Highly fluorescent BF2 complexes of hydrazine-schiff base linked bispyrrole

Article abstract of DOI:10.1021/ol501162f

A series of BF₂ complexes of hydrazine-Schiff base linked bispyrrole have been prepared from a simple two-step reaction from commercially available substances and are highly fluorescent in solution, film, and solid states with larger Stokes shift and excellent photostabilities comparable or even super to those of their BODIPY analogues. These resultant fluorescent dyes are highly susceptible to the postfunctionalization, as demonstrated in this work via the Knoevenagel condensation to introducing functionalities or tether groups to the chromophore.

Full text of DOI:10.1021/ol501162f

Letter
pubs.acs.org/OrgLett
Highly Fluorescent BF2 Complexes of Hydrazine−Schiff Base Linked
Bispyrrole
Changjiang Yu, Lijuan Jiao,* Ping Zhang, Zeya Feng, Chi Cheng, Yun Wei, Xiaolong Mu,
and Erhong Hao*
Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, School of
Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China
S
* Supporting Information
ABSTRACT: A series of BF2 complexes of hydrazine−Schiff base
linked bispyrrole have been prepared from a simple two-step
reaction from commercially available substances and are highly
fluorescent in solution, film, and solid states with larger Stokes
shift and excellent photostabilities comparable or even super to
those of their BODIPY analogues. These resultant fluorescent
dyes are highly susceptible to the postfunctionalization, as
demonstrated in this work via the Knoevenagel condensation to
introducing functionalities or tether groups to the chromophore.
Scheme 1. Syntheses of Complexes 1a−e and 3a,b
luorescent dyes have attracted increasing research interest in
Fhighly diverse research fields, for example, as molecular
probes in biomedical labeling and analysis and as organic
electronics in material science.1 Among those, BODIPY (4,4-
difluoro-4-bora-3a,4a-diaza-s-indocene) derivatives as the well-
known organoboron complexes have attracted wide research
efforts with remarkable achievements due to their rich chemistry
and their excellent photophysical properties.2,3 Despite their
intense fluorescence in solution, most BODIPYs have weak
fluorescence in the solid state due to self-absorption closely
associated with their narrow Stokes shift, which limits their
further applications as optoelectronic devices. Many organo-
boron complexes,4−7 such as the N,N π-conjugated boron(III)
complexes (B−D8 in Figure 1), have been prepared as BODIPY
analogues to overcome this problem.
boron(III) atom to form either the six-membered-ring BF2-
chelating complex 1 or the five-membered-ring chelating
complex A (Figure 1). Among those, complex 1 as the
hydrazine-inserted BODIPY would afford a larger Stokes shift
while maintaining the easy postfunctionalization ability and the
easily tunable photophysical properties of their BODIPY
analogues. While this manuscript was in preparation, Ziegler’s
group reported the synthesis of two symmetrical pyrrole−BF2
complexes.9 Herein, we report the efficient synthesis, the
characterizations, and the property studies of a series of BF2
complexes of hydrazine−Schiff base linked bispyrrole 1, which
are highly fluorescent in solution, film, and solid states via a
simple two-step reaction from commercially available substances.
The BF2 complexation of compound 2a with BF3·OEt2 in
dichloromethane yields a mixture of two products according to
TLC (a strong fluorescence one and a weak fluorescence one).
The former was later confirmed to be the diborate complex 1a,
and the later was indentified to be monoborate complex 3a as
Recently, we have prepared several hydrazine−Schiff base
linked bispyrrole 2 (Scheme 1) from the simple condensation of
commercially available 2-formylpyrrole with hydrazine in high
yields. We rationalized that compound 2 would chelate with
Figure 1. Chemical structures of BODIPY, the six-membered-ring BF2
chelating complex 1, and the five-membered-ring BF2 chelating A and
some representative literature-reported π-conjugated N,N-boron(III)
complexes B−D.
Received: April 22, 2014
Published: May 21, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
confirmed by the X-ray diffraction results. Although several five-
membered-ring chelating examples such as B−D (Figure 1) have
been reported,8 the variation of reaction conditions in our system
could not afford any of the other expected five-membered-ring
chelating product A or even its monoborate analogue.
