Evaluating the intramolecular charge transfer in novel meso-alkoxyphenyl and β-ethynylphenolic BODIPY derivatives

Article abstract of DOI:10.1016/j.molstruc.2020.127774

Two new BODIPY derivatives (BDP1 and BDP3) were synthetized and characterized using photophysical, electrochemical and theoretical methods. Twisted intramolecular charge transfer (TICT) and intramolecular charge transfer (ICT) phenomena were evidenced from solvatochromism effects in THF/toluene mixtures and supported by theoretical DFT calculations. The different results suggest that TICT and ICT processes are directly related to the meso-alkoxyphenyl and β-ethynylphenolic substitutions linked to the BODIPY core. The functionalization with the β-ethynylphenolic group favors the ICT process due to its coplanarity with the BODIPY moiety, while the functionalization with an alkoxy-phenyl unit, which lacks this coplanarity, exhibited a TICT process instead. The higher electronic conjugation of BDP3 in comparison to BDP1, leads to more pronounced cathodic and anodic shifts. Phenolic moiety has a strong contribution in HOMO orbitals, while LUMO orbital has a contribution for the BODIPY core.

Full text of DOI:10.1016/j.molstruc.2020.127774

Journal of Molecular Structure 1206 (2020) 127774
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Evaluating the intramolecular charge transfer in novel meso-
alkoxyphenyl and b-ethynylphenolic BODIPY derivatives
Juan S. Rocha-Ortiz , Alberto Insuasty , Braulio Insuasty c, Alejandro Ortiz a c
a
b
a
,
, , *
a
Heterocyclic Compounds Research Group, Department of Chemistry, Universidad del Valle, A.A. 25360, Cali, Colombia
Institut des Sciences Mol ꢀe culaires de Marseille (iSm2), Aix-Marseille Universit ꢀe , 13013, Marseille, France
Center for Research and Innovation in Bioinformatics and Photonics-CIBioFi, Calle 13 No. 100-00, Edificio E20, No. 1069, Cali, Colombia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 November 2019
Received in revised form
Two new BODIPY derivatives (BDP1 and BDP3) were synthetized and characterized using photophysical,
electrochemical and theoretical methods. Twisted intramolecular charge transfer (TICT) and intra-
molecular charge transfer (ICT) phenomena were evidenced from solvatochromism effects in THF/
toluene mixtures and supported by theoretical DFT calculations. The different results suggest that TICT
20 January 2020
Accepted 20 January 2020
Available online 21 January 2020
and ICT processes are directly related to the meso-alkoxyphenyl and
b-ethynylphenolic substitutions
linked to the BODIPY core. The functionalization with the -ethynylphenolic group favors the ICT process
b
due to its coplanarity with the BODIPY moiety, while the functionalization with an alkoxy-phenyl unit,
which lacks this coplanarity, exhibited a TICT process instead. The higher electronic conjugation of BDP3
in comparison to BDP1, leads to more pronounced cathodic and anodic shifts. Phenolic moiety has a
strong contribution in HOMO orbitals, while LUMO orbital has a contribution for the BODIPY core.
© 2020 Elsevier B.V. All rights reserved.
Keywords:
BODIPY
Intramolecular charge transfer
Twisted intramolecular charge transfer
Donor-acceptor
Molecular materials
1. Introduction
However, they still do not supply the requirements of high effi-
ciencies for their soon application. In this sense, other less common
The design and synthesis of new molecules with donor-acceptor
D-A) moieties, which exhibit a push-pull behavior, have been
architectures such as T [5,6], V [7], H [8], L [9] and star-like [10e13]
shapes have been also explored presenting notable efficiencies.
This work is focused on linear and a L-like D-A architectures
because there are a few studies about L-like systems, while a linear
shape is a well-known architecture that allows to evaluate the
charge transfer between both systems.
(
investigated among last decades due to their wide use in OLEDs,
nonlinear optics (NLO) and organic photovoltaics (OPV) [1e3]. The
latter being highly desirable among the scientific community in its
effort towards the replacement of fossil fuels. However, one of the
main drawbacks in the commercialization of OPV is the low power
conversion efficiencies (PCE) of organic cells in comparison with
other more developed solar technologies. Thus, big efforts for
enhancing the performance of organic photovoltaics through de-
vice engineering and material design have been devoted over the
last years. Such materials are usually based on D-A molecules with
a highly efficient charge transfer process pretending to emulate
photosynthesis [4]. Therefore, the study of such phenomena using
different electroactive units and different molecular architectures,
in order to achieve efficient materials, is quite desirable.
