Synthesis and photoluminescence properties of rhenium(i) complexes based on 2,2′:6′,2″-terpyridine derivatives with hole-transporting units

Article abstract of DOI:10.1039/c2dt32154h

Based on 2,2′:6′,2″-terpyridine ligands (L1), five terpyridine derivatives, namely 4′-carbazol-9-yl-2,2′:6′, 2″-terpyridine (L2), 4′-diphenylamino-2,2′:6′,2″- terpyridine (L3), 4′-bis(4-tert-butylphenyl)amino-2,2′:6′, 2″-terpyridine (L4), 4′-[naphthalen-1

Full text of DOI:10.1039/c2dt32154h

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Synthesis and photoluminescence properties of
rhenium(I) complexes based on 2,2’:6’,2’’-terpyridine
derivatives with hole-transporting units†
Cite this: Dalton Trans., 2013, 42, 2716
Dong Wang,a Qiu-Lei Xu,a Song Zhang,a Hong-Yan Li,a Cheng-Cheng Wang,a
Tian-Yi Li,a Yi-Ming Jing,a Wei Huang,a You-Xuan Zheng*a and Gianluca Accorsi*b
Based on 2,2’:6’,2’’-terpyridine ligands (L1), five terpyridine derivatives, namely 4’-carbazol-9-yl-2,2’:
6’,2’’-terpyridine (L2), 4’-diphenylamino-2,2’:6’,2’’-terpyridine (L3), 4’-bis(4-tert-butylphenyl)amino-
2,2’:6’,2’’-terpyridine (L4), 4’-[naphthalen-1-yl-(phenyl)amino]-2,2’:6’,2’’-terpyridine (L5), 4’-[naphthalen-
2-yl(phenyl)amino]-2,2’:6’,2’’-terpyridine (L6) and their corresponding Re(I) complexes ReLn(CO)3Cl (n =
1–6) have been synthesized and characterized by elemental analysis and 1H NMR spectroscopy. The X-ray
crystal structure of ReL3(CO)3Cl has also been obtained. The luminescence spectra of ReL2(CO)3Cl–ReL5
(CO)3Cl, obtained in CH2Cl2 solution at room temperature, show strong dπ (Re) → π* (diimine) MLCT char-
acter (λmax ∼ 600 nm) and a small red shift relative to ReL1(CO)3Cl. This, confirmed by the study of the
triplet energy levels of the L1–L6 ligands at low temperature (77 K rigid matrix), indicates that the intro-
duction of electron-donating moieties on the terpyridine unit decreases the triplet levels of the ligands,
leading to a reduction of the energy gap between d and π* orbitals. In the solid state, upon MLCT exci-
tation, all the complexes show an even stronger emission and a blue spectral shift (λmax ∼ 550 nm) com-
pared to those obtained in solution.
Received 24th April 2012,
Accepted 15th November 2012
DOI: 10.1039/c2dt32154h
www.rsc.org/dalton
diodes (OLEDs)2 and in other fields such as solar energy con-
version,3 supramolecular chemistry,4 catalysis,5 and medicinal
Introduction
Bipyridine (bpy) complexes obtained from Re(CO)5Cl and its chemistry.6
derivatives have attracted considerable attention over the
2,2′:6′,2″-Terpyridine (terpy) also forms many stable com-
years.1 This is mainly due to the interesting photochemical plexes with a variety of transition metal ions, which are useful
properties of the Re(I) d6 low-spin centre which may show in catalysis and molecular electronics, as well as in supramole-
metal-to-ligand-charge-transfer (MLCT) luminescence owing to cular chemistry.7 Nevertheless, the reaction product of
the presence of the bipyridine group.2 This class of complexes Re(CO)5Cl with 2,2′:6′,2″-terpyridine, made by Juris et al.,8 and
exhibits good phosphorescence properties thanks to the strong characterized as fac-Re(σ2-terpy)(CO)3Cl,9 was found to be non-
spin–orbit coupling induced by the Re ion that enhances luminescent in solution at room temperature. To enhance the
the singlet–triplet mixing, affording a fairly long-lived excited application potential of this type of complex, there are several
state and appreciable emission quantum efficiencies, poten- scientific studies describing the possibility of improving the
tially exploitable for applications in organic light-emitting emission performance by insertion of terpy ligand structural
modifications.10 However, the resulting Re(I) complexes have
been rarely employed in electroluminescent devices due to
aState Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of
triplet–triplet annihilation (TTA) that leads to low efficiency at
high current density.1d,2 Accordingly, many efforts have been
Microstructures, Center for Molecular Organic Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China.
