Excited-state photodynamics of perylene-porphyrin dyads. 5. tuning light-harvesting characteristics via perylene substituents, connection Motif, and three-dimensional architecture

Article abstract of DOI:10.1021/jp910705q

Seven perylene-porphyrin dyads were examined with the goal of identifying those most suitable for components of light-harvesting systems. The ideal dyad should exhibit strong absorption by the perylene in the green, undergo rapid and efficient excited-sta

Full text of DOI:10.1021/jp910705q

J. Phys. Chem. B 2010, 114, 14249–14264
14249
Excited-State Photodynamics of Perylene-Porphyrin Dyads. 5. Tuning Light-Harvesting
Characteristics via Perylene Substituents, Connection Motif, and Three-Dimensional
Architecture†
Christine Kirmaier,‡ Hee-eun Song,‡ Eunkyung Yang,‡ Jennifer K. Schwartz,‡ Eve Hindin,‡
James R. Diers,§ Robert S. Loewe,| Kin-ya Tomizaki,| Fabien Chevalier,| Lavoisier Ramos,
Robert R. Birge,⊥ Jonathan S. Lindsey,*,| David F. Bocian,*,§ and Dewey Holten*,‡
Department of Chemistry, Washington UniVersity, St. Louis, Missouri, 63130-4889, Department of Chemistry,
UniVersity of California, RiVerside, California, 92521-0403, Department of Chemistry, North Carolina State
UniVersity, Raleigh, North Carolina, 27695-8204, and Department of Chemistry, UniVersity of Connecticut,
Storrs, Connecticut, 06269-3060
ReceiVed: NoVember 10, 2009; ReVised Manuscript ReceiVed: December 20, 2009
Seven perylene-porphyrin dyads were examined with the goal of identifying those most suitable for
components of light-harvesting systems. The ideal dyad should exhibit strong absorption by the perylene in
the green, undergo rapid and efficient excited-state energy transfer from perylene to porphyrin, and avoid
electron-transfer quenching of the porphyrin excited state by the perylene in the medium of interest. Four
dyads have different perylenes at the p-position of the meso-aryl group on the zinc porphyrin. The most
suitable perylene identified in that set was then incorporated at the m- or o-position of the zinc porphyrin,
affording two other dyads. An analogue of the o-substituted architecture was prepared in which the zinc
porphyrin was replaced with the free base porphyrin. The perylene in each dyad is a monoimide derivative;
the perylenes differ in attachment of the linker (either via a diphenylethyne linker at the N-imide or an
ethynylphenyl linker at the C9 position) and the number (0-3) of 4-tert-butylphenoxy groups (which increase
solubility and slightly alter the electrochemical potentials). In the p-linked dyad, the monophenoxy perylene
with an N-imide diphenylethyne linker is superior in providing rapid and essentially quantitative energy transfer
from excited perylene to zinc porphyrin with minimal electron-transfer quenching in both toluene and
benzonitrile. The dyads with the same perylene at the m- or o-position exhibited similar results except for
one case, the o-linked dyad bearing the zinc porphyrin in benzonitrile, where significant excited-state quenching
is observed; this phenomenon is facilitated by close spatial approach of the perylene and porphyrin and the
associated thermodynamic/kinetic enhancement of the electron-transfer process. Such quenching does not
occur with the free base porphyrin because electron transfer is thermodynamically unfavorable even in the
polar medium. The p-linked dyad containing a zinc porphyrin attached to a bis(4-tert-butylphenoxy)perylene
via an ethynylphenyl linker at the C9 position exhibits ultrafast and quantitative energy transfer in toluene;
the same dyad in benzonitrile exhibits ultrafast (<0.5 ps) perylene-to-porphyrin energy transfer, rapid (∼5
ps) porphyrin-to-perylene electron transfer, and fast (∼25 ps) charge recombination to the ground state.
Collectively, this study has identified suitable perylene-porphyrin constructs for use in light-harvesting
applications.
I. Introduction
energy-transfer characteristics for a wide variety of multipor-
phyrin arrays have been examined.2 Although porphyrins absorb
blue light quite strongly, they have rather limited absorption
across the remainder of the visible spectrum. Improvements in
the overall light-harvesting efficiency can be realized by the
introduction of accessory pigments.
Multipigment arrays that absorb visible light and funnel the
resulting excited-state energy rapidly and efficiently to a
designated site are of great interest for probing the mechanisms
of electronic energy migration among pigments1 and may
provide the basis for new types of molecular constructs for solar-
energy applications. Among various pigments, members of the
porphyrin family are attractive candidates for light-harvesting
systems due to their high absorption coefficients and facile
excited-state energy-transfer characteristics. In this regard, the
The ideal features3 of an accessory pigment for use with
porphyrins appear to be largely satisfied by perylene-imide
chromophores, which exhibit the following attributes: (1)
perylene-imides absorb light strongly in the region between the
porphyrin Soret and Q bands.4 (2) A perylene-imide typically
displays a long and monophasic singlet excited-state lifetime,5
and typically has a fluorescence quantum yield near unity.6,7
(3) Perylene-imides are exceptionally stable.6 (4) Perylene-
imides can be derivatized with synthetic handles in a variety of
ways for use in modular building-block approaches.8–10
‡ Washington University.
§ University of California, Riverside.
| North Carolina State University.
⊥ University of Connecticut.
† Part of the “Michael R. Wasielewski Festschrift”.
* To whom correspondence should be addressed. E-mail: holten@wuchem.
wustl.edu (D.H.); David.Bocian@ucr.edu (D.F.B); jlindsey@ncsu.edu
(J.S.L.).
The literature concerning perylene is vast (“perylene” itself
elicits >4000 citations on Web of Science). A key focus for
10.1021/jp910705q  2010 American Chemical Society
Published on Web 01/29/2010

14250 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
CHART 2: Perylene-monoimides Studied Herein
CHART 1: Previously Studied Perylene-imides
much work with perylene-imides concerns perylene-bisimides,
which are excellent electron acceptors and, with appropriate
N-imide substituents, readily aggregate to give higher-order
assemblies.11 Far less work has been carried out with perylene-
monoimides, which are much poorer electron acceptors but still
aggregate in the absence of solubilizing substituents.12 Repre-
sentative perylene-bisimide and perylene-monoimides are
PDI-m and PMI-1, respectively, shown in Chart 1.
Both types of perylene-imides have strong absorption (ελmax
∼50 000 M-1cm-1) in the region between the porphyrin Soret
and Q bands. Introduction of bulky groups at the N-aryl moieties
is necessary to achieve solubility. For example, the perylene
bearing tert-butyl groups on the aryl rings (PDI-m) is readily
soluble in organic solvents, whereas the analogue (not shown)
with unsubstituted phenyl substituents is not.13 The N-aryl group
can also be substituted with 2,6-diisopropyl groups as a means
of achieving high solubility, as shown for perylene-monoimide
PMI-2 and other perylene-monoimides examined herein (Chart
2).14 The absorption and emission characteristics of the perylene-
imides are little affected by the presence of solubilizing
substituents at the imide position because of the nodes present
at the imide nitrogen in both the HOMO and LUMO.15,16 Further
increases in solubility are achieved upon introduction of alkoxy
substituents directly at the perimeter of the perylene nucleus.8–10
On the other hand, the introduction of alkoxy groups for
solubilization (Chart 2) or 9-ethynyl groups for joining with
dyads (e.g., PMI-e in Chart 1) also results in bathochromic shifts
in the absorption spectrum.
derivatized with bulky substituents to achieve high solubility
in organic solvents, thereby suppressing self-aggregation. Initial
studies concerned dyads containing a porphyrin and a perylene-
bisimide.24,25 To achieve efficient energy transfer with minimal
competing electron transfer, we then focused on perylene-
monoimides containing bulky substituents.24,27–31,33 Although
synthetic access to perylene-monoimides initially was limited,
scalable routes have since been developed that provide robust
entre´e to perylene-monoimide building blocks.35
Chart 3 displays two such dyad motifs containing a porphyrin
and a perylene-imide, as well as benchmark porphyrins for
comparison purposes. One dyad utilizes a perylene-bisimide
(PDI) joined to the porphyrin at the N-imide position of the
A substantial amount of prior work has been done concerning
arrays composed of tetrapyrrole macrocycles and perylene-
imides,17–23 with the focus also chiefly centering on perylene-
bisimides, whereupon the perylene serves as an electron acceptor
upon photoexcitation and often directs formation of higher-order
assemblies. In our own work, we have prepared a large number
of dyads and multads composed of tetrapyrrole macrocycles and
perylene-imides.24–35 In each case, the perylene-imide was

