Ligand Based Dual Fluorescence and Phosphorescence Emission

Article pubs.acs.org/IC

Ligand Based Dual Fluorescence and Phosphorescence Emission from BODIPY Platinum Complexes and Its Application to Ratiometric Singlet Oxygen Detection Fabian Geist, Andrej Jackel, and Rainer F. Winter* Fachbereich Chemie der Universität Konstanz, Universitätsstraße 10, D-78464 Konstanz, Germany S Supporting Information *

ABSTRACT: Four new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-8-yl (BODIPY) platinum(II) complexes of the type cis-/ trans-Pt(BODIPY)Br(PR3)2 (R = Et or Ph) were synthesized and characterized by NMR, electronic absorption, and luminescence spectroscopy. Three of the complexes were also studied by single crystal X-ray diffraction. The absorption profiles of the four complexes feature intense HOMO → LUMO π → π* transitions with molar extinction coefficients ε of ca. 50 000 M−1cm−1 at around 475 nm and vibrational progressions that are characteristic of BODIPYs. Most remarkably, most complexes exhibit dual emissions through fluorescence at ca. 490 nm and phosphorescence at ca. 650 nm that originate from Pt-perturbed BODIPY-centered 1ππ* or 3ππ* states, respectively. Electronic absorption and luminescence spectroscopy data are in good agreement with our TD-DFT calculations. While the emission of the cis-complexes is dominated by fluorescence, their trans-isomers emit predominantly through phosphorescence with a phosphorescence quantum yield for trans-Pt(BODIPY)Br(PEt3)2 (trans-1) of 31.2%. trans-1 allows for ratiometric one-component oxygen sensing in fluid solution up to atmospheric concentration levels and exhibits a remarkably high Stern−Volmer constant for the quenching of the excited triplet state by oxygen of ca. 350 bar−1 as determined by changes in phosphorescence intensity and lifetime.

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INTRODUCTION During the past three decades, 4-bora-3a,4a-diaza-s-indacene (boron dipyrromethene, BODIPY) dyes have evolved as an important class of powerful light harvesters1−6 with applications in light to energy conversion schemes,7−10 in laser technology,11,12 or in photodynamic therapy.13,14 Due to their chemical robustness, BODIPYs lend themselves for versatile synthetic strategies, which allow for the synthesis of elaborate derivatives and their incorporation into complex architectures, including multichromophoric arrays.2,3,15−18 Such modification also allows to tune the energy of their prominent HOMO− LUMO band from the mid-visible (Vis) to the near-infrared (NIR) by introduction of suitable substituents or by extending their conjugated π-system. Besides their typically sharp and intense absorption with high molar extinction coefficients of generally >50 000 M−1 cm−1, BODIPYs also show bright, intense fluorescence with quantum yields of mostly >50% with small Stokes shifts.15,19 Room-temperature phosphorescence emission from BODIPY dyes has, however, only rarely been observed. 10 Diiodination or -bromination at the 2- and 6-positions was reported to enhance the intersystem crossing (ISC) rate from the 1ππ* to the 3ππ* state due to the heavy atom effect of the halogen atom.9,13,20,21 The enhanced ISC of such halogenated derivatives makes them useful as photosensitizers,8,14,22,23 where formation of a long-lived triplet state is crucial for © XXXX American Chemical Society

triggering efficient photocatalytic processes. In the absence of a triplet energy acceptor, relaxation from the 3ππ* state of a BODIPY to the ground state, however, usually proceeds by nonradiative processes rather than through phosphorescence.10,24 A more promising approach to access BODIPY-centered phosphorescence is the attachment of these dyes to triplet photosensitizers based on Ru(II), Ir(III), or Pt(II) polyimine complexes.17,18,25−29 Excitation into the metal-to-ligand chargetransfer (MLCT) band of these complexes usually promotes rapid ISC to the 3MLCT state owing to the high spin−orbit coupling constants of the heavy metal atoms. ISC may be followed by triplet energy transfer to the BODIPY-centered 3 ππ* state, provided the latter energy transfer process is exergonic. Due to the long lifetime of the BODIPY 3ππ* state of up to tens of milliseconds, no phosphorescence emission is usually observed at room temperature and occasionally not even in frozen matrices at 77 K.17,18,25−30 Notable exceptions are the Ir(ppy)2(L) and Ru(bipy)2 (L) (ppy = 2-phenylpyridine; bipy = 2,2′-bipyridine; L = BODIPY-functionalized bipy ligand) complexes shown in Chart 1. Both complexes feature the same BODIPY-functionalized bipy ligand, which Received: August 28, 2015

A

DOI: 10.1021/acs.inorgchem.5b01969 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Chart 1. Molecular Structures of Room-Temperature Phosphorescent BODIPY Complexes Ru-BDP, Ir-BDP, and Pt-BDP

Figure 1. ORTEP diagrams of (a) cis-2, (b) trans-2, and (c) trans-1. Only one of the two independent molecules per unit cell of trans-1 is shown. Ellipsoids are drawn at a 40% probability level. Hydrogen atoms are omitted for clarity reasons.

2-bromothioxanthone (Br-Tx) to Pt(PEt3)4 to give transPtBr(PEt3)2(Tx) enhances the ISC efficiency of the thioxanthone and leads to simultaneous Tx based fluorescence and long-lived phosphorescence emission after excitation into the HOMO−LUMO band.36 It was further shown that the transPt(PEt3)2X (X = Cl, Br, CN, or I) fragment increases the rate for phosphorescence decay to the ground state (i.e., decreases the phosphorescence lifetime) while simultaneously decreasing the rate for thermal deactivation of the excited triplet state.35,37 This resulted in quantum yields of up to 19% for Tx based phosphorescence at room temperature in fluid solution.36 On the basis of these findings we anticipated that direct attachment of a BODIPY to platinum might lead to efficient BODIPY based room-temperature phosphorescence. Our findings along these lines are reported herein.

provides an uninterrupted conjugation path between the ethynylated BODIPY and the 2,2′-biypridine chelate. In each case, room-temperature phosphorescence from the BODIPY 3 ππ* state was observed, however, with a very low quantum yield of 0.03% for the iridium complex Ir-BDP and an even lower one for the ruthenium complex Ru-BDP.26,27 Remarkably, their immediate relatives, where the BODIPY is connected to the bipy via its meso-position, do not display any BODIPY based phosphorescence, thus underlining the importance of the site of attachment and of integrating the BODIPY π-system into that of the metal based chromophore. In fact, quantum chemical calculations on the complexes in Chart 1 have shown that their HOMOs, while largely based on the BODIPY dye, extend onto the bipyridine segment of the corresponding ligand and the metal atom, while they are entirely restricted to the BODIPY in their meso-platinated counterparts. Electronic coupling between the BODIPY and the metal polyimine moiety not only enhances energy transfer from MLCT states to the BODIPY 3ππ* state, but also triggers ISC within the BODIPY moiety itself after exciting the BODIPY-centered 1 ππ* state.17,26,28 In general, a large heavy atom effect is crucial not only for triggering the otherwise forbidden ISC but also for an efficient relaxation from the triplet state T1 to the S0 ground state by phosphorescence. Therefore, a close proximity between the heavy atom and the emissive center is beneficial in order to increase the phosphorescence quantum yield.31,32 Following these lines, Zhao and co-workers have recently demonstrated that attachment of the ethynyl groups of a 2,6-diethynyl BODIPY derivative to Pt(N^C^N) moieties (Pt-BDP, Chart 1) leads to long-lived room-temperature BODIPY-centered NIR phosphorescence with a quantum yield of 3.5%, which seems to be the best reported value until now.33 Inspired by the elegant work of Sharp and his co-workers on platinum aryl complexes,34,35we have recently shown that platination of a thioxanthone (Tx) dye via oxidative addition of

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EXPERIMENTAL SECTION

Materials and General Methods. The oxidative addition reactions were performed under N2 atmosphere by standard Schlenk techniques. Workup and cis−trans isomerization were conducted in air. Benzene was distilled over sodium and degassed by three freeze− pump−thaw cycles. C6D6 was supplied under argon from Eurisotop, stored under argon, and used without further purification. All other solvents were used as received. Pt(PPh3)4 was purchased from ABCR and Br-BODIPY,38 cis-Pt(Et)2(PEt3)239 and cis-Pt(η2-C2H4) (PPh3)240 were synthesized according to literature protocols. NMR experiments were carried out on a Varian Unity Inova 400, a Bruker Avance III DRX 400, or a Bruker Avance DRX 600 spectrometer. 1H and 13C NMR spectra were referenced to the solvent signal, while 31P and 195Pt NMR spectra were referenced to the appropriate external standard (85% H3PO4 or a saturated solution of K2PtCl6 in D2O, respectively). NMR spectral data are given as follows: chemical shift (δ ppm), multiplicity (m, multiplet; s, singlet; d, doublet; t, triplet; vt, virtual triplet; br, broad), coupling constant (Hz), integration. Unequivocal signal assignments were achieved by 2D NMR experiments (1H,1H gCOSY, 1H,13C gHSQC, 1H,13C gHMBC, and 1H,31P gHMBC). The numbering of the nuclei follows that of the crystal structures in Figure 1. Combustion analysis was conducted with a Elementar vario MICRO cube CHN-analyzer from Heraeus. B