made on both the ligand structure and rigidity of the com-
E-mail: yxzheng@nju.edu.cn; Fax: +86 25 83314502; Tel: +86 25 83596775
bMolecular Photoscience Group, Istituto per la Sintesi Organica e la Fotoreattività,
plexes to enhance the excited state properties. Basically, there
are two feasible strategies to overcome the above mentioned
shortcoming: (i) the synthesis of novel diimine ligands with
different ligand-field (molecular design)11 which would affect
the molecular orbital energy in the Re(I) complexes and (ii) the
addition of groups with electron-donating or -accepting prop-
erties into the diimine ligands2a,d,12 to avoid the triplet–triplet
Consiglio Nazionale delle Ricerche (CNR-ISOF), Via P. Gobetti, 101, 40129 Bologna,
Italy. E-mail: gianluca.accorsi@isof.cnr.it; Fax: +39 051 6399844;
Tel: +39 051 6399816
†Electronic supplementary information (ESI) available: Photoluminescence life-
time decay measurement of Re(I) complexes in solid at room temperature. CCDC
reference number 826740. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt32154h
2716 | Dalton Trans., 2013, 42, 2716–2723
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annihilation thanks to the steric hindrance effect. Accordingly, where Φunk and Φstd are the luminescence quantum yield of
we have already synthesized Re(I) complexes with carbazole the unknown sample and air-equilibrated acetonitrile solution
and oxadiazole moieties showing enhanced luminescence of [Ru(bpy)3]2+·2Cl−, respectively. The Iunk and Istd are the inte-
properties.13
grated emission intensities of the unknown sample and
Here we report the synthesis of five 2,2′:6′,2″-terpyridine [Ru(bpy)3]2+·2Cl− solution, respectively. The Aunk and Astd are
derivative ligands with carbazole and diphenylamine deriva- the absorbance of the unknown sample and [Ru(bpy)3]2+·2Cl−
tives, 4′-carbazol-9-yl-2,2′:6′,2″-terpyridine (L2), 4′-diphenyl- acetonitrile solution at their excitation wavelengths (A < 0.1),
amino-2,2′:6′,2″-terpyridine (L3), 4′-bis(4-tert-butyl-phenyl)- respectively. The ηunk and ηstd terms represent the refractive
amino-2,2′:6′,2″-terpyridine (L4), 4′-[naphthalen-1-yl(phenyl)- indices of the corresponding solvents (pure solvents were
amino]-2,2′:6′,2″-terpyridine (L5), 4′-[naphthalen-2-yl(phenyl)- assumed). The Φstd of air-equilibrated acetonitrile solution of
amino]-2,2′:6′,2″-terpyridine (L6) and their corresponding [Ru(bpy)3]2+·2Cl− has been revealed to be 0.018.16
Re(I) complexes ReLn(CO)3Cl (n = 2–6). The ligand L2 has been
reported in our former publication14 and ligands L3–L6 are
Crystallography
novel. Similar to that shown previously,9–11 here only two
The crystal structures were determined on a Siemens (Bruker)
SMART CCD diffractometer using monochromated Mo Kα
N atoms are coordinated to the same Re(I) ion despite the
three pyridine rings in the 2,2′:6′,2″-terpyridine derivatives.
