New 2,6-distyryl-substituted BODIPY isomers: Synthesis, photophysical properties, and theoretical calculations

Article abstract of DOI:10.1002/chem.201303291

A 2,6-distyryl-substituted boradiazaindacene (BODIPY) dye and a new series of 2,6-p-dimethylaminostyrene isomers containing both α- and β-position styryl substituents were synthesized by reacting styrene and p-dimethylaminostyrene with an electron-rich diiodo-BODIPY. The dyes were characterized by X-ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6-distyryl-substituted BODIPYs, relative to analogous 3,5-distyryl- and 1,7-distyryl-substituted BODIPYs. Although the fluorescence of the compound with β-position styryl substituents on both pyrrole moieties and one with both β- and α-position substituents was completely quenched, the compound with only α-position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6-p-dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non-emissive charge-separated S₁ states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects. Copyright

Full text of DOI:10.1002/chem.201303291

DOI: 10.1002/chem.201303291
Full Paper
&
Dyes/Pigments
New 2,6-Distyryl-Substituted BODIPY Isomers: Synthesis,
Photophysical Properties, and Theoretical Calculations**
Lizhi Gai,[a, b] John Mack,[c] Hua Lu,*[a] Hiroko Yamada,[d] Daiki Kuzuhara,[d] Guoqiao Lai,[a]
Zhifang Li,*[a] and Zhen Shen*[b]
Abstract:
A
2,6-distyryl-substituted boradiazaindacene
both b- and a-position substituents was completely
quenched, the compound with only a-position substituents
exhibits weak emission in polar solvents, but moderately in-
tense emission with a quantum yield of 0.49 in hexane. Pro-
tonation studies have demonstrated that these 2,6-p-dime-
thylaminostyrene isomers can be used as sensors for
changes in pH. Theoretical calculations provide strong evi-
dence that styryl rotation and the formation of non-emissive
charge-separated S1 states play a pivotal role in shaping the
fluorescence properties of these dyes. Molecular orbital
theory is used as a conceptual framework to describe the
electronic structures of the BODIPY core and an analysis of
the angular nodal patterns provides a reasonable explana-
tion for why the introduction of substituents at different po-
sitions on the BODIPY core has markedly differing effects.
(BODIPY) dye and a new series of 2,6-p-dimethylaminosty-
rene isomers containing both a- and b-position styryl sub-
stituents were synthesized by reacting styrene and p-dime-
thylaminostyrene with an electron-rich diiodo-BODIPY. The
dyes were characterized by X-ray crystallography and NMR
spectroscopy and their photophysical properties were inves-
tigated and analyzed by carrying out a series of theoretical
calculations. The absorption spectra contain markedly red-
shifted absorbance bands due to conjugation between the
styryl moieties and the main BODIPY fluorophore. Very low
fluorescence quantum yields and significant Stokes shifts are
observed for 2,6-distyryl-substituted BODIPYs, relative to
analogous 3,5-distyryl- and 1,7-distyryl-substituted BODIPYs.
Although the fluorescence of the compound with b-position
styryl substituents on both pyrrole moieties and one with
the chemical environment.[1] BODIPYs have been widely used
in a variety of organic functional materials, such as labeling re-
agents, chemosensors and laser dyes, and in applications such
as photodynamic therapy.[2] Recently, there has been an in-
creasing focus on the synthesis, functionalization, and photo-
physical properties of BODIPYs. One area of particular interest
has been the design of new dyes with absorption and emis-
sion bands shifted towards the near-infrared (NIR) region and
into the optical window in biological tissue.[3] Despite the lack
of a cyclic perimeter, BODIPYs possess many of the characteris-
tics of heteroaromatic compounds, such as planarity and high
chemical stability, due to the manner in which the BF2 moiety
interacts with the p system. A redshift of the main absorption
band into the NIR or red region can be achieved by preparing
substituted BODIPY structures in moderate yield through tran-
sition-metal-catalyzed cross-coupling reactions,[4] Knoevenagel
condensations reactions,[5] and nucleophilic substitution reac-
tions.[6] The introduction of styryl group is one of the most effi-
cient ways to achieve an extension p-conjugation system, and
hence a significant redshift of the main spectral bands. For ex-
ample, the introduction of styryl substituents at the 3,5-posi-
tions leads to a profound redshift of approximately 120 nm rel-
ative to the corresponding tetramethyl-BODIPY compound.[7]
In contrast, the shift induced by styryls at the 1,7-positions is
only 70 nm.[8] Since the substituent position clearly has
a strong influence on the spectroscopic properties, a logical
Introduction
Boradiazaindacenes (BODIPYs) are a well-known type of highly
fluorescent dye with unusually intense absorption and emis-
sion bands, high photochemical stability, and low sensitivity to
[a] L. Gai, Dr. H. Lu, Prof. G. Lai, Prof. Z. Li
Key Laboratory of Organosilicon Chemistry
and Material Technology, Ministry of Education
Hangzhou Normal University, Hangzhou, 310012 (P.R. China)
Fax: (+86)571-2886-8529
E-mail: hualu@hznu.edu.cn
zhifanglee@hznu.edu.cn
[b] L. Gai, Prof. Z. Shen
State Key Laboratory of Coordination Chemistry
Nanjing National Laboratory of Microstructures
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing, 210093 (P.R. China)
Fax: (+86)25-8331-4502
E-mail: zshen@nju.edu.cn
[c] Dr. J. Mack
Department of Chemistry, Rhodes University
Grahamstown 6140 (South Africa)
[d] Prof. H. Yamada, D. Kuzuhara
Graduate School of Materials Science
Nara Institute of Science and Technology
Ikoma, 630-0192 (Japan)
[**] BODIPY=4-bora-3a,4a-diaza-s-indacene.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303291.
