Synthesis and reactivity of 4,4-dialkoxy-bodipys: An experimental and computational study

Article abstract of DOI:10.1021/ic502821m

A series of boron-disubstituted O-BODIPYs were synthesized, and their structures and spectroscopic properties were investigated using both computational and experimental methods. Three methods were investigated for the preparation of 4,4-dimethoxy-BODIPYs bearing electron-donating or electron-withdrawing 8-aryl groups: method A employs refluxing in the presence of NaOMe/MeOH, method B uses AlCl₃ in refluxing dichloromethane followed by addition of methanol as nucleophile, and method C involves activation of the BODIPYs using TMSOTf in refluxing toluene followed by addition of methanol. The yields obtained depend on the method used and the structure of the starting BODIPYs; for example, 1a and 3a were most efficiently prepared using method C (98 and 70%, respectively), while 2a was best prepared by method A (50%). Methods B and C were employed for the synthesis of seven new 4,4-dialkoxy-BODIPYs. 4,4-Dipropargyloxy-BODIPY 1e reacted under Cu(I)-catalyzed alkyne-azide Huisgen cycloaddition conditions to produce 4,4-bis(1,2,3-triazole)-BODIPY 4 in 78% yield. The substitution of the fluorides for alkoxy groups on the BODIPYs had no significant effect on the absorption and emission wavelengths but altered their fluorescence quantum yields. Among this series of dialkoxy-BODIPYs, the 4,4-dipropargyloxy 1e and its corresponding bis(1,2,3-triazole) 4 show the largest quantum yields in toluene and THF, respectively.

Full text of DOI:10.1021/ic502821m

Article
pubs.acs.org/IC
Synthesis and Reactivity of 4,4-Dialkoxy-BODIPYs: An Experimental
and Computational Study
Alex L. Nguyen,† Petia Bobadova-Parvanova,‡ Melissa Hopfinger,‡ Frank R. Fronczek,† Kevin M. Smith,†
and M. Graca̧
H. Vicente*,†
†Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
‡Department of Chemistry, Rockhurst University, Kansas City, Missouri 64110, United States
S
* Supporting Information
ABSTRACT: A series of boron-disubstituted O-BODIPYs were synthesized, and their structures
and spectroscopic properties were investigated using both computational and experimental
methods. Three methods were investigated for the preparation of 4,4-dimethoxy-BODIPYs bearing
electron-donating or electron-withdrawing 8-aryl groups: method A employs refluxing in the
presence of NaOMe/MeOH, method B uses AlCl3 in refluxing dichloromethane followed by
addition of methanol as nucleophile, and method C involves activation of the BODIPYs using
TMSOTf in refluxing toluene followed by addition of methanol. The yields obtained depend on the
method used and the structure of the starting BODIPYs; for example, 1a and 3a were most
efficiently prepared using method C (98 and 70%, respectively), while 2a was best prepared by method A (50%). Methods B and
C were employed for the synthesis of seven new 4,4-dialkoxy-BODIPYs. 4,4-Dipropargyloxy-BODIPY 1e reacted under Cu(I)-
catalyzed alkyne−azide Huisgen cycloaddition conditions to produce 4,4-bis(1,2,3-triazole)-BODIPY 4 in 78% yield. The
substitution of the fluorides for alkoxy groups on the BODIPYs had no significant effect on the absorption and emission
wavelengths but altered their fluorescence quantum yields. Among this series of dialkoxy-BODIPYs, the 4,4-dipropargyloxy 1e
and its corresponding bis(1,2,3-triazole) 4 show the largest quantum yields in toluene and THF, respectively.
of the fluorides of BODIPY.6,7 Using the latter methodology,
the synthesis of 4,4-dialkyl-, 4,4-diaryl-, and 4,4-dialkynyl-
INTRODUCTION
■
In the last two decades, 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (known as BODIPY) dyes have become an attractive
synthetic target for many chemists because of their numerous
BODIPYs have been reported using Grignard or organolithium
reagents, in one or two steps.8−16 Relatively milder conditions
have been investigated for the preparation of 4,4-dialkoxy- and
useful properties.1−3 Their high thermal and photochemical
4,4-diaryloxy-BODIPYs.17−26 For example, both the mono- and
stability, strong absorptions in the visible spectral region, and
dimethoxy (up to 52% yield) BODIPYs were obtained upon
sharp fluorescence with high quantum yields make them
desirable for various applications, particularly as fluorescent
refluxing BODIPY 1 in methanol and sodium methoxide for 14
h18 or by using a microwave reactor at 92 °C for 40 min.19
A
labels and sensors.3−5 The spectroscopic and photophysical
properties of BODIPY dyes can be tuned via the introduction
of certain groups and functionalities at the peripheral positions
of the dipyrromethene core, allowing for example the
preparation of BODIPYs with extended π systems that emit
in the near-IR region. On the other hand, functionalization of
the BODIPY at the boron center (position 4 of the BODIPY
core) has little or no effect on the absorption and emission
wavelengths but can be used to increase the Stokes shifts,
improve aqueous solubility, and modulate the fluorescence
properties, stability, and redox behavior of this type of
molecule. Nevertheless, boron functionalization has lagged far
behind developments at the carbon atoms in BODIPYs. In
particular, substitution of the fluorides with alkoxy or aryloxy
groups has led to fluorescent BODIPY systems with improved
aqueous solubility and with decreased oxidation and reduction
potentials that are promising for applications as biological
probes and in the construction of donor−acceptor systems.
Two methods can be employed for the synthesis of 4,4-
functionalized BODIPYs: (1) by reaction of dipyrromethenes
with functionalized boron compounds and (2) by substitution
milder method for the preparation of 4,4-disubstituted
BODIPYs involves the activation of the B−F bonds with
aluminum trichloride, followed by reaction with excess alcohols.
