Synthesis, photoluminescence and computational study of rhenium(I) diimine complexes with [1,3,4]oxadiazole substituted 2,2′-bipyridine ligands

Article abstract of DOI:10.1016/j.jorganchem.2013.06.024

Four bipyridine ligands substituted by the [1,3,4]oxadiazole moieties at different positions, 4,4′-bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,2′- bipyridine (L1), 4,4′-bis[5-(4-tert-butyl-phenyl)-[1,3,4] oxadiazol-2-yl]-2,2′-bipyridine (L2), 5,5′-bis(5-phenyl-[1,3,4] oxadiazol-2-yl)-2,2′-bipyridine (L3), and 5,5′-bis [5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-2,2′-bipyridine (L4), and their corresponding rhenium(I) Re(CO)3Cl(L) complexes (1e4) were synthesized and characterized by the elemental analysis, IR, 1H NMR and UVevis spectroscopy. The complex 2 is also determined by the crystal structure analysis. In the UV evis spectra complexes 1, 2 and 4 show intense spin-allowed intraligand (φ → φ*) transitions and weak metal to ligand charge-transfer dp (Re) → φ* (diimine) (MLCT) bands in THF solution. Complexes 1, 2 and 3 show efficient photoluminescent emission assigned to dp (Re) → φ* (diimine) MLCT phosphorescence at ambient temperature, and the different substitution position of [1,3,4]oxadiazole moieties in bipyridine ligands has a great effect on both absorption and emission properties of complexes. Among complexes 1, 2 and 3, the 4,4′-disubstituted complex 1 exhibits the strongest phosphorescence emission both in aerated THF solution (a quantum yield of 1.1%) and solid state. Meanwhile, the ground-state electronic structure and absorption spectra of 1e4 are investigated theoretically by DFT and TD-DFT methods.

Full text of DOI:10.1016/j.jorganchem.2013.06.024

Journal of Organometallic Chemistry 743 (2013) 37e43
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, photoluminescence and computational study of rhenium(I)
diimine complexes with [1,3,4]oxadiazole substituted 2,20-bipyridine
ligands
,
Hong-Yan Li a b, Tian-Yi Li a, Quan Liu a, Qiu-Lei Xu a, Cheng-Cheng Wang a, Song Zhang a,
,
c,
**
Chen Lin a, Wei Huang a, You-Xuan Zheng a , Xiu-Qiang Wang
*
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering,
Center for Molecular Organic Chemistry, Nanjing University, Nanjing 210093, PR China
b Engineering Research Center of Seawater Utilization Technology, Ministry of Education, Hebei University of Technology,
Tianjin 300130, PR China
c School of Life Science, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Four bipyridine ligands substituted by the [1,3,4]oxadiazole moieties at different positions, 4,40-
bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,20-bipyridine (L1), 4,40-bis[5-(4-tert-butyl-phenyl)-[1,3,4] oxadia-
zol-2-yl]-2,20-bipyridine (L2), 5,50-bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,20-bipyridine (L3), and 5,50-bis
[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-2,20-bipyridine (L4), and their corresponding rhenium(I)
Re(CO)3Cl(L) complexes (1e4) were synthesized and characterized by the elemental analysis, IR, 1H NMR
and UVevis spectroscopy. The complex 2 is also determined by the crystal structure analysis. In the UV
Article history:
Received 15 March 2013
Received in revised form
13 June 2013
Accepted 21 June 2013
Keywords:
Rhenium(I)
[1,3,4]Oxadiazole
Photoluminescence
Computational calculation
evis spectra complexes 1, 2 and 4 show intense spin-allowed intraligand (
metal to ligand charge-transfer d (Re)/ * (diimine) (MLCT) bands in THF solution. Complexes 1, 2 and
3 show efficient photoluminescent emission assigned to d (Re)/ * (diimine) MLCT phosphorescence at
p/p*) transitions and weak
p
p
p
p
ambient temperature, and the different substitution position of [1,3,4]oxadiazole moieties in bipyridine
ligands has a great effect on both absorption and emission properties of complexes. Among complexes 1,
2 and 3, the 4,40-disubstituted complex 1 exhibits the strongest phosphorescence emission both in
aerated THF solution (a quantum yield of 1.1%) and solid state. Meanwhile, the ground-state electronic
structure and absorption spectra of 1e4 are investigated theoretically by DFT and TD-DFT methods.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
complexes possessing long-lived excited states exhibit strong
phosphorescence [11], which can lead to their applications in
The photophysical and photochemical properties of diimine
complexes with d6 metal ions, such as rhenium(I), ruthenium(II)
and osmium(II) have drawn tremendous attentions due to their
theoretic value for basic science as well as for promising practical
applications [1e4]. Among them, Re(I) tricarbonyl complexes
with the type of fac-[Re(CO)3(N^N)(X)], where N^N ¼ diimine
and X ¼ halide, are widely studied for their easy synthesis,
chemical stability and excellent photophysical and photochem-
ical properties [5e10]. Because of their properties of strong spine
orbit coupling and enhanced singlet-triplet mixing, the Re(I)
organic light emitting diodes (OLEDs) [12] or in other fields such
as solar energy conversion, supramolecular chemistry, catalysis,
and medical chemistry [13e16].
