Synthesis and excited-state photodynamics of perylene-porphyrin dyads. 1: Parallel energy and charge transfer via a diphenylethyne linker

Article abstract of DOI:10.1021/jp010335i

The photophysical properties of a perylene-porphyrin dyad have been examined with the aim of using this construct for molecular photonics applications. The dyad consists of a perylene-bis(imide) dye (PDI) connected to a zinc porphyrin (Zn) via a diphenylethyne linker (pep). In both polar and nonpolar solvents, the photoexcited perylene unit (PDI*) decays very rapidly (lifetimes of 2.5 (toluene) and 2.4 ps (acetonitrile)) by energy transfer to the porphyrin, forming PDI-pep-Zn* in high yield (80%, toluene; 70% acetonitrile), and hole transfer to the porphyrin, forming PDI^--pep-Zn⁺ in lesser yield (20%, toluene; 30% acetonitrile). In both toluene and acetonitrile, the Zn* excited state subsequently decays with a lifetime of 0.4 ns primarily (80%) by electron transfer to the perylene (forming PDI^--pep-Zn⁺). In the nonpolar solvent (toluene), the PDI^--pep-Zn⁺ charge-transfer product has a lifetime of > 10 ns and decays by charge recombination primarily to the ground state but also by thermal repopulation of the Zn* excited state. The occurrence of the latter process provides a direct experimental measure of the energy of the charge-separated state. In the polar solvent (acetonitrile), the PDI^--pep-Zn⁺ charge-separated state decays much more rapidly (<0.5 ns) and exclusively to the ground state. In general, the complementary perylene and porphyrin absorption properties together with very fast and efficient PDI*-pep-Zn a?? PDI-pep-Zn* energy transfer suggest that perylenes have significant potential as accessory pigments in porphyrin-based arrays for light-harvesting and energy-transport applications. Furthermore, the finding of fast energy transfer initiated in PDI*, charge-transfer reactions that can be elicited either in PDI* or Zn*, and a charge-separated state (PDI^--pep-Zn⁺) that can be long- or short-lived depending on solvent polarity, indicates the versatility of the perylene-porphyrin motif for a variety of applications in molecular photonics.

Full text of DOI:10.1021/jp010335i

J. Phys. Chem. B 2001, 105, 8237-8248
8237
Synthesis and Excited-State Photodynamics of Perylene-Porphyrin Dyads. 1. Parallel
Energy and Charge Transfer via a Diphenylethyne Linker
Sreedharan Prathapan,†,‡ Sung Ik Yang,§ Jyoti Seth,| Mark A. Miller,† 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: January 25, 2001; In Final Form: June 6, 2001
The photophysical properties of a perylene-porphyrin dyad have been examined with the aim of using this
construct for molecular photonics applications. The dyad consists of a perylene-bis(imide) dye (PDI) connected
to a zinc porphyrin (Zn) via a diphenylethyne linker (pep). In both polar and nonpolar solvents, the photoexcited
perylene unit (PDI*) decays very rapidly (lifetimes of 2.5 (toluene) and 2.4 ps (acetonitrile)) by energy transfer
to the porphyrin, forming PDI-pep-Zn* in high yield (80%, toluene; 70% acetonitrile), and hole transfer to
the porphyrin, forming PDI--pep-Zn+ in lesser yield (20%, toluene; 30% acetonitrile). In both toluene and
acetonitrile, the Zn* excited state subsequently decays with a lifetime of 0.4 ns primarily (80%) by electron
transfer to the perylene (forming PDI--pep-Zn+). In the nonpolar solvent (toluene), the PDI--pep-Zn+
charge-transfer product has a lifetime of >10 ns and decays by charge recombination primarily to the ground
state but also by thermal repopulation of the Zn* excited state. The occurrence of the latter process provides
a direct experimental measure of the energy of the charge-separated state. In the polar solvent (acetonitrile),
the PDI--pep-Zn+ charge-separated state decays much more rapidly (<0.5 ns) and exclusively to the ground
state. In general, the complementary perylene and porphyrin absorption properties together with very fast
and efficient PDI*-pep-Zn f PDI-pep-Zn* energy transfer suggest that perylenes have significant potential
as accessory pigments in porphyrin-based arrays for light-harvesting and energy-transport applications.
Furthermore, the finding of fast energy transfer initiated in PDI*, charge-transfer reactions that can be elicited
either in PDI* or Zn*, and a charge-separated state (PDI--pep-Zn+) that can be long- or short-lived depending
on solvent polarity, indicates the versatility of the perylene-porphyrin motif for a variety of applications in
molecular photonics.
Introduction
Thus, our first-generation molecular photonic wire2 and opto-
electronic gates3 employed a boron-dipyrrin dye input unit,
The development of molecular photonic devices requires the
creation of molecular arrays that absorb light of specific
wavelengths and undergo excited-state energy- and/or charge-
transfer reactions. The versatile optical, redox, and photochemi-
cal properties of porphyrins makes them ideally suited as
components of molecular devices. The ability to introduce pho-
toexcitation at a specific site in a multiporphyrin array requires
the use of accessory pigments. The ideal accessory pigment for
use with porphyrins would exhibit the following features: (1)
absorb strongly in the trough between the B and Q bands, (2)
exhibit a singlet-excited-state lifetime (preferably monophasic)
this is sufficiently long to support highly efficient energy
transfer, (3) undergo energy transfer without deleterious excited-
state quenching reactions, (4) exhibit a high level of stability,
(5) provide compatibility with the synthetic building block
approach, and (6) exhibit sufficient solubility for chemical
processing. In considering the advantages and disadvantages of
various pigments for use with porphyrins, including carotenoids,
coumarins, cyanines, and xanthenes, we originally turned to the
boron-dipyrrin dyes as the most suitable accessory pigment.1
and several light-harvesting arrays were constructed that em-
ployed from one to eight boron-dipyrrin accessory pigments.4
The boron-dipyrrin dyes were found to exhibit a modest
fluorescence yield as well as two excited-state conformers. The
existence of the latter caused the analysis of the excited-state
dynamics of extended arrays to be unduly complicated. We
subsequently considered the use of perylene dyes as accessory
pigments in conjunction with porphyrins. The N,N′-bis(2,5-di-
tert-butylphenyl)-3,4,9,10-perylenedicarboximide dye, for ex-
ample, has strong absorption between the porphyrin Soret (B)
and Q bands (ꢀ ) 58 000 M-1cm-1 at 490 nm in CHCl3),5 a
high fluorescence quantum yield (Φf ∼1),5,6 a long fluorescence
lifetime (∼4 ns),7 and very high photostability.6 Though many
perylene bis(imide) derivatives have poor solubility in common
organic solvents, their solubility is considerably enhanced by
the introduction of bulky substituents.5-9 The absorption and
emission characteristics of perylene-imide dyes are not affected
significantly by the presence of bulky substituents at the imide
nitrogen.10
The perylene dyes have been employed in diverse applications
ranging from paint pigments to use in solar cells.11 In solar cells,
perylene bis(imide) dyes have served multiple functions in
addition to light absorber, including energy-transfer partici-
pant12,13 and charge carrier.14 Despite the wide use of perylene
* To whom correspondence should be addressed.
† North Carolina State University.
‡ Permanent Address: Department of Applied Chemistry, Cochin
University of Science and Technology, Cochin, India 682022.
§ Washington University.
| University of California.
10.1021/jp010335i CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/31/2001