0
Our molecular design is based on the use of BODIPY (4,4 -
difluoro-4-bora-3a,4a-diaza-s-indacene) units, which have found a
promising use in several applications. BODIPY molecules show
interesting properties such as: thin absorption and emission bands
around 500 nm, high molar extinction coefficients, high fluores-
cence quantum yields, long lifetimes of excited states, chemical
robustness, good photostability, high solubility in common organic
solvents, capability of change their donor or acceptor properties
and resistance towards self-aggregation [14e18]. All these proper-
ties make BODIPYs excellent molecules for different applications
such as tunable laser dyes [19e21], electroluminescent devices
[22e24], biological labeling and imaging [25e27], energy transfer
cassettes [28e30], fluorescent chemosensors [31e33] and photo-
dynamic therapy [34e36]. All these possible applications are
encouraging researchers to synthesize new BODIPY derivatives
with tunable properties. Several BODIPYs functionalized with
Currently, a large set of linear-shape D-A systems have been
commonly developed with notable power conversion efficiencies.
*
Corresponding author. Faculty of Science, Department of Chemistry, Uni-
versidad del Valle, Santiago de Cali, Colombia.
E-mail address: alejandro.ortiz@correounivalle.edu.co (A. Ortiz).
https://doi.org/10.1016/j.molstruc.2020.127774
0
022-2860/© 2020 Elsevier B.V. All rights reserved.

2
J.S. Rocha-Ortiz et al. / Journal of Molecular Structure 1206 (2020) 127774
phenolic groups at different positions (meso and 3-5) have shown
excellent electron transfer properties and good interaction with
metals that modulate their physical and optical properties, how-
ever BODIPYs with an alkenyl phenolic substitution at the
b posi-
tion have not been reported [37e41]. The aim of this work is to
develop new BODIPY molecules with different architectures, linear
(BDP1) and L-like (BDP3) shapes (Fig. 1), which will be achieved by
the functionalization with meso-alkoxy phenyl and -alkenyl
b
phenolic substituents. These systems are compared with similar
compounds (A and B, Fig. 2) reported in the literature to evaluate
their relevance.
2
. Experimental
2.1. Materials and methods
Chemicals were purchased from commercial suppliers and used
without further purification. Compounds A [42], B, F1 and F2
43,44] were prepared according to the literature. All oxygen or
ꢂ5
Fig. 2. Absorption spectra of B, A, BDP1 and BDP3 in THF at 1 ꢁ 10 M. Spectra of B
[
and BDP1 are not normalized to avoid their overlap.
moisture sensitive reactions were performed under an argon at-
mosphere using dried solvents prepared according to recom-
1
13
mended standard procedures. H NMR (400 MHz), and C NMR
100 MHz) spectra were recorded on the Bruker Avance (III)
00 MHz, using CHCl -d as solvent. Chemical shifts are reported in
parts per million (ppm) relative to the residual solvent peak (CHCl
2.1.1. Compound B1
(
2 3
A solution of vanillin (4.000 g, 26.3 mmol, 1 eq), K CO (5.450 g,
9.4 mmol, 1.5 eq) and 1-bromohexane (6.6 mL, 47.0 mmol, 1.8 eq)
in 20 mL of DMF, was heated at 135 C and stirred for 27 h. Then, the
hot reaction mixture was added on crushed ice and stirred for
0 min. Finally, the solid formed was filtered and washed with
4
3
3
:
ꢀ
3
1
13
7
.26 ppm for H and 77.16 ppm for C). Multiplicities are given as: s
(
singlet), d (doublet), t (triplet), q (quartet), dd (doublet of dou-
2
blets), m (multiplet), and the coupling constants J are given in Hz.
Mass spectra were carried out at 70 eV on a SHIMADZU GCMS-QP
distilled water and a mixture of ethanol: water (1:5). Beige solid
1
(
6.093 g, 98.2%). H NMR (400 MHz, CDCl
3
) d: 9.83 (s, 1H), 7.42 (dd,
2
010 Bruker mass spectrometer. UVeVis absorption spectra of all
J ¼ 8.2, 1.9 Hz, 1H), 7.40 (d, J ¼ 1.9 Hz, 1H), 6.96 (d, J ¼ 8.1 Hz, 1H),
compounds were recorded on a JASCO V-730 UVeVIS Spectro-
photometer. Fluorescence spectra of all the compounds were
recorded on a JASCO FP-8500 spectrophotometer. Index of refrac-
tion were measured using a refractometer ATAGO Abbe-NAR-2T.
Electrochemical measurements were performed at room temper-
ature in a potentiostate/galvanostate Autolab PGSTAT302N. Mea-
surements were carried out in a home-built one-compartment cell
with a three-electrode configuration, containing 0.1 M tetrabuty-
4
(
.09 (t, J ¼ 6.8 Hz, 2H), 3.92 (s, 3H), 1.87 (q, J ¼ 7.0 Hz, 2H), 1.53e1.41
13
m, 2H), 1.36e1.31 (m, 4H), 0.95e0.85 (m, 3H) ppm; C NMR
100 MHz, CDCl -d) : 191.0 (CHO), 154.3(C), 150.0 (C), 130.0 (C),
26.9 (CH), 111.5 (CH), 109.4 (CH), 69.3 (CH ), 56.1 (CH ), 31.6 (CH ),
9.0 (CH ), 25.7 (CH ), 22.6 (CH ), 14.1 (CH ) ppm. MS (EI): m/z
calcd for [C14 ] : 236.14; found: 236.45. Anal. calcd for
) C, 71.16; H, 8.53. Found C, 71.27; H, 8.47.
