Rational molecular design towards Vis/NIR absorption and fluorescence by using pyrrolopyrrole aza-BODIPY and its highly conjugated structures for organic photovoltaics

Article abstract of DOI:10.1002/chem.201405761

Pyrrolopyrrole aza-BODIPY (PPAB) developed in our recent study from diketopyrrolopyrrole by titanium tetrachloride-mediated Schiff-base formation reaction with heteroaromatic amines is a highly potential chromophore due to its intense absorption and fluor

Full text of DOI:10.1002/chem.201405761

DOI: 10.1002/chem.201405761
Full Paper
&
Fluorescence
Rational Molecular Design towards Vis/NIR Absorption and
Fluorescence by using Pyrrolopyrrole aza-BODIPY and its Highly
Conjugated Structures for Organic Photovoltaics**
Soji Shimizu,*[a] Taku Iino,[b] Akinori Saeki,*[c] Shu Seki,[c] and Nagao Kobayashi*[b]
Abstract: Pyrrolopyrrole aza-BODIPY (PPAB) developed in
our recent study from diketopyrrolopyrrole by titanium tet-
rachloride-mediated Schiff-base formation reaction with het-
eroaromatic amines is a highly potential chromophore due
to its intense absorption and fluorescence in the visible
region and high fluorescence quantum yield, which is great-
er than 0.8. To control the absorption and fluorescence of
PPAB, particularly in the near-infrared (NIR) region, further
molecular design was performed using DFT calculations. This
results in the postulation that the HOMO–LUMO gap of
PPAB is perturbed by the heteroaromatic moieties and the
aryl-substituents. Based on this molecular design, a series of
new PPAB molecules was synthesized, in which the largest
redshifts of the absorption and fluorescence maxima up to
803 and 850 nm, respectively, were achieved for a PPAB con-
sisting of benzothiazole rings and terthienyl substituents. In
contrast to the sharp absorption of PPAB, a PPAB dimer,
which was prepared by a cross-coupling reaction of PPAB
monomers, exhibited panchromatic absorption across the
UV/Vis/NIR regions. With this series of PPAB chromophores
in hand, a potential application of PPAB as an optoelectronic
material was investigated. After identifying a suitable PPAB
molecule for application in organic photovoltaic cells based
on evaluation using time-resolved microwave conductivity
measurements, a maximized power conversion efficiency of
1.27% was achieved.
Introduction
cal modification of the ketone moieties of DPP has great po-
tential for extension of the conjugated systems to create new
functional chromophore molecules, most of the previous ef-
forts have been devoted to chemical modification of the aryl
substituents on DPP by using cross-coupling reactions to fabri-
cate donor–acceptor small molecules or polymers for applica-
tion in organic photovoltaics.[6,7] In addition, another reason for
this lies in the inherent low reactivity of the carbonyl carbon
atoms towards nucleophilic substitution reactions, which was
pointed out in the initial research on DPP by Iqbal et al.[4a,b]
Only two examples of nucleophilic substitution reactions had
been reported before the recent pioneering study by Zum-
busch and Daltrozzo. Both of the nucleophilic substituted
products were synthesized through activation of the ketone
moiety with phosphorous pentasulfide (P4S10) or phosphoryl
chloride (POCl3) into a thioketone species or a mono-phos-
phorylated species.[4a,b] Zumbusch et al. further developed the
latter method to extend the conjugated systems through me-
thine carbons to the heteroaromatic ring units and finally con-
verted DPP into new pyrrolopyrrole-bridged dimeric boron di-
pyrromethene (BODIPY) analogues, which they referred to as
pyrrolopyrrole cyanines.[8] Recently we found that titanium tet-
rachloride-mediated Schiff-base formation reaction can provide
another new route for nucleophilic substitution reactions of
DPP and successfully synthesized dimeric aza-BODIPY ana-
logues 1–3, which are referred to as pyrrolopyrrole aza-BODIPY
(PPAB).[9,10]
Creation of functional chromophore molecules, whose optical
properties can be controlled by simple derivatization such as
introduction of substituents or replacement of part of the
structure has recently become increasingly important, due to
the potential utilization of these types of chromophore mole-
cules in the fields of bio-imaging, electroluminescence, photo-
chemical sensing, and organic photovoltaics.[1,2] Diketopyrrolo-
pyrrole (DPP)[3–5] is a fascinating bicyclic lactam, widely utilized
in industry as a red colorant. In recent years, this unique mole-
cule has been focused on as an optoelectronic material due to
its excellent photophysical properties such as photostability
and intense absorption in the visible region.[6] Although chemi-
[a] Prof. Dr. S. Shimizu
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
Fukuoka 819-0395 (Japan)
E-mail: ssoji@cstf.kyushu-u.ac.jp
[b] T. Iino, Prof. Dr. N. Kobayashi
Department of Chemistry, Graduate School of Science
Tohoku University, Sendai 980-8578 (Japan)
E-mail: nagaok@m.tohoku.ac.jp
[c] Prof. Dr. A. Saeki, Prof. Dr. S. Seki
Department of Applied Chemistry, Graduate School of Engineering
Osaka University, Suita, 565-0871 (Japan)
E-mail: saeki@chem.eng.osaka-u.ac.jp
[**] BODIPY=boron dipyrromethene.
PPAB exhibited intense absorption and fluorescence with
fluorescence quantum yields of over 0.8. The gradual redshift
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405761.
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Results and Discussion
Molecular design based on DFT and TDDFT calculations
The PPAB molecules 1–3[9] exhibited intriguingly intense ab-
sorption and fluoresce in the lower-energy visible region (1:
labs =638 nm, lem =661 nm, 2: labs =655 nm, lem =672 nm, 3:
labs =671 nm, lem =692 nm (labs and lem denote absorption
and fluorescence maxima)). The fluorescence quantum yields
of these compounds were exceptionally high (1: FF =0.87, 2:
FF =0.81, 3: FF =0.83). Since a redshift of fluorescence is in
general accompanied by a significant decrease in the fluores-
cence quantum yield, which is one of the reasons why NIR flu-
orophores have remained very limited, it is a reasonable strat-
egy to modify highly fluorescent compounds in the visible
region and shift their fluorescence into the NIR region. In this
sense, PPAB appears to have high potential. The TDDFT calcula-
tions (B3LYP/6-31G(d)) performed in the previous study of
PPAB demonstrated that the main absorption of the PPAB mol-
ecules arises from a p–p* transition between the highest occu-
pied molecular orbital (HOMO) and the lowest unoccupied mo-
lecular orbital (LUMO).[9] The redshift of the main absorption
from 1 to 2 and further to 3 was well reproduced based on
the TDDFT calculations (1: 580 nm, 2: 600 nm, 3: 613 nm), and
the calculated HOMO–LUMO energy gap (DH–L) decreased in
this order (1: 2.20 eV, 2: 2.16 eV, 3: 2.10 eV). Although the tran-
sition energies were overestimated, relative shift of the absorp-
tion caused by modification of the PPAB chromophore systems
can be qualitatively estimated based on DFT and TDDFT calcu-
lations at the B3LYP/6-31G(d) level.
of the absorption and fluorescence observed in the order from
1 to 2 and to 3 was indicative of significant perturbation of
the electronic structure of PPAB by the heteroaromatic ring
moieties. Striking structural difference of PPAB from pyrrolopyr-
role cyanines[8] is the high co-planarity of the phenyl-substitu-
ents on the pyrrolopyrrole moiety with respect to the PPAB
core structure, which is due to the lower steric hindrance of
the imine bridging compared to the methine carbon bridges
with cyano-substituents in the structures of pyrrolopyrrole cya-
nines. This structural feature implies non-negligible substituent
effects on the optical properties of PPAB. Changes in both the
heteroaromatic ring units and substituents on the pyrrolopyr-
role moiety can, therefore, be utilized to control the absorption
and fluorescence of PPAB molecules. In this study, shift of the
absorption and fluorescence even further into the near infrared
(NIR) region was successfully achieved through survey of po-
tential combinations of the heteroaromatic ring units and
types of substituent based on the DFT and time-dependent
DFT (TDDFT) calculations.
