Biphenyl- and Fluorene-Based o-Carboranyl Compounds: Alteration of Photophysical Properties by Distortion of Biphenyl Rings

Article abstract of DOI:10.1021/acs.organomet.7b00093

Four biphenyl- and fluorene-based o-carboranyl compounds, 4-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]biphenyl (1B), 4,4″-bis[2-(p-n-butylphenyl)-1-o-carboran-1-yl]biphenyl (2B), 2-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]fluorene (1F), and 2,7-bis[2-(p-n-buty

Full text of DOI:10.1021/acs.organomet.7b00093

Article
pubs.acs.org/Organometallics
Biphenyl- and Fluorene-Based o‑Carboranyl Compounds: Alteration
of Photophysical Properties by Distortion of Biphenyl Rings
Nara Shin, Seokhyeon Yu, Ji Hye Lee, Hyonseok Hwang, and Kang Mun Lee*
Department of Chemistry and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon,
Gangwon 24341, Republic of Korea
S
* Supporting Information
ABSTRACT: Four biphenyl- and fluorene-based o-carboranyl
compounds, 4-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]-
biphenyl (1B), 4,4″-bis[2-(p-n-butylphenyl)-1-o-carboran-1-
yl]biphenyl (2B), 2-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]-
fluorene (1F), and 2,7-bis[2-(p-n-butylphenyl)-1-o-carboran-1-
yl]fluorene (2F), were prepared and fully characterized by
multinuclear NMR spectroscopy and elemental analysis. The
crystal structures of 1B and 2B, analyzed by single-crystal X-
ray diffraction, exhibited distinct distortions of the central
biphenyl rings with dihedral angles of 44.2 and 33.1°. In
photoluminescence measurements, fluorene-based carboranyl
compounds in the rigid state (e.g., in solution at 77 K and as films) exhibited a noticeable emission in the low-energy region
below 400 nm. 1F displayed a low-energy emissive trace in solution at ambient temperature, whereas biphenyl-based carboranes
mainly exhibited high-energy emissions above 400 nm. TD-DFT calculations on the first excited singlet (S1) state of each
compound suggested that the emission in the low-energy region involves intramolecular charge transfer (ICT) between the
carborane and central phenylene rings, indicating that the radiative decay of these ICT bands could be enhanced by preventing
the distortion of the central biphenyl groups. Furthermore, less distortion on the biphenyl rings of the biphenyl-based analogues
at S1 in the optimized structures supports that these phenylenes must be flat for the formation of stable excited states.
Comparison of the quantum efficiency (ΦPL) and radiative decay constants (kr) of the o-carboranes also reveals definitively that
the structural features of the biphenyl groups can have a decisive effect on those photophysical properties.
Chart 1. Examples of o-Carborane-Based Luminophores
INTRODUCTION
■
Icosahedral carboranes, C2B10H12, are polyhedral boron cluster
compounds that are well-known as three-dimensional (3D)
analogues of various aromatic derivatives and have recently
aroused academic and technological interest in materials
science for their photonic characteristics.1 Such unique
properties such as high Lewis acidity arising from their high
boron content and thermal/electrochemical stability have
garnered much attention for carboranes as functional materials
for a wide range of photonic applications.2,3 In particular, o-
carborane (closo-1,2-C2B10H12)-appended π-conjugated deriva-
tives have recently attracted significant interest for their
potential optoelectronic applications as light-emitting materials
due to their specific luminescent properties evoked from the
intrinsic nature of the o-carborane units.2,4,5 The strong
electron-withdrawing properties of the C atoms of the cage
and highly polarizable σ-aromatic character1,6 lead to intra-
molecular charge transfer (ICT) between the π-conjugated
aromatic groups and o-carborane.7 Thus, organic luminophores
including o-carborane can be regarded as a donor−acceptor
dyad system, in which the o-carborane cage acts as an acceptor.
In this context, Kang and co-workers have recently reported
donor−acceptor dyads (I in Chart 1) based on dicarbazole
moieties as the donor, demonstrating that specific charge-
separated excited states originated from efficient photoinduced
charge transfer (CT).8,9 Furthermore, extensive studies on the
fascinating photophysical properties originating from the ICT
of o-carboranyl aromatic complexes have been steadily
performed currently.4,5,7−13 For example, Chujo and co-
workers reported that ICT-based aggregation-induced emission
Received: February 6, 2017
© XXXX American Chemical Society
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(AIE) can be activated in various 1,2-diaryl-o-carboranes (II)
and their emission color can be fine-tuned by introducing
various substituents on the aryl groups.10 Similarly, Lee’s group
also found that through-space ICT between the terminal aryl
group (HOMO) and the central phenylene ring (LUMO) can
lead to emission color tuning of di-o-carboranylbenzene
compounds (III) according to the type of substituents
positioned on the terminal aryl groups.11 In addition to the
discovery of these specifically luminescent characters of
carborane-based organic complexes, recent reports on o-
carborane complexes containing aromatic rings have empha-
sized that the emissive band origin can be attributed to the
molecular geometry and formation of the structure. Fox’s group
has reported C-diazaboryl-o-carboranes as donor−acceptor
dyad systems (IV, diazaboryl group as the donor), showing
dual emissions from both locally excited (LE) high-energy
states and low-energy ICT states, depending on the molecular
geometry.12 This study distinctly showed that the rotation of
the diazaboryl group could prevent the emissive ICT
mechanism, resulting in concentration on the formation of
LE states only. Additionally, Lee’s group has reported
iridium(III) cyclometalates in which two carboranes on a
ligand were tethered by an alkylene linker to restrict the
rotation of the o-carborane (V), resulting in enhancement of
phosphorescence efficiency.13 Such results indicate that the
photophysical and electronic characteristics of o-carborane-
appended compounds are closely related to those structural and
geometrical features. Although these emission behaviors in π-
conjugated o-carborane derivatives and the relationship with
these structural variations, such as the rotation of carborane
cages or appended aromatic groups, have been extensively
examined, a detailed investigation of the effects of the structural
variations of appended aromatic groups on the photophysical
properties has not been reported.
