Photochemistry of hybrid organic-inorganic triarylborane-o-carboranes

Article abstract of DOI:10.1016/j.jorganchem.2015.04.025

Para- and meta-substituted o-carboranes with a dimesityl(phenyl)borane (dmpb) group, namely, p-1, p-2, m-1, and m-2, were prepared and their electronic natures were evaluated by steady-state photophysical methods. It was found that excited states were gre

Full text of DOI:10.1016/j.jorganchem.2015.04.025

Journal of Organometallic Chemistry xxx (2015) 1e7
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photochemistry of hybrid organiceinorganic
triarylborane-o-carboranes
,
, *
So-Yoen Kim a, Ah-Rang Lee b, Yang-Jin Cho a, Ho-Jin Son a *, Won-Sik Han b
,
,
*
Sang Ook Kang a
a Department of Advanced Materials Chemistry, Korea University, Sejong 339-700, Republic of Korea
b Department of Chemistry, Seoul Women's University, Seoul 139-774, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Para- and meta-substituted o-carboranes with a dimesityl(phenyl)borane (dmpb) group, namely, p-1, p-
2, m-1, and m-2, were prepared and their electronic natures were evaluated by steady-state photo-
physical methods. It was found that excited states were greatly influenced by the linking position of the
dimesityl-borane group(s) at the phenyl unit of diphenyl-o-carboranes. While intramolecular charge-
transfer (ICT) prevails in both para- and meta-regioisomers, para-isomers show much enhanced ICT
character, as evidenced by observation of a new emission band at lmax z 570 nm in dichloromethane
(DCM) solution. Furthermore, the solvatochromic shifts, Dyf, from n-hexane to DCM were significantly
different for para- and meta-isomers, which showed 8105, 8184, 1834, and 1895 cmꢀ1 for p-1, p-2, m-1,
and m-2, respectively. The difference in dipole moment between ground and excited states for para-
(~31.8 D) and meta-isomers (~15.0 D) by using MatagaeLippert plot confirmed that the ICT occurs from
dimesityl-borane to o-carborane in para-isomers while the ICT occurs only from dimesityl-borane to the
bridged phenyl group in meta-isomers. Excited state estimation by electrochemical and DFT studies
corroborated well to the ICT character, and even with engagement of the strong electron withdrawing
dimesityl-borane group o-carborane unit still acts as an electron accepter as found in the para-isomers,
p-1 and p-2.
Received 12 March 2015
Received in revised form
14 April 2015
Accepted 15 April 2015
Available online xxx
Keywords:
Carborane photochemistry
Intramolecular charge-transfer
Hybrid organiceinorganic boron complexes
© 2015 Elsevier B.V. All rights reserved.
Introduction
distinctive electron donoreacceptor dyads were formed to show a
low energy charge-transfer (CT) transition band in their photo-
Tri-coordinated organoborane with its electron deficient nature
of the central boron atom, is an important electron-acceptor for
conjugated materials and has drawn much attention as potential
organic electronic materials applicable to blue-light fluorescence,
anion sensing, and nonlinear optics [1]. One way to stabilize an
electron deficient boron atom is to replenish electrons to the cen-
luminescent spectra [5].
p-
Within the context of further exploration of potential photo-
functional materials in the realms of organoboranes, electron
deficient inorganic cluster boranes are introduced in this work.
Recently, o-carborane derivatives have shown to be efficient
electron acceptors if they were coordinated to the aryl groups [6].
Even though the nature of the -electron acceptor character be-
p-
tral boron through the
p
-electron channels from neighboring
p
organic functional groups. In most cases, organic functional groups
tween organoboranes and o-carboranes differs greatly, combina-
tion of these two units into a single molecule would create new
types of hybrid organiceinorganic boron complexes otherwise
unable to predict their electronic nature. In this regard, incorpo-
ration of o-carboane and triarylborane has been reported as a
fluoride ion sensor [7], but their detailed photophysical properties
were not reported. To establish further structureeelectronic
property relationship, we have prepared a series of triarylborane-o-
carboranes in which dimesitylboranes were attached to the para-
and meta-position of diphenyl-o-carborane, as shown in Scheme 1.
It has been found that incorporation of organoboranes into the
were aryl or alkylated aryl groups facilitating pp / pp orbital
*
interaction from aryl groups to the boron atom [2e4]. Depending
on the electron donating power of the aryl substituent, much
enhanced
p-type interaction was foreseeable. When strong elec-
tron donating (ED) amine groups were engaged to the
p-aryls,
* Corresponding authors.
