Azine-imidazole aza-BODIPY analogues with large Stokes shift

Article abstract of DOI:10.1016/j.dyepig.2016.11.009

A series of azine-imidazole aza-BODIPY analogues has been prepared by a simple synthesis from 2-azinecarboxylic acids and imidazole N-oxides. The new fluorescent complexes exhibit large Stokes shifts (up to 10?000?cm^?1), fluorescence in crystal

Full text of DOI:10.1016/j.dyepig.2016.11.009

Dyes and Pigments 137 (2017) 312e321
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Azine-imidazole aza-BODIPY analogues with large Stokes shift
,
*
Patrycja Bukowska a, Joanna Piechowska b, Rafał Loska a
a Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland1
b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2016
Received in revised form
24 October 2016
Accepted 8 November 2016
Available online 9 November 2016
A series of azine-imidazole aza-BODIPY analogues has been prepared by a simple synthesis from 2-
azinecarboxylic acids and imidazole N-oxides. The new fluorescent complexes exhibit large Stokes
shifts (up to 10 000 cmꢀ1), fluorescence in crystalline phase, high stability and fluorescence
solvatochromism.
© 2016 Elsevier Ltd. All rights reserved.
Keywords:
Boron dipyrromethene
Solid-state fluorescence
Stokes shift
N-oxides
Cycloaddition
1. Introduction
meso atom. Such complexes offer wide possibilities in terms of
structural variation and often exhibit large Stokes shifts, particu-
Boron dipyrromethenes (BODIPYs) and their aza analogues have
become one of the most important and useful classes of fluorescent
dyes [1e5]. Apart from their excellent photophysical and chemical
properties, important limitations of BODIPYs are (i) small Stokes
shift values causing self absorption and interference with fluores-
cence measurements, (ii) aggregation-induced fluorescence
quenching in the solid state. Therefore, a great research effort has
been directed towards development of unsymmetrical BODIPY
analogues, in which relaxation of molecular geometry in the
excited state allows to achieve large Stokes shift [6], and stacking of
larly those incorporating an azine unit [29,32,35e37]. Efficient solid
state fluorescence has been also reported for triazaborines [42] and
di- and triazaanthracene BF2 complexes, which can be considered
BODIPY analogues containing two azine rings [43e48]. In partic-
ular, Kubota, Matsui and co-workers described a series of unsym-
metrically substituted BF2 pyridomethenes, which exhibited
fluorescence in the solid state, even though the shift between ab-
sorption and fluorescence maxima was relatively small (up to
22 nm) [43]. The parent difluoroboron-triazaanthracene absorbs
and emits in the blue region of the visible light spectrum and ex-
hibits high lasing efficiency [44].
p-conjugated chromophores one above another is avoided or
diminished in condensed phase. A straightforward approach to this
problem is desymmetrisation of the standard BODIPY core by
appending donor and acceptor substituents [7e11] or condensation
of aromatic rings to one of the pyrrole rings [12,13]. In recent years a
rapidly developing area is construction of N-B-N and N-B-O com-
plexes in which the boron atom is chelated by a nitrogen hetero-
cycle and imido or imine nitrogen [14e21] or enolate/phenolate
oxygen [22e28], or between two different heterocyclic systems
other than pyrrole, with [29e37] or without [38e41] a bridging
Herein we report that novel unsymmetrical BF2 complexes 1
exhibiting very good optical properties, such us very high Stokes
shift values and fluorescence in solid state, can be assembled in a
straightforward manner from two types of very common hetero-
cyclic systems, an azine (pyridine, quinoline, etc.) and an imidazole
(Scheme 1). The chemistry of the derivatives of both types of het-
erocycles is very rich, which opens diverse possibilities of further
functionalization of chromophores 1.
Synthesis of complexes 1 requires preparation of unsymmetrical
bis(heteroaryl)amine chelating ligands with appropriately placed
imidazole and azine units. Bis(heteroaryl)amines are important as
ligands [49e54] and pharmaceuticals [55] and a few general
methods of their synthesis have been described: (i) nucleophilic
* Corresponding author.
E-mail address: rafal.loska@icho.edu.pl (R. Loska).
http://www.icho.edu.pl.
1
http://dx.doi.org/10.1016/j.dyepig.2016.11.009
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
313
Scheme 1. Synthesis of azine-imidazole boron difluoride complexes.
aromatic substitution of halogen or hydrogen in azines with 2-
2.2. Synthesis and characterisation
aminoheterocycles [56e58] or cine substitution in N-oxides [59],
(ii) Cu [60] or Pd-catalyzed [61e64] CeN bond formation, and (iii)
assembly from acyclic precursors [65].
2.2.1. General procedure for preparation of bis(heteroaryl)amines 2
2-Pyridinecarboxylic, 2-quinolinecarboxylic or 1-
isoquinolinecarboxylic acid (1.5 mmol) and NEt3 (1.5 mmol,
152 mg, 209 L) were added to dry NMP (2.1 mL) in a Schlenk flask
under Ar atmosphere. At
ꢁC, diphenyl phosphoryl azide
(1.6 mmol, 440 mg, 345 L) was added drop-wise and the reaction
Considering the importance of bis(heteroaryl)amines and as a
continuation of our studies on functionalisation of heteroaromatic
rings by 1,3-dipolar cycloaddition of N-oxides of azines and azoles
[66], we chose to investigate and develop cycloaddition between N-
oxides and heteroaryl isocyanates as an alternative approach to
constructing the requisite bis(heteroaryl)amine ligands 2 in a
flexible manner from readily available substrates and without the
use of transition metal catalysis.
m
0
m
mixture was stirred at 35 ꢁC for 1 h. N-Oxide (1 mmol) was then
added in one portion and the reaction mixture was stirred at 70 ꢁC
for 20 h. The mixture was then poured into water (50 mL) and
extracted with AcOEt (3 ꢂ 20 mL). Combined organic extracts were
washed with brine (5 ꢂ 30 mL) dried over anhyd. Na2SO4 and
evaporated. Products 2 were purified by column chromatography
on silica gel using hexaneseAcOEt 2:1 or tolueneeAcOEt 2:1, then
AcOEt as eluent.
2. Experimental
2.1. Materials and instruments
2.2.1.1. 1-Benzyl-4,5-dimethyl-2-(2-pyridylamino)imidazole
Obtained in 148 mg (53%) yield as yellow oil. IR (CH2Cl2):
(2a).
Analytical grade solvents were used as received. Hexanes used
for extraction and chromatography were distilled before use.
