Platinum(II) diimine complexes of acetylides containing 7-azaindolyl and 2,2′-dipyridylamino functional groups

Article abstract of DOI:10.1039/b305408j

Four Pt(II) diimine complexes containing acetylide ligands that are functionalized by 7-azaindolyl or 2,2′-dipyridylamino groups have been synthesized and fully characterized. Complexes 1 and 2 have the general formula Pt(bpy)(C≡C-C₆H₄-R)₂ where bpy = 2,2′-bipyridine, R = N-7-azaindolyl for 1 and 2,2′-dipyridylamino for 2, while complexes 3 and 4 have the general formula Pt(phen)(C≡C-C₆H₄-R)₂ where phen = 1, 10-phenanthroline, R = N-7-azaindolyl for 3 and 2,2′-dipyridylamino for 4. The structures of 1 and 4 have been determined by single-crystal X-ray diffraction analysis. All four complexes display orange or green emission when irradiated by UV light at ambient temperature. The emission energy of 2 and 4 in solution is concentration-dependent, attributable to excimer formation at high concentrations. The utility of the 2,2′-dipyridylamino functions groups in complexes 2 and 4 was demonstrated by the interaction of 2 with Zn(II) ions via the dipyridyl chelating site, that leads to a change in emission intensity.

Full text of DOI:10.1039/b305408j

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Platinum(II) diimine complexes of acetylides containing
7-azaindolyl and 2,2Ј-dipyridylamino functional groups†
Youngjin Kang, Junghyun Lee, Datong Song and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Received 14th May 2003, Accepted 31st July 2003
First published as an Advance Article on the web 18th August 2003
Four Pt() diimine complexes containing acetylide ligands that are functionalized by 7-azaindolyl or 2,2Ј-dipyridyl-
amino groups have been synthesized and fully characterized. Complexes 1 and 2 have the general formula Pt(bpy)-
᎐
(C᎐C–C H -R) where bpy = 2,2Ј-bipyridine, R = N-7-azaindolyl for 1 and 2,2Ј-dipyridylamino for 2, while complexes
᎐
6
4
2
᎐
3 and 4 have the general formula Pt(phen)(C᎐C–C H -R) where phen = 1, 10-phenanthroline, R = N-7-azaindolyl for
᎐
6
4
2
3 and 2,2Ј-dipyridylamino for 4. The structures of 1 and 4 have been determined by single-crystal X-ray diffraction
analysis. All four complexes display orange or green emission when irradiated by UV light at ambient temperature.
The emission energy of 2 and 4 in solution is concentration-dependent, attributable to excimer formation at high
concentrations. The utility of the 2,2Ј-dipyridylamino functions groups in complexes 2 and 4 was demonstrated
by the interaction of 2 with Zn() ions via the dipyridyl chelating site, that leads to a change in emission
intensity.
2,2Ј-bipyridine, 1,10-phenanthroline, X = N-7-azaindole, 2,2Ј-
Introduction
dipyridylamine. Herein, we report the syntheses, crystal struc-
tures and luminescent properties of the new platinum diimine
arylacetylides.
There has been considerable interest in transition metal
σ-acetylide complexes and polymers due to their various appli-
cations in non-linear optics, molecular photochemical devices,
as well as supramolecular chemistry.1 Among numerous transi-
tion metal σ-acetylide complexes, square planar platinum()
diimine bis(acetylide) complexes, in particular, have attracted
much attention recently. It has been established by Che and co-
workers2a and many others that Pt(diimine)(arylacetylide)
complexes often show bright emission in fluid solution and the
Experimental
All experiments were carried out under a dry nitrogen atmos-
phere using standard Schlenk techniques. Benzene, hexane and
THF were freshly distilled over sodium and benzophenone. Di-
chloromethane was dried and distilled over P2O5. All starting
materials were purchased from either Aldrich or Strem and
3
solid state with a long-decay life time, arising from MLCT.
Some of the platinum() diimine bis(acetylide) complexes
exhibit tunable orange–red emission with high quantum effi-
ciency and long-lived MLCT excited-states,3 making them
potential emitters in electroluminescent (EL) devices and also
luminescent sensors in photovoltaic cells.4 Notably, such Pt()
complexes have been demonstrated to have potential as
chromophores in the conversion of light-to-chemical energy.
For example, Eisenberg and co-workers have designed and
developed dyad or triad systems, which contain either one- or
two-electron donor/acceptor components in a platinum diimine
chromophore.5 These platinum diimine complexes have been
shown to have long-lived excited states, attributed to metal-to-
ligand charge transfer (MLCT) with contribution from Pt (dπ)
and diimine (π*) orbitals.
