A Di-Nuclear Ag(I) Complex Designed for Highly Efficient Thermally

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A Di-Nuclear Ag(I) Complex Designed for Highly Efficient Thermally Activated Delayed Fluorescence Marsel Z. Shafikov, Alfiya Suleymanova, Alexander Schinabeck, and Hartmut Yersin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03160 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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The Journal of Physical Chemistry Letters

A Di-nuclear Ag(I) Сomplex Designed for Highly Efficient Thermally Activated Delayed Fluorescence Marsel Z. Shafikov†,‡,*, Alfiya F. Suleymanova†, Alexander Schinabeck †, and Hartmut Yersin†,* †

University of Regensburg, Institute of Physical Chemistry, Universitätsstr. 31,

D-93053 Regensburg, Germany. ‡

Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia.

ABSTRACT

The di-nuclear Ag(I) complex has been designed to show thermally activated delayed fluorescence (TADF) оf high efficiency. Strongly electron-donating terminal ligands are introduced to destabilize the d-orbitals of the Ag

+

ions.

Consequently, the orbitals distinctly contribute to the HOMO, whereas the LUMO is localized on the bridging ligand. This ensures charge transfer character of the lowest excited singlet S1 and triplet T1 state. Accordingly, a small energy gap ∆E(S1─T1) is obtained, being essential for TADF behavior. Photophysical investigations show that at ambient temperature the complex exhibits TADF reaching a quantum yield of ФPL = 70 % with the decay time of only τ = 1.9 µs, manifesting one of the fastest TADF decays observed so far. Such an outstanding TADF efficiency is based on a small value of ∆E(S1─T1) = 480 cm-1 combined with a large radiative transition rate of k(S1→S0) = 2.2107 s-1.

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KEYWORDS: thermally activated delayed fluorescence, TADF, Ag(I) complex, di1 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

nuclear Ag(I) complex, OLED

Compounds exhibiting thermally activated delayed fluorescence (TADF) are attractive as emitter materials for OLEDs.1 These compounds can harvest the energy of all excitons formed, singlets and triplets,2 in the lowest singlet state S1 which then decays radiatively to the ground state S0. This mechanism, called Singlet Harvesting, allows to achieve up to 100% of internal efficiency in OLEDs.1,3-14 The key requirement for a compound to show TADF is a small energy gap between the lowest singlet and triplet excited states ∆E(S1─T1)1,3-11,15-22, so that the intersystem crossing (ISC) S1→T1 and, more importantly, the temperature dependent up-ISC T1→S1 (sometimes also called reverse ISC, RISC) are effective near ambient temperature.1,23,24 For a small ∆E(S1─T1), the unpaired electrons must have a weak exchange interaction25,26, which can be achieved when the states are of charge transfer character. Compounds with the states S1 and T1 in thermal equilibrium at small ∆E(S1─T1) usually show TADF since the radiative rate of the S1→S0 transition kr(S1→S0) is typically orders of magnitude higher than the rate of the spin-forbidden T1→S0 transition, kr(T1→S0). The state T1 serves as a relatively long-living reservoir supplying excitation energy to the emitting state S1. Exploiting low-cost TADF materials and the Singlet Harvesting Mechanism in OLEDs is advantageous over the alternative approach via the Triplet Harvesting Mechanism exploiting expensive phosphorescent complexes based on Ir(III), Pt(II), or Os(II)27-32. Indeed, there are numerous examples of comparatively inexpensive Cu(I) complexes1,4,5,10-


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The Journal of Physical Chemistry Letters 12,15,16,33-53