Commercially available dry NMP (Aldrich) was used for cycload-
dition reactions. Flash chromatography was performed using silica
gel 60 (0.040e0.063 mm). Analytical thin layer chromatography
(TLC) was performed using pre-coated silica gel plates (Merck,
0.20 mm thickness) and visualized under a UV lamp. NMR spectra
were recorded at 25 ꢁC in CDCl3 at the spectrometer frequency
indicated in the description of each compound, using Bruker 400
and Varian 500 spectrometers. Chemical shifts are given in ppm
relative to TMS for 1H and 13C NMR spectra and CFCl3 for 19F NMR
spectra. Mass spectra were obtained using electron impact (EI)
ionisation (Waters AutoSpec Premier) or electrospray (ESI) ionisa-
tion (Waters MaldiSYNAPT G2-S HDMS). IR spectra were obtained
using a Perkin-Elmer 1640 FT-IR spectrometer. Elemental analysis
was performed on a Perkin-Elmer 240 Elemental Analyzer (C, H, N)
nmax ¼ 3217, 3062, 3031, 2923, 2097, 1579, 1540, 1464, 1438, 1355,
1311, 1149, 775, 733, 697 cmꢀ1. 1H NMR (400 MHz, CDCl3):
d
¼ 2.00
(3H, s), 2.14 (3H, s), 5.03 (2H, s), 6.59 (1H, t, 3JHH ¼ 6.1 Hz), 6.95 (1H,
d, 3JHH ¼ 8.5 Hz), 7.07 (2H, d, 3JHH ¼ 7.1 Hz), 7.25 (3H, m), 7.40 (1H,
ddd, 3JHH ¼ 8.7 Hz, 7.0 Hz, 4JHH ¼ 1.8 Hz), 8.01 (1H, dd, 3JHH ¼ 5.1 Hz,
4JHH ¼ 1.1 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d
¼ 8.9, 12.0, 46.1,
112.5, 114.0, 119.1, 126.4, 127.4, 128.7, 137.2, 137.3, 137.6, 143.2, 146.0,
157.5 ppm. HRMS (ESI) calcd for C17H19N4 ([MþH]þ), 279.1610;
found, 279.1605.
2.2.1.2. 5-(4-Chlorophenyl)-1-n-propyl-2-(2-pyridylamino)imidazole
(2b). Obtained in 200 mg (64%) yield as yellow oil. IR (KBr):
nmax ¼ 3217, 3037, 2968, 1694, 1592, 1549, 1515, 1471, 1440, 1319,
1161, 1136, 1089, 994, 963, 839, 810, 770, 618, 522 cmꢀ1 1H NMR
.
3
(400 MHz, CDCl3):
d
¼ 0.75 (3H, t, JHH ¼ 7.4 Hz), 1.58 (2H, m,
€
or using Schoniger method (titration with Th(NO3)4; F).
3JHH ¼ 7.5 Hz), 3.91 (2H, t, 3JHH ¼ 7.5 Hz), 6.68 (1H, m), 6.90 (1H, s),
Heteroarylcarboxylic acids (2-pyridyl, 2-quinolinyl and 1-
isoquinolinyl) are commercially available and were used as
received. Preparation and characterization of imidazole N-oxides
has been reported by us recently [66]. 1-Benzyl-4,5-
dimethylimidazole 3-oxide was prepared according to a literature
procedure [67]. 4,5-Dimethyl-1-p-tolylimidazole 3-oxide was pre-
pared from 1,3,5-tris(p-tolyl)hexahydro-sym-triazine [68]. 5-(4-
3
7.25 (1H, m), 7.32 (2H, dm, JHH ¼ 8.5 Hz), 7.40 (2H, dm,
3JHH ¼ 8.5 Hz), 7.49 (1H, t, 3JHH ¼ 7.1 Hz), 8.02 (1H, m) ppm. 13C NMR
(100 MHz, CDCl3):
d
¼ 10.9, 23.3, 44.9, 112.6, 112.6, 114.5, 123.5,
129.0, 129.3, 129.7, 133.7, 137.9, 145.2, 145.2, 155.4 ppm. HRMS (ESI)
calcd for C17H18N4Cl ([MþH]þ), 313.1220; found, 313.1216.
Chlorophenyl)-1-n-propylimidazole
3-oxide
and
5-(4-
2.2.1.3. 5-(4-Methoxyphenyl)-1-n-propyl-2-(2-pyridylamino)imid-
azole (2c). Obtained in 107 mg (35%) yield as yellow oil. IR (CH2Cl2):
nmax ¼ 3211, 2963, 1599, 1578, 1465, 1438, 1249, 1177, 1030, 836,
methoxyphenyl)-1-n-propylimidazole 3-oxide were obtained
from the corresponding a-ketooximes [69] using cyclisation
method described in the literature [70].
772 cmꢀ1. 1H NMR (400 MHz, CDCl3):
d
¼ 0.72 (3H, t, 3JHH ¼ 7.4 Hz),

314
P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
1.56 (2H, m, 3JHH ¼ 7.5 Hz), 3.84 (3H, s), 3.89 (2H, t, 3JHH ¼ 7.5 Hz),
6.71 (1H, t, 3JHH ¼ 6.1 Hz), 6.84 (1H, s), 6.96 (2H, dm, 3JHH ¼ 8.7 Hz),
7.30 (3H, m), 7.52 (1H, m), 8.05 (1H, d, 3JHH ¼ 4.1 Hz) ppm. 13C NMR
820, 781 cmꢀ1. UVeVis (MeCN): lmax(log ε) ¼ 265 (4.02), 289 (4.02),
300 (4.01), 385 (4.11) nm. 1H NMR (400 MHz, CDCl3):
d
¼ 1.96 (3H,
s), 2.16 (3H, s), 2.35 (3H, s), 6.25 (1H, d, 3JHH ¼ 7.0 Hz), 7.03 (1H, d,
3JHH ¼ 7.0 Hz), 7.19 (5H, m), 7.25 (1H, d, 3JHH ¼ 7.7 Hz), 7.35 (1H, t,
3JHH ¼ 7.7 Hz), 8.14 (1H, d, 3JHH ¼ 8.0 Hz) ppm. 13C NMR (100 MHz,
(100 MHz, CDCl3):
d
¼ 10.8, 23.1, 44.7, 55.2, 108.6, 113.9, 114.1, 114.6,
122.2, 123.1, 130.0, 130.2, 137.7, 148.0, 155.7, 159.3 ppm. HRMS (ESI)
calcd for C18H21N4O ([MþH]þ), 309.1715; found, 309.1707.
CDCl3):
d
¼ 9.8, 12.8, 21.2, 105.8, 118.8, 125.6, 126.2, 126.5. 127.4,
127.6, 127.9, 128.2, 129.2, 130.7, 134.6, 135.8, 136.7, 149.8, 150.4 ppm.
HRMS (ESI) calcd for
329.1764.
2.2.1.4. 1-Benzyl-4,5-dimethyl-2-(2-quinolinylamino)imidazole (2d).
Obtained in 132 mg (40%) yield as orange crystals (isooctane-
CH2Cl2), mp 126e127 ꢁC. IR (KBr): nmax ¼ 3023, 2909, 1635, 1599,
C
21H21N4 ([MþH]þ), 329.1766; found,
1545, 1469, 1435, 1341, 1138, 821, 732 cmꢀ1
.
