Intersystem crossing versus electron transfer in porphyrin-based donor-bridge-acceptor systems: Influence of a paramagnetic species

Article abstract of DOI:10.1021/ja0370488

We have investigated how the spin state of an acceptor influences the photophysical processes in a donor-bridge-acceptor (D-B-A) system. The system of choice has zinc porphyrin as the electron donor and high- or low-spin iron(III) porphyrin as the acceptor. The spin state of the acceptor porphyrin is switched simply by coordinating imidazole ligands to the metal center. The D-A center-center distance is 26. A, and the bridging chromophore varies from π-conjugated to a σ-bonded system. The presence of a high-spin iron(III) porphyrin in such systems has previously been shown to significantly enhance intersystem crossing in the remote zinc porphyrin donor, whereas no significant electron transfer to the iron porphyrin acceptor was observed, even though the thermodynamics would allow for photoinduced electron transfer. Here, we demonstrate that by switching the acceptor to a low-spin state, the dominating photophysical process is drastically changed; the low-spin system shows long-range electron transfer on the picosecond time-scale, and intersystem crossing occurs at its normal rate.

Full text of DOI:10.1021/ja0370488

Published on Web 05/11/2004
Intersystem Crossing versus Electron Transfer in
Porphyrin-Based Donor-Bridge-Acceptor Systems:
Influence of a Paramagnetic Species
Karin Pettersson,† Kristine Kilså,‡,§ Jerker Mårtensson,† and Bo Albinsson*,†
Contribution from the Department of Chemistry and Bioscience,
Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and
Beckman Institute, MC 139-74, California Institute of Technology, Pasadena, California 91125
Received July 3, 2003; E-mail: balb@chembio.chalmers.se
Abstract: We have investigated how the spin state of an acceptor influences the photophysical processes
in a donor-bridge-acceptor (D-B-A) system. The system of choice has zinc porphyrin as the electron
donor and high- or low-spin iron(III) porphyrin as the acceptor. The spin state of the acceptor porphyrin is
switched simply by coordinating imidazole ligands to the metal center. The D-A center-center distance
is 26 Å, and the bridging chromophore varies from π-conjugated to a σ-bonded system. The presence of
a high-spin iron(III) porphyrin in such systems has previously been shown to significantly enhance
intersystem crossing in the remote zinc porphyrin donor, whereas no significant electron transfer to the
iron porphyrin acceptor was observed, even though the thermodynamics would allow for photoinduced
electron transfer. Here, we demonstrate that by switching the acceptor to a low-spin state, the dominating
photophysical process is drastically changed; the low-spin system shows long-range electron transfer on
the picosecond time-scale, and intersystem crossing occurs at its “normal” rate.
Introduction
bridge-acceptor (D-B-A) systems with a high-spin iron(III)
porphyrin as the acceptor (Figure 1), we initially expected
Excitation energy transfer and photoinduced electron transfer
are processes of great interest in many research areas. The most
important energy- and electron-transfer processes occur in
natural photosynthesis. Light-harvesting molecules transfer
energy to the reaction center where a sequence of electron-
transfer reactions takes place. Several research groups are trying
to design systems capable of artificial photosynthesis. To achieve
this, a detailed understanding of the energy and the electron-
transfer processes is of considerable value. Also, for the
development of molecular scale electronic components, such
as switches, transistors, and rectifiers, etc., these processes are
of great importance. Several review articles have been published
on both of these subjects.1-9
electron transfer to be the dominating deactivation pathway for
the lowest singlet excited state of the donor moiety.14 It turned
out, however, not to be the case. The singlet excited state of
the donor was significantly quenched, but by an enhanced
intersystem crossing process rather than electron transfer. The
proximity between the donor porphyrin and the high-spin
iron(III) porphyrin was suggested to be responsible for the
enhanced intersystem crossing. The purpose of the present study
is to investigate if the spin state of the paramagnetic iron(III)
porphyrin influences the rate of the different deactivation
processes, in particular electron transfer and donor intersystem
crossing. In the iron(III) porphyrins, the spin state is sensitive
to ligands that coordinate to the metal, and it has been suggested
that Lewis base ligands such as imidazole induce a low-spin (S
) 1/2) electron configuration whereas a chloride ligand induces
The present study is part of an ongoing effort to investigate
how the rate of energy and electron transfer could be modulated
by the intervening medium.10-14 When designing the donor-
5
a high-spin (S ) /2) situation.15
† Chalmers University of Technology.
The studied systems (Figure 1) consist of three separate
chromophores: donor, bridge, and acceptor. The donor is
a zinc(II) 5,15-diphenyl-2,8,12,18-tetraethyl-3,7,13,17-tetra-
methylporphyrin (ZnP). The corresponding acceptor porphyrin
is either a high-spin iron(III) porphyrin (Fe(Cl)P) or a low-spin
‡ California Institute of Technology.
§ Present address: Department of Physical Chemistry, Box 579, Uppsala
University, SE-75123 Uppsala, Sweden.
(1) Speiser, S. Chem. ReV. 1996, 96, 1953-1976.
(2) Turro, N. J. Pure Appl. Chem. 1977, 49, 405-429.
(3) Ward, M. D. Chem. Soc. ReV. 1997, 26, 365-375.
(4) Closs, G. L.; Miller, J. R. Science 1988, 240, 440-447.
(5) Gust, D.; Moore, T. A.; Moore, A. L. IEEE Eng. Med. Biol. Magn. 1994,
13, 58-66.
(6) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(7) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143-150.
(8) Burrell, A. K.; Wasielewski, M. R. J. Porphyrins Phthalocyanines 2000,
4, 401-406.
(9) Anderson, H. L. Chem. Commun. 1999, 2323-2330.
(10) Kilså, K.; Kajanus, J.; Mårtensson, J.; Albinsson, B. J. Phys. Chem. B 1999,
103, 7329-7339.
(11) Andre´asson, J.; Kajanus, J.; Mårtensson, J.; Albinsson, B. J. Am. Chem.
Soc. 2000, 122, 9844-9845.
(12) Andre´asson, J.; Zetterqvist, H.; Kajanus, J.; Mårtensson, J.; Albinsson, B.
J. Phys. Chem. A 2000, 104, 9307-9314.
(13) Kilså, K.; Kajanus, J.; Macpherson, A. N.; Mårtensson, J.; Albinsson, B.
J. Am. Chem. Soc. 2001, 123, 3069-3080.
(14) Kilså, K.; Kajanus, J.; Larsson, S.; Macpherson, A. N.; Mårtensson, J.;
Albinsson, B. Chem.-Eur. J. 2001, 7, 2122-2133.
(15) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543-555.