radiation (λ = 0.71073 Å) at 291 K. Cell parameters were
The choice of appending carbazole and diphenylamine deriva-
retrieved using SMART software and refined using SAINT17 on
tives moieties in ReLn(CO)3Cl (n = 2–6) has been dictated by
all observed reflections. Data was collected using a narrow-
frame method with scan widths of 0.30° in ω and an exposure
their ability to act as light-harvesting systems, sensitizing and
protecting the metal emitting states, together with their intrin-
time of 10 s per frame. The highly redundant data sets were
sic charge-transporting capability, making such molecules
reduced using SAINT17 and corrected for Lorentz and polari-
potentially interesting for the fabrication of OLEDs.2a–c,12a–c
zation effects. Absorption corrections were applied using
SADABS18 supplied by Bruker. Structures were solved by direct
methods using the program SHELXL-97.19 The positions of
Experimental section
Materials
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 posi-
Rhenium pentacarbonyl chloride, 2,2′:6′,2″-terpyridine (L1),
4-chloro-2,2′:6′,2″-terpyridine, diphenylamine, bis(4-tert-butyl-
phenyl)amine,
N-phenylnaphthalen-1-amine,
N-phenyl-
tion and refined as riding atoms with a uniform value of Uiso
.
naphthalen-2-amine and 1,3-dimethyl-3,4,5,6-terahydro-2(1H)-
pyrimidinone (DMPU) were bought from Alfa Aesar Co. (China).
Carbazole, CuI, 18-Crown-6, K2CO3 were purchased from Yuan
Hang Reagent Co. and Sinopharm Chemical Reagent Co.
(China), respectively. All of them were used as received. The sol-
vents were used without further purification, except for toluene.
Synthesis
The chemical structures of the materials used in this work and
the synthetic routes are depicted in Scheme 1. The ligand L2
was synthesized from 4-chloro-2,2′:6′,2″-terpyridine and carba-
zole with our reported procedure.14 Ligands L3–L6 were syn-
thesized using 4-chloro-2,2′:6′,2″-terpyridine and diphenylamine
Spectroscopy
1H NMR spectra were measured on a Bruker AM 500 spectro-
meter. Elemental analyses for C, H, and N were performed on
an Elementar Vario MICRO analyzer. Mass spectra were deter-
mined with an Autoflex IITM instrument for MALDI-TOF-MS.
Absorption and photoluminescence spectra were measured on
a UV-3100 spectrophotometer and a Hitachi F-4600 lumine-
scence spectrophotometer, respectively. The photoluminescence
lifetimes were measured with an Edinburgh Instruments
FLS920P fluorescence spectrometer. The 77 K phosphorescence
spectra were measured by Perkin-Elmer LS-50 spectrofluoro-
meter equipped with a pulsed xenon lamp with variable repetition
rate. The luminescence quantum efficiencies were calculated by
comparison of the emission intensities (integrated areas) of a
standard sample (air-equilibrated acetonitrile solution of [Ru-
(bpy)3]2+·2Cl−) and the unknown sample according to eqn (1):15
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Aunk
ηunk
ηstd
Φunk ¼ Φstd
ð1Þ
Scheme 1 Synthetic routes for ligands and corresponding Re(I) complexes.
Dalton Trans., 2013, 42, 2716–2723 | 2717
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derivatives as the starting materials by a modified Ullmann (450.5333): C 82.64, H 4.92, N 12.44. Found: C 82.69, H 5.06,
reaction.20 The Re(I) complexes ReLn(CO)3Cl (n = 1–6) were N 12.41.
obtained with a modification of previously published pro-
General synthesis of ReL1(CO)3Cl–ReL6(CO)3Cl. Re(CO)5Cl
cedures by reacting Re(CO)5Cl with the appropriate ligand in (0.035 g, 0.10 mmol) was refluxed with an equimolar quantity
toluene.2,13,21
of appropriate ligand in toluene for 6 h under nitrogen. After
the mixture was cooled to RT, the solvent was distilled off. The
resulting yellow solid was purified by recrystallization from
ethyl acetate and petroleum ether.