To shift the absorption and fluorescence of PPAB to the NIR
region, we envisioned the modification of two potential com-
ponents in the PPAB structure: 1) Heteroaromatic ring moieties
and 2) Substituents on the pyrrolopyrrole moiety. The redshift
caused by the former has been confirmed both in our previous
study and in the study of pyrrolopyrrole cyanines by Zumbuch
and Daltrozzo.[8] The latter effect also appeared to be possible
due to the co-planarity of the phenyl substituents with respect
to the PPAB core structure with dihedral angles of 30–408 ob-
served in the crystal structures of 1–3.[9] In the case of pyrrolo-
pyrrole cyanines, the substituent effect is rather marginal due
to the nearly perpendicular orientation of the aryl substituents
to the pyrrolopyrrole moiety caused by the steric hindrance of
the cyano groups.[8h] Taking these into consideration, potential
structures of PPAB for NIR absorption were surveyed by using
DFT and TDDFT calculations (Figure 1 and Table 1).
In the course of the synthesis of PPAB with various aryl sub-
stituents on the pyrrolopyrrole moiety using cross-coupling re-
actions, we envisioned extension of the conjugated system of
this chromophore by dimerization. Under the conditions of
palladium-catalyzed borylation and in situ cross-coupling reac-
tion, a dimer of PPAB can be easily synthesized. Surprisingly,
this compound exhibited panchromatic absorption across the
UV/Vis/NIR regions.
In this manuscript, the synthesis and molecular design of
these new PPAB molecules and their intriguing optical proper-
ties are described. In addition, with this series of compounds
in hand, their application as optoelectronic materials in organic
photovoltaics with bulk heterojunction architectures was also
investigated. After optimization of the solubility of PPAB and
the device fabrication conditions based on an electrochemical
study on PPAB molecules and time-resolved microwave photo-
conductivity measurements using white-light pulse,[11] a promis-
ing performance as the first application of these compounds in
organic photovoltaics was achieved.
Since PPAB attained a redshift and narrow DH–L as the het-
eroaromatic moieties became polycyclic in the previous
study,[9] a model structure of PPAB consisting of benzoindole
units (M-2) was estimated, and the theoretical absorption max-
imum and the DH–L values were compared with those of
a model structure of 2 (M-1). The redshift of the absorption
maximum by 89 nm from M-1 and the narrower DH–L of M-2
(1.88 eV) based on the DFT and TDDFT calculations implied
that this modification is effective. Next, the substituent effect
was estimated. When the phenyl substituents on the pyrrolo-
pyrrole moiety of M-1 was replaced with more electron-donat-
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DPP-2 was utilized as a DPP unit
with
N,N-dialkylaminophenyl
substituents. DPP and heteroaro-
matic amines were treated in
the presence of titanium tetra-
chloride (TiCl4) and triethylamine
in toluene under reflux,[16] fol-
lowed by addition of borontri-
fluoride diethyl ether complex
(BF3·OEt2) to provide PPAB 4, 5,
6a, and 6b in 1.0, 5.8, 39, and
30% yields, respectively. The
comparatively high yields of 6a
and 6b were due to the high re-
activity of HA-3a and HA-3b to-
wards DPP-3, whereas the ex-
ceptionally low yield of 4 was as-
cribed to the low reactivity of
HA-1 towards DPP-1.
To further extend the conjuga-
tion, compound 6a was bromi-
nated with N-boromosuccini-
Figure 1. Frontier molecular orbitals of PPAB model structures, M-1 to M-4 (B3LYP/6-31G(d)).
mide (NBS) in chloroform to
form 7a, which was then con-
Table 1. Summary of DFT and TDDFT calculations of PPAB model struc-
tures, M-1 to M-4 (B3LYP/6-31G(d)).
verted into bithienyl and terthienyl-substituted PPAB 8 and 9
by palladium-catalyzed coupling reactions with thiophene-2-
boronic acid and 5’-hexyl-2,2’-bithiophene-5-boronic acid pina-
col ester.[17,18] The yields of 8 and 9 from 7a were 45 and 25%,
respectively.
Compound HOMO LUMO DH–L Energy f[a]
[eV] [eV] [eV] [nm]
Wavefunction[b]
!
M-1
M-2
M-3
M-4
ꢀ5.40 ꢀ3.24 2.16 600
ꢀ5.28 ꢀ3.40 1.88 689
ꢀ4.69 ꢀ2.75 1.94 666
ꢀ5.26 ꢀ3.23 2.03 628
0.81 +0.711jL H> +…
!
All these compounds were unambiguously characterized by
high-resolution matrix-assisted laser desorption/ionization
Fourier transform ion cyclotron resonance mass spectrometry
0.97 +0.712jL H> +…
!
0.75 +0.713jL H> +…
!
0.68 +0.711jL H> +…
1
[a] Oscillator strength. [b] The wavefunctions are based on the eigenvec-
tors predicted by TDDFT. H and L represent the HOMO and LUMO, re-
spectively.
(HR-MALDI-FT-ICR-MS) and H NMR spectroscopy. The structure
of 6b was further confirmed by X-ray single crystallographic
analysis on crystals obtained from a vapor diffusion of hexane
into a toluene solution of 6b (Figure 2).
ing N,N-dimethylaminophenyl substituents (M-3) and thienyl
substituents (M-4), the theoretical absorption maximum of M-
1 at 600 nm shifted to 666 nm for M-3 and to 628 nm for M-4
accompanied by decrease in the DH–L from 2.16 eV of M-1 to
1.94 eV of M-3 and 2.03 eV of M-4. These results also indicated
effective modification of the substituents on the pyrrolopyrrole
moiety towards NIR absorption.
Figure 2. X-ray crystal structure of 6b, top view (left) and side view (right).
The thermal ellipsoids were scaled to the 50% probability level. Hydrogen
atoms, 2-hexyldecyloxy substituents, and a set of the disordered thiophene
rings were omitted for clarity in both views.
Synthesis and characterization
Based on the above-discussed molecular design supported by
the theoretical calculations, new PPAB molecules 4–9 were syn-
thesized (Scheme 1 and 2). Precursory DPP (DPP-1–3)[8e,12,13]
and heteroaromatic amines (HA-1[14] and HA-2) were synthe-
sized according to the literature procedures. HA-3a and HA-
3b with an octyloxy substituent and a 2-hexyldecyloxy sub-
stituent at the 6-position were newly derivatized from 2-
amino-6-hydroxybenzothiazole[15] in this study to attain suita-
ble solubilities of the thienyl substituted PPAB 6a and 6b (see
the Supporting Information). Due to its synthetic availability,
In the crystal structure, the thienyl substituents were disor-
dered into two orientations: In one orientation the sulfur atom
of the thienyl substituent points towards the boron atom of
the BF2 unit, whereas in the other orientation it faces towards
the bridging nitrogen atom. In both orientations, the dihedral
angles between the thienyl substituents and the core structure
of PPAB in 6b are about 278, which is smaller than those ob-
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aminophenyl
substituents.[8h]
Upon extension of the thienyl
substituents of 6a to bithienyl
(8) and further to terthienyl (9)
substituents, a gradual redshift
by about 50 nm per thiophene
unit was observed (8: 756 nm, 9:
803 nm). In the series of PPAB
molecules in this study, 9 exhib-
ited the largest redshift of the
absorption, in which the tail of
the absorption extended to
900 nm.
