Effect of the Molecular Conformation on Excitation Energy Transfer in Conformationally Constrained Boryl-BODIPY Dyads

Article abstract of DOI:10.1021/acs.inorgchem.0c02739

We studied the dual emission characteristics of a series of boryl-BODIPYs (1-6) comprised of triarylborane (TAB) as an energy donor and BODIPY as an energy acceptor. The molecular conformations of dyads 1-6 were systematically tuned by judiciously changing the spacer that bridged the boryl and BODIPY moieties. Frontier molecular orbitals (FMOs) are localized in 3, 4, and 6 with a twisted molecular conformation. In contrast, FMOs are significantly delocalized in 1, 2, and 5 with the least-twisted molecular conformation. Dyads 1-6 showed dual emission features when they were excited at the TAB-dominated absorption band. However, the ratio between the two emission bands in 1-6 significantly varied depending on the molecular conformations. Systematic photoluminescence (PL) studies (both steady-state and time-resolved PL) together with computational, crystal structure, and anion binding studies established that the frustrated excited-state energy transfer from borane to BODIPY is the cause of the dual emission features in these molecular dyads. These studies also revealed that the energy transfer from borane to BODIPY can be elegantly tuned by modulating the dihedral angle between these two moieties.

Full text of DOI:10.1021/acs.inorgchem.0c02739

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Article
Effect of the Molecular Conformation on Excitation Energy Transfer
in Conformationally Constrained Boryl-BODIPY Dyads
Rajendra Prasad Nandi, Chinna Ayya Swamy P,* Pandi Dhanalakshmi, Santosh Kumar Behera,
and Pakkirisamy Thilagar*
Cite This: Inorg. Chem. 2021, 60, 5452−5462
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ABSTRACT: We studied the dual emission characteristics of a
series of boryl-BODIPYs (1−6) comprised of triarylborane (TAB)
as an energy donor and BODIPY as an energy acceptor. The
molecular conformations of dyads 1−6 were systematically tuned
by judiciously changing the spacer that bridged the boryl and
BODIPY moieties. Frontier molecular orbitals (FMOs) are
localized in 3, 4, and 6 with a twisted molecular conformation.
In contrast, FMOs are significantly delocalized in 1, 2, and 5 with
the least-twisted molecular conformation. Dyads 1−6 showed dual
emission features when they were excited at the TAB-dominated
absorption band. However, the ratio between the two emission
bands in 1−6 significantly varied depending on the molecular
conformations. Systematic photoluminescence (PL) studies (both
steady-state and time-resolved PL) together with computational, crystal structure, and anion binding studies established that the
frustrated excited-state energy transfer from borane to BODIPY is the cause of the dual emission features in these molecular dyads.
These studies also revealed that the energy transfer from borane to BODIPY can be elegantly tuned by modulating the dihedral angle
between these two moieties.
have been developed to understand the underlying mechanism
in the EET process.36−43 In recent decades, the study of how
external stimuli affect EET in covalently linked D−A systems
has received a lot of attention due to their potential sensing
applications. However, knowledge of external stimuli that affect
EET in covalently coupled D−A systems is limited partly
because of the difficulties in designing a versatile D−A system
that can exhibit different optical signatures in the presence and
absence of a stimulus.
Recently, a new type of D−A system comprised of tri- and
tetracoordinate boron, such as triarylborane (TAB) and
difluoroborondipyrromethene (BODIPY), has received con-
siderable attention. The boron center in TAB is coordinatively
unsaturated, inherently electron-deficient, and a Lewis acid.
TAB can expand its coordination from three to four and
effectively modify its structure and optical properties in the
presence of suitable nucleophiles.7,44,45 On the other hand,
BODIPY is a strongly luminescent dye that behaves as an
INTRODUCTION
■
In recent decades, dual-emissive organic and organometallic
compounds have received a lot of research interest, owing to
their potential applications in displays, optical devices, sensing,
and bioimaging.1−7 Therefore, an understanding of the
underlying fundamental aspects that cause dual emission in
organic and organometallic molecules is crucial to advance
their functional application. Dual emissions such as dual
phosphorescence,8,9 fluorescence phosphorescence,8,10−13
TADFTphosphorescence,14−16 and dual TADF emission17
involving both singlet and triplet states have increasingly been
reported in recent literatures. Continuous research efforts have
uncovered that photoinduced electronic processes, such as
intramolecular charge transfer (ICT),18,19 twisted ICT
(TICT),20−22 excited-state intramolecular proton transfer
(ESIPT),23−25 excited energy transfer (EET),26−28 trigger
dual emission in organic and organometallic compounds.29
Among all these, the excited-state energy transfer (EET)
process has been extensively exploited to develop broad
emissive molecules and materials.30−32 The rate of energy
transfer from an excited donor (D) to an energy acceptor (A)
is governed by parameters such as the spectral overlap between
the absorption and emission of A and D, the distance between
D and A, the interchromophore orientation factor (k),
etc.27,33−35 In the last two decades, numerous D−A systems
Received: September 14, 2020
Published: April 8, 2021
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electron-donor or an electron-acceptor depending on the
substituents attached.46−49 Covalently linked TAB and
BODIPY conjugates would be ideal systems to understand
the effect of external stimuli on the EET process because of the
presence of a receptor (coordinatively unsaturated TAB) and
strongly luminescent reporter BODIPY moieties. As proof of
concept, we and others independently demonstrated the effect
of nucleophiles, such as fluoride or cyanide, binding on the
EET in different TAB-BODIPY dyads.7,50−57 During these
studies, we have developed dual-, tricolor-, and white-light-
emissive TAB-BODIPY conjugates.7,55,58,59 As a part of the
ongoing program, we aim to investigate how finely one can
tune the EET process in TAB-BODIPY dyads.60 In this work,
we have designed and synthesized a series of dyads (Chart 1)
Scheme 1. Synthesis and Chemical Structures of 1−6
Chart 1. Chemical Structures of the Newly Synthesized
Boryl-BODIPY dyads 1−6 and Reference Compounds R1
and R259
and 19F) and HRMS techniques (Figure S1−S46). The
molecular structures of all the compounds except 5 were
characterized by single-crystal X-ray diffraction studies.
As expected, all the compounds showed two distinct 11B
NMR signals corresponding to tri- and tetracoordinate boron
in the regions at ∼65−70 and ∼0−2 ppm, respectively. The
11B resonances in 3, 4, and 6 were shifted upfield compared to
those in 1, 2, and 5. In the 19F NMR spectra, 1−6 showed a
typical quartet for the boron-bound fluorines in BODIPY at ca.
−146 ppm. The 13C signal corresponding to the carbons
attached to the boron could not be observed due to the fast
relaxation of the quadrupolar boron nuclei. The number of 13
C
and 1H NMR signals and the corresponding integration values
were consistent with these compounds’ molecular structures.
