Scanning-tunneling-spectroscopy-directed design of tailored deep-blue emitters

Article abstract of DOI:10.1002/anie.201407439

Frontier molecular orbitals can be visualized and selectively set to achieve blue phosphorescent metal complexes. For this purpose, the HOMOs and LUMOs of tridentate Pt^II complexes were measured using scanning tunneling microscopy and spectroscopy. The introduction of electron-accepting or -donating moieties enables independent tuning of the frontier orbital energies, and the measured HOMO-LUMO gaps are reproduced by DFT calculations. The energy gaps correlate with the measured and the calculated energies of the emissive triplet states and the experimental luminescence wavelengths. This synergetic interplay between synthesis, microscopy, and spectroscopy enabled the design and realization of a deep-blue triplet emitter. Finding and tuning the electronic "set screws" at molecular level constitutes a useful experimental method towards an in-depth understanding and rational design of optoelectronic materials with tailored excited state energies and defined frontier-orbital properties.

Full text of DOI:10.1002/anie.201407439

Angewandte
Chemie
DOI: 10.1002/anie.201407439
Molecular Set Screws
Scanning-Tunneling-Spectroscopy-Directed Design of Tailored Deep-
Blue Emitters**
Jan Sanning, Pascal R. Ewen, Linda Stegemann, Judith Schmidt, Constantin G. Daniliuc,
Tobias Koch, Nikos L. Doltsinis, Daniel Wegner,* and Cristian A. Strassert*
Abstract: Frontier molecular orbitals can be visualized and
selectively set to achieve blue phosphorescent metal complexes.
For this purpose, the HOMOs and LUMOs of tridentate PtII
complexes were measured using scanning tunneling microsco-
py and spectroscopy. The introduction of electron-accepting or
-donating moieties enables independent tuning of the frontier
orbital energies, and the measured HOMO–LUMO gaps are
reproduced by DFT calculations. The energy gaps correlate
with the measured and the calculated energies of the emissive
triplet states and the experimental luminescence wavelengths.
This synergetic interplay between synthesis, microscopy, and
spectroscopy enabled the design and realization of a deep-blue
triplet emitter. Finding and tuning the electronic “set screws” at
molecular level constitutes a useful experimental method
towards an in-depth understanding and rational design of
optoelectronic materials with tailored excited state energies and
defined frontier-orbital properties.
devices. Depending on the substitution pattern, they can be
employed either as monomeric[10] or as aggregated species.[11]
For the monomeric species, the emission occurs from metal-
perturbed ligand-centered triplet states (3MP-LC), whereas
the aggregates emit from excimeric or metal–metal-to-ligand
charge-transfer states (3MMLCT).[9] However, the search for
stable, deep-blue-emitting species is still the subject of
intensive research.[12] Moreover, precise knowledge of the
frontier orbital localization and the associated energies is
required for the appropriate design and implementation of
hosts and dopants with matching highest occupied (HOMO)
and lowest unoccupied molecular orbital (LUMO) levels. The
proper choice of materials facilitates the optimization of hole
and electron injection and recombination processes, as well as
the exciton confinement to achieve efficient optoelectronic
devices.[1]
Recently, we have used scanning tunneling microscopy
(STM) and spectroscopy (STS) to spectroscopically map (in
energy and spatial distribution) molecular orbitals of a tri-
dentate PtII complex (C1), ranging from the HOMOÀ3 to the
LUMO + 2.[13] We found that the ligand-centered HOMOs
and LUMOs are localized at specific units of the tridentate
chelate, and are not affected by hybridization with the metal
T
riplet emitters have found application as dopants in
optoelectronic devices, such as organic light-emitting
diodes[1] (OLEDs) and light-emitting electrochemical cells
(LEECs).[2] Apart from IrIII,[3] ReI,[4] and CuI[5] complexes,
coordination compounds featuring PtII[6] and AuIII[7] cations
have been described. The latter possess a d8 electronic
configuration and a mostly square-planar coordination envi-
ronment. Both bidentate[8] as well as tridentate[9] ligands have
been employed, yielding highly luminescent coordination
compounds. We have also recently shown that neutral PtII
complexes bearing dianionic tridentate N^N^N ligands can
be used as triplet emitters, particularly in electroluminescent
2
substrate, unlike the platinum-centered dz orbitals. The
HOMO appeared mainly associated to the triazole unit of
the chelating luminophore, whereas the LUMO is predom-
inantly linked to the central pyridine ring. In this work, we
assessed the utility of this knowledge to fine-tune the energy
of the frontier orbitals and to correlate it with the energy of
the emissive triplet state.
