Synthesis, photophysical, electrochemical, and electrogenerated chemiluminescence studies. multiple sequential electron transfers in BODIPY monomers, dimers, trimers, and polymer

Article abstract of DOI:10.1021/ja2010219

Synthesis of the C⁸ BODIPY monomers, dimers, and trimers, a C⁸ polymer, and N⁸ aza-BODIPY monomer and dimer was carried out. Methyl and mesityl C⁸-substituted monomers, dimers, and trimers were used. Dimers, trimers, and polymer were formed chemically through the β-β (2/6) positions by oxidative coupling using FeCl₃. A red shift of the absorbance and fluorescence is observed with addition of monomer units from monomer to polymer for C⁸ dyes. The aza-BODIPY dye shows red-shifted absorbance and fluorescence compared with the C⁸ analogue. Cyclic voltammetry shows one, two, and three one-electron waves on both reduction and oxidation for the monomer, dimer, and trimer, respectively, for the C⁸ BODIPYs. The separation for the reduction peaks for the C⁸ dimers is 0.12 V compared with 0.22 V for the oxidation, while the trimers show separations of 0.09 V between reduction peaks and 0.13 V for oxidation peaks. The larger separations between the second and third peaks, 0.25 V for the oxidation and 0.2 V for the reduction, are consistent with a larger energy to remove or add a third electron compared with the second one. The BODIPY polymer shows the presence of many sequential one-electron waves with a small separation. These results provide evidence for significant electronic interactions between different monomer units. The aza-BODIPY dye shows a reduction peak 0.8 V more positive compared to the C⁸ compound. Aza-BODIPY dimer shows the appearance of four waves in dichloromethane. The separation between two consecutive waves is around 0.12 V for reduction compared with 0.2 V for oxidation, which is comparable with the results for the C ⁸ dyes. Electrogenerated chemiluminescence (ECL) of the different species was obtained, including weak ECL of the polymer.

Full text of DOI:10.1021/ja2010219

ARTICLE
pubs.acs.org/JACS
Synthesis, Photophysical, Electrochemical, and Electrogenerated
Chemiluminescence Studies. Multiple Sequential Electron Transfers
in BODIPY Monomers, Dimers, Trimers, and Polymer
Alexander B. Nepomnyashchii,† Martin Br€oring,‡ Johannes Ahrens,‡ and Allen J. Bard*,†
†Center for Electrochemistry, Chemistry and Biochemistry Department, The University of Texas at Austin, Austin, Texas 78712,
United States
‡Institut f€ur Anorganische und Analytische Chemie, Technische Universit€at Carolo-Wilhelmina, Hagenring 30, 38106 Braunschweig,
Germany
S Supporting Information
b
ABSTRACT: Synthesis of the C8 BODIPY monomers, dimers, and
trimers, a C8 polymer, and N8 aza-BODIPY monomer and dimer was
carried out. Methyl and mesityl C8-substituted monomers, dimers,
and trimers were used. Dimers, trimers, and polymer were formed
chemically through the βꢀβ (2/6) positions by oxidative coupling
using FeCl3. A red shift of the absorbance and fluorescence is observed with addition of monomer units from monomer to polymer
for C8 dyes. The aza-BODIPY dye shows red-shifted absorbance and fluorescence compared with the C8 analogue. Cyclic
voltammetry shows one, two, and three one-electron waves on both reduction and oxidation for the monomer, dimer, and trimer,
respectively, for the C8 BODIPYs. The separation for the reduction peaks for the C8 dimers is 0.12 V compared with 0.22 V for the
oxidation, while the trimers show separations of 0.09 V between reduction peaks and 0.13 V for oxidation peaks. The larger
separations between the second and third peaks, 0.25 V for the oxidation and 0.2 V for the reduction, are consistent with a larger
energy to remove or add a third electron compared with the second one. The BODIPY polymer shows the presence of many
sequential one-electron waves with a small separation. These results provide evidence for significant electronic interactions between
different monomer units. The aza-BODIPY dye shows a reduction peak 0.8 V more positive compared to the C8 compound. Aza-
BODIPY dimer shows the appearance of four waves in dichloromethane. The separation between two consecutive waves is around
0.12 V for reduction compared with 0.2 V for oxidation, which is comparable with the results for the C8 dyes. Electrogenerated
chemiluminescence (ECL) of the different species was obtained, including weak ECL of the polymer.
metal ions, anions, or neutral molecules can be made by changing
the structure of the BODIPY dye. Such probes already have been
reported for the determination of Fe3þ, Ca2þ, Hg2þ, Cd2þ, CNꢀ,
nitroxyls (HNO), phosphorylated amino acids, and others,6,7,16ꢀ20
including sensitive and selective fluorescent probes.
1. INTRODUCTION
BODIPY dyes are of wide importance and have a broad use as
laser dyes in biological sensing, electrogenerated chemilumines-
cence (ECL), and other possible applications.1ꢀ11 Considerable
work has also been carried out on their photophysical proper-
ties.1,2,6,7 The absorption and fluorescence properties depend on
the position of substitution (see Scheme 1), where addition of a
donor substituent to position 8 does not cause a substantial change
of the photophysical behavior,1,12 but bulky donor groups in
positions 2 and 6 cause a large red shift in the absorbance and
fluorescence.1 The effect of an acceptor group is different, with a
red shift of the absorbance and fluorescence with addition of the
acceptor, like a cyano group, to position 8.1 Addition of a cyano
group to positions 2 and 6 causes a smaller change of absorbance
and fluorescence wavelength, but it may have a large effect on the
fluorescence quantum yield.1 The presence of bromine atoms in
positions 2 and 6 also does not have a substantial effect on the
absorbance and fluorescence wavelength.13 Formation of con-
jugated systems via positions 2 and 6 also has an effect on the
photophysical properties of the BODIPY dyes, showing red-shifted
behavior with an increasing degree of conjugation.14,15 Multiple
fluorescence probes for determination of the concentration of
The electrochemical and ECL properties of these dyes are also
of interest.21ꢀ28 BODIPY compounds blocked by alkyl or aromatic
groups show one-electron nernstian reduction and oxidation waves
for the first electron transfer, where the chemical reversibility de-
pends on the character of substitution.21,25 The stability of anion
radicals depends on substitution in position 8, which is subject to
nucleophilic attack, while the cation radicals are stabilized by
substitution at the 2 and 6 positions, preventing electrophilic
attack.
In this paper we discuss the synthesis of C8 BODIPY mono-
mers, dimers, trimers, and polymer and aza-BODIPY monomer
and dimer (Scheme 2) and investigation of their photophysical,
electrochemical, and ECL properties. The monomer, dimer, and
trimer of C8 BODIPY dyes in this study have similar structures, with
Received: February 9, 2011
Published: May 12, 2011
r
2011 American Chemical Society
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either methyl or mesityl groups in the meso-position(Scheme 2aꢀf).
The polymer (Scheme 2g) was synthesized with the mesityl sub-
stitution to improve its solubility in dichloromethane (DCM) and
tetrahydrofuran (THF). Comparative studies of the cyclic voltam-
metry (CV) of the monomer, dimer, trimer, and polymer can pro-
vide information about the electronic interactions among the mono-
meric units in the reduced and oxidized species,29ꢀ33 as previously
studied, for example, with oligothiophenes and oligoanthrylenes.34ꢀ36
Results for the anthracene dimers also show the presence of sub-
stantial interaction between different units with separation of
about 0.2ꢀ0.3 V.37 Study of the photophysical and structural
properties for the anthracene dimers was also carried out to com-
plement the redox data.38 Electron spin resonance was also shown
to be useful for characterization of the oligomers and polymers, as
it allows distinguishing between paramagnetic and diamagnetic
states, which is very important in conductivity studies.39,40 The
monomer and dimer of aza-BODIPY dyes (Scheme 2h,i), with a
nitrogen in position 8 instead of the carbon in C8 BODIPY, were
also studied. These molecules show red or near-infrared fluores-
cence, as well as less negative reduction, making it easier to study
the electrochemistry of the molecules.41ꢀ46 Aza-BODIPY dyes
are good candidates for NIR biological probes because of their
relatively high quantum yields; few red or NIR probes currently
exist.47,48 pH-responsive fluorescent probes based on N8 BOD-
IPY have been developed for imaging.49
During preparation of this manuscript, we learned that linear
dimers and trimers of the BODIPY dyes were synthesized and
characterized independently by the Ziessel research group using
a different synthetic strategy.50
2. EXPERIMENTAL SECTION
2.1. Synthetic Details. 2.1.1. Chemicals and Instrumentation.
All reagents were purchased from Sigma-Aldrich (Germany) and used as
received unless stated otherwise. NMR spectra were obtained with
Bruker DRX 400 and Bruker Avance 300 spectrometers. Chemical shifts
(δ) are given in ppm relative to residual proton solvent resonances (1H,
13C NMR spectra) or to external standards (BF3 Et2O for 11B and
3
CFCl3 for 19F NMR spectra). High-resolution ESI and APCI mass
spectra were recorded with a Finnigan LTQ FT. Molecular weight
determination by gel permeation chromatography (GPC) relative to a
polystyrene standard was carried out in THF through a column of
polystyrene sulfonate with a pore size of 5 μm. The pump was a Knauer
HPLC 64 with a flow rate of 1 mL minꢀ1 and the refractive index
detector was a Knauer RI.
