Impact of deboronation on the electronic characteristics ofcloso-o-carborane: intriguing photophysical changes in triazole-appended carboranyl luminophores

Article abstract of DOI:10.1039/d0dt04038j

5-Phenyl-1,2,4-triazole-appendedcloso- (CB1andCB2) andnido-o-carboranyl (nido-CB1andnido-CB2) compounds were prepared and fully characterized using multinuclear NMR spectroscopy and elemental analysis. The solid-state molecular structures of bothcloso-compounds were analyzed by X-ray crystallography. Although thecloso-compounds exhibited dual emissive patterns in the rigid state (in THF at 77 K), which were assignable to a π-π* local excitation (LE)-based emission (λ_em=ca. 380 nm) on the triazole moieties and to an intramolecular charge transfer (ICT)-based emission (ca. 460 nm) in which theo-carborane units acted as the acceptor (A), at 298 K in THF, the LE-based emission dominated. In contrast, thenido-compounds exhibited an intensive emission originating from ICT transitions in which theo-carborane units reversibly acted as the donor (D). In particular, the positive solvatochromic effects of bothnido-compounds and the results of theoretical calculations for theo-carboranyl compounds supported the electronic role of theo-carboranyl unit in each compound. Investigation of the radiative decay mechanism of thecloso- andnido-compounds using their quantum efficiency (Φ_em) and decay lifetime (τ_obs) suggested that the ICT-based radiative decay ofnido-compounds occurred more efficiently than the LE-based decay ofcloso-compounds. These results implied that emission from thecloso-compounds was drastically enhanced by the deboronation reaction upon exposure to an increasing concentration of fluoride anions, and finally became similar to the emission color (sky-blue) of thenido-compounds.

Full text of DOI:10.1039/d0dt04038j

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Impact of deboronation on the electronic
characteristics of closo-o-carborane: intriguing
photophysical changes in triazole-appended
carboranyl luminophores†
Cite this: Dalton Trans., 2021, 50,
3207
Mingi Kim,‡ Sehee Im,‡ Chan Hee Ryu, Seok Ho Lee, Ju Hyun Hong and
Kang Mun Lee
*
5-Phenyl-1,2,4-triazole-appended closo- (CB1 and CB2) and nido-o-carboranyl (nido-CB1 and nido-
CB2) compounds were prepared and fully characterized using multinuclear NMR spectroscopy and
elemental analysis. The solid-state molecular structures of both closo-compounds were analyzed by
X-ray crystallography. Although the closo-compounds exhibited dual emissive patterns in the rigid state
(in THF at 77 K), which were assignable to a π–π* local excitation (LE)-based emission (λem = ca. 380 nm)
on the triazole moieties and to an intramolecular charge transfer (ICT)-based emission (ca. 460 nm) in
which the o-carborane units acted as the acceptor (A), at 298 K in THF, the LE-based emission dominated.
In contrast, the nido-compounds exhibited an intensive emission originating from ICT transitions in which
the o-carborane units reversibly acted as the donor (D). In particular, the positive solvatochromic effects
of both nido-compounds and the results of theoretical calculations for the o-carboranyl compounds
supported the electronic role of the o-carboranyl unit in each compound. Investigation of the radiative
decay mechanism of the closo- and nido-compounds using their quantum efficiency (Φem) and decay
lifetime (τobs) suggested that the ICT-based radiative decay of nido-compounds occurred more efficiently
than the LE-based decay of closo-compounds. These results implied that emission from the closo-com-
pounds was drastically enhanced by the deboronation reaction upon exposure to an increasing concen-
tration of fluoride anions, and finally became similar to the emission color (sky-blue) of the nido-
compounds.
Received 25th November 2020,
Accepted 3rd February 2021
DOI: 10.1039/d0dt04038j
rsc.li/dalton
when it is directly linked with aromatic organic groups.21–47
Such dyad molecules exhibit distinct intramolecular charge
Introduction
The icosahedral boron-cluster component, closo-carborane transfer
(ICT)
transitions
during
excitation
and
(1,2-dicarba-closo-dodecaborane), is a well-known three-dimen- relaxation.3,4,21–24,29–47 It has also been reported that closo-o-
sional (3D) variant.1–5 Recently, it has attracted widespread carborane (A)-containing π-conjugated aromatic organic lumi-
attention as a functional unit because o-carborane-appended nophores (D) can exhibit specific luminescence characteristics
π-aromatic fluorophores can exhibit unique photophysical based on the ICT transition. Furthermore, the ICT-based radia-
properties.3–17 For example, its strong electron withdrawing tive decay mechanism in such D–A systems is severely affected
character through the carbon atoms of the cluster,18–20 which by conformational changes involving twisting between the D
arises from the polarizability of its σ-aromaticity, allows the and A units in the excited state, which is known as twisted
formation of electron donor (D)–acceptor (A) dyad systems, intramolecular charge transfer (TICT).31–37 In the context of
o-carborane possessing π-conjugated dyad luminophores, our
group has also demonstrated that ICT-based emission can be
controlled by the structural orientation, such as any distortion
Department of Chemistry, Institute for Molecular Science and Fusion Technology,
or planarity,41–45 in addition to the rotational regulation of the
Kangwon National University, Chuncheon, Gangwon 24341, Republic of Korea.