The
fluorescence
spectra
showed a mirror-imaged emis-
sion with a Stokes shift of 159–
855 cmꢀ1
Table 2).
(Figure 3b
and
The fluorescence
maxima also exhibited a redshift
in the same order as observed
for the redshift of the absorp-
tion, with the exception of 5.
The larger Stokes shift of 5 com-
pared with the other com-
pounds can be explained in
terms of the intramolecular
charge-transfer contribution. All
of the compounds exhibited ex-
ceptionally high fluorescence
quantum yields (FF) relative to
other conventional fluorescent
compounds in the correspond-
ing Vis/NIR regions due the rigid
Scheme 1. Synthesis of PPAB molecules 4–6 from DPP (DPP-1–3) and heteroaromatic amines (HA-1–3).
served for the phenyl-substituted PPAB 1–3 (30–398)[9] due to
the less bulky thienyl substituents.
Absorption and fluorescence spectra of PPAB
All of the absorption spectra of PPAB prepared in this study ex-
hibited similar absorption spectra, featuring an intense band
and two or three following weaker bands in the shorter wave-
length region, but the position of the intense band varied de-
pending on the heteroaromatic ring moieties and substituents
on the pyrrolopyrrole moiety (Figure 3a and Table 2). With re-
spect to the absorption maximum of 2 at 655 nm, replacement
of benzothiazole rings with benzoindole rings from 2 to 4 re-
sulted in a redshift by 92 nm. When the phenyl substituents
were replaced with p-piperidinophenyl substituents (5) or
thienyl substituents (6a), redshifts of the absorption were also
observed (5: 733 nm, 6a: 699 nm). In addition, compound 5
exhibited a significant broadening of the main absorption with
disappearance of the shoulder absorption, whereas a new
Figure 3. (a) UV/Vis/NIR absorption and (b) fluorescence spectra of PPAB
molecules 4 (red), 5 (purple), 6a (aqua blue), 8 (green), 9 (blue), and 11
(pink), in CHCl3. UV/Vis absorption and fluorescence spectra of 2 in CHCl3
are shown as a reference (black).
band developed at 516 nm. This band can be assigned as an
intramolecular charge-transfer band, which was also reported
in the case of pyrrolopyrrole cyanines with N,N-disubstituted
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ents. The FF and tF values then
decreased when the substituents
were further elongated to ter-
thienyl in the case of 9.
Synthesis of PPAB dimer
Since the substituents on the
pyrrolopyrrole moiety give sig-
nificant perturbation of the opti-
cal properties of PPAB, further
enhanced interchromophore in-
teractions could be expected for
dimer or oligomer systems of
PPAB. The PPAB dimer 10 was
successfully synthesized in 3.6%
yield from a homo-coupling re-
action of 7b through Pd-cata-
lyzed borylation with bis(pinaco-
[19]
lato)diboron (pinB)2 and an in
situ cross-coupling sequence
(Scheme 3).[18] Compound 7b
was used instead of 7a to im-
prove the solubility of the dimer
molecule.
The structure of 10 was con-
firmed by HR-MALDI-FT-ICR-MS,
which demonstrated a molecular
ion peak at m/z=2280.0281
([M+];
calcd
for
C
120H154B4F8N12O4S8: 2280.0278).
Scheme 2. Synthesis of PPAB molecules 7–9. Reagents and conditions: (i) thiophene-2-boronic acid, [Pd(PPh3)4],
Na2CO3, Aliquat336, toluene, water, reflux, 1 h; (ii) 5’-hexyl-2,2’-bithiophene-5-boronic acid pinacol ester,
[PdCl2(dppf)], KOAc, DMF, 808C, 15 min.
In the 1H NMR spectrum in
CD2Cl2 at room temperature,
peaks corresponding to the
thienyl substituents and benzo-
Table 2. Summary of optical properties of PPAB molecules in CHCl3.
thiazole units were observed at d=9.32 (2H), 9.06
(2H), 7.96 (2H), 7.83 (4H), 7.67 (2H), 7.28 (2H), 7.15
(2H), and 7.07 (6H) ppm, reflecting the free-rotation
of the bithienyl moiety at this temperature and the
chemically identical natures of the monomer units.
Surprisingly, compound 10 showed panchromatic ab-
sorption across the UV/Vis/NIR regions with four ab-
sorption maxima at 374, 580, 680, and 839 nm, which
is not a simple redshift as expected from the above
mentioned results (Figure 4).
[a]
Compound labs
loge lem
Stokes
FF[b] tF
[ns] [ꢁ108 sꢀ1
kr
knr
[c]
[d]
[nm]
[nm] shift [cmꢀ1
]
]
[ꢁ108 sꢀ1
]
2
655
5.04 672 386
0.81 5.29 1.53
0.36
3.83
1.95
14.6
3.29
8.04
–
4
5
6a
8
9
747
733
699
756
803
839
638
5.09 756 159
4.88 782 855
5.08 712 261
5.04 773 291
5.11 847 647
0.18 2.14 0.84
0.35 3.34 1.05
0.11 0.61 1.80
0.24 2.31 1.04
0.14 1.07 1.31
10
11
4.89
–
–
–
–
–
4.92 654 383
0.75 6.00 1.25
0.42
Since only a very small difference in the absorption
spectrum was reported in the case of the bithienyl-
bridged porphyrin dimer, the contribution from exci-
ton interactions would be negligible in the case of
10.[21] To give an in-depth insight into these signifi-
cant changes in absorption from monomer to dimer,
[a] Only the longest absorption maxima are shown. [b] Absolute fluorescence quan-
tum yields. [c] Rate constant for radiative decay: kr =FF/tF. [d] Rate constant for non-
radiative decay: knr =(1ꢀFF)/tF.
structure of PPAB. The slightly small FF value and short fluores-
cence lifetime (tF) of 6a (FF =0.11, tF =0.61 ns) was due to an
enhanced non-radiative decay process reflecting the excited-
state dynamics of this molecule. Upon elongation of the thien-
yl substituents to bithienyl from 6a to 8, the FF and tF values
recovered due to suppression of the flexibility of the substitu-
DFT and TDDFT calculations were performed on model struc-
tures M-5a (s-cis), M-5b, and M-5c (s-trans) with dihedral
angles of 0, 90, and 1808, respectively. In all the cases, the four
frontier orbitals, HOMOꢀ1 and HOMO, and LUMO and LUMO+
1, were generated by a linear combination of the HOMO of M-
1 and the LUMO of M-1, respectively (Figure 5). Based on the
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interaction between the PPAB
moieties due to the effective ex-
tension of the conjugation
through the bithienyl bridge.
