Dual Room-Temperature Fluorescent and Phosphorescent Emission in8-Quinolinolate Osmium(II) Carbonyl Complexes: Rationalization andGeneralization of Intersystem Crossing Dynamics
Yi-Ming Cheng, Yu-Shan Yeh, Mei-Lin Ho, and Pi-Tai Chou*
Department of Chemistry, National Taiwan UniVersity, Taipei 10764, Taiwan
Po-Shen Chen and Yun Chi*
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan
Received April 7, 2005
A new series of quinolinolate osmium carbonyl complexes were synthesized and characterized by spectroscopicmethods. Single-crystal X-ray diffraction studies indicate that these complexes consist of an octahedral ligandarrangement with one chelating quinolinolate, one tfa or halide ligand, and three mutually orthogonal terminal COligands. Variation of the substituents on quinolinolate ligands imposes obvious electronic or structural effects, whilechanging the tfa ligand to an electron-donating iodide slightly increases the charge density on the central osmiumatom. These Os(II) complexes show salient dual emissions consisting of fluorescence and phosphorescence, thespectral properties and relaxation dynamics of which have been studied comprehensively. The results, in combinationwith the theoretical approaches, lead us to propose that the emission mainly originates from the quinolinolate ππ*state. Both experimental and theoretical approaches generalize various types of intersystem crossing versus thoseof the tris(quinolinolate) iridium Ir(Q)3, and their relative efficiencies were accessed on the basis of the associatedfrontier orbital configurations. Our results suggest that ⟨1dππ*|Hso|3ππ*⟩ (or ⟨3dππ*|Hso|1ππ*⟩) in combination witha smaller ∆ES1-T1 gap (i.e., increasing the MLCT (dππ*) character) is the main driving force to induce the ultrafastS1 f T1 intersystem crossing in the third-row transition metal complexes, giving the strong phosphorescent emission.
1. Introduction
Since the first report of efficient electroluminescence from abilayer organic light-emitting device (OLED), metal quinolino-lates, such as Al(Q)3 with Q ) 8-quinolinolate, have beenthe central focus for the successful development of newtechnologies such as the next generation of flat panel displaysand other emissive systems.1 Work on derivations of Al-(Q)3 was also conducted in an attempt to fine-tune the emis-sion color for OLED applications.2 Concurrently, other metalquinolinolates were synthesized and tested for applications
such as the emitting materials or the electron transportinghosts in OLEDs. These materials typically contain at leastone quniolinolate or substituted quinolinolate chelate that isdirectly coordinated to a main group element, such as a Li,Zn, B, or Ga metal ion.3 Moreover, these complexes exhibitvery high fluorescence quantum efficiency in both solid andfluid states, for which both absorption and emission bandsare ascribed to the ligand-centered singletππ* transitionsor the intraligand charge transfer (ILCT) transitions incor-porating the highest-occupied molecular orbital (hom*o),lying mainly on the phenoxide ring, and the lowest-unoc-cupied molecular orbital (LUMO), predominantly on thenitrogen atom.4 For the tris-substituted complexes, such as* To whom correspondence should be addressed. E-mail:
[emailprotected] (Y.C.); [emailprotected] (C.T.-T.).(1) Tang, C. W.; VanSlyke, S. A.Appl. Phys. Lett. 1987, 51, 913. (b)
Mitschke, U.; Bauerle, P.J. Mater. Chem.2000, 10, 1471. (c) Kelley,T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase,M. A.; Vogel, D. E.; Theiss, S. D.Chem. Mater.2004, 16, 4413. (d)Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A.Chem. Mater.2004, 16, 4556.
(2) Pohl, R.; Montes, V. A.; Shinar, J.; Anzenbacher, P., Jr.J. Org. Chem.2004, 69, 1723. (b) Montes, V. A.; Li, G.; Pohl, R.; Shinar, J.;Anzenbacher, P., Jr.AdV. Mater. 2004, 16, 2001. (c) Colle, M.;Gmeiner, J.; Milius, W.; Hillebrecht, H.; Brutting, W.AdV. Funct.Mater. 2003, 13, 108.
Inorg. Chem. 2005, 44, 4594−4603
4594 Inorganic Chemistry, Vol. 44, No. 13, 2005 10.1021/ic0505347 CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 05/25/2005
Al(Q)3, subsequent MO calculations indicated that configu-ration interactions would lead to extensive mixing of allπsystems in the low-energy transitions resulting in a substantialred shift for both absorption and emission bands, relative tothose of the metal complex with a single quinolinolate ligand.5
To our knowledge, surprisingly little is known about thenature of the triplet excited states of the parent 8-qunioli-nolate fragment (Q) because of a very inefficient S1 f T1
intersystem crossing process.6 Despite this limitation, pulseradiolysis of a benzene solution containing chelate complexAl(Q)3 has been shown to produce the triplet states by energytransfer from appropriate sensitizers, while the phosphores-cent emission of Al(Q)3 was successfully recorded in an ethyliodide matrix at 77 K.7 Obviously, in this approach, theexternal heavy iodine atom is the key to promoting theintersystem crossing and allowing the direct observation ofan otherwise typically spin forbidden transition. Alternatively,one would expect that the related quinolinolate complexeswith an internal heavy transition metal would render a moresuitable model for investigating the respective triplet excitedstates as well as the phosphorescent emission. This delinea-tion has led to the synthesis and characterization of a fewheavy metal quinolinolate complexes of formula M(Q)n, withn ) 2 and M) Pt(II) and Pb(II) orn ) 3 and M) Bi(III)and Ir(III),8 together with the recently characterized high-oxidation-state Re(VII) complex ReO3(Q)9 and Re(I) car-bonyl complexes of formulas Re2(CO)6(Q)2 and Re(CO)3-(Q)(solv), where solv) pyridine and acetonitrile.10 Aparticularly interesting feature is that the emission propertiesof these metal complexes differ greatly in their spectralpatterns and relative intensities. This result implies that thereare substantial variations in terms of their fundamentalphotophysical properties when the central metal ion and thenature of the ancillary ligands are changed.
With this goal in mind, we have undertaken the determi-nation of the structural characterization and photophysicalproperties of the heretofore unknown 8-quinolinolate Os(II)carbonyl complexes Os(CO)3(X)(Q), where X) trifluoro-acetate (tfa) and iodide, which were obtained in good yieldsusing a direct solid pyrolysis method.11 To our surprise, thesecomplexes showed rarely observed dual fluorescence andphosphorescence in solution at room temperature, which canbe readily differentiated by oxygen quenching experiments.The results, in combination with relaxation dynamics andtheoretical approaches, allowed us to gain detailed insightsinto the Os(CO)3(X)(Q) properties. More importantly, gen-eralizations for various types of intersystem crossing andhence their corresponding rates were made on an empiricalbasis of frontier orbital configurations. Details of the resultsand a discussion appear in the following sections.
2. Experimental Section
General Experiments. Commercially available chemical re-agents were used without further purification. The reactions weremonitored using Merck precoated glass TLC plates (0.20 mm withfluorescent indicator UV254). Compounds were visualized with UVlight irradiation at 254 and 365 nm. Column chromatography wascarried out using silica gel purchased from Merck (230-400 mesh).Mass spectra were obtained on a JEOL SX-102A instrumentoperating in an electron impact (EI) mode or a fast atom bombard-ment (FAB) mode.1H and 13C NMR spectra were recorded on aVarian INOVA-500 instrument; chemical shifts are given withrespect to the internal standard, tetramethylsilane, for1H and13CNMR data. Elemental analysis was carried out with a HeraeusCHN-O Rapid Elementary Analyzer.
