Switching on the phosphorescence of pyrene by cycloplatination

Article abstract of DOI:10.1021/om800410m

1-Diphenylphosphinopyrene (1-PyP) and 1,6-bis(diphenylphosphino)pyrene (1,6-PyP) can be metalated at C5 and C10 to give cyclometalated complexes [M(L)(1-PyP-H)]⁺ and [M₂(L)₂(1,6-PyP)] ²⁺ (M = Pd or Pt, L = dipho

Full text of DOI:10.1021/om800410m

Organometallics 2009, 28, 51–59
51
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, and
Department 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 metalated
at C5 and C10 to give cyclometalated complexes [M(L)(1-PyP-H)]+ and [M2(L)2(1,6-PyP)]2+ (M ) Pd
or 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 the
pyrenyl 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 the
phosphorescence.
enhancing the phosphorescence by mixing the spin parentage
of the excited states.1,5 Many studies focused on the external
(intermolecular) heavy atom effect of metal ions, solvent
molecules, or surfactants.4b,6 However, a more direct, effective
way to induce the emission is to attach heavy metal atoms
covalently to the organic ring.4a,7 Recently, Gray et al. have
demonstrated gold(I) ion can induce the phosphorescence of
pyrene.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 the
metal ions is additive and position-dependent. For example, our
results showed that 1,8-dimetalated pyrenes are less phospho-
rescent than the 1,6-metalated isomers.9
Bromopyrenes or their corresponding lithiated pyrenes were
employed in the syntheses of the Au(I) and Pt(II) complexes.8-10
A major limitation of this method is that bromination of pyrene
happens only at the C1, C3, C6, and C8 positions, and
consequently only these positions have been metalated so far.
Introduction
Pyrene has been commonly used as a fluorescent probe
because of its intense fluorescence,1 excimeric formation,1,2 and
fluorescence anisotropy.3 Most of the applications of pyrene
focus on its lowest energy singlet state (S1), but the correspond-
ing triplet excited state (T1) and its phosphorescence have rarely
been harnessed in chemical sensing and photochemical
reactions.4a A major obstacle is the low quantum yield of T1
formation, as the S1 f T1 intersystem crossing is spin-forbidden.
Furthermore, because of the strong exchange interactions, the
S1 and T1 states are widely separated, resulting in a large
Franck-Condon barrier for the intersystem crossing. Heavy
atoms with their large spin-orbit coupling are capable of
* To whom correspondence should be addressed. E-mail: chmyiphk@
nus.edu.sg. Fax: 65-67791691.
‡ National University of Singapore.
† The Hong Kong Polytechnic University.
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10.1021/om800410m CCC: $40.75
2009 American Chemical Society
Publication on Web 11/26/2008

52 Organometallics, Vol. 28, No. 1, 2009
Hu et al.
Scheme 1
used as received. Solvents used for spectroscopic measurements
were purified according to the literature procedures. 1-Bromopy-
rene12 and 1,6-dibromopyrene9 were prepared according to literature
methods. Bis(diphenylphosphino)methane (dppm), 1,2-bis(diphe-
nylphosphino)ethane (dppe), and chlorodiphenylphosphine were
obtained from Aldrich. Pt(dppm)Cl2 and Pd(dppe)Cl2 was prepared
by substitution of MeCN by dppm from PtCl2(MeCN)2 and
PdCl2(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 on
a Bruker ACF 300 spectrometer. All chemical shifts (δ) are reported
in ppm and coupling constants (J) in Hz. 1H NMR chemical shifts
are relative to SiMe4, and the resonance of residual protons of
solvents was used as an internal standard. 31P{1H} NMR chemical
shifts were relative to 85% H3PO4. The UV-vis absorption and
emission spectra of the complexes were recorded on a Shimadzu
UV-1601 UV-visible spectrophotometer and a Perkin-Elmer
LS50B luminescence spectrophotometer, respectively. Emission
lifetime measurements were performed with a Quanta Ray DCR-3
pulsed Nd:YAG laser system (excitation wavelength ) 355 nm,
pulse width ) 8 ns). The decays were analyzed by means of
DataStation software v2.3. The emission quantum yields were
measured with rhodamine 101 perchlorate as standard. Error limits
were 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. Elemental
analyses of the complexes were carried out at the Chemical,
Molecular and Materials Analysis Centre (CMMAC), National
University 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 freshly
distilled diethyl ether, and the solution was kept at 0 °C. Butyl
lithium (16.7 mL of 1.6 M solution in hexane, 26.7 mmol) was
added dropwise to the solution, and the mixture was stirred for 10
min before addition of chlorodiphenylphosphine (4.00 mL, 22.3
mmol). The resulting mixture was stirred for 12 h at room
temperature. A pale yellow solid was filtered and washed with water
and diethyl ether; it was then dried under vacuum and stored under
N2. 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).