Taking into consideration the
free-rotational behavior of the
bithienyl unit in 10 proposed
1
based on the H NMR spectrum,
the observed absorption spec-
trum of 10 can be explained in
terms of the presence of con-
formers with different dihedral
angles in solution. In contrast to
the unique absorption spectrum,
compound 10 is non-fluorescent
Scheme 3. Synthesis of PPAB dimer 10. Reagents and conditions: (i) Bis(pinacolate)diboron, [PdCl2(dppf)], KOAc,
DMF, 808C, 90 min.
due to its flexible structure, which may enhance non-radiative
decay processes.
Electrochemistry
To experimentally estimate the HOMO and LUMO energy
levels, the redox potentials of PPAB were measured by cyclic
voltammetry in o-dichlorobenzene containing 0.1m tetra-n-bu-
tylammonium perchlorate as
a
supporting electrolyte
(Figure 6). All of the PPAB compounds exhibited one or two re-
versible oxidations and reductions. The redox potentials as
well as the experimental HOMO and LUMO energy levels de-
termined by the first oxidation and reduction potentials (Ox1
and Red1) are summarized in Table 4. Both Ox1 and Red1
shifted positively when the heteroaromatic ring moieties
became larger from monocyclic pyridine (1: 0.43 V and ꢀ1.33 V
vs. Fc+/Fc) to bicyclic benzothiazole and quinoline (2: 0.62 V
and ꢀ1.06 V and 3: 0.51 V and ꢀ1.15 V). The potential differ-
ence between Ox1 and Red1 (DE) of 2 (1.68 V) and 3 (1.66 V)
decreased relative to 1 (1.76 V) due to the greater extent of
the positive shift of the Red1 than that of Ox1. In contrast,
Figure 4. UV/Vis/NIR spectrum of PPAB dimer 10 in CHCl3.
TDDFT calculations, the main absorption in the lowest energy
region consists of transitions between these orbitals, and the
transition energies vary depending on the dihedral angles
(Table 3). The theoretical absorption of M-5b appears to be
similar to that of M-1, indicating small interactions between
the PPAB moieties in this conformation. In contrast, a batho-
chromic shift of the main absorption band was predicted for
M-5a at 872 nm and for M-5c at 837 nm compared with that
of M-5b at 644 nm. These results are indicative of enhanced
Figure 5. Frontier molecular orbitals of PPAB dimer model structures, M-5a–M-5c (B3LYP/6-31G(d)). q is defined by the dihedral angle of S1ꢀC1ꢀC2ꢀS2. 2-Hex-
yldecyloxy substituents were omitted in the model structures for simplicity.
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creasing the number of thiophene units. These re-
sults and variation of the DE values are in good
agreement with the redshift of the absorption of
these PPAB molecules and broadly reproduced by
the theoretical calculations (Figure 1).
Table 3. Summary of TDDFT calculations of PPAB dimer model structures, M-5a–M-5c
(B3LYP/6-31G(d)).
Compound
q
[8][a]
Energy
[nm]
f[b]
Wavefunction[c]
!
M-5a
0
872
628
553
483
459
644
631
473
0.45
0.95
0.53
0.42
0.18
1.11
0.16
0.21
+0.709jL H> +…
Cyclic voltammetry measurements were also per-
formed on the dimer 10, demonstrating Ox1 at
0.36 V and Red1 at ꢀ0.95 V. This result is very compa-
rable to that of 8 with bithienyl substituents, indicat-
ing that the redox processes may occur independent-
ly on each PPAB moiety.
!
!
+0.561jL+1 H>ꢀ0.433jL H-1> +…
!
!
+0.667jL H-2> +0.190jL+1 H-1> +…
!
+0.686jL H-4> +…
!
+0.678jL+2 H> +…
!
!
M-5b
M-5c
90
+0.585jL+1 H-1>ꢀ0.403jL H> +…
!
!
+0.523jL+1 H>ꢀ0.479jL H-1> +…
!
!
+0.500jL H-3> +0.410jL+1 H-2>
!
!
ꢀ0.209jL H-3>ꢀ0.110jL+1 H-4> +…
!
!
453
0.87
+0.499jL H-5> +0.413jL+1 H-4>
!
!
Potential application of PPAB in bulk heterojunc-
tion photovoltaic cells
+0.201jL H-5> +0.147jL+1 H-2> +…
!
180
837
641
592
554
478
455
1.12
0.16
0.36
0.55
0.39
0.17
+0.709jL H> +…
!
!
+0.565jL+1 H>ꢀ0.429jL H-1> +…
!
!
Because of the intense absorption in the Vis/NIR re-
gions and the highly conjugated planar structures,
PPAB are potential candidates for bulk heterojunction
organic photovoltaics (BHJ-OPV). Based on the
HOMO and LUMO energy levels estimated experi-
mentally by cyclic voltammetry (Table 4), possible p-
type PPAB molecules with an appropriate LUMO
+0.675jL+1 H-1> +0.201jL H-2> +…
!
!
+0.666jL H-2>ꢀ0.195jL+1 H-1> +…
!
!
+0.675jL H-4>ꢀ0.129jL+1 H-3> +…
!
!
+0.675jL+2 H>ꢀ0.106jL H-4> +…
[a] Dihedral angle of S1ꢀC1ꢀC2ꢀS2 in Figure 5. [b] Oscillator strength. [c] The wavefunc-
tions are based on the eigenvectors predicted by TDDFT. H and L represent the
HOMO and LUMO, respectively.
Table 4. Summary of electrochemical properties of PPAB molecules[a] and
experimental HOMO and LUMO energy levels (EHOMO and ELUMO).
[b]
ox
[b]
red
[d]
[e]
Compound
E1/2
E1/2
DE[c]
EHOMO
ELUMO
[V]
[V]
[V]
[eV]
[eV]
1
2
3
5
6a
8
9
0.43
0.62
0.51
0.19, 0.29
0.61
0.39
0.15
0.36
ꢀ1.33
1.76
1.68
1.66
1.33
1.61
1.34
1.12
1.31
ꢀ5.22
ꢀ5.42
ꢀ5.31
ꢀ4.99
ꢀ5.41
ꢀ5.19
ꢀ4.95
ꢀ5.16
ꢀ3.47
ꢀ3.74
ꢀ3.65
ꢀ3.66
ꢀ3.80
ꢀ3.83
ꢀ3.80
ꢀ3.85
ꢀ1.06, ꢀ1.67
ꢀ1.15, ꢀ1.67
ꢀ1.14, ꢀ1.70
ꢀ1.00
ꢀ0.95
ꢀ0.97
10
ꢀ0.95
[a] Electrochemical measurements on 4 could not be performed due to
the low yield of 4. [b] Determined by cyclic voltammetry (conditions:
0.5 mm o-DCB solutions of analyte containing 0.1m tetrabutylammonium
perchlorate as a supporting electrolyte at a scan rate of 100 mVsꢀ1). Po-
tentials are given relative to the ferrocenium/ferrocene couple. [c] DE: dif-
ference between the Ox1 and Red1. [d] EHOMO =ꢀ(Ox1+4.8) eV.
