Articles

Switching on the Phosphorescence of Pyrene by Cycloplatination

Jian Hu,‡ John H. K. Yip,*,‡ Dik-Lung Ma,† Kwok-Yin Wong,† and Wai-Hong Chung†

Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore, 117543, andDepartment of Applied Biology and Chemical Technology, The Hong Kong Polytechnic UniVersity,

Hung Hom, Kowloon, Hong Kong SAR, People’s Republic of China

ReceiVed May 8, 2008

1-Diphenylphosphinopyrene (1-PyP) and 1,6-bis(diphenylphosphino)pyrene (1,6-PyP) can be metalatedat C5 and C10 to give cyclometalated complexes [M(L)(1-PyP-H)]+ and [M2(L)2(1,6-PyP)]2+ (M ) Pdor Pt, L ) diphosphines). The π f π* transitions of the pyrenyl ring are strongly perturbed by the PPh2

groups at C1/6, while the perturbation of the metal ions at C5/10 is weak. The phosphorescence of thepyrenyl ring is strongly enhanced by the heavy atom effect of the Pt ion up to a quantum yield of 1.5 ×10-2. Direct coordination of the Pt ion to the pyrenyl ring is needed for enhancement of thephosphorescence.

Introduction

Pyrene has been commonly used as a fluorescent probebecause of its intense fluorescence,1 excimeric formation,1,2 andfluorescence anisotropy.3 Most of the applications of pyrenefocus on its lowest energy singlet state (S1), but the correspond-ing triplet excited state (T1) and its phosphorescence have rarelybeen harnessed in chemical sensing and photochemicalreactions.4a A major obstacle is the low quantum yield of T1

formation, as the S1f T1 intersystem crossing is spin-forbidden.Furthermore, because of the strong exchange interactions, theS1 and T1 states are widely separated, resulting in a largeFranck-Condon barrier for the intersystem crossing. Heavyatoms with their large spin-orbit coupling are capable of

enhancing the phosphorescence by mixing the spin parentageof the excited states.1,5 Many studies focused on the external(intermolecular) heavy atom effect of metal ions, solventmolecules, or surfactants.4b,6 However, a more direct, effectiveway to induce the emission is to attach heavy metal atomscovalently to the organic ring.4a,7 Recently, Gray et al. havedemonstrated gold(I) ion can induce the phosphorescence ofpyrene.8 Our recent work showed that both Au(I) and Pt(II)ions can increase Φp up to 3.9 × 10-3, and the effect of themetal ions is additive and position-dependent. For example, ourresults showed that 1,8-dimetalated pyrenes are less phospho-rescent than the 1,6-metalated isomers.9

Bromopyrenes or their corresponding lithiated pyrenes wereemployed in the syntheses of the Au(I) and Pt(II) complexes.8-10

A major limitation of this method is that bromination of pyrenehappens only at the C1, C3, C6, and C8 positions, andconsequently only these positions have been metalated so far.

* To whom correspondence should be addressed. E-mail: chmyiphk@nus.edu.sg. Fax: 65-67791691.

‡ National University of Singapore.† The Hong Kong Polytechnic University.(1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:

London, 1970.(2) (a) Winnik, F. M. Chem. ReV. 1993, 93, 587. (b) Yang, R.-H.; Chan,

W.-H.; Lee, A. W. M.; Xia, P.-F.; Zhang, H.-K.; Li, K. A. J. Am. Chem.Soc. 2003, 125, 2884. (c) Langenegger, S. M.; Haner, R. Chem. Commun.2004, 2792. (d) Nishizawa, S.; Kato, Y.; Teramae, N. J. Am. Chem. Soc.1999, 121, 9463. (e) Bodenant, B.; Fages, F.; Delville, M.-H. J. Am. Chem.Soc. 1998, 120, 7511. (f) Kim, S. K.; Lee, S. H.; Lee, J. Y.; Lee, J. Y.;Bartsch, R. A.; Kim, J. S. J. Am. Chem. Soc. 2004, 124, 16499. (g) Moon,S.-Y.; Youn, N. J.; Park, S. M.; Chang, S.-K. J. Org. Chem. 2005, 70,2394. (h) Martınez, R.; Espinosa, A.; Tarraga, A.; Molina, P. Org. Lett.2005, 7, 5869. (i) Fujimoto, K.; Muto, Y.; Inouye, M. Chem. Commun.2005, 4780. (j) Okamoto, A.; Ichiba, T.; Saito, I. J. Am. Chem. Soc. 2004,126, 8364. (k) Rogers, C. W.; Wolf, M. O. Angew. Chem., Int. Ed. 2002,41, 1898. (l) Matkovich, K. M.; Thorne, L. M.; Wolf, M. O.; Pace, T. C. S.;Bohne, C.; Patrick, B. O. Inorg. Chem. 2006, 45, 4610.

(3) (a) Shyamala, T.; Sankararaman, S.; Mishra, A. K. Chem. Phys. 2006,330, 469. (b) Sigman, M. E.; Read, S.; Barbas, J. T.; Ivanov, I.; Hagaman,E. W.; Buchanan, A. C., III; Dabestani, R.; Kidder, M. K.; Britt, P. F. J.Phys. Chem. A 2003, 107, 3450. (c) Sharma, J.; Tleugabulova, D.;Czardybon, W.; Brennan, J. D. J. Am. Chem. Soc. 2006, 128, 5496.

(4) (a) Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.;Castellano, F. N. Inorg. Chem. 2003, 42, 1394. (b) Omary, M. A.; Kassab,R. M.; Haneline, M. R.; Elbjeirami, O.; Gabbaı, F. P. Inorg. Chem. 2003,42, 2176.

(5) (a) Kessinger, M.; Michl, J. Excited States and Photochemistry ofOrganic Molecules; Wiley-VCH: New York, 1995. (b) Suzuki, H. ElectronicAbsorption Spectra and Geometry of Organic Molecules: an Applicationof Molecular Orbital Theory; Academic Press: New York, 1967. (c) Turro,N. J. Modern Molecular Photochemistry; Benjamin: Menlo Park, NJ, 1978.

(6) (a) Blatt, E.; Launikonis, A.; Mau, A. W.-H.; Sasse, W. H. F. Aust.J. Chem. 1987, 40, 1. (b) Humphry-Baker, R.; Moroi, Y.; Gratzel, M. Chem.Phys. Lett. 1978, 58, 207. (c) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.;Kuo, E. W.; Corbin, D. R. J. Am. Chem. Soc. 1992, 114, 3882. (d)Mohamed, A. A.; Rawashdeh-Omary, M. A.; Omary, M. A.; Fackler, J. P.,Jr. Dalton Trans. 2005, 2597. (e) Burress, C.; Elbjeirami, O.; Omary, M. A.;Gabbaı, F. P. J. Am. Chem. Soc. 2005, 127, 12166.

(7) (a) Che, C.-M.; Chao, H.-Y.; Miskowski, V. M.; Li, Y.; Cheung,K.-K. J. Am. Chem. Soc. 2001, 123, 4985. (b) Hong, X.; Cheung, K.-K.;Guo, C.-X.; Che, C.-M. J. Chem. Soc., Dalton Trans. 1994, 1867. (c) Li,D.; Hong, X.; Che, C.-M.; Lo, W.-C.; Peng, S.-M. Chem. Soc., Dalton Trans.1993, 2929. (d) Chao, H.-Y.; Lu, W.; Li, Y.; Chan, M. C. W.; Che, C.-M.;Cheung, K.-K.; Zhu, N. J. Am. Chem. Soc. 2002, 124, 14696. (e) Osawa,M.; Hoshino, M.; Akita, M.; Wada, T. Inorg. Chem. 2005, 44, 1157.

