Synthesis, spectroscopy, crystal structure, electrochemistry, and quantum chemical and molecular dynamics calculations of a 3-anilino difluoroboron dipyrromethene dye

Article abstract of DOI:10.1021/jp8077584

An asymmetrically substituted fluorescent difluoroboron dipyrromethene (BODIPY) dye, with a phenylamino group at the 3-position of the BODIPY chromophore, has been synthesized by nucleophilic substitution of 3,5-dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-3a

Full text of DOI:10.1021/jp8077584

J. Phys. Chem. A 2009, 113, 439–447
439
Synthesis, Spectroscopy, Crystal Structure, Electrochemistry, and Quantum Chemical and
Molecular Dynamics Calculations of a 3-Anilino Difluoroboron Dipyrromethene Dye
Wenwu Qin,†,‡ Volker Leen,† Taoufik Rohand,† Wim Dehaen,† Peter Dedecker,† Mark Van der
Auweraer,† Koen Robeyns,† Luc Van Meervelt,† David Beljonne,§ Bernard Van Averbeke,§
John N. Clifford,† Kris Driesen,† Koen Binnemans,† and Noe¨l Boens*,†
Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200fsbus 02404, 3001 LeuVen, Belgium,
College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou 730000, China, and Laboratory for
Chemistry of NoVel Materials, UniVersity of Mons-Hainaut, Place du Parc 20, 7000 Mons, Belgium
ReceiVed: September 1, 2008; ReVised Manuscript ReceiVed: NoVember 7, 2008
An asymmetrically substituted fluorescent difluoroboron dipyrromethene (BODIPY) dye, with a phenylamino
group at the 3-position of the BODIPY chromophore, has been synthesized by nucleophilic substitution of
3,5-dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. The solvent-dependent spectroscopic and
photophysical properties have been investigated by means of UV-vis spectrophotometry and steady-state
and time-resolved fluorometry and reflect the large effect of the anilino substituent on the fluorescence
characteristics. The compound has a low fluorescence quantum yield in all but the apolar solvents cyclohexane,
toluene, and chloroform. Its emission maxima in a series of solvents from cyclohexane to methanol are red-
shifted by approximately 50 nm in comparison to classic BODIPY derivatives. Its oxidation potential in
dichloromethane is at ca. 1.14 V versus Ag/AgCl. The absorption bandwidths and Stokes shifts are much
larger than those of typical, symmetric difluoroboron dipyrromethene dyes. The values of the fluorescence
rate constant are in the (1.4-1.7) × 108 s-1 range and do not vary much between the solvents studied. X-ray
diffraction analysis shows that the BODIPY core is planar. Molecular dynamics simulations show that there
is no clear indication for aggregates in solution.
Introduction
and emission bands as a function of solvent polarity/polariz-
ability. Recently, we described that the easily obtained 3,5-
dichloro-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene derivatives19
can be substituted with a wide range of oxygen-, carbon-,
nitrogen-, and sulfur-centered nucleophiles6,11 and that the
reaction conditions can be adjusted to have either mono or
disubstitution.11 The solvent-dependent photophysical charac-
teristics of four BODIPY derivatives with uncommon substit-
uents such as chloro, methoxy, and malonate at the 3,5-positions
were investigated by means of UV-vis spectrophotometry and
steady-state and time-resolved fluorometry.20 Their absorption
(from 508 to 517 nm) and emission (from 519 to 530 nm)
maxima are very similar to those of conventional BODIPY dyes,
showing small Stokes shifts (between 440 and 490 cm-1) and
Because of their favorable photophysical and optoelectronic
properties, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene1 (difluo-
roboron dipyrromethene or BODIPY) derivatives have found
widespread application as fluorescent labels in bioanalytics, as
fluorophores in new chemosensors, or as laser dyes.2-6 The
excellent qualities of BODIPY comprise relatively high fluo-
rescence quantum yields Φf (often approaching 1.0) and molar
absorption coefficients (ε > 50 000 L mol-1 cm-1), narrow
emission bandwidths with high peak intensities, excellent
thermal and photochemical stability, good solubility, chemical
robustness, negligible triplet-state formation, and excitation/
emission wavelengths in the visible region. Moreover, their
building block synthesis allows one to develop many different
BODIPY analogues with emission wavelengths tunable from
500 to over 700 nm, by attaching supplementary units at the
pyrrole,7 meso,8-10 and N-ortho positions.6,11-14 An interesting
new class of BODIPY-related fluorophores is obtained by
functionalization of the boron atom in the difluoroboradiaza-
indacene unit via substitution of the F atoms with C- and
O-centered groups.2,15 The photophysical properties of several
BODIPY dyes in solution have been studied by means of
UV-vis absorption and fluorescence techniques.10,16-18
narrow absorption (full width at half-maximum, fwhmabs
,
between 710 and 820 cm-1) and emission bands.