General synthesis route for ligands L3–L6
A mixture of diphenylamine or derivatives, 4-chloro-2,2′:6′,2″-
ReL1(CO)3Cl. Yield: 61%. M.p.: 265–267 °C. 1H NMR
terpyridine, potassium tert-butoxide and DMPU was put into a (500 MHz, CDCl3, ppm) δ 9.13 (d, J = 4.9 Hz, 1H), 8.85 (d, J =
reactor, then heated at 205 °C for 12 h under nitrogen. After 4.1 Hz, 1H), 8.29 (d, J = 8.1 Hz, 1H), 8.27 (d, J = 8.3 Hz, 1H),
cooling to room temperature, the mixture was dissolved in 8.17 (t, J = 7.9 Hz, 1H), 8.10 (t, J = 7.8 Hz, 1H), 7.95 (d, J =
dichloromethane, washed with water 3 times, and the solid 5.9 Hz, 2H), 7.80 (d, J = 7.5 Hz, 1H), 7.55 (dd, J = 13.2, 6.5 Hz,
obtained was purified with column chromatography on SiO2 2H). MS (MALDI-TOF): m/z 476.451 [M+]. Anal. Calcd for
using ethyl acetate and petroleum ether (v/v = 1 : 2) as eluent, ReC18H11N3O3Cl (538.9581): C 40.11, H 2.06, N 7.80. Found:
and then the resulting solid was purified again by recrystalliza- C 40.03, H 2.08, N 7.88.
tion from an ethyl acetate–petroleum ether solution to yield
ligands L3–L6.
ReL2(CO)3Cl. Yield: 41%. M.p.: 263–266 °C. 1H NMR
(500 MHz, CDCl3, ppm) δ 9.15 (s, 1H), 8.89 (s, 1H), 8.47 (s,
1H), 8.21 (s, 1H), 8.11 (d, J = 32.0 Hz, 6H), 7.74 (s, 2H), 7.59 (s,
The brown solid was purified.
4-Diphenylamino-2,2′:6′,2″-terpyridine (L3). Diphenylamine 2H), 7.52 (s, 2H), 7.41 (s, 2H). MS (MALDI-TOF): m/z 641.961
(0.50 g, 3.0 mmol), 4-chloro-2,2′:6′,2″-terpyridine (0.22 g, [M+]. Anal. Calcd for ReC30H18N4O3Cl (668.6961): C 51.17,
0.82 mmol), potassium tert-butoxide (0.22 g, 2.0 mmol) and H 2.58, N 7.96. Found: C 51.12, H 2.60, N 7.99.
1
DMPU (0.1 mL), L3: 0.16 g (yield: 50%). M.p.: 213–218 °C. H
ReL3(CO)3Cl. Yield: 68%. M.p.: 272–275 °C. 1H NMR
NMR (500 MHz, CDCl3, ppm) δ 8.60 (dd, J = 10.6, 6.1 Hz, 4H), (500 MHz, CDCl3, ppm) δ 9.06 (s, 1H), 8.75 (s, 1H), 7.91 (d, J =
8.02 (s, 2H), 7.84 (s, 2H), 7.39 (s, 4H), 7.28 (d, J = 8.4 Hz, 6H), 38.5 Hz, 3H), 7.75 (s, 1H), 7.49 (s, 7H), 7.35 (s, 6H), 7.01 (s,
7.22 (s, 2H). MS(ESI): m/z 401.05 [M]+. Anal. Calcd for 1H). MS (MALDI-TOF): m/z 643.638 [M+]. Anal. Calcd for
C27H20N4 (400.4747): C 80.98, H 5.03, N 13.84. Found: C 80.84, ReC30H20N4O3Cl (706.1647): C 51.03, H 2.85, N 7.93. Found:
H 5.03, N 13.99.
4-Bis(4-tert-butylphenyl)amino-2,2′:6′,2″-terpyridine (L4). Bis(4-
C 50.99, H 2.87, N 7.98.
ReL4(CO)3Cl. Yield: 31%. M.p.: >300 °C. H NMR (500 MHz,
1
tert-butylphenyl)amine (0.88 g, 3.1 mmol), 4-chloro-2,2′:6′,2″- CDCl3, ppm) δ 9.07 (d, J = 5.1 Hz, 1H), 8.76 (d, J = 4.5 Hz, 1H),
terpyridine (0.20 g, 0.73 mmol), potassium tert-butoxide 7.95 (t, J = 7.4 Hz, 1H), 7.86 (t, J = 7.1 Hz, 1H), 7.80 (d, J = 7.7 Hz,
(0.20 g, 1.8 mmol) and DMPU (0.1 mL), L4: 0.19 g (yield: 51%). 1H), 7.74 (d, J = 8.1 Hz, 1H), 7.54–7.40 (m, 7H), 7.24 (t, J = 11.3
1
M.p.: 208–213 °C. H NMR (500 MHz, CDCl3, ppm) δ 8.62 (s, Hz, 4H), 6.98 (d, J = 2.3 Hz, 1H), 1.37 (s, 18H). MS (MALDI-TOF):
2H), 8.56 (s, 2H), 7.95 (s, 2H), 7.85 (s, 2H), 7.38 (s, 4H), m/z 755.753 [M+]. Anal. Calcd for ReC38H36N4O3Cl (818.3773):
7.33–7.29 (m, 2H), 7.20 (s, 4H), 1.36 (s, 18H). MS(ESI): m/z C 55.77, H 4.43, N 6.85. Found: C 55.72, H 4.46, N 6.89.