E-mail addresses: hjson@korea.ac.kr (H.-J. Son), wshan@swu.ac.kr (W.-S. Han),
sangok@korea.ac.kr (S.O. Kang).
http://dx.doi.org/10.1016/j.jorganchem.2015.04.025
0022-328X/© 2015 Elsevier B.V. All rights reserved.
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S.-Y. Kim et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
Scheme 1. Synthetic routes for p-1, p-2, m-1, and m-2.
inorganic boron cluster produced a new type of electron acceptor
featuring extended electronic conjugation between organic and
inorganic boranes. Among the series, the para-isomers showed
AgjAgNO3 (0.1 M) were used as the working, counter, and
reference electrodes, respectively. All potentials were calibrated
to the ferrocene/ferrocenium (FcjFcþ) redox couple. Dimesi-
tylboron fluoride and n-BuLi (2.5 M solution in n-hexanes) were
purchased from Aldrich and used as obtained without any
further purification. Decarborane (B10H14) was purchased from
Katchem and used after sublimation. 1-bromo-4-(phenylethynyl)
benzene (1), 1,2-bis(4-bromophenyl)ethyne (2), 1-bromo-3-
(phenylethynyl)benzene (3), and 1,2-bis(3-bromophenyl)ethyne
(4) were synthesized according to the literature procedure [8]. 1-
notable
p-electronic communication between dimesityl-borane
and o-carborane through the phenyl bridge. Steady-state photo-
chemistry of the para-isomers revealed that intramolecular charge-
transfer (ICT) process was a dominant feature found in polar
organic solvents.
Experimental section
bromo-4-(o-carboranyl)benzene (5) and p-17a
,
1,2-bis(4-
bromophenyl)-o-carborane (6)6b were also prepared according
General procedures
to the literature procedure.
All manipulations were carried out under a dry nitrogen or
argon atmosphere using standard Schlenk techniques. THF was
distilled freshly over sodium benzophenone. The 1H, 11B, and 13C
NMR spectra were recorded on a Varian Mercury 300 spec-
trometer operating at 300.1, 96.3, and 75.4 MHz, respectively. All
proton and carbon chemical shifts were measured relative to the
Synthesis of 1-bromo-3-(o-carboranyl)benzene (7)
The mixture of compound 3 (0.38 g, 1.50 mmol) and decarbor-
ane (0.21 g, 1.73 mmol) was dissolved in dry toluene (15 mL) at
room temperature under N2 atmosphere. N,N-Dimethylaniline
(0.34 mL, 2.70 mmol) was added, and the mixture was refluxed for
5 h. After cooling to room temperature, the solvent was separated
from the solid and evaporated. The residue was subjected to silica
gel column chromatography with hexane as an eluent (Rf ¼ 0.56).
Recrystallization from chloroform and ethanol provided compound
7 as a colorless crystal (0.34 g, 0.92 mmol, 61%). 1H NMR (CDCl3):
internal residual benzene from the lock solvent (99.5% CDCl3). 11
B
NMR chemical shifts were referenced to BF3$O(C2H5)2 (0.0 ppm),
with a negative sign indicating an up-field shift. The elemental
analyses were performed using a Carlo Erba Instruments CHNSO
EA 1108 analyzer. High Resolution Tandem Mass Spectrometry
(Jeol LTD JMS-HX 110/110A) was performed by the Korean Basic
Science Institute. Cyclic voltammetry (CV) was performed in an
electrolyte containing 1 mM of the electro-active compounds and
0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) at
room temperature under a N2 atmosphere using a BAS 100B
electrochemical analyzer. Glassy carbon, platinum wire, and
d
7.56 (t, J ¼ 2.1 Hz, 1H, AreH), 7.44 (m, 2H, AreH), 7.35 (m, 2H,
AreH), 7.24 (m, 1H, AreH), 7.18 (m, 2H, AreH), 7.00 (t, J ¼ 8.1 Hz,
1H), 1.50e3.70 (br, 10H, CBꢀBH). The ESI-MS calculated for
C
C
14H19B10Br was 376.1601. Found: 377.2872 [M þ H]þ. Calcd for
14H19B10Br: C, 44.80; H, 5.10. Found: C, 44.75; H, 5.09.