[e] ELUMO =ꢀ(Red1+4.8) eV.
energy level for n-type phenyl-C61-butylic acid methyl ester
(PC61BM) were examined. Since slightly higher LUMO energy
levels of donor molecules by 0.2–0.3 eV relative to those of ac-
ceptor molecules are necessary for efficient charge separation,
compound 1 with its LUMO at ꢀ3.47 eV has high potential
within this series of PPAB molecules. All of the other PPAB mol-
ecules including the dimer 10 possess a low-lying LUMO at
ꢀ3.65–ꢀ3.85 eV, which is fairly close to the LUMO energy level
of PC61BM at ꢀ3.8 eV. Further necessary information before
device fabrication, such as a p/n blend ratio, can be deduced
based on time-resolved microwave conductivity (TRMC) meas-
urements[21–23] using monochromatic light pulse from a nano-
second laser (laser-flash)[24–27] or white-light pulse from a Xe
flash-lamp (Xe-flash)[11] as photoexcitation sources. The laser-
Figure 6. Cyclic voltammograms of PPAB molecules (0.5 mm) in o-dichloro-
benzene containing 0.1m tetra-n-butylammonium perchlorate as a support-
ing electrolyte at a scan rate of 100 mVsꢀ1. Values are half-wave potentials.
changing the substituents on the pyrrolopyrrole moiety
caused a negative shift of Ox1. Compound 5 with p-piperidi-
nophenyl substituents exhibited Ox1 at 0.19 V, which is signifi-
cantly shifted to the negative with respect to 2, whereas
a gradual negative shift of Ox1 was observed in the order
from 6a (0.61 V) to 8 (0.39 V) and further to 9 (0.15 V) upon in-
Chem. Eur. J. 2014, 20, 1 – 13
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flash-TRMC measurements demonstrated that the maximum
transient conductivity, fSmmax, in which f and Sm represent
charge carrier generation efficiency and the sum of hole and
electron mobilities, respectively, was significantly enhanced for
1 upon blending with PC61BM (=1:1 in weight fraction (w/w))
compared with those of the other PPAB molecules 2, 6a, and
8 (Figure 7a). This result also supports the potential utility of
Table 5. OPV performance of PPAB molecules.
Blend
Solvent
L[a]
[nm]
Anneal Jsc
[mAcmꢀ2
Voc FF PCE
] [V] [%]
1:PC61BM CF[b] 0.7% v/v DIO 140
11:PC71BM CF[b] 0.5% v/v DIO 200
11:PC71BM CF[b] 0.5% v/v DIO 200 1008C 4.60
1.16
3.62
0.49 0.35 0.20
0.72 0.41 1.07
0.66 0.42 1.27
The inverted cell structure (ITO/ZnO/BHJ/MoO3/Ag) was employed.
[a] Active layer thickness. [b] CF represents chloroform.
served by optical microscopy and surface-topography of AFM
(see the Supporting Information, Figures S1 and S2).
To improve the solubility of 1, compound 11 bearing 2-eth-
ylhexyloxy substituents instead of the octyloxy substituents
was synthesized (Figure 7e). A similar device structure with an
as-cast active layer of 11 and phenyl-C71-butylic acid methyl
ester (PC71BM) (=1:1 (w/w)), which was used instead of PC61BM
to improve the photoabsorption in the visible region,[28] exhib-
ited a better PCE of 1.07% after optimization of the device fab-
rication conditions, such as the spin-coat temperature, the
amount of additive DIO, and the thickness of the active layer.
Further improvement in PCE up to 1.27% with Voc of 0.66 V, Jsc
of 4.60 mAcmꢀ2, and FF of 0.42 was achieved by annealing the
BHJ layer at 1008C (Figure 7c,d and Table 5). AFM measure-
ments on this annealed BHJ layer demonstrated improvement
in the surface roughness, indicating the formation of a BHJ
structure having nanometer dimension (see the Supporting In-
formation, Figure S2).
Figure 7. (a) Maximum TRMC signals (fSmmax) of pristine PPAB molecule and
PPAB/PC61BM (1:1 (w/w)) drop-cast films from chloroform solutions mea-
sured by laser-flash TRMC (ex. 355 nm); (b) Dsmax of Xe-flash TRMC of drop-
cast films of 1:PC61BM plotted as a function of PC61BM content. 100wt%
PC61BM represents pure PC61BM; (c) J/V curves of the PPAB/PC61(71)BM (1:1
(w/w)) (g=1/PC61BM, spin-coated from a chloroform solution containing
0.7 v/v% DIO, without thermal annealing; b=11/PC71BM, spin-coated
from a chloroform solution containing 0.5 v/v% DIO, without thermal an-
nealing; solid line: 11:PC71BM, spin-coated from a chloroform solution con-
taining 0.5 v/v% DIO, with 1008C annealing); (d) EQE spectra of the devices
of (c); (e) Structure of 11.
Conclusion
The TiCl4-mediated Schiff-base formation reaction of DPP
opened access to a series of PPAB molecules containing a varie-
ty of heteroaromatic ring moieties and substituents on the pyr-
rolopyrrole moiety, which were designed based on theoretical
calculations with the aim of shifting the main intense absorp-
tion and fluorescence to the red. The fluorescence quantum
yields of these PPAB molecules were comparatively high rela-
tive to other conventional fluorophores in the corresponding
regions. The results of this study indicate that the introduction
of electron-donating substituents to the pyrrolopyrrole moiety,
as exemplified by oligothienyl substituted PPAB molecules 6a,
8, and 9, appears to be a very practical strategy towards NIR
absorption and fluorescence emission. Extension of the conju-
gated systems was further achieved in the dimeric system,
which unexpectedly exhibited panchromatic absorption in the
UV/Vis/NIR regions due to the presence of conformers with
various dihedral angles. With this series of PPAB molecules in
hand, their application in BHJ-OPV devices was investigated.
Based on electrochemical and TRMC screening, compound
1 was selected as a potential candidate for this purpose. By
optimizing several conditions including the solubility of the
PPAB, the choice of fullerene derivatives as an acceptor, and
the device fabrication conditions, a PCE of 1.27% with Voc of
0.66 V, Jsc of 4.60 mAcmꢀ2, and FF of 0.42 was achieved using
a blend film of 11 and PC71BM (=1:1 (w/w)) as an active layer.
1 as a p-type material in BHJ-OPV using PC61BM as an acceptor.
Following this, Xe-flash TRMC measurements on blend films of
1 and PC61BM were performed to optimize the p/n blend ratio.
The Xe-flash TRMC provided photoconductivity maxima
(Dsmax), which includes information about not only the yield
and local mobility of charge carriers, but also their lifetime be-
cause of the lower photoirradiation density and time resolu-
tion than those of laser-flash TRMC. From the maximum of the
Dsmax located at 50 wt% of PC61BM, the optimized blend ratio
was determined as 1:1 (w/w) of 1 and PC61BM (Figure 7b). A
BHJ-OPV device with a Glass/ITO/ZnO/BHJ/MoO3/Ag inverted
cell structure was fabricated using an active layer spin-coated
from a chloroform solution of this ratio of 1 and PC61BM con-
taining 0.5 volume% of solvent additive 1,8-diiodooctane
(DIO). This device gave a power conversion efficiency (PCE) of
0.20% with an open-circuit voltage (Voc) of 0.49 V, a short-cir-
cuit current density (Jsc) of 1.16 mAcmꢀ2, and a fill factor (FF)
of 0.35 under AM1.5G illumination (100 mWcmꢀ2; Figure 7c,d
and Table 5). The low performance was explained by rough-
ness of the active layer due to aggregation of 1, which was ob-
&
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least squares technique (SHELXL-2013).[29] Yadokari-XG software
was used as a GUI for SHELXL-2013.[30] CCDC-1021750 contains the
supplementary crystallographic data for 6b. This data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Further improvement of the OPV performance can be expect-
ed, if the photoabsorption of PPAB in the higher energy visible
region can be enhanced. In this respect, the dimer 10 should
be highly promising. However, most of the PPAB molecules
synthesized to date are not appropriate due to their low-lying
LUMO energy levels, which originate from the PPAB structures
consisting of benzothiazole heteroaromatic ring moieties.
