Linker conjugation effects in rhenium(I) bifunctional hole-transport/ emitter molecules

Article abstract of DOI:10.1002/chem.200802373

Spectroscopic, electrochemical and density functional theory (DFT) methods have been employed to investigate a group of [Re(CO)₃(HT)(phen)] ⁺ complexes (phen = 1,10-phenanthroline), and in particular the level of electronic communica

Full text of DOI:10.1002/chem.200802373

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DOI: 10.1002/chem.200802373
Linker Conjugation Effects in Rhenium(I) Bifunctional Hole-Transport/
Emitter Molecules
Deidre M. Cleland,[a] Garth Irwin,[a] Pawel Wagner,[b] David L. Officer,[b] and
Keith C. Gordon*[a]
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Chem. Eur. J. 2009, 15, 3682 – 3690

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FULL PAPER
Abstract: Spectroscopic, electrochemi-
cal and density functional theory
(DFT) methods have been employed
time studies and DFT calculations sug-
gest that multiple dp(Re)!p*(phen)
metal-to-ligand charge transfers
(MLCTs) exist for each complex, two
of which significantly absorb at about
340 and 385 nm, and one that emits at
approximately 540 nm. In the com-
plexes containing more-conjugated HT
ligands, non-emissive intraligand transi-
tions (IL(HT)) exist with energies be-
tween the ground and MLCT excited
states. The overlap of these IL(HT) tran-
sitions and the absorbing MLCT of
lowest energy deactivates emission re-
sulting from about 385 nm excitation,
and lowers the quantum yield and ex-
cited-state lifetimes of these complexes.
Cyclic voltammetry experiments indi-
cate that throughout the series investi-
gated, the highest occupied molecular
orbital (HOMO) of each complex is
centred on the HT ligand, while the oc-
cupied molecular orbitals localised on
the rhenium are lower in energy.
AHCTUNGTRENNUNG
to
investigate
a
group
complexes
of
[Re(CO)3(HT)ACHTUNGTRENNUNG
(phen)]+
(phen=1,10-phenanthroline), and in
particular the level of electronic com-
munication between various hole-trans-
porting (HT) ligands and the rhenium
centre. Here, the HT ligand consists of
a coordinating pyridine connected to
dimethylaniline group through a sin-
gle-, double- or triple-bond-connecting
system. Electronic absorption, reso-
nance Raman, and steady-state emis-
sion spectroscopy combined with life-
Keywords: electrochemistry · elec-
tronic structure · light-emitting di-
odes · UV/Vis spectroscopy · transi-
tion metals
Introduction
This study looks at an alternate approach to the design of
OLED materials in which the hole-transporting (HT), elec-
tron-transporting (ET) and emissive layers shown in
Figure 1 are reduced to functional groups within a trifunc-
tional molecule (Figure 2a). The hypothesis is that the tri-
Since the first organic light emitting diode (OLED) was
shown to exhibit high emission efficiency, fast response and
a low turn on voltage,[1] considerable research has focused
on developing OLED materials that further optimise these
parameters. The majority of those investigated have been
based on the multilayer device structure employed by Tang
and VanSlyke (similar to Figure 1).[1,2] However, while some
of these exhibit promising characteristics, the best often in-
volve layered structures more complex than that of Figure 1,
and there is still room for improvement in regards to the
key operation parameters.[3,4]
Figure 1. The basic structure of a typical multilayer OLED device. Elec-
trons and holes move through the electron-transporting (ET) and hole-
transporting (HT) layers, respectively, to recombine on the emitting
layer, form an exciton and radiatively decay.
Figure 2. The function of each moiety within a trifunctional OLED mole-
cule.
[a] D. M. Cleland, Dr. G. Irwin, Prof. K. C. Gordon
Department of Chemistry and
functional molecules act in a similar fashion to a multilayer
device. When a potential is applied, electrons are injected
into the lowest unoccupied molecular orbital (LUMO) of
the ET moiety and holes into the highest occupied molecu-
lar orbital (HOMO) of the HT moiety. These charges then
travel towards the emissive functional group where they re-
combine, form an exciton, and decay to produce the ob-
served light emission (Figure 2b).[5]
MacDiarmid Institute for Advanced Materials and Nanotechnology
University of Otago, Union Place
PO Box 56, Dunedin (New Zealand)
Fax : (+64)3-479-7906
E-mail: kgordon@chemistry.otago.ac.nz
[b] Dr. P. Wagner, Prof. D. L. Officer
ARC Centre of Excellence for Electromaterials Science
Intelligent Polymer Research Institute
University of Wollongong
Use of these trifunctional compounds will reduce the
number of layers required in OLEDs, resulting in simpler
Wollongong NSW 2522 (Australia)
Chem. Eur. J. 2009, 15, 3682 – 3690
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K. C. Gordon et al.
device fabrication. Also, in principle these compounds may
