Design Strategy for Ag(I)-Based Thermally Activated Delayed Fluorescence Reaching an Efficiency Breakthrough

Article abstract of DOI:10.1021/acs.chemmater.6b05175

A design strategy for the development of Ag(I)-based materials for thermally activated delayed fluorescence (TADF) is presented. Although Ag(I) complexes usually do not show TADF, the designed material, Ag(dbp)(P₂-nCB) [dbp = 2,9-di-n-butyl-1,10-phenanthroline, and P₂-nCB = nido-carborane-bis(diphenylphosphine)], shows a TADF efficiency breakthrough exhibiting an emission decay time of τ(TADF) = 1.4 μs at a quantum yield of Φ_PL = 100%. This is a consequence of three optimized parameters. (i) The strongly electron-donating negatively charged P₂-nCB ligand destabilizes the 4d orbitals and leads to low-lying charge (CT) states of MLL′CT character, with L and L′ being the two different ligands, thus giving a small energy separation between the lowest singlet S₁ and triplet T₁ state of ΔE(S₁-T₁) = 650 cm^-1 (80 meV). (ii) The allowedness of the S₁ → S₀ transition is more than 1 order of magnitude higher than those found for other TADF metal complexes, as shown experimentally and by time-dependent density functional theory calculations. Both parameters favor a short TADF decay time. (iii) The high quantum efficiency is dominantly related to the rigid molecular structure of Ag(dbp)(P₂-nCB), resulting from the design strategy of introducing n-butyl substitutions at positions 2 and 9 of phenanthroline that sterically interact with the phenyl groups of the P₂-nCB ligand. In particular, the shortest TADF decay time of τ(TADF) = 1.4 μs at a Φ_PL value of 100%, reported so far, suggests the use of this outstanding material for organic light-emitting diodes (OLEDs). Importantly, the emission of Ag(dbp)(P₂-nCB) is not subject to concentration quenching. Therefore, it may be applied even as a 100% emission layer.

Full text of DOI:10.1021/acs.chemmater.6b05175

Subscriber access provided by NEW MEXICO STATE UNIV
Article
Design strategy for Ag(I)-based thermally activated
delayed fluorescence reaching an efficiency breakthrough
Marsel Z. Shafikov, Alfiya F. Suleymanova, Rafal Czerwieniec, and Hartmut Yersin
Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 19 Jan 2017
Downloaded from http://pubs.acs.org on January 20, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth
Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 24
Chemistry of Materials
1
2
3
4
5
Design strategy for Ag(I)-based thermally activated delayed
fluorescence reaching an efficiency breakthrough
6
7
Marsel Z. Shafikov†‡, Alfiya F. Suleymanova†§, Rafał Czerwieniec†*, and Hartmut Yersin†*
8
9
†
Institut für Physikalische und Theoretische Chemie, Universität Regensburg,
Universitätsstrasse 31, Dꢀ93053 Regensburg, Germany
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
‡
Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia
§
I. Postovsky Institute of Organic Synthesis, Ekaterinburg, 620041, Russia.
Abstract
A design strategy is presented for the development of Ag(I)ꢀbased materials for
thermally activated delayed fluorescence (TADF). Although Ag(I) complexes usually do
not show TADF, the designed material, Ag(dbp)(P2ꢀnCB) (with dbp = 2,9ꢀdiꢀnꢀbutylꢀ
1,10ꢀphenanthroline and P2ꢀnCB = nidoꢀcarboraneꢀbisꢀ(diphenylphosphine)), shows a
TADF efficiency breakthrough exhibiting an emission decay time of τ(TADF) = 1.4 ꢁs at
a quantum yield of ΦPL = 100 %. This is a consequence of three optimized parameters:
(i) The strongly electronꢀdonating negatively charged P2ꢀnCB ligand destabilizes the 4dꢀ
orbitals and leads to low lying charge (CT) states of MLL’CT character, with L and L’
being the two different ligands, thus, giving a small energy separation between the
lowest singlet S1 and triplet T1 state of ∆E(S1ꢀT1) = 650 cm−1 (80 meV). (ii) The
allowedness of the S1→S0 transition is more than one order of magnitude higher than
found for other TADF metal complexes, as shown experimentally and by TDꢀDFT
calculations. Both parameters favor short TADF decay time. (iii) The high quantum
efficiency is dominantly related to the rigid molecular structure of Ag(dbp)(P2ꢀnCB),
resulting from the design strategy of introducing nꢀbutyl substitutions at the 2,9ꢀ
positions of phenanthroline which sterically interact with the phenyl groups of the P2ꢀ
nCB ligand. In particular, the shortest TADF decay time of τ(TADF) = 1.4 ꢁs at ΦPL
=
100 % reported so far suggests the use of this outstanding material for OLEDs.
1
ACS Paragon Plus Environment

Chemistry of Materials
Page 2 of 24
1
2
3
4
Importantly, the emission of Ag(dbp)(P2ꢀnCB) is not subject to concentration quenching.
Therefore, it may be applied even as a 100% emission layer.
5
6
7
1. Introduction
8
9
Fundamental research focusing on photophysical and chemical properties of organoꢀ
transition metal complexes is strongly stimulated by potential applications, in particular,
in the field of electroꢀluminescent devices, such as emitters for OLEDs1ꢀ15 or LEECs.7,16ꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
19
For the emitters, it is crucial that all generated singlet and triplet excitons20 are
harvested and transferred into light. This can be achieved by two different mechanisms:
(i) Complexes that exhibit high spinꢀorbitꢀcoupling (SOC) with respect to the lowest
excited triplet state T1, representing triplet emitters, allow to harvest all excitons in the
lowest triplet state.21ꢀ23 (ii) Compounds that show thermally activated delayed
fluorescence (TADF) harvest also all excitons, but the emission occurs essentially via
the thermally activated singlet state S1.24ꢀ28
Development of TADF compounds is currently under heavy research, since such
materials can be realized with lowꢀcost and environmentally friendly Cu(I)
complexes9,24,25,29ꢀ44 as well as with purely organic molecules.26,45,46 For organoꢀ
transition metal compounds, the occurrence of TADF is crucially related to metalꢀtoꢀ
ligand charge transfer (MLCT) states having frontier orbitals, HOMO and LUMO, that
are spatially well separated. This leads to a small exchange interaction47,48 between the
involved electrons and hence, to a small splitting ∆E(S1ꢀT1) between the lowest excited
singlet state S1 and triplet state T1. A small ∆E(S1ꢀT1) value is a necessary condition for
obtaining a short (radiative) TADF decay time, being important for minimizing rollꢀoff
effects and stability problems in OLEDs. Additionally, the allowedness of the transition
from the S1 state to the ground state (S0), S1→S0, that is thermally activated from the
lower lying triplet state plays a crucial role. However, basic quantum chemical
considerations suggest that the corresponding rate k(S1→S0) and ꢀE(S1ꢀT1) are
2
ACS Paragon Plus Environment

