Light-Emitting Tridentate Cyclometalated Platinum(II) Complexes Containing σ-Alkynyl Auxiliaries: Tuning of Photo- and Electrophosphorescence

Published on Web 03/25/2004

Light-Emitting Tridentate Cyclometalated Platinum(II)

Complexes Containing σ-Alkynyl Auxiliaries: Tuning of

Photo- and Electrophosphorescence

Wei Lu,^†Bao-Xiu Mi,^‡Michael C. W. Chan,^†Zheng Hui,^†Chi-Ming Che,*^,†

Nianyong Zhu,^†and Shuit-Tong Lee^‡

Contribution from the Department of Chemistry and HKU-CAS Joint Laboratory on

New Materials, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong SAR, China, and

Centre of Super-Diamond and AdVanced Films and Department of Physics and Materials

Science, City UniVersity of Hong Kong, Kowloon, Hong Kong SAR, China

Received December 17, 2003; E-mail: cmche@hku.hk

Abstract: The synthesis and structural, photophysical, electrochemical, and electroluminescent properties

of a class of platinum(II) complexes bearing σ-alkynyl ancillary ligands, namely [(C^∧N^∧N)Pt(CtC)_nR]

[H(C^∧N^∧N) ) 6-aryl-2,2′-bipyridine; n ) 1-4; R ) aryl, alkyl, or trimethylsilyl], have been studied. Substituents

with different steric and electronic properties were introduced into the tridentate cyclometalating and

arylacetylide ligands, and the π-conjugation length of the oligoynyl moiety was homologously extended

from ethynyl to octatetraynyl. The X-ray crystal structures of several derivatives confirm the Pt-(CtC)

ligation and reveal various intermolecular interactions, such as π-π, Pt‚‚‚Pt, and C-H‚‚‚F-C. The complexes

display good thermal stability and intense phosphorescence in fluid and glassy solutions with high quantum

yields and microsecond lifetimes. Their emission energies are sensitive to solvent polarity, the electronic

affinities of the substituents on both the cyclometalating and arylacetylide groups, and the length of the

oligoynyl ligand. By choosing appropriate cyclometalating and σ-alkynyl ligands, the emission color of this

class of platinum(II) complexes can be tuned from green-yellow to saturated red. In addition to ³MLCT

3

[Pt(5d) f π*(C^∧N^∧N)] and IL(C^∧N^∧N), intriguing IL(alkynyl) excited states localized on -(CtC)₄- and

-(CtCpyrenyl-1) moieties that afford narrow-bandwidth emissions have been observed. Selected Pt(II)

complexes were doped into the emissive region of multilayer, vapor-deposited organic light-emitting diodes.

The tunable electrophosphorescence energy resembles that recorded in fluid solutions for these emitters,

and the devices exhibit high luminance and efficiencies (up to 4.2 cd A^-1).

Introduction

Recently, Raithby and co-workers reported a blue-luminescent

[Pt(PⁿBu₃)₂]-based polymer with acetylide ligands containing

The scope and diversity of studies on transition metal

σ-alkynyl complexes in the realm of material science have

continued to expand since the initiate report on soluble [Pt-

(PⁿBu₃)₂]-bearing oligoyne polymers in 1975.¹Acetylide and

polyyne moieties, as rigid π-conjugated bridging components,

facilitate a wide range of photorelated processes, including triplet

energy transfer, electron (or hole) transfer, photon migration,

and electron delocalization, while maintaining strict stereo-

chemical integrity.²There has been particular interest in

platinum(II) σ-alkynyl complexes for nonlinear optics, liquid

crystals, low-dimension conductors, photovoltaic devices, and

supramolecular hosts applications due to their chemical and

structural stability. However, investigations into the light-

emitting characteristics of these materials are partly hindered

by the lack of luminescence under ambient conditions for Pt-

(II) σ-alkynyl derivatives supported by phosphine ligands.³

low-lying π* orbitals in the main chain, and the emission was

assigned to triplet intraligand (IL) charge-transfer excited state

of the bridging acetylide group.⁴Ligation of oligopyridines,

(2) (a) Manna, J.; John, K. D.; Hopkins, M. D. AdV. Organomet. Chem. 1995,

38, 79-154. (b) Tour, J. M. Chem. ReV. 1996, 96, 537-554. (c) Paul, F.;

Lapinte, C. Coord. Chem. ReV. 1998, 178-180, 431-509. (d) Lavastre,

O.; Plass, J.; Bachmann, P.; Guesmi, S.; Moinet, C.; Dixneuf, P. H.

Organometallics 1997, 16, 184-189. (e) Touchard, D.; Haquette, P.;

Daridor, A.; Romero, A.; Dixneuf, P. H. Organometallics 1998, 17, 3844-

3852. (f) John, K. D.; Hopkins, M. D. Chem. Commun. 1999, 589-590.

(g) Long, N. J. In Optoelectronic Properties of Inorganic Compounds;

Roundhill, D. M., Fackler, J. P., Jr., Eds.; Plenum Press: New York, 1999,

Chapter 4. (h) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999,

99, 1863-1934. (i) Nguyen, P.; Gomez-Elipe, P.; Manners, I. Chem. ReV.

1999, 99, 1515-1548. (j) Martin, R. E.; Diederich, F. Angew. Chem., Int.

Ed. Engl. 1999, 38, 1350-1377. (k) Swager, T. M. Acc. Chem. Res. 1998,

31, 201. (l) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644. (m) Long,

N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42, 2586-2617.

(3) (a) Lewis, J.; Khan, M. S.; Kakkar, A. K.; Johnson, B. F. G.; Marder, T.

B.; Fyfe, H. B.; Wittmann, F.; Friend, R. H.; Dray, A. E. J. Organomet.

Chem. 1992, 425, 165-176. (b) Frapper, G.; Kertesz, M. Inorg. Chem.

1993, 32, 732-740. (c) Bunten, K. A.; Kakkar, A. K. Macromolecules

1996, 29, 2885-2893. (d) Chawdhury, N.; Ko¨hler, A.; Friend, R. H.; Wong,

W. Y.; Lewis, J.; Younus, M.; Raithby, P. R.; Corcoran, T. C.; Al-

Mandhary, M. R. A.; Khan, M. S. J. Chem. Phys. 1999, 110, 4963-4970.

(e) Wilson, J. S.; Ko¨hler, A.; Friend, R. H.; Al Suti, M. K.; Al -Mandhary,

M. R. A.; Khan, M. S.; Raithby, P. R. J. Chem. Phys. 2000, 113, 7627-

7634.

^†The University of Hong Kong.

^‡City University of Hong Kong.

(1) (a) Fujikura, Y.; Sonogashira, K.; Hagihara, N. Chem. Lett. 1975, 1067-

1070. (b) Sonogashira, K.; Takahashi, S.; Hagihara, N. Macromolecules

1977, 10, 879-880.

9

4958

J. AM. CHEM. SOC. 2004, 126, 4958-4971

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

Scheme 1. Excited-State Tuning for Phosphorescent Ir(III)-

which also feature low-lying π* orbitals, is another way to

harness the luminescence of Pt(II) σ-alkynyl species. In 1994,

our research group reported the first luminescent Pt(II) acetylide

complex with aromatic diimine ligands, namely [Pt(phen)(Ct

CPh)₂] (phen ) 1,10-phenanthroline), which exhibits intense

triplet [5d(Pt) f π*(phen)] metal-to-ligand charge transfer

(MLCT) emission in fluid solution at room temperature.⁵

Subsequent studies have improved the synthetic procedure and

probed the excited-state properties of [Pt(R-diimine)(CtCAr)₂]

complexes through systematic modification of both the diimine

and acetylide auxiliaries.⁶Intriguingly, these complexes have

recently been shown to be promising materials in organic light-

emitting diodes (OLEDs)⁷as well as photoinduced charge-

separation systems.⁸

(predominantly IL) and Pt(II)- (MLCT and IL) Based Complexes

through Ligand Modification

There has been tremendous impetus to develop OLEDs using

organic or metal-organic compounds as emitting materials since

the report in 1987 by Tang and Van Slyke of a electrolumi-

nescent (EL) device using Alq₃(q ) 8-quinolinate) as the

fluorescent emitter.⁹Recent progress has shown that phospho-

rescent materials should in principle be superior to fluorescent

substrates for small-molecule OLED applications.¹⁰This is

because efficient phosphorescent emitters doped into fluorescent

host materials can potentially harvest both singlet and triplet

excitons upon electron-hole recombination from electrical

excitation. The [Ir(C^∧N)₃] (HC^∧N ) 2-phenylpyridine) complex

and its derivatives has been one of the most widely studied class

of electrophosphorescent emitters containing a heavy transition

metal since it was first described in 2000 by Baldo, Thompson,

and Forrest.¹¹Efficiencies of OLEDs were dramatically in-

creased to, for example, 15.4 ( 0.2% and almost 100% for

external and internal quantum efficiency respectively, by

employing this green emitter in a multilayer heterostructure.^10e

Furthermore, it has been reported that chemical modifications

of tris- and bis-cyclometalated iridium(III) derivatives can afford

substantial improvements.^10dFor instance, fluorinated analogues

can reduce triplet-triplet annihilation processes and increase

their sublimability,¹²introduction of bulky groups can decrease

the self-quenching of emitters at high doping levels,¹³the use

of fluorene-modified (C^∧N) ligands yields crystallization-

resistant materials that can be incorporated into single layer

OLEDs,¹⁴expanding the π-conjugation of the (C^∧N) ligands

can result in enhanced device performances,¹⁵and the employ-

ment of facial isomers leads to blue-shifted emissions and higher

quantum efficiencies compared to meridional analogues.^10f

Color-tuning of the emission is possible and blue to red

electrophosphorescence with excellent efficiencies based on

these derivatives have been achieved.^10dAll endeavors to tune

the photophysical and electroluminescent properties of the bis-

and tris-cyclometalated iridium complexes have been focused

on varying the donor and/or acceptor groups on the (C^∧N)

ligands whereas the third bidentate auxiliary, such as acetyl-

acetonate (acac), is apparently ineffective in modifying the triplet

intraligand-dominated excited state of Ir(III) complexes (Scheme

1).

Our present study aims to develop high-efficiency emitters

based on Pt(II) σ-alkynyl complexes. The tridentate cyclom-

etalated Pt(II) lumophores [Pt(C^∧N^∧N)]¹⁶are chosen because

of their superior emissive properties compared to the 2,2′:6′,2′′-

terpyridine (tpy)¹⁷and (C^∧N^∧C) (HC^∧N^∧CH ) 2,6-diphen-

ylpyridine)¹⁸congeners. The anionic σ-alkynyl ligand affords

(4) Younus, M.; Ko¨hler, A.; Cron, S.; Chawdhury, N.; Al -Mandhary, M. R.

A.; Khan, M. S.; Lewis, J.; Long, N. J.; Friend, R. H.; Raithby, P. R. Angew.

Chem., Int. Ed. Engl. 1998, 37, 3036-3039.

(5) Chan, C. W.; Cheng, L. K.; Che, C. M. Coord. Chem. ReV. 1994, 132,

87-97.

(6) (a) Connick, W. B.; Geiger, D.; Eisenberg, R. Inorg. Chem. 1999, 38, 3264-

3265. (b) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.;

Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-

457. (c) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S.

Inorg. Chem. 2001, 40, 4053-4062. (d) Wadas, T. J.; Lachicotte, R. J.;

Eisenberg, R. Inorg. Chem. 2003, 42, 3772-3778. (e) Pomestchenko, I.

