Synthesis, electronic structure, and electron transfer dynamics of (aryl)ethynyl-bridged donor-acceptor systems

Article abstract of DOI:10.1021/ja021278p

The ET dynamics of a series of donor-spacer-acceptor (D-Sp-A) systems featuring (porphinato)zinc(II), (aryl)ethynyl bridge, and arene diimide units were investigated by pump-probe transient absorption spectroscopy. Analysis of these data within the context of the Marcus-Levich-Jortner equation suggests that the π-conjugated (aryl)ethynyl bridge plays an active role in the charge recombination (CR) reactions of these species by augmenting the extent of (porphinato)zinc(II) cation radical electronic delocalization; this increase in cation radical size decreases the reorganization energy associated with the CR reaction and thereby attenuates the extent to which the magnitudes of the CR rate constants are solvent dependent. The symmetries of porphyrin-localized HOMO and HOMO-1, the energy gap between these two orbitals, and D-A distance appear to play key roles in determining whether the (aryl)ethynyl bridge simply mediates electronic superexchange or functions as an integral component of the D and A units.

Full text of DOI:10.1021/ja021278p

Published on Web 06/27/2003
Synthesis, Electronic Structure, and Electron Transfer
Dynamics of (Aryl)ethynyl-Bridged Donor-Acceptor Systems
Naomi P. Redmore, Igor V. Rubtsov, and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received October 17, 2002; Revised Manuscript Received January 30, 2003; E-mail: therien@a.chem.upenn.edu
Abstract: The ET dynamics of a series of donor-spacer-acceptor (D-Sp-A) systems featuring
(porphinato)zinc(II), (aryl)ethynyl bridge, and arene diimide units were investigated by pump-probe
transient absorption spectroscopy. Analysis of these data within the context of the Marcus-Levich-Jortner
equation suggests that the π-conjugated (aryl)ethynyl bridge plays an active role in the charge recombination
(CR) reactions of these species by augmenting the extent of (porphinato)zinc(II) cation radical electronic
delocalization; this increase in cation radical size decreases the reorganization energy associated with
the CR reaction and thereby attenuates the extent to which the magnitudes of the CR rate constants
are solvent dependent. The symmetries of porphyrin-localized HOMO and HOMO-1, the energy gap
between these two orbitals, and D-A distance appear to play key roles in determining whether the (aryl)-
ethynyl bridge simply mediates electronic superexchange or functions as an integral component of the
D and A units.
Introduction
dynamics derive from the π-conjugated nature of the intervening
(aryl)ethynyl bridge and the electronic structure of the high-
lying filled orbitals of the porphyryl and diimide units.
Electron transfer (ET) reactions play fundamental roles in
solar energy conversion. Mechanistic studies of photoinduced
charge separation (CS) and thermal charge recombination (CR)
reactions in covalently linked donor-acceptor (D-A) arrays
have probed how varying magnitudes of thermodynamic
driving force (∆G°), reorganization energy (λ), and electronic
coupling (HAB) control such ET processes.1,2 While there exist
some notable exceptions,3-7 the vast majority of such investiga-
tions involving porphyrin-containing donor-spacer-acceptor
(D-Sp-A) compounds have focused on systems in which the
D, Sp, and A units remain electronically distinct within the
excited and charge-transfer states pertinent to the CS and CR
reactions. Hence, while the D, Sp, and A units of these
assemblies generally feature extensive π-conjugation, design
criteria have typically ensured weak electronic coupling between
these conjugated components;1,2 as a result, relatively little is
known regarding the photophysics and ET dynamics of D-Sp-A
assemblies in which strong electronic coupling mixes D, Sp,
and A electronic states effectively.
This contribution details the potentiometric properties, photo-
physics, and the photoinduced CS and thermal CR dynamics
for a series of ET assemblies comprising a (porphinato)zinc(II)
donor, an (aryl)ethynyl bridge (Sp), and either a pyromellitimide
(PI) or naphthylic diimide (NI) acceptor unit (Figure 1). These
D-Sp-A arrays feature D-to-Sp connectivity in which a meso-
porphyryl-ethyne substituent directly links the conjugated macro-
cycle to the aromatic spacer units.8 We show that this
combination of D, Sp, and A moieties gives rise to atypical
dynamics following the CS events in these systems; these
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J. AM. CHEM. SOC. 2003, 125, 8769-8778
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A R T I C L E S
Redmore et al.
Figure 1. Rigid D-Sp-A systems featuring (aryl)ethynyl bridging units.
Experimental Section
Pump-Probe Transient Absorption Spectroscopic Mea-
surements. 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; a 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 noise level in these
transient absorption experiments corresponded to ∼0.2 mOD/s
of signal accumulation. The time resolution is 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 has recently been
reported.10 All experiments were carried out at room temperature
(23 ( 1 °C).
Materials. All manipulations were carried out as previously
reported;9 synthetic details are described in the Supporting
Information and in an earlier contribution.9
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 250 MHz AC-250, or 360 MHz Bru¨ker
spectrometers. Cyclic voltammetric responses were recorded on
an EG&G Princeton Applied Research model 273A potentiostat/
galvanostat.
(2) (a) Miller, S. E.; Lukas, A. S.; Marsh, E.; Bushard, P.; Wasielewski, M. R.
J. Am. Chem. Soc. 2000, 122, 7802-7810. (b) Napper, A. M.; Read, I.;
Waldeck, D. H. J. Am. Chem. Soc. 2000, 122, 5220-5221. (c) Sykora,
M.; Maxwell, K. A.; DeSimone, J. M.; Meyer, T. J. Proc. Natl. Acad. Sci.
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Y.; Okada, T.; Tamaki, K.; Sakata, Y. J. Am. Chem. Soc. 2000, 122, 2279-
2288. (e) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617-6628. (f)
Kilså, K.; Kajanus, J.; Macpherson, A. N.; Mårtensson, J.; Albinsson, B.
J. Am. Chem. Soc. 2001, 123, 3069-3080. (g) 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.
Electronic Structure Calculations. Frontier orbital energies
for the [5,15-bis[(4′-substituted-aryl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) compounds were determined using CAChe
ZINDO with standard INDO-1 semiempirical parameters at a
configuration interaction (CI) level ) 20.11 For all the model
complexes, the central zinc metal atoms were assigned a dsp2
(3) Foley, M. J.; Singer, L. A. J. Phys. Chem. 1994, 98, 6430-6435.
(4) Biswas, M.; Nguyen, P.; Marder, T. B.; Khundkar, L. R. J. Phys. Chem. A
1997, 101, 1689-1695.
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7, 3134-51.
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566-570.
(7) Pourtois, G.; Belijonne, D.; Cornil, J.; Ratner, M. A.; Bre´das, J. L. J. Am.
Chem. Soc. 2002, 124, 4436-4447.
