Dipyrrolylquinoxaline difluoroborates with intense red solid-state fluorescence

Article abstract of DOI:10.1039/c5dt02012c

A set of organic fluorescent dyes of dipyrrolylquinoxalines (PQs 4-6) and their BF₂ complexes (BPQs 1-3) were synthesized from commercial reagents, and were characterized by their X-ray structural analysis, and optical and electrochemical properties. BPQs 1-3 showed intense broad absorption in the visible region in the solution-state. In comparison with that of PQs 4-6, there is an over 110 nm red-shift of the absorption maximum in the BPQs 1-3 (up to 583 nm). Interestingly, dyes 1-6 all exhibit red solid-state fluorescence with moderate to high fluorescence quantum yields except for PQ 4 which showed bright yellow solid-state fluorescence. X-ray structures of BPQs 2-3 showed the planar structure of quinoxaline with one pyrrole unit via the BF₂ chelation and the almost perpendicular orientation of the uncoordinated pyrrole to the NBN core plane (the dihedral angle of 70-73°). The extended π-conjugation was in good agreement with the observed red-shift of the spectra. These dyes formed well-ordered intermolecular packing structures via the intermolecular hydrogen bonding between the N atoms of quinoxaline moieties and the NH units of adjacent pyrroles. The lack of π-π stacking in their crystal packing structures may explain the interestingly intense solid-state fluorescence of these dyes.

Full text of DOI:10.1039/c5dt02012c

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Dipyrrolylquinoxaline difluoroborates with intense
red solid-state fluorescence†
Cite this: DOI: 10.1039/c5dt02012c
Changjiang Yu, Erhong Hao,* Tingting Li, Jun Wang, Wanle Sheng, Yun Wei,
Xiaolong Mu and Lijuan Jiao*
A set of organic fluorescent dyes of dipyrrolylquinoxalines (PQs 4–6) and their BF2 complexes (BPQs 1–3)
were synthesized from commercial reagents, and were characterized by their X-ray structural analysis,
and optical and electrochemical properties. BPQs 1–3 showed intense broad absorption in the visible
region in the solution-state. In comparison with that of PQs 4–6, there is an over 110 nm red-shift of the
absorption maximum in the BPQs 1–3 (up to 583 nm). Interestingly, dyes 1–6 all exhibit red solid-state
fluorescence with moderate to high fluorescence quantum yields except for PQ 4 which showed bright
yellow solid-state fluorescence. X-ray structures of BPQs 2–3 showed the planar structure of quinoxaline
with one pyrrole unit via the BF2 chelation and the almost perpendicular orientation of the uncoordinated
pyrrole to the NBN core plane (the dihedral angle of 70–73°). The extended π-conjugation was in good
agreement with the observed red-shift of the spectra. These dyes formed well-ordered intermolecular
packing structures via the intermolecular hydrogen bonding between the N atoms of quinoxaline moieties
and the NH units of adjacent pyrroles. The lack of π–π stacking in their crystal packing structures may
explain the interestingly intense solid-state fluorescence of these dyes.
Received 28th May 2015,
Accepted 22nd June 2015
DOI: 10.1039/c5dt02012c
www.rsc.org/dalton
Introduction
Organic fluorescent dyes have found wide applications in bio-
medical and materials science, for example as organic light-
emitting diodes (OLED), solid-state organic lasers, bio-mole-
cular labels and molecular probes.1 Boron complexation has
been documented as an efficient strategy to enhance the rigid-
ity and the fluorescence intensity of the molecules, and many
organoboron complexes have been used as organic solid-state
fluorescent dyes in optoelectronics.1,2 However, some organo-
boron complexes suffer from the quenching of fluorescence at
high concentrations or in the solid state. For example, BODIPY
(A in Fig. 1) dyes,3 as among the most popular and intriguing
dyes, have been extensively studied in the last two decades due
to their outstanding chemical and photophysical properties,
such as their strong absorption in the visible and NIR region,
high fluorescence quantum yield and excellent photochemical
stability, while they show very small Stokes shifts and extre-
Fig. 1 Chemical structures of the BODIPY core (A), pyrrolylpyridine BF2
scaffold (B), the BF2 complex of hydrazine-linked bispyrrole (C), and the
dipyrrolylquinoxaline (PQ) and the two possible BF2 complexes of PQ
(BDPQ and BPQ).
mely low fluorescence in concentrated solutions or in the
solid-state. In addition, most of these reported solid emitting
dyes that reached the red region generally showed quite low
fluorescence quantum yields.