11B and 19F resonances of these compounds are comparable
with those of TAB-BODIPY dyads reported else-
where.50,51,56,59
Theoretical Modeling Studies. Density functional theory
(DFT) computational studies were performed on 1−6 to
investigate the effect of their molecular conformations on the
electronic communication between the boryl and BODIPY
chromophores. DFT calculations were carried using a B3LYP
hybrid functional and a 6-31G(d) basis set for all the atoms as
incorporated in the Gaussian 09 software.62 All the molecules
were optimized to their minimum potential-energy surface in
the ground state, and corresponding frequency calculations
were also carried out.
Figure 1 shows the frontier molecular orbitals (FMOs) of
dyads 1−6 in their ground-state optimized geometries. The
dihedral angle between the meso-phenyl ring (spacer) and the
BODIPY core is an essential factor that determines the nature
of the electronic communication between the boryl and
BODIPY. The relative dihedral angles between the aryl spacer
and the BODIPY ring were correlated to the extent of rigidity
and conjugation around the BODIPY fluorophore. The
calculated dihedral angles for 3, 4, and 6 were close to 90°
due to two methyl groups on the spacer that were closer to the
meso-carbon of BODIPY. In contrast, smaller angles were
observed in 1, 2, and 5 with less sterically demanding spacers.
In compounds 3, 4, and 6 that have larger dihedral angles, the
highest occupied molecular orbitals (HOMOs) and the lowest
unoccupied molecular orbitals (LUMOs) are mainly localized
over BODIPY, indicating that the lowest electronic transition
is centered on the BODIPY core. Meanwhile, the HOMO − 1
and LUMO + 1 orbitals are mainly localized over the borane
moiety, suggesting the second possible transition centered on
the TAB moiety. However, in compounds 1, 2, and 5 without
methyl substituents in the spacer closer to the meso-carbon of
where the distance between the energy donor borane and the
energy acceptor BODIPY was kept constant (−C6H4−).
However, the molecular conformations were perturbed
systematically by changing the dihedral angle between the D
and A moieties by introducing steric crowding at the donor,
acceptor, or spacer moieties. This study revealed that dyads
with less sterically demanding spacers show dual emissions
with a different PL intensity, whereas dyads with large sterically
demanding spacers show predominantly a single emission.
These intriguing results are summarized in this manuscript.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Boryl-benzaldehydes
a1−a4 were synthesized following the procedure reported
elsewhere59,61 (see the Supporting Information for the detailed
synthesis procedures). Boryl-BODIPY dyads 1−6 were
prepared analogously from the respective boryl-benzaldehydes
following the method recently reported by us.66 In short, Lewis
acid-mediated condensation of boryl-benzaldehydes (a1-a4)
with pyrrole (pyrrole for 1−4 and 2-methyl pyrrole for 5 and
6), followed by oxidation with DDQ and subsequent
complexation with BF3•Et2O in the presence of Et3N, gave
the desired compound (Scheme 1). Analytically pure 1−6
were obtained by passing the crude products through a neutral
alumina column using hexane and ethyl acetate mixtures
(9.8:0.2 ratio) as the eluent. Compounds 1−6 were purified as
red-colored solids and characterized by NMR (1H, 13C, 11B,
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Figure 1. FMOs of 1−6 (left to right) obtained from the DFT level of theory using the B3LYP hybrid functional and the 6-31G(d) basis set for all
the atoms as incorporated in the Gaussian 09 software (iso value of 0.02).62 Atom color codes are as follows: C, black; B, orange; and N, blue.
Hydrogen atoms are omitted for clarity.
BODIPY, the LUMO and LUMO + 1 orbitals are delocalized
compared to their HOMO (localized on BODIPY) and
HOMO − 1 (localized on borane) orbitals. Orbital coefficient
distributions in FMOs of 1, 2, and 5 are comparable with R1
devoid of substituents on the phenyl spacer, whereas 3, 4, and
6 behave like R2 with larger twist angles between the TAB and
BODIPY moieties. Compounds 1, 2, and 5 with diffused
FMOs are expected to show red-shifted absorption bands
compared to those of 3, 4, and 6 with localized FMOs.
These results collectively imply that the molecular
conformation significantly affects the FMOs of these dyads.
Further, the noncrossing FMOs in 3, 4, and 6 are expected to
show better energy transfer from TAB to BODIPY than 1, 2,
and 5 with diffused FMOs. Thus, much like R1 and R2, the
UV−vis absorbance spectra of 1−6 are expected to be like the
additive spectrum of their respective isolated TAB and
BODIPY moieties. However, emission properties may vary
depending upon the modification in the excited states.
To further understand the nature of the electronic structure
and molecular conformations of dyads, an analysis of the bond
lengths was highly informative (Table 1). The C(spacer)−
C(BODIPY) bond length is useful to qualitatively understand
the conformational flexibility of the BODIPY core compared to
that of the aryl spacer. As depicted in Table 1 for the DFT-
optimized structures of 1, 3, 6, R1, and R2, C(Boryl)−
C(spacer) distances (∼1.575 Å) are shorter than those of 2, 4,
and 5 (∼1.595 Å). At the same time, C(spacer)−C(BODIPY)
distances in 1, 2, 5, and R1 (∼1.482 Å) are much more
compressed than those in 3, 4, 6 (two methyl groups on the
spacer toward BODIPY), and R2 (∼1.497 Å). Compound 4
shows a significant elongation in both the C(spacer)−
C(Boryl) (1.598 Å) and C(spacer)−C(BODIPY) (1.499 Å)
bonds due to the sterically hindered duryl spacer. Thus, the
presence of additional methyl groups twisted the molecules
and consequently localized the FMOs in these systems.
DFT calculations were also carried out for all the molecules
in the solvated state (in dichloromethane) using a B3LYP
hybrid functional and a 6-31G(d) basis for all the atoms as
incorporated in Gaussian 09 software.62 Corresponding
frequency calculations confirmed the optimization in their
minimum potential-energy surface in the ground state. The
comparison of metric parameters and FMO energy levels
between the gas- and solution-phase calculations indicates
minimum changes, as shown in the Supporting Information
(Table S8−S12).
Single-Crystal X-ray Diffraction Studies. Single crystals
of 1, 2, 3, 4, and 6 were obtained from the slow evaporation of
their respective solutions in a pentane and chloroform mixture
(5:1). Compounds 1, 2, and 4 were crystallized in the triclinic
̅
crystal system with the P1 space group, whereas 3 and 6 were
crystallized in the monoclinic crystal lattice with P21/n and
P21/c space groups, respectively (Figures 2 and S53−S58,
Tables S1 and S2). The structural parameters obtained from
the single-crystal X-ray analysis are very similar to those of the
DFT optimized structures (Table 1), and similar inferences
can be drawn by analyzing these parameters. The slight
deviation in the values of the bond parameters and dihedral
angles may be due to solid-state packing and the associated
intermolecular interactions in the crystal lattice. The
tricoordinate boron center adopted trigonal planar geometry
in all the compounds with a sum of the three C−B−C angles
being 360°. For 3, 4, and 6, the dihedral angle between the
spacer and BODIPY is ∼88°, whereas much smaller angles
were observed in 1 (58.18°) and 2 (51.27°). This result clearly
indicated that the methyl substituent on the spacer and the
BODIPY moiety orthogonalized the BODIPY and TAB
moieties. In summary, the combined crystallographic and
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DFT analyses suggest that 1 and 2 are relatively more flexible
than 3, 4, and 6. The systematic steric perturbation is highly
useful for fine-tuning the molecular conformation and the
FMOs of the dyads.