(8) Partyka, D. V.; Esswein, A. J.; Zeller, M.; Hunter, A. D.; Gray, T. G.Organometallics 2007, 26, 3279.

(9) Heng, W. Y.; Hu, J.; Yip, J. H. K. Organometallics 2007, 26, 6760.(10) (a) Yam, V. W.-W.; Choi, S. W.-K.; Cheung, K.-K. J. Chem. Soc.,

Dalton Trans. 1996, 3411. (b) Weisemann, C.; Schmidtberg, G.; Brune,H.-A. J. Organomet. Chem. 1989, 365, 403.

Organometallics 2009, 28, 51–59 51

10.1021/om800410m CCC: $40.75 2009 American Chemical SocietyPublication on Web 11/26/2008

Our previous studies showed that Pt(II) ions can be anchoredon an anthracene ring via facile cyclometalation betweenPtL(OTf)2 [L ) bis(diphenylphosphino)methane (dppm)] and9,10-bis(diphenylphosphino)anthracene.11a Demonstrated in thiswork is how cyclometalation allows C5 and C10 to be metalated.Our method started with the syntheses of the ligands 1-diphe-nylphoshinopyrene (1-PyP) and 1,6-bis(diphenylphosphino)py-rene (1,6-PyP), which possess PPh2 groups at C1/C6 (Scheme1). Coordination of Pd(dppe)(OTf)2 (dppe ) bis(diphenylphos-phino)ethane) and Pt(dppm)(OTf)2 to the phosphines woulddirect the attack of the metal ions at the C-H bonds at the C5and C10 positions, leading to the cyclometalated complexes[Pt(dppm)(1-PyP-H)](OTf) (Pt), [Pd(dppe)(1-PyP-H)](OTf)(Pd), [Pt2(dppm)2(1,6-PyP-H2)](OTf)2 (Pt2), and [Pd2(dppe)2(1,6-PyP-H2)](OTf)2 (Pd2) (Scheme 1). Like many cycloplatinatedcomplexes,11 the present complexes are emissive. The com-plexes [Pt(dppm)Cl(1-PyP)](OTf) (PtCl) and [Pt2(dppm)2Cl2(1,6-PyP)](OTf) (Pt2Cl2), which consist of dangling Pt ions, werealso prepared for comparison. Our results not only showeddrastic enhancement of the phosphorescence by internal heavyatom effect but also provide insights into the interactionsbetween the metal ions and pyrene.

Experimental Section

General Methods. All syntheses were carried out in a dry N2

atmosphere using standard Schlenk techniques unless otherwisestated. All reagents and HPLC grade solvents for syntheses were

used as received. Solvents used for spectroscopic measurementswere purified according to the literature procedures. 1-Bromopy-rene12 and 1,6-dibromopyrene9 were prepared according to literaturemethods. Bis(diphenylphosphino)methane (dppm), 1,2-bis(diphe-nylphosphino)ethane (dppe), and chlorodiphenylphosphine wereobtained from Aldrich. Pt(dppm)Cl2 and Pd(dppe)Cl2 was preparedby substitution of MeCN by dppm from PtCl2(MeCN)2 andPdCl2(MeCN)2, respectively.13 Pt(dppm)(OTf)2 and Pd(dppe)(OTf)2

were prepared in situ based on modified literature procedures.13

Physical Measurements. NMR experiments were performed ona Bruker ACF 300 spectrometer. All chemical shifts (δ) are reportedin ppm and coupling constants (J) in Hz. 1H NMR chemical shiftsare relative to SiMe4, and the resonance of residual protons ofsolvents was used as an internal standard. 31P{1H} NMR chemicalshifts were relative to 85% H3PO4. The UV-vis absorption andemission spectra of the complexes were recorded on a ShimadzuUV-1601 UV-visible spectrophotometer and a Perkin-ElmerLS50B luminescence spectrophotometer, respectively. Emissionlifetime measurements were performed with a Quanta Ray DCR-3pulsed Nd:YAG laser system (excitation wavelength ) 355 nm,pulse width ) 8 ns). The decays were analyzed by means ofDataStation software v2.3. The emission quantum yields weremeasured with rhodamine 101 perchlorate as standard. Error limitswere estimated as follows: λ ((1 nm); τ ((10%); Φ ((10%).Deaerated solutions used for emission spectra and lifetime measure-ments were obtained by four freeze-pump-thaw cycles. Elementalanalyses of the complexes were carried out at the Chemical,Molecular and Materials Analysis Centre (CMMAC), NationalUniversity of Singapore.

Synthesis of 1-(Diphenylphosphino)pyrene (1-PyP). The com-pound was synthesized following literature methods.14 1-Bromopy-rene (6.26 g, 22.3 mmol) was dissolved in 150 mL of freshlydistilled diethyl ether, and the solution was kept at 0 °C. Butyllithium (16.7 mL of 1.6 M solution in hexane, 26.7 mmol) wasadded dropwise to the solution, and the mixture was stirred for 10min before addition of chlorodiphenylphosphine (4.00 mL, 22.3mmol). The resulting mixture was stirred for 12 h at roomtemperature. A pale yellow solid was filtered and washed with waterand diethyl ether; it was then dried under vacuum and stored underN2. Yield: 6.00 g, 70%. 1H NMR (CDCl3, 300 MHz, δ/ppm): 8.77(dd, 3JH-H ) 9 Hz, 4JH-P ) 5 Hz, 1H, H10), 8.21-8.18 (m, 2H, H6

and H8), 8.12-7.99 (m, 5H, H3,4,5,7,9), 7.56 (dd, 3JH-H ) 8 Hz, 3JH-P

) 4 Hz, 1H, H2) and 7.35-7.34 (m, 10H, phenyl). 31P{1H} NMR(CDCl3, 121.5 MHz, δ/ppm): -13.85 (s).

Synthesis of 1,6-Bis(diphenylphosphino)pyrene (1,6-PyP). 1,6-Dibromopyrene (3.38 g, 9.39 mmol) was suspended in freshlydistilled diethyl ether (100 mL) and kept at 0 °C. Butyl lithium(14.1 mL of 1.6 M solution in hexane, 22.56 mmol) was then addeddropwise to the suspension, which was then stirred for 10 min beforethe addition of chlorodiphenylphosphine (4.14 g, 9.39 mmol). Theresulting mixture was stirred for 12 h at room temperature. Thepale yellow solid obtained was then filtered and washed with water,methanol, and diethyl ether. The product was further purified bycolumn chromatography on silica gel with CH2Cl2 as the eluent. Itwas dried under vacuum and stored under N2. Yield: 4.00 g, 75%.Melting point: 278-282 °C. Anal. Calcd (%) for C, 84.20; H, 4.95.Found (%): C, 84.35; H, 5.01. 1H NMR (CDCl3, 300 MHz, δ/ppm):8.81 (dd, 3JH-H ) 9 Hz, 4JH-P ) 5 Hz, 2H, H5,10), 8.07-8.01 (m,4H, H3,8 and H4,9), 7.56 (dd, 3JH-H ) 8 Hz, 3JH-P ) 4 Hz, 2H,

