Luminescent metal complexes featuring photophysically innocent boron cluster ligands

Article abstract of DOI:10.1039/c6sc01146b

We report the synthesis and characterization of a series of d⁸ metal complexes featuring robust and photophysically innocent strong-field chelating 1,1′-bis(o-carborane) (bc) ligand frameworks. A combination of UV-Vis spectroscopy, single crystal X-ray structural analysis, and DFT calculations of these species suggest that the dianionic bc ligand does not contribute to any visible metal-to-ligand charge transfer (MLCT) transitions, yet it provides a strong ligand field in these complexes. Furthermore, a bc-based Pt(ii) complex containing a 4,4′-di-tert-butyl-2,2′-bipyridine ligand (dtb-bpy) has been prepared and was found to display blue phosphorescent emission dominated by MLCT from the Pt(ii) center to the dtb-bpy ligand. Importantly, the bulky three-dimensional nature of the bc ligand precludes intermolecular Pt(ii)?Pt(ii) interactions in the solid state where the resulting compounds retain their emission properties. This study opens a potentially new avenue for designing organic light-emitting diode (OLED) materials with tunable properties featuring photophysically innocent boron-rich cluster ligands.

Full text of DOI:10.1039/c6sc01146b

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Luminescent metal complexes featuring
photophysically innocent boron cluster ligands†
Cite this: DOI: 10.1039/c6sc01146b
Kent O. Kirlikovali, Jonathan C. Axtell, Alejandra Gonzalez, Alice C. Phung,
Saeed I. Khan and Alexander M. Spokoyny*
8
We report the synthesis and characterization of a series of d metal complexes featuring robust and
0
photophysically innocent strong-field chelating 1,1 -bis(o-carborane) (bc) ligand frameworks.
A
combination of UV-Vis spectroscopy, single crystal X-ray structural analysis, and DFT calculations of
these species suggest that the dianionic bc ligand does not contribute to any visible metal-to-ligand
charge transfer (MLCT) transitions, yet it provides a strong ligand field in these complexes. Furthermore,
0
0
a bc-based Pt(II) complex containing a 4,4 -di-tert-butyl-2,2 -bipyridine ligand (dtb-bpy) has been
prepared and was found to display blue phosphorescent emission dominated by MLCT from the Pt(II)
center to the dtb-bpy ligand. Importantly, the bulky three-dimensional nature of the bc ligand precludes
intermolecular Pt(II)/Pt(II) interactions in the solid state where the resulting compounds retain their
emission properties. This study opens a potentially new avenue for designing organic light-emitting
diode (OLED) materials with tunable properties featuring photophysically innocent boron-rich cluster
ligands.
Received 24th February 2016
Accepted 18th April 2016
DOI: 10.1039/c6sc01146b
www.rsc.org/chemicalscience
Icosahedral dicarba-closo-dodecaboranes (C
ane) are robust, charge-neutral boron clusters that are oen
2 10
B H12, carbor-
Introduction
Many square planar platinum complexes containing chelating viewed as 3D aromatic analogues of arenes. Unfunctionalized
biaryl ligands have been established as efficient phosphores- carborane species have an extremely large HOMO–LUMO gap
5
cent emitters, making them desirable dopants in organic light- (ꢀ8 eV, see ESI†), making them potentially useful building
1,2
emitting diodes (OLEDs). The emissive properties of these blocks for probing their photophysical innocence in the context
Pt(II) complexes can be tuned by varying the electronic nature of of metal-based phosphorescent emitters. This is especially
the ligands surrounding the metal center; however, complete appealing given the available functionalization routes through
control over desired photophysical properties is still limited. either carbon or boron vertices in these clusters, enabling the
For example, these square planar complexes are susceptible to synthesis of tailored ligand frameworks for transition metal
6
aggregation as a consequence of the lack of steric bulk above complexes. For example, Lee and co-workers have recently
2
7a
and below the square plane, leading to intermolecular Pt(II)/ demonstrated that k -C,N-bound 1-(2-pyridyl)-o-carboranyl
2
i
7b
Pt(II) interactions that bring about luminescence quenching and k -C,P-bound 1-( Pr
2
PCH
2
)-o-carboranyl can be strong
2,3
and red-shiing of the solid-state emission frequency.