[*] Dipl.-Chem. J. Sanning,[+] M. Sc. P. R. Ewen,[+]
M. Sc. L. Stegemann,[+] J. Schmidt,[+] Dr. D. Wegner,[+]
Dr. C. A. Strassert[+]
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Wilhelm-Klemm-Strasse 10, 48149 Mꢁnster (Germany)
[+] J.Sa. synthesized and characterized the complexes; P.R.E. per-
formed the STM and STS experiments; L.S. carried out the
photophysical characterization; J.Sch. and C.D. performed the
mass-spectrometric and X-ray diffractometric analysis, respectively;
T.K. and N.D. carried out the DFT calculations; D.W. and C.A.S.
conceived the experiments, discussed the results, and wrote the
manuscript.
Physikalisches Institut—Center for Nanotechnology
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Heisenbergstrasse 11, 48149 Mꢁnster (Germany)
E-mail: ca.s@wwu.de
M. Sc. P. R. Ewen,[+] Dr. D. Wegner[+]
Institute for Molecules and Materials
Radboud University Nijmegen
Heyendaalseweg 135, 6525 AJ Nijmegen (Netherlands)
E-mail: d.wegner@science.ru.nl
Dr. C. G. Daniliuc[+]
[**] Financial support from the DFG (SFB-TRR 61, projects B13 and
C07) is gratefully acknowledged.
Supporting information for this article (details regarding the
synthesis and characterization of ligands and complexes, STM/STS
methods and results, photophysical characterization and spectro-
scopic data, DFT methods and results) is available on the WWW
under http://dx.doi.org/10.1002/anie.201407439.
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 7, 48149 Mꢁnster (Germany)
B. Sc. T. Koch,[+] Prof. Dr. N. L. Doltsinis[+]
Institut fꢁr Festkçrpertheorie und
Center for Multiscale Theory and Computation
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Scheme 1. Syntheses of complexes C1–C9.
We started by replacing the s-electron-withdrawing CF3
moieties of the blue–green complex C1 by tert-butyl groups
(complex C2), in an attempt to destabilize the HOMO
(Scheme 1). This should shift the emission into the green
region of the electromagnetic spectrum by lowering the
HOMO–LUMO gap and thus the energy of the emissive
triplet state. The complexes were synthesized and fully
characterized as described in the Supporting Information.
STS measurements indeed indicated that the replacement of
the electron withdrawing CF3 group by a tert-butyl unit causes
the HOMO level of C2 to lie higher than for C1, but without
significantly affecting the LUMO (Figure 1b). Therefore, the
HOMO can be selectively tuned by varying the substitution
pattern at the triazole ring.
In a further step, we also considered the introduction of
a p-electron-donating substituent on the central pyridine ring
of the tridentate luminophore of C2 to destabilize the LUMO
(Scheme 1). For this purpose, a straightforward synthetic
route was developed to insert a OCH3 substituent on the
central pyridine unit. Briefly, chelidamic acid was converted
into the corresponding methyl ester and treated with hydra-
zine followed by reaction with a substituted amidine to yield
the desired luminophore (see Scheme S1 and the correspond-
ing description in the Supporting Information). This synthesis
resulted in complex C3. STS measurements showed that the
LUMO of C3 appears destabilized compared to C2, without
any noticeable shift of the HOMO (Figure 1c). Thus, the
LUMO can be selectively tuned as well, but by varying the
substitution pattern at the pyridine unit.
The demonstrated selective tunability of the frontier
orbitals encouraged us to maximize the HOMO–LUMO gap
of the luminophore by both stabilizing the HOMO and
destabilizing the LUMO, by introduction of CF3 and OCH3
groups on the triazole and pyridine rings, respectively
(Scheme 1). To obtain the desired complex C4, a convenient
synthetic strategy was developed. In short, chelidamic acid
was converted into the corresponding dinitrile, which was
treated with ammonia and then reacted with hydrazine and
ethyl trifluoroacetate to yield the desired chromophore (see
Scheme S1 and the corresponding description in the Support-
ing Information). As revealed by STM and STS, the HOMO
of the resulting complex C4 lies energetically as deep as that
of C1, whereas the LUMO lies as high as that of C3
Figure 1. a)–d) STM images of C1–C4 (left) and corresponding STS
spectra (right) locally measured at two different positions within the
tridentate ligand (triazole or pyridine). e) Calculated and measured
spatial distributions of the HOMO (left) and LUMO (right) of C2,
representative of all other complexes.