(11) (a) Hu, J.; Lin, R.; Yip, J. H. K.; Wong, K.-Y.; Ma, D.-L.; Vittal,J. J. Organometallics 2007, 26, 6533. (b) Kui, S. C. F.; Chui, S. S.-Y.;Che, C.-M.; Zhu, N. J. Am. Chem. Soc. 2006, 128, 8297. (c) Kui, S. C. F.;Sham, I. H. T.; Cheung, C. C. C.; Ma, C.-W.; Yan, B.; Zhu, N.; Che, C.-M.; Fu, W.-F. Chem.-Eur. J. 2007, 13, 417. (d) Yip, J. H. K.; Suwarno,Vittal, J. J. Inorg. Chem. 2000, 39, 3537. (e) Williams, J. A. G.; Beeby,A.; Davies, E. S.; Weinstein, J. A.; Wilson, C. Inorg. Chem. 2003, 42,8609. (f) Ma, B.; Djurovich, P. I.; Thompson, M. E. Coord. Chem. ReV.2005, 249, 1501. (g) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky,S.; Thompson, M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622. (h)Song, D.; Wu, Q.; Hook, A.; Kozin, I.; Wang, S. Organometallics 2001,20, 4683. (i) Bai, D.-R.; Wang, S. Organometallics 2006, 25, 1517. (j)Ionkin, A. S.; Marshall, W. J.; Wang, Y. Organometallics 2005, 24, 619.(k) Koo, C.-K.; Lam, B.; Leung, S.-K.; Lam, M. H.-W.; Wong, W.-Y. J. Am.Chem. Soc. 2006, 128, 16434. (l) Koo, C.-K.; Ho, Y.-M.; Chow, C.-F.;Lam, M. H.-W.; Lau, T.-C.; Wong, W.-Y. Inorg. Chem. 2007, 46, 3603.(m) Fernandez, S.; Fornies, J.; Gil, B.; Gomez, J.; Lalinde, E. Dalton Trans.2003, 822.

(12) Vyas, P. V.; Bhatt, A. K.; Ramachandraiah, G.; Bedekar, A. V.Tetrahedron Lett. 2003, 44, 4085.

(13) (a) Davies, J. A.; Hartley, F. R.; Murray, S. G. J. Chem. Soc., DaltonTrans. 1979, 11, 1705. (b) Fallis, S.; Anderson, G. K.; Rath, N. P.Organometallics 1991, 10, 3180.

(14) (a) Muller, T. E.; Green, J. C.; Mingos, D. M. P.; Mcpartlin, C. M.;Whittingham, C.; Williams, D. J.; Woodroffe, T. M. J. Organomet. Chem.1998, 551, 313. (b) Yip, J. H. K.; Prabhavathy, J. Angew. Chem., Int. Ed.2001, 40, 2159.

Scheme 1

52 Organometallics, Vol. 28, No. 1, 2009 Hu et al.

H2,7), 7.35-7.34 (m, 20H, phenyl). 31P{1H} NMR (CDCl3, 121.5MHz, δ/ppm): -13.72 (s). EI-MS: m/z(relative abundance)570.2(10%) [M]+, 385(40%) [M - PPh2]+.

Synthesis of [Pt(dppm)(1-PyP-H)](OTf) (Pt). Pt(dppm)Cl2 (228mg, 0.35 mmol) and AgOTf (180 mg, 0.70 mmol) were mixed inCH2Cl2(40 mL)/CH3CN(20 mL) and stirred for 1 day in the dark.The resulting mixture was filtered, and 1-PyP (135 mg, 0.35 mmol)was added to the filtrate. The mixture was stirred for 1 day, followedby filtration. The filtrate was concentrated to ∼5 mL by rotaevapo-ration, followed by adding of excess diethyl ether to precipitatethe pale yellow product, which was filtered, washed with diethylether, and dried under vacuum. The product was purified byrecrystallization from a CH2Cl2/Et2O solution. Yield: 0.19 g, 50%.Anal. Calcd (%) for C54H40F3O3P3PtS: C, 58.22; H, 3.62. Found(%): C, 57.88; H, 3.35. 1H NMR (CDCl3, 300 MHz): δ/ppm8.32-7.81 (unresolved m, 9H, pyrenyl H’s), 7.58-7.16 (m, 30H,phenyl rings), 5.20-5.13 (m, 2H, CH2). 31P{1H} NMR (CDCl3,121.5 MHz): δ/ppm 44.41 (dd, P1), -25.93 (dd, P2), -33.23 (dd,P3); 1JP1-Pt ) 2800 Hz, 1JP2-Pt ) 2419 Hz, 1JP3-Pt ) 1442 Hz;2JP1-P2 ) 362 Hz, 2JP1-P3 ) 15 Hz, 2JP2-P3 ) 42 Hz. ESI-MS (m/z): 964.5 [M - OTf]+.

Synthesis of [Pd(dppe)(1-PyP-H)](OTf) (Pd). AgOTf (179 mg,0.696 mmol) was added to a 30 mL CH2Cl2 solution of Pd(dppe)Cl2

(182 mg, 0.316 mmol). The mixture was stirred for 21 h, and AgClwas removed by filtration. To the filtrate was added 1-PyP (122mg, 0.316 mmol), and the mixture was stirred for 1 day. Theresulting mixture was filtered and concentrated. Addition of excessEt2O precipitated the pale yellow product. Single crystals suitablefor X-ray diffraction were grown by slow diffusion from CH2Cl2/Et2O. Yield: 0.28 g, 85%. Anal. Calcd (%) for C55H42F3O3P3PdS:C, 63.56; H, 4.07. Found (%): C, 63.25; H, 4.19. 1H NMR (CDCl3,300 MHz): unresolved multiplets, 8.18-7.19 (4H), 7.96-7.81 (6H),7.66-7.53 (7H), 7.43-7.37 (5H), 7.30-7.19 (16H); 2.78-2.58 (m,4H, PCH2CH2P). 31P{1H} NMR (CDCl3, 121.5 MHz): δ/ppm 58.81(P1), 55.51 (P2), 41.45 (P3). ABX system, 2JP1-P2 ) 333 Hz, 2JP1-P3

) 30 Hz, JP2-P3 ) 30 Hz. ESI-MS (m/z): 889.4 [M - OTf]+.Synthesis of [Pt2(dppm)2(1,6-PyP-H2)](OTf)2 (Pt2). AgOTf (95

mg, 0.370 mmol) was added into a 50 mL CH2Cl2 solution ofPt(dppm)Cl2 (120 mg, 0.185 mmol), and the mixture was stirredfor 6 h in the dark. The resulting mixture was filtered, and 1,6-PyP(53 mg, 0.093 mmol) was added to the filtrate. The mixture wasstirred for 36 h, and the product was then precipitated by addingEt2O to the concentrated reaction solution. The product was purifiedby recrystallization from CH2Cl2/Et2O by slow diffusion. Yield:152 mg, 94%. Anal. Calcd (%) for C92H70F6O6P6Pt2S2 · 2CH2Cl2:C, 51.42; H, 3.40. Found (%): C, 51.45; H, 3.52. 1H NMR (CD3CN,300 MHz): δ/ppm 8.29-8.25, 8.06-8.00, 7.96-7.89, 7.61-7.16,unresolved multiples; 5.10-5.03 (m, 4H, CH2). 31P{1H} NMR(CD3CN, 121.5 MHz, δ/ppm): 46.50 (dd, P1), -24.46 (dd, P2),-31.92 (dd, P3); 1JP1--Pt ) 2812 Hz, 1JP2-Pt ) 2418 Hz, 1JP3-Pt )1434 Hz; 2JP1-P2 ) 347 Hz, 2JP1-P3 ) 15 Hz, 2JP2-P3 ) 42 Hz.ESI-MS (m/z): 863.8 [M - 2OTf]2+.

