Structural diversity of new solid-state luminophores based on quinoxaline-β-ketoiminate boron difluoride complexes with remarkable fluorescence switching properties

Article abstract of DOI:10.1039/c4cc08958h

A series of structurally simple yet highly tunable organoboron luminophores was designed and synthesized. The solid-state fluorescence quantum yields exhibit nearly exponential growth by decorating the luminophore with additional sterically demanding substituents. Uniquely, the luminescence of these organoboron dyes can be easily switched on/off by acidic/basic vapors, yielding a solid-state fluorescence switching function. This journal is

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Structural diversity of new solid-state luminophores based on
quinoxaline-β-ketoiminate boron difluoride complexes with remarkable
fluorescence switching properties
Chia-Wei Liao, Rajeswara Rao M, and Shih-Sheng Sun*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
50 complexes with high solid-state emissions along with large
Stokes shifts aiming to gain meaningful insights and establish in-
depth structure-property relationship, which would pave the ways
for practical applications. In combination of the unique
A series of structurally simple yet highly tunable organoboron
luminophores was designed and synthesized. The solid-state
fluorescence quantum yields exhibit nearly exponential
aggregation-induced
emission
enhancement
(AIEE)
10 growth by decorating the luminophore with additional steric
demanding substituents. Uniquely, the luminescence of these
organoboron dyes can be easily switched on/off by
55 phenomenon14 and the intriguing properties associated with
unsymmetry boron complexes possessing enhanced solid-state
emission and large Stokes shifts,15 we report here a series of
quinoxaline-β-ketoiminate derivatives and their corresponding
boron complexes (Scheme 1) which show remarkable
60 luminescent properties in both solid state as well as in solution.
As envisaged, phenyl substituted derivatives exhibited strong
solid-state luminescence due to the structural twist exerted by the
additional phenyl substitution.5n Moreover, the luminescence of
these organoboron compounds can be easily switched on/off by
65 acidic/basic vapors, yielding a solid-state acid/base fluorescence
indicator.16
acidic/basic vapors, yielding
switching function.
a
solid-state fluorescence
15 Organic solid-state luminescent materials have received
increasing attentions in recent years because of their potential
applications in optoelectronics,1, 2 optically pumped lasers,3 and
fluorescent sensors.4 Of special note, external stimuli-responsive
tuning of solid-state fluorescence based on the phase
20 transformation of single substrate, which render them the
possibility of versatile applications in the field of optical
recording material and security inks.5 Known to possess
intriguing optical properties such as high absorption coefficients
and fluorescence quantum yields, organoboron complexes are one
25 of the most crucial fluorescent dyes which show tunable
absorption/emission profiles as well as excellent chemical and
photochemical stability.6 In the perspective of these attractive
features, considerable efforts have been devoted to the
development of various fluorescent organoboron complexes with
30
a
variety of applications spanning from bio-labeling7 to
photovoltaics,8 laser dyes,9 and molecular probes.10
However, the common boron dipyrromethene (BODIPY)
derivatives normally suffer from drawbacks such as feeble solid-
state emission attributed to the notorious π−π interactions leading
35 to formation of excimers and exciplexes11 and small Stokes shifts
(400-600 cm-1 in most cases) which results in self-absorption of
its own fluorescence. Moreover, the interference by excitation
and scattering light renders the fluorescence signal difficult to
separate from the background noise in a bioassay.12 While a few
40 boron complexes were reported towards addressing these issues,13
it is still highly desirable to develop next-generation organoboron
Scheme 1. Synthetic routes of quinoxaline-β-ketoimnate ligands 1a-5a and their
70 corresponding boron difluoride complexes 1-5. Inset depicts the overlapped crystal
structures of 1 (pink), 2 (orange), and 4 (green).
Institute of Chemistry, Academia Sinica, Taipei, Taiwan, Republic of
China. Fax: 886 2 27831237; Tel: 886 2 27898596;
45 E-mail: sssun@chem.sinica.edu.tw
† Electronic Supplementary Information (ESI) available: experimental
details, characterization data, UV-vis, fluorescence spectra, details of
crystal analysis and photographs. See DOI:10.1039/b000000x/
All new quinoxaline-β-ketoimnate ligands and their
75 corresponding boron complexes were fully characterized by
various spectroscopic techniques. Details of the experimental
procedures and data for structural characterization are described
in the Supplementary Information. The key precursors,
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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Page 2 of 4
quinoxaline-β-ketoiminate derivatives 1a-5a, were obtained by
shifted band in the range of 500-550 nm attributed to an
the reaction of quinoxaline derivatives 6-8 with the corresponding
intramolecular charge transfer (ICT) transition between the
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aryl ester in the presence of sodium hydride. Noticeably, these
triphenylamine donor group and the acceptor BF2 moiety (Fig.
