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Cite this: Chem. Commun., 2014, 50, 11043 Received 20th June 2014, Accepted 27th July 2014
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Red-luminescent biphosphine stabilized ‘Cu12S6’ cluster molecules† d Xiao-Xun Yang,ab Ibrahim Issac,bc Sergej Lebedkin,c Michael Ku ¨ hn, cd abc ace ace Florian Weigend, Dieter Fenske, Olaf Fuhr and Andreas Eichho ¨ fer*
DOI: 10.1039/c4cc04702h www.rsc.org/chemcomm
The synthesis, molecular structures and luminescence properties of two ‘Cu12S6’ cluster molecules with stabilizing bidentate phosphine ligands are described. Both display in the solid state at ambient temperature high photoluminescence quantum yields (Z48%) and good stability towards oxygen.
Luminescent mononuclear Cu(I) complexes, which demonstrate photoluminescence (PL) quantum yields as high as up to B90% in the solid state at ambient temperature, have gained increasing interest for their application in organic light emitting diodes (OLEDs).1 It has been suggested that increasing structural rigidity can reduce radiationless deactivation in mononuclear Cu(I) complexes, thus leading to strongly enhanced quantum yields.2 Such effect may also be expected for polynuclear (cluster) Cu(I) species which might display additional useful properties, e.g. with regard to photostability. Although light emission has been observed for a number of polynuclear Cu(I) metal complexes like [Cu4(m4-E)(dppm)4]2+ (E = S, Se; dppm = bis(diphenylphosphino)methane),3a [Cu6(mtc)6] (mtc = di-n-propylmonothio-carbamate),3b [Cu6(btt)6] (btt = 2-benzothiazolethiolate),3c [Cu3(dppm)3(m3-SR)2]+ 3d and [Cu4( p-S–C6H4–NMe2)4(dppm)2]3e their luminophore efficiency has not been quantified. A large number of
a
Lehn Institute of Functional Materials, Sun Yat-Sen University, Guangzhou 510275, China b ¨r Anorganische Chemie, Karlsruher Institut fu ¨r Technologie (KIT), Institut fu ¨d, Engesserstr. 15, 76131 Karlsruhe, Germany Campus Su c ¨r Nanotechnologie, Karlsruher Institut fu ¨r Technologie (KIT), Campus Institut fu Nord, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. E-mail: [email protected] d ¨r Physikalische Chemie, Abteilung fu ¨r Theoretische Chemie, Karlsruher Institut fu ¨r Technologie (KIT), Campus Su ¨d, Fritz-Haber-Weg 2, 76131 Karlsruhe, Institut fu Germany e Karlsruhe Nano Micro Facility (KNMF), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany † Electronic supplementary information (ESI) available: Experimental section, computational details, detailed structure parameters, powder XRD, time dependent PL, UV-Vis absorption spectra. CCDC 994711 and 994712. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c4cc04702h
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phosphine stabilized Cu(I) chalcogenide clusters which comprise bridging E2 ligands (E = S, Se and Te) are known,4 however luminescence studies on these clusters have not been reported so far. Here we report on two larger polynuclear Cu(I) molecules with ‘Cu12S6’ cores stabilized by bidentate phosphine ligands, which demonstrate PL quantum yields as high as Z48% in the solid state at ambient temperature. The red-coloured cluster complexes [Cu12S6(dpppt)4] (1) and [Cu12S6(dppo)4] (2) were synthesized by the reaction of copper acetate with S(SiMe3)2 in toluene by the use of bidentate bis(diphenylphosphino)pentane (dpppt = Ph2P(CH2)5PPh2) or bis(diphenylphosphino)octane (dppo = Ph2P(CH2)8PPh2), respectively.‡ 1 crystallizes in the tetragonal space group P42/ncm.§ The molecular structure is best described as consisting of an almost ideal octahedron of nonbonding sulfur atoms (S(1), S(2), S(3) and symmetry equivalent positions; S S: 431.6–439.5 pm) with the copper atoms (Cu(1)–Cu(4) and symmetry equivalent positions) bridging the twelve edges (Cu–S: 215.5–237.8 pm) (Fig. 1). The atoms S(2), S(3), Cu(3) and Cu(4) comprise ..m site symmetry whereas a twofold axis is running through S(1) and S(1 0 ) with the whole cluster residing on an inversion center. The copper atoms thus form two face sharing tetragonal antiprisms (Cu–Cu: 2.626–2.941 pm). The bidentate phosphine ligands (P(1), P(2) and symmetry equivalent positions) bridge Cu(1) and Cu(2) and symmetry equivalent positions at the upper and lower square planar face. The ‘Cu12S6P8’ cluster core in 1 is isostructural to those reported for [Cu12S6(L)8] (L = PEt3, PEtPh2).5 This also holds for the cluster core in 2 which crystallizes in the triclinic space group P1% . Characteristic atom distances are quite similar to those observed in 1 (Cu–S: 217.0–237.2, Cu Cu: 257.1–298.1 and S S: 433.9– 437.7 pm). Opposite to 1, in 2 each of the four bidentate dppo ligands bridges one copper atom of the upper and one of the lower square planar face. Instead of bridging those copper atoms lying above each other (e.g. Cu(3) and Cu(7)) the diphosphine ligands connect those which are rotated by 901 against each other along an axes through S(1) and S(6) (e.g. Cu(3) and Cu(6)).