ancillary ligands that contribute to the electronic stabilization
Furthermore, in prototypical mixed ligand complexes, (L)(L ) of bis(heteroleptic) Ir(III) species (see ESI† for molecular struc-
0
Pt(II), the HOMO is generally both metal- and ligand-based (Pt(II) tures), leading to an arylpyridine-dominant phosphorescent
0
4
and L) while the LUMO is located on the remaining ligand (L ).
emission. From DFT calculations and analysis of the emission
This ultimately complicates the predictability of ligand effects spectra, these authors determined the C-bound o-carboranyl
on emission properties. A class of compounds that could unit remains uninvolved in electronic transitions and that
kinetically stabilize the Pt(II) square planar framework without phosphorescent emission results from MLCT of the Ir(III)-based
participating in electronic transitions would prove very useful HOMO to the arylpyridine-based LUMO. Furthermore, several
for creating improved phosphorescent emitters for the next groups have functionalized biaryl ligands with C-connected
generation of OLED devices.
carboranyl moieties (ortho, meta, para, and nido) to tune lumi-
nescent properties (see ESI† for molecular structures). To the
8
best of our knowledge, attempts to design a tunable, exclusively
Department of Chemistry and Biochemistry, University of California, 607 Charles E.
Young Drive East, Los Angeles, California 90025-1569, USA. E-mail: spokoyny@
chem.ucla.edu; Web: https://www.organomimetic.com
carborane-based ligand scaffold for phosphorescent emitter
9
molecules have not been explored thus far. Such a ligand
†
Electronic supplementary information (ESI) available: See ESI for full framework would be an ideal system for a rational design of
experimental details. CCDC 1446940–1446945. For ESI and crystallographic data
metal-based luminescent complexes (vide supra).
in CIF or other electronic format see DOI: 10.1039/c6sc01146b
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transition metal complexes of the type M(bc)(dppe) (M ¼ Ni(II),
Pd(II), Pt(II); dppe ¼ 1,2-bis(diphenylphosphino)ethane; Fig. 2).
The dilithio salt of bc (Li [bc]) was generated in THF and
2
ꢂ
transferred into a slurry of M(dppe)Cl in THF at ꢁ80 C. The
2
reaction was allowed to slowly warm to room temperature
overnight, yielding a dark brown solution. Pure compounds
were isolated in the yields reported in Fig. 2 as crystalline
yellow-orange solids aer purication using column chroma-
0
Fig. 1 Oxidative coupling of o-carborane (1) yields 1,1 -bis(o-carbor- tography on silica or alumina (see ESI†). All reactions can be
3
1
ane) (2). Deprotonation of 2 yields the dianionic bc ligand, which can
be conceptually thought of as a 3D aromatic analogue of a dianionic
biphenyl ligand (bph).
easily monitored via P NMR spectroscopy. For example, 3a
and 3b exhibit a signicant (>10 ppm) upeld shi in their
3
1
observed P NMR singlet resonance compared to the starting
metal-based precursors (see ESI†). These results are also
consistent with the recent work by Welch and co-workers who
independently synthesized 3a.
values for 3c change signicantly compared to the starting
0
In 1964, Hawthorne reported the rst synthesis of 1,1 -bis(o-
11a
1
Observed
JPt–P coupling
10a
carborane) (2), effectively a 3D analogue of biphenyl (Fig. 1),
and in 1973, Zakharkin showed that the oxidative coupling of
1
material, exhibiting a decrease of 1133 Hz ( JPt–P value changes
from 3624 Hz to 2491 Hz). The substantial reduction in the
10d
two o-carboranes (1) through carbon vertices yields 2. Later,
Hawthorne demonstrated that the deprotonation of 2 results in
a dianionic species bc, which was shown to bind several tran-
1
magnitude of the J
coupling for 3c can be attributed to the
Pt–P
13
strong ligand eld of bc versus that of the chloride ligands.
10b–f
sition metals in bidentate or monodentate fashions.
