Tuning the Colors of the Dark Isomers of Photochromic Boron Compounds with Fluoride Ions: Four-State Color Switching

Article abstract of DOI:10.1021/acs.orglett.6b02308

Combining a three-coordinated boron (BMes₂) moiety with a four-coordinated photochromic organoboron unit leads to a series of new diboron compounds that undergo four-state reversible color switching in response to stimuli of light, heat, and fluoride ions. Thus, these hybrid diboron systems allow both convenient color tuning/switching of such photochromic systems, as well as visual fluoride sensing by color or fluorescent emission color change.

Full text of DOI:10.1021/acs.orglett.6b02308

Letter
pubs.acs.org/OrgLett
Tuning the Colors of the Dark Isomers of Photochromic Boron
Compounds with Fluoride Ions: Four-State Color Switching
Soren K. Mellerup,† Ying-Li Rao,† Hazem Amarne,§ and Suning Wang*,†,‡
†Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
‡Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of
Technology, Beijing 1000081, P. R. China
§Department of Chemistry, The Hashemite University, P.O. Box 330117, Zarqa, 13133, Jordan
S
* Supporting Information
ABSTRACT: Combining a three-coordinated boron (BMes2)
moiety with a four-coordinated photochromic organoboron
unit leads to a series of new diboron compounds that undergo
four-state reversible color switching in response to stimuli of
light, heat, and fluoride ions. Thus, these hybrid diboron
systems allow both convenient color tuning/switching of such
photochromic systems, as well as visual fluoride sensing by
color or fluorescent emission color change.
Scheme 1. Four-State Switching and Structures of the
Diboron Molecules
olecular systems capable of undergoing discrete and
Mreadily reversible interconversion between various states
are of great interest in materials science owing to their potential
applicability in optical memory devices/switches1 and molec-
ular recognition.2 Aside from well-known classes of chemical
entities such diarylethenes (DTEs)3 or spiropyrans,4 boron-
containing π-materials have begun to attract significant research
attention within these fields as a result of their diverse
reactivities and rich optoelectronic properties.5 We have
recently demonstrated that four-coordinated N,C-organoboron
chelates are capable of displaying efficient photochromic
switching wherein a thermally reversible intramolecular C−C
bond-forming/breaking reaction generates intensely colored
B,N-embedded bisnorcornadienes with a general structure
analogous to diboron-dark shown in Scheme 1.6 This reactivity
stands in stark contrast to most other photochromic systems,
which tend to undergo ring opening/closing processes upon
exposure to light, and remains as one of only two7 examples of
photochromic switching at a boron core. Nevertheless,
organoboron subunits have found use as substituents in several
DTE derivatives, wherein the boron fragments either allow the
effective tuning of the DTE photochemical switching8 or
display varying Lewis acidity in each available state.9 Based on
these prior works and the propensity for triarylboron to bind
with small anions such as fluoride,10 we postulated that
substitution of our N,C-organoboron systems with BMes2
could provide a simple and efficient strategy for modulating
the colors of the dark isomers, achieving four-state switching as
shown in Scheme 1. The dark isomers are in general stable
toward water, which is often associated with fluoride ions,
making it possible to use fluoride ions to tune/switch the colors
of the photochromic boron-based systems. To demonstrate
this, four diboron compoundspreviously reported B2 and
three new molecules with different backbones (Scheme 1)
were prepared to establish the impact of the acceptor unit and
its substitution position on the color switching of the boron
system.
Compounds B2O, B2S, and B2N (B2X) were prepared in
moderate yields via a one-step lithium−halogen exchange and
deprotonation of the appropriate precursor (pre-B2X),
followed by the addition of 2 equiv of FBMes2 as shown in
Scheme 2. B2 was prepared as previously reported.6a B2X
compounds were fully characterized by 1H, 13C, and 11B NMR,
Received: August 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02308
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Letter
Scheme 2. Syntheses of the Diboron Compounds
HRMS, and elemental analysis (see Supporting Information,
SI). The crystal structure of B2O was determined by single-
crystal X-ray diffraction and is shown in Figure 1.
Figure 2. UV−vis (top) and fluorescence (normalized, bottom)
spectra of B2X and B2, and their fluoride adducts in toluene at 10−5
M. Inset: Photographs showing the color and fluorescence color
change before and after fluoride addition.
Table 1. Photophysical Properties of the Diboron
Compounds
a
a
b
compd
λabs (nm) (∈, M−1 cm−1
)
λem (nm)
Φ
Figure 1. Crystal structure of B2O. H atoms were omitted for clarity.