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Fig. 1 Molecular structure of (a) [Cu12S6(dpppt)4] (1) and (b) [Cu12S6(dppo)4] (2) in the crystal (50% ellipsoids for Cu, S and P atoms, H atoms are omitted for clarity). Symmetry transformations in 1 used to generate equivalent atoms: x + 1, y, z; y + 1/2, x 1/2, z; y + 1/2, x + 1/2, z.
Fig. 2 Room temperature photoluminescence excitation (PLE) and emission (PL) spectra of a suspension of freshly prepared microcrystals in toluene of [Cu12S6(dpppt)4] (1) (down) and [Cu12S6(dppo)4] (2) (up) measured in the integrating sphere. The inset shows the colours of the suspensions of microcrystal under white LED light.
This results in a helical arrangement of the bidentate ligands around the ‘Cu12S6’ cluster core. The measured X-ray powder patterns of 1 and 2 show a good agreement with those simulated based on the single crystal data (Fig. S1 and S3, ESI†), if measured as a suspension of crystals in the mother liquor. Upon subsequent drying the loss of toluene leads to a decrease of the long range order (Fig. S2 and S4, ESI†) indicated by notable changes of the diffraction patterns. Solid complexes 1 and 2 show bright red emission when excited in the UV-Vis spectral range. The PL peaks at 648 and 665 nm, respectively, when the complexes are measured as suspension of freshly prepared microcrystals in toluene at ambient temperature (Fig. 2). Decay times of 6.1 and 6.5 ms for 1 and 2, respectively (Fig. S5, ESI†), are indicative for emission from triplet states, i.e. phosphorescence. The fact that the decay is relatively short and monoexponential is advantageous for applications.2 The PL quantum efficiencies at ambient temperature, jPL, of 0.48 and 0.67 were determined for 1 and 2, respectively, using an integrating sphere (see ESI†). Due to significant overlaps of the emission and excitation (PLE) spectra, i.e. very small Stokes shifts, the actual efficiency can even be somewhat higher, taking into account reabsorption of the short-wavelength emission. A small Stokes shift has also been observed for other polynuclear Cu(I) complexes.3e Such high PL efficiency may probably be expected also for other ‘Cu12S6’ cluster molecules. Note that the PLE spectra in Fig. 2 appear ‘‘saturated’’ due to optically ‘‘thick’’ samples inserted into the integrating sphere. Drying of crystalline 1 and 2 is accompanied by removal of intercalated solvent molecules and disorder of the crystal structure as evidenced by powder XRD (see ESI†). Interestingly, the
drying has a little effect on the PL of 2 and jPL values of solid 1 and 2, but results in a significantly broader emission and complicated, emission-wavelength-dependent PL kinetics of 1 (Fig. S6 and S7, ESI†). The latter indicates the presence of several luminescent substates/configurations of 1 and energy transfer/relaxation processes between them. Both complexes 2 and 1 dissolve in small amounts in thf to give a pink-red and orange-red solution, respectively. A comparison of the UV-Vis absorption spectra in solution and of a mull of microcrystals in nujol indicates at least partial decomposition of 1 in thf in agreement with the solution PL/PLE spectra which show the presence of different species in solution (Fig. S8 and S10, ESI†). In difference, the optical properties of 2 in thf are similar to those in the solid state (Fig. S9 and S11, ESI†). Calculations of the 60 lowest singlet excitations (Fig. 3, Tables S4 and S5, ESI†) with time dependent density functional theory (TDDFT, level B3-LYP/def2-SV(P))6 yield distinct spectra for 1 and 2 (for computational details see ESI†). Both spectra exhibit a low intensity band at lower energies (1: 2.57 and 2: 2.33 eV). Going to higher energies, 2 displays a well separated band at 2.52 eV with a shoulder at 2.62 eV; after a gap at 2.75 eV, the