These conditions are compatible with several functional groups,
including aldehydes, esters, and amines.20−25 Various alcohols
have been investigated, including methanol, benzyl alcohol,
phenol, catechol, polyethylene glycol, and ethanediol, produc-
ing the corresponding 4,4-dialkoxy- or 4,4,-diaryloxy-BODIPYs,
in yields ranging from 20% to quantitative.20−25 However, the
low yields often reported using this methodology, particularly
using phenol,20 catechol,7 and polyethylene glycol25 as
nucleophiles, in part due to difficult purifications due to the
need for using a large excess (ca. 278 equiv) of alcohol, have
sparked renewed interest in the development of alternative
functionalization strategies. One such strategy involves a two-
step reaction using boron trichloride to form a reactive 4,4-
dichloro-BODIPY derivative, followed by reaction with
Received: November 25, 2014
© XXXX American Chemical Society
A
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Article
alkoxide, phenoxide, Grignard, or organolithium reagents to
produce the corresponding 4,4-functionalized BODIPYs.26−28
A milder method recently reported involves the substitution of
one or both fluorides with acetoxy groups in up to 37% yield,
with trimethylsilylacetoxy groups, prepared in situ from TMSCl
Scheme 1. Synthesis of 4,4-Dimethoxy-BODIPYs 1a−3a
using Methods A−C
and acetic acid.29 This methodology has also been used for 19
F
radiofluorination of BODIPYs.30,31 A similar strategy was
previously employed for the synthesis of a 4,4-dicyano-
BODIPY using trimethylsilyl cyanide in the presence of BF3·
OEt2 as catalyst.13 Alternatively, activation of the BODIPY
using trimethylsilyltrifluoromethanesulfonate (TMSOTf) fol-
lowed by addition of an alcohol as nucleophile is reported to
produce the corresponding 4-monoalkoxy-BODIPYs in 38−
70% yields.32 This methodology has also been investigated for
boron functionalization in subphthalocyanines;33 this reaction
presumably occurs via an activated triflate−subphthalocyanine
intermediate, which is reactive toward nucleophilic attack at the
boron atom.
Herein we further develop these boron-functionalization
methodologies and extend them to the synthesis of 4,4-
dialkoxy-BODIPYs, including seven new 4,4-dialkoxy-BODI-
PYs. We also investigated the reactivity of 4,4-dipropargyloxy-
BODIPY 1e under Cu(I)-catalyzed alkyne−azide click
conditions to produce the first boron-bis(1,2,3-triazole)-
functionalized BODIPY. The structural (including six X-ray
structures), spectroscopic, and fluorescence properties of all
synthesized BODIPYs were investigated and are compared and
discussed.
containing an electron-withdrawing methoxycarbonyl group on
the meso-phenyl ring produced 4,4-dimethoxy-BODIPY 3a in
only 4% yield, while 53% of starting material was recovered.
The presumably lower reactivity of BODIPY 3 under these
conditions led us to explore alternative methodologies for the
synthesis of 4,4-dialkoxy-BODIPYs (methods B and C in
Scheme 1; vide infra). DFT calculations at the B3LYP/6-
31+G(d,p) level for evaluation of the natural population
analysis (NPA) atomic charges on BODIPYs 1−3 indicate that
the charges on the 12-atom BODIPY core are very similar and
are not affected by the electron-donating or -withdrawing
nature of the 8-aryl group (see the Supporting Information).
This is probably due to the large dihedral angles between the 8-
aryl and the BODIPY core as a result of 1,7-dimethyl
substitution as seen in Figure 1 (vide infra), which prevents
electronic communication between the two.
Methods B and C involve a two-step one-pot reaction, in
which the BODIPY is first activated, using AlCl3 or TMSOTf,
respectively, followed by reaction with a milder alcohol
nucleophile that is compatible with a wider variety of
functionalized BODIPYs. In method B (Scheme 1), 2 equiv
of AlCl3 in refluxing dichloromethane was used to activate the
BODIPY, followed by addition of a large excess of alcohol (278
equiv), as originally reported.20 On the other hand, the
activation of BODIPYs with TMSOTf followed by reaction
with an alcohol has only been recently investigated for the
synthesis of mono-4-alkoxy-BODIPYs; the reported conditions
use 5 equiv of TMSOTf followed by 100 equiv of alcohol and
10 equiv of DIPEA.32 We modified this method and extended it
to the preparation of 4,4-dialkoxy-BODIPYs, inspired by a
procedure reported for boron functionalization of subphthalo-
cyanines.33 The optimized procedure uses 2.5 equiv (rather
than 5 equiv) of TMSOTf in toluene, followed by addition of 5
equiv (rather than 100 equiv) of alcohol and 5 equiv of DIPEA
(method C, Scheme 1); the smaller amount of nucleophile used
in this procedure facilitates the purification and isolation of the
products. Under these conditions, BODIPY 1a was produced in
52% (method B) and 98% yields (method C), BODIPY 2a in
11% (method B) and 8% yields (method C), and BODIPY 3a
in 58% (method B) and 70% yields (method C) (Table 1).
Therefore, the newly developed method C produced BODIPYs
1a and 3a in the highest yields of the three methods
investigated, providing an alternative synthetic route to this
type of compound.
RESULTS AND DISCUSSION
■
Synthesis. The three 8-aryl BODIPYs 1−3 were prepared
using a three-step one-pot procedure, as previously described,
from commercially available 2,4-dimethylpyrrole and the
corresponding aryl aldehyde.34 Electron-donating methoxy or
electron-withdrawing methoxycarbonyl groups were introduced
at the 8-phenyl group of BODIPYs 2 and 3, respectively, to
investigate their effect on the boron-substitution reaction.
4,4-Dimethoxy-BODIPYs 1a−3a were obtained in yields
ranging from 4% to 98% (Table 1) using the three
Table 1. Isolated Yields of 4,4-Dimethoxy-BODIPYs 1a−3a
using Methods A−C
yield (%)
BODIPY
method A
method B
method C
1a
2a
3a
66
50
4
52
11
58
98
8
70
methodologies shown in Scheme 1. Method A employs a
one-step reaction using an excess (6 equiv) of sodium
methoxide as nucleophile in refluxing methanol, as previously
described.18 Using this methodology, BODIPY 1a was isolated
as the main product in 66% optimized yield, after 12 h of
refluxing. The monosubstituted O-BODIPY was an intermedi-
ate product under these conditions, as indicated by TLC and as
previously observed.6 The reaction conditions were applied to
BODIPYs 2 and 3 to investigate the applicability of this
methodology to functionalized BODIPYs. When BODIPY 2
containing two electron-donating methoxy groups in positions
3 and 4 of the meso-phenyl group was subjected to the above
reaction conditions, the 4,4-dimethoxy-BODIPY 2a was
produced in 50% yield. On the other hand, BODIPY 3
The wide range of yields (4−98%) obtained for BODIPYs
1a−3a under the three methodologies motivated us to
investigate the intermediates formed upon fluoride dissociation
B
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Figure 1. Molecular structures of BODIPYs 1a−c,e, 3a, and 4 from X-ray crystal structure analysis, with 50% probability ellipsoids. Only one of the
independent molecules is shown for those structures with Z′ > 1.
from the BODIPY under each of the reaction conditions, using
computational modeling. These substitution reactions likely
proceed via formation of the boronium cation intermediate
INT1 (Scheme 2); such boronium cations have been previously
dichloromethane (56 kcal/mol), and it is the hardest to achieve
in toluene (92 kcal/mol). This tendency is apparently due to
the relative dielectric permittivities of the three solvents: 32.6
for methanol, 8.93 for dichloromethane, and 2.37 for toluene.