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next step would be to explore the effect of introduc-
ing styryl substituents at the 2,6-positions. In this
study, 2,6-distyryl BODIPYs have been synthesized by
using the analogous diiodo-BODIPY through Heck re-
actions.[9] Surprisingly, the use of p-dimethylaminos-
tyrene results in the synthesis of three geometric iso-
mers with markedly different photophysical proper-
ties. The dimethylaminostyryl group was selected for
detailed study due to its strong electron-donor prop-
erties, since this is likely to result in an enhanced red-
shift of the spectral bands. Protonation could also
lead to applications as sensors for pH.[5e,18b] In this
study, we report the synthesis and characterization of
2,6-distyryl-substituted BODIPY dyes and the use of
DFT and time-dependent (TD)-DFT calculations to ex-
plore the structure–property correlations.
Results and Discussion
Scheme 1. Synthesis of 2,6-distyryl-substituted BODIPY derivatives.
Synthesis
1
The 2,6-distyryl-substituted BODIPYs were synthesized from an
electron-rich 2,6-diiodo-BODIPY by a reaction with styrene in
dimethylformamide and triethylamine. Palladium diacetate was
used as a catalyst. The 2,6-distyryl-substituted BODIPY 1 was
formed in 41% yield. The structure of 1 was determined by
1H NMR spectroscopy and HRMS. Two doublet signals with J=
16 Hz at d=6.88 and 6.62 ppm were assigned to the protons
on the ethene moiety. Similar signals were reported in the
1H NMR spectra of the analogous BODIPYs with styryl group at
the 3,5- or 1,7-positions.[7,8] The use of p-dimethylaminostyrene
provides scope for introducing
at d=4.89 and 5.70 ppm are observed in the H NMR spectra
of 2b (Figure 1) due to the terminal olefin protons, split peaks
are observed for the methyl protons of the amino groups be-
tween d=2.95–2.98 ppm and of the main BODIPY moiety at
around d=1.21, 1.46, 2.40, and 2.72 ppm due to the unsym-
metrical structure. The pair of peaks at highfield can be as-
signed to the a-position styryl substituent, since the corre-
sponding signals for 2c lie at d=1.21 and 2.40 ppm (Figure 1).
Our research confirms that geminal substitution (to form the
a-position substituents) is strongly favored by electron-donat-
electron-donating groups, which
can be used to further fine-tune
the redox and optical properties.
When the synthesis was carried
out, however, three new BODIPY
isomers
were
isolated
(Scheme 1); one with b-position
styryl substituents on both pyr-
role moieties (2a), another with
both b- and a-position substitu-
ents (2b), and another with only
a-position substituents (2c). The
same molecular ion peak is ob-
served at m/z 614 (calcd for
[M]+ =614 amu) in the MALDI-
TOF mass spectra of 2a–c. In the
1H NMR spectra of 2a (Figure 1),
the proton signals of the styryl
moiety and methyl groups are
shifted to highfield compared
with those of 1. This is probably
related to shielding effects
caused by the electron-donating
properties of the dimethylamino
groups. Although singlet peaks Figure 1. 1H NMR spectra of 2a–c.
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ing substituents. This can be rationalized based on a consider-
ation of the different cationic transition states that can be
formed after Pd insertion into the double bond (see
Scheme S1 in the Supporting Information).[10]
Table 1. Spectroscopic and photophysical properties of 1 and 2a–c in
hexane and CH2Cl2 at 298 K, and those reported previously for 3 and 4.
Solvent
labs
lem
Demꢀabs
F
tf
[nm]
[nm]
[cmꢀ1
]
[ns]
A single crystal of 2b was obtained by the slow diffusion of
hexane into a dichloromethane solution. As shown in Figure 2,
the boron atom is coordinated in a tetrahedral geometry by
1
hexane
CH2Cl2
hexane
CH2Cl2
CH2Cl2
hexane
CH2Cl2
CH2Cl2
hexane
CH2Cl2
CH2Cl2
MeOH
toluene
toluene
575
575
603
616
578
564
570
550
529
529
524
572
582
629
628
633
n.d[a]
n.d[a]
619
n.d[a]
n.d[a]
597
559
548
545
588
603
641
1470
1590
–
–
1150
–
0.02
0.01
–
–
0.01
–
2.08
1.14
–
–
2.46
–
2a
2a+H+[b]
2b
–
–
–
2b+H+[b]
2c
1430
1010
660
740
480
600
300
0.02
0.49
<0.001
0.46
0.72
0.44
0.59
4.22
6.51
4.65
5.05
n.a[c]
n.a[c]
4.6
2c+H+[b]
3[8]
4[7a]
[a] Not detected. [b] After the addition of 100 equivalents of TFA. [c] Not
available.
the 570–620 nm region than are typically observed for the
main BODIPY absorption band (Figure 4).[7c,11] In CH2Cl2, a red-
shift of 41 nm is observed for the band center of the lowest-
Figure 2. Front (top) and side (bottom) ORTEP views of the molecular struc-
tures of 2b with the thermal ellipsoids set at 50% probability.
two nitrogen and two fluorine atoms and the meso (or 8)-
phenyl ring lies almost orthogonal to the indacene plane with
torsion angles of 88.68 (Figure 2). The indacene moiety is
highly planar with an average root-mean-square (rms) devia-
tion of 0.0166 ꢁ. The C9ꢀC10 and C30ꢀC31 bond lengths (1.335
and 1.316 ꢁ, respectively) are consistent with the double-bond
character of terminal and styryl olefins. Head-to-tail p–p stack-
ing interactions are observed in the molecular packing dia-
grams with interplanar separations of approximately 3.9 ꢁ
(Figure 3). There is a significant dihedral angle of 37.28 be-
tween the b-position styryl substituent and the main BODIPY
fluorophore, due to the steric interactions with the methyl sub-
stituents at the 1- and 3-positions.