(Figure 1d). Besides yielding precise information about
HOMO–LUMO localization and energies (see Table 1), the
analysis of individual molecules appears advantageous in
comparison to macroscopic cyclic voltammetry (CV): PtII
complexes frequently show irreversible oxidation waves or
hardly measurable oxidation processes in CV. Moreover, CV
usually requires concentrations between 10À4 m and 10À3 m in
the presence of a supporting electrolyte, thus favoring
aggregation processes.
The PtII complexes C1–C4 bearing a planar pyridine
ancillary ligand tend to aggregate (see below); therefore, we
decided to introduce P(phenyl)3 for the formation of efficient
non-stacking emitters (Scheme 1). As the HOMOs and the
LUMOs are mainly localized on the tridentate chromophores,
the photophysical properties are not significantly affected
upon replacement of pyridine by P(phenyl)3 as the ancillary
2
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Table 1: Comparison of HOMO, LUMO, HOMO–LUMO gap, and lowest triplet state (ET1) energies obtained by STS, DFT, and photoluminescence
spectroscopy. Excited state lifetimes in frozen matrices as well as photoluminescence quantum yields of the corresponding solids are also indicated.
t (77 K) [ms][d]
ET1 [eV] (nm)[e] FL
[f]
EHOMO [eV]
STS[a] DFT[b]
ELUMO [eV]
STS[a] DFT[b]
DEHOMO-LUMO [eV]
STS[a] DFT[b]
ET1 [eV] (nm)
DFT[c] 77 K[d]
for the solids
C1
C2
C3
C4
C5
C6
C7
C8
C9
À2.11Æ0.03 À2.21 1.98Æ0.03 2.06 4.09Æ0.05 4.27
À1.54Æ0.03 À1.68 1.94Æ0.07 2.21 3.48Æ0.08 3.89
À1.39Æ0.11 À1.63 2.36Æ0.08 2.47 3.75Æ0.14 4.09
À1.98Æ0.07 À2.12 2.21Æ0.16 2.37 4.19Æ0.16 4.50
2.75 (451) 2.73 (455)
2.43 (510) 2.52 (492)
2.59 (479) 2.67 (465)
3.00 (413) 2.86 (434)
2.73 (454) 2.73 (454)
2.43 (510) 2.53 (491)
2.57 (482) 2.66 (466)
2.85 (435) 2.86 (434)
2.46 (504) 2.59 (478)
8.1
12.8
8.8
2.23 (557)
2.46 (503)
2.65 (467)
2.26 (549)
2.70 (460)
2.41 (515)
2.56 (485)
2.81 (441)
2.53 (490)
0.48
0.15
0.19
0.49
0.65
0.47
0.59
0.48
0.31
4.6
–
–
–
–
–
À2.08
À1.65
À1.61
À2.01
À1.40
–
–
–
–
–
2.16
2.25
2.52
2.46
2.39
–
–
–
–
–
4.24
3.90
4.13
4.48
3.79
13.8
18.7
18.8
14.8
23.2
[a] Measured by STS for a molecular monolayer on Au(111). [b] At the optimized singlet ground-state geometry in the gas phase and energy-shifted to
facilitate comparison to experimental numbers on gold substrate (see the Supporting Information). [c] At the optimized geometry in the lowest triplet
state. Energies are given relative to the optimized ground state. [d] Obtained from the highest-energy emission maximum in frozen glassy matrices.
Amplitude-averaged lifetimes are given when multiexponetial decays are observed. Values Æ0.1 ms. [e] Measured on powders or crystals at room
temperature. [f] Measured at room temperature in an integrating sphere. Values Æ0.02.
ligand. Moreover, defined crystals can be grown for X-ray
diffraction analysis (see the Supporting Information). All of
the complexes were characterized in terms of absorption
spectroscopy, and the results are shown in the Supporting
Information. Emission spectroscopy in the solid state (pow-
ders and crystals, neat films and doped into PMMA), as well
as in solution at room temperature and in frozen matrices at
77 K was carried out, including absolute photoluminescence
quantum yields and time-resolved luminescence decays to
assess the energy of the emissive triplet states and their
lifetimes. The absorption, excitation, and emission spectra as
well as the luminescence decay curves of each complex are
depicted in the Supporting Information, along with the
relevant photophysical data such as photoluminescence
quantum yields, radiative and non-radiative rate constants.