Although there have been extensive studies on luminescence
and the properties of excited states of Pt(diimine)bis(acetylide)
complexes, the variation of such systems — for example, the
introduction of a highly emissive and functional group into the
ligand framework — remains virtually unexplored. Our recent
studies have shown that 7-azaindolyl and 2,2Ј-dipyridylamino
groups are highly emissive and capable of binding to metal ions
and protons.6 Attachment of these two functional groups to the
acetylide ligands would allow us to obtain new Pt() diimine
bis(acetylide) complexes that have vacant binding sites for
potential expansion to supramolecular structures and interest-
ing luminescent properties as well. Based on these consider-
ations, we carried out the synthesis of several new platinum
diimine bis(arylacetylide) complexes that contain blue lumin-
1
used without further purification. H, and 13C NMR spectra
were recorded on a Bruker Avance 300- or 400-MHz spectro-
meter. UV/Vis spectra were obtained on a Hewlett Packard
8562A diode array spectrophotometer with all sample con-
centrations in the range 10–50 µM. Excitation and emission
spectra were recorded on a Photon Technologies International
QuantaMaster Model 2 spectrometer. Photoluminescent life-
times were measured on a Photon Technology International
Phosphorescent lifetime spectrometer, Timemaster C-631F
equipped with a xenon flash lamp, and digital emission photon
multiplier tube using a band path of 5 nm for both excitation
and emission. All solutions for photophysical experiments were
degassed with more than three repeated freeze–pump–thaw
cycles on a vacuum line. Elemental analyses were performed by
Canadian Microanalytical Service Ltd., Delta, British Colum-
bia, Canada. Melting points were determined on a Fisher-
Johns melting point apparatus. Emission quantum yields of the
complexes 1–4 were obtained relative to tris(2,2Ј-bipyridyl)-
ruthenium() in degassed acetonitrile (Φr = 0.062) at 298 K, and
calculated by using previously reported procedures.7 The
absorbance of all the samples and the standard at the excitation
wavelength were approximately 0.1. Cyclic voltammetry was
performed using a BAS CV-50W analyzer with scan rates of
50 m V sϪ1. The electrolytic cell used was a conventional three
compartment cell, with a Pt working electrode, Pt auxiliary
electrode, and Ag/AgCl reference electrode. All experiments
were performed at room temperature using 0.10 M tetrabutyl-
ammonium hexafluorophosphate (TBAP)/acetonitrile as the
supporting electrolyte. The ferrocenium/ferrocene couple (Ep =
0.46 V, ∆Ep = 86 mV) was used as the internal standard.
Pt(bpy)Cl2,8 Pt(phen)Cl2,9 4-(N-7-azaindolyl)phenylacetylene10
and 4-(2,2Ј-dipyridylamino)phenylacetylene10 were prepared by
previously reported procedures.
escent ligands such as 7-azaindole or 2,2Ј-dipyridylamine with
the general formula Pt(diimine)(C᎐CC H -X) with diimine =
᎐
᎐
6
4
2
† Electronic supplementary information (ESI) available: emission spec-
tra of 1–4 in the solid state at 298 and 77 K; structures and packing
diagrams of 1 and 4. See http://www.rsc.org/suppdata/dt/b3/b305408j/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 4 9 3 – 3 4 9 9
3493

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Preparation of platinum diimine bis(acetylide) complexes
in CD2Cl2 (δ, ppm, 25 ЊC): 10.0 (dd, J = 5.1, 1.2 Hz, 2H), 8.73
(dd, J = 8.4, 1.2, 2H), 8.31 (ddd, J = 4.8, 1.8, 0.6 Hz, 4H), 8.08
(s, 2H), 8.0 (dd, J = 8.1, 5.1 Hz, 2H), 7.62 (ddd, J = 15.6, 7.5, 2.1
Hz, 4H), 7.55, 7.12 (AAЈBBЈ, JAB = 8.7 Hz, 4H, 4H), 7.07 (d,
J = 8.1 Hz, 4H), 6.98 (ddd, J = 7.2, 5.1, 0.9 Hz, 4H). 13C NMR
in CDCl3 (δ, ppm, 25 ЊC): 165.8, 158.8, 148.9, 145.2, 137.9,
133.5, 130.5, 128.6, 128.2, 127.4, 126.0, 123.2, 118.7, 117.6,
Complexes 1–4 were synthesized by using Sonogashira
coupling.11
Method A. To a Schlenk flask containing Pt(diimine)Cl2 and
a small excess of the corresponding arylacetylene was added
CuI (10 mol%), NEt3 (5 mL) and 20 mL of degassed CH2Cl2.
The reaction mixture was refluxed for 24 h. The initial yellow
suspension became a dark red solution. After cooling, addition
of water (5 mL) led to a bright orange precipitate. The solid was
filtered off, washed with water and diethyl ether. Pure title com-
plexes were isolated by using column chromatography workup
on alumina (eluent: CH2Cl2 for 1 and 3, CH2Cl2/THF (1 : 1,v/v)
for 2 and 4.)
