Difference in the photophysical properties of a perylenetetracarboxylic diimide dimer and a hexamer linked by the same hexaphenylbenzene group

Article abstract of DOI:10.1002/cphc.201300602

A perylenetetracarboxylic diimide hexamer (6PDI) and a dimer (2PDI) linked with the same hexaphenylbenzene group were prepared, and the structures were fully characterized by ¹H NMR spectroscopy, mass spectrometry, and elemental analysis. Due to the similar molecular structure of these two compounds, similar interactions between/among the PDI subunits as well as similar photophysical properties are expected. However, the stationary UV/Vis absorption spectra reveal that the interactions among/between the PDI subunits in 6PDI are significantly stronger than those in 2PDI. This can be attributed to blocked rotation along the long axis of the PDI subunits in 6PDI due to steric hindrance of the two neighboring PDI subunits. The stronger interactions among the PDI subunits in 6PDI lead to long-wavelength emission, which can be assigned to excimer-like excited states. A similar conclusion can also be deduced from the fluorescence quantum yields and the fluorescence lifetimes. Electrochemical studies revealed that interactions between/among the PDI subunits in both 2PDI and 6PDI are still in the range of weak interactions. Ultrafast transient anisotropy decay dynamics revealed that excitation delocalization between the PDI subunits within 2PDI and 6PDI is quick and efficient. More interestingly, delocalization is faster in 6PDI than in 2PDI, probably because of the stronger interactions among the PDI subunits in the former. Interacting with Diimides: A perylenetetracarboxylic diimide (PDI) dimer and a hexamer linked with the same hexaphenylbenzene group are prepared. Despite the similar relative orientation of the neighboring PDI units in these two compounds, stronger interactions among the PDI units within the hexamer are found. The excitation delocalization between the PDI subunits is faster in the hexamer than in the dimer.

Full text of DOI:10.1002/cphc.201300602

CHEMPHYSCHEM
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DOI: 10.1002/cphc.201300602
Difference in the Photophysical Properties of
a Perylenetetracarboxylic Diimide Dimer and a Hexamer
Linked by the Same Hexaphenylbenzene Group
Lin Xue, Yan Shi, Liangliang Zhang, and Xiyou Li*[a]
A perylenetetracarboxylic diimide hexamer (6PDI) and a dimer
(2PDI) linked with the same hexaphenylbenzene group were
prepared, and the structures were fully characterized by
1H NMR spectroscopy, mass spectrometry, and elemental analy-
sis. Due to the similar molecular structure of these two com-
pounds, similar interactions between/among the PDI subunits
as well as similar photophysical properties are expected.
However, the stationary UV/Vis absorption spectra reveal that
the interactions among/between the PDI subunits in 6PDI are
significantly stronger than those in 2PDI. This can be attributed
to blocked rotation along the long axis of the PDI subunits in
6PDI due to steric hindrance of the two neighboring PDI sub-
units. The stronger interactions among the PDI subunits in
6PDI lead to long-wavelength emission, which can be assigned
to “excimer-like” excited states. A similar conclusion can also
be deduced from the fluorescence quantum yields and the
fluorescence lifetimes. Electrochemical studies revealed that in-
teractions between/among the PDI subunits in both 2PDI and
6PDI are still in the range of weak interactions. Ultrafast transi-
ent anisotropy decay dynamics revealed that excitation deloc-
alization between the PDI subunits within 2PDI and 6PDI is
quick and efficient. More interestingly, delocalization is faster
in 6PDI than in 2PDI, probably because of the stronger interac-
tions among the PDI subunits in the former.
