Highly Efficient Thermally Activated Delayed Fluorescence in

Sep 22, 2016 - Abstract: The complexes [Cu(I)(POP)(dmbpy)][BF4] (1) and ... as a consequence of a small energy separation between the S1 (singlet) and...
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Highly Efficient Thermally Activated Delayed Fluorescence in Dinuclear Ag(I) Complexes with a Bis-Bidentate Tetraphosphane Bridging Ligand Jin Chen,† Teng Teng,‡ Liju Kang,‡ Xu-Lin Chen,‡ Xiao-Yuan Wu,‡ Rongmin Yu,*,‡ and Can-Zhong Lu*,‡ †

College of Life Science, University of Fujian Agriculture and Forestry, Fuzhou, Fujian 350002, China Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China

‡

S Supporting Information *

ABSTRACT: A series of highly emissive neutral dinuclear silver complexes [Ag(PPh3)(X)]2(tpbz) (tpbz = 1,2,4,5tetrakis(diphenylphosphanyl)benzene; X = Cl (1), Br (2), I (3)) was synthesized and structurally characterized. In the complexes, the silver atoms with tetradedral geometry are bridged by the tpbz ligand, and the ends of the molecules are coordinated by a halogen anion and a terminal triphenylphosphine ligand for each silver atom. These complexes exhibit intense white-blue (λmax = 475 nm (1) and 471 nm (2)) and green (λmax = 495 nm (3)) photoluminescence in the solid state with quantum yields of up to 98% (1) and emissive decay rates of up to 3.3 × 105 s−1 (1) at 298 K. With temperature decreasing from 298 to 77 K, a red shift of the emission maximum by 9 nm for all these complexes is observed. The temperature dependence of the luminescence for complex 1 in solid state indicates that the emission originates from two thermally equilibrated charge transfer (CT) excited states and exhibits highly efficient thermally activated delayed fluorescence (TADF) at ambient temperature. At 77 K, the decay time is 638 μs, indicating that the emission is mainly from a triplet state (T1 state). With temperature increasing from 77 to 298 K, a significant decrease of the emissive decay time by a factor of almost 210 is observed, and at 298 K, the decay time is 3.0 μs. The remarkable decrease of the decay time indicates that thermal population of a short-lived singlet state (S1 state) increases as the temperature increases. The charge transfer character of the excited states and TADF behavior of the complexes are interrogated by DFT and TDDFT calculations. The computational results demonstrate that the origin of TADF can be ascribed to 1,3(ILCT + XLCT+ MLCT) states in complexes 1 and 2 and 1,3(XLCT) states mixed with minor contributions of MLCT and ILCT in complex 3.

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tetrahedrally coordinated cuprous complexes [Cu(NN) (PP)]+ possessing a diimine (NN) ligand and a bis-phosphine (PP) chelating ligand are perhaps the most well-known.5 For [Cu(NN) (PP)]+ complexes, the highest occupied molecular orbital (HOMO) has a predominant metal d character, partially mixed with contributions from the bisphosphine, while the lowest unoccupied molecular orbital (LUMO) is essentially localized on the π* orbital of the diimine.6 The composition of the frontier orbitals reveals that their photoluminescence involves mainly charge transfer transitions, including ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT), and the latter is enhanced by both the low oxidation potential of the Cu(I) core and the absence of a higher-lying metal centered dd-transition. The spatial separation of the HOMO and LUMO orbitals leads

INTRODUCTION Highly emissive transition metal complexes continue to receive a great deal of attention because of their various applications, especially in the field of optoelectronic devices, such as organic light-emitting devices (OLEDs) and light-emitting electrochemical cells (LEECs).1 Most previous studies on luminescent materials for OLEDs have focused on phosphors of heavy transition metals including iridium, platinum, and osmium.1i,2 However, the natural resources of these metals are very limited, and they are usually highly expensive. Under this situation, it is demanding to find alternative materials with reliable resources and affordable cost.3 Over the past decade, coinage metal emissive materials have immerged with promising electroluminescent properties and cost advantages, and have been extensively studied as emitting dopants for use in OLEDs.3,4 Many materials with excellent electroluminescence performance comparable to the standardsetting iridium materials have been found, among which © XXXX American Chemical Society