Page 3 of 24
Chemistry of Materials
1
correlated. Small splitting ꢀE(S1ꢀT1) requires that the exchange interaction between the
unpaired electrons is small. For this, small overlap of HOMO and LUMO is
advantageous. At the same time, small HOMOꢀLUMO overlap leads to a small
oscillator strength of the S1→S0 transition, and thus, to small k(S1→S0) and long
fluorescence decay time. Indeed, experimental studies on Cu(I) complexes showing
TADF reveal that such a correlation exists for a large number of compounds.34 Thus,
engineering of TADF materials with shorter TADF decay times is a challenge. For
instance, TADF decay times of less than a few ꢁs have not been reported so
far29,32,36,39,49 In particular, there are no reports on (radiative) decay times shorter than 3
ꢁs.34,49,50
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In this investigation, we focus on a new Ag(I) complex that opens a breakthrough in
designing a TADF material with a significantly shorter radiative TADF decay time. In
contrast to Cu(I) based compounds, reports on Ag(I) based TADF materials are
scarce.37,50,51 This is a consequence of the higher oxidation potential of the Ag+ ion
compared to the Cu+ ion.52 Accordingly, the 4dꢀorbitals of Ag(I) complexes mostly lie
3
energetically below ligandꢀcentered (LC) orbitals. This leads to lowꢀlying states of LC
character.53ꢀ56 Therefore, Ag(I) complexes often do not show TADF, but longꢀlived
phosphorescence and sometimes even slow intersystem crossing (ISC) resulting in
dual emission.55 Obviously, designing Ag(I) complexes that exhibit efficient and shortꢀ
lived TADF represents an optimization challenge. For this aim, it is required to
destabilize the energetically deepꢀlying 4dꢀorbitals by an organic ligand with good
electronꢀdonating properties. Accordingly, we choose a silver complex containing a
chelating nidoꢀcarboraneꢀbisꢀ(diphenylphosphine) (P2ꢀnCB) ligand57 (structure shown in
Scheme 1). The phosphine coordination induces substantial electronꢀdonating
character, which is further strongly enhanced by the negative charge of the nidoꢀ
carborane moiety. In combination with the chromophoric 2,9ꢀdiꢀnꢀbutylꢀ1,10ꢀ
3
ACS Paragon Plus Environment

Chemistry of Materials
Page 4 of 24
1
2
3
4
5
6
7
8
phenanthroline (dbp) ligand, we obtain the neutral Ag(dbp)(P2ꢀnCB) complex.
Moreover, this compound is relatively rigid due to the rigid nidoꢀcarborane cage and, in
particular, the sterically demanding 2,9ꢀdiꢀnꢀbutyl substitutions at the phenanthroline
ligand. This allows us to expect high emission quantum yield.31 Here we present the
synthesis of the complex, characterize it by TDꢀDFT calculations and by detailed
photophysical measurements investigating properties over a wide temperature range
(20 K ≤ T ≤ 300 K). In particular, it will be shown that the resulting Ag(dbp)(P2ꢀnCB)
complex exhibits 100 % emission quantum yield and an extraordinarily short radiative
TADF decay time, being significantly shorter than reported for any other TADF material
so far.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2. Synthesis and molecular structure
Ag(dbp)(P2ꢀnCB) was obtained by reacting equimolar amounts of silver
hexafluorophosphate (AgPF6), orthoꢀcarboraneꢀbis-(diphenylphosphine) (P2ꢀoCB),58
and 2,9ꢀdiꢀnꢀbutylꢀ1,10ꢀphennanthroline (dbp) in refluxing ethanol, in analogy to
literature procedures57,59,60 (Scheme 1). Under these reaction conditions, orthoꢀ
carborane of the cationic Ag(dbp)(P2ꢀoCB)+ complex (not isolated) undergoes partial
deboronation leading to the respective nidoꢀcarborane (nCB) anion,59 thus affording the
neutral Ag(dbp)(P2ꢀnCB) complex. Synthetic procedure and structural characterization
of Ag(dbp)(P2ꢀnCB) are given in the ESI.
EtOH
Ph
N
P
N
Et2O
nꢀBuLi
AgPF6
dbp
Li
Li
H
H
P
C
C
C
C
P(Ph2)Cl
C
C
Ph
Ph
Ag
Ph
Ph
Ph
Ph
−B
P
C
P
C
Ph
= B
Ph = C6H5
dbp = 2,9ꢀdiꢀnꢀbutylꢀ1,10ꢀphenanthroline
P2ꢀoCB
Ag(dbp)(P2ꢀnCB)
Scheme 1. Synthetic route to complex Ag(dbp)(P2ꢀnCB)
4
ACS Paragon Plus Environment

Page 5 of 24
Chemistry of Materials
1
2
3
4
5
6
7
8
Xꢀray single crystal diffraction study reveals the Ag(dbp)(P2ꢀnCB) molecule (Figure 1) in
which the silver ion is chelated by the two ligands, dbp and P2ꢀnCB, in a distorted
tetrahedral coordination. The mean bond lengths to the metal center PꢀAg and NꢀAg of
2.45 Å and 2.32 Å, respectively, are unremarkable compared to other silver
complexes52,54,57,61,62 and are larger than in the analogous Cu(I) complexes due to the
larger ion size of Ag+ as compared to Cu+. The bite angles PꢀAgꢀP and NꢀAgꢀN of 86°
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
and 73°, respectively, are smaller than in analogous Cu(I) complexes.63 Voids between
the phenyl groups of P2ꢀnCB are partly occupied by the nꢀbutyl chains of dbp. Owing to
such steric effects, the arrangement of the ligand side groups is expected to rigidify the
structure and thus, to restrain excitedꢀstate geometry distortions.64ꢀ70
Figure 1. Chemical structure and perspective view (OLEXꢀ271 plot with
50% probability thermal ellipsoids) of Ag(dbp)(P2ꢀnCB). Hydrogen atoms
are omitted for clarity. For further data see the ESI.
3. Theoretical calculations based on density functional theory (DFT) and time
dependent DFT (TD-DFT) methods
By use of DFT and TDꢀDFT calculations, we can obtain an insight into the electronic
structure of Ag(dbp)(P2ꢀnCB). The calculations were carried out at the M0672/def2SVP73
level of theory for geometry optimizations and at the M062X72/def2SVP level for timeꢀ
dependent calculations (See the ESI for details). The optimized geometry of the ground
5
ACS Paragon Plus Environment

Chemistry of Materials
Page 6 of 24
1
2
3
4
5
6
7
8
sate (S0) is found to be in a good agreement with the experimental XꢀRay geometry
(Table S1 in the ESI). Calculations performed for both geometries, S0 and T1, reveal
that the lowest electronic transitions, corresponding to states S1 and T1, involve
electron density shifts from the metal, the phosphorus atoms, and the nidoꢀcarboranyl
anion of the P2ꢀnCB ligand to the phenanthroline core of the dbp ligand. This charge
transfer results in a flattening distortion of the pseudoꢀtetrahedral ground state
geometry as frequently described for Cu(I) complexes.64ꢀ70,74,75 Since the orbital origins
of the states T1 and S1 are similar from the perspective of the ground state and mainly
emission properties are discussed, we focus on the T1 state geometry. In this geometry,
the S0→S1 excitation is dominated by the HOMO→LUMO transition (92 %) perturbed
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
by HOMOꢀ1→LUMO character (4 %). (Figure 2 and Table S2 in the ESI) The HOMO is
essentially composed of the metal (13 %) and phosphorus orbitals (47 %) (engaged in
bonding to Ag), while the LUMO represents a π* orbital of the dbp ligand (Figure 2 and
Table S3 in the ESI). The T1 state is derived from almost the same oneꢀelectron
excitations as the state S1. Thus, the theoretical predictions allow us to assign the two
lowest excited states as 1,3(MLL’CT) states, wherein L and L’ represent P2ꢀnCB and
dbp, respectively. The calculated energy separation ꢀE(S1ꢀT1), obtained as the
difference of vertical excitation energies amounts to 0.15 eV (≈ 1200 cm−1) and thus is
somewhat larger than the experimentally determined activation energy (Section 4).
6
ACS Paragon Plus Environment

