Electron-donating abilities and luminescence properties of tolane-substituted: Nido -carboranes

Article abstract of DOI:10.1039/c7nj02438j

The luminescence properties of nido-carborane (7,8-dicarba-nido-carborate anion)-substituted tolane derivatives were investigated. It was shown that the nido-carborane unit acted as an electron-donating group in all derivatives, and their emission colors

Full text of DOI:10.1039/c7nj02438j

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Electron-donating abilities and luminescence
properties of tolane-substituted nido-carboranes†
Cite this:DOI: 10.1039/c7nj02438j
Kenta Nishino, Yasuhiro Morisaki,‡ Kazuo Tanaka
and Yoshiki Chujo*
The luminescence properties of nido-carborane (7,8-dicarba-nido-carborate anion)-substituted tolane
derivatives were investigated. It was shown that the nido-carborane unit acted as an electron-donating
group in all derivatives, and their emission colors can be tuned by the substituents at the tolane moiety.
According to theoretical investigation, it was proposed that frontier orbitals were distinctly separated by
the twisted structure of the methyl substituents at the nido-carborane unit. Thus, in the absorption
spectra, drastic changes were obtained because the HOMO–LUMO transition was forbidden. These
structural and electrical substituent effects should serve as good guidelines for designing optoelectronic
materials based on nido-carboranes.
Received 7th July 2017,
Accepted 3rd August 2017
DOI: 10.1039/c7nj02438j
rsc.li/njc
prepared and applied as BNCT materials46 and catalysts
composed of metal complexes with ruthenium, titanium, and
Introduction
p-Conjugated systems involving ‘‘element-blocks’’, which are zirconium.47 In recent years, emissive materials concerning nido-
defined as a minimum functional unit composed of heteroatoms, carboranes were reported.48 Based on the above elimination reaction
are attractive candidates for receiving unique optically-functional of the boron atom from o-carborane followed by drastic changes in
materials.1 From this viewpoint, a series of boron clusters2–6 have the luminescence color, fluoride sensing was accomplished. We
gathered much attention especially for obtaining luminescent also have synthesized nido-species obtained from diaryl-fused
materials.7–34 As a representative example, o-carborane (C2H12B10) dibenzocarboranes.49 They showed strong emission both in the
derivatives in combination with conjugated molecules showed solution and crystal states.49 However, there are still very few
bright emission only in the aggregation state (aggregation- examples that afford nido-carborane anion-based optical materials.
induced emission, AIE).35,36 Owing to the bulkiness of o-carborane, In particular, the electronic properties of the nido-carborane unit in
aggregation-caused quenching (ACQ), which is often observed from the p-conjugated system has been veiled. Therefore, it should be of
commodity organic luminescent dyes, can be suppressed.37–39 great significance to collect systematic information on the electronic
In addition, because of the strong electron-deficient nature of interaction of nido-carborane anions with conjugation units for
o-carborane, intense emission from the intramolecular charge realizing advanced optically-functional materials based on the
transfer (ICT) state can be often obtained even in the solid nido-carborane unit as a versatile ‘‘element-block’’.
state.40 Furthermore, by controlling the morphology in the
Herein, synthesis and characteristics of nido-carborane-
solid sample, luminescence color tuning and stimuli-responsive substituted tolane are presented. From a series of optical
luminescence chromism were achieved.41 Therefore, comprehension measurements and theoretical investigations with the synthesized
of electronic structures and luminescence mechanism of compounds, contribution of electronic interaction was examined.
o-carborane-containing conjugated molecules is essential.42–44 In particular, it was suggested that the electron-donating ability of
o-Carborane can be transformed into nido-carborane species the nido-carborane unit should critically depend on the dihedral
in the presence of strong nucleophiles. For example, a 7,8-dicarba- angle toward the tolane moiety. To obtain systematic information
nido-undecaborate anion (nido-carborane) was obtained by on this phenomenon, further molecules were designed and
eliminating the boron atom with a fluoride anion.45 Based on synthesized. Finally, modulation of electronic interaction based
this reaction, a series of nido-carborane derivatives has been on the tunable electron-donating ability of the nido-carborane unit
was demonstrated.
