Synthesis, photophysical properties and theoretical studies of new bis-quinolin curcuminoid BF2-complexes and their decomplexed derivatives

Article abstract of DOI:10.1016/j.saa.2020.118065

This paper presents the synthesis and characterization of two series of new bis-quinolin curcuminoid BF₂-complexes 11 and their respective decomplexed bis-quinolin curcuminoid derivatives 12, in an attempt to understand their optical properties. The synthesized compounds showed interesting fluorescent characteristics in both solution and in solid-state. The characteristic of the electronic transitions involved in these systems were measured via Uv-vis spectroscopy and fluorescence spectroscopy. Results revealed that the absorption and emission bands are dependent of the structure of compounds 11 and 12 but also of the type of substituent, even showing a push-pull behavior in those derivatives substituted with methyl group. These findings were also confirmed through computational calculations at DFT level via simulations of the Uv-vis spectra and determining the topology of the border orbitals responsible for light absorption.

Full text of DOI:10.1016/j.saa.2020.118065

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Synthesis, photophysical properties and theoretical studies of new
bis-quinolin curcuminoid BF2-complexes and their decomplexed
derivatives
Daniel Insuasty, Lorena Cabrera, Alejandro Ortiz, Braulio
Insuasty, Jairo Quiroga, Rodrigo Abonia
PII:
S1386-1425(20)30042-1
https://doi.org/10.1016/j.saa.2020.118065
SAA 118065
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 1 November 2019
Revised date:
11 January 2020
11 January 2020
Accepted
date:
Please cite this article as: D. Insuasty, L. Cabrera, A. Ortiz, et al., Synthesis, photophysical
properties and theoretical studies of new bis-quinolin curcuminoid BF2-complexes and
their decomplexed derivatives, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy(2018), https://doi.org/10.1016/j.saa.2020.118065
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Synthesis, photophysical properties and theoretical studies of new bis-
quinolin curcuminoid BF2-complexes and their decomplexed
derivatives
Daniel Insuasty a, Lorena Cabrera b, Alejandro Ortiz b,c, Braulio Insuasty b,c, Jairo Quiroga b,c,
Rodrigo Abonia b,c*
a Departamento de Química y Biología, División de Ciencias Básicas, Universidad del Norte, Km 5 vía Puerto Colombia,
Barranquilla 081007, Colombia
b
Research Group of Heterocyclic Compounds, Department of Chemistry, Universidad del Valle, A.A. 25360 Cali,
Colombia
c Center for Bioinformatics and Photonics-CIBioFI, Universidad del Valle, Calle 13 No. 100-00, Edificio 320, No. 1069,
Cali, Colombia
* Corresponding author. Tel: +(57)-2-3393248. E-mail addresses: rodrigo.abonia@correounivalle.edu.co
GRAPHICAL ABSTRACT:
Abstract: This paper presents the synthesis and characterization of two series of new bis-quinolin curcuminoid BF2-

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complexes 11 and their respective decomplexed bis-quinolin curcuminoid derivatives 12, in an attempt to understand their
optical properties. The synthesized compounds showed interesting fluorescent characteristics in both solution and in solid-
state. The characteristic of the electronic transitions involved in these systems were measured via Uv-vis spectroscopy and
fluorescence spectroscopy. Results revealed that the absorption and emission bands are dependent of the structure of
compounds 11 and 12 but also of the type of substituent, even showing a push-pull behavior in those derivatives
substituted with methyl group. These findings were also confirmed through computational calculations at DFT level via
simulations of the Uv-vis spectra and determining the topology of the border orbitals responsible for light absorption.
Keywords: Quinolines, Curcumin, absorption spectroscopy, fluorescence spectroscopy, chromophores, BF2-
complexes.
1.
Introduction
The last decades there were an increase in the design and synthesis of new organic pigments with outstanding electronic
and optical properties [1], due to its broad spectrum of application in the field of nonlinear optics [2], photodynamic
therapy [3], organic lasers [4], bioimages [5], organic light-emitting diodes (OLEDs) [6], sensors [7] and photovoltaic [8].
Recently, different curcuminoid derivatives and their boron complexes emerged as an attractive building block for the
design of new pigments with enhanced photophysical properties [9–11]. Consequently, different chromophores were used
to replace the aryl rings, which acts as electrodonor groups and are directly linked to the enolate [12,13]. For example,
previous studies of difluoroboron curcuminoids [14] 1, showed that an electron-withdrawing substituent in the aromatic
ring induced a hypsochromic shift in both emission and absorption bands, while an electrodonor substituent induced a
bathochromic shift. Sachin et al. [12] reported the synthesis of difluoroboron bis-coumarin analog complexes 2, in which
the quantum fluorescence performance, the absorption, and emission wavelength were sensitive to the polarity of the
solvent. This finding was associated with the rigidity of the structure of compounds 2 (thanks to the BF2-1,3-diketone
moiety), and by the electron-withdrawing effect of the BF2 functionality. Additionally, curcuminoid BF2-complexes are
related with biomedical applications, such as compound 3 that has properties as a bioimage detector [5].
Different study fields also presented an increasing interest in quinolines and their oxo-derivatives due to their numerous
practical applications, such as medical and pharmaceutical agents [15,16]. Additionally, quinolines play an important role
in optoelectronic applications due to their high thermal and chemical stability, high fluorescence, electron transport, and
their possibility of modifications [17–19]. Figure 1 presents some examples of compounds (4-6) with optical properties

