Thermally Activated Delayed Fluorescence from Ag(I) Complexes: A Route to 100% Quantum Yield at Unprecedentedly Short Decay Time

Article abstract of DOI:10.1021/acs.inorgchem.7b02002

The four new Ag(I) complexes Ag(phen)(P₂-nCB) (1), Ag(idmp)(P₂-nCB) (2), Ag(dmp)(P₂-nCB) (3), and Ag(dbp)(P₂-nCB) (4) with P₂-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S₁ → S₀ transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S₁ and T₁ states. The oscillator strength of the S₁ ? S₀ transition f(S₁ ? S₀) - an important parameter for the TADF efficiency being proportional to the radiative rate - can be increased from f(S₁ ? S₀) = 0.0258 for Ag(phen)(P₂-nCB) (1) to f(S₁ ? S₀) = 0.0536 for Ag(dbp)(P₂-nCB) (4), as calculated for the T₁ state optimized geometries. This parameter governs the radiative TADF decay time (τ^r) at ambient temperature, found to be τ^r = 5.6 μs for Ag(phen)(P₂-nCB) (1) but only τ^r = 1.4 μs for Ag(dbp)(P₂-nCB) (4) - a record TADF value. In parallel, the photoluminescence quantum yield (Φ_PL) measured for powder samples at ambient temperature is boosted up from Φ_PL = 36% for Ag(phen)(P₂-nCB) (1) to Φ_PL = 100% for Ag(dbp)(P₂-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P₂-nCB) (1), Ag(idmp)(P₂-nCB) (2), and Ag(dmp)(P₂-nCB) (3) to Ag(dbp)(P₂-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S₁ state from the state T₁, ΔE(S₁-T₁). Experimentally it is determined for the complexes Ag(dmp)(P₂-nCB) (3) and Ag(dbp)(P₂-nCB) (4) to be of moderate size of ΔE(S₁-T₁) = 650 cm^-1.

Full text of DOI:10.1021/acs.inorgchem.7b02002

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Thermally Activated Delayed Fluorescence from Ag(I) Complexes: A
Route to 100% Quantum Yield at Unprecedentedly Short Decay Time
Marsel Z. Shafikov,†,‡ Alfiya F. Suleymanova,†,§ Rafał Czerwieniec,† and Hartmut Yersin*,†
†Institut fur Physikalische und Theoretische Chemie, Universitat Regensburg, Universitatsstrasse 31, D-93053 Regensburg, Germany
̈
̈
̈
‡Ural Federal University, Mira 19, Ekaterinburg 620002, Russia
§I. Postovsky Institute of Organic Synthesis, Ekaterinburg 620041, Russia
S
* Supporting Information
ABSTRACT: The four new Ag(I) complexes Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), Ag(dmp)(P2-nCB) (3), and
Ag(dbp)(P2-nCB) (4) with P2-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-
dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were
designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence
(TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via
influencing the radiative rate of the S1 → S0 transition and (ii) structurally by rigidifying the molecular geometry with respect to
geometry changes occurring in the lowest excited S1 and T1 states. The oscillator strength of the S1 ↔ S0 transition f(S1 ↔ S0)
an important parameter for the TADF efficiency being proportional to the radiative ratecan be increased from f(S1 ↔ S0) =
0.0258 for Ag(phen)(P2-nCB) (1) to f(S1 ↔ S0) = 0.0536 for Ag(dbp)(P2-nCB) (4), as calculated for the T1 state optimized
geometries. This parameter governs the radiative TADF decay time (τr) at ambient temperature, found to be τr = 5.6 μs for
Ag(phen)(P2-nCB) (1) but only τr = 1.4 μs for Ag(dbp)(P2-nCB) (4)a record TADF value. In parallel, the
photoluminescence quantum yield (ΦPL) measured for powder samples at ambient temperature is boosted up from ΦPL
=
36% for Ag(phen)(P2-nCB) (1) to ΦPL = 100% for Ag(dbp)(P2-nCB) (4). This is a consequence of a cooperative effect of both
decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P2-nCB) (1),
Ag(idmp)(P2-nCB) (2), and Ag(dmp)(P2-nCB) (3) to Ag(dbp)(P2-nCB) (4). Another parameter important for the TADF
behavior is the activation energy of the S1 state from the state T1, ΔE(S1−T1). Experimentally it is determined for the complexes
Ag(dmp)(P2-nCB) (3) and Ag(dbp)(P2-nCB) (4) to be of moderate size of ΔE(S1−T1) = 650 cm−1.