The Key Laboratory of Functional Molecular Solids, Ministry of Education; Anhui
Laboratory of Molecule-Based Materials; School of Chemistry and Materials Science,
Anhui Normal University, Wuhu, Anhui 241000, China.
E-mail: haoehong@mail.ahnu.edu.cn, jiao421@mail.ahnu.edu.cn
†Electronic supplementary information (ESI) available: Additional UV-vis, fluo-
rescence spectra, copies of NMR spectra and high resolution mass spectra for all
Lately some elegant research studies on the unsymmetrical
bidentate nitrogen ligands2,4,5 have already demonstrated the
essential roles of suitable ligands in the development of novel
(family of) fluorescent dyes with a set of desired properties
like the large Stokes-shift and the high solid-state fluorescence
new compounds, CCDC 1403580–1403583 for dyes 2, 3, 4 and 5. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt02012c
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addressed here. As part of our effort toward large Stokes-shift 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was used as the base,
and highly fluorescent solid materials, we recently have the BF2 complexation of PQ 4 smoothly proceeded in toluene
designed a hydrazine-linked bispyrrolic ligand C6 upon BF2 under refluxing conditions, from which a major product was
complexation, which shows an improved Stokes-shift up to obtained in 50% isolated yield (Scheme 2). Theoretically, there
around 70 nm over comparable BODIPY dyes and high solid- are two possible products for the five-membered-ring BF2 com-
state fluorescence. Note that a set of dipyrrolylquinoxaline plexation (Fig. 1): the mono-chelated BPQ and the dichelated
(PQ, Fig. 1) derivatives have been developed by Oddo and Behr BDPQ. The major product from this reaction was confirmed by
and have recently been applied as colorimetric anion sensors X-ray diffraction to be BPQ 1. No BDPQ was isolated from this
by Sessler and coworkers,7,8 while few efforts have been reaction with a variation of the reaction conditions, including
devoted to the investigation of their fluorescence properties.9 a further increase of the amount of boron trifluoride etherate
Besides, the BF2 complexes of pyrrolylpyridines10 (B, Fig. 1) (BF3·OEt2) and the usage of a stronger base. This may result
show a large Stokes-shift (up to 100 nm) and acceptable fluo- from the poor stability of BDPQ which readily undergoes
rescence quantum yield (ϕ = 0.22 in THF) with strong absorp- decomposition upon formation.11
max
abs
tion in the ultraviolet region (λ
= 325 and 405 nm). Taking
To test the versatility of this reaction and to facilitate
the structural advantage of PQ derivatives (possessing both further investigation of the substituent effect on the optical
the pyrrolic NH unit and the aromatic quinoxaline N atom), we and electronic properties of the resulting dyes, we further
rationalized that the BF2 complexation of suitable PQ deriva- extended this reaction condition to the BF2 complexation of
tives would render the desired red-shift of the absorption PQs 6–7, from which the desired BPQs 2 and 3 were smoothly
spectra with the extended conjugation via the annulation of a generated in 56% and 62% yields, respectively and were fully
benzene moiety onto the chromophore, while maintaining the characterized by HRMS, NMR and single crystal structural
large Stokes-shift due to the asymmetrical structure of the analysis.
complex. Herein, we report the efficient synthesis of a set of
As shown in Fig. 2a and summarized in Table 1, PQs 4–6
BF2 complexes of PQ (BPQs 1–3) with intense red solid-state each showed intense absorption centered in the range of
fluorescence, and the comparative investigation of their X-ray
structures, optical and electrochemical properties of these
dyes with their corresponding PQ ligands.
Results and discussion
Dipyrrolyldiketones 7–9 as the key synthetic precursors for
PQs 4–6 were prepared in 35%–45% yields using a literature
procedure7a,8a via the condensation of oxalyl chloride with a
stoichiometric amount of a commercial pyrrole, 2,4-dimethyl-
pyrrole or 3-ethyl-2,4-dimethylpyrrole, in the presence of dry
pyridine (Scheme 1). These resulting dipyrrolyldiketones were
applied for the subsequent condensation with an excess
amount of 1,2-phenylenediamine by following a modified lit-
erature procedure,7,8 from which the target PQs 4–6 were pre-
pared in 69%–80% yields.