(
3
d
1
2
3
2
2
2
2
2
3
þ
20 3
H O
14 20 3
(C H O
6
lammonium hexafluorophosphate (TBAPF ) as supporting electro-
2
.1.2. Compound BDP1
In a 500 mL round bottom flask, a drop of trifluoro acetic acid
lyte. A glassy carbon (GCE) was used as the working electrode, a
platinum wire as the counter electrode and, and Ag wire as the
pseudo-reference electrode. Prior to each voltammetric measure-
ment the cell was degassed under an argon atmosphere for about
was added to an Ar degassed solution of B1 (583 mg, 2.5 mmol, 1
eq) in 200 mL of CH Cl . After 15 min of stirring at rt, 2,4-dimethyl
pyrrole (0.54 mL, 5.3 mmol, 2.1eq) was added and the final mixture
was stirred at rt for 24 h under Ar atmosphere protected from light.
Afterwards, the reaction was cooled down to 0 C and chloranil
2
2
2
0 min. The electrochemical measurements were performed by
using a concentration of approximately 1.0 mM of the corre-
ꢀ
þ
sponding compound, and the ferrocene/ferrocenium (Fc/Fc ) redox
(677 mg, 2.75 mmol, 1.1 eq) was added and stirred for 10 min. Then,
couple was added as an internal reference. The elemental analyses
were obtained using a LECO CHNS900 elemental analyzer and the
values are within ±0.4% of the theoretical values.
the reaction was warmed to room temperature and stirred for
further 30 min. Then TEA was added (3.5 mL, 25.0 mmol, 10 eq) and
after 10 min BF
reaction was stirred for 4 h. Then, the reaction mixture was washed
three times with saturated aqueous solution of NaHCO
SO . The solvent was
3 2
.OEt (3.0 mL, 25 mmol, 10 eq) was added and the
3
(
3 ꢁ 100 mL) and dried over anhydrous Na
2
4
evaporated and the residue was purified by silica gel column
chromatography using hexane: AcOEt (5:1) as the eluant. Orange
1
solid (138 mg, 38%). H NMR (400 MHz, CDCl
3
-d)
d: 7.00 (d,
J ¼ 7.8 Hz,1H), 6.82 (d, J ¼ 8.0 Hz, 2H), 6.02 (s, 2H), 4.09 (t, J ¼ 6.9 Hz,
2
H), 3.87 (s, 3H), 2.58 (s, 6H), 1.92 (p, J ¼ 7.0 Hz, 2H), 1.51 (s, 6H),
13
1
.46e1.24 (m, 6H), 0.98e0.92 (m, 3H) ppm; C NMR (100 MHz,
CDCl -d) : 155.5 (C), 150.3 (C), 149.3 (C), 143.3 (C), 141.9 (C), 136.5
C), 127.2 (C), 121.2 (CH), 120.5 (CH), 113.2 (CH), 111.6 (CH), 69.3
3
d
(
(
CH
2
), 56.3 (CH
3
), 31.8 (CH
2
), 29.3 (CH
2 2 2
), 25.8 (CH ), 22.7 (CH ), 14.7
Fig. 1. Structure of molecules A and B.
(CH
3
), 14.6 (CH
3
), 14.2 (CH )
3
ppm. MS (EI): m/z calcd for

J.S. Rocha-Ortiz et al. / Journal of Molecular Structure 1206 (2020) 127774
3
þ
[
(
C
C
26
H
33BF
33BF
2
N
N
2
O
O
2
] : 454.26; found: 454.64. Anal. calcd for
) C, 68.73; H, 7.32; N, 6.17. Found C, 68.41; H, 6.98;
(CH
(CH
2
), 25.8 (CH
) ppm. MS (EI): m/z calcd for [C36
49BF
2
), 22.07 (CH
2
), 14.2 (CH
3
), 13.7 (CH
3
), 13.6 (CH
3
), 0.2
þ
26
H
2
2
2
3
H49BF
2
N
2
O
2
Si
2
] : 646.34;
N, 6.35.
found: 646.51. Anal. calcd for (C36
H
2
N
2
O
2
Si
2
) C, 66.85; H, 7.64;
N, 4.33. Found C, 66.56; H, 7.81; N, 4.50.
