Synthesis, structure and photophysics of binuclear gold(I) and mercury(II) complexes derived from 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole

b h Bap Alkynyl Crystal structures Mercury try een es D)(p Me ation reaction of the corresponding metal chloride precursors with HC„ C(p-C H )(OXD)(p-C H ) these metallaynes. The structural properties of 1 have been studied by X-ray crystallography, in which no short-contact aurophilic interaction is observed. The influence of the heavy metal atom on the inter- gn of ributio e past meta cal methods. Among these, rigid-rod metallaynes of late transition metals represent an important class of new materials in which there is great scope for chemical modification of the conjugated units [8]. Their interesting photoluminescence and electronic prop- erties depend on the transition metal, the central organic spacer in the main chain and the symmetry of the molecules [9]. oxadiazole compounds such as 1,3,4-oxadiazoles have been inves- tigated as electron-transporting materials within multilayered OLED devices because the five-membered oxadiazole blocks show high electron affinity in organic small-molecules and polymers [12]. Consequently, in keeping with the ongoing research interests of our group pertaining to the structure–property correlation study of metallaynes, we report here the first examples of group 11–12 metal acetylide complexes containing the 1,3,4-oxadiazole linker and study their spectroscopic, structural and photophysical behav- ior. The nature of the lowest singlet and triplet excited states will be characterized according to the selection of the terminal late q In memory of my mentor, Prof. F. Albert Cotton, for his remarkable contribution to inorganic and organometallic chemistry throughout his life. * Corresponding author. Tel.: +852 34117074; fax: +852 34117348. Journal of Molecular Structure 890 (2008) 150–156 Contents lists availab Journal of Molec lse E-mail address: [email protected] (W.-Y. Wong). be applied in various domains of optoelectronic devices [1] such as organic light-emitting diodes (OLEDs) [2], photovoltaic cells [3], field-effect transistors [4], sensors [5], and non-linear optical systems [6]. In fact, harvesting of triplet excitons using metal-con- taining complexes is still one of the most promising avenues of making high-efficiency OLEDs [7]. Strong spin–orbit coupling in- duced by a heavy-atom effect mixes the singlet and triplet excited states through efficient intersystem crossing (ISC) so that phospho- rescence (or triplet emission) can be measured readily using opti- nyl complexes are essential for designing and making technologically useful materials for OLEDs and photovoltaic cells [11]. Along this line, the trade-off problems for phosphorescence efficiency/optical bandgap have been addressed and elucidated [11]. In contrast to the library of work on luminescent metallaynes with simple electron-rich aromatic and heterocyclic units such as phenylene, anthryl, fluorenyl and thienyl rings [8,11], there are no reports of similar systems possessing p-conjugated electron- deficient oxadiazole group in the main skeleton so far. Aromatic Oxadiazole Phosphorescence 1. Introduction Advances in the molecular desi materials have made significant cont ment of organic electronics. Over th ors have been made to develop new 0022-2860/$ - see front matter � 2008 Elsevier B.V. A doi:10.1016/j.molstruc.2008.03.003 system crossing rate from the S1 singlet excited state to the T1 triplet excited state in these metal alkynyl systems and the spatial extent of the lowest singlet and triplet excitons is systematically characterized. Our investigations indicate that harvesting of the organic triplet emissions by the heavy-atom effect of group 11–12 transition metals generally follows the order Au(I) > Hg(II). � 2008 Elsevier B.V. All rights reserved. modern optoelectronic ns toward the develop- decades, many endeav- l–organic complexes to In the past two decades, there has been immense experimental and theoretical attention focusing on the energy levels of singlet and triplet states in conjugated polymers and materials [10]. In this regard, metallaynes offer good prospects for the development. A concise consideration and a good compromise of the triplet photo- physics and the corresponding bandgaps in these metalated alky- Keywords: Gold 6 4 6 4 C„ CH at room temperature. We report the optical absorption and photoluminescence properties of Synthesis, structure and photophysics of complexes derived from 2,5-bis(ethynylp Wai-Yeung Wong *, Yan-He Guo Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong a r t i c l e i n f o Article history: Received 21 January 2008 Received in revised form 6 March 2008 Accepted 6 March 2008 Available online 13 March 2008 a b s t r a c t The organometallic