compounds exhibiting thermally activated delayed fluorescence
(TADF). In TADF compounds the energy of all the excitons is
emitted via the lowest excited singlet state (S1); hence, the
mechanism is called the singlet harvesting mechanism.7−12 This
mechanism is based on a fast thermal equilibration between the
singlet and the triplet state and is related to fast down- and up-
ISC (up-ISC is also called RISC) processes. The efficiency of
T1 → S1 up-ISC strongly depends on the activation energy that
can be approximated to the energy gap between the states,
ΔE(S1−T1). Therefore, a small ΔE(S1−T1) value or at least a
INTRODUCTION
■
A material applied as an emitter in OLEDs should be able to
harvest and convert all singlet and triplet excitons formed into
light.1 There are two main classes of compounds that meet this
requirement. The first class is represented by complexes of
heavy transition metals, which harvest all the excitons in the
lowest triplet state (T1) and emit via a phosphorescence
pathway, according to the triplet harvesting mechanism.2−6 This
mechanism can be realized due to heavy-metal-induced strong
spin−orbit coupling (SOC), which provides fast intersystem
crossing (ISC) from the singlet state to the triplet manifold and
opens the spin-forbidden pathways of the T1 → S0 transitions
(phosphorescence).2,5 The second class is represented by
Received: August 8, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02002
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Figure 1. Structural formulas of the investigated Ag(I) complexes.
Scheme 1. Synthetic Route to the Studied Ag(I) Complexes
moderately small value is of high importance for a compound
to exhibit efficient TADF. A small energy gap ΔE(S1−T1)
requires a small exchange interaction between the unpaired
electrons, which can be achieved when these electrons are
spatially well separated.13−15 This condition can be realized
when the lowest excited states (S1 and T1) are of charge
transfer character. For instance, complexes of Cu(I) with metal
to ligand charge transfer (MLCT) or halide (X) and metal to
ligand charge transfer (XMLCT) character of the lowest excited
states often show TADF.8−10,16−35 Moreover, also pure organic
compounds with charge transfer character of the lowest excited
states exhibit TADF behavior and can be applied in OLEDs,
giving up to 100% internal efficiency.11,36−41
Cu(I) compounds50−59 are also expected to occur in analogous
Ag(I) complexes. Such distortions increase the nonradiative
relaxation rate (knr) of the excited state due to the increased
vibrational overlap of excited state and ground state vibrational
wave functions, having the consequence of reducing the
emission quantum yield.19,50,56,60 Therefore, in the design of
TADF materials it is important to make the molecular
structures rigid, along with fulfilling the electronic requirements
discussed above. Having these in mind (structural rigidity, high
transition rate kr(S1 → S0), and small ΔE(S1−T1)), we
synthesized and investigated a series of Ag(I) complexes,
namely Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2),
Ag(dmp)(P2-nCB) (3), and Ag(dbp)(P2-nCB) (4) (Figure
1) with P2 -nCB
= negatively charged 1,2-bis-
Interestingly, although there are numerous examples of
Cu(I) complexes exhibiting TADF, analogous Ag(I) complexes
rarely follow this trend.25,42−44 Frequently, they exhibit long-
lived phosphorescence from ligand-centered (LC) excited
(diphenylphosphine)-nido-carborane ligand,61 phen = 1,10-
phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp
= 1,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-
1,10-phenanthroline. All of these complexes are electroneutral
and share a similar structure motif, which comprises a
negatively charged P2-nCB ligand and a 1,10-phenanthroline-
based ligand coordinated to a silver cation (Figure 1).
