Terthiophene-perylene diimides: Color tuning via architecture variation

Article abstract of DOI:10.1021/jp911800q

N, Na?2-Bisoctylperylene diimides (PDIs) have been functionalized in the 1,7-position with terthiophenes of varying architecture giving three new donor-acceptor (D-A) compounds of the same molecular weight. Different conjugation lengths, arrangements, and connections of the thiophene units within themselves and toward the PDI core have strong effects on the optical, electronic, and photochemical properties of the D-A compounds. Like jigsaw pieces joined together to give different pictures, the terthiophenes are linked to PDIs to achieve different colors. These insights into tuning color and energy levels can open new possibilities for tailoring chromophores to their desired applications, e.g., organic photovoltaics or organic field effect transistore. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp911800q

J. Phys. Chem. B 2010, 114, 14343–14347
14343
Terthiophene-Perylene diimides: Color Tuning via Architecture Variation†
‡
§
§
,‡
,§
Henrike Wonneberger, Chang-Qi Ma, Martina A. Gatys, Chen Li,* Peter B a¨ uerle,* and
,
‡
Klaus M u¨ llen*
Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, and
Institute of Organic Chemistry II and AdVanced Materials, UniVersity of Ulm,
Albert-Einstein-Allee 11, 89081 Ulm, Germany
ReceiVed: December 14, 2009; ReVised Manuscript ReceiVed: March 29, 2010
N,N′-Bisoctylperylene diimides (PDIs) have been functionalized in the 1,7-position with terthiophenes of
varying architecture giving three new donor-acceptor (D-A) compounds of the same molecular weight.
Different conjugation lengths, arrangements, and connections of the thiophene units within themselves and
toward the PDI core have strong effects on the optical, electronic, and photochemical properties of the D-A
compounds. Like jigsaw pieces joined together to give different pictures, the terthiophenes are linked to PDIs
to achieve different colors. These insights into tuning color and energy levels can open new possibilities for
tailoring chromophores to their desired applications, e.g., organic photovoltaics or organic field effect transistors.
Chemical compounds, which can absorb and reflect light of
different wavelengths, stain the world and make it beautiful.
To achieve materials in a myriad of colors, organic chemists
are still attracted by the design and synthesis of new dyes or
knowledge, this is the first report varying the color of perylene
dyes by only changing the arrangement of three thiophenes in
the 1,7-positions of the perylene core.
All three compounds were prepared by imidization of 1,7-
dibromoperylene-3,4:9,10-tetracarboxylic acid dianhydride with
n-octylamine as described in literature11 and subsequent metal
catalyzed coupling reactions of the N,N′-bisoctyl-1,7-dibromop-
erylene-3,4:9,10-tetracarboxylic acid diimide with the three
different terthiophene moieties (5′-(4,4,5,5-tetramethyl [1,3,2]
1,2
pigments where perylene dyes are key chromophores. Nowa-
days they are being used not only as pure colorants for aesthetic
purposes but also as active components in high technologies,
such as organic electronics, biology, and sensing.6 By
functionalization using innumerable synthetic pathways, perylenes
of various colors can be achieved possessing unique optical and
electrochemical properties. Generally, the color of chromophores
depends on the structure of the chemicals, which can be tuned
by (1) enlargement of the π-system, (2) transition from aromatic
to quinoidal structures, (3) polymerization, and (4) introduction
of donor-acceptor substituents. Among all these methods, the
last one is most efficient because of abundant donors and
acceptors.7
3
,4
5
12,13
dioxaborolan-2-yl)-5,5′-bis(trimethylsilyl)-2,2′:3′,2′-terthiophene
(5), 3′-(4,4,5,5-tetramethyl[1,3,2]dioxaborolan-2-yl)-5,5′′-bis(t-
rimethylsilyl)-2,2′:5′,2′′-terthiophene (7), and 5-(4,4,5,5-
tetramethyl[1,3,2]dioxaborolan-2-yl)-2,2′:5′,2′′-terthiophene (9),
respectively) under Suzuki conditions followed by removal of
the trimethylsilyl groups with tetra-n-butylammonium fluoride
(TBAF) in the cases of 1 and 2 (Figure 1). Terthiophene 7 was
synthesized starting from 3-bromo-2,5-diiodothiophene (11) and
trimethyl(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-
In this article, we present three N,N′-bisoctylperylene diimide
PDI) derivatives (1-3, Figure 1) with terthiophene moieties
2
-yl)silane (13), which were prepared according to modified
(
14,15
protocols (see Supporting Information) described in literature
on the perylene core. Thiophene-based materials are well-known
as p-type semiconductors and exhibit extraordinary charge
transport properties that find broad application in optoelectronic
(
Scheme 1). Suzuki coupling of 11 with 2 equiv of 13 yielded
3
′-bromo-5,5′′-bis(trimethylsilyl)-2,2′:5′,2′′-terthiophene (14).
8
Subsequent metal-halogen exchange of 14 with n-butyllithium
and quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane yielded 3′-(4,4,5,5-tetramethyl[1,3,2]dioxaborolan-
devices. By coupling two terthiophenes as electron-donating
groups in the 1,7-position of PDI, we obtained bay-function-
alized PDI compounds for which the first oxidation potential is
shifted considerably into the negative direction. Whereas the
focus of other research groups so far has mainly been set on
2
-yl)-5,5′′-bis(trimethylsilyl)-2,2′:5′,2′′-terthiophene, 7. Detailed
information on the synthesis can be found in the Supporting
Information.
9
,10
varying the chain length of the thiophene moiety,
we will
Both 3 and 2 consist of linear terthiophenes based on R-R
connection of the three thiophene units. For 3, the terthiophene
and the PDI core are joined via the R-position of the terminal
thiophene. In 2, however, it is the middle thiophene unit of the
terthiophene chain that is linked to the PDI core via its
compare three PDIs bearing terthiophene units of different
architecture in the 1,7-bay positions of the perylene core,
displayed in Figure 1. Moreover, due to the variation in the
architecture of the three terthiophenes, these terthiophene-PDIs
all (Figure 1) have the same molecular weight, consist of a PDI
and three thiophene units, but interestingly, are of different color,
i.e., bluish green, purple, and dark green, respectively. To our
ꢀ
-position. Different from 3 and 2, 1 comprises two branched
terthiophenes containing an R-R and an R-ꢀ connection of
the thiophene units within the terthiophene building block. The
terthiophene itself is R-linked to the PDI core at the middle
thiophene unit.
†
Part of the “Michael R. Wasielewski Festschrift”.
*
To whom correspondence should be addressed. E-mail: muellen@
mpip-mainz.mpg.de.
‡
To investigate their different intramolecular charge transfer
(ICT) characters caused by the terthiophenes, the absorption
Max-Planck Institute for Polymer Research.
University of Ulm.
§
1
0.1021/jp911800q  2010 American Chemical Society
Published on Web 04/23/2010