On the basis of these values, it is likely that the direct
dissociation of fluoride ion from BODIPY 1 occurs in method
A, since the reaction involves reflux in methanol at ca. 70−75
°C, which allows the reaction barrier to be overcome for the
formation of INT1. However, this is not the case in method B,
since 56 kcal/mol would need to be overcome. Our calculations
indicate that the Lewis acid AlCl3 significantly stabilizes the
boronium cation INT1 due to the formation of the counterion
AlCl3F−. This is in agreement with the milder conditions
required for this reaction. Moreover, our calculations suggest
that the reaction could occur at lower temperature. Indeed,
performing the reaction entirely at room temperature resulted
in the formation of BODIPY 1a, albeit in lower yield (44 vs
52%) after 12 h. In the case of method C, the abstraction of
fluoride likely leads to the formation of INT1 along with triflate
anion and trimethylsilyl fluoride; the potential energy barrier of
25 kcal/mol for this reaction can be easily overcome under the
conditions of reflux in toluene. Indeed, such a boronium species
bearing a triflate ligand has been previously characterized by
NMR and X-ray crystallography.36 Moreover, our calculations
also suggest similar Gibbs free energies and stabilities for INT1
from all starting BODIPYs 1-3 (ΔG values are 37.4 kcal/mol
for 1, 37.5 kcal/mol for 2, and 38.7 kcal/mol for 3 following
method A, within 1 kcal/mol calculation error).
Scheme 2. Boronium Intermediate INT1 and BODIPYs 1a−
e Formed using Methods B and C
isolated and characterized by 19F NMR and X-ray crystallog-
raphy.35 Using DFT modeling, the Gibbs free energies for the
formation of INT1 from BODIPY 1 were calculated, and the
results obtained are shown in Table 2. The compounds were
modeled in methanol for method A, dichloromethane in
method B, and toluene in method C. As expected, the
dissociation of fluoride ion from BODIPY 1 requires the least
amount of energy in methanol (37 kcal/mol), followed by
Table 2. Calculated ΔG Energies for the Formation of INT1
via Methods A−C
method
modeled solvent
anion
ΔG (kcal/mol)
A
B
B
C
C
CH3OH
CH2Cl2
CH2Cl2
PhCH3
PhCH3
F−
37
56
While for method A the formation of INT1 with a fluoride
ion is most likely the rate-determining step, this is not the case
for method B. Detailed mechanistic analyses are currently
underway37 and will be published elsewhere. In brief, it appears
that deprotonation of the alcohol following nucleophilic attack
F−
AlCl3F−
F−
OTf−
−22
92
25
C
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on boron is the rate-determining step in method B, which also
partially explains the need for a large excess of nucleophile in
this case. In method C, the use of DMAP in place of DIPEA
significantly lowered the yield of BODIPY 1a, probably due to
chelation of DMAP to the boron atom, as previously
observed.30,31,35,36 The reaction yields were also affected by
the temperature and solvent used; when the reaction was
performed at room temperature, BODIPY 1a was isolated in
50% (in toluene), 25% (in dichloromethane), and 29% yields
(in chloroform) and it was not formed at all (only starting
BODIPY 1 was recovered) when THF was used as the solvent.
In some cases, unreacted BODIPY 1 and the mono-4-methoxy-
BODIPY were also isolated as side products, in ca. 20 and 17%
yields, respectively, when the reaction was performed at room
temperature for 2 h in toluene or dichloromethane or overnight
in chloroform.
yield from 1e, using in situ generated copper(I) catalyst from
copper(0) and copper(II), in the presence of sodium ascorbate
and benzyl azide in aqueous THF. To our knowledge, this is
the first report of a click cycloaddition reaction performed on a
BODIPY boron functionalized with an alkyne group.
Structural Characterization. All BODIPYs were charac-
terized by 1H, 13C, and 11B NMR, HRMS, UV−vis, and
fluorescence and in the case of 1a−c,e, 3a, and 4 also by X-ray
crystallography (see Table 4, Figure 1, and the Supporting
Table 4. Calculated Relative Gibbs Free Energies, HOMO
and LUMO Energies, and HOMO−LUMO Gap in THF for
the Symmetric (1a−e, 4) and Unsymmetric (1a′−e′, 4′)
BODIPY Conformers
BODIPY
HOMO (au)
LUMO (au)
gap (eV)
ΔG (kcal/mol)
1a
−0.20599
−0.2044
−0.09520
−0.09280
−0.09440
−0.09245
−0.09436
−0.09117
−0.09608
−0.09077
−0.09941
−0.09635
−0.09602
−0.09638
3.01
3.04
3.02
3.03
3.02
3.05
3.02
3.01
3.01
3.03
3.02
3.03
0
The milder methods B and C were employed for the
synthesis of new 4,4-dialkoxy-BODIPYs 1b−e, as shown in
Scheme 2. The isolated yields obtained for the targeted
BODIPYs are given in Table 3. While method B gave moderate
1a′
1b
1b′
1c
6.76
0
−0.20542
−0.20393
−0.20548
−0.20315
−0.20694
−0.20143
−0.21009
−0.20776
−0.20686
−0.20778
8.39
0
1c′
1d
1d′
1e
9.92
0
Table 3. Isolated Yields of 4,4-Dimethoxy-BODIPYs 1a−e
using Methods B and C
7.82
0
yield (%)
1e′
4
7.45
0
BODIPY
R
method B
method C
1a
1b
1c
1d
1e
Me
Et
52
64
26
57
52
98
18
0
4′
6.55
iPr
Information). The 11B NMR spectra clearly indicated the
formation of the targeted 4,4-dialkoxy-BODIPY, via the
disappearance of the triplet due to the BF2 (at ca. 0.6 ppm)
and appearance of the singlet attributed to B(OR)2 at ca. 2
allyl
30
40
propargyl
yields (26−64%) for all targeted BODIPYs, method C was
highly sensitive to the bulkiness of the alcohol, likely as a result
of the close proximity of the triflate anion to the boron
center.37 Indeed, no product was formed when isopropyl
alcohol was used as the nucleophile under method C, and the
steric demands of the tert-butyl alcohol/tert-butoxide system
has resulted in an effective strategy for BODIPY deborona-
tion.19 In the case of BODIPY 1b, the yield could be increased
to up 37% when a greater excess of nucleophile was used; the
mono-4-ethoxy-BODIPY was also isolated in 20% yield under
these conditions. Our results show that consideration of the
structures of the starting BODIPY and the nucleophile are
needed prior to selection of the methodology, leading to the
highest yield of the targeted 4,4-dialkoxy-BODIPYs.
