Dinuclear copper(I) complexes containing diimine and phosphine ligands: Synthesis, copper–copper separation and photophysical properties

Inorganica Chimica Acta 362 (2009) 2492–2498

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Dinuclear copper(I) complexes containing diimine and phosphine ligands: Synthesis, copper–copper separation and photophysical properties Yong Chen a, Ji-Shu Chen b, Xin Gan c, Wen-Fu Fu a,c,* a

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Department of Chemistry and Life Science, Qujing Normal College, Qujing 655011, PR China c College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, PR China b

a r t i c l e

i n f o

Article history: Received 7 July 2008 Received in revised form 25 August 2008 Accepted 9 November 2008 Available online 18 November 2008 Keywords: Copper(I) complexes CuI  CuI distances Spectral properties

a b s t r a c t Dinuclear copper(I) complexes with bridging bis(dicyclohexylphosphino)methane (dcpm) or bis(diphenylphosphino)methane (dppm) and 2,20 -bipyridine or 2-[N-(2-pyridyl)methyl]amino-5,7-dimethyl-1,8naphthyridine (L), [Cu2(bpy)2(dppm)2](BF4)2 (1), [Cu2(bpy)2(dcpm)](BF4)2 (2), [Cu2(L)(dppm)](BF4)2 (3) and [Cu2(L)(dcpm)](BF4)2 (4) were prepared, and their structures were determined by X-ray crystal analysis. Two-, three-, and four-coordinate copper(I) centers are found in these complexes. Compounds 3 and 4 show close CuI  CuI separations of 2.664(3) and 2.674(1) Å, respectively, whereas an intramolecular copper–copper distance of 3.038 Å is found in 2 having only dcpm as an additional bridge. Powdered samples of 1, 3, and 4 display intense and long-lived phosphorescence with kmax at 533, 575, and 585 nm at room temperature, respectively. In the solid state, 2 exhibits only a weak emission at 555 nm. The time-resolved absorption and emission spectra of these complexes were investigated. The difference in the emission properties among complexes 1–4 suggests that both CuI  CuI distances and coordination environment of the copper(I) centers affect the excited-state properties. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction It is well documented in the literature that copper(I) complexes with phenanthroline-based diimine ligands exhibit low-energy metal-to-ligand charge-transfer (MLCT) transition, and a subtle change in the electronic property of polypyridine ligand has been reported to alter the nature of the lowest excited state in these complexes [1,2]. However, these complexes only with phenanthroline ligand that possesses alkyl or aryl substituents at the 2- and 9-positions display weak photoluminescence in the visible region. In general, bulky substituents in the phenanthroline ring can lead to an unexpected increase in emissive quantum yield. The unusual effect was considered in terms of its role by both inhibiting the flattening distortion and suppressing the ligand-addition reactions in the excited state [3]. Recently, much attention has been focused on the nature of the emissive excited state for dinuclear/polynuclear copper(I) complexes with varied CuI  CuI contact distances which affect the spectroscopic properties [4]. Though an average CuI  CuI distance of 2.35 Å for trinuclear copper(I) complex [5] and a remarkably short ligand-unsupported CuI  CuI contact of 2.81 Å [6] have been found for a long time, the existence of a bonding interaction between copper(I) atoms has been both supported * Corresponding author. Address: Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China. Fax: +86 10 6255 4670. E-mail address: [email protected] (W.-F. Fu). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.11.007

[7] and refuted [8] at various levels of theory. Previous reports have suggested that the close intramolecular metal–metal and metal– anion/solvent contacts as well as the coordination number around the copper(I) center have important influences on the photoproperties of di- and trinuclear coinage metal complexes with aliphatic phosphine ligands [9]. We are also interested in the photoluminescent properties of mixed–ligand copper(I) complexes of aromatic imines and bridging phosphines with variable coordinate number, and try to explore the effects of the coordinate configuration of metal center and CuI  CuI contact on photophysical properties. In the present study, four dinuclear copper(I) complexes with 2,20 -bipyridine or substituted 1,8-naphthyridine and phosphinebridged ligand were prepared and crystallographically characterized. Spectroscopic investigations involving time-resolved emission and transient absorption are presented. These studies suggest that CuI  CuI contacts play key roles in affecting the emissive spectra of these complexes. 2. Results and discussion 2.1. Synthesis 2-[N-(2-Pyridyl)methyl]amino-5,7-dimethyl-1,8-naphthyridine (L) was prepared from 2,4-dimethyl-7-amino-1,8-naphthyridine [10,11] and 2-bromopyridine according to the modified literature methods (Scheme 1) [12,13]. Reactions of Cu(CH3CN)4BF4 with

