Ethyne-bridged (porphinato)zinc(II)-(porphinato)iron(III) complexes: Phenomenological dependence of excited-state dynamics upon (porphinato)iron electronic structure

Article abstract of DOI:10.1021/ja061388m

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy) phenyl)porphinato]zinc(II)-[5′-[(10′,20′- di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chl

Full text of DOI:10.1021/ja061388m

Published on Web 07/20/2006
Ethyne-Bridged (Porphinato)Zinc(II)-(Porphinato)Iron(III)
Complexes: Phenomenological Dependence of Excited-State
Dynamics upon (Porphinato)Iron Electronic Structure
Timothy V. Duncan, Sophia P. Wu, and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received February 28, 2006; E-mail: therien@sas.upenn.edu
Abstract: We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical
studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)-[5′-[(10′,20′- di-((4-ethyl
ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn-PFe-Cl), along with a series of related
supermolecules ([PZn-PFe-(L)1,2
) pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)).
Relevant monomeric [(porphinato)iron-(ligand)1,2
+ ([PFe(L)1,2]+) benchmarks have also been synthesized
]
+ species) that possess a range of metal axial ligation environments (L
]
and fully characterized. Ultrafast pump-probe transient absorption spectroscopic experiments that
+
interrogate the initially prepared electronically excited states of [PFe(L)1,2
]
species bearing nonhindered
axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state.
Comparative pump-probe transient absorption experiments that interrogate the initially prepared excited
states of PZn-PFe-Cl, [PZn-PFe-(py)2]+, [PZn-PFe-(4-CN-py)2]+, [PZn-PFe-(collidine)]+, and [PZn-
PFe-(2,6-lutidine)]+ demonstrate that the spectra of all these species are dominated by a broad, intense
NIR S1 f Sn transient absorption manifold. While PZn-PFe-Cl*, [PZn-PFe-(py)2]+*, and [PZn-PFe-(4-
CN-py)2]+* evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared
electronically excited states to the ground state, the excited-state dynamics observed for [PZn-PFe-(2,6-
lutidine)]+* and [PZn-PFe-(collidine)]+* show fast relaxation to a [PZn+-PFe(II)] charge-separated state
having a lifetime of nearly 1 ns. Potentiometric data indicate that while ∆GCS for [PZn-PFe-(L)1,2]+* species
is strongly influenced by the PFe+ ligation state [ligand (∆GCS): 4-cyanopyridine (-0.79 eV) < pyridine
(-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV); solvent ) CH2Cl2], the pump-probe transient
absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not
correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the
ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate
PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large
magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus
make electron transfer a competitive excited-state deactivation pathway.
Introduction
exploit biologically inspired chromophoric building blocks.
Strong interpigment coupling is a common feature of such
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of energy transduction and for the eventual design and develop-
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9
10.1021/ja061388m CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 10423-10435
10423

A R T I C L E S
Duncan et al.
to use. Absolute ethanol (Fisher Scientific), as well as NMR solvents,
was used as received. ZnCl2 was dried by heating under vacuum and
stored under N2. The catalysts, tetrakis(triphenylphosphine)palladium
(Pd(PPh3)4), tris(dibenzylideneacetone)dipalladium (Pd2dba3), and triph-
enylarsine (AsPh3) were purchased from Strem Chemicals.
on interrogating the photophysics of covalently linked as-
semblies of porphyrins and related polypyrrolic macrocycles
where chromophore-chromophore electronic interactions give
rise to extensively delocalized electronically excited
states.12,13,35,36,43-62
Chemical shifts for 1H NMR spectra are reported relative to residual
protium in the deuterated solvents (CDCl3, δ ) 7.24 ppm; pyridine-d5,
δ ) 7.19 ppm, C6D6, δ ) 7.15 ppm). The number of attached protons
is found in parentheses following the chemical shift value. Flash column
chromatography was performed on the benchtop, using silica gel (230-
400 mesh) obtained from EM Science. High-resolution electrospray
mass spectra were acquired at the Mass Spectrometry Center at the
University of Pennsylvania. MALDI-TOF mass spectroscopic data were
obtained with a Perspective Voyager DE instrument in the Laboratory
of Dr. Virgil Percec (Department of Chemistry, University of Penn-
sylvania). Samples were prepared as micromolar solutions in THF, and
dithranol (Aldrich) was utilized as the matrix. Fast atom bombardment
(FAB) mass spectrometry analyses were performed at the Mass
Spectrometry Center at Drexel University.
While considerable progress has been made with respect to
understanding the fundamental excited-state dynamics of strongly
coupled porphyrinoid compounds,35,49,62 little is known regarding
the corresponding excited-state delocalization-, electron-, and
energy-transfer dynamics of related systems that possess open
d-electron shells. Here, we report the synthesis, spectroscopy,
and potentiometric and excited-state dynamical studies of
5-[(10,20-di-(((4-ethyl ester)methylene-oxy)phenyl)porphinato)-
zinc(II)]-[5′-[(10′,20′- di-(((4-ethyl ester)methylene-oxy)phe-
nyl)porphinato)iron(III)-chloride]ethyne (PZn-PFe-Cl), along
with a series of related compounds that possess a range of metal
axial ligation environments. Application of femtosecond (fs)
transient absorption spectroscopy shows that the evolution of
the initially prepared electronically excited states of these
supermolecular, meso-to-meso ethyne-bridged PZn-PFe(III)
compounds is governed by the axial ligation state of the
(porphinato)iron center.
Instrumentation. Electronic spectra were recorded on an OLIS UV/
vis/near-IR spectrophotometry system that is based on the optics of a
Cary 14 spectrophotometer. NMR spectra were recorded on either 250
MHz AC-250 or 360 MHz DMX-360 Bru¨ker spectrometers. Cyclic
voltammetric experiments were performed with an EG&G Princeton
Applied Research model 273A potentiostat/galvanostat. A single-
compartment electrochemical cell with glassy carbon working electrode
was used. Steady-state fluorescence emission spectra were recorded
on a SPEX Fluorolog 3 spectrometer that utilized a T-channel
configuration and PMT/InGaAs/Extended-InGaAs detectors; these
spectra were corrected using the spectral output of a calibrated light
source supplied by the National Bureau of Standards.
Experimental Section
Materials. All manipulations were carried out under nitrogen
previously passed through an O2 scrubbing tower (Schweitzerhall R3-
11 catalyst) and a drying tower (Linde 3 Å molecular sieves) unless
otherwise stated. Air-sensitive solids were handled in a Braun 150-M
glovebox. Standard Schlenk techniques were employed to manipulate
air-sensitive solutions. Methylene chloride (CH2Cl2) and tetrahydrofuran
(THF) were distilled from CaH2 and K/4-benzoylbiphenyl, respectively,
under N2. Nitrogenous ligands were purchased from Aldrich and used
as received. Acetone was dried over K2CO3 overnight and filtered prior
Magnetic Susceptibility Measurements and Ferric Spin State
Determination. The spin states of iron(III) in the monomeric complexes
were determined by the Evans method.63 These measurements were
carried out in 5-mm diameter NMR tubes that featured 1-mm diameter
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9
10424 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

Ethyne-Bridged PZn(II)−PFe(III) Complexes
A R T I C L E S
inner tubes. The inner tube contained solvent, the complex, and a
standard, while the outer region contained only solvent and the standard.
The magnetic susceptibility per gram, øi, was determined by the
expression:
were brought together in a 2-L round-bottom flask containing 1.5 L of
dry methylene chloride and a magnetic stir bar under N2. Trifluoroacetic
acid (0.16 mL, 2.1 mmol) was added to this mixture; the flask was
then covered with aluminum foil and stirred at room temperature for
14 h. DDQ (2,3-dichloro-4,5-dicyanoquinone, 3.58 g, 15.75 mmol) was
added to the methylene chloride solution, and the mixture was allowed
to stir for 15 min before the solvent was removed under reduced
pressure. The reaction mixture was then purified by silica chromatog-
raphy using 99:1 CH2Cl2/methanol as the eluant, to give 1.5 g of the
isolated product (43% yield based on 1.54 g of the dipyrrylmethane
starting material). Vis (CH2Cl2) λmax nm (log ꢀ): 408 (5.37), 504 (4.17),
ν0 - νi
-3
4mπ νs
øi )
+ ø0
(1)
(
)
In this equation, ν0 and νi are the respective chemical shift values,
expressed in Hertz, of the standard in the absence (outer tube) and
presence (inner tube) of the paramagnetic species; νs is the resonance
frequency of the spectrometer (500 MHz); m is the concentration of
paramagnetic species in the inner tube expressed in grams per cubic
centimeter; and ø0 is the magnetic susceptibility per gram of the
standard. The magnetic susceptibility per mole of the paramagnetic
species, øm, is determined by multiplying øi by the molar mass. Finally,
the magnetic moment (units of Bohr magneton) can be determined from
the molar susceptibility by a formulation of Curie’s Law:
1
539 (3.79), 577 (3.75), 632 (3.37). H NMR (250 MHz, CDCl3): δ
10.29 (s, 2 H), 9.37 (d, 4 H, J ) 4.65 Hz), 9.07 (d, 4 H, J ) 4.65 Hz),
8.16 (d, 4 H, J ) 7.95 Hz), 7.28 (d, 4 H, J ) 7.95 Hz) 5.53 (s, 4 H),
4.40 (q, 4 H, J ) 7.28 Hz), 1.42 (t, 6 H, J ) 7.30 Hz), -3.02 (broad
s, 2 H). HRMS (MH+): 667.2573 (calcd 667.2537).
5-Bromo-[10,20-di((4-ethyl ester)methylene-oxy)phenyl]porphy-
rin (3). In a 1.0-L round-bottomed flask, 2 (600 mg, 0.9 mmol) was
dissolved in 500 mL of methylene chloride and cooled to 0 °C.