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containing oxo-quinolin fragments in their structures [7,20,21].
Figure 1. Difluoroboron curcuminoid complexes 1-3 and oxo-quinolin hibrids 4-6 with interesting photophysical and
biological properties.
In that scenario, the present study designed, synthesized, and evaluated the absorption and emission properties of new bis-
quinolin-curcuminoid analogs 11 and 12 containing varied substituted quinolin-moieties as donor fragments and 1,3-
diceto (in its enolic form) or 1,3-diketodifluoroborate functionalities as acceptors framed within a push-pull concept
(Figure 2). For this study, a set of three O-alkyl quinolin- and three N-butylquinolone-based structures derived of 11 and
their decomplexed analogues 12, containing an electron donor group (R = CH3), an electron acceptor group (R = Cl) and a
neutral group (R = H) (see Figure 2), was selected, in order to evaluate the behavior of these groups over different
photophysical properties. Additionally, the current study presented a solvatochromic study of compounds 11 and 12 in a
binary solvent mixture, and a computational analysis using the DFT method.
Figure 2. General structure of target molecules 11 and 12
2.
Experimental
2.1. Material and methods
Melting points were measured using a Stuart SMP10 melting point apparatus and are uncorrected. ATR-FTIR spectra
were recorded on a Shimadzu IRAffinity–1. 1H and 13C NMR spectra were recorded on a Bruker Avance 400

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spectrophotometer operating at 400 MHz and 100 MHz respectively, using DMSO–d6 and CDCl3 as solvent and
tetramethylsilane as internal standard. Chemical shifts are reported in parts per million (ppm) and multiplicities are given
as: s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublets), m (multiplet) and the coupling constants J are
given in Hz. Mass spectra were run on a SHIMADZU-GCMS 2010-DI-2010 spectrometer (equipped with a direct inlet
probe) operating at 70 eV. HPLC-HRMS data were obtained on an Agilent Technologies Q-TOF 6520 spectrometer via
an electrospray ionization (ESI, 4000 V). UV–Vis absorption spectra of all compounds were recorded on a JASCO V-730
UV–VIS Spectrophotometer. Fluorescence spectra of all the compounds were recorded on a JASCO FP-8500
spectrophotometer. Chemicals and the required solvents were purchased from Sigma-Aldrich, Fluka and Merck
(analytical grade reagent), and used without further purification.
2.2. General procedure for the synthesis of the bis-quinolin curcuminoid BF2-complexes 11b-f: A mixture of 2,4-
pentanodione (1 equiv.) and BF3.Et2O (1.5 equiv.) in toluene (1.5 mL) was heated at 80°C for 2 h. After cooling, a mixture
of formylquinolines 10a-f, (2 equiv.), N-butylamine (0.2 equiv.) and tributyl borate (1 equiv.) in toluene was added to the
AA-BF2 solution and was stirred at room temperature for 5 h. The solid formed was filtered and washed repeatedly with
toluene, affording pigments 11b-f.
3.3'-((1E,1'E)-(2,2-Difluoro-2H-1λ3,3,2λ4-dioxaborinin-4,6-diyl)bis(etheno-2,1-diyl))bis(2-butoxy-6-methylquinoline)
1
11b. Orange solid, yield: 94%; m.p. 235-237 °C. FTIR (ATR):  = 2965 (=C-H), 1602 (C=O), 1508 (C=C) cm-1. H
NMR (400 MHz, CDCl3) δ = 1.10 (t, J = 7.4 Hz, 6H), 1.57 – 1.66 (m, 4H), 1.92 – 2.00 (m, 4H), 2.53 (s, 6H), 4.61 (t, J =
6.7 Hz, 4H), 6.14 (s, 1H), 7.25 (d, J = 15.6 Hz, 2H), 7.52 – 7.54 (m, 4H), 7.72 (d, J = 9.1 Hz, 2H), 8.15 (s, 2H), 8.22 (d, J
= 15.6 Hz, 2H). 13C NMR (400 MHz, CDCl3) δ = 14.0, 19.6, 21.3, 31.0, 66.5, 103.2, 119.1, 123.7, 124.8, 126.8, 127.4,
133.8, 134.4, 141.0, 142.5, 145.8, 159.5, 180.4 ppm. ESI-QTOF (positive ionization) M+H calc. for C35H37BF2N2O4:
599.2893; found: 599.2888.
3,3'-((1E,1'E)-(2,2-Difluoro-2H-1λ3,3,2λ4-dioxaborinin-4,6-diyl)bis(etheno-2,1-diyl))bis(2-butoxyquinoline) 11b. Orange
solid, yield: 75%. m.p. 185-187 °C. FTIR (ATR):  = 2955 (=C-H), 1624 (C=O), 1518 (C=C) cm-1. 1H NMR (400 MHz,
CDCl3) δ = 1.10 (t, J = 7.4 Hz, 6H), 1.56 – 1.66 (m, 4H), 1.91 – 1.98 (m, 4H), 4.60 (t, J = 6.7 Hz, 4H), 6.15 (s, 1H), 7.21
– 7.30 (m, 4H), 7.42 (t, J = 7.4 Hz, 2H), 7.68 (t, J = 7.6 Hz, 2H), 7.75 (d, J = 8.1 Hz, 2H), 8.17 – 8.21 (m, 4H). 13C NMR
(400 MHz, CDCl3) δ = 14.0, 19.6, 31.0, 66.7, 103.2, 119.3, 123.9, 124.8, 124.8, 127.1, 128.3, 128.4, 131.7, 141.5, 142.3,
159.9, 180.4 ppm. ESI-QTOF (positive ionization) M+H calc. for C33H33BF2N2O4: 571.1502; found 571.1508.
3,3'-((1E,1'E)-(2,2-Difluoro-2H-1λ3,3,2λ4-dioxaborinin-4,6-diyl)bis(etheno-2,1-diyl))bis(2-butoxy-6-chloroquinoline)
11c. Orange solid, yield: 85%. m.p. 261-263 °C. FTIR (ATR):  = 2957 (=C-H), 1609 (C=O), 1520 (C=C) cm-1. 1H NMR
(400 MHz, CDCl3) δ = 1.10 (t, J = 7.4 Hz, 6H), 1.58 – 1.64 (m, 4H), 1,92 – 2.00 (m, 4H), 4.62 (t, J = 6.7 Hz, 4H), 6.16
(s, 1H), 7.25 (d, J = 15.7 Hz, 2H), 7.63 (dd, J = 8.8, 2.4 Hz, 2H), 7.74 – 7.77 (m, 4H), 8.14 (s, 2H), 8.21 (d, J = 15.7 Hz,
2H). NMR (101 MHz, 5) δ = 14.0. 19.6, 31.1, 66.5, 102.7, 106.3, 120.3, 122.5, 125.5, 127.2, 128.2, 135.5, 137.6, 142.2,
156.4, 158.7, 183.5 ppm. ESI-QTOF (positive ionization) M+H calc. for C33H32BCl2F2N2O4: 639.1800; found: 639.1795.
3,3’-((1E,1’E)-(2,2-Difluoro-2H-1λ3,3,2λ4-dioxaborinin-4,6-diyl)bis(etheno-2,1-diyl))bis(1-butylquinolin-2(1H)-ona)
1
11d. red solid, yield: 80%. M.p. 215-217 °C. FTIR (ATR):  = 2959 (=C-H), 1645 (2 x C=O), 1516 (C=C) cm-1. H
NMR (400 MHz, CDCl3) δ = 1.07 (t, J = 7.3 Hz, 6H), 1.53 – 1.60 (m, 4H), 1.77 – 1.84 (m, 4H), 4.38 (t, J = 7.8 Hz, 4H),