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and even of purely organic materials8,17,24,54-58 exhibiting TADF. Interestingly,

although silver and copper are elements of the same group in the periodic table and have similar outer electronic shells, examples of Ag(I) complexes showing TADF are rare and appeared only recently.15,19,59-61 Instead, Ag(I) complexes typically show long-living phosphorescence stemming from ligand centered excited state(s) (3LC).62-65 This is because of a higher ionization potential of the Ag+ ion (second ionization potential of the Ag atom) compared to that of the Cu+ ion.62 Accordingly, for Ag(I)-based complexes, the lowest excited states of metal-to-ligand charge transfer (MLCT) character are usually at higher energy than the 3LC excited states.66 Therefore, it is a challenge to design a Ag(I) complex that shows efficient TADF. However, it could be shown recently that by use of strongly +

electron-donating ligands, such as phosphines, the d-orbitals of Ag could be sufficiently destabilized to render the lowest states to exhibit distinct charge transfer character. This can lead to the required TADF behavior of Ag(I) complexes.59 Indeed, it could be shown recently that Ag(I) complexes containing 1,10-phenanthroline-based ligands in combination with 1,2-bis(diphenylphosphine)-nido-carborane (P2-nCB) ligand represent efficient TADF materials.59 In this situation, the electron-donating property of the phosphines is enhanced by the negative charge of the nido-carborane moiety. Besides, it is believed that the rigid structure of the nido-carborane cage in the P2-nCB ligand disfavors non-radiative relaxation thus, increasing the TADF quantum yield. Therefore, using the P2-nCB ligand at designing TADF Ag(I) complexes should be advantageous not only for electronic but also for structural reasons.



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In this work, we present a di-nuclear TADF Ag(I) complex. This new compound consists 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

of

two

Ag+

ions

bridged

tetrakis(diphenylphosphino)benzene

by

the

(tpbz)

electroneutral

ligand,

whereby

tetra-dentate each

Ag(I)

1,2,4,5center

is

coordinated by a terminal P2-nCB ligand, thus, giving the di-nuclear Ag2(tpbz)(P2-nCB)2 complex (Figure 1). This complex is designed to have the HOMO distributed over the negatively charged P2-nCB ligands and the metal centers, whereas the LUMO is supposed to be localized on the electroneutral tpbz ligand. A DFT/TDDFT investigation, as presented below, will show that the lowest excited singlet and triplet states are of (M+L)L’CT character, whereby L and L’ represent the P2-nCB and the tpbz ligand, respectively. Such an electronic structure is a very good prerequisite for the complex to exhibit TADF. Indeed, in this contribution we show that Ag2(tpbz)(P2-nCB)2 exhibits highly efficient TADF reaching a photoluminescence quantum yield of ФPL = 70% with a TADF decay time of only τ = 1.9 µs. Given the high TADF quantum yield, this short TADF decay time is remarkable, especially, taking into account that Cu(I) compounds known so far show TADF decay times longer than 3 µs.10,18 It should be noted further that luminescent materials with short emission decay times (at high emission quantum yields) are of high value for OLED applications, in order to reduce roll-off and saturation effects in the devices and to increase the emitter stability.


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Figure

1.

investigated

Chemical dinuclear

structure Ag(I)

of

the

complex

Ag2(tpbz)(P2-nCB)2.

The bridging central ligand 1,2,4,5-tetrakis(diphenylphosphino)benzene (tpbz) was synthesized through a reaction of commercially available 1,2,4,5-tetrafluorobenzene with sodium diphenylphosphanide obtained from chlorodiphenylphosphine (Scheme 1).67 The proligand 1,2-bis(diphenylphosphine)-ortho-caborane (P2-oCB) was synthesized by double lithiation of ortho-caborane (1,2-dicarba-closo-dodecaboran) with n-butyllithium. The lithiated intermediate product was isolated and coupled with chlorodiphenylphosphine (Scheme 1).59,68,69 In the next step the proligand P2-oCB was coupled with silver hexafluorophosphate (AgPF6) in ethanol for an hour, and then the bridging ligand tpbz was added. The stochiometric ratio of the reactants, P2-oCB : AgPF6 : tpbz, was kept to 2 : 2 : 1. After complexation under refluxing ethanol the proligand P2-oCB undergoes partial deboration giving the negatively charged 1,2-bis(diphenylphosphine)-nido-caborane (P2nCB) ligand,70 thus affording the aimed dinuclear and electroneutral Ag(I) complex Ag2(tpbz)(P2-nCB)2 (Figure 1). For the synthetic procedures and characterizations see the Supporting Information (SI).