Department of Polymer Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: chujo@chujo.synchem.kyoto-u.ac.jp
Results and discussion
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj02438j
‡ Present address: Department of Applied Chemistry for Environment, School of
Synthesis of the nido-carborane derivative is shown in Scheme 1.
4-Bromo-o-carboranylbenzene was obtained by the addition of
Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo
669-1337, Japan.
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Scheme 1 Synthesis of the nido-carborane-substituted tolanes.
Table 1 Optical properties of nido-carboranes in THF solutionsa
decaborane with p-bromoethynylbenzene. Next, the Sonogashira–
Hagihara coupling reaction was performed, and o-carboranyltolane
1a was obtained in 28% yield. Then, 1a was reacted with n-BuLi and
MeI to obtain (1-methyl-o-carboran-2-yl)tolane 1b. Finally, one boron
atom was eliminated from the o-carborane unit by treating with a
THF solution of tetrabutylammonium fluoride, and nido-carborane
was obtained. All compounds were characterized using 1H, 11B and
13C NMR spectra and high resolution mass measurements. In the
1H NMR spectra of 2, there was a specific peak at À2 ppm (see
the ESI†). These peaks were assigned to the bridging hydrogens
b
lab
e
Eg
lPL
t
kr
knr
(nm) (MÀ1 cmÀ1
)
(eV) (nm) FPL (ns) (108 sÀ1
)
(108 sÀ1
)
c
2a 313
29 000
37 000
35 000
23 000
3.50 447
3.51 474
3.56 430
3.21 525
0.59 4.5 1.31
0.29 7.2 0.40
0.45 3.3 1.36
0.11 1.1 1.00
0.91
0.99
1.67
8.09
2b 289
2c 317
2d 339
a
b
Measured using 10 Â 10À5 M. Calculated from the absorption edge.
c
Determined as an absolute value.
in the nido-caborane unit according to the literature.50–52 From extinction coefficient of about 37 000 MÀ1 cmÀ1. Moreover, the
these characterization data, we concluded that the products extinction coefficient of 2b at 313 nm was too small to be
should have desired structures. The nido-carboranyltolane compared with that of 2a although 2b showed a higher coeffi-
anions with a tetraalkylammonium cation were stable to heat, cient at 289 nm than 2a. However, the energy gaps between the
oxygen, moisture and light, while the anion with a potassium highest occupied molecular orbital (HOMO) and the lowest
cation was relatively reactive toward oxygen and light. It was unoccupied molecular orbital (LUMO) which were calculated
assumed that steric hindrances of counter anions might contribute from the absorption band edge were almost identical (2a: 3.50 eV,
to improving their stability. Thus, all measurements were performed 2b: 3.51 eV). This result suggests that there should be little
using ammonium salts in this study.
differences in energy levels and be sure differences in transition
To gather information on their electronic properties in the probabilities.
ground state, UV-vis absorption spectra were recorded in the
In the photoluminescence (PL) spectra, 2b showed a red-shifted
THF solutions at room temperature (Fig. 1a and Table 1). 2a emission compared to 2a (Fig. 1b). However, the quantum
showed a broad absorption band with peaks at 287 and 313 nm, efficiency (FPL) of 2b was smaller than that of 2a (Table 1). To
while 2b showed a specific band with a peak at 289 nm with an quantitatively evaluate kinetics in the decay processes, the rate
constants of radiative (kr) and non-radiative decay processes
(knr) were estimated using FPL and fluorescence lifetime (t)
values according to the following equations:
kr = FPL/t
(1)
(2)
knr = (1 À FPL)/t
As shown in Table 1, knr values were almost identical, while
kr of 2a was about three times larger than that of 2b. This result
suggests that low FPL and long t of 2b were caused by low kr.
