Synthesis and investigation of BODIPYs with restricted meso -8-aryl rotation

Article abstract of DOI:10.1142/S1088424619501967

Three BODIPYs bearing 1,3,5,7-tetramethyl substituents and a meso-8-aryl group were synthesized and investigated, both experimentally and computationally. The presence of the 1,7-methyl groups and of ortho-substituents on the meso-8-aryl ring prevent free

Full text of DOI:10.1142/S1088424619501967

This is a JAM - Just Accepted Manuscript
Journal of Porphyrins and Phthalocyanines
Article Title:
Synthesis and Investigation of BODIPYs with Restricted meso-8-Aryl
Rotation
Author(s):
DOI:
Guanyu Zhang, Maodie Wang, Caroline Ndung'U, Petia Bobadova-
Parvanova, Frank R. Fronczek, Kevin M. Smith, M. Graca H. Vicente
10.1142/S1088424619501967
To be cited as:
Guanyu Zhang, Maodie Wang, Caroline Ndung'U, Petia Bobadova-Parvanova, Frank R.
Fronczek, Kevin M. Smith, M. Graca H. Vicente, Synthesis and Investigation of BODIPYs with
Restricted meso-8-Aryl Rotation, Journal of Porphyrins and Phthalocyanines, doi:
1
0.1142/S1088424619501967
Received: 15 October, 2019
Accepted: 9 December, 2019
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Synthesis and Investigation of BODIPYs with Restricted meso-8-Aryl Rotation
†
†
†
‡,
Guanyu Zhang, Maodie Wang, Caroline Ndung'U, Petia Bobadova-Parvanova, * Frank R.
†
†
†,
Fronczek, Kevin M. Smith, and M. Graça H. Vicente *
†
Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
‡
Department of Chemistry, Rockhurst University, Kansas City, MO 64110, USA
Received
Accepted
Dedicated to Professor Roberto Paolesse on the occasion of his 60th birthday
ABSTRACT: Three BODIPYs bearing 1,3,5,7-tetramethyl substituents and a meso-8-aryl
group were synthesized and investigated, both experimentally and computationally. The
presence of the 1,7-methyl groups and of ortho-substituents on the meso-8-aryl ring prevent
free rotation of the meso-8-aryl group, resulting in high fluorescence quantum yields.
Substitution at the 2,6-positions of these BODIPYs with chlorine atoms causes pronounced
red-shifted absorptions and emissions, and in the case of 2,6-dichloro-1,3,5,7-tetramethyl-8-
(2,4,6-triphenylphenyl)-BODIPY 2c increases its fluorescence quantum yields to 0.93 in
dichloromethane and 0.98 in toluene. The X-ray structure of 1,3,5,7-tetramethyl-8-(2,4,6-
triphenylphenyl)-BODIPY shows increase deviation from planarity and smaller dihedral angle
of the meso-8-aryl group compared with the meso-8-phenyl- and meso-8-mesityl-BODIPY
analogs.
KEYWORDS: BODIPY, aryl-rotation, fluorescence
*Correspondence to: M. Graça H. Vicente, email: vicente@lsu.edu, tel: +225-578-7405

Accepted manuscript to appear in JPP
INTRODUCTION
,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (known as boron dipyrromethene or BODIPY) is an
4
important class of fluorophore with multiple applications in fluorescence imaging due to their excellent
spectroscopic properties, including large molar extinction coefficients, high fluorescent quantum
yields, and high photostability [1-10]. The rapid increase in research interest of this type of dye has
also arisen from its highly tunable physicochemical properties, that have resulted in the synthesis of
BODIPY derivatives with a broad range of absorption and emission wavelengths. The modification of
BODIPY has been explored by introducing various functional groups at the periphery of the dye, on
both the carbon and boron atoms, using for example, electrophilic or nucleophilic substitution reactions
[11-15], metal-mediated cross-couplings [16-17], and Knoevenagel condensations [18-19]. In
particular, functionalization at the meso-8-position has been achieved by Pd(0)-catalyzed cross-
coupling reactions on 8-halogenated BODIPYs [17, 20-21]. The presence of an aryl group at the meso-
8-position of the BODIPY usually has no significant effect on the absorption and emission
wavelengths, due to the large dihedral angle it forms with the BODIPY core to minimize steric
interactions with the 1,7-hydrogen atoms, but it can provide dyes with decreased fluorescence quantum
yields limiting their application in fluorescence imaging. This problem is largely attributed to the free
rotation of the meso-aryl group, which reduces the loss of energy from the excited states via non-
irradiative decay [20, 22]. To solve this problem, BODIPYs bearing meso-8-phenyl groups with ortho
substituents (for example a mesityl group) and/or methyl groups at the 1,7-positions of the BODIPY
have been synthesized to reduce the rotation of the meso-aryl group, showing restored emission, as
shown in Figure 1 [20, 22-23]. On the other hand, this drawback has also been utilized in designing
off-on BODIPY fluorescent probes for viscosity or temperature in biological research through rotation-
restriction induced emission enhancement mechanisms [24-26]. Herein, we report the synthesis and
investigation of new sterically hindered BODIPYs bearing bulky meso-8-(2,4,6-triphenylphenyl)
groups and 1,7-methyl substituents with restricted rotation of the meso-aryl group. The spectroscopic
properties of these BODIPYs were investigated and compared with those of the meso-8-phenyl- and
meso-8-mesityl-BODIPY derivatives, both experimentally and computationally.
RESULTS AND DISCUSSION
Synthesis