Page 7 of 24
Chemistry of Materials
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Isoꢀsurface contour plots (isoꢀvalue = 0.05) of frontier orbitals of Ag(dbp)(P2ꢀ
nCB), calculated at the M062X/Def2ꢀSVP level of theory in the T1 excited state
geometry. Hydrogen atoms are omitted for clarity. For further data see the ESI.
The calculations performed for the triplet state optimized geometry reveal further that
the S2 and S3 states result mainly from the HOMO→LUMO+1 and HOMOꢀ1→LUMO
transitions, respectively. Since S2 carries the same type of metal 4d character as T1
(and as S1) and since S3 is largely of ligandꢀtoꢀligand charge transfer character with no
significant contributions from the metal, spinꢀorbit admixtures of these states (S2 and
S3) to the T1 state are expected to be small.76ꢀ78 The nearest singlet state with metal 4d
contribution different from that in the T1 state is S4 (largely resulting from HOMOꢀ2 →
LUMO). It lies 1.57 eV above the T1 level. For such a large energy difference, SOC is
expected to be very small. (Compare refs. 34,49,76ꢀ78) Thus, a very long phosphorescence
decay time is expected to occur. This was experimentally observed, as shown in
Section 4.
Importantly, the calculations predict a relatively high oscillator strength for the S0→S1
transition of ƒ = 0.0536 (Table S1 in the ESI). For comparison, the ƒ values for chargeꢀ
transfer transitions of Cu(I) complexes are usually more than one order of magnitude
7
ACS Paragon Plus Environment

Chemistry of Materials
Page 8 of 24
1
2
3
smaller.42,49 Indeed, the high allowedness of the S0↔S1 transition correlates well with
4
5
the very high fluorescence rate as determined experimentally (Section 4).
6
7
8
9
4. Photophysical characterization of Ag(dbp)(P2-nCB)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4.1 Spectroscopic introduction
In Figure 3, we present a spectroscopic introduction to properties of Ag(dbp)(P2ꢀnCB).
The absorption spectrum shows strong absorption bands in the UV region with maxima
at 232 nm (57100 M−1—cm−1) and 273 nm (37700 M−1—cm−1) and a weaker band
centered at 385 nm (2100 M−1—cm−1). The short wavelength transitions are dominated
by πꢀπ* transitions within the dbp and P2ꢀnCB ligands, while the long wavelength
absorption is assigned, in analogy to mixedꢀligand phenanthrolineꢀCu(I)
complexes,36,63,79ꢀ84 to transitions with distinct charge transfer (CT) character. TDꢀDFT
calculations predict a MLL’CT type of transition (Section 3).
Figure 3. Absorption spectrum of Ag(dbp)(P2ꢀnCB) measured in dichloromethane
(DCM) at 300 K (black line) and emission spectra of Ag(dbp)(P2ꢀnCB) shown in
8
ACS Paragon Plus Environment

Page 9 of 24
Chemistry of Materials
1
colored lines (λexc = 410 nm) measured at different conditions. The DCM solutions
were of ≈ 10−5 M concentration. The PMMA film was doped with ≈ 1 wt.% of the
complex.
2
3
4
5
6
7
8
9
The emission spectra displayed in Figure 3 are broad and unstructured even if
cooled to T = 40 K (or 1.6 K, not reproduced). The emission of Ag(dbp)(P2ꢀnCB) doped
in PMMA (λmax = 535 nm) and dissolved in DCM (λmax = 585 nm) is redꢀshifted
compared to the emission measured in powder by 9 nm and 59 nm, respectively. This
is related to a flattening distortion of Ag(dbp)(P2ꢀnCB) induced by the CT excitation, as
predicted by the theoretical calculations presented in Section 3. In particular, the data
show that the distortionꢀrelated red shift becomes more pronounced with decreasing
rigidity of the environment. (The blue shift of 14 nm as observed on cooling is a
consequence of the TADF effect and is discussed below.) Moreover, usually such
structural changes are connected with an increase of nonꢀradiative decay rates or
reductions of the emission quantum yields due to increasing FranckꢀCondon (FC)
factors of higher excited vibrational wavefunctions of the electronic ground state and of
energetically lower vibrational wavefunctions of the electronically excited state.48,85
Indeed, this is also observed for Ag(dbp)(P2ꢀnCB), for which the quantum yield
decreases (under degassed conditions) from ФPL(powder) = 100% to ФPL(PMMA) = 85
% and to ФPL(DCM) = 3 %. Similar trends have already been reported for Cu(I)
compounds24,34, though the decrease of ФPL is usually more distinct than found for
Ag(dbp)(P2ꢀnCB). Obviously, the title compound is rather rigid due to its specific
molecular structure. The data of the emission peak positions and the quantum yields
are summarized in Table 1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4.2 Thermally activated delayed fluorescence and drastic increase of the
emission decay rate
For a deeper photophysical characterization of the TADF properties, investigation of the
broad emission spectra is not promising. However, the alternative spectroscopic
method, based on temperatureꢀ and timeꢀresolving measurements can be applied.
Presumably this is the only procedure that allows us to obtain detailed insight into
electronic structures in such cases. Corresponding studies are carried out for
Ag(dbp)(P2ꢀnCB) powders.86
9
ACS Paragon Plus Environment

Chemistry of Materials
Page 10 of 24
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Left: luminescence decay times (τ) of Ag(dbp)(P2ꢀnCB) powder measured
at different temperatures; Right: The luminescence decay times (τ) plotted versus
temperature. The values of k(S1→S0) = 5.6—107 s−1 (18 ns) and ꢀE(S1−T1) = 650 cm−1
result from a fit of eq. 1 to the experimental τ(T) values, with τ(T1) fixed to 1570 ꢁs as
determined directly for T < 60 K (plateau).
Figure 4 (left) displays emission decay curves measured at different temperatures. By
use of the emission quantum yields and the decay times (Table 1), one can determine
that the radiative decay rate kr = ФPL/τ increases from kr(40 K) = 5.5—102 s−1 (assuming
the same quantum yield as found for T = 77) to kr(300 K) = 7.1—105 s−1. This
corresponds to an increase by factor of almost 1300. Obviously, such changes have to
be related to the involvement of different electronic transitions at low and high
temperature, respectively. They correspond to emissions from the T1 and S1 state to the
ground state, respectively. Moreover, the decays are monoꢀexponential. Thus, we can
conclude that the ISC time is fast (compare also refs.66,69,87) and that the two states are
thermally equilibrated faster than the measured decay times. In this situation, the
emission decay time τ(T) of the system of two states can be expressed by:9,25,34,78,88
ꢁ
ꢏ
ꢂ
ꢌ
∆ꢊ ꢋ ꢍꢎ
ꢌ
ꢄꢅꢆꢇꢈ ꢉ
ꢎ
ꢐ
ꢁ ꢂ
ꢀ T ꢃ
(1)
ꢁ
ꢂ
ꢌ
∆ꢊ ꢋ ꢍꢎ
ꢌ
ꢁ
ꢂ
ꢁ
ꢂ
ꢌ
ꢄꢑ ꢒ ꢅꢑ ꢓ ꢆꢇꢈ ꢉ
ꢌ
ꢏ
ꢎ
ꢐ
wherein k(T1) = 1/τ(T1) and k(S1) = 1/τ(S1) are the decay rates with the decay times
τ(T1) and τ(S1) of the triplet and singlet excited state, respectively, ꢀE(S1−T1) is the
energy separation between the S1 and T1 state and kB is the Boltzmann constant.
In Figure 4 (right), the decay time is plotted versus temperature. At low temperature, 20
K ≤ T ≤ 60 K, a constant value of 1570 ꢁs is observed (plateau). It represents the
phosphorescence decay time τ(T1) for the T1→S0 transition. Phosphorescence decay
10
ACS Paragon Plus Environment

