Effect of Branching on the Delayed Fluorescence and Phosphorescence of Simple Borylated Arylamines

Article abstract of DOI:10.1021/acs.inorgchem.9b03446

A donor-π-acceptor strategy is being well exploited in several fields in view of their robust optical properties. However, the impact of branching in quadrupolar [A-(π-D)₂] and octupolar [A-(π-D)₃] molecules in comparison to parent d

Full text of DOI:10.1021/acs.inorgchem.9b03446

pubs.acs.org/IC
Article
Effect of Branching on the Delayed Fluorescence and
Phosphorescence of Simple Borylated Arylamines
Sudhakar Pagidi, Neena K. Kalluvettukuzhy, and Pakkirisamy Thilagar*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A donor-π-acceptor strategy is being well exploited in several fields in view
of their robust optical properties. However, the impact of branching in quadrupolar [A-(π-
D)2] and octupolar [A-(π-D)3] molecules in comparison to parent dipolar (A-π-D)
molecules on the delayed fluorescence and phosphorescence properties is seldom explored.
We have presented herein the distinct and contrasting optical properties of a tridurylborane
core bearing −NH2 (1−3) and −NMe2 (4−6) donor moieties, wherein the number of
donors is increased systematically. Because of propeller molecular architecture, the donor
and acceptor are weakly coupled, and the frontier molecular orbitals are spatially localized.
All of the compounds show delayed fluorescence under ambient conditions and persistent
phosphorescence at low temperature. Solvent-dependent studies and temperature-
dependent luminescence measurements established that quadrupolar (2 and 5) and
octupolar (3 and 6) compounds underwent symmetry breaking in the excited state.
Curiously, delayed fluorescence and phosphorescence spectra are found to be blue-shifted
and follow the same trend as the fluorescence upon an increase in the branches. The highest quantum yield was observed for dipolar
compounds. Besides, the phosphorescence lifetime decreases with an increase in the number of branches. These interesting
experimental observations are further supported by quantum-mechanical calculations.
(TADF) phenomenon.9−11 Still, there is a lot of room to
understand the delayed fluorescence (DF) in molecules with
subtle perturbations in electronic and steric factors, in
particular, the branching effects on the DF.
INTRODUCTION
Harnessing the electronic interactions between the electron
donors and acceptors is of paramount importance in view of
■
their potential applications in optoelectronics, sensors, and
bioimaging.1−3 Electron delocalization is further manipulated
The outstanding optical properties of the π-conjugated
boron (B) compounds arise from pπ−π* conjugation of the
by increasing the number of donors or acceptors, thereby
generating quadrupolar and octupolar systems. Such com-
vacant p orbital on boron with the π* orbital of the conjugated
π system.12 Triarylborane (TAB)-containing π-conjugated
pounds find suitable applications in two-photon absorption
and solar cells.4−6 It has been established that multipolar
compounds show enhanced efficiencies over the parent dipolar
systems. Most of the reported multipolar molecules rely on the
triphenylamine core with different acceptor units.7,8 The
effects of branching (arm) in the case of quadrupolar [D-(π-
A)2] and octupolar [D-(π-A)3] molecules on the photophysical
properties are investigated in detail.4−8,11,15b,c The more
pronounced solvatochromic features of these compounds
were explained on the basis of a charge-delocalized/localized
intramolecular charge-transfer (ICT) process. In nonpolar
solvents, the charge delocalized to all of the arms, whereas in
polar solvents, the charge localized on one of the branches,
pointing to symmetry breaking in the excited state prior to the
emission.7,8,17 The effects of branching on the fluorescence
emission of various types of compounds are well studied;
however, phosphorescence is relatively ill-defined. In recent
times, delayed fluorescence materials have gained rapid
momentum in view of their potential applications in
optoelectronics.9 Numerous publications have boosted our
understanding of the thermally activated delayed fluorescence
donor−acceptor systems have been effectively used in organic
light-emitting diodes (OLEDs),13 nonlinear optics,14 organic
field-effect transistors,15 fluorescent thermometers,16 sensors,17
and bioimaging.18 TAB compounds with multidimensional
ICT properties are of particular interest in view of their
promising photophysical properties and robust applica-
tions.14,18b,c Even though the chemistry of TAB-based D−A
systems is well studied and explored in almost all the fields of
materials science and biology, the impact of branching on their
photophysical properties, in particular DF and phosphor-
escence, has seldom been observed.11−18 Most of the reported
TAB-based multipolar [D-(π-A)n; n = 1, 2, or 3] compounds
showed a bathochromic shift in the absorption and emission
bands with increasing electron-donating strength of the
Received: December 2, 2019
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 1. Synthesis of TDB-Based Molecules 1−6
substituents.19−21 However, most of the triphenylamine-based
multipolar [D-(π-A)n; n = 1, 2, or 3] systems show a blue shift
in the absorption and emission spectra with increasing
branches/peripheral acceptor units.7d In order to understand
such anomalies as well as the effects of branching on the
absorption, fluorescence and the unexplored DF and
phosphorescence in these multipolar systems with a systematic
increase from 1D to 2D to 3D counterparts will be an
interesting topic to be studied. Owing to the promising and
upcoming applications of TAB in optoelectronics, we have
chosen sterically demanding tridurylborane (TDB) as the
electron acceptor moiety and simple −NH2 units as donors
(1−3). In another series (4−6), the −NH2 donors were
replaced with −NMe2 moieties to further see the effect of
steric and electronic factors on their optical attributes.22a We
found that these compounds exhibit distinct and contrasting
optical properties; in particular, the effects of branching on the
DF and phosphorescence are prominent. Besides, symmetry
breaking in quadrupolar and octupolar compounds is further
confirmed by variable-temperature and time-resolved lumines-
cence measurements apart from regular solvent-dependent
studies. These interesting optical outcomes are discussed in
detail in this Article.
estimated from the temperature-dependent H NMR reso-
nances of compounds in the region 1.80−2.20 ppm
(corresponding to the −CH3 groups attached to the duryl
moieties). The coalescence temperature (Tc) and free energy
of activation (ΔG)23,24 were calculated for 1 (Tc = 303 K and
ΔG = 19 kcal/mol), 2 (Tc = 303 K and ΔG = 18.8 kcal/mol),
4 (Tc = 303 K and ΔG = 19.5 kcal/mol), and 5 (Tc = 313 K
and ΔG = 20.5 kcal/mol). The calculated activation energies of
1, 2, 4, and 5 are in good agreement with the values reported
for D−A systems with TAB as the core.23,24 Powder X-ray
diffraction analysis revealed that the pristine samples of 1, 2, 4,
and 5 are amorphous, while 3 and 6 are crystalline in nature
(Figure S33).
1
Molecular Structures. Single crystals of 3 and 6 suitable
for X-ray diffraction studies were obtained by the slow
evaporation of dichloromethane (DCM) and tetrahydrofuran
(THF) solutions, respectively, under ambient conditions
(Table S1). Multiple attempts for the crystallization of other
compounds under various conditions were unsuccessful.
Compound 3 crystallized in triclinic crystal system with P1
̅
space group and compound 6 in monoclinic crystal system
with C2/c space group. In the crystal structure of 6, the
crystallographic C2 axis coincides with the molecular axis along
the B1−C13 bond. Compounds 3 and 6 adopt a propeller-like
structure in which the boron center possesses a trigonal-planar
geometry, with the sum of the C−B−C angle around boron
being ∼360° (Figure 1). The peripheral nitrogen atoms of the
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Tris(bromoduryl)-
borane (3a) was synthesized by following the literature
procedure reported by Yamaguchi et al., which involves
selective monolithiation of 1,4-dibromodurene with n-
butyllithium in diethyl ether, followed by quenching with
BF3·OEt2.16 Further, selective mono- or dilithiation of 3a with
n-butyllithium, followed by quenching with water, yielded 1a
and 2a, respectively (Scheme S1). Compounds 1−6 were
prepared according to a recently reported procedure.22b The
synthesis of 1−3 involves the heating of a mixture of sodium
azide, L-proline, copper(I) iodide, and the corresponding p-
bromo(tridurylborane) derivatives 1a−3a, respectively, at 100
°C in dimethyl sulfoxide (DMSO). Selective para-N,N-
dimethylation of 1−3 using sodium hydride and methyl iodide
quantitatively yielded 4, 5 and 6, respectively (Scheme 1). All
of the compounds are stable under ambient conditions.