2.1.2. Synthesis and Characterization. Compounds monomer 151
and aza-BODIPY monomer45,52 were prepared according to standard
procedures. All other compounds are new, and their synthesis has not
been previously published. A schematic of the synthetic procedure is
presented in Scheme 3, and the procedures are described below.
Preparation51 of monomer 1 (1,3,5,7,8-Pentamethyl-4,4-di-
fluoro-4-bora-3a,4a-diaza-s-indacene). To a solution of 2,4-dimeth-
yl-1H-pyrrole (5.4 mL, 52.0 mmol) in dry CH2Cl2 (20 mL) is added
acetyl chloride (8.7 mL, 121.4 mmol) dropwise at room temperature
over 30 min. The deep red solution is heated to reflux for 1 h. The
mixture is poured into n-hexane (100 mL) after cooling and con-
centrated to dryness on a rotary evaporator. The resulting dipyrrin
hydrochloride is used without any further purification. To a solution
of it in dry CH2Cl2 (240 mL) is added NEt3 (20.9 mL, 150 mmol),
Scheme 1. Structural Representation of the Core of the
BODIPY Dyes
Scheme 2. Structural Representation of the BODIPY Monomers, Dimers, and Trimersa
a (a,d) monomer 1 and monomer 2; (b, e) dimer 1 and dimer 2; (c, f) trimer 1 and trimer 2; (g) polymer; (h) aza-BODIPY monomer; and (i) aza-
BODIPY dimer.
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Scheme 3. Synthetic Procedure for the Compounds Used in the Studiesa
a (a) Synthesis of monomer 1; (b) synthesis of monomer 2; (c) synthesis of C8 dimers, trimers, and polymer; (d) synthesis of aza-BODIPY monomer;
and (e) synthesis of aza-BODIPY dimer.
and the solution is stirred for 10 min at room temperature. BF3 Et2O
14.6 (t, J = 2 Hz, 2C; 3/5-CCH3). 19F NMR (376 MHz, CDCl3):
δ = ꢀ146.6 (q, JBF = 33 Hz, 2F; BF2). 11B NMR (128 MHz, CDCl3):
δ = 1.30 (t, JBF = 33 Hz, 1B; BF2). HRMS (ESIþ): m/z calcd for
C14H17BF2N2Na [MþNa]þ, 285.1345; found, 285.1348.
3
(27.8 mL, 225 mmol) is added dropwise and stirred for 1 h at room
temperature. The deep red solution is washed with saturated aqueous
Na2CO3 solution (4 ꢁ 100 mL), dried over Na2SO4, and concen-
trated on a rotary evaporator. The red, oily residue is purified by
column chromatography on silica with n-pentane/CH2Cl2 = 1:1. The
orange-green fluorescing product fraction is dried, and the residue is
recrystallized from CH2Cl2/MeOH to yield a red-orange, crystalline
solid. Yield: 4.924 g, 72%.
Preparation of dimer 1 and trimer 1. To a solution of 1,3,5,7,8-
pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (monomer 1)
(195 mg, 0.744 mmol) in dry CH2Cl2 (35 mL) is added anhydrous
FeCl3 (422 mg, 2.60 mmol) at room temperature. The orange solution
rapidly turns deep green-red-violet. The reaction is quenched after 20
min stirring by addition of MeOH (50 mL) and then stirred for an
additional 30 min. The organic phase is washed with H2O (2 ꢁ 50 mL),
dried over Na2SO4, and concentrated to dryness on a rotary evaporator.
1H NMR (400 MHz, CDCl3): δ = 6.05 (s, 2H; 2/6-CH), 2.57 (s, 3H;
8-CCH3), 2.52 (s, 6H; CH3), 2.41 (s, 6H; CH3). 13C NMR (100 MHz,
CDCl3): δ = 153.8, 141.6, 141.1, 132.2, 121.4 (2C; 2/6-CH), 17.4, 16.5,
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The solid residue is separated by column chromatography on silica with
CH2Cl2. First the residual educt is eluted as an orange-yellow fraction of
greenish yellow fluorescence (100 mg, 51% of recovered educt mono-
mer 1). The first product, dimer 1, is eluted as an orange fraction of
orange-yellow fluorescence followed by the second product, trimer 1, as
a violet-red fraction of orange fluorescence. Concentrated to dryness,
dimer 1 gives a red solid and trimer 1 a deep red solid.
Dimer 2 (2,20-bi-(8-mesityl-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-
3a,4a-diaza-s-indacenyl)):Yield:21mg, 10%.1H NMR (300 MHz, CDCl3):
δ = 6.95 (s, 2H; 2 ꢁ mesityl-CH), 6.91 (s, 2H; 2 ꢁ mesityl-CH), 5.97 (s,
2H; 6/60-CH), 2.57 (s, 6H; CH3), 2.36 (s, 6H; CH3), 2.32 (s, 6H; CH3),
2.12 (s, 6H; CH3), 2.05 (s, 6H; CH3), 1.39 (s, 6H; CH3), 1.13 (s, 6H;
CH3). 13C NMR (100 MHz, CDCl3): δ = 155.7, 154.4, 142.8, 141.8,
140.4, 138.8, 135.1, 134.9, 131.4, 131.0, 130.6, 129.3 (2C; 2 ꢁ mesityl-
CH), 129.1 (2C; 2 ꢁ mesityl-CH), 124.5, 121.2 (2C; 6/60-CH), 21.3,
19.7, 19.7, 14.8 (br s, 2C; 5/50-CCH3), 13.6, 13.5 (br s, 2C; 3/30-CCH3),
12.1. 19F NMR (376 MHz, CDCl3): δ = ꢀ146.7 (q, JBF = 33 Hz, 4F; 2 ꢁ
BF2). 11B NMR (128 MHz, CDCl3):δ=0.57(t, JBF =33Hz, 2B;2ꢁ BF2).
HRMS (APCIþ): m/z calcd for C44H49B2F4N4 [MþH]þ, 731.4085;
found, 731.4083.
Dimer 1 (2,20-bi-(1,3,5,7,8-pentamethyl-4,4-difluoro-4-bora-3a,4a-
diaza-s-indacenyl)): Yield: 55 mg, 27%. 1H NMR (400 MHz, CD2Cl2):
δ = 6.13 (s, 2H; 6/60-CH), 2.67 (s, 6H; CH3), 2.51 (s, 6H; CH3), 2.46 (s,
6H; CH3), 2.32 (s, 6H; CH3), 2.23 (s, 6H; CH3). 13C NMR (100 MHz,
CD2Cl2): δ = 154.7, 153.4, 142.6, 142.5, 140.1, 133.0, 132.7, 125.2, 122.0
(2C; 6/60-CH), 17.8, 17.3, 15.9, 14.8 (br s, 2C; 5/50-CCH3), 13.5 (br s, 2C;
3/30-CCH3). 19F NMR (376 MHz, CD2Cl2): δ = ꢀ146.7 (q, JBF = 33 Hz,
4F; 2 ꢁ BF2). 11B NMR (128 MHz, CD2Cl2): δ = 0.37 (t, JBF = 33 Hz, 2B;
2 ꢁ BF2). HRMS (APCIþ): m/z calcd for C28H33B2F4N4 [MþH]þ,
523.2822; found, 523.2830.
Trimer 2 (2,20,60,200-tri-(8-mesityl-1,3,5,7-tetramethyl-4,4-difluoro-
4-bora-3a,4a-diaza-s-indacenyl)): Yield: 6 mg, 3%. 1H NMR (300 MHz,
CDCl3): δ = 6.95 (s, 2H; 2 ꢁ mesityl-CH), 6.92 (s, 4H; 4 ꢁ mesityl-CH),
5.98 (s, 2H; 6/600-CH), 2.56 (s, 6H; CH3), 2.37 (s, 6H; CH3), 2.34 (s,
6H; CH3), 2.32 (s, 6H; CH3), 2.31 (s, 3H; CH3), 2.12 (s, 6H; CH3), 2.07
(s, 6H; CH3), 2.04 (s, 6H; CH3), 1.39 (s, 6H; CH3), 1.13 (s, 12H; CH3).