Page 11 of 24
Chemistry of Materials
1
2
3
4
5
6
7
8
times of the order of milliseconds were found also for other Ag(I) and Cu(I)
compounds.29,34,49,51,74 This long decay displays the spinꢀforbiddenness of this
transition. Obviously, SOC to adequate singlet states is weak. According to the
discussion presented in Section 3, the next singlet state (S4) that exhibits a different 4d
orbital character than the T1 state and that can induce SOC (following general quantum
mechanical rules76ꢀ78) is energetically far, with ꢀE(S4ꢀT1) = 1.57 eV. Hence, the singlet
character mixed into the T1 state is very small.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
For T > 60 K, the decay time becomes shorter with increasing temperature due to a
thermal population of the higher lying S1 state. A plot of the τ values versus
temperature has a characteristic form of an sꢀshaped curve with the point of maximum
slope at T = 90 K (τ = 870 ꢁs) (Figure 4). For T > 200 K, the τ(T) values decrease only
slightly and reach at ambient temperature τ(300 K) = 1.4 ꢁs. The τ(T) data can be
perfectly fitted to eq. (1) (Figure 4) giving the activation energy of ꢀE(S1ꢀT1) = 650 cm−1
and the radiative rate of the prompt fluorescence of kr(S1→S0) = 5.6—107 s−1 formally
corresponding to the prompt fluorescence decay time of τ(S1) = 18 ns. It is remarked
that the related prompt fluorescence is not directly observed, since the ISC processes
from S1 to T1 are about three orders of magnitude faster (Compare refs.66,69,87).
The experimental value of k(S1→S0) = 5.6—107 s−1 found for the prompt fluorescence
rate is remarkably large. Cu(I) complexes investigated so far (and that have
comparable ꢀE(S1ꢀT1) splittings) exhibit only rates being more than one order of
magnitude smaller.34 Thus, the large (prompt) fluorescence rate is identified as the key
factor leading to the exceptionally fast TADF decay time of Ag(dbp)(P2ꢀnCB). This
behavior fits perfectly to the large oscillator strength that is calculated for the S1→S0
transition (Section 3). The theoretical value of oscillator strength is also more than one
order of magnitude larger than for any other TADF complex reported so far (Section
3).42,49
11
ACS Paragon Plus Environment

Chemistry of Materials
Page 12 of 24
1
2
3
4
5
6
7
8
Table 1 summarizes the emission data that characterize Ag(dbp)(P2ꢀnCB). Due to the
extraordinarily large radiative TADF decay rate of kr(300 K) = 7.1—105 s−1, the emission
quantum yield reaches ФPL(TADF) = 100 %. Interestingly at T = 77 K, the quantum yield
amounts only to ФPL(77 K) = 87 %, since at that temperature the decay time is as long
as τ(77 K) = 1300 ꢁs giving a radiative rate of kr(77 K) = 6.7—102 s−1. According to knr =
(1ꢀФPL)/τ, the nonꢀradiative rate is determined to knr = 1—102 s−1. Thus, at T = 77 K the
nonꢀradiative process can moderately compete with the radiative process, but not at
ambient temperature.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Emission data of Ag(dbp)(P2ꢀnCB) in
different environments.
powder
526 nm
100 %
PMMA
535
CH2Cl2
585
λmax (300 K)
ФPL (300 K)
τ(300 K)
85 %
3 %
1.4 ꢂs
kr(300 K)
ФPL (77 K)
τ(77 K)
7.1—105 s−1
87 %
1300 ꢂs
6.7—102 s−1
1—102 s−1
1570 ꢂs
kr(77 K)
knr(77 K)
τ(T1, 40 K)
kr(S1→S0)α
∆E(S1→T1)α
5.6—107 s−1
(18 ns)
650 cm−1
α
determined from the fit of experimental
luminescence decay times according to eq. 1,
measured for a powder sample of Ag(dbp)(P2ꢀnCB)
at different temperatures.
4.3 TADF highlights
12
ACS Paragon Plus Environment

Page 13 of 24
Chemistry of Materials
1
2
3
4
5
6
7
8
In Figure 5, we highlight the Ag(dbp)(P2ꢀnCB) material´s TADF properties. At low
temperature (T < 60 K), one observes only longꢀliving phosphorescence as T1→S0
transition decaying with τ = 1570 ꢁs. With temperature increase (kBT), upꢀISC (or
reverse ISC, RISC) leads to a population of the S1 state that lies higher in energy than
T1. Accordingly, a blue shifted TADF emission of 14 nm (≈ 500 cm−1) is found (Figure
3). This value corresponds well to the activation energy of ꢀE(S1ꢀT1) = 650 cm−1
resulting from the decay times’ analysis (Figure 4).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Since, the S1→S0 transition rate is much higher than of any other organoꢀmetallic TADF
material, the TADF decay time drops to the extremely small value of τ(TADF, 300 K) =
1.4 ꢁs. As a consequence of the related high radiative TADF decay rate, the emission
quantum yield reaches 100% at ambient temperature.
Figure 5. Photophysical properties of Ag(dbp)(P2ꢀnCB) as a powder sample shown
on a simplified energy diagram. Sometimes upꢀISC is also called reverse ISC
(RISC).
5. Summarizing conclusions
Thermally activated delayed fluorescence is known at least since the pioneering work of
Parker et al.89 But only recently, a deeper scientific interest was stimulated by
applicability of TADF materials as emitters in OLEDs, as proposed first in the patent
literature.27 In particular, it became a challenge to develop efficient TADF materials
based on purely organic molecules26,45,46 as well as on organoꢀtransition metals
13
ACS Paragon Plus Environment

Chemistry of Materials
Page 14 of 24
1
2
3
4
5
6
7
8
complexes. Within the manifold of transition metal compounds, so far mainly Cu(I)33,34
complexes with distinct MLCT character in the lowest excited states were subject of
detailed investigations. Only a few TADF reports exist for Pd(0),90 Pt(0),90,91 and
Ag(I)37,50,51 based compounds. In all cases, lowꢀlying chargeꢀtransfer (CT) transitions
result in relatively small exchange splittings and hence, relatively small singletꢀtriplet
energy separations ꢀE(S1ꢀT1) as required for efficient TADF materials.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
It is a challenge to design Ag(I) complexes with lowꢀlying CTꢀtype excited states that
involve significant metal 4dꢀcharacter, since most compounds exhibit lowꢀlying 1,3ππ*
states (that do not show TADF).53 This problem could be resolved by designing a Ag(I)
complex with a strongly electronꢀdonating negatively charged diphosphineꢀnidoꢀ
carborane (P2ꢀnCB) ligand.
For TADF materials, in particular, if applied in OLEDs, it is a further challenge to
minimize the TADF decay time (at high emission quantum yield). Eq. (1) shows that
three parameters govern this decay time: (i) ꢀE(S1ꢀT1) should be small, as is frequently
discussed.26,33,34,37,43,46,49,50,92,93 (ii) The phosphorescence decay rate k(T1→S0) should
be high, which can be obtained for compounds that show high SOC with respect to the
lowest triplet state.30,32ꢀ34 (iii) The allowedness of the S1↔S0 transition, described by the
rate of the prompt fluorescence k(S1→S0) or the corresponding oscillator strength
should be high, however, without increasing ꢀE(S1ꢀT1). For the first time this problem is
addressed and solved. For the title compound, experimental as well as TDꢀDFT
calculations reveal that the radiative rate kr(S1→S0) (or the oscillator strength) are more
than one order of magnitude higher than found for any other organoꢀmetallic TADF
material with similar ꢀE(S1ꢀT1) splitting.
In conclusion, the high S1→S0 allowedness found for the title compound leads to the
attractive emission properties of Ag(dbp)(P2ꢀnCB), exhibiting a quantum yield of ФPL
=
100 % at a decay time of τ(TADF) = 1.4 ꢁs. This represents the shortest radiative
14
ACS Paragon Plus Environment

