Long-lived emission from platinum(II) terpyridyl acetylide complexes

Article abstract of DOI:10.1021/ic025580a

Photoluminescence with high quantum yield and long lifetime from a triplet metal-to-ligand charge transfer (MLCT) excited state in fluid solution at room temperature has been observed for a series of platinum(II) 4′-p-tolyl-terpyridyl acetylide complexes.

Full text of DOI:10.1021/ic025580a

Inorg. Chem. 2002, 41, 5653−5655
Long-Lived Emission from Platinum(II) Terpyridyl Acetylide Complexes
Qing-Zheng Yang, Li-Zhu Wu,* Zi-Xin Wu, Li-Ping Zhang, and Chen-Ho Tung*
Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences,
Beijing 100101, P.R. China
Received March 11, 2002
Chart 1. Structure of Complexes 1-6
Photoluminescence with high quantum yield and long lifetime from
a triplet metal-to-ligand charge transfer (MLCT) excited state in
fluid solution at room temperature has been observed for a series
of platinum(II) 4′-p-tolyl-terpyridyl acetylide complexes.
The platinum(II) terpyridyl complexes have attracted
considerable attention in recent years, mainly because of their
interesting spectroscopic behavior1-6 and useful physical and
biological properties.7,8 Indeed, these complexes have been
established as important DNA intercalators7,9 and protein
probes.10 The spectroscopic properties and low-energy
absorptions generally arise from metal-to-ligand charge
transfer (MLCT) transitions. However, the development of
the photochemistry of these complexes is limited by their
nonemissive or short-lived MLCT excited states in solution
at room temperature.1 This lack of emission originates from
low-lying d-d excited states, which provide facile nonra-
diative deactivation pathways via molecular distortion. For
these complexes, the d-d and MLCT excited states are very
close in energy, resulting in fast internal conversion from
MLCT to d-d excited states. To obtain emissive platinum-
(II) terpyridyl complexes with long-lived excited states and
high emission quantum yields, many efforts have been
performed.1-6 The main strategy to construct long-lived and
emissive platinum(II) terpyridyl complexes involves utilizing
substituted terpyridyl ligands with low-lying LUMO and/or
ancillary ligands with large electron-donating ability to raise
the HOMO of the metal center, hence resulting in the
reduction of the MLCT excited state energy. As a result,
the energy difference between the MLCT and the d-d states
increases, and the nonradiative deactivation via d-d states
decreases. However, apart from some 4′-substituted [Pt(trpy)-
Cl]+ complexes bearing large aryl groups (the emission of
which mainly arises from their intraligand excited states
rather than MLCT states1), there is no example of a platinum-
3
(II) terpyridyl complex which exhibits MLCT emission in
fluid solution with a lifetime greater than 1 µs and emission
quantum yield greater than 1 × 10-2. Here, we are able to
vary the energy of MLCT states of platinum(II) terpyridyl
complexes by using a series of substituted acetylide ligands
and a 4′-tolyl substituted terpyridyl ligand (Chart 1), and we
found that these complexes exhibit long-lived emission from
3
the MLCT states with high emission quantum yields. In
* To whom correspondence should be addressed. E-mail: chtung@
ipc.ac.cn.
(1) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193.
dichloromethane solution at room temperature, two of these
complexes show lifetimes greater than 10 µs and emission
quantum yields greater than 0.25.
Complexes 1-5 (Chart 1) were synthesized by two steps.
First, the starting material, [Pt(4′-p-tolyl-trpy)Cl]Cl (trpy )
2,2′:6′,2′′-terpyridine), was prepared by a literature method11
using 4′-p-tolyl-terpyridine and K2PtCl4 as the reagents. This
material was then reacted with 2 equivalent amounts of
acetylide in DMF in the presence of catalyst CuI and
triethylamine at room temperature. After metathesis reaction
by NaClO4 and recrystallization of the crude product by
vapor diffusion of diethyl ether into an acetonitrile solution,
complexes 1-5 were obtained as orange crystals with ca.