1
ligands showed a distinctively downfield (~13 ppm) shifted H
1a). Akin to the absorption spectra, the emisDsiOoIn: 1m0.a10x3im9/aC4oCf C30a8n9d58H
5
NMR signal for the H-bonded enol proton. Unlike previously 70 5 were also shifted toward longer wavelength than those of 1, 2,
reported pyrazine-based β-ketoiminates,13b ligands 1a-5a did not
exhibit keto-enol tautomerism in the 1H NMR except for
triphenylamine substituted derivatives. Treatment of the resulting
quinoxaline-β-ketoiminate ligands 1a-5a with boron trifluoride
and 4 (Fig. 1b) due to the presence of low-lying ICT transition,
which also resulted in lower fluorescence quantum yields for
complexes 3 and 5 than others. In sharp contrast to the minor
solvent dependent spectral features of 1, 2, and 4, the emission
10 diethyl etherate in the presence of triethylamine in 75 spectra of the donor-acceptor molecules, 3 and 5, exhibited
dichloromethane gave the desired boron complexes 1-5 in high
yields (see SI). In the H NMR spectroscopy, the disappearance
prominent solvatochromic effect with a monotonic bathochromic
shift upon increasing the solvent polarity indicative of large
dipole moment in the excited state (Table S2).
1
of the intramolecular H-bonded enol proton signal at ~13 ppm
readily supports the BF2 complexation which is further confirmed
15 through mass spectra. The 11B spectra appear well-defined triplets
due to the coupling with the two 19F nuclei and the chemical
shifts are in the range of 2 to 2.5 ppm. The resonances for the
fluorine atoms appear as a 1:1:1:1 quartet at ~ –126 ppm in the
19F NMR spectra due to the coupling with boron (see SI).
20
The redox property of complexes 1-5 was investigated by
cyclic voltammetry (CV) in dichloromethane (Table S1). All
boron complexes exhibited a single reversible reduction between
–1.07 and –1.20 V (vs. Fc/Fc+), which is comparable to the
reduction of the structurally similar BODIPY core.17 The
80
Fig. 1 Absorption (a) and emission (b) spectra of 1-5 recorded in CHCl3 (5 × 10-6
M).
25 reduction is consistently and moderately correlated to the aryl
substituents (R’) depending on their electron
85 Table 1 Photophysical data of complexes 1-5 recorded in CHCl3 solution (5 × 10-6
M) and solid state
donating/withdrawing ability. One irreversible oxidation at ~1.5
V (vs. Fc/Fc+) for all boron complexes was observed except for
complexes 3 and 5 which showed an additional reversible
30 oxidation at ~0.6 V (vs. Fc/Fc+) ascribed to the oxidation of TPA
(Fig. S1). The suitable LUMO and HOMO energies for all
compounds estimated by CV may find potential applications in
organic photovoltaics.9
All new boron complexes were structurally characterized by
35 X-ray crystallography. The overlapped crystal structures of 1, 2
and 4 are depicted in Scheme 1. The bond distances of B1-O1 are
around 0.13 Å longer than those of B1-N1 in crystal structures
presumably in reflection of the electronegativity of the oxygen
atom and the unsymmetrical nature at the boron center. The
Dyes
λabs, nm
(log ε)
λem
(nm)
Δ
τ
(ns)
λem
Φ
Φ
(cm-1)
b
a
b
(nm)
(%)
(%)
1
2
410 (4.2),
434 (4.4),
459 (4.4)
405 (4.3)
427 (4.5),
452 (4.4)
510 (4.6)
417 (4.3),
439 (4.5),
466 (4.4)
475
467
733
711
4.13
2.75
92
81
2.0
9.1
580
537
3
4
605
482
3079
713
2.92
2.50
30
78
1.8
22
624
523
5
525 (4.6)
617
2840
2.44
10
9.3
620
40 crystal structures of 1,
2 and 3 reveal a nearly planar
a
Quantum yields in solution were determined by using 9,10-diphenylanthracene (1,
conformation between the quinoxaline unit and the
phenyl/triphenylamino substituent (R’). Compared to the
unsubstituted and methyl substituted analogues, the presence of
additional phenyl substituents results in obvious non-planarity in
45 the molecular structures of 4 and 5 with dihedral angles of
~57.89°, ~44.14°, respectively (Table S3). We are able to further
deduce the structure-property correlations of H-, methyl- and
phenyl-substituted boron complexes in solution and solid state by
the analysis of their photophysical properties and crystal packing
50 patterns.