In a previous report, it was shown that aromatic molecule-
connecting o-carboranes presented significant emission from
the ICT state.40 To examine ICT characters of 2a and 2b,
Lippert–Mataga plots were prepared according to the empirical
Fig. 1 (a) UV-vis absorption and (b) PL spectra of 2a (solid line) and 2b
(dashed line) in THF solutions (1.0 Â 10À5 M). Excitation wavelengths for
recording PL spectra were at 300 nm.
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Fig. 2 PL spectra of 2a (solid line) and 2b (dashed line) in (a) 2-MeTHF
solutions (1.0 Â 10À5 M) at r.t. (black) and 77 K (gray) and (b) the crystalline
state.
formula (see the ESI†). By changing solvents, UV-vis absorption
and PL spectra were monitored, and the degree of peak shifts in
PL spectra was evaluated. The degree of ICT can be estimated
from the slope of a fitting line (Fig. S1 and Tables S1–S3, ESI†).
In addition, the dipole moments in the excited state were
approximately calculated using hypothetical Onsager radii
and estimated dipole moments in the ground state. Significant
Fig. 3 Optimized structures in the ground state, where j is the dihedral
angle between nido-carborane and tolane, and l is the C–C bond length in
the nido-carborane unit, and molecular orbitals of (a) 2a and (b) 2b
in major absorption transitions. Calculations were performed at the
CAM-B3LYP/6-31+G(d,p)//B3LYP/6-31G(d,p) level.
dependencies of the Stokes shift (Dn~) values were observed delocalized through the entire molecule. Most of the HOMO
using a solvent polarity parameter (Lippert polarity parameter; existed on the nido-carborane unit, and some of the orbital
Df ). These data mean that the PL properties of 2a and 2b should was located at the tolane moiety. Moreover, the LUMO was
include the ICT characters because of the electron-donating located at the tolane moiety. Because of the elongation of the
ability of the nido-carborane unit. The PL spectra of 2 were HOMO to the tolane moiety, the high oscillator strength of 2a
obtained in the frozen and crystalline states (Fig. 2). The for the S0 - S1 transition was obtained (0.9612). Furthermore,
emission spectra were blue-shifted at 77 K compared to those S0 - S1 and S0 - S2 transitions of 2b also corresponded to the
at room temperature in 2-methyl-THF (2-MeTHF) (Fig. 2a). HOMO - LUMO and HOMOÀ1 - LUMO, and the locations of
Moreover, these blue-shifted emission bands were also observed HOMOÀ1 and LUMO were similar to those of 2a. However, the
in the crystalline-state PL spectra compared to the emission in HOMO of 2b was localized at the nido-carborane unit because
the THF solution (Fig. 2b). These blue-shifted emissions could of its highly twisted structure. Hence, the oscillator strength
have originated from a decrease of the re-orientation energy of of 2b for the S0 - S1 transition was smaller than that of 2a, and
solvents as is often observed in the ICT luminophores. Hence, indeed the extinction coefficient of 2b at 320 nm should be
from these data, it was suggested that emission of 2 should be extremely small.
obtained from the ICT state.
The optimized structure in the ES is shown in Fig. 4.
To obtain deeper insight into their optical properties, quan- Interestingly, the optimized structure of 2a in the ES was
tum calculations were performed for estimating the molecular changed compared with that in the GS. The dihedral angle
orbitals by using density functional theory (DFT) for the ground was 881, and this value was identical to that of 2b (881). This
state (GS) and time dependent-DFT (TD-DFT) for the excited structural change in the dihedral angle in the ES was also
state (ES). The calculations were performed at the CAM-B3LYP/ observed in the calculation results with other aryl-substituted
6-31+G(d,p)//B3LYP/6-31G(d,p) level.53 The optimized structures o-carborane dyads (Table S5, ESI†).37–39 The calculated emission
in the GS are shown in Fig. 3. In this figure, j means the properties of 2a and 2b were 459 and 469 nm, respectively, and
dihedral angle between the C–C bond in the nido-carborane unit these corresponded to the experimental results. The molecular
and that in the benzene ring, and l means the C–C bond in the orbitals in the emission transition are also presented in Fig. 4.