Our initial attempt to form the BF2 complex of PQ 4 using
triethylamine as the base produced only a small amount of
Scheme 2 Syntheses and yields of BPQs 1–3.
product, from which most of the PQ 4 was recovered. When
Scheme 1 Syntheses and yields of PQ dyes 4–6.
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410–450 nm with high extinction coefficients (up to 4.18 × 104
M−1 cm−1) in dichloromethane. Their fluorescence emission
centered at around 490–550 nm in dichloromethane with the
fluorescence quantum yield between 0.13 and 0.20 (Fig. 2b).
Under daylight irradiation conditions, their dichloromethane
solutions show a pale yellow color, which changes to an
intense blue for PQ 4 and light green for PQs 5–6 under hand-
held UV lamp irradiation (365 nm) conditions (Fig. 2c).
BPQ 1 gave a broad absorption band centered at 492 nm
with two shoulders at around 465 and 520 nm in dichloro-
methane in the visible region and a comparably high extinc-
tion coefficient (4.27 × 104 M−1 cm−1) to most organoboron
complexes. In comparison with that of PQ 4, there is an
approximately 80 nm red-shift of the absorption in BPQ 1
(Fig. 2). Similar absorption patterns in the visible region were
observed for BPQs 2 and 3 containing electron-donating alkyl
substituents in dichloromethane with the absorption
maximum centered at 529 and 547 nm, respectively (Fig. 2).
BPQs 1–3 showed their fluorescence emission maximum
centered in the range of 490–550 nm in dichloromethane with
fluorescence quantum yields between 0.07 and 0.23. In com-
parison with that of BPQ 1, a significant red-shift of the fluo-
rescence emission was also observed in BPQs 2–3: BPQ 1
showed a fluorescence emission maximum centered at 552 nm
with a shoulder at 583 nm in dichloromethane, while the fluo-
rescence emission maximum of BPQ 3 was red-shifted to
632 nm with a shoulder extended to 675 nm. Similar red-shifts
of the fluorescence emission were also observed in other sol-
vents (Table 1).
Under daylight irradiation conditions, the dichloromethane
solutions are pale yellow for BPQ 1, pink for BPQ 2 and purple
for BPQ 3. The same colors were observed for BPQs 1–2 under
handheld UV lamp irradiation conditions, while a red solution
color was observed for BPQ 3 under these conditions (Fig. 2c).
The solvent-dependent optical properties for BPQs 1–3 were
investigated and are summarized in Table 1 and Fig. S1–S6 in
the ESI.† The fluorescence emission maximum of these dyes
varies with the variation of the solvent polarity from hexane to
acetonitrile. A gradual decrease of the fluorescence quantum
yields was observed for BPQs 1–3 with the increase of the
solvent polarity. For example, the fluorescence quantum yields
for BPQ 3 were reduced from 0.18 to 0.13, 0.07 and 0.04 with
the increasing solvent polarity from hexane to toluene, dichloro-
methane and acetonitrile. This may be attributed to the
strong intramolecular charge transfer (ICT) process12 from the
uncoordinated pyrroles to the BF2 complex of the pyrrolyl-
quinoxaline moiety in BPQs 1–3. In addition, a pH sensitive
absorption was also observed for BPQs 1–3. A red-shift of the
absorption maximum in the visible region was observed with
the addition of TFA to the dichloromethane solutions of these
dyes (Fig. S4–6 in the ESI†) resulting from the protonation of
lone pairs of the nitrogen atom in the quinoxaline unit.
Fig. 2 Overlaid and normalized absorption (a), fluorescence emission
(b) spectra and the colors (c) of the dichloromethane solutions of dyes
1–6 under daylight (top) and handheld UV lamp irradiation (365 nm)
(bottom) conditions.