E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano, F. N. Inorg. Chem.

2003, 42, 1394-1396. (f) Fernandez, S.; Fornies, J.; Gil, B.; Gomez, J.;

Lalinde, E. J. Chem. Soc., Dalton Trans. 2003, 822-830.

(7) Chan, S. C.; Chan, M. C. W.; Che, C. M.; Wang, Y.; Cheung, K. K.; Zhu,

N. Y. Chem. Eur. J. 2001, 7, 4180-4190.

(8) McGarrah, J. E.; Kim, Y. J.; Hissler, M.; Eisenberg, R. Inorg. Chem. 2001,

40, 4510-4511.

(9) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 193-195.

(10) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;

Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154. (b) Ma, Y.

G.; Zhang, H. Y.; Shen, J. C.; Che, C. M. Synth. Met. 1998, 94, 245-248.

(c) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys.

Lett. 2000, 77, 904-906. (d) Lamansky, S.; Djurovich, P.; Murphy, D.;

Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P.; Forrest, S. R.;

Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304-4312. (e) Adachi,

C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E. Kwong,

R. C. Appl. Phys. Lett. 2001, 78, 1622-1624. (f) Tamayo, A. B.; Alleyne,

B. D.; Djurovich, P.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.;

Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377-7387. (g)

Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.; Zuppiroli,

L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790-8797.

(11) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750-

753.

(12) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V.

A.; Wang, Y. Chem. Commun. 2001, 1494-1495.

(13) Xie, H. Z.; Liu, M. W.; Wang, O. Y.; Zhang, X. H.; Lee, C. S.; Hung,

L..S.; Lee, S. T.; Teng, P. F.; Kwong, H. L.; Hui, Z.; Che, C. M. AdV.

Mater. 2001, 13, 1245-1248.

(14) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C. Chem.

Commun. 2002, 784-785.

(15) (a) Duan, J. P.; Sun, P. P.; Cheng, C. H. AdV. Mater. 2003, 15, 224-228.

(b) Su, Y. J.; Huang, H. L.; Li, C. L.; Chien, C. H.; Tao, Y. T.; Chou, P.

T.; Datta, S.; Liu, R. S. AdV. Mater. 2003, 15, 884-888. (c) 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, 12 971-12 979.

(16) (a) Chan, C. W.; Lai, T. F.; Che, C. M.; Peng, S. M. J. Am. Chem. Soc.

1993, 115, 11 245-11 253. (b) Cheung, T. C.; Cheung, K. K.; Peng, S.

M.; Che, C. M. J. Chem. Soc., Dalton Trans. 1996, 1645-1651. (c) Neve,

F.; Ghedini, M.; Crispini, A. J. Chem. Soc., Chem. Commun. 1996, 2463-

2464. (d) Lai, S. W.; Chan, M. C. W.; Cheung, T. C.; Peng, S. M.; Che,

C. M. Inorg. Chem. 1999, 38, 4046-4055. (e) Lai, S. W.; Chan, M. C.

W.; Cheung, K. K.; Che, C. M. Organometallics 1999, 18, 3327-3336.

(f) Yip, J. H. K.; Suwarno; Vittal, J. J. Inorg. Chem. 2000, 39, 3537-

3543.

(17) (a) Arena, G.; Calogero, G.; Campagna, S.; Scolaro, L. M.; Ricevuto, V.;

Romeo, R. Inorg. Chem. 1998, 37, 2763-2769. (b) Michalec, J. F.; Bejune,

S. A.; McMillin, D. R. Inorg. Chem. 2000, 39, 2708-2709. (c) Yam, V.

W. W.; Tang, R. P. L.; Wong, K. M. C.; Ko, C. C.; Cheung, K. K. Inorg.

Chem. 2001, 40, 571-574.

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4959

A R T I C L E S

Lu et al.

Chart 1

neutrality to the [Pt(C^∧N^∧N)] moiety and is envisaged to

modulate their photophysical properties by (1) destabilizing

nonradiative ligand-field transitions that are inherent to this class

of compounds and (2) modifying the properties of the triplet

excited state [normally Pt f π*(cyclometalating ligand)]

through π-bonding conjugation between Pt(II) and the σ-alkynyl

auxiliary. We have previously communicated that these materials

can be vacuum-deposited into multilayer OLEDs to emit yellow

to red light with excellent efficiencies.¹⁹We now report that

extensive modifications of both the tridentate and σ-alkynyl

ligands in these complexes can result in subtle tuning of their

structural, photophysical and luminescent characteristics and

provide a novel family of electrophosphorescent emitters. In

2002, Thompson and Forrest reported the employment of the

blue luminescent Pt(II) complex [4′,6′-F₂-(C^∧N)Pt(acac)] [4′,6′-

F₂-(HC^∧N) ) 2-(4′,6′-difluorophenyl)pyridine], which displays

orange-red excimeric emission, as a dopant in white light

OLEDs.²⁰

Scheme 2. Synthesis of the Cyclometalated Pt(II) σ-Alkynyl

Complexes

6-aryl-2,2′-bypyridine ligands were obtained using Kro¨hnke’s

syntheses for oligopyridines,²²that is, condensation between

acalkylpyridinium salts and R,â-unsaturated ketones in the

presence of ammonium acetate in methanol. 4-Ethoxycarbonyl-

6-phenyl-2,2′-bipyridine was prepared by refluxing 4-hydroxy-

carbonyl-6-phenyl-2,2′-bipyridine²³in ethanol in the presence

of concentrated sulfuric acid.

The cyclometalated Pt(II) chloride precursors were synthe-

sized by refluxing the 6-aryl-2,2′-bypyridine ligands and K₂-

PtCl₄in CH₃CN/H₂O (1/1, v/v), a method originally developed

by Constable and co-workers²⁴(Scheme 2). In this work, Pt(II)

acetylide complexes (Chart 1) derived from these precursors

were prepared by employing Sonogashira’s conditions (terminal

alkynes, CuI/ⁱPr₂NH/CH₂Cl₂), which resulted in simple workup

and purification procedures. The terminal oligoynes required

for 12 and 13 were prepared in situ due to their instability toward

heat and light. Thus, the compound Me₃Si(CtC)_nSiMe₃(n )

Results

Synthesis. In this work, substituents with a variety of steric

and electronic properties were introduced into the potentially

tridentate cyclometalating ligands. The sterically encumbered

6-phenyl-4,4′-bis(tert-butyl)-2,2′-bipyridine was prepared from

4,4′-bis(tert-butyl)-2,2′-bipyridine through nucleophilic addition

of phenyllithium and subsequent oxidation by MnO₂.²¹The other

(18) Lu, W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M. Organometallics

2001, 20, 2477-2486.

(21) Goodman, M. S.; Hamilton, A. D.; Weiss, J. J. Am. Chem. Soc. 1995, 117,

8447-8455.

(19) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Zhu, N. Y.; Lee, S. T.; Che,

C. M. Chem. Commun. 2002, 206-207.

(22) Kro¨hnke, F. Synthesis 1976, 1-24.

(23) Neve, F.; Crispini, A.; Di Pietro, C.; Campagna, S. Organometallics 2002,

21, 3511-3518.

(24) (a) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.

Chem. Soc., Chem. Commun. 1990, 513-515. (b) Constable, E. C.; Henney,

R. P. G.; Leese, T. A.; Tocher, D. A. J. Chem. Soc., Dalton Trans. 1990,

443-449. (c) Constable, E. C.; Henney, R. P. G.; Raithby, P. R.; Sousa,

L. R. J. Chem. Soc., Dalton Trans. 1992, 2251-2258.

(20) (a) D′Andrade, B. W.; Brooks, J.; Adamovich, V.; Thompson, M. E.;

Forrest, S. R. AdV. Mater. 2002, 14, 1032-1036. (b) Brooks, J.; Babayan,

Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E.

Inorg. Chem. 2002, 41, 3055-3066. (c) Adamovich, V.; Brooks, J.;

Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D’Andrade, B. W.; Adachi,

C.; Forrest, S. R.; Thompson, M. E. New J. Chem. 2002, 26, 1171-1178.

9

4960 J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

Figure 1. Crystal packing diagrams of 1 (a), 2 (b), and 8 (c), illustrating intermolecular contacts.

3,²⁵4,²⁶respectively) was monodesilylated with MeLi in THF

in the presence of CuI at -78 °C, and the resultant mixture

(presumably containing Cu(CtC)_nSiMe₃) was further treated

with [(C^∧N^∧N)PtCl] to give [(C^∧N^∧N)Pt(CtC)₃SiMe₃] (12) and

[(C^∧N^∧N)Pt(CtC)₄SiMe₃] (13) with a product yield of 56%

and 4%, respectively. A similar protocol was previously used

to prepare (η⁵-C₅Me₅)(NO)(PPh₃)Re(CtC)_nCu intermediates.²⁶

All complexes are air-stable and soluble in CH₂Cl₂, CHCl₃,

acetone, THF, and DMSO, except 13, 18, and 22 which display

low solubility in common organic solvents. All ¹H and ¹³C (and

¹⁹F where applicable) NMR spectra are well resolved. Thermal

analysis by TGA in N₂(see Supporting Information) show that

the decomposition temperature is around 400 °C for 1, 2, 28,

and 29 and around 330 °C for 8, 10, and 11 (the residual

decomposition product is composed of metallic platinum). This

thermal stability is remarkable for σ-alkynylmetal complexes

and especially for oligoynyl derivatives, i.e., the butadiynyls 8

and 11.

plane of the [(C^∧N^∧N)Pt] moiety for 1, 2, 7, and 8 is 56, 7, 5

and 44°, respectively.

For the structures of 10-12, the oligoyne moiety is system-

atically lengthened from trimethylsilyl-capped ethynyl to buta-

diynyl and hexatriynyl. As a direct consequence of this

extension, the deviation of the acetylenic unit from linearity

increases. Although each of the bond angles at the acetylenic

carbon vertexes (172-178°) remains close to 180°, the ac-

cumulation of this effect for six angles in the crystal structure

of 12 makes the hexatriynyl moiety appear like an arch. These

curved structures have been previously observed in sym-

metrically and unsymmetrically endcapped oligoyne crystals and

are presumably caused by crystal packing factors.²⁸The Si-

C(acetylide) distance also increases marginally from 1.800(8)

Å for 10 and 1.827(8) Å for 11 to 1.89(4) Å for 12 with the

extension of the oligoyne chain.