513.69 [M]+. Anal. Calcd for C35H36N4 (512.6873): C 81.99,
H 7.08, N 10.93. Found: C 81.97, H 7.07, N 10.89.
ReL5(CO)3Cl. Yield: 37%. M.p.: 215–218 °C. 1H NMR
(500 MHz, CDCl3, ppm) δ 9.04 (d, J = 4.6 Hz, 1H), 8.72 (dd, J =
4-(Naphthalen-1-yl(phenyl)amino)-2,2′:6′,2″-terpyridine (L5). N- 14.3, 4.1 Hz, 1H), 7.98 (td, J = 16.8, 8.1 Hz, 4H), 7.83 (td, J =
Phenylnaphthalen-1-amine (0.89 g, 4.0 mmol), 4-chloro- 11.8, 4.9 Hz, 2H), 7.75 (t, J = 8.4 Hz, 1H), 7.64–7.53 (m, 5H),
2,2′:6′,2″-terpyridine (0.29 g, 1.1 mmol), potassium tert-butoxide 7.43 (dd, J = 15.7, 4.9 Hz, 8H). MS (MALDI-TOF): m/z 693.110
(0.30 g, 2.7 mmol) and DMPU (0.1 mL), L5: 0.17 g (yield: 34%). [M+]. Anal. Calcd for ReC34H22N4O3Cl (756.2233): C 54.00,
1
M.p.: 237–239 °C. H NMR (500 MHz, CDCl3, ppm) δ 8.56 (t, H 2.93, N 7.41. Found: C 53.97, H 2.95, N 7.44.
J = 7.3 Hz, 4H), 8.01 (d, J = 8.5 Hz, 1H), 7.92 (q, J = 7.9 Hz, 4H),
ReL6(CO)3Cl. Yield: 45%. M.p.: 214–217 °C. 1H NMR
7.81 (td, J = 7.8, 1.5 Hz, 2H), 7.57 (t, J = 7.7 Hz, 1H), 7.54–7.47 (500 MHz, CDCl3, ppm) δ 9.07 (d, J = 5.0 Hz, 1H), 8.73 (d, J =
(m, 2H), 7.40 (t, J = 7.4 Hz, 1H), 7.32 (dt, J = 15.7, 7.8 Hz, 4H), 4.3 Hz, 1H), 7.97 (d, J = 8.7 Hz, 1H), 7.91 (s, 2H), 7.86–7.77 (m,
7.26 (dd, J = 6.4, 5.1 Hz, 2H), 7.14 (t, J = 7.1 Hz, 1H). MS(ESI): 4H), 7.73 (d, J = 8.1 Hz, 1H), 7.56 (dd, J = 11.1, 6.5 Hz, 4H),
m/z 451.53 [M]+. Anal. Calcd for C31H22N4 (450.5333): C 82.64, 7.51–7.47 (m, 2H), 7.45–7.36 (m, 6H). MS (MALDI-TOF): m/z
H 4.92, N 12.44. Found: C 82.78, H 4.95, N 12.42.
693.331 [M+]. Anal. Calcd for ReC34H22N4O3Cl (756.2233):
4-[Naphthalen-2-yl(phenyl)amino]-2,2′:6′,2″-terpyridine (L6). N- C 54.00, H 2.93, N 7.41. Found: C 53.98, H 2.94, N 7.46.
Phenylnaphthalen-2-amine (0.75 g, 3.4 mmol), 4-chloro-
2,2′:6′,2″-terpyridine (0.25 g, 0.93 mmol), potassium tert-butoxide
(0.25 g, 2.2 mmol) and DMPU (0.1 mL), L6: 0.09 g (yield: 22%).