113.5, 102.4, 89.1. IR (KBr, pellet, cmϪ1): ν(C᎐C) 2109,
᎐
᎐
2120(sh). UV/Vis (CH3CN): λmax/nm (ε/MϪ1 cmϪ1) 392 (8800),
304 (42 900), 274 (sh) (38 900). Anal. Calcd for C48H32N8Pt: C,
62.94; H, 3.49; N, 12.23. Found: C, 62.96; H, 3.54; N, 11.64%.
X-Ray crystallographic analysis
Single crystals of 1 were obtained from slow vapor diffusion of
hexane into a solution of 1 in CH2Cl2/benzene. Single crystals
of 4 were obtained from slow vapor diffusion of hexane/toluene
into a solution of 4 in CH2Cl2. Data were collected on a Bruker
P4 single-crystal X-ray diffractometer with a CCD-1000
detector and graphite-monochromated Mo-Kα radiation,
operating at 50 kV and 30 mA at 25 ЊC. The data collection 2θ
ranges were 4.50–56.6Њ for both 1 and 4. No significant decay
was observed during data collection. Data were processed on a
PC using a Bruker SHELXTL software package12 (version
5.10) and are corrected for Lorentz and polarization effects.
Both compounds 1 and 4 crystallize in the triclinic space group
Method B. A sample of Pt(diimine)Cl2, CuI (10 mol%), NEt3
(5 mL) and the corresponding arylacetylene (small excess) in
CH2Cl2 (20–30 mL) were sonicated for 4 h. During the soni-
cation, a bright orange precipitate was formed. The solid was
filtered off, and washed with diethyl ether. Pure title complexes
were also isolated by the same workup as described in method
A.
᎐
Pt(bpy)(C᎐CC H -N-7-azaindol) (1). The procedure fol-
᎐
6
4
2
lowed was either method A or B. However, method B gives
better yields than method A. Yield: 62%; mp 148–150 ЊC. H
1
¯
P1. All structures were solved by direct methods. Empirical
NMR in CD2Cl2 (δ, ppm, 25 ЊC): 9.86 (d, J = 5.7 Hz, 2H), 8.39
(dd, J = 4.5, 1.5 Hz, 2H), 8.22 (m, overlap, 2H, 2H), 8.02 (dd,
J = 7.8, 1.8 Hz, 2H), 7.78, 7.68 (AAЈBBЈ, JAB = 8.7 Hz, 4H, 4H),
7.71 (m, overlap, 2H), 7.63 (d, J = 3 Hz, 2H), 7.18 (dd, J = 7.8,
4.5 Hz, 2H), 6.69 (d, J = 3.6 Hz, 2H). 13C NMR in CD2Cl2
(δ, ppm, 25 ЊC): 157.0, 152.2, 144.1, 139.7, 133.1, 129.5, 128.4,
126.6, 123.9, 123.3, 122.2, 120.7, 118.6, 117.2, 110.7, 105.2,
absorption corrections were applied to all crystals. Solvent mol-
ecules were located in 1 (0.5 benzene and 0.5 hexane per mole-
cule of 1). Solvent molecules were also located in the lattice of 4
(1 CH2Cl2 and 0.5 hexane per molecule of 4). The hexane mole-
cule in 1 is disordered over two sites related by an inversion
center of symmetry with a 50% occupancy for each site. It was
modeled and refined successfully with each of the disordered
atoms being assigned a 0.5 occupancy factor. All non-hydrogen
atoms except the disordered solvent molecules were refined
anisotropically. The positions of hydrogen atoms except those
of the disordered solvent molecules were calculated, and their
contributions to structural factor calculations were included.
Crystal data for 1 and 4 are summarized in Table 1. Selected
bond lengths and angles for 1 and 4 are given in Table 2.
CCDC reference numbers 210583 and 210584.
102.1, 88.9. IR (KBr, pellet, cmϪ1): ν(C᎐C) 2111, 2120(sh).
᎐
᎐
UV/Vis (CH3CN): λmax/nm (ε/MϪ1 cmϪ1) 388 (7900), 306 (45
400). Anal. Calcd for C40H26N6Pt: C, 61.41; H, 3.31; N, 10.70.
Found: C, 61.39; H, 3.40; N, 10.46%.
᎐
Pt(bpy)(C᎐CC H -2,2Ј-dipyridylamine) (2). The procedure
᎐
6
4
2
followed was either method A or B. However, method B also
gives better yields than method A. Yield: 75%; mp 223–225 ЊC.