Ligand
To demonstrate that the bc framework is structurally similar
to the biphenyl (bph) framework, we compared bond distances,
angles, and molecular geometries of 3a–3c to those of a series of
cyclometalated M(bph)(L^L) in reported X-ray crystal structures,
and determined that bc does not inuence the intramolecular
geometry much differently than the bph ligand. Furthermore,
the M–P bond lengths in 3a–3c are also consistent with the
strong-eld ligand nature of bc in these complexes (see ESI† for
specic discussion).
bc possesses similar electronic and physical properties as the
parent o-carborane (see ESI†), and behaves as a robust transi-
tion metal ligand. More recently, several groups have improved
the synthesis of 2 (ref. 10j) and further expanded the series of
11
heteroleptic late-transition metal complexes containing bc.
However, fundamental electronic characterization and poten-
tial applications for these compounds as electronic materials
have yet to be disclosed.
Two monoanionic bidentate ligands are commonly used in
Pt(II) architectures employed for OLEDs, but there are few
examples of Pt(II)-based emitters containing a dianionic
UV-Vis spectroscopic measurements were performed on 3a–
3c and revealed strong transitions in the UV region below
360 nm corresponding to p–p* transitions on the ligand.
12
bidentate ligand and a neutral bidentate ligand. One such
UV-Vis spectra of 3a–3c also feature weaker intensity transitions
in the visible region ranging from 400–500 nm that result from
M(II)-(dppe) MLCT. DFT calculations were performed on the
geometry optimized X-ray crystal structure inputs of 3a–3c in
order to conrm the nature of the observed electronic transi-
tions (see ESI†). Our computational studies reveal an almost
entirely metal-based HOMO and dppe ligand-based LUMO with
negligible contribution from the bc ligand in all frontier
1
2a–c
0
example is Pt(bph)(bpy)
(where bph ¼ biphenyl, bpy ¼ 2,2 -
bipyridine, see ESI† for a molecular structure). We hypothesized
that employing bc in place of bph could introduce sufficient
steric bulk above and below the square plane to eliminate
intermolecular Pt(II)/Pt(II) interactions, which are oen
responsible for non-radiative decay pathways that lower lumi-
2,3
nescent efficiency.
Additionally, the lack of an exposed
p-aromatic system in bc should help in reducing these unde-
sirable intermolecular interactions and unwanted substitution
and degradation pathways. Furthermore, the high-lying LUMO
of bc should be inaccessible for orbital mixing and MLCT
processes. Therefore, the bc ligand was hypothesized to provide
kinetic stability while maintaining photophysical innocence in
the context of designing OLEDs. Finally, emission originating
from a single ligand will yield greater color purity, as mixing of
emissions from multiple delocalized excited states will not be
possible. Together, these properties should allow for the
rational tuning of other ancillary ligands without electronic
interference from bc.
8
orbitals. As we hypothesized previously, bc chelated to the d
transition metals in our model complexes remains uninvolved
in all MLCT-based visible transitions, suggesting its photo-
physical innocence in the UV-Vis region.
Fig. 2 Left: Synthesis of M(bc)(dppe), where M ¼ Ni (3a), Pd (3b), Pt
Results and discussion
(
3c). The synthesis of 2 was adapted from ref. 10j. Right: Stacked X-ray
crystal structures of compounds 3a–3c (CCDC 1446940–1446942),
illustrating structural similarity down the group. Red ¼ 3a, green ¼ 3b,
To test our hypothesis, we rst proceeded to evaluate the pho-
8
tophysical behavior of bc as a ligand in a series of d model blue ¼ 3c (see ESI† for thermal ellipsoid plots of 3a–3c).
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0
Next, we sought to synthesize a Pt(II)-bpy (bpy ¼ 2,2 -bipyr- authors explained this B–Ru bond formation results from
idine) complex chelated by bc, as Pt(II) complexes containing a competitive B–H activation process, which we think is remi-
this class of ligands are known to exhibit phosphorescent niscent of the observed formation of 4b in this work.
1
2a–c
emission.