9
6710
J. AM. CHEM. SOC. 2004, 126, 6710-6719
10.1021/ja0370488 CCC: $27.50 © 2004 American Chemical Society

Porphyrin-Based Donor−Bridge−Acceptor Systems
A R T I C L E S
Figure 1. Structures of the low-spin iron(III) porphyrin, Zn(Im)P-RB-Fe(Im)2P+, the high-spin iron(III) porphyrin, ZnP-RB-Fe(Cl)P, and the bridges
RB ) OB, BB, NB, and AB.
iron(III) porphyrin (Fe(Im)2P+) with two imidazole ligands
coordinated to the iron. Please note, when imidazole is present
it also coordinates to the zinc porphyrin (Zn(Im)P), thus also
altering the donor properties. The four bridging chromophores
are 1,4-bis(phenylethynyl)bicyclo-octane (OB), 1,4-bis(phenyl-
ethynyl)benzene (BB), 1,4-bis(phenylethynyl)naphthalene (NB),
and 9,10-bis(phenylethynyl)anthracene (AB). The methyl groups
in the porphyrin â-positions cause sterical hindrance and force
the porphyrin to be nearly perpendicular to the bridging
chromophore, thus preventing the chromophores from being
π-conjugated.
D-A center-to-center distance, no electron transfer was ob-
served. In lack of evidence for electron transfer, we have shown
that enhanced intersystem crossing and energy transfer are the
major deactivation routes.14 Enhanced intersystem crossing in
zinc or free base porphyrin donors linked to paramagnetic
copper(II) porphyrins has also been observed by Aseno-Someda
and co-workers.23-25
The present work focuses on the deactivation channels for
the series of D-B-A systems with a low-spin Fe(Im)2P+
acceptor (Figure 1). Furthermore, the quenching properties of
the D-B-A systems with a high-spin Fe(Cl)P are compared
to those containing a low-spin Fe(Im)2P+ acceptor.
Studies on related donor-acceptor systems with zinc and
iron(III) porphyrins have shown that electron transfer is the
major deactivation route at short D-A distances.16-22 For our
ZnP-RB-Fe(Cl)P system (Figure 1), which all have a 26 Å
Experimental Section
Materials. The syntheses of the D-B-A systems as well as the
relevant reference compounds D-B are described elsewhere.10,14,26 All
solvents - 2-methyl-tetrahydrofuran (2Me-THF), methylene chloride
(CH2Cl2), N,N-dimethylformamide (DMF), chloroform (CHCl3) - were
(16) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992, 114, 6227-
6238.
(17) Osuka, A.; Tanabe, N.; Kawabata, S.; Yamazaki, I.; Nishimura, Y. J. Org.
Chem. 1995, 60, 7177-7185.
(18) Osuka, A.; Maruyama, K.; Mataga, N.; Asahi, T.; Yamazaki, I.; Tamai, N.
J. Am. Chem. Soc. 1990, 112, 4958-4959.
(22) de Rege, P. J. F.; Williams, S. A.; Therien, M. J. Science 1995, 269, 1409-
1413.
(19) Tamiaki, H.; Nomura, K.; Maruyama, K. Bull. Chem. Soc. Jpn. 1994, 67,
1863-1871.
(23) Asano-Someda, M.; Kaizu, Y. Inorg. Chem. 1999, 38, 2303-2311.
(24) Toyama, N.; Asano-Someda, M.; Ichino, T.; Kaizu, Y. J. Phys. Chem. A
2000, 104, 4857-4865.
(20) Portela, C. F.; Brunckova, J.; Richards, J. L.; Schollhorn, B.; Iamamoto,
Y.; Magde, D.; Traylor, T. G.; Perrin, C. L. J. Phys. Chem. A 1999, 103,
10540-10552.
(25) Asano-Someda, M.; Jinmon, A.; Toyama, N.; Kaizu, Y. Inorg. Chim. Acta
2001, 324, 347-351.
(21) Mataga, N.; Yao, H.; Okada, T.; Kanda, Y.; Harriman, A. Chem. Phys.
1989, 131, 473-480.
(26) Kilså, K.; Macpherson, A. N.; Gillbro, T.; Mårtensson, J.; Albinsson, B.
Spectrochim. Acta, Part A 2001, 57, 2213-2227.
9
J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004 6711

A R T I C L E S
Pettersson et al.
used as purchased. Imidazole (Im) was purchased from Fluka AG and
used as received. To form the low-spin complexes, the high-spin
complexes were dissolved in a solution containing imidazole in excess
to ascertain full conversion to Zn(Im)P and Fe(Im)2P+ (vide infra).
Ground-state absorption spectroscopy was performed with a Cary
4 Bio spectrophotometer. The spectral bandwidth was 0.5 nm, and the
scanning rate was 300 nm/min. Spectrophotometric titrations of FeP
with imidazole were performed in CHCl3, 2Me-THF, DMF, and
CH2Cl2.
block the pump every second pulse. Subsequently, the pump beam was
sent through a computer-controlled optical delay line (Aerotech) and
then focused with reflective optics on the sample at a small angle
relative to the probe beam. The polarization of the pump beam relative
to the polarization of the probe beam was set to the magic angle by a
Berek compensator (New Focus), and the pulse energy at the sample
in a typical experiment was 1.4 µJ/pulse. The intensity of the probe
beam was reduced by two neutral density filters (one OD ) 4 and one
variable 0-2), before it was focused into a thin sapphire plate to
generate a white light continuum. A second beam splitter (50/50) was
used to split the generated white light continuum into the probe and
the reference beams. Both beams were focused on the sample, with
the probe beam overlapping the pump beam. After the sample, both
probe and reference beams were focused onto the slit of a computer-
controlled monochromator (ISA, TRIAX 180). Three photodiodes were
used to monitor the intensity of probe, reference, and pump beams,
respectively. The signals were gated by boxcar integrators (SR250,
Stanford Research Systems), fed into a PC-based A-D-card, and
averaged by a LabView program.
Steady-State Fluorescence Spectroscopy. The fully corrected
emission spectra were recorded with a SPEX Fluorolog 3 or a SPEX
Fluorolog τ2 spectrofluorimeter. The absorbance at the excitation
wavelength was kept low, approximately 0.05 (corresponding to a
concentration of approximately 2.5 µM), to avoid inner filter effects
and intermolecular interactions. The systems were excited at the
maximum of the donor Q-band absorption (545-555 nm, depending
on solvent and coordinating species). Steady-state fluorescence mea-
surements were performed in 2Me-THF, DMF, and CH2Cl2.
Time-resolved fluorescence spectroscopy was carried out using
the time-correlated single photon counting (TCSPC) method. The
excitation source was the second harmonic of a Ti:Sapphire oscillator
(Tsunami, Spectra Physics), pumped by a continuous-wave frequency-
doubled diode pumped Nd:YVO4 laser (Millennia Vs, Spectra Physics).