By optimizing the reaction conditions, complex 1a was
generated exclusively in toluene at 110 °C, while the monoborate
complex 3a was mainly obtained in chloroform at 60 °C (Scheme
1). A similar reaction was observed for the BF2 complexation of
compound 2b to give dyes 1b and 3b in 40% and 35% yields,
respectively. To test the versatility of this reaction, hydrazine−
Schiff bases 2c−e were also prepared from the condensation of
their corresponding 2-formylpyrrole derivatives and even their
analogue 2-formyl(3-methyl-1H-indole) with hydrazine and
were used for the subsequent BF2 complexation in toluene at
110 °C, from which the desired complexes 1b−e in 21−43%
isolated yields (Scheme 1).
These resultant complexes 1 are susceptible to the
postfunctionalization. For example, complexes 1b and 1c show
reactivities comparable to those of their BODIPY analogues in
Knoevenagel condensation reaction10 with aromatic aldehyde,
like 4-N,N-dimethylbenzadeldehyde demonstrated in this work
(Scheme 2), to introducing various functionalities or tether
Scheme 2. Postfunctionalization of 1b and 1c through
Knoevenagel Condensation To Generate 4a and 4b
Figure 2. X-ray structures of 1a (a), 1b (b), 1c (c), 3a (d), and 3b (e).
Key: C, light gray; H, gray; N, blue; B, dark yellow; F, bright green.
hydrogen atoms at Cm are significantly lower for the latter (7.94
and 7.01 ppm, repectively).
Complexes 1a−d exhibit excellent optical properties with a
strong absorption and emission (fluorescence quantum yield
close to unity) in visible regions in several solvents studied as
summarized in Figure 3, Table 1, and Table S2 and Figures S5−
S8 in the Supporting Information. A gradual red-shift of the
absorption and emission was observed with the installation of
alkyl groups on the pyrrolic position of the chromophore. For
example, the pyrrolic-unsubstituted complex 1a gave a strong
absorption and fluorescence emission at 423 nm (ε = 3.98 × 104
M−1·cm−1) and 468 nm in dichloromethane, respectively, which
were red-shifted to 478 nm (ε = 5.24 × 104 M−1·cm−1) and 504
nm, respectively, for complex 1c with alkylation at the pyrrolic
position of the chromophore. The absorption and emission
maxima of complexes 1a−1d are only slight solvent dependent
with an around 2 ns of fluorescence lifetime, comparable to those
of classical BODIPYs. Unlike 1a−d, a low and solvent-dependent
fluorescence (ϕ = 0.06 in dichloromethane and ϕ = 0.45 in
hexane, Table S2, Supporting Information) was observed for
complex 1e similar to those previously reported indole-derived
dyes.12 In comparison with complexes 1a,b, their monoborate
analogues 3a,b exhibited relative short wavelength absorption
with low fluorescence quantum yields.
As expected, the installation of electron-donating substituents
(NMe2) at the p-phenyl position of the chromophore to form
4a,b via the Knoevenagel condensation leads to a significant red-
shift of the absorption and emission maxima (Figure 1 and Table
1). For example, 4a shows a 90 and 80 nm red-shift in their
absorption and fluorescence emission spectra, respectively, with
respect to 1b. A similar red-shift was observed for 4b with respect
to 1c. Unlike their starting materials 1b,c, both 4a and 4b show
strong solvent-dependent fluorescence. As shown in Table S2
and Figures S10 and S11 (Supporting Information), both 4a and
4b show large bathochromic shifts and fluorescence quenching in
groups to the chromophore. All these resultant complexes were
characterized by NMR and HRMS. Among those, complexes 1a,
1b, 1c, 3a, and 3b were further characterized by X-ray
crystallographic analysis (Figure 2).