8238 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Prathapan et al.
CHART 1
SCHEME 1
dyes as intense absorbers and energy-transfer donors, perylene
has rarely been employed as an energy-transfer donor in
covalently linked arrays with porphyrins. There is only one study
we are aware of in which a perylene bis(imide) dye and a
porphyrin are linked: Wasielewski’s group prepared a molecular
optical switch comprised of a perylene bis(imide) dye attached
to two free base porphyrins via N-phenyl linkers.15 Excitation
of the porphyrin resulted in electron transfer from the porphyrin
to the perylene. These results showed that a perylene bis(imide)
would participate as an electron acceptor with a porphyrin joined
via a short linker. Perylene bis(imide) derivatives have also been
used in arrays with energy donors,13 other perylenes,16-18
phthalocyanines,19 or carotenoids.20 The only examples of ener-
gy transfer in any type of perylene-porphyrin array that we
are aware of involved a perylene monoimide in a molecular
switch21 devised by Wasielewski’s group and a molecular wire22
that we developed upon extension of the work reported herein.
The molecular switch is comprised of a dialkylamino-bis-
(alkoxy)perylene monoimide derivative attached to a zinc
porphyrin; the perylene has extensive charge-transfer character
and serves primarily as an electrochromic sensor but also can
transfer excited-state energy (∼370 ps) to the zinc porphyrin.
perylene-diimide component with a monoimide, changing the
linker to the phenylethyne unit and altering the site of linker
attachment to the perylene.24 Collectively, these studies along
with Wasielewski’s work15,21,25 lay a foundation for utilizing
tunable perylene-porphyrin units in multichromophore arrays
for a variety of applications in molecular photonics.
We sought to investigate the use of perylene-imide dyes as
accessory pigments (i.e., energy-transfer donors) with porphy-
rins, using a longer linker than in Wasielewski’s phenyl-linked
system15 in an attempt to suppress electron-transfer quenching
while maintaining energy transfer. The perylene-porphyrin
dyads are comprised of a perylene-diimide joined at an N-imide
position to the porphyrin (magnesium, zinc, or free base) via a
diphenylethyne (designated pep) linker (Chart 1). As anticipated,
a variety of excited-state processes are indeed observed for the
perylene-porphyrin dyads. However, the diversity in the
behavior exceeded our expectations and includes (1) energy
transfer from the photoexcited perylene to the porphyrin, (2)
competing hole transfer from the photoexcited perylene to the
porphyrin, (3) electron transfer from the excited porphyrin to
the perylene, (4) charge recombination of transient products
derived from the perylene excited state forming the porphyrin
excited state, and (5) charge recombination of products derived
from the porphyrin excited state either reforming the excited
state or deactivating to the ground state.
Results
Synthesis. A monoethynyl porphyrin (ZnU′) was selected as
the porphyrin building block.26 We have previously employed
the diarylethyne unit to bridge chromophores in various mo-
lecular devices.27 These linkages are semirigid and maintain a
relatively fixed center-to-center distance between chromophores.28
The perylene dye building block was prepared building on the
foundation established by Langhals and co-workers.5,6,8,10,16,17,29
The 2,5-di-tert-butylphenyl group was selected as one N-aryl
substituent to enhance the solubility, chemical stability, and light
fastness.5-8 The N,N′-bis(2,5-di-tert-butylphenyl)-3,4,9,10-
perylenedicarboximide dye (PDI-m)5 was partially hydrolyzed
to give 1 (Scheme 1). Subsequent condensation with 4-iodo-
aniline gave the desired iodo-substituted perylene 2. This route
is superior to the earlier route to this class of compounds which
required successive imidation of perylene-3,4,9,10-tetracarboxy-
lic dianhydride.30,31 The diphenylethyne-linked perylene-por-
phyrin dyad PDI-pep-Zn was prepared by the Pd(0)-mediated
coupling of the iodo- and ethynyl-substituted pigments, using
the conditions developed initially for porphyrin-porphyrin
coupling reactions.26 Thus, reaction of ethynyl porphyrin ZnU′
and the iodo-perylene 2 afforded PDI-pep-Zn in 34% yield
The results of our studies are divided into four articles. In
this first paper, the photoinduced energy- and charge-transfer
processes that occur in either the excited perylene or porphyrin
are discussed for the dyad containing the zinc porphyrin (PDI-
pep-Zn). The second, companion paper extends the studies to
two dyads that incorporate a magnesium or free base (Fb)
porphyrin in place of the zinc porphyrin.23 The differences in
excited-state and redox properties of these three porphyrins
afford considerable latitude in manipulating the energy- versus
charge-transfer processes for specific applications. The third and
fourth articles describe even greater tunability in the rates and
yields of the various excited-state processes upon replacing the
1
following chromatographic purification (Scheme 2). The H
NMR, IR, and mass spectral data were consistent with the
expected structure.
Redox Potentials. The electrochemical data for PDI-pep-
Zn and its monomeric component parts are summarized in Table

Synthesis and Photodynamics of Dyads
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8239
SCHEME 2
TABLE 1: Half-Wave Potentials (E1/2) for the
Perylene-Containing Compoundsa
oxidation potentials
porphyrin
reduction potentials
perylene porphyrin
perylene E1/2 (1) E1/2 (2) E1/2 (1) E1/2 (2) E1/2 (1) E1/2 (2)
PDI-pep-Zn +1.40 +0.52 +0.90 -0.77 -1.06 -1.71 -2.12
PDI-m
+1.36
-0.81 -1.07
a Obtained in butyronitrile containing 0.1 M TBAH. 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.
1. The E1/2 values were obtained with square wave voltammetry
(frequency 10 Hz). The isolated perylene-diimide dyes exhibit
one oxidation wave and two reduction waves in the +1.5 to
-2.0 V range. The E1/2 values of the perylene unit in the PDI-
pep-Zn dyad are quite similar to those of the benchmark PDI-m
monomer and to those found previously for the isolated dye
and related perylene-imide molecules.32,33 The zinc porphyrin
unit in the dyad exhibits two oxidation and two reduction waves
in the +1.5 to -2.0 V range with E1/2 values that are similar to
those of other tetraarylporphyrins.34 These data are indicative
of relatively weak ground-state electronic interactions between
the perylene and porphyrin constituents of the dyad.
Absorption Spectra. The electronic ground-state absorption
spectrum of PDI-pep-Zn in toluene at room temperature is
shown in Figure 1 (panel A, solid spectrum). This figure also
gives the spectra of the ZnU′ and PDI-m reference compounds.
The spectrum of the dyad closely matches the sum of the spectra
of the component parts. In particular, the porphyrin Soret (B)
band of PDI-pep-Zn is essentially unchanged in position or
width (424 nm, fwhm 12 nm) from that of ZnU′, and the same
is true of the weaker porphyrin Q(1,0) and Q(0,0) bands at 550
and 590 nm, respectively. Likewise, the perylene absorption
bands of PDI-pep-Zn are, except for ∼1 nm shifts, superim-
posable on those of PDI-m (consisting of a progression of four
features at 528, 491, 460, and 434 nm of decreasing intensity
that will be denoted the (0,0), (1,0), (2,0), and (3,0) bands,
respectively). The essentially identical spectral features of the
dyad and its component chromophores indicates that the
perylene-porphyrin ground-state electronic interactions are
weak (but not necessarily insignificant), in agreement with the
Figure 1. Electronic absorption spectra (solid) and fluorescence spec-
tra (dashed and dotted) for the dyad PDI-pep-Zn (A) and the
monomers ZnU′ (B) and PDI-m (C) in toluene at room temperature.
The porphyrin absorption in the 450-650 nm region in panels A and
B have been multiplied by the factors shown. The porphyrin emis-
sion in the dyad in panel A was obtained using perylene excitation at
490 nm (dashed) or porphyrin excitation at 420 nm (dotted). All spec-
tra have been normalized to the same amplitude in the respective
regions. The extinction coefficient of the porphyrin Soret band at
424 nm is 468 000 M-1 cm-1 and that for the perylene at the Q(1,0)
band at 490 nm is 43 900 M-1 cm-1. Fluorescence yields are given in
Table 2.
conclusion drawn from the redox data. A very attractive feature
of the PDI-pep-Zn dyad is that the perylene unit absorbs
strongly in the region between the porphyrin Soret and Q bands

8240 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Prathapan et al.
by porphyrin fluorescence (Figure 1A, dashed spectrum). Only
a small amount of PDI* fluorescence is observed (e.g., the (0,0)
emission at 530 nm). Comparison of optically matched solutions
of PDI-pep-Zn and PDI-m (at 490 nm) reveals that the
perylene emission is diminished by ∼1000-fold in the dyad to
Φf ∼ 0.001. Collectively, these results are consistent with sub-
stantial energy transfer from the photoexcited perylene (PDI*)
to the ground-state zinc porphyrin to produce the porphyrin
excited-state Zn*. This process is illustrated in Figure 3.
The PDI-pep-Zn dyad was also illuminated at 420 nm
where the zinc porphyrin absorption is >100-fold that of the
perylene. As expected, emission occurs essentially exclusively
from the porphyrin with only a trace of perylene fluorescence
(Figure 1A, dotted spectrum). However, the yield of Zn*
emission in the dyad is reduced considerably from that in the
reference compound. In particular, comparison of optically
matched solutions of PDI-pep-Zn and ZnU′ in toluene (at λexc
at 420, 550, or 585 nm) shows that the Zn* fluorescence yield
in the dyad in toluene is decreased to 0.010 compared with 0.035
in the monomer (Table 2). The most likely process by which
Zn* is quenched in the dyad relative to the monomer is electron
transfer to the perylene unit to form the PDI--pep-Zn+ state
(Figure 3). The alternative charge-transfer product PDI+-pep-
Zn- (formed by hole transfer from Zn* to the perylene) lies at
significantly higher energy based on the redox data (Table 1).
Direct support for the PDI-pep-Zn* f PDI--pep-Zn+
process is provided by the transient absorption data (vide infra).
The electron-transfer quenching of Zn* to form PDI--pep-
Zn+ will also necessarily occur when Zn* is formed from PDI*
via energy transfer rather than by direct excitation. In particular,
this Zn* quenching process will diminish the observed Zn*
fluorescence yield obtained upon excitation of the perylene or
upon excitation of the porphyrin. Thus, comparison of the Zn*
emission intensity with perylene versus porphyrin excitation will
give a measure of the yield of PDI*-pep-Zn f PDI-pep-
Zn* energy transfer. (The intensity of the Zn emission using
perylene excitation is the product of the quantum yield of energy
transfer from the perylene to the porphyrin and the quantum
yield of emission from the porphyrin.) Predominant excitation
of the perylene at 515 nm gives an apparent emission quantum
yield from Zn* of 0.008, which is 20% lower than the value of
0.010 found with porphyrin excitation (Table 1). These results
indicate that energy transfer from PDI* to the porphyrin has a
quantum yield of ∼80% in the PDI-pep-Zn dyad in toluene.
This value together with the low (<0.1%) PDI* fluorescence
yield in the dyad (and the fact that PDI fluorescence dominates
the inherent decay pathways in this dye) indicate that the other
∼20% of the PDI* decay in the dyad must reflect a pathway
not present in the monomer. The most likely such process is
hole transfer from PDI* to the porphyrin to form PDI--pep-
Zn+ (Figure 3; vide infra).
Figure 2. Room temperature fluorescence spectra. Panel A shows the
spectra in toluene obtained using perylene excitation at 420 nm for the
PDI-pep-Zn dyad (solid), the PDI-m monomer (dashed) normalized
at the (0,0) emission maximum near 530 nm, and the spectrum for the
dyad minus that of the monomer (dotted). The difference spectrum is
virtually identical to the spectrum of ZnU′ (Figure 1B). These data
afford one estimate of the fluorescence yield of the perylene component
of the dyad (see text and Table 2). Panel B compares the fluorescence
spectra of PDI-pep-Zn in toluene (solid) and acetonitrile using
excitation of the porphyrin at 420 nm.
(allowing preferential excitation of the dye) but has minimal
absorption in the Soret region (allowing preferential excitation
of the porphyrin). For example, at 490 nm, the perylene (ꢀ )
43 900 M-1cm-1) absorbs 125 times that of the zinc porphyrin,
whereas at 423 nm, the zinc porphyrin (ꢀ ) 468 000 M-1cm-1
)
absorbs 140 times that of the perylene dye (based on PDI-m
and ZnU′ in toluene).
Fluorescence Spectra and Quantum Yields. The fluores-
cence spectra of PDI-pep-Zn and the ZnU′ and PDI-m
reference compounds in toluene are shown in Figure 1 (dashed
and dotted spectra). Some of these data are reproduced in Figure
2 along with the emission spectrum of the dyad in acetonitrile.
In the following, emphasis is given to the results in toluene.
For both monomers, the emission features are in approximate
mirror symmetry to the corresponding absorption features. The
fluorescence spectrum of ZnU′ consists principally of the Q(0,0)
and Q(0,1) bands at 595 and 645 nm, respectively (Figure 1B).35
The fluorescence spectrum of PDI-m consists of a progression
extending from the (0,0) band near 530 nm through the (0,3)
band at 700 nm (Figure 1C). The fluorescence quantum yield
of PDI-m in toluene is found to be Φf ) 0.97, in excellent
agreement with the literature value of 0.96.6 The fluorescence
yield of ZnU′ in toluene is 0.035.35 These values are listed in
Table 2.
Very similar fluorescence behavior is found for PDI-pep-
Zn in acetonitrile or dimethyl sulfoxide (Table 2). The major
difference compared to toluene is that the positions and intensity
ratio of the porphyrin fluorescence bands are changed in the
polar solvents (Figure 2B). These effects are attributable to
coordination of a solvent molecule to the central zinc ion of
the porphyrin.
Fluorescence Lifetimes. The lifetime of the Zn* excited state
of PDI-pep-Zn was measured by fluorescence modulation
(phase shift) spectroscopy. The dyad in toluene exhibits a dual-
exponential fluorescence decay (λexc ) 420 or 550 nm). The
lifetime components are 0.4 ( 0.1 and 10 ( 2 ns with a 80/20
amplitude ratio. Only the shorter lifetime component is observed
The emission of PDI-pep-Zn was monitored using excita-
tion at several wavelengths where either the perylene or the
porphyrin primarily absorbs. Upon illumination of PDI-pep-
Zn in toluene at 490 nm, where the perylene absorption is >100-
fold that of the porphyrin, the emission spectrum is dominated

