Synthesis and excited-state photodynamics of perylene-bis(imide)-oxochlorin dyads. A charge-separation motif

Article abstract of DOI:10.1021/jp0269423

Three perylene-oxochlorin dyads have been prepared and characterized with the goal of identifying charge-injection or molecular-switching motifs for use in molecular photonics. Each dyad consists of a perylene-bis(imide) dye (PDI) joined at the 10-position of a magnesium, zinc, or free base (Fb) oxochlorin via a diphenylethyne linker. Each dyad has been studied in both polar and nonpolar media using static and time-resolved optical spectroscopy and electrochemical techniques. Dyad PDI-MgO is an excellent charge-separation unit in which the excited perylene (a?? 3.5 ps lifetime) or the excited oxochlorin (lifetimes of 0.5 ns in benzonitrile and 1.0 ns in toluene) give rise to state PDI^- MgO⁺ in high overall yield (>90%); the charge-separated state has a lifetime of a?￥ 1 ns in both toluene and benzonitrile. The pathway for generating PDI^- MgO⁺ from the excited perylene (PDI*) involves both hole transfer and energy transfer to the oxochlorin followed by electron transfer from the resulting MgO* to PDI. Similar decay of PDI* by energy transfer and hole transfer is found for dyads PDI-ZnO and PDI-FbO. However, electron-transfer quenching of the excited oxochlorin in these two dyads either does not occur or occurs to a much lesser degree than for PDI-MgO in both polar and nonpolar solvents. For PDI-FbO the decay of the charge-separated state occurs significantly by charge recombination to give the excited oxochlorin, making this a good light-harvesting system even though the early stages of the dynamics include charge separation/recombination. The observed differences in the extent of the possible excited-state processes (energy, hole, and electron transfer) among the dyads and in polar versus nonpolar media are consistent with the estimated energy ordering of the excited- and charge-separated states. This study has provided a new class of arrays containing perylene accessory pigments and oxochlorin chromophores that can be utilized for applications in light harvesting and molecular optoelectronics.

Full text of DOI:10.1021/jp0269423

J. Phys. Chem. B 2003, 107, 3443-3454
3443
Synthesis and Excited-State Photodynamics of Perylene-Bis(Imide)-Oxochlorin Dyads. A
Charge-Separation Motif
Christine Kirmaier,† Eve Hindin,† Jennifer K. Schwartz,† Igor V. Sazanovich,† James R. Diers,‡
Kannan Muthukumaran,§ Masahiko Taniguchi,§ David F. Bocian,*,‡
Jonathan S. Lindsey,*,§ and Dewey Holten*,†
Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4899, Department of Chemistry,
UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry, North Carolina State
UniVersity, Raleigh, North Carolina 27695-8204
ReceiVed: September 8, 2002; In Final Form: January 2, 2003
Three perylene-oxochlorin dyads have been prepared and characterized with the goal of identifying charge-
injection or molecular-switching motifs for use in molecular photonics. Each dyad consists of a perylene-
bis(imide) dye (PDI) joined at the 10-position of a magnesium, zinc, or free base (Fb) oxochlorin via a
diphenylethyne linker. Each dyad has been studied in both polar and nonpolar media using static and time-
resolved optical spectroscopy and electrochemical techniques. Dyad PDI-MgO is an excellent charge-separation
unit in which the excited perylene (∼3.5 ps lifetime) or the excited oxochlorin (lifetimes of 0.5 ns in benzonitrile
and 1.0 ns in toluene) give rise to state PDI- MgO+ in high overall yield (>90%); the charge-separated state
has a lifetime of g1 ns in both toluene and benzonitrile. The pathway for generating PDI- MgO+ from the
excited perylene (PDI*) involves both hole transfer and energy transfer to the oxochlorin followed by electron
transfer from the resulting MgO* to PDI. Similar decay of PDI* by energy transfer and hole transfer is found
for dyads PDI-ZnO and PDI-FbO. However, electron-transfer quenching of the excited oxochlorin in these
two dyads either does not occur or occurs to a much lesser degree than for PDI-MgO in both polar and
nonpolar solvents. For PDI-FbO the decay of the charge-separated state occurs significantly by charge
recombination to give the excited oxochlorin, making this a good light-harvesting system even though the
early stages of the dynamics include charge separation/recombination. The observed differences in the extent
of the possible excited-state processes (energy, hole, and electron transfer) among the dyads and in polar
versus nonpolar media are consistent with the estimated energy ordering of the excited- and charge-separated
states. This study has provided a new class of arrays containing perylene accessory pigments and oxochlorin
chromophores that can be utilized for applications in light harvesting and molecular optoelectronics.
Introduction
diphenylethyne linker is an excellent light-harvesting motif:
energy is transferred rapidly and effectively quantitatively from
the perylene component to the oxochlorin unit, the excited state
of which subsequently decays with characteristics comparable
to those of the isolated chromophore.37 In the present paper,
we utilize instead a perylene-bis(imide) accessory pigment, the
prototypical unit being PDI-1 shown in Chart 1. We have
previously utilized this dye in diphenylethyne-linked dyads
containing a zinc, magnesium, or free base porphyrin (PDI-
ZnP, PDI-MgP, and PDI-FbP; Chart 1). In the present paper,
we have prepared and studied the analogous oxochlorin dyads
PDI-ZnO, PDI-MgO, and PDI-FbO (Chart 1). The goal is
to utilize the favorable redox characteristics of both units and
the enhanced red absorption of the oxochlorins (compared to
porphyrins) to elicit both strong light absorption over a broad
region and efficient generation of long-lived charge-separated
states. The dyads have been characterized by electrochemistry,
static absorption, and fluorescence spectroscopy, and time-
resolved fluorescence and transient absorption spectroscopy.
Taken together, the studies described herein provide fundamental
insight into the design and function of dyad architectures for
specific applications in molecular photonics.
One theme of our recent work has been the design, synthesis,
and characterization of molecular architectures containing
porphyrins or porphyrins plus accessory pigments that undergo
facile energy- and/or charge-transfer processes in a controlled
manner.1 Efficient excited-state energy transfer is critical for
light harvesting while charge-transfer processes underlie opto-
electronic gating. Recently we have shown that arrays containing
perylenes and porphyrins can be designed to select for excited-
state energy or charge transfer by tuning the redox and
photophysical properties of the components.2-12 We have also
begun to extend our studies of tetrapyrroles by developing new
synthetic routes to chlorins13-15 and oxochlorins,16 an area of
active research in recent years.17 A number of dyads containing
chlorins/oxochlorins or one of these chromophores with an
accessory pigment or electron acceptor have been prepared.18-36
In the preceding paper, we showed that a dyad consisting of
a perylene-monoimide dye and a zinc oxochlorin joined by a
* To whom correspondence should be addressed. (Dewey Holten)
E-mail: holten@wuchem.wustl.edu. (Jonathan S. Lindsey) E-mail: jlindsey@
ncsu.edu. (David F. Bocian) E-mail: David.Bocian@ucr.edu.
† Department of Chemistry, Washington University.
‡ Department of Chemistry, University of California.
§ Department of Chemistry, North Carolina State University.
10.1021/jp0269423 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/20/2003

3444 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Kirmaier et al.
