf]. Re-
(CO)3BrBpy-ZnTPP (0.10 g, 0.081 mmol), synthesized from Bpy-
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A R T I C L E S
Gabrielsson et al.
Scheme 1. Synthesis of [Re(CO)3(pic)Bpy-MTPP][OTf] (M ) Zn, Mg)
1
ZnTPP (see Supporting Information) according to the method of Aspley
et al.,76,78 was dissolved in a mixture of tetrahydrofuran (10 cm3, dry);
3-picoline (0.5 cm3) and AgOTf (0.1 g, 0.4 mmol) were added. The
reaction mixture was heated under argon at reflux for 1 h. The solvent
was removed in vacuo, and the solid was redissolved in dichloromethane
(3 cm3) and purified by column chromatography (Si-60, dichlo-
romethane with 20% v/v tetrahydrofuran), to yield [Re(CO)3(Pic)Bpy-
ZnTPP][OTf] (0.10 g, 0.071 mmol, 88%).
50%). The H NMR spectrum of Bpy-MgTPP was best resolved in
pyridine-d5 (Table S1, Supporting Information).
1H NMR (500 MHz, pyridine-d5): δ 2.15 (3 H, s, Bpy CH3); 7.02
(1 H, d, J 4.64 Hz, Bpy), 7.66 (9 H, m, p-, m-phenyl); 8.14 (1 H, d, J
5.00 Hz, Bpy); 8.30 (6 H, m, o-phenyl); 8.35 (2 H, d, J 8.30 Hz,
m-bridging phenyl); 8.43 (1 H, s, Bpy); 8.48 (2 H, d, J 8.30 Hz,
m-bridging phenyl); 8.55 (1 H, d, J 4.9 Hz, 1 Bpy); 8.87 (1 H, d, J
4.64 Hz, Bpy); 9.02 (4 H, s, â-pyrrolic); 9.02 (2 H, d, J 4.64 Hz,
â-pyrrolic); 9.06 (2 H, d, J 4.64 Hz, â-pyrrolic); 9.48 (1 H, s, Bpy);
11.80 (1 H, s, CONH)
IR (ν/cm-1) (THF) 2031 (s), 1927 (s, broad) (ν(CO)).
1H NMR (400 MHz, CD2Cl2): δ 2.30 (3 H, s, Bpy CH3); 2.75 (3 H,
3-picoline CH3); 7.27 (1 H, dd, J 7.8 and 5.9 Hz, 3-picoline H2); 7.60
(1 H, d, J 5.9 Hz, Bpy H5′); 7.67 (1 H, d, J 7.8 Hz, 3-picoline H4);
7.80 (9 H, m, 9 m-, p-phenyl); 7.95 (1 H, d, J 5.9 Hz 3-picoline H6);
8.14 (1 H, s, 3-picoline H2); 8.27 (10 H, m, 6 o-phenyl and 4 bridging
C4H6); 8.38 (1 H, d, J 5.9 Hz, Bpy H5); 8.72 (1 H, s, Bpy H3′); 8.92 (4
H, s, â-pyrrolic Ha and Hb); 8.94 (2 H, d, J 4.9 Hz, â-pyrrolic Hc);
9.00 (1 H d, J 5.9 Hz Bpy H6′ overlaps with doublet at 9.01 ppm);
9.01 (2 H, d, J 4.9 Hz, â-pyrrolic Hd); 9.19 (1 H, s, Bpy H3); 9.33 (1
H, d, J 5.9 Hz, Bpy H6) 10.66 (1 H, s, br, CONH).
ESI-MS: m/z ) 849 (100% MH+).
High-Resolution FAB-MS: For [C56H37N7OMg]+, observed mass
847.2907, calculated mass 847.2907, difference 0.0 mDa.
5-{4-[Rhenium(I)tricarbonylbromide-4-methyl-2,2′-bipyridine-
4′-carboxyamidyl]phenyl}-10,15,20-triphenylporphyrinatomagne-
sium(II): Re(CO)3BrBpy-MgTPP. Rhenium pentacarbonyl bromide
(0.030 g, 0.073 mmol) and Bpy-MgTPP (0.050 g, 0.059 mmol) were
heated in toluene (30 cm3, dry) at 70 °C under argon for 5 h. After a
black precipitate had formed, the reaction mixture was left overnight
before the solvent was removed in vacuo. The residue was washed
with methanol (10 cm3) and diethyl ether (10 cm3) yielding a dark purple
product that was used without further purification in the next step.
5-{4-[Rhenium(I)tricarbonyl(3-picoline)-4-methyl-2,2′-bipyridine-
4′-carboxyamidyl]phenyl}-10,15,20-triphenylporphyrinatomagne-
sium(II) Trifluoromethanesulfonate: [Re(CO)3(Pic)Bpy-MgTPP]-
[OTf]. Re(CO)3BrBpy-MgTPP (0.070 g, 0.059 mmol) was suspended
in dry tetrahydrofuran (10 cm3); 3-picoline (0.2 cm3) and silver
trifluoromethanesulfonate (0.075 g, 0.295 mmol) were added. The
reaction mixture was heated under argon at reflux for 1 h before the
solvent was removed, and the solid was redissolved in tetrahydrofuran
(10 cm3). The solution was filtered through a plug of alumina (eluted
with tetrahydrofuran with 0.5% v/v triethylamine), and the solvent was
removed to give [Re(CO)3(Pic)Bpy-MgTPP][OTf] (0.068 g, 0.05
mmol, 85%).
ESI-MS: m/z ) 1253 (100% [M, major contribution from
C65H44N8O4187Re66Zn]+), 1160 (10% [M - 3-picoline]+).
High-resolution ESI-MS: For isotopomer, [C65H44N8O4185Re64Zn]+
observed mass, 1249.2301, calculated mass1249.2298, difference 0.3
mDa
5-[4-(4-Methyl-2,2′-bipyridine-4′-carboxyamidyl)phenyl]-10,15,-
20-triphenylporphyrinatomagnesium(II): Bpy-MgTPP. A literature
procedure was adapted for the insertion of magnesium into Bpy-
H2TPP.79,80 Bpy-H2TPP (0.200 g, 0.24 mmol) was dissolved in
trichloromethane (30 cm3); diisopropylethylamine (1 cm3) and mag-
nesium bromide (anhydrous 0.44 g, 2.4 mmol) were added. The reaction
mixture was stirred at room temperature, and a purple precipitate formed
after a few minutes; the precipitate dissolved upon addition of diethyl
ether (5 cm3, dry). The reaction was stirred for 20 min at room
temperature, diluted with dichloromethane (50 cm3), washed with
sodium carbonate solution (5%, 150 cm3), and dried (MgSO4), and the
solvent was removed in vacuo. The blue-purple product was purified
by column chromatography (alumina neutral Brockman grade 1, poured
in dichloromethane and eluted with dichloromethane/tetrahydrofuran,
2/1 v/v) to give Bpy-MgTPP as a blue-purple solid (0.1 g, 0.12 mmol,
IR (ν/cm-1) (THF) 2031(s), 1927 (s, broad). (PrCN): 2033 (s), 1927
(s, broad) (ν(CO)).
1H NMR (500 MHz, CD2Cl2): δ 2.30 (3 H, s, Bpy CH3); 2.75 (3 H,
3-picoline CH3); 7.26 (1 H, dd, J 7.8 and 5.9 Hz, 3-picoline H2); 7.61
(1 H, d, J 5.9 Hz, Bpy H5′); 7.67 (1 H, d, J 7.8 Hz, 3-picoline H4);
7.80 (9 H, m, 9 m-, p-phenyl); 7.95 (1 H, d, J 5.9 Hz, 3-picoline H6);
8.14 (1 H, s, 3-picoline H2); 8.27 (10 H, m, 6 o-phenyl and 4 bridging
C4H6); 8.38 (1 H, d, J 5.9 Hz, Bpy H5); 8.72 (1 H, s, Bpy H3′); 8.94 (4
(79) Lindsey, J. S.; Woodford, J. N. Inorg. Chem. 1995, 34, 1063-1069.
(80) Oshea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J. S. Inorg. Chem.
1996, 35, 7325-7338.
9
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Charge Separation in Rhenium-Appended Porphyrin
A R T I C L E S
H, s, â-pyrrolic Ha and Hb); 8.96 (2 H, d, J 4.9 Hz, â-pyrrolic Hc);
9.00 (1 H, d, J 5.9 Hz Bpy H6′ overlaps with doublet at 9.03 ppm):
9.03 (2 H, d, J 4.9 Hz, â-pyrrolic Hd); 9.19 (1 H, s, Bpy H3); 9.33 (1
H, d, J 5.9 Hz, Bpy H6) 10.66 (1 H, s, br, CONH).