The crystals suitable for X-ray analysis were obtained from the
slow evaporation of the dichloromethane solutions of these
complexes. There are four rigid planar ring structures in the
chromophore of complexes 1a−c: two five-membered pyrrole
units at the periphery and two BF2-containing six-membered
rings in the center, with only the fluorine atoms and the alkyl
substituents (1b and 1c) deviating from these plane
chormophore. The dihedral angles of two pyrrole rings in the
new formed chromophore are less than 2.7° (Table S1,
Supporting Information), indicating the existence of an inversion
center (C2h symmetry) in complexes 1a−c. Similarly, the
formation of one six-membered-ring BF2-chelating structure
was also observed in the monoborate complexes 3a and 3b. The
bond lengths in the pyrrolic units of these complexes 1a−c and
3a,b are similar to those observed in BODIPY derivatives,11
indicating the remaining of the aromaticity of the peripheral
pyrrole units at the edge.
Intramolecular C−H···F hydrogen bonds between F atoms
and various hydrogen atoms (Ca and Cm in Table S1, Supporting
Information) are formed due to the strong electron negativity of
the F atom.11a The Cm−F distances range from 2.88 to 3.09 Å,
while the Ca−F distances are significantly longer (3.10−3.52 Å).
In the 1H NMR spectra, the chemical shifts for hydrogen atoms
at Ca are both at around 2.50 ppm for 1b and the corresponding
1,3,5,7-tetramethylBODIPY,11b while the chemical shifts for
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Letter
Figure 4. Normalized absorption of 1c in dichloromethane (black), thin
film (red), and powder solid (blue) states and normalized fluorescence
emission spectra of 1c in dichloromethane (magenta), thin film (olive),
and powder solid (navy) states.
S22 in the Supporting Information), while BODIPY dyes barely
exhibit fluorescence in their solid state.10a The broader
absorption spectra and fluorescence emission bands in the
solid states for all complexes are red-shifted with respect to
corresponding absorption and emission bands in solutions. For
example, complexes 1b and 1c show strong fluorescence at 550
nm (ϕ = 0.28) and 605 nm (ϕ = 0.19) in the solid powder state,
respectively (Figure 4 and Table 1). More importantly,
complexes 4a and 4b show relatively strong near-infrared
fluorescence at 754 nm (ϕ = 0.14) and 725 nm (ϕ = 0.11) in
their solid powder state (Figures S21−S22 in the Supporting
Information), which are extremely rare for organoboron
materials.13 The high solid state fluorescence of dyes 1a-c is in
agreement with their crystal-packing structures. These dyes all
give well ordered packing structures due to multiple
intermolecular C−H···F hydrogen bonds (Figures S1−S3 in
the Supporting Information). However, no significant π−π
interactions were observed from crystal packing structures of
dyes 1a and 1b. Slipped dimers are formed in crystal packing
structure of 1c (Figures S3) which are typical J-aggregates.6a
The cyclic voltammetry of complexes 1a−c were performed in
deoxygenated dichloromethane at room temperature containing
tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supportive electrolyte. As shown in Figure 5, complexes 1a, 1b,
and 1c display an irreversible reduction wave with Epc at −1.14,
Figure 3. Normalized absorption (a) and fluorescence emission (b)
spectra of complexes 1a−e, 3a,b, and 4a,b in dichloromethane.
Table 1. Photophyscial Properties of Complexes 1a−e, 3a,b,
and 4a,b at Room Temperature in Dichloromethane, Thin
Film, and Solid Powder States
dichloromethane
thin film
solid powder
λemmax/nm
λemmax/nm
λemmax/nm
b
c
c
c
λabsmax/nm (lg ε)
(τ /ns, ϕ )
(ϕ )
(ϕ )
a
1a
1b
1c
1d
1e
3a
3b
4a
4b
423 (4.60), 442
468 (2.24, 1.00)
487 (2.66, 1.00)
497 (2.66, 1.00)
504 (2.58, 1.00)
580 (1.06, 0.06)
438 (2.13, 0.04)
530 (2.29, 0.03)
667 (1.44, 0.20)
537 (0.15)
534 (0.12)
558 (0.08)
575 (0.12)
667 (0.04)
522 (0.05)
585 (0.03)
726 (0.02)
710 (0.01)
543 (0.12)
550 (0.28)
605 (0.19)
576 (0.10)
673 (0.07)
526 (0.06)
587 (0.05)
754 (0.14)
725 (0.11)
a
444, 467 (4.67)
a
453, 478 (4.72)
a
455 (4.58), 480
a
498 (4.67), 522
410 (4.49)
441 (4.56)
a
526, 557 (4.80)
a
536, 557 (4.59)
670 (1.61, 0.21)
a
b
c
Shoulder peak. Fluorescence lifetime. Fluorescence quantum yield;
see the Supporting Information for details.