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14251
CHART 3: Previously Studied Perylene-Porphyrin Dyads and Benchmark Porphyrins
perylene via a diphenylethyne (pep) linker. The second motif
utilizes a perylene-monoimide (PMI) joined to the porphyrin
at the C9 position of the perylene via an ethynylphenyl (ep)
linker.25 In both cases, the porphyrin is the free base (Fb),
magnesium chelate (Mg), or zinc chelate (Zn).27 The excited-
state photochemical properties of these dyads are illustrated via
consideration of the zinc chelates PMI-ep-Zn and PDI-pep-
Zn. PMI-ep-Zn in toluene undergoes ultrafast (<0.5 ps)
perylene-to-porphyrin energy transfer, followed by decay of the
excited porphyrin as in the isolated pigment.27 In acetonitrile,
ultrafast electron transfer and charge recombination dominate
the photochemical behavior.29 The ultrafast dynamics are derived
from the strong through-bond (linker-mediated) perylene-por-
phyrin electronic coupling indicated by the progressive red shift
of the ground-state absorption of the perylene-monoimide (505
nm; PMI-1; Chart 1) upon successive addition of the ethyne
(517 nm; PMI-e; Chart 1), phenyl (530 nm; PMI-ep; Chart 1),
and porphyrin (548 nm; PMI-ep-Zn; Chart 3).27 In comparison,
PDI-pep-Zn in toluene or acetonitrile exhibits somewhat slower
but still rapid (∼2.5 ps) parallel energy transfer (70-80%) and
hole transfer (20-30%) from the photoexcited perylene to the
porphyrin.25,26 The excited porphyrin then decays (τ ∼4 ns) by
electron transfer to the perylene. The lifetime of the charge-
separated product varies from <0.5 ns to >10 ns depending on
solvent polarity.
perylene in each dyad is a monoimide derivative, the linker is
attached to the N-imide or C9 position, and zero to three 4-tert-
butylphenoxy groups (hereafter referred to as aryloxy groups)
are incorporated for tailoring solubility and possible alteration
of the electrochemical potentials. These dyads were designed
to address two questions. To address the important question
concerning the type of perylene-monoimide suitable for light-
harvesting applications, four dyads were prepared each with a
different perylene joined at the p-aryl position of a zinc
porphyrin (Chart 4). A second question concerns the three-
dimensional architecture of the dyad construct. To address this
question, the perylene-monoimide found to be most suitable at
the p-aryl position of the porphyrin was employed in dyads that
utilized the m- or o-aryl position of a zinc porphyrin (Chart 5).
One free base porphyrin also was prepared to explore possible
mitigation of competing electron-transfer quenching processes.
All of the dyads were examined by electrochemistry and static
and time-resolved optical spectroscopy. Density functional
theory (DFT) calculations were also performed on the various
perylene-monoimides to examine the characteristics of the
frontier molecular orbitals. Collectively, these studies identified
suitable perylene-porphyrin motifs for use in multipigment
light-harvesting arrays in polar and nonpolar media.
II. Experimental Section
In this paper, we present the photophysical and redox
properties of seven additional dyads containing a porphyrin and
a perylene-monoimide. The dyads employ the best design
features identified during studies of prior dyads. Thus, the
A. Literature Compounds. Five of the seven perylene-
porphyrin dyads32 (p-PMI-aep-Zn, p-PMI(OR)-aep-Zn, p-PMI-
(OR)3-aep-Zn, o-PMI(OR)-aep-Zn, and m-PMI(OR)-aep-Zn),
all but one of the perylene benchmarks (PMI-2,14,36 PMI(OR),32

14252 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
CHART 4: Dyads with Distinct Perylene-monoimides and a Common Porphyrin
PMI(OR)-e,32 PMI(OR)3-e,32,35 PMI(OR)3,35 and PMI(OR)3-135),
and the porphyrin benchmarks (ZnU,37 FbU38) were synthesized
as described previously.
1,6-Bis(4-tert-butylphenoxy)-9-[2-(trimethylsilyl)ethynyl]-N-
(2,5-di-tert-butylphenyl)-3,4-perylenedicarboximide [PMI(OR)2-e-
TMS]. A mixture of PMI(OR)2-Br (150 mg, 170 µmol),
Pd2(dba)3 (7.8 mg, 8.5 µmol), PPh3 (13.3 mg, 50.9 µmol), and
CuI (3.2 mg, 17 µmol) in toluene/triethylamine [6 mL, (5:1)]
was placed into an oil bath at 60 °C, and (trimethylsilyl)acety-
lene (28.7 µL, 203 µmol) was added. The mixture was stirred
for 2.5 h and then passed over a silica column (4 × 25 cm,
B. Synthesis. Three new compounds were prepared for the
work described herein. Perylene building block17 PMI(OR)2-Br
was treated with (trimethylsilyl)acetylene under Pd-catalyzed cross-
coupling conditions with copper cocatalysis to afford the perylene
benchmark PMI(OR)2-e-TMS in 99% yield (Scheme 1). Analo-
gous reaction of PMI(OR)2-Br and the porphyrin building block
5,10,15-trimesityl-20-(4-ethynylphenyl)porphyrin (ZnU′)39 under
copper-free39 Pd-catalyzed cross-coupling conditions gave the
ethynylphenyl-linked perylene-porphyrin dyad PMI(OR)2-ep-Zn
in 60% yield. The dyad containing a free base porphyrin (o-
PMI(OR)-aep-Fb) was synthesized by demetalation of the cor-
responding zinc porphyrin construct upon treatment with trifluo-
roacetic acid in chloroform at room temperature.
1
CHCl3) to afford a red solid (152 mg, 99%): mp >300 °C; H
NMR (300 MHz, CDCl3) δ 0.35 (s, 9H), 1.24 (s, 9H), 1.28 (s,
9H), 1.329 (s, 9H), 1.332 (s, 9H), 6.93 (d, 1H) [splittings are
due to regioisomers], 7.04-7.09 (m, 4H), 7.39-7.43 (m, 5H),
7.53 (d, J ) 8.4 Hz, 1H), 7.68 (t, J ) 7.8 Hz, 1H), 7.77 (d, J
) 8.1 Hz, 1H), 8.28 (d, J ) 3.3 Hz, 2H), 8.46 (d, J ) 7.5 Hz,
1H), 9.28 (d, J ) 8.1 Hz, 1H), 9.40 (d, J ) 6.9 Hz, 1H); FAB-

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14253
CHART 5: Dyads for Comparison of Architectural Distinctions and Energetic Differences
MS obsd 901.4571, calcd 901.4526 (C61H63NO4Si); λabs (log ε)
421 (3.8), 487 (4.4), 520 (4.6) nm; λem (λex ) 520 nm) 551,
587(sh) nm.
passed through a silica column (CHCl3, 3 × 10 cm) to afford
a magenta solid (9.1 mg, 91%): 1H NMR (400 MHz, CDCl3) δ
-1.64 (s, 2H), -0.21 (dd, J1 ) 6.4 Hz, J2 ) 1.2 Hz, 12H),
1.36 (s, 9H), 1.80 (s, 3H), 1.82 (s, 6H), 1.84 (s, 6H), 1.86 (s,
3H), 1.90 (m, 2H), 2.58 (s, 3H), 2.62 (s, 6H), 5.51 (s, 2H),
6.87 (d, J ) 8.4 Hz, 1H), 7.07 (d, J ) 8.4 Hz, 2H), 7.20 (s,
1H), 7.25 (brs, 5H), 7.43 (d, J ) 8.8 Hz, 2H), 7.57 (t, J ) 7.6
Hz, 1H), 7.70 (t, J ) 7.2 Hz, 1H), 7.80 (t, J ) 8.0 Hz, 1H),
8.01 (d, J ) 7.2 Hz, 1H), 8.07 (d, J ) 8.4 Hz, 1H), 8.14 (d, J
) 6.8 Hz, 1H), 8.19 (d, J ) 8.8 Hz, 1H), 8.23-8.32 (m, 3H),
8.35 (d, J ) 8.4 Hz, 1H), 8.38 (d, J ) 7.6 Hz, 1H), 8.56-8.59
(m, 4H), 8.67 (d, J ) 4.4 Hz, 2H), 8.74 (d, J ) 4.4 Hz, 2H);
LD-MS obsd 1391.0, calcd average mass 1392.77 (C99H85N5O3);
λabs 420, 485, 513, 593, 649 nm; λem (λex ) 513 nm) 649,
719 nm.
9-[2-(4-(5,10,15-Trimesitylporphinatozinc(II)-20-yl)phenyl)-
ethynyl]-1,6-bis(4-tert-butylphenoxy)-N-(2,5-di-tert-butylphe-
nyl)-3,4-perylenedicarboximide [PMI(OR)2-ep-Zn]. Following
a standard procedure for copper-free Sonogashira coupling,39
samples of 5,10,15-trimesityl-20-(4-ethynylphenyl)porphina-
tozinc(II) (ZnU′, 20.0 mg, 24.2 µmol) and PMI(OR)2-Br (21.4
mg, 24.2 µmol) were coupled using Pd2(dba)3 (3.3 mg, 3.6 µmol)
and P(o-tol)3 (8.8 mg, 29 µmol) in degassed toluene/triethyl-
amine [9.6 mL (5:1)] at 60 °C for 2 h. The reaction mixture
was then concentrated and passed over a silica column [CH2Cl2/
hexanes (1:1)]. Preparative SEC (THF) followed by column
chromatography [silica, CH2Cl2/hexanes (2:1)] afforded a purple
1
solid (23.6 mg, 60%): H NMR (300 MHz, CDCl3) δ 1.28 (s,
C. Electrochemistry. The electrochemical studies were
performed in butyronitrile (Burdick and Jackson) using previ-
ously described instrumentation.40 The supporting electrolyte
was 0.1 M tetrabutylammonium hexafluorophosphate (Aldrich;
recrystallized three times from methanol and dried at 110 °C
in vacuo). The electrochemical cell was housed in a Vacuum
Atmospheres glovebox (model HE-93) equipped with a Dri-
Train (model 493). The E1/2 values were obtained with square
wave voltammetry (frequency 10 Hz).
D. Spectroscopy and Analysis. The transient-absorption
measurements were conducted on the arrays in toluene or
benzonitrile (PhCN) at 295 K (5-30 µM solutions in 2 mm
path cells). These measurements employed ∼130 fs, 25-30 µJ,
excitation flashes (focused to ∼1 mm diameter) at 490 nm and
white-light probe pulses of comparable duration.27 The excitation
flashes were attenuated to ∼8 µJ to avoid exciton annihilation,41,42
9H), 1.30 (s, 9H), 1.35 (s, 9H), 1.36 (s, 9H), 1.85 (s, 18H),
2.63 (s, 9H), 6.96 (d, J ) 8.4 Hz, 1H), 7.11-7.15 (m, 4H),
7.28 (s, 6H), 7.41-7.47 (m, 5H), 7.55 (d, J ) 8.4 Hz, 1H),
7.81 (t, J ) 8.1 Hz, 1H), 7.99 (d, J ) 8.1 Hz, 1H), 8.05 (d, J
) 8.1 Hz, 2H), 8.29 (d, J ) 8.1 Hz, 2H), 8.34 (s, 2H), 8.69-8.91
(m, 9H), 9.43 (d, J ) 8.1 Hz, 1H), 9.49 (d, J ) 7.5 Hz, 1H);
LD-MS obsd 1633.09; calcd average mass 1632.40
(C111H99N5O4Zn); λabs 424, 549, 591 nm; λem (λex ) 550 nm)
599, 646 nm.
5-[2-(2-(4-(9-(4-tert-Butylphenoxy)-3,4-perylenedicarbox-
imido)-3,5-diisopropylphenyl)ethynyl)phenyl]-10,15,20-trimesi-
tylporphyrin [o-PMI(OR)-aep-Fb]. A sample of o-PMI(OR)-
aep-Zn (10.5 mg, 7.21 µmol) in CHCl3 (10 mL) at room
temperature was treated with TFA (8.3 mL, 110 µmol). After
stirring for 15 min, TEA (200 µL) was added. The mixture was

