Synthesis and Photochemical Properties of 2,3;5,6-bis(cyclohexano)-BODIPY

Article abstract of DOI:10.1007/s10895-017-2201-4

The boron-dipyrromethene (BODIPY) dye containing an annelated cyclohexyl rings at the 2,3 and 5,6-positions of pyrroles has been synthesized and characterized. Photochemical properties of the obtained compound have been investigated in different individua

Full text of DOI:10.1007/s10895-017-2201-4

Journal of Fluorescence
https://doi.org/10.1007/s10895-017-2201-4
ORIGINAL ARTICLE
Synthesis and Photochemical Properties of 2,3;5,6-bis(cyclohexano)-
BODIPY
Anna Yu. Kritskaya1 · Natalia A. Bumagina1 · Elena V. Antina1 · Alexander A. Ksenofontov1 · Mikhail B. Berezin1 ·
Alexander S. Semeikin2
Received: 23 August 2017 / Accepted: 7 December 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
The boron-dipyrromethene (BODIPY) dye containing an annelated cyclohexyl rings at the 2,3 and 5,6-positions of pyrroles
has been synthesized and characterized. Photochemical properties of the obtained compound have been investigated in dif-
ferent individual solvents. 2,3;5,6-Bis(cyclohexano)-BODIPY exhibits intense chromophore properties with maximum of
So → S1 band in the 543–549 nm (A from 66000 to 96000 L/mol·cm). The complex is a fluorophore with a quantum yield up
to ~100%. The influence of solvent polarity on the spectral properties was evaluated. To better understand the spectroscopic
results, quantum chemical calculations were carried out. Photostability of dye was studied.
Keywords BODIPY · Spectral properties · Solvatochromism · Photostability · Quantum chemical calculations
Introduction
Interestingly, the first report about BODIPYs appeared about
50 years ago (in 1968) [2], but their unique luminescent
properties at that time were not studied. As a result, over
the next 20 years, these dyes did not attract the interest of
scientists.
At present, the family of 4,4-difluoro-4-boron-3a, 4a-diaza-
s-indacene dyes of (known under the BODIPY trademark
(Fig. 1a)) is the subject of active research. According to the
data, presented in the review by Jorge Banuelos in 2016,
the publications number devoted to chromophores of this
class is increased exponentially in the past few years [1].
The first results, confirming the prospects of using
BODIPYs as active media of tunable lasers, were appeared
only in the early 1990s of the last century as a result of
research by Boyer et al. [3–5]. Optimum laser performance
of these dyes is due to their high chemical and thermal sta-
bility, low propensity to photodestruction and unique photo-
physical properties that are more dependent on the molecular
structure [6–12]. Due to the first encouraging results since
the 1990s, the direction of BODIPY dyes has received spe-
cial attention from the scientific community and by now has
turned into several rapidly and successfully developing areas
of science and technology [13–16]. The most important
and intensively developing field of application of BODIPY
fluorophores is sensorics (fluorescent sensors, switches and
probes) [17–19], biochemical sciences (fluorescent mark-
ers for bio-recognition) [20–22], medicine (singlet-oxygen
generators in photodynamic therapy) [23, 24]. BODIPYs are
also promising in light-storage arrays for antenna systems
[25, 26], in photovoltaics [27, 28] and other directions.
Most BODIPY chromophores are relatively well soluble
in most organic solvents. The main reason for the success
and universality of BODIPYs is its chromophoric core,
* Alexander A. Ksenofontov
ivalex.09@mail.ru
Anna Yu. Kritskaya
aynikonova@mail.ru
Natalia A. Bumagina
27angel07@mail.ru
Elena V. Antina
eva@isc-ras.ru
Mikhail B. Berezin
mbb@isc-ras.ru
Alexander S. Semeikin
semeikin@isuct.ru
1
G.A. Krestov Institute of Solution Chemistry of Russian
Academy of Sciences (ISC RAS), 1 Akademicheskaya st.,
153045 Ivanovo, Russian Federation
2
Ivanovo State University of Chemistry and Technology
(ISUCT), 7 Sheremetevskij prosp., 153000 Ivanovo,
Russian Federation
Vol.:(0123456789)
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Journal of Fluorescence
Scheme 1 Synthesis of 2,3;5,6-bis(cyclohexano)-dipyrromethene hydrobromide
Fig. 1 Structure of BODIPY (a) and 2,3;5,6-bis(cyclohexano)-BODIPY 1 (b)
due to which their solutions are characterized by intense
absorption and fluorescence in the visible and near infra-
red regions of the spectrum. The molar absorption coeffi-
cient at the maximum of the intense band reaches 105 M−1
cm− 1. The fluorescence quantum yield can reach 100%
and weakly depends on the properties of the environment
[9, 13, 29].
we present the experimental results of the detailed
investigation of the spectral-luminescent properties of
2,3;5,6-bis(cyclohexano)-BODIPY 1 (Fig. 1b) in wide
range of organic solvents together with the results of
1H NMR analysis of related compounds and the results
of quantum chemical calculations (molecular structure
optimization, spectroscopic characteristics and FMOs
energies), and the results of analysis of the spectral char-
acteristics depending on the solvatochromic parameters
(Lippert and Dimroth-Reichardt). Additionally, photo-
stability of complex by UV irradiation was studied.