N-Bromosuccinimide (128 mg, 0.72 mmol) was then added to the
stirring porphyrin solution. After 15 min, the reaction was quenched
by the addition of 30 mL of acetone. The solvents were removed, and
the recovered residue was purified by silica chromatography using
methylene chloride as the eluant. Three purple bands were observed;
the second band was isolated as the desired product (369 mg, 68.7%
yield based on 128 mg of NBS). Vis (CH2Cl2) λmax (log ꢀ): 417 (5.36),
µ ) 2.84 ø T
(2)
x
M
Here, T is the temperature in K. Since the magnetic moment is also
given by
µ ) 2 S(S + 1)
(3)
x
1
513 (4.12), 549 (3.76), 590 (3.61) 646 (3.41). H NMR (250 MHz,
the spin state of the paramagnetic species can easily be estimated from
the experimentally determined value.
CDCl3): δ 10.14 (s, 1 H), 9.70 (d, 2 H, J ) 4.875 Hz), 9.25 (d, 2 H,
J ) 4.7 Hz), 8.90 (q, 4 H, J ) 4.6), 8.1 (d, 4 H, J ) 8.75 Hz), 7.3 (d,
4 H, J ) 8.55 Hz), 5.30 (s, 4 H), 4.40 (q, 4 H, J ) 7.28 Hz), 1.42 (t,
6 H, J ) 7.30 Hz), -3.02 (broad s, 2 H). HRMS (MH+): 745.1651
(calcd 745.1661).
For these experiments, trimethylorthoformate (1,1,1-trimethoxy-
methane) was chosen as a standard because of its low molecular weight
and its sharp NMR singlet (δ ≈ 3.3 ppm in CHCl3, 9H), which resides
far away from solvent, porphyrin, and ligand proton resonances. While
the ø0 value of trimethylorthoformate to our knowledge has not been
experimentally determined, a molar susceptibility of 50 × 10-6 mol/
cm3 (ø0 ≈ 4.72 × 10-7 g/cm3) was estimated from the molar
susceptibilities of similar compounds listed in the Handbook of
Chemistry and Physics, 74th ed. This standard comprised 2% of the
total solvent volume in the inner and outer tubes in these experiments.
Due to the necessity of using large excesses of axial ligand to guarantee
a homogeneous coordination environment in these experiments (vide
infra), the use of high (mM) (porphinato)iron(III) complex concentra-
tions (to ensure large magnitude ∆ν values) was precluded. In these
experiments, the concentrations of paramagnetic complexes were ∼0.7
mg/mL, which provided ∆ν values on the order of 30 Hz, which were
large enough to accurately measure at a field strength of 500 MHz.
(4-Formyl-phenoxy)acetic Acid Ethyl Ester (1). This precursor
synthesis is analogous to that reported in the literature.64 A 200-mL
round-bottom flask was charged with of 4-hydroxybenzaldehyde (6.5
g, 53 mmol), ethyl bromoacetate (9.78 g, 58.5 mmol), potassium
carbonate (8.08 g, 58.5 mmol), and 100 mL of acetone. The reaction
mixture was brought to a gentle reflux overnight. It was then diluted
with water and extracted thrice with ethyl ether. The combined ether
layers were then washed three times with 2 M KOH solution and once
with water. The organic layer was then dried over Na2SO4 and
evaporated to yield a yellow oil (15.3 g, 90% yield based on the mass
5-Bromo-[(10,20-di((4-ethyl ester)methylene-oxy)phenyl)porphi-
nato]zinc(II) (4). Compound 3 (250 mg, 0.34 mmol) and zinc acetate
(735 mg, 3.35 mmol) were refluxed in chloroform (400 mL) in a 1-L
round-bottom flask equipped with a magnetic stir bar and a reflux
condenser. The reaction was monitored by electronic absorption
spectroscopy and was complete within 2 h. Following evaporation of
solvent, the residue was passed down a short silica column using
methylene chloride as the eluent. The desired product eluted first and
was collected in quantitative yield. Vis (CH2Cl2) λmax (log ꢀ): 418
1
(5.47), 547 (4.09), 586 (3.28). H NMR (250 MHz, CDCl3): δ 10.05
(s, 1 H), 9.70 (d, 2 H, J ) 4.7 Hz), 9.24 (d, 2 H, J ) 4.4 Hz), 8.9 (q,
4 H, J ) 4.6 Hz), 8.0 (d, 4 H, J ) 7.95 Hz), 7.26 (d, 4 H, J ) 7.95
Hz) 4.89 (s, 4 H), 4.40 (q, 4 H, J ) 7.28 Hz), 1.39 (t, 6 H, J ) 6.9
Hz). HRMS (M+): 806.0768 (calcd 806.0718).
5-Trimethylsilylethynyl-[(10,20-di((4-ethyl ester)methylene-oxy)-
phenyl)porphinato]zinc(II) (5). Trimethylsilylacetylene (0.30 mL, 2.5
mmol) and 5 mL of THF were brought together in a 50-mL Schlenk
tube. The solution was deaerated and cooled to -78 °C. n-Butyllithium
(0.84 mL of a 2.5 M hexanes solution, 2.1 mmol) was added dropwise,
following which the Schlenk tube was removed from the dry ice/acetone
bath and slowly warmed to room temperature. A THF solution of zinc
chloride (429 mg, 3.15 mmol) was cannula transferred into the solution
containing the lithiated ethynyl(porphinato)zinc(II) compound, upon
which the reaction mixture turned cloudy. A separate Schlenk tube was
charged with 4 (170 mg, 0.21 mmol), Pd(PPh3)4 (24 mg, 0.021 mmol),
and 30 mL of THF. After deaerating the porphyrin solution by three
freeze-pump-thaw-degas cycles, the contents of both Schlenk tubes
were combined and stirred in a 50 °C oil bath under N2 overnight. The
reaction mixture was then diluted with ethyl acetate and washed twice
with NH4Cl and once with water. The organic phase was dried over
Na2SO4 and evaporated. The resulting mixture was purified via silica
chromatography using 30% THF in hexanes as eluant. The fluorescent
purple band was isolated, giving the desired product (165 mg, 86.8%
yield based on 170 mg of compound 4). Vis (CH2Cl2) λmax (log ꢀ):
1
of 4-hydroxybenzaldehyde). H NMR (250 MHz, CDCl3): δ 9.88 (s,
1 H), 7.82 (d, 2 H, J ) 4.6 Hz), 7.0 (d, 2 H, J ) 4.6 Hz), 4.69 (s, 2
H), 4.25 (q, 2 H, J ) 7.0 Hz), 1.25 (t, 3 H, J ) 7.0 Hz). HRMS (M+):
208.0734 (calcd 208.0735).
[5,15-Di((4-ethyl ester)methylene-oxy)phenyl]porphyrin (2). 2,2′-
Dipyrrylmethane65,66 (1.54 g, 10.5 mmol) and 1 (2.19 g, 10.5 mmol)
(64) Lottner, C.; Bart, K.-C.; Bernhardt, G.; Brunner, H. J. Med. Chem. 2002,
45, 2079-2089.
(65) Lin, V. S.-Y.; Iovine, P. M.; DiMagno, S. G.; Therien, M. J.; Malinak, S.;
Coucouvanis, D. Inorg. Synth. 2002, 33, 55-61.
(66) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea, D. F.;
Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.
1
425 (5.34), 555 (4.18), 596 (3.85) nm. H NMR (250 MHz, CDCl3):
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10425

A R T I C L E S
Duncan et al.
δ 10.11 (s, 1 H), 9.76 (d, 2 H, J ) 4.73 Hz), 9.27 (d, 2 H, J ) 4.55
Hz), 9.00 (d, 2 H, J ) 4.88 Hz), 8.98 (d, 2 H, J ) 4.60 Hz), 8.11 (d,
4 H, J ) 8.55 Hz), 7.30 (d, 4 H, J ) 8.60 Hz), 4.91 (s, 4 H), 4.40 (q,
4 H, J ) 7.23 Hz) 1.42 (t, 6 H, J ) 7.15 Hz), 0.62 (s, 9 H).
phenyl)porphinato]iron(II)-(pyridine)2]ethyne (PZn-PFe-(py)2).
Mossy zinc (5 g, 76.5 mmol), previously washed with 0.1 N HCl and
rinsed to neutral pH using deionized (DI) water, was placed in a 125-
mL Erlenmeyer flask containing 25 mL of DI water. A solution of
HgO (166 mg in ∼1 mL of concentrated HCl) was added dropwise to
the mossy zinc with continuous swirling. Once H2 evolution had ceased,
the resulting Zn/Hg amalgam was filtered and rinsed thoroughly with
DI water. The amalgam was then dried under vacuum for several hours.