The Journal of Physical Chemistry Letters 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

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Scheme 1. Synthetic routes of the ligand tpbz, the proligand P2-oCB, and the dinuclear

Ag(I) complex Ag2(tpbz)(P2-nCB)2 shown in Figure 1.

The complex was characterized and its structure was confirmed by means of single crystal x-ray diffraction analysis (Figure 2), elemental analysis (C,H,N) and

31

P NMR

spectroscopy. It is noted that 1H NMR in this case is not informative due to a large number of flexible phenyl groups interacting with each other. Crystals of Ag2(tpbz)(P2-nCB)2 suitable for x-ray diffraction analysis were obtained by slow diffusion of methanol into CH2Cl2 (DCM, dichloromethane) solution of the complex. The x-ray analysis gave coordination bond lengths of Ag─P: 2.48–2.54 Å, which are unremarkable and similar to those of complexes with the same type of coordination reported earlier.15,19,59,64,66,71-75 Further x-ray obtained structure data are summarized in Table S1 in the Supporting Information (SI).


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Figure 2. Perspective view (OLEX-2 plots

with 50% probability thermal ellipsoids) of the molecular structure of Ag2(tpbz)(P2nCB)2 obtained by single crystal x-ray diffraction analysis.

Theoretical investigations were carried out using the DFT approach at the M0676/def2-SVP77,78 theory level for geometry optimization and at the M062X76/def2-SVP level for time-dependent calculations (TDDFT), all utilizing the Gaussian 09 D.01 code79. The geometry parameters of the Ag2(tpbz)(P2-nCB)2 molecule in the electronic ground state (S0) optimized for gas phase conditions agree fairly well with those obtained by x-ray diffraction analysis (Table S1 in the SI). Interestingly, in the lowest triplet state T1 the calculated coordination geometry at the Ag1 atom is notably twisted from a tetrahedral geometry towards a planar one as compared to that in the ground state. For comparison, the coordination geometry at the Ag2 atom in the T1 state keeps the tetrahedral geometry as the ground state (S0). (Table S1 in the SI). This is due to the charge transfer character of the T1 state involving the Ag1 atom. Such geometry reorganizations at the coordination center were also reported for mononuclear Ag(I) complexes, and experimentally shown for mono-nuclear Cu(I) complexes with charge transfer character of the T1 state.80-89 Since we



are interested in the emissive properties of Ag2(tpbz)(P2-nCB)2, further discussions are 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

related to results obtained for the optimized T1 state geometry. According to TDDFT calculations, the two lowest excited states S1 and T1 are by 85% and 89% of HOMO→LUMO character, respectively (Table S2 in the SI) and therefore, the potential energy hypersurfaces of these states are expected to be rather similar. A plot of the HOMO shows that it is mainly localized on the phosphines P1 and P2 (atom numbering corresponds to that in Figure 2) with a contribution from Ag1, the phosphines P3 and P4, and the nido-carborane moiety, whereas the LUMO is localized on the benzene ring of the tpbz ligand. (Figure 3, Table S3) This allows us to assign the states S1 and T1 as being of metal+ligand(L)-to-ligand(L’) charge transfer ((M+L)L’CT) character. Therefore, one can expect that the exchange interaction of the unpaired electrons is weak. Consequently, ∆E(S1─T1) is small enough for TADF to occur at near-ambient temperatures. Indeed, a coarse estimate using the energy difference between calculated vertical (Franck-Condon) transitions S0→S1 and S0→T1 gives a proximate value of ∆E(S1─T1) = 1400 cm─1. In fact, Ag2(tpbz)(P2-nCB)2 exhibits very efficient TADF at temperatures above 100 K with an experimentally determined activation energy of 480 cm-1 (60 meV) as will be shown and discussed below. It is noted that the calculated value of the oscillator strength ƒ(S0→S1) of the S0→S1 transition amounts to 0.1077. This is a large value for a charge transfer transition, if compared, for example, to those in mono-nuclear Ag(I) complexes.59,61 A large ƒ-value, being proportional to the radiative rate kr(S1→S0), is advantageous for obtaining a short TADF decay time (see eq. 1, below). However, the value may strongly vary even with


