Synthesis and characterization of phenanthrylphosphine gold complex: Observation of Au-induced blue-green phosphorescence at room temperature

Article abstract of DOI:10.1021/ic048538j

A new 9-diphenylphosphinophenanthrene ligand (9DPP, 1), its oxide (9DPPO, 2), and its gold complex [(AuCl(9DPP)] (3) were synthesized. The Au(I) complex 3 was found to exhibit intense blue-green, room-temperature phosphorescence (Φ_p = 0.06 and τ_T = 22.7 μs) originating in the locally excited triplet of the phenanthrene moiety (³LE) in degassed 2-methyltetrahydrofuran solution. On the assumption that Φ_ST = 1.0 for 3, the radiative rate constant (k_r) in the triplet state is calculated to be 2.6 × 10³ s^-1. This value is 4 orders of magnitude larger than the radiative rate constant of the triplet phenanthrene (0.26 s^-1). Thus, the coordinated Au(I) atom is concluded to have a markedly large heavy-atom effect on k_r of the phenanthrene chromophore in 3.

Full text of DOI:10.1021/ic048538j

Inorg. Chem. 2005, 44, 1157−1159
Synthesis and Characterization of Phenanthrylphosphine Gold
Complex: Observation of Au-Induced Blue-Green Phosphorescence at
Room Temperature
Masahisa Osawa,*,†,‡ Mikio Hoshino,†,‡ Munetaka Akita,‡ and Tatsuo Wada†
RIKEN (The Institute of Physical and Chemical Research), Wako-shi, Saitama 351-0198, Japan,
and Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received October 19, 2004
A new 9-diphenylphosphinophenanthrene ligand (9DPP, 1), its
oxide (9DPPO, 2), and its gold complex [(AuCl(9DPP)] (3) were
synthesized. The Au(I) complex 3 was found to exhibit intense
blue-green, room-temperature phosphorescence (Φp ) 0.06 and
A limited number of examples which emit visual phos-
phorescence from LE at room-temperature include metal
3
complexes3a with a heavy metal such as Pd, Pt, or Rh, in
which the metal acts as a heavy atom perturber. In general,
phosphorescence of polyaromatics including free porphyrins
is usually not observed at room temperature because of the
fact that the nonradiative rate constant, knr ()102-103 s-1),
of the aromatics is much larger than the radiative rate
τT ) 22.7
phenanthrene moiety (3LE) in degassed 2-methyltetrahydrofuran
solution. On the assumption that ΦST 1.0 for 3, the radiative
rate constant (kr ) in the triplet state is calculated to be 2.6
103
µs) originating in the locally excited triplet of the
)
×
constant, kr ()10-1-10-2 s-1), of the T1 state.4 With the use
s-1. This value is 4 orders of magnitude larger than the radiative
rate constant of the triplet phenanthrene (0.26 s-1). Thus, the
coordinated Au(I) atom is concluded to have a markedly large
heavy-atom effect on kr of the phenanthrene chromophore in 3.
-1
of the triplet yield, ΦST, and the lifetime, τT ()kT
)
1/(kr + knr)), the phosphorescence yield ΦP, is expressed as
ΦP ) ΦSTkrτT (1)
Equation 1 implies that a large kr value is favorable for the
observation of visual room-temperature phosphorescence.
The increase in kr in the T1 state of the aromatics can be
attained by either external5 and/or internal3,4 heavy atom
effects. Although enhanced spin-orbit coupling by heavy
atom effects affords an increase in not only kr and ΦST, but
also knr, the increase in kr and ΦST is essentially important
for observation of room-temperature phosphorescence from
the aromatics. This paper reports on room-temperature
phosphorescence from an aromatic ligand observed with
gold(I) complex, in which the heavy atom, Au(I), is
coordinated to a phosphorus atom in a newly synthesized
diphenylphosphino-phenanthrene6 ligand.
In recent years, phosphorescent heavy metal complexes
have attracted considerable interest1 partly because of their
importance as efficient luminescent dopants in organic light-
emitting diodes (OLEDs).2 These complexes exhibit strong
room-temperature phosphorescence, mostly originating in
terms of the 3MLCT (metal to ligand charge-transfer) excited
state, which is located in energy below the locally excited
3
triplet state of the organic ligand, LE.
* To whom correspondence should be addressed. E-mail: osawa@
postmsn.riken.jp.