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S.-Y. Kim et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
3
Synthesis of 1,2-bis(3-bromophenyl)-o-carborane (8)
Found: 795.2367 [M þ H]þ. Calcd for C50H62B12: C, 89.02; H, 7.77; B,
3.21. Found: C, 89.00; H, 7.76.
The compound was synthesized according to a procedure
similar to that for 7, using 4 (0.50 g, 1.50 mmol) instead of 3 to give
Crystal structure determination
colorless crystal (0.46 g, 1.01 mmol, 67%).1H NMR (CDCl3):
d 7.58 (t,
J ¼ 2.1 Hz, 2H, AreH), 7.38 (m, 4H, AreH), 7.06 (t, J ¼ 8.1 Hz, 2H,
Single crystals of p-2, m-1, and m-2 were sealed in glass capil-
laries under argon and mounted a diffractometer. The preliminary
examination and data collection were performed on a Bruker
SMART CCD detector system single-crystal X-ray diffractometer
equipped with a sealed-tube X-ray source (50 kV ꢂ 30 mA) using
AreH), 1.50e3.70 (br, 10H, CBꢀBH). The ESI-MS calculated for
C
C
14H18B10Br2 was 454.0706. Found: 455.0536 [M þ H]þ. Calcd for
14H18B10Br2: 37.02; H, 3.99. Found: C, 37.04; H, 4.00.
Synthesis of 1,2-bis(4-(Dimesitylboryl)phenyl)-o-carborane
(p-2)
graphite-monochromated Mo Ka radiation (
l
¼ 0.71073 Å). The
preliminary unit cell constants were determined using a set of 45
narrow-frame (0.3ꢁ in
u
) scans. A double-pass method of scanning
A hexane solution of n-BuLi (2.5 M, 0.56 mL, 1.39 mmol) was
added at ꢀ78 ꢁC to a solution of 6 (0.29 g, 0.63 mmol) in THF
(20 mL), and the mixture was stirred for 1 h at that temperature. A
solution of dimesitylboron fluoride (0.41 g, 90%, 1.39 mmol) in THF
(20 mL) was added to the mixture. The reaction mixture was stirred
for 1 h at ꢀ78 ꢁC, allowed to slowly warm to room temperature, and
stirred overnight. After quenching by saturated aqueous NH4Cl
(30 mL), the aqueous layer was extracted further with dichloro-
methane. The combined organic layers were washed with water
(3 ꢂ 30 mL), dried over MgSO4, and concentrated under reduced
pressure. The residue was suspended with a EtOAc/MeOH mixed
solvent. The insoluble part was collected by filtration and washed
with MeOH to afford the desired product p-2 as a white solid
was used to exclude any noise. The collected frames were inte-
grated using an orientation matrix determined from the narrow-
frame scans. The SMART software package was used for data
collection and SAINT was used for frame integration [9]. Final cell
constants were determined by a global refinement of the xyz cen-
troids of the reflections harvested from the entire data set. Struc-
ture solution and refinement were carried out using the SHELXTL-
PLUS software package [10].
Absorption and emission spectra
The absorption and photoluminescence spectra were recorded
on an SHIMADZU UV-3101PC UVeVISeNIR scanning spectropho-
tometer and a VARIAN Cary Eclipse fluorescence spectrophotom-
eter, respectively.
(0.24 g, 48%). 1H NMR (CDCl3):
d
7.33 (d, J ¼ 8.4 Hz, 4H,
Mes2BꢀPhꢀCH), 7.21 (d, J ¼ 7.8 Hz, 4H, PhꢀCH), 6.77 (s, 8H,
MesꢀCH), 2.29 (s, 12H, MesꢀCH3), 1.84 (s, 24H, MesꢀCH3),
1.50e3.70 (br, 10H, CBꢀBH). 13C NMR (CDCl3):
d
147.98, 141.42,
Thermal property
140.83, 139.39, 135.67, 133.65, 130.22, 128.51, 127.11, 84.87, 23.59,
21.44. 11B NMR (CDCl3):
d
ꢀ0.44, ꢀ8.13. The ESI-MS calculated for
Thermal properties were measured using differential scanning
calorimetry (PerkineElmer/Pyris Diamond DSC). A heating rate of
10 ꢁC/min was used after first melting the compound, followed by a
rapid cooling rate of 40 ꢁC/min to room temperature.