nido-carborane unit. The dihedral angles in 2a and 2b were Both of their HOMOs were located at the nido-carborane unit,
calculated to be 1581 and 1071, respectively. The much twisted and LUMOs were located at the tolane moiety. Since the HOMO
conformation of 2b was originated from the steric effect of the of 2b was also located at the methyl group, the energy level of
methyl group at the nido-carborane unit. The calculated absorp- the HOMO became larger than that of 2a. Thus, the band gap
tion transitions are summarized in Table S4 (ESI†). Accordingly, energy of 2b could become smaller, and the red-shifted emission
the absorption peaks at 313 and 287 nm of 2a were assigned was indeed compared to 2a. It can be summarized from these
to S0 - S1 and S0 - S2 transitions, respectively. Therefore, results that the nido-carborane unit should work as an electron-
these transitions corresponded to the HOMO - LUMO and donating substituent.
HOMOÀ1 - LUMO transitions, respectively. The molecular
To evaluate the degree of electron-donating ability and to
orbitals are also shown in Fig. 3. The HOMOÀ1 of 2a was demonstrate color tuning, additional substituent groups were
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Fig. 5 (a) UV-vis absorption and (b) PL spectra of 2c and 2d in the THF
solutions (1.0 Â 10À5 M). Excitation wavelengths for recording PL spectra
were at 300 nm.
demonstrated that the optical properties of nido-carborane-
connected aryl groups can be tuned by the substituent effect.
Conclusion
The optical properties of the nido-carborane-substituted tolane
derivatives are discussed. The methyl group at the carbon
position in the nido-carborane unit showed steric effects and
inhibited the rotation of the tolane unit in the ground state.
Hence, the extinction coefficient drastically decreased as theoretical
investigation suggested. Moreover, the radiative deactivation was
suppressed, and the lifetime of the excited species was extended.
From the PL spectra in various solvents, the substituent effect and
calculated results, the ICT emission originating from the electron-
donating ability of the nido-carborane unit was observed. These
results could be useful for designing luminescent and charge
separation materials based on the nido-carborane unit.
Fig. 4 Optimized structures in the excited state, where j is the dihedral
angle between nido-carborane and tolane, and l is the C–C bond length in
the nido-carborane unit, and molecular orbitals of (a) 2a and (b) 2b
in major emission transitions. Calculations were performed at the
CAM-B3LYP/6-31+G(d,p)//B3LYP/6-31G(d,p) level.
introduced into the opposite end in the tolane moiety toward
nido-carborane. Based on this idea, 2a derivatives were designed
and synthesized (Scheme 2). The electron-donating (methoxy,
2c) and electron-withdrawing (trifluoromethyl, 2d) groups were
attached to the tolane moiety. The UV-vis absorption and PL
spectra were recorded (Fig. 5 and Table 1). It was shown that the
absorption peaks and edges and emission peaks were red-
shifted from 2d. It is likely that electronic interaction would
be enhanced via the strong electron-donating and withdrawing
system. As a result, bathochromic shifts were detected in optical
spectra. In 2c, the emission band was observed at a similar
position to that in 2a in the PL spectrum. This fact implies that a
similar degree of electron donation might occur in 2c with 2a.
This result means that the nido-carborane unit is capable of
working as an electron-donating unit. Furthermore, it was
Acknowledgements
This work was partially supported by the Konica Minolta
Science and Technology Foundation (for K. T.) and a Grant-in-
Aid for Scientific Research on Innovative Areas ‘‘New Polymeric
Materials Based on Element-Blocks (No. 2401)’’ (JSPS KAKENHI
Grant Number JP24102013).
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