As shown in Fig. 3 and Table 1, powders of BPQs 1–3
showed strong broad solid-state absorption in the UV-visible
region and intense solid-state fluorescence emission (centered
at 636, 654 and 655 nm, respectively) with the absolute
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Table 1 Photophysical properties of dyes 1–6 in various solvents studied and in the solid-state at room temperature
Stokes-shift
(nm)
a
Solvents
λambasx (nm) (log εmax
)
λmemax (nm)
ϕfb
1
2
3
Hexane
Toluene
Dichloromethane
Acetonitrile
Solid
Hexane
Toluene
Dichloromethane
Acetonitrile
Solid
Hexane
Toluene
Dichloromethane
Acetonitrile
Solid
Dichloromethane
Solid
Dichloromethane
Solid
Dichloromethane
Solid
304(4.33), 330(4.20), 470(3.73), 492(4.27), 528(4.22)
301(4.31), 334(4.16), 467(3.71), 494(4.23), 526(4.12)
304(4.31), 332(4.19), 465(3.77), 492(4.22), 520(4.09)
301(4.32), 326(4.19), 484(4.22)
300–650
311(4.78), 345(4.59), 495(3.99), 521(4.69), 558(4.62)
310(4.72), 346(4.51), 495(4.04), 527(4.66), 564(4.57)
311(4.74), 344(4.56), 496(4.17), 529(4.65), 562(4.14)
309(4.74), 339(4.52), 521(4.63)
300–670
314(4.60), 355(4.36), 507(3.84), 537(4.45), 577(4.37)
316(4.57), 355(4.30), 510(3.84), 544(4.44), 583(4.34)
316(4.58), 353(4.33), 511(3.92), 547(4.40), 582(3.94)
314(4.58), 347(4.26), 539(4.39)
300–670
300(5.12), 340(3.61), 412(4.18)
300–530
536, 555, 574
552, 581(sh)
552, 583(sh)
556
63
58
60
72
—
67
73
82
79
—
74
78
85
83
—
0.39
0.28
0.23
0.04
0.09
0.33
0.13
0.10
0.05
0.21
0.18
0.13
0.07
0 04
0.19
0.19
0.13
0.20
0.09
0.13
0.16
636
588, 634
600, 636(sh)
611, 648(sh)
600
664
611, 656(sh)
622, 666(sh)
632, 675(sh)
622
665
487
532
535
614
552
4
5
6
75
—
95
—
102
—
307(4.83), 325(4.12), 440(3.99)
300–670
309(4.89), 331(4.08), 450(4.12)
300–620
603
a Molar absorption coefficients are corresponding to the maximum absorption of the dye. b The absolute quantum yields (ϕf) are determined
using an Edinburgh Instrument FLS920 spectrofluorometer by using calibrating sphere systems, excited at 500 nm for 1, at 520 nm for 2 and 3,
and at 400 nm for 4–6. All ϕf values are corrected for changes in refractive indexes of different solvents. The standard errors are less than 5%.
fluorescence quantum yields of 0.09, 0.21 and 0.19, respecti- boron atoms of BPQs 2 and 3 both showed a tetrahedral geo-
vely. Notably, PQs 4–6 each showed an intense solid-state fluo- metry and the plane defined by F–B–F atoms is almost perpen-
rescence emission maximum centered at 532, 614 and 603 nm, dicular to that of the NBN core structure of these two dyes.
respectively with
a comparable solid-state fluorescence The B1–N1 bond distances of BPQs 2 and 3 (1.53 Å, 1.51 Å,
quantum yield (0.13, 0.09 and 0.16, respectively). In compari- respectively) are about 0.08 Å shorter than their B1–N2 bond
son with their solution-state, a red-shift was observed in the distances (1.61 Å, 1.59 Å), which indicates the asymmetrical
solid-state fluorescence emission maximum for these dyes structural features of these molecules.
(Table 1). Under daylight irradiation conditions, PQs 4–6 each
Multiple intramolecular and intermolecular C–H⋯F hydro-
showed a dark yellow, brown and orange color respectively, gen bonds between F atoms and various hydrogen atoms are
which changed into orange, dark red and brown colors accord- formed in BPQs 2–3 due to the strong electronegativity of the F
ingly under handheld UV lamp irradiation conditions (Fig. 3c). atom. The strong intermolecular hydrogen bonding also helps
By contrast, a deep red color was observed for the powders of the establishment of the crystal packing structures of these
BPQs 1–3 under daylight irradiation conditions, which dyes in the solid-state (Fig. 8 and S7 in the ESI†). Slipped
changed into an intense bright red color under handheld UV “head to tail” dimers are formed in the crystal packing struc-
lamp irradiation conditions. This may result from the aggrega- ture of 3 (Fig. 8). The plane of the two molecules is almost par-
tion-induced CT states13 which makes dyes 1–6 ideal bright allel (the angle between the mean planes of the C2N2 cores is
yellow and red solid emitting materials.