chemis and mercury(II) salts has b nyl) binuclear complex [(PR3)AuC„ C(p-C6H4)(OX C6H4)C„ CHgR] (R = Ph, 3; journal homepage: www.e ll rights reserved. inuclear gold(I) and mercury(II) enyl)-1,3,4-oxadiazoleq tist University, Waterloo Road, Kowloon Tong, Hong Kong, PR China of the dialkyne ligand 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole with gold(I) investigated. Four new group 11 gold(I) and group 12 mercury(II) bis(alky- functionalized with electron-deficient oxadiazole (OXD) spacer -C6H4)C„ CAu(PR3)] (R = Ph, 1; Me, 2) and [RHgC„ C(p-C6H4)(OXD)(p- , 4) were prepared in good yields by the base-catalyzed dehydrohalogen- le at ScienceDirect ular Structure vier .com/locate /molstruc olec transition metal ions and the results are subsequently compared to the corresponding biphenylene congeners. 2. Experimental 2.1. General Solvents were predried and distilled from appropriate drying agents. All chemicals, unless otherwise stated, were obtained from commercial sources and used as received. All reactions were car- ried out under nitrogen atmosphere with the use of standard Schlenk techniques, but no special precautions were taken to ex- clude oxygen during work-up. Preparative TLC was performed on 0.7 mm silica plates (Merck Kieselgel 60 GF254) prepared in our lab- oratory. Infrared spectra were recorded in CH2Cl2 on a Perkin-El- mer FTIR 550 spectrometer, using CaF2 cells with a 0.5 cm path length. NMR spectra were measured in appropriate solvents on a JEOL EX270 or a Varian INOVA 400 MHz FT-NMR spectrometer, with 1H and 13C NMR chemical shifts quoted relative to tetrameth- ylsilane and 31P chemical shifts relative to an 85% H3PO4 external standard. Fast atom bombardment (FAB) mass spectra were re- corded on a Finnigan MAT SSQ710 mass spectrometer. Absorption spectra were obtained with a Hewlett–Packard 8453 UV–vis spec- trometer. For emission spectral measurement, the 325 nm line of a He–Cd laser was used as an excitation source. The luminescence spectra were analyzed by a 0.25 m focal length double monochro- mator with a Peltier cooled photomultiplier tube and processed with a lock-in-amplifier. For low temperature measurements, sam- ples were mounted in a closed-cycle cryostat (Oxford CC1104) in which the temperature can be adjusted from 10 to 330 K. The fluo- rescence quantum yields (rF) were determined in dichloromethane solutions at 293 K against the quinine sulfate in 1.0 N H2SO4 (rF = 0.54) [13]. Caution: Organomercurials are toxic, and all exper- imentation involving these reagents should be carried out in a well-vented hood. 2.2. Synthesis of ligands 2.2.1. 2,5-Bis(4-bromophenyl)hydrazine This compound was prepared according to a slightly modified literature method [14]. 4-Bromobenzoic chloride (2.40 g, 11.0 mmol) and hydrazine (0.25 g, 7.80 mmol) were dissolved in chloroform (50 mL). Triethylamine (2.20 g, 21.8 mmol) was then added at 0 �C and the solution was stirred for 30 min. After warm- ing to room temperature (rt), the solution was allowed to react overnight. A white precipitate then came out and was collected, washed several times sequentially with 10% NaHCO3 and water to obtain bis(4-bromophenyl)hydrazine (1.40 g, 64%) for the next step. This intermediate (1.40 g, 3.53 mmol) was added to POCl3 (30 mL) and the solution was refluxed for 8 h under N2. The solu- tion was then reduced to 5 mL in vacuo. Upon cooling, the solution was poured into ice/water, and a white solid precipitated out. The white precipitate was filtered off, washed with water and cold eth- anol. The pure white needles were obtained by recrystallization from ethanol to give 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (1.30 g, 97%). 1H NMR (CDCl3): d (ppm) 8.00 (d, 4H, J = 8.4 Hz, phe- nyl), 7.68 (d, 4H, J = 8.4 Hz, phenyl). FAB-MS: m/z 380 [M]+. Calcd for C14H8N2Br2O: C, 44.25; H, 2.12; N, 7.37. Found: C, 44.20; H, 2.01; N, 7.42%. 2.2.2. 2,5-Bis(trimethylsilylethynylphenyl)-1,3,4-oxadiazole To 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (0.38 g, 1.00 mmol) in triethylamine (50 mL) were added CuI (25.0 mg), Pd(OAc)2 W.-Y. Wong, Y.-H. Guo / Journal of M (25.0 mg) and PPh3 (75.0 mg) under N2. After the mixture was stir- red at rt for 15 � 20 min, trimethylsilylacetylene (0.28 mL, 2.00 mmol) was added. The mixture was stirred for a further 15– 20 min at rt before it was refluxed overnight under N2. The comple- tion of the reaction was confirmed by IR and TLC. The reaction mix- ture was filtered and the filtrate evaporated to dryness on a rotary evaporator. The residue was dissolved in CH2Cl2 and washed suc- cessively with 10% HCl, water, saturated NaHCO3 and water. The dichloromethane solution was dried over MgSO4 and evaporated to dryness under vacuum. Purification was effected by silica col- umn chromatography using hexane/CH2Cl2 (1:1, v/v) to afford the title product as a yellow solid in 77% yield (0.32 g). IR (CH2Cl2): 2160 cm�1 (mC„C); 1H NMR (CDCl3): d (ppm) 8.07 (d, 4H, J = 8.6 Hz, phenyl), 7.61 (d, 4H, J = 8.6 Hz, phenyl), 0.28 (s, 18H, SiMe3); 13C NMR (CDCl3): d (ppm) 164.10, 132.50, 126.66, 126.63, 123.26 (aro- matic C), 103.90, 97.88 (C„ C), �0.035 (SiMe3); FAB-MS: m/z 415 [M]+. Calcd for C24H26N2OSi2: C, 69.52; H, 6.32; N, 6.76. Found: C, 69.34; H, 6.15; N, 6.58%. 