Synthesis, structures and spectroscopic properties of platinum(II), copper(I) and zinc(II) complexes bearing 4-(p-dimethylaminophenyl)-6-phenyl-2,2′-bipyridine ligand

Article abstract of DOI:10.1016/j.ica.2008.11.020

The reactions of 4-(p-dimethylaminophenyl)-6-phenyl-2,2′-bipyridine (HL) with three metal salts of platinum(II), copper(I) and zinc(II) provide the new complexes [Pt(L)(PPh₃)]ClO₄ (1), [Cu(HL)₂]BF₄ (2), [Cu(HL)(

Full text of DOI:10.1016/j.ica.2008.11.020

Inorganica Chimica Acta 362 (2009) 2529–2536
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Inorganica Chimica Acta
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Synthesis, structures and spectroscopic properties
of platinum(II), copper(I) and zinc(II) complexes bearing
4-(p-dimethylaminophenyl)-6-phenyl-2,20-bipyridine ligand
Quan-Qing Xu a,b, De-Hui Wang c, Shao-Ming Chi c, Xin Gan c, Wen-Fu Fu a,b,c,
*
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC CAS, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Zhong Guan Cun Bei Yi Tiao 2, Beijing 100190, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
c College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, PR China
a r t i c l e i n f o
a b s t r a c t
The reactions of 4-(p-dimethylaminophenyl)-6-phenyl-2,20-bipyridine (HL) with three metal salts of
platinum(II), copper(I) and zinc(II) provide the new complexes [Pt(L)(PPh3)]ClO4 (1), [Cu(HL)2]BF4 (2),
[Cu(HL)(PPh3)]BF4 (3) and [Zn(HL)2](ClO4)2 (4). All the structures of these four complexes have been char-
acterized by single crystal X-ray diffraction, and their spectroscopic properties were investigated. Espe-
cially for complex 1, upon protonation, the excited state can be tuned from the intraligand charge transfer
(ILCT) to the metal-to-ligand charge transfer (MLCT), and such switching in the excited state is acid/base
reversible. The time-dependent density functional theory (TD-DFT) calculation was used to interpret the
absorption spectra of complex 1, and the calculated result is consistent with those of experiments results.
In contrast with 1, the lowest energy absorption at 410–650 nm of complexes 2 and 3 can be assigned to
MLCT excited state. In solid state or solution complex 4 exhibits intense photoluminescence attributed to
a ILCT transition in nature.
Article history:
Received 14 May 2008
Received in revised form 13 November 2008
Accepted 13 November 2008
Available online 6 December 2008
Keywords:
Pt(II) and Cu(I) complexes
Cyclometalated ligand
Crystal structures
Charge transfer
Proton sensor
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
the host molecules. The multi-excited states of these complexes,
such as metal-to-ligand charge transfer (MLCT), ligand-to-ligand
There have been great interests in designing host molecules
which show notable and reversible changes in color and/or lumi-
nescence upon exposure to specific guest molecules. In this field,
platinum(II) and copper(I) complexes might be two better candi-
dates. The latter have attracted considerable attention for their
being environmentally friendly, less expensive, intriguing coordi-
nation modes, rich photochemical and photophysical properties
[1–4]. The square-planar molecular configuration and electronic
arrangements, coordinatively unsaturated polypyridyl platinum(II)
complexes bearing cyclometalated 6-phenyl-2,20-bipyridine
(HC^N^N) and related ligands generally exhibit improved photo-
charge transfer (LLCT) and intraligand charge transfer (ILCT) tran-
sitions depend very much on the 4-phenyl substitution.
Recently, electron-donating or withdrawing group substituent
effects on the spectroscopic properties of platinum(II) terpyridyl
systems have been extensively studied, such as switching of ex-
cited states by pH and functionalized platinum(II) terpyridyl com-
plex as colorimetric or luminescence pH sensors [10–14]. However,
reports on the amino electron-donating group (–NR2) introduced
into the p-position at the 4-phenyl substitution of (HC^N^N) ligand
to form platinum(II) as well as copper(I) and zinc(II) complexes are
rare. Herein, we report the structures and spectroscopic properties
of the new complexes [Pt(L)(PPh3)]ClO4 (1) (HL = 4-(p-dimethyl-
aminophenyl)-6-phenyl-2,20-bipyridine), [Cu(HL)2]BF4 (2), [Cu(HL)
(PPh3)]BF4 (3) and [Zn(HL)2](ClO4)2 (4). To the best of our knowl-
edge, these compounds are first example investigating the tunable
excited states based on HC^N^N derivatives for proton sensors.