(2) Lai, S. W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M. Inorg. Chem.
1999, 38, 4262.
(3) Bu¨chner, R.; Cunningham, C. T.; Field, J. S.; Haines, R. J.; McMillin,
D. R.; Summerton, G. C. J. Chem. Soc., Dalton Trans. 1999, 711.
(4) Bu¨chner, R.; Field, J. S.; Haines, R. J.; Cunningham, C. T.; McMillin,
D. R. Inorg. Chem. 1997, 36, 3952.
(5) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000,
39, 2708.
(6) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Cheung, K. K.
Organometallics 2001, 20, 4476.
(7) Lippard, S. J. Acc. Chem. Res. 1978, 11, 211.
(8) Peyratout, C. S.; Aldridge, T. K.; Crites, D. K.; McMillin, D. R. Inorg.
Chem. 1995, 34, 4484.
(9) Arena, G.; Monsu´ Scolaro, L.; Pasternack, R. F.; Romeo, R. Inorg.
Chem. 1995, 34, 2994.
(10) Ratilla, E. M. A.; Brothers, H. M., II.; Kostic´, N. M. J. Am. Chem.
Soc. 1987, 109, 4592.
(11) Yip, H. K.; Cheng, L. K.; Cheung, K. K.; Che, C. M. J. Chem. Soc.,
Dalton Trans. 1993, 2933.
10.1021/ic025580a CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/28/2002
Inorganic Chemistry, Vol. 41, No. 22, 2002 5653

COMMUNICATION
Table 2. Electrochemical Data for Complexes 1-5a
oxidationb
reductionc
complex
Epa/V vs SCE
E
1/2/V vs SCE
1
2
3
4
5
+1.56
+1.50
+1.17
+1.10
+1.26
-0.88, -1.30
-0.84, -1.35
-0.85, -1.33
-0.83, -1.31
-0.83, -1.29
a In DMF solution with 0.1 M nBu4NPF6 (TBAH) as supporting
electrolyte at room temperature; scan rate 200 mV s-1
anodic peak potential for the irreversible oxidation waves. E1/2 ) (Epa
Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials, respectively.
.
b Epa refers to the
c
+
π*(trpy) MLCT transition. However, interpretation of the
MLCT bands is complicated by the fact that these bands
clearly are composed of two distinct transitions. This
observation is precedented for platinum(II) diimine bis-
(acetylide) complexes.13 It is noteworthy that as the electron-
donating ability of the acetylide ligands increases, the MLCT
absorption bands red shift by ca. 2900 cm-1 over the series
1-5. This phenomenon is consistent with the notion that
the HOMO is predominantly metal-based. An electron-rich
acetylide ligand would render the metal center more electron-
rich and hence raise the dπ(Pt) orbital energy, leading to
lower energy absorption. To support this proposal, we
performed cyclic voltammetry studies, and the data are listed
in Table 2. All the complexes exhibit two quasireversible
couples at ca. -0.85 and -1.33 V versus SCE. The
insensitivity of the potentials toward the acetylide ligands
suggests that these couples originate mainly from the
terpyridyl reductions with some mixing of the Pt(II) metal
character.6 Irreversible anodic waves were observed at ca.
+1.10 to +1.56 V versus SCE. With reference to the
previous studies on other platinum complexes,6,14 metal-
centered oxidation from Pt(II) to Pt(III) is tentatively
assigned. The oxidations of complexes 1 and 2 occur at more
positive potentials than the other three. This observation
could be attributed to the electron delocalization within the
aryl and acetylide groups in the aryl-acetylide ligands,
resulting in larger electron-donating ability compared to that
of the alkyl-acetylide ligands.