ESI-MS: m/z ) 1211 (100% M+), (30% [M - (3-Pic)]+).
High-Resolution FAB-MS: For [C65H44N8O4Mg185Re]+ isotopomer,
observed mass 1209.2865, calculated mass 1209.2866, difference 0.1
mDa.
ment, and in no case was any photodecomposition observed. The
transient decay was fitted to a single-exponential function.
Nanosecond Absorption Experiments. The York nanosecond
absorption equipment has been described previously.84 The solutions
were excited with the second harmonic of a Quanta-Ray 3 Nd:YAG
laser (pulse length 10 ns) and detected with a pulsed arc lamp,
monochromator, and photomultiplier linked to a digital oscilloscope.
Ultrafast Infrared Experiments. Time-resolved infrared (TRIR)
measurements were performed on the PIRATE equipment that has been
described in detail previously.68-70,85,86 In brief, the sample solution
was excited at 560 or 600 nm with an OPA pumped by frequency-
doubled pulses from a Ti:sapphire laser of ∼150 fs duration (fwhm)
and ca. 1 µJ energy focused to a spot of ca. 150 µm. The samples
were probed with IR (∼150 fs) pulses obtained by difference-frequency
generation. The IR probe pulses cover spectral ranges ca. 200 cm-1
wide. The sample solutions in MeCN were flowed through a CaF2 IR
cell, which was rastered in two dimensions. Path lengths of 0.5 to 1
mm were used. All spectral and kinetic fitting procedures were
performed using Microcal Origin 7 software. A germanium filter was
placed in front of the detector to remove scattered excitation light.
Steady-State Spectroscopy. Infrared spectra were recorded on a
Mattson RS FTIR instrument with the solvent spectrum as background.
FAB mass spectra (both low and high resolution) were recorded on a
VG AutoSpec mass spectrometer and an ESI mass spectra on a Finnigan
LCQ instrument. High-resolution ESI spectra were recorded by the
EPSRC National Mass Spectrometry Service Centre in Swansea, UK.
NMR spectra were recorded on a JEOL EX 270, Bruker MSL 300, or
Bruker AMX 500 spectrometer. UV-visible spectra were recorded on
a Perkin-Elmer Lambda 7 or Hewlett-Packard 8452A spectrophotom-
eter. Emission spectra were recorded with a Jobin Yvon-Spex Fluo-
romax-2 spectrometer with right angle illumination (typically slit widths
for both emission and excitation 2 nm). Spectra were corrected for
photomultiplier response by using the correction file supplied by the
manufacturer. When spectra were recorded under inert atmosphere,
samples were placed in a cuvette equipped with a bulb attached via
sidearm and a ptfe stopcock and were degassed by three consecutive
freeze-pump-thaw cycles and backfilled with argon.
Electrochemistry. Cyclic voltammograms were recorded with a
PAR EG&G model 283 potentiostat, using an airtight single-compart-
ment cell placed in a Faraday cage. The working electrode was a Pt
microdisk (0.42 mm2 apparent surface area), polished with a 0.25 µm
diamond paste between the scans. Coiled Pt and Ag wires served as
an auxiliary and pseudoreference electrode, respectively. The concentra-
tion of the analyte was 10-3 mol dm-3. Ferrocene (Fc) was used as the
internal standard.81 The supporting electrolyte, [Bu4N][PF6], was
recrystalized twice from absolute EtOH and dried overnight under
vacuum at 80 °C before use.
Results
Synthetic Methodology. The synthetic strategy for [Re(CO)3-
(Pic)Bpy-MTPP][OTf] (M ) Zn, Mg) is depicted in Scheme
1. The synthesis of Re(CO)3BrBpy-ZnTPP has been described
previously,76,78 but an altered and higher yielding procedure is
presented for its precursors Bpy-H2TPP (see Experimental) and
Bpy-ZnTPP (see Supporting Information). The rhenium-
appended complex Re(CO)3BrBpy-ZnTPP was converted to
the 3-picoline derivative [Re(CO)3(Pic)Bpy-ZnTPP][OTf] by
a standard method of reaction with 3-picoline and silver triflate
in THF at reflux.
Magnesium ions were inserted into Bpy-H2TPP by the
heterogeneous method developed by Lindsey et al. yielding Bpy-
MgTPP under mild conditions.79,80 Bpy-MgTPP is extremely
acid-sensitive, and chromatography over silica regenerated the
free-base compound Bpy-H2TPP quantitatively. The rhenium
chelate Re(CO)3BrBpy-MgTPP was made by heating the
ligand and Re(CO)5Br in benzene or toluene.48,76,78 Since Re-
(CO)3BrBpy-MgTPP was only soluble in coordinating solvents
such as DMF and pyridine, it was converted directly to the
3-picoline derivative [Re(CO)3(pic)Bpy-MgTPP][OTf] without
purification by the same method as that for the zinc porphyrin.
Ground-State Spectroscopy. NMR Spectroscopy. 1H NMR
spectra of [Re(CO)3(Pic)Bpy-ZnTPP][OTf] were assigned with
the aid of NOE and COSY experiments (Supporting Informa-
tion). The labeling and assignment for the spectrum in CD2Cl2
are shown in Figure 1. Not surprisingly, the magnesium
derivative displays a very similar spectrum to that of the zinc
complex. The two methyl groups resonate at δ 2.30 and 2.75,
and the amide N-H proton appears at δ 10.70. The â-pyrrolic
protons are shifted by 0.2 ppm downfield relative to the zinc
derivative. There were no resonances at negative shift where
the free-base inner N-H protons of any residual free-base
porphyrin appear, nor were there any resonances due to picoline
complexed to magnesium.
IR and UV/vis spectroelectrochemistry at room temperature was
conducted with an Optically Transparent Thin-Layer Electrochemical
(OTTLE) cell.82 The concentrations of the solutions were 10-1 mol
dm-3 in the supporting electrolyte and 5 × 10-3 mol dm-3 (IR) or
10-3 mol dm-3 (UV/vis) in the analyte. The working electrode potential
of the spectroelectrochemical cells was controlled with a PA4 poten-
tiostat (EKOM, Polna´, Czech Republic). The IR spectra were recorded
with a Bio-Rad FTS-7 spectrometer (16 scans, 2 cm-1 spectral
resolution), and the UV/vis spectra, with a HP 8453A diode array
spectrophotometer.
Ultrafast Visible Absorption Experiments. The time-resolved
visible (TRVIS) absorption experiments were performed with the
University of Amsterdam ultrafast spectroscopy setup with a pump
wavelength of 560 nm.83 The energies of the pump and probe pulses
were <5 µJ per pulse and 5 nJ per pulse, respectively, and the angle
between the pump and probe pulses was between 7° and 10°. To avoid
local heating, the circular cell (Hellma d ) 18 mm, l ) 1 mm) was
placed in a homemade rotating mount (1000 rpm). Between 200 and
500 pulses were averaged to provide one spectrum at a particular time.
The wavelength interval between points was 1.4 nm. Data were
collected for 300 delay times. The total instrument rise-time was
approximately 200 fs.83 The samples were prepared with an optical
density of ca. 0.9 at the excitation wavelength. Experiments were
performed under the magic angle condition in order to circumvent
effects from the reorientation of the solute. The ground-state electronic
absorption spectra were recorded before and after the ultrafast experi-
UV/vis Absorption Spectra. The UV/vis absorption spectrum
of Bpy-ZnTPP is very similar to that of ZnTPP, with one very
strong absorption peak observed at 424 nm (Soret band) and
two strong transitions at 556 and 596 nm (Q-bands, Table 1).
(81) Gritzner, G.; Ku˚ta, J. Pure Appl. Chem 1984, 56, 461-466.
(82) Krejcˇik, M.; Daneˇk, M.; Hartl, F. J. Electroanal. Chem. Interfacial
Electrochem. 1991, 317, 179-187.
(83) Vergeer, F. W.; Kleverlaan, C. J.; Stufkens, D. J. Inorg. Chim. Acta 2002,
327, 126-113.
(84) Nicasio, M. C.; Perutz, R. N.; Tekkaya, A. Organometallics 1998, 17,
5557-5564.
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4257

A R T I C L E S
Gabrielsson et al.
romethane has little effect on the carbonyl stretching frequencies
(less than 2 cm-1).