14254 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
TABLE 1: Half-Wave Potentials and Molecular-Orbital
SCHEME 1: Synthesis of Perylene-Porphyrin Dyad
PMI(OR)2-ep-Zn and Control Monomer
PMI(OR)2-e-TMS
Energies for Perylene Monomersa
reduction potentials molecular-orbital energies
perylene
monomer
E1/2 (1)
(V)
E1/2 (2)
(V)
HOMO
(eV)
LUMO
(eV)
PMI-1
PMI-e
PMI-2
PMI(OR)
PMI(OR)2-e-TMS -1.13
PMI(OR)3
PMI(OR)3-e
-1.24b
-1.16b
-1.21
-1.21
-1.79b
-1.65b
-1.81
-1.78
-1.61
-1.78
-1.77
-5.52
-5.53
-5.54
-5.31
-5.23
-5.04
-5.10
-2.78
-2.92
-2.81
-2.67
-2.68
-2.47
-2.54
-1.32
-1.30
a Obtained in butyronitrile containing 0.1 M TBAH. E1/2 vs Ag/
Ag+; E1/2 of FeCp2/FeCp2 ) +0.19 V. (Add +0.26 V to obtain the
+
values versus SCE.) The E1/2 values are obtained from square wave
voltammetry (frequency ) 10 Hz). Values are (0.01 V. The first
and second oxidation potentials were also measured for PMI(OR)3
and are +0.81 and +1.19 eV, respectively. Essentially the same
values were found for PMI(OR)3-e (+0.82 and +1.19 eV). b From
ref 27.
TABLE 2: Half-Wave Potentials (E1/2) for the
Perylene-Porphyrin Dyadsa
porphyrin
perylene
oxidation potentialsb reduction potentials
E1/2 (1)
(V)
E1/2 (2)
(V)
E1/2 (1)
(V)
E1/2 (2)
(V)
dyad
PMI-ep-Znc
p-PMI-aep-Zn
p-PMI(OR)-aep-Zn
m-PMI(OR)-aep-Zn
o-PMI(OR)-aep-Zn
PMI(OR)2-ep-Zn
p-PMI(OR)3-aep-Zn
+0.50
+0.52
+0.52
+0.52
+0.51
+0.52
+0.52
+0.88
+1.08
+0.96
+0.96
+0.96
+0.96
+0.92
-1.14
-1.19
-1.19
-1.19
-1.20
-1.13
-1.30
-1.60
-1.81d
-1.73d
-1.73d
-1.76d
-1.57
-1.76d
a Obtained in butyronitrile containing 0.1 M TBAH. E1/2 vs Ag/
Ag+; E1/2 of FeCp2/FeCp2 ) +0.19 V. (Add +0.26 V to obtain the
+
values versus SCE.) The E1/2 values are obtained from square wave
voltammetry (frequency ) 10 Hz). Values are (0.01 V. b The
potential is approximate because the second oxidation wave of the
porphyrin partially overlaps the first oxidation wave of the perylene.
The second perylene oxidation potential was measured for
p-PMI(OR)3-aep-Zn (+1.19 V). c Taken from ref 27. d The
potential is approximate because the second reduction wave of the
perylene strongly overlaps the first reduction wave of the perylene.
which if present adds a short 1-3 ps component to the
photodynamics. For each dyad and solvent, a data set was
obtained that encompassed wavelengths 505-750 nm and times
from -20 ps to 3.9 ns. Global analysis of the transient-
absorption data for a given array was performed using IgorPro6
(Wavemetrics) for data sets in which ∆A in each spectrum was
averaged in 5 nm steps across the range 505-680 nm. The
resulting kinetic traces for pump-probe delay times from -20
to 200 ps or 3.9 ns (depending on sample) were globally fit
using the convolution of the instrument response with function
consisting of one or more exponentials plus a constant. Some
curve fitting of individual kinetic traces utilized Origin8
(Microcal).
E. Molecular Orbital Calculations. Density-functional-
theory (DFT) calculations were performed with Spartan ‘06 for
Windows (Wave function, Inc.) on a PC (Dell Optiplex GX270)
equipped with a 3.2 GHz CPU and 3 GB of RAM.43 The hybrid
B3LYP functional and a 6-31G* basis set were employed. The
equilibrium geometries were fully optimized using the default
parameters of the Spartan ‘06 program. The TMS group of
PMI(OR)2-e-TMS was replaced with H to simplify the
calculations.
in Table 1. The data for the perylene-porphyrin dyads are given
in Table 2. The data in Table 1 focus on the two reduction waves
of the perylene, the first being in the range from -1.1 to -1.3
V and the second typically being in the range from -1.6 to
-1.8 V. The first and second oxidation waves for the two
perylene monomers bearing three aryloxy groups were also
measured and were found to be about +0.8 and +1.2 V (see
Table 1 footnotes). These results are consistent with our previous
studies of related perylene compounds.25,27
The redox characteristics of the perylene units in the dyads
are essentially identical to those of the isolated chromophores.
In all cases, the E1/2 values of the perylenes in the dyads are
within a few mV of those of the benchmarks. The zinc porphyrin
unit in each dyad exhibits two oxidation waves and two
reduction waves in the +1.0 to -2.0 V range; the oxidation
potentials are listed in Table 2 because they are most relevant
to the studies herein. The E1/2 values for zinc porphyrins are
similar among the dyads and essentially identical to those of
structurally related tetraarylporphyrin monomers.44 These data
are indicative of relatively weak ground-state electronic interac-
tions between the perylene and porphyrin constituents of the
dyad.
III. Results
A. Electrochemical Characteristics. The electrochemical
data for the benchmark perylene-monoimides are summarized

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14255
Figure 1. Electron density distributions and energies for the frontier molecular orbitals of perylene monomers, obtained from DFT calculations.
One particularly noteworthy feature of the redox properties
of the perylene compounds is that the addition of one aryloxy
group has a minimal effect, and up to three aryloxy groups only
a modest effect, on the redox potentials. For example, the first
reduction potentials for PMI-2, PMI(OR), and PMI(OR)3 are
-1.21, -1.21, and -1.32 V, respectively. A similar trend is
observed for the second reduction potentials and for the set
PMI-e, PMI(OR)2-e-TMS, and PMI(OR)3-e (Table 1). Like-
wise, for the dyads, the perylene first reduction potentials for
p-PMI-aep-Zn, p-PMI(OR)-aep-Zn, and p-PMI(OR)3-aep-
Zn are -1.19, -1.19, and -1.30 V, respectively, and similarly
for the second reduction potentials.
The observation that the addition of an alkoxy group to the
C9 position of the perylene (PMI(OR) versus PMI-2) has very
little effect on the redox potentials is surprising in view of the
fact that the addition of an alkynyl group at this position (PMI-e
versus PMI-1) shifts the reduction potentials positively by an
average of 0.1 V (Table 1). Possibly, the electron-donating
property of the alkoxy group is balanced by a nearly equal and
opposite inductive effect. However, there is a small incremental
effect when two additional aryloxy groups are added to the
perylene 1- and 6-positions. On the basis of the first reduction
potentials for the perylene monomers, the addition of three
aryloxy groups makes the perylene harder to reduce by an
average of 0.12 V. This prediction is realized in the dyads,
wherein p-PMI(OR)3-aep-Zn is harder to reduce than p-PMI-
aep-Zn by 0.11 V. Thus, the addition of three aryloxy groups
to the perylene to increase solubility has the added benefit of
rendering the dyad slightly less susceptible to electron-transfer
quenching. This is so because the lowest-energy electron-transfer
process for a photoexcited perylene-porphyrin dyad will
generally involve reduction of the perylene (and oxidation of
the porphyrin) on the basis of the redox potentials given in
Tables 1 and 2 and prior work.25–27
and that for PMI(OR)3 is -2.47 eV. A similar trend is observed
for PMI-e, PMI(OR)2-e-TMS, and PMI(OR)3-e. A less nega-
tive reduction potential implies that the molecule is easier to
reduce. Thus, although the directions of the shifts in orbital
energies track the shifts of the reduction potentials, the effect
of the aryloxy groups on the orbital energies is (about 3-fold)
larger than that on the redox potentials. This difference likely
arises in part from the effect of solvation to modulate the effects
of the aryloxy groups on the redox potentials measured for the
perylenes in solution compared to the orbital energies calculated
for the same molecules in the gas phase.
C. Photophysical Properties of Perylene Benchmark
Monomers. 1. Absorption and Fluorescence Spectra, Fluo-
rescence Yields, and Singlet Excited-State Lifetimes. The
structures of perylene monomers investigated here are shown
in Chart 2. The absorption spectra of the perylene-monoimide
monomers bearing no aryloxy substituents (PMI-2), one aryloxy
group [PMI(OR)], or three aryloxy groups [PMI(OR)3] all
display a broad absorption manifold (fwhm ∼75 nm) consisting
of a series of substantially overlapping vibronic transitions. The
presence of one or three aryloxy substituents results in a
bathochromic shift of approximately 30 nm, such that the (0,0)
band moves from 506 nm in PMI-2 to 532 nm in PMI(OR),
532 nm in PMI(OR)3, and 536 nm in PMI(OR)3-e in toluene.
A similar trend is observed for the same compounds in PhCN
except that in each case there is a 3-10 nm red-shift compared
to that in toluene. For benchmark perylenes having an ethyne
directly attached to the perylene C9 position, the (0,0) band
moves from 505 nm in PMI-1 to 517 nm in PMI-e (Chart 1)
as found previously,27 to 527 nm for PMI(OR)2-e-TMS (Chart
2). Collectively, these observations indicate that the dominant
contribution to the spectral shift stems from substitution at the
9-position of the perylene; further substitution of two aryloxy
groups at the flanking positions (1-, 6-) of the perylene or
incorporation of the ethyne group on the aryl ring at the N-imide
position generally has a more modest effect on the spectral
properties.
B. Molecular Orbital Characteristics. The molecular orbital
characteristics (energies and electron-density distributions) for
the benchmark perylenes were examined to gain further insights
into the redox potentials (Figure 1). Paralleling the comparisons
of reduction potentials given above, the LUMO energy for
PMI-2 is -2.81 eV, whereas that for PMI(OR) is -2.67 eV
Each of the perylene-monoimide monomers has a broad
fluorescence contour that is approximately a mirror image to
the absorption manifold. In analogy to the absorption spectrum,