π-conjugated aromatic organic luminophore (D).46,47
E-mail: kangmunlee@kangwon.ac.kr
†Electronic supplementary information (ESI) available: Multinuclear NMR
The closo-o-carborane cage can also be readily converted
spectra, X-ray crystallographic data, spectroscopic data, and computational
into a nido-o-carborane (a nest-like structure, where one boron
results. CCDC 2045310 for CB1 and 2045311 for CB2. For ESI and crystallo-
atom is removed from the icosahedron) via a deboronation
graphic data in CIF or other electronic format see DOI: 10.1039/d0dt04038j
reaction in the presence of a weak base, such as halide
‡These authors are equally contributed to this work.
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anions.46,48–55 Interestingly, the intrinsic characteristics of the species and alteration of the intrinsic photophysical pro-
o-carborane unit are dramatically altered to the extent that it perties, we prepare two N-phenyl-containing 1,2,4-triazole-
becomes electron donating, and this can be attributed to the based closo-o-carboranyl compounds, CB1 and CB2, in
anionic character of the nido-species.48–60 Such a change can addition to their corresponding nido-species, nido-CB1, and
therefore provide unprecedented photophysical properties to nido-CB2 for comparison of their photophysical characteristics.
nido-o-carboranyl organic luminophores. Indeed, triaryl- Detailed synthesis procedures, characterization, and investi-
borane-appended nido-o-carboranyl compounds have been gation of the optical properties (with theoretical calculations)
demonstrated to form a donor (nido-o-carborane)–acceptor of these compounds are also described, as is the application of
(triarylborane) dyad system, which exhibits thermally-activated closo-o-carboranyl 1,2,4-triazole-based-conjugates in sensing
delayed fluorescence (TADF).53,55 Furthermore, the optical materials.
characteristics of various closo-o-carboranyl organic lumino-
phores have been found to change dramatically during conver-
sion to the nido-o-carborane structure in the presence of weak
Results and discussion
Synthesis and characterization
bases and halides, thereby demonstrating their potential appli-
cation as sensory materials for such anionic species.48–54,59,60
To further investigate such significant changes in the elec- The synthesis routes employed for the preparation of the
tronic properties of D–A-type π-conjugated systems through the 5-phenyl-1,2,4-triazole based closo-o-carboranyl compounds
conversion of closo-o-carborane compounds to their anionic (CB1 and CB2) and their respective nido-o-carborane com-
nido-o-equivalents, we herein explore whether o-carboranyl pounds (nido-CB1 and nido-CB2) are outlined in Fig. 1. The
conjugate systems based on the 5-phenyl-1,2,4-triazole-deriva- acetylene-precursor for CB1, namely A1, was synthesized via a
tive, which efficaciously acts as a donor, can exhibit altered Sonogashira coupling reaction between Br1 (the bromo-precur-
photoluminescence properties upon deboronation to their sor for A1, prepared according to a reported procedure45) and
corresponding nido-carborane structure. We also investigate trimethylsilylacetylene, and further treated with a mild base
the potential of this system as a candidate for a sensory- (K2CO3) to deprotect the trimethylsilyl group in an excellent
material scaffold to detect anionic species. In fact, our group total yield (98%). A further Sonogashira coupling reaction
previously elucidated the structural impact of the structural between 1-bromo-4-iodobenzene and trimethylsilylacetylene
orientation (planarity) on the ICT-based radiative decay for produced Br2 in a high yield (85%). In addition, compound B2
1,2,4-triazole-appended o-carboranyl compounds.45 Thus, to was obtained by the boronic acidification of Br2 using tri-
investigate the electronic effect of deboronation on these methylborate. A Suzuki–Miyaura coupling between the com-
Fig. 1 Synthesis routes to 1,2,4-triazole-based closo-o-carboranyl compounds (CB1 and CB2) and their respective nido-o-carboranyl compounds
(nido-CB1 and nido-CB2). Reaction conditions: (i) Trimethylsilylacetylene, CuI, Pd(PPh3)2Cl2, toluene/NEt3, 90 °C, 24 h; (ii) K2CO3, MeOH, 25 °C, 2 h;
(iii) B10H14, Et2S, toluene, 120 °C, 24 h; (iv) TBAF, THF, 25 °C, 1 min; (v) trimethylsilylacetylene, CuI, Pd(PPh3)2Cl2, THF/NEt3, 60 °C, 30 min; (vi) n-BuLi,
B(OCH3)3, THF, 25 °C, 12 h; and (vii) 5-bromo-1-(2,6-diisopropylphenyl)-3-methyl-1H-1,2,4-triazole, K2CO3, Pd(PPh3)4, toluene/H2O, 120 °C, 12 h.
The X-ray crystal structures of CB1 (left) and CB2 (right) (50% thermal ellipsoids) are shown in the inset; the solvent molecules present in the unit
cell and the H atoms have been omitted for clarity.