physical properties compared with
a-diimine platinum(II) com-
plexes [5,6] for their MLCT excited state. Also, compared with
terpyridyl ligand (N^N^N), the o-C-deprotonated 6-phenyl-2,20-
bipyridine ligand favors better the planar geometry and depresses
the D2d distortion [7,8] thus to slow down radiationless decay [9].
These characters provide us the ground to choose complexes of
platinum(II) and copper(I) based on the HC^N^N derivatives as
2. Experimental
* Corresponding author. Address: Key Laboratory of Photochemical Conversion
and Optoelectronic Materials, TIPC CAS, Technical Institute of Physics and Chem-
istry, Chinese Academy of Sciences, Zhong Guan Cun Bei Yi Tiao 2, Beijing 100190,
PR China. Tel.: +86 10 82543519; fax: +86 10 64879375.
2.1. General
All reactions were performed under a nitrogen atmosphere.
4-(p-Dimethylaminophenyl)-6-phenyl-2,20-bipyridine (HL) and
E-mail address: wf_fu@yahoo.com.cn (W.-F. Fu).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.11.020

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Q.-Q. Xu et al. / Inorganica Chimica Acta 362 (2009) 2529–2536
[Cu(CH3CN)4]BF4 were prepared according to the literature meth-
ods [15,16]. Other reagents were of analytical grade and were used
as received. All solvents were purified by standard procedures be-
fore use.
6.93 (s, 5H), 6.87 (d, J = 8.8 Hz, 3H), 3.04 (s,6H). Anal. Calc. for
42H36ClCuN3O4P: C, 64.95; H, 4.67; N, 5.41. Found: C, 64. 82; H,
4.60; N, 5.37%.
C
2.3.5. Synthesis of [Zn(HL)2](ClO4)2 (4)
2.2. Instrumentation
A
mixture of Zn(ClO4)2 ꢀ 6H2O (0.0372 g, 0.1 mmol) and
L(0.0704 g, 0.2 mmol) in 30 mL CH2Cl2 was stirred for 2 h at room
temperature, then filtered and concentrated in vacuum to ca.5 mL.
Diffusion of diethyl ether into the resulting orange solution yielded
yellow crystals of 4. Yield: 0.012 g, 40%. 1H NMR (DMSO-d6,
400 MHz,): d = 8.75 (d, J = 4.1 Hz, 2H), 8.61 (d, J = 8.0 Hz, 2H), 8.57
(s, 2H), 8.34 (d, J = 7.2 Hz, 4H), 8.21 (s, 2H), 8.01 (t, J = 8.0 Hz,
2H), 7.88 (d, J = 8.8 Hz, 4H), 7.56 (t, J = 7.2 Hz, 4H), 7.49–7.51 (m,
4H), 6.87 (d, J = 8.8 Hz, 4H), 3.01 (s, 12H). Anal. Calc. For
C48H42Cl2N6O8Zn: C, 59.61; H, 4.38; N, 8.69. Found: C, 60.92; H,
4.51; N, 8.68%.
1H NMR spectra were measured on a Bruker Avance DPX-
400 MHz resonance spectrometer, using TMS (SiMe4) as an internal
reference at room temperature. Elemental analysis were per-
formed with a Carlo Erba 1106 element analysis instrument. UV–
Vis absorption spectra were recorded using Hitach U-3010 spectro-
photometer. Emission spectra were obtained on a Hitach F-4500
fluorescence spectrofluorometer.
2.3. Synthesis
2.3.1. Synthesis of 4-(p-dimethylaminophenyl)-6-phenyl-2,20-
bipyridine (HL)
2.4. Calculation details
HL was prepared according to the literature methods [18]. 1H
NMR (CDCl3, 400 MHz): d = 8.73 (d, J = 4.7 Hz, 1H), 8.69 (d,
J = 7.6 Hz, 1H), 8.63 (s, 1H), 8.20 (d, J = 7.4 Hz, 2H), 7.97 (s, 1H),
7.86 (t, J = 7.6 Hz, 1H), 7.80 (d, J = 8.8 Hz, 2H), 7.53 (t, J = 7.4 Hz,
2H), 7.45 (t, J = 7.4 Hz, 1H), 7.34 (d, J = 4.7, 7.6 Hz, 1H), 6.84 (d,
J = 8.8 Hz, 2H), 3.05 (s, 6H). m.p. 154–155 °C.
