Outstanding luminescence from neutral copper(i) complexes with pyridyl-tetrazolate and phosphine ligands

Article abstract of DOI:10.1039/c3cc42280a

Strongly luminescent, neutral copper(i) complexes bearing 5-(2-pyridyl)tetrazolate and various phosphine ligands were synthesized. While the cationic copper(i) precursors 1b-4b do not exceed photoluminescence quantum yields (PLQY) of 4-46%, the neutral co

Full text of DOI:10.1039/c3cc42280a

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Outstanding luminescence from neutral copper(I)
complexes with pyridyl-tetrazolate and phosphine
ligands†
Cite this: Chem. Commun., 2013,
49, 6501
Received 28th March 2013,
Accepted 21st April 2013
Larissa Bergmann,ab Jana Friedrichs,b Mathias Mydlak,b Thomas Baumann,b
Martin Niegerc and Stefan Brase*ad
¨
DOI: 10.1039/c3cc42280a
www.rsc.org/chemcomm
Strongly luminescent, neutral copper(I) complexes bearing 5-(2-pyridyl)-
tetrazolate and various phosphine ligands were synthesized. While the
cationic copper(I) precursors 1b–4b do not exceed photoluminescence
quantum yields (PLQY) of 4–46%, the neutral complexes 1a–4a show
PLQYs of up to 89%.
Luminescent Cu(I) complexes have gained increasing interest for
their application in organic light emitting devices (OLEDs), as they
offer a new path to efficient emitters that can make use of both
singlet and triplet excitons.1 Their structural diversity and easy
accessibility allow for designing emitters with desired properties like
emission colour or additional functionalities while maintaining high
efficiencies.1b,2 In contrast to their charged counterparts, mono-
nuclear, neutral Cu(I) complexes are favourable emitters for OLED
applications due to their superior emission characteristics3 and the
lack of mobile counterions, which can have a negative influence on
the device performance.4 So far, only a few examples of neutral Cu(I)
complexes have proven their suitability for OLEDs,5 but they still
suffer from disadvantages. On the one hand, their high sensitivity
towards oxygen complicates synthesis,3c,d,6 while on the other hand
their susceptibility to structural changes in the excited state and a
subsequent non-radiative decay result in low quantum yields.7
Herein, we present four neutral, mononuclear Cu(I) complexes
1a–4a with high PLQYs (fem) and good stability towards oxygen, and
compare them to their cationic precursors 1b–4b, which show only
moderate luminescence properties (Fig. 1). All neutral Cu(I) com-
plexes contain 5-(2-pyridyl)tetrazolate (PyrTet) as a chromophoric
N^N ligand, which has been chosen as a model ligand since its soft
Fig. 1 Molecular structures of 1a–4a and 1b–4b. For 2b the molecular formula
[(DPEPhos)2Cu2(PyrTet)2H]BF4 was determined using elemental and X-ray analysis.
base character favours strong binding to the soft Cu(I) centre (HSAB
concept),8 leading to reduced sensitivity towards oxygen, while
offering protonation sites at the same time. Tetrazole-containing
ligands have been used for the preparation of cationic Cu(I) com-
plexes lately,2a,9 but not yet for neutral ones. In addition to afore-
mentioned advantages, tetrazoles show high acidity due to their
mesomeric structure, which simplifies the synthesis of the com-
plexes, due to the need for only mild bases for deprotonation. While
PyrTet was used as the chromophoric ligand, several neutral and
bulky phosphine ligands were tested, which are also soft Lewis
bases. They provide mostly steric advantages,3b inhibiting flattening
of the complex in the excited state. Either commercially available or
easily accessible phosphines with different steric demands and
rigidity were used to synthesize the presented Cu(I) complexes:
triphenylphosphine (PPh3), bis[2-(diphenylphosphino)phenyl]ether
(DPEPhos), 9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene
(Xantphos) and bis[2-(diphenylphosphino)-p-tolyl]ether (PTEPhos).