The physico-chemical properties of BODIPYs can
vary considerably depending on the nature of the func-
tional substituents in their chromophore core. There are
many variants of the functionalization of the dipyrrome-
thene ligand, which makes it possible to obtain a variety
of BODIPY derivatives [13, 15, 16]. The main ways of
modification of the BODIPY structure include the replace-
ment of hydrogen atoms in the 1,2,3,5,6,7,8-positions of
the dipyrromethene core by different functional groups;
replacement of methine meso-spacer by nitrogen atom; the
replacement of the anion fluoride at the boron(III) atom
by other ligands.
Experimental
Materials
Propanol-1 (UV-IR-HPLC-HPLC preparative PAI) and
cyclohexane (Panreac, Barcelona) were used without fur-
ther purification. Other solvents were purified according to
the procedures described in [36, 37].
Another method is based on annelation of cyclic
aromatic or aliphatic rings to the pyrrole fragments
of the dipyrromethene core. Unlike BODIPY with an
extended π-system [30, 31], information about BODIPYs
with cyclic aliphatic substituents is extremely lim-
ited. The first data about BODIPYs with cyclic ali-
phatic substituents were appeared only in 2007 in
a review by Loudet and Burgess [13] and the article
by Thompson et al. [32]. The properties of 3-chloro-
5,6-cyclohexano-BODIPY, obtained by the authors
from Belgium [33], were also remained unexplored. In
2008, Liangxing Wu and Kevin Burgess in their paper
[34] presented the synthesis results and characteris-
tics of absorption and fluorescence electron spectra of
2,3;5,6-bis(cyclohexano)-BODIPY in methylene chlo-
ride. And only in 2016 I. A. Boldyrev et al. in their work
described 2,3;5,6-bis(cyclohexano)-BODIPY as novel
fluorescent membrane (phosphatidylcholine) probe [35].
However, photophysical properties of this complex were
poorly investigated. A detailed study of the physico-
chemical properties is primarily necessary to describe
the prospects for the practical using new cyclohexano-
BODIPYs. Therefore, in this paper for the first time
Synthesis
2,3;5,6-Bis(cyclohexano)-dipyrromethene hydrobro-
mide (C17H21N2Br; М 333.27) was prepared by reacting
2,3-cyclohexanopyrrole with 2,3-cyclohexano-5-formylpyr-
role in a methanol solution in the presence of hydrobromic
acid according to Scheme 1.
With mixing and room temperature, the 2 ml of concen-
trated hydrobromic acid were added to solution of 1.8 g
(14.8 mmol) of 2,3-cyclohexanopyrrole (M 121.18) and
2.01 g (13.45 mmol) of 2,3-cyclohexano-5-formylpyrrole (M
149.19) in a 20 ml of methanol (Scheme 1). The mixture was
stirred for 2 h. The precipitate was filtered off, washed on the
filter with water, methanol and air-dried. Yield: 14.5 mmol
1
(89%). λmax = 496; 345 nm (CH2Cl2). Н NMR spectrum
(CDCl3), δ, 13.25 s (2H, NH), 6.99 s (1H, ms-H), 6.80 s (2H,
1,7-H), 3.13 t (4H, CH2), 2.57 t (4H, CH2), 1.82–1.86 m
1 3

Journal of Fluorescence
(4H, CH2), 1.75–1.79 m (4H, CH2). Found, %: C 61.22, H
6.24, N 8.31. C17H21N2Br. Calculated, %: C 61.27, H 6.35,
N 8.41.
solution was added to pH=7. The solution was cooled, the
precipitate was filtered off and air-dried. Yield: 88 g (78%).
2,3-Cyclohexanopyrrole (C8H11N; M 121.18)
Synthesis of 2,3;5,6-bis(cyclohexano)-dipyrromethene dif-
luoroborate (C17H19BF2N2; М 300.16) was based on the
exchange reaction of 2,3;5,6-bis(cyclohexano)-dipyrrome-
thene hydrobromide with boron(III) trifluoride etherate as
complexing agent (Scheme 2).
To solution of 1.87 g (5.61 mmol) of 2,3;5,6-bis(cyclohexano)-
dipyrromethene hydrobromide in 50 ml of methylene chloride,
the 5.67 g (56.1 mmol) of triethylamine and 7.96 g (56.1 mmol)
of boron trifluoride etherate were added at room temperature
and mixing (Scheme 2). The solution was kept under stirring
for 2 h, then was washed with water. The solvent was distilled
off, and the complex was chromatographed on silica gel (meth-
ylene chloride as eluent). The solvent was evaporated. The com-
plex was precipitated with methanol under cooling. Yield: 0.6 g
(1.99 mmol; 35.6%). λmax =547; 361 nm (CH2Cl2). 1Н NMR
spectrum (CDCl3), δ, 6.93 s (1H, ms-H), 6.64 s (2H, 1,7-H), 3.06
t (4H, J=6.2 Hz, CH2), 2.57 t (4H, J=6.2 Hz, CH2), 1.84–1.91 m
(4H, CH2), 1.73–1.80 m (4H, CH2). 13C{1H} NMR (CDCl3),
δ, 22.3, 22.8, 23.2, 24.7, 76.8, 77.0, 77.3, 125.3, 125.9, 129.3,
134.0, 158.0. 11B NMR (CDCl3): δ 0.68 (t). Found, %: C 67.93,
H 6.24, N 9.18. C17H19N2BF2. Calculated, %: C 68.03, H 6.38,
N 9.33. Mass spectrum MALDI-TOF: calculated (C17H19N2BF2)
m/z=300.16, found m/z=300.1651.