† RIKEN.
‡ Tokyo Institute of Technology.
(1) For selected examples, see: (a) Sprouse, S.; King, K. A.; Spellane, P.
J.; Watts, R. J. J. Am. Chem. Soc. 1984, 106, 6647. (b) Yam, V. W.
W.; Li, C. K.; Chan, C. L. Angew. Chem., Int. Ed. 1998, 37, 2857. (c)
Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong,
R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg.
Chem. 2001, 40, 1704. (d) Yam, V. W. W.; Chan, C.-L.; Li, C.-K.;
Wong, K. M.-C. Coord. Chem. ReV. 2001, 216-217, 173. (e) Lee,
Y.-A.; McGarrah, J. E.; Lachicotte, R. J.; Eisenberg, R. J. Am. Chem.
Soc. 2002, 124, 10662.
(2) For selected examples, see: (a) Baldo, M. A.; O’Brien, D. F.; You,
Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature
1998, 395, 151. (b) Baldo, M. A.; Lamansky, S.; Burrows, P. E.;
Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (c)
Ma, Y.; Che, C.-M.; Chao, H.-Y.; Zhou, X.; Chan, W.-H.; Shen, J.
AdV. Mater. 1999, 11, 852. (d) Lamansky, S.; Djurovich, P.; Murphy,
D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest,
S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (e) Che,
C.-M.; Chan, S.-C.; Xiang, H.-F.; Chan, M. C. W.; Liu, Y.; Wang, Y.
Chem. Commun. 2004, 1484.
The syntheses of the ligand and the related compounds
are as follows. Lithiation of 9-bromophenanthrene with
(3) For the internal heavy atom approach, see: (a) Gouterman, M. The
Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol.
III, Part A, Chapter 1. (b) Tzeng, B.-C.; Li, D.; Peng, S.-M.; Che,
C.-M. J. Chem. Soc., Dalton Trans. 1993, 2365. (c) Knor, G.; Strausser,
A. Inorg. Chem. Commun. 2002, 5, 993. (d) Che, C.-M.; Chao, H.-
Y.; Miskowski, V. M.; Li, Y.; Cheung, K.-K. J. Am. Chem. Soc. 2001,
123, 4985. (e) Chao, H.-Y.; Lu, W.; Li, Y.; Chan, M. C. W.; Che,
C.-M.; Cheung, K.-K.; Zhu, N. J. Am. Chem. Soc. 2002, 124, 14696.
(4) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:
New York, 1970.
(5) For the external heavy atom approach, see: (a) Scypinski, S.; Cline
Love, L. J. Anal. Chem. 1984, 56, 322. (b) Hamai, S. J. Am. Chem.
Soc. 1989, 111, 3954. (c) Hamai, S. J. Chem. Soc., Chem. Commun.
1994, 2243.
10.1021/ic048538j CCC: $30.25
Published on Web 02/01/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 5, 2005 1157

COMMUNICATION
Figure 2. Absorption spectra of phenanthrene (- - -), 1 (red), 2 (yellow),
and 3 (blue) in CH2Cl2.
Figure 1. The structures of 1, 2, and 3.
n-BuLi in THF took place smoothly and was followed by
addition of an equimolar amount of PClPh2. After chromato-
graphic purification, the desired ligand (9DPP, 1) was
obtained as a white powder. Compound 1 is fairly stable in
CHCl3 and does not react with oxygen in the dark. However,
upon illumination with UV light, 1 is readily converted to
its oxide (9DPPO, 2). An authentic sample of 2 was prepared
by oxidation of 1 with hydrogen peroxide.6a
The Au complex (3) was prepared by the reaction of AuCl-
(Me2S)7 with a stoichiometric amount of 1 in dichloro-
methane to give a white powder in high yield. Analytically
pure compounds (1-3) have been characterized by 1H NMR,
31P NMR, and MS spectroscopy.
The structures of 2 and 3 determined by X-ray crystal-
lography are shown in Figure 1.8 The two structures including
the geometry near the P atom are very similar. The bond
lengths of Au-P and Au-Cl are 2.236(1) and 2.302(1) Å,
respectively. These values are identical with those of the
structural analogue, [AuCl(PR3)] (R ) Ph,9a Me,9b iPr9b).