Pt(bpy)Cl
Starting with the addition of Li [bc] to a slurry of
A mixture of 4a and 4b was dissolved in hot 2-MeTHF and
2
2
, a large amount of emissive, insoluble product was allowed to cool to room temperature, yielding single crystals of
obtained. The extremely low solubility of this product in 4a (Fig. 3C). The molecule adopts a minimally distorted square
ꢂ
common organic solvents hampered its characterization. In planar geometry with the C–Pt–Ntrans angles at 174.2(7) and
ꢂ
order to potentially circumvent this issue, we then chose the 173.0(8) . The bulky bc ligand forces the molecule to pack
0
0
4
,4 -di-tert-butyl-2,2 -bipyridine (dtb-bpy) ligand as an alterna- “head-to-tail” with Pt(II)/Pt(II) distances ranging between
˚
˚
tive, anticipating more favorable solubility properties. Using the 5.87(0) A and 5.52(0) A (see ESI†), which far exceed the reported
˚
same synthetic route yielded, again, a largely insoluble, emis- 3.15–3.76 A expected for intermolecular Pt(II)/Pt(II) interac-
2,3
sive solid (Fig. 3A). Aer dissolving the crude product in hot 1,2- tions (see ESI† for further discussion about the single crystal
diuorobenzene and passing the solution through a Celite plug, X-ray structure of 4a).
a non-emissive solid was le on the Celite; a yellow solid that
Given that the bc framework is amenable to substitution, we
emitted blue-green under UV excitation (365 nm) remained hypothesized that functionalizing this scaffold with alkyl
1
aer all volatiles were removed in vacuo. Surprisingly, H NMR groups would increase the solubility of the resulting Pt(II)
spectroscopic data suggests that the isolated product consists of complexes, ultimately allowing us to better characterize these
a mixture of two species (4a/4b) with a bc ligand chelated to the emissive species. We therefore installed ethyl groups at the B(9)
2
2
Pt(II) center in both k -C,C-bound (4a) and k -B,C-bound modes and B(12) positions of the parent 9,12-B-diiodo-o-carborane (5)
(
2
4b) (Fig. 3). The k -C,C-bound species 4a, derived from the using Kumada cross-coupling conditions producing bis(alky-
symmetric binding of the bc ligand, is consistent with the lated) species 6 (Fig. 4A and ESI†). Compound 6 was then sub-
presence of three resonances of equal integration in the jected to Cu-mediated oxidative coupling conditions, ultimately
1
aromatic region (dtb-bpy ligand) of the H NMR spectrum producing the tetralkylated-bc (7) in 50% isolated yield (Fig. 4A
1
0j
(
Fig. 3B, label A). The six remaining resonances in the aromatic and ESI†). Compound 7 was dilithiated and added to Pt(dtb-
1
region (Fig. 3B, label B) of the H NMR spectrum are consistent bpy)Cl
2
in a similar manner as with 2 (Fig. 4A). Surprisingly,
with an asymmetric k -B,C-bound bc species 4b (vide infra). aer the reaction mixture was stirred for a day at 60 C,
From the relative integration of these two sets of resonances, we predominantly a k -B,C-bound isomer 8 was observed by H and
estimate that the produced mixture contained a ratio of 1.4 : 1.0 2D C– H HSQC NMR spectroscopy (>80%). Purication of the
of 4a to 4b. Repeated attempts to optimize this reaction
2
ꢂ
2
1
1
3
1
produced the same mixture in varying ratios of 4a and 4b (see
ESI†). Attempts to drive the formation of one isomer from the
mixture of isomers while heating under forcing conditions
1
11
produced no observable change in both H and B NMR
spectra. Notably, during the preparation of this manuscript,
Welch and co-workers reported the synthesis of a series of Ru(II)
2
11c
complexes chelated by the k -B,C-bound bc ligand.
The
Fig. 3 (A) Synthesis of the Pt(bc)(dtb-bpy) complex leads to a mixture
2
0
0
0
containing two product isomers featuring a k -C,C-bound bc (4a) and Fig. 4 (A) Synthetic route to 9,9 ,12,12 -tetraethyl-1,1 -bis(o-carbor-
2
1
k -B,C-bound bc (4b). (B) H NMR spectrum of the aryl region for the ane) (7), syntheses of 5 and 6 from ref. 6k. (B) X-ray crystal structure of
isolated mixture of 4a (label A) and 4b (label B). (C) Single crystal X-ray 8 (CCDC 1446944) with thermal ellipsoids drawn at 50% probability, H
structure of 4a drawn with 50% thermal ellipsoid probability (CCDC atoms omitted for clarity. (C) Stacking of 8 with Pt(II)/Pt(II) distances of
1
446943). H atoms are omitted for clarity.