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The synthetic routes to BODIPYs 1a-c and 2a-c are shown in Scheme 1. All BODIPYs were
prepared, in 20-40% overall yields, by condensation of 2,4-dimethylpyrrole with the corresponding
aryl aldehyde in the presence of trifluoroacetic acid (TFA), followed by oxidation and boron
complexation using boron trifluoride dietherate under basic conditions [27]. Treatment of BODIPYs
1a-c with 0.75 equivalent of trichlorocyanuric acid (TCCA) [17, 28-29] produced the corresponding
2,6-dichloro-BODIPYs 2a-c in 72-76% yields. The chlorination of BODIPYs 1a-c was cleary indicated
1
by H-NMR, due to the disappearance of the 2,6-hydrogen atoms at ca. 5.9 ppm. The structures of all
1
13
11
BODIPYs were characterized by high resolution mass spectrometry and by H- C- and B-NMR
specroscopy (see Supporting Information, Figures S1-S9).
X-Ray analysis
A crystal of BODIPY 1c suitable for X-ray analysis was obtained by slow evaporation of
dichloromethane and pentane, and the result is shown in Figure 2. The 12-atom BODIPY core is
slightly nonplanar, having a mean deviation of 0.058 Å and a slight twist about its longitudinal axis,
such that the four methyl groups lie alternately above and below the plane by an average of 0.26 Å. The
3 2
six atoms of the central C N B core of the BODIPY ring have a mean deviation from their best plane
of 0.040 Å. The meso-phenyl ring forms a dihedral angle of 74.7˚ with the six-atom central BODIPY
core. This dihedral angle is slightly smaller than the more nearly orthogonal ones seen in 1a [16], 81.8˚
and 1b [30], 85.5˚. The performed computational studies confirm this effect; the dihedral angles in 1a
3 2
and 1b are 90.0˚ and 90.1˚, respectively, while that of 1c is smaller at 79.8˚. The central C N B rings of
1a and 1b are also more planar than those of 1c, both with mean deviations of 0.021 Å.
Spectroscopic properties
The spectroscopic properties for BODIPYs 1a-c and 2a-c in were investigated in dichloromethane,
toluene and in a protic solvent (methanol), and the results obtained are summarized in Table 1 and
shown in Figure 3 (see also the Supporting Information, Figures S10 and S11). All BODIPYs exhibited
a typical BODIPY absorption profile, characterized by a strong S0 → S1 transition peak centered at
around 500 nm for 1a,b and 528 nm for 2a,b, along with a blue-shifted shoulder representing the
vibrational transitions. BODIPYs 1c and 2c showed about 10 nm red-shifted absorptions relative to
1a,b and 2a,b, respectively, due to their lower HOMO-LUMO energy gap (see Figure 4). The 2,6-
dichlorinated BODIPYs 2a-c displayed significant red-shifted absorptions and emissions, by 21-27 nm,
with slightly increased Stokes shift by 3-5 nm relative to BODIPYs 1a-c, as previously observed for
other 2,6-dichloro substituted BODIPYs [17, 29, 31]. This is attributted to the electron-withdrawing