1H NMR in CD2Cl2 (δ, ppm, 25 ЊC): 9.83 (d, J = 4.8 Hz, 2H),
8.30 (dd, J = 5.1, 1.2, 4H), 8.25–8.15 (m, overlap, 2H, 2H), 7.70
(dd, J = 7.2, 1.5 Hz, 2H), 7.61 (ddd, J = 15.6, 7.5, 2.1 Hz, 4H),
7.15, 7.10 (AAЈBBЈ, JAB = 8.4 Hz, 4H, 4H), 6.95–7.04 (m, over-
lap, 2H, 2H, 4H). 13C NMR in CDCl3 (δ, ppm, 25 ЊC): 166.2,
158.8, 148.9, 145.8, 139.6, 137.9, 133.5, 131.6, 128.8, 128.3,
See http://www.rsc.org/suppdata/dt/b3/b305408j/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
Syntheses and structures of the complexes
127.5, 125.8, 118.7, 117.7, 102.7, 88.5. IR (KBr, pellet, cmϪ1):
ν(C᎐C) 2112, 2121(sh). UV/Vis (CH CN): λmax/nm (ε/MϪ1 cmϪ1
)
᎐
The functionalized acetylene molecules 4-(N-7-azaindolyl)-
phenylacetylene and 4-(2,2Ј-dipyridylamino)phenylacetylene
were synthesized using a procedure reported recently by our
group.10 The corresponding complexes Pt(diimine)(arylacetyl-
ide)2, 1–4, where diimine = 2,2Ј-bipyridine or 1,10-phen-
anthroline, were synthesized according to previous synthetic
methodology3a reported by Eisenberg et al. by the reaction of
᎐
3
396 (5400), 308 (48 300). Anal. Calcd for C46H32N8Pt: C, 61.94;
H, 3.59; N, 12.56. Found: C, 61.52; H, 3.59; N, 12.12%.
᎐
Pt(phen)(C᎐CC H -N-7-azaindol) (3). The procedure fol-
᎐
6
4
2
lowed was method B. Yield: 51%; mp 220–222 ЊC. 1H NMR in
CD2Cl2 (δ, ppm, 25 ЊC): 10.03 (dd, J = 5.1, 1.2 Hz, 2H), 8.72
(dd, J = 8.1, 1.2 Hz, 2H), 8.39 (dd, J = 4.5, 1.2 Hz, 2H), 8.08 (s,
Pt(diimine)Cl2, diimine
= 2,2Ј-bipyridine and 1,10-phen-
2H), 8.01 (m, overlap, 2H, 2H), 7.80, 7.73 (AAЈBBЈ, JAB
=
anthroline, with the corresponding arylacetylene in the pres-
8.7 Hz, 4H, 4H), 7.63 (d, J = 3.6 Hz, 2H), 7.18 (dd, J = 7.8, 4.8
Hz, 2H), 6.69 (d, J = 3.6 Hz, 2H). 13C NMR in CD2Cl2 (δ, ppm,
25 ЊC): 157.6, 152.7, 148.2, 144.0, 138.7, 137.1, 133.1, 131.5,
129.5, 128.3, 128.1, 126.8, 126.6, 123.9, 123.0, 122.1,
ence of CuI/NEt3 under either heating or sonication, as
᎐
shown Scheme 1. Complexes Pt(bpy)(C᎐CC H -N-7-aza-
᎐
6
4
᎐
indole) , 1 and Pt(phen)(C᎐CC H -N-7-azaindole) , 3 were
᎐
2
6
4
2
obtained in moderate yields by using both reaction conditions.
117.2, 102.1, 90.2. IR (KBr, pellet, cmϪ1): ν(C᎐C) 2113,
᎐
However, the sonication procedure afforded a better yield for
᎐
2123(sh). UV/Vis (CH3CN): λmax/nm (ε/MϪ1 cmϪ1) 392 (6500),
᎐
complexes Pt(bpy)(C᎐CC H -2,2Ј-dipyridylamine) ,
2 and
᎐
6
4
2
᎐
308 (sh) (37 000), 280 (43 400). Anal. Calcd for C42H26N6Pt
0.5CH2Cl2: C, 59.89; H, 3.17; N, 9.86. Found: C, 60.13; H, 3.11;
N, 9.62%. The presence of 0.5CH2Cl2 in the crystalline form
was determined by 1H NMR spectroscopy.