1H NMR (400 MHz,
2.2.1.9. 5-(4-Chlorophenyl)-1-n-propyl-2-(1-isoquinolinylamino)
CDCl3):
d
¼ 2.00 (3H, s), 2.21 (3H, s), 5.23 (2H, s), 6.72 (1H, d,
imidazole (2i). Obtained in 61 mg (17%) yield as dark yellow oil. IR
3
3JHH ¼ 9.4 Hz), 7.08 (1H, tm, JHH ¼ 8.2 Hz), 7.14 (2H, d,
(CH2Cl2): nmax ¼ 2960, 2928, 1625, 1497, 1092, 781 cmꢀ1. 1H NMR
3JHH ¼ 7.0 Hz), 7.20 (1H, m), 7.28 (3H, m), 7.40 (3H, m) ppm. 13C NMR
(400 MHz, CDCl3):
d
¼ 0.85 (3H, t, JHH ¼ 7.4 Hz), 1.74 (2H, m,
3
(100 MHz, CDCl3):
d
¼ 9.0, 12.6, 45.5, 116.3, 118.4, 121.2, 121.8, 124.5,
3JHH ¼ 7.5 Hz), 4.18 (2H, t, 3JHH ¼ 7.5 Hz), 6.44 (1H, d, 3JHH ¼ 7.0 Hz),
3
3
126.8, 127.0, 127.3, 127.4, 128.5, 129.6, 134.9, 138.5, 138.6, 150.3,
151.1 ppm. HRMS (ESI) calcd for C21H21N4 ([MþH]þ), 329.1766;
found, 329.1776. Elem. analysis calcd for C21H20N4: C, 76.80; H, 6.14;
N, 17.06. Found: C, 75.73; H, 5.90; N, 16.97.
6.95 (1H, s), 7.15 (1H, t, JHH ¼ 7.0 Hz), 7.35 (2H, d, JHH ¼ 8.6 Hz),
7.40 (2H, d, 3JHH ¼ 8.6 Hz), 7.46 (2H, m), 7.57 (1H, t, 3JHH ¼ 7.4 Hz),
8.65 (1H, d, 3JHH ¼ 7.9 Hz) ppm. 13C NMR (100 MHz, CDCl3):
¼ 11.3,
d
23.7, 44.6, 106.3, 122.7, 125.93, 125.94, 126.8, 127.1, 127.4, 128.2,
128.9, 129.5, 130.0, 131.1, 133.2, 136.0, 150.3, 152.9 ppm. HRMS (ESI)
calcd for C21H20N4Cl ([MþH]þ), 363.1376; found, 363.1371.
2.2.1.5. 4,5-Dimethyl-2-(2-quinolinylamino)-1-p-tolylimidazole (2e).
Obtained in 122 mg (37%) yield as orange crystals (isooctane-
CH2Cl2), mp 138e140 ꢁC. IR (KBr): nmax ¼ 3039, 2913, 2853, 1636,
1599, 1550, 1518, 1500, 1429, 1386, 1339, 1246, 1215, 1139, 1012, 953,
2.2.2. Preparation of BF2 complexes 1
Bis(heteroaryl)amine 2 (1.0 mmol) was dissolved in dry toluene
(7 mL) under Ar atmosphere and i-Pr2NEt (4.0 mmol 0.52 g,
0.70 mL) and BF3,OEt2 (6.0 mmol, 0.85 g, 0.74 mL) were added. The
reaction mixture was stirred vigorously for 72 h at rt. It was then
diluted with CH2Cl2 (5 mL) and washed with saturated aqueous
NaHCO3 (10 mL) and brine (10 mL). After drying over anhyd. Na2SO4
and evaporation, products 1 were purified by column chromatog-
raphy on silica gel with hexaneseAcOEt 2:1 or 5:1. After evapora-
tion they were crystallized from isooctaneeCH2Cl2 by slow
evaporation of solvent (tolueneeCH2Cl2 gives comparable results)
and washed with n-pentane.
821, 741, 592 cmꢀ1. 1H NMR (400 MHz, CDCl3):
d
¼ 2.01 (3H, s), 2.28
3
(3H, s), 2.42 (3H, s), 6.62 (1H, d, JHH ¼ 9.5 Hz), 7.09 (1H, t,
3JHH ¼ 7.4 Hz), 7.20 (2H, d, 3JHH ¼ 8.1 Hz), 7.30 (3H, m), 7.39 (3H, m)
ppm. 13C NMR (100 MHz, CDCl3):
d
¼ 9.7, 12.7, 21.2, 116.2, 119.2,
121.2, 121.9,124.9,127.3,127.9,129.4,129.4,129.6,134.3,134.6,137.1,
138.3, 150.6, 151.1 ppm. HRMS (ESI) calcd for C21H21N4 ([MþH]þ)
329.1766; found, 329.1772.
2.2.1.6. 5-(4-Chlorophenyl)-1-n-propyl-2-(2-quinolinylamino)imid-
azole (2f). Obtained in 140 mg (38%) yield as yellow (isooctane-
CH2Cl2), mp 144e145 ꢁC. IR (KBr): nmax ¼ 3338, 2956, 2867, 1876,
1638, 1598, 1542, 1469, 1429, 1339, 1274, 1151, 1090, 947, 825, 795,
753 cmꢀ1. UVeVis (MeCN): lmax(log ε) ¼ 198 (4.25), 306 (3.94), 402
2.2.2.1. Complex 1a. Obtained in 52% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 168e169 ꢁC. IR (KBr): nmax ¼ 3066, 3030, 2991,
2930, 1972, 1900, 1639, 1566, 1530, 1495, 1447, 1393, 1352, 1328,
1185, 1097, 1012, 955, 857, 766, 729, 711 cmꢀ1. UVeVis (MeCN):
lmax(log ε) ¼ 223 (3.63), 250 (3.65), 300 (4.06), 368 (3.96) nm. 1H
(3.80) nm. 1H NMR (400 MHz, CDCl3):
d
¼ 0.79 (3H, t, 3JHH ¼ 7.4 Hz),
3
3
1.63 (2H, m, JHH ¼ 7.5 Hz), 4.09 (2H, t, JHH ¼ 7.6 Hz), 6.80 (1H, d,
3JHH ¼ 9.4 Hz), 6.98 (1H, s), 7.12 (1H, t, 3JHH ¼ 7.4 Hz), 7.33 (3H, m),
7.42 (4H, m), 7.49 (1H, d, 3JHH ¼ 9.5 Hz), 14.26 (1H, s) ppm. 13C NMR
NMR (500 MHz, CDCl3):
d
¼ 1.98 (3H, s), 2.26, (3H, s), 5.16 (2H, s),
(100 MHz, CDCl3):
d
¼ 11.1, 23.5, 44.3,116.1,121.1, 122.2,122.9,124.5,
6.59 (1H, td, 3JHH ¼ 6.7 Hz, 4JHH ¼ 1.1 Hz), 6.91 (1H, d, 3JHH ¼ 8.8 Hz),
7.18 (2H, d, 3JHH ¼ 7.0 Hz), 7.25 (1H, m), 7.30 (2H, m), 7.41 (1H, ddd,
3JHH ¼ 8.8 Hz, 7.0 Hz, 4JHH ¼ 1.8 Hz), 7.91 (1H, d, 3JHH ¼ 6.0 Hz) ppm.