Since PPAB with pyridine rings has a suitable LUMO energy
level for PC61(71)BM, control of the photoabsorption based on
this PPAB structure by changing the substituents on the pyrro-
lopyrrole moiety or by homo-oligomerization will be the next
topic to be pursued with the aim of achieving a high PCE in
the PPAB-based BHJ-OPV. Furthermore, from a synthetic point
of view, the TiCl4-mediated Schiff-base formation reaction is
a versatile synthetic method to convert other conjugated bicy-
clic lactam molecules to dimeric aza-BODIPY analogues. Re-
search along both of these directions is currently being under-
taken and will be reported in due course.
Computational methods
The Gaussian 09 software package[31] was used to carry out DFT
and TDDFT calculations using the B3LYP functional with 6-31G(d)
basis set. Structural optimization was performed on M-1–4 as
model compounds of 2, 4, 5, and 6a, respectively.
Time-resolved microwave conductivity
A resonance cavity was used to obtain a high degree of sensitivity
in the conductivity measurement. The resonance frequency and
microwave power were set at about 9.1 GHz and 3 mW, respective-
ly, so that the electric field of the microwave was sufficiently small
not to disturb the motion of charge carriers. Nanosecond laser
pulse at third-harmonic generation (THG; 355 nm) of a Nd:YAG
laser (Continuum, Surelite II, 5–8 ns pulse duration, 10 Hz) or micro-
second white-light pulse from a Xe flash lamp was used as an exci-
tation source. The photoconductivity Ds was obtained by DPr/
(APr), in which DPr, A, and Pr are the transient power change of re-
flected microwave from a cavity, the sensitivity factor, and the re-
flected microwave power, respectively. The power of the white-
light pulse was 0.3 mJcmꢀ2 pulseꢀ1. The samples were drop-cast
onto a quartz plate from solutions of PPAB and PC61BM and dried
in a vacuum oven. The TRMC experiments were performed under
ambient conditions at room temperature.
Experimental Section
General procedures
Electronic absorption spectra were recorded on a JASCO V-570
spectrophotometer. Fluorescence spectra were measured on a Hita-
chi F-4500 spectrofluorimeter or on a Horiba Fluorolog-3 spectro-
fluorimeter. Absolute fluorescence quantum yields were measured
on a Hamamatsu Photonics C9920-03G calibrated integrating
sphere system. The fluorescence lifetime was measured with a pico-
second light pulser (Hamamatsu Photonics C4725: 408 nm, 59 ps
FWHM) and a streak-scope (Hamamatsu Photonics C4334-02).
1H NMR spectra were recorded on a Bruker AVANCE III 500 spec-
Organic photovoltaic cell
A ZnO layer was fabricated onto a cleaned ITO layer by spin-coat-
ing of ZnO precursor solution (0.1 gmLꢀ1 zinc acetate dihydrate
and 0.028 gmLꢀ1 ethanolamine in 2-methoxyethanol). The sub-
strate was annealed on a hot plate at 2008C for 30 min. An active
layer consisting of the PPAB and PC61(71)BM (purchased from Fron-
tier Carbon Inc.) was cast on top of the ZnO layer in a nitrogen
glovebox by spin-coating. The thickness was controlled by chang-
ing the rotation speed and was measured by a surface profiler, an
ULVAC model Dektak 150. An anode consisting of 10 nm MoO3 and
100 nm Ag layers was sequentially deposited through a shadow
mask on top of the active layers by thermal evaporation in
a vacuum chamber. The resulting device configuration was ITO
(120–160 nm)/ZnO (30 nm)/active layer (140–200 nm)/MoO3
(10 nm)/Ag (100 nm) with an active area of 7.1 mm2. Current–volt-
age (J/V) curves were measured using a source-measure unit
(ADCMT Corp., 6241 A) under AM1.5G solar illumination at
100 mWcmꢀ2 (1 sun, monitored by a calibrated standard cell,
Bunko Keiki SM-250 KD) from a 300 W solar simulator (SAN-EI
Corp., XES-301S). EQE spectra were measured using a Bunko Keiki
model BS-520BK equipped with a Keithley model 2401 source
meter. The monochromated light power was calibrated with a sili-
con photovoltaic cell, Bunko Keiki model S1337-1010BQ.
1
trometer (operating at 500.133 MHz for H) using residual solvent
as an internal reference for 1H (d=7.26 ppm for CDCl3 and d=
5.32 ppm for CD2Cl2). High-resolution mass spectra were recorded
on a Bruker Daltonics solariX 9.4T spectrometer. Cyclic voltammo-
grams were recorded on a Hokuto Denko HZ5000 potentiostat
under a nitrogen atmosphere in o-dichlorobenzene solutions with
0.1m tetra-n-butylammonium perchlorate as a supporting electro-
lyte. Measurements were made with a glassy carbon electrode
(area=0.07 cm2), an Ag/AgCl reference electrode, and a Pt-wire
counter electrode. The concentration of the solution was fixed at
0.5 mm and the sweep rates were set to 100 mVsꢀ1. The ferroceni-
um/ferrocene (Fc+/Fc) couple was used as an internal standard.
Preparative separations were performed by using alumina and
silica gel column chromatography (Wako) and recycling preparative
GPC-HPLC (JAI LC-9201 with preparative JAIGEL-2H and ꢀ2.5H col-
umns using chloroform as an eluent). Thin-layer chromatography
(TLC) was performed with aluminum sheet silica gel 60 F254
(Merck). Toluene, which was used for the synthesis of PPAB, was
distilled over CaH2 prior to use. All the other reagents and solvents
were of commercial reagent grade and were used without further
purification except where noted.
Crystallographic data collection and structure refinement
Synthesis of PPAB
Suitable crystals of 6b for X-ray analysis were obtained from slow
diffusion of hexane into a toluene solution. Data collection for 6b
was carried out at ꢀ173(2)8C on a Bruker APEXII CCD diffractome-
ter with MoKa radiation (l=0.71073 ꢂ). The structures were solved
by a direct method (SHLEXS-97)[29] and refined using a full-matrix
Synthesis of 4: To a toluene solution (7 mL) of DPP-1 (96 mg,
0.18 mmol) and HA-1 (301 mg, 1.78 mmol) heated at reflux were
added TiCl4 (0.25 mL, 2.3 mmol) and triethylamine (1.0 mL,
7.2 mmol). After consumption of DPP-1, which was confirmed by
TLC analysis, BF3·OEt2 (1.0 mL, 8.1 mmol) was added, and resulting
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solution was heated at reflux for 4 h. The reaction mixture was
poured into 300 mL of water, and the organic phase was extracted
with CHCl3. The organic layer was dried over sodium sulfate, and
the solvent was removed. The crude product was purified by silica
gel column chromatography using CHCl3 as an eluent. Recrystalli-
zation from chloroform and methanol provided 4 as a brown
(130000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS: m/z calcd for
C44H44B2Br2F4N6O2S4: 1074.0871 [M+]; found: 1074.0871.
