Meso-Pyridyl BODIPYs with tunable chemical, optical and electrochemical properties

Article abstract of DOI:10.1039/c3nj00426k

A series of meso-pyridyl substituted BODIPY molecules has been synthesized, characterized and their optical and electrochemical properties compared. By utilizing ethanol and dichloromethane during the initial condensation reactions, there is a significant increase in the isolated yields compared to standard protocols. The properties of the highly fluorescent BODIPYs could be tuned by modifying the substituents of the pyridine, leading to pyridyl BODIPY as prospective ligands for future metal complexes. Furthermore, the presented BODIPY derivatives are shown to be applicable for fluorescence pH sensing over selective pH ranges.

Full text of DOI:10.1039/c3nj00426k

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meso-Pyridyl BODIPYs with tunable chemical, optical
and electrochemical properties†
Cite this: DOI: 10.1039/c3nj00426k
Juergen Bartelmess,a Walter W. Weare,*a Narah Latortue,b Christina Duongb and
Daniel S. Jonesb
A series of meso-pyridyl substituted BODIPY molecules has been synthesized, characterized and their
optical and electrochemical properties compared. By utilizing ethanol and dichloromethane during the
initial condensation reactions, there is a significant increase in the isolated yields compared to standard
protocols. The properties of the highly fluorescent BODIPYs could be tuned by modifying the
substituents of the pyridine, leading to pyridyl BODIPY as prospective ligands for future metal
complexes. Furthermore, the presented BODIPY derivatives are shown to be applicable for fluorescence
pH sensing over selective pH ranges.
Received (in Porto Alegre, Brazil)
23rd April 2013,
Accepted 15th June 2013
DOI: 10.1039/c3nj00426k
www.rsc.org/njc
In this study, we present an improved synthesis (up to a sixfold
increase in isolated yield) for meso-pyridyl BODIPYs. These
Introduction
A large number of pyridine containing metal complexes have
been reported, including metal complexes of porphyrins or
phthalocyanines with pyridine as an axial substituent.1–3 For
instance, ruthenium phthalocyanines bearing substituted pyr-
idine ligands, have shown to significantly influence the optical
and redox properties of the phthalocyanine complex.4 Cobalt
complexes are also known to incorporate pyridine ligands and
are useful components for applications such as photocatalytic
water-splitting, valence tautomers for magnetoelectronic mate-
rials, or as redox mediators in dye sensitized solar cells.5–7
Chromophore–metal complexes contain additional functionality,
including increased absorption and emission features and can be
applied for energy or electron transfer applications,8–11 or for
sensing and biological marking.12,13
compounds have interest as prospective ligands for future
coordination complexes. For example, a Ru-complex containing
the unsubstituted pyridyl BODIPY 1 has been recently reported,
which shows the potential for this class of compounds as
ligands for coordination chemistry.24 Several other meso-sub-
stituted BODIPY derivatives containing pyridines, usually incor-
porating bipyridine or- terpyridine, have been reported.25,26
Based on these BODIPY derivatives a variety of metal complexes
have been prepared, leading examples include metal centers
such as Zn,27 Cu,28,29 La,30 Ru31 or Pt.32 The application of
BODIPY as the axial substituent of metal complexes has not
been widely employed, however, some examples include the
axial functionalization of Sb(V)33 and Sn(IV)34 porphyrins or
Si(IV)11 phthalocyanines. The linkage is usually made with
alkoxide, phenolate or benzoate bridges.
Boron dipyrromethenes (BODIPY) have been and continue
to be studied extensively for applications in a multitude of
fields.14–22 Recently, we reported a synthesis for water-soluble
cationic BODIPYs, based on a meso-pyridyl substituted core.23
In addition, BODIPY 1 is also known as fluorescent pH sensor,35
a concept we have extended to additional pH ranges with the
described BODIPY derivatives. Several other BODIPY based fluores-
cent pH sensors have been reported, mainly based on BODIPY
bound dimethylaniline, phenol or carboxylic acid groups.36 In 2007,
Harriman et al. published a detailed spectroscopic study investi-
gating the processes a meso-pyridyl BODIPY undergoes upon proto-
nation or methylation of the pyridine group.37 The tunability of the
redox properties of BODIPY dyes has been elucidated in several
studies, and their application in electrochemoluminescence has
been described. Such examples show the versatility of the BODIPY
chromophore and the many possibilities for fine-tuning their
electrochemical potentials.20,26,38–40 To our knowledge, the substi-
tuted pyridyl BODIPYs described here have not been reported.