their fluorescence spectra with the increase of the solvent
polarity. For example, 4a emits at 581 nm with fluorescence
quantum yield of 0.45 in hexane which was red-shifted to 721 nm
with a reduced fluorescence quantum yield of 0.01 in acetonitrile.
This indicates the presence of an intramolecular charge transfer
(ICT) process10 in complexes 4a,b. In addition, complexes 4a,b
are sensitive to the pH variation of the system. The addition of
TFA to their dichloromethane solutions induced a blue-shift of
absorption from 672 nm to 485 and 512 nm for 4a and from 667
to 540 nm for 4b (Figures S12 and S13, Supporting Information)
due to the protonation of NMe2 group.
Figure 5. Cyclic voltammograms of 1 mM 1a-c measured in
dichloromethane solution, containing 0.1 M TBAPF6 as the supporting
electrolyte at room temperature. Glassy carbon electrode as a working
electrode, and the scan rate at 50 mV s−1.
Most of these complexes also show strong fluorescence
emission in the thin film and solid powder states, covering the
range of visible to near IR regions (Figure 4 and Figures S14−
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−1.03, and −0.97 V. One irreversible oxidation wave for 1a and
reversible oxidation waves for 1b and 1c were observed with Epa
at 1.40 V (1a) and half-wave potentials at 1.46 and 1.33 V (vs
SCE) (1b and 1c), respectively. HOMO energy levels of −5.80,
−5.74, and −5.56 eV and LUMO energy levels of −3.57, −3.80,
and −3.66 eV were estimated for complexes 1a−c, respectively,
based on their onset potential of the first oxidation and reduction
waves. Thus, the installation of alkyl groups on the pyrrolic
position indeed helps the decrease the LUMO level of the
chromophore and the decrease of the energy band gaps.
Electrochemical energy band gaps for complexes 1a−c were
calculated to be 2.23, 1.94, and 1.90 eV, respectively, which is in
well correlation with their optical band gaps.
To test their potential practical applications as novel dyes, we
also studied the photostabilities of these resultant complexes in
toluene under continuous irradiation with a 500 W Xe lamp
(Figure S23, Supporting Information). As demonstrated by
complexes 1a and 1d in Figure S23a (Supporting Information) ,
both complexes show excellent photostabilities during the period
of strong irradiation (60 min): more than 98% amount of 1a and
1d remained, while only 76% of the well-known commercialized
1,3,5,7-tetramethylBODIPY (4,4-difluoro-1,3,5,7-tetramethyl-4-
bora-3a,4a-diaza-s-indocene) left under the same condition. In
addition, these dyes are also stable in aqueous solution as shown
in Figure S23c (Supporting Information). The photostability of
1a in aqueous DMSO solution is much better than that of
fluorescein in 0.1 M NaOH solution.
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In summary, we have developed an efficient synthesis of a
series of BF2 complexes of hydrazine−Schiff base linked
bispyrrole via a simple two-step reaction from commercially
available substances. These resultant complexes are highly
fluorescent in solution, film, and solid states with excellent
photostabilities over the well-known commercialized 1,3,5,7-
tetramethylBODIPY. The photophysical properties of these
novel dyes are easily tunable through the structural variation of
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AUTHOR INFORMATION
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Corresponding Authors
(12) (a) Wakamiya, A.; Murakami, T.; Yamaguchi, S. Chem. Sci. 2013,
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*E-mail: jiao421@mail.ahnu.edu.cn.
*E-mail: haoehong@mail.ahnu.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work is supported by the National Nature Science
Foundation of China (Grants Nos. 21072005, 21272007 and
21372011) and the Research Culture Funds of Anhui Normal
University (Grant No. 160-791310).
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