0.0°). The mean distance between the planes of two neighbor-
For a better understanding of the strong solid-state fluore- ing cores is 3.96 Å. Further packing of these dimers was
scence of these dyes, the crystal structures of these dyes and achieved via hydrogen bonding, and no π–π stacking between
their packing features were investigated and the crystal para- the adjacent dimers was observed in these dyes. This typical
meters are summarized in Table S1 in the ESI.† Crystals of J-aggregate type packing14 of BPQs 2 and 3 might explain their
BPQs 2 and 3 and PQs 4 and 5 suitable for X-ray analysis were good fluorescence quantum yields in the solid-state.
obtained via the slow diffusion of hexane into their dichloro-
PQs 4 and 5 both showed a slightly different inverted con-
methane solutions (Fig. 4 and 5). As shown in Fig. 4, both formation for the two pyrrole rings with NH of each pyrrole
BPQs 2 and 3 contain an almost planar NBN core structure pointing toward the quinoxaline nitrogen atoms (Fig. 5). For
formed between a pyrrole and the quinoxaline unit via BF2 PQ 5, the dihedral angles between the two pyrrole and quin-
chelation, in which a small dihedral angle was observed oxaline units are similar (35.6° and 40.7°, respectively). In
between these two moieties (10.1° for BPQ 2 and 6.3° for BPQ addition, the intramolecular N–N bond lengths between the N
3, respectively). The other uncoordinated pyrrole in these two atoms of quinoxaline and the two NH on the adjacent pyrroles
dyes stays almost perpendicular to this NBN core (dihedral are also similar (2.79 Å and 2.82 Å, respectively). By contrast,
angle of 66.1° for BPQ 2 and 68.7° for BPQ 3, respectively). The the two pyrrole rings on PQ 4 took a different spatial orien-
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Fig. 5 Top view (a, c) and front view (b, d) of X-ray structures of PQs 4
and 5. C, light gray; H, gray; N, blue.
Fig. 6 Crystal-packing patterns of PQ 4 that show the lack of π–π
stacking between the adjacent interlayered crystals; C, light gray; H,
gray; N, blue. Intermolecular N–N: 3.08 Å; intramolecular N–N: 2.68 Å
and 3.24 Å. Hydrogen atoms have been removed for clarity.
Fig. 3 Normalized absorption (a), fluorescence emission (b) spectra
and the powder fluorescence images under daylight (top) and handheld
UV (365 nm) lamp (bottom) irradiation conditions (c) of BPQs 1–6.
tation with one pyrrole nearly coplanar (dihedral angle of 5.6°)
and the other pyrrole almost orthogonal (dihedral angle of
86.1°) to the quinoxaline plane. Different intramolecular N–N
bond lengths (2.68 Å and 3.24 Å, respectively) were observed
between the N atoms of quinoxaline and the NH of the two
adjacent pyrroles. These conformational differences induce a
remarkable difference in the crystal packing structures of PQs
4 and 5 (Fig. 6 and 7). For PQ 5, both pyrroles participate in
the hydrogen bonding with their neighboring quinoxaline
molecules with a mean distance of 3.01 Å. These hydrogen
bondings lead to an extended two-dimensional array packing
structure for PQ 5. By contrast, only the orthogonal pyrrole in
PQ 4 participates in the hydrogen bonding with its neighbor-
ing quinoxaline molecule with a distance of 3.08 Å, while the
other pyrrole coplanar to the quinoxaline moiety only partici-
pates in an intramolecular hydrogen bonding with quinoxaline
(the distance of 2.68 Å). In both cases, no π–π stacking was
observed for these PQ dyes which may explain the interestingly
strong solid fluorescence of these dyes.
Fig. 4 Top view (a, c) and front view (b, d) of the X-ray structures of
BPQs 2 and 3. C, light gray; H, gray; N, blue; B, dark yellow; F, green.
