Spectroscopic and photophysical properties of mono- and dithiosubstituted BODIPY dyes

Article abstract of DOI:10.1016/j.jphotochem.2019.112206

A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes, substituted at the 3 and 5 positions was synthesized and their spectral and photophysical properties were investigated in solvents of different polarity and polarizability. All compound

Full text of DOI:10.1016/j.jphotochem.2019.112206

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Spectroscopic and photophysical properties of mono- and
dithiosubstituted BODIPY dyes
´
Katarina Zlatic, Hicham Ben El Ayouchia, Hafid Anane, Branka
´
´
Mihaljevic, Nikola Basaric, Taoufik Rohand
PII:
S1010-6030(19)31468-6
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112206
JPC 112206
Reference:
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Journal of Photochemistry & Photobiology, A: Chemistry
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Accepted Date:
26 August 2019
9 October 2019
28 October 2019
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Please cite this article as: Zlatic K, Ayouchia HBE, Anane H, Mihaljevic B, Basaric N, Rohand
T, Spectroscopic and photophysical properties of mono- and dithiosubstituted BODIPY dyes,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
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Spectroscopic and photophysical properties of mono- and dithiosubstituted BODIPY
dyes
Katarina Zlatić,a Hicham Ben El Ayouchia,b Hafid Anane,b Branka Mihaljević,c Nikola
Basarića* and Taoufik Rohandb*
a
Laboratory for Synthetic Organic Chemistry, Department of Organic Chemistry and
Biochemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia. E-mail:
nbasaric@irb.hr.
b
Laboratory of analytical and molecular Chemistry, Faculty Polydisciplinaire of Safi, Route
Sidi Bouzid BP 4162, 46000 Safi, University Cadi Ayyad Marrakech, Morocco. E-mail:
taoufik.rohand@uca.ac.ma.
c
Laboratory for Radiation Chemistry and Dosimetry, Department of Material Chemistry,
Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia.
Graphical abstract
Highlights
 A series of BODIPY dyes, substituted at the 3 and 5 positions with Cl or thioalkyl
groups was synthesized
 BODIPY compounds are characterized by strong absorption bands at 500-580 nm,
corresponding to S0→S1 transition
1

 Emission spectra of BODIPY compounds are mirror images to absorption with
maxima at 520-600 nm.
 Substitution of the BODIPY by Cl or S-alkyl induces batochromic shifts in absorption
and emission for 50-60 nm and higher Φf (0.1-0.7).
Estimated singlet excited state lifetimes (τ = 2-6 ns) reveal that kR and non kNR do not vary
with changes in solvent polarity/polarizability
Abstract
A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes, substituted at the 3
and 5 positions was synthesized and their spectral and photophysical properties were
investigated in solvents of different polarity and polarizability. All compounds are
characterized by strong absorption bands at 500-580 nm, corresponding to S0→S1 transition,
the typical for the BODIPY chromophore. Emission spectra are mirror images to absorption
with maxima at 520-600 nm. Substitution of the BODIPY chromophore by Cl or thioalkyl
substituents induces batochromic shifts in both absorption and emission spectra for 50-60 nm
and induces higher quantum yields of fluorescence (Φf = 0.1-0.7). The findings were
rationalized by TD-DFT computations. The dyes exhibit weak solvatochromic properties
which could not be correlated to solvent polarizability, contrary to majority of BODIPY
derivatives. Furthermore, estimated singlet excited state lifetimes (τ = 2-6 ns) reveal that
radiative and non-radiative rate constants for deactivation from S1 do not vary with changes in
solvent polarity/polarizability. However, the highest values of Φf and τ are generally found in
solvents of high polarizability. Presented spectroscopic and photophysical study is important
for the use of dyes as chemodosimeters for thiols and cystein, as well as for the rational
design of new molecular probes.
2

Key words: BODIPY; fluorescence spectroscopy; solvatochromism
1. Introduction
Derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene,[1] better known as BODIPY dyes,
have become popular fluorophores in life sciences, often used in the design of molecular
probes, or in fluorescent microscopy for staining different intracellular organelles.[2] The
outstanding properties of BODIPY dyes include good thermal and photochemical stability,
good solubility in many organic solvents, excitation/emission spectra in the visible region and
narrow emission bandwidths with high peak intensities, high fluorescence quantum yields
(Φf) and molar absorption coefficients (ε > 50 000 M−1 cm−1), and usually negligible quantum
yields of intersystem crossing (ISC) and triplet-state formation.[3] In recent years, an
increasing number of reports has focused on the syntheses and functionalization reactions of
BODIPY dyes, providing
a
plethora of new molecules with tunable
spectroscopic/photophysical properties, which was a subject of several review articles.[4]
However, a pursuit for even better dyes with different properties suitable for various
applications is not over. It is particularly important to develop different BODIPY analogues
with emission wavelengths tunable from 500 nm to over 700 nm, which can be used as
fluorescence FRET pairs with applications in chemistry and biology. One approach to
differently substituted BODIPY derivatives include substitution of chlorine on the BODIPY
core, which can be facilitated by nucleophiles,[5] or in Pd-catalyzed arylations.[6]
Furthermore, Jiao et al. recently introduced a better method for the chlorination[7] and
bromination of BODIPY derivatives.[8] However, recent synthetic advances allowed for the
nucleophilic substitution on the BODIPY without chlorine substitution,[9] as well as direct C-
H arylation reactions,[10] significantly shortening the synthetic pathways towards new dyes.
3

Moreover, Jiao et al. also described transition-metal-free regioselective C-H/S-H
crosscoupling of BODIPY with thiols.[11]
Different substituents at the BODIPY positions 3 and 5 provided new dyes whose
photophysical properties have been investigated.[12] These dyes mostly exhibit fluoro-
solvatochromic properties that were related to polarizability of solvent.[12] However, a
striking difference in the spectral and photophysical properties was observed between mono-
[13] and bis-anilino-substituted BODIPY dyes,[14] and rationalized in view of the typical
properties of polymethine dyes.[13,14,15] On the other hand, solvatochromic properties of
certain dyes could not be explained by solvent polarizability and multiple-parameter approach
introduced by Catalan was used.[16] Here we report a systematic study of spectroscopic and
photophysical properties for 5 BODIPY dyes that bear different subsitution at the 3 and 5
positions (Figure 1). Two dyes are symmetrically substituted (Cl2 and S2), two are
monosubstituted (H-Cl and H-S) and one is asymmetrically substituted (S-Cl). Our
spectroscopic investigation is initiated due to recent applications of Cl-substituted BODIPY
dyes or carbaldehydes as chemosensors for cysteine,[17] or glutathione.[18] Therefore, we
disclose a thorough spectroscopic and photophysical investigation of S-substituted BODIPY
dyes in comparison to Cl-substituted dyes. Synthesis of S-substituted BODIPY dyes has been
reported,[19] as well as some preliminary investigation of their photophysical properties.[20]
However, a thorough photophysical investigation and comparison between the Cl- and S-
subsitited dyes has hitherto not been disclosed.
4