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6.19 (s, 1H), 1.07 (t, J = 7.5 Hz, 2H), 7.42 (d, J = 8.6 Hz, 2H), 7.67 – 7.72 (m, 4H), 7.86 (d, J = 15.2 Hz, 2H), 7.99 – 8.02
(m, 4H). 13C NMR (400 MHz, CDCl3) δ = 13.9, 20.4, 29.6, 42.8, 104.0, 114.4, 120.5, 122.7, 125.1, 125.2, 130.4, 132.9,
139.6, 143.0, 144.4, 160.5, 180.8 ppm. ESI-QTOF (positive ionization) M+H calc. for C33H33BF2N2O4: 571.1502; found:
571.1498.
3,3’-((1E,1’E)-(2,2-Difluoro-2H-1λ3,3,2λ4-dioxaborinin-4,6-diil)bis(etheno-2,1-diiy))bis(1-butyl-6- methylquinolin
-
2(1H)-one) 11e. red solid, yield: 85%. M.p. 234-236 °C. FTIR (ATR):  = 2953 (=C-H), [1647, 1630] (C=O), 1520
(C=C) cm-1. 1H NMR (400 MHz, CDCl3) δ = 1.06 (t, J = 7.3 Hz, 6H), 1.52 – 1.60 (m, 4H), 1.75 – 1.83 (m, 4H), 2.48 (s,
6H), 4.36 (t, J = 7.8 Hz, 4H), 6.18 (s, 1H), 7.32 (d, J = 8.5 Hz, 2H), 7.49 – 7.51 (m, 4H), 7.87 (d, J = 15.3 Hz, 2H), 7.95
(s, 2H), 8.00 (d, J = 15.3 Hz, 2H). 13C NMR δ = 14.1, 17.7, 19.7, 31.0, 66.2, 102.5, 119.8, 124.1, 124.8, 125.8, 127.0,
130.8, 135.2, 135.5, 138.7, 145.6, 159.0, 183.6 ppm. ESI-QTOF (positive ionization) M+H calc C35H37BF2N2O4:
599.2893; found: 599.2884.
3,3’-((1E,1’E)-(2,2-Difluoro-2H-1λ3,3,2λ4-dioxaborinin-4,6-diyl)bis(etheno-2,1-diyl))bis(1-butyl-6-chloroquinolin-2(1H)-
one) 11f. red solid, yield: 91%. M.p. 256-258 °C. FTIR (ATR):  = 2961 (=C-H), 1645 (2 x C=O), 1520 (C=C) cm-1. 1H
NMR (400 MHz, CDCl3) δ = 1.07 (t, J = 7.3 Hz, 6H), 1.52 – 1.59 (m, 4H), 1.76 – 1.80 (m, 4H), 4.33 – 4.37 (m, 4H), 6.22
(s, 1H), 7.36 (d, J = 9.1 Hz, 2H), 7.62 (dd, J = 9.1, 2.4 Hz, 2H), 7.69 (d, J = 2.4 Hz, 2H), 7.88 (d, J = 15.3 Hz, 2H), 7.94
(s, 2H), 8.00 (d, J = 15.3 Hz, 2H). Compound 11f is barely soluble in CDCl3 and dimethyl sulfoxide, thus, made the
registration of a high resolution 13C NMR spectrum impossible. ESI-QTOF (positive ionization) M+H calc. for
C33H31BCl2F2N2O4: 639.1800; found: 639.1806.
2.3. General procedure for the synthesis of the bis-quinoline decomplexed derivatives 12a-f. A solution of bis-
quinolin curcuminoid BF2-complexes 11b-f (200 mg) in MeOH/DMSO (1:1, 5 mL) was heated under reflux for 1 h.
After completion (checked by TLC), the solids formed were collected by filtration, washed cold methanol (2 x 3 mL) and
dried in air. No further purification was required.
(1E,4Z,6E)-1,7-bis(2-Butoxy-6-methylquinolin-3-yl)-5-hydroxyhepta-1,4,6-trien-3-one 12a. Yellow solid, yield: 80%.
M.p. 165-168 °C. FTIR (ATR):  = 2959 (=C-H), 1614 (C=O), 1562 (C=C) cm-1. 1H NMR (400 MHz, CDCl3) δ = 1.09
(t, J = 7.4 Hz, 6H), 1.58 – 1.67 (m, 4H), 1.91 – 1.98 (m, 4H), 2.52 (s, 6H), 4.61 (t, J = 6.6 Hz, 4H), 5.90 (s, 1H), 7.03 (d,
J = 15.9 Hz, 2H), 7.48 – 7.52 (m, 4H), 7.77 (d, J = 8.4 Hz, 2H), 7.91 (d, J = 15.9 Hz, 2H), 8.12 (s, 2H), OH was not
observed. 13C NMR (400 MHz, CDCl3) δ = 14.0, 19.6, 21.3, 31.1, 66.6, 102.6, 120.2, 124.9, 126.6, 127.0, 127.1, 132.8,
134.2, 135.5, 138.1, 144.9, 159.6, 183.8 ppm. EI MS (70 eV): m/z (%): 550 (M+, 30), 212 (88), 184 (100).
(1E,4Z,6E)-1,7-bis(2-Butoxyquinolin-3-yl)-5-hydroxyhepta -1,4,6-trien-3-one 12b. Yellow solid, yield: 70%. M.p. 120-
123 °C. FTIR (ATR):  = 2957 (=C-H), 1628 (C=O), 1560 (C=C) cm-1. 1H NMR (400 MHz, CDCl3) δ = 1.10 (t, J = 7.4
Hz, 6H), 1.58 – 1.68 (m, 4H), 1.92 – 1.99 (m, 4H), 4.63 (t, J = 6.6 Hz, 4H), 5.91 (s, 1H), 7.04 (d, J = 15.9 Hz, 2H), 7.41
(t, J = 7.4 Hz, 2H), 7.66 (t, J = 7.4 Hz, 2H), 7.76 (d, J = 7.9 Hz, 2H), 7.86 (d, J = 8.3 Hz, 2H), 7.93 (d, J = 15.9 Hz, 2H),
8.21 (s, 2H), OH was not observed. 13C NMR (400 MHz, CDCl3) δ = 14.0, 19.6, 31.1, 66.6, 102.6, 120.4, 124.6, 125.0,
126.9, 127.2, 128.0, 130.6, 135.3, 138.5, 146.6, 160.0, 183.5 ppm. EI MS (70 eV): m/z (%): 341 (4), 81 (48), 69 (100).
(1E,4Z,6E)-1,7-bis(2-Butoxy-6-chloroquinolin-3-yl)-5-hydroxyhepta-1,4,6-trien-3-one 12c. Yellow solid, yield: 75%.