3208 | Dalton Trans., 2021, 50, 3207–3215
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5-bromo-1-(2,6-diisopropylphenyl)-3- led to conversion of the closo-carborane species to the nido-
methyl-1H-1,2,4-triazole and B2, followed by the subsequent species, i.e. nido-CB1 (= [nido-form of CB1]·[NBu4]) and nido-
deprotection of the trimethylsilyl group, afforded the acety- CB2 (= [nido-form of CB2]·[NBu4]) in moderate yields (56 and
lene-precursor for CB2, namely A2, in a good total yield (92%). 71%, respectively, Fig. 1).
Finally, the 1,2,4-triazole-appended closo-o-carboranyl com-
All compounds were fully characterized by multinuclear
pounds CB1 and CB2 were prepared from A1 and A2, respect- NMR spectroscopy (Fig. S1–S12 in the ESI†) and elemental
1
ively, using a decaborane (B10H14)-promoted cage-forming analysis. The H{11B decoupled} spectra of the closo- and nido-
reaction in the presence of diethyl sulfide.61–63 The synthetic o-carboranyl compounds exhibited the expected resonances
yields for both the o-carboranyl compounds were low (i.e., assignable to the aryl rings, the 5-phenyl-1,2,4-triazole groups,
22 and 32%, respectively) due to steric hindrance from the and the terminal N-phenyl rings. The broad peaks present in
bulky 5-phenyl-1,2,4-triazole moieties hindering the cage the upfield region (above −2.4 ppm) for nido-CB1 and nido-
formation.
CB2, which were not observed for the neutral closo-com-
The subsequent treatment with an excess amount of a weak pounds, are characteristic of the B–H–B bridge protons of nido-
base, namely n-tetrabutylammonium fluoride (TBAF, NBu4F), o-carboranyl cages. In addition, the observation of five broad
peaks between −1 and −14 ppm in the 11B{1H-decoupled}
NMR spectra of CB1 and CB2 confirmed the presence of a
closo-o-carborane cage. Furthermore, the 11B{1H} NMR spectra
of nido-CB1 and nido-CB2 showed nine broad peaks between
−9 and −37 ppm; these signals are significantly shifted upfield
due to the anionic character of the nido-o-carborane structure,
therefore clearly indicating the presence of the nido-o-carbor-
ane cage.
The solid-state molecular structures of closo-compounds
CB1 and CB2 were then determined by X-ray crystallography,
and are shown in the inset of Fig. 1; detailed parameters,
including selected bond lengths and angles, are provided in
Tables S1 and S2 of the ESI†. As shown, both structures
contain the perfect pentagonal ring of the 1,2,4-triazole group
(Σ[five complementary angles]
=
540°, Table S2†).
Furthermore, the N-diisopropylphenyl ring of CB2 was
found to be orthogonally oriented (ca. 82°) relative to the tri-
azole plane due to the steric hindrance from the diisopropyl
groups.
Analysis of the photophysical properties of the o-carboranyl
compounds based on theoretical calculations
Fig. 2 UV-vis absorption and PL spectra of (a) CB1, (b) CB2, (c) nido-
CB1, and (d) nido-CB2. Black: absorption spectra in THF (50 μM), blue:
PL spectra in THF (50 μM) at 298 K, green: PL spectra in THF (50 μM) at
77 K, and orange: PL spectra in the films (5 wt% doped with PMMA) at
298 K. The insets show the emission colors of the different states upon
irradiation with a hand-held UV lamp (λex = 254 nm).
The photophysical properties of the closo-o- (CB1 and CB2)
and nido-o-carboranyl compounds (nido-CB1 and nido-CB2)
were investigated by performing UV-vis absorption and photo-
Table 1 Photophysical data for the 5-phenyl-1,2,4-triazole-based o-carboranyl compounds
a,d
λem/nm
Φem
λabs a/nm
(ε/10−3
a
e
f
h
λex/
nm
n-
τobs
Filmc THFb Filmc ns
/
kr
/
knr
/
HOMOg/ Eg
eV
/
LUMOi/
eV
M−1 cm−1
)
hexb THFb MeCNb 77 Ka
108 s−1 109 s−1 eV
CB1
CB2
271 (11.8)
269 (16.7)
290
303
387
368
389
370
389
371
385,
459
365,
461
424
401
380
360
0.05
0.01
0.08
0.03
0.83
0.60
0.22
1.14
2.15
−6.53
−6.69
3.98 −2.55
4.02 −2.67
0.46
nido-CB1 304 (10.8)
nido-CB2 304 (10.6)
290
303
417
388
466
440
491
464
424
415
0.30
0.24
0.35
0.28
3.39
2.27
0.88
1.06
0.21
0.33
−5.38
−5.43
3.50 −1.88
3.52 −1.91
a
c = 50 μM in THF. b c = 50 μM, observed at 298 K. c Measured in the film state (5 wt% doped on PMMA) at 298 K. d Absolute PL quantum yield.
kr = Φem/τobs. f knr = kr(1/Φem−1). g Calculated from the oxidation onset potential vs. a ferrocene/ferrocenium+ couple in CV cycles. h Band gap cal-
e
culated from the absorption onset energy. i Calculated from HOMO level + Eg.