A summary of the most relevant photophysical data is
presented in Table 1.
exponential luminescence decays. The introduction of bulky
P(phenyl)3 ligands, on the other hand, completely suppresses
aggregation phenomena, and to a great extent avoids other
bimolecular quenching processes. Thus, high photolumines-
cence quantum yields are achieved in the solid state (see
Table 1 and Figure 2), even for the deep-blue emitting C8
(48%). To further assess the directing character of the
tridentate luminophore, a OCH3 substituted complex with
a resulting destabilized LUMO was also synthesized (com-
plex C9). The extended conjugation originated by the phenyl
units at the triazole rings only marginally affects the energy of
the emissive triplet state, as compared to C7 bearing tert-butyl
groups on the triazole rings and a OCH3 moiety on the central
pyridine. The extended conjugation, however, prolongs the
excited state lifetime due to the enhanced LC character.
The DFT phosphorescence wavelengths have been
derived from all-electron energy differences between the
ground state and the lowest triplet state, and are in excellent
quantitative agreement with the experimental data (Table 1).
Although the HOMO–LUMO gap constitutes a simple one-
electron approximation, it also provides a good measure for
the prediction of the energy level gaps measured by STS.
Remarkably, the trends regarding the HOMO–LUMO gaps
exhibit a good correlation with the singlet–triplet gaps.
Moreover, the DFT calculations regarding the lowest singlet
state (ground state) and the lowest triplet state (emissive
state) indicate that the HOMO and the corresponding
HSOMO+1 display a node at the center of the pyridine
ring, which explains the minor influence of the pyridine
substitution pattern on these frontier orbitals (Figure 1e and
the Supporting Information). They also show that the LUMO
and the HSOMO have a nodal plane at the substituted atom
of the triazole rings on the tridentate chromophore, which in
turn explains the lack of influence of the introduced OCH3
group on this set of orbitals. It is worth noting that all the
frontier orbitals involve the PtII center, which explains the
spin–orbit coupling required to enable the phosphorescence,
and supports the assignment of the emissive state as 3MP-LC.
In summary, we have shown that STM and STS can be
used to locate the frontier orbitals of triplet emitters and to
The long excited-state lifetimes and vibrational progres-
sions observed in frozen matrices at 77 K indicate that the
3
emission occurs from MP-LC excited states. Complexes C1
and C5 show blue–green emission, whereas C2 and C6 display
green phosphorescence as the alkyl substituents destabilize
the HOMO compared with the CF3 moieties. Complexes C3
and C7 appear blue-shifted with respect to complexes C2 and
C6, owing to the destabilized LUMO caused by the intro-
duction of the OCH3 group at the central pyridine ring of the
luminophore. As predicted by STS, complexes C4 and C8
show the bluest phosphorescence as the HOMO is stabilized
by the CF3 substituent and the LUMO is destabilized by the
OCH3 unit. These trends are also followed at room temper-
ature, as can be clearly observed for complexes C5–C9. It
should be noted that both complexes C1 and C4 show
a characteristic broad, red-shifted luminescence that is
attributed to stacked species, except in dilute solutions
where the phosphorescence of the monomers can be identi-
fied by their characteristic vibrational progression. The
emission spectra of complexes C2 and C3 indicate that
aggregation is suppressed by the bulky tert-butyl groups, even
though other bimolecular processes, such as triplet–triplet
annihilation, cannot be excluded, as suggested by the multi-
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optoelectronic devices.
Received: July 21, 2014
Published online: && &&, &&&&
Keywords: blue triplet emitters · density functional theory ·
.
frontier orbitals · photophysics ·
scanning tunneling spectroscopy
4
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

.
Angewandte
Communications
Communications
Molecular Set Screws
J. Sanning, P. R. Ewen, L. Stegemann,
J. Schmidt, C. G. Daniliuc, T. Koch,
N. L. Doltsinis, D. Wegner,*
C. A. Strassert*
&&&& — &&&&
Scanning-Tunneling-Spectroscopy-
Directed Design of Tailored Deep-Blue
Emitters
Seeing is believing: Frontier orbitals of
PtII complexes have been visualized and
measured by scanning tunneling spec-
troscopy. Moreover, they have been tuned
with the aid of targeted synthetic strat-
egies to yield a deep-blue triplet emitter.
This approach of finding and tuning the
right electronic set screws at the molec-
ular level constitutes a new strategy to
design and to realize tailored optoelec-
tronic materials.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!