Our previous studies showed that Pt(II) ions can be anchored
on an anthracene ring via facile cyclometalation between
PtL(OTf)2 [L ) bis(diphenylphosphino)methane (dppm)] and
9,10-bis(diphenylphosphino)anthracene.11a Demonstrated in this
work 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 (Scheme
1). Coordination of Pd(dppe)(OTf)2 (dppe ) bis(diphenylphos-
phino)ethane) and Pt(dppm)(OTf)2 to the phosphines would
direct the attack of the metal ions at the C-H bonds at the C5
and 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 cycloplatinated
complexes,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, were
also prepared for comparison. Our results not only showed
drastic enhancement of the phosphorescence by internal heavy
atom effect but also provide insights into the interactions
between the metal ions and pyrene.
Synthesis of 1,6-Bis(diphenylphosphino)pyrene (1,6-PyP). 1,6-
Dibromopyrene (3.38 g, 9.39 mmol) was suspended in freshly
distilled 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 added
dropwise to the suspension, which was then stirred for 10 min before
the addition of chlorodiphenylphosphine (4.14 g, 9.39 mmol). The
resulting mixture was stirred for 12 h at room temperature. The
pale yellow solid obtained was then filtered and washed with water,
methanol, and diethyl ether. The product was further purified by
column chromatography on silica gel with CH2Cl2 as the eluent. It
was 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):
Experimental Section
General Methods. All syntheses were carried out in a dry N2
atmosphere using standard Schlenk techniques unless otherwise
stated. All reagents and HPLC grade solvents for syntheses were
(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) Ferna´ndez, S.; Fornie´s, J.; Gil, B.; Go´mez, J.; Lalinde, E. Dalton Trans.
2003, 822.
3
4
8.81 (dd, JH-H ) 9 Hz, JH-P ) 5 Hz, 2H, H5,10), 8.07-8.01 (m,
3
3
4H, H3,8 and H4,9), 7.56 (dd, JH-H ) 8 Hz, JH-P ) 4 Hz, 2H,
(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., Dalton
Trans. 1979, 11, 1705. (b) Fallis, S.; Anderson, G. K.; Rath, N. P.
Organometallics 1991, 10, 3180.
(14) (a) Mu¨ller, 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.

Switching on the Phosphorescence of Pyrene
Organometallics, Vol. 28, No. 1, 2009 53
H2,7), 7.35-7.34 (m, 20H, phenyl). 31P{1H} NMR (CDCl3, 121.5
MHz, δ/ppm): -13.72 (s). EI-MS: m/z(relative abundance)
570.2(10%) [M]+, 385(40%) [M - PPh2]+.
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 (107
mg, 0.416 mmol) in a mixture of CH3CN (25 mL) and CH2Cl2 (25
mL) was added Pt(dppm)Cl2 (271 mg, 0.417 mmol). The mixture
was stirred for 2 h in the dark, followed by filtration. 1-PyP (161
mg, 0.416 mmol) was added to the filtrate, and the solution was
stirred for 1 h before being filtered. The filtrate was concentrated
by rotaevaporation, and excess Et2O was added to precipitate the
pale yellow product. Single crystals suitable for X-ray diffraction
were obtained from CH2Cl2/Et2O. Yield: 110 mg, 86%. Anal. Calcd
(%) for C54H41ClF3O3P3PtS · CH2Cl2: C, 53.47; H, 3.51. Found (%):
Synthesis of [Pt(dppm)(1-PyP-H)](OTf) (Pt). Pt(dppm)Cl2 (228
mg, 0.35 mmol) and AgOTf (180 mg, 0.70 mmol) were mixed in
CH2Cl2(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, followed
by filtration. The filtrate was concentrated to ∼5 mL by rotaevapo-
ration, followed by adding of excess diethyl ether to precipitate
the pale yellow product, which was filtered, washed with diethyl
ether, and dried under vacuum. The product was purified by
recrystallization 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/ppm
8.32-7.81 (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,
1
C, 53.79; H, 3.40. H 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) ) 3031
Hz; 2J(P1-P2) ) 423 Hz, 2J(P1-P3) ) 23 Hz, J(P2-P3) ) 65 Hz.
