Distance dependence of electron transfer in rigid, cofacially compressed, π-stacked porphyrin-bridge-quinone systems

Article abstract of DOI:10.1021/ja012504i

The electron-transfer (ET) dynamics of a series of unusually rigid π-stacked porphyrin-quinone (P-Q) systems, in which sub-van der Waals interplanar distances separate juxtaposed porphyryl, aromatic bridge, and quinonyl components of these assemblies, are

Full text of DOI:10.1021/ja012504i

Published on Web 06/21/2002
Distance Dependence of Electron Transfer in Rigid, Cofacially
Compressed, π-Stacked Porphyrin-Bridge-Quinone Systems
Youn K. Kang, Igor V. Rubtsov, Peter M. Iovine, Jianxin Chen, and
Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received November 8, 2001. Revised Manuscript Received February 13, 2002
Abstract: The electron-transfer (ET) dynamics of a series of unusually rigid π-stacked porphyrin-quinone
(P-Q) systems, in which sub-van der Waals interplanar distances separate juxtaposed porphyryl, aromatic
bridge, and quinonyl components of these assemblies, are reported. The photoinduced charge separation
(CS) and thermal charge recombination (CR) ET reactions of [5-[8′-(2′′,5′′-benzoquinonyl)-1′-naphthyl]-
10,20-diphenylporphinato]zinc(II) (1a-Zn), [5-[8′-(4′′-[8′′′-(2′′′′,5′′′′-benzoquinonyl)-1′′′-naphthyl]-1′′-phenyl)-
1′-naphthyl]-10,20-diphenylporphinato]zinc(II) (2a-Zn), and [5-(8′-[4′′-(8′′′-[4′′′′-(8′′′′′-[2′′′′′′,5′′′′′′-benzoquinonyl]-
1′′′′′-naphthyl)-1′′′′-phenyl]-1′′′-naphthyl)-1′′-phenyl]-1′-naphthyl)-10,20-diphenylporphinato]zinc(II) (3a-Zn)
in CH2Cl2 were investigated by pump-probe transient absorption spectroscopy. Analyses of these
data show that the phenomenological ET distance dependence (â) for both the CS and CR reactions
in these systems is soft (âCS ) 0.43 Å-1; âCR ) 0.35 ( 0.16 Å-1). This work demonstrates that simple
aromatic building blocks such as benzene, which are characterized by highly stabilized filled molecular
orbitals and large HOMO-LUMO gaps, can provide substantial D-A electronic coupling when organized
within a π-stacked structural motif that features a modest degree of arene-arene interplanar compression.
Introduction
distances (∼3.5 Å). In contrast to the rapidly expanding body
of information regarding double-helical DNA-mediated electron-
Focused experimental efforts over the past several years have
provided considerable mechanistic insight into the nature of
electronic coupling mediated by DNA-based oligonucleotide
scaffolds,1-14 which typically feature nucleobase-nucleobase
interplanar separations corresponding to van der Waals contact
transfer (ET) reactions, relatively little is known regarding such
reactions in other classes of π-stacked structures.
We have synthesized several examples of unusually rigid
π-stacked porphyrin-quinone (P-Q) systems via an approach
that exploits metal-catalyzed cross-coupling methodologies,15-21
porphyryl boronate synthons,22 and a 1,8-naphthyl23,24 pillaring
motif; these donor-spacer-acceptor (D-Sp-A) compounds
define a new class of porphyrin-based supramolecular structures
in which sub-van der Waals interplanar separations between
juxtaposed porphyryl, aromatic bridge, and quinonyl components
of these assemblies are strictly enforced.25,26
* To whom correspondence may be sent. E-mail: therien@
a.chem.upenn.edu.