rings, which differed from the case for the biphenyl-based
compounds 1B and 2B. The synthesis, characterization,
structural features, and photophysical properties of 1B, 2B,
1F, and 2F, along with theoretical calculation results, are herein
presented in detail.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The reaction of
decaborane (B10H14) with the mono- or bis-acetylene
precursors (1Ba, 2Ba, 1Fa, and 2Fa), obtained by the
palladium-catalyzed coupling of bromobiphenyl or -fluorene
groups with 1-butyl-4-ethynylbenzene, afforded the o-carborane
compounds 4-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]-biphenyl
(1B), 4,4″-bis[2-(p-n-butylphenyl)-1-o-carboran-1-yl]biphenyl
(2B), 2-[2-(p-n-butylphenyl)-1-o-carboran-1-yl]fluorene (1F),
and 2,7-bis[2-(p-n-butylphenyl)-1-o-carboran-1-yl]fluorene
(2F), respectively, in moderate yields (38−50%) (Scheme 1).
All of the resulting compounds were fully characterized by
multinuclear NMR spectroscopy (Figures S16−S23 in the
Supporting Information), elemental analysis, and X-ray
1
diffraction analysis (Figure 1). While the H and 13C NMR
To examine the change in photophysical and electronic
features induced by the structural variation of appended
aromatic groups to o-carborane, we herein report biphenyl-
and fluorene-based (in which the biphenyl rings are anchored
by a CH2) mono-o-carboranyl (Scheme 1, 1B and 1F) and bis-
o-carboranyl (2B and 2F) compounds. Interestingly, fluorene-
based compounds 1F and 2F revealed that emissive ICT
between the carborane and central fluorene group could be
enhanced by maintaining the flatness of the central biphenyl
Figure 1. X-ray crystal structures of (top) 1B, (middle) 2B, and
(bottom) 2F (50% thermal ellipsoids). The H atoms and n-hexane
(for 2B) are omitted for clarity.
spectra contain the expected resonances corresponding to the
biphenyl or fluorene and aryl o-carboranyl moieties, two (in 2B,
1F, and 2F) or three (in 1B) broad 11B NMR peaks centered at
δ values ranging from −2 to −11 ppm in approximately a 2:8 or
3:2:5 ratio, respectively, confirmed the presence of closo-
carborane cages. Furthermore, the solid-state molecular
structures of 1B, 2B, and 2F were determined by single-crystal
X-ray diffraction analysis, as shown in Figure 1, with the
corresponding structural parameters given in Table S1 in the
Supporting Information. Crystal structures of these aromatic-
based carboranyl compounds showed reasonable bond lengths
and angles (Table S2 in the Supporting Information), and the
biphenyl rings of 1B and 2B were distorted with respect to the
plane of the other phenyl ring, resulting in dihedral angles of
44.2 and 33.1°, respectively. Such structural features are
similarly shown in the ground state (S0) optimized structures
of 1B and 2B, as calculated by time-dependent density
functional theory (TD-DFT) (see below). In particular, the
distorted biphenyl rings of 1B and 2B each became significantly
flattened in the first excited (S1) optimized structure, as did the
fluorene derivatives, indicating that the ground state of
Scheme 1. Synthetic Scheme for Biphenyl- and Fluorene-
a
Based o-Carboranes
a
Reagents and conditions: (i) 1-butyl-4-ethynylbenzene, CuI,
PdCl2(PPh3)2, NEt3/toluene (v/v, 9/1), 90 °C, 24 h; (ii) B10H14, 5
equiv of Et2S, toluene, 110 °C, 72 h.
B
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biphenyl-based o-carboranyl compounds (1B and 2B) could be
stabilized with the appropriate dihedral angles and the stable
excited state could be embodied in the flatness of the biphenyl
rings. Additionally, 1B, 1F, 2B, and 2F are air- and moisture-
stable, similar to the case for other aromatic-based closo-o-
carboranyl derivatives.
Photophysical Properties. The photophysical properties
for all four described derivatives were investigated by UV−vis
absorption and photoluminescence (PL) experiments in both
solution and film states (Figure 2). The biphenyl o-carboranyl
imately 320 nm that tailed off at 350 nm (Figure 2a). These
bands could be assigned to the transition originating from an
ICT between the biphenyl or fluorene group and the o-
carborane moiety (see Theoretical Calculations). Such ICT
character in the absorption bands of carboranyl compounds
could be confirmed by a solvatochromic effect (Figure S1 of the
Supporting Information). Indeed, the absorption spectra of all
carboranyl compounds exhibited an apparent solvent depend-
ence of the fluorescence band. With increasing solvent polarity
(cyclohexane < THF < dimethyl sulfoxide (DMSO)), the
bands were slightly red shifted, confirming solvatochromic
behavior and further suggesting the occurrence of ICT
transition character in the absorption bands.
The emissive features of the o-carboranyl derivatives were
examined by PL measurements (Figure 2b−d and Table 1).