Compounds 1−6 were characterized by NMR (1H, 13C, and
11B) and high-resolution mass spectrometry (Figures S1−S28).
1H NMR analysis revealed that compounds 3 and 6 show a
simple NMR spectral pattern indicating higher symmetry,
while compounds 1, 2, 4, and 5 show a complex pattern
pointing to the lower symmetry in a CDCl3 solution at 25 °C.
1H NMR spectra of 1, 2, 4, and 5 were recorded in toluene-d8
at different temperatures between 213 and 323 K (Figure
S29−S32). The rotation barrier around the B−C bond was
Figure 1. ORTEP diagrams of the molecular structures of 3 (left) and
6 (right) with thermal ellipsoids drawn at 50% probability. Color
code: carbon, gray; nitrogen, blue; boron, pink. All of the hydrogen
atoms, except that of the −NH2 unit in 3, are omitted for clarity. The
asymmetric unit in the crystal of 3 contains one molecule of 3 and a
THF molecule, which is removed for clarity.
amines in both 3 and 6 adopt a pyramidal geometry with sums
of the total bond angle of ∼357° and ∼351°, respectively.
Replacement of the hydrogen atoms with CH3 groups on the
nitrogen atom in 6 induces a little pyramidalization for steric
reasons. The dihedral angle between the B−C2 plane and the
duryl spacer was found to be in the ranges of 47−59° for 3 and
48−52° for 6, indicating a propellerlike arrangement of the
B
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 2. Absorption (left) and fluorescence emission (right) spectra of 1−6 (solvent = DCM; concentration = 10 μM; λex = 370 nm for 1−3 and
330 nm for 4−6).
three duryl groups. The sterically demanding CH3 group on
the nitrogen leads to elongation of the C(duryl)−N bond in 6
(1.432 Å) compared to that in 3 (1.389 Å). The structural data
are in line with the results reported elsewhere by Yamaguchi et
al.19 and Song et al.24 for TDB derivatives.
case of 6 corresponds to the S0 → S4 (f = 0.0114; λcal. = 339
nm) transition. The calculated highest occupied molecular
orbital (HOMO)−lowest unoccupied molecular orbital
(LUMO) energy gap followed the same trend as that observed
in the experimental UV−vis absorption spectra (vide infra).
Yamaguchi and co-workers observed a bathochromic shift in
absorption and fluorescence emission with increasing number
of donors in a series of tris(phenylethynylduryl)borane
derivatives, which was explained by the extent of π
conjugation.19 Later in 2006, Lambert et al. compared the
optical and electrochemical properties of mono- and tris-
(carbazole)-substituted TABs. Charge-transfer (CT) absorp-
tion and fluorescence emission of the compounds showed a
bathochromic shift with increasing number of carbazole units,
attributed to the electronic coupling between the subchromo-
phores of the trimer. Further, the absorption and fluorescence
emission spectra showed negative and positive solvatochrom-
ism, respectively.20 On the basis of these literature reports and
our experimental observations, we tentatively concluded that
the bathochromic shift observed in 4−6 with increasing donor
units can be attributed to the exciton coupling between the
branches.
Solvent-dependent absorption properties of 1−6 were
investigated to understand the effect of branching in the
ground state by varying the solvent dielectrics (Figure S34 and
Table S2). The absorption bands of compounds were sensitive
to the changes in solvent polarity, indicating that they originate
from the CT transition arising from amine to the boryl unit.
For 1−3, the solvatochromic features of CT absorption bands
were more profound compared to those of 4−6. Apparently,
there seems to be symmetry breaking for quadrupolar (2) and
octupolar (3) compounds in the ground state. Very weak
intramolecular D−A interactions in 4−6 in the ground state
are indicated by their negligible dependence of absorption
features upon variation of the solvent polarity.
UV−Vis Absorption and Emission Properties. In the
UV−vis absorption spectra, 1 exhibits two clearly separated
absorption bands at ∼321 and ∼362 nm, while 2 shows a band
at ∼366 nm with an unresolved shoulder peak at ∼345 nm,
whereas a broad absorption band with an absorption maximum
at ∼362 nm is observed for 3. There are no profound changes
in the energies of absorption maxima. Only an increase in the
molar extinction coefficient (ε) values observed upon an
increase in the number of branches in 1−3 reveals their
independent behavior (Table S2). Compounds 4−6 exhibited
a broad absorption band within the region 300−400 nm
(Figure 2). A weak absorption band in the tailing region (360−
375 nm) was observed for 4−6 in addition to major broad
absorption bands at ∼325 nm for 4, ∼330 nm for 5, and ∼345
nm for 6. Unlike 1−3, the absorption maxima showed a
bathochromic shift with increasing numbers of donors in the
case of 4−6. Density functional theory (DFT)/time-dependent
DFT (TD-DFT) calculations were carried out to understand
the nature of electronic transitions involved in the UV−vis
absorption features of the compounds, and the major
transitions corresponding to the absorption bands are depicted
in Table S9. The long-wavelength absorption band of 1 at
∼362 nm is assigned to the S0 → S1 (f = 0.1597; λcal. = 368
nm) transition, while high-energy band at ∼321 nm is assigned
to the combined electronic transitions of S0 → S2(f = 0.0083;
λcal. = 323 nm) and S0 → S3 (f = 0.0121; λcal. = 324 nm). For 2,
the absorption band at ∼366 nm is attributed to the S0 → S1(f
= 0.1832; λcal. = 373 nm) electronic transition and the shoulder
band at 345 nm closely matches the S0 → S2 (f = 0.0968; λcal.
=
354 nm) electronic transition. The broad absorption band for
3 around ∼362 nm is ascribed to the combined electronic
transitions of S0 → S1 (f = 0.1834; λcal. = 373 nm) and S0 → S2
(f = 0.2016; λcal. = 362 nm). In the case of 4−6, the unresolved
band in the tailing region of major absorption bands were
attributed to S0 → S1 (f = 0.0548; λcal. = 369 nm) in the case of
4, S0 → S1 (f = 0.0895; λcal. = 374 nm) and S0 → S2 (f =
0.0217; λcal. = 368 nm) in the case of 5, and S0 → S1 (f =
0.0912; λcal. = 377 nm) and S0 → S2 (f = 0.0787; λcal. = 373
nm) in the case of 6. The broad absorption bands in the high-
energy region were assigned to electronic transitions such as S0
→ S2 (f = 0.0122; λcal. = 334 nm) and S0 → S3 (f = 0.0109; λcal.
= 332 nm) for 4 and S0 → S3 (f = 0.0097; λcal. = 336 nm) and
S0 → S4 (f = 0.0140; λcal. = 336 nm) for 5, whereas that in the
It is well-known that the −NH2 unit is a weaker electron
donor than the −NMe2 unit; hence, one should expect more
bathochromic shift for the absorption bands of 4−6 than those
of 1−3. Surprisingly, the CT absorption bands of 1−3 showed
a bathochromic shift compared to those of 4−6 probably
because of the imposed steric repulsions between the methyl
groups on nitrogen and the duryl rings in 4−6, which further
reduce the electronic coupling between the amines and boron.
These results clearly show that not only electronic features but
also steric factors are playing a role in controlling their optical
properties.30 These observations are in line with the calculated
ground-state dipole moments of 1−6 (Table S3).
C
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 3. Temperature-dependent emission spectra of compounds (a) 1 and (b) 4 (solvent = MTHF; concentration = 10 μM; λex = 370 nm for 1
and 330 nm for 4).
Unlike absorption spectra, a structureless broad emission
band was observed for 1−6 in a DCM solution upon excitation
at their corresponding absorption maxima, which could be
attributed to the emission stem from a CT state (Figure 2).