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9
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J. AM. CHEM. SOC. 2002, 124, 8275-8279
8275

A R T I C L E S
Kang et al.
dimethoxyphenyl]-1′′′′′-naphthyl)-1′′′′-phenyl]-1′′′-naphthyl)-1′′-phen-
yl]-1′-naphthyl)-10,20-diphenylporphinato]zinc(II) (3b-Zn), 5-(8′-[4′′-
(8′′′-[4′′′′-(8′′′′′-[2′′′′′′,5′′′′′′-benzoquinonyl]-1′′′′′-naphthyl)-1′′′′-phenyl]-
1′′′-naphthyl)-1′′-phenyl]-1′-naphthyl)-10,20-diphenylporphyrin (3a),
and [5-(8′-[4′′-(8′′′-[4′′′′-(8′′′′′-[2′′′′′′,5′′′′′′-benzoquinonyl]-1′′′′′-naph-
thyl)-1′′′′-phenyl]-1′′′-naphthyl)-1′′-phenyl]-1′-naphthyl)-10,20-diphen-
ylporphinato]zinc(II) (3a-Zn). Chromatographic purification (Silica Gel
60, 230-400 mesh, EM Science) of all compounds was performed on
the benchtop. Chemical shifts for 1H NMR spectra are relative to
residual protium in the deuterated solvents (CDCl3, δ ) 7.24 ppm).
All coupling constants are reported in hertz.
Instrumentation. Electronic spectra were recorded on an OLIS UV/
vis/NIR spectrophotometry system that is based on the optics of a Cary
14 spectrophotometer. NMR spectra were recorded on either a 500- or
250-MHz AC-Bruker spectrometer. Mass spectral data were obtained
at the University of Pennsylvania Mass Spectrometry Laboratory located
within the Department of Chemistry.
Pump-Probe Transient Absorption Spectroscopic Measure-
ments. Transient absorption spectra were obtained using standard
pump-probe methods. Optical pulses, centered at 775 nm, were
generated using a Ti:sapphire laser (Clark-MXR, CPA-2001). Optical
parametric amplifiers (near-IR and visible OPAs, Clark-MXR) generate
excitation pulses tunable in wavelength from the UV through the near-
IR region; white light continuum served as the probe beam. After
passing through the sample, the probe light was focused onto the
entrance slit of the computer-controlled image spectrometer (SpectraPro-
150, Acton Research Corp.). The baseline fluctuation in these transient
absorption experiments corresponded to ∼0.2 mOD per second of signal
accumulation. The time resolution was probe wavelength dependent;
in these experiments, the fwhm of the instrument response function
varied between 140 and 200 fs. A detailed description of the transient
optical apparatus will appear in an upcoming publication.28 All
experiments were carried out at room temperature (23 ( 1 °C).
Compounds 1a-Zn, 2a-Zn, and 3a-Zn (note: in the drawings
the porphyryl 20-phenyl group has been omitted for clarity)
exemplify structures within this family of D-Sp-A assemblies
that feature compressed, stacked π manifolds. NMR data evince
that these systems display an unusual degree of conformational
homogeneity in the condensed phase; furthermore, a rigorous
solution NMR structural determination for the free base analogue
of 2a-Zn shows that the internuclear distances separating the
C1 and C1′ carbon atoms of the 1-quinonyl and 1′-phenyl
substituents of the upper naphthalene pillar, as well as the
C5 and C4′ carbon atoms of the 5-porphyryl and 4′-phenyl
substituents of lower naphthyl unit, correspond to 2.97 Å. These
solution structural studies are thus consistent with X-ray
crystallographic data which show that the separation of the
C1- and C1′-phenyl carbons of 1,8-diphenylnaphthalene is
2.99 Å.23,24
We report herein the photoinduced electron-transfer (ET) and
thermal charge recombination (CR) dynamics in (1-3)a-Zn and
demonstrate that these structures exhibit an unusual phenom-
enological distance dependence for both of these classes of
charge-transfer reactions.