Synthesis and Photodynamics of Dyads
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8241
TABLE 2: Photophysical Properties of PDI-pep-Zn and Reference Monomersa
perylene
excited-state decay
porphyrin
excited-state decay
CSS decayh
τ (ns)e
Φff
ΦET
τ(ns)
c
d
g
compound
Monomers
solvent
τ (ps)
Φfb
ΦENT
ΦHT
ZnU′
toluene
CH3CN
toluene
2.4
0.035
0.05i
2.2i
PDI-m
3600
0.97j
Dyad
PDI-pep-Zn
toluene
CH3CN
2.5
2.4
0.001k
0.001
0.8l
0.7
0.2m
0.3
0.4n
0.5n
0.008o
0.007q
0.8p
0.8p
>10
<0.5
a All data were taken at room temperature. The errors in the lifetimes are (10% as are those for the fluorescence yields except for values <0.01,
which have errors (0.001. b Perylene fluorescence yield. The value for PDI was measured as in footnote f and those for the dyads relative to
PDI-m, using perylene excitation at 490 nm. c Yield of energy transfer to the porphyrin. d Yield of hole transfer to the porphyrin. e The lifetime for
ZnU′ is (0.2 ns (determined by fluorescence decay), and those in the dyads are (0.1 ns (average of fluorescence lifetime and transient absorption
measurements). f Porphyrin fluorescence yield referenced to a value of 0.033 for zinc tetraphenylporphyrin (ZnTPP),44 obtained using porphyrin
Soret excitation. g Yield of electron transfer to the perylene. h Decay of the charge-separated species PDI--pep-Zn+. i We have obtained a range
of values for the lifetime and fluorescence yield for ZnU′ and related molecules such as zinc tetraphenylporphyrin in acetonitrile and other coordinating
solvents, affording average values of τ ) 2.2 ( 0.6 ns and Φf ) 0.05 ( 0.01. These values imply that the radiative rate kf is somewhat greater in
these solvents than in toluene, as is inferred also from the stronger Q(0,0) band in the absorption and emission spectra in the coordinating solvents.
j Essentially the same value was measured previously in a variety of solvents. k Composite value from measurements using reduction in perylene
emission in dyad versus the PDI-m monomer, integrated PDI versus porphyrin emission in the dyad, and reduction in the PDI* lifetime in the dyad
versus the monomer. l The same value was deduced from the energy-transfer yield obtained from the yield of porphyrin fluorescence with perylene
versus porphyrin excitation and 1 - ΦHT in the following column. m The same value is obtained from the amplitude of the transient absorption
features and from 1 - ΦENT in the previous column. n Composite value from transient absorption and fluorescence lifetime measurements, with the
latter also containing a slower, delayed component. o The measured emission yield determined relative to the ZnTPP monomer using porphyrin
excitation is 0.010. However, this value contains a 20% contribution of delayed fluorescence, so the actual yield of prompt fluorescence is 0.008.
A yield of 0.008 was also determined using perylene excitation, and this value similarly must be corrected down by 20% to ∼0.006. The value with
perylene excitation is lower than that obtained from porphyrin excitation because of hole transfer (20%) competing with energy transfer (80%) to
the porphyrin. p Similar values were obtained from the reduction in porphyrin fluorescence yield in the dyad versus monomer, the porphyrin lifetime
in the dyad versus the monomer, and the amplitudes of the features in the transient absorption spectra. q The same result is found in dimethyl
sulfoxide.
emission decay in the dyad in toluene can be ascribed to delayed
fluorescence resulting from thermal repopulation of Zn* via
charge recombination of PDI--pep-Zn+ (Figure 3). The
repopulation process and thus the magnitude of the slow
fluorescence component is greatly diminished as the charge-
separated species is energetically stabilized below that of Zn*
in the more polar media. (3) The yield of prompt fluorescence
from Zn* in PDI-pep-Zn in toluene is 0.008 (with porphyrin
excitation) because the measured value of 0.010 contains a 20%
contribution from the delayed component (Table 2). This low
fluorescence yield is essentially the same as that found in
acetonitrile or dimethylsolfoxide, validating this analysis and
giving consistency to the results.
Figure 3. State diagram and kinetic scheme for the photoinduced
Time-Resolved Absorption Spectra. Transient absorption
spectra were acquired for PDI-pep-Zn in toluene and in
acetonitrile. The spectral changes were measured at time delays
extending to about 3.5 ns following excitation of either the
perylene or the porphyrin with a 130 fs flash. The assignments
processes for the PDI-pep-Zn dyad in toluene and acetonitrile. The
excited-state energies were obtained from the static absorption and
emission spectra and the energy of the charge-separated state from the
delayed fluorescence data (see text). The lifetimes and yields are given
in Table 2 and the rate constants in Table 3. Note the scale break on
the energy (eV) axis.
of the absorption difference spectra (Atransient-state - Aground-state
)
TABLE 3: Summary of Kinetic Data for the PMI-pep-Zn
are aided by comparison of the features with those in the static
absorption and fluorescence spectra given in Figure 1. Consider
first the spectra observed using excitation of PDI-pep-Zn in
toluene at 550 nm, which primarily excites the porphyrin
component (Figure 4B). The spectrum at 0.2 ps after the flash
can be ascribed predominantly to the photoexcited porphyrin
Zn*. The spectrum contains bleaching of the zinc porphyrin
Q(1,0) ground-state absorption band at 550 nm and of the
weaker Q(0,0) band at 590 nm. The feature at 590 nm in the
transient spectrum is ∼50% Q(0,0) stimulated emission from
Zn* (emission stimulated by the white-light probe pulse that
occurs at approximately the position of the corresponding
static-fluorescence band). The trough at 650 nm is the corre-
sponding Q(0,1) stimulated emission band. The ratios of all of
these features are those expected based on the static optical
Dyada
-1
(k0)-1 (k1)-1 (k2)-1 (k3)-1 (k-3
)
(k4)-1 (k5)-1
solvent
(ns)
(ps)
(ps)
(ns)
(ns)
(ns)
(ns)
toluene
CH3CN
3.6
3.6
3.1
3.4
12.5
8.0
0.5
0.5
∼1.5
2.4
2.2
>10
<0.5
NA
a The rate constants refer to the processes shown in Figure 3.
for PDI-pep-Zn in acetonitrile or dimethyl sulfoxide. Three
points regarding these observations are noteworthy. (1) The
shortening of the Zn* lifetime to 0.4 ns for the PDI-pep-Zn
dyad from 2.4 ns for the ZnU′ monomer, like the corresponding
diminution in the fluorescence yield, is indicative of a process
(such as electron transfer from Zn* to the perylene) that is not
present in the monomer. (2) The 10 ns component to the Zn*

