Perylenetetracarboxylic diimide derivatives linked with spirobifluorene

Article abstract of DOI:10.1021/ol300985h

A series of perylenetetracarboxylic diimide (PDI) compounds linked with spirobifluorene have been prepared. The orthogonal configuration of the PDI subunits efficiently hindered their molecular aggregation in solution. Energy transfer from a 1,7-diphenoxyl group substituted PDI (PO-PDI) to a 1,7-dipyrrolidinyl group substituted PDI (PY-PDI) occurred with a large efficiency when PO-PDI was selectively excited, despite the orthogonal orientation of the two units. This observation was in direct conflict with predictions derived from the Foerster theory. More interestingly, this efficient energy transfer also occurred in the solid state.

Full text of DOI:10.1021/ol300985h

ORGANIC
LETTERS
2012
Vol. 14, No. 12
3052–3055
Perylenetetracarboxylic Diimide
Derivatives Linked with Spirobifluorene
Yanan Du,† Lilin Jiang,‡ Jun Zhou,† Guiju Qi,† Xiyou Li,*,† and Yanqiang Yang*,‡
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Department
of Chemistry, Shandong University, Jinan 250100, China, and Center for Condensed
Matter Science and Technology, Department of Physics, Harbin Institute of Technology,
Harbin 150001, China
xiyouli@sdu.edu.cn; yqyang@hit.edu.cn
Received April 27, 2012
ABSTRACT
A series of perylenetetracarboxylic diimide (PDI) compounds linked with spirobifluorene have been prepared. The orthogonal configuration of the
PDI subunits efficiently hindered their molecular aggregation in solution. Energy transfer from a 1,7-diphenoxyl group substituted PDI (PO-PDI) to
a 1,7-dipyrrolidinyl group substituted PDI (PY-PDI) occurred with a large efficiency when PO-PDI was selectively excited, despite the orthogonal
€
orientation of the two units. This observation was in direct conflict with predictions derived from the Forster theory. More interestingly, this
efficient energy transfer also occurred in the solid state.
Significant effort has recently been devoted to the
synthesis of highly ordered organic materials based on
perylenetetracarboxylic diimide (PDI) by utilizing weak
intermolecular forces.1 The photophysical properties of
individual molecular components in the molecular assem-
bly were, however, foundtobe dramaticallyvariedbecause
of the strong intermolecular interactions. More impor-
tantly, changes in the photophysical properties were some-
times unpredictable. The development of covalently
connected PDI oligomers with rigid conformations be-
tween the PDI units, with the aim of revealing the relation-
ship between the structure and photophysical properties of
the PDI assembly, is therefore necessary.2 PDI molecules
arranged in a parallel manner have been reported by
† Shandong University.
‡ Harbin Institute of Technology.
(1) (a) Langhals, H. Helv. Chim. Acta 2005, 88, 1309–1343.
(b) Wasielewski, M. R. Acc. Chem. Res. 2009, 42, 1910–1921. (c) Chen,
€
Z. J.; Baumeister, U.; Tschierske, C.; Wurthner, F. Chem.;Eur. J. 2007,
13, 450–465. (d) Rybtchinski, B. ACS Nano 2011, 5, 6791–6818.
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(e) Yagai, S.; Seki, T.; Karatsu, T.; Kitamura, A.; Wurthner, F. Angew.
Chem., Int. Ed. 2008, 47, 3367–3371. (f) Zang, L.; Che, Y.; Moore, J. S.
Acc. Chem. Res. 2008, 41, 1596–1608.
r
10.1021/ol300985h
Published on Web 06/05/2012
2012 American Chemical Society