To a mixture of 95 g (1.7 mol) of KOH and 27 g
(0.24 mol) of cyclohexanone oxime in 150 ml of DMSO, the
41.3 g (0.48 mol) of 1,2-dichloroethane in 38 ml of DMSO
were added dropwise with stirring and heating (120 °C). The
mixture was then stirred for an additional 30 min, cooled,
poured into water and distilled with water vapor. The distil-
late was extracted with ether. The ether was distilled off,
and the pyrrole was distilled under vacuum, collecting the
fraction≈123 °C. Yield 7 g (27%).
2,3-Cyclohexano-5-formylpyrrole (C9H11NO; M 149.19)
The 2,3-cyclohexanopyrrole and 2,3-cyclohexano-5-for-
mylpyrrole were prepared as follows.
Cyclohexanone oxime (C6H11NO; M 113.16)
The 3.0 ml (0.032 mol) of phosphorus oxychloride was added
dropwise to a solution of 2.89 g (0.024 mol) of 2,3-cyclohex-
anonopyrrole in 18 ml of dried DMF with stirring and cooling
(temperature below 20 °C). The mixture was then stirred for
1.5 h at room temperature, poured into 100 ml of water, filtered
and basified to an alkaline reaction with 20% sodium hydroxide
solution (~9 g of NaOH) with cooling. After a few hours, the
precipitate was filtered off, washed with water and dried at room
temperature. Yield 2.8 g (78%).
The 70 g (1.01 mol) of hydroxylamine hydrochloride
(NH2OH·HCl) in 300 ml of water was added portionwise to
98 g (103.4 ml, 1.0 mol) of cyclohexanone. Then, the KOH
Scheme 2 Synthesis of
2,3;5,6-bis(cyclohexano)-
BODIPY
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Journal of Fluorescence
quantum yield and fluorescence lifetime according to the
following equations:
Spectroscopy
1H NMR, 13C NMR and 11B NMR (CDCl3) spectra were car-
ried in the centre for joint use of scientific equipment “The
upper Volga region centre of physico-chemical research” on
the spectrometer Bruker 500 (Germany) [38, 39].
Mass spectrum (MALDI-TOF) of 2,3;5,6-bis(cyclohexano)-
BODIPY was performed on AXIMA Confidence (Shimadzu)
MALDI-TOF mass spectrometer.
knr = (1 − ꢀ)∕ꢁ.
The function of universal interactions (Δf), used as a
solvent polarity parameter, was calculated according to the
equation of Lippert (Table 1) [42]:
n2D − 1
(
)
−
(
)
=
ꢀ − 1
2
Δf = f(ꢀ) − f nD
,
2n2D + 1
2ꢀ + 1
Electronic absorption spectra (EAS) of solutions of
compound 1 were recorded on SM 2203 spectrofluorimeter
(SOLAR) in the range of molar concentrations from 1·10−5
to 2·10−5 mol/l and thickness of absorbing layer is 10 mm
at T=25 0.1 °C.
where ε is dielectric constant, nD is the refractive index of
2
the solvent. The f(ε) is total polarization and f(nD ) is induc-
tion polarization.
The Dimroth-Reichardt parameter (ET) takes into
account the polarity and polarizability of the solvent. We
used a dimensionless normalized (ETN) Dimroth-Reichardt
parameter:
Fluorescence spectra were obtained on spectrofluorimeter
SM 2203 (SOLAR) at an optical density not higher than
0.1 at the excitation wavelength (495–500 nm).
Stokes shift was calculated as the difference between the
values of maxima of the intense bands in the fluorescence
and absorption spectra:
ET(solv) − ET(tms)
ET(water) − ET(tms)
ET(solv) − 30, 7
ETN =
=
,
32, 4
( )
and Δvst cm−1 = v − vmfl ax
.
max
Δꢀ(nm) = ꢀflmax − ꢀ
abs
abs
max
Full width at half maximum (FWHM, cm−1) in the elec-
tronic absorption and fluorescence spectra was determined
as the difference between the maximum and minimum val-
ues of the wavelengths, taken at half of intensity of So →
S1 band.
ET (tms)- polarity parameter of the tetramethylsilane; ET
(water)- polarity parameter of the water; ET (solv)- polarity
parameter of the solvent. Tetramethylsilane and water are
standard solvents with extreme polarities.ETN varies from 0,0
(tetrametylsilane is the least polar solvent) to 1,0 (water is
the most polar solvent).
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 (Rhodamine 6G in ethanol with
φ=94% [40, 41]), A denotes the absorbance of sample and
standard at the excitation wavelength, and n denotes the
refractive index of the solvent.
Quantum Chemical Calculations
All DFT, TD-DFT theoretical calculations were performed
using the PC GAMESS v.12 program package [45]. Geom-
etry optimization of BODIPY 1 in the ground state (S0) were
performed on the basis of density functional theory with the
B3LYP [46, 47] functional and the 6-311G (d,p) [48, 49]
basis set in the gas phase. The excited state geometries were
The fluorescence lifetime (τ) was estimated based on
the spectral-luminescent characteristics. In accordance
with φ = krad/(krad + knr) and τ = 1/(krad + knr), fluorescence
lifetime was determined by: τ = φ / krad, where knr is the
rate of non-radiative processes, krad is rate of radiative
processes (radiation constant). The rate constant of radia-
tive deactivation was estimated from the characteristics
of the electronic absorption spectra in accordance with
the [42, 43]:
Table 1 Polarity parameters of
EN
Solvent
Cyclohexane
Hexane
Benzene
Toluene
Chloroform
THF
Propanol-1
Ethanol
DMF
Δf
T
organic solvents
0.002
0.001
0.003
0.014
0.150
0.210
0.270
0.290
0.280
0.01
0.01
0.11
0.10
0.26
0.21
0.62
0.65
0.40
2
krad = 2.9 ⋅ 10−9 ⋅ [(9nD2)∕(nD2 + 2)2] ⋅ vmax ⋅ Amax ⋅ Δv1∕2
,
where nD is the refractive index of the solvent, ν is wave
number of absorption band maximum (cm−1); Δν1/2 is half-
width of the absorption band (cm−1); A is extinction coef-
ficient of intense absorption band (l/mol·cm).