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O O CH3CCH2CCH3 H2N

N

NH2 HOAc, H SO , reflux 2 4

N N

N

N

Br

NH2 toluene, reflux

N

N

N H

Scheme 1.

2,20 -bipyridine or 1,8-naphthyridine derivative followed by addition of phosphine ligand in dry CH2Cl2 under N2 atmosphere afforded dinuclear complexes 1–4. Attempts to obtain [Cu2(bpy)2(dcpm)2]2+ as that in the case of [Cu2(bpy)2(dppm)2]2+ were unsuccessful due to the steric hindrance of dcpm ligand. Complexes 1–4 are stable in the solid state with respect to air and moisture.

contacts found in the analogous structural dinuclear gold(I) complexes, in which AuI  AuI interactions are well documented,

2.2. Crystal structures The molecular structures of complexes 1–4 with different bridging ligands have been characterized by X-ray crystal analysis. The ORTEP diagrams along with the atom-numbering scheme are illustrated in Figs. 1–4. Selected crystallographic data are listed in Tables 1 and 2. The coordination geometry of the Cu atom of 1 was best described as distorted tetrahedron with each Cu atom coordinated to bipyridine–nitrogen atoms as well as to two dppm ligands (Fig. 1). The two N2Cu+ fragments are bridged by two dppm ligands to form an eight-membered ring. The average Cu–N and Cu–P distances are 2.095 and 2.249 Å, respectively, which are consistent with those of [Cu(bpy)(PPh3)2]+ and [Cu(phen)(PPh3)2]+ [14]. The N–Cu–N angle of 78.56(1)° is comparable to that observed in other copper(I) complexes containing 2,20 -bipyridine ligand [15], whereas the P–Cu–P angle of 133.33(4)° is slightly larger than that for other pseudotetrahedral bis(phosphine) molecules [16]. In contrast, 2 exhibits two planar trigonal copper(I) centers wrapped around by bipyridine and one dcpm ligand (Fig. 2), and N2Cu+ fragments are bridged only by a bulky dcpm ligand, resulting in the formation of a clip structure. The two pyridyl rings of 2,20 -bipy ligand are almost coplanar with the dihedral angles being 3.9°. We note that the two 2,20 -bipy ligands are slightly twisted with a dihedral angle of 8° and that the intramolecular p–p interaction between them is separated by 3.340 Å. Copper– copper distance of 3.028 Å is found. In contrast, the AuI  AuI

Fig. 1. ORTEP diagram of complex 1 representing the thermal ellipsoids with 30% probability. The hydrogen atoms and anions are omitted for clarity.

Fig. 2. ORTEP diagram of complex 2 representing the thermal ellipsoids with 30% probability. The hydrogen atoms and anions are omitted for clarity.

Fig. 3. ORTEP diagram of complex 3 representing the thermal ellipsoids with 30% probability. The hydrogen atoms and anions except for F7 atom are omitted for clarity.

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Y. Chen et al. / Inorganica Chimica Acta 362 (2009) 2492–2498 Table 2 Selected bond lengths (Å) and angles (°) for 1–4. Bond lengths (Å) Complex 1 Cu(1)–Cu(2) Cu(1)–N(1) Cu(1)–N(2) Cu(1)–P(1) Cu(1)–P(2)#1

Fig. 4. ORTEP diagram of complex 4 representing the thermal ellipsoids with 30% probability. The hydrogen atoms and anions except for F2 atom are omitted for clarity.