ESI-MS (m/z): 1001.3(100%) [M - OTf]+.
1
1
1
P3); JP1-Pt ) 2800 Hz, JP2-Pt ) 2419 Hz, JP3-Pt ) 1442 Hz;
2JP1-P2 ) 362 Hz, JP1-P3 ) 15 Hz, JP2-P3 ) 42 Hz. ESI-MS (m/
2
2
z): 964.5 [M - OTf]+.
[Pt(dppm)Cl]2(1,6-PyP)(OTf)2 (Pt2Cl2). It was prepared by the
same procedure for PtCl, except that 0.5 equiv of 1,6-PyP was
used instead of 1-PyP. Yield: 0.39 g, 90%. Anal. Calcd (%) for
C92H82Cl2F6O6P6Pt2S2: C, 52.66; H, 3.94. Found (%): C, 52.46; H,
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 AgCl
was removed by filtration. To the filtrate was added 1-PyP (122
mg, 0.316 mmol), and the mixture was stirred for 1 day. The
resulting mixture was filtered and concentrated. Addition of excess
Et2O precipitated the pale yellow product. Single crystals suitable
for 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 of
Pt(dppm)Cl2 (120 mg, 0.185 mmol), and the mixture was stirred
for 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 was
stirred for 36 h, and the product was then precipitated by adding
Et2O to the concentrated reaction solution. The product was purified
by 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),
1
3.58. H 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 were
carried out on a Bruker AXS SMART CCD 3-circle diffractometer
with a sealed tube at 23 °C using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å). The software used were SMART15a
for collecting frames of data, indexing reflection, and determination
of lattice parameters; SAINT15a for integration of intensity of
reflections and scaling; SADABS15b for empirical absorption
correction; and SHELXTL15c for space group determination,
structure solution, and least-squares refinements on F2. The crystals
were mounted at the end of glass fibers and used for the diffraction
experiments. Anisotropic thermal parameters were refined for the
rest of the non-hydrogen atoms. The hydrogen atoms were placed
in their ideal positions.
Results and Discussion
Synthesis. The ligands 1-(diphenylphosphino)pyrene (1-PyP)
and 1,6-bis(diphenylphosphino)pyrene (1,6-PyP) were prepared
by 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 with
Pt(dppm)(OTf)2 and Pd(dppe)(OTf)2 to give mononuclear
complexes [Pt(dppm)(1-PyP-H)](OTf) (Pt) and [Pd(dppe)(1-
PyP-H)](OTf) (Pd), respectively. Similarly, the reactions of the
diphosphine ligand 1,6-PyP with Pt(II) and Pd(II) gave the
doubly 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 by
ESI-MS, 31P{1H} NMR, and X-ray crystallography. The dan-
gling complexes PtCl and Pt2Cl2 were prepared by reacting
Pt(dppm)(OTf)Cl with the phosphines. The complexes decom-
pose slowly in diluted CH2Cl2 solution, as adventitious chloride
ions in the solvent can react with the Pt ions to form the stable
cis-Pt(dppm)Cl2.
1
1
1
-31.92 (dd, P3); JP1--Pt ) 2812 Hz, JP2-Pt ) 2418 Hz, JP3-Pt
)
2
2
2
1434 Hz; JP1-P2 ) 347 Hz, JP1-P3 ) 15 Hz, JP2-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 of
Pd(dppe)Cl2 (201 mg, 0.349 mmol), and the mixture was stirred
for 1 day in the dark. Subsequently, the resulting mixture was
filtered. 1,6-PyP (101 mg, 0.177 mmol) was added to the filtrate,
and the mixture was stirred for 36 h. The resulting solution was
concentrated to ∼5 mL, and excess Et2O was added to precipitate
the product as a pale yellow solid. The product was purified by
recrystallization 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 of
pyrenyl ring), 7.89-7.83 (m, 8H, Ph), 7.73-7.67 (m, 2H, H3, 8 of
pyrenyl 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 pyrenyl
ring, Ph), 2.61-2.46 (m, 8H, CH2). 31P{1H} NMR (CD2Cl2, 121.5
Structures. The complexes were all characterized by single-
crystal X-ray diffractions. The structures of Pt · CH2Cl2, Pd, Pt2,

54 Organometallics, Vol. 28, No. 1, 2009
Hu et al.