CHART 1
Results and Discussion
absorption spectroscopy. The demetalation of PDI-ZnO using
trifluoroacetic acid (TFA) in CH2Cl2 afforded the corresponding
free-base dyad PDI-FbO in 76% yield. Treatment of PDI-
FbO with MgI2 in CH2Cl2 in the presence of N,N-diisopropyl-
ethylamine (DIEA) at room temperature (heterogeneous method
for magnesium insertion)38 afforded the Mg-containing dyad
PDI-MgO in 45% yield.
Several oxochlorin benchmarks were prepared from ZnO1.
Demetalation of ZnO1 on treatment with TFA in CH2Cl2
afforded the free-base chlorin FbO1 in 94% yield (Scheme 2).
Magnesium insertion was achieved by treating FbO1 with MgI2
and DIEA to afford MgO1 in 92% yield. Each oxochlorin in
this series bears one ethyne group.
Synthesis. The rational synthesis of C-alkylated chlorins
provides ready access to a variety of chlorin building blocks.13-15
We recently developed a simple procedure for oxidation of the
methylene group in the reduced ring of the chlorin, forming
the corresponding oxochlorin.16 Thus, treatment of zinc chlorin
Zn113,14 with basic alumina (activity I) in toluene at 50 °C in
the presence of air gave a mixture of the zinc hydroxychlorin
and a trace amount of the zinc oxochlorin. The mixture was
then treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) in toluene for 5 min to give the desired zinc oxochlorin
ZnO1 in 47% overall yield (Scheme 1).
The perylene-Zn-oxochlorin dyad was synthesized by the Pd-
catalyzed coupling of the ethynylphenyl-substituted zinc oxochlo-
rin and the iodo-substituted perylene building block. The
coupling reactions were carried out under similar conditions
developed for joining porphyrins,2,3 which have been used
recently for attaching perylenes to porphyrins.4,6,7,8 The condi-
tions employ dilute solutions of the reactants in the presence
of tris[dibenzylideneacetone]-dipalladium(0) [Pd2(dba)3] and tri-
o-tolylphosphine [P(o-tol)3] in a solution of toluene/triethylamine
(TEA) at 35 °C. Thus, the Pd-mediated reaction of ethyne-
substituted ZnO1 and N-(2,5-di-tert-butylphenyl)-N′-(4-iodo-
phenyl)-3,4,9,10-perylenetetracarboximide (PDI-2)6 afforded
the perylene-bis(imide)-Zn-oxochlorin dyad PDI-ZnO in 51%
yield (Scheme 1). The dyad PDI-ZnO was characterized by
laser desorption mass spectrometry, 1H NMR spectroscopy and
Electrochemical Characterization. The electrochemical data
for dyad PDI-ZnO and benchmark perylene dye PDI-1 are
summarized in Table 1. The E1/2 values were obtained with
square wave voltammetry (frequency 10 Hz). The isolated
perylene dyes exhibit one oxidation and two reduction waves
in the +1.4 to -2.0 V range. As we have previously reported,5,7
the redox potentials of the perylene-bis(imide) dyes studied here
are anodically shifted by ∼0.4 V relative to those of the
perylene-monoimide dyes studied in the preceding article.37 The
E1/2 values for the perylene units in the dyads are generally
similar to those of the isolated dyes. The oxochlorin unit in
each of the dyads exhibits two oxidation and two reduction
waves in the +1.4 to -2.0 V range; the E1/2 values are similar
to those of the monomeric macrocycle (ref 16 and J. R. Diers

A Perylene-Oxochlorin Charge-Separation Motif
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3445
SCHEME 1
SCHEME 2
a
TABLE 1: Half-Wave Potentials (E1/2
)
oxidation potentials
reduction potentials
perylene oxochlorin
perylene E1/2 (1) E1/2 (2) E1/2 (1) E1/2 (2) E1/2 (1) E1/2 (2)
+1.36 -0.81 -1.07
PDI-ZnO +1.40c +0.51 +0.75 -0.76 -1.04 -1.56 -1.94
ZnOd -1.50 n.m.e
+0.59 +0.89
oxochlorin
PDI-1b
a Obtained in butyronitrile containing 0.1 M (n-Bu)4NPF6. E1/2 vs
Ag/Ag+; E1/2 of FeCp2/FeCp2+ ) 0.19 V. The E1/2 values are obtained
from square wave voltammetry (frequency ) 10 Hz). Values are (
0.01 V. b Taken from ref 7. c Value is approximate because the
maximum in the redox wave is outside the electrochemical window.
d For an oxochlorin monomer described in the preceding paper (ZnO9).37
e Not measured.
in photophysical properties of a given dyad in the two solvents
is that benzonitrile provides greater energy stabilization of the
charge-separated states (e.g., involving the oxidized oxochlorin
and reduced perylene), which can alter the rates and yields of
the excited-state processes. These effects are illustrated in Figure
1, which summarizes the excited-state processes that have been
deduced from this study. Reference to this summary diagram
will aid in the presentation and discussion of the results.
Absorption Spectra. The ground-electronic-state absorption
spectrum of each perylene-oxochlorin dyad is essentially given
by the sum of the spectra of the subunits, reflecting relatively
weak perylene-oxochlorin interactions. This point is illustrated
in Figure 2 for PDI-ZnO in toluene (solid spectra). The
absorption spectrum is dominated by the oxochlorin near-UV
Soret band at ∼425 nm, the oxochlorin Qy(0,0) band (S0 f S1)
at ∼610 nm, and the perylene absorption contour (S0 f S1)
between 450 and 550 nm. The spectrum of PDI-ZnO and the
PDI-1 reference monomer contains a clear vibronic progression
with an origin at 527 nm. Spectra analogous to those shown in
Figure 2 are given in the Supporting Information for PDI-ZnO
(and its reference monomers) in benzonitrile as well as for PDI-
MgO and PDI-FbO in both solvents. For comparison purposes,
spectra focused on the PDI and oxochlorin origin bands (500-
660 nm) for the three dyads and both solvents are shown in
Figure 3. In all three dyads, the blue-green absorption of the
and D. F. Bocian, unpublished results). The similarity of the
redox potentials of the components of the dyads with that of
the isolated components is indicative of the relatively weak
ground-state electronic interactions between the dyad compo-
nents.
Photophysical Properties. Complete photophysical charac-
terization of each of the three perylene-oxochlorin dyads
depicted in Chart 1, as well as of the reference monomers, was
carried out in toluene and in benzonitrile at room temperature.
Studies were performed in these two solvents because potential
applications may utilize the arrays in either nonpolar or polar
media. The static absorption and emission spectra of each dyad
and all of the photophysical properties of each reference
compound generally differ only modestly in the two solvents.
These effects are due mainly, but not exclusively, to the
coordination of benzonitrile to the central metal ion in the Zn-
and Mg-containing oxochlorins. The most significant difference

3446 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Kirmaier et al.
Figure 1. Summary diagram showing the excited-state processes in the dyads at room temperature. The values in parentheses are the estimated
free energies of the states in eV (see text).
perylene accessory pigment complements the blue and red
absorption of the oxochlorin, thereby giving broad spectral
coverage.
as indicated by the bands at ∼540 and ∼580 nm (Figure 2A,
dotted) that lie at the same positions as in the PDI-1 monomer
(Figure 2C). This perylene emission in PDI-ZnO is reduced
by orders of magnitude from that in the monomer, reflecting
the fact that PDI* decays predominantly by a combination of
energy and hole transfer to the oxochlorin, with relative yields
that are best determined from the transient absorption data
described below (see Figure 1C). The fact that residual perylene
emission is observed in the dyad despite the substantial
quenching derives from the high intrinsic emission yield of this
chromophore (Table 2), as we described in the previous article
for PMI-ZnO.37
The oxochlorin emission yields for PDI-ZnO, PDI-MgO,
and PDI-FbO in toluene and benzonitrile were determined from
the spectra obtained with direct oxochlorin excitation (see Figure
2A, dashed), with subtraction of the residual perylene emission
followed by comparison to a fluorescence standard (FbTPP),
as described in the previous paper.37 The resulting yields are
given in Table 2, along with those for the benchmark monomers.