Figure 4. Absorption spectra of 1 and 2a–c in dichloromethane. Inset: Pho-
tograph of 1 and 2a–c in dichloromethane.
The UV/Vis absorption and emission spectra of 1 and 2a–
c were measured in hexane and CH2Cl2. The photophysical
properties are summarized in Table 1. The absorption spectra
of 1 and 2a,b exhibit broader and more structureless bands in
energy absorption band of 2a relative to that of 1. Absorption
bands are observed at 529 and 570 nm for 2c and 2b, respec-
tively, when there is a-position substitution of the styryl unit
(Scheme 1). This represents a blueshift of 87 and 46 nm relative
to the absorption band of 2a. This can be readily attributed to
weaker conjugation between the a-position-substituted styryl
moiety and the main BODIPY fluorophore. The absorption
spectra of 1 and 2c are largely unaffected by solvent polarity,
as is normally the case with BODIPY chromophores.[12] In con-
trast, the band maxima of 2a,b are blueshifted by 13 nm when
the solvent is changed from hexane to CH2Cl2. An increase in
the ground state dipole moment would be anticipated based
on the presence of the dimethylamino group on a b-position
styryl substituent.
The emission band of 1 at 633 nm exhibits mirror symmetry
with the absorption band. There is a significant Stokes shift of
Figure 3. View of the p–p stacking interactions of 2b.
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1590 cmꢀ1, which is much larger
than those typically observed for
BODIPYs due to the rigid struc-
ture of the fluorophore.[13] Com-
pounds 2a,b do not exhibit any
fluorescence, whereas only very
weak emission is observed for 1.
This suggests that there is
a greater degree of conforma-
tional flexibility in the S1 excited
state, because strong emission is
observed
for
3
and
4
Figure 5. Molecular structures of distyryl-substituted BODIPY dyes.
(Figure 5).[14] In contrast, com-
pound 2c exhibits weak emis-
sion in polar solvents, but mod-
erately intense emission with
a
quantum yield of 0.49 in
hexane (Table 1). In both of the
solvents investigated the fluores-
cence decay profiles of 1 and 2c
could be described by a single-
exponential fit on a nanosecond
timescale. The low fluorescence
quantum yields of styryl-substi-
tution at the 2,6-position of the
BODIPY core are reflected in the
rates of nonradiative decay knr
(knr =(1ꢀF)/t=8.7ꢂ108 sꢀ1
in
CH2Cl2), which increase around
10-fold relative to the corre-
sponding
3,5-disubstituted
BODIPY (knr =0.89ꢂ108 sꢀ1 in tol-
uene).[7b]
The absorption spectra of 2a–
c
exhibit interesting changes
during titrations with trifluoro-
acetic acid (TFA; Figure 6). Sever-
al isosbestic points are observed.
There is a hypsochromic shift of
the maximum absorption band
and a slight increase of the ex-
tinction coefficient. The ob-
served blueshift is consistent
with the elimination of the
mesomeric interaction between
the dimethylamino group and
the p system. The main emission
band gradually increases in in-
tensity with an almost un-
changed wavelength when the
electron-donating properties of
dimethylamino substituents are
eliminated upon protonation.[15]
This is related primarily to a sig-
Figure 6. Absorption (left) and fluorescent (right) spectra of 2a–c in CH2Cl2 by addition of TFA. lex is 540 (for 2a),
530 (for 2b), and 510 nm (for 2c).
nificant change in the fluorescence quantum yield (Table 1).
The TFA titration spectra demonstrate that 2a–c could be used
as “turn-on” fluorescence sensors for changes in pH despite
the relatively low quantum yield values (F=0.01 for 2a and
0.02 for 2b) when 100 equivalents of TFA are added.
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one-electron transition but also contain a minor HOMOꢀ2!
LUMO component, which arises from an MO associated primar-
ily with the styryl substituents. When the systematic over-esti-
mation of the energies of the spectral bands is taken into con-
sideration, the HOMOꢀ2!LUMO one-electron transition is
predicted to result in a second weaker absorption band with
significant intensity close to the blue end of the absorption
spectrum (Table 3), as is observed experimentally in the 350–
450 nm region (Figure 4). The MOs derived from the HOMOꢀ1
and HOMOꢀ2 of the parent unsubstituted BODIPY dye (com-
pound 5) are not predicted to play a significant role in the visi-
ble region spectral bands of 1, 3, and 4. A marked destabiliza-
tion of these MOs is predicted in the p systems of 2a,b relative
to the LUMO (Figure 8). This may account for the intensity ob-
served in the 300–350 nm region of the absorption spectra of
these compounds (Figure 4 and Table 3).
Discussion
Theoretical calculations have been used to gain greater insight
into the electronic structures and to account for the observed
photophysical properties. The B3LYP functional of the Gaussi-
an 09 software package[16] was used with 6-31G(d) basis sets to
optimize the structures of 1, 2a–c, 3, 4, an unsubstituted
meso-phenyl-BODIPY model complex (5), and a 3,5-distyryl-
substituted analogue of 2a (6a), without symmetry restraints.
The B3LYP optimized geometries were also used to carry out
a second set of TD-DFT calculations using the Coulomb-attenu-
ated B3LYP functional (CAM-B3LYP) functional with 6-31G(d)
basis sets, Tables 2 and 3, since the results of TD-DFT calcula-
Table 2. Observed and calculated wavelengths for the main absorption
bands of 1, 2a–c, 3, and 4 calculated by using the B3LYP and CAM-
B3LYPs functional with 6-31G(d) basis sets, both in the gas phase and di-
chloromethane using the polarizable continuum model (PCM).