are in the range of 3.256–3.284 Å [17]. The Cu atom adopts slightly distorted Y-shaped trigonal configuration with N(1)–Cu(1)–N(2), N(1)–Cu(1)–P(1) and N(2)–Cu(1)–P(1) bond angles of 81.04(2)°, 141.36(1)° and 136.23(1)°, respectively. The bond lengths of Cu(1)–N(1) (2.011(4) Å) and Cu(1)–P(1) (2.1622(2) Å) are slightly shorter than those of 1. As shown in Figs. 3 and 4, in complexs 3 and 4, there are two kinds of different coordinated copper(I) centers that are bridged through phosphine ligands, respectively. One features a planar trigonal configuration with the copper atom bonded to N atoms from pyridine and naphthyridine rings and to the P atom of phosphine ligand, the other is two-coordinate with a N atom from naphthyridine and a P atom from phosphine ligand. For the former,

Complex 2 Cu(1)–Cu(2) Cu(1)–N(1) Cu(1)–N(2) Cu(1)–P(1) Cu(2)–N(3) Cu(2)–N(4) Cu(2)–P(2) Complex 3 Cu(1)–Cu(2) Cu(1)–N(1) Cu(1)–N(4) Cu(1)–P(1) Cu(2)–N(2) Cu(2)–P(2) Cu(1)–F(7) Complex 4 Cu(1)–Cu(2) Cu(1)–N(1) Cu(1)–N(3) Cu(1)–P(1) Cu(2)–N(2) Cu(2)–P(2) Cu(2)–F(2)

Angles (°) 4.671 2.080(3) 2.109(3) 2.272(1) 2.227(1)

N(1)–Cu(1)–N(2) N(1)–Cu(1)–P(2)#1 N(2)–Cu(1)–P(2)#1 N(1)–Cu(1)–P(1) N(2)–Cu(1)–P(1) P(2)#1–Cu(1)–P(1)

78.56(1) 121.20(9) 106.75(9) 100.25(9) 101.31(9) 133.33(4)

3.028(1) 2.011(4) 2.041(4) 2.162(2) 2.032(4) 2.015(4) 2.156(2)

N(1)–Cu(1)–N(2) N(1)–Cu(1)–P(1) N(2)–Cu(1)–P(1) N(3)–Cu(2)–N(4) N(3)–Cu(2)–P(2) N(4)–Cu(2)–P(2)

81.04(2) 141.36(1) 136.23(1) 81.13(2) 137.84(1) 139.99(1)

2.664(3) 2.020(1) 1.969(1) 2.162(4) 1.937(1) 2.136(5) 2.559

N(1)–Cu(1)–N(4) N(1)–Cu(1)–P(1) N(4)–Cu(1)–P(1) N(2)–Cu(2)–P(2)

93.1(5) 141.2(4) 125.7(4) 173.7(4)

2.674(1) 2.055(5) 2.016(5) 2.205(2) 1.932(5) 2.178(2) 2.716

N(1)–Cu(1)–N(3) N(1)–Cu(1)–P(1) N(3)–Cu(1)–P(1) N(2)–Cu(2)–P(2)

94.4(2) 138.68(2) 124.54(2) 166.07(2)

Symmetry code of 1: #1 x + 2, y + 1, z.

the N–Cu–P bond angles vary from 125.7(4)° to 141.2(4)°, and N–Cu–N bond angle is 93.1(5)° for 3, while N–Cu–P bond angles of 4 vary from 124.54(2)° to 138.68(2)°, and N–Cu–N bond angle is 94.4(2)°. In the case of latter, the N–Cu–P bond angles are 173.7(4)° in 3 and 166.07(2)° in 4 rather than 180° as expected for an idealized two-coordinate linear geometry. This is due to short Cu  Cu distance and/or weak anion  CuI interaction, as signified by the nearest Cu  F interactions of 2.559 Å for 3 and

Table 1 Summary of X-ray crystallographic data for complexes 1–4.