Further derivatization of the 4,4-dipropargyloxy-BODIPY 1e
using click conditions previously investigated for the con-
jugation of alkyne-functionalized BODIPYs with carbohy-
drate38 and folate39 moieties was performed (Scheme 3), to
demonstrate the tolerance of the 4-propargyloxy groups under
the Cu(I)-catalyzed Huisgen cycloaddition conditions. The 4,4-
bis(N-benzyl-1,2,3-triazole)-BODIPY 4 was formed in 78%
1
ppm. In the case of 4, the H NMR clearly indicated the
formation of the 1,2,3-triazoles with two singlets at 7.50 and
5.43 ppm for the triazole and benzyl CH2 protons, respectively.
Crystals suitable for X-ray analysis were grown from slow
diffusion of hexane in dichloromethane (for 1a−c and 4),
acetone (for 1e), and acetone (for 3a). The structures, shown
in Figure 1, all have at least approximate 2-fold symmetry.
BODIPY 1b has crystallographic C2 symmetry, and BODIPY
1a has one molecule on a 2-fold axis and another in a general
position. The central six-membered C3N2B rings of all five
compounds deviate little from planarity, with respective mean
and maximum deviations of 0.021 and 0.057 Å for 1a, 0.024
and 0.037 Å for 1b, 0.003 and 0.005 Å for 1c, 0.008 and 0.021
Å for 1e, 0.035 and 0.075 Å for 3a, and 0.014 and 0.032 Å for 4.
In all cases, the phenyl plane forms a large dihedral angle with
the best plane of the C9N2B BODIPY core; these dihedral
angles are 84.2 and 83.8° (two independent molecules) for 1a,
86.5° for 1b, 84.7° for 1c, 79.8 and 82.1° (two independent
molecules) for 1e, 84.0 and 84.6° (two independent molecules)
for 3a, and 78.9 and 79.1° (two independent molecules) for 4.
The coordination of the boron atoms is tetrahedral, and 18 B−
O bond distances over the six compounds have lengths in the
range 1.420(4)−1.444(5) Å, with an average value of 1.432 Å.
These distances are in good agreement with those in the p-
iodophenyl analogue of BODIPY 1a: 1.420(4) and 1.437(3)
Å.20 As in that compound, the conformations of the B−OR
groups in all six compounds have the α-C atoms of both alkyl
groups folded symmetrically toward the BODIPY core.
Scheme 3. Click Reaction of BODIPY 1e
The geometries of BODIPYs 1a−e and 4 were also
optimized computationally. In agreement with the X-ray data,
D
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the results showed a nearly tetrahedral geometry around the
boron centers and the 8-aryl groups oriented nearly
perpendicular to the BODIPY plane. The computational
optimization of the structures also revealed the existence of
two conformers for all BODIPYs. The most stable conformer is
symmetric, with the two B−OR groups symmetrically
positioned on both sides of the BODIPY plane (see Figure
2a for 1a) and the other has the B(OR)2 group tilted sideways
show absorption and emission spectra characteristic for this
type of compound, with strong absorption bands (log ε = 4.4−
5.7) in the range 501−506 nm and emission bands in the range
505−517 nm in THF and toluene. These results are in
agreement with previous studies showing that substitution of
fluorides of BODIPY dyes with alkoxy groups produces only
minor changes in the absorption and emission wave-
lengths.20,21,25 On the other hand, the bis(1,2,3-triazole)-
BODIPY 4 showed a slightly blue shifted absorption (λmax ca.
495 nm) and the largest Stokes shifts in both solvents.
Computational modeling indicates similar HOMO−LUMO
gaps for the 4,4-dialkoxy-BODIPYs 1a−e and 4 (Table 4) in
agreement with the observed experimental values. Interestingly,
the HOMO−LUMO gaps for the symmetric and the tilted
conformers of each compound (Figure 2) are very similar,
suggesting that these would not be possible to distinguish
spectroscopically.
The fluorescence quantum yields decreased for the 4,4-
dimethoxy-BODIPY derivatives 1a−3a relative to the starting
BODIPYs 1−3 in both solvents, likely due to the higher
rotational freedom of the methoxy group in comparison with
that of fluoride, therefore increasing the amount of energy lost
to nonradiative decay to the ground state. This trend was also
observed for the 4,4-diethoxy-BODIPY derivative 1b in THF
and for all 4,4-alkoxy-BODIPYs in toluene. However, a similar
fluorescence quantum yield in THF was determined for
BODIPY 1d and significant enhancements were observed in
the case of BODIPYs 1c,e, bearing the isopropoxy and
propargyloxy groups, respectively, relative to BODIPY 1. This
enhancement of fluorescence has been previously observed for
BODIPY derivatives containing isopropoxy and benzyloxy
groups.20 The formation of the bis(1,2,3-triazole)-BODIPY
resulted in further enhancement of the fluorescence quantum
yield in THF but not in toluene. These results suggest that
boron functionalization with alkoxy groups tends to decrease
the quantum yields in toluene, although enhancement of the
quantum yields for certain BODIPYs can be observed in a more
polar solvent such as THF. Further derivatization of the
propargyloxy groups on boron via click chemistry leads to the
formation of a bis(1,2,3-triazole)-BODIPY that shows larger
Stokes shifts and enhanced fluorescence quantum yield in THF
in comparison with its precursor.
Figure 2. Molecular structures of (a) the symmetric conformer 1a and
(b) the unsymmetric conformer 1a′ from B3LYP/6-31+G(d,p)
optimization.