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Fig. 7 Crystal-packing patterns of PQ 5 that show the lack of π–π
stacking between the adjacent interlayered crystals. C, light gray; H,
gray; N, blue. Intermolecular N–N: 2.98 Å, 3.01 Å, 3.05 Å; intramolecular
N–N: 2.79 Å and 2.82 Å. Hydrogen atoms have been removed for clarity.
Fig. 9 Cyclic voltammograms of 1 mM BPQs 1–3 measured in dichloro-
methane solution, containing 0.1 M TBAPF6 as the supporting electrolyte
at room temperature, a glassy carbon electrode as a working electrode,
and the scan rate at 50 mV s−1
.
Table 2 Electrochemical data acquired at 50 mV s−1 and HOMO–
LUMO gaps determined using spectroscopy for BPQs 1–3 a
Dyes
E2°B/B (V)
E1°B/B− (V)
LUMO (eV)
HOMO (eV)
Eg (eV)
1
2
3
−0.97
−1.10
−1.15
−3.49
−3.38
−3.32
−5.49
−5.41
−5.29
2.00
2.03
1.97
−1.59
−1.60
onset
a E°B/B = reversible reduction potential; E
= the onset reduction
Fig. 8 Crystal-packing patterns of BPQ 3 that show the lack of π–π
stacking between the adjacent interlayered crystals; C, light gray; N,
blue; B, dark yellow; F, green. Hydrogen atoms have been removed for
clarity.
red
onset
potentials; ELUMO = −e(E
+ 4.4); Eg = bandgap, obtained from the
intercept of the absorption spectra; EHOMO = ELUMO − Eg.
red
Conclusions
Finally, the electronic states (HOMO/LUMO levels) of BPQs
1–3 were investigated by cyclic voltammetry, performed in deoxy-
genated dichloromethane at room temperature with tetrabutyl-
ammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte. As shown in Fig. 9 and summarized in Table 2,
BPQs 2 and 3 both display an irreversible oxidation wave (Epa at
1.11 V and 1.00 V, respectively) and two reversible reduction
waves (half-wave potentials at −1.10 V and −1.59 V for BPQ 2
and −1.15 V and −1.60 V for BPQ 3). Only one reversible
reduction wave was observed for BPQ 1 with Epc at −1.02 V and
a half-wave potential at −0.97 V. Based on their onset potential
of the first oxidation and reduction waves, the HOMO–LUMO
energy levels were estimated for BPQs 1–3 (HOMO energy levels
of −5.49, −5.41 and −5.29 eV and LUMO energy levels of −3.49,
−3.38 and −3.32 eV, respectively). Thus, the installation of alkyl
groups on the pyrrolic position of these dyes indeed helps the
decrease of the LUMO and the increase of the HOMO energy
levels of the chromophore, and leads to the decrease of the
energy band gaps. Electrochemical energy band gaps for BPQs
1–3 were calculated to be 2.00, 2.03 and 1.97 eV respectively,
which are in good correlation with their optical band gaps.
In summary, a set of PQ ligands and their BF2 complexes
(BPQs) have been efficiently synthesized from a commercial
reagent and showed intense yellow to deep red solid-state fluo-
rescence. Their interesting solid-state fluorescence was inter-
preted by their X-ray packing structures. BPQs 2–3 both
formed well ordered packing structures (slipped dimers) due
to the multiple intermolecular C–H⋯F hydrogen bonding. No
intermolecular π–π stacking interaction was observed in their
crystal packing structures. Electrochemical energy band gaps
calculated for BPQs 1–3 are in good correlation with the
optical band gaps of these dyes.
Experimental section
General
Reagents and solvents were used and received from commer-
cial suppliers unless noted otherwise. Anhydrous toluene was
obtained by distillation of commercial analytical grade toluene
over sodium. All reactions were performed in oven-dried or
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flame-dried glassware, and were monitored by TLC using removed under vacuum. The crude product was purified by
0.25 mm silica gel plates with an UV indicator (60F-254). 1H silica gel column chromatography (hexane/dichloromethane =
and 13C NMR spectra were recorded on a 300 MHz or 500 MHz 2 : 1, v/v).