2.2.3. 2,5-Bis(ethynylphenyl)-1,3,4-oxadiazole L 2,5-Bis(trimethylsilylethynylphenyl)-1,3,4-oxadiazole (0.32 g, 0.77 mmol) and K2CO3 (20.0 mg, 0.15 mmol) were combined in MeOH/Et2O (40 mL, 1:1, v/v) and the mixture was stirred at rt for 20 h. Infrared spectroscopy showed that the starting material had been consumed. Upon removal of solvent under reduced pressure, the crude product was subjected to column chromatography on sil- ica eluting with hexane/CH2Cl2 (1:1, v/v) to give L as a yellow solid in 95% yield (0.20 g, 0.74 mmol). IR (CH2Cl2): 3295 (m„CH), 2107 cm�1 (mC„C); 1H NMR (CDCl3): d (ppm) 8.10 (d, 4H, J = 8.1 Hz, phenyl), 7.65 (d, 4H, J = 8.1 Hz, phenyl), 3.27 (s, 2H, C„ C); 13C NMR (CDCl3): d (ppm) 164.02, 132.68, 126.70, 125.63, 123.62 (aromatic C), 82.63, 80.19 (C„ C); FAB-MS: m/z 270 [M]+. Calcd for C18H10N2O: C, 79.99; H, 3.73; N, 10.36. Found: C, 80.08; H, 3.66; N, 10.16%. 2.3. Complex syntheses 2.3.1. Gold(I) complex 1 Triphenylphosphinegold(I) chloride (99.0 mg, 0.20 mmol) was dissolved in MeOH/CH2Cl2 (6 mL, 5:1, v/v) and L (27.0 mg, 0.10 mmol) was added. Then NaOH in MeOH (2 mL, 0.2 M) was added to the mixture, and the mixture was allowed to stir at rt un- der nitrogen for 2 h. The product was precipitated out and centri- fuged. The solvent was removed and the product was air-dried to obtain a yellow solid of 1 (77.0 mg, 65%). IR (CH2Cl2): 2116 cm�1 (mC„C); 1H NMR (CDCl3): d (ppm) 8.02 (d, 4H, J = 8.4 Hz, phenyl), 7.64 (d, 4H, J = 8.4 Hz, phenyl), 7.59–7.43 (m, 30H, PPh3); 13C NMR (CDCl3): d (ppm) 164.20, 134.28, 134.08, 132.75, 131.55, 131.52, 129.87, 129.17, 129.05, 126.45 (aromatic C), 128.50, 121.69 (C„ C); 31P {1H} NMR (CDCl3): d (ppm) 43.13; FAB-MS: m/z 1187 [M]+. Calcd for C54H38N2OP2Au2: C, 54.65; H, 3.23; N, 2.36. Found: C, 54.49; H, 3.28; N, 2.28%. 2.3.2. Gold(I) complex 2 Similar to 1, complex 2 was prepared from Au(PMe3)Cl instead of Au(PPh3)Cl and collected as a yellow solid in 87% yield (28.0 mg from 25.0 mg of Au(PMe3)Cl). IR (CH2Cl2): 2104 cm�1 (mC„C); 1H NMR (CDCl3): d (ppm) 7.98 (d, 4H, J = 8.4 Hz, phenyl), 7.57 (d, 4H, J = 8.4 Hz, phenyl), 1.54 (s, 18H, Au(PMe3)); 13C NMR (CDCl3): d (ppm) 164.09, 132.62, 132.19, 128.43, 126.32 (aromatic C), 128.08, 121.53 (C„ C), 15.91 (Au(PMe3)); 31P {1H} NMR (CDCl3): d (ppm) 2.08; FAB-MS: m/z 814 [M]+. Calcd for C24H26N2OP2Au2: C, 35.40; H, 3.22; N, 3.44. Found: C, 35.23; H, 3.19; N, 3.27%. 2.3.3. Mercury(II) complex 3 Phenylmercury(II) chloride (51.0 mg, 0.16 mmol) was dissolved ular Structure 890 (2008) 150–156 151 in MeOH/CH2Cl2 (6 mL, 5:1, v/v) and L (87.8 mg, 0.07 mmol) was added. Then 0.20 M basic NaOH in MeOH (2 mL) was subsequently added to give a pale-yellow suspension. Then, the mixture was mixture at rt. Geometric and intensity data were collected at 293 K using graphite-monochromated Mo-Ka radiation (k = 0.71073 Å) olec on a Bruker Axs Smart 1000 CCD area detector diffractometer. The collected frames were processed with the software SAINT [15a], and an absorption correction was applied (SADABS) [15b] to the collected reflections. The structure was solved by the direct methods (SHELXTL) [16] in conjunction with standard difference Fourier techniques and subsequently refined by full-matrix least- squares analyses on F2. All non-hydrogen atoms were assigned with anisotropic displacement parameters. Crystal data for 1�CHCl3: C55H39N2Cl3OP2Au2,M = 1306.10, monoclinic, space group P21/c, a = 6.9195(9), b = 24.577(3), c = 29.218(4) Å, b = 91.891(2)�, U = 4966(1) Å3, Z = 4, T = 293 K, l(Mo-Ka) = 6.169 mm�1, 24,638 reflections measured, 8696 unique, Rint = 0.0783, final R1 = 0.0545, wR2 = 0.1184 for 4534 [I > 2r(I)] observed reflections. 3. Results and discussion 3.1. Synthesis All of the metal alkynyl complexes were obtained in good yields by the general reaction routes as described in Scheme 1. The organic precursor Lwas chosen as a versatile synthon in the present study to form a series of group 11–12 metal acetylide complexes by adapta- tion of the base-catalyzed dehydrohalogenation procedures re- ported in the literature [17,18]. We have prepared the d10 digold(I) diacetylide complexes 1 and 2 by reaction of Au(PPh3)Cl or Au(P- Me3)Cl with L under ambient conditions in a mole ratio of 2:1 in the presence of NaOH inMeOH [17]. Likewise,mercuration of Lwith twomolar equivalents of PhHgCl orMeHgCl usingmethanolicNaOH as a base at rt gave yellow solids identified as 3 and 4, respectively, which are the isoelectronic and isolobal analogues of the gold(I) counterparts [18]. The yields of these transformations are high in each case (ca. 65–87%). All of the new complexes are air-stable pale yellow solids and can be storedwithout demanding any special pre- cautions. They generally exhibit good solubility in chlorocarbons such as CH2Cl2 and CHCl3, but are insoluble in hydrocarbons. 