Results and Discussion
Experimental Section
Protected-quinone derivatives of (1-3)a-Zn ([5-[8′-(2′′,5′′-
dimethoxyphenyl)-1′-naphthyl]-10,20-diphenylporphinato]-
zinc(II) (1b-Zn), [5-[8′-(4′′-[8′′′-(2′′′′,5′′′′-dimethoxyphenyl)-
1′′′-naphthyl]-1′′-phenyl)-1′-naphthyl]-10,20-diphenylporphin-
ato]zinc(II) (2b-Zn), and [5-(8′-[4′′-(8′′′-[4′′′′-(8′′′′′-[2′′′′′′,5′′′′′′-
dimethoxyphenyl]-1′′′′′-naphthyl)-1′′′′-phenyl]-1′′′-naphthyl)-1′′-
phenyl]-1′-naphthyl)-10,20-diphenylporphinato]zinc(II) (3b-Zn))
served as key spectroscopic benchmarks for this study. The
electronic spectra of the respective singlet-excited states of these
species are similar.
The photoinduced charge separation (CS) and thermal charge
recombination (CR) ET dynamics of (1-3)a-Zn in CH2Cl2 were
investigated by pump-probe transient absorption spectroscopy;
Figure 1 displays representative data obtained for compounds
1a-Zn and 2a-Zn. A transient absorption near 450 nm,
characteristic of the porphyrin S1 excited state, is evident at
early delay times for 2a-Zn (Figure 1B) and 3a-Zn; this signal
is identical to that observed in the excited-state spectra of the
corresponding protected-quinone derivatives of these species.
In contrast, an excited-state spectrum cannot be clearly resolved
for 1a-Zn; fitting the signal observed at 700 nm (Figure 1A) as
a biexponential convoluted with the instrument response function
(IRF, 150 ( 5 fs) enables the magnitude of τCS to be estimated
(τCS e 20 fs).
Materials. All manipulations were carried out under nitrogen
prepurified by passage through an O2 scrubbing tower (Schweizerhall
R3-11 catalyst) and a drying tower (Linde 3-Å molecular sieves) unless
otherwise noted. Air-sensitive solids were handled in a Braun 150-M
glovebox. Standard Schlenk techniques were employed to manipulate
air-sensitive solutions. All solvents utilized in this work were obtained
from Fisher Scientific (HPLC grade). Tetrahydrofuran (THF) and 1,2-
dimethoxyethane (DME) were dried over K/benzoylbiphenyl, while
CH2Cl2 was dried over CaH2; these solvents were subsequently distilled
from these reagents under nitrogen. Anhydrous dimethylformamide
(DMF) (Aldrich) was stirred over MgSO4 prior to distillation under
vacuum. Deionized water was used without any further purification.
Ba(OH)2‚8H2O (Aldrich) was recrystallized from H2O. Zn(OAc)2‚
2(H2O) and K3PO4‚nH2O were used as received from Fisher Scientific.
The catalyst, Pd(PPh3)4, was obtained from Strem. 1,8-Diiodonaph-
thalene was prepared using the procedure developed by House.23 1,4-
Di(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)benzene was pre-
pared via the esterification of 1,4-phenylenebisboronic acid (Aldrich)
with pinacol (Aldrich) in methanol in the presence of MgSO4, followed
by a recrystallization from hexanes.27 (1-Iodo-8-[4′-(8′′-[2′′′,5′′′-dimethoxy-
phenyl]-1′′-naphthyl)-1′-phenyl]naphthalene and [5-(4′,4′,5′,5′-tetramethyl-
[1′,3′,2′[dioxaborolan-2′-yl)-10,20-diphenylporphinato]zinc(II) were pre-
pared as reported previously.22,25 Compounds 1a-Zn and 2a-Zn were
prepared from their corresponding free-base porphyrin derivatives; the
syntheses and characterization data for these materials have been
reported previously.25,26 Characterization and analytical data for 1a-Zn
and 2a-Zn are available in the Supporting Information, along with the
descriptions of the syntheses of [5-(8′-[4′′-(8′′′-[4′′′′-(8′′′′′-[2′′′′′′,5′′′′′′-
For all three compounds, the transient absorption (∼700 nm)
corresponding to the (porphinato)zinc(II) cation radical is clearly
(27) Todd, M. H.; Balasubramanian, S.; Abell, C. Tetrahedron Lett. 1997, 38,
6781-6784.