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slight geometry changes in the emitting state, which also depends on the rigidity of the 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

environment.90

Figure 3. Contour plots of the

frontier orbitals (M062X/def2-SVP) of Ag2(tpbz)(P2-nCB)2 (iso-value = 0.05) calculated for the gas phase optimized (M06/def2-SVP) T1 state geometry.

The absorption spectrum of Ag2(tpbz)(P2-nCB)2 consists of overlapping absorption bands in the 225 – 350 nm region and a band of significantly lower absorptivity centered at about 380 nm (Figure 4). According to the discussed calculations and in analogy to other tetraphosphine coordinated Ag(I) complexes.91, the lower energy band is assigned to the MLL’CT transition. Overlapping bands of higher energy are assigned to π→π* transitions centered at the tpbz19 ligand and at the P2-nCB ligand. The absorption spectrum of 9 ACS Paragon Plus Environment


Ag2(tpbz)(P2-nCB)2 is shown in Figure 4 with a non-scaled absorption axis because of the 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

very low solubility of the compound in CH2Cl2 or other solvents, which did not allow us to prepare a solution of defined concentration. The ambient-temperature emission spectrum of Ag2(tpbz)(P2-nCB)2 features unstructured shape and is centered at 555 nm for a powder sample (Figure 4). It is noted that a broad and unstructured spectral shape is typical for a charge transfer emission.1,4,10,16,18,59,61 This is due to a significant distortion of the molecular geometry in the emitting state compared to the ground state and consequently, to a large number of vibrational Franck-Condon transitions involved.92

Figure

4.

Absorption

spectrum

of

Ag2(tpbz)(P2-nCB)2 measured for CH2Cl2 solution (black line) and emission spectra (colored lines) of a powder sample and a doped in PMMA matrix with the dopant concentration of c < 1wt %, respectively.

The ambient-temperature emission spectrum of Ag2(tpbz)(P2-nCB)2 doped in PMMA (poly(methyl methacrylate) is red shifted compared to that of a powder sample and appears centered at 575 nm (Figure 4). The photoluminescence quantum yield drops from ФPL = 70 10 ACS Paragon Plus Environment

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% for the powder sample to ФPL = 35 % for the doped PMMA film. (Table 1) The two 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

effects, the red shift of emission and the drop of quantum yield, are related to a larger extend of geometry reorganization in the less rigid environment of the PMMA film compared to the rigid powder.4,59 The more distinct reorganization in PMMA results in a more stabilized emitting state, giving the red shift, and a stronger vibrational coupling with the ground state, increasing the rate of non-radiative relaxation.25,92 Such an influence of environment’s rigidity on the emission quantum yield has also been reported for other Cu(I) 4,10,61

and Ag(I)19,59,61,90 TADF complexes. The emission of the Ag2(tpbz)(P2-nCB)2 powder

measured at ambient temperature (300 K) is blue shifted compared to that measured at cryogenic temperatures, for example, at T = 30 K (Figure 4). This is expected for an occurrence of TADF and will be discussed further.10,11,19,59,61 Table 1. Emission data of Ag2(tpbz)(P2-

nCB)2 in different environments. powder

PMMA

λmax (300 K)

555 nm

575

ФPL (300 K)

70 %

35 %

τ(300 K)

1.9 µs

kr(300 K)