5.981 A˚ and 7.979 A˚ .
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2
resulting mixture further afforded pure k -B,C-bound species as
a pale orange solid which exhibits blue-green emission in the
solid state and, as hypothesized, is extremely soluble in the
majority of common organic solvents. To our knowledge, this is
the rst reported example of a functionalized bc bound to
10i
a metal.
Crystals of 8 suitable for X-ray analysis were grown by slow
evaporation of diethyl ether over the course of one week. The
diffraction study conrmed the presence of the asymmetric
isomer with one Pt–C(1) bond and one Pt–B(4) bond (Fig. 4B). At
+
Fig. 5 (A) Cyclic voltammogram of 8 versus Fc/Fc in MeCN with
ꢁ1
a glassy carbon working electrode, scan rate ¼ 0.1 V s . (B) UV-Vis
absorption spectrum in CH Cl (dotted line) and emission spectra of 8
Pt–C(1) bond. Furthermore, the greater trans inuence of the in 2-MeTHF at 77 K (blue), 2 wt% PMMA film (red), and neat solid (black),
˚
˚
2
.07(3) A, the Pt–B(4) bond is slightly longer than the 2.03(6) A
2
2
14
carborane-based boryl moiety can be seen in the elongation of
l
exc ¼ 380 nm; inset: picture of 8 in 2-MeTHF at 77 K (lexc ¼ 365 nm).
˚
the Pt–N bond lengths: the Pt–N(1) bond is 2.17(5) A, whereas the
˚
Pt–N(2) bond is only 2.05(3) A. As a result of the asymmetric
a neutral bidentate ligand are scarce, Table 1 presents redox
potentials for two such examples, Pt(bph)(bpy) and Pt(bph)(en)
binding of the bc-based ligand in 8, one carborane cage rotates
and forces the ethyl group about 30 out of the plane created by
ꢂ
15b
(
where en ¼ 1,2-ethylenediamine). The reduction potential
C(1)–Pt–B(4), whereas the other 3 ethyl groups sit in the square
plane (Fig. 4C). This protruding ethyl group likely forces the dtb-
bpy out of the square plane, causing the molecule to adopt
a slightly distorted square planar structure; however, bond
angles of 176.5(5) for C(1)–Pt–N(2) and 168.8(2) for B(4)–Pt–
N(1) are well within the range of corresponding angles in previ-
ously reported 4-coordinate Pt(bph)(N^N) compounds (see ESI†).
Importantly, the intermolecular Pt(II)/Pt(II) distances were
augmented even more in the solid-state than in 4a through the
introduction of ethyl groups, yielding Pt(II)/Pt(II) distances of
.891 A (when ethyl groups face away from each other) and
.979 A (when ethyl groups point towards each other), effectively
preventing any potential intermolecular Pt(II)/Pt(II) interac-
tions (Fig. 4C). Furthermore, the solid-state packing adopts
a “head-to-tail” arrangement such that dtb-bpy lies above and
below the bc-based ligand in the crystal lattice, eliminating the
potential for any p–p stacking interactions, which have also
been reported to result in deleterious non-radiative emission
for 8 is similar to the other two compounds (Table 1, column 2);
however, the oxidation potential of 8 is signicantly greater
(Table 1, column 3). This is consistent with the strong eld
ligand character of the bc-based framework, which should
make it more difficult to remove an electron from the Pt(II)
HOMO level in 8.
ꢂ
ꢂ
Similar to 4a/4b, we observed that 8 emits an intense blue-
green color upon irradiation with a table-top UV lamp at 365 nm
at room temperature. Given the improved solubility properties
of 8, we were able to carry out a detailed series of photophysical
measurements in order to ascertain the efficiency and nature of
this luminescent behaviour. The UV-Vis and phosphorescent
emission spectra for 8 are presented in Fig. 5B with corre-
sponding data in Table 2. The absorption spectrum reveals
strong transitions in the UV region (#330 nm) that arise from
p–p* transitions on the dtb-bpy ligand. The broad, lower
intensity band from 340–420 nm can be assigned to both singlet
˚
5
7
˚
1
and triplet metal-to-ligand charge transfers ( MLCT and
2c,3d
quenching.