Some work has been done toward developing BODIPY
derivatives with absorption and emission spectra shifted to
longer wavelengths by increasing the conjugation at the
3,5-positions.4,5,18,21,22 In the present work, we describe the
detailed procedure for the synthesis of a 3-anilino-5-chloro
substituted BODIPY dye (2) via the recently described substitu-
tion reaction11 of 3,5-dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene (1) with aniline as nucleophile. In
addition, we investigate the spectroscopic and photophysical
characteristics of 2 in several solvents by UV-vis absorption
spectrophotometry and steady-state and time-resolved fluores-
cence spectroscopy. From these experiments we could determine
the wavelengths of the spectral maxima [λabs(max), λex(max),
λem(max)], the full width at half-maximum of the absorption
band (fwhmabs, in cm-1), the full width at half-maximum of the
fluorescence emission band (fwhmem, in cm-1), the Stokes shifts
Although there are several positions on the BODIPY chro-
mophore where functionalization can be carried out, suitable
substituents at the 3- (and 5-) position(s) can shift the excitation
* To whom correspondence should be addressed. E-mail: Noel.Boens@
chem.kuleuven.be.
† Katholieke Universiteit Leuven.
‡ Lanzhou University.
§ University of Mons-Hainaut.
10.1021/jp8077584 CCC: $40.75
2009 American Chemical Society
Published on Web 12/19/2008

440 J. Phys. Chem. A, Vol. 113, No. 2, 2009
Qin et al.
SCHEME 1: Synthesis of the BODIPY Derivative 2a
Crystal Structure Determination. Compound 2 was crystal-
lized from cyclohexane/chloroform (9:1, v/v), yielding red
crystals with an average size of 0.2 × 0.1 × 0.05 mm3. The
crystals belonged to the monoclinic space group P21/n (number
14) with cell dimensions a ) 12.134(8) Å, b ) 12.926(8) Å, c
) 12.543(7) Å, ꢀ ) 107.85(2)°, V ) 1873(2) Å3, Z ) 4, Fcalc
) 1.446 g cm-3, 2θmax ) 142.2°, µ(Cu KR) ) 2.090 cm-1
.
The data were measured on a Bruker SMART 6000 detector,
Cu KR (λ ) 1.54178 Å) with crossed Go¨bel mirrors. The
crystals were cryocooled to 100 K. A total of 18 737 reflections
were measured of which 3559 are unique reflections. The data
were corrected for Lorentz, absorption (Bruker SADABS), and
polarization effects. The structure was solved by direct methods.
Full-matrix least-squares refinement based on F2 (SHELXL97),23
277 parameters, with hydrogen atoms placed at calculated
positions with temperature factors 20% higher than parent atom
and the hydrogen bond distances free to refine resulted in R1 )
0.0428 [for 2946 with I > 4σ(I)], ωR2 ) 0.1052, max/min
a Reaction conditions: (a) rt/aniline/CH3CN/14 h.
(∆νj, in cm-1), the fluorescence quantum yields (Φf) and
lifetimes (τ), and the rate constants of fluorescence (kf) and
nonradiative (knr) deactivation. Next, the electrochemical proper-
ties of 2 have been measured, and furthermore, its crystal-
lographic structure has been determined. Finally, quantum
chemical and molecular dynamics calculations have been applied
to elucidate the UV-vis absorption spectra of 2.
residual electron density 0.30/-0.24 e- Å-3
.
Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-622572. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (fax: +44(0)-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk).
Cyclic Voltammetry. Electrochemical data were obtained
using an Autolab PGSTAT12 potentiostat and a standard three-
electrode cell (platinum working and counter electrodes and an
Ag/AgCl reference) at a scan rate of 200 mV s-1. The
voltammograms were recorded at room temperature in a solution
of 0.1 mol L-1 tetrabutylammonium hexafluorophosphate as the
supporting electrolyte in dry dichloromethane. All solutions were
purged with argon prior to measurement.
UV-Vis Steady-State Spectroscopy. UV-vis absorption
spectra were recorded on a Perkin-Elmer Lambda 40 UV-vis
spectrophotometer, whereas for the corrected steady-state excita-
tion and emission spectra, a SPEX Fluorolog was employed.
All absorption and fluorescence excitation/emission spectra were
recorded at 20 °C using nondegassed samples (degassing did
not influence the fluorescence quantum yield). For the dye in a
specific solvent, three or four absorption, excitation, and
emission spectra were measured as a function of dye concentra-
tion. Since the spectra are recorded digitally and the peaks are
relatively narrow, the maxima can be determined not only from
visual inspection but also via the analysis-calculus-integrate
menu of the Origin software. The error on the wavelength of
the maximum is usually 1-2 nm. For the determination of the
relative fluorescence quantum yields (Φf), only dilute solutions
with an absorbance below 0.1 at the excitation wavelength λex
were used. Rhodamine 6G in H2O (λex ) 488 nm, Φf ) 0.76)
was used as fluorescence standard.24 The Φf values reported in
this work are the averages of multiple (generally three or four),
fully independent measurements. The majority of the Φf
determinations was done on nondegassed samples. To check
the influence of dissolved O2 on Φf, a few samples, degassed
via successive freeze-pump-thaw cycles, were measured. The
obtained Φf values were within experimental error equal for
aerated and degassed samples, in accordance with results for
other BODIPY compounds.10a In all Φf determinations, correc-
tion for the solvent refractive index was applied.25
Experimental Section
Materials. All solvents for the spectroscopic measurements
were of spectroscopic grade and were used without further
purification. The chemicals for the synthesis were of reagent
grade quality, procured from commercial sources, and used as
received.