1. Introduction
In the bacterial photosynthetic antenna of light-harvesting
complexes 1 and 2 (LH1 and LH2), photosynthetic tetrapyrrolic
pigments are deliberately positioned in a wheel-like arrange-
ment.[1–3] The absorbed light energy is transferred through
intra- or interwheel excitation energy transfer (EET) to the reac-
tion center. Inspired by these perfect architectures, cyclic and
dendritic molecular arrays of organic dyes have been explored,
and intramolecular EET has been studied with the goal of ap-
plying these molecular arrays to molecular photonic devices or
artificial light-harvesting systems.[4–9] The most popular organic
dyes investigated in this field are porphyrins because their mo-
lecular structures and photophysical properties are similar to
those of natural bacteriochlorophyll a (Bchl a).[10–23] However,
the use of light-harvesting systems in molecular photonic devi-
ces requires different types of molecular arrays with different
properties. Therefore, molecular arrays of organic dyes other
than porphyrin were also developed in the past several de-
cades.[24–30] Among the numerous organic dyes employed in ar-
tificial light-harvesting studies, perylenentetracarboxylic di-
imide (PDI) compounds play an important role, because of
their excellent stability towards heat and light as well as their
high fluorescence quantum yields (almost 100%) and large ex-
tinction coefficients in the visible-light region.[31–38] Most impor-
tantly, PDI molecules can be easily modified and decorated at
the imide nitrogen atoms or the peripheral positions, and thus,
their properties can be conventionally tuned to meet the
needs of different applications.[39,40]
In these numerous PDI-based molecular arrays, one group of
compounds, which are composed of several PDI subunits with
precisely controlled relative orientations, are designed and pre-
pared with the goal of revealing the effects of molecular inter-
actions between the PDI subunits on their photophysical prop-
erties. For instance, Wasielewski and co-workers reported a PDI
dimer linked by a xanthene spacer[41] that showed a blueshifted
band in its absorption spectrum and excimer emission in the
long-wavelength region in its fluorescence spectrum. Similar
properties were also presented by PDI dimers with flexible link-
ages.[42,43] The photophysical properties of these dimeric PDIs
can be explained as the result of the interaction between two
transitions that occur parallel to each according to exciton
theory. A more rigid structure of the PDI dimers results in
higher emission efficiency of the excimer as well as a longer
fluorescence lifetime.[44] Decorating the PDI rings with electron-
donating amino groups at the bay positions in a PDI dimer
leads to significant quenching of the emission of the excimer.
A charge-separated state is observed for this PDI dimer, which
is similar with that found for the special pair in natural photo-
synthesis.[45] PDI trimers with face-to-face stacked structures
were also reported recently by Wasielewski and co-work-
ers.[46,47] The corresponding lowest excited singlet states of the
PDI trimers are dimerlike in nature, and this suggests that
[a] L. Xue,+ Y. Shi,+ L. Zhang, Prof. X. Li
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education of
China, Department of Chemistry, Shandong University
Shanda nan lu, #27, Jinan, Shandong, 250100 (China)
Fax: (+86)531-88564464
E-mail: xiyouli@sdu.edu.cn
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201300602.
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structural distortions that accompany the formation of the
trimers are sufficient to confine the electronic interaction
within two chromophores of these systems. Some PDI arrays
with relatively weak interactions between/among the PDI sub-
units were also reported. Matak and co-workers reported a PDI
hexamer linked by a hexaazatriphenylene group that shows
a tendency to undergo aggregation due to strong p–p interac-
tions between the molecules. No significant changes in the
photophysical properties of the hexamer were identified in
comparison with those of the monomer because of the large
distances between the PDI subunits.[48] A similar phenomenon
was also reported for a PDI trimer.[49] A perylenemonocarboxyl-
ic imide (PMI) hexamer linked with a hexaperihexabenzocoro-
nene (HBC) group was reported by Mꢁllen. Intramolecular
energy or electron transfer between HBC and PMI was revealed
by steady-state absorption and fluorescence spectroscopy.[50]
A PDI hexamer linked with HBC was found to form different
aggregates in solution as well as in solid films. The different
aggregation behavior of HBC and PDI leads to different photo-
induced processes.[51] More interestingly, molecular arrays of
PDI with relative orthogonal orientations have also been re-
ported. Although the ground-state interactions between the
PDI subunits are weak due to the relative orthogonal orienta-
tion of neighboring transitions, energy transfer can occur with
high efficiency and a high rate constant.[52,53] Some flexible PDI
arrays can also be found in the literature. Because the flexible
structure brings more complicated interactions between the
neighboring PDI subunits, reliable structure–property relation-
ships cannot be established in these molecules.[54,55] In summa-
ry, some primary relationships between the structures and the
photophysical properties of the PDI arrays have been estab-
lished, but they are far from being fully understood. To pro-
mote the use of PDI arrays as antenna systems in artificial pho-
tosynthesis, further detailed studies on structure–property rela-
tionships are necessary.