The photophysical and photochemical properties of the Re(I)
tricarbonyl complexes can be tuned by varying their ligand struc-
tures. 2,20-Bipyridine is a planar bidentate ligand with powerful
binding abilities towards Re(I) metal ion and it can be easily
modified by the introduction of substituent groups at its different
positions. The introduction of substituent groups such as electron-
withdrawing or -donating moieties are helpful to tune the energy
levels of the novel 2,20-bipyridine ligand [17,18].
In our previous work, we have introduced the electro-donating
and hole-transporting unit of carbazole into the 5,50-positions of
2,20-bipyridine, and consequently, the corresponding Re(I) complex
shows relatively efficient emission [19]. In this paper, we introduce
the electron-withdrawing [1,3,4]oxadiazole group into 2,20-
* Corresponding author. Tel.: þ86 25 83596775; fax: þ86 25 83314502.
** Corresponding author.
E-mail addresses: yxzheng@nju.edu.cn (Y.-X. Zheng), xqwang@nju.edu.cn
(X.-Q. Wang).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.024

38
H.-Y. Li et al. / Journal of Organometallic Chemistry 743 (2013) 37e43
bipyridine at 4,40-positions and 5,50-positions to obtain ligands L1e
L4 and their corresponding Re(I) complexes 1e4 (Scheme 1). By the
comparative analysis of emission properties of the 2,20-bipyridine
Re(I) complexes with the [1,3,4]oxadiazole at different positions,
we found that the different substitution position of [1,3,4]oxadia-
zole at 2,20-bipyridine ligand has a great effect on both emission
intensity and peak wavelength.
solution of [Ru(bpy)3]Cl2, respectively. The Iunk and Istd are the in-
tegrated emission intensities of the unknown sample and
[Ru(bpy)3]Cl2 solution, respectively. The Aunk and Astd are the
absorbance of the unknown sample and [Ru(bpy)3]Cl2 acetonitrile
solution at their excitation wavelengths, respectively. The hunk and
hstd terms represent the refractive indices of the corresponding
solvents (pure solvents were assumed). The Fstd values of air-
equilibrated and deaerated acetonitrile solution of [Ru(bpy)3]Cl2
have been revalued to be 0.018 and 0.059, respectively [21].
2. Experimental section
2.2. Crystallography
2.1. Materials and measurements
The crystal structure of complex 2 was determined on a Siemens
Rhenium pentacarbonyl chloride (Re(CO)5Cl), benzoylhydrazine,
4-tert-butylbenzhydrazide, 2,20-bipyridinyl-4,40-dicarboxylic acid
and 2,20-bipyridinyl-5,50-dicarboxylic acid were used as received.
The solvents except toluene were used without further purification.
The IR spectra were taken on a Vector22 Bruker spectropho-
tometer (400e4000 cmꢀ1) with KBr pellets. 1H NMR spectra were
measured on a Bruker AM 500 spectrometer. Mass spectra were
determined with an Autoflex II TM instrument for MALDI-TOF-MS.
Elemental analyses for C, H, and N were performed on an Elementar
Vario MICRO analyzer. Absorption and photoluminescence spectra
were measured on a UV-3100 spectrophotometer and a Hitachi
F4600 luminescence spectrophotometer, respectively. For the solid
state samples we prepared the solid film samples by using the
tabletting method with the same amount to make sure the surfaces
are smooth and similar. The luminescence quantum efficiencies
were calculated by comparison of the emission intensities (inte-
grated areas) of a standard sample (air-equilibrated and deaerated
acetonitrile solutions of [Ru(bpy)3]Cl2) and the unknown sample
according to the equation [20].
(Bruker) SMART CCD diffractometer using monochromated Mo K
a
ꢀ
radiation (
l
¼ 0.71073 A) at 291 K. Cell parameters were retrieved
using SMART software and refined using SAINT [22] on all observed
reflections. Data were collected using a narrow-frame method with
scan widths of 0.30ꢁ in
u and an exposure time of 10 s/frame. The
highly redundant data sets were reduced using SAINT [22] and
corrected for Lorentz and polarization effects. Absorption correc-
tions were applied using SADABS [23] supplied by Bruker. Structure
was solved by direct methods using the program SHELXL-97 [24].