Synthesis of 7b: Compound 7b was similarly synthesized and pu-
rified to the case of 7a. Compound 7b was obtained as a green
power in 93% yield (133 mg). 1H NMR (500 MHz, CDCl3): d=8.94
(d, J=4.3 Hz, 2H), 7.95 (d, J=8.9 Hz, 2H), 7.25 (d, J=4.3 Hz, 2H),
7.17 (s, 2H), 7.11 (d, J=9.1 Hz, 2H), 3.90 (d, J=5.7 Hz, 4H), 1.83–
1.30 (m, 50H), 0.91–0.87 ppm (m, 12H). HR-MALDI-FT-ICR-MS: m/z
calcd for C60H76B2Br2F4N6O2S4: 1298.3375 [M+]; found: 1298.3375.
1
powder (1.7 mg, 1.0%). H NMR (500 MHz, CDCl3): d=8.73 (d, 4H,
J=8.5 Hz), 8.26 (d, 2H, J=7.0 Hz), 8.17 (d, 2H, J=8.0 Hz), 7.82 (dd,
2H, J1 =J2 =7.6 Hz), 7.77 (d, 2H, J=7.6 Hz), 7.74 (d, 2H, J=6.8 Hz),
7.62 (dd, 2H, J1 =J2 =7.7 Hz), 7.17 (d, 4H, 8.6 Hz), 4.18 (t, 4H, J=
6.5 Hz), 1.91–1.25 (m, 24H), 0.92 ppm (t, 6H, J=6.5 Hz); UV/Vis
(CHCl3): lmax (e)=355 (13000), 440 (23000), 497 (16000), 684
(33000), 747 nm (120000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS:
m/z calcd for C56H54B2F4N6O2: 990.4425 [M+]; found: 990.4428.
Synthesis of 8: Compound 7a (24 mg, 0.022 mmol) and thio-
phene-2-boronic acid (14 mg, 0.11 mmol) were dissolved in toluene
(0.6 mL), and a small amount of Aliquat 336 and 2.0m aqueous so-
lution of Na2CO3 (0.4 mL) was added. After degassing under nitro-
gen bubbling for 30 min, tetrakis(triphenylphosphine)palladium
(5.0 mg, 4.3 mmol) was added, and the resulting solution was
heated at reflux for 1 hour. After cooling to room temperature, the
resulting mixture was poured into methanol (20 mL), and the pre-
cipitate was collected by filtration. Purification of the filtrate by
silica gel chromatography using CHCl3 as an eluent and recrystalli-
zation from CHCl3 and methanol provided 8 as a pink powder
Synthesis of 5: Compound 5 was synthesized under similar reac-
tion conditions to 4 from DPP-2 (102 mg, 0.22 mmol), HA-2 (330 g,
2.2 mmol), TiCl4 (0.30 mL, 2.7 mmol), triethylamine (1.0 mL,
7.2 mmol), and BF3·OEt2 (0.75 mL, 6.1 mmol). The reaction mixture
was purified similarly to 4, to give 5 as a violet powder (10.4 mg,
1
5.8%). H NMR (500 MHz, CDCl3): d=8.54 (broad, 4H), 7.91 (d, J=
1
8.2 Hz, 2H), 7.72 (d, J=8.0 Hz, 2H), 7.50 (dd, J1 =J2 =8.0 Hz, 2H),
7.38 (dd, J1 =J2 =7.7 Hz, 2H), 7.31 (broad, 4H), 3.58 (broad, 8H),
1.91 (broad, 8H), 1.77 ppm (broad, 4H); UV/Vis (CHCl3): lmax (e)=
(11 mg, 45%). H NMR (500 MHz, CDCl3): d=9.18 (d, J=4.2 Hz, 2H),
7.99 (d, J=8.8 Hz, 2H), 7.49 (d, J=3.6 Hz, 2H), 7.41 (d, J=4.2 Hz,
2H), 7.39 (d, J=5.1 Hz, 2H), 7.18 (s, 2H), 7.12–7.10 (m, 4H), 4.03 (t,
J=6.5 Hz, 4H), 1.83 (m, 4H), 1.51–0.89 (m, 20H), 0.90 ppm (t, J=
6.9 Hz, 6H); lmax (e)=324 (30000), 389 (36000), 520 (28000), 551
(28000), 687 (40000), 756 nm (110000 molꢀ1 dm3 cmꢀ1); HR-MALDI-
FT-ICR-MS: m/z calcd for C52H50B2F4N6O2S6: 1080.2438 [M+]; found:
1080.2437.
329
(27000),
354
(29000),
516
(44000),
733 nm
(75000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS: m/z calcd for
C42H36B2F4N8S2: 814.2622 [M+]; found: 814.2622.
Synthesis of 6a: Compound 6a was synthesized under similar re-
action conditions to 4 from DPP-3 (199 mg, 0.66 mmol), HA-3a
(1.0 g, 3.6 mmol, for the synthesis of HA-3a, see the Supporting In-
formation), TiCl4 (0.50 mL, 4.6 mmol), triethylamine (2.5 mL,
18 mmol), and BF3·OEt2 (2.0 mL, 16 mmol). The reaction mixture
was purified similarly to 4, to give 6a as a green powder (235 mg,
Synthesis of 9: Compound 7a (19 mg, 0.018 mmol), 5’-hexyl-2,2’-
bithiophene-5-boronic acid pinacol ester (32 mg, 0.085 mmol),
[1,1’-bis(diphenylphosphino)ferrocene]palladium(II) dichloride di-
chloromethane adduct ([PdCl2(dppf)], 8.1 mg, 9.9 mmol), and potas-
sium acetate (23 mg, 0.23 mmol) were dissolved in dry DMF
(0.6 mL), and the resulting solution was stirred at 808C for 15 min.
After cooling to room temperature, the resulting mixture was
poured into water, and the precipitate was collected by filtration.
Purification of the filtrate by silica gel chromatography using CHCl3
as an eluent and GPC-HPLC and recrystallization from CHCl3 and
methanol provided 9 as a dark-blue powder (6.3 mg, 25%).
1H NMR (500 MHz, CD2Cl2): d=9.24 (d, J=4.2 Hz, 2H), 7.90 (d, J=
9.2 Hz, 2H), 7.35 (d, J=3.7 Hz, 2H), 7.31 (d, J=3.9 Hz, 2H), 7.15 (s,
2H), 7.11–7.09 (m, 6H), 6.76 (d, J=3.6 Hz, 2H), 4.00 (t, J=6.5 Hz,
4H), 2.84 (t, J=7.7 Hz, 4H), 1.82–1.27 (m, 40H), 0.93–0.90 ppm (m,
12H); lmax (e)=364 (49000), 424 (37000), 583 (37000), 728
(46000), 803 nm (130000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS:
m/z calcd for C72H78B2F4N6O2S8: 1412.4070 [M+]; found: 1412.4070.
1
39%). H NMR (500 MHz, CDCl3): d=9.07 (d, J=4.0 Hz, 2H), 7.95 (d,
J=9.1 Hz, 2H), 7.85 (d, 2H), 7.33 (dd, J1 =4.0 Hz, J2 =5.0 Hz, 2H),
7.18 (s, 2H), 7.10 (d, J=9.1 Hz, 2H), 4.02 (t, J=6.5 Hz, 4H), 1.82 (m,
4H), 1.49–1.30 (m, 20H), 0.89 ppm (t, J=6.9 Hz, 6H); lmax (e)=348
(35000),
462
(20000),
638
(43000),
699 nm
(120000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS: m/z calcd for
C44H46B2F4N6O2S4: 916.2683 [M+]; found: 916.2683.