Synthesis.
3,5-Dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (1)20 is the starting compound for the synthesis of
BODIPY derivative 2 (Scheme 1).
5-Chloro-3-phenylamino-8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene (2). To a stirred solution of 1 (100 mg, 0.29
mmol) in acetonitrile (50 mL) under nitrogen atmosphere was
added aniline (104 µL, 1.20 mmol). The reaction mixture was
stirred at room temperature for 14 h. Then the reaction mixture
was poured in water, and the product was extracted with
dichloromethane (2 × 50 mL). The combined extracts were
washed with water and brine, dried over MgSO4, and evaporated
to dryness under reduced pressure. The solid residue was
chromatographed on silica gel with heptane/dichloromethane
7:3 (v/v) as eluent to yield 81 mg (69% yield) of brick-red
crystals. Mp 183 °C. 1H NMR (CDCl3) δ 8.10 (s, broad, NH),
7.44-7.33 [m, 4H (tolyl)], 7.28-7.24 [m, 5H (Ph)], 6.93 [d,
1H (H-6), J ) 4.6 Hz), 6.44 [d, 1H (H-1), J ) 3.7 Hz], 6.42 [d,
1H (H-7), J ) 4.6 Hz], 6.23 [d, 1H (H-2), J ) 3.7 Hz], 2.44 [s,
3H, CH3 (tolyl)]; 13C NMR (CDCl3) δ 159.1 (s), 140.0 (s), 137.7
(d), 135.9 (s), 134.3 (s), 132.4 (s), 131.6 (s), 130.6 (d), 129.4
(d), 126.5 (d), 122.9 (d), 122.1 (s), 113.7 (d), 111.9 (d), 20.9
[q, CH3 (tolyl)]; LRMS (EI, 70 eV) m/z 407 (M+ 100), 409
(M+ 31); HRMS: calcd for C22H17BClF2N3 407.1172, found
407.1191.
1H and 13C NMR spectra were recorded at room temperature
on a Bruker Avance 300 instrument operating at a frequency
of 300 MHz for 1H and 75 MHz for 13C. 1H NMR spectra were
referenced to tetramethylsilane (0.00 ppm) as an internal
standard. Chemical shift multiplicities are reported as s )
singlet, d ) doublet, q ) quartet, and m ) multiplet. 13C NMR
spectra were referenced to the CDCl3 (77.67 ppm) signal. Mass
spectra were recorded on a Hewlett-Packard 5989A mass
spectrometer (EI mode and CI mode). High-resolution mass data
were obtained with a Kratos MS50TC instrument. Melting points
were taken on a Reichert Thermovar and are uncorrected.
Time-Resolved Fluorescence Spectroscopy. Fluorescence
decay traces of BODIPY derivative 2 were recorded at several
emission wavelengths by the single-photon timing method.26

3-Anilino Difluoroboron Dipyrromethene Dye
J. Phys. Chem. A, Vol. 113, No. 2, 2009 441
Details of the instrumentation used and experimental procedures
have been described elsewhere.27 The samples were excited at
488 nm with a repetition rate of 8.10 MHz, using the frequency-
doubled output from an OPO pumped by a picosecond Ti:
Sapphire laser. Fluorescence decay histograms were collected
at magic angle in 4096 channels with a time increment per
channel of 1.8 ps, using 10 × 10 mm2 optical path length
cuvettes. The absorbance at the excitation wavelength was
always below 0.1. All lifetime measurements were performed
on samples that were degassed by consecutive freeze-pump-
thaw cycles. This is common practice in our laboratory because
we want to guarantee excluding the effect of dissolved oxygen
on the fluorescence lifetimes. After analysis of the initial
fluorescence decay curves indicates that degassing is not
necessary, we usually start collecting the remaining fluorescence
decay traces using nondegassed samples. The fluorescence
lifetime of 2 in the three solvents used did not change upon
degassing. Histograms of the instrument response function were
recorded using a LUDOX scatterer. The fwhm of the instrument
response function was ∼60 ps. All measurements were done at
20 °C.
The fitting parameters were determined by minimizing the
global, reduced chi-square ꢁg :
Figure 1. PLATON/Ortep (Spek, A. L. J. Appl. Crystallogr. 2003,
36, 7-13) representation of compound 2 with displacement ellipsoids
at the 50% probability level. The intramolecular hydrogen bond between
the NH group and the fluorine atom is indicated by a dashed line, and
the BODIPY bond lengths are shown in gray.
2
q
2
g
ꢁ )
w (yo - yc)2/ν
(1)
∑∑
li li
li
l
i
where the index l sums over q experiments, and the index i
sums over the appropriate channel limits for each individual
experiment. yoli and ylci denote, respectively, the observed and
calculated (i.e., fitted) values corresponding to the ith channel
of the lth experiment, and wli is the corresponding statistical
weight. ν represents the number of degrees of freedom for the
entire multidimensional fluorescence decay surface.