13C NMR (100 MHz, CDCl3): δ = 155.9, 155.0, 154.3, 142.9, 141.8,
141.7, 140.8, 140.4, 138.9, 138.8, 135.1, 134.9, 134.9, 131.5, 131.4, 131.1,
130.8, 130.6, 129.3 (4C; 4 ꢁ mesityl-CH), 129.1 (2C; 2 ꢁ mesityl-CH),
124.8, 124.4, 121.2 (2C; 6/600-CH), 21.3, 19.8, 19.8, 19.7, 14.8 (br s, 2C;
5/500-CCH3), 13.6, 13.6 (br s) 13.5 (br s), 12.1, 12.1. 19F NMR (376 MHz,
CDCl3): δ = ꢀ146.8 (m, 6F; 3 ꢁ BF2). 11B NMR (128 MHz, CDCl3):
δ = 0.63 (pseudo t, JBF = 33 Hz, 3B; 3 ꢁ BF2). HRMS (APCIþ): m/z
calcd for C66H72B3F6N6 [MþH]þ, 1095.6007; found, 1095.6027.
Preparation of polymer. To a solution of 1,3,5,7-tetramethyl-8-
mesityl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (monomer 2) (116 mg,
0.318 mmol) in dry CH2Cl2 (30 mL) is added anhydrous FeCl3 (207 mg,
1.27 mmol) at room temperature. H2O (100 mL) is added to the re-
action mixture after stirring for 22 h at room temperature. The se-
parated organic phase is dried over Na2SO4 and concentrated to dryness
on a rotary evaporator. The blue, solid residue is separated by column
chromatography on neutral aluminum oxide (Brockmann activity IV).
The smaller units are washed away with CH2Cl2. The polymer is finally
eluted as a blue fraction with CH2Cl2/MeOH = 1:1 and pure MeOH. Phase
separation from the coeluted water and filtration leads after evaporation
to the product as a violet solid.
Trimer 1 (2,20,60,200-tri-(1,3,5,7,8-pentamethyl-4,4-difluoro-4-bora-
3a,4a-diaza-s-indacenyl)): Yield: 17 mg, 9%. 1H NMR (400 MHz,
CD2Cl2): δ = 6.14 (s, 2H; 6/600-CH), 2.75 (s, 3H; 80-CCH3), 2.68 (s,
6H; CH3), 2.51 (s, 6H; CH3), 2.47 (s, 6H; CH3), 2.35 (s, 6H; CH3), 2.33
(s, 6H; CH3), 2.26 (s, 6H; CH3), 2.24 (s, 6H; CH3). 13C NMR: Due to
the low solubility no analyzable 13C NMR spectrum could be recorded.
19F NMR (376 MHz, CD2Cl2): δ = ꢀ146.6 (m, 6F; 3 ꢁ BF2). 11B NMR
(128 MHz, CD2Cl2): δ = 0.59 (t, JBF = 31 Hz, 3B; 3 ꢁ BF2). HRMS
(APCIþ): m/z calcd for C42H48B3F6N6 [MþH]þ, 783.4118; found,
783.4120.
Preparation of monomer 2 (8-Mesityl-1,3,5,7-tetramethyl-4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene). To a solution of 2,4,6-tri-
methylbenzaldehyde (0.73 mL, 5 mmol) and 2,4-dimethyl-1H-pyrrole
(1.28 mL, 12.5 mmol) in dry CH2Cl2 (250 mL) is added a solution of
trifluoroacetic acid (50 μL, 0.65 mmol) in dry CH2Cl2 (2.5 mL) slowly
at room temperature. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (1.128 g,
5 mmol) is added after 3 h stirring under ice bath cooling and stirred for
10 min. The solution is stirred for an additional 1 h at room temperature.
NEt3 (10 mL, 72 mmol) is added, followed by slow addition of BF3 Et2O
3
(10 mL, 81 mmol). The reaction mixture is washed after 2 h of stirring at
room temperature with saturated aqueous Na2CO3 solution (3 ꢁ 50 mL),
dried over Na2SO4, and concentrated on a rotary evaporator. The brown,
oily residue is purified by column chromatography on silica with n-pentane/
CH2Cl2 = 5:1, then 2:1, then pure CH2Cl2. The product fraction with
greenish fluorescence is dried to yield a red-brown solid.
Yield: 18 mg. 1H NMR (300 MHz, CD2Cl2): δ = 6.94 (br s; mesityl-
CH), 6.00 (s; CH), 3.09ꢀ2.80 (br m), 2.56ꢀ0.88 (br m). 19F NMR
(376 MHz, CD2Cl2): δ = ꢀ146.6 (m; BF2). 11B NMR (128 MHz,
CD2Cl2): δ = 1.08 (t; BF2). GPC (THF, 23 °C, polystyrene standard):
Mn = 8732, Mw = 17635, D(Mw/ Mn) = 2.02.
Yield: 1.698 g, 93%. 1H NMR (300 MHz, CDCl3): δ = 6.94 (s, 2H;
2 ꢁ mesityl-CH), 5.96 (s, 2H; 2/6-CH), 2.56 (s, 6H; CH3), 2.33 (s, 3H;
CH3), 2.10 (s, 6H; CH3), 1.38 (s, 6H; CH3). 13C NMR (75 MHz, CDCl3):
δ = 155.2, 142.4, 141.8, 138.7, 135.1, 131.3, 130.8, 129.1 (2C; 2 ꢁ
mesityl-CH), 120.9 (2C; 2/6-CH), 21.3, 19.6, 14.8 (br s, 2C; 3/5-CCH3),
13.5. 19F NMR (376 MHz, CDCl3): δ = ꢀ146.5 (q, JBF = 33 Hz, 2F;
BF2). 11B NMR (128 MHz, CDCl3): δ = 0.69 (t, JBF = 33 Hz, 1B; BF2).
HRMS (ESIþ): m/z calcd for C22H25BF2N2Na [MþNa]þ, 389.1971;
found, 389.1983.
Preparation45,52 of aza-BODIPY monomer. To a solution of tetra-
phenyl azadipyrromethene (414 mg, 0.92 mmol) in dry CH2Cl2 (160 mL)
is added dry i-Pr2EtN (1.8 mL, 10.1 mmol), and the mixture is stirred
for 15 min at room temperature. Freshly destilled BF3 Et2O (1.8 mL,
3
14.3 mmol) is slowly added and stirred for 24 h at room temperature.
The solution is washed with H2O (3 ꢁ 100 mL), dried over Na2SO4, and
concentrated on a rotary evaporator. The product is dried under vacuum
to yield a black-blue solid.
Preparation of dimer 2 and trimer 2. To a solution of 1,3,5,7-
tetramethyl-8-mesityl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (mono-
mer 2) (227 mg, 0.64 mmol) in dry CH2Cl2 (30 mL) is added anhydrous
FeCl3 (402 mg, 2.48 mmol) at room temperature. The orange solution
rapidly turns deep green-red-violet. The reaction is quenched after 25 min
stirring by addition of MeOH (25 mL). The organic phase is washed with
H2O (3 ꢁ 100 mL), dried over Na2SO4, and concentrated to dryness on a
rotary evaporator. The solid residue is separated by column chromatog-
raphy on silica with CH2Cl2. First the residual educt is eluted as greenish
yellow fluorescing fraction. The first product, dimer 2, is eluted as an
orange fluorescing fraction, followed by the second product, trimer 2, as a
reddish fluorescing fraction. Concentrated to dryness, dimer 2 gives a red
solid and trimer 2 a violet solid.
Yield: 468 mg, 100%. 1H NMR (300 MHz, CDCl3): δ = 8.17ꢀ7.98 (m,
8H), 7.56ꢀ7.35 (m, 12H), 6.99 (s, 2H; 2 ꢁ β-CH). 13C NMR (100 MHz,
CDCl3): δ = 159.7 (2C; aza-BODIPY-C), 145.7 (2C; aza-BODIPY-C),
144.3 (2C; aza-BODIPY-C), 132.4, 131.7, 131.0, 129.7 (t, J = 3 Hz),
129.6, 129.5, 128.8, 128.7, 119.3 (2C; aza-BODIPY-CH). 19F NMR
(376 MHz, CDCl3): δ = ꢀ133.6 (q, JBF = 31 Hz, 2F; BF2). 11B NMR
(128 MHz, CDCl3): δ = 0.57 (t, JBF = 31 Hz, 1B; BF2). HRMS
(APCIþ): m/z calcd for C32H23BF2N3 [MþH]þ, 498.1951; found,
498.1948.
