Dinuclear Ag(I) Complex Designed for Highly Efficient Thermally

Jan 19, 2018 - Such an outstanding TADF efficiency is based on a small value of ΔE(S1–T1) = 480 cm–1 combined with a large transition rate of k(S...
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Cite This: J. Phys. Chem. Lett. 2018, 9, 702−709

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

Institute of Physical Chemistry, University of Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany Ural Federal University, Mira 19, Ekaterinburg 620002, Russia

‡

S Supporting Information *

ABSTRACT: The dinuclear Ag(I) complex has been designed to show thermally activated delayed fluorescence (TADF) of 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 states. Accordingly, a small energy gap ΔE(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 ΦPL = 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 ΔE(S1−T1) = 480 cm−1 combined with a large transition rate of k(S1 → S0) = 2.2 × 107 s−1.

ompounds 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 one to achieve up to 100% 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 ΔE(S 1 − 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 ΔE(S1− T1), the unpaired electrons must have a weak exchange interaction,25,26 which can be achieved when the states are of charge transfer character. Compounds with the states S1 and T1 in thermal equilibrium at small ΔE(S1−T1) usually show TADF because the radiative rate of the S1 → S0 transition kr(S1 → S0) is typically orders of magnitude higher than the rate of the spinforbidden 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) complexes1,4,5,10−12,15,16,18,33−53 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

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© XXXX American Chemical Society

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 was shown that Ag(I) complexes containing 1,10-phenanthroline-based ligands in combination with a 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 nCB moiety. Besides, it is believed that the rigid structure of the nCB cage in the P2nCB ligand disfavors nonradiative relaxation, thus increasing the TADF quantum yield. Therefore, using the P2-nCB ligand to design TADF Ag(I) complexes should be advantageous not only for electronic but also for structural reasons. In this work, we present a dinuclear TADF Ag(I) complex. This new compound consists of two Ag+ ions bridged by the Received: November 29, 2017 Accepted: January 19, 2018 Published: January 19, 2018 702

DOI: 10.1021/acs.jpclett.7b03160 J. Phys. Chem. Lett. 2018, 9, 702−709

Letter

The Journal of Physical Chemistry Letters

1).59,68,69 In the next step, the proligand P2-oCB was coupled with silver hexafluorophosphate (AgPF6) in ethanol for 1 h, 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 P2oCB underwent partial deboration, giving the negatively charged P2-nCB 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 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 31P NMR

electroneutral tetra-dentate 1,2,4,5-tetrakis(diphenylphosphino)benzene (tpbz) ligand, whereby each Ag(I) center is coordinated by a terminal P2-nCB ligand, thus giving the dinuclear Ag2(tpbz)(P2-nCB)2 complex (Figure 1).

Figure 1. Chemical structure of the investigated dinuclear Ag(I) complex Ag2(tpbz)(P2-nCB)2.

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 ΦPL = 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. The bridging central ligand 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 oCB (1,2-dicarba-closo-dodecaboran) with n-butyllithium. The lithiated intermediate product was isolated and coupled with chlorodiphenylphosphine (Scheme

Figure 2. Perspective view (OLEX-2 plots with 50% probability thermal ellipsoids) of the molecular structure of Ag2(tpbz)(P2-nCB)2 obtained by single-crystal X-ray diffraction analysis.

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 structural data are summarized in Table S1 in the SI. Theoretical investigations were carried out using the DFT approach at the M0676/def2-SVP77,78 theory level for geometry

Scheme 1. Synthetic Routes to the Ligand tpbz, to the Proligand P2-oCB, and to the Dinuclear Ag(I) Complex Ag2(tpbz)(P2nCB)2 (Shown in Figure 1)

703

DOI: 10.1021/acs.jpclett.7b03160 J. Phys. Chem. Lett. 2018, 9, 702−709

Letter

The Journal of Physical Chemistry Letters optimization and at the M062X76/def2-SVP level for timedependent calculations (TDDFT), all utilizing the Gaussian 09 D.01 code.79 The geometry parameters of the Ag2(tpbz)(P2nCB)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 toward 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 mononuclear Cu(I) complexes with charge transfer character of the T1 state.80−89 Because we are interested in the emissive properties of Ag2(tpbz)(P2-nCB)2, further discussions are 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 nCB 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

for a charge transfer transition if compared, for example, to those in mononuclear Ag(I) complexes.59,61 A large f value, being proportional to the radiative rate kr(S1 → S0), is advantageous for obtaining a short TADF decay time (see eq 1). However, the value may strongly vary even with slight geometry changes in the emitting state, which also depends on the rigidity of the environment.90 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

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 sample in a PMMA matrix with a dopant concentration of c < 1 wt %, respectively.

analogy to other tetraphosphine-coordinated Ag(I) complexes,91 the lower-energy band is assigned to the (M +L)L′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 Ag2(tpbz)(P2-nCB)2 is shown in Figure 4 with a nonscaled absorption axis because of the 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 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 that in the ground state and, consequently, to a large number of vibrational Franck−Condon transitions involved.92 The ambient-temperature emission spectrum of Ag2(tpbz)(P2-nCB)2 doped in PMMA (poly(methyl methacrylate) is redshifted compared to that of a powder sample and appears centered at 575 nm (Figure 4). The photoluminescence quantum yield drops from ΦPL = 70% for the powder sample to ΦPL = 35% for the doped PMMA film. (Table 1) The two effects, the red shift of emission and the drop of quantum yield, are related to a larger extent 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 nonradiative relaxation.25,92 Such an influence of the 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

Figure 3. Contour plots of the frontier orbitals (M062X/def2-SVP) of Ag2(tpbz)(P2-nCB)2 (isovalue = 0.05) calculated for the gas-phaseoptimized (M06/def2-SVP) T1 state geometry.

S1 and T1 as being of metal(M)+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, ΔE(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 an approximate value of ΔE(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 f(S0 → S1) of the S0 → S1 transition amounts to 0.1077. This is a large value 704

DOI: 10.1021/acs.jpclett.7b03160 J. Phys. Chem. Lett. 2018, 9, 702−709

Letter

The Journal of Physical Chemistry Letters

transition and a weak spin−orbit coupling (SOC) with the states of singlet manifold. A comparably long decay time of the T1 state has also been reported for other Ag(I) 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 ΔE(S1−T1), and (ii) 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 ΔE(S1−T1), can be determined by fitting the experimental decay time data by a Boltzmann-type equation (eq 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.

Table 1. Emission Data of Ag2(tpbz)(P2-nCB)2 in Different Environments λmax(300 K) ΦPL(300 K) τ(300 K) kr(300 K) ΦPL(77 K) τ(77 K) kr(77 K) knr(77 K)a τ(T1, 40 K)b plateau kr(T1, 40 K) kr(S1 → S0)c ΔE(S1−T1)c

powder

PMMA

555 nm 70% 1.9 μs 3.7 × 105 s−1 ∼100% 675 μs 1.5 × 103 s−1