8242 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Prathapan et al.
ponent (Figure 4A). In this case, we expect to see the behavior
just described for Zn* but preceded by behavior associated with
PDI*. The spectrum at 0.2 ps (solid) is due predominantly to
the photoexcited perylene PDI*. This assignment is made on
the following basis. The spectrum contains bleaching of the
perylene (0,0) and (1,0) ground-state absorption bands near 530
and 490 nm (see Figure 1C). About one-half of the negative-
going feature near 530 nm also derives from PDI* (0,0)
stimulated emission, and the troughs at 580 and 635 nm
correspond to (0,1) and (0,2) stimulated emission (compare with
monomer fluorescence spectra in Figure 1C). The positive
absorption at ∼690 nm derives from PDI*.
As time proceeds, the PDI* spectrum decays into the transient
difference spectrum shown at 20 ps in Figure 4A. This spectrum
is due predominantly to Zn* formed via energy transfer from
PDI*. This assignment is based on the spectral characteristics
expected for Zn* and also found, as discussed above, upon direct
excitation of the porphyrin component of the dyad (see the 0.2
ps spectrum in Figure 4B). However, the 20 ps spectrum
produced by decay of PDI* also contains features associated
with the perylene component. The latter features include the
same characteristics (bleaching in the regions of the ground-
state bands at 530 and 490 nm, no PDI* stimulated emission,
and a broad absorption band at 710 nm) that were assigned
above to the PDI--pep-Zn+ species formed (with τ ∼ 400
ps) from Zn* upon direct porphyrin excitation (see the 1 ns
spectrum in Figure 4B). Thus, the perylene-associated spectral
features in the 20 ps spectrum in Figure 4A must represent a
small amount of PDI--pep-Zn+. In this case, the PDI--pep-
Zn+ species forms in <20 ps by hole transfer from PDI* to the
porphyrin, in parallel with energy transfer from PDI* to the
porphyrin to form Zn* (Figure 3). On the basis of characteristics
such as the magnitude of the 530 nm perylene bleaching
(referenced to the broad background transient absorption) at 20
ps versus 0.2 ps, the PDI*-pep-Zn f PDI--pep-Zn+ hole-
transfer process in toluene has a yield of 20%. Thus, the
predominant (80%) decay pathway of the photoexcited perylene
is the PDI*-pep-Zn f PDI-pep-Zn* energy-transfer pro-
cess. Subsequently, the spectral features at 20 ps associated with
Zn* evolve into those shown at 1 ns (Figure 4A). This evolution
includes a dramatic increase in the perylene-associated char-
acteristics that can be assigned to the PDI--pep-Zn+ species,
as described above for the spectrum taken 1 ns after direct
porphyrin excitation (Figure 4B). Subsequently, the PDI--pep-
Zn+ spectrum does not change appreciably over the 3.5 ns time
scale of the transient absorption measurements.
Figure 4. Time-resolved absorption difference spectra for PDI-pep-
Zn in toluene at room temperature obtained using predominant
excitation of the perylene unit at 490 nm (A) or the porphyrin at 550
nm (B) with a 130 fs flash.
spectra in Figure 1B. The 0.2 ps transient spectrum also contains
small features at 530 and 490 nm that can be assigned to
bleaching in the perylene (0,0) and (1,0) ground-state absorption
bands, respectively (see Figure 1C). The latter features are
present because of minor direct excitation of the perylene
component.
As time progresses, the Zn* spectral features give way to
the absorbance changes shown at 1 ns in Figure 4B. This
spectrum retains bleaching of the porphyrin ground-state bands
at 550 and 590 nm, but the stimulated emission features are no
longer present (evident by the ratio of the features). These
findings indicate that the zinc porphyrin has not returned to the
ground state but is no longer in the emissive Zn* excited state.
The nature of the species present at 1 ns is revealed by the
presence of substantial bleaching in the perylene (0,0) and (1,0)
ground-state absorption bands at 530 and 490 nm, respectively.
Because the corresponding perylene stimulated emission features
are absent (see the 0.2 ps spectrum in Figure 4A where these
features contribute at 530 and 580 nm), the perylene component
of the dyad is not in the PDI* excited state at 1 ns. Instead, the
broad transient absorption with a distinct maximum near 710
nm indicates that the perylene unit is in the anionic form PDI-.33
Thus, the 1 ns spectrum in Figure 4B can be assigned as the
PDI--pep-Zn+ species formed by electron transfer from Zn*
to the ground-state perylene unit (Figure 3). A yield of 80%
for the latter process is estimated from the amplitude of the
PDI bleachings at 490 and 530 nm in the 1 ns spectrum relative
to the amplitude of the bleachings in the 0.2 ps Zn* spectrum.
The time evolution of the spectral features gives a Zn* lifetime
of 400 ( 50 ps for PDI-pep-Zn in toluene (Table 2; vide
infra).
Representative kinetic data for PDI-pep-Zn in toluene
associated with the spectra in Figure 4a are shown in Figure 5.
A PDI* lifetime of 2.5 ps is found from the average time
constant determined from the evolution of the spectral features
associated with either the perylene or the porphyrin. These
features include the formation of the Zn* absorption at 465 nm,
substantial decay of bleaching in the perylene (1,0) ground-
state absorption band at 490 nm (together with formation of
Zn* absorption), and decay of the perylene (0,1) stimulated
emission at 570 nm. The subsequent evolution of the absorbance
changes are associated with the decay of Zn*, which has an
average time constant of 370 ( 70 ps (insets to Figure 5). These
features include decay of the Zn* excited-state absorption at
465 nm and formation of perylene bleachings (because of
PDI--pep-Zn+) at 490 and 530 nm. The reason the associated
time constants appear somewhat wavelength dependent may
derive from the equilibrium involving the Zn* and PDI--pep-
Zn+ species depicted in Figure 3 (which is also reflected in the
Consider next the transient absorption data for PDI-pep-
Zn in toluene obtained upon excitation of the perylene com-

Synthesis and Photodynamics of Dyads
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8243
Figure 6. (A) Time-resolved absorption difference for PDI-pep-Zn
in acetonitrile at room temperature obtained using predominant excita-
tion of the perylene unit at 490 nm. The inset gives an expanded view
of the data at 20 ps and 2.8 ns on the same wavelength scale as the
main panel. (B) Kinetic data for the growth of the Zn* transient
absorption at 460 nm and its subsequent decay. The dual exponential
fit (solid line) gives time constants of 2.6 ( 0.2 and 390 ( 30 ps for
the two components. The inset shows data at 530 nm and a single-
exponential fit with a time constant of 2.3 ( 0.2 ps. (These values and
those obtained at other wavelengths afford the average values given in
Table 2.)
Figure 5. Kinetic profiles at selected wavelengths for PDI-pep-Zn
in toluene using predominant excitation of the perylene component at
490 nm. For the insets, which emphasize the longer-time behavior, data
before and during the excitation flash are not shown for clarity;
the ∆A ) 0 line is the horizontal axis in panel A and the horizontal
line in panels B and C. The solid lines through the data are fits using
a dual-exponential function giving the time constants shown. (Average
values for the time constants at these and other wavelengths are given
in Table 2.)
slow component to the Zn* fluorescence decay noted above).
Similar results are found when the porphyrin is excited directly
(Zn* lifetime of 400 ( 50 ps). Note that a Zn* lifetime of ∼400
ps derived from the transient absorption data is in good
agreement with the value of 400 ( 100 ps for the fast, primary
component to the Zn* fluorescence decay. Following the decay
of Zn*, the absorbance changes associated with PDI--pep-
Zn+ do not decay on the time scale of these measurements,
indicating a lifetime for the charge-transfer product of >10 ns
(Figure 5, insets).
Transient absorption measurements were also performed on
PDI-pep-Zn in acetonitrile. The spectra at 0.2 and 20 ps after
excitation of the perylene at 490 nm (Figure 6A) are very similar
to those obtained under similar conditions in toluene (Figure
4A) and can be similarly assigned. The 0.2 ps spectrum in
acetonitrile can be ascribed to PDI*, which is found to decay
with a lifetime of 2.4 ( 0.2 ps (similar to the 2.5 ps value in
toluene) from the average of measurements at several wave-
lengths (see Figure 6B). Following PDI* decay, the spectrum
at 20 ps has features that can be assigned to a combination of
Zn* (decreased slightly to 70% from 80% in toluene) and
PDI--pep-Zn+ (increased to 30% from 20% in toluene).
However, the subsequent behavior has both similarities and
differences from that found in toluene. The main difference is
that the 20 ps spectrum in acetonitrile decays uniformly to zero
with a time constant of 400 ( 100 ps (Figure 6), whereas in
toluene the spectrum assigned to PDI--pep-Zn+ increased in
magnitude over this time frame and then remained unchanged
to the 3.5 ns limit of the measurements. The 0.4 ns decay time
in acetonitrile is similar to the Zn* lifetime of 0.5 ( 0.1 ns
determined from fluorescence decay in this solvent (and to the
Zn* lifetime of 0.4 ns found in toluene). Again, these values
are much shorter than the excited-state lifetime of monomeric
zinc porphyrins (Table 2). Collectively, these results suggest
the following: (1) The Zn* lifetime in the dyad is dominated
by electron transfer to form PDI--pep-Zn+ in both solvents
(80% yield as described above in toluene). (2) PDI--pep-
Zn+ decays by charge recombination to the ground state in <0.5
ns in acetonitrile, compared to the >10 ns lifetime of this species
in toluene.
Discussion
The state diagram and kinetic model in Figure 3 summarizes
the primary photophysical processes observed for PDI-pep-
Zn in toluene and acetonitrile. The microscopic rate constants
for the various processes can be obtained in terms of this model
using the corresponding yields and the lifetimes of the transient
states derived from the time-resolved and static optical measure-
ments (Tables 2 and 3). In the following, we draw together the
various results to (1) verify their consistency, (2) derive the
kinetic parameters, and (3) allow insights into the mechanisms
of photoinduced processes in this dyad. These considerations
and the resulting conclusions not only give a comprehensive
description of the excited-state pathways operable in the PDI-
pep-Zn array but also form a benchmark for the varied behavior
observed for the other perylene-porphyrin dyads described in
subsequent articles, including the following paper.