Absorption spectra of 1-3 in CH2Cl2 are shown in Figure
2. In comparison with the absorption spectrum of phenan-
threne (PT), the phosphorus substitution in 1-3 commonly
Figure 3. Emission spectra A-D in degassed 2-MeTHF (λexc ) 300 nm).
The inset shows the decay profile of emission observed for 3 after 266 nm
laser pulse at 297 K. The initial spike observed immediately after the pulse
is due to fluorescence. The picture displays luminescence of degassed
2-MeTHF solutions of B and C at 297 K.
induces a red shift of the absorption bands of the phenan-
threne chromophore. Because of the absence of lone pair
electrons at the phosphorus atom, the absorption spectra of
2 and 3 are very similar. The molecular absorption coef-
ficients (ꢀ’s) for the first absorption bands of 2 and 3 (320-
360 nm) are ca. 2 times that of PT.
(6) Several diphenylphosphino substituted polyaromatic fluorescent ma-
terials have been reported, see: (a) Akasaka, K.; Suzuki, T.; Ohrui,
H.; Meguro, H. Anal. Lett. 1987, 731. (b) Heuer, L.; Schmultzler, R.
J. Fluorine Chem. 1988, 39, 197. (c) Padmaperuma, A. B.; Schmett,
G.; Forgarty, D.; Washton N.; Nanayakkara, S.; Sapochak, L.;
Ashworth, K.; Madrigal, L.; Reeves, B.; Spangler, C. W. Mater. Res.
Soc. Symp. Proc. 2000, 621(Electron-Emissive materials, Vacuum
Microelectronics and Flat-Panel Displays), Q3.9.1-Q3.9.6. (d) Yip,
J. H. K.; Probhavathy, J. Angew. Chem., Int. Ed. 2001, 40, 2159. (e)
Fei, Z.; Kocher, N.; Mohrschladt, C. J.; Ihmels, H.; Stalke, D. Angew.
Chem., Int. Ed. 2003, 42, 783.
Figure 3 shows emission spectra of 1-3 observed in
degassed 2-methyltetrahydrofuran (2-MeTHF) (1-3, at 297
K; 2, at 77 K). Fluorescence of 1 at 297 K shows a weak
and broad band centered at ca. 550 nm, and the Stokes’ shift
is as large as ca. 14000 cm-1 (A in Figure 3). This featureless
fluorescence is tentatively assigned to the ICT (intramolecular
charge-transfer) fluorescence from the diphenylphosphino
group to the PT moiety on the basis of the marked red-shift
in polar solvents.10,11 In sharp contrast to 1, 2 and 3 do not
display the ICT fluorescence.
(7) Mu¨ller, T. E.; Green, J. C.; Mingos, D. M. P.; McPartlin, C. M.;
Wittingham, C.; Williams, D. J.; Woodroffe, T. M. J. Organomet.
Chem. 1998, 551, 313.
(8) X-ray data were collected at 100.2 K on a Rigaku RAXIS-CS Imaging
Plate diffractometer with Mo KR radiation. Structure solutions were
obtained by standard Patterson and difference Fourier methods and
refined on F2. 9DPPO: a ) 9.0031(6) Å, b ) 10.5751(8) Å, c )
11.1424(7) Å, R ) 80.126(2)°, â ) 73.148(2)°, γ ) 75.032(2)°, V )
975.5(1) Å3, triclinic P1h, Z ) 2, θmin/max ) 1.90/30.3°, 5180 unique
observed reflections were used to refine 253 atomic parameters and
gave a final R factor of 0.0674. AuCl(9DPP): a ) 8.9688(5) Å, b )
10.9280(5) Å, c ) 11.7414(7) Å, R ) 91.210(3)°, â ) 109.454(2)°,
The fluorescence spectrum of 2 observed at 298 K (B in
Figure 3) is almost identical with that of PT suggesting that
a strong emission band in the region of 360-410 nm is
attributed to fluorescence from the locally excited singlet
state of the PT moiety in 2, 1LE. The fluorescence quantum
γ ) 97.417(3)°, V ) 1073.5(1) Å3, triclinic P1h, Z ) 2, θmin/max
)
1.80/30.3°, 5781 unique observed reflections were used to refine 262
atomic parameters and gave a final R factor of 0.0423.
(10) See the Supporting Information Figure S1. (a) Lippert, E. Z. Elektro-
chem. 1957, 61, 962. (b) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull.
Chem. Soc. Jpn. 1956, 29, 465.