1
4344 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Wonneberger et al.
Figure 1. Synthesis scheme of the terthiophene substituted perylenedicarboximides 1-3: (a) Pd(PPh
days; yields 47%, 30%, 44%, respectively; (b) TBAF, THF, 30-40 min, RT-40 °C; yield 78%; (i) NIS (N-iodosuccinimide); (ii) 1 ) BuLi, 2 )
-isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane; (iii) Pd (dba) CHCl /HP(tBu) BF /K PO ; (iv) 1 ) Mg, diethyl ether, 2 ) 2,3-dibromothiophene,
Ni(dppp)Cl ]; (v) Bu NF, THF.
3 4 2 3 2
) , K CO , toluene, H O, ethanol, 80 °C, 3
2
2
3
3
4
4
3
4
[
2
4
and fluorescence emission spectra of 1-3 were measured and
are displayed in Figure 2. Going from 2 f 1 f 3, a monotonic
decrease of the absorption coefficient and a red shift of the
absorption onset are observed, which indicates varying donor
strength due to the different architectures of the three ter-
thiophenes. Whereas the branched terthiophene compound (1)
and the linear R-connected terthiophene compound (3) both
show broad, structureless, and intense low-energy charge transfer
3 possesses three R-connected thiophenes, resulting in the
longest conjugation, which is also reflected in the bathochro-
mically shifted absorption maximum. This red shift is noticeably
smaller for 1 with only two R-conjugated thiophenes around
the PDI core. 2 with the ꢀ-connected terthiophene on the other
hand shows a hypsochromic shift (λmax ) 512 nm) that is due
to the lack of conjugation (ꢀ-connected with PDI core) between
the PDI core and the terthiophene moiety. The even hypso-
chromic shift of the absorption maximum of 2 compared to a
naked PDI (λmax ) 526 nm for N,N′-bis(1-heptyloctyl)-3,4:9,10-
absorption bands (transition from the ground state S
excited charge transfer singlet state S ) with a maxima at λ )
91 and 610 nm, respectively, this S f S band has almost
vanished in the linear ꢀ-connected terthiophene compound
0
to the
1
18
5
0
1
perylenetetracarboxdicarboximide), however, results from the
electronic S f S transition.
0
2
1
6,17
(
2).
This effect corresponds to the decreasing length of the
Regarding the fluorescence emission, 1 and 3 show emission
bands of different intensities at λmax ) 738 and 782 nm,
R-conjugated oligothiophene around the PDI core (3 > 1 > 2).