In the present work, we designed a covalently linked PDI
hexamer with a hexaphenylbenzene group as a linkage (i.e.
6PDI, Scheme 1). For the sake of comparison, a dimer linked
with same linkage at the ortho positions was also prepared,
that is, 2PDI. The most important characteristic of these two
molecules is that the relative orientation of the neighboring
PDI molecules is neither parallel nor orthogonal; instead, the
PDI molecules form a dihedral angle of 608. Another important
feature is the wheel-like structure of the hexamer, which
mimics the B850 cycle in LH2. With these two compounds, we
can estimate the effects of the relative orientation and the
wheel structure on the migration of the excitation energy in
these molecules.
Scheme 1. Synthesis of the monomer, 2PDI, and 6PDI
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2. Results and Discussion
Molecular Design and Synthesis
The linkage we chose for these two compounds is
a popular building block for similar compounds such
as the porphyrin hexamer[56] and ferrocene hexa-
mer.[57] The synthesis is shown in Scheme 1. Inter-
mediate 1 was prepared by the reaction of 1,7-di(4-
tert-butylphenoxyl)-N-butyl-perylene-3,4-dicarboxylic
imide-9,10-dicarboxylic anhydride with di(4-amino-
phenyl)ethyne[58] in reasonable yield. 2PDI was pre-
pared by the reaction of 1 with tetraphenylcyclopen-
tadienone in diphenyl ether under an atmosphere of
nitrogen. 6PDI was prepared by the trimerization of
1 in refluxing 1,4-dioxane with Co2(CO)8 as the cata-
lyst. To prevent the influence of air, the catalytic reac-
tion was performed in a glove box. The product was
~
&
*
Figure 1. Absorption spectra of 6PDI ( ), 2PDI ( ), and the monomer ( ) in CH2Cl2
(1ꢂ10À6 molLÀ1). The inset shows the normalized (at 541 nm) absorption spectra of the
purified by column chromatography on silica gel first monomer, 2PDI, and 6PDI.
and then further purified by preparative thin-layer
chromatography on silica gel. The product gave a cor-
Table 1. Parameters of the absorption and fluorescence spectra of the
monomer, 2PDI, and 6PDI.
rect MALDI-TOF mass spectrum and elemental analysis results.
1
However, its H NMR spectrum shows only broad peaks with-
out any fine structure. This is because 6PDI is actually a mixture
of isomers, which results from hindered free rotation of the
single bond between the imide nitrogen atom and the carbon
atom of the phenyl ring due to steric hindrance. Because the
phenoxy groups at the 1,7-positions of the perylene core are
not symmetrically connected, hindered rotation results in
many isomers of 6PDI. All of the other new compounds were
Compound
labs [nm]
lf [nm]
Ff[a]
tf [ns] (%)[b]
(eꢂ104 [molÀ1 LcmÀ1])
monomer
2PDI
6PDI
543 (5.44), 509(3.59)
544 (10.4), 509(7.90)
544 (24.2), 509(19.6)
575
578
588
1.0
0.42
0.08
4.5 (100)
4.3(85), 6.7 (15)
4.1(72), 8.3 (28)
[a] With monomer as a reference (Ff =1). [b] The lifetimes were measured
for fluorescence at 600 nm.