Synthesis of Os(CO)3(tfa)(Q). A 15 mL Carius tube was chargedwith finely pulverized Os2(tfa)2(CO)6 (100 mg, 0.13 mmol) and8-quinolinol (Q, 41 mg, 0.28 mmol). The tube was degassed andsealed under a vacuum, and then it was placed into a 190°C ovenfor 35 min. After the reaction was completed, the contents weresubjected to sublimation at 80°C and 200 mTorr yielding 74 mgof a yellow product, Os(CO)3(tfa)(Q) (1, 0.14 mmol, 55%).Recrystallization was conducted in methanol at room temperature.It is noted that TLC should not be employed for sample purificationbecause it would cause a marked decomposition of the sample uponcontact with silica gel.
Spectral data of1. MS (FAB, 192Os): m/z 533 [M+], 420 [M+
- CF3CO2]. IR (CH2Cl2): ν(CO) 2126 (s), 2051 (s), 2026 (s) cm-1.1H NMR (500 MHz, CDCl3): δ 8.84 (dd,JHH ) 5.0, 1.0 Hz, 1H),8.46 (dd,JHH ) 8.3, 1.3 Hz, 1H), 7.59 (t,JHH ) 8.0 Hz, 1H), 7.52(dd, JHH ) 8.5, 5.0 Hz, 1H), 7.21 (d,JHH ) 7.0 Hz, 1H), 7.17 (d,JHH ) 8.0 Hz, 1H).13C NMR (100 MHz, CDCl3): δ 176.0 (CO),169.1 (CO), 168.0 (CO), 167.0 (C), 162.7 (q,JCF ) 30 Hz, CO),149.8 (CH), 131.7 (CH), 130.8 (C), 145.0 (CH), 131.7 (CH), 130.8(C), 121.9 (CH), 117.2 (CH), 114.3 (q,JCF ) 230 Hz, CF3), 114.0(CH). 19F NMR (470 MHz, CDCl3): δ -74.53 (s, 3F). Anal. Calcdfor C14H6F3NO6Os: C, 31.64; H, 1.14; N, 2.64. Found: C, 31.74;H, 1.28; N, 2.53.
Synthesis of Os(CO)3(Cl)(Q). A mixture of Os(CO)3(tfa)(Q)(100 mg, 0.19 mmol) and NaCl (45 mg, 0.77 mmol) in methanol(25 mL) was heated to reflux for 4 h. Then, the solvent wasevaporated in a vacuum, and the residue was washed with water,
(3) Wu, Q.; Esteghamatian, M.; Hu, N.-X.; Popovic, Z.; Enright, G.; Tao,Y.; D’Iorio, M.; Wang, S.Chem. Mater.2000, 12, 79. (b) Schmitz,C.; Schmidt, H.-W.; Thelakkat, M.Chem. Mater.2000, 12, 3012. (c)Leung, L. M.; Lo, W. Y.; So, S. K.; Lee, K. M.; Choi, W. K.J. Am.Chem. Soc.2000, 122, 5640. (d) Sapochak, L. S.; Padmaperuma, A.;Washton, N.; Endrino, F.; Schmett, G. T.; Marshall, J.; Fogarty, D.;Burrows, P. E.; Forrest, S. R.J. Am. Chem. Soc.2001, 123, 6300. (e)Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio,K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E.J.Am. Chem. Soc.2002, 124, 6119. (f) Middleton, A. J.; Marshall, W.J.; Radu, N. S.J. Am. Chem. Soc.2003, 125, 880. (g) Qiao, J.; Wang,L. D.; Duan, L.; Li, Y.; Zhang, D. Q.; Qiu, Y.Inorg. Chem.2004,43, 5096. (h) Cui, Y.; Liu, Q.-D.; Bai, D.-R.; Jia, W.-L.; Tao, Y.;Wang, S.Inorg. Chem. 2005, 44, 601.
(4) Chen, C. H.; Shi, J.Coord. Chem. ReV. 1998, 171, 161. (b) Curioni,A.; Boero, M.; Andreoni, W.Chem. Phys. Lett. 1998, 294, 263. (c)Anderson, S.; Weaver, M. S.; Hudson, A. J.Synth. Met.2000, 111-112, 459.
(5) Sapochak, L. S.; Burrows, P. E.; Garbuzov, D.; Ho, D. M.; Forrest,S. R.; Thompson, M. E.J. Phys. Chem.1996, 100, 17766. (b) Halls,M. D.; Schlegel, H. B.Chem. Mater.2001, 13, 2632.
(6) Corsby, G. A.; Whan, R. E.; Alire, R. M.J. Chem. Phys. 1961, 34,743. (b) Goldman, M.; Wehry, E. L.Anal. Chem. 1970, 42, 1178.
(7) Burrows, H. D.; Fernandes, M.; Seixas de Melo, J.; Monkman, A. P.;Navaratnam, S.J. Am. Chem. Soc.2003, 125, 15310.
(8) Ballardini, R.; Indelli, M. T.; Varani, G.; Bignozzi, C. A.; Scandola,F. Inorg. Chim. Acta1978, 31, L423. (b) Ballardini, R.; Varani, G.;Indelli, M. T.; Scandola, F.Inorg. Chem.1986, 25, 3858. (c) Donges,D.; Nagle, J. K.; Yersin, H.Inorg. Chem.1997, 36, 3040.
(9) Kunkely, H.; Vogler, A.Inorg. Chem. Commun.2000, 3, 645.(10) Kunkely, H.; Vogler, A.Inorg. Chem. Commun.1998, 1, 398. (b)
Czerwieniec, R.; Kapturkiewicz, A.; Anulewicz-Ostrowska, R.; Nowac-ki, J. J. Chem. Soc., Dalton Trans.2001, 2756.
(11) Gong, J.-H.; Hwang, D.-K.; Tsay, C.-W.; Chi, Y.; Peng, S.-M.; Lee,G.-H. Organometallics1994, 13, 1720. (b) Yu, H.-L.; Chi, Y.; Liu,C.-S.; Peng, S.-M.; Lee, G.-H.Chem. Vap. Deposition2001, 7, 245.
8-Quinolinolate Os(II) Carbonyl Complexes
Inorganic Chemistry, Vol. 44, No. 13, 2005 4595
filtered, and dried. The dried mass was further purified byrecrystallization from CH2Cl2 and hexane yielding 60 mg of yellowsolid Os(CO)3(Cl)(Q) (2, 0.13 mmol, 71%).
Spectral data of2. MS (FAB, 192Os): m/z 533 [M+], 420 [M+
- Cl]. IR (CH2Cl2): ν(CO) 2122 (s), 2046 (s), 2020 (s) cm-1. 1HNMR (500 MHz, CDCl3): δ 8.78 (dd,JHH ) 5.0, 1.0 Hz, 1H),8.40 (dd,JHH ) 8.3, 1.3 Hz, 1H), 7.56 (t,JHH ) 8.0 Hz, 1H), 7.49(dd, JHH ) 8.3, 4.8 Hz, 1H), 7.20 (d,JHH ) 8.5 Hz, 1H), 7.13 (d,JHH ) 8.0 Hz, 1H).13C NMR (125 MHz, CDCl3): δ 170.2 (CO),169.5 (CO), 167.1 (C), 165.5 (CO), 148.7 (CH), 142.7 (C), 140.4(CH), 131.5 (CH), 131.0 (C), 122.1 (CH), 117.5 (CH), 113.7 (CH).Anal. Calcd for C12H6ClNO4Os: C, 31.76; H, 1.33; N, 3.09.Found: C, 31.76; H, 1.68; N, 3.23.
Synthesis of Os(CO)3(I)(Q). A 15 mL Carius tube was chargedwith finely pulverized Os2I2(CO)6 (100 mg, 0.12 mmol) and8-quinolinol (Q, 45 mg, 0.27 mmol). The tube was degassed andsealed under a vacuum, and then it was placed into a 190°C ovenfor 30 min. After the reaction was completed, the contents wereextracted into CH2Cl2, followed by sublimation at reduced pressure,yielding 75 mg of a yellow product, Os(CO)3(I)(Q) (3, 0.14 mmol,55%). Crystalline samples were obtained from the slow cooling ofa saturated methanol solution at room temperature.