A 50-mL Schlenk-style round-bottomed flask equipped with a magnetic
stir bar was charged with PZn-PFe-Cl and the Zn/Hg amalgam. Once
the flask had been thoroughly deaerated, 10 mL of 10:1 C6D6/d5-
pyridine was added via cannula. The flask was warmed to 45 °C, and
the reduction was allowed to proceed over 2 h. The solution was then
filtered and transferred to a NMR tube under N2. Optical and 1H NMR
data were consistent with homogeneous conversion of PZn-PFe-Cl
to [PZn-PFe-(py)2]. Vis (C6D6/pyridine-d5, 10:1): 428, 476, 536, 572,
5-Ethynyl-[(10,20-di((4-ethyl ester)methylene-oxy)phenyl)porphi-
nato]zinc(II) (6). Compound 5 (165 mg, 0.182 mmol) was dissolved
in 150 mL of a 1:1 THF/EtOH solution. Potassium carbonate (252 mg,
1.82 mmol) was added, and the reaction was stirred in the dark at
ambient temperature overnight. Once the reaction was complete, the
solvents were removed, and the recovered residue was purified by
column chromatography via a short silica gel column using 3:2 hexanes/
THF as the eluant. The fluorescent purple band was collected and
concentrated. The resulting solid was further purified by recrystallization
from THF/hexanes to give 125 mg of the title compound (82% yield,
based on 165 mg of the (porphinato)zinc(II) starting material). Vis (CH2-
Cl2) λmax (log ꢀ): 422 (5.33), 552 (4.33), 591 (3.85) nm. 1H NMR (250
MHz, 19:1 CDCl3/d5-pyridine): δ 10.15 (s, 1 H), 9.75 (d, 2 H, J )
4.82 Hz), 9.30 (d, 2 H, J ) 4.75 Hz), 9.01 (d, 2 H, J ) 4.41 Hz), 8.98
(d, 2 H, J ) 4.41 Hz), 8.10 (d, 4 H, J ) 8.53 Hz), 7.30 (d, 4 H, J )
8.78 Hz), 4.91 (s, 4 H), 4.40 (q, 4 H, J ) 7.26 Hz), 4.13 (s, 1 H), 1.43
(t, 6 H, J ) 7.13 Hz). ESI-MS (M+): 752.10 (calcd 752.16).
1
684 nm. H NMR (360 MHz, C6D6/pyridine-d5, 10:1): δ 10.8 (m, 4
H), 10.05 (s, 1 H), 9.55 (s, 1 H), 9.23 (m, 4 H), 9.20 (m, 4 H), 9.00
(m, 4 H), 8.10 (m, 8 H), 7.60 (m, 8 H), 4.55 (m, 8 H), 4.10 (m, 8 H),
1.46 (m, 12 H).
[5,15-Di[((4-ethyl ester)methylene-oxy)phenyl]porphinato]iron-
(III)-chloride (PFe-Cl). Iron metalation of the free-base macrocycle
followed a previously reported procedure.67 Compound 2 (100 mg,
0.150 mmol), dissolved in 20 mL of EtOH and 60 mL of CHCl3 with
FeCl2‚4H2O (149 mg, 0.750 mmol), was refluxed under Ar for 2 h,
during which time the color changed from purple to brown. After
electronic absorption spectroscopy indicated the reaction was complete,
the solvent was evaporated, and excess ferrous chloride salt was
removed from the residue via chromatography on a short neutral
alumina plug using THF as the eluant. The product, recovered as the
µ-oxo-bridged bis[(porphinato)Fe(III)] complex, was dissolved in CHCl3
and washed thrice with 1 M HCl using a separatory funnel, followed
by three water washings. Following removal of chloroform, 105 mg
of the desired product was isolated (93% yield, based on 100 mg of
compound 2). Vis (CH2Cl2) λmax (log ꢀ): 377(4.62), 408(4.81), 502-
(4.03), 574(3.46), 641(3.40), 674(3.32) nm. FAB-MS (M+ - axial Cl-):
720.6 (calcd 720.2).
Determination of Ligand Binding Constants. Electronic absorption
spectral changes were recorded upon the addition of known amounts
of nitrogenous Lewis base to 5 × 10-5 M solutions of [5,15-di[((4-
ethyl ester)methylene-oxy)phenyl]porphinato]iron(III)-chloride (PFe-
Cl) in CH2Cl2. The solution was stirred vigorously after each addition
of axial ligand. The change in total solution volume between the start
and finish of these titration experiments was less than 10%. Equilibrium
constants K1 and â2, as defined by eqs 4-6, were determined by
methods established previously by Walker.68,69
[5-[(10,20-Di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]-
zinc(II)]-[5′-[(10′,20′-di-(4-ethyl ester)methylene-oxy)phenyl]por-
phyryl]ethyne (7). A 100-mL Schlenk tube was charged with 6 (70
mg, 0.084 mmol), 3 (75 mg, 0.101 mmol), triphenylarsine (50 mg,
0.164 mmol), the catalyst Pd2dba3 (19 mg, 0.021 mmol), and 50 mL
of dry THF. Following the addition of 2 mL of triethylamine, the
reaction was deaerated by three freeze-pump-thaw cycles and stirred
at 35 °C under N2 for 2 d. The solution was then diluted with EtOAc
and washed thrice with water, and the combined organic layer was
dried over MgSO4. After the organic solvents were removed, the
recovered green solid was purified by column chromatography on silica
gel using 3:2 hexanes/THF as the eluant. Following elution of
monomeric porphyrin species and a butadiyne-bridged (porphinato)-
zinc(II) impurity, the eluant polarity was increased to 100% THF. The
title compound was isolated and further purified by recrystallization
from THF/hexanes to give pure 7 (85 mg, 71% yield based on 70 mg
of the (porphinato)zinc(II) starting material). Vis (CH2Cl2) λmax (log
ꢀ): 416 (5.33), 473 (5.31), 521 (4.32), 567 (4.30), 644 (4.39), 708 (4.55)
1
nm. H NMR (250 MHz, 19:1 CDCl3/d5-pyridine): δ 10.40 (d, 2H, J
) 4.63 Hz), 10.35 (d, 2H, J ) 4.63 Hz), 10.07 (s, 1 H), 10.05 (s, 1 H),
9.21 (t, 4 H, J ) 4.10 Hz), 9.11 (d, 2 H, J ) 4.65 Hz), 9.07 (d, 2 H,
J ) 4.80 Hz), 8.92 (t, 4 H, J ) 4.48 Hz), 8.13 (m, 8 H), 7.26 (m, 8 H),
4.88 (s, 8 H), 4.37 (q, 8 H, J ) 7.08 Hz), 1.35 (t, 12 H, J ) 6.95 Hz),
-2.27 (broad s, 2 H). MALDI-TOF MS (M+): 1418.87 (calcd 1418.82).
5-[(10,20-Di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]-
zinc(II)-[5′-[(10′,20′-di-(((4-ethyl ester)methylene-oxy)phenyl)por-
phinato]iron(III)-chloride]ethyne (PZn-PFe-Cl) (8). 5-[(10,20-Di-
(((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)]-5′-(10′,20′-
di-(((4-ethyl ester)methylene-oxy)phenyl)porphyryl)ethyne (10 mg,
0.007 mmol) was dissolved in 20 mL of ethanol and 60 mL of CHCl3.
Ferrous chloride tetrahydrate (7 mg, 0.035 mmol) was added to the
stirring porphyrin solution, and the reaction was refluxed under N2.
The metal insertion reaction was monitored by electronic absorption
spectroscopy and was complete after 2 h. The solvents were evaporated,
and the recovered residue was loaded onto a short neutral alumina plug
(Fluka, type H). The product was eluted with THF and concentrated.
The recovered solid was then dissolved in CH2Cl2 and washed thrice
with 1 M HCl and three times with water, and was dried over CaCl2
and evaporated. Recrystallization of the isolated bis[(porphinato)metal]-
complex from THF/hexanes gave 10 mg of compound 8 (quantitative
yield). Vis (CH2Cl2) λmax (log ꢀ): 423 (4.96), 467 (4.92), 512 (4.35),
607 (4.30), 680 (4.15), 731 (4.08) nm. MALDI-TOF MS (M+ - axial
Cl-): 1472.66 (calcd 1472.65).
K1
PFeCl + L y
\
z PFeClL
(4)
(5)
K2
PFeClL + L y\
z PFeL2+Cl-
â2
â2 ) K1K2
PFeCl + 2L y
z PFeL2+Cl-
(6)
Results and Discussion
Synthesis. Using methodology described previously,12,13,55,61,70
PZn-PFe-Cl was synthesized by the route outlined in Scheme
(67) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg. Chem.
2001, 40, 3423-3434.
(68) Walker, F. A.; Lo, M.-W.; Ree, M. T. J. Am. Chem. Soc. 1976, 98, 5552-
5560.
(69) Koerner, R.; Wright, J. L.; Ding, X. D.; Nesset, M. J. M.; Aubrecht, K.;
Watson, R. A.; Barber, R. A.; Mink, L. M.; Tipton, A. R.; Norvell, C. J.;
Skidmore, K.; Simonis, U.; Walker, F. A. Inorg. Chem. 1998, 37, 733-
745.
(70) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Org. Chem. 1993, 58,
5983-5993.
[5-[(10,20-Di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]-
zinc(II)-pyridine]-[5′-[(10′,20′-di-((4-ethyl ester)methylene-oxy)-
9
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Ethyne-Bridged PZn(II)−PFe(III) Complexes
A R T I C L E S
Scheme 1. Synthetic Route to PZn-PFe-Cl
1. This synthesis of a meso-to-meso ethyne-bridged mixed-metal
bis(porphyrin) complex utilized (5-ethynylporphinato)zinc(II)
and 5-bromoporphyrin precursors (Scheme 1). This strategy
avoids the presence of a redox-active, axially ligated metal center
in the reductive metal-catalyzed cross-coupling reaction condi-
tions, and produces an intermediate meso-to-meso ethyne-
bridged (porphinato)zinc(II)-porphyrin (PZn-PH2) com-
pound55 that can be isolated and characterized prior to subsequent
metalation.67
structures have been discussed extensively,12,13,46,49,55,61 a number
of distinct characteristics of the PZn-PFe-Cl electronic absorp-
tion spectrum are worth noting. Relative to PZn-PZn species,
PZn-PFe-Cl displays a Soret region absorption band maximum
that is less intense than that manifested by its PZn-PZn
counterpart. Further, the spectral breadth of the PZn-PFe-Cl
S2-state absorption manifold (fwhm ≈ 7000 cm-1) exceeds that
established for PZn-PZn (∼4600 cm-1).12,13,46,49,55,61 In this
regard, these S2-state absorption manifold characteristics
resemble those described for [(5,-10,20-bis[3,5-bis(3,3-dimethyl-
1-butyloxy)phenyl]porphinato)zinc(II)]-[(5′,-15′-ethynyl-10′,-
20′-bis[10,20-bis(heptafluoropropyl)]porphinato)zinc(II)]-
ethyne,61,62 an electronically asymmetric meso-to-meso ethyne-
bridged bis[(porphinato)zinc(II)] complex, which is composed
of a 10,20-aryl-substituted macrocycle and a second (porphi-
nato)zinc(II) unit that features σ-electron-withdrawing perfluo-
ropropyl groups at the 10 and 20 positions.71 As in this
previously studied electronically asymmetric highly conjugated
bis[(porphinato)metal] complex,61,62 PZn-PFe-Cl features a
low-energy Qx absorption band that is both broader and tails
more significantly to the red, than that observed for PZn-
PZn,12,13,46,49,55,61,62 suggestive of increased charge-resonance
character of this transition.