14256 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
TABLE 3: Photophysical Properties of Perylene-Porphyrin
Dyads and Reference Monomersa
toluene
τS (ns)
PhCN
Φf
Dyads
Φf
τS (ns)
p-PMI-aep-Zn
0.042
0.046
0.048
0.036
0.038
0.082
0.070
2.4
2.4
2.4
2.4
2.4
0.040
0.043
0.041
0.023
0.041
0.085
e0.003
2.0
1.9
1.8
1.4
2.1
p-PMI(OR)-aep-Zn
p-PMI(OR)3-aep-Zn
o-PMI(OR)-aep-Zn
m-PMI(OR)-aep-Zn
o-PMI(OR)-aep-Fb
PMI(OR)2-ep-Zn
13.6
2.4
13.3
0.005
Monomers
PMI-2
0.91b
0.82b
0.86b
0.97
5.0b
4.8b
5.0b
5.3
0.99
0.79
0.92
0.96
0.74
0.97
0.90
0.041
0.096
5.2
4.8
5.2
5.2
4.6
PMI(OR)
PMI(OR)3-e
PMI(OR)3
PMI(OR)-e
PMI(OR)2-e-TMS
PMI(OR)3-1
ZnU
0.72
0.90
5.0
5.0
5.0
0.85
5.5
5.4
0.034b
0.090
2.4b
13.3c
2.0b
13.3
FbU
a All data at room temperature in Ar-purged (deaerated) solution.
b From ref 30. c From ref 46.
Figure 2. Time-resolved absorption difference spectra at 0.5 ps (solid)
and 20 ps (dashed) after excitation with a 130 fs 490 nm excitation
flash for PMI(OR) in toluene (A) and PhCN (B). Ground-state
absorption (solid) and fluorescence (dotted) features for PMI(OR) in
PhCN are given for comparison (C).
the fluorescence spectrum red-shifts upon incorporation of the
aryloxy substituents. On the other hand, the presence of aryloxy
groups does not significantly affect the fluorescence quantum
yield of the perylene-monoimide (Table 3). The high (>0.7)
fluorescence yields of the perylene-monoimide monomers both
in toluene and PhCN indicate that the excited singlet state of
each of these chromophores has only minor nonradiative decay
competing with the fluorescence emission process. These
perylene monomers also have long (∼5 ns) fluorescence
lifetimes (Table 3). Similar properties have been reported for
other perylene-monoimides.26,27,30,45 The spectroscopic properties
of the porphyrin benchmarks ZnU and FbU have been examined
previously30,46 and are included in the table for comparison.
2. Time-ResolWed Absorption Spectra and Excited-State
Relaxation Dynamics. Transient absorption spectra were ac-
quired for perylene-monoimide monomers (bearing zero, one,
or three aryloxy substituents) in toluene and in PhCN utilizing
130 fs excitation flashes at 490 nm and a time delay of 3.5 ns.
Representative spectra for PMI(OR) in toluene and in PhCN
are given in parts A and B of Figure 2, respectively. For
comparison, ground-state absorption spectra (solid) and fluo-
rescence spectra (dotted) are given in Figure 2C. The absorption
difference spectrum for PMI(OR) in PhCN at 0.5 ps includes
bleaching of the broad ground-state absorption band at ∼530
nm (Figure 2B). This negative-going contribution is overlapped
with singlet-excited-state stimulated emission (fluorescence
stimulated by the white-light probe pulse) that also gives a
negative-going contribution to the difference spectrum. The 0.5
ps spectrum also shows a broad excited-state absorption
(positive-going contribution) at wavelengths >570 nm.
The spectrum at 20 ps then partially decays in a uniform
manner (no change in shape) out to the 3.9 ns limit of the
experiment and with a time constant of ∼5 ns (not shown).
These findings on PMI(OR) in PhCN are consistent with (i)
the 4.8 ns singlet excited-state lifetime derived from the
fluorescence measurements (Table 3) and (ii) the decay of the
perylene lowest singlet excited state being dominated by return
to the ground state (largely by fluorescence) with little formation
of a longer lived triplet excited state. A similar time evolution
of the spectrum is observed for PMI(OR) in toluene (Figure
2A).
Representative kinetic profiles (555 and 665 nm) for the
spectral evolution for PMI(OR) in PhCN are shown in Figure
3A. The solid lines are fits to convolution of the instrument
response plus the single-exponential function Aexp(-t/τ) + C.
The globally fit time constant is 6.4 ps. Analogous spectral and
kinetic data were acquired for the other perylene monomers in
toluene and PhCN (see the Supporting Information). The time
constants for the excited-state evolution for different perylene/
solvent combinations are in the range 3.2-6.6 ps (Table 4).
The simplest interpretation of the spectral time evolution in
each case is as follows. The spectra at 0.5 ps for each perylene-
monoimide monomer are assigned to the initial Franck-Condon
form of the singlet excited state (denoted PMI**; formed by
single-photon absorption and not a two-photon process). This
initial excited-state form may differ conformationally, vibra-
tionally, and/or electronically from the relaxed, lowest-energy
singlet excited-state form (denoted PMI*). State PMI* for each
perylene is responsible for the observed spontaneous fluores-
cence spectrum and the fluorescence quantum yield and singlet-
excited-state lifetime listed in Table 3. The stimulated emission
(and presumably spontaneous fluorescence) from PMI** occurs
at shorter wavelengths than that from PMI*.
At 20 ps, there is an overall diminution of the combined
bleaching plus stimulated-emission feature between 510 and 570
nm, giving way to increased net excited-state absorption in this
region. In contrast, there is a net decrease in the excited-state
absorption at wavelengths >570 nm. This latter characteristic
is due at least in part to the growth of stimulated emission in
this region that is expected to have the general profile of steady-
state spontaneous fluorescence (Figure 2C, dotted line). This
combination results in a net negative-going feature between 650
and 730 nm in the 20 ps spectrum (Figure 2B, dashed line).
Thus, the time evolution of the spectrum that occurs for each
perylene-monoimide during the first 10 ps after excitation
reflects a “dynamic Stokes shift”. This evolution likely involves
a change in internal coordinates (conformational/vibrational/

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14257
latter. Such motions in turn would modulate the electron
distribution involving the perylene and aryloxy groups and thus
the relative extent of charge-transfer character in PMI* com-
pared to PMI** and compared to the ground electronic state.
Differences in charge-transfer character between the two excited-
state forms would also cause a change in the interactions of the
perylene with the solvent. Whatever the origin, the presence of
two excited-state forms for the PMI has consequences for the
energy-transfer dynamics in the dyads, as described below.
3. The Effect of SolWent on the Optical Properties. To
further probe the issues raised above concerning the excited-
state dynamics of the perylenes, the absorption and fluorescence
spectra of PMI-2, PMI(OR), and PMI(OR)3-e were measured
in a range of solvents having different refractive indices (n)
and dielectric constants (ε). These values for each solvent and
the spectra are given in the Supporting Information. As noted
above, the spectra are characterized by an intense band in the
400-600 nm region. Vibronic structure is typically resolved in
nonpolar solvents such as hexane but not in polar solvents such
as methanol. Figure 4A,C,E shows plots of the wavenumber
position of the absorbance maximum as a function of solvent
polarizability, R(n) ) (n2 - 1)/(n2 + 2). There is a good linear
correlation between this function and peak position for samples
in nonpolar solvents but not in polar media. The same is true
for the peak position of the fluorescence (Figure 4G,I,K). Figure
4B,D,F shows that the peak absorption position also varies with
solvent polarity, P(ε) ) (ε - 1)/(ε + 2). This relationship is
linear for the samples in nonpolar solvents, and the same is
true except for PMI(OR)3-e in polar solvents. Figure 4H,J,L
shows similar plots for the emission position. In this case, a
linear relationship is observed for all three compounds in both
polar and nonpolar media. In terms of both the dielectric constant
and the index of refraction of the solvent, the shifts are more
pronounced for fluorescence than for absorbance position. The
solvent dependence of the optical properties depicted in Figure
4 may be due to dispersive interactions between the solvent
and the perylene. The collective data suggest that the excited
state (PMI**, PMI*, or both) has a larger dipole moment than
the ground state.50
Figure 3. Representative kinetic profiles for (A) perylene monomer
PMI(OR) in PhCN at 555 nm (open circles) and 665 nm (closed
circles), (B) perylene-porphyrin dyad o-PMI(OR)-aep-Zn in PhCN
at 520 nm (open circles) and 616 nm (closed circles), and (C)
o-PMI(OR)-aep-Zn in toluene at 545 nm (open circles) and 660 nm
(closed circles) obtained using 130 fs excitation flashes at 490 nm.
The inset in panel B shows the data on an expanded time scale. The
solid lines are fits to a function consisting of the instrument response
convolved with one (A), two (C), or four (B) exponentials plus a
constant.
D. Photophysical Properties of Diarylethyne-Linked
Perylene-Porphyrin Dyads. 1. Absorption and Fluorescence
Spectra, Fluorescence Yields, and Singlet Excited-State Life-
times. Figure 5 shows ground-state absorption spectra of
perylene-porphyrin dyad p-PMI-aep-Zn and reference com-
pounds PMI-2 and ZnU in toluene (solid lines). The spectrum
of the dyad closely matches the sum of the spectra of the compo-
nent parts. This comparison indicates that the perylene-porphyrin
electronic interactions in the ground and lowest singlet excited
state are weak (but not necessarily insignificant). This conclusion
is consistent with that drawn above about the ground-state
perylene-porphyrin interactions on the basis of the redox data.
The same conclusions are drawn from the optical and redox
data for all of the perylene-porphyrin arrays that employ the
diarylethyne (-aep-) linker (Chart 4).
TABLE 4: Rate Constants for Fast Relaxation in Perylene
Monomers
(krel)-1 (ps)
perylene monomer
toluene
PhCN
PMI-2
3.2
5.3
5.1
4.8
6.6
6.4
6.0
6.5
PMI(OR)
PMI(OR)3-e
PMI(OR)3
electronic) of the perylene as well as a change in the interaction
of the perylene with the medium. The dependence of the
relaxation rate (Table 4) and the associated evolution of the
spectra in Figure 2 suggest that the process depends on
the presence and number of the aryloxy groups. The nature of
the relaxation process may have some analogies to the dynamics
that we have described previously for difluoroboron-dipyrrin47,48
and bis(dipyrrinato)zinc(II)49 chromophores. For those two
classes, following excitation, there is a picosecond time scale
relaxation to a lower energy excited-state form involving
changes in the structure and electronic character of the chro-
mophore. For the perylenes studied here, the dynamics may be
in part connected with internal rotation of the aryloxy groups
with respect to the perylene core along with distortions of the
The fluorescence spectra of dyad p-PMI-aep-Zn and refer-
ence monomers PMI-2 and ZnU in toluene are also shown in
Figure 5 (dashed lines). The emission of p-PMI-aep-Zn was
measured using excitation at several wavelengths at which either
the perylene or the porphyrin primarily absorbs. Upon illumina-
tion of p-PMI-aep-Zn in toluene in 490 nm, where the perylene
absorption is >100-fold that of the porphyrin, the emission is
dominated by fluorescence from the porphyrin (Figure 5A,
dashed spectrum). This finding is consistent with substantial
energy transfer from the photoexcited perylene (PMI* and