Received: January 11, 2016

A

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

complexes. Herein, we present a class of neutral binuclear emissive silver complexes with this ligand as linker. These complexes have been found to be highly efficient TADF materials at ambient temperature with very high PLQYs in solid state (up to 98%), which are the highest values for Ag(I) complexes reported so far. The emissive decay times of these complexes are very short and comparable to those found in benchmark Ir(III) emissive compounds.1i,11

to a small exchange integral and consequently to a small energy gap ΔE(S1−T1) between the lowest triplet state T1 and the lowest singlet state S1. The better the separation, the smaller the ΔE(S1−T1) is, resulting in a Boltzmann-governed intersystem conversion of the S1 states and the T1 state. Therefore, the Cu(I) emissive materials can emit a very efficient thermally activated delayed fluorescence (TADF) that overcomes the poor efficiency and long decay time of triplet emission (phosphorescence) of Cu(I) phosphorescent materials due to the relatively small effectiveness of spin−orbit coupling of the copper atom. Because TADF materials can convert excitons from the triplet state to the singlet state, such materials can harvest both singlet and triplet excitons in the process of electroluminescence. Indeed, highly efficient OLEDs with external quantum efficiency over 20% using Cu(I) TADF materials have already been realized.4d Although there are many reports on luminescent silver complexes, most reported to date are multinuclear complexes that have argentophilic bonding, an analogue of aurophilic bonding known as “ closed-shell (d10−d10) interactions ”.2d,7 Emissive Ag(I) complexes with tetrahedral coordination geometry similar to the Cu(I) complexes have been seldom studied possibly because of their potential photosensitivity and limited luminescent properties.8 As the metal atom goes down from Cu(I) to Ag(I), the increase in effective nuclear charge leads to a stronger net attracting force, and the d orbitals of Ag(I) are lower in energy, which, in turn, results in an increase of the metal oxidation potential. As a result, the Ag(I) ion is more inert to oxidation than Cu(I), and unlike the cases of mononuclear Cu(I) emissive compounds, the highest occupied molecular orbital (HOMO) of mononuclear Ag(I) compounds often contains no silver contribution. Therefore, in the lowest lying singlet and triplet excited states, no MLCT contribution is found, and the luminescent Ag(I) complexes show emission usually due to a ligand-centered ππ* transition.8f Recently, luminescent properties of a series of electronically and structurally diverse Ag(I)-bis(diphosphine) complexes (diphosphine = 1,2-bis(diphenylphosphino)benzene (dppb) and its analogues and derivatives) have been investigated using electrochemical and temperature-dependent emission measurements.8a−c,h These Ag(I) complexes have interesting emission properties. Emissive properties of the complexes and the computational results show that these Ag(I)-bis(diphosphine) complexes have excited states with evident MLCT character, which is remarkable for Ag(I) complexes.8f Moreover, Yersin and co-workers reported a highly emissive dinuclear silver [Ag2Cl2(dppb)2] that demonstrates very efficient TADF at ambient temperature.9 These studies highlight the promising potential to develop new Ag(I) emitters with TADF and MLCT character. In our studies, we chose 1,2,4,5-tetrakis (diphenylphosphino)benzene (tpbz) as a bis-bidentate linker for the design of [Ag-L-Ag] dimers because an analogous bidentate ligand, 1,2-bis(diphenylphosphino)benzene, and its derivatives have been used in the syntheses of emissive Ag(I) complexes.8a−c The tpbz ligand contains four coordinative sites, and can be used as an interesting and versatile ligand in the syntheses of coordination compounds and polymers. However, the chemistry of this ligand and its compounds has barely been explored.10 To the best of our knowledge, very few examples of emissive compounds with this ligand have been reported so far.10g Recently, this tetraphosphane ligand has been used by us in the synthesis of various emissive Cu(I) and Ag(I) binuclear

