Photophysical properties of an alkyne-bridged bis(zinc porphyrin)-perylene bis(dicarboximide) derivative

Article abstract of DOI:10.1021/jp905214g

We report the synthesis, electrochemistry, and photophysical properties of a new donor-acceptor-donor molecule in which the meso carbon atoms of two zinc porphyrin (POR) units are linked through ethynylene bridges to the 1,7-positions of a central perylen

Full text of DOI:10.1021/jp905214g

10826
J. Phys. Chem. A 2009, 113, 10826–10832
Photophysical Properties of an Alkyne-Bridged Bis(zinc porphyrin)-Perylene
Bis(dicarboximide) Derivative
Susan A. Odom,† Richard F. Kelley,‡ Shino Ohira,† Trenton R. Ensley,§ Chun Huang,†
Lazaro A. Padilha,§ Scott Webster,§ Veaceslav Coropceanu,† Stephen Barlow,†
David J. Hagan,§ Eric W. Van Stryland,§ Jean-Luc Bre´das,† Harry L. Anderson,|
Michael R. Wasielewski,‡ and Seth R. Marder*,†
Department of Chemistry and Biochemistry and The Center for Organic Photonics and Electronics (COPE),
Georgia Institute of Technology, Atlanta, Georgia, 30332-0400, Department of Chemistry and
Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern UniVersity, EVanston,
Illinois 60208-3113, CREOL and Department of Physics, UniVersity of Central Florida, Orlando, Florida
32816-2700, and Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford,
Mansfield Road, Oxford OX1 3TA, United Kingdom
ReceiVed: June 3, 2009; ReVised Manuscript ReceiVed: August 7, 2009
We report the synthesis, electrochemistry, and photophysical properties of a new donor-acceptor-donor
molecule in which the meso carbon atoms of two zinc porphyrin (POR) units are linked through ethynylene
bridges to the 1,7-positions of a central perylene-3,4:9,10-bis(dicarboximide) (PDI). In contrast to previously
studied systems incorporating POR and PDI groups, this alkyne-based derivative shows evidence of through-
bond electronic coupling in the ground state; the new chromophore exhibits absorption features similar to
those of its constituent parts as well as lower energy features (at wavelengths up to ca. 1000 nm), presumably
arising from donor-acceptor interactions. Transient absorption measurements show that excitation at several
visible and near-IR wavelengths results in the formation of an excited-state species with a lifetime of 290 ps
in 1% (v/v) pyridine in toluene. The absorption spectrum of this species resembles the sum of the spectra for
the chemically generated radical cation and radical anion of the chromophore. The chromophore shows moderate
two-photon absorption cross sections (2000-7000 GM) at photon wavelengths close to the onset of its low-
energy one-photon absorption feature.
Introduction
the POR units are covalently linked through the nitrogen atoms
or the 1,7-positions (as in the compounds in Figure 1) of the
PDI, the ground-state absorption spectra of these chromophores
typically exhibit no evidence for electronic coupling between
POR and PDI moieties; i.e., the spectra generally resemble the
sum of those of the constituent chromophores. The relatively
weak coupling can be attributed to the presence of nodal planes
through the imide nitrogen atoms in both the HOMO and LUMO
of PDIs in the case of N,N′-linked systems,25 orsin the case of
a zinc POR-PDI D-A-D derivative (4, Figure 1) in which
the meso position of the POR units and the 1,7-positions of the
PDI are linked by p-phenylene bridges21sto the twist anticipated
between the phenylene bridge and the donor and acceptor
moieties.
Porphyrins (PORs) and other tetrapyrrolic macrocycles have
attracted interest as sensitizers in photovoltaic cells,1,2 hole-
transporting materials,3 two-photon absorbing dyes4 for photo-
dynamic therapy,5 and chromophores for optical pulse suppres-
sion6 because of their spectroscopic and photophysical properties.
Zinc POR derivatives have two sets of absorption maxima: the
Soret bands (observed at ca. 425 nm for simple D4h-symmetric
zinc porphyrins such as tetraphenyl zinc porphyrin) and Q-bands
(at ca. 560 and 600 nm).7 Simple perylene-3,4:9,10-bis(dicar-
boximide)s (PDIs) without substitution on any of the perylene
positions are characterized by an absorption showing well-
defined vibronic structure and a maximum at around 525 nm,
efficient fluorescence, and successive reversible reductions to
the corresponding mono- and dianions. PDI derivatives have been
studied as fluorescent probes,8,9 electron-transport materials,10,11
electron acceptors in conjugated donor-acceptor systems for
light-harvesting chromophores,12,13 sensitizers for photovolta-
ics,14 and two-photon absorbing dyes.15-18
The objective of the present work is to examine the effect of
stronger electronic coupling between POR and PDI moieties,
achieved through the incorporation of a different conjugated
linker group, namely, ethynylene groups in the new chro-
mophore 1 (synthesized as shown in Scheme 1; see details in
the Supporting Information), in place of the phenylene groups
of 4, on the photophysical properties of D-A-D chromophores.