Terthiophene-Perylene diimides
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14345
Figure 2. (a) Absorption spectra (in CH
2
Cl
2
) and (b) fluorescence emission spectra (in toluene) of 1-3; excitation wavelengths according to the
-
5
legend (x ) λex, c ) 2 × 10 mol/L). (c) Absorption spectra of 1-3 in toluene after sunlight irradiation and (d) fluorescence emission spectra in
toluene of 1 before and after sunlight irradiation (λex ) 400 nm): *, before sunlight irradiation; **, after sunlight irradiation, not further purified.
respectively. As the emission bands are in the near-infrared
region and very broad without the typical PDI fine structure,
these emission bands can be assigned to the intramolecular
charge transfer between the terthiophene donor and the PDI
acceptor.19 Moreover, both compounds possess large Stokes
shifts (147 and 172 nm, respectively). The emission intensity
of 3 is much lower than that of 1. A similar fluorescence
quenching in the presence of a thiophene donor was detected
respectively), it is coherent that no obvious solvatochromic
absorption is observed. Regarding the fluorescence, the quench-
ing in more polar solvents can be explained by a higher
thermodynamic driving force helping the photoinduced electron
13
transfer from the donor to the acceptor moiety.
Due to their different optical behaviors, 1-3 should also
exhibit various highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energies. HOMO
and LUMO levels of the three terthiophene-PDI compounds
were determined via cyclovoltammetry using the onset potentials
and are shown in Table 1. Moreover, the bandgaps were deduced
from the cyclovoltammetric results and compared with the
optical bandgaps inferred from the onsets of the longest
wavelength absorption bands. In general, we can see a good
agreement of the electrochemical and optical results. Only 2
shows a much smaller electrochemical than optical bandgap.
As the LUMO level is mainly determined by the PDI core and
resembles the LUMO level of bay-unsubstituted PDI (LUMO
20
by Huang et al. for dithienothiophene-PDI copolymers.
Moreover, a decrease in the fluorescence quantum yield cor-
related to an increasing number of R-connected thiophene units
9
,10
in the 1,7-position of PDI was observed. Correspondingly,
the quantum yields of 1 and 3 could be determined as 16% and
2
1
8
%, respectively, using cresyl violet (Φ ) 54% ) as a reference.
Compared to the S f S emission band in 1, the one in 3 is
1
0
bathochromically shifted. This red shift in emission results just
like the red shift in absorption from the longer conjugation. 2
does not show any significant fluorescence. This indicates that
the charge transfer state of 2 could be quenched by electron
transfer and thus supports the presence of a photoinduced
electron transfer (PET) fluorescence quenching mechanism.
Contrary to the absorption where only weak solvent sensitivity
is observed (Figure S3: absorption (normalized) of 1 in CH Cl ,
2 2
acetone, toluene, and DMSO, Supporting Information), the
fluorescence of 1 and 3 is thoroughly quenched in DMSO
-
3.8 eV and HOMO -5.8 eV for N,N′-bis(1-ethylpropyl)-
22
3
,4,9,10-perylenetetracarboxylic acid diimide ), the HOMO is
strongly influenced by the donor and the donor-acceptor
interactions, respectively. Because of the ꢀ-connection of the
terthiophene and a strong twist of the donor (D, terthiophene)
with respect to the acceptor (A, perylene) plane around the D-A
bond, the donor is decoupled in 2 and the HOMO in the D-A
compound is mainly affected by the high HOMO of the
terthiophene itself. DFT calculations carried out by Bouzinne
et al. resulted in a HOMO level energy of -5.18 eV for a linear
terthiophene.23 It is the same decoupling effect that determines
the large optical bandgap as for the orbital partitioning between
the HOMO and LUMO the intensity of the intramolecular
(
Figure S2: fluorescence emission spectra of 1-3 in DMSO,
Supporting Information). This solvent effect confirms the
assumption that these low energy emission bands in fact result
from charge transfer. As all three compounds are symmetrical
and according to quantum mechanical calculations (Figure S1:
molecular geometries of 1-3, Supporting Information) exhibit
no strong dipole (0.82, 1.86, and 2.45 debye for 3, 2, 1,