Spectral data of3. MS (FAB, 192Os): m/z 547 [M+], 420 [M+
- I]. IR (CH2Cl2): ν(CO) 2117 (s), 2043 (s), 2019 (s) cm-1. 1HNMR (500 MHz, CDCl3): δ 8.81 (d,JHH ) 5.0 Hz, 1H), 8.35 (d,JHH ) 8.5 Hz, 1H), 7.54 (t,JHH ) 8.0 Hz, 1H), 7.47 (dd,JHH )8.8, 5.0 Hz, 1H), 7.15 (d,JHH ) 8.0 Hz, 1H), 7.10 (d,JHH ) 8.5Hz, 1H).13C NMR (125 MHz, CDCl3): δ 170.0 (CO), 169.6 (CO),167.3 (C), 163.3 (CO), 149.2 (CH), 143.4 (C), 140.2 (CH), 131.4(CH), 131.9 (C), 122.4 (CH), 117.7 (CH), 113.4 (CH). Anal. Calcdfor C12H6INO4Os: C, 26.43; H, 1.11; N, 2.57. Found: C, 26.75;H, 1.45; N, 2.65.
Synthesis of Os(CO)3(tfa)(MQ). A 15 mL Carius tube wascharged with finely pulverized Os2(tfa)2(CO)6 (100 mg, 0.13 mmol)and 2-methyl-8-quinolinol (MQ, 45 mg, 0.28 mmol). The tube wasdegassed and sealed under a vacuum, and then it was placed intoa 190°C oven for 45 min. After the reaction was completed, thecontents were subjected to sublimation at 110°C and 450 mTorr,yielding 56 mg of a yellow product, Os(CO)3(tfa)(MQ) (4, 0.10mmol, 40%). Crystals suitable for X-ray diffraction studies wereobtained from the slow cooling of a saturated methanol solution atroom temperature.
Spectral data of4. MS (FAB, 192Os): m/z547 [M+], 434 [M+ -CF3CO2]. IR (CH2Cl2): ν(CO) 2125 (s), 2049 (s), 2025 (s) cm-1. 1HNMR (500 MHz, CDCl3): δ 8.27 (d,JHH ) 8.5 Hz, 1H), 7.47 (t,JHH ) 7.8 Hz, 1H), 7.43 (d,JHH ) 9.0 Hz, 1H), 7.14 (d,JHH ) 7.5 Hz,1H), 7.08 (d,JHH ) 7.5 Hz, 1H), 3.13 (s, 3H).13C NMR (125 MHz,CDCl3): δ 170.4 (CO), 168.1 (CO), 168.0 (CO), 166.3 (C), 162.7(q, JCF ) 38 Hz, CO), 159.8 (C), 142.5 (C), 141.2 (CH), 130.2(CH), 128.9 (C), 123.2 (CH), 117.2 (CH), 114.3 (q,JCF ) 287 Hz,CF3), 114.1 (CH), 30.4 (CH3). Anal. Calcd for C15H8F3NO6Os: C,33.03; H, 1.48; N, 2.57. Found: C, 33.06; H, 1.66; N, 2.75.
Synthesis of Os(CO)3(tfa)(FQ). A 15 mL Carius tube wascharged with finely pulverized Os2(tfa)2(CO)6 (100 mg, 0.13 mmol)and 5-fluoro-8-quinolinol (FQ, 46 mg, 0.28 mmol). The tube wasdegassed and sealed under a vacuum, and then it was placed intoa 190°C oven for 20 min. After the reaction was completed, thecontents were subjected to sublimation at 110°C and 450 mTorr,yielding 74 mg of a light brown product, Os(CO)3(tfa)(FQ) (5, 0.13mmol, 52%). Further purification was carried out by recrystallizationfrom a methanol solution at room temperature.
Spectral data of5. MS (FAB, 192Os): m/z 551 [M+], 438 [M+
- CF3CO2]. IR (CH2Cl2): ν(CO) 2127 (s), 2053 (s), 2027 (s) cm-1.
1H NMR (500 MHz, CDCl3): δ 8.88 (dd,JHH ) 5.0, 1.5 Hz, 1H),8.67 (dd,JHH ) 8.8, 1.3 Hz, 1H), 7.58 (dd,JHH ) 8.0, 5.0 Hz,1H), 7.34 (dd,JHF ) 9.8 Hz,JHH ) 8.8 Hz, 1H), 7.21 (dd,JHH )8.5 Hz,JHF ) 4.0 Hz, 1H).13C NMR (100 MHz, CDCl3): δ 169.8(CO), 168.7 (CO), 167.8 (CO), 163.2 (C), 162.7 (q,JCF ) 30 Hz,CO), 150.6 (CH), 148.8 (C), 146.9 (C), 141.0 (d,JCF ) 3.6 Hz,C), 135.0 (CH), 120.4 (d,JCF ) 17 Hz, C), 120.0 (CH), 114.9 (d,JCF ) 168 Hz, CH), 114.7 (d,JCF ) 62 Hz, CH), 114.3 (q,JCF )230 Hz, CF3). 19F NMR (470 MHz, CDCl3): δ -74.5 (s, 3F),-139.3 (dd,JHF ) 8.5, 4.2 Hz). Anal. Calcd for C14H5F4NO6Os:C, 30.61; H, 0.92; N, 2.55. Found: C, 31.22; H, 1.15; N, 2.47.
Structural and Photophysical Measurements. Single-crystalX-ray diffraction data were measured on a Bruker SMART ApexCCD diffractometer using (Mo KR) radiation (λ ) 0.71073 Å).Data collection was executed using the SMART program. Cellrefinement and data reduction were performed with the SAINTprogram. The structure was determined using the SHELXTL/PCprogram and refined using full-matrix least squares. All non-hydrogen atoms were refined anisotropically, whereas hydrogenatoms were placed at the calculated positions and included in thefinal stage of refinements with fixed positional parameters. Thecrystallographic refinement parameters for complexes4 and5 aresummarized in Table 1, and selected bond distances and angles ofthese complexes are listed in Tables 2 and 3, respectively.
Steady-state absorption and emission spectra were recorded witha Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920)fluorometer, respectively. DCM in methanol withΦ ∼ 0.44 servedas the standard for the calculation of the emission quantum yield.Nanosecond lifetime studies were performed with an EdinburghFL 900 photon-counting system with a hydrogen-filled or nitrogenlamp as the excitation source. The setup for the picoseconddynamical measurements consists of a femtosecond Ti-Sapphire
Table 1. Crystal Data and Structure Refinement Parameters forComplexes4 and5
4 5
empirical formula C15H8F3NO6Os C14H5F4NO6Osfw 545.42 549.39temp 295(2) K 295(2) Kcryst syst monoclinic monoclinicspace group C2/c C2/ca 13.5579(5) Å 19.2840(6) Åb 9.3166(4) Å 10.5425(3) Åc 25.7751(10) Å 16.8868(5) Åâ 100.979(1)° 117.088(1)°vol, Z 3196.2(2) Å3, 8 3056.5(2) Å3, 8density (calcd) 2.267 mg/m3 2.388 mg/m3
abs coeff 8.046 mm-1 8.424 mm-1
F(000) 2048 2048cryst size 0.20× 0.15× 0.10 mm3 0.28× 0.25× 0.23 mm3
θ ranges 2.67-27.50° 2.27-27.50°independent
reflns3679 [R(int) ) 0.0406] 3511 [R(int) ) 0.0332]
data/restraints/parameters
3679/0/262 3511/0/236
GOF onF2 1.039 1.054final R indices
[I > 2σ(I)]R1 ) 0.0239,R2 ) 0.0507 R1 ) 0.0206,R2 ) 0.0429
R indices(all data)
R1 ) 0.0295,R2 ) 0.0528 R1 ) 0.0245,R2 ) 0.0441
largest diffpeak and hole
0.596 and-0.463 e Å-3 0.512 and-0.703 e Å-3
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complex5
Os-N(1) 2.113(3) Os-O(4) 2.053(2)Os-O(5) 2.094(2) Os-C(1) 1.924(4)Os-C(2) 1.908(4) Os-C(3) 1.891(4)
O(4)-Os-N(1) 79.44(9) O(4)-Os-C(2) 174.47(12)N(1)-Os-C(1) 173.66(12) O(5)-Os-C(3) 174.77(12)
Cheng et al.