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luminescence (PL) measurements (Fig. 2 and Table 1). closo- able continuum model (CPCM) was also used to include the
Compounds CB1 and CB2 mainly exhibited their lowest effect of THF as the solvent.64,65 The computational results for
absorption bands at λabs = ca. 270 nm. These bands were the closo-compounds in the S0 state revealed that the lowest-
assigned to the spin-allowed π–π* locally excited (LE) transition energy electronic transitions were mainly associated with those
in the 5-phenyl-1,2,4-triazole-centered aromatic region and from the highest occupied molecular orbital (HOMO) to the
slight typical ICT transitions between the phenyl–triazole rings lowest unoccupied molecular orbital (LUMO). While the
and the o-carborane cages (see the theoretical calculation HOMOs are significantly occupied by the appended phenyl–tri-
results below). In contrast, the nido-compounds showed domi- azole aromatic region (N-phenyl group + 5-phenyl-1,2,4-triazole
nant absorption bands at λabs = ca. 304 nm (Table 1), which moiety, >94%, Tables S4 and S6†), the LUMOs are mainly loca-
were significantly red-shifted compared to those of the closo- lized on the o-carborane cages (>12%, Tables S4 and S6†), as
compounds. The theoretical calculation results showed that well as being typically occupied by the phenyl–triazole aro-
these absorption bands could be largely associated with the matic region. Such results indicate that the lowest-energy elec-
ICT transitions between the nido-carboranyl cages and the tri- tronic absorption for closo-compounds CB1 and CB2 could be
azole moieties.
assigned to the π–π* LE transitions of the appended triazole
To gain further insight into the origin of the electronic tran- aromatic moiety, and to the sizable ICT transitions between
sitions, all carboranyl compounds were examined by time- the aromatic moieties (D) and the o-carboranyl cages (A). In
dependent density functional theory (TD-DFT) calculations. contrast, their corresponding nido-carboranyl compounds
The ground (S0) and first excited (S1) states of these com- demonstrated completely different electronic transition pro-
pounds were optimized at the B3LYP/6-31G(d) level of theory perties. More specifically, although the major absorption tran-
(Fig. 3), and these calculations were based on the solid-state sitions for both nido-CB1 and nido-CB2 were also assignable to
molecular structures of CB1 and CB2. A conductor-like polariz- the HOMO → LUMO transitions (Fig. 3), the HOMOs and
Fig. 3 Frontier molecular orbitals of (a) CB1, CB2, (b) nido-CB1, and nido-CB2 in the ground state (S0) and the first excited singlet state (S1) where
the relative energies were obtained from DFT calculations (isovalue 0.04). The transition energy (in nm) was calculated using the TD-B3LYP method
with the 6-31G(d) basis set.
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LUMOs are perfectly distributed over the o-carboranyl cages than those (35% and 28%) in film, indicating that the radiative
(>95%, Tables S8 and S10†) and the appended triazole aro- efficiency for the ICT transition could be slightly decreased by
matic moiety (>98%), respectively. Thus, the major electronic the structural motion in solution at 298 K. Furthermore, the
absorption transitions of nido-CB1 and nido-CB2 were attribu- relatively high Φem values (32% and 25%) for both nido-CB1
ted to the entire ICT transitions between the nido-carborane and nido-CB2 in n-hexane compared to those in THF also
cages (D) and the triazole moieties (A). These results imply imply that the non-polar solvent can slightly inhibit the struc-
that the deboronation reaction from the closo- to the nido- tural motion in the excited states formed by the ICT transition,
species dramatically alters the electronic roles of the o-carbora- resulting in a more blue-shifted emission than in THF.
nyl cages, i.e. changing them from an acceptor (A) to a donor
Based on the computational results for the S1 states of
(D).
closo-compounds CB1 and CB2, their high-energy emissions
Subsequently, the emission properties of the closo- and centered at ca. 380 nm and their low-energy emission at ca.
nido-o-carboranyl compounds were examined by PL measure- 400–450 nm were considered to be dominated by LUMO+1 and
ments under various conditions (Fig. 2 and Table 1). In THF at LUMO → HOMO transitions (Fig. 3a), respectively. Although
298 K, closo-carboranyl compounds CB1 and CB2 exhibited the HOMOs and LUMO+1 are significantly localized in the
emission bands centered at 389 and 370 nm, respectively, appended phenyl–triazole aromatic region (>90%, Tables S4
while the nido-compounds showed a considerably red-shifted and S6†), the LUMOs are mostly centered on the o-carborane
emission in the region of λem = 440–466 nm. The PL experi- cages (>80%). These results strongly support the assumption
ments for the various carboranyl compounds performed in sol- that the high- and low-energy emissions originate from the LE
vents with different polarities (n-hexane and acetonitrile) pro- emission of a π–π* transition on the triazole-based aromatic
vided further insight into the origin of the intrinsic emissive groups and from an ICT-based emission corresponding to the
characteristics. Although the emissive centers of the closo-com- o-carborane unit (A), respectively. On the other hand, the cal-
pounds were very slightly altered upon variation in the solvent culated results for nido-compounds nido-CB1 and nido-CB2
polarity (Δλem < 3 nm, Table 1 and Fig. S14†), those of the demonstrated that the LUMO → HOMO transitions are the
nido-compounds changed by >74 nm (Table 1 and Fig S14†), major contributors to the emissive energy in both compounds
resulting in a solvatochromic effect. These features strongly (Fig. 3b). In this case, the LUMOs and HOMOs are fully occu-
indicate that the emissions of the closo- and nido-compounds pied over the appended phenyl–triazole aromatic region (>96%
are correlated with local-excited (LE) emission and CT-based Tables S8 and S10†) and the nido-o-carborane units (∼99%)
emission, respectively. The theoretical calculation results on respectively, clearly indicating that emission from the nido-
the S1 states of all carboranyl compounds also confirmed the compounds originates from the radiative mechanism of the
characteristics of the electronic transitions (vide infra).