The PL spectra of all compounds except for 1F in toluene at
298 K exhibited an intense emission in the high-energy region
ranging from 350 to 400 nm, while that of 1F specifically
showed an emissive trace in the low-energy region below 400
nm. The sharply pointed band shape in the high-energy region
supports an emission originating from a π−π* transition in the
central phenylene groups. Additionally, the emissive nature of
o-carboranyl aromatic compounds is also commonly involved in
ICT transitions between carborane and aromatic rings,7−12
tentatively indicating that the emission in the high-energy
region is associated not only with π−π* transitions on the
central phenylene groups but also with the ICT band from
carborane to a terminal butylphenyl ring. The low-energy
emission could originate from an ICT between carborane and
the central biphenyl or fluorene group (see the DFT results
below). Interestingly, the PL spectra examined in solution at 77
K of bis-o-carboranyl compounds 2B and 2F displayed a
distinctly intensified emission in the low-energy region below
400 nm along with the familiar high-energy emission (Figure
2c), resulting in dual emission patterns (λem 359 and 426 nm
for 2B and λem 377 and 470 nm for 2F; Table 1). In
comparison, 1B and 1F exhibited only slightly enhanced
emissions in the high- and low-energy regions, respectively.
These emissive features measured in the solution state at 298
and 77 K distinctly indicate that the luminescent mechanism of
bis-o-carboranyl compounds depends on the ICT from
carborane to both the terminal phenyl and central phenylene
rings, whereas that of mono-o-carboranyl compounds 1B and
1F is associated only with the ICT between carborane and the
terminal phenyl or central phenylene rings, respectively. These
ICT characters of carboranyl compounds were well demon-
strated in PL spectra measured in various organic solvents at
298 K (Figure S2 of the Supporting Information). The PL
spectra of 1B and 1F exhibited a marginal red shift of each
emission maximum (Δλem = 9 nm for 1B and 10 nm for 1F) as
the polarity of the solvents increased from cyclohexane to
Figure 2. (a) UV−vis absorption in THF (3.0 × 10−5 M). PL spectra
in toluene (3.0 × 10−5 M) at (b) 298 K and (c) 77 K. (d) PL spectra
in the film state (PMMA film doped with 5 wt % of the respective
compounds). Inset figures show the emission color in each state under
a hand-held UV lamp (λex 254 nm).
compounds 1B and 2B displayed major absorption bands in the
region of 260−280 nm (λmax 268 nm for 1B and 278 nm for
2B; Table 1). The structureless absorption features of 1B and
2B suggested that these bands could be assigned not only to a
π−π* transition on the biphenyl rings but also to ICT
transitions between the carborane and terminal phenyl rings or
central biphenyl rings. The absorption bands of bis-o-carboranyl
compound 2B were red-shifted by about 10 nm in comparison
to those of monocarborane 1B, indicating that the LUMO level
of 2B was slightly lower than that of 1B due to the LUMO
stabilization effect of o-carborane.14 These absorption features
were also present in both fluorene-based compounds, 1F and
2F, which exhibited major absorption bands from 280 to 320
nm (λmax 282, 306 nm for 1F and 289, 311 nm for 2F; Table 1)
mainly involving a π−π* transition of a fluorene ring. The
absorption of 2F was red-shifted by nearly 6 nm in comparison
to that of 1F. In particular, the absorption spectra of all of the
derivatives featured low-energy absorption bands at approx-
Table 1. Photophysical Data for o-Carboranyl Derivatives
bd
,
b
be
kr (ns−1
,
λem (nm) in toluene
ΦPL
τobs (ns)
)
a
ab
ab
bc
,
a
c
a
c
a
c
,
,
λabs (nm) in THF (ε × 10−4
)
298 K
77 K
385
λem (nm) PMMA
toluene
PMMA toluene
PMMA
toluene
PMMA
1B
2B
1F
2F
268 (2.6)
386
363
503
366
383
460
490
479
0.245
0.040
0.009
0.060
0.336
f
0.75
1.1
0.81
f
3.27 × 108
3.64 × 107
2.50 × 107
1.28 × 107
4.15 × 108
278 (4.8)
359, 426
478
282 (3.0), 306 (2.8)
289 (5.2), 311 (5.7)
0.366
0.368
0.36
4.7
0.37
4.7
9.89 × 108
7.83 × 107
377, 470
a
b
c
c = 3.0 × 10−5 M. Excitation for emission, λex (nm): 1B, 318; 2B, 312; 1F, 315; 2F, 320. Spin-coated PMMA film doped with the respective
d
e
f
compounds (5 wt %). Absolute PLQY. kr = ΦPL/τobs. Not observed.
C
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DMSO, suggesting that the emission originated from the
radiative ICT transitions. Similarly, the high-energy emissions
of 2B and 2F were red-shifted by as much as 19 nm for 2B and
16 nm for 2F with increasing solvent polarity. However, the
interesting observation was that the low-energy emissive traces
of 2B and 2F were detected in the highly polar solvent DMSO
(Figure S2b,d). These results indicate that the radiative decay
of bis-o-carboranyl compounds inherently exist in both high-
and low-energy regions but that the low-energy emission
associated with the ICT transition between the carborane and
central phenylene rings can only be seen in a highly polar
solvent due to its ability to stabilize the ICT transition.
The PL spectra of the film states (PMMA film doped with 5
wt % of each carborane compound) at room temperature also
showed emissive patterns similar to those in solution at 77 K
(Figure 2d). The low-energy emissions of fluorene-based
compounds 1F and 2F were distinctly observed, while the
emissive traces of 1B in only the high-energy region were
marginally detected and those of 2B were barely visible.