45−48
3
states of ππ* character.
This behavior is explained by
the higher ionization potential of Ag+ (second ionization
potential of the silver atom) in comparison to that of Cu+
because of the greater atomic number of silver.49 This results in
more stabilized d orbitals of Ag(I) complexes. Thus, these are
not involved in transitions that lead to the lowest excited states.
Therefore, for a complex of Ag(I) to show TADF, the d orbitals
should be destabilized and highly contribute to the frontier
occupied orbitals (HOMO, HOMO-1, and HOMO-2) of the
complex. This can be fulfilled using ligands of highly electron
donating character, such as phosphines, and the situation is
better if they carry a negative charge, which along with
enhancing the electron-donating properties would additionally
make the complex electroneutral.
The variation of the energy gap, ΔE(S1−T1), in the series of
complexes seems to be moderate, as the experimentally
obtained values for 3 and 4 are about 650 cm−1. However,
the change in oscillator strength, f(S1 → S0), in the series is
significant. This shows, how the TADF properties can be
strongly modulated by, at first sight, very insignificant structural
changes. In particular, the complex with the most advanced
TADF properties, Ag(dbp)(P2-nCB) (4), exhibits an out-
standingly short TADF decay time of τ = 1.4 μs and
photoluminescence quantum yield reaching unity, as measured
for a powder sample under ambient conditions.
Although an appropriate electronic structure is essential for a
compound to show TADF, it is not enough for a material to be
used as an efficient TADF material for OLED devices. High
quantum yield of emission and short decay time are also
required. It is well-known that emission quantum yields
strongly depend on the molecular rigidity of the com-
pound.19,50 In particular, extensive flattening distortions in
the excited states of charge transfer character as shown for
SYNTHESIS AND STRUCTURES
■
A series of Ag(I) complexes was synthesized in two steps
according to a procedure reported earlier.42,61−63 In the first
step, silver hexafluorophosphate (AgPF6) or silver tetrafluor-
oborate (AgBF4) was stirred in ethanol with o-carborane-
bis(diphenylphosphine) (P2-oCB) prepared in advance.42,64 In
B
DOI: 10.1021/acs.inorgchem.7b02002
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Figure 2. Perspective views (OLEX-265 plots with 50% probability thermal ellipsoids) of the newly synthesized complexes. Hydrogen atoms are
omitted for clarity.
Figure 3. Coordination core geometries of complexes Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), Ag(dmp)(P2-nCB) (3), and Ag(dbp)(P2-
nCB) (4)42 as found by single-crystal X-ray diffraction analysis. The given interplanar angles characterize the deviations from the tetrahedral
geometry (φ = 90°) (not to be confused with the valence angle of 109° in a tetrahedral molecule such as methane).
the second step, the phenanthroline-based ligand is added to
the mixture, which then was stirred under reflux conditions for
1 h. The variety of the phenanthrolines used in the second step
was represented by 1,10-phenanthroline (phen), 4,7-dimethyl-
1,10-phenanthroline (idmp), 2,9-dimethyl-1,10-phenanthroline
(dmp), and 2,9-di-n-butyl-1,10-phenanthroline (dbp). Impor-
tantly, refluxing ethanol is a necessary reaction condition at the
second step for o-carboranebis(diphenylphosphine) to undergo
partial deboration and transform to the negatively charged nido-
carboranebis(diphenylphosphine) (P2-nCB) ligand, which
finally makes the obtained complexes electroneutral.62,63
According to the variety of ligands with a 1,10-phenanthroline
core, the complexes Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-
nCB) (2), and Ag(dmp)(P2-nCB) (3) were obtained (Scheme
1). Ag(dbp)(P2-nCB) (4) was synthesized analogously and was
reported earlier, along with its characterization.42 The obtained
complexes are stable in air under day light and ambient
conditions. Samples under photophysical investigation did not
show any sign of decomposition in 48 h. Synthetic procedures
to give o-carborane-bis(diphenylphosphine) and the newly
obtained complexes and structural characterizations of the
compounds are given in the Supporting Information.
complexes are summarized in Table S2 in the Supporting
Information.
