Effect of solvent nature on spectral properties of blue-emitting meso-propargylamino-BODIPY

Article abstract of DOI:10.1016/j.molliq.2019.04.058

The spectral and luminescent properties of meso?propargylamino?boron (III) dipyrromethenate (BODIPY 1) were studied in a wide range of organic solvents of different polarities, electron- and proton-donating abilities. Effect of organic solvents on the properties of the BODIPY 1 spectral characteristics was analyzed in details. It was shown that the fluorescence of BODIPY 1 is characterized by a high sensitivity to the nature of the solvent. It was found that the complex, which emits in the blue-green region (433–486 nm), has a high quantum yield of fluorescence (~100%) in non-polar media (cyclohexane, benzene, toluene), while in proton- and especially electron donor media, sharp fluorescence quenching (2.2–31.2 times) of the dye is observed. For a better understanding of the spectral analysis results, a quantum-chemical analysis of the molecular structure, electronic structure, and spectral characteristics of BODIPY 1 was carried out. For the first time, it was shown that BODIPY 1 fluorescence quenching in proton and electron donor media is caused by the formation of a supramolecular dye complex due to specific interactions of solvent molecules with atoms of the meso?propargylamine group. Due to practically useful spectral properties, including the sensitivity of the fluorescence to the nature of the media, BODIPY 1 is recommended as a new effective fluorescent sensor of specific electron and proton donor molecules.

Full text of DOI:10.1016/j.molliq.2019.04.058

Accepted Manuscript
Effect of solvent nature on spectral properties of blue-emitting
meso-propargylamino-BODIPY
Galina B. Guseva, Alexander A. Ksenofontov, Elena V. Antina,
Mikhail B. Berezin, Anatoliy I. Vyugin
PII:
S0167-7322(18)36523-1
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2019.04.058
MOLLIQ 10781
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
12 December 2018
9 April 2019
11 April 2019
Please cite this article as: G.B. Guseva, A.A. Ksenofontov, E.V. Antina, et al., Effect
of solvent nature on spectral properties of blue-emitting meso-propargylamino-BODIPY,
Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.04.058
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ACCEPTED MANUSCRIPT
Effect of solvent nature on spectral properties of blue-emitting meso-propargylamino-BODIPY
Galina. B. Guseva, Alexander A. Ksenofontov, Elena V. Antina, Mikhail B. Berezin,
Anatoliy I. Vyugin
G.A. Krestov Institute of Solution Chemistry of Russian academy of sciences (ISC RAS)
1 Akademicheskaya st., 153045, Ivanovo, Russian Federation
Abstract
The spectral and luminescent properties of meso-propargylamino-boron(III) dipyrromethe-
nate (BODIPY 1) were studied in a wide range of organic solvents of different polarities, electron-
and proton-donating abilities. Effect of organic solvents on the properties of the BODIPY 1 spectral
characteristics was analyzed in details. It was shown that the fluorescence of BODIPY 1 is charac-
terized by a high sensitivity to the nature of the solvent. It was found that the complex, which emits
in the blue-green region (433–486 nm), has a high quantum yield of fluorescence (~ 100%) in non-
polar media (cyclohexane, benzene, toluene), while in proton- and especially electron donor media,
sharp fluorescence quenching (2.2–31.2 times) of the dye is observed. For a better understanding
of the spectral analysis results, a quantum-chemical analysis of the molecular structure, electronic
structure, and spectral characteristics of BODIPY 1 was carried out. For the first time, it was shown
that BODIPY 1 fluorescence quenching in proton and electron donor media is caused by the for-
mation of a supramolecular dye complexes due to specific interactions of solvent molecules with
atoms of the meso-propargylamine group. Due to practically useful spectral properties, including
the sensitivity of the fluorescence to the nature of the media, BODIPY 1 is recommended as a new
effective fluorescent sensor of specific electron and proton donor molecules.
Keywords
BODIPY
Meso-propargylamino-boron(III) dipyrromethenate
Spectral properties
Solvent-dependent
Fluorescence quenching
Quantum chemical analysis
One parameter and multi parameter analysis
Introduction
The active interest of scientists to fluorescent molecules is primarily due to prospects of
their application in the technical and medical fields [1]. An analysis of the scientific literature cur-
 Corresponding Author: gbg@isc-ras.ru
1