Article

Inorganic Chemistry

298 K): δ 7.59 (br s, 2H, H5/H9), 7.30 (d, 3JHH = 3.94 Hz, 2H, H3/ H7), 6.40 (dd, 3JHH = 3.94 Hz, 3JHH = 2.04 Hz, 2H, H4/H8), 2.11 (m, 6H, P2−CH2−), 1.75 (m, 6H, P1−CH2−), 1.21 (dt, 3JPH = 16.31 Hz, 3 JHH = 7.61 Hz, 9H, P2−CH2−CH3), and 1.01 (dt, 3JPH = 17.31 Hz, 3 JHH = 7.59 Hz, 9H, P1−CH2−CH3). 31P NMR (161.8 MHz, CDCl3, 298 K): δ 5.89 (d, 2JPP = 18.86 Hz, with satellites 1JPtP = 1858 Hz, P1), 0.32 (d, 2JPP = 18.86 Hz, with satellites 1JPtP = 3765 Hz, P2). 13C NMR (100.53 MHz, CDCl3, 298 K): δ 189.6 (dd, 2JPC,cis = 111.6 Hz, 2JPC,trans = 7.3 Hz, C1, Pt satellites were not detected due to low S/N ratio), 143.1 (s, with satellites 2JPtC = 21.1 Hz, C2/C6), 137.4 (s, C5/C9), 134.0 (s, with satellites 3JPtC = 34.1 Hz, C3/C7), 116.1 (s, C4/C8), 17.1 (dd, 1JPC = 37.7 Hz, 3JPC = 2.0 Hz, with satellites 2JPtC = 36.5 Hz, P1−CH2−), 15.3 (d, 1JPC = 30.5 Hz, with satellites 2JPtC = 20.8 Hz, P2−CH2−), 8.5 (d, 2JPC = 1.5 Hz, with satellites 3JPtC = 15.2 Hz, P2− CH2−CH3), and 8.2 (d, 2JPC = 2.8 Hz, with satellites 3JPtC = 27.7 Hz, P1−CH2−CH3). 195Pt NMR (85.56 MHz, CD2Cl2, 298 K): δ −4498 (dd, 1 JPtP = 1858 Hz, 1J PtP = 3765 Hz). Anal. Calcd for C21H36BBrF2N2P2Pt: C, 35.92; H, 5.17; N, 3.99. Found: C, 35.77; H, 5.33; N, 4.17%. trans-Bromo(4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-8-yl)bis(triethylphosphine)-platinum(II) (trans-1). An NMR tube was charged with cis-Pt(BODIPY)Br(PEt3)2 (cis-1) (30 mg, 42.7 μmol), AgOTf (15 mg, 58.4 μmol), and 0.6 mL of CD2Cl2. The mixture was heated to reflux for 5 min. 31P and 1H NMR experiments indicated a complete abstraction of bromide and the formation of the trans-triflato complex. The precipitate was filtered off, and the filtrate was added to NaBr (8.7 mg, 85 μmol) dissolved in 2 mL of methanol. The orange suspension was stirred for 45 min at room temperature. Then, the solvent was removed, and the product was extracted with CH2Cl2 (3 × 3 mL). The solvent was evaporated, and the product was extracted with diethyl ether/pentane = 1/1. The solvents were removed in vacuo, and the obtained solid was washed with pentane (3 × 0.6 mL), which gave the analytically pure product. Yield: 57%. 1H NMR (600.3 MHz, CD2Cl2, 298 K): δ 7.65 (br s, 2H, H5/H9), 7.43 (d, 3JHH = 3.95 Hz, 2H, H3/H7), 6.46 (dd, 3JHH = 3.95 Hz, 3JHH = 2.05 Hz, 2H, H4/H8), 1.80 (m, 12H, P−CH2−), 1.05 (dt, 3JHH = 7.66 Hz, 3JPH = 16.70 Hz, 18H, P−CH2−CH3). 31P NMR (161.8 MHz, CD2Cl2, 298 K): δ 8.89 (s, with satellites 1JPtP = 2480 Hz). 13C NMR (151.0 MHz, CD2Cl2; 298 K) δ 179.1 (t, 2JPC = 7.9 Hz, with satellites 1JPtC = 995 Hz, C1), 143.2 (s, with satellites 2JPtC = 47.1 Hz, C2/C6), 137.3 (s, C5/C9), 133.8 (s, with satellites 3JPtC = 50.8 Hz, C3/C7), 116.6 (s, C4/C8), 14.8 (m, P−CH2−), 8.3 (s, with satellites 3JPtC = 22.2 Hz, P−CH2− CH3). 195Pt NMR (85.6 MHz, CD2Cl2, 298 K): δ −4266 (t, 1JPtP = 2480 Hz). Crystals suitable for single crystal X-ray diffraction were grown by layering a concentrated CH2Cl2 solution of trans-1 with npentane. Anal. Calcd for C21H36BBrF2N2P2Pt: C, 35.92; H, 5.17; N, 3.99. Found: C, 36.02; H, 4.94; N, 4.51%. cis-Bromo(4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-8-yl)-bis(triphenylphosphine)-platinum(II) (cis-2). Starting from Pt(PPh3)4. A 100 mL Schlenk tube was charged with Pt(PPh3)4 (400 mg, 321 μmol), 8-bromo-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (87 mg, 321 μmol), and 20 mL of benzene. The dark green suspension was heated at 80 °C. After 1 h, a black solid precipitated (most likely due to decomposition of the Br-BODIPY induced by the released phosphine). After 4 h at 80 °C the solvent was removed, and the remaining dark red solid was purified by column chromatography on neutral deactivated alumina with a solvent gradient from 2/1 to 5/1 CH2Cl2/n-pentane. The obtained orange-brown solid was further purified by washing with toluene (3 × 4 mL), which gave the pure orange product. Yield: 38%. Starting from Pt(η2-C2H4)(PPh3)2. A 100 mL Schlenk tube was charged with Pt(η2-C2H4)(Ph3P)2 (216 mg, 289 μmol) and 8-bromo4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (78.4 mg, 289 μmol), and 5 mL of benzene were added. The reaction mixture was stirred for 10 min. Immediately after benzene addition, the color turned from red to orange, and an orange solid precipitated. The mixture was layered with pentane and subsequently filtered. The orange solid was washed with pentane (3 × 10 mL) and dried in vacuo yielding the product as a benzene hemisolvate. Yield: 84%. 1H NMR (399.8 MHz, CD2Cl2, 303 K): δ 7.57 (d, 3JHH = 3.92 Hz, 2H, H5/H9), 7.45 (br vt, J = 9.46 Hz,