Although the radiative ICT band can be intensified by the
restriction of the C−C bond variation on the o-carborane cage
in the rigid state, distinctly different emissive patterns between
biphenyl- and fluorene-based compounds indicate that the
emissive ICT related to the central phenylene rings is largely
due to their structural formations. Consequently, these emissive
results imply that the structural distortion between biphenyl
rings disturbs the emissive ICT among them, but the
persistence of flatness on the biphenyl rings, such as in the
fluorene moiety, induces the ICT to remain concentrated
between the carborane and central fluorene rings. Indeed, the
structural variation of the biphenyl group of 1B is immediately
clear from the 44.2° dihedral angle visible in the solid-state X-
ray diffraction analysis, which is slightly larger than that of 2B
(33.1°). As a result, 1B exhibited only high-energy emissions in
all of the studied states, assignable either to the ICT band from
carborane to a terminal phenyl ring or to the π−π* transition
on the biphenyl group.
the film states (2.50 × 107 → 9.89 × 108 ns−1 for 1F and 1.28 ×
107 → 7.83 × 107 ns−1 for 2F; Table 1), respectively, while that
of 1B increased only by a factor of 1.3 (3.27 × 108 → 4.15 ×
108 ns−1). These results suggest that, if the structural variation
can be restricted in the rigid state, the radiative decay of ICT
from the carboranes to the central phenylene groups is
considerably influenced by the distortion of the phenylene
rings. Thus, the relatively high ΦPL and kr values of 1F and 2F
in the film state were due to the flatness of the fluorene groups.
On the other hand, the exclusively high energy emission of 1B
and the insignificant difference of its ΦPL and kr values between
the solution and film states indicate that ICT to the biphenyl
rings does not contribute to its emissive nature, only the π−π*
transition on the biphenyl group or the ICT band from the
carborane to a terminal phenyl ring.
Theoretical Calculations. To elucidate the electronically
optimized structures of the four carborane derivatives and the
origin of their electronic transitions and photophysical
properties, TD-DFT calculations on the ground state (S0)
and first singlet excited state (S1) were carried out using the
B3LYP functional and 6-31G(d) basis sets (Table 2, Figure 3,
Table 2. Major Low-Energy Electronic Transitions for the
First Singlet Excited State (S1) in Toluene from the TD-DFT
a
Calculations
λcalc (nm)
fcalc
assignt
1B
2B
1F
2F
520.45
393.18
485.64
386.84
546.53
389.33
517.10
395.57
0.5017
0.5196
1.1285
0.5043
0.4815
0.5140
0.6722
0.4832
HOMO → LUMO (99.3%)
HOMO-1 → LUMO (98.6%)
HOMO → LUMO (98.2%)
HOMO-1 → LUMO (95.3%)
HOMO → LUMO (99.5%)
HOMO-1 → LUMO (64.7%)
HOMO → LUMO (99.2%)
HOMO-1 → LUMO (92.2%)
a
Singlet energies for the vertical transition were calculated for
optimized S1 geometries.
The study of the absolute quantum efficiencies (ΦPL) and
radiative decay constants (kr) for all carboranes provided
supporting information for the relationship between the
photophysical properties and structural features. The ΦPL
value of 1B in solution at 298 K was substantially high
(24.5%, Table 1), while those of the other carboranyl
compounds were quite low (ΦPL = 4.0% for 2B, 0.9% for 1F,
and 6.0% for 2F; Table 1), since the C−C bond variation of the
o-carborane cage in solution at ambient temperature prevents
the radiative decay originating from the ICT.4b,10−12 However,
the ΦPL value of the fluorene-based compounds (1F and 2F)
was dramatically enhanced to around 37% in the film state,
which was 41 and 6 times the values of those of the solution
states, respectively (Table 1). On the other hand, those of the
biphenyl-based compounds only increased by a factor of 1.4
(ΦPL = 24.5 → 33.6% for 1B) or became too diminished to be
estimated in the case of 2B. These features were verified by the
determined radiative decay constants (kr = ΦPL/τobs). The
decay lifetimes of the carborane compounds in toluene (3.0 ×
10−5 M) and film (PMMA matrices doped with 5 wt % of each
compound on quartz plates) at 298 K were obtained from
single- or double-exponential curve fittings of each emission
decay curve (Figures S3−S6 in the Supporting Information).
These were calculated to be a few nanoseconds (0.7−4.7 ns;
Table 1), which is characteristic of a typical fluorescence. The kr
values of 1F and 2F in solution were 40 and 6 times those of
and Figure S7 in the Supporting Information). The structural
geometries used for the calculations were optimized from the
X-ray structures of 1B, 2B, 1F, and 2F. To include the solvent
effects of toluene, the conductor-like polarizable continuum
model (CPCM) was used.15
The results of the calculated transition in the S0 optimized
geometries exhibit that the lowest energy absorption mainly
involves the HOMO → LUMO transition (>96%, Figure S5
and Tables S3, S5, S7, and S9 in the Supporting Information).