Cyclic Voltammetry and Spectroelectrochemistry. Data for
cyclic voltammetry are listed in Table 2. The oxidation of [Re-
(CO)3(Pic)Bpy-ZnTPP][OTf] and Bpy-ZnTPP occurs at the
same electrode potential, consistent with porphyrin-based one-
electron oxidation. The potential for oxidation of Bpy-MgTPP
is 140 mV lower. Both Bpy-MgTPP and Bpy-ZnTPP show
chemically reversible reduction (V ) 100 mV s-1), whereas the
reduction of [Re(CO)3(Pic)Bpy-ZnTPP][OTf] in THF is partly
chemically irreversible and occurs at a potential 510 mV less
negative than that for Bpy-ZnTPP. This behavior is consistent
with reduction localized at the rhenium bipyridine moiety and
some replacement of picoline by the donor solvent.23 In
butyronitrile (PrCN), on the other hand, the reduction of [Re-
(CO)3(Pic)Bpy-ZnTPP][OTf] is fully reversible. The radical
product is further reduced to the corresponding anion in a
chemically irreversible step at Ep,c ) -1.77 V vs Fc/Fc+ (not
in Table 2), which partly overlaps with the reversible reduction
of the remote ZnTPP moiety at -1.83 V.
UV/vis spectroelectrochemistry of Bpy-ZnTPP in THF was
employed to examine the effect of oxidation on the UV/vis
spectrum (See Figure S5 in Supporting Information). The
spectrum revealed loss of the Q-band at 596 and 556 nm and
gain of absorption in the 450 nm region and a very broad band
centered at 630 nm. We can exploit the changes in absorbance
to deduce that ∆ꢀ650/∆ꢀ470 ≈ 0.6 and ∆ꢀ470 lies in the range ∆ꢀ
) 1.8 × 104 to 3.4 × 104 dm3 mol-1 s-1 by employing the
value of ꢀ at 559 nm for Bpy-ZnTPP (∆ꢀ450 refers to the change
in molar absorption coefficient on oxidation at 450 nm, etc.;
the two values correspond to extreme values for the absorption
coefficient of the radical cation at 559 nm.)
1
Figure 1. Labeling and chemical shifts for the H NMR spectra of [Re-
(CO)3(Pic)Bpy-ZnTPP][OTf] in CD2Cl2 shown in two parts.
Table 1. Absorption Maxima (nm) and Molar Absorption
Coefficients (dm3 mol-1 cm-1) of Metalloporphyrin Derivatives
compound
Q1
Q2
Q3
Soret
Bpy-ZnTPP
596
(14 100) (22 900) (4340) (470 000)
556 517 424
(21 300) (3800) (474 000)
570 531 430
556
517
424
IR spectroelectrochemistry of [Re(CO)3(Pic)Bpy-ZnTPP]+
was carried out in PrCN at ambient temperature. On reduction,
the ν(CO) bands of the precursor at 2035 and 1933 cm-1 lost
intensity and new bands appeared at 2012 and 1902 cm-1; the
latter is very broad like the corresponding band of the precursor
(Figure 2). These two bands are assigned to the radical species
[Re+(CO)3(Pic)Bpy•--ZnTPP] and illustrate the characteristic
shift to low frequency (-23 cm-1 for the upper band and ca.
-31 cm-1 for the lower unresolved bands). The thin-layer cyclic
voltammogram recorded in the course of the experiment did
not reveal any significant substitution of picoline with PrCN,
which would otherwise shift the electrode potential more
negatively by ca. 100 mV (see Table 2). The changes in
absorbance (Figure 2) allow us to estimate that the molar
absorption coefficient of the high frequency band of the singly
reduced species (2012 cm-1) is approximately 57% that of the
corresponding band of the precursor (2035 cm-1). Subsequent
irreversible reduction of [Re+(CO)3(Pic)Bpy•--ZnTPP] resulted
in a mixture of two anionic species [Re(CO)3(PrCN)Bpy-
ZnTPP]- (major) and [Re(CO)3Bpy-ZnTPP]- (minor) char-
[Re(CO)3(Pic)Bpy-ZnTPP]+ 596
(9100)
614
(16 400) (19 300) (5200) (490 000)
[Re(CO)3(Pic)Bpy-MgTPP]+ 614
570 531 430
Bpy-MgTPP
(12 200) (13 800) (4500) (356 000)
The rhenium-appended complex shows only small changes in
these porphyrin-based bands. The rhenium-bpy MLCT band and
IL absorption are expected around 370 and 300 nm, respec-
tively.50,56 Although the transitions are masked by the porphyrin
transitions, the molar absorption coefficient of [Re(CO)3(Pic)-
Bpy-ZnTPP][OTf] is higher in this region than that of Bpy-
ZnTPP. The absorption spectra in different solvents were
investigated in an attempt to resolve the rhenium complex
transitions from those of the zinc porphyrin. No clear feature
assignable to either the IL or MLCT bands of the Re(CO)3-
(pic)(bpy) unit emerged upon substituting the solvent or by
coordination of N-methylimidazole to the zinc porphyrin. The
magnesium porphyrin analogues show a long wavelength shift
of all bands (Table 1, see Figure S4 in Supporting Information).
acterized by high-frequency bands at 1983 and 1948 cm-1
respectively, and overlapping broad low-frequency bands be-
tween 1900 and 1840 cm-1 63-65
,
.
IR Spectra. The ground-state infrared spectra of [Re(CO)3-
(Pic)Bpy-ZnTPP][OTf] and [Re(CO)3(Pic)Bpy-MgTPP][OTf]
display two bands at 2031 and 1927 cm-1 in THF. The a′′ and
a′(2) bands merge into one broad absorption at 1927 cm-1
reflecting the pseudo-C3V geometry around the transition metal.
Variation of the solvent from THF to butyronitrile or dichlo-
In agreement with cyclic voltammetry, the reduction of [Re-
(CO)3(Pic)Bpy-ZnTPP]+ in THF at room temperature is also
irreversible when monitored in situ by IR spectroscopy. The
parent complex (ν(CO) at 2033 and 1928 cm-1) converts to
the corresponding radical (ν(CO) at 2011, 1906, and 1892 cm-1
)
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4258 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

Charge Separation in Rhenium-Appended Porphyrin
A R T I C L E S
Table 2. Cyclic Voltammetry Data, E1/2/V vs Fc+/Fca
E1 2/(red)
/
E1 2/(red)
/
compound
E1 2(ox)
/
(bpy-centered)
(porphyrin-centered)
Zn(TPP)
Bpy-ZnTPP
Bpy-MgTPP
[Re(CO)3(Pic)Bpy-ZnTPP][OTf]
[Re(CO)3(Pic)Bpy-ZnTPP][OTf]/PrCN
[Re(CO)3(Pic)(bpy)][OTf]
[Re(CO)3(PrCN)(bpy)][OTf]
[Re(CO)3(THF)(bpy)][OTf]b
0.43
0.38
0.24
0.38
0.36
-1.84
-1.95
-1.76
-1.44b
-1.41
-1.47c
-1.58
-1.69d
-1.83
a Conditions: in THF unless otherwise stated containing [Bu4N][PF6] (10-1 mol dm-3), working electrode Pt disk, T ) 298 K. b Partly chemically irreversible.
c Reference 23. d Reference 63.
Figure 2. IR spectroelectrochemistry of [Re(CO)3(Pic)Bpy-ZnTPP][OTf]
Figure 3. Emission spectra (uncorrected) of Bpy-MgTPP (black line) and
in PrCN at room temperature, recorded with an OTTLE cell. Green
[Re(CO)3(Pic)Bpy-MgTPP][OTf] (red broken line) in MeCN, 560 nm
excitation.
spectrum: [Re(CO)3(Pic)Bpy-ZnTPP]+; blue spectrum [Re+(CO)3(Pic)-
Bpy•--ZnTPP]. This one-electron-reduced radical is stable under these
conditions.
The porphyrin fluorescence of Bpy-MgTPP is identical in
band position and intensity to that of MgTPP with two intense
peaks at 610 and 664 nm (612 and 665 nm corrected) in MeCN.7
The emission spectrum of [Re(CO)3(Pic)Bpy-MgTPP][OTf]
excited at 560 nm exhibits two sharp bands at the same
wavelengths as those for the rhenium-free analogue. However,
the intensity in MeCN is only 2.5% of the emission intensity
of Bpy-MgTPP (Figure 3). The excitation wavelength does not
influence the relative emission yield.
No evidence of the broad featureless rhenium MLCT emission
could be detected in any of the above experiments.48 Excitation
spectra also show features characteristic of the metalloporphyrins
only. Strikingly, at 77 K in a methanol-ethanol (4/5 v/v) glass,
the rhenium-appended porphyrin emits with similar intensity
to that of the rhenium-free analogue (72% at 560 nm excitation
and 92% at 600 nm excitation). Such dependence of quenching
behavior on the medium is characteristic of electron-transfer
processes (see Discussion).
