Highly efficient thermally activated fluorescence of a new rigid Cu(i) complex [Cu(dmp)(phanephos)]+

Article abstract of DOI:10.1039/c3dt51006a

The rigid [Cu(dmp)(phanephos)]⁺ complex displays a high luminescence quantum yield of 80% at ambient temperature. In contrast to the long-lived phosphorescence of 240 μs at T < 120 K with a radiative rate of k_r = 3 × 10³ s^-1, the ambient-temperature emission represents a thermally activated delayed fluorescence (DF) with a decay time of only 14 μs and a radiative rate of k_r(DF) = 6 × 10⁴ s^-1. Evidence for the involvement of the excited singlet state in the emission process is presented. This material has high potential to be applied in efficient OLEDs taking advantage of the singlet harvesting mechanism. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt51006a

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Highly efficient thermally activated fluorescence of
a new rigid Cu(I) complex [Cu(dmp)(phanephos)]+†
Cite this: DOI: 10.1039/c3dt51006a
Received 16th April 2013,
Accepted 13th May 2013
Rafał Czerwieniec,*a Konrad Kowalski*b and Hartmut Yersin*a
DOI: 10.1039/c3dt51006a
www.rsc.org/dalton
The rigid [Cu(dmp)(phanephos)]+ complex displays a high lumi- compounds. It allows one to achieve much higher OLED
nescence quantum yield of 80% at ambient temperature. In con- efficiencies than obtainable with typical organic fluorescent
trast to the long-lived phosphorescence of 240 µs at T < 120 K (singlet) emitters by which only 25% of the total number of
with a radiative rate of kr = 3 × 103 s−1, the ambient-temperature excitons can be exploited. However, the triplet emitters are
emission represents a thermally activated delayed fluorescence based on high-cost platinum group metals. Therefore, alterna-
(DF) with a decay time of only 14 μs and a radiative rate of tive materials, such as Cu(I) complexes,5,6,10–14 came into the
kr(DF) = 6 × 104 s−1. Evidence for the involvement of the excited focus of research.
singlet state in the emission process is presented. This material
At first sight, Cu(I) complexes seem to exhibit substantial
has high potential to be applied in efficient OLEDs taking advan- problems with regard to OLED applications: (1) compared to Ir
tage of the singlet harvesting mechanism.
or Pt, Cu as a 1st row transition metal induces much weaker
spin–orbit coupling.15 As a consequence, transitions between
the excited triplet state and the singlet ground state are largely
forbidden. Thus, long phosphorescence decay times of several
100 μs are found.5,6 Therefore, in an OLED, strong satu-
ration effects would result. (2) Cu(I) complexes with reducible
ligands, i.e. with energetically low-lying π* orbitals, such as
aromatic diimines, often display distinct low-energy metal-to-
ligand charge-transfer (MLCT) transitions in the visible part of
the spectrum. With this type of electronic excitation, a flatten-
ing distortion of the molecular structure takes place.13a,16–18
Such structural rearrangements are usually connected with an
increase of non-radiative deactivation or even quenching of the
emission due to an increase of the Franck–Condon factors that
couple the excited state and the ground state.19,20 This is
especially distinct in non-rigid environments.21
This contribution presents a new, adequately designed
Cu(I) complex that may be used to overcome both problems.