oscillator strength continuously increases. For 1 the first intense band at higher energies is centred at 2.77 eV directly followed by a weaker maximum at 2.9 eV and a featureless increase of the oscillator strength (for a comparison with the experimental absorption spectra see Fig. S12, ESI†). Visualization of the character of the peaks using the non-relaxed transition densities (see ESI†) indicates that all excitations in 1 result from a charge transfer from the ‘Cu12S6’ core (red) to the biphosphine ligands (blue). For 2, this is also true for the excitations with energies greater than 2.75 eV, but, surprisingly, the excitations below obviously do not involve the ligands, but rather
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configuration in solid complex 1. However, similar energetic shifts of the peak positions can also be observed for the triplet spectra with respect to the corresponding singlet transitions and also if a larger basis set (def2-TZVP instead of def2-SV(P)) is used. Further investigations are therefore required to clarify why the removal of solvent from the crystal lattice only affects the optical properties of 1 but not those of 2 and where the different optical behavior (i.e. the different character of the orbitals involved in the lowest energy transitions) of 1 and 2 originates from. As mentioned above, cluster complex 2 is also stable in solution under inert atmosphere gas conditions with an almost unchanged emission behaviour compared to the solid state. Usually copper chalcogenide cluster molecules are found to decompose in solution, as observed for 1, e.g. due to Cu–P bond dissociation and aggregation of the clusters. Therefore the possibility of solution processing of 2 should be advantageous for further investigations with respect to potential applications of this class of compounds. Fig. 3 Calculated singlet excitation energies and oscillator strengths of the lowest 60 transitions of [Cu12S6(dpppt)4] (1) (up) and [Cu12S6(dppo)4] (2) (down) plotted as vertical lines (green) as well as by superimposed Gaussians of FWHM = 0.1 eV (black) to simulate the experimental spectrum (level B3-LYP/def2-SV(P)). The character of the peaks was visualized using the non-relaxed transition densities (see ESI†). The contributions of occupied orbitals are plotted in red, those of the unoccupied orbitals in blue.
are transitions within the cluster core. Triplet spectra (Fig. S13, Tables S4 and S5, ESI†) are similar to the respective singlet spectra in terms of energy and character of the transitions. The ‘red-shift’ of the onset of the calculated lowest energy transitions by comparing 1 and 2 is mirrored in the experimental PL and PLE spectra. These latter observations together with the small Stokes shifts in the luminescence spectra suggest that the same types of orbitals as the calculated ones might also be involved in the respective triplet emissions of 1 and 2. The reasons for the different characters of the lowest energy excitations in 1 and 2, despite the similar ‘Cu12S6’ cluster cores and almost identical electron donating and accepting properties of the two bidentate phosphines, are not completely understood at the moment. TDDFT calculations for different structure parameters were done – for reasons of economy – at the BP86 level of theory. They also reproduced the different characters of the lowest energy transitions found for 1 and 2 at the B3-LYP level of theory except the overall shift in their energetic positions by approximately 0.8 eV (Fig. S14–S16, ESI†). The different character of the lowest energy excitations in 1 and 2 might correlate with the different effect of the crystal drying on the PL properties. Removal of solvent molecules, which leads to changes in the crystal and probably also molecular structure, e.g. configuration