Key bond lengths (Å): B(1)−C(1) 1.631(7), B(1)−C(14) 1.628(7),
B(1)−C(23) 1.641(8), B(1)−N(1) 1.668(6), B(2)−C(12) 1.574(7),
B(2)−C(32) 1.566(8), B(2)−C(41) 1.554(8).
B2S
B2O
B2N
B2
348, 426 (1.95 × 104)
488
489
495
525
0.18
0.79
0.56
0.33
345, 434 (3.14 × 104)
328, 376, 422 (0.691 × 104)
352, 410 (0.443 × 104)
a
b
1.0 × 10−5 M in toluene at 298 K. QY in solution, determined
As shown in Figure 2 and summarized in Table 1, the
diboron compounds all possess multiple intense absorption
bands in the UV−vis spectra. Similar to previously reported
N,C-organoboron chelates,6 all four molecules display a low
energy band at ca. 410−430 nm. For B2S and B2O, the lower
energy band is intense and well resolved while those of B2 and
B2N appear as shoulder peaks. From TD-DFT calculations, the
primary contribution to the S1 vertical excitation of B2, B2O,
and B2S is HOMO (Mes of chelate boron; π) to LUMO
(backbone plus the tricoordinated boron unit; π*), which is
assigned to their low energy absorption band. In contrast, for
B2N, the S1 state (422 nm band) is mainly from HOMO (Mes
of chelate boron; π) to LUMO+1 (backbone, π*) with little
contributions from the triarylboron unit, while the S2 is
dominated by HOMO → LUMO (BMes2Ph, π*) and
responsible for the band at 376 nm. Consistent with this TD-
DFT data, the addition of fluoride (tetrabutylammonium
fluoride, TBAF) to B2, B2O, and B2S caused quenching of
the low energy absorption band while, for B2N, it led to
quenching of the band at 376 nm instead of the 410 nm
shoulder band. Consequently, B2N experienced little color
change after fluoride addition, while the other three diboron
compounds all became colorless (Figure 2). The fluoride
addition also caused a distinct change in their fluorescence
relative to quinine hemisulfate.11
spectra. For B2, B2O, and B2S the fluorescence spectra
experienced a hypsochromic shift with fluoride addition, to λem
= ∼420 nm and emission color change from blue-green/green
to blue. For B2N, its fluorescence spectrum displayed a small
hypsochromic shift of ∼15 nm and the emission color changed
from blue-green to green (Figure 2, bottom). In all cases, the
F− binding constants were determined to fall within (2.1−2.7)
× 104 ( 0.5) M−1, which are consistent with previous reported
values of triarylboron compounds.10
All three B2X compounds undergo photoisomerization in
the same manner as B2 upon UV irradiation at 350 nm yielding
their respective dark isomers B2O-dark, B2S-dark, and B2N-
dark, as evidenced by UV−vis spectroscopy (Figure 3 and SI)
which shows the appearance of new low-lying absorption bands
between 650 and 750 nm and distinct colors (green, B2O;
yellow, B2S; turquoise, B2N; brown, B2). In accordance with
previous findings,6 TD-DFT calculations suggest that all four
bands of the diboron dark isomers originate from HOMO (π)
to LUMO (π*-backbone-BMes2) CT transitions, with the
HOMO located on the boracycle and the cyclohexadienyl ring
(see SI). Due to the similar HOMO orbitals of all four dark
B
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SI). As a result, the fluoride ion has a minimal impact on the
color of B2N-dark. This supports that direct attachment of a
BMes2 unit at the 5-position on the pyridyl ring is an effective
approach in achieving color tuning of the dark isomers using
fluoride ions. B2X-F-dark and B2−F-dark can be readily
converted to B2X-dark and B2-dark by simply washing the
toluene/benzene solution of the F-bound dark isomers with
water, which removes the fluoride ion (see SI). Furthermore, as
in the case of the non-F bound dark isomers, the F-dark
isomers can thermally revert back to the colorless isomers upon
heating.