Synthesis of [Pd2(dppe)2(1,6-PyP-H2)](OTf)2 (Pd2). AgOTf(197 mg, 0.767 mmol) was added to a 30 mL CH2Cl2 solution ofPd(dppe)Cl2 (201 mg, 0.349 mmol), and the mixture was stirredfor 1 day in the dark. Subsequently, the resulting mixture wasfiltered. 1,6-PyP (101 mg, 0.177 mmol) was added to the filtrate,and the mixture was stirred for 36 h. The resulting solution wasconcentrated to ∼5 mL, and excess Et2O was added to precipitatethe product as a pale yellow solid. The product was purified byrecrystallization from CH2Cl2/Et2O. Yield: 0.32 g, 96%. Anal. Calcd(%) for C94H74F6O6P6Pd2S2: C, 60.17; H, 3.97. Found (%): C, 59.70;H, 4.22. 1H NMR (CD2Cl2, 300 MHz): 8.06-8.00 (m, 2H, H2,7 ofpyrenyl ring), 7.89-7.83 (m, 8H, Ph), 7.73-7.67 (m, 2H, H3, 8 ofpyrenyl ring), 7.65-7.60 (m, 4H, Ph on P1), 7.55-7.51 (m, 8H,Ph), 7.44-7.35 (m, 8H, Ph), 7.28-71.8 (m, 34H, H4, 9 of pyrenylring, Ph), 2.61-2.46 (m, 8H, CH2). 31P{1H} NMR (CD2Cl2, 121.5

MHz): δ/ppm 58.83 (P1), 55.57 (P2), 42.09 (P3). ABX system,2JP1-P2 ) 333 Hz, 2JP1-P3 ) 30 Hz, JP2-P3 ) 30 Hz. ESI-MS (m/z):789.8 [M - 2OTf]2+.

[Pt(dppm)Cl(1-PyP)]OTf (PtCl). To a solution of AgOTf (107mg, 0.416 mmol) in a mixture of CH3CN (25 mL) and CH2Cl2 (25mL) was added Pt(dppm)Cl2 (271 mg, 0.417 mmol). The mixturewas stirred for 2 h in the dark, followed by filtration. 1-PyP (161mg, 0.416 mmol) was added to the filtrate, and the solution wasstirred for 1 h before being filtered. The filtrate was concentratedby rotaevaporation, and excess Et2O was added to precipitate thepale yellow product. Single crystals suitable for X-ray diffractionwere obtained from CH2Cl2/Et2O. Yield: 110 mg, 86%. Anal. Calcd(%) for C54H41ClF3O3P3PtS · CH2Cl2: C, 53.47; H, 3.51. Found (%):C, 53.79; H, 3.40. 1H NMR (CDCl3, 300 MHz): δ/ppm 8.68 (dd,3JH-H ) 9 Hz, 3JH-P ) 1.7 Hz, 1H, H10 of pyrene ring), 8.35-7.94(m, 10H, unresolved), 7.75-7.69 (m, 2H unresolved), 7.49-7.21(m, 26H, unresolved), 5.23-4.99 (m, 2H, CH2). 31P{1H} NMR(CDCl3, 121.5 MHz): 17.72 (dd, P1), -52.34 (dd, P2), -54.07 (dd,P3); 1J(P1-Pt) ) 2345 Hz, 1J(P2-Pt) ) 2090 Hz, 1J(P3-Pt) ) 3031Hz; 2J(P1-P2) ) 423 Hz, 2J(P1-P3) ) 23 Hz, J(P2-P3) ) 65 Hz.ESI-MS (m/z): 1001.3(100%) [M - OTf]+.

[Pt(dppm)Cl]2(1,6-PyP)(OTf)2 (Pt2Cl2). It was prepared by thesame procedure for PtCl, except that 0.5 equiv of 1,6-PyP wasused instead of 1-PyP. Yield: 0.39 g, 90%. Anal. Calcd (%) forC92H82Cl2F6O6P6Pt2S2: C, 52.66; H, 3.94. Found (%): C, 52.46; H,3.58. 1H NMR (CD2Cl2/CD3CN, 300 MHz): δ/ppm 8.77 (m, 2H,pyrenyl H’s), 8.17 (m, 2H, pyrenyl H’s), 7.82-7.74 (m, 4H, Ph),7.60-7.29 (m, 60H, Ph and pyrenyl H’s), 4.79-4.72 (m, 4H, CH2).31P{1H} NMR (CD2Cl2/CD3CN, 121.5 MHz): δ/ppm 18.76 (dd,P1), -50.88 (dd, P2), -53.07 (dd, P3); 2J(P1-P2) ) 403 Hz,1J(P1-P3) ) 40 Hz, 2J(P2-P3) ) 65 Hz. ESI-MS (m/z): 900.7 (30%)[M - 2(OTf)]2+, 1185.0 (100%) [M - Pt(dppm)Cl - 2(OTf)]+.

X-ray Crystallography. The diffraction experiments werecarried out on a Bruker AXS SMART CCD 3-circle diffractometerwith a sealed tube at 23 °C using graphite-monochromated Mo KRradiation (λ ) 0.71073 Å). The software used were SMART15a

for collecting frames of data, indexing reflection, and determinationof lattice parameters; SAINT15a for integration of intensity ofreflections and scaling; SADABS15b for empirical absorptioncorrection; and SHELXTL15c for space group determination,structure solution, and least-squares refinements on F2. The crystalswere mounted at the end of glass fibers and used for the diffractionexperiments. Anisotropic thermal parameters were refined for therest of the non-hydrogen atoms. The hydrogen atoms were placedin their ideal positions.

Results and Discussion

Synthesis. The ligands 1-(diphenylphosphino)pyrene (1-PyP)and 1,6-bis(diphenylphosphino)pyrene (1,6-PyP) were preparedby lithiation of 1-bromopyrene and 1,6-dibromopyrene, respec-tively, followed by the reaction with PPh2Cl. The monophos-phine ligand 1-PyP underwent facile cyclometalation withPt(dppm)(OTf)2 and Pd(dppe)(OTf)2 to give mononuclearcomplexes [Pt(dppm)(1-PyP-H)](OTf) (Pt) and [Pd(dppe)(1-PyP-H)](OTf) (Pd), respectively. Similarly, the reactions of thediphosphine ligand 1,6-PyP with Pt(II) and Pd(II) gave thedoubly cyclometalated complexes [Pt2(dppm)2(1,6-PyP-H2)]-(OTf)2 (Pt2) and Pd2(dppe)2(1,6-PyP-H2)](OTf)2 (Pd2), respec-tively (Scheme 1). The complexes were fully characterized byESI-MS, 31P{1H} NMR, and X-ray crystallography. The dan-gling complexes PtCl and Pt2Cl2 were prepared by reactingPt(dppm)(OTf)Cl with the phosphines. The complexes decom-pose slowly in diluted CH2Cl2 solution, as adventitious chlorideions in the solvent can react with the Pt ions to form the stablecis-Pt(dppm)Cl2.

Structures. The complexes were all characterized by single-crystal X-ray diffractions. The structures of Pt · CH2Cl2, Pd, Pt2,

Switching on the Phosphorescence of Pyrene Organometallics, Vol. 28, No. 1, 2009 53

Pd2 · 3CH2Cl2, PtCl · CH2Cl2, and Pt2Cl2 · 2CH3CN are depictedin Figures 1-6, respectively (see Supporting Information forselected bond lengths and angles). The structures are inaccordance with the ESI-MS and 31P{1H} NMR data. ForPt · CH2Cl2 and Pd, the metal ion is bonded to the C10 of thepyrenyl ring. On the other hand, the dicyclometalated Pt2 andPd2 · 3CH2Cl2 have two metal centers attached to the C5 andC10 of the pyrenyl ring.

For the cyclometalated complexes, all structures consist offive-membered chelate rings formed between the metal ions andthe pyrenyl ring. The formation of the chelate rings can be oneof the driving forces for the facile cyclometalation. Bothbimetallic cations of Pt2 and Pd2 display an approximate C2h

symmetry. For all complexes, the metal centers show distortedsquare-planar coordination and are coplanar with the pyrenylrings.

