Phosphorescence Tuning through Heavy Atom Placement in Unsymmetrical Difluoroboron β-Diketonate Materials

Article abstract of DOI:10.1002/chem.201703513

Difluoroboron β-diketonates (BF₂bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl–phenyl dyes (BF₂nbm) (1–6) were prepared to test heavy-atom placement effects. The BF₂nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetime—important features for designing O₂ sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S₁ and T₁ states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S₁ to T₁ and emission from T₁ to S₀ are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.

Full text of DOI:10.1002/chem.201703513

A Journal of
Accepted Article
Title: Phosphorescence Tuning via Heavy Atom Placement in
Unsymmetrical Difluoroboron b-Diketonate Materials
Authors: Tiandong Liu, Guoqing Zhang, Ruffin Evans, Carl Trindle,
Zikri Altun, Christopher DeRosa, Fang Wang, Meng Zhuang,
and Cassandra L. Fraser
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201703513
Link to VoR: http://dx.doi.org/10.1002/chem.201703513
Supported by

10.1002/chem.201703513
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Phosphorescence Tuning via Heavy Atom Placement in
b-
Unsymmetrical Difluoroboron Diketonate Materials
Tiandong Liu,[a] Guoqing Zhang,[a,b] Ruffin E. Evans,[a,c] Carl O. Trindle, [a] Zikri Altun,[d] Christopher A.
DeRosa,[a] Fang Wang,[a] Meng Zhuang,[a] and Cassandra L. Fraser *[a]
Abstract: Difluoroboron b-diketonates (BF2bdks) show both
fluorescence (F) and room-temperature phosphorescence (RTP)
when confined to a rigid matrix such as poly(lactic acid). These
materials have been utilized as optical oxygen sensors (e.g. tumors,
wounds and cells). Spectra features include charge transfer (CT)
from the major aromatic donor to the bdk acceptor. A series of
naphthyl-phenyl dyes (BF₂nbm) (1-6) were prepared to test heavy-
atom placement effects. The BF₂nbm dye (1) was substituted with Br
on naphthyl (2), phenyl (3), or both rings (4) to tailor the F/P ratio
and RTP lifetime—important features for designing O2 sensing dyes
via the heavy atom effect. Computational studies identify the
naphthyl ring as the major donor. Thus, Br substitution on the
naphthyl ring produced greater effects on the optical properties such
as increased RTP intensity and decreased RTP lifetime compared to
phenyl substitution. However, for electron-donating piperidyl-phenyl
dyes (5), the phenyl aromatic is the major donor. As a result, Br
substitution on the naphthyl ring (6) did not alter the optical
properties significantly. Experimental data and computational
modelling show the importance of Br position. The S1 and T1 states
are described by two singly occupied MOs (SOMOs). When both of
these SOMOs have substantial amplitude on the heavy atom,
passage from S1 to T1 and emission from T1 to S0 are both favoured.
This shortens the excited state lifetimes and enhances
phosphorescence.
when confined to a rigid matrix, such as a crystal, where the
dyes are “shielded” from collisional quenching by oxygen.5,6
Notably, dyes such as difluoroboron diketonates (BF2bdks) show
both fluorescence and phosphorescence at room temperature
when embedded into a rigid polymer, such as poly(lactic acid)
(PLA).
When exposed to air, the phosphorescence is
completely quenched, but the fluorescence is unaffected.7 Such
phenomena have been exploited for ratiometric oxygen
sensing.8,9
Previous reports established that when difluoroboron
dibenzoylmethane (BF2dbm) was embedded in poly(lactic acid),
whether covalently attached or blended, it exhibited room
temperature phosphorescence (RTP) in addition to intense
fluorescence.7,10 Due to the long phosphorescence lifetime of the
dye, and the permeability of oxygen through PLA, the RTP is
highly sensitive to oxygen. Through nanoprecipitation methods,
dual-emissive BF2bdkPLA materials can be converted into
nanoparticles for use as ratiometric oxygen probes for in vivo
hypoxia imaging.1,11–13 Given the many benefits of BF2dbm triplet
emission for oxygen sensing and imaging in biological contexts
and other uses, it is important to develop systematic methods to
tune both fluorescence and RTP, not just in the blue-green
region but other wavelength ranges as well, for multiplexing and
greater tissue penetration of light.14 An important first step is to
understand the nature of the excited states, which, for example,
could be from intramolecular charge transfer or delocalized π –
π* states.15 Recent investigations revealed that both π-
conjugation and intramolecular charge transfer (ICT) contribute
to the photophysical properties of the BF2bdk dyes.16–18 When
the two aryl groups of the diketone have identical or comparable
electron donating abilities, the dye is prone to excitation through
π – π* transitions for the lowest excited state and its intrinsic
fluorescence lifetime is typically short (~2 ns). If one of the aryls
is replaced by a stronger donor, some ICT character in the
transition can be expected. Increasing the disparity in electron
donating ability of the two aromatic donors favors ICT character
relative to a π – π* transition, and the lifetime increases (e.g.
anthryl-phenyl dye, BF2abm, in PLA: t = ~10 ns).16 Altering the
influence of the ICT process could be an effective method to
adjust emission properties, thereby achieving desired optical
properties by tuning lifetimes and altering the fluorescence-to-
phosphorescence ratio.
Introduction
Phosphorescence emission has garnered much interest
over the years, particularly for light emitting diode (LED)
applications and optical oxygen sensing.1–3 Recently, organic
compounds have attracted renewed interest as alternatives to
metal-activated phosphors and inorganic salts. However,
organic materials that show phosphorescence are typically very
sensitive to oxygen.4 Some organic materials only phosphoresce
[a]
Carl O. Trindle, Christopher A. DeRosa, Fang Wang, Meng Zhuang,
Cassandra L. Fraser*
Department of Chemistry
University of Virginia
McCormick Road, Charlottesville, VA 22904, USA
E-mail: fraser@virginia.edu
In this study, the heavy atom effect was employed as a
tool to screen the modulation of the ICT process, particularly in
triplet emission. Previous reports on this class of compounds
[b]
[c]
[d]
Guoqing Zhang
Hefei National Laboratory for Physical Sciences at the Microscale
University of Science and Technology of China
Hefei, Anhui, 230026 China
Ruffin Evans
Department of Physics
Harvard University
Cambridge, MA 02138, USA
Prof. Zikri Altun
covalently
linked
to
PLA,
not
blended,
revealed
phosphorescence tunable by heavy atom placement and
polymer molecular weight.19 Here, we have designed a panel of
six
difluoroboron
naphthoylbenzoylmethane
complexes
(BF2nbm), as shown in Figure 1, to study the influences of
charge transfer and fluorescence to phosphorescence (F/P) ratio
tuning in depth. To assess whether phosphorescence properties
could be controlled by arene ring sizes and substituents,
Department of Physics
Marmara University Göztepe Kampus
Istanbul, Turkey, 34772
Supporting information for this article is given via a link at the end of
the document.