With increasing number of donors on the TDB core, a
hypsochromic shift in the fluorescence emission spectra was
observed in both the −NH2 (1−3) and −NMe2 (4−6) series.
Compounds with −NMe2 substituents (4−6) showed a red-
shifted emission compared to their corresponding −NH2
derivatives (1−3, respectively), opposite to the trend observed
in their UV−vis absorption spectra. Compounds 4−6 showed
higher Stokes shifts compared to their corresponding N-
unsubstituted 1−3 analogues. A gradual decrease in the Stokes
shift was also observed with increasing number of branches in
both 1−3 and 4−6. The hypsochromic shift with increasing
number of branches due to the repulsive dipole−dipole
interactions between branches results in a slight increase of
the S0 → S1 energy gap. Compounds 1−6 showed positive
solvatofluorochromism with increasing solvent dielectric
parameter (Figure S35 and Table S2). The strong solvent-
dependent characteristics of 2, 3, 5, and 6 clearly indicate
symmetry breaking in the excited state for quadrupolar and
octupolar compounds, giving rise to a dipolar state. The
formation of the dipolar CT state is confirmed by the large
Stokes shifts and higher exited-state dipole moments compared
to that of the ground state (Table S3). To obtain a further
understanding of the emissive states, time-resolved fluores-
cence measurements were carried out in nonpolar (hexane),
medium-polar (DCM), and high-polar (DMSO) solvents. The
fluorescence lifetime is found to increase with increasing
solvent polarity, further confirming that the emissive state
possesses CT characteristics (Table S4).
Compounds are brightly emissive in the solid state (Figure
S36). Notably, with increasing number of branches, a
significant hypsochromic shift was observed in the solid state
for both −NH2 and −NMe2 derivatives. Very recently, Yang et
al. incorporated mono-, di-, and tris(phenoxazine) (PXZ)
donor units into TDB and then exploited them for OLED
applications. They observed a red shift in the emission and an
increase in the luminescence quantum yield upon an increase
in the number of donors in the thin-film state owing to the
increased electronic delocalization among the branches.21
Compounds 1−6 showed red-shifted fluorescence emission
compared to their respective emission in hexane solutions,
which can be attributed to the effect of intermolecular
interactions in the solid state. The absolute solid-state
quantum efficiencies of 1−6 are found to be 28%, 10%, 8%,
29%, 19%, and 10%, respectively, which shows that the
increase of branches elevates the nonradiative decay channels.
The above inference was further supported by the gradual
increase in the magnitude of nonradiative decay rate constants
with increasing number of branches in both the 1−3 and 4−6
series (Table S5).
Temperature-Dependent Luminescence. In order to
further confirm the symmetry breaking at the excited state and
the formation of the dipolar charge-transfer state, we carried
out temperature-dependent (298−77 K) steady-state lumines-
cence measurements for 1−6 in a 2-methyltetrahydrofuran
(MTHF) solution (Figures 3 and S37 and 38) because the
charge-transfer state is sensitive to the environment.
Compound 1 showed a structureless broad emission band
(fwhm = ∼96 nm) at ∼520 nm in a MTHF solution at 298 K
(λex = 370 nm). As the temperature decreased from 298 to 140
K, a gradual red shift of ∼32 nm was observed. This can be
attributed to an increase in the solvent dielectric constant and
thus stabilization of the CT state. Cooling to the glass
transition temperature of MTHF (∼95 K) resulted in a blue
shift of the emission, ascribed to the slight increase of the
solvent relaxation time.25,26 A further cooling (<95 K) showed
a pronounced blue-shifted emission (λem = 462 nm), and no
significant changes in the emission spectra could be seen even
after cooling to 77 K. Notably, the emission (λem = 462 nm) at
77 K is comparable to the emissions in the nonpolar toluene
solvent (λem = 467 nm) and solid state (λem = 470 nm). This is
due to the fact that the fluorescence time scales are much faster
than the solvent relaxation times in the nonpolar frozen glassy
matrix of MTHF below 95 K.25,26 Interestingly, we could see a
broad emission band (fwhm = ∼65 nm) even at 77 K,
indicating CT characteristics of the emissive state in the frozen
nonpolar matrix.
Under similar experimental conditions, the temperature-
dependent luminescence spectral features of 2−6 are the same
as that of 1, except in the frozen matrix (Figures 3 and S37 and
S38). Below 95 K, three emission peaks were observed for 2
and 3, while there were two emission bands for 5 and 6 in
MTHF, which are attributed to the combination of
fluorescence and phosphorescence emissions in the steady
state (Figure S37). For compounds 2 and 3, the phosphor-
escence spectra appear to be slightly vibronic in structure with
an average spacing of ca. 1200 cm−1; similar kinds of spectral
D
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 4. Prompt and delayed emission spectra of compounds (a) 1 and (b) 4 at 77 and 298 K (solvent = MTHF; concentration = 10 μM; λex
370 nm for 1 and 330 nm for 4). The insets show phosphorescence emissions of compounds under UV-light illumination.
=
Figure 5. Phosphorescence spectra of compounds (a) 1−3 and (b) 4−6 at 77 K in the solid state (λex = 370 nm for 1 and 330 nm for 4).
features were also observed by Marder et al. for [4-(tert-
butyl)phenyl]bis(fluoromesityl)borane below 95 K in
MTHF.26 The temperature-dependent luminescence measure-
ments undoubtedly revealed that the dipolar nature of the
excited states of quadrupolar (2 and 5) and octupolar (3 and
6) compounds support symmetry breaking at the excited state.
Both steady-state and time-resolved emission measurements
were carried out at 298 and 77 K to probe their fluorescence,
phosphorescence, and DF characteristics in both solution and
thin-film states for all compounds (Figures 4 and S39 and
Table S6). In the case of compound 1, the emission is located
at ∼462 nm with a lifetime (τ) of ∼13.4 ns can be ascribed to
fluorescence (prompt fluorescence) at 77 K. The correspond-
ing delayed component was around ∼503 nm with τ =
∼957.74 ms; such a red-shifted and long-lived emission
compared to prompt fluorescence could be phosphorescence.
At 298 K, the prompt (τ = ∼12.86 ns) and delayed (τ = ∼5.14
μs) emission spectra were the same with maxima around ∼520
nm. Compounds 2−6 also showed similar kinds of emission
feature at 298 and 77 K. Notably, a gradual increase in the
emission intensity can be noted when the temperature
increases from 250 to 298 K in compounds 4−6 (Figure
S40), presumably due to the contribution of DF.9 The
calculated singlet and triplet splitting energy (ΔEST;
determined from the peak position) values lie in the range of
0.15−0.29 eV, indicating that TADF might be in operation,
thus the increase in the emission intensity with an increase in
the temperature for compounds 4−6; however, for 1−3, a
decrease in the emission intensity can be noted even when the
ΔEST values were in the range 0.21−0.27 eV, pointing to the
elevation of the nonradiative channels with increasing
temperature (Figure S40 and Table S8).31 The phosphor-
escence lifetime decreases with increasing number of branches
at 77 K (Table S6). For instance, compound 1 (τ = ∼957.74
ms) showed a longer-lived lifetime than 2 (τ = ∼666.24 ms)
and 3 (τ = ∼494.45 ms). A similar observation was noted for 4
(τ = 928.00 ms), 5 (τ = 795.23 ms), and 6 (τ = 665.50 ms) as
well. Involvement of triplets in DF is further supported by the
decrease of DF under oxygen-aerated MTHF solutions (Figure
S41).