Page 15 of 24
Chemistry of Materials
1
2
3
4
5
6
7
decay time of any TADF material reported so far. Moreover, this is the first TADF
material with a radiative decay time being comparable to Ir(III) complexes78,94 applied in
OLEDs.
8
9
ASSOCIATED CONTENT
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
*Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website DOI:
Experimental details, synthetic procedures, compound characterizations of the
synthesized ligand and complex, photophysical instrumentation, Xꢀray data,
computational details, calculated geometries in XYZ format, excitedꢀstate energies,
dominant orbital excitations from TDꢀDFT calculations, contour plots of the molecular
orbitals that participate in the formation of the discussed excited states, calculated
molecular orbital compositions (PDF) (TXT) (CIF)
Acknowledgements
The authors thank the German Ministry of Education and Research for financial support
in the scope of the cyCESH project (FKN 13N12668). MZS gratefully acknowledges
Professor Duncan Bruce (York) and The University of York for providing computational
facilities. RC thanks the European Research Council (ERC) for support in the
framework of the MSCA RISE Project no. 645628. AS acknowledges the German
Academic Exchange Service (DAAD) for support.
References
(1) Yersin, H., Highly Efficient OLEDs with Phosphorescent Materials. WileyꢀVCH:
Weinheim, 2008.
(2) Brütting, W.; Adachi, C., Physics of Organic Semiconductors. WileyꢀVCH: Weinheim,
2012.
(3) Kim, Y.; Park, S.; Lee, Y. H.; Jung, J.; Yoo, S.; Lee, M. H. Homoleptic TrisꢀCyclometalated
Iridium Complexes with Substituted oꢀCarboranes: Green Phosphorescent Emitters for Highly
Efficient SolutionꢀProcessed Organic LightꢀEmitting Diodes. Inorg. Chem. 2016, 55, 909ꢀ917.
15
ACS Paragon Plus Environment

Chemistry of Materials
Page 16 of 24
1
2
3
4
5
6
7
8
(4) Liu, Z.; Qiu, J.; Wei, F.; Wang, J.; Liu, X.; Helander, M. G.; Rodney, S.; Wang, Z.; Bian,
Z.; Lu, Z.; Thompson, M. E.; Huang, C. Simple and High Efficiency Phosphorescence Organic
LightꢀEmitting Diodes with Codeposited Copper(I) Emitter. Chem. Mater. 2014, 26, 2368ꢀ2373.
(5) Zhang, X. Q.; Xie, Y. M.; Zheng, Y.; Liang, F.; Wang, B.; Fan, J.; Liao, L. S. Highly
phosphorescent platinum(II) complexes based on rigid unsymmetric tetradentate ligands. Org.
Electron. phys. mater. appl. 2016, 32, 120ꢀ125.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(6) Kim, J.; Lee, K. H.; Lee, S. J.; Lee, H. W.; Kim, Y. K.; Kim, Y. S.; Yoon, S. S. Red
Phosphorescent BisꢀCyclometalated Iridium Complexes with Fluorineꢀ, Phenylꢀ, and
FluorophenylꢀSubstituted 2ꢀArylquinoline Ligands. Chem. Eur. J. 2016, 22, 4036ꢀ4045.
(7) Wong, M. Y.; Hedley, G. J.; Xie, G.; Kölln, L. S.; Samuel, I. D. W.; Pertegás, A.; Bolink, H.
J.; ZysmanꢀColman, E. LightꢀEmitting Electrochemical Cells and SolutionꢀProcessed Organic
LightꢀEmitting Diodes Using Small Molecule Organic Thermally Activated Delayed
Fluorescence Emitters. Chem. Mater. 2015, 27, 6535ꢀ6542.
(8) Minaev, B.; Baryshnikov, G.; Agren, H. Principles of phosphorescent organic light emitting
devices. Phys. Chem. Chem. Phys. 2014, 16, 1719ꢀ1758.
(9) Yersin, H.; Rausch, A. F.; Czerwieniec, R., Organometallic Emitters for OLEDs. Triplet
Harvesting, Singlet Harvesting, Case Structures, and Trends. In Physics of Organic
Semiconductors, 2 ed.; Brütting, W.; Adachi, C., Eds. WileyꢀVCH: Weinheim, 2012; pp 371ꢀ
425.
(10) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Management
of singlet and triplet excitons for efficient white organic lightꢀemitting devices. Nature 2006,
440, 908ꢀ912.
(11) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% internal
phosphorescence efficiency in an organic light emitting device. J. Appl. Phys. 2001, 90, 5048ꢀ
5051.
(12) Lamansky, S.; Djurovich, P.; Murphy, D.; AbdelꢀRazzaq, F.; Lee, H. E.; Adachi, C.;
Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Highly phosphorescent bisꢀcyclometalated
iridium complexes: Synthesis, photophysical characterization, and use in organic light emitting
diodes. J. Am. Chem. Soc. 2001, 123, 4304ꢀ4312.
(13) Kim, S. Y.; Jeong, W. I.; Mayr, C.; Park, Y. S.; Kim, K. H.; Lee, J. H.; Moon, C. K.;
Brütting, W.; Kim, J. J. Organic lightꢀemitting diodes with 30% external quantum efficiency
based on a horizontally oriented emitter. Adv. Func. Mater. 2013, 23, 3896ꢀ3900.
(14) Schmidt, T. D.; Reichardt, L. J.; Rausch, A. F.; Wehrmeister, S.; Scholz, B. J.; Mayr, C.;
Wehlus, T.; Ciarnáin, R. M.; Danz, N.; Reusch, T. C. G.; Brütting, W. Extracting the emitter
16
ACS Paragon Plus Environment

Page 17 of 24
Chemistry of Materials
1
2
3
4
orientation in organic lightꢀemitting diodes from external quantum efficiency measurements.
Appl. Phys. Lett. 2014, 105, DOI: 10.1063/1.4891680.
5
6
7
8
(15) Cherpak, V.; Stakhira, P.; Minaev, B.; Baryshnikov, G.; Stromylo, E.; Helzhynskyy, I.;
Chapran, M.; Volyniuk, D.; TomkutéꢀLukšiené, D.; Malinauskas, T.; Getautis, V.;
Tomkeviciene, A.; Simokaitiene, J.; Grazulevicius, J. V. Efficient "warmꢀwhite" OLEDs based
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
on the phosphorescent bisꢀcyclometalated iridium(III) complex. J. Phys. Chem. C 2014, 118
,
11271ꢀ11278.
(16) Yang, C. H.; Beltran, J.; Lemaur, V.; Cornil, J.; Hartmann, D.; Sarfert, W.; Fröhlich, R.;
Bizzarri, C.; De Cola, L. Iridium metal complexes containing Nꢀheterocyclic carbene ligands for
blueꢀlightꢀemitting electrochemical cells. Inorg. Chem. 2010, 49, 9891ꢀ9901.
(17) Henwood, A. F.; ZysmanꢀColman, E. Luminescent Iridium Complexes Used in Lightꢀ
Emitting Electrochemical Cells (LEECs). Top. Curr. Chem. 2016, 374, DOI: 10.1007/s41061ꢀ
016ꢀ0036ꢀ0.
(18) Pal, A. K.; Cordes, D. B.; Slawin, A. M. Z.; Momblona, C.; Ortĺ, E.; Samuel, I. D. W.;
Bolink, H. J.; ZysmanꢀColman, E. Synthesis, Properties, and LightꢀEmitting Electrochemical
Cell (LEEC) Device Fabrication of Cationic Ir(III) Complexes Bearing ElectronꢀWithdrawing
Groups on the Cyclometallating Ligands. Inorg. Chem. 2016, 55, 10361ꢀ10376.
(19) Ertl, C. D.; Bolink, H. J.; Housecroft, C. E.; Constable, E. C.; Ortí, E.; Junqueraꢀ
Hernández, J. M.; Neuburger, M.; Shavaleev, N. M.; Nazeeruddin, M. K.; Vonlanthen, D. Bisꢀ
Sulfoneꢀ and BisꢀSulfoxideꢀSpirobifluorenes: Polar Acceptor Hosts with Tunable Solubilities for
BlueꢀPhosphorescent LightꢀEmitting Devices. Eur. J. Org. Chem. 2016, 2016, 2037ꢀ2047.
(20) Helfrich, W.; Schneider, W. G. Transients of VolumeꢀControlled Current and of
Recombination Radiation in Anthracene. J. Chem. Phys. 1966, 44, 2902ꢀ2909.
(21) Rausch, A. F.; Thompson, M. E.; Yersin, H. Matrix effects on the triplet state of the OLED
emitter Ir(4,6ꢀdFppy)2(pic) (FIrpic): investigations by highꢀresolution optical spectroscopy.
Inorg. Chem. 2009, 48, 1928ꢀ1937.
(22) Lamansky, S.; Kwong, R. C.; Nugent, M.; Djurovich, P. I.; Thompson, M. E. Molecularly
doped polymer light emitting diodes utilizing phosphorescent Pt(II) and Ir(III) dopants. Org.
Electron. 2001, 2, 53ꢀ62.
(23) Yersin, H. Triplet emitters for OLED applications. Mechanisms of exciton trapping and
control of emission properties. Top. Curr. Chem. 2004, 241, 1ꢀ26.
(24) Czerwieniec, R.; Yu, J.; Yersin, H. Blueꢀlight emission of Cu(I) complexes and singlet
harvesting. Inorg. Chem. 2011, 50, 8293ꢀ8301.
(25) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D.
J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. Eꢀtype delayed fluorescence
17
ACS Paragon Plus Environment

