Carborane photochemistry triggered by aryl substitution: Carborane-based dyads with phenyl carbazoles

Article abstract of DOI:10.1002/anie.201109069

A bright combination: A new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts (see picture). Copyright

Full text of DOI:10.1002/anie.201109069

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Chemie
DOI: 10.1002/anie.201109069
Carborane Clusters
Carborane Photochemistry Triggered by Aryl Substitution: Carborane-
Based Dyads with Phenyl Carbazoles**
Kyung-Ryang Wee, Won-Sik Han, Dae Won Cho, Soonnam Kwon, Chyongjin Pac,* and
Sang Ook Kang*
Since the discoveries of boranes and carborane clusters,[1]
relentless effort has been made to understand their electronic
structures,[2] particularly those of carborane compounds,
because of their unique cage structures.[3] However, there
have been few investigations on the excited-state properties
and photochemical behavior of carborane compounds,[4]
probably because the parent carboranes and their derivatives
without p substituent show little or no absorption at
> 250 nm, nor any emission. Even for their p-substituted
derivatives, furthermore, no detailed investigations have been
performed, particularly into how the carborane cages affect
their excited-state behavior.[5] Recently, a limited number of
reports appeared on the unique fluorescence of polymers
containing diaryl-substituted 1,2-carborane (o-Cab) units,
which was inferred from their aggregation-induced emission[6]
and charge transfer.[7] However, no detailed analysis has been
performed on the electronic nature of the fluorescence from
the o-Cab unit or the possible roles of the o-Cab cage. As an
important part of our research projects on carborane chemis-
try,[8] we performed a detailed investigation on the excited-
state properties of a series of aryl-substituted carboranes with
the carbazole (Cz) end group (Scheme 1), using steady-state
and time-resolved spectroscopic methods. The Cz group was
used as an effective chromophore for spectroscopic studies,
because rich spectroscopic data of Cz are available and
because they are typical molecules acting as an electron
donor. We report herein the observations on the emission
behavior of the Cab compounds and discuss the implications
of the effects of the carborane cage on the electronic and
material properties of the molecules.
Scheme 1. Structures of the carborane compounds. The solid and
open circles in the cage represent the carbon and boron atoms,
respectively.
including the X-ray structures of o-1, o-2, and p-1 (see the
Supporting Information for details). As shown by the ORTEP
structures of o-1, o-2, and p-1 in Figure S1 in the Supporting
Information, the Cz ring is disposed perpendicularly to the
phenylene ring and the C–C distance of o-1 (1.725 ꢀ) is
significantly longer than that of the nonsubstituted o-Cab
(1.629 ꢀ).[9]
As shown in Figure 1a and Table 1, the absorption spectra
of o-1, m-1, and p-1 are similar, commonly showing the
longest absorption maximum at around 336 nm characteristic
of the p,p* transition of the Cz chromophore.[10] This can be
reasonably understood from the structures that indicate little
conjugation between the Cz ring and the perpendicularly
situated phenylene ring. However, a slight difference can be
found in the 300–330 nm region; the intense band of o-1 at
around 324 nm seems to arise as a result of the red shift of the
band of m-1 and p-1 at around 305 nm, possibly because of the
slight electronic perturbations caused by the deformation of
the o-Cab cage. By contrast, the emission of o-1 is highly
different from those of m-1 and p-1 which are almost identical
each other with the spectra, lifetimes, and quantum yields
characteristic of the excited-singlet Cz emission. In the case of
o-1, a broad emission appears at 531 nm along with extensive
quenching of the Cz fluorescence. This broad red-shifted
emission can be attributed to the charge-transfer (CT) state,
as shown by the remarkable solvatochromic shifts of the
emission displayed in Figure 1b. Similar solvent-dependent
CT emissions were observed for the other o-Cab compounds,
o-2 and o-3 (see Figures S3 and S4 in the Supporting
Information).
