Synthesis and excited-state photodynamics of A perylene-monoimide-oxochlorin dyad. A light-harvesting array

Article abstract of DOI:10.1021/jp026941a

A perylene-oxochlorin dyad has been prepared and characterized for potential use as a new light-harvesting motif. The dyad (PMI-ZnO) consists of a perylene-monoimide dye (PMI) joined at the 5-position of a zinc oxochlorin (ZnO) via a diphenylethyne linker. The dyad and its subunits have been studied in both polar and nonpolar media using static and time-resolved optical spectroscopy and electrochemical techniques. Energy flows very rapidly from both unrelaxed (vibrationally, conformationally, or electronically) and relaxed forms of the excited perylene (PMI*) to the ground-state oxochlorin with an effective time constant of a??4 ps and an efficiency of 99% in both toluene and benzonitrile. Subsequently, there is little or no quenching of the excited oxochlorin (ZnO*) in either solvent. These findings are consistent with the expectation that all charge-separated states such as PMI^- ZnO⁺ lie energetically above both PMI* and ZnO* in both polar and nonpolar media. Furthermore, light absorption by the perylene and oxochlorin occurs in complementary regions, giving broad spectral coverage. Thus, the new perylene-oxochlorin dyad is an excellent light-harvesting unit that can be incorporated into more elaborate architectures for use in solar-energy and molecular-photonics applications.

Full text of DOI:10.1021/jp026941a

J. Phys. Chem. B 2003, 107, 3431-3442
3431
Synthesis and Excited-State Photodynamics of A Perylene-Monoimide-Oxochlorin Dyad. A
Light-Harvesting Array
Kannan Muthukumaran,† Robert S. Loewe,† Christine Kirmaier,‡ Eve Hindin,‡
Jennifer K. Schwartz,‡ Igor V. Sazanovich,‡ James R. Diers,§ David F. Bocian,*,§
Dewey Holten,*,‡ and Jonathan S. Lindsey*,†
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4899, and Department of
Chemistry, UniVersity of California, RiVerside, California 92521-0403
ReceiVed: September 8, 2002; In Final Form: January 2, 2003
A perylene-oxochlorin dyad has been prepared and characterized for potential use as a new light-harvesting
motif. The dyad (PMI-ZnO) consists of a perylene-monoimide dye (PMI) joined at the 5-position of a zinc
oxochlorin (ZnO) via a diphenylethyne linker. The dyad and its subunits have been studied in both polar and
nonpolar media using static and time-resolved optical spectroscopy and electrochemical techniques. Energy
flows very rapidly from both unrelaxed (vibrationally, conformationally, or electronically) and relaxed forms
of the excited perylene (PMI*) to the ground-state oxochlorin with an effective time constant of ∼4 ps and
an efficiency of 99% in both toluene and benzonitrile. Subsequently, there is little or no quenching of the
excited oxochlorin (ZnO*) in either solvent. These findings are consistent with the expectation that all charge-
separated states such as PMI- ZnO+ lie energetically above both PMI* and ZnO* in both polar and nonpolar
media. Furthermore, light absorption by the perylene and oxochlorin occurs in complementary regions, giving
broad spectral coverage. Thus, the new perylene-oxochlorin dyad is an excellent light-harvesting unit that
can be incorporated into more elaborate architectures for use in solar-energy and molecular-photonics
applications.
Introduction
CHART 1
As part of a program in molecular photonics, we have been
working to prepare synthetic architectures that perform func-
tions such as light harvesting and optoelectronic gating.1 Light
harvesting requires efficient energy transfer while optoelectronic
gating requires efficient hole/electron transfer. Although por-
phyrins have provided the foundation for almost all of our work
to date, we have recently begun employing perylene dyes2,3 in
conjunction with porphyrins to achieve the desired functions.4-12
The two types of perylene dyes of interest include perylene-
bis(imide) dyes and perylene-monoimide dyes, exemplified by
PDI-1 and PMI-1, respectively (Chart 1). A principal difference
between these two classes is that when joined to porphyrins
the perylene-bis(imide) dyes generally undergo excited-state
charge-transfer reactions more readily than their perylene-
monoimide counterparts owing to a significant difference in
reduction potentials. We have exploited this difference in
perylene reduction potential, differences in oxidation potential
of zinc, magnesium, and free-base porphyrins (ZnP, MgP, and
FbP), and the different structures of the linker motif (involving
aryl and ethyne groups) to tune the absolute and relative yields
of energy- and hole/electron-transfer processes in perylene-
porphyrin dyads.
PMI-ep-MgP; Chart 2)7,13 can be employed as very efficient
light-harvesting motifs: the perylene moiety absorbs strongly
in the trough between the Soret band and the Q-band of the
porphyrin, thereby complementing the weak absorption of the
porphyrin in this spectral region. Following absorption of light
in nonpolar media, perylene-to-porphyrin excited-state energy
transfer occurs rapidly and almost quantitatively; there is no
subsequent quenching of the excited porphyrin, making the
harvested energy available for further utilization. Through proper
choice of the porphyrin and linker (e.g., PMI-ZnP), this light-
harvesting behavior is essentially retained even in polar media
as required by some applications.
For example, dyads comprised of a perylene-monoimide dye
and a porphyrin (PMI-ZnP, PMI-ep-ZnP, PMI-ep-FbP, and
* To whom correspondence should be addressed. (Jonathan S. Lindsey)
E-mail: jlindsey@ncsu.edu. (Dewey Holten) E-mail: holten@
wuchem.wustl.edu. (David F. Bocian) E-mail: David.Bocian@ucr.edu.
† Department of Chemistry, North Carolina State University.
‡ Department of Chemistry, Washington University.
On the other hand, dyads comprised of a perylene-bis(imide)
and a porphyrin undergo facile hole/electron-transfer reaction
§ Department of Chemistry, University of California.
10.1021/jp026941a CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/20/2003

3432 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Muthukumaran et al.
CHART 2
initiated in either the excited perylene or the excited porphyrin
(PDI-ZnP, PDI-FbP, and PDI-MgP; Chart 2).5,6 The rates
and yields of the processes can be tuned by choice of oxidation
potential of the porphyrins. The excited-state electron-transfer
properties of these dyads suggest application as efficient charge-
separation units for molecular-switching or hole/electron-
injection applications.
In a separate line of inquiry, we have developed rational
synthetic routes to chlorins14-16 and oxochlorins.17 The synthetic
routes provide chlorins or oxochlorins bearing substituents at
designated sites at the perimeter of the hydroporphyrin macro-
cycle. Chlorins and oxochlorins differ from porphyrins in having
more strongly allowed absorption in the red in addition to the
strong absorption in the blue (Soret band). The important
photochemical properties of chlorins and oxochlorins have
motivated the development of a number of new syntheses of
these green pigments in the past few years.18
Chlorin/oxochlorin dyads are of particular interest as bench-
marks for assessing the extent of electronic communication (i.e.,
energy transfer and/or electron transfer) involving the corre-
sponding chlorin/oxochlorin monomers. A number of arrays
containing chlorin/oxochlorins with various linkages and sites
of attachment have been prepared, including the following: (1)
chlorins/oxochlorins with electron acceptors such as quinones19-25
or C60,26-29 (2) chlorins with carotenoid accessory pig-
ments,30-32 (3) chlorin/oxochlorin dyads for energy-transfer
studies,33-35 and (4) chlorins/oxochlorins plus porphyrins and
pyromellitimides exhibiting energy and charge transfer.36,37 We
have focused on the oxochlorins owing to their greater stability
compared with that of chlorins.
In this paper, we combine these two areas of research to
prepare perylene-oxochlorin dyads as shown in Chart 3. One
type of dyad incorporates a perylene-monoimide and a zinc
oxochlorin (PMI-ZnO) for use in light-harvesting, and is the