All calculations were carried out at the GAUSSIAN 03 package in
Virtual Laboratory for Computational Chemistry, CNIC, CAS at the
B3LYP level [18,19]. The basis sets which contained in G03 library
used for C, P, N and H atoms was 6-31G(d) while effective core
potentials with a LanL2DZ basis set of G03 were employed for tran-
sition metals. The 6-311G (d) basis set for I, Cl was downloaded
from the EMSL basis set library [20]. The contour plots of MOs were
obtained with the GAUSS VIEW 3.07 program.
2.3.2. Synthesis of [Pt(L)(PPh3)]ClO4 (1)
A modification of Constable’s method was used [17]. A mixture
of K2PtCl4 (0.3272 g, 0.79 mmol) and HL (0.2767 g, 0.79 mmol) in
30 mL mixed solvent of CH3CN/H2O (1/1, v/v) was refluxed for
36 h to give a deep red solution. The solution was evaporated to
dryness, resulting product was extracted with CH2Cl2. Upon con-
centration by rotary evaporation to about 5 mL solvent, excess Li-
ClO4 was added to yield the product as a red precipitate which was
collected and washed with diethyl ether and then dried. The red
solid was mixed with equal mol of PPh3 in CH3CN (30 mL). After
stirring for 12 h the solution was filtered, and diffusion of diethyl
ether into the filtrate yielded red crystals of 1. Yield: 0.046 g,
44%. 1H NMR (CD3CN, 400 MHz): d = 8.16 (d, J = 7.8 Hz, 1H), 8.04
(s, 1H), 7.86–7.96 (m, 8H), 7.80 (d, J = 8.9 Hz, 2H), 7.61 (t,
J = 6.6 Hz, 3H), 7.48–7.54 (m, 7H), 7.01(t, J = 6.6 Hz, 1H), 6.87 (t,
J = 7.8 Hz, 1H), 6.70 (d, J = 8.9 Hz, 2H), 6.51–6.56 (m, 2H), 6.39 (s,
1H), 2.96 (s, 6H). Anal. Calc. for C42H35N3O4ClPPt: C, 55.60; H,
3.89; N, 4.63. Found: C, 55.26; H, 3.42; N, 4.84%.
2.5. X-ray crystallography
Crystal of complex 1 was obtained by vapor diffusion of diethyl
ether into CH3CN solution, while subsequent diethyl ether diffu-
sion into the CH2Cl2 solution 2–4 afforded crystals of different col-
ors. Crystal data and details of data collection as well as refinement
are summarized in Table 1. X-ray data were collected with graphite
monochromatized Mo K
a (k = 0.071073 nm) either on a Rigaku
Saturn for 1 and 2 or a Bruker SMART diffractometer for 3 and 4.
The structures were solved by direct methods using the program
SHELXS 97 and refined by full-matrix least squares on F2 [21]. The
non-hydrogen atoms were refined anisotropically.
3. Results and discussion
3.1. Crystal structure determination
2.3.3. Synthesis of [Cu(HL)2]BF4 (2)
Complexes 1–4 are red, brown, orange and yellow crystalline
solids, respectively, and soluble in organic solvents, such as CH2Cl2
and CH3CN. Perspective drawings of 1–4 with atomic numbering
schemes are depicted in Figs. 1–4, respectively. The selected bond
lengths and bond angles were listed in Table 2. All the complexes
have a mononuclear structure. The X-ray crystal analysis shows
that for complex 1, the cyclometalated ligand HL and the auxiliary
ligand PPh3 are arranged in a distorted square-planar geometry
A mixture of [Cu(CH3CN)4]BF4 (0.0788 g, 0.25 mmol) and HL
(0.1755 g, 0.5 mmol) in 30 mL CH2Cl2 was stirred for 2 h at room
temperature, and then filtered and concentrated in vacuum to ca.