In particular, we compare the linear and transient absorption
spectra of 1 to those of previously reported related compounds
4 and 5, the latter being the only other reported compound in
which the 1,7-positions of a PDI are linked through ethynylene
bridges to macrocyclic donors (specifically to the 2-positions
of zinc phthalocyanines, Pc’s).26 Since extended conjugated
architectures of zinc POR derivatives have been reported to
exhibit very large two-photon absorption (2PA) cross sections,4
The photophysical properties of several chromophores con-
taining both POR donors (D’s) and PDI acceptors (A’s) have
previously been reported,19-24 with photoinduced D to A electron
transfer being studied in some examples. Regardless of whether
* To whom correspondence should be addressed. E-mail: seth.marder@
chemistry.gatech.edu.
† Georgia Institute of Technology.
‡ Northwestern University.
§ University of Central Florida.
| University of Oxford.
10.1021/jp905214g CCC: $40.75  2009 American Chemical Society
Published on Web 09/15/2009

Photophysical Properties of a POR-PDI-POR Derivative
J. Phys. Chem. A, Vol. 113, No. 40, 2009 10827
Figure 1. Previously studied compounds whose photophysical properties we compare to those of the new chromophore 1: a phenylene-linked
POR-PDI-POR chromophore21 (4, left) and an alkyne-linked Pc-PDI-Pc chromophore (5, right).26
SCHEME 1: Synthesis of Alkyne-Linked POR-PDI-POR (1) Using in Situ Alkyne Deprotection and Sonogashira
Couplinga
a 3b is used as a 3:1 ratio of the 1,7-isomer (shown) and the 1,6-isomer, but 1 is obtained isomerically pure after chromatography.
which are of interest for applications including fluorescence
imaging,27 microfabrication,28 and optical pulse suppression,29
we have also investigated the 2PA properties of 1 in the vicinity
of the one-photon absorption edge. These photophysical studies
are further supplemented by electrochemical and quantum-
chemical studies.
of the MOs suggests that the transitions are likely to be strongly
localized on either POR or PDI.
Linear Absorption Spectra. The UV-vis-NIR absorption
spectrum for compound 1 is compared to those of its constituent
unitssmodel compounds 2 and 3sin Figure 4.31 Pyridine (1%,
v/v) was used to prevent aggregation of the POR moieties. The
UV-vis absorption spectrum of 421 is essentially the sum of
individual POR and PDI absorption spectra. However, although
the absorption spectrum of 1 shows a strong POR-like absorption
at ca. 430 nm, the lower energy features in the 470-630 nm
range are less clearly identifiable as PDI-based and POR Q-band
absorptions than those observed in compound 4. Moreover, a
relatively strong feature in the NIR region, at lower energy than
the absorptions of any of the building blocks, may be attributable
to an interaction between the donor and acceptor moieties. The
spectrum of 1 can also be compared with those of other ethynyl-
substituted PDIs. That of 526 is consistent with the superposition
of Pc-based absorptions and the absorption bands expected for
a PDI chromophore with extended conjugation, possibly also
in combination with additional low-energy transitions, although
a well-separated charge-transfer band is not clearly distinct from
the Pc-based transitions. For 1,7-bis(arylethynyl)-PDI deriva-
tives, weak donor and acceptor aryl groups lead to red-shifted
absorptions retaining some vibronic structure, while stronger
π-donating aminoaryl donors lead to structureless charge-
transfer-type absorption bands.32 The differences between 1 and
4 likely result from increased donor-acceptor coupling in the
alkyne-bridged species relative to the phenylene-bridged species.
Results and Discussion
Molecular Structure and Frontier Orbitals. The DFT
calculations for both 1 and 4 indicate that there are two low-
energy conformers for each compound (one of which is similar
to the conformation previously observed in a disubstituted PDI
derivative11) differing in energy from one another by 0.08-0.18
eV.30 DFT calculations (see the Supporting Information for
complete details) suggest that the frontier molecular orbitals of
4 are localized on either POR or PDI units (with the HOMO
and LUMO being POR- and PDI-based, respectively) with little
evidence for any significant donor-acceptor interaction (see
Figures 2 and 3). In contrast, compound 1 shows effective con-
jugation between the POR and PDI units, with both the HOMO
and LUMO exhibiting significantly mixed POR/PDI character,
although the HOMO retains more POR character and the LUMO
more PDI character, consistent with the primarily donor and
acceptor character of these respective moieties. The conjugation
between POR and PDI in 1 suggests the possibility of a direct
low-energy HOMO-to-LUMO optical transition for 1, since
there is significant spatial overlap between the relevant orbitals.
On the other hand, this is not the case for 4, in which the nature

10828 J. Phys. Chem. A, Vol. 113, No. 40, 2009
Odom et al.
Figure 4. UV-vis-NIR absorption spectra of 1 (blue), 2 (green), and
3a (red) in 1% pyridine in toluene.