1
fully characterized by H NMR spectroscopy, MALDI-TOF mass
spectrometry, and elemental analysis.
and the intensity of the bands at 510 and 545 nm can be di-
rectly correlated to the ratio of aggregates to the monomer in
solution as well as to the strength of the interactions between
the PDI subunits.[51,61] On the basis of the molecular structure
shown in Scheme 1, the relative orientation of the PDI subunits
in 2PDI and 6PDI is the same and interactions between them
should also be theoretically identical. The difference between
the absorption spectra of 2PDI and 6PDI may result from inter-
molecular aggregation of 6PDI. To verify this point, we record-
ed the temperature-dependent and concentration-dependent
absorption spectra of 6PDI (Supporting Information) and found
that the relative intensities of the 0–0 and 0–1 vibration bands
of 6PDI are temperature and concentration independent. This
excludes the possibility of intermolecular aggregation of 6PDI
in solution, and the difference between the absorption spectra
of 2PDI and 6PDI must be the result of intramolecular
interactions.
Absorption and Fluorescence Spectra
UV/Vis absorption spectroscopy is sensitive to interchromo-
phore distances and orientation, and thus it has been widely
used to evaluate inter- and intramolecular interactions be-
tween large p systems.[59,60] The absorption spectra of 2PDI,
6PDI, and the monomer are shown in Figure 1, and the corre-
sponding spectral parameters are summarized in Table 1. Two
absorption peaks at about 545 and 510 nm are observed in
the absorption spectra of both 2PDI and 6PDI, and this is simi-
lar that observed for the monomer. The two absorption bands
can be assigned to the 0–0 and 0–1 vibration bands of the
S0–S1 transition, respectively. Although there is no clear differ-
ence in the wavelengths of the bands in the absorption spec-
tra of 2PDI, 6PDI, and the monomer, the intensity ratio of the
0–0 and 0–1 bands clearly changes as the number of the PDI
subunits in one molecule increases. For the monomer, the in-
tensity of the 0–1 vibration band is very small, but in the spec-
trum of 2PDI, the relative intensity is clearly increased, and in
the spectrum of 6PDI, the intensity is increased even further.
These changes in the absorption spectra suggest the presence
of ground-state interactions between the PDI subunits in both
2PDI and 6PDI. Following a previous report, the 0–0 and
0–1 transitions should reverse in intensity upon p–p stacking,
The optimized structures, obtained with the AM1 method in
the Gaussian03 software package, of these two compounds
are shown in Figure 2, and the related parameters are labeled.
The center-to-center distances between the neighboring PDI
subunits in 2PDI are the same as those in 6PDI, but the dihe-
dral angle (a) between the PDI plane and the central phenyl
ring in 2PDI is significantly smaller than that in 6PDI (25.66 vs
35.778). As a result of the larger dihedral angle between the
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quantum yields of these three compounds were also mea-
sured, and the results are summarized in Table 1.
With the monomer as the standard, the fluorescence quan-
tum yield of 2PDI and 6PDI was found to be 0.42 and 0.08, re-
spectively, and these are both much smaller than the quantum
yield of the monomer. This result suggests that there are inter-
actions between or among the PDI subunits in 2PDI and 6PDI.
The interactions induce some nonradiative decay in the excited
states and thus reduce the fluorescence quantum yields signifi-
cantly. This phenomenon has also been found for other PDI
aggregates.[43,44] Moreover, the fluorescence quantum yield of
6PDI is smaller than that of 2PDI, which indicates that interac-
tions among the PDI subunits in 6PDI are stronger than those
in 2PDI. This is consistent with the results obtained from the
absorption spectra and can be explained as the aforemen-
tioned difference in the relative orientation of the PDI subunits
in the minimized structures of 2PDI and 6PDI.
Figure 2. Minimized structures of 2PDI and 6PDI. d1 =center-to-center dis-
tance, d2 =edge-to-edge distance between neighboring PDI subunits, a=di-
hedral angle between the two six-member rings highlighted in light gray.