3.2. Spectroscopic properties allowed to stir at rt under nitrogen atmosphere for 2 h. The crude product was precipitated out and centrifuged. The solvent was dec- anted and a yellow solid was washed with MeOH (2� 20 mL) and air-dried to afford a pure sample of 3 (32.0 mg, 69%). IR (CH2Cl2): 2134 cm�1 (mC„C); 1H NMR (CDCl3): d 8.10 (d, 4H, J = 8.1 Hz, phe- nyl), 7.65 (d, 4H, J = 8.1 Hz, phenyl), 7.43–7.26 (m, 10H, HgPh); FAB-MS: m/z 824 [M]+. Calcd for C30H18N2OHg2: C, 43.75; H, 2.20; N, 3.40. Found: C, 43.80; H, 2.10; N, 3.44%. 2.3.4. Mercury(II) complex 4 A procedure similar to that used for 3 was employed to obtain the title compound in 76% yield. IR (CH2Cl2):2130 cm�1 (mC„C); 1H NMR (CDCl3): d 8.11–8.04 (m, 4H, phenyl), 7.66–7.58 (m, 4H, phenyl), 0.74 (s, 6H, 2JHg–H = 148.5 Hz, HgMe); 13C NMR (CDCl3): d 163.98, 146.76, 132.49, 126.57, 123.60 (aromatic C), 122.67, 104.06 (C„ C), 7.12 (HgMe); FAB-MS: m/z 700 [M]+. Calcd for C20H14N2OHg2: C, 34.34; H, 2.02; N, 4.00. Found: C, 34.40; H, 2.08; N, 3.89%. 2.4. X-ray crystallography Colorless crystals of 1 suitable for X-ray diffraction experiment were grown by slow evaporation of its solution in a CHCl3/hexane 152 W.-Y. Wong, Y.-H. Guo / Journal of M The IR, NMR and MS data of our compounds shown in Section 2 agree with their chemical structures. The solution IR spectra are characterized by a single sharp m(C„ C) absorption band at ca. 2104–2116 cm�1 (for Au complexes) and 2130–2134 cm�1 (for Hg complexes). The IR spectrum of each compound shows no char- acteristic „CAH stretching band in the range 3200–3300 cm�1, thus confirming that 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole is capped by metal groups via r bonds. The room temperature 31P NMR spectrum of 1 and 2 each displays a sharp singlet at d 43.13 and 2.08, respectively, which indicates a symmetrical arrangement of PAuC„ C groups in solution. 1H NMR resonances arising from the protons of the organic moieties were observed. The low solubil- ity of 3 in chlorinated solvents led to some difficulties for its com- plete 13C NMR characterization. The formulas of the metal complexes were successfully established by the presence of in- tense molecular ion peaks in their respective positive FAB mass spectra. 3.3. Crystal structure analyses The three-dimensional molecular structure of 1 was analyzed by X-ray crystallography and a perspective view of 1 is shown in Fig. 1. Pertinent bond distances and angles are given in Table 1. The crystal structure consists of discrete binuclear molecules in which two terminal organometallic Au(PPh3)+ groups are linked by the 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole moiety to afford the rigid molecular structure. The dihedral angles made between the central five-membered oxadiazole plane with the two neigh- boring inner C6H4 rings are ca. 7.2� and 7.9�. The coordination geometry is linear about the gold centers. The C„ C bond lengths in the ethynyl bridge of 1.198(11)–1.200(11) Å are fairly typical of metal-alkynyl r-bonding [11c,17,18]. The bond angles for the metalAC„ C units are close to linearity and conform to the rigid-rod nature of the compound. No apparent short intermolecular contacts or p-stacking interactions are observed in 1. In contrast to many Au(I) compounds, where there fre- quently exists short Au� � �Au contacts, aurophilicity is absent in the crystal lattice of 1 [19]. 3.4. Electronic absorption and photoluminescence spectra The photophysical data of the new compounds 1–4measured in CH2Cl2 are shown in Table 2. All of the metal alkynyls display sim- ilar structured absorption bands in the near UV region (Figs. 2–5). The lowest energy transitions are predominantly intraligand in nature consisting of both acetylenic and aromatic 1(pp*) character, possibly mixed with some admixture of metal orbitals. The 0–0 absorption peak is assigned as the S0? S1 transition from the high- est occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which are mainly delocalized p and p* orbitals [17,18]. As compared to the band at kabs = 307 nm in CH2Cl2 for L (Fig. 6), we find that the position of the lowest energy absorption band for 1–4 is red-shifted after the inclusion of metal fragment. This reveals that p-conjugation is preserved through the metal site by mixing of the frontier orbitals of metal and the ligand. In energy terms, the experimentally determined HOMO–LUMO en- ergy