Figure 1. ORTEP diagram of Pt · CH2Cl2 (50% thermal ellipsoids).
H atoms, the anion, and solvent molecule are omitted for clarity.
Pd2 · 3CH2Cl2, PtCl · CH2Cl2, and Pt2Cl2 · 2CH3CN are depicted
in Figures 1-6, respectively (see Supporting Information for
selected bond lengths and angles). The structures are in
accordance with the ESI-MS and 31P{1H} NMR data. For
Pt · CH2Cl2 and Pd, the metal ion is bonded to the C10 of the
pyrenyl ring. On the other hand, the dicyclometalated Pt2 and
Pd2 · 3CH2Cl2 have two metal centers attached to the C5 and
C10 of the pyrenyl ring.
For the cyclometalated complexes, all structures consist of
five-membered chelate rings formed between the metal ions and
the pyrenyl ring. The formation of the chelate rings can be one
of the driving forces for the facile cyclometalation. Both
bimetallic cations of Pt2 and Pd2 display an approximate C2h
symmetry. For all complexes, the metal centers show distorted
square-planar coordination and are coplanar with the pyrenyl
rings.
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, the
Pt(1)-P(3) (2.3262(12) Å) bond is longer than the other Pt-P
bonds (2.2848(12)-2.3169(12) Å). This is in accordance with
the fact that P(3) and P(3A) are trans to the strongly σ-donating
carbanions. Pd and Pd2 have identical Pd-C bond lengths
(2.080(5) Å). The bonds are significantly longer than those found
in 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 shortening
compared 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 multiple
bonding was attributed to the Pd(II)-to-aryl π back-donation.
In this work, the auxiliary ligands on Pd(II) are tertiary
phosphines (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 trans
phosphine ligands may account for the lengthening of Pd-C
bonds. Similar lengthening of the Pd-C bond due to the
Figure 2. (a) ORTEP diagram of Pd (50% thermal ellipsoids). H
atoms and the anion are omitted for clarity. (b) Packing diagram
showing the π-π stacking and edge-to-face interactions. The
thermal ellipsoids are large presumably due to the high temperature
(293 K) for data collection.
presence of trans phosphine ligands was reported.18 The Pd-P
bond lengths (2.2978(13)-2.3424(13) Å) are normal.
No π-π stacking between pyrenyl rings is observed in the
crystal of Pt · CH2Cl2. On the other hand, the cations of Pd
aggregate into dimers via π-π stacking of pyrenyl rings (Figure
2b), as they are parallel and partially overlap with an interplanar
distance 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 toward
the pyrenyl ring of an adjacent ion, and the distance between
the phenyl H atoms and the centroid of the pyrenyl ring
(d(CH · · · π)) is 2.724 Å, which is typical for the secondary
hydrogen bonding.
(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.; Ferna´ndez,
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.; Ferna´ndez,
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.; Ferna´ndez,
A.; Ortigueira, J. M.; Ferna´ndez, A.; Bailey, N. A.; Adams, H. J. Organomet.
Chem. 1994, 471, 259. (h) Vila, J. M.; Gayoso, M.; Pereira, M. T.; Torres,
M. L.; Ferna´ndez, J. J.; Ferna´ndez, A.; Rivero, B. E. J. Organomet. Chem.
1996, 510, 51.
(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 Empirical
Absorption Corrections; University of Go¨ttingen: 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.; Lo´pez-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.; Ferna´ndez, J. J.;
Ferna´ndez, A. J. Organomet. Chem. 1998, 556, 31.
(18) (a) Vila, J. M.; Gayoso, M.; Lo´pez-Torres, M.; Ferna´ndez, J. J.;
Ferna´ndez, 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.; Lo´pez-Torres, M; Castineiras, A.; Suarez, A.; Ferna´ndez,
J. J.; Ferna´ndez, A. J. Organomet. Chem. 1997, 547, 297.

Switching on the Phosphorescence of Pyrene
Organometallics, Vol. 28, No. 1, 2009 55
Figure 3. (a) ORTEP diagram of Pt2 (50% thermal ellipsoids). (b) Packing diagram showing edge-to-face interactions between the phenyl
and 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, and
Due to the steric hindrance of the bulky phenyl groups on
the P atoms, no π-π stacking of pyrenyl rings is observed
in the crystals of Pt2 and Pd2 · 3CH2Cl2; however, inspection
of the packing of the cations reveals extensive CH · · · π
interactions. Figure 3b shows the crystal packing of Pt2. The
cations are aligned with their pyrenyl rings being parallel to
each other. Four phenyl rings are sandwiched between the
pyrenyl rings, forming edge-to-face CH · · · π interactions with
d(CH · · · π) of 2.807-2.873 Å. Similar CH · · · π interactions are
observed in the crystal of Pd2 · 3CH2Cl2.