Mechanistic studies of electron transfer, as well as excitation
delocalization and excitation migration processes in mixed-metal
multiporphyrin systems have typically examined assemblies that
feature spectroscopically identifiable chromophoric entities that
function as energy donors and acceptors. For multi(porphinato)-
metal systems that feature d1-d9 metal centers, the metal d
manifold typically affords a multiplicity of states lower in energy
than the (porphinato)zinc(II) S1 state.72-77 In weakly coupled
(porphinato)zinc(II)-spacer-(porphinato)iron(III) (PZn-Sp-
PFe) systems, both energy- and electron-transfer migration to
Figure 1. Ground-state electronic absorption spectra of PZn-PFe-Cl
(black line) and PZn-PFe-Cl in the presence of 5000-fold molar excess
of specific nitrogenous Lewis bases (L) in CH2Cl2 solvent [L ) pyridine
(blue line), 4-cyanopyridine (magenta line), collidine (red line), and 2,6-
lutidine (green line)].
Ground-State Electronic Absorption Spectra. Figure 1
shows the electronic absorption spectrum of PZn-PFe-Cl. The
gross spectral features of this complex resemble those delineated
for meso-to-meso ethyne-bridged bis[(porphinato)zinc(II)] (PZn-
PZn) compounds,12,13,46,49,55,59,61,62 featuring two dominant
absorption manifolds derived from the classic porphyrin B
(Soret)- (S0 f S2) and Q-band (S0 f S1) transitions. It is
noteworthy that the electronic absorption spectrum of PZn-
PFe-Cl is not superimposable on the combined electronic
absorption spectra of its monomeric precursors, reflecting the
extensive interpigment excitonic interactions and extensive π
electronic delocalization characteristic of this class of super-
molecular chromophores. While the optical spectroscopic
features of this class of highly conjugated bis(chromophore)
(71) While Fe d character is manifest in the ground- and excited-state wave
functions, we utilize S0, S1, S2, and Sn labels for these ground and initially
prepared excited states to facilitate comparisons to the established
spectroscopy and dynamics of closely related diamagnetic analogues of
PZn-PFe(III).
(72) Brookfield, R. L.; Ellul, H.; Harriman, A. J. Chem. Soc., Faraday Trans.
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9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10427

A R T I C L E S
Duncan et al.
upon pyridine ligation; such spectral changes are diagnostic of
a bis-axially ligated (porphinato)iron(III) [PFe(III)] center.90,92-94
4-Cyanopyridine, a poor σ-donor, is known to have a minimal
impact on PFe-Cl electronic spectral features;91 congruent with
this, the PZn-PFe-Cl spectrum is modulated little by the
presence of large excesses of 4-cyanopyridine in solution (Figure
1). Likewise, substantial steric interactions between the por-
phyrin macrocycle and encumbered axial ligand substituents
manifest predictable optical absorption spectral changes relative
to the PZn-PFe-Cl spectrum. It has been demonstrated that
axial metal ligation by bulky nitrogenous bases such as 2-tert-
butylimidazole and 2-ethylimidazole effect a red-shift of the
Q-band absorptions and intensification of an absorption band
centered around 570 nm relative to that manifested for the
corresponding ferric chloride complex;92 these ligation-depend-
ent electronic absorption spectral changes are mirrored in
analogous experiments chronicled in Figure 2 for a benchmark
PFe(III) complex and correspondingly bulky Lewis bases
[collidine (2,4,6-trimethylpyridine) and 2,6-lutidine (2,6-di-
methylpyridine)]. Bulky axial ligands related to collidine and
2,6-lutidine have been established to stabilize high-spin PFe-
(III) electronic states;95,96 the Figure 1 and Figure 2 spectra
suggest that, for PZn-PFe(III) compounds, these axial ligands
drive similar high-spin Fe(III) electronic structures.
Nature of Collidine Ligation and the Resulting Spin State
at Ferric Porphyrin Centers. The binding constants for a wide
range of nitrogenous Lewis bases at ferrous and ferric porphyrin
centers have been determined;97 of the ligands highlighted in
the spectra of Figures 1-2, only binding affinity data for
collidine at PFe(III) sites remain sparse. While factors dictating
ligation stoichiometry at PFe centers have been extensively
studied,68,97-100 cases where steric interactions play the dominant
role in such interactions are few.101,102 While it is well
established that the ferrous ions of many biological hemes and
meso-tetraphenylporphyrins (TPPs) are pentacoordinate with
bulky axial ligands such as 2-methylimidazle103-107 and 2,6-
lutidine,108 2-methylimidazole has been shown to form a bis-
axially ligated species at more sterically congested meso-
(tetramesitylporphinato)iron(II) (TMPFe(II)).101,102 Despite the
established pentacoordinate binding stoichiometry of 2-meth-
ylimidazole, 1-methylbenzimidazole, and 2-methylpyrazines at
TPPFe(II), these ligands nonetheless form hexacoordinated
Figure 2. Ground-state electronic absorption spectra of PFe-Cl (black line)
and PFe-Cl in the presence of 5000-fold molar excess of specific
nitrogenous Lewis bases (L) in CH2Cl2 solvent [L ) pyridine (blue line),
collidine (red line), and 2,6-lutidine (green line)]. The spectrum for L )
4-cyanopyridine is omitted for clarity, as it is nearly identical to that for
PFe-Cl.
the PFe(III) chromophore from the PZn-localized singlet excited
state have been observed.73,77-89 Such PZn-Sp-PFe assemblies
have utilized hydrogen-bonded interfaces,84 as well as rigid
phenyl,79-82,86,87 and (phenylethynyl)-arene73,85,88,89 bridging
moieties, to connect the porphyrin-based pigments; for such
linkage motifs, excitation delocalization is negligible, as these
Sp structures afford only weak electronic coupling between the
two (porphinato)metal units.
Given the extensive pigment-pigment coupling driven by a
meso-to-meso ethyne bridge, the nature of metal d manifold
electronic states would be expected to play a major role in
+
determining the dynamics of [PZn-PFe-(ligand)1,2
]
excited
states. Consistent with this, distinct spectral changes in the
PZn-PFe-Cl electronic absorption spectrum are observed upon
the addition of nitrogenous Lewis bases (Figure 1). These
spectral changes are analogous to those observed for [5,15-di-
[((4-ethyl ester)methylene-oxy)phenyl]porphinato]iron chloride
(PFe-Cl; Figure 2) upon addition of identical ligating axial
Lewis bases.90-92 For instance, classic intensification of the
Q-band R- and â-absorptions are clearly evident in Figures 1-2
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9
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Ethyne-Bridged PZn(II)−PFe(III) Complexes
A R T I C L E S
Figure 4. Determination of binding constants K1 and â2 for collidine at
[5,15-di[((4-ethyl ester)methylene-oxy)phenyl]porphinato]iron(III). Experi-
mental conditions: [PFe-Cl] ) 53 µM, solvent ) CH2Cl2, T ) 20 °C.
Figure 3. Collidine concentration dependent electronic absorption spectra
for [5,15-di[((4-ethyl ester)methylene-oxy)phenyl]porphinato]iron(III) in
CH2Cl2 solvent. Experimental conditions: [PFe-Cl] ) 53 µM, T ) 20 °C;
[collidine]: 0 mM (black line), 26 mM (light red line), 52 mM (yellow
line), 83 mM (blue-green line), 103 mM (magenta line), 127 mM (dark red
line), 170 mM (green line), 200 mM (dark blue line), 311 mM (light blue
line).
Table 1. Equilibrium Ligand Binding Constants Determined for
Nitrogenous Bases at (Porphinato)Iron(III) Centers via Electronic
Absorption Spectroscopya
1
2
PFe(III) complex
base
pKa
solvent
log K1 (M-
)
log â )
2 (M-
PFe-Cl
collidine
2-Me-Im
N-Me-Im
pyridine
7.43
7.56
7.33
5.28
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
-1.3
1.3
1.9
-5.0
3.6
4.0
TPPFe-Cl68
TPPFe-Cl68
TPPFe-Cl68
species at TPPFe-(III);109-112 similarly, contrary to simple
expectations based on established axial ligation at TMPFe(II),
TMPFe(III) centers with one axially ligated imidazole have been
characterized.92
-0.7
-0.3
a [porphyrin] ≈ 10-5 M; experimental data obtained at ambient temper-
ature; Im ) imidazole.