The Cab compounds were prepared in moderate yields
(31–35%) using explored synthetic protocols and their
structures were firmly identified by their spectroscopic data,
[*] K.-R. Wee, W.-S. Han, D. W. Cho, S. Kwon, C. Pac, S. O. Kang
Department of Materials Chemistry, Korea University
Chungnam 339-700 (South Korea)
E-mail: jjpac@korea.ac.kr
sangok@korea.ac.kr
[**] This research was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science, and Technology (MEST) of
Korea (grant number 2011-0018595) and by the Center for Next
Generation Dye-Sensitized Solar Cells (grant number 2011-
0001055). We thank Prof. Tetsuro Majima, Associate Prof. Mamoru
Fujistuka, and Takumi Kimura for their assistance during the pulse
radiolysis studies in SANKEN. We also acknowledge Youlchon
Chemical Co. for its financial support to C.P.
Figure 2a shows the Mataga–Lippert plots for the solva-
tochromic shifts of the CTemissions of o-1 and o-2 versus the
solvent-polarity parameter (Df). From the slopes of the linear
plots, the differences in the dipole moments (Dm) between the
ground-state and CT states were estimated to be around
28.0 D for o-1 and around 60 D for o-2. Taking the dipole
moment (mg) in the ground state to be 2.0 D for o-1 and 1.1 D
for o-2, as obtained by theoretical calculations,[11] the dipole
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109069.
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Figure 2. Mataga–Lippert plots for CT emissions of o-1 (black) and o-2
(red).
occur in the excited state of o-1 and o-2. This is again true for
o-3 as shown by the solvent-dependent shifts of the CT
emission.
To provide explicit evidence for the above arguments
based on the Mataga–Lippert plots, we conducted laser-flash
photolysis experiments for o-1. As shown in Figure 3a, two
characteristic absorptions appeared at 415 and 830 nm upon
irradiation of laser pulses at 355 nm. Both the 415 and 830 nm
transients decayed commonly within a few nanoseconds,
roughly in accord with the observed CT emission lifetime.
Therefore, the transient spectrum must be due to the excited
state of o-1. The 830 nm transient can be attributed to the
+
radical cation of the Cz unit (CzC ), because the radical cations
of various carbazole compounds generally show characteristic
absorptions at around 800 nm.[12] This assignment was firmly
supported by the pulse radiolysis of o-1 in 1-chlorobutane,
which showed a transient at 830 nm (see Figure S11 in the
Supporting Information). To assign the 415 nm bands, spec-
troelectrochemical (SEC) measurements were performed for
o-1 under constant voltage conditions at À1.8 V, a potential
corresponding to the one-electron reduction of o-1. As shown
in Figure 3b, a 415 nm peak appeared under keeping the
spectral shape after prolonged electrolysis, being thus assign-
able as the spectrum of the radical anion of o-1 (o-1CÀ). A
similar spectrum was also obtained by pulse radiolysis of o-
1 in DMF (see Figure S12 in the Supporting Information).
This absorption is closely related to the spectrum reported for
the radical anion of 1,2-diphenyl-o-Cab (Ph2-o-CabCÀ).[13]
These spectroscopic observations strongly indicate that the
excited state of o-1 is a charge-separated state consisting of
Figure 1. a) Absorption (<350 nm) and emission (>350 nm) spectra
of o-1 (black), m-1 (red), and p-1 (blue) in toluene. b) Emission spectra
of o-1 in various solvents.
Table 1: Photophysical properties of the Cab compounds.
[a]
Cab labs [nm][a]
lem [nm][a,b]
tF [ns][a] FF
Eox/Ered[c] [V]
o-1
o-2
o-3
293, 325, 338 360, 410, 531 2.83
291, 326, 338 365, 583 3.41
293, 324, 338 350, 360, 595 1.28
0.06 0.72/À1.88
0.05 0.71/À1.79
0.04 0.73/–[d]
0.32 0.74/–[d]
0.33 0.64/–[d]
m-1 293, 308, 339 349, 361
p-1 293, 310, 339 350, 362
6.26
6.37
[a] Measured in Ar-saturated toluene solution (5 mm) at room temper-
ature. [b] Taken by excitation at 290 nm. For lifetimes monitored at
different wavelengths, see Table S3 in the Supporting Information.