Interesting intermolecular interactions and conformations have

been observed in these crystal lattices (Figures 1-3). For crystals

1, 2, 8, and 10-12, neighboring complex molecules are stacked

into pairs in a head-to-tail fashion, with π-π separations of

ca. 3.5 Å. For crystal 7 (Figure 3), there are extensive

C-H‚‚‚F-C interactions²⁹(with typical H‚‚‚F distances in the

2.55-2.78 Å range) between the peripheral hydrogen atoms of

the (C^∧N^∧N) ligand and the fluorine atoms of the neighboring

pentafluorophenylacetylide moiety; these weak noncovalent

contacts arrange the complex molecules into stacked sheets with

interlayer distances of ca. 3.5 Å. For crystal 12 (Figure 2c), the

intermolecular Pt-Pt contact of 3.466 Å between adjacent

molecules is comparable to those reported for [(tpy)₂Pt₂(µ-pz)]-

(ClO₄)₃(3.432 Å; Hpz ) pyrazole)³⁰and related diphosphine-

bridged derivatives.^16d

Crystal Structures. Single crystals of 1, 2, 7, 8, and 10-

12, obtained from their DMSO solutions, were solved by X-ray

crystallography (see Supporting Information for crystal data and

selected bond lengths and angles; the structures of 2 and 8 were

reported in our earlier communication¹⁹). In each structure, the

Pt atom resides in a distorted square planar geometry that is

comparable to those found for previously reported [(C^∧N^∧N)-

PtL]ⁿ⁺derivatives. The Pt-C(acetylide) distances range from

1.957(9) to 1.98(1) Å, which are similar to the values of 1.94

to 1.96 Å for [(tpy)PtCtCPh](PF₆)²⁷and [(R-diimine)Pt(Ct

CAr)₂] complexes.^6a,7For 1, 2, 7, and 8, the acetylenic units

have alternating CtC and C-C distances and deviate slightly

from linear geometry, as indicated by the angles at the acetylenic

carbon atoms (177 and 178° for 1, 174 and 172° for 2, 179 and

173° for 7, and 175, 176, 177 and 177° for 8, respectively).

The dihedral angle between the acetylenic phenyl ring and the

Tuning of Spectroscopic Properties. To facilitate compari-

son, all absorption and emission spectra are recorded in CH₂-

(28) (a) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J. Am.

Chem. Soc. 2000, 122, 810-822. (b) Mohr, W.; Stahl, J.; Hampel, F.;

Gladysz, J. A. Inorg. Chem. 2001, 40, 3263-3264. (c) Bruce, M. I.; Hall,

B. C.; Kelly, B. D.; Low, P. J.; Skelton, B. W.; White, A. H. J. Chem.

Soc., Dalton Trans. 1999, 3719-3728. (d) Guillemot, M.; Toupet, L.;

Lapinte, C. Organometallics 1998, 17, 1928-1930. (e) Sakurai, A.; Akita,

M.; Moro-oka, Y. Organometallics 1999, 18, 3241-3244.

(25) Rubin, Y.; Lin, S. S.; Knobler, C. B.; Anthony, J.; Boldi, A. M.; Diederich,

F. J. Am. Chem. Soc. 1991, 113, 6943-6949.

(26) Weng, W.; Bartik, T.; Brady, M.; Bartik, B.; Ramsden, J. A.; Arif, A. M.;

Gladysz, J. A. J. Am. Chem. Soc. 1995, 117, 11 922-11 931.

(27) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Cheung, K. K.

Organometallics 2001, 20, 4476-4482.

(29) Thalladi, V. R.; Weiss, H. C.; Bla¨ser, D.; Boese, R.; Nangia, A.; Desiraju,

G. R. J. Am. Chem. Soc. 1998, 120, 8702-8710.

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4961

A R T I C L E S

Lu et al.

Figure 2. Crystal packing diagrams of 10 (a), 11 (b), and 12 (c), illustrating intermolecular contacts.

Figure 3. Crystal packing diagram of 7, illustrating intermolecular contacts.

Cl₂at 298 K and in alcoholic glass (CH₃OH/C₂H₅OH ) 1/4)

at 77 K except otherwise stated, and these spectroscopic data

are summarized in Table 1. All complexes except 3, 31, and

32 are highly emissive in fluid solutions at 298 K with emission

maxima in the 560-644 nm range and lifetimes in the

microsecond regime. The solid-state emissions of these com-

plexes vary from λ_max564 to 715 nm but no distinct relationship

is evident between molecular structures and emission energies.

(1) Variation of Arylacetylide Substituents. The electronic

absorption spectra of 1-6 in CH₂Cl₂solution at 298 K (see

Supporting Information) show broad low-energy bands in the

410-470 nm range (ꢀ ≈ 5 × 10³dm³mol^-1cm^-1), which are

red-shifted (and more intense) compared to those of [(C^∧N^∧N)-

PtL]⁺(L ) pyridine, phosphines, etc., λ_max380-420 nm, ꢀ e

1.5 × 10³dm³mol^-1cm^-1; assigned as ¹MLCT transitions).^16d

These low-energy absorption peak maxima appear at 415 to

465 nm and systematically red-shift in accordance with the

electron-donating ability of the para-substituent on the phenyl-

acetylide ligand. The high-energy absorption bands of 1-5

located at λ_max330-370 nm are attributed to intraligand (IL)

charge-transfer transitions of the (C^∧N^∧N) ligand. For 6, there

is an additional intense absorption band at λ_max375 nm (ꢀ ≈

2.5 × 10⁴dm³mol^-1cm^-1), which is characteristic of the ¹IL-

(4-nitrophenylacetylide) charge-transfer transition.^6bExcitation

of complexes 1-6 in fluid solution at room temperature results

in long-lived (τ in µs scale) yellow to red luminescence (Table

1, Figure 4). The emission energies of the [(C^∧N^∧N)PtCt

CC₆H₄-4-R] series are red-shifted from the [(C^∧N^∧N)PtCl]

precursor, in line with the absorption studies. The emission

maxima in dichloromethane at 298 K varies from 630 nm for

3 to 560 nm for 6, a range of 1980 cm^-1, and is affected by the

para-substituent in the order (λ_max, nm): MeO > Me > F ≈ H

≈ Cl > NO₂. The radiative decay rate constants k_nrin CH₂Cl₂

solution at 298 K for these emissions are in the 10⁴-10⁵s^-1

range and the plot of ln k_nragainst emission peak energy affords

good linearity (Figure 4, inset), and is thus in agreement with

the energy gap law. Complex 1 was chosen to examine the effect

of solvent upon the low-energy absorption and emission spectra

(see Supporting Information). When the polarity of the solvent

decreases from ethanol and acetonitrile to CH₂Cl₂and benzene,

the low-energy broad absorption band red-shifts in energy, and

in the cases of CH₂Cl₂and benzene, two distinguishable

absorption peak maxima are observed. In addition, the emission

λ_maxslightly red-shifts from 580 to 585 nm, while the lifetime

and quantum yield changes from 0.3 to 0.6 µs and 0.03 to 0.07,

respectively.

Except for 3, the emission quantum yields of these complexes

in CH₂Cl₂are comparable to that of [Ru(bpy)₃]²⁺(bpy ) 2,2′-

bipyridine) salts (0.062 in acetonitrile). As previously observed

for related Pt(II) lumophores,^6a,16the self-quenching constants

k_qof the emissions of 1-6 in CH₂Cl₂range from 3.1 × 10⁹s^-1

dm^-1mol^-1for 6 to 5.7 × 10⁹s^-1dm^-1mol^-1for 2 at room

temperature (Table 1). The solid-state emissions of 1-6 at 298

K are broad with λ_maxin the 590-720 nm range. However, a

(30) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32, 369-

370.

9

4962 J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

Table 1. Spectroscopic Data

emission in fluid

solution:^b

absorption:^a

_max/nm

λ

_max/nm (τ/µs; φ);

emission in

solid-state

emission (298 K):

_max/nm (τ/µs)

solid-state

emission (77 K):

λ_max/nm (τ/µs)

λ

k_q/10⁹s^-1dm^-1mol^-1

k_nr/10⁶s^-1

;

alcoholic glass:^c

λ

3

complex

(ꢀ/× 10³dm mol^-1cm^-1

)

_max/nm (τ/µs)

λ

[(C^∧N^∧N)PtCtCC₆H₅] (1)

229 (29.7), 281 (38.3),

335 (14.0), 366 (9.98),

434 (5.18), 455 (4.94)

245 (39.4), 281 (54.6),

335 (18.0), 366 (11.4),

440 (5.09), 460 (4.95)

245 (27.6), 280 (34.7),

335 (13.4), 366 (8.51),

440 (4.20), 465 (4.22)

230 (43.8), 283 (65.9),

334 (23.3), 365 (15.5),

432 (8.67), 455 (8.31)

247 (26.5), 266 (31.4),

279 (34.6), 334 (13.7),

368 (8.97), 433 (4.88),

453 (4.76)

237 (39.1), 281 (27.9),

338 (19.8), 375 (24.6),

415 (12.9)

240 (28.2), 283 (38.7),

362 (10.4), 334 (15.2),

436 (5.46), 421 (5.45)

231 (32.0), 281 (24.0),

316 (18.7), 366 (7.79),

430 (4.97)

582 (0.4; 0.04);

4.7; 2.5

540, 574 (7.8)

534, 575 (7.8)

564 (8.8)

592, 630 (sh)

(0.5)

593, 639 (1.5)

589, 620 (0.6)

710 (e 0.1)

710 (0.9)

[(C^∧N^∧N)PtCtCC₆H₄-4-

CH₃] (2)

600 (0.2; 0.02);

5.7; 5.8

590, 615 (sh)

(e 0.1)

[(C^∧N^∧N)PtCtCC₆H₄-4-

OCH₃] (3)

630 (e 0.1; e 10^-3

)

720 (e 0.1)

705 (e 0.1)

nonemissive

[(C^∧N^∧N)PtCtCC₆H₄-4-

Cl] (4)

578 (0.5; 0.08);

4.9; 1.7

529, 565 (8.2)

542, 580 (7.1)

[(C^∧N^∧N)PtCtCC₆H₄-4-

F] (5)

585 (0.3; 0.03);

4.0; 2.4

810 (e 0.1)

[(C^∧N^∧N)PtCtCC₆H₄-4-

NO₂] (6)

560 (0.9; 0.08);

3.1; 1.0

537, 572 (7.9)

531, 575 (5.9)

535, 572 (9.7)

540, 578 (8.0)

531, 568 (7.7)

616, 650 (sh) (0.4)

627 (e 0.1)

610, 650 (sh) (1.4)

586, 620 (sh) (1.5)

568, 646 (e 0.1)

585, 630 (2.2)

[(C^∧N^∧N)PtCtCC₆F₅] (7)

560 (0.6; 0.06)

571 (1.0; 0.08)

584 (0.4; 0.04)

[(C^∧N^∧N)Pt(CtC)₂C₆H₅] (8)

[(C^∧N^∧N)PtCtCC(CH₃)₃] (9)

[(C^∧N^∧N)PtCtCSi(CH₃)₃] (10)

647 (e 0.1)

281 (25.0), 335 (13.9),

372 (7.40), 436 (3.69),

460 (3.40)

242 (36.3), 263 (29.4),

281 (35.0), 334 (19.7),

366 (11.8), 427 (5.49),

450 (4.02)

598 (0.31), 640 (sh)

578, 615 (sh) (0.2)

570 (0.3; 0.04);

2.8; 3.1

575, 625 (1.1)

[(C^∧N^∧N)Pt(CtC)₂Si(CH₃)₃] (11)

[(C^∧N^∧N)Pt(CtC)₃Si(CH₃)₃] (12)

[(C^∧N^∧N)Pt(CtC)₄Si(CH₃)₃] (13)

[4-Ph(C^∧N^∧N)PtCtCC₆H₅] (14)

230 (48.0), 244 (4.39),

280 (30.4), 336 (17.5),

366 (10.8), 426 (6.40)

246 (65.2), 260 (75.0),

336 (17.2), 368 (10.8),

431 (8.87)

234 (37.3), 282 (82.8),

337 (17.4), 364 (11.8),

415 (6.34), 445 (8.61)

229 (36.8), 285 (54.7),

337 (18.3), 370 (12.0),

441 (7.54), 462 (sh, 6.85)

230 (37.1), 284 (49.2),

332 (sh, 19.0), 370 (12.4),

442 (8.29), 460 (sh, 8.14)

229 (44.2), 282 (51.7),

331 (32.1), 366 (sh, 16.4),

441 (9.94), 460 (sh, 9.18)

230 (38.2), 287 (53.0),

339 (17.9), 373 (12.2),

446 (8.13), 465 (sh, 7.89)

244 (39.9), 291 (54.4),

339 (25.3), 365 (sh, 16.4),

446 (8.41)

247 (30.2), 283 (39.8),

330 (15.2), 360 (9.52),

426 (5.95), 432 (5.93)

239 (31.5), 281 (40.1),

329 (16.8), 357 (11.2),

413 (6.33), 439 (6.38)

236 (64.8), 283 (45.6),

293 (46.5), 330 (18.0),

368 (30.9), 382 (30.2),

399 (24.8), 457 (broad, 8.6),

505 (sh, 4.2)

256 (36.6), 283 (42.4),

347 (15.7), 376 (10.8),

458 (5.89), 484 (5.79)

252 (32.5), 285 (46.6),

347 (17.9), 373 (12.8),

441 (6.98), 463 (7.99)

228 (30.4), 251 (29.0),

282 (39.0), 337 (14.1),

367 (8.62), 437 (5.10),

460 (sh, 4.83)

229 (29.8), 282 (40.5),

338 (13.7), 369 (10.5),

436 (4.61), 455 (sh, 4.32)

561 (0.2; 0.03)

557 (0.8; 0.07)

532, 570 (7.0)

705 (0.3)

688 (0.7)

n. d.