Accepted manuscript to appear in JPP
effect of the electronegative chlorine atoms, which removes electron density from the BODIPY core.
Using the more polar solvent methanol instead of dichloromethane results in small (3-4 nm) blue
(hypsochromic) shifts in both the maximum absorption wavelength and maximum emission
wavelength, consistent with previous studies [32-33].
As expected, BODIPYs 1b and 1c bearing sterically hindered meso-aryl groups showed higher
fluorescence quantum yields compared with BODIPY 1a due to increased restricted rotation of their
meso-aryl groups compared with phenyl [22-23, 30]. On the other hand, BODIPY 1c displayed lower
fluorescence quantum yields than BODIPY 1b probably due to additional vibration freedom of 1c
compared with 1b, as suggested by the zero-point vibrational enegy calculations (see below).
Chlorination of the 2,6-positions increased the fluorescence quantum yields of BODIPYs 2a-c relative
to 1a-c [17, 29, 31], with exception of BODIPY 1b which showed very high quantum yields in the
absence of the 2,6-chlorines and slight lower yields in the dichlorinated derivative. The electron-
withdrawing effect of the chlorine atoms in general increases the fluorescence quantum yields due to
the decrease in electronic charge density on the BODIPY, particularly at the meso-position [31]. In
addition, dichlorinated BODIPY 2c showed very high fluorescence quantum yields in dichloromethane
and toluene, but moderate in methanol, likely due to its lower solubility in this solvent (although
slightly higher than 1c). These results are in agreement with previous reports showing that the
nonradiative rate constant decreases significantly with each chlorine atom introduced on the BODIPY
[31].
Computational studies
To complement the experimental characterization, all BODIPYs were modeled computationally. The
energies and shapes of the calculated frontier molecular orbitals in dichloromethane are presented in
Figure 4. The energy gap is also given. The shapes of HOMO and LUMO are similar in the entire
series with little electron density located on the meso-substituent. The HOMOs of the 2,6-dichlorinated
BODIPYs show electron density on the chlorine atoms that correspond to chlorine lone pairs. The
orbital energies differ only slightly but are able to explain several experimental trends as described
below.
Using dichloromethane as a solvent, the calculations confirm the experimentally observed red-shifts
of BODIPYs 2a-c as compared to BODIPYs 1a-c (Table 1). As expected, the electron-withdrawing
chlorine atoms at the 2,6-positions stabilize both the HOMO and LUMO, but the effect on the LUMO
is more pronounced. As a result, the HOMO-LUMO gaps for 2a-c are about 0.1 eV smaller than those

Accepted manuscript to appear in JPP
for 1a-c, which explains the observed red-shifts of the absorption maxima wavelenghts. The observed
red shifts and HOMO-LUMO stabilizations are in agreement with previous studies [29, 31].
The effect of the meso-aryl substituion on orbital energies is very small. Still, the predicted red-shift
from 1a to 1b to 1c and 2a to 2b to 2c confirms the experimental findings and is in agreement with the
slightly decreased band gap, as seen in Figure 4.
Using methanol instead of dichloromethane as the solvent, stabilizes both HOMO and LUMO
(Figure 4). The effect on both orbitals is very similar but results in a slight increase in the HOMO-
LUMO gap. This is in agreement with the observed small (3-4 nm) hypsochromic shifts in the
maximum absorption wavelengths in methanol versus dichloromethane (Table 1). In general, the
solvatochromic shifts are complex in BODIPYs, and could involve contribution from polarity,
polarizability, H-bond formation abilities, and electron-donor capacities of the solvent [32, 34].
The performed calculations suggest slight lowering of the oscillator strenghs from 1a to 1b to 1c and
2a to 2b to 2c, which is in agreement with the experimental molar absorptivities. The same trend is
predicted for the fluorescence abilities of the synthesized BODIPYs. Indeed, experimentally, 1c is less
fluorescent than 1b in dichloromethane and toluene. On the other hand, the oscillator strengths are
smaller in methanol than in dichloromethane, which might explain the lower fluorescence quantum
yields observed for 1a,b and 2a-c in methanol. Polar solvents such as methanol are expected to
decrease the fluorescence quantum yields due to increased solute-solvent interactions and higher
probability for external conversion. Indeed, all BODIPYs from the series are polar with dipole
moments ranging from 5.9 to 6.8 D (Table 1).
The lower fluorescence quantum yield observed for BODIPY 1c relative to 1b (Table 1) might be
explained with the increased vibrational freedom of the larger size molecule. Indeed, zero-point
vibrational energies increase from 0.3437 a.u. for 1a, to 0.4263 for 1b and 0.5859 a.u. for 1c. As
discussed above, restricting the rotational and vibrational freedom of the meso-aryl substituent can play
a substantial role in the observed fluorescence quantum yield due to decreased probability for non-
radiative decay. The calculated rotational barrier for BODIPY 1a is 24.7 kcal/mol and involves
distortion of the planarity of the BODIPY core (Figure 5). 2,6-Dichloro substitution does not change
the barrier significantly; it is calculated to be 23.5 kcal/mol for BODIPY 2a. Without the distortion
(keeping the BODIPY core planar) the barrier for 1a is high at 46 kcal/mol, so likely distortion occurs
upon rotation, since the distortion of the BODIPY core allows easier rotation of the phenyl group.
Rotation of around 50° keeps the core planar but a distortion is observed after that and increases as the
dihedral angle increases. Room temperature kinetic energy is enough to overcome barriers of roughly
20 kcal/mol. Therefore, the meso-phenyl group of BODIPY 1a most likely does not rotate completely