The photophysical properties of boron complexes 1-5 were
studied in solvents of varying polarity. The absorption spectra of
1, 2, and 4 were hardly affected by solvent polarity (Table S2).
The emission maximum displayed slightly bathochromic shift
55 when solvent was changed from nonpolar hexane to polar
CH3CN. The insensitivity of the absorption and emission spectra
toward solvent polarity suggests negligible change of the dipole
moments between their ground and both Franck-Condon and
relaxed excited states. However, the absorption spectra were red-
60 shifted about ~10 nm switching from methyl (2) to phenyl
substitution (4). The absorption spectra of 1, 2, and 4 recorded in
CHCl3 showed distinct fine structures between 400-450 nm (Fig.
1a), which are characteristic of typical π−π* transitions with
notable vibronic features and high extinction coefficients (∼104
65 M-1cm˗1). In a sharp contrast, 3 and 5 exhibited a broad and red-
b
2, 4) or Rhodamine 6G (3, 5) as the references. Measured by the integrated sphere
method in solid state.
90
All boron complexes 1-5 showed red-shifted emission in
solid state with respect to the corresponding emissions in solution,
which is attributed to the increasing intermolecular interactions
(Table 1). The bulkiness of the substituent (R) on the quinoxaline
95 exhibits an apparent correlation to the solid-state quantum yield
whereas no positive correlation exists for such substituent effect
with R’ groups. The complexes with phenyl substituent (R)
showed higher solid-state emission quantum yields in comparison
to their unsubstituted and methyl substituted analogues. In CHCl3
100 solution, fluorescence quantum yields follow the order of 1 (92%)
> 2 (81%) > 4 (78%) whereas a reverse order of fluorescence
quantum yields was observed in solid state with 4 (22%) > 2
(9.1%) > 1 (2.0%). Moreover, the emission maxima in solid state
also follows a trend in which 1 (580 nm) > 2 (537 nm) > 4 (523
105 nm) (Table 1). In order to pinpoint the structure-property
relationship of distinct photophysical properties of 1, 2 and 4, a
comparison of the molecular arrangements in the crystal
structures was investigated. Various secondary bonding
interactions (π−π, C-H⋅⋅⋅F, C-H⋅⋅⋅π, and C-H⋅⋅⋅N interactions) play
110 critical roles in molecular stacking assemblies in crystal
structures 1-5. Crystal 1 is organized in two independent crystal
stacks and the molecules are arranged as dimers in a head-to-tail
2 | Journal Name, [year], [vol], 00–00
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manner. The π−π distance is 3.588 Å in the dimer of Column B
an extra excited-state energy deactivation channel to result in a
whereas the counterpart is 3.885 Å in Column A (Fig. S7). Apart
from the strong π−π interactions, the interplays of eight C-H⋅⋅⋅F
hydrogen bonds in the range of 3.263-3.545 Å (∠C-H⋅⋅⋅F =
lower solid-state emission quantum yield than complex 4 despite
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the intermolecular π−π distances in complexes 4 and 5 are
DOI: 10.1039/C4CC08958H
50 comparable. Notably, the AIEE-active ICT emitter in complex 5
was supported by the UV and emission spectra in THF-water
mixture (Fig. S11). The Mie scattering effect observed in the
visible absorption spectra of 5 implied that the nanoparticles of
the aggregated process were formed in the mixtures with water
55 fraction up to 80%. The slightly blue-shifted emission (80%
water fraction) could be attributed to the formation of
nanoaggregates with twisted conformation and suppressed the
ICT character.