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EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under air atmosphere unless specified. Chemicals were purchased from commercial sources and used without further purification. The complex 1,2,4,5-tetrakis(diphenylphosphino)benzene was prepared according to a literature procedure.12 Elemental analyses (C, H, N) were carried out with an Elementar Vario EL III elemental analyzer. Preparation of [Ag(PPh3) (Cl)]2(tpbz)·6DMF (1·6DMF). A mixture of AgCl (29 mg, 0.2 mmol)) and PPh3 (262 mg, 1 mmol) in 10 mL of DMF was stirred for about 1 h to give a clear yellow solution, and then tpbz (81 mg, 0.1 mmol) was added. The resulting mixture was stirred further for 2 h. After filtration, the filtrate was allowed to stand undisturbed. Yellow crystals of the products appeared gradually over a period of 2 weeks. Yield: 146 mg, 71%. Anal. Calcd for C90H72Ag2Cl2P6 (%): C, 66.46; H, 4.43. Found: C, 66.36; H, 4.51. FTIR (KBr pellet, cm−1): 3047m, 1585w, 1479s,1434s,1184w, 1158w, 1093s, 1027w, 997w, 757s, 739s, 691s, 621s, 605s, 521s, 514s, 504s, 487s, 473s. Preparation of [Ag(PPh3) (Br)]2(tpbz) (2). A procedure similar to that used for compound 1 was followed but with AgBr (38 mg, 0.2 mmol) instead of AgCl. Yield: 135 mg, 79%. Anal. Calcd for C90H72Ag2Br2P6 (%): C, 63.01; H, 4.20. Found: C, 62.79; H, 4.31. FTIR (KBr pellet, cm−1): 3049m, 1585w, 1479s,1434s,1185w, 1158w, 1094s, 1027w, 997w, 756s, 740s, 691s, 622s, 606s, 521s, 515s, 504s,487s, 445s. Preparation of [Ag(PPh3)(I)]2(tpbz) (3). A procedure similar to that used for compound 1 was followed but with AgI (47 mg, 0.2 mmol) instead of AgCl. Yield: 145 mg, 80%. Anal. Calcd for C90H72Ag2I2P6 (%): C, 59.73; H, 3.98. Found: C, 60.06; H, 4.15. FTIR (KBr pellet, cm−1): 3050m, 1584w, 1478s,1434s,1184w, 1159w, 1094s, 1026w, 998w, 745s, 739s, 692s, 620s, 603s, 541s, 516s, 494s, 485s, 457s. X-ray Crystallographic Analysis. Diffraction data were collected on a SuperNova, Dual, Cu at zero, Atlas diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods with SHELXS and refined by full-matrix least-squares methods with the SHELXL program package.13 All the non-hydrogen atoms were located with successive difference Fourier synthesis and refined anisotropically. Hydrogen atoms were added in idealized positions. The crystallographic data of complexes 1−3 are summarized in Table S1, and selected bond distances and angles are listed in Table S2. Photophysical Measurements. The powder samples were obtained by drying the crystals in vacuum and were measured without further treatment. Photoluminescence spectra at room temperature and 77 K were recorded on a HORIBA Jobin-Yvon FluoroMax-4 spectrometer and an Edinburgh Analytical instrument FLS920, respectively. The lifetimes of powder samples were measured on an Edinburgh Analytical instrument FLS920 with a Picosecond Laser Diode. The powder PL quantum yields were defined as the number of photons emitted per photon absorbed by the systems and measured by an Edinburgh FLS920 fluorescence spectrometer equipped with an integrating sphere. The infrared spectra in the region of 4000−400 cm−1 were obtained in KBr discs with a PerkinElmer FT-IR Spectrum One. Cyclic voltammetry was carried out in a gas-tight singlecompartment three-electrode cell with a BAS Epsilon Electrochemical Analyzer at room temperature. A glassy carbon disk and a platinum wire were used as working and auxiliary electrodes, respectively; the reference electrode was Ag/Ag+ (0.1 M of AgNO3 in acetonitrile). The CV measurements were performed in anhydrous and nitrogensaturated dichloromethane solutions with 0.1 M n-tetrabutylammoB

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry nium perchlorate (TBAP) and about 0.01 mM silver complexes. Under the present experimental conditions, the E1/2 for the Fc+/Fc couple consistently occurred at +680 mV in CH2Cl2. The UV−vis absorption spectra were recorded with a Perkin-Elmer Lambda 45 UV−vis spectrophotometer. Computational Methodology. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed with the Gaussian 09 program package,14 using the hybrid Becke three-parameter Lee−Yang−Parr (B3LYP) functional level.15 The input structures were extracted from the X-ray crystallographic data. In all calculations, the relativistic effective core potential (RECP) and the associated basis set Lanl08 (f) and Lanl08(d),16 which are the revised version of the original Hay-Wadt basis set, were employed for the Ag(I), Br, and I atoms. An all-electron basis set of 6-31G* was used for Cl, P, C, and H. Visualization of the optimized structures and frontier molecular orbitals was performed with GaussView. The partition orbital compositions were analyzed by the Multiwfn 2.4 program.17