Although very low fluorescence quantum yields are ob-
served for the 2,6-distyryl-substituted BODIPY 1 (Table 1), the
1,7- and 3,5-distyryl-substituted BODIPYs 3 and 4 have high
quantum yields of over 0.40. Both the HOMOs and LUMOs of 3
and 4 have significant MO coefficients on the styryl moieties
(Figure 7), so there is only limited scope for charge-transfer
character in the HOMO!LUMO transition. However, while the
HOMO of 1 has a broadly similar set of nodal patterns to those
of 3 and 4, the LUMO is centered almost exclusively on the
BODIPY core, so there is significant charge transfer from the
styryl groups to the main BODIPY fluorophore. This charge-
transfer character is further enhanced in 2a–c by the introduc-
tion of electron-donating amino groups. Although the rotation
of styryl groups at the 2,6-positions of the BODIPY moiety of
2a–c could result in fast internal conversion to the ground
state, this would not account for the higher quantum yield ob-
served for 2c in non-polar solvents. In CH2Cl2, the emission in-
tensity of 2c increases markedly after the addition of
100 equivalents of TFA (F=0.46) due to the protonation of
the para substituents (Table 1). This suggests that charge trans-
fer between the substituents and main BODIPY fluorophore
plays a significant role. The fluorescence intensity can be fur-
ther quenched in polar solvents as is observed with 2c due to
an enhancement of the rate of nonradiative decay through
what are often referred to as intramolecular charge transfer
(ICT) states.[5e,18] These so-called ICT states are derived from the
S1 state of the fluorophore based on a separation of charge on
different portions of the structure upon electronic excitation.
The rate of nonradiative decay is enhanced by the formation
of conical intersections between this low-lying non-emissive S1
state and the ground state.[18,19] In the context of 1 and 2a–c,
the quenching of the fluorescence may be further enhanced
by the steric interactions between the styryl groups and the
methyl substituents at the 1- and 3-positions (Figure 9 and
Table 4), since it has been demonstrated that the charge-sepa-
rated character of non-emissive S1 state is enhanced when
there is a twisting of the bond linking the donor and acceptor
moieties, similar to those predicted in the structure of 1 and
2a (Figures 7 and 9, and Table 4).[19]
1
2a
2b
2c
3
4
Experimental
B3LYP (PCM)
B3LYP
CAM-B3LYP (PCM)
CAM-B3LYP
575[a]
547
537
486
475
616[a]
669
626
514
499
564[a]
636
594
488
475
529[a]
588
541
453
441
582[b]
560
535
519
497
629[b]
590
556
555
525
[a] Dichloromethane. [b] Toluene.
tions are known to be problematic when significant charge-
transfer character is involved.[17] The CAM-B3LYP functional in-
cludes a long-range correction of the exchange potential,
which incorporates an increasing fraction of Hartree–Fock (HF)
exchange as the interelectronic separation increases.
If the systematic underestimation of the energies of the
transitions is set aside, the general trend predicted for the
wavelengths of the main spectral bands of the 2,6-p-dimethy-
laminostyryl-substituted compounds 2a–c relative to 1, 3, and
4 in calculations with the CAM-B3LYP functional is in closer
agreement with the experimental data (Table 2 and Figure 4)
than those obtained with the B3LYP functional. The lowest-
energy band of 2a is predicted to lie at 499 nm (f=1.13) and
to arise almost exclusively from the HOMO!LUMO one-elec-
tron transition (Table 3). The HOMO and LUMO of 2a are both
destabilized relative to those of 1 due to the introduction of
the electron-donating amino substituents (Figure 8).
There is significant mixing between the HOMO of the
BODIPY fluorophore and an MO introduced by the styryl sub-
stituents, which is the HOMOꢀ2 of 1 and 2a–c (Figure 7).
There is a significant redshift of the absorption bands of 2a,
since the destabilization of the HOMO is greater than that of
the LUMO, which has very small MO coefficients on the styryl
substituents (Figure 7 and 8). The a-position styryl substituents
of 2b,c are predicted to stabilize the HOMO (Figure 8), but to
have little effect on the LUMO, thus increasing the HOMO–
LUMO gap, resulting in a significant blueshift of the main ab-
sorption band relative to that of 2a. The lowest-energy transi-
tions of 1 and 2b,c are also dominated by the HOMO!LUMO
Small Stokes shifts of around 600 and 300 cmꢀ1 are observed
for the 1,7- and 3,5-distyryl-substituted BODIPYs 3 and 4 in tol-
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Figure 7. The frontier occupied and virtual MOs for an unsubstituted meso-phenyl-BODIPY model complex (5), 1, 2a–2c, 3, and 4 at the CAM-B3LYP/6-31G(d)
level of theory using an isosurface value of 0.05 a.u.
uene, respectively, whereas the 2,6-distyryl-subsitituted BODIPY
1 has a much larger Stokes shift of 1590 cmꢀ1 (Table 1). The ge-
ometry of the potential energy surfaces involved in the relaxa-
tion of the S1 excited state after vertical excitation from the S0
state plays a vital role in determining the size of the Stokes
shift.[14,20] To rationalize the relative sizes of the Stokes shifts,
the geometries of the ground and S1 excited states of 1, 2a, 3,
and 4 were optimized by using the Gaussian software pack-
age.[16] The most prominent difference between the S0 and S1
geometries is the dihedral angle between the styryl and
BODIPY moieties. The dihedral angles in the ground state for
b-position styryl substituents are 44.3, 42.4, 37.9, 17.9, and 5.28
for 1, 2a, 2b, 3, and 4, respectively. In each case, the two moi-
eties are predicted to have a significantly greater level of co-
planarity in the S1 state. The dihedral angles are predicted to
be much larger in the S1 states of 1, 2a and 2b, however, com-
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large Stokes shift observed for
1 and 2a–c (Table 1), but also for
why no quenching of the fluo-
rescence of 3 and 4 has been re-
ported (Table 1), since there is
significantly less scope for coni-
cal intersections to form be-
tween
a charge-separated S1
state and the ground state when
the styryl and BODIPY moieties
effectively form a single near
planar p-conjugation system
(Table 4).