127.3, 128.2, 128.9, 129.4, 129.9, 129.9, 133.1, 135.4, 137.9, 151.2,
153.1 ppm. HRMS (ESI) calcd for C21H20N4Cl ([MþH]þ), 363.1376;
found, 363.1380. Elem. analysis calcd for C21H19N4Cl: C, 69.51; H,
5.28; N, 15.44; Cl, 9.77. Found: C, 69.63; H, 5.33; N, 15.29; Cl, 9.85.
13C NMR (100 MHz, CDCl3):
d
¼ 8.3, 9.2, 45.2, 111.3, 119.5, 119.6,
121.8, 126.9, 127.6, 128.7, 136.4, 136.6, 138.0, 147.3, 154.7 ppm. 19F
NMR (376 MHz, CDCl3):
d
¼ ꢀ134,5 (q, 1JFB ¼ 29.4 Hz) ppm. 11B NMR
2.2.1.7. 5-(4-Methoxyphenyl)-1-n-propyl-2-(2-quinolinylamino)
imidazole (2g). Obtained in 118 mg (32%) yield as orange crystals
(isooctane-CH2Cl2), mp 103e105 ꢁC. IR (KBr): nmax ¼ 2954, 2829,
1634, 1600, 1546, 1499, 1469, 1431, 1251, 1175, 1033, 826, 792, 750,
(160 MHz, CDCl3):
C
calcd for C17H17N4BF2: C, 62.60; H, 5.25; N, 17.18; F, 11.65. Found: C,
62.56; H, 5.29; N, 17.07; F, 11.59.
d
¼ 1.41 (t, 1JBF ¼ 29.1) ppm. HRMS (ESI) calcd for
17H18N4BF2 ([MþH]þ), 327.1593; found, 327.1594. Elem. analysis
607 cmꢀ1. 1H NMR (400 MHz, CDCl3):
d
¼ 0.78 (3H, t, 3JHH ¼ 7.4 Hz),
1.64 (2H, m, 3JHH ¼ 7.5 Hz), 3.84 (3H, s), 4.06 (2H, t, 3JHH ¼ 7.6 Hz),
2.2.2.2. Complex 1b. Obtained in 65% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 113e115 ꢁC. IR (KBr): nmax ¼ 3155, 2971, 2883,
3
6.81 (1H, d, JHH
¼
9.4 Hz), 6.92 (1H, m), 6.96 (2H, dm,
3JHH ¼ 8.7 Hz), 7.10 (1H, t, 3JHH ¼ 7.6 Hz), 7.32 (3H, m), 7.41 (2H, m),
1900, 1810, 1640, 1567, 1523, 1475, 1082, 979, 836, 773 cmꢀ1
.
7.47 (1H, d, 3JHH ¼ 9.5 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d
¼ 11.1,
UVeVis (MeCN): lmax(log ε) ¼ 195 (4.67), 294 (4.31), 361 (4.23) nm.
23.4, 44.2, 55.3, 114.1, 116.3, 121.2, 121.8, 122.0, 123.8, 124.4, 127.3,
129.3, 129.8, 129.8, 135.2, 138.2, 151.1, 152.3, 159.1 ppm. HRMS (ESI)
calcd for C22H23N4O ([MþH]þ), 359.1872; found, 359.1878. Elem.
analysis calcd for C22H22N4O: C, 73.72; H, 6.19; N, 15.63. Found: C,
73.78; H, 6.18; N, 15.36.
1H NMR (400 MHz, CDCl3):
d
¼ 0.81 (3H, t, 3JHH ¼ 7.4 Hz), 1.63 (2H,
3
3
m, JHH ¼ 6,1 Hz), 3.97 (2H, t, JHH ¼ 7.6 Hz), 6.68 (1H, td,
3JHH ¼ 6.7 Hz, 4JHH ¼ 1.2 Hz), 6.91 (1H, s), 7.00 (1H, d, 3JHH ¼ 8.9 Hz),
7.35 (2H, dm, 3JHH ¼ 8.7 Hz), 7.45 (2H, dm, 3JHH ¼ 8.7 Hz), 7.51 (1H,
ddd, 3JHH ¼ 8.8 Hz, 6.9 Hz, 4JHH ¼ 1.8 Hz), 7.94 (1H, d, 3JHH ¼ 6.4 Hz)
ppm. 13C NMR (100 MHz, CDCl3):
d
¼ 11.0, 22.7, 44.2, 111.5, 111.9,
2.2.1.8. 4,5-Dimethyl-2-(1-isoquinolinylamino)-1-p-tolylimidazole
(2h). Obtained in 138 mg (42%) yield as dark yellow oil. IR (KBr):
nmax ¼ 2915, 1638, 1599, 1516, 1492, 1437, 1400, 1250, 1228, 1135,
122.0, 127.3, 128.9, 19.3, 130.2, 135.1, 136.6, 138.6, 148.6, 155.1 ppm.
19F NMR (376 MHz, CDCl3):
d
¼ ꢀ137.8 (q, JFB ¼ 28.1 Hz) ppm.
1
HRMS (ESI) calcd for C17H17N4ClBF2 ([MþH]þ), 361.1203; found,

P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
315
361.1212. Elem. analysis calcd for C17H16N4ClBF2 C, 56.62; H, 4.47; N,
15.54; Cl, 9.83; F, 10.54. Found: C, 56.71; H, 4.57; N, 15.61; Cl, 9.74; F,
11.04.
calcd for C21H19N4ClBF2 ([MþH]þ), 411.1359; found, 411.1366. Elem.
analysis calcd for C21H18N4ClBF2: C, 61.42; H, 4.42; N, 13.64; F, 9.25
Cl, 8.63. Found: C, 61.46; H, 4.36; N, 13.61; F, 9.12; Cl, 8.40.
2.2.2.3. Complex 1c. Obtained in 45% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 95e96 ꢁC. IR (CH2Cl2): nmax ¼ 3465, 2964, 1641,
2.2.2.7. Complex 1g. Obtained in 45% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 155e156 ꢁC. IR (KBr): nmax ¼ 2965, 2837, 1640,
1574, 1552, 1498, 1442, 1410, 1299, 1247, 1177, 1107, 1073, 1025, 964,
831, 746 cmꢀ1. UVeVis (MeCN): lmax(log ε) ¼ 198 (4.73), 223 (4.42),
1571, 1528, 1495, 1293, 1251, 1168, 1110, 1023, 980, 837, 768 cmꢀ1
.