5 mL. Diffusion of diethyl ether into the resulting deep purple solu-
tion gave brown crystals of 2. Yield: 0.077 g, 40%. 1H NMR (CDCl3,
400 MHz): d = 8.01–8.24 (broad, 6H), 7.66 (d, J = 7.4 Hz, 7H), 7.36
(m, 4H), 7.01 (s, 3H), 6.84 (d, J = 7.8 Hz, 10H), 3.08 (s, 12H). Anal.
Calc. for C48H42BCuF4N6: C, 67.57; H, 4.96; N, 9.85. Found: C,
67.41; H, 4.93; N 9.82%.
about the platinum atom (Fig. 1). The Pt–N bond length of
0
2.148(2) ÅA trans to the phenyl group is noticeably longer than that
0
of 2.005(2) ÅA trans to the PPh3 ligand, which is similar to that ob-
served for the reported complex [Pt(L)(Ppa)]PF6 (HL = 4-(P-tolyl)-
6-phenyl-2,20-bipyridine; Ppa = diphenylphosphinopropanoic acid)
[22]. The bite angles C12–Pt1–N2 and N1–Pt1–P1 are 81.88(9) and
95.30(7)°, respectively. The planes C17–C22 and C6–C10 are nearly
coplanar with the dihedral angle being 4.5°, while the dihedral an-
gle of 9.3° between planes C1–C5 and C6–C10 is slight bigger than
that of 5.5° for the C11–C16 and C6–C10 planes.
2.3.4. Synthesis of [Cu(HL)(PPh3)]BF4 (3)
A mixture of [Cu(CH3CN)4]ClO4 (0.0351 g, 0.1 mmol) and HL
(0.0352 g, 0.1 mmol) in 20 mL CH2Cl2 was stirred for 2 h at room
temperature, and PPh3 (0.0262 g, 0.1 mmol) in CH2Cl2 (5 mL) was
then added to the brown solution. After stirring the solution for
2 h, the solvent was removed. Orange single crystals were obtained
by diffusion of diethyl ether into a CH2Cl2 solution. Yield: 0.038 g,
65%. 1H NMR (DMSO-d6, 400 MHz): d = 8.84 (s,1H), 8.72 (s,1H),
8.48 (s,1H), 8.21 (s,2H), 8.03 (d, J = 8.8 Hz, 3H), 7.85 (s, 2H), 7.59–
7.64 (broad, 2H), 7.44 (t, J = 7.4 Hz, 4H), 7.33 (t, J = 7.5 Hz, 6H),
The copper(I) core in complex 2 is coordinated by two ligands
via their N(bipyridine)-donor sites to form a distorted tetrahedron

Q.-Q. Xu et al. / Inorganica Chimica Acta 362 (2009) 2529–2536
2531
Table 1
Summary of X-ray crystallographic data for complexes 1–4.
Complexes
1
2
3
4
Formula
C42H35ClN3O4PPt
907.24
113(2)
0.71070
Triclinic
P
0.32 ꢂ 0.20 ꢂ 0.10
8.526(2)
10.015(3)
21.052(6)
101.269(7)
101.720(5)
90.233(3)
1724.3(8)
2
C48H42BCuF4N6
853.25
113(2)
C42H36ClCuN3O4P
776.70
298(2)
0.71073
Triclinic
C48H42Cl2N6O8Zn
967.15
298(2)
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Crystal size (mm)
a (Å)
0.71070
Monoclinic
C2/c
0.26 ꢂ 0.22 ꢂ 0.20
10.765(3)
25.229(7)
14.887(4)
90
101.132(4)
90
3967.1(19)
4
0.71073
Monoclinic
P2/c
0.52 ꢂ 0.21 ꢂ 0.04
7.471(3)
9.847(3)
29.784(4)
90
90.520(3)
90
2191.0(11)
2
P
0.32 ꢂ 0.18 ꢂ 0.12
10.083(3)
12.659(4)
15.569(5)
100.164(4)
90.519(4)
110.574(4)
1825.9(9)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc. (mg/m3)
h range (°)
1.747
2.08–27.88
16167
1.429
1.61–27.92
18679
1.413
1.75–25.01
9578
1.466
1.37–25.01
9755
Total reflection
Unique reflection [R(int)
]
8072
4.243
4742
0.614
6336
0.763
3494
0.746
l
(mmꢃ1
)
Maximum and minimum transmission
Parameters
0.6763 and 0.3438
471
0.8871 and 0.8567
297
0.9140 and 0.7924
506
0.9708 and 0.6977
321
Goodness-of-fit (GOF)
F(000)
1.018
900
0.0198, 0.0447
0.0233, 0.0454
1.066
1768
0.0401, 0.0993
0.0502, 0.1049
1.017
804
0.0465, 0.0923
0.0937, 0.1131
1.166
1000
0.1356, 0.3293
0.2273, 0.4005
a
R1a, wR2
R1, wR2 (all data)
P
P
P
P
2
2
1=2
||Fo| ꢃ |Fc||/ |Fo|; wR2 ¼ f ½wðF2o ꢃ Fc2Þ ꢄ= ½wðFo2Þ ꢄg
.
