Harnessing Fluorescence versus Phosphorescence Branching Ratio

Apr 23, 2013 - Marsel Z. Shafikov , Dmitry N. Kozhevnikov , Michael Bodensteiner , Fabian Brandl , and Rafał Czerwieniec. Inorganic Chemistry 2016 55...
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Harnessing Fluorescence versus Phosphorescence Branching Ratio in (Phenyl)n‑Bridged (n = 0−5) Bimetallic Au(I) Complexes Yuh-Chia Chang,†,⊥ Kuo-Chun Tang,†,⊥ Hsiao-An Pan,† Shih-Hung Liu,† Igor O. Koshevoy,*,‡ Antti J. Karttunen,§ Wen-Yi Hung,*,∥ Ming-Hung Cheng,∥ and Pi-Tai Chou*,† †

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan R. O. C. Department of Chemistry, University of Eastern Finland, Joensuu 80101, Finland § Department of Chemistry, University of Jyväskylä, PO Box 35, FI-40014 Jyväskylä, Finland ∥ Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan R. O. C. ‡

S Supporting Information *

ABSTRACT: We have designed and synthesized a series of Au(I) complexes bearing either an alkynyl−(phenylene)n−diphosphine (A-0−A-3) or a (phenylene)n−diphosphine (B-1−B-5) bridge, among which the effective distance between Au(I) and the center of the emitting ππ* chromophore can be fine-tuned via the insertion of various numbers of phenylene spacers. We then demonstrated for the first time in a systematic manner the decrease of rate constant for S1 → T1 intersystem crossing (ISC) kisc as the increase of the effective distance. The results also unambiguously showed that the phosphorescence could be harvested via higher S0 → Sn (n > 1) electronic excitation, followed by fast Sn → Tm ISC and then the population at T1 state, bypassing the relatively slow S1 → T1 ISC. The results unify a recent report on higher excited-state relaxation dynamics for the late transition metal complexes (J. Am. Chem. Soc. 2012, 134, 7715−7724). The dual, far separated fluorescence and phosphorescence of the titled complexes make feasible the white light generation in a single molecule unit, as successfully demonstrated using complex B-3 as a dopant to fabricate organic light emitting diodes.

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INTRODUCTION Transition metal complexes have been attracting considerable attention in the emerging field of organic light emitting diodes (OLEDs). In this application, the metal core serves as a key element to facilitate the rate of intersystem crossing (ISC) and the subsequent T1 → S0 radiative decay rate constant, giving rise to highly intensive phosphorescence.1 This, together with theoretically 100% exciton recombination in the triplet manifold, makes the phosphorescent transition metal complex commonly outperforming those of fluorescent materials in lighting application. From the fundamental point of view, the rate constant of S1 → T1 intersystem crossing kisc and T1 → S0 radiative decay kpr are two key elements to harvest the phosphorescence; their underlying fundamentals are expressed as k isc ∝

k rp ∝

an uncommon condition when the T2 state is thermally or symmetrically involved), and μS1 is the S0 → S1 transition dipole. These bases lead to several realizations in regard to harnessing the phosphorescence emission.2 In general, if the triplet state T1 and the singlet state S1 involve the same orbitals, the spin−orbit matrix element |⟨ψT1| HSO|ψS1⟩| becomes zero or very small. Therefore, for one case, the spin−orbit coupling of T1 with a different higher lying singlet state may yield effective intersystem crossing. 3 Furthermore, when the metal dπ orbital is directly involved in the electronic transition, such as dπ → π*, known as the metal to ligand charge transfer (MLCT) or vice versa ligand to metal charge transfer (LMCT), giving rise to a state electronic configuration of dπ*, the influence of core metal atom to the kisc and kpr is mainly two-fold. First, as for the intersystem crossing involving S1(dπ*) → T1(ππ*) (and vice versa S1(ππ*) → T1(dπ*)), the changes of orbital angular momentum (d → π) coupled with the spin flipping greatly enhance the spin− orbit coupling matrix term |⟨ψT1|HSO|ψS1⟩|. Second, the dπ* configuration significantly diminishes the electron exchange

|⟨ψT |HSO|ψS ⟩|2 1

1

(ΔES1− T1)2

(1)

|⟨ψS |HSO|ψT ⟩|2 μS 2 1

1

1

(ΔES1− T1)2

(2) Received: March 17, 2013 Revised: April 19, 2013 Published: April 23, 2013

where HSO denotes the spin−orbit operator, ΔES1−T1 is the energy difference between S1 and T1 (or ψT2 and ΔES1−T2 under © 2013 American Chemical Society