of the ligands, is apparently more relevant for the PL of 1 than of 2 (see above). Note that the single crystal XRD and optimized DFT molecular structures differ slightly for both 1 and 2. The calculated energetic changes in the spectra resulting from the geometric differences amount up to 0.1 eV. This is in the range of the emission broadening observed for 1 upon drying (emission bandwidth in suspension: 0.15 eV; in dried crystals: 0.26 eV). Correspondingly, the differences in the molecular structure as indicated in Fig. S14 (ESI†) may reflect the magnitude of the effect of drying on the ligand
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Notes and references ‡ Synthesis of [Cu12S6(dpppt)4] (1): dpppt (0.45 g, 1.02 mmol) and CuOOCCH3 (0.125 g, 1.02 mmol) were dissolved in 30 ml of toluene. S(SiMe3)2 (0.11 mL, 0.51 mmol) was then added at 40 1C to yield a clear colourless solution. Then, after warming up to +2 1C overnight orangered crystals of 1 appeared. After three days they were collected and washed two times with 10 mL of toluene and one time with 10 mL of Et2O and dried in vacuum to give a final yield of 0.145 g (62.8%). C116H120Cu12P8S6 (2716.9): calcd C 51.3, H 4.5, S 7.1 found C 52.0, H 4.3, S 6.8%. Synthesis of [Cu12S6(dppo)4] (2): dppo (0.244 g, 0.5 mmol) and CuOOCCH3 (0.062 g, 0.5 mmol) were dissolved in 15 mL toluene. S(SiMe3)2 (0.053 mL, 0.25 mmol) was then added at 0 1C to yield a clear red solution. Then, after warming up to +2 1C overnight in a fridge red crystals of 2 appeared. After one week they were collected and washed with toluene to give a final yield of 0.064 mg (52.1%). C128H144Cu12P8S6 (2885.25): C 53.30; H 5.03; S 6.67 found: C 53.56; H 5.08; S 6.38%. § Crystal data for 1: C116H120Cu12P8S62C6H5CH3, red cubic crystals, 0.34 0.32 0.30 mm, Mr = 2956.92, tetragonal, space group P42/ncm (No. 138), a = b = 2608.1 pm, c = 1931.3(4) pm, V = 13 137(4) 106 pm3, Z = 4, Dc = 1.467 g cm3, m(Mo-Ka) = 2.138 mm1 giving final R1 value of 0.0326 for 348 parameters and 5578 unique reflections with I Z 2s(I) and wR2 of 0.0859 for all 52 835 reflections (Rint = 0.0556). For 2: C128H144Cu12S6P83.5C6H5CH3, pink-red blocks, 0.08 0.063 0.03 mm, Mr = 3207.5, triclinic, space group P1% (No. 2), a = 1780.3(4) pm, b = 1852.2(4) pm, c = 2371.3(5) pm, a = 83.46(3)1, b = 80.88(3)1, g = 71.93(3)1, V = 7322(3) 106 pm3, Z = 2, Dc = 1.455 g cm3, m(Mo-Ka) = 1.925 mm1 giving final R1 value of 0.0434 for 1509 parameters, and 23 988 unique reflections with I Z 2s(I) and wR2 of 0.1153 for all 117 021 reflections (Rint = 0.0514) (see also Tables S1–S3, ESI†). 1 (a) L. Bergmann, J. Friedrichs, M. Mydlak, T. Baumann, M. Nieger ¨se, Chem. Commun., 2013, 49, 6501–6503; (b) S. Igawa, and S. Bra M. Hashimoto, I. Kawata, M. Yashima, M. Hoshino and M. Osawa, J. Mater. Chem. C, 2013, 1, 542–551. 2 H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer, Coord. Chem. Rev., 2011, 255, 2622–2652. 3 (a) V. W.-W. Yam, K. K.-W. Lo, W. K.-M. Fung and C.-R. Wang, Coord. Chem. Rev., 1998, 171, 17–41; (b) P. C. Ford and A. Vogler, Acc. Chem. Res., 1993, 26, 220–226; (c) C. Yue, C. Yan, R. Feng, M. Wu, L. Chen, F. Jiang and M. Hong, Inorg. Chem., 2009, 48, 2873–2879; (d) V. W.-W. Yam, C.-H. Lam, W. K.-M. Fung and K.-K. Cheung, Inorg. Chem., 2001, 40, 3435–3442; (e) R. Langer, M. Yadav, B. Weinert, D. Fenske and O. Fuhr, Eur. J. Inorg. Chem., 2013, 3623–3631. 4 O. Fuhr, S. Dehnen and D. Fenske, Chem. Soc. Rev., 2013, 42, 1871–1906. ¨fer, D. Fenske and R. Ahlrichs, Angew. Chem., 1994, 5 S. Dehnen, A. Scha ¨fer, D. Fenske and R. Ahlrichs, 106, 786–790; S. Dehnen, A. Scha Angew. Chem., Int. Ed. Engl., 1994, 33, 764–768. 6 TURBOMOLE Version 6.5, TURBOMOLE GmbH 2013. TURBOMOLE is a development of University of Karlsruhe and Forschungszentrum Karlsruhe 1989–2007, TURBOMOLE GmbH since 2007.