a
I > 2r(I), R1
=
1.997 (2) Å and 2.099 (2) Å, respectively, and the distortion from
tetrahedral geometry is evidenced by the wider N1–Cu1–N1A an-
gle of 131.33(8)° and the narrower N1–Cu1–N2 angle of
80.32(6)°. The dihedral angle between C17–C22 and C6–C10 is
20.1°, while the dihedral angles of C1–C5 and C6–C10 as well as
N2–Cu1–N1 and N2A–Cu1–N1A planes are 16.8° and 75.8°,
respectively.
In contrast, complex
3 exhibits trigonal copper(I) center
wrapped around by HL and one PPh3 ligands (Fig. 3). The two pyr-
idyl rings of 2,20-bipyridine ligand are almost coplanar with dihe-
dral angles being 8.4° and the average Cu–N bond length is
2.044 Å, while planes C17–C22 and C1–C5 is almost coplanar with
dihedral angle of 3.1°. Corresponding N1–Cu1–N2, N1–Cu1–P1 and
N2–Cu1–P1 bond angles are 80.9(1), 151.89(9) and 123.40(9)°,
respectively. Complex 4 is a monomeric four-coordinate complex
with two HL ligands and has a similar structure to complex 2 ex-
cept that the metal core is zinc and the counter ion is ClO4 (Fig. 4).
3.2. Electronic absorption and emission spectra
3.2.1. Electronic absorption and emission spectra of HL
The absorption and emission spectra recorded for free HL in
CH2Cl2 as a function of H+ concentration are shown in Figs. 5–7.
The absorption spectrum of HL exhibits multiple peaks at
<350 nm which can be assigned to intraligand charge transfer
(ILCT) transition pPh
!
pCꢁ
(Fig. 5). Upon addition HBF4 to the
^N^
N
CH2Cl2 solution of HL, low-energy absorption was red-shifted to
482 nm. The process is characterized by isosbestic points of
absorption spectra at 270, 285 and 382 nm. The absorption spectra
with kmax at 342 nm decreased but with a concomitant develop-
ment of the absorption centered at 482 nm. However, the subse-
quent addition of HBF4 led to the decrease in low-energy
absorption intensity (Fig. 6). To verify a tentative assignment of
the absorption spectra, DFT calculation carried out at the B3LYP/
6-31G(d,p) level was adopted to determine the binding site of H+
with the ligand. The calculated total energies of the molecules in
which H+ was associated with N1, N2 and N3 of ligand HL were
Fig. 1. The cation structure of 1, the hydrogen atoms are omitted for clarity.
configuration. The asymmetric unit of complex 2 consists of half
the molecule and the whole molecule is generated by inversion,
as shown in Fig. 2. The bond lengths of Cu1–N1 and Cu1–N2 are

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Q.-Q. Xu et al. / Inorganica Chimica Acta 362 (2009) 2529–2536
Fig. 2. (a) The cation structure of 2, the hydrogen atoms are omitted for clarity and (b) view of the crystal packing of 2.
ꢃ1091.9128, ꢃ1091.9263, and ꢃ1091.8695 a.u., respectively.