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M.p. 205-208 °C. FTIR (ATR):  = 2963 (=C-H), 1628 (C=O), 1564 (C=C) cm-1. 1H NMR (400 MHz, CDCl3) δ = 1.10
(t, J = 7.3 Hz, 6H), 1.58 – 1.67 (m, 4H), 1.92 – 1.99 (m, 4H), 4.65 (t, J = 5.9 Hz, 4H), 5.92 (s, 1H), 7.02 (d, J = 15.9, 2H),
7.58 – 7.60 (m, 2H), 7.74 (s, 2H), 7.83 (d, J = 8.4 Hz, 2H), 7.91 (d, J = 15.9 Hz, 2H), 8.11 (s, 2H), OH was not observed.
Compound 12c is barely soluble in CDCl3 and dimethyl sulfoxide, thus, made the registration of a high resolution 13C
NMR spectrum impossible. EI MS (70 eV): m/z (%): 425 (12), 367 (22), 69 (100).
3,3’-((1E,3Z,6E)-3-Hidroxy-5-oxohepta-1,3,6-trien-1,7-diyl)bis(1-butylquinolin-2(1H)-one) 12d. Orange solid, yield:
1
68%. M.p. 218-220 °C. FTIR (ATR):  = 2959 (=C-H), [1649, 1605] (C=O), 1555 (C=C) cm-1. H NMR (400 MHz,
CDCl3) δ = 1.06 (t, J = 7.3 Hz, 6H), 1.52 – 1.61 (m, 4H), 1.75 – 1.83 (m, 4H), 4.63 (t, J = 7.8 Hz, 4H), 5.96 (s, 1H), 7.26
– 7.31 (m, 2H), 7.39 (d, J = 8.6 Hz, 2H), 7.50 (d, J = 15.7 Hz, 2H), 7.60 – 7.66 (m, 4H), 7.73 (d, J = 15.7 Hz, 2H), 7.92
(s, 2H), OH was not observed. 13C NMR (400 MHz, CDCl3) δ = 13.9, 20.4, 29.6, 42.7, 103.4, 114.2, 120.6, 122.4, 126.3,
128.2, 129.8, 131.6, 136.1, 139.1, 140.2, 160.7, 183.8 ppm. EI MS (70 eV): m/z (%): 522 (M+, 3), 129 (23), 43 (100).
3,3’-((1E,3Z,6E)-3-Hydroxy-5-oxohepta-1,3,6-trien-1,7-diyl)bis(1-butyl-6-methylquinolin-2(1H)-one) 12e. Orange solid,
1
yield: 75%. M.p. 224-226 °C. FTIR (ATR):  = 2951 (=C-H), [1647, 1600] (C=O), 1560 (C=C) cm-1. H NMR (400
MHz, CDCl3) δ = 1.06 (t, J = 7.3 Hz, 6H), 1.51 – 1.60 (m, 4H), 1.75 – 1.83 (m, 4H), 2.46 (s, 6H), 4.35 (t, J = 7.7 Hz,
4H), 5.97 (s, 1H), 7.28 (d, J = 9.7 Hz, 2H), 1.41 – 7.43 (m, 4H), 7.47 (d, J = 15.9 Hz, 2H), 7.73 (d, J = 15.7 Hz, 2H), 7.85
(s, 2H), ), OH was not observed. 13C NMR (400 MHz, CDCl3) δ = 13.7, 20.4, 29.6, 42.6, 63.6, 102.9, 114.1, 120.7, 126.4,
128.1, 129.3, 131.8, 132.9, 136.1, 137.3, 139.6, 160.6, 183.8 ppm. EI MS (70 eV): m/z (%): 341 (8), 81 (48), 69 (100).
3,3’-((1E,3Z,6E)-3-Hydroxy-5-oxohepta-1,3,6-trien-1,7-diyl)bis(1-butyl-6-chloroquinolin-2(1H)-one) 12f. Yellow solid,
1
yield: 80%. M.p. 268-270 °C. FTIR (ATR):  = 2961 (=C-H), [1653, 1612], (C=O), 1548 (C=C) cm-1. H NMR (400
MHz, CDCl3) δ = 1.09 (t, J = 7.4 Hz, 6H), 1.59 – 1.64 (m, 4H), 1.90 – 1.97 (m, 4H), 4.60 (t, J = 6.6 Hz, 4H), 5.88 (s,
1H), 7.01 (d, J = 15.9 Hz, 2H), 7.35 (dd, J = 8.4, 1.6 Hz, 2H), 7.66 (d, J = 8.6 Hz, 2H), 7.84 (d, J = 1.8 Hz, 2H), 7.87 (d,
J = 15.9 Hz, 2H), 8.13 (s, 2H), OH was not observed. 13C NMR (101 MHz, CDCl3) δ = 14.0, 19.5, 31.0, 66.8, 102.8,
120.5, 123.3, 125.4, 126.3, 127.5, 129.0, 135.0, 136.5, 137.9, 147.2, 160.6, 183.3 ppm. EI MS (70 eV): m/z (%): 590/592
(M+, 46/35), 260 (45).
3.
Results and discussion
3.1.
Synthesis of new bis-quinolin curcuminoid BF2-complexes and their decomplexed derivatives
Scheme 1 presents the synthetic route for the preparation of the new bis-quinolin based difluoroboron-curcuminoid
complexes 11b-f. Initially, we obtained the donor units 3-formyl-2-alkoxy-quinolines (O-butylated) 10a-c and 3-formyl-
2-oxo-quinolines (N-butylated) 10d-f via Meth-Cohn reactions, mediated by acid hydrolysis of chloro-derivatives 8 and
O-, C-alkylation of intermediates 9 [18]. Subsequently, the synthesis of the new BF2-complexes 11b-f was performed
following the methodology reported by Liu et al. [22]. In this approach, the acetylacetone boronite (AA-BF2) was
generated in situ, by treating acetylacetone with BF3Et2O at 80°C for 2h. After cooling, the condensation reaction was
carried out by adding a mixture of the formylquinolines 10a-f, n-butylamine (n-BuNH2) and tributyl borate B(OBu)3 in
toluene, to the AA-BF2 solution. The thin-layer chromatography (TLC) analysis showed the formation of a single