a Department of Chemistry, North Carolina State University, Campus Box 8204,
Raleigh, NC, 27695-8204, USA. E-mail: wwweare@ncsu.edu;
Fax: +1-919-515-8920; Tel: +1-919-515-1746
b Department of Chemistry, The University of North Carolina at Charlotte,
9201 University City Blvd., Charlotte, NC 28223-0001, USA.
E-mail: djones@uncc.edu; Fax: +1-704-687-0960; Tel: +1-704-687-0992
† Electronic supplementary information (ESI) available: Additional experimental
details for all new compounds and methods. pH dependant fluorescent spectra.
ATR-FT IR, 1H and 13C NMR data for new compounds 2–5, 19F NMR spectra.
Crystallographic procedures and data for 2 and 3. CCDC 935208 and 935207. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3nj00426k
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Results and discussion
Synthetic aspects
this to be necessary to optimize the yield of the isolated product.
This can be illustrated by comparing the yield we report for BODIPY
1 (30% following procedure A, 3 days reaction time for the first
step) with other studies that have much shorter reaction times
(16 h = 13% yield).43 A rational explanation for the long reaction
time is that the basic pyridine starting material can undergo
protonation by the trifluoroacetic acid catalyst, reducing the active
catalyst concentration and thus extending the reaction time.
NMR spectroscopy of 1H, 13C and 19F has been carried out in
order to characterize the synthesized compounds. While the 1H
and 13C NMR spectra are unremarkable, 19F NMR spectra
showed a distinct influence of the pyridyl substituents. It has
been previously described that small changes on the symmetry
of the molecule, even in large distance to the 19F core, significantly
alter the NMR spectra of BODIPY molecules.44–46 We found that
the symmetrical BODIPY derivatives 1 and 3 show a single quartet,
while the derivatives with lowered symmetry (2 and 4) show a 14
line spectrum – see Fig. S8 and S9 and the ESI† for further details.
This matches exactly the observations reported by Benniston
et al. in 2008 for unsymmetric meso-quinone substituted
BODIPY derivatives.44
The synthesis of pyridyl BODIPY 1 (1,3,5,7-tetramethyl-8-(4-
pyridyl)-4,40-difluoroboradiaza-indacene) has been described
earlier23,35 and is representative of the standard protocol for
the preparation of BODIPY derivatives with substituents on the
pyridyl group (in the following denoted as procedure A). It
includes the condensation of a pyrrole with a benzaldehyde
derivative in dichloromethane in the presence of trifluoroacetic
acid. 2,4-Dimethylpyrrole and most of the pyridine-4-carboxalde-
hydes in this work are commercially available, except 3-methyl-
pyridine-4-carboxaldehyde, which was prepared by the oxidation
of 3,4-lutidine with SeO2, following a published procedure.41
Condensation is followed by chemical oxidation with chloranil
and subsequent complexation with borontrifluoride etherate in
the presence of an amine base (e.g. diisopropylethylamine
(DiPEA)) in a sequential one-pot reaction – Scheme 1. Pyridyl
BODIPYs 2, 4 and 5, bearing just one substituent on the pyridine,
were synthesized, following this general procedure, in typical
yields of around 20%. However, we found that procedure A for
sterically hindered dichloro-substituted pyridyl BODIPY 3 results
in a 3% yield. Earlier, Lindsey and Wagner reported that for the
preparation of sterically demanding meso-mesityl porphyrins,
Crystallography
the use of ethanol as co-solvent dramatically improved yields In this study, we present the crystal structures of derivatives 2
and afforded high yields of dipyrromethanes.42 Thus, we used and 347 and compare them to the crystal structure of 1.35 This
a 1 : 14 (v/v) solvent mixture of ethanol : dichloromethane for allows the investigation of the structural influences of
the condensation and oxidation steps. To avoid undesirable the introduction of one or two chloro substituents on the
reactions of borontrifluoride with ethanol, the solvent mixture pyridine – Fig. 1 and Table 1. In contrast to 1, which has two
has to be removed under vacuum after the oxidation step, at independent molecular structures in the unit cell,35 com-
which time the crude reaction mixture was re-dissolved in pounds 2 and 3 have only one independent molecule in the
pure dichloromethane for the remaining synthetic procedure, unit cell. As expected, comparing the bond lengths and angles
completing a sequential one-pot reaction similar to procedure among these structures allows us to conclude that the molecular
A. This modified synthetic protocol is denoted as procedure B structures of compounds 2 and 3 are closely related to the structure
in Scheme 1, and increases the yield of BODIPY 3 to 20%. The of 1. Any differences of bond lengths and angles, including the
use of ethanol as co-solvent was tested with 1–5, and was found dihedral angle of the pyridine plane to the BODIPY core plane are
to increase the isolated yield in all cases. Even though the small. The only difference is observed when comparing the angles
approach presented here differs from Lindsey and Wagner, we C2–C3–C6 and C4–C3–C6 respectively. This angle represents the
feel that their thorough explanation for the catalytic effects of ability of the pyridyl group to bend relative to the BODIPY plane.