The statistical criteria for assessing the quality of the fit
included both graphical and numerical tests and have been
described elsewhere.26c,28 Decays were analyzed first individually
by a (multi)exponential decay law in terms of decay times τi
and their associated pre-exponential factors Ri. The final curve-
fitting was done by global analysis in which decays recorded
at six different emission wavelengths λem (550, 560, 570, 600,
620, and 650 nm) were described by a single-exponential decay
function with linked lifetime τ and local (nonlinked) pre-
exponentials R. The quality of the fit was judged for each
fluorescence decay trace separately as well as for the global
fluorescence decay surface. All curve-fittings presented here had
ꢁ2 values below 1.1.
accounted for through molecular dynamics (MD) simulations
of one BODIPY molecule embedded in a solvent box (in our
simulations we used tetrahydrofuran, THF), using the DRE-
IDING force field.32 MD simulations were run for a period of
100 ps within the canonical ensemble (using fixed temperature,
pressure, and number of particles) at room temperature. A total
of 30 snapshots were extracted from the MD trajectory and
considered further for INDO/SCI excited-state calculations. We
also took into account the possible formation of small aggregates
in solution. To do so, the same MD and INDO/SCI simulations
were performed for two molecules (facing each other in the
initial configuration) in the solvent bath.
Results and Discussion
Compound 2 was synthesized by an efficient nucleophilic
substitution reaction of compound 1.6,11 This synthetic meth-
odology is extremely useful for obtaining nonsymmetrically and
symmetrically substituted boradiazaindacene dyes with substit-
uents at the 3- (and 5-) position(s) that are difficult to realize
otherwise. The crystal structure of 2 has been determined, and
its photophysical properties in various solvents and its electro-
chemical properties in dichloromethane have been investigated.
Crystallographic Structure. In compound 2 (Figure 1), the
BODIPY ring system is flat (max deviation 0.051 Å for boron
atom), with the two fluorine atoms above and under the plane.
This is in contrast with the majority of BODIPY structures found
in the Cambridge Structural Database (CSD)33 where the angle
between the two pyrrole moieties is on average 5.4° (range
0-18°) and where a maximum displacement of 0.34 Å from
the BODIPY plane is observed for the boron atom. The phenyl
ring at the meso position makes an angle of 54.8° with the
BODIPY core, which is in the range of most BODIPY
derivatives (40.3-90°), but is smaller than the average value
of 74.5° found in the CSD.
For the single-exponential fluorescence decays, the rate
constants of fluorescence (kf) and nonradiative (knr) deactivation
can be calculated from the measured fluorescence quantum yield
Φf and lifetime τ according to eqs 2 and 3:
kf ) Φf/τ
(2)
(3)
knr ) (1 - Φf)/τ
Computational Details. Molecular properties in the gas phase
were obtained using semiempirical and ab initio quantum
chemical methods. The ground-state (respectively, excited-state)
geometries of compound 2 were computed at the AM1
(respectively, AM1/CI)29 and DFT/B3LYP30 levels. These
structures were subsequently used as input for electronic excited-
state calculations performed at the INDO/SCI level.31 The
highest 30 occupied and 30 unoccupied molecular orbitals were
included in the SCI active space. Conformational fluctuations
and their impact on the spectroscopic properties were also
At least one intramolecular hydrogen bond is found between
the NH group and one of the fluorine atoms; the interaction
with the second fluorine atom is probably weak or absent

442 J. Phys. Chem. A, Vol. 113, No. 2, 2009
Qin et al.
[d(N16H16) ) 0.86 Å, d(H16 · · ·F15) ) 2.43 Å, ang(N16H16 · · ·F15)
) 124.3°, d(H16 · · ·F14) ) 2.77 Å, ang(N16H16 · · ·F14) ) 112.4°].
The nonsymmetric substitution pattern is reflected in the pyrrole
bond distances.
Electrochemistry. The oxidation potential of 2 estimated
from the midpoints of the forward and reverse peaks of the scan
in the cyclic voltammogram appears at ca. 1.14 V versus Ag/
AgCl. Upon scanning in the cathodic direction, a precipitate
can be clearly seen to form on the surface of the working
electrode, which could be due to instability of the oxidized form
of 2. This observation manifests itself with the reverse scans
displaying smaller current amplitudes than the forward scans.
Upon moving to faster scan rates this effect appears less
pronounced; however, fully reversible forward and reverse scans
are still not observed. We did not observe any reduction
electrochemistry for 2, and this could be because the redox
potential is beyond the accessible range of dichloromethane.
Even extending this range in the cathodic direction using
solvents as THF and acetonitrile did not lead to the observation
of a reduction wave.