CH3
N
N
B
R1
R2
F
F
H-Cl R1= H, R2 = Cl
Cl2
H-S
R1= R2 = Cl
R1= H, R2 = SCH2COOCH2CH3
S-Cl R1= Cl, R2 = SCH2COOCH2CH3
S2
R1= R2 = SCH2COOCH2CH3
Figure 1. BODIPY derivatives
2. Materials and Methods
2.1.General
1H and 13C NMR spectra were recorded at 300, or 600 MHz (75 MHz and 150 MHz) at 25 °C
13
using TMS as a reference and chemical shifts were reported in ppm. C NMR spectra were
recorded by APT or complete (COM) techniques. HRMS were obtained on a MALDI
TOF/TOF instrument. Chemicals were purchased from the usual commercial sources and
were used as received. Silica gel (0.05–0.2 mm) was used for chromatographic purifications.
Solvents for chromatographic separations were used as delivered from the supplier (p.a. or
HPLC grade) or purified by distillation (CH2Cl2). Dry CH2Cl2 was obtained after standing of
commercial product over anhydrous MgSO4 overnight, then filtered and stored over 4Å
molecular sieves. Dry THF was obtained by distillation from Na, and stored over Na.
Synthetic procedure and characterization of known intermediates and known BODIPY
compounds Cl2, H-Cl, S-Cl and S2 is reported in the Supporting Information.
2.2. Synthesis of 4,4-Difluoro-8-(4-methylphenyl)-3-(2-ethoxy-2-oxoethyl)thio-4-bora-
3a,4a-diaza-s-indacene (H-S)
5

BODIPY compound H-Cl (20 mg, 0.063 mmol) was dissolved in CH3CN (10 mL). To the
solution ethyl mercaptoacetate (8 µL, 0.069 mmol) and triethylamine (TEA, 10 µL, 0.069
mmol) were added. The reaction mixture was stirred at rt 4 h. When the reaction was
completed, H2O (20 mL) was added and an extraction with CH2Cl2 (2× 20 mL) was carried
out. The extracts were dried over anhydrous Na2SO4, filtered, and the solvent was removed on
a rotary evaporator. The residue was chromatographed on a column of silica gel using CH2Cl2
as eluent to afford the pure product (10 mg, 40%).
violet crystals; mp 142-143 °C; 1H NMR (CDCl3, 300 MHz): δ = 7.79 (s, 1H), 7.42 (d, 2H, J
= 8.0 Hz), 7.31 (d, 2H, J = 7.9 Hz), 6.95 (d, 1H, J = 4.2 Hz), 6.80 (d, 1H, J = 4.2 Hz), 6.57 (d,
1H, J = 4.5 Hz), 6.48 (dd, 1H, J = 1.9, 2.1 Hz), 4.24 (q, 2H, J = 7.2, Hz,), 2.46 (s, 3H), 1.29 (t,
3H, J = 7.2 Hz) ppm; 13C NMR (CDCl3, 150 MHz, COM): δ = 168.3 (s), 157.7 (s), 143.0 (s),
141.0 (d), 140.9 (s), 136.7 (s), 134.2 (s), 132.3 (d), 130.9 (s), 130.5 (d), 129.1 (d), 128.8 (d),
118.0 (d), 117.4 (d), 62.2 (t), 34.7 (t), 21.4 (q), 14.1 (q) ppm; HRMS (MALDI-TOF/TOF)
calculated for C20H19BF2N2O2S 400.1228; found 400.1217.
2.3. UV-vis and fluorescence spectroscopy
UV-vis spectra were recorded on a PG T80/T80+, or a Varian Cary 100 Bio
spectrophotometer at rt. Fluorescence measurements were performed on an Agilent Cary
Eclipse fluorometer by use of slits corresponding to the bandpass of 2.5 nm for the excitation
and 5.0 nm for the emission. The samples were excited at 480, 490 and 500 nm, and for S-H,
S-Cl and S2 also at 520 nm. The emission was detected in the range of 490-800 nm. Solvents
for photophysical measurements were of HPLC or spectroscopic grade purity. Fluorescence
quantum yields were determined by use of rhodamine B in CH3OH (Φf = 0.66) as a
reference.[21] Quantum yields Φf were calculated according to Eq. S1 in the SI. One
fluorescence measurement was performed by exciting sample at three or four different
6

wavelengths, and the average value was calculated. Prior to the measurements, the solutions
were purged with Ar for 15 min. The measurement was performed at rt (25 °C).
2.4. Time-resolved fluorescence
Time-resolved florescence measurements were performed on a LP980 Edinburgh Instruments
spectrometer. For the excitation the second harmonic of a Qsmart Q450 Quantel YAG laser
was used. The energy of the laser pulse at 543 nm was set to <5 mJ and it was adjusted prior
to each measurement not to saturate the detector. The pulse duration was 7 ns. Absorbances at
the excitation wavelength were set to < 0.1. The solutions were purged for 15 min with Ar
prior to the measurements. The decays were analyzed by fitting to sums of exponentials (Eq.
S2 in the SI) using Gaussian-weighted non-linear least-squares fitting based on Marquardt-
Levenberg minimization implemented in the software package from Edinburgh Instruments.
The fitting parameters (decay times and pre-exponential factors) were determined by
minimizing the reduced chi-square χ2 and graphical methods were used to judge the quality of
the fit that included plots of the weighted residuals vs. channel number. For each sample,
three decays were measured at three emission wavelengths, and the average value is reported.
2.5. Computational methods
The DFT computations were performed using the Gaussian 09 program package.[22] All
stationary point of BODIPY derivatives Cl2, S-Cl and S2 depicted in Fig. 5 were optimized
using the B3LYP functional,[23] together with the 6-31G(d,p) basis set [24] in the gas phase.
The UV-vis absorption spectra in CH3CN were simulated using Time-Dependent density
functional theory (TD-DFT) [25] employing CAM-B3LYP/6-31G(d,p) [26] and the conductor-
like polarizable continuum model (CPCM) [27] by single point calculations on the ground
state optimized geometries.
7

3. Results and discussion
3.1. Synthesis
Synthetic procedure for the preparation of BODIPY dyes is based on the standard procedure
for the preparation of the BODIPY core substituted with chlorines, [28] which can be
substituted by nucleophiles.[5] However, contrary to previous report,[28] chlorination of
dipyrromethane afforded a mixture of mono- and bis-chloro substituted molecules which were
not separated, but they were transformed to a mixture of BODIPY derivatives H-Cl and Cl2
(Scheme 1), which are more stable so it was easier to separate them. Note that chlorination of
BODIPY, as reported by Jiao et al. provides chlorinated derivatives in higher yields.[7]
Nevertheless, nucleophilic substitution of H-Cl and Cl2 with ethyl 2-mercaptoacetate
afforded dyes H-S, SCl and S2 in good yields.
CH3
CH3
CH3
CH3
CH3
TFA
NClS
THF
+
+
N
H
CHO
NH HN
NH HN
NH HN
N
F
N
B
Cl
Cl
1) DDQ
Cl
Cl
F
S
O
O
t
r
,
S-Cl
O
O
N
C
3
2) TEA, BF3OEt2
H
HS
C
,
CH3
A
CH3
E
CH3
T
CH3
O
O
2
HS
HS
O
O
+
TEA, CH3CN
TEA, CH3CN
N
F
N
N
F
N
B
F
B