(28) Rubtsov, I. V.; Kang, Y. K.; Rubtsov, G. I.; Therien, M. J. Manuscript in
preparation.
9
8276 J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002

ET in π-Stacked Porphyrin−Bridge−Quinone Systems
A R T I C L E S
Figure 2. Transient decay kinetics determined at 650 nm for (A) 1a-Zn,
(B) 2a-Zn, and (C) 3a-Zn. The best biexponential function fit (convoluted
in the 1a-Zn and 2a-Zn cases with the IRF) is depicted by the thin line
(spectral contributions of the hot ground state are negligible at this
wavelength). Experimental conditions: λex ) 557 nm, methylene chloride
solvent, temperature ) 23 ( 1 °C.
Figure 1. Transient absorption spectra of (A) 1a-Zn and (B) 2a-Zn in
methylene chloride. Time delays are shown as insets. Experimental
conditions: λex ) 557 nm, temperature ) 23 ( 1 °C. Note that the porphyryl
20-phenyl group has been omitted for clarity.
observed (Figure 1),29 signaling the formation of the charge-
separated state, D+-Sp-A-. The rate constants for CS and
CR reactions were determined from a multiwavelength global
fit of the visible spectral domain transient dynamics using a
three-exponential function. The respective CS time constants
for 2a-Zn and 3a-Zn were determined to be 0.62 ( 0.02 and
3.1 ( 0.3 ps; note that the observed charge-separated states
for 1a-Zn, 2a-Zn, and 3a-Zn undergo rapid charge recombina-
tion reactions (τCR ) 1.6 ( 0.2, 1.7 ( 0.1, and 20 ( 2 ps,
respectively), to produce vibrationally hot ground states. These
hot ground states display characteristic absorptions at ∼450
and 570 nm, and cool with a time constant of ∼15 ps,30-33
similar to that observed for strongly coupled PQ benchmarks.30,31
Figure 2 highlights the comparative transient decay kinetics
Figure 3. Anisotropic transient absorption spectra obtained for [5-[8′-
(4′′-[8′′′-(2′′′′,5′′′′-dimethoxyphenyl)-1′′′-naphthyl]-1′′-phenyl)-1′-naphthyl]-
10,20-diphenylporphinato]zinc(II) (2b-Zn) measured at time delays of 230
(solid lines) and 550 fs (dashed lines) for probe light polarization parallel
(|) and perpendicular ( ) to the pump pulse polarization. The inset shows
(A) representative excited-state anisotropy (r) decay dynamics measured
at a probe wavelength (λpr) ) 700 nm and (B) the anisotropy dynamics
representative of the S0 f S1 electronic transition, which is constructed
using I|′ and I ′ signal intensities for | and polarizations, where I|′ )
I|(531 nm) - I|(558 nm) and I ′ ) I (531 nm) - I (558 nm). This intensity
difference between the signals measured at 531 and 558 nm eliminates
contributions to the anisotropy deriving from the S1 f Sn transition, allowing
estimation of the anisotropy associated with the S0 f S1 absorption.
Experimental conditions: λex ) 600 nm, methylene chloride solvent,
temperature ) 23 ( 1 °C.
for (1-3)a-Zn at long wavelength (λex ) 557 nm, λprobe
)
650 nm), where absorptive contributions from the hot ground
state are negligible.
Given that the time scales of the CS reactions in (1-3)a-Zn
are faster or of a similar magnitude to the established anisotropy
dephasing times for (porphinato)metal compounds,30,34 pump-
probe transient anisotropy studies were performed in order to
establish whether electronic dephasing determines the magnitude
of the electron-transfer rate constant. Figure 3 shows exemplary
anisotropic transient absorption spectra obtained for 2b-Zn.