Pt(phen)(C᎐CC H -2,2Ј-dipyridylamine) , 4 than that obtained
᎐
6
4
2
under heating, due to the low solubility of Pt(phen)Cl2 in
CH2Cl2. All complexes are soluble in common organic solvents
and are stable in the solid state and in solution for several days
at room temperature in the absence of light. Slow degradation
of the complexes upon exposure to light was observed. Com-
plexes 1–4 were fully characterized by NMR, IR and elemental
᎐
Pt(phen)(C᎐CC H -2,2Ј-dipyridylamine) (4). The procedure
᎐
6
4
2
followed was method B. Yield: 65%; mp 238–240 ЊC. 1H NMR
D a l t o n T r a n s . , 2 0 0 3 , 3 4 9 3 – 3 4 9 9
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Table 1 Crystallographic data for 1 and 4
1
4
Formula
C40H26N6Ptؒ0.5(C6H6)ؒ0.5(C6H14)
C48H32N8Ptؒ(CH2Cl2)ؒ0.5(C6H14)
M
867.90
P1
1043.93
P1
¯
¯
Space group
a/Å
b/Å
c/Å
α/Њ
6.6498(12)
16.203(3)
17.395(4)
88.544(3)
83.276(4)
83.331(4)
1847.6(6)
2
10.154(3)
13.057(3)
19.080(5)
70.846(4)
81.790(5)
72.416(6)
2275.4(10)
2
β/Њ
γ/Њ
V/Å3
Z
Dcalc/Mg cmϪ3
1.559
3.837
1.524
3.247
µ/mmϪ1
No. of rflns measured
13567
13823
No. of rflns used (Rint
)
8536 (0.0527)
0.0489, 0.0729
0.1488, 0.1722
0.695
9491 (0.0549)
0.0589, 0.1171
0.0862, 0.1437
0.850
Final R[I>2σ(I )]: R1, wR2
R (all data): R1, wR2
Goodness of fit on F 2
Table 2 Selected bond lengths (Å) and angles (Њ) for 1 and 4
Compound 1
Pt(1)–C(30)
Pt(1)–N(5)
N(1)–C(8)
C(14)–C(15)
1.934(9)
2.054(7)
1.445(10)
1.183(10)
Pt(1)–C(15)
Pt(1)–N(6)
N(3)–C(23)
C(29)–C(30)
1.946(9)
2.055(7)
1.443(9)
1.207(10)
C(30)–Pt(1)–C(15)
C(15)–Pt(1)–N(5)
C(15)–Pt(1)–N(6)
C(1)–N(1)–C(8)
C(35)–N(5)–Pt(1)
C(14)–C(15)–Pt(1)
90.3(3)
96.4(3)
174.0(4)
123.5(10)
116.8(7)
177.5(8)
C(30)–Pt(1)–N(5)
C(30)–Pt(1)–N(6)
N(5)–Pt(1)–N(6)
C(22)–N(3)–C(23)
C(31)–N(5)–Pt(1)
C(29)–C(30)–Pt(1)
173.1(4)
95.6(3)
77.6(3)
127.7(8)
124.5(7)
176.8(8)
Compound 4
Pt(1)–N(1)
Pt(1)–C(13)
N(4)–C(18)
C(13)–C(14)
2.059(8)
Pt(1)–N(2)
Pt(1)–C(31)
N(7)–C(36)
C(31)–C(32)
2.077(9)
1.903(12)
1.414(13)
1.222(15)
1.980(14)
1.415(13)
1.194(14)
C(13)–Pt(1)–C(31)
C(31)–Pt(1)–N(1)
N(1)–Pt(1)–N(2)
C(14)–C(13)–Pt(1)
95.7(4)
170.2(4)
79.5(4)
C(13)–Pt(1)–N(1)
C(13)–Pt(1)–N(2)
C(12)–N(1)–Pt(1)
C(32)–C(31)–Pt(1)
93.8(4)
173.4(4)
114.7(7)
171.6(10)
175.1(10)
analyses. The IR spectra for complexes 1–4 exhibit two bands at
about 2110 and 2120 cmϪ1, which correspond to the symmetric
᎐
(ν ) and asymmetric (ν ) C᎐C vibrational stretches of the
᎐
s
as
acetylides in a cis-configuration. The crystal structures of 1 and
4 have been established by single-crystal X-ray diffraction
analyses.
Scheme 1
As shown in Fig. 1, the molecule of 1 has an approximate C2
symmetry. The Pt atom in 1 has a somewhat distorted square
planar geometry as indicated by the bond angles C(15)–Pt(1)–
N(6), 174.0(4)Њ and C(30)–Pt(1)–N(5), 173.1(4)Њ (see Table 2).
The Pt–N and Pt–C bond lengths are typical and similar to
those of previously reported Pt() diimine acetylide com-
plexes.2,3 The acetylide ligands are coordinated to the Pt()
center linearly as indicated by the Pt(1)–C(30)–C(29) and
Pt(1)–C(15)–C(14) bond angles, 176.8(8)Њ and 177.5(8)Њ,
respectively. The acetylene portion of the ligand does not
appear to have significant conjugation with the phenyl ring, as
indicated by the C(11)–C(14) bond length (1.438(10) Å) and
C(26)–C(29) bond length (1.441(11) Å). The 7-azaindolyl group
is not co-planar with the phenyl ring as indicated by the
dihedral angles of 40.6Њ and 43.5Њ between the 7-azaindolyl ring
and the phenyl ring of the two ligands. In the crystal lattice,
molecules of 1 stack via the flat Pt(bpy) portion as shown in
Fig. 2. The shortest intermolecular Pt–Pt separation is 4.966(2)
Å. The shortest atomic separation between two neighboring
bpy units is 3.32 Å, indicating the presence of significant π–π
Fig. 1 A diagram showing the molecular structure of 1 with labeling
schemes and 50% thermal ellipsoids.