ICT transition from the triazole unit (A) to the nido-carborane
Intriguingly, the PL spectra of both closo-compounds in (D). These results therefore suggested that the intramolecular
THF at 77 K exhibited additional emissive bands centered at electronic role of the o-carborane cages could be switched
ca. 460 nm (Fig. 2a and Table 1). These features result from from the acceptor (A, in the closo-species) to the donor (D, in
the restricted motion around the o-carborane cages in the the nido-species) through the deboronation of the carborane
rigid states, such as fluctuation of the C–C bond distance and units in these molecules.
rotational motion of the o-carborane unit, which prevents the
To gain further insight into the origin of the red-shifted
radiative decay mechanism based on ICT emission of the nido-o-compounds (λem = 466 nm for nido-CB1
transitions.21–23,31–33,45–47,66–68 Indeed, the PL spectra recorded and 440 nm for nido-CB2) compared to those of the closo-o-
in the film state (5 wt% doped on poly(methyl methacrylate) compounds (λem = 389 nm for CB1 and 370 nm for CB2), cyclic
(PMMA), solid state) also showed slightly enhanced emissive voltammetry (CV) measurements were performed (Fig. 4 and
patterns in the region between 450 and 550 nm, which is typi- Table 1). Indeed, the oxidative CV results obtained for the
cally assignable to the ICT-based emission. The absolute closo- and nido-compounds confirmed such red-shifted emis-
quantum efficiencies (Φem) of both CB1 and CB2 in the film sive features following a deboronation reaction. More specifi-
were estimated to be 8% and 3% respectively, which were cally, the experimental HOMO levels calculated from the oxi-
2–3% larger than those in THF at 298 K (5% and 1%, respect- dative onset potentials (Eox, onset, Fig. 4) for the closo-com-
ively, Table 1); this distinctly implies the enhancement of ICT- pounds were greatly shifted upon deboronation to the nido-
based emission in the film state.
species, i.e., −6.53 eV for CB1 → −5.38 eV for nido-CB1,
In contrast, both nido-carboranyl compounds exhibited a ΔEHOMO = 1.14 eV; and −6.69 eV for CB2 → −5.43 eV for nido-
blue-shifted single-emissive trace in the rigid state (THF at CB2, ΔEHOMO = 1.31 eV. Furthermore, the LUMO levels could
77 K and in the film state) between 400 and 450 nm, which are be measured from the band gaps (Eg, Table 1) calculated from
similar to those observed in n-hexane. The observations the absorption onset energy and the HOMO levels, i.e., −2.55
suggest that the excited states of the nido-compounds origi- eV for CB1 → −1.88 eV for nido-CB1, ΔELUMO = 0.67 eV; and
nated from the ICT transition are significantly affected by the −2.67 eV for CB2 → −1.91 eV for nido-CB2, ΔELUMO = 0.76 eV.
structural motions of the different physical and polar states. These findings indicate that deboronation to the nido-carbor-
The Φem values of both nido-CB1 and nido-CB2 in THF at ane compounds elevated the HOMO levels to a greater extent
298 K (30% and 24%, respectively, Table 1) were much lower than the LUMO levels, thereby narrowing the band-gap, and
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the efficiency of the radiative decay mechanism. Consequently,
the deboronation reaction, which resulted in conversion of the
electronic role of the o-carborane unit from an acceptor (A) to
a donor (D), gave an efficient radiative emission via the ICT
transition taking place in the nido-o-carboranes.
The distinctly enhanced radiative characteristics resulting
from the conversion to the nido-species indicated that the
closo-o-carboranyl compounds have the potential for appli-
cation in sensory materials due to their color change and turn-
on emissive features observed upon exposure to a weak base
such as the fluoride anion. To clarify the changes in the PL
properties exhibited during the conversion of CB1 and CB2 to
their corresponding nido-species, we investigated the changes
in the emissive patterns of the closo-compounds in THF (5.0 ×
10−5 M) as a function of an increasing number of TBAF equiva-
lents. As shown in Fig. 5, upon the addition of large amounts
of TBAF (0–3.0 equiv.) into the respective solutions of CB1 and
CB2 at ambient temperature (ca. 298 K), the PL spectra
promptly exhibited turn-on emissive features centered at ca.