588 (2.1), 717 (1.2)

709 (1.6)

530, 567 (10.2)

584, 667 (11.5)

539, 580 (sh) (14)

550, 590 (sh) (13.6)

544, 580 (sh) (14)

547, 580 (sh) (12.7)

558, 597 (15.5)

513, 550 (7.0)

589, 671 (2.7; 0.01)

597 (0.8; 0.07)

597 (0.8; 0.09)

594 (1.0; 0.09)

603 (0.7; 0.08)

592 (0.5; 0.10)

571 (0.8; 0.05)

550 (0.9; 0.07)

664, 723 (21; 0.01)

n. d.

613 (0.4)

593 (0.5)

595 (0.2)

590 (e 0.1)

601, 645 (sh) (3.1)

584, 630 (4.7)

587, 634 (5.2)

588, 635 (2.2)

[4-(4-CH₃C₆H₄)

(C^∧N^∧N)PtCtCC₆H₅] (15)

[4-(4-CH₃OC₆H₄)

(C^∧N^∧N)PtCtCC₆H₅] (16)

[4-(4-ClC₆H₄)

(C^∧N^∧N)PtCtCC₆H₅] (17)

[4-(4-NO₂C₆H₄)

615, 710 (sh)

(e 0.1)

604, 710 (sh)

(e 0.1)

(C^∧N^∧N)PtCtCC₆H₅] (18)

[4,4′-^tBu₂(C^∧N^∧N)

PtCtCC₆H₅] (19)

543, 570,

610 (sh) (0.6)

549, 580 (8.9)

545, 585 (5.0)

[4,4′-^tBu₂(C^∧N^∧N)

PtCtCC₆F₅] (20)

510, 546 (5.5);

715 (2.0)

563, 588,

640 (sh) (0.9)

[4,4′-^tBu₂(C^∧N^∧N)PtCtC-

pyrenyl-1] (21)

653 (114), 669,

710, 727

669 (weak)

669 (0.8), 687,

704, 717, 740,

[EtO₂C(C^∧N^∧N)PtCtCC₆H₅]

(22)

622 (0.3; 0.03)

593 (1.6; 0.08)

590 (0.4; 0.02)

568 (9.1)

749 (e 0.1)

590 (1.0), 642

591 (1.8), 642

596, 645 (2.0)

[EtO₂C(C^∧N^∧N)PtCtCC₆F₅]

(23)

558 (7.4), 599

542 (6.1), 585

601 (0.6)

[5′′-CH₃(C^∧N^∧N)PtCtCC₆H₅]

(24)

601, 654 (sh)

(e 0.1)

[4′′-CH₃(C^∧N^∧N)PtCtCC₆H₅]

(25)

579 (0.4; 0.03)

534 (6.6), 578

604, 653

(e 0.1), 705

563, 603 (e 0.1),

649

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4963

A R T I C L E S

Lu et al.

Table 1 (Continued)

emission in fluid

solution:^b

absorption:^a

_max/nm

λ_max/nm (τ/µs; φ);

emission in

solid-state

emission (298 K):

_max/nm (τ/µs)

solid-state

emission (77 K):

_max/nm (τ/µs)

λ

k_q/10⁹s^-1dm^-1mol^-1

k_nr/10⁶s^-1

;

alcoholic glass:^c

λ

3

complex

(ꢀ/× 10³dm mol^-1cm^-1

)

_max/nm (τ/µs)

λ

[4′′-CF₃(C^∧N^∧N)PtCtCC₆H₅]

(26)

229 (31.4), 268 (34.6),

183 (38.3), 329 (16.7),

365 (sh, 8.30), 435 (5.55),

454 (5.65)

229 (29.4), 253 (33.6),

280 (40.8), 332 (13.3),

355 (sh, 9.61), 441 (4.76)

256 (24.7), 280 (32.6),

321 (19.1), 371 (14.9),

436 (4.97), 460 (4.49)

259 (26.9), 279 (35.0),

323 (19.2), 373 (15.2),

442 (5.01), 465 (4.80)

229 (22.1), 252 (18.8),

282 (28.0), 314 (1.81),

369 (13.9), 421 (4.90),

437 (4.88)

233 (31.9), 281 (40.4),

310 (sh, 22.3), 364 (17.0),

434 (5.80), 457 (5.34)

229 (33.0), 278 (39.7),

367 (14.6), 432 (5.91),

465 (sh, 5.47)

230 (32.2), 283 (40.8),

376 (15.4), 417 (6.72),

432 (sh, 6.40)

590 (0.5; 0.05)

519 (9.9), 542

nonemissive

[2′′,4′′-F₂(C^∧N^∧N)PtCtCC₆H₅]

(27)

582 (0.7; 0.07)

515 (18.9)

564, 603 (0.4)

630, 660 (0.3)

625, 655 (0.5)

∼630, 664 (0.4)^d

559, 608, 654 (4.2)

628, 684 (1.5)

624, 678 (1.5)

627 (1.1), 672

[(S^∧N^∧N)PtCtCC₆H₅] (28)

615, 660 (sh) (1.0; 0.05)

616, 660 (sh) (0.9; 0.04)

616, 660 (sh) (1.1, 0.04)

595, 644 (8.7)

[(S^∧N^∧N)PtCtCC₆H₄-4-CH₃]

(29)

595, 646 (8.1)

[(S^∧N^∧N)PtCtCC₆F₅] (30)

594 (8.9), 646, 713 (3.1)

[(O^∧N^∧N)PtCtCC₆H₅] (31)

∼640 (e 0.1; e 10^-3

)

613, 664 (4.1)

610, 664 (4.3)

610, 664 (4.8)

715 (e 0.1)

755 (e 0.1)

d

[(O^∧N^∧N)PtCtCC₆H₄-4-CH₃]

(32)

∼720 (e 0.1)^d

nonemissive

629 (0.15); 757

(e 0.1)

d

[(O^∧N^∧N)PtCtCC₆F₅] (33)

644, 680 (sh) (0.9, 0.02)

nonemissive

^aMeasured in CH₂Cl₂solution at 298 K. ^bMeasured in degassed CH₂Cl₂solution at 298 K (concentrations ∼1 × 10^-5mol dm^-3). ^cMeasured in CH₃OH/

C₂H₅OH ) 1/4 at 77 K. ^dVery weak (n.d. ) not determined).

Figure 5. UV-vis absorption and normalized emission (λ_ex) 350 nm)

spectra of complexes 1 and 8 in CH₂Cl₂solution at 298 K.

Figure 4. Normalized emission spectra of complexes 1-6 in CH₂Cl₂

solution at 298 K (λ_ex) 350 nm). Inset: plot of ln k_nrversus emission

12 in CH₂Cl₂solutions at 298 K are similar and poorly resolved

(see Supporting Information for spectra). The emission quantum

yields and lifetimes of these three complexes are comparable.

The emission energies in CH₂Cl₂solution at 298 K (Figure 6)

decrease in the order: triynyl (λ_max557 nm) > diynyl (λ_max

561 nm) > monoynyl (λ_max570 nm), but the differences are

minor, as are those found in glassy alcoholic solutions at 77 K.

maximum (legends indicate 4-arylacetylide substituents in the [(C^∧N^∧N)-

PtCtCC₆H₄-4-R] series).

correlation between the solid-state emission and substituents on

the arylacetylide ligand is not apparent.

(2) Extension of Oligoyne Ligand. The effects of the

π-conjugated length of the oligoyne ligands upon the emission

of [(C^∧N^∧N)Pt-(CtC)_n-R] have been examined. When the

alkynyl ligand is extended from phenyl-capped ethynyl (1) to

butadiynyl (8), the lowest absorption peak maximum of the

broad MLCT transition band in CH₂Cl₂solution blue-shifts from

∼460 to ∼430 nm, while the corresponding emission λ_max

partially blue-shifts by ca. 330 cm^-1from 582 to 571 nm (Figure

Stark changes in the absorption and emission spectra are

observed when the trimethylsilyl-capped oligoyne ligand is

further extended to octatetraynyl (13), the next homologue. In

the absorption spectrum of 13 in CH₂Cl₂solution at 298 K (see

Supporting Information for spectra), the low-energy bands are

well resolved into two distinct peak maxima at 411 and 444

nm. Saliently, 13 exhibits highly structured, narrow-bandwidth

emission at 298 K, with peak maxima at λ_max589 and 671 nm

(Figure 6) and emission lifetime of 2.7 µs (complex concentra-

tion ≈ 1.5 × 10^-5mol dm^-3). The vibronic progression of ca.

2100 cm^-1signifies that the -(CtC)₄- unit is involved in the

5). In the 77 K alcoholic glass spectra, the emission λ_max(λ_0-0

)

blue-shifts by only ca. 170 cm^-1from 540 nm for 1 to 535 nm

for 8 (see Supporting Information for spectra).

From 10 to 12, the alkynyl ligand is homologously lengthened

from trimethylsilyl-capped ethynyl to hexatriynyl. The low-

energy bands beyond 400 nm in the absorption spectra of 10-

9

4964 J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

Figure 8. Emission spectra of 19, 20, 22, and 23 in CH₂Cl₂solution at

298 K (λ_ex) 350 nm, concentration ∼1 × 10^-5mol dm^-3).

nm were recorded for the former, while a band at λ_max428 nm

was detected for the latter.

Figure 6. Emission spectra of 10-13 in CH₂Cl₂solution at 298 K (λ_ex

350 nm).

)

Additionally, complex 9 with a tert-butylacetylide ligand,

which is the C-congener of the trimethylsilyl acetylide 10, was

prepared. The lower-energy absorption band of 9 in CH₂Cl₂

solution is slightly red-shifted from that of 10 by ca. 650 cm^-1

.