gaps (Eg) as measured from the onset absorption wavelength are also tabulated in Table 2. According to the type of the metal groups, the optical energy gaps of the polyynes follow the experi- mental order 3 > 1 and 4 > 2. At 293 K, all of the complexes in CH2Cl2 solutions emit purple-blue 1(pp*) fluorescence (S1? S0) peak near 400 nm that is characterized by the small Stokes shift between the bands in the absorption and the emission spectra. At 77 K, each of them shows dual emission peaks of different intensities with the high- ular Structure 890 (2008) 150–156 er-energy peak due to fluorescence and the lower-energy one due to phosphorescence. The triplet emission peak occurs at ca. 512, 513, 508 and 517 nm for 1–4, respectively, in frozen CH2Cl2 N 3 olec Cl O Br Br Br N N O NH2NH2.H2O Et3N / CHCl3 Me3Si NEt3 N Br0~r.t. overnight+ CuI, Pd(OAc)2, PPh K2CO3 / MeOH W.-Y. Wong, Y.-H. Guo / Journal of M (Figs. 2–5). The large Stokes shifts of these lower-lying emission peaks from the dipole-allowed absorptions (1.15–1.49 eV, see Figs. 2–5), plus the long emission lifetimes (sP) in the microsecond re- gime are indicative of their triplet parentage, and they are thus as- signed to the 3(pp*) excited states of the diethynylheteroarylene core (i.e. T1? S0 emission). Such assignment can be further sup- ported by the observed temperature dependence of the emission data for 1 (see Fig. 7), in accordance with earlier work on metal diy- nes and polyynes [11]. From 290 to 11 K, the singlet emission peak intensity increases only by a factor of 3.0 but the intensity of the lower-lying triplet emission drastically increases by a factor of >100 which is accompanied by a well-resolved vibronic structure, and such an increase in intensity indicates a long-lived triplet ex- NaOH / MeOH Au(PR3)Cl (2 equiv.) O O N N (R3P)Au O N N Au(PR3) R = Ph 1 R = Me 2 L Scheme 1. Synthetic routes to binuclear g Fig. 1. A perspective drawing of 1 with the thermal ellipsoids drawn at the 25% prob Table 1 Selected bond lengths (Å) and angles (�) for 1 Au(1)AP(1) 2.277(2) Au(2)AP(2) 2.264(2) Au(1)AC(19) 1.989(9) Au(2)AC(36) 1.998(9) C(19)AC(20) 1.198(11) C(35)AC(36) 1.200(11) P(1)AAu(1)AC(19) 177.1(2) P(2)AAu(2)AC(36) 175.2(2) Au(1)AC(19)AC(20) 174.0(8) Au(2)AC(36)AC(35) 173.8(7) NHNH O Br O O N N SiMe3Me3Si HgRCl (2 equiv.) NaOH / MeOH POCl3 N N RHg N N HgR N2 / reflux ular Structure 890 (2008) 150–156 153 cited state that is more sensitive to thermally activated non-radia- tive decay mechanisms [20]. As the temperature is lowered from 293 to 77 K, there is a notable red shift in the PL maximum for 3 and 4which is presumably a manifestation of the solid-state aggre- gation effect in the frozen phase at 77 K [21]. However, this solid- state effect is not significant for the Au(I) complexes 1 and 2. Based on the absorption and photoluminescence data, we can obtain experimental values of the lower-lying excitations (Table 3) and construct an energy scheme as shown in Fig. 8 for 1–4 that provides clearly the spatial extent of the singlet and triplet exci- tons. The energy values are absolute values with respect to the S0 ground state. Values of DE(So � T1) (energy gap between So and T1) were compiled to be 2.40–2.44 eV. The measured DE(S1 � T1) values are 0.78, 0.79, 0.71 and 0.69 eV for 1–4, respectively, and they correspond well with the S1 � T1 energy gap of 0.7 ± 0. l eV for similar p-conjugated Pt(II), Au(I) and Hg(II) acetylides [9a,17b,18b]. We attribute such a constant DE(S1 � T1) value to the exchange energy and possibly some additional constant contri- bution due to the admixture of the metal orbitals [22]. From the S1 energy levels obtained by absorption studies, it is clear that the S1 states are notably lower for the d10 Au(I) than their Hg(II) congeners, i.e. the order of p-delocalization through the metal OO R = Ph 3 R = Me 4 old(I) and mercury(II) complexes 1–4. ability level. The labels on the phenyl ring carbon atoms are omitted for clarity. 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 Photolum inescence (a.u.) A bs o rb an ce (a .u. ) olec Table 2 Photophysical data for the gold(I) and mercury(II) alkynyl complexes kabs (nm)a Eg (eV)b kem (nm) CH2Cl2 CH2Cl2 (290 K)c Frozen CH2Cl2 (77 K)d 1 276 sh (2.5) 3.35 366 sh 388 (S1) 299 sh (4.5) 387 (0.28, 1.1) 404 sh 332 (8.6) 405 sh 412 sh 340 (8.6) 512 (13.4) (T1) 360 sh (4.4) 542 sh 557 sh 2 297 sh (3.1) 3.31 365 sh 386 sh (S1) 331 (6.3) 385 (0.34, 1.2) 513 (15.9) (T1) 154 W.-Y. Wong, Y.-H. Guo / Journal of M chromophore is Au(I) > Hg(II) [17d]. We observe that the S1 energy level and the bandgap for 1 and 4 are higher relative to the biphen- ylene-spaced gold(I) and mercury(II) analogues [(PPh3)AuC„ C(p- C6H4)(p-C6H4)C„ CAu(PPh3)] (kabs = 326 nm, Eg = 3.24 eV) and [MeHgC„ C(p-C6H4)(p-C6H4)C„ CHgMe] (kabs = 309 nm, Eg = 3.31 eV), respectively [23]. This may be attributed to the partial conjugation interruption of the conjugated chain in the presence of an electron-accepting oxadiazole ring which results in an angu- lar geometry. 