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RESULTS AND DISCUSSION

Syntheses and Crystal Structures. The studied complexes were prepared by the reaction of each corresponding silver halide and excess amount of PPh3 in DMF with the bisbidentate tetraphosphane ligand. The mixtures were stirred until a clear solution was obtained, and then filtered. Analytically pure crystalline products of the compounds grew gradually from the filtrate over a period of 2 weeks. The addition of an excess amount of PPh3 facilitated the formation of homogeneous reaction mixtures. Once the solid complexes are formed, they are only slightly soluble in common organic solvents, including CH2Cl2, CHCl3, and DMF. It is interesting that the presence of a small amount of PPh3 improves the solubility of the complexes in CH2Cl2 and CHCl3. Silver complexes are notorious for their light sensitivity and usually decompose easily to silver metal. The studied complexes are relatively of high stability that may be a direct result of the stabilization effect of the bis-bidentate tetraphosphane ligand. The crystals obtained are suitable for X-ray diffraction. Respective ORTEP representations are shown in Figure 1. As shown in Figure 1, these complexes have very similar molecular structures which can be viewed as containing two neutral [AgX(PPh3)] units and a tpbz tetraphosphane bridging ligand. The silver atoms are four-coordinated in a distorted tetrahedral geometry defined by two P atoms from the tetraphosphane bridging ligand, one halide anion and one P atom from a terminal triphenylphosphine ligand. The Ag−X and Ag−P distances are in the normal ranges. The small P(1)− Ag(1)−P(2) angles in these complexes are due to the steric rigidity and small bite angle of the tpbz ligand. The dihedral angles between the X−Ag(1)−P(3) plane and the P(1)− Ag(1)−P(2) plane for 1 and 2 (89.58(4)° and 89.84(1)° for 1 and 2, respectively) are very close to 90° for an ideal tetrahedron, while the angle for 3 (84.44 (4)°) deviates more as compared to those in 1 and 2. The central phenylene ring and the phosphine atoms of the tpbz ligand are essentially planar. The two Ag(I) atoms in each molecule are located above and below the (P2C6H2P2) mean plane, respectively, leading to the formation of a chairlike conformation for the [Ag](P2C6H2P2)[Ag] fragment. The distances away from the (P2C6H2P2) mean plane for the Ag(I) atoms are 0.745(1), 1.2961(9), and 1.204(3) Å for 1, 2, and 3, respectively. The dihedral angles between the P(1)−Ag(1)−P(2) plane and the (P2C6H2P2) mean plane that reveals the magnitude of the butterfly conformation deviating from the planar conformation

Figure 1. ORTEP diagrams of complexes 1−3 with thermal ellipsoids at the 50% probability level. Solvent molecules, the anion, and H atoms are omitted for clarity.

are 23.55(4)°, 37.81(3)°, and 41.55(2)° for 1, 2, and 3, respectively. The intramolecular distance between the Ag(I) centers in these complexes is about 9.0 Å. Complex 1 exhibits the largest Ag···Ag distance (9.1313(4) Å), which is a result of the relatively more planar structure of the [Ag](P2C6H2P2)[Ag] core of this molecule. A comparison of the interatomic distances of these complexes reveals that the lengths of the silver halide bonds (Ag−X) increase from 1 (2.5513(5) Å) to 2 (2.6838(3) Å) to 3 (2.8184(6) Å), which is due to the larger ionic radius of the heavier halide atom. Photophysical Properties. The photophysical data of the complexes are summarized in Table 1. Absorption spectra and emission spectra of the complexes and free tpbz ligand in degassed CH2Cl2 solutions are shown in Figure 2. As shown in Figure 2, the absorption spectrum of the free tpbz ligand exhibits an intense band at 290 nm that is assigned to a mixed transition of n → π* and π → π* character.10g The C

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Absorption and Emission Data of Complexes 1−3 solution complex 1 2 3

λabs/nm (ε × 10

−3

film (5.0% in PMMAd)

crystalline powder M

−1

−1

cm )

268(47.3), 379(4.6)a 267, 384b 272, 400b

λemc

τ/μs

475 471 495

3.0 2.9 2.5

Φ (%)

λemc

τ/μs

Φ (%)

98 91 74

531 519 517

5.3 4.5 4.0

34 40 16

a c ≈ 2 × 10−5 M in degassed CH2Cl2. bSolubility of the complexes 2 and 3 in CH2Cl2 is very poor, and the concentrations of their solutions cannot be calculated precisely. cExcitation wavelengths for the emission spectra are 369, 370, and 370 nm for complexes 1, 2, and 3, respectively. d Concentration is calculated in weight.

are shown in Figure 3. All the complexes exhibit broad and unstructured emission bands with a maximum at 471−495 nm,

Figure 2. Absorption spectra of complexes 1−3 and free tpbz ligand in CH2Cl2.

Figure 3. Emission spectra of complexes 1−3 in powder states at 77 K (dashed lines) and 298 K (solid lines). The excitation wavelengths for the emission spectra are 369, 370, and 370 nm for complexes 1, 2, and 3, respectively.