14258 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
Figure 5. Ground-state absorption spectra (solid) and fluorescence
spectra (dashed) of diarylethyne-linked perylene-porphyrin dyad (A),
benchmark perylene (B), and benchmark porphyrin (C) in toluene.
PMI** as discussed below) to the ground-state zinc porphyrin
to produce the porphyrin lowest singlet excited state (Zn*). The
dyad was also illuminated at 560 nm where the zinc porphyrin
is primarily excited. As expected, emission occurs essentially
exclusively from the porphyrin.
The fluorescence quantum yield (Φf ∼0.04) for p-PMI-aep-
Zn is essentially the same (i) using excitation of the perylene
or the porphyrin, (ii) for the dyad in toluene or PhCN, and (iii)
as reference monomer ZnU (Table 3). Similarly, the singlet
excited-state lifetime (τS) of p-PMI-aep-Zn in toluene or PhCN
is the same as that for ZnU in toluene (2.4 ns) or PhCN (2.0
ns). These data indicate that (i) energy transfer from perylene
to porphyrin is basically quantitative and (ii) there is no
appreciable quenching of the Zn* due to the presence of the
perylene (or of PMI* due to the porphyrin), even in polar media.
Inspection of Table 3 indicates that similar conclusions can
be drawn concerning quantitative perylene to porphyrin energy
transfer without any significant electron-transfer quenching (of
either the excited perylene or excited porphyrin) for dyads
p-PMI(OR)-aep-Zn and m-PMI(OR)-aep-Zn in toluene or
PhCN, and for dyads p-PMI(OR)3-aep-Zn and o-PMI(OR)-
aep-Zn in toluene. In the case of p-PMI(OR)3-aep-Zn in PhCN,
there is a hint of excited-state quenching (e10%) based on a
slightly diminished porphyrin τS value of 1.8 ns compared to
2.0 ns for ZnU in PhCN; however, because this difference is at
the margins of the (0.1 ns error in these values, the quenching
is not significant if it does occur.
The exception to the behavior described above is for dyad
o-PMI(OR)-aep-Zn in PhCN. Although the fluorescence yield
is again about the same for excitation of perylene or porphyrin
(indicative of quantitative energy transfer to the porphyrin), in
both cases, the Φf value (0.023) is lower than that for ZnU in
this medium (0.041). Similarly, the τS value for the dyad (1.4
ns) is smaller than that for ZnU (2.0 ns). These combined results
reflect efficient perylene-to-porphyrin energy transfer but with
Figure 4. Peak positions of the ground state absorption maxima (A-F)
and fluorescence maxima (G-L) for perylene monomers as a function
of the solvent polarizability (left panels) and polarity (right panels).
Solvent abbreviations: ben ) benzene, chex ) cyclohexane, dec )
decalin, DMF ) dimethylformamide, DMSO ) dimethylsulfoxide, EA
) ethyl acetate, hex ) hexane, IE ) isopropyl ether, MeCN )
acetonitrile, MeOH ) methanol, pen ) isopentane, PrCN ) propioni-
trile, PrOH ) 2-propanol, tol ) toluene.

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14259
for PMI(OR) at 20 ps and are attributed to the relaxed perylene
singlet excited state PMI* (Figure 2A, dashed line). The 2 ps
spectra for the dyads also show a small bleaching in the
porphyrin Q(1,0) ground-state absorption band at 550 nm (see
Figure 5C), indicating that some energy has moved from the
perylene to the porphyrin. The porphyrin Q(1,0) bleaching at
550 nm is fully developed by 50 ps (Figure 6C), and is
accompanied by the expected (see Figure 5C) small Q(0,0)
bleaching plus stimulated emission at 585 nm and the pro-
nounced Q(0,1) stimulated emission at 650 nm. All of these
features indicate that the spectrum of each of the three dyads
in toluene at 50 ps can be attributed to Zn*. These features
have uniformly and partially decayed by 2 ns (Figure 6D) and
further still by the 3.9 ns limit of the measurement.
The same behavior is also observed for toluene solutions of
all the other dyads employing a diarylethyne linker attached at
the N-imide position of the perylene: p-PMI-aep-Zn, p-PMI-
(OR)3-aep-Zn, and o-PMI(OR)-aep-Fb. The only differences
among these cases are the detailed kinetics (described below)
and differences in the porphyrin excited-state spectra at 50 ps.
The latter reflect the changes in the positions of the ground-
state absorption bands (and thus bleaching features in the
transient spectra) and spontaneous fluorescence bands (and thus
stimulated-emission features in the transient spectra) due to (i)
the number of aryloxy groups on the perylene and (ii) the
porphyrin being a zinc chelate (and formation of Zn*) or the
free base (formation of Fb*).
Figure 6E-H shows representative time-resolved absorption
spectra for the trio of architecturally distinct dyads o-PMI(OR)-
aep-Zn, m-PMI(OR)-aep-Zn, and p-PMI(OR)-aep-Zn in
PhCN. Similar data were also found for p-PMI-aep-Zn,
p-PMI(OR)3-aep-Zn, and o-PMI(OR)-aep-Fb in PhCN. In all
six cases, the spectral evolution from 0.5 to 2 ps and then to 50
ps (Figure 6E-G) is similar to that described above for the
dyads in toluene (Figure 6A-C). Again, the only differences
for a particular compound in PhCN versus toluene are the
detailed kinetic profiles and, for dyads containing a zinc
porphyrin, the positions and relative intensities of the various
bleaching and stimulated-emission features due to coordination
of solvent molecules to the central zinc ion, as expected on
the basis of the ground-state absorption and spontaneous-
fluorescence spectra. For all of these dyads in PhCN except
o-PMI(OR)-aep-Zn, the spectra (due to Zn* or Fb*) at 50 ps
then decay uniformly and partially by 2 ns (Figure 6H, dotted
and dashed lines) and then to the 3.9 ns limit of the measure-
ment. This behavior reflects the ∼2 ns lifetime of Zn* or ∼13
ns lifetime of Fb* obtained from fluorescence decays (Table
3).
Figure6. Time-resolvedabsorptiondifferencespectraforperylene-porphyrin
dyads in toluene and PhCN obtained using 130 fs 490 nm excitation
flashes.
subsequent quenching of Zn* by the perylene. The fact that
this quenching occurs in PhCN but not toluene suggests that
electron transfer is the source of the quenching. The rate constant
and yield (∼30%) for this quenching process will be derived
below.
When the zinc porphyrin of o-PMI(OR)-aep-Zn is replaced
by the free base in analogue o-PMI(OR)-aep-Fb, no quenching
is observed in either toluene or PhCN. Compared to zinc
porphyrins, the free base porphyrins in the dyads have larger
Φf values (0.09) and τS values (∼13 ns) (Table 3), just like the
FbU reference monomer, both in toluene and PhCN. These
results are again indicative of quantitative perylene-to-porphyrin
energy transfer with no indication of excited-state electron-
transfer quenching in nonpolar and polar media.
2. Time-ResolWed Absorption Spectra. Figure 6A-D shows
representative time-resolved absorption spectra for the trio of
architecturally distinct dyads o-PMI(OR)-aep-Zn, m-PMI(OR)-
aep-Zn, and p-PMI(OR)-aep-Zn in toluene. In each case, the
perylene was preferentially excited with a 130 fs flash at 490
nm. The same general behavior is observed in each case. The
spectrum at 0.5 ps (Figure 6A) is basically identical to that for
the benchmark perylene monomer PMI(OR) immediately after
excitation (Figure 2A, solid line) and is thus attributed to the
unrelaxed perylene singlet excited state PMI**. Modest changes
in the spectra occur by 2 ps (Figure 6B) that reflect those seen
The only exception to the behavior for the diarylethyne-linked
dyads in PhCN described above is found in the spectra at times
longer than 50 ps for o-PMI(OR)-aep-Zn in PhCN. In that case,
the spectrum at 2 ns (Figure 6H, solid line) is markedly different
from the others. In particular, the spectrum shows a relatively
broad but pronounced absorption band at ∼620 nm along with
perylene bleaching (520 nm) and porphyrin bleaching (550 nm)
but no porphyrin stimulated emission (e.g., 670 nm). This
spectrum is assigned to the electron-transfer product state
PMI- Zn+, as is described in more detail below along with the
kinetics for formation and decay of this state.
3. Global Kinetic Analysis for Dyads Exhibiting No Electron-
Transfer Quenching. For all of the cases except o-PMI(OR)-
aep-Zn in PhCN (described in the next subsection), global
analysis for each dyad/solvent was performed on the data subset
encompassing 510-680 nm and -20 to 200 ps using the