It is worth noting that completely photoconverting the
diboron compounds to their dark isomers first and
subsequently adding TBAF generates the same F-bound dark
isomers B2-F-dark/B2X-F-dark as those obtained from the
direct photoisomerization of the F-bound diboron compounds
B2 and B2X (see SI). Thus, reversible four-state switching can
be achieved for these diboron compounds. This unusual
multistate color switching facilitated by light, heat, and fluoride
ions is illustrated in Figure 4 using B2 as an example. All
Figure 3. UV−vis spectra of the B2X and B2 dark isomers (left) and
B2X-F and B2-F dark isomers (right) in toluene at 10−4 M. Inset:
Photographs showing the colors of the dark isomers.
isomers, their unique solution colors are caused by the
variations in their LUMO energy levels, which are governed
1
by the π* orbitals of the chelating ligands. The H and 11B
NMR spectra showed that all diboron compounds undergo
clean and quantitative transformation to the dark isomers (see
SI) with photoisomerization quantum efficiencies (ΦPI) similar
to their monoboron counterparts6a,d (<0.10 for B2X and 0.83
for B2). The low ΦPI of B2X compounds is in agreement with
the previous findings, which showed that extending the π-
backbone of the chelating ligand results in stabilization of the
excited state, hence decreasing their photoreactivity.6b All
diboron dark isomers readily undergo thermal conversion back
to their original light colored state when heated at 70 °C
(several hours at typical NMR concentrations).
Next we examined the photoswitching of the fluoride adducts
of B2X and B2 in solution. All four fluoride adducts undergo
photoisomerization in the same manner as the nonfluoride
bound parent molecules, as confirmed by UV−vis and NMR
spectroscopy (see SI). Most interesting is that the colors of the
dark isomers of the fluoride adducts are distinctly different from
those of the parent molecule with the exception of B2N-F-dark
and B2N-dark which have a similar turquoise color (Figure 3).
As shown in Figure 3 and Table 2, F-bound dark isomers of the
three compounds with BMes2 substituted on the pyridine ring
display a dramatic hypsochromic shift in their low energy
absorption λmax compared to the non-F bound dark isomers.
This is rationalized whereby the formation of the fluoride
adducts results in destabilization of the LUMO energy level,
creating larger optical gaps (Eg) which manifest themselves as
higher absorption energies. Conversely, the absorption band of
B2N-F-dark shows only a 10 nm bathochromic shift of its
absorption band compared to B2N-dark. In fact, the colors of
B2N-F-dark and B2N-dark are similar to the related
monoboron compound, lacking the three-coordinated
BMes2.6c This difference can be explained by the contributions
of the BMes2 unit to the LUMO level. For B2O-dark, B2S-
dark, and B2-dark, the pπ orbital of the B atom dominates the
π* (LUMO) orbital, while its contribution to the LUMO is far
less pronounced in B2N-dark with the phenyl linker of the
BMes2 unit lying almost perpendicular to the indolyl ring (see
Figure 4. Four-state reversible switching of B2: UV−vis spectra of the
four diboron species involved in the four-state color switching of B2.
Inset: A diagram showing the distinct colors of the four states based on
B2 in polystyrene (PS) films (coated on the inside of a cuvette) and in
toluene (vials).
diboron compounds and their F-bound adducts also undergo
photoisomerization when doped in polystyrene matrices,
generating the same dark isomers as in solution (Figure 4
and SI).
To demonstrate the stability of the diboron systems toward
repeated photo- and thermal switching, B2, B2-F, B2N, and
B2N-F were cycled through their photochromic states several
times as representative examples for this class of molecules. As
shown in Figure 5, both B2 and B2-F compounds are capable
of undergoing several rounds of photo- and thermal switching
with the B2-F derivative exhibiting worse fatigue following
Table 2. Photophysical Properties of the Dark Isomers
a
a
compd
λabs (nm)
color
yellow
compd
λabs (nm)
color
B2S-dark
B2O-dark
B2N-dark
B2-dark
736
704
614
715
B2S-F-dark
B2O-F-dark
B2N-F-dark
B2-F-dark
575
548
604
502
blue-gray
purple
turquoise
red
yellow-green
turquoise
brown
a
All data acquired in toluene at 10−4 M.
C
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02308.
(h) Rao, Y.-L.; Horl, C.; Braunschweig, H.; Wang, S. Angew. Chem., Int.
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Ed. 2014, 53, 9086−9089.
Figures and tables of all characterization data, TD-DFT
calculation data, additional UV−vis and fluorescence data
of B2 and B2X, their dark isomers, and X-ray data for
B2O (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: wangs@chem.queensu.ca.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Natural Science and Engineering
Research Council (NSERC, RGPIN1193993-2013) of Canada
for financial support. S.K.M. and Y.L.R. thank the Canadian
Government for their Vanier Scholarships.
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