PDI plane and the central phenyl ring in 6PDI, the relative ori-
entation of the neighboring PDI subunits is more of a “face-to-
face” stacking mode. Moreover, the PDI subunits in 2PDI can
rotate freely along the molecular long axis, but in 6PDI, the ro-
tation of the PDI subunits along the molecular long axis is
blocked due to steric hindrance of the two neighboring sub-
units. We suggest that the larger dihedral angle and the
blocked free rotation of the PDI subunits in 6PDI are responsi-
ble for the different absorption spectra of 6PDI and 2PDI.
The normalized fluorescence spectra of the monomer, 2PDI,
and 6PDI are shown in Figure 3. The maximum emission peak
for 2PDI and 6PDI is close to that of the monomer, which sug-
The fluorescence lifetimes of these three compounds were
also measured by time-correlated single photon counting
(TCSPC) at 600 nm. The lifetime of the monomer is 4.5 ns,
which is similar to literature results.[44] However, the fluores-
cence of 6PDI and 2PDI show biexponential decay, and thus,
two fluorescence lifetimes can be measured. The short one,
with a lifetime of 4.3 ns for 2PDI and 4.1 ns for 6PDI, can be as-
cribed to monomeric fluorescence. The long fluorescence life-
time component might be attributed to the emission of “exci-
mer-like” states. “Excimer-like” states of PDIs always show life-
times of fluorescence that are longer than those of the mono-
mer. The longest lifetime reported so far is 28 ns, which was
presented by a cyclic PDI dimer with a relatively rigid struc-
ture.[44] However, the “excimer-like” state of a PDI dimer with
a relatively flexible structure was also reported to show a fluo-
rescence lifetime of around 18 ns.[43] Therefore, the fluores-
cence lifetime of PDI “excimer-like” states varies dramatically
between compounds with different structures. The rigid struc-
ture with the strictly face-to-face stacked model will lead to
a longer fluorescence lifetime for the “excimer-like” states of
PDI, and weak interactions or a flexible PDI dimer may cause
a relatively short fluorescence lifetime for the “excimer-like”
states. The lifetimes of the “excimer-like” states in 6PDI and
2PDI are relatively short, which suggests that the interactions
between or among the PDI subunits in these compounds are
relatively weak.[43]
Electrochemical Properties
&
*
Figure 3. Normalized fluorescence spectra of 6PDI ( ), 2PDI ( ), and mono-
mer ( ) in CH2Cl2 (1.0ꢂ10À6 molLÀ1).
~
Weak ground-state interactions between/among the PDI sub-
units were also revealed by electrochemical experiments.
Differential pulse voltammetry was performed for these three
compounds, and the results are shown in Figure 4 and the ex-
perimental data are summarized in Table 2. The electrochemi-
cal behavior of these three compounds is very similar. One oxi-
dation peak appears at a relatively high positive voltage and
two reduction peaks appear in the negative region. However,
tiny differences can still be identified among these three com-
pounds. The first reduction occurs at À0.61 V for the mono-
mer, but this peak moves to À0.63 and À0.65 V for 2PDI and
gests that the emission is dominated by the fluorescence of
the monomeric PDI. However, in the emission band of both
2PDI and 6PDI, a tail extending to longer wavelengths can be
identified. This tail is more significant in the spectrum of 6PDI
than in the spectrum of 2PDI, which corresponds well with the
stronger interactions among the PDI subunits in 6PDI, as re-
vealed by the absorption spectra. The emission tail in the
long-wavelength region can be assigned to “excimer-like”
states due to face-to-face interactions.[43] The fluorescence
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with constant interactions between the neighboring chloro-
phyll units have been well documented.[62–65]
In addition to the TAA experiments, femtosecond transient
absorption (TA) spectra were recorded for the three com-
pounds, and the results are shown in the Supporting Informa-
tion. The TA spectra of the three compounds are similar and
feature three groups of peaks. The first group of peaks results
from ground-state bleaching (GSB), and they present as two
negative peaks around 510 and 550 nm. The second group of
signals results from excited-state absorption (ESA), and they
appear around 710 nm as broad positive peaks. This ESA band
may actually extend into the lower wavelength region, but this
cannot be discerned because of strong overlap with the more-
pronounced GSB band. Besides the GSB and ESA bands,
a stimulated emission (SE) band can be found in the 550–
650 nm region in the TA spectra of the monomer and 2PDI.