Further, the prompt and delayed emission spectra and decay
profiles were collected at 298 and 77 K in the thin-film state
(Figures S42 and S43 and Table S7). In the case of 1, prompt
fluorescence was observed at 478 nm (τ = 8.07 ns), while
phosphorescence was located at 517 nm (τ = 329.33 ms) at 77
K. Compounds 2−6 also showed phosphorescence character-
istics with millisecond lifetime at 77 K. Further, to confirm the
phosphorescence emission characteristics, time-resolved decay
profiles of 4 at different temperatures were collected as
representative examples. An increase in the excited-state
lifetime upon decreasing temperature further supports the
phosphorescence nature of the emission (Figure S43). It is
worth mentioning that phosphorescence of TABs at 77 K with
different lifetimes was reported independently by Marder et
al.,26 Yamguchi et al.,27 Wagner et al.,28 and Wang et al.29
Remarkably, the phosphorescence spectra, like fluorescence,
are blue-shifted with increasing number of branches (Figure
5). Compounds also showed DF with microsecond lifetime at
E
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 6. Frontier molecular orbitals (HOMO and LUMO) and their associated energies for compounds 1−6 (energies are not up to the scale).
298 K in the solid state and followed the same trend as that of
phosphorescence and fluorescence, i.e., hypsochromic shift
with increasing number of branches in the series 1−3 and 4−6
(Figure S42). The involvement of triplet states in the emission
features of the compounds was confirmed by measuring the
delayed emission and delayed emission decay profiles under an
oxygen atmosphere at 298 K. Compounds 1−6 showed a
significant quenching of the luminescence intensity and
excited-state lifetime in the presence of oxygen (Figures S45
and S46). Further, a decrease in the emission quantum yields
(from 28% to 25% for 1, from 10% to 9% for 2, from 8% to 6%
for 3, from 29% to 27% for 4, from 19% to 16% for 5, and from
10% to 7% for 6) was observed in the presence of oxygen.
Upon an increase in the donor strength from −NH2 to
−NMe2, the triplet energy levels did not perturb much;
however, the singlet energy levels were significantly destabi-
lized and showed blue-shifted fluorescence at 77 K (Figures 5
and S47 and Table S8).
Theoretical Modeling. To further obtain insight into the
electronic structure and photophysical properties, theoretical
calculations were performed using a DFT/B3LYP/6-31G(d)
approach.32 Vertical transition energies were estimated on
ground-state-optimized geometries using the TD-DFT meth-
odology (Table S9). The molecular orbital coefficients of the
HOMOs for 1−3 reside on the NH2 units with large
contribution from the amine-attached duryl moieties (Figure
6). The HOMO level is slightly destabilized with increasing
number of donors. The orbital coefficients of the LUMOs for
1-3 are delocalized over the entire molecule, with significantly
large contribution from the boron atom. Notably, the LUMO
level was destabilized with an increment of ∼0.17 eV per each
amine incorporation; however, no such type of trend was
observed for the HOMO levels for compounds 1−3. On the
other hand, the frontier molecular orbital distribution patterns
in 4−6 are different. For 4 and 5, the HOMOs are localized on
the amine units with a slight contribution from the duryl
moieties attached to amines. In contrast, the HOMO is
exclusively localized on one of the NMe2 groups, with some
contribution from its attached duryl ring; there is a minor
contribution from the other NMe2 groups in 6. The LUMOs
are delocalized over the entire molecule except on the amines.
The increase in the number of donors has a negligible effect on
the LUMO energy levels. The extent of frontier molecular
orbital destabilization in 4−6 is less compared to that in 1−3
with increasing donors. The energy gap in 4−6 is slightly lower
than that in 1−3, which is in good agreement with the
experimental observations.
Remarkably, the LUMO level for 4−6 is more stabilized
than that of the 1−3 series, indicating the stronger electron-
accepting properties of boron as a result of weak/negligible
electronic interaction between the amine and boron in the
ground state, which is well illustrated by their insignificant
absorption solvatochromic characteristics. The estimated
difference in vertical transition energies between the singlet
(S1) and triplet (T1) excited states is in the range of 0.3−0.6
eV, supporting the trend of ΔEST determined from the
experiments (Tables S8 and S9).
CONCLUSIONS
■
In conclusion, the design, synthesis, and intriguing optical
properties of a series of TDB-based dipolar (1 and 4),
quadrupolar (2 and 5), and octupolar (3 and 6) compounds
were reported. The degree of CT and energy of the CT states
were fine-tuned by systematically varying the number and
strength of the donors as well as the symmetry of the molecular
systems. Because of the propeller molecular architecture, the
donor and acceptor are weakly coupled, and the frontier
molecular orbitals are specially localized. These arrangements
lowered the energy gap between the S1 and T1 excited states
and facilitated reverse intersystem crossing in compounds 1−6.
Thus, all of the compounds show DF under ambient
conditions and persistent phosphorescence at low temperature.
The DF and phosphorescence emissions were gradually blue-
shifted with an increase in the number of donor branches. The
detailed optical and computational studies on these com-
pounds established that symmetry breaking in the excited state
is responsible for the observed intriguing optical properties. Of
all of the compounds, the dipolar molecules 1 and 4 exhibited
the highest solid-state quantum yields. To the best of our
knowledge, these are the simplest boron-based donor−
acceptor molecules showing DF and persistent phosphor-
escence.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03446.
■
sı
Synthesis and characterization details, crystallographic
structural refinement data, and supporting optical
F
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
characterization details and DFT calculation results
pubs.acs.org/IC
Article
Emission Spanning from Green to Red for Solution-Processable
Organic Light-Emitting Devices. Angew. Chem., Int. Ed. 2018, 57 (19),
5463−5466. (h) Lee, C.-H.; Tang, M.-C.; Cheung, W.-L.; Lai, S.-L.;
Chan, M.-Y.; Yam, V. W.-W. Highly luminescent phosphine oxide-
containing bipolar alkynylgold(iii) complexes for solution-processable
organic light-emitting devices with small efficiency roll-offs. Chem. Sci.
2018, 9 (29), 6228−6232.
(2) (a) Jin, S.; Furukawa, K.; Addicoat, M.; Chen, L.; Takahashi, S.;
Irle, S.; Nakamura, T.; Jiang, D. Large pore donor-acceptor covalent
organic frameworks. Chem. Sci. 2013, 4 (12), 4505−4511. (b) Zhang,
J.; Chen, W.; Kalytchuk, S.; Li, K. F.; Chen, R.; Adachi, C.; Chen, Z.;
Rogach, A. L.; Zhu, G.; Yu, P. K. N.; Zhang, W.; Cheah, K. W.;
Zhang, X.; Lee, C.-S. Self-Assembly of Electron Donor-Acceptor-
Based Carbazole Derivatives: Novel Fluorescent Organic Nanoprobes
for Both One- and Two-Photon Cellular Imaging. ACS Appl. Mater.
(PDF)
Accession Codes
CCDC 993251 and 993252 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_-
request/cif, or by emailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
AUTHOR INFORMATION
Corresponding Author
■
Pakkirisamy Thilagar − Department of Inorganic and Physical
Chemistry, Indian Institute of Science (IISc), Bangalore
560012, India; orcid.org/0000-0001-9569-7733;
Email: thilagar@iisc.ac.in
̈
Interfaces 2016, 8 (18), 11355−11365. (c) Schafer, J.; Holzapfel, M.;
Schmiedel, A.; Steiner, U. E.; Lambert, C. Fine tuning of electron
transfer and spin chemistry parameters in triarylamine-bridge-
naphthalene diimide dyads by bridge substituents. Phys. Chem.
Chem. Phys. 2018, 20 (42), 27093−27104. (d) Pachfule, P.;
Authors
Acharjya, A.; Roeser, J.; Sivasankaran, R. P.; Ye, M.-Y.; Bruckner,
̈
Sudhakar Pagidi − Department of Inorganic and Physical
Chemistry, Indian Institute of Science (IISc), Bangalore
560012, India
Neena K. Kalluvettukuzhy − Department of Inorganic and
Physical Chemistry, Indian Institute of Science (IISc),
Bangalore 560012, India
A.; Schmidt, J.; Thomas, A. Donor-acceptor covalent organic
frameworks for visible light induced free radical polymerization.