T
N
N
N
N
S
F
S
B
B
S
O
F
H-Cl
F
Cl
O
Cl
F
F
Cl
O
Cl2
O
S2
O
H-S
O
Scheme 1. Synthesis of BODIPY derivatives (∆T - heating).
3.2. Spectroscopic and photophysical properties
Absorption and fluorescence spectra for the set of 5 BODIPY dyes were measured in eight
solvents characterized by different polarity and polarizability. Table 1 compiles spectroscopic
properties of the dyes. Absorption spectra are characterized by the typical BODIPY sharp
band with a maximum at 500-579 nm due to the 0-0 S0→S1 transition, a shoulder at higher
8

energy side due to 0-1 vibrational band of the same transition, and a weaker broad band at
300-400 nm, populating higher excited states. The half width of the intense absorption band
(fwhmabs) is small 700-900 cm-1, regardless of substituents, contrary to the pronounced effects
observed for amino substituted BODIPY dyes that exhibit much larger fwhmabs.[13,14
]
Emission spectra are small Stokes-shifted mirror images of the intense absorption bands with
maxima at 510-590 nm. The substitution of the BODIPY moiety with Cl, or more pronounced
with thiol groups, shifts both absorption and fluorescence spectra batochromically. Figure 2
shows absorption and fluorescence spectra of the dyes in cyclohexane, and Fig. S1 in
acetonitrile (CH3CN). In cyclohexane, a difference in the maximum of absorption between H-
Cl and S2 is 55 nm, similar to the difference in fluorescence spectra of 59 nm. Such a trend
hardly changes with the solvent polarity. Thus, similar differences of 56 nm in the absorption
and 66 nm in fluorescence spectra were observed and between H-Cl and S2 in CH3CN. The
origin of the batochromic shifts is in a significant electron density on the Cl and S atoms in
HOMO and LUMO orbitals (vide infra), giving rise to a higher degree of electron
delocalization. In addition to batochromic shifts, substitution of the BODIPY moiety gives
rise to higher fluorescence quantum yields (Φf). The highest Φf was measured for S2, S-Cl
and Cl2 molecules, whereas H-Cl and HS are generally less fluorescent (Table 1).
1.0
Cl2
H-Cl
H-S
S-Cl
S2
Cl2
0.8
0.6
0.4
0.2
0.0
H-Cl
H-S
S-Cl
S2
500
550
600
650
300
400
500
600
Wavelength / nm
Wavelength / nm
Figure 2. Absorption (left) and fluorescence spectra (right) of BODIPY dyes in cyclohexane.
9

Table 1. Spectral and photophysical properties of BODIPY derivatives.
e
Comp Solvent
λabs / nm fhwmabs/cm-1 b λem / nm c Stokes / cm-1 d Φf
a
Cl2
Cyclohex. 514
724
790
524
529
371
514
0.24±0.03
0.69±0.01
0.63[20]
toluene
515
CH2Cl2
EtOAc
THF
512
509
511
803
775
769
524
521
523
447
452
449
0.42±0.05
0.42±0.02
0.47±0.01
0.37[20]
DMSO
512
879
527
556
0.087±0.0
01
CH3CN
CH3OH
H-Cl Cyclohex. 505
508
508
819
819
784
856
803
877
869
904
519
520
518
522
518
516
517
521
417
454
497
528
226
540
499
608
0.41±0.01
0.38±0.01
0.10±0.02
0.27±0.01
0.14±0.01
0.13±0.01
0.16±0.01
0.080±0.0
01
toluene
CH2Cl2
EtOAc
THF
508
512
502
504
505
DMSO
CH3CN
CH3OH
Cyclohex. 533
toluene
CH2Cl2
EtOAc
500
501
884
842
778
805
817
824
514
515
546
553
547
546
545
543
447
608
515
553
0.11±0.01
0.11±0.01
0.07±0.01
0.18±0.01
0.09±0.01
0.078±0.0
01
H-S
535
532
530
THF
532
535
851
912
549
553
582
608
0.083±0.0
02
0.130±0.0
03
DMSO
CH3CN
CH3OH
528
528
830
830
545
542
591
489
0.08±0.01
0.064±0.0
02
S-Cl
Cyclohex. 541
684
746
757
799
793
892
842
805
884
768
812
553
557
553
552
555
560
551
550
583
588
582
401
429
435
506
501
593
543
510
422
476
456
0.20±0.02
0.52±0.01
0.30±0.03
0.36±0.03
0.28±0.01
0.33±0.02
0.25±0.01
0.26±0.02
0.65±0.05
0.75±0.01
0.50±0.05
toluene
CH2Cl2
EtOAc
THF
DMSO
CH3CN
CH3OH
544
540
537
540
542
535
535
S2
Cyclohex. 569
toluene
CH2Cl2
572
567
10

EtOAc
THF
DMSO
CH3CN
CH3OH
566
569
573
564
564
814
806
897
791
824
582
588
592
580
580
486
568
560
489
489
0.63±0.03
0.62±0.01
0.50±0.05
0.66±0.01
0.61±0.02
a Maximum in absorption spectrum. b Half width of the absorption band, fhwmabs.c Maximum
d
e
in emission spectrum. Stokes shift. Quantum yield of fluorescence measured by use of
Rhodamine B in CH3OH as a reference (Φf = 0.66).[21] The Reported values represent average
from at least three measurements, and the error is maximal deviation.
Increase of solvent polarity generally induces small blue shifts in the absorption and
batochromic shifts in the emission spectra for all dyes. Normalized absorption and emission
spectra for S-Cl and S2 are shown in Figs 3 and 4 (for other molecules see Figs S2-S4 in the
SI). However, dependence of the maxima in absorption, emission, or Stokes shifts could not
be well correlated to polarity parameter f(ε), polarizability f(n2), Lippert (f(ε)- f(n2)), ET(30)
(for correlations see figs S5-S8 in the SI). Albeit somewhat better correlation was found for
the dependence of the Stokes shift on the Lippert parameter for S2 (r = 0.76) and ET(30) for
Cl2 (r = 0.85), general lack of the correlation suggests that Cl and thiol substituted BODIPY
dyes, particularly those which are asymmetrically substituted, do not exhibit an increase of
dipole moment between S0 and S1. Therefore, there is no dependence on fluorosolvatochromic
properties on the Lippert parameter or ET(30). A blue shift in the absorption spectra upon
increase of solvent polarity actually suggests a larger dipole moment in S0 then in S1. As for
the dependence of the solvatochromicity on the solvent polarizability f(n2), fitting gave poor
correlations, with the notable exceptions for the dependence of νabs and νem for H-Cl (r = 0.75
and 0.74, respectively). These findings indicate that the Wan der Waals and excitonic
interactions with polarizable solvent are masked with solvent-solute specific interactions,
resulting in no clear dependence of the maxima in absorption or emission on f(n2). Such a
solvatochromic behavior has been documented for asymmetrically substituted BODIPY,[13]
11