Fluorescence Spectra and Quantum Yields. Fluorescence
emission spectra for PDI-ZnO and its reference monomers in
toluene at room temperature are shown in Figure 2 (dashed and
dotted spectra). Fluorescence spectra for all the dyads and
reference monomers in both toluene and benzonitrile are given
in the Supporting Information. Excitation of the perylene subunit
of PDI-ZnO in toluene at 480 nm produces a substantial amount
of oxochlorin emission, namely, the (0,0) band at 612 nm and
the (0,1) band at 670 nm (Figure 2A, dotted spectrum) that are
at the same positions found for the ZnO1 monomer (Figure 2B).
The level of the oxochlorin emission is a sizable fraction of
that found with direct oxochlorin excitation at 392 nm (Figure
2A, dashed). This finding is indicative of significant energy
transfer from the excited perylene (PDI*) to the ground-state
oxochlorin to form ZnO*. The emission spectrum obtained using
primarily perylene excitation also contains perylene emission,

A Perylene-Oxochlorin Charge-Separation Motif
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3447
TABLE 2: Photophysical Data for Perylene-Oxochlorin Dyads and Reference Compoundsa
PDI* decay
O* decay
τ (ns)e
PDI- O+ decay
c
f
compound
dyads
PDI-MgO
PDI-ZnO
PDI-FbO
solvent
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
τ (ps)b
ΦHT
Φfd
ΦET
τ (ns)g
3.5
3.5
3.3
3.6
3.0
3.0
0.60
0.65
0.25
0.30
0.50
0.55
0.032
0.011
0.030
0.018
0.10
0.9 ( 0.2
0.5 ( 0.2
0.6 ( 0.2
0.4 ( 0.1
8.4 ( 0.5
7.2 ( 0.5
0.70 ( 0.05
0.85 ( 0.05
<0.2
>1.5
1.0 ( 0.3
>1.5
0.4 ( 0.2
<0.2
0.6 ( 0.2
>1.5
0.20 ( 0.05
0.080
<0.2
monomers
MgO1
ZnO1h
FbO1
PDI-1
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
0.13
0.14
0.037
0.030
0.13
0.13
0.97i
0.99
3.1 ( 0.2
3.0 ( 0.2
0.7 ( 0.2
0.7 ( 0.2
9.0 ( 0.5
9.1 ( 0.5
3600i
3800
benzonitrile
a All data at room temperature. b Lifetime of the perylene lowest excited singlet state; the values for the dyads were determined by transient
absorption spectroscopy (average single-exponential fit through the PDI bleaching region) and those in the monomers by fluorescence decay ((200
ps). c Yield of hole transfer to the oxochlorin ((5%); the corresponding yield of energy transfer is ΦENT ∼ 1 - ΦHT
.
d Oxochlorin fluorescence
yield ((15%) determined using Soret (392 nm) excitation and free base tetraphenylporphyrin (FbTPP) in toluene (Φf ) 0.11) as a standard.41 The
yield for PDI-1 in benzonitrile was referenced to the value in toluene. e Oxochlorin excited-singlet-state lifetime determined by averaging the
f
values determined by transient absorption kinetics and fluorescence decay. Yield of electron transfer from excited oxochlorin to perylene determined
by averaging the values obtained from transient absorption spectra, excited-state lifetimes, and fluorescence yields (see text). g Decay of the charge-
separated state involving the reduced perylene and oxidized oxochlorin. h Similar values are obtained for other zinc oxochlorin monomers.37 i From
ref 5.
Figure 2. Absorption spectra (solid) and fluorescence spectra (dashed,
dotted) at room temperature. Emission for the dyad was obtained using
Figure 3. Ground-state absorption spectra in the region of the perylene
and oxochlorin origin bands in toluene (solid) and benzonitrile (dashed)
at room temperature.
primarily excitation of the porphyrin at 392 nm (dashed) or perylene
at 480 nm (dotted). The emission intensities in parts A and B have
been corrected for the absorbance at the excitation wavelength and on
the same scale so that they can be compared. The intensities in C have
been reduced by an order of magnitude for clarity of presentation.
from comparison of the excited-singlet-state lifetime with that
of the appropriate reference compound in the same solvent. The
excited-state lifetimes can be determined by both fluorescence
and time-resolved absorption techniques; the latter measurements
are described in the following section. Both methods were used
for each of the metallooxochlorin-containing dyads and for
monomer ZnO1 (in both toluene or benzonitrile), and the
average values for each solvent are given in Table 2. The
lifetimes for these compounds derived from multiple determina-
A substantial reduction in fluorescence yield in the dyad
compared to the monomer is found for PDI-MgO in both
solvents. As indicated by the analysis given below, this
quenching is indicative of substantial electron transfer from
MgO* to the perylene in this dyad (Figure 1A,B).
Fluorescence Lifetimes. A second assay of the extent of
oxochlorin charge-transfer quenching in each dyad is obtained

3448 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Kirmaier et al.
(see dashed spectrum in Figure 2C). The PDI* spectrum also
shows a broad excited-state absorption band centered at ∼690
nm.
The early-time dynamics of PDI* in PDI-MgO in both
toluene and benzonitrile shows evidence for both relaxation
(conformational, vibrational, or electronic) and subsequent decay
of the relaxed form of the excited state. This behavior is
analogous to that described in the preceding article for the
excited perylene PMI* in dyad PMI-ZnO (although there are
differences in detail).37 One indication of these two contributions
to the PDI* dynamics is a probe wavelength variation in a simple
single-exponential fit through the data at times <50 ps (τ ∼
2-7 ps), as is shown for PDI-MgO in benzonitrile (Figures
5A-C). It is possible that both the relaxed and unrelaxed forms
of PDI* in PDI-MgO participate in energy/hole transfer to the
oxochlorin, which itself may undergo excited-state relaxation
on the 1-10 ps time scale.16 Thus, in assessing the overall rates
of these processes, we will utilize an effective PDI* lifetime of
3.5 ps derived from the PDI bleaching decay (Table 2).
After PDI* has decayed, the perylene feature at ∼530 nm
has diminished (but not disappeared) and the oxochlorin (0,0)
bleaching plus stimulated emission feature at ∼620 nm has
developed to its maximum amplitude (Figure 4A,B, dashed).
The spectrum at 30 ps in either toluene or benzonitrile cannot
be ascribed solely to the excited oxochlorin (MgO*) formed
by perylene-to-oxochlorin energy transfer because the latter state
would show no perylene bleaching. Hence, the residual perylene
bleaching must reflect partial decay of PDI* by perylene-to-
oxochlorin hole transfer to give PDI- MgO+ (which will give
perylene bleaching). The amplitude of the perylene (0,0)
bleaching at 30 ps compared to the (approximately equal)
bleaching plus stimulated emission for PDI* at 0.5 ps indicates
that the yield of PDI* hole-transfer is ∼60% in toluene and
slightly larger in benzonitrile. The energy-transfer pathway
primarily governs the other fraction of PDI* decay (Figure 1A,B
and Table 2). The relative amplitudes of the perylene and
oxochlorin features in the transient difference spectra of PDI-
MgO (Figure 4) are consistent with these interpretations and
with the features in the static optical spectra (see Figure 3 and
Supporting Information) when allowance is made for modest
transient absorption in the 500-630 nm region (no larger than
that seen at >700 nm).