These data, and analogous spectra obtained for 1b-Zn and
3b-Zn (data not shown), evince that the transient anisotropy
dephasing times exhibited by protected-quinone compounds
(1-3)b-Zn are <150 fs, which is the approximate fwhm of
the instrument response function. No change in the magnitude
of the anisotropy was observed at any recorded wavelength
after a time delay of ∼200 fs (Figure 3); furthermore, the
magnitude of the observed anisotropy value at this time
was ∼0.1, which corresponds to the expected value for the
doubly degenerate excited state of a pseudo-D4h-symmetric
(porphinato)metal compound after in-plane dephasing is
complete.30 Hence, for (2-3)a-Zn, excited-state electronic
dephasing cannot play a role in determining the photoinduced
charge separation time constant.
(29) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem.
Soc. 1970, 92, 3451-3459.
(30) Wynne, K.; LeCours, S. M.; Galli, C.; Therien, M. J.; Hochstrasser, R. M.
J. Am. Chem. Soc. 1995, 117, 3749-3753.
(31) Rodriguez, J.; Kirmaier, C.; Johnson, M. R.; Friesner, R. A.; Holten, D.;
Sessler, J. L. J. Am. Chem. Soc. 1991, 113, 1652-1659.
(32) Wynne, K.; Galli, C.; Hochstrasser, R. M. J. Chem. Phys. 1994, 100, 4797-
4810.
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566-570.
(34) Galli, C.; Wynne, K.; LeCours, S. M.; Therien, M. J.; Hochstrasser, R. M.
Chem. Phys. Lett. 1993, 206, 493-499.
A variety of investigators have fit the distance dependence
of nonadiabatic ET rate constants to an empirical exponential
9
J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002 8277

A R T I C L E S
Kang et al.
and reduction potentials (vs SCE) of benzene (E1/20/+ ) 2.30 V;
-/0
E1/2 ) -3.35 V)45,46 differ significantly from those deter-
-/0
mined for nucleobases [e.g., E1/20/+(guanine) ) 1.24 V; E1/2
(cytosine) ) -2.42 V],47 the comparatively soft decay of CS
and CR rate constants in the (1-3)a-Zn series (Figure 4) with
increasing D-A distance, relative to that observed for D-Sp-A
assemblies based upon oligonucleotide scaffolds, is exceptional
in light of the McConnell relation,48 as the tunneling energy
lies several electronvolts from the medium frontier orbitals.49
(3) Reaction energetics50,51 suggest that these CS and CR
reactions are near barrierless only for 2a-Zn.52,53 This fact,
coupled with the expected distance dependence of λS,54-57
suggests that the pure electronic â values should be further
diminished with respect to the phenomenological âCS and
â
CR values extracted from the data shown in Figure 4. (4) The
Figure 4. Distance dependence of the CS and CR rate constants for
(1-3)a-Zn in methylene chloride at 23 ( 1 °C. Slopes of the straight
line plots shown give âCR ) 0.35 ( 0.16 Å-1 (CR) and âCS ) 0.43 Å-1
(CS) [R0 ) 2.97 Å; k0 ) 1.1 × 1012 s-1 (CR); k0 ) 8.5 × 1013 s-1 (CS)].
Given the adiabatic nature of CS in 1a-Zn, âCS was computed using the
relevant data points for 2a-Zn and 3a-Zn only. Error bars for each
experimental rate constant are shown. Note that the lower limit of the
error bar for the CS rate constant of 1a-Zn corresponds to the fastest rate
constant that we can resolve (5 × 1013 s-1); the width of this error bar is
arbitrary. The error reported in âCR was determined via standard linear
regression analysis.
magnitude of kCS/kCR is unusual (>60) for 1a-Zn, given
that the driving forces for CS and CR are predicted to be
approximately isoergic.52
(41) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton,
M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119,
10563-10564.
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1998, 396, 60-63.
(43) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992, 114, 6227-
6238.
(44) Osuka, A.; Maruyama, K.; Mataga, N.; Asahi, T.; Yamazaki, I.; Tamai, N.