The SE band is not found in the TA spectrum of 6PDI because
the relatively strong interactions between the PDI subunits en-
hance the nonradiative decay pathway for the singlet excited
states of the PDIs. This corresponds well with the results ob-
tained from the stationary absorption and fluorescence
spectra.
Figure 4. Differential pulse voltammetry of a) the monomer, b) 2PDI, and
c) 6PDI in CH2Cl2 containing 0.1m [Bu4N+][ClO4À].
Figure 5 presents the time dependence of the anisotropy (r)
of the monomer, 2PDI, and 6PDI probed at 710 nm, in which
the ESA signal dominates the absorption difference spectra,
followed by the 400 nm excitation. The fitting results are sum-
marized in Table 3. Notably, after the initial 10 ps, the anisotro-
py is nearly constant for all three compounds. The anisotropy
dynamics of 2PDI and 6PDI are significantly different from
those of the monomer. Upon photoexcitation of the S2 state
(400 nm), all three compounds showed ultrafast anisotropy
decay from a value near 0.4 to a negative value. This is reason-
Table 2. Half-wave redox potentials [V vs SCE] of the monomer, 2PDI,
and 6PDI in CH2Cl2 containing 0.1m [Bu4N][ClO4].
Compound
Oxd1 [V]
Red1 [V]
Red2 [V]
DE0 [a] [V]
1/2
monomer
2PDI
6PDI
1.48
1.48
1.50
À0.61
À0.63
À0.65
À0.79
À0.79
À0.78
2.09
2.11
2.15
[a] DE01/2 =Ooxd ÀRred
1
1
6PDI, respectively. This negative shift in the first reduction po-
tential implies that it is more difficult to reduce 2PDI and 6PDI
than the monomer. Because the reduction process is closely re-
lated to the energy levels of the LUMO, the more negative first
reduction potential of 6PDI and 2PDI indicates that their
LUMOs lie at a higher energy level. The shift in the first reduc-
tion potential suggests the presence of interactions between/
among the PDI subunits in 6PDI and 2PDI, but the significance
of these interactions is small.
Ultrafast Transient Anisotropy Decay Dynamics
To further explore the relaxation dynamics of the excited-state
interactions, we performed femtosecond transient absorption
anisotropy (TAA) experiments on these three compounds.
Anisotropy decay implies a dependence on the relative orien-
tations of the pump and probe polarizations, which is caused
by reorientation of excitonically coupled transition dipole mo-
ments or by differently oriented transition dipole moments if
excitation of an energy hopping process (i.e. EET) exists.[14,56]
Studies on anisotropy depolarization of natural light-harvesting
systems (i.e. LH1 and LH2) that have circular arrangements
Figure 5. Transient absorption anisotropy decay profiles of a) the monomer,
b) 2PDI, and c) 6PDI in CH2Cl2.
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tween the PDI subunits and finally affect the excitation energy
delocalization process.