Most interestingly, the boron complexes 1-5 possess unusual
60 acidochromic behavior triggered by acid vapor, which holds the
potential to develop into a solid-state fluorescence switching
material. After exposure to trifluoroacetic acid (TFA) vapors, the
colors of 1-5 turned into obscure and absorption spectra were red-
shifted accompanied by strong quenching of luminescence. The
65 fluorescence can be switched between “on” and “off” by treating
with triethylamine (TEA)/TFA vapors in spite of slight
attenuation of luminescence (Fig. 3 and Fig. S12-S13). The
fluorescence quenching upon acid fuming is attributed to the
synergistic effects: (i) the protonation of nitrogen of pyrazine
70 segment induces push-pull effect and further enhances the ICT
transition and (ii) the changes of intermolecular packing and
molecular conformation upon acid protonation may transform the
crystalline powder into an amorphous state through enhanced
intermolecular interactions judged by the powder X-ray
75 diffraction patterns (Fig. S14).16
5
138.66o-166.00o) and additional C-H⋅⋅⋅N and B-F⋅⋅⋅π
(quinoxaline) dipole-dipole interactions govern the overall crystal
packing in complex 1 (Table S3). Methyl analogue 2 exhibits an
intermolecular π−π distance of 3.620 Å and three C-H⋅⋅⋅F
interactions in the range of 3.153-3.473 Å (∠C-H⋅⋅⋅F = 119.56o-
10 164.88o). Furthermore, an additional C-H⋅⋅⋅N interaction also
contributes to the overall crystal packing in 2. Interestingly,
phenyl substituted derivative 4 displays the longest π−π distances
of 3.649 and 3.717 Å and one of the intermolecular H-bond (C18-
H18⋅⋅⋅F1, 3.244 Å, 129.1o) constrain the rotation which may
15 further uplift π−π distance due to its vertical direction. The subtle
difference in the crystal packing patterns by changing the
substitution from –H to –Me to –Ph effectively minimizes the
intermolecular interactions. The decrease in the extent of π−π
overlap in the structural packing of complex 4 compared to
20 complexes 1 and 2 (Fig. S10) leads to a hypsochromic emission
in solid state along with high fluorescence quantum yields instead
of producing detrimental excimer-like emission (Fig. 2). The π–π
interactions and dihedral angles summarized in Table
unambiguously support our conclusion.
2
25
Fig. 3 (a) Images of 4 taken under naked eye and UV lamp (λex = 365 nm). The
samples were deposited on a slide glass. (b) Fluorescence spectra of 4 between “on”
80 and “off” by reversibly treated with TEA/TFA vapors in solid state. (c) Recycling of
the fluorescence switching of solid powder 4 upon fuming with TFA and TEA
vapors (I0 is the original PL intensity and I is the PL intensity of 4 after fuming with
TFA or TEA).
Fig. 2 Packing diagrams of crystals (a) 1, (b) 2, (c) 4. (d) Solid-state emission
spectra of 1-5.
30
Table 2. The π–π interactions and dihedral angles in single crystals 1-5
85
In summary, we have developed new quinoxaline-β-
ketoiminate boron difluoride complexes possessing tunable and
fascinating photophysical properties by simple decorating the
luminophore with various substitutions. These complexes
displayed high fluorescence quantum yields in solution. In
Å
Dyes
π–π (
)
Dihedral angles (o)a, b
1
2
3
4
5
3.589, 3.885
3.620
3.625, 3.656, 3.660, 3.698
A-4.18a, B-3.57a
5.37a
12.06a
3.649, 3.717
3.646, 3.725
12.72a, 57.89b
22.05a, 44.14b
90 particular, the solid-state fluorescence of these complexes is
enhanced by an order of magnitude upon changing the substituent
from –H to –Me to –Ph. Moreover, a large Stokes shift has been
achieved for triphenylamine substituted analogues. Such results
demonstrate that these complexes are of paramount significant for
95 the design of efficient solid-state emitter and suitable for
promising applications in solar cells and organic light-emitting
transistors.18 Most uniquely, complexes 1-5 exhibit a reversible
on–off solid-state luminescence switching by acid/base fuming
processes, which might pave the way for future fluorescent
100 molecular switches.
[a] The dihedral angle between the quinoxaline-BF2 core and phenyl/triphenylamino
substituent (R’). [b] The dihedral angle between the quinoxaline-BF2 core and
additional phenyl substituent (R).
35
A similar trend was observed for the –Me (3) and –Ph (5).
Compared to complex 3, complex
5 has a longer π−π
intermolecular distance in the dimer and the degree of overlap
between adjacent dimers are quite different in spite of π−π
40 interactions are also formed in adjacent dimers (Fig. S9). The
twisted conformation of crystal 5 significantly impedes the
intermolecular interactions and results in minor red shift in solid-
state fluorescence with high quantum yield (Table S3).
Nevertheless, the head-to-tail arrangement of the molecular
45 packing mode in complex
intermolecular charge transfer interaction, which likely become
5
undesirably introduces
Acknowledgements
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We are grateful to the Ministry of Science and Technology of
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at Institute of chemistry, Academia Sinica and Mr. Ting-Shen
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DOI: 10.1039/C4CC08958H
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