3.7105 s−1

ФPL (77 K)

≈ 100 %

τ(77 K)

675 µs

kr(77 K)

1.5103 s−1

knr(77 K)a

< 4.410 s−1

τ(T1, 40 K)b plateau kr(T1, 40 K) kr(S1→S0)c

∆E(S1─T1)

c

1845 µs 5.4·102 s-1 2.2107 s−1 (45 ns) 480 cm−1

a. determined as knr = (1-ФPL)/τ, assuming 11 ACS Paragon Plus Environment


an error of the quantum yield measurement of 3%, i. e. assuming ФPL(77K) = 97%. b. The same value is found for the temperature range 15 < T < 50 K. c. determined from the fit of experimental luminescence decay times according to eq. 1, measured for a powder sample of Ag2(tpbz)(P2-nCB)2 at different temperatures.

To gain a profound understanding of photophysical properties of Ag2(tpbz)(P2-nCB)2, we investigated the change of emission decay time in a temperature range from 15 K to 300 K (Figure 5). In the temperature range of 15 K ≤ T ≤ 50 K, the decay time does not change and forms a plateau with the value of τ = 1845 µs. In the range 50 K ≤ T ≤ 150 K the emission decay time drops strongly and with further increase of temperature from 150 K to 300 K another plateau with τ ≈ 2 µs reaching τ = 1.9 µs at 300 K is observed. Such a significant decrease of the emission decay time with temperature increase above 50 K is explained by a drastic increase of the radiative rate calculated as kr = ФPL/τ. In fact, assuming that at temperatures below 77 K the photoluminescence quantum yield does not significantly vary (ФPL(77 K) ≈ 100%), the radiative rate increases from kr(15 K) = 5.4102 s─1 to kr(300 K) = 3.7105 s─1, which is an increase of about three orders of magnitude. At low temperature (T ≤ 50 K) the decay time does not vary because the emission stems solely from T1→S0 transition (or, more exactly, from the related triplet substates) with a decay time of τ(T1) = 1845 µs. Such a long decay of the T1 state reflects the spin-forbiddenness of the T1→S0 transition and a weak spin-orbit coupling (SOC) with the states of singlet manifold. A 12 ACS Paragon Plus Environment

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comparably long decay times of the T1 state has also been reported for other Ag(I) 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

complexes.15,19,59-61 At temperatures above 50 K, the S1 state is being thermally populated from the T1 state. As a consequence, an additional emission path due to the S1→S0 transition, representing the TADF, is activated. It is noted that the radiative rate of the S1→S0 transition is significantly higher than that of the spin-forbidden T1→S0 transition. Therefore, the additional and much faster decay path leads to a drastic decrease of the decay time. Thus, with increasing temperature and efficient thermal population of the S1 state a very steep increase of the activated part (TADF) of the total emission intensity is observed.

At temperatures

above 150 K,

TADF strongly

dominates

over the

phosphorescence and the decay time forms the high-temperature plateau (Figure 5). Therefore, at near-ambient temperature the state T1 hardly emits, instead it serves as a reservoir supplying its excitation energy to the slightly higher lying S1 state. This change of the emitting state from T1 at temperatures below 50 K to S1 at ambient temperature is also reflected in the slight blue shift of the emission spectrum, as shown for the powder sample in Figure 4. The efficiency of TADF is mainly defined by two factors: (i) the efficiency of thermal population of the S1 state from the T1 state, depending on the energy gap ∆E(S1─T1) and (ii) on the radiative rate of the S1→S0 transition, representing the prompt fluorescence. However usually, the prompt fluorescence cannot be measured directly because of much faster ISC(S1→T1) processes occurring in the picosecond time scale, as reported for Cu(I) complexes exhibiting TADF.84,89,93 Instead, both parameters, kr(S1) and ∆E(S1─T1), can be



determined by fitting the experimental decay time data by a Boltzmann type equation (eq. 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

1).1,5,10,30,33,94 This equation refers to the decay time of emission from the thermally equilibrated states T1 and S1 as a function of temperature. ∆ ∆

T

(1)

Herein τ(T) is the emission decay time at a given temperature T, k(T1) = k(T1→S0) = 1/τ(T1) and k(S1) = k(S1→S0) = 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.