4596 Inorganic Chemistry, Vol. 44, No. 13, 2005
oscillator (82 MHz, Spectra Physics). The fundamental train ofpulses was pulse-selected (Neos, model N17389) to reduce itsrepetition rate to typically 0.8-8 MHz and then used to producesecond harmonics (375-425 nm) as an excitation light source. Apolarizer was placed in the emission path to ensure that thepolarization of the fluorescence was set at the magic angle (54.7°)with respect to that of the pump laser to eliminate the fluorescenceanisotropy. An Edinburgh OB 900-L time-correlated single-photon-counting system was used as the detection system, rendering atemporal resolution of∼15 ps. Data were analyzed using thenonlinear least-squares procedure in combination with an iterativeconvolution method. The long-lived phosphorescence spectrum wasmeasured in a direct laser flush experiment. Briefly, an Nd:YAG(355 nm, 8 ns, Continuum Surelite II) pumped optical parametricoscillator coupled with a second harmonic device served as a tunableexcitation source. The time-resolved emission was detected bygating an intensified charge-coupled detector (ICCD, PrincetonInstruments, model 576G/1) at different delay times with respectto the excitation pulse.
3. Computational Methodology
Time-dependent DFT calculations based on the geometry takenfrom X-rays of complexes1-5 were carried out using a hybridB3LYP method,12 while a double-ú quality basis set consisting ofHay and Wadt’s effective core potentials (LANL2DZ)13 wasemployed for iodine and osmium atoms; a 6-31G* basis set14 wasemployed for the H, C, N, F, Cl, and O atoms. A relativisticeffective core potential (ECP) replaced the inner core electrons ofOs(II), leaving the outer core (5s25p6) electrons and the 5d6 valenceelectrons. Typically, the lowest 10 triplet and singlet roots of thenonhermitian eigenvalue equations were obtained to determine thevertical excitation energies. Oscillator strengths were deduced fromthe dipole transition matrix elements (for singlet states only). Theexcited-state TDDFT calculations were carried out using Gaussi-an03.15
In the frontier region, neighboring orbitals are often closelyspaced. In such cases, consideration of only the hom*o and LUMOmay not yield a realistic description. For this reason, partial densityof states (PDOS) diagrams, which incorporate a degree of overlapbetween the curves convoluted from neighboring energy levels, cangive a more representative picture. The contribution of a group toa molecular orbital was calculated within the framework of Mullikenpopulation analysis. The PDOS spectra were created by convolutingthe molecular orbital information with Gaussian curves of unitheight and fwhm of 0.5 eV. The PDOS diagrams for1 and Ir(Q)3,shown in this work, are generated using the AOMix program.16
4. Results
Preparation and Characterization. Three distinctivequinolinolate osmium complexes Os(CO)3(tfa)(Q) (1), Os-(CO)3(tfa)(MQ) (4), and Os(CO)3(tfa)(FQ) (5) were synthe-sized in moderate yields from osmium carbonyl compound[Os(CO)3(tfa)2]2 and the quinolinol ligand using the solid-state pyrolysis technique (Chart 1).17 For each experiment,the reactants were finely pulverized and mixed thoroughlybefore being loaded into a Carius tube. The tube was thensealed under a vacuum and placed into a preheated ovenmaintained at the desired temperatures. The reactions couldbe visually monitored by gradual changes in color. After thereaction was completed, the tube was opened and the contentswere subjected to vacuum sublimation to remove nonvolatilecomponents, followed by recrystallization from warm metha-nol to yield bright yellow crystalline solids.
To further extend the synthetic scope of our approach,complex 1 was treated with NaCl in refluxing methanol,which led to the isolation of the respective chloride-substituted complex Os(CO)3(Cl)(Q) (2). On the other hand,the iodide-substituted analogue Os(CO)3(I)(Q) (3) wasprepared by heating the iodide dimer [Os(CO)3(I)2]2
18 andthe quinolinol ligand using the solid-state pyrolysis methodmentioned above. Chart 1 depicts the molecular drawing ofOs metal complexes1-5. It should be noted that dissolutionof the tfa-substituted samples1, 4, and5 into a chlorinatedsolvent, such as CH2Cl2, would produce dark brown solutionswithin hours. TLC analysis showed the formation of smallamounts of a black intractable material that stayed at theorigin of the silica gel plate, together with a fast eluting paleyellow spot from the remaining osmium complexes. Ac-
(12) Becke, A. D.Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.;Parr, R. G.Phys. ReV. B 1988, 37, 785. (c) Miehlich, B.; Savin, A.;Stoll, H.; Preuss, H.Chem. Phys. Lett.1989, 157, 200.
(13) Hay, P. J.; Wadt, W. R.J. Chem. Phys.1985, 82, 270. (b) Wadt, W.R.; Hay, P. J.J. Chem. Phys.1985, 82, 284. (c) Hay, P. J.; Wadt, W.R. J. Chem. Phys.1985, 82, 299.
(14) Hariharan, P. C.; Pople, J. A.Mol. Phys.1974, 27, 209.(15) Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.(16) Gorelsky, S. I.AOMix, revision 6.1; http://www.sg-chem.net/. (b)
Gorelsky, S. I.; Lever, A. B. P.J. Organomet. Chem.2001, 635, 187.
(17) Chen, Y.-L.; Sinha, C.; Chen, I.-C.; Liu, K.-L.; Chi, Y.; Yu, J.-K.;Chou, P.-T.; Lu, T.-H.Chem. Commun.2003, 3046. (b) Gong, J.-H.;Hwang, D.-K.; Tsay, C.-W.; Chi, Y.; Peng, S.-M.; Lee, G.-H.Organometallics1994, 13, 1720.
(18) Rosenberg, S.; Herlinger, A. W.; Mahoney, W. S.; Geoffroy, G. L.Inorg. Synth.1989, 25, 187.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complex4
Os-N(1) 2.139(3) Os-O(4) 2.052(2)Os-O(5) 2.102(3) Os-C(1) 1.915(4)Os-C(2) 1.919(4) Os-C(3) 1.897(5)
O(4)-Os-N(1) 79.47(11) O(4)-Os-C(1) 175.86(15)N(1)-Os-C(2) 171.68(14) O(5)-Os-C(3) 173.15(13)
Chart 1. Molecular Structures of Relevant Compounds Prepared andDiscussed in This Study
8-Quinolinolate Os(II) Carbonyl Complexes
Inorganic Chemistry, Vol. 44, No. 13, 2005 4597
cordingly, sample purifications were best conducted in a non-chlorinated organic solvent, such as methanol or acetone,while the IR as well as NMR spectra were recordedimmediately after the sample was added to the chlorinateddeuterated solvents to minimize the effect of decomposition.
For the product characterization, the FAB MS analysissupported the anticipated molecular ion (M+) as well as thedaughter ion products by loss of the weakly bonded anionssuch as the trifluoroacetate (tfa), chloride, or iodide ligands.The1H NMR spectra exhibited all of the characteristic signalsderived from quinolinolate ligands, while the13C NMRanalyses showed three additional signals at the downfieldregion (δ 170-68) from the coordinated CO ligands. Inaddition, IR analyses of all of the complexes in solution gavethree intenseν(CO) stretching bands of equal intensity,implying that they could adopt the mutually orthogonal facialarrangement. It is also notable that theν(CO) stretchingfrequencies of tfa-substituted complexes1, 4, and 5 wereessentially identical, suggesting that variations of the sub-stituent on the quinolinolate ligands had a negligible influ-ence on the central metal ion. In contrast, replacement ofthe tfa ligand of1 with the chloride and iodide ligands,forming 2 and 3, shifted theν(CO) stretching bands to alower-energy side by 4-6 and 7-9 cm-1, respectively. Thissystematic trend indicates a betterπ-donor strength for thehalide ligands which then increases the electron density onthe central Os(II) cation and, in turn, results in an enhancedbackπ bonding to the CO ligands.