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(11) ZINDO software was provided by CAChe Scientific, Beaverton, OR.
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(Aryl)ethynyl-Bridged Donor−Acceptor Systems
A R T I C L E S
(square planar) geometry, and the 10- and 20-meso phenyl
groups were fixed at 90° with respect to the porphyrin plane.
Geometry optimizations were carried out using the MOPAC-
AM1 method. The convergence criteria for these restricted
Hartree-Fock (RHF) self-consistent field (SCF) calculations
required the root-mean-squared (rms) difference in the elements
of the density matrix to be below 10-6 on two successive SCF
cycles.
gel chromatography (CH2Cl2) and filtration through a 3-µm glass
filter to remove suspended particulates. Complete details
of the synthesis and characterization of [5,15-bis[4′-(N-
(N′-octyl)pyromellitic diimide)phenyl)ethynyl]-10,20-diphenyl-
porphinato]zinc(II) (PZn[( )PI]2), [5,15-bis[4′-(N-(N′-octyl)-
naphthylic diimide)phenyl]ethynyl]-10,20-diphenylporphinato]-
zinc(II) (PZn[( )NI]2), and [5,15-bis[3′-methyl-4′-(N-(N′-octyl)-
naphthylic diimide)phenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II) (PZn[(CH3)NI]2) can be found in the Supporting
Information.
General Procedure for the Preparation of N-(4′-Halo-
phenyl)-N′-(octyl)diimide Complexes. The precursors, N-octyl-
benzene-1,2-dicarboxyanhydride-4,5-dicarboximide (1) and N-
octylnaphthalene-1,8-dicarboxyanhydride-4,5-dicarboximide (2),
were prepared by cyclization of 1,2,4,5-pyromellitic dianhydride
or 1,8,4,5-naphthylic dianhydride with n-octylamine in refluxing
DMF solution.12 Either 1 or 2 (∼200 mg, 530 µmol) was added
under N2 to a 50-mL round-bottomed flask which was equipped
with a reflux condenser. DMF (10 mL) was added to the flask,
and the mixture was heated to 80 °C and stirred rapidly to form
a fine suspension. CaO (5 mg) and a 4-haloaniline (∼125 mg,
580 µmol) were then added to the mixture. After refluxing for
12 h, the hot reaction mixture was filtered, and the collected
solid was washed with hot hexanes. The residual solid was
collected as product. Complete details of the synthesis and
characterization of [5-[4′-(N-(N′-octyl)pyromellitic diimide)-
phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (PZn( )PI),
[5-[4′′-(N-(N′-octyl)naphthylic diimide)phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) (PZn( )NI), [5-[2′,5′-difluoro-4′-(N-
(N′-octyl)naphthylic diimide)phenyl)ethynyl]-10,20-diphenyl-
porphinato]zinc(II) (PZn(F2)NI), [5,15-bis[4′-(N-(N′-octyl)-
pyromellitic diimide)phenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II), and [5-[4′-[4′′-(N′(N′′-octyl)pyromellitic diimide)-
phenyl)ethynyl]phenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II) (PZn( )( )PI) can be found in the Supporting Information.
General Procedure for the Preparation of) [5-[4′-(N-(N′-
octyl)pyromellitic/naphthylic diimide)aryl)ethynyl]-10,20-
diphenylporphinato]zinc(II) Complexes. [5-Ethynyl-10,20-
diphenylporphinato]zinc(II) (55 mg, 100 µmol),13,14 the desired
N-(4′-halophenyl)-N′-(octyl)diimide species (∼50 mg, 90 µmol),
CuI (2 mg, 12 µmol), Pd(Ph3)2Cl2 (4 mg, 5 µmol) were
combined in a 25-mL Schlenk tube under N2. An anhydrous,
degassed 10-mL mixture of 1:1:1 diisopropylamine:THF:
acetonitrile was added to the Schlenk tube via cannula transfer.
The reaction mixture was stirred at 40 °C for 12-24 h, until
TLC (7:3 hexanes:THF) indicated that the starting material had
been consumed. The solution was cooled to room temperature,
diluted with chloroform (100 mL), washed with saturated
aqueous ammonium chloride (50 mL), distilled water (50 mL),
and saturated aqueous sodium chloride (50 mL), and dried
over magnesium sulfate. After filtration to remove drying agent,
the filtrate was passed through a plug of silica gel (CHCl3),
concentrated, and purified by size exclusion chromatography
(THF). The first green fraction eluted from the column
was concentrated under vacuum to yield the desired [5-(4′-(N-
(N′-octyl)pyromellitic/naphthylic diimide)aryl)ethynyl]-10,20-
diphenylporphinato]zinc(II) complex. Samples suitable for
transient optical studies were prepared by subsequent silica
Results and Discussion
Synthesis. The synthesis of PZn[( )NI]2 is illustrative of the
synthesis of these (phenyl)ethynyl-bridged D-A systems.
Naphthylic dianhydride was converted to the corresponding
imide-anhydride precursor 2 (N-(N′-octyl)-naphthalene-1,8-
dicarboxyanhydride-4,5-dicarboximide), using methodology de-
scribed by Wasielewski.12 N-(4-iodophenyl)-N′-octyl diimide can
serve as a substrate in a palladium-catalyzed cross-coupling
reaction with either [5-ethynyl-10,20-diphenylporphinato]zinc(II)
or [5,15-diethynyl-10,20-diphenylporphinato]zinc(II)13,14 to yield
the desired D-Sp-A species (Scheme S1, Supporting Informa-
tion). [5-[4′-[4′′-(N′-(N′′-Octyl)pyromellitic diimide)phenyl)-
ethynyl]phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (PZn-
( )( )PI) (Figure 1), which features an extended conjugated
bridge, was prepared by the one-pot palladium-catalyzed cross-
coupling reaction between N-(4′-iodophenyl)-N-(n-octyl)pyro-
mellitic diimide, 1,4-diethynylbenzene, and 5-bromo-(10,20-
diphenylporphinato)zinc(II) (Scheme S2, Supporting Information).
Transient Absorption Spectra. PZnPI. The ET dynamics
for PZn-PI have been reported previously.9 Photoexcitation
of PZn-PI in dichloromethane results in the formation of a
porphyrin-based singlet excited state 1(PZn)*-PI which under-
goes a CS reaction with a time constant (τCS) of 770 fs, followed
by a CR reaction with τCR ) 5.2 ps. The signature transient
absorption for both the PI anion radical (sharp absorption at
∼710 nm) and the PZn cation radical (broad absorptions at ∼470
and 700 nm) can be identified clearly in the pump-probe
spectroscopic experiments. These spectra serve as a benchmark
for comparison to those manifest by the D-Sp-A systems in
which a conjugated bridging unit links the PZn and PI/NI
moieties.