TABLE 1: Absorption Maxima (nm) for the Lowest Energy
Absorption Feature for D and A Components and for
Compounds 1, 4,21 and 526 in 1% Pyridine in Toluene
compd
D component
A component
D-A-D
1
4
5
623
614
681
527
548
527
852
613
716
The calculated HOMOs and LUMOs of these compounds (see
above) provide some insight into the differences between the
spectra of 4 and 1. As pointed out earlier, there is little spatial
overlap between the HOMO and LUMO of 4, suggesting that
a direct HOMO f LUMO excitation would have a negligible
transition dipole moment; the significant spatial wave function
overlap in 1 leads to the possibility of a low-energy HOMO f
LUMO band with significant intensity and having some qua-
drupolar POR-to-PDI charge-transfer character.
The quantum-chemical calculations indicate that, in contrast
to 4, most transitions in 1 show a significant mixture of local
and charge-transfer excitations. In particular, INDO calculations
also predict a low-energy transition (at 716 nm), which, given
the method used, is reasonably consistent with the experimental
maximum of the low-energy peak. The transition is characterized
by a transition dipole moment of 9.6 D, which compares very
well with the experimental value of 9.7 D, estimated by
integration over the range 650-950 nm in pyridine (1%, v/v)
in dichloromethane. No comparable allowed low-energy transi-
tion is predicted for 4, which is consistent with previously
published experimental results.21
Figure 2. Illustration of the DFT wave functions for the frontier orbitals
HOMO - 1 through LUMO + 2 for compounds 1 and 4.
Electrochemistry. In addition to the evidence for ground-
state electronic coupling between the donor and acceptor from
the absorption spectra of 1, the electrochemical potentials are
also supportive of extended conjugation. Cyclic voltammetry
(CV) of 1 (Figure 5) shows two one-electron oxidations and
three one-electron reductions (the third reduction is not shown
in Figure 5); the nonbrominated PDI model compound 3a shows
two reductions, and the model POR derivative 2 shows one
oxidation and one reduction. The redox potentials are sum-
marized in Table 2. The separation between the two oxidations
of 1 suggests some interaction between the two isolated POR
centers. Several factors contribute to separations between
electrochemical potentials of nominally equivalent redox centers
including through-space electrostatic effects, through-bond
inductive effects, and electronic coupling effects.37 The large
distance between the two redox centers in 1 means that the first
two contributions are likely to be minor and that the separation
seen here reflects electronic coupling between the two centers;
this result would be consistent with formation of a radical cation
delocalized over both POR units and the bridge, consistent with
the HOMO obtained from DFT calculations (Figure 2). The
Figure 3. Molecular orbital energies for 1 and 4 computed at the
B3LYP/6-31G** level. Orbitals are assigned to the donor and acceptor
depending on whether POR or PDI makes the larger contribution,
although the orbitals of 1 are more delocalized than those of 4. Orbitals
dominated by POR are shown in black, while those localized on the
PDI are shown in blue.
Stronger coupling through ethynylene versus p-phenylene
bridges has previously been observed in several other classes
of compounds, for example, ferrocene/ferrocenium mixed
valence species.33,34 It also constitutes a general feature of meso-
linked porphyrin systems due to the steric effect of the
neighboring ꢀ-protons;35,36 indeed, the case of 4, values of about
60° are calculated for the torsion angles between the planes of
the phenylene bridge and the PDI core. The lowest energy
absorption maxima for all three sets of donors, acceptors, and
D-A-D chromophores are shown in Table 1.

Photophysical Properties of a POR-PDI-POR Derivative
J. Phys. Chem. A, Vol. 113, No. 40, 2009 10829
n
Figure 5. Cyclic voltammogram of 1 in 0.1 M Bu4PF6 in dichlo-
romethane at 50 mV/s, shown with ferrocene as the internal reference
at 0 V.
TABLE 2: Half-Wave Redox Potentials (V) of 1, 2, and 3a
n
Figure 6. Transient absorption spectra of 1 in 1% pyridine in toluene,
following excitation with 414 nm, 120 fs laser pulses. Inset: transient
absorption kinetics for compound 1 monitored at 451 nm (blue) and
645 nm (red). Nonlinear least-squares fits to the data are also shown.
in 0.1 M BuN4PF6 in Dichloromethane, All Relative to
Cp2Fe+/0 at 0 V
2+/1+
+/0
0/-
1-/2-
2-/3-
compd
E1/2
+0.34
E1/2
E1/2
E1/2
E1/2
-1.37a
1
2
3a
+0.19
+0.25
-1.00
-1.73
-1.06
-1.19
TABLE 3: Kinetics for Compounds 1, 4, and 5 from Fitting
of Transient Absorption Experiments in 1% Pyridine in
Toluene (PhMe)
-1.26
a Not displayed in the voltammogram in Figure 5.
excitation
compd, solvent
wavelength (nm)
τR (ps)
τD (ps)
77 ( 3
result can be contrasted with 1,7-bis(arylethynyl)-PDIs with
amine-containing aryl groups in which the two amine oxidations
take place at experimentally indistinguishable potentials32 and
the HOMOs are more strongly donor-localized than in 1.38
Additionally, compound 1 is slightly easier to oxidize (by ca.