(shown in Figure 2b for 1a′). The deviation from planarity for
the nonsymmetric conformers of the BODIPYs ranges between
15 and 17° and the relative Gibbs free energies between the
two conformers calculated in THF (solvent used for
spectroscopic investigations) are given in Table 4. Since the
Gibbs energy differences are very small, rapid transition
between the two conformers in THF solution freely occurs at
room temperature for all BODIPYs. Although only the
symmetric conformer is seen in the crystal structures presented
in Figure 1, we have previously observed the BODIPY
conformer with the boron lying out of the C3N2 plane for
related BF2 systems.40
Spectroscopic Studies. The spectroscopic properties and
fluorescence quantum yields of all BODIPYs in THF and
toluene are summarized in Table 5 and shown for three
representative BODIPYs in Figure 3 (see also the Supporting
Information). The 4,4-dialkoxy-BODIPYs 1a−e, 2a, and 3a
CONCLUSIONS
■
Three different methods were investigated for the preparation
of 4,4-alkoxy-BODIPYs. Method A (NaOMe in refluxing
methanol for 12 h) gave the highest yield (50%) of BODIPY
Table 5. Spectroscopic Properties of BODIPYs in THF and Toluene (in Parentheses) at Room Temperature
BODIPY
abs λmax (nm)
emission λmax (nm)
Stokes shift (nm)
Φf
log ε (M−1 cm−1
)
1
501 (504)
502 (504)
502 (504)
501 (504)
503 (505)
504 (506)
501 (505)
503 (505)
503 (506)
504 (506)
494 (496)
505 (511)
507 (512)
507 (512)
505 (509)
507 (511)
508 (513)
505 (509)
507 (512)
508 (515)
510 (517)
511 (512)
4 (7)
5 (8)
0.56 (0.96)
0.44 (0.73)
0.46 (0.70)
0.63 (0.70)
0.56 (0.57)
0.62 (0.82)
0.93 (0.87)
0.42 (0.57)
0.43 (0.51)
0.31 (0.34)
0.72 (0.56)
4.96 (5.33)
4.86 (5.28)
4.54 (5.24)
4.70 (5.48)
4.66 (5.47)
4.74 (5.04)
5.00 (5.66)
4.38 (5.06)
4.42 (5.62)
4.73 (5.48)
4.80 (4.94)
1a
1b
1c
1d
1e
2
5 (8)
4 (5)
4 (6)
4 (7)
4 (4)
2a
3
4 (7)
5 (9)
3a
4
6 (11)
17 (16)
E
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Inorganic Chemistry
Article
Figure 3. Normalized absorption (full lines) and emission (dashed lines) spectra of BODIPYs 1c (red), 1e (blue), and 4 (purple) in THF at room
temperature.
General Procedure for Synthesis of BODIPYs 1−3.34,41−43
2a, bearing an 8-(3,4-dimethoxyphenyl) group. On the other
Aryl aldehyde (9.81 mmol) was suspended in anhydrous CH2Cl2 (300
mL). 2,4-Dimethylpyrrole (2.12 mL, 20.6 mmol) was added to the
reaction mixture, followed by 6 drops of BF3·OEt2. After the mixture
was stirred for 48 h and TLC indicated no more aryl aldehyde in the
reaction mixture, DDQ (1.113 g, 4.91 mmol) in CH2Cl2 was added.
After 1.5 h of stirring at room temperature, NEt3 (10.3 mL, 73.58
mmol) was added and after 1 h BF3·OEt2 (12.1 mL, 98.1 mmol) was
hand, BODIPYs 1a and 3a were most efficiently prepared using
method C (TMSOTf in refluxing toluene, followed by 5 equiv
of methanol and DIPEA), obtained in 98 and 70% yields,
respectively. However, for alcohols other than methanol,
method C produced lower yields of the targeted 4,4-
disubstituted BODIPYs, likely due to steric hindrance caused
by the triflate ion. In this case, method B (AlCl3 in refluxing
dichloromethane, followed by a large excess of alcohol)
produced the targeted 4,4-dialkoxy-BODIPYs consistently in
moderate yields (26−64%). The 4,4-dipropargyloxy-BODIPY
was converted to the corresponding bis(1,2,3-triazole)-
BODIPY via a Cu(I)-catalyzed Huisgen cycloaddition reaction,
demonstrating its stability under these conditions.
The 4,4-dialkoxy-BODIPYs showed absorption and emission
spectra similar to those of the parent BODIPY dyes, with only
slightly red shifted absorption and emission bands in THF.
However, introduction of two large N-benzyl-(1,2,3-triazole)
groups on boron induced blue-shifted absorption in both THF
and toluene. The fluorescence quantum yields of all 4,4-
dialkoxy-BODIPYs decreased in toluene, likely due to increased
energy loss to nonradiative decay to the ground state. However,
in THF the quantum yields were enhanced for the 4,4-
diisopropyloxy-, 4,4-dipropargyloxy-, and the bis(1,2,3-tria-
zole)-BODIPYs relative to the parent BF2 dye. Furthermore,
the last BODIPY showed the largest Stokes shifts in both
solvents.
added. The final mixture was stirred at room temperature for 6 h and
then poured into water (50 mL). The organic layer was washed with
water (2 × 50 mL). The combined organic layers were dried over
anhydrous Na2SO4 and concentrated under reduced pressure. The
product was purified by silica gel flash column chromatography using
mixtures of petroleum ether and dichloromethane for elution.
4,4-Difluoro-8-phenyl-1,3,5,7-tetramethyl-BODIPY (1): obtained
1
as a green-orange solid (20% yield, 650 mg), mp 168−169 °C; H
NMR (CDCl3, 400 MHz) δ 7.47−7.26 (m, 5H), 5.98 (s, 2H), 2.56 (s,
6H), 1.38 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 155.57, 143.29,
141.87, 135.14, 131.57, 129.26, 129.26, 128.08, 121.34, 14.71, 14.46;
11B NMR (CDCl3, 128 MHz) δ 0.65 (t, J = 33.4 Hz); HRMS (ESI-
TOF) m/z 324.1718 [M + H]+ calculated for C19H20BF2N2 324.1728.
These data are in agreement with those previously reported.34,41
4,4-Difluoro-8-(3,4-dimethoxyphenyl)-1,3,5,7-tetramethyl-BODI-
PY (2): obtained as a red solid (6% yield, 229 mg), mp 182−183 °C;
1H NMR (CDCl3, 400 MHz) δ 6.97 (d, J = 8.1 Hz, 1H), 6.84 (d, J =
8.1 Hz, 1H), 6.80 (s, 1H), 6.00 (s, 2H), 3.96 (s, 3H), 3.86 (s, 3H),
2.56 (s, 6H), 1.49 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 155.35,
149.80, 149.53, 143.14, 141.59, 131.71, 127.09, 121.12, 120.44, 111.57,
111.10, 56.10, 55.88, 14.53, 14.42; 11B NMR (CDCl3, 128 MHz) δ
0.62 (t, J = 33.0 Hz); HRMS (ESI-TOF) m/z 384.1930 [M + H]+
calculated for C21H24BF2N2O2 384.1939. These data are in agreement
with those previously reported.34,42
4,4-Difluoro-8-(4-methoxycarbonylphenyl)-1,3,5,7-tetramethyl-
BODIPY (3): obtained as a bright red solid (26% yield, 483 mg), mp
181−183 °C; 1H NMR (CDCl3, 400 MHz) δ 8.19 (d, J = 8.2 Hz, 2H),
δ 7.42 (d, J = 8.1 Hz, 2H), 5.99 (s, 2H), 3.98 (s, 3H), 2.57 (s, 6H),
1.37 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 166.58, 156.10, 143.01,
140.35, 139.94, 131.04, 130.94, 130.48, 128.50, 121.62, 52.50, 14.71,
14.61; 11B NMR (CDCl3, 128 MHz) δ 0.61 (t, J = 32.8 Hz); HRMS
(ESI-TOF) m/z 382.1773 [M + H]+ calculated for C21H22BF2N2O2
382.1752. These data are in agreement with those previously
reported.34,43
EXPERIMENTAL SECTION
■
General Considerations. All of the reagents and solvents were
purchased from Sigma-Aldrich without further purification. All
reactions were carried out with oven-dried glassware under a dry
argon atmosphere. All of the reactions were monitored using Sorbent
Technologies 0.2 mm silica gel TLC plates with UV 254 nm indicator.