Preparation of aza-BODIPY dimer. To a solution of aza-BODIPY
monomer (50 mg, 0.100 mmol) in dry CH2Cl2 (20 mL) is added an-
hydrous FeCl3 (65 mg, 0.401 mmol) at room temperature. After stirring
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for 30 min, argon-saturated H2O (40 mL) is added to the reaction mix-
ture and then stirred for an additional 1 h. The blue organic phase is
washed with H2O (2 ꢁ 40 mL) and concentrated to dryness on a rotary
evaporator. The blue, solid residue is separated by column chromatog-
raphy on silica with n-pentane/CH2Cl2= 1:1, then 1:2. First the residual
educt is eluted as a blue fraction of red fluorescence, and then the pro-
duct, aza-BODIPY dimer, is eluted as blue fraction. Concentrated to
dryness, aza-BODIPY dimer is a dark blue powder.
1
Yield: 21 mg, 43%. H NMR (300 MHz, CD2Cl2): δ = 8.03ꢀ7.90
(m, 8H), 7.54ꢀ7.42 (m, 8H), 7.42ꢀ7.34 (m, 6H), 7.32ꢀ7.03 (m, 20H).
13C NMR: Due to the low solubility no analyzable 13C NMR spectrum
could be recorded. 19F NMR (376 MHz, CD2Cl2): δ = ꢀ130.9 (dq, JBF
=
30 Hz, 2F; 2 ꢁ BFF), ꢀ131.9 (dq, JBF = 30 Hz, 2F; 2 ꢁ BFF). 11B NMR
(128 MHz, CD2Cl2): δ = 0.54 (t, JBF = 30 Hz, 2B, 2 ꢁ BF2). HRMS
(APCIþ): m/z calcd for C64H43B2F4N6 [MþH]þ, 993.3685; found,
993.3666.
2.2. Photophysical and Electrochemical Details. 2.2.1.
Chemicals. Electrochemical grade DCM, ferrocene, and 10-methylphe-
nothiazine were obtained from Aldrich Chemical Co. (Milwaukee, WI)
and used without further purification. Supporting electrolytes tetra-n-
butylammonium hexafluorophosphate (TBAPF6) and benzoyl peroxide
were obtained from Fluka.
2.2.2. Apparatus and Methods. UVꢀvis and fluorescence investiga-
tions were carried out in DCM as solvent in air. A DU640 spectrophoto-
meter (Beckman, Fullerton, CA) was used for the absorbance measurements.
Fluorescence experiments were carried out under the same conditions
using a double-beam QuantaMaster spectrofluorimeter (Photon Tech-
nology International, Birmingham, NJ) with a 70 W xenon lamp and slit
width of 0.5 mm. Quantum yield calculations were carried out using
fluorescein as a standard and compared with known literature results. Elec-
trochemical experiments were done using a three-electrode setup with a
0.0314 cm2 platinum disk working electrode, platinum wire as a counter
electrode, and silver wire as a quasi-reference electrode. The electrode
potential was determined using ferrocene as a standard reference material
and assuming its potential equal to 0.342 V vs SCE.53 Larger electrodes
with an area of 0.2 cm2 were used for ECL polymer experiments. A
straight platinum electrode was used for all CV measurements, and a
bent L-type electrode was used for the ECL investigations. The working
electrode was polished for 5 min with 0.3 μm alumina and sonicated for
5 min in EtOH prior to the experiment. The glassware was dried at
120 °C in an oven before transferring to the vacuum chamber of a glovebox
(Vacuum Atmospheres Corp., Hawthorne, CA). All solutions for elec-
trochemical measurements were prepared in the glovebox under inert
conditions and sealed with a Teflon cap. Three metals rods were drilled
through the cap to achieve electrode contact. CV and chronoampero-
metry pulsing experiments were done using a CH Instruments 660 (Austin,
TX) electrochemical workstation. Scan rates 1.0, 0.5, 0.25, and 0.1 V/s
were used. Chronoamperometry and scan rate measurements were used
to determine the diffusion coefficient values for all species. ECL spectra
were generated by stepping the potential to 80 mV past a given peak at a
frequency of 10 Hz with 1 min duration. When benzoyl peroxide was
used as a co-reactant, stepping was from 0 V to 80 mV after half-wave
reduction potential. ECL spectra were recorded with a Princeton
Instruments Spec 10 CCD camera (Trenton, NJ) cooled with liquid
nitrogen to ꢀ100 °C with an Acton SpectPro-150 monochromator.
ECL transient measurements were carried out prior to obtaining the
ECL spectra. The electrochemical experiments in this case used a
multichannel Eco Chemie Autolab PGSTAT100 potentiostat
(Utrecht, The Netherlands). The simultaneous ECL-CV signal was re-
corded with a Hamamatsu (Tokyo, Japan) photomultiplier tube, read
with a Keithley electrometer (model 6517, Keithley Instruments, Inc.,
Cleveland, OH). The relative ECL quantum yield was with reference to
Ru(bpy)32þ under similar conditions. Digital simulations for the CV of
Figure 1. Absorbance and fluorescence spectra of 2 μM BODIPY dyes: (a,
b) monomer 1 and 2;(c,d) dimer 1 and 2;(e) angular dimer;(f,g) trimer 1
and 2; (h) polymer; (i) aza-BODIPY monomer; (j) aza-BODIPY dimer.
monomer, dimer, and trimer were carried out with the Digisim software
package (Bioanalytical Systems, West Lafayette, IN).54ꢀ57
3. RESULTS AND DISCUSSION
3.1. Synthetic Strategy. The synthetic aim of the project was
to obtain new oligomeric and polymeric structures of BODIPYs,
i.e. dimers, trimers, and small polymers. The best method found
is oxidative coupling with anhydrous FeCl3 (∼3.5 equiv relative
to the BODIPY monomer) in CH2Cl2 of double β-free penta-
substituted BODIPYs through the 2/6-position. Formation of
dimers and trimers (and different higher oligomers) is observed
in one pot at room temperature. The reaction is quenched by
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Table 1. Photophysical Properties of the Studied BODIPY
Compounds
Scheme 4. Structural Representation of the Angular Dimer
λmax (nm)
dye
abs
fluor ε (104 Mꢀ1 cmꢀ1
)
Φfluor Es (eV)
monomer 1
monomer 2
dimer 1
353, 497 512
359, 501 513
368, 526 563
368, 535 573
370, 550 587
372, 562 596
378, 590 614
0.75, 8.8
0.74, 8.7
1.5, 15.0
1.6, 15.7
2.3, 22.5
2.35, 23.0
11.0, 155.0
0.60, 8.5
0.97
0.97
0.66
0.66
0.60
0.60
0.35
0.30
2.43
2.43
2.20
2.17
2.13
2.09
2.02
1.82
dimer 2
trimer 1
trimer 2
polymer
has also red-shifted fluorescence of around 70 to 80 nm com-
pared with the linear dimer.
aza-BODIPY 468, 647 682
monomer
The trimers show behavior similar to the dimers and the absence
of the substantial exciton splitting (Figure 1f,g). Comparison of
the monomers, dimers, and trimers shows a red shift of the wave-
length for the absorbance going from monomer to dimer to trimer,
corresponding to the interactions inside a linear alignment of the
same chromophors according to the exciton model of Kasha62
and also partially due to a higher degree of conjugation. Absorbance
maxima are red-shifted around 29ꢀ34 nm in the case of the tran-
sition from monomer to dimer and around 24ꢀ27 nm in the case
of the transition from dimer to trimer, which shows a smaller change
in absorbance properties with addition of consecutive BODIPY
units. Fluorescence results show a similar trend, with a large change
of around 50 nm in the case of transition from monomer to dimer
and only around 20 nm for the transition to trimer. These pho-
tophysical properties correspond with an increase of the Stokes
shift from 12 to 16 nm for monomers compared with 37 to 38 nm
for the dimers and 34 to 37 nm for the trimers. This increase of
the Stokes shift corresponds with higher nonradiative decay and a
smaller value of the fluorescence quantum yield for the dimer and
trimer compared with the monomer. The trimers show a similar
Stokes shift to the dimers, probably due to the diminished influence
of the interactions with addition of more and more similar units.
There is also a smaller change of the quantum yield on going
from dimer to trimer relative to the transition from monomer to
dimer. The polymer shows the appearance of S1ꢀS0 absorbance
transition at 590 nm and fluorescence maximum at 614 nm, which is
slightly larger than for the trimer, at 596 nm (Figure 1h). The
linear polymer shows a fluorescence maximum still blue-shifted
compared with that of the angular dimer.