Time-resolved Spectra. Picosecond and Nanosecond Vis-
ible Absorption Spectroscopy. Time-resolved visible absorp-
tion (TRVIS) spectroscopy was used to determine the spectra
and lifetimes of transient excited states and intermediates. The
transient lifetimes of the rhenium-appended porphyrins are
reduced dramatically compared to their rhenium-free counter-
parts, but their spectra are identical.
The picosecond TRVIS spectrum of Bpy-ZnTPP in PrCN
excited at 560 nm (Figure 4a) is very similar to those of ZnTPP
and MgTPP7,87 with the typical zinc porphyrin S1 features of a
strong broad absorption at 460 nm and stimulated emission at
610 and 660 nm. The transient decays negligibly on the time
scale of hundreds of picoseconds.
which gradually undergoes picoline substitution with solvent
molecules, producing the cation [Re(CO)3(THF)Bpy-ZnTPP]+
(ν(CO) at 2019, 1914, and 1897 cm-1). The latter is reduced at
a more negative potential than the parent cationic complex
(Table 2).
Steady-State Emission Spectroscopy. Zinc and magnesium
porphyrins emit strongly from the first singlet-excited state;
simple rhenium tricarbonyl bipyridine complexes are also
emissive but exhibit distinctly different spectra.7,48 As a result,
steady state emission spectroscopy is a valuable tool for probing
potential perturbations of the excited state of one part of the
supramolecule in the presence of the other. Solvent studies can
help elucidate the mechanism of emission quenching. If pho-
toinduced electron transfer occurs, a large dipole change ensues
and the solvent polarity is expected to have a major impact on
the quenching. In contrast, energy transfer is usually less
dependent on the solvent polarity. Quenching may also occur
through the heavy atom effect, but this does not involve a change
in dipole moment and consequently is unaffected by the solvent
polarity.
The porphyrin fluorescence of Bpy-ZnTPP was found to be
identical in band position and intensity to that of ZnTPP.7,48
The emission spectrum of [Re(CO)3(Pic)Bpy-ZnTPP][OTf]
exhibits two sharp bands at 602 and 650 nm (604 and 655 nm
when corrected for instrument response) like that of the rhenium-
free analogue. However, the emission quantum yield of [Re-
(CO)3(Pic)Bpy-ZnTPP][OTf] in toluene solution was reduced
to 15% of the emission intensity of Bpy-ZnTPP, further
decreasing to 6.6% in THF and 4.5% in PrCN as the solvent
polarity increased.
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4259

A R T I C L E S
Gabrielsson et al.
Table 3. Observed Lifetimes (τ) Determined by Fitting the TRVIS
Decay to a Single Exponential Function
complex
solvent
τ/ps
[Re(CO)3(Pic)Bpy-ZnTPP]+
[Re(CO)3(Pic)Bpy-ZnTPP]+
[Re(CO)3(Pic)Bpy-MgTPP]+
THF
PrCN
PrCN
53 ( 3
23.7 ( 0.2
16.7 ( 0.7
no bands in the region from 650 to 800 nm for Bpy-MTPP (M
) Mg, Zn) or for their rhenium complexes.91 The Bpy radical
anion also absorbs weakly in the 650-800 nm region but was
not detected.92,93
Nanosecond time-resolved spectroscopy of zinc porphyrins
probes the first triplet state, T1. As expected, this T1 state was
detected as an intense transient on excitation of Bpy-ZnTPP
in THF solution (excitation wavelength 532 nm, monitoring
wavelength 470 nm). In a corresponding experiment with [Re-
(CO)3(Pic)Bpy-ZnTPP][OTf], the transient had only 6% of the
intensity observed with Bpy-ZnTPP and a much longer lifetime.
The reduced yield accords with the steady-state emission and
the picosecond absorption experiments. The increased lifetime
may be associated with a reduction in the rate of triplet-triplet
annihilation because of the low concentration of triplet.
Picosecond Time-Resolved Infrared Spectroscopy. Tran-
sient visible absorption spectroscopy is only suited to study the
porphyrin part of the molecule due to the strong excited-state
absorption. Moreover, the S1 state absorption spectrum consists
of broad bands that are very similar to the absorption spectra
of both the T1 state and the radical cation, making it difficult to
distinguish a charge-separated state from singlet or triplet states.
Our compounds are designed to provide a spectroscopic handle
in the infrared through CO-stretching frequencies. Consequently,
picosecond time-resolved infrared (TRIR) spectroscopy should
be an ideal tool for probing charge or energy transfer.
Figure 4. Ultrafast TRVIS spectra excited at 560 nm. Spectra recorded at
-0.2, 1 ps, then every 4 ps to 41 ps, then 48, 58, 68, and 78 ps. (a) Bpy-
ZnTPP in PrCN, (b) [Re(CO)3(Pic)Bpy-ZnTPP][OTf] in PrCN.
Figure 5. Decay of the TRVIS transient monitored at 463 nm following
excitation of [Re(CO)3(Pic)Bpy-ZnTPP][OTf] at 560 nm. Circles, experi-
mental points; line, single-exponential fit. The inset shows an expansion
for the first picosecond.
The TRIR spectrum of [Re(CO)3(Pic)Bpy-MgTPP][OTf] in
MeCN was obtained by excitation into the Q-bands of the
porphyrin at 600 nm. It is characterized by two negative
absorption bands (bleaches), one sharp at 2033 cm-1 and one
broad at ∼1931 cm-1, due to depletion of the ground state
(Figure 6). Importantly, the negative absorption bands are
present immediately after excitation, i.e., formed on a subpi-
cosecond time scale. (They are at a maximum intensity already
in the first spectrum, delay of 1.5 ps.) The bleach recovers
The TRVIS spectra of [Re(CO)3(Pic)Bpy-MTPP][OTf] (M
) Mg, Zn) in PrCN strongly resemble those of Bpy-MTPP
with a broad absorption at 460 nm and stimulated emission at
610 nm (Figure 4b). In stark contrast to the rhenium-free
metalloporphyrins, the transient species derived from [Re(CO)3-
(Pic)Bpy-MTPP][OTf] in PrCN decay with lifetimes of 23.7
( 0.2 ps (M ) Zn) and 16.7 ( 0.7 ps (M ) Mg) leaving very
small residual absorption (Figure 4b and Figure 5). The signal
rises within 200 fs (see Figure 5 inset) and decays obeying good
first-order kinetics. The transient kinetics are strongly solvent
dependent, with faster decay in PrCN than in THF (Table 3).
The rate of decay of the transient from [Re(CO)3(Pic)Bpy-
MgTPP][OTf] is more than 2 orders of magnitude greater than
that of Bpy-MgTPP.
completely with a lifetime of 34 ( 4 ps (measured at 1928 cm-1
,
errors as 95% confidence limits, Figure 7a). In the spectrum
measured after a 1.5 ps delay, there are product bands (labeled
CS for charge-separated state; see Discussion) at 2008 and 1900
cm-1 and a positive shoulder overlapping the bleach at ca. 2028
cm-1. The CS bands lie close to the positions of the one-electron
reduced species detected by IR spectroelectrochemistry. The
features labeled CS are at their maximum on the initial spectrum
at 1.5 ps. They decay with a lifetime of 20 ( 2 ps (measured
at 1893 cm-1, Figure 7b) revealing after ca. 6 ps a pair of bands
(labeled HG for hot ground state; see Discussion) at 2023 and
1908 cm-1. The decay of this pair was fitted to a single
exponential function with a lifetime of approximately 35 ( 8
ps (measured at 2023 cm-1, Figure 7c) that is identical within
It is difficult to distinguish the spectrum of the S1 excited
state from that of the porphyrin radical cation in the region 450-
650 nm. However, radical cations of simple porphyrins exhibit
a characteristic feature at about 700 nm.21,88-90 We observed
(85) Matousek, P.; Towrie, M.; Ma, C.; Kwok, W. M.; Phillips, D.; Toner, W.
T.; Parker, A. W. J. Raman Spectrosc. 2001, 32, 983-988.
(86) Liard, D. J.; Busby, M.; Farrell, I. R.; Matousek, P.; Towrie, M.; Vlcˇek,
A., Jr. J. Phys. Chem. A 2004, 108, 2363-2369.
(87) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111, 6500-
6506.
(88) Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.; Bottomley, L. A. Inorg. Chem.
1981, 20, 1274-1277.
(89) Kadish, K. M.; Rhodes, R. K. Inorg. Chem. 1981, 20, 2961-2966.
(90) Kadish, K. M.; Shiue, L. R. Inorg. Chem. 1982, 21, 3623-3630.
(91) TRVIS spectra of [Re(CO)3(Br)Bpy-ZnTPP] excited at 400 nm show
transient absorption in the 650-750 nm region contrasting with [Re(CO)3-
(Pic)Bpy-ZnTPP][OTf].