PZn( )PI. The ET dynamics that follow electronic excitation
of PZn( )PI, in which a (4′-phenyl)ethynyl bridge separates D
from A, differ markedly from that exhibited by PZn-PI. The
transient optical spectra obtained in CH2Cl2 show evidence for
vibrational relaxation and hot ground-state formation in addition
to dynamics associated with the CS and CR reactions (Figure
2). Photoinduced CS and thermal CR rate constants were
determined from a multiwavelength global fit of the visible
domain transients. (Representative wavelength ranges that are
particularly useful in this regard include ∼470-530 and ∼680-
740 nm, where the radical anions of NI and PI, respectively,
have strong absorbances, and wavelengths near 500 nm, which
can be utilized to track the relative populations of the excited
PZn singlet state and its corresponding porphyrin cation radical,
(PZn)•+.)
(12) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R. J. Am.
Chem. Soc. 1996, 118, 6767-6777.
The PZn( )PI transient absorption spectra observed im-
mediately following electronic excitation (e.g., 260 fs time delay,
Figure 2) are dominated by absorptive and emissive contribu-
(13) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105-
1111.
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A R T I C L E S
Redmore et al.
Table 1. Charge Separation and Charge Recombination Dynamical Data Determined from Pump-Probe Transient Absorption
Spectroscopic Experiments
compound
solvent
τCS (ps)
kCS (s-1
)
τCR (ps)
kCR (s-1
)
τCR/τCS
PZnPI
PZn( )PI
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
benzene
CH3CN
CH2Cl2
benzene
polystyrene
0.77
24.4
65
64
29
29
33
23
12
1.3 × 1012
4.1 × 1010
1.5 × 1010
1.6 × 1010
3.5 × 1010
3.5 × 1010
3.0 × 1010
4.3 × 1010
8.3 × 1010
2.2 × 1010
∼1.0 × 1011
6.5 × 1010
2.8 × 1011
5.2
1.05
564
7
5.7
4.4
2.2
6
430
4.8
1.9 × 1011
9.5 × 1011
1.8 × 109
1.4 × 1011
1.8 × 1011
2.3 × 1011
4.5 × 1011
1.6 × 1011
2.3 × 109
2.1 × 1011
∼1.0 × 1011
1.4 × 1010
1.1 × 109
6.8
0.043
8.7
0.11
0.20
0.15
0.053
0.6
36
0.07
0.59
4.8
PZn( )( )PI
PZn[( )PI]2
PZn( )NI
PZn(F2)NI
PZn[( )NI]2
PZn[(CH3)NI]2
45
∼10
15.5
3.6
∼10
74
900
250
Figure 2. Transient absorption spectra of PZn( )PI in methylene chloride,
with labeled time delays. Inset shows transient kinetics measured at 723
nm. Experimental conditions: λex ) 630 nm, temperature ) 23 ( 1 °C.
Figure 3. Fluorescence decay profile of PZn( )PI in acetonitrile solvent
acquired by the time-correlated single-photon counting method. Experi-
mental conditions: λex ) 588 nm, λfl ) 622 nm.
tions associated with the porphyrin S1-excited state, along with
the expected ground-state bleach signatures; these spectra are
analogous to those obtained for model compounds which lack
an acceptor unit. Interestingly, (PZn( )PI)* exhibits stimulated
emission (∼680 nm) coincident with that observed in the steady-
state fluorescence spectrum; note that Figure 2 displays no
appreciable transient absorptions that can be assigned to either
the porphyrin cation radical or to the pyromellitimide anion
radical species and that decay of the excited state spectrum
occurs concomitantly with the recovery of the ground-state
bleach component of the absorbance spectrum.
Characteristic times of 1.05 and 24.4 ps were obtained from
a biexponential fit of the PZn( )PI transient data at 480 and
∼725 nm; as the charge-separated state does not dominate the
spectrum, the 24.4 and 1.05 ps time constants were assigned to
the CS and CR reactions, respectively (Table 1). These
assignments were verified by fluorescence lifetime measure-
ments determined by time-correlated single-photon counting
spectroscopy (Figure 3) carried out in acetonitrile solvent. The
τCS value determined by this method (154 ( 5 ps) is in good
agreement with the CS time constant measured in CH3CN
solvent by pump-probe transient absorption spectroscopy (τCS
) 150 ps, see Supporting Information).
of these species are listed in Table 1. The spectra obtained over
time domains ranging from approximately 100 fs to ∼10 ps
consist largely of transient signals associated with the S1 state,
which is consistent with the observation of stimulated emission
over this time domain. Rapid quenching of the S1-excited state
in these systems causes the difference spectra to decay quickly;
similar behavior is seen for PZn( )PI (Figure 2).
For nonadiabatic ET reactions, as solvent polarity decreases,
the dipolar state becomes less stabilized, decreasing and
increasing ∆G° for the CS and CR reactions (∆G°CS and
∆G°CR), respectively (tabulated thermodynamic and metrical
data for these compounds can be found in Table 2). At the same
time, the diminished solvation of the charge-separated state in
a less polar solvent decreases the total reorganization energy
(λT) for CS reactions. This decrease in λT parallels a diminution
in thermodynamic driving force in less polar solvents, causing
(∆G + λT) and thus kCS to exhibit weak solvent dependences.15-18
For CR reactions in the Marcus inverted region, decreasing
solvent polarity increases the energy of the charge-separated
state; the concomitant decrease in the solvent reorganization
energy (λS) makes the quantity (∆G + λT) more negative,
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Transient Absorption Spectra of PZn( )NI, PZn(F2)NI,
PZn( )NI, and PZn( )PI2. The general features of the pump-
probe transient absorption spectra obtained for PZn( )NI, PZn-
(F2)NI, PZn[( )PI]2, and PZn[( )NI]2 (Figure 1, see Supporting
Information) resemble those elucidated for PZn( )PI (Figure
2); the time constants calculated for the CS and CR reactions
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13148-13168.
9
8772 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

(Aryl)ethynyl-Bridged Donor−Acceptor Systems
Table 2. Thermodynamic and Metrical Data
A R T I C L E S
ox a
red
o
b,c
o
E
1/2
E
1/2
RDA
−∆G CS
−∆G CR
compound
PZnPI
PZn( )PI
PZn( )( )PI
PZn[( )PI]2
mV
mV
Å
eV
eV
solvent
880
650
820
850
855
780
780
860
880
820
-860
8.3
0.55
0.67
0.54
0.57
0.50
0.86
0.80
0.83
0.75
0.81
0.75e
0.85
0.78e
1.55
1.43
1.56
1.53
1.60
1.24
1.30
1.27
1.35
1.29
1.43e
1.25
1.40e
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Bzd
CH2Cl2
Bz
CH2Cl2
Bz
CH2Cl2
Bz
CH2Cl2
Bz
-890 15
-810 21.7
-790 15
-785
PZn( )NI
-560 15.2
-560
PZn(F2)NI
PZn[( )NI]2
PZn[(CH3)NI]2
-510 15.3
-510
-570 15.2
-570
810
-545 15.2
-550
a Experimental conditions: [porphyrin] ) 1.0 mM; [TBAClO4] ) 0.1
M; reference electrode ) SCE; working electrode ) glassy carbon; auxiliary
electrode ) Pt wire. E1/2ox and E1/2red values correspond respectively to the
PZn0/+ and PI/NI-/0 potentials. These data are reported relative to SCE;
the ferrocene/ferrocenium redox couple (0.43 V vs SCE) was used as the
internal standard. b These values have been corrected for the Coulombic
work term, ∆G(ꢀ). The D-A center-to-center distances were estimated from
their MOPAC-determined minimum energy geometries. c The porphyrin
lowest excited singlet state energy, E(0,0), was determined from the overlap
of the absorbance and emission spectra. d Bz ) benzonitrile. e Assuming
no significant change in the (D/D+) potential from that determined in
methylene chloride.