Although BODIPY dyes do not
formally fit Hꢃckel’s rule for aro-
maticity, their properties are
broadly similar to those of an ar-
omatic p-system, since the coor-
dination of the boron atom
holds the dipyrromethene ligand
in a rigidly planar conformation.
The p-MOs associated with the
indacene plane can be com-
pared to those of an aromatic
Figure 8. The frontier MO energies and HOMO–LUMO gaps of an unsubstituted meso-phenyl-BODIPY model com-
plex (5), 1, 2a–c, 3, 4 and a 3,5-disubstituted analogue of 2a (6a) calculated using the CAM-B3LYP functional with
6-31G(d) basis sets. Diamonds, circles, squares, and triangles are used to denote MOs that exhibit the same nodal
patterns as the frontier p-MOs of the parent BODIPY fluorophore.
Table 3. Calculated electronic excitation energies (<4.0 eV), oscillator strengths and the related wave functions
for 5, 1, 2a–c, 3, and 4 calculated by using the CAM-B3LYP functional with 6-31G(d) basis sets.
[a]
#
E[b]
l
f[c]
Wave function[d]
[eV]
[nm]
2ꢀ
C12H12 cyclic perimeter, which
5
S1
S2
S3
S4
S1
S2
S3
S1
S2
S3
S1
S2
S3
S4
3.14
3.97
4.19
4.30
2.62
3.47
3.55
2.50
3.18
3.44
2.62
3.34
3.54
3.92
397
314
297
290
475
359
351
499
392
362
475
373
352
317
0.43
0.09
0.00
0.29
1.10
0.05
0.22
1.22
0.04
0.31
1.00
0.15
0.10
0.11
H!L (95%), Hꢀ1!L (4%)
has MOs arranged in a ML =0, ꢁ
1, ꢁ2, ꢁ3, ꢁ4, ꢁ5, 6 sequence
in ascending energy terms
(Figure 10). The angular nodal
patterns for the p-MOs of
a BODIPY core follow a similar
sequence, but the introduction
of the BF2 moiety, the cross-links
and pyrrole-nitrogen atoms re-
sults in a marked lifting of the
degeneracies of the MO energies
due to the C2v symmetry. This re-
sults in a HOMO and a LUMO
that are well-separated in energy
terms from the other p-MOs.
Theoretical calculations predict
that the lowest-lying S0!S1 tran-
sition is associated almost 100%
with the HOMO!LUMO transi-
tion. The a2 MOs with a short-
axis nodal plane passing through
the boron atom are largely unaf-
fected in energy terms relative
Hꢀ1!L (95%), H!L (4%)
Hꢀ3!L (87%), Hꢀ2!L (11%)
Hꢀ2!L (86%), Hꢀ3!L (11%)
1
H!L (95%)
Hꢀ1!L (87%), Hꢀ6 (Hꢀ3)!L (10%)
Hꢀ2!L (87%), Hꢀ5 (Hꢀ1)!L (10%)
H!L (87%), Hꢀ2!L (8%)
2a
2b
Hꢀ1!L (85%), Hꢀ4 (Hꢀ3)!L (13%)
Hꢀ2!L (66%), Hꢀ3 (Hꢀ1)!L (29%)
H!L (82%), Hꢀ2!L (12%)
Hꢀ1!L (46%), Hꢀ2!L (31%), Hꢀ4 (Hꢀ3)!L (13%), H!L (6%)
Hꢀ1!L (40%), Hꢀ2!L (38%), Hꢀ3 (Hꢀ1)!L (10%), Hꢀ4 (Hꢀ2)!L (10%)
Hꢀ4 (Hꢀ3)!L (51%), Hꢀ3 (Hꢀ1)!L (17%), Hꢀ2!L (8%), Hꢀ1!L (7%),
Hꢀ8!L (6%)
2c
S1
S2
S3
S4
S5
S1
S2
S3
S1
S2
S3
S4
S5
2.83
3.55
3.59
3.87
4.08
2.51
3.04
3.18
2.37
3.75
3.80
4.08
4.11
441
350
347
321
305
497
409
392
525
332
327
305
303
0.84
0.02
0.07
0.05
0.03
1.01
0.00
0.38
1.00
0.04
0.50
0.02
1.31
H!L (72%), Hꢀ2!L (25%)
Hꢀ1!L (89%), Hꢀ4 (Hꢀ3)!L (9%)
Hꢀ2!L (60%), Hꢀ3 (Hꢀ1)!L (24%), H!L (15%)
Hꢀ3 (Hꢀ1)!L (68%), Hꢀ2!L (11%), H!L (12%), Hꢀ9!L (5%)
Hꢀ4 (Hꢀ2)!L (74%), Hꢀ8!L (17%), Hꢀ1!L (8%)
H!L (98%)
3
4
Hꢀ2!L (93%)
Hꢀ1!L (94%)
H!L (97%)
Hꢀ2 (Hꢀ1) ! L (65%), Hꢀ3!L (30%)
Hꢀ1!L (89%)
Hꢀ6!L (82%), H!L+1 (7%), Hꢀ4 (Hꢀ3)!L (5%)
H!L+1 (84%), Hꢀ6!L (5%)
[a] Excited state. [b] Energy of excited state. [c] Oscillator strength. [d] MOs involved in the transitions. One-
electron transitions between MOs associated primarily with the BODIPY p-system are highlighted in italics and
where necessary the corresponding MO of the parent meso-phenyl-BODIPY model compound 5 is provided in
parentheses. Only one-electron transitions that provide a contribution of greater than 5% are included.
to
a C12N2H10 heteroaromatic
model compound, whereas the
b2 MOs with nodal planes
aligned with the long-axis are
significantly stabilized due to
pared with those predicted for 3 and 4. This greater conforma-
tional flexibility is due to the steric interactions between the
styryl substituents and the methyl groups (Figure 9 and
Scheme 1). This provides an rationalization not only for the
stronger bonding interactions between the two pyrrole moiet-
ies, larger MO coefficients on the electronegative nitrogen
atoms, and the cross-linking between atoms on the outer pe-
rimeter due to the incorporation of the two pyrrole moieties.