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rently available shows a rapid increase in publications devoted to the group of open-chain oli-
gopyrroles based on 4,4-difluoro-4-boron-3a, 4a-diaza-s-indacene (BODIPY) [2, 3]. Due to the ex-
cellent photophysical properties and great amenability to structural modification, these fluoro-
phores are popular and find practical application as fluorescent markers [4, 5], fluorescent sensors
[6], active media of tunable lasers [7]. The advantage of BF2-dipyrromethenates in comparison with
the known commercial dyes, for example, derivatives of the coumarin series [8], is their high photo-
and chemostability [7, 9]. The main unique feature of the BODIPY structure is the completely flat
and rigid chromophore core. Such a molecular structure provides a high probability of radiation
deactivation and low non-radiative relaxation of the system. In addition, the dye core functionali-
zation can be carried out in various ways [10, 11]. One of the alternative ways of influence on the
chromophore and luminescent properties of this class of fluorophores is the structural modification
of the substituents in the meso-position of the dipyrromethene core (Fig. 1 a). The introduction of
different nature substituents into the meso-position makes it possible to obtain fluorophores with
the spectral desired properties in a wide range of the visible spectrum [10, 11].
Reactive site
a
b
Fig. 1. Representation of molecular structures: a) BODIPY and its IUPAC numbering system, the 8-
position is often also designated meso; and b) 8-propargylamino 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (BODIPY 1).
Most of the known BODIPYs absorb and emit light in the yellow-green and red-orange re-
gions of the visible spectrum [12, 13], although the need to create new efficient and photostable
fluorophores working in the blue-green spectral region is very high. Analysis of the literature data
[14] showed that a significant hypsochromic shift of the absorption and fluorescence bands maxima
can be achieved by introducing nitrogen or oxygen containing substituents into the 8-position of
the BODIPY. The recorded spectral bands show that the hypsochromic shift depends on the elec-
tronegativity of the attached heteroatom [3, 14]. For example, the binding of methylamine and
methoxy groups led to a large hypsochromic shift of the spectral bands (i.e. 85 and 50 nm in ab-
sorption) compared to the unsubstituted dyes. Using quantum chemical calculations, the authors
of [15] showed that the observed blue shift of the electronic spectra of meso-amino-BODIPYs is
caused by a significant redistribution of electron density on atoms of the chromophore π-system of
the fluorophore in the excited state compared to the ground state (Fig. 2).
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It is known that the BODIPY dyes substituted to 8-position by strongly electron-donating
groups such as amines are highly sensitive to the proton- and electron donation properties of sol-
vents, leading to low fluorescence quantum yields as a consequence of intramolecular electron
transfer [16]. Such a specific functionalization of the BODIPY core opens up alternative ways for the
development of the new highly sensitive to the properties of the medium blue-emitting sensors.
Fig. 2. Main resonant structures of the meso-amino-BODIPY.
Thus, we proceeded to carry out research of the meso-propargylamino-boron(III) dipyrro-
methenate (BODIPY 1) (Fig. 1 b) and study in detail the dependence of its spectral properties, linking
on their sensitivity to the nature of the medium, in order to enhance their practical application. We
paid special attention to the study of the influence of the properties of the medium on the spectral
characteristics of BODIPY 1 in a series of organic solvents of different aromaticity, proton- and elec-
tron donating ability. In addition, we carried out a detailed quantum chemical analysis of the mo-
lecular structure, electronic structure, and spectral parameters and, for the first time, showed the
reasons of the dye fluorescence quenching in proton and electron donor media.
Experimental
Materials. Сyclohexane, DMF, DMSO, DEA (Panreac, Barcelona) and propanol-1 (UV-IR-
HPLC-HPLC preparative PAI) were used without further purification. Other solvents (benzene, tolu-
ene, chloroform, ethanol, butanol-1) were purified according to the procedures described in [17,
18].
Synthesis. 8-Propargylamino 4, 4-difluoro-4-bora-3a, 4a-diaza-s-indacene was prepared
using the techniques described in [19]. Synthesis was performed according to Scheme 1.
Scheme 1. Synthesis of 8-propargylamino 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene.
Bis-(pyrrol-2-yl)-methanethione. A solution of pyrrole (1.0 g, 14.0 mmol) in dry ether (20
ml) was added dropwise to a vigorously stirred solution of thiophosgene (0.8 g, 7.0 mmol) in dry
toluene (20 ml) at 0°C. After 10 min, aqueous methanol (10%) (25 ml) was added and the mixture
stirred for further 30 min at rt. The residue was dried under vacuum, then dissolved in chloroform
3

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and chromatographed on neutral aluminum oxide. The collected fraction, after removing the sol-
vents under reduced pressure, gives a thioketone as a crystalline orange solid. The collected frac-
tion, after removing the solvents under reduced pressure, yieldes a thioketone as a crystalline or-
1
ange solid (0,95 g, 5.4 mmol, 63%). H NMR (CDCl3): δ 6,43 (2H, CH, q, J=6.5 Hz); 7.07 (2H, CH, t,
J=6.5 Hz); 7.24 (2H, CH, s); 9.81 (2H, NH, br s). Mass spectrum (EI): m/z 176 (M+); Anal. Calcd for
C9H8N2S: C 61.34, H 4.58, N 15.90. Found: C 61,42; H, 4,78; N, 15,77.
2-[Methyl sulfanyl-(1H-pyrrol-2-yl)]-2H-pyrrolium iodide. To a solution of bis-(1H-pyrrol-2-
yl)-methanethione (0.60 g, 3.4 mmol) in anhydrous dichloromethane (20 ml) was added methyl
iodide (0.96 ml, 15 mmol). The reaction mixture was stirred for 24 h for completion (TLC monitor-
ing). Solvent was removed under reduced pressure to obtain brown colored gummy solid. The com-
pound was used without further purification for the next reaction. Mass spectrum (EI): m/z 190.3
(M+).
8-(Thiomethyl)4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. To a solution of 2-[methyl sul-
fanyl-(1H-pyrrol-2-yl)]-2H-pyrrolium iodide (0.90 g, 4.7 mmol) in anhydrous dichloromethane (25
ml) under argon atmosphere at rt was added triethylamine (3.3 ml, 7.0 mmol). After stirring for 5
min BF3·OEt2 (3.14 ml, 25.0 mmol) was added. The mixture was stirred for 60 min at room temper-
ature. After the evaporation of solvents under vacuum, the crude product was chromatographed
on silica gel (L60/100, using 5% MeOH in dichloromethane), yielded compound as a dark red solid
1
(0.67 g; 60%). H NMR (CDCl3): δ 2.88 (s, 3H), 6.50-6.53 (m, 2H), 7.39-7.40 (m, 2H), 7.76-7.78 (m,
2H). Mass spectrum (EI): m/z 238 (M+); Anal. Calcd for C10H9BF2N2S: C 50.45; H 3.81; N 11.77. Found:
C 50.38; H 3.78; N 11.67.
8-Propargylamino 4, 4-difluoro-4-bora-3a,4a-diaza-s-indacene. To a solution of 0.5 g (2.1
mmol) of 8-(thiomethyl)4,4-difluoro-4-bora-3a,4a-diaza-s-indacene in 10 ml of dried methylene
chloride was added 0.22 ml (3.15 mmol) of propargylamine. The mixture was stirred for 2 hours.
The precipitate was filtered off, washed with methylene chloride and dried. The remaining solution
was chromatographed on silica gel (5% methanol in chloroform), the solvent was distilled off, and
the precipitate was filtered. The total yield of the product in the form of light yellow crystals was
0.38 g (1.63 mmol, 78%). λmax= 415, 317 nm (CH2Cl2). 1H NMR (CDCl3) δ: 2.57 (1H, CH, t, J=2.5Hz,);
4.49 (2H, CH2, dd J=2.5 Hz, 1J=5.6 Hz,); 6.52 bs, 7.13 bs, 7.67 bs (2 x 3H, β-H,). Mass spectrum (EI):
m/z 245 (M+); Anal. Calcd for C12H10BF2N3: C 58.82, H 4.11, N 17.15. Found: C 58.38; H 4.09; N 17.07.
1H NMR (CDCl3) spectra were recorded on the spectrometer Bruker 500 (Germany).
Electronic absorption spectra of solutions of BODIPY 1 were recorded on CM 2203 spectro-
fluorimeter (SOLAR) in the range of molar concentrations from 1·10–5 to 2·10–6 mol/l and thickness
of absorbing layer is 10 mm at t = 25 ± 0.1 °C.
Fluorescence spectra were obtained on CM 2203 spectrofluorimeter (SOLAR) at an optical
density not higher than 0.1 at the excitation wavelength (370–400 nm).
Stokes shift was calculated as the difference between the values of maxima of the intense
bands in the fluorescence and absorption spectra:
fl
abs
max
abs
max
fl
max
Δλ (nm) =  max – 
and st (cm-1) = 
– 
.
Full width at half maximum (FWHN, cm-1) in the electronic absorption and fluorescence
spectra was determined as the difference between the maximum and minimum values of the wave-
lengths, taken at half of intensity of So → S1 band.
4