Chemistry of Materials
Page 18 of 24
1
2
3
4
of a phosphineꢀsupported cu 2(ꢁꢀnar 2) 2 diamond core: Harvesting singlet and triplet excitons
in OLEDs. J. Am. Chem. Soc. 2010, 132, 9499ꢀ9508.
5
6
7
(26) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic lightꢀ
emitting diodes from delayed fluorescence. Nature 2012, 492, 234ꢀ238.
8
9
(27) Yersin, H.; Monkowius, U. Komplexe mit kleinen SingulettꢀTriplettꢀEnergieꢀAbständen
zur Verwendung in optoꢀelektronischen Bauteilen (SingulettꢀHarvestingꢀEffekt). DE 10 2008
033563, 2008.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(28) Hong, X.; Wang, B.; Liu, L.; Zhong, X. X.; Li, F. B.; Wang, L.; Wong, W. Y.; Qin, H. M.;
Lo, Y. H. Highly efficient blue–green neutral dinuclear copper(I) halide complexes containing
bidentate phosphine ligands. J. Lumin. 2016, 180, 64ꢀ72.
(29) Leitl, M. J.; Küchle, F.ꢀR.; Mayer, H. A.; Wesemann, L.; Yersin, H. Brightly Blue and
Green Emitting Cu(I) Dimers for Singlet Harvesting in OLEDs. J. Phys. Chem. A 2013, 117
,
11823ꢀ11836.
(30) Leitl, M. J.; Krylova, V. A.; Djurovich, P. I.; Thompson, M. E.; Yersin, H.
Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling SingletꢀTriplet
Splitting in Brightly Emitting and Sublimable Cu(I) Compounds. J. Am. Chem. Soc. 2014, 136
,
16032ꢀ16038.
(31) Linfoot, C. L.; Leitl, M. J.; Richardson, P.; Rausch, A. F.; Chepelin, O.; White, F. J.;
Yersin, H.; Robertson, N. Thermally Activated Delayed Fluorescence (TADF) and enhancing
photoluminescence quantum yields of [CuI(diimine)(diphosphine)]+ complexes ꢀ Photophysical,
structural, and computational studies. Inorg. Chem. 2014, 53, 10854ꢀ10861.
(32) Hofbeck, T.; Monkowius, U.; Yersin, H. Highly Efficient Luminescence of Cu(I)
Compounds: Thermally Activated Delayed Fluorescence Combined with ShortꢀLived
Phosphorescence. J. Am. Chem. Soc. 2015, 137, 399ꢀ404.
(33) Leitl, M. J.; Zink, D. M.; Schinabeck, A.; Baumann, T.; Volz, D.; Yersin, H. Copper(I)
Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device
Properties. Top. Curr. Chem. 2016, 374, 25.
(34) Czerwieniec, R.; Leitl, M. J.; Homeier, H. H. H.; Yersin, H. Cu(I) complexes – Thermally
activated delayed fluorescence. Photophysical approach and material design. Coord. Chem. Rev.
2016, 325, 2ꢀ28.
(35) Chen, X. L.; Yu, R.; Wu, X. Y.; Liang, D.; Jia, J. H.; Lu, C. Z. A strongly greenishꢀblueꢀ
emitting Cu4Cl4 cluster with an efficient spinꢀorbit coupling (SOC): Fast phosphorescence:
Versus thermally activated delayed fluorescence. Chem. Commun. 2016, 52, 6288ꢀ6291.
18
ACS Paragon Plus Environment

Page 19 of 24
Chemistry of Materials
1
2
(36) Ohara, H.; Kobayashi, A.; Kato, M. Simple and extremely efficient blue emitters based on
3
4
mononuclear Cu(I)ꢀhalide complexes with delayed fluorescence. Dalton Trans. 2014, 43
,
5
6
17317ꢀ17323.
(37) Osawa, M.; Kawata, I.; Ishii, R.; Igawa, S.; Hashimoto, M.; Hoshino, M. Application of
neutral d10 coinage metal complexes with an anionic bidentate ligand in delayed fluorescenceꢀ
type organic lightꢀemitting diodes. J. Mater. Chem. C 2013, 1, 4375ꢀ4383.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(38) Bergmann, L.; Friedrichs, J.; Mydlak, M.; Baumann, T.; Nieger, M.; Bräse, S. Outstanding
luminescence from neutral copper(I) complexes with pyridylꢀtetrazolate and phosphine ligands.
Chem. Commun. 2013, 49, 6501ꢀ6503.
(39) Armaroli, N.; Accorsi, G.; Holler, M.; Moudam, O.; Nierengarten, J. F.; Zhou, Z.; Wegh,
R. T.; Welter, R. Highly luminescent CuꢀI complexes for lightꢀemitting electrochemical cells.
Adv. Mater. 2006, 18, 1313ꢀ1316.
(40) Tsuboyama, A.; Kuge, K.; Furugori, M.; Okada, S.; Hoshino, M.; Ueno, K. Photophysical
Properties of Highly Luminescent Copper(I) Halide Complexes Chelated with 1,2ꢀ
Bis(diphenylphosphino)benzene. Inorg. Chem. 2007, 46, 1992ꢀ2001.
(41) Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. Simple Cu(I)
complexes with unprecedented excitedꢀstate lifetimes. J. Am. Chem. Soc. 2002, 124, 6ꢀ7.
(42) Kang, L.; Chen, J.; Teng, T.; Chen, X. L.; Yu, R.; Lu, C. Z. Experimental and theoretical
studies of highly emissive dinuclear Cu(I) halide complexes with delayed fluorescence. Dalton
Trans. 2015, 44, 11649ꢀ11659.
(43) Zink, D. M.; Bächle, M.; Baumann, T.; Nieger, M.; Kühn, M.; Wang, C.; Klopper, W.;
Monkowius, U.; Hofbeck, T.; Yersin, H.; Bräse, S. Synthesis, structure, and characterization of
dinuclear copper(I) halide complexes with P^N ligands featuring exciting photoluminescence
properties. Inorg. Chem. 2013, 52, 2292ꢀ2305.
(44) Volz, D.; Chen, Y.; Wallesch, M.; Liu, R.; Fléchon, C.; Zink, D. M.; Friedrichs, J.; Flügge,
H.; Steininger, R.; Göttlicher, J.; Heske, C.; Weinhardt, L.; Bräse, S.; So, F.; Baumann, T.
Bridging the efficiency gap: Fully bridged dinuclear Cu(I)ꢀcomplexes for singlet harvesting in
highꢀefficiency OLEDs. Adv. Mater. 2015, 27, 2538ꢀ2543.
(45) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient blue organic
lightꢀemitting diodes employing thermally activated delayed fluorescence. Nat. Photon. 2014, 8
,
326ꢀ332.
(46) Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo, S.; Komino, T.; Oiwa,
H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Purely organic electroluminescent
material realizing 100% conversion from electricity to light. Nat. Commun. 2015, 6, 8476.
(47) Atkins, P. W., Quanta: A Handbook of Concepts. Oxford University Press: 1991; p 367.
19
ACS Paragon Plus Environment