D a l t o n T r a n s . , 2 0 0 3 , 3 4 9 3 – 3 4 9 9
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Fig. 4 A diagram showing the π stacked dimer of 4.
Electrochemical properties
Complexes 1–4 display a reversible reduction wave at E
p
a
c
(1/2(Ep ϩ Ep )) = Ϫ0.80 V (∆E = 0.87 mV), Ϫ0.706 (∆Ep =
p
0.87 mV), Ϫ0.76 V (∆Ep = 0.60 mV), and Ϫ0.78 V (∆Ep = 0.81
mV), respectively (versus Ag/AgCl), which can be attributed to
a one-electron reduction of the diimine ligand.2,3,13 For com-
pounds 3 and 4 a second reduction wave at Ep = Ϫ1.09 V (∆Ep =
0.51 mV) and Ϫ1.03 V (∆Ep = 0.50 mV), respectively, was also
observed, which may be caused by the further reduction of the
coordinated phenanthroline ligand.13 Compound 4 displayed
an irreversible oxidation wave at 1.23 V, which is likely due to
the one-electron oxidation of the Pt() ion to a Pt() ion. The
electrochemical properties of complexes 1–4 are consistent with
those of previously known Pt() diimine complexes.2,3,13
Fig. 2 A diagram showing the intermolecular π stacking of 1.
stacking interactions. Benzene and hexane solvent molecules
were located in the void channels of the crystal lattice of 1 (see
ESI†). Crystals of 1 lose the lattice solvent molecules slowly
upon removal from solution.
Molecules of 4 also have an approximate C2 symmetry as
shown in Fig. 3. In contrast to the structure of 1, the Pt–
acetylide portion deviates significantly from linearity, as
indicated by the bond angles of Pt(1)–C(13)–C(14) and
Pt(1)–C(31)–C(32), 175.1(10) and 171.6(10)Њ, respectively,
which is clearly caused by the increased crowdedness of
molecule 4 due to the presence of the 2,2Ј-dipyridylamino unit.
The amino nitrogen atom lone pair electrons do not appear to
conjugate with the phenyl ring, as indicated by the dihedral
angles between the C(17) phenyl ring and the C(26)–N(4)–
C(21) plane (121.2Њ) and the C(38) phenyl ring and the C(43)–
N(7)–C(44) plane (54.7Њ). The dihedral angles between the
phenyl ring and the pyridyl rings range from 57.7–102.3Њ. The
molecules of 4 stack in pairs through the flat Pt(phen) portion
in the crystal lattice as shown in Fig. 4 with the intermolecular
Pt–Pt separation between the pair being 4.767(2) Å. The short-
est atomic separation between the two neighboring phen-
anthroline ligands is 3.44 Å, again indicating the presence of
significant π–π stacking interactions. CH2Cl2 and hexane
solvent molecules are located in the crystal lattice of 4. Crystals
of 4 slowly lose the lattice solvent molecules at ambient
temperature.
Luminescence properties
᎐
᎐
᎐
The free ligands p-HC᎐CC H -N-7-azaindole and p-HC᎐CC -
᎐
6
4
6
H4-2,2Ј-dipyridylamine have fluorescence emissions in the
370–400 nm region when irradiated by UV light. The 2,2Ј-bi-
pyridine and 1,10-phenanthroline ligands also display an
emission peak at λmax ≈ 380 nm in solution. These emissions are
attributed to π–π* transitions. In contrast, the Pt() acetylide
complexes 1–4 display quite different luminescence properties
(see Table 3).
As shown in Fig. 5, the UV/Vis spectra of 1–4 all display a
weak and broad MLCT band in the region of 350–500 nm,
characterized by a characteristic solvachromic shift, in addition
to the intense π–π* transition absorption bands of the ligands.
The absorption maximum of the MLCT band for compounds
1–4 in benzene, CH2Cl2, THF, acetone, methanol, DMF and
CH3CN is summarized in Table 4. The solvent column in Table
4 is arranged in the order of increasing dipole moment (e.g.
benzene has a zero dipole moment while DMF has the highest
dipole moment (3.86) among the solvents listed). In general, the
MLCT band for the four complexes appears to shift toward a
shorter wavelength with an increase in the dipole moment of
the solvent with the exception of the DMF solutions.
The emission spectra of 1–4 in CH2Cl2 solution at 298 K and
in a CH2Cl2 glass 77 K are shown in Fig. 6. For complexes 1 and
3 orange phosphorescent emission was observed in solution at
ambient temperature and green emission was observed in the
solid state at ambient temperature. Interestingly, upon the
removal of the solvent molecules from the crystal lattice of 1
under reduced pressure, the emission color of 1 changed from
green to orange. At 77 K, the emission band of 1 and 3 is blue
shifted to the green region, which is likely caused by the
increased rigidity of the solution medium at 77 K and the slow-
ing down of solvent interactions with the complex. A blue shift
of the emission band with a decrease in temperature has also
been observed in some other luminescent complexes and is
described as “luminescence rigidochromism”.14a–c An excellent
account on solvent and temperature effects on luminescence
can be found in Lakowicz’s book.14d Based on the results of
a previous study on 2,2-bipyridine and 1,10-phenanthroline
Fig. 3 A diagram showing the molecular structure of 4 with labeling
schemes and 50% thermal ellipsoids.