2.1.3. Compound BDP2
To a 50 mL flat-bottomed flask containing a 20 mL CH
2
Cl
2
so-
2.1.6. Compound BDP5
lution of BDP1 (227 mg, 0.5 mmol, 1 eq), N-iodosuccinimide
450 mg, 2.0 mmol, 4 eq). was added and the mixture was stirred at
room temperature for 12 h, then the mixture was washed with
saturated Na aqueous solution and was extracted with CH Cl
The organic layer was dried over anhydrous Na SO , and then the
solvent was evaporated under reduced pressure. The residue was
purified by silica gel column chromatography using CHCl : hexane
3:2) as the eluant. Red solid (346 mg, 98.0%). H NMR (400 MHz,
CDCl
: 7.02 (d, J ¼ 8.1 Hz, 1H), 6.79 (dd, J ¼ 8.1, 2.0 Hz,1H), 6.76 (d,
BDP4 (19 mg, 0.03 mmol, 1 eq) and K
1.5 eq) were dissolved in 10 mL of methanol, the mixture was
stirred at rt for 5 h, then 40 mL of CH Cl was added, the resulting
mixture was washed with brine and was extracted with CH Cl . The
organic layer was dried over anhydrous Na SO , and then the sol-
vent was evaporated under reduced pressure. The residue was
purified by silica gel column chromatography using CHCl : hexane
(1:1) as the eluant. Orange solid (14 mg, 95.1%). H NMR (400 MHz,
CDCl
2 3
CO (6 mg, 0.043 mmol,
(
2
2
2
S
2
O
3
2
2
.
2
2
2
4
2
4
3
3
1
1
(
3
)
d
3
)
d
6.99 (d, J ¼ 8.1 Hz, 1H), 6.76 (dd, J ¼ 8.1, 2.1 Hz, 1H), 6.73 (d,
J ¼ 2.0 Hz, 1H), 4.11 (t, J ¼ 6.9 Hz, 2H), 3.87 (s, 3H), 2.68 (s, 6H), 1.94
J ¼ 2.0 Hz, 1H), 4.07 (t, J ¼ 6.9 Hz, 2H), 3.83 (s, 3H), 3.32 (s, 2H), 2.64
(
p, J ¼ 7.0 Hz, 2H), 1.52 (s, 6H), 1.45e1.36 (m, 4H), 1.29 (s, 2H),
(s, 6H), 1.90 (p, J ¼ 7.0 Hz, 2H), 1.57 (s, 6H), 1.53e1.46 (m, 2H),
13
13
0
.99e0.92 (m, 3H) ppm; C NMR (100 MHz, CDCl
3
)
d: 156.8 (C),
1.40e1.34 (m, 4H), 0.95e0.89 (m, 3H) ppm; C NMR (100 MHz,
1
1
2
50.6 (C), 149.7 (C), 145.5 (C), 141.6 (C), 136.5 (C), 131.8 (C), 126.8 (C),
20.4 (CH), 113.4 (CH), 111.2 (CH), 69.3 (CH ), 56.4 (CH ), 31.8 (CH ),
9.2 (CH ), 25.8 (CH ), 22.7 (CH ), 17.2 (CH ), 14.2 (CH
ppm. MS (EI): m/z calcd for [C26
06.47. Anal. calcd for (C26 31BF
Found C, 43.97; H, 4.69; N, 4.12.
CDCl
131.4 (C), 126.3 (C), 120.4 (CH), 113.3 (CH), 111.2 (CH) 84.2 (C), 76.1
(CH), 69.3 (CH ), 56.3 (CH ), 31.8 (CH ), 29.2 (CH ), 25.8 (CH ), 22.7
(CH ), 14.2 (CH ), 13.5 (CH ) ppm. MS (EI): m/z calcd for
3
) d 158.9 (C), 150.5 (C), 149.7 (C), 145.7 (C), 143.4 (C), 136.5 (C),
2
3
2
2
2
2
3
), 16.2 (CH
3
3
)
2
3
2
2
2
þ
H
31BF
2
I
2
N
2
O
2
] : 706.05; found:
2
3
), 13.6 (CH
3
3
þ
7
H
2
I
2
N
2
O
2
) C, 44.22; H, 4.43; N, 3.97.
[C30
H33BF
33BF
2
N
N
2
O
O
2
] : 502.26; found: 502.61. Anal. calcd for
) C, 71.72; H, 6.62; N, 5.58. Found C, 71.24; H, 6.91;
(C30H
2
2
2
N, 5.73.
2
.1.4. Compound BDP3
To a solution of BDP2 (179 mg, 0.25 mmol, 1 eq), CuI (10 mg,
.05 mmol, 0.2 eq), Pd(PPh Cl (18 mg, 0.025 mmol, 0.1 eq) and
(13 mg, 0.025 mmol, 0.1 eq) in 15 mL of dry i-Pr NH, a solution
NH was added
3. Results and discussion
0
PPh
3
)
2
2
3
2
3.1. Synthesis
of F2 (120 mg, 1.01 mmol, 4 eq) in 10 mL of dry i-Pr
2
ꢀ
dropwise. The mixture was stirred at 65 C for 18 h under Ar at-
mosphere protected from light. Then mixture was filtered and the
solvent was evaporated under reduced pressure. The residue was
Target compounds (BDP1 and BDP3) were synthesized
following the synthetic route shown in Scheme 1.