0.06 V) and to reduce (by ca. 0.06 V) than its isolated POR (2)
and PDI (3a) components, which again is presumably due to
the more delocalized nature of the HOMO and LUMO in 1.
The difference in the first reduction potential between the
unsubstituted N,N′-dialkyl-PDI (3a) and the bis(porphyrin-
ethynyl)PDI (1) is similar to differences previously seen in
reduction potentials of an unsubstituted N,N′-diaryl-PDI and its
bis(arylethynyl)-PDI derivatives (with both donor and acceptor
aryl groups), with differences ranging from 0.04 to 0.11 V for
these examples.32
Transient Absorption Spectra. Transient absorption spectra
have been used to study charge separation in a wide range of
donor-substituted perylene bis(dicarboximide) derivatives; these
include many species with porphyrin donors,19-22,24,32 including
compound 4.21 However, only a few such studies have involved
systems in which there is evidence for the coupling of donors
and acceptors in the ground-state absorption spectrum,26,32,39,40
only one of whichsthe study of compound 5sinvolves a
macrocyclic donor.26 To examine whether a similar charge-
separated state is formed in 1 and to probe its lifetime, transient
absorption spectra were recorded for 1 in 1% pyridine in toluene
using 120 fs pulses at 414, 600, and 832 nm. Figure 6 shows
the transient absorption spectra (plotted as the difference in
absorbance) obtained on excitation at 414 nm; the spectra
obtained using 600 and 832 nm excitations are essentially
identical. Bleaching of the POR Soret band at 450 nm is
accompanied by the appearance of peaks with maxima around
475, 600, and 650 nm. In contrast to the transient spectra of 4,
those for 1 are rather broad, and the features are not clearly
assignable to POR•+ and PDI•- moieties (or to 2•+ and 3b•-
moieties; see the Supporting Information), precluding description
of the state ultimately formed on excitation of 1 as a fully
charge-separted POR•+-PDI•--POR state with minimal cat-
ion-anion interaction. This is presumably related to the more
delocalized orbital structure in 1, i.e., the stronger coupling
between POR and PDI units. Indeed the transient spectra
4, 1% pyridine in PhMe
5, 1% pyridine in PhMe
1, 1% pyridine in PhMea
1, 1% pyridine in PhMe
1, 1% pyridine in PhMe
612
387
414
600
832
10.1 ( 0.5
not provided 517 ( 52
6.5 ( 0.5
6.7 ( 0.5
6.4 ( 0.5
294 ( 4
291 ( 2
278 ( 8
a An additional feature in the kinetic fitting was observed with τ
) 0.3 ( 0.1 ps, which is possibly due to the relaxation of the
vertical excited state to the lowest excited state.
observed for 1 are more reminiscent of those obtained for
5sanother strongly coupled D-A-D chromophore.
Also shown in Figure 6, the transient absorption kinetics were
monitored at 451 and 645 nm and were fitted using a nonlinear
least-squares fit to a general sum-of-exponentials function with
an added Gaussian instrument response function. The same time
constants were observed following excitation at 400, 600, and
832 nm.41 The time constants characterizing the rise and decay
of the state ultimately formed by 1, τR and τD, respectively, are
compared to those for 4 and 5 in Table 3. The value of τR for
1 is a little lower than that for 4, perhaps due to the increased
donor-acceptor coupling. The lifetime of the state formed by
1 also falls within the range of lifetimes of species attributed to
charge-separated states in other strongly coupled donor-
substituted PDIs,32,39,40 and is considerably shorter than the
lifetimes of the (presumably non-charge-separated) states formed
by PDIs substituted with weak π-donors or with π-acceptors.32
In the next section the relationship between the transient spectra
of 1 and the radical ion spectra is examined in more detail.
Radical Ion Spectra. As discussed above, the transient
absorption spectra are not clearly interpreted as the sum of the
radical ion spectra of isolated POR and PDI units. Moreover,
the molecular orbitals and electrochemical data suggest that 1•+
is likely to be a rather delocalized species, rather than resembling
an isolated POR•+ unit. Accordingly, we obtained the radical
ion spectra to establish the extents to which (i) the spectrum of
the state with a lifetime of ca. 290 ps observed in transient
absorption experiments resembles the sum of spectra for 1•+
and 1•- and (ii) the radical ion spectra for 1 resemble those for
isolated POR and PDI units. Figure 7 shows the vis-NIR
absorption spectra of 1 and the spectra obtained upon oxidation

10830 J. Phys. Chem. A, Vol. 113, No. 40, 2009
Odom et al.