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Table 3 Photoluminescence data for 1–4a
Compound
Absorptionb MLCT (λmax/nm)
Emission (λmax/nm)
Quantumc yield (Φem
)
Life time/µs
Conditions
1
402 (7900)
589
533
542
546
571
523
565
565
576
542
548
542
590
538
573
567
0.067
4.9
239
8.3
183
1.3
234
4.2
221
3.9
189
5.8
164
6.1
210
6.0
212
Solution, 298 K
Solution, 77 K
Solid, 298 K
Solid, 77 K
Solution, 298 K
Solution, 77 K
Solid, 298 K
Solid, 77 K
Solution, 298 K
Solution, 77 K
Solid, 298 K
Solid, 77 K
Solution, 298 K
Solution, 77 K
Solid, 298 K
Solid, 77 K
2
3
4
406 (5400)
398 (6500)
398 (8800)
0.016
0.045
0.005
a Concentration: [M] = 4.5 × 10Ϫ5 MϪ1
tris(2,2Ј-bipyridyl)ruthenium().
.
b In CH2Cl2 solution at 298 K. c The quantum yields of all compounds were obtained by comparing to
Table 4 Absorption and emission for MLCT of 1–4 in various solvents
1
2
3
4
λem a/nm
a
Solvent
λabs/nm
λem/nm
λabs/nm
λem /nm
λabs/nm
λem/nm
λabs/nm
Benzene
THF
CH2Cl2
Acetone
CH3OH
CH3CN
DMF
424
416
402
402
388
388
398
588
593
589
586
565
581
572
424
416
406
404
384
396
402
520
544
523
530
532
540
537
418
410
398
398
388
392
398
582
590
576
587
545
581
582
424
410
398
398
384
392
397
526
563
538
550
536
546
536
a At 77K.
Fig. 5 Top: UV/Vis absorption spectra of complexes 1–4 in aceto-
nitrile at 298 K. Bottom: UV/Vis spectra showing the MLCT band of 1
in various solvents at 298 K.
Fig. 6 Top: emission spectra of 1–4 in CH2Cl2 at 298 K. Bottom:
emission spectra of 1–4 in CH2Cl2 at 77 K.
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complexes of Pt() containing acetylide ligands,2,3 we believe
that the phosphorescent emission of 1 and 3 likely originates
from MCLT transitions. Complexes 2 and 4 also display green
emission in the solid state at ambient temperature. An emission
color change from green to orange was also observed for the
solid of 4, when the solvent molecules were driven out of the
crystal lattice under reduced pressure. An intriguing phen-
omenon is that in solution, the emission energy of 2 and 4 at
ambient temperature is concentration-dependent (Fig. 7). At
tration to [Pt(4,7-diphenyl-1,10-phenthroline)(CN)2] to ligand
centered π–π* transitions and the emission band at relatively
high concentrations to an excimer emission involving either
intermolecular π–π interactions or direct Pt–Pt interactions. In
the intermediate concentration range, Volger et al. suggested
that metal-centered, ligand-centered and MLCT transitions are
all likely to contribute to the lowest energy absorption and
emission bands. Based on our crystal structure data, it seems
that the excimer emissions for 2 and 4 are likely caused by inter-
ligand (bipy or phen) π–π interactions. At 77 K, the emission
bands of 2 and 4 in a frozen solution resemble those of 1 and 3
and do not change with concentration. The 77 K emission
bands of 1–4 in frozen CH2Cl2 solutions can be obtained using
the MLCT excitation energy (≈400 nm). Therefore, we believe
that they originate from MLCT transitions. The long emission
lifetimes (190–240 µs) of compounds 1–4 in CH2Cl2 at 77 K
(Table 3) are consistent with those of previously reported
MLCT emissions of Pt(diimine)X2 complexes.2,3,13
One important feature of molecules 1–4 that distinguishes
them from the previously known Pt(diimine)X2 compounds is
the presence of a 7-azaindolyl or a 2,2Ј-dipyridylamino func-
tional group. These functional groups bear lone pair electrons,
and hence can coordinate or interact with a metal ion. Such
interactions may lead to a change of luminescence. To confirm
this, we examined the interaction of ZnCl2 with complex 2 in
solution by monitoring the emission spectrum of 2 either in a
low concentration solution (1.0 × 10Ϫ5 M) at ambient temper-
ature or in a frozen solution (1.0 × 10Ϫ5 M, CH2Cl2, 77 K). As
shown in Fig. 7, the phosphorescent emission peak (at 77 K)
was gradually quenched with an increase in ZnCl2 content. In
contrast, however, the ligand-centered fluorescent emission of 2
at ambient temperature gained intensity substantially, as the
ZnCl2 content was increased. In addition, the shape of the
fluorescent emission band also changes with an increase in
ZnCl2. The interaction of the ZnCl2 unit with complex 2 clearly
has an opposite effect on the low energy emission (dominating
at 77 K, or at a high concentration at ambient temperature) and
very low concentration (<10Ϫ5 M), the emission band is in the
᎐
blue-violet region, resembling that of the free p-HC᎐CC H -
᎐
6
4
2,2Ј-dipyridylamine ligand, the bpy ligand and the phen ligand.