several groups in the literature.3 PDIs aggregated with
relative orthogonal orientations, however, have rarely
been reported.4 Therefore, further investigation of the
effects of the orthogonal orientation between neighboring
PDI units on the photophysical properties is required.
of a series of novel PDI compounds connected by a spiro-
bifluorene core, Spiro-PDI 1, Spiro-PDI 2, and Spiro-PDI
3 (Scheme 1). For the sake of comparison, one other model
compound, F-PDI 4, containing the same components but
with a different linkage, was also prepared. The distances
and angles between the PDI units were fixed by the spiro-
bridge because of its rigid molecular structure,6 and these
compounds were therefore expected to behave as ideal
models for studying the interactions between PDI units
with orthogonally arranged orientations. Detailed syn-
thetic procedures and the corresponding structural char-
acterizations are shown in the Supporting Information
(SI). All of the new compounds were fully characterized
by 1H and 13C NMR and MALDI-TOF mass spectra or
elemental analysis.
Scheme 1. Molecular Structures of the Compounds
The absorption spectra of Spiro-PDI 1, Spiro-PDI 2,
and Spiro-PDI 3 in solution showed two absorption
maxima at 545 and 510 nm, which were attributed to
the absorption of PO-PDI (Figure 1). Another absorp-
tion band that was centered at 688 nm in the absorption
spectra of Spiro-PDI 2 and Spiro-PDI 3 was assigned to
the absorption of PY-PDI. The absorption spectra of the
PO-PDI and PY-PDI units in these compounds were
identical to those of their monomeric counterparts PO-
PDI 5 and PY-PDI 6, suggesting that there are no
significant ground state interactions within these com-
pounds between the PO-PDI or PY-PDI subunits. More
importantly, the absorption spectra of Spiro-PDIs re-
vealed no changes with increasing concentration, sug-
gesting that Spiro-PDI compounds do not self-assemble
into molecular aggregates within the concentration range
tested (1 Â 10À6À1 Â 10À4 mol LÀ1), The absorption
3
spectra of F-PDI 4, however, changed dramatically with
increasing concentration, as shown in the SI, indicating
directly the formation of molecular aggregates in a
concentrated solution. These results clearly revealed the
importance of the orthogonal orientation on hindering
the formation of molecular aggregates in a concentrated
solution.
The linking of conjugated components with a spiro-
bridge represents a sophisticated strategy for achieving
an orthogonal orientation between two conjugated
components.5 Herein, we present the design and synthesis
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Figure 1. Absoprtion (solid symbol) and fluorescence (open
symbol) spectra of Spiro-PDI 1 (black), Spiro-PDI 2 (red),
and Spiro-PDI 3 (blue) in toluene.
Org. Lett., Vol. 14, No. 12, 2012
3053