Spectroscopic Properties. Figure 2a shows the absorption
and steady-state fluorescence emission spectra of 2 in cyclo-
hexane. The absorption spectrum is of comparable shape as
those of described BODIPY dyes,10,16-20,34 with an intense
absorption band somewhat above 500 nm and a less pronounced
shoulder at shorter wavelengths. The absorption spectra of 2 in
other solvents (Figure 2b) are analogous to that in cyclohexane,
and they all show two absorption bands: (1) in the visible
wavelength region, the 0-0 band of a strong S0-S1 transition
with a maximum ranging from 498 to 529 nm (Table 1) and a
(more or less pronounced) shoulder on the high-energy side,
which is attributed to the 0-1 vibrational band of the same
transition. The wavelengths of these absorption maxima
λabs(max) (Table 1) are very similar to those of other typical
BODIPY derivatives.10,16,17,20,34 The absorption spectra are
dependent on solvent polarity; the maximum being blue-shifted
(by ∼30 nm) when the solvent is changed from cyclohexane
(529 nm) to methanol (498 nm). (2) Additionally, a weaker,
broad absorption band is found in the UV at about 350 nm, the
position of which is not appreciably affected by solvent polarity.
This broader and weaker absorption band is attributed to the
S0-S2 transition. The fwhmabs of the intense absorption band
around 500 nm is in the 2500-3500 cm-1 range (Table 1),
which is significantly more than generally found for BODIPY
derivatives substituted at the 3- and 5-positions by weaker
electron donors or electron-withdrawing groups (e.g., fwhmabs
for 1 is in the 650-780 cm-1 range).20 One should note that
already in an apolar solvent as cyclohexane the fwhmabs is
already several times larger than that observed for other common
BODIPY derivatives. This indicates that upon excitation a
significant change in bond lengths or angles occurs.
Figure 2. (a) Absorption and fluorescence emission spectra of 2 in
cyclohexane normalized to 1.0. (b) Analogously normalized absorption
spectra of 2 in different solvents. (c) Corresponding normalized
fluorescence emission spectra (excitation at 495 nm).
Compound 2 in cyclohexane shows an emission spectrum
with a maximum at 567 nm, shaped like the mirror image of
the absorption spectrum, displaying the typical emission features
of boron dipyrromethene.10,16,17,20,34 Besides the 0-0 transition
at 567 nm, there is the 0-1 transition, visible as a shoulder
peak located at 615 nm. The difference (1380 cm-1) matches
the frequency of a conjugated double bond. This is to be
expected because small changes in bond length of the different
bonds in the conjugated system can occur upon excitation. The
maximum wavelength of the fluorescence band is hardly affected
by solvent polarity (Figure 2c, Table 1): there is only a 9 nm
difference between the wavelengths of the emission maxima
λem(max) in toluene (575 nm) and methanol (566 nm). In more
polar solvents the broadening of the individual vibronic bands
due to solvent interactions blurs out the vibrational progression
observed in cyclohexane. The wavelength of the emission
maximum of 2 is red-shifted by ∼50 nm compared to other
described BODIPY dyes10,16,17,34 (e.g., λem(max) for 1 varies
between 529 nm in toluene and 519 nm in methanol).20 The
fwhmem varies between 1815 cm-1 in cyclohexane and 2384
cm-1 in THF (Table 1). There is an excellent parallel between
the corresponding values of fwhmabs and fwhmem for each
solvent. The best linear fit of fwhmem (y-values) as a function

3-Anilino Difluoroboron Dipyrromethene Dye
J. Phys. Chem. A, Vol. 113, No. 2, 2009 443
TABLE 1: Photophysical Data of Dye 2 in Several Solvents
solventa
λabs(max)/nm
λ
em(max)/nm λex(max)/nm fwhmabs/cm-1 fwhmem/cm-1 ∆νj/cm-1
Φf
τ/nsb kf/108 s-1 knr/108 s-1
1
2
3
4
5
6
7
8
9
cyclohexane
1,4-dioxane
toluene
diethyl ether
chloroform
ethyl acetate
THF
529
508
524
509
519
504
505
514
498
567
573
575
570
572
572
574
570
566
529
508
524
509
519
504
505
514
498
2545
3199
2860
3105
2983
3453
3212
3098
3292
1815
2334
2094
2308
2109
2375
2384
2200
2365
1267
2233
1693
2103
1785
2359
2380
1911
2412
0.28
0.026
0.35
0.008
0.26
0.01
0.008
0.044
0.003
1.96
2.03
1.62
1.4
1.7
1.6
3.7
3.2
4.6
1-decanol
methanol
a The solvents are classified according to increasing polarity (as measured by the dielectric constant ε). b The standard errors on all lifetimes
τ are e10 ps.
of fwhmabs (x-values) gives y ) 126 + 0.679x (in cm-1) with
a correlation coefficient of 0.952. When compared to other,
classic BODIPY derivaties,10,16-18 2 shows a much larger fwhm
in both absorption and emission and a considerable larger Stokes
shift of the 0-0 vibronic transition. This indicates that along a
coordinate with a small force constant (torsions and/or solvation)
and hence with a separation of vibrational levels (hνvib) smaller
than kT, there is a significant shift between the minima of the
potential energy surfaces in the ground state and the excited
state.35 When only low-frequency vibrations (hνvib < kT) are
considered, the Stokes shift, ∆νj, and bandwidth, fwhm (all in
cm-1), are proportional to λ, the reorganization energy, and ꢀλ,
respectively. Indeed, the Stokes shift is equal to ∆νj ) 2λ,
Figure 3. Major Lewis structural formulas contributing to the real
structure of compound 2.