Complexes 1-5 show well-resolved vibronic structured
emission at 510-700 nm in methanol/ethanol (1:1) glass at
77 K. Figure 2 gives the emission spectrum of 4 upon
excitation at 430 nm and the excitation spectrum for this
emission. The emission mirrors the excitation spectrum. The
vibronic internal is ca. 1270 cm-1, which corresponds to the
aromatic vibrational mode of the terpyridyl ligands.6 All these
complexes display intense photoluminescence in fluid solu-
tion at room temperature. Figure 3 shows the emission
spectra of these complexes in degassed dichloromethane at
room temperature, obtained on a Perkin Elemer LS50B
spectrofluorometer. In addition, the photoluminescence quan-
tum yields (Φem) were determined by the optical dilute
method15 using a degassed acetonitrile solution of [Ru(bpy)3]-
Figure 1. Absorption spectra of complexes 1-5 measured in CH2Cl2
solution.
Table 1. Photphysical Properties of Complexes 1-6 in CH2Cl2 at 298
K
complex
λab/nm (ꢀ/dm3 mol-1 cm-1
)
λem τ (µs) Φem
1
274 (35900), 289 (34300), 315 (18500),
328 (18000), 342 (19300), 430 (7260)
276 (37800), 290 (44300), 317 (28100),
345 (28800), 420sh (8250), 441 (10200)
273 (47400), 316 (21500), 342 (21500),
430 (7770), 477 (7280)
266 (45300), 289 (37000), 317 (23900),
342 (23900), 433 (8270), 482 (7340)
268 (43000), 287 (34500), 317 (21700),
343 (20600), 432 (7530), 492 (6660)
287 (26700), 318 (15600), 329 (13500),
345 (13000), 422 (4570), 477 (4290)
552 14.6 0.30
2
3
4
5
6
580 10.3 0.25
611 4.7 0.071
619 4.6 0.052
639 0.8 0.0076
618 1.9 0.036
76% yields. Complex 6 was synthesized by similar proce-
dure. The identities of all of the complexes were confirmed
by 1H NMR spectroscopy, HR-MS spectrometry, and satis-
factory elemental analyses, which are given in the Supporting
Information. All of the complexes are soluble in organic
solvents (DMF, CH3CN, CH2Cl2) and very stable both as
solids and in solution.
UV-vis spectra were measured on a Shimadzu UV-1601
PC spectrophotometer. All the complexes in dichloromethane
solution exhibit intense vibronic-structured absorption bands
at λ < 350 nm with extinction coefficients (ꢀ) on the order
of 104 dm3 mol-1 cm-1, and less intense band at 400-500
nm with ꢀ on the order of 103 dm3 mol-1 cm-1. The
absorption spectral properties were found to follow Beer’s
Law below saturated concentration (ca. 5 × 10-5 mol dm-3)
in this solvent, suggesting no any significant complex
aggregation occurred. Table 1 summarizes the band maxima
and the extinction coefficients. Figure 1 shows the long-
wavelength region of the spectra. With reference to previous
spectroscopic work on platinum(II) terpyridyl complexes,1-6
the absorption bands at λ < 350 nm are assigned to the
intraligand (IL) transition of terpyridyl and acetylide ligands
as well as the charge transfer transition involved in the Pts
CtCR moieties,12 while the low energy absorption bands
at 400-500 nm are tentatively assigned to the dπ(Pt) f
(13) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053.
(14) Blanton, C. B.; Murtaza, Z.; Shaver, R. J.; Rillema, D. P. Inorg. Chem.
1992, 31, 3230.
(15) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(12) Masai, H.; Sonogashiram, K.; Hagihara, N. Bull. Chem. Soc. Jpn. 1971,
44, 2226.