The fluorescence spectra of Spiro-PDI 1 bear a close
resemblance to those of monomeric PO-PDI 5 with a
strong emission band at 580 nm and a 96% fluorescence
quantum yield (λex 530 nm). The fluorescence lifetime was
measured to be 4.3 ns, which was also similar to that of
standard PO-PDI 5 (Figure 1, Table 1).7 Taken together,
these results further demonstrate that there is no ground
state interaction between the PO-PDI units within com-
pound Spiro-PDI 1.
Whenthe PO-PDI units are selectivelyexcited at530nm,
the fluorescence spectra of Spiro-PDI 2, Spiro-PDI 3, and
F-PDI 4 show one broad band centered at 720 nm, which
was attributed to the fluorescence of PY-PDI.3d,8 The
fluorescence from PO-PDI at 570 nm was too weak to be
detected. This result revealed that the fluorescence of PO-
PDI was efficiently quenched by PY-PDI in Spiro-PDI 2,
Spiro-PDI 3, and F-PDI 4. The fluorescence quenching
efficiencies (Φq) calculated from the fluorescence quantum
yields were close to 100%. The appearance of the fluores-
cence of PY-PDI when PO-PDI was selectively excited
suggested the presence of an energy transfer from PO-PDI
to PY-PDI within Spiro-PDI 2, Spiro-PDI 3, and F-PDI 4.
Due to no ground state interactions between donor and
Figure 2. Transient absorption spectra of Spiro-PDI 2 in toluene
after excitation at 530 nm with a 120 fs pulse of laser. The inset
shows the dynamic decay of the bleaching at 545 nm and the
fitting results.
(688 nm), the difference between these two spectra in the
range of 350À580 nm, where only PO-PDI absorbs light,
reflects the efficiency of the energy transfer. The ΦEnt
values calculated in this way for Spiro-PDI 2 and Spiro-
PDI 3 were ∼100%.
€
acceptor as revealed by the absorption spectra, the Forster
type energy transfer is expected. The presence of an energy
transfer was also supported by the excitation spectra of
these three compounds. When the fluorescence intensity of
PY-PDI at 720 nm was monitored, the absorption of PO-
PDI in the range of 350À580 nm contributed significantly
to the fluorescence of PY-PDI (SI, Figure S1).
This efficient energy transfer between PO-PDI and PY-
PDI within Spiro-PDI-2 and Spiro-PDI-3 was surprising
€
because the Forster theory predicts that the resonance
energy transfer between a pair of orthogonally arranged
transition dipole moments is a forbidden process. The
transition dipole moments of PDIs are oriented parallel
to the connection line between the nitrogen atoms of the
imides for both absorption and fluorescence.10 The ob-
served efficient energy transfers between PO-PDI and PY-
PDI within Spiro-PDI 2 and Spiro-PDI 3 are therefore
extraordinary. Only one example can be found in the
literature involving an efficient energy transfer between a
pair of perpendicularly arranged transition dipoles,4 in
which the energy transfer was attributed to thermally
populated ground-state vibrations breaking the orthogo-
nal arrangement.
The kinetics of the energy transfer within Spiro-PDI 2
were investigated using femtosecond transient absorption
measurements by selectively exciting the PO-PDI units at
530 nm (120 fs in pulse width) and probing them with a
white light supercontinuum probe (460À750 nm). The
transient spectra of Spiro-PDI 2 in pure toluene taken at
different delay times are shown in Figure 2. The major
features centered at 460 to 610 nm were attributed to the
ground state bleaching of PO-PDI.11 Excitation at 530 nm
generated the immediate bleaching of PO-PDI, with the
bleaching of PY-PDI appearing subsequently at 700 nm
along with the decay of the PO-PDI bleaching. Kinetic
analysis on the bleaching of PO-PDI at 545 nm revealed
Table 1. Fluorescence Properties of the Compounds
Φf (%)a
τ (ns)
b
c
λem
ΦEnt
Φq
compound
(nm) PO-PDI PY-PDI PY-PDI
(%)
(%)
Spiro-PDI 1 569
Spiro-PDI 2 713
Spiro-PDI 3 715
96
;
;
;
;
0.14
0.12
0.00
1.78
2.66
3.61
2.31
3.35
4.40
100
100
100
99.9
99.9
100
F-PDI 4
714
a With PO-PDI 5 in toluene (Φf = 100%) as standard. b Calculated
from the fluorescence quantum yields (Φf). c Φq = 1 À Φf/Φ0.9
The energy transfer efficiency (ΦEnt) can be estimated
from the excitation and absorption spectra of the com-
pounds. After normalizing the absorption and excitation
spectra at the wavelength of the maximum absorption
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Org. Lett., Vol. 14, No. 12, 2012