The LUMO levels of all carboranes are delocalized not only
over the center phenylene rings (∼60−70%, Tables S4, S6, S8,
and S10 in the Supporting Information) but also over the
carborane moiety (∼25−30%), whereas the central phenylene
rings are the major contributors to the HOMOs (>90%). These
orbital distribution results suggest that the absorption spectra of
all four of the studied carboranes could be largely attributed to
the π → π* transition on the central phenylene rings, with only
a small contribution of a CT from the center phenylene to
carborane moieties. These calculation results match well with
the experimental UV−vis absorption spectra. The major
electronic transitions on the S1 optimized geometries can be
divided into the high-energy region around 350−400 nm and
the low-energy region below 400 nm, corresponding to the dual
emission pattern of the experimental PL spectra in solution at
77 K. The calculated transition in the low-energy emission
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Figure 3. Frontier molecular orbitals in the first singlet excited state (S1) optimized geometries in toluene and the lowest-energy electronic
transitions from the TD-DFT calculations (isovalue 0.04).
region could be assigned entirely to the HOMO → LUMO
transition (λcalc 520.45 nm for 1B, 485.64 nm for 2B, 546.53
nm for 1F, and 517.10 nm for 2F; Table 2) with a large
contributor (fcalc ≥ 0.5). While the HOMO of all carboranes is
localized mainly on the central phenylene rings (>82%, Tables
S4, S6, S8, and S10), the contribution of the carborane moieties
to their LUMOs (>70%) is drastically increased in comparison
to those on the S0 optimized geometries, indicating that the
origin of the low-energy emission below 400 nm is completely
due to ICT between the carborane and central phenylene rings.
On the other hand, the major contribution (fcalc ≈ 0.50, Table
2) in the high-energy region is dominated by the HOMO-1 →
LUMO transition (λcalc 393.18 nm for 1B, 386.84 nm for 2B,
389.33 nm for 1F, and 395.57 nm for 2F; Table 2). The
HOMO-1 of all compounds is mainly localized on the terminal
butylphenyl ring (∼70−89%; Tables S4, S6, S8, and S10),
resulting in high-energy emissions that are significantly affected
by ICT from the carborane to the terminal phenyl rings.
Interestingly, the calculated structures of 1B and 2B
optimized in the ground state (S0) resulted in dihedral angles
on the biphenyl rings of 42.1 and 36.3°, respectively, which
were similar to those found in the X-ray structures (44.2° for
1B and 33.1° for 2B). The biphenyl rings of 1B and 2B became
flattened (21.9° dihedral angle of 1B and 13.9° for 2B) in the
first singlet excited state (S1) optimized structures, indicating
that the flatness of the biphenyl group has an enormous effect
on the formation of stable excited states in 1B and 2B. The
slightly larger solid-state dihedral angle of 1B implies that its
biphenyl group, in all states, can be considerably distorted from
one to another, and the low-energy emissive mechanism of 1B
cannot be induced by the formation of structurally unstable
excited states. The unique and intense low-energy emission of
1F (Figure 2b−d) also indicates that the flatness of the central
phenylene groups is distinctly associated with the emissive ICT
band between the carborane and central phenylene rings. Such
a correlation between the dihedral angles and emissive ICT
characters in the low-energy region can definitively elucidate
the huge difference in the absolute quantum efficiency between
2B and 2F in the film state (ΦPL of 2B < 0.01 and ΦPL of 2F =
0.368; Table 1). It could be reasoned that the sustainable
flatness of the biphenyl rings in 2F embodied the stable excited
states of the ICT band and its efficient radiative decay in the
low-energy region, resulting in the drastically enhanced ΦPL
and kr values of 2F in comparison with those of 2B in the film
state. Consequently, the prevention of distortion of the
phenylene rings can certainly enhance the ICT emissive
character to the phenylene groups and the efficiency of the
radiative mechanism in o-carboranyl complexes.
CONCLUSIONS
■
We have synthesized and characterized the biphenyl- and
fluorene-based mono- and bis-o-carboranyl compounds 1B, 2B,
1F, and 2F with terminal phenyl rings, and the solid-state
structures of all complexes were analyzed by single-crystal X-ray
diffraction. In particular, the structures of 1B and 2B showed
distortions on the central biphenyl group with dihedral angles
of 44.2 and 33.1°, respectively. Fluorene-based carboranyl
compounds exhibited a noticeable emission in the low-energy
region below 400 nm, while biphenyl-based carboranes showed
a high-energy emission in the region above 400 nm and little
(2B) to no (1B) low-energy emissive trace in the rigid state
(e.g., in solution at 77 K or as a film). The theoretical
calculation results for the first excited singlet (S1) state of each
compound suggested that emission in the low-energy region
distinctly involved ICT between the carborane and central
phenylene rings, indicating that the radiative decay of these
ICT bands could be enhanced by the prevention of distortion
of the central biphenyl groups. A smaller degree of distortion of
the biphenyl rings of 1B and 2B in the S1 optimized structures
in comparison with that of the ground states strongly implies
that these phenylenes must become flattened for the formation
of stable excited states and, therefore, for efficient radiative
mechanisms. The high quantum efficiencies (ΦPL) and radiative
decay constants (kr) of 1F and 2F in the rigid state in
comparison to those of 1B and 2B also definitively support the
relationship between the emissive and structural features of
each o-carborane compound.
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed under an
inert nitrogen atmosphere using standard Schlenk and glovebox
techniques. Anhydrous grade solvents (Aldrich) were dried by passing
through an activated alumina column and stored over activated
molecular sieves (5 Å). Spectrophotometric-grade solvents (THF,
E
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Article
cyclohexane, and DMSO) were used as received from Alfa Aesar.
Commercial reagents were used without any further purification after
purchase from Aldrich (bis(triphenylphosphine)palladium(II) dichlor-
ide, copper(I) iodide, diethyl sulfide, triethylamine, 4-butylphenylace-
tylene, 4-bromobiphenyl, 4,4″-dibromobiphenyl, 2-bromofluorene,
2,7-dibromofluorene) and KatChem (B10H14, decaborane). CDCl3
from Cambridge Isotope Laboratories was used after drying over
activated molecular sieves (5 Å). NMR spectra of compounds were
(2.0 mL, 11.5 mmol) afforded 2Fa as a gray solid without purification.