absorption spectra of complexes 1−3 are more complicated and broader than that of the free tpbz ligand. All complexes have an intense broad absorption band centered at about 270 nm, and a less intense shoulder in the range of 360 to 400 nm. The bands at about 270 nm are close to the intense band observed in the spectrum of the tpbz ligand, suggesting that these bands involve intraligand (IL) character. The weaker absorption bands from 360 to 400 nm are not observed for the free tpbz ligand, and are assigned to transitions affected by the Ag(I) atoms and halide atoms. These assignments are in accordance with theoretical results of TDDFT and DFT (see below). The excitation spectra of the complexes have also been measured. The spectra are shown in Figure S2. These complexes have two obvious peaks at about 270 and 370 nm. The excitation spectrum of complex 3 shows an extra peak at 414 nm, where the spectra of 1 and 2 have small bumps. The excitation spectra fit well with the absorption spectra in CH2Cl2. The emission spectra of complexes 1−3 were measured with powder samples. The solid state samples usually are not appropriate for detailed investigation of decay properties because concentration quenching processes, such as triplet− triplet annihilation or energy transfer between adjacent molecules, can strongly influence deactivation mechanisms. However, for the studied dinuclear silver complexes (1−3), such processes do not seem to be of importance as indicated by the very high φPL values in the solid state. Similar observations have been made in many emissive Cu(I) and Ag(I) complexes with low-lying 1,3MLCT states in which because of self-trapping mechanisms, the emission displays largely isolated molecular properties even of the compounds embedded in a crystalline cage of the neat material.1e,18 The studied complexes show intense white-blue (1, 2) and green (3) luminescence under UV excitation. Their emission spectra at ambient temperature and liquid nitrogen temperature

which can be ascribed to intramolecular charge-transfer (CT) transitions. At room temperature, the photoluminescence quantum yields (PLQY) of these complexes are 98%, 91%, and 74% for 1, 2, and 3, respectively. The transient photoluminescence decay characteristics of these complexes show one component decays with decay lifetimes of 3.0, 2.9, and 2.5 μs for complexes 1−3, respectively. It is noteworthy to point out that there is no prompt fluorescence with nanosecond decay lifetime observed for these complexes. According to kr = ΦPLτ−1, the corresponding radiative rates kr at room temperature can be calculated to be 3.3 × 105, 3.1 × 105, and 3.0 × 105 s−1 for complexes 1−3, respectively. The magnitudes of the lifetimes and radiative rates indicate that the photoluminescence transitions have a triplet nature. The PLQYs of these binuclear silver complexes are much higher than those of mononuclear silver complexes reported in the literature. For example, mononuclear complexes [Ag(X) (PPh3) (dppb)] (dppb = 1,2-bis(diphenylphosphanyl)benzene; X = Cl, Br, I) which have a very similar coordination environment to 1−3 have much weaker luminescence.19 The PLQYs for [Ag(X) (PPh3) (dppb)] are only 5.5% (X = Cl), 8.3% (X = Br), and 28% (X= I), respectively. The emissive properties of complexes 1−3 were also investigated in PMMA with a concentration of 5.0%.19 As shown in Table 1, the luminescence properties of complexes 1−3 strongly depend on the environment. A red shift of the emission accompanied by a considerable reduction of the emission quantum yield is observed for all the complexes. These observations can be explained by changes of the molecular geometry in excited states from pseudotetrahedral to a more flattened structure.8c Distortion of the molecular geometry results in a smaller energy gap between the ground state and the excited state, and an increase of radiationless D

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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temperature indicate that the complex at room temperature emits via thermally activated delayed fluorescence (TADF), which has been observed often from emissive Cu(I) complexes and organic donor−acceptor compounds with small S1−T1 energy gaps.4f,8d,23 According to the relatively long decay time, it is obvious that the emissions are phosphorescent (T1 → S0) dominantly at low temperature, and the corresponding emissive transition has a long decay time because the transition from the lowest triplet state to the singlet ground state is spin-forbidden. With increasing temperature, thermal population of the upperlying state occurs gradually. The transition corresponding to the higher energy state is significantly more likely than the lowtemperature phosphorescence as shown by the decrease of decay time upon temperature increase. At ambient temperature, the higher-lying state is significantly populated via efficient thermally activated conversion from the T1 state and in turn emits efficient thermally activated delayed fluorescence (TADF). Analogous to the reported results of TADF, the higher-lying state is assigned as the S1 state. For thermally equilibrated states, the observed decay time (τobs) of the two excited states can be expressed as a function of the temperature as shown in eq 1 that has been used generally in the literature.18b,23a

deactivation because of a better overlap of ground state and excited state vibrational wave functions.20 Such a distortion is affected by many factors, including the rigidity of the matrix.21 The distortions of the studied complexes are suppressed largely in the crystalline state, and thus high emission quantum yields are observed. Similar observations have often been reported for emissive Cu(I) and Ag(1) complexes.4d,8c,22 The emission spectra of these complexes in the solid state were also measured at 77 K. With temperature increasing from 77 to 298 K, a blue shift of the emission maximum by 9 nm for all these complexes was observed. To further understand the emission proprieties of the complexes, the observed lifetimes (τobs) of complex 1 at varied temperatures from 77 to 298 K were investigated and the temperature-dependent results are shown intuitively in Figure 4. For complex 1, the observed