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product. The solids formed were filtered under reduced pressure and washed repeatedly with toluene, affording pigments
1
1
11b-f in 75-94% yield. All BF2-complexes 11 were characterized by H and 13C NMR and mass spectrometry. The H
NMR spectrum of the BF2-complex 11b, run in CDCl3, showed a singlet at 6.14 ppm, corresponding to the proton H-4´,
and two doublets at 7.25 and 8.22 ppm (J = 15.6 Hz), assigned to protons 1´ and 2´ of the α,β-unsaturated moiety.
Therefore, confirming a satisfactory condensation reaction between AA-BF2 and 10, as well as, E configuration of the
new double bonds formed. In the 13C NMR spectrum, the total expected signals (i.e. 13) for BF2-complex 11b were
observed (see supplementary Information). The mass spectrum confirmed the formation of the desired BF2-complex 11b
1
and allowed to observe the peak of the molecular ion with m/z 598. The H, 13C NMR spectra for the remaining BF2-
complexes 11b-f were similar, confirming the generality of the process (supplementary Information).
Scheme 1. Synthesis of bis-quinolin curcuminoid BF2-complexes 11b-f.
Once the BF2-complexes 11 were obtained, subsequently, we performed the decomplexation in order to obtain their
corresponding bis-quinolin curcuminoid derivatives 12a-f. Thus, the treatment of BF2-complexes 11 with a mixture
MeOH/DMSO (1:1 ratio) under reflux for 1h [22], afforded the decomplexed derivatives 12a-f as yellow solids in 68-
1
85%, as shown in Scheme 2. The H and 13C NMR spectra showed that decomplexed derivatives 12a-f exist in their
1
enolic tautomers, as usual [23], instead of their diketonic ones. In consequence, in the H NMR spectrum of the
decomplexed derivative 12a a singlet was observed at 5.90 ppm assigned to the proton H-4’, confirming the formation
of the enolic tautomer, and two doublets at 7.03 and 7.91 ppm (J = 15.9 Hz), corresponding to the protons 1´ and 2´ of
the α,β-unsaturated moiety, confirming the E configuration of the new double bonds formed in the decomplexed
derivatives 12. In the 13C NMR spectrum, the total expected signals (18) were observed for the decomplexed
derivative 12a (including the absence of aliphatic carbon CH2 due to the preferred enol tautomeric form). Finally, the

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mass spectrum of 12a corroborated the decomplexation process, observing the peak of the expected molecular ion
with m/z 550.
Scheme 2. Synthesis of the bis-quinoline decomplexed derivatives 12a-f.
3.2.
Absorption and emission properties
Figure 3 and Table 1 show the results of the absorption and emission measures of the BF2-complexes 11b-f, as well as,
their corresponding decomplexed derivatives 12a-f, performed at a concentration of 1x10-5 M, using dichloromethane as
solvent. The absorption spectra showed intense bands between 413-512 nm, likely due to CT-type transitions and weak
bands around 300 nm, which are attributed to n-π* transitions (Figure 3a,b). In turn, we observed in all studied
compounds a high absorption intensity, as reflected in their values of ε (molar absorptivity), ranging between 3.41x104 -
9.99x104 (see Table 1). The emission spectra (Figure 3c,d) were measured at 355 nm as the excitation wavelength of
prodan provided the fluorescence standard.

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Figure 3. CH2Cl2 absorption spectra of: (a) BF2-complexes 11b-f; (b) decomplexed derivatives 12a-f and emission
spectra of: (c) BF2-complexes 11b-f; (d) unpacked decomplexed derivatives 12a-f.
Table 1. Measured values of absorption and emission spectra in CH2Cl2 as the solvent, for BF2-complexes 11b-f and
decomplexed derivatives 12a-f; at an approximate concentration of 1x10-5
M
b
1abs(nm)/(M-1cm-1) x10-4 2abs(nm)/(M-1cm-1) x10-4 1ems(nm)a 2ems(nm)a  cm-1)
F
Compound
461/6.44
458/8.62
451/9.99
478/6.52
486/7.50
476/3.41
418/9.82
458/8.72
413/3.95
432/5.71
485/6.50
533
525
524
549
560
548
475
469
466
483
-
-
208333
222222
208333
217391
208333
222222
175438
909090
188679
196078
8.85x10-4
1.17x10-3
1.89 x10-3
9.48x10-4
8.07x10-4
6.98x10-4
7.83x10-4
2.30x10-4
1.74x10-4
2.68x10-3
11b
11b
11c
11d
11e
11f
480/8.59
476/9.59
-
503/6.56
-
512/6.95
-
503/3.39
-
-
-
-
-
499
494
493
507
12a
12b
12c
12d