Table 3. Fitting results of the anisotropic decay at 710 nm for the mono-
mer, 2PDI, and 6PDI.[a]
Compound
t1 [fs]
t2 [fs]
t3[b] [ps]
Experimental Section
monomer
2PDI
6PDI
97
100
100
–
371
168
>10
>10
>10
General Information
1H NMR spectra were recorded with a Bruker 300 or 400 MHz NMR
spectrometer. Chemical shifts are reported relative to tetramethylsi-
lane as an internal standard. MALDI-TOF mass spectra were record-
ed with a Bruker/ultra flex instrument. Absorption spectra were
measured with a Hitachi U-4100 spectrophotometer. Fluorescence
spectra and fluorescence lifetimes were measured with an ISS K2
system. The fluorescence lifetimes were measured with a phase
modulation model with a scattering sample as standard. Electro-
chemical measurements were performed with a BAS CV-50W vol-
tammetric analyzer. The cell comprised inlets for a glassy carbon
disk working electrode that was 2.0 mm in diameter and a silver-
wire counterelectrode. The reference electrode was Ag/Ag+, which
was connected to the solution by a Luggin capillary, the tip of
which was placed close to the working electrode. It was corrected
for junction potentials by referencing internally to the ferroceni-
um/ferrocene (Fc+/Fc) couple [E1/2 (Fc+/Fc)=501 mV vs SCE]. Typi-
cally, a 0.1 molLÀ1 solution of [Bu4N][ClO4] in CH2Cl2 containing
0.5 mmolLÀ1 of sample was purged with nitrogen for 15 min, and
then the voltammograms were recorded at ambient temperature.
The scan rate was 10 mVsÀ1 for differential pulse voltammetry. The
transient absorption spectra and anisotropic decay were measured
by using a homemade femtosecond broadband pump–probe
setup. Experimental details can be found in the Supporting
Information.
[a] The value of c2 for each fitting was in the range of 1.05–1.24. [b] Long
component in the TAA decay, which was longer than the time window of
the experiment.
able for PDI derivatives, for the dipole moment of the S0–S2
electronic transition is perpendicular to that of the S0–S1 transi-
tion.[66,67] By fitting the decay profile of the monomer with
a biexponential function, two time constants were obtained.
The short one (172 fs) can be assigned to the relaxation from
S2 to S1, in which the long lifetime component (beyond the ex-
perimental time window, >10 ps) is due to the rotation diffu-
sion of the molecules. Three time constants were obtained
from the best fits of the profiles of the TAA spectra of 2PDI
and 6PDI. The shortest (100 fs for both 2PDI and 6PDI) and
longest (>10 ps) components are similar to those of the mo-
nomer and, therefore, can be assigned to S2 to S1 internal con-
version and rotation diffusion, respectively. Another fast com-
ponent (371 fs for 2PDI and 168 fs for 6PDI) was found in the
TAA decay of 2PDI and 6PDI. On the basis of previous observa-
tions of the different absorption spectra, a delocalized exciton
state is formed in branched 2PDI and 6PDI, and it is reasonable
to attribute this second fast component in 2PDI and 6PDI to
excitation delocalization between/among the different PDI
branches.[68] From the fitting results, delocalization of the excit-
ed states in 6PDI (168 fs) is faster than that in 2PDI (371 fs),
which supports the idea that the intramolecular interactions in
6PDI are stronger than those in 2PDI. This behavior is also
seen in photosynthetic light-harvesting antenna pigment
systems.[69]
Synthesis and Characterization
Compound 1: A 100 mL, three-necked flask was charged with
N-butyl-1,7-di(4-tert-butyl)phenoxyl-perylene-3,4-dicarboxylic imide-
9,10-dicarboxylic anhydride (148 mg, 0.2 mmol), bis(4-aminopheny-
l)ethyne (20 mg, 0.1 mmol), toluene (20 mL), and imidazole (1.5 g).
The mixture was heated to 1168C and kept at this temperature for
about 10 h. The progress of the reaction was monitored by thin-
layer chromatography (TLC). Upon complete consumption of the
reactant as revealed by TLC, the reaction mixture was evaporated
to dryness under reduced pressure. The residue was dissolved in
chloroform and washed with water repeatedly to remove imida-
zole. The organic layer was dried with anhydrous magnesium sul-
fate overnight, and the solvent was then evaporated. The residue
was purified by column chromatography on silica gel (chloroform/
methanol=98:2). Compound 1 was collected as a red solid
(39.8 mg, 24%). 1H NMR (300 MHz, CDCl3): d=9.11–9.25 (d, 4H),
8.23–8.42 (d, 4H), 8.11 (d, 4H), 7.66–7.68 (d, 4H), 7.44–7.48 (t, 4H),
7.36 (m, 8H), 6.84–6.94 (m, 8H), 3.2 (t, 4H), 1.59–1.64 (t, 4H), 1.32–
1.34 (m, 40H), 0.98 ppm (m, 6H). MS (MALDI-TOF): m/z: 1660.17
[M]+ (C110H90N4O12 calcd 1659.96). Elemental analysis calcd (%) for
C110H90N4O12 (1659.66): C 79.59, H 5.47, N 3.38; found: C 79.32, H
5.36, N 3.36.