A Perylene-Oxochlorin Light-Harvesting Array
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3433
CHART 3
SCHEME 1
subject of the present paper. The other type incorporates a
perylene-bis(imide) with a variation in the oxochlorin (PDI-
ZnO, PDI-MgO, and PDI-FbO) for applications requiring
charge-transfer processes; this set of arrays is examined in the
accompanying paper.38 In both cases, the perylene dye provides
strong absorption in the green region, complementing the two
major absorption bands of the oxochlorin. The attachment of
the PMI unit at the meta-position of the oxochlorin meso-phenyl
ring provides an architecture of the type that we have used
previously with porphyrins in large light-harvesting arrays to
suppress cofacial aggregation of the porphyrin macrocycles and
thereby achieve enhanced solubility.10-12 The dyads have been
characterized by electrochemistry, static absorption and fluo-
rescence 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.
Results and Discussion
Synthesis. The synthesis of oxochlorin building blocks relies
on the acid-catalyzed condensation of a tetrahydrodipyrrin
(Western half) and a 1-bromodipyrromethane-monocarbinol
(Eastern half). The resulting tetrahydrobilene-a intermediate is
subjected to metal-mediated oxidative cyclization forming the
chlorin.14 A regiospecific oxidation then is used to convert the
chlorin to the oxochlorin.
The synthesis of the Eastern half begins with a dipyr-
romethane, which is monoacylated upon reaction with an S-2-
pyridyl thioate derivative.39 The reaction of 3-bromobenzoyl
chloride and 2-mercaptopyridine afforded the acylating agent
1 in 51% yield. Treatment of 5-mesityldipyrromethane (2)40 with
ethylmagnesium bromide followed by 1 at -78 °C afforded
the desired monoacylated dipyrromethane 3 (Scheme 1). Bro-
mination of monoacyldipyrromethane 3 with N-bromosuccin-
imide afforded the Eastern-half precursor 4. This compound was
treated with NaBH4 to afford the monocarbinol (Eastern half).
The monocarbinol was reacted with tetrahydrodipyrrin 5
(Western half)14 under the standard conditions for chlorin
formation. The first step entails acid-catalyzed condensation at
0.1 M to afford the tetrahydrobilene-a intermediate. The reaction
mixture is then diluted 10-fold and treated with the reagents

3434 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Muthukumaran et al.
SCHEME 2
CHART 4
similarly substituted porphyrins.41 The conditions employ dilute
solutions of the reactants in the presence of tris[dibenzyli-
deneacetone]dipalladium(0) [Pd2(dba)3] and tri-o-tolylphos-
phine [P(o-tol)3] in a solution of toluene/triethylamine (TEA)
at 60 °C. Thus, the Pd-mediated coupling reaction of bromo-
substituted ZnO6 and N-(2,6-diisopropyl-4-ethynylphenyl)-9-
(4-tert-butylphenyloxy)-3,4-perylenedicarboximide (PMI-2)10
afforded the corresponding dyad PMI-ZnO in 54% yield
(Scheme 2). This dyad was purified by column chromatography
(silica), preparative size exclusion chromatography (SEC), and
column chromatography (silica). The dyad PMI-ZnO was
characterized by mass spectrometry, 1H NMR spectroscopy, and
absorption spectroscopy. As reference compounds, we prepared
three zinc oxochlorin monomers, ZnO7,38 ZnO8,17 and ZnO917
(Chart 4).
for metal-mediated oxidative cyclization, employing zinc acetate,
silver triflate (AgOTf), and 2,2,6,6-tetramethylpiperidine (TMPi)
in refluxing acetonitrile exposed to air. In this manner, the
desired zinc chlorin Zn6 was obtained in 37% yield.
Electrochemical Characterization. The electrochemical data
for the dyad PMI-ZnO and benchmark perylene dye PMI-1
are summarized in Table 1. The E1/2 values were obtained with
square wave voltammetry (frequency 10 Hz). The isolated
perylene dye exhibits one oxidation and two reduction waves
in the +1.4 to -2.0 V range. The E1/2 values for the perylene
unit in the dyad are generally similar to those of the isolated
dye. The oxochlorin unit in the dyad 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 isolated macrocycle (ref 17
and J. R. Diers and D. F. Bocian, unpublished results). The
similarity of the redox potentials of the constituents of the dyad
with those of the isolated monomers is indicative of the
We recently developed a simple procedure for oxidation of
the methylene group in the reduced ring of the chlorin, forming
the corresponding oxochlorin.17 Thus, treatment of Zn6 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 ZnO6 in 49% overall
yield (Scheme 2).
The perylene-Zn-oxochlorin dyad was synthesized by the Pd-
catalyzed coupling of an ethynyl-substituted perylene and a
bromo-substituted zinc oxochlorin. The coupling reaction was
carried out under similar conditions developed for joining

A Perylene-Oxochlorin Light-Harvesting Array
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3435
a
TABLE 1: Half-Wave Potentials (E1/2
)
oxidation potentials
oxochlorin
reduction potentials
perylene
oxochlorin
compound
perylene
E
1/2 (1)
E1/2 (2)
+0.77
E1/2 (1)
E1/2 (2)
E1/2 (1)
-1.57
E1/2 (2)
-1.92
PMI-ZnO
PMI-1
ZnO9
+1.05
+0.54
-1.18
-1.21
-1.75
-1.78
+1.07b
+0.59c
+0.89
-1.50
n.m.d
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 This value is for an analogue that lacks the aryloxy group at the perylene
9-position.13 c The same value was reported in ref 17. d Not measured.
+
Figure 2. Absorption spectra (solid) and fluorescence spectra (dashed,
dotted) in toluene at room temperature. Emission for the dyad was
obtained using 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
Figure 1. Summary diagram showing the excited-state processes in
wavelength and on the same scale so that they can be compared. The
dyad PMI-ZnO at room temperature. The values in parentheses are
intensities in C have been reduced by an order of magnitude for clarity
the estimated free energies of the states in eV (see text).
of presentation.
relatively weak ground-state electronic interactions between the
dyad components.
contour (S0 f S1) between 450 and 550 nm. The weak
oxochlorin Qy(1,0) band and Qx features (S0 f S2) observed in
the spectrum of the isolated chromophore (Figure 2B) underlie
the broad perylene absorption contour in the dyad. The latter
consists of several unresolved features with an apparent origin
at ∼540 nm in both the dyad and reference monomer (Figure
2A,C), as is typical for phenoxy-substituted perylene-monoimide
dyes.7 Similar spectra are observed in benzonitrile, except for
a bathochromic shift of ∼6 nm in the oxochlorin bands and
∼10 nm in the perylene contour (see Supporting Information).
For the dyad in either solvent, the blue-green absorption of the
perylene accessory pigment complements the blue and red
absorption of the oxochlorin, thereby giving broad spectral
coverage.
Fluorescence Spectra and Quantum Yields. Fluorescence
emission spectra for PMI-ZnO and its reference monomers in
toluene at room temperature are shown in Figure 2 (dashed and
dotted spectra). The fluorescence spectra in benzonitrile are
given in the Supporting Information. For the dyad in either
solvent, an emission spectrum was recorded using primarily
oxochlorin excitation at 392 nm and primarily perylene excita-
tion at 480 nm. The 392 and 480 nm excitation wavelengths
Photophysical Properties. Complete photophysical charac-
terization of dyad PMI-ZnO and reference monomers PMI-1
and ZnO7, ZnO8, ZnO9 was carried out in both toluene and
benzonitrile at room temperature. Studies were performed in
these two solvents because potential applications may utilize
the arrays in either polar or nonpolar media. The photophysical
properties of both the dyad and monomers differ only modestly
in the two solvents. To aid in the presentation and discussion
of the results, Figure 1 summarizes the understanding obtained
in this study of the light-driven processes of the perylene-
oxochlorin dyad PMI-ZnO.
Absorption Spectra. The ground-electronic-state absorption
spectrum of PMI-ZnO is essentially given by the sum of the
spectra of the subunits (Figure 2, solid). This finding reflects
the relatively weak ground- and excited-state electronic interac-
tions between the perylene and oxochlorin, complementing the
conclusion drawn from the electrochemical measurements. The
absorption spectrum of PMI-ZnO is dominated by the oxochlo-
rin near-UV Soret band (S0 f S3) at ∼424 nm, the oxochlorin
Qy(0,0) band (S0 f S1) at ∼610 nm, and the perylene absorption