The positions of metal atoms and their first coordination spheres
were located from direct-methods E-maps; other non-hydrogen
atoms were found in alternating difference Fourier syntheses and
least-squares refinement cycles and, during the final cycles, refined
anisotropically. Hydrogen atoms were placed in calculated position
and refined as riding atoms with a uniform value of Uiso.
2.3. Computational details
All computations were carried out using Gaussian09 programs
package [25]. The ground-state geometries were optimized by the
density functional theory (DFT) method. The excited states calcu-
lations were performed by time-dependent DFT (TD-DFT)
approach. The B3LPY functional [26] is used in both DFT and TD-
DFT calculations. The same combination of basis sets was
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Aunk
hunk
hstd
Funk
¼
Fstd
where Funk and Fstd are the luminescence quantum yields of the
unknown sample and air-equilibrated or deaerated acetonitrile
R
R
R
R
HOOC
COOH
+ H2NHN
O
i
ii
O
O
R
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Re
OC
OC
1, 2
Cl
L1, L2
CO
L1, 1: R = H; L2, 2: R = C(CH3)3
O
O
O
i
HOOC
COOH
O
+
H2NHN
R
R
R
N
N
N
N
N
N
N
N
L3, L4
O
ii
R
R
L3, 3: R = H; L4, 4: R = C(CH3)3
N
N
N
N
N
N
Re
CO
3, 4
OC
OC
Cl
(i) POCl3, reflux; (ii) Re(CO)5Cl, toluene, reflux.
Scheme 1. Synthetic routes for ligands L1eL4 and complexes 1e4.

H.-Y. Li et al. / Journal of Organometallic Chemistry 743 (2013) 37e43
39
employed in all the calculations. 6e31G** and Lan2LDZ were
adopted respectively for lighter nonmetal atoms and Re atom. In
order to make theoretical calculations more comparable to the
experimental results, solvent effect of THF was taken into account
by the conductor-like polarizable continuum model (C-PCM)
[27,28] in all the calculations.
GaussView5.0.8 and ChemCraft were used to visualize the
frontier molecular orbitals of the complexes. GaussSum 2.2 pro-
gram was used for the stimulations and analysis of UVevis spectra
(FWHM ¼ 3000 cmꢀ1). MQForge program was used to give accurate
percentage data of frontier molecular orbitals.
5,50-Bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,20-bipyridine
(L3).
Yield: 14.6%. IR (KBr, cmꢀ1): 1600, 1485, 1455, 1075, 1020, 847, 720,
691. 1H NMR (CDCl3, 500 MHz, ppm):
d 9.71 (s, 2H), 9.14 (d,
J ¼ 8.2 Hz, 2H), 8.71 (d, J ¼ 8.3 Hz, 2H), 8.21 (d, J ¼ 7.5 Hz, 4H), 7.73 (t,
J ¼ 7.4 Hz, 2H), 7.65 (t, J ¼ 7.5 Hz, 4H). MS(MALDI-TOF): m/z 445.354
[M þ H]þ. Anal. calcd for C26H16N2O2 (444.13): C 70.26, H 3.63, N
18.91. Found: C 70.17, H 3.74, N 18.90.
5,50-Bis[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-2,20-
bipyridine (L4). Yield: 12.7%. IR (KBr, cmꢀ1): 1605, 1490, 1456, 1074,
1014, 843, 745, 719. 1H NMR (DMSO-d6, 500 MHz, ppm)
d 9.71 (s,
2H), 9.09 (dd, J ¼ 8.4, 1.2 Hz, 2H), 8.71 (d, J ¼ 8.4 Hz, 2H), 8.13 (d,
J ¼ 8.4 Hz, 4H), 7.66 (d, J ¼ 8.5 Hz, 4H), 1.41 (s, 18H). MS(MALDI-
TOF): m/z 557.481 [M þ H]þ. Anal. calcd for C34H32N6O2 (556.26):
C 73.36, H 5.79, N 15.10. Found: C 73.21, H 5.74, N 15.21.
2.4. Synthesis
The chemical structures of the materials used in this work and
the synthetic routes are depicted in Scheme 1.
2.4.2. General synthesis of Re(CO)3Cl(L) (1e4)
Re(CO)5Cl (0.18 g, 0.5 mmol) was refluxed with equimolar
quantity of appropriate ligand in toluene for 6 h under N2. After the
mixture was cooled to RT, the solvent was distilled out. The
resulting yellow solid was purified by silica gel column chroma-
tography with dichloromethane and petroleum ether (v/v ¼ 2:1, a
little Et3N was added) as eluant.