4. Concluding remarks This report describes the use of 2,5-bis(ethynylphenyl)-1,3,4- oxadiazole in the production of a new series of luminescent bime- 347 (4.7) 403 sh 531 358 sh (3.6) 558 sh 577 sh 3 288 sh (2.1) 3.44 369 (0.28, 1.3) 380 sh 320 (4.8) 394 (S1) 345 sh (1.9) 508 (13.6) (T1) 533 sh 552 sh 4 280 sh (1.9) 3.48 369 (0.47, 1.2) 318 sh 319 (4.8) 401 (S1) 340 sh (2.2) 419 sh 517 (11.4) (T1) 561 sh L 264 sh (1.8) 3.59 338 sh 307 (5.1) 357 (0.64, 1.4) 372 a Extinction coefficients (104 M�1 cm�1) are shown in parentheses. b Estimated from the onset wavelength of the optical absorption. c Fluorescence quantum yields (%) and lifetimes (ns) shown in parentheses (rF, sF) are measured in CH2Cl2 relative to 1.0 M quinine sulfate in H2SO4 (rF = 0.54). d Phosphorescence lifetimes sP (ls) at 77 K for the peak maxima are shown in parentheses. sh, shoulders or weak bands. 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Photolum inescence (a.u.) A bs or ba nc e (a. u.) Wavelength (nm) Fig. 2. Optical absorption spectrum at 293 K (� � �) and PL spectra at both 293 (—) and 77 K (---) of 1 in CH2Cl2. 1.0 1.0 ular Structure 890 (2008) 150–156 tallic materials. The group 11–12 transition elements in these oxa- diazole-phenylene derivatives can exert heavy-atom effects to trigger the phosphorescence emission at low temperatures. Future work in this direction should elucidate a direct evaluation of the role of a metal center on the properties of conjugated compounds 300 400 500 600 700 0.0 0.0 Wavelength (nm) Fig. 3. Optical absorption spectrum at 293 K (� � �) and PL spectra at both 293 (—) and 77 K (---) of 2 in CH2Cl2. 300 400 500 600 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Photolu m in escen ce (a.u.) A bs o rb an ce (a .u. ) Wavelength (nm) Fig. 4. Optical absorption spectrum at 293 K (� � �) and PL spectra at both 293 (—) and 77 K (---) of 3 in CH2Cl2. 300 400 500 600 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Photolu m in escen ce (a.u.) A bs or ba nc e ( a.u .) Wavelength (nm) Fig. 5. Optical absorption spectrum at 293 K (� � �) and PL spectra at both 293 (—) and 77 K (---) of 4 in CH2Cl2. olec 0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0 Photolum inescence A bs or ba nc e (a. u.) W.-Y. Wong, Y.-H. Guo / Journal of M and such an investigation is desirable for optoelectronic applica- tions that utilize the T1 state for light emission through light-har- vesting techniques. Work is underway to extend the method to the electroluminescent metallopolymeric systems desirable for poly- mer LEDs with improved charge-transporting properties so that more efficient EL devices can be developed. 5. Supplementary material Crystallographic data (comprising hydrogen atom coordinates, thermal parameters and full tables of bond lengths and angles) for the structural analysis of 1 has been deposited with the Cam- 300 400 500 600 0.0 0.2 0.0 0.2 (a.u.) Wavelength (nm) Fig. 6. Optical absorption (� � �) and photoluminescence (PL) spectra (—) of 2,5-bi- s(ethynylphenyl)-1,3,4-oxadiazole L in CH2Cl2 at 293 K. Fig. 7. Temperature dependence of the PL spectra of 1. Table 3 Experimental values of various transition energies among the S0, S1 and T1 levels and intersystem crossing efficiencies of gold(I) and mercury(II) bis(alkynyl) complexes containing oxadiazole spacer Energy (eV) 1 2 3 4 S0? S1a 3.65 3.55 3.87 3.89 S1? S0 3.20 (3.20)b 3.21 (3.22)b 3.15 (3.36)b 3.09 (3.36)b T1? S0 2.42 2.42 2.44 2.40 S1? T1 0.78 0.79 0.71 0.69 DE(T1? So, S1? So)c 0.89 9.41 0.11 0.16 a Obtained from the absorption data in CH2Cl2. b The energy values determined from the S0? S1 emission data at 293 K in CH2Cl2 are shown in parentheses. c Ratio of the intensities of triplet emission to singlet emission at 77 K. bridge Crystallographic Centre (Deposition No. CCDC-675080). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: + 44 1223 336 033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk). Acknowledgments Financial support from a CERG Grant from the Hong Kong Re- search Grants Council of the Hong Kong SAR, PR China (Project No. HKBU 2024/04P) and a Faculty Research Grant from the Hong Kong Baptist University (FRG/06-07/II-63) is gratefully acknowl- edged. We also thank Prof. K.-W. Cheah for the access of the facil- ities for variable temperature photoluminescence measurements. References [1] (a)J.L. Brédas, R.R. Chance (Eds.), Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics and Molecular Electronics, Academic Publishers, Dordrecht, 1990; (b) Y. Shirota, J. Mater. Chem. 10 (2000) 1; (c) U. Mitschke, P. Bäuerle, J. Mater. Chem. 10 (2000) 1471; (d) C.D. Entwistle, T.B. Marder, Chem. Mater. 