Figure 8. Transient absorption spectra of compound 1 at 24 ps in 1%
pyridine in toluene following excitation with 414 nm, 120 fs laser pulses
(green, solid), difference spectrum from chemical oxidation with
acetylferrocenium tetrafluoroborate in dichloromethane (blue, dotted),
and difference spectrum from chemical reduction with cobaltocene in
tetrahydrofuran (red, dashed).
maximum of the disappearance of the Soret band of the POR
at ca. 440 nm. The sum of the spectra obtained upon oxidation
and reduction is not an exact match to the result from the
transient absorption spectra. However, DFT results suggest
delocalization of both the hole and electron over both the donor
and acceptor, which would be anticipated to lead to strong
hole-electron interactions in the excited state. Moreover,
differences in molecular geometries and in solvation45 between
excited states and the radical ions may be expected to lead to
differences in the spectra. Inevitably the stronger donor-acceptor
interactions in 1 and the more delocalized character of both the
radical anion and cation make the transient absorption spectra
more difficult to interpret, and preclude definitive assignment
of the ca. 290 ps state as a charge-separated species. However,
the difference spectra broadly show that the overall changes
observed in the transient absorption experiments resemble the
changes observed in a combination of the chemical oxidation
and reduction experiments, suggesting that the long-lived state
has significant charge-transfer character.
Two-Photon Absorption. Bis-POR diynes and bis-POR
diethynylarenes have previously been shown to exhibit peak
2PA cross sections into a state lying slightly higher in energy
than the Soret-type band state, with cross sections up to 500
times greater than those of isolated porphyrins. These cross
sections range from 3000 to 17000 GM at photon wavelengths
of ca. 820-890 nm, i.e., close to the one-photon absorption
edge (near double resonance with the Q-band states),4,46,47 and
have potential applications in two-photon-excited photodynamic
therapy.5 In addition, PDI derivatives and related compounds
have been shown to have large 2PA cross sections at photon
energies close to the one-photon edge.15-18,32 Moreover, we have
recently shown that a chromophore in which POR groups are
coupled through ethynylene linkers to a bis(indolinylidenem-
ethyl)squaraine core exhibits a 2PA spectrum that substantially
differs from the sum of 2PA spectra of the constituent building
blocks.48 In view of the ethynylene-mediated donor-
acceptor interactions evident in the linear spectrum of 1 and in
its calculated molecular orbitals (see above), the 2PA spectrum
of 1 was investigated, both theoretically and experimentally.
INDO/MRD-CI calculations of the 2PA spectrum of 1 predict
the presence of 2PA active states with cross sections greater
than 3500 GM at a state energy of ca. 2.58 eV due to strong
coupling between POR and PDI moieties, while the lack of such
Figure 7. (A) Chemical oxidation with acetylferrocenium tetrafluo-
roborate in dichloromethane: 1-0 (black), neutral 1; 1-X1 (blue) through
1-X4 (red), gradual appearance of 1+. (B) Reduction with cobaltocene
in THF (bottom) of compound 1: 1-0 (black), neutral 1; 1-R1 (blue)
through 1-R5 (fuchsia), gradual appearance of 1-. Insets: expansion
of the region from 450 to 1100 nm (top) and from 450 to 1500 nm
(bottom).
with additions of successive aliquots of acetylferrocenium
tetrafluoroborate (E1/2(FcAc+/0) ) +0.27 V versus Cp2Fe+/0 42
)
in dichloromethane.43 Chemical reduction was accomplished by
+/0
addition of successive portions of cobaltocene (E1/2(CoCp2
)
) -1.33 V versus Cp2Fe+/0 in 0.1 M nBu4NPF6/dichlo-
romethane42) in tetrahydrofuran to a solution of 1 in tetrahy-
drofuran.44
While the oxidation of 1 results in new features, some of
which occur at energies similar to those of the absorptions of
an isolated POR or ethynyl-susbtituted POR radical cation (i.e.,
the absorption features at ca. 640 and 890 nm, see the Supporting
Information for the spectrum of oxidized 2), it is difficult to
assign the new bands to specific POR•+ features. However, some
of the features associated with the bands assigned to neutral
POR-based transitions have decreased in intensity. The lack of
a close correspondence to a monomeric POR radical cation
implies that the radical cation is not localized on a single POR
moiety, consistent with the electrochemical data. Similarly, the
reduction of 1 results in new peaks, some of which are at
energies similar to those of PDI radical anions; 1,7-bis(aryl-
ethynyl)-PDI derivatives have been shown to have radical anion
absorption features at ca. 730, 820, and 1000 nm,32 with some
variation in wavelength depending on the nature of the aryl
substituent. The spectrum of the reduction of 1 shows additional
peaks, such as the feature centered at ca. 1300 nm. Also, the
peak assigned to the POR Soret band decreases in intensity,
indicating that reduction also affects at least one of the POR
moieties; this confirms that the excess electron is not restricted
to the PDI ring system, consistent with minor POR contributions
seen in the DFT-calculated LUMO (Figure 2).