C
C
50H62B12 was 794.5968. Found: 795.4572 [M þ H]þ. Calcd for
50H62B12: C, 89.02; H, 7.77; B, 3.21. Found: C, 89.01; H, 7.78.
Synthesis of dimesityl(3-(o-carboranyl)phenyl)borane (m-1)
DFT calculations
The compound was synthesized according to a procedure
similar to that for p-2, using 7 (0.35 g, 0.92 mmol) instead of 6 with
n-BuLi (2.5 M, 0.40 mL, 1.01 mmol) and dimesitylboron fluoride
(0.30 g, 90%, 1.01 mmol) to give 0.26 g of white powder (52% yield).
Theoretical calculations for the derivatives were conducted on
the Gaussian 09 package [11]. The ground-state geometries of p-1,
p-2, m-1, and m-2 were optimized by using the B3LYP density
functional theory (DFT) and 6e31G(d,p) basis set. Time-dependent
DFT (TDDFT) calculations were then performed with the same
functional and basis set to estimate the energies and oscillator
strengths of the derivatives. The contours of the electron density
were plotted by using Chem3D ver.10.
1H NMR (CDCl3):
d
7.60 (d, J ¼ 7.5 Hz, 1H, Mes2B-Ph-CH), 7.35 (m,
3H, Mes2BꢀPhꢀCH), 7.42 (s, 1H, Mes2BꢀPhꢀCH), 7.24e7.07 (m, 4H,
PhꢀCH), 6.79 (s, 4H, MesꢀCH), 2.32 (s, 6H, MesꢀCH3), 1.78 (s, 12H,
MesꢀCH3), 1.50e3.70 (br, 10H, CBꢀBH). 13C NMR (CDCl3):
d 146.37,
141.18, 140.82, 139.34, 137.75, 137.69, 134.02, 130.86, 130.45, 130.24,
128.51, 128.46, 128.29, 85.36, 85.22, 23.53, 21.49. 11B NMR (CDCl3):
d
ꢀ0.76, ꢀ8.73. The ESI-MS calculated for C32H41B11 was 546.4232.
Results and discussion
Found: 546.9825 [M]þ. Calcd for C32H41B11: C, 70.57; H, 7.59; B,
21.84. Found: C, 70.55; H, 7.60.
Synthesis and structure characterization
Synthesis of 1,2-bis(3-(Dimesitylboryl)phenyl)-o-carborane
(m-2)
A series of para- and meta-regioisomers, p-1, p-2, m-1, and m-2,
were prepared in modest yields by the reaction between the
lithium salts derived from 1-bromo-x-(o-carboranyl)benzene (x ¼ 4
for 5 and 3 for 7), 1,2-bis(x-bromophenyl)-o-carborane (x ¼ 4 for 6
and 3 for 8) and fluorodimesitylborane, as shown in Scheme 1.
Except p-1, the single crystals of p-2, m-1, and m-2 were grown
from a dichloro-methane/n-hexane mixed solution and their
crystal structures are shown in Fig.1. The single crystal structure for
p-1 was reported previously [7a]. Selected crystal data are sum-
marized in Table 1 and full crystal data with collection parameters
are in Table S1 in the supporting data. Among the crystal structures,
p-2 was crystallized in the orthorhombic crystal system with Pbcn
space group while m-1 and m-2 were crystallized in the triclinic
crystal system with P-1 space group. The adjacent phenyl rings to o-
The compound was synthesized according to a procedure
similar to that for p-1, using 8 (0.29 g, 0.63 mmol) instead of 6 with
n-BuLi (2.5 M, 0.56 mL, 1.39 mmol) and dimesitylboron fluoride
(0.41 g, 90%, 1.39 mmol) to give 0.21 g of white powder (43% yield).
1H NMR (CDCl3):
d
7.56 (s, 2H, Mes2BꢀPhꢀCH), 7.43 (d, J ¼ 7.8 Hz,
2H, Mes2BꢀPhꢀCH), 7.35 (d, J ¼ 7.8 Hz, 2H, Mes2BꢀPhꢀCH), 7.10 (t,
J ¼ 7.8 Hz, 2H, PhꢀCH), 6.80 (s, 8H, MesꢀCH), 2.32 (s, 12H,
MesꢀCH3), 1.80 (s, 24H, MesꢀCH3), 1.50e3.70 (br, 10H, CBꢀBH). 13
C
NMR (CDCl3): d 146.27, 141.17, 140.84, 139.40, 138.61, 137.89, 133.91,
130.57, 128.56, 128.22, 85.60, 23.60, 21.49. 11B NMR (CDCl3):
ꢀ0.53, ꢀ8.61. The ESI-MS calculated for C50H62B12 was 794.5968.