5654 Inorganic Chemistry, Vol. 41, No. 22, 2002

COMMUNICATION
The large Stokes shift and lifetime in the microsecond
range for the photoluminescence of 1-5 suggest that the
emission originates from a triplet parentage, which is
3
tentatively assigned as arising from the MLCT state. An
excimeric emission assignment is not preferred because the
emission maxima are independent of the complex’s concen-
tration below saturation. For comparison, the photolumines-
cence of a chloride derivative [Pt(4′-p-tolyl-trpy)Cl]+ 11 in
dichloromethane at room temperature was measured. The
3MLCT excited state of this complex possesses higher energy
(emission occurs at λmax ) 540 nm, τ ) 0.54 µs) than those
of complexes 1-5. Furthermore, over the series 1-5, the
emission maxima shift to longer wavelengths as the electron-
donating ability of the acetylide ligands increases (Table 1),
which is consistent with the observations by Eisenberg,17
Yam,6 and others13 on the aromatic diimine, terpyridyl, and
cyclometalated platinum(II) acetylide complexes. These
observations further demonstrate that one can use an electron-
rich ancillary ligand to raise the HOMO energy of the metal
Figure 2. Emission and excitation spectra of complex 4 in methanol/
ethanol (1:1) glass at 77 K.
3
center, and hence to decrease the energy of the MLCT
excited states. Significantly, 1-5 show high photolumines-
cence quantum yields and long lifetimes in solution at room
temperature. Compounds 1 and 2, the complexes with alkyl-
substituted acetylide ligands, are even more unique in their
emission lifetimes and quantum yields. For example, complex
1 shows emission with lifetime of ca. 15 µs and quantum
yield of ca. 0.30 in degassed dichloromethane at room
temperature. To our knowledge, this is the first example of
Pt(II) terpyridyl complexes which exhibit photoluminescence
with such a long lifetime and high quantum yield. This
observation may be rationalized by the fact that the strong
electron-donating acetylide ligands raise the HOMO of the
metal center, thus reducing the MLCT excited state energy.
On the other hand, the strong ligand field of the CtCR
moiety increases the energy of the d-d excited state. As a
result, the energy difference between the MLCT and the d-d
states increases, and the radiationless decay of the MLCT
state via d-d state is less prevalent. However, inspection of
the data in Table 1 reveals that the emission lifetime and
quantum yield decrease with the increase in the electron-
donating ability of the acetylide ligand across series 1-5.
This trend is in accord with the energy gap law.18
Figure 3. Emission spectra of complexes 1-5 measured in deoxygenated
CH2Cl2 solution.
(PF6)2 as the reference (Φem ) 0.062).16 The photolumines-
cence decay lifetimes were measured using a conventional
laser system. The excitation source was 355 nm output (third
harmonic, 10 ns of pulsed Nd:YAG laser, 10 Hz). The
photoluminescence quantum yields and lifetimes for com-
plexes 1-5 are listed in Table 1. Yam and co-workers6
reported the photoluminescence quantum yields and lifetimes
in acetonitrile solution of a series of platinum(II) terpyridyl
acetylide complexes, [Pt(trpy)CtCR]PF6, where R is a
substituted phenyl group. These complexes show photolu-
minescence quantum yields smaller than 0.012 and lifetimes
shorter than 0.5 µs in acetonitrile at room temperature. For
comparison, we chose one of these complexes, [Pt(trpy)-
(CtCC6H5)]ClO4 (6), which exhibits the greatest emission
quantum yield and the longest lifetime in acetonitrile among
these complexes, and we measured its emission quantum
yield and lifetime in dichloromethane. The data are also listed
in Table 1.
Acknowledgment. Financial support from the Ministry
of Science and Technology of China (Grants G2000078104
and G2000077502) and the National Science Foundation of
China is gratefully acknowledged. We also thank Mr. Jun-
Feng Xiang for the lifetime measurements and Dr. Jia-Hong
Zhou for the cyclic voltammetry measurements.
Supporting Information Available: The data of 1H NMR
spectroscopies, HR-MS spectrometries, elemental analyses, and
infrared spectroscopies of complexes 1-5. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC025580A
(17) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachiocotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447.
(18) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
(16) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.;
Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5655