Photophysical Properties. Compounds 1−6 exhibited
similar absorption spectral features, with two major peaks at
∼350 and ∼520 nm and a shoulder peak at ∼475 nm (Figures
3, S58, and S59). Model compounds (Chart 2) M4−M7
showed absorption peaks at ∼310−330 nm, whereas M1−M3
showed absorption peaks at ∼350 and ∼500 nm with a
shoulder at ∼455 nm (Figures 4, S66, and S83).
Despite the direct attachment of TAB with the BODIPY
moiety, the electronic interactions between these moieties
were weak, and the absorption bands corresponding to
individual molecular subunits can be recognized in their
spectra of dyads. Thus, the absorption spectra of 1−6 are
additive spectra of both TAB and BODIPY model compounds
(M1−M3 and M4−M7, respectively (Chart 2)). These results
indicated that linking TAB units at the meso-carbon of
BODIPY does not affect the ground and Franck−Condon
excited states of the individual chromophores. Similar
absorption features have been reported for R1 and R2 with
similar molecular constituents.
Even though the absorption bands may be ascribed to the
distinct molecular components of the dyads, the electronic
communications among them are not negligible. The
absorption bands in 1, 2, 5, R1, and R2, which are devoid of
−CH3 substituents closer to the meso-carbon of the BODIPY
unit, are red-shifted compared to M1-M3, indicating the
electronic communication between BODIPY and the attached
TAB unit. In contrast, a blue shift was observed in 3, 4, and 6,
which have −CH3 substituents closer to the meso-carbon of the
BODIPY unit. A robust blue shift was observed in 4, which has
a sterically demanding duryl spacer. Furthermore, there were
no additional bands corresponding to the electronic transitions
of the different mixed electronic levels of the molecular
partners, as observed for thiophene-substituted boryl-BODIPY
triads reported elsewhere.7
Based on these results, the higher energy bands in the region
from 300 to 400 nm can be attributed to the π → π* and π →
π(B)* transition of the TAB moiety, and the intense band at a
lower energy (∼520 nm) could be arise from the HOMO to
LUMO transition centered in the BODIPY moiety. The TD-
DFT results indicated two major electronic transitions in all
the compounds (Table S8 and S9), the HOMO → LUMO
transition centered on the BODIPY unit and the HOMO − 1
→ LUMO + 1 transition centered on the TAB moiety. The
absorption spectral features of these dyads revealed that TAB
and BODIPY could be combined to form dyads in such a
manner that the molecular partners retain the characteristics of
their electronic levels to a large extent. Further, an important
fact of this investigation on the intramolecular energy transfer
is that the selective excitation of the TAB group in the region
of ∼300−350 nm is possible for all dyads.
Normalized emission spectra of 1−6 with R1 and R2 are
depicted in Figure 3, and all the photophysical data are
summarized in Table 2. Upon excitation at 350 nm, 1, 2, and 5
showed two emission bands at ∼410 and ∼520 nm,
respectively; however, the BODIPY-centered band at 520 nm
is slightly broader than the PL band observed for model
compounds M1−M3 (Figure 3). Compounds 3, 4, and 6 with
two methyl groups adjacent to the BODIPY meso-carbon also
exhibit dual PL bands at ∼410 and ∼520 nm, respectively,
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Figure 2. Molecular structures of 1−6. All the hydrogens are omitted for clarity.
Figure 3. Normalized absorption (left) and emission (right) spectra of 1−6, R1, and R2 in dichloromethane (concentration of 10 μM, λex at 350
nm).
Chart 2. Chemical Structures of the Model Compounds (M1−M7)
under similar conditions. The width of the lower-energy band
for all the molecules is comparable to those of the bands
observed in M1−M3; in sharp contrast, the ∼410 nm PL band
in 3, 4, and 6 was noticably weaker than the band observed in
1 (spacer devoid of −CH3 substituents), 2, and 5 (with two
methyl groups adjacent to the Mes2B− unit). Compound R2
has two methyl groups on the BODIPY moiety adjacent to the
meso-carbon and show PL features comparable to those of 1, 2,
and 5 that significantly differ from those of 3, 4, and 6. In
contrast, R1 is devoid of substituents on both the phenyl
spacer, and the first and seventh positions of the BODIPY
moiety show dual emissions with equal intensities. These
results strongly suggested that the position of the methyl
groups in the spacer plays a vital role in controlling the energy
transfer from TAB to the BODIPY moiety.
From the PL spectra of 1−6, it is clear that the emitting S1
state is primarily localized on the BODIPY moiety. The weak
bathochromic shift and line broadening of the BODIPY
fluorescence band can be attributed to the weak electronic
coupling between the TAB and BODIPY moieties. The
presence of emission from the TAB chromophore in these
dyads points to the fact that the segmental features found in
their ground state have prevailed in the excited state. Evidently,
the weak coupling between the TAB and BODIPY and the
rigidity imposed by additional methyl groups on the aryl spacer
in 3, 4, and 6 facilitate faster energy transfer than the excited-
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Figure 4. Normalized absorption and emission spectra of 1, 6, and the respective model compounds M1, M3M4, and M6 in DCM (λex at 350 nm,
concentration of 10 μM).
Table 2. Absorption Band Maxima (λmabasx, nm) with Molar Extinction Coefficients (ε) and Fluorescence Maxima (λemmax, nm) of
Dyads (In Dichloromethane, 10 μM Concentration)
a
compound
λabs (nm)
ε (M−1 cm−1
)
λem (nm)
Stokes shift (cm−1
)
Φ
1
2
3
4
5
6
R1
R2
359, 504
387, 500
309, 502
323, 499
338, 510
313, 512
355, 512
350, 501
5.67 × 104, 1.02 × 105
1.12 × 104, 4.43 × 104
1.71 × 104, 5.39 × 104
1.25 × 104, 3.88 × 104
1.98 × 104, 5.07 × 104
1.18 × 104, 5.31 × 104
2.51 × 104, 7.21 × 104
2.49 × 104, 8.87 × 104
418, 523
445, 514
514
511
425, 524
523
5263
7143
8333
8333
7143
9091
4762
7143
0.38
0.34
0.81
0.84
0.76
0.86
0.49
0.42
410, 531
405, 515
a
The PL quantum yields were calculated for the dichloromethane solution using the following equation: Φ = ΦR × (I/IR) × (AR/A) × (η2/
2
ηR ),here Φ is the quantum yield, I is the area under the PL band, A is the absorbance at λex, and η is the refractive index of the solvent. Standard
0.1 M H2SO4 quinine sulfate (ΦR = 0.577) was used. The emission wavelength used was λex = 350 nm.
state molecular rearrangements. Interestingly, although the
electronic coupling between the D and A moieties in 1, 2, and
5 is larger than in those in 3, 4, and 6, the excited state
molecular conformational changes occur faster than the energy
transfer process because of molecular flexibility; consequently,
well-separated dual-emission peaks were observed. TD-DFT-
computed electronic transitions revealed that electronic
transitions in 1, 2, and 5 involving the LUMO have a
contribution from the vacant p-orbital of tricoordinate boron;
in contrast, such a contribution is missing in 3, 4, and 6. As a
result, 1, 2, and 5 show emissions in the boryl region, which
are nearly absent in 3, 4, and 6.