UVeVis (MeCN): lmax(log ε) ¼ 293 (4.30), 363 (4.19). 1H NMR
3
(400 MHz, CDCl3):
d
¼ 0.80 (3H, t, JHH ¼ 7.4 Hz), 1.62 (2H, m,
289 (4.45), 378 (4.31) nm. 1H NMR (400 MHz, CDCl3):
d
¼ 0.82 (3H,
3JHH ¼ 6.1 Hz), 3.85 (3H, s), 3.95 (2H, m), 6.64 (1H, td, 3JHH ¼ 6.6 Hz,
t, 3JHH ¼ 7.4 Hz), 1.65 (2H, m, 3JHH ¼ 7.5 Hz), 3.85 (3H, s), 3.99 (2H, t,
3JHH ¼ 7.7 Hz), 6.99 (4H, m), 7.29 (1H, m), 7.34 (2H, dm,
3JHH ¼ 6.7 Hz), 7.57 (2H, m), 7.72 (1H, d, 3JHH ¼ 9.1 Hz), 8.43 (1H, dm,
4JHH
¼
1.2 Hz), 6.85 (1H, s), 6.98 (3H, m), 7.32 (2H, dm,
3JHH ¼ 8.8 Hz), 7.48 (1H, ddd, 3JHH ¼ 6.9 Hz, 6.7 Hz, JHH ¼ 1.8 Hz),
4
7.93 (1H, d, 3JHH ¼ 6.2 Hz). 13C NMR (100 MHz, CDCl3):
d
¼ 10.9, 22.6,
3JHH ¼ 8.7 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d
¼ 11.0, 22.8, 44.1,
4
44.0, 55.3, 110.6, 111.6, 114.3, 120.9, 121.9, 129.9, 130.4, 136.5, 138.3,
55.3, 111.4, 114.4, 120.7, 121.0 (t, JCF ¼ 7.5 Hz), 123.4, 123.5, 123.7,
148.1, 155.0, 160.1 ppm. 19F NMR (376 MHz, CDCl3):
d
¼ ꢀ137.9 (q,
127.9, 130.3, 130.5, 130.5, 138.1, 139.4, 147.1, 155.4, 160.2 ppm. 19F
1JFB ¼ 28.0 Hz) ppm. HRMS (ESI) calcd for C18H20N4OBF2 ([MþH]þ),
357.1698; found, 357.1691. Elem. analysis calcd for C18H19N4OBF2 C,
60.70; H, 5.38; N,15.73; F, 10.67. Found: C, 60.71; H, 5.27; N,15.74; F,
10.77.
NMR (376 MHz, CDCl3):
d
¼ ꢀ133.8 (q, 1JFB ¼ 31.1 Hz) ppm. HRMS
(ESI) calcd for
C
22H21N4ONaBF2 ([MþNa]þ), 429.1674; found,
429.1677. Elem. analysis calcd for C22H21N4OBF2: C, 65.04; H, 5.21;
N, 13.79; F, 9.35. Found: C, 64.89; H, 5.10; N, 13.71; F, 9.29.
2.2.2.4. Complex 1d. Obtained in 93% yield, yellow crystals, mp
215e217 ꢁC. IR (KBr): nmax ¼ 3027, 2920, 1812, 1637, 1573, 1547,
1521, 1504, 1481, 1442, 1389, 1360, 1330, 1134, 1077, 1037, 972, 826,
741 cmꢀ1. UVeVis (MeCN): lmax(log ε) ¼ 191 (4.76), 225 (4.36), 290
2.2.2.8. Complex 1h. Obtained in 65% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 155e156 ꢁC. IR (KBr): nmax ¼ 3051, 2918, 1636,
1536, 1504, 1341, 1279, 1200, 1153, 1090, 1013, 799, 684 cmꢀ1
.
UVeVis (MeCN): lmax(log ε) ¼ 216 (4.66), 288 (4.14), 299 (4.16), 384
(4.35), 382 (4.25) nm. 1H NMR (400 MHz, CDCl3):
d
¼ 2.02 (3H, s),
(4.34). 1H NMR (400 MHz, CDCl3):
d
¼ 2.03 (3H, s), 2.39 (3H, s), 2.47
3
3
2.34 (3H, s), 5.20 (2H, s), 6.93 (1H, d, JHH ¼ 9.2 Hz), 7.21 (2H, d,
(3H, s), 6.84 (1H, d, JHH ¼ 7.2 Hz), 7.27 (2H, m), 7.36 (3H, m), 7.50
3JHH ¼ 6.8 Hz), 7.30 (4H, m), 7.53 (2H, m), 7.67 (1H, d, 3JHH ¼ 9.2 Hz),
(1H, m), 7.57 (1H, m), 7.69 (1H, d, JHH ¼ 6.7 Hz), 8.42 (1H, d,
3
3
8.43 (1H, dm, JHH ¼ 8.7 Hz) ppm. 13C NMR (100 MHz, CDCl3):
3JHH ¼ 8.2 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d
¼ 9.1, 9.4, 21.2,
d
¼ 8.4, 9.3, 45.4, 120.4, 120.5, 121.0 (4JCF ¼ 7.8 Hz), 123.4, 123.5,
110.5, 120.5, 120.7, 125.6, 125.6, 126.9, 127.3, 127.6, 129.7, 129.9,
123.7, 127.0, 127.8, 127.9, 128.8, 130.1, 136.4, 137.8, 139.5, 146.2,
131.8, 132.0, 136.0, 138.4, 147.0, 153.4 ppm. 19F NMR (376 MHz,
155.0 ppm. 19F NMR (376 MHz, CDCl3):
d
¼ ꢀ130.3 (q, 1JFB ¼ 32.6 Hz)
CDCl3):
C
d
¼ ꢀ133.9 (q, JFB ¼ 29.3 Hz) ppm. HRMS (ESI) calcd for
1
ppm. HRMS (ESI) calcd for C21H20N4BF2 ([MþH]þ), 377.1749; found,
377.1742. Elem. analysis calcd for C21H19N4BF2: C, 67.04; H, 5.09; N,
14.89; F, 10.10. Found: C, 66.96; H, 5.12; N, 14.86; F, 10.26.
21H20N4BF2 ([MþH]þ), 377.1749; found, 377.1744.
2.2.2.9. Complex 1i. Obtained in 90% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 169e172 ꢁC. IR (KBr): nmax ¼ 2967, 1638, 1537,
1504, 1464, 1355, 1279, 1155, 1106, 1071, 994, 797, 750, 686. UVeVis
(MeCN): lmax(log ε) ¼ 194 (4.67), 218 (4.60), 296 (4.29), 376 (4.36)
2.2.2.5. Complex 1e. Obtained in 77% yield, yellow crystals, mp
240e242 ꢁC. IR (KBr): nmax ¼ 2918, 1635, 1531, 1496, 1444, 1390,
1341, 1294, 1197, 1075, 828, 756 cmꢀ1. UVeVis (MeCN): nmax
(log ε) ¼ 193 (4.77), 291 (4.29), 384 (4.25) nm. 1H NMR (400 MHz,
nm. 1H NMR (400 MHz, CDCl3):
d
¼ 0.88 (3H, t, 3JHH ¼ 7.5 Hz), 1.75
(2H, m). 4.12 (2H, t, 3JHH ¼ 7.4 Hz), 6.93 (1H, d, 3JHH ¼ 7.2 Hz), 7.01
3
3
CDCl3):
d
¼ 2.00 (3H, s), 2.43 (6H, s), 6.82 (1H, d, 3JHH ¼ 9.2 Hz), 7.24
(1H, s), 7.37 (2H, dm, JHH ¼ 8.6 Hz), 7.46 (2H, dm, JHH ¼ 8.6 Hz),
7.56 (1H, tm, 3JHH ¼ 8.3 Hz), 7.60 (1H, d, 3JHH ¼ 7.8 Hz), 7.69 (1H, m),
7.72 (1H, dd, 3JHH ¼ 7.3 Hz, 4JHH ¼ 2.0 Hz), 8.79 (1H, d, 3JHH ¼ 8.3 Hz).