Figure 4. Frontier molecular orbitals of (A) PZn[(F2)NI]2 and (B) PZn-
( )NI.
the PZn 5- and 15-(aryl)ethynyl substituents are electronically
asymmetric.23 Large magnitude splittings of the Qx and Qy
transitions are also evident in such systems, with the Qx(0,0)
state lying several hundred wavenumbers lower in energy with
respect to its Qy(0,0) counterpart.8,23 Interestingly, simplex
spectral fitting analyses (data not shown) demonstrate that the
magnitudes of the energetic splittings between the Qx and Qy
transitions for these [5-[4′-(N-(N′-octyl)pyromellitic/naphthylic
diimide)aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) and
[5,15-bis[4′-(N-(N′-octyl)pyromellitic/naphthylic diimide)aryl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) complexes are di-
minished with respect to the previously investigated 5,15-bis-
[(4′-substituted-aryl)ethynyl]-10,20-diphenylporphinato]zinc(II)
benchmarks8,20-23 and are on the order of a few hundred
wavenumbers (vide infra).
Electronic Structure Calculations. The frontier orbital (FO)
energies for PZn( )NI and PZn[(F2)NI]2 were determined using
the ZINDO method at a CI level of 20 (Experimental Section);
FOs for these species are displayed in Figure 4. Interestingly,
in contrast to [5,15-bis[(4′-substituted-aryl)ethynyl]porphinato]-
zinc(II) complexes (4′-substituents ) F, MMe2, H, OCH3, CH3,
NO2), which possess a2u-derived HOMOs,8 these ZINDO-based
computational studies predict an a1u-derived HOMO for PZn-
( )NI and PZn[(F2)NI]2. Consistent with the fact that the a1u
orbital possesses a node at the meso-carbon position, an
examination of the dependence of the FO energies upon the
relative donor, bridge, and acceptor torsional angles (Supporting
Information) reveals that the extent of D-Sp-A conjugative
interactions influence the relative energies of the HOMO-1 and
lowest-lying porphyrin-localized unoccupied orbitals. Note,
however, that the porphyrin-localized HOMO and LUMO levels
for PZn( )NI differ by no more than ∼0.02 eV at maximal and
minimal degrees of D-Sp-A π conjugation.
typically effecting a decrease the magnitude of the CR rate
constant (kCR). The CR reactions of covalently linked D-A
dyads have, almost without exception,19 been found to have
negative (∆G +λ) values, indicating that these reactions lie
within the Marcus inverted region. Consistent with this, (i) the
magnitudes of kCR reported for these D-Sp-A compounds are
larger in more polar solvents, and (ii) the transient absorption
spectra acquired for these species in highly nonpolar solvents
such as benzene display stronger absorption signals from the
CS state (Table 1).
Steady-State Electronic Absorption Spectra. PZn( )PI,
PZn( )NI, PZn(F2)NI, PZn[( )PI]2, PZn( )PI PZn[( )NI]2,
PZn[(CH3)NI]2, and PZn( )( )PI display broadened and red-
shifted Soret and Q-transitions in all solvents relative to the
directly linked D-A compound PZn-PI, (see Supporting
Information). Appending a cylindrically π-symmetric ethyne to
the macrocycle meso-carbon position effects a significant
splitting of the normally degenerate porphyrin x- and y-polarized
excited states. This perturbation from the electronic spectra of
conventional PZn complexes has been discussed previously.8,20-24
For example, 5,15-diethynyl-10,20-diarylporphyrin manifests a
broadened Soret transition as the x- and y-polarized B-state
transitions are no longer isoenergetic; simplex fitting of the
B-band shows that the splitting of the Bx and By states is on the
order of ∼300 cm-1 24
.
meso-(Aryl)ethynyl substituents have
been observed to effect larger splittings of the x- and y-polarized
B states relative to an ethyne group,8,13,20-23 with the energy
gap between the Bx(0,0) and By(0,0) levels >2000 cm-1 when
(19) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607-2617.
(20) LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang, C. H.; Therien, M.
J. J. Am. Chem. Soc. 1996, 118, 1497-1503.
(21) Priyadarshy, S.; Therien, M. J.; Beratan, D. N. J. Am. Chem. Soc. 1996,
118, 1504-1510.
(22) LeCours, S. M.; Phillips, C. M.; dePaula, J. C.; Therien, M. J. J. Am. Chem.
Soc. 1997, 119, 12578-12589.
Electron Paramagnetic Resonance Spectroscopy. The
nature of the PZn cation radical ([PZn]•+) species in these
D-Sp-A systems was probed using electron paramagnetic
resonance (EPR) spectroscopy. The cation radical states of both
(23) Karki, L.; Vance, F. W.; Hupp, J. T.; LeCours, S. M.; Therien, M. J. J.
Am. Chem. Soc. 1998, 120, 2606-2611.
(24) Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C.; Hupp, J. T.;
Angiolillo, P. J.; Therien, M. J. J. Am. Chem. Soc. 2000, 122, 7017-7033.
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8773

A R T I C L E S
Redmore et al.
nance effects contribute strongly to the magnitude of NO2 group
σp value, the σp parameter for cyclic imides is determined largely
by the field (F) effect,26 suggesting that the observed b1u HOMO
for [5,15-bis(4′-nitrophenyethynyl)-10,20-diphenylporphinato]-
zinc(II) is not caused solely by a simple Hammett substituent
effect but also from supplemental π-conjugative interactions
between the porphyrin core and the nitrophenyl unit that
destabilize the a2u-derived orbital. Regardless of the precise
origin of the au-symmetric HOMO observed for these D-Sp-A
compounds (Figure 1), this electronic structural perturbation
plays a key role in determining the nature of the ET dynamics
delineated in these systems28 as well as the relative magnitudes
of the CS and CR rate constants (vide infra).