Similarly, 9 exhibits intense orange emission with λ_maxat 586

nm in CH₂Cl₂solution, which is red-shifted from the emission

displayed by 10 (λ_max570 nm). When the polarity of the solvent

decreases from ethanol and acetonitrile to CH₂Cl₂and benzene

(see Supporting Information), red shifts are detected for both

the low-energy absorption band maximum (from 422 to 479

nm) and the emission λ_max(from 579 to 587 nm), while the

lifetime and quantum yield changes from 0.3 to 0.4 µs and 0.02

to 0.04, respectively.

(3) Derivatization of Tridentate Cyclometalating Ligand.

Modifications of the substituents on the (C^∧N^∧N) ligand have

been undertaken in order to facilitate the fine-tuning of the

emission energy for the [(C^∧N^∧N)PtCtC-R] complexes.

Attachment of a para-substituted aryl ring at the 4-position of

the (C^∧N^∧N) ligand afforded Pt(II) complexes that are more

robust but less soluble in common organic solvents. In CH₂Cl₂

solution at 298 K, the low-energy absorption band of 18, bearing

an electron-withdrawing nitro group at the said para-position,

contains a distinct peak maximum at 446 nm. This transition is

blue-shifted from that for 14-17, which occurs as a broad band

in the 400-550 nm range (see Supporting Information for

spectra). However, 14-18 exhibit similar orange-red emissions

with λ_maxaround 595 nm in CH₂Cl₂solution at 298 K, and the

emission energies and quantum yields are comparable to those

for the parent complex [(C^∧N^∧N)PtCtCPh] (1). This implies

that the impact of the 4-aryl ring is relatively small with regards

to the emission properties of this class of emitters.

The introduction of two tert-butyl groups to the bipyridine

moiety of the (C^∧N^∧N) ligand affects the emission properties

of 19 and 20 (Figure 8). The electron-donating nature of the

tert-butyl groups leads to blue-shifted emissions for 19 (λ_max

571 nm) and 20 (λ_max550 nm) compared to the parent

complexes 1 (λ_max582 nm) and 7 (λ_max560 nm), respectively,

in CH₂Cl₂solution at 298 K. The bulky tert-butyl groups

presumably also minimize solid-state intermolecular interactions

between neighboring [(C^∧N^∧N)Pt] planes. Accordingly, the

bright yellow color of crystalline 19 and 20, which is in contrast

to the deep red/brown color of the [(C^∧N^∧N)PtCtCC₆H₄-4-R]

series in the solid state, is likely to be a consequence of blue-

Figure 7. Site-selective emission [vibronic progression (cm^-1) in paren-

theses] and excitation spectra of 13 in alcoholic glassy solution at 77 K

(concentration ∼1 × 10^-5mol dm^-3).

electronic transition (see section on ethynylpyrene complex 21).

The emission maximum slightly red-shifts from 588 to 589, 591

and 594 nm when the solvent polarity decreases from ethanol

to CH₂Cl₂, THF and toluene, respectively. This emission is

similar in energy and shape to the acetylenic ³ππ* emission of

[(Cy₃P)Au(CtC)₄Au(PCy₃)]³¹(λ_0-0574 nm with vibronic

progression of 2120 cm^-1in CH₂Cl₂at 298 K), except for a

minor red-shift of ∼440 cm^-1. In alcoholic glassy solution at

77 K, complex 13 exhibits multiple and site-selective emissions.

Structured emission is observed upon excitation at 350 nm

(Figure 7), which resembles overlapping of three emissions with

different origins, namely: (1) λ_0-0523 nm with vibronic

progression of ca. 1360 cm^-1, (2) λ_0-0568 nm with vibronic

progression of 2150 cm^-1, and (3) λ_0-0584 nm with vibronic

progression of 2115 cm^-1. Upon excitation into a lower-energy

band (443 nm), the emission energies do not alter but the relative

intensity of the 523 nm band decreases. Moreover, the excitation

spectra monitored at λ_em583 and 523 nm are different in the

400-500 nm spectral range; two peak maxima at 411 and 433

(31) Lu, W.; Xiang, H. F.; Zhu, N.; Che, C. M. Organometallics 2002, 21, 2343-

2346.

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4965

A R T I C L E S

Lu et al.

shifted electronic transitions due to the lack of inter-

molecular interactions. Conversely, the electron-withdrawing

ethoxycarbonyl group on the 4-position of the (C^∧N^∧N) ligand

dramatically red-shifts the emission maximum for 22 (620 nm)

and 23 (593 nm) compared to 1 (582 nm) and 7 (560 nm),

respectively, in CH₂Cl₂solution (Figure 8).

Incorporation of methyl, CF₃and F substituents onto the

phenyl ring of the (C^∧N^∧N) ligand result in relatively small

differences in the low-energy absorption (see Supporting

Information for spectra) and emission properties of the com-

plexes. Hence, complexes 24-27 exhibit intense orange emis-

sions (λ_max≈ 585 nm) with comparable quantum yields and

lifetimes in CH₂Cl₂solution at 298 K (Table 1).

With the development of red emitters as a clear objective,

the [(S^∧N^∧N)PtCtCR] (28-30) and [(O^∧N^∧N)PtCtCR] (31-

33) (R ) Ph, C₆H₄-4-Me, and C₆F₅respectively) complexes

containing the thienyl or furyl moiety as cyclometalating carbon

donor, respectively, were synthesized. The absorptions in the

300-400 nm range in the absorption spectra of 28-33 (see

Figure 9. Emission spectra of 1, 28 and 31 in alcoholic glass at 77 K

(λ_ex) 350 nm, concentration ∼1 × 10^-5mol dm^-3).

1

Supporting Information for spectra) are assigned to IL transi-

tions of the (S^∧N^∧N) and (O^∧N^∧N) ligand, respectively, and

they are red-shifted from those of the corresponding (C^∧N^∧N)

complexes. The low-energy absorption for 28-33 is comparable

to those of the (C^∧N^∧N) congeners 1, 2, and 7, respectively,

but the mild tendency to red-shift from (C^∧N^∧N)- to (S^∧N^∧N)-

and (O^∧N^∧N)-based lumophores is discernible. For example,

the edge of the lower-energy absorption band partially red-shifts

from 1 (∼460 nm) to 28 (∼470 nm) and 31 (∼475 nm). Upon

excitation with UV light, 28-30 gives intense red emission in

CH₂Cl₂at λ_max616 nm (shoulder 660 nm), irrespective of the

arylacetylide substituents, and the spectral profile of this

emission is similar to that of [Pt(S^∧N)₂] (HS^∧N ) 2-(2-thienyl)-

pyridine)³²except that the emission maximum is red-shifted by

860 cm^-1. In the solid state, the emission shows well-resolved

vibronic structure with a progression of ∼1300 cm^-1and slightly

red-shifts to λ_max630 nm, while in alcoholic glassy solutions,

the emission maximum blue-shifts to 595 nm. In glassy alcoholic

solution at 77 K, 31-33 emits strongly at λ_max610 nm with a

shoulder at 664 nm, again irrespective of the substituents on

the arylacetylide ligand. Complexes 31 and 32 are weakly

emissive in fluid solutions, while 33 bearing a pentafluorophe-

nylacetylide ligand is emissive [λ_max644 (shoulder 680) nm]

in CH₂Cl₂solution at 298 K. The effects of solvent upon the

spectroscopic properties of 28 and 33 have been examined (see

Supporting Information for spectra). When the solvent changes

from ethanol to acetonitrile, CH₂Cl₂and benzene, the low-energy

broad absorption band splits into two distinct peaks and the edge

of this absorption manifold red-shifts from ca. 450 to 500 nm

for 28 and from ca. 420 to 450 nm for 33. Nevertheless, the

emission maximum only undergoes a slight red-shift from 608

to 619 nm for 28 and from 634 to 647 nm for 33, and the

lifetime (quantum yield) remains relatively constant at ca.

1.0 µs (0.04) for 28 and ca. 0.4 µs (0.02) for 33.

Figure 10. Emission spectra of 7, 30 and 33 in CH₂Cl₂solution at 298 K

(λ_ex) 350 nm, concentration ∼1 × 10^-5mol dm^-3, * denotes instrumental

artifact).

cyclometalating ligand is modified from (C^∧N^∧N) to (S^∧N^∧N)

and (O^∧N^∧N). The Huang-Rhys ratios (S ) I_1-0/I_0-0, estimated

using relative peak intensities) of the emissions recorded in

glassy solutions appear to decrease in the sequence of 1 (ca.

0.6) > 31 (ca. 0.5) > 28 (ca. 0.4), and similar values were also

recorded in fluid solutions. While 1 is a yellow light emitter,

the excited-state properties of 28-30 and 33 have been adjusted

to yield orange-red and saturated red emissions, respectively.

(4) Red-Shifted and Narrow-Width Emission from [4,4′-

^tBu₂-(C^∧N^∧N)PtCtCpyrenyl-1] (21). When 1-ethynylpyrene,

which has a low-lying triplet excited state,³³was employed as

auxiliary acetylide ligand in complex 21, a low-energy and

narrow-bandwidth emission (Figure 11) was observed at λ_0-0

(λ_max) 664 nm with major vibronic progression, lifetime and

quantum yield of 1230 cm^-1, 20.8 µs and 0.01, respectively, in

CH₂Cl₂solution at 298 K. This emission is similar in energy to

that (λ_maxca. 660 nm) observed for [(^tBu₂bpy)Pt(CtCpyrenyl-

1)₂] (^tBu₂bpy ) 4,4′-bis-tert-butyl-2,2-bipyridine)^6ebut slightly

lower than the ³ππ* emission recorded for [(Cy₃P)AuCt

Cpyrenyl-1] (λ_0-0654 nm in the solid-state at 298 K, see

For comparison, the emission spectra of 1, 28, and 31 [(Ct

CPh) complexes] in alcoholic glass at 77 K and 7, 30, and 33

[(CtCC₆F₅) complexes] in CH₂Cl₂solution at 298 K are shown

in Figures 9 and 10, respectively. For the same arylacetylide

ligand, the emission maximum substantially red-shifts when the

(32) Maestri, M.; Sandrini, D.; Balzani, V.; Chassot, L.; Jolliet, P.; von Zelewsky,

(33) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the

Triplet State; Prentice Hall: Englewood NJ, 1969.

A. Chem. Phys. Lett. 1985, 122, 375-379.

9

4966 J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

Table 3. Electroluminescence Data

maximum maximum

a

EL λ_max,/nm

turn-on doping luminance/ efficiency/ maximum

-2

complex (CIE coordinates^a[x, y]) voltage/V ratio (%) cd m

cd A^-1

η_ext/%

1

564

(0.480, 0.484)

580

(0.508, 0.466)

548

(0.440, 0.507)

567

(0.480, 0.469)

545

(0.419, 0.526)

608, 656

(0.549, 0.330)

612, 656

(0.553, 0.305)

3.6

4.2

4.5

4.3

4.5

2^b

4

9900^e

7800^d

3900^e

2100^e

5600^f

9800^e

5800^c

4.2^h

2.4^h

1.4^g

0.6^g

1.5^h

3.2^g

1.4^g

1.6

0.9

0.6

0.3

1.1

0.6

2

6

7

2^b

4

9

5

20

28

29

4

6

5500^d

2090^d

2000^e

3100^f

1800^e

1.9^g

0.5ⁱ

0.7^h

0.6^h

1.0^g

0.5ⁱ

0.1

0.2

0.6

0.5

0.8

0.7

2^b

4

6

Figure 11. Absorption and emission spectra of 21 in various solvents at

298 K (λ_ex) 350 nm, concentration ∼1 × 10^-5mol dm^-3).

^aDoping level 4%, at 10 V. ^bWeak electrofluorescence at ∼440 nm

. . .