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integral so that ΔE between S1(dπ*) and T1(dπ*) becomes rather small.4 For many emissive late transition metal complexes, both S1 and T1 possess a state mixing between dπ* and ππ*. Therefore, the combination of the above two mechanisms is fully operative, that is, the enlargement of |⟨ψT1| HSO|ψS1⟩| and reduction of ΔES1−T1, resulting in large facilitation of kisc and kpr .5 Clearly, ΔES1−T1 is not directly relevant to how heavy the metal atom is but plays no less important role than the heavy atom effect. This concept has recently been realized and exploited to the first row transition metals such as Cu(I) complexes. Despite the much lighter Cu(I) core atom than that of Os(II), Ir(III), and Pt(II), etc., the decrease of ΔES1−T1 by tuning S1 to the dπ* enriched configuration substantially increases both kisc and krp, resulting in highly intensive phosphorescence for a number of Cu(I) complexes.6 Recently, exploiting the small ΔES1−T1, Adachi and co-workers would be able to efficiently generate fluorescence (delayed fluorescence) thermally repopulated from the triplet manifold, the net result of which breakdowns the spin statistics restriction (25%) for the fluorescence, resulting in the high performance of OLEDs.7 On the other hand, when the transition such as S1 (ππ*) → T1 (ππ*) ISC and T1 (ππ*) → S0 radiative decay takes place, both of which lack contribution from the metal dπ orbital, the negligible changes of orbital angular momentum result in a small |⟨ψT1|HSO|ψS1⟩| coupling matrix. This, together with large ΔES1−T1 due to vast electron exchange integral, renders a drastic reduction of kisc and kpr . In this case, the heavy atom effect emerges and manipulates the spin−orbit coupling, the influence of which is mainly governed by two factors: (1) how heavy the metal atom is, i.e., the atomic number Z; (2) how far the distance is between the metal atom and the designated ππ* chromophore involved in the transition. An oversimplified approach using a hydrogen-like (one-electron) atom deduces the spin−orbit coupling to be proportional to Z4/r3 (and hence kisc ∝ Z8/r6).2a Although this relationship may not be applicable in the many-electron atoms because of the electron shielding effect, in a qualitative manner, the proportionality to (Zeff)n and (1/r)m, in which Zeff denotes the effective atomic number and n and m are arbitrary positive numbers (n, m > 1), may still hold. In a recent contribution, we have inferred ISC involving those dπ* and pure ππ* to the internal and external metal atom effect, respectively.6 For the former case, tuning the percentage of metal dπ* contribution, i.e., MLCT% (or LMCT), in S1 by altering the ligand HOMO π energy level (relative to the metal dπ orbital) has demonstrated that the increase of MLCT% facilitates the rate of ISC and the subsequent phosphorescence radiative decay rate. For many third-row transition metal complexes having a substantial MLCT contribution S1 → T1, kisc can even be ≫1012 s−1 with a T1 → S0 phosphorescence radiative lifetime as short as a few microseconds.8 The latter external metal atom effect in the ππ* dominated emitting transition metal complexes has also received great attention recently due to its relatively slow ISC and hence the existence of fluorescence and phosphorescence dual emission potentially suited for white light generation in a single molecule. For the latter case, tuning kisc and kpr and hence the ratio for fluorescence versus phosphorescence intensity may theoretically be achieved via altering either the core metal atom or the distance between the emitting chromophore (ππ*) and the metal atom. While replacing the metal core element is

synthetically challenging and even formidable, the rational design of the modifiable organic constituent of a metal complex offers a more facile way for the systematic investigation of the heavy atom effect on the photophysical properties of emissive materials. In this respect, Au(I) compounds are the ideal candidates due to the small or even negligible dπ (i.e., MLCT) contribution in the lowest lying transitions.6a Due to this fact, Au(I) ion affects the intraligand (3ππ*) emission mainly as the external heavy atom. A significant amount of work done by other groups has been devoted to the studies of intraligand luminescence perturbed by the coordination of various metal ions, and of d10 gold centers in particular.9 A variety of the organic ligands such as alkynes, isonitriles, and N-donor heterocycles, which contain chromophore aromatic units, were successfully used for the preparation of the families of gold luminescent complexes and adducts, exhibiting a distinct heavy atom effect induced by close proximity of Au(I) ions to the organic emitter.9a−e,10 However, incorporation of the chromophore fragments into the P-donor ligands and their subsequent use for the preparation of metalcontaining luminophores is a much less common technique. Nevertheless, it is recognized that phosphines and their derivatives serve as important building blocks for the construction of materials, which exhibit attractive light emissive properties.12 Moreover, it is well-documented that binding of Au(I) ions to the phosphines strongly affects intraligand emission, often leading to an appearance of phosphorescence or dual luminescence.11h,i,13 Particularly, it was suggested by Yam et al.13i that the intraligand transitions within the phenylene spacer of the PPh2-C6H4−PPh2 bridging group play a key role in luminescence of the corresponding gold compound. Taking these observations into account, we recently developed the series of diphosphine ligands based on the conjugated backbones of variable length.13a,14 The preliminary investigation of some of their Au(I) complexes proved the high importance of the ligand-to-ligand charge transfer in the photoemission and tunability of the heavy atom effect. Herein, we report two families of Au(I) compounds of general composition, which are endowed with stepwise distance-tuning ability between the metal atom and the emitting ππ* chromophore. The results, together with the state-dependent branching ratio for the fluorescence versus phosphorescence, establish a full spectrum of excited-state photophysics for the lowest lying ππ* configured transition metal complexes.