X-ray diffraction analysis was performed at 100 K on a STOE IPDSII diffractometer equipped with a graphite-monochromated radiation source (λ = 0.710 73 Å) and an image plate detection system. A crystal mounted on a fine glass fiber with silicon grease was employed. If not indicated otherwise, the selection, integration, and averaging procedure of the measured reflex intensities, the determination of the unit cell dimensions, and a least-squares fit of the 2θ values as well as data reduction, LP-correction, and space group determination were performed using the X-Area software package delivered with the diffractometer. A semiempirical absorption correction was performed.41 All structures were solved by the heavy-atom methods (SHELXS-97, SHELXS-2013, or SHELXS-2014).42 Structure solutions were completed with difference Fourier syntheses and full-matrix least-squares refinement using SHELXL-97, SHELXS-2013, or SHELXS-2014,42 minimizing ω(Fo2 − Fc2)2. The weighted R factor (wR2) and the goodness of fit GOF are based on F2. All non-hydrogen atoms were refined with anisotropic displacement parameters, while hydrogen atoms were treated in a riding model. Molecular structures in this work are plotted with ORTEP 32.43 CIF files of trans-1, cis-2, and trans-2 have been deposited with the Cambridge Structure Data Base as CCDC 1419987 (cis-2), 1419988 (trans-2), and 1419989 (trans-1) and can be obtained free of charge via www.ccdc.cam.ac.uk/ conts/retrieving.html or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax (+44) 1223336-033, or at [email protected]. UV−vis absorption spectra were obtained on a TIDAS fiberoptic diode array spectrometer MCS from j&m in HELLMA quartz cuvettes with 1 cm optical path lengths. Luminescence spectra and lifetimes were measured in CH 2 Cl 2 on a PicoQuant FluoTime 300 spectrometer. Emission spectroscopy under inert gas atmosphere and the preparation of samples with defined O2-concentrations for the recording of O2 dependent emission spectra were conducted in a quartz cuvette modified with an angle valve from Normag. O2 concentrations were adjusted by completely degassing the sample and subsequent injection of adequate volumes of air and nitrogen by syringe. Quantum yields were measured using a Hamamatsu Absolute PL Quantum Yield Measurement System C9920-02 equipped with an integrating sphere. Computational Details. The ground state electronic structures were calculated by density functional theory (DFT) methods using the Gaussian 0944 program packages. Quantum chemical studies were performed without any symmetry constraints. Open shell systems were calculated by the unrestricted Kohn−Sham approach (UKS).45 Geometry optimization followed by vibrational analysis was made either in vacuum or in solvent media. The quasirelativistic WoodBoring small-core pseudopotentials (MWB)46,47 and the corresponding optimized set of basis functions48 for Pt and the 6-31G(d) polarized double-ζ basis set49 for the remaining atoms were employed together with the Perdew, Burke, Ernzerhof exchange and correlation functional (B3LYP).50,51 Solvent effects were accounted for by the Polarizable Conductor Continuum Model (PCM)52−55 with standard parameters for dichloroethane. Absorption spectra and orbital energies were calculated using time-dependent DFT (TD-DFT)56 with the same functional/basis set combination mentioned above. For easier comparison with the experiment, the obtained absorption and emission energies were converted into wavelengths and broadened by a Gaussian distribution (full width at half-maximum =3000 cm−1) using the program GaussSum.57 Molecular orbitals were visualized with the GaussView program.58 Experimental Details. cis-Bromo(4,4-difluoro-4-bora-3a,4adiaza-s-indacen-8-yl)-bis(triethylphosphine)-platinum(II) (cis-1). In a Young tube, cis-Pt(Et)2(PEt3)2 (140 mg, 286 μmol) was dissolved in 0.6 mL of C6D6. The colorless solution was frozen, and the Young tube was evacuated. The mixture was heated to 113 °C for 1 h resulting in a light brown solution. After cooling to room temperature, 8-bromo-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (85 mg, 313 μmol) was added, whereupon a dark orange solid precipitated. The suspension was kept at room temperature for 15 min. The orange solid was filtered off and washed with benzene (2 × 3 mL), giving the analytically pure product. Yield: 62%. 1H NMR (399.8 MHz, CDCl3, C


Article

Inorganic Chemistry Scheme 1. Synthesis of the BODIPY Complexes

also used for the combustion analysis. Anal. Calcd for C45H36BBrF2N2P2Pt·CD2Cl2: C, 51.28; H, 3.74; N 2.60. Found: C, 51.58; H, 3.97; N, 2.83%.

6H, P1-ortho-CH), 7.38−7.32 (m, 11H, P1-para-CH, P2-ortho-CH and H3/H7), 7.30 (t, 3JHH = 7.51 Hz, 3H, P2-para-CH), 7.23 (br vt, 3 JHH = 7.72 Hz, 6H, P1-meta-CH), 7.10 (br dvt, 4JPH = 2.18 Hz, 3JHH = 7.60 Hz, 6H, P2-meta-CH), 6.39 (dd, 3JHH = 3.92 Hz, 3JHH = 1.99 Hz, 2H, H4/H8). 31P NMR (161.8 MHz, CD2Cl2, 303 K): δ 14.94 (d, 2JPP = 18.01 Hz, with satellites 1JPtP = 1874 Hz, P1), 14.45 (d, 2JPP = 18.01 Hz, with satellites 1JPtP = 3982 Hz, P2). 13C NMR (151.0 MHz, CD2Cl2, 298 K): δ 188.6 (dd, 2JPC,cis = 114.3 Hz, 2JPC,trans = 6.8 Hz, with satellites 1JPtC = 755 Hz, C1), 142.2 (s, C2/C6), 137.8 (s, C3/ C7), 135.6 (d, 2JPC = 10.1 Hz, P1-ortho-C), 134.4 (br d, 2JPC = 8.3 Hz, P2-ortho-C), 134.0 (s, with satellites 4JPtC = 27.8 Hz, C5/C9), 131.8 (d, 5JPC = 2.0 Hz, P2-para-C), 131.6 (s, P2−C), 131.2 (s, P1−C), 130.9 (d, 5JPC = 2.2 Hz, P1-para-C), 128.5 (d, 3JPC = 10.13 Hz, P1meta-C), 128.30 (d, 3JPC = 11.4 Hz, P2-meta-C), 116.3 (s, C4/C8). 195 Pt NMR (85.56 MHz, CD2Cl2, 298 K): δ −4552 (dd, 1JPtP = 1874 Hz, 1JPtP = 3982 Hz). Crystals suitable for X-ray crystallography were obtained by direct crystallization of cis-2 from the reaction mixture. Anal. Calcd for C45H36BBrF2N2P2Pt: C, 54.57; H, 3.66; N 2.83. Found: C, 54.05; H, 4.05; N, 2.98%. trans-Bromo(4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-8-yl)bis(triphenylphosphine)-platinum(II) (trans-2). A 25 mL Schlenk tube was charged with cis-Pt(BODIPY)Br(PPh3)2 (cis-2) (60 mg, 60.5 μmol), AgBF4 (12 mg, 61.6 μmol), and 2 mL of CH2Cl2. The mixture was heated to reflux for 5 min and stirred at room temperature for 2 h. Precipitated AgBr was filtered off, and the filtrate was added to a methanolic solution of NaBr (12.7 mg, 123.2 μmol in 3 mL of MeOH). The reaction mixture was stirred at ambient temperature for 45 min. The solvent was removed, and the crude product was purified by column chromatography (pentane/ethyl acetate = 4/1, silica gel). Further purification by washing the dried eluate with acetone (3 × 2.5 mL) provided 29 mg of the analytically pure product. Yield: 48%. 1H NMR (399.8 MHz, CD2Cl2, 298 K): δ 7.57−7.50 (m, 12H, P-orthoCH), 7.42−7.37 (m, 6H, P-para-CH), 7.35−7.28 (m, 12H, P-metaCH), 7.22 (d, 3JHH = 3.95 Hz, 2H, H3/H7), 7.20 (br s, 2H, H5/H9), 6.05 (dd, 3JHH = 3.95 Hz, 3JHH = 2.02 Hz, 2H, H4/H8). 31P NMR (161.8 MHz, CD2Cl2, 298 K): δ 20.28 (s, with satellites 1JPtP = 2767 Hz). 13C NMR (100.5 MHz, CD2Cl2, 298 K): δ 142.3 (s, C2/C6), 137.2 (s, C5/C9), 135.0 (vt, 2JPC = 4JPC = 6.0 Hz, P-ortho-C), 133.1 (s, with satellites 3JPtC = 41.8 Hz, C3/C7), 131.4 (s, P-para-C), 129.3 (t, 1 JPC = 29 Hz with satellites 2JPtC = 26.6 Hz, P−C), 128.5 (vt, 3JPC = 5 JPC = 5.4 Hz, P-meta-CH), 115.9 (s, C4/C8), the resonance signal of C1 was not detected. 195Pt NMR (86.6 MHz, CD2Cl2, 298 K): δ −4424 (t, 1JPtP = 2767 Hz). Crystals suitable for X-ray crystallography were obtained by slow evaporation of a CD2Cl2 solution of trans-2, and contained one molecule of CD2Cl2 per complex unit. They were

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RESULTS AND DISCUSSION Synthesis and NMR Characterization. Complexes cis-1 and cis-2 were synthesized in moderate to good yields via oxidative addition of Br-BODIPY to either cis-Pt(η2-C2H4)(PEt3)2, cis-Pt(η2-C2H4)(PPh3)2, or Pt(PPh3)4, respectively (Scheme 1). cis-Pt(η2-C2H4)(PEt3)2 was obtained by heating Pt(Et)2(PEt3)239 in C6D6 for 1 h at 113 °C according to a literature protocol.59 Br-BODIPY reacts readily with the ethylene complexes within 10−15 min at ambient temperature, whereas the oxidative addition of Br-BODIPY to Pt(PPh3)4 required much longer reaction times of 4 h at 80 °C. Furthermore, the BF2-fragment of the Br-BODIPY is sensitive toward the PPh3 released from Pt(PPh3)4, which is reflected in the much lower yield of 38% following the latter procedure, as compared to 84% when using cis-Pt(η2-C2H4)(PPh3)2 as the Pt(0) precursor complex. The trans-isomers trans-1 and trans-2 were obtained in good to moderate yields by bromide abstraction from the respective cis-isomers with either Ag+OTf− or Ag+BF4− and subsequent addition of a methanolic solution of NaBr with concomitant substitution of the weakly coordinating OTf− or BF4− ligand from the Pt center. 31 P NMR spectra of the cis-complexes show two doublets each with 2JPP coupling constants of ca. 18.5 Hz. Each doublet is flanked by 195Pt satellites with one small coupling constant 1 JPtP of ca. 1850 Hz for the phosphine ligand trans to the BODIPY and a larger one of ca. 3800 Hz for the phosphine ligand trans to the bromo ligand. These coupling constants are also observed in the 195Pt NMR spectra, where the resonance signal is accordingly split into a doublet of doublet. These results are in agreement with the crystallographically determined structure of cis-2 (vide inf ra), where the two Pt− P bond lengths differ appreciably from each other as a result of the strong trans-influence of the σ-bonded BODIPY ligand. The 31 P NMR spectra of the trans-complexes show sharp singlets flanked by 195Pt satellites with a coupling constant 1JPtP of 2767 Hz for trans-2 and 2480 Hz for trans-1. The corresponding D