The aza-BODIPY monomer shows red-shifted absorbance
and fluorescence compared with the same C8 dye (Figure 1i),
with the characteristic BODIPY dye S2ꢀS0 and S1ꢀS0 transi-
tions. Fluorescence studies show a substantial fluorescence signal for
the monomer, although with less efficiency compared with the C8 dye
(Figure 1j). The absorption of the aza-BODIPY dimer is red-shifted
about 40 nm compared to the monomer. A much higher quenching
effect is seen for the dimer, with a quantum yield of less than 0.01.
3.3. Electrochemical Results. Electrochemical studies of the
C8 BODIPY monomers show the presence of one-electron reduc-
tion and oxidation waves with separation of around 2.5 V between
peaks from first reduction and first oxidation, which is character-
istic for green-emitting compounds (Figure 2). All half-wave
potentials for the reduction and oxidation and the diffusion co-
efficients are summarized in Table 2. Electrochemical properties
of monomer 1 were studied previously in acetonitrile and DCM.21,25
Digital simulations were also carried out to determine the me-
chanism of the electron transfer (Figures S1 and S2 in the
aza-BODIPY 488, 696 720
dimer
1.2, 16.0
<0.01
1.72
addition of methanol and/or water after 20ꢀ30 min. Although
the yields of the dimers (27% and 10%) and trimers (9% and 3%)
are low, the unreacted monomers (monomer 1 and monomer 2)
can be easily recovered and the oligomers separated by column
chromatography. So no additional reaction steps are needed to
set up simple monomeric BODIPY for direct coupling. In con-
trast to this oxidative CꢀC bond formation via the β-free positions
with FeCl3, the reductive coupling and palladium-mediated cross
coupling would need an introduction of halogen substituents. 58
The series with the mesityl group at the meso-position is much
more soluble than the series with the methyl group. Also, short
polymers are available if the reaction time is extended to 22 h
under otherwise the same conditions. The obtained polymers
from monomer 2 are soluble, and one size of them was even se-
parated by column chromatography to yield polymer with a
given structure. GPC analysis for this violet compound corresponds
to an average of 24 repeating monomer units. To test the versatility
of this oxidative coupling over the 2/6-position, the reaction pro-
cedure was further extended to aza-BODIPY monomer,52 from
which the dimer (aza-BODIPY dimer) was obtained in 43% yield
despite the steric effect of the phenyl groups.
3.2. Photophysical Results. The photophysical characterization
of all the compounds was done in a solution of CH2Cl2. Monomer 1
shows the usual behavior for 2/6-unsubstituted BODIPYs, with a
narrow absorption peak at 497 nm and fluorescence with a max-
imum at 512 nm (Figure 1a). Monomer 2 shows similar behavior
with slightly red-shifted absorbance and fluorescence (Figure 1b).
The results are summarized in Table 1. Changing the donor sub-
stituent by varying the size of the alkyl or aromatic group in the
meso-position 8 does not have a significant effect on the fluores-
cence wavelength. As an example, addition of the bulky amide
group, instead of a methyl group, to position 8 causes a shift of
the fluorescence of just several nanometers.59
The dimers show characteristic S1ꢀS0 and S2ꢀS0 transitions
common for the BODIPY dyes (Figure 1c,d). The mesityl-sub-
stituted dye also shows slightly red-shifted absorbance and fluores-
cencecompared with the methyl one, similar to monomers. How-
ever, there is a huge difference in the behavior of this linear dimer
compared with angular dimer (Scheme 4).60,61 The linear dimers
show a very small degree of exciton splitting in the absorbance
compared with the angular dimer with a high degree of splitting
and visible presence of the two absorption peaks instead of that
seen for the S1ꢀS0 transition (Figure 1e). The angular dimer
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Supporting Information). Previous studies show a nernstian one-
electron transfer for reduction and one-electron oxidation fol-
lowed by formation of the dimer, with a dimerization constant of
simulations for the monomer 2 (mesityl-substituted dye), which
shows similar oxidation and reduction behavior to monomer 1
(Figure S2 in the Supporting Information). A larger dimerization
constant of 2000 Mꢀ1 sꢀ1 was used in simulations.
400 Mꢀ1 sꢀ1 25 This electrochemical oxidation is consistent with
.
the formation of the dimer chemically using oxidative agent
FeCl3 described above. The same mechanism was used in digital
The mechanism of the dimerization can be represented as
RꢀH2 f RꢀH2 þ þ eꢀ
E1=2 ¼ 1:14 V
ð1Þ
1
3
þ
2þ
RꢀH2 þ þ RꢀH2 f H2RꢀRH2
3
3
kdim ¼ 2000 Mꢀ1 sꢀ1 ð2Þ
H2RꢀRH2 f HR ꢀ RH þ 2Hþ ðfastÞ k > 104s-1 ð3Þ
2þ
þ
2
HRꢀRH f HRꢀRH2þ þ eꢀ E1=2 ¼ 1:36 V ð4Þ
3
HR ꢀ RH f HRꢀRH þ þ eꢀ
E1=2 ¼ 1:10 V ð5Þ
d
3
The mesityl-substituted dye shows a slightly more negative re-
duction potential compared with the methyl-substituted dye that
correlates with a slight red shift of the fluorescence and absorbance.
The dimers show the presence of two one-electron waves on
both reduction and oxidation. The fact that the second waves
occur as separate ones at more extreme potentials is consistent
with a significant interaction between the two BODIPY units
(Figure 3aꢀf). The reduction peak potentials for the dimer 1
are ꢀ1.17 and ꢀ1.29 V, compared with the oxidation potentials
which are 1.09 and 1.31 V. There is no evidence of substantial
dimerization or other chemical processes for both oxidation and
reduction products of the dimers, so a simple EE mechanism with
two nernstian electrochemical waves was assumed for the
simulation (Supporting Information Figures S3 and S4). A small
amount of instability on oxidation, however, results in some film
formation on repeated cycling, suggesting possible slow cou-
pling, consistent with the absence of substitution in the positions
where chain propagation occurs. Dimer 2 shows similar electro-
chemical behavior as dimer 1 with about the same oxidation and
reduction potentials. The degree of the separation between reduction
Figure 2. Cyclic voltammograms of (a) 1.1 mM monomer 1 and (d)
1.0 mM monomer 2; (b) and (e) scan rate dependence while scanning
in negative and (c) and (f) in positive direction. Solvent, DCM;
supporting electrolyte, TBAPF6; platinum electrode area, 0.0314 cm2.
Table 2. Electrochemical Studies of the BODIPY Compounds
E1/2 (V vs SCE)
a
dye
monomer 1
A/Aꢀ
A/Aþ
λmax (ECL) (nm)
ΦECL
ΔHs (eV)
D (cm2/s)
ꢀ1.21
ꢀ1.19
ꢀ1.17
ꢀ1.29
ꢀ1.15
ꢀ1.27
ꢀ1.15
ꢀ1.24
ꢀ1.43
ꢀ1.13
ꢀ1.23
ꢀ1.43
many
1.12
1.14
1.09
1.31
1.10
1.37
1.04
1.17
1.42
1.11
1.24
1.50
many
1.14
1.10
1.24
545
538
587
0.006
0.007
0.008
2.24
2.24
2.18
7.0 ꢁ 10ꢀ6
7.0 ꢁ 10ꢀ6
5.2 ꢁ 10ꢀ6
monomer 2
dimer 1
dimer 2
trimer 1
596
607
0.008
0.011
2.16
2.09
5.2 ꢁ 10ꢀ6
4.8 ꢁ 10ꢀ6
trimer 2
608
0.016
2.08
4.8 ꢁ 10ꢀ6
polymer
620
695
ꢀ
<0.001
<0.001
ꢀ
1.90
1.40
ꢀ
1.4 ꢁ 10ꢀ6
7.0 ꢁ 10ꢀ6
5.2 ꢁ 10ꢀ6
aza-BODIPY monomer
aza-BODIPYdimer
ꢀ0.44
ꢀ0.37
ꢀ0.5
a Relative to Ru(bpy)32þ under similar conditions.
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Figure 4. Cyclic voltammograms of (a) 0.1 mM trimer 1 and (d) 0.24 mM
trimer 2; scan rate dependence while scanning (b,e) in negative and (c,f) in
positive direction. Solvent, DCM; supporting electrolyte, TBAPF6; platinum
electrode area, 0.0314 cm2.
between the two oxidation and the two reduction peaks is not
well understood but can probably be related with the extent of
delocalization of electron density in charged states and the
nature of the electrostatic interactions.26,63 Work is in progress
with oligomers of different monomers to try to resolve this
issue.