2.4.1. General synthesis procedures of L1eL4 with [1,3,4]oxadiazole
moieties
A mixture of 2,20-bipyridinyl-4,40-dicarboxylic acid or 2,20-
bipyridinyl-5,50-dicarboxylic acid (2 mmol) and benzoylhydrazine
or 4-tert-butylbenzhydrazide (4 mmol) was refluxed in 30 mL
POCl3 under nitrogen for 5 h. After cooling to room temperature,
the excessive POCl3 was distilled out, and the residue was poured
into water. The crude solid product was collected by filtration and
purified by silica gel column chromatography with dichloro-
methane and methanol (v/v ¼ 50/1) as eluant.
Re(CO)3Cl(L1) (1). Yield: 35.0%. IR (KBr, cmꢀ1): 2019, 1906. 1H
NMR (DMSO-d6, 500 MHz, ppm)
d
9.64 (s, 2H), 9.33 (d, J ¼ 5.7 Hz,
2H), 8.44 (d, J ¼ 4.5 Hz, 2H), 8.34 (d, J ¼ 6.4 Hz, 4H), 7.73e7.77 (m,
6H). MS(MALDI-TOF): m/z 715.095 [M ꢀ Cl]þ. Anal. Calcd for
C29H16ClN6O5Re (750.04): C 46.43, H 2.15, N 11.20. Found: C 46.33,
4,40-Bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,20-bipyridine
(L1).
Yield: 23.9%. IR (KBr, cmꢀ1): 1606, 1542, 1485, 1454, 1066, 723, 687.
1H NMR (CDCl3, 500 MHz, ppm):
H 2.15, N 11.12.
Re(CO)3Cl(L2) (2). Yield: 63.1%. IR (KBr, cmꢀ1): 2023, 1927, 1902.
d
9.42 (s, 2H), 9.25 (d, J ¼ 5.4 Hz,
1H NMR (CDCl3, 500 MHz, ppm)
d
9.34 (d, J ¼ 5.7 Hz, 2H), 9.30 (s,
2H), 8.58 (d, J ¼ 5.0 Hz, 2H), 8.25 (d, J ¼ 7.8 Hz, 4H), 7.78 (t,
J ¼ 7.3 Hz, 2H), 7.69 (t, J ¼ 7.6 Hz, 4H). MS(MALDI-TOF): m/z 445.388
[M þ H]þ. Anal. calcd for C26H16N2O2 (444.13): C 70.26, H 3.63, N
18.91. Found: C 70.17, H 3.66, N 18.94.
2H), 8.28 (d, J ¼ 5.5 Hz, 2H), 8.16 (d, J ¼ 8.3 Hz, 4H), 7.70 (d,
J ¼ 8.4 Hz, 4H), 1.43 (s, 18H). MS(MALDI-TOF): m/z 827.065
[M ꢀ Cl]þ. Anal. calcd for C37H32ClN6O5Re (862.35): C 51.53, H 3.74,
N 9.74. Found: C 51.51, H 3.84, N 9.78.
4,40-Bis[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-2,20-
bipyridine (L2). Yield: 15.6%. IR (KBr, cmꢀ1): 1606, 1489, 1454,
1390, 1265, 1067, 1016, 847, 744, 716. 1H NMR (CDCl3, 500 MHz,
Re(CO)3Cl(L3) (3). Yield: 50.9%. IR (KBr, cmꢀ1): 2019, 1928, 1903.
1H NMR (DMSO-d6, 500 MHz, ppm)
d
9.80 (s, 2H), 9.04 (d, J ¼ 8.5 Hz,
2H), 8.61 (d, J ¼ 8.6 Hz, 2H), 8.24 (d, J ¼ 7.3 Hz, 4H), 7.78 (t, J ¼ 7.5 Hz,
2H), 7.70 (t, J ¼ 7.6 Hz, 4H). MS(MALDI-TOF): m/z 714.970 [M ꢀ Cl]þ.
Anal. calcd for C29H16ClN6O5Re (750.04): C 46.43, H 2.15, N 11.20.
Found: C 46.34, H 2.19, N 11.24.
ppm):
d
9.37 (s, 2H), 9.25 (d, J ¼ 5.5 Hz, 2H), 8.56 (dd, J ¼ 5.5,
1.2 Hz, 2H), 8.17 (d, J ¼ 8.5 Hz, 4H), 7.70 (d, J ¼ 8.5 Hz, 4H), 1.43 (s,
18H). MS(MALDI-TOF): m/z 557.463 [M þ H]þ. Anal. calcd for
C
34H32N6O2 (556.26): C 73.36, H 5.79, N 15.10. Found: C 73.28, H
Re(CO)3Cl(L4) (4). Yield: 45.6%. IR (KBr, cmꢀ1): 2023, 1920, 1896.