Flash column chromatography was performed using Sorbent
Technologies 60 Å silica gel (230−400 mesh). Prep-TLC 60 Å silica
gel 20 × 20 cm (210−270 μm) was used. All H, 13C, and 11B NMR
1
spectra were collected on an AV-400 spectrometer in deuterated
chloroform. The CDCl3 chemical shifts (δ) are reported in ppm using
as reference 7.27 ppm for proton and 77.16 ppm for carbon, and BF3·
OEt2 was used as reference (δ 0.00) for boron NMR. Coupling
constants are reported in hertz (Hz). All of the mass spectra were
collected using a Agilent 6210 ESI-TOF mass spectrometer.
General Procedure for O-BODIPYs using Method A. In an
oven-dried flask, BODIPY (20.0 mg, 0.062 mmol) and NaOMe (21.0
mg, 0.389 mmol) were dissolved in anhydrous methanol (2 mL). The
mixture was refluxed for 12 h and then cooled to room temperature.
F
DOI: 10.1021/ic502821m
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Inorganic Chemistry
Article
1
The reaction mixture was poured into ice water (5.0 mL), extracted
with CH2Cl2 (3 × 5.0 mL), washed with brine (1 × 5.0 mL), and dried
over anhydrous Na2SO4. The crude product was concentrated under
reduced pressure, and the resulting residue was purified by TLC, using
ethyl acetate for elution.
General Procedure for O-BODIPYs using Method B. In an
oven-dried flask, BODIPY (20.0 mg, 0.062 mmol) and AlCl3 (16.4 mg,
0.123 mmol) were dissolved in anhydrous CH2Cl2 (5 mL). The
mixture was refluxed for 15 min before alcohol (17.15 mmol, 278.0
equiv) was added. The final reaction mixture was refluxed for 15 min.
The mixture was cooled to room temperature and then concentrated
under reduced pressure. The resulting residue was purified by TLC,
using the eluents indicated below.
hexane/dichloromethane: mp 140−145 °C; H NMR (CDCl3, 400
MHz) δ 7.49−7.30 (m, 5H), 5.96 (s, 2H), 3.40−3.34 (septuplet, J =
6.24 Hz, 2H), 2.61 (s, 6H), 1.39 (s, 6H), 0.89 (d, J = 6.08 Hz, 12H);
13C NMR (CDCl3, 100 MHz) δ 156.26, 141.50, 141.42, 135.96,
132.28, 129.06, 128.78, 128.46, 121.16, 63.01, 24.93, 15.76, 14.65; 11B
NMR (CDCl3, 128 MHz) δ 1.30 (s); HRMS (ESI-TOF) m/z
426.2564 [M + Na]+ calculated for C25H33BN2NaO2 426.2575.
4,4-Diallyloxy-8-phenyl-1,3,5,7-tetramethyl-BODIPY (1d). This
BODIPY was produced in yields of 57% by method B and 30% by
method C. The crude product was purified by TLC (eluent 80/20
hexane/EtOAc) to give an orange solid that was recrystallized from 1/
1
1 hexane/dichloromethane: mp 70−73 °C; H NMR (CDCl3, 400
MHz) δ 7.52−7.29 (m, 5H), 5.95 (s, 2H), 5.92−5.82 (m, 2H), 5.15
(d, J = 1.84 Hz, 1H), 5.11 (d, J = 1.88 Hz, 1H), 4.92 (d, 1H), 4.89 (d,
1H), 3.63 (d, J = 5.24 Hz, 4H), 2.56 (s, 6H), 1.38 (s, 6H); 13C NMR
(CDCl3, 100 MHz) δ 156.06, 141.53, 138.70, 135.77, 132.78, 129.13,
128.85, 128.34, 128.23, 121.18, 113.42, 63.41, 15.03, 14.59; 11B NMR
(CDCl3, 128 MHz) δ 2.01 (s); HRMS (ESI-TOF) m/z 422.2251 [M
+ Na]+ calculated for C25H29BN2NaO2 422.2254.
General Procedure for O-BODIPYs using Method C. In an
oven-dried flask, BODIPY (20 mg, 0.06 mmol) and TMSOTf (28.0
μL, 0.154 mmol) were dissolved in anhydrous toluene (5 mL) and the
mixture was refluxed for 30 min. The solution was cooled to room
temperature, and alcohol (0.308 mmol, 5.0 equiv) was added, followed
by DIPEA (27.0 μL, 0.154 mmol, 2.5 equiv). After 1 h of stirring at
room temperature, the crude product was concentrated under reduced
pressure and purified by TLC, using the eluents indicated below.
4,4-Dimethoxy-8-phenyl-1,3,5,7-tetramethyl-BODIPY (1a). This
BODIPY was produced in yields of 66% by method A, 52% by method
B, and 98% by method C. The crude product was purified by TLC
(eluent EtOAc) to give an orange solid that was recrystallized from 1/
4,4-(Diprop-2-yn-1-yloxy)-8-phenyl-1,3,5,7-tetramethyl-BODIPY
(1e). This BODIPY was produced in yields of 52% by method B and
40% by method C. The crude product was purified via TLC (eluent
80/20 hexane/EtOAc) to give an orange solid that was recrystallized
from 1/1 hexane/dichloromethane: mp 70−73 °C; 1H NMR (CDCl3,
400 MHz) δ 7.49−7.27 (m, 5H), 5.97 (s, 2H), 3.86 (s, 4H), 2.61 (s,
6H), 1.52 (s, 2H), 1.40 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ
156.62, 142.22, 141.42, 135.58, 132.79, 129.16, 128.92, 128.26, 121.52,
83.16, 70.83, 49.96, 15.19, 14.58; 11B NMR (CDCl3, 128 MHz) δ 1.78
(s); HRMS (ESI-TOF) m/z 418.1938 [M + Na]+ calculated for
C25H25BN2NaO2 418.1934.