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435/10.7
434/7.89
355/1.79
-
-
-
-
513
508
-
128205
192307
-
7.58x10-3
4.38x10-3
0.98
12e
12f
486
439
Prodan
aMeasurements in exc of PRODAN (355 nm), used as a fluorescence standard.b Values relative to the quantum
fluorescence performance of the standard, s = 0.98.
The BF2-complexes 11d-f showed a better chromophore system compared to BF2-complexes 11b-c, as suggested by the
analysis of absorption spectra of the synthesized BF2-complexes 11b-f (Figure 3a) and the fluorescence emission spectra
(Figure 3c), which revealed that the corresponding wavelengths for the BF2-complexes of N-butylquinolones are displaced
towards the red compared to the BF2-complexes of O-butylquinolines. We also observed that for BF2-complexes 11b and
11e, which contain an electrodonor group such as -CH3 in the heterocyclic ring, the absorption, and emission bands
undergo a slight bathochromic displacement in comparison with the unsubstituted BF2-complexes 11b and 11d, whereas
the displacement is hypsochromic in the presence of an electron-withdrawing group such as -Cl (see Table 1). On the
other hand, we observed non-regular tendency in the absorption (Figure 3b) and emission (Figure 3d) spectra of
decomplexed derivatives 12a-f (see Table 1), suggesting the presence of a non-rigid conjugate system and a weaker
acceptor behavior than the BF2-complexes. This behavior is consistent with the photophysical characteristics of several
BF2-complex and their curcumin analogues reported in the literature [24].
The emission spectra of BF2-complexes 11b-f evidenced an intramolecular charge transfer (ICT) process from the
quinolinic ring (donor fragment) toward the moiety that contains the BF2 functionality (acceptor fragment). This process
is observed via deactivation of the fluorescence intensity when passing from an electron-withdrawing substituent, such as
–Cl (f: 1.89 x10-3), to an electron-donor substituent, such as –CH3 (f: 8.85 x10-4). Therefore, a donor substituent favors
deactivation by an ICT process, whereas an acceptor substituent favors deactivation to a lesser extent by the ICT process
and mostly by fluorescence. Also, the N-butylated BF2-complexes 11d-f presented lower fluorescence intensity (see f in
Table 1), than their corresponding O-butylated ones 11b-c, indicating that the N-butyl substituent behave as a better
electron-donor than the O-butyl substituent. It is important to note that derivatives 11b-c (quinolines), have a higher
aromatic character compared to derivatives 11d-f (quinolones). In other words, the heterocyclic fragments greatly affect
the spectroscopic behavior of the resultant curcumin-BF2 complexes. Table 1 also shows the smaller values of Stoke´s
displacements in BF2-complexes 11 (: 222222-208333 cm-1) when comparing with decomplexed derivatives 12 (:
909090-128205 cm-1). This finding is associated with a greater flexibility in decomplexed derivatives 12, which consume
the energy absorbed in motion and other non-radiative processes, whereas more rigid BF2-complexes 11 lose less energy
due to non-radiative processes only.

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Additionally, the emission spectra of BF2-complexes 11b-f showed a single emission band, in contrast to the
decomplexed derivatives 12a-f with their two deactivation pathways (Figure 3d). These results are likely an outcome of
the decomplexed derivatives 12a-f that can be deactivated from two close excitonic states of energy found in a keto-
enolic equilibrium, as shown in Figure 4. Interestingly, this behavior does not correspond to Kasha's rule [25]. Therefore,
a titration was performed with trifluoroacetic acid (TFA) (Figure 5) to disfavor the keto-enolic equilibrium shown in
Figure 4. To achieve that, we selected compounds 12b and 12d that have two emission bands, (see Figure 3d). In Figure
5, we decreased the fluorescence intensity of those two compounds by increasing the number of TFA equivalents. In turn,
compound 12d showed a single band without a shoulder (Figure 5b) and a marked deactivation of the emission of the
protonated species, an expected behavior when the keto-enolic equilibrium is disrupted. The latter behavior can be clearly
attributed to excited state intramolecular proton transfer (ESIPT), which was described by Joo et al. [26] for similar
systems, but also could be associated to two close vibronic structures.
Figure 4. Keto-enolic equilibrium for decomplexed derivatives 12a-f
Figure 5. Fluorescence spectra of decomplexed derivatives (a) 12b; (b) 12d at a concentration of 5x10-5 M, titrated
with TFA
Fluorescence Quantum yield (ΦF) were calculated in Equation 1, using Prodan as standard in dichloromethane (DCM)
(ΦFstd = 0.98) [27].

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Eq. (1)
Where F and Fstd are the areas under the fluorescence curves of products 11b-f, 12a-f, and the standard, respectively. A
and Astd correspond to the absorbance of the sample and the standard, I and Istd are the relative intensities of the excitation
light and n2 and n2std are the refractive indices of the solvents used for the sample and the DCM standard. This estimation
showed that the fluorescence quantum yield (ΦF) reached relatively good values (see Table 1).
Subsequently, a solvatochromic study was performed for BF2-complexes 11b, 11e and 11c and derivatives 12b and 12e,
using a binary mixture of CH2Cl2/MeOH as solvent. The selection of these compounds occurred because: (i) their
structural characteristics and (ii) the possibility to compare their structure under a change in polarity of the solvent
mixture. For BF2-complex 11b (O-butyl, R = -CH3), we noticed that when the percentage of MeOH increases the
fluorescence intensity decrease (see ΦF in Table 2). Consequently, there is an intramolecular charge transfer, since 11b
derivative are strongly dependent on solvent polarity; the emission intensity of the 11b decreases progressively with
increasing solvent polarity, which is consistent with the stabilization of the charge separation state in the emissive state
when polar solvents are used. On the other hand, Figure 6b shows that ems for this BF2-complex did not vary
significantly when the polarity of the solvent varies. The same analysis applied to the decomplexed derivative 12b (O-
butyl, R = -CH3) revealed that the fluorescence intensity behaves randomly (Figure 7a), this suggests that the charge
separation state in the emissive state did not depend on the solvent polarity effect, possibly due to the non-rigid bis-
quinoline-curcumine bonds compared to the rigid BF2-complex 11b. In the normalized fluorescence spectrum for this
compound (Figure 7b), a slight bathochromic shift is distinguished by increasing the polarity of the solvent system from
95/5% CH2Cl2/MeOH (475 nm) to 75/25% CH2Cl2/MeOH (503 nm) (see Table 2), thus suggesting the presence of a
weak intramolecular charge transfer process or an excited state intramolecular proton transfer (ESIPT).