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Figure 9. Optimized geometries of 3, 1, and 4 calculated with the 6-31G(d) basis set at the B3LYP level.
(Figure 7). This leads to a narrowing of the HOMO–LUMO gap
due to a significant destabilization of the HOMO (Figure 8) and
accounts for the marked redshift of the main BODIPY band of
4 to 629 nm. This band lies about 54 nm to the red of the cor-
responding bands in the spectra of 2,6- and 1,7-distyryl-substi-
tuted dyes, 1 and 3. It should be noted, however, that the rea-
sons for the redshift of the main absorption band of 4 relative
to that of 1 may also be related to the large dihedral angles
between the styryl moieties and the BODIPY core in the struc-
tures of 1 and 2a (Table 4), which limit the extent of conjuga-
tion across the entire molecule and lead to significant differen-
ces in the sizes of the Stokes shifts (Table 1).
Table 4. Dihedral angles for optimized geometric structures of 1, 2a, 2b,
3, and 4.
Dihedral angles[a] [8]
3
1
2a
2b
4
qGS
qES
17.9
5.6
44.3
31.6
42.4
33.0
37.9
22.6
5.2
0.0
[a] qGS is the average angle in the ground state between the two styryl
group and fluorophore core. qES is the average angle between the two
styryl group and the fluorophore core in the excited state. The dihedral
angles were calculated using the Mercury 2.3 program.[21]
From the standpoint of obtaining a redshift of the main ab-
sorption and emission spectral band, a structural modification
must alter the energies of the two frontier p-MOs in a manner
that narrows the HOMO–LUMO gaps. In the context of the
cyclic perimeters associated with heteroaromatic cyclic poly-
enes such as porphyrins, Michl has demonstrated that the in-
ductive effect of the substituent on the energies of the HOMO
and LUMO will usually be similar, but the mesomeric effects
can differ significantly.[22] Meso substituents have only a minor
effect on the energy of the HOMO, since there is a nodal plane
at this position, but changes to the para substituent on
a phenyl substituent can have a significant impact on the
energy of the LUMO, since there is a large MO coefficient on
the meso-carbon. The effects of substituents at the 1,7-, 2,6-
and 3,5-positions of the BODIPY structure can be rationalized
in a similar manner. There is a nodal plane near the 2,6-posi-
tions in the LUMO but not in the HOMO, while there are nodal
planes near the 1,7-positions in the HOMO but not in the
LUMO. Although there are no nodal planes, there are larger
MO coefficients at the 3,5-positions in the HOMO than in the
LUMO. In each case, therefore, there is scope for narrowing the
HOMO–LUMO gap based on the mesomeric effects of substitu-
ents when there is an extension of the p-conjugation system.
For example, substitution at the 3,5-positions results in a signifi-
cant extension of the p-conjugation system, since there are
significant MO coefficients across the entire structure
A relatively minor mesomeric effect would be predicted for
the styryl substituent based on the published Hammett param-
eters (sm =0.03, and sp =ꢀ0.07),[23] so only minor mesomeric
effects are expected for 1, 3 and 4, in which there is no elec-
tron-donating para substituents. The large torsion angle in the
structure of 1, therefore, probably does not have a significant
effect on the wavelength of the main absorption band of 1 rel-
ative to those of 3 and 4. Similar trends in the wavelengths of
the main spectral bands have been reported for phenylethyn-
yl-substituted compounds in which the torsion angles are sig-
nificantly smaller.[8,24] The structure of 2a is more problematic
from this standpoint, however, since a larger mesomeric effect
would be anticipated. B3LYP geometry optimizations and TD-
DFT calculations (Table 5) were carried out for a series model
complexes comprised of analogues of 1, 2a, 4, and 6a with no
methyl substituents, so that the effect of substitution position
on the energies of the frontier p-MOs could be further as-
sessed. As would be anticipated, substantial destabilizations of
the HOMO energies are predicted relative to that of the parent
meso-phenyl-BODIPY compound 5 (the Supporting Informa-
tion, Figure S5), due to the introduction of styryl substituents
at positions in which there are significant MO coefficients.
When the B3LYP functional is used, the mesomeric effect asso-
ciated with the styryl moieties is predicted to lead to a greater
narrowing of the HOMO–LUMO gap of 2a relative to 6a, and
hence a larger redshift in the wavelength predicted for the
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Figure 10. MO energies and angular nodal patterns of a C12H122ꢀ cyclic perimeter, BODIPY and a C12N2H10 model compound (consisting of two pyrrole ring
moieties linked by two sp2 hybridized carbon atoms). It is noteworthy that the nodal patterns are broadly similar in each case and that the rigid planarity of
the BODIPY p system due to coordination by the BF2 moiety results in sets of MO coefficients of comparable magnitude on the two pyrrole moieties at the
3,5-, 2,6- and 1,7-positions, due to the two-fold symmetry axis of the BODIPY chromophore.