4,4′-(Di-N-benzyl-1,2,3-triazolo-4-methylenoxy)-8-phenyl-
1,3,5,7-tetramethyl-BODIPY (4). 4,4-Dipropargyloxy-BODIPY 1e
(15.7 mg, 0.040 mmol, 1.0 equiv) and Cu(0) (10.1 mg, 0.158 mmol,
4.0 equiv) were dissolved in a mixture of THF and water (8 mL, 3/1)
under an inert gas atmosphere. Benzyl azide (0.074 mL, 0.594 mmol,
5.0 equiv) was added dropwise to the reaction mixture. A solution of
CuSO4·5H2O (7.4 mg, 0.030 mmol, 0.75 equiv) and sodium ascorbate
(15.7 mg, 0.079 mmol, 2.0 equiv) in THF/water (8 mL, 3/1) was
added (after sonication for 30 min) and the reaction mixture was
heated for 1.5 h at 70 °C. Once TLC indicated completion of the
reaction, the mixture was cooled to room temperature and partitioned
between ethyl acetate (30 mL) and water (30 mL). The combined
organic layers were dried over Na2SO4 and then concentrated under
reduced pressure. The crude product was purified via prep-TLC with
20/80 hexane/EtOAc as eluent and recrystallized from 3/1 hexane/
dichloromethane to afford the bis(1,2,3-triazole)-BODIPY 4 as a dark
1
1 hexane/dichloromethane: mp 125−130 °C; H NMR (CDCl3, 400
MHz) δ 7.50−7.30 (m, 5H), 5.97 (s, 2H), 2.97 (s, 6H), 2.54 (s, 6H),
1.39 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 155.77, 141.71, 141.38,
135.81, 133.01, 129.10, 128.82, 128.35, 121.05, 49.24, 14.70, 14.58; 11B
NMR (CDCl3, 128 MHz) δ 2.52 (s); HRMS (ESI-TOF) m/z
370.1938 [M + Na]+ calculated for C21H25BN2NaO2 370.1923. These
data are in agreement with literature data.18
4,4-Dimethoxy-8-(3,4-dimethoxyphenyl)-1,3,5,7-tetramethyl-
BODIPY (2a). This BODIPY was produced in yields of 50% by method
A, 11% by method B and 8% by method C. The crude product was
purified by TLC (eluent 50:50 hexane/EtOAc) to afford a dark
orange/red solid, that was recrystallized from acetone: mp 100−105
°C dec; 1H NMR (CDCl3, 400 MHz) δ 6.97 (d, J = 8.1 Hz, 1H), 6.84
(d, J = 8.1 Hz, 1H), 6.78 (s, 1H), 5.98 (s, 2H), 3.96 (s, 3H), 3.89 (s,
3H), 2.98 (s, 3H), 2.95 (s, 3H), 2.54 (s, 6H), 1.49 (s, 6H); 13C NMR
(CDCl3, 100 MHz) δ 155.79, 149.85, 149.53, 141.54, 141.37, 133.30,
128.02, 120.96, 120.86, 111.58, 111.53, 56.31, 56.06, 49.33, 49.26,
14.72; 11B NMR (CDCl3, 128 MHz) δ 2.52 (s); HRMS (ESI-TOF)
m/z 430.2149 [M + Na]+ calculated for C23H29BN2NaO4 430.2136.
4,4-Dimethoxy-8-(4-methoxycarbonylphenyl)-1,3,5,7-tetrameth-
yl-BODIPY (3a). This BODIPY was produced in yields of 4% by
method A, 58% by method B, and 70% by method C. The crude
product was purified by TLC (eluent EtOAc) to afford an orange solid
1
orange solid (20.4 mg, 78% yield): H NMR (CDCl3, 400 MHz) δ
7.50 (s, 2H), 7.36−7.27 (m, 15H), 5.84 (s, 2H), 5.43 (s, 4H), 4.28 (s,
4H), 2.40 (s, 6H), 1.36 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ
155.63, 149.42, 142.17, 135.48, 134.93, 129.21, 129.15, 128.75, 128.35,
128.30, 121.68, 121.48, 56.80, 54.15, 31.06, 29.84, 29.41, 14.96, 14.57;
11B NMR (CDCl3, 128 MHz) δ 2.06 (s); HRMS (ESI-TOF) m/z
684.3218 [M + Na]+ calculated for C39H39BN8NaO2 684.3229.
Molecular Structures. Crystal structures were determined using
low-temperature data from a Bruker Kappa APEX-II DUO
diffractometer with either Mo Kα or Cu Kα radiation. For all
structures, H atoms were located from difference maps but constrained
in calculated positions during refinement. For 1a, Z′ = 3/2, for 1b, Z′
= 1/2, and for 1e, 3a, and 4, Z′ = 2. 3a was a hemihydrate and a
nonmerohedral twin, and 4 was also a nonmerohedral twin. Crystal
data for 1a: C21H25BN2O2, Mr = 348.24, monoclinic, a = 20.2592(14)
Å, b = 11.2886(8), Å, c = 12.7820(8) Å, β = 93.968(3)°, U =
2916.2(3) Å3, T = 100 K, space group P2/c, Z = 6, Dc = 1.190 g cm−3,
μ(Cu Kα) = 0.597 mm−1, 24457 reflections measured, θmax = 68.8°,
5251 unique reflections (Rint = 0.032). The final R = 0.040 (4321 I >
2σ(I) data), Rw(F2) = 0.105 (all data), CCDC 1035578. Crystal data
for 1b: C23H29BN2O2, Mr = 376.29, monoclinic, a = 9.1588(4) Å, b =
11.4850(7) Å, c = 10.1066(4) Å, β = 96.860(2)°, U = 1055.49(9) Å3,
T = 100 K, space group P2/n, Z = 2, Dc = 1.184 g cm−3, μ(Mo Kα) =
0.075 mm−1, 33635 reflections measured, θmax = 29.4°, 3096 unique
1
that was recrystallized from acetone: mp 100−105 °C dec; H NMR
(CDCl3, 400 MHz) δ 8.19 (d, J = 8.2 Hz, 2H), δ 7.42 (d, J = 8.1 Hz,
2H), 5.98 (s, 2H), 3.98 (s, 3H), 2.97 (s, 6H), 2.57 (s, 6H), 1.37 (s,
6H); 13C NMR (CDCl3, 100 MHz) δ 166.70, 156.34, 141.17, 140.70,
140.21, 132.48, 130.72, 130.37, 128.79, 121.40, 52.50, 49.24, 14.76,
14.73; 11B NMR (CDCl3, 128 MHz) δ 2.51 (s); HRMS (ESI-TOF)
m/z 428.1992 [M + Na]+ calculated for C23H27BN2NaO4 428.1981.