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bromide and amine groups were attached on naphthyl or phenyl
groups to modulate the electronic properties of the donor
moieties. The rationale is as follows. In order to obtain strong
phosphorescence, efficient passage from S1àT1 and bright
emission from T1àS0 must both occur. A heavy atom is known
to exert strong spin-orbit coupling. Triplet-singlet mixing is
enhanced by strong spin-orbit coupling, a small singlet-triplet
gap, and substantial amplitude at the heavy atom in both
coupled SOMOs. Charge transfer transitions typically have small
S1-T1 energy gaps, so if the S0-S1 transition has substantial
charge transfer character, ISC to T1 can be enhanced .20,21 Near
unit ISC yield could be expected for an almost pure CT, and if
emission from the T1 state is not rapid, delayed fluorescence is
expected.22–24 Both the CT character of the transition (which
affects the first ISC process) and the impact of the placement of
the heavy atom (which can influence both the first ISC and the
rate of emission from T1) play roles in determining the total
emission spectra. In this study, we combine experimental
observation and computational characterization of states
participating in the photophysics for BF2bdk dyes 1-6. To the
best of our knowledge, no previous study has described the
interplay of CT and heavy atom effects on emission spectra in
this manner.
Dyes 1-4 are yellow solids and show intense fluorescence under
UV light, however, luminescence from orange solids 5 and 6 is
very weak and can barely be observed by the naked eye.
Scheme 1. Synthesis of Naphthyl-Phenyl b-Diketone Ligands and Boron
Complexes.
Optical Properties in Solution. The optical properties were
measured in dilute CH2Cl2 (Table 1). The absorption spectra of
dyes 1-6 in CH2Cl2 solution are shown in Figure 2. The
absorption maxima of 1-4 range from 377-411 nm. Absorption
maxima of BF2nb(pip)m (5) and BF2n(Br)b(pip)m (6) are at 477
and 482 nm respectively in addition to a small absorption peak
at around 370 nm. The considerable bathochromic shifts of 5
and 6 in a polar solvent indicate an extended conjugation in the
π system and a significant contribution from enhanced ICT to
the transition.25–27
Figure 1. Chemical modifications to difluoroboron naphthyl-phenyl b-
diketonate complexes.
Results and Discussion
Synthesis. The β-diketonate ligands were prepared by Claisen
condensation, shown in Scheme 1. The ketone precursors in 1a-
4a were deprotonated by NaH before reaction with the desired
esters. In 5a and 6a, due to the existence of electron donating
piperidine, which renders the ketone less acidic, a stronger base
is required to facilitate the reaction. Using LDA, the ligands 5a
and 6a were obtained in moderate yield. The boron compounds
1-6 were obtained from
a previously reported method in
moderate to good yields.16 The boronation reaction was carried
out with boron trifluoride etherate and the corresponding β-
diketones in dichloromethane. Immediately upon the addition of
the boron reagent, intense fluorescence is observed. After ~2 h
under refluxing, the ligand typically cannot be observed on thin-
layer chromatography (TLC) plate and the reaction was
considered complete. The yields were up to 70% after
recrystallization or over 80% yield after column chromatography.
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Table 1. Optical Properties of BF2nbm Complexes 1-6 in CH2Cl2
dropped.28,31 However, compared to BF2nbm (1), BF2nb(Br)m (3)
showed a slight decrease in lifetime, from 4.8 ns to 4.6 ns,
whereas the lifetimes of BF2n(Br)bm (2) and BF2n(Br)b(Br)m (4)
decreased to 0.86 and 0.80, which was about 1/6 of the lifetimes
for BF2n(Br)bm and BF2n(Br)b(Br)m. As previously described,
the optical properties of the charge transfer emitting species are
dominated by the major donor, naphthyl over phenyl.16
Therefore, halogen atoms effects are predicted to be stronger
when they are attached to naphthyl groups and show little
influence when they are substituents on the less powerful arene
donors, based on the measured lifetime values. The lifetime
measurements of 1-4, are accurately fit by a single exponential
decay, which implies only one excited species existed in each
case. However, triple and double exponential decay profiles
were required to fit data for BF2nb(pip)m (5) and
BF2n(Br)b(pip)m (6). This suggests that excited-state twisted
isomers may exist for piperidine systems, and that there may be
an impact of twisted intramolecular charge transfer (TICT) on the
optical properties.26,32 A near-planar conformation and a more
significantly twisted conformation may be accessible to the
excited state.25,33
[a]
[c]
[d]
ε[b]
λem
(nm)
λF
Stokes
Shift
[g]
λmax
(nm)
trad
[f]
(M-1cm-1
)
Sample
ΦF
(ns)
(ns)
(cm-1)
377[h]
399
37100
37900
1
482
4.82
0.50
0.17
0.31
9.68
4320
2990
4280
2820
382[h]
403
39000
42900
2
3
460
490
465
0.86
4.58
0.80
5.06
14.8
383[h]
405
39000
39000
4
5
411[i]
34000
0.18
4.44
44.0
341[h]
477
19600
80100
444[h]
561
1.42
1.10
6800
3140
0.025
344[h]
482
16900
65400
459[h]
553
0.88
7280
2660
6
0.0093
300
2.79[e]
[a] Absorption maxima. [b] Extinction coefficients calculated at the absorption
maxima. [c] Fluorescence emission maximum. [d] Fluorescence lifetime
excited with
a 369 nm LED monitored at the emission maxima. All
fluorescence lifetimes are fitted with single-exponential decays unless
indicated. [e] Double exponential decay. [f] Relative fluorescence quantum
yield using quinine sulfate in 0.1M H2SO4 as a standard. [g] Radiative lifetime,
where trad = tF/FF. [h] Band is present as a shoulder or second maximum. [i]
Broad high-energy shoulder but no clearly identifiable second maximum.
Figure 3. Normalized emission spectra of BF2nbm complexes 1-6 in CH2Cl2
(λex = 369 nm).
The quantum yield measurements were carried out with quinine
sulfate in 0.1 M H2SO4.
The standard BF2nbm dye (1)
possesses the highest fluorescence quantum yield of 50% (ΦF =
0.50). Due to the heavy atom effect, the presence of bromide
causes the diminished fluorescence via an increased
intersystem crossing rate.34,35 Thus, the quantum yields of 2-4
are all decreased relative to BF2nbm. Placement of a single Br
on the phenyl fragment reduced the quantum yield to 31%.