improve the efficiency and balance of charge injection into
an OLED device, while promoting recombination at the ap-
propriate emissive site.[6–9] In order to be effective, trifunc-
tional molecules must be designed so that the orbital ener-
gies of each functional group encourage the required charge
transfer, while maintaining an appropriate level of electronic
communication across the molecule. At present, the molecu-
lar characteristics that result in these properties are poorly
understood.[7] This study looks at a group of hole-transport-
ing/emissive centre molecules shown in Figure 3 (1–3), in an
attempt to provide some insight into the nature of the HT
ligand/emissive centre interaction. This study is not directly
concerned with producing the optimal OLED materials.
ACHTUNGTRENNUNG
centre.
Results and Discussion
To establish that the DFT computational method is accurate
in modelling the molecules investigated here, the IR and
FT-Raman spectra were calculated from an optimised mo-
lecular geometry, and compared to experimentally obtained
data. The level of correlation was then quantified by using
the mean absolute deviation (MAD) between the frequen-
cies of the most intense peaks.[15] Calculated frequencies
were scaled by 0.970, as this value produced the lowest
MADs; this scale factor has been found to be optimal for
scaling polypyridyl vibrations.[16] An example of a calculat-
ed/experimental FT-Raman comparison is shown for 2 in
Figure 4. This data is considered to show good correlation
with regards to the position and intensity of peaks, and has
a MAD of 8 cmꢀ1.
Figure 3. The combination of emissive centre and HT ligand for the mol-
ecules investigated in this project.
Figure 4. Comparison between the experimental and calculated FT-
Raman spectra of 2. The calculated spectrum was obtained using the
B3LYP/6-31G(d) DFT computational method (with the LANLDZ basis
set on the Re atom) and frequencies were scaled by 0.970. The experi-
mental spectrum was recorded from solid-state 2 with lex =1064 nm.
A rhenium(phenanthroline) {ReACTHUNTGRNEUNG(phen)} group was chosen
as the emissive centre becuase of its high quantum
yield.[10,11] The associated carbonyls are also useful spectro-
scopic tools in studies of this nature.[12] The tertiary amine
group provides a site for hole injection,[13,14] and this is con-
nected to a coordinating pyridine through a bonding net-
work of variable conjugation. It is expected that the level of
conjugation between the HT group and metal centre will
have some effect on the charge communication across the
two functionalities, although the extent and consequences of
this effect are unknown and thus the focus of this investiga-
tion.
Spectroscopic and electrochemical techniques were em-
ployed to examine the vibrational, electronic and redox
properties of the bifunctional complexes. DFT calculations
were also performed to aid in the interpretation of physical
results. To help identify a HT-ligand/emissive-centre interac-
tion when present, each of the bifunctional compounds were
additionally compared to isolated HT and emissive systems.
These were represented by the uncoordinated HT ligands of
the respective complexes and [Re(CO)3(4-methylpyridine)-
One anomaly in these results was in the poorer agreement
of theoretical spectra with experimental data for the singly
bonded systems. This situation was not improved by altering
the starting geometries. The disparity was attributed to the
tendency of the B3LYP level of theory to overestimate the
importance of conjugation.[17] However, experimental data
from complex 1 is very similar to that of 4. This suggests
that with the singly bonded connecting system, the tertiary
amine substituent of 1 has a minimal effect on the molecule.
Empirical comparisons to calculations on complex 4 can
therefore be made to compensate for any inaccuracies in the
DFT results of 1.
The low-energy UV/Vis transitions observed for the free
ligands and complexes are given in Table 1. In addition to
these, an intense absorption (eꢁ4ꢁ104 Lmolꢀ1 cmꢀ1) is evi-
dent in the spectra of all Re complexes at around 277 nm.
From comparison to literature, this was assigned to an intra-
ligand p!p* phenanthroline (IL(phen)) transition.[18] The
wavelength invariance of this transition with the various HT
3684
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ligands suggests that any effect the change in conjugation on
the Re moiety does not appear to extend to the phen ligand.
The low-energy shoulder at 338 nm in the spectrum of 4
has been previously assigned to a dp(Re)!p*
ACHTUNGTREN(NUNG phen) metal-
to-ligand charge transfer (MLCT).[18] The similarity of this
to the shoulder in the spectrum of complex 1, with regards
to wavelength and extinction coefficients, suggests that 1
possesses a similar dp(Re)!p*
ACHTUNGTRENNUNG