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The fluorescence quantum yields (φ) of BODIPY 1 were obtained with the following equation:
φsample = φstandard · (Ssample/Sstandard) · (Astandard/Asample) · (nsample/nstandard)2,
where S denotes the area under the fluorescence band of sample (BODIPY 1) and standard (Cou-
marin 102), A denotes the absorbance of sample and standard at the excitation wavelength, and n
denotes the refractive index of the solvent. The standard for measuring the fluorescence quantum
yield is Coumarin 102 in ethanol (φ = 76.4%) [20, 21].
The fluorescence lifetime () was estimated based on the spectral-luminescent characteris-
tics. In accordance with φ = krad/(krad + knr) and  = 1/(krad + knr), fluorescence lifetime was deter-
mined by:  = φ/krad, where knr is the rate of non-radiative processes, krad is rate of radiative pro-
cesses (radiation constant), the fluorescence lifetime error is ~10–15%. The rate constant of radia-
tive processes was estimated from the characteristics of the electronic absorption spectra in ac-
2
2
cordance with the [20]: krad = 2.9·10–9 · [(9nD )/(nD +2)2 ] · max2 · max · 1/2, where nD is the refrac-
tive index of the solvent,  is wave number of absorption band maximum (cm-1); 1/2 is half-width
of the absorption band (cm–1);  is extinction coefficient of intense absorption band. The rate con-
stant of nonradiative deactivation (knr) was calculated from experimentally measured fluorescence
quantum yield and fluorescence lifetime according to the following equations: knr = (1 – φ)/.
The function of universal interactions (Δf), used as a solvent polarity parameter, was calcu-
lated according to the equation of Lippert [21]:
nD2 1
 1
2 1




2
Δf = f(ε) – f(nD ) =

, where ε is dielectric constant, nD is the refractive index of the
2nD2 1


2
solvent. The f(ε) is total polarization and f(nD ) is induction polarization.
Table 1. Polarity/polarizability, acidity, basicity parameters and Gutman's electron-donor and ac-
ceptor numbers of organic solvents
ETN
DN
AN
SPP
SB
SA
a
b
π*
Solvent
ε
μ
Δf
cyclohex-
ane
benzene
toluene
–
0
0.557 0.07
0
0
0
0
2.024
2.3
0
0
0.002 0.01
0.003 0.11 0.1
2.38 0.31 0.014 0.10 0.001
4.9 1.15 0.150 0.26 4.8
17.1 1.68 0.263 0.6 19.5 36.8 0.84 0.81 0.34 0.84 0.84 0.47
20 37.3 0.847 0.78 0.37 0.84 0.9 0.52
24.3 1.66 0.290 0.65 19.6 37.9 0.853 0.66 0.4 0.86 0.75 0.54
36.7 3.86 0.280 0.40 26.6
45.0 3.96 0.263 0.44 29.8 19.3 1.0 0.65 0.07
12.5 0.92 0.252 0.15 50 9.4
8.2 0.667 0.12
0.655 0.13
0
0
0
0
0.10 0.59
0.11 0.54
–
chloroform
butanol-1
propanol-1 20.3 1.68 0.270 0.62
ethanol
DMF
DMSO
DEA
23.1 0.786 0.07 0.05 0.2 0.1 0.58
16 0.954 0.61 0.03
0
0
0.69 0.88
0.76
1
-
-
-
0.3 0.7 0.24
ETN
Notes: ε is dielectric constant, μ – dipole moment, (D), Δf – empirical Lippert parameter [20],
– normalized Dimroth-Reichardt parameter [22]; a, b, π* – Kamlet-Taft solvent parametrs; SPP –
polarity/polarizability, SB – basicity and SA – acidity Catalán solvent parameters [22], DN and AN –
Gutman's electron-donor and acceptor numbers [24].
Quantum chemical calculations. All the quantum mechanical calculations were performed
by means of the PC GAMESS US v. 12 program package [25]. Ground (GS) and excited (ES) state
5