The above analysis shows that PDI* decays to give both
MgO* and PDI- MgO+ for PDI-MgO in both solvents. Thus,
one expects to observe the decay of both of these states, with
the former possibly giving rise to an additional amount of the
latter by electron transfer (depending on the relative energy
ordering of the states). The changes in the transient absorption
spectra between 30 ps and 3.4 ns give evidence for these
processes. Note that the decay of MgO* and PDI- MgO+ will
have the same sign of ∆∆A at most wavelengths while the
growth of the latter state will generally have the opposite sign.
Kinetic analysis shows that the MgO* and PDI- MgO+ lifetimes
are similar. The combination of these phenomena give rise to
complex kinetic profiles that require dual-exponential fitting at
times >50 ps at some wavelengths, and a probe wavelength
dependence of a single-exponential fit at other wavelengths. The
representative kinetic traces shown in Figure 5 illustrate these
points. Detailed analysis gives best dual-exponential fits across
the 500-730 nm region with time constants of 0.9 ns and g1.5
ns for PDI-MgO in toluene and 0.5 and 1.0 ns in benzonitrile.
Figure 4. Transient absorption spectra at selected pump-probe delay
times for dyad PDI-MgO in (A) toluene and (B) benzonitrile following
excitation with a 130 fs flash at 495 nm.
tions using fluorescence modulation spectroscopy are as fol-
lows: PDI-ZnO in toluene (0.6 ( 0.1 ns) and benzonitrile
(0.4 ( 0.1); PDI-MgO in toluene (0.9 ( 0.2 ns) and
benzonitrile (0.6 ( 0.2 ns); ZnO1 in toluene (0.8 ( 0.2 ns) and
benzonitrile (0.7 ( 0.2 ns). These fluorescence lifetimes are
generally somewhat longer than those obtained by transient
absorption, but agree within the error limits of the average values
given in Table 2. The excited-state lifetimes listed in Table 2
for PDI-FbO, FbO1, and MgO1 were obtained from fluores-
cence measurements alone. It should be noted that the fluores-
cence measurements on some dyads/solvents indicate the
presence of a second, minor component. This component
generally could be attributed to minor (free) perylene emission
(τ ) 3-5 ns). However, in some cases this second component
may represent the decay of a charge-separated state (involving
oxidized oxochlorin and reduced perylene) by charge recom-
bination to give in part the oxochlorin excited singlet state (see
Figure 1).
Time-ResolVed Absorption Spectra. The presentation of the
transient absorption data for the three perylene-oxochlorin dyads
will be facilitated by reference to Figure 1, which summarizes
the collective results on the excited-state photodynamics of these
systems.
PDI-MgO. Figure 4 shows time-resolved absorption differ-
ence spectra for PDI-MgO in toluene (part A) and benzonitrile
(part B) at selected pump-probe time delays. The results are
generally similar for this dyad in the two solvents, so the two
cases will be presented together. In each solvent, the initial (0.5
ps) spectrum was constructed from a series of spectra acquired
at very closely spaced time intervals in order to account for the
time-dispersion of wavelengths in the white-light probe pulse
(Figure 4A,B, solid). This initial spectrum can be associated
primarily with the excited perylene (PDI*) produced by direct
perylene excitation at 495 nm. The pronounced feature at ∼530
nm contains bleaching of the perylene ground-state (0,0)
absorption band (see solid spectrum in Figure 2C) and (0,0)
stimulated (by the white-light probe pulse) emission from PDI*
that occurs in the same region as the spontaneous fluorescence
The most consistent assignment based on the spectra and the
fluorescence lifetime and yields is that the shorter time constant
reflects the lifetime of MgO* and that the longer value is due

A Perylene-Oxochlorin Charge-Separation Motif
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3449
Figure 6. Transient absorption spectra at selected pump-probe delay
times for dyad PDI-FbO in (A) toluene and (B) benzonitrile following
excitation with a 130 fs flash at 483 nm.
Figure 5. Kinetic traces and multiexponential fits at selected wave-
lengths for PDI-MgO. Parts A-C show results from the same
measurements in Figure 4B and in part D from those in Figure 4A.
At this point the analogies between the behavior of PDI-
FbO and PDI-MgO end. In both toluene and benzonitrile, the
PDI bleaching present at 30 ps (due to state PDI- FbO+) decays
substantially by 3.4 ns, but the oxochlorin bleaching plus
stimulated emission at ∼640 nm changes only modestly. This
observation suggests that the charge-separated state PDI- FbO+
that is formed from PDI* decays largely by charge recombina-
tion to form the excited oxochlorin (FbO*) with only modest
return to the ground state. Based on the PDI bleaching decay
at ∼530 nm that occurs between ∼30 ps and 3.4 ns (and the
kinetics in other regions such as ∼700 nm where PDI- absorbs),
the time constant for charge recombination is ∼200 ps in
benzonitrile and >1.5 ns (likely 2-4 ns) in toluene (Figures
7A and 7B and Table 2). The spectral features remaining at 3.4
ns can be ascribed primarily to FbO*, which according to the
fluorescence measurements decays with a time constant only
slightly shorter than the lifetime of 9 ns for the isolated
oxochlorin (Table 2).
PDI-ZnO. Figure 8 shows selected transient absorption data
for dyad PDI-ZnO using primarily excitation of the perylene
at 495 nm. The elucidation of the excited-state processes for
this dyad proved to be the most difficult of all the arrays because
the data ultimately show a significantly higher yield of perylene-
to-oxochlorin energy transfer and an intermediate yield of
electron-transfer quenching of the excited oxochlorin. There-
fore, studies were also carried out using excitation flashes at
540 nm (excites both the perylene and oxochlorin components)
and 585 nm (excites primarily oxochlorin). The results and
conclusions summarized in the following are most consistent
with all the measurements. The PDI* decay again shows
evidence for two components (τ ∼ 2.5-7.0 ps) associated with
decay of the unrelaxed and relaxed forms (see Figure 7C,D),
with an average PDI* lifetime of ∼3.5 ps determined by single-
exponential decays of the PDI bleaching in both toluene and
benzonitrile (Table 2). The extent of PDI bleaching (∼530 nm)
at 30 ps compared to bleaching plus stimulated emission at 0.5
ps affords a yield of ∼25% for PDI* ZnO f PDI- ZnO+ hole
transfer in toluene, giving a yield of 75% for PDI* ZnO f
to decay of PDI- MgO+. The data also suggest that the latter
state forms in high yield from the former in both solvents,
complementing the amount of the charge-separated state pro-
duced directly from PDI* (Figure 1A,B). The fact that the PDI-
MgO+ requires g1.5 ns to decay in toluene is seen directly
from the transient difference spectrum at 3.4 ns in Figure 4A
(dotted). This spectrum shows a significant amount of both
perylene and oxochlorin bleaching and thus cannot be assigned
simply to ZnOT (formed from ZnO*), which would show no
perylene bleaching. In benzonitrile, more of the spectrum has
collapsed to the baseline by 3.4 ns (Figure 4B, dotted). This
result indicates that MgO* has been severely quenched by
electron transfer to the perylene and that charge recombination
of PDI- MgO+ is almost complete by 3.4 ns in the polar
medium. The rate constants and yields of the various processes
consistent with all of the time-resolved and static optical data
for PDI-MgO will be derived below (Table 2).