Initially, vanillin was alkylated using 1-bromohexane in pres-
ence of potassium carbonate in N,N-dimethylformamide [45]. BDP1
was obtained by a double condensation of B1 with two molecules of
2,4-dimethylpyrrole using trifluoroacetic acid as catalyst, yielding
to the formation of dipyrromethane which is oxidized using
chloranil giving a dipyrromethene derivative that is treated with
triethylamine and boron trifluoride diethyl etherate affording
finally BDP1. The treatment of BDP1 with N-iodosuccinimide as
iodinating reagent for beta pyrrolic positions gives BDP2, which is
suitable for cross coupling reactions.
purified by silica gel column chromatography using hexane: AcOEt
1
(
3:1) as the eluant. Purple solid (38 mg, 21.4%). H NMR (400 MHz,
CDCl
3
)
d
7.36e7.31 (m, 2H), 6.99 (d, J ¼ 8.2 Hz, 1H), 6.81e6.73 (m,
4
2
H), 5.47 (s, 1H), 4.08 (t, J ¼ 6.9 Hz, 2H), 3.84 (s, 3H), 2.69 (s, 3H),
.65 (s, 3H), 1.90 (p, J ¼ 7.0 Hz, 2H), 1.66 (s, 3H), 1.59 (s, 3H),
1.41e1.33 (m, J ¼ 3.5, 3.0 Hz, 4H), 1.30e1.21 (m, 2H), 0.95e0.89 (m,
3
H) ppm; 13C NMR (100 MHz, CDCl
3
) d 167.2 (C), d 156.0 (C), 150.5
(
(
C), 150.0 (C), 149.6 (C), 144.8 (C), 142.0 (C), 136.5 (C) 133.1
CH),132.2 (C), 131.3 (C), 131.2 (C), 126.7 (CH), 120.5 (C),116.8 (C),
1
15.7 (CH), 113.4 (CH), 111.4 (CH), 96.6 (C), 80.1 (C), 69.4 (CH
2
), 56.4
), 25.8 (CH ), 16.1
3
), 13.7 (CH ) ppm. MS (EI): m/z calcd for
Alkyne F2 was obtained from 4-iodophenol performing a
Sonogashira reaction coupling using TMSA as alkyne source
yielding to F1. Then TBAF was employed to deprotect the terminal
alkyne, which was used as obtained with no further purification in
the next reaction step. A Sonogashira cross coupling reaction be-
tween F2 and BDP2 leads to the formation of compound BDP3.
Although the formation of the diphenolic compound BDP6 was
envisaged (Scheme 1), only the monophenolic derivative BDP3 was
obtained, this result can be explained probably by the low stability
of precursor F2.
(
(
[
(
CH
CH
C
C
3
3
), 31.8 (CH
), 14.2 (CH
2
), 29.2 (CH
2
2 2 3
), 22.7 (CH ), 17.1 (CH
3
), 13.9 (CH
3
þ
34
H
H
36BF
36BF
2
IN
IN
2
O
3
] : 696.18; found: 696.29. Anal. calcd for
34
2
2
3
O ) C, 58.64; H, 5.21; N, 4.02. Found C, 58.85; H, 5.43;
N, 4.30.
2.1.5. Compound BDP4
To a solution of BDP2 (70 mg, 0.1 mmol, 1 eq), TMSA (0.5 mL,
0
3 2 2
.3 mmol, 1 eq), CuI (4 mg, 0.021 mmol, 0.21 eq) and Pd(PPh ) Cl
(
7 mg, 0.01 mmol, 0.1 eq) in 15 mL of dry degassed THF, a solution of
An alternative synthetic route to obtain BDP6 was contem-
plated, consisting in a Sonogashira reaction between BDP2 and
TMSA giving BDP4, which after deprotection with K CO and
2 3
TEA (0.4 mL, 2.9 mmol, 29 eq) was added dropwise. The mixture
was stirred at rt for 26 h under Ar atmosphere protected from light.
Then mixture was filtered and the solvent was evaporated under
reduced pressure. The residue was purified by silica gel column
methanol, yields to BDP5. A final cross coupling between BDP5 and
4-iodophenol would lead to BDP6 but unfortunately after several
reaction conditions a complex mixture of products was always
obtained.