1
4346 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Wonneberger et al.
TABLE 1: HOMO and LUMO Levels, Band Gap, Absorption and Fluorescence Maxima, and MALDI-TOF Data for 1-3
MALDI-TOF
before sunlight
irradiation
MALDI-TOF
compound
HOMOCV
LUMOCV
bandgapCV
bandgapopt
λ
maxAbs(in toluene)
λ
maxEm(in toluene)
after sunlight
a
irradiation
1
2
3
-5.7
-5.5
-5.4
-3.9
-3.9
-3.9
1.8
1.6
1.5
1.8
1.9
1.6
593
509
608
739
782
1107
1107
1107
1103
decomposed
1107
a
No further purification.
branched terthiophene-PDI 1 before and after sunlight, Sup-
porting Information). The color and the loss of mass indicate
the loss of four hydrogens and photoinduced fusion of the
terthiophene moiety as it is known in literature for various
branched terthiophenes. According to Jayasuriya et al. the
2
6
.2′3′,2′′-terthiophene always fuses to the benzo[1,2-b:3,4-b′:
25
,5-b′′]trithiophene, not yielding any other isomers. We
therefore suggest the formation of 4 according to the structure
shown in Figure 3. The fusion of the branched terthiophene
leads to a new and more stable chromophore donor, which
strongly shifts the optical properties.
Upon fusion, 1 experiences a strong hypsochromic shift of
the absorption, the longest wavelength absorption band showing
a λmax of only 567 nm (Figure 2). This blue shift can also be
observed in the fluorescence of 154 nm to λmax ) 584 nm
(
Figure 2), hence, fluorescence in the PDI range. The hypso-
chromic shift in absorption would implicate a rather large
bandgap. As the LUMO in the fused product of 1 should remain
unchanged as the PDI core is not altered, a larger bandgap would
correspond to a more negative HOMO level energy. Molecular
geometry calculations (included in the Supporting Information)
show a strong twist of the donor and the PDI acceptor, the two
planes being almost perpendicular to each other (∼97°), which
leads to a stronger decoupling of the donor from the acceptor.
A similar case, though even enhanced by the ꢀ-connection, is
Figure 3. Solution colors (in toluene) and molecular structures of 1-3
before and after light irradiation (suggested structure 4).
2
, in which we found a strong influence of the donor moiety on
the HOMO level energy. In fact, DFT calculations of Taerum
et al. predicted the HOMO of terthienobenzene, hence the donor
moiety in the fused compound, to be as low as -5.93 eV.
2
6
charge transition is very low and hence no clear ICT absorption
band can be observed.
2
4
This corresponds to the result of the larger optical bandgap.
For the conjugated thiophene-PDI D-A systems the band-
gap diminished with every additional thiophene unit (bandgaps
for N,N′-bis(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic acid
diimide and its thiophene (T)-substituted derivatives: PDI, 2.31
eV; 1T-PDI-1T, 1.97 eV; 2T-PDI-2T, 1.73 eV; 3T-PDI-3T, 1.63
eV). The decreased electrochemical bandgaps of 1 and 3
correspond to the bathocromic shifts in absorption and hence
the optical bandgaps. Thus, both decoupling and sizable
conjugation, which on first glance seem to be contrary effects,
can result in a similarly small bandgap like in 2 and 3,
respectively.
In summary, we showed how different terthiophene archi-
tectures can influence optical and photochemical properties of
D-A terthiophene-PDIs. The syntheses of three different bay-
substituted donor-acceptor terthiophene-PDIs were presented.
The target compounds were characterized by UV-vis absorp-
tion, fluorescence, and cyclovoltammetric measurements, and
their photochemical stability was tested. The optical properties
showed that the color can be adjusted and energy levels tuned
by variation of conjugation, assembly, and connection of the
thiophene moieties within the terthiophene and toward the PDI
core. The photochemical stability differs and varies strongly
1
0
As mentioned above, 1 and 3 show substantial charge transfer
whereas 2 exhibits strong photoinduced electron transfer. These
characteristics can also influence the photochemical stability of
the three compounds. To investigate their photochemical stabil-
ity, solutions in toluene were exposed to sunlight for several
days and interestingly all three compounds show different
behavior, as presented in Figure 3. Whereas 3 is stable and
shows the same mass in MALDI-TOF (Table 1) and the same
absorption bands as before (Figure 2), 2 changes color from
purple to yellow and, according to MALDI-TOF as well as
absorption measurements, is completely decomposed. 1 under-
goes a color change from green to red and shows a decrease in
mass of 4 according to MALDI-TOF (Figure S5: MALDI of
(
charge transfer or electron transfer) and photoinduced fusion
of the branched terthiophene moiety offers a way to synthesize
new chromophores. The tuning of color and energy levels of
these terthiophene-PDIs is of vital interest for the potential
application in organic electronics, especially for organic pho-
tovoltaics, as they show promising absorption in their broadness
in the UV-vis region (especially 1 and 3) and their high
absorption coefficients (especially 2). The terthiophene-PDIs
1-3 all possess suitable energy levels for the application as
the acceptor component in bulk heterojunction cells in combina-
tion with P3HT (LUMO, -3.2 eV; HOMO, -5.2 eV). Photo-
voltaic studies will be performed in the future.

Terthiophene-Perylene diimides
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14347
Acknowledgment. We acknowledge the financial support
from the Deutsche Forschungsgemeinschaft (DFG) BA 880/3-
, ME 1246/15-1, MU 334/28-1, the Bundesministerium f u¨ r
Bildung und Forschung (BMBF), and BASF SE.
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spectra in DMSO, absorption spectra of 1 in different solvents,
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JP911800Q