Chem. Commun., 2014, 50, 11043--11045 | 11045
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Cite this: Chem. Commun., 2014, 50, 11043 Received 20th June 2014, Accepted 27th July 2014
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Red-luminescent biphosphine stabilized ‘Cu12S6’ cluster molecules† d Xiao-Xun Yang,ab Ibrahim Issac,bc Sergej Lebedkin,c Michael Ku ¨ hn, cd abc ace ace Florian Weigend, Dieter Fenske, Olaf Fuhr and Andreas Eichho ¨ fer*
DOI: 10.1039/c4cc04702h www.rsc.org/chemcomm
The synthesis, molecular structures and luminescence properties of two ‘Cu12S6’ cluster molecules with stabilizing bidentate phosphine ligands are described. Both display in the solid state at ambient temperature high photoluminescence quantum yields (Z48%) and good stability towards oxygen.
Luminescent mononuclear Cu(I) complexes, which demonstrate photoluminescence (PL) quantum yields as high as up to B90% in the solid state at ambient temperature, have gained increasing interest for their application in organic light emitting diodes (OLEDs).1 It has been suggested that increasing structural rigidity can reduce radiationless deactivation in mononuclear Cu(I) complexes, thus leading to strongly enhanced quantum yields.2 Such effect may also be expected for polynuclear (cluster) Cu(I) species which might display additional useful properties, e.g. with regard to photostability. Although light emission has been observed for a number of polynuclear Cu(I) metal complexes like [Cu4(m4-E)(dppm)4]2+ (E = S, Se; dppm = bis(diphenylphosphino)methane),3a [Cu6(mtc)6] (mtc = di-n-propylmonothio-carbamate),3b [Cu6(btt)6] (btt = 2-benzothiazolethiolate),3c [Cu3(dppm)3(m3-SR)2]+ 3d and [Cu4( p-S–C6H4–NMe2)4(dppm)2]3e their luminophore efficiency has not been quantified. A large number of
a
Lehn Institute of Functional Materials, Sun Yat-Sen University, Guangzhou 510275, China b ¨r Anorganische Chemie, Karlsruher Institut fu ¨r Technologie (KIT), Institut fu ¨d, Engesserstr. 15, 76131 Karlsruhe, Germany Campus Su c ¨r Nanotechnologie, Karlsruher Institut fu ¨r Technologie (KIT), Campus Institut fu Nord, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. E-mail: [email protected] d ¨r Physikalische Chemie, Abteilung fu ¨r Theoretische Chemie, Karlsruher Institut fu ¨r Technologie (KIT), Campus Su ¨d, Fritz-Haber-Weg 2, 76131 Karlsruhe, Institut fu Germany e Karlsruhe Nano Micro Facility (KNMF), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany † Electronic supplementary information (ESI) available: Experimental section, computational details, detailed structure parameters, powder XRD, time dependent PL, UV-Vis absorption spectra. CCDC 994711 and 994712. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c4cc04702h
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phosphine stabilized Cu(I) chalcogenide clusters which comprise bridging E2 ligands (E = S, Se and Te) are known,4 however luminescence studies on these clusters have not been reported so far. Here we report on two larger polynuclear Cu(I) molecules with ‘Cu12S6’ cores stabilized by bidentate phosphine ligands, which demonstrate PL quantum yields as high as Z48% in the solid state at ambient temperature. The red-coloured cluster complexes [Cu12S6(dpppt)4] (1) and [Cu12S6(dppo)4] (2) were synthesized by the reaction of copper acetate with S(SiMe3)2 in toluene by the use of bidentate bis(diphenylphosphino)pentane (dpppt = Ph2P(CH2)5PPh2) or bis(diphenylphosphino)octane (dppo = Ph2P(CH2)8PPh2), respectively.‡ 1 crystallizes in the tetragonal space group P42/ncm.§ The molecular structure is best described as consisting of an almost ideal octahedron of nonbonding sulfur atoms (S(1), S(2), S(3) and symmetry equivalent positions; S S: 431.6–439.5 pm) with the copper atoms (Cu(1)–Cu(4) and symmetry equivalent positions) bridging the twelve edges (Cu–S: 215.5–237.8 pm) (Fig. 1). The atoms S(2), S(3), Cu(3) and Cu(4) comprise ..m site symmetry whereas a twofold axis is running through S(1) and S(1 0 ) with the whole cluster residing on an inversion center. The copper atoms thus form two face sharing tetragonal antiprisms (Cu–Cu: 2.626–2.941 pm). The bidentate phosphine ligands (P(1), P(2) and symmetry equivalent positions) bridge Cu(1) and Cu(2) and symmetry equivalent positions at the upper and lower square planar face. The ‘Cu12S6P8’ cluster core in 1 is isostructural to those reported for [Cu12S6(L)8] (L = PEt3, PEtPh2).5 This also holds for the cluster core in 2 which crystallizes in the triclinic space group P1% . Characteristic atom distances are quite similar to those observed in 1 (Cu–S: 217.0–237.2, Cu Cu: 257.1–298.1 and S S: 433.9– 437.7 pm). Opposite to 1, in 2 each of the four bidentate dppo ligands bridges one copper atom of the upper and one of the lower square planar face. Instead of bridging those copper atoms lying above each other (e.g. Cu(3) and Cu(7)) the diphosphine ligands connect those which are rotated by 901 against each other along an axes through S(1) and S(6) (e.g. Cu(3) and Cu(6)).