However, when a bromide is attached on the naphthyl side, the
dyes showed more dramatic decreases in quantum yield, to 17%
for 2 and 18% for 4. The quantum yields of piperidyl-substituted
phenyl systems 5 and 6 in dichloromethane are ~1.0% and 2.8%
respectively. The low quantum yields of 5 and 6 are presumably
correlated to strong CT character in the transition which permits
large, energy-dissipating nuclear reorganization.36–38
Figure 2. Absorption spectra of BF2nbm complexes 1-6 in CH2Cl2.
The absorptions show
a
linear dependence on
concentration for each of the boron diketonate dyes in CH2Cl2 in
the 5-20 µM range. These results suggest that only ground-state
monomeric species for 1-6 absorption occur under the tested
conditions. The molar extinction coefficients of 1-6 are listed in
Table 1. The introduction of bromine atoms to BF2n(Br)bm and
BF2nb(Br)m induces an increase of the extinction coefficients.28
An anomaly in this trend is when bromides are linked to both
sides of the diketone (BF2n(Br)b(Br)m; 4), as the absorption
slightly decreased to 34,000 M-1 cm-1. For piperidine complexes
5 and 6, the extinction coefficient shows a significant, greater
than twofold enhancement compared to those of 1-4, which can
also be attributed to the increased electron donor ability of the
piperidine unit on the phenyl group.16,29,30
The emission spectra of BF2nbm complexes 1-6 are
provided in Figure 3. We also examined the fluorescence
lifetimes of 1-6 in dichloromethane solution. The results are
recorded in Table 1. It is widely accepted that heavy atoms can
efficiently shorten lifetimes of organic dyes, so it is not surprising
that the lifetimes of 2-4, which contain bromide, have all
Computational Studies. To explain the observed optical
properties, time-dependent density functional theory (TD-DFT)
calculations at the optimized geometries with the model
wB97XD/6-311+G(d) and Tomasi solvent provide estimates of
S0-S1 absorption (Table 2). The frontier molecular orbital
diagrams including highest energy occupied molecular orbitals
(HOMOs) and lowest energy unoccupied molecular orbitals
(LUMOs) are shown in Figures 4 and 5. The absorption spectra
for species 1-6 can be divided into two categories. The phenyl-
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piperidyl species, 5 and 6, absorb at longer wavelengths
compared to species 1-4 (450–520 nm or 2.4–2.8 eV for 5 and 6,
350–425 nm or 2.9–3.5 eV for 1-4). The phenyl-piperidyl species
5 and 6 absorb more strongly than species 1-4. Phenyl-piperidyl
systems 5 and 6 also have a lower-intensity absorption at 320–
350 nm or 3.5–3.9 eV. There is some hint that a second
absorption may lie below 300 nm (beyond 4.0 eV) for species 1-
4. The calculated spectra are provided in the Supporting
Information (Figures S1-S6).
Figure 4. Calculated frontier molecular orbital diagrams for compounds 1-4.
Calculated energies are provided in Table S1.
All features of the experimental spectra are captured by
modeling. Computations on species 5 and 6 place the reddest
transition at about 3.3 eV, while the computed transition
energies for the reddest transitions of species 1-4 lie near 3.6 eV.
The singlet TD-DFT calculations seem to have a systematic
error, shifting transitions to the blue by roughly 0.4 eV. The
greater intensity of absorption for 5 and 6 relative to 1-4 is
captured as well. Computed oscillator strengths for 5 and 6 are
near 1.7, while for 1-4 the computed oscillator strengths range
from 1.1 to 1.5. The variation is associated with the pattern of Br
substitution: the unsubstituted species
1 has the smallest
oscillator strength. Bromide substitution on naphthyl enhances
the oscillator strength more effectively than substitution on the
phenyl. The shorter wavelength and weaker band near 3.8 eV
clearly evident in the experimental spectra of 5 and 6 is placed
at about 3.9 eV in modeling, with oscillator strengths less than
0.05 for both piperidyl-substituted systems. The shorter
wavelength (bluer) transitions computed for 1-4 are near 4.0 eV;
in contrast to the behavior for the longer wavelength absorption,
bromo-substitution on the naphthyl fragment reduces the
oscillator strength to ~ 0.15 (2 and 4) from ~ 0.25 (1 and 3).
Figure 5. Calculated frontier molecular orbital diagrams for
Calculated energies are provided in Table S1.
5 and 6.
Calculations provide a pictorial view of the transitions in
the absorption spectra, based on the molecular orbital (MO)
composition of the excitations. The longest wavelength
transitions are almost entirely single excitations from the HOMO
to the LUMO. The highest energy occupied MO localized on
isolated naphthalene is higher in energy than the highest energy
occupied MO on unsubstituted benzene. Bromo- and piperidyl-
substitution on such aromatic rings raise the energies of these
MOs. Numerical details are provide in Table S1 and
accompanying discussion in supplementary material. Piperidyl
substitution has a greater effect, so for species 5 and 6 the
HOMO is local to the phenyl-piperidyl fragment. In contrast, for
1-4 the HOMO is always local to the naphthyl fragment,
regardless of Br substitution. The consequence is that, for the
ISC process S1àT1, Br substitution on naphthyl has only a minor
effect on the transitions for 5 and 6, while Br substitution on
naphthyl has a more substantial effect for species 1-4. Since the
phenyl fragment does not participate extensively in the spectra
for species 1-4, Br substitution on phenyl has a less dramatic
impact on spectra for these systems.
The red feature in the spectra for 1-3 seems to be
composed of two components. We considered the possibility
that in solution there may coexist two rotamers, differing in the
torsion angle around the bond connecting the naphthyl fragment
to the b-diketonate core. The computed energies put these
isomers at virtually identical energy (within 0.5 kcal/mol), so an
equilibrium between the two forms seems possible. The
computed absorption spectra for the two isomers are almost
indistinguishable and do not represent the directions of the small
shifts in the absorption maxima. These results do not allow us to
reach a conclusion on the presence of the two rotamers. The
absorption spectrum for 4 seems anomalous. Our modeling
suggests the oscillator strength f for the red band for 4 should be
greater than those values for 1-3, and we have no explanation
for the breadth or shape of this band.