(3H, m), 7.32 (2H, d, 3JHH ¼ 8.1 Hz), 7.50 (1H, dm, 3JHH ¼ 7.8 Hz), 7.55
3
4
(1H, ddd, JHH ¼ 8.7 Hz, 5.7 Hz, JHH ¼ 1.4 Hz), 7.61 (1H, d,
3JHH ¼ 9.2 Hz), 8.44 (1H, dm, JHH ¼ 8.8 Hz) ppm. 13C NMR
13C NMR (100 MHz, CDCl3):
d
¼ 11.2, 22.9, 44.6, 111.0, 112.0, 125.5,
3
(100 MHz, CDCl3):
d
¼ 9.0, 9.4, 21.2, 120.6, 121.0 (4JCF ¼ 7.3 Hz), 121.1,
126.0, 127.1. 127.1, 127.2, 129.3, 129.3, 129.9, 130.2, 132.1, 135.3, 136.2,
123.4, 123.7, 123.7, 127.6, 127.9, 130.0, 130.1, 131.6, 137.7, 138.8, 139.4,
148.3, 153.8. 19F NMR (376 MHz, CDCl3):
d
¼ ꢀ137.1 (q,
146.4, 155.1 ppm. 19F NMR (470 MHz, CDCl3):
d
¼ ꢀ130.0 (q,
1JFB ¼ 28.3 Hz) ppm. HRMS (ESI) calcd for C21H18N4ClBF2Na
([MþNa]þ), 433.1179; found, 433.1179.
1JFB ¼ 32.5 Hz) ppm. 11B NMR (160 MHz, CDCl3):
d
¼ 2.40 (t,
1JBF ¼ 32.6 Hz) ppm. HRMS (ESI) calcd for C21H20N4BF2 ([MþH]þ),
377.1749; found, 377.1745. Elem. analysis calcd for C21H19N4BF2: C,
67.04; H, 5.09; N, 14.89; F, 10.10. Found: C, 66.88; H, 5.15; N, 14.85; F,
10.13.
2.3. Fluorescence measurements
Solvents which were used for spectroscopic measurements: n-
hexane (HEX), ethyl acetate (EA), acetonitrile (ACN) were of spec-
troscopic quality. Absorption spectra were recorded with Shimadzu
UV 3100 spectrophotometer, and the measurements of emission
spectra were carried out using Edinburgh Analytical Instruments
FS900 spectrofluorimeter. Luminescence spectra were subse-
quently corrected by subtraction of the background due to the
solvent (e.g., Raman lines, excitation line passed as second order of
monochromator grating). Both absorption and fluorescence spectra
were recorded as function of wavelength. In order to compare both
kind of spectra we divided the former by factor of l and the later by
l5 [71]. The samples were excited in the first absorption band.
Luminescence quantum yields were determined using quinine
sulfate in 0.1 N H2SO4 as a standard (FF ¼ 0.51) [72] and corrected
for the refractive index of the solvent [73]. For quantum yield
2.2.2.6. Complex 1f. Obtained in 74% yield, yellow crystals (isooc-
tane-CH2Cl2), mp 161e163 ꢁC. IR (KBr): nmax ¼ 3128, 2971, 1636,
1611,1573,1541,1520,1502,1477,1443,1408,1350,1298,1253,1165,
1109, 1075, 1012, 966, 825, 800, 745 cmꢀ1. UVeVis (MeCN):
lmax(log ε) ¼ 196 (4.79), 223 (4.50), 289 (4.49), 389 (4.36) nm. 1H
NMR (400 MHz, CDCl3):
d
¼ 0.82 (3H, t, 3JHH ¼ 7.5 Hz), 1.65 (2H, m,
3
3JHH ¼ 7.5 Hz), 4.00 (2H, t, JHH ¼ 7.6 Hz), 7.01 (2H, m), 7.30 (1H, t,
3JHH ¼ 7.5 Hz), 7.36 (2H, dm, JHH ¼ 8.5 Hz), 7.45 (2H, dm,
3
3JHH ¼ 8.5 Hz), 7.59 (2H, m), 7.54 (1H, d, 3JHH ¼ 9.2 Hz), 8.43 (1H, dm,
3JHH ¼ 8.8 Hz) ppm. 13C NMR (100 MHz, CDCl3):
¼ 11.0, 22.8, 44.3,
d
112.2, 121.1 (t, JCF ¼ 7.4 Hz), 123.3, 123.7, 123.7, 127.0, 128.0, 129.3,
129.5, 130.2, 130.4, 135.2, 138.4, 139.4, 147.6, 155.5 ppm. 19F NMR
(376 MHz, CDCl3):
d
¼ ꢀ133.5 (q, 1JFB ¼ 29.9 Hz) ppm. HRMS (ESI)

316
P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
determinations, optical density of both the sample and the stan-
dard were identical at the wavelength of excitation.
2a-i containing pyridine, quinoline and isoquinoline units con-
nected to C-2 position of imidazoles with different substitution
pattern were all obtained successfully in moderate yields.
Formation of boron complexes 1 was achieved in good yields by
stirring amines 2 with BF3,OEt2 in the presence of i-Pr2NEt in
toluene at room temperature. Dichloromethane which is often used
in similar transformation gave significantly inferior results. Chro-
matographic purification and crystallization from isooctane-DCM
or toluene-DCM by slow evaporation gave boron difluoride com-
plexes 1a-i as yellow-greenish crystals exhibiting strong fluores-
cence upon irradiation with 365 nm UV lamp. Complexes 1a-i are
highly soluble in common organic solvents and they are very stable
thermally, as the samples after melting point measurements (in
some cases above 200 ꢁC) show no decomposition as determined
by their 1H NMR spectra. 1a-i have been characterised by their 1H,
13C, 19F and in some cases 11B NMR spectra, as well as COSY, HSQC
and HMBC NMR experiments, IR and UVeVis spectra, HRMS mass
spectrometry and elemental analysis. 19F NMR spectra contain
2.4. Protonation experiments
Absorption and fluorescence spectra of protonated 1b have been
recorded. Solution I: 1b (2.4,10ꢀ5 M) in EtOH; Solution II: 1b
(2.4,10ꢀ5 M) and HClO4 (0.1 M) in EtOH; Solution III: Solution
II þ EtONa (1.1 equiv with respect to HClO4, as a 21% solution in
EtOH). Reference cuvettes: I: EtOH; II: EtOH
þ
HClO4; III:
EtOH þ HClO4 þ EtONa. Fluorescence spectrum measurement:
isosbestic point has been determined by titration of 1b sample with
HClO4 (335 nm), the sample was excited at 335 nm.