J. Am. Chem. Soc. 1990, 112, 4958-4959.
expression that reflects the expected rapid decay of electronic
coupling with increasing donor (D)-acceptor (A) distance.2,6,13,14
Figure 4 highlights the distance dependence of the CS and CR
rate constants for the 1a-Zn, 2a-Zn, and 3a-Zn ET systems,
analyzed in terms of a simplified Marcus-Levich-Jortner
equation (eq 1).35-38 Here, the ET rate constant (kET) is assumed
(45) Osa, T.; Yildiz, A.; Kuwana, T. J. Am. Chem. Soc. 1969, 91, 3994-3995.
(46) Meerholz, K.; Heinze, J. J. Am. Chem. Soc. 1989, 111, 2325-2326.
(47) Seidel, C. A. M.; Schulz, A.; Sauer, M. H. M. J. Phys. Chem. 1996, 100,
5541-5553.
(48) McConnell, H. M. J. Chem. Phys. 1961, 35, 508-515. The McConnell
relation predicts that â (1/R) ln(∆E/T), where R is the nearest-neighbor
spacing, ∆E is the tunneling energy gap, and T is the nearest-neighbor
electronic interaction.
(49) Lee, M.; Shephard, M. J.; Risser, S. M.; Priyadarshy, S.; Paddon-Row, M.
N.; Beratan, D. N. J. Phys. Chem. A 2000, 104, 7593-7599.
(50) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.
(51) Weller, A. Z. Phys. Chem. N. F. 1982, 133, 93-98.
(52) The Gibbs energies for charge separation (∆GCS) and charge recombination
(∆GCR) were estimated using the following equations,
kET ) k0 exp[-â(RDA - R0)]
(1)
to be dependent upon the magnitude of the maximal rate
constant at D-A contact (k0), an exponential decay parameter
(â), and the difference between the D-A separation distance
(RDA) and the smallest possible reactant separation (R0), which
for these systems is taken as 2.97 Å.26 Note that because the
magnitude of the ET rate constant for CS in 1a-Zn greatly
exceeds that of the fastest component of solvent relaxation
(∼1013 s-1),39 it was considered adiabatic and thus was not
factored into our determination of âCS (Figure 4).
-(∆GCS) ) E(0,0)(MP) - E1/2(MP/MP+) + E1/2(A-/A) - ∆G(ꢀ) (i)
∆G(ꢀ) ) (e2/4πꢀ0)[(1/ꢀS){1/(2RD) + 1/(2RA) - 1/RDA} -
(1/ꢀT){1/(2RD) + 1/2RA)}] (ii)
-(∆GCR) ) E(0,0) + ∆GCS
(iii)
where E(0,0) is the energy of the lowest excited singlet state [(1a-3a)-Zn )
1.88, 2.05, and 2.05 eV, respectively],
1/2(MP/MP+) is the one-
E
electron oxidation potential of the PZn unit [(1a-3a)-Zn ) 0.83, 0.84, and
0.82 V, respectively], E1/2(A-/A) is the Q one-electron reduction potential
[(1a-3a)-Zn ) -0.60, -0.58, and -0.56 V, respectively], and ꢀT is
the static dielectric constant for the high-dielectric solvent in which
the potentiometric measurements were obtained. The estimated energetics
for these CS and CR reactions are thus ∆GCS(1a-Zn) ) -0.90 eV,
∆GCR(1a-Zn) ) -0.98 eV; ∆GCS(2a-Zn) ) -0.87 eV, ∆GCR(2a-Zn) )
-1.18 eV; ∆GCS(3a-Zn) ) -0.82 eV, ∆GCR(3a-Zn) ) -1.23 eV.