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EXPERIMENTAL SECTION (AuC2Ph)n15 and the diphosphine ligand PPh2(C6H4)5PPh211c were obt ained according t o literat ure methods. PPh2(C6H4)4PPh2 was prepared analogously to PPh2(C6H4)5PPh2 from 4,4‴-dibromoquaterphenyl. The complexes A-0−A-3 14 and B-1−B-3 13i,16 were synthesized previously. Tetrahydrofuran was distilled over Na-benzophenoneketyl under a nitrogen atmosphere prior to use. Other reagents and solvents were used as received. The solution 1D 1 H, 31P NMR, and 1H−1H COSY spectra were recorded on a Bruker Avance 400 spectrometer. Microanalyses were carried out in the analytical laboratory of the University of Eastern Finland. (AuC2Ph)2PPh2(C6H4)4PPh2 (B-4). (AuC2Ph)n (100 mg, 0.336 mmol) was suspended in CH2Cl2 (10 mL), and PPh2(C6H4)4PPh2 (120 mg, 0.178 mmol) was added. The reaction mixture was stirred for 1 h in the absence of light to give a nearly colorless transparent solution. It was diluted with 9624

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toluene (5 cm3), passed through Al2O3 (0.5 cm × 2 cm, neutral, ∼150 mesh), and concentrated to ca. 5 cm3. A pale-yellow solid was precipitated by centrifugation, washed with toluene (5 cm3) and diethyl ether (2 × 5 cm3), and vacuum-dried. Recrystallization by gas-phase diffusion of diethyl ether into chloroform/methanol (6:1 v/v) solution of B-4 at +5 °C gave a pale-yellow microcrystalline solid (187 mg, 88%). 31P{1H} NMR (CD2Cl2; δ): 41.2 (s, br). 1H NMR (CD2Cl2; δ): 7.83− 7.76 (m, 12H), 7.73−7.62 (m, 12H), 7.62−7.52 (m, 12H), 7.44 (m, br, 4H), 7.31−7.21 (m, 6H). Anal. Calcd for C64H46Au2P2: C, 60.48; H, 3.65. Found: C, 60.12; H, 3.75. (AuC2Ph)2PPh2(C6H4)5PPh2 (B-5). B-5 was prepared analogously to B-4 except using PPh2(C6H4)5PPh2. Recrystallization by slow evaporation of dichloromethane/hexane solution of B-5 at +5 °C gave a nearly colorless microcrystalline material (86%). 31P{1H} NMR (CD2Cl2; δ): 41.2 (s, br). 1H NMR (CD2Cl2; δ): 7.84−7.76 (m, 16H), 7.74−7.63 (m, 12H), 7.62−7.52 (m, 12H), 7.44 (m, br, 4H), 7.31−7.21 (m, 6H). Anal. Calcd for C70H50Au2P2: C, 62.42; H, 3.74. Found: C, 62.09; H, 3.91. Photophysical Measurements. Steady-state absorption and emission measurements in solution were recorded with a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920) fluorometer, respectively. Both the wavelengthdependent excitation and emission response of the fluorometer have been calibrated. To determine the photoluminescence quantum yield in solution, the samples were degassed by three freeze−pump−thaw cycles. Quinine sulfate monohydrate in sulfuric acid and coumarin 480 in methanol, with a quantum yield of ∼0.57 and ∼0.87, respectively, served as the standard for measuring the quantum yield. Nanosecond lifetime studies were performed with an Edinburgh FL 900 photon-counting system using a hydrogen-filled lamp as the excitation source. The emission decays were fitted by the sum of exponential functions with a temporal resolution of ∼300 ps via the deconvolution of instrument response function. Nanosecond− picosecond lifetime studies were performed with an Edinburgh OB 900-L time-correlated single photon counting (TCSPC) system coupled with a femtosecond Ti-sapphire oscillator (80 MHz, Spectra-Physics) and a regenerative amplifier (Spitfire Pro, Spectra-Physics) that produce 120 fs laser pulses at 710− 800 nm with a 1 kHz repetition rate. This fundamental pulse was then employed to produce a second harmonic (400 nm) as the excitation pulse. A polarizer was placed in the emission path to ensure that the polarization of the fluorescence was set at the magic angle (54.7°) with respect to that of the pump laser to eliminate the fluorescence anisotropy. The resolution of the TCSPC system is limited by the detector response of ∼50 ps. The fluorescence decays were analyzed by the sum of exponential functions with an iterative convolution method which allows partial removal of the instrument time broadening and consequently renders a temporal resolution of ∼30 ps. The phosphorescence lifetime (>μs) was detected by an optical spectrum analyzer, consisting of an intensified charge-coupled device (ICCD) in conjunction with a spectrograph (SP2300i, Acton). The gate channel of the ICCD was open at 50 ns and synchronized with the firing time of the excitation pulse. Typically, the second harmonic (400 nm, fwhm 95% contributed by the interligand π(phenyl-ethyne) → π*(oligophenylene) transition (see Tables S1−S10 in the Supporting Information). The slow rates of intersystem crossing for the titled Au(I) complexes are manifested by the distinct dual emission bands observed for both A and B series, shown in Figure 1. In degassed CH2Cl2, the emission consists of a short wavelength (330−400 nm, the F band) and a long wavelength band (>430 nm, the P band), in which the P band is drastically quenched by O2. For example, complex B-3 in fully degassed CH2Cl2 reveals a ∼2:1 (F-band:P-band) ratio at the peak wavelength, as shown in Figure 1b. The P-band however vastly diminishes if degassing is not complete and becomes negligible upon aeration. Thus, the assignment of the P-band to phosphorescence is unambiguous for all titled complexes. Clearly, for both A and B series, upon increasing the number of bridging phenylene rings, the intensity ratio for fluorescence versus phosphorescence increases (see Figure 1). Pertinent steadystate peak wavelengths for fluorescence and phosphorescence as well as their corresponding emission yields are listed in Table 1. The dominant ππ* configuration is subject to a large electron exchange integral and hence a large separation between S1 and T1 states. This is also experimentally supportive by the observed far separation between fluorescence and phosphor-