The very long emission decay time of the lowest excited
triplet state T1 does not become effective if the energy sepa-
ration between this state and the higher lying singlet state S1,
ΔE(S1–T1), is sufficiently small to thermally populate the S1
state from the lower lying triplet state by fast up-intersystem
crossing at ambient temperature. Accordingly, a thermally
In the past decade, substantial investigations were carried out
to develop novel materials for organic light emitting diodes
(OLEDs),1 in particular, to increase the device efficiency. In
this respect, luminescent materials and excitation mecha-
nisms play a crucial role. A breakthrough was reached by
applying organometallic complexes based on the 3rd-row tran-
sition metals iridium and platinum.2–6 These substances fre-
quently display high phosphorescence quantum yields
approaching even 100% and short emission decay times of a
few μs.7 Of special importance is the fact that by an electro-
luminescent excitation all triplet and singlet excitons formed in
the emissive layer of an OLED can be utilized for light gen-
eration, whereby the emission stems from the lowest triplet
state. This mechanism, representing the triplet harvesting
effect,8,9 is based on the properties of the organometallic
aUniversität Regensburg, Institut für Physikalische und Theoretische Chemie,
Universitätstr. 31, D-93040 Regensburg, Germany. E-mail: rafal.czerwieniec@ur.de,
hartmut.yersin@ur.de
bUniversity of Łódź, Faculty of Chemistry, Department of Organic Chemistry,
Tamka 12, PL-91-403 Łódź, Poland. E-mail: konkow@chemia.uni.lodz.pl
†Electronic supplementary information (ESI) available: X-ray crystallographic
data files (CIF), instrumentation, experimental procedures, synthesis details, activated delayed S1 → S0 fluorescence22 (TADF; compare also
crystal structures of [Cu(dmp)(Rp-phanephos)](PF6) and [Cu(dmp)(Sp-phane-
ref. 5, 6, 13, 21–25) occurs and the overall emission decay time
becomes much shorter than that of the T1 → S0 phosphor-
escence. With respect to electroluminescent applications, this
phos)] (PF6), lowest electronic transitions according to TD-DFT calculations, and
estimation of the radiative decay rate of the S1 → S0 electronic transition
(prompt fluorescence) from an analysis of the absorption spectrum. CCDC
mechanism can thus also be used to harvest all triplet and
singlet excitons for light generation, though in the excited
921267 and 921268. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt51006a
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singlet state. Therefore, this mechanism is termed “singlet
harvesting”.5,6,24,25 The problem of the molecular flattening
upon electronic excitation is addressed by the use of ligands
with high steric demands. For instance, sterically demanding
groups on the 2 and 9 positions of 1,10-phenanthroline can
effectively hamper the flattening tendency in homo- and hetero-
leptic diimine Cu(I) complexes.18,21 Applying this ligand in
combination with a chelating ligand that exhibits a wide bite
angle,26 the molecular structure of the complex can be dis-
tinctly further rigidified. It will be shown that the title com-
pound overcomes the problems discussed above.
The studied complex was synthesized by reacting [Cu-
(CH3CN)4](PF6) with equimolar amounts of two chelat-
ing ligands: dmp = 2,9-dimethyl-1,10-phenanthroline and
phanephos = 4,12-bis(diphenylphosphino)-[2.2]paracyclophane
(Scheme 1). The molecular structures of both enantiomers27 of
[Cu(dmp)(phanephos)]+ are displayed in Fig. S2 (ESI†) and
bond lengths and angles are listed in Table S1.† The X-ray
investigations reveal a strongly distorted tetrahedral coordi-
nation of the four-coordinated metal center due to distinctly
different steric requirements of the dmp and phanephos
ligands. In particular, the phanephos ligand with the bulky
paracyclophane backbone and the two diphenylphosphino
functions separated by a P⋯P distance of about 3.9 Å and the
relatively wide P–Cu–P bite angle of 116° form a rigid “semi-
cage” for the metal ion and the dmp ligand. Apparently, both
ligands provide a firm framework around the metal ion, sup-
pressing strong molecular geometry changes and shielding the
metal center from the environment.
Fig. 1 Ambient temperature absorption and luminescence spectra of [Cu-
(dmp)(phanephos)]+ recorded for a diluted (c ≈ 3 × 10−5 M−1) dichloromethane
solution28 (solid lines) and
a powder sample (dashed line) of [Cu(dmp)-
(phanephos)](PF6). LC and MLCT describe ligand centered (π–π*) and metal-to-
ligand charge-transfer (d–π*) transitions, respectively.
emission properties were studied in degassed dichloromethane,
in PMMA, and in the solid phase (as powder) (Fig. 1). Emission
data are summarized in Table 1. Independent of the matrix,
the emission spectra are broad and unstructured even down to
20 K. This is in accordance with the MLCT nature of the emit-
ting state. At ambient temperature, the emission bands
measured as powder, in PMMA, and in CH2Cl2 are centered at
530, 535, and 558 nm, respectively. This red shift is associated
with a lowering of the matrix rigidity and an increasing
freedom for changes of the molecular geometry upon MLCT
excitation. In addition to this bathochromic shift, the geome-
try change results in an increase of radiationless deactivation
according to a better overlap of the vibrational wave functions
of the ground and the excited state19,20 and thus to a decrease
of the emission quantum yield from ϕPL = 80% (powder) to
65% (PMMA) and to 40% (dissolved in CH2Cl2). Interestingly,
the change of the luminescence properties of [Cu(dmp)-
(phanephos)]+ with a variation of the matrix rigidity is much
less distinct than is found for other Cu(I) complexes.