three dynamic decay processes with time constants of
τ1 = 212 fs (78.2%), τ2 = 2.10 ps (20.2%), and τ3 = 401 ps
(1.6%). The two fastest components of the decay were
attributed to the energy transfer from PO-PDI to PY-PDI,
whereas the minor slower component was attributed to
another fluorescence quenching process, which was as-
sumed to be an electron transfer between PO-PDI and PY-
PDI.12
of the thin solid films of Spiro-PDI 3 (SI, Figure S5). This
finding provided us with a new idea for the design of novel
fluorescent solid materials.
Thin solid films of compounds Spiro-PDI 1 and Spiro-
PDI 2 were prepared by simply spreading drops of toluene
solution (1 Â 10À5 mol LÀ1) of these compounds onto the
surface of a substrate. Following evaporation of the
solvents at rt in a closed container, the absorption and
fluorescence spectra of the thin solid films were recorded
(Figure 3). The absorption spectra of Spiro-PDI 1 in solid
film showed two absorption bands at 512 and 545 nm,
respectively, with the one at 512 nm having the maximum
intensity. Both of the bands were red-shifted by 5 nm
relative to those of monomeric PO-PDI in solution. The
fluorescence spectra contained a red-shifted broad band at
∼650 nm, which was assigned to an excimer emission.3c,7b
The results of both absorption and fluorescence spectra
revealed effective πÀπ interactions between the PO-PDI
units in the thin solid films, which might be the result of the
sub-branch interactions of different molecules in the solid
films.
The absorption spectra of the thin solid film of Spiro-
PDI 2 showed two broad absorption bands, which were
similar with those of Spiro-PDI 1. But these two absorp-
tion bands further red-shifted to 519 and 551 nm in
comparison with that of Spiro-PDI 1. The band at 551
nm had the largest oscillation strength, suggesting the
presence of “J-type” πÀπ interactions between the PO-
PDI units of different molecules in the thin solid films of
Spiro-PDI 2.13 The fluorescence spectra (λex 530 nm),
however, showed only the emission of PY-PDI, which
slightly blue-shifted relative to the monomeric PY-PDI 6,
and no emission of PO-PDI could be detected. This result
demonstrated that the energy transfer from PO-PDI to
PY-PDI in this thin solid film occurred with a large
efficiency. It is worthy of note that the PY-PDI is normally
not fluorescent in the aggregated state, as reported in the
literatures.3b,d The observed emission of PY-PDI in the
thin solid films of Spiro-PDI 2 indicated that the PY-PDI
of Spiro-PDI 2 does not form aggregates in the thin solid
films likely because of the spiro-bridge. This result has also
been supported by the absorption and fluorescence spectra
Figure 3. Normalized fluorescence (open symbol) and absorp-
tion (solid symbol) spectra of the thin solid films of Spiro-PDI 1
(black) and Spiro-PDI 2 (red).
In summary, a series of PDI compounds, Spiro-PDI 1,
Spiro-PDI 2, and Spiro-PDI 3, containing PO-PDI and PY-
PDI units linked by a spiro-bridge, were designed and
prepared. An unexpected efficient and quick photoinduced
energy transfer from PO-PDI to a perpendicularly arranged
PY-PDI was found in these compounds, which was in direct
€
conflict with the predictions of the Forster theory. Due to
Langhal’s report of a similar energy transfer in a different
system,4 we suggest that this kind of “unexpected” energy
transfer might be a common phenomenon in orthogonally
configured systems. More importantly, energy transfer
from PO-PDI to PY-PDI was also observed in the thin
solid films of Spiro-PDI 2. We believe that this finding will
be useful for the design of new artificial photosynthetic
systems and also helpful in guiding the design of organic
fluorescent materials for molecular electronic devices.
Acknowledgment. Financial support from the Natural
Science Foundation of China (21073112, 21173136) and
the Natural Science Foundation of Shandong Province
(ZR2010EZ007) is gratefully acknowledged.
Supporting Information Available. Details of synthesis
and characterizations, excitation fluorescence spectra,
concentration dependent absorption spectra, time re-
sovled emission traces. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) The estimated free energy changes (ΔG) of the electron transfer
between PO-PDI and PY-PDI within Spior-PDI 2 following the
RehmÀWeller equation is À0.16 eV. The electron transfer is a thermal
dynamically favorable reaction.
(13) Wu, H.; Xue, L.; Shi, Y.; Chen, Y.; Li, X. Langmuir 2011, 27,
3074–3082.
The authors declare no competing financial interest.
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