450 nm, resulting in a vivid emission color change from a
weak deep-blue color to strong sky-blue (Fig. 5, inset figures).
In particular, the emission intensities and band shapes for the
final PL response (in the presence of 3.0 equiv. of TBAF) were
similar to those (blue-solid lines in Fig. 5) of each nido-o-car-
boranyl compound. These results indicate that the conversion
from the closo- to the nido-species led to the emission turn-on
and color change response. These conversion processes were
clearly evidenced from the changes in the specific peaks of the
1H NMR spectra of the closo-compounds in the presence of 3.0
equiv. of TBAF in THF-d8 (Fig. S16†). More specifically, the aryl
protons of both closo-compounds in the region between 8.0
and 7.0 ppm shifted to the upfield region upon conversion,
while broad singlet peaks at δ = −2.0 and −3.0 ppm, which
were assigned to the B–H–B bridge protons of the nido-o-car-
borane, were clearly observed. These results therefore indicate
Fig. 4 Cyclic voltammograms (CV) of the o-carboranyl compounds
showing oxidation (50 mM in acetonitrile, scan rate = 100 mV s−1 for
oxidation).
resulting in red-shifting of the emission patterns. These experi-
mental results correspond well to the computational calcu-
lations; the band-gaps (the energy gap between the HOMO and
LUMO levels in the S1-states) of the nido-compounds (2.93 eV
for nido-CB1 and 3.10 eV for nido-CB2) are certainly smaller
than those of the corresponding closo-compounds (3.39 eV for
CB1 and 3.32 eV for CB2).
Emission color changes and turn-on features of the closo-o-
carboranyl compounds upon deboronation
To elucidate the efficiency of the radiative decay process for almost full conversion under these conditions. Overall, the
the closo- and nido-o-carboranyl compounds, Φem and decay above results suggest that closo-o-carboranyl compounds CB1
lifetimes (τobs) were investigated at 298 K in THF (Table 1 and and CB2, which exhibit the LE-based emission, can cause dra-
Fig. S15†). The emission decay lifetimes for all compounds matic ‘turn-on’ emission color changes via deboronation of
were found to be 0.46–3.39 ns, demonstrating the fluorescence
feature. In particular, the Φem values of nido-compounds nido-
CB1 (30%, Table 1) and nido-CB2 (24%) were considerably
higher than those of their corresponding closo-compounds
(5% for CB1 and 1% for CB2). Furthermore, the radiative (kr)
and non-radiative decay constants (knr) calculated from the
Φem and τobs values (Table 1) suggested that the radiative
decay mechanism of the nido-compounds is significantly more
efficient than that of the closo-compounds. Although the kr
values for each closo-compound (6.0 × 107 s−1 for CB1 and 2.2
× 107 s−1 for CB2, Table 1) are lower than those for each nido-
compound (8.8 × 107 s−1 for nido-CB1 and 10.6 × 107 s−1 for
nido-CB2), the knr values for the closo-compounds (1.1 × 109
Fig. 5 Spectral changes in the emission of (a) CB1 (5.0 × 10−5 M, λex
=
s−1 for CB1 and 2.2 × 109 s−1 for CB2) are at least five times
larger than those of the nido-compounds (2.1 × 108 s−1 for
nido-CB1 and 3.3 × 108 s−1 for nido-CB2). Such results strongly
290 nm) and (b) CB2 (5.0 × 10−5 M, λex = 303 nm) in the presence of
different amounts of TBAF at ambient temperature. The insets show
figures of the closo-compounds before and after heating with 3.0 equiv.
suggest that the deboronation process significantly enhances of TBAF, with irradiation using a UV lamp (λex = 295 nm).
3212 | Dalton Trans., 2021, 50, 3207–3215
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the o-carborane cage, owing to the new ICT transitions corres- Data for CB1
ponding to the nido-o-carborane structure.
B10H14 (0.62 g, 5.1 mmol), Et2S (1.36 mL, 12.6 mmol), and A1
(1.09 g, 4.2 mmol) in toluene afforded CB1 as a white solid.
The crude product was purified by column chromatography on
silica gel (ethyl acetate : n-hexane = 1 : 4, v/v). Yield = 22%
(0.35 g). 1H{11B} NMR (CDCl3): δ 7.44 (m, 7H), 7.32 (dd, J = 6.4,
3.0 Hz, 2H), 3.95 (s, 1H, CB–H), 2.54 (br s, 2H, CB–BH), 2.50 (s,
Experimental
General considerations
All operations were performed under an inert nitrogen atmo- 3H, –CH3), 2.45 (br s, 3H, CB–BH), 2.31 (br s, 3H, CB–BH), 2.25
sphere using standard Schlenk and glove box techniques. (br s, 2H, CB–BH). 13C NMR (CDCl3): δ 161.11, 152.53, 138.01,
Anhydrous solvents (toluene, chloroform, tetrahydrofuran 135.01, 129.77, 129.50, 129.36, 129.09, 127.86, 125.55, 75.68
(THF), n-hexane, ethanol (EtOH), and methanol (MeOH); (CB–C), 60.10 (CB–C), 13.96 (–CH3). 11B{1H} NMR (CDCl3): δ
Sigma-Aldrich) were dried by passage through an activated −1.93 (br s, 1B), −3.99 (br s, 1B), −8.80 (br s, 2B), −10.89 (br s,
alumina column and stored over activated molecular sieves 3B), −12.68 (br s, 3B). Anal. calcd for C17H23B10N3: C, 54.09; H,
(5 Å). Spectrophotometric-grade solvents (THF, toluene, di- 6.14; N, 11.13. Found: C, 53.88; H, 6.01; N, 11.07.