With respect to respect to encumbered pyridyl axial ligands,
a 1:1 adduct of 2,6-dimethylpyridine with ferric protoporphyrin
IX (PPIXFe) has been characterized in the solid state;95 likewise,
within a protoheme dimethylester ligand environment, ligand
titration experiments show Fe(II) binds 2,6-lutidine in a 1:1
stoichiometry.108 However, similar studies by Momenteau
suggests that TPPFe(II) manifests hexacoordination in the
presence of excess 2,6-lutidine.113 Given these limited and
perhaps contradictory observations regarding the role played
by ligand steric factors in determining heme coordination
environments, axial ligand titration studies were carried out to
determine the nature of 2,6-dimethylpyridyl adducts at PFe-
(III); Figure 3 shows the Q-band region electronic absorption
spectral changes that accompany increasing equivalents of
collidine ligand added to a CH2Cl2 solution of PFe-Cl. Clear
isosbestic behavior is observed at 486 nm (Figure 3). Crossover
points centered around 530 and 615 nm spread over an
approximate 10-nm range as collidine concentration is steadily
increased (Figure 3); these spectral features resemble those
observed previously for titrations involving other hindered
bases68 at PFe(III) complexes. Similar [collidine]-dependent
spectral changes are observed in analogous titrations involving
CH2Cl2 solutions of PZn-PFe-Cl (data not shown).
concentration. Such plots, fashioned at four wavelengths, are
shown in Figure 4. The formation constant of the pentacoor-
dinated species, -log K1, was determined from the intercept of
wavelength-dependent log((A - A0)/(Ac - A)) vs log[L] plots
which displayed a linear relationship and a slope of unity;
similarly, -1/2 log â2 was evaluated from wavelength-dependent
linear log((A - A0)/(Ac - A)) vs log[L] relationships that
displayed slopes of two. This analysis of the Figure 4 data gives
K1 ) 5.3 × 10-2 M-1 and â2 ) 1.0 × 10-5 M-2. Comparative
equilibrium constants for ligand binding at PFe(III) are listed
in Table 1. Note that the â2 value evaluated for collidine at
PFe(III) is ∼5 and 9 orders of magnitude smaller, respectively,
than that determined for pyridine and 2-methylimidazole,68,114
despite the fact that the 2,4,6-trimethylpyridine (collidine) pKa
value is comparable to those of the imidazole ligands. These
data thus indicate that unlike the case for the binding of
unencumbered pyridyl ligands to the PFe(III) centers of simple
meso-arylporphyrins, ferric porphyrin centers remain nearly
exclusively pentacoordinate throughout the collidine concentra-
tion regimes chronicled in the Figure 4 data.
The magnetic moment determined by Hambright for solid-
state samples of PPIXFe(III)-(2,6-dimethylpyridine) and PPIXFe-
(III)-(2,4-dimethylpyridine) indicate a high-spin ferric heme
at ambient temperature.95 While these studies suggest a common
S ) 5/2 spin state for pentacoordinate hemes featuring sterically
encumbered, methyl-substituted pyridine and imidazole Lewis
bases at ambient temperature, recent work demonstrates that
the magnetic properties of such PFe(III)-L species are sensitive
to solvent, temperature, and ligand electronic structure.92
Extensive experimental data compiled by Nakamura for a series
of (porphinato)Fe(III)-(imidazole) compounds show that the
quantum mechanical spin-admixed state of high and intermediate
Collidine ligand binding equilibrium constants were deter-
mined from data analyzed using eqs 4-6 (Experimental
Section); plots of log((A - A0)/(Ac - A)) vs log[L] were
constructed. Here, A is the absorbance at the wavelength of
interest, A0 is the absorbance in the absence of ligand, Ac is
that of the fully ligated complex, and [L] is the ligand
(109) Scheidt, W. R.; Kirner, J. F.; Hoard, J. L.; Reed, C. A. J. Am. Chem. Soc.
1987, 109, 1963-1968.
(110) Satterlee, J. D.; La Mar, G. N.; Bold, T. J. J. Am. Chem. Soc. 1977, 99,
1088-1093.
(111) Satterlee, J. D.; La Mar, G. N.; Frye, J. S. J. Am. Chem. Soc. 1976, 98,
7275-7282.
(112) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106, 6888-
6898.
(114) Coyle, C. L.; Rafson, P. A.; Abbott, E. H. Inorg. Chem. 1973, 12, 2007-
(113) Momenteau, M.; Lavalette, D. J. Am. Chem. Soc. 1978, 100, 4322-4324.
2010.
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J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10429

A R T I C L E S
Duncan et al.
Table 2. Comparative Cyclic Voltammetric Data for TPPFe and
PZn-PFe Systemsa
potential (V)
/
III
/
III
b
E1 2(FeII
)
E1 2(FeII
)
log(KFe(III)/KFe(II)
PZn PFe
)
/
/
axial ligand
pKa
TPPFe
PZn−
PFe
−
Cl-
-0.300121
-0.260122
0.050120
-0.215
-0.170
0.145
collidinec
pyridinec
4-CNpyc
7.43
5.28
1.86
-0.30
-6.2
-10
0.300121
0.385
a Experimental conditions: [PZn-PFe] ) 1 mM; [TBAP] ) 0.1 M;
[axial ligand] ) 1 M; solvent ) CH2Cl2; scan rate ) 100 mV/s; working
electrode ) Pt disk. Potentials reported are relative to SCE. b Binding
constants calculated as described by Kadish.119,123 c Generated from the
corresponding ferric chloride complex.
Figure 5. Cyclic voltammetric responses of PZn-PFe-Cl and PZn-PFe-
(pyridine)2 recorded in methylene chloride solvent highlighting the revers-
ible FeIII/II redox couples. Experimental conditions are described in Table
2.
As Kadish has shown previously,118,119,123,124 the relative
pyridine binding affinities for PFeII and PFeIII species can be
evaluated from potentiometric data obtained as a function of
exogenous axial ligand concentration. This relationship between
ligand concentration and redox potential can be expressed in a
modified version of the Nernst equation (eq 7):
5
spins (S )
/
and S )3/2) can be accessed at ambient
2
temperature.92 For example, magnetic moments determined at
25 °C for a series of sterically encumbered TMPFe(III)-(L)
compounds (L ) 2- or 5-substituted imidazole) show that the
3
S ) /2 contribution to the admixed spin state can range from
1 to 70% as function of the imidazole substituents. Further, the
3
S ) /2 contribution to the admixed spin state was established
KFe(III)
n
n
(E1/2)s ) (E1/2)c -
log
-
log [L]p-q (7)
to be sensitive to solvent polarity, diminishing precipitously with
decreasing solvent dielectric strength.92,115 Given the macrocycle
electronic characteristics and axial ligand σ-donor and steric
properties, these data raise the possibility that [PFe-(collidine)]+
exists in the quantum mechanical spin-admixed (S ) /2, /2)
state at ambient temperature92,95,115,116 with a spin composition
dependent upon solvent polarity (vide infra). To ascertain the
spin state of the pentacoordinate [PFe-(collidine)]+, a series of
NMR experiments was carried out to determine its magnetic
moment. Using the experimental protocol established by
Evans,63 the monocollidine ligated ferric porphyrin center was
0.059
KFe(II)
0.059
Here, (E1/2)s and (E1/2)c are half wave reduction potentials of
the fully ligated and ligand-free species, n is the number of
electrons transferred, KFe(III) and KFe(II) are the formation
constants of the ligated ferric and ferrous species, [L] is the
ligand concentration, and p and q are number of ligands bound
to the oxidized and reduced species, respectively.119 The
magnitude of the potential difference between the FeII/III redox
couples for high- and low-spin FeIII species (determined from
the reductive electrochemistry of the PZn-PFe-Cl and [PZn-
PFe-(ligand)1,2]+ complexes) allows the evaluation of the ratio
of the Fe(II) and Fe(III) axial ligand binding constants (Table
2). This analysis indicates that, for these pyridyl ligands, ferrous
center stability correlates with the axial Lewis base pKa value.
Note also that the relative preference for maintaining a bis-
axially ligated ferrous center for the 4-cyanopyridine ligand
(Table 2) is 4 orders of magnitude greater [log(KFe(II)/KFe-
(III)) ) 10] than that for pyridine [log(KFe(II)/KFe(III)) ) 6.2].
The fact that the singly axial ligated collidine stabilizes poorly
the ferrous porphyrin center relative to sterically unencumbered
pyridyl ligands correlates with the low evaluated formation
3
5
5
determined to exist solely as a high spin species (S ) /2) at
ambient temperature in chloroform solvent.
Cyclic Voltammetric Studies Probing the FeII/III Redox
Couple. Representative cyclic voltammetric responses recorded
in CH2Cl2 solvent for PZn-PFe compounds are shown in
Figure 5. The current vs potential profiles are consistent with
previously investigated iron porphyrin species at solid working
electrodes.117-120 Table 2 lists the E1/2 (FeII/III) values determined
for TPPFe and PZn-PFe structures as a function of the
(porphinato)iron axial ligand. Note that for all the PFe axial
ligands examined in this study, the E1/2 (FeII/III) value determined
for a given Fe-ligated conjugated bis(porphyrin) compound,
PZn-PFe, is ∼100 mV more positive than that exhibited by
its corresponding TPPFe-(ligand)1,2 benchmark; thus, with
respect to the Fe center, the (5-ethynylporphinato)zinc(II) moiety
behaves as an electron-withdrawing substituent. As previously
constant for bis-ligand coordination (Table 1).
Excited-State Dynamics of [PZn-PFe-(L)1,2
mophores. Pump-probe transient absorption spectroscopy was
employed to examine the impact of axial ligation and solvent
excited-state relaxation dynamics.
Representative transient absorption spectra obtained for PZn-
PFe-Cl in methylene chloride solvent recorded at several time
+
]
Chro-
+
upon [PZn-PFe(L)1,2
]
observed for TPPFe compounds (Table 2),102,121 the E1/2(FeII/III
values determined for PZn-PFe species decrease with increas-
)
ing acidity of the ligating pyridyl Lewis base.
delays are shown in Figure 6. The early time spectrum (tdelay
)
400 fs) displays four pronounced features: (i) a high oscillator
strength bleach in the Soret (S0 f S2) band region, (ii) a transient
absorption between the Soret and x-polarized Q-state (S0 f S1)
transition in the ∼500-650 nm spectral domain (partially
(115) Sakai, T.; Ohgo, Y.; Hoshino, A.; Ikeue, T.; Saitoh, T.; Takahashi, M.;
Nakamura, M. Inorg. Chem. 2004, 43, 5034-5043.