Chem. Sci. 2019, 10 (36), 8316−8322. (e) He, J.; Rauch, F.; Friedrich,
A.; Sieh, D.; Ribbeck, T.; Krummenacher, I.; Braunschweig, H.; Finze,
M.; Marder, T. B. N-Heterocyclic Olefins as Electron Donors in
Combination with Triarylborane Acceptors: Synthesis, Optical and
Electronic Properties of D-π-A Compounds. Chem. - Eur. J. 2019, 25
(60), 13777−13784. (f) Kunchala, D.; Sa, S.; Nayak, P.; Ponniah S, J.;
Venkatasubbaiah, K. Tetrahydrodibenzophenanthridine-Based Boron-
Bridged Polycyclic Aromatic Hydrocarbons: Synthesis, Structural
Diversity, and Optical Properties. Organometallics 2019, 38 (4), 870−
878.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03446
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
̈
(3) (a) Parab, K.; Venkatasubbaiah, K.; Jakle, F. Luminescent
Triarylborane-Functionalized Polystyrene: Synthesis, Photophysical
Characterization, and Anion-Binding Studies. J. Am. Chem. Soc. 2006,
128 (39), 12879−12885. (b) Xu, B.; Wu, X.; Li, H.; Tong, H.; Wang,
L. Selective Detection of TNT and Picric Acid by Conjugated
Polymer Film Sensors with Donor-Acceptor Architecture. Macro-
molecules 2011, 44 (13), 5089−5092. (c) Dhanunjayarao, K.;
Mukundam, V.; Venkatasubbaiah, K. Tetracoordinate Imidazole-
Based Boron Complexes for the Selective Detection of Picric Acid.
Inorg. Chem. 2016, 55 (21), 11153−11159. (d) Narayanaswamy, K.;
Yadagiri, B.; Chowdhury, T. H.; Swetha, T.; Islam, A.; Gupta, V.;
Singh, S. P. Impact of A-D-A-Structured Dithienosilole- and
Phenoxazine-Based Small Molecular Material for Bulk Heterojunction
and Dopant-Free Perovskite Solar Cells. Chem. - Eur. J. 2019, 25,
16320.
(4) (a) Terenziani, F.; Le Droumaguet, C.; Katan, C.; Mongin, O.;
Blanchard-Desce, M. Effect of Branching on Two-Photon Absorption
in Triphenylbenzene Derivatives. ChemPhysChem 2007, 8 (5), 723−
734. (b) Makarov, N. S.; Mukhopadhyay, S.; Yesudas, K.; Bredas, J.-
L.; Perry, J. W.; Pron, A.; Kivala, M.; Mullen, K. Impact of Electronic
Coupling, Symmetry, and Planarization on One- and Two-Photon
Properties of Triarylamines with One, Two, or Three Diarylboryl
Acceptors. J. Phys. Chem. A 2012, 116 (15), 3781−3793. (c) Poronik,
Y. M.; Hugues, V.; Blanchard-Desce, M.; Gryko, D. T. Octupolar
Merocyanine Dyes: A New Class of Nonlinear Optical Chromo-
phores. Chem. - Eur. J. 2012, 18 (30), 9258−9266. (d) Yan, L.; Chen,
X.; He, Q.; Wang, Y.; Wang, X.; Guo, Q.; Bai, F.; Xia, A.; Aumiler, D.;
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.T. thanks the Science and Engineering Research Board
(SERB), New Delhi, India, and IISc for financial support. S.P.
and N.K.K. thank the SERB, New Delhi, India, for financial
support.
REFERENCES
■
(1) (a) Duan, L.; Qiao, J.; Sun, Y.; Qiu, Y. Strategies to Design
Bipolar Small Molecules for OLEDs: Donor-Acceptor Structure and
Non-Donor-Acceptor Structure. Adv. Mater. 2011, 23 (9), 1137−
1144. (b) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C.
Highly efficient organic light-emitting diodes from delayed
fluorescence. Nature 2012, 492 (7428), 234−238. (c) Xu, L.; Zhu,
H.; Long, G.; Zhao, J.; Li, D.; Ganguly, R.; Li, Y.; Xu, Q.-H.; Zhang,
Q. 4-Diphenylamino-phenyl substituted pyrazine: nonlinear optical
switching by protonation. J. Mater. Chem. C 2015, 3 (35), 9191−
9196. (d) Cao, X.; Zhang, D.; Zhang, S.; Tao, Y.; Huang, W. CN-
Containing donor-acceptor-type small-molecule materials for ther-
mally activated delayed fluorescence OLEDs. J. Mater. Chem. C 2017,
5 (31), 7699−7714. (e) Olivier, Y.; Moral, M.; Muccioli, L.; Sancho-
García, J.-C. Dynamic nature of excited states of donor-acceptor
TADF materials for OLEDs: how theory can reveal structure-property
relationships. J. Mater. Chem. C 2017, 5 (23), 5718−5729. (f) Ryu, J.
Y.; Lee, J. M.; Van Nghia, N.; Lee, K. M.; Lee, S.; Lee, M. H.; Stang,
P. J.; Lee, J. Supramolecular Pt(II) and Ru(II) Trigonal Prismatic
Cages Constructed with a Tris(pyridyl)borane Donor. Inorg. Chem.
2018, 57 (18), 11696−11703. (g) Tang, M.-C.; Lee, C.-H.; Ng, M.;
Wong, Y.-C.; Chan, M.-Y.; Yam, V. W.-W. Highly Emissive Fused
Heterocyclic Alkynylgold(III) Complexes for Multiple Color
́
̈
́
Vdovic, S.; Lin, S. Localized Emitting State and Energy Transfer
Properties of Quadrupolar Chromophores and (Multi)Branched
Derivatives. J. Phys. Chem. A 2012, 116 (34), 8693−8705.
(e) Wang, Y.; Yin, S.; Liu, J.; Yao, L.; Wang, G.; Liu, D.; Jing, B.;
Cheng, L.; Zhong, H.; Shi, X.; Fang, Q.; Qian, S. Probing ultrafast
excited state dynamics and nonlinear absorption properties of three
star-shaped conjugated oligomers with 1,3,5-triazine core. RSC Adv.
2014, 4 (21), 10960−10967.
G
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
́
̂
̀
(5) (a) Roquet, S.; Cravino, A.; Leriche, P.; Aleveque, O.; Frere, P.;
Roncali, J. Triphenylamine−Thienylenevinylene Hybrid Systems with
Internal Charge Transfer as Donor Materials for Heterojunction Solar
Cells. J. Am. Chem. Soc. 2006, 128 (10), 3459−3466. (b) He, G. S.;
Tan, L.-S.; Zheng, Q.; Prasad, P. N. Multiphoton Absorbing
Materials: Molecular Designs, Characterizations, and Applications.
Chem. Rev. 2008, 108 (4), 1245−1330. (c) Kim, H. M.; Sik Seo, M.;
Jeon, S.-J.; Rae Cho, B. Two-photon absorption properties of hexa-
substituted benzene derivatives. Comparison between dipolar and
octupolar molecules. Chem. Commun. 2009, No. 47, 7422−7424.
(d) Sissa, C.; Manna, A. K.; Terenziani, F.; Painelli, A.; Pati, S. K.
Activated Delayed Fluorescent Organic Light-Emitting Diodes. Adv.
Funct. Mater. 2018, 28 (43), 1802558.
(10) (a) Numata, M.; Yasuda, T.; Adachi, C. High efficiency pure
blue thermally activated delayed fluorescence molecules having 10H-
phenoxaborin and acridan units. Chem. Commun. 2015, 51 (46),
9443−9446. (b) Suzuki, K.; Kubo, S.; Shizu, K.; Fukushima, T.;
Wakamiya, A.; Murata, Y.; Adachi, C.; Kaji, H. Triarylboron-Based
Fluorescent Organic Light-Emitting Diodes with External Quantum
Efficiencies Exceeding 20%. Angew. Chem., Int. Ed. 2015, 54 (50),
15231−15235. (c) Hatakeyama, T.; Shiren, K.; Nakajima, K.;
Nomura, S.; Nakatsuka, S.; Kinoshita, K.; Ni, J.; Ono, Y.; Ikuta, T.