but it contrasts majority of symmetrically substituted BODIPY dyes such as phenoxy-[12] and
anilino-derivatives [14] whose solvatochromic properties depend on the polarizability of
solvent.
Solvatochromic properties in absorption and emission spectra, and the Stokes shifts were
fitted also to multiparameter Catalán equation (Eq. 1), which includes solvent acidity (SA),
basicity (SB), polarizability (SP) and dipolarity (SdP).[29] Estimated fitting parameters can be
found in the SI (Table S1). Contrary to previously demonstrated applicability of the Catalán
approach for the estimation of spectral properties of BODIPY dyes,[16,30] the best correlation
was found for the dependence of the Stokes shift for Cl2 (r = 0.85), and νabs and νem for S-Cl
(r = 0.89 and 0.88) and S2 (r = 0.95 and 0.96), whereas for the other dyes the Catalán
approach is not applicable. Furthermore, for S2 for which the best correlation was observed
we investigated which parameters significantly affect the νabs and νem and found out that none
of the parameters can be excluded. Fitting to SP and SdP only (two independent parameters),
gave poor correlations with r ≈ 0.1, whereas including SA or SB (three independent
parameters) increased the quality of the fit with the correlation of r ≈ 0.5. Consequently, the
set of investigated BODIPY derivatives shows mild and not specific solvatochromic
properties.
푦 = 푦0 + 푎SASA + 푏SBSB + 푐SPSP + 푑SdPSdP
(Eq. 1)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
cyclohexane
toluene
CH2Cl2
EtOAc
THF
CH3CN
DMSO
cyclohexane
toluene
CH2Cl2
EtOAc
THF
CH3CN
DMSO
CH3OH
0.8
0.6
0.4
0.2
0.0
CH3OH
480 500 520 540 560 580 600 620
460
480
500
520
540
560
580
Wavelength / nm
Wavelength / nm
Figure 3. Absorption spectra of S-Cl (left) and S2 (right) in different solvents.
12

cyclohexane
toluene
CH2Cl2
EtOAc
cyclohexane
toluene
CH2Cl2
EtOAc
THF
THF
CH3CN
DMSO
CH3OH
CH3CN
DMSO
CH3OH
550
600
650
550
600
650
700
Wavelength / nm
Wavelength / nm
Figure 4. Emission spectra of S-Cl (left) and S2 (right) in different solvents.
Singlet excited state lifetimes for BODIPY derivatives were measured on a laser flash
photolysis (LFP) setup (λex = 534 nm), giving only approximate lifetime values (τ, Table 2,
for some representative decays see Figs S9-S11 in the SI). However, these approximate values
together with Φf allowed for the approximation of rate constant for radiative (kR) and non-
radiative deactivation from S1 (kNR). Note a general trend that all BODIPY derivatives exhibit
the highest values of Φf and τ in highly polarizable solvents such as CH2Cl2 or toluene. Such
a behavior has been reported for different BODIPY dyes.[3,12,13,14,15,20] The estimated values
of kR are (3-6)×107 s-1 for H-S, (6-12) ×107 for S-Cl and ≈ 108 s-1 for S2, and similarly to kNR,
they do not vary much with changes in solvent polarity/polarizability. On the hand, it is
interesting to note that kNR exhibits almost an order of magnitude lower values for more
fluorescent S2 BODIPY compared to less fluorescent derivatives H-S, or S-Cl, indicating that
kNR has a more profound effect on Φf then kR. A significant effect of BODIPY molecular
structure on kNR and Φf has been reported, particularly in the examples for the N-substituted
dyes which undergo photoinduced charge transfer.[12,13,14] However, for the BODIPY
derivatives investigated herein, charge transfer is not plausible, as reflected by weak
solvatochromic properties.
13

Table 2. Photophysical properties of BODIPY compounds.
τ / ns a kR/107 s-1 b kNR/107 s-1 c
Comp. Solvent
H-S
S-Cl
S2
Cyclohex. 2.3±1
3.0
2.9±0.3 6.2
2.1±0.8 4.3
3.0±0.5 2.6
2.2±0.5 3.7
2.1±0.6 6.2
40
28
43
31
42
41
32
37
24
13
20
22
30
20
22
26
6.6
4.3
8.2
6.3
6.8
9.4
5.9
6.7
toluene
CH2Cl2
EtOAc
THF
DMSO
CH3CN
CH3OH
2.9±1
2.5±0.5 2.5
2.8
Cyclohex. 3.3±0.5 6.0
toluene
CH2Cl2
EtOAc
THF
DMSO
CH3CN
CH3OH
3.8±0.3 14
3.5±0.2 8.6
2.9±0.5 12
2.4±0.3 12
3.3±0.4 10
3.4±0.3 7.4
2.8±0.3 9.3
Cyclohex. 5.3±0.3 12
toluene
CH2Cl2
EtOAc
THF
DMSO
CH3CN
CH3OH
5.8±0.4 13
6.1±0.5 8.2
5.9±0.5 11
5.6±0.4 11
5.3±0.3 9.4
5.8±0.4 11
5.8±0.3 10
a Estimated lifetime measured by LFP spectrometer.
b Radiative rate constant for deactivation from S1.
c Nonradiative rate constant for deactivation from S1.
3.3. Molecular modeling
To rationalize spectral and photophysical observations, computations were performed for
molecules Cl2, S-Cl and S2. Figure 5 shows the optimized structures and orbital distributions.
The HOMO is delocalized over both pyrrole rings as π-bonding orbitals, while the LUMO
expands all over the BODIPY moiety as π*-antibonding orbital. Note that both HOMO and
LUMO have electron density on the S and Cl atoms, contributing to the increased electron
delocalization and observed red-shifts in absorption and fluorescence spectra compared to
non-substituted dyes. Furthermore, promotion of an electron from HOMO to LUMO increases
14

electron density on the meso-phenyl ring. However, excitation does not lead to a significant
change in dipole moment, which was reflected in modest fluorosolvatochromic properties that
do not depend on the Lippert or ET(30) parameters.
Comp.
Optimized Geometry
HOMO-Orbital
LUMO-Orbital
Cl2
15

S-Cl
S2
Figure 5. Optimized geometry for BODIPY derivatives Cl2, S-Cl and S2 at B3LYP/6-31G (d,
p) level of theory, and the corresponding HOMO and LUMO orbitals.
Calculated energies of HOMO and LUMO are compiled in Table 3. The substitution of Cl by
thiols causes a small increase in the energy level of the HOMO and a slight increase in the
energy level of the LUMO, thus decreasing the energy gap between these frontier orbitals
from 2.94 eV to 2.74 eV for S-Cl and 2.6 eV for S2 in good agreement with the measured
UV-vis spectra (Fig. 3, for computed spectra see Fig S12 in the SI). The experimentally
observed absorption maxima at 508 nm for Cl2, 535 nm for S-Cl and 564 nm for S2 nm
originate mainly from the HOMO→LUMO transitions, and they are comparable to computed
maxima at 427 nm, 449nm, and 479nm, respectively.
Table 3.Calculated energy levels (in eV) of the HOMO, LUMO and energy gap (∆E) at
B3LYP/6-31G(d,p) level of theory.
EHOMO
ELUMO
∆Ea
16