Formula Fw Space group Crystal system a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) qcalcd (g cm3) h Range (°) l (mm1) Goodness-of-fit Number of unique Rint Number of parameters R1a wR2a Maximum, minimum peaks (e Å3) a

I > 2r(I). R1 =

12 C2H5OC2H5

21/2 C2H5OC2H5

3

4CH2Cl2

C78H80B2Cu2F8N4O2P4 1530.04 P2(1)/n monoclinic 14.942(5) 13.432(4) 19.734(6) 90 108.970(6) 90 3745(2) 4 293(2) 1.357 1.87–25.01 0.722 1.043 6602 0.0427 534 0.0447 0.1054 0.399, 0.311

C47H67B2Cu2F8N4O0.50P2 1058.69 P2(1)/n monoclinic 11.245(5) 18.481(9) 25.530(1) 90 96.530(9) 90 5271(4) 4 293(2) 1.334 1.36–25.01 0.932 1.043 9252 0.0436 667 0.0604 0.1570 1.074, 0.594

C80H72B4Cu4F16N8P4 1870.74  P1

C41H62B2Cl2Cu2F8N4P2 1044.49 P2(1)/n monoclinic 20.479(4) 11.234(2) 22.286(5) 91.110(2) 111.515(3) 91.010(2) 4770.1(2) 4 298(2) 1.454 1.70–25.01 1.136 1.058 8341 0.0605 550 0.0593 0.1436 1.074, 0.594

P P P P ||Fo|  |Fc|| |Fo|. wR2 ¼ ½wðF 2o  F 2c Þ2 = ½wðF 2o Þ2 1=2 .

triclinic 11.112(3) 19.093(3) 20.347(3) 72.433(2) 79.023(3) 77.143(3) 3976.9(1) 2 298(2) 1.562 1.75–25.01 1.224 1.025 12918 0.1802 1040 0.1440 0.3134 1.814, 1.042

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2.716 Å for 4. The intramolecular Cu  Cu distances in 3 and 4 are 2.664(3) and 2.6742(1) Å, respectively, which are noticeably shorter than those in 1 and 2. 2.3. Electronic absorption spectra The photophysical data of 1–4 are listed in Table 3. The electronic absorption spectra of 1 and 2 in CH2Cl2 at room temperature are depicted in Fig. 5. The intense absorptions below 330 nm for 1 and 2 are assigned to intraligand (IL p?p*) transitions in terms of their similarities with those of free bipyridine and phosphine ligand in shape and position. In view of the CuI  CuI separations of 4.671 and 3.028 Å, which are long enough to neglect CuI  CuI interaction, the weak absorptions in the range of 330–450 nm are attributed to MLCT (1[3d(Cu)?p*(bipy)]) transitions, as their mononuclear analogue [Cu(phen)(PPh3)2]+ [16]. As shown in Fig. 6, the electronic absorption spectra of 3 and 4 in CH2Cl2 are similar with that of free ligand L, except that a noticeable shoulder forms in the long-wavelength region. The low-energy broad absorption bands with a peak around 365 nm are presumably as-

Fig. 6. Absorption spectra of 3 and 4, as well as L in CH2Cl2 solution at room temperature.

cribed to the MLCT (1[3d(Cu)?p*(N-heteroaromatic ligand)]) transitions, together with some mixing of the Cu d?s character. Table 3 Spectroscopic and photophysical properties of 1–4.

2.4. Emission spectra

Complex

Medium (T [K])

kabs [nm] (e [mol1 dm3 cm1])

kem [nm]/s [ls]

kabs [nm]/s [ls]

1

CH2Cl2(298)

335(sh)(2870) 295(sh)(22450) 244(47770)

no emission

390/0.14 450/0.11

solid (298) 2

CH2Cl2(298)

533/6.65 346(sh)(2280) 304(17720) 244(27180)

solid (298) 3

CH2Cl2(298)

555/1.83 364(15180) 285(sh)(19940) 240(43500)

solid (298) 4

CH2Cl2(298)

solid (298)

no emission

620/0.12

500/0.12

575/0.93 380(sh)(12230) 366(14420) 285(17400) 254(30140)

610/0.04

440/0.04

580/0.27

Fig. 5. Absorption spectra of 1 and 2 in CH2Cl2 solution at room temperature.