ThePt-C(2.051(3)-2.055(4)Å)andPt-P(2.2596(8)-2.3323(8)Å) bond lengths of Pt and Pt2 are normal.11a For Pt2, thePt(1)-P(3) (2.3262(12) Å) bond is longer than the other Pt-Pbonds (2.2848(12)-2.3169(12) Å). This is in accordance withthe fact that P(3) and P(3A) are trans to the strongly σ-donatingcarbanions. Pd and Pd2 have identical Pd-C bond lengths(2.080(5) Å). The bonds are significantly longer than those foundin related cyclopalladated complexes, in which the Pd-C bonds(1.934(14)-2.017(4)Å) were assumed to have partial multiple-bond character on account of significant Pd-C bond shorteningcompared to the sum of covalent radii (2.05-2.08 Å) of carbon(0.77 Å) and palladium (1.31 Å).16,17 The partial multiplebonding was attributed to the Pd(II)-to-aryl π back-donation.In this work, the auxiliary ligands on Pd(II) are tertiaryphosphines (dppe) instead of halogens, acetates, or acetylac-etonate, as found in those complexes with shorter Pd-C bonds,and the stronger σ-donating and π-accepting abilities of the transphosphine ligands may account for the lengthening of Pd-Cbonds. Similar lengthening of the Pd-C bond due to the

presence of trans phosphine ligands was reported.18 The Pd-Pbond lengths (2.2978(13)-2.3424(13) Å) are normal.

No π-π stacking between pyrenyl rings is observed in thecrystal of Pt · CH2Cl2. On the other hand, the cations of Pdaggregate into dimers via π-π stacking of pyrenyl rings (Figure2b), as they are parallel and partially overlap with an interplanardistance of 3.566 Å. In addition, there are possible phenyl(edge)-to-pyrenyl(face) CH · · · π interactions between neighboring ions:one of the phenyl rings of dppe of a cation is directed towardthe pyrenyl ring of an adjacent ion, and the distance betweenthe phenyl H atoms and the centroid of the pyrenyl ring(d(CH · · · π)) is 2.724 Å, which is typical for the secondaryhydrogen bonding.

(15) (a) SMART & SAINT Software Reference Manuals, Version 4.0;Siemens Energy & Automation, Inc., Analytical Instrumentation: Madison,WI, 1996. (b) Sheldrick, G. M. SADABS: a Software for EmpiricalAbsorption Corrections; University of Gottingen: Germany, 2001. (c)SHELXTL Reference Manual, Version 5.03; Siemens Energy & Automation,Inc., Analytical Instrumentation: Madison, WI, 1996.

(16) (a) Churchill, M. R.; Wasserman, H. J.; Young, G. J. Inorg. Chem.1980, 19, 762. (b) Selbin, J.; Abboud, K.; Watkins, S. F.; Gutierrez, M. A.;Fronczek, F. R. J. Organomet. Chem. 1983, 241, 259. (c) Navarro-Ranninger, C.; Zamora, F.; Lopez-Solera, I.; Monge, A.; Masaguer, J. R.J. Organomet. Chem. 1996, 505, 149. (d) Vila, J. M.; Pereira, M. T.;Ortigueira, J. M.; Torres, M. L.; Califano, S.; Lata, D.; Fernandez, J. J.;Fernandez, A. J. Organomet. Chem. 1998, 556, 31.

(17) (a) Caygill, G. B.; Steel, P. J. J. Organomet. Chem. 1987, 327,115. (b) Vila, J. M.; Gayoso, M.; Pereira, M. T.; Romar, A.; Fernandez,J. J. J. Organomet. Chem. 1991, 401, 385. (c) Vila, J. M.; Gayoso, M.;Pereira, M. T.; Rodriguez, M. C.; Ortigueira, J. M.; Thornton-Pett, M. J.Organomet. Chem. 1992, 426, 267. (d) Vila, J. M.; Gayoso, M.; Fernandez,A.; Bailey, N. A.; Adams, H. J. Organomet. Chem. 1993, 448, 233. (e)Fuchita, Y.; Yoshinaga, K.; Hanaki, T.; Kawano, H.; Kinoshita-Nagaoka,J. J. Organomet. Chem. 1999, 580, 273. (f) Phillips, I. G.; Steel, P. J. J.Organomet. Chem. 1991, 410, 1991. (g) Vila, J. M.; Gayoso, M.; Fernandez,A.; Ortigueira, J. M.; Fernandez, A.; Bailey, N. A.; Adams, H. J. Organomet.Chem. 1994, 471, 259. (h) Vila, J. M.; Gayoso, M.; Pereira, M. T.; Torres,M. L.; Fernandez, J. J.; Fernandez, A.; Rivero, B. E. J. Organomet. Chem.1996, 510, 51.

(18) (a) Vila, J. M.; Gayoso, M.; Lopez-Torres, M.; Fernandez, J. J.;Fernandez, A.; Ortigueira, J. M.; Bailey, N. A.; Adams, H. J. Organomet.Chem. 1996, 511, 129. (b) Vila, J. M.; Gayoso, M.; Pereira, M. T.;Ortigueira, J. M.; Lopez-Torres, M; Castineiras, A.; Suarez, A.; Fernandez,J. J.; Fernandez, A. J. Organomet. Chem. 1997, 547, 297.

Figure 1. ORTEP diagram of Pt · CH2Cl2 (50% thermal ellipsoids).H atoms, the anion, and solvent molecule are omitted for clarity.

Figure 2. (a) ORTEP diagram of Pd (50% thermal ellipsoids). Hatoms and the anion are omitted for clarity. (b) Packing diagramshowing the π-π stacking and edge-to-face interactions. Thethermal ellipsoids are large presumably due to the high temperature(293 K) for data collection.

54 Organometallics, Vol. 28, No. 1, 2009 Hu et al.

Due to the steric hindrance of the bulky phenyl groups onthe P atoms, no π-π stacking of pyrenyl rings is observedin the crystals of Pt2 and Pd2 · 3CH2Cl2; however, inspectionof the packing of the cations reveals extensive CH · · · πinteractions. Figure 3b shows the crystal packing of Pt2. Thecations are aligned with their pyrenyl rings being parallel toeach other. Four phenyl rings are sandwiched between thepyrenyl rings, forming edge-to-face CH · · · π interactions withd(CH · · · π) of 2.807-2.873 Å. Similar CH · · · π interactions areobserved in the crystal of Pd2 · 3CH2Cl2.

The molecular structure of PtCl · CH2Cl2 (Figure 5) shows aPt ion coordinated to one Cl- and three P atoms, one from 1-PyPand the other two from dppm. The Pt ion has a distorted square-

planar geometry, and the Pt-Cl (2.3411(12) Å)19 and Pt-Pbond lengths are normal. The Pt(1)-P(3) bond (2.2437(11) Å)is shorter than the other two Pt-P bonds (Pt(1)-P(1) 2.3619(11)Å, Pt(1)-P(2) 2.2971(12) Å) because P(3) is trans to a weakσ-donor Cl-, whereas the other two are trans to each other.The torsional angle between the Pt ion and the pyrene plane is127.50°.

The structure of Pt2Cl2 · 2CH3CN (Figure 6) shows two Ptions, each coordinated to one Cl- ion and three P atoms from

(19) Ho, K.-C.; McLaughlin, M.; McPartlin, M.; Robertson, G. ActaCrystallogr. 1982, B32, 421.

Figure 3. (a) ORTEP diagram of Pt2 (50% thermal ellipsoids). (b) Packing diagram showing edge-to-face interactions between the phenyland the pyrenyl rings. H atoms and anions are omitted for clarity.

Figure 4. ORTEP diagram of Pd2 · 3CH2Cl2 (50% thermal el-lipsoids). H atoms and solvent molecules are omitted for clarity.

Figure 5. ORTEP plot of PtCl · CH2Cl2. H atoms, anion, andsolvent molecule are omitted for clarity. Thermal ellipsoids areshown at 30% probability.