Concentration-dependent absorption and emission spectra
of all the molecules were recorded in different concentrations
(10−5 and 10−6 M, Figures S61 and S62, respectively). When
the compound concentration decreased, the intensity of
absorption and emission bands decreased without a shift in
peak maxima. The ratio of both the emission bands in 1−6
remains constant in all concentrations. Excitation spectra of all
the compounds were recorded by monitoring the decay of the
corresponding PL bands. Aside from compounds 1 and 5, all
the other compounds excitation spectra are similar to their
respective compound’s absorption spectra (Figure S65). The
difference in the excitation and absorption spectra of 1 and 5
can be attributed to the difference in their ground- and excited-
state structures. The solvent polarity-dependent absorbance
and PL of all the compounds were studied in detail. The results
indicated that the emissive states in these compounds are
dominated by nonpolar excited states (Figures S57 and S58),
which is also supported by the calculated excited-state dipole
moment values (Table S6).
Model compounds M1−M3 showed a single PL band
corresponding to the BODIPY unit, whereas M4−M7 showed
a band at 390 nm. The larger energy gap (∼5557 cm−1)
between these two bands in 1−6 suggests that the emissions
come from different electronic states, i.e., Franck−Condon
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(FC) excited states (the molecular identity is preserved) and a
relaxed S1 state (energy-transfer state, see below). It has been
well-established that a substantial overlap between the
emission spectrum of the donor and the absorption spectrum
of the acceptor invites Forster resonance energy transfer
(FRET) between the donor and the acceptor (Figure 5). Upon
Table 3. TAB → BODIPY Energy-Transfer Efficiency
(ETE) in 1−6, R1, and R2 and the Corresponding Energy-
Transfer Rate
compounds
1
2
3
4
5
6
R1
R2
ETE (%)
ETE rate
54
53
95
1293
100
100
81
290
100
16
9
78
161
(107 s−1
)
2, and 6 (Table S3). The lifetime of all the compounds was in
the nanosecond range, indicating that the emissions are from a
singlet excited state. The longer lifetimes (ca. ∼ 6 to 8 ns) in 3,
4, 5, and 6 indicate that nonradiative decay in these
compounds with a rigid structure is lower than for other
compounds with a flexible molecular structure. These results
are also consistent with the PL quantum yields of 1−6.
The PL quantum yield (PLQY), radiative rate, and
nonradiative rate constants were also calculated (Table S3)
for all the dyads. The PLQYs of 3, 4, 5, and 6 are higher
(almost double) than those obtained for 1, 2, R1, and R2. A
higher quantum yield and a longer lifetime for longer
wavelength bands suggest that the energy transfer is more
efficient in compounds 3, 4, 5, and 6. On the other hand, the
results obtained for 1, 2, R1, and R2 suggested that energy
transfer is less efficient in these systems. From these results,
one can conclude that all the newly synthesized compounds
show dual-emission characteristics because of incomplete
fluorescence resonance energy transfer from the donor to
acceptor moieties.
Cyclic Voltammetry Studies. Cyclic voltammetry studies
were performed for 1−6 and R1 and R2 to gain a detailed
insight of their electronic structures (Figures S94 and S95 and
Table S7). The experiments were performed in a dichloro-
methane solution (1 mM) for oxidation and in a THF solution
(1 mM) for reduction with TBAPF6 (100 mM) as the
supporting electrolyte at 20 °C using a Pt disk as a working
electrode, a Pt wire as a counter electrode, and a Ag wire as a
reference electrode. The potential scales are referenced to the
ferrocene/ferrocenium couple. During the anodic scan, only 6
and R2 showed single reversible oxidation processes at 0.91
and 0.80 V, respectively. All other molecules show single
reversible oxidation processes in the region from 1.20 to 1.25
V, which is very similar to the reported data for M1 (1.19 V).67
The lower values for 6 and R2 could be due to the electron-
donating effect of the extra methyl groups present in the
molecules. During the cathodic scan, all the molecules show
two reduction processes. Reversible reduction processes were
observed in the range from −1.2 to −1.6 V. 1 has the lowest
first-reduction potential at −1.18 V, which is comparable to the
reported value −1.15 V for M1,67 whereas 6 has the highest
value of −1.64 V. As 5, 6, and R2 have extra methyl groups on
the bodipy core, they have higher reduction potentials
compared to those of the others. Although R1 has the extra
methyl groups on the BODIPY core, the effect of the TAB
group probably accounts for the reduction potential at −1.34
V. The second reduction potential was observed from −1.8 to
−2.5 V. This is also minimum for 1 (−1.79 V) and a maximum
for 6 (−2.48 V). Reduction peaks corresponding to the
triarylboron center would be ca. −2.5 V,68 although they were
not observed for these molecules even when the scans were
done up to −3.5 V (with respect to Fc/Fc+). HOMO and
LUMO energies were also calculated using the first-oxidation
and first-reduction potentials and were comparable to the
Figure 5. (Left) Schematic representation of the excitation energy
transfer from the donor (TAB) to the acceptor (BODIPY). (Right)
Comparison of the emission spectrum of TAB (M1) and the
absorption spectrum of BODIPY (M4).
excitation, the molecule reaches the FC state where within the
time window the energy transfer takes place from D to A,
going to a relaxed S1 state. Furthermore, favorable orientations
of the donor−acceptor chromophores, the closeness between
the donor and the acceptor, and the nonmixing FMOs of two
chromophores are salient criteria for FRET process. All the
requirements mentioned above were fulfilled in dyads 1−6;
thus, FRET is operative in these dyes. Therefore, dual emission
in these dyads is due to the simultaneous emission from the
FC state (boryl emission) and the energy-transfer state (FRET
emission). The difference in the ratio of the dual-emission
bands in these compounds indicated that the EET operates
differently in these compounds depending on the molecular
conformations.