Unambiguous confirmation of the proposed molecularstructures was provided by a single-crystal X-ray analysisof the fluorine-substituted quinolinolate complex5. Thecorresponding ORTEP diagram is displayed in Figure 1,while the bond distances and angles are listed in Table 2. Itis pertinent to note that the Os metal atom showed a slightlydistorted octahedral ligand arrangement, encapsulated by achelating quinolinolate ligand, together with a tfa ligand andthree facial carbonyl ligands. The gross structure can bereferred to that found in the closely related derivative Os-
(CO)3(tfa)(hfac), where hfac) hexafluoroacetylacetonate,19
and hom*oleptic complexes, such as Os(CO)3(tfa)2 and Os-(CO)3(pyS)2, where pyS) pyridine-2-thionate, of which oneligand takes the chelating mode, while the second adoptsthe monodentate bonding mode in forming the six-coordinategeometry.20 Moreover, we also conducted the X-ray structuralanalysis of the 2-methyl-substituted complex,4, to studyminor variations of the ligand effect. A comparison of theORTEP diagram (Figure 2) and the metric parameters (Tables2 and 3) with those of5 shows no significant deviation,except for a slight lengthening of the unique Os-N distancefrom 2.113(3) Å, as observed for5, to 2.139(3) Å, asobserved for4. This observation agrees with the nearlyidentical IRν(CO) data from the solution state.
Photophysical Properties. The room-temperature UV-vis and emission spectra of the first three complexes1, 4,and5 in toluene are shown in Figure 3, while those of1, 2,and3 are depicted in Figure 4. The separated figures allowus to clearly visualize the spectral changes associated withthe three quinolinolate ligands as well as to understand theeffect of the ancillary ligands. Table 4 summarizes the peakwavelengths of the lowest-energy absorption and otherimportant photophysical data. It is obvious that the strongabsorption in the UV region (e365 nm) with the vibronicfine structure is derived from a typical ligand-centeredππ*transition because the corresponding transition was docu-mented for the free quinolinol ligands. A popular assignmentof metal-to-ligand charge transfer (MLCT) transition for theabsorption band near 420-450 nm seems unlikely to occuron the basis of the following arguments. (i) The peakwavelength with anε value of g3000 cm-1 M-1 leads tothe assignment of a generally weaker MLCT transition,which is quite inappropriate. (ii) Although the MLCT
(19) Yu, H.-L.; Chi, Y.; Liu, C.-S.; Peng, S.-M.; Lee, G.-H.Chem. Vap.Deposition2001, 7, 245.
(20) Deeming, A. J.; Meah, M. N.; Randle, N. P.; Hardcastle, K. I.J. Chem.Soc., Dalton Trans.1989, 2211.
Figure 1. ORTEP diagram of complex5 with thermal ellipsoids shownat 30% probability. Figure 2. ORTEP diagram of complex4 with thermal ellipsoids shown
at 30% probability.
Cheng et al.
4598 Inorganic Chemistry, Vol. 44, No. 13, 2005
transition might take place in the same energy region forsuch metal carbonyl complexes, its major involvement canbe ruled out because of the occurrence of equivalentabsorption in the closed shell metal complexes, such as Al-(Q)3, for which the MLCT transition is strictly prohibited.(iii) MLCT should reveal greater dependence on the elec-tronic character of a central metal ion. However, this effectwas not observed in the case of iodide complex3, eventhough the IR ν(CO) spectrum has shown significantincreases in electron density (vide supra).
Alternatively, it is more plausible to assign this band tothe lowestππ* transition with a significant contribution fromthe intraligand charge transfer (ILCT) transition. Thisdelineation shows good agreement with the∼20 nm batho-chromic shifting of the lowest-energy absorption band ofcomplex5, for which fluorine substitution at the 5 positionis expected to markedly lower theππ* gap of the quinoli-nolate fragment because of the resonance (mesomeric) effect.Further support of these viewpoints will be elaborated in theDiscussion.
Figures 3 and 4 depict the emission spectra of complexes1-5 in degassed toluene, while the associated steady-stateand dynamics data are listed in Table 4. Except for thefluorine-substituted complex,5, the emission spectra ofcomplexes1-3 revealed two distinct bands, the intensityratios for which varied according to the quinolinol ligandsand the monoanionic ancillary ligands. The excitation spectramonitored at these two bands were identical, which, withinexperimental error, also matched the absorption spectra,indicating that the dual emission originated from the sameground-state precursor. The shorter-wavelength band, show-ing characteristics of short lifetime (,2 ns, see Table 4), isa mirror image with respect to the lowest-ππ*/ILCT bandand is classified as fluorescent emission, while the long-wavelength band can be assigned to the respective phos-phorescence on the basis of its relatively much longer lifetime(>1 µs) and drastic quenching in intensity under the presenceof oxygen (not shown here). In comparison, remarkablydifferent spectral features were observed in complex5, inwhich only fluorescence with a lifetime of 1.1 ns could beresolved. Apparently, the incorporation of a fluorine atomon the quniolinolate ligand plays a key role for the corre-sponding relaxation dynamics. Furthermore, Figure 4 showsthe UV-vis spectra and the corresponding emission spectraof complexes1-3, which allow us to delineate the majorinfluences of tfa and halide ligands on the overall photo-physical properties. The chloride substituent of2 imposesonly a small variation compared to that of the tfa ligand byshowing very similar absorption and emission spectra.However, in the case of iodide complex3, despite negligiblechanges in absorption and emission peak wavelengths, thephosphorescence was significantly increased, plausibly aresult of the additional iodide heavy atom effect (vide infra).21
With regard to metal-quniolinolate complexes, dualluminescence at 77 K in absolute ethanol has been docu-mented in the main group complexes, such as Bi(Q)3 andPb(Q)2.8b In contrast, because of the more efficient spin-orbit interaction for the second- and third-row transition metalanalogues, such as Rh(Q)3, Pd(Q)2, Ir(Q)3, Pt(Q)2, and MeHg-(Q),8b,22 the fluorescent emission was completely quenchedresulting in a unique strong phosphorescence. Thus, theobservation of dual emission in1-4 and even the lack ofphosphorescence in5 are of great fundamental interest. Theresults imply that the heavy atom effect of the central osmiumatom in these complexes only transmits partially to theemitting chromophore, the net result of which is insufficient
(21) Kunkely, H.; Vogler, A.Chem. Phys. Lett.2003, 376, 226.
Figure 3. UV-vis absorption and normalized emission spectra of1 (blacksquares),4 (blue triangles), and5 (red circles) in toluene at roomtemperature.
Figure 4. UV-vis absorption and normalized emission spectra of1 (blacksquares),2 (red circles), and3 (blue triangles) in toluene at roomtemperature; the excitation wavelength is 500 nm.
Table 4. Photophysical Properties of Complexes1-5 in DegassedToluene at Room Temperature
abs,λmax (nm)(ε [M-1 cm-1]) PL, λmax (nm) Φa (%) fluorescence phosphorescenceb
1 424 (3300) 526, 635 1.4 0.55 ns 33.1µs2 426 (3100) 520, 635 1.3 0.50 ns, 35.6µs3 430 (4100) 520, 650 2.4 0.15 ns 3.8µs4 421 (3400) 520, 650 0.2 0.21 ns 21.6µs5 443 (2900) 560, 689 1.1 1.10 ns NA
a The solution was degassed with at least three freeze-pump-thawcycles. The reportedΦ value is the sum of fluorescence and phosphores-cence.b The phosphorescence lifetime was measured by a direct laser flashexperiment.