The photons were collected by a microchannel plate photomultiplier
tube (MCP-PMT R3809U-50, Hamamatsu) and fed into a multichannel
analyzer with 4096 channels yielding a time resolution of about 10 ps
(fwhm). A diluted silica sol scattering solution was used to collect the
instrument response signal. Further, the collected crude decay curves
were iteratively convoluted and evaluated using the software package
F900 (Edinburgh Instruments). The excitation wavelength was in the
Soret band region (400-420 nm) for technical reasons. In all experi-
ments, the absorption at the excitation wavelength was set to 0.1-0.4,
and at least 10 000 photons were collected in the top channel within 5
min. Time-resolved fluorescence was performed in 2Me-THF, DMF,
and CH2Cl2.
Nanosecond transient absorption kinetic traces were recorded by
using the signal from an OPO (Surelite OPO, Continuum) pumped by
a Nd:YAG laser (fwhm ) 6 ns, Surelite II-10, Continuum). A xenon
arc lamp was used as probe light source followed by a conventional
monochromator photomultiplier system (symmetrical Czerny-Turner
arrangements and a five-stage Hamamatsu R928). Data acquisition was
performed via a Tektronix TDS 2022 digital oscilloscope interfaced to
a PC-based A-D-card and averaged in a LabView program.
All samples were degassed either by bubbling argon gas for 10
minutes or by six freeze-pump-thaw cycles. The ground-state
absorbance at the excitation wavelength was below 0.05, and 2Me-
THF was used as solvent. The systems were excited at the maximum
of the donor Q-band absorption (545-555 nm, depending on solvent
and coordinating species). Further, the kinetic traces were collected at
the maximum donor triplet-triplet absorption (470 nm for both ZnP
and Zn(Im)P), and 64 decay traces were averaged to form the kinetic
traces.
Transient absorption spectra were recorded at different delay times
with a wavelength step of 2 nm and a monochromator bandwidth of
14 nm. Transient absorption decays were recorded at different
wavelengths with a monochromator bandwidth of 14 nm. The sample
was held in a static 1 mm path length cuvette, and the optical density
at the excitation wavelength was approximately 1. The decay traces
were fitted to a sum of exponentials with the Matlab software package.
2Me-THF was used as solvent.
Electron paramagnetic resonance (EPR) X-band spectra were
measured on a Bruker EMX instrument with a standard TE102 cavity.
In all experiments, the modulation amplitude was 3 G, the microwave
power was 2 mW, and the frequency was 9.378 GHz. Observed g-values
are found as g ) hν/µBH0, where h is Planck’s constant, µB is the Bohr
magneton, ν is the operating frequency, and H0 is the applied magnetic
field value used to observe the EPR feature. The solvent was 2Me-
THF, and the spectra were recorded at 20 K. The spectra were corrected
for the cavity background measured for a sample of 2Me-THF.
Results and Discussion
The purpose of this study is to investigate if and how the
photophysical properties of a D-B-A system are influenced
by the spin state of a remote acceptor. Furthermore, the
electronic structure of the bridge unit varies in the series of
D-B-A systems, and this is expected to have a marked
influence on the electronic communication between the donor
and acceptor. To this end, we will compare the photophysics
of a series of low-spin iron(III) porphyrin complexes (Zn(Im)P-
RB-Fe(Im)2P+) with the corresponding high-spin iron(III)
porphyrin complexes (ZnP-RB-Fe(Cl)P) and discuss the
differences (Figure 1). This section will be organized as
follows: First, EPR and UV/vis measurements that confirm that
the spin state of the iron porphyrin changes from high- to low-
spin by introducing imidazole are presented. Second, the total
quenching efficiencies for the excited states of the zinc porphyrin
donors based on steady-state and time-resolved fluorescence
measurements are presented. Here, we show that the low-spin
iron(III) porphyrin complexes behaves quite differently from
the corresponding high-spin iron(III) porphyrin complexes. Next,
we demonstrate that increased intersystem crossing previously
observed as the dominating deactivation pathway for the singlet
excited zinc porphyrin in the high-spin complexes14 is only a
minor contribution to the quenching of the low-spin iron(III)
porphyrin complexes. Finally, we show that the quenching in
the low-spin iron(III) porphyrin complexes is dominated by
For femtosecond transient absorption measurements, the pump-
probe technique was employed. The sample was excited at the Q-band
maximum (545-555 nm, coordination dependent) with the second
harmonic of the signal from a TOPAS (Light Conversion Ltd.). The
TOPAS was pumped by a Ti:Sapphire regenerative amplifier (Spitfire,
Spectra Physics) at 1 kHz repetition rate. The regenerative amplifier
was pumped by a frequency-doubled diode-pumped Nd:YLF laser
(Evolution-X, Spectra Physics) and seeded by a mode-locked femto-
second Ti:Sapphire laser (Tsunami, Spectra Physics). The seed laser
was pumped by a continuous-wave frequency-doubled diode-pumped
Nd:YVO4 laser (Millennia Vs, Spectra Physics). Further, the output
from the regenerative amplifier (∼130 fs) was split into two beams
with a beam splitter (70/30), the pump beam and the probe beam. The
pump beam (the output from the TOPAS) was chopped at 500 Hz to
9
6712 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004

Porphyrin-Based Donor−Bridge−Acceptor Systems
A R T I C L E S
spectrum is in agreement with a similar bis-(N-methyl imidazole)
octaethyl tetraphenyl iron(III) porphyrin and is typical of low-
spin Fe(III) porphyrin complexes.30 Although the information
about the actual structure of the Fe(Cl)P complex is not
conclusive,27 the ground-state electronic absorption and EPR
measurements reported above clearly show that Fe(Cl)P in 2Me-
THF exists as a high-spin complex provided no imidazole is
added. The measurement also shows that addition of imidazole
to the 2Me-THF solution of the high-spin ferric complex
efficiently converts it to a low-spin Fe(Im)2P+ complex. The
same results according to UV/vis measurements are obtained
for the other two solvents used in this study (CH2Cl2, DMF).31
Therefore, although Fe(Cl)P might exist as differently coordi-
nated species due to solvation and solvent coordination, neither
of the solvents used in this study have sufficiently high ligand-
field strength to cause a change in the spin state of the ferric
porphyrin from high- to low-spin.
In Figure 3, the absorption spectra of the low-spin iron(III)
porphyrin complex Zn(Im)P-NB-Fe(Im)2P+ and of the high-
spin iron(III) porphyrin complex ZnP-NB-Fe(Cl)P are shown
together with the donor reference compounds, Zn(Im)P-NB
and ZnP-NB, and the acceptors, Fe(Cl)P and Fe(Im)2P+. It is
possible to resolve the dimer spectrum into the reference and
the acceptor spectra, indicating that we have electronically
separate chromophores. Introducing imidazole red-shifts and
narrows the spectrum, indicating a more symmetric porphyrin
because the central iron atom changes from five-coordinated
to six-coordinated.15 In addition, the peak at 650 nm which is
characteristic for a high-spin iron(III) porphyrin disappears when
switching to a low-spin iron(III) porphyrin (Figure 2).