Ultrapure Blue Thermally Activated Delayed Fluorescence Molecules:
Efficient HOMO-LUMO Separation by the Multiple Resonance
Effect. Adv. Mater. 2016, 28 (14), 2777−2781. (d) Wu, T.-L.; Huang,
M.-J.; Lin, C.-C.; Huang, P.-Y.; Chou, T.-Y.; Chen-Cheng, R.-W.; Lin,
H.-W.; Liu, R.-S.; Cheng, C.-H. Diboron compound-based organic
light-emitting diodes with high efficiency and reduced efficiency roll-
off. Nat. Photonics 2018, 12 (4), 235−240.
̈
Beyond the Forster formulation for resonance energy transfer: the
role of dark states. Phys. Chem. Chem. Phys. 2011, 13 (28), 12734−
12744. (e) Ongungal, R. M.; Sivadas, A. P.; Kumar, N. S. S.; Menon,
S.; Das, S. Self-assembly and mechanochromic luminescence switch-
ing of trifluoromethyl substituted 1,3,4-oxadiazole derivatives. J.
Mater. Chem. C 2016, 4 (40), 9588−9597.
(6) (a) Walker, B.; Kim, C.; Nguyen, T.-Q. Small Molecule Solution-
Processed Bulk Heterojunction Solar Cells. Chem. Mater. 2011, 23
(3), 470−482. (b) Zhang, J.; Yu, J.; He, C.; Deng, D.; Zhang, Z.-G.;
Zhang, M.; Li, Z.; Li, Y. Solution-processable star-shaped photovoltaic
organic molecules based on triphenylamine and benzothiadiazole with
longer pi-bridge. Org. Electron. 2012, 13 (1), 166−172. (c) Kength-
anomma, T.; Thamyongkit, P.; Gasiorowski, J.; Ramil, A. M.;
Sariciftci, N. S. On the potential of porphyrin-spiked triarylamine
stars for bulk heterojunction solar cells. J. Mater. Chem. A 2013, 1
(35), 10524−10531.
(11) (a) Wang, S.; Yang, D.-T.; Lu, J.; Shimogawa, H.; Gong, S.;
Wang, X.; Mellerup, S. K.; Wakamiya, A.; Chang, Y.-L.; Yang, C.; Lu,
Z.-H. In Situ Solid-State Generation of (BN)2-Pyrenes and
Electroluminescent Devices. Angew. Chem., Int. Ed. 2015, 54 (50),
15074−15078. (b) Kitamoto, Y.; Namikawa, T.; Ikemizu, D.; Miyata,
Y.; Suzuki, T.; Kita, H.; Sato, T.; Oi, S. Light blue and green thermally
activated delayed fluorescence from 10H-phenoxaborin-derivatives
and their application to organic light-emitting diodes. J. Mater. Chem.
C 2015, 3 (35), 9122−9130. (c) Chen, D.-G.; Lin, T.-C.; Chen, C.-L.;
Chen, Y.-T.; Chen, Y.-A.; Lee, G.-H.; Chou, P.-T.; Liao, C.-W.; Chiu,
P.-C.; Chang, C.-H.; Lien, Y.-J.; Chi, Y. Optically Triggered
Planarization of Boryl-Substituted Phenoxazine: Another Horizon of
TADF Molecules and High-Performance OLEDs. ACS Appl. Mater.
Interfaces 2018, 10 (15), 12886−12896. (d) Neena, K. K.; Sudhakar,
P.; Thilagar, P. Catalyst- and Template-Free Ultrafast Visible-Light-
Triggered Dimerization of Vinylpyridine-Functionalized Tetraaryla-
minoborane: Intriguing Deep-Blue Delayed Fluorescence. Angew.
Chem., Int. Ed. 2018, 57 (51), 16806−16810. (e) Gao, Y.; Pan, Q.-Q.;
Zhao, L.; Geng, Y.; Su, T.; Gao, T.; Su, Z.-M. Realizing performance
improvement of blue thermally activated delayed fluorescence
molecule DABNA by introducing substituents on the para-position
of boron atom. Chem. Phys. Lett. 2018, 701, 98−102.
̀
(7) (a) Porres, L.; Mongin, O.; Katan, C.; Charlot, M.; Pons, T.;
Mertz, J.; Blanchard-Desce, M. Enhanced Two-Photon Absorption
with Novel Octupolar Propeller-Shaped Fluorophores Derived from
Triphenylamine. Org. Lett. 2004, 6 (1), 47−50. (b) Easwaramoorthi,
S.; Thamaraiselvi, P.; Duraimurugan, K.; Beneto, A. J.; Siva, A.; Nair,
B. U. Charge instability of symmetry broken dipolar states in
quadrupolar and octupolar triphenylamine derivatives. Chem.
Commun. 2014, 50 (52), 6902−6905. (c) Cvejn, D.; Michail, E.;
Polyzos, I.; Almonasy, N.; Pytela, O.; Klikar, M.; Mikysek, T.;
̌
Giannetas, V.; Fakis, M.; Bures, F. Modulation of (non)linear optical
properties in tripodal molecules by variation of the peripheral cyano
acceptor moieties and the π-spacer. J. Mater. Chem. C 2015, 3 (28),
7345−7355. (d) Cvejn, D.; Michail, E.; Seintis, K.; Klikar, M.; Pytela,
O.; Mikysek, T.; Almonasy, N.; Ludwig, M.; Giannetas, V.; Fakis, M.;
Bures, F. Solvent and branching effect on the two-photon absorption
properties of push-pull triphenylamine derivatives. RSC Adv. 2016, 6
(16), 12819−12828.
(12) (a) Doty, J. C.; Babb, B.; Grisdale, P. J.; Glogowski, M.;
Williams, J. L. R. Boron photochemistry: IX. Synthesis and fluorescent
properties of dimesityl-phenylboranes. J. Organomet. Chem. 1972, 38
̌
́
(2), 229−236. (b) Pron, A.; Baumgarten, M.; Mullen, K. Phenylene
̈
Bridged Boron−Nitrogen Containing Dendrimers. Org. Lett. 2010, 12
(19), 4236−4239. (c) Hudnall, T. W.; Chiu, C.-W.; Gabbaï, F. P.
Fluoride Ion Recognition by Chelating and Cationic Boranes. Acc.
Chem. Res. 2009, 42 (2), 388−397. (d) Mukherjee, S.; Thilagar, P.
Stimuli and shape responsive ‘boron-containing’ luminescent organic
materials. J. Mater. Chem. C 2016, 4 (14), 2647−2662. (e) Ren, Y.;
(8) (a) Parthasarathy, V.; Fery-Forgues, S.; Campioli, E.; Recher, G.;
Terenziani, F.; Blanchard-Desce, M. Dipolar versus Octupolar
Triphenylamine-Based Fluorescent Organic Nanoparticles as Brilliant
One- and Two-Photon Emitters for (Bio)imaging. Small 2011, 7
(22), 3219−3229. (b) Cvejn, D.; Achelle, S.; Pytela, O.; Malval, J.-P.;
̌
Spangenberg, A.; Cabon, N.; Bures, F.; Robin-le Guen, F. Tripodal
̈
Jakle, F. Merging thiophene with boron: new building blocks for
molecules with triphenylamine core, diazine peripheral groups and
extended π-conjugated linkers. Dyes Pigm. 2016, 124, 101−109.
(9) (a) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng,
C.; Zhang, L.; Huang, W. Thermally Activated Delayed Fluorescence
Materials Towards the Breakthrough of Organoelectronics. Adv.
Mater. 2014, 26 (47), 7931−7958. (b) Godumala, M.; Choi, S.; Cho,
M. J.; Choi, D. H. Thermally activated delayed fluorescence blue
dopants and hosts: from the design strategy to organic light-emitting
diode applications. J. Mater. Chem. C 2016, 4 (48), 11355−11381.