Table 2. Calculated Energies of Absorption, Fluorescence and
Phosphorescence.[a]
Triplet
Sample
Ground State[b]
Singlet Excited State[c]
Excited
State[d]
S0
S0
Absorption
(f)
S1
S1
Emission
(f)
T1
Absorption
(eV)
Emission
(eV)
Emission
(eV)
3.57
1.09
0.34
3.20
3.89
1.17
0.29
4.03[e]
1
2
3
4
5
6
1.73
1.77
1.76
1.74
1.98
2.00
3.64
1.33
0.14
3.20
3.75
1.47
0.09
4.00[e]
3.57
1.22
0.28
3.18
3.66
1.40
0.17
3.87[e]
3.60
1.46
0.16
3.23
3.77
1.60
0.07
3.98[e]
3.23
1.77
0.04
2.91
3.83
1.88
0.03
4.02[e]
3.25
1.68
0.01
2.94
3.84
1.89
0.05
3.98[e]
[a] Energies and oscillator strengths of dyes 1-6. (See Supporting information
for calculated UV/Vis and Cartesian coordinates of all excited states.) [b]
Absorption energy (eV) and corresponding oscillator strengths at the optimized
ground state geometry. [c] Fluorescence energy (eV) and corresponding
oscillator strengths at the optimized singlet excited-state geometry. [d]
Phosphorescence energy (eV) and corresponding oscillator strengths at the
optimized triplet excited-state geometry. [e] Minor absorption of contributing
orbitals.
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TD-DFT calculations of the emission energies from the
lowest excited state for species 1-6 were performed using the
optimized excited state geometries. In dichloromethane solution,
species 5 and 6 display two emission signals attributed to
fluorescence, at about 2.2 and 2.8 eV. The fluorescence
quantum yield is very small, and the lifetime for the longer
wavelength emission is hardly affected by bromo substitution.
For the shorter wavelength emission, bromo substitution halves
the fluorescence lifetime. The Stokes shifts, about 3000 cm-1 for
the red emission and 7000 cm-1 for the blue emission, are not
strongly affected by bromo substitution. Systems 1-4 fluoresce
over a range from 2.7 to 2.5 eV. According to the calculations,
bromo substitution on the naphthyl fragment shifts the
fluorescence to the red, reduces the fluorescence quantum yield,
and shortens the fluorescence lifetime. Bromo substitution on
phenyl has relatively minor effects. For 1 and 3, the Stokes shift
is about 4300 cm-1 while for 2 and 4 (with bromo substitution on
naphthyl) have Stokes shifts of about 2900 cm-1. The systematic
error in modeling excited states is typically greater than the error
in describing ground states, and the error in emission properties
will be greater than the error for absorption properties. For
species 5 and 6 the longer wavelength emissions are predicted
to lie at about 2.9 eV, an overestimate of about 0.7 eV. The
computed oscillator strengths are large (~1.9) and almost
identical. With due caution, we observe that the S2-S0 energy
difference is about 3.8 eV (~325 nm), far from the second
experimental feature at 2.8 eV (~450 nm). This transition has a
much smaller computed oscillator strength (<0.1) than the S1-S0
transition (Table 2).
Modeling of triplet states can be done in two distinct ways.
First, the optimized geometry of the triplet can be obtained by
direct computation. The S0-T1 gap can be estimated by the
energy difference between S0 and T1 at that geometry (the
“DSCF” method), or by TD-DFT including singlet and triplet
excitations from S0 at the optimized T1 geometry. Since TD-DFT
does not include all energy terms in the SCF calculations,
results will differ in principle. In practice, the discrepancy is no
more than 0.1 eV. In either case, the predicted
phosphorescence energy for 5 and 6 are near 2.0 eV, 620 nm.
For 1-4 the computed phosphorescence covers a narrow range
from 1.72 to 1.76 eV, with the longer wavelength emissions for 2
and 4, and the shorter wavelength emissions for 1 and 3. This is
consistent with the effects on spectroscopic features of bromo
substitution on the naphthyl fragment seen throughout this
discussion. In this case, the systematic error red-shifts the
transition by about 0.5 eV, probably because the unrestricted
DFT method is more flexible for the triplet than the restricted
DFT used for the ground state singlet. Restricted open shell
calculations on the triplets may reduce the systematic error.
The lowest triplet orbitals for 2-4 are shown in Figure 6,
which are related to the second ISC process, i.e., T1àS0. From
the amplitude distribution in these orbitals, it can be seen that for
both 2 and 4, substantial contribution from the bromine atom can
be found for both SOMOs; while for 3, only one of the SOMOs
has significant bromine contribution. The implication is that the
radiative decay from the T1 state will be enhanced more by the
heavy atom effect for 2 and 4 than for 3, given that either SOMO
electron spin results in ISC. The results are, in fact, consistent
with the fluorescence lifetime data measured in solution, where
much shorter lifetimes are seen for 2 and 4. We can also predict
that the relative intensity ratio of phosphorescence-to-
fluorescence (IP/IF) will be higher for 2 than for 3.
Figure 6. Singly occupied molecular orbitalss (SOMOs) at the T1 geometry
show that substantial amplitude is found on Br atoms for species 2 and 4,
where the heavy atom substitution is on the naphthyl fragment. In contrast,
species 3 has appreciable amplitude on bromine only for one of the SOMOs.
Optical Properties in PLA Blends. Dyes were also studied in
polymer matrices to activate dual emission, both fluorescence
and room temperature phosphorescence. Dyes 1-6 were
dispersed in poly(lactic acid) (1.3 weight percent), and the
resulting mixture was coated as thin films on vials. Solid-state
emission data under air and N2 are provided in Table 3, and
Figures 7 and 8. The fluorescence of compounds 1-4 in PLA
showed similar blue emission to dyes in dichloromethane
solution. In general, the emission in PLA is slightly blue-shifted
versus solution (e.g. 1 lF (nm); CH2Cl2 = 482, PLA = 444). This
could be linked to the polarity of the environment, as dyes with
ICT often show solvatochromism.39,40 For compounds 5 and 6,
the blue emission disappears. Aggregation in the PLA may
induce energy transfer to the lowest energy species.41,42 Also
similar to the pattern of optical properties observed for systems
in solution, the fluorescence lifetimes (tF) are most greatly
influenced by the heavy atom placement. When the bromide is
placed on the major donor (1-4: naphthyl), the fluorescence
lifetimes decrease substantially (e.g. tF (ns); 1 = 3.29, 2 = 0.78).
The increased rate of intersystem crossing depopulates the
singlet state, decreasing the fluorescence lifetime.31
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Table 3. Optical Properties of BF2nbm Complexes 1-6 in PLA at Room
Temperature [a]
[b]
[c]
[d]
[e]
lF
lP
tF
tP
[f]
Sample
IP/IF
(nm)
(nm)
(ns)
(ms)
1
2
3
4
5
6
444
541
3.29
813
0.27
0.73
0.44
437
449
448
558
559
554
544
561
0.78
2.52
0.75
1.88
3.46
13.6
341
12.9
0.80
580[g]
[h]
[h]
[h]
[h]
[h]
[a] Dye loading: 1.3 wt% in PLA. [b] Fluorescence emission maxima, excited
at 369 nm. [c] Phosphorescence emission maxima, excited at 369 nm. [d]
Fluorescence lifetimes fitted with single-exponential decay. Excitation source:
369 nm LED monitored at the emission maxima. [e] Room temperature
phosphorescence pre-exponential weighted lifetime fit to triple exponential
decay. Excitation source: xenon flash lamp. [f] The relative intensity of
phosphorescence over fluorescence at the emission maxima under N2. [g]
Emission maximum estimated from weak broad signal with noise. [h] Signal
too weak to be detected.