compounds B-4 and B-5 were obtained similarly to other complexes of these series by depolymerization of the gold acetylide (AuC2Ph)n with a slight excess of the corresponding diphosphine (see the Experimental Section), which is a general and convenient synthetic method for the preparation of Au alkynyl−phosphine species.13i,15,22 In the complexes obtained, a diphosphine unit bears two equivalent metal centers, each of which has a typical two-coordinate linear geometry.9d,f,13i,23 All compounds were characterized in solution by 1H and 31P NMR spectroscopy and elemental analyses. The 31P NMR spectra of the complexes under study display singlet resonances around 16 and 41−42 ppm for A and B series, respectively. The low field shift of the signals in comparison to the free phosphine ligands (over 40 ppm) clearly points to the binding of AuC2Ph units to PPh2 moieties. The amounts of the signals observed in the spectral patterns together with their relative intensities completely match the proposed structures and indicate that the novel compounds B-4 and B-5 exist in their molecular forms in solution, analogously to their previously described congeners A0−A-3 and B-1−B-3. Therefore, according to the NMR data, no Au−Au intermolecular bonding is detected in solution. Photophysical Properties of the S1 State. The UV−vis absorption spectra shown in Figure 1 reveal a trend of red shift of the S0 → S1 peak wavelength upon increasing number of phenylene rings from n = 0 (A-0) to n = 3 (A-3) and n = 1 (B1) to n = 5 (B-5), respectively. This correlation is well explained by the decrease of the LUMO π* energy level upon elongation of the oligophenylene spacer through stabilization of 9626

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Table 1. Photophysical Properties of the Fluorescence and the Phosphorescence for the Titled Complexes in CH2Cl2a complex A-0 A-1 A-2 A-3 B-1 B-2 B-3 B-4 B-5

λf, λp (nm) 370, 373, 361, 372, 316, 333, 360, 373, 380,

455 482 525 550 424 457 510 534 549

ΔES1−T1 (kcal·mol−1) 14.4 17.3 22.1 24.8 23.0 23.2 23.3 23.1 23.1

QYf 0.0003 0.001 0.077 0.4 0.001 0.006 0.16 0.53 0.84

τfobs (ps)

knr (s−1)

13 20 65 450 1.2 4.5 75 213 303

× × × × × × × × ×

7.7 5.0 1.4 1.3 1.0 2.5 1.2 2.2 5.3

k′nr (s−1)

10

10 1010 1010 109 1012 1011 1010 109 108

1.1 4.1 7.6 1.2 1.4 1.2 2.6

× × × × × × ×

109 108 108 109 109 108 108

QYp 0.002 0.006 0.034 0.020 0.11 0.27 0.08 0.120 14 kcal/mol for all titled complexes. Moreover, ΔES1−T1 for A2, A-3, and B-2−B-5 are in the same range of 22−25 kcal/mol, similar to many aromatic compounds,25 indicating the dominant ππ* configuration in the lowest lying excited states. The above experimental results are also confirmed by the time-dependent DFT (TD-DFT) calculation, showing ΔES1−T1 to be >15 kcal/mol except for A-0 (see Table S1 in the Supporting Information). Empirically, it has been known that the substantial involvement of MLCT (e.g., >10%) in the S1 state, owing to its less electron exchange integral, should reduce ΔES1−T1 substantially to 1) ISC that competes with the Sj internal conversion (IC), giving direct population at the triplet state and the resulting phosphorescence that bypasses the slow S1 → T1 ISC. Along this line, both A and B series with dual emission provide ideal cases in point to systematically explore the state-dependent branching ratio for the phosphorescence versus fluorescence. Using A-3 as a prototype, shown in Figure 4a, the intensity ratio for the phosphorescence versus fluorescence clearly reveals excitation energy dependence in the CH2Cl2 solution, being increased upon tuning the excitation wavelength toward the higher energy states. This is also clearly unfolded via the excitation spectrum in which the higher energy excitation contributes more to the phosphorescence. The results, in a

Figure 3. The plot of (kisc)1/2ΔES1−T1 vs 1/reff3 for (a) A series and (b) B series. Also shown is the linear fitting of the B series (R2 = 0.987, see text for details). 9628

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phosphorescence (∼6:1) in the solid film to be interplayed with the spin statistics (1:3 for singlet versus triplet excitons) and hence to ingeniously generate whitelight for the electroluminescence (EL, see Figure S3 in the Supporting Information). TCTA is a well-known hole transporting material with a good hole mobility of 3.0 × 10−4 cm2/(V s) and suitable HOMO/LUMO levels of 5.7/2.4 eV to confine the B-3 in the emissive layer.26 After coating EML, 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI, 50 nm) as an electron transporting layer (ETL) was thermally deposited at a base pressure of 10−6 Torr. PEDOT:PSS and LiF served as hole- and electron-injecting layers, respectively. In our preliminary test, the J−V−L characteristic of the device exhibits turn-on voltages of 4 V (defined as the voltage at which the EL is rapidly enhanced); a maximum brightness of 290 cd m−2 at 13.5 V (760 mA cm−2) is shown in Figure 5a.