8-Quinolinolate Os(II) Carbonyl Complexes
Inorganic Chemistry, Vol. 44, No. 13, 2005 4599
to induce an efficient intersystem crossing from the singletto the triplet states and vice versa. Detailed mechanistic basesare elaborated in the Discussion
5. Discussion
Theoretical Approaches. Theoretical confirmation of theunderlying basis for the photophysical properties of theseosmium complexes was provided by the ab initio MOcalculations. The results for complexes1-5 are shown inFigure 5 and Table 5. Figure 5 depicts the features of thehighest-occupied (hom*o) and the lowest-unoccupied(LUMO) frontier orbitals mainly involved in the lower-lyingtransition, while the descriptions and the energy gaps of eachtransition are listed in Table 5. Apparently, the electrondensities of the hom*o are located mainly on the phenolate
fragment of the quinolinolate ligand, whereas those of theLUMO are distributed over the entire quinolinolate moiety.The results clearly indicate that the lowest-electronic transi-tion in both S0 f S1 and S0 f T1 manifolds is dominatedby ππ* in combination with ILCT (phenolate site (π) fpyridine site (π*)) character. The latter transition, in part,incorporates the transfer of electron densities from the oxygenatom of the phenolate fragment to theπ* orbitals of the fusedring system. Moreover, for the hom*o of complex5, the pπ
orbital in the fluorine substituent conjugates with thequinolinolateπ moiety and is also involved in the lowest-energy transition. Because the fluorine atom possesses bothinductive and resonant properties, the results are reminiscentof the mesomeric effect,23 in which once the substituent islocated withinπ conjugation, either electron-donating orelectron-accepting substituents result in the decrease ofelectronic transition energy with a consequent bathochromicshift in the absorption and emission bands. This happensbecause when∆hom*o is normally larger than∆LUMO,both are positive with electron-donor substituents, and when∆hom*o is smaller than∆LUMO, both are negative withelectron-acceptor substituents. The variations in the energylevels caused by the mesomeric effect are fully consistentwith the red-shifted absorption in complex5 compared tothat in complex1.
The lowest singlet-state transitions of 436 and 486 nmcalculated for4 and5, respectively, are in good agreementwith those (4, 421 nm; 5, 443 nm) obtained from theabsorption spectra. Likewise, the estimated S0 f T1 transitionof 583 nm for complex4 is qualitatively consistent withobserved 650 nm phosphorescence. The deviation of thecurrent theoretical approach from the experimental resultsmay be partly explained by the underestimation of the mixingof the high-density low-lying states or the less-extensive basisset used for the Os(II) atom. Moreover, a comparison of theenergy gap calculated from the S0 f T1 transition (in thegas phase) with that obtained from phosphorescence (insolution) may not be fair because of the role of the solventpolarity effect, which normally leads to a further spectralred shift. Nevertheless, the result qualitatively predicts thetendencies of the relative energy levels of these lower-lyingexcited states. Despite the lack of phosphorescence, thecalculation predicts the S0 f T1 transition for complex5 tobe∼651 nm. Taking into account the solvation effect, it isreasonable to predict the phosphorescence to appear at∼690nm. Thus, the lack of observable room-temperature phos-phorescence in complex5 may be rationalized by therelatively slow intersystem crossing in combination with thesmall T1-S0 energy gap delineated as follows.
Intersystem Crossing.As listed in Table 4, the fluores-cence lifetime was measured to be,2 ns for complexes1-5in toluene. In comparison, the metal-free quinolinolate anionexhibits a fluorescence decay time as long as 4.1 ns (kobs )
(22) Donges, D.; Nagle, J. K.; Yersin, H.J. Lumin.1997, 72-74, 658. (b)Yersin, H.; Donges, D.; Nagle, J. K.; Sitters, R.; Glasbeek, M.Inorg.Chem.2000, 39, 770. (c) Kunkely, H.; Vogler, A.J. Photochem.Photobiol. A2001, 144, 69.
(23) Klessinger, M.; Michl, J.Excited States and Photochemistry of OrganicMolecules; VCH: New York, 1995.
Figure 5. hom*o and LUMO of complexes1, 5, and Ir(Q)3. Note thatthat the lowest-lying state in both singlet and triplet manifolds is dominatedby the hom*of LUMO transition. To avoid redundancy, only complexes1 and5 are shown for the quinolinolate Os(II) complexes.
Table 5. Calculated Energy Levels of the S1 and T1 Transitions forComplexes1-5
nm E (eV) f assignments
1T1 614.2 2.02 ∼0 hom*o f LUMOS1 450.2 2.75 0.064 hom*of LUMO
2T1 625.2 1.98 ∼0 hom*o f LUMOS1 456.5 2.72 0.0542 hom*of LUMO
3T1 632.7 1.96 ∼0 hom*o f LUMOS1 461.9 2.68 0.0594 hom*of LUMO
4T1 583.2 2.13 ∼0 hom*o f LUMOS1 436.3 2.84 0.0525 hom*of LUMO
5T1 651.0 1.90 ∼0 hom*o f LUMOS1 486.8 2.55 0.0465 hom*of LUMO
Cheng et al.
4600 Inorganic Chemistry, Vol. 44, No. 13, 2005
2.44× 108 s-1). The observed rate of decay for complexes1-5 can be expressed as
wherekisc is the intersystem crossing rate constant andkr
andknr denote the radiative and nonradiative decay rate con-stants, respectively, excluding the intersystem crossing. As-suming a relatively much smallerkisc for the quinolinolate an-ion, kisc for complexes1-5 can therefore be estimated viakisc(1-5) ) kobs(1-5) - kobs(quinolinolate ion). As listed inTable 4, thekisc of 6.43 × 109 s-1 calculated for3 is thelargest among the complexes studied. Conversely, thekisc of6.65× 108 s-1 for complex5 represents the smallest value.Moreover, for similar analogues,1 and4, thekisc (4.52× 109
s-1) in 4 is ∼3-fold larger than that of1 (1.57× 109 s-1).To rationalize the results, one might consider the spin-for-
bidden nature and hence the very small S1 f Tn intersystemcrossing (ISC) rate in theππ* configuration of the quinolino-late ion. Thus, perturbations, such as vibronic coupling andspin-orbit coupling, are necessary to break-down the spin-forbidden nature and enhance the intersystem crossing rate.To simplify the approach, assuming that the S1 f T1 transi-tion is thedominant ISCprocess,on thebasisofFermi’sgoldenrule, the rate of intersystem crossing can be expressed as24
whereΦS1 andΦT1 denote the electronic wave functions ofthe singlet and triplet states, respectively. The density of thefinal vibronic states (T1) has the same energy as the initialstates (S1), represented byF(ET1 ) ES1). Hso is the Hamil-tonian for the spin-orbit coupling, andφv denotes thevibrational wave function.
For a many-electron system,25 the spin-orbit couplingHamiltonian could be expressed as
wherem is the mass of the electron,c is the velocity of light,SB andLB are the electron spin and orbital angular momentumoperators, respectively,r is the electron-nuclear distance,andV is the potential. As for an oversimplified approach, ifone neglects the electron shielding effect, assuming the coreatom to be hydrogen-like,Hso can be expressed as
wheree andm are the charge and mass of the electron, re-spectively,Z denotes the nuclear charge of the atom,n andl are the principle and orbital angular momentum quantumnumbers, respectively, for the electron of concern, andr isthe electron-nuclear distance. Apparently, in this expressionof the heavy atom effect, the rate of intersystem crossing isproportional toZ8 and inversely proportional tor6.