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Figure 6. Fluorescence emission spectrum of BF2-complex 11b at different ratios of the CH2Cl2/MeOH solvent
system (a) without normalization and (b) normalized
Figure 7. Fluorescence emission spectrum of the decomplexed derivative 12b, at different ratios of the CH2Cl2/MeOH
solvent system (a) without normalization and (b) normalized
Table 2. Measured absorption and emission values in CH2Cl2/MeOH as solvent, for compounds 11b, 11c, 11e, 12a and
12e at an approximate concentration of 1x10-5 M
BF2-complex
BF2-complex 11e
BF2-complex 11c
Dec. deriv. 12b
Dec. deriv. 12e
11b
λ1
ems
(nm)a
489
% MeOH
λ1
λ1
λ1
λ1
ems
ems
/λ2
ems
ems
ems
b
b
b
b
b
(nm)a
535
ΦF
(nm)a
562
ΦF
(nm)a
525
ΦF
ΦF
(nm)a
ΦF
5
8.87x10-4
3.57x10-4
1.87x10-3
7.38x10-4
1.11x10-3
1.19x10-3
1.02x10-3
1.19x10-3
521
523
526
528
528
3.65x10-3
5.10x10-3
4.49x10-3
4.18x10-3
5.13x10-3
10
15
20
25
536
537
537
538
8.8x10-4
565
564
565
565
3.25x10-4
1.95x10-4
1.66x10-4
7.80x10-5
525
527
527
528
1.20x10-3
1.44x10-3
1.24x10-3
1.17x10-3
501
501
502
503
4.34x10-4
2.78x10-4
1.93x10-4
Measured in exc of PRODAN (355 nm), used as a fluorescence standard. b. Values relative to the quantum
fluorescence performance of the standard, s = 0.98.
Likewise, the solvatochromic study performed for the BF2-complex 11e (N-butyl, R = -CH3) showed a marked
deactivation of the fluorescence band (Figure 8a) (see ΦF in Table 2). Based in this observation, the N-alkylamine
functionality in 11e behaves as a moiety with better electron-donor characteristic (Figure 9). To confirm the above,
solvatochromic experiments were carried out for the decomplexed derivative 12e (N-butyl, R = -CH3). Figure 10a shows
that the emission intensity of derivatives 12e and 12a did not show a dependent relationship with the polarity of the
solvent system, likely an outcome of the weak electron-acceptor capacity of the flexible bis-quinoline-curcuminoid
moiety. Interestingly, in this case, when the polarity of the solvent system increases, the em moves towards red (~ 14 nm)

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(see Table 2) (Figure 10b), thus confirming the greater electron-donor force of the N-butylamine functionality. To better
show the solvatochromic behavior of the measured compounds, a ratio between emission intensity and absorbance was
made, where it is evidenced that the behavior of the quantum yield with respect to the variation of the polarity of the
solvent is in accordance with the phenomena described before [26] (see table 3). For instance, for compound 11b the
emission intensity/absorbance ratio decreases with respect to system solvent polarity. On the other hand, for compound
12b the values of intensity/absorbance ratio are randomly distributed in accordance with the system solvent polarity.
Table 3. Values for intensity of emission/absorption ratios in CH2Cl2/MeOH as solvent, for compounds 11b, 11c, 11e,
12b and 12e at an approximate concentration of 1x10-5 M.
BF2-complex
BF2-complex 11e
BF2-complex 11c
Dec. deriv. 12b
Dec. deriv. 12e
11b
Ratio int em/Abs
84.65
% MeOH
Ratio int em/Abs
65.23
Ratio int em/Abs
146.35
Ratio int em/Abs
78.52
Ratio int em/Abs
173.06
5
10
15
20
25
98.41
58.47
46.64
39.48
70.32
52.60
55.09
29.90
100.75
134.01
104.42
105.17
104.05
104.81
99.17
224.36
169.97
191.01
228.32
106.38
Figure 8. Fluorescence emission spectrum of BF2-complex 11e, at different ratios of the CH2Cl2/MeOH solvent
system (a) without normalization and (b) normalized

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Figure 9. Comparison of the electron-donor capacity between the different quinolinic moieties of BF2-complexes 11b
and 11e, according to the photophysical measurements.
Subsequently, solvatochromic studies were performed for BF2-complex 11c, which has chlorine atoms in the C-6 position
of the quinoline rings. Figure 10c shows that the emission intensity did not depend on the polarity of the solvent system,
but the normalization of all the emission bands (Figure 10d) revealed a slight red shift (~ 1 nm) (Table 2). This result
indicates the presence of an intramolecular charge transfer but attenuated by the chlorine atom with its characteristic
electron-withdrawing effect, which decreased the electron-donor capability of the quinolinic system.
Figure 10. Fluorescence emission spectra of (I): decomplexed derivative 12e, (a) without normalization and (b)
normalized. (II) BF2-complex 11c, (c) without normalization and (d) normalized, both at different ratios of the
CH2Cl2/MeOH solvent system
3.3.
Theoretical calculations
A computational analysis allowed to determine the characteristics of the most significant electronic transitions; for this