(92) Krejcˇik, M.; Vlcˇek, A. A. J. Electroanal. Chem. 1991, 313, 243-257.
(93) Noble, B. C.; Peacock, R. D. Spectrochimica Acta 1990, 46A, 407-412.
9
4260 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

Charge Separation in Rhenium-Appended Porphyrin
A R T I C L E S
Figure 8. TRIR spectra of [Re(CO)3(Pic)Bpy-ZnTPP][OTf] in PrCN
following excitation at 560 nm. Time intervals are displayed in ps; the thick
line is at 1 ps. Arrows indicate the rise and decay of the transients. A function
has been subtracted from all spectra, corresponding to the baseline drift
measured at 2046 cm-1 where there is no significant absorption. Experi-
Figure 6. TRIR spectra of [Re(CO)3(Pic)Bpy-MgTPP][OTf] in MeCN
observed following 600 nm excitation. Time delays are displayed in
picoseconds, the thick line is at 1.5 ps. Arrows indicate rise and decay of
the transients. A function has been subtracted from all spectra, corresponding
to the baseline drift measured at 2046 cm-1 where there is no significant
mental points are separated by 4-5 cm-1
.
absorption. The experimental points are separated by 4-5 cm-1
.
Figure 7. Transient kinetics observed by IR spectroscopy following
600 nm excitation of [Re(CO)3(Pic)Bpy-MgTPP][OTf] in MeCN (a)
bleach recovery at 1928 cm-1, (b) decay at 1893 cm-1, (c) decay at 2023
cm-1. The lines show single-exponential fits to the experimental points
shown.
Figure 9. TRIR kinetics of [Re(CO)3(Pic)Bpy-ZnTPP][OTf] in PrCN
following 560 nm excitation. (a) Bleach at 1932 cm-1, (b) CS state at 2007
cm-1, (c) HG state at 2021 cm-1. The lines show single-exponential fits to
the points shown.
Table 4. Lifetimes of Transients Observed for
[Re(CO)3(Pic)Bpy-MgTPP][OTf] in MeCN (CS Charge-Separated
State, HG ) Hot Ground State, 95% Confidence Limits on Last
Digit in Brackets)
The TRIR spectrum of [Re(CO)3(Pic)Bpy-MgTPP][OTf]
was also recorded in PrCN; the more viscous solvent did not
affect the spectral shape or decay kinetics of the transient
significantly. Furthermore, the transient IR spectrum was
recorded with both 560 and 600 nm excitation corresponding
to the Q0,0 and Q0,1 transition in order to investigate the role of
excess vibrational energy of the excited state. The decay kinetics
and spectral shapes were found to be identical.
1
TRVIS
λ/nm
TRIR
ν
/cm-
transient
decay
bleach
bleach
CS
CS
decay
HG
HG
method
recovery
recovery
decay
decay
decay
probe
τ/ps
469
16.7 (7)
1928
34 (4)
2032
37(4)
1893 2008 1906 2023
20(2) 21(4) 43(8) 35(8)
The TRIR spectra of [Re(CO)3(Pic)Bpy-ZnTPP][OTf] in
PrCN show a bleach at 2031 and 1932 cm-1 formed within 1
ps of excitation (the first spectrum after the laser flash) that
grows to reach its maximum at about 10 ps and then decays
with an average lifetime of 55 ( 8 ps (Figures 8 and 9). A
product spike, labeled ππ*, is observed at 2026 cm-1 on the
lower energy side of the bleach immediately after excitation (2
ps); the low-frequency partner of this band is expected to
contribute to the broad maximum at 1906 cm-1. These bands
decay within 10 ps while a new band appears at 2007 cm-1
experimental error to that of the bleach recovery. Lifetime data
are collected in Table 4.94 The first product state (CS) shows
considerable shifts to lower frequency compared to the starting
material, consistent with charge transfer from porphyrin to
rhenium bipyridine (see below).
(94) Figure 5 shows a small positive spike at 2028 cm-1 that overlaps with the
high-frequency bleach of the parent molecule. This feature may represent
the ππ* state that precedes formation of the CS state as is observed for
[Re(CO)3(Pic)Bpy-ZnTPP][OTf]. However, there are too few data points
to be sure that it is significant. The spike is present for the first 6 ps.
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4261

A R T I C L E S
Gabrielsson et al.
Table 5. Lifetimes of Transients Observed for
[Re(CO)3(Pic)Bpy-ZnTPP][OTf] in MeCN (CS Charge-Separated
State, HG ) Hot Ground State, 95% Confidence Limits on Last
Digit in Brackets)
Table 6. Electrochemical Data in THF vs Fc/Fc+, Low
Temperature Emission Data in MeOH/EtOH 4/5 Glass at 77 K,a
and Derived Excited State Oxidation Potential for Bpy-MTPP
/
/
porphyrin
E
ox/V
E00(S1)/eV
Eox/Vb
1
TRVIS
λ/ nm
TRIR ν
/cm-
Bpy-ZnTPP
Bpy-MgTPP
0.38
0.24
2.06
2.00
-1.68
-1.76
transient
decay
bleach
bleach
ππ
decay
*
CS
HG
method
recovery
recovery
decay
decay
a We use measurements made at 77 K in order to minimize the effect of
thermal excitation,7 although the differences from the room-temperature
probe
τ/ps
463
23.7 (2)
2031
58 (7)
1932
52 (9)
2026
ca. 5
2007
40 (4)
2021
45 (8)
/
measurements are negligible. Eox is given by Eox - E/ .
b
00
and the low-frequency maximum shifts to ∼1896 cm-1. The
new bands, labeled CS, reach their maximum at about 10 ps.
The decay constant of the band at 2007 cm-1 is 40 ( 4 ps.
Product CS gives way to a third product species by 40 ps after
excitation with maxima shifted to 2021 and 1905 cm-1; the
decay of this product, labeled HG, is nearly complete at 100
ps. Transient kinetics are collected in Table 5.
estimate of the free energy change of electron transfer can be
obtained from the porphyrin emission energy in combination
with the electrochemical data. Equations 1 and 2 describe the
free energy expressions for oxidative and reductive electron
transfer, respectively.96 The correction term for Coulombic
charge stabilization is omitted because the charge distribution
is not well defined. Equation 1a shows the charge-transfer pro-
cess from metalloporphyrin to rhenium bipyridine. The corre-
Discussion
sponding free energy is shown in eq 1b, where E° (MTPP) and
ox
E° (ReBpy) are the standard potentials for oxidation of the
red
Ground-State Spectroscopy, Emission Quenching Mech-
anism, and Energetics of Electron Transfer. The rhenium-
appended metalloporphyrin complexes [Re(CO)3(Pic)Bpy-
MTPP][OTf] (M ) Zn, Mg) exhibit ground-state IR and UV/
vis absorption spectra characteristic of the rhenium carbonyl
moiety and the metalloporphyrin moiety, respectively. No new
transition unique to the assembly is detectable, even at
wavelengths exceeding 500 nm where the spectrum is not
congested. The photophysical measurements were conducted
in nitrile solvents which dissolve the compounds very well. We
anticipate that the nitrile coordinates to the zinc and magnesium
in the axial positions to form five- or six-coordinated metal-
loporphyrins. The coordination prevents aggregation at the
concentrations that are used in the photophysical experiments.
The emission spectra of [Re(CO)3(Pic)Bpy-MTPP][OTf] show
the two characteristic porphyrin bands at about 600 nm, but
their intensities are reduced drastically (by >95% in polar
solvents) relative to the bipyridine-appended porphyrins Bpy-
MTPP. For cationic rhenium bipyridine complexes, a broad
MLCT emission band is normally observed at ca. 550 nm,48
but no such emission could be detected for [Re(CO)3(Pic)Bpy-
MTPP][OTf].
donor (metalloporphyrin) and reduction of the acceptor (rhenium
bipyridine) and F is the Faraday; E/00 is the emission energy of
the porphyrin in eV which approximates to the energy of its
excited state.
*[Re+(CO)3(Pic)Bpy-MTPP] f
[Re+(CO)3(Pic)Bpy-•-MTPP+•] (1a)
∆G° ) FE° (MTPP) - FE° (ReBpy) - E/00 (1b)
ox
ox
red
Equation 2a shows the charge-transfer process from rhenium
bipyridine to metalloporphyrin. The corresponding free energy
is shown in eq 2b, where E° (ReBpy) and E° (MTPP) are the
ox
red
standard potentials for oxidation of the donor (rhenium bipy-
ridine) and reduction of the acceptor (metalloporphyrin); E/00 is
unchanged.