Based on these results, the site in the first place to be binded
was expected to be the N2 atom of ligand HL (Fig. 1). Therefore,
function of concentration of HBF4 is shown in Fig. 7 and with the
addition HBF4 decreased and then enhanced, which indicates the
emissions observed can be ascribed to ILCT excited state in nature.
the interaction between H+ and N2 atom lowers the
p
*(C^N^N)
orbital energy level, resulting in the change in low-energy absorp-
tion from 342 to 482 nm. The subsequent addition of H+ promoted
hydrogen ion associated to N3 at amino-substituted group to de-
3.2.2. Acid/Base-controlled Molecular Switching based on complex 1
The absorption spectra of complex 1 in CH3CN are presented in
Fig. 8. With reference to the previous spectroscopic work on cyclo-
metalated platinum(II) complexes [23–26], the absorption bands of
1 at wavelengths below 350 nm are assigned to the local excitation
(LE) transition of C^N^N, while the intense low-energy absorption
press
p(Ph) orbital energy level such that longer wavelength
absorption was lowered. Compound HL exhibits an intense emis-
sion with kmax at 450 nm in CH2Cl2 upon excitation at 350 nm at
room temperature. The luminescence intensity measured as a
with kmax at 443 nm is ascribed to an ILCT transition from the
p

Q.-Q. Xu et al. / Inorganica Chimica Acta 362 (2009) 2529–2536
2533
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1–4.
1
Pt1–N2
Pt1–N1
N2–Pt1–C12
C12–Pt1–N1
C12–Pt1–P1
2.005(2)
2.148(2)
80.88(9)
158.25(9)
95.30(7)
Pt1–C12
Pt1–P1
N2–Pt1–N1
N2–Pt1–P1
N1–Pt1–P1
2.023(3)
2.2465(8)
77.80(8)
175.79(6)
106.13(6)
2
Cu1–N1
N1–Cu1–N2
N1–Cu1–N1#
1.997(2)
80.32(6)
131.33(8)
Cu1–N2
N1–Cu1–N2#
N2–Cu1–N2#
2.099(2)
131.12(6)
107.11(8)
3
Cu1–N1
Cu1–P1
N1–Cu1–N2
N2–Cu1–P1
2.020(3)
2.1770(1)
80.9(1)
Cu1–N2
2.067(3)
N1–Cu1–P1
151.89(9)
123.40(9)
4
Zn1–N2
N2–Zn1–N1
N1–Zn1–N1#
1.96(1)
82.0(4)
99.3(5)
Zn1–N1
N2–Zn1–N2#
N2–Zn1–N1#
2.042(10)
124.9(6)
137.6(4)
Symmetry transformations used to generate equivalent atoms, 2: #1 ꢃx + 2, y,
ꢃz + 3/2 4: #1 ꢃx, y, ꢃz + 1/2.
approximations because of the possible extensive orbital mixing in
the complex 1.
Upon addition of HBF4 to a solution of 1 in CH3CN, the low-en-
ergy absorption centered at 443 nm decreases monotonically
throughout the addition, and at the end of the acid titration a less
intense band was observed in the region 400–550 nm (Fig. 8). This
observation may be attributed to the protonation of the amino
group on the ligand part. Since the sites of N1 and N2 have been
binded by the Pt atom, thus only the N3 site in the amino group
was left without occupied. With the amino group protonated, the
Fig. 3. The cation structure of 3, the hydrogen atoms are omitted for clarity.
energy level of pPh is decreased, and the pPh
!
pCꢁ
band shifts
^N^
N
orbital of the amino-substituted phenyl group to the p* orbital of
to higher energy, and hence the MLCT transition becomes the low-
est energy transition [27]. Complex 1 is nonemissive in CH3CN
solution, but in the acidic solution, complex 1 exhibits moderately
intense 3MLCT emission with kmax at 550 nm and a lifetime of
183 ns. Worthy of being mentioned, these changes in the absorp-
tion and emission spectra for 1 are fully reversible. Addition of tri-
ethylamine to an acidic solution of 1 leads to the recurrence of
luminescence quenching and the recovery of the ILCT band in the
absorption spectra.
the metal-bound C^N^N moiety, probably with some admixture
*
of d(Pt)-
p (C^N^N) character.
The TD-DFT calculation also indicated that the lowest energy
absorption of 1 is the excitation (Ph) ?