Trimer 1 shows three one-electron transitions for both ox-
idation and reduction (Figure 4). The oxidation shows three
clear peaks while on reduction the first two peaks are merged to
produce a two-electron wave with a shoulder. The reduction half-
wave potentials for 0.14 mM of the trimer 1 are at ꢀ1.15, ꢀ1.24,
and ꢀ1.43 V and oxidation half-wave potentials are at 1.04, 1.17,
and 1.42 V, as obtained from digital simulations assuming an EE
mechanism with three nernstian waves (Supporting Information
Figures S5 and S6). Trimer 2 shows very similar behavior. The
trimers show, as the dimers, a smaller separation between the first
and second reduction peaks compared with the oxidation. The
extent of separation between the two reduction and two oxida-
tion peaks decreases from dimer to trimer. As expected, it is
harder to withdraw or add a third electron compared with the
second one because of the greater electrostatic repulsion. Similar
effects have been seen, e.g., for truxene-oligofluorene compounds64
and many others. The polymer shows the presence of multiple
one-electron peaks corresponding to a series of waves, as ex-
pected from the results with dimer and trimer (Figure 5). This is
especially clear for the experiment in THF, where appearance of
about 20 consecutive reduction waves is noticed. A diffusion
coefficient of 1.4 ꢁ 10ꢀ6 cm2/s was estimated by using eq 629
Figure 3. Cyclic voltammograms of (a) 0.14 mM dimer 1 and
(d) 0.3 mM dimer 2; scan rate dependence while scanning (b,e)
in negative and (c,f) in positive direction; (gꢀi) comparison of
the oxidation and reduction potential between 0.2 mM dimer 1
and 0.15 angular dimer, where in (g) full scan is shown and in (h) and
(i) parts for the reduction (h) and oxidation (i) are highlighted.
Solvent, DCM; supporting electrolyte, TBAPF6; platinum electrode
area, 0.0314 cm2.
and oxidation waves for both compounds is also substantially less
than that of the angular dimer (Figure 3gꢀi), which is electro-
chemical evidence for a smaller degree of interaction between
BODIPY units in case of the dimer formed through position 2/6
compared with position 3. The angular dimer also shows the
same phenomenon of larger peak separation on oxidation com-
pared with reduction, which seems to be a characteristic feature
of these dyes.28 As a result the larger degree of separation on ox-
idation cannot be explained by the small instability of the oxi-
dation product as the angular dimer has all positions substituted.
The reason for the difference in the different amount of separation
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Figure 6. Cyclic voltammograms of 0.6 mM aza-BODIPY monomer at
a scan rate of 0.1 V/s in DCM at platinum working electrode. Area,
0.0314 cm2; supporting electrolyte, 0.1 M TBAPF6. (a) Forward scan to
the negative direction; (b) forward scan to the positive direction; scan
rate dependence for oxidation (c) and reduction (d); (e) scan for 0.8 mM
aza-BODIPY monomer in THF.
reversible reduction wave for this species in DCM (Figure 6aꢀ
c,e). The separation between the two reduction waves is around
0.82 V, which is smaller than 1.09 V seen for the C8 system
(which shows unusually large separations between the first two
electron additions).25 The separation between consecutive peaks
is, however, still larger than that in 9,10-diphenylanthracene and
other polycyclic hydrocarbons (about 0.5 V).65ꢀ67 A slight de-
crease in the separation between the two reduction waves is seen
with addition of the acceptor group, i.e., 1.09 V for alkyl-sub-
stituted dye and 0.98 V for the 8-cyano-substituted dye,25 com-
pared to that with the acceptor atom nitrogen, i.e., 0.82 V for aza-
BODIPY. Reduction of the aza-BODIPY monomer in THF at
more negative potentials shows the absence of any electroche-
mical processes up to ꢀ3.0 V (Figure 6e). Digital simulation con-
firmed the reversibility of the second electron reduction (Sup-
porting Information Figure S7aꢀd). As with the C8 BODIPY,
oxidation of the aza-compound also shows some dimer forma-
tion. Simulations including formation of the dimer on oxidation
were carried out and show a rate of dimerization about the same
as with the analogous C8 BODIPY (Supporting Information Figure
S7eꢀh). The aza-BODIPY dimer shows four reduction waves in
DCM. The first two transitions correspond to the addition of one
electron to each monomer unit and the second one to the addition
of an additional electron to each unit (Figure 7). An experiment
with the analogous C8 dimer 2 in THF at room temperature also
shows the presence of four peaks at more negative potentials, but
the reversibility is much poorer under these conditions (Figure 7e).
Figure 5. Cyclic voltammograms of 0.15 mM polymer in DCM during
scan in (a) negative and (b) positive direction; scan rate dependence
during scan in (c) negative and (d) positive direction, for 1 (blue), 0.5
(green), 0.25 (red), and 0.1 V/s (black). (eꢀg) Cyclic voltammograms
of polymer in THF: (e) scan rate dependence for 0.15 mM polymer,
for 1 (blue), 0.5 (green), 0.25 (red), and 0.1 V/s (black); (f) scan rate
0.01 V/s; (g) scan to ꢀ2.5 V. Electrode area, 0.0314 cm2; supporting
electrolyte, TBAPF6.
from the overall limiting current values.
0:55
ðDp=DmÞ ¼ ðMm=MpÞ
ð6Þ
It is difficult to simulate the polymer CV behavior because of
the polydispersity of the polymer and also the influence of many
peak splittings and rate constants on the electrochemical reduc-
tion process. This can also cause a deviation of the number of
electron transfers from the number expected from the gel permea-
tionstudies, 24, and the expected diffusion current. The oxidation
also shows multiple electron transfers, but observation of all multiple
transitions is limited by the potential window.
The aza-BODIPY monomer shows a nernstian reduction
wave with a peak potential shifted positive by about 0.8 V com-
pared to the C8 BODIPY (Figure 6). It is possible to see a second
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Figure 8. Electrogenerated (red line) and fluorescence (black line) of
investigated monomers. Annihilation spectrum for (a) 1.0 mM mono-
mer 1 and (b) 1.1 mM monomer 2. (c,d) Spectra generated in the
presence of 5 mM of benzoyl peroxide. (e) Comparative spectra of the
annihilation results (green) and in the presence of co-reactant (black).
Solvent, DCM; supporting electrolyte, 0.1 M TBAPF6; platinum
electrode area, 0.0314 cm2.
Figure 7. Cyclic voltammograms of 0.9 mM aza-BODIPY dimer at a
scan rate of 0.1 V/s in DCM at platinum working electrode. Area,
0.0314 cm2; supporting electrolyte, 0.1 M TBAPF6. (a) forward scan to
the negative direction; (b) forward scan to the positive direction; scan
rate dependence for (c) oxidation and (d) reduction; (e) reduction of
0.3 mM of dimer 2 in THF.
Digital simulations can be fit to the experimental results with some
deviation for waves 3 and 4, probably because of kinetic complica-
tions and instability of the 3- and 4-species (Supporting Informa-
tion Figure S8). The separation between the first two sequential
waves (and also the third and fourth wave) was ∼0.12 V, very close
to the results for the C8 BODIPY dimers. The separation between
the first two waves on oxidation is larger than that for the reduction.
The separation between the first and third reduction wave was
∼1.0 V, which is similar to the results for the monomer. The reason
for that was determined to be steric in the aza-BODIPY core.25
3.4. ECL Results. ECL of both the monomers showed relatively
weak emission while stepping in both positive and negative direc-
tions (Figure 8).21,25 However, the intensity was sufficient to be
able to obtain ECL spectra (Figure 8a,b). The annihilation ECL
mechanism can be represented as
cation. The ECL efficiency could be enhanced by using oxidative
correactant benzoyl peroxide (BPO), which allows light genera-
tion without electrochemical oxidation of the dye:68ꢀ70
BODIPY þ eꢀ f BODIPY
ð11Þ
ð12Þ
ð13Þ
ꢀ
3
ꢀ
ꢀ
3
3
BODIPY þ BPO f BODIPY þ BPO
ꢀ
BPO f C6H5CO2ꢀ þ C6H5CO2
3
3
BODIPY ꢀ þ C6H5CO2 f BODIPY þ C6H5CO2 ð14Þ
ꢀ
ꢂ
3
3
The ECL emission was about 15 times larger with the co-reactant
and also showed a much higher stability with time compared with
the annihilation ECL (Figure 8e). The ECL annihilation spec-
trum can be generated only for a few minutes, while in the pre-
sence of the benzoyl peroxide it was stable for more than an hour.