[c] Oxidation and reduction potentials measured for Ar-saturated
dichloromethane solution by cyclic voltammetry (V versus SCE). [d] No
wave observed.
+
the carbazole radical cation (CzC ) and the acceptor radical
anion (Ph2-o-CabCÀ), an assignment in accord with the argu-
ments based on the Mataga–Lippert plots. This assignment is
again in line with the TD-DFT calculation results which show
the localized populations of the highest singly occupied
molecular orbital (HSOMO) on the Cz units in the oxidized
cationic state and on the Ph2-o-Cab moiety in the reduced
anionic state (Figure 3a inset).
The CT emissions of o-1 to o-3 show similar solvent-
dependent behavior, such that increasing the solvent polarity
decreases their intensity to virtually nil in acetonitrile,
moment (me) in the excited state should be around 30 D for o-
1 and around 62 D for o-2. If we take 4.8 D as the dipole
moment between the unit charges separated by 1 ꢀ, the
excited-state dipole moment values correspond to the com-
pletely charge-separated states with dipole lengths of around
6.3 ꢀ for o-1 and around 13 ꢀ for o-2, which are comparable
to the distances found between the nitrogen atom of Cz and
À
the center of the C C bond of o-Cab (6.8 ꢀ for o-1 and 11.1 ꢀ
for o-2). This implies that complete charge separation should
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is completely inert under the usual photochemical and
electrochemical conditions, the bonding of the p-electronic
system (i.e. the phenyl group) with one or two of the carbon
atoms of o-Cab causes a drastic change in its electronic
nature, making the molecule an efficient electron acceptor.[16]
As shown in Figure S11 in the Supporting Information, we
confirmed that 1,2-Ph2-o-Cab quenches the fluorescence of
ethylcabzole at a diffusion-controlled rate, an observation in
accord with the extensive quenching of the Cz fluorescence
observed with o-1. However, the intermolecular fluorescence
quenching is not accompanied by any new emission, indicat-
ing that the emission from the CT state would require
a structure favorable for electronic coupling between the
positively and negatively charged units. We are presently
conducting a further investigation on the excited states of the
present and other diaryl-substituted o-Cab molecules using
more sophisticated spectroscopic methods and theoretical
calculations. In contrast to the o-Cab cage, the m- and p-
counterparts act as an insulator that completely separates the
two Cz-Ph electronic systems. This is, conversely, beneficial to
the use of m-1 and p-1 as host materials for phosphorescent
organic light-emitting diodes, because the solid films have
suitable glass-transition temperatures, reasonable hole mobi-
lities, and sufficiently high triplet-energy levels. We intend to
apply m-1 and p-1 to blue-phosphorescent devices.
Experimental Section
Synthesis: 1,2-Bis(4-(N-carbazolyl)phenyl)-ortho-carborane (o-1),
1,2-bis(4-(N-carbazolyl)biphenyl)-ortho-carborane (o-2), 1-(4-(N-
Carbazolyl)phenyl)-ortho-carborane (o-3), 1,7-bis(4-(N-carbazolyl)
phenyl)-meta-carborane (m-1), and 1,12-bis(4-(N-carbazolyl)-
phenyl)-para-carborane (p-1) were synthesized from 9-(4-bromophe-
nyl)carbazole, as shown in Scheme S1 in the Supporting Information.