*[Re+(CO)3(Pic)Bpy-MTPP] f
[Re2+(CO)3(Pic)Bpy-MTPP-•] (2a)
∆G° ) FE° (MTPP) - FE° (ReBpy) + E/00 (2b)
red
red
ox
Although the steady-state emission spectra showed no
contribution assigned to the rhenium bipyridine subunit, it was
evident that the presence of rhenium affects the fluorescence
intensity and therefore the lifetime of the first excited singlet
state, S1 significantly. For supramolecular assemblies involving
porphyrins, emission quenching is usually ascribed to energy
transfer or electron transfer.1,7 Given the strong spin-orbit
coupling of the rhenium atom, enhanced intersystem crossing
must also be considered as a possible reason for the reduction
in emission intensity. Since the emission of the porphyrin lies
to long wavelengths of the emission of related cationic rhenium
complexes, energy transfer would be an endergonic process and
can be ruled out. The heavy atom effect that might lead to a
reduction in lifetime via spin-orbit coupling can also be
excluded since the free-base analogue [Re(CO)3(Pic)Bpy-
H2TPP][OTf] showed negligible emission quenching.95
To analyze the possibility of photoinduced electron transfer,
it is essential to consider the energetics of electron transfer. An
We use the data in Table 2 for [Re(CO)3(Pic)Bpy-ZnTPP]-
[OTf]. Table 6 shows oxidation potentials and low-temperature
emission data for Bpy-ZnTPP and Bpy-MgTPP together with
the derived excited-state oxidation potential. For the rhenium
moiety, we employed [Re(CO)3(Pic)(bpy)]+ as a model which
has an oxidation potential of 1.41 V.97
We consider four processes: (a) oxidative quenching of the
S1 state by electron transfer from the porphyrin to the Re(bpy)
moiety; (b) reductive quenching from Re(bpy) to the porphyrin
S1 state; (c) oxidative quenching from the porphyrin T1 state to
Re(bpy); (d) reductive quenching from the porphyrin T1 state
to Re(bpy). Processes b, c, and d may be excluded since they
are calculated to be strongly endergonic. For instance, ∆G° is
calculated as +1.2 eV for process b in both [Re(CO)3(Pic)-
Bpy-MgTPP][OTf] and its zinc analogue. In contrast, process
a is calculated to be an exergonic process with ∆G° ) -0.32
(96) Rehm, D.; Weller, A. Isr. J. Chem 1970, 8, 259-271.
(97) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; Degraff,
B. A. Inorg. Chem. 1990, 29, 4335-4340.
(95) Gabrielsson, A. Ph.D. Thesis, University of York, York, U.K., 2002.
9
4262 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

Charge Separation in Rhenium-Appended Porphyrin
A R T I C L E S
eV for [Re(CO)3(Pic)Bpy-MgTPP][OTf] and -0.24 eV for
its Zn analogue. Since, the chemical irreversibility of the
reduction process makes an accurate measurement of the
reduction potential impossible, these values should be considered
approximate. In conclusion, energetic arguments support electron
transfer from metalloporphyrin to rhenium bipyridine as the most
likely quenching mechanism. The reduced quenching in a low
temperature glass also suggests an electron transfer mechanism
since electron transfer processes are usually suppressed in rigid
media.
Picosecond Spectroscopy and Photochemical Mechanism.
Transient Visible Absorption Spectra. The picosecond TRVIS
spectra of Bpy-MgTPP and Bpy-ZnTPP show typical mag-
nesium porphyrin S1 features. In PrCN, the S1 excited states of
Bpy-MTPP (M ) Mg, Zn) decay on a longer time scale than
the instrument is capable of measuring. It is reasonable to
assume that the lifetime of Bpy-MTPP is similar to that of
MTPP, viz. ca. 3 ns in polar solvents.7
The presence of the rhenium moiety in [Re(CO)3(Pic)Bpy-
MTPP][OTf] affects the porphyrin TRVIS kinetics drastically,
and the lifetime of the transient is reduced to 23.7 ( 0.2 and
16.7 ( 0.7 ps for M ) Zn and Mg, respectively (PrCN solvent).
The transient spectrum is affected very little by the rhenium
moiety, and product bands at very long wavelengths charac-
teristic of a radical cation are absent. If the spectrum arises from
a CS state rather than an S1 state, there should also be changes
in the absorption coefficient (and hence intensity) in the 470-
nm region. We employ the absorption coefficients to obtain
additional evidence to distinguish whether the signal arises from
the S1 or the CS species. The change upon excitation in the
molar absorption coefficient, ∆ꢀ, in the S1 state of ZnTPP has
been estimated as ca. 7.1 × 104 dm3 mol-1 s-1 at 470 nm.98
The corresponding value for the oxidation of Bpy-ZnTPP was
estimated from our spectroelectrochemical data to lie in the
range ∆ꢀ ) 1.8 × 104 to 3.4 × 104 dm3 mol-1 s-1 (see above).
The maximum value is about half that of the S1 state. The
spectroelectrochemical data also show that ∆ꢀ at 650 nm is ca.
60% of ∆ꢀ at 470 nm. When we compare the picosecond spectra
of Bpy-ZnTPP and [Re(CO)3(Pic)Bpy-MTPP][OTf], we see
a loss of ca. 30% in intensity but no change in band shape. If
the charge-shifted state were the only species present on
irradiation of [Re(CO)3(Pic)Bpy-MTPP][OTf], the reduction
in intensity should be far greater and the cation features in the
region 650-750 nm should be detectable. We infer from these
considerations that the ultrafast visible spectra are dominated
by excited state(s) of [Re(CO)3(Pic)Bpy-MTPP][OTf] of S1-
(ππ*) type.
Transient IR Spectra. Transient IR spectra of Re(CO)3(bpy)
complexes typically show a shift of the ν(CO) bands to high
frequency because of population of the MLCT excited
state68,73,74,99,100 that creates a positive charge at the metal center,
reducing the Re-to-CO π-back-bonding and increasing ν(CO).
Intraligand excitation is expected to result in only small negative
shifts in ν(CO). For instance, Dattelbaum et al.73 have observed
shifts of -5 and -10 cm-1 for [Re(CO)3(4-Et-py)(Me2-dppz)]+
(dppz ) dipyridophenazine). Comparable shifts were observed
in other examples such as [Re(CO)3(PPh3)(dppz)]+ (average shift
-8 cm-1), [Re(CO)3(4-styrylpyridine)(bpy)]+ (shift -7 cm-1),
and [Re(CO)3(py)(11,12-dichloro-dppz)]+ (shifts -8 and -14
cm-1).67,72,101,102 In contrast, electron transfer from the metal-
loporphyrin to Re(CO)3(bpy) would be expected to cause a low-
frequency shift characteristic of increased Re-to-CO π-back-
bonding. TRIR spectra of such reduced photoproducts are
expected to be similar to IR spectra of electrochemically
generated [Re(CO)3(L)(bpy•-)] complexes.63-65
A. TRIR Spectra of Magnesium Porphyrin. Picosecond
transient infrared spectroscopy of [Re(CO)3(Pic)Bpy-MgTPP]-
[OTf] in MeCN provides an unequivocal demonstration of an
ultrafast charge transfer, since (a) the maximum bleach is
detected at the first measurement after the flash (1.5 ps) and
(b) the product ν(CO) bands lie considerably to low wavenum-
bers of the precursor. It should be stressed that the rhenium
moiety does not absorb at the excitation wavelengths (560 or
600 nm) and therefore Re(bpy) excited states cannot be
populated directly. Likewise, two-photon absorption was ex-
cluded by attenuating the excitation power. The signal intensity
was found to decrease linearly with the pump energy, rather
than the square of pump energy as would be expected for two-
photon absorption.
The product bands labeled CS are shifted to lower frequency
with respect to the precursor and decay with a lifetime of 20 (
2 ps. The shifts are -24 cm-1 for the high-frequency band and
-35 cm-1 for the low-frequency band. To test whether the CS
state contains a reduced [Re+(CO)3(Pic)Bpy•-] unit, we
compare the TRIR spectra with the spectrum of electrochemi-
cally generated [Re(CO)3(Pic)Bpy-ZnTPP]. For quantitative
comparison, the shift between the cation and radical forms of
electrochemically reduced [Re(CO)3(Pic)Bpy-ZnTPP][OTf] is
-23 cm-1 for the high frequency and -31 cm-1 for the low-
frequency band. Thus the bands labeled as CS are consistent
with a charge-separated excited state in which the Re(bpy) unit
has been converted from cation to radical and the Mg(TPP) from
neutral to radical cation, [Re+(CO)3(Pic)Bpy•--MgTPP•+]. The
spectroelectrochemical data were used to estimate that the
absorption coefficient of the high frequency band of the radical
anion is about 57% that of the parent. If the same applies to the
CS state, we can compare the intensities of the high frequency
bands of the bleach and the CS state at early times in order to
estimate the proportion of the product in the CS state. This
method indicates that at least half the product detected by TRIR
spectroscopy is in the CS state at the early times.