ethanol addition is relevant to this synthesis and does not Compound 1 shows a much larger flexibility, illustrated by
warrant further discussion here.42 The reaction time for the structure 1(B) with the difference of the aforementioned bond
first step of the BODIPY condensation is 3 days, which is long angles of 5.8(5)1. The introduction of bulky chloro substituents
compared to other BODIPY derivatives.14,15 However, we found (one or two) reduces this flexibility and leads to a more linear
Scheme 1 Numbering and synthesis of the BODIPY derivatives. (A) Dichloromethane. (B) Ethanol : dichloromethane 1 : 14 (v/v). (i) Trifluoroacetic acid. (ii) Chloranil.
(iii) (for B: remove solvent mixture and dissolve in pure dichloromethane) DiPEA, BF3 ꢀ Et2O.
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Fig. 1 Crystal structure for compounds 2 (left) and 3 (right). Hydrogen atoms
omitted for clarity.
Fig. 2 Normalized absorption (full line) and fluorescence (dotted) spectra of
compounds 1–5 recorded in dichloromethane.
Table 1 Selected bond lengths [Å] and angles [deg] of compounds 1, 2, and 3
Compound
1a
2
3
and 10 nm for two substituents (compound 3). The methyl
substituent in compound 5 does not influence the absorption
band of the BODIPY. The molar extinction coefficients of all
compounds are between 70 000 and 80 000 Mꢁ1 cmꢁ1. The
Stokes shift for these compounds is not significantly altered
by incorporation of substituted pyridyl groups and is between
10 and 14 nm for all BODIPYs. The fluorescence quantum yields,
however, increase for all of the BODIPYs with a substituted pyridine
(2–5). While compound 1 has a quantum yield of 0.30, the
quantum yields for all other molecules are much higher, between
0.78 (4) and approaching unity for BODIPY 3. Quantum yields of
2–5 were measured relative to BODIPY 1 based on previously
published values.23 A rational explanation for this observation is
based on earlier reports, which show that an increase in the rigidity
of a BODIPY molecule, especially when blocking the rotation of
a meso-substituent relative to the BODIPY core, leads to an increase
of the fluorescence quantum yield.48–50 In addition, the fluores-
cence lifetimes of compounds 1–5 were determined upon excita-
tion of the samples in dichloromethane at 457 nm. These
measurements reveal that an increase of the fluorescence lifetime
is correlated with the aforementioned increase of the fluorescence
quantum yield. Compound 1, for example, has a fluorescence
lifetime of 1.73 ns and a fluorescence quantum yield of 0.30, while
compound 3, with the highest fluorescence quantum yield
observed (0.98), shows a much longer fluorescence lifetime of
6.30 ns. These changes of the fluorescence lifetimes corrobo-
rate earlier studies investigating the photophysics of sterically
hindered BODIPY derivatives in comparison to their sterically
unhindered analogues and are a typical consequence for
increased rigidity in BODIPY molecules.48,50 The photophysical
investigations support the conclusions obtained by the crystal-
lographic studies, which suggest a more rigid molecular struc-
ture for compounds 2–5 compared to compound 1.