5.80, N 15.19.
1H NMR (CDCl3, 500 MHz, ppm)
d
9.75 (d, J ¼ 1.5 Hz, 2H), 8.84 (d,
Fig. 1. Left: ORTEP view of complex 2 with the atom-numbering scheme. Hydrogen atoms and solvent molecule are omitted for clarity. Ellipsoids are drawn at the 30% probability
level. Right: the molecular packing of complex 2.

40
H.-Y. Li et al. / Journal of Organometallic Chemistry 743 (2013) 37e43
Table 1
nitrogen atmosphere with an equimolar amount of Re(CO)5Cl in a
ꢁ
ꢀ
Crystallographic data, selected bond lengths (A) and angles ( ) for complex 2.
non-polar solvent such as hexane and toluene, where toluene was
used in our case. The purity and composition of the ligands and the
complexes were confirmed by IR, 1H NMR, MS and elemental
analyses.
ꢀ
Crystallographic
data for 2$CH3OH
Selected bond lengths (A) and
angles (ꢁ) for 2$CH3OH
Formula
FW
T (K)
Wavelength (A)
Cryst syst
C38H36ClN6O6Re
894.38
291(2) K
0.71073
Triclinic
Pꢀ1
Re(1)eC(1)
Re(1)eC(2)
Re(1)eC(3)
Re(1)eN(1)
Re(1)eN(2)
Re(1)eCl(1)
1.974(12)
1.974(12)
1.926(12)
2.194(8)
2.188(9)
2.510(5)
87.0(5)
88.3(5)
87.8(4)
94.2(4)
98.9(4)
171.5(5)
95.3(4)
172.9(3)
99.0(4)
74.3(3)
177.3(3)
95.5(3)
3.2. Crystallography
ꢀ
Using the single-crystal X-ray diffraction method, the crystal
structure of complex 2 was obtained and the ORTEP diagram is
shown in Fig. 1. The crystal data, selected bond distances and angles
are presented in Table 1. One CH3OH solvent molecule is found in
the crystal structure of complex 2.
Space group
ꢀ
a (A)
10.62(2)
11.33(2)
19.34(6)
93.64(4)
96.08(4)
115.15(2)
2079(9)
2
1.429
3.305
892
2.00e24.39
C(1)eRe(1)eC(2)
C(1)eRe(1)eC(3)
C(2)eRe(1)eC(3)
C(1)eRe(1)eN(1)
C(2)eRe(1)eN(1)
C(3)eRe(1)eN(1)
C(1)eRe(1)eN(2)
C(2)eRe(1)eN(2)
C(3)eRe(1)eN(2)
N(2)eRe(1)eN(1)
C(1)eRe(1)eCl(1)
C(2)eRe(1)eCl(1)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(deg)
(deg)
(deg)
For complex 2, the Re atom is in a distorted octahedral coordi-
nation environment with three carbonyl ligands in a fac arrange-
ment, two N atoms from two pyridine rings of the ligand, and a
3
ꢀ
V (A )
Z
rcalcd (g/cm3)
ꢀ
chlorine atom. The average ReeN bond distance is 2.191(8) A, which
m
(Mo K
a
) (mmꢀ1
)
is consistent with the similar Re(I) tricarbonyl complexes with
other polypyridine ligands [19]. The bond angels between adjacent
CO carbon atoms are 87.0(5)e88.3(5)ꢁ, which is close to 90ꢁ, indi-
cating that CO ligands are linearly coordinated. While the NeReeN
angle is 74.3(3)ꢁ, which is much less than 90ꢁ, owing to the steric
effect from the chelating ligands. The dihedral angle between the
two pyridine rings is 2.9ꢁ, and the dihedral angle between the
[1,3,4]oxadiazole rings and their parent pyridine planes are 15.8ꢁ
and 1.0ꢁ, respectively.
F (000)
Range of transm
factors (deg)
Reflns collected
Unique
9615
6641
1.073
0.0557, 0.1567
0.0692, 0.1643
844032
P
C(3)eRe(1)eCl(1)
N(1)eRe(1)eCl(1)
N(2)eRe(1)eCl(1)
93.0(4)
82.3(2)
82.1(2)
GOF on F2
Ra1, wRb2 (I > 2
s(I))
Ra1, wRb2 (all data)
CCDC no.
P
P
P
R1a
¼
jjFoj ꢀ j:Fcj:j= Foj: wRb2 ¼ ½ wðFo2 ꢀ Fc2Þ2= wðFo2Þꢂ1=2
.