solvent molecule are omitted for clarity. Thermal ellipsoids are
shown at 30% probability.
planar geometry, and the Pt-Cl (2.3411(12) Å)19 and Pt-P
bond 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 weak
σ-donor Cl-, whereas the other two are trans to each other.
The torsional angle between the Pt ion and the pyrene plane is
127.50°.
The structure of Pt2Cl2 · 2CH3CN (Figure 6) shows two Pt
ions, each coordinated to one Cl- ion and three P atoms from
The molecular structure of PtCl · CH2Cl2 (Figure 5) shows a
Pt ion coordinated to one Cl- and three P atoms, one from 1-PyP
and the other two from dppm. The Pt ion has a distorted square-
(19) Ho, K.-C.; McLaughlin, M.; McPartlin, M.; Robertson, G. Acta
Crystallogr. 1982, B32, 421.

56 Organometallics, Vol. 28, No. 1, 2009
Hu et al.
The absorption spectrum of pyrene displays four vibronic
bands, 1Lb, 1La, 1Bb, and 1Ba, peaked at 352, 337, 274, and 243
1
1
1
nm. The La, Bb, and Ba bands are very intense (ꢀmax ) 4-8
5a
× 104 M-1cm-1), but the Lb band is very weak (ꢀmax ) 500
M-1 cm-1) because the corresponding transition is pseudoparity
forbidden.1,20 The absorption spectra of the complexes and the
ligands show three or four intense bands. Although the bands
are broader and their vibronic features not resolved, their
positions and intensity suggest that they are mainly pyrene-
centered. Accordingly, they are labeled as I, II, III, and IV in
1
1
1
1
1
correspondence to the Ba, Bb, La, and Lb bands of pyrene,
respectively.
The absorption bands become broadened in the spectra of
1-PyP and 1,6-PyP (Figure 7). Notably, bands III of 1-PyP and
1,6-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) have
similar energies and shapes to those of the corresponding
ligands; especially the energies of bands III of the complexes
and the ligands are very close. Notably, bands IV of the two
complexes are very intense (PtCl: λmax ) 380 nm, ꢀmax ) 1.23
× 104 M-1 cm-1; Pt2Cl2: λmax ) 391 nm, ꢀmax) 2.10 × 104
M-1 cm-1).
Figure 6. Molecular diagram of Pt2Cl2 · 2CH3CN (50% thermal
ellipsoids). H atoms, anions, and solvent molecules are omitted for
clarity.
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 absorptions
are close to those of 1-PyP and PtCl. Bands IV of the complexes
(Pt: λmax ) 384 nm, ꢀmax ) 9.6 × 103 M-1 cm-1, Pd: λmax
)
387 nm, ꢀmax ) 9.0 × 103 M-1 cm-1) appear as shoulders in
the tails of bands III and are close to that of PtCl (λmax ) 380
nm, ꢀmax) 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 (Figure
8). Both spectra show four intense bands, I (230-270 nm), II
(280-320 nm), III (340-390 nm), and IV (390-400 nm), of
similar energy and intensity. Interestingly, band IV is more
intense than band III. Furthermore, the intensities of bands IV
of the binuclear complexes are higher than those of the
mononuclear complexes and 1,6-PyP. Notably, the absorption
energies of Pt2, Pd2, and 1,6-PyP are rather close.
Figure 7. UV-vis absorption spectra of 1,6-PyP (red), 1-PyP (blue),
Pt2Cl2 (green), PtCl (orange), and pyrene (black) in CH2Cl2.
The absorption bands of all the complexes are not sensitive
to the polarity of the solvent, as no significant difference is
observed when the solvent is changed from CH2Cl2 to CH3CN.
This is consistent with the fact that the transitions are mainly
ligand centered. The metal-to-ligand charge-transfer transitions
(ꢀ ≈ 103 M-1 cm-1) cannot be located, probably being covered
by the intense ligand-centered transition.