Analyses of the ET Dynamical Data. The ET rates
determined from the pump-probe transient absorption spec-
troscopic studies were analyzed using the semiclassical Marcus-
Levich-Jortner eq 1.29-32
Figure 5. EPR spectrum of [PZn( )PI]•+ recorded in 19:1 methylene
chloride:THF. Experimental conditions: temperature ) 23 °C; modulation
frequency ) 100 Hz.
2
HAB
Scm
∞
2π
kET
)
exp(-S )
×
∑
c
p
m)0 m!
4πλ k T
x
S
B
[PZn( )PI]•+ and [PZn[( )NI]2]•+ were generated by the reaction
of the neutral complexes with tris(4-bromophenyl)aminium
hexachloroantimonate under oxygen-free conditions;25 a repre-
sentative EPR spectrum is shown in Figure 5. Note that no
resolvable hyperfine splittings are evident in the EPR spectra
of either [PZn( )PI]•+ or [PZn[( )NI]2]•+, indicating that the
HOMOs of these species possess a1u-derived symmetry.
For [5,15-bis[(4′-substituted-aryl)ethynyl]porphinato]zinc(II)
complexes, electronic interactions between the aryl and por-
phyryl units facilitated by the cylindrically π-symmetric ethyne
moiety raise the energy of the a2u-derived bu/b1u orbital above
that of the a1u-derived au orbital.8 While strongly electron-
withdrawing (4′-aryl)ethynyl substituents attenuate the magni-
tude of the bu/b1u orbital destabilization, an a2u-like HOMO
remains manifest even for [5,15-bis(4′-nitrophenyethynyl)-10,20-
diphenylporphinato]zinc(II).8 The D-Sp-A systems of Figure
1 feature meso-(aryl)ethynyl-derivatized PZn chromophores that
possess electron-withdrawing 4′-NI/PI substituents, with experi-
ment verifying the computational prediction of an a1u-derived
HOMO for these species. For [5,15-bis[(4′-substituted-aryl)-
ethynyl]porphinato]zinc(II) complexes, the magnitude of the
bu/b1u-au energy gaps, as well as the absolute HOMO energy,
track predictably with the Hammett σp value.26 These data,
coupled with an approximate Hammett σp value for PI and NI
(0.33)26 predict correctly the computed HOMO energy for these
meso-[(4′-(NI/PI)-aryl)ethynyl]-derivatized PZn compounds with
respect to the previously established benchmarks. While the
bu/b1u-au splittings for 5,15-bis[(4′-substituted-aryl)ethynyl]-
porphinato]zinc(II) compounds decrease progressively from 0.27
to 0.1 eV with increasing values of the σp parameter [NMe2
(-0.83), OMe (-0.27), H (0.0), F (0.06), NO2 (0.78)],8 a change
in HOMO symmetry from bu/b1u to au for PZn( )PI and PZn-
[( )NI]2 is not predicted in such an analysis. One possible
explanation for this spectroscopic perturbation evolves from
Swain-Lupton dissection of the σp value into its inductive (F)
and resonance (R) contributions.27 While inductive and reso-
(∆G + λs + mp ω )2
4λskBT
exp -
(1)
(
)
In eq 1, p is Planck’s constant, HAB is the electronic coupling,
kB is Boltzman’s constant, λs is the solvent reorganization
energy, Sc is the Huang-Rhys factor (Sc ) λi/[p ω ]), where λi
is the inner sphere reorganization energy, ω is the averaged
frequency of the high-frequency vibrations in the reactant state,
and m is the number of vibrational levels. The ∆G values were
determined using the standard Weller eqs 2 and 3;33 (Table 2).
The solvent reorganization energy (λS) was calculated by the
Marcus relation (eq 4).29
e2
∆GCS ) e(Eox - Ered) - E0,0
e2
-
+
4πꢀoꢀsRDA
1
1
ꢀsref
1
1
ꢀsref
1
1
2RA
-
-
+
(2)
(3)
(4)
4πꢀ ꢀ
ꢀ
2R
(
)(
)(
)
o
s
s
D
∆GCR ) -∆GCS - E0,0
e2
4πꢀ0 n
1
2
1
1
1
1
λs )
-
+
-
(
)(
)
ꢀs 2RD 2RA RDA
The D (RD) and A (RA) reactant sizes, and the center-to-center
distance (RDA) between D and A were determined from
D-Sp-A conformations optimized by the MOPAC-AM1
method. HAB and λi were the only adjustable parameters used
in the global fit of the dynamical data; in this computational
analysis, λi was assigned a value of 0.3 eV,34 a reasonable value
for these D-Sp-A systems, given literature precedent,9,35 and
was allowed to range within (0.1 eV of this initial estimate.
(28) Strachan, J.-P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191-11201.
(29) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.
(30) Levich, V. G. AdV. Electrochem. Electrochem. Eng. 1966, 4, 249-371.
(31) Ulstrup, J.; Bixon, M. J. Chem. Phys. 1975, 63, 4358-4368.
(32) Efrima, S.; Bixon, M. Chem. Phys. 1976, 13, 447-460.
(33) Weller, A. Z. Phys. Chem. N. F. 1983, 133, 93-98.
(34) Rubtsov, I. V.; Shirota, H.; Yoshihara, K. J. Phys. Chem. A 1999, 103,
1801-1808.
(25) Shultz, D. A.; Lee, H.; Gwaltney, K. P. J. Org. Chem. 1998, 63, 7584-
7585.
(26) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(35) Osuka, A.; Noya, G.; Taniguchi, S.; Okada, T.; Nishimura, Y.; Yamazaki,
I.; Mataga, N. Chem.sEur. J. 2000, 6, 33-46.
(27) Swain, C. G.; Lupton, E. C., Jr. J. Am. Chem. Soc. 1968, 90, 4328-4337.
9
8774 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

(Aryl)ethynyl-Bridged Donor−Acceptor Systems
A R T I C L E S
Table 3. Comparison of Experimentally Determined Photoinduced CS Rates with Those Predicted by the Marcus-Levitch-Jortner
Equation
benzene
methylene chloride
CS(exp) CS(calc)
(ps-1 (ps-1
acetonitrile
CS(exp)
(ps-1
RD
(Å)
R
(Å)
RDA
(Å)
H
k
CS(exp)
k
CS(calc)
k
k
k
k
CS(calc)
A
AB
(cm-1
compound
PZn( )PIa
PZn[( )NI]2
PZn[(CH3)NI]2
)
(ps-1
)
(ps-1
)
)
)
)
(ps-1
)
4.0
4.0
4.0
3.6
3.8
3.8
15
15.2
15.2
50
50
50
0.0093
0.083
0.065
3.1 × 10-6
0.041
0.043
0.100
0.018
0.076
0.067
0.0067
0.030
0.022
0.008
0.034
0.031
a
0.018
b
0.039
a λi ) 0.3 eV. b λi ) 0.35 eV.