8244 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Prathapan et al.
The lowest excited singlet state of an isolated chromophore
such as the perylene-diimide (PDI*) or the zinc porphyrin (Zn*)
decays by fluorescence and internal conversion to give the
ground electronic state, as well as intersystem crossing to form
the excited triplet state. The rate constants for these three
processes are kf, kic, and kisc, respectively. Thus, the isolated
perylene and porphyrin chromophores have a lifetime of their
respective lowest excited singlet state, τm, and a fluorescence
yield, Φmf , given by eqs 1 and 2:
with direct porphyrin excitation, 0.008. These considerations
demonstrate the consistency of the various measurements and
combine to show that the fate of the photoexcited perylene unit
in the PDI-pep-Zn dyad is overwhelmingly dictated (>99%)
by processes involving the porphyrin that are not operable in
the isolated perylene dye.
The fluorescence and transient absorption data further indicate
that the primary processes of the perylene excited state in the
dyad are energy transfer (PDI*-pep-Zn f PDI-pep-Zn*)
and hole transfer (PDI*-pep-Zn f PDI--pep-Zn+). These
processes have rate constants k1 and k2, respectively (Figure
3). Thus, k1 + k2 . k0. Furthermore, energy-transfer from PDI*
to the porphyrin dominates over hole transfer (k1 > k2). In
particular, a yield of 80% for the PDI*-pep-Zn f PDI-pep-
Zn* energy-transfer process is estimated from the Zn* fluores-
cence yield in the dyad obtained using predominantly perylene
excitation versus that with porphyrin excitation (Table 2). This
finding implies a corresponding yield of 20% for the PDI*-
pep-Zn f PDI--pep-Zn+ hole-transfer process. The same
20% yield of hole transfer is estimated from the amplitudes of
the features in the transient absorption spectra, again indicating
an 80% energy-transfer yield. Using the model in Figure 3, these
yields and the measured PDI* lifetime of 2.5 ps for PDI-pep-
Zn in toluene give k1/(k1 + k2) ) 0.8, k2/(k1 + k2) ) 0.2, and
k1 + k2 ) (2.5 ps)-1. Thus, the rate constants of the parallel
energy- and hole-transfer pathways of PDI* are k1 ) (3.1 ps)-1
and k2 ) (12.5 ps)-1 (Table 3). Generally similar values are
obtained from the analysis of the measurements on PDI-pep-
Zn in acetonitrile, except for a slightly faster hole-transfer rate
for PDI* than in toluene. These rate constants again reflect the
fact that energy transfer to the porphyrin is the primary fate of
the photoexcited perylene dye in the PDI-pep-Zn dyad.
τm ) (kf + kic + kisc)-1
Φmf ) kf /(kf + kic + kisc)
(1)
(2)
It is reasonable to assume that the rate constants for the inherent
decay pathways of PDI* and Zn* in the PDI-pep-Zn dyad
are well approximated by those for the isolated monomers. This
view is supported by the findings that the redox potentials and
static optical spectra of the dyad are essentially given by (the
sum of) those of the isolated reference compounds, indicating
essentially no perturbation of the perylene because of the
presence of the porphyrin (and linker) and vice versa. However,
in the dyad, there are additional decay channels for the excited
states not possible in the monomer. The fluorescence and
transient absorption data indicate that these channels for the
PDI-pep-Zn dyad are energy transfer, hole transfer, and
electron transfer (rate constants kENT, kHT, and kET, respectively).
The rate constant by which either the PDI* or Zn* excited state
in the dyad decays by one or a combination of these quenching
processes can be denoted kq. Thus, in analogy with eqs 1 and
2 for the isolated pigments, the lifetime of the lowest excited
singlet state, τ, and the fluorescence yield, Φf, of either PDI*
or Zn* in the dyad are given by eqs 3 and 4:
Photodynamics of the Excited Porphyrin. The Zn* excited
state of PDI-pep-Zn can be formed via energy transfer from
the excited perylene (in 80% yield) or via direct excitation. As
expected, the course of decay of Zn* is identical in the two
cases. The Zn* lifetime in the isolated zinc porphyrin ZnU′ is
τm ) 2.4 ns. Following the logic given above (negligible change
in the inherent decay rates of the porphyrin), k4 ) (2.4 ns)-1
for the PDI-pep-Zn dyad (Figure 3 and Table 3). The primary
fate of the Zn* in the isolated chromophore is intersystem
crossing (Φmisc ∼ 0.9) with only a small fluorescence contribu-
tion (Φmf ) 0.035). The yield of prompt fluorescence yield is
reduced in the dyad (Φf ) 0.008). This reduction occurs not
because of energy transfer to the perylene (which is energetically
uphill) but rather because of PDI-pep-Zn* f PDI--pep-
Zn+ electron transfer. A yield of 77% for this latter process is
τ ) (kf + kic + kisc + kq)-1
Φf ) kf /(kf + kic + kisc + kq)
(3)
(4)
The yield of the energy-transfer pathway, for example, is
ΦENT ) kENT /(kf + kic + kisc + kq)
(5)
where again kq is the rate constant for the energy-transfer
channel plus that for any other process (hole or electron transfer)
not operable in the monomer that contributes to the dynamics
of the associated excited state in the dyad.
Photodynamics of the Excited Perylene in PDI-pep-Zn.
The excited state of the isolated perylene dye (PDI*) has a
lifetime τm ) 3.6 ns. Thus, k0 ) (3.6 ns)-1 in the kinetic model
in Figure 3 for PDI-pep-Zn, following the considerations given
above concerning minimal perturbation of the inherent properties
of the perylene in the array. The isolated perylene dye decays
almost exclusively by fluorescence (Φmf ) 0.97). Although
fluorescence remains the major contributor to k0 in the dyad,
the yield is reduced dramatically to Φf ) 0.001, as determined
by direct measurement of this quantity. The same value (Φf )
0.001) is calculated from the corresponding reduction in the
PDI* lifetime in the dyad (τ ) 2.5 ps in toluene) versus the
monomer via the expression Φf ) Φmf τ/τm (obtained by
manipulation of eqs 1-4). A similar value (Φf ) 0.0015) is
obtained from the product of the following three factors: (a)
the ratio of integrated perylene versus porphyrin fluorescence
spectra measured with perylene excitation, 0.26, (b) the
measured yield of perylene-to-porphyrin energy transfer, 0.8,
and (c) the yield of porphyrin prompt fluorescence measured
calculated using the expression ΦET ) 1 - Φf/Φm (derived
f
from manipulation of eqs 2, 4, and 5). The yield of this electron-
transfer process is also measured to be 80% from the amplitudes
of the features in the transient absorption spectra (Figure 3B).
A yield of 83% also can be calculated using the Zn* lifetime τ
) 0.4 ns in the dyad (composite value from fluorescence and
transient absorption) and the lifetime τm ) 2.4 ns in the
monomer using the expression ΦET ) 1 - τ/τm (derived from
manipulation of eqs 1, 3, and 5). Thus, there is consistency in
the various results regarding the yield of the electron-transfer
pathway of Zn* in the dyad (80%), which dominates the excited-
state dynamics.
The rate constant (k3) for the PDI-pep-Zn* f PDI--pep-
Zn+ electron-transfer reaction can be obtained by several means.
The most rigorous is to utilize the expressions for the time-
evolving populations of Zn* and PDI--pep-Zn+ derived for
the kinetic model in Figure 3, which includes thermal repopu-