Figure 8. Normalized emission spectra BF2nbm complexes (1-6) (1.3 wt%) in
PLA films (λex = 369 nm). A) Total emission spectra in air, highlighting
fluorescence, with phosphorescence largely quenched by oxygen. B) Total
emission spectra under N2 showing fluorescence and unquenched
phosphorescence. C) Delayed emission spectra under N2 showing
phosphorescence (2 ms delay). Note: Total emission of compounds 5 and 6
did not change between air (A) and N2 (B).
Under N2, compounds 1-4 show two emission bands,
which correspond to their singlet and triplet emissions,
respectively. Dyes BF2nb(pip)m and BF2n(Br)b(pip)m only
showed one emission band which is assigned to the singlet
excited state emission. This band showed considerable
bathochromic shift compared to singlet emissions of other
BF2nbm derivatives and is assigned to a charge transfer state.
As shown in Table 3, the presence of bromine enhanced the
relative phosphorescence intensity in BF2n(Br)bm, BF2nb(Br)m
and BF2n(Br)b(Br)m. The relative phosphorescence intensity
(IP/IF) was moderately increased from 0.27 for 1 (BF2nbm) to
0.44 for
3 [BF2nb(Br)m] when a bromine atom replaced
hydrogen on the phenyl group, which is defined as the minor
donor in this dye. But when a bromide was attached on the
naphthyl group, the relative intensity jumped over two-fold to
0.73 for [BF2n(Br)bm]. Introducing an additional bromide on the
phenyl unit forming [BF2n(Br)b(Br)m], 4, induced only a slight
increase to 0.80. Positioning the substituent on the major donor
has a dramatic influence on triplet species populations. The
heavy atom effect is significantly amplified if the substituent is
present on the major electron-donating group. The long lifetime
triplet species will be more likely to return to their ground states
via either delayed fluorescence or a non-radiative pathway
rather than phosphorescence.43,22,44,45
Figure 7. Images of dyes 1-6 (in order, left to right) in PLA films inside vials in
air (fluorescence), N2 (fluorescence plus phosphorescence), and under N2 with
the UV source turned off (Delay; phosphorescence) (λex = 369 nm). Note:
Compounds 5 and 6 do not change between air and nitrogen. Dyes 2 and 4
have short lived phosphorescence emissions that are not captured with the
camera. Note: The green left edge for sample 2 is a reflection from sample 1.
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The isolated phosphorescence spectra of dyes 1-4 are Conclusions
shown in Figure 8. Dyes 1-3 exhibit phosphorescence with
maxima around 540 nm and a shoulder around 560 nm. The
phosphorescence emission maximum for BF2n(Br)b(Br)m, 4, is
at 559 nm. Compound 5 showed weak phosphorescence, and
no obvious phosphorescence was observed for BF2n(Br)b(pip)m,
6. Because there is no significant heavy atom effect in 5 or 6,
the relevant comparisons are with 1 or 3. If, as the MO diagrams
in Figures 4 and 5 suggest, there is greater charge transfer
character in the transitions of 5 and 6 than in transitions of 1 and
3, HOMO and LUMO are more nearly disjoint for 5 and 6. Then
the spin orbit coupling integrals mixing T1 and S0 for 5 and 6
would be smaller, and phosphorescence would be weaker. A
non-emissive transformation between the twisted and coplanar
structures of the aminophenyl moiety could also account for the
weak phosphorescence emission.26
From the measurements of quantum yields, triplet state
populations and phosphorescence lifetimes of dyes 1-6, it is
concluded that if two donors are present in ICT type
fluorophores, the position of the substituted heavy atom, such as
a bromine atom in this case, can have substantial effect on the
phosphorescence yield. Unlike predominantly local p-p*
transition, charge transfer precludes certain regions and
moieties in the molecule from participating in a given electronic
transition. Thus, when the heavy atom is precluded, its effect is
trivial compared to when it is included. Furthermore, when the
heavy atom does participate in the electronic transition,
substantial amplitude can be found on it in only one or both
SOMOs. This allows for easy modulation of the fluorescence-to-
phosphorescence ratio by simply changing the position of heavy
atom substitution. For example, comparing 2 and 3, the bromine
on the phenyl ring contributes only to the S1àT1 transition while
the bromine on the naphthyl ring contributes both to the S1àT1
and T1àS0 transitions. Consequently, the phosphorescence
yield of 2 is visibly higher than that of 3. The optical properties of
BF2nb(pip)m (5) and BF2n(Br)b(pip)m (6) seem to depend most
strongly on the interaction between piperidinophenyl and
difluoroboron. Thus, the modification with bromine on the
naphthyl versus phenyl side had little influence on the overall
emission due to the exclusion of bromine participation in the
electronic transition, which explains the high similarity between
BF2nb(pip)m and BF2n(Br)b(pip)m emissions.
The phosphorescence lifetimes of dyes 1-4 were also
listed in Table 3. We hypothesized that when the groups on both
sides of the b-diketonates are not identical, then the group with
more electron donating character contributes more to the
luminescence properties of the dyes, termed the “major donor”
in this report. Modifications on the major donor are expected to
show a greater influence on the dye emission properties than
substituents on the less important donor. Compound 1, BF2nbm,
has the longest triplet state emission lifetime of 813 ms. The
lifetimes of Br-substituted 2-4 are all shorter than that of BF2nbm.