Perturbations of Substituents. A substituent can perturb the
frontier orbitals of pyrene by its inductive effect, its π-accepting/
donating ability, or hyperconjugation. The four absorption bands
of 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 rise
Figure 8. UV-vis absorption spectra of Pt2 (red), Pd2 (blue), Pt
(green), and Pd (black).
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 of
PtCl · CH2Cl2. The Pt(1)-P(3) bond (2.242(2) Å) is shorter than
the 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 the
pyrene plane is 122.12°.
1
1
to the La band, while the HOMO-1 f LUMO (1Ag f B2u)
and HOMO f LUMO+1 transitions lead to two degenerate
1
1B2u states, which mix strongly, resulting in a high-energy Bb
1
1
state and a Lb state that is lower in energy than the La state.
However, the 1Lb band is very weak due to the forbidden nature
of the corresponding transition. Our recent study demonstrated
that substituting pyrene with Au(I) or Pt(II) ions at C1, C6, or
C8 led to red-shifts of the absorptions and an increase in the
The 31P and 1H NMR spectra of the complexes are in accord
with their X-ray crystal structures. A detailed discussion of the
NMR spectroscopy of the complexes is given in the Supporting
Information.
Electronic Spectroscopy. The UV-vis absorption spectra
of the complexes and the ligands are shown in Figures 7 and 8,
and the spectral data summarized in Table 2.
9
1
intensity of the Lb band up to 2.2-33 × 103 M-1 cm-1. The
(20) (a) Clar, E. Spectrochim. Acta 1950, 4, 116. (b) Yoshinaga, T.;
Hiratsuka, H.; Tanzaki, Y. Bull. Chem. Soc. Jpn. 1977, 50, 3096.

Switching on the Phosphorescence of Pyrene
Organometallics, Vol. 28, No. 1, 2009 57
latter is due to the uplifting of the degeneracy of the 1B2u states
and mixing of the 1La and 1Lb states due to lowering of
1
symmetry. The red-shift of the La 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 gap
between the HOMO and the LUMO.
The red-shifts of band III, which corresponds to the 1La band
of the unperturbed system, displayed by 1-PyP (1900 cm-1) and
1,6-PyP (3800 cm-1) indicate strong perturbation of the PPh2
group. The perturbation is additive with respect to the number
of PPh2 groups, as shown by the fact that the absorption of 1,6-
PyP is shifted two times more than that of 1-PyP. The
perturbations can arise from the donation of the lone pair and/
or the inductive effect of the P atom. However, the absorption
spectra of PtCl and Pt2Cl2 do not show significant shifts from
their corresponding ligands. As the lone-pairs of the PPh2 groups
are donated to the metal ions in the complexes, the result
suggests that the lone-pair donation does not play a significant
role in perturbing the pyrenyl rings in the ligands. The
perturbation 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 are
responsible for the red-shift of the absorptions of trimethylsilyl-
substituted pyrenes.21
The high resemblance between the spectra of Pt and Pd, and
Pt2 and Pd2, is rather unexpected, as one would expect that
given the different electronic properties of the two metals, the
extent of their perturbations should be different. The energies
of the other four bands of Pt2 and Pt are surprisingly close to
those of Pt2Cl2 and PtCl, respectively, suggesting that for the
cyclometalated complexes the perturbation of the metal ions at
C5/C10 is weaker than that of the coordinated PPh2 groups at
C1/6. It was shown that the extent of perturbations of a
substituent is proportional to the square of the LCAO coefficient
5a
2
(cVi ) of the carbon atom at the point of attachment.
A
2
semiempirical calculation on pyrene shows that the cVi at C1
and C6 (0.12) are larger than that at C5 and C10 (0.08 in the
HOMO), while similar carbon atoms have similar cVi2 values in
the LUMO. This could be one of the reasons for the perturbation
of the coordinated PPh2 groups at C1/6 being stronger than that
of the metal ions at C5/10. That the bands III (1La) of Pt2 and
Pt are very close in energy to those of Pd2 and Pd suggests
that the pyrenyl ring is not strongly perturbed by the metal-ligand
π interactions. Nonetheless, the drastic increase in the intensity
of band IV in the cyclometalated complexes indicates a lowering
of the symmetry of the pyrene ring due to the metal-pyrene
interactions.