Synthesis and Photodynamics of Dyads
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8245
CHART 2
minimal contribution of the Fo¨rster through-space (TS) process.
The empirical evidence for this assessment includes the
dependence of the rate on factors such as porphyrin orbital
characteristics and porphyrin-linker torsional constraints that
primarily modulate the linker-mediated inter-porphyrin elec-
tronic coupling. Additionally, the calculated Fo¨rster rates are
far slower than the measured values. For example, the measured
rate of energy transfer from Zn* to Fb in the Zn-pep-Fb dyad
is (24 ps)-1, which is ∼30-fold faster than the calculated Fo¨rster
rate of ∼(750 ps)-1 (Table 4).37 The measured rate is even faster
for energy transfer from Zn* to Mg in the analogous dyad Zn-
pep-Mg (9 ps)-1 even though the calculated Fo¨rster rate is
slower than for Zn-pep-Fb because of poorer spectral overlap
(Table 4). Indeed, faster energy transfer in Zn-pep-Mg versus
Zn-pep-Fb can be traced largely to enhanced TB coupling
derived from the differences in the electron density in the
molecular orbitals at the site of linker attachment in the Mg
versus Fb porphyrin acceptor.40 A Fo¨rster calculation for PDI-
pep-Zn using the same formalism (Appendix B) gives a TS
rate of (17 ps)-1, which is 5.5-fold slower than the experimental
value of (3.1 ps)-1 (Tables 3 and 4). This comparison suggests
that the TS mechanism makes a modest contribution to the
energy transfer from the photoexcited perylene to the porphyrin
in PDI-pep-Zn but that the TB mechanism again plays the
principal role. This behavior is consistent with that in a related
dyad BDPY-pep-Zn, in which energy transfer occurs from a
boron-dipyrrin pigment to a zinc porphyrin via a diphenyl-
ethyne linker (Chart 2).4 In the latter dyad, energy transfer occurs
with rate constants of ∼(20 ps)-1 and ∼(2 ps)-1 (70:30 amp-
litude ratio) that are associated with two excited states produced
by a photoinduced conformational change in the BDPY chro-
mophore, a process that also exists in the isolated dye.4
lation of the former species from the latter (see Appendix A).
This close equilibrium is indicated by the finding of a 10 ns
(delayed) component to the Zn* fluorescence decay in toluene,
in addition to the dominant 0.4 ns faster (prompt) component.
In order for the kinetic model to reproduce the data, a rate
constant k3 ∼ (0.5 ns)-1 for the PDI-pep-Zn* f PDI--pep-
Zn+ electron-transfer process is required. (Similar values of k3
are obtained in a more straightforward manner from the lifetime
or fluorescence data and expressions obtained by manipulation
of eqs 1-5 that ignore the thermal repopulation process in
Figure 3.36) This modeling also returns a rate constant of the
reverse process of k-3 ∼ (1.5 ns)-1. This value implies that
PDI--pep-Zn+ lies only ∼kT below Zn*, which must be the
case to observe the delayed component to the Zn* decay. The
modeling also indicates that charge recombination of PDI--
pep-Zn+ to directly give the ground state (and perhaps the
triplet excited state of the perylene) may have a rate constant
k5 ∼ (50 ns)-1. However, given the observed lack of decay of
the charge-transfer product over the 3.5 ns span of the transient
absorption measurements, a more conservatively estimate of k5
< (10 ns)-1 is more appropriate. For PDI-pep-Zn in aceto-
nitrile, the same primary decay processes of Zn* are operable,
although the rates and yields differ. The most notable of these
is that the decay of PDI--pep-Zn+ is measured to be much
faster than in toluene (Table 3). Additionally, thermal repopu-
lation of Zn* from PDI--pep-Zn+ is negligible (k-3 , k3),
and k3 is estimated simply from the lifetime and fluorescence
data (Table 3).
Collectively, these results and considerations lead to a self-
consistent description of the primary excited-state processes in
the PDI-pep-Zn dyad, as summarized in Figure 3 and Tables
2 and 3. The dynamics are dominated by energy transfer from
the photoexcited perylene diimide to the zinc porphyrin (80%
yield) on the time scale of a few picoseconds and subsequent
electron-transfer back from the excited porphyrin to the perylene
(80% yield) on the time scale of several hundred picoseconds.
Mechanisms of Energy and Electron Transfer. We have
carried out extensive studies of excited-state energy transfer
between porphyrins joined by the same diphenylethyne linker
used in the perylene-porphyrin dyad PDI-pep-Zn.37-40 One
example is Zn-pep-Fb (previously denoted ZnFbU), which
contains zinc and free base porphyrins (Chart 2). We found that
porphyrin-porphyrin energy transfer in this architecture is
dominated by a through-bond (TB) mechanism, with only a
Implications for Use of the PDI-pep-Zn Dyad in Mo-
lecular Photonic Devices. Comparison of the results described
above for the PDI-pep-Zn and BDPY-pep-Zn dyads delin-
eates several advantages of the perylene-diimide dye versus
boron-dipyrrin pigment as a light-input unit in porphyrin-based
arrays. These favorable characteristics include faster rates of
energy transfer, single-exponential excited-state behavior, and
enhanced absorption in the spectral window between the
porphyrin Soret and Q bands. A drawback of the PDI-pep-
Zn unit is that the yield of energy transfer from PDI* to the
porphyrin is not quite as high (80%) as that which occurs from
BDPY* to the porphyrin in BDPY-pep-Zn (>99%).4 The
ultrafast rate of ∼(3 ps)-1 for PDI*-pep-Zn f PDI-pep-
Zn* energy transfer would clearly support the same near-
quantitative yield were it not for the competing PDI*-pep-
Zn f PDI--pep-Zn+ hole transfer process, which has a rate
of ∼(13 ps)-1. Thus, one means of improving the energy-transfer
efficiency in this dyad is to decrease the rate of hole-transfer
by making the energetics of the latter process less favorable.
This can be achieved by altering the redox potentials of the
porphyrin as described in the companion paper.
The substantial electron-transfer quenching of Zn* that occurs
once the energy has arrived from PDI* presents a limitation of
PDI-pep-Zn as a light-input element in light-harvesting arrays.
This PDI-pep-Zn* f PDI--pep-Zn+ quenching process has
a yield of 80% and a rate of ∼(0.5 ns)-1. Thus, for use as a
light-input element, the energy-transfer rate from Zn* to the
next constituent in an array would have to be rapid compared
with ∼(0.5 ns)-1. However, as noted above, we have previously
found that energy transfer from Zn* to the Fb porphyrin in Zn-
pep-Fb has a rate of (24 ps)-1 37
pep-Zn-pep-Fb triad, energy transfer would be the preferable
. Thus, in a hypothetical PDI-

8246 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Prathapan et al.
TABLE 4: Calculated Fo1rster Energy-Transfer Rate for Several Porphyrin-Containing Dyadsa
d
calc
g
dyad
R (Å)b
κ2
Φfc
τ (ns)c
J (cm6 mmol-1
)
(kENT
)
-1 (ps)e
(kENT
)
-1 (ps)f
ΦENT
PDI-pep-Zn
BDPY-pep-Zn
BDPY-pep-Fb
Zn-pep-Fbj
21.1
18
18
20.1
20.1
2
0.25
0.25
1.125
1.125
0.96
3.7
5.8 × 10-14
1.3 × 10-13
7.0 × 10-14
2.9 × 10-14
1.6 × 10-14
17
53
99
3.1
20/2*
20/2*
24
0.80h
0.98
0.95
0.99
0.99
0.058
0.058
0.033
0.033
0.52
0.52
2.4
745
Zn-pep-Mgk
2.4
9
a All data were obtained in toluene at room temperature. b Center-to-center distance. The distance in PDI-pep-Zn was estimated at 21.1 Å
using Discover 3 in INSIGHT II from Molecular Simulations, Inc. c Fluorescence yield and excited-state lifetime for the energy-transfer donor in
the absence of the acceptor (e.g., Figure 1). d The Fo¨rster spectral overlap term (J) was computed using selected monomers which best approximate
those units in the dyad. The extinction coefficients used for PDI-pep-Zn are as follows: ZnU′, 490 nm (350 M-1 cm-1), 423 nm (468 000 M-1
cm-1); PDI-m, 490 nm (43 900 M-1 cm-1), 423 nm (3300 M-1 cm-1). e Calculated Fo¨rster rate. f Measured rate. g Measured yield. h The energy-
transfer yield for PDI-pep-Zn is reduced from >0.99 to 0.80 because of competition with ultrafast hole transfer to the porphyrin (see Figure 3
and text). i These complexes exhibit dual-exponential behavior with time constants of ∼20 and ∼2 ps with 70:30 relative amplitudes (see text).4
j Complex was previously denoted ZnFbU.37 k Complex was previously denoted MgZnU.40
pathway (99.5% yield) if it were in competition with electron
transfer (rate ∼ (0.5 ns)-1). The yield of energy transfer would
be even higher (than in PDI-pep-Zn-pep-Fb) if the Fb
porphyrin output element were replaced with a magnesium
porphyrin or a phthalocyanine, which exhibit energy-transfer
The observation of delayed fluorescence from Zn* in PDI-
pep-Zn (in toluene), which occurs via thermal repopulation of
the excited state from the PDI--pep-Zn+ charge-transfer
product, indicates that the energy of this latter species lies ∼kT
(0.026 eV at 298 K) below Zn* (Figure 3, vide supra). This
information along with the absolute energies of the PDI* and
Zn* excited states (as indicated by the static absorption and
emission properties of these constituents) provides a starting
point for a detailed analysis of the charge-transfer behavior of
the perylene-porphyrin dyads. In the accompanying paper,23 the
data for the benchmark PDI-pep-Zn dyad are used along with
those for analogous arrays containing Mg and Fb porphyrins to
construct a complete and fully self-consistent picture of the
excited-state properties of this class of dyads.
rates from the neighboring Zn* of ∼(9 ps)-1 and ∼(3 ps)-1
,
respectively.40,41 Thus, the electron-transfer quenching that
occurs following formation of Zn* from PDI* could be
circumvented by appropriate molecular design. We have recently
utilized these ideas, together with the insights gained from the
results given in the two other papers in this series on perylene-
porphyrin dyads,23,24 to prepare a perylene-bis(porphyrin)-
phthalocyanine wire that exhibits enhanced energy-transfer
properties relative to our first-generation wires.22
For applications such as molecular switching, the high yield
of electron transfer that occurs from Zn* to the perylene
component of PDI-pep-Zn can be exploited. The use of
perylene-based dyes as electron acceptors when attached to
porphyrins has been explored previously but with different
linkers connected to the N-imide position of the perylene unit
than the diphenylethyne linker used in PDI-pep-Zn.15,21,25 One
example is the phenyl-linked dyad PDI-p-Fb in which electron
transfer from Fb* to the perylene occurs with a rate constant of
∼(10 ps)-1 in pyridine, followed by charge recombination to
Experimental Section
General. 1H NMR spectra were collected at 300 MHz.
Products were analyzed by fast-atom bombardment (FAB) or
by laser desorption mass spectrometry (LD-MS) in the absence
of a matrix.43 All reagents were obtained from Aldrich Chemical
Co., and all solvents were obtained from Fisher Scientific.
N-(2,5-Di-tert-butylphenyl)-3,4,9,10-perylenetetracar-
boxylic-3,4-anhydride-9,10-imide (1). Following a slight modi-
fication of method II described by Kaiser et al.,16 N,N′-bis(2,5-
di-tert-butylphenyl)-3,4,9,10-perylenebis(dicarboximide)5 (4.0 g,
5.2 mmol) was suspended in 100 mL of tert-butyl alcohol.
Powdered KOH (80%, 1 g) was added, and the mixture was
refluxed with vigorous stirring for 2 h. After cooling to room
temperature, the reaction mixture was treated with a mixture of
HCl (50 mL, 2 M) and glacial acetic acid (100 mL) with
vigorous stirring. The resulting mixture was refluxed for 1 h,
cooled, and filtered. The residue obtained was suspended in 100
mL of 10% K2CO3 and refluxed for 0.5 h and then filtered using
a frittered-glass funnel under vacuum to separate the soluble
monopotassium salt of 3,4,9,10-perylenetetracarboxylic acid.
(Note: The residue should not be washed with water). The
residue obtained was washed twice with 10% K2CO3 solution.
The funnel was then filled with 2% HCl, kept for 0.5 h and
filtered. This process was repeated twice. The residue was then
washed with distilled water until the washings were neutral and
then suspended in 10% triethylamine in water (50 mL) and
refluxed for 0.5 h. The mixture was filtered to separate the
insoluble residue of unreacted starting material. The filtrate was
acidified with a minimum amount of 10% HCl, and the product
was separated by filtration (0.45 g, 15%).
the ground state with a time constant of ∼(120 ps)-1 15
.
It is
noteworthy that the forward electron-transfer and charge-
recombination rates initiated in Fb* in PDI-p-Fb are both
faster than the values of ∼(500 ps)-1 and <(10 ns)-1 observed
for PDI-pep-Zn in toluene. This difference can be ascribed
predominantly to greater perylene-porphyrin electronic cou-
pling via the phenyl (p) versus diphenylethyne (pep) linker.
(Along the same lines, we have found that excited-state energy
transfer from Zn to Fb porphyrins is also faster across a phenyl
versus diphenylethyne linker: (3.5 ps)-1 versus (24 ps)-1
,
respectively.42) We have also found that the charge recombina-
tion in PDI--pep-Zn+ is enhanced in polar solvents such as
acetonitrile; this effect no doubt reflects a more favorable
balance between the free-energy change for the process (smaller)
and the reorganization energy (larger) than in toluene, as is
discussed in more detail in the companion paper that follows.
The availability of perylene-porphyrin dyads with tunable rates
of formation and decay of the electron-transfer product is
potentially useful for applications involving molecular switching.
Finally, we note that in order to design molecular devices
for applications in which it is desirable to either optimize or
minimize charge-transfer processes, it is essential to have a firm
understanding of the rate versus free-energy relationship for the
charge-transfer processes. It is also highly desirable to have
direct experimental information on the absolute energies of all
of the species involved, including the charge-separated products.
N-(4-Iodophenyl)-N′-(2,5-di-tert-butylphenyl)-3,4,9,10-
perylenebis(dicarboximide) (2). A mixture of 1 (0.45 g, 0.78
mmol), p-iodoaniline (0.42 g, 2.0 mmol), imidazole (2.5 g), and
anhydrous zinc acetate (0.15 g) was stirred in an oil bath at