Article

Inorganic Chemistry

P1−Pt1−P2

178.6(3) 178.6(3) 83.92(12) 178.54(14) 175.01(11) 176.02(11) 99.59(4) 177.37(5) 87.28(8) 88.16(8) 173.25(4) 91.36(4)

P2−Pt1−Br1 P1−Pt1−Br1

87.73(8) 88.41(8) 85.50(3) 88.69(4) 91.3(3) 90.5(3) 91.47(12) 89.11(13)

C1−Pt1−P2 C1−Pt1−P1

93.7(3) 92.9(3) 167.66(12) 90.91(13) 2.327(3) 2.324(3) 2.2456(12) 2.3206(13)

P2−Pt1 P1−Pt1 C1−Pt1

1.984(10) 1.964(11) 2.031(4) 1.992(5) cis-2 trans-2

molecule 1 molecule 2 compd

trans-1

Br1−Pt1

2.315(3) 2.320(3) 2.3476(11) 2.3113(13)

bond angles bond lengths

Table 1. Selected Bond Lengths [Å] and Angles [deg] of trans-1, cis-2, and trans-2 E

2.4973(12) 2.5118(12) 2.4829(5) 2.5134(5)

Pt NMR spectra show a triplet with the same 1JPtP coupling constants. Direct attachment of the BODIPY moiety to Pt was confirmed by the appearance of platinum satellites for atoms C1, C2/C6, and C3/C7 (the numbering corresponds to that of the single crystal X-ray structures, see Figure 1) in the 13C NMR spectra with coupling constants 1JPtC ranging from 755 to 995 Hz and 2JPt−C and 3JPt−C couplings of ca. 40 Hz. Reproductions of the spectra can be found as Figures S1− S16 in the Supporting Information (SI). Single Crystal X-ray Diffraction. Figure 1 displays the ORTEP diagrams of complexes cis-2, trans-2 and trans-1. Relevant bond lengths and bond angles can be taken from Table 1 while Table S1 of the SI summarizes the crystal and refinement data. Crystals of cis-2 and trans-2 suitable for single crystal X-ray diffraction were obtained by slow evaporation of a benzene or CD2Cl2 solution of the corresponding complex while crystals of trans-1 were obtained by layering a concentrated CH2Cl2 solution of trans-1 with n-pentane. Crystals of cis-2 and trans-2 contain half a molecule of benzene or one disordered molecule of CD2Cl2 (disorder over two positions, the positions of the chlorine atoms of the disordered CD2Cl2 molecule are almost identical whereas the methylene group is either oriented toward or away from the complex molecule) per complex unit (Figure 1). cis-2, trans-1, and trans-2 crystallize in the monoclinic space group P21/c, P21/a, or C2/c, respectively. The unit cell of trans-1 contains two independent molecules with different sets of bond lengths and bond angles (see Table 1). The crystal structures show the expected square-planar coordination of the Pt atom. While the bond angles at the coordination center of trans-2 are close to ideal, minor deviations in trans-1 arise from a slight bending of ca. 2° of the phosphine ligands toward the bromine atom. More substantial deviations from the ideal square-planar structure are, however, observed for cis-2. As a consequence of the steric demands of the cis-disposed PPh3 ligands, the angle P1−Pt−P2 is rather obtuse at 99.59(4)°. This in turn causes a compression of the angles involving the bromo ligand as the one with the smallest steric demand to 83.92(12)° (C1−Pt−Br1) and 85.50(3)° (P1−Pt1−Br1). The remaining angle C1−Pt−P2 of 91.47(12)° adopts an almost ideal value. For all structures, the BODIPY ligand shows a slight folding of 3.89° or 5.32° for the two independent molecules of trans-1 or of 7.05° or 13.34° for trans-2 and cis-2, respectively, along the C1−B1 vector. The best plane through the atoms of that ligand is oriented almost perpendicular with respect to the coordination plane at the platinum atom (interplanar angles range from 78.7° to 88.4°). A slight bending of the BF2 fragment by ca. 10° out of the plane of the pyrrole rings is observed in all structures. As inferred from the Pt−Br and Pt−P bond lengths of the three structures, the σ-aryl bonded BODIPY ligand exerts a strong trans-influence. Thus, the Pt−Br bond lengths of the trans-complexes are by about 0.03 Å longer compared to that of cis-2. In addition, the Pt−P bond lengths in cis-2 differ appreciably from each other, such that the Pt−P1 bond opposite the σ-bonded BODIPY ligand is by 0.1 Å longer than the Pt−P2 bond trans to the bromo ligand (see Table 1). The most important bond parameter of these complexes is probably the Pt−C1 bond length. For the three available structures, that bond elongates in the order trans-1 < trans-2 < cis-2. These trends are similar to those reported by Sharp and co-workers for the cis- and trans-isomers of closely related complexes of dibenz[a,c]anthracene.60As we will see later in this paper, that

C1−Pt1−Br1

195


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Inorganic Chemistry bond length seems to deeply impact the efficiency of ISC and hence the emission properties of these complexes.31,32 In the solid state, complexes trans-2 and cis-2 display weak intramolecular π−π interactions that involve one or even three phenyl rings of the PPh3 ligands (see Figures S17d and S18d of the SI). In both cases, one phenyl ring is oriented parallel to the central plane of the BODIPY ligand formed by atoms C1, C2, C6, N1, N2 with a distance of 3.435 or 3.516 Å between these planes, respectively. cis-2 possesses a second intramolecular π−π interaction involving two parallel-displaced phenyl rings belonging to different PPh3 ligands with a stacking distance of 3.364 Å (see Figure S17d of the SI). Moreover, all crystallographically investigated complexes show various kinds of intermolecular association motifs. In all cases short intermolecular contacts due to hydrogen bonding between aryl (PPh3) or alkyl (PEt3) protons and the fluorine atoms of the BF2 group are found. For trans-2, individual molecules are oriented parallel to the crystallographic b axis and associate by two hydrogen bonds F1···H32 and F2···H24 of 2.403 and 2.601 Å to chains that likewise run along the b axis. The CD2Cl2 solvate molecules in the voids of the packing contribute to chain formation by making additional short contacts between atom F2 and one of the deuterium atoms of CD2Cl2 and between a Cl atom and H7 (see Figures S18a and S18c of the SI). These chains are interconnected along the a axis by CD2Cl2 molecules that reside either above or below the ab plane. For cis-2, short CH···F contacts between atoms F1 and H12 and between atoms F2 and H13 of 2.451 and 2.466 Å lead to the formation of zigzag chains that run parallel to the crystallographic c axis. These chains are further connected along the a axis by additional F···H contacts between atoms F2 and H25 of 2.390 Å and along the b axis via H···Br contacts of 3.022 and 2.991 Å to hydrogen atoms H3 and H30 (see Figures S17a−cof the SI). Similar CH···F contacts are also found in the structure of the PEt3 complex trans-1. Here, they likewise contribute to chainlike association of molecules A (dF1···H13c = 2.659 Å) and to the formation of a 2D-sheet structure by additional interactions between crystallographically independent molecules A and B via F···H contacts in the range 2.404−2.618 Å (see Figures S19a−c of the SI). Additional interactions involve atoms Br1 and H5 with a H···Br contact of 3.007 Å and C− H···π interactions between atoms H39A and C29 and C30 with distances of 2.800 and 2.813 Å, respectively, which both augment the aforementioned chains (see Figures S17a−c of the SI). Electronic Absorption Spectroscopy. Figure 2 depicts the electronic absorption spectra of Br-BODIPY and the four complexes in CH2Cl2 solutions at concentrations of ca. 10−5 M. As was already reported in the literature, meso-substituted BrBODIPY exhibits several medium intense transitions in the range 240−400 nm and one characteristic, prominent Vis band, which is located at 506 nm in CH2Cl2 (see Figure 2). While most of these bands, albeit shifted, are readily discerned in the UV−vis absorption spectra of the Pt complexes, those in the range 250−310 nm are hidden underneath more intense absorptions that are obviously due to transitions within the PtBr(PR3)2 entity or involve charge-transfer between the BODIPY and the PtBr(PR3)2 moieties (vide infra). Of note is a blue shift of the characteristic Vis band by 25 to 38 nm (1030 to 1605 cm−1) on platinum attachment along with some reduction in absorptivity. Still, molar extinction coefficients are