Figure 4. (a) Absorption, emission, and excitation spectra of A-3 in degassed CH2Cl2. (b) Emission spectra of A-3 in solid film, which was prepared by casting the A-3 (in CH2Cl2) on a quartz glass substrate. In both parts a and b, the emission spectra are normalized at the fluorescence peak. Also shown in part a are the % of major MLCT/ LMCT transitions in the higher lying states with its magnitude indicated by the vertical bar (see Table S11 in the Supporting Information for detail).

qualitative manner, are also consistent with the computation approach, which (see Figure 4a, the vertical bar) shows increase of %MLCT or %LMCT contribution in the higher electronic excited states, ranging from 250 to 350 nm (see Table S11, Supporting Information). Both MLCT and LMCT transition assist the progress of ISC and result in the increase of phosphorescence branching upon higher energy excitation. A similar excitation-energy-dependent intensity ratio for phosphorescence versus fluorescence was observed for the titled complexes exhibiting dual emission, and the results are summarized in Figure S2 of the Supporting Information. In view of practical application in, e.g., OLEDs (vide infra), we also explored the titled complexes in solid film and observed the same photophysical properties as that in solution. Figure 4b reveals a typical example of A-3 in pure solid film, in which the dual emission is evident and the intensity ratio for phosphorescence versus fluorescence also exhibits remarkable excitation-wavelength dependence, affirming that the MLCT harvesting triplet-state mechanism2b is also operative in the solid state. Electroluminescence Measurement. In an aim to test if the dual emission is suited for white light generation, we have then fabricated solution processed devices using 4,4′,4″tris(carbazol-9-yl)-triphenylamine (TCTA) as a host with complex B-3 as the dopant for an EML in the structure ITO/PEDOT:PSS (30 nm)/EML (50 nm)/TPBI (50 nm)/ LiF (0.5 nm)/Al (100 nm). B-3 was selected for the reason of its photoluminescent intensity ratio for fluorescence versus

Figure 5. (a) Current density−voltage−luminance (J−V−L) characteristics, (b) external quantum (ηext) and power efficiencies (ηP) as a function of brightness, and (c) EL spectra for the device using B-3 as a dopant with different voltages. Inset: The EL color appearance with an applied voltage of 13 V.

The maximum external quantum efficiency, current, and power efficiency were 0.22%, 0.31 cd A−1, and 0.25 lm W−1, respectively (see Figure 5b). The EL spectra of the device display the additional TCTA emission at 380 nm and the dominant emission of B-3 at ∼465 and 610 nm, which cover all visible wavelengths from 380 to 750 nm to demonstrate white emission with high color-rendering index (CRI = 89.2−92.2), as shown in Figure 5c. With increasing driving voltage from 10 to 13 V, the relative intensity of deep-blue emission decreased and red emission slightly increased, which led to a slight shift of 9629

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Role of Spin-Orbit Coupling: A Study Based on High-Resolution Optical Spectroscopy. Top. Organomet. Chem. 2010, 29, 193−235. (4) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno, T.; Ishii, Y.; Sakai, K.; Haga, M. Syntheses and Properties of Emissive Iridium(III) Complexes with Tridentate Benzimidazole Derivatives. Inorg. Chem. 2005, 44, 4737−4746. (5) (a) Chou, P. T.; Chi, Y.; Chung, M. W.; Lin, C. C. Harvesting Luminescence via Harnessing the Photophysical Properties of Transition Metal Complexes. Coord. Chem. Rev. 2011, 255, 2653− 2665. (b) Cheng, Y. M.; Li, E. Y.; Lee, G. H.; Chou, P. T. Strategic Design and Synthesis of Osmium(II) Complexes Bearing a Single Pyridyl Azolate π-Chromophore: Achieving High-Efficiency Blue Phosphorescence by Localized Excitation. Inorg. Chem. 2007, 46, 10276−10286. (6) (a) 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 Metal-Structure-Photophysics Relationship of Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light Emitting Devices. J. Am. Chem. Soc. 2011, 133, 12085−12099. (b) 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(μ-NAr2)2 Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs. J. Am. Chem. Soc. 2010, 132, 9499−9508. (c) Czerwieniec, R.; Yu, J.; Yersin, H. Blue-Light Emission of Cu(I) Complexes and Singlet Harvesting. Inorg. Chem. 2011, 50, 8293−8301. (7) (a) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (b) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient Up-Conversion of Triplet Excitons into a Singlet State and its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98, 083302−083303. (8) (a) Tang, K. C.; Liu, K. L.; Chen, I. C. Rapid Intersystem Crossing in Highly Phosphorescent Iridium Complexes. Chem. Phys. Lett. 2004, 386, 437−441. (b) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C. V.; McCusker, J. K. Femtosecond Dynamics of Excited-State Evolution in [Ru(bpy)3]2+. Science 1997, 275, 54−57. (9) (a) Yam, V. W. W.; Wong, K. M. C. Luminescent Metal Complexes of d6, d8 and d10 Transition Metal Centres. Chem. Commun. (Cambridge, U. K.) 2011, 47, 11579−11592. (b) Lu, W.; Kwok, W. M.; Ma, C. S.; Chan, C. T. L.; Zhu, M. X.; Che, C. M. Organic Triplet Excited States of Gold(I) Complexes with Oligo (o- or m-phenyleneethynylene) Ligands: Conjunction of Steady-State and Time-Resolved Spectroscopic Studies on Exciton Delocalization and Emission Pathways. J. Am. Chem. Soc. 2011, 133, 14120−14135. (c) Tong, G. S. M.; Chow, P. K.; Che, C. M. Where is the HeavyAtom Effect? Role of the Central Ligand in Tetragold(I) Ethynyl Complexes. Angew. Chem., Int. Ed. 2010, 49, 9206−9209. (d) Lin, Y.; Yin, J.; Yuan, J. J.; Hu, M.; Li, Z. Y.; Yu, G. A.; Liu, S. H. Synthesis, Characterization, and Properties of Binuclear Gold(I) Phosphine Alkynyl Complexes. Organometallics 2010, 29, 2808−2814. (e) Wong, W. Y.; Ho, C. L. Di-, oligo- and Polymetallaynes: Syntheses, Photophysics, Structures and Applications. Coord. Chem. Rev. 2006, 250, 2627−2690. (f) Chao, H. Y.; Lu, W.; Li, Y. Q.; Chan, M. C. W.; Che, C. M.; Cheung, K. K.; Zhu, N. Y. Organic Triplet Emissions of Arylacetylide Moieties Harnessed through Coordination to [Au(Pcy3)]+. Effect of Molecular Structure upon Photoluminescent Properties. J. Am. Chem. Soc. 2002, 124, 14696−14706. (10) (a) Yam, V. W. W.; Cheng, E. C. C. Highlights on the Recent Advances in Gold Chemistry - a Photophysical Perspective. Chem. Soc. Rev. 2008, 37, 1806−1813. (b) Yam, V. W.-W.; Cheng, E. C.-C. In Photochemistry and Photophysics of Coordination Compounds II; Balzani, V., Campagna, S., Eds.; Springer: Berlin, Heidelberg, 2007; pp 269− 309. (c) Yam, V. W. W. Molecular Design of Transition Metal Alkynyl Complexes as Building Blocks for Luminescent Metal-Based Materials: Structural and Photophysical Aspects. Acc. Chem. Res. 2002, 35, 555− 563. (d) Yam, V. W. W.; Choi, S. W. K. Synthesis, Photophysics and