16 (2004) 4574; 0 2.0 2.5 3.0 3.5 4.0 S1 PL 3.36S1 PL 3.36 S1 PL 3.22S1 PL 3.20 4321 T1 PL 2.42 T1 PL 2.42 T1 PL 2.44 T1 PL 2.40 S1 PL 3.21S1 PL 3.20 S1 PL 3.09 S1 PL 3.15 S1 Abs. 3.89 S1 Abs 3.57 S1 Abs. 3.87 S1 Abs 3.63 S0S0S0S0 En er gy / eV Fig. 8. Electronic energy level diagram of 1–4 determined from absorption and PL data. For the S1 PL levels, the dashed lines represent those obtained from the PL data at 293 K in CH2Cl2 whereas the solid lines those from the PL data at 77 K in frozen CH2Cl2. The S0 levels are arbitrarily shown to be of equal energy. ular Structure 890 (2008) 150–156 155 (e) C.H. Chen, J. Shi. Coord. Chem. Rev. 171 (1998) 161; (f) B.J. Coe, N.R.M. Curati, Comments Inorg. Chem. 25 (2004) 147; (g)T.A. Skotheim, J.R. Reynolds, R.L. Elsenbaumer (Eds.), Handbook of Conducting Polymers, second ed., Marcel Dekker, New York, 1998. [2] (a) J.H. Burroughs, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature (London) 347 (1990) 539; (b) A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 402; (c) R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Brédas, M. Lögdlund, W.R. Salaneck, Nature (London) 397 (1999) 121. [3] (a) J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature (London) 376 (1995) 498; (b) G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789; (c) N. Chawdhury, A. Köhler, R.H. Friend, W.-Y. Wong, J. Lewis, M. Younus, P.R. Raithby, T.C. Corcoran, M.R.A. Al-Mandhary, M.S. Khan, J. Chem. Phys. 110 (1999) 4963; (d) A. Köhler, H.F. Wittmann, R.H. Friend, M.S. Khan, J. Lewis, Synth. Met. 67 (1994) 245; (e) W.K. Chan, C.S. Hui, K.Y.K. Man, K.W. Cheng, H.L. Wong, N. Zhu, A.B. Djurišic´, Coord. Chem. Rev. 249 (2005) 1351; (f) N. Chawdhury, M. Younus, P.R. Raithby, J. Lewis, R.H. Friend, Opt. Mater. 9 (1998) 498; (g) M. Younus, A. Köhler, S. Cron, N. Chawdhury, M.R.A. Al-Mandhary, M.S. Khan, J. Lewis, N.J. Long, R.H. Friend, P.R. Raithby, Angew. Chem. Int. Ed. 37 (1998) 3036; (h) W.-Y. Wong, X.-Z. Wang, Z. He, A.B. Djurišic´, C.-T. Yip, K.-Y. Cheung, H. Wang, C.S.K. Mak, W.-K. Chan, Nat. Mater. 6 (2007) 521; (i) W.-Y. Wong, X.-Z. Wang, Z. He, K.-K. Chan, A.B. Djurišic´, K.-Y. Cheung, C.-T. Yip, A.M.-C. Ng, Y.Y. Xi, C.S.K. Mak, W.-K. Chan, J. Am. Chem. Soc. 129 (2007) 14372; (j) W.-Y. Wong, Macromol. Chem. Phys. 209 (2008) 14. [4] (a) Q.-Z. Yang, L.-Z. Wu, H. Zhang, B. Chen, Z.-X. Wu, L.-P. Zhang, C.-H. Tung, Inorg. Chem. 43 (2004) 5195; (b) P.K.M. Siu, S.W. Lai, W. Lu, N. Zhu, C.M. Che, Eur. J. Inorg. Chem. (2003) 2749; (c) I.-B. Kim, B. Erdogan, J.N. Wilson, U.H.F. Bunz, Chem. Eur. J. 10 (2004) 6247. [5] (a) F. Garnier, R. Hajlaoui, A. Yasser, P. Svwastra, Science 265 (1994) 1684; (b) H. Sirringhaus, N. Tissler, R.H. Friend, Science 280 (1998) 1741. [6] (a) N.J. Long, Angew. Chem. Int. Ed. Engl. 34 (1995) 21; (b) S.R. Marder, in: D.W. Bruce, D. O’Hare (Eds.), Inorganic Materials, Wiley, Chichester, 1996, p. 121; (c) S. Barlow, D. O’Hare, Chem. Rev. 97 (1997) 637; (d) I.R. Whittal, A.M. McDonagh, M.G. Humphrey, Adv. Organomet. Chem. 42 (1998) 291; (e) G.-J. Zhou, W.-Y. Wong, Z. Lin, C. Ye, Angew. Chem. Int. Ed. 45 (2006) 6189; (f) G.-J. Zhou, W.-Y. Wong, C. Ye, Z. Lin, Adv. Funct. Mater. 17 (2007) 963. [7] (a) J.S. Wilson, A.S. Dhoot, A.J.A.B. Seeley, M.S. Khan, A. Köhler, R.H. Friend, Nature (London) 413 (2001) 828; (b) W. Lu, B.-X. Mi, M.C.W. Chan, Z. Hui, C.-M. Che, N. Zhu, S.-T. Lee, J. Am. Chem. Soc. 126 (2004) 4958; (c) C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Lett. 90 (2001) 5048; (d) A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971; (e) M.K. Nazeeruddin, R. Humphry-Baker, D. Berner, S. Rivier, L. Zuppiroli, M. (i) W.-Y. Wong, L. Liu, S.-Y. Poon, K.-H. Choi, K.-W. Cheah, J.-X. Shi, Macromolecules 37 (2004) 4496; (j) W.-Y. Wong, S.-Y. Poon, A.W.-M. Lee, J.-X. Shi, K.-W. Cheah, Chem. Commun. (2004) 2420; (k) S. Szafert, J.A. Gladysz, Chem. Rev. 103 (2003). 4175 and references therein; (l) V.W.W. Yam, Acc. Chem. Res. 35 (2002). 555 and references therein; (m) E.E. Silverman, T. Cardolaccia, X. Zhao, K.-Y. Kim, K. Haskins-Glusac, K.S. Schanze, Coord. Chem. Rev. 249 (2005) 1491. [10] A. Köhler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701. [11] (a) W.-Y. Wong, C.-L. Ho, Coord. Chem. Rev. 250 (2006) 2627; (b) W.-Y. Wong, Dalton Trans. (2007) 4495; (c) W.-Y. Wong, J. Inorg. Organomet. Polym. Mater. 15 (2005) 197. [12] (a) C. Wang, M. Kilitziraki, L.-O. Pälsson, M.R. Bryce, A.P. Monkman, I.D.W. Samuel, Adv. Funct. Mater. 11 (2001) 47; (b) Q. Pei, Y. Yang, Chem. Mater. 7 (1995) 1568; (c) X.-B. Zhang, B.-C. Tang, P. Zhang, M. Lin, W.-J. Tian, J. Mol. Struct. 846 (2007) 55; (d) K.L. Paik, N.S. Baek, H.K. Kim, J.-H. Lee, Y. Lee, Macromolecules 35 (2002) 6782; (e) F. Lang, L. Wang, D. Ma, X. Jing, F. Wang, Appl. Phys. Lett. 81 (2002) 4; (f) Z. Peng, J. Zhang, Chem. Mater. 