*(C^N^N) mixed with
some MLCT contribution. As shown in Fig. 9, the HOMO and the
LUMO for L are mainly localized on the pph and pCꢁ
moieties,
p
p
^N^
N
respectively. The calculated energy gap between HOMO and LUMO
is 2.55 eV, nearly in accord with the measured absorption energy of
2.81 eV. Accordingly, we tentatively attributed the low-energy
absorption to be from a mixed-orbital parentage including ILCT
and MLCT character. However, the assignments of electronic tran-
sitions between metal and/or ligand localized orbits are only rough
3.2.3. Electronic absorption and emission spectra of complex 2–4
The electronic absorption spectra of complexes 2–4 in CH2Cl2
solutions at room temperature exhibit broad low-energy absorp-
Fig. 4. The cation structure of 4, the hydrogen atoms are omitted for clarity.

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Q.-Q. Xu et al. / Inorganica Chimica Acta 362 (2009) 2529–2536
Fig. 8. Changes in the absorption spectra of 1 (3.0 ꢂ 10ꢃ5 mol dmꢃ3) upon addition
of various concentrations of HBF4 (0–6.0 ꢂ 10ꢃ5 mol dmꢃ3) in CH3CN.
Fig. 5. The electronic absorption spectra of HL, 2, 3 and 4 in CH2Cl2 at room
temperature.
tion bands between 410 and 650 nm with kmax at 560, 416 and
440 nm, respectively (Fig. 5). The relatively high-energy absorp-
tions centered at <400 nm are assigned to ILCT transition which
exhibits significant MLCT character in the case of copper(I) com-
plexes 2 and 3, while the absorption band at 410–650 nm for the
complexes are characteristic for metal-to-ligand charge transfer
(MLCT) transition, which can be compared with previous spectro-
Fig. 6. Changes in the absorption spectra of HL (2.0 ꢂ 10ꢃ5 mol dmꢃ3
) upon
addition of various concentrations of HBF4 in CH2Cl2 [HBF4]: 0–
3.0 ꢂ 10ꢃ5 mol dmꢃ3; inset: [HBF4]: 4–6.0 ꢂ 10ꢃ5 mol dmꢃ3
.
Fig. 7. Changes in the emission spectra of HL (2.0 ꢂ 10ꢃ5 mol dmꢃ3) upon addition
of various concentrations of HBF4 in CH2Cl2. [HBF4]: 0–3.0 ꢂ 10ꢃ5 mol dmꢃ3; inset:
[HBF4]: 4–6.0 ꢂ 10ꢃ5 mol dmꢃ3
.
Fig. 9. Contour plots and orbital energies of the HOMO and LUMO for 1.

Q.-Q. Xu et al. / Inorganica Chimica Acta 362 (2009) 2529–2536
2535
4. Conclusion
In summary, four new complexes [Pt(L)(PPh3)]ClO4 (1),
[Cu(HL)2]BF4 (2), [Cu(HL)(PPh3)]BF4 (3) and [Zn(HL)2](ClO4)2 (4)
have been successfully synthesized and characterized. X-ray crys-
tal analysis reveals that 2 and 4 have a similar configuration. The
studies on spectroscopic properties of the complexes and HL show
dominating ILCT excited state with the admixture of MLCT state,
and the excited states can be tuned by micro environmental
changes such as pH in the case of 1. The spectral assignments of
1 were further supported by pH sensing properties of HL and TD-
DFT calculation, and the calculating result was found to be consis-
tent with that of the experiment. Compared with the excited state
of 1, those of complexes 2 and 3 exhibiting multi-coordination
model have MLCT character. The intense photoluminescence of 4
in solid state or solution was assigned to ILCT transition in nature.
The results would provide a new insight into the development of
amino group (–NR2) substituted HC^N^N derivatives for lumines-
cence sensor applications.
Fig. 10. Changes in the absorption spectra of ligand (2.0 ꢂ 10ꢃ5 mol dmꢃ3) or 2
(inset, 2.0 ꢂ 10ꢃ5 mol dmꢃ3
)
upon addition of various concentrations of
[Cu(CH3CN)4]BF4 in CH2Cl2 solution.
Supplementary material
CCDC 684609, 684610, 684611 and 684612 contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
The work was supported by the National Basic Research Pro-
gram of China (973 Program 2007CB613304), the National Natural
Science Foundation of China (NSFC Grant Nos. 20671094,
90610034 and 20761006).
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