Feeble luminescence from TAB components indicated that
the energy transfer from TAB to BODIPY is very efficient in
these dyads. The energy-transfer efficiencies (ETEs)63−65 in
1−6 were calculated from their respective PL spectra using the
equation ETE = A − D, where D is the emission intensity of the
A
donor TAB and A is the emission intensity of the acceptor
BODIPY dye. Higher ETEs were calculated for 2, 3, 4, 5, and
6, which have additional methyl substituents on the aryl spacer
either close to the meso-carbon or the Mes2B- moiety, than for
1, R1 and R2, which are devoid of substituents on the phenyl
spacer. These results indicated that the identity of the spacer
connecting the donor and the acceptor is crucial in altering the
EET and PL quantum yield in these types of compounds. The
energy-transfer rates for 1, 2, 5, R1, and R2 were calculated
(see the Supporting Information for details, eq S1), and the
results are summarized in Table 3.66 The calculated energy-
transfer rate amply corroborated the ETE, as the compounds
with higher ETEs show higher energy-transfer rates and vice
versa.
Time-resolved decay kinetics were performed on all the
compounds (Figure S67−S82) in dichloromethane, and the
results are summarized in Table S3. In all the compounds, the
higher-energy band at ∼410 nm showed a biexponential decay
with a short and a long lifetime. The lower-energy band at
∼510 nm showed a single exponential decay for 3, 4, and 5 like
in R1 and R2, while a biexponential decay was obtained for 1,
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Figure 6. Changes in absorption and emission spectra of 1 and 6 (in DCM, 1.0 × 10−5 M) upon the incremental addition of TBAF (in DCM, 1.0 ×
10−3 M). (A) Absorption of 1, (B) emission of 1, (C) absorption of 6, and (D) emission of 6 (λex = 350 nm).
energies obtained from DFT calculations, as depicted in Table
S7.
observation directly corroborates the results obtained in optical
studies.
Fluoride binding strongly quenched the PL of 1, 3, and 6;
however, very minimal PL quenching was observed in 2, 4, and
5 (Figures 6 and S84). This result indicated that fluoride
binding altered the energy transfer from the donor (TAB) to
the acceptor (BODIPY) moiety. Interestingly, with the
successive addition of fluoride ions, the intensities of both
the emission bands in 1 gradually decreased (Figure 6).
However, a rapid decrease in the intensity of the longer-
wavelength band compared to that of the lower wavelength
band was observed for 6. It was found that the addition of
fluoride ions did not quench the BODIPY emission in M1, M3,
and the boryl emission of M8 (boron is much more sterically
protected) (Chart S1 and Figure S91). However, F− binding
altered the absorption and extinguished the boryl emission in
M4 (boron is less sterically protected). The fluorescence
excitation spectra (λem = 520 nm) of 1, 3, and 6 were recorded
in the presence of TBAF (Figure S85). In accordance with the
quenching of the BODIPY emission band in the presence of
fluoride, the intensities of both the excitation bands also
decreased. Thus, the absence of BODIPY emission in fluoride-
bound adducts of 1, 3, and 6 upon being excited at the TAB
domain indicates a decrease in the energy transfer from TAB
→ BODIPY.
Effect of External Chemical Stimuli on EET in 1−6. As
discussed, vide supra, the boron center in the TAB moiety can
expand its coordination from three to four in the presence of a
suitable anion and consequently modify the EET from TAB to
the BODIPY moiety. Thus, anion binding studies on dyads 1−
6 were carried out in detail to understand the effect of
molecular conformation on the sensing and EET from the
donor (TAB) to the acceptor (BODIPY). Upon fluoride
addition, the absorption bands in 1, 3, and 6 were blue-shifted
with an isosbestic point, indicating that the fluoride binding
disrupts the electronic conjugation between the TAB and
BODIPY moieties through an empty p-orbital on boron in the
TAB fragments (Figures 6 and S84). The optical changes were
more apparent in 1 than in 3 and 6, with a methyl group close
to the meso-carbon of the BODIPY unit. Compounds 2, 4, and
5 with sterically hindered tricoordinate boron (an aryl spacer
with methyl groups around the Mes2B− moiety) do not show
appreciable changes in their absorption spectra in the presence
of fluoride, indicating that the steric crowding around TAB
significantly affects the fluoride binding in these compounds.
These observations are comparable to the fluoride binding
results of TABs reported elsewhere.7,59 The fluoride binding
events in these compounds were also studied using 19F NMR
spectroscopic methods (Figures S86−S88 for 1, 3, and 6,
respectively). In the presence of fluoride, compounds 1, 3, and
6 showed a 19F signal corresponding to the (Ar)3BF moiety at
ca. −172 ppm. In contrast, compounds 2, 4, and 5 did not
show a 19F signal corresponding to the (Ar)3BF moiety. These
results indicated that the fluoride ions bind to the less sterically
hindered tricoordinate boron in 1, 3, and 6 and not to the
sterically demanding tricoordinate boron in 2, 4, and 5. This
Like fluoride, TABs bind equally to cyanide and show
different optical features. The addition of a cyanide ion brings
about significantly fewer changes in the optical characteristics
of 1, 3, and 6 and no changes in cases of 2, 4, and 5 (Figure
S89). Furthermore, the optical feature of 1−6 did not change
in the presence of Cl−, Br−, I−, ClO4 , HSO4 , H2PO4 , NO3 ,
−
−
−
−
−
and PF6 (10 equiv or excess) (Figure S90), indicating that
these probes are selective to only fluoride ions. The limits of
detection (LOD) for 1, 3, and 6 were calculated to be 1.34
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( 0.06), 1.00 ( 0.05), and 0.07 ( 0.03) ppm, respectively.
These results are comparable with the values reported
elsewhere for similar TAB compounds.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Finally, the reversibility of fluoride binding in 1, 3, and 6 was
studied using BF3·Et2O as an external Lewis acid (Figure S94).
The addition of BF3·Et2O to the solutions of 1 + F−, 3 + F−,
and 6 + F−, restored the PL features of the free Lewis acids (1,
3, and 6), indicating that BF3·Et2O abstracted the boron-
bound fluoride in these compounds and subsequently
regenerated 1, 3, and 6. Thus, these TAB-BODIPY dyads
can also be used for ON/OFF and OFF/ON fluorescence
switching in the presence and absence of fluoride ions,
respectively.