Therefore, we were pursued to study BDP3, which on one hand,
it is an excellent compound as a building block to obtain more
complex molecules, and on the other hand this compound has a L-
shape donor-acceptor-donor system composed by alkoxyphenyl-
BODIPY-alkenylphenol units. Such type of system is quite inter-
esting and, in this work, we study its photophysical and
chromatography using CHCl
3
: hexane (1:1) as the eluant. Orange
solid (42 mg, 65.7%). H NMR (400 MHz, CDCl
7.01 (d, J ¼ 8.1 Hz,
H), 6.78 (dd, J ¼ 8.1, 1.9 Hz, 1H), 6.75 (d, J ¼ 2.0 Hz, 1H), 4.10 (t,
J ¼ 6.9 Hz, 2H), 3.86 (s, 3H), 2.66 (s, 6H), 1.93 (p, J ¼ 7.0 Hz, 2H), 1.59
1
3
) d
1
13
(
s, 6H), 1.44e1.32 (m, 6H), 0.97e0.93 (m, 3H), 0.24 (s, 18H) ppm;
NMR (100 MHz, CDCl 158.7 (C), 150.5 (C), 149.6 (C), 145.1 (C),
35.4(C),132.4 (C),128.6 (C), 126.5 (C), 120.4 (CH), 113.3 (CH), 111.3
CH), 101.8 (C), 97.3 (C), 69.3 (CH ), 56.3 (CH ), 31.8 (CH ), 29.4
C
3
) d
1
(
2
3
2

4
J.S. Rocha-Ortiz et al. / Journal of Molecular Structure 1206 (2020) 127774
ꢀ
Scheme 1. Synthetic path to obtain compounds BDP1-BDP5. a) n-C
6
H13Br, K
2
CO
3
, DMF, 135 C (27 h); b) CF
3
CO
2
H, 2 4-dimethylpyrrole, CH
2
ꢀ
Cl
2
, Ar, Chloranil, Et
3
3
N, BF
3 2
∙Et O; c) NIS,
ꢀ
ꢀ
ꢀ
CH
2
Cl
2
, 25 C, (14 h); d) Pd(PPh
3
)
2
Cl
2
, CuI, TEA, THF, TMSA, 25 C, (24h); e) TBAF, THF, 25 C, (1h); f) F2, Pd(PPh
3
)
2
Cl
2
, CuI, i-Pr
2
NH, PPh
3
, 65 C, (18h); g) Pd(PPh
)
2
2
Cl , CuI, TEA, THF,
ꢀ
ꢀ
TMSA, 25 C, (26h); h) K
2 3
CO , MeOH, 25 C, (5h); i) Pd(PPh
3
)
2
Cl
2
, 4-iodophenol, CuI, TEA, THF.
electrochemical properties complemented by theoretical calcula-
tions. The study is focused on comparing its properties with BDP1
presenting a linear architecture with only an alkoxyphenyl group as
substituent.
0 1
the transition S / S decreases its energy due to the stabilization
of the lower unoccupied molecular orbital (LUMO), causing them to
be at longer wavelengths with respect to B and BDP1. If the bands
of A are compared with those of BDP3, it is observed that the band
associated with the electronic transition
p / p * of the latter
compound is of higher energy with respect to A, this could be
explained due to a weaker intramolecular charge transfer (ICT)
3.2. Photophysical properties
character for BDP3 presenting only one substituent at the
b posi-
Absorption and emission spectra of BDP1 and BDP3 were
tion together with a decrease in the conjugation of the system.
For a complete analysis, the emission spectra of compounds
BDP1, A, BDP3 and B in THF were measured (see Table 1). In order
to attest the existence of ICT in the pigments, additional emission
spectra in THF-toluene mixtures for all the compounds were per-
formed. Fig. 4 shows the emission spectra of the compounds BDP1,
A and BDP3 (a, b and c, respectively). For B this experiment is not
shown as it does not present solvatochromic effect.
In molecules that undergo ICT processes, the emission bands are
useful to qualitatively characterize this process, which is stabilized
when the solvent turns polar. This is reflected in a reduction of the
emission when apolar solvents are used, favoring the recombina-
tion processes in the state of separation of charges, emitting the
absorbed energy [53]. Fig. 4 shows a decrease in the emission when
the polarity of the solvent is increased, favoring ICT processes in
compounds BDP1, A and BDP3. Such a behavior has been already
observed for compound A [43], which is in analogy to our com-
pounds BDP1 and BDP3, confirming the existence of an ICT process
in our systems. Compounds A and BDP3 show a marked deactiva-
tion of the fluorescence with respect to BDP1, which does not
recorded in THF (Figs. 3 and 4). Compounds A and B were used as
references to compare their properties with our target compounds
BDP1 and BDP3 (Fig. 2) [43].
Table 1 shows absorption and emission data for reference
compounds A and B.
The absorption spectra of compounds A, B, BDP1 and BDP3 are
shown in Fig. 3. Compounds BDP1 and B have a similar absorption
behavior with a main band around 500 nm, BDP1 however shows
lower molar absorptivity. Such band corresponds to the electronic
transition S
this band, at 477 nm, corresponds to a more energetic vibrational
transition of the band S / S [49].
0 1
/ S (which are of character p / p*). The shoulder of
0
2
As expected, the absorption spectra of compounds A and BDP3
have similar characteristic bands. Compound A has an absorption
band at 586 nm which is attributed to a transition S
corresponds to a * transition type, and another one at
14 nm associated with a transition S / S *) that cor-
0 1
/ S , this
p
/ p
4
0
2
(p / p
responds to the phenylethynyl moiety, which can be comparable to
other BODIPY dyes reported in the literature [38,43,50e52]. Com-
pound BDP3 presents similar bands at 424 and 500 nm. It is worth
present substituents at
b positions. It is important to remark that
to note that by increasing the conjugation at the b-BODIPY position,

J.S. Rocha-Ortiz et al. / Journal of Molecular Structure 1206 (2020) 127774
5
Fig. 3. Emission spectra of a) BDP1, b) A and c) BDP3 in THF-Toluene mixtures.