Luminescence. Room-temperature emission spectra of de-
aerated MeCN solutions of the complexes are shown in Figures
9 (PtCl and Pt2Cl2), 10 (Pd and Pd2), and 11 (Pt and Pt2), and
the 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 arises
from the electronic transition from the lowest singlet excited
state S1 (1Lb) to the ground state S0. Pt2Cl2 shows a very weak
emission around 650 nm (Φp < 10-5), which could be
phosphorescence from the lowest triplet excited state T1 (3Lb)
to S0.
Pd and Pd2 display S1 f S0 fluorescence at 400 and 429
nm, 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.

58 Organometallics, Vol. 28, No. 1, 2009
Hu et al.
Table 2. Electronic Absorption Spectral Data of the Complexes, 1,6-PyP, 1-PyP, and Pyrene
λ
max/nm (ꢀ, × 104 M-1 cm-1
)
bands
compound
I
II
III
IV
384 (0.96)
387 (0.90)
400 (5.21)
Pt
Pd
Pt2
243 (9.72)
240 (9.50)
247 (13.11)
277 (4.33), 288 (4.05)
352sh (2.24), 365 (2.78)
342 (2.09), 357 (2.50), 365sh (1.45)
356sh (1.93), 373 (3.54), 379 (3.93),
386 (2.86)
275 (5.29), 287sh (4.98)
301 (7.11)
Pd2
252 (11.51)
303 (6.54)
352 (2.18), 367 (2.70), 377 (2.94),
383 (2.38)
350 (1.97), 357 (2.05) 367 (2.54)
365 (1.91), 379 (2.06)
337sh (1.92), 360 (2.58)
370 (2.83)
398 (4.18)
380 (1.23)
PtCl
238 (7.70)
274 (3.82) 285 (4.57)
278 (6.19), 291 (5.43)
274 (2.57), 282 (2.88)
282 (2.79), 289 (2.78)
254 (1.16), 264 (2.48),
274 (4.58) (1Bb)
Pt2Cl2
1-PyP
1,6-PyP
pyrene
250sh (8.86)
245 (6.05)
391 (2.10)
a
249 (6.55)
∼390
243 (7.24) (1Ba)
297 (0.45), 308 (1.17), 322 (2.88),
352 (0.08), 357 (0.05),
364 (0.04), 373
(0.03) (1Lb)
337 (4.48) (1La)
a Band IV is covered by band III.
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.
Figure 9. Emission spectra of PtCl (red) and Pt2Cl2 (black) in
deaerated CH3CN solution at room temperature. Excitation wave-
length ) 320 nm. Filter: 350 nm cutoff.
and strong red phosphorescence peaking at 611 nm (Pt) and
627 nm for (Pt2), which corresponds to a T1 f S0 transition.
The phosphorescence has a vibronic shoulder, which is separated
from the peak by ∼1350 cm-1, as expected for pyrenyl-centered
emission. Similar vibronic emissions are commonly observed
for π-conjugated compounds and ligands. The spin-forbidden
nature of the red emission is supported by its relatively long
lifetime of 31.3 µs for Pt and 63.7 µs for Pt2 and the large
Stokes shift from the corresponding bands IV (∼9500 cm-1).
The large energy separation between the fluorescence and the
phosphorescence (∼9000 cm-1) is consistent with the strong
exchange interactions in excited state, which is mainly ligand-
centered. Oxygen can quench the phosphorescence: in the
aerated solutions, the intensity of the phosphorescence decreases;
the phosphorescence returns upon deaeration. This observation
lends further support to the triplet parentage of the red emission
because oxygen is an efficient quencher for triplet emission.
The fluorescence of Pt (Φf ) 3.2 × 10-4) is more intense than
that of Pt2 (Φf ) 7.9 × 10-5), while the former is less
phosphorescent than the latter. Φp of Pt (4.4 × 10-3) is close
to that of diaurated and diplatinated pyrenes that contain two
Pt(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 the
metalated pyrenes. No excimeric emission is observed for all
the complexes even at high concentrations (>10-3 M). Possibly
formation of excimer is sterically hindered by the bulky
phosphine ligands.
Figure 10. Emission spectra of Pd (red) and Pd2 (black) in
deaerated CH3CN solution at room temperature. Excitation wave-
length ) 320 nm. Filter: 350 nm cutoff.
A weak red emission at ∼620 nm (quantum yield Φp < 10-5
)
is also observed for the deaerated solutions of Pd2. Due to the
low intensity of the emission, its lifetime, which lies in the
microsecond domain, cannot be determined accurately. But
the large Stokes shift and energy of the emission suggest it is
the T1 f S0 phosphorescence.