TABLE 2: Spectroscopic Properties of Compound 2 in the
Gas Phasea
whereas the bandwidth amounts to fwmh ) 4((ln2) λ kT)1/2
.
λ
abs(max)/nm
λ
em(max)/nm
533
Substitution of λ, obtained from the experimental Stokes shift,
in the formula for fwhm always underestimates fwhm. The
larger value of the experimental fwhm is due to the 0-1 vibronic
transition of a high-frequency vibration in the spectrum. This
relation also explains why the Stokes shift and fwhm evolve in
parallel upon increasing the solvent polarity.
AM1//INDO/SCI
DFT(B3LYP)//INDO/SCI
504
503
a Compare these spectroscopic data with the experimental ones in
solvents (Table 1).
Contrary to the near-invariance of the emission wavelength
maximum λem(max) in different solvents, the fluorescence
quantum yield Φf of 2 is strongly solvent-dependent. The Φf
values are moderately high in cyclohexane (0.28), toluene (0.35),
and chloroform (0.26), whereas in all other solvents used the
emission intensity is strongly quenched (Φf < 0.044). Similar
solvent-dependent Φf changes have been observed for BODIPY
analogues with a 4-N,N-dimethylaminophenyl substituent at the
meso position.10,17 But unlike for these latter compounds, no
red-shifted, broad, and solvent polarity-dependent fluorescence
band originating from an intramolecular charge-transfer state
(ICT) was found in the emission spectrum of 2. Indeed, there
is no direct spectral indication for two separate emission bands
(from the locally excited state and an ICT state) in the emission
spectra of 2. The Stokes shift ∆νj ) νjabs - νjem [with νjabs and
νjem corresponding to the wavelengths λabs(max) and λem(max),
respectively] increases nearly 2-fold between cyclohexane (1267
cm-1) and methanol (2412 cm-1) and is significantly larger than
the Stokes shifts observed for 3,5-dichloro-BODIPY 1 (440 (
30 cm-1).20 This agrees with the increased electron phonon
coupling suggested by the fwhmabs of 2 in cyclohexane. This
can perfectly be understood in the framework developed for
cyanine dyes.36 For symmetric BODIPYs there are two equiva-
lently contributing Lewis structural formulas, and all bonds
along the conjugated chain have a similar length which only
increases slightly and homogeneously upon excitation. For 2,
however, the two delocalized structures A and B (Figure 3) are
not equivalent any longer, with structure B being more stable.
Moreover, one can write one extra stable, delocalized, Lewis
structural formula (C in Figure 3) in which the anilino nitrogen
donates a lone electron pair to the conjugated chain. As a
consequence bond alternation starts to occur in the ground state,
and this alternation will be inverted upon excitation, leading to
increased electron phonon coupling. Also the solvent depen-
dence of Φf and ∆νj may be interpreted as indication of a small
contribution of a dipolar resonance form to the ground or excited
state of 2. The blue shift of the absorption spectrum from
cyclohexane to THF or ethyl acetate can hardly be attributed
to a difference in solvent polarizability. If it is attributed to
dipolar interactions, this blue shift suggests that the dipole
moment of the ground state is larger than that of the excited
state, as supported by the quantum chemical calculations, vide
infra. The latter also explains why the increased Stokes shift in
methanol (compared to cyclohexane) is related to a blue shift
of the absorption rather than to a red shift of the emission
spectrum. This would suggest that the increase of the electron
density in the conjugated BODIPY system by the anilino moiety
is less outspoken in the excited state than in the ground state.
Plots of the maxima of absorption (νjabs) and emission (νjem)
of 2 in all solvents versus f(n2) ) (n2 - 1)/(2n2 + 1) (Figure
S1, Supporting Information) indicate an inadequate linear
relationship (correlation coefficient r ) -0.784 for νjabs and r
) -0.657 for νjem). A similar poor relationship (r ) 0.687,
Figure S2, Supporting Information) was found for the Stokes
shift ∆νj of 2 in all solvents versus the Lippert solvent parameter
∆f ) f(ε) - f(n2) ) [(ε - 1)/(2ε + 1)] - [(n2 - 1)/(2n2 +
1)].37,38 The relationship between the Stokes ∆νj of 2 and ∆f is
equally poor for the aprotic solvents only (r ) 0.620). The
reason is probably that although shifts induced by solvent
polarity and solvent polarizability are of the same magnitude

444 J. Phys. Chem. A, Vol. 113, No. 2, 2009
Qin et al.
while the Stokes shift is nearly independent of solvent polarity.
Hence, in contrast to these BODIPYs the charge density on the
conjugated chain of 2 changes upon excitation. The blue shift
from cyclohexane over diethyl ether to methanol clearly suggests
a larger dipole moment in the ground state compared to the
excited state. This is also supported by the quantum chemical
calculations, vide infra.
To investigate the fluorescence dynamics of 2, fluorescence
decay traces in cyclohexane, toluene, and chloroform were
collected as a function of emission wavelength. All fluorescence
decay profiles of 2 could be described by a single-exponential
fit in the solvents used. The lifetimes τ estimated via single
curve analysis were independent of the observation wavelength.