11 (1999) 1138; (g) P. Wang, C. Chai, Y. Chuai, F. Wang, X. Chen, X. Fan, Y. Xu, D. Zou, Q. Zhou, Polymer 48 (2007) 5889. [13] Y. Kuwabara, H. Ogawa, H. Inada, N. Noma, Y. Shirota, Adv. Mater. 6 (1994) 156 W.-Y. Wong, Y.-H. Guo / Journal of Molecular Structure 890 (2008) 150–156 Graetzel, J. Am. Chem. Soc. 125 (2003) 8790; (f) X. Gong, J.C. Ostrowski, G.C. Bazan, D. Moses, A.J. Heeger, M.S. Liu, A.K.-Y. Jen, Adv. Mater. 15 (2003) 45; (g) A.S. Dhoot, N.C. Greenham, Adv. Mater. 14 (2002) 1834; (h) P.K.H. Ho, J.S. Kim, J.H. Burroughes, H. Becker, S.F.Y. Li, T.M. Brown, F. Cacialli, R.H. Friend, Nature (London) 404 (2000) 481; (i) Y. Cao, I.D. Parker, G. Yu, C. Zhang, A.J. Heeger, Nature (London) 397 (1999) 414; (j) V. Cleave, G. Yahioglu, P. Le Barny, R.H. Friend, N. Tessler, Adv. Mater. 11 (1999) 285; (k) M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature (London) 395 (1998) 151; (l) M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature (London) 403 (2000) 750; (m) S.-C. Chan, M.C.W. Chan, Y. Wang, C.-M. Che, K.-K. Cheung, N. Zhu, Chem. Eur. J. 7 (2001) 4180. [8] (a) N.J. Long, C.K. Williams, Angew. Chem. Int. Ed. 42 (2003) 2586; (b) P. Nguyen, P. Gómez-Elipe, I. Manners, Chem. Rev. 99 (1999) 1515. [9] (a) J.S. Wilson, N. Chawdhury, M.R.A. Al-Mandhary, M. Younus, M.S. Khan, P.R. Raithby, A. Köhler, R.H. Friend, J. Am. Chem. Soc. 123 (2001) 9412; (b) M.S. Khan, M.R.A. Al-Mandhary, M.K. Al-Suti, B. Ahrens, M.F. Mahon, L. Male, P.R. Raithby, C.E. Boothby, A. Köhler, Dalton Trans. (2003) 74; (c) M.S. Khan, M.R.A. Al-Mandhary, M.K. Al-Suti, A.K. Hisahm, P.R. Raithby, B. Ahrens, M.F. Mahon, L. Male, E.A. Marseglia, E. Tedesco, R.H. Friend, A. Köhler, N. Feeder, S.J. Teat, J. Chem. Soc., Dalton Trans. (2002) 1358; (d) J.S. Wilson, A. Köhler, R.H. Friend, M.K. Al-Suti, M.R.A. Al-Mandhary, M.S. Khan, P.R. Raithby, J. Chem. Phys. 113 (2000) 7627; (e) N. Chawdhury, A. Köhler, R.H. Friend, M. Younus, N.J. Long, P.R. Raithby, J. Lewis, Macromolecules 31 (1998) 722; (f) W.-Y. Wong, C.-K. Wong, G.-L. Lu, A.W.-M. Lee, K.-W. Cheah, J.-X. Shi, Macromolecules 36 (2003) 983; (g) W.-Y. Wong, G.-L. Lu, K.-H. Choi, J.-X. Shi, Macromolecules 35 (2002) 3506; (h) W.-Y. Wong, K.-H. Choi, G.-L. Lu, Macromol. Rapid Commun. 22 (2001) 461; 677. [14] X. Zhan, Y. Liu, X. Wu, S. Wang, D. Zhu, Macromolecules 35 (2002) 2529. [15] (a) SAINT+, ver. 6.02a, Bruker Analytical X-ray System, Inc., Madison, WI, 1998.; (b) G.M. Sheldrick, SADABS, Empirical Absorption Correction Program, University of Göttingen, Germany, 1997. [16] G.M. Sheldrick, SHELXTLTM, Reference manual, ver. 5.1, Madison, WI, 1997. [17] (a) H.-Y. Chao, W. Lu, Y. Li, M.C.W. Chan, C.-M. Che, K.-K. Cheung, N. Zhu, J. Am. Chem. Soc. 124 (2002) 14696; (b) W. Lu, H.-F. Xiang, N. Zhu, C.-M. Che, Organometallics 21 (2002) 2343; (c) C.-M. Che, H.-Y. Chao, V.M. Miskowski, Y. Li, K.-K. Cheung, J. Am. Chem. Soc. 123 (2001) 4985; (d) W.-Y. Wong, K.-H. Choi, G.-L. Lu, J.-X. Shi, P.-Y. Lai, S.-M. Chan, Z. Lin, Organometallics 20 (2001) 5446; (e) B. Li, B. Ahrens, K.-H. Choi, M.S. Khan, P.R. Raithby, P.J. Wilson, W.-Y. Wong, Cryst. Eng. Commun. 4 (2002) 405. [18] (a) W.-Y. Wong, K.-H. Choi, G.-L. Lu, Z. Lin, Organometallics 21 (2002) 4475; (b) W.-Y. Wong, L. Liu, J.-X. Shi, Angew. Chem. Int. Ed. 42 (2003) 4064; (c) W.-Y. Wong, Coord. Chem. Rev. 251 (2007) 2400. [19] (a) R.J. Puddephatt, Chem. Commun. (1998) 1055. and references cited therein; (b) R.J. Puddephatt, Coord. Chem. Rev. 216–217 (2001) 313; (c) (c) H. Schmidbaur, Nature 413 (2001) 31. and references cited therein. [20] D. Beljonne, H.F. Wittmann, A. Köhler, S. Graham, M. Younus, J. Lewis, P.R. Raithby, M.S. Khan, R.H. Friend, J.L. Brédas, J. Chem. Phys. 105 (1996) 3868. [21] U.H.F. Bunz, Chem. Rev. 100 (2000) 1605. [22] (a) D. Hertel, S. Setayesh, H.G. Nothofer, U. Scherf, K. Mullen, H. Bässler, Adv. Mater. 13 (2001) 65; (b) A.P. Monkamn, H.D. Burrows, L.J. Hartwell, L.E. Horsburgh, I. Hamblett, S. Navaratnam, Phys. Rev. Lett. 86 (2001) 1358; (c) Y.V. Romanovskii, A. Gerhard, B. Schweitzer, U. Scherf, R.I. Personov, H. Bässler, Phys. Rev. Lett. 84 (2000) 1027. [23] L. Liu, S.-Y. Poon, W.-Y. Wong, J. Organomet. Chem. 690 (2005) 5036. Synthesis, structure and photophysics of binuclear gold(I) and mercury(II) complexes derived from 2,5-bis(ethynylphenyl)-1,3,4-oxadiazole Introduction Experimental General Synthesis of ligands 2,5-Bis(4-bromophenyl)hydrazine 2,5-Bis(trimethylsilylethynylphenyl)-1,3,4-oxadiazole 2,5-Bis(ethynylphenyl)-1,3,4-oxadiazole L Complex syntheses Gold(I) complex 1 Gold(I) complex 2 Mercury(II) complex 3 Mercury(II) complex 4 X-ray crystallography Results and discussion Synthesis Spectroscopic properties Crystal structure analyses Electronic absorption and photoluminescence spectra Concluding remarks Supplementary material Acknowledgments References