For a fixed core heavy atom like Os(II) for complexes1-5, the largestkisc value in3 manifests the additional iodineheavy atom effect, enhancing the rate of the S1 f T1
intersystem crossing. However, replacing iodine with chlo-rine, forming complex2, shows negligible enhancement incomparison to parent complex1 (cf., kisc ≈ 1.76× 109 s-1
vs that of 1.57× 109 s-1 in complex1) because of the lighterCl atom. Relative to complex1, the addition of a 2-methylgroup in 4 enhancingkisc by 3-fold may tentatively berationalized by the increase in the density of the final vibronicstate (T1), which has the same energy as the initial states(S1) (i.e., the increase ofF(ET1 ) ES1) in eq 1), although aquantitative approach is pending for resolution. For complex5, the additional conjugation effect introduced by the fluorineatom at the 5 position of the quinolinolate chromophore, ina qualitative manner, elongates the electron-core (Os(II))distance r on average, resulting in the retardation ofintersystem crossing and hence the reduction ofkisc.
Likewise, a similar argument can be applied to the T1 fS0 transition dipole (i.e., the radiative decay rate) which,based on an approximation of zero-order perturbation, cantheoretically be expressed as
whereES1 andET1 are the energies of S1 and T1, respectively.Similar to the derivation in eq 3, it can thus be perceivedthat the T1 f S0 transition moment, and hence the phos-phorescence radiative decaykr, in addition to borrowing theintensity from the S1 f S0 transition, is also qualitativelyproportional toZ8/r6. Thus, with increases in the rate of theS1 f T1 intersystem crossing, the T1 f S0 radiative decayrate increases accordingly. This viewpoint can be qualita-tively supported by the increase of the intensity ratio for thephosphorescence versus the fluorescence askisc increases inthe same analogues, such as in4 and1 (or 2 and3). As for5, a theoretical approach predicts a T1-S0 energy gap (∼651nm, Table 5) much smaller than that for the rest of thecomplexes because of the extensiveπ electron delocalization.Thus, the T1 f S0 decay dynamics may be dominated bythe radiationless deactivation, a mechanism known as theenergy gap law.26 This, in combination with the smallest S1
f T1 kisc, leads to the lack of phosphorescence for5 in thesteady-state approach.
In a sharp contrast to complexes1-5, the third-rowtransition metal-quinolinolate complexes, such as Ir(Q)3, Pt-(Q)2, and MeHg(Q), exhibit a unique room-temperaturephosphorescence,8b,22and hence an ultrafastkisc is expected.To gain detailed insight into the associated mechanism, wethus performed a similar theoretical approach for the me-
(24) Becker, R. S.Theory and Interpretation of Fluorescence andPhosphorescence; Wiley-Interscience: New York, 1969. (b) Siddique,Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K.Inorg. Chem.2003, 42,6366.
(25) Fayer, M. D.Elements of Quantum Mechanics; Oxford UniversityPress: Oxford, U.K., 2001.
kobs) kr + knr + kisc
kisc ) 2πp
|⟨ΦT1|Hso|ΦS1
⟩|2|⟨φv′|φv⟩|2F(ET1) ES1
) (1)
Hso )1
2m2c∑
i[1
ri
∂V(ri)
∂ri]LBiSBi
) ú∑i
LBiSBi (2)
Hso ) ( e2h2
2m2c2r3)[ Z4
n3(l + 1)(l + 12)l] (3)
⟨ΦS0|er|ΦT1
⟩ ){⟨ΦS1
|Hso|ΦT1⟩ ⟨ΦS1
|er|ΦS0⟩}
ES1- ET1
(4)
8-Quinolinolate Os(II) Carbonyl Complexes
Inorganic Chemistry, Vol. 44, No. 13, 2005 4601
ridional isomer of Ir(Q)3, and the results are shown in Figure5 and Table 6. In this calculation, the meridional isomer waschosen to be the target molecule (Chart 1) because itrepresents the kinetic product of the reaction between IrCl3
and the cyclometalated ligands,27 and the respective DFTcalculations on the relatedmer- andfac-isomers of Ir(ppz)3,where ppz) 1-phenylpyrazole, gave a similar picture forthe hom*o and LUMO orbitals, particularly the degree ofinvolvement of the metal d-orbital.27a Interestingly, insteadof the dominantππ*/ILCT transition, resolved from com-plexes1-5, for Ir(Q)3, the lowest transition in both thesinglet and triplet manifolds involves mainly mixing ofMLCT andππ* character. The transition characters for both1 and Ir(Q)3 can be clearly perceived from the PDOS spectra(see Experimental Section) shown in Figure 6, in which theelectron densities contributed to each Os (or Ir) atom andquinolinolate, tfa, and CO ligand are specified. Apparently,the difference in the transition properties between1-5 andIr(Q)3 lies in different ligand fields, in that the electronwithdrawing property of CO, because of its provision ofπback-bond donation, further stabilizes the dπ orbital of thecenter Os(II) atom, rendering the lowest transition possessingsolely aππ*/ILCT character in1-5. Conversely, as shownin Figure 5 and Table 6, for Ir(Q)3, bothπ-conjugated orbitalsin quinolinolate ligand and dπ orbitals in the Ir(III) coreparticipate in hom*o, whereas LUMO mainly possesses thequinolinolateπ* property, giving rise to a mixing ofππ*/MLCT character in both singlet and triplet manifolds.
On the basis of the above stance, complexes1-5 mainlyundergo1ππ* f 3ππ* intersystem crossing, in which thecoupling between the orbital and spin angular momentumshould be small because the transition involves negligiblechanges of the orbital angular momentum. Thus, there is nofirst-order spin-orbit coupling to enhance the intersystemcrossing. In contrast, as for Ir(Q)3, mixing of ππ* and MLCTin the lowest singlet and triplet states leads to the S1 f T1
intersystem crossing, in part incorporating a1dππ* f 3ππ*or 1ππ* f 3dππ* transition. Since the net effect generatesthe change of orbital angular momentums (i.e., dπ f πcoupled with the flip of the electron spin) the transition has
an appreciable first-order spin-orbit coupling term, resultingin a drastic enhancement of the intersystem crossing. In goodagreement with this theoretical prediction, photophysicalproperties of the high-oxidation-state metal complex ReO3-(Q) or even the actinide metal complex Th(MQ)4 have beenreported.9,28 Both showed dominant fluorescent emission,while the corresponding phosphorescence is comparablymuch weaker at room temperature. The lack of a strongspin-orbit interaction in these heavy metal complexes mustbe related to the contracted d0 electron configuration, andthe interaction between the dπ orbital of the central metalion and theπ electron system of the quinolinolate ligand isthus suppressed.
Another key feature is related to the S1-T1 energy gap.29
The energy difference between singlet (S1) and triplet (T1)states, ∆ES1-T1, lies mainly in the matrix element,J,associated with the electron repulsion from the electronexchange, so that
where theJ value is essentially equivalent to the overlapintegral between the occupied electron wave functions in S1
and T1. In the case of1-5 with pureππ* configurations inthe lowest singlet and triplet states,J can be expressed as
where the numbers refer to the electrons occupying theseorbitals and e/r12 represents the repulsion between the
(26) Johnson, S. R.; Westmoreland, T. D.; Caspar, J. V.; Barqawi, K. R.;Meyer, T. J.Inorg. Chem.1988, 27, 3195. (b) Caspar, J. V.; Kober,E. M.; Sullivan, B. P.; Meyer, T. J.J. Am. Chem. Soc.1982, 104,630. (c) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene,F. R.; Meyer, T. J.Inorg. Chem.1996, 35, 2242. (d) Perkins, T. A.;Pourreau, D. B.; Netzel, T. L.; Schanze, K. S.J. Phys. Chem.1989,93, 4511.
(27) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba,I.; Ho, N. N.; Bau, R.; Thompson, M. E.J. Am. Chem. Soc.2003,125, 7377. (b) Yang, C.-H.; Fang, K.-H.; Chen, C.-H.; Sun, I.-W.Chem. Commun.2004, 2232.
(28) Kunkely, H.; Vogler, A.Chem. Phys. Lett.1999, 304, 187.(29) Yersin, H.; Strasser, J.Coord. Chem. ReV. 2000, 208, 331. (b) Yersin,
H.; Donges, D.Top. Curr. Chem.2001, 214, 81. (c) Yersin, H.Top.Curr. Chem.2004, 241, 1.