chloromethane (DCM), acetonitrile (MeCN), and n-hexane)
Data for CB2
were used as received from Alfa Aesar. Commercial reagents
were used without any further purification after purchase from
Sigma-Aldrich (4-bromobenzoyl chloride, benzoyl chloride,
ethyl acetimidate, phenyl hydrazine, phenylboronic acid, tri-
methylsilylacetylene, 1-bromo-4-iodobenzene, trimethylborate
(B(OCH3)3), potassium carbonate (K2CO3), sodium carbonate
(Na2CO3), magnesium sulfate (MgSO4), ammonium chloride
B10H14 (70.0 mg, 0.6 mmol), Et2S (0.26 mL, 2.4 mmol), and A1
(0.16 g, 0.5 mmol) in toluene afforded CB2 as a white solid.
The crude product was purified by column chromatography on
silica gel (DCM). Yield = 32% (71.4 mg). 1H{11B} NMR
(CD2Cl2): δ 7.52 (t, J = 7.8 Hz, 1H), 7.44 (dd, J = 8.0, 3.6 Hz,
2H), 7.38 (dd, J = 8.0, 3.2 Hz, 2H), 7.30 (d, J = 7.8 Hz, 2H), 4.02
(s, 1H, CB–H), 2.51 (br s, 2H, CB–BH), 2.46 (s, 3H, –CH3), 2.41
(br s, 3H, CB–BH), 2.32 (m, 2H, –CH(CH3)2), 2.25 (br s, 5H,
CB–BH), 1.10 (d, J = 6.8 Hz, 6H, –CH(CH3)2), 0.87 (d, J = 6.9 Hz,
6H, –CH(CH3)2). 13C NMR (CD2Cl2): δ 161.09, 153.68, 146.48,
134.88, 134.78, 131.14, 129.77, 128.20, 128.07, 124.87, 76.33
(CB–C), 60.70 (CB–C), 28.86 (–CH(CH3)2), 24.78 (–CH(CH3)2),
22.82 (–CH(CH3)2), 14.16 (–CH3). 11B{1H} NMR (CD2Cl2): δ
−3.23 (br s, 2B), −5.33 (br s, 2B), −9.98 (br s, 2B), −11.99 (br s,
2B), −13.76 (br s, 2B). Anal. calcd for C23H35B10N3: C, 59.84; H,
7.64; N, 9.10. Found: C, 59.73; H, 7.48; N, 9.01.
(NH4Cl),
tetrakis(triphenylphosphine)palladium(0)
(Pd
(PPh3)4), bis(triphenylphosphine)palladium(II) dichloride (Pd
(PPh3)2Cl2), copper(I) iodide (CuI), triethylamine (NEt3),
diethyl sulfide (Et2S), and poly(methyl methacrylate) (PMMA)),
Acros (1.6 M n-butyllithium in hexane (n-BuLi), 1.0 M n-tetra-
butylammonium fluoride in THF (TBAF)), and Alfa Aesar
(decaborane (B10H14), 5-bromo-1-(2,6-diisopropylphenyl)-3-
methyl-1H-1,2,4-triazole). The bromo-precursor Br1 was pre-
pared according to
a
previously reported procedure.45
Deuterated solvents, CDCl3 and CD2Cl2, were purchased from
Cambridge Isotope Laboratories and were dried over activated
molecular sieves (5 Å). All nuclear magnetic resonance (NMR)
spectra were recorded on a Bruker Avance 400 spectrometer
General synthesis method for 1,2,4-triazole-based nido-o-
carborane compounds, nido-CB1 (= [nido-form of CB1]·[NBu4])
and nido-CB2 (= [nido-form of CB1]·[NBu4])
1
(400.13 MHz for H, 100.62 MHz for 13C and 128.38 MHz for
11B) at ambient temperature. Chemical shifts are given in ppm
and are referenced against external tetramethylsilane (1H and
13C) or boron trifluoride–diethyl ether (BF3·Et2O) (11B).
Elemental analyses were performed on an EA3000 instrument
(EuroVector) at the Central Laboratory of Kangwon National
University. Synthesis procedures for the precursors (A1, Br2,
B2, and A2), various measurements, and theoretical calcu-
lation are described in the ESI.†
The prepared closo-o-carborane compound was dissolved in
THF at room temperature. After the addition of an excess of
1.0 M n-tetrabutylammonium fluoride (TBAF, NBu4F) in THF
(4.0 equiv.), the reaction mixture was stirred for 1 min. The
resulting mixture was then treated with DCM (10.0 mL × 3);
the organic layer was separated, dried over anhydrous MgSO4,
filtered, and concentrated to dryness, affording a white
residue. Purification by recrystallization from diethyl ether
gave the corresponding nido-o-carborane compound as a white
solid.