the CIE coordinates from (0.28, 0.30) to (0.29, 0.31). Though pending full optimization, the result showing the ratiometric increase in intensity for phosphorescence versus fluorescence is intriguing. It may reflect the harvest of phosphorescence at higher lying singlet states at higher voltage. However, because parameters affecting the device performance are complicated, comprehensive investigation of the device properties is necessary before the conclusion can be made. Nonetheless, our preliminary result proves the concept of latent potential of the dual emission for white light generation.

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CONCLUSION In sum, we report a comprehensive study of the (phenylene)n bridged bimetallic Au(I) complexes, the results of which demonstrate several unprecedented trends of photophysical properties. First, we are able to elongate the effective distance (reff) in a systematic manner so as to modulate the rate of S1 → T1 ISC and T1 → S0 phosphorescence radiative decay rate constant, rendering a decreasing trend of the intensity ratio for the phosphorescence versus fluorescence. We then further demonstrate the possibility of harvesting energy in triplet manifolds; that is, the higher lying electronic excitation with increasing %MLCT character leads to a higher ratio of phosphorescence emission via the fast Sm → Tn (m, n > 1) ISC, i.e., the harvest of the phosphorescence, unifying the recent proposed phosphorescence harvesting theorem.17 The fine-tuning (phenyl)n-bridged bimetallic Au(I) complexes lead to the capability of harnessing fluorescence versus phosphorescence via distance- and state-dependent intersystem crossing.

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ASSOCIATED CONTENT

S Supporting Information *

Additional computational, spectroscopic, and experimental details and complete ref 18 are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: igor.koshevoy@uef.fi (I.O.K); [email protected]. edu.tw (W.-Y.H.); [email protected] (P.-T.C). Author Contributions ⊥

Y.-C.C., K.-C.T.: These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the National Science Council, Taiwan (NSC-100-2923-M-002-005-MY3), and the strategic funding of the University of Eastern Finland (Spearhead project) and the Academy of Finland (grant 138560/2010, A.J.K.).

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REFERENCES

(1) Chou, P. T.; Chi, Y. Phosphorescent Dyes for Organic LightEmitting Diodes. Chem.Eur. J. 2007, 13, 380−395. (2) (a) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: Englewood Cliffs, NJ, 1969; p 189, Chapter 5. (b) Chi, Y.; Chou, P. T. Contemporary Progresses on Neutral, Highly Emissive Os(II) and Ru(II) Complexes. Chem. Soc. Rev. 2007, 36, 1421−1431. (3) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the 9630