Comparing the steady-state fluorescence spectra of the high-
and low-spin complexes immediately indicates some important
differences (Figure 3). Because the iron(III) porphyrin is
nonfluorescent in both of its spin states, only the ZnP emission
is detected. The fluorescence of ZnP shows the expected red-
shift and a slight change in shape due to imidazole coordination.
The important difference, though, is seen when comparing the
series of different bridging chromophores. The quenching is
clearly due to introduction of the acceptor unit in all cases, but
while the high-spin complexes all show a high degree of
quenching the low-spin series shows a much larger variation
among the members of the series. Furthermore, as will be shown
quantitatively below, the small quenching observed in Zn(Im)P-
OB-Fe(Im)2P+ is entirely due to singlet energy transfer
Zn(Im)P f Fe(Im)2P+. These differences - yet only indications
- will be shown to be due to different quenching mechanisms
for the low- and high-spin complexes.
Total Quenching Efficiencies. The fluorescence lifetimes
(τ) of the molecules measured by single photon counting and
the total quenching efficiencies (EDBA) are presented in Table
1. To calculate EDBA ) 1 - τDBA/τDB, the decay traces of the
D-B (τDB) and the D-B-A (τDBA) compounds were recorded
while observing at the maximum emission wavelength. The
decays were fitted to biexponential expressions, and the second
component was a long-lived species (∼1 ns, most likely traces
of zinc porphyrin dimers) with a small preexponential factor
Figure 2. Spectrophotometric titration on FeP (∼10 µM) with imidazole.
The arrows indicate an increasing amount of imidazole (0 f 0.2 M). Solvent
2Me-THF.
electron transfer in contrast to the deactivation in the high-spin
complexes. A corresponding set of measurements using the
related free-base porphyrin donor showed similar results.
Because these results just confirm the observations made with
the zinc porphyrin donor, they were omitted from this Article.
Steady-State Absorption, Emission, and Spin-State Char-
acterization. Because the purpose of this study is to investigate
the effect of the acceptor spin state on the relative rates of the
possible deactivation pathways in a D-B-A system, a tool to
change this spin state is essential. Ligand-field strength, one of
the major factors determining the ground-state electronic
configurations of iron(III) porphyrins, has been recognized as
an efficient and reliable tool to access different spin states of
ferric porphyrins. Addition of a very strong-field ligand, such
as imidazole, to high-spin ferric porphyrin complexes can be
regarded as a classic way to obtain the corresponding low-spin
complexes.15 In Figure 2, a spectrophotometric titration on
Fe(Cl)P in 2Me-THF with imidazole is shown. The presence
of several well-defined isosbestic points indicates that two
species are present - Fe(Cl)P27 and Fe(Im)2P+. Moreover,
analyses of binding isotherms26 (not shown) confer that two
imidazole ligands coordinate to FeP in accordance to what has
been shown for tetraphenyl and octaethyl Fe(III) porphyrins.15
The EPR spectra of Fe(Cl)P and Fe(Im)2P+ show distinct
differences in accordance with a change in spin state of the iron
center (Figure S1, Supporting Information). The spectrum of
Fe(Cl)P in 2Me-THF clearly resembles that of a high-spin (S
5
) /2) Fe(III) species, showing a high-intense axial symmetry
signal with a g-value around 6.0 and a smaller signal with g )
2.0.28 Such a signal is typical for a five-coordinated ferric
compound with a moderate-field ligand such as a halide15 and
is essentially identical to spectra found for tetraphenyl and
methyl-substituted tetraphenyl iron(III) porphyrins.29 Adding the
strong-field ligand imidazole to the 2Me-THF solution of
Fe(Cl)P accomplishes a change in the spin state, from the high-
1
spin to the low-spin (S ) /2), of the ferric complex as can be
seen in the EPR spectrum. The EPR spectrum of Fe(Im)2P+ in
2Me-THF shows g-values of 2.8, 2.4, and 1.6. This rhombic
(27) In the following text, the abbreviation Fe(Cl)P is used to denote a high-
spin ferric porphyrin acceptor, irrespective of its actual composition and
structure. Although always in the high-spin state, the coordination sphere
of iron(III) will vary between the various solvents used due to their different
solvation and coordination abilities.
(28) Palmer, G. In Iron Porphyins. Part II; Lever, A. B. P., Gray, H. B., Eds.;
Addison-Wesley Publishing Co.: Reading, MA, 1983; pp 43-88.
(29) Manso, C.; Neri, C. R.; Vidoto, E. A.; Sacco, H. C.; Ciuffi, K. J.; Iwamoto,
L. S.; Iamamoto, Y.; Nascimento, O. R.; Serra, O. A. J. Inorg. Biochem.
1999, 73, 85-92.
(30) Ogura, H.; Yatsunyk, L.; Medforth, C. J.; Smith, K. M.; Barkigia, R. M.;
Renner, M. W.; Melamed, D.; Walker, F. A. J. Am. Chem. Soc. 2001, 123,
6564-6578.
(31) Gouterman, M. In Iron Porphyins. Part III; Lever, A. B. P., Gray, H. B.,
Eds.; Addison-Wesley Publishing Co.: Reading, MA, 1983; pp 69-74.
9
J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004 6713

A R T I C L E S
Pettersson et al.
Figure 3. Upper: Absorption spectra. Lower: Emission spectra. Solvent 2Me-THF.
Table 1. Fluorescence Lifetimes of the Molecules (τ) and the Total Quenching Efficiencies (EDBA) in 2Me-THF at Room Temperature
sample
τ (ps)
EDBAa (%)
sample
τ (ps)
EDBAa (%)
ZnP
1480
119
1356
262
1389
303
1390
486
Zn(Im)P
1347
57
1092
345
1268
519
1243
1007
1311
ZnP-AB-Fe(Cl)P
91 ( 1
81 ( 2
78 ( 2
67 ( 3
Zn(Im)P-AB-Fe(Im)2P+
Zn(Im)P-AB
95 ( 1
73 ( 2
58 ( 4
23 ( 8
ZnP-AB
ZnP-NB-Fe(Cl)P
ZnP-NB
Zn(Im)P-NB-Fe(Im)2P+
Zn(Im)P-NB
ZnP-BB-Fe(Cl)P
ZnP-BB
Zn(Im)P-BB-Fe(Im)2P+
Zn(Im)P-BB
ZnP-OB-Fe(Cl)P
ZnP-OB
Zn(Im)P-OB-Fe(Im)2P+
Zn(Im)P-OB
1465
a The error margins for EDBA are based on an estimated lifetime variation of (5% (ø2 < 1.4).
(less than 3%). Nevertheless, the long-lived species has a
pronounced impact on the steady-state emission spectra, and
this makes the efficiencies calculated from time-resolved more
reliable than those estimated from steady-state measurements.