Comp. -5.871936
-2.931072
-2.687632
-2.473296
2.94
2.74
2.60
S-Cl
S2
-5.433744
-5.07416
a∆E=ELUMO-EHOMO
4. Conclusion
A series of BODIPY dyes, symmetrically or asymmetrically substituted at the 3 and 5
positions was synthesized. Photophysical properties of the dyes were investigated in solvents
of different polarity and polarizability. In contrast to majority of BODIPY derivatives, here
presented dyes exhibit weak solvatochromic properties that cannot be correlated to solvent
polarizability. Substitution of the BODIPY chromophore by Cl or thioalkyl substituents
induces batochromic shifts in both absorption and emission spectra for 50-60 nm and induces
higher quantum yields of fluorescence. Thus, the highest Φf was measured for Cl2, S-Cl and
S2, whereas H-Cl and H-S are less fluorescent. Similarly to weak solvatochromic properties,
Φf, singlet excited state lifetimes, kR and kNR, are insignificantly affected by changes in
solvent polarity/polarizability. Nevertheless, the highest values of Φf and τ were found in
solvents of the highest polarizability. The findings were rationalized by TD-DFT
computations. Presented study is important for the use of chloro-substituted BODIPY dyes as
chemodosimeters for thiols and cystein, as well as for the rational design of new dyes.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgement
17

These materials are based on work financed by the Croatian Science Foundation (HRZZ IP-
2014-09-6312). Professor Dr. Taoufik Rohand thanks the Laboratory of Analytical and
Molecular Chemistry of the Faculty Polydisciplinaire Safi for the financial support.
ORCID
Hicham Ben El Ayouchia: 0000-0002-4713-4837
Nikola Basarić: 0000-0001-9412-9734
Taoufik Rohand: 0000-0002-7370-9358
References
1 A. Treibs, F.-H. Kreuzer, Difluorboryl‐Komplexe von Di‐ und Tripyrrylmethenen, Liebigs
Ann. Chem. 718 (1968) 208-223.
2 R. P. Haugland, The Handbook. A Guide to Fluorescent Probes and Labeling Technologies,
10th ed.; Molecular Probes, Inc.: Eugene, Oregon, USA, 2005.
3 The following represents a non-exhaustive list of early and recent BODIPY papers with
spectroscopic/photophysical data: (a) E. Vos de Wael, J. A. Pardoen, J. A. van Koeveringe, J.
Lugtenburg, Pyrromethene‐BF2 complexes (4,4′‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacenes).
Synthesis and luminescence properties. Recl. Trav. Chim. Pays-Bas 96 (1977) 306-309. (b) J.
Karolin, L. B.-Å. Johansson, L. Strandberg and T. Ny, Fluorescence and Absorption
Spectroscopic Properties of Dipyrrometheneboron Difluoride (BODIPY) Derivatives in
Liquids, Lipid Membranes, and Proteins. J. Am. Chem. Soc. 116 (1994) 7801-7806. (c) M.
Kollmannsberger, K. Rurack, U. Resch-Genger, J. Daub, Ultrafast Charge Transfer in Amino-
Substituted Boron Dipyrromethene Dyes and Its Inhibition by Cation Complexation:ꢀ A New
Design Concept for Highly Sensitive Fluorescent Probes. J. Phys. Chem. A 102 (1998)
18

10211-10220. (d) T. López Arbeloa, F. López Arbeloa, I. López Arbeloa, I. García-Moreno,
A. Costela, R. Sastre, F. Amat-Guerri, Correlations between photophysics and lasing
properties of dipyrromethene-BF2 dyes in solution. Chem. Phys. Lett. 299 (1999) 315-321.
(e) K. Rurack, M. Kollmannsberger, J. Daub, A highly efficient sensor molecule emitting in
the near infrared (NIR): 3,5-distyryl-8-(p-dimethylaminophenyl)difluoroboradiaza-s-
indacene. New. J. Chem. 25 (2001) 289-292. (f) K. Rurack, M. Kollmannsberger, J. Daub,
Molecular Switching in the Near Infrared (NIR) with a Functionalized Boron-Dipyrromethene
Dye. Angew. Chem. Int. Ed. 40 (2001) 385-387. (g) F. Bergström, I. MikhalyovP. Hägglöf,
R. Wortmann, T. Ny, L. B.-Å. Johansson, Dimers of Dipyrrometheneboron Difluoride
(BODIPY) with Light Spectroscopic Applications in Chemistry and Biology. J. Am. Chem.
Soc. 124 (2002) 196-204. (h) A. Costela, I. García-Moreno, C. Gomez, R. Sastre, F. Amat-
Guerri, M. Liras, F. Lόpez Arbeloa, J. Bañuelos Prieto, I. Lόpez Arbeloa, Photophysical and
Lasing Properties of New Analogs of the Boron-Dipyrromethene Laser Dye PM567 in Liquid
Solution. J. Phys. Chem. A 106 (2002) 7736-7742. (i) F. Lόpez Arbeloa, J. Bañuelos Prieto,
V. Martínez Martínez, T. Arbeloa Lόpez, I. Lόpez Arbeloa, Intramolecular Charge Transfer in
Pyrromethene Laser Dyes: Photophysical Behaviour of PM650. ChemPhysChem 5 (2004)
1762-1771. (j) Y. Gabe, Y. Urano, K. Kikuchi, H. Kojima, T. Nagano, Highly Sensitive
Fluorescence Probes for Nitric Oxide Based on Boron Dipyrromethene ChromophoreRational
Design of Potentially Useful Bioimaging Fluorescence Probe. J. Am. Chem. Soc. 126 (2004)
3357-3367. (k) J. Bañuelos Prieto, F. Lόpez Arbeloa, V. Martínez Martínez, T. Arbeloa
Lόpez, F. Amat-Guerri, M. Liras, I. Lόpez Arbeloa, Photophysical properties of a new 8-
phenyl analogue of the laser dye PM567 in different solvents: internal conversion
mechanisms. Chem. Phys. Lett. 385 (2004) 29-35. (l) W. Qin, M. Baruah, M. Van der
Auweraer, F. C. De Schryver, N. Boens, Photophysical Properties of Borondipyrromethene
Analogues in Solution. J. Phys. Chem. A 109 (2005) 7371-7384. (m) N. Basarić, M. Baruah,
19