main absorption band. This suggests that in the absence of
the tilting of the styryl moiety due to steric effects (Table 4),
the incorporation of para substituents at the 2,6-positions,
which introduce strongly electron-donating or -withdrawing
mesomeric interactions with the BODIPY p-system may prove
to be a better strategy for fine-tuning the magnitude of the
redshift of the main spectral bands over a wide range of wave-
lengths than substitution at the 3,5-positions. The presence of
a nodal plane near these positions in the LUMO (Figure 11),
means that mesomeric effects are insignificant where the
energy of the LUMO is concerned, resulting in an enhanced
narrowing of the HOMO–LUMO gap on moving from 1 to 2a
relative to moving from 4 to 6a, since there is a smaller desta-
bilization of the LUMO. Although these trends are not present
in calculations with the CAM-B3LYP functional (Table 5 and Fig-
ure S5 in the Supporting Information), it should be noted that
the CAM-B3LYP functional calculations underestimate the
wavelength range spanned by 2a–c relative to the experimen-
tal values and those predicted by the B3LYP functional
(Table 2), so they may underestimate the importance of the
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thesis by the Heck reaction proved to be problematic. The
most likely rationalization is that the CꢀH bonds of the pyrrole
rings also participate in the reaction resulting in a complex
mixture of products. Efforts are underway to identify a protocol
that would enable the isolation of significant yields of 2,6-dis-
tyryl-meso-aryl-BODIPYs.
Table 5. Predicted wavelengths in TD-DFT calculations with the B3LYP
and CAM-B3LYP functionals for the main absorption bands of 1, 2a–c, 4,
meso-phenyl-BODIPY (5), and 1-planar, 2a-planar, 4-planar, and 6a-planar
model compounds that have no methyl substituents but are otherwise
identical to 1, 2a, 4, and 6a, both in the gas phase and dichloromethane
using the polarizable continuum model (PCM).
Conclusion
A new family of 2,6-distyryl-substituted BODIPY dyes has been
synthesized and characterized by X-ray crystallography and
1H NMR spectroscopy. The similar absorption band and red-
shifts of emission bands were found to be comparable to
those observed in the spectra of the analogous 1,7-disubstitut-
ed compound 3, but significantly smaller than those observed
for the main absorption and emission bands of the 3,5-disub-
stituted compound 4. The photophysical properties of the
three new p-dimethylamino-substituted isomers 2a–c have
been analyzed by carrying out a series of theoretical calcula-
tions. Although the fluorescence quantum yields of 2a,b are
negligible, a strong solvent polarity dependence is observed in
the quantum yields of 2c, which has only a-position styryl sub-
stituents. Compound 2c is, therefore, potentially suitable for
use as a turn-on fluorescence sensor in this regard. There is
strong pH dependence in the absorption and emission bands
of all three isomers, which could also lead to applications as
sensors. Theoretical calculations provide strong evidence that
styryl rotation and the formation of non-emissive charge-sepa-
rated S1 states play a pivotal role in shaping the weak fluores-
cence properties of 1 and 2a,b. MO theory has been used as
a conceptual framework to describe the electronic structures
of the BODIPY core. The markedly differing effects of substitu-
tion at different positions on the BODIPY core can be readily
accounted for through a comparison of the angular nodal pat-
terns of the frontier p-MOs of the BODIPY core. Calculations
with model compounds demonstrate that results derived from
TD-DFT studies need to be treated with caution when the
main absorption band has significant charge-transfer character
due to the extension of the p system with styryl substituents.
l [nm]
5
1-planar 2a-planar 4-planar 6a-planar
B3LYP (PCM)
B3LYP
CAM-B3LYP (PCM)
CAM-B3LYP
411 619
402 611
409 514
397 507
804
744
568
552
593
563
554
527
704
639
612
570
Experimental Section
Materials and instrumentations
All reagents were obtained from commercial suppliers and used
without further purification unless otherwise indicated. All air- and
moisture-sensitive reactions were carried out under a nitrogen at-
mosphere. Glassware was dried in an oven at 1008C and cooled
under a stream of inert gas before use. Dichloromethane and trie-
thylamine were distilled over calcium hydride. Dry THF was distilled
from sodium metal using benzophenone as an indicator under a ni-
trogen atmosphere. 1H NMR spectra were recorded on a Bruker
DRX400 spectrometer and referenced to the residual proton sig-
nals of the solvent. HR-MS were recorded on a Bruker Daltonics mi-
croTOF-Q II spectrometer. Mass spectra were measured with
a Bruker Daltonics AutoflexII TM MALDI-TOF spectrometer. Melting
points was measured on a WRS-1 A Melting-Point Apparatus. The
melting points of all dyes exceed 2008C.
Figure 11. Nodal patterns of the HOMO and LUMO of an unsubstituted
BODIPY model compound at an isosurface of 0.07 a.u.
charge transfer character arising from the mesomeric effect as-
sociated with the p-dimethylamino substituents. The prepara-
tion of 2,6-distyryl compounds without methyl substituents
was attempted to explore this question further, but their syn-
Chem. Eur. J. 2014, 20, 1091 – 1102
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Synthesis
X-ray structure determination
Compound 1: 2,6-diiodo-BODIPY (57.6 mg, 0.10 mmol) was dis-
solved in DMF (10 mL), and triethylamine (0.4 mmol, 56 mL), styrene
(0.30 mmol, 30 mL), and [Pd(OAc)2] (10 mmol, 2 mg) were then
added. The mixture was stirred under a nitrogen atmosphere at
658C until the reaction was complete. The reaction mixture was
poured into dichloromethane (50 mL) and washed with water. The
organic layer was dried over MgSO4, filtered, and evaporated to
dryness. The crude product was purified by silica-gel flash column
chromatography (60% Hexane/CH2Cl2) and recrystallized from
The X-ray diffraction data were collected on a Bruker Smart Apex
CCD diffractometer with graphite monochromated MoKa radiation
(l=0.71073 ꢁ) using the w-2q scan mode. The structure was
solved by direct methods and refined on F2 by full-matrix least-
squares methods using the SHELX-2000 program.[26] All calculations
and molecular graphics were carried out using the SHELX-2000
and Diamond program packages.