Table 4. Comparison of Experimentally Determined Thermal CR Rates with Those Predicted by the Marcus-Levitch-Jortner Equation
benzene methylene chloride acetonitrile
CR(exp) CR(calc) CR(exp) CR(calc)
RD
(Å)
R
(Å)
RDA
(Å)
H
k
CR(exp)
k
CR(calc)
k
k
k
k
A
AB
(cm-1
compound
)
(ps-1
)
(ps-1
)
(ps-1 (ps-1 (ps-1 (ps-1
)
)
)
)
PZn( )PIa
PZn[( )NI]2
PZn[(CH3)NI]2
3.6
3.8
3.8
4.0
4.0
4.0
15
15.2
15.2
75
50
50
0.005
0.0023
0.014
3.5 × 10-6
7.2 × 10-6
1.8 × 10-5
0.95
0.16
0.10
0.97
0.41
0.39
0.37
0.45
0.21
0.15
0.034
0.027
a
b
a λi ) 0.3 eV. b λi ) 0.35 eV.
Table 5. Comparison of Experimentally Determined Photoinduced CS Rates with Those Predicted by the Marcus-Levitch-Jortner
Equation Using Expanded RD and RA
benzene
methylene chloride
CS(exp) CS(calc)
(ps-1 (ps-1
acetonitrile
RD
(Å)
R
(Å)
H
k
CS(exp
k
CS(calc)
k
k
k
CS(exp)
k
CS(calc)
A
AB
(cm-1
compound
)
(ps-1
)
(ps-1
)
)
)
(ps-1
)
(ps-1
)
PZn( )PIa,b
PZn[( )NI]2
PZn[(CH3)]NI2
7.7
7.7
7.7
3.6
3.8
3.8
10
15
15
0.0093
0.083
0.065
0.017
0.030
0.022
0.041
0.043
0.10
0.020
0.035
0.032
0.0067
0.030
0.022
0.020
0.039
0.037
a
c
b
a λi ) 0.3 eV. RDA ) 7.7 Å. c λi ) 0.35 eV.
Table 6. Comparison of Experimentally Determined Thermal CR Rates with Those Predicted by the Marcus-Levitch-Jortner Equation
Using Expanded RD and RA
benzene
methylene chloride
CR(exp) CR(calc)
(ps-1 (ps-1
acetonitrile
RD
(Å)
R
(Å)
H
k
CR(exp)
k
CR(calc)
k
k
k
CR(exp)
k
CR(calc)
A
AB
(cm-1
compound
PZn( )PI
PZn[( )NI]2
PZn[(CH3)NI]2
)
(ps-1
)
(ps-1
)
)
)
(ps-1
)
(ps-1
)
3.6
3.8
3.8
7.7
7.7
7.7
100
55
40
0.005
0.0023
0.014
0.002
0.003
0.007
0.95
0.16
0.10
0.16
0.20
0.17
0.37
0.45
0.21
0.35
0.42
0.26
The ET rate constants calculated in this manner, obtained
using D, A, and RDA sizes from geometry optimized structures,
agree well with the kCS values determined via pump-probe
transient optical spectroscopic measurements (Table 3). The
experimentally determined CR rate constants, however, differ
dramatically from the values predicted by the classic Marcus-
Levich-Jortner analysis (Table 4). Among the possible expla-
nations for this disparity is that the (aryl)ethynyl bridging unit
facilitates augmented delocalization of the CS state with respect
to the ground and singlet-excited states, rather than playing a
passive role in the ET dynamics of these conjugated D-Sp-A
arrays. The notion that a highly π-conjugated bridging unit
can augment the magnitude of donor or acceptor electronic
delocalization to form a “superdonor” or “superacceptor”15 has
been discussed previously. Singer has shown in a series of
p-polyphenylamines that aromatic bridging units can act as an
extension of the D or A moieties in charge-transfer reactions,3
and Khundkar has reported the involvement of a π-conjugated
bridge in the excited state CR reactions of asymmetrically
substituted di-(aryl)ethynyl molecules.4 More recently, Bre´das
has implicated the formation of superdonors/superacceptors in
the ET dynamics of a series of compounds in which p-phenylene
vinylene bridges link a tetracene donor to a pyromellitic diimide
acceptor.5,7
Accordingly, the ET rate constants were recalculated, treating
the D and A reactant sizes in the excited and CS states as
adjustable parameters. While such a procedure has little impact
on the quality of the fit obtained for the CS ET rate data,
considerably better fits of the CR dynamical data were gener-
ated; this improvement in fit quality was most dramatic for the
symmetric D-Sp-A systems PZn[( )NI]2 and PZn[(CH3)NI]2
(Tables 5 and 6). The reactant size of either (or both) the
[(PZn)•+] acceptor or the PI•-/NI•- donor were increased to
include the (aryl)ethynyl bridging unit; the extent to which these
theoretical and experimental results correlate can be depicted
graphically. Figure 6 compares CR rate constants computed via
the Marcus-Levich-Jortner equation using both restricted and
expanded radical ion reactant sizes to those measured experi-
mentally for PZn[( )NI]2. The CR rate constants, calculated with
one enlarged radical ion size, fit the experimental data well
(Figure 6); when the radical ion reactant sizes were restricted
to the PZn•+ and NI•- units, the quality of the fit dropped
dramatically (Figure 6). The CS rate constants calculated for
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8775

A R T I C L E S
Redmore et al.
NI]2 should be evident in the fit of the ET dynamics. The
Marcus-Levich-Jortner equation, however, predicts only a
small change in the magnitude of kCS in these systems, which
is consistent with the experimental observation (Table 3), and
indicates that Sp-A electronic delocalization is not a determinant
of the magnitude of kCS in these systems.
An analogous examination can be made of the PZn[(CH3)-
NI]2 CR reaction. An identical theoretical analysis predicts that
bridge methylation should have modest impact on the PZn-
[(CH3)NI]2 CR dynamics with respect to that observed for
PZn[( )NI]2 in nonpolar solvents such as benzene. Again, theory
and experiment correlate (Table 4). While the absolute magni-
tude of kCR for these compounds is not modeled well by these
Marcus-Levich-Jortner calculations, note that theory does
predict the trend in the relative magnitudes of kCR for these
species; this rules out the possibility that enhanced Sp-A
electronic delocalization within the CS state plays a role in the
CR dynamics. However, because augmented reactant size is
crucial for a reasonable fit of the CR rate data, these data imply
that it is the PZn cation radical that delocalizes onto the (aryl)-
ethynyl unit in the CS state.