The description of the product state as charge-separated (CS)
represents a simplification that neglects the coupling between
the components of the molecule. Alternative descriptors are
redox-separated (RS)73 or a shift of positive charge from the
Re(bpy) unit to the metalloporphyrin unit. The minimum
distance between donor and acceptor centers may be taken as
the edge-to-edge separation of the porphyrin and bpy units,
which is seven CC or CN bonds.
Further interpretation is assisted by combining the information
from TRIR and TRVIS spectra. The TRVIS spectra decay (τ
) 16.7 ( 0.7 ps) at a rate that is close to the decay rate of CS
bands (τ ) 20 ( 2 ps) but faster than the bleach recovery in
the TRIR spectra (τ ≈ 35 ps). However, a further product (HG)
(99) Dattelbaum, D. M.; Meyer, T. J. J. Phys. Chem. A 2002, 106, 4519-4524.
(100) Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin, R. L.;
Meyer, T. J. Inorg. Chem. 2002, 41, 6071-6079.
(101) Schoonover, J. R.; Strouse, G. F.; Dyer, R. B.; Bates, W. D.; Chen, P.
Y.; Meyer, T. J. Inorg. Chem. 1996, 35, 273-274.
(102) Busby, M.; Matousek, P.; Towrie, M.; Vlcˇek, A. J. Phys. Chem. A 2005,
109, 3000-3008.
(98) Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1281-1291.
9
J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4263

A R T I C L E S
Gabrielsson et al.
Scheme 2. Photochemical Pathways for
is observed in the TRIR spectra that decays with a lifetime of
ca. 39 ps. This species has no counterpart in the TRVIS spectra,
suggesting that it does not correspond to an electronically excited
state. It is not possible to obtain accurate decay kinetics for the
HG state from the TRIR spectra because the bands of the two
products overlap, but the lifetime of HG is similar to that of
the bleach recovery. The HG state has broad bands that are still
shifted to low wavenumbers but are considerably closer to those
of the ground state. The kinetic and spectroscopic evidence
strongly suggests that the HG state is the intermediate between
the CS state and the bleach recovery. It is assigned to a Hot
Ground state, i.e., a vibrationally excited state.
[Re(CO)3(Pic)Bpy-MTPP][OTf] (lifetimes M ) Mg, red; M) Zn,
black)a
B. TRIR Spectra of Zinc Porphyrin. The behavior of the
Zn complex, [Re(CO)3(Pic)Bpy-ZnTPP][OTf], is qualitatively
the same as that of the magnesium complex but quantitatiVely
different. In the TRIR spectra, a spike that overlaps with the
high frequency bleach of the parent is observed for the first
five picoseconds (Figure 8). It is provisionally assigned as an
initial excited state, ππ*, of the parent but may represent only
a reactive conformer of this state (see below). The bleach and
the CS state reach a maximum only after 5 ps. The kinetics of
the CS state could not be obtained as accurately as for the
magnesium complex because the problem of overlap with the
HG state was more severe. The slower rise of the CS state may
be associated with the reduced free energy of charge separation
for the zinc complex.
a The times indicate measured lifetimes with 95% confidence limits.
for the decay of the CS state may correspond to the combination
of rate constants for back-electron transfer of different conform-
ers. Rate constants for back electron transfer for individual
conformers may be appreciably faster than the measured rate
constant. Detailed kinetic modeling will await studies on related
systems.
In an alternative model (fast back electron transfer) it is
postulated that the kinetics observed by transient visible
spectroscopy correspond to the charge-separation process, but
the back electron transfer to HG is considerably faster. Simula-
tion of this model indicates that the concentration of the CS
state would be no more than 5% of the S1 state, if the CS state
is to reach its peak 1 ps after the flash (M ) Mg) or 5 ps after
the flash (M ) Zn). This figure is inconsistent with the estimate
above that the proportion of CS state contributing at early times
corresponds to at least 50% of the bleach of the TRIR spectrum.
Since, we could only reconcile this model with the data if a
large proportion of the molecules in the excited state make no
contribution to the TRIR spectrum, we favor the multiple
conformation model. Two other models are summarized in the
footnotes.103
Therien et al. have studied the formation of charge-separated
states of zinc porphyrins linked to quinones in supramolecular
assemblies by both transient absorption and transient IR
spectroscopy with subpicosecond time resolution. They have
observed that charge separation occurs in the time range of 20
fs to 3 ps according to the donor-acceptor distance.20-22 They
have also used TRIR spectroscopy to monitor variations in the
mixing of the donor and acceptor states. Therien et al. also
Mechanistic Conclusions. The conclusions from this analysis
can be summarized as follows. (a) The transient UV/vis spectra
are dominated by excited states of S1 (ππ*) type, but the clean
first-order kinetics belie considerable complexity that is revealed
by the transient IR spectra. (b) The transient IR spectra inform
us that there is a substantial proportion of CS state from the
earliest time delays measured. The CS state is at its maximum
for the magnesium complex with a delay of 1 ps, while the CS
state maximizes at ca. 5 ps for the zinc complex. (c) The CS
state decays initially to vibrationally excited levels of the
electronic ground state. (d) The deductions from the TRVIS
spectra can be reconciled with the TRIR data if there is
branching between two pathways, such that there are substantial
proportions of both the S1 state and the CS state populated within
1 (M ) Mg) or 5 (M ) Zn) ps. The latter accounts for at least
50% of promptly formed photoproducts. The photochemical
observations derived above for [Re(CO)3(Pic)Bpy-MTPP]-
[OTf] (M ) Mg, Zn) are summarized in Scheme 2.
We have considered several kinetic models that could account
for the data. The most satisfactory model (multiple conforma-
tions) postulates that the rates of forward and back electron-
transfer vary with the torsional angles between the porphyrin
plane and the C6H4 plane and that several conformers with
different torsional angles are present; twisted conformations will
be more populated, but there is restricted overlap between the
bridge and the porphyrin π-system that reduces the rate of
electron transfer when compared to conformers that are close
to coplanar. Optical excitation of the reactive (coplanar)
conformer A yields the excited-state A* which is responsible
for the subpicosecond formation of the CS state (Scheme 3).
The apparent rate constant of the S1 decay corresponds to the
combination of the rate constants of conformational changes
and/or of charge separation in less favorable conformations B*,
C*,... of the excited state. Similarly, the apparent rate constant
(103) We have also considered two further models. (a) Ultrafast Equilibrium:
excitation leads to an ultrafast equilibrium between the CS and S1 states,
implying that the CS and S1 states are much closer in energy than the
electrochemical calculations indicate. (b) Delocalized CS State: the CS
state branches from the S1 precursor state on a subpicosecond time scale
for M ) Mg and 5 ps time scale for M ) Zn and is the major species
during the first 5 ps; it decays in 20 ps (Mg) or 40 s (Zn) by back electron
transfer. This model also requires a channel for conversion of the residual
S1 state into CS either by the conformational mechanism or by an ultrafast
equilibrium. It differs from these mechanisms in containing an additional
postulate, namely that the UV/vis spectrum of the CS state resembles
that of the S1 state because it is highly delocalized at least in its own
upper state. As a result the contributions to the TRVIS spectrum from
CS and the S1 state resemble one another. This idea chimes with Therien’s
proposal for enhanced delocalization of the CS state compared to the S1
state.104
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4264 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006

Charge Separation in Rhenium-Appended Porphyrin
A R T I C L E S
Scheme 3. Model for Formation of CS State from Several Conformers of Parent Complex; the Observed Kinetic Behavior Corresponds to
the Sum of the Contributions from the Various Conformers
studied porphyrins linked to diimines via an ethynyl bridge and
proposed that charge recombination is faster than forward
electron transfer.104 This analysis includes a demonstration that
different conformations have a small effect on the energetics
of electron transfer. One of the closest examples to [Re(CO)3-
(Pic)Bpy-MTPP][OTf] reports photophysical data for zinc and
magnesium porphyrins linked via an ethynyl bridge to square
planar platinum centers.33 As in our own examples, these
complexes show extensive quenching of the porphyrin-based
excited state but no sign of the signature of the radical cation
in the TRVIS spectrum. The authors suggest that back electron
transfer is faster than forward electron transfer but do not have
the benefit of TRIR spectroscopy to test their proposal.
product (HG) is thus in accordance with the surprisingly fast
back reaction which exceeds the rate of vibrational cooling of
the ground state. We are not aware of a previous observation
of a hot ground state in an electron-transfer reaction in the
Marcus inverted region that could be used to test the theory.