In CH2Cl2 solution, 1 and 2 are non-emissive, while 3 and 4 display weak emissions with kmax at 620 and 610 nm, respectively (Table 3). Fig. 7 presents the solid-state emission spectra of 1–4 at room temperature. Upon excitation at 350 nm, crystalline 1 shows broad and asymmetric band with kmax at 533 nm. In comparison with 1, complex 2 exhibits weak solid-state emission centered at 555 nm in the same conditions, whereas the emission maxima of 3 and 4 are red-shifted to 575 and 580 nm in the solid state at room temperature, respectively. The large Stokes shifts and the emission lifetimes in the microsecond range imply phosphorescence emissions from the triplet excited state. In analogy to earlier results with the CuðPhenÞðPPh3 Þ2 þ system, the ambient temperature solid-state emissions of 1 and 2 are attributed to 3 MLCT transitions, which are almost completely quenched in solution largely because of solvent-induced exciplex quenching [18]. However, the emissions of 3 and 4 are less affected in solution despite their coordinative unsaturated copper(I) centers. These differences should be related to the close CuI  CuI contacts of

Fig. 7. Normalized solid-state emission spectra of 1–4 with kex = 350 nm at room temperature.

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2.664(3) and 2.674(1) Å in 3 and 4, respectively, which are shorter than the sum of van der Waals radii of two copper atoms [19]. Therefore, the long-lived luminescence of 3 and 4 would be better described as emissions from cluster-centered excited states, with mixed MLCT and d?s characters. 2.5. Time-resolved emission and absorption spectra Time-resolved emission measurements of solid samples of 1, 3, and 4 revealed that the emissions follow a single-exponential decay with microsecond magnitude of lifetimes (Figs. 8 and 9 and Fig. S1). The difference-absorption spectra of degassed CH2Cl2 solution of 1, 3 and 4 were measured following nanosecond-pulsed excitation at 355 nm. Strong absorptions were observed in the near-UV and visible region (Figs. 10 and 11 and Fig. S2). The spectrum of 1 shows the absorption peaks with kmax at 390 and 450 nm, the decay lifetimes of which occur on the same scale (139 and 112 ns, respectively), indicating that the parentage of the single type of excited state is responsible for the experimental observation. For complexes 3 and 4, the transient difference-absorption

spectra are basically similar, and show an intense absorption band in the 400–600 nm region and a strong bleach at about 650 nm, which nearly coincides with the emission maximum at 610– 620 nm (Table 3). The decay lifetimes of the absorption signal are consistent with those of the emission, suggesting that the transient absorption and emission come from the same triplet excited state of cluster-centered transition [20]. 3. Conclusion The structural investigations of two-, three- and four-coordinate dinuclear copper(I) complexes [Cu2(bpy)2(dppm)2](BF4)2, [Cu2(bpy)2(dcpm)] (BF4)2, [Cu2(L)(dppm)](BF4)2, and [Cu2(L)(dcpm)](BF4)2 show that both CuI  CuI distances and coordination environment around the copper(I) center play key roles in affecting the spectroscopic properties of these complexes. Close CuI  CuI contacts of 2.664(3) and 2.674(1) Å are found in the complexes. In particular with regard to the clip structure of [Cu2(bpy)2(dcpm)](BF4)2 having only dcpm as an additional bridge, it is important to note the intramolecular copper–copper distance of 3.038 Å, which can be compared with the weak AuI  AuI interactions (3.256–3.284 Å) found in analogous structural dinuclear gold(I) complexes. Time-resolved emission measurements of the complexes revealed that the emissions follow a single-exponential decay with microsecond magnitude of lifetimes, tentatively being assigned to 3MLCT excited state for 1 and to cluster-centered excited state for 3 and 4 in nature. The obtained transient difference-absorption spectra in the range of 390–600 nm, in which the decay lifetimes of the absorption signal are consistent with those of the emission, suggest that the transient absorption and emission come from the same triplet excited state.

4. Experimental 4.1. Materials

Fig. 8. Time-resolved emission spectra of 1 in the solid state at room temperature (kex = 350 nm). The spectra are developed by measurement of the decays at individual wavelengths.

