Boosting the triplet activity of heavy-atom-free difluoroboron dibenzoylmethane via sp3 oxygen-bridged electron donors

Article abstract of DOI:10.1039/c8cc08346k

Purely organic phosphors have important applications in imaging, sensing, informatics and illumination. Methoxy-substituted difluoroboron dibenzoylmethane (BF₂dbm) complexes exhibit intense fluorescence with an almost unity quantum yield. Here

Full text of DOI:10.1039/c8cc08346k

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Boosting the triplet activity of heavy-atom-free
difluoroboron dibenzoylmethane via sp3
oxygen-bridged electron donors†
Cite this: Chem. Commun., 2019,
55, 67
Received 19th October 2018,
Accepted 23rd November 2018
Wenhuan Huang,a Xuepeng Zhang,a Biao Chen, *a Hui Miao,a Carl O. Trindle,b
a
a
Yucai Wang,
Yi Luoa and Guoqing Zhang
*
DOI: 10.1039/c8cc08346k
rsc.li/chemcomm
Purely organic phosphors have important applications in imaging, heavy atoms, nor does it require functional groups strongly
sensing, informatics and illumination. Methoxy-substituted difluoro- enhancing spin–orbit coupling (SOC) such as carbonyl, nitro or
boron dibenzoylmethane (BF2dbm) complexes exhibit intense fluores- amine substitution. Instead, the sp3 oxygen-bridged electron
cence with an almost unity quantum yield. Here we show that by donor moiety significantly boosts the triplet state yield via a
simply introducing an sp3 oxygen-bridged methoxylphenyl group as a combination of SOC related to transition angular momentum and
pendant to BF2dbm, the boron complex exhibits a triplet quantum charge-transfer (CT) mediated intersystem crossing (ISC).28,29
yield of 0.16, a more than 100-fold increase compared to that of
BF2dbm.
Three BF2dbm model complexes 1–3 (Fig. 1a) were readily
synthesized by Claisen condensation and subsequent coordina-
tion with boron trifluoride in high yield (overall yields 450%,
Photoluminescent organoboron molecules are routinely inves- Scheme S1; for synthetic procedure and characterization, see
tigated for their exceptional photophysical properties and Fig. S10–S18 in ESI†), where the oxygen atom is linked to methyl,
have found applications in optical sensing1,2 and light-emitting phenyl and p-methoxylphenyl to produce 1, 2 and 3, respectively,
devices.3–7 Among these reports on organoboron materials, and provides a twist angle between the BF2dbm core and the
difluoroboron diketonates (BF2bdks)8–10 have received tremen-
dous attention due to their strong interaction with visible light,
high fluorescence quantum yields, tunable emission features,
long-lived phosphorescence in polymer matrices, and mechano-
chromic luminescence (ML) in the solid state.11–15 In order to
increase the relative quantum yield of the oxygen-sensitive,
emissive triplet excited state of BF2bdks, halogen-substitution
has been used in various applications such as tumour oxygen
imaging,16–20 wound recovery monitoring21,22 and ML quench-
ing23–27 due to the enhanced heavy-atom effect with boosted
triplet activity. Otherwise, the nearly unity fluorescence from
BF2bdks would have precluded most applications based on a
balanced ratio between fluorescence and phosphorescence.
In addition, the halogen atom can be photochemically unstable,
toxic and unfriendly to the environment, which makes heavy-
atom free BF2dbm with high triplet activity an attractive sub-
stitute. Here we show a new methodology, where the fluorescent
species can be transformed into phosphorescent ones by minor
alterations in the molecular structure. The strategy employs no
Fig. 1 (a) Chemical structures of BF2dbm model complexes 1–3; (b) UV/Vis
absorbance and steady-state emission (lex = 372 nm) spectra of model
complexes 1–3 in m-THF at room temperature, inset: image showing
PL emission in m-THF at RT at an identical concentration of 4.95 mM;
(c) steady-state emission spectra of model complexes 1–3 in m-THF at low
temperature (77 K, LT) (lex = 372 nm), inset: image showing steady-state PL
emission in m-THF at 77 K; (d) delayed (Dt = 100 ms) emission spectra of
model complexes 1–3 in m-THF at 77 K, inset: image showing afterglow
100 ms after excitation (using a 365 nm handheld lamp) had ceased with the
aid of a mechanical shutter.
a Hefei National Laboratory for Physical Sciences at the Microscale,
University of Science and Technology of China, Hefei, Anhui 230026, China.
E-mail: gzhang@ustc.edu.cn, biaochen@ustc.edu.cn
b Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cc08346k
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Chem. Commun., 2019, 55, 67--70 | 67