Chemistry of Materials
Page 20 of 24
1
2
3
4
5
6
7
(48) Barltrop, J. A.; Coyle, J. D., Excited states in organic chemistry. Wiley: 1975; p 87.
(49) Czerwieniec, R.; Yersin, H. Diversity of Copper(I) Complexes Showing Thermally
Activated Delayed Fluorescence: Basic Photophysical Analysis. Inorg. Chem. 2015, 54, 4322ꢀ
4327.
8
9
(50) Chen, J.; Teng, T.; Kang, L.; Chen, X.ꢀL.; Wu, X.ꢀY.; Yu, R.; Lu, C.ꢀZ. Highly Efficient
Thermally Activated Delayed Fluorescence in Dinuclear Ag(I) Complexes with a BisꢀBidentate
Tetraphosphane Bridging Ligand. Inorg. Chem. 2016, 55, 9528ꢀ9536.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(51) Yersin, H.; Leitl, M. J.; Czerwieniec, R., TADF for singlet harvesting ꢀ Next generation
OLED materials based on brightly green and blue emitting Cu(I) and Ag(I) compounds. In
Proceedings of SPIE - The International Society for Optical Engineering, 2014; Vol. 9183.
(52) Kaeser, A.; Moudam, O.; Accorsi, G.; Séguy, I.; Navarro, J.; Belbakra, A.; Duhayon, C.;
Armaroli, N.; DelavauxꢀNicot, B.; Nierengarten, J. F. Homoleptic copper(I), silver(I), and
gold(I) bisphosphine complexes. Eur. J. Inorg. Chem. 2014, 1345ꢀ1355.
(53) Hsu, C.ꢀW.; Lin, C.ꢀC.; Chung, M.ꢀW.; Chi, Y.; Lee, G.ꢀH.; Chou, P.ꢀT.; Chang, C.ꢀH.;
Chen, P.ꢀY. Systematic Investigation of the MetalꢀStructure–Photophysics Relationship of
Emissive d10ꢀComplexes of Group 11 Elements: The Prospect of Application in Organic Light
Emitting Devices. J. Am. Chem. Soc. 2011, 133, 12085ꢀ12099.
(54) Igawa, S.; Hashimoto, M.; Kawata, I.; Hoshino, M.; Osawa, M. Photoluminescence
Properties, Molecular Structures, and Theoretical Study of Heteroleptic Silver(I) Complexes
Containing Diphosphine Ligands. Inorg. Chem. 2012, 51, 5805ꢀ5813.
(55) Hsu, C.ꢀC.; Lin, C.ꢀC.; Chou, P.ꢀT.; Lai, C.ꢀH.; Hsu, C.ꢀW.; Lin, C.ꢀH.; Chi, Y. Harvesting
Highly Electronically Excited Energy to Triplet Manifolds: StateꢀDependent Intersystem
Crossing Rate in Os(II) and Ag(I) Complexes. J. Am. Chem. Soc. 2012, 134, 7715ꢀ7724.
(56) Kunkely, H.; Vogler, A. Optical properties of Ag(tripod)X with tripod = 1,1,1ꢀ
tris(diphenylꢀphosphinomethyl)ethane and X− = Cl− and I−: Intraligand and ligandꢀtoꢀligand
charge transfer. Inorg. Chim. Acta. 2006, 359, 388ꢀ390.
(57) Crespo, O.; Gimeno, M. C.; Jones, P. G.; Laguna, A. Silver complexes with the nidoꢀ
diphosphine [7,8ꢀ(PPh2)2ꢀ7,8ꢀC2B9H10]. Dalton Trans. 1996, 4583ꢀ4588.
(58) Alexander, R. P.; Schroeder, H. Chemistry of DecaboraneꢀPhosphorus Compounds. IV.
Monomeric, Oligomeric, and Cyclic Phosphinocarboranes. Inorg. Chem. 1963, 2, 1107ꢀ1110.
(59) Teixidor, F.; Vinas, C.; Abad, M. M.; Kivekas, R.; Sillanpaa, R. The formation of nido
[7,8ꢀ(PR2)2ꢀ7,8ꢀC2B9H10]ꢀ from closo 1,2ꢀ(PR2)2ꢀ1,2ꢀC2B10H10 (R = Ph, Et, iPr or OEt): a
process enhanced by complexation. J. Organomet. Chem. 1996, 509, 139ꢀ150.
20
ACS Paragon Plus Environment

Page 21 of 24
Chemistry of Materials
1
2
3
4
5
6
7
8
(60) Teixidor, F.; Viñas, C.; Mar Abad, M.; Nuñez, R.; Kivekäs, R.; Sillanpäa, R. Procedure for
the degradation of 1,2ꢀ(PR2)2ꢀ1,2ꢀdicarbaꢀclosoꢀdodecaborane(12) and 1ꢀ(PR2)ꢀ2ꢀR′ꢀ1,2ꢀ
dicarbaꢀclosoꢀdodecaborane(12). J. Organomet. Chem. 1995, 503, 193ꢀ203.
(61) Balakrishna, M. S.; Venkateswaran, R.; Mobin, S. M. Silver(I) complexes of bis[2ꢀ
(diphenylphosphino)phenyl] ether. Polyhedron 2008, 27, 899ꢀ904.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(62) Balakrishna, M. S.; Venkateswaran, R.; Mobin, S. M. Mixedꢀligand silver(I) complexes
containing bis[2ꢀ(diphenylphosphino)phenyl]ether and pyridyl ligands. Inorg. Chim. Acta. 2009,
362, 271ꢀ276.
(63) Cheng, G.; So, G. K.ꢀM.; To, W.ꢀP.; Chen, Y.; Kwok, C.ꢀC.; Ma, C.; Guan, X.; Chang, X.;
Kwok, W.ꢀM.; Che, C.ꢀM. Luminescent zinc(II) and copper(I) complexes for highꢀperformance
solutionꢀprocessed monochromic and white organic lightꢀemitting devices. Chem. Sci. 2015, 6
,
4623ꢀ4635.
(64) Chen, L. X.; Jennings, G.; Liu, T.; Gosztola, D. J.; Hessler, J. P.; Scaltrito, D. V.; Meyer,
G. J. Rapid ExcitedꢀState Structural Reorganization Captured by Pulsed Xꢀrays. J. Am. Chem.
Soc. 2002, 124, 10861ꢀ10867.
(65) Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.; Meyer, G.
J.; Coppens, P. MLCT State Structure and Dynamics of a Copper(I) Diimine Complex
Characterized by Pump−Probe Xꢀray and Laser Spectroscopies and DFT Calculations. J. Am.
Chem. Soc. 2003, 125, 7022ꢀ7034.
(66) Iwamura, M.; Watanabe, H.; Ishii, K.; Takeuchi, S.; Tahara, T. Coherent Nuclear
Dynamics in Ultrafast Photoinduced Structural Change of Bis(diimine)copper(I) Complex. J.
Am. Chem. Soc. 2011, 133, 7728ꢀ7736.
(67) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A., Photochemistry and Photophysics of
Coordination Compounds: Copper. In Photochemistry and Photophysics of Coordination
Compounds I, Balzani, V.; Campagna, S., Eds. Springer Berlin Heidelberg: Berlin, Heidelberg,
2007; pp 69ꢀ115.
(68) LavieꢀCambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.; Bassani, D. M.;
McClenaghan, N. D. Improving the photophysical properties of copper(I) bis(phenanthroline)
complexes. Coord. Chem. Rev. 2008, 252, 2572ꢀ2584.
(69) Iwamura, M.; Takeuchi, S.; Tahara, T. Ultrafast ExcitedꢀState Dynamics of Copper(I)
Complexes. Acc. Chem. Res. 2015, 48, 782ꢀ791.
(70) Garakyaraghi, S.; Danilov, E. O.; McCusker, C. E.; Castellano, F. N. Transient Absorption
Dynamics of Sterically Congested Cu(I) MLCT Excited States. J. Phys. Chem. A 2015, 119
,
3181ꢀ3193.
21
ACS Paragon Plus Environment