AUTHOR INFORMATION
■
Corresponding Authors
Pakkirisamy Thilagar − Department of Inorganic and
Physical Chemistry, Indian Institute of Science, Bangalore
560012, India; orcid.org/0000-0001-9569-7733;
Email: thilagar@iisc.ac.in
Chinna Ayya Swamy P − Department of Chemistry, National
Institute of Technology Calicut, Kozhikode 673601, India;
orcid.org/0000-0001-7875-9517; Email: swamy@
nitc.ac.in
CONCLUSION
■
Authors
We have demonstrated a versatile design strategy for
developing a new class of dual-emissive dyads. Dyads 1−6
were constructed with similar molecular building blocks, and
their molecular conformations were systematically fine-tuned
by judiciously varying the number of methyl substituents on
both TAB and BODIPY moieties. Despite the direct
attachment of TAB to the BODIPY moiety, the electronic
interactions are weak, and the absorption bands corresponding
to individual molecular subunits can be recognized in the
spectra of the dyads. The covalent linking of TAB unit at the
meso-carbon of BODIPY does not affect the ground and
Franck−Condon excited states of the individual chromo-
phores. Furthermore, a weak coupling between TAB and
BODIPY accounts for the slight red-shift and broadening of
the BODIPY fluorescence band. The residual TAB emission in
these dyads points to the fact that the segmental features found
in their ground state prevail in the excited state. The weak
coupling between TAB and BODIPY combined with the steric
hindrance imposed by methyl groups on the aryl spacer in 3, 4,
and 6 enables faster energy transfer than the excited-state
molecular reorganization. Though the electronic coupling
between the donor and acceptor moieties in 1, 2, and 5 is
significantly larger than that in 3, 4, and 6, the excited-state
molecular reorganization occurs faster than the energy transfer
because the molecular flexibility in the latter set of dyads leads
to intriguing dual-emission features. This study revealed that
TAB and BODIPY may be combined to form dyads in such a
manner that the molecular partners retain their characteristic
electronic levels to a large extent. An important fact of this
investigation on intramolecular energy transfer is that the
selective excitation of the TAB group in the region ∼300−350
nm is possible for all dyads. Further, external stimuli, such as
fluoride, can be used as probes to understand the EET in these
dyads.
Rajendra Prasad Nandi − Department of Inorganic and
Physical Chemistry, Indian Institute of Science, Bangalore
560012, India; orcid.org/0000-0001-9597-9466
Pandi Dhanalakshmi − Department of Inorganic and Physical
Chemistry, Indian Institute of Science, Bangalore 560012,
India
Santosh Kumar Behera − Department of Inorganic and
Physical Chemistry, Indian Institute of Science, Bangalore
560012, India; orcid.org/0000-0003-2066-4712
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02739
Funding
SERB, New Delhi, India. File no. EMR/2015/000572 dtd.
21.3.16
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.T. thanks SERB New Delhi, India for financial support;
C.A.S.P. and R.P.N. thank IISc for a research fellowship; S.K.B.
thanks UGC, New Delhi, India for a Kothari postdoctoral
fellowship; and P.D. thanks SERB for a National Postdoctoral
fellowship (file no. PDF/2018/000099) New Delhi, India.
Authors thank the IPC Department and IISc Bangalore, India
for infrastructure and instrumental facilities.
REFERENCES
■
(1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.;
Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Bredas,
J. L.; Logdlund, M.; Salaneck, W. R. Electroluminescence in
conjugated polymers. Nature 1999, 397, 121−128.
(2) Organic Light-Emitting Devices; Shinar, J., Ed.; Springer-Verlag
New York: New York, NY, 2004.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02739.
(3) Goncalves, M. S. Fluorescent labeling of biomolecules with
■
sı
organic probes. Chem. Rev. 2009, 109, 190−212.
(4) Ding, D.; Li, K.; Liu, B.; Tang, B. Z. Bioprobes based on AIE
fluorogens. Acc. Chem. Res. 2013, 46, 2441−53.
(5) Liu, Y.; Pan, M.; Yang, Q.-Y.; Fu, L.; Li, K.; Wei, S.-C.; Su, C.-Y.
Dual-Emission from a Single-Phase Eu-Ag Metal-Organic Framework:
An Alternative Way to Get White-Light Phosphor. Chem. Mater.
2012, 24, 1954−1960.
Detailed synthesis procedure, characterization data,
additional photophysical studies, NMR studies, and
theoretical calculation data (PDF)
(6) P, P. C. S.; Mukherjee, S.; Thilagar, P. Multichannel-emissive V-
shaped boryl-BODIPY dyads: Synthesis, structure, and remarkably
diverse response toward fluoride. Inorg. Chem. 2014, 53, 4813−4823.
(7) Sarkar, S. K.; Thilagar, P. A borane-bithiophene-BODIPY triad:
intriguing tricolor emission and selective fluorescence response
towards fluoride ions. Chem. Commun. 2013, 49, 8558−60.
Accession Codes
CCDC 1994506−1994510 contain the supplementary crys-
tallographic 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
5460
https://doi.org/10.1021/acs.inorgchem.0c02739
Inorg. Chem. 2021, 60, 5452−5462

Inorganic Chemistry
pubs.acs.org/IC
Article
(8) Zhou, C.; Zhang, S.; Gao, Y.; Liu, H.; Shan, T.; Liang, X.; Yang,
B.; Ma, Y. Ternary Emission of Fluorescence and Dual Phosphor-
escence at Room Temperature: A Single-Molecule White Light
Emitter Based on Pure Organic Aza-Aromatic Material. Adv. Funct.
Mater. 2018, 28, 1802407.
(9) He, Z. K.; Zhao, W. J.; Lam, J. W. Y.; Peng, Q.; Ma, H. L.; Liang,
G. D.; Shuai, Z. G.; Tang, B. White light emission from a single
organic molecule with dual phosphorescence at room temperature.
Nat. Commun. 2017, 8, 416.
(10) Yang, J.; Ren, Z.; Xie, Z.; Liu, Y.; Wang, C.; Xie, Y.; Peng, Q.;
Xu, B.; Tian, W.; Zhang, F.; et al. AIEgen with fluorescence-
phosphorescence dual mechanoluminescence at room temperature.
Angew. Chem., Int. Ed. 2017, 56, 880−884.
(11) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. A
dual-emissive-materials design concept enables tumour hypoxia
imaging. Nat. Mater. 2009, 8, 747−51.
(12) Gong, Y.; Zhao, L.; Peng, Q.; Fan, D.; Yuan, W. Z.; Zhang, Y.;
Tang, B. Z. Crystallization-induced dual emission from metal- and
heavy atom-free aromatic acids and esters. Chem. Sci. 2015, 6, 4438−
4444.
(13) Li, X.; Baryshnikov, G.; Deng, C.; Bao, X.; Wu, B.; Zhou, Y.;
Agren, H.; Zhu, L. A three-dimensional ratiometric sensing strategy
on unimolecular fluorescence-thermally activated delayed fluores-
cence dual emission. Nat. Commun. 2019, 10, 731.
(14) Takeda, Y.; Kaihara, T.; Okazaki, M.; Higginbotham, H.; Data,
P.; Tohnai, N.; Minakata, S. Conformationally-flexible and moderately
electron-donating units-installed D-A-D triad enabling multicolor-
changing mechanochromic luminescence, TADF and room-temper-
ature phosphorescence. Chem. Commun. 2018, 54, 6847−6850.
(15) Bhattacharjee, I.; Acharya, N.; Bhatia, H.; Ray, D. Dual
Emission through Thermally Activated Delayed Fluorescence and
Room-Temperature Phosphorescence, and Their Thermal Enhance-
ment via Solid-State Structural Change in a Carbazole-Quinoline
Conjugate. J. Phys. Chem. Lett. 2018, 9, 2733−2738.
fluorescent molecular probes and luminescent materials. Phys. Chem.