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geometries were fully optimized by wB97X-D/6-311++G (d,p) [26–28]. Absorption transitions were
simulated using TD-CAMB3LYP/6-311++G(d,p) [29]. The polarizable continuum model (PCM) apply-
ing dielectric constants of cyclohexane, chloroform, benzene, toluene, ethanol, propanol, butanol,
DMF, DMSO, and DEA allowed to evaluate solvation effects [30]. The ChemCraft 1.8 was used for
analyses of results and molecular graphics [31].
Results and discussion
The spectral characteristics of the BODIPY 1 in a series of organic solvents are presented in
Table 2, the electronic absorption and emission spectra are shown in Fig.3.
Table 2. Spectral characteristics of BODIPY 1 in organic solvents
fl
max
abs
max
krad∙10-7,
τ, ns
FWHN,
cm–1
Abs
λ
(ε),
knr∙10-7,
Δλst
φ (λex)
S1 → S0
λ
,
s–1
s–1
Solvent
S0 → S1
S0 → S2
S1→S0
426(41316)
318(18075)
422(43771)
321(18913)
421(42912)
322(20113)
418(42228)
320(22180)
406(32496)
319(18137)
408(34962)
318(19223)
406(33530)
318(16444)
404(34821)
319(15790)
404(35641)
320(17396)
405(33921)
319(17298)
1695
1843
1781
1997
2040
2097
2186
2255
2323
2357
0.999
(400)
0.924
(400)
0.92
(395)
0.829
(395)
cyclohexane
benzene
toluene
486
485
485
485
463
467
468
458
463
433
60
63
64
67
57
59
62
54
59
28
15.72
14.72
14.02
15.66
13.12
14.36
14.48
14.82
16.11
15.89
6.36
6.28
6.56
5.29
6.02
5.16
4.14
2.98
1.73
0.20
0.02
1.21
1.22
chloroform
butanol-1
propanol-1
ethanol
3.23
0.790
3.49
0.741
(395)
0.600
(395)
0.442
(395)
0.278
(395)
0.032
(370)
5.02
9.66
DMF
18.73
41.73
48.16
DMSO
DEA
abs
max
Notes: λ λex – absorption, excitation and fluorescence maxima, respectively, nm; ε – molar ab-
sorption coefficient (L mol cm-1); FWHN is full width at half maximum in the electronic absorption
spectrum (cm-1); Δλst – Stokes shift, nm; φ – fluorescence quantum yield; krad, knr – rate constants
of radiative process and nonradiative deactivation, respectively, s–1; τ – fluorescence lifetime, ns.
6

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a
b
Fig. 3. Normalized electronic absorption spectra (a) and electronic emission spectra (b) of BODIPY
1 solution in organic solvents.
The absorption bonds of BODIPY 1 are placed in the blue-green region in all solvents studied
(Table 2, Fig. 3 a). The high-intensity S0 → S1 band is in the range of 404–426 nm. A less intense S0
→ S2 band with shoulders on the right and left slopes is recorded in the shorter wavelength region
(315–322 nm) (Fig. 3). The molar extinction coefficient of the S0 → S1 band (33530–43771 l/mol ·
cm) (Table 2) is almost 1.5–2 times lower than for the alkyl-substituted BODIPYs [32].
The introduction of the propargylamino group into the meso-position shifts the absorption
and fluorescence bands maxima of BODIPY 1 by almost 100 nm to the shortwave region (Table. 2,
3) compared with the meso-unsubstituted analog 3,3',4,4'- tetramethylalkylated BODIPY 2 [32] and
boron(III) dipyrromethenates with meso-substituents: –CH2–N(C2H5)2 in BODIPY 3 [33] and –Ph–
NH2 in BODIPY 4 [34], in which the electron pair of the unhybridized p-orbital of an amino group
nitrogen atom is isolated from the conjugated chromophore system due to –CH2– and –Ph–frag-
ments:
[
]
32
[
BODIPY 4[34]
CH2Cl2
33]
BODIPY 2
BODIPY 3
C2H5OH
503 (512)
0.88
C2H5OH
497 (513)
0.015
fl
max
abs
max
507(523)
0.198.
λ
( λ
), nm
ϕ
The fluorescence spectra (after excitation at 370-395 nm) of the meso-propargylamino-
BODIPY 1 are the mirror reflections of the high-intensity S0 → S1 bands with a significant Stokes shift
7

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Δλst = 28–67 nm, (Fig. 3 b, Table 2), while for alkyl-substituted analogs, the value Δλst is usually small
and fluctuates in the range of 5–10 nm [32]. The observed large Stokes shift and the broadened
shape of the emission band (Table 2, Fig. 3 b) indicate a significant difference in the geometry of
the excited state relative to the ground state of BODIPY 1.
Fig. 4. Absorption and normalized fluorescence spectra of BODIPY 1 in toluene (λex= 395 nm).
Analysis of the solvatochromic and solvatofluoric effects showed a high sensitivity of the spectral
characteristics of BODIPY 1 to the properties of the medium, which is not typical for most alkyl-
substituted BODIPY [35, 36].
The effect of solvent nature on the position of the maximum of the intense absorption and
fluorescence bands (ν, cm-1), Stokes shift (Δν, cm-1), fluorescence quantum yield (φ) and lifetime (τ)
of BODIPY 1 were analyzed using polarity/polarizability, acidity (SA), basicity (SB) parameters, Gut-
man's acceptor and electron-donor numbers of the solvent (Table 2, 3). Figure 5 shows the depend-
ences of the spectral characteristics on the Lippert, Dimrot-Reichardt, Catalan parameters and Gut-
man's acceptor and electron-donor numbers.
a
b
8

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c
d
e
f
g
h
Fig. 5. Dependences of spectral characteristics of BODIPY 1 on parameters of solvent polarity Lip-
EN
pert (Δf), Dimroth-Reichardt (
), Catalan (SPP, BS, AS) and Gutman's acceptor and electron-donor
T
EN
numbers (DN and AN): ν= Δf (a); ν = SPP (b); ν = DN (c); ν = AN (d); Δν =
(e); φ = DN (f); φ = SA
T
(g); τ = DN (h); (where ● – cyclohexane, ● – benzene, ● – toluene, ● – chloroform, ● – butanol-1, ●
– propanol-1, ● – ethanol, ● – DMF, ● – DMSO, ● – DEA).
9