3.3. UVevis absorption and photoluminescence spectra
J ¼ 8.4 Hz, 2H), 8.48 (d, J ¼ 8.5 Hz, 2H), 8.11 (d, J ¼ 8.4 Hz, 4H), 7.63
(d, J ¼ 8.4 Hz, 4H), 1.42 (s, 18H). MS(MALDI-TOF): m/z 827.130
[M ꢀ Cl]þ. Anal. calcd for C37H32ClN6O5Re (862.35): C 51.53, H 3.74,
N 9.74. Found: C 51.54, H 3.80, N 9.94.
The electronic absorption spectra of 1, 2 and 4 in THF are illus-
trated in Fig. 2a and their spectral data are given in Table 2. Due to
the poor solubility in common organic solvents, the absorption
spectrum of 3 was not measured in solution. After the introduction
of tert-butyl units, the solubility of complex 4 becomes much better.
In Fig. 2a, all complexes 1, 2 and 4 display intense absorption bands
at ca. 220e420 nm (220e360 nm for 1 and 2, 220e420 nm for 4)
with extinction coefficients on the order of 104 Mꢀ1 cmꢀ1, which
3. Results and discussion
3.1. Synthesis and characterization
Ligands L1eL4 and their corresponding Re(I) complexes 1e4
were prepared according to the modified literature methods [29].
The substitution of [1,3,4]oxadiazole into L1eL4 was conducted in
POCl3 which worked as dehydration and cyclization solvent. The
main effect of tert-butyl substituent at the phenyl ring was to
improve the solubility of the ligands and complexes. Re(I) com-
plexes of the type fac-[Re(CO)3(N^N)Cl] could be easily synthesized
by reacting the appropriate N^N-donor ligands under an inert
can be assigned to spin-allowed intraligand (p/p*) transitions.
With reference to previous studies on related Re(I) system
[6,29,30], the absorption at low-energy bands (390e550 nm for 1
and 2, 420e520 nm for 4) are attributed to the metal to ligand
charge-transfer d
p
(Re)/
p
* (diimine) (MLCT). Compared 1 with 2,
the maximal peaks of
p/p* transitions (321 nm and 327 nm,
respectively) and the extinction coefficients are close, indicating
that the tert-butyl substituent at the phenyl ring has a negligible
1.0
1
1
9x10
(a)
(b)
2
3
4
2
8x10
7x10
6x10
5x10
4x10
3x10
2x10
1x10
4
0.8
0.6
0.4
0.2
0.0
0
250
300
350
400
450
500
550
600
250
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 2. UVevis absorption of Re(I) complexes 1, 2, 4 in THF solution (2 ꢃ 10ꢀ5 mol Lꢀ1) at room temperature (a) and the computational calculated absorption spectra of the
complexes 1e4 (b).

H.-Y. Li et al. / Journal of Organometallic Chemistry 743 (2013) 37e43
41
Table 2
Absorption and photoluminescence data of Re(I) complexes at room temperature.
Complex
Medium
Absorption [
l
, nm (ε, 103 Mꢀ1 cmꢀ1)]
lexca nm
lem, maxb nm
Fc (deaerated) %
1
THF
Solid
THF
Solid
Solid
THF
266 (28.5), 321 (41.3), 438 (6.72)
269 (30.2), 327 (43.7), 438 (7.00)
469
369
461
372
607
605
564
627
1.1 (3.6)
0.1 (0.4)
2
3
4
365
604
d
d
d
261 (34.6), 68 (46.6), 459 (1.90)
a
Excitation wavelength.
Emission maxima from not corrected spectra.
b
c
Calculated by Fs
¼
Fstd(IsAstdh2s)/(Istd shs2td) using air-equilibrated and deaerated [Ru(bpy)3]2þ$2Clꢀ acetonitrile solution as a standard sample, respectively, the error in this
A
method is estimated to be approximately 10% of the measured value.
d
No luminescence.
effect on the absorbance. However, in complex 4, which is based on
the highest among complexes 1e4 both in solution and solid state,
and its quantum yield reaches to 1.1% in air-equilibrated THF so-
lution and 3.6% in deaerated THF solution at room temperature. The
efficiency enhancement suggests that the excited states of the Re(I)
complexes can be quenched by O2. But when tert-butyl units were
introduced to phenyl rings, the emission intensity of complex 2 was
decreased greatly and the quantum yield is 10 times lower than
that of complex 1, which can be attributed to the higher vibration
energy loss of tert-butyl substituents. CeH vibrations of the tert-
butyl units in the molecules can provide an efficient nonradiative
pathway for relaxation of the luminescent state of the emitting
compounds by vibronic coupling, since they can be effectively
mediated by the ubiquitous molecular vibrations, which probably
weaken the luminescence of complexes [33]. And it is also can be
observed that by comparison with the solution sample, the emis-
sion wavelength of 2 in solid state showed obvious red shift from
564 nm to 605 nm. From the molecular packing of complex 2 (Fig. 1,
right) it can be found that the distances between CO units to the
benzene groups of [1,3,4]oxadizole in adjacent complexes are 3.36
the 5,50-disubstituted 2,20-bipyridine by Refs. [1,3,4]oxadiazole
moieties, the ligand centered p/p* transition red shifts to 368 nm.