Bromides on the arenes strongly influence the triplet decay
lifetimes. The trend matches the pattern of the fluorescence
lifetimes in CH2Cl2 solutions. Again, because the naphthyl group
is the dominant donor in 1-4, the modification of naphthyl will
lead to a greater change in lifetime. The phosphorescence
lifetime of BF2n(Br)bm (2) was dramatically reduced to 13.6 ms,
which is around one sixtieth of that for BF2nbm. In comparison,
the lifetime of BF2nb(Br)m (3), with bromide on the phenyl ring,
was only decreased to 341 ms. The additional bromide on
BF2n(Br)b(Br)m (4) induced a slight lifetime decrease to 12.9 ms
from 13.6 ms for BF2n(Br)bm (2). These results demonstrate that
fluorescence and phosphorescence lifetimes of ICT dyes can be
tuned with substituents on aryl electron donors. These are
important considerations for tailoring the sensitivity and
designing fluorophores for lifetime imaging, such as
fluorescence lifetime imaging microscopy and phosphorescence
lifetime imaging microscopy (FLIM and PLIM).46–48 For lifetime-
based imaging, sensors like BF2nbmPLA,49–51 a compound with
similar properties to compounds 1 and 3 in this study with long
phosphorescence lifetimes, can be dynamically quenched by
oxygen more quickly, and exhibit a linear response to O2 within a
small range (0.0-0.3% O2) The large Stern-Volmer constant
makes the sensors sensitive to minor changes in O2
concentration. This type of sensor may be suitable for
measurements in environments depleted of oxygen, such as
tumors.11,52,53 Shorter phosphorescence lifetime sensors such as
BF2nbm(Br)PLA, analogous to compound (2) with a broader
linear sensing range (0-3%) are capable of monitoring O2 in
environments more rich in oxygen, such as wounds and
tissues.50
Based on these findings, we can tune the emission
properties of ICT dyes either greatly or slightly by introducing
substituents on various donors. If the emission properties need
to be adjusted only slightly, then substituents can be assigned
on minor donors. With substituents on major electron donors,
some properties, such as lifetimes, triplet state populations and
quantum yields will be greatly affected. These findings may be
exploited for the design of oxygen imaging and sensing agents.
Experimental Section
Materials. 3,6-Dimethyl-1,4-dioxane-2,5-dione (D, L-lactide, PURAC
Biomaterials, 99.8%) was stored in
a
drybox under
a
nitrogen
atmosphere and used without further purification.
Tin(II) 2-
ethylhexanoate (Sn(oct)2, Spectrum), ethylene glycol (Sigma-Aldrich,
99.8%), BF3·OEt2 (Sigma-Aldrich, purified and redistilled) and all other
reagents were obtained from Sigma-Aldrich and used as received.
Anhydrous tetrahydrofuran (THF) and dichloromethane (CH2Cl2) for
reactions were prepared by passage through alumina columns under
argon. Polylactide was prepared as previously described.54
Methods. 1H NMR (300 MHz) spectra were recorded on a Varian
UnityInova spectrometer in CDCl3 unless otherwise indicated.
Resonances were referenced to the signal for residual protiochloroform
at 7.260 ppm. 1H NMR coupling constants are given in hertz. UV/vis
spectra were recorded on
a Hewlett-Packard 8452a diode-array
spectrophotometer in CH2Cl2. High resolution mass spectra (HRMS)
were obtained with a Micromass Q-TOF Ultima spectrometer using
electrospray ionization (ESI). Additional mass spectra were recorded
using an Applied Biosystems 4800 spectrometer with a MALDI TOF
analyzer. Steady-state fluorescence emission spectra were recorded on
a Horiba Fluorolog-3 Model FL3-22 spectrofluorometer (double-grating
excitation
and
double-grating
emission
monochromators).
Phosphorescence spectra were recorded with the same instrument
except that a pulsed xenon lamp (λex = 369 nm; duration < 1 ms) was
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used for excitation and spectra were collected with a 1 ms delay after
excitation. Time-correlated single-photon counting (TCSPC) fluorescence
lifetime measurements were performed with a NanoLED-370 (369 nm)
excitation source and DataStation Hub as the SPC controller.
Phosphorescence lifetimes were measured with a 500 ns multi-channel
scalar (MCS) excited with a pulsed xenon lamp (λex = 369 nm; duration <
1 ms). Lifetime data were analyzed with DataStation v2.4 software from
Horiba Jobin Yvon. Fluorescence quantum yields, ΦF, for all boron dyes
in CH2Cl2 were calculated versus quinine sulfate in 0.1 M sulfuric acid
aqueous solution as a standard using the following values: ΦF quinine
2-Naphthoyl 6-bromobenzoyl methane, nb(Br)m, 3a. The ligand nb(Br)m
was prepared by method A, but with 4-bromo acetophenone (1.00 g, 5.00
mmol), methyl 2-naphthoate (1.16 g, 6.03 mmol) and sodium hydride
(200 mg, 7.92 mmol) in THF (40 mL). 2-Naphthoyl 6-bromobenzoyl
methane was obtained after recrystallization from acetone as a pale
yellow solid (968 mg, 55%). 1H NMR (300 MHz, CDCl3) δ 16.91 (s, 1H,
ArCOH), 8.54 (s, 1H, 1′-ArH), 8.03 – 7.88 (m, 6H, 3′, 4′, 5′, 8′, 3″, 5″-ArH),
7.66 – 7.53 (m, 4H, 6′, 7′, 2″, 6″-ArH), 6.96 (s, 1H, COCHCO); MS
(MALDI): m/z calculated for C19H14BrO2 [M+H]+ 353.02, found 353.00.
20
20
sulfate = 0.54,8 nD H2O = 1.33, nD CH2Cl2 = 1.42. Optically dilute
CH2Cl2 solutions of boron diketonates and the 0.1M sulfuric acid quinine
sulfate standard solution were prepared in 1 cm path length quartz
cuvettes with absorbances ≈ 0.1. Quantum yield measurements were
performed with excitation, lex = 350 nm, and an emission integration
range = 365-695 nm. Boron-dye/PLA blend films were fabricated by a
simple casting method: PLA (15 mg, with 1.3 wt% dye content) was
dissolved in methylene chloride (3 mL) and a portion of the solution (0.5
mL) was transferred to a borosilicate cover slide and dried in vacuo for
10-12 h at RT. The photographs were taken with a Canon Powershot
SD890IS Digital Elph camera on the automatic setting (no flash).
6-Bromo-2-naphthoyl 4-bromobenzoyl methane, n(Br)b(Br)m, 4a. The
ligand n(Br)b(Br)m was prepared by method A, but with 4-bromo
acetophenone (626 g, 3.08 mmol), methyl 6-bromo-2-naphthoate (1.00 g,
3.70 mmol) and sodium hydride (117 mg, 4.62 mmol) in THF (40 mL). 6-
Bromo-2-naphthoyl 4-bromobenzoyl methane was obtained as a yellow
solid after recrystallization (2X) from dichloromethane (675 mg, 51%). 1
H
NMR (300 MHz, CDCl3) δ 16.86 (s, 1H, ArCOH), 8.50 (s, 1H, 1′-ArH),
8.07 (s, 1H, 5′-ArH), 8.03 (dd, J = 1.8, J = 8.7, 1H, 4′-ArH), 7.91 – 7.84 (m,
4H, 3′, 8′, 3″, 5″-ArH), 7.67 – 7.63 (m, 3H, 7′, 2″, 6″-ArH), 6.94 (s, 1H,
COCHCO); MS (MALDI): m/z calculated for C19H13Br2O2 [M+H]+ 432.91,
found 432.93.