Accepted manuscript to appear in JPP
at room temperature. However, a partial rotation to 80-85° is possible, in agreement with the dihedral
angle observed by X-ray crystallography [16]. Since rotation of the meso-aryl group significantly
influences the extent of the π-electron conjugation system [23], this rotational freedom is associated
with lower quantum yields and likely explains the lower values observed for 1a in comparison with 1b
and 1c. The presence of bulkier meso-8-aryl substituents, as in BODIPYs 1b and 1c makes the rotation
more difficult, as demonstrated with the increased rotational barriers of 47.0 and more than 60
kcal/mol, respectively. Again, full rotation is not possible at room temperature, however, partial
rotation at around 60-65° for 1b and at around 50-55° for 1c can be accomplished. The BODIPY core
distortion starts around dihedral angle of 50° for 1b and 40° for 1c. Interestingly, the newly-
synthesized BODIPY 1c excibits an “umbrella-like” distortion as a result of the rotation. As the meso-
8-(2,4,6-triphenylphenyl) group rotates, the 2,4,6-phenyls move up (away from the BODIPY core) to
avoid the steric interaction. The calculated potential energy surface shows that increasing the dihedral
angle from -80 to +60° still does not reach the energy maximum.
Previous studies [31] have suggested that the removal of electron density from the BODIPY core by
electron-withdrawing substituents should be “beneficial for the fluorescence capabilites of the
chlorinated derivatives because it should significantly reduce the deleterous influence of the phenyl
free rotation on their photophysics”. However, our studies of the potential energy surfaces of BODIPYs
1a-2c (see Figure 5) demonstrate that the 2,6-dichloro substituion does not significanly affect the
rotational barrier.
EXPERIMENTAL
1. Synthesis
General: All commercially available reagents and solvents were purchased and directly used
without purification. The reactions were monitored by analytical thin-layer chromatography using
plastic backed TLC plates 254 (precoated, 200 µm) from Sorbent Technologies Co. Column
chromatography was performed on silica gel 60 (230 × 400 mesh). NMR characterization of all
1
11
products were conducted on an AVIII-400-3 Bruker spectrometer (400 MH for H, 128 MHz for B)
1
3
and an AV500-PRODIGY Bruker spectrometer (126 MHz for C). Chemical shifts were recorded in
1
13
1
13
δ (ppm) relative to CDCl
3
7.26 ppm ( H), 77.0 ppm ( C); CD
0.00 ppm ( B). BODIPYs 1a [27, 35], 1b [30, 36-37], and 2a [31, 38] were prepared as
previously reported.
2 2
Cl 5.30 ppm ( H), 53.4 ppm ( C) and
1
1
3 2
BF OEt