Torsion angle
BODIPY plane – pyridine
(C4 or C2–C3–C6–C13)
Corresponding dihedral angle (A): 92.0
(B): 98.4
Angle C2–C3–C6
Angle C4–C3–C6
Distance C3–C6
(A): 90.2(4) 100.5(2)
(B): 98.5(5)
99.6(2)
100.3(4)
99.4(3)
(A): 119.3(4) 121.9(2)
(B): 116.8(4)
(A): 121.8(4) 121.2(2)
(B): 122.6(3)
(A): 1.483(5) 1.493(2)
(B): 1.499(5)
Monoclinic Monoclinic Monoclinic
P21/c P21/n C2/c
121.9(1)
122.4(1)
1.490(2)
Crystal system
Space group
a
Data derived from crystal structure in ref. 35.
molecular structure. The difference of the aforementioned
bond angles in 2 is 0.7(3)1 and 0.5(1)1 for 3. Thus, the pyridine
is interlocked between the methyl substituents of the BODIPY
plane and the BODIPY plane itself. Despite this finding, the
introduction of substituents on the pyridine leads to only
minor changes in the rest of the molecular structure, leaving
the BODIPY core largely unaffected.
Photophysical properties
The photophysical data for all compounds, recorded in dichloro-
methane, is summarized in Table 2, while normalized absorption
and fluorescence spectra are shown in Fig. 2. The most intense
absorption band at around 500 nm undergoes a red-shift
upon introduction of electron withdrawing substituents on
the pyridine. This shift is 5 nm for one (compounds 2 and 4)
Table 2 Overview over the photophysical data of compounds 1–5. All data was
recorded in dichloromethane
Compound lAbs [nm] e [ꢀ103 Mꢁ1 cmꢁ1
]
lEm [nm] FF
tF [ns]
1a
2
3
4
5
505
510
515
510
505
76.8
83.2
73.4
75.9
75.4
516
522
526
524
515
0.30 1.73
0.90 5.56
0.98 6.30
0.78 4.66
0.91 5.33
Electrochemistry
The electrochemical data for the BODIPY compounds is sum-
marized in Table 3 and visualized in Fig. 3. Each BODIPY shows
one reversible reduction potential at around ꢁ1.4 V (vs. Fc/Fc+ =
0 V as standard). In addition, the BODIPYs have two oxidation
a
Data (except FF) derived from ref. 23.
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Table 3 Oxidation and reduction potentials of compounds 1–5 (in V vs. Fc/Fc+
=
0 as internal standard). Data recorded in degassed, dry acetonitrile with 0.1 M
tetrabutylammonium hexafluorophosphate electrolyte at 200 mV sꢁ1
a
b
a
Compound
Ered [V]
Eox1 [V]
Eox2 [V]
1
2
3
4
5
ꢁ1.46
ꢁ1.40
ꢁ1.35
ꢁ1.39
ꢁ1.47
0.77
0.85
0.90
0.85
0.79
0.90
0.94
0.99
0.94
0.91
a
b
Reversible potential. Irreversible potential.
Fig. 4 Plot of the normalized integrated fluorescence intensity of compounds
1–5 vs. pH in different buffer solutions (solvent: 25% H2O–75% methanol; buffer:
trifluoroacetic acid/trifluoroacetate or acetic acid/acetate).
protonation/methylation have on a related BODIPY derivative,
explaining the fluorescence quenching in detail, was published
by Harriman et al. in 2007.37 Since solubility required that these
measurements were performed in a 25: 75 H2O : MeOH mixture,
we refrain from reporting specific pKa values for these com-
pounds with this method. However, a clear trend for the
basicity of compounds 1–5 is demonstrated during pH depen-
dent fluorescence measurements. The basicity of compounds
1–5 is dependent on the functionalization of the pyridyl group
of the BODIPY – see Fig. 4 and Fig. S3 (ESI†). The strongest
basicity is seen for compound 5, bearing an electron donating
methyl group on the pyridine. This is followed by unsubstituted
compound 1, halogenated compounds 2–4, with electron with-
drawing substituents show much weaker basicity. This trend
matches the expectation for such acid–base equilibria,51 since
protonation of the pyridine creates a positive charge that is
stabilized in electron rich systems (1 and 5) and destabilized in
electron poor systems (2–4). A similar trend has been shown for
the basicity of 3-substituted pyridines.52 The BODIPY core is
identical in 1–5 and acts only as the fluorescence sensor in
this experiment. The series of pyridyl BODIPY derivatives 1–5
presented in this study allows for fluorescence pH sensing
in acidic media in a pH range between B1 and B4. Further
experimental details are contained within the ESI.†
Fig. 3 Cyclic voltammograms of compounds 1–5 recorded in degassed, dry
acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate. Electrode setup:
glassy carbon working electrode, platinum counter electrode, silver wire as quasi-
reference electrode. Ferrocene/ferrocenium (Fc/Fc+) was used as internal reference
and all graphs were referenced for Fc/Fc+ to be at 0 V. Scan rate 200 mV sꢁ1
.