Switching on the Phosphorescence of Pyrene Organometallics, Vol. 28, No. 1, 2009 55

1,6-PyP and dppm. The two metal centers adapt an anti-orientation. The Pt(II) ion has a distorted square-planar geom-etry. The bond lengths and angles are very similar to those ofPtCl · CH2Cl2. The Pt(1)-P(3) bond (2.242(2) Å) is shorter thanthe other two Pt-P bonds (Pt(1)-P(1) 2.366(2) Å, Pt(1)-P(2)2.300(2) Å). The torsional angle between the Pt ion and thepyrene plane is 122.12°.

The 31P and 1H NMR spectra of the complexes are in accordwith their X-ray crystal structures. A detailed discussion of theNMR spectroscopy of the complexes is given in the SupportingInformation.

Electronic Spectroscopy. The UV-vis absorption spectraof the complexes and the ligands are shown in Figures 7 and 8,and the spectral data summarized in Table 2.

The absorption spectrum of pyrene displays four vibronicbands, 1Lb, 1La, 1Bb, and 1Ba, peaked at 352, 337, 274, and 243nm. The 1La, 1Bb, and 1Ba bands are very intense (εmax ) 4-8× 104 M-1cm-1),5a but the 1Lb band is very weak (εmax ) 500M-1 cm-1) because the corresponding transition is pseudoparityforbidden.1,20 The absorption spectra of the complexes and theligands show three or four intense bands. Although the bandsare broader and their vibronic features not resolved, theirpositions and intensity suggest that they are mainly pyrene-centered. Accordingly, they are labeled as I, II, III, and IV incorrespondence to the 1Ba, 1Bb, 1La, and 1Lb bands of pyrene,respectively.

The absorption bands become broadened in the spectra of1-PyP and 1,6-PyP (Figure 7). Notably, bands III of 1-PyP and1,6-PyP show red-shifts of 1900 and 3800 cm-1, respectively.In addition, band IV of 1,6-PyP appears as a peak at 390 nm.

The absorption bands of PtCl and Pt2Cl2 (Figure 7) havesimilar energies and shapes to those of the correspondingligands; especially the energies of bands III of the complexesand the ligands are very close. Notably, bands IV of the twocomplexes are very intense (PtCl: λmax ) 380 nm, εmax ) 1.23× 104 M-1 cm-1; Pt2Cl2: λmax ) 391 nm, εmax) 2.10 × 104

M-1 cm-1).Pt and Pd have very similar absorption spectra (Figure 8);

the bands of Pt are only slightly red-shifted by 300-600 cm-1

from those of Pd. In addition, the energies of the absorptionsare close to those of 1-PyP and PtCl. Bands IV of the complexes(Pt: λmax ) 384 nm, εmax ) 9.6 × 103 M-1 cm-1, Pd: λmax )387 nm, εmax ) 9.0 × 103 M-1 cm-1) appear as shoulders inthe tails of bands III and are close to that of PtCl (λmax ) 380nm, εmax) 1.23 × 104 M-1 cm-1) in energy.

Analogous to Pd and Pt complexes, there is a high resem-blance between the absorption spectra of Pt2 and Pd2 (Figure8). Both spectra show four intense bands, I (230-270 nm), II(280-320 nm), III (340-390 nm), and IV (390-400 nm), ofsimilar energy and intensity. Interestingly, band IV is moreintense than band III. Furthermore, the intensities of bands IVof the binuclear complexes are higher than those of themononuclear complexes and 1,6-PyP. Notably, the absorptionenergies of Pt2, Pd2, and 1,6-PyP are rather close.

The absorption bands of all the complexes are not sensitiveto the polarity of the solvent, as no significant difference isobserved when the solvent is changed from CH2Cl2 to CH3CN.This is consistent with the fact that the transitions are mainlyligand centered. The metal-to-ligand charge-transfer transitions(ε ≈ 103 M-1 cm-1) cannot be located, probably being coveredby the intense ligand-centered transition.

Perturbations of Substituents. A substituent can perturb thefrontier orbitals of pyrene by its inductive effect, its π-accepting/donating ability, or hyperconjugation. The four absorption bandsof pyrene arise from electronic transitions from the HOMO-1(b2g) and the HOMO (b3g) to the LUMO (au) and the LUMO+1(b1u). The HOMO f LUMO (1Ag f 1B3u) transition gives riseto the 1La band, while the HOMO-1 f LUMO (1Ag f 1B2u)and HOMO f LUMO+1 transitions lead to two degenerate1B2u states, which mix strongly, resulting in a high-energy 1Bb

state and a 1Lb state that is lower in energy than the 1La state.However, the 1Lb band is very weak due to the forbidden natureof the corresponding transition. Our recent study demonstratedthat substituting pyrene with Au(I) or Pt(II) ions at C1, C6, orC8 led to red-shifts of the absorptions and an increase in theintensity of the 1Lb band up to 2.2-33 × 103 M-1 cm-1.9 The

(20) (a) Clar, E. Spectrochim. Acta 1950, 4, 116. (b) Yoshinaga, T.;Hiratsuka, H.; Tanzaki, Y. Bull. Chem. Soc. Jpn. 1977, 50, 3096.

Figure 6. Molecular diagram of Pt2Cl2 · 2CH3CN (50% thermalellipsoids). H atoms, anions, and solvent molecules are omitted forclarity.

Figure 7. UV-vis absorption spectra of 1,6-PyP (red), 1-PyP (blue),Pt2Cl2 (green), PtCl (orange), and pyrene (black) in CH2Cl2.

Figure 8. UV-vis absorption spectra of Pt2 (red), Pd2 (blue), Pt(green), and Pd (black).

56 Organometallics, Vol. 28, No. 1, 2009 Hu et al.

latter is due to the uplifting of the degeneracy of the 1B2u statesand mixing of the 1La and 1Lb states due to lowering ofsymmetry. The red-shift of the 1La band is particularly pro-nounced and can be used to gauge the extent of the perturba-tions, as it is closely related to a change in the energy gapbetween the HOMO and the LUMO.

The red-shifts of band III, which corresponds to the 1La bandof the unperturbed system, displayed by 1-PyP (1900 cm-1) and1,6-PyP (3800 cm-1) indicate strong perturbation of the PPh2

group. The perturbation is additive with respect to the numberof PPh2 groups, as shown by the fact that the absorption of 1,6-PyP is shifted two times more than that of 1-PyP. Theperturbations can arise from the donation of the lone pair and/or the inductive effect of the P atom. However, the absorptionspectra of PtCl and Pt2Cl2 do not show significant shifts fromtheir corresponding ligands. As the lone-pairs of the PPh2 groupsare donated to the metal ions in the complexes, the resultsuggests that the lone-pair donation does not play a significantrole in perturbing the pyrenyl rings in the ligands. Theperturbation of the PPh2 group possibly comes from pyrene(pπ)-to-P(dπ) back-bonding and/or σ*(P-C)-π*(pyrenyl) interac-tions. It is known that σ*(Si-C)-π*(pyrenyl) interactions areresponsible for the red-shift of the absorptions of trimethylsilyl-substituted pyrenes.21

The high resemblance between the spectra of Pt and Pd, andPt2 and Pd2, is rather unexpected, as one would expect thatgiven the different electronic properties of the two metals, theextent of their perturbations should be different. The energiesof the other four bands of Pt2 and Pt are surprisingly close tothose of Pt2Cl2 and PtCl, respectively, suggesting that for thecyclometalated complexes the perturbation of the metal ions atC5/C10 is weaker than that of the coordinated PPh2 groups atC1/6. It was shown that the extent of perturbations of asubstituent is proportional to the square of the LCAO coefficient(cVi

2) of the carbon atom at the point of attachment.5a Asemiempirical calculation on pyrene shows that the cVi

2 at C1and C6 (0.12) are larger than that at C5 and C10 (0.08 in theHOMO), while similar carbon atoms have similar cVi

2 values inthe LUMO. This could be one of the reasons for the perturbationof the coordinated PPh2 groups at C1/6 being stronger than thatof the metal ions at C5/10. That the bands III (1La) of Pt2 andPt are very close in energy to those of Pd2 and Pd suggeststhat the pyrenyl ring is not strongly perturbed by the metal-ligandπ interactions. Nonetheless, the drastic increase in the intensityof band IV in the cyclometalated complexes indicates a loweringof the symmetry of the pyrene ring due to the metal-pyreneinteractions.