Table 6. Calculated Energy Gaps of the Lowest Transition forComplexes1-5 and Ir(Q)3
complexES-T (calcd)(kcal/mol)
ES-T (exp)(kcal/mol)
major configurationscontributed to S1 f T1 ISC
1 16.8 9.3 ILCT (ππ*)2 17.1 10.0 ILCT (ππ*)3 16.7 11.0 ILCT (ππ*)4 16.4 11.0 ILCT (ππ*)5 15.0 9.6 ILCT (ππ*)Ir(Q)3 9.9 N/A MLCT (30.8%)/LLCT (ππ*, 69.2%)
Figure 6. Partial density of state plots of1 (upper) and Ir(Q)3 (lower).For 1, the contributions from the Os atom (black), quinolinolate (blue), tfa(green), and the CO ligands (red) are shown. For Ir(Q)3, the contributionsfrom the Ir atom (black), Q1 (blue), Q2 (green), and Q3 (blue) are shown.
∆ES1-T ) E(S1) - E(T1) ≈ 2J
J ) ⟨π(1)π*(2)| er12
|π(2)π*(1)⟩ (5)
Cheng et al.
4602 Inorganic Chemistry, Vol. 44, No. 13, 2005
exchanging electrons. As a simplified approach, the lattercan be factored out so thatJ is qualitatively perceived to beproportional to the overlap of the participating orbitals andcan be expressed as
Conversely, for the case of Ir(Q)3 which has mixed MLCT/ππ* character, the MLCT contribution toJ can be expressedas
Thus, it is clear thatJπ,π* is in general larger thanJdπ,π*
because the integral expressed in eq 6 is greater than that ineq 7 as a result of the better orbital overlap betweenππ*than that of dππ*. As a result,∆ES1-T1 for 1-5 is expectedto be significantly larger than that of Ir(Q)3. This viewpointcan be further supported by the theoretical approach. Asshown in Table 6, the∆ES1-T1 values were calculated to be>15 kcal/mol for1-5. If one neglects the solvation effect,the results for1-4 are qualitatively in agreement with the∆ES1-T1 value of>9 kcal/mol obtained from the differencein peak maxima between fluorescence and phosphorescence.Conversely, although∆ES1-T1 cannot be resolved experi-mentally for Ir(Q)3, its theoretically estimated value of 9.9kcal/mol is smaller than that calculated for1-5 by at least6 kcal/mol.
Since the term of the density factor,F(ET1 ) ES1), shownin eq 1 is inversely proportional to∆ES1-T1
2, the intersystemcrossing of Ir(Q)3 is enhanced not only by the spin-orbitcoupling but also by the much lower∆ES1-T1 gap. Con-versely, for complexes1-5, kisc is expected to be muchsmaller, relatively, because of its much weaker spin-orbitand vibronic couplings, consistent with the experimentalresults. These results are also qualitatively in agreement withthose reported for the Ir(III) complex, Ir(ppy)3, and in therelated complexes Ir(ppy)2(acac) and Ir(ppy)2(bza), whereppy ) 2-phenylpyridine, acac) acetylacetonate, and bza)benzylacetonate.30 The DFT calculation showed that all ofthe low-lying transitions are categorized as MLCT transitionsand, as expected, the metal orbitals involved in the transitionshave about 50% metal 5d character, along with a significantadmixture of the ligandπ character. As a result,kisc isexpected to be ultrafast, resulting in a unity population inthe T1 state.
6. Conclusion
In conclusion, we have synthesized a new series ofquinolinolate osmium carbonyl complexes,1-5. These Oscomplexes show salient dual emissions, consisting of fluo-rescence and phosphorescence, the spectral properties andrelaxation dynamics of which have been studied. The results,in combination with theoretical approaches, lead us topropose that both fluorescence and phosphorescence originatemainly from the quinolinolateππ* state. Both the experi-mental and theoretical approaches generalize various types
of intersystem crossing and hence their relative efficiencieson the basis of the mechanism incorporating spin-orbit andvibronic coupling. The S1-T1 intersystem crossing incomplexes1-5 with the solely1ππ* f 3ππ* transition,because of its unfavorable spin-orbit and vibronic couplingand larger∆ES1-T1 gap, is much slower in rate than that ofIr(Q)3, which involves the1dππ* f 3ππ*, 1ππ* f 3dππ*,or both types of intersystem crossing, in combination with asmaller ∆ES1-T1 gap (i.e., increasing the MLCT (dππ*)character). It is also believed that a similar argument holdsfor the case of the mixing of LMCT (ligand-to-metal chargetransfer) andππ* characters. On the basis of this standpoint,it is reasonable to predict that third-row transition complexespossessing purelyππ* character in both S1 and T1 may notenhance the intersystem crossing because of the lack ofchanges in orbital angular momentums, and hence the smallspin-orbit coupling (i.e., a small⟨1dππ* |Hso|3dππ* ⟩ term orvice versa), despite the vibronic coupling term, may befavorable because of the small∆ES1-T1. It should be notedthat in the aforementioned approach, we have neglected theintersystem crossing via the higher-lying triplet states (i.e.,the S1-Tm (m > 1) pathways). The breakdown of eq 1 isexpected when the intersystem crossing involves Tn states(n g 1). In this case, the spin-orbit coupling term shouldbe replaced by∑n)1 ⟨ΦTn|Hso|ΦS1⟩, and the resulting mech-anism is very complicated.
Finally, in comparison to most third-row transition metalcomplexes utilizing the unique phosphorescence property inOLEDs,31 the intrinsic dual emission in complexes such as1- 5 renders certain perspectives in view of white lightgeneration. As an ideal approach, if the efficiency of theS1-T1 intersystem crossing is 25%, the intensity ratio forfluorescence versus phosphorescence should be 3:1, if oneneglects other radiationless transition pathways (triplet-triplet annihilation included) in both S1 and T1 states.Accordingly, after the exciton recombination, an equalintensity for fluorescence and phosphorescence should begenerated statistically, achieving white light generation ifboth peak wavelengths are tuned optimally.
Acknowledgment. This work was funded by the NationalScience Council of Taiwan (Grants NSC 93-2113-M-007-012 and NSC 93-2752-M-002-002-PAE).
Supporting Information Available: X-ray crystallographic datafile (CIF) of complexes4 and5 and the complete list of authorsfor ref 15. This material is available free of charge via the Internetat http://pubs.acs.org.
IC0505347
(30) Hay, P. J.J. Phys. Chem. A2002, 106, 1634.
(31) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E.J.Am. Chem. Soc.2001, 123, 4304. (b) Zalis, S.; Farrell, I. R.; Vlcek,A. J. Am. Chem. Soc.2003, 125, 4580. (c) Tamayo, A. B.; Alleyne,B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.;Thompson, M. E.J. Am. Chem. Soc.2003, 125, 7377. (d) Tsuboyama,A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.;Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno,K. J. Am. Chem. Soc.2003, 125, 12971. (e) Song, Y.-H.; Yeh, S.-J.;Chen, C.-T.; Chi, Y.; Liu, C.-S.; Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.;Peng, S.-M.; Lee, G.-H.AdV. Funct. Mater. 2004, 14, 1221. (f)Kavitha, J.; Chang, S.-Y.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.;Peng, S.-M.; Lee, G.-H.; Tao, Y.-T.; Chien, C.-H.; Carty, A. J.AdV.Funct. Mater.2005, 15, 223.
Jπ,π* ∝ ⟨π(1)π*(2)|π(2)π*(1)⟩ (6)
Jdπ,π* ∝ ⟨dπ(1)π*(2)|dπ(2)π*(1)⟩ (7)
8-Quinolinolate Os(II) Carbonyl Complexes
Inorganic Chemistry, Vol. 44, No. 13, 2005 4603