potentials, an irreversible one at around 0.8 V and a reversible
one at around 0.9 V. These results are in line with previously
published data on a largely similar pyridyl BODIPY derivative,
bearing two additional ethyl substituents on the BODIPY core.37
The authors of the aforementioned study do observe the irrever-
sible oxidation peak in dichloromethane, but not in acetonitrile,
which is in contrast to our data. However, since the substitution
patterns of the BODIPY derivatives in this report and ref. 37
differ, observations of differential solvent effects are not unex-
pected. In general, insertion of electron withdrawing groups on
the pyridine shifts the potentials toward more positive values.
The influence of pyridyl substituents on the BODIPYs redox
potentials are moderate (B0.1 V) and consistent with the relative
electron withdrawing capabilities of the meso-pyridyl group. This is
in contrast to direct substitution of the BODIPY core, which has
larger effects (B0.7 V).38
Conclusions
In this paper we describe the preparation and characterization of
novel pyridyl BODIPYs and demonstrate that the introduction of
electron donating and withdrawing groups on the pyridine allow
pH dependent fluorescence
Determining the basicity of the pyridyl group on 1–5 is impor- for moderate tuning of the properties of the BODIPY core, while
tant for predicting their reactivity with metal centers in future significantly altering the basicity of the pyridine substituent. In
coordination complexes. Additionally, this property allows for addition, we were able to significantly increase the isolated yield of
these compounds to be considered as tuneable fluorescent pH pyridyl BODIPYs by using a solvent mixture (ethanol : dichloro-
sensors. It has been previously demonstrated that the fluores- methane, 1 : 14) instead of pure dichloromethane during the initial
cence emission of pyridyl BODIPY 1 can be reversibly quenched condensation reactions. A modulated rigidity of the molecular
by protonation of the pyridine substituent, which is corroborated structure leads to an increase of the observed fluorescence quan-
by our findings.35,37 A spectroscopic study of the effects that tum yields as well as of the fluorescence lifetimes. We expect these
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novel pyridyl BODIPYs to be promising ligands for future metal respectively. Crystal structures were measured on an Agilent
complexes that incorporate BODIPY functionality (e.g. increased Gemini Ultra diffractometer with Cu Ka radiation (l = 1.54184 Å)
absorbance as well as fluorescence features). Additionally, meso- at 100(1) K. Crystal structures were solved by direct methods
pyridyl BODIPY derivatives 1–5 can be applicable as fluorescent pH (SHELXS-97) and expanded using difference Fourier techniques.
sensors in a pH range from B1 to B4.
The structures were refined with SHELXL-97 using full-matrix
least-squares calculations. CCDC reference numbers: 2 – 935208,
3 – 935207.
Experimental
Materials
General synthetic procedure B
Diisopropylethylamine (DiPEA), borontrifluoride etherate, 3-chloro-
pyridine-4-carboxaldehyde and 3,4-lutidine were purchased from
Alfa Aesar, chloranil from TCI America, 2,4-dimethylpyrrole and
selenium dioxide from Sigma-Aldrich, 3-fluoro-pyridine-4-carbox-
aldehyde and 3,5-dichloro-pyridine-4-carboxaldehyde from Frontier
Scientific. All solvents were ACS grade and used as received, unless
otherwise noted. Acetonitrile for the electrochemical experiments
was dried over CaH2 and distilled prior to use. Reactions and
measurements were carried out under ambient conditions, unless
otherwise noted. Deuterated chloroform for NMR experiments was
purchased from Cambridge Isotope Laboratories, Inc.