PDI-FbO. Transient absorption difference spectra for PDI-
FbO in toluene and in benzonitrile obtained using primarily
excitation of the perylene at 483 nm are shown in Figure 6. In
analogy to the spectra for PDI-MgO (Figure 4), the spectrum
in each solvent at 0.5 ps can be ascribed to PDI* (Figure 6,
solid). Relaxations within this excited state and decay of the
relaxed form give rise to two components in the early-time (<50
ps) dynamics (Figure 7A,B), with an average PDI* lifetime of
∼3 ps in each solvent determined from single-exponential fits
to the PDI bleaching (Table 2). Also in analogy to PDI-MgO,
the spectrum at 30 ps shows both perylene and oxochlorin
bleachings that arise from a combination of FbO* and PDI-
FbO+ formed from PDI* decay. The amplitude of the PDI
feature at ∼530 nm at 30 ps versus 0.5 ps indicates that the
yield of the charge-separated state via the PDI* hole-transfer
pathway is ∼50% in toluene and slightly higher in benzonitrile,
with the other fraction of the PDI* decay occurring by the
energy-transfer route (Table 2 and Figure 1E,F).

3450 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Kirmaier et al.
also decays on this time scale, so that the spectral changes
contain a convolution of the decay of the oxochlorin excited
state, decay of PDI- ZnO+ formed from PDI*, and possibly
the formation and subsequent decay of the PDI- ZnO+ formed
from ZnO* (see Figure 1C,D). Figure 7C,D show two examples
of the resulting kinetic traces that either clearly show two kinetic
components or an intermediate time constant from a single-
exponential fit (which is probe wavelength dependent) on the
30 ps to 3.4 ns time scale. Combined analysis of the transient
absorption and fluorescence measurements give a ZnO* lifetime
for PDI-ZnO of 0.6 ns in toluene and 0.4 ns in benzonitrile
(Table 2).
Energies of the Electronic States of the Dyads. In con-
structing the state diagrams shown in Figure 1, the energies of
the excited perylene and porphyrin components were taken
directly from the spectroscopic data for each dyad. In particular,
the average energies of the (0,0) absorption and fluorescence
bands of the two components of each array were used to obtain
the energies of the lowest excited singlet states (Figure 1). The
free energies of the charge-separated states were derived using
an experimentally determined5 reference value of 2.05 eV for
state PDI- ZnP+ in the perylene-porphyrin dyad PDI-ZnP in
toluene (the porphyrin analogue of PDI-ZnO) along with
appropriate differences in ground-state redox potentials (using
the assumptions and methods described previously6) as follows
(Figure 1). (1) State PDI- ZnO+ in PDI-ZnO is 80 meV higher
than PDI- ZnP+ in PDI-ZnP due to the ∼80 mV more positive
oxidation potential of zinc oxochlorins versus zinc porphyrins
(Table 1 and refs 6 and 16). (2) State PDI- FbO+ in PDI-FbO
and PDI- MgO+ in PDI-MgO are 150 meV higher and lower,
respectively, than PDI- ZnO+ in PDI-ZnO using the +150
mV and -150 mV typical differences in oxidation potentials
of Fb and Mg porphyrins, respectively, versus Zn porphyins.6
(3) The charge-separated species will be stabilized by 0.26 eV
in benzonitrile versus toluene; this value was estimated using
the simple Coulomb interaction e2/(ꢀsr), where e2 ) 14.45
eV‚Å), r is the oxochlorin-perylene center-to-center separation
(∼21 Å), and ꢀs is the static dielectric constant (2.38 for toluene
and 25.2 for benzonitrile).
Figure 7. Kinetic traces and multiexponential fits at selected wave-
lengths for PDI-FbO from the same measurements shown in (A) Figure
6A or (B) Figure 6B or for PDI-ZnO from the same measurements
shown in (C and D) Figure 8B.
Rates and Yields of the Energy- and Charge-Transfer
Processes. The transient absorption data described above have
provided the relative yields of the two principal decay pathways
of the excited perylene (PDI*) in each dyad, namely energy or
hole transfer to the oxochlorin (Figure 1 and Table 2). These
pathways cause the excited perylene in each dyad to have a
lifetime that is about 3 orders of magnitude shorter than the
lifetime of ∼5 ns for the isolated dye, with an average value
(unrelaxed and relaxed forms) of 3-4 ps (Table 2). The rate
constants and yields for decay of the excited perylene in the
PDI-based dyads can be obtained by applying standard methods,
taking into account the branching ratio for energy versus hole
transfer.5
Figure 8. Transient absorption spectra at selected pump-probe delay
times for dyad PDI-ZnO in (A) toluene and (B) benzonitrile following
excitation with a 120 fs flash at 495 nm.
For PDI-MgO in toluene, the transient absorption data
indicate that the fractional yields of the energy- and hole-transfer
pathways are 0.40 and 0.60, respectively. Using the PDI*
lifetimes for the dyad and MgO1 monomer in Table 2, the
overall rate constant for combined quenching of PDI* by energy
PDI ZnO* energy transfer; in benzonitrile, the hole transfer yield
is slightly larger and the energy transfer slightly less than in
toluene (Table 2). In toluene, there is little decay of the PDI
bleaching at ∼530 nm between 30 ps and 3.4 ns (Figure 8A,
dashed and dotted spectra), indicating that PDI- FbO+ has a
lifetime >1.5 ns (likely 3 ns). The PDI- ZnO+ bleaching decays
with a time constant of 600 ( 200 ps in benzonitrile, as deduced
from measurements across the spectrum (500-800 nm) using
495 or 540 nm excitation. One of the complexities is that ZnO*
plus hole transfer is kQ ) [(3.5 ps)-1 - (4500 ps)-1 -1
]
∼ (3.5
ps)-1 with an overall quenching yield of ΦQ ) 1-3.5/4500 ∼
99.9%. Thus the rate constant and quantum yield for PDI*
MgO f PDI MgO* energy transfer are kENT ) [(3.5 ps)-1][0.40]
) (8.7 ps)-1 and ΦENT ) (0.40)(0.999)(100) ) 40%, while the
values for PDI* MgO f PDI- MgO+ hole transfer are kHT
)
[(3.5 ps)-1][0.60] ) (5.8 ps)-1 and ΦHT ) (0.60)(0.999)(100)

A Perylene-Oxochlorin Charge-Separation Motif
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3451
) 60%. The rate constants and yields for the principal decay
pathways for the other PDI-based dyads in toluene or benzoni-
trile are similarly obtained (Figure 1 and Table 2). The finding
that each of the PDI-based dyads (PDI-FbO, PDI-ZnO, and
PDI-MgO) exhibits significant hole transfer is consistent with
the energy estimates given above in which the charge-separated
state is placed lower in free energy than PDI* (Figure 1). This
behavior contrasts that observed in the preceding article for
PMI-ZnO, which showed little or no evidence of hole transfer,
consistent with the prediction that PMI- ZnO+ is above PMI*
in free energy.37
(Table 2). This level of electron transfer is not sufficient for an
effective charge-separation unit, although the finding of electron
transfer is consistent with the energy estimates that place PDI-
ZnO+ slightly below ZnO* in benzonitrile. The excited-state
lifetimes and fluorescence yields for PDI-ZnO in toluene and
PDI-FbO in either solvent are very similar in the dyad and
respective reference monomers, indicating that if electron-
transfer quenching occurs, the yield is <0.2 in each case. These
findings are consistent with (1) the energy estimates that place
the charge-separated state in each case slightly above the excited
oxochlorin, with the largest energy gap being for PDI-FbO in
toluene (Figure 1), and (2) the transient absorption data, which
suggest that a significant fraction of the PDI- FbO+ decay
occurs by charge recombination to make FbO*, thereby
complementing the amount of the excited free base oxochlorin
formed by energy transfer from PDI*.