General synthesis method for 1,2,4-triazole-based closo-o-
carborane compounds, CB1 and CB2
Data for nido-CB1
To a toluene solution (40.0 mL) of decaborane (B10H14, 1.2
equiv.) and the acetylene precursor was added an excess of CB1 (0.19 g, 0.5 mmol), THF (6.0 mL), and a 1.0 M TBAF THF
diethyl sulfide (Et2S, 3 equiv.) at 25 °C. After heating to reflux, solution (2.0 mL, 2.0 mmol) afforded nido-CB1 as a white
1
the reaction mixture was stirred for further 24 h. The solvent solid. Yield = 56% (0.17 g). H{11B} NMR (CD2Cl2): δ 7.42 (d,
was then removed under vacuum, and the resulting solid was J = 5.1 Hz, 3H), 7.32 (t, J = 7.8 Hz, 2H), 7.19 (q, J = 9.1 Hz, 4H),
filtered prior to redissolving in toluene. The crude product was 3.09 (t, J = 8.4, 8H, n-butyl-CH2), 2.42 (s, 3H, –CH3), 2.29 (br s,
purified by column chromatography on silica gel to obtain the 1H, CB–CH), 2.05 (br s, 2H, CB–BH), 1.79 (br s, 3H, CB–BH),
corresponding closo-o-carborane compound as a white solid.
1.59 (m, 8H, n-butyl-CH2), 1.42 (m, 8H, n-butyl-CH2), 1.27 (br s,
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4H, CB–BH), 1.02 (t, J = 7.3, 12H, n-butyl-CH3), −2.44 (br s, 1H, was found to result in a ratiometric change in the emission
B–H–B). 13C NMR (CD2Cl2): δ 160.76, 154.56, 148.33, 139.08, color from deep-blue (∼380 nm) to sky-blue (∼450 nm), in
129.66, 128.87, 128.12, 126.82, 125.80, 124.66, 59.40 (n-butyl- addition to intense turn-on responses.
CH2), 24.27 (n-butyl-CH2), 20.10 (n-butyl-CH2), 14.05 (–CH3),
13.75 (n-butyl-CH3). 11B{1H} NMR (CD2Cl2): δ −9.73 (br s, 1B),
−11.28 (br s, 1B), −14.43 (br s, 1B), −17.51 (br s, 1B), −19.02
(br s, 1B), −20.49 (br s, 1B), −23.68 (br s, 1B), −33.67 (br s, 1B),
Conflicts of interest
−36.74 (br s, 1B). Anal. calcd for C33H59B9N4: C, 65.07; H, 9.76;
N, 9.20. Found: C, 65.07; H, 9.69; N, 9.05.
There are no conflicts of interest to declare.
Data for nido-CB2
Acknowledgements
CB2 (47.7 mg, 0.1 mmol), THF (1.0 mL), and a 1.0 M TBAF
THF solution (0.41 mL, 0.4 mmol) afforded nido-CB2 as a
This work was supported by the National Research Foundation
white solid. Yield = 71% (50.6 mg). 1H{11B} NMR (CD2Cl2): δ
of Korea (NRF) grant (NRF-2020R1A2C1006400 and
7.49 (t, J = 7.7 Hz, 1H), 7.27 (d, J = 7.3 Hz, 2H), 7.20 (d, J = 7.2
NRF-2016M3A7B4909246 for K. M. Lee) funded by the Ministry
Hz, 2H), 7.09 (d, J = 7.4 Hz, 2H), 3.09 (t, J = 8.5 Hz, 8H, n-butyl-
of Science and ICT and the Ministry of Education. We thank
CH2), 2.44 (s, 3H, –CH3), 2.36 (m, 2H, –CH(CH3)2), 2.24 (br s,
Dr Dongwook Kim (Center for Catalytic Hydrocarbon
1H, CB–CH), 2.01 (br s, 3H, CB–BH), 1.73 (br s, 2H, CB–BH),
Functionalizations, Institute for Basic Science (IBS)) for assist-
1.59 (m, 8H, n-butyl-CH2), 1.42 (m, 8H, n-butyl-CH2), 1.27 (br s,
ance with X-ray diffraction experiments.
4H, CB–BH), 1.09 (d, J = 6.7 Hz, 6H, –CH(CH3)2), 0.99 (d, J = 7.3
Hz, 12H, n-butyl-CH3), 0.88 (d, J = 6.8 Hz, 6H, –CH(CH3)2,
−2.48 (br s, 1H, B–H–B). 13C NMR (CD2Cl2): δ 161.41, 155.40,
148.34, 146.52, 135.27, 130.72, 127.06, 126.77, 124.60, 124.28,
59.38 (n-butyl-CH2), 28.80 (–CH(CH3)2), 24.75 (–CH(CH3)2),
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