3436 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Muthukumaran et al.
TABLE 2: Photophysical Data for Perylene-Oxochlorin
Dyads and Reference Compoundsa
After subtraction of the perylene emission as outlined above,
the intensity of oxochlorin emission in the dyad is comparable
to that obtained using primarily oxochlorin excitation (compare
dotted and dashed spectra in Figure 2A, using the PMI monomer
emission in Figure 2C as a reference). This observation reflects
a high yield of perylene-to-oxochlorin excited-state energy
transfer. (2) The perylene emission obtained using perylene
excitation is about ∼100-fold less than in the isolated dye. This
result also reflects substantial energy transfer from PMI* to the
oxochlorin. [Although hole transfer would also quench the
perylene emission, this process is insignificant in this case
(Figure 1A), as will be described below.] (3) The perylene
emission that is observed derives largely from the high intrinsic
fluorescence yield (Φf ∼ 1) of this chromophore. Thus, residual
perylene emission in the dyad is expected to be of the same
order as that from the oxochlorin (Φf ∼0.04) even if the perylene
emission is quenched in the dyad due to energy (or hole) transfer
to the oxochlorin with an efficiency of >95%. A similar
situation applies to perylene-porphyrin dyads that we have
studied previously.5-7 (4) Due to high/low intrinsic perylene/
oxochlorin fluorescence, it follows that even trace perylene
impurity (e.g., from minor decomposition) can compromise the
use of the static emission data to quantitate the yield of perylene-
to-oxochlorin energy transfer. Thus, the energy-transfer yields
will be derived from the time-resolved absorption measurements
described below.
PMI* decay
ZnO*
decay
compound
solvent
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
τ (ps)b
φfc
τ (ns)d
PMI-ZnO
3.6
4.5
0.041
0.047
0.037
0.030
0.040
0.031
0.044
0.039
0.8 ( 0.2
0.6 ( 0.3
0.7 ( 0.2
0.7 ( 0.2
0.7 ( 0.2
0.6 ( 0.2
0.9 ( 0.2
0.9 ( 0.2
ZnO7
ZnO8
ZnO9
PMI-1
benzonitrile
toluene
4800e
4500f
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 PMI bleaching region) and those in the monomers by fluorescence
decay ((200 ps). c Oxochlorin fluorescence yield ((15%) determined
using Soret (392 nm) excitation. d Oxochlorin excited singlet state
lifetime determined by averaging the values from transient absorption
kinetic profiles and fluorescence decay. e From ref 11; the fluorescence
yield is 0.82. f The fluorescence yield is 0.71.
were also employed for the oxochlorin and perylene monomers,
respectively, and for the free base meso-tetraphenylporphyrin
(FbTPP) reference compound (ΦF ) 0.11)42 used for the flu-
orescence yield determinations. These excitation wavelengths
were chosen to provide a good match in absorption (typically
0.04-0.08) within and among the compounds, selectivity in
excitation of each component, and appropriate level of fluo-
rescence so that comparisons of relative intensities would be
as reliable as possible.
Fluorescence Lifetimes. A second assay of possible oxochlo-
rin charge-transfer quenching in dyad PMI-ZnO is obtained
from comparison of the excited-singlet-state lifetime with that
for a zinc oxochlorin reference compound in the same solvent.
A ZnO* lifetime of 0.8 ( 0.2 ns was measured for PMI-ZnO
in toluene and 0.7 ( 0.2 ns in benzonitrile using fluorescence
detection. The excited-state lifetimes (( 0.2 ns) for reference
monomers ZnO7, ZnO8, and ZnO9 in toluene are 0.8, 0.6, and
0.9 ns, respectively, while the values in benzonitrile are 0.9,
0.7, and 0.9 ns. The lifetimes measured by transient absorption
were comparable to or somewhat shorter than those determined
by fluorescence. Average values from the two methods are given
in Table 2. Some of the fluorescence measurements on the dyad
indicated the presence of a second component that may represent
a trace amount of (free) perylene emission (τ ) 3-5 ns).
Time-ResolVed Absorption Spectra. Figure 3 shows time-
resolved absorption difference spectra for PMI-ZnO in toluene
(part A) and benzonitrile (part B) at selected pump-probe time
delays. The results are generally similar in the two solvents, so
the two cases will be presented together. In each solvent, the
initial spectrum (0.5 ps; solid) was constructed from a series of
spectra acquired at very closely spaced time intervals to account
for the time dispersion of wavelengths in the white-light probe
pulse. This initial spectrum can be associated primarily with
the excited perylene (PMI*) produced by direct perylene
excitation (483 or 495 nm). The broad trough from 500 to 570
nm reflects bleaching of the perylene ground-state absorption
contour (see solid spectrum in Figure 2C) and stimulated (by
the white-light probe pulse) emission from PMI* that occurs in
the same region as the spontaneous fluorescence (see dashed
spectrum in Figure 2C). The expected vibronic-overtone stimu-
lated-emission features underlie the opposing, pronounced PMI*
transient absorption that has a maximum between 600 and 620
nm and extends to ∼700 nm. The small dip at ∼610 nm on the
top of the PMI* transient absorption of PMI-ZnO in either
toluene or benzonitrile (Figure 3A) can be attributed to a
combination of (1) oxochlorin ground-state Qy(0,0) bleaching
due to perylene-oxochlorin electronic coupling in the dyad that
The fluorescence spectrum for PMI-ZnO in toluene using
oxochlorin excitation is identical in shape to that for the isolated
chromophore (Figure 2, dashed spectra). The spectrum contains
a (0,0) band at 612 nm and a (0,1) band at 670 nm. In the dyad,
there is only trace perylene emission (due to minor excitation
of this moiety), which can be seen from the trace PMI (0,0)
emission near 570 nm (compare dashed spectra in Figure 2).
To obtain the oxochlorin fluorescence yield in the dyad, the
perylene monomer emission spectrum was normalized to the
dyad spectrum in the PMI (0,0) band at ∼570 nm, and then
subtracted to obtain the perylene-free dyad emission spectrum
(only a few percent correction in this case). Integration of the
resulting spectrum (590-750 nm) and comparison with the
FbTPP fluorescence standard (Φf ) 0.11)42 gives an oxochlorin
fluorescence yield of Φf ) 0.041 for PMI-ZnO in toluene. This
value is the same within experimental error as Φf ) 0.037 to
0.044 measured for the monomers ZnO7-ZnO9 (Table 2). This
result can be seen from visual inspection of the emission spectra
for the dyad and monomer in Figure 2 (dashed spectra), which
have been corrected for small differences in the absorbance at
the excitation wavelength. Similarly, the fluorescence yield for
the dyad in benzonitrile is comparable to or slightly larger than
those for the reference compounds (Table 2). The finding of
comparable oxochlorin emission for the PMI-ZnO dyad and
the reference monomers indicates that the excited oxochlorin
(ZnO*) is not quenched appreciably if at all by charge (electron)
transfer to the perylene component of the dyad in either toluene
or benzonitrile (see Figure 1).
Excitation of PMI-ZnO in toluene using perylene excitation
at 480 nm gives the dotted spectrum in Figure 2A (corrected
for absorbance at the excitation wavelength). The perylene
emission is larger in this case (compared to using 392 nm
excitation), as indicated by the increased PMI (0,0) emission at
∼570 nm. There are several key aspects of these results. (1)