Yield: 2.02 g, 91%. 1H NMR (CDCl3): δ 7.72 (d, J = 8.0 Hz, 2H), 7.68
(s, 2H), 7.54 (d, J = 7.6 Hz, 2H), 7.44 (d, J = 8.0 Hz, 4H), 7.15 (d, J =
8.4 Hz, 4H), 3.90 (s, 2H), 2.61 (t, J = 15.6 Hz, 4H), 1.60 (m, 4H),
1.36 (m, 4H), 0.92 (t, J = 14.8 Hz, 6H). 13C NMR (CDCl3): δ 143.50,
143.36, 141.08, 131.48, 130.59, 128.49, 128.10, 121.97, 120.48, 120.00,
89.97 (acetylene-C), 89.41 (acetylene-C), 36.56, 35.61, 33.39, 22.31,
13.93. Anal. Calcd for C37H34: C, 92.84; H, 7.16. Found: C, 92.98; H,
6.89.
1
recorded on a Bruker Avance 400 spectrometer (400.13 MHz for H,
100.62 MHz for 13C, 128.38 MHz for 11B) at ambient temperature.
Chemical shifts are given in ppm and are referenced against external
Me4Si (1H, 13C) and BF3·Et2O (11B). Elemental analyses were
performed on an EA3000 (Eurovector) in the Central Laboratory of
Kangwon National University. UV/vis absorption and PL spectra were
recorded on a Varian Cary 100 and a HORIBA FluoroMax-4P
spectrophotometer, respectively. Fluorescence decay lifetimes were
measured using a time-correlated single-photon counting (TCSPC)
spectrometer (FLS920-EDINBURGH Instruments, at the Central
Laboratory of Kangwon National University) equipped with a EPL-
375 ps pulsed semiconductor diode laser as an excitation source and a
microchannel plate photomultiplier tube (MCP-PMT, 200−850 nm)
as a detector at 298 K.
General Synthesis of Acetylene Precursors. All acetylene
precursors (1Ba, 1Fa, 1Ba, and 2Fa) were synthesized by the
following scheme with an adequate amount of starting materials,
respectively. Toluene and triethylamine (v/v 1/9) were added via
cannula to the mixture of bromo-aromatic precursors, copper iodide,
and Pd(PPh3)2Cl2 at room temperature. After the mixture was stirred
for 15 min, 4-butylphenylacetylene (1.2 equiv per bromide) was added
to the resulting dark brown slurry. The reaction mixture was then
refluxed for 24 h. The volatiles were removed with a rotary evaporator,
affording a dark gray residue. After washing with MeOH followed by n-
hexane, the remaining solid was purified by column chromatography,
which afforded solid acetylene precursors.
Data for 1Ba. 4-Bromobiphenyl (1.5 g, 5.36 mmol), copper iodide
(80 mg), Pd(PPh3)2Cl2 (120 mg), and 4-butylphenylacetylene (1.2
mL, 6.4 mmol) afforded 1Ba as pale gray solid which was purified by
column chromatography (eluent: only n-hexane). Yield: 1.06 g, 64%.
1H NMR (CDCl3): δ 7.59 (m, 2H), 7.57 (s, 4H), 7.43 (m, J = 18.0 Hz,
4H), 7.36 (m, 1H), 7.15 (d, J = 8.4 Hz, 2H), 2.62 (t, J = 15.2 Hz, 2H),
1.59 (m, 2H), 1.36 (m, 2H), 0.92 (t, J = 14.8 Hz, 3H). 13C NMR
(CDCl3): δ 143.43, 140.75, 140.41, 131.96, 131.50, 128.84, 128.48,
127.57, 127.00, 126.98, 122.43, 120.40, 90.30 (acetylene-C), 88.63
(acetylene-C), 35.60, 33.39, 22.30, 13.93. Anal. Calcd for C24H22: C,
92.86; H, 7.14. Found: C, 92.64; H, 7.00.
General Synthesis of o-Carborane Compounds 1B, 2B, 1F,
and 2F. To a toluene solution (100 mL) of decaborane (B10H14, 1.1
equiv per acetylene moiety) and acetylene precursors (1Ba, 1Fa, 1Ba,
and 2Fa) was added an excess amount of diethyl sulfide (Et2S, 5 equiv
of an acetylene precursor) at room temperature. After it was heated to
reflux, the reaction mixture was further stirred for 3 days. The solvent
was removed under vacuum, and MeOH (50 mL) was added. The
resulting yellow solid was filtered and redissolved in toluene. The
solution was purified by passing through an alumina column, and the
solvent was removed in vacuo, affording o-carborane compounds as a
white solid.
Data for 4-[2-(p-n-Butylphenyl)-1-o-carboran-1-yl]biphenyl (1B).
Decaborane (0.59 g, 4.8 mmol), Et2S (1.2 mL, 11 mmol), and 1Ba
(1.03 g, 2.2 mmol) in toluene (100 mL) afforded 1B as a white solid.
Recrystallization from a acetone/MeOH mixture gave 0.47 g of 1B
1
(50.0%). H NMR (CDCl3): δ 7.45 (d, J = 8.8 Hz, 4H), 7.38 (t, J =
14.4 Hz, 2H), 7.33 (m, 5H), 6.91 (d, J = 8.4 Hz, 2H), 3.30−1.50 (br,
10H, Car-BH), 2.45 (t, J = 15.2 Hz, 2H), 1.45 (m, 2H), 1.18 (m, 2H),
0.81 (t, J = 14.8 Hz, 3H). 13C NMR (CDCl3): δ 145.31, 142.63,
139.31, 139.28, 131.02, 130.53, 129.76, 128.85, 128.28, 128.00, 126.92,
126.63, 85.68 (CB-C), 85.05 (CB-C), 34.92, 32.89, 22.08, 13.78. 11B
NMR (CDCl3): δ −2.61 (br s, 3B), −9.27 (br s, 2B), −10.46 (br s,
5B). Anal. Calcd for C24H32B10: C, 67.25; H, 7.53. Found: C, 67.22; H,
7.80.