^gAt 20 mA cm^{-2 h}At 30 mA cm^{-2 i}At 50 mA cm^-2

from CBP was also observed. ^cAt 10 V. ^dAt 11 V. ^eAt 12 V. ^fAt 13 V.

Table 2. Electrochemical Data^aof Selected Complexes

reduction

^1/2(V)^b

anodic

-1.93 V versus Cp₂Fe^+/0; this couple presumably corresponds

to the one-electron reduction of the (C^∧N^∧N) ligand. Substitution

at the (C^∧N^∧N) ligand exerts a more substantial effect upon

the E^1/2value than at the arylacetylide ligand. The electron-

withdrawing ethoxycarbonyl group in 22 shifts the first reduction

wave to -1.60 V, while this reduction occurs at -1.93 V for

19 which bears two electron-donating tert-butyl groups. Com-

plex 6, with a nitro group on the para-position of the

arylacetylide ligand, shows a higher E^1/2value of -1.61 V for

the reduction of the (C^∧N^∧N) moiety. The conjugation length

of the oligoynyl ligand does not significantly affect the (C^∧N^∧N)

reduction. The (C^∧N^∧N), (S^∧N^∧N), and (O^∧N^∧N) derivatives

containing the pentafluorophenylacetylide ligand show compa-

rable reduction waves.

complex

E

peak E_pa(V)^b

[(C^∧N^∧N)PtCtCPh] (1)

-1.81

-1.77

-1.84

-1.61

-1.78

-1.87

-1.82

-1.77

-1.76

-1.70

-1.93

-1.60

-1.80

-1.77

0.04

0.01

0.08

0.12

0.09

0.29

0.47

0.28

0.22

c

[(C^∧N^∧N)PtCtCC₆H₄-4-CH₃] (2)

[(C^∧N^∧N)PtCtCC₆H₄-4-OCH₃] (3)

[(C^∧N^∧N)PtCtCC₆H₄-4-NO₂] (6)

[(C^∧N^∧N)PtCtCC₆F₅] (7)

[(C^∧N^∧N)PtCtCC(CH₃)₃] (9)

[(C^∧N^∧N)PtCtCSi(CH₃)₃] (10)

[(C^∧N^∧N)Pt(CtC)₂Si(CH₃)₃] (11)

[(C^∧N^∧N)Pt(CtC)₃Si(CH₃)₃] (12)

[(C^∧N^∧N)Pt(CtC)₄Si(CH₃)₃] (13)

[4,4′-^tBu₂(C^∧N^∧N)PtCtCPh] (19)

[EtO₂C(C^∧N^∧N)PtCtCPh] (22)

[(S^∧N^∧N)PtCtCC₆F₅] (30)

0.29

0.05

0.12

0.18

[(O^∧N^∧N)PtCtCC₆F₅] (33)

n

^aDetermined in CH₂Cl₂at 298 K with 0.1 M Bu₄NPF₆as supporting

electrolyte; scanning rate: 50 mV s^-1

.

^bValue versus E^1/2(Cp₂Fe^+/0) [0.11-

Except 13, all complexes show an irreversible oxidation wave

at 0.01-0.47 V versus Cp₂Fe^+/0(0.41-0.87 V versus NHE).

Complex 10, which features the shortest conjugation length at

the acetylide moiety, exhibits the most positive oxidation peak

potential of 0.47 V. The ethoxycarbonyl group in 22 shifts the

first oxidation wave to 0.05 V while this oxidation occurs at

0.29 V for the bis(tert-butyl)-substituted 19.

0.13 V versus Ag/AgNO₃(0.1 M in CH₃CN) reference electrode]. ^cBeyond

the spectral window of CH₂Cl₂solvent.

Supporting Information for spectra) by ca. 230 cm^-1. The

emission maximum slightly red-shifts from 662 to 664, 668 and

669 nm when the solvent polarity decreases from ethanol to

CH₂Cl₂, THF and toluene, respectively. In contrast, the low-

energy band in the absorption spectrum of 21 exhibits more

prominent solvatochromism, thus the low-energy shoulder shifts

from ca. 450, to 475, 520 and 550 nm when the solvent polarity

decreases from ethanol to CH₂Cl₂, THF and toluene, respectively

(left of Figure 11).

Time-Resolved Absorption Spectroscopy. The triplet excited-

state absorption spectra of 1 and 28 in CH₃CN solution have

been recorded (see Supporting Information for spectra). In both

cases, the decay of the absorption signals corresponds to the

emission lifetime, indicating that the absorption originated from

the emissive excited state. The salient features in these spectra

are, for 1, strong bleaching in the near UV region (400-460

nm), absorption in the 460-520 nm range and intense lumi-

nescence bleaching in the 520-750 nm range, and for 28, strong

absorption in the 380-570 nm range (λ_maxca. 400, 490 and

530 nm) and intense luminescence bleaching in the 570-750

nm range.

Electrophosphorescence: Application as OLED Emitters.

Several platinum complexes were incorporated into multilayer

OLEDs and yellow to red electroluminescence was generated.

The devices in the present study were fabricated on indium-

tin-oxide (ITO) glass using the vacuum deposition method. For

comparison purposes, the multilayer device configuration

reported by Thompson and Forrest was adopted.^10cGenerally,

NPB (N,N′-di-1-naphthyl-N,N′-diphenyl-benzidine) and Alq₃

were used as the hole- and electron-transporting layers, respec-

tively. BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,

bathocuproine) was used to confine excitons within the lumi-

nescent zone, and magnesium silver alloy was used as the

cathode. The Pt(II) σ-alkynyl materials were doped into the

conductive host material CBP (4,4′-N,N′-dicarbazole-biphenyl)

with mass ratios of 2 to 6%.

The performances of the OLEDs using Pt(II) σ-alkynyl

complexes as electrophosphorescent materials are listed in Table

3 (see Figure 12 and Supporting Information for details of

current density, voltage and luminance characteristics). Upon

stimulation of positive bias voltage above ∼4 V for devices

with emitter ratios of 4 and 6%, intense orange to red emission

Cyclic Voltammetry. The electrochemical data of several

Pt(II) complexes are summarized in Table 2. In general, the

cyclic voltammograms in CH₂Cl₂solutions at 298 K exhibit

one reversible reduction couple with E^1/2in the range -1.60 to

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4967

A R T I C L E S

Lu et al.

y ) 0.341), the maximum luminance of 3100 cd m^-2at 12 V

and efficiency of 1.0 cd A^-1(η_ext0.8%) at 30 mA cm^-2are

observed using 4% of 29 in CBP. Generally, higher doping ratios

led to lower efficiencies and inferior performance, apparently

due to aggregate-induced quenching.

Discussion

General Remarks. The synthetic methodology to the tri-

dentate cyclometalated Pt(II) σ-alkynyl complexes in the present

study involves three steps. Hence, Kro¨hnke’s method for

oligopyridines,²²Constable’s cyclometalation process²⁴and

Sonogashira’s metal-alkynyl formation reaction¹have been

adopted to prepare the cyclometalating ligand, its cyclometalated

Pt(II) chloride derivative and the σ-alkynyl complexes, respec-

tively. These procedures are generally efficient and tolerate a

variety of substituents on the cyclometalating ligand, and such

versatility facilitate the rational design and derivatization of this

class of materials.

Figure 12. Current density, voltage and luminance characteristics (inset:

external quantum efficiency vs current density) for OLED using 7 as emitter

at 4% doping level.

The structural determinations confirm the molecular structures

of these complexes and in particular the presence of the Pt-

alkynyl linkages. Intermolecular interactions in the crystal

lattices of Pt(II) complexes are known to play a crucial role in

their solid-state emission properties.³⁴A variety of weak

contacts, including π-π stacking, Pt‚‚‚Pt and C-H‚‚‚F-C

interactions, are revealed in the crystal structures of 1, 2, 7, 8,

and 10-12, and these may be invoked to rationalize the lower-

energy solid-state emissions displayed by these complexes.

Consequence of the σ-Alkynyl Moiety: π-Conjugation

with Pt(II). There are numerous reports on the phenomenon of

π-conjugation in monomeric and oligomeric/polymeric trans-

Pt(CtCR)₂moieties.³⁵In the present work, we found that the

acetylide ligand can affect the electronic structure of the

complex, as reflected by both electrochemical and photophysical

data. Complex 6 is an illustrative example, whereby the

introduction of a nitro group at the 4-position of phenyl-

acetylide leads to an increase in the E^1/2value of the first

reduction wave (by 0.2 V) that occurs primarily at the (C^∧N^∧N)

ligand. If there is no communication between the arylacetylide

and (C^∧N^∧N) groups, such an effect would be unlikely. We

conceive that there is a degree of interaction between the π

orbitals of the (C^∧N^∧N) and arylacetylide moieties via the Pt

dπ orbitals, and hence the electron-withdrawing capability of

the nitro group extends across the arylacetylide moiety and Pt-

(II) center and influences the redox nature of the (C^∧N^∧N)

ligand. In the crystal structures, the small dihedral angles (5-

7°) between the (C^∧N^∧N) and phenylacetylide planes in 2 and

7 suggest that favorable π-overlap can occur, while the larger

dihedral angles for 1 (56°) and 8 (44°) still allow the possibility

for partial π-overlap, although crystal packing effects cannot

be disregarded.

Figure 13. Normalized electroluminescence spectra for 1, 2, 7, 9, 20, and

29 (4%) operated at 10 V.

is observed for 1, 2, 7, 20, 28, and 29, while blue emission

from the host and hole-transporting layers is negligible. For 1,

2, 7, 28, and 29, the emission maxima are independent of the

doping level and applied voltage (for current density up to 600

mA cm^-2). As shown in Figure 13, the EL red-shifts in the

order 7, 1, 2, 9, which is consistent with that observed for the

photoluminescence (PL) in solution. For 1, 2, 7, and 20, the

EL maximum is consistently blue-shifted by ca. 500 cm^-1from

the PL values recorded in CH₂Cl₂solution at 298 K. For 7 and

20, both of which bear the pentafluorophenylacetylide ligand,

doping levels of greater than 5% afford an additional broad

emission at λ_maxca. 710 nm in the EL spectra. For 33, both the

blue emission of the host material CBP at λ_maxca. 440 nm and

red emission with peak maxima at 620 and 670 nm were

observed at 6% doping level.

The highest luminance (9800 cd m^-2at 12 V; λ_max548 nm;

CIE coordinates x ) 0.440, y ) 0.506) and EL efficiency [3.2

cd A^-1(η_ext1.1%) at 20 mA cm^-2] are achieved by using emitter

7 at 4% doping level. The luminance of 7800 cd m^-2at 11 V

and efficiency of 2.4 cd A^-1(η_ext0.9%) at 30 mA cm^-2are

obtained for an orange OLED (λ_max564 nm; CIE coordinates x

) 0.480, y ) 0.484) using 1 at 4% doping level. At 2% ratio

of 1, improved luminance (9900 cd m^-2at 12 V) and efficiency

[4.2 cd A^-1(η_ext1.6%) at 30 mA cm^-2] are observed, but weak

emission from CBP at λ_maxca. 440 nm is also detected. For the

red OLED (λ_max612 (max), 656 nm; CIE coordinates x ) 0.594,

There are apparently two transitions embedded in the allowed

low-energy broad absorption band (in the 400-550 nm range)

(34) (a) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529-

1533. (b) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30, 4446-

4452. (c) Houlding, V. H.; Miskowski, V. M. Coord. Chem. ReV. 1991,

111, 145-152. (d) Miskowski, V. M.; Houlding, V. H.; Che, C. M.; Wang,

Y. Inorg. Chem. 1993, 32, 2518-2524. (e) Bailey, J. A.; Hill, M. G.; Marsh,

R. E.; Miskowski, V. M.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995,

34, 4591-4599.