A Perylene-Oxochlorin Light-Harvesting Array
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3437
Figure 3. Transient absorption spectra at selected pump-probe delay
times for dyad PMI-ZnO in (A) toluene and (B) benzonitrile following
excitation with a 130 fs flash at (A) 495 nm or (B) 483 nm.
Figure 4. Kinetic traces and multiexponential fits at selected wave-
lengths for PMI-ZnO from the same measurements depicted in
Figure 3B.
leads to effects on the oxochlorin when PMI* forms, (2)
oxochlorin bleaching or excited-state stimulated-emission caused
by direct minor excitation of the oxochlorin, or (3) bleaching
and stimulated emission due to the initial stage of rapid energy
transfer from PMI* to the oxochlorin.
these lines, the apparent lack of PMI bleaching (500-550 nm)
following PMI* decay in toluene (Figure 3A, dashed spectrum)
indicates that PMI* ZnO f PMI- ZnO+ hole transfer has not
occurred to a measurable degree, as the charge-separated state
would give PMI bleaching. This finding indicates that the
charge-separated state must lie above PMI* in free energy for
PMI-ZnO in toluene, as is indicated in Figure 1A. [The redox
potentials indicate that PMI+ ZnO- must lie substantially higher
in energy than PMI- ZnO+.] Similarly, there is possibly only a
very small PMI bleaching at 30 ps in benzonitrile (Figure 3B,
dashed), indicating that PMI* ZnO f PMI- ZnO+ hole transfer
has a yield of at most ∼20% in this case. Thus, even with
solvent stabilization in the polar medium, state PMI- ZnO+ lies
above or close to PMI* in free energy, as is shown in Figure
1B. Similarly, as ZnO* subsequently decays (and ZnOT forms),
there is no indication that PMI bleaching develops in either
solvent (Figure 3A and 3B, dotted). The latter result indicates
that PMI ZnO* f PMI- ZnO+ electron transfer is insignificant,
consistent with the charge-separated state lying well above ZnO*
in both solvents (Figure 1). These conclusions are supported
by additional analysis given below.
Although the above description of the key features in the
transient absorption spectra presents a straightforward analysis
of the excited-state behavior of PMI-ZnO in polar and nonpolar
media, there is actually quite significant underlying complexity.
This behavior is illustrated by the series of kinetic traces for
PMI-ZnO in benzonitrile shown in Figure 4. Part A shows
decay of PMI bleaching plus stimulated emission at 510 nm,
along with a single-exponential fit (on the time scale shown)
with τ ) 5.5 ps and a marginally better dual-exponential fit
with τ1 ) 2.0 ps and τ2 ) 7.8 ps. Panel B shows the growth of
oxochlorin bleaching plus stimulated emission at 615 nm and a
fit with τ ) 6.0 ps, while part D shows that τ ) 2.1 ps at 630
nm. The kinetic profile at 565 nm (part C) unequivocally
demonstrates the presence of two kinetic components (2.6 and
By ∼30 ps for PMI-ZnO in either toluene or benzonitrile,
the PMI ground-state bleaching and PMI* stimulated-emission
features have basically disappeared and have been replaced by
features that can be associated primarily with the lowest excited
singlet state (ZnO*) of the zinc oxochlorin (Figure 3, dashed
spectra). The feature at ∼610 nm in the 30 ps spectra contains
comparable contributions of Qy(0,0) ground-state bleaching and
ZnO* stimulated emission, the small dip at ∼670 nm is Qy(0,1)
stimulated emission, and the small dip at ∼570 nm is Qy(1,0)
bleaching; all of these contributions are in keeping with the
positions and relative amplitudes of the features of the static
absorption and fluorescence spectra (Figure 2B). The excited
state ZnO* also has a relatively weak and featureless transient
absorption across the entire region displayed.17
The ZnO* stimulated-emission contributions at ∼30 ps decay
substantially as time proceeds so that by ∼3.4 ns the major
feature is oxochlorin Qy(0,0) ground-state bleaching at ∼610
nm, along with Qy(1,0) bleaching at ∼570 nm and a relatively
weak, featureless transient absorption across the spectrum
(Figure 3, dotted spectra). The 3.4 ns spectrum for PMI-ZnO
in either toluene or benzonitrile can be assigned primarily to
the oxochlorin lowest excited triplet state (ZnOT). The fact that
the ZnOT Qy(0,0) bleaching is roughly one-half the magnitude
of the combined ZnO* Qy(0,0) bleaching plus stimulated-
emission indicates that the yield of ZnO* f ZnOT intersystem
crossing (the triplet yield) is very high, as we have found for
zinc oxochlorin monomers (see Figure 5C).17
In summary, the transient absorption difference spectra are
consistent with the PMI* ZnO f PMI ZnO* energy transfer
followed by PMI ZnO* f PMI ZnOT intersystem crossing (plus
some internal conversion and fluorescence) being the primary
excited-state processes that occur following photoexcitation of
PMI-ZnO in either toluene or benzonitrile (Figure 1). Along

3438 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Muthukumaran et al.
constant of this PMI* relaxation is ∼3 ps for PMI-1 in
benzonitrile. The nature of the relaxation (conformational,
vibrational, and electronic) for a series of perylene dyes is under
investigation via experiment and theory and will be reported
on in detail elsewhere.13
The finding of such complexity in the photophysics of even
the simple, isolated perylene dye prompted the use of a kinetic
model in the global analysis for dyad PMI-ZnO in benzonitrile
(Figures 3 and 4) in which there are two forms of PMI*, one
unrelaxed and the other relaxed, both of which can undergo
energy transfer to make ZnO*, which in turn decays to make
ZnOT (and the ground state). [The amplitude (preexponential
factor) spectra from the global analysis assuming three simple
independent exponentially decaying components is given in the
Supporting Information.] The resulting species-associated spec-
trum of the 2 ps component (Figure 5A, black) and 8 ps
component (red) match the respective experimental spectra at
0.5 and 40 ps for the PMI-1 monomer (Figure 5B) in terms of
shape and the peak wavelengths of the major features. Thus,
the early-time behavior for the dyad involves both a relaxation
(conformational/vibrational/electronic) within PMI* as well as
the population decay of this electronic state. It is significant
that the peak at ∼570 nm in the 40 ps spectrum for the perylene
monomer (Figure 5B, red) is slightly larger than in the peak at
∼600 nm in the 0.5 ps spectrum (black), whereas the reverse is
true in the corresponding species-associated spectra for the dyad
(Figure 5A). This difference can be ascribed to energy transfer
from PMI* to the oxochlorin in the dyad already occurring to
some degree during the 2 ps kinetic phase (in competition with
relaxation within PMI*), with the remainder occurring during
the 8 ps kinetic phase (primarily electronic decay of the relaxed
excited state). Thus, energy transfer during the 2 ps phase
siphons off part of the PMI* population, so that spectrum of
the relaxed form of the state in the dyad has a smaller magnitude
(relative to the unrelaxed form) compared to the situation in
the isolated dye. Thus, in assessing the rate of energy transfer
for PMI-ZnO in benzonitrile, we will use an average time
constant of ∼4.5 ps obtained from single-exponential fits across
the PMI bleaching (500-530 nm), which provides a measure
of the return of PMI* (relaxed plus unrelaxed forms) to the
ground state as electronic energy shifts to the oxochlorin to
produce ZnO*. [Assessing the early time dynamics is further
complicated because ZnO* may also have a relaxation process
that occurs on the time scale of 10-15 ps, as judged from studies
on an isolated oxochlorin.17]
Figure 5. (A) Species-associated spectra from multiexponential global
analysis of the same data set depicted for dyad PMI-ZnO in Figures
3B and 4, using the model described in the text. Transient absorption
spectra at selected pump-probe delay times for samples in benzonitrile
at room temperature are shown in (B) for perylene monomer PMI-1
(483 nm excitation) and in part C for oxochlorin monomer ZnO8 (540
nm excitation).
11 ps) on this time scale because at this wavelength the
components enter with opposite signs (one is a growth and the
other a decay), and similar results are found at 645 nm (part
E). Global analysis was performed on the kinetic profiles at
the above five wavelengths and about 30 others obtained across
the entire spectral region shown in Figure 3 (500-730 nm),
spanning times before and during the excitation flash and
extending to ∼3.5 ns (using a function containing three
exponentials and “constant” long-time asymptote (τ > 3 ns)
plus the instrument response). On the short time scale (<50
ps) shown in Figure 4, the best-fit time constants are τ1 ) 2 ps
and τ2 ) 8 ps; on the longer time scale (25 ps to 3.5 ns) a third
component has τ3 ∼ 400 ps (see inset to Figure 4B). Although
there are clearly three kinetic components, and two contributing
at times <50 ps, the simple scheme described above (Figure 1)
indicates there should be only two states that decay on the time
scale of the measurements, namely, PMI* and ZnO*.
The origin of the three kinetic components is demonstrated
in Figure 5, which shows species-associated spectra derived from
the global analysis of the data set for the PMI-ZnO in
benzonitrile (part A) along with key spectra for the reference
monomers PMI-1 (part B) and ZnO8 (part C) in the same
solvent. (Spectra similar to those for ZnO8 were found for
ZnO7.) A central aspect of this analysis derives from the finding
that the PMI-1 monomer exhibits two distinct spectra at early
times (<50 ps) after excitation (Figure 5B) even though the
lowest excited singlet state (PMI*) does not decay over the
course of the transient absorption measurements because this
state has a lifetime of ∼5 ns (as deduced by fluorescence decay;
Table 2). As we will report elsewhere,13 both monomer transient
difference spectra can be assigned to PMI* but differ in shape
due to a shift of the stimulated-emission contribution with time
(analogous to a dynamic Stokes shift) plus possible additional
changes in the transient absorption contribution. The time
The species-associated spectrum of the 400 ps component
for the dyad (Figure 5A, green) and the subsequent spectrum
(blue) that does not decay on the time scale of the transient
absorption experiment (τ > 3 ns) are readily interpreted. These
calculated spectra are basically the same as the experimental
spectra observed at early and long times for the ZnO8 monomer
(part C) and other reference monomers (not shown), and at 40
ps and 3.4 ns for the dyad (Figure 3B, dashed and dotted) that
were described above. Thus, the species-associated spectra for
the third and “constant” kinetic components for the dyad can
be assigned to the oxochlorin excited states ZnO* and ZnOT.
The average ZnO* lifetime from the transient absorption and
fluorescence experiments for PMI-ZnO in benzonitrile is 0.6
ns (Table 2).
Similar complexity in the early-time dynamics is found for
PMI-ZnO in toluene, although the spectral changes during the
PMI* relaxation phase are less pronounced than in benzonitrile.
These differences are in accord with the PMI* relaxation effects