Figure 2. Electronic absorption spectra of Br-BODIPY, cis-1, cis-2, trans-1, and trans-2 in ca. 10−5 M CH2Cl2 solution.

high with values of 44 500 to 58 600 M−1 cm−1. In all cases that band maintains its characteristic sharpness and vibrational structuring with higher energy progressions of decreasing intensities. This strongly suggests that the prominent BODIPY based π → π* band is only moderately perturbed by the PtBr(PR3)2 fragment. In further keeping with the above assignment, the prominent Vis band displays a slight negative solvatochromism on changing the solvent from THF to CH2Cl2 or CH3CN of very similar magnitude as observed for BrBODIPY itself (see Figures S20−S23 of the SI).61 The above views are strongly supported by our DFT and TD-DFT calculations. Results are compiled together with the experimental data in Table 2, while Figure 3 provides a graphical account for trans-2 as a representative example. Equivalent compilations for the other complexes are presented as Figures S24−S36 of the SI. Coordinates of the optimized structures in their S0 and T1 states are provided in Tables S5− S12 of the SI while Table S13 collects the most important structure parameters. Thus, the immediate frontier MOs of the present complexes strongly resemble those of purely organic BODIPYs.61−64 The compositions of the crucial frontier orbitals as calculated by Mulliken analysis compiled in Table 3 show that the HOMO and LUMO of every complex are mainly based on the BODIPY ligand with only small Pt contributions. As for other BODIPYs, the HOMO has a nodal plane directed along the Pt1−C1−B1 axis, perpendicular to the plane of the BODIPY ligand. The weak interaction between the Pt coordination center and the BODIPY present in the HOMO involves a Pt dδ-orbital and can thus be described as a δ*-type interaction. The meso-position, however, contributes to the LUMO and engages in an antibonding fashion with a Pt dπ orbital. That interaction, together with the net electrondonating character of the PtBr(PR3)2 fragment, causes a preferential destabilization of the LUMO, which explains the blue shift of the HOMO → LUMO transitions of the complexes by ca. 1300 cm−1 compared to Br-BODIPY itself. The same reasoning also rationalizes the fact that the HOMO → LUMO transitions of the PEt3 complexes are slightly blueshifted with respect to those of their PPh3 counterparts by the stronger electron donation from the PEt3 ligand. Just like Br-BODIPY itself, the complexes possess two weak absorptions at ca. 360 and 310 nm with molar extinction coefficients of 5 000 and 10 000 M−1 cm−1, respectively, that envelope the entire spectral range between 300 and 400 nm. The fact that these bands are common to Br-BODIPY and all complexes and the independence of λmax from solvent polarity let us conclude that these two bands are also BODIPYcentered, higher-lying π → π* transitions. This is also borne out by our TD-DFT calculations, although some additional F


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Inorganic Chemistry

Table 2. Absorption Data of Br-BODIPY and Complexes cis-1, trans-1, cis-2, and trans-2 of ca. 10−6 M CH2Cl2 Solutions at 298 K and TD-DFT Calculations in DCEa TD-DFT data compd cis-1

absorption data λmax/nm (ε × 10−3/M−1 cm−1)

λ/nm

major contribution

fb

278 (27.8), 303 (19.2), 349 (5.7), 476 (58.6)

262

H − 9 → LUMO (13%) H − 6 → L + 1 (52%) H − 8 → LUMO (42%) H − 7 → LUMO (15%) H − 6 → LUMO (15%) H − 3 → L + 1 (13%) H − 4 → LUMO (58%) HOMO → L + 1 (38%) HOMO → LUMO (96%) H − 9 → LUMO (47%) H − 10 → LUMO (46%) H − 6 → LUMO (95%) H − 5 → LUMO (89%) H − 4 → LUMO (90%) HOMO → LUMO (96%) H − 7 → L + 1 (17%)

0.097

transition delocalized over entire molecule

0.19

(Et3P)2BrPt → BODIPY CT

0.078

π → π*/BODIPY → PtBr(PEt3)2 CT

0.39 0.056

π → π* (Et3P)2BrPt → BODIPY CT

0.13 0.096 0.045 0.38 0.12

π → π* (Et3P)2Pt → BODIPY CT π → π* π → π* BODIPY → PtBr(PPh3)cis CT/PtBr(PPh3)2centered transition

0.13

(Ph3P)2BrPt → BODIPY CT

0.046

π → π*/minor (PPh3)cisPt → BODIPY CT

0.34 0.45

π → π* PtBr(PPh3)2-centered transition

0.033 0.033 0.031 0.32

π→π*/phenyl → BODIPY CT π → π* (Ph3P)2BrPt → BODIPY CT π → π*

287

329

trans-1

cis-2

254 (7.5), 322 (11.7), 359 (5.9), 468 (52.6)

401 261

274 (41.1), 354 (7.3), 481 (52.9)

306 325 331 398 255

279

323

trans-2

Br-BODIPY a

253 (31.2), 320 (9.6), 355 (6.1), 477 (44.5)

249 (14.4), 277 (10.1), 320 (11.5), 366 (4.9), 389 (3.5), 506 (64.7)

403 276 320 333 339 402 n.c.

H − 6 → L + 1 (48%) H − 13 → LUMO (12%) H − 11 → LUMO (20%) H − 10 → LUMO (19%) H − 4 → LUMO (38%) H − 3 → LUMO (47%) HOMO → LUMO (96%) H − 4 → L + 1 (73%) H − 2 → L + 1 (15%) H − 6 → LUMO (94%) H − 5 → LUMO (96%) H − 4 → LUMO (91%) HOMO → LUMO (97%)

assignment

n.c. = not calculated. bOscillator strength.

with clearly detectable vibrational progressions similar to those of Br-BODIPY itself. As it is documented in panels a, b, and e of Figure 5, the low energy emission is strongly quenched on exposure to oxygen, whereas the one at higher energy is essentially unaffected under these conditions. Excitation spectra of the complexes recorded at the maximum of either the high or low energy emissions coincide with the electronic absorption spectra, ruling out the possibility that any of the emissions originate from impurities (see Figure 6 and Figures S27−S29 of the SI). The high energy emissions of the PPh3 complexes and of cis1 decay within approximately 0.6 ns, while the high energy emission of trans-1 is too low in intensity and decays too rapidly to allow for a precise determination of the luminescence lifetime with our equipment, which has a detection limit of 200 ps (Table 4; for the decay of the individual emissions of the complexes with time see Figures S30−S34 of the SI). On the basis of the insensitivity against oxygen, the short lifetimes, and the identical appearance to the fluorescence emission of the BrBODIPY precursor itself including the vibrational progressions, we assign the high energy emission band of each complex as fluorescence from the BODIPY-centered 1ππ* state. We also note that the blue shift of the absorption bands of the complexes with respect to Br-BODIPY also translates into a

bands involving CT from either the PtBr(PR3)2 fragment to BODIPY or vice versa are computed in that part of the spectrum (see Figure 3 and Figures S24−S26 of the SI). The spectral region between 240 and 300 nm features one intense band at ca. 280 nm for the cis complexes, which is shifted to the edge of the spectral window at around 250 nm in their transisomers. While this band shows no negative solvatochromism for trans-1, trans-2, and cis-2, that of cis-1 is blue-shifted from 287 nm in CH2Cl2 or THF to outside the detectable region in CH3CN. Our TD-DFT calculations predict two transitions of the complexes in that region: one CT band from either the PtBr(R3P)2 fragment to BODIPY or vice versa and one transition which involves MOs that spread over the entire molecule. Therefore, we assign the band of cis-1 at 287 nm as a PtBr(PEt3)2 → BODIPY CT band and those of trans-1, trans2, and cis-2 as transitions that involve strongly delocalized MOs. Luminescence Spectroscopy. Figure 4 displays the luminescence spectra of cis-1, trans-1, cis-2, and trans-2 in nitrogen saturated CH2Cl2 solutions and of Br-BODIPY in aerated CH2Cl2 solution at concentrations of ca. 10−6 M. The most remarkable result is that the trans-configured complexes trans-1 and trans-2 and, very weakly, also cis-1 show dual emissions with bands centered at ca. 485 and 640 nm, both G


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Inorganic Chemistry

Figure 3. Energies and graphical representations of the relevant molecular orbitals along with calculated electronic transitions of trans-2.