d
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S.-Y. Kim et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
Fig. 1. Single crystal structures of p-2 (a), m-1 (b), and m-2 (c) with 50% thermal ellipsoid. The hydrogen atoms are omitted for clarity.
carborane are disposed orthogonally to the CeC bond of the car-
borane cage with dihedral angles of 71.6(2)ꢁ, 75.0(1)ꢁ, and 85.8(2)ꢁ
for p-2, m-1, and m-2, respectively. The CeC bond distance of o-
carborane cages (1.721(3) Å for p-2, 1.725(2) Å for m-1, and 1.728(3)
Å for m-2) were significantly longer than that of the nonsubstituted
o-carborane (1.629 Å) [12] which indicates electron-withdrawing
(EW) character of p-2, m-1, and m-2 because it is known that the
CeC bond distance in carborane cage is elongated when o-carbor-
ane accepts one-electron [13].
Photophysical properties
Fig. 2 shows the electronic absorption and fluorescence spectra
of p-1, p-2, m-1, and m-2 in dichloromethane (DCM) solution and
their spectroscopic data are summarized in Table 2. All of the
prepared compounds exhibited similar absorption spectra with
two main absorption bands in the range of 250e365 nm and the
spectra were characterized by broad lowest energy and relatively
structured higher energy absorption bands. The higher energy
absorption band observed for each compound can be ascribed to
Fig. 2. UV/Vis absorption and fluorescence spectra of dimesityl(phenyl)borane-o-car-
boranes, p-1, p-2, m-1, and m-2, for 10 M solution in dichloromethane (DCM).
m
In the fluorescence spectra, on the other hand, para- and meta-
regioisomers showed significantly different behaviors. In general,
the fluorescence of triarylborane derivatives comes from p(aryl)e
p(B) CT excited states which is evidenced by fluoride-addition
experiments [3]. For meta-isomers, typical emission from
p(aryl)ep(B) CT excited states were observed at lmax z 400 nm in
DCM solution. In contrast to meta-isomers, para-isomers showed
a broad emission band at lmax z 570 nm along with relatively
weak emission intensity, as shown in Fig. 2. It should be noted
that the emission from p(aryl)ep(B) CT excited states disappeared
and only the new emission band was observed. This broad red-
shifted emission can be attributed to the strong intramolecular
charge-transfer (ICT) state derived from the presence of o-car-
borane group at the para-position, as shown by the remarkable
the
pep* transition of the aryl group [14]. The lowest energy
absorption bands of each compound are assigned to CT transitions
between p orbitals of the aryl group, p(aryl), and vacant the p
orbitals of the central boron atom, p(B). The extinction coef-
ficiencies ( ) of p-2 and m-2 with two dimesityl(phenyl)borane
3
(dmpb) units are approximately double those with one dmpb unit
in p-1 and m-1, as summarized in Table 2. Also, all compounds
showed close similarities of the absorption energy and spectral
band shapes with the dmpb. These results indicate that all com-
pounds have negligible electronic coupling between dmpb and o-
carborane moieties in the ground state. Therefore, the boron atom
in the dmpb groups for all compounds act as an electron acceptor,
while the o-carborane group is simply a spectator in the ground
state.
Table 1
Summary of crystal data for p-2, m-1, and m-2.
Identification code
Empirical formula
Formula weight
p-2
m-1
C32H41B11
544.56
m-2
C50H62B12
792.71
C50H62 B12
792.71
Wavelength
0.71073 Å
0.71073 Å
0.71073 Å
Crystal system, space group
Unit cell dimensions
Orthorhombic, Pbcn
a ¼ 30.290(4) Å
b ¼ 12.4277(15) Å
c ¼ 26.818(3) Å
10095(2) Å3
Triclinic, P-1
a ¼ 10.914(2) Å
b ¼ 11.236(2) Å
c ¼ 27.191(5) Å
3315.4(11) Å3
Triclinic, P-1
a ¼ 12.174(3) Å
b ¼ 14.430(3) Å
c ¼ 15.288(3) Å
2457.7(9) Å3
a
b
g
¼ 94.359(4)ꢁ
¼ 94.151(4)ꢁ
¼ 90.688(4)ꢁ
a
b
g
¼ 66.980(4)ꢁ
¼ 84.020(5)ꢁ
¼ 88.816(5)ꢁ
Volume
Final R indices [I > 2
s
(I)]
R1 ¼ 0.0668,a wR2 ¼ 0.1637b
R1 ¼ 0.0572,a wR2 ¼ 0.1561b
R1 ¼ 0.1396,a wR2 ¼ 0.2076b
R1 ¼ 0.0652,a wR2 ¼ 0.1699b
R1 ¼ 0.1474,a wR2 ¼ 0.2258b
R indices (all data)
R1 ¼ 0.1586,a wR2 ¼ 0.2385b
P
a
R1
¼
jjFoj ꢀ jFcjj (based on reflections with Fo2 > 2sF2).