M.p.: 210–212 °C. H NMR (500 MHz, CDCl3, ppm) δ 8.59 (s,
4H), 8.09 (s, 2H), 7.90–7.81 (m, 4H), 7.74–7.69 (m, 1H), 7.65 (s,
Results and discussion
Crystallography
1
1H), 7.44 (d, J = 38.5 Hz, 5H), 7.30 (d, J = 16.7 Hz, 4H), 7.23 (s, The crystals of ReL1(CO)3Cl and ReL3(CO)3Cl were obtained
1H). MS(ESI): m/z 451.53. [M]+. Anal. Calcd for C31H22N4 from the CH2Cl2/CH3OH and CH2Cl2/C2H5OH solution
2718 | Dalton Trans., 2013, 42, 2716–2723
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Table 1 Crystallographic data, selected bond lengths (Å) and angles (°) for
ReL3(CO)3Cl
ReL3(CO)3Cl·C2H5OH
Bond lengths (Å) and
angles (°)
Crystallographic data
Formula
FW
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C32H26N4O4ClRe Re(1)–C(28)
1.922(10)
1.884(9)
1.894(9)
2.185(6)
2.209(6)
2.481(2)
1458.37
293(2)
0.71073
Monoclinic
P2(1)/c
Re(1)–C(29)
Re(1)–C(30)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–Cl(1)
7.0443(14)
26.056(5)
17.159(3) A
90.00
107.024(7)
90.00
3011.5(10)
2
1.608
C(29)–Re(1)–C(30) 86.0(4)
C(29)–Re(1)–C(28) 90.1(4)
C(30)–Re(1)–C(28) 85.2(3)
C28 Re1 N1
C(29)–Re(1)–N(1)
C30 Re1 N1
C28 Re1 N2
C29 Re1 N2
C30 Re1 N2
175.1(3)
93.7(3)
98.0(3)
101.7(3)
97.3(3)
172.3(3)
γ (°)
V (Å3)
Z
ρcalcd (g cm−3
)
μ (Mo Kα) (mm−1
F (000)
)
4.163
1428
2.48–25.00
C(28)–Re(1)–Cl(1) 91.8(2)
C(29)–Re(1)–Cl(1) 178.2(3)
C(30)–Re(1)–Cl(1) 94.4(3)
Range of transm
factors (°)
Reflns collected
Unique
13 536
5311
1.031
0.0497, 0.1275
0.0592, 0.1331
826740
N(1)–Re(1)–Cl(1)
N(2)–Re(1)–Cl(1)
N(1)–Re(1)–N(2)
84.44(17)
82.14(16)
74.9(2)
GOF on F2
R1a, wR2b [I > 2σ(I)]
R1a, wR2b (all data)
CCDC No.
Fig. 1 Top: ORTEP view of complex ReL3(CO)3Cl with the atom-numbering
scheme. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 30%
probability level. Down: The molecular packing of ReL3(CO)3Cl.
a R1 = Σ||Fo| − |Fc||/ΣFo|. b wR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)]1/2
.
respectively. By single-crystal X-ray diffraction method, their
structures were analysed. The crystal structure of ReL1(CO)3Cl
is similar to those reported with and without solvent (CCDC
No.: 666615).9,22 For ReL3(CO)3Cl, the relative ORTEP dia-
grams and the molecular packing are shown in Fig. 1, the
crystal data and selected bond distances, angles are presented
in Table 1.
In ReL3(CO)3Cl, the rhenium atoms are in a distorted octa-
hedral coordination environment with three carbonyl ligands
in a fac arrangement, two N atoms from two pyridine rings of
the ligand and a chlorine atom. Solvent molecules are found
in complexes ReL1(CO)3Cl (one CH3OH) and ReL3(CO)3Cl (one
C2H5OH). In complex ReL3(CO)3Cl, the distances of Re–C are
1.884(9)–1.922(10) Å, the average Re–N bond distance is 2.197(6)
Å. The bond angles between adjacent CO carbon atoms are
85.2(3)–90.1(4)°, which is close to 90°, indicating that CO
ligands are linearly coordinated, while the N–Re–N angle is
74.9(2). The dihedral angle between the free pyridine ring and
the coordinated ones is 54.3°, and that between the two co-
ordinated pyridine rings is 14.2°. From the molecular packing
of ReL3(CO)3Cl (Fig. 1, down), there are strong π–π interactions
between benzene and pyridine rings of adjacent complexes
with the centroid-to-centroid separation of 3.774 Å.