(

3 + exp τobs =

3 τT

+

1 τS

−ΔEST kB T

(

× exp

)

−ΔEST kB T

)

(1)

where kB is the Boltzmann constant, τS and τT are the decay lifetimes of the S1 and T1 states, respectively, and ΔEST is the energy gap between the S1 and T1 states. With the use of a weighted linear least-squares fitting method, the parameters, τT, τS, and ΔEST in eq 1 are determined as shown in the inset of Figure 4. An analysis of these data allows us to have a better understanding of the emission process. For complex 1, the τT fitted value of 680 μs is close to the measured value of 638 μs at 77 K, which confirms that the dominated excited state is T1 at this temperature as mentioned above. The fit values of ΔEST of 0.065 eV is in good agreement with the value (0.10 eV) determined from the emission spectra. The small energy gap between the S1 and T1 states facilitates thermally activated conversion of T1 → S1 and therefore leads to pronounced TADF at ambient temperature. The fit τS value of 180 ns is 3 orders of magnitude smaller than the fit values of τT. These results support the presented model in which the higher- and lower-energy states are assigned to a spin-allowed S1 and a spinforbidden T1 state, respectively. At room temperature, the

Figure 4. (a) Temperature-dependent emission lifetimes of complexes 1. The solid line is a fit curve according to eq 1. (b) Energy levels for ground state and lowest excited state of complexes 1. Herein, τDF, τP, and ΔEst represent the decay time of the delayed fluorescence at room temperature, the decay time of the transition of T 1 → S 0 (phosphorescence), and the energy gap between S1 and T1 state, respectively.

lifetime was reduced drastically by about 230 times from 638 μs at 77 K to 3.0 μs at 298 K. The severe decrease of the emission lifetimes indicates that the emissions of these complexes may arise from two interconvertible excited states in thermal equilibrium. The change of the emission lifetimes along with the blue shift of the emission maxima with increasing

Figure 5. Orbital contour maps of HOMO and LUMO for 1−3 with structures optimized in the ground state. E

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Natural transition orbital pairs for (A) the lowest singlet excited state and (B) the lowest triplet excited states of 1 in the optimized S0 structure. The generation probabilities are 98% and 97%, respectively.

the HOMO → LUMO transition possesses significant chargetransfer (CT) character. The transition energies from TDDFT calculations, oscillator strengths, and configurations for the most relevant singlet excited states in each complex are listed in Table S4. The calculated S0 → S1 values in these complexes are found at 396, 403, and 423 nm for 1, 2, and 3, respectively. Although the calculations do not consider solvent effects on the complexes (gas phase calculations), the results are in good agreement with the trend of the experimental values of 379 (1), 384 (2), and 400 (3) nm in the absorption spectra of these complexes in CH2Cl2. The S0 → S1 transition comes mainly from the HOMO → LUMO. The calculations also reveal that the HOMO → LUMO transition contributes largely to the T1 → S0 transition. The configurations of HOMO → LUMO to S1 and T1 are 98 and 93% in 1, 96 and 94% in 2, and 75 and 50% in 3, respectively, as listed in Table S4. Natural transition analyses (NTO) show that the lower-lying transitions for these complexes all contain substantial CT character. Maps of the hole and electron in the S1 and T1 states of 1 are illustrated in Figure 6. The maps of S1 and T1 for complex 1 are similar to each other, because the S1 and T1 states are composed essentially of the HOMO → LUMO transition (Figure 6). The hole in 1 is mainly confined to the orbitals of the silver atoms, the Cl atoms and the phosphorus atoms; and the remaining is distributed over the phenyl groups of the tpbz ligand. The electron distribution lies mainly on the phenylene and the phosphorus atom of the tpbz ligand. The contributions from CT and π → π* transitions to fluorescence (S1 → S0) were calculated to be 60% and 28%, respectively. The CT transition in S1 → S0 contains contributions from MLCT (14%), XLCT (15%), and ILCT(32%). The contributions from CT and π → π* transitions to phosphorescence (T1 → S0) were calculated to be 56% and 31%, respectively. The CT transition in T1 → S0 consists of contributions from MLCT (14%), XLCT (10%), and ILCT(32%). NTO maps and origins of photoluminescence for complexes 2 and 3 are shown in Figures S8 and S9. The contributions from the CT transition in S1 → S0 and T1 → S0 are 63% and 58% for 2, and 73% and 66% for 3, respectively. According to the results of NTO, the lowest excited states of complex 1 and 2 can be assigned as 1,3(ILCT + XLCT+ MLCT) states, while those of complex 3 are assigned as