Figure 5. Left panel: Emission decay curves of Ag2(tpbz)(P2-nCB)2 powder measured at

different temperatures. Right panel: Emission decay time of Ag2(tpbz)(P2-nCB)2 plotted versus temperature (black dots) and fitted with eq.1 (red line). The values of k(S1→S0) = 2.2107 s−1 (45 ns) and ∆E(S1−T1) = 480 cm−1 (60 meV) result from a fit of eq. 1 to the experimental τ(T) values, with τ(T1) fixed to 1845 µs as determined directly for T < 50 K (plateau).


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The fit of eq.1 to experimental emission decay times (Figure 5) allows us to determine the 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

energy gap of ∆E(S1−T1) = 480 cm─1 (60 meV). This energy separation is relatively small as compared to other Cu(I) or Ag(I) complexes1,10,61 and explains the onset of TADF at comparatively low temperature.18 Additionally, the fit reveals the rate of S1→S0 transition to be of k(S1) = 2.2107 s─1 corresponding to a prompt fluorescence decay time only of τ(S1) = 45 ns. This rate is notably higher than of any TADF Cu(I) complex with a similarly small ∆E(S1−T1) value10 reported so far. Accordingly, the ambient-temperature TADF decay time becomes as short as 1.9 µs (radiative decay time: 2.7 µs). This result agrees well with the calculated rather high value of the oscillator strength ƒ(S1→S0) (section 3) and demonstrates the suitability of using Ag(I) as central metal ion at designing TADF materials.

The investigations carried out revealed that Ag2(tpbz)(P2-nCB)2 represents a highly efficient TADF emitter at near-ambient temperatures. The states S1 and T1 are in thermal equilibrium due to fast ISC and up-ISC processes. At temperatures below 50 K, this equilibrium is totally shifted to the lower lying and long-living T1 state. Accordingly, at those temperatures only long-living phosphorescence (T1→S0 transition) with τ(T1) =1845 µs is observed. However, with increase of temperature, already near 50 K, the S1 state gets noticeably thermally populated by up-ISC and an additional emission path via a much faster S1→S0 transition is opened, representing the TADF path. The related rate amounts to k(S1→S0) = 2.2107 s─1, which corresponds formally to a decay time of only 45 ns. This is more than four orders of magnitude shorter than the phosphorescence decay time. As the temperature increases, the higher lying state S1 is populated to a higher ratio, leading to a



range of emission that consists of TADF and phosphorescence with gradually decreasing 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

emission decay time. Already at 150 K, fast TADF dominates strongly over the long-living phosphorescence (Figure 5). At ambient temperature, the emission of Ag2(tpbz)(P2-nCB)2 is represented solely by TADF with a remarkably short decay time of only τ(300K, TADF) = 1.9 µs. The photophysical behavior of Ag2(tpbz)(P2-nCB)2 is summarized schematically in an energy diagram as shown in Figure 6.

Figure 6. Simplified energy level diagram and decay

properties of Ag2(tpbz)(P2-nCB)2 (powder).