A Perylene-Oxochlorin Light-Harvesting Array
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3439
found for the PMI-1 monomer in toluene versus benzonitrile.13
An effective PMI* lifetime of ∼3.6 ps is obtained from the
single-exponential fits across the PMI bleaching (Table 2), which
primarily reflects energy transfer to the oxochlorin (from both
the unrelaxed and relaxed forms of the excited perylene). It
should be noted that without correlation of comprehensive data
for the dyad and monomer, the finding of more pronounced
spectral evolution for the dyad in polar medium could have been
taken to imply greater participation of a charge-separated state
such as PMI- ZnO+. Such a possibility would have been further
attractive because the electrochemically generated PMI-1 anion
has a pronounced band at ∼600 nm;13 however, the excited-
state feature at approximately the same position has comparable
intensity (see Figure 3B, solid, and 5B). These combined data
make it difficult to use such absorption features alone as a
diagnostic for the formation of a charge-separated state.
HOMO (the a1u-like orbital) in this macrocycle has much lower
electron density at the meso-carbon linker-attachment site than
the HOMO (the a2u orbital) in the porphyrin-containing dyad
and (2) the meta-phenyl position to which the ethyne group of
the linker is attached has less electron density than the para-
phenyl position utilized in the porphyrin-containing dyad.43
Efficient excited-state energy transfer also has been observed
from a porphyrin (or other tetrapyrrole chromophore) to a
chlorin/oxochlorin.33-37 Energy transfer from a carotenoid
accessory pigment to a chlorin is also quite fast (∼14 ps) and
moderately efficient (∼50%) at an inter-chromophore separation
of ∼2 Å using attachment at a â-pyrrole position that is
contained in the conjugation path of the macrocycle, but is very
slow (>1 ns) and inefficient (<10%) at a distance of ∼5 Å
using attachment at the reduced ring of the hydroporphyrin.31,32
Collectively, these studies show that rapid and efficient pho-
toinduced energy transfer to chlorins/oxochlorins can be achieved,
and with characteristics that depend on the chromophores and
linker motifs employed.
In an ideal light-harvesting array, the harvested energy
transferred to the acceptor chromophore should not be wasted
by charge-transfer processes involving the accessory pigment.
The transient absorption spectra for PMI-ZnO in toluene or
benzonitrile show no evidence for electron transfer to the
perylene, implying that ZnO* decays primarily by the normal
routes open to the isolated chromophore (mainly intersystem
crossing to the excited triplet state). The same conclusion is
reached upon 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 The ZnO*
fluorescence quantum yields in the dyad are comparable to or
slightly larger than in the reference monomers in both solvents,
and similarly the average (fluorescence and transient absorption)
ZnO* lifetime in the dyad and monomer in toluene (0.8 ( 0.2
ns) is the same within experimental error, both results indicating
no excited-state quenching. In benzonitrile the slightly shorter
average lifetime of 0.6 ( 0.2 ns in the dyad versus 0.7 ( 0.2
ns in the monomers suggests that there may be some electron-
transfer quenching of ZnO* with a yield ΦET ) [1 - 0.6/0.7]‚
100 ∼ 15% in this polar medium. However, the yield could be
zero within experimental uncertainty by this calculation, in
agreement with the other measurements. Additionally, the
analysis given above, showing no evidence for PMI* hole
transfer and with PMI- ZnO+ lying energetically above PMI*,
would also place the charge-separated state well above ZnO*,
making quenching of the latter state unlikely. Taken as a whole,
the results indicate that PMI ZnO* f PMI- ZnO+ electron
transfer is not significant for PMI-ZnO in either toluene or
benzonitrile (Figure 1). Thus, ZnO* has decay characteristics
comparable to those of the isolated chromophore in both polar
and nonpolar media. It follows that the composition of the dyad
provides no impediment for use of the electronic energy placed
in the excited oxochlorin either by direct excitation or efficient
energy transfer from the perylene accessory pigment.
Energies of the Electronic States of the Dyads. In con-
structing the state diagram shown in Figure 1, the energies of
the excited perylene and porphyrin components were taken
directly from the spectroscopic data for the 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 state in both solvents.
The free energy of the charge-separated state PMI- ZnO+ was
derived using an experimentally determined6 reference value
of 2.05 eV for state PDI- ZnP+ in the perylene-porphyrin dyad
PDI-ZnP in toluene, which differs from dyad PMI-ZnO
studied here in both the perylene and tetrapyrrole components
but has a similar linker. In particular, the state PMI- ZnO+ in
PMI-ZnO is higher than the state PDI- ZnP+ in PDI-ZnP by
a total of 0.48 eV from two sources: (1) the ∼80 mV more
positive oxidation potential of zinc oxochlorins versus zinc
porphyrins (Table 1)6,17 and (2) the 0.4 eV more negative
reduction potential of the perylene-monoimide versus perylene-
bis(imide) dye (Table 1).5 Finally, the charge-separated state
will be stabilized by 0.26 eV in benzonitrile versus toluene
(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)).
Rates and Yields of the Energy- and Charge-Transfer
Processes. The transient absorption data described above
indicate that the excited perylene (PMI*) in the dyad decays
primarily by energy transfer to the oxochlorin, with hole transfer
being minimal (Figure 1). Energy transfer causes PMI* to have
a lifetime that is about 3 orders of magnitude shorter than the
lifetime of ∼5 ns in the isolated dye, with an average value
(unrelaxed and relaxed forms) of 3.5-5 ps (Table 2). The rate
constant and quantum yield of PMI* ZnO f PMI ZnO* energy
transfer in toluene are obtained by applying standard methods5
to the PMI* lifetimes for the PMI-ZnO dyad and ZnO7
reference monomer in Table 2. The results are kENT
)
)
[(3.6 ps)-1 - (4800 ps)-1 -1
]
∼ (3.6 ps)-1 and ΦENT
[1 - 3.6/4800]‚100 >99%. The energy-transfer yield for PMI-
ZnO is >99% in benzonitrile as well. It is interesting to note
that even though PMI-ZnO energy transfer is very rapid and
basically quantitative, it is about 40% slower than we observed
previously for dyad PDI-ZnP (Chart 2) that contains a perylene-
bis(imide) dye and a zinc porphyrin employing the same linker
motif.5 This difference is likely due to the predominant through-
bond (Dexter) nature of energy transfer in these systems1,5
together with a weaker electronic coupling in PMI-ZnO
compared to PMI-ZnP. A reduced through-bond coupling in
the oxochlorin-containing dyad is anticipated1 because (1) the
Conclusions and Outlook
We have synthesized and characterized a new perylene-
oxochlorin dyad for potential use in light-harvesting applications.
This dyad is a first step beyond our previous studies of perylene-
porphyrin arrays in combining accessory pigments and hydro-
porphyrins to elicit desirable photophysical properties. The
incorporation of the oxochlorin was intended to simultaneously
provide stronger absorption in the red spectral region compared
to porphyrins while maintaining similar redox characteristics.