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Fig. 1 Molecular structure of (a) [Cu12S6(dpppt)4] (1) and (b) [Cu12S6(dppo)4] (2) in the crystal (50% ellipsoids for Cu, S and P atoms, H atoms are omitted for clarity). Symmetry transformations in 1 used to generate equivalent atoms: x + 1, y, z; y + 1/2, x 1/2, z; y + 1/2, x + 1/2, z.
Fig. 2 Room temperature photoluminescence excitation (PLE) and emission (PL) spectra of a suspension of freshly prepared microcrystals in toluene of [Cu12S6(dpppt)4] (1) (down) and [Cu12S6(dppo)4] (2) (up) measured in the integrating sphere. The inset shows the colours of the suspensions of microcrystal under white LED light.
This results in a helical arrangement of the bidentate ligands around the ‘Cu12S6’ cluster core. The measured X-ray powder patterns of 1 and 2 show a good agreement with those simulated based on the single crystal data (Fig. S1 and S3, ESI†), if measured as a suspension of crystals in the mother liquor. Upon subsequent drying the loss of toluene leads to a decrease of the long range order (Fig. S2 and S4, ESI†) indicated by notable changes of the diffraction patterns. Solid complexes 1 and 2 show bright red emission when excited in the UV-Vis spectral range. The PL peaks at 648 and 665 nm, respectively, when the complexes are measured as suspension of freshly prepared microcrystals in toluene at ambient temperature (Fig. 2). Decay times of 6.1 and 6.5 ms for 1 and 2, respectively (Fig. S5, ESI†), are indicative for emission from triplet states, i.e. phosphorescence. The fact that the decay is relatively short and monoexponential is advantageous for applications.2 The PL quantum efficiencies at ambient temperature, jPL, of 0.48 and 0.67 were determined for 1 and 2, respectively, using an integrating sphere (see ESI†). Due to significant overlaps of the emission and excitation (PLE) spectra, i.e. very small Stokes shifts, the actual efficiency can even be somewhat higher, taking into account reabsorption of the short-wavelength emission. A small Stokes shift has also been observed for other polynuclear Cu(I) complexes.3e Such high PL efficiency may probably be expected also for other ‘Cu12S6’ cluster molecules. Note that the PLE spectra in Fig. 2 appear ‘‘saturated’’ due to optically ‘‘thick’’ samples inserted into the integrating sphere. Drying of crystalline 1 and 2 is accompanied by removal of intercalated solvent molecules and disorder of the crystal structure as evidenced by powder XRD (see ESI†). Interestingly, the
drying has a little effect on the PL of 2 and jPL values of solid 1 and 2, but results in a significantly broader emission and complicated, emission-wavelength-dependent PL kinetics of 1 (Fig. S6 and S7, ESI†). The latter indicates the presence of several luminescent substates/configurations of 1 and energy transfer/relaxation processes between them. Both complexes 2 and 1 dissolve in small amounts in thf to give a pink-red and orange-red solution, respectively. A comparison of the UV-Vis absorption spectra in solution and of a mull of microcrystals in nujol indicates at least partial decomposition of 1 in thf in agreement with the solution PL/PLE spectra which show the presence of different species in solution (Fig. S8 and S10, ESI†). In difference, the optical properties of 2 in thf are similar to those in the solid state (Fig. S9 and S11, ESI†). Calculations of the 60 lowest singlet excitations (Fig. 3, Tables S4 and S5, ESI†) with time dependent density functional theory (TDDFT, level B3-LYP/def2-SV(P))6 yield distinct spectra for 1 and 2 (for computational details see ESI†). Both spectra exhibit a low intensity band at lower energies (1: 2.57 and 2: 2.33 eV). Going to higher energies, 2 displays a well separated band at 2.52 eV with a shoulder at 2.62 eV; after a gap at 2.75 eV, the oscillator strength