(11) Ching, K. C.; Lequan, M.; Lequan, R. M.; Grisard, A.; Markovitsi,
D. J. Chem. Soc., Faraday Trans. 1991, 87, 2225.
(9) (a) Baenziger, N. C.; Bennett, W. E.; Soborof, D. M. Acta Crystallogr.
1976, B32, 962. (b) Angermaier, K.; Zeller, E.; Schmidbaur, H. J.
Organomet. Chem. 1994, 472, 371.
1158 Inorganic Chemistry, Vol. 44, No. 5, 2005

COMMUNICATION
yield of 2 (ΦF ) 0.1712 in 2-MeTHF) at 297 K is higher
than that of PT (ΦF ) 0.0612 in 2-MeTHF). At 77 K, the
phosphorescence spectrum with peaks at 470, 502, and 540
nm is observed for 2 (D in Figure 3), and the phosphores-
cence lifetime is determined to be ca. 5 s. The phosphores-
cence spectrum is very similar to that of PT, indicating that
phosphorescence originates in the locally excited triplet of
chromophore at T1 due to partial removal of the spin-
forbidden nature of the T1-S0 radiative transition.
From eq 1, the radiative rate constant, kr, at T1 is
formulated as
kr ) ΦP/(τTΦST)
(2)
The Au(I) complex (3) affords Φp ) 0.0612 and τT ) 22.7
µs at 298 K. On the assumption that ΦST ) 1.0 for 3, kr is
calculated to be 2.6 × 103 s-1. This value is 4 orders of
magnitude larger than the radiative rate constant of the triplet
phenanthrene (0.26 s-1).14a Thus, the coordinated Au(I) atom
is found to give a markedly large heavy atom effect on kr of
the phenanthrene chromophore in 3, and the use of Au(I)
atom is more effective in increasing kr than halogen substit-
uents which are expected to have a significant heavy atom
effect (kr values of 9-bromo- and 9-iodophenanthrene are
92.6 and 578 s-1, respectively).14b
The present results are summarized as follows: (1)
molecule 1 gives ICT fluorescence, (2) molecule 2 exhibits
fluorescence much stronger than PT, and (3) molecule 3
emits blue-green room-temperature phosphorescence due to
the effects of Au(I) atom.
3
the PT moiety in 2, LE.
The Au complex (3) in degassed 2-MeTHF demonstrates
blue-green emission at ca. 500 nm at 297 K (C in Figure 3).
This emission is attributed to room-temperature phos-
phorescence from phenanthrene chromophore (3LE) of 3 on
the basis of the following observations: (1) the lifetime of
the emission is as long as 22.7 µs, and (2) the emission
spectrum with the vibronic structure (intervals are ca. (1.3-
1.4) × 103 cm-1) agrees well with that of the phosphores-
cence spectrum of PT at 77 K. It is noteworthy that
fluorescence from the phenanthrene chromophore in 3 is very
weak.
As mentioned above, room-temperature phosphorescence
of the Au(I) complex (3) in 2-MeTHF occurs from the
phenenthrene chromophore. Presumably, the MLCT state of
the Au complex (3) is located higher in terms of energy than
The detailed photophysical studies of these compounds
as well as the synthesis of related heavy atom metal
complexes are in progress.
3
the LE state. It is suggested that the strong spin-orbit
coupling caused by the Au atom (Z ) 79)13 in 3 leads to (1)
an efficient intersystem crossing from the singlet to the triplet
excited state, and (2) the large kr value of the phenanthrene
Supporting Information Available: Synthetic procedures,
characterization data, X-ray crystallographic data (PDF and CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) The fluorescence quantum yields were determined by standard methods
with anthracene in EtOH (ΦF ) 0.27 for 2) and quinine sulfate in 1
N H2SO4 (ΦF ) 0.546 for 3) as references: Eaton, D. F. Pure Appl.
Chem. 1988, 60, 1055.
(13) The related Cu(I) complex did not exhibit room-temperature phos-
phorescence, to be submitted.
IC048538J
(14) (a) Miller, J. C.; Breakstone, K. U.; Meek, J. S.; Strickler, S. J. J. Am.
Chem. Soc. 1977, 99, 1142. (b) Miller, J. C.; Meek, J. S.; Strickler, S.
J. J. Am. Chem. Soc. 1977, 99, 8175.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1159