Mechanism of Photosubstitution at Remote Site in the
Presence of NEt3. In this section, we demonstrate how the
photoreactions that we observed previously may be reconciled
with the results reported here. We showed earlier that long
wavelength irradiation (λ > 450 nm) of [Re(CO)3(Pic)Bpy-
ZnTPP][OTf] in the presence of halide ions leads to replace-
ment of coordinated picoline by halide. Photolysis in the absence
of halide leads to substitution of picoline by the solvent (THF).
If excess 3-picoline is added in addition to Et3N, the 1e-reduced
species [Re+(CO)3(Pic)Bpy•--ZnTPP] is formed.23 Similar
observations have been made for [Re+(CO)3(Pic)Bpy•--
MgTPP].95 The experiments reported here show that the IR
bands of the CS state rise and decay in the 1-40 ps time domain.
(Because of the rapid quenching of the singlet state, triplet
photochemistry is not significant.) It follows that any electron
transfer from triethylamine to the metalloporphyrin must be
complete within the lifetime of the CS state, implying that the
Et3N donor must be coordinated to the Mg or Zn prior to
excitation. Coordination of nitrogen to zinc porphyrins to form
5- and 6-coordinate complexes is well established.113,114 The
energetics and dynamics of coordination are also well docu-
mented.115 Following photoinduced electron transfer from the
metalloporhyrin to the rhenium bipyridine center, irreversible
electron transfer from the coordinated triethylamine to the
metalloporphyrin can fill the hole at the porphyrin and the
oxidized triethylamine will decoordinate. This process results
in a long-lived radical, [Re+(CO)3(Pic)Bpy•--MTPP], that is
stable in the presence of excess 3-picoline but undergoes
substitution of 3-picoline by halide or solvent in its absence.
Back-Electron Transfer and Formation of the Hot Ground
State. We conclude above that back electron-transfer results in
formation of a hot ground (HG) state prior to return to the initial
state. Here we set this hypothesis in context. Photodissociation
of metal carbonyls can result in initial formation of metal
carbonyl intermediates in their ground electronic state but
excited vibrational states. Two distinguishable situations have
been observed: (a) population of the V ) 1 ν(CO) level
manifested by separate bands for the ν(CO) 0f1 and 1f2
transitions, the latter shifted by ca. -15 cm-1 105
(b) excitation
;
of low frequency vibrational modes that are anharmonically
coupled to ν(CO) modes, resulting in a downshift of all ν(CO)
modes. In this situation, the hot bands are not well resolved,
and their population decreases as the vibrational levels relax
resulting in a high-frequency shift.106 Vibrational spectra of
excited states of metal carbonyls have also revealed downshifts
in ν(CO) due to population of coupled low-frequency modes
similar to that summarized in (b).102,107 In [Re(CO)3(Pic)Bpy-
MTPP][OTf], there is sufficient excess energy to populate such
hot ground states after photoinduced electron transfer, and our
metal carbonyl system is well adapted to observe this effect
(free energies for back-electron transfer are calculated as -1.68
and -1.76 eV for Mg and Zn, respectively; see Table 6).
The back-electron transfer in [Re(CO)3(Pic)Bpy-MTPP]-
[OTf] takes place in the Marcus inverted region where the
driving force exceeds the reorganization energy. Theory predicts
that population of higher vibrational levels, as postulated in the
formation of the HG state, will speed up a reaction in the Marcus
inverted region. The electron transfer populates vibrations which
accept some of the excess free energy released during the
reaction, effectively reducing the barriers and increasing the
rate.108-112 The spectroscopic detection of the vibrationally hot
Conclusions
In this paper, we have reported the synthesis and photophysics
of magnesium and zinc porphyrins covalently linked via an
amide bond to a rhenium(bipyridine)tricarbonyl unit. The design
of the assembly allows for probing of the porphyrin moiety via
absorption and emission spectroscopy and the rhenium bipyri-
dine moiety via IR spectroscopy. The conjugated bridge and
(109) Sutin, N.; Creutz, C.; Fujita, E. Comments Inorg. Chem. 1997, 19, 67-
92.
(110) Jortner, J.; Bixon, M. J. Chem. Phys. 1988, 88, 167-170.
(111) Omberg, K. M.; Chen, P. Y.; Meyer, T. J. AdV. Chem. Phys. 1999, 106,
553-570.
(112) Bixon, M.; Jortner, J. AdV. Chem. Phys. 1999, 106, 35-202.
(113) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin), 1987; Vol. 64.
(114) Senge, M. O.; Kalisch, W. W. Inorg. Chem. 1997, 36, 6103-6116.
(115) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.; Hawley,
J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 3.
(104) Redmore, N. P.; Rubtsov, I. V.; Therien, M. J. J. Am. Chem. Soc. 2003,
125, 8769-8778.
(105) Dougherty, T. P.; Heilweil, E. J. J. Chem. Phys. 1994, 100, 4006-4009.
(106) Lian, T. Q.; Bromberg, S. E.; Asplund, M. C.; Yang, H.; Harris, C. B. J.
Phys. Chem. 1996, 100, 11994-12001.
(107) Liard, D. J.; Busby, M.; Matousek, P.; Towrie, M.; Vlcˇek, A., Jr. J. Phys.
Chem. A 2004, 108, 2363-2369.
(108) Brunschwig, B. S.; Sutin, N. Comments Inorg. Chem. 1987, 6, 209-235.
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J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006 4265

A R T I C L E S
Gabrielsson et al.
the electron accepting properties of the [Re+(CO)3(Pic)Bpy]
unit make this design well adapted for fast electron transfer.
Although there is no discernible interaction between the
rhenium(bipyridine)tricarbonyl and the metalloporphyrin moi-
eties in the ground electronic state, the systems exhibit remark-
able photophysical and photochemical properties.
absorption and (b) electron transfer from the metalloporphyrin
to the bipyridine is followed rapidly by irreversible electron
transfer from the triethylamine to the metalloporphyrin.
The experiments graphically demonstrate benefits of the
incorporation of carbonyl ligands on the electron acceptor, since
the sequence of charge separation and back electron transfer
can only be identified correctly with the aid of the time-resolved
IR data. They emphasize the utility of the [Re+(CO)3(Pic)Bpy]
unit as an electron acceptor and the rapidity of photoinduced
electron-transfer processes even in quite large and complex
molecules.
When the assembly is excited at long wavelengths in a region
where all absorption relates to the porphyrin, electron transfer
from porphyrin to rhenium bipyridine occurs on an ultrafast
time scale. In the case of the magnesium porphyrin, the IR bands
of the charge-separated excited state are observed after 1.5 ps
and decay in ca. 20 ps. For the zinc analogue, the corresponding
rise takes ca. 5 ps and the decay 40 ps. The charge-separated
excited state may be approximated by the description [Re+-
(CO)3(Pic)Bpy--MTPP•+]. A further state is identified in the
IR spectra at a stage when the transient absorption signal has
disappeared. It is characterized by a reduced low-frequency shift
of the ν(CO) bands and is assigned to a species that remains
vibrationally excited but is in the ground electronic state. The
assembly returns to its initial (vibrational and electronic ground
state) with a time constant of 35-55 ps. This rapid charge
separation and back electron transfer process provides a mech-
anism for quenching of the porphyrin emission that is solvent
dependent.
Acknowledgment. We acknowledge support from COST D14
and D35, EPSRC, CCLRC, and the University of York.
Furthermore, we thank Mr. Taasje Mahabiersing and Mr.
Michiel Groeneveld (University of Amsterdam) for their help
with the TRVIS experiments. We also acknowledge Dr. Trevor
Dransfield of the University of York mass spectrometry service
and the EPSRC National Mass Spectrometry Service Centre in
Swansea. We appreciated the helpful comments of the referees
and of Dr. John Moore.
Supporting Information Available: Synthesis of Bpy-
1
ZnTPP; Table S1, H NMR data for Bpy-ZnTPP and Bpy-
1
MgTPP; Figures S1-S3, H NMR spectra of [Re(CO)3(Pic)-
The system is reversible in the absence of an added electron
donor but undergoes irreversible reaction at the reduced rhenium
bipyridine center in the presence of added triethylamine. The
observation of productive reaction at the rhenium site on
irradiation into the absorption of the metalloporphyrin site is
compatible with ultrafast back electron transfer provided that
(a) triethylamine coordinates to zinc or magnesium prior to
Bpy-MTPP][OTf] (M ) Mg, Zn); Figure S4, Electronic
absorption spectra of Bpy-MgTPP and [Re(CO)3(Pic)Bpy-
MgTPP][OTf]. Figure S5, UV/vis spectroelectrochemistry of
Bpy-ZnTPP. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0539802
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4266 J. AM. CHEM. SOC. VOL. 128, NO. 13, 2006