Simultaneous (or global) curve-fitting of the fluorescence decay
surface with τ linked as a function of the observation wavelength
confirmed that the decays are indeed single-exponential and do
not depend on the emission wavelength. The estimated fluo-
rescence lifetimes τ are compiled in Table 1.
Figure 4. Transition density distribution calculated at the INDO/SCI
level for the first optical transition of compound 2. The blue spheres
depict the negative charges, whereas red spheres indicate the presence
of a positive charge.
Using the experimental Φf and τ values, one can calculate
(according to eqs 2 and 3) values for the rate constants of radiative
(kf) and radiationless (knr) decay. The values of kf are in the
(1.4-1.7) × 108 s-1 range for the three investigated solvents
(cyclohexane, toluene, and chloroform), indicating that kf is nearly
independent of the low-polarity solvent used. However, the absence
of lifetime data in more polar solvents (due to the very low Φf
values) makes it difficult to determine the trend of kf and knr in
more polar solvents. The kf values obtained for 2 are very
comparable to those obtained for compound 1 (kf between 1.4 ×
108 and 1.8 × 108 s-1).20 As before,10,20 the largest kf value is found
in toluene. The knr values as a function of solvent are in the
(3.2-4.6) × 108 s-1 range, comparable to what was found before
for BODIPY derivatives.10,16-18,20
Quantum Chemical and Molecular Dynamics Calcula-
tions. The existence of ground-state dimers of a few BODIPY
analogues, especially in molecularly confined environments, has
been reported.39 Using quantum chemical and MD calculations,
we want to investigate if evidence for ground-state dimers of
dye 2 can be uncovered.
Molecule 2 in the Gas Phase. The spectroscopic properties
(absorption and emission vertical transition energies) calculated
at INDO/SCI level for both AM1 and DFT(B3LYP) optimized
structures of compound 2 in the gas phase are compiled in Table
2 and should be compared to the experimental ones in solvents
(reported in Table 1).
Figure 5. Snapshot at 30 ps of a molecular dynamics run for compound
2 in THF.
in 2, those properties do not vary in a parallel way over the
series of solvents used. In this way 2 differs significantly from
most typical BODIPYs where the absorption (νjabs) and emission
(νjem) maxima shift to lower energy linearly with increasing f(n2)
Figure 6. Angle, ꢀ, between the molecular plane and the intermolecular axis passing through the geometric centers (left) and center-to-center
distance (right), as followed along an MD run performed for two molecules 2 embedded in a THF box.

3-Anilino Difluoroboron Dipyrromethene Dye
J. Phys. Chem. A, Vol. 113, No. 2, 2009 445
TABLE 3: INDO/SCI State Dipole Moments (in debye, D)
in the Ground State (S0) and Excited State (S1) of 2 and 3a
2/D
3/D
S0
S1
8.76
5.65 (5.84)
6.47
8.30 (8.95)
a The numbers in between parentheses are the dipoles calculated
for the relaxed excited-state geometry.
Figure 7. INDO/SCI absorption spectra of a single molecule and a
dimer of compound 2 in THF. Ensemble spectra are averaged over 30
snapshots.
Figure 9. Structure of BODIPY dye 3.
(consistent with a decrease of ꢀ from ∼90° to ∼30°) ac-
companied by an increased center-to-center separation (from
∼5 to ∼9 Å), as time goes by. It is also worth noting that these
geometrical parameters fluctuate significantly in time, likely
because of the nonplanar conformation of the molecules (the
anilino group is out of the plane formed by the BODIPY core).
Thus, the MD simulations suggest that aggregates of 2 are very
unlikely in THF at room temperature. Yet, to check the effect
of intermolecular interactions, the absorption spectra have been
computed for both a single molecule and a dimer of 2 in THF.
The ensemble (averaged over 30 snapshots) spectra displayed
in Figure 7 are consistent with the structural analysis above.
Except for a slight red shift (due to the formation of a weak
J-type aggregate, with the molecular transition dipole moments
shifted along the molecular axis), there is no clear sign for
aggregation.
Figure 8. INDO/SCI (ZDO) changes in atomic charge density when
going from the ground state to the lowest electronic excited state of 2
(left). For comparison, the corresponding charge redistribution is shown
for compound 3 (right). Red circles correspond to positive partial
charges, and blue circles correspond to negative charges. The solid line
arrows represent the ground-state (S0) and excited-state (S1) dipole
moment; the dotted arrows show the dipole moment difference.
State Dipole Measurements. From the INDO/SCI calcula-
tions obtained from AM1 geometries we extracted the state
dipole and the corresponding charge distribution in ground- and
excited-state geometries. This information can provide a better
insight of the “negative” solvatochromic effect observed
experimentally.
Figure 8 pictures the changes in atomic charges when going
from the ground state to the lowest electronic excited state of
2 (as calculated at the INDO/SCI level in the ground-state
geometry). The corresponding state dipole moments are listed
in Table 3. Note that very similar results are obtained when
relaxing the geometry in the excited state.