Δf is empirical Lippert parame-
The rate constant of nonradiative deactivation (knr) was
calculated from experimentally measured fluorescence
ter [42], EN is normalized Dim-
T
roth-Reichardt parameter [44].
1 3

Journal of Fluorescence
determined by means of TD-DFT with the CAM-B3LYP
[50] functional and the 6-311G (d,p) basis set. Harmonic
vibrational frequencies were computed at the same level of
theory for S0 state in order to characterize the stationary
points as true minima, representing equilibrium structures
on the potential energy surfaces. Frontier molecular orbit-
als (FMOs) diagrams were constructed on the optimized
structures. The polarizable continuum model (PCM) apply-
ing dielectric constants of cyclohexane, hexane, benzene,
toluene, chloroform, propan-1-ol, ethanol, DMF, and THF
allowed to evaluate solvation effects.
1 was monitored. The relative absorption intensity (D/Do)
was calculated as the ratio of the absorption intensities at
the maximum of the characteristic band of EAS after and
before each irradiation.
The half-life (τ1/2) of the complex was determined from
the D/Do = f(t) dependence, as the irradiation time at which
the optical density at the intense band maximum of the
electronic absorption spectrum of the complex solution was
decreased by 2.
Rate constant of the photodestruction as determined
according to the following equations: ln(Do/Dt)=kt, where
Do is the absorbance in maximum wavelength before
the irradiation, Dt is the absorbance in maximum wave-
length after the irradiation. The slope, k, is the rate of the
photodestruction.
Photostability
In order to determine the photostability of the
2,3;5,6-bis(cyclohexano)-BODIPY 1, its solution in
cyclohexane (8.6·10−6 mol/l) was subjected to the irradiation
by UV light (mercury lamp DRL 250 with lighting power
W=6.900 mW/cm2, irradiation by a light filter with a trans-
mission of 365 nm). Electronic absorption spectra of the
complex solution were recorded before and after irradiation
at regular intervals (60 min). Depending on the irradiation
time, the absorption at the maximum 546 nm of the charac-
teristic band of EAS of 2,3;5,6-bis (cyclohexano)-BODIPY
Results and Discussion
1H NMR Spectra Description
1
In this section, the H NMR spectra of complex 1 and
2,3;5,6-bis(cyclohexano)dipyrromethene hydrobromide
are compared to confirm the structure of the synthesized
Fig. 2 1H NMR spectra of
a
2,3;5,6-bis(cyclohexano)-dipy-
rromethene hydrobromide (a)
and 2,3;5,6-bis(cyclohexano)-
BODIPY 1 (b)
CDCl3
1.82–1.86 m
1.75–1.79 m
2.57 t
4H, CH2-
cyclohexane
3.13 t
4H, CH2-
cyclohexane
8H, CH2-
cyclohexane
6.80 s
2H, 1,7-CH
6.99 s
1H, CHmeso
13.25 s
2H, NH
14.0
13.0
7.0
6.0
3.0
2.5
2.0
b
CDCl3
2.57 t
3.06 t
1.84–1.91 m
1.73–1.80 m
4H, CH2-
cyclohexane
4H, CH2-
cyclohexane
8H, CH2-
6.64 s
cyclohexane
2H, 1,7-CH
6.93 s
1H, CHmeso
14.0
13.0
7.0
6.0
3.0
2.5
2.0
1 3

Journal of Fluorescence
Table 2 Spectral-
Solvent
Spectral-luminescent characteristics
luminescent characteristics
of 2,3;5,6-bis(cyclohexano)-
BODIPY 1
ꢀabs , nm
ꢀabs
,
max
ꢀfl
,
ꢀfl
,
max
A, l/mol·cm
S0 → S1
FWHM
cm−1
Abs
FWHM ΔλSt, ΔνSt,
max
max
nm
S1→S0 S1→S0
cm−1
FL
nm
cm−1
S0 → S1
S0→S1(sh)
S0 → S2
cm−1
cm−1
S0→S1(sh)
S0 → S2
S0→S1
C6H12
C6H14
C6H6
546
84,587
30,086
7087
95,948
32,586
7483
82,205
31,285
7513
79,834
29,511
6795
79,130
30,373
7470
76,640
31,305
7261
77,921
32,347
8133
74,033
31,977
7976
18,315 541
18,349 580
18,248 706
18,248 709
18,215 741
18,382 827
18,382 863
18,416 866
18,416 896
554
555
559
558
559
556
554
553
555
18,051 707
8
265
331
359
327
326
396
332
333
398
517
366–371
545
18,018 766
10
11
10
10
12
10
10
12
516
364–371
548
17,889 757
17,921 760
17,889 727
17,986 766
18,051 678
18,083 712
18,018 738
521
366–370
C6H5-CH3 548
521
366–371
CHCl3
THF
549
521
364–370
544
517
362–369
PrOH-1
EtOH
DMF
544
517
360–369
543
516
362–368
543
66,135
29,465
7771
519
364–371