Procedure A is similar to our previously published synthetic
protocol23 and can be found in detail in the ESI,† as well as
purification methods and the characterization of all new com-
pounds. The optimized general procedure B for compounds 1–5
is as follows (one equivalent is typically 5.36 mmol):
Two equivalents 2,4-dimethylpyrrole (approx. 1.1 g) and one
equivalent of the pyridine-4-carboxaldehyde derivative were dis-
solved in a deoxygenated 1 : 14 (v/v) mixture of ethanol (200 proof)
and dichloromethane. Several drops of trifluoroacetic acid were
added and the reaction mixture was stirred for 3 days under N2
atmosphere. Following this, one equivalent of chloranil was
added and the mixture was stirred for additional 2 hours. Next,
the solvent was removed under high vacuum und the remaining
crude solids were dissolved in dichloromethane. Subsequent
addition of DiPEA and borontrifluoride followed by stirring for
30 min and 3 hours, respectively, completed the synthesis.
Workup is identical to procedure A and all compounds show
identical properties after purification – see ESI.†
Methods and instrumentation
NMR spectra were recorded on a 400 MHz Varian Unity Inova
spectrometer. Values are given in ppm, relative to the solvent
1
signal of CDCl3 (7.26 ppm for H NMR (measured at 400 MHz)
and 77.16 ppm for 13C NMR (measured at 100 MHz)). 19F NMR
spectra were recorded at 376 MHz, values are given in ppm relative
to hexafluorobenzene as external reference (ꢁ162.23 ppm). Absorp-
tion spectra were recorded on an Agilent 8453 Diode Array
Spectrophotometer, fluorescence spectra on a QuantaMastert
Acknowledgements
40 spectrofluorometer (Photon Technology International, PTI). This work was supported in part by startup funding from North
Fluorescence quantum yields of new compounds were recorded Carolina State University. Mass spectra were obtained at the
relative to BODIPY 1 with a reported fluorescence quantum yield NCSU Department of Chemistry Mass Spectrometry Facility,
of 0.30.23 Fluorescence lifetimes were measured on a Horiba Jobin which is supported by the North Carolina Biotechnology Center
Yvon Fluorolog 3 spectrofluorometer. Samples were excited by a and the NCSU Department of Chemistry. We thank Dr Jonathan
Jobin Yvon NanoLED 460 with an excitation maximum at 457 nm, Lindsey and Dr Ana Soares (NCSU – Department of Chemistry)
emission for time-correlated single photon counting (TCSPC) was for assistance with instrumentation and helpful discussion and
detected at 530 nm for all samples. ATR/FT-IR (Attenuated Total Dr Achmed El-Shafei and Hammad Cheema (NCSU – College of
Reflectance/Fourier Transformed Infrared) spectra were measured Textiles) for additional instrumental support. WW acknowl-
on a Bruker Vertex 80V FT-IR spectrometer equipped with a edges MG, SC and SK from the #Scifund Challenge for support.
Platinum ATR accessory. pH measurements were performed This work was supported in part by funds provided by the
utilizing a Fisher Scientific accumet pH meter. Electrochemical University of North Carolina at Charlotte.
measurements were performed on a BioLogic SP-200 potentiostat/
galvanostat using a glassy carbon working electrode, a platinum
counter electrode and a silver wire as quasi-reference electrode. All
Notes and references
measurements were carried out in degassed, dry acetonitrile with
a 0.1 M tetrabutylammonium hexafluorophosphate electrolyte.
Fc/Fc+ was used as internal reference and all graphs were refer-
enced for Fc/Fc+ to be at 0 V. High resolution exact mass spectro-
metry measurements-ESI (HRMS-ESI) were carried out on an
Agilent Technologies (Santa Clara, California) 6210 LC-TOF mass
spectrometer. Samples were diluted in methanol and analyzed via a
1 mL minꢁ1 flow injection at 300 mL minꢁ1 in a water : methanol
mixture (75 : 25 v/v) with 0.1% formic acid. The mass spectrometer
was operated in positive-ion mode with a capillary voltage of 4 kV,
the fragmentor and skimmer voltages were 180–210 V and 60 V,
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New J. Chem.
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