Lately, materials exhibiting TADF are of huge interest in the fields of scientific research and as efficient emitters to be applied in OLEDs. Like phosphorescent materials based on Pt(II),


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Ir(III), or Os(II), TADF materials can harvest all excitons, singlets and triplets, formed in the 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

emitting layer of an OLED. However, TADF materials can be designed on the basis of much cheaper metals, such as Cu or Ag. In fact, a huge number of TADF related work has been published since the discovery of the occurrence of efficient TADF for Cu(I) complexes. The best Cu(I)-based examples show radiative emission decay times τr of 4-5 µs.10,18,61 Note that short emission decay time is important to reduce saturation and efficiency roll-off effects and to increase the OLED device life. It was discovered recently that using Ag(I) instead of Cu(I) can help to significantly decrease the radiative TADF decay time. Thus, it was even possible to obtain a material that exhibits a slightly shorter decay time than the phosphorescence decay of Ir(ppy)3.32,59 Complexes of Ag(I), however, typically show only long-living phosphorescence because of the low-energy positions of the d-orbitals of Ag(I), unlike in the situation of Cu(I). This peculiarity kept the attention of researchers for long away from Ag(I)-based complexes to realize TADF materials, until it was unraveled only recently by energetically destabilizing the d-orbitals through chemical design. In this work, a di-nuclear complex Ag2(tpbz)(P2-nCB)2 that bears two strongly electron-donating negatively charged P2-nCB ligands for destabilizing the d-orbitals is presented. Besides, the complex contains 16 spatially confined phenyl groups for rigidifying the molecular structure. Such a molecular design provides a small energy gap of ∆E(S1−T1) = 480 cm-1 (60 meV), while keeping a relatively high radiative rate kr(S1→S0). These properties result in a short-living TADF at a quantum yield as high as ФPL = 70 %. In fact, Ag2(tpbz)(P2-nCB)2 is one of a handful TADF materials with a radiative decay time below 3 µs.19,59,61



ASSOCIATED CONTENT 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

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details, synthetic procedures, characterizations of the synthesized complexes, photophysical instrumentation, X-ray data, computational details, excited state energies, dominant orbital excitations from TD-DFT calculations, contour plots of the molecular orbitals that participate in the formation of the discussed excited states, calculated compositions of the frontier molecular orbitals, and geometries of the represented complex optimized in the states S0 and T1 (PDF) Crystallographic data (CIF)

Author information

Corresponding Authors *E-mail: [email protected] (M.Z.S.), [email protected] (H.Y.). ORCID Marsel Z. Shafikov: 0000-0003-0495-0364 Alfiya F. Suleymanova: 0000-0003-3064-5427 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors thank the German Ministry of Education and Research (BMBF) for financial support in the scope of the cyCESH project (FKN 13N12668). M.Z.S. is grateful to Professor Duncan Bruce (York, UK) for help with computations facilities. A.F.S. acknowledges the German Academic Exchange Service (DAAD) for support. REFERENCES (1) Yersin, H., Highly efficient OLEDs – Materials based on Thermally Activated Delayed Fluorescence. WileyVCH: Weinheim, Germany, 2018; p 400. (2) Helfrich, W.; Schneider, W. G. Transients of Volume-Controlled Current and of Recombination Radiation in Anthracene. J. Chem. Phys. 1966, 44, 2902-2909. (3) Yersin, H.; Monkowius, U. Komplexe mit kleinen Singulett-Triplett-Energie-Abständen zur Verwendung in opto-elektronischen Bauteilen (Singulett-Harvesting-Effekt). Internal patent filing, University of Regensburg 2006. Patent DE 10 2008 033563, 2008. (4) Czerwieniec, R.; Yu, J.; Yersin, H. Blue-light emission of Cu(I) complexes and singlet harvesting. Inorg. Chem. 2011, 50, 8293-8301. (5) 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. E-type delayed fluorescence of a phosphine-supported Cu2(μ18 ACS Paragon Plus Environment

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(TOC) A new Ag(I) complex of unique di-nuclear design is reported. The compound exhibits highly efficient TADF with 70% quantum yield at a decay time of only 1.9 µs representing one of the shortest TADF decays reported so far. Such outstanding properties result from the specific chemical design that leads to (i) a small singlet-triplet splitting, (ii) a rigid molecular structure, and (iii) a very high S1→S0 transition rate.


A Di-Nuclear Ag(I) Complex Designed for Highly Efficient Thermally

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