(116) Quinn, R.; Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1982, 104,
2588-2595.
(117) Kadish, K. M.; Morrison, M. M.; Constant, L. A.; Dickens, L.; Davis, D.
G. J. Am. Chem. Soc. 1976, 98, 8387-8390.
(118) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1981, 20, 1348-1357.
(119) Kadish, K. M. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.;
Addison-Wesley: MA, 1983; Vol. 2, pp 161-249.
(122) Baker, J. A. ; University of Pennsylvania: Philadelphia, PA, 2000; p 241.
(123) Kadish, K. M.; Bottomley, L. A.; Beroiz, D. Inorg. Chem. 1978, 17, 1124-
(120) Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999,
121, 5196-5209.
1129.
(124) Kadish, K. M.; Bottomley, L. A. J. Am. Chem. Soc. 1977, 99, 2380-
(121) Kadish, K. M.; Bottomley, L. A. Inorg. Chem. 1980, 19, 832-836.
2382.
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Ethyne-Bridged PZn(II)−PFe(III) Complexes
A R T I C L E S
Figure 6. PZn-PFe-Cl transient absorption spectra determined at magic
angle polarization; delay times ) 0.4, 1.0, 4.0, and 10 ps. Experimental
conditions: solvent ) methylene chloride, λex ) 605 nm. A scaled steady-
state absorption spectrum is shown inverted (solid pink line) for comparative
purposes.
Figure 7. [PZn-PFe-(collidine)]+ transient absorption spectra determined
at magic angle polarization; delay times ) 0.33, 1.0, 4.3, 10.3, 101, and
1000 ps. Experimental conditions: solvent ) 10:1 CH2Cl2/collidine, λex
)
570 nm. A scaled steady-state absorption spectrum is shown (dotted line)
for comparative purposes. (Inset) Transient absorption spectra normalized
to the Soret band bleaching signature at delay times of 0.3, 1.0, and 10.0
ps showing the clear growth of a transient feature centered at approximately
680 nm.
obscured by scattering from the 605 nm excitation pump pulse),
(iii) a bleach in the Q-band region, and (iv) an intense near-
infrared (NIR) transient S1 f Sn (λmax: PZn-PFe-Cl (S1 f
Sn) ≈ 870 nm; Sn ) a higher-lying delocalized singlet excited
state) that possess an integrated oscillator strength of magnitude
comparable to the B- and Q-state bleaching transitions.71 This
low-energy feature of the PZn-PFe-Cl excited-state absorption
spectrum resembles that manifested by meso-to-meso ethyne-
bridged bis[(porphinato)zinc(II)] (PZn-PZn) compounds.62 The
intense, broad NIR S1 f Sn transient absorption manifold is a
spectroscopic hallmark of this class of highly conjugated
multichromophoric compounds:62,125,126 these excited-state tran-
sient absorption signatures are unique to strongly coupled bis-
[(porphinato)metal] chromophores, and underscore the globally
delocalized nature of the low-lying excited states of these
systems.62 Relative to the S1 f Sn transient absorption manifold
interrogated for bis[(5,5′-10,20-bis[3,5-bis(3,3-dimethyl-1-bu-
tyloxy)phenyl]porphinato)zinc(II)]ethyne (λmax: PZn-PZn (S1
f Sn) ≈ 985 nm; fwhm ) 655 cm-1),62 the PZn-PFe-Cl (S1
f Sn) manifold is significantly broader (fwhm ) 2700 cm-1),
signaling a substantial charge resonance contribution to this
transition.
Following excitation at 605 nm, the PZn-PFe-Cl transient
absorption spectrum decays within several picoseconds back
to the ground-state. The dynamics of the PZn-PFe-Cl excited-
state decay are multiexponential, with the two largest amplitude
decay component time constants possessing lifetimes of τ )
400 fs and 1.4 ps, though these values may reflect averages of
several additional components. Note that the transient absorption
spectrum recorded at 10 ps time delay shows evidence for a
hot ground-state transition centered at ∼500 nm (Figure 6); this
absorption decays with an 18 ps time constant. None of the
fast excited-state decay components are associated with sig-
nificant changes to the spectral shape of the transient signals,
as indicated by the clear isosbestic point behavior at ∆A ) 0
mOD, the corresponding decay-associated spectra (see Sup-
porting Information), and the fact that spectra recorded at 0.4
and 1 ps are essentially superimposable (Figure 6) when
normalized to uniform ∆A amplitude. These PZn-PFe-Cl
excited-state dynamics are distinct from those delineated for
PZn-PZn chromophores,49,62,125 which are characterized by S1-
state lifetimes of ∼1 ns; in this regard, the PZn-PFe-Cl
excited-state lifetime bears similarity to those determined for
simple monomeric PFe(III) compounds (vide infra), where the
partially filled ferric ion d-orbital shell gives rise to a multiplicity
of states that facilitate subpicosecond and picosecond time scale
deactivation of the initially prepared electronically excited state
(Supporting Information).127-129 Congruent with this picture,
pump-probe transient absorption spectra acquired following
electronic excitation of [PZn-PFe-(py)2]+ and [PZn-PFe-(4-
CN-py)2]+ (see Supporting Information) feature NIR S1 f Sn
absorption manifolds and excited-state decay time constants
nearly identical to those described above for PZn-PFe-Cl. The
data shown in Figure 6 and in the Supporting Information for
PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-PFe-(4-CN-
py)2]+ thus suggest that the PZn-PFe E1/2 (FeII/III) value (Table
2), which varies by more than 600 mV in these three strongly
conjugated bis[(porphinato)metal] compounds, exerts little influ-
ence on the observed excited-state dynamical behavior.
Excited-state dynamical data obtained following electronic
excitation of [PZn-PFe-(collidine)]+, however, challenge this
simplistic picture. Transient absorption spectral data obtained
for this compound in 10:1 CH2Cl2/collidine are shown in Figure
7. Note that the early time spectrum (tdelay ) 300 fs) displays a
NIR transition manifold (λmax ) 970 nm) significantly red-
shifted (∆E ) 1185 cm-1) with respect to the PZn-PFe-Cl
NIR transient signal λmax shown in Figure 6. The [PZn-PFe-
(collidine)]+ NIR excited-state transition manifold exhibits
significant spectral breadth, ranging from ∼750 to 1050 nm;
the fwhm of this manifold (>3500 cm-1) exceeds that for the
analogous transient absorption of electronically excited PZn-
PFe-Cl by over 800 cm-1 (Figures 6 and 7). These facts,
coupled with the observation that the [PZn-PFe-(collidine)]+
(125) Duncan, T. V.; Susumu, K.; Sinks, L. E.; Therien, M. J. J. Am. Chem.
Soc. 2006, 128, 9000-9001.
(126) Susumu, K.; Duncan, T. V.; Therien, M. J. J. Am. Chem. Soc. 2005, 127,
5786-5195.
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J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10431

A R T I C L E S
Duncan et al.
and PZn-PFe-Cl excited-state NIR transition manifolds exhibit
disparate spectral shapes, suggest that electronically excited
[PZn-PFe-(collidine)]+ possesses augmented charge resonance
character as well as additional distinct underlying transitions
within its NIR S1 f Sn absorption manifold relative to that
observed for electronically excited PZn-PFe-Cl (Figure 6).
The Figure 7 transient absorption spectral data obtained for
[PZn-PFe-(collidine)]+ possess two further striking aspects.
The first includes the clear formation of a transient absorption
band centered at approximately 680 nm during the first few
picoseconds following photoexcitation. While this feature is
partially obscured by the S0 f S1 (Q-state) band bleaching
signature, distinct rising signals are evident in the tailing regions
of the S0 f S1 bleach at ∼620 and 740 nm. Growth of the
transient signals at 620 and 740 nm occurs with concomitant
loss of intensity in the transient NIR absorption band; normalized
spectra highlighting this fact are shown in the Figure 7 inset.
The dynamics of the growth and decay of this feature are
multiexponential, but a global fit of the dynamics yield major
decay constants of τ1 ≈ 0.3 ps, τ2 ≈ 1.3 ps, and τ3 ≈ 878 ps
(probe-wavelength-dependent relative amplitudes: λprobe ) 950
nm, A(τ1):A(τ2):A(τ3) ≈ 1:1:0; λprobe ) 620, A(τ1):A(τ2):A(τ3)
≈ 1:1:-2). The dynamics associated with the τ1 relaxation time
effect a substantial (∼50%) diminution of the S1 f Sn NIR
transition moment and a concomitant rise in the transient ab-
sorption band centered at ∼680 nm (Supporting Information,
Figure S3); the dynamics associated with τ2 primarily contribute
to ground-state recovery, as the decay associated spectrum
connected with this process is similar in shape to the ground-
state bleaching signature. Consequently, the remaining 50% of
the initial NIR transient absorption intensity decays via the τ2
process. The ∼880 ps characteristic relaxation time corresponds
to the intrinsic decay time constant of the 680 nm-absorbing
state back to the ground-state. Additional decay components
with time constants of 13 and 96 ps, respectively, are evident
but possess small associated amplitudes; both components con-
tribute to a red-shift of the ground-state bleaching signature after
formation of the absorption feature centered at 680 nm (Figure
S3). Given the similarity of these time-dependent spectral chan-
ges and their associated time constants to established processes
relevant to the excited-state dynamics of related PZn-PZn
compounds,62 these small amplitude decay components are like-
ly associated with dynamics that lead to enhanced PZn-to-PFe
conjugation (i.e., torsional relaxation around the ethyne bridge).