Accepted manuscript to appear in JPP
BODIPY 1c: To a mixture of 2,4,6-triphenylbenzaldehyde (100 mg, 0.3 mmol) and 2,4-
dimethylpyrrole (68 mg, 0.72 mmol) dissolved in dichloromethane (15 mL), was added a catalytic
amount of TFA under argon and the final solution was stirred at room temperature for 8 h. 2,3-
Dichloro-5,6-dicyano-1,4-benzoquinone or DDQ (68 mg, 0.3 mmol) was then added to the mixture in
small portions. After 2 h stirring, triethylamine (0.63 ml, 4.5 mmol) and BF3•OEt
were added and the mixture left to react for 12 h before quenching with sat. NaHCO
organic phase was washed with water and brine, dried over anhydrous Na SO , and the solvent
2
(0.44 ml, 4.5 mmol)
3
solution. The
2
4
evaporated under vacuum. The crude product was purified by silica gel column chromatography using
dichloromethane/hexane (1:1) as eluent. The title product was dried under vacuum for 2 h and obtained
1
in 24% (40 mg) yield. H NMR (400 MHz, CD
2
Cl
2
) δ 7.89 – 7.68 (m, 4H), 7.49 (t, J = 7.5 Hz, 2H),
1
3
7
.41 (t, J = 7.3 Hz, 1H), 7.34 – 7.14 (m, 10H), 5.91 (s, 2H), 2.39 (s, 6H), 1.68 (s, 6H). C NMR (126
MHz, CD Cl ) δ 155.03, 142.13, 141.87, 141.11, 140.21, 139.64, 139.41, 132.84, 130.16, 129.08,
2
28.93, 128.51, 128.01, 127.53, 127.05, 121.07, 14.28, 13.87. B NMR (128 MHz, CD ) δ 0.31 (t,
2
2
1
1
1
2
Cl
+
J = 32.8 Hz). MS (MALDI- TOF) m/z [M-F] 415.13235, calcd for C22H23BCl FN , 415.13139.
2 2
BODIPY 2b: To a stirred dichloromethane solution (5 ml) of BODIPY 2a (20 mg, 0.054 mmol),
was added trichlorocyanuric acid (9.5 mg, 0.041 mmol) and the color of the solution changed
immediately. The reaction was quenched with sat. NaHCO
starting material to the desired product. The organic phase was washed with water and brine and dried
over anhydrous Na SO . The organic solvent was removed under reduced pressure and the resulting
3
after TLC showed the full conversion of
2
4
residue was purified by silica gel column chromatography using dichloromethane/hexane (1:4) for
1
elution. The title product was dried under vacuum and obtained in 76% (18 mg) yield. H NMR (400
1
3
MHz, CDCl
3
) δ 6.98 (s, 2H), 2.58 (s, 6H), 2.36 (s, 3H), 2.08 (s, 6H), 1.38 (s, 6H). C NMR (126 MHz,
CDCl ) δ 152.19, 142.67, 139.26, 137.21, 134.82, 130.33, 129.24, 128.86, 122.24, 21.24, 19.48, 12.43,
3
1
1
+
1
0.83. B NMR (128 MHz, CDCl
3
) δ 0.35 (t, J = 31.9 Hz). MS (ESI- TOF) m/z [M+H] 552.26327,
calcd for C37 , 552.26275.
2 2
H31BF N
BODIPY 2c: This compound was synthesized as described above using BODIPY 1c (20 mg, 0.036
mmol) and trichlorocyanuric acid (6.3 mg, 0.027 mmol) to afford the title product in 72% (16 mg)
1
yield. H NMR (400 MHz, CD
2
Cl
2
) δ 7.79 (d, J = 8.8 Hz, 4H), 7.51 (t, J = 7.5 Hz, 2H), 7.43 (t, J = 7.3
1
3
Hz, 1H), 7.36 – 7.11 (m, 10H), 2.43 (s, 6H), 1.68 (s, 6H). C NMR (126 MHz, CD
2
2
Cl ) δ 151.96,
1
1
42.65, 141.21, 140.68, 139.81, 139.38, 136.74, 131.02, 129.30, 129.24, 129.00, 128.40, 128.22,
1
1
3
27.77, 127.11, 122.40, 12.16, 11.55. B NMR (128 MHz, CDCl ) δ -0.02 (t, J = 31.7 Hz). MS (ESI-
+
TOF) m/z [M-F] 601.18141, calcd for C37
H29BCl
2
F
2
N
2
, 601.17858.