On the other hand, the MLCT transition absorbance of 4 is two times
weaker than that of 1 and 2. The results suggested that the sub-
stitution position of the [1,3,4]oxadiazole unit on 2,20-bipyridine
has a great impact on the absorption spectrum, such as the red shift
of the absorbance peaks and the dramatically lowered intensity of
MLCT bands.
The emission spectra and photophysical data for complexes 1e4
at room temperature are listed in Fig. 3 and summarized in Table 2,
respectively. Due to the poor solubility in common organic solvents,
the emission spectrum of 3 was not obtained in solution. Excitation
at the MLCT maxima absorption peaks, the emission spectra of
complexes show broad bands between 500 nm and 750 nm (Fig. 3
and Table 2) both in solution (1 and 2) and solid state (1, 2 and 3),
which are assigned to dp(Re)/p* (diimine) MLCT phosphores-
cence emission. For the emission measurements in the solid states
it is a good method to prepare the powder samples by the milling of
the single crystals to assess the effect of reabsorption and induced
structural defects [31,32] to keep reproducibility. But in our paper it
is hard to control the temperature to melt the complexes without
decomposition. Therefore, we prepared the solid film samples by
using the tabletting method with the same amount to make sure
the surfaces are smooth and similar. Obviously, the emission
spectra of these complexes show variation both in intensity and
peak position due to the different substitution positions of [1,3,4]
oxadiazole groups. It is obvious that the emission intensity of
complex 1, which is based on 4,40-disubstituted 2,20-bipyridine, is
ꢀ
and 3.85 A, respectively. The short distances suggesting that there
are strong interactions between the adjacent molecules, which
perhaps affect the luminescence wavelengths of the complex 2 in
the solid state. The 5,50-disubstituted 2,20-bipyridine Re(I) complex
3 which has the same [1,3,4]oxadiazole moieties as 1 shows weaker
emission intensity than 1, even lower than that of 2. Complex 4
with the same [1,3,4]oxadiazole substituent as complex 2 attached
to 5,50-positions of bpy ligand, shows negligible luminescence
either in solution or solid state under the excitation wavelength of
200e800 nm region at room temperature.
1400
1500
1
1
(b)
(a)
2
2
1200
3
1200
900
600
300
0
1000
800
600
400
200
0
500
550
600
650
700
750
800
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Fig. 3. Emission spectra of Re(I) complexes in deaerated THF solution (5 ꢃ 10ꢀ5 mol Lꢀ1, 1: lexc ¼ 469 nm, 2: lexc ¼ 461 nm) (a) and in solid state (1: lexc ¼ 369 nm, 2: lexc ¼ 372 nm,
3: lexc ¼ 365 nm) (b) at room temperature.

42
H.-Y. Li et al. / Journal of Organometallic Chemistry 743 (2013) 37e43
Fig. 4. Graphical representation of the frontier orbitals of complexes 1e4.
In summary, the complexes 1 and 2 of 4,40-disubstituted by
and TD-DFT methods [34]. They concluded that the DFT calcula-
tions overestimate halide contributions in metalehalide com-
plexes, especially if a low-valent metal atom is involved, although
DFT approaches are remarkably good in the case of non-halide
complexes. On the other hand, Stufkens et al. have noted that the
electronic transitions to the R-diimine have strongly mixed MLCT/
LLCT character due to the interaction of M-dp and X-pp orbitals in
the experiment [34,35].
[1,3,4]oxadiazole moieties exhibit much better luminescence than
that of 5,50-disubstituted complexes with the same substitution
groups, respectively. Therefore, the photophysical behaviors of
Re(I) complexes are ligand dependent and can be tuned by varying
the substituents of their chelate ligands. In our series of Re(I)
complexes, the coordination field is changed by the different sub-
stitution positions of [1,3,4]oxadiazole on bipyridine ligand, and the
4,4-disubstituted 2,20-bipyridine ligand by [1,3,4]oxadiazole moi-
eties adjust the Re centered MLCT emission leading to the better
luminescence.