3
MLCT). Compound 8 is non-emissive in solution at room
Cyclic voltammetry (CV) of 8 reveals a reversible, one-elec-
Red
temperature, suggesting emission might be thermally
quenched through interaction with solvent molecules.
However, at 77 K in 2-MeTHF, bright blue phosphorescence is
tron reduction (E1/2 ¼ ꢁ1.92 V) and an irreversible one-electron
Ox
oxidation (E1/2 ¼ 0.85 V), as shown in Table 1 and Fig. 5A. This
electrochemical behavior is consistent with other square planar
Pt(II) species undergoing a reversible ligand-centered reduction
2–4,12a,c,15a
and irreversible metal-centered oxidation.
calculations support these data (vide infra).
Further, DFT
a
Table 2 Photophysical data of 8 from solution-based and solid-
b
state measurements
Though electrochemical characterization for heteroleptic
Pt(II) complexes bound by a dianionic bidentate ligand and
c
d
f
4
ꢁ1
f
4 ꢁ1
Medium
7 K
PMMA lm 497
Neat solid
Em. lmax (nm)
f
s (ms)
k
r
(10 s
)
k
nr (10 s
)
7
456, 486, 514
—
11.4 e
—
—
0.07 4.24e 1.67
22.1
103.1
Table 1 Electrochemical data for 8 and related compounds from
a
476, 505, 540 0.03 0.94 3.20
literature
a
Solutions at room temperature were non-emissive, and 77 K spectra
Red
1/2
Ox
1/2
E
E
b
were measured in 2-MeTHF. PMMA lm was prepared as 2 wt%,
neat solid was 8 in powder form. Quantum yields were measured
using an integrating sphere under N
c
Compound
(V)
(V)
Solvent
Reference
d
2
.
77 K lifetime was measured
c
c
c
d
8
ꢁ1.92
ꢁ1.87
ꢁ2.13
0.85
MeCN
MeCN
CH Cl
2 2
This work in 2-MeTHF, PMMA lm and neat solid lifetimes were measured in
b
d
e
Pt(bph)(bpy)
Pt(bph)(en)
ꢁ0.33
12a
12c
the absence of air. Values obtained from the weighted average of
f
b
d
a multi-exponential decay. Calculated according to the equations k
f/s and knr ¼ (1 ꢁ f)/s, where k is the radiative rate constant, knr is
r
0.25
¼
r
a
+
Values reported relative to the ferrocene/ferrocenium couple (Fc/Fc ). the non-radiative rate constant, f is the quantum yield, and s is the
Values were corrected according to ref. 15b. Reversible. Irreversible. luminescence lifetime.
b
c
d
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observed (lmax ¼ 485 nm, s ¼ 11.4 ms). The well-dened vibra-
tional features suggest ligand-centered emission resulting from
1a
an MLCT.
Similarly, the neat solid 8 also exhibits an emission prole
with a resolved vibronic ne structure, further suggesting the
ligand-centered emission. Compared to the emission prole
from the neat solid, emission from the solution at 77 K is
hypsochromically shied by roughly 20 nm. This shi is ex-
pected as vibrational relaxations to a lower energy excited
state will not be favorable at lower temperatures, resulting in
a higher energy, blue-shied emission observed for 8 at 77 K
in 2-MeTHF. When 8 is doped in a PMMA matrix (2 wt%), the
emission prole is broadened and the peak is blue-shied by
about 8 nm versus the emission of the neat solid. The excited-
state lifetime (s) for 8 increases as the environment becomes
more rigid. This increase is signicant, going from 0.94 ms as
a neat solid, to 4.24 ms doped in PMMA, further to 11.4 ms at
7
7 K. This evidence suggests that decreasing vibrational
motion through a more rigid and ordered surrounding envi-
ronment can preserve the excited state, possibly by mini-
mizing the energy loss via non-radiative relaxation
1
a
pathways.
From the measured excited state lifetimes and quantum
yields for 8 doped in the PMMA matrix and neat solid, the
r
radiative rate constant (k ) and non-radiative rate constant (knr)
could be calculated (Table 2). Though k for the doped PMMA
r
Fig. 6 Frontier orbitals of 8 based on optimized geometries of S
states (BP86-D3, TZP).