Luminescence. Room-temperature emission spectra of de-aerated MeCN solutions of the complexes are shown in Figures9 (PtCl and Pt2Cl2), 10 (Pd and Pd2), and 11 (Pt and Pt2), andthe emission data are shown in Table 3.

The dangling complexes show fluorescence around 410 nm.Pt2Cl2 (Φf ≈ 10-3) is more emissive than PtCl (Φf ≈ 10-5).Comparing the emission with that of pyrene suggests it arisesfrom the electronic transition from the lowest singlet excitedstate S1 (1Lb) to the ground state S0. Pt2Cl2 shows a very weakemission around 650 nm (Φp < 10-5), which could bephosphorescence from the lowest triplet excited state T1 (3Lb)to S0.

Pd and Pd2 display S1 f S0 fluorescence at 400 and 429nm, respectively. The emissions are weak, with quantum yields(Φf) of 6.6 × 10-5 and 6.3 × 10-5 for Pd and Pd2, respectively.

(21) Maeda, H.; Inoue, Y.; Ishida, H.; Mizuno, K. Chem. Lett. 2001,22, 1224.

Tab

le1.

Cry

stal

logr

aphi

cD

ata

ofth

eC

ompl

exes

Pt·C

H2C

l 2Pd

Pt2

Pd2·3

CH

2Cl 2

PtC

l·C

H2C

l 2Pt

2Cl 2·2

CH

3CN

form

ula

C55

H42

Cl 2

F 3O

3P3P

tSC

55H

42F 3

O3P

3PdS

C92

H70

F 6O

6P6P

t 2S 2

C97

H80

Cl 6

F 6O

6P6P

d 2S 2

C55

H43

Cl3

F 3O

3P3P

tSC

96H

78C

l 2F 6

N2O

6P6P

t 2S 2

crys

tsi

ze(m

m3 )

0.44

×0.

40×

0.20

0.32

×0.

20×

0.14

0.20

×0.

20×

0.10

0.60

×0.

24×

0.10

0.46

×0.

14×

0.10

0.14

×0.

10×

0.10

crys

tsy

stem

mon

oclin

ictr

iclin

ictr

iclin

ictr

iclin

ictr

iclin

ictr

iclin

icsp

ace

grou

pP

2 1/n

P1j

P1j

P1j

P1j

P1j

latti

cepa

ram

a)

13.5

802(

6)Å

,b)

17.4

404(

8)Å

,c)

21.2

316(

9)Å

a)

9.38

73(4

)Å

,b)

14.5

499(

6)Å

,c)

17.6

888(

7)Å

a)

15.5

793(

7)Å

,b)

16.7

840(

7)Å

,c)

19.0

982(

8)Å

a)

9.51

18(1

0)Å

,b)

15.8

255(

16)

Å,

c)

16.5

373(

17)

Å

a)

11.0

345(

5)Å

,b)

13.3

791(

6)Å

,c)

17.3

341(

8)Å

a)

10.9

217(

5)Å

,b)

13.3

248(

7)Å

,c)

16.9

365(

9)Å

R)

90°,

�)

106.

640(

1)°,

γ)

90°

R)

85.3

75(1

)°,

�)

82.7

11(1

)°,

γ)

80.3

32(1

)°R)

110.

538(

1)°,

�)

110.

094(

1)°,

γ)

98.7

70(1

)°R)

76.0

36(2

)°,

�)

74.3

66(2

)°,

γ)

89.6

60(2

)°R)

94.6

56(1

)°,

�)

100.

037(

1)°,

γ)

97.4

73(1

)°.

R)

69.5

66(1

)°,

�)

86.9

11(2

)°,

γ)

81.6

16(1

)°V

(Å3 )

4818

.0(4

)23

58.1

0(17

)41

35.1

(3)

2321

.7(4

)24

84.3

(2)

2285

.0(2

)Z

42

21

21

dens

ity(c

alc)

(g/c

m3 )

1.65

31.

464

1.62

71.

524

1.65

11.

585

abso

rpco

eff

(mm

-1 )

3.22

40.

596

3.61

40.

774

3.18

13.

334

F(0

00)

2384

1060

2004

1080

1228

1082

tota

lno

.of

refln

s34

091

3143

354

307

3082

917

711

1633

0in

dex

rang

es-

17e

he

16,-

15e

ke

22,-

27e

le

24-

12e

he

2,-

18e

ke

8,-

22e

le

22-

20e

he

20,-

21e

ke

21,-

24e

le

24-

12e

he

12,-

20e

ke

20,-

21e

le

21-

14e

he

12,-

17e

ke

15,-

22e

le

22-

14e

he

13,-

17e

ke

-17

,-

21′le

172θ

rang

efo

rda

taco

llect

ion

(deg

)3.

90to

55.0

02.

84to

55.0

02.

54to

55.0

02.

64to

55.0

03.

08to

55.0

03.

30to

54.9

4

inde

pre

flns

1106

7(R

int)

0.02

81)

1081

3(R

int)

0.05

00)

1894

8(R

int)

0.06

06)

1066

0(R

int)

0.06

30)

1133

8(R

int)

0.02

19)

1041

0(R

int)

0.05

04)

no.

ofpa

ram

sva

ried

634

595

1027

577

694

545

final

Rin

dice

sR

1)

0.03

02,

wR

2)

0.07

72R

1)

0.07

16,

wR

2)

0.17

04R

1)

0.04

18,

wR

2)

0.07

67R

1)

0.05

92,

wR

2)

0.13

18R

1)

0.04

10,

wR

2)

0.10

40R

1)

0.06

53,

wR

2)

0.14

31go

odne

ssof

fit(G

OF)

1.05

71.

113

0.98

21.

042

1.03

81.

026

larg

est

diff

peak

and

hole

(eÅ

-3 )

1.36

3an

d-

0.88

71.

160

and-

0.81

01.

356

and-

0.62

30.

927

and-

0.68

51.

777

and-

0.93

42.

157

and-

1.17

5

Switching on the Phosphorescence of Pyrene Organometallics, Vol. 28, No. 1, 2009 57

A weak red emission at ∼620 nm (quantum yield Φp < 10-5)is also observed for the deaerated solutions of Pd2. Due to thelow intensity of the emission, its lifetime, which lies in themicrosecond domain, cannot be determined accurately. Butthe large Stokes shift and energy of the emission suggest it isthe T1 f S0 phosphorescence.