The energy levels of HOMO for compounds 1e4 are 6.13, 6.17,
6.22 and 6.20 eV (Fig. 4, Table S1), respectively, and that of LUMO
are 3.02, 3.04, 3.18 and 3.15 eV (Fig. 4, Table S1), respectively. The
corresponding energy gaps of HOMO to LUMO are 3.11, 3.13, 3.04
and 3.05 eV for compounds 1e4, respectively. The variation of the
energies of the HOMO and LUMO is correlated to the nature and
positions of substituents in complexes. Compared with complexes
1 and 2, we observed that the HOMO and LUMO energy levels
decrease by some extent in 3 and 4 based on 5,50-disubstituted 2,20-
bipyridine and the variation range is larger in the LUMO orbitals,
resulting in the decreased band gap which then leads to an increase
in the lmax values in 3 and 4. Moreover, there are big differences
between the 4,40-disubsitution and 5,50-disubsitution complexes in
the absorption intensities of MLCT bands (Fig. 2b). The MLCT ab-
sorption bands are much lower in 5,50-disubsitution Re(I) com-
plexes (3 and 4). It is probably the weak absorption that caused the
decrease in the emission intensities of 5,50-disubstitution Re(I)
complexes. On the other words, the strong MLCT absorption makes
a major contribution to the high luminescence emission in complex
1 where 4,40-disubstituted 2,20-bipyridine is a coordinated ligand.
3.4. Computational study
In order to study the influence of the substituents on the
luminescence of 2,20-bipyridine based complexes in theory, the
theoretical calculations on complexes 1e4 were carried out by
using DFT and TD-DFT methods. Fig. 4 and Figs. S1 and S2 show the
plots of the most representative molecular frontier orbitals for 1e4.
Tables S1 and S2 give the spin-allowed electronic transitions
calculated with the TD-DFT method and electron density distri-
butions, respectively.
It will be useful to study the highest occupied (HOMO) and
lowest (LUMO) virtual orbitals for these Re(I) complexes to
examine the electronic transitions and the transition character. The
assignment of the type of each MO is made on the basis of its
composition. In complexes 1e4, the two highest occupied orbitals,
HOMO, HOMO-1, are all a mixture of rhenium and chlorine char-
acter and the two lowest LUMOs are calculated to be almost
exclusively located on diimine 2,20-bipyridine moieties. Thus, the
low-energy absorption bands in the UVevis spectra which mainly
originated from HOMO/LUMO and HOMO-1/LUMO transitions
can be assigned to a mixture of MLCT and LLCT (Fig. 2b). Further-
more, the other two obvious absorption bands in the stimulated
spectra, which range from 280 nm to 440 nm for 1 and 2 and from
250 nm to 460 nm for 3 and 4, can be safely assigned to the mix-
4. Conclusion
We have successfully synthesized four new neutral 4,40-disub-
stituted and 5,50-disubstituted 2,20-bipyridine ligands L1eL4 and
their corresponding Re(I) complexes 1e4 with electron-
withdrawing groups of [1,3,4]oxadiazole. All the complexes
except for 4 show efficient photoluminescent emission at room
temperature. The strongest phosphorescence emission was
observed in complex 1 both in solution and solid states and its
quantum yield of 1.1% was also detected, which is due to the
introduction of [1,3,4]oxadiazole moieties at 4,40-positions of 2,20-
bipyridine ligand. In comparison, the emission intensities are much
weak in complexes 3 and 4 which are 5,50-disubstituted 2,20-
bipyridine by [1,3,4]oxadiazole groups. These observation implies
that the modification of 2,20-bipyridine ligand in Re(I) carbonyl
tures of
from lower occupied MOs, which is mainly consisted of the ligands
orbitals (such as HOMO-3 and HOMO-4), to unoccupied MOs that
localize on the ligand
pep* LLCT and ILCT. They are generated by the transitions
p
p
* orbitals (such as LUMOþ1 and LUMOþ2).
The assignment is to be some deviations from the experimental
result, which is that experiment more stressed the contribution of
metal than calculation. Turki et al. calculated the UVevisible ab-
sorption spectra of Ru complex containing Cl with CASSCF/CASPT2

H.-Y. Li et al. / Journal of Organometallic Chemistry 743 (2013) 37e43
43
complexes with suitable electron-withdrawing moieties at 4,40-
positons can lead to efficient phosphorescence emission and high
luminescence quantum efficiencies.
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Acknowledgments
We gratefully acknowledge the financial support of the National
Natural Science Foundation of China (20971067, 21021062) and the
Major State Basic Research Development Program of China
(2011CB808704, 2013CB922101).
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