Computational Methods. We employed a modern functional wB97XD
which incorporates range-correction and empirical dispersion,55 and a
flexible Pople basis 6-311+G(d)56 to compute optimized geometries for
the ground singlet state (S0), excited single state (S1) and the lowest
energy triplet state (T1). The Tomasi model represented the effects of
dichloromethane solvent.57 Optimization of the lowest singlet excited
state was accomplished by use of the time-dependent density functional
theory (TD-DFT)58 and the wB97XD density functional but a smaller
Pople basis set 6-31G(d),59 along with the Tomasi model for
dichloromethane. TD-DFT calculations at these optimized geometries
with the model wB97XD/6-311+G(d) and Tomasi solvent provide
estimates of S0-S1 absorption and S1-S0 emission frequencies and
oscillator strengths for species in solution, and similar calculations
incorporating both singlets and triplets give estimates of T1-S0 emission
energies.
β-Diketone Synthesis: Method B. The β-diketonate ligands nb(pip)m
5a and n(Br)b(pip)m 6a were prepared using lithium diisopropylamide
(LDA) instead of NaH as the base. A representative synthesis, given for
nb(pip)m 5a, is as follows. 4-Piperidinoacetophenone (836 mg, 4.00
mmol), methyl 2-naphthoate (891 mg, 4.78 mmol) and THF (20 mL) were
added sequentially to a 50 mL round bottom flask. After stirring for 10
min, a solution of lithium diisopropylamide (LDA) (1.73 M in hexanes,
3.00 mL, 6.59 mmol) was added dropwise at -78 °C under N2. The
mixture was maintained at -78 °C for 4 h, before it was allowed to warm
to room temperature and stirred for an additional 4 h. The reaction was
then quenched with saturated aqueous NH4Cl (10 mL). The aqueous
phase was extracted with CH2Cl2 (3 × 20 mL). The combined organic
layers were washed with water twice (2 × 10 mL) and brine (10 mL), and
dried over Na2SO4 before concentration in vacuo. The residue was
purified by column chromatography on silica gel with hexanes/ethyl
acetate (8:1) to give 2-naphthoyl 4-piperidinobenzoyl methane as a
brown solid (770 mg, 54%). 1H NMR (300 MHz, CDCl3) δ 17.28 (s, 1H,
ArCOH), 8.56 (s, 1H, 1′-ArH), 8.06 – 7.91 (m, 6H, 3′, 4′, 5′, 6′, 7′, 8′-ArH),
7.64 – 7.55 (m, 2H, 2″, 6″-ArH), 6.99 – 6.95 (m, 3H, 3″, 5″-ArH,
COCHCO), 3.42 (m, 4H, 2, 6-piperidino H), 1.72 (s, 6H, 3, 4, 5-piperidino
H); MS (MALDI): m/z calculated for C24H23NO2 [M]+ 357.17, found 357.14.
β-Diketone Synthesis: Method A. The β-diketonate ligands nbm,
n(Br)bm, nb(Br)m, n(Br)b(Br)m (1a-4a) were prepared by Claisen
condensation in the presence of NaH as previously described.16
A
representative synthesis, given for 2a, is as follows. Acetophenone (400
mg, 3.30 mmol), methyl 6-bromo-2-naphthoate (1.16 g, 4.28 mmol) and
THF (20 mL) were added sequentially to a 50 mL round bottom flask.
After stirring the mixture for 10 min, a suspension containing sodium
hydride (NaH) (125 mg, 4.95 mmol) in THF (10 mL) was added dropwise
at room temperature under N2. The mixture was stirred for 20 h before
saturated aqueous NaHCO3 (1 mL) was added to quench the reaction.
THF was removed in vacuo before 1 M HCl (20 mL) was added. The
aqueous phase was extracted with CH2Cl2 (3 × 20 mL). The combined
organic layers were washed with distilled water (2 × 10 mL) and brine (10
mL), and dried over Na2SO4 before concentration in vacuo. The residue
was purified by column chromatography on silica gel eluting with
hexanes/ethyl acetate (6:1) to give 6-bromo-2-naphthoyl benzoyl
methane as a grey solid (755 mg, 65%). 1H NMR (300 MHz, CDCl3) δ
16.92 (s, 1H, COCHCOH), 8.50 (s, 1H, 1′-ArH), 8.06 - 8.02 (m, 4H, 3′, 4′,
5′, 8′-ArH), 7.84 (dd, J = 2.7, J = 8.7, 2H, 2″, 6″-ArH), 7.57 - 7.49 (m, 4H,
7′, 3″, 4″, 5″-ArH), 6.98 (s, 1H, COCHCO); MS (MALDI): m/z calculated
for C19H14BrO2 [M+H]+ 353.02, found 352.99.
6-Bromo-2-naphthoyl 4-piperidinobenzoyl methane, n(Br)b(pip)m, 6a.
The ligand n(Br)b(pip)m was prepared by Method B as described above
for 5a but with 4-piperidonoacetophenone (646 mg, 3.08 mmol), methyl
6-bromo-2-naphthoate (1.00 g, 3.70 mmol) and LDA (1.30
M in
heptanes/tetrahydrofuran/ethylbenzene, 2.85 ml, 3.70 mmol) in THF (40
mL). Crude 6-bromo-2-naphthoyl 4-piperidinobenzoyl methane obtained
after recrystallization from acetone was used for the next step to prepare
complex 6 without further purification (968 mg, 55%). Pure 1f was
obtained by hydrolysis of difluoroboron from 6 in methanol.9 1H NMR
(300 MHz, CDCl3) δ 17.22 (s, 1H, ArCOH), 8.48 (s, 1H, 1′-ArH), 8.05 (s,
1H, 5′-ArH), 8.02 (dd, J = 1.8, J = 8.7, 1H, 8′-ArH), 7.95 (d, J = 9.0, 2H, 2″,
6″-ArH), 7.85 – 7.81 (m, 2H, 3′, 7′-ArH), 7.62 (dd, J = 1.8, J = 8.7, 1H, 4′-
ArH), 6.92 (d, J = 9.0, 2H, 3″, 5″-ArH), 6.89 (s, 1H, COCHCO), 3.39 (m,
4H, 2, 6-piperidino H), 1.68 (s, 6H, 3, 4, 5-piperidino H);; MS (MALDI):
m/z calculated for C24H22BrNO2 [M]+ 435.08, found 435.06.