P
P
wR2 ¼ ½ ½wðFo2 ꢀ Fc2Þ2ꢃ= ½wðFo2Þ2ꢃꢃ1=2
;
w ¼ 1=½s2ðFo2Þ þ ð0:095PÞ2ꢃ; P ¼ ½maxðFo2; 0Þ þ 2Fc2ꢃ=3ðalso with Fo2 > 2sF2Þ.
b
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S.-Y. Kim et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
5
Table 2
Optical, photophysical, and thermal properties of the compounds in DCM.
Comp.
Abs. lmax (nm), (
3
, Mꢀ1cmꢀ1
)
Em. lmax (nm)
Stokes shift (cmꢀ1
)
Ered (V)a
EHOMO (eV)a
ELUMO (eV)b
Tg (ꢁC)
p-1
p-2
m-1
m-2
268 (51600), 321 (31400)
268 (89300), 322 (60900)
260 (29000), 323 (25500)
260 (58400), 322 (54200)
566
575
405
406
13,485
13,665
6268
ꢀ1.67
ꢀ1.65
ꢀ1.75
ꢀ1.80
ꢀ6.47
ꢀ6.46
ꢀ6.46
ꢀ6.36
ꢀ3.13
ꢀ3.15
ꢀ3.05
ꢀ3.00
83
130
68
6425
109
a
b
red
The HOMO and LUMO levels were determined using the following equations: EHOMO (eV) ¼ ꢀe(ELUMO ꢀ Eogpt). ELUMO (eV) ¼ ꢀe(E
þ 4.8).
onset
Fig. 3. Solvent dependent fluorescent spectra of p-1 (a), p-2 (b), m-1 (c), and m-2 (d). The fluorescence intensities are normalized to those at the maximum energies. Asterisk mark
indicates the second order peak of the excitation wavelength at 322 or 327 nm.
solvatochromic shifts of the emission which depicted in Fig. 3. The
fluorescence maximum energies of the derivatives (yf) shifted to
lower energy as the solvent polarity increased. Such solvent ef-
fects have been observed for an ICT-type compound and are due
to stabilization of the fluorescent ICT excited-state energy. In the
present case, the solvatochromic shift of yf from n-hexane to DCM,
Dyf, was dependent on the nature of the regioisomer structure, as
shown in Fig. 3 and Table 3. Mataga and Lippert showed that the
solvent sensitivity to polarity can be analyzed in terms of differ-
ence in the dipole moments in the ground and excited states [15].
Thus, the difference in dipole moment between ground and
excited states were measured for para- and meta-regioisomers by
using a MatagaeLippert plot which is essentially a plot of the
Stokes shift of the fluorescence emission versus the solvent
polarity. As can be seen in Fig. 4, the Stokes shift of para-isomers
changes linearly in response to the solvent polarity, which cor-
relates highly with an increase in the dipole moments (Dm) of
31.80 and 31.77 D for p-1 and p-2, respectively. On the other hand,
meta-isomers showed much smaller dipole moments of 14.87 and
15.00 D for m-1 and m-2, respectively. When one electron
transfers the distance of 1 Å, the change of the dipole moment is
4.8 D. Therefore, observed dipole moment changes in para-iso-
mers indicate that one-electron (unit charge) transfers the dis-
tance of ~6.6 Å, which corresponds to the distance between the
dimesityl-donor and the carborane-acceptor. For meta-isomers,
one-electron transfers the distance of ~3.1 Å, the equivalent dis-
tance between the dimesityl group and the bridged phenyl group.