3. Results and discussion
3.1. Synthesis of BF2 complexes
1:1:1:1 quartets in the range ꢀ130.0
e
ꢀ137.9 ppm
In general, cycloaddition of N-oxides and isocyanates is known
to produce 2-aminoazines and 2-aminoazoles [74]. An isolated
example of in situ generation of moderately stable pyridyl iso-
cyanates from the corresponding acyl azides and subsequent
(1JBF ¼ 28.0e32.6 Hz) [16], and 11B spectra recorded for 1a and 1e
contain a triplet at 1.41 and 2.40 ppm, respectively.
cycloaddition with
a
pyridine N-oxide derivative has been
3.2. UVeVis absorption and emission spectroscopy
demonstrated by Fiksdahl and co-workers [75].
The synthesis of ligands 2 was based on cycloaddition of 2-
heteroaryl isocyanates, generated upon the action of diphenyl-
phosphoryl azide (DPPA) on 2-heteroarylcarboxylic acids and
subsequent Curtius rearrangement occurring at elevated temper-
ature, in the presence of the second partner for cycloaddition, an
imidazole N-oxide (Scheme 1). After some experimentation with
proportions of the reactants and the duration and temperature of
the two steps, the following optimal conditions were established:
DPPA (1.6 equiv.), NEt3 (1.5 equiv.) and carboxylic acid (1.5 equiv) in
NMP at 35 ꢁC for 1 h, then addition of N-oxide (1.0 equiv), 70 ꢁC for
20 h. The use of NMP as solvent was found to be critical as solvents
with similar properties such as DMF, DMAc or tetramethylurea gave
only traces of 2a under the same conditions. Bis(heteroaryl)amines
Photophysical properties of complexes 1a-i (Table 1) and ab-
sorption and fluorescence spectra obtained in solvents of different
polarity for three representative complexes bearing
a 4-
chlorophenyl substituent 1b, 1f, 1i are shown in Fig. 1 (for details
and plots for all complexes 1a-i, see SI). The more red-shifted of the
two absorption bands and the fluorescence band are mirror images
and in hexane they both exhibit a well resolved vibrational struc-
ture. For all complexes, the position of absorption bands is virtually
independent on polarity of the medium, while fluorescence bands
while fluorescence bands are red-shifted with increasing polarity.
This fluorescence solvatochromism effect is the largest for
pyridine-derived complexes 1aec, with 56e74 nm difference be-
tween hexane and MeCN.
Table 1
Photophysical properties of 1a-i in solution.
Solvent
Hexane
labs (nm)
log εmax
lem (nm)
Dn (cmꢀ1
)
FF
1a
1b
1c
1d
1e
1f
1g
1h
1i
1a
1b
1c
1d
1e
1f
1g
1h
1i
1a
1b
1c
1d
1e
1f
1g
1h
1i
302, 370
294, 363
298, 366
291, 402
292, 404
292, 394
290, 396
299, 386
297, 378
298, 367
296, 362
294, 364
291, 383
292, 385
291, 392
291, 379
299, 386
297, 378
297, 366
298, 362
293, 363
291, 382
292, 384
290, 390
293, 378
299, 384
297, 376
446
435
444
461
464
454
463
440
433
503
468
491
516
516
477
506
457
446
520
491
511
527
527
501
522
465
453
6107
5950
6269
4715
4713
4805
5132
3232
3414
8718
7667
8740
8157
8085
6028
6897
5457
5377
9813
8711
9379
8688
8491
6884
7813
6014
5735
0.19
0.46
0.42
0.22
0.27
0.56
0.41
0.53
0.56
0.03
0.14
0.07
0.03
0.03
0.14
0.07
0.48
0.58
0.01
0.06
0.03
0.01
0.01
0.04
0.02
0.40
0.57
AcOEt
MeCN
3.96
4.23
4.30
4.25
4.25
4.36
4.31
4.34
4.36

P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
317
character. A good linear correlation (Fig. 2) has been found between
the magnitude of the Stokes shift value and solvent polarity func-
tion in accordance with the Lippert-Mataga equation [76] (Eq. (1)).
.ꢀ
mGÞ2 hca3
ꢀ
ꢁ
vA ꢀ vF ꢃ 2
D
f ðmE
ꢀ
; Df
ꢁ.ꢀ
ꢁ
¼ ðε ꢀ 1Þ=ð2ε þ 1Þ ꢀ n2 ꢀ 1
2n2 þ 1
(1)
where nA and nF are wavenumbers of absorption and emission
maxima, mE and mG are dipole moments of excited and ground state,
h is Planck's constant, c is the speed of light, a is the cavity radius
and
Df is the orientation polarizability function defined by the
dielectric constant ε and the refractive index n.
For the series of 5-chlorophenylimidazole complexes 1b, 1f, and
1i the change of dipole moment upon excitation has been esti-
mated as 8.4 D, 9.4, D and 9.7 D, respectively, when 50% of the size
of conjugated chromophore was assumed as the effective cavity
radius, and 11.1 D, 11.7 D and 12.0 D, respectively, using 40% of the
long axis of the molecules [77,78]. Molecular geometries have been
estimated using quantum mechanical calculations at HF 6e31 level
performed in Gaussian 03, rev. B.05 program (for full reference, see
SI).
Complexes 1 exhibit fluorescence observable fluorescence in
crystalline phase upon irradiation with a 365 nm UV lamp. Absolute
photoluminescence quantum yields determined using integrating
sphere method [79] for polycrystalline samples of a representative
series of 5-chlorophenylimidazole complexes 1b, 1f, and 1i are
22.4%, 14.8% and 10.2% respectively. The fluorescence spectra
(Fig. 3) show a batochromic shift with the expansion of the
conjugated system from pyridine to isoquinoline and quinoline.
p
3.3. Theoretical calculations
Theoretical calculations performed for the three model struc-
tures Py, Q, IQ optimized at DFT B3LYP 6e31g(d) level and con-
taining Me groups instead of benzyl, aryl and N-propyl substituents
present in 1 support the notion of ICT character of the excited state
of complexes 1 (Fig. 4). In each case, the HOMO orbital is delo-
calized over the whole conjugated ring system, whereas the LUMO
is concentrated mainly in the azine part of the molecule, similar to
earlier examples of azine-imidazole BODIPY analogues reported in
the literature [34,40]. This shift of electron density upon excitation
is probably associated with reorganisation of molecular structure in
the excited state of 1 and their large Stokes shift values. The HOMO-
LUMO energy gaps in model structures Py, Q, IQ (3.97 eV, 3.70 eV,
3.84 eV) correspond to the trend observed in the more batochromic
absorption maxima of the UVeVis spectra in Fig. 1.
Fig. 1. The absorption and emission spectra of 1b, 1f and 1i in Hexane, AcOEt and
MeCN.