Synthesis of 6b: Compound 6b was synthesized under similar re-
action conditions to 4 from DPP-3 (113 mg, 0.38 mmol), HA-3b
(742 mg, 1.9 mmol for the synthesis of HA-3b, see the Supporting
Information), TiCl4 (0.25 mL, 2.3 mmol), triethylamine (1.0 mL,
7.2 mmol), and BF3·OEt2 (1.0 mL, 8.1 mmol). The reaction mixture
was purified similarly to 4, to give 6b as a green powder (128 mg,
1
30%). H NMR (500 MHz, CDCl3): d=9.07 (d, J=3.9 Hz, 2H), 7.95 (d,
Synthesis of 10: Compound 7b (88 mg, 0.068 mmol), bis(pinacola-
te)diboron (0.24 g, 0.94 mmol), potassium acetate (0.15 g,
1.5 mmol) and a catalytic amount of [PdCl2(dppf)] were dissolved
in dry DMF (5 mL), and the resulting solution was stirred at 808C
for 90 min. After cooling to room temperature, the resulting mix-
ture was poured into methanol, and the precipitate was collected
by filtration. Purification of the filtrate by silica gel chromatography
using CHCl3 as an eluent and GPC-HPLC, and recrystallization from
CHCl3 and methanol provided 10 as a violet powder (2.8 mg,
3.6%). 1H NMR (500 MHz, CDCl3): d=9.29 (d, J=4.0 Hz, 2H), 9.08
(d, J=3.3 Hz, 2H), 8.00 (d, J=8.7 Hz, 2H), 7.90 (d, J=8.3 Hz, 2H),
7.79 (d, J=4.8 Hz, 2H), 7.66 (d, J=4.1 Hz, 2H), 7.11 (s, 2H), 7.05 (m,
6H), 3.65 (m, 8H), 1.83–1.20 (m, 100H), 0.90 (m, 24H); (500 MHz,
CD2Cl2): d=9.32 (brs, 2H), 9.06 (brs, 2H), 7.96 (d, J=6.0 Hz, 2H),
7.83 (m, 4H), 7.67 (brs, 2H), 7.28 (brs, 2H), 7.15 (brs, 2H), 7.07
(brs, 6H), 3.87 (m, 8H), 1.83–1.20 (m, 100H), 0.90 ppm (m, 24H);
UV/Vis (CHCl3): lmax (e)=374 (68000), 581 (64000), 679 (99000),
J=9.1 Hz, 2H), 7.85 (d, J=5.0 Hz, 2H), 7.33 (dd, J1 =J2 =4.2 Hz,
2H), 7.19 (s, 2H), 7.11 (d, J=9.1 Hz, 2H), 3.90 (d, J=5.6 Hz, 4H),
1.81–1.29 (m, 50H), 0.90–0.87 ppm (m, 12H); HR-MALDI-FT-ICR-MS:
m/z calcd for C60H78B2F4N6O2S4: 1140.5187 [M+]; found: 1140.5187;
Synthesis of 7a: N-bromosuccinimide (51.6 mg, 0.29 mmol) was
added slowly to
a CHCl3 solution (15 mL) of 5a (130 mg,
0.14 mmol), and the resulting solution was stirred for three days.
The mixture was then poured into an aqueous solution of sodium
thiosulfate, and the organic phase was extracted with CHCl3. The
organic layer was dried over sodium sulfate, and the solvent was
removed. Recrystallization from CHCl3 and methanol provided 7a
1
as a green powder (138 mg, 92%). H NMR (500 MHz, CDCl3): d=
8.94 (d, J=4.3 Hz, 2H), 7.95 (d, J=9.2 Hz, 2H), 7.29 (d, J=4.3 Hz,
2H), 7.18 (s, 2H), 7.12 (d, J=9.1 Hz, 2H), 4.02 (t, J=6.5 Hz, 4H),
1.83 (m, 4H), 1.52–1.30 (m, 20H), 0.90 ppm (t, J=7.0 Hz, 6H); lmax
(e)=355 (42000), 475 (28000), 650 (48000), 713 nm
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839 nm (78000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS: m/z calcd
for C120H154B4F8N12O4S8: 2280.0278 [M+]; found: 2280.0281.
Glodkowska-Mrowka, T. Stoklosa, D. T. Gryko, Org. Lett. 2012, 14, 2670–
2673; c) W. Yue, S. L. Suraru, D. Bialas, M. Muller, F. Wꢄrthner, Angew.
Chem. 2014, 126, 6273–6276; Angew. Chem. Int. Ed. 2014, 53, 6159–
6162.
Synthesis of 11: Compound 11 was synthesized under similar reac-
tion conditions to 4 from 3,6-bis-(4-(2-ethylhexyloxy)phenyl)-2,5-di-
hydropyrrolyl[3,4-c]pyrrole-1,4-dione (DPP-4, 109 mg, 0.18 mmol),
2-aminopyridine (169 mg, 1.8 mmol), TiCl4 (0.20 mL, 1.8 mmol), trie-
thylamine (1.0 mL, 7.2 mmol), and BF3·OEt2 (1.0 mL, 8.1 mmol). The
reaction mixture was purified by alumina gel chromatography
using CHCl3 as an eluent, to give 11 as a blue powder (27 mg,
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17%). H NMR (500 MHz, CDCl3): d=8.44 (d, J=6.9 Hz, 4H), 8.22 (d,
J=6.6 Hz, 2H), 7.79 (dd, J1 =J2 =7.8 Hz, 2H), 7.06–7.03 (m, 6H),
3.97 (d, J=4.0 Hz, 4H), 1.78–1.34 (m, 18H), 0.98–0.92 ppm (m,
12H); UV/Vis (CHCl3): lmax (e)=316 (23000), 414 (28000), 591
(36000), 638 nm (83000 molꢀ1 dm3 cmꢀ1); HR-MALDI-FT-ICR-MS:
m/z calcd for C44H50B2F4N6: 792.4112 [M+]; found: 792.4113.
Acknowledgements
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This work was partly supported by a Grant-in-Aid for Scientific
Research on Innovative Area, “New Polymeric Materials Based
on Element-Blocks (No. 25102503), from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology (MEXT) and
a Grant-in-Aid for Young Scientists A (No. 26708003) from
Japan Society for the Promotion of Science (JSPS). The authors
thank Prof. Takeaki Iwamoto and Dr. Shintaro Ishida, Tohoku
University for the X-ray measurement.
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Keywords: chromophores · density functional calculations ·
fluorescence spectroscopy
spectroscopy
· fluorescent probes · UV/Vis
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Published online on && &&, 2014
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FULL PAPER
Fluorescence under control: The ra-
tional control of the absorption and
fluorescence of pyrrolopyrrole aza-
BODIPY (PPAB) molecules in the visible
and near infrared regions and panchro-
matic absorption of the PPAB dimer
were achieved (see figure). In addition,
application of PPAB molecules as optoe-
lectronic materials was investigated for
the first time.
&
Fluorescence
S. Shimizu,* T. Iino, A. Saeki,* S. Seki,
N. Kobayashi*
&& – &&
Rational Molecular Design towards
Vis/NIR Absorption and Fluorescence
by using Pyrrolopyrrole aza-BODIPY
and its Highly Conjugated Structures
for Organic Photovoltaics
Chem. Eur. J. 2014, 20, 1 – 13
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