14260 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
TABLE 5: Summary of Kinetic Data for the
Perylene-Porphyrin Dyads
observed
time
calculated
rate
constantsa
constantsb
-1
-1
τ1
τ2
(kENT1
(ps)
)
(kENT2
(ps)
)
dyad
solvent (ps) (ps)
o-PMI(OR)-aep-Zn toluene 3.1 15
PhCN 2.3 15
m-PMI(OR)-aep-Zn toluene 3.0 16
PhCN 2.8 15
p-PMI(OR)-aep-Zn toluene 2.7
PhCN 1.4
toluene nrc
PhCN 1.8
p-PMI(OR)3-aep-Zn toluene 1.4
PhCN 1.9
7.5
3.6
6.9
5.0
5.5
1.8
nrc
2.5
2.0
2.7
15
15
16
15
9.7
7.2
4.5
3.5
6.2
7.2
9.7
7.2
4.5
3.5
6.2
7.2
p-PMI-aep-Zn
a Time constants measured for the dyad by transient absorption
spectroscopy. Time constant τ1 reflects the decay of the unrelaxed
(upper) perylene singlet excited state (PMI**) by energy transfer in
competition with electronic/conformational/vibrational relaxation to
the lowest excited state (PMI*), and τ2 reflects decay of PMI* by
energy transfer competing with the perylene intrinsic
(monomer-like) decay pathways (fluorescence, internal conversion,
and intersystem crossing); see Figure 8. b The value of kENT1 is
obtained using the formula kENT1 ) (τ1)-1 - krel, where the value of
krel is the inverse of the measured time for PMI** f PMI*
relaxation for the relevant perylene monomer given in Table 4. The
value of kENT2 is obtained using the formula kENT2 ) (τ2)-1
-
-1, where τ
is the PMI* lifetime of the relevant perylene
mon
mon
(τ
)
per
per
monomer given in Table 3. c A fast kinetic component is not
resolved.
convolution of the instrument response with the dual-exponential
function A1 exp(-t/τ1) + A2 exp(-t/τ2) + C. These cases are
m-PMI(OR)-aep-Zn, p-PMI(OR)-aep-Zn, p-PMI-aep-Zn,
p-PMI(OR)3-aep-Zn, and o-PMI(OR)-aep-Fb in toluene and
in PhCN, and o-PMI(OR)-aep-Zn in toluene. A representative
kinetic profile for the latter case is shown in Figure 3C.
The dual-exponential fits to the data set for a given dyad/
solvent give a fast component with a time constant (τ1) that
ranges from 1.4 to 3.1 ps and a slower component with a time
constant (τ2) in the range 3.5-24 ps (Table 5). For dyad
o-PMI(OR)-aep-Zn in toluene, the spectra of the preexponen-
tial-amplitude factors A1 and A2 associated with the two lifetime
components (τ1 ) 3.1 ps, τ2 ) 15 ps) and the constant asymptote
C are shown in Figure 7A; such spectra are referred to as
“amplitude” or “decay-associated” spectra. [Note that a negative
feature (∆A < 0) in these (preexponential) amplitude spectra
(also known as decay-associated spectra) reflects either the decay
of a bleaching or the growth of an excited-state absorption, and
similarly a positive feature reflects either the growth of a
bleaching or decay of an excited-state absorption.] Figure 7C,D
gives static optical data for the benchmark perylene and
porphyrin monomers for comparison. Like the measured 0.5
ps spectrum for this dyad (Figure 6A, solid line), the A1
amplitude spectrum (Figure 7A, red circles) is basically identical
to the 0.5 ps spectrum for the benchmark monomer PMI(OR)
in toluene (Figure 2A, solid line) assigned to PMI**. Similarly,
the A2 amplitude spectrum for the dyad (Figure 7A, blue squares)
is basically identical to the 20 ps spectrum for PMI(OR) (Figure
2A, dashed line). The A2 spectrum shows a net blue shift in the
key features from the A1 spectrum and is assigned to PMI*.
Both the A1 and A2 amplitude spectra contain bleaching of the
perylene ground-state absorption (Figure 7C, black line) at
wavelengths <560 nm, consistent with the assignment to
perylene-associated excited states. The C amplitude spectrum
Figure 7. Amplitude spectra obtained from global analysis of the time
evolution of the absorbance difference spectra (see Figure 6) for dyad
o-PMI(OR)-aep-Zn in toluene (A) or PhCN (B). (C) Ground state
absorption features for the perylene monomer PMI(OR) (black) and
its π-anion radical (red) in PhCN. (D) Ground state absorption (black)
and fluorescence (blue) features for porphyrin monomer ZnU in toluene.
(Figure 7A, open black circles) has the characteristics expected
for Zn*. These features include bleaching of the Q(1,0) ground-
state absorption band at 550 nm (Figure 7D, black line), Q(0,1)
stimulated emission at ∼650 nm that coincides with the
corresponding band in the spontaneous fluorescence spectrum
(Figure 7D, blue line), and a small combined Q(0,0) bleaching
plus stimulated-emission feature at ∼580 nm. The C component
has not decayed appreciably by 200 ps (and is treated as a
constant on this time scale in this analysis) consistent with
the 2.4 ns Zn* lifetime obtained using fluorescence methods
(Table 3). The same analysis applies to m-PMI(OR)-aep-Zn,
p-PMI(OR)-aep-Zn, p-PMI-aep-Zn, p-PMI(OR)3-aep-Zn, and
o-PMI(OR)-aep-Fb in toluene and in PhCN. The only differ-
ences are in the τ1 and τ2 values, which are listed in the fifth
and sixth columns of Table 5.
Analysis of the photodynamics in these dyads is facilitated
by use of the kinetic model presented in Figure 8. In this model,
the unrelaxed perylene excited state PMI** decays by two
pathways: (i) relaxation (electronic/vibrational/conformational)
to the relaxed form PMI* with rate constant krel, and (ii) energy
transfer to the porphyrin with rate constant kENT1. Thus, the
PMI** lifetime is τ1 ) (krel + kENT1)
-1. Subsequently, PMI*