The general trend observed from the steady-state measure-
ments prevails; the degree of quenching increases down the
series of bridges, OB < BB < NB < AB. Furthermore, the
different bridges have a larger effect on the quenching efficien-
cies for the imidazole complexed series than in the high-spin
cases. This, again, indicates different mechanisms for quenching
in the high- and low-spin complexes.
Nanosecond transient absorption was performed to study
how the paramagnetic iron(III) porphyrin acceptor influences
the intersystem crossing rate in the donor porphyrin. In Figure
4, the transient absorption traces of the excited zinc porphyrin
triplet state - dimer and reference substances - are shown
together with the expected traces, which are simulated assuming
the intersystem crossing rate constants of the dimer (D-B-
A), kisc, and reference substance (D-B), kisc0, to be equal.
Previously, we have shown that the high-spin iron(III) porphyrin
enhances the intersystem crossing rate in the donor porphyrin.14
In the low-spin iron(III) porphyrin systems, the intersystem
crossing rate in the donor porphyrin is not enhanced. To reach
a quantitative result, kisc and kisc0, respectively, are compared.
The quantum yields for intersystem crossing and fluorescence
in the absence of the acceptor (i.e., for the reference substances)
are given by:
0
Φisc0 ) kisc0‚τDB
Φf 0 ) kf ‚τDB
(1)
0
where kf and τDB are the rate constant for fluorescence and
singlet lifetime of the reference substance, respectively. Like-
wise, the quantum yields for intersystem crossing and fluores-
cence in the presence of the acceptor (i.e., for the dimers) are
given by:
Φisc ) kisc‚τDBA
Φf ) kf‚τDBA
(2)
where kf and τDBA are the rate constant for fluorescence and
9
6714 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004

Porphyrin-Based Donor−Bridge−Acceptor Systems
A R T I C L E S
ment of the intersystem crossing rate (dashed line “expected
trace”). For the high-spin complex (bottom spectra), the
measured ∆ADBA is larger than the “expected trace”; that is,
the intersystem crossing rate is enhanced. On the other hand,
the ∆ADBA for the low-spin complex (top spectra) is at the same
absolute ∆A-value as the expected trace; that is, there is no
enhancement of the intersystem crossing rate. The ∆ADBA values
for the π-bridged systems are more difficult to analyze
quantitatively than those for the OB-bridged system, due to
donor triplet state quenching in these cases (vide infra).
Nevertheless, the qualitative difference between the low- and
high-spin systems seems to hold.
The triplet lifetimes of ZnP-OB, ZnP-OB-Fe(Cl)P,
Zn(Im)P-OB, and Zn(Im)P-OB-Fe(Im)2P+ are all approxi-
mately the same (∼5 µs), indicating that there is no triplet energy
transfer from the donor to the acceptor for the OB-bridged
systems.14,32 At first sight, the uniform 5 µs triplet lifetime of
the OB-bridged systems, which seems to be independent of the
spin state of the acceptor, appears to be puzzling. However, for
the zinc porphyrin used in this study, that is, a 5,15-diphenyl-
2,3,7,8,12,13,17,18-octalkyl-substituted porphyrin, it has been
shown that the triplet lifetime is many orders of magnitude
shorter than for the corresponding tetraphenyl or octaalkyl
porphyrins.12 This is due to a conformational relaxation that
leads to a nonplanar distortion of the porphyrin macrocycle in
the excited triplet state.33 This leads to a dramatic shortening
of the triplet lifetime, and the distortion was shown to be
thermally activated and consequently the lifetime is strongly
temperature dependent. This might be the reason for not
observing an enhancement of the intersystem crossing process
(T1 f S0) from the excited triplet to the ground state; the excited
state is already heavily quenched by the conformational distor-
tion, and the much smaller influence from the spin state of the
acceptor is not possible to observe.
Figure 4. Kinetic traces (at 470 nm where 3ZnP* has a maximum
absorption) of dimer and reference substances for the low-spin acceptor
(top diagram) and for the high-spin acceptor (bottom diagram). The expected
traces for the dimers using the observed quenching rate constants and
assuming kisc ) kisc are shown with dashed lines. The black curves are
monoexponential fittings corresponding to 5 µs. Solvent 2Me-THF.
0
lifetime of the dimer, respectively. Now, because the absorption
spectrum (Figure 3) of the donor part (ZnP) is unaffected by
appending the acceptor (FeP), it can be concluded that the
radiative rate constants are equal for the donor fluorescence,
0
that is, kf ) kf . By combining eqs 1 and 2, the ratio of the rate
constants for intersystem crossing is given by:
Femtosecond transient absorption was primarily performed
to detect the ZnP radical cation (ZnP•+) which is one of the
products resulting from intramolecular electron transfer. It would
be satisfying if signatures of the FeP radical also could be
detected, but they are probably too weak and/or broad and
therefore hidden among the other absorbing species in the
transient absorption spectra, which makes the assignment
difficult. As an example, the transient absorption spectra of the
low-spin complex (Zn(Im)P-BB-Fe(Im)2P+), the high-spin
complex (ZnP-BB-Fe(Cl)P), and the corresponding reference
substances (Zn(Im)P-BB and ZnP-BB) recorded at 1 ns delay
time are shown in Figure 5. The strongest triplet absorption
(3ZnP*) is centered around 470 nm, and the singlet absorption
(1ZnP*) is centered around 500 nm.12,26,34-36 At 1 ns delay time,
3ZnP* absorption dominates the spectra between 450 and 520
nm for the reference and high-spin complex but is significantly
weaker for the low-spin complex. Larger ∆A values for the high-
spin complexes around 470 nm indicate that more triplets are
formed, supporting the conclusion of enhanced intersystem
0
kisc Φisc/Φisc
∆ADBA/∆ADB
)
)
(3)
0
Φf/Φf0
τDBA/τDB
kisc
The right-hand side of eq 3 needs further comments. ∆ADBA
and ∆ADB are the measured maximum triplet absorption of the
donor in the dimer and reference substance estimated from the
nanosecond transient absorption trace at 470 nm (ZnP and
Zn(Im)P). In other words, we are measuring the ZnP triplet
formation with nanosecond transient absorption to estimate the
0
ratio kisc/kisc (eq 3). The values of ∆ADBA and ∆ADB are
measured at “time zero”; however, as the triplet lifetimes of
the reference and dimer substances (vide infra) are equal, the
ratio between ∆ADBA and ∆ADB can be calculated at any time.
From the ∆A values, the kisc/kisc0 ratio is calculated (eq 3) using
the measured singlet lifetimes (τDBA and τDB) in Table 1. The
ratios between the intersystem crossing rate constants (eq 3)
are 2.0 for ZnP-OB-Fe(Cl)P and 1.0 for Zn(Im)P-OB-
Fe(Im)2P+. These measurements show that while intersystem
crossing in the high-spin complexes is significantly enhanced
the low-spin complexes have “normal” intersystem crossing rates
and, consequently, the quenching of the singlet excited donor
porphyrin must be caused by different processes in the two
cases. In Figure 4, we have visualized the differences between
the high- and low-spin complexes by indicating how the
D-B-A trace would look if kisc/kisc0 ) 1, that is, no enhance-
(32) Andre´asson, J.; Kyrychenko, A.; Mårtensson, J.; Albinsson, B. Photochem.