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pendant group. The absorption and emission spectra in dilute
solutions (2-methyl-tetrahydrofuran, m-THF, 4.95 mM) are pre-
sented in Fig. 1b, where complex 1 has been investigated by
many research groups including us, both in solutions and in
the solid state.13,30 All three compounds have almost the same
absorption between 350 and 420 nm (lmax = 396, 393 and
395 nm, respectively); the slight blue shift for 2 is likely due
to the electronic conjugation between the oxygen lone pair and
the phenyl group, which reduces the donating ability of the
lone pair. Both 1 and 2 emit strong blue photoluminescence
(PL, lem = 420 nm, FF = 0.97 and 0.81) at room temperature
under UV irradiation at 372 nm, whereas, interestingly, 3
exhibits extremely attenuated blue PL as shown in the inset
(FF o 0.001, Fig. 1b and Table 1). It is not uncommon for
PL molecules to show reduced fluorescence yield with more
substituent groups due to extra vibrational and rotational
modalities that could dissipate the energy in the excited state.
In this case, however, the reduction in PL quantum yield from 2
to 3 (from nearly unity to nothing) is far too significant to
attribute to nuclear motions. A more reasonable explanation
would be fluorescence quenching from a dark, CT-mediated
ISC process, as has been noted from other donor-sp3-acceptor
systems.28 As shown in Fig. 1c, 3 exhibited strong photo-
luminescence at 77 K with a strong phosphorescence peak at
B500 nm (tP = 1.41 s) and a calculated phosphorescence yield
of 0.16, while the value for 1 was almost negligible.
Fig. 2 Steady-state emission spectra of model complexes 1–3 in thin
PMMA films at room temperature in air (a) and under vacuum (b) (lex
=
372 nm); (c) time-resolved emission decay curves of 3 at 510 nm in air and
under vacuum; delayed (Dt = 100 ms) emission spectra of model com-
Purely organic, heavy-atom-free RTP molecules have wide plexes in PMMA at RT (d) and 77 K (e) and emission spectra of model
complexes 1 (f), 2 (g), and 3 (h) in PMMA at RT (Ss: steady state emission, DE
in air: delayed emission in air (Dt = 50 ms), DE in vacuum: delayed emission
in vacuum (Dt = 50 ms)).
applications in optical sensing, bioimaging and illumina-
tion.31–41 Many applications require that small molecular phos-
phors be embedded into rigid polymer matrices as an efficient
approach to achieve RTP due to suppression of molecular
motions in these systems (restriction of non-radiative relaxa-
Table 2 Luminescence properties of 1, 2 and 3 in PMMA films (0.1% by
tions).42,43 All three molecules were dissolved in PMMA films
weight) at room temperature
(film thickness B200 mm, solute 0.1% by weight) and the
a
b
c
d
e
lF (nm)
lRTP (nm)
tF (ns)
tRTP (ms)
tRTP (s)
steady-state PL emissions in air and under vacuum were mea-
sured as shown in Fig. 2a and b, respectively. The PL emissions of
1 and 2 are similar with a single fluorescence peak at 433–434 nm
(tF = 2.2–2.3 ns, Table 2) in PMMA films while that of 3 is
significantly broadened in the lower energy region (450–650 nm).
We ascribed the broadening to the enhanced ISC and sub-
sequently boosted RTP with polymer protection. Indeed, a life-
1
2
3
434
433
440
510
510
510
2.27
2.15
2.00
—
—
250
0.70
0.58
0.31
a
Fluorescence emission maximum from steady-state spectra (lex
=
=
b
372 nm) in PMMA at room temperature. RTP maximum at RT (lex
c
372 nm). FL lifetime fitted from single-exponential decay (lex = 372 nm,
d
nanoLED) at RT. RTP lifetime fitted from single-exponential decay (lex
=
time of 250 ms (Fig. 2c inset) was recorded for 3 in the low energy 370 nm, spectraLED) at RT in air, note that RTP emissions of 1 and 2 in air
e
are too weak (if any) to measure. RTP lifetime fitted from single-
exponential decay (lex = 370 nm, spectraLED) at RT in vacuum.
Table 1 Luminescence properties of 1, 2 and 3 in m-THF
shoulder band area (500–550 nm). Under vacuum, the relative
a
b
c
d
e
f
g
lF (nm) lP (nm) tF (ns) tLP (s) FPL
FPL FP
intensity of the broad shoulder band is further increased, which
confirms the oxygen-sensitive nature of the RTP from 3. The RTP
lifetime exhibits a concomitant increase by more than 1000 fold
to 310 ms (Fig. 2c). Although the relative RTP intensity is much
lower for 1 and 2, their lifetimes are nonetheless much longer,
which can be explained by Fig. 2d and e. The delayed emission
1
2
3
420
414
415
502
497
539
1.75
1.60
1.25
1.30
1.41
0.97
0.81
o0.001 0.30
0.97 o0.01
0.82
0.045
0.16
h
—
a
Fluorescence emission maximum from steady-state emission spectra
(lex = 372 nm) in m-THF at 77 K. LP maximum at 77 K (lex = 372 nm).
FL lifetime fitted from single-exponential decay (lex = 374 nm, nanoLED)
b
c
d
at 77 K Phosphorescence lifetime fitted from single-exponential decay spectra recorded not only show the main RTP band at 510 nm
e
but also the thermally activated delayed fluorescence (TADF)44–47
(lex = 372 nm, spectraLED) at 77 K. Absolute quantum yield from
400–600 nm at RT. Absolute quantum yield from 400–600 nm at 77 K.