Replacing the Pd ion with its heavy congener greatly enhances
phosphorescence. Both Pt and Pt2 show dual emissions in
degassed solutions with a weak fluorescence at 370-450 nm
Intersystem crossing between singlet and triplet ππ* excited
states is very inefficient, as there is little change in the angular

Switching on the Phosphorescence of Pyrene
Organometallics, Vol. 28, No. 1, 2009 59
Table 3. Emission Data of the Complexes
complex
fluorescence λmax (nm)
Φf
phosphorescence λmax (nm)
Φp
phosphorescence lifetime (µs)
Pt
Pd
Pt2
Pd2
PtCl
Pt2Cl2
398
400
420
429
404
408
3.2 × 10-4
6.6 × 10-5
7.9 × 10-5
6.3 × 10-5
611
4.4 × 10-3
31.3
627
620
1.5 × 10-2
63.7
a
<10-5
10-5
b
a
10-3
650
<10-5
b
a The intensity of the emission is too weak for accurate lifetime determination. b The quantum yields cannot be determined accurately, as the
complexes tend to dissociate at concentration e 10-5 M.
momentum. The relatively strong phosphorescence displayed
by the Pt complexes is due to the heavy atom effect of the metal
ion, which increases the rate of S1 f T1 intersystem crossing.
It should be noted that not only the metal ions but also P and
Cl in the complexes are conventionally considered as heavy
atoms. However, our results show that the effect of the metal
ions is dominating. With its large spin-orbit coupling, the Pt
ion can enhance phosphorescence by mixing the spin parentage
of the singlet and triplet excited states. The Hamiltonian operator
of the spin-orbit perturbation HSO is given as equation 1, where
Z and r are the atomic number of the heavy atom and the
distance between the nucleus of the atom and the chromophore,
and li and si are angular momentum and spin operators,
respectively.22
Although the lifetime of the phosphorescence of the Pt
complexes is much longer than that of fluorescence, it is very
much shorter than that of pyrene (∼0.44 s).23 This is again due
to the spin-orbit coupling of the Pt ions, which introduces
singlet character into the triplet excited state T1, making the T1
f S0 transition less spin-forbidden.
Conclusion
By comparing the electronic spectra of the cyclometalated
and dangling complexes, we deduce that the pyrenyl rings in
these complexes are mainly perturbed by the PPh2 group at C1/
6. A surprising result is the similarity between the spectra of
Pt and Pd or Pt2 and Pd. This suggests weak π interactions
between the metal ions and the pyrenyl ring. On the other hand,
the forbidden 1Lb band is intensified, mostly likely due to
Ze2
1
1
1
increased mixing between La and Lb 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 case
of Pt2. Finally, our study demonstrated that direct bonding of
the metal ions to the pyrenyl ring is needed for efficient
enhancement of the phosphorescence.
HSO
)
lisi
(1)
∑
4m2c2 r3
i
It is clear from eq 1 that the spin-orbit coupling increases
with the atomic number of the perturbing atom. This explains
the fact that the phosphorescence of the Pt complexes is much
stronger than that of the Pd complexes (Z of Pt and Pd are 78
and 46, respectively). Furthermore, the effect is additive, as it
increases with the number of heavy atoms. This accounts for
the fact that the phosphorescence of Pt2 is stronger than that of
Pt, and Pd2 displays weak phosphorescence despite the weak
spin-orbit coupling of the metal. Equation 1 also shows that
the heavy atom effect decreases rapidly with the distance r;
however, only very weak phosphorescence is observed for
Pt2Cl2 despite the fact that the dangling Pt ions are not very far
from the pyrenyl ring, as the shortest distance between the metal
ions and the carbon atoms of the ring is only ∼3.391 Å. This
suggests that the metal-ligand orbital (σ and π) interactions
are crucial to the manifestation of the heavy atom effect.
Acknowledgment. J.H.K.Y. would like to thank National
University of Singapore for financial support and Miss Tan
Geok Kheng and Professor Kop Lip Lin for solving the X-ray
crystal structures.
Supporting Information Available: CIF files of the crystal
structures and selected bond lengths and angles, and NMR spectra
of the complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM800410M
(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.
(22) Lower, S. K.; El-Sayed, M. A. Chem. ReV. 1966, 66, 199.