decay time of the delayed fluorescence is 3.0 μs, i.e., about 18 times larger than the fit τS value. The electrochemical properties of these complexes were investigated by cyclic voltammetry (CV). Irreversible oxidation waves were observed at 999, 1001, and 766 mV vs Ag/Ag+ for complexes 1, 2, and 3, respectively, as compared to the irreversible oxidation wave at 905 mV for the tpbz ligand (Figure S3). The oxidation process for complex 3 occurs at a lower potential than those of complexes 1 and 2, indicating that the HOMO for 3 is higher in energy than in the latter. From the onsets of the oxidation waves, the energies of the HOMOs, −5.00, −4.97, and −4.74 eV for complexes 1, 2 and 3, respectively, were estimated. This result is consistent with the fact that the increasing electron-donating character of the heavier halide destabilizes the HOMO of the complexes. The solid reflectance spectra of the complexes were measured, and the spectra are shown in Figure S4. From the spectra, the energy gaps (ΔEg) of HOMO−LUMO (2.73 eV for 1, 2.70 eV for 2, and 2.66 eV for 3, respectively) are estimated. From the energy values of HOMO and ΔEg, the energies of the LUMOs, −2.27, 2.27, and 2.08 eV for complexes 1, 2, and 3, respectively, are calculated. DFT and TDDFT calculations provide useful information on the nature of the electronic structure and electronic excited states, although the absolute value of the charge populations depends sensitively upon the choice of the basic set. The electronic ground states of these complexes were calculated based on the structures optimized in the ground state. As shown in Figure 5, the contributions in the lowest unoccupied molecular orbital (LUMO) of these complexes are very similar to each other, and are mainly composed of the π*-antibonding orbital of the phenylene group in the tpbz ligand. The contribution in the highest occupied molecular orbital (HOMO) is mainly associated with the Ag, P, and halide atoms, and the halogen ions directly bound to the silver atoms evidently have an influence on the electronic structures of the frontier orbitals. In 1, the contributions from Ag, P, and Cl atoms are 14.60%, 37.22%, and 21.98%. In 2, the contributions from Ag, P, and Br atoms are 10.89%, 25.23%, and 45.35%. In 3, the contributions from Ag, P, and halogen atoms are 6.41%, 6.97%, and 77.14%. The contributions from Ag and P atoms are in the order of 1 > 2 > 3, while the contributions of halogen atoms are in the order of 1 < 2 < 3. The electron density of the HOMO in 3 is mainly from I ions. These results indicate that F

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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the Chinese Academy of Sciences (KJCX2-YW-319), and the National Natural Science Foundation of China (21373221, 21521061, 51672271, 21671190).

(XLCT) because the contribution of XLCT in complex 3 is much bigger than those of MLCT and ILCT. As mentioned above, the lowest excited states, S1 and T1, have substantial charge transfer character that induces distinct spatial separations of the involved orbitals, leading to relatively small exchange integrals and consequently to small energy gaps ΔE(S1 − T1) between T1 and S1. The TDDFT calculations based on the optimized S0 structures reveal that the energy gaps between the S1 and T1 states are very small (0.12, 0.09, and 0.04 eV for complexes 1, 2, and 3, respectively), which are consistent with the experimental data calculated from the emission spectra of the complexes at 77 and 298 K as shown in Figure 3. The ΔE(S1−T1) values have also been calculated based on the optimized T1 structures and X-ray structures, respectively. The obtained ΔE(S1−T1) (as shown in Table S6) are smaller than 0.2 eV. The results of the calculations confirm further the major CT character of the emissive processes from the S1 and T1 excited states, and indicate theoretically that these complexes can show very efficient thermally activated delayed fluorescence.