Data for 4,4″-Bis[2-(p-n-butylphenyl)-1-o-carboran-1-yl]biphenyl
(2B). Decaborane (0.59 g, 4.8 mmol), Et2S (1.2 mL, 11 mmol), and
2Ba (1.03 g, 2.2 mmol) in toluene (100 mL) afforded 2B as a white
solid. Recrystallization from a CH2Cl2/n-hexane mixture gave 0.72 g of
2B (46.6%). 1H NMR (CDCl3): δ 7.41 (d, J = 6.8 Hz, 2H), 7.29 (d, J
= 8.4 Hz, 2H), 7.21 (d, J = 6.9 Hz, 2H), 6.89 (d, J = 8.4 Hz, 2H),
3.30−1.50 (br, 20H, Car-BH), 2.43 (t, J = 15.3, 4H), 1.42 (m, 4H),
1.17 (m, 4H), 0.79 (t, J = 14.6 Hz, 6H). 13C NMR (CDCl3): δ 145.34,
140.70, 131.10, 130.59, 130.50, 128.31, 127.92, 126.51, 85.65 (CB-C),
84.61 (CB-C), 34.91, 32.87, 22.08, 13.81. 11B NMR (CDCl3): δ −3.03
(br s, 4B), −10.83 (br s, 16B). Anal. Calcd for C36H54B20: C, 61.50; H,
7.74. Found: C, 61.81; H, 7.96.
Data for 2Ba. 4,4″-Dibromobiphenyl (1.54 g, 4.93 mmol), copper
iodide (80 mg), Pd(PPh3)2Cl2 (120 mg), and 4-butylphenylacetylene
(2.1 mL, 11.8 mmol) afforded 2Ba as a pale gray solid without a
purification. Yield: 2.14 g, 93%. 1H NMR (CDCl3): δ 7.58 (br s, 8H),
7.44 (d, J = 10.8 Hz, 4H), 7.15 (d, J = 10.9 Hz, 4H), 2.61 (t, J = 20.5
Hz, 4H), 1.59 (m, 4H), 1.34 (m, 4H), 0.92 (t, J = 19.5 Hz, 6H). 13C
NMR (CDCl3): δ 143.45, 139.82, 131.99, 131.47, 128.45, 126.78,
122.72, 120.28, 90.54 (acetylene-C), 88.53 (acetylene-C), 35.57, 33.35,
22.26, 13.89. Anal. Calcd for C36H34: C, 92.66; H, 7.34. Found: C,
92.35; H, 7.40.
Data for 1Fa. 2-Bromofluorene (0.98 g, 4.00 mmol), copper iodide
(80 mg), Pd(PPh3)2Cl2 (120 mg), and 4-butylphenylacetylene (0.9
mL, 4.8 mmol) afforded 1Fa as a gray solid which was purified by
column chromatography (eluent: only n-hexane). Yield: 0.85 g, 66%.
1H NMR (CDCl3): δ 7.77 (d, J = 10.0 Hz, 1H), 7.72 (t, J = 23.2 Hz,
2H), 7.54 (d, J = 9.6 Hz, 2H), 7.45 (d, J = 11.2 Hz, 2H), 7.37 (t, J =
18.8 Hz, 1H), 7.30 (m, 1H), 7.15 (d, J = 11.2 Hz, 2H), 3.90 (s, 2H),
2.61 (t, J = 20.4 Hz, 2H), 1.60 (m, 2H), 1.35 (m, 2H), 0.92 (t, J = 19.6
Hz, 3H). 13C NMR (CDCl3): δ 143.55, 143.28, 143.18, 141.69,
141.16, 131.47, 130.44, 128.48, 128.10, 127.09, 126.90, 125.08, 121.56,
120.58, 120.15, 119.76, 89.63 (acetylene-C), 89.50 (acetylene-C),
36.76, 35.62, 33.41, 22.33, 13.94. Anal. Calcd for C25H22: C, 93.12; H,
6.88. Found: C, 92.68; H, 6.40.
Data for 2-[2-(p-n-Butylphenyl)-1-o-carboran-1-yl]fluorene (1F).
Decaborane (0.34 g, 2.8 mmol), Et2S (1.3 mL, 12 mmol), and 1Fa
(0.81 g, 2.5 mmol) in toluene (100 mL) afforded 1F as a white solid.
Recrystallization from a acetone/MeOH mixture gave 0.43 g of 1F
(38.8%). 1H NMR (CDCl3): δ 7.67 (d, J = 6.8, 1H), 7.56 (s, 1H), 7.49
(d, J = 8.0, 2H), 7.42 (d, J = 6.4, 1H), 7.31 (m, 5H), 6.87 (d, J = 8.8,
2H), 3.74 (s, 2H), 3.30−1.50 (br, 10H, Car-BH), 2.40 (t, J = 15.2,
2H), 1.45 (m, 2H), 1.18 (m, 2H), 0.81 (t, J = 14.8, 3H). 13C NMR
(CDCl3): δ 145.25, 143.79, 143.45, 142.99, 140.21, 130.52, 129.68,
129.10, 128.25, 128.10, 127.65, 127.28, 126.98, 125.13, 120.44, 119.28,
86.07 (CB-C), 85.89 (CB-C), 36.75, 34.93, 32.87, 22.16, 13.80. 11B
NMR (CDCl3): δ −2.42 (br s, 2B), −10.29 (br s, 8B). Anal. Calcd for
C25H32B10: C, 68.14; H, 7.32. Found: C, 68.01; H, 6.98.