It is noted that in Ag(dmp)(P2-nCB) (3) the steric
interaction of the P2-nCB ligand with alkyl substituents of the
dmp ligand, similar to the case for Ag(dbp)(P2-nCB) (4),42
stabilizes the coordination center at a geometry closer to a
tetrahedron, in comparison to the complexes Ag(phen)(P2-
nCB) (1)61 and Ag(idmp)(P2-nCB) (2). The significantly
weaker steric interaction involving H atoms instead of the alkyl
substituents in the 2- and 9-positions of phenanthroline in both
Ag(idmp)(P2-nCB) (2) and Ag(phen)(P2-nCB) (1) results in a
more flexible geometry at the coordination center. Con-
sequently, the latter two complexes show geometries strongly
deviating from that of a tetrahedron. This effect can be clearly
seen from comparison of angles between planes drawn through
N−Ag−N and P−Ag−P atoms (Figure 3).
It will be shown below that these structural properties and
the related structural flexibility, as concluded on the basis of X-
ray geometries and voids around the coordination centers, can
notably affect the TADF behavior. In particular, according to
TDDFT calculations, the positions of the ligands relative to the
coordination center correlate with the oscillator strengths of the
S1 ↔ S0 transitions, being one of the primary parameters for the
TADF efficiency.
Single crystals for X-ray analysis were grown from CH2Cl2
(DCM) solutions by slow solvent evaporation.
X-ray diffraction analyses of the obtained crystals revealed
molecular structures of complexes 1−4 (Figure 2) with mean
bond lengths of Ag−P and Ag−N of 2.44 and 2.31 Å,
respectively, being unremarkable in comparison to Ag(I)
complexes reported earlier.47,49,61,66−68 In fact, the structural
data obtained for Ag(phen)(P2-nCB) (1) agree very well with
the data reported earlier by Crespo et al.61 The structural data
characterizing the coordination centers of the studied
TADF DECAY TIMES AND CRUCIAL PARAMETERS
■
Before investigating photophysical properties of the complexes
presented above, we briefly discuss the key points important for
TADF characteristics, such as emission decay time and
quantum yield. The occurrence of TADF is due to efficient
down- and up-ISC between the singlet and triplet states. In
complexes of Cu(I) exhibiting TADF the processes of down-
C
DOI: 10.1021/acs.inorgchem.7b02002
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Figure 4. Coordination core geometries of the complexes Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), Ag(dmp)(P2-nCB) (3), and
Ag(dbp)(P2-nCB) (4) taken from the structures optimized (M06/def2-SVP, gas phase) in the electronic ground state (S0) and in the lowest excited
triplet state (T1) for comparison. The interplanar angles between the P−Ag−P (orange) and N−Ag−N (blue) planes are marked.
ISC are very fast52,53,55,56,69−71 (several picoseconds). A similar
behavior is expected to occur for Ag(I) complexes. Fast ISC
processes lead to a fast equilibration between the singlet and
triplet manifolds, usually represented by the S1 and T1 states.
Consequently, monoexponential decay of the emission can
occur. It is important that the process of T1 → S1 up-ISC,
unlike S1 → T1 down-ISC, entails thermal activation from the
T1 state to a vibrational state that is isoenergetic with a
vibrational state of the S1 state. Therefore, the efficiency of T1
→ S1 up-ISC, apart from SOC, depends on the energy gap
ΔE(S1−T1). Accordingly, a smaller ΔE(S1−T1) value favors a
higher T1 → S1 up-ISC rate at a given temperature. This
dependence is explicitly reflected in eq 1, defining the emission
decay time from equilibrated T1 and S1 states at a given
temperature.5,10,16,22,72
T1) was shown for a large number of TADF Cu(I)
complexes.22 Therefore, designing a compound with high
TADF efficiency often means finding the optimal relation
between the k(S1) and ΔE(S1−T1) values.
