Pyrrolopyrrole aza-BODIPY analogues: A facile synthesis and intense fluorescence

Article abstract of DOI:10.1039/c3cc38452g

Pyrrolopyrrole aza-BODIPY analogues were synthesized from diketopyrrolopyrrole and heteroaromatic amines in the presence of titanium tetrachloride. These novel compounds exhibit intense absorption in the visible region and strong emission with high fluorescence quantum yields greater than 0.8. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38452g

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Pyrrolopyrrole aza-BODIPY analogues: a facile
synthesis and intense fluorescence†
Soji Shimizu,*a Taku Iino,a Yasuyuki Arakib and Nagao Kobayashi*a
Cite this: Chem. Commun., 2013,
49, 1621
Received 24th November 2012,
Accepted 8th January 2013
DOI: 10.1039/c3cc38452g
www.rsc.org/chemcomm
Pyrrolopyrrole aza-BODIPY analogues were synthesized from this position. In place of a direct nucleophilic substitution reaction,
diketopyrrolopyrrole and heteroaromatic amines in the presence two methods are known to activate the carbonyl carbon atoms.
of titanium tetrachloride. These novel compounds exhibit intense Thioketone was easily formed from the reaction of DPP with P4S10,
absorption in the visible region and strong emission with high which was further converted into a more reactive S-alkylated species
fluorescence quantum yields greater than 0.8.
toward nucleophilic alkylation reaction. Secondly reaction of DPP
with phosphoryl chloride provided a mono-phosphorylated species
Diketopyrrolopyrrole (DPP, Scheme 1) is a conventional red as a stable intermediate, which was converted into an aminophenyl
pigment due to its thermal and chemical stability.1,2 Recently substituted derivative. Recently Zumbusch et al. used the phosphoryl
its derivatives with oligothiophene or donor–acceptor units at chloride-activation method to synthesize a series of pyrrolopyrrole
the a-positions and their polymers have been widely studied in cyanines, and reported intense visible–near-infrared absorption and
the fields of optoelectronics and molecular electronics, such emission of these compounds.6 The structures of pyrrolopyrrole
as organic thin layer transistors,3 light emitting diodes,4 and cyanines can be recognized as a kind of dimeric boron dipyrro-
dye-sensitized solar cells.5
methene (BODIPY) analogue linked by a pyrrolopyrrole unit, and
From a synthetic point of view, a pyrrolopyrrole moiety in expansion of the p-conjugated system through the pyrrolopyrrole
DPP is a fascinating unit as a fundamental structure for unit plays a crucial role in the significant red-shift of both
creating novel p-conjugated systems. However, chemical the absorption and emission bands. It is also generally known that
modification of DPP has been less investigated, and introduc- aza-BODIPY analogues, in which a carbon atom at the meso-position
tion of substituents is almost exclusively limited to the nitrogen in a BODIPY structure is replaced by a nitrogen atom, exhibit
atoms or pyrrolic a-positions (Scheme 1). Low reactivity of bathochromic shifts relative to their carbon congeners.7,8 With these
the carbonyl carbon atoms toward nucleophilic substitution points in mind, dimeric BODIPY analogues containing both
reaction, which was revealed in the initial research on DPP by bridging meso-nitrogen atoms and a pyrrolopyrrole unit are
Iqbal et al.,2a,b hampered expansion of p-conjugated systems at fascinating functional chromophores. We therefore investigated
imination reactions of DPP for the first time to find that in the
presence of titanium tetrachloride heteroaromatic amines were
easily reacted with DPP to provide novel chromophore molecules
with intense fluorescence, the structures of which can be referred to
as pyrrolopyrrole aza-BODIPY analogues (PPABs). The availability of
this titanium-mediated imination reaction of DPP in the synthesis of
a series of PPABs as well as their unique optical properties are
Scheme 1 Structure of diketopyrrolopyrrole.
described in detail in this manuscript.