Compound 2b: C39H41BF2N4; a purple block-like crystal with ap-
proximate 0.13ꢂ0.14ꢂ0.15 mm3 dimensions was selected for mea-
surement. Space group P21/n, a=10.4511(11) ꢁ, b=15.7350(16) ꢁ,
c=20.686(2) ꢁ, a=908, b=91.512(2)8, g=908, V=3400.6(6) ꢁ3,
Z=4, F(000)=1304.0, 1=1.200 Mgmꢀ3, R1 =0.0504, wR2 =0.1808,
GOF=1.038, residual electron density between 0.264 and
ꢀ0.274 eꢁꢀ3. CCDC-951445 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
1
CH2Cl2/hexane to provide 1 as red crystals (21.6 mg, 41%). H NMR
(400 MHz, CDCl3): d=7.54–7.52 (m, 3H) 7.44 (d, J=7.6 Hz, 4H),
7.36–7.32 (m, 6H), 7.24–7.23 (m, 2H), 6.88 (d, J=16.4 Hz, 2H), 6.65
(d, J=16.8 Hz, 2H), 2.73 (s, 6H), 1.47 ppm (s, 6H); UV/Vis (CH2Cl2):
lmax (e)=575 nm (31000mꢀ1 cmꢀ1); MALDI-TOF: m/z calcd: 528.3
[C35H31BF2N2]+; found: 527.9 [M]+, 508.8 [MꢀF]+; HRMS: m/z calcd
for C35H31BF2N2: 528.2542; found: 528.2542 [M]+
C
.
Compounds 2a–c: Compounds 2a, 2b, and 2c were obtained by
following a procedure similar to that of 1. Compound 2a was ob-
1
tained in 9% yield as light-blue crystals. H NMR (400 MHz, CDCl3):
Computational details
d=7.52–7.49 (m, 3H), 7.34–7.32 (m, 6H), 6.81–6.72 (m, 4H), 6.67
(d, J=16.8 Hz, 2H), 6.56 (d, J=16.4 Hz, 2H), 2.98 (s, 12H), 2.71 (s,
6H), 1.45 ppm (s, 6H); UV/Vis (CH2Cl2): lmax (e)=616 nm (58
900mꢀ1 cmꢀ1). Compound 2b was obtained as purple crystals in
Geometry optimization calculations were carried out on the sun
cluster at the Centre for High Performance Computing in Cape
Town, South Africa by using the B3LYP functional of the Gaussi-
an 09 software package[16] with 6–31G(d) basis sets. The B3LYP and
CAM-B3LYP functionals were used with 6–31G(d) basis sets during
TD-DFT calculations of the B3LYP geometries. The calculations were
made both in the gas phase and in dichloromethane using the po-
larizable continuum model (PCM). CIS calculations were carried out
to optimize the geometry of the S1 states.
1
31% yield. H NMR (400 MHz, CDCl3): d=7.48–7.47 (m, 3H), 7.35–
7.33 (m, 4H), 7.21–7.18 (m, 2H), 6.70–6.55 (m, 6H), 5.69 (s, 1H),
4.89 (s, 1H), 2.98 (s, 6H), 2.95 (s, 6H), 2.72 (s, 3H), 2.40 (s, 3H), 1.46
(s, 3H), 1.21 ppm (s, 3H); UV/Vis (CH2Cl2): lmax (e)=570 nm (54
000mꢀ1 cmꢀ1). 2c was obtained as red crystals in 14% yield.
1H NMR (400 MHz, CDCl3): d=7.45–7.44 (m, 3H), 7.35–7.34 (m, 2H),
7.20–7.19 (m, 4H), 6.64 (brs, 4H), 5.71 (d, J=0.8 Hz, 2H), 4.89 (d,
J=0.8 Hz, 2H), 2.96 (s, 12H), 2.40 (s, 6H), 1.22 ppm (s, 6H); UV/Vis
(CH2Cl2): lmax (e)=529 nm (49500mꢀ1 cmꢀ1); MALDI-TOF: m/z calcd: Acknowledgements
614 [C39H41BF2N4]+; found: 614 [M]+, 595 [MꢀF]+; HRMS: m/z calcd
for C39H41BF2N4: 615.3465 [M+H]+; found 614.3402 [M]+ (for 2a),
C
We are thankful to the NSFC (nos. 21101049 and 21021062) for
their financial support.
615.3418 [M+H]+ (for 2b), 637.3235 [M+Na]+, 615.3404 [M+H]+,
and 595.3361 [MꢀF]+ (for 2c).
Keywords: density functional calculations · dyes/pigments ·
fluorescence · NMR spectroscopy · photophysics · synthetic
methods
Spectroscopic measurements
UV/Visible absorption spectra were recorded on a Shimadzu UV-
2550 spectrophotometer, whereas fluorescence spectra were mea-
sured on a Hitachi F-2700 FL spectrophotometer equipped with
a xenon arc lamp as the light source. The fluorescence lifetimes of
the sample were measured with a Horiba Jobin Yvon Fluorolog-3
spectrofluorometer. Samples for absorption and emission measure-
ments were contained in 1ꢂ1 cm quartz cuvettes. For all measure-
ments, the temperature was kept constant at (298ꢁ2) K. Dilute
solutions with absorbance of less than 0.05 at the excitation wave-
length were used for the measurement of fluorescence quantum
yields. Rhodamine 6G was used as the standard (FF =0.88 in etha-
nol).[25] The quantum yield (F) was calculated by using Equa-
tion (1):
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ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Isample
Istd
nsample
nstd
Astd
Asample
ð1Þ
Fsample ¼ Fstd
in which the “sample” and “std” subscripts denote the sample and
standard, respectively, I is the integrated emission intensity, A
stands for the absorbance, and n is refractive index.
Chem. Eur. J. 2014, 20, 1091 – 1102
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Received: August 22, 2013
Published online on December 16, 2013
Chem. Eur. J. 2014, 20, 1091 – 1102
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