Solvent Dependence of CR in PZn[(CH3)NI]2. On the basis
of the Marcus-Levich-Jortner analysis of the CS reaction rate
data, the initially prepared CS state is not highly delocalized
(vide supra); the key remaining issue involves whether the CS
state undergoes a change, either in geometry or in polarization,
before the CR reaction occurs. Analysis of the CR rate data
obtained for PZn[(CH3)NI]2 and PZn[( )NI]2 argue that if any
augmented delocalization of the CS state occurs, it does not
involve an appreciable change in the nature of Sp-A electronic
interactions. To further support the hypothesis that the initially
formed PZn•+ delocalizes onto the (4′-phenyl)ethynyl bridge,
we examine the CR dynamics for PZn[(CH3)NI]2 in viscous
solvent; an increase in the time constant for diffusional rotation
of the Sp phenyl ring is expected with increased solvent
viscosity.36-41 The ET dynamics exhibited by PZn[(CH3)NI]2
in polystyrene were compared to those established in benzene,
as these two solvents feature similar dielectric constants but
highly disparate viscosities.
τCR for PZn[(CH3)NI]2 in polystyrene is 900 ps, more than
10-fold larger than the analogous time constant determined in
benzene (Table 1). This substantial difference in the magnitudes
of kCR in polystyrene and benzene supports the propositions
that: (i) the angle formed between the phenyl and porphyrin
rings is an important determinant of CR dynamics, and (ii) the
time scale for PZn•+-Sp electronic delocalization is likely a
function of aryl ring rotational time constant in these (phenyl)-
ethynyl bridged D-A arrays.
Pump-Wavelength Dependence of the Magnitude of kCS
in PZn[( )NI]2 and PZn[(CH3)NI]2. The proposition that the
extent of CS state electronic delocalization depends at least in
part upon aryl ring rotational dynamics, suggests, together with
the Marcus-Levich-Jortner-based ET dynamical analyses, that
Figure 6. Graphical comparison of experimentally determined PZn[( )-
NI]2 CR rate constants (b) with those computed theoretically via the
Marcus-Levich-Jortner equation using D- and A-localized radical ion sizes
(() and with a PZn•+ acceptor that is delocalized over the (aryl)ethynyl
spacer (9).
PZn[( )NI]2 using either small (Table 3) or large (Table 5) ion
sizes correlate well with the experimental CS rates (data not
shown); note that expansion of D and A reactant sizes to include
the bridge in the analysis of the CS reaction dynamics requires
a concomitant drop in electronic coupling.
While expanded reactant sizes are clearly implicated in the
CR reactions of these D-Sp-A assemblies, key questions that
need to be addressed include: (1) To what extent are the
electronically excited states of these ET arrays delocalized? and
(2) Is it the PZn cation radical or the diimide anion radical (or
both) that is delocalized over the bridge in the charge-separated
state? Some insight into this issue can be obtained by examining
how reducing π-overlap of either D or A with the conjugated
bridge affects the observed ET dynamics, as attenuated conjuga-
tion should diminish excited- and CS-state delocalization.
Comparative ET Dynamics in PZn[(CH3)NI]2 and PZn-
[( )NI]2: Determining the Degree of Sp-A Delocalization. We
first examine the degree of delocalization between the (aryl)-
ethynyl bridge and the diimide acceptor in the excited and CS
states. Introducing additional steric hindrance between the Sp-
phenyl ring ortho substituents and the A diimide amide oxygens
increases the energy of Sp-A conformeric populations having
small phenyl-PI/NI interplanar torsional angles and thereby
reduces the extent of π-conjugation between Sp and A in the
ground, excited, and charge-separated states. Molecular model-
ing studies (Experimental Section, Supporting Information)
suggest that the ortho-methyl substituent in PZn[(CH3)NI]2 is
sufficient to ensure a significant reduction in the range of
thermally accessible torsional angles, θ, between the NI acceptor
and the Sp-phenyl ring; notably, this bridge ortho-methyl
substitution has negligible impact on the ET thermodynamics
(Table 2).
If bridge ortho-methylation modulates the ET dynamics in
PZn[(CH3)NI]2 relative to PZn[( )NI]2 to an extent greater than
that predicted by theory on purely thermodynamic grounds, such
data would lend support to the hypothesis that augmented Sp-A
electronic delocalization plays a role in the ET reactions of the
systems. With respect to the CS reactions for these species, if
Sp-A electronic delocalization were important, the impact of
attenuated conjugation in PZn[(CH3)NI]2 relative to PZn[( )-
(36) LaChapelle, M.; Belleteˆte, M.; Poulin, M.; Godbout, N.; LeGrand, F.;
He´roux, A.; Brisse, F.; Durocher, G. J. Phys. Chem. 1991, 95, 9764-9772.
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H. J. Chem. Phys. 1995, 103, 6414-6425.
(38) Chen, Y.; Wu, S.-K. J. Photochem., Photobiol., A 1997, 102, 203-206.
(39) Yee, W. A.; O’Neil, R. H.; Lewis, J. W.; Zhang, J. Z.; Kliger, D. S. J.
Phys. Chem. A 1999, 103, 2388-2393.
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2878-2887.
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9
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(Aryl)ethynyl-Bridged Donor−Acceptor Systems
A R T I C L E S
interesting that these data are incongruent with the established
photophysics of [5,15-bis[(aryl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) compounds.8,20-23 This result likely derives
in part from differences in the configuration expansion that
describes the S1-excited state for PZn[( )NI]2 and PZn[(CH3)-
NI]2 with respect to previously studied [5,15-bis[(aryl)ethynyl]-
10,20-diphenylporphinato]zinc(II) compounds;8,20,22 in contrast
to these benchmarks, [5,15-bis[4′-(N-(N′-octyl)pyromellitic/
naphthylic diimide)aryl)ethynyl]-10,20-diphenylporphinato]zinc-
(II) complexes possess a1u-derived HOMOs. Importantly, how-
ever, the relatively minor dependence that CS rate constants
have upon λex is consistent with the expectation derived from
the theoretical analysis of the ET dynamical data (vide supra)
that suggests that the excited states of these D-Sp-A as-
semblies are largely PZn-localized and that (aryl)ethynyl bridge
participation in electronic delocalization augmentation is con-
fined to the CS state.
Figure 7. Transient absorption spectra of PZn[(CH3)NI]2 in benzene, with
labeled time delays. Inset shows transient kinetics measured at 738 and
620 nm, which probe the formation of the CS state. Experimental
conditions: λex ) 672 nm, temperature ) 23 ( 1 °C.
Conclusions
The solvent-dependent ET dynamics of a series of [5-
[4′-(N-(N′-octyl)pyromellitic/naphthylic diimide)aryl)ethynyl]-
10,20-diphenylporphinato]zinc(II) and [5,15-bis[4′-(N-(N′-octyl)-
pyromellitic/naphthylic diimide)aryl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) complexes have been determined via time-
resolved transient absorption spectroscopy. Marcus-Levich-
Jortner analysis of these data shows that, in contrast to the
charge-separation reaction dynamics, the magnitudes of the
experimentally determined CR rate constants cannot be fit using
this model if the CS states of these D-Sp-A systems feature
conventional D- and A-localized reactant sizes. This analysis,
coupled with electronic structure calculations, potentiometric
experiments, and additional pump-wavelength- and solvent-
viscosity dynamical data, indicates that the initially formed CS
states of these species relax to more delocalized CS states in
which the porphyrin macrocycle and (4′-phenyl)ethyne bridge
form a π-conjugated “super acceptor”;3,4,7,42 thus, while the
(aryl)ethynyl Sp simply mediates electronic superexchange for
photoinduced charge separation, it plays a nonpassive role in
thermal CR processes.