ꢀabs is maximum S0 → S1 and S0 → S2 bands of absorption (nm); sh is shoulder, which is attributed to
max
max
vibrational component of S0 →S1 band; A is absorption molar coefficient (l/mol·cm); ꢀfl
is S1 → S0
band maximum in the fluorescence spectrum (nm); FWHM is full width at half maximum in the electronic
absorption and fluorescence spectrum (cm−1); ΔλSt (nm) and ΔνSt (cm−1) is Stokes shift
Table 3 Spectral-luminescent characteristics of 2,3;5,6-bis(cyclohexano)-
BODIPY 1
Solvent
Spectral-luminescent characteristics
φ
krad, s−1
knr, s−1
τ, ns
S1 → S0
(λex, nm)
(·107 s−1
)
(·107 s−1
)
C6H12
C6H14
C6H6
C6H5-CH3
CHCl3
THF
PrOH-1
EtOH
DMF
0.92 (495)
0.83 (495)
~1.00 (495)
0.97 (495)
0.97 (495)
0.89 (500)
0.98 (495)
0.92 (500)
0.82 (495)
5.01
6.11
6.28
6.41
6.34
6.98
7.41
7.08
6.55
0.39
1.24
0
0.21
0.20
0.85
0.17
0.62
1.43
18.44
13.61
15.92
15.10
15.29
12.77
13.19
12.98
12.53
φ is fluorescence quantum yield; krad is rate constant of radiative pro-
cess (·107 s−1); knr is rate constant of non-radiative process (·107 s−1);
τ is fluorescence lifetime (ns)
Fig. 3 Normalized electronic absorption and emission spectra of
2,3;5,6-bis(cyclohexano)-BODIPY 1 in cyclohexane
1 3

Journal of Fluorescence
Table 4 BODIPYs 2–8 with aliphatic acyclic and cyclic substituents
BODIPY 2–4 with aliphatic acyclic substituents
BDP
2[13]
3[13]
4[55]
0.80 (EtOH)
505 nm
0.70 (EtOH)
517 nm
0.91 (EtOH)
529 nm
abs
max
fl
max
516 nm
546 nm
540 nm
λ
BODIPY 5–8 with aliphatic cyclic substituents
BDP
5[33, 56]
6[13]
7[32]
8[13]
0.69 (MeCN),
0.78 (MeOH),
0.84 (EtOH)
–
0.81 (EtOH)
0.92 (THF),
0.86 (toluene)
abs
514 nm (MeCN),
515 nm (MeOH),
517 nm (THF),
523 nm (toluene)
527 nm (MeCN),
527 nm (MeOH),
529 nm (THF),
534 nm (toluene)
535 nm
560 nm
–
–
512 nm
535 nm
max
fl
max
λ
BODIPY 1. The 1H NMR spectrum of ligand (Fig. 2a) con-
tains several peaks corresponding to the chemical shifts of
protons of different groups. The signals of protons of six-
membered aliphatic rings are in the region of 1.75–3.13 ppm.
The bands in the form of singlets at 6.99 and 6.80 ppm,
respectively, refer to the proton signals of a methine meso-
spacer and two symmetrically arranged protons at the
1,7-positions of pyrroles. The pyrrole and pyrrolenine N–H
protons in the form of singlet were located at 13.26 ppm.
From 1H NMR of complex 1 (Fig. 2b), the disappearance
of the signal at 13.25 ppm for N–H protons, and also shifting
of proton signals to a strong field: the protons of cyclohexane
groups in the region of 1.73–3.06 ppm, protons of the meso-
spacer and 1.7-protons of the pyrrole rings, respectively, at
6.93 and 6.64 ppm confirms the formation of complex 1. The
proton signals of individual groups in the 1H NMR spectrum
of the obtained complex retain a high characteristic nature
and do not overlap (Fig. 2).
The observed regularities of the arrangement of the
signals of individual proton groups in 1H NMR spectrum
of BODIPY 1 correspond to those of other structurally
related BODIPYs and other dipyrromethene dyes [51–53].
Thus, unlike cyclohexano-substituted dipyrromethene, in
the 1H NMR spectrum of the aliphatic acyclic derivative, for
1 3

Journal of Fluorescence
example, 4,4′-diethyl-3,3′,5,5′-tetramethyldipyrromethene
hydrobromide, [53] the proton signal of NH groups was
shifted to a stronger field by ~0.4 ppm (δ=12.87 ppm). Con-
versely, the proton signal of the methine meso-spacer was
shifted to a weaker (by 0.05 ppm) field at 7.04 ppm. Sig-
nals of the protons of alkyl groups in the form of singlets,
triplets and quartets are in the range 1.07–2.65 ppm. When
complexing 4,4′-diethyl-3,3′,5,5′-tetramethyldipyrromethene
hydrobromide with boron(III), as in the case of BODIPY 1, a
strong-field shift of all proton signals is observed.
Spectral‑Fluorescent Properties
Spectral-fluorescent characteristics of 2,3;5,6-bis(cyclohexano)-
BODIPY 1 were obtained in a series of organic solvents of dif-
ferent polarity (Tables 2 and 3).