(53) The solvent reorganization energy (λS) and the total reorganization energy
(λT) were calculated using
A number of aspects of these data deserve comment. (1)
Taking the extensive data set of distance- and driving force-
dependent ET rate data compiled by Lewis and Wasielewski
as benchmarks,6-10 it is clear that the magnitude of the
phenomenological ET distance dependence (â) for both the
CS and CR reactions in these systems (âCS ) 0.43 Å-1
;
âCR ) 0.35 ( 0.16 Å-1) is approximately a factor of 2
smaller than that determined in D/A-modified DNA hairpin
structures (âCS ) 0.7 Å-1; âCR ) 0.9 Å-1);10 interestingly, the
magnitude of these decay parameters is reminiscent of those
determined for several classes of highly conjugated organic
structures.40-44 (2) Considering that the one-electron oxidation
λS ) (e2/4πꢀ0)({1/(2RD) + 1/(2RA) - 1/RDA})[(1/ꢀOP) - (1/ꢀS)] (iv)
λT ) λS + λi
(v)
where RD is the PZn radius (5.5 Å), RA is the Q radius (3.2 Å), RDA is the
porphyrin plane-to-quinonyl centroid distance (1a-Zn ) 3.3 Å; 2a-Zn )
6.8 Å; 3a-Zn ) 10.5 Å), and ꢀOP and ꢀS are the optical and static dielectric
constants. This analysis estimates λS values for 1a-Zn, 2a-Zn, and 3a-Zn
as 0.24, 0.96, and 1.24 eV, respectively). The total inner-sphere reorganiza-
tion energy (λi) for (1a-3a)-Zn was estimated to be 0.4 eV using standard
methods (see, for example: Rubtsov, I. V.; Shirota, H.; Yoshihara, K. J.
Phys. Chem. A 1999, 103, 1801-1808).
(35) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-978.
(36) Levich, V. G. AdV. Electrochem. Eng. 1966, 4, 249.
(37) Jortner, J. J. Chem. Phys. 1976, 64, 4860-4867.
(54) Tavernier, H. L.; Fayer, M. D. J. Phys. Chem. B 2000, 104, 11541-11550.
(55) Bixon, M.; Jortner, J. J. Phys. Chem. B 2000, 104, 3906-3913.
(56) Berlin, Y. A.; Burin, A. L.; Ratner, M. A. J. Phys. Chem. A 2000, 104,
443-445.
(57) Tong, G. S. M.; Kurnikov, I. V.; Beratan, D. N. J. Phys. Chem. B 2002,
106, 2381-2392.
(38) Bixon, M.; Jortner, J. AdV. Chem. Phys. 1999, 106, 35-202.
(39) Reynolds, L.; Gardecki, J. A.; Frankland, S. J. V.; Horng, M. L.; Maroncelli,
M. J. Phys. Chem. 1996, 100, 10337-10354.
(40) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.;
Chidsey, C. E. D.; Feldberg, S. W. Science 2001, 291, 1519-1523.
9
8278 J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002

ET in π-Stacked Porphyrin−Bridge−Quinone Systems
A R T I C L E S
Conclusion
Acknowledgment. This work was supported by a grant from
the Division of Chemical Sciences, Office of Basic Energy
Research, U.S. Department of Energy (DE-FG02-94ER14494).
M.J.T. thanks the Office of Naval Research (N00014-97-0317)
and the MRSEC Program of the National Science Foundation
(DMR-00-79909) for equipment grants for transient optical
instrumentation.
In summary, we have (i) carried out the first ET experiments
in π-stacked D-Sp-A systems in which sub-van der Waals
interplanar distances (closest atom-atom contacts ) 2.97 Å)
separate juxtaposed D, Sp, and A moieties and (ii) determined
the phenomenological distance dependence of the photoinduced
charge separation and thermal charge recombination reactions
in the species over D-A distances ranging between 3.3 and
10.5 Å (porphyrin plane to quinonyl centroid). This work shows
that simple aromatic building blocks such as benzene, which
are characterized by highly stabilized filled molecular orbitals
and large HOMO-LUMO gaps, can provide substantial D-A
electronic coupling when organized within a π-stacked structural
motif that features a modest degree of arene-arene interplanar
compression.
Supporting Information Available: Optical spectra of
(1-3)a-Zn, along with characterization data and synthetic
procedures for 1a-Zn, 2a-Zn, 3b-Zn, 3a, 3a-Zn and key
naphthyl-containing precursors (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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