The transient absorption spectra have provided insights into
whether the excited oxochlorin in each dyad/solvent decays by
electron transfer to the perylene in addition to the normal routes
open to the isolated chromophore (mainly intersystem crossing
to the excited triplet state). However, assessment of the extent
of electron transfer also can be provided by comparison of the
lifetime and fluorescence yield of the excited oxochlorin in the
dyad compared to the values in the reference monomers using
standard equations.5
Comparison with Previous Studies. Photoinduced charge-
separation processes have been studied previously in arrays
containing chlorins/oxochlorins with other tetrapyrrole chro-
mophores,21,35,36 carotenoids,18 quinones,19-25 fullerenes,26-29
and pyromellitimides.35,36 In a carotenoid-pheophorbide-quinone
triad, electron transfer from the central tetrapyrrole chromophore
to the quinone followed by hole transfer to the carotenoid
produces a charge-separated state spanning the triad that lives
for ∼120 ns.30 In triads containing a chlorin/oxochlorin plus a
porphyrin and either a quinone21 or pyromellitimide36 electron
acceptor, sequential electron transfer and charge-shift reactions
on the time scale of ∼10-100 ps also have been used to
generate array-spanning charge-separated states that have
lifetimes extending to hundreds of nanoseconds.21,36 Charge-
recombination reactions are generally shorter (∼100 ps to
several nanoseconds) in corresponding dyads due to the shorter
distance between the oxidized and reduced components.21,23,26
Rapid (subpicoseconds to hundreds of picoseconds) excited-
state electron-transfer followed by slower (subnanoseconds to
milliseconds) charge- recombination reactions have been achieved
in dyads comprised of chlorins and fullerene.26a,e,h,27 In many
of these systems, like the perylene-oxochlorin dyads studied
here, the rates of the charge separation and charge recombination
reactions have been manipulated using factors such as solvent
polarity and the redox characteristics of the components,
including the metalation state of the tetrapyrrole chromophores.
The excited oxochlorin in PDI-MgO undergoes extensive
electron transfer to the perylene in both polar and nonpolar
media. The MgO* lifetime for the dyad in toluene (0.9 ns)
compared to the MgO1 reference monomer (3.1 ns) gives an
electron-transfer yield for PDI MgO* f PDI- MgO+ of
ΦET ) 1-0.9/3.1 ) 0.71 (Table 2). A value of ΦET ) 1-0.032/
0.13 ) 0.75 is obtained from the corresponding fluorescence
yields (Table 2), giving a conservative estimate of 0.70. For
PDI-MgO in benzonitrile, the corresponding calculations based
on the MgO* lifetimes and fluorescence yields afford ΦET
)
1-0.5/3.0 ) 0.83 and ΦET ) 1-0.011/0.13 ) 0.91 giving a
conservative estimate of 0.85 (Table 2). It should be noted that
the high yield of the charge-separated state from MgO*
supplements the yield of 60% in toluene and 65% in benzonitrile
from PDI* (with the remaining fraction of PDI* forming
MgO*). Thus, the ultimate yield of PDI- MgO+ beginning in
PDI* is 60% + (40% ‚ 0.7) ∼ 88% in toluene and 65% +
(35% ‚ 0.85) ∼ 95% in benzonitrile. The high yields of electron
transfer (PDI MgO* f PDI- MgO+) are generally consistent
with the estimates that place the latter state comparable in energy
to the former in toluene and well below it in benzonitrile (Figure
1). In addition to the high yields of electron transfer, the transient
absorption data indicate that PDI- MgO+ has a lifetime of
∼1 ns or longer in both solvents. Thus, PDI-MgO is well suited
for incorporation into more elaborate arrays as a molecular-
switching or charge-injection unit.
The PDI-containing dyads (PDI-MgO and PDI-ZnO)
studied here differ in several respects from the PMI-containing
dyad PMI-ZnO studied in the previous paper.37 The difference
in reduction potentials of the PDI and PMI units is the primary
factor that facilitates excited-state charge-transfer processes in
the former arrays but minimizes these processes in the latter
array so that energy-transfer dominates the photodynamics.
However, the two types of arrays differ in two other charac-
teristics that may affect perylene-oxochlorin electronic com-
munication (and thus the rates of both through-bond energy and
charge transfer if the processes are thermodynamically allowed).
First, as described in the preceding article,37 slightly reduced
electronic communication is expected in PMI-ZnO compared
to PDI-ZnO (and PDI-MgO and PDI-FbO) because the linker
utilizes an ethyne group connected to the meta- versus para-
positions of the phenyl ring joined to the perylene. Second, the
PMI-based array has the linker connected to the oxochlorin
5-position whereas the PDI-based arrays utilize the 10-position.
In principle, differences in electron density at the two oxochlorin
sites could affect electronic communication (and thus energy/
charge transfer rates) with the perylene. To explore this point,
a series of analogous arrays differing only in this connection
The excited perylene in PDI-ZnO and PDI-FbO also serves
effective roles as a parallel light-harvesting and hole-transfer
agent. Using the PDI* lifetimes and the yields of the two
pathways given in Table 2, rate constants comparable to those
found above for PDI-MgO are obtained. This is expected given
that each of the charge-separated states PDI- ZnO+ and PDI-
FbO+ lies below PDI*, consistent with the energy estimates
(Figure 1). One of the principal differences among the dyads is
the extent to which ZnO* in PDI-ZnO or FbO* in PDI-FbO
in toluene or benzonitrile is quenched by electron transfer to
the perylene to also produce the charge-separated state. On the
bases of fluorescence quantum yields and excited-state lifetimes
in Table 2, the only one of these four cases (two dyads and two
solvents) for which some electron transfer is clear is PDI-ZnO
in benzonitrile. Using the same analysis methodology described
above, the quantum yield of electron-transfer based on the
lifetime or fluorescence yield in the dyad versus monomer is
ΦET ) 1 - 0.4/0.7 ) 0.4 or ΦET ) 1 - 0.018/0.030 ) 0.4

3452 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Kirmaier et al.
motif would need to be prepared and investigated, which is
beyond the scope of this study. However, on symmetry grounds
one might expect that the orbital coefficients at the oxochlorin
5- and 10-positions are not vastly different. This point seems
to be borne out by the fact that the rates of perylene-to-
oxochlorin energy transfer are similar ((3-9 ps)-1) in the two
types of arrays. These considerations suggest that connection
to the 5- versus 10-positions of the oxochlorin makes a far less
substantial difference in the excited-state processes than accrues
from the use of the different perylene pigments (PMI, PDI).
high-resolution fast atom bombardment mass spectrometry
(FAB-MS) and by laser desorption mass spectrometry (LD-MS)
in the absence of a matrix.39 Absorption and emission spectra
were collected in toluene at room temperature unless noted
otherwise. Silica gel (Baker, 40 µm average particle size) was
used for column chromatography. Alumina activity grade I
(Fisher) was deactivated to grade V for chromatography of
magnesium-porphyrinic compounds.40 Preparative SEC was
performed using BioRad Bio-Beads SX-1 (200-400 mesh)
beads. Analytical SEC was performed using a 1000 Å column
(flow rate ) 0.800 mL/min; solvent ) THF; quantitation at
420 and 520 nm; reference at 475 and 590, respectively; oven
temperature 40 °C; or quantitation at 520 and 610 nm; reference
at 575 and 670, respectively; oven temperature 25 °C). Toluene
and triethylamine were freshly distilled from CaH2 and sparged
of oxygen prior to use. All other solvents were used as received.