a
b
νmax, cm–1
FWHM, cm–1
Abs
FL
∆f
∆f
c
d
∆ν, cm–1
ϕ
∆f
∆f
e
τ, ns
∆f
Fig. 4 Dependencies of spectral characteristics of BODIPY 1 on Lippert parameter: ꢀ
(Δf), d; τ=f(Δf), e
= f (Δf), a; FWHM=f (Δf), b; Δν=f (EN), c; φ=f
max
T
1 3

Journal of Fluorescence
Fig. 5 Dependencies of spectral
characteristics of BODIPY 1 on
Dimroth-Reichardt parameter:
a
b
FWHM, cm–1
νmax, cm–1
ν
max= f (EN), a; FWHM=f
T
(EN), b; Δν=f (EN), c; φ=f
T
T
(EN), d; τ=f(EN), e
Abs
T
T
FL
N
N
ET
ET
c
d
ϕ
∆ν, cm–1
N
N
ET
ET
e
τ, ns
N
ET
In the general case, the electronic absorption spectrum
of 1 is similar in character to the EAS of other structurally
related compounds and is typical for dyes of the 4,4-difluoro-
3a,4a-diaza-s-indacene family. The absorption spectrum of
1 in organic solvents contains one intense long-wavelength
band in the region with a maximum of 543–549 nm due to
the S0 → S1 electron transition (Fig. 3). The molar absorp-
tion coefficient of the S0 → S1 band is 66,135–95,948 L/
mol·cm (Table 2) and is comparable to that of other structur-
ally related BODIPYs.
transition of the S0 → S1 band [54]. The second low-intensity
broadened S0 → S2 band is located in the near-UV region
at 360–371 nm.
The fluorescence spectra of 2,3;5,6-bis(cyclohexano)-
BODIPY are a mirror image of their EASs with a Stokes
shift of 8–12 nm (Fig. 3). The fluorescence band of the
complex is recorded in the region with a maximum (ꢀfl
)
max
at 554–559 nm (excitation at 495–505 nm). The fluores-
cence quantum yield (φ) of 2,3;5,6-bis(cyclohexano)-
BODIPY 1 reaches 0.82–1.00 (Table 3) and is comparable
with the values for many other alkyl-substituted BODIPYs
[55].
The shoulder at 516–521 nm is recorded on the left
slope of the first band, which is attributed to the vibrational
1 3

Journal of Fluorescence
spectral range as for BODIPY 2–4 with acyclic aliphatic
substituents on the periphery of pyrrole rings.
Unfortunately, data on the spectral characteristics of
BODIPY 7 are not available in the original article [32].
Solvatochromism
Analysis of spectral-luminescent characteristics of
2,3;5,6-bis(cyclohexano)-BODIPY 1 in nine different
Table 5 Calculated spectroscopic characteristics and FMOs energies
of BODIPY 1, calculated in solvents
Solvent
Transitions
f
E0−0, eV
λ, nm
Fig. 6 Normalized electronic absorption spectra of 2,3;5,6-bis(cyclohexano)-
BODIPY 1in organic solvents; fragment of spectra in the enlarged scale (insert)
Cyclohexane
H–L
0.808
0.131
0.172
0.145
0.164
2.704
3.932
5.341
6.207
6.736
459
315
232
200
184
H-1–L
H-3–L
H–L+1
H–L+2
Hexane
Toluene
Benzene
THF
H–L
0.797
0.132
0.167
0.141
0.159
0.821
0.130
0.179
0.149
0.170
0.823
0.130
0.180
0.149
0.171
0.796
0.136
0.168
0.120
0.146
0.807
0.134
0.173
0.139
0.159
0.799
0.137
0.170
0.097
0.142
0.789
0.138
0.165
0.068
0.132
0.783
0.139
0.162
0.081
0.124
2.718
3.935
5.348
6.211
6.738
2.687
3.931
5.333
6.201
6.734
2.686
3.930
5.332
6.201
6.733
2.719
3.954
5.354
6.201
6.748
2.706
3.946
5.346
6.201
6.743
2.716
3.958
5.354
6.196
6.749
2.728
3.960
5.360
6.199
6.751
2.735
3.963
5.364
6.199
6.753
456
315
231
200
184
461
315
232
200
184
462
315
233
200
184
456
313
233
200
184
458
314
232
200
184
456
313
232
200
184
454
313
231
200
184
The constants of radiative (krad) and nonradiative (knr)
processes for BODIPY 1 were 4.63·107–7.41·107 s− 1 and
0–1.43·107 s−1, respectively, depending on the solvent nature
(Table 3). The fluorophore lifetime (Table 3) in the excited
state reaches values from 12.53 ns in DMF to 18.44 ns in
cyclohexane.
H-1–L
H-3–L
H–L+1
H–L+2
H–L
H-1–L
H-3–L
H–L+1
H–L+2
It should be noted, that the currently obtained BODIPY
derivatives with aliphatic rings, annelated to pyrrole frag-
ments, (Table 4, compounds 1, 5–8) are more conforma-
tionally restricted, than BODIPYs containing acyclic ali-
phatic substituents (Table 4, compounds 2–4) [13, 32, 33,
55, 56].
H–L
H-1–L
H-3–L
H–L+1
H–L+2
Formally, conjugated bonds systems in BODIPY core
with acyclic (compounds 2–4) and cyclic (compounds 1,
5–8) aliphatic substituents are similar. The molar absorp-
tion coefficients of these two groups of complexes are in the
range 70,000–90,000 L/mol·cm.
H–L
H-1–L
H-3–L
H–L+1
H–L+2
CHCl3
H–L
Compared with alkylated BODIPY 4, the “closing” of
H-1–L
H-3–L
H–L+1
H–L+2
alkyl substituents in the cyclohexyl rings gives a signifi-
max
cant bathochromic shift of the ꢀabs (up to 6–16 nm) and
ꢀfl
max
(12–21 nm) in the spectra of the bis(cyclohexano)-
DMF
H–L
BODIPY 1 and 6 solutions, while maintaining a high (up
to ~ 100%) fluorescence quantum yield (Tables 3 and 4)
[13, 34]. Moreover, the largest red shift is observed in the
emission spectra. The authors of Ref [35]. explain such a
red shift by the influence of cyclohexyl fragments, which
give greater rigidity to the BODIPY molecule.