Cationic complexes [(PPh3)2Cu(PyrTetH)]BF4 1b, [(DPEPhos)2-
Cu2(PyrTet)2H]BF4 2b, [(Xantphos)Cu(PyrTetH)] BF4 3b, and [(PTE-
Phos)Cu(PyrTetH)]BF4 4b were synthesized from the appropriate
ligands and [Cu(CH3CN)4]BF4 as a copper source in a 2:1:1 or
1 : 1 : 1 ratio, respectively. The neutral complexes [(PPh3)2Cu(PyrTet)]
1a, [(DPEPhos)Cu(PyrTet)] 2a, [(Xantphos)Cu(PyrTet)] 3a, and
[(PTEPhos)Cu(PyrTet)] 4a were obtained by deprotonation of
the tetrazole moieties of the cationic precursors with potassium
hydroxide in dry methanol solutions. All complexes are stable under
a Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT),
Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany. E-mail: braese@kit.edu
b cynora GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany. E-mail: info@cynora.com
c Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, Fi-00014,
Finland. E-mail: Martin.Nieger@helsinki.fi
d Karlsruhe Institute of Technology (KIT), Institute of Toxicology and Genetics,
Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
† Electronic supplementary information (ESI) available. CCDC 922230–922235.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc42280a
c
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stabilization of the molecular orbitals involved in the LC transitions.
Additional shoulders around 339–341 nm for neutral complexes 1a–4a,
which are not observed for the free ligands, are assigned to metal-to-
ligand charge transfer (MLCT) transitions (e= 2.6–5.0 ꢀ 103 Mꢁ1 cmꢁ1).
The corresponding MLCT bands of the cationic complexes 1b–4b are
red-shifted to 347–362 nm (e = 1.7–3.0 ꢀ 103 Mꢁ1 cmꢁ1).
To explain these experimental results, compounds 1a and 1b were
chosen as model complexes and studied by means of density functional
theory (DFT) calculations using the BP86 functional.10 As observed in
similar complexes,9 the highest occupied molecular orbital (HOMO) of
Fig. 2 Molecular structures of 1a (left) and 1b (right) (displacement parameters cationic 1b is mainly located on the metal centre and the phosphorous
are drawn at 50% probability level). The counterion is omitted for clarity.
atoms, while the lowest unoccupied molecular orbital (LUMO) is
located on the PyrTetH ligand. Consequently, the absorption band at
ambient conditions in the solid state, and even for days in aerated 348 nm is assigned to a MLCT transition. The neutral complex 1a shows
solution, which is a major improvement for neutral Cu(I)emitters.3c,d,6 a different localization of the frontier orbitals, with the HOMO mainly
The complexes were characterized using elemental analysis, 1H and confined to Cu(I) and the tetrazole moiety with small contributions
31P NMR and X-ray diffraction if suitable crystals were obtained.
from the pyridine-N atom, and the LUMO located primarily on the
The crystal structures of 1a and 1b are shown in Fig. 2, demon- pyridine ring and to a smaller extent on the tetrazole ring (Fig. S3, ESI†).
strating the tetrahedral geometry of the complexes, typical for mono- The strong intraligand charge transfer (ILCT) character of these transi-
nuclear Cu(I) complexes.7d The comparison of the neutral and cationic tions is further confirmed by the hardly detectable MLCT maximum in
species reveals several differences, which arise due to the loss of a the absorption spectra of the neutral complex 1a. Interestingly the
proton and the counterion: the Cu–N distances in the neutral com- phosphines do not contribute to the frontier orbitals, i.e. they should
plexes are shorter than those in the cationic ones, which can be not influence the emission wavelength directly.