NMR spectrometer at room temperature. Chemical shifts (δ)
BPQ 1 was obtained as a red powder in 50% yield (77 mg)
are given in ppm relative to CDCl3 (7.26 ppm for 1H and from dipyrrolylquinoxalines 4 (130 mg, 0.5 mmol). 1H NMR
77 ppm for 13C) or to internal TMS. High-resolution mass (300 MHz, CDCl3) δ 9.63 (s, 1H), 8.12 (d, J = 8.1 Hz, 1H), 7.87
spectra (HRMS) were obtained using APCI-TOF in positive (d, J = 6.9 Hz, 1H), 7.68–7.55 (m, 2H), 7.38–7.25 (m, 3H), 7.10
mode. Electrochemical studies by cyclic voltammetry were per- (s, 1H), 6.40 (s, 2H). 13C NMR (75 MHz, CDCl3) δ 143.0, 140.3,
formed with a conventional 3-electrode system using a 100 W 139.9, 131.6, 130.9, 130.1, 129.5, 129.2, 128.7, 127.3, 123.2,
electrochemical analyzer in deoxygenated and anhydrous 120.1, 118.0, 116.0, 113.9, 110.8. HRMS (APCI) Calcd for
dichloromethane at room temperature.
working electrode, a saturated calomel electrode (SCE) refer-
A
glassy carbon C16H12BF2N4 [M + H]+ 309.1113, found 309.1117.
BPQ 2 was obtained as a red solid in 56% yield (102 mg)
ence electrode, a platinum auxiliary electrode, and the sample from dipyrrolylquinoxalines 5 (158 mg, 0.5 mmol). 1H NMR
solutions containing 1 mM sample and 0.1 M tetrabutyl- (300 MHz, CDCl3) δ 8.47 (s, 1H), 8.01 (d, J = 7.8 Hz, 1H), 7.89
ammonium hexafluorophosphate as a supporting electrolyte (d, J = 8.1 Hz, 1H), 7.64 (t, J = 8.1 Hz, 1H), 7.52 (t, J = 7.8 Hz,
were used. Argon was bubbled for 10 min before each 1H), 5.89 (s, 1H), 5.85 (s, 1H), 2.40 (s, 3H), 2.29 (s, 3H), 2.16
measurement.
(s, 3H), 1.60 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 145.4, 145.2,
141.8, 140.4, 132.7, 132.0, 131.6, 130.8, 129.9, 129.4, 128.2,
127.4, 124.3, 119.9, 119.4, 111.5, 13.5, 13.0, 12.9, 12.5. HRMS
Fluorometric analysis
UV-visible absorption and fluorescence emission spectra were (APCI) Calcd for C20H20BF2N4 [M + H]+ 365.1735, found
recorded on commercial spectrophotometers (Shimadzu 365.1739.
UV-2450 and Edinburgh FLS920 spectrometers). All measure-
BPQ 3 was obtained as a red solid in 62% yield (130 mg)
ments were made at 25 °C, using 5 × 10 mm cuvettes. The from dipyrrolylquinoxalines 6 (186 mg, 0.5 mmol). 1H NMR
absolute quantum yields of the samples in solutions and solid (300 MHz, CDCl3) δ 8.34 (s, 1H), 7.86 (d, J = 8.1 Hz, 1H), 7.86
states were measured using an Edinburgh Instrument FLS920 (d, J = 8.1 Hz, 1H), 7.60 (t, J = 7.5 Hz, 1H), 7.48 (t, J = 7.5 Hz,
spectrofluorometer by using calibrating sphere systems,6a 1H), 2.43 (q, J = 7.5 Hz, 1H), 2.37 (s, 3H), 2.31 (q, J = 7.5 Hz,
excited at 500 nm for 1, at 520 nm for 2 and 3, and at 400 nm 1H), 2.24 (s, 3H), 2.12 (s, 3H), 1.50 (s, 3H), 1.08 (t, J = 7.5 Hz,
for 4–6.
1H), 1.02 (t, J = 7.5 Hz, 1H). 13C NMR (125 MHz, CDCl3)
δ 145.5, 143.3, 141.9, 140.2, 131.9, 131.6, 130.9, 130.1, 129.3,
127.8, 127.8, 127.0, 124.2, 123.6, 122.5, 119.7, 17.9, 15.7, 15.2,
X-ray crystallography for dyes 2, 3, 4 and 5
Crystals of dyes 2–5 suitable for X-ray analysis were obtained 12.0, 11.7, 10.8, 10.6. HRMS (APCI) Calcd for C24H28BF2N4
by slow diffusion of hexane into their dichloromethane solu- [M + H]+ 421.2370, found 421.2371.