Chem. Phys. 2012, 14, 8803−17.
(26) Dexter, D. L. A Theory of Sensitized Luminescence in Solids. J.
Chem. Phys. 1953, 21, 836−850.
(27) Forster, T. 10th Spiers Memorial Lecture - Transfer
Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959,
27, 7−17.
(28) Scandola, F.; Balzani, V. Energy-transfer processes of excited
states of coordination compounds. J. Chem. Educ. 1983, 60 (10), 814.
(29) Valeur, B. Molecular Fluorescence: Principles and Applications;
Wiley-VCH Verlag GmbH: Weinheim, Germany, 2001.
(30) Wang, J.; Huang, J.; Du, L.; Lan, Z. Photoinduced Ultrafast
Intramolecular Excited-State Energy Transfer in the Silylene-Bridged
Biphenyl and Stilbene (SBS) System: A Nonadiabatic Dynamics Point
of View. J. Phys. Chem. A 2015, 119, 6937−48.
(31) Zhao, W.; Cheung, T. S.; Jiang, N.; Huang, W.; Lam, J. W. Y.;
Zhang, X.; He, Z.; Tang, B. Z. Boosting the efficiency of organic
persistent room-temperature phosphorescence by intramolecular
triplet-triplet energy transfer. Nat. Commun. 2019, 10, 1595.
(32) Cravcenco, A.; Hertzog, M.; Ye, C.; Iqbal, M. N.; Mueller, U.;
Eriksson, L.; Borjesson, K. Multiplicity conversion based on
intramolecular triplet-to-singlet energy transfer. Sci. Adv. 2019, 5,
No. eaaw5978.
(33) Selvin, P. R. The renaissance of fluorescence resonance energy
transfer. Nat. Struct. Biol. 2000, 7, 730−4.
(34) Stryer, L.; Haugland, R. P. Energy transfer: a spectroscopic
ruler. Proc. Natl. Acad. Sci. U. S. A. 1967, 58, 719−26.
(35) Clegg, R. M. Fluorescence resonance energy transfer. Curr.
Opin. Biotechnol. 1995, 6, 103−10.
(36) Takakusa, H.; Kikuchi, K.; Urano, Y.; Sakamoto, S.; Yamaguchi,
K.; Nagano, T. Design and synthesis of an enzyme-cleavable sensor
molecule for phosphodiesterase activity based on fluorescence
resonance energy transfer. J. Am. Chem. Soc. 2002, 124, 1653−7.
(37) Albers, A. E.; Okreglak, V. S.; Chang, C. J. A FRET-based
approach to ratiometric fluorescence detection of hydrogen peroxide.
J. Am. Chem. Soc. 2006, 128, 9640−1.
(38) Han, Z. X.; Zhang, X. B.; Li, Z.; Gong, Y. J.; Wu, X. Y.; Jin, Z.;
He, C. M.; Jian, L. X.; Zhang, J.; Shen, G. L.; Yu, R. Q. Efficient
fluorescence resonance energy transfer-based ratiometric fluorescent
cellular imaging probe for Zn(2+) using a rhodamine spirolactam as a
trigger. Anal. Chem. 2010, 82, 3108−13.
(39) Zhang, X.; Xiao, Y.; Qian, X. A ratiometric fluorescent probe
based on FRET for imaging Hg2+ ions in living cells. Angew. Chem.,
Int. Ed. 2008, 47, 8025−9.
(40) Guliyev, R.; Coskun, A.; Akkaya, E. U. Design strategies for
ratiometric chemosensors: modulation of excitation energy transfer at
the energy donor site. J. Am. Chem. Soc. 2009, 131, 9007−13.
(41) Laquai, F.; Park, Y. S.; Kim, J. J.; Basche, T. Excitation energy
transfer in organic materials: from fundamentals to optoelectronic
devices. Macromol. Rapid Commun. 2009, 30, 1203−31.
(42) Hardin, B. E.; Sellinger, A.; Moehl, T.; Humphry-Baker, R.;
Moser, J. E.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M.; McGehee, M.
D. Energy and hole transfer between dyes attached to titania in
cosensitized dye-sensitized solar cells. J. Am. Chem. Soc. 2011, 133,
10662−7.
(43) Zhang, D.; Cai, M.; Zhang, Y.; Bin, Z.; Zhang, D.; Duan, L.
Simultaneous Enhancement of Efficiency and Stability of Phosphor-
escent OLEDs Based on Efficient Forster Energy Transfer from
Interface Exciplex. ACS Appl. Mater. Interfaces 2016, 8, 3825−32.
(44) Yamaguchi, S.; Akiyama, S.; Tamao, K. Colorimetric fluoride
ion sensing by boron-containing pi-electron systems. J. Am. Chem. Soc.
2001, 123, 11372−5.
(16) Chen, C. J.; Huang, R. J.; Batsanov, A. S.; Pander, P.; Hsu, Y.
T.; Chi, Z. G.; Dias, F. B.; Bryce, M. R. Intramolecular Charge
Transfer Controls Switching Between Room Temperature Phosphor-
escence and Thermally Activated Delayed Fluorescence. Angew.
Chem., Int. Ed. 2018, 57, 16407−16411.
(17) Cho, Y. J.; Jeon, S. K.; Chin, B. D.; Yu, E.; Lee, J. Y. The design
of dual emitting cores for green thermally activated delayed
fluorescent materials. Angew. Chem., Int. Ed. 2015, 54, 5201−4.
(18) Lippert, E.; Luder, W.; Moll, F.; Nägele, W.; Boos, H.; Prigge,
̈
H.; Seibold-Blankenstein, I. Conversion of electron excitation energy.
Angew. Chem. 1961, 73, 695−706.
(19) Zachariasse, K. A.; Grobys, M.; vonderHaar, T.; Hebecker, A.;
Il'ichev, Y. V.; Jiang, Y. B.; Morawski, O.; Kuhnle, W. Intramolecular
charge transfer in the excited state. Kinetics and configurational
changes. J. Photochem. Photobiol., A 1996, 102, 59−70.
(20) Rotkiewicz, K.; Grellmann, K.; Grabowski, Z. Reinterpretation
of the anomalous fluorescense of pn, n-dimethylamino-benzonitrile.
Chem. Phys. Lett. 1973, 19, 315−318.
́
(21) Siemiarczuk, A.; Grabowski, Z. R.; Krówczynski, A.; Asher, M.;
Ottolenghi, M. Two emitting states of excited p-(9-anthryl)-n, n-
dimethylaniline derivatives in polar solvents. Chem. Phys. Lett. 1977,
51, 315−320.
(22) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A. Dual
fluorescence of donor-acceptor molecules and the Twisted Intra-
molecular Charge Transfer (TICT) states. J. Lumin. 1979, 18−19,
420−424.