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In the general case, with the increase of Δf, SPP and DN, the absorption and fluorescence
bands maxima are shifted towards short waves and the relative quantum yields and fluorescence
lifetime decrease (Fig. 4 a-c, f and h). Sufficiently high values of the approximation coefficients were
obtained for the dependences of the spectral characteristics on the Lippert and Catalan parameters
(R2 = 0.9306–0.7437), which indicates a significant contribution of specific interactions to the solv-
atofluoric and solvatochromic effects of BODIPY 1. The best correlation of the dependences of the
spectral characteristics is observed with the Gutman's electron-donor number (R2 up to 0.9543, Fig.
5 c). The R2 values for the spectral characteristics dependences on AN are significantly lower and
do not exceed ~ 0.2060 (Fig. 5 d). The good correlation is observed while using the SA parameter,
which takes into account the acid-base interactions of dissolved compound with the solvent. The
group of alcohols (ethanol, propanol-1, butanol-1) is allocated in a separate area on this depend-
ence (Fig. 5 g). It should be noted that on the dependences of Δν values on the considered solvent
EN
parameters, including
(Fig. 5 e), three areas for non-polar and weakly polar (cyclohexane, ben-
T
zene, toluene, chloroform), electron-donor (DMSO, DMF, DEA) and proton-donor (ethanol, propa-
nol-1, butanol-1) solvents are released. The obtained results suggest significant differences in the
geometry of the excited and ground state of BODIPY 1 caused by the specific solvation of the lumi-
nophore due to acid-base interactions of electron-donor heteroatoms with the acetylene proton
(Solv)O:∙∙∙H-C≡C– and proton of –OH group of alcohols with an electron-donating nitrogen pair of
the amino group (Solv)OH∙∙∙:N≤. This assumption was confirmed by the results of quantum chemi-
cal calculations, which are presented in the next section. Analyzing the obtained data, it can be
noted that the strongest influence on the spectral characteristics of BODIPY 1 is provided by sol-
vents with pronounced electron donor properties. Narrowing of the intense band of the electronic
absorption spectrum is observed in the transition from polar to nonpolar media (Table 2). The
FWHM values for the complex in DEA are 2357 cm–1 and decrease by more than 662 cm–1 in cyclo-
hexane (1695 cm–1). The transfer from inert cyclohexane to electron donor DMF, DMSO and DEA is
accompanied by a short-wave shift of the maxima of the absorption (fluorescence) bands on ~ 21–
22 (23–53) nm and sharp quenching of the dye fluorescence (φ decreases in ~2.2–31.2 times) (Table
2). A smaller decrease in the fluorescence quantum yield is observed in alcohols (in ~1.3–1.7 times).
This indicates a higher sensitivity of BODIPY 1 fluorescence to the presence, especially, of electron-
donating compounds.
The fluorescence lifetime of the BODIPY 1 in the excited state is significantly reduced (from
6.36 to 1.73 ns) when replacing non-polar media with proton- and electron-donor solvents (Table
2). At the same time, the value of the radiative processes constant (krad) has little dependence on
the properties of the medium (Table 2). Changing the nature of the medium has significant effect
on the values of nonradiative deactivation constants of BODIPY 1. For example, the knr values in-
crease in ~2400 times when replacing an inert cyclohexane with electron donor diethylamine.
A more correct description of effect of solvent nature on spectral properties should be car-
ried out using a multiparameter solvent scales, taking into account solvent polarity, polarizability,
proton and electron donating properties, with the linear regression method:
X = X0 + aA + bB + cC +…,
where X – physical or chemical characteristic depending on the solvent (νabs, νfl and Δν); X0 – value
of the physical or chemical property in the gas phase, or ‘‘standard’’ (inert) solvent; A, B and C –
10

ACCEPTED MANUSCRIPT
independent but complementary solvent parameters (polarity, polarizability proton or electron do-
nating and other properties), describing various mechanisms of interaction of the solvent with
the solute; a, b and c – coefficients characterizing the contribution of the parameters A, B and C of
the solvent in the property studied.
Application of the linear regression analysis method allows determining the contributions
of the individual parameters of the solvents in the photophysical characteristics of the solute, taking
into account their impact quantitatively. The most commonly used parameter sets are the param-
eters proposed by Kamlet and Taft – the scale of polarity/polarizability of the solvent (π*), scale of
proton-donor properties of the solvent (a) and electron-donor properties scale (b) [37], as well as
a set of Catalan parameters – the scale of polarity/polarizability of the solvent (SPP), acidity solvent
scale (SA) and basic solvents scale (SB) [23]. The values of each parameter vary from 0 to 1. X0
parameter is the free term of the linear equation; it was taken as the property of the investigated
molecules in the gas phase.
Higher coefficients of determination of linear correlation were obtained for used solvent
parameter sets in comparison with the one-parameter correlations (Fig. 5), that means the applica-
bility of the taken model and chosen set of parameters (Table 3).
Table 3. Calculated parameter values in gas phase (Xo), correlation coefficients of the linear regres-
sion (a, b, c), and (R2) are values of coefficients of determination
Kamlet and Taft parameters
X
X0,
а(π*)
b(a)
c(b)
R2
φ
, cm–1
0.820±0.165
0.322±0.308
0.707±0.277
-1.115±0.31
0.5672
0.9047
0.6931
0.5536
abs
max
λ
23561.65±124.12 152.26±232.14 -252.56±208.33 1444.37±233.61
21129.95±347.081546.70±649.15 1492.63±582.56 3009.35±653.26
2434.70±260.99 1698.97±488.14 1240.09±438.06 -1564.99±491.22
fl
max
λ
, cm–1
Δλst, cm–1
Catalan parameters
X
X0,
а(SPP)
b(SA)
c(SB)
R2
1.914±0.282
-1.497±0.440
0.430±0.359
-0.137±0.284
0.8454
0.9772
0.9469
0.6998
φ
, cm–1
abs
max
22267.34±231.482070.36±361.40 99.24±94.82 710.33±233.41
20008.41±356.49 685.99±556.57 1107.67±454.02 1705.68±359.46
2258.93±224.43 1384.38±350.39 1206.95±285.83 -995.37±226.30
λ
λ
fl
max
, cm–1
Δλst, cm–1
Better linear regression accuracy could be achieved in the case of the Catalan parameters.
Nature of the spectral characteristics dependence on the parameters of solvents is complex. Neg-
ative values of the a and SA parameters for the absorption and fluorescence maxima indicate a
hypsochromic shift in the spectra with an increase in the electrophilic properties of the solvent.
Strengthening the proton- and electron donor properties of the solvent leads to a hypsochromic
shift of the absorption maxima and reducing the Stokes shift in comparison with inert solvents.
Linear regression analysis allows us to conclude that parameters b and SB make a greater contribu-
tion in the spectral characteristics for obtaining the best correlation. Solvent parameters a and SA
11