Chloroform contained ethanol (0.8%) as a stabilizer.
Conclusions and Outlook
In the preceding article, we showed that incorporation of a
perylene-monoimide accessory pigment with an oxochlorin
(PMI-ZnO) gives an excellent light-harvesting array in which
perylene-to-oxochlorin excited-state energy transfer is essentially
quantitative and there is effectively no subsequent quenching
of the excited oxochlorin (ZnO*). Here we have shown that
perylene-bis(imide) dyes are also excellent light harvesters,
transferring energy to the oxochlorin with 35-75% yields in
the three dyads PDI-MgO, PDI-ZnO, and PDI-FbO in
toluene and benzonitrile. The only reason that perylene-to-
oxochlorin energy transfer is not quantitative is because these
PDI-containing arrays were designed to bring the charge-
separated state (PDI- O+, involving reduced perylene and
oxidized oxochlorin) down in energy, thereby facilitating charge-
transfer reactions. Indeed, the other fraction of the PDI* that
does not undergo energy transfer to make the excited oxochlorin
(O*) affords the desired charge-separated state PDI- O+. Thus,
in these dyads, the perylene-(bis)imide component serves a dual
function: light harvesting and charge separation.
The desired fate of the excited oxochlorin O* (formed either
from PDI* or direct excitation) is to subsequently undergo
electron transfer to the largest extent possible and thereby
generate (additional) PDI- O+. The yield of electron transfer
depends very strongly on the oxidation potential of the oxochlo-
rin, which affects the energy of the PDI- O+ state. In PDI-
ZnO, we found some potential quenching of ZnO* by the
perylene, but not at a level sufficient for a useful charge-
separation unit. In the free base analogue PDI-FbO, electron-
transfer quenching in the dyad is basically shut off, consistent
with the expected higher free energy of the charge-separated
state PDI- FbO+ above the excited oxochlorin FbO*. In PDI-
MgO, where the energy of PDI- MgO+ drops well below that
of MgO*, the yield of oxochlorin-to-perylene electron-transfer
rises to ∼70% in toluene and to ∼85% in benzonitrile. When
one adds the amount of PDI- MgO+ that is made from PDI*
when the accessory pigment is excited (in addition to energy
transfer to make MgO*), the overall yield of the charge-
separated state elevates to 88% in toluene and 95% in benzoni-
trile, thereby producing the desired photoproduct in high yield.
Furthermore, PDI- MgO+ has a lifetime g1 ns in both polar
and nonpolar media, making the state suitable for injecting
charge into another stage, or substrate, or acting as a molecular-
switching unit that can interrupt excited-state energy flow in
an adjoining molecular wire. Collectively, the studies described
in this paper and the preceding article have provided a set of
fully characterized perylene-oxochlorin dyads and have eluci-
dated design criteria that can be exploited for a variety of
applications in solar energy and molecular photonics.
10-[4-Ethynylphenyl]-17,18-dihydro-18,18-dimethyl-5-(4-
methylphenyl)-17-oxoporphinatozinc(II) (ZnO1). Following
a literature method,16 a solution of Zn113,14 (98.2 mg, 165 µmol)
and basic alumina (activity I, 7.5 g) in 6 mL toluene was stirred
for 16 h at 60 °C in an air atmosphere. TLC analysis of the
reaction mixture showed that all of the starting material was
consumed. The alumina was concentrated by filtration and was
washed with CH2Cl2/CH3OH (19:1) until the washings were
colorless, then the filtrate was removed under vacuum. The
residue was dissolved in 80 mL of toluene and 2 equiv of DDQ
(75.0 mg, 331 µmol) was added. The mixture was stirred for 5
min and then 0.1 mL of triethylamine was added. The solvent
was removed under vacuum. Chromatography of the residue
(silica, CH2Cl2) afforded a bluish-purple solid (46.9 mg, 47%):
1H NMR δ: 2.06 (s, 6H), 2.68 (s, 3H), 3.30 (s, 1H), 7.50-
7.54 (m, 2H), 7.84-7.86 (m, 2H), 7.95-7.98 (m, 2H), 8.05-
8.08 (m, 2H), 8.58 (d, J ) 4.4 Hz, 1H), 8.61 (d, J ) 4.4 Hz,
1H), 8.76 (d, J ) 4.4 Hz, 1H), 8.86 (d, J ) 4.4 Hz, 1H), 8.93
(d, J ) 4.4 Hz, 1H), 8.96-9.00 (m, 2H), 9.60 (s, 1H). LD-
MS observed: 604.80. FAB-MS observed: 606.1404. Calcd:
606.1398 (C37H26N4OZn). λabs (log ꢀ)/nm 424 (5.48), 563 (3.99),
and 609 (4.73).
10-[4-Ethynylphenyl]-17,18-dihydro-18,18-dimethyl-5-(4-
methylphenyl)-17-oxoporphyrin (FbO1). A solution of ZnO1
(9.1 mg, 15 µmol) in CH2Cl2 (1 mL) was treated with TFA (12
µL, 150 µmol). The reaction mixture was stirred at room
temperature and monitored by absorption spectroscopy. After
2 h, the reaction mixture was diluted with CH2Cl2 and washed
with saturated aqueous NaHCO3. The organic layer was dried
(Na2SO4) and concentrated. Chromatography on silica (CH2-
Cl2) afforded a purple solid (7.6 mg, 94%): 1H NMR δ: -2.44
to -2.41 (br, 1H), -2.29 to -2.26 (br, 1H), 2.11 (s, 6H), 2.70
(s, 3H), 3.32 (s, 1H), 7.55 (d, J ) 7.6 Hz, 2H), 7.86-7.90 (m,
2H), 8.00-8.04 (m, 2H), 8.10-8.14 (m, 2H), 8.60 (d, J ) 4.4
Hz, 1H), 8.63 (d, J ) 4.4 Hz, 1H), 8.88 (d, J ) 4.4 Hz, 1H),
8.96 (d, J ) 4.4 Hz, 1H), 9.08-9.10 (m, 1H), 9.15-9.18 (m,
1H), 9.24 (s, 1H), 9.86 (s, 1H). LD-MS observed: 543.14. FAB-
MS observed: 544.2233. Calcd: 544.2263 (C37H28N4O). λabs
416, 512, 548, 593, and 642 nm.