Photobiol. Sci. 2002, 1, 111-119.
(33) Kyrychenko, A.; Andre´asson, J.; Mårtensson, J.; Albinsson, B. J. Phys.
Chem. B 2002, 106, 12613-12622.
(34) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.;
Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617-6628.
(35) Brun, A. M.; Harriman, A.; Heitz, V.; Sauvage, J. P. J. Am. Chem. Soc.
1991, 113, 8657-8663.
(36) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111, 6500-
6506.
9
J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004 6715

A R T I C L E S
Pettersson et al.
complex, Zn(Im)P-OB-Fe(Im)2P+, shows no build-up time
at 680 nm but only a decay on the nanosecond time-scale.
Transient absorption traces at 500 nm were also recorded for
all dimers. By and large, the lifetime of the singlet excited ZnP
plus a longer lifetime, presumably due to ZnP triplet decay,
could be fitted to the data.
In summary, we have detected the zinc porphyrin radical
cation, ZnP•+, formed by electron transfer in the low-spin
complexes with the π-conjugated bridges (AB, NB, BB). This
is in marked contrast to the corresponding high-spin complexes
where enhanced intersystem crossing induced by the remote
1
FeP(Cl) is the main source of ZnP* quenching. As a result,
simply adding imidazole switches the present D-B-A system
from a situation where the excitation energy is localized on the
donor (increased intersystem crossing) to a system that performs
electron transfer on the picosecond time-scale. Because the low-
spin complex with the OB-bridge does not show the ZnP•+
absorption peak, and as no enhancement of intersystem crossing
is detected by the nanosecond transient absorption measure-
ments, we can conclude that singlet energy transfer is the only
feasible deactivation channel. It is also reassuring that the
estimated Fo¨rster rate constant is of the same order as the
quenching rate constant in this case (vide infra). In this respect,
these findings are in line with the results obtained before with
similar systems bearing a gold(III) porphyrin acceptor (ZnP-
RB-AuP+).13
Figure 5. Transient absorption spectra at 1 ns delay time. The spectra of
ZnP-BB-Fe(Cl)P and Zn(Im)P-BB-Fe(Im)2P+ are enlarged 3 times to
make them comparable to the reference spectra. Solvent 2Me-THF.
crossing in the high-spin complexes. Furthermore, in the low-
spin complex spectrum, a distinct peak around 680 nm is
evident, which is assigned to ZnP•+.26,37,38 In contrast, in the
high-spin complex spectrum, no distinct peak around 680 nm
is found, and its spectrum is interpreted as broad overlapping
1ZnP* and 3ZnP* bands with negative contributions from
stimulated emission at 580 and 640 nm (emission maxima of
the high-spin complex).
Rate Constants. To quantitatively compare the different
systems, the rate of donor fluorescence quenching is calculated
(eq 4) and collected in Table 2. The relevant energy levels and
potential relaxation pathways for the ZnP/Fe(III)P systems are
also shown in Figure 7. The total quenching rate constant, kq,
is equal to the sum of all additional processes that contributes
to the increased deactivation rate of the donor, and in eq 4 this
has been expressed as the sum of rate constants for the Fo¨rster
energy transfer (kFo¨rster), the increased intersystem crossing (kisc
- kisc0), and a rest-term (krest).
In Figure 6, transient absorption decay traces monitored at
680 nm are shown for the high- and low-spin complexes of the
four different bridged systems. The fast component that
dominates the decays at short times (<2 ps) is due to the
relaxation of the FeP excited state which is directly excited by
the laser.14 The donor, ZnP, cannot be excited exclusively, and
with 545 nm excitation 20-50% (solvent and coordination
species dependent) of the incident light excites FeP.
At longer times, the differences between the high- and low-
spin complexes are striking; for example, for ZnP-NB-
Fe(Cl)P, a monotonic decaying transient is observed, while for
the Zn(Im)P-NB-Fe(Im)2P+ complex, the transient absorption
at 680 nm builds up before it decays. The decays at longer decay
times at 680 nm of the high-spin complexes can accurately be
fitted to the same lifetimes as the fluorescence decays of the
dimers. Further, the low-spin complexes with fully conjugated
bridges (complexes with AB-, NB-, and BB-bridges) exhibit a
build-up time followed by a slower decay at 680 nm. The build-
up times in the transients of the low-spin complexes Zn(Im)P-
NB-Fe(Im)2P+ and Zn(Im)P-BB-Fe(Im)2P+ are the same as
the fluorescence lifetimes; that is, the formation of ZnP•+ is
directly linked to the decay of the singlet excited ZnP. In
contrast, the rise time in the transient of Zn(Im)P-AB-
Fe(Im)2P+ is significantly faster (10-20 ps) than the fluorescent
lifetime (57 ps), which might reflect an equilibrium situation
between the excited ZnP and the intermediate charge-separated
species, Zn(Im)P•+-AB•--Fe(Im)2P+. The potential competi-
tion between a stepwise and direct electron-transfer mechanism
is presently being studied in the related systems with a gold(III)
porphyrin acceptor.39 In addition, the low-spin OB-bridged
1
1
kq )
-
) kFo¨rster + (kisc - kisc0) + krest (4)
τDBA τDB
Please observe that krest represents different deactivation pro-
cesses - electron transfer or enhanced intersystem crossing -
depending on which system is discussed. The rate constant for
intersystem crossing in the reference compound is calculated
from eq 1 where the intersystem crossing quantum yield is
assumed to be equal to the value for zinc tetraphenylporphyrin,
0.90.40 The rate constant for intersystem crossing in the dimer
compounds (kisc), with the OB-bridge, is calculated from the
0
measured ratio kisc/kisc (eq 3). The enhancement of the
intersystem crossing rate (S1 f T1) of the ZnP donor is probably
influenced by the bridge in the same fashion as has been
observed for electron transfer and excitation energy transfer for
similar systems.10,13 As a consequence, kisc is expected to
increase going from OB, BB, NB to the AB-bridge. However,
∆ADBA values for the π-bridged systems are more difficult to
analyze quantitatively due to donor triplet state quenching in
these cases. Therefore, we estimate kisc for the π-bridged systems
(37) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada,
T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771-11782.
(38) Chosrowjan, H.; Tanigichi, S.; Okada, T.; Takagi, S.; Arai, T.; Tokumaru,
K. Chem. Phys. Lett. 1995, 242, 644-649.
(39) Winters, M.; Pettersson, K.; Mårtensson, J.; Albinsson, B., manuscript in
preparation.