For example, for the complexes [Cu(dmp)(pop)]+ 26a–c and
Fig. 1 shows electronic absorption and emission spectra of
the studied complex recorded at ambient temperature. [Cu-
(dmp)(phanephos)]+ dissolved in dichloromethane28 displays
intense high energy absorption bands with maxima at
≈230 nm (ε ≈ 74 000 M−1 cm−1) and ≈270 nm (ε ≈ 36 000 M−1
cm−1), which are assigned to π–π* ligand centered (LC) tran-
sitions of dmp and phanephos. Between ≈320 and ≈440 nm
distinctly weaker absorptions (ε(380 nm) ≈ 3900 M−1 cm−1
)
occur. These are attributed to metal-to-ligand charge-transfer
(MLCT) excited singlet states involving mainly the metal and
the dmp ligand.18 The MLCT character of the lowest electronic
transitions is also supported by the results of TD-DFT calcu-
lations (ESI†).
The studied Cu(I) complex displays intense green-yellow
luminescence at ambient temperature even in solution. The
Table 1 Luminescence properties of [Cu(dmp)(phanephos)](PF6) in dichloro-
methane, in PMMA (poly(methyl methacrylate)), and as powder
T = 300 K
T = 77 K
λmax c [nm] τd [μs] ϕPL e [%] λmax c [nm] τd [μs] ϕPLe [%]
a
CH2Cl2
558
10
20
14
40
65
80
548
567
562
130
170
240
60
60
70
PMMAb 535
Powderb 530
a The CH2Cl2 solution was degassed by repeated freeze–thaw cycles.
b Measured in a nitrogen gas atmosphere. c Emission maximum.
d Emission decay time (error 5%) measured after pulsed diode laser
excitation (λexc = 372 nm, pulse width 100 ps). e Photoluminescence
quantum yield (relative error 10% at 300 K; 20% at 77 K).
Scheme 1 Synthesis of [Cu(dmp)(phanephos)](PF6).
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[Cu(pop)(pz2Bph2)],5,24 with pop = bis[2-(diphenylphosphino)-
The temperature dependence of τ = τ(T), as displayed in
phenyl]ether and pz2Bph2 = diphenyl-bis(pyrazol-1-yl)borate, Fig. 2, can be interpreted in terms of a three state kinetic
showing ϕPL values in rigid powders up to 90%, the emission model involving the electronic ground state S0, the lowest
quantum yields measured in CH2Cl2 amount to only 15 and triplet state T1, and the singlet state S1. Under the assumption
8%, respectively. Thus, the title compound belongs to the of a fast thermalization between the T1 and S1 states (assuming
strongest emitting Cu(I) complexes (even in solution) reported temperature-independent decay rates), the decay time at a
so far.5,10,13c,24,26a–c,29 Obviously, due to the relatively stiff given temperature can be expressed by5,6,24,25
“semi-cage” provided by the combined steric effects of the two
ligands, phanephos and dmp, the excited state geometry
rearrangements are less distinct than found for other Cu(I)
complexes. This result may be of high significance with
respect to optoelectronic applications.
3 þ expðꢀΔEðS1–T1Þ=kBTÞ
τðTÞ ¼
ð1Þ
3=τðT1Þ þ 1=τðS1Þ expðꢀΔEðS1–T1Þ=kBTÞ
Herein kB is the Boltzmann constant, τ(T1) and τ(S1) are the
The emission decay time is strongly dependent on tempera- individual decay times of the triplet (phosphorescence) and
ture (Fig. 2). Between 20 K and about 120 K, the decay time of the singlet (spontaneous fluorescence) excited states, and
the powder sample is almost constant and as long as τ ≈ ΔE(S1–T1) is the energy separation (activation energy) between the
240 μs. Therefore, we assign this emission to a T1 → S0 phos- two emitting states. By fitting the above equation to the
phorescence. Obviously, no other decay mechanism is acti- measured decay times and inserting the measured decay time
vated in this temperature range of ≈100 K corresponding to a τ(T1) = 240 μs (plateau at 20 K < T < 120 K), values of τ(S1) =
thermal energy range of ≈70 cm−1. However, with further 40 ns and ΔE(S1–T1) = 1000 cm−1 are obtained (see the inset of
temperature increase, a steep decrease of the decay time is Fig. 2). It is remarked that ΔE(S1–T1) of 1000 cm−1 found for
observed (Fig. 2). This is connected with a drastic increase of the powder material corresponds well to the spectral blue shift
the radiative decay rate kr. According to the data given in observed for the emission maximum with temperature
Table 1 and the relation kr = ϕPL/τ, the rates are determined to increase from T = 77 K (λmax = 562 nm) to 300 K (λmax
=
be kr = 3 × 103 s−1 at 77 K and kr = 6 × 104 s−1 at ambient temp- 530 nm) (Table 1). This correspondence between the activation
erature, respectively. This corresponds to an increase of the energy and the spectral shift upon temperature increase rep-
radiative rate by a factor of twenty. These drastic changes with resents further support for the model presented.