ꢀ
þ
Fig. 4. Cyclic voltammogram of a) cathodic scan and b) anodic scan for A, BDP3, BDP1 and B at 100 mV/s in dry THF and 25 C (V vs Fc/Fc ).
BDP1 does not present a conventional ICT process since the
alkoxyphenyl group is not coplanar with respect to the BODIPY
core, this especial feature gives rise to a twisted intramolecular
charge transfer (TICT) [54], in which the charge transfer process
goes from the alkyloxy groups in phenyl group to the BODIPY
moiety that is in other plane. The ICT process in this case is much
less effective in comparison to a normal ICT process of two coplanar
entities [54]. Although, a TICT process can be also given for BDP3,

6
J.S. Rocha-Ortiz et al. / Journal of Molecular Structure 1206 (2020) 127774
Table 1
Table 2
Photophysical properties of synthetized compounds.
Electrochemical properties of synthesized compounds.
ꢂ1
ꢂ1
Fa
Compound E1Red/V E1Ox/V
Compound
l
abs/nm (ε/M cm
)
lem/nm
l
s
/nm
E
LUMO (eV)
E
HOMO (eV)
E
gap (eV)
E0-0 (eV)
BDP1
BDP3
A
500 (81775)
550 (21985)
586 (66517)
500 (94515)
514
605
622
515
13.1
55.7
35.3
14.2
0.808
<0.001
0.032
0.720
B
ꢂ1.727 0.895
ꢂ1.948 0.911
ꢂ1.646 0.760
ꢂ1.459 0.734
ꢂ2.673
ꢂ2.452
ꢂ2.754
ꢂ2.941
ꢂ5.295
ꢂ5.311
ꢂ5.160
ꢂ5.134
2.622
2.859
2.406
2.193
2.446
2.448
2.086
2.137
BDP1
BDP3
A
B
a
Quantum yields were measured by a reported method using rhodamine B as a
standard [46e48].
compounds A and BDP3.
Compound A has a greater reduction potential (0.187 V) in
comparison with BDP3, this could be due to a higher conjugated
system in the case of A. In addition, the effect of conjugation to
support a positive charge is reflected when comparing the oxida-
tion potential between A and B or between BDP1 and BDP3. These
results show the existence of ICT processes in the compounds ob-
tained in this work; the oxidation potential for all compounds were
attributed to ionization energy of BODIPY core and the electronic
effect of their substituent groups in the different positions.
The HOMO and LUMO energy values are shown in Table 2 in two
ways: the first is the optics (E0-0), which was obtained using the
crossing wavelength between the absorption and emission bands;
then using the Planck-Maxwell equation, the energy was calcu-
lated; the second is the electronics (Egap), this was found from the
difference between the energies HOMO and LUMO which were
found through equations (1) and (2).
the contribution of such type of ICT is much weaker in comparison
with the ICT coming from the alkyne-phenol group which is con-
jugated in the same plane of the BODIPY core.
3.3. Electrochemical properties
The energies associated with the reduction potentials (Ered) and
the oxidation potentials (Eox) correspond to the energies of the
lower unoccupied molecular orbital (LUMO) and higher occupied
molecular orbital (HOMO), respectively. On this basis, the cyclic
voltammograms of compounds A, BDP3, BDP1 and B were
measured (Fig. 5). Table 2 summarizes the electrochemical prop-
erties of the compounds mentioned above.
The dashed black arrows in Fig. 5 shows the direction of the
sweeping. Comparing the voltammograms of the different de-
rivatives in Fig. 5a, all show a semireversible reduction wave (this
was found based on the difference of the peaks of the reduction
wave, which were greater than 60 mV) associated with the process
of reduction of the BODIPY fragment in all cases.
ELUMO ¼ ꢂ ðEred þ 4:4Þ
EHOMO ¼ ꢂ ðEox þ 4:4Þ
(1)
(2)
Regarding Fig. 5b, all the compounds present irreversible
oxidation potentials meaning that when the compound is oxidized
it does not return to its neutral form.
BDP1 has a lower reduction potential (0.221 V) with respect to
3 2 5 3
B, probably due to the presence of -OCH and -O(CH ) CH groups
in BDP1, which donate charge to the central nucleus making it less
prone to accept the electrons. The oxidation potential of BDP1
showed a slightly higher value than B (þ0.016 V), therefore, there is
no significant effect of the methoxy- and hexyloxy-groups in the
oxidation processes. This last behavior is also observed for
Between the values of E0-0 and Egap, a small difference is found
which is normal due to the coulombic interaction between the
hollow-electron pairs [55]. If the values of E0-0 and Egap are
compared for compounds BDP1 and BDP3, it is found that for the
latter this difference is smaller. This could be reasoned as an in-
crease in the conjugation leads to a decrease in the energy of the
border orbitals, which means that less energy is needed to make
the transition. This effect is seen in the same way for A and B.