Figure 8 compares examples of difference spectra from the
transient absorption, chemical oxidation, and chemical reduction
experiments. The three sets of spectra were normalized to the

Photophysical Properties of a POR-PDI-POR Derivative
J. Phys. Chem. A, Vol. 113, No. 40, 2009 10831
analogue, which closely resembles the sum of the spectra of
localized POR and PDI radical ions. However, similarities of
the transient spectrum of 1 to the spectra of the chemically
generated radical cations and anions suggest significant charge-
transfer character to this excited state. Large 2PA cross sections
are found for 1 at wavelengths of ca. 1.06-1.20 µm, at which
isolated POR and PDI chromophores do not absorb. Quantum-
chemical calculations indicate that this 2PA can be attributed
to the presence of frontier molecular orbitals with mixed POR
and PDI character and that comparably long-wavelength strong
2PA is not expected for the more weakly coupled compound 4.
In summary, the differences in photophysical properties between
phenylene- and alkyne-based D-A-D derivatives can be
understood in terms of greater electronic coupling between D
and A moieties, which can allow for tuning of the optical
properties for different potential applications.
Figure 9. Two-photon absorption at 1064, 1120, and 1200 nm of
compound 1 plotted versus the transition wavelength, measured by
Z-scan, shown with its linear absorption spectrum, all in 1% pyridine
in dichloromethane.
Acknowledgment. We thank the Office of Naval Research
(through a MURI, Grants N00014-05-1-0021 and N00014-03-
1-0793, and through the DARPA MORPH Program, Grant
N00014-06-1-0897), the Army Research Laboratory (Grant
W911NF0420012), the U.S. Army Research Office (Grants
W911NF0610283 and 5037-CH-MUR), the National Science
Foundation (through the STC Program under Agreement
Number DMR-0120967, through Grant ECS-0217932, and
through a Graduate Research Fellowship to S.A.O.), and the
EPSRC. We thank Xuan Zhang for providing compound 3b as
a starting material and Dr. Dirk Guldi, Dr. Tomas Torres, and
co-workers for providing spectroscopic data for compound 5.
Finally, we thank Dr. Craig Smith and Dr. Michael Frampton
for insightful conversations.
coupling in 4 leads to the corresponding transitions being 2PA-
inactive. Thus, the POR-PDI coupling plays a crucial role in
affording significant 2PA cross section at lower energies than
are observed for individual POR and PDI chromophores.
2PA cross sections for compound 1 were measured using the
degenerate Z-scan technique at photon wavelengths of 1064,
1120, and 1200 nm (Figure 9); for consistency with previously
measured POR monomers and dimers,4 the solvent chosen for
2PA measurements was 1% pyridine in dichloromethane. The
2PA cross sections range from 2100 to 7000 GM, with the
largest error bar at 1064 nm, close to the onset of the linear
absorption and where contributions from ESA (both 2PA- and
1PA-induced, with ESA leading to reverse saturable absorption)
could not be ruled out. The measured 2PA cross sections are
on the same order of magnitude as those of other POR dimer
and PDI derivatives, although the observable 2PA state occurs
at lower energy for compound 1 than for previous POR and
PDI systems (with the exception of our previously reported
porphyrin-squaraine chromophore48). In particular, strong 2PA
is observed in 1 at lower energy than the Soret-type state, while
in previously studied POR dimers, the strongly 2PA-allowed
state lies higher in energy than the Soret-type state, which
presumably implies that a different type of state is involved.
Thus, the incorporation of PDI into a conjugated bis-POR
architecture enables the tuning of the 2PA absorption of POR
derivatives into the near-IR.
Supporting Information Available: Experimental details,
synthetic details, additional UV-vis absorption spectra of
chemically oxidized and reduced model compounds, and ad-
ditional transient absorption spectra. This information is avail-
able free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Kim, H.-S.; Kim, C.-H.; Ha, C.-S.; Lee, J.-K. Synth. Met. 2001,
117, 289.
(2) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh,
P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.;
Graetzel, M.; Officer, D. L. J. Phys. Chem. C 2007, 111, 11760.
(3) Savenije, T. J.; Goossens, A. Phys. ReV. B 2001, 64, 115323/1.
(4) Drobizhev, M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane,
A.; Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 2004, 126, 15352.
(5) Collins, H. A.; Khurana, M.; Moriyama, E. H.; Mariampillai, A.;
Dahlstedt, E.; Balaz, M.; Kuimova, M. K.; Drobizhev, M.; Yang, V. X. D.;
Phillips, D.; Rebane, A.; Wilson, B. C.; Anderson, H. L. Nat. Photonics
2008, 2, 420.
(6) Calvete, M.; Ying Yang, G.; Hanack, M. Synth. Met. 2004, 141,
231.
(7) Harada, K.; Fujitsuka, M.; Sugimoto, A.; Majima, T. J. Phys. Chem.
A 2007, 111, 11430.
(8) Kohl, C.; Weil, T.; Qu, J.; Muellen, K. Chem.sEur. J. 2004, 10,
5297.
(9) Lippitz, M.; Hubner, C. G.; Christ, T.; Eichner, H.; Bordat, P.;
Herrmann, A.; Mu¨llen, K.; Basche, T. Phys. ReV. Lett. 2004, 92, 100301.