temperature increase are assigned to be related to a growing
The same trend holds also for the compound doped into
involvement of the higher lying S1 singlet state with its much PMMA. On the other hand, in fluid solution, temperature
higher decay rate. The S1 state is thermally activated from the reduction leads to a blue shift of the emission, which does not
lower lying T1 state. Hence, the ambient temperature emission seem to agree with the discussed model. However, in this situ-
represents a thermally activated delayed fluorescence (DF) that ation the usual rigidochromic blue shift, related to the phase
exhibits a decay time of τ(DF) = 14 μs at 300 K.
transition of CH2Cl2 at 176 K, obviously prevails.
The time constant τ(S1) = 40 ns of the S1 → S0 process
(corresponding to a spontaneous fluorescence under the
assumption of vanishing intersystem crossing) resulting from
the fit procedure gives a radiative decay rate of kr(S1 → S0, fit) =
2 × 107 s−1 (50 ns; if it is taken into account that the emission
quantum yield amounts to ϕPL = 80%, Table 1). Related infor-
mation can be independently determined from an analysis of
the absorption spectrum.30 According to Fig. 1, the absorption
peak of the lowest energy centered at 380 nm and showing a
slight spectral overlap with the emission is assigned to rep-
resent this S0 → S1 transition. Thus, a (radiative) transition
rate for the related emission can be directly estimated to be
kr(S1 → S0, abs) = 1.2 × 107 s−1 (corresponding to ≈80 ns) (for
details see the ESI†). This value corresponds to a moderately
1
allowed transition, as is expected for a S0 → MLCT type tran-
sition. With respect to the errors of the two different and inde-
pendent methods of determination/estimates, both values for
the transition rate are in good agreement. This is an important
Fig. 2 Emission decay time of [Cu(dmp)(phanephos)](PF6) powder versus
temperature. The sample was excited with a pulsed UV laser at λexc = 355 nm
(pulse width 7 ns). The emission was detected at λdet = 550 nm. The solid line
represents a fit of eqn (1) to the experimental data with the phosphorescence result since it strongly supports the model presented above
decay time τ(T1) = 240 μs measured at 77 K. The resulting fit parameters are
with respect to the involvement of the singlet S1 state in the
emission process at ambient temperature, being the basis for
the thermally activated short-lived emission. Moreover, it is
ΔE(S1–T1) = 1000 cm−1 and τ(S1) = 40 ns, representing the magnitude of the
singlet–triplet splitting and the decay time of the spontaneous S1 → S0 fluore-
scence, respectively. The spontaneous fluorescence is not observed directly due
concluded that the still slightly occurring geometry change in
the excited state relative to the electronic ground state does not
to much faster intersystem crossing. τ(DF) = 14 μs is the decay time of the
delayed fluorescence at ambient temperature.
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significantly alter the S0 → S1 transition rate, determined from
the absorption (unrelaxed geometry), as compared to the S1 → S0
rate, determined for the relaxed emitting states.
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The phanephos ligand can be successfully applied to engineer
highly luminescent Cu(I) complexes. According to the large
P–Cu–P bite angle, the robustness of the paracyclophane core,
and the overall bulkiness of the phanephos ligand in combi-
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tion is observed. At ambient temperature, the luminescence of
[Cu(dmp)(phanephos)](PF6) is dominated by a thermally acti-
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excited molecules. As a consequence, the decay time τ is
reduced by a factor of nearly twenty from 240 μs at 77 K to
14 μs at ambient temperature. Since both emissions, the phos-
phorescence at lower temperatures and the delayed fluo-
rescence at higher temperatures, display high quantum yields,
this shortening of the decay time is related to a drastic
increase of the radiative rate. These properties suggest high
potential of [Cu(dmp)(phanephos)]+ and related compounds
as emitter materials in opto-electronic applications, such as
OLEDs, taking advantage of the singlet harvesting mechanism.
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Acknowledgements
The authors gratefully acknowledge Ms S. Stempfhuber and
Dr M. Zabel for the single-crystal X-ray structural analyses.
R.C. and K.K. thank the German Academic Exchange Service
(DAAD) and its program “Ostpartnerschaften” for travel
support.
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solution as checked by NMR spectroscopy at least for the
time necessary for sample preparation and photophysical
measurements. All measurements were done with freshly
prepared samples. Coordinating solvents have been avoided
in order to prevent ligand exchange reactions.26a,28b
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Dalton Trans.