3,6-Bis(4-octyloxyphenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-
dione 1 (p-octyloxyphenyl-substituted DPP) was synthesized
according to the literature procedure2a and used in this study
because of the moderate solubility of this starting material in
common organic solvents. In the presence of titanium tetra-
chloride and triethylamine,9 1 was reacted with 2-aminopyridine
2a in refluxing toluene, and the mixture was further reacted
with BF3ÁOEt2 to provide 3a in 21% yield (Scheme 2). 2-Benzo-
thiazoleamine 2b and 2-aminoquinoline 2c were successfully
a Department of Chemistry, Graduate School of Science, Tohoku University, Sendai
980-8578, Japan. E-mail: ssoji@m.tohoku.ac.jp, nagaok@m.tohoku.ac.jp;
Fax: +81 22 795 7728; Tel: +81 22 795 7728
b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Sendai 980-8577, Japan. Fax: +81 22 217 5610; Tel: +81 22 217 5609
† Electronic supplementary information (ESI) available: Syntheses, spectroscopic
data, and theoretical calculations. CCDC 96328–96330. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3cc38452g
c
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pyrrolopyrrole cyanines is relatively smaller dihedral angles between
the phenyl substituents at the a-positions and the pyrrolopyrrole
moieties (361 for 3a, 391 for 3b, and 301 for 3c) due to the lower steric
congestion around the meso-positions: pyrrolopyrrole cyanines pos-
sess cyano groups at the meso-positions, which hinder co-planarity
of the aryl substituents at the a-positions.6
PPABs exhibit an intense band at around 650 nm with broad and
weak bands at around 330 and 430 nm, the latter of which is
characteristic of PPABs and not observed for the pyrrolopyrrole
cyanines (Fig. 2). The main absorption band shifts to the red upon
changing the heteroaromatic ring units from pyridine (3a: 638 nm)
to benzothiazole (3b: 655 nm) and quinoline (3c: 671 nm). The
fluorescence emission spectra are also red-shifted in this order
(661 nm (3a), 676 nm (3b), and 692 nm (3c)), whereas the Stokes
shifts become gradually smaller (545 cmÀ1 (3a), 474 cmÀ1 (3b), and
452 cmÀ1 (3c)). The fluorescence quantum yields determined by the
absolute method were 0.87 (3a), 0.81 (3b), and 0.83 (3c), and the life
times were 6.11 ns for 3a, 5.29 ns for 3b, and 4.91 ns for 3c,
respectively.
Scheme 2 Synthesis of a series of PPABs: (i) TiCl4, triethylamine, toluene, reflux;
(ii) BF3ÁOEt2.
In order to provide in-depth insight into the electronic
structures of PPABs, theoretical calculations based on the
DFT method at the B3LYP/6-31G(d) level10 were carried out
using the crystal structures as model structures, in which the p-
octyloxyphenyl substituents were replaced by phenyl substitu-
ents for simplicity. Based on the TDDFT calculations, the main
absorption bands of PPABs at around 650 nm are attributed to
transitions from the HOMO to the LUMO, whereas the higher-
energy broad bands at around 430 nm mainly are attributed to
transitions from the HOMO À 2 to the LUMO and from the
HOMO to the LUMO + 2 (see the ESI†). Density distribution
patterns in these frontier molecular orbitals, which appear to
be similar in all the cases, reveal delocalization of molecular
orbitals on entire molecules of PPABs (Fig. 3 and for more
details see the ESI†). In addition relatively large electron
densities are observed on the phenyl substituents due to their
co-planarity to the pyrrolopyrrole moieties. There is no clear
trend in the energies of the HOMO and the LUMO between
these compounds, but the HOMO–LUMO energy gap decreases
reacted with DPP under similar reaction conditions to give 3b
and 3c in 20% and 18% yields, respectively. These PPABs were
initially characterized by the high-resolution MALDI mass spec-
trometry and the NMR analysis. In the 1H NMR spectra in
CD2Cl2, all the compounds exhibited peak patterns typical of
C2 symmetric molecular structures: four, four, and six proton
signals due to the heteroaromatic ring units were observed for
3a (8.22, 7.86, 7.31, and 7.11 ppm), 3b (7.94, 7.74, 7.52, and
7.41 ppm), and 3c (8.56–8.53, 8.12, 7.76, 7.71, 7.49, and 7.26 ppm),
respectively. Coordination of boron difluoride units was also
confirmed by a triplet signal in the 11B NMR spectra (3a: 1.2 ppm,
3b: 1.2 ppm, 3c: 2.2 ppm) and a quartet signal in the 19F NMR
spectra (3a: À110 ppm, 3b: À111 ppm, 3c: À104 ppm). Finally
the structures were explicitly elucidated by single crystal X-ray
analysis.‡ All the compounds exhibit broadly similar planar
structures comprising two BODIPY-like moieties bridged
by a pyrrolopyrrole unit (Fig. 1). A striking difference in the
structures of PPABs from those of structurally resembling
Fig. 1 X-ray crystal structures of (a) 3a, (b) 3b, and (c) 3c: top view (top) and side view (bottom). Hydrogen atoms and p-octyloxy substituents were omitted for clarity
in both views. The thermal ellipsoids were scaled to the 50% probability level.