explained by a stronger coordination of the deprotonated N^N ligand
All of the neutral complexes 1a–4a show strong luminescence in
to the metal centre (1a: Cu–N1 2.043, Cu–N11 2.118, 1b: Cu–N1 2.079, the green spectral region from 502–545 nm with PLQYs of 76–89%
Cu–N11 2.140 Å). Associated with this shortening of Cu–N bonds is a and lifetimes of 17.8–26.6 ms in the solid state (powder) when excited
decrease in the P1–Cu1–P2 angle along with an increased torsion of at 350 nm (Table 1); a representative emission spectrum of 2a is
the tetrazole ring versus the pyridine ring. The Cu–N, Cu–P bond shown in Fig. 3 (Fig. S4, ESI†). The cationic complexes 1b–4b exhibit
lengths and N1–Cu1–N11 angles of 2a–4a are similar to the values of red-shifted emission bands from 518–569 nm, in accordance with
1a, but the P1–Cu1–P2 angles are slightly smaller due to lower steric the DFT calculations (Table S2, ESI†), and significantly decreased
hindrance of the bis(phosphines) compared to that of two PPh3 PLQYs of 4–46%. A shortening of lifetimes in cationic complexes as
ligands.7c Two isomers with different proton positions are possible seen for 1b–4b (5.2–15.3 ms) is also observed in a similar study with
for each of the cationic complexes 1b–4b. The proton of 1b is located 2-(20-quinolyl)benzimidazole as an N^N ligand.5b Especially the
on N4 probably due to steric reasons. In the case of 2b, the proton is complexes 3b and 4b show strongly red-shifted emission bands
bound to N2 and bridges one cationic and one neutral complex via accompanied by a strong drop in the PLQY. In addition, the very
hydrogen bonding in each unit cell (Fig. S1 and Table S1, ESI†).
broad PL emission bands of 3b and 4b (full width at half maximum
All complexes exhibit broad and intense absorption bands between in nm, 1b: 122; 2b: 130; 3b: 145; 4b: 169) and emission decays with
260–281 nm (e= 1.7–2.5 ꢀ 104 Mꢁ1 cmꢁ1) in dichloromethane solution, non-resolvable short-lived components indicate two emissive states.
which can be attributed to ligand-centred (LC) transitions of the When comparing absorption spectra and calculated excitation ener-
phosphine and PyrTet ligands (Fig. S2, ESI†). A small redshift of these gies (Fig. S2 and Table S2, ESI†), the experimentally found MLCT
latter bands in neutral complexes (1a, 3a, 4a) relative to the charged bands of 1b and 2b fit very well to the calculated values of 1b(N4) and
compounds can be explained with the stronger coordination of the 2b(N2), for which the position of the proton was determined by X-ray
PyrTet ligand to Cu(I) after deprotonation, leading to an energetic analysis. The excitation of the other possible isomers 1b(N2) and
Table 1 Photophysical properties of 1a–4a and 1b–4b
Emissionb
Absorbancea
Complex
lmax [nm] (e [104 Mꢁ1 cmꢁ1])
lmax [nm]
fem ꢂ 0.05
tave [ms]
kr [sꢁ1
]
knr [sꢁ1
]
c
1a
2a
3a
4a
260 (2.49), 273 (sh, 2.27), 339 (sh, 0.26)
265 (2.16), 280 (sh, 2.15), 341 (sh, 0.56)
278 (2.39), 341 (0.44)
512
510
545
502
0.85
0.78
0.76
0.89
20.6
19.9
26.6
17.8
4.1 ꢀ 104
3.9 ꢀ 104
2.9 ꢀ 104
5.0 ꢀ 104
7.3 ꢀ 103
1.1 ꢀ 104
9.0 ꢀ 103
6.2 ꢀ 103
276 (1.92), 340 (sh, 0.50)
1b
2b
3b
4b
260 (2.47), 348 (0.17)
265 (1.78), 281 (1.69), 347 (sh, 0.30)
274 (2.48), 353 (0.23)
522
518
559
569
0.46
0.21
0.06
0.04
13.5
15.3
5.9d
5.2d
3.4 ꢀ 104
1.4 ꢀ 104
1.0 ꢀ 104
7.7 ꢀ 103
4.0 ꢀ 104
5.2 ꢀ 104
1.6 ꢀ 105
1.9 ꢀ 105
265 (2.22), 274 (sh, 2.12), 362 (0.25)
a
b
c
In CH2Cl2, 10ꢁ5 mol Lꢁ1
.
In the solid state. PL lifetime is composed of two components. For simplicity, a weighted-average lifetime was used
P
P
d
(tave) and calculated by the equation tave
=
Aiti/ Ai with Ai as the pre-exponential factor for the lifetime. With short-lived components.
c
6502 Chem. Commun., 2013, 49, 6501--6503
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increase significantly to 111.5 ms (Fig. 3 and Fig. S5, ESI†), which
indicates the effect of thermally activated delayed fluorescence
(TADF), which has already been described as Singlet Harvesting for
Cu(I) complexes.1,3a,13 Emission at 293 K occurs from a higher lying
singlet state S1, which is in thermal equilibrium with a lower lying
triplet state T1, and is repopulated by means of thermal energy kBT.