The expansion of A cation size within the CS state (i) causes
the effective D-A edge-to-edge distance in these (aryl)ethynyl-
bridged PZn-PI/NI compounds to differ little from that defined
by directly linked PZnPI (Figure 1) and (ii) attenuates severely
the dependence of the magnitudes of the CR rate constants
upon solvent. The established CR dynamics, coupled with the
solvent dependence of the Marcus-Levich-Jortner-computed
(∆G°CR + λT) values, underscore this latter point: as solvent
polarity is varied from acetonitrile to benzene, analyses which
utilize D- and A-localized reactant states show that the quantity
(∆G°CR + λT) takes on values that range from 0.7 to -1.9 eV.
In contrast, when the effect of a conjugation-expanded PZn
cation radical is considered, (∆G°CR + λT) spans -0.25 to -1.28
eV between these extremes of solvent polarity.
Table 7. Charge Separation and Charge Recombination
Dynamical Data Determined from Pump-Probe Transient
Absorption Spectroscopic Measurements at Varying Excitation
Wavelengths (λex) in Benzene Solvent
compound
λex (nm)
τCS (ps)
kCS (s-1
)
τCR (ps)
kCR (s-1
)
τCR/τCS
PZn[( )NI]2
645
672
16
12
18
16
13
6.3 × 1010 500
2 × 109 32
8.3 × 1010 430 2.3 × 109 36
PZn[(CH3)]NI2 645
5.6 × 1010
6.3 × 1010
7.7 × 1010
71 1.4 × 1010
74 1.4 × 1010
71 1.4 × 1010
4.0
4.8
5.4
655
672
the initially prepared excited-states of these complexes are
largely PZn-localized. Whether or not the Franck-Condon state
initially formed upon excitation undergoes geometrical changes
leading to the formation of a lower-energy, more coplanar state
that facilitates augmented D-Sp delocalization can be determined
from an examination of the pump-wavelength dependence of
the CS dynamics.
The lowest-lying singlet-excited state of meso-(aryl)ethynyl-
substituted porphyrins has been determined to be polarized along
the x-axis; the spectral heterogeneity of the Qx-transition
envelope of these species derives from conformeric populations
that differ in the extent of conjugation between the (aryl)ethynyl
arene and the porphyrin core.8 Exciting a population of ground-
state molecules in which the geometry resembles the low-energy
excited state (red-edge excitation of the Qx(0,0) envelope) should
effect an increase in CS rate constant; likewise pump wave-
lengths that correspond to the blue-edge of the Qx(0,0) manifold
selectively excite a population of less conjugated conformers.
Figure 7 displays the relevant transient absorption spectra for
PZn[(CH3)NI]2, while Table 7 tabulates the excitation-
wavelength-dependent dynamical data obtained for PZn[( )NI]2
and PZn[(CH3)NI]2.
The pump wavelength dependences of the CS dynamics for
PZn[( )NI]2 and PZn[(CH3)NI]2 show that the average τCS
measured in benzene decreases only slightly with longer
excitation wavelength (λex), varying from 17.9 to 13.2 ps for
PZn[(CH3)NI]2 and from 15.7 to 10.8 ps for PZn[( )NI]2 for
λex ranging from 645 to 672 nm (Table 7), suggesting that the
extent of electronic delocalization in the initially prepared S1
state varies only modestly with the excitation wavelength. It is
In these D-Sp-A assemblies, the σ-electron-withdrawing
4′-(aryl)ethynyl substituents influence ground-state electronic
structure; both EPR spectroscopy and theory show that these
D-Sp-A assemblies possess a1u-derived HOMOs. Electronic
delocalization in the CS state, however, is unlikely to be
(42) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature
1998, 396, 60-63.
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8777

A R T I C L E S
Redmore et al.
facilitated by an a1u-derived HOMO, as it possesses a node at
the meso-position. Given the close D-A distances in these
D-Sp-A systems, the nature of the Sp electronic structure,
and the fact that the classic Gouterman a1u- and a2u-derived FOs
are predicted to lie very close in energy (Figure 4 and Supporting
Information), Coulombic interactions between D+ and A- likely
drive an a1u-a2u orbital inversion in the CS state. The analysis
of the CR rate data obtained in viscous media suggests that an
electronic structural rearrangement is likely coupled tightly to
diffusive rotation of the Sp unit aryl ring. Consistent with such
an analysis, the CS and CR reactions for PZn( )( )PI, which
has a larger D-A distance and therefore reduced D+/A-
Coulombic interaction, does not display the reverse kinetics (kCR
. kCS) associated with the remainder of this series of com-
pounds (Table 1).
Finally, while these data indicate that simple (aryl)ethynyl
units can facilitate augmented CS-state hole delocalization over
the D-A length scales probed in this study, it is important to
emphasize that this work, coupled with earlier investigations,8,21-23
highlights as well that the extent of singlet-excited-state wave
function amplitude on the arylethynyl moiety is a sensitive
function of arene electronic structure; hence, engineering the
extent to which the (aryl)ethyne bridge delocalizes D- or
A-centered electron density, in both photoinduced CS and
thermal CR reactions within this class of compounds, should
be amenable to modulation via molecular design.
Acknowledgment. We thank Professor Paul J. Angiolillo (St.
Joseph’s University) and Dr. Thomas Troxler (NIH Regional
Laser Laboratory, University of Pennsylvania) for their respec-
tive assistance with EPR and time-correlated single-photon
counting spectroscopic measurements. This work was supported
by a grant from the Division of Chemical Sciences, Office of
Basic Energy Research, U.S. Department of Energy (DE-FGO2-
02ER15299). 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.
Supporting Information Available: Complete details of the
synthesis and characterization of the series of compounds, and
synthetic schemes. Transient- and steady-state optical spectra
of PZn(F2)NI, PZn( )PI, PZn[( )PI]2, and PZn[( )NI]2 in
methylene chloride, transient optical spectra of PZn( )PI and
PZn[(CH3)NI]2 in acetonitrile, and tabulated cyclic voltammetric
data. Both tabulated and graphic HOMO-1, HOMO, LUMO,
and LUMO+1 energies for PZn( )NI. and MOPAC-calculated
phenyl ring rotational barriers for PZn[( )NI]2 and PZn[(CH3)-
NI]2 (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA021278P
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