Calculated phosphorescence quantum yield at 77 K by spectral sub-
traction. Signal too weak for accurate measurement.
f
g
band at around 430 nm, which disappears at 77 K (Fig. 2e). The
RTP bands for all three complexes 1–3 are almost super-imposable,
h
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indicating the identical origin from the localized p–p* state and Tables S1–S3 (ESI†). Briefly speaking, the energy gaps
from the BF2dbm core structure, as has been previously between the S1 state (localized p–p*) and the neighbouring lowest
reported.13,30,48 The fact that 3 alone exhibits the most intense triplet state for both 1 and 2 are almost identical (B0.22 eV), while
TADF peak strongly suggests multiple energy pathways for ISC, that of 3 (1CT) exhibits a sharp decrease to 0.025 eV, readily
most likely a combination of localized (B430 nm) and other bifurcating to T3 and T2. Specifically, MO analysis (Fig. 3a) shows
mediating states (4440 nm), such as CT, in between 1p–p* and little if any CT character in 1’s reddest transition. There is also
3p–p*.44,45 Higher TADF ratios generally promise shorter RTP substantial splitting between S1 and T1 dominated by the
lifetimes, irrespective of thermal activation or triplet–triplet HOMO–LUMO excitation. This is consistent with a large
annihilation. In addition, the similarity in TADF peak intensity exchange integral K (HOMO–LUMO). Strong fluorescence emis-
of 1 and 2 also points to the fact that vibrational coupling sion ( f = 1.04, experimentally FF B 1) probably also helps to
caused by the additional phenyl ring plays a minor role in hinder effective passage to the nearest triplet state. When
effectuating the ISC process. Fig. 2f–h compare the steady-state O-phenyl replaces O-methyl to form 2, the reddest (HOMO–
and delayed emission spectra of 1–3, respectively. It is evident LUMO) transition in the absorption spectrum is hardly shifted,
that RTP is dramatically more sensitive to oxygen quenching vs. and the MOs (Fig. 3b) involved are still confined to the core
TADF given that air quenches most RTP while TADF persists. defined by 1. Little CT character is seen in the transition. The
We were also able to record the TADF spectra in different further substitution of a para MeO-fragment moves the MO on
solvents (Fig. S33 and S34, ESI†); it was found that the TADF the O-phenyl-OMe up in energy; which becomes the new HOMO
intensity is the lowest in the most polar solvent. This observa- (Fig. 3c). The LUMO is still located at the BF2dbm core, so the
tion is attributed to increased non-radiative decay rate in polar HOMO–LUMO transition has a substantial CT character. This is
solvents as has been previously reported by Adachi et al.49
illustrated by the small exchange splitting between the singlet
To further understand the photophysics of the sp3-bridged and triplet states defined by those two MOs (only 0.025 eV) and
donor effects, we performed quantum chemical calculations the relative weakness of the lowest singlet-state transition
and modelled their molecular geometries in the ground and ( f = 0.016). These features of 3 are preserved in the excited S1
excited states (singlet and triplet) using a previously established state (Fig. 3c, splitting 0.01 eV and f o 0.01). This defines a
method.50,51 Theoretical modeling for this sequence was diradical character and implies a long lifetime for the S1 state.
accomplished with the functional B3PW91 and the basis All these features suggest efficient passage to the triplet state
6-311+G(d), proved by Barlow and co-workers to be effective and subsequent strong RTP emission of 3.
in describing the charge-transfer excitations.52 The medium
In summary, we have successfully synthesized three difluoro-
was modeled by the polarizable continuum defined by Tomasi, boron dibenzoylmethane derivatives with the methyl, phenyl and
with parameters for THF.53 Charge transfer plays a crucial role p-methoxylphenyl as the ‘‘pedant’’ groups to the BF2dbm core,
in populating the triplet excited states as indicated in Fig. 3a–c respectively. Despite the subtle structural change, the molecules
Fig. 3 Calculated molecular orbitals for the lowest singlet-state transition for all three boron complexes. Schematic diagrams of the TD-DFT calculated
energy levels and possible ISC channels of 1 (a), 2 (b) and 3 (c) at the singlet (S1) and triplet (Tn) states. H and L represent the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. The insets are the Kohn–Sham frontier orbitals obtained from
all three complexes; and (d) calculated molecular geometries at the ground state (S0), first triplet excited state (T1) and first singlet excited state (S1) for all
three complexes.
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We thank the National Key R&D Program of China
(2017YFA0303500 to G. Z. and Y. L.) and the Fundamental
Research Funds for the Central Universities (WK2340000068
to G. Z.) for their financial support.
Conflicts of interest
There are no conflicts to declare.
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