(35) (a) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus, M.;

Khan, M. S.; Raithby, P. R.; Ko¨hler, A.; Friend, R. H. J. Am. Chem. Soc.

2001, 123, 9412-9417 and references therein. (b) Liu, Y.; Jiang, S.; Glusac,

K.; Powell, D. H.; Anderson, D. F.; Schanze, K. S. J. Am. Chem. Soc.

2002, 124, 12 412-12 413.

9

4968 J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

3

of these tridentate cyclometalated Pt(II) σ-alkynyl complexes,

and one of these is likely to be the ¹MLCT (L ) cyclometalating

ligand) transition. The other transition apparently involves the

σ-alkynyl group, since [(C^∧N^∧N)PtCl] and related derivatives

without the acetylide auxiliary show only one transition in this

spectral region with smaller extinction coefficients. Because the

¹ππ* transitions of arylacetylide metal complexes usually occur

at significantly higher energies, the ‘second’ low-energy transi-

tion in these complexes is tentatively attributed to acetylide-

to-(C^∧N^∧N) L′LCT or acetylide-to-platinum(II) LMCT transi-

tions. We believe that the π-interaction across the Pt(II) center

is crucial to the unusual physical and spectroscopic nature of

these complexes.

solution (λ_max560 nm) for 6 is close to the reported IL(4-

nitrophenylacetylide) emission of [(4,4-^tBu₂bpy)Pt(CtCC₆H₄-

4-NO₂)₂] at λ_max570 nm. Therefore a triplet excited state

composed of an admixture of MLCT (L ) C^∧N^∧N) and IL-

(4-nitrophenylacetylide) is assigned for 6. Although complexes

6 and [(C^∧N^∧N)PtCtCC₆F₅] (7) exhibit emissions at a similar

energy (both at λ_max560 nm) in CH₂Cl₂at 298 K, the

3

contribution from the MLCT (L ) C^∧N^∧N) excited state to

3

the emission is expected to be greater for 7, because the IL-

(CtCC₆F₅) emission of [(Cy₃P)AuCtCC₆F₅] occurs at λ_max

ca. 485 nm in alcoholic glasses at 77 K, and this complex is

nonemissive in fluid solution.³⁹

The impact of the substituent at the 4-position of the (C^∧N^∧N)

ligand upon the emission energy of the Pt(II) complexes has

been evaluated. The emission is blue-shifted by electron-

donating groups and red-shifted by electron-withdrawing moi-

eties, and this is perfectly consistent with a MLCT (L )

cyclometalating ligand) assignment for the excited state. At-

tachment of substituted phenyl groups at the 4-position (14-

18) and functional groups on the cyclometalating phenyl moiety

(24-27) does not dramatically alter the emission properties of

these complexes. The minimal differences observed for the

phenylacetylide derivatives 14-18 and 24-27 contrast directly

with the remarkable substituent effects reported for the [Ir-

(C^∧N)₂(acac)] system.^10dThe origins for these emissions are

different; the photoluminescence of the former in fluid solution

is derived from ³MLCT excited states while that of the latter is

Nature of Excited States and Substituent Effects. In our

previous studies,¹⁶the emission of the [(C^∧N^∧N)PtCl] precursor

and the cationic [(C^∧N^∧N)PtL]⁺(L ) pyridine, phosphine or

isocyanide) species are assigned to ³MLCT (L ) C^∧N^∧N)

excited states and the broad low-energy absorption bands in

the 380-420 nm range are attributed to ¹MLCT transitions. The

introduction of an acetylide ligand into the [(C^∧N^∧N)Pt]⁺moiety

makes such assignments more complicated, since arylacetylide

and oligoynyl ligands also feature low-lying π and π* orbitals

that can participate in low-energy electronic transitions. The

lowest-energy transitions of R-diimine platinum(II) bis(acetylide)

3

complexes are typically ascribed to MLCT [Pt(5d) f π*(R-

diimine)], but the involvement of ³IL transitions for bis(4-

nitrophenylacetylide) and bis(σ-phenylbutadiynyl) derivatives

have also been proposed by Eisenberg^6band Che.⁷Recent

studies on related luminescent terpyridine platinum(II)³⁶and

R-diimine rhenium(I)³⁷complexes with extended σ-oligo-

ynyl groups, plus theoretical calculations³⁸on the R-diimine

platinum(II) bis(acetylide) system, suggest that acetylide-to-

diimine L′LCT transitions may contribute toward their emissive

excited states. In the present work, we ascribe the lower-energy

band in the absorption spectra to an allowed MLCT (L )

diimine) transition. This assignment is consistent with the fact

that the absorption band is red-shifted by electron-rich acetylide

and electron-deficient (C^∧N^∧N) moieties and exhibits negative

solvatochromism (i.e., transition energy increases with greater

solvent polarity). Here, it is assumed that the energies of

nonbonding and weakly π-bonding metal orbitals will increase

with the donor strength of the arylacetylide ligands and that

the ground state is more polar than the MLCT excited state.

Accordingly, the emissions of 1-27 (see exceptions below) in

3

predominantly IL in nature.

A change in the emissive origin is apparent for lumophores

supported by the heteroatom-containing (S^∧N^∧N) and (O^∧N^∧N)

ligands. The fluid emissions [λ_max616 and 644 nm in CH₂Cl₂

for the (S^∧N^∧N) (28-30) and (O^∧N^∧N) (31-33) series,

respectively] occur at lower energies than their related (C^∧N^∧N)

complexes. More importantly, the salient difference compared

with the (C^∧N^∧N) series is that these emissions are insensitiVe

to the substituent(s) on the arylacetylide ligand, although

negative solvatochromic effects are detected (see Supporting

Information for spectra of 28 and 33). The excited states for

these complexes are therefore tentatively assigned to ³IL (L )

S^∧N^∧N and O^∧N^∧N, respectively) charge-transfer transitions.

Narrow-Bandwidth Acetylenic ³ππ* Emissions. Upon

ascending the homologous series for [(C^∧N^∧N)Pt(CtC)_nSiMe₃]

(n ) 1-4, 10-13), participation of the acetylide manifold in

the electronic transitions is enhanced. Although the σ-donating

abilities of the oligoynyl moieties in 10-13 are assumed to be

comparable, the extension of the π-conjugation length may

afford an energetically accessible acetylenic ³ππ* state localized

on the [-(CtC)₄-] unit, the energy of which may be close to

or even lower than that of the [Pt(C^∧N^∧N)] ³MLCT state

(typically ca. λ_max560 nm). The intriguing vibronically struc-

tured narrow-width emission from 13 in fluid solution is similar

to that of [(Cy₃P)Au(CtC)₄Au(PCy₃)]³¹in CH₂Cl₂and identical

to that of [(^tBu₃tpy)Pt(CtC)₄Pt(^tBu₃tpy)](PF₆)₂(^tBu₃tpy )

4,4′,4′′-tris-tert-butyl-2,2′:6′,2′′-terpyridine) in CH₃CN, both in

energy and vibronic progression, and this clearly indicates an

acetylenic ³ππ* excited state originating from the octatetraynyl

ligand. Observation of localized acetylenic ³ππ* emission from

the -(CtC)₄- moiety is feasible since it is lower in energy

than the ³MLCT state of the [Pt(C^∧N^∧N)] lumophore. The

energy difference between these two triplet excited states is

3

fluid solution are assigned to MLCT [Pt(5d) f π*(cyclo-

metalating ligand)] excited states in nature, in view of their

solvent-sensitive emission energies and microsecond-regime

lifetimes. It is noteworthy that, (1) the emission for [(C^∧N^∧N)-

PtCtCC₆H₄-4-NO₂] (6) in alcoholic glass at 77 K (λ_max537

nm; structured with vibronic progression of ∼1200 cm^-1

)

resembles the ³IL(4-nitrophenylacetylide) emission of [(Cy₃P)-

AuCtCC₆H₄-4-NO₂] in CH₂Cl₂solution which appears at λ_max

524 nm;³⁹and (2) the emission energy recorded in CH₂Cl₂

(36) Yam, V. W. W.; Wong, K. M. C.; Zhu, N. Angew. Chem., Int. Ed. 2003,

42, 1400-1403.

(37) Yam, V. W. W.; Chong, S. H. F.; Ko, C. C.; Cheung, K. K. Organometallics

2000, 19, 5092-5097.

(38) (a) Klein, A.; van Slageren, J.; Za´lisˇ, S. Inorg. Chem. 2002, 41, 5216-

5225. (b) van Slageren, J.; Klein, A.; Za´lisˇ, S. Coord. Chem. ReV. 2002,

230, 193-211. (c) Klein, A.; van Slageren, J.; Za´lisˇ, S. Eur. J. Inorg. Chem.

2003, 1927-1938.

(39) See Supporting Information and: Lu, W.; Zhu, N.; Che, C. M. J. Am. Chem.

Soc. 2003, 125, 16 081-16 088.

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4969

A R T I C L E S

Lu et al.

evidently small and hence three types of progressions (1360,

2150 and 2115 cm^-1) corresponding to the ground-state vibra-

tions of the (C^∧N^∧N) and octatetraynyl ligands are detected in

the emission spectrum of 13 in alcoholic glassy solution at 77

K (Figure 7). The observation of multiple and site-selective

emissions signify that the octatetraynyl ³ππ* and ³MLCT (L )

C^∧N^∧N) excited states may not be coupled in glassy solutions

and their relative energies and decay pathways are presumably

affected by the local micro-environment.

cm^-1) were observed, while the EL energies for (S^∧N^∧N)-based

emitters are virtually identical (a slight blue shift of less than

100 cm^-1) to the PL values recorded in solution. It is well-

known that ³MLCT excited states are environment-sensitive and

exhibit hypsochromism in a rigid matrix.⁴¹If we regard the

emitting layer of an electroluminescent device as a rigid matrix

of dispersed phosphorescent dopant molecules, the blue shift

of the EL energy relative to the PL is not surprising.

Summary and Prospects

Interestingly, a red-shifted narrow-bandwidth emission is also

observed for complex 21 in degassed fluid solutions, and this

Our results demonstrate that the present class of tridentate

3

is attributed to a ππ* excited state localized on the 1-eth-

cyclometalated Pt(II) σ-alkynyl complexes exhibit highly emis-

3

ynylpyrene ligand that is even lower in energy compared to

the octatetraynyl unit. Nevertheless, as in the case of complex

13, a small contribution from Pt(5d) f π*(alkynyl) excited-

state cannot be excluded, in view of the red-shift from the gold-

(I) congener [(Cy₃P)AuCtCpyrenyl-1] and the observed mild

solvatochromism. Hence, the ligation of the octatetraynyl or

1-ethynylpyrene unit, both of which exhibit triplet π f π*-

(alkynyl) excited states that are lower in energy compared to

[(C^∧N^∧N)Pt] moieties, substantially changes the nature of the

emissive excited state for the [(C^∧N^∧N)Pt(CtC)_nR] lumophores.