(c) Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi,
Z.; Aldred, M. P. Recent advances in organic thermally activated
delayed fluorescence materials. Chem. Soc. Rev. 2017, 46 (3), 915−
1016. (d) Furue, R.; Matsuo, K.; Ashikari, Y.; Ooka, H.; Amanokura,
N.; Yasuda, T. Highly Efficient Red-Orange Delayed Fluorescence
Emitters Based on Strong π-Accepting Dibenzophenazine and
Dibenzoquinoxaline Cores: toward a Rational Pure-Red OLED
Design. Adv. Opt. Mater. 2018, 6 (5), 1701147. (e) Cai, X.; Su, S.-
J. Marching Toward Highly Efficient, Pure-Blue, and Stable Thermally
conjugated materials. Dalton Trans. 2016, 45 (36), 13996−14007.
(f) Ji, L.; Griesbeck, S.; Marder, T. B. Recent developments in and
perspectives on three-coordinate boron materials: a bright future.
Chem. Sci. 2017, 8 (2), 846−863. (g) Li, S.-Y.; Sun, Z.-B.; Zhao, C.-H.
Charge-Transfer Emitting Triarylborane π-Electron Systems. Inorg.
Chem. 2017, 56 (15), 8705−8717. (h) Mellerup, S. K.; Wang, S.
Boron-based stimuli responsive materials. Chem. Soc. Rev. 2019, 48
(13), 3537−3549.
(13) (a) Lee, Y. H.; Park, S.; Oh, J.; Shin, J. W.; Jung, J.; Yoo, S.;
Lee, M. H. Rigidity-Induced Delayed Fluorescence by Ortho Donor-
Appended Triarylboron Compounds: Record-High Efficiency in Pure
Blue Fluorescent Organic Light-Emitting Diodes. ACS Appl. Mater.
Interfaces 2017, 9 (28), 24035−24042. (b) Turkoglu, G.; Cinar, M.
E.; Ozturk, T. Triarylborane-Based Materials for OLED Applications.
Molecules 2017, 22 (9), 1522. (c) Matsui, K.; Oda, S.; Yoshiura, K.;
Nakajima, K.; Yasuda, N.; Hatakeyama, T. One-Shot Multiple
Borylation toward BN-Doped Nanographenes. J. Am. Chem. Soc.
2018, 140 (4), 1195−1198. (d) Brend’amour, S.; Gilmer, J.; Bolte,
H
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
M.; Lerner, H.-W.; Wagner, M. C-Halogenated 9,10-Diboraanthra-
cenes: How the Halogen Load and Distribution Influences Key
Optoelectronic Properties. Chem. - Eur. J. 2018, 24 (63), 16910−
16918. (e) Pershin, A.; Hall, D.; Lemaur, V.; Sancho-Garcia, J.-C.;
Muccioli, L.; Zysman-Colman, E.; Beljonne, D.; Olivier, Y. Highly
emissive excitons with reduced exchange energy in thermally activated
delayed fluorescent molecules. Nat. Commun. 2019, 10 (1), 597.
(14) (a) Yuan, Z.; Collings, J. C.; Taylor, N. J.; Marder, T. B.; Jardin,
C.; Halet, J.-F. Linear and Nonlinear Optical Properties of Three-
Coordinate Organoboron Compounds. J. Solid State Chem. 2000, 154
C. R.; Gabbaï, F. P. Cyanide and Azide Anion Complexation by a
Bidentate Stibonium-Borane Lewis Acid. Z. Naturforsch., B: J. Chem.
Sci. 2014, 69b, 1199−1205. (d) Yin, X.; Liu, K.; Ren, Y.; Lalancette,
̈
R. A.; Loo, Y.-L.; Jakle, F. Pyridalthiadiazole acceptor-functionalized
triarylboranes with multi-responsive optoelectronic characteristics.
Chem. Sci. 2017, 8 (8), 5497−5505. (e) Sudhakar, P.; Neena, K. K.;
Thilagar, P. Borylated perylenediimide: self-assembly, photophysics
and sensing application. Dalton Trans. 2019, 48 (21), 7218−7226.
(f) Teerasarunyanon, R.; Wilkins, L. C.; Park, G.; Gabbaï, F. P.
Synthesis, structure and anion binding properties of 1,8-bis-
(dimesitylboryl)anthracene and its monoborylated analog. Dalton
Trans. 2019, 48 (39), 14777−14782.
(18) (a) Pagidi, S.; Kalluvettukuzhy, N. K.; Thilagar, P. Triarylboron
Anchored Luminescent Probes: Selective Detection and Imaging of
Thiophenols in the Intracellular Environment. Langmuir 2018, 34
(28), 8170−8177. (b) Griesbeck, S.; Michail, E.; Wang, C.;
Ogasawara, H.; Lorenzen, S.; Gerstner, L.; Zang, T.; Nitsch, J.;
Sato, Y.; Bertermann, R.; Taki, M.; Lambert, C.; Yamaguchi, S.;
Marder, T. B. Tuning the π-bridge of quadrupolar triarylborane
chromophores for one- and two-photon excited fluorescence imaging
of lysosomes in live cells. Chem. Sci. 2019, 10 (20), 5405−5422.
(c) Griesbeck, S.; Ferger, M.; Czernetzi, C.; Wang, C.; Bertermann,
R.; Friedrich, A.; Haehnel, M.; Sieh, D.; Taki, M.; Yamaguchi, S.;
Marder, T. B. Optimization of Aqueous Stability versus π-
Conjugation in Tetracationic Bis(triarylborane) Chromophores:
Applications in Live-Cell Fluorescence Imaging. Chem. - Eur. J.
2019, 25 (32), 7679−7688.
(19) Yamaguchi, S.; Shirasaka, T.; Tamao, K. Tridurylboranes
Extended by Three Arylethynyl Groups as a New Family of Boron-
Based π-Electron Systems. Org. Lett. 2000, 2 (26), 4129−4132.
(20) Stahl, R.; Lambert, C.; Kaiser, C.; Wortmann, R.; Jakober, R.
Electrochemistry and Photophysics of Donor-Substituted Triarylbor-
anes: Symmetry Breaking in Ground and Excited State. Chem. - Eur. J.
2006, 12 (8), 2358−2370.
(21) Liu, Y.; Xie, G.; Wu, K.; Luo, Z.; Zhou, T.; Zeng, X.; Yu, J.;
Gong, S.; Yang, C. Boosting reverse intersystem crossing by increasing
donors in triarylboron/phenoxazine hybrids: TADF emitters for high-
performance solution-processed OLEDs. J. Mater. Chem. C 2016, 4
(20), 4402−4407.
(22) (a) Sudhakar, P. Rational design and facile synthesis of boryl
anilines: intriguing aggregation induced emission and external stimuli
responsive properties. Ph.D. Dissertation, Indian Institute of Science,
Bangalore, India, 2015. (b) Sudhakar, P.; Neena, K. K.; Thilagar, P.
H-Bond assisted mechanoluminescence of borylated aryl amines:
tunable emission and polymorphism. J. Mater. Chem. C 2017, 5,
6537−6546.
(23) Yoshino, J.; Nakamura, Y.; Kunitomo, S.; Hayashi, N.; Higuchi,
H. Synthesis of tris(4-amino-2,6-dimethylphenyl)borane and facile
extension of its π-conjugated system by utilizing the reactivity of the
amino groups. Tetrahedron Lett. 2013, 54 (22), 2817−2820.
(24) Mao, M.; Ren, M.-G.; Song, Q.-H. Thermodynamics and
Conformations in the Formation of Excited States and Their
Interconversions for Twisted Donor-Substituted Tridurylboranes.
Chem. - Eur. J. 2012, 18 (48), 15512−15522.