3.4. Theoretical calculations
Computational calculations were performed at the RDFT level
with a functional hybrid B3LYP and a base set 3-21G þþ. The ge-
ometry of the BDP3 molecule was optimized. Subsequently, an
energy calculation was performed with the TD-RDFT model and the
continuous polarization method (PCM), which let us to simulate the
UVeVis spectrum of this compound, characterize the molecular
orbitals involved and confirm the characteristics of the vertical
electronic transition bands. Table 3 shows the calculated data.
In the first electronic transition that corresponds to 550 nm,
HOMO and LUMO orbitals are involved, and is assigned to a S
transition. The absorption band at 424.4 nm corresponds to elec-
tronic transition S / S in which HOMO-2, HOMO-3 and LUMO
0 1
/S
0
3
orbitals are involved. The calculated energy values for the elec-
tronic transitions present in the UVeVis spectrum are close to those
obtained experimentally (Fig. 3). In addition, the prediction of the
wavelengths of the transitions is very close to the experimental
values.
Fig. 3 shows the electron density maps of the frontier molecular
orbitals involved in the electronic transitions for BDP3. It is
observed that the phenolic unit at the
b position undergoes an
intramolecular charge transfer to the core of BODIPY favored by the
coplanarity between both units. The opposite occurs with the
methoxy and hexyloxy substituents on the meso ring which is
Fig. 5. Electron density maps of the frontier molecular orbitals for BDP3.

J.S. Rocha-Ortiz et al. / Journal of Molecular Structure 1206 (2020) 127774
7
Table 3
Theoretical calculations data for BDP3.
Compound
Excited state
Composition
Energy (eV)
l
abs (nm)
Oscillator strength
BDP3
S1
S3
HOMO/LUMO (0.69522)
HOMO-3/LUMO (0.26739)
HOMO-2/LUMO (0.61543)
2.283
2.962
543.1
418.6
0.4735
0.3623
perpendicular to the BODIPY core causing a loss in the conjugation.
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4
. Conclusions
Two new BODIPY derivatives (BDP1 and BDP3) with linear and
L-like architectures were synthesized with high reaction yields.
Theoretical and experimental studies confirmed ICT processes
where methoxy, hexyloxy and phenolic groups undergo charge
transfer to the BODIPY core. The methoxy and hexyloxy sub-
stituents (BDP1) have a minimal ICT phenomenon because the ring
to which they are attached is not coplanar with the BODIPY core.
The effect of the phenolic substituent (BDP3) is strongly marked
due to its coplanarity with the BODIPY unit. Finally, the value of the
theoretical difference between the border orbitals for the BDP3
compound is between E0-0 and Egap, which it is assumed that the
experimental results correspond with the theoretical value.
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[
Declaration of competing interest
[
The authors declare no conflict of interest.
CRediT authorship contribution statement
[14] M. Benstead, G.H. Mehl, R.W. Boyle, 4,4’-Difluoro-4-bora-3a,4a-diaza-s-inda-
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Juan S. Rocha-Ortiz: Investigation, Methodology, Writing -
original draft, Visualization, Writing - review & editing. Alberto
Insuasty: Investigation, Methodology, Formal analysis, Writing -
original draft, Visualization, Writing - review & editing. Braulio
Insuasty: Writing - original draft, Visualization, Writing - review &
editing. Alejandro Ortiz: Conceptualization, Supervision, Writing -
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Acknowledgments
10.1039/B617972J.
19] J. Ba n~ uelos, V. Martín, C.F.A. G oꢀ mez-Dur aꢀ n, I.J.A. C oꢀ rdoba, E. Pe n~ a-Cabrera,
The authors gratefully acknowledge financial support from the
Universidad del Valle and Center for Research and Innovation in
Bioinformatics and Photonics (CIBioFi) (BPIN 2013000100007) for
financial support.
I. García-Moreno, Aꢀ . Costela, M.E. P eꢀ rez-Ojeda, T. Arbeloa, ꢀI .L. Arbeloa, New 8-
Amino-BODIPY derivatives: surpassing laser dyes at blue-edge wavelengths,
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[
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Appendix A. Supplementary data
21] D. Zhang, V. Martín, I. García-Moreno, A. Costela, M.E. P eꢀ rez-Ojeda, Y. Xiao,
Development of excellent long-wavelength BODIPY laser dyes with a strategy
that combines extending p-conjugation and tuning ICT effect, Phys. Chem.
Supplementary data to this article can be found online at
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Chem. Phys. 13 (2011) 13026e13033, https://doi.org/10.1039/c1cp21038f.
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