(10) Chen, H. Z.; Ling, M. M.; Mo, X.; Shi, M. M.; Wang, M.; Bao, Z.
Chem. Mater. 2007, 19, 816.
(11) Jones, B. A.; Fachetti, A.; Wasielewski, M. R.; Marks, T. J. AdV.
Funct. Mater 2008, 18, 1329.
(12) Ishi-i, T.; Murakami, K.; Imai, Y.; Mataka, S. Org. Lett. 2005, 7,
3175.
Conclusions
In conclusion, we have synthesized an ethynylene-linked
D-A-D derivative, 1, with ethynylene-linked POR and PDI
moieties; to the best of our knowledge this is first example of
a photophysical study of a strongly coupled POR-PDI chro-
mophore.49 In contrast to that of a related p-phenylene-bridged
system, 4, the UV-vis-NIR absorption spectrum of 1 deviates
significantly from the sum of spectra of isolated POR and PDI
units, demonstrating more extensive donor-acceptor interaction.
This finding is consistent with quantum-chemical calculations
showing more extensive delocalization of frontier orbitals over
both the donor and acceptor, and with electrochemical data.
Transient absorption spectra show that a state with a lifetime
of ca. 290 ps is formed on a ca. 6 ps time scale following
photoexcitations at multiple wavelengths in the vis-NIR region;
the lifetime of this state is slightly longer than that of the state
formed by its p-phenylene analogue,21 but shorter than that in
an alkyne-bridged Pc-PDI-Pc system.26 The spectrum of the
excited state with a lifetime of 290 ps is qualitatively different
from that of the state ultimately formed by its phenylene
(13) Weiss, E. A.; Ahrens, B.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 5577.
(14) Shin, W. S.; Jeong, H.-H.; Kim, M.-K.; Jin, S.-H.; Kim, M.-R.;
Lee, J.-K. J. W.; Gal, Y.-S. J. Mater. Chem. 2006, 16, 384.
(15) Oliveira, S. L.; Corrêa, D. S.; Misoguti, L.; Zilio, S. C.; Aroca,
R. F.; Constantino, C. J. L.; Mendonça, C. R. AdV. Mater. 2005, 17, 1890.

10832 J. Phys. Chem. A, Vol. 113, No. 40, 2009
Odom et al.
(16) Corrêa, D. S.; Oliveira, S. L.; Misoguti, L.; Zilio, S. C.; Aroca,
R. F.; Constantino, C. J. L.; Mendonça, C. R. J. Phys. Chem. A 2006, 110,
6433.
(17) Boni, L. D.; Constantino, C. J. L.; Misoguti, L.; Aroca, R. F.; Zilio,
S. C.; Mendonça, C. R. Chem. Phys. Lett. 2003, 371, 744.
(18) Gao, B.; Lu, C.; Xu, J.; Meng, F.; Cui, Y.; Tian, H. Chem. Lett.
2006, 35, 1416.
(19) Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem.
Soc. 1998, 120, 5118.
(20) Hayes, R. T.; Wasielewski, M. R.; Gosztola, M. J. J. Am. Chem.
Soc. 2000, 122, 5563.
(21) Kelley, R. F.; Shin, W. S.; Rybtchinski, B.; Sinks, L. E.;
Wasielewski, M. R. J. Am. Chem. Soc. 2007, 129, 3173.
(22) Prodi, A.; Chiorboli, C.; Scandola, F.; Iengo, E.; Alessio, E.;
Dobrawa, R.; Wu¨rthner, F. J. Am. Chem. Soc. 2005, 127, 1454.
(23) Chen, Y.; Lin, Y.; EI-Khouly, M. E.; Zhuang, X.; Araki, Y.; Ito,
O.; Zhang, W J. Phys. Chem. C 2007, 111, 16096.
(24) van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.; Weiss,
E. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124, 9582.
(25) Kazmaier, P. M.; Hoffmann, R. J. J. Am. Chem. Soc. 1994, 116,
9684.
(34) Patoux, C.; Coudret, C.; Launay, J. P.; Joachim, C.; Gourdon, A.
Inorg. Chem. 1997, 36, 5037.
(35) Frampton, M. J.; Akdas, H.; Cowley, A. R.; Rogers, J. E.; Slagle,
J. E.; Fleitz, P. A.; Drobizhev, M.; Rebane, A.; Anderson, H. L. Org. Lett.
2005, 7, 5365.
(36) Screen, T. E. O.; Blake, I. M.; Rees, L. H.; Clegg, W.; Borwick,
S. J.; Anderson, H. L. J. Chem. Soc., Perkin Trans. 1 2002, 320.
(37) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 3125.
(38) Shoaee, S.; Eng, M. P.; An, Z.; Zhang, X.; Barlow, S.; Marder,
S. R.; Durrant, J. R. Chem. Commun. 2008, 40, 4915.
(39) Huang, J.; Fu, H.; Wu, F.; Chen, S.; Shen, F.; Zhao, X.; Liu, Y.;
Yao, J. J. Phys. Chem. C 2008, 112, 2689.