MLCT Phosphors as OLED Emitters. If an emitter is

intended for incorporation into a high-performance OLED where

the thickness of each layer requires precise control to obtain

charge balance, this material must tolerate the harsh conditions

during vacuum-deposition processes. The tridentate cyclometa-

lated Pt(II) complexes in the present study possess superior

thermal stability when compared to bidentate aromatic R-diimine

Pt(II) bis(arylacetylides) and other metal-alkynyl complexes that

are also emissive but which easily decompose during vacuum

deposition.⁴⁰We suggest that π-conjugation of the tridentate

aromatic ligand with the Pt-acetylide moiety may impart

sublimability and additional stability to these neutral materials.

In devices with emitter ratios above 2% for 1, 2, 7, 9, 20,

28, and 29, intense orange to red emission is observed while

blue emission from the host and hole-transporting layers is

negligible, implying efficient energy transfer from singlet to

triplet excitons. However, for those devices with emitter ratio

of 2% and for 33 as emitter at doping levels as high as 6%,

both the blue EL of CBP at ca. 440 and orange-red EL are

observed. This may be accredited to incomplete energy transfer

from the CBP host to the triplet emitter. An additional broad

EL band at λ_max710 nm for devices with 7 and 20 as emitter at

doping levels higher than 5% is tentatively ascribed to exciplex

formation through C-H‚‚‚F-C interactions between pentafluo-

rophenylacetylide moieties and the host CBP molecules.

sive excited states that can be MLCT, ππ*(alkynyl) and/or

³IL(cyclometalating ligand) in nature, and the relative energies

of the Pt d-orbitals and the π and π* orbitals of the cyclo-

metalating and σ-alkynyl ligands ultimately govern their emis-

sive characteristics. It is noteworthy that the σ-alkynyl compo-

nent in our complexes can act as photophysically ‘active’ ligands

and constitute an integral part of the electronic structure through

Pt-(alkynyl) π-conjugation, so that another approach to fine-

tune the emissive properties of metal-based phosphorescent

emitters becomes available. Significantly, low-energy narrow-

bandwidth emissions from acetylenic ³ππ* excited states

localized on the -(CtC)₄- and 1-ethynylpyrene auxiliaries are

observed in fluid solutions.

The tridentate cyclometalated Pt(II) σ-alkynyl complexes are

promising light-emitting materials, especially as phosphorescent

dopants for OLED applications, due to their intense, readily

adjustable triplet emissions under ambient conditions and their

thermal stability. For the ³MLCT emitters of the series, the EL

is slightly blue-shifted from the PL recorded in fluid solution,

and this should be taken into account when designing colors

for a multicolor OLED display. Because EL energies can be

fine-tuned through chemical modification of both the tridentate

cyclometalating and σ-alkynyl auxiliaries, it may be feasible to

develop high-efficiency and -brightness multicolor OLEDs based

on these organometallic emitters. A high-throughput combina-

torial approach⁴²is anticipated to be a more efficient method

to achieve high-performance emitters based on the chemistry

and photo- and electroluminescence reported here. Indeed, there

remains significant scope for optimization of device fabrication

and OLED performance.

Experimental Section

General Procedures and Materials. All starting materials were

purchased from commercial sources and used as received unless stated

otherwise. The solvents used for synthesis were of analytical grade.

The compounds 4-nitrophenylacetylene,⁴³pentafluorophenylacetylene,⁴⁴

Ph(CtC)₂H,⁴⁵Me₃Si(CtC)₂H,⁴⁶Me₃Si(CtC)_nSiMe₃(n ) 3,²⁵4²⁶),

1-ethynylpyrene⁴⁷and [(C^∧N^∧N)PtCl] (HC^∧N^∧N ) 6-phenyl-2,2′-

bipyridine)²⁴and [(S^∧N^∧N)PtCl] (HS^∧N^∧N ) 6-(2-thienyl)-2,2′-bipy-

ridine)²⁴were prepared following literature methods. The compound

3

The color of the MLCT (L ) C^∧N^∧N) emission for the

[(C^∧N^∧N)PtCtCR] series can be tuned from green to orange-

red by chemical modification of both the tridentate cyclometa-

lating and arylacetylide ligands, while the ³IL (L ) S^∧N^∧N and

O^∧N^∧N) emissions occur in the red region. The emission

tunability, microsecond-scale lifetime plus their superior thermal

stability renders this class of lumophores suitable as electro-

phosphorescent dopants in OLED applications. The resultant

electroluminescent spectra resemble their photoluminescent

counterparts recorded in solution. For devices employing

(C^∧N^∧N)-based emitters, blue-shifted EL energies (by ca. 500

(41) (a) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998-1003.

(b) Lees, A. J. Comment. Inorg. Chem. 1995, 17, 319-346.

(42) Lavastre, O.; Illitchev, I.; Jegou, G.; Dixneuf, P. H. J. Am. Chem. Soc.

2002, 124, 5278-5279.

(43) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980,

627-630.

(44) Zhang, Y. D.; Wen, J. X. Synthesis 1990, 727-728.

(45) Kende, A. S.; Smith, C. A. J. Org. Chem. 1988, 53, 2655-2657.

(46) Bartik, B.; Dembinski, R.; Bartik, T.; Arif, A. M.; Gladysz, J. A. New J.

Chem. 1997, 21, 739-750.

(40) Ma, Y. G.; Chan, W. H.; Zhou, X. M.; Che, C. M. New J. Chem. 1999,

263-265.

(47) Inouye, M.; Hyodo, Y.; Nakazumi, H. J. Org. Chem. 1999, 64, 2704-

2710.

9

4970 J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004

Tuning of Photo- and Electrophosphorescence

A R T I C L E S

1-(2-furyl)-3-dimethylaminopropan-1-one hydrochloride was obtained

by refluxing a mixture of 2-acetylfuran, Me₂NH‚HCl and paraformal-

dehyde in ethanol. ¹H and ¹³C NMR spectra were recorded on a Bruker

Avance 400 or 300 DRX FT-NMR spectrometer (referenced to residual

solvent) at 298 K. ¹⁹F NMR spectra were recorded on a Bruker Avance

400 (trifluoroacetic acid reference) at 298 K. Mass spectra (FAB) were

obtained on a Finnigan MAT 95 mass spectrometer. Elemental analyses

were performed by Beijing Institute of Chemistry, Chinese Academy

of Sciences. X-ray crystallographic data were collected on a MAR

diffractometer with a 300 mm image plate detector using monochro-

matized Mo KR radiation (λ ) 0.710 71 Å). Details of solvent treatment

for photophysical studies and cyclic voltammetric studies have been

described earlier.⁷

UV-vis spectra were recorded on a Perkin-Elmer Lambda 19 UV-

vis spectrophotometer. Solution samples for emission measurements

were degassed by at least four freeze-pump-thaw cycles. Emission

spectra were obtained on a SPEX Fluorolog-2 Model F111 fluorescence

spectrophotometer. Emission lifetime measurements were performed

with a Quanta Ray DCR-3 pulsed Nd:YAG laser system (pulse output

355 nm, 8 ns). Luminescent quantum yields were referenced to [Ru-

(bpy)₃](ClO₄)₂in acetonitrile (φ ) 0.062) and extrapolated to infinite

dilution (estimated error ( 15%). Self-quenching studies were per-

formed by luminescence lifetime measurements over a range of complex

concentrations (5 × 10^-6to 5 × 10^-4M) and fitted by a modified

Stern-Volmer expression (eq 1)⁴⁸

spectra were performed with the mentioned Nd:YAG laser system with

pulse-width of 8 ns. 355 nm output (third harmonic) was used. The

monitoring light was from a 300 W continuous wave tungsten-halogen

or xenon lamp oriented orthogonally to the direction of the laser pulse.

Transient species was monitored by the monochromatic light from

tungsten or xenon lamp. The transient absorption signals at individual

wavelengths were fed to the Tektronix 2430 or TDS 350 oscilloscope.

The entire optical difference spectrum was generated point to point by

monitoring at each wavelength.

OLED Fabrication and Characterization. Devices were fabricated

with the following configuration: glass/indium tin oxide (ITO: 30

Ω/square)/NPB (400 Å)/CBP doped with tridentate cyclometalated Pt-

(II) σ-alkynyl complex (60 Å)/Alq₃(200 Å)/Mg:Ag (2000 Å). The ITO

glass substrate was cleaned with detergent and deionized water and

dried in an oven for 2 h. It was then treated with UV-ozone for 25 min

before loading into a deposition chamber. The organic films of NPB,

CBP:Pt complex, BCP and Alq₃were sequentially deposited onto the

ITO substrate at a pressure of around 1.0 × 10^-5mbar. The pressure

during deposition of the Pt(II) complex was maintained below 9.0 ×

10^-6mbar to minimize decomposition. The current-voltage-luminance

characteristics and EL spectra were recorded with a computer-controlled

DC power supply and a Spectrascan PR650 photometer at room

temperature. The emission area of the devices is 0.1 cm², as defined

by the overlapping area of the anode and cathode. The external quantum

efficiencies η_extwere calculated based on the method described by

Okamoto et al.⁴⁹using luminance, EL spectrum and current density.

τ₀/τ_obs) 1 + k_qτ₀[Pt]

(1)

Acknowledgment. We are grateful for financial support from

The University of Hong Kong, the Research Grants Council of

Hong Kong SAR, China [HKU 7039/03P and CityU 8730009],

the Croucher Foundation (Hong Kong), and the Innovation and

Technology Commission of the Hong Kong SAR Government

(ITS/053/01). W.L. acknowledges the receipt of his studentship

from The University of Hong Kong. We thank Mr. Wai-Lim

Chan for assistance in device fabrication and Dr. Kung-Kai

Cheung for solving the crystal structure of complex 10.

where k_qis the self-quenching rate constant, [Pt] is the concentration

of Pt complex, and τ₀and τ_obsare the lifetime of the lumophore in

infinite dilution and known concentration, respectively. The radiative

and nonradiative rate constants were calculated using eq 2 and 3,

respectively

-1

k_r) φ₀τ₀

(2)

(3)

k_nr) k_r(φ₀^-1- 1)

Supporting Information Available: Synthesis and charac-

terization data; TGA curves for 1 and 28; additional crystal-

lographic plots and CIFs; supplementary absorption and emis-

sion spectra; cyclic voltammograms; configuration, current

density, voltage and luminance characteristics of selected

OLEDs. This material is available free of charge via the Internet

at http://pubs.acs.org.

where k_rand k_nrare the radiative and nonradiative rate constant,

respectively, φ₀and τ₀are the luminescence quantum yield and lifetime

of the lumophore in infinite dilution, respectively. Transient absorption

(48) Vincze, L.; Sandor, F.; Pem, J.; Bosnyak, G. J. Photochem. Photobiol. A

1999, 120, 11-14.

(49) Okamoto, S.; Tanaka, K.; Izumi, Y.; Adachi, H.; Yamaji, T.; Suzuki, T.

Jpn. J. Appl. Phys. 2001, 40, L783-L784.

JA0317776

9

J. AM. CHEM. SOC. VOL. 126, NO. 15, 2004 4971

Article Doi

Light-Emitting Tridentate Cyclometalated Platinum(II) Complexes Containing σ-Alkynyl Auxiliaries: Tuning of Photo- and Electrophosphorescence

DOI: 10.1021/ja0317776

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ja0317776

Full text of DOI:10.1021/ja0317776

Products guided by the article

R&D Labs maybe for 477891-37-5

Relevant to this article

Hot Product