ACCEPTED MANUSCRIPT
also contribute to the spectral characteristics of the luminophore under study, but to a much
smaller extent.
The observed high fluorescence sensitivity of BODIPY 1 to electron and proton donor com-
pounds favorably distinguishes the synthesized fluorophore from most alkyl analogs [32, 35], what
is of considerable interest in the development of new fluorescent sensors based on meso-amino-
BODIPYs to the presence of specifically solvating electron- and proton-donor molecules.
Computational modeling. To better understanding of spectroscopic measurements results,
computational modeling was carried out. The computed absorption spectra of BODIPY 1 show four
bands of different intensity caused by the S0-Sn electronic transitions (Table S1, Fig. 6).
The most intense S0–S1 absorption band has a maximum in the range of 351–364 nm (de-
pending on the solvent). This band caused by the HOMO–LUMO transition. The HOMO is the elec-
tron density localization at the BODIPY core. The LUMO is localized at the BODIPY core and the
nitrogen atom of the propargylamine group. In the range of 268–285 nm there are the bands be-
longing to S0–S2, S0–S3 electronic transition (HOMO-1–LUMO, HOMO-2–LUMO, respectively); in the
range of 253–257 nm the S0–S4 band (HOMO-3–LUMO) is registered.
H
H-1
H-2
H-3
Fig. 6. FMOs of BODIPY 1.
L
The intense S0–S1 absorption band is most sensitive to the solvent nature (Table S1). This
band is shifted to the blue region at the transition from non-polar to polar solvents (Table S1). It is
shown fluorescence of BODIPY 1 is quenched in proton- and electron-donor solvents. Guo et al have
showed fluorescence quenching of BODIPYs with electron-donor groups in meso-position (in CHCl3
and ethanol) is due to photoinduced electron transfer from the electron-donor group to the BODIPY
core [38]. Thus in BODIPY 1, the HOMO of propargylamine group is higher in energy than the HOMO
12

ACCEPTED MANUSCRIPT
of BODIPY core. When photoexcited, the electron transfer from the HOMO of propargylamine
group to the HOMO of BODIPY core is energetically favorable (Fig. 7).
Fig. 7. Photoinduced electron transfer mechanism of BODIPY 1.
However, this paper [38] does not consider mechanism of the influence of the solvent molecules
on the BODIPYs fluorescence quenching. It can be assumed the BODIPY 1 fluorescence quenching
is associated with solvation increase by proton- and electron-donor solvents of BODIPY 1 in the
excited state. The reason for this phenomenon is the excess negative charge on the nitrogen atom
of the amino group and the positive charge on the hydrogen atom of methine group of BODIPY 1
in the excited state. This creates the conditions for the formation of BODIPY 1 supramolecular com-
plexes with proton- and electron-donor molecules ([BODIPY 1·X], where X – chloroform, ethanol,
propanol, butanol, DMF, DMSO, DEA) (Fig. 8). Molecular modeling of [BODIPY 1·X] showed the pro-
ton-donor molecules (chloroform, ethanol, propanol) are coordinated on the nitrogen atom of
BODIPY 1 propargylamine group to form H⋯N hydrogen bond while the electron-donor molecules
are coordinated on the acetylene group hydrogen atom of BODIPY 1 propargylamine group to form
the H⋯О hydrogen bond (DMF, DMSO) and to form the H⋯N hydrogen bond (DEA) (Fig. 8, Table 4).
It is important to note, in contrast to ethanol and propanol, in the case of butanol, it is not the
proton that participates in the interaction with the luminophor, but the oxygen atom of the hy-
droxyl group. This is evidenced by the relatively small distance between the oxygen of the OH-group
of butanol and the proton of the acetylene BODIPY 1 residue (it is comparable to that in solvate
complexes with DMF and DMSO ([BODIPY 1·X]) (Fig. 8, Table 4). Obviously, this type of interaction
is realized by reducing the acidic properties of the butanol hydroxyl group in comparison with pre-
cursors with a shorter carbon chain [39]. In addition, the possibility of the formation of weak ori-
entational interactions between the nitrogen atom of BODIPY 1 propargylamine group and the OH-
group hydrogen atom of butanol remains (Fig. 8, Table 4). Supramolecular complexation increases
the nonradiative transitions probability. It leads to the BODIPY 1 fluorescence quenching in proton-
and electron-donor solvents.
13

ACCEPTED MANUSCRIPT
a
b
c
d
Fig. 8. Structures of supramolecular complexes formed by BODIPY 1 with сhloroform (a), ethanol
(b), DMF (c), and butanol (d).
The dipole moments of the ground (GS) and excited (ES) state of BODIPY 1 and supramolecular
complexes formed by BODIPY 1 with proton- and electron-donor molecules ([BODIPY 1·X], where X
– chloroform, ethanol, propanol, butanol, DMF, DMSO, and DEA) are given in Table 4.
Table 4. Calculated the structural parameters of the GS and ES of BODIPY 1 and [BODIPY 1·X]
dipole moments (μ, D) and distance between the X molecule and BODIPY 1 (l, Å).
GS
ES
System
l, Å
μ
l, Å
μ
BODIPY 1
-
7.64
6.16
7.24
5.60
-
7.62
5.95
7.29
6.71
(chloroform)H⋯N(BODIPY 1)
(ethanol)H⋯N(BODIPY 1)
(propanol)H⋯N(BODIPY 1)
(butanol)H⋯N(BODIPY 1)
(butanol)O⋯H(BODIPY 1)
(DMF)O⋯H(BODIPY 1)
(DMSO)O⋯H(BODIPY 1)
(DEA)N⋯H(BODIPY 1)
2.373
3.684
2.973
4.104
2.825
2.274
1.998
3.686
2.391
3.512
3.644
4.173
2.911
2.119
1.991
3.783
6.08
5.99
7.44
6.64
7.05
7.59
6.39
6.97
The supramolecular complexes formation results in BODIPY 1 chromophore system depo-
larization. This cause to the stabilization of BODIPY 1 in the ground and excited state in proton- and
electron-donor solvents. The stabilization effect of BODIPY 1 is accompanied by a blue shift of the
14