Table 3. Calculated Mulliken Parameters of cis-1, trans-1, cis-2, and trans-2a PR3

BODIPY

Pt

Br

compd

HOMO

LUMO

spin densityb

HOMO

LUMO

spin densityb

HOMO

LUMO

spin densityb

HOMO

LUMO

spin densityb

cis-1 trans-1 cis-2 trans-2

1 1 1 1

1 2 3 3

0.002 0.011 0.011 0.025

98 98 98 98

94 94 93 94

1.989 1.979 1.982 1.974

1 1 1 1

3 4 2 3

−0.005 0.010 −0.008 0.006

0 0 1 0

2 0 2 0

0.014 −0.005 0.015 −0.005

a

Percent contributions of the fragments PR3 (R = Ph or Et), BODIPY, Pt, and Br to the respective HOMO, LUMO, and spin densities. bSpin density contribution of the respective fragment to the spin density surface.

much more intense and, in the absence of oxygen, decay with a lifetime of 162 μs for trans-1 and 268 μs for trans-2. Due to the long lifetimes in concert with the entire appearance and the vibrational progressions as well as the large Stokes shifts of ca. 5670 cm−1, the low energy bands are assigned as phosphorescence from a BODIPY-centered 3ππ* state. Although small Stokes shifts are observed for the fluorescence of BODIPYs due to the small structural deformation in the excited singlet state, the Stokes shifts for BODIPY-centered phosphorescence are usually in the range of 4000 to 6000 cm−1,10,17,18,24,29,33 very similar to what is found here. This assignment also agrees with the calculated spin density surfaces of the T1 state illustrated in Figure 7. Our calculations place almost the entire spin density on the BODIPY moiety with only minor Pt dπ contributions (Table 3). Such a spin density distribution is the ideal scenario for dual emission, because such moderate Pt contributions render the ISC rate constants similar to those for fluorescence emission from the complexes.32,36,65 In further keeping with a 3 ππ* excited state the calculated bond parameters around the Pt center are almost identical for the T1 state and the S0 ground state (see Table S13 of the SI). ISC is in general much more efficient in the trans complexes as can be concluded from the lower fluorescence and the

Figure 4. Luminescence spectra of cis-1, trans-1, cis-2, and trans-2 in N2 saturated CH2Cl2 and of Br-BODIPY in aerated CH2Cl2 solutions upon irradiation into their lowest energy absorption band at concentrations of ca. 10−6 M.

blue shift of the fluorescence emission, albeit in the usual, somewhat attenuated fashion (980 to 1535 cm−1 as compared to 1.030 cm−1 to 1605 cm−1).61 As it was already mentioned, the intensity of the low energy emission band of the cis complexes is very low, which precludes an accurate determination of their emission lifetimes. The low energy emission bands of the trans complexes are, however, H


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Inorganic Chemistry

Figure 7. Spin density surfaces of the T1 state of cis-1, trans-1, cis-2, and trans-2.

significantly higher phosphorescence quantum yields. Thus, cis1 and cis-2 show almost exclusively fluorescence with QYs of 26% and 19.5%, respectively. Phosphorescence QYs for these complexes are only 0.9% or ca. 0.2%. For their trans-isomers the situation is basically reversed. Thus, the fluorescence QY (ϕFl) of trans-2 is reduced to 7.2%, whereas the phosphorescence QY (ϕPh) reaches a value 14.2%. The figures of trans-1 are even more impressive with ϕFl = 1.1% and ϕPh = 31.2%. In this case ISC from the S1 to the T1 state is so efficient that the fluorescence lifetime is decreased to below 200 ps. The quantum yield for room-temperature phosphorescence of trans-1 is by almost 1 order of magnitude larger than the best reported value for a BODIPY dye that we are presently aware of.33 Taken together with our single crystal X-ray analyses our results seem to indicate that a shortening of the Pt−C1 bond aids in rendering ISC more efficient.31 For pure ligand-centered ππ* excited states, the metal ion essentially acts as an external heavy atom. In such a case, the ISC rate scales with d−6, where d is the distance between the Pt atom and the center of the emitting chromophore.66 That hypothesis and the reversal of the fluorescence and phosphorescence QYs from the cis- to the trans-isomers are also supported by our TD-DFT calculations. The latter predict that the Pt contribution to the spin densities in the T1 state of the complexes, while generally small (Table 3 and Figure 7), is larger in the trans complexes than for their cis isomers. The better performance of the PEt3 complexes compared to their PPh3 counterparts (compare the fluorescence QY of cis-1 versus that of cis-2 and the phosphorescence QY of trans-1 to that of trans-2) can most probably be traced to the larger ligand-field splitting induced by the PEt3 ligand, which reduces the detrimental influence of other, nonemissive deactivation channels via thermal population of higher lying,

Figure 5. (a) Emission spectra of trans-1 in CH2Cl2 at different O2 concentrations (irradiation at 404 nm). (b) Emission spectra of Pt−Cl in CH2Cl2 at different O2 concentrations (irradiation at 400 nm). (c) Stern−Volmer plot of I0,Ph/IPh for trans-1 at 637 nm. (d) Stern− Volmer plot of I0,Ph/IPh of Pt−Cl at 520 nm. (e) Phosphorescence decay traces of trans-1 in CH2Cl2 at different O2 concentrations (irradiation at 404 nm). (f) Stern−Volmer plot of (e) τ0,Ph/τPh (τ0,Ph = phosphorescence lifetime in nitrogen saturated solution; τPh = phosphorescence lifetime at different O2 concentrations) for trans-1.

Figure 6. Excitation spectra detected at λem = 490 nm (orange) or at λem = 637 nm (blue) and electronic absorption spectrum (red) of trans-1 in N2 saturated CH2Cl2 solution.

Table 4. Luminescence Data of cis-1, trans-1, cis-2, and trans-2 in Nitrogen Saturated CH2Cl2 and of Br-BODIPY in Aerated CH2Cl2 Solutions at Concentrations of ca. 10−6 Ma λmax,Fl/nm (Stokes shift/cm−1) Br-BODIPY cis-1 trans-1 cis-2 trans-2 a

517(421) 487(474) 479(491) 492(465) 490(556)

λmax,Ph/nm (Stokes shift/cm−1) 652(5671) 637(5669) 660(5638) 657(5743)

ϕFl 0.496 0.260 0.011 0.195 0.072

ϕPh

τFl/ns

τPh/μs

0.009 0.312 ∼0.0017 0.142

4.63 0.586 n.d. 0.686 0.700

162

b

268

n.d. = not detectable. bValue is taken from ref 38 in THF solution. I


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Inorganic Chemistry

concentration of ca. 7.5%, whereas that of trans-1 still shows weak phosphorescence in fully aerated solutions (21% of O2) at room temperature. Concomitantly, the phosphorescence lifetime of trans-1 rapidly decreases by increasing the O2 concentration (Figure 5e). Figure 5c,d depicts the corresponding I0,Ph/IPh ratios as a function of the partial pressure of oxygen (p(O2)). They are described by the Stern−Volmer equation (I0/I = 1 + KSV[O2], where I0 is the luminescence intensity under exclusion of oxygen, I is the luminescence intensity at a specific oxygen concentration, and KSV is the Stern−Volmer quenching constant). The Stern−Volmer plots show satisfactory linearity with KSV of 121 ± 13 bar−1 for Pt−Cl and 293 ± 40 bar−1 for trans-1. Due to the much longer phosphorescence lifetime of trans-1, its phosphorescence is more readily quenched by oxygen, which is reflected in the higher quenching constant. On the other hand, its superior phosphorescence QY allows for O2 detection up to aerobic oxygen levels. Although the accuracies of both quenching constants suffer from rather large standard deviations rooted in the somewhat crude method used for adjusting the O2 concentrations (see Experimental Section) and remaining trace amounts of oxygen in the degassed solutions, the magnitudes of these constants in combination with the corresponding lifetimes rank the present complexes among the best published Pt(II) compounds used for ratiometric oxygen sensing.65,85,86 Additionally, when plotting the τ0,Ph/τPh-ratio (τ0,Ph = phosphorescence lifetime under nitrogen, τPh = phosphorescence lifetime at different amounts of oxygen) of trans-1 against p(O2) (Figure 5f), a similar quenching constant of 445 ± 45 bar−1 is obtained, confirming the reliability of the obtained results.

nonemissive metal-centered excited states.32,67−70 In addition, emissive relaxation to the ground state is further favored by the shorter fluorescence and phosphorescence lifetime of the PEt3 complexes, which makes emission even more competitive to other, nonradiative deactivation channels. Moreover, ISC from the S1 to the T1 and from the T1 to the S0 states is more efficient for trans-1 than for its PPh3 counterpart trans-2 as is reflected in the lower value of ϕFl as well as the shorter phosphorescence lifetime of 162 μs compared to 268 μs. We note, however, that emission quantum yields and lifetimes are subject to a delicate balance of many factors, including ISC from the S1 to the T1 state, ISC from the T1 to the S0 state, the rate of nonradiative decay by internal conversion, the interference of thermally accessible 3MC states, and even intermolecular interactions.32,71−82 A recent study of tris(imine) and cyclometalated arylbis(imine) platinum alkynyl complexes has masterfully demonstrated that even seemingly slight modifications can influence the luminescence properties of such platinum complexes in a rather profound and a priori not fully foreseeable manner as they simultaneously affect several of the contributing and interrelated factors.83 The effects brought about by changes of coordination geometry (here cis or trans) or of the substitution of PPh3 by PEt3 ligands seen here are just another manifestation of such intricacies. We have recently published the Pt complex trans-PtCl(PEt3)2(Tx) (Pt−Cl, Tx = thioxanthon-2-yl, Figure 8), which