Bis(dicyclohexylphosphino)methane (dcpm), bis(diphenylphosphino)- methane (dppm), 2,20 -bipyridine, 2,6-diaminopyridine, and 2-bromopyridine were purchased from Alfa Aesar and used as received. [Cu(CH3CN)4](BF4) was prepared according to the literature methods [21]. CH2Cl2 for synthesis was distilled using standard techniques and was saturated with N2 atmosphere before use. All the reactions were performed under an inert atmosphere. All the solvents used for photophysical studies were of HPLC grade. 4.2. Instrumentation and physical measurements UV–Vis absorption spectra were recorded using Hitach U-3010 spectrophotometer. Emission spectra were obtained on a Hitach F4500 fluorescence spectrofluorometer. 1H NMR spectra were recorded on Bruker DPX-400 multinuclear FT-NMR spectrometers with chemical shift (in ppm) relative tetramethylsilane. Timeresolved emission spectra and emission lifetime measurements were performed by a single photon counting technique on an Edinburgh Instrument F900. In the transient absorption spectroscopy experiments, the samples were purged with nitrogen for 30 min. Excitation was provided by using an Nd:YAG laser (third harmonic, 10 ns) at 355 nm. The detector was a xenon lamp on the Edingburge LP920 apparatus from Analytical Instruments. 4.3. X-ray structure analysis

Fig. 9. Time-resolved emission spectra of 3 in the solid state at room temperature (kex = 350 nm). The spectra are developed by measurement of the decays at individual wavelengths.

Crystals of 1–4 suitable for X-ray structure analysis were obtained by vapor diffusion of diethyl ether into CH2Cl2 solution over a period of several days. The diffraction data were collected

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with graphite monochromatized Mo Ka radiation (k = 0.71073 Å) on a Bruker SMART CCD area detector. An absorption correction was applied by correction of symmetry-equivalent reflections using the SADABS program [22]. The structure was solved by direct methods using the SHELXS 97 program and was refined by fullmatrix least squares on F2 using the SHELXL 97 software [23,24]. The hydrogen atoms were added using ideal geometries. 4.4. 2-[N-(2-Pyridyl)methyl]amino-5,7-dimethyl-1,8-naphthyridine (L) [11] A suspension of 2,4-dimethyl-7-amino-1,8-naphthyridine (1.73 g, 0.01 mol), 2-bromopyridine (1.58 g, 0.01 mol), and powdered KOH (1.12 g, 0.02 mol) in toluene was refluxed for 24 h. Upon removal of the solvent, the residue was washed with water until the washing was neutral. The product was purified by column chromatography over silica gel column using CHCl3/CH3CH2OH as the eluent. 0.93 g (37%); 1H NMR (400 MHz, CDCl3, 25 °C): d = 2.60 (s, 3H, CH3), 2.70 (s, 3H, CH3), 6.95 (t, J = 6.7 Hz, 1H), 7.02 (s, 1H), 7.42 (d, J = 8.8 Hz, 1H), 7.73 (t, J = 7.8 Hz, 1H), 8.14 (d, J = 8.9 Hz, 1H), 8.33 (d, J = 4.2 Hz, 1H), 8.42 (s, 1H), 10.90 (s, 1H, NH).