ACCEPTED MANUSCRIPT
absorption and emission spectra maxima of BODIPY 1 in proton- and electron-donor solvents.
Conclusion
Thus, the spectral analysis showed that the synthesized meso-propargylamino-BODIPY 1 ex-
hibits a high sensitivity of the spectral characteristics to the properties of the medium, this is not
typical for most of the known alkyl-substituted analogues. It was found that the complex, emitting
in the blue-green region (458–486 nm), has a high fluorescence quantum yield (almost ~ 100%) in
non-polar media (cyclohexane, benzene, toluene), while in proton- and especially in electron do-
nating media, sharp quenching of the fluorescence (2.2–31.2 times) of the dye is observed. One
parameter and multi parameter approaches were used for describing solvent influence on spectral
properties of the BODIPY 1. Quantum-chemical analysis showed that the reason for quenching of
fluorescence of the synthesized fluorophore in proton- and electron-donor media is the formation
of supramolecular complexes due to acid-base interactions of molecules of electron-donor solvents
and alcohols with atoms of the meso-propargylamine group. The obtained results allow us to pro-
pose the BODIPY 1 as a potential highly sensitive sensor of electron and proton donor compounds.
Acknowledgements
We are grateful to the center "The upper Volga region centre of physico-chemical research" for the
1
conduct of the H NMR analysis of the samples and JSCC RAS (Moscow) for providing MBC 100 K
cluster resources.
References
[1] L.M. Wysocki, L.D. Lavis. Advances in the chemistry of small molecule fluorescent probes, Curr.
Opin. in Chem. Biol. 15 (2011) 752–759. doi:10.1016/j.cbpa.2011.10.013.
[2] V. Lakshmi, R. Sharma, M. Ravikanth. Functionalized boron-dipyrromethenes and their appli-
cations, Rep. in Org. Chem. 6 (2016) 1–24. doi:org/10.2147/ROC.S60504.
[3] I. Esnal, I. Valois-Escamilla, C.F. Gόmez-Durán, A. Urίas-Benavides, M. L. Betancourt-Mendiola,
I. Lόpez-Arbeloa, J. Bañuelos, I. Garcίa-Moreno, A. Costela, and E. Peña-Cabrera. Blue-to-Or-
ange Color-Tunable Laser Emission from Tailored Boron-Dipyrromethene Dyes, Chem-
PhysChem. 14 (2013) 4134 – 4142. doi: 10.1002/cphc.201300818.
[4] N. Boens, V. Leen, W. Dehaen. Fluorescent indicators based on BODIPY, Chem. Soc. Rev., 41
(2012) 1130–1172. doi: 10.1039/C1CS15132K.
[5] A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung, K. Burgess. BODIPY dyes in photody-
namic therapy, Chem. Soc. Rev. 42 (2013) 77–88. doi:10.1039/c2cs35216h.
[6] S. Zhu, J. Zhang, J. Janjanam, G. Vegesna, F.-T. Luo, A. Tiwari, H. Liu. Highly water-soluble BOD-
IPY-based fluorescent probes for sensitive fluorescent sensing of zinc(II). J. Mater. Chem. B. 1
(2013) 1722–1728. doi:0.1039/C3TB00249G.
[7] A. Costela, I. García-Moreno, M. Pintado-Sierra, F. Amat-Guerri, R. Sastre, M. Liras, López Ar-
beloa F., J. BañuelosPrieto, I. López Arbeloa. New analogues of the BODIPY dye PM597: photo-
physical and lasing properties in liquid solutions and in solid polymeric matrices, J. Phys. Chem.
A. 113 (2009) 8118-8124. doi:10.1021/jp902734m.
15

ACCEPTED MANUSCRIPT
[8] N.G. Bryantseva, I.V. Sokolova, A.B. Tsyrenzhapova, N.I. Selivanov, V.P. Khilya, Ya.L. Garazd Flu-
orescent
characteristics
of
coumarin
photosensitizers,
J. of App. Spec. 75 (2008) 700-705. doi:10.1007/s10812-008-9100-z.
[9] D.A. Merkushev, S.D. Usoltsev, Yu.S. Mar, A.P. Pushkarev, D. Volyniuk, J.V. Grazulevicius, E.V.
Rumyantsev. BODIPY associates in organic matrices: Spectral properties, photostability and
evaluation as OLED emitters, Mat. Chem. and Phys. 187 (2017) 104–111.
doi:org/10.1016/j.matchemphys.2016.11.053.
[10] V. Lakshmi, R. Sharma, M. Ravikanth. Functionalized boron-dipyrromethenes and their appli-
cations, Rep. in Org. Chem. 6 (2016) 1–24. doi:org/10.2147/ROC.S60504.
[11] G. Ulrich, R. Ziessel, A. Harriman, The Chemistry of Fluorescent Bodipy Dyes: Versatility Unsur-
passed. Angew, Chem. Int. Ed. 47 (2008) 1184–1201. doi:10.1002/anie.200702070.
[12] A.Yu. Nikonova, R.T. Kuznetsova, Iu.V. Aksenova, E.N. Tel’minov, G.V. Mayer, N.A. Dudina, E.N.
Nuraneeva, and E.V. Antina. Optical Properties of Zinc(II) and Boron(III) Dipyrrinates with Dif-
ferent Structures, Opt. and Spec. 120 (2016) 395–402. doi: 10.1134/S0030400X16030176.
[13] H. Lu, J. Mack, Y. Yang and Z. Shen. Structural modification strategies for the rational design of
red/NIR region BODIPYs, Chem. Soc. Rev. 43 (2014) 4778– 4823. doi: 10.1039/c4cs00030g.
[14] Yu Gabe, Y. Urano, K. Kikuchi, H. Kojima, and T. Nagano. Highly Sensitive Fluorescence Probes
for Nitric Oxide Based on Boron Dipyrromethene Chromophores Rational Design of Potentially
Useful Bioimaging Fluorescence Probe, J. Am. Chem. Soc. 126 (2004) 3357-3367. doi:
10.1021/ja037944j.
[15] J. Baсuelos, V. Martуn, C.F.A. Gуmez-Durán, I.J.A. Cόrdoba, E. Peсa-Cabrera, I. Garcίa-Moreno,
Á. Costela, M.E. Pérez-Ojeda, T. Arbeloa, Н. Lόpez-Arbeloa. New 8-Amino-BODIPY Derivatives:
Surpassing Laser Dyes at Blue-Edge Wavelengths. J. Chem. Eur. 17 (2011) 7261-7270.
doi:10.1002/chem.201003689.
[16] R.I. Roacho, A. Metta-Magaña, M.M. Portillo, E. Peña-Cabrera, and K.H. Pannell. 8‑Amino-BOD-
IPYs: Structural Variation, Solvent-Dependent Emission, and VT NMR Spectroscopic Properties
of 8‑R2N‑BODIPY, J. Org. Chem. 78 (2013) 4245−4250. doi:org/10.1021/jo302758a.
[17] Gordon, R. Ford. Sputnik Chimica. Mir, Moscow. (1976) 529.
[18] J.A. Riddick, W.B. Bunger, T.K. Sakano. Organic solvents: physical properties and methods of
purification. (1986) 1325.
[19] T.V. Gouda, A. Tutar, J.F. Biellmann. Synthesis of 8-heteroatom-substituted 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene dyes (BODIPY), Tetrahedron. 62 (2006) 5084-5091. doi:
10.1016/j.tet.2006.03.036.
[20] M. Fischer, J. Georges. Fluorescence quantum yield of rhodamine 6G in ethanol as a function
of concentration using thermal lens spectrometry, Chem. Phys. Lett. 260 (1996) 115–118. doi:
org/10.1016/0009-2614(96)00838-X.
[21] O.S. Wolfbeis. Standardization and Quality Assurance in Fluorescence Measurements Tech-
niques, Springer Ser. Fluoresc. (2008) 101–145.
[22] J.R. Lakowicz. Principles of fluorescence spectroscopy. Springer Science & Business Media,
(2006) 954.
[23] J. Catalan, V. Lopez, P. Perez. Use of the SPP scale for the analysis of molecular systems with
dual emissions resulting from the solvent polarity, J. Fluoresc. 6 (1996) 15–22. doi:
10.1007/BF00726722.
16