Electrochemical data obtained from cyclic voltammetry are
in good agreement with the experimentally observed high
stabilities towards oxygen for neutral complexes 1a–4a. For 2a
the irreversible oxidation potential is 1.07 eV, which is assumed
to be the oxidation of the phosphine ligand. An irreversible
reduction peak was found at ꢁ0.91 eV (Fig. S6, ESI†).
Fig. 3 Absorption spectrum in CH2Cl2 at room temperature and emission
spectra in the solid state at different temperatures of 2a.
In conclusion, four mononuclear, neutral Cu(I) complexes
with 5-(2-pyridyl)tetrazolate and various phosphine ligands
were studied and their photophysical properties were compared
to those of their cationic precursor complexes. The neutral
compounds show efficient emission from mixed (ML + IL)CT
states, with virtually no electronic contribution from the phos-
phine ligands, which only influence the rigidity of the mole-
cules. Singlet harvesting was shown for 2a illustratively.
We acknowledge financial support from KIT. We also thank the
Deutsche Forschungsgemeinschaft (DFG) for support (project B2 of
SFB/TRR 88), and Dr. B. Rudat for electrochemical measurements.
2b(N4) is predicted to be red-shifted by more than 50 nm, but has
not been observed in the experiments. In contrast, the absorption
spectrum of 4b is extended to much longer wavelengths, thus
indicating a mixture of isomers. Furthermore, the trend of longer
emission wavelengths of 3b and 4b compared to those of 1b and 2b
is confirmed by calculated phosphorescence energies. All these
observations lead to the assumption that 3b and 4b are present as
mixed forms 3b(N2) and 3b(N4), or 4b(N2) and 4b(N4), respectively.
Based on these findings, one can assume that the proton in the
cationic complexes not only leads to different emissive states (MLCT
vs. (ML + IL)CT) but also to less efficient luminescence due to
vibrational quenching of the N–H bond on the tetrazole ring,11
which has already been observed for similar complexes with 2-(20-
quinolyl)benzimidazole ligands.5b The fact that two energetically
similar complex isomers are possible with the PyrTetH ligand might
even lead to lower PLQY. Additionally, counterions in cationic
complexes can have a negative influence on the emission properties,
when they are in close proximity to the chromophoric moieties.12
In general, the neutral Cu(I) complexes 1a–4a show superior
PLQYs due to a strong binding of the anionic PyrTet ligand with soft
base character, leading to a rigid complex structure. Similar emis-
sion wavelengths for 1a–4a indicate that the various phosphines
hardly have any influence on the emissive states, as predicted by
DFT. Their influence is confined to steric aspects, which are mostly
reflected in the non-radiative rate constants knr. While complexes 1a,
with a high mutual steric hindrance of two PPh3 molecules, and 4a,
where the phosphines are linked via an ether bridge for rigidity,
exhibit low knr values, complex 3a shows knr values almost 50%
larger. The most important distortion of tetrahedral Cu(I) complexes
in the excited state is considered to be a flattening of the tetrahedral
coordination geometry. This flattening is often represented by the
change in the dihedral angle between the Cu–N–N and the Cu–P–P
planes in the excited state versus the ground state. The largest
flattening value has been calculated for the least efficient neutral
complex 3a (S0 90.01, T1 55.71), but it has to be kept in mind that the
torsion of the N^N ligand out of plane is not taken into account in
this method, which is especially pronounced for 2a and 4a (Table S1,
ESI†). In any case, there seems to be a correlation between the PLQY
and the ligand movement, like in other Cu(I) emitters.
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Low temperature measurements were conducted with complex
2a in the solid state in order to gain a deeper understanding of the
emissive states of the neutral species. Upon cooling from 293 K to
77 K, a strong redshift from 510 to 549 nm is observed, and lifetimes
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6501--6503 6503