■

CONCLUSION We report here on four platinum complexes where a BODIPY ligand is linked via its meso-position to a cis-/trans-PtBr(PR3)2 (R = Et, Ph) fragment. Three of these complexes have been characterized by X-ray crystallography. The most important finding of this study is the observation of dual fluorescence and phosphorescence emissions from excited BODIPY 1ππ* and 3 ππ* states, whose relative intensities are reverted between the cis- and trans-isomers. Thus, for the PEt3 substituted complex trans-1 the quantum yield (QY) of fluorescence emission is decreased to only 1.1% while that of the phosphorescence reaches an impressive value of 31.2% at room temperature in fluid solution. Owing to the large phosphorescence QY and the comparatively small phosphorescence lifetime of 162 μs, trans1 acts as an efficient sensor for the ratiometric detection of oxygen up to the atmospheric level with a high quenching constant of ca. 350 bar−1. We note here that the modular assembly of the complexes cis-/trans-Pt(BODIPY)Br(PR3)2 from a brominated BODIPY (here 4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-8-yl) and a Pt(PR3)2 fragment (introduced best via an ethylene or a bis(ethyl) precursor), along with the ease of further modification by a sequence of bromide abstraction from the kinetic cis-isomer and subsequent (re)addition of Na+ X− to give the trans-isomers trans-Pt(BODIPY)(PR3)2X, offer ample opportunities for their further modification and improvement. Besides variations of the BODIPY dye and of the anionic ligand X, the PR3 ligands also seem to exert a profound influence on the emission properties with appreciably faster ICS rates, lower emission lifetimes, and higher phosphorescence quantum yields for the PEt3 complex. The results described herein provide

Figure 8. Molecular structure of trans-PtCl(PEt3)2(Tx) (Pt−Cl, Tx = thioxanthon-2-yl).

displayed similar behavior to that of complexes trans-1 and trans-2, i.e., thioxanthonyl based dual emission through fluorescence and phosphorescence with a phosphorescence QY and lifetimes of 19% and 2.2 μs.36 It thus appears that the trans-Pt(PEt3)2X (X = Br or Cl) fragment is particularly wellsuited for inducing dye based dual emissions originating from the dye’s 1ππ* and 3ππ* states. Owing to their rather intense phosphorescence and still moderate fluorescence emissions, Pt−Cl and trans-1 are promising candidates for the ratiometric detection of triplet molecules including oxygen, and for 1O2 generation. Dually emissive dyes are highly advantageous for such applications, because they obviate the need to account for the different bleaching rates of the dyes used for oxygen sensing (the phosphorescent dye) or as the fluorescent standard.65,84 Complexes trans-1 and Pt−Cl are particularly promising candidates for such purpose, because their long phosphorescence lifetimes guarantee a sensitive bleach of the phosphorescence band upon small changes of O2 concentration levels, and because their well-separated emissions allow for a precise determination of the fluorescence and phosphorescence intensities. Therefore, we investigated the applicability of trans1 and Pt−Cl toward the quantitative ratiometric 3O2 detection. Figure 5a,b depicts the normalized emission spectra of trans-1 and Pt−Cl at different oxygen concentrations. While the fluorescence of both complexes remains unaffected by increasing O2 concentration levels, the phosphorescence is successively quenched. Due to the lower phosphorescence QY of Pt−Cl, its phosphorescence is completely quenched at an O2 J


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Inorganic Chemistry

(11) Costela, A.; García-Moreno, I.; Barroso, J.; Sastre, R. Appl. Phys. B: Lasers Opt. 2000, 70, 367−373. (12) Shah, M.; Thangaraj, K.; Soong, M.-L.; Wolford, L. T.; Boyer, J. H.; Politzer, I. R.; Pavlopoulos, T. G. Heteroat. Chem. 1990, 1, 389− 399. (13) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K. Chem. Soc. Rev. 2013, 42, 77−88. (14) Awuah, S. G.; You, Y. RSC Adv. 2012, 2, 11169−11183. (15) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891−4932. (16) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184−1201. (17) Galletta, M.; Campagna, S.; Quesada, M.; Ulrich, G.; Ziessel, R. Chem. Commun. 2005, 4222−4224. (18) Rachford, A. A.; Ziessel, R.; Bura, T.; Retailleau, P.; Castellano, F. N. Inorg. Chem. 2010, 49, 3730−3736. (19) Lu, H.; Mack, J.; Yang, Y.; Shen, Z. Chem. Soc. Rev. 2014, 43, 4778−4823. (20) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. J. Am. Chem. Soc. 2005, 127, 12162−12163. (21) Zhang, C.; Zhao, J.; Wu, S.; Wang, Z.; Wu, W.; Ma, J.; Guo, S.; Huang, L. J. Am. Chem. Soc. 2013, 135, 10566−10578. (22) Li, W.; Li, L.; Xiao, H.; Qi, R.; Huang, Y.; Xie, Z.; Jing, X.; Zhang, H. RSC Adv. 2013, 3, 13417−13421. (23) McDonnell, S. O.; Hall, M. J.; Allen, L. T.; Byrne, A.; Gallagher, W. M.; O'Shea, D. F. J. Am. Chem. Soc. 2005, 127, 16360−16361. (24) Zhang, X.-F.; Yang, X.; Niu, K.; Geng, H. J. Photochem. Photobiol., A 2014, 285, 16−20. (25) Nastasi, F.; Puntoriero, F.; Campagna, S.; Diring, S.; Ziessel, R. Phys. Chem. Chem. Phys. 2008, 10, 3982−3986. (26) Sun, J.; Zhong, F.; Yi, X.; Zhao, J. Inorg. Chem. 2013, 52, 6299− 6310. (27) Wu, W.; Sun, J.; Cui, X.; Zhao, J. J. Mater. Chem. C 2013, 1, 4577−4589. (28) Nastasi, F.; Puntoriero, F.; Campagna, S.; Olivier, J.-H.; Ziessel, R. Phys. Chem. Chem. Phys. 2010, 12, 7392−7402. (29) Galletta, M.; Puntoriero, F.; Campagna, S.; Chiorboli, C.; Quesada, M.; Goeb, S.; Ziessel, R. J. Phys. Chem. A 2006, 110, 4348− 4358. (30) Lazarides, T.; McCormick, T. M.; Wilson, K. C.; Lee, S.; McCamant, D. W.; Eisenberg, R. J. Am. Chem. Soc. 2011, 133, 350− 364. (31) Chen, Y.-L.; Li, S.-W.; Chi, Y.; Cheng, Y.-M.; Pu, S.-C.; Yeh, Y.S.; Chou, P.-T. ChemPhysChem 2005, 6, 2012−2017. (32) Chou, P.-T.; Chi, Y.; Chung, M.-W.; Lin, C.-C. Coord. Chem. Rev. 2011, 255, 2653−2665. (33) Wu, W.; Zhao, J.; Guo, H.; Sun, J.; Ji, S.; Wang, Z. Chem. - Eur. J. 2012, 18, 1961−1968. (34) Lee, H. B.; Sharp, P. R. Organometallics 2005, 24, 4875−4877. (35) Wang, B.-Y.; Karikachery, A. R.; Li, Y.; Singh, A.; Lee, H. B.; Sun, W.; Sharp, P. R. J. Am. Chem. Soc. 2009, 131, 3150−3151. (36) Geist, F.; Jackel, A.; Winter, R. F. Dalton Trans. 2015, 44, 3974− 3987. (37) Heng, W. Y.; Hu, J.; Yip, J. H. K. Organometallics 2007, 26, 6760−6768. (38) Leen, V.; Yuan, P.; Wang, L.; Boens, N.; Dehaen, W. Org. Lett. 2012, 14, 6150−6153. (39) McCarthy, T. J.; Nuzzo, R. G.; Whitesides, G. M. J. Am. Chem. Soc. 1981, 103, 3396−3403. (40) Nagel, U. Chem. Ber. 1982, 115, 1998−1999. (41) Herrendorf, W.; Bärnighausen, W. HABITUS. Program for the Optimization of the Crystal Shape for Numerical Absorption Correction in X-SHAPE, Version 1.06; Fa. Stoe, Darmstadt: Karlsruhe, Gießen, 1999. (42) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (43) Farrugia, L. J. Appl. Crystallogr. 2012, 45, 849−854. (44) Frisch, M. J.; Trucks, G.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Blonio, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;

useful guidelines for the further optimization of the emissive properties of such complexes starting from this initial hit, even including the perspective of obtaining efficient phosphorescent NIR emitters.33

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ASSOCIATED CONTENT

S Supporting Information *

Excitation spectra of the four BODIPY complexes as well as 1H, 13 C, 31P, and 195Pt NMR spectra, fluorescence and phosphorescence decay traces of the BODIPY complexes and the results of (TD-)DFT calculations with representations of the MOs involved in the transitions as well as atom positioning and important structure parameters of the geometry-optimized structures of the S0 and T1 states of the complexes. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01969. Excitation, NMR, and fluorescence spectra; phosphorescence decay traces, calculational results, and crystallographic data (PDF) Cystallographic data in CIF format of cis-2 (CIF) Cystallographic data in CIF format of trans-2 (CIF) Cystallographic data in CIF format of trans-1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG, Grant Wi12612/11-1). We also thank the state of Baden-Württemberg and the Deutsche Forschungsgemeinschaft for providing us with the computational facilities of the bwUni Cluster, Karlsruhe.