́
(1), 5−12. (b) Yuan, Z.; Entwistle, C. D.; Collings, J. C.; Albesa-Jove,
D.; Batsanov, A. S.; Howard, J. A. K.; Taylor, N. J.; Kaiser, H. M.;
Kaufmann, D. E.; Poon, S.-Y.; Wong, W.-Y.; Jardin, C.; Fathallah, S.;
Boucekkine, A.; Halet, J.-F.; Marder, T. B. Synthesis, Crystal
Structures, Linear and Nonlinear Optical Properties, and Theoretical
Studies of (p-R-Phenyl)-, (p-R-Phenylethynyl)-, and (E)-[2-(p-R-
Phenyl)ethenyl]dimesitylboranes and Related Compounds. Chem. -
Eur. J. 2006, 12 (10), 2758−2771. (c) Griesbeck, S.; Michail, E.;
Rauch, F.; Ogasawara, H.; Wang, C.; Sato, Y.; Edkins, R. M.; Zhang,
Z.; Taki, M.; Lambert, C.; Yamaguchi, S.; Marder, T. B. The Effect of
Branching on the One- and Two-Photon Absorption, Cell Viability,
and Localization of Cationic Triarylborane Chromophores with
Dipolar versus Octupolar Charge Distributions for Cellular Imaging.
Chem. - Eur. J. 2019, 25 (57), 13164−13175. (d) Griesbeck, S.;
Michail, E.; Wang, C.; Ogasawara, H.; Lorenzen, S.; Gerstner, L.;
Zang, T.; Nitsch, J.; Sato, Y.; Bertermann, R.; Taki, M.; Lambert, C.;
Yamaguchi, S.; Marder, T. B. Tuning the π-bridge of quadrupolar
triarylborane chromophores for one- and two-photon excited
fluorescence imaging of lysosomes in live cells. Chem. Sci. 2019, 10
(20), 5405−5422.
(15) (a) Hatakeyama, T.; Hashimoto, S.; Seki, S.; Nakamura, M.
Synthesis of BN-Fused Polycyclic Aromatics via Tandem Intra-
molecular Electrophilic Arene Borylation. J. Am. Chem. Soc. 2011, 133
(46), 18614−18617. (b) Hatakeyama, T.; Hashimoto, S.; Oba, T.;
Nakamura, M. Azaboradibenzo[6]helicene: Carrier Inversion Induced
by Helical Homochirality. J. Am. Chem. Soc. 2012, 134 (48), 19600−
19603. (c) Wang, X.-Y.; Lin, H.-R.; Lei, T.; Yang, D.-C.; Zhuang, F.-
D.; Wang, J.-Y.; Yuan, S.-C.; Pei, J. Azaborine Compounds for
Organic Field-Effect Transistors: Efficient Synthesis, Remarkable
Stability, and BN Dipole Interactions. Angew. Chem., Int. Ed. 2013, 52
(11), 3117−3120. (d) Miyamoto, F.; Nakatsuka, S.; Yamada, K.;
Nakayama, K.-i.; Hatakeyama, T. Synthesis of Boron-Doped
Polycyclic Aromatic Hydrocarbons by Tandem Intramolecular
Electrophilic Arene Borylation. Org. Lett. 2015, 17 (24), 6158−
6161. (e) Katayama, T.; Nakatsuka, S.; Hirai, H.; Yasuda, N.; Kumar,
J.; Kawai, T.; Hatakeyama, T. Two-Step Synthesis of Boron-Fused
Double Helicenes. J. Am. Chem. Soc. 2016, 138 (16), 5210−5213.
(16) (a) Feng, J.; Tian, K.; Hu, D.; Wang, S.; Li, S.; Zeng, Y.; Li, Y.;
Yang, G. A Triarylboron-Based Fluorescent Thermometer: Sensitive
Over a Wide Temperature Range. Angew. Chem., Int. Ed. 2011, 50
(35), 8072−8076. (b) Neena, K. K.; Sudhakar, P.; Dipak, K.;
Thilagar, P. Diarylboryl-phenothiazine based multifunctional molec-
ular siblings. Chem. Commun. 2017, 53 (26), 3641−3644. (c) Li, S.-
Y.; Hui, Y.-J.; Sun, Z.-B.; Zhao, C.-H. A triarylborane-based biphenyl
exhibiting abrupt fluorescence enhancement at a specific high
temperature. Chem. Commun. 2017, 53 (24), 3446−3449. (d) Shi,
Y.-g.; Wang, J.-w.; Li, H.; Hu, G.-f.; Li, X.; Mellerup, S. K.; Wang, N.;
Peng, T.; Wang, S. A simple multi-responsive system based on
aldehyde functionalized amino-boranes. Chem. Sci. 2018, 9 (7),
1902−1911. (e) Sun, Z.-B.; Liu, J.-K.; Yuan, D.-F.; Zhao, Z.-H.; Zhu,
X.-Z.; Liu, D.-H.; Peng, Q.; Zhao, C.-H. 2,2′-Diamino-6,6′-diboryl-
1,1′-binaphthyl: A Versatile Building Block for Temperature-Depend-
ent Dual Fluorescence and Switchable Circularly Polarized
Luminescence. Angew. Chem., Int. Ed. 2019, 58 (15), 4840−4846.
(17) (a) Huh, J. O.; Do, Y.; Lee, M. H. A BODIPY−Borane Dyad
for the Selective Complexation of Cyanide Ion. Organometallics 2008,
27 (6), 1022−1025. (b) Yamaguchi, S.; Akiyama, S.; Tamao, K.
Colorimetric Fluoride Ion Sensing by Boron-Containing π-Electron
Systems. J. Am. Chem. Soc. 2001, 123 (46), 11372−11375. (c) Wade,
(25) Zoon, P. D.; Brouwer, A. M. A push-pull aromatic
chromophore with a touch of merocyanine. Photochem. Photobiol.
Sci. 2009, 8 (3), 345−353.
(26) Zhang, Z.; Edkins, R. M.; Nitsch, J.; Fucke, K.; Steffen, A.;
Longobardi, L. E.; Stephan, D. W.; Lambert, C.; Marder, T. B. Optical
and electronic properties of air-stable organoboron compounds with
strongly electron-accepting bis(fluoromesityl)boryl groups. Chem. Sci.
2015, 6 (1), 308−321.
(27) Kushida, T.; Camacho, C.; Shuto, A.; Irle, S.; Muramatsu, M.;
Katayama, T.; Ito, S.; Nagasawa, Y.; Miyasaka, H.; Sakuda, E.;
Kitamura, N.; Zhou, Z.; Wakamiya, A.; Yamaguchi, S. Constraint-
induced structural deformation of planarized triphenylboranes in the
excited state. Chem. Sci. 2014, 5 (4), 1296−1304.
(28) Januszewski, E.; Lorbach, A.; Grewal, R.; Bolte, M.; Bats, J. W.;
Lerner, H.-W.; Wagner, M. Unsymmetrically Substituted 9,10-
I
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Dihydro-9,10-diboraanthracenes as Versatile Building Blocks for
Boron-Doped π-Conjugated Systems. Chem. - Eur. J. 2011, 17 (45),
12696−12705.
(29) Jia, W.-L.; Song, D.; Wang, S. Blue Luminescent Three-
Coordinate Organoboron Compounds with a 2,2‘-Dipyridylamino
Functional Group. J. Org. Chem. 2003, 68 (3), 701−705.
̈
(30) Venkatasubbaiah, K.; Pakkirisamy, T.; Lalancette, R. A.; Jakle,
F. Tuning the electronic structure of diboradiferrocenes. Dalton Trans.
2008, No. 33, 4507−4513.
(31) Dobek, K.; Karolczak, J. The Influence of Temperature on
C153 Steady-State Absorption and Fluorescence Kinetics in Hydro-
gen Bonding Solvents. J. Fluoresc. 2012, 22 (6), 1647−1657.
(32) (a) Becke, A. D. Density-functional thermochemistry. III. The
role of exact exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652.
(b) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.;
Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi,
R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar,
S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox,
J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, O.;
Farkas, A. D.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009.
(c) Becke, A. D. Density-functional exchange-energy approximation
with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys.
1988, 38 (6), 3098−3100. (d) Lee, C.; Yang, W.; Parr, R. G.
Development of the Colle-Salvetti correlation-energy formula into a
functional of the electron density. Phys. Rev. B: Condens. Matter Mater.
Phys. 1988, 37 (2), 785−789.
J
https://dx.doi.org/10.1021/acs.inorgchem.9b03446
Inorg. Chem. XXXX, XXX, XXX−XXX