(23) Kasha, M. Proton-Transfer Spectroscopy - Perturbation of the
Tautomerization Potential. J. Chem. Soc., Faraday Trans. 2 1986, 82,
2379−2392.
(24) Formosinho, S. J.; Arnaut, L. G. Excited-state proton transfer
reactions II. Intramolecular reactions. J. Photochem. Photobiol., A 1993,
75, 21−48.
(45) Sudhakar, P.; Neena, K. K.; Thilagar, P. Borylated
perylenediimide: self-assembly, photophysics and sensing application.
Dalton Trans. 2019, 48, 7218−7226.
(46) Boens, N.; Leen, V.; Dehaen, W. Fluorescent indicators based
on BODIPY. Chem. Soc. Rev. 2012, 41, 1130−72.
(25) Zhao, J.; Ji, S.; Chen, Y.; Guo, H.; Yang, P. Excited state
intramolecular proton transfer (ESIPT): from principal photophysics
to the development of new chromophores and applications in
5461
https://doi.org/10.1021/acs.inorgchem.0c02739
Inorg. Chem. 2021, 60, 5452−5462

Inorganic Chemistry
pubs.acs.org/IC
Article
(47) Loudet, A.; Burgess, K. BODIPY dyes and their derivatives:
syntheses and spectroscopic properties. Chem. Rev. 2007, 107, 4891−
932.
(48) Ulrich, G.; Ziessel, R.; Harriman, A. The chemistry of
fluorescent bodipy dyes: versatility unsurpassed. Angew. Chem., Int.
Ed. 2008, 47, 1184−201.
(49) Filatov, M. A. Heavy-atom-free BODIPY photosensitizers with
intersystem crossing mediated by intramolecular photoinduced
electron transfer. Org. Biomol. Chem. 2020, 18, 10−27.
(50) Lu, J. S.; Ko, S. B.; Walters, N. R.; Wang, S. Decorating
BODIPY with three- and four-coordinate boron groups. Org. Lett.
2012, 14, 5660−3.
(51) Huh, J. O.; Do, Y.; Lee, M. H. A BODIPY-Borane Dyad for the
Selective Complexation of Cyanide Ion. Organometallics 2008, 27,
1022−1025.
(52) Fu, G. L.; Pan, H.; Zhao, Y. H.; Zhao, C. H. Solid-state emissive
triarylborane-based BODIPY dyes: photophysical properties and
fluorescent sensing for fluoride and cyanide ions. Org. Biomol.
Chem. 2011, 9, 8141−6.
(53) Sun, H. B.; Dong, X. C.; Liu, S. J.; Zhao, Q.; Mou, X.; Yang, H.
Y.; Huang, W. Excellent BODIPY Dye Containing Dimesitylboryl
Groups as PeT-Based Fluorescent Probes for Fluoride. J. Phys. Chem.
C 2011, 115, 19947−19954.
(63) Sakamoto, K.; Takashima, Y.; Hamada, N.; Ichida, H.;
Yamaguchi, H.; Yamamoto, H.; Harada, A. Selective photoinduced
energy transfer from a thiophene rotaxane to acceptor. Org. Lett.
2011, 13, 672−5.
(64) Atilgan, S.; Ozdemir, T.; Akkaya, E. U. Selective Hg(II) sensing
with improved Stokes shift by coupling the internal charge transfer
process to excitation energy transfer. Org. Lett. 2010, 12, 4792−5.
(65) Liu, J. Y.; Yeung, H. S.; Xu, W.; Li, X.; Ng, D. K. Highly
efficient energy transfer in subphthalocyanine-BODIPY conjugates.
Org. Lett. 2008, 10, 5421−4.
(66) Hildebrandt, N., How to Apply FRET: From Experimental
̈
Design to Data Analysis. In FRET - Forster Resonance Energy Transfer;
Medintz, I., Hildebrandt, N., Eds.; Wiley-VCH Verlag GmbH & Co.
KGaA: Weinheim, Germany, 2013; pp 105−163.
(67) Richards, V. J.; Gower, A. L.; Smith, J. E.; Davies, E. S.; Lahaye,
D.; Slater, A. G.; Lewis, W.; Blake, A. J.; Champness, N. R.; Kays, D.
L. Synthesis and characterisation of BODIPY radical anions. Chem.
Commun. 2012, 48, 1751.
(68) Cummings, S. A.; Iimura, M.; Harlan, C. J.; Kwaan, R. J.; Trieu,
I. V.; Norton, J. R.; Bridgewater, B. M.; Jäkle, F.; Sundararaman, A.;
Tilset, M. An Estimate of the Reduction Potential of B(C6F5)3from
Electrochemical Measurements on Related Mesityl Boranes. Organo-
metallics 2006, 25, 1565−1568.
(54) Yin, Z.; Tam, A. Y.; Wong, K. M.; Tao, C. H.; Li, B.; Poon, C.
T.; Wu, L.; Yam, V. W. Functionalized BODIPY with various sensory
units–a versatile colorimetric and luminescent probe for pH and ions.
Dalton Trans. 2012, 41, 11340−50.
(55) P, P. C. S.; Priyanka, R. N.; Thilagar, P. Triarylborane-
dipyrromethane conjugates bearing dual receptor sites: the synthesis
and evaluation of the anion binding site preference. Dalton Trans.
2014, 43, 4067−4075.
(56) Swamy P, C. A.; Priyanka, R. N.; Mukherjee, S.; Thilagar, P.
Panchromatic Borane-aza-BODIPY Conjugate: Synthesis, Intriguing
Optical Properties, and Selective Fluorescent Sensing of Fluoride
Anions. Eur. J. Inorg. Chem. 2015, 2015, 2338−2344.
(57) Misra, R.; Jadhav, T.; Dhokale, B.; Mobin, S. M. Colorimetric
and fluorimetric detection of fluoride and cyanide ions using tri and
tetra coordinated boron containing chromophores. Dalton Trans.
2015, 44, 16052−16060.
(58) Sarkar, S. K.; Kumar, G. R.; Thilagar, P. White light emissive
molecular siblings. Chem. Commun. 2016, 52, 4175−8.
(59) Swamy, C. A.; Mukherjee, S.; Thilagar, P. Dual emissive
borane-BODIPY dyads: Molecular conformation control over
electronic properties and fluorescence response towards fluoride
ions. Chem. Commun. 2013, 49, 993−995.
(60) Swamy, C. A. P. Design and Syntheses of Triarylborane Decorated
Luminescent Dyes: Intriguing Optical Properties and Anion Sensing
Applications. PhD Thesis, Indian Institute of Science, Bangalore, India,
2014.
(61) Kumar, G. R.; Thilagar, P. Triarylborane conjugated acacH
ligands and their BF 2 complexes: facile synthesis and intriguing
optical properties. Dalton Trans. 2014, 43, 3871−3879.
(62) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,
G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.
V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;
Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson,
T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.;
Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Throssell, K.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov,
V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J.
Gaussian 09; Gaussian, Inc.: Wallingfor, CT, 2009.
5462
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