3440 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Muthukumaran et al.
The latter factors led to the choice of oxochlorins compared to
their close chlorin relatives, the former macrocycle being harder
to oxidize and more stable than the latter but with comparable
red absorption.
130.33, 130.71, 136.64, 137.23, 138.11, 150.54, 150.62, 188.00.
Anal. Calcd for C12H8BrNOS: C, 49.00; H, 2.74; N, 4.76.
Found: C, 49.07; H, 2.75; N, 4.78.
1-(3-Bromobenzoyl)-5-mesityldipyrromethane (3). Follow-
ing a general procedure,39 a solution of EtMgBr in THF (10
mL, 10 mmol, 1.0 M) was added to a solution of 5-mesityl-
dipyrromethane (2, 1.06 g, 4.00 mmol) in dry THF (4 mL) at
room temperature. After stirring for 10 min, the flask was cooled
to -78 °C and a solution of 1 (1.18 g, 4.00 mmol) in THF (4
mL) was added. The cooling bath was removed and the mixture
was stirred for 30 min. The reaction was quenched with saturated
aqueous NH4Cl. The mixture was extracted with CH2Cl2. The
organic layer was washed with water, dried (Na2SO4) and
concentrated to give a dark residue. Column chromatography
[silica, CH2Cl2 f CH2Cl2/ethyl acetate (95:5)] afforded a pale
brown foamlike solid. Recrystallization from hexanes provided
a pale yellow solid (1.21 g, 68%): mp 208 °C (dec). 1H NMR
δ: 2.09 (s, 6H), 2.30 (s, 3H), 5.95 (s, 1H), 6.10-6.16 (m, 2H),
6.20-6.24 (m, 1H), 6.67-6.71 (m, 1H), 6.81-6.84 (m, 1H),
6.90 (s, 2H), 7.32 (t, J ) 8.0 Hz, 1H), 7.64-7.69 (m, 1H),
7.74-7.79 (m, 1H), 7.80-7.88 (br, 1H), 7.96 (t, J ) 1.6 Hz,
1H), 9.15-9.28 (br, 1H); 13C NMR δ 20.75, 20.89, 38.72,
107.34, 109.07, 110.41, 117.04, 120.95, 122.53, 127.44, 128.90,
129.52, 129.94, 130.71, 131.80, 132.95, 134.53, 137.42, 137.51,
140.43, 141.62, 182.32. Anal. Calcd for C25H23BrN2O: C,
67.12; H, 5.18; N, 6.26. Found: C, 67.00; H, 5.25; N, 6.22.
The results demonstrate that the new perylene-oxochlorin
dyad (PMI-ZnO) studied here has many favorable qualities as
a light-harvesting unit: energy flows rapidly (∼4 ps) and
efficiently (99%) from perylene to oxochlorin, which then shows
excited-state decay characteristics comparable to those of the
isolated chromophore in both polar and nonpolar media. Thus,
incorporation of the perylene-monoimide as an accessory
pigment to complement the absorption of the oxochlorin has
not resulted in unwanted charge-transfer decay pathways. Such
processes would compromise the availability of the harvested
energy for subsequent transfer from the excited oxochlorin to
another stage in a larger architecture. On the other hand, charge
transfer can be elicited through appropriate tuning of energy
levels of the perylene and/or oxochlorin, thereby converting a
light-harvesting motif into a charge-separation unit for switching
or charge-injection applications in molecular optoelectronics.
A description of perylene-oxochlorins wherein charge separation
is the dominant excited-state process is provided in the
companion paper.
Experimental Section
1
General. H NMR (400 MHz) and 13C NMR (100 MHz)
1-Bromo-9-(3-bromobenzoyl)-5-mesityldipyrromethane (4).
Following a general procedure,14,15 a solution of 3 (895 mg,
2.00 mmol) in 20 mL of dry THF was cooled to -78 °C under
argon. A sample of N-bromosuccinimide (NBS, 356 mg, 2.00
mmol) was added and the reaction mixture was stirred for 1 h
at -78 °C. Hexanes (50 mL) and water (50 mL) were added
and the mixture was allowed to warm to room temperature. The
mixture was extracted with CH2Cl2. The organic layer was dried
(Na2SO4) and concentrated under reduced pressure without
heating. Column chromatography [silica, hexanes/ethyl acetate
(9:1) f (4:1)] afforded a pale yellow powder (930 mg, 88%):
1H NMR δ: 2.10 (s, 6H), 2.30 (s, 3H), 5.88 (s, 1H), 6.00-
6.03 (m, 1H), 6.10-6.17 (m, 2H), 6.80-6.83 (m, 1H), 6.91 (s,
2H), 7.34 (t, J ) 8.0 Hz, 1H), 7.67 (d, J ) 8.0 Hz, 1H), 7.70-
7.80 (br, 1H), 7.76 (d, J ) 8.0 Hz, 1H), 7.97 (s, 1H), 9.11-
9.21 (br, 1H). This compound was unstable and decomposed
while recording a 13C NMR spectrum. However, adequate
stability over the course of 2 days was achieved upon storage
as a solid at 0 °C.
spectra were recorded in CDCl3. Mass spectra were obtained
by high-resolution fast atom bombardment mass spectrometry
(FAB-MS) and by laser desorption mass spectrometry (LD-MS)
in the absence of a matrix.44 Absorption and emission spectra
were collected in toluene at room temperature unless noted
otherwise. Melting points are uncorrected. Alumina (activity
grade I) was obtained from Fisher. Silica gel (Baker, 40 µm
average particle size) was used for column chromatography.
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 )
tetrahydrofuran (THF); quantitation at 420 and 520 nm; refer-
ence 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).
Solvents. Toluene and triethylamine were freshly distilled
from CaH2 and sparged of oxygen prior to use. THF was
distilled from sodium benzophenone ketyl as required. CH3CN
was distilled from CaH2 and stored over molecular sieves.
Anhydrous methanol was prepared by drying over CaH2 for 12
h followed by distillation. All other solvents were used as
received.
5-(3-Bromophenyl)-17,18-dihydro-10-mesityl-18,18-di-
methylporphinatozinc(II) (Zn6). Following a standard pro-
cedure,14 a solution of 4 (526 mg, 1.00 mmol) in THF/methanol
[40 mL (4:1)] was treated with NaBH4 (0.380 mg, 10.0 mmol).
After 15 min (TLC indicated complete reaction), the reaction
mixture was quenched with cold water and extracted with
CH2Cl2. The organic layer was dried (K2CO3) and concentrated
under reduced pressure without heating to afford the mono-
carbinol. The foamlike residue was dissolved in 10 mL of
CH3CN. Then 5 (190 mg, 1.0 mmol) was added followed by
TFA (77 µL, 1.0 mmol). The reaction mixture was stirred at
room temperature for 30 min, and then diluted with 90 mL of
CH3CN. Samples of AgOTf (771 mg, 3.00 mmol), Zn(OAc)2
(2.75 g, 15.0 mmol), and 2,2,6,6-tetramethylpiperidine (5.10 mL,
30.0 mmol) were added. The resulting mixture was refluxed
for 18 h. The reaction mixture was concentrated. The residue
thus obtained was chromatographed [silica, hexanes/CH2Cl2 (2:
1)] to afford a blue solid (252 mg, 37%): 1H NMR δ: 1.85 (s,
6H), 2.01-2.04 (m, 6H), 2.59 (s, 3H), 4.52 (s, 2H), 7.21 (s,
Noncommercial Compounds. The syntheses of dipyr-
romethane 2,40 Western half 5,14 and perylene PMI-210 have
been described previously.
S-2-Pyridyl 3-bromobenzothioate (1). Following a general
procedure,39 a solution of 2-mercaptopyridine (11.1 g, 0.100
mol) in CH2Cl2 (400 mL) was treated with a solution of
3-bromobenzoyl chloride (13.2 mL, 0.100 mol) in CH2Cl2 (100
mL) via a pressure equalizing dropping funnel over 20 min. A
solid precipitated when about half of the acid chloride solution
was added. Then CH2Cl2 (300 mL) and 2N aqueous NaOH
solution were added. The organic phase was separated, washed
with water, dried (MgSO4), and concentrated. The solid obtained
was crystallized from hexanes to afford a colorless solid (15.1
g, 51%): mp 63-65 °C. 1H NMR δ: 7.32-7.42 (m, 2H), 7.68-
7.76 (m, 2H), 7.77-7.84 (m, 2H), 8.11-8.14 (m, 1H), 8.66-
8.72 (m, 1H); 13C NMR δ 122.96, 123.80, 126.05, 130.27,