Expansion of the conjugated
p electron system by replacing
pyridine ring with quinoline (1d-g) or isoquinoline (1h, i) ring
system causes a batochromic shift of the more red-shifted ab-
sorption band. The influence on fluorescence bands is more com-
plex: fluorescence maxima of quinoline complexes 1f,g containing
an aryl substituent at C-5 position of the imidazole ring are slightly
red-shifted with respect to 1a-c, whereas for isoquinoline-derived
complexes 1h,i they are blue-shifted by 40e50 nm and significantly
narrower. Owing to the trends described above, the largest Stokes
shift values (above 9000 cmꢀ1) are observed for pyridine-based BF2
complexes in the most polar medium, MeCN (Table 1). Fluorescence
quantum yields FF are relatively high for all complexes in hexane,
ranging from 0.19 (1a) to 0.56 (1f, 1i). In more polar AcOEt and
MeCN FF of 1aeg drops considerably, though the values for chlor-
ophenyl derivatives remain higher than those for compounds with
different substitution pattern. Interestingly, FF of isoquinoline-
derived complexes 1h,i remains high in polar media (Table 1).
The Stokes shift values recorded for these two complexes in MeCN
are the lowest in the investigated series 1a-i, but still relatively high
(6014 cmꢀ1 for 1h, 5735 cmꢀ1 for 1i).
3.4. Crystal structure of complex 1i
Further insight into the structure of complexes 1 has been ob-
tained by X-ray diffraction analysis of 1i crystals (Fig. 5; CCDC
1475937). In the crystal structure, four molecules of 1i occupy one
cell unit (space group C1c1; see Fig. S8 in SI). The two NeB bonds
differ in length by 0.037 Å and the conjugated core of the molecule
including isoquinoline, imidazole and triazaborine rings is signifi-
cantly distorted from planarity: the boron atom and C(6) of the
isoquinoline ring lay 0.432 Å and 0.208 Å, respectively, above the
mean plane drawn through seventeen C, N and B atoms of the four
conjugated rings, while C(2) of isoquinoline and N(4) of imidazole
ring are located 0.249 Å and 0.274 Å, respectively, below that plane.
The deviation of B(1) atom from the mean plane is associated with
nonequivalence of the two F atoms, which form 74.7ꢁ and 45.5ꢁ
average torsion angles with C(1)eN(1) and C(11)eN(3) bonds, and
The batochromic shift of fluorescence maxima with increasing
medium polarity suggests that the dipole moment of the excited
state of complexes 1a-i is higher than that of the ground state, and
that the excitation has an intramolecular charge transfer (ICT)

318
P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
Fig. 2. Lippert-Mataga relationships between Stokes shift value (nA
e nF) and orientation polarizability function Df of hexane, AcOEt and MeCN.
with torsion angle of 16.6ꢁ between the mean planes of the azine
and azole rings. Similar distortion from planarity has been observed
recently in the crystal structure of a pyridine-phthalazine BF2
complex [48].
The F(1) atom closer to the plane of the molecule forms a close
contact (2.281 Å, CeH/F angle 160.0ꢁ) with the C(11)eH atom of
the imidazole ring of another molecule (Figs. S8 and SI). 4-
Chlorophenyl e imidazole torsion angle is 30.4ꢁ. The adjacent
molecules are not stacked one above another. Instead, 4-
chlorophenyl ring of one molecule is placed above the all-carbon
ring of isoquinoline unit of another molecule in a head-to-tail
arrangement, with the interplanar distance of ca. 3.75 Å. These
intermolecular interactions are probably responsible for the non-
planar arrangement of 1i chromophore in crystalline phase. Theo-
retical calculations mentioned above and performed in vacuum
predict the molecules of complexes 1 to be completely planar. Apart
from large Stokes shifts of complexes 1a-i, non-planar shape of
molecules together with little overlap between chromophore cores
and CeH/F interactions that rigidify the crystal structure may all
account for significant fluorescence of these complexes in crystal-
line phase.
3.5. Protonation experiments
Since the meso nitrogen atom in complexes 1 contains a free
electron pair, they can potentially serve as optical sensors for Hþ

P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
319
Fig. 3. Crystalline phase fluorescence spectra of complexes 1b, 1f, and 1i excited with
406 nm laser irradiation.
Fig. 5. Crystal structure of complex 1i (CCDC 1475937). Selected bond lengths (Å),
bond angles (ꢁ) and dihedral angles (ꢁ): B1eN1 1.549, B1eN3 1.512, N1eC9 1.365,
N3eC10 1.342, C9eN2 1.332, C10eN2 1.343, B1eF1 1.387, B1eF2 1.373, F1eB1eN3
111.33, F2eB1eN3 110.70, F1eB1eN1 110.17, F2eB1eN1 110.75, F1eB1eF2 108.34,
C9eN2eC10 116.20, N1eB1eN3 105.57, F1eB1eN3eC11 74.33, F2eB1eN3eC11 46.23,
C11eC12eC13eC14 29.69.
Fig. 6. Absorption and fluorescence spectra of 1b in EtOH (black curves), spectra of
1b,Hþ (green) and absorption spectrum after neutralisation of HClO4 (red). (For
interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
4. Conclusions
We prepared a set of simple boron difluoride chromophores,
each containing an imidazole and an azine unit bridged by a meso
nitrogen atom. These new BODIPY analogues are characterised by
high chemical stability, large Stokes shift, good fluorescence
quantum yields and fluorescence in crystalline phase. Within this
small set of relatively simple molecules we were able to observe a
significant dependence of florescence efficiency in solvents of
various polarity on the structure of chromophore (pyridine and
quinoline vs. isoquinoline systems). This property, together with
fluorescence solvatochromism and sensitivity of absorption prop-
erties to protonation (and perhaps coordination of metal ions) of
meso nitrogen make these complexes promising substrates for
construction of fluorescent probes and chemosensors. The com-
plexes are also distinguished by their simple, modular synthesis.
We have demonstrated usefulness of cycloaddition of heterocyclic
Fig. 4. HOMO (left) and LUMO (right) orbitals calculated for model structures Py (a), Q
(b), IQ (c) and meso-protonated Py (d).
and other Lewis-acidic species [4]. Indeed, protonation of meso
nitrogen influences optical properties, as demonstrated in Fig. 6 for
compound 1b. Protonation of 1b with an excess of strong acid
(HClO4) in EtOH solution caused a hipsochromic shift of both ab-
sorption maxima by almost 50 nm. The intensity of the fluores-
cence band was slightly diminished, but its position remained
unchanged. Similar behaviour has been reported for a triazaan-
thracene-BF2 complex [44]. The protonation-induced changes of
absorption properties are reversible, as neutralization of acid with
EtONa restored the original absorption spectra (Fig. 6, red curve).

320
P. Bukowska et al. / Dyes and Pigments 137 (2017) 312e321
dx.doi.org/10.1039/c5nj03217b.
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helpful discussions, and Professor Tomasz Martynski (Poznan
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