ACCEPTED MANUSCRIPT
[24] V. Gutmann, Solvent effects on the reactivities of organometallic compounds, Coord. Chem.
Rev. 18 (1976) 225–255.
[25] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N.
Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14
(1993) 1347. doi:org/10.1002/jcc.540141112.
[26] J.-D. Chai and M. Head-Gordon. Long-range corrected hybrid density functionals with damped
atom-atom
dispersion
corrections,
Chem.
Phys.
10
(2008)
6615-6620.
doi:10.1039/B810189B.
[27] A.D. McLean and G.S. Chandler. Contracted Gaussian-basis sets for molecular calculations. 1.
2nd row atoms, Z=11-18. J. Chem. Phys. 72 (1980) 5639-48. doi:10.1063/1.438980.
[28] K. Raghavachari, J. S. Binkley, R. Seeger, and J. A. Pople. Self-Consistent Molecular Orbital Meth-
ods. 20. Basis set for correlated wave-functions. J. Chem. Phys. 72 (1980) 650-54.
doi:10.1063/1.438955.
[29] T. Yanai, D. Tew, and N. Handy. A new hybrid exchange-correlation functional using the Cou-
lomb-attenuating method (CAM-B3LYP). Chem. Phys. Lett. 393 (2004) 51-57. doi:
10.1016/j.cplett.2004.06.011.
[30] G. Scalmani and M.J. Frisch. Continuous surface charge polarizable continuum models of solv-
ation. I. General formalism, J. Chem. Phys. 132 (2010) 110–114. doi:10.1063/1.3359469.
[31] http://www.chemcraftprog.com.
[32] N.A. Bumagina, E.V. Antina, M.B. Berezin, A.A. Kalyagin. Influence of structural and solvation
factors on the spectral-fluorescent properties of alkyl-substituted BODIPYs in solutions, Spec-
trochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 173 (2017) 228–234. DOI:
10.1016/j.saa.2016.09.026.
[33] B. Guo, X. Peng, A. Cui, Y. Wu, M. Tian, L. Zhang, X. Chen, Y. Gao. Synthesis and spectral prop-
erties of new boron dipyrromethene dyes, Dyes and Pigments. 73 (2007) 206-210.
org/10.1016/j.dyepig.2005.11.007.
[34] Y. Chen, L. Wan, D. Zhang, Y. Bian, J. Jiang. Modulation of the spectroscopic property of Bodipy
derivates through tuning the molecular configuration, Photochem. and Photobiol. Scien. 10
(2011) 1030–1038. doi:10.1039/C1PP00001B.
[35] A. Loudet, K. Burgess, BODIPY dyes and their derivatives: syntheses and spectroscopic
properties., Chem. Rev. 107 (2007) 4891–932. doi:10.1021/cr078381n.
[36] E.V. Antina, M.B. Berezin, N.A. Dudina, S.L. Burkova, and A.Yu. Nikonova. Synthesis, Spectral-
Luminescent Properties of B(III) and Zn(II) Complexes with Alkyl- and Aryl- Substituted Dipyrrins
and Azadipyrrins, Rus. J. of Inorg. Chem. 59 (2014) 1187–1194. doi:
10.1134/S0036023614100027.
[37] C. Reichardt, T. Welton, Solvents and Solvent Effects in Organic Chemistry, fourth ed., Wiley-
VCH Verlag GmbH & Co. KGaA, 2011.
[38] B. Guo, X. Peng, A. Cui, Y. Wu, M. Tian, L. Zhang, X. Chen, Y. Gao, Synthesis and spectral prop-
erties of new boron dipyrromethene dyes, Dye. Pigment. 73 (2007) 206–210.
doi:10.1016/J.DYEPIG.2005.11.007.
[39]J. Rumble CRC handbook of chemistry and physics, 99th Edition. – CRC press. (2018) 1550 p.
17

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Graphical abstract
[40]
18

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Highlights
 Meso-propargylamino-BODIPY 1 demonstrates the high sensitivity to the nature of the media.
 The dye fluorescence quenching increases with increasing the solvent proton and electron donor
ability.
 Supramolecular complexation leads to the BODIPY 1 fluorescence quenching in polar medium.
 BODIPY 1 is recommended as a new fluorescent sensor of specific electron and proton donor
molecules.
19