0
and

lm is half that of the neat solid, knr for the doped PMMA lm
T
1
decreased by about a factor of 5, which supports the trend seen
for measured s values (vide supra). Additionally, the doped
PMMA lm exhibits a quantum yield (f) more than twice that of
the neat solid, as well as a lifetime that is about 4.5 times
greater. Based on these data, it is likely that the PMMA lm
decreases access to a non-radiative decay pathway through its
behavior as a rigid matrix.
To further our understanding of the photophysical proper-
ties of 8, we performed a DFT computational study at the BP86-
D3 level using the TZP basis set (Fig. 6). The optimized geometry
of the singlet state displays a slightly distorted square planar
structure, which is in agreement with the obtained single crystal
X-ray structure. The frontier orbital diagram indicates a HOMO
and HOMOꢁ1 almost completely localized on the Pt(II) with
negligible contribution from the bc fragment. Both the LUMO
and LUMO+1 are isolated on dtb-bpy, which corroborates the
In general, phosphorescent blue OLEDs suffer from short
lifetimes (ꢀ600 hours) relative to their red and green counter-
6
16a
parts (10 hours). Several groups have previously investigated
the degradation of blue OLEDs and found that the decomposi-
tion of the phosphorescent dopant molecules occurs during
regular use, greatly inhibiting the overall lifetime and efficiency
1
6b–e
of the device.
Thermogravimetric analysis of 8 suggests the
ꢂ
bc ligand framework remains intact upon heating to 500 C (see
ESI†). This observation suggests that chelating boron cluster
scaffolds may be potentially appropriate ligands that can
ameliorate previously described stability issues in OLED devices.
Conclusions
observed ligand-centered phosphorescence of
8 without
observed contribution from the bc fragment. The optimized In conclusion, for the rst time, we have presented a detailed
0
geometry of the triplet state, however, reveals an almost tetra- study on the behavior of 1,1 -bis(o-carborane) as a dianionic
hedral structure that is extremely distorted from the favorable ligand (bc) in group 10 metal complexes. From the synthesis and
square planar geometry seen in the ground state (Fig. 6). In the characterization of model complexes, we have discovered that
excited state, the complex twists via a non-radiative decay unlike the structurally reminiscent biphenyl ligand, bc displays
pathway, resulting in a large value for k . This hypothesis a unique photophysical innocence and remains uninvolved in
nr
supports the observed decrease in knr from the pure solid to the relevant photophysical transitions when bound to the group 10
PMMA matrix: as the rigidity of the environment increases, the transition metals. Furthermore, since bc introduces sufficient
geometry of the molecule will be more difficult to distort. These steric bulk above and below the square plane of the metal center,
calculations suggest that future molecular designs should it effectively shuts down undesired intermolecular interactions in
incorporate a large degree of steric bulk to potentially minimize the solid-state. For Pt(II) complexes featuring phosphorescent
this excited state distortion, thereby improving phosphores- emission properties, this unique ligand design aspect allowed
cence efficiency in these compounds.
us to remove any possible Pt(II)/Pt(II) interactions, which
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commonly lead to luminescence quenching. The organo-
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6e,17
mimetic
properties of carboranes in general, and bc in
particular, enabled us to rationally tune the processability of
a blue phosphorescent emitting Pt(II) species. We are currently
investigating the substitution of carborane-based ligands with
bulkier functional groups that will minimize the excited state
distortion. This work opens a new avenue in designing lumi-
nescent materials with improved properties incorporating robust
and photophysically innocent multidentate ligand platforms.
7
035–7062; (b) J. M. Oliva, P. v. R. Schleyer, G. Aull ´o n,
J. I. Burgos, A. Fern ´a ndez-Barbero and I. Alkorta, Phys.
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Acknowledgements
We would like to thank the Department of Chemistry and
Biochemistry at UCLA for start up funds. Portions of this
material are based upon work supported by the National
Science Foundation (CHE-1048804). We are indebted to Prof.
Peter I. Djurovich and Prof. Mark E. Thompson for assistance
with photophysical measurements and helpful discussions. A.
G. thanks the UCLA CARE Fellows & Scholars Program for
funding. We thank Dr Liban M. A. Saleh for helpful comments
and Rafal M. Dziedzic for assistance with the syntheses of 5 and
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application covering the content of this manuscript was led by
UCLA on February 18, 2016.
1
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