THEORETICAL INVESTIGATIONS
■
In this section, we present a theoretical introduction of
electronic structures and excited state characteristics of the
complexes presented in this work. DFT computations were
carried out using a DFT approach at the M0676/def2-SVP77
level of theory for geometry optimizations and at the
M062X76/def2-SVP77 level for time-dependent (TD-DFT)
computations, all under gas-phase conditions and utilizing the
Gaussian 0978 code. The calculated ground state geometries of
the complexes, in particular, characteristic angles at coordina-
tion centers, are sensitive to the substituents on the
phenanthroline ligand and their positions. The two alkyl
groups on positions 2 and 9 of the phenanthroline ligands in
the complexes Ag(dbp)(P2-nCB) (4) and Ag(dmp)(P2-nCB)
(3) represent strongly hindering anchors, which lead to ground
state geometries being closer to a tetrahedron in comparison to
complexes Ag(idmp)(P2-nCB) (2) and Ag(phen)(P2-nCB)
(1). This result agrees well with the results from X-ray analyses
discussed above.
3 + exp − ΔE(S −T)
1
1
(
)
1 1
kBT
τ(T) =
3k(T) + k(S1) exp − ΔE(S −T)
1
(
)
kBT
(1)
Herein τ(T) is the emission decay time as a function of
temperature, k(S1) (k(S1 → S0)) and k(T1) are the rates of the
S1 → S0 and T1 → S0, transitions, respectively, ΔE(S1−T1) is
the energy gap between the equilibrated emitting states S1 and
T1, kB is the Boltzmann constant, and T is the temperature.
Thus, according to eq 1 the population of the S1 state from the
T1 state depends exponentially on the ΔE(S1−T1) energy gap.
Consequently, a higher TADF efficiency is expected for a
smaller ΔE(S1−T1) value, as was already discussed many
times.5,10,11,20−22,25,31,37,43,73−75 A small ΔE(S1−T1) energy gap
requires a small exchange interaction between the unpaired
electrons in the excited states. Therefore, the TADF effect
frequently occurs for molecules that exhibit low-lying states of
charge transfer character.
The triplet state (T1) relaxed geometries of the presented
complexes are twisted toward planarization, in comparison to
the ground state geometries. Similar structural changes in the
excited state were also reported for Cu(I) complexes with
charge transfer character of the lowest excited states.50−56,59
This effect is more pronounced for the complexes Ag(phen)-
(P2-nCB) (1) and Ag(idmp)(P2-nCB) (2) with more flexible
molecular geometries than for the relatively rigid Ag(dmp)(P2-
nCB) (3) and Ag(dbp)(P2-nCB) (4) complexes (Figure 4 and
Tables S3 and S4 in the Supporting Information).
Thus, as calculated for gas-phase conditions, the change of
angle between P−Ag−P and N−Ag−N planes (Δφ) from the
ground state to the T1 state is significantly larger in complexes 1
(Δφ ≈ 34.7°) and 2 (Δφ ≈ 34.3°) than in complexes 3 (Δφ ≈
8.4°) and 4 (Δφ ≈ 11.8°). This result agrees well with the
conclusion drawn on the basis of structures obtained by X-ray
analyses showing that complexes 1 and 2 have more flexible
molecular structures in comparison to 3 and 4. Such structural
flexibility and extensive reorganizations in the emitting state
Moreover, a high radiative rate of the S1 → S0 transition
kr(S1) favors a faster depletion of the S1 state, resulting in a
shorter TADF decay time. This parameter is explicitly included
in eq 1 as a pre-exponential factor. A large k(S1) value means a
good overlap of the orbitals involved in the transition, such as
the HOMO and LUMO. However, a good overlap of the
orbitals increases the singlet−triplet splitting ΔE(S1−T1),
consequently decreasing the up-ISC rate and the TADF
efficiency. Correlation of the parameters k(S1) and ΔE(S1−
D
DOI: 10.1021/acs.inorgchem.7b02002
Inorg. Chem. XXXX, XXX, XXX−XXX