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REFERENCES

(1) Ziessel, R.; Ulrich, G.; Haefele, A.; Harriman, A. J. Am. Chem. Soc. 2013, 135, 11330−11344. (2) Lazarides, T.; McCormick, T. M.; Wilson, K. C.; Lee, S.; McCamant, D. W.; Eisenberg, R. J. Am. Chem. Soc. 2011, 133, 350− 364. (3) Whited, M. T.; Djurovich, P. I.; Roberts, S. T.; Durrell, A. C.; Schlenker, C. W.; Bradforth, S. E.; Thompson, M. E. J. Am. Chem. Soc. 2011, 133, 88−96. (4) Sabatini, R. P.; Zheng, B.; Fu, W.-F.; Mark, D. J.; Mark, M. F.; Hillenbrand, E. A.; Eisenberg, R.; McCamant, D. W. J. Phys. Chem. A 2014, 118, 10663−10672. (5) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H.; Singh, D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1998, 120, 10001−10017. (6) Lammi, R. K.; Ambroise, A.; Balasubramanian, T.; Wagner, R. W.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 2000, 122, 7579−7591. (7) Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.; Roncali, J. Chem. Commun. 2009, 1673−1675. (8) Sabatini, R. P.; McCormick, T. M.; Lazarides, T.; Wilson, K. C.; Eisenberg, R.; McCamant, D. W. J. Phys. Chem. Lett. 2011, 2, 223− 227. (9) Huang, L.; Zhao, J.; Guo, S.; Zhang, C.; Ma, J. J. Org. Chem. 2013, 78, 5627−5637. (10) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. Chem. Soc. Rev. 2015, DOI: 10.1039/C5CS00364D. K


Article

Inorganic Chemistry Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, J. A. M., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.01, Gaussian Inc.: Pittsburgh, PA, 2009. (45) Gunnarsson, O.; Lundqvist, B. I. Phys. Rev. B 1976, 13, 4274− 4298. (46) Küchle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535−7532. (47) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730− 1734. (48) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123−141. (49) Hariharan, P. H.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213− 222. (50) Stephens, P. J.; Devlin, F. J.; Cabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627. (51) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (52) Cancés, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032−3041. (53) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151−5158. (54) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669−681. (55) Scalmani, G.; Frisch, M. J. J. Chem. Phys. 2010, 132, 114110− 114124. (56) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997−1000. (57) O'Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. J. Comput. Chem. 2008, 29, 839−845. (58) Keith, T.; Millam, J. GaussView, Version 3; Semichem Inc.: Shawnee Mission, KS, 2009. (59) Nuzzo, R. G.; McCarthy, T. J.; Whitesides, G. M. Inorg. Chem. 1981, 20, 1312−1314. (60) Singh, A.; Sharp, P. R. J. Am. Chem. Soc. 2006, 128, 5998−5999. (61) Boens, N.; Wang, L.; Leen, V.; Yuan, P.; Verbelen, B.; Dehaen, W.; Van der Auweraer, M.; De Borggraeve, W. D.; Van Meervelt, L.; Jacobs, J.; Beljonne, D.; Tonnelé, C.; Lazzaroni, R.; Ruedas-Rama, M. J.; Orte, A.; Crovetto, L.; Talavera, E. M.; Alvarez-Pez, J. M. J. Phys. Chem. A 2014, 118, 1576−1594. (62) Gomez-Duran, C. F. A.; Garcia-Moreno, I.; Costela, A.; Martin, V.; Sastre, R.; Banuelos, J.; Lopez Arbeloa, F.; Lopez Arbeloa, I.; PenaCabrera, E. Chem. Commun. 2010, 46, 5103−5105. (63) Bañuelos, J.; Martín, V.; Gómez-Durán, C. F. A.; Córdoba, I. J. A.; Peña-Cabrera, E.; García-Moreno, I.; Costela, Á .; Pérez-Ojeda, M. E.; Arbeloa, T.; Arbeloa, I.́ L. Chem. - Eur. J. 2011, 17, 7261−7270. (64) Osorio-Martínez, C. A.; Urías-Benavides, A.; Gómez-Durán, C. F. A.; Bañuelos, J.; Esnal, I.; López Arbeloa, I.; Peña-Cabrera, E. J. Org. Chem. 2012, 77, 5434−5438. (65) Liu, Y.; Guo, H.; Zhao, J. Chem. Commun. 2011, 47, 11471− 11473. (66) Chi, Y.; Chou, P.-T. Chem. Soc. Rev. 2007, 36, 1421−1431. (67) Tolman, C. A. Chem. Rev. 1977, 77, 313−348. (68) Bennett, M. A.; Clark, R. J. H.; Goodwin, A. D. J. Inorg. Chem. 1967, 6, 1625−1631. (69) Que, L.; Pignolet, L. H. Inorg. Chem. 1973, 12, 156−163. (70) Winkel, R. W.; Dubinina, G. G.; Abboud, K. A.; Schanze, K. S. Dalton Trans. 2014, 43, 17712−17720. (71) Freeman, G.; Williams, J. A. G. Metal Complexes of Pincer Ligands: Excited States, Photochemistry, and Luminescence. In Organometallic Pincer Chemistry; van Koten, G., Milstein, D., Eds.; Springer: Berlin, 2013; Vol. 40, pp 89−129. (72) Yam, V. W.-W.; Wong, K. M.-C. Chem. Commun. 2011, 47, 11579−11592.

(73) Sun, Q.; Mosquera-Vazquez, S.; Suffren, Y.; Hankache, J.; Amstutz, N.; Lawson Daku, L. M.; Vauthey, E.; Hauser, A. Coord. Chem. Rev. 2015, 282−283, 87−99. (74) Aliprandi, A.; Genovese, D.; Mauro, M.; De Cola, L. Chem. Lett. 2015, 44, 1152−1169. (75) Chia, Y. Y.; Tay, M. G. Dalton Trans. 2014, 43, 13159−13168. (76) Rausch, A.; Homeier, H. H.; Yersin, H. Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the Role of Spin−Orbit Coupling: A Study Based on High-Resolution Optical Spectroscopy. In Photophysics of Organometallics; Lees, A. J., Ed.; Springer: Berlin, 2010; Vol. 29, pp 193−235. (77) Rausch, A. F.; Murphy, L.; Williams, J. A. G.; Yersin, H. Inorg. Chem. 2012, 51, 312−319. (78) Bossi, A.; Rausch, A. F.; Leitl, M. J.; Czerwieniec, R.; Whited, M. T.; Djurovich, P. I.; Yersin, H.; Thompson, M. E. Inorg. Chem. 2013, 52, 12403−12415. (79) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. Coord. Chem. Rev. 2011, 255, 2622−2652. (80) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205−268. (81) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.; Rajapakse, N. Coord. Chem. Rev. 2006, 250, 1819−1828. (82) Balzani, V.; Bergamini, G.; Campagna, S.; Puntoriero, F. Photochemistry and Photophysics of Coordination Compounds: Overview and General Concepts. In Photochemistry and Photophysics of Coordination Compounds I, Balzani, V.; Campagna, S., Eds.; Springer: Berlin, 2007; Vol. 280, pp 1−36. (83) Lam, W. H.; Lam, E. S.-H.; Yam, V. W.-W. J. Am. Chem. Soc. 2013, 135, 15135−15143. (84) Kostov, Y.; Rao, G. Sens. Actuators, B 2003, 90, 139−142. (85) Van Houten, K. A.; Heath, D. C.; Barringer, C. A.; Rheingold, A. L.; Pilato, R. S. Inorg. Chem. 1998, 37, 4647−4653. (86) Xiang, H.; Zhou, L.; Feng, Y.; Cheng, J.; Wu, D.; Zhou, X. Inorg. Chem. 2012, 51, 5208−5212.

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Ligand Based Dual Fluorescence and Phosphorescence Emission

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