H-1–L
H-3–L
H–L+1
H–L+2
propan-1-ol
ethanol
H–L
H-1–L
H-3–L
H–L+1
H–L+2
In addition, the spectral-luminescent properties of
bis(cyclohexano)-BODIPYs depend on the site of attach-
ment of the aliphatic rings and, as well as the number
of carbon atoms in the aliphatic ring. For example, the
absorption and emission bands maxima of compound
8, containing aliphatic 5-membered rings in the 1,2 and
6,7-positions of BODIPY pyrroles, correspond to the same
H–L
453
H-1–L
H-3–L
H–L+1
H–L+2
313
231
200
184
1 3

Journal of Fluorescence
Fig. 7 The optimized geometry and FMOs (a) and theoretical UV/Vis spectra (ethanol (black) and benzene (red)) of BODIPY 1
organic solvents (Tables 2 and 3) allowed to study its
solvatochromism (dependence of its spectral properties on
solvent). The maxima positions of the So → S1 band in the
electronic absorption and fluorescence spectra, the width of
the bands of spectra, the values of the Stokes shifts, and the
fluorescence quantum yield of compound 1 were analyzed
using solvatochromic parameters (Table 1). In Figs. 4 and
5 the dependences of the spectral characteristics on the
Lippert and Dimroth–Reichardt parameters are presented.
The effect of the medium nature in the EAS is mani-
fested in the red (2–6 nm) shift of the intense band (Fig. 6)
and the increase in the extinction coefficient (by ~30,000 l/
mol·cm) upon transition from the group of polar (THF,
alcohols, DMF) to the group of nonpolar (cyclohexane,
hexane) solvents (Table 2).
A similar tendency of a red shift of the maximum
position ꢀfl
of the band with a decreasing polarity of
max
the solvent is not observed in the fluorescence spectra
(Table 2).
However, in the electronic absorption and fluorescence
spectra of the complex in weakly polar chloroform and non-
polar aromatic solvents (benzene, toluene) in comparison
with nonpolar saturated hydrocarbons (cyclohexane, hex-
ane) and polar (THF, propanol-1, ethanol, DMF) solvents,
a red shift (up to 5–6 nm) of the So → S1 band maximum
is observed (Table 2). This can be caused by the specific
solvation of the fluorine and nitrogen atoms of BODIPY by
protons of a chloroform molecule or by π-π-stacking inter-
actions of π-systems of dipyrromethene molecules and aro-
matic solvents. This behavior is typical for other BODIPYs
[9, 55].
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 DMF are 896 cm− 1 and decrease
by more than 350 cm− 1 in cyclohexane (541 cm− 1). In
Figs. 4b and 5b show the dependencies FWHM = f (Δf)
and FWHM = f (ETN ) with the correlation coefficients
R2 = 0.8303 and 0.7165, respectively. The FWHM val-
ues in the fluorescence spectra do not correlate with
the medium polarity (Table 2) and vary in the range
Wavelength (nm)
Fig. 8 Changes in the electronic absorption spectra of complex 1 in
678–766 cm− 1
.
cyclohexane at irradiation by UV light (365 nm)
1 3

Journal of Fluorescence
Fig. 9 Dependence of the rela-
tive optical density on the UV
irradiation time (a) and logarith-
mic dependence of the relative
optical density Ln(Do/Dt) on the
UV irradiation time (b)
a
b
Ln(Do/Dt)
Irradiation time (min)
Irradiation time (min)
The Stokes shift value does not correlate with the polar-
ity parameters of the solvents. The correlation coefficients
for these dependences (Figs. 4c and 5c) do not exceed
R2 = 0.2352.
S0–S1 absorption band has a maximum in the range of
462–453 nm (depending on the solvent), and caused by the
HOMO–LUMO transition. In the range of 313–213 nm there
are the bands belonging to S0–S2, S0–S3 electronic transi-
tion (HOMO-1–LUMO, HOMO-3–LUMO, respectively);
in the range of 200–184 nm the S0–S4 and S0–S5 bands
(HOMO–LUMO + 1, HOMO–LUMO + 2) are registered.
All electronic transitions are caused by the redistribution of
the electron density in the BODIPY 1 core.
The fluorescence of complex 1 is weakly sensitive to
the medium polarity. Almost in all the studied solvents
(cyclohexane, benzene, toluene, THF, chloroform, alco-
hols), the values of φ vary from 0.89 to ~1 (Figs. 4d, 5d;
Table 3). A decrease in the fluorescence quantum yield (by
~ 15%) is observed only in hexane (φ = 0.83) and DMF
(φ = 0.82). The lack of fluorescence selectivity to the
medium polarity distinguishes BODIPY 1, as well as most
other BODIPY derivatives [57], from Zn(II) complexes with
dipyrromethenes [58, 59].
The intense S0–S1 absorption band is most sensitive to
the solvent polarity (Fig. 7b). This band is shifted to the
red region at the transition from polar to non-polar sol-
vents (Fig. 7b; Table 5). Also, the pattern of increasing the
energy gap (E0−0) of HOMO–LUMO transition is observed
in a series of solvents: benzene – toluene – cyclohexane
–CHCl3 – DMF – hexane – THF – propan-1-ol – ethanol
(Table 5). Thus, improving spectral-luminescent properties
of BODIPY 1 can be expected when replacing non-polar to
polar solvents.