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4.5. [Cu2(bpy)2(dppm)](BF4)2 (1) A mixture of Cu(CH3CN)4BF4 (0.11 g, 0.35 mmol) and 2,20 -bipyridine (0.055 g, 0.35 mmol) in CH2Cl2 (20 mL) was stirred at room temperature for 4 h under N2 atmosphere, and then a solution of dppm (0.135 g, 0.35 mmol) in CH2Cl2 (20 mL) was added dropwise via a constant filter to the above red-brown solution. The resulting solution was stirred for 4 h and then filtered. Slow diffusion of diethyl ether into the filtrate yielded yellow crystals (0.17 g, 71%). 1H NMR (400 MHz, CDCl3, 25 °C): d = 3.21 (s, 4H, PCH2P), 6.95 (t, J = 7.5 Hz, 16H, ph-H), 7.08–7.13 (m, 24H, ph-H), 7.57 (broad s, 4H, py-H), 8.10 (t, J = 6.7 Hz, 4H, py-H), 8.44 (d, J = 7.8 Hz, 4H, py-H), 8.89 (s, 4H, py-H); 31P{1H} NMR (162 MHz, DMSO-d6): d = 5.78. 4.6. [Cu2(bpy)2(dcpm)2](BF4)2 (2) Compound 2 was prepared according to the same procedure as that for 1, except that dcpm (0.143 g, 0.35 mmol) was used instead of dppm. 0.11 g (64%); 1H NMR (400 MHz, DMSO-d6, 25 °C): d = 1.19–1.38 (m, 24H, dcpm-H), 1.68–2.00 (m, 20H, dcpm-H), 2.31 (t, J = 9.4, 2H, PCH2P), 7.58 (broad s, 4H, py-H), 8.10 (broad s, 4H, py-H), 8.21 (broad s, 4H, py-H), 8.28 (broad s, 4H, py-H); 31 1 P{ H} NMR (162 MHz, DMSO-d6): d = 21.40. 4.7. [Cu2(L)(dppm)](BF4)2 (3) A mixture of Cu(CH3CN)4BF4 (0.13 g, 0.41 mmol) and L (0.052 g, 0.21 mmol) in CH2Cl2 (20 mL) was stirred at room temperature for 4 h under N2 atmosphere, and then a solution of dppm (0.081 g, 0.21 mmol) in CH2Cl2 (20 mL) was added dropwise via a constant filter to the above red solution. The resulting solution was stirred for 8 h and then filtered. Slow diffusion of diethyl ether into the filtrate yielded yellow crystals (0.22 g, 58%). 1H NMR (400 MHz, DMSO-d6, 25 °C): d = 2.58 (s, 3H, CH3), 2.67 (s, 3H, CH3), 3.17 (s, 2H, CH2), 7.06–7.12 (m, 2H, py-H), 7.24 (broad s, 1H, naph-H), 7.30–7.32 (broad s, 12H, ph-H), 7.40 (s, 1H, naph-H), 7.46 (broad s, 1H, py-H), 7.60 (broad s, 8H, ph-H), 7.98 (broad s, 1H, py-H), 8.66 (broad s, 1H, naph-H); 31P{1H} NMR (162 MHz, DMSO-d6): d = 9.09. 4.8. [Cu2(L)(dcpm)](BF4)2 (4)

Fig. 10. Time-resolved difference-absorption spectra of 1 in degassed CH2Cl2 at room temperature (kex = 355 nm). The spectra are developed by monitoring the signal at individual wavelengths.

Compound 4 was prepared according to the same procedure as that for 3, except that dcpm (0.086 g, 0.21 mmol) was used instead of dppm. 0.14 g (68%); 1H NMR (400 MHz, DMSO-d6, 25 °C): d = 1.11–2.24 (m, 46H, dcpm-H), 2.74 (s, 3H, CH3), 2.99 (s, 3H, CH3), 7.43 (broad s, 2H, py-H), 7.50 (d, J = 8.6 Hz, 1H, naph-H), 7.63 (s, 1H, naph-H), 8.14 (broad s, 1H, py-H), 8.46 (broad s, 1H, py-H), 8.72 (d, J = 8.6 Hz, 1H, naph-H), 11.36 (s, 1H, NH); 31P{1H} NMR (162 MHz, DMSO-d6): d = 19.4. Acknowledgments This work was supported by the National Basic Research Program of China (973 program 2005CCA06800, 2007CB613304). We are grateful to the National Natural Science Foundation of China (NSFC Grant Nos. 20761006, 20671094, 90610034) for the financial support.

Appendix A. Supplementary material Fig. 11. Time-resolved difference-absorption spectra of 4 in degassed CH2Cl2 at room temperature (kex = 355 nm). The spectra are developed by monitoring the signal at individual wavelengths. Inset: decay traces of transient differenceabsorption spectra of 4 monitored at 450 nm.

CCDC 631707, 643409, 643410 and 643411 contain the supplementary crystallographic data for 1, 2, 3 and 4. These data can be obtained free of charge from The Cambridge Crystallographic Data

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