tions. CCDC 1403580 (2), 1403581 (3), 1403582 (4) and
General procedure for PQs 4–6
1403583 (5) contain the supplementary crystallographic data
for this paper. Data were collected using a diffractometer To dipyrrolyldiketone (1 mmol) in toluene (40 mL) were added
equipped with a graphite crystal monochromator situated in 1,2-phenylenediamine (216 mg, 2 mmol) and a catalytic
the incident beam for data collection at room temperature. amount of HOAc. The mixture was refluxed for 10 h, cooled
Cell parameters were retrieved using SMART15 software and down to room temperature, poured into water and extracted
refined using SAINT16 on all observed reflections. The determi- with ethyl acetate. Organic layers were combined, dried over
nation of unit cell parameters and data collections were per- anhydrous sodium sulfate and removed under vacuum. The
formed with Mo Kα radiation (λ) at 0.71073 Å. Data reduction crude product was purified by silica gel column chromato-
was performed using the SAINT software, which corrects graphy (hexane/dichloromethane = 1 : 1, v/v) to give the desired
for Lp and decay. The structure was solved by direct products.
methods using the SHELXS-97 program and refined by the
PQ 4 8a was synthesized as a green powder in 80%
least squares method on F2, SHELXL-97,17 incorporated in yield (208 mg) from dipyrrolyldiketone 7 (188 mg, 1 mmol).
SHELXTL V5.10.18
1H NMR (300 MHz, CDCl3) δ 9.74 (s, 2H), 7.84 (s, 2H),
7.54–7.53 (m, 2H), 6.95–6.91 (m, 4H), 6.26 (s, 2H). 13C NMR
(75 MHz, CDCl3) δ 143.7, 139.7, 129.1, 128.9, 128.0, 121.2,
General procedure for the synthesis of BPQs 1–3
To dipyrrolylquinoxalines (0.5 mmol) in toluene (30 mL) was 112.9, 110.0.
added 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (1 mL,
PQ 5 was obtained as a brown powder in 74% yield
6.5 mmol). The mixture was refluxed for 10 min, and distilled (233 mg) from dipyrrolyldiketone 8 (244 mg, 1 mmol).
boron trifluoride etherate (1.2 mL, 9.5 mmol) was added 1H NMR (300 MHz, CDCl3) δ 8.76 (s, 2H), 7.93 (q, J = 3.3 Hz,
into the reaction mixture. The reaction mixture was refluxed 2H), 7.57 (q, J = 3.3 Hz, 2H), 5.80–5.79 (m, 2H), 2.28 (s, 6H),
for 4 h, cooled down to room temperature, poured into 1.89(s, 6H). 13C NMR (75 MHz, CDCl3) δ 145.4, 139.7, 131.5,
water and extracted with ethyl acetate. Organic layers 129.1, 128.1, 125.2, 124.2, 111.6, 13.5, 12.7. HRMS (APCI)
were combined, dried over anhydrous sodium sulfate and Calcd for C20H21N4 [M + H]+ 317.1761, found 317.1760.
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PQ 6 was obtained as an orange powder in 69% yield
2014, 43, 4778; (f) Y. Ni and J. Wu, Org. Biomol. Chem.,
2014, 12, 3774.
(256 mg) from dipyrrolyldiketone 9 (300 mg, 1 mmol).
1H NMR (300 MHz, CDCl3) δ 8.52 (s, 2H), 7.88 (q, J = 3.3 Hz,
2H), 7.3 (q, J = 3.3 Hz, 2H), 2.37 (q, J = 7.5 Hz, 4H), 2.22
(s, 6H), 1.86 (s, 6H), 1.04 (t, J = 7.5 Hz, 6H). 13C NMR (75 MHz,
CDCl3) δ 145.8, 139.8, 128.4, 128.1, 126.6, 124.1, 123.3, 121.3,
17.5, 15.6, 11.3, 10.1. HRMS (APCI) Calcd for C24H29N4
[M + H]+ 373.2383, found 373.2387.
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Acknowledgements
This work was supported by the National Nature Science
Foundation of China (Grant No. 21372011, 21402001 and
21472002), the Nature Science Foundation of Anhui Province
(Grant No. 1508085J07) and the Special and Excellent Research
Fund of Anhui Normal University.
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