2-Naphthoyl benzoyl methane, nbm, 1a. The ligand nbm was prepared
by method A but with acetophenone (500 mg, 4.12 mmol), methyl 2-
naphthoate (1.03 g, 5.36 mmol) and sodium hydride (167 mg, 6.61 mmol)
in THF (40 mL). Column chromatography on silica gel eluting with
hexanes/ethyl acetate (6:1) to give 2-naphthoyl benzoyl methane as a
pale yellow solid (700 mg, 62%).. Data are in accord with the literature.2
Boron Complex Synthesis. Difluoroboron 2-naphthoyl benzoyl methane,
BF2nbm, 1. Boron trifluoride diethyl etherate (88 µL, 0.70 mmol) was
added at room temperature under N2 to a solution of 2-naphthoyl benzoyl
methane 1a (191 mg, 0.70 mmol) in CH2Cl2 (20 mL). The mixture was
refluxed for 2 h. The precipitate was filtered and recrystallized in acetone
to give difluoroboron naphthoyl benzoyl methane as a yellow solid (147
mg, 65%). Data are in accord with the literature.2
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10.1002/chem.201703513
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Difluoroboron 6-bromo-2-naphthoyl benzoyl methane, BF2n(Br)bm, 2.
The same method was employed as for 1, but with 6-bromo-2-naphthoyl
benzoyl methane 2a (208 mg, 0.59 mmol) and BF3·OEt2 (74 µL, 0.59
mmol) in CH2Cl2 (20 mL). Complex 2 was obtained as a yellow powder
after recrystallization in acetone (183 mg, 78%). 1H NMR (300 MHz,
CDCl3) δ 8.76 (s, 1H, 1′-ArH), 8.22 (s, 1H, 5′-ArH), 8.20 (m, 1H, 4′-ArH),
8.14 – 8.11 (m, 2H, 3′, 8′-ArH), 7.90 (d, 2H, J = 8.7, 2″, 6″-ArH), 7.76 –
7.69 (m, 2H, 3″, 5″- ArH), 7.62 – 7.57 (m, 2H, 3′, 7′-ArH), 7.32 (s, 1H,
COCHCO); MS (MALDI): m/z calculated for C19H11BBrF2O2Na [M+Na]+
422.99. Found 423.03. HRMS (ESI, TOF) m/z calculated for
C19H12BBrF2O2Na, 422.9979 [M + Na]+; found 422.9981.
grant number R01CA167250.
The Goldwater Scholarship
Foundation is acknowledged for a fellowship to Ruffin E. Evans.
The research fund BABKO of Marmara University provided
support to Zikri Altun. We thank Nguyen D. Nguyen and Alan D.
Chien for their assistance, and Prof. J. N. Demas for helpful
discussions.
Keywords: Boron Complexes • Phosphorescence • Heavy Atom
• Charge Transfer • Oxygen Sensing
Difluoroboron 2-naphthoyl 4-bromobenzoyl methane, BF2nb(Br)m, 3. The
same method was followed as for 1, but with 2-naphthoyl 4-
bromobenzoyl methane (191 mg, 0.54 mmol) and BF3·OEt2 (68 µL, 0.54
mmol) in CH2Cl2 (20 mL). Complex 3 was obtained as a yellow powder
after recrystallization (2×) in acetone (64 mg, 30%). 1H NMR (300 MHz,
CDCl3) δ 8.80 (s, 1H, 1′-ArH), 8.12 – 7.92 (m, 6H, 3′, 4′, 5′, 6′, 7′, 8′-ArH),
7.75 – 7.61 (m, 4H, 2″, 3″, 5″, 6″-ArH), 7.30 (s, 1H, COCHCO); MS
(MALDI): m/z calculated for C19H11BBrF2O2Na [M+Na]+ 422.99. Found
422.96. HRMS (ESI, TOF) m/z calculated for C19H12BBrF2O2Na,
422.9979 [M + Na]+; found 422.9986.
References
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4-bromobenzoyl
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[6]
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bromo-2-naphthoyl 4-bromobenzoyl methane (405 mg, 0.94 mmol) and
BF3·OEt2 (178 µL, 1.41 mmol) in CH2Cl2 (50 mL). Complex 4 was
obtained as a grey powder after recrystallization (3×) in dichloromethane
(272 mg, 61%). 1H NMR (300 MHz, CDCl3) δ 8.75 (s, 1H, 1′-ArH), 8.18 –
8.04 (m, 4H, 3′, 4′, 5′, 8′-ArH), 7.91 – 7.88 (d, J = 8.4, 2H, 2″, 6″-ArH),
7.75 – 7.72 (d, J = 8.4, 2H, 3″, 5″-ArH), 7.28 (s, 1H, COCHCO); MS
(MALDI): m/z calculated for C19H10BBr2F2O2Na [M+Na]+ 502.90. Found
502.81. HRMS (ESI, TOF) m/z calculated for C19H11BBr2F2O2Na,
500.9085 [M + Na]+; found 500.9106.
C. Wu, B. Bull, K. Christensen, J. McNeill, Angew. Chem. Int. Ed. 2009,
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Difluoroboron 2-naphthoyl 4-piperidinobenzoyl methane, BF2nb(pip)m, 5
The same method was followed as for 2, but with 2-naphthoyl 4-
piperidinobenzoyl methane (134 mg, 0.38 mmol) and BF3·OEt2 (118 µL,
0.98 mmol) in CH2Cl2 (30 mL). The crude product was purified with
column chromatography (silica gel, hexanes-EtOAc, 2:1) to afford 5 as
an orange solid (132 mg, 88%). 1H NMR (300 MHz, CDCl3) δ 8.71 (s, 1H,
1′-ArH), 8.10 – 7.88 (m, 5H, 3′, 4′, 5′, 6′, 8′-ArH), 7.65 – 7.54 (m, 3H, 6′-
ArH, 2″, 6″-ArH), 7.13 (s, 1H, COCHCO), 6.88 (d, J = 9.3, 2H, 3″, 5″-ArH),
3.53 (m, 4H, 2, 6-piperidino H), 1.72 (s, 6H, 3, 4, 5-piperidino H); MS
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Entry for the Table of Contents
FULL PAPER
Tiandong Liu, Prof. Dr. Guoqing
Zhang, Ruffin E. Evans, Prof. Dr.
Carl O. Trindle, Prof. Dr. Zikri
Altun, Prof. Dr. Christopher A.
DeRosa, Fang Wang, Meng
Zhuang, and Prof. Dr. Cassandra
L. Fraser*
Page No. – Page No.
Phosphorescence Tuning via
Heavy Atom Placement in
Unsymmetrical Difluoroboron
Diketonate Materials
b
-
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