Therefore, we can conclude that para- and meta-regioisomers
Table 3
Solvent parameters and spectroscopic properties of p-1, p-2, m-1, and m-2 in various solvents.
Solvent
Solvent parameter
p-1
p-2
m-1
m-2
Ds
n
lem (nm/cmꢀ1
)
lem (nm/cmꢀ1
)
lem (nm/cmꢀ1
)
lem (nm/cmꢀ1
)
n-Hexane
1.904
4.19
6.11
9.14
1.3749
1.3527
1.3724
1.4246
388/25773
483/20704
547/18282
566/17668
391/25575
498/20080
560/17857
575/17391
377/26525
391/25575
401/24938
405/24691
377/26525
396/25253
402/24876
406/24630
Diethylether
Ethyl acetate
Dichloromethane
Dyfa
178/8105
184/8184
28/1834
29/1895
a
From n-hexane to DCM.
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6
S.-Y. Kim et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
have completely different excited states and o-carborane in para-
isomers only acts as a strong electron-accepting (EA) group,
thereby opening the channel for electronic communication from
dmpb to o-carborane.
Electrochemical properties
To confirm the different electronic states of the regioisomers,
the electrochemical properties were investigated by using cyclic
voltammogram (CV); their spectra are shown in Fig. 5 and the
reduction potentials are summarized in Table 2. The reduction
red
waves (E
) for p-1 and p-2 are observed at ꢀ1.67 and ꢀ1.65 V
onset
(versus Fc/Fcþ), respectively. On the other hand, m-1 and m-2
showed their reduction waves (E
respectively, slightly shifted to negative potential. The LUMO levels
red
) at ꢀ1.75 and ꢀ1.80 V,
onset
were determined by the following equation; ELUMO
red
(eV) ¼ ꢀe(E
þ 4.8). The calculated LUMO levels of para-isomers
onset
(ꢀ3.13 eV for p-1 and ꢀ3.15 eV for p-2) are considerably lower than
those of meta-isomers (ꢀ3.05 eV for m-1 and ꢀ3.00 eV for m-2).
Such a trend is in good agreement with emission property in which
only para-isomers allow electronic communication between dmpb
and the o-carborane unit.
Fig. 4. MatagaeLippert plots for p-1, p-2, m-1, and m-2. Increases in the dipole mo-
ments (Dm) are 31.80, 31.77, 14.87, and 15.16 D for p-1, p-2, m-1, and m-2, respectively.
Theoretical calculations
To gain insight into the electronic states of p-1, p-2, m-1, and m-
2, DFT calculations were carried out using the B3LYP/6-31G(d,p)
method with the Gaussian 09 package [11]. As shown in Fig. 6, all
compounds showed that their HOMOs are mainly localized at the
mesityl groups in the dmpb unit. On the other hand, distributions of
LUMOs for para- and meta-regioisomers are significantly different.
LUMOs of para-isomers are delocalized from the vacant p orbital on
the boron atom to the o-carborane whereas LUMOs of meta-iso-
mers are primarily localized at the vacant p orbital on the boron
atom with little participation of phenyl units. Similar to results
obtained from optical and electrochemical experiments, the LUMO
levels (obtained by DFT calculation) of para-isomers are found to be
substantially lower than those of meta-isomers: the LUMO levels of
p-1, p-2, m-1, and m-2 are ꢀ2.10, ꢀ2.15, ꢀ1.93, and ꢀ1.99 eV,
respectively. The difference of the calculated HOMO levels were
relatively small, depending on the position of dimesitylborane to
diphenyl-o-carborane (ꢀ6.18 eV for p-1 and p-2, ꢀ6.15 eV for p-1
and p-2). Furthermore, time-dependent DFT (TD-DFT) calculations
with the same functional and basis set confirm that the lowest-
energy absorption transitions are mainly associated with the
HOMO / LUMO transition. The calculated five lowest-energy
Fig. 5. Cyclic voltammograms for 0.1 mM DCM solution of p-1, p-2, m-1, and m-2
containing 0.1 M TBAP taken at a scan rate of 0.1 V/s. Glassy carbon electrode is the
working electrode, and a platinum wire and Ag/AgNO3 is the counter and reference
electrodes, respectively.
Fig. 6. Spatial distributions for the frontier orbitals of p-1, p-2, m-1, and m-2.
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S.-Y. Kim et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
7
absorption transitions of the compounds are summarized in
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Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.04.025.
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