The fluorophore lifetime (Table 3) in the excited
state reaches 18.44 ns in nonpolar cyclohexane. With an
increase in the medium polarity parameter, the lifetime
is slightly decreased (τ = 12.53 ns in DMF). The correla-
tion coefficients for the dependences of the lifetime on
the polarity parameters in Figs. 4e and 5e reach 0.531
and 0.372.
Photochemical Stability
Quantum Chemical Calculations
The spectral-luminescent properties of BODIPYs in organic
solvents remain unchanged for several months of their stor-
age in the light. On the other hand, studies [59, 60] show that
the molecular structure of dipyrromethene dyes, including
BODIPY, is more sensitive to UV radiation.
To better understand the results of the spectroscopic meas-
urements, quantum chemical calculations were carried out.
The optimized structure of isolated BODIPY 1 obtained
from DFT calculations using the B3LYP/6–311G (d,p)
method is shown in Fig. 7. Computed absorption energies for
the excited states of BODIPY 1 are shown in Table 5. The
absorption energies have been calculated using the CAM-
B3LYP/6–311G(d,p) and cyclohexane, hexane, toluene, ben-
zene, THF, CHCl3, DMF, propan-1-ol, ethanol as a solvent.
The computed absorption spectra of BODIPY 1 show
five bands of different intensity caused by the S0-Sn elec-
tronic transitions (Table 5; Fig. 7b). The most intense
The change in the EAS of a complex 1 solution in
cyclohexane under UV irradiation is shown in Fig. 8. Pho-
todestruction of complex 1 in the process of UV irradiation
(365 nm) is accompanied by a decrease in the absorption of
the characteristic S0 → S1 band of the complex in the visible
region of the spectrum (Fig. 8).
With UV irradiation, the S0 → S1 band of EAS of complex
1 disappears completely only after about 30 h of irradiation
1 3

Journal of Fluorescence
chemiluminescent properties of selected dipyrromethene – BF2
dyes. J Phys Chem B 107:5036–5042. https://doi.org/10.1021/
jp034578h
of the solution. From the data, shown in Fig. 9a, the half-
life (τ1/2) under the action of UV-irradiation was 980 min.
The rate constant of the photodestruction of compound 1
under the UV irradiation of the solution, determined from
the dependence in Fig. 9b, was 6.75·10−4 mol/min.
8. Lopez Arbeloa F, Banuelos J, Martınez V, Arbeloa T, Lopez Arbe-
loa I (2005) Structural, photophysical and lasing properties of
pyrromethene dyes. Int Rev Phys Chem 24:339–374. https://doi.
org/10.1080/01442350500270551
The value of the rate constant of the destruction of com-
plex 1 is comparable to the data for structurally related
meso-phenyl substituted BODIPYs [60, 61].
9. Qin W, Baruah M, Auweraer M, Schryver F, Boens N (2005)
Photophysical properties of borondipyrromethene analogues in
solution. J Phys Chem A 109:7371–7384. https://doi.org/10.1021/
jp052626n
10. Garcia O, Sastre R, del Agua D, Costela A, Garcia-Moreno I,
Lopez Arbeloa F, Banuelos Prieto J, Lopez Arbeloa I (2007)
Laser and physical properties of BODIPY chromophores in new
fluorinated polymeric materials. J Phys Chem C 111:1508–1516.
https://doi.org/10.1021/jp065080t
Conclusions
11. Susdorf T, del Agua D, Tyagi A, Penzkofer A, Ggarcia O,
Sastre R, Costela A, Garcia-Moreno I (2007) Photophysical
characterization of pyrromethene 597 laser dye in silicon-con-
taining organic matrices. Appl Phys B 86:537–545. https://doi.
org/10.1007/s00340-006-2439-z
Thus, the study of the properties of 2,3;5,6-bis(cyclohexano)-
BODIPY 1 showed that the dye has intense chromophore
properties (λmax = 543–549 nm, A = 66,135–95,948 L/
mol·cm), comparable to alkyl-substituted BODIPYs dyes.
The 2,3;5,6-bis(cyclohexano)-BODIPY is an effective fluo-
rophore with a quantum yield of 82–100%. The fluorescence
quantum yield of the dye is weakly depends on the medium
polarity. Structure of 2,3;5,6-bis(cyclohexano)-BODIPY
is stable in visible light and more sensitive to UV irradia-
tion. With UV irradiation (365 nm), the complex undergoes
complete destruction with a half-life of 980 min and a rate
constant of 6.75·10−4 mol/min.
12. Thorat KG, Kamble P, Mallah R, Ray AK, Sekar N,, and Theory
TD-DFT. (2015) Congeners of Pyrromethene-567 dye: perspec-
tives from synthesis, photophysics, photostability, laser. J Org
Chem 80:6152–6164. https://doi.org/10.1021/acs.joc.5b00654
13. Loudet A, Burgess K (2007) BODIPY dyes and their deriva-
tives: syntheses and spectroscopic properties. Chem Rev
107:4891–4932. https://doi.org/10.1021/cr078381n
14. Ziessel R, Ulrich G, Harriman A (2007) The chemistry of
BODIPY: a new El Dorado for fluorescence tools. New J Chem
31:496–501. https://doi.org/10.1039/B617972J
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Acknowledgements The reported study was funded by RFBR accord-
ing to the research project No. 16-33-00611 mol_a and 16-33-00852
mol_a. We are grateful to the Interdisciplinary Supercomputer Center
of the Russian Academy of Sciences (Moscow) for providing MBC
100 K cluster resources.
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based on BODIPY. Chem Soc Rev 41:1130–1172. https://doi.
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