(40) Huang, J.; Wu, Y.; Fu, H.; Zhan, X.; Yao, J.; Barlow, S.; Marder,
S. R. J. Phys. Chem. A 2009, 113, 5039.
(41) From kinetic fitting of the data transient absorption spectra of 1 in
1% pyridine in dichloromethane with excitation at 414 nm, the following
time constants were obtained: τR ) 3.3 ( 0.5 ps and τD ) 12.5 ( 3.0 ps.
(42) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(43) Toluene, the solvent primarily used for the transient absorption
spectra, is a poor solvent for ionic species, while dichloromethane was
chosen due to our previous observations that some radical cations are
considerably more persistent in chlorinated solvents than other polar
solvents. See: Coropceanu, V.; Gruhn, N. E.; Barlow, S.; Lambert, C.;
Durivage, J. C.; Bill, T. G; No¨ll, G.; Marder, S. R; Bre´das, J.-L. J. Am.
Chem. Soc. 2004, 126, 2727.
(44) The reduction of cobaltocenium species to the corresponding
cobaltocenes is not fully reversible in dichloromethane. See: Barlow, S.
Inorg. Chem. 2001, 40, 7047.
(45) Additionally, as evidenced by the solvent dependence of the spectra
of the neutral compound 1 (see Figure S3 in the Supporting Information),
it is expected that some peaks have different shapes and energies of
maximum absorption due to the different solvent environments used for
each experiment.
(26) Jime´nez, A. J.; Spa¨nig, F.; Salome Rodr´ıguez-Morgade, M.;
Ohkubo, K.; Fukuzumi, S.; Guldi, D. M.; Torres, T. Org. Lett. 2007, 9,
2481.
(27) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73.
(28) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.;
Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I.-Y. S.;
McCord-Maughon, D.; Qin, J.; Rockel, H.; Rumi, M.; Wu, X.-L.; Marder,
S. R.; Perry, J. W. Nature 1999, 398, 51.
(29) Spangler, C. W. J. Mater. Chem. 1999, 9, 2013.
(30) In both conformers, the PDI core is nonplanar, and the two POR
groups are noncoplanar with the PDI, presumably due to steric interactions
between the core and the substituents; one conformer is referred to as “bent”
and the other as “twisted” (Figure S6, Supporting Information). Since the
choice of conformer has only a limited effect on the calculated properties
(such as the orbital structure and energies) and the twisted conformer
resembles the experimental geometry of a disubstituted PDI derivative, the
specific results discussed below refer to the twisted geometry (see the
Supporting Information for the details of both geometries).
(31) The energy at which the low-energy absorption feature of 1 is
observed is relatively insensitive to the solvent; however, the line shape
does vary with the solvent. Three representative spectra (acquired in solvents
containing 1% pyridine to prevent aggregation of the POR moieties) are
shown in Figure S3, Supporting Information.
(46) Karotki, A.; Drobizhev, M.; Dzenis, Y.; Taylor, P. N.; Anderson,
H. L.; Rebane, A. Phys. Chem. Chem. Phys. 2004, 6, 7.
(47) Drobizhev, M.; Stepanenko, V.; Dzenis, Y.; Karotki, A.; Rebane,
A.; Taylor, P. N.; Anderson, H. L. J. Phys. Chem. B 2005, 109, 7223.
(48) Odom, S. A.; Webster, S.; Padilha, L. A.; Peceli, D.; Hu, H.; Nootz,
G.; Chung, S.-J.; Ohira, S.; Matichak, J. D.; Przhonska, O. V.; Kachkovski,
A. D.; Barlow, S.; Bre´das, J. L.; Anderson, H. L.; Hagan, D. J.; Van
Stryland, E. W.; Marder, S. R. J. Am. Chem. Soc. 2009, 131, 7510.
(49) Examples of D-A POR-perylenemonoimide derivatives exhibiting
significant ground-state coupling have been studied: Kirmaier, C.; Yang,
S. I.; Prathapan, S.; Miller, M. A.; Diers, J. R.; Bocian, D. F.; Lindsey,
J. S.; Holten, D. Res. Chem. Intermed. 2002, 28, 719–740. Yang, S. I.;
Lammi, R. K.; Prathapan, S.; Miller, M. A.; Seth, J.; Diers, J. R.; Bocian,
D. F.; Lindsey, J. S.; Holten, D. J. Mater. Chem. 2001, 11, 2420–2430.
(32) An, Z.; Odom, S. A.; Kelley, R. F.; Huang, C.; Zhan, X.; Barlow,
S.; Padilha, L. A.; Fu, J.; Webster, S.; Hagan, D. J.; Van Stryland, E. W.;
Wasielewski, M. R.; Marder, S. R. J. Phys. Chem. A 2009, 113, 5585.
(33) LeVanda, C.; Bechgaard, K.; Cowan, D. O. J. Org. Chem. 1976,
41, 2700.
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