Figure 8. [PZn-PFe-(collidine)]+ transient absorption spectra determined
at magic angle polarization in (A) 10:1 benzonitrile/collidine and (B) 10:1
toluene/collidine; delay times ) 0.43, 1.1, 4.3,11.6, 103, and 1000 ps, λex
) 605 nm.
population, as only a fraction of electronically excited [PZn-
PFe-(collidine)]+ species relax via an electron-transfer pathway.
It should be noted that while the photoinduced charge separation
time constant is close to the experimentally determined longi-
tudinal relaxation times of methylene chloride (τL ) 0.33 and
0.56 ps),133,134 identical experiments that probe [PZn-PFe-
(collidine)]+ excited states in benzonitrile solvent (τL ) 6.9 and
5.1 ps) evince almost identical excited-state relaxation dynamics
(vide infra). We thus conclude that either (i) solvent relaxation
dynamics have only a minimal effect on the excited-state decay
processes of [PZn-PFe-(collidine)]+ or (ii) the local chro-
mophore solvation environment is dominated by collidine, which
makes up 10% of the solvent mixture in these experiments,
thereby masking dynamics associated with longitudinal CH2Cl2/
benzonitrile relaxation.
In contrast to PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-
PFe-(4-CN-py)2]+ which evince subpicosecond and picosecond
time-scale relaxation of their respective initially prepared
electronically excited states to the ground state, [PZn-PFe-
(collidine)]+ excited-state dynamics show ultrafast relaxation
to a [(PZn-P)+• FeII-(collidine)]+ charge-separated state having
a lifetime of nearly 1 ns. Figure 8 displays complimentary
transient absorption spectral data obtained for [PZn-PFe-
(collidine)]+ in 10:1 benzonitrile/collidine and 10:1 toluene/
collidine. [PZn-PFe-(collidine)]+ excited-state dynamics evi-
dent in the benzonitrile solution (Figure 8A) resemble qualitatively
those determined in the methylene chloride solution (Figure 7).
Fast (t ≈ 1 ps) loss of the NIR S1 f Sn transition manifold
Given the observed spectral characteristics of the 680 nm
transient absorption band and substantial literature precedent,
this signal is assigned to a (porphinato)zinc(II) cation radi-
cal.130,131,132 Of the two fastest dynamical components obtained
from a global analysis of the transient spectral data, only τ1
contributes significantly to the growth of the 680 nm transient
absorption feature, and thus a charge separation time constant
of 0.3 ps is concluded. The τ2 time constant corresponds to the
nonradiative decay of the initially prepared excited-state directly
back to the ground-state, presumably via empty low-lying metal-
centered states. Considering the difference in magnitude between
τ1 and τ2, and the therefore disproportionably modest yield of
the electron-transfer product (∼50%) relative to the ground
electronic state following internal conversion, it is reasonable
to conclude that excited-state spectral dynamics following
photoexcitation do not arise from a homogeneous excited-state
(127) Cornelius, A. W.; Steele, R. M.; Chernoff, D. A.; Hochstrasser, R. M.
Chem. Phys. Lett. 1981, 82, 9-14.
(128) Adar, F.; Gouterman, M.; Aronowitz, S. J. Phys. Chem. 1976, 80, 2184-
2191.
(129) Lim, M.; Jackson, T. A.; Anfinrud, P. A. J. Phys. Chem. 1996, 100,
12043-12051.
(130) Kang, Y. K.; Rubtsov, I. V.; Iovine, P. M.; Chen, J.; Therien, M. J. J.
Am. Chem. Soc. 2002, 124, 8275-8279.
(131) Redmore, N. P.; Rubtsov, I. V.; Therien, M. J. J. Am. Chem. Soc. 2003,
125, 8769-8778.
(132) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am.
Chem. Soc. 1970, 92, 3451-3459.
(133) Grampp, G.; Landgraf, S.; Rasmussen, K. J. Chem. Soc., Perkin Trans. 2
1999, 1897-1899.
(134) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys.
Chem. 1995, 99, 17311-17337.
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10432 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

Ethyne-Bridged PZn(II)−PFe(III) Complexes
A R T I C L E S
intensity occurs concomitantly with the growth in the cation-
radical feature centered at 680 nm. As in 10:1 methylene
chloride/collidine solution, the [PZn-PFe-(collidine)]+* dy-
namics in 10:1 benzonitrile/collidine are multiexponential;
analyses of these data reveal t1, t2, and t3 time constants nearly
identical in magnitude to those determined in 10:1 CH2Cl2/
collidine. Consistent with this finding, the time-dependent
spectral changes that follow excitation of [PZn-PFe-(colli-
dine)]+ in these two solvent systems are similar, indicating that,
at least for polar aprotic solvents, modulation of solvent
dielectric does not substantially influence the observed electron-
transfer dynamics. Transient absorption spectra recorded at time
delays longer than 20 ns (Supporting Information, Figure S9)
rule out the presence of a long-lived excited-state having a high
spin multiplicity as a contributing factor to the excited-state
relaxation dynamics exhibited by this species.
Figure 9. Excited-state decay profiles probed at 650 nm for PFe-Cl (O)
in methylene chloride solvent and [PFe-(collidine)]+ (0) in 10:1 methylene
chloride/collidine; λex ) 595 nm. The solid lines were determined from
multiexponential fits of the transient data. (Inset) Excited-state decay profiles
under identical conditions for [PFe-(collidine)]+ (0) in 10:1 methylene
chloride/collidine and PFe-PF6 (4) in methylene chloride solvent.
The dynamics and transient absorption spectral features
exhibited by electronically excited [PZn-PFe-(collidine)]+ in
10:1 toluene/collidine (Figure 8b), interestingly, resemble those
established for PZn-PFe-Cl (Figure 6), [PZn-PFe-(py)2]+,
and [PZn-PFe-(4-CN-py)2]+ (Supporting Information) in polar
solvents. Like the time-dependent transient absorption spectral
features exhibited by electronically excited PZn-PFe-Cl in the
methylene chloride solution (Figure 6), no cation radical spectral
signature is manifest in the [PZn-PFe-(collidine)]+ excited-
state dynamics recorded in 10:1 toluene/collidine; all the
observed dynamical processes contribute to fast or ultrafast
recovery of the ground electronic state. Also noteworthy is the
fact that while the ground-state electronic absorption spectrum
of [PZn-PFe-(collidine)]+ differs substantially from that
determined for PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-
PFe-(4-CN-py)2]+ in methylene chloride solvent, the electronic
absorption spectra of PZn-PFe-Cl in methylene chloride
solvent and [PZn-PFe-(collidine)]+ in 10:1 toluene/collidine
solution are nearly identical (Supporting Information, Figure
S8). This observation is thus consistent with the finding that
transient dynamics evident for [PZn-PFe-(collidine)]+ in 10:1
toluene/collidine solution (Figure 8B) resemble those manifested
by PZn-PFe-Cl in CH2Cl2 (Figure 6).
(collidine)]+ in polar solvents with respect to that evident for
PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-PFe-(4-CN-
py)2]+, comparative excited-state dynamical studies were carried
out for [PFe-(collidine)]+, PFe-Cl, [PFe-(py)2]+, and [PFe-
(4-CN-py)2]+. Figure 9 contrasts the excited-state decay dynam-
ics of [PFe-(collidine)]+and PFe-Cl in dichloromethane solvent.
For PFe-Cl, the initially prepared electronically excited-state
decays with a time constant τavg ≈ 750 fs, consistent with
previously delineated heme photophysics.128,129 Identical to the
trends observed in the excited-state dynamics of the [PZn-
+
PFe-(L)1,2
]
complexes (vide supra), ligation of pyridine and
4-cyanopyridine results in no change of the transient spectral
signature and observed decay dynamics characteristic of PFe-
Cl (data not shown). On the other hand, [PFe-(collidine)]+
displays unique excited-state dynamics: following photoexci-
tation, this compound exhibits a fast relaxation component (τ
≈ 1 ps, A ≈ 72%), as well as longer characteristic relaxation
times of ∼7 ps (A ≈ 18%) and 655 ps (A ≈ 10%); such long-
lived ferric porphyrin excited-states are without precedent.
Additionally, [PFe-(collidine)]+ excited-state dynamics follow
the same solvent-dependent trends established for [PZn-PFe-
(collidine)]+: nonradiative lifetimes determined for [PFe-
(collidine)]+ in toluene are consistent with data indicating that
collidine is incapable of displacing Cl- from PFe-Cl in nonpolar
solvents.
Scheme 2 summarizes the excited-state relaxation dynamics
of [PZn-PFe-(L)1,2]+. Excited-state deactivation of [PZn-
PFe] is both solvent and PFe+ ligation-(spin)-state dependent;
these factors determine the relative magnitudes of charge
separation (kcs) and nonradiative(knr) rate constants. The driving
force for charge-separation, ∆Gcs, can be estimated from the
Weller equation:136
The fact that increasing the solvent polarity from methylene
chloride to benzonitrile results in almost no change in the [PZn-
PFe-(collidine)]+* electron-transfer dynamics, while use of
toluene solvent completely suppresses photoinduced charge
separation, can be rationalized from solvent-dependent NMR
spectroscopic data. The NMR spectrum of PFe(III) in 10:1 CD2-
Cl2/collidine manifests two aromatic collidine resonances [δ ≈
6.8 ppm (free collidine) and δ ≈ 8.0 ppm (ligated collidine)],
whereas similar spectra recorded in 10:1 toluene-d8/collidine
manifest only one collidine signal [δ ≈ 6.46 ppm]. These data
indicate that dissolution in toluene inhibits collidine binding,
even when this ligand is present at a several 1000-fold molar
excess. This result likely derives from inadequate solvation of
free chloride ions in a nonpolar medium.135
+
Excited-State Dynamics of [PFe-(L)1,2
]
Species. To pro-
e2
1
1
vide added insight into the origin of the unique excited-state
dynamics manifested by electronically excited [PZn-PFe-
∆Gcs ) e[E(ox)D - E(red)A] - E0,0
-
-
-
(
4πꢀoꢀs RDA 2RD
1
(135) The similarity of electron-transfer dynamics between the methylene
chloride and benzonitrile experiments is not as simple to rationalize. This
result may derive in part from a local solvation environment dominated
by collidine, or from the large driving force for charge separation (∼-
1.3 eV).