dx.doi.org/10.1021/jp402666r | J. Phys. Chem. C 2013, 117, 9623−9632

The Journal of Physical Chemistry C

Article

Photochemistry of Alkynylgold(I) Phosphine Complexes. J. Chem. Soc., Dalton Trans. 1996, 4227−4232. (11) (a) Elbjeirami, O.; Rawashdeh-Omary, M. A.; Omary, M. A. Phosphorescence Sensitization via Heavy-Atom Effects in d10 Complexes. Res. Chem. Intermed. 2011, 37, 691−703. (b) Li, X. L.; Zhang, K. J.; Li, J. J.; Cheng, X. X.; Chen, Z. N. Dual Luminescent Dinuclear Gold(I) Complexes of Terpyridyl-Functionalized Alkyne Ligands and Their Efficient Sensitization of Eu-III and Yb-III Luminescence. Eur. J. Inorg. Chem. 2010, 3449−3457. (c) Koshevoy, I. O.; Karttunen, A. J.; Shakirova, J. R.; Melnikov, A. S.; Haukka, M.; Tunik, S. P.; Pakkanen, T. A. Halide-Directed Assembly of Multicomponent Systems: Highly Ordered Au-I-Ag-I Molecular Aggregates. Angew. Chem., Int. Ed. 2010, 49, 8864−8866. (d) Mullice, L. A.; Thorp-Greenwood, F. L.; Laye, R. H.; Coogan, M. P.; Kariuki, B. M.; Pope, S. J. A. The Coordination Chemistry of Fluorescent Pyridinyl- and Quinolinyl-Phthalimide Ligands with the {AuI-PPh3} Cationic Unit. Dalton Trans. 2009, 6836−6842. (e) Aly, S. M.; Ho, C. L.; Wong, W. Y.; Fortin, D.; Harvey, P. D. Intrachain Electron and Energy Transfers in Metal Diynes and Polyynes of Group 10−11 Transition Elements Containing Various Carbazole and Fluorene Hybrids. Macromolecules 2009, 42, 6902−6916. (f) Liu, L.; Wong, W. Y.; Poon, S. Y.; Shi, H. X.; Cheah, K. W.; Lin, Z. Y. Effect of Acetylenic Chain Length on the Tuning of Functional Properties in FluoreneBridged Polymetallaynes and Their Molecular Model Compounds. Chem. Mater. 2006, 18, 1369−1378. (g) Omary, M. A.; Mohamed, A. A.; Rawashdeh-Omary, M. A.; Fackler, J. P. Photophysics of Supramolecular Binary Stacks Consisting of Electron-Rich Trinuclear Au(I) Complexes and Organic Electrophiles. Coord. Chem. Rev. 2005, 249, 1372−1381. (h) Lin, R.; Yip, J. H. K.; Zhang, K.; Koh, L. L.; Wong, K. Y.; Ho, K. P. Self-Assembly and Molecular Recognition of a Luminescent Gold Rectangle. J. Am. Chem. Soc. 2004, 126, 15852− 15869. (i) Irwin, M. J.; Vittal, J. J.; Puddephatt, R. J. Luminescent Gold(I) Acetylides: From Model Compounds to Polymers. Organometallics 1997, 16, 3541−3547. (j) Nguyen, M. H.; Yip, J. H. K. Gold(I) and Platinum(II) Tetracenes and Tetracenyldiacetylides: Structural and Fluorescence Color Changes Induced by σ-Metalation. Organometallics 2010, 29, 2422−2429. (12) (a) Liu, C.; Gu, Y.; Fu, Q.; Sun, N.; Zhong, C.; Ma, D. G.; Qin, J. G.; Yang, C. L. Nondoped Deep-Blue Organic Light-Emitting Diodes with Color Stability and Very Low Efficiency Roll-Off: Solution-Processable Small-Molecule Fluorophores by Phosphine Oxide Linkage. Chem.Eur. J. 2012, 18, 13828−13835. (b) Han, C. M.; Zhang, Z. S.; Xu, H.; Yue, S. Z.; Li, J.; Yan, P. R.; Deng, Z. P.; Zhao, Y.; Yan, P. F.; Liu, S. Y. Short-Axis Substitution Approach Selectively Optimizes Electrical Properties of Dibenzothiophene-Based Phosphine Oxide Hosts. J. Am. Chem. Soc. 2012, 134, 19179−19188. (c) Padmaperuma, A. B.; Sapochak, L. S.; Burrows, P. E. New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence: 2,7-Bis(diphenylphosphine oxide)-9,9-dimethylfluorene. Chem. Mater. 2006, 18, 2389−2396. (d) Baumgartner, T.; Reau, R. Organophosphorus π-Conjugated Materials. Chem. Rev. 2006, 106, 4681−4727. (13) (a) Camara, J.; Crespo, O.; Gimeno, M. C.; Koshevoy, I. O.; Laguna, A.; Ospino, I.; Smirnova, E. S.; Tunik, S. P. Emission Tuning in Dinuclear Gold Complexes with Diphosphanes Containing Alkyne and/or Oligophenylene Spacers. Dalton Trans. 2012, 41, 13891− 13898. (b) Mangalum, A.; Smith, R. C. Bifunctional Cross-Conjugated Luminescent Phosphines and Phosphine Derivatives: Phospha-Cruciforms. Dalton Trans. 2010, 39, 5145−5151. (c) Su, H. C.; Fadhel, O.; Yang, C. J.; Cho, T. Y.; Fave, C.; Hissler, M.; Wu, C. C.; Reau, R. Toward Functional π-Conjugated Organophosphorus Materials: Design of Phosphole-Based Oligomers for Electroluminescent Devices. J. Am. Chem. Soc. 2006, 128, 983−995. (d) Osawa, M.; Hoshino, M.; Akita, M.; Wada, T. Synthesis and Characterization of Phenanthrylphosphine Gold Complex: Observation of Au-Induced Blue-Green Phosphorescence at Room Temperature. Inorg. Chem. 2005, 44, 1157−1159. (e) Bardaji, M.; de la Cruz, M. T.; Jones, P. G.; Laguna, A.; Martinez, J.; Villacampa, M. D. Luminescent Dinuclear Gold Complexes of Bis(Diphenylphosphano) Acetylene. Inorg. Chim.