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(1) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395 (6698), 151−154. (b) Kessler, F.; Costa, R. D.; Di Censo, D.; Scopelliti, R.; Orti, E.; Bolink, H. J.; Meier, S.; Sarfert, W.; Gratzel, M.; Nazeeruddin, M. K.; Baranoff, E. Near-UV to Red-Emitting Charged Bis-Cyclometallated Iridium(III) Complexes for Light-Emitting Electrochemical Cells. Dalton Trans 2012, 41 (1), 180−191. (c) Du, B.-S.; Liao, J.-L.; Huang, M.-H.; Lin, C.-H.; Lin, H.-W.; Chi, Y.; Pan, H.-A.; Fan, G.-L.; Wong, K.-T.; Lee, G.-H.; Chou, P.-T. Os(II) Based Green to Red Phosphors: A Great Prospect for Solution-Processed, Highly Efficient Organic Light-Emitting Diodes. Adv. Funct. Mater. 2012, 22 (16), 3491−3499. (d) Lin, C.-H.; Hsu, C.-W.; Liao, J.-L.; Cheng, Y.-M.; Chi, Y.; Lin, T.-Y.; Chung, M.-W.; Chou, P.-T.; Lee, G.H.; Chang, C.-H.; Shih, C.-Y.; Ho, C.-L. Phosphorescent OLEDs Assembled using Os(II) Phosphors and a Bipolar Host Material Consisting of Both Carbazole and Dibenzophosphole Oxide. J. Mater. Chem. 2012, 22 (21), 10684−10694. (e) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. The Triplet State of OrganoTransition Metal Compounds. Triplet Harvesting and Singlet Harvesting for Efficient OLEDs. Coord. Chem. Rev. 2011, 255 (21− 22), 2622−2652. (f) Baranoff, E.; Fantacci, S.; De Angelis, F.; Zhang, X.; Scopelliti, R.; Gratzel, M.; Nazeeruddin, M. K. Cyclometalated Iridium(III) Complexes Based on Phenyl-Imidazole Ligand. Inorg. Chem. 2011, 50 (2), 451−462. (g) Murawski, C.; Liehm, P.; Leo, K.; Gather, M. C. Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll-Off of Phosphorescent Organic Light-Emitting Diodes. Adv. Funct. Mater. 2014, 24 (8), 1117−1124. (h) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. High-Efficiency Fluorescent Organic Light-Emitting Devices using a Phosphorescent Sensitizer. Nature 2000, 403 (6771), 750−753. (i) Costa, R. D.; Orti, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Luminescent Ionic TransitionMetal Complexes for Light-Emitting Electrochemical Cells. Angew. Chem., Int. Ed. 2012, 51 (33), 8178−8211. (j) Chi, Y.; Chou, P. T. Transition-Metal Phosphors with Cyclometalating Ligands: Fundamentals and Applications. Chem. Soc. Rev. 2010, 39 (2), 638−655. (k) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X. Recent Progress in Metal-Organic Complexes for Optoelectronic Applications. Chem. Soc. Rev. 2014, 43 (10), 3259−3302. (l) Tang, M.; Tsang, D. P.; Wong, Y.; Chan, M.; Wong, K.; Yam, V. W. Bipolar Gold(III) Complexes for Solution-Processable Organic Light-Emitting Devices with a Small Efficiency Roll-Off. J. Am. Chem. Soc. 2014, 136 (51), 17861−17868. (2) (a) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the Role of Spin−Orbit Coupling: A Study Based on High-Resolution Optical Spectroscopy. Top. Organomet. Chem. 2010, 29, 193−235. (b) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems. Adv. Mater. 2009, 21 (44), 4418−4441. (c) Marin, V.; Holder, E.; Hoogenboom, R.; Schubert, U. S. Functional Ruthenium(II)- and Iridium(III)-containing Polymers for Potential Electro-Optical Applications. Chem. Soc. Rev. 2007, 36 (4), 618−635. (d) Visbal, R.; Gimeno, M. C. N-Heterocyclic Carbene Metal Complexes: Photoluminescence and Applications. Chem. Soc. Rev. 2014, 43 (10), 3551−3574. (3) Barbieri, A.; Accorsi, G.; Armaroli, N. Luminescent Complexes beyond the Platinum Group: the d10 Avenue. Chem. Commun. (Cambridge, U. K.) 2008, 19, 2185−2193. (4) (a) Zhang, Q.; Zhou, Q.; Cheng, Y.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Highly Efficient Electroluminescence from Green-LightEmitting Electrochemical Cells Based on CuI Complexes. Adv. Funct. Mater. 2006, 16 (9), 1203−1208. (b) Zhang, Q.; Ding, J.; Cheng, Y.;

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CONCLUSION A series of neutral binuclear silver complexes with tetrahedral geometry, [Ag(PPh3)(X)]2(tpbz) (X = Cl (1), Br (2), I (3)), has been synthesized. The crystalline powders of the complexes emit strong white−blue or green light under irradiation with UV light. Photophysical and theoretical studies of these complexes indicate that their luminescence in the solid state at ambient temperature results from TADF. Heavier halogen ligands with electrodonating character slightly reduce the contribution of metal orbitals to the HOMOs of the complexes, but apparently reduce the contribution of ILCT to the HOMOs of the complexes. Consequently, the origin of TADF changes from 1,3(ILCT + XLCT+ MLCT) states in complexes 1 and 2 to 1,3(XLCT) states mixed with a minor contribution of MLCT and ILCT in complex 3. This study shows that the introduction of tpbz as a bridging ligand to synthesize dinuclear silver emissive complexes is an effective way to improve the photoluminescence of silver complexes in the solid state.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00068. Experimental details, crystallographic data, theoretical calculation (PDF) Crystallographic data 1 (CIF) Crystallographic data 2 (CIF) Crystallographic data 3 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), G

DOI: 10.1021/acs.inorgchem.6b00068 Inorg. Chem. XXXX, XXX, XXX−XXX

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