As the concentration increases, the emission intensity decreases
and the emission energy shifts toward longer wavelength. For
example, at ≈10Ϫ4 M, the emission of 2 is dominated by a very
weak phosphorescent emission band with λmax at 570 nm. A
similar phenomenon was also observed for 4. The concen-
tration-dependent behavior of 2 and 4 is reminiscent of that
of [Pt(4,7-diphenyl-1,10-phenthroline)(CN)2] investigated by
Volger et al.,15 who attributed the emission band at low concen-
Fig. 7 Top: emission spectra of complex 2 at 0.16 mM and 0.016 mM.
Middle: emission spectral change of 2 in frozen CH2Cl2 at 77 K (1.0 ×
10Ϫ5 M) in the presence of ZnCl2. Bottom: emission spectral change of
2 at 298 K in CH2Cl2 (1.0 × 10Ϫ5 M) in the presence of ZnCl2.
Scheme 2
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447; (b) E. C. Whittle, J. A. Weinstein, M. W. George and K. S.
the high energy emission band (dominating at a low concen-
tration at ambient temperature). The details are yet to be
understood. Nonetheless, we believe that the chelating inter-
action as depicted in Scheme 2 must be responsible for the
emission quenching/enhancing phenomenon and that such co-
ordination interaction may be useful for the potential appli-
cations of complexes 1–4 as luminescent sensors for metal ions.
Further investigation on the interactions of 1–4 with various
metal ions is currently being conducted.
Schanze, Inorg. Chem., 2001, 40, 4053; (c) W. B. Connick, D. Geiger
and D. R. Eisenberg, Inorg. Chem., 1999, 38, 3264; (d ) Y. Ng,
C. M. Che and S. M. Peng, New J. Chem., 1996, 20, 781.
4 (a) M. Hissler, J. E. McGarrah, W. B. Connick, D. K. Geiger, S. D.
Cummings and R. Eisenberg, Coord. Chem. Rev., 2000, 208, 115;
(b) S. C. Chan, M. C. W. Chan, Y. Wang, C. M. Che, K. K. Cheung
and N. Zhu, Chem. Eur. J., 2001, 7, 4180.
5 J. E. McGarrah, Y. J. Kim, M. Hissler and R. Eisenberg,
Inorg. Chem., 2001, 40, 4510.
6 (a) Y. Kang, C. Seward, D. Song and S. Wang, Inorg. Chem., 2003,
42, 2789; (b) Y. Kang, D. Song, H. Schmider and S. Wang,
Organometallics, 2002, 21, 2413; (c) S. Wang, Coord. Chem. Rev.,
2001, 215, 79; (d ) Q. Liu, L. Thorne, I. Kozin, D. Song, C. Seward,
M. D’Iorio, Y. Tao and S. Wang, J. Chem. Soc., Dalton Trans., 2002,
16, 3234; (e) S. F. Liu, Q. Wu, H. L. Schmider, H. Aziz, N. X. Hu,
Z. Popovic and S. Wang, J. Am. Chem. Soc., 2000, 122, 3671;
( f ) J. Ashenhurst, G. Wu and S. Wang, J. Am. Chem. Soc., 2000,
122, 2541; (g) Q. Wu, M. Esteghamatian, N. X. Hu, Z. Popovic,
G. Enright, S. R. Breeze and S. Wang, Angew. Chem., Int. Ed., 1999,
38, 985; (h) J. Pang, Y. Tao, X. P. Yang, M. D’Iorio and S. Wang,
J. Mater. Chem., 2002, 12, 206; (i) Q. Wu, Q. A. Hook and S. Wang,
Angew. Chem., Int. Ed., 2000, 39, 3933; (j) D. Song, Q. Wu,
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Mater., 2001, 13, 71.
Conclusions
In summary, four novel Pt() diimine complexes containing
2,2Ј-dipyridylamino and 7-azaindolyl functionalized arylacetyl-
ide ligands have been synthesized and fully characterized. These
complexes display MLCT charge transfer and orange–green
phosphorescence in solution and in the solid state. They are
potentially useful as luminescent sensors/probes for metal ions.
Acknowledgements
We thank the Natural Sciences and Engineering Research
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Council of Canada for financial support.
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