W. Qin, B. Metten, M. Smet, W. Dehaen, N. Boens, Synthesis and spectroscopic
characterisation of BODIPY® based fluorescent off-on indicators with low affinity for
calcium. Org. Biomol. Chem. 3 (2005) 2755-2761. (n) K. Yamada, Y. Nomura, D. Citterio,
N. Iwasawa, K. Suzuki, Highly Sodium-Selective Fluoroionophore Based on Conformational
Restriction of Oligoethyleneglycol-Bridged Biaryl Boron−Dipyrromethene. J. Am. Chem.
Soc. 127 (2005) 6956-6957. (o) J.-P. Malval, I. Leray, B. Valeur, A highly selective
fluorescent molecular sensor for potassium based on a calix[4]bisazacrown bearing boron-
dipyrromethene fluorophores. New. J. Chem. 29 (2005) 1089-1094. (p) A. Coskun, E. U.
Akkaya, Ion Sensing Coupled to Resonance Energy Transfer:ꢀ A Highly Selective and
Sensitive Ratiometric Fluorescent Chemosensor for Ag(I) by a Modular Approach. J. Am.
Chem. Soc. 127 (2005) 10464-10465. (q) Y. Wu, X. Peng, B. Guo, J. Fan, Z. Zhang, J. Wang,
A. Cui, Y. Gao, Boron dipyrromethene fluorophore based fluorescence sensor for the
selective imaging of Zn(II) in living cells. Org. Biomol. Chem. 3 (2005) 1387-1392. (r) M.
Baruah, W. Qin, C. Flors, J. Hofkens, R. A. L. Vallée, D. Beljonne, M. Van der Auweraer, W.
M. De Borggraeve, N. Boens, Solvent and pH Dependent Fluorescent Properties of a
Dimethylaminostyryl Borondipyrromethene Dye in Solution. J. Phys. Chem. A 110 (2006)
5998-6009. (u) W. Qin, M. Baruah, W. M. De Borggraeve, N. Boens, Photophysical
properties of an on/off fluorescent pH indicator excitable with visible light based on a
borondipyrromethene-linked phenol. J. Photochem. Photobiol. A: Chem. 183 (2006) 190-197.
(v) Z. Dost, S. Atilgan, E. U. Akkaya, Distyryl-boradiazaindacenes: facile synthesis of novel
near IR emitting fluorophores. Tetrahedron 62 (2006) 8484-8488. (w) Y. Mei, P. A. Bentley,
W. Wang, A selective and sensitive chemosensor for Cu2+ based on 8-hydroxyquinoline.
Tetrahedron Lett. 47 (2006) 2447–2449. (x) H. J. Kim, J. S. Kim, BODIPY appended cone-
calix[4]arene: selective fluorescence changes upon Ca2+ binding. Tetrahedron Lett. 47 (2006)
7051-7055. (y) X. Qi, E. J. Jun, L. Xu, S. J. Kim, J.S. J. Hong, Y. J. Yoon, J. Yoon, New
20

BODIPY Derivatives as OFF-ON Fluorescent Chemosensor and Fluorescent
Chemodosimeter for Cu2+:ꢀ Cooperative Selectivity Enhancement toward Cu2+. J. Org. Chem.
71 (2006) 2881-2884. (z) X. Peng, J. Du, J. Fan, J. Wang, Y. Wu, J. Zhao, S. Sun, T. Xu, A
Selective Fluorescent Sensor for Imaging Cd2+ in Living Cells. J. Am. Chem. Soc. 129 (2007)
1500-1501. (ab) Z. Li, R. Bittman, Synthesis and Spectral Properties of Cholesterol- and
FTY720-Containing Boron Dipyrromethene Dyes. J. Org. Chem. 72 (2007) 8376-8382. (ac)
W. Qin, T. Rohand, W. Dehaen, J. N. Clifford, K. Driessen, D. Beljonne, B. Van Averbeke,
M. Van der Auweraer, N. Boens, Boron Dipyrromethene Analogs with Phenyl, Styryl, and
Ethynylphenyl Substituents:ꢀ Synthesis, Photophysics, Electrochemistry, and Quantum-
Chemical Calculations. J. Phys. Chem. A 111 (2007) 8588-8597. (ad) Z. Ekmekci, M. D.
Yilmaz, E. U. Akkaya, A Monostyryl-boradiazaindacene (BODIPY) Derivative as
Colorimetric and Fluorescent Probe for Cyanide Ions. Org. Lett. 10 (2008) 461-464. (ae) L.
Li, J. Han, B. Nguyen, K. Burgess, Syntheses and Spectral Properties of Functionalized,
Water-Soluble BODIPY Derivatives. J. Org. Chem. 73 (2008) 1963-1970. (af) W. Qin, M.
Baruah, M. Sliwa, M. Van der Auweraer, W. M. De Borggraeve, D. Beljonne, B. Van
Averbeke, N. Boens, Ratiometric, Fluorescent BODIPY Dye with Aza Crown Ether
Functionality: Synthesis, Solvatochromism, and Metal Ion Complex Formation. J. Phys.
Chem. A 112 (2008) 6104-6114. (ag) K. Krumova, G. Cosa, Bodipy Dyes with Tunable
Redox Potentials and Functional Groups for Further Tethering: Preparation, Electrochemical,
and Spectroscopic Characterization. J. Am. Chem. Soc. 132 (2010) 17560-17569. (ah) R.
Lincoln, L. E. Greene, C. Bain, J. O. Flores-Rizo, D. S. Bohle, G. Cosa, When Push Comes to
Shove: Unravelling the Mechanism and Scope of Nonemissive meso-Unsaturated BODIPY
Dyes. J. Phys. Chem. B 119 (2015) 4758-4765. (ai) P. P. Goswami, A. Syed, C. L. Beck, T.
R. Albright, K. M. Mahoney, R. Unash, E. A. Smith, A. H. Winter, BODIPY-Derived
Photoremovable Protecting Groups Unmasked with Green Light, J. Am. Chem. Soc. 137
21

(2015) 3783-3786. (aj) M.-N. Belzile, R. Godin, A. M. Durantini, G. Cosa, Monitoring
Chemical and Biological Electron Transfer Reactions with a Fluorogenic Vitamin K
Analogue Probe. J. Am. Chem. Soc. 138 (2016) 16388-16397. (ak) A. M. Durantini, L. E.
Greene, R. Lincoln, S. R. Martínez, G. Cosa, Reactive Oxygen Species Mediated Activation
of a Dormant Singlet Oxygen Photosensitizer: From Autocatalytic Singlet Oxygen
Amplification to Chemicontrolled Photodynamic Therapy, J. Am. Chem. Soc. 138 (2016)
1215-1225. (al) E. Palao, T. Slanina, L. Muchovꢁ, T. ꢂolomek, L. Vítek, P. Klꢁn, Transition-
Metal-Free CO-Releasing BODIPY Derivatives Activatable by Visible to NIR Light as
Promising Bioactive Molecules. J. Am. Chem. Soc. 138 (2016) 126-133. (am) T. Slanina, P.
Shrestha, E. Palao, D. Kand J. A. Peterson, A. S. Dutton, N. Rubinstein, R. Weinstain, A. H.
Winter, P. Klꢁn, In Search of the Perfect Photocage: Structure-Reactivity Relationships in
meso-Methyl BODIPY Photoremovable Protecting Groups. J. Am. Chem. Soc. 139 (2017)
15168-15175. (an) J. A. Peterson, C. Wijesooriya, E. J. Gehrmann, K. M. Mahoney, P. P.
Goswami, T. R. Albright, A. Syed, A. S. Dutton, E. A. Smith, A. H. Winter, Family of
BODIPY Photocages Cleaved by Single Photons of Visible/Near-Infrared Light. J. Am.
Chem. Soc. 140 (2018) 7343-7346.
4 (a) R. Ziessel, G. Ulrich, A. Harriman, The chemistry of Bodipy: A new El Dorado for
fluorescence tools. New J. Chem. 31 (2007) 496-501. (b) A. Loudet, K. Burgess, BODIPY
Dyes and Their Derivatives:ꢀ Syntheses and Spectroscopic Properties. Chem. Rev. 107 (2007)
4891-4932. (c) G. Ulrich, R. Ziessel, A. Harriman, The chemistry of fluorescent bodipy dyes:
versatility unsurpassed. Angew. Chem. Int. Ed. 47 (2008) 1184-1201. (d) M. Benstead, G. H.
Mehl, R. W. Boyle, 4,40-Difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) as components
of novel light active materials, Tetrahedron 67 (2011) 3573-3601. (e) N. Boens, V. Leen, W.
Dehaen, Fluorescent indicators based on BODIPY, Chem. Soc. Rev. 41 (2012) 1130-1172.
22