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14261
absorption spectrum at 2 ns (Figure 6H, solid line) that differs
substantially from the other cases (including the same dyad in
toluene) and can simply be ascribed to Zn*. For this case, global
analysis was performed on the data set encompassing 510-680
nm and -20 ps to 3.9 ns using the convolution of the instrument
response with the multiexponential function A1 exp(-t/τ1) +
A2 exp(-t/τ2) + A3 exp(-t/τ3) + A4 exp(-t/τ4) + C. The value
of τ3 was fixed at the 1.4 ns Zn* lifetime determined from
fluorescence decay (Table 3), thereby eliminating one free
parameter in the fit. The time constants returned from the global
fits are τ1 ) 2.3 ps, τ2 ) 15 ps, and τ4 ) 2.7 ns. A representative
kinetic profile for the latter case is shown in Figure 3B.
Figure 8. State diagram indicating the processes that occur in
perylene-porphyrin dyads following photoexcitation. PMI** is the
unrelaxed (vibrationally, conformationally, and/or electronically) singlet
excited state of the perylene. PMI* is the relaxed, lowest energy singlet
excited state of the perylene. The perylene monomer-like decay rate
constant kPMI is dominated (>90%) by fluorescence to the ground state
along with some internal conversion and intersystem crossing. The
porphyrin monomer-like decay rate constant kPOR is dominated (∼80%)
by intersystem crossing to the triplet excited state with some internal
conversion plus fluorescence to the ground state.
In analogy with the analysis for the other cases given above,
the kinetic components for o-PMI(OR)-aep-Zn in PhCN with
lifetimes τ1 and τ2 are assigned to PMI** and PMI*, respec-
tively. In keeping with this assignment, the A1 and A2 amplitude
spectra for (Figure 7B red and blue) are similar to those for the
other cases (Figure 7A). The τ1 and τ2 values for this dyad
together with the values for krel and kPMI for the benchmark
compounds gives kENT1 ) (3.6 ps)-1 and kENT2 ) (15 ps)-1 for
the rate constants for energy transfer from PMI** and PMI*,
respectively, to Zn*.
decays by two pathways: (i) decay to the ground state (and
perhaps a small amount to the triplet excited state) with
combined monomer-like rate constant kPMI and (ii) energy
transfer to the porphyrin with rate constant kENT2. Thus, the PMI*
The Zn* lifetime of 1.4 ns for o-PMI(OR)-aep-Zn in PhCN
is shorter than the value of 2.0 ns for the ZnU reference
porphyrin in this solvent (Table 3), which is consistent with
some electron-transfer quenching. On the basis of the redox
potentials described above, the lowest energy electron-transfer
product would be PMI- Zn+. This state must lie below (or
slightly above and thermally accessible from) Zn* in this case
(Figure 8). The τ3 ) 1.4 ns kinetic component in the global
analysis reflects the decay of Zn* and has the spectral
characteristics expected for formation of PMI- Zn+. The A3
amplitude spectrum (Figure 7B, green triangles) is well repre-
sented by subtraction of the ground-state absorption spectrum
of the PMI(OR) monomer (Figure 7C, black line) from the
spectrum for the PMI(OR) anion prepared electrochemically
(Figure 7C, red line). Expressed in different terms, the negative
feature at 620 nm in the A3 amplitude spectrum represents
formation of the perylene anion band, and the positive feature
at wavelengths <560 nm represents growth of bleaching of the
perylene ground-state absorption. Neither of these features is
present in the measured absorption difference spectra at earlier
times, which are due to Zn* [e.g., 50 ps spectrum in Figure 6G
(solid line)].
lifetime is τ2 ) (kPMI + kENT2)
-1. Because Zn* (formed by
perylene-to-porphyrin energy transfer) is not quenched in the
diarylethyne-linked dyads being considered here (i.e., all cases
except o-PMI(OR)-aep-Zn in PhCN), the perylene-porphyrin
charge-separated state that would result from electron transfer
either lies above Por* (Zn* or Fb* depending on the dyad) and
does not form on thermodynamic grounds or lies below Por*
and does not form because the electron-transfer rate is too slow
compared to the monomer-like decay pathways of the excited
porphyrin. This issue will be considered further in the Discussion
section.
The values for krel are the inverse of the lifetimes obtained
from the time-resolved spectral studies of the benchmark
perylenes (Table 4). The values for kPMI are determined from
decay of fluorescence from the benchmark monomer [∼(5 ns)-1;
Table 3]. These values for krel and kPMI, the values of τ1 and τ2
derived from the global analysis (Table 5), and the formulas
given above allow calculation of the rate constants kENT2 and
kENT1 depicted in Figure 8. For example, for o-PMI(OR)-aep-
Zn in toluene, substitution into the expression for the PMI**
decay, τ1 ) (krel + kENT1) +
-1, gives (3.1 ps) ) [(5.3 ps)-1
kENT1]-1, from which kENT1 ) (7.5 ps)-1 is obtained. These values
indicate that ∼40% of the PMI** decay occurs by energy
transfer to the porphyrin (producing Zn*) and the other ∼60%
Clear features associated with the zinc porphyrin in the A3
amplitude spectrum owing to the PMI Zn* f PMI- Zn+ are
not observed. This is expected because the porphyrin bleaching
present in the measured Zn* spectrum (e.g., 570 nm in Figure
6G, solid) does not change appreciably as PMI- Zn+ forms (e.g.,
570 nm in Figure 6H, solid line). This observation follows
because any Zn* that does not lead to PMI- Zn+ (the yield will
be derived below) will decay by the normal Zn* monomer-like
pathways, namely, ∼80% formation of the excited triplet state
ZnT, which also exhibits ground-state bleaching, with little
ground-state recovery (by fluorescence and internal conversion).
Thus, the PMI Zn* f PMI- Zn+ electron-transfer process is
expected to show little net signal change associated with
porphyrin bleaching, and thus little contribution to the preex-
ponential-factor amplitude spectrum. The features associated
with decay of the Zn* stimulated emission as PMI- Zn+ forms
overlap with the formation of the perylene anion band and are
obscured. Once formed, the PMI- Zn+ state decays by charge
recombination to give the ground state, with the τ4 ) 2.7 ns
lifetime obtained from the global analysis. The A4 amplitude
of the PMI** decay occurs by relaxation to PMI*. Substitution
-1
into the expression for the PMI* decay, τ2 ) (kPMI + kENT2
) ,
gives (15 ps) ) [(4.8 ns)-1 + kENT2
]
-1, from which kENT2 ≈ (15
ps)-1 is obtained. These values indicate that >99% of the PMI*
decay occurs by energy transfer to the zinc porphyrin. The
values of the energy-transfer rate constants obtained in this
manner for o-PMI(OR)-aep-Zn in toluene are listed in the last
two columns of Table 5. The same is true for the other
diarylethyne-linked dyads in toluene or PhCN. The only
exception is p-PMI-aep-Zn in toluene, for which a fast (τ1)
kinetic component (PMI** decay) in the dyad could not be
resolved. For the latter case, the τ2 lifetime of 4.5 ps reflects
essentially quantitative energy transfer from PMI* to the
porphyrin to produce Zn*, in keeping with the findings on the
other diarylethyne-linked dyads.
4. Global Kinetic Analysis for o-PMI(OR)-aep-Zn in PhCN.
The one case not covered in the previous subsection is
o-PMI(OR)-aep-Zn in PhCN. This dyad has a transient-

14262 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
spectrum (Figure 7B, open orange triangles) is essentially a
mirror image of that associated with the formation of this state
from Zn* (Figure 7B, closed green triangles). This relationship
is that expected on the basis of the spectral analysis given above.
The one aspect of the photophysics of o-PMI(OR)-aep-Zn
in PhCN that has not been discussed concerns the rate constant
(kET) and yield (ΦET) for the PMI Zn* f PMI- Zn+ electron-
transfer process. These values can be calculated from the
excited-state lifetimes for this dyad and the ZnU monomer
(Table 3) as follows: kET ) (1.4 ns)-1 - (2.0 ns)-1 ) (4.7 ns)-1
and ΦET ) kET (1.4 ns) ) (4.7 ns)-1 (1.4 ns) ) 0.3. Thus,
although electron-transfer quenching of Zn* occurs, the yield
is only 30%.
E. Photophysical Properties of PMI(OR)2-ep-Zn. All of
the dyads described above contain a diarylethyne linker con-
nected to the N-imide position of the perylene. These dyads all
have absorption spectra that are basically the composite of those
of the reference perylene and porphyrin (e.g., Figure 5). The
situation is different for dyad PMI(OR)2-ep-Zn, in which the
ethyne moiety of the phenylethyne linker is attached to the C9
position of the perylene core (Chart 4). In this case, substantial
deviations from the ground-state absorption spectra of the
component parts are found. These differences are analogous to
those found previously and described above for PMI-ep-Zn
(Chart 3).27 These effects reflect the very strong linker-mediated
perylene-porphyrin electronic interactions.
The porphyrin fluorescence yield of PMI(OR)2-ep-Zn (0.070)
in toluene (using excitation of perylene or porphyrin) is about
twice that of isolated porphyrin ZnU (Table 3), although the
Zn* lifetime is the same (2.4 ns). We have seen similar effects
in prior studies of phenylethyne-linked porphyrins, which can
be attributed to a perturbed porphyrin radiative rate constant
(kf) as a result of the linker-mediated electronic coupling with
the perylene. That Φf is affected much more strongly than τS
follows from the formulas for these two quantities: Φf ) kf/(kf
+ kIC + kISC) and τS ) 1/(kf + kIC + kISC), where kIC and kISC
are the rate constants for decay of the singlet excited state by
internal conversion to the ground state and intersystem crossing
to the triplet excited state, respectively. Because τS is dominated
by kISC, a 2-fold perturbation in kf has little effect. However, a
2-fold perturbation in kf alters Φf by about 2-fold.
The τS for PMI(OR)2-ep-Zn in toluene remains at the value
of 2.4 ns found for ZnU, consistent with essentially no
quenching of Zn* in the dyad compared to the isolated
porphyrin. Thus, the incorporation of the 1- and 6-aryloxy
groups on the perylene in this dyad to enhance solubility has
not had a deleterious effect on the porphyrin photophysics, at
least in nonpolar media. However, the situation is dramatically
different for PMI(OR)2-ep-Zn in PhCN. Now the fluorescence
yield and Zn* lifetime are both substantially reduced (Table
3). The Zn* lifetime in this case is so short that it was
determined by transient absorption measurements, as described
in the following.
Figure 9A gives representative time-resolved absorption
spectra for PMI(OR)2-ep-Zn in PhCN acquired using excitation
of the perylene at 490 nm. The spectrum at 0.5 ps after excitation
is dominated by Zn*. This is evidenced by the Q(1,0) bleaching
at 570 nm and the combined Q(0,0) bleaching and stimulated
emission at ∼620 nm. Thus, energy transfer from the excited
perylene (PMI** and PMI*) to the zinc porphyrin (Figure 8)
occurs in <0.5 ps. By 8 ps, bleaching in the perylene absorption
at wavelengths <560 nm has developed, the Q-band bleachings
of the zinc porphyrin remain, and a broad absorption feature
centered at ∼700 nm has developed. We interpret the latter as
Figure 9. Representative time-resolved absorbance difference spectra
(A) and kinetic profile at 700 nm (B) for dyad PMI(OR)2-ep-Zn
obtained using 130 fs excitation flashes at 490 nm. The fit in part B
represents the convolution of the instrument response plus two
exponentials plus a constant asymptote, giving the time constants
indicated.
the perylene anion, which is red-shifted from its position at ∼620
nm for PMI(OR) in Figure 7C due primarily to the attachment
of the ethyne group to the C9 position of perylene unit. Thus,
the 8 ps spectrum for PMI(OR)2-ep-Zn in PhCN (Figure 9A,
dashed line) is assigned to the electron-transfer product
PMI- Zn+. This state has decayed by 133 ps (Figure 9A, dotted
line). A representative kinetic profile is shown in Figure 9B.
Analysis of such kinetic traces across the spectral region shown
in Figure 9A gives time constants of 6 ( 1 ps (Zn* lifetime)
and 25 ( 3 ps (PMI- Zn+). Compared to the Zn* lifetime of 2
ns in the benchmark porphyrin, the 6 ps lifetime in this dyad
indicates that electron transfer to the perylene is essentially
quantitative for PMI(OR)2-ep-Zn in PhCN.
IV. Discussion
A. Molecular Design. Each perylene employed in this study
is a monoimide construct. The monoimide derivatives were
chosen over the bisimides because the former are less prone to
electron-transfer quenching reactions.27,29 One dyad, p-PMI-
aep-Zn, incorporates a diarylethyne linker which joins the
perylene and porphyrin via the p-position at the two ends of
the linker. This linker motif was chosen because it should afford
diminished through-bond electronic communication, thereby
suppressing electron-transfer quenching reactions. Another dyad,
PMI(OR)2-ep-Zn, utilizes an ethynylphenyl linker at the C9
position of the perylene (which enhances electronic communica-
tion). For this perylene, two aryloxy substituents were incor-
porated both to enhance solubility and to render the chro-
mophore more electron-rich and less susceptible to excited-state
reduction with the adjoining porphyrin. Two other dyads,