(40) Harriman, A.; Porter, G.; Wilowska, A. J. Chem. Soc., Faraday Trans. 2
1983, 79, 807-816.
9
6716 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004

Porphyrin-Based Donor−Bridge−Acceptor Systems
A R T I C L E S
Figure 6. Kinetic traces of the dimers with a low-spin acceptor (4) and a high-spin acceptor (0) at 680 nm where ZnP•+ and 1ZnP* dominate the absorption.
Solvent 2Me-THF.
Table 2. Total Quenching Rate Constant (kq), Rate Constants for Intersystem Crossing (ISC) in the Absence (kisc0) and in the Presence of
a
the FeP Acceptor (kisc), Calculated Fo¨rster Rate Constant (kFo¨rster), and Rest-Term, krest
dominating
quenching process
sample
bkq (ns-1
)
ck 0 (ns-1
)
dkisc (ns-1
)
kFo¨rster (ns-1
)
ekrest (ns-1
)
isc
ZnP-AB-Fe(Cl)P
7.7 ( 0.5
3.1 ( 0.2
2.6 ( 0.2
1.4 ( 0.2
17 ( 1.0
2.1 ( 0.2
1.1 ( 0.2
0.23 ( 0.09
0.66 ( 0.05
0.65 ( 0.05
0.65 ( 0.05
0.61 ( 0.05
0.82 ( 0.05
0.71 ( 0.05
0.72 ( 0.05
0.69 ( 0.05
g1.3 ( 0.1
g1.3 ( 0.1
g1.3 ( 0.1
1.2 ( 0.1
0.8 ( 0.05
0.7 ( 0.05
0.7 ( 0.05
0.7 ( 0.05
0.5 ( 0.1
0.5 ( 0.1
0.5 ( 0.1
0.5 ( 0.1
0.2 ( 0.04
0.2 ( 0.04
0.2 ( 0.04
0.2 ( 0.04
e6.5 ( 0.5
e2.0 ( 0.2
e1.5 ( 0.2
0.3 ( 0.2
16 ( 1.0
1.9 ( 0.2
0.9 ( 0.2
0
ISC/EET
ISC/EET
ISC/EET
ISC/EET
ET
ET
ET
EET
ZnP-NB-Fe(Cl)P
ZnP-BB-Fe(Cl)P
ZnP-OB-Fe(Cl)P
Zn(Im)P-AB-Fe(Im)2P+
Zn(Im)P-NB-Fe(Im)2P+
Zn(Im)P-BB-Fe(Im)2P+
Zn(Im)P-OB-Fe(Im)2P+
a The dominating quenching process is suggested in the last column, that is, ISC (enhancement of the ISC rate as compared to the reference substance),
electron transfer (ET), or excitation energy transfer (EET). Solvent 2Me-THF. The error margins are calculated using an estimated lifetime variation of
0
(5%. b Equation 4. c Equation 1. d The ratio kisc/kisc is measured for the OB-bridged systems and estimated for the π-bridged systems; see text for details.
e krest ) kq - (kisc - kisc0) - kFo¨rster. Please observe that krest accounts for different deactivation processes for different systems; see text for details.
to be of the same magnitude or larger than kisc for the OB-
bridged system (Table 2).
the increased intersystem crossing and energy-transfer terms
both contribute significantly. This is most obvious for ZnP-
OB-Fe(Cl)P, where the enhanced intersystem crossing and the
estimated energy-transfer rates contribute equally to the mea-
sured donor quenching. The situation is more complicated when
interpreting the π-bridged high-spin complexes because in these
cases the rest-term makes a significant contribution. However,
from studies on similarly bridged systems, we know that the
rate of excitation energy transfer is strongly affected by the
electronic structure of the bridging chromophore.10,11,13 This is
a consequence of the bridge-mediated electronic coupling which
causes both energy transfer and potentially the intersystem
crossing to be much faster. In the corresponding ZnP-RB-
H2P systems, we found that the bridge-mediated energy transfer
The Fo¨rster energy-transfer rate constant, kFo¨rster, is calculated
from the structural and spectroscopic properties of the D-B-A
systems.14 It is seen in Table 2 that energy transfer dominates
the donor quenching in Zn(Im)P-OB-Fe(Im)2P+ while for the
other low-spin complexes with π-conjugated bridges (AB, NB,
BB) the rest-term dominates (eq 4). Because the transient
absorption measurements clearly show that electron transfer is
the most important process connected to the excited-state
relaxation in the low-spin complexes, we interpret this rest-
term to be dominated by electron transfer.
The last column of Table 2 gives a summary of the
dominating deactivation pathways. For the high-spin complexes,
9
J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004 6717

A R T I C L E S
Pettersson et al.
Table 3. Calculated Driving Forces (∆G°) and Reorganization
Energies (λ) in Different Solventsa
∆G°
λ
∆G° + λ
sample
solvent
(eV)
(eV)
(eV)
ZnP-RB-Fe(Cl)P
ZnP-RB-Fe(Cl)P
ZnP-RB-Fe(Cl)P
2Me-THF -1.01
0.98 -0.03
1.03 -0.05
1.22 -0.07
CH2Cl2
DMF
-1.08
-1.29
Zn(Im)P-RB-Fe(Im)2P+ 2Me-THF -0.97 ( 0.1 0.98
0.01 ( 0.1
Zn(Im)P-RB-Fe(Im)2P+ CH2Cl2
Zn(Im)P-RB-Fe(Im)2P+ DMF
-1.05 ( 0.1 1.03 -0.02 ( 0.1
-1.29 ( 0.1 1.22 -0.07 ( 0.1
a Estimated uncertainties for the imidazole complexes; see text for details.
spin complexes. As seen in Table 3 for the ZnP/Fe(Cl)P pair,
the driving force is about 1 eV yielding an estimated barrier
free electron-transfer reaction (∆G° + λ ≈ 0) in all three
solvents (eq 5). For a barrier free electron-transfer reaction, a
shift in the driving force by 100 meV up or down can solely
result in a slower electron-transfer rate (by 60% with 1 eV
reorganization energy). Because the opposite is observed, the
driving force alone cannot explain the fact that electron transfer
is more feasible in the low-spin complex, and according to eq
5 the observed rate increase must be caused by a larger
electronic coupling, V. Of importance might be the changes in
distortion from planarity of the porphyrin ring upon changing
the coordination of the iron porphyrin. In going from a five-
coordinated iron in Fe(Cl)P to a six-coordinated complex
Fe(Im)2P+ with two axial ligands on opposite sides of the
porphyrin ring, the changes in ring distortion are expected to
be associated with alterations in both orbital symmetries and
orbital energies.47,48 Such variations could have a large effect
on the electronic coupling between the acceptor and bridge
units.49 In addition, as compared to the out-of-plane five-
coordinated system, an increased electronic coupling between
the iron center and the rest of the D-B-A system is expected
for the six-coordinated low-spin system where the iron atom is
located in the plane of the porphyrin.50 This may be of
importance because the reduction of iron(III) porphyrins is
suggested to take place on the iron center.28,51,52 Portela et al.
also report a much larger ET rate for their low-spin acceptor
dimer (ZnP/FeP) as compared to a high-spin acceptor dimer
and suggest that it is due to that the iron is in the porphyrin
plane in the low-spin case and out-of-plane in the high-spin
case.20
Figure 7. Energy level diagram for ZnP-RB-FeP in 2Me-THF. The
energy of the lowest FeP excited state is not experimentally determined
and is therefore indicated by a dashed line.
increased in the series BB < NB < AB, in accordance with the
trend observed for the rest-term found in this study. Moreover,
from the femtosecond transient absorption measurements, we
can rule out electron transfer to be responsible for the rest-term
in the high-spin complexes.