(8)
)
2RA
where E(ox)D and E(red)A are the oxidation and reduction
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10433

A R T I C L E S
Duncan et al.
with those delineated to [PFe-(collidine)]+ (Figure 9, inset),
suggesting that the combination of doming and a weak FeIII
axial ligand interaction may commonly give rise to [PFe-
(ligand)1]+ complexes with unusually long excited-state life-
times. Whether the genesis of this effect is due to a reduction
in the degree of overlap between the partly filled iron d-orbitals
and porphyrin π-symmetric molecular orbitals beyond some
critical amount, or to more obscure factors such as an altered
landscape of accessible excited-state nuclear relaxation modes,
is not presently known.
Scheme 2. Energy Level Diagram Showing the Major Decay
+
Pathways of [PZn-PFe-(L)1,2
]
Excited States
Whatever the origin of this effect, it is clear that the significant
reduction in the magnitude of knr observed for [PFe-(colli-
dine)]+* (and for [PFe-(2,6-lutidine)]+*, [PFe]+*[PF6]-*)
relative to that evinced for all other electronically excited [PFe-
(ligand)1,2]+ complexes plays an important role in determining
the observed [PZn-PFe-(collidine)]+* dynamics. Once knr is
significantly diminished, charge separation becomes a competi-
tive excited-state deactivation pathway in [PZn-PFe-(colli-
dine)]+*. Thus, despite the fact that ∆GCS (4-cyanopyridine)
> ∆GCS (pyridine) > ∆GCS (collidine) > ∆GCS (chloride), the
collidine-ligated species exhibits photoinduced charge separation
due to the apparent inability of the iron open d shell to provide
an efficient pathway for fast, nonradiative decay.
potentials of the porphyrin and iron center, respectively. The D
(RD) and A (RA) reactant sizes and the center-to-center distance
(RDA) determined from ZINDO optimized structures are RD )
RA ≈ 5.7 Å and RDA ≈ 11.9 Å. Using these parameters and the
potentiometric data reported in Table 1 highlights that ∆Gcs for
[PZn-PFe-(L)1,2]+* is strongly influenced by the PFe+ ligation
state [ligand (∆GCS): 4-cyanopyridine (-0.79 eV) < pyridine
(-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV);
solvent ) CH2Cl2]. Note that the pump-probe transient
absorption dynamics elucidated for [PZn-PFe-(L)1,2]+ species
are not correlated with the thermodynamic driving force for
photoinduced charge separation, as the [(PZn-P)+• FeII(L)]+
transient signal was only observed following optical excitation
of [PZn-PFe-(collidine)]+.
Conclusion
We have synthesized and examined the spectroscopy, poten-
tiometric properties, and transient dynamics of strongly coupled
meso-to-meso ethyne-bridged bis[(porphinato)metal] complexes
that possess Zn(II) and Fe(III) centers in a variety of axial ligand
(L) environments [pyridine (py), 4-cyanopyridine (4-CN-py),
2,6-dimethylpyridine (2,6-lutidine), and 2,4,6-trimethylpyridine
(collidine)]. The excited-state transient dynamics of [PZn-PFe-
+
(L)1,2
]
supermolecules as well as several [(porphinato)iron-
(ligand)1,2]+ ([PFe-(L)1,2]+) benchmarks were interrogated using
pump-probe spectroscopy. Femtosecond time scale transient
For PFe(II) and PFe(III) complexes, it has been established
previously that knr is fast [>(1 × 10-12) s-1].128,129 It would
+
thus be expected that strongly conjugated [PZn-PFe-(L)1,2
]
absorption spectroscopic experiments show that monomeric
+
complexes PFe-Cl, [PFe-(py)2]+, and [PFe-(4-
supermolecules would possess low quantum yields for charge
separation regardless of the PFe+ ligation state, due to the
prominent role played by the Fe open d shell in facilitating fast
excited-to-ground-state internal conversion. Collidine (and 2,6-
lutidine, not shown) coordination to [PFe]+ followed by optical
excitation clearly causes a significant fraction of [PFe]+* states
[PFe(L)1,2
]
CN-py)2]+ demonstrate subpicosecond-to-picosecond relaxation
dynamics; the observed ultrafast ground-state recovery is
consistent with the expectation that the partially occupied heme
d-manifold provides a multiplicity of states that facilitate large-
magnitude nonradiative decay rate constants.127-129 Interestingly,
the electronically excited states of [PFe-(collidine)]+, [PFe-
(2,6-lutidine)]+, and [PFe]+[PF6]- all manifest dynamics that
feature a significant, slow ground-state recovery component with
a time constant in excess of 0.5 ns; such long-lived (porphinato)-
iron(III) electronically excited states have heretofore been
without precedent.
to decay via a nonradiative process that is unusually slow with
+
respect to all other [PFe-(ligand)1,2
]
systems studied to date
(Figure 9). The origins of this effect remain unclear, but
differences in spin state, iron oxidation potential, and a chloride-
derived heavy-atom effect can be eliminated as possible
explanations by the above-described experiments. In contrast
to the other ligands studied, collidine’s 2- and 6-methyl groups
give rise to significant steric interactions with the macrocycle
plane. Such steric parameters are known to significantly distort
the planarity of the ferric porphyrin macrocycle107 and also to
contribute to ferric ion doming.106,107,137 To assess whether
ferric-ion doming and weak metal axial ligand interactions may
be partly responsible for the unique excited-state decay dynamics
of [PFe-(collidine)]+, [PFe]+[PF6]- was prepared by ligand
metathesis from PFe-Cl. Remarkably, the excited-state decay
dynamics of [PFe]+[PF6]-* are almost exactly superimposable
Comparative pump-probe transient absorption experiments
that interrogate the initially prepared excited states of PZn-
PFe-Cl, [PZn-PFe-(py)2]+, [PZn-PFe-(4-CN-py)2]+, [PZn-
PFe-(collidine)]+, and [PZn-PFe-(2,6-lutidine)]+ demonstrate
that the spectra of all these species are dominated by a broad,
intense NIR S1 f Sn transient absorption manifold. This
absorptive feature is a spectroscopic hallmark of this class of
highly conjugated multichromophoric compounds:62,125,126 such
signals are absent from transient absorption spectra recorded
for monomeric porphyrinoid species and from less-strongly
coupled multiporphyrin ensembles.62 While PZn-PFe-Cl*,
[PZn-PFe-(py)2]+*, and [PZn-PFe-(4-CN-py)2]+* evince
(136) Weller, A. Z. Phys. Chem. N. F. 1982, 133, 93-98.
(137) Scheidt, W. R. Acc. Chem. Res. 1977, 10, 339-345.
9
10434 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

Ethyne-Bridged PZn(II)−PFe(III) Complexes
A R T I C L E S
subpicosecond and picosecond time-scale relaxation of their
respective initially prepared electronically excited states to the
ground state, the excited-state dynamics observed for [PZn-
PFe-(2,6-lutidine)]+* and [PZn-PFe-(collidine)]+* show fast
relaxation to a [PZn+-PFe(II)] charge-separated state having
a lifetime of nearly 1 ns. Potentiometric data indicate that while
∆GCS for [PZn-PFe-(L)1,2]+* species is strongly influenced
by the PFe+ ligation state [ligand (∆GCS): 4-cyanopyridine
(-0.79 eV) < pyridine (-1.04 eV) < collidine (-1.35 eV) <
chloride (-1.40 eV); solvent ) CH2Cl2], the pump-probe
transient absorption dynamical data demonstrate that the nature
of the dominant excited-state decay pathway is not correlated
with the thermodynamic driving force for photoinduced charge
separation but depends on the ferric ion ligation mode. Sterically
bulky axial ligands (that drive a pentacoordinate PFe center and
a weak metal axial ligand interaction) serve to sufficiently
suppress the normally large-magnitude, nonradiative decay rate
constants characteristic of (porphinato)iron(III) complexes, and
thus make electron transfer a competitive excited-state deactiva-
tion pathway for [PZn-PFe-(L)1]+ supermolecules. While
ascertaining the genesis of these effects will require further
study, the data reported herein suggest that the combination of
metal doming and a weak FeIII axial ligand interaction may
commonly give rise to pentacoordinate [PFe-L1]+ complexes
with uncharacteristically long excited-state lifetimes. Such ligand
environments for (porphinato)iron(III) centers will enable
fabrication of PFe(III)-based strongly coupled multipigment
materials and supramolecular ensembles in which charge-
transfer dynamics play an unusually prominent role.
Acknowledgment. This work was supported by the National
Institutes of Health. T.V.D. thanks the Department of Education
for a GAANN pre-doctoral fellowship. M.J.T. is indebted to
the Office of Naval Research and the MRSEC Program of the
National Science Foundation for equipment grants for transient
optical instrumentation. We are grateful to Dr. H. C. Fry for
the preparation of the [PFe]+[PF6]- complex.
Supporting Information Available: Transient spectral data
and decay-associated spectra for electronically excited [PZn-
PFe(L)1,2]+ and [PFe(L)1,2]+ species. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA061388M
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J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10435