4,4-Diethoxy-8-phenyl-1,3,5,7-tetramethyl-BODIPY (1b). This
BODIPY was produced in yields of 64% by method B and 18% by
method C. The crude product was purified via TLC (eluent 80/20
hexane/EtOAc) to give an orange solid that was recrystallized from 1/
1
1 hexane/dichloromethane: mp 140−145 °C; H NMR (CDCl3, 400
MHz) δ 7.50−7.29 (m, 5H), 5.95 (s, 2H), 3.07 (q, J = 7.0 Hz, 4H),
2.58 (s, 6H), 1.38 (s, 6H), 1.07 (t, J = 7.0 Hz, 6H); 13C NMR (CDCl3,
100 MHz) δ 155.87, 141.61, 141.25, 135.85, 132.84, 129.09, 128.80,
128.37, 121.00, 56.64, 17.83, 14.94, 14.59; 11B NMR (CDCl3, 128
MHz) δ 1.82 (s); HRMS (ESI-TOF) m/z 398.2251 [M + Na]+
calculated for C23H29BN2NaO2 398.2264.
4,4-Diisopropyloxy-8-phenyl-1,3,5,7-tetramethyl-BODIPY (1c).
This BODIPY was produced in yields of 26% by method B and 0%
by method C. The crude product was purified via TLC (eluent 80/20
Hex/EtOAc) to give an orange solid that was recrystallized from 1/1
G
DOI: 10.1021/ic502821m
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Inorganic Chemistry
Article
reflections (Rint = 0.045). The final R = 0.043 (2411 I > 2σ(I) data),
Rw(F2) = 0.119 (all data), CCDC 1035579. Crystal data for 1c:
C25H33BN2O2, Mr = 404.34, monoclinic, a = 9.0299(11) Å, b =
16.252(2) Å, c = 16.2554(19) Å, β = 102.241(7)°, U = 2331.3(5) Å3,
T = 100 K, space group P21/c, Z = 4, Dc = 1.152 g cm−3, μ(Mo Kα) =
0.072 mm−1, 12118 reflections measured, θmax = 25.9°, 4417 unique
reflections (Rint = 0.091). The final R = 0.058 (2457 I > 2σ(I) data),
Rw(F2) = 0.134 (all data), CCDC 1035580. Crystal data for 1e:
C25H25BN2O2, M = 396.28, monoclinic, a = 7.9474(8), b =
11.8269(10), c = 45.453(4) Å, β = 92.521(6)°, U = 4268.2(7) Å3, T
= 100 K, space group P21/n, Z = 8, Dc = 1.233 g cm−3, μ(Cu Kα) =
0.611 mm−1, 18058 reflections measured, θmax = 67.4°, 7057 unique
reflections (Rint = 0.045). The final R = 0.080 (5633 I > 2σ(I) data),
Rw(F2) = 0.180 (all data), CCDC 1035581. Crystal data for 3a:
C23H27BN2O4·0.5H2O, Mr = 415.28, triclinic, a = 7.6310(6) Å, b =
15.4871(12) Å, c = 19.4805(16) Å, α = 72.585(4)°, β = 84.215(4)°, γ
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE-1362641). The authors thank the Louisiana Optical
Network Initiative (LONI) for the use of their computational
facilities and Dr. Svetlana Pakhomova for assistance with the X-
ray structure determination for BODIPY 4.
REFERENCES
■
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= 89.809(4)°, U = 2184.7(3) Å3, T = 100 K, space group P1, Z = 4, D
̅
c
= 1.263 g cm−3, μ(Cu Kα) = 0.703 mm−1, 15119 reflections measured,
θmax = 66.9°, 7555 unique reflections (Rint = 0.108). The final R =
0.067 (5951 I > 2σ(I) data), Rw(F2) = 0.196 (all data), CCDC
1035582. Crystal data for 4: C39H39BN8O2, Mr = 662.59, triclinic, a =
11.1659(8) Å, b = 16.0899(12) Å, c = 19.5309(13) Å, α = 90.127(5)°,
β = 92.245(5)°, γ = 96.332(5)°, U = 3484.7(4) Å3, T = 90 K, space
group P1, Z = 4, D = 1.263 g cm−3, μ(Cu Kα) = 0.639 mm−1, 33229
̅
c
reflections measured, θmax = 67.7°, 11371 unique reflections (Rint
=
0.062). The final R = 0.049 (8767 I > 2σ(I) data), Rw(F2) = 0.123 (all
data), CCDC 1051095.
Computational Methods. The reaction mechanism was studied
by electronic structure calculations at the B3LYP/6-31+G(d,p) level.
The hybrid Becke three-parameter DFT functional was used.44,45 The
solvent effects were taken into account using the polarized continuum
model (PCM).46,47 Atomic charges were calculated following the NPA
method.48 The geometries of all structures were optimized without
symmetry constraints. The stationary points on the potential energy
surface were confirmed with frequency calculations. All calculations
were performed using the Gaussian 09 program package.49
Spectroscopic Studies. The photophysical properties of these
compounds 1, 1a−e, 2, 2a, 3, 3a, and 4 were determined by preparing
a stock solution with a concentration of 5 × 10−5 M that was diluted to
the appropriate concentrations for absorbance and emission spectra
measurements. All absorption spectra were obtained using a Varian
Cary 50 UV/visible spectrophotometer. The optical densities, ε, were
obtained by preparing solution concentrations between 7.5 × 10−6 and
2.5 × 10−5 M to give λmax values between 0.5 and 1.0. Emission spectra
were measured on a PTI QuantaMaster4/2006SE spectrofluorometer
with the slit width set at 3 nm. All absorbance and emission spectra
were acquired within 3 h of solution preparation, at room temperature.
BODIPYs 1−3 were used as standards in THF for calculating the
fluorescence quantum yields with excitation at 480 nm (BODIPY 1 Φf
= 0.56,41 BODIPY 2 Φf = 0.93,34 BODIPY 3 Φf = 0.43),34 as
previously reported. Rhodamine 6G (Φf = 0.95 in EtOH) was used as
the reference for BODIPYs 1, 1a−e, 2, 2a, 3, 3a, and 4 in toluene. A
spectrophotometric cell with a path length of 10 mm was used.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures and CIF files giving X-ray data for 1a−c,e, 3a, and 4
1
and H, 13C, and 11B NMR spectra and normalized absorption
and fluorescence spectra of all BODIPYs. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
248−251.
(30) Hendricks, J. A.; Keliher, E. J.; Wan, D.; Hilderbrand, S. A.;
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Corresponding Author
*E-mail for M.G.H.V.: vicente@lsu.edu.
Notes
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The authors declare no competing financial interest.
Goze, C. Chem. - Eur. J. 2014, 20, 12933−12944.
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