o-1 was obtained in two steps, through the reaction of di(4-(N-
carbazolyl)phenyl)acetylene with decaborane, in an overall yield of
35%. o-2 was prepared by the Suzuki coupling reaction between 1,2-
bis(4-bromophenyl)-ortho-carborane and two equivalents of 4-(car-
bazol-9-yl)phenylboronic acid. On the other hand, o-3, m-1, and p-
1 were prepared in a single step in moderate yields through the
coupling reaction of 9-(4-bromophenyl)carbazole with the lithium
salt of o-carborane or m-carborane or p-carborane, in the presence of
copper(I) halides. All products were isolated by flash column
chromatography and further purified by train sublimation in moder-
ate yields. The formation of o-1, o-2, o-3, m-1, and p-1 was confirmed
by X-ray single-crystal structure, high-resolution mass spectrometry,
and elemental analyses. The compounds showed the expected signals
Figure 3. a) Transient absorption spectra of o-1 in Ar-purged CH2Cl2
obtained at a 10 ns-delay after 355 nm laser excitation (Inset: cationic
and anionic highest singly occupied molecular orbitals (HSOMOs) of
o-1). b) Spectroelectrochemical (SEC) spectra of o-1 taken by electrol-
ysis at À1.8 V and cyclic voltammetry (CV) spectrum of o-1.
whereas the lifetime is initially lengthened to a maximum in
toluene and then shortened as the solvent polarity is further
increased. This behavior might be interpreted in terms of the
solvation-induced electronic changes of the CT state, which
would lead to different solvent dependencies of the radiative
and nonradiative rate constants.[14]
Another notable observation is that both the Cz fluores-
cence and excimer emission[15] can be detected for o-1, though
they are very weak or masked by the CT emission in the low-
polarity solvents. This means that the CT state is not formed
instantaneously upon excitation of the Cz chromophore, but
by way of the excited-singlet Cz (1Cz*). If this is the case, the
rate constant for the formation of the CT state estimated from
the Cz fluorescence lifetime of o-1 (120 Æ 10 ps) is around 8 ꢁ
109 sÀ1, which is not far from the value for electron transfer
over a distance of around 7 ꢀ with a driving force of around
1 eV.
1
in their H, 11B and 13C NMR spectra for the characteristic phenyl-
carbazole and carborane groups.
Cyclic voltammetry and spectroelectrochemistry: The cyclic
voltammetry experiments were performed using a BAS 100 electro-
chemical analyzer. A three-electrode cell system containing a plati-
num disk, a platinum wire, and Ag/AgNO3 as the working, counter,
and reference electrodes, respectively, was used. All data were
obtained for an Ar-purged CH2Cl2 solution containing 0.1m tetrabu-
tylammonium perchlorate (TBAP) at a scan rate of 0.1 VsÀ1
.
Spectroelectrochemical[17] (SEC) measurements were performed to
measure the UV/Vis absorption spectra of the one-electron-reduced
species (OER) of o-1 using strands of 0.1 mm diameter Pt wire. In
a porous glass tube (2 mm inside diameter, 3 mm outside diameter,
40 mm length) as the working electrode, a Pt wire coiled around the
porous glass tube as the counter electrode, and an Ag/AgNO3
reference electrode. A THF solution containing o-1 (0.5 mm) and
The unique excited behavior of the o-Cab compounds
reported herein implies the existence of unusual effects of the
o-Cab cage on the electronic systems. While the parent o-Cab
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TBAP (0.1m) in a quartz cell was purged with Ar for 10 min.
Immediately after electrolysis using a BAS 100 electrochemical
analyzer, the UV/Vis absorption spectra were measured.
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photolysis. The third harmonic generation (THG, 355 nm) of a Q-
switched Nd:YAG laser (Continuum, Surelite II, pulse width of 4.5 ns
full width at half maximum, fwhm) was used as the excitation light. A
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(Ando Technology, iStar) with a gate time of 1.6 ns at 10 ns time delay.
The reported signals were the averages of 500 events. The sample
solutions were saturated with argon.
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Keywords: boranes · cage compounds · electron transfer ·
photophysics
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