A Perylene-Oxochlorin Light-Harvesting Array
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3441
2H), 7.54 (t, J ) 7.6 Hz, 1H), 7.82-7.87 (m, 1H), 8.00-8.03
(m, 1H), 8.20-8.24 (m, 1H), 8.26 (d, J ) 4.4 Hz, 1H), 8.31 (d,
J ) 4.4 Hz, 1H), 8.51 (d, J ) 4.4 Hz, 1H), 8.58-8.60 (m, 1H),
8.63 (d, J ) 4.4 Hz, 1H), 8.65 (s, 1H), 8.68 (d, J ) 4.4 Hz,
2H). LD-MS observed: 674.62. FAB-MS observed: 674.0999.
Calcd: 674.1024 (C37H31BrN4Zn). λabs 412 and 609 nm.
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 NSF.
Supporting Information Available: Absorption and emis-
sion spectra for PMI-ZnO and its component monomers in
benzonitrile, amplitude-associated spectra from transient absorp-
tion measurements on the dyad in benzonitrile, and complete
spectral data (1H NMR, LD-MS) for all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
5-(3-Bromophenyl)-17,18-dihydro-10-mesityl-18,18-di-
methyl-17-oxoporphinatozinc(II) (ZnO6). Following a stan-
dard procedure,17 a solution of Zn6 (156 mg, 0.230 mmol) in
toluene (12 mL) was treated with basic alumina (activity I, 10
g). The mixture was heated to 50 °C with constant stirring under
an air atmosphere. After 66 h, TLC examination showed
complete consumption of starting material. The mixture was
filtered. The filtered material was washed with CH2Cl2/methanol
(19:1) until the washings were colorless. The filtrate was
concentrated under reduced pressure to give a bluish-green solid.
The latter was dissolved in toluene (115 mL) and treated with
DDQ (104 mg, 0.460 mmol). The reaction mixture was stirred
at room temperature for 5 min and quenched with triethylamine.
The reaction mixture was concentrated under reduced pressure
and chromatographed (silica, CH2Cl2) to afford a bluish-purple
solid (78 mg, 49%): 1H NMR δ: 1.82 (s, 6H), 2.01 (s, 6H),
2.61 (s, 3H), 7.24 (s, 2H), 7.57 (t, J ) 7.6 Hz, 1H), 7.80-7.90
(m, 1H), 8.04 (d, J ) 7.6 Hz, 1H), 8.24-8.27 (m, 1H),
8.40-8.55 (m, 2H), 8.67 (d, J ) 4.4 Hz, 1H), 8.78 (d, J ) 4.4
Hz, 1H), 8.92-8.99 (m, 3H), 9.50-9.52 (m, 1H). Anal. LD-
MS observed: 688.94. FAB-MS observed: 688.0829. Calcd:
688.0816 (C37H29BrN4OZn). λabs 422 and 609 nm.
References and Notes
(1) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002,
35, 57-69.
(2) Zollinger, H. Color Chemistry; VCH: Weinheim, 2nd ed., 1991.
(3) Langhals, H. Heterocycles 1995, 40, 477-500.
(4) Miller, M. A.; Lammi, R. K.; Prathapan, S.; Holten, D.; Lindsey,
J. S. J. Org. Chem. 2000, 65, 6634-6649.
(5) Prathapan, S.; Yang, S. I.; Seth, J.; Miller, M. A.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2001, 105, 8237-8248.
(6) Yang, S. I.; Prathapan, S.; Miller, M. A.; Seth, J.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2001, 105, 8249-8258.
(7) Yang, S. I.; Lammi, R. K.; Prathapan, S.; Miller, M. A.; Seth, J.;
Diers, J. R.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Mater. Chem. 2001,
11, 2420-2430.
(8) Ambroise, A.; Kirmaier, C.; Wagner, R. W.; Loewe, R. S.; Bocian,
D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2002, 67, 3811-3826.
(9) Kirmaier, C.; Yang, S. I.; Prathapan, S.; Miller, M. A.; Diers, J.
R.; Bocian, D. F.; Lindsey, J. S.; Holten, D. Res. Chem. Intermed. 2002,
28, 719-740.
5-[3-[2-[4-[9-(4-tert-Butylphenyloxy)perylene-3,4-dicarbox-
imido]-3,5-diisopropylphenyl]ethynyl]phenyl]-17,18-dihydro-
10-mesityl-18,18-dimethyl-17-oxoporphinatozinc(II) (PMI-
ZnO). Following a standard procedure,41 samples of ZnO6 (35.0
mg, 50.0 µmol), PMI-2 (33.0 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 60 °C with stirring for 1 h. The
reaction mixture was cooled to room temperature and filtered
through a pad of Celite. The filtered material 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,
CHCl3), preparative SEC (THF), and chromatography (short
silica column, CHCl3). The resulting solid was washed with
methanol, affording a pale-purple solid (34 mg, 54%) that was
homogeneous by analytical SEC: 1H NMR δ: 1.17 (d, J )
6.4 Hz, 12H), 1.37 (s, 9H), 1.84 (s, 6H), 2.06 (s, 6H), 2.61 (s,
3H), 2.70-2.78 (m, 2H), 6.96 (d, J ) 8.4 Hz, 1H), 7.08-7.14
(m, 2H), 7.25 (s, 2H), 7.42-7.49 (m, 2H), 7.53 (s, 2H), 7.64-
7.74 (m, 2H), 7.94-7.98 (m, 1H), 8.06-8.09 (m, 1H), 8.25-
8.36 (m, 3H), 8.43-8.46 (m, 2H), 8.49-8.57 (m, 4H), 8.58-
8.68 (m, 3H), 8.85 (d, J ) 4.8 Hz, 1H), 8.92-9.00 (m, 2H),
9.59 (s, 1H). LD-MS observed: 1264.66. FAB-MS observed:
1261.44. Calcd: 1261.45 (C83H67N5O4Zn). λabs 423, 513, and
610 nm.
(10) Loewe, R. S.; Chevalier, F.; Tomizaki, K.-Y.; Lindsey, J. S. J.
Porphyrins Phthalocyanines. In press.
(11) Tomizaki, K.-Y.; Loewe, R. S.; Kirmaier, C.; Schwartz, J. K.;
Retsek, J. L.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2002,
67, 6519-6534.
(12) Loewe, R. S.; Tomizaki, K.-Y.; Youngblood, W. J.; Bo, Z.; Lindsey,
J. S. J. Mater. Chem. 2002, 12, 3438-3451.
(13) Kirmaier, C.; Schwartz, J. K.; Hindin, E.; Diers, J. R.; Loewe, R.
S.; Tomizaki, K.-Y.; Birge, R. R.; Bocian, D. F.; Lindsey, J. S.; Holten, D.
Manuscripts in preparation.
(14) Taniguchi, M.; Ra, D.; Mo, G.; Balasubramanian, T.; Lindsey, J.
S. J. Org. Chem. 2001, 66, 7342-7354.
(15) (a) Strachan, J.-P.; O’Shea, D. F.; Balasubramanian, T.; Lindsey,
J. S. J. Org. Chem. 2000, 65, 3160-3172. (b) Strachan, J.-P.; O’Shea, D.
F.; Balasubramanian, T.; Lindsey, J. S. J. Org. Chem. 2001, 66, 642.
(16) Balasubramanian, T.; Strachan, J.-P.; Boyle, P. D.; Lindsey, J. S.
J. Org. Chem. 2000, 65, 7919-7929.
(17) Taniguchi, M.; Kim, H.-J.; Ra, D.; Schwartz, J. K.; Kirmaier, C.;
Hindin, E.; Diers, J. R.; Prathapan, S.; Bocian, D. F.; Holten, D.; Lindsey,
J. S. J. Org. Chem. 2002, 67, 7329-7342.
(18) (a) Montforts, F.-P.; Gerlach, B.; Ho¨per, F. Chem. ReV. 1994, 94,
327-347. (b) Jacobi, P. A.; Lanz, S.; Ghosh, I.; Leung, S. H.; Lo¨wer, F.;
Pippin, D. Org. Lett. 2001, 3, 831-834. (c) Silva, A. M. G.; Tome´, A. C.;
Neves, M. G. P. M. S.; Cavaleiro, J. A. S. Tetrahedron Lett. 2000, 41,
3065-3068. (d) Shea, K. M.; Jaquinod, L.; Khoury, R. G.; Smith, K. M.
Tetrahedron 2000, 56, 3139-3144. (e) Burns, D. H.; Shi, D. C.; Lash, T.
D. Chem. Commun. 2000, 299-300. (f) Krattinger, B.; Callot, H. J. Eur. J.
Org. Chem. 1999, 1857-1867. (g) Johnson, C. K.; Dolphin, D. Tetrahedron
Lett. 1998, 39, 4619-4622. (h) Mironov, A. F.; Efremov, A. V.; Efremova,
O. A.; Bonnett, R.; Martinez, G. J. Chem. Soc., Perkin Trans. 1 1998, 3601-
3608.
(19) Lindsey, J. S.; Delaney, J. K.; Mauzerall, D. C.; Linschitz, H. J.
Am. Chem. Soc. 1988, 110, 3610-3621.
(20) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. (c)
Wasielewski, M. R. In Chlorophylls; Scheer, H., Ed.; CRC Press: Boca
Raton, FL, 1991; pp 269-286.
Characterization. The electrochemical and spectroscopic
studies were conducted using instrumentation and techniques
previously described.5 Transient absorption measurements used
5-10 µM samples at room-temperature excited with ∼130 fs,
20-30 µJ, and 480-600 nm pulses.
(21) (a) Johnson, D. G.; Niemczyk, M. P.; Minsek, D. W.; Wiederrecht,
G. P.; Svec, W. A.; Gaines, G. L., III; Wasielewski, M. R. J. Am. Chem.
Soc. 1993, 115, 5692-5701. (b) Wasielewski, M. R.; Wiederrecht, G. P.;
Svec, W. A.; Niemczyk, M. P. Solar Energy Mater. Solar Cells 1995, 38,
127-134.
(22) (a) Abel, Y.; Montforts, F.-P. Tetrahedron Lett. 1997, 38, 1745-
1748. (b) Kutzki, O.; Montforts, F.-P. Synlett 2001, 53-56.
(23) Tkachenko, N. V.; Tauber, A. Y.; Grandell, D.; Hynninen, P. H.;
Lemmetyinen, H. J. Phys. Chem. A 1999, 103, 3646-3656.
Acknowledgment. This research was supported by the NSF
(CHE-9988142). Mass spectra were obtained at the Mass
Spectrometry Laboratory for Biotechnology at North Carolina