Data for 2,7-Bis[2-(p-n-butylphenyl)-1-o-carboran-1-yl]fluorene
(2F). Decaborane (0.59 g, 4.8 mmol), Et2S (1.2 mL, 11 mmol), and
2Fa (1.05 g, 2.2 mmol) in toluene (100 mL) afforded 2F as a white
solid. Recrystallization from a acetone/MeOH mixture gave 0.60 g of
2F (38.2%). 1H NMR (CDCl3): δ 7.54 (s, 2H), 7.39 (s, 4H), 7.29 (d, J
= 8.4 Hz, 4H), 6.86 (d, J = 8.0 Hz, 4H), 3.60 (s, 2H), 3.30−1.50 (br,
20H, CB-BH), 2.39 (t, J = 15.2 Hz, 4H), 1.39 (m, 4H), 1.12 (m, 4H),
0.75 (t, J = 14.4 Hz, 6H). 13C NMR (CDCl3): δ 145.29, 143.54,
141.88, 130.48, 130.12, 129.76, 128.27, 127.98, 127.40, 119.85, 85.84
(CB-C), 85.49 (CB-C), 36.59, 34.89, 32.83, 22.09, 13.78. 11B NMR
(CDCl3): δ −2.69 (br s, 4B), −10.37 (br s, 16B). Anal. Calcd for
C37H54B20: C, 62.15; H, 7.61. Found: C, 61.96; H, 7.43.
Data for 2Fa. 2,7-Dibromoofluorene (1.55 g, 4.78 mmol), copper
iodide (80 mg), Pd(PPh3)2Cl2 (120 mg), and 4-butylphenylacetylene
F
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Organometallics XXXX, XXX, XXX−XXX

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Article
X-ray Crystallography. Single crystals of 1B, 2B, and 2F were
coated with Paratone oil and mounted onto a glass capillary. The
crystallographic measurements were performed on a Bruker SMART
Apex II CCD area detector diffractometer with graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by
direct methods, and all non-hydrogen atoms were subjected to
anisotropic refinement by a full-matrix least-squares method on F2 by
using the SHELXTL/PC package, resulting in X-ray crystallographic
data of 1B, 2B, and 2F in CIF format (CCDC 1458111, 1458110, and
1458112). Hydrogen atoms were placed at their geometrically
calculated positions and refined riding on the corresponding carbon
atoms with isotropic thermal parameters. Detailed crystallographic
data are given in Tables S1 and S2 in the Supporting Information.
UV−Vis Absorption and Photoluminescence Emission
Experiments. UV−vis absorption spectra in THF and PL spectra
in toluene (3.0 × 10−5 M) were recorded on Varian Cary 100 and
HORIBA FluoroMax-4P spectrophotometers, respectively. The
absolute photoluminescence quantum efficiencies (PLQY) of toluene
solutions (3.0 × 10−5 M) and film (PMMA matrices doped with 5 wt
% for carborane compounds on quartz plates) at 298 K were obtained
using a 3.3 in. integrating sphere equipped on a Quantaurus-QY
(absolute PL quantum yield spectrometer C11347-11, Hamamatsu
Photonics). Fluorescence decay lifetimes were measured using a time-
correlated single-photon counting (TCSPC) spectrometer (FLS920,
Edinburgh Instruments) equipped with a EPL-375 ps pulsed diode
laser as an excitation source and a microchannel plate photomultiplier
tube (MCP-PMT, 200−850 nm) as a detector at 298 K. The PL
experiments were performed with the following excitations: λex for 1B
318 nm, 2B 312 nm, 1F 315 nm, and 2F 320 nm.
Theoretical Calculations. The geometries of the ground (S0) and
lowest-lying singlet excited (S1) state of 1B, 2B, 1F, and 2F were
optimized using the density functional theory (DFT) method with the
B3LYP16 functional and the 6-31G(d)17 basis sets. Time-dependent
density functional theory (TD-DFT)18 measurements based on the
hybrid B3LYP functional (TD-B3LYP) were utilized for calculation of
the electronic transition energies, which also included an account of
electron correlation. To include the solvent effects of THF, the
conductor-like polarizable continuum model (CPCM) was used.15
The calculations were performed by the GAUSSIAN 09 program.19
The percent contribution of a group in a molecule to each molecular
orbital was calculated by the GaussSum 3.0 program.20
2015R1D1A1A01057396 for K.M.L.) through the National
Research Foundation of Korea (NRF) funded by the Ministry
of Science, ICT & Future Planning and the Ministry of
Education. This study was supported by a 2016 Research Grant
from Kangwon National University (No. 520160510).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
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X-ray crystallographic data, computational details, and
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X-ray crystallographic data (CIF)
X-ray crystallographic data (CIF)
X-ray crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.M.L.: kangmunlee@kangwon.ac.kr.
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ORCID
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Kang Mun Lee: 0000-0001-8547-558X
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was supported by the Nano·Material Technology
Development Program (NRF-2016M3A7B4909246 for
K.M.L.) and Basic Science Research Program (NRF-
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DOI: 10.1021/acs.organomet.7b00093
Organometallics XXXX, XXX, XXX−XXX