5 T. Rohand, M. Baruah, W. Qin, N. Boens, W. Dehaen, Functionalisation of fluorescent
BODIPY dyes by nucleophilic substitution, Chem. Commun. (2006) 266-268.
6 (a) T. Rohand, W. Qin, N. Boens, W. Dehaen, Palladium-Catalyzed Coupling Reactions for
the Functionalization of BODIPY Dyes with Fluorescence Spanning the Visible Spectrum.
Eur. J. Org. Chem. (2006) 4658-4663. (b) E. Palao, G. Duran-Sampedro, S. de la Moya, M.
Madrid, C. García-Lꢃpez, A. R. Agarrabeitia, B. Verbelen, W. Dehaen, N. Boens, M. J. Ortiz,
Exploring the Application of the Negishi Reaction of HaloBODIPYs: Generality,
Regioselectivity, and Synthetic Utility in the Development of BODIPY Laser Dyes. J. Org.
Chem. 81 (2016) 3700-3710.
7 X. Zhou, Ch. Yu, Z. Feng, Y. Yu, J. Wang, E. Hao, Y. Wei, X. Mu, L. Jiao, Highly
Regioselective α-Chlorination of the BODIPY Chromophore with Copper(II) Chloride, Org.
Lett. 17 (2015) 4632-4635.
8 L. Jiao, W. Pang, J. Zhou, Y. Wei, X. Mu, G. Bai, E. Hao, Regioselective Stepwise
Bromination of Boron Dipyrromethene (BODIPY) Dyes, J. Org. Chem. 76 (2011) 9988-9996.
9 V. Leen, V. Zaragozí Gonzalvo, W. M. Deborggraeve, N. Boens, W. Dehaen, Direct
functionalization of BODIPY dyes by oxidative nucleophilic hydrogen substitution at the 3-
or 3,5-positions. Chem. Commun. 46 (2010) 4908-4910.
10 (a) L. Wang, B. Verbelen, C. Tonnelé, D. Beljonne, R. Lazzaroni, V. Leen, W. Dehaen, N.
Boens, UV-vis spectroscopy of the coupling products of the palladium-catalyzed C-H
arylation of the BODIPY core. Photochem. Photobiol. Sci. 12 (2013) 835-847. (b) B.
Verbelen, L. C. Dias Rezende, S. Boodts, J. Jacobs, L. Van Meervelt, J. Hofkens, W. Dehaen,
Radical C-H Alkylation of BODIPY Dyes Using Potassium Trifluoroborates or Boronic
Acids. Chem. Eur. J. 21 (2015) 12667-12675. (c) J. Wang, Y. Li, Q. Gong, H. Wang, E. Hao,
P.-C. Lo, L. Jiao, β‑Alkenyl BODIPY Dyes: Regioselective Synthesis via Oxidative C-H
23

Olefination, Photophysical Properties, and Bioimaging Studies. J. Org. Chem. 84 (2019)
5078-5090.
11 F. Lv, B. Tang, E. Hao, Q. Liu, H. Wang, L. Jiao, Transition-metal-free regioselective
cross-coupling of BODIPYs with thiols, Chem. Commun. 55 (2019) 1639-1642.
12 (a) W. Qin, T. Rohand, M. Baruah, A. Stefan, M. Van der Auweraer, W. Dehaen, N.
Boens, Solvent-dependent photophysical properties of borondipyrromethene dyes in solution,
Chem. Phys. Lett. 420 (2006) 562-568. (b) T. Rohand, J. Lycoops, S. Smout, E. Braeken, M.
Sliwa, M. Van der Auweraer, W.Dehaen, W. M. De Borggraeve, N. Boens, Photophysics of
3,5-diphenoxy substituted BODIPY dyes in solution Photochem. Photobiol. Sci. 6 (2007)
1061-1066. (c) K. Cieślik-Boczula, K. Burgess, L. Li, B. Nguyen, L. Pandey, W. M. De
Borggraeve, M. Van der Auweraer, N. Boens, Photophysics and stability of cyano-substituted
boradiazaindacene dyes. Photochem. Photobiol. Sci. 8 (2009) 1006-1015.
13 W. Qin, V. Leen, T. Rohand, W. Dehaen, P. Dedecker, M. Van der Auweraer, K. Robeyns,
L. Van Meervelt, D. Beljonne, B. Van Averbeke, J. N. Clifford, K. Driesen, K. Binnemans, N.
Boens, Synthesis, Spectroscopy, Crystal Structure, Electrochemistry, and Quantum Chemical
and Molecular Dynamics Calculations of a 3-Anilino Difluoroboron Dipyrromethene Dye, J.
Phys. Chem. A 113 (2009) 439-447.
14 W. Qin, V. Leen, W. Dehaen, J. Cui, C. Xu, X. Tang, W. Liu, T. Rohand, D. Beljonne, B.
Van Averbeke, J. N. Clifford, K. Driesen, K. Binnemans, M. Van der Auweraer, N. Boens,
3,5-Dianilino Substituted Difluoroboron Dipyrromethene: Synthesis, Spectroscopy,
Photophysics, Crystal Structure, Electrochemistry, and Quantum-Chemical Calculations. J.
Phys. Chem. C 113 (2009) 11731-11740.
15 (a) L.G. S. Brooker, G. H. Keyes, W. Williams, Color and Constitution. V.1 The
Absorption of Unsymmetrical Cyanines. Resonance as a Basis for a Classification of Dyes, J.
Am. Chem. Soc. 64 (1942) 199-210. (b) L. G. S. Brooker, G. H. Keyes, R. H. Sprague, R. H.
24