The ECL spectral maximum was similar to that of the fluores-
cence with the slight difference due to an inner filter effect. No
features that can be assigned to formation of the dimer, trimer or
some other species were seen. The same relatively weak annihila-
tion intensity was found with the dimers and trimers (Figures 9
and 10), with dimers showing perhaps a bit higher ECL annihilation
emission, and trimers slightly higher compared with the mono-
mer (Figures 9a,b and 10a,b). There is also an increase in ECL
intensity with the addition of the benzoyl peroxide for both
dimers and trimers (Figures 9c,d and 10c,d).21 The ECL maxima
BODIPY þ eꢀ f BODIPY
ð7Þ
ð8Þ
ꢀ
3
BODIPY ꢀ eꢀ f BODIPY ꢀ ðunstableÞ
3
ꢀ
ꢀ
3
ꢂ
3
BODIPY þ BODIPY f BODIPY
ð9Þ
ꢂ
BODIPY f BODIPY þ hν
ð10Þ
The low intensity of the ECL is related to absence of the
substitution in positions 2 and 6, causing instability of the radical
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Figure 11. (a) Simultaneous ECL-CV measurement for 0.1 mM poly-
mer in the presence of 1 mM benzoyl peroxide at a scan rate of 1 V/s.
(b) ECL spectra generated in the presence of 5 mM benzoyl peroxide.
Solvent, CH2Cl2; supporting electrolyte, 0.2 M TBAPF6; electrode area,
0.12 cm2.
Figure 9. Electrogenerated (red) and fluorescence spectra (black) for
the investigated dimers. Annihilation spectrum for 0.14 mM dimer 1 (a)
and 0.2 mM dimer 2 (b). (c,d) Spectra generated in the presence of
5 mM of benzoyl peroxide. Solvent, DCM; supporting electrolyte, 0.1 M
TBAPF6; platinum electrode area, 0.0314 cm2.
Figure 12. Simultaneous ECL-CV cyclic voltammograms of the
0.5 mM of the aza-BODIPY monomer during scans into the (a) neg-
ative and (b) positive direction at a scan rate of 1 V/s; (c) in the
presence of 3.0 mM benzoyl peroxide; (d) in the presence of 3.0 mM
10-MP. (e) ECL spectra for 0.5 mM (red) and fluorescence spectra
(black) for 2 mM aza-BODIPY monomer in the presence of 5.0 mM
10-MP. ECL spectrum was generated from 80 mV of the peaks of the
reduction of aza-BODIPY monomer and oxidation of 10-MP.
Solvent, CH2Cl2; supporting electrolyte, 0.1 M TBAPF6; electrode
area, 0.0314 cm2.
wavelengths for the dimers and trimers is blue-shifted compared
with angular dimer in agreement with the fluorescence and elec-
trochemical results (Figure 10e). The mesityl- substituted dyes
showed slightly red-shifted ECL compared with the methyl-sub-
stituted dyes in agreement with the fluorescence. The energies of
annihilation, ΔHann are close to the Es values as for all of the
BODIPY dyes suggesting direct singlet population or triplet
formation followed by tripletꢀtriplet annihilation (the ST route)
Figure 10. Electrogenerated spectra of the corresponding trimers.
Annihilation spectrum for 0.1 mM trimer 1 (a) and 0.24 mM trimer
2 (b). (c,d) Spectra generated in the presence of 5 mM of benzoyl
peroxide. (e) Comparison of the ECL spectra for 1.0 mM monomer 1
(black), 0.14 mM dimer 1 (green), 0.1 mM trimer 1 (red), and 0.5 mM
angular dimer (blue) in the presence of 5 mM benzoyl peroxide. All
spectra were normalized to the same height. Solvent, DCM; supporting
electrolyte, 0.1 M TBAPF6; platinum electrode area, 0.0314 cm2.
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Journal of the American Chemical Society
ARTICLE
is possible.21,22,25,28 The polymer produced detectable ECL
with co-reactant, but not by annihilation ECL (Figure 11).
ECL studies of the aza-BODIPY monomer and aza-BODIPY
dimer did not show substantial ECL by annihilation or by using
benzoyl peroxide as a co-reactant (Figure 12). This can be attributed
to two factors: one of them is a decrease in the photoluminescence
quantum efficiency with a shift of the reduction potential to the
positive direction compared with the C8 dye and the other one is
instability on oxidation. The first factor is probably more important,
as there was substantial annihilation ECL signal for C8 BODIPY.
A small ECL signal can be produced by a mixed system, with
10-methylphenothiazine (10-MP) as the radical cation precursor
(Figure 12c,d).71ꢀ73 In this case the ECL mechanism can be
presented as
’ AUTHOR INFORMATION
Corresponding Author
ajbard@mail.utexas.edu
’ ACKNOWLEDGMENT
We thank the Center for Electrochemistry, Roche, Inc., the
Robert A. Welch Foundation (F-0021), and Deutsche Forschungs-
gemeinschaft for support of this research and Andy Tennyson for
help in THF purification.
’ REFERENCES
(1) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932.
(2) Sathyamoorthi, G.; Boyer, J. H.; Allik, T. H.; Chandra, S.
Heteroat. Chem. 1994, 5, 403–407.
(3) Pavlopoulos, T. G.; Shah, M.; Boyer, J. H. Appl. Opt. 1988,
27, 4998–4999.
ꢀ
aza-BODIPY þ eꢀ f aza-BODIPY
ð15Þ
ð16Þ
3
þ
10-MP ꢀ eꢀ f 10-MP
3
(4) Pavlopoulos, T. G.; Shah, M.; Boyer, J. H. Opt. Commun. 1989,
70, 425–427.
ꢀ
þ
3
3
(5) Wang, D.; Miyamoto, R.; Shiraishi, Y.; Hirai, T. Langmuir 2009,
25, 13176–13182.
(6) Kennedy, D. P.; Kormos, C. M.; Burdette, S. J. Am. Chem. Soc.
2009, 131, 8578–8586.
(7) Rosenthal, J.; Lippard, S. J. J. Am. Chem. Soc. 2009,
132, 5536–5537.
(8) Lee, Y. C.; Hupp, J. T. Langmuir 2010, 26, 3760–3765.
(9) Godoy, J.; Vives, G.; Tour, J. M. Org. Lett. 2010, 12, 1464–1467.
(10) Nierth, A.; Kobitski, A. Y.; Nienhaus, G. U.; J€aschke, A. J. Am.
Chem. Soc. 2010, 132, 2646–2654.
(11) Frein, S.; Camerel, F.; Ziessel, R.; Barbera, J.; Deschenaux, R.
Chem. Mater. 2009, 21, 3950–3959.
(12) Ba~nuelos, J.; Arbeloa, F. L.; Arbeloa, T.; Sallares, S.; Amat-Guerri,
F.; Liras, M.; Arbeloa, I. L. J. Phys. Chem. A 2008, 112, 10816–10822.
(13) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. J. Am.
Chem. Soc. 2005, 127, 12162–12163.
3
aza-BODIPY þ 10-MP f aza-BODIPY þ 10-MP ð17Þ
3
3
1
ꢂ
aza-BODIPY þ aza-BODIPY f aza-BODIPY
þ aza-BODIPY
ð18Þ
ð19Þ
1
ꢂ
aza-BODIPY f aza-BODIPY þ hν
The presence of the ECL signal in this case can be explained by
higher stability of the 10-MP radical cation. The aza-BODIPY
dimer does not show any substantial ECL signal under any con-
ditions, which is consistent with its low fluorescence intensity and
also the relatively positive potentials for reduction.
(14) Meng, G.; Velayudham, S.; Smith, A.; Luck, R.; Liu, H.
Macromolecules 2009, 42, 1995–2001.
’ CONCLUSIONS
Monomeric, dimeric, trimeric and polymeric structures of C8
BODIPY and monomeric and dimeric aza-BODIPY were synthe-
sized. Dimers and trimers were synthesized through oxidative
coupling between the 2/6-position with FeCl3. Electrochemical
studies show correlation of the number of active units with the
amount of electrochemical peaks from monomer to oligomer.
Cyclic voltammetry shows that the interaction, and hence the
potential splitting between consecutive peaks increases in the order
monomer < dimer < trimer, with a larger separation between two
consecutive peaks for the oxidation process compared with the
reduction one. The same phenomenon is seen for the aza-BODIPY
monomer and dimer. ECL spectra were generated for all C8 sys-
tems, which show a small increase in the annihilation efficiency from
monomer to dimer to trimer. A substantial increase in the ECL ef-
ficiency occurred when benzoyl peroxide was used as a co-reactant.
Polymer and aza-BODIPY monomer show very small or no ECL
due to the reactivity of the radical ions. No ECL is found for the
aza-BODIPY dimer.
(15) Thivierge, C.; Bandichhor, R.; Burgess, K. Org. Lett. 2007,
9, 2135–2138.
(16) Kamiya, M.; Johnsson, K. Anal. Chem. 2010, 82, 6472–6479.
(17) Ojida, A.; Sakamoto, T.; Inoue, M.; Fujishima, S.; Lippens, G.;
Hamachi, I. J. Am. Chem. Soc. 2009, 131, 6543–6548.