c
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This work was partly supported by Grant-in-Aids for Scientific
Research (B) (no. 23350095) and (C) (no. 23550040) from JSPS, and
a
Grant-in-Aid for Scientific Research on Innovative Areas
(no. 20108007, ‘‘pi-Space’’) from MEXT. S.S. is also grateful to
Mitsubishi Chemical Corporation Fund for the financial support.
The authors thank Prof. Takeaki Iwamoto and Dr Shintaro Ishida,
Tohoku University for the X-ray measurements.
Fig. 2 UV/vis absorption (left) and fluorescence (right) spectra of 3a (light blue),
3b (green), and 3c (blue) in CHCl3.
Notes and references
‡ Crystallographic data for 3a: C44H50B2N6O2F4, Mw = 792.52, triclinic,
%
space group P1 (no. 2), a = 7.0735(15), b = 9.584(2), c = 15.633(3) Å, a =
101.780(3)1, b = 99.884(3)1, g = 98.418(3)1, V = 1003.8(4) Å3, Z = 1, rcalcd
=
1.311 g cmÀ3, T = À173(2) 1C, 4761 measured reflections, 3464 unique
reflections (Rint = 0.0150), R = 0.0364 (I > 2s(I)), Rw = 0.0939 (all data),
goodness-of-fit on |F|2 = 1.024, largest diff. peak/hole 0.198/À0.256 e Å3,
CCDC 96328. Crystallographic data for 3b: C48H50B2N6O2F4S2, Mw
=
%
904.68, triclinic, space group P1 (no. 2), a = 9.389(2), b = 11.178(3), c =
11.344(3) Å, a = 70.195(3)1, b = 81.592(3)1, g = 83.582(3)1, V =
1105.7(4) Å3, Z = 1, rcalcd = 1.359 g cmÀ3, T = À173(2) 1C, 10 421
measured reflections, 3888 unique reflections (Rint = 0.0424), R =
0.0423 (I > 2s(I)), Rw = 0.1179 (all data), goodness-of-fit on |F|2
=
1.044, largest diff. peak/hole 0.496/À0.240 e Å3, CCDC 96329. Crystal-
lographic data for 3c: C54H56B2N6O2F4Cl6, Mw = 1131.37, monoclinic,
space group C2/c (no. 15), a = 19.017(9), b = 14.773(7), c = 19.603(9) Å, b =
104.419(5)1, V = 5334(4) Å3, Z = 4, rcalcd = 1.409 g cmÀ3, T = À173(2) 1C,
24 331 measured reflections, 4705 unique reflections (Rint = 0.0465), R =
0.0545 (I > 2s(I)), Rw = 0.1731 (all data), goodness-of-fit on |F|2 = 1.084,
largest diff. peak/hole 0.500/À0.509 e Å3, CCDC 96330.
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from 3a (2.20 eV) to 3b (2.16 eV) and 3c (2.10 eV),
which corresponds well to the red-shift of the lowest-energy
absorption in this order. These results indicate that the main
absorption bands of PPABs can be controlled by heteroaromatic
units, and that the extent of the shift of the absorption band
can be predicted by theoretical calculations.
In summary, novel PPABs have been synthesized from
DPP and heteroaromatic amines using titanium tetrachloride,
showing intense absorption in the lower-energy visible region
and emission with relatively high quantum yields. A significant
perturbation of the energy levels of the frontier orbitals by the
heteroaromatic units was inferred from the red-shift of the
main absorption bands of the PPABs with respect to those
units. In addition aryl substituents at the a-positions also
appear to play a crucial role in determining the energy levels of
the frontier orbitals due to the co-planarity to the pyrrolopyrrole
moieties. The absorption ranges of PPABs would be thus finely
tuned by changing both the heteroaromatic units and the sub-
stituents at the a-positions. Considering the facile syntheses of
PPABs and their future prospects for broader ranges of absorption
and emission in the visible and near-infrared regions, PPABs are
potential candidates not only in the fields of molecular electronics
and optoelectronics but also in the field of molecular probes, and
research along these directions is currently being undertaken in
our laboratory.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1621--1623 1623