Synthesis and Photodynamics of Dyads
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8247
130 °C for 1 h. The mixture was allowed to cool to room
temperature, and 25 mL of 60% ethanol was added, stirred at
room temperature for 2 h, and filtered. The residue was treated
with 25 mL of 10% aqueous triethylamine, and the resulting
mixture was refluxed. After cooling, the suspension was filtered
and the residue was washed several times with 60% ethanol,
then with ethanol, and finally with hexanes. The solid product
obtained was dried in an oven and chromatographed over silica
gel (CH2Cl2) to yield 0.45 g (84%) of the pure product: mp
>270 °C; IR (KBr) 1708, 1670 cm-1; 1H NMR (CDCl3) δ 1.30
(s, 9 H), 1.334 (s, 9 H), 7.084 (d, J ) 2 Hz, 1H), 7.137 (d, J )
8 Hz, 2 H), 7.488 (dd, JAX ) 2 Hz, JAM ) 8 Hz, 1 H, AMX),
7.615 (d, J ) 8 Hz, 1 H, AMX), 7.915 (d, J ) 8 Hz, 2 H),
8.644-8.790 (m, 8 H); FAB-MS obsd 781.1587, calcd exact
mass 781.1563 (C44H33N2O4I); λabs (toluene) 432, 460, 491, 528
nm. Alternatively, a mixture of perylene-3,4,9,10-tetracarboxylic
dianhydride (1 equiv), 2,5-di-tert-butylaniline (3 equiv), p-
iodoaniline (2 equiv), zinc acetate, and imidazole was heated
under nitrogen at 180-190 °C for 2 h. The reaction mixture
was cooled to room temperature, diluted with 60% ethanol and
allowed to stand for 2 h. The resulting precipitate was collected
by filtration and washed with ethanol and then suspended in
10% KOH solution, refluxed, and filtered. The residue was taken
up in CH2Cl2, and the insoluble diimide was separated by
filtration. The filtrate was concentrated and chromatographed
over silica gel. The product was collected as the second fraction
(2% overall yield).
were measured relative to the appropriate reference monomer
(ZnTPP and H2TPP).44
Time-Resolved Fluorescence. Fluorescence lifetimes were
obtained on samples that had concentrations of 0.5-10 µM and
were deaerated by bubbling with N2. Lifetimes were determined
by fluorescence modulation (phase shift) techniques using a
Spex τ2 spectrometer. Samples were excited at various wave-
lengths and detected through using the appropriate colored glass
filters. Modulation frequencies from 20 to 300 MHz were
utilized, and both the fluorescence phase shift and modulation
amplitude were analyzed.
Time-Resolved Absorption. Transient absorption data were
acquired as follows.41,42 Samples (∼10 µM in toluene) in 2 mm
path length cuvettes at room temperature were excited at 10
Hz with ∼130 fs, 4-7 µJ pulses at the appropriate wavelength
and probed with white-light probe pulses of comparable
duration.
Acknowledgment. This research was supported by the NSF
(CHE-9707995 and CHE-9988142) (D.F.B., D.H., and J.S.L).
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.
P.S. thanks Cochin University of Science and Technology for
a sabbatical leave.
Appendix A. Kinetic Model for Equilibrium Involving the
Zn* Excited State
Perylenediimide-Diphenylethyne-Zinc Porphyrin Dyad
(PDI-pep-Zn). Following a method for porphyrin-porphyrin
coupling reactions,26 samples of 2 (0.021 g, 0.031 mmol, 2.5
mM), ZnU′ (0.028 g, 0.03 mmol, 2.5 mM),26 Pd2dba3 (0.004 g,
4.4 µmol, 15 mol %), and tri-o-tolylphosphine (0.011 g, 0.036
mmol, 4:1 per mol Pd) were weighed into a 50 mL Schlenk
flask. The reaction vessel was purged with Ar and then 13 mL
of distilled and degassed toluene/triethylamine (5:1) was added
under inert conditions. The reaction mixture was heated to 35
°C and stirred for 1 h. The crude reaction mixture was filtered
through Celite, rotary evaporated to dryness, chromatographed
on silica (CH2Cl2/hexanes, 3:1), chromatographed by SEC
(THF), and recrystallized (ethanol) to afford a purple solid
The following expression describes the time evolution of the
concentration of the photoexcited zinc porphyrin (Zn*) in the
equilibrium kinetic model of Figure 3:
1
2
1
2
[Zn*](t) ) A exp - (a + b)t + B exp - (a - b)t (1)
[
]
[
]
1
A ) (-2bok-3 + aok3 - k -3 - k5 + k4 + b) (2a)
2b
1
B ) (2bok-3 - aok3 + k-3 + k5 - k4 + b)
2b
(2b)
(3a)
a ) k3 + k-3 + k4 + k5
1
(0.025 g, 57%). H NMR (CDCl3) δ 1.32 (s, 9 H), 1.36 (s, 9
H), 1.56 (s, 18 H), 2.65 (s, 9 H), 7.05 (d, 1 H), 7.28-7.29 (6
H), 7.46 (d, J ) 8 Hz, 2 H), 7.50 (dd, 1 H), 7.63 (d, J ) 8 Hz,
1 H), 7.90 (d, J ) 8 Hz, 2 H), 7.97 (d, J ) 8 Hz, 2 H), 8.26 (d,
J ) 8 Hz, 2 H), 8.77-8.91 (m, 8 H); FAB-MS obsd 1480.8,
calcd exact mass 1478.5 (C99H78N6O4Zn); λabs (toluene) 424,
460, 492, 528, 548 (s), 589 nm.
b ) (k + k-3 + k4 + k5)2 - 4[k3k5 + (k-3 + k5)k4] (3b)
x
3
ao ) [Zn*](0)
(4a)
(4b)
bo ) [PDI--pep-Zn+](0)
Electrochemistry. The electrochemical studies were per-
formed in butyronitrile (Burdick and Jackson) using previously
described instrumentation.38 The supporting electrolyte was 0.1
M tetrabutylammonium hexafluorophosphate (Aldrich; recrys-
tallized 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 observed dual exponential fluorescence Zn* decay with
components having time constants of 0.4 and 10 ns comprising
80 and 20% of the decay, respectively, gives
[Zn*](t) ) 0.8 exp[-(0.4 ns)-1t] + 0.2 exp[-(10 ns)-1t]
(5)
Equating eqs A1 and A2 affords (a + b) ) (0.2 ns)-1, (a - b)
) (5 ns)-1, A ) 0.8, and B ) 0.2. The relative yields of Zn*
and PDI--pep-Zn+ from PDI* determined from the transient
absorption data gives ao ) 0.8 and bo ) 0.2. The Zn* lifetime
Static Absorption and Emission. Static absorption (HP8451A,
Cary 3 and 219) and fluorescence (Spex Fluoromax or Fluorolog
II) measurements were performed as described previously.41,42
Nondeaerated samples with an absorbance e0.15 at λexc (0.3-
0.8 µM) were used for the key emission measurements, although
more concentrated samples were investigated; the detection band
pass was 4-5 nm, and the spectra were corrected for the
detection-system spectral response. Emission quantum yields
in the ZnU′ monomer affords k4 ) (2.4 ns)-1
.
A consistent set of results is given by k3 ∼ (0.5 ns)-1, k-3
∼
(1.6 ns)-1, and k5 ∼ (50 ns)-1. The value of k3 is slightly longer
than the measured 0.4 ns Zn* fluorescence-decay component,
as expected from the equilibrium involved. The values of k-3