continuously increases. For 1 the first intense band at higher energies is centred at 2.77 eV directly followed by a weaker maximum at 2.9 eV and a featureless increase of the oscillator strength (for a comparison with the experimental absorption spectra see Fig. S12, ESI†). Visualization of the character of the peaks using the non-relaxed transition densities (see ESI†) indicates that all excitations in 1 result from a charge transfer from the ‘Cu12S6’ core (red) to the biphosphine ligands (blue). For 2, this is also true for the excitations with energies greater than 2.75 eV, but, surprisingly, the excitations below obviously do not involve the ligands, but rather
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configuration in solid complex 1. However, similar energetic shifts of the peak positions can also be observed for the triplet spectra with respect to the corresponding singlet transitions and also if a larger basis set (def2-TZVP instead of def2-SV(P)) is used. Further investigations are therefore required to clarify why the removal of solvent from the crystal lattice only affects the optical properties of 1 but not those of 2 and where the different optical behavior (i.e. the different character of the orbitals involved in the lowest energy transitions) of 1 and 2 originates from. As mentioned above, cluster complex 2 is also stable in solution under inert atmosphere gas conditions with an almost unchanged emission behaviour compared to the solid state. Usually copper chalcogenide cluster molecules are found to decompose in solution, as observed for 1, e.g. due to Cu–P bond dissociation and aggregation of the clusters. Therefore the possibility of solution processing of 2 should be advantageous for further investigations with respect to potential applications of this class of compounds. Fig. 3 Calculated singlet excitation energies and oscillator strengths of the lowest 60 transitions of [Cu12S6(dpppt)4] (1) (up) and [Cu12S6(dppo)4] (2) (down) plotted as vertical lines (green) as well as by superimposed Gaussians of FWHM = 0.1 eV (black) to simulate the experimental spectrum (level B3-LYP/def2-SV(P)). The character of the peaks was visualized using the non-relaxed transition densities (see ESI†). The contributions of occupied orbitals are plotted in red, those of the unoccupied orbitals in blue.
are transitions within the cluster core. Triplet spectra (Fig. S13, Tables S4 and S5, ESI†) are similar to the respective singlet spectra in terms of energy and character of the transitions. The ‘red-shift’ of the onset of the calculated lowest energy transitions by comparing 1 and 2 is mirrored in the experimental PL and PLE spectra. These latter observations together with the small Stokes shifts in the luminescence spectra suggest that the same types of orbitals as the calculated ones might also be involved in the respective triplet emissions of 1 and 2. The reasons for the different characters of the lowest energy excitations in 1 and 2, despite the similar ‘Cu12S6’ cluster cores and almost identical electron donating and accepting properties of the two bidentate phosphines, are not completely understood at the moment. TDDFT calculations for different structure parameters were done – for reasons of economy – at the BP86 level of theory. They also reproduced the different characters of the lowest energy transitions found for 1 and 2 at the B3-LYP level of theory except the overall shift in their energetic positions by approximately 0.8 eV (Fig. S14–S16, ESI†). The different character of the lowest energy excitations in 1 and 2 might correlate with the different effect of the crystal drying on the PL properties. Removal of solvent molecules, which leads to changes in the crystal and probably also molecular structure, e.g. configuration of the ligands, is apparently more relevant for the PL of 1 than of 2 (see above). Note that the single crystal XRD and optimized DFT molecular structures differ slightly for both 1 and 2. The calculated energetic changes in the spectra resulting from the geometric differences amount up to 0.1 eV. This is in the range of the emission broadening observed for 1 upon drying (emission bandwidth in suspension: 0.15 eV; in dried crystals: 0.26 eV). Correspondingly, the differences in the molecular structure as indicated in Fig. S14 (ESI†) may reflect the magnitude of the effect of drying on the ligand