Replacing the Pd ion with its heavy congener greatly enhancesphosphorescence. Both Pt and Pt2 show dual emissions indegassed solutions with a weak fluorescence at 370-450 nm

and strong red phosphorescence peaking at 611 nm (Pt) and627 nm for (Pt2), which corresponds to a T1 f S0 transition.The phosphorescence has a vibronic shoulder, which is separatedfrom the peak by ∼1350 cm-1, as expected for pyrenyl-centeredemission. Similar vibronic emissions are commonly observedfor π-conjugated compounds and ligands. The spin-forbiddennature of the red emission is supported by its relatively longlifetime of 31.3 µs for Pt and 63.7 µs for Pt2 and the largeStokes shift from the corresponding bands IV (∼9500 cm-1).The large energy separation between the fluorescence and thephosphorescence (∼9000 cm-1) is consistent with the strongexchange interactions in excited state, which is mainly ligand-centered. Oxygen can quench the phosphorescence: in theaerated solutions, the intensity of the phosphorescence decreases;the phosphorescence returns upon deaeration. This observationlends further support to the triplet parentage of the red emissionbecause oxygen is an efficient quencher for triplet emission.The fluorescence of Pt (Φf ) 3.2 × 10-4) is more intense thanthat of Pt2 (Φf ) 7.9 × 10-5), while the former is lessphosphorescent than the latter. Φp of Pt (4.4 × 10-3) is closeto that of diaurated and diplatinated pyrenes that contain twoPt(PEt3)2Br groups at the C1 and C6/C8 positions (Φp ) 3.9 ×10-3),9 but Φp (1.5 × 10-2) of Pt2 is highest among all themetalated pyrenes. No excimeric emission is observed for allthe complexes even at high concentrations (>10-3 M). Possiblyformation of excimer is sterically hindered by the bulkyphosphine ligands.

Intersystem crossing between singlet and triplet ππ* excitedstates is very inefficient, as there is little change in the angular

Table 2. Electronic Absorption Spectral Data of the Complexes, 1,6-PyP, 1-PyP, and Pyrene

λmax/nm (ε, × 104 M-1 cm-1)

bands

compound I II III IV

Pt 243 (9.72) 277 (4.33), 288 (4.05) 352sh (2.24), 365 (2.78) 384 (0.96)Pd 240 (9.50) 275 (5.29), 287sh (4.98) 342 (2.09), 357 (2.50), 365sh (1.45) 387 (0.90)Pt2 247 (13.11) 301 (7.11) 356sh (1.93), 373 (3.54), 379 (3.93),

386 (2.86)400 (5.21)

Pd2 252 (11.51) 303 (6.54) 352 (2.18), 367 (2.70), 377 (2.94),383 (2.38)

398 (4.18)

PtCl 238 (7.70) 274 (3.82) 285 (4.57) 350 (1.97), 357 (2.05) 367 (2.54) 380 (1.23)Pt2Cl2 250sh (8.86) 278 (6.19), 291 (5.43) 365 (1.91), 379 (2.06) 391 (2.10)1-PyP 245 (6.05) 274 (2.57), 282 (2.88) 337sh (1.92), 360 (2.58) a

1,6-PyP 249 (6.55) 282 (2.79), 289 (2.78) 370 (2.83) ∼390pyrene 243 (7.24) (1Ba) 254 (1.16), 264 (2.48),

274 (4.58) (1Bb)297 (0.45), 308 (1.17), 322 (2.88),

337 (4.48) (1La)352 (0.08), 357 (0.05),

364 (0.04), 373(0.03) (1Lb)

a Band IV is covered by band III.

Figure 9. Emission spectra of PtCl (red) and Pt2Cl2 (black) indeaerated CH3CN solution at room temperature. Excitation wave-length ) 320 nm. Filter: 350 nm cutoff.

Figure 10. Emission spectra of Pd (red) and Pd2 (black) indeaerated CH3CN solution at room temperature. Excitation wave-length ) 320 nm. Filter: 350 nm cutoff.

Figure 11. Emission spectra of deaerated MeCN solutions of Pt(red) and Pt2 (black) at room temperature. Excitation wavelength) 320 nm. Filter: 350 nm cutoff.

58 Organometallics, Vol. 28, No. 1, 2009 Hu et al.

momentum. The relatively strong phosphorescence displayedby the Pt complexes is due to the heavy atom effect of the metalion, which increases the rate of S1 f T1 intersystem crossing.It should be noted that not only the metal ions but also P andCl in the complexes are conventionally considered as heavyatoms. However, our results show that the effect of the metalions is dominating. With its large spin-orbit coupling, the Ption can enhance phosphorescence by mixing the spin parentageof the singlet and triplet excited states. The Hamiltonian operatorof the spin-orbit perturbation HSO is given as equation 1, whereZ and r are the atomic number of the heavy atom and thedistance between the nucleus of the atom and the chromophore,and li and si are angular momentum and spin operators,respectively.22

HSO )∑i

Ze2

4m2c2

1

r3lisi (1)

It is clear from eq 1 that the spin-orbit coupling increaseswith the atomic number of the perturbing atom. This explainsthe fact that the phosphorescence of the Pt complexes is muchstronger than that of the Pd complexes (Z of Pt and Pd are 78and 46, respectively). Furthermore, the effect is additive, as itincreases with the number of heavy atoms. This accounts forthe fact that the phosphorescence of Pt2 is stronger than that ofPt, and Pd2 displays weak phosphorescence despite the weakspin-orbit coupling of the metal. Equation 1 also shows thatthe heavy atom effect decreases rapidly with the distance r;however, only very weak phosphorescence is observed forPt2Cl2 despite the fact that the dangling Pt ions are not very farfrom the pyrenyl ring, as the shortest distance between the metalions and the carbon atoms of the ring is only ∼3.391 Å. Thissuggests that the metal-ligand orbital (σ and π) interactionsare crucial to the manifestation of the heavy atom effect.

Although the lifetime of the phosphorescence of the Ptcomplexes is much longer than that of fluorescence, it is verymuch shorter than that of pyrene (∼0.44 s).23 This is again dueto the spin-orbit coupling of the Pt ions, which introducessinglet character into the triplet excited state T1, making the T1

f S0 transition less spin-forbidden.

Conclusion

By comparing the electronic spectra of the cyclometalatedand dangling complexes, we deduce that the pyrenyl rings inthese complexes are mainly perturbed by the PPh2 group at C1/6. A surprising result is the similarity between the spectra ofPt and Pd or Pt2 and Pd. This suggests weak π interactionsbetween the metal ions and the pyrenyl ring. On the other hand,the forbidden 1Lb band is intensified, mostly likely due toincreased mixing between 1La and 1Lb states. The phosphores-cence of pyrene can be switched on by the heavy atom effect.The quantum yield can be as high as 1.5 × 10-2, as in the caseof Pt2. Finally, our study demonstrated that direct bonding ofthe metal ions to the pyrenyl ring is needed for efficientenhancement of the phosphorescence.

Acknowledgment. J.H.K.Y. would like to thank NationalUniversity of Singapore for financial support and Miss TanGeok Kheng and Professor Kop Lip Lin for solving the X-raycrystal structures.

Supporting Information Available: CIF files of the crystalstructures and selected bond lengths and angles, and NMR spectraof the complexes. This material is available free of charge via theInternet at http://pubs.acs.org.

OM800410M

(22) Lower, S. K.; El-Sayed, M. A. Chem. ReV. 1966, 66, 199.

(23) (a) Kellogg, R. E.; Wyeth, N. C. J. Chem. Phys. 1966, 45, 3156.(b) Kropp, J. L.; Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1969,73, 1747.

Table 3. Emission Data of the Complexes

complex fluorescence λmax (nm) Φf phosphorescence λmax (nm) Φp phosphorescence lifetime (µs)

Pt 398 3.2 × 10-4 611 4.4 × 10-3 31.3Pd 400 6.6 × 10-5

Pt2 420 7.9 × 10-5 627 1.5 × 10-2 63.7Pd2 429 6.3 × 10-5 620 <10-5 a

PtCl 404 10-5b

Pt2Cl2 408 10-3b 650 <10-5 a

a The intensity of the emission is too weak for accurate lifetime determination. b The quantum yields cannot be determined accurately, as thecomplexes tend to dissociate at concentration e 10-5 M.

Switching on the Phosphorescence of Pyrene Organometallics, Vol. 28, No. 1, 2009 59