3442 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Muthukumaran et al.
(24) Malinen, P. K.; Tauber, A. Y.; Helaja, J.; Hynninen, P. H. Liebigs
Ann. 1997, 1801-1804.
(33) Zenkevich, E. I.; Knyukshto, V. N.; Shulga, A. M.; Kuzmitsky, V.
A.; Gael, V. I.; Levinson, E. G.; Mironov, A. F. J. Lumin. 1997, 75, 229-
244.
(34) Tamiaki, H.; Miyatake, T.; Tanikaga, R.; Holzwarth, A. R.;
Schaffner, K. Angew. Chem. Int. Ed. 1996, 35, 772-774.
(35) (a) Osuka, A.; Wada, Y.; Maruyama, K.; Tamiaki, H. Heterocycles
1997, 44, 165-168. (b) Osuka, A.; Marumo, S.; Maruyama, K. Bull. Chem.
Soc. Jpn. 1993, 66, 3837-3839.
(36) Osuka, A.; Marumo, S.; Maruyama, K.; Mataga, N.; Tanaka, Y.;
Taniguchi, S.; Okada, T.; Yamazaki, I.; Nishimura, Y. Bull. Chem. Soc.
Jpn. 1995, 68, 262-276.
(37) Osuka, A.; Marumo, S.; Mataga, N.; Taniguchi, S.; Okada, T.;
Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc. 1996,
118, 155-168.
(38) Kirmaier, C.; Hindin, E.; Schwartz, J. K.; Sazanovich, I. V.; Diers,
J. R.; Muthukumaran, K.; Taniguchi, M.; Bocian, D. F.; Lindsey, J. S.;
Holten, D. J. Phys. Chem. B. 2003, 107, 3443.
(39) Rao, P. D.; Littler, B. J.; Geier, G. R., III.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 1084-1092.
(40) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.
(41) Loewe, R. S.; Lammi, R. K.; Diers, J. R.; Kirmaier, C.; Bocian, D.
F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 1530-1552.
(42) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1-13.
(43) (a) Li, J.; Ambroise, A.; Yang, S. I.; Diers, J. R.; Seth, J.; Wack,
C.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1999, 121,
8927-8940. (b) Lammi, R. K.; Ambroise, A.; Balasubramanian, T.; Wagner,
R. W.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 2000,
122, 7579-7591. (c) Ferrante, C.; Kensey, U.; Dick, B. J. Phys. Chem.
1993, 97, 13457-13463.
(25) Mo¨ssler, H.; Wittenberg, M.; Niethammer, D.; Mudrassagam, R.
K.; Kurreck, H.; Huber, M. Magn. Reson. Chem. 2000, 38, 67-84.
(26) (a) Tkachenko, N. V.; Rantala, L.; Tauber, A. Y.; Helaja, J.;
Hynninen, P. H.; Lemmetyinen, H. J. Am. Chem. Soc. 1999, 121, 9378-
9387. (b) Helaja, J.; Tauber, A. Y.; Abel, Y.; Tkachenko, N. V.;
Lemmetyinen, H., Kilpela¨inen, I.; Hynninen, P. H. J. Chem. Soc., Perkin
Trans. 1 1999, 121, 2403-2408. (c) Tkachenko, N. V.; Vuorimaa, E.; Kesti,
T.; Alekseev, A. S.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. J.
Phys. Chem B 2000, 104, 6371-6379. (d) Tkachenko, N. V.; Guenther,
C.; Imahori, H.; Tamaki, K.; Sakata, Y.; Shunichi, F.; Lemmetyinen, H.
Chem. Phys. Lett. 2000, 326, 344-350. (e) Vehmanen, V.; Tkachenko, N.
V.; Tauber, A. Y.; Hynninen, P. H.; Lemmetyinen, H. Chem. Phys. Lett.
2001, 345, 213-218. (f) Efimov, A.; Tkachenko, N. V.; Lemmetyinen, H.
J. Porphyrins Phthalocyanines 2001, 5, 835-838. (g) Alekseev, A. S.;
Tkachenko, N. V.; Tauber, A. Y.; Hynninen, P. H.; Osterbacka, R.; Stubb,
H.; Lemmetyinen, H. Chem. Phys. 2002, 275, 243-251. (h) Vehmanen,
V.; Tkachenko, N. V.; Efimov, A.; Damlin, P.; Ivaska, A.; Lemmetyinen,
H. J. Phys. Chem. A. 2002, 106, 8029-8038.
(27) Zheng, G.; Dougherty, T. J.; Pandey, R. K. Chem. Commun. 1999,
2469-2470.
(28) Fukuzumi, S.; Ohkubo, K.; Imahori, H.; Shao, J.; Ou, Z.; Zheng,
G.; Chen, Y.; Pandey, R. K.; Fujitsuka, M.; Ito, O.; Kadish, K. M. J. Am.
Chem. Soc. 2001, 123, 10676-10683.
(29) (a) Kutzki, O.; Walter, A.; Montforts, F.-P. HelV. Chim. Acta 2000,
83, 2231-2245. (b) Montforts, F.-P.; Kutzki, O. Angew. Chem. Int. Ed.
2000, 39, 599-601.
(30) Liddell, P. A.; Nemeth, G. A.; Lehman, W. R.; Joy, A. M.; Moore,
A. L.; Bensasson, R. V.; Moore, T. A.; Gust, D. Photochem. Photobiol.
1982, 36, 641-645.
(31) Liddell, P. A.; Barrett, D.; Makings, L. R.; Pessiki, P. J.; Gust, D.;
Moore, T. A. J. Am. Chem. Soc. 1986, 108, 5350-5352.
(44) (a) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J.
Porphyrins Phthalocyanines 1997, 1, 93-99. (b) Srinivasan, N.; Haney,
C. A.; Lindsey, J. S.; Zhang, W.; Chait, B. T. J. Porphyrins Phthalocyanines
1999, 3, 283-291.
(32) Wasielewski, M. R.; Liddell, P. A.; Barrett, D.; Moore, T. A.; Gust,
D. Nature 1986, 322, 570-572.