Electron Transfer versus Intersystem Crossing. According
to the Marcus theory, the rate constant for diabatic electron
transfer in the high-temperature limit is:
(∆G° + λ)2
π
2
kET
)
|V| exp -
(5)
p2λk T
[
]
4λkBT
x
B
where kB is the Boltzmann constant, T is the temperature, V is
the electronic coupling, and λ is the total reorganization energy.
The thermodynamic driving force ∆G° and the reorganization
energy λ can be calculated using redox potentials for the
interacting species together with solvent dielectrical properties
and donor/acceptor radii.41-44
e2
∆G0 ) e(Eox - Ered) - E00
-
+
4πꢀ0ꢀsRDA
e2
1
1
1
2R
1
2RA
-
+
(6)
(7)
0
s
ꢀsref
D
4πꢀ ꢀ
(
)(
)
e2
1
1
1
1
2
1
ꢀs
λ ) λi + λs ) λi +
+
-
-
(
)(
)
4πꢀ0 2RD 2RA RDA
n
It is interesting to compare the electron-transfer rates for the
present ZnP-RB-FeP+ series with the corresponding series
with gold(III) porphyrin as the acceptor, ZnP-RB-AuP+.13
With the ZnP/AuP redox couple, the driving force for electron
transfer is significantly smaller (∆G° ) -0.61 eV in 2Me-THF).
Nevertheless, the observed electron-transfer rates are similar to
those reported here for the low-spin complexes. This again
suggests that the electronic coupling rather than the thermody-
namic driving force is the important factor that explains the
Relevant numerical values for all parameters have been reported
previously,14 except for the reduction potential of Fe(Im)2P+
which is fairly difficult to accurately determine due to irrevers-
ible redox processes. However, by comparison to other ferric
porphyrins where the reduction potentials for high- and low-
spin complexes have been compared, it can safely be concluded
that they are within 100 meV.45,46 The same redox potentials
are used for the high- and low-spin redox pairs; as a conse-
quence, only the E00 shift changes the driving force between
the high- and low-spin complexes. In Table 3, ∆G°, λ, and ∆G°
+ λ are reported for three different solvents (2Me-THF, CH2Cl2,
DMF), with the uncertainty of (100 meV included for the low-
(47) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am. Chem.
Soc. 1988, 110, 7566-7567.
(48) Poveda, L. A.; Ferro, V. R.; Garcia de la Vega, J. M.; Gonzalez-Jonte, R.
H. Phys. Chem. Chem. Phys. 2000, 2, 4147-4156.
(49) Yang, S. I.; Seth, J.; Balasubramanian, T.; Kim, D.; Lindsey, J. S.; Holten,
D.; Bocian, D. F. J. Am. Chem. Soc. 1999, 121, 4008-4018.
(50) Ghosh, A.; Halvorsen, I.; Nilsen, H. J.; Steene, E.; Wondimagegn, T.; Lie,
R.; van Caemelbecke, E.; Guo, N.; Ou, Z.; Kadish, K. M. J. Phys. Chem.
B 2001, 105, 8120-8124.
(41) Marcus, R. A. Can. J. Chem. 1959, 37, 155-163.
(42) Marcus, R. A. J. Chem. Phys 1965, 43, 679-701.
(43) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834-839.
(44) Weller, A. Z. Phys. Chem. NF 1982, 133, 93-98.
(45) O’Brien, P.; Sweigart, D. A. Inorg. Chem. 1985, 24, 1405-1409.
(46) Doeff, M. M.; Sweigart, D. A.; O’Brien, P. Inorg. Chem. 1983, 22, 851-
852.
(51) Johansson, M. P.; Blomberg, M. R. A.; Sundholm, D.; Wikstro¨m, M.
Biochim. Biophys. Acta 2002, 1553, 183-187.
(52) Goeff, H. M. In Iron Porphyins. Part I; Lever, A. B. P., Gray, H. B., Eds.;
Addison-Wesley Publishing Co.: Reading, MA, 1983; pp 237-281.
9
6718 J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004

Porphyrin-Based Donor−Bridge−Acceptor Systems
A R T I C L E S
difference between the high- and low-spin ferric complexes.
The Zn(Im)P-RB-Fe(Im)2P+ and ZnP-RB-AuP+ series also
behave quite similar with respect to variation of the electron-
transfer rates as a function of bridging chromophore. In both
series, no electron transfer is observed for the bridges with
broken π-conjugation (OB), and the rate increases as the energy
gap between the relevant donor and bridge states decreases. This
is perfectly in line with the superexchange model, but because
this has been thoroughly discussed and quantitatively investi-
gated in a previous paper, we will refrain from repeating it
here.13
porphyrin to decrease to its normal rate. Second, the suggested
change in structure of the acceptor porphyrin47,48,50 turns it into
a better electron acceptor due to larger electronic coupling to
the rest of the system. Thus, larger electronic coupling and less
competition from other deactivation processes turns a poor
electron transducing D-B-A system into a good one. As
observed before, when the bridge is not π-conjugated, electron
transfer is not observed, and energy transfer is the dominating
deactivation pathway for the low-spin OB-bridged system.
Acknowledgment. This work was supported by grants from
the Swedish Research Council, the Knut and Alice Wallenberg
Foundation, and the Hasselblad Foundation. We thank Dr. Angel
J. Di Bilio for skillful assistance with recording of the EPR
spectra.
Concluding Remarks
By changing the spin state of the paramagnetic acceptor from
5
1
high-spin (S ) /2) to low-spin (S ) /2), the deactivation of
the excited donor porphyrin changes from enhanced intersystem
crossing to electron transfer in the systems connected by fully
π-conjugated bridges. Two opposing mechanisms are expected
to cause this dramatic dependence on imidazole coordination.
First, the magnitude of the acceptor spin angular momentum is
decreased which causes the rate of triplet formation in the donor
Supporting Information Available: EPR spectra of Fe(Cl)P
and Fe(Im)2P+ in 2Me-THF at 20 K. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA0370488
9
J. AM. CHEM. SOC. VOL. 126, NO. 21, 2004 6719