Photodynamics of Perylene-Porphyrin Dyads
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14263
p-PMI(OR)-aep-Zn and p-PMI(OR)3-aep-Zn, also incorporate
bis-para attached diarylethyne linkers; however, these dyads
bear one or three aryloxy groups attached to the perylene,
respectively. In two other dyads, m-PMI(OR)-aep-Zn and
o-PMI(OR)-aep-Zn, linker attachment to the porphyrin is at
the meso- or ortho-position of the phenyl group of the
diarylethyne linker. These two linkers also contain a single
aryloxy group at the C9 position of the perylene. Again, the
C9 aryloxy group was incorporated in an effort both to increase
solubility and to render the perylene less susceptible to excited-
state reduction with the adjoining porphyrin.
aep-Zn and m-PMI(OR)-aep-Zn, is that the closer perylene-por-
phyrin distance (Chart 4) would give a larger Coulomb
interaction (-e2/εR). [Here, e is the electron charge, ε is the
dielectric constant of the solvent, and R is the separation of the
charges (e.g., perylene-porphyrin center-to-center distance).]
However, the magnitude of this effect is unclear.
Another reason why electron-transfer quenching may occur
for o-PMI(OR)-aep-Zn and not the other dyads such as
p-PMI(OR)-aep-Zn and m-PMI(OR)-aep-Zn in PhCN is that
the closer distance between the perylene and the porphyrin using
the o-linkage motif would give a larger solvent reorganization
energy, in addition to a possible difference in the free energy
change for the reaction. The Franck-Condon factor and thus
the rate constant for electron-transfer quenching is dictated by
the relationship between these two quantities, which in turn
reflects the relationship between the potential energy surfaces
of Zn* and PMI- Zn+. Furthermore, the perylene and porphyrin
may become sufficiently close in o-PMI(OR)-aep-Zn to allow
substantial orbital overlap. In this case, this “through-space”
interaction would enhance the electron transfer compared to the
other connection motifs, wherein only linker-mediated “through-
bond” transfer can occur.
In the analysis presented in the Results section, it was found
that the PMI Zn* f PMI- Zn+ electron-transfer process for
o-PMI(OR)-aep-Zn in PhCN is rather slow, with a rate constant
of (4.7 ns)-1. When compared to the normal monomer-like
combined decay rate of (2.0 ns)-1 for Zn* in this medium, this
accounts for the modest yield of 0.3 for the process. Thus,
although quenching of Zn* is observed for o-PMI(OR)-aep-
Zn in the highly polar medium PhCN, the extent is only 30%
and leaves a modestly long (1.4 ns) excited-state lifetime for
utilization of the harvested light energy. Additionally, this
modest quenching can be readily eliminated. For example, the
quenching does not occur for this same dyad in toluene because
the reduced solvent stabilization moves the electron-transfer
product state PMI- Zn+ to higher energy. Similarly, quenching
is not observed for o-PMI(OR)-aep-Fb in either toluene or
PhCN because the free base porphyrin is harder to oxidize than
the zinc porphyrin.
D. Phenylethyne-Linked Dyads. The direct attachment of
the ethyne group of the phenylethyne linker to the perylene C9
position (e.g., PMI(OR)2-ep-Zn) greatly increases the rate of
energy transfer from perylene to porphyrin compared to use of
a diarylethyne linker attached to the N-imide of the perylene
(e.g., p-PMI(OR)-ep-Zn), namely, <0.5 ps versus 5-25 ps
depending on the dyad. Because energy transfer is competing
with an ∼5 ns inherent lifetime for the excited perylene, the
energy-transfer yield is essentially quantitative in both systems.
Thus, the enhanced rates for the phenylethyne linker have not
paid a dividend in terms of yield compared to the diarylethyne
linker. The net cost of increasing the energy-transfer rate is that
the perylene ethyne attachment makes the perylene easier to
reduce, and thus the associated dyads more susceptible to
electron-transfer quenching. We had observed this behavior
previously for PMI-ep-Zn (Chart 3)27 and found that the same
is true here for PMI(OR)2-ep-Zn, in which the aryloxy groups
were incorporated for enhanced solubility. Both systems are
excellent light-harvesting motifs in nonpolar media such as
toluene but undergo ultrafast electron-transfer quenching of the
excited porphyrin followed by rapid (tens of picoseconds) charge
recombination to give back the ground state in highly polar
media such as PhCN.
The m- and o-linked dyads differ from the p-linked dyads in
that the perylene lies out of the plane of the porphyrin. An
intriguing possibility is that the architectures of the m- and
o-linked dyads could suppress cofacial aggregation of the
porphyrins, thereby affording increased solubility compared with
that of the porphyrin alone. While the solubility of the dyads is
of little interest, poor solubility often is a major limitation in
the synthesis and characterization of larger multipigment arrays.
Thus, the m- and o-linked dyads were of interest as possible
motifs that could provide advantageous light-harvesting and
solubility features upon incorporation in larger constructs. In
this regard, a set of organic-soluble m-linked perylene-porphyrin
light-harvesting rods has been prepared by oligomerization of
the corresponding perylene-porphyrin monomeric unit.31
B. Efficient Energy Transfer in the Absence of Electron-
Transfer Quenching. In all of the diarylethyne-linked perylene-
porphyrin dyads investigated herein (Charts 4 and 5), energy
transfer from the perylene to the porphyrin (zinc chelate or free
base) is facile and efficient. Even starting in the unrelaxed
perylene excited state PMI**, energy transfer to the porphyrin
is sufficiently fast (∼2-8 ps) that it has a yield of about 50%.
Energy transfer for the relaxed excited state PMI* is also rapid
(∼5-25 ps) and effectively quantitative because it is competing
with the monomer-like decay time of ∼5 ns. Once the energy
reaches the porphyrin, no quenching is observed for all but one
of the diarylethyne-linked perylene-porphyrin dyads studied,
and that quenching occurs only to a modest degree in PhCN.
That one exception is o-PMI(OR)-aep-Zn in PhCN, which is
discussed further below. Thus, all of these dyad motifs are
excellent constructs as ancillary light-harvesting units in syn-
thetic or biohybrid constructs for solar-energy conversion in both
nonpolar and highly polar media. The fact that electron-transfer
quenching does not occur even in highly polar media is quite
favorable for many applications, such as solution-based or solid-
state solar cells where the medium may have a high dielectric
to enhance ion mobility or charge transfer.
C. Modest Electron-Transfer Quenching for o-PMI(OR)-
aep-Zn in PhCN. The one case investigated where a perylene-
porphyrin dyad exhibits quenching of the porphyrin excited state
compared to the isolated porphyrin is o-PMI(OR)-aep-Zn in
PhCN. For this dyad, the electron-transfer product state generi-
cally denoted PMI- Por+ in Figure 8 (PMI- Zn+ in this case)
must lie either below the excited porphyrin generically denoted
Por* in Figure 8 (Zn* in this case) or within thermal energy
(kT) above Por*. For the other diarylethyne-linked dyads, the
exact energetic positioning of PMI- Por+ relative to Por* in
PhCN is not entirely clear. The most straightforward interpreta-
tion for the absence of electron transfer is that PMI- Por+ is
above Por*. Regardless of the details of the energetics of
PMI- Por+ versus Por* in PhCN, it is clear that the former state
lies higher in energy in toluene. A possible reason why
PMI- Por+ might lie lower in energy for o-PMI(OR)-aep-Zn
in PhCN compared to the other dyads, such as p-PMI(OR)-

14264 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Kirmaier et al.
(20) Ahrens, M. J.; Kelley, R. F.; Dance, Z. E. X.; Wasielewski, M. R.
Phys. Chem. Chem. Phys. 2007, 9, 1469–1478.
V. Conclusions
The studies described herein have identified perylene-
porphyrin dyads that represent excellent motifs for use in light-
harvesting architectures. The dyads employ diarylethyne linkers
with attachment to the N-imide position of the perylene-
monomide. These constructs give rapid and quantitative perylene-
to-porphyrin energy transfer and long porphyrin excited-state
lifetimes that are unchanged from those in the isolated porphyrin.
The incorporation of up to three aryloxy groups to impart
increased solubility to the perylene has the added benefit of
making the dyad somewhat less prone to electron-transfer
quenching. A diarylethyne-linked dyad that has close approach
of the perylene and porphyrin due to the use of o-positions of
an aryl ring of the linker also has excellent light-harvesting
characteristics in nonpolar media but suffers from electron-
transfer quenching in polar media. A construct that utilizes a
phenylethyne linker attached to the C9 position of the perylene
gives even faster (<0.5 ps) perylene-to-porphyrin energy transfer
but is also not satisfactory for light-harvesting applications due
to facile electron transfer and ultrafast charge recombination in
polar media. Collectively, the studies described herein provide
concepts and architectures that should be useful in the design
of molecular solar energy conversion systems.
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Acknowledgment. This work was supported by grants from
the Division of Chemical Sciences, Geosciences and Biosciences
Division, Office of Basic Energy Sciences, of the U.S. Depart-
ment of Energy to D.F.B. (DE-FG02-05ER15660), D.H. (DE-
FG02-05ER15661), and J.S.L. (DE-FG02-96ER14632).
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Supporting Information Available: Transient absorption
spectra of perylene monomers, absorption and fluorescence
spectra of perylene monomers in a variety of solvents, and a
table of solvent properties. This material is available free of
charge via the Internet at http://pubs.acs.org.
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1995, 60, 5266–5273.
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