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Notes and references ‡ Synthesis of [Cu12S6(dpppt)4] (1): dpppt (0.45 g, 1.02 mmol) and CuOOCCH3 (0.125 g, 1.02 mmol) were dissolved in 30 ml of toluene. S(SiMe3)2 (0.11 mL, 0.51 mmol) was then added at 40 1C to yield a clear colourless solution. Then, after warming up to +2 1C overnight orangered crystals of 1 appeared. After three days they were collected and washed two times with 10 mL of toluene and one time with 10 mL of Et2O and dried in vacuum to give a final yield of 0.145 g (62.8%). C116H120Cu12P8S6 (2716.9): calcd C 51.3, H 4.5, S 7.1 found C 52.0, H 4.3, S 6.8%. Synthesis of [Cu12S6(dppo)4] (2): dppo (0.244 g, 0.5 mmol) and CuOOCCH3 (0.062 g, 0.5 mmol) were dissolved in 15 mL toluene. S(SiMe3)2 (0.053 mL, 0.25 mmol) was then added at 0 1C to yield a clear red solution. Then, after warming up to +2 1C overnight in a fridge red crystals of 2 appeared. After one week they were collected and washed with toluene to give a final yield of 0.064 mg (52.1%). C128H144Cu12P8S6 (2885.25): C 53.30; H 5.03; S 6.67 found: C 53.56; H 5.08; S 6.38%. § Crystal data for 1: C116H120Cu12P8S62C6H5CH3, red cubic crystals, 0.34 0.32 0.30 mm, Mr = 2956.92, tetragonal, space group P42/ncm (No. 138), a = b = 2608.1 pm, c = 1931.3(4) pm, V = 13 137(4) 106 pm3, Z = 4, Dc = 1.467 g cm3, m(Mo-Ka) = 2.138 mm1 giving final R1 value of 0.0326 for 348 parameters and 5578 unique reflections with I Z 2s(I) and wR2 of 0.0859 for all 52 835 reflections (Rint = 0.0556). For 2: C128H144Cu12S6P83.5C6H5CH3, pink-red blocks, 0.08 0.063 0.03 mm, Mr = 3207.5, triclinic, space group P1% (No. 2), a = 1780.3(4) pm, b = 1852.2(4) pm, c = 2371.3(5) pm, a = 83.46(3)1, b = 80.88(3)1, g = 71.93(3)1, V = 7322(3) 106 pm3, Z = 2, Dc = 1.455 g cm3, m(Mo-Ka) = 1.925 mm1 giving final R1 value of 0.0434 for 1509 parameters, and 23 988 unique reflections with I Z 2s(I) and wR2 of 0.1153 for all 117 021 reflections (Rint = 0.0514) (see also Tables S1–S3, ESI†). 1 (a) L. Bergmann, J. Friedrichs, M. Mydlak, T. Baumann, M. Nieger ¨se, Chem. Commun., 2013, 49, 6501–6503; (b) S. Igawa, and S. Bra M. Hashimoto, I. Kawata, M. Yashima, M. Hoshino and M. Osawa, J. Mater. Chem. C, 2013, 1, 542–551. 2 H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer, Coord. Chem. Rev., 2011, 255, 2622–2652. 3 (a) V. W.-W. Yam, K. K.-W. Lo, W. K.-M. Fung and C.-R. Wang, Coord. Chem. Rev., 1998, 171, 17–41; (b) P. C. Ford and A. Vogler, Acc. Chem. Res., 1993, 26, 220–226; (c) C. Yue, C. Yan, R. Feng, M. Wu, L. Chen, F. Jiang and M. Hong, Inorg. Chem., 2009, 48, 2873–2879; (d) V. W.-W. Yam, C.-H. Lam, W. K.-M. Fung and K.-K. Cheung, Inorg. Chem., 2001, 40, 3435–3442; (e) R. Langer, M. Yadav, B. Weinert, D. Fenske and O. Fuhr, Eur. J. Inorg. Chem., 2013, 3623–3631. 4 O. Fuhr, S. Dehnen and D. Fenske, Chem. Soc. Rev., 2013, 42, 1871–1906. ¨fer, D. Fenske and R. Ahlrichs, Angew. Chem., 1994, 5 S. Dehnen, A. Scha ¨fer, D. Fenske and R. Ahlrichs, 106, 786–790; S. Dehnen, A. Scha Angew. Chem., Int. Ed. Engl., 1994, 33, 764–768. 6 TURBOMOLE Version 6.5, TURBOMOLE GmbH 2013. TURBOMOLE is a development of University of Karlsruhe and Forschungszentrum Karlsruhe 1989–2007, TURBOMOLE GmbH since 2007.
Chem. Commun., 2014, 50, 11043--11045 | 11045