Van Dyke, E. Van Lare, G. Van Zandt, F. L. White, H. W. J. Cressman, S. G. Dent, Color and
Constitution. X.1 Absorption of the Merocyanines, J. Am. Chem. Soc. 73 (1951) 5332-5350.
16 A. Filarowski, M. Kluba, K. Cieślik-Boczula, A. Koll, A. Kochel, L. Pandey, W. M. De
Borggraeve, M. Van der Auweraer, J. Catálan, N. Boens, Generalized solvent scales as a tool
for investigating solvent dependence of spectroscopic and kinetic parameters. Application to
fluorescent BODIPY dyes. Photochem. Photobiol. Sci. 9 (2010) 996-1008.
17 (a) L.-Y. Niu, Y.-S. Guan, Y.-Z. Chen, L.-Z. Wu, C.-H. Tung, Q.-Z. Yang, BODIPY-
Based Ratiometric Fluorescent Sensor for Highly Selective Detection of Glutathione over
Cysteine and Homocysteine, J. Am. Chem. Soc. 134 (2012) 18928-18931. (b) S. Madhu, R.
Gonnade, M. Ravikanth, Synthesis of 3,5-Bis(acrylaldehyde) Boron-dipyrromethene and
Application in Detection of Cysteine and Homocysteine in Living Cells, J. Org. Chem. 78
(2013) 5056-5060. (c) F. Wang, Z. Guo, X. Li, X. Li, C. Zhao, Development of a Small
Molecule Probe Capable of Discriminating Cysteine, Homocysteine, and Glutathione with
Three Distinct Turn-On Fluorescent Outputs, Chem. Eur. J. 20 (2014) 11471-11478. (d) A.
Poirel, A. De Nicola, R. Ziessel, Thiazolidine Derivatives from Fluorescent Dithienyl-
BODIPYcarboxaldehydes and Cysteine. J. Org. Chem. 79 (2014) 11463-11472. (e) F. Wang,
L. Zhou, C. Zhao, R. Wang, Q. Fei, S. Luo, Z. Guo, H. Tian, W.-H. Zhu, A dual-response
BODIPY-based fluorescent probe for the discrimination of glutathione from cystein and
homocystein, Chem. Sci., 6 (2015) 2584-2589. (f) R. Lincoln, L. E. Greene, W. Zhang, S.
Louisia, G. Cosa, Mitochondria Alkylation and Cellular Trafficking Mapped with a
Lipophilic BODIPY-acrolein Fluorogenic Probe, J. Am. Chem. Soc. 139 (2017) 16273-
16281.
18 D. Gong, S.-C. Han, A. Iqbal, J. Qian, T. Cao, W. Liu, W. Liu, W. Qin, H. Guo, Fast and
Selective Two-Stage Ratiometric Fluorescent Probes for Imaging of Glutathione in Living
Cells, Anal. Chem. 89 (2017) 13112-12119.
25

19 a) Ö. Dilek, S. L. Bane, Synthesis, spectroscopic properties and protein labeling of water
soluble 3,5-disubstituted boron dipyrromethenes, Bioorg. Med. Chem. Lett. 19 (2009) 6911-
6913. b) J. Han, O. Gonzalez, A. Aguilar-Aguilar, E. Peña-Cabrera, K. Burgess, 3- and 5-
Functionalized BODIPYs via the Liebeskind-Srogl reaction, Org. Biomol. Chem. 7 (2009)
34-36. c) R. I. Roacho, A. Metta-Magaña, E. Peña-Cabrera, K. Pannell, Unprecedented one-
pot sequential thiolate substitutions under mild conditions leading to a red emissive BODIPY
dye 3,5,8-tris(PhS)-BODIPY, Org. Biomol. Chem. 13 (2015) 995-999. d) J. Ahrens, A.
Scheja, R. Wicht, M. Bröring, Excitonic Coupling in Acyclic and Cyclic Dithioaryl-Linked
BODIPY DYEmers, Eur. J. Org. Chem. (2016) 2864-2870. e) E. A. Leushina, I. A. Usol’tsev,
S. I. Bezzubov, A. A. Moiseeva, M. V. Terenina, A. V. Anisimov, I. V. Taydakov, A. V.
Khoroshutin, BODIPY dyes with thienyl- and dithienylthiosubstituents - synthesis, redox and
fluorescent properties, Dalton Trans. 46 (2017) 17093-17100. f) L. C. D. Rezende, S. M. G.
Melo, S. Boodts, B. Verbelen, F. S. Emery, W. Dehaen, Thiocyanation of 3-substituted and
3,5-disubstituted BODIPYs and its application for the synthesis of new fluorescent sensors.
Dyes and Pigments 154 (2018) 155-163.
20 E.Fron, E. Coutiño-Gonzalez, L. Pandey, M. Sliwa, M. Van der Auweraer, F. C. De
Schryver, J. Thomas, Z. Dong, V. Leen, M. Smet, W. Dehaen, T.Vosch. Synthesis and
photophysical characterization of chalcogen substituted BODIPY dyes, New J. Chem. 33
(2009) 1490-1496.
21 N. Boens, W. Qin, N. Basarić, J. Hofkens, M. Ameloot, J. Pouget, J.-P. Lefèvre, B. Valeur,
E. Gratton, M. vandeVen, N. D. Silva, Jr., Y. Engelborghs, K. Willaert, A. Sillen, G.
Rumbles, D. Phillips, A. J. W. G. Visser, A. van Hoek, J. R. Lakowicz, H. Malak, I.
Gryczynski, A. G. Szabo, D. T. Krajcarski, N. Tamai, A. Miura, Fluorescence Lifetime
Standards for Time and Frequency Domain Fluorescence Spectroscopy, Anal. Chem. 79
(2007) 2137-2149.
26

22 M. J. Frisch et al (2009) GAUSSIAN 09, Revision E 01, Gaussian Inc. Wallingford CT.
23 a) A. D. Becke, Density-functional thermochemistry. III. The role of exact Exchange. J.
Chem. Phys. 98 (1993) 5648-5652. b) C. Lee, W.Yang, R. G. Parr, Development of the Colle-
Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B. 37
(1998) 785-789. (c) L. Simón, J. M. Goodman, How reliable are DFT transition structures?
Comparison of GGA, hybrid-meta-GGA and meta-GGA functionals. Org Biomol Chem. 9
(2011) 689-700.
24 W. J. Hehre, L. Radom, P. V. R. Schleyer, J. Pople, Ab initio Mol Orbital Theory.Wiley,
New York, 1986.
25 E. K. U, Gross, W, Kohn, Time-Dependent Density-Functional Theory. Adv. Quant.
Chem. 21 (1990) 255-291.
26 T, Yanai, D. P, Tew, N. C. A, Handy, A new hybrid exchange correlation functional using
the Coulomb-attenuating method (CAM-B3LYP). Chem. Phys. Lett, 393 (2004) 51-57.
27 a) J. Tomasi, M Persico, Molecular Interactions in Solution: An Overview of Methods
Based on Continuous Distributions of the Solvent. Chem. Rev, 94 (1994) 2027-2094. b) B. Y.
Simkin B Y, I. Sheikhet, Quantum chemical and statistical theory of solutions computational
approach. Ellis Horwood, London. (1995)
28 M. Baruah, W. Qin, N. Basarić, W. M. De Broggraeve, N. Boens, BODIPY-Based
Hydroxyaryl Derivatives as Fluorescent pH Probes, J. Org. Chem. 70 (2005) 4152-4157.
29 J. Catálan, Toward a Generalized Treatment of the Solvent Effect Based on Four Empirical
Scales: Dipolarity (SdP, a New Scale), Polarizability (SP), Acidity (SA), and Basicity (SB) of
the Medium, J. Phys. Chem. B 113 (2009) 5951-5960.
30 L. Jiao, C. Yu, J. Wang, E. A. Briggs, N. A. Besley, D. Robinson, M. J. Ruedas-Rama, A.
Orte, L. Crovetto, E. M. Talavera, J. M. Alvarez-Pez, M. Van der Auweraer, N. Boens,
27

Unusual spectroscopic and photophysical properties of meso-tert-butylBODIPY in
comparison to related alkylated BODIPY dyes, RSC Adv. 5 (2015) 89375-89388.
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