3. Conclusions
In summary, a PDI dimer and a wheel-like hexamer linked by
the same hexaphenylbenzene group were prepared. Due to
greater steric hindrance in 6PDI caused by the two neighbor-
ing PDI subunits, rotation of the PDI subunits along the molec-
ular long axis is blocked, whereas the PDI subunits in 2PDI can
rotate freely. Thus, interactions among the PDI subunits in
6PDI are relatively stronger than those in 2PDI, as revealed by
stationary absorption and fluorescence spectra. Quick and effi-
cient excitation delocalization between/among the PDI subu-
nits was found in both 6PDI and 2PDI. Excitation energy deloc-
alization in 6PDI is faster than that in 2PDI, which can be as-
cribed to stronger ground-state interactions among the PDI
subunits in 6PDI. This result demonstrates successfully that the
photophysical properties of 2PDI and 6PDI are significantly dif-
ferent from each other even though they have similar molecu-
lar structures. The subtle difference in the flexibility of the mol-
ecules may cause a dramatic change in the interactions be-
2PDI: A 100 mL, three-necked flask was charged with 1 (166 mg,
0.1 mmol) and tetraphenylcyclopentadienone (38 mg, 0.1 mmol);
diphenyl ether (100 mL) was then added. The reaction mixture was
heated at reflux for 12 h, and the reaction mixture was then
cooled to room temperature and poured into methanol (100 mL).
The solid was separated by filtration and dried at room tempera-
ture. The product was purified by column chromatography on
silica gel (dichloromethane). 2PDI was collected as a red solid
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 3319 – 3326 3324

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
1
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(10 mg, 5%). H NMR (300 MHz, CDCl3): d=9.53 (d, 4H), 8.51–8.53
(d, 2H), 8.42–8.44 (d, 2H), 8.31 (s, 2H), 8.21 (s, 2H), 7.39–7.41 (d,
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6PDI: Under an atmosphere of nitrogen, a three-necked flask was
charged with 1 (166 mg, 0.1 mmol), dry 1,4-dioxane (20 mL), and
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12h. The solvent was then evaporated to dryness under reduced
pressure. The residue was purified by column chromatography on
silica gel (dichloromethane), and 6PDI was collected as a dark red
solid (5.94 mg, 4%). 1H NMR (300 MHz, CDCl3): d=9.28–9.42 (br,
12H), 8.40–8.60 (br, 12H), 8.12–8.35 (br, 12H), 7.60–7.80 (m, 24H),
6.89–7.01 (m, 24H), 6.72–6.86 (m, 24H), 4.08–4.13 (d, 12H), 1.56 (t,
12H), 1.26–1.35 (m, 120H), 0.87–0.97 ppm (m, 18H). MS (MALDI-
TOF): m/z: 4980.7 [M]+ (C330H270N12O36 calcd 4979.89). Elemental
analysis (%) calcd for C330H270N12O36 (4979.89): C 79.59, H 5.47, N
3.38; found: C 79.47, H 5.42, N 3.27.
Acknowledgements
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We thank the Natural Science Foundation of China (Grant Nos.
21073112, 21173136, and 91233108), the National Basic Research
Program of China (973 Program: 2012CB93280), and the Natural
Science Foundation of Shandong Province (ZR2010EZ007) for fi-
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Published online on July 24, 2013
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