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purpose, the structures of compounds 11b, 11c, 11e and 12b were optimized at DFT level with B3LYP hybrid functional
and 6-31G++ basis set, in vacuum. Optimized structures helped to establish the different electronic properties associated
with the structure. For example, to study the differences (i) between 11b and 11e induced, respectively, by the O-
substitution and the N-substitution and (ii) between 11b and 12b with the influence of –BF2 functionality on the π-
conjugate system. Finally, the optimized structures contributed to deduce the effect of the methyl substituent (electron-
donor) in 11b in comparison to the chlorine substituent (electron-withdrawing) in 11c. Additionally, we simulated the
electronic transitions of the absorption spectra and a continuous polarization model (PCM) with dichloromethane as
solvent and estimated the characteristics of absorption modes (Table 4).
Table 4. Calculated characteristics of linear absorption transitions for compounds 11b, 12a, 11e, and 11c.
Excited
Status
S1
Energy
(eV)
abs
(nm)
499.6
Compound
Composition
Oscillator Force
HOMO → LUMO (0.67786)
2.482
1.8711
0.0356
11b
12a
11e
HOMO-1 → LUMO (0.67526)
HOMO → LUMO (0.18874)
HOMO-1 → LUMO (0.19887)
HOMO → LUMO (0.66753)
S2
2.568
482.8
S1
S1
S2
S1
S2
3.111
2.245
2.414
2.525
2.728
398.6
552.4
513.6
491.0
454.4
0.9288
0.7852
0.0643
1.8821
0.1574
HOMO → LUMO (0.65354)
HOMO-1 → LUMO (0.65261)
HOMO → LUMO (0.25646)
HOMO → LUMO (0.69744)
11c
HOMO-1 → LUMO (0.69493)
HOMO → LUMO (0.16984)
The calculated values of abs are very close to the experimental values of abs for the compounds 11b, 11c, and 11e, and
for the abs value in 12b (Table 2). These results combined with the transitions of border molecular orbitals as described
in Table 4, demonstrated that the transitions of maximum absorption for 11b, 11c, and 11e should be observed as two
wide bands resulting from the sum of S1 (HOMO→LUMO) with S2 (HOMO-1→LUMO; HOMO→LUMO). On the other
hand, the behavior for maximum absorption transitions for compound 12b must be presented as broadband with the
presence of one or more shoulders product of S1, which is composed of the combination of two transitions (HOMO-
1→LUMO; HOMO→LUMO) in different ratios. To describe these behaviors, Figure 11 shows the topology of the
border orbitals involved in the electronic transitions.

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Figure 11. Topology of the border orbitals calculated for a) 11b, b) 12a, c) 11e and d) 11c with a Bohr tolerance.
The topological analysis of the boundary orbitals calculated for compound 11b used as a reference took into account that:
the transition HOMO→LUMO and HOMO-1→LUMO have an ICT character from the quinolinic rings to the boron
complex suggesting the presence of a D-A type system. Besides, for compound 12b the two electronic transitions
HOMO-1→LUMO and HOMO→LUMO had a strong character given the weaker electron-acceptor force of the
αβunsaturated system and the free rotation thereof. In the case of compound 11e, a strong ICT character become
evident since the electronic distribution in HOMO and the HOMO-1 is mainly located in the quinoline rings that act as
electron-donors. The little participation of the cyclic system of the BF2-complex in the above occupied molecular orbitals
contrasts to LUMO topology, which has an electronic distribution with greater participation of the BF2 functionality and a
small participation on quinoline rings. In summary, N-butylated quinolines act more effectively as electron-donors than
the O-butylated ones, and their molecules have a D-A-D type system. Finally, compound 11c showed similar HOMO-1,
HOMO, and LUMO topology than for compound 11b, but with an attenuated ICT character of the HOMO-1→LUMO
transition due to the electron-withdrawing action of the chlorine atoms attached to the quinolinic rings, as evidenced in
the LUMO by the significant electronic participation on the quinolinic rings.
4.
Conclusions
In summary, we synthesized new series of bis-quinolin curcuminoid BF2-complexes 11b-f and and their decomplexed

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derivatives 12a-f, through a convergent synthetic methodology with good reaction yields, providing an alternative source
of chromophoric organic systems susceptible to structural characteristics. After measuring their absorption and
fluorescence spectra, the N-butylated derivatives showed a higher chromophore character and better push-pull
characteristics than the O-butylated analogues. In addition, we found that the methyl group attached to the quinolinic rings
of the 11b, 11e, 12a, and 12e structures, favored the charge transfer processes, whereas this process is disfavored in
compounds that have an electron-withdrawing group such as chlorine attached to the same rings (i.e. 11c, 11f, 12c, and
12f) when comparing with the unsubstituted derivatives (11b, 11d, 12a, and 12d). These results were confirmed via
computational calculations at DFT level that optimized the TD-DFT structures, thus allowing the simulation of electronic
transitions. For compounds (11b, 11e, 12a, and 12e) the HOMO and HOMO-1 are mainly located between the quinolinic
rings, while the LUMO is located on the BF2 functionality of the BF2-complexes (i.e. 11b, 11e), as well as, on the 1,2-
dicarbonyl system for the decomplexed derivatives (i.e. 12a, 12e).
Conflicts of Interest
None
Acknowledgement. The authors gratefully acknowledge COLCIENCIAS, CIBioFi, Universidad del Valle, and
Universidad del Norte for financial support.
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Credit Author Statement:
R.A.: Conceptualization, Supervision. D.I., L.C. and A.O.: Investigation, Methodology.
A.O.: Formal analysis. R.A., D.I., L.C., A.O., B.I. and J.Q.: Writing - Original draft
preparation, Visualization, Writing - Reviewing and Editing.

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Declaration of interests
☒ The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this
paper.
☒The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
None

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Highlights
 The synthesis and characterization of two series of new bis-quinolin curcuminoid BF2-complexes and
their decomplexed derivatives is reported.
 Synthesized compounds displayed interesting optical properties.
 BF2-complex and their decomplexed derivatives showed fluorescent properties both in solution and in
solid state.
 Studies revealed that the absorption and emission bands for both series of compounds are dependent of
their structures but also of the type of substituent.
 Experimental data were confirmed through computational calculations at DFT level via simulations of
the Uv-vis spectra and determining the topology of the border orbitals responsible for light absorption.