Acta 2005, 358, 1365−1372. (f) Pintado-Alba, A.; de la Riva, H.; Nieuwhuyzen, M.; Bautista, D.; Raithby, P. R.; Sparkes, H. A.; Teat, S. J.; Lopez-de-Luzuriaga, J. M.; Lagunas, M. C. Effects of Diphosphine Structure on Aurophilicity and Luminescence in Au(I) Complexes. Dalton Trans. 2004, 3459−3467. (g) Pawlowski, V.; Kunkely, H.; Vogler, A. Synthesis and Optical Properties of Binuclear Gold(I) Complexes with Bridging Phosphine Ligands: Luminescence from Intraligand and Metal-Centered Excited States. Inorg. Chim. Acta 2004, 357, 1309−1312. (h) Yip, J. H. K.; Prabhavathy, J. A Luminescent Gold Ring That Flips Like Cyclohexane. Angew. Chem., Int. Ed. 2001, 40, 2159−2162. (i) Yam, V. W. W.; Choi, S. W. K.; Cheung, K. K. Synthesis and Design of Novel Tetranuclear and Dinuclear Gold(I) Phosphine Acetylide Complexes. First X-Ray Crystal Structures of a Tetranuclear ([Au4(tppb)(CCPh)4]) and a Related Dinuclear ([Au2(dppb)(CCPh)2]) Complex. Organometallics 1996, 15, 1734−1739. (14) Koshevoy, I. O.; Lin, C. L.; Hsieh, C. C.; Karttunen, A. J.; Haukka, M.; Pakkanen, T. A.; Chou, P. T. Synthesis, Characterization and Photophysical Properties of PPh2−C2−(C6H4)n−C2−PPh2 Based Bimetallic Au(I) Complexes. Dalton Trans. 2012, 41, 937−945. (15) Coates, G. E.; Parkin, C. Gold(I) Alkynyls and Their CoOrdination Complexes. J. Chem. Soc. 1962, 3220−3226. (16) Koshevoy, I. O.; Koskinen, L.; Haukka, M.; Tunik, S. P.; Serdobintsev, P. Y.; Melnikov, A. S.; Pakkanen, T. A. Self-Assembly of Supramolecular Luminescent Au(I)-Cu(I) Complexes: “Wrapping” an Au6Cu6 Cluster in a [Au3(Diphosphine)3]3+ “Belt”. Angew. Chem., Int. Ed. 2008, 47, 3942−3945. (17) 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: State-Dependent Intersystem Crossing Rate in Os(II) and Ag(I) Complexes. J. Am. Chem. Soc. 2012, 134, 7715− 7724. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (19) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid ExchangeCorrelation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (20) (a) Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 82, 284−298. (b) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299−310. (c) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition-Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270− 283. (d) Harihara, P . C.; Pople, J. A. Accuracy of AHn Equilibrium Geometries by Single Determinant Molecular-Orbital Theory. Mol. Phys. 1974, 27, 209−214. (21) (a) Houjou, H.; Inoue, Y.; Sakurai, M. Physical Origin of the Opsin Shift of Bacteriorhodopsin. Comprehensive Analysis Based on Medium Effect Theory of Absorption Spectra. J. Am. Chem. Soc. 1998, 120, 4459−4470. (b) Mineva, T.; Russo, N. Solvent Effects Computed with the Gaussian Density Functional Method. Int. J. Quantum Chem. 1997, 61, 665−671. (22) Ferrer, M.; Gutierrez, A.; Rodriguez, L.; Rossell, O.; Lima, J. C.; Font-Bardia, M.; Solans, X. Study of the Effect of the Phosphane Bridging Chain Nature on the Structural and Photophysical Properties of a Series of Gold(I) Ethynylpyridine Complexes. Eur. J. Inorg. Chem. 2008, 2899−2909. (23) Lu, W.; Zhu, N. Y.; Che, C. M. Polymorphic Forms of a Gold(I) Arylacetylide Complex with Contrasting Phosphorescent Characteristics. J. Am. Chem. Soc. 2003, 125, 16081−16088. (24) Koshevoy, I. O.; Karttunen, A. J.; Tunik, S. P.; Haukka, M.; Selivanov, S. I.; MeInikov, A. S.; Serdobintsev, P. Y.; Khodorkovskiy, M. A.; Pakkanen, T. A. Supramolecular Luminescent Gold(I)Copper(I) Complexes: Self-Assembly of the AuxCuy Clusters inside 9631

dx.doi.org/10.1021/jp402666r | J. Phys. Chem. C 2013, 117, 9623−9632

The Journal of Physical Chemistry C

Article

the [Au3(diphosphine)3]3+ Triangles. Inorg. Chem. 2008, 47, 9478− 9488. (25) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Books: Sausalito, CA, 2010; p 306, Chapter 5. (26) Park, J. J.; Park, T. J.; Jeon, W. S.; Pode, R.; Jang, J.; Kwon, J. H.; Yu, E. S.; Chae, M. Y. Small Molecule Interlayer for Solution Processed Phosphorescent Organic Light Emitting Device. Org. Electron. 2009, 10, 189−193.

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dx.doi.org/10.1021/jp402666r | J. Phys. Chem. C 2013, 117, 9623−9632