The quantum chemical calculations reproduce qualitatively
the overall trends observed experimentally. The transition
density distribution for the first optical transition of compound
2 is depicted in Figure 4. The calculations underestimate the
Stokes shift, which might be due to solvent effects not accounted
for at this stage.
Molecule 2 in a Solvent Box. A snapshot of two molecules
2 embedded in a THF bath is shown in Figure 5.
The aniline moiety plays the role of a donor in both the
ground and excited state of 2, which results in the appearance
of a dipole pointing with its negative head toward the BODIPY
core. However, the INDO/SCI calculations suggest that the
ground-state dipole moment in 2 is larger than the excited-state
dipole moment so that the dipole moment difference points from
the BODIPY to the anilino moiety, see Figure 8. This is
responsible for the negative solvatochromic effect observed for
this dye: the ground-state absorption spectrum shows a large
blue shift with increasing solvent polarity (the inertial contribu-
tion to the dielectric response stabilizes the ground state to a
larger extent than the excited state, hence the blue shift), whereas
The simulations show that, starting from a closely spaced,
cofacial-like arrangement, the two molecules tend to separate
from each other, which we associate mostly to electrostatic
repulsion between the chlorine and fluorine atoms bearing larger
negative charges. This can be clearly seen by following along
the MD run the geometric center-to-center distance as well as
the angle, ꢀ, between the molecular plane and the intermolecular
axis passing through the molecular centers (see Figure 6).
The time evolution of the structural parameters suggests a
sliding of one molecular plane with respect to the second one

446 J. Phys. Chem. A, Vol. 113, No. 2, 2009
Qin et al.
a weaker effect is observed in photoluminescence (due to the
smaller excited-state dipole). It is interesting to note that this
behavior is different from that observed in related BODIPY
dyes, such as molecule 3 (Figure 9), in which the amine donor
group is separated from the BODIPY core by a styryl spacer.
In dye 3, the charge transfer from the side group to the BODIPY
unit is increased upon going from the ground state to the
optically allowed electronic excited state. A positive solvato-
chromic shift is thus predicted and in fact measured22 for 3,
with a red shift of the fluorescence spectrum in high-polarity
solvents and a weak dependence of the absorption spectrum on
solvent polarizability (electronic part of the dielectric response).
+ 1) as a function of solvent, plot of the Stokes shifts of 2
versus the Lippert solvent parameter ∆f, and 1H NMR spectrum
of 2. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Conclusion
A fluorescent difluoroboron dipyrromethene dye (2) with a
phenylamino group at the 3-position of the BODIPY core has
been synthesized by nucleophilic substitution of 3,5-dichloro-
BODIPY (1) with aniline. This asymmetrically substituted dye
breaks the general symmetry of the BODIPY chromophore. The
solvent-dependent photophysical properties have been investi-
gated by means of UV-vis spectrophotometry and steady-state
and time-resolved fluorometry. This study demonstrates the large
effect of the phenylamino substituent on the fluorescence
characteristics of the BODIPY dye. Compound 2 has a low
fluorescence quantum yield in most solvents except cyclohexane,
toluene, and chloroform. Although its emission maxima [λem
-
(max)] in a series of solvents from cyclohexane to methanol
are red-shifted by approximately 50 nm when compared to
typical, symmetric BODIPY derivatives, its absorption maxima
wavelengths are not affected by the phenylamino substituent.
The fwhmabs and Stokes shifts ∆νj of BODIPY 2 are much larger
than for classic BODIPY derivatives. The values of the
fluorescence rate constant of 2 are in the (1.4-1.7) × 108 s-1
range and do not vary much between the solvents studied. The
oxidation potential of 2 in dichloromethane is about 1.14 V
versus Ag/AgCl. X-ray analysis shows that the BODIPY core
is planar, which is in contrast to the majority of BODIPY
compounds found in the CSD database. Moreover, an intramo-
lecular hydrogen bond is observed between the NH group and
a fluorine atom. We also performed theoretical simulations of
the spectroscopic behavior. Molecular dynamics simulations
reveal that there is no clear sign for aggregation of molecule 2
in solution but confirms that the spectral behavior of 2 originates
from a negative solvatochromic effect (the inertial contribution
to the dielectric response stabilizes the ground state to a larger
extent than the excited state).
Acknowledgment. The Belgian IAP-VI/27 program is thanked
for continuing support and a fellowship to W.Q. and T.R. The
authors are grateful to the Research Council of the K. U. Leuven
for Grant GOA 2006/2 and a Junior Fellowship to J.N.C. K.R.
thanks the K. U. Leuven for financial support. The Fonds voor
Wetenschappelijk Onderzoek-Vlaanderen is thanked for Grant
G.0320.00, a predoctoral fellowship to P.D., and a postdoctoral
fellowship to K.D. The Instituut voor de aanmoediging van
innovatie door Wetenschap en Technologie in Vlaanderen (IWT)
is acknowledged for Grant ZWAP 04/007 and for a fellowship
to V.L. D.B. is a research associate of the Belgian Fonds
National de la Recherche Scientifique (FNRS). B.V.A. thanks
the Fonds pour la Recherche en Industrie et en Agriculture
(FRIA) for financial support.
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