Accepted manuscript to appear in JPP
2
. Spectroscopic properties
All absorption spectra were measured on a Varian Cary 50 spectrophotometer. Emission spectra
were recorded on a PerkinElmer LS55 spectrophotometer. The measurement was carried on using
HPLC-grade CH Cl and toluene and quartz cuvettes (10 mm path length) at room temperature. The
2
2
molar extinction coefficients was determined from the plots of absorbance (A = 0.05−0.8) vs
concentration. Dilute solution (0.01<A<0.05) was used to determine the relative fluorescence quantum
yields (Ф
f f f
). Fluorescein (Ф = 0.90 in 0.1 M NaOH) and rhodamine 6G (Ф = 0.94 in anhydrous
ethanol) were employed as the external reference.
3. X-ray Crystallographic data
Diffraction data for 1c were collected at T=90K on a Bruker Kappa Apex-II DUO diffractometer
equipped with a CuKα microfocus source (λ=1.54184 Å). Refinement was by full-matrix least squares,
with H atoms in idealized positions, guided by difference maps. Crystal data: C37 , monoclinic
space group P2 /c, a=11.082(3), b=14.305(5), c=18.404(6) Å, β=102.01(2)˚, Z=4, ρcalcd=1.286 g cm ,
µ(CuKα)=0.66 mm . A total of 30,118 data was collected at θmax=60.2˚. R=0.080 for 1973 data with
H
2 2
31BF N
-
3
1
-
1
2
2
o o
F >2σ(F ) of 4234 unique data and 383 refined parameters, CCDC 1957273.
4. Computational studies
The geometries of all BODIPYs were optimized at the B3LYP/6-31G(d,p) level [39-40] without
symmetry constraints. The stationary points on the potential energy surface were confirmed with
frequency calculations. The solvent effects were taken into account using the Polarized Continuum
Model (PCM) [41]. The UV-vis absorption data were calculated using the TD-DFT method [42] at the
M06-2X/6-31+G(d,p) level [43] as recommended in the literature [44]. All calculations were
performed using the Gaussian 09 program package [45]. The meso-aryl rotational energies were
obtained by scanning the aryl dihedral angle and optimizing the remaining degrees of freedom using
B3LYP/6-31+G(d,p) level and redundant internal coordinates.
CONCLUSIONS
A novel 1,3,5,7-tetramethyl-8-(2,4,6-triphenylphenyl)-BODIPY 1c and its 2,6-dichloro analog 2c
were synthesized and their spectroscopic properties compared with those of meso-8-phenyl, meso-8-

Accepted manuscript to appear in JPP
mesityl and their 2,6-dichloro BODIPY derivatives. The structures of all the BODIPYs were confirmed
1
11
13
by H, B and C-NMR spectroscopy, and by high-resolution mass spectrometry. In addition, for the
new BODIPY 1c the X-ray structure was obtained, showing this molecule to be more distorted from
planarity than 1a and 1b, and bearing a smaller meso-aryl dihedral angle of ca. 75˚. The spectroscopic
properties of the BODIPYs were conducted in dichloromethane, toluene, and methanol solutions. The
introduction of 2,6-dichloro substituents on BODIPYs 1a-c stabilizes the HOMO and LUMO, causing
about 25 nm red-shifted absorptions and emissions of BODIPYs 2a-c, while reducing the molar
absortivities. On the other hand, 2,6-dichloro substitution tends to increase the fluorescence quantum
yields, particularly in the case of new BODIPY 2c, which displayed the highest quantum yields in
dichloromethane and toluene of all BODIPYs investigated.
ACKNOWLEDGEMENTS
This work was supported by the National Science Foundation (CHE-1800126). The authors thank
the Louisiana Optical Network Initiative (www.loni.org) for the use of their computer facilities.
SUPPORTING INFORMATION
1
13
11
Supporting Information includes H-, C- and B-NMR spectra, UV-vis and fluorescence emission
spectra for BODIPYs, and optimized cartezian coordinates for the ground and excited states of 1c and
2c. Crystallographic results in CIF format have been deposited with the Cambridge Crystallographic
Data Centre.
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Accepted manuscript to appear in JPP
TABLES
Table 1. Experimental and calculated spectroscopic properties of BODIPYs in dichloromethane
DCM), toluene, and methanol (MeOH) at room temperature. All parameters are calculated at M06-
(
2
X/6-31+G(d,p) level
.
The leading transition is HOMO→LUMO for all calculated molecules.
*
BODIPY Solvent
Dipole
λabs (nm)
ε
Oscillat
or
λem (nm)
Φf
Oscillat
or
Stokes shift
(nm)
-
1
-1
moment
(M cm )
(
Debye)
strength
strength
exp
calc
435
exp
calc
exp
calc
478
exp
0.64
0.68
0.52
0.94
0.97
0.90
0.71
0.66
ND
calc
exp
10
11
10
9
calc
1
1
a
CH
Toluene
CH OH
CH Cl
Toluene
CH OH
CH Cl
Toluene
CH OH
CH Cl
Toluene
CH OH
CH Cl
Toluene
CH OH
CH Cl
Toluene
CH OH
2
Cl
2
6.33
502
504
499
503
505
500
512
515
509
527
531
524
529
531
525
539
541
535
95200
104900
82900
61700
98500
82700
35900
36800
31800
55600
29900
44100
44600
37900
45700
36100
37600
36300
0.638
512
515
509
512
514
509
522
525
518
542
545
538
542
544
538
552
554
548
0.774
43
3
6.56
431
437
0.614
0.599
488
480
0.818
0.744
57
43
b
2
2
6.31
9
3
6.57
5.91
434
444
0.574
0.510
489
488
0.793
0.624
9
55
44
1
c
2
2
10
10
9
3
6.30
6.50
441
450
0.485
0.668
499
492
0.729
0.787
55
42
2
a
2
2
0.78
0.80
0.73
0.83
0.96
0.81
0.93
0.98
0.48
15
14
14
13
13
13
13
13
13
3
6.73
6.51
446
452
0.643
0.635
490
493
0.829
0.764
40
41
2
b
2
2
3
6.75
6.28
448
456
0.610
0.575
502
504
0.814
0.685
54
48
2
c
2
2
3
6.62
452
0.550
499
0.750
58
*
Fluorescein (0.90 in 0.1 M NaOH) and rhodamine 6G (Фf = 0.94 in anhydrous ethanol) were used as the standard for BODIPYs
a-c and 2a-c, respectively. ND = not determined due to poor solubility in the solvent.
1