Chemistry of Materials
Page 22 of 24
1
2
3
4
5
6
(71) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2:
a complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339ꢀ
341.
(72) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group
thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and
transition elements: two new functionals and systematic testing of four M06ꢀclass functionals
and 12 other functionals. Theor. Chem. Acc. 2007, 120, 215ꢀ241.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(73) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and
quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem.
Chem. Phys. 2005, 7, 3297ꢀ3305.
(74) Matsumoto, K.; Shindo, T.; Mukasa, N.; Tsukuda, T.; Tsubomura, T. Luminescent
Mononuclear Ag(I)−Bis(diphosphine) Complexes: Correlation between the Photophysics and
the Structures of Mononuclear Ag(I)−Bis(diphosphine) Complexes. Inorg. Chem. 2010, 49, 805ꢀ
814.
(75) Osawa, M.; Hoshino, M. Photochemistry and photophysics of the tetrahedral silver(i)
complex
with
diphosphine
ligands:
[Ag(dppb)2]PF6
(dppb
=
1,
2ꢀ
bis[diphenylphosphino]benzene). Chem. Commun. 2008, 6384ꢀ6386.
(76) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. Organometallic Pt(II) and Ir(III) triplet
emitters for OLED applications and the role of spinꢀorbit coupling: A study based on highꢀ
resolution optical spectroscopy. Top. Organomet. Chem. 2010, 29, 193ꢀ235.
(77) Rausch, A. F.; Homeier, H. H. H.; Djurovich, P. I.; Thompson, M. E.; Yersin, H., Spinꢀ
orbit coupling routes and OLED performance ꢀ Studies of blueꢀlight emitting Ir(III) and Pt(II)
complexes. In Proceedings of SPIE - The International Society for Optical Engineering, 2007;
Vol. 6655.
(78) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. The triplet state of
organoꢀtransition metal compounds. Triplet harvesting and singlet harvesting for efficient
OLEDs. Coord. Chem. Rev. 2011, 255, 2622ꢀ2652.
(79) Chen, J.ꢀL.; Guo, Z.ꢀH.; Luo, Y.ꢀS.; Qiu, L.; He, L.ꢀH.; Liu, S.ꢀJ.; Wen, H.ꢀR.; Wang, J.ꢀY.
Luminescent monometallic Cu(i) triphenylphosphine complexes based on methylated 5ꢀ
trifluoromethylꢀ3ꢀ(2′ꢀpyridyl)ꢀ1,2,4ꢀtriazole ligands. New J. Chem. 2016, 40, 5325ꢀ5332.
(80) McCusker, C. E.; Castellano, F. N. Efficient Visible to NearꢀUV Photochemical
Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex. Inorg. Chem. 2015, 54
,
6035ꢀ6042.
22
ACS Paragon Plus Environment

Page 23 of 24
Chemistry of Materials
1
2
3
4
5
6
7
(81) Liang, D.; Chen, X.ꢀL.; Liao, J.ꢀZ.; Hu, J.ꢀY.; Jia, J.ꢀH.; Lu, C.ꢀZ. Highly Efficient
Cuprous Complexes with Thermally Activated Delayed Fluorescence for SolutionꢀProcessed
Organic LightꢀEmitting Devices. Inorg. Chem. 2016, 55, 7467ꢀ7475.
(82) McCusker, C. E.; Castellano, F. N. Design of a LongꢀLifetime, EarthꢀAbundant, Aqueous
8
9
Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects. Inorg. Chem. 2013, 52
,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8114ꢀ8120.
(83) Wang, B.; Shelar, D. P.; Han, X.ꢀZ.; Li, T.ꢀT.; Guan, X.; Lu, W.; Liu, K.; Chen, Y.; Fu,
W.ꢀF.; Che, C.ꢀM. LongꢀLived Excited States of Zwitterionic Copper(I) Complexes for
Photoinduced CrossꢀDehydrogenative Coupling Reactions. Chem. Eur. J. 2015, 21, 1184ꢀ1190.
(84) Wang, Z.; Zheng, C.; Wang, W.; Xu, C.; Ji, B.; Zhang, X. Synthesis, Structure, and
Photophysical Properties of Two FourꢀCoordinate Cu(I)ꢀNHC Complexes with Efficient
Delayed Fluorescence. Inorg. Chem. 2016, 55, 2157ꢀ2164.
(85) Turro, N. J., Modern Molecular Photochemistry. University Science Books: 1991.
(86) For most compounds, the decay behavior measured of powder materials is modified, for
example, by processes of energy transfer or tripletꢀtriplet annihilation. However, in analogy to
investigations of Cu(I) TADF materials [9, 24, 29, 34], also the lowꢀlying CT states of
Ag(dbp)(P2ꢀnCB) exhibit geometry distortions even in the relatively rigid crystalline
environment. These induce a localization (selfꢀtrapping). Thus, the emission of the powder
material displays largely molecular properties. Accordingly, concentration quenching does not
occur and the decay behavior does not show any distinctive features with concentration increase.
(87) Bergmann, L.; Hedley, G. J.; Baumann, T.; Bräse, S.; Samuel, I. D. Direct observation of
intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid
state. Sci. Adv. 2016, 2, e1500889.
(88) Azumi, T.; O'Donnell, C. M.; McGlynn, S. P. On the multiplicity of the phosphorescent
state of organic molecules. J. Chem. Phys. 1966, 45, 2735ꢀ2742.
(89) Parker, C. A.; Hatchard, C. G. Tripletꢀsinglet emission in fluid solutions. Phosphorescence
of eosin. T. Faraday Soc. 1961, 57, 1894ꢀ1904.
(90) Tsubomura, T.; Ito, Y.; Inoue, S.; Tanaka, Y.; Matsumoto, K.; Tsukuda, T. Strongly
luminescent palladium(0) and platinum(0) diphosphine complexes. Inorg. Chem. 2008, 47, 481ꢀ
486.
(91) AbedinꢀSiddique, Z.; Ohno, T.; Nozaki, K.; Tsubomura, T. Intense Fluorescence of Metalꢀ
toꢀLigand Charge Transfer in [Pt(0)(binap)2] [binap = 2,2‘ꢀBis(diphenylphosphino)ꢀ1,1‘ꢀ
binaphthyl]. Inorg. Chem. 2004, 43, 663ꢀ673.
23
ACS Paragon Plus Environment

Chemistry of Materials
Page 24 of 24
1
2
3
4
5
6
(92) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. Highly
Efficient Green Organic LightꢀEmitting Diodes Containing Luminescent ThreeꢀCoordinate
Copper(I) Complexes. J. Am. Chem. Soc. 2011, 133, 10348ꢀ10351.
(93) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C.
Efficient upꢀconversion of triplet excitons into a singlet state and its application for organic light
emitting diodes. Appl. Phys. Lett. 2011, 98, 083302.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(94) Hofbeck, T.; Yersin, H. The Triplet State of facꢀIr(ppy)3. Inorg. Chem. 2010, 49, 9290ꢀ
9299.
(TOC)
It is shown, how to design a new Ag(I) complex that exhibits 100 % emission quantum
yield at the shortest TADF decay time reported so far. This is a consequence of (i) a
small singletꢀtriplet splitting, (ii) a rigid molecular structure, and most importantly (iii) a
very high S1→S0 transition rate.
(Graphical Abstract)
24
ACS Paragon Plus Environment