8248 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Prathapan et al.
and k3 together suggest that PDI--pep-Zn+ lies ∼1.2kT below
Zn*, which is consistent with the finding of a measurable
delayed fluorescence component. The value of k5 is consistent
with the finding of essentially no decay of PDI--pep-Zn+ over
the 3.5 ns time scale of the experiments. However, given the
time span of the measurements, it is more appropriate to use
(19) Liu, S.-G.; Liu, Y.-Q.; Xu, Y.; Jiang, X.-Z.; Zhu, D.-B. Tetrahedron
Lett. 1998, 39, 4271-4274.
(20) Wasielewski, M. R.; Gosztola, D.; Svec, W. A. Mol. Cryst. Liq.
Cryst. 1994, 253, 289-296.
(21) Just, E. M.; Wasielewski, M. R. Superlattices Microstr. 2000, 28,
317-328.
(22) Miller, M. A.; Lammi, R. K.; Prathapan, S.; Holten, D.; Lindsey,
J. S. J. Org. Chem. 2000, 65, 6634-6649.
an upper limit k5 < (10 ns)-1
.
(23) 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.
(24) (a) 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.,
in press. (b) Yang, S. I.; Prathapan, S.; Miller, M. A.; Seth, J.; Diers, J. R.;
Bocian, D. F.; Lindsey, J. S.; Holten, D. Manuscript in preparation.
(25) Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem.
Soc. 1998, 120, 5118-5119.
Appendix B. Fo1rster Calculations
The Fo¨rster rate is calculated using the standard formulation
based on the point dipole-dipole approximation (eq B1), using
the program PhotochemCAD.45
(26) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266-5273.
kFo¨rster ) (8.8 × 1023)κ2ΦfmJn-4R-6(τm)-1
(B1)
(27) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc.
1996, 118, 11166-11180.
Here, κ2 is the orientation factor (obtained by dynamic averaging
about the pigment-linker bonds), Φmf is the fluorescence yield
of the donor in the absence of the acceptor, J is the spectral
overlap term (in cm6 mmol-1) obtained from the optical spectra
of the monomers, n is the refractive index (1.496 for toluene),
R6 is the donor-acceptor center-to-center distance (in Å)
obtained from molecular modeling, and τm is the lifetime of
the donor in the absence of the acceptor. For other Fo¨rster
calculations of perylene-bis(imide) dyes, see ref 46.
(28) Bothner-By, A. A.; Dadok, J.; Johnson, T. E.; Lindsey, J. S. J.
Phys. Chem. 1996, 100, 17551-17557.
(29) Langhals, H. Heterocycles 1995, 40, 477-500.
(30) Nagao, Y.; Misono, T. Bull. Chem. Soc. Jpn. 1981, 54, 1191-
1194.
(31) Nagao, Y.; Misono, T. Dye Pigments 1984, 5, 171-188.
(32) Salbeck, J.; Kunkely, H.; Langhals, H.; Saalfrank, R. W.; Daub, J.
Chimia 1989, 43, 6-9.
(33) Ford, W. E.; Hiratsuka, H.; Kamat, P. V. J. Phys. Chem. 1989, 93,
6692-6696.
(34) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. V, pp 53-126.
(35) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D.;
Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines
1999, 3, 117-147.
References and Notes
(1) Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996, 68, 1373-
1380.
(2) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759-
9760.
(3) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian,
D. F. J. Am. Chem. Soc. 1996, 118, 3996-3997.
(4) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III; Singh,
D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J.
Am. Chem. Soc. 1998, 120, 10001-10017.
(5) Langhals, H. Chem. Ber. 1985, 118, 4641-4645.
(6) (a) Rademacher, A.; Ma¨rkle, S.; Langhals, H. Chem. Ber. 1982,
115, 2927-2934. (b) Ebeid, E. M.; El-Daly, S. A.; Langhals, H. J. Phys.
Chem. 1988, 92, 4565-4568.
(7) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373-6380.
(8) (a) Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225-230.
(b) Feiler, L.; Langhals, H.; Polborn, K. Liebigs. Ann. 1995, 1229-1244.
(9) Cormier, R. A.; Gregg, B. A. Chem. Mater. 1998, 10, 1309-1319.
(10) Langhals, H.; Demmig, S.; Huber, H. Spectrochim. Acta 1988, 44A,
1189-1193.
(11) Zollinger, H. Color Chemistry: Syntheses, Properties, and Ap-
plications of Organic Dyes and Pigments; VCH: Weinheim, Germany,
1991.
(12) Kerp, H. R.; Donker, H.; Koehorst, R. B. M.; Schaafsma, T. J.;
van Faassen, E. E. Chem. Phys. Lett. 1998, 298, 302-308.
(13) Tian, H.; Liu, P.-H.; Zhu, W.; Gao, E.; Wu, D.-J.; Cai, S. J. Mater.
Chem. 2000, 10, 2708-2715.
(14) (a) Gregg, B. A. Chem. Phys. Lett. 1996, 258, 376-380. (b)
Petritsch, K.; Dittmer, J. J.; Marseglia, E. A.; Friend, R. H.; Lux, A.;
Rozenberg, G. G.; Moratti, S. C.; Holmes, A. B. Sol. Energy Mater. Sol.
Cells 2000, 61, 63-72.
(36) One calculation of k3 in Figure 3 based on the Zn* lifetime in the
dyad and the yield of PDI--pep-Zn+ gives 0.8/(0.4 ns) ∼ (0.5 ns)-1. The
same value is obtained solely from the Zn* lifetime in the dyad versus the
monomer of k3 ) (τ)-1 - (τm)-1 ) (0.4 ns)-1 - (2.4 ns)-1 ∼ (0.5 ns)-1
.
Last, a value of ∼(0.7 ns)-1 is estimated from the fluorescence yield and
lifetime data via the expression k3 ) [(Φmf /Φf) - 1]/τm, using values of
Φmf ) 0.033 and Φf ) 0.008. (The latter value is used instead of the
measured value of 0.010 because only the prompt component of the
fluorescence is relevant to the calculation.) These estimates for k3 are in
good agreement with the value of k3 ∼ (0.5 ns)-1 obtained from the
equilibrium kinetic model for the decay of Zn* in Figure 3 using the lifetime
and yield data (see Appendix A).
(37) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.;
Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian,
D. F.; Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181-11193.
(38) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191-
11201.
(39) Yang, S. I.; Seth, J.; Balasubramanian, T.; Kim, D.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1999, 121, 4008-4018.
(40) Hascoat, P.; Yang, S. I.; Lammi, R. K.; Alley, J.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. Inorg. Chem. 1999, 38, 4849-4853.
(41) Yang, S. I.; Li, J.; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Mater. Chem. 2000, 10, 283-296.
(42) Yang, S. I.; Lammi, R. K.; Seth, J.; Riggs, J. A.; Arai, T.; Kim,
D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. 1998, 102, 2,
9426-9436.
(43) (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.
(15) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines,
G. L., III; Wasielewski, M. R. Science 1992, 257, 63-65.
(16) Kaiser, H.; Lindner, J.; Langhals, H. Chem. Ber. 1991, 124, 529-
535.
(44) Seybold, P. G.; Gouterman, M. P. J. Mol. Spectrosc. 1969, 31,
1-13.
(17) (a) Langhals, H.; Gold, J. J. Prakt. Chem. 1996, 338, 654-659.
(b) Langhals, H.; Gold, J. Liebigs Ann. 1997, 1151-1153. (c) Langhals,
H.; Jona, W. Angew. Chem. 1998, 37, 952-955.
(45) Du, H.; Fuh, R.-C. A.; Li, J.; Corkan, L. A.; Lindsey, J. S.
Photochem. Photobiol. 1998, 68, 141-142.
(46) Johansson, L. B.-Å.; Langhals, H. Spectrochim. Acta 1991, 47A,
857-861.
(18) Wu¨rthner, F.; Sautter, A. Chem. Commun. 2000, 445-446.