10-[4-Ethynylphenyl]-17,18-dihydro-18,18-dimethyl-5-(4-
methylphenyl)-17-oxoporphinatomagnesium(II) (MgO1). Fol-
lowing a standard procedure,38 a solution of FbO1 (7.1 mg, 13
µmol) in CH2Cl2 (1 mL) was treated with DIEA (46 µL, 260
µmol) and MgI2 (36 mg, 13 µmol). The reaction mixture was
stirred at room temperature and monitored by fluorescence
Experimental Section
General. 1H NMR (400 MHz) spectra were recorded in
CDCl3 unless noted otherwise. Mass spectra were obtained by

A Perylene-Oxochlorin Charge-Separation Motif
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3453
excitation spectroscopy. After 10 min, another batch of MgI2
(36 mg, 13 µmol) was added and the reaction mixture was stirred
for 30 min. Then the reaction mixture was diluted with CHCl3
and washed with saturated aqueous NaHCO3. The organic layer
was dried (Na2SO4) and concentrated. Chromatography [basic
alumina, grade V, CHCl3/methanol (98:2)] afforded a green solid
(6.8 mg, 92%): 1H NMR (THF-d8) δ: 2.03 (s, 6H), 2.67 (s,
3H), 3.77 (s, 1H), 7.52 (d, J ) 7.6 Hz, 2H), 7.82 (d, J ) 8.0
Hz, 2H), 7.97 (d, J ) 8.0 Hz, 2H), 8.09 (d, J ) 8.4 Hz, 2H),
8.40-8.45 (m, 2H), 8.62 (d, J ) 4.4 Hz, 1H), 8.66 (d, J ) 4.4
Hz, 1H), 8.83 (d, J ) 4.4 Hz, 1H), 8.91 (d, J ) 4.4 Hz, 1H),
8.95 (s, 1H), 9.46 (s, 1H). LD-MS observed: 566.26. FAB-
MS observed: 566.1965. Calcd 566.1957 (C37H26MgN4O). λabs
426, 567, and 616 nm.
N-(4-[2-[4-[17,18-Dihydro-18,18-dimethyl-5-(4-methylphen-
yl)-17-oxoporphinatomagnesium(II)-10-yl]phenyl]ethynyl]-
phenyl)-N′-(2,5-di-tert-butylphenyl)-3,4,9,10-perylene-bis-
(dicarboximide) (PDI-MgO). Following a standard procedure,38
a solution of PDI-FbO (4.0 mg, 3.3 µmol) in CH2Cl2 (500 µL)
was treated with DIEA (23 µL, 130 µmol) and MgI2 (18 mg,
6.6 µmol). The reaction mixture was stirred at room temperature
and monitored by fluorescence excitation spectroscopy. After
3 h, the fluorescence excitation spectrum of a sample from the
reaction mixture showed the presence of PDI-FbO. Therefore,
the reaction mixture was stirred overnight. Then, the reaction
mixture was diluted with CHCl3 and washed with saturated
aqueous NaHCO3. The organic layer was dried (Na2SO4) and
concentrated. Chromatography (basic alumina, grade V, CHCl3)
afforded a purple solid (1.8 mg, 45%): 1H NMR (THF-d8) δ:
1.25-1.32 (m, 18H), 2.03 (s, 6H), 2.70 (s, 3H), 7.28-7.30 (m,
1H), 7.40-7.45 (m, 2H), 7.50-7.65 (m, 4H), 7.82 (d, J ) 7.6
Hz, 2H), 7.93 (d, J ) 7.6 Hz, 2H), 7.98 (d, J ) 7.6 Hz, 2H),
8.16 (d, J ) 7.6 Hz, 2H), 8.48 (d, J ) 4.4 Hz, 1H), 8.51 (d,
J ) 4.4 Hz, 1H), 8.60-8.70 (m, 6H), 8.75-8.85 (m, 5H), 8.92
(d, J ) 4.4 Hz, 1H), 8.95 (s, 1H), 9.47 (s, 1H). LD-MS
observed: 1219.92. FAB-MS observed: 1218.42. Calcd: 1218.43
(C81H58MgN6O5). λabs 427, 460, 492, 528, 589, and 617 nm.
Characterization. The electrochemical and spectroscopic
studies were conducted using instrumentation and techniques
previously described.5 Transient absorption measurements used
5-10 mM samples at room-temperature excited with ∼130 fs,
20-30 mJ, and 480-600 nm pulses.
N-(4-[2-[4-[17,18-Dihydro-18,18-dimethyl-5-(4-methylphen-
yl)-17-oxoporphinatozinc(II)-10-yl]phenyl]ethynyl]phenyl)-
N′-(2,5-di-tert-butylphenyl)-3,4,9,10-perylene-bis(dicarbox-
imide) (PDI-ZnO). Following a standard procedure,2,3 samples
of ZnO1 (30.4 mg, 50.0 µmol), PDI-26 (39.0 mg, 50.0 µmol),
Pd2(dba)3 (7.3 mg, 8.0 µmol), and P(o-tol)3 (19.5 mg, 64.0 µmol)
were placed in a Schlenk flask and pump-filled with argon three
times. A solution of toluene/triethylamine [21 mL (5:1)] was
then added. The mixture was heated to 35 °C. After 4 h, another
identical batch of catalyst was added. The reaction was
continued for another 7 h. The reaction mixture was cooled to
room temperature and filtered through a pad of Celite. The Celite
was washed with CHCl3 until the washings were colorless. The
filtrate was concentrated under reduced pressure to afford a dark
purple solid. Purification was achieved by chromatography
[silica, CH2Cl2 f CH2Cl2/methanol (98:2)], preparative SEC
(THF), and chromatography [short silica column, CH2Cl2/
methanol (98:2)]. The resulting solid was washed with methanol,
affording a brown-purple solid (32 mg, 51%): 1H NMR δ 1.31
(s, 9H), 1.35 (s, 9H), 2.06 (s, 6H), 2.69 (s, 3H), 7.07-7.09 (m,
1H), 7.41 (d, J ) 8.0 Hz, 2H), 7.45-7.51 (m, 1H), 7.54 (d,
J ) 8.0 Hz, 2H), 7.60-7.63 (m, 1H), 7.88 (d, J ) 8.0 Hz, 2H),
7.94 (d, J ) 8.0 Hz, 2H), 7.98 (d, J ) 8.0 Hz, 2H), 8.11 (d,
J ) 8.0 Hz, 2H), 8.55-8.66 (m, 6H), 8.66-8.70 (m, 2H), 8.73-
8.76 (m, 2H), 8.81 (d, J ) 4.4 Hz, 1H), 8.85 (d, J ) 4.0 Hz,
1H), 8.92 (d, J ) 4.4 Hz, 1H), 8.96 (s, 1H), 8.98 (d, J ) 4.4
Hz, 1H), 9.59 (s, 1H). LD-MS observed: 1255.87. FAB-MS
observed: 1258.37. Calcd: 1258.38 (C81H58N6O5Zn). λabs 424,
459, 491, 528, 561, and 609 nm.
Acknowledgment. This research was supported by the NSF
(CHE-9988142). Mass spectra were obtained at the Mass
Spectrometry Laboratory for Biotechnology at North Carolina
State University. Partial funding for the Mass Spectrometry
Laboratory for Biotechnology at North Carolina State University
was obtained from the North Carolina Biotechnology Center
and the National Science Foundation.
Supporting Information Available: Complete spectral data
1
(absorption, H NMR, and LD-MS) for all new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References and Notes
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N-(4-[2-[4-[17,18-Dihydro-18,18-dimethyl-5-(4-methylphen-
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-2.39 (br, 1H), -2.28 to -2.24 (br, 1H), 1.28-1.38 (m, 18H),
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nm.
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