Accepted manuscript to appear in JPP
FIGURES LEGENDS AND SCHEME CAPTIONS
Figure 1. BODIPYs with restricted meso-aryl rotation.
Figure 2. X-ray crystal structure of BODIPY 1c; (A) top view and (B) side view with 50% ellipsoids.

Accepted manuscript to appear in JPP
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1a
1b
1c
2a
2b
2c
ꢀ1a
ꢀ1b
ꢀ1c
ꢀ2a
ꢀ2b
ꢀ2c
0.8
0.6
0.4
0.2
0.0
4
50
500
550
600
500
550
600
650
Waveleng thꢀ(nm)
Waveleng thꢀ(nm)
Figure 3. Normalized UV-vis (left) and fluorescence (right) emission spectra of BODIPYs 1a-c and
a-c in dichloromethane at room temperature.
2
-
2.09
-2.08
-2.15
(-2.12)
(-2.11)
(-2.21)
-6.87
-6.85
-6.86
(
-6.90)
(-6.88)
(-6.92)
E =4.78
E =4.77
E =4.71
g
g
g
(4.79)
(4.77)
(4.71)
1
a
1b
1c
-
2.37
-2.36
-2.40
(-2.39)
(-2.38)
(-2.44)
-
7.06
-7.05
-7.04
(-7.08)
(-7.07)
(-7.08)
E =4.69
E =4.68
E =4.64
g
g
g
(
4.70)
(4.69)
(4.65)
2
a
2b
2c
Figure 4. Front view and side view of the frontier orbitals of BODIPYs 1a-c and 2a-c. Orbital and gap
energies in eV. Modeled in dichloromethane (methanol).

Accepted manuscript to appear in JPP
70
60
50
40
30
20
10
0
1a
1b
1c
2a
2b
2c
-90 -80 -70 -60 -50 -40 -30 -20 -10
0
10 20
Dihedral angle, θ
Figure 5. Potential energy surface for the ground states of BODIPYs 1a-c and 2,6-dichloro-BODIPYs
a-c as a function of the meso-aryl dihedral angle. Modeled in dichloromethane.
2

Accepted manuscript to appear in JPP
SCHEME
R
R
B
R
R
R
R
R
1.
, TFA
N
TCCA
H
Cl
Cl
N
N
N
N
R
R
2. DDQ
. Et N, BF •OEt
2
B
F
F
F
F
CHO
3
3
3
1
1
1
a: R = H
b: R = Me
c: R = Ph
2a: R = H
2b: R = Me
2c: R = Ph
R = H, Me or Ph
Scheme 1. Synthesis of BODIPYs 1a-c and 2a-c.