(18) Zhang, X.; Xiao, Y.; Qian, X. Angew. Chem., Int. Ed. 2008,
47, 8025–8029.
(19) Cheng, T.; Xu, Y.; Zhang, S.; Zhu, W.; Qian, X.; Duan, L. J. Am.
Chem. Soc. 2008, 130, 16160–16161.
(20) Huh, J. O.; Do, Y.; Lee, M. H. Organometallics 2008,
27, 1022–1025.
(21) Lai, R. Y.; Bard, A. J. J. Phys. Chem. B 2003, 107, 5036–5042.
(22) Sartin, M. A.; Camerel, F.; Ziessel, R.; Bard, A. J. J. Phys. Chem.
C. 2008, 112, 10833–10841.
(23) Trieflinger, C.; R€ohr, H.; Rurack, K.; Daub, J. Angew. Chem., Int.
Ed. 2005, 44, 6943–6947.
(24) Benniston, A. C.; Copley, G.; Harriman, A.; Howgego, D.;
Harrington, R. W.; Clegg, W. J. Org. Chem. 2010, 75, 2018–2027.
(25) Nepomnyashchii, A. B.; Cho, S.; Rossky, P. J.; Bard, A. J. J. Am.
Chem. Soc. 2010, 132, 17550–17559.
(26) Rachford, A. A.; Ziessel, R.; Bura, T.; Retailleau, P.; Castellano,
F. N. Inorg. Chem. 2010, 49, 3730–3736.
(27) Dumas-Verdes, C.; Miomandre, F.; Lepicier, E.; Galangau, O.;
Vu, T. T.; Clavier, G.; Meallet-Renault, R.; Audebert, P. Eur. J. Org.
Chem. 2010, 2525–2535.
These electrochemical and spectroscopic characterizations of
small oligomers of BODIPYs should help in development of new
polymeric dyes for new applications.
’ ASSOCIATED CONTENT
(28) Nepomnyashchii, A. B; Br€oring, M.; Ahrens, J.; Kr€uger, R.;
Bard, A. J. J. Phys. Chem. C 2010, 114, 14453–14460.
(29) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.
Soc. 1978, 100, 4248–4253.
S
Supporting Information. Additional figures and experi-
b
mental information. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Journal of the American Chemical Society
ARTICLE
(30) Astruc, D.; Ornelas, C.; Ruiz, J. Chem.—Eur. J. 2009, 15,
8936–8944.
(31) Wang, A.; Ornelas, C.; Astruc, D.; Hapiot, P. J. Am. Chem. Soc.
2009, 131, 6652–6653.
(32) Ornelas, C.; Ruiz, J.; Belin, C.; Astruc, D. J. Am. Chem. Soc.
2009, 131, 590–601.
(67) M€ullen, K. Chem. Rev. 1984, 84, 603–646.
(68) Chandross, E. A.; Sonntag, F. I. J. Am. Chem. Soc. 1966, 88,
1089–1096.
(69) Akins, D. L.; Birke, R. L. Chem. Phys. Lett. 1974, 29, 428–435.
(70) Santa-Cruz, T. D.; Akins, D. L.; Birke, R. L. J. Am. Chem. Soc.
1976, 98, 1677–1682.
(33) Amatore, C.; Bouret, Y.; Maisonhaute, E.; Goldsmith, J. I.;
Abru~na, H. D. Chem.—Eur. J. 2001, 7, 2206–2226.
(34) Guay, J.; Kasai, P.; Diaz, A.; Wu, R.; Tour, J. M.; Dao, L. H.
Chem. Mater. 1992, 4, 1097–1105.
(71) Freed, D. J.; Faulkner, L. R. J. Am. Chem. Soc. 1971, 93,
3565–3568.
(72) Keszthelyi, C. P.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc.
1972, 94, 1522–1527.
(35) Mitshke, U.; B€auerle, P. J. Chem. Soc., Perkin Trans. 2001, 1,
(73) Slaterbeck, A. F.; Meehan, T. D.; Gross, E. M.; Wightman, R. M.
740–753.
J. Phys. Chem. B 2002, 106, 6088–6095.
(36) M€uller, U.; Baumgarten, M. J. Am. Chem. Soc. 1995, 117,
5840–5850.
(37) Itaya, K.; Bard, A. J.; Szwarc, M. Z. Phys. Chem. NF 1978, 112,
1–9.
(38) Grabner, G.; Rechthaler, K.; K€ohler, G. J. Phys. Chem. A 1998,
102, 689–696.
(39) Baumgarten, M.; Gheral, L.; Friedrich, J.; Jurczok, M.; Rettig,
W. J. Phys. Chem. A 2000, 104, 1130–1140.
(40) Hill, M. G.; Penneau, J.-F.; Zinger, B.; Mann, K.; Miller, L. L.
Chem. Mater. 1992, 4, 1106–1113.
(41) Allik, T. H.; Hermes, R. E.; Sathyamoorthi, G.; Boyer, J. H. Proc.
SPIE-Int. Soc. Opt. Eng. 1994, 2115, 240–248.
(42) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher,
W. M.; O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619–10631.
(43) McDonnell, S. O.; O’Shea, D. F. Org. Lett. 2006, 8, 3493–3496.
(44) Bouit, P. A.; Kamada, K.; Feneyrou, P.; Berginc, G.; Toupet, L.;
Maury, O.; Andraud, C. Adv. Mater. 2009, 21, 1151–1154.
(45) Loudet, A.; Bandichhor, R.; Wu, L.; Burgess, K. Tetrahedron
2008, 64, 3642–3654.
(46) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008,
47, 1184–1201.
(47) Sasaki, E.; Kojima, H.; Nishimatsu, H.; Urano, Y.; Kikuchi, K.;
Hirata, Y.; Nagano, T. J. Am. Chem. Soc. 2005, 127, 3684–3685.
(48) Lee, H.; Berezin, M. Y.; Guo, K.; Kao, J.; Achilefu, S. Org. Lett.
2009, 11, 29–32.
(49) Murtagh, J.; Frimannsson, D. O.; O’Shea, D. F. Org. Lett. 2009,
11, 5386–5389.
(50) Rihn, S.; Erdem, M.; De Nicola, A.; Retailleau, P.; Ziessel, R.
Org. Lett. 2011, 13, 1916–1919.
(51) Shah, M.;Thangaraj, K.;Soong, M.-L.; Wolford, L. T.;Boyer, J. H.;
Politzer, I. R.; Pavlopoulos, T. G. Heteroat. Chem. 1990, 1, 389–399.
(52) Sathyamoorthi, G.; Soong, M.-L.; Ross, T. W.; Boyer, J. H.
Heteroat. Chem. 1993, 4, 603–608.
(53) Sahami, S.; Weaver, M. J. Electroanal. Chem. 1981,
122, 155–170.
(54) Rudolph, M. J. Electroanal. Chem. 1991, 314, 13–22.
(55) Rudolph, M. J. Electroanal. Chem. 1992, 338, 85–98.
(56) Mocak, J.; Feldberg, S. W. J. Electroanal. Chem. 1997, 378, 31–37.
(57) Feldberg, S. W.; Goldstein, C. I.; Rudolph, M. J. Electroanal.
Chem. 1996, 413, 25–36.
(58) Negishi, E., Ed. Handbook of Organopalladium Chemistry for
Organic Synthesis; John Wiley & Sons: New York, 2002.
(59) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am. Chem.
Soc. 2006, 128, 4548–4549.
(60) Br€oring, M.; Kr€uger, R.; Link, S.; Kleeberg, C.; K€ohler, S.; Xie,
X.; Ventura, B.; Flamigni, L. Chem.—Eur. J. 2008, 14, 2976–2983.
(61) Ventura, B.; Marconi, G.; Br€oring, M.; Kr€uger, R.; Flamigni, L.
New J. Chem. 2009, 33, 428–438.
(62) Kasha, M.; Rawls, R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965,
11, 371–392.
(63) Prieto, J. B.; Arbeloa, F. L.; Martinez, V. M.; Lopez, T. A.;
Arbeloa, I. L. Phys. Chem. Chem. Phys. 2004, 6, 4247–4253.
(64) Omer, K. M.; Kanibolotsky, A. L.; Skabara, P. J.; Perepichka,
I. F.; Bard, A. J. J. Phys. Chem. B 2007, 111, 6612–6619.
(65) Bard, A. J. Pure Appl. Chem. 1971, 25, 379–394.
(66) Meerholz, K.; Heinze, J. J. Am. Chem. Soc. 1989, 111, 2325–2326.
8645
dx.doi.org/10.1021/ja2010219 |J. Am. Chem. Soc. 2011, 133, 8633–8645