Fig. 2 UV-vis absorption spectra of Re(I) complexes in CH2Cl2 solution (1 ×
10−5 mol L−1) at room temperature.
220–330 nm spectral range (extinction coefficients of the order
of 104 M−1 cm−1) that can be assigned to the spin-allowed
intraligand (π → π*) transitions (IL) as suggested by the com-
parison with the absorption bands of the free ligands L1–L6
(Fig. S1, ESI†) as well as those of the closely related metal com-
plexes.2 By contrast, the broad bands at lower energy in the
330–480 nm region are attributed to the dπ (Re) → π* (diimine)
metal-to-ligand charge-transfer (MLCT) transitions. Notably,
the weaker MLCT absorption of ReL1(CO)3Cl (without substitu-
ent on the terpyridine unit) relative to ReLn(CO)3Cl (n = 2–6)
Photophysical properties
The absorption spectra of Re(I) complexes in dichloromethane
solution (1 × 10−5 mol L−1) at room temperature are depicted
in Fig. 2. All of them display intense absorption bands in the
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ReL6(CO)3Cl in solution is too weak to be measured under
these conditions) and that of the reference ReL1(CO)3Cl show
broad bands centred at ca. 530–610 nm, respectively, (Fig. 3a,
Table 2) assigned to dπ (Re) → π* (diimine) MLCT phosphore-
scence. The highest emission intensity obtained by exciting
the MLCT states directly, indicates that the orbital overlap
between the higher π → π* and the lower MLCT states is very
efficient, and the major contribution of the observed emission
is from the latter one.2e,j,23a,24
From Table 2, it has also to be noted that compared to the
reference sample ReL1(CO)3Cl, the introduction of carbazole
(ReL2(CO)3Cl), diphenylamine (ReL3(CO)3Cl), bis(4-tert-butyl-
phenyl)amine (ReL4(CO)3Cl), and N-phenyl-naphthalen-
1-amine (ReL5(CO)3Cl) units enhanced the luminescence
performances and the quantum efficiency (Φp) of the corres-
ponding Re(I) complexes since the hole-transporting moieties
favour the energy transfer process from the substituents to the
central terpyridine as already found in similar carbazole con-
taining rhenium and ruthenium complexes.13,25 These results
confirm that antenna substitutes can be profitably used to
increase the light harvesting efficiency of the Re(I) complexes.
In addition, due to the various ligand-field strength caused by
the substitutes for the present cases, thermally accessible d–d
MC levels could provide different non-radiative path for dispo-
sal of the excitation energy in the Re(I) complexes reported
here. This behavior is well known for derivatives of [Ru-
(bipy)3]2+ in which the ligand-field strength around the metal
is reduced by steric distortions26 and it has recently been
demonstrated for a series of Re(I)–tricarbonyl-diimine com-
plexes25 although a range of other non-radiative decay path-
ways are in principle available.27,28 For instance, from Fig. 3(a)
and Table 2 it can be observed that ReL4(CO)3Cl (with (4-tert-
butylphenyl)amine) shows the strongest emission intensity of
the series, followed by ReL5(CO)3Cl (N-phenyl-naphthalen-
1-amine) and ReL3(CO)3Cl (diphenylamine), respectively. Their
emission performances largely exceeded that of the reference
sample ReL1(CO)3Cl. Surprisingly, for ReL6(CO)3Cl (N-phenyl-
naphthalen-2-amine), structurally very similar to ReL5(CO)3Cl,
only a weak luminescence has been detected under the same
conditions.
By contrast, in solid state all the complexes show strong
phosphorescent emission centred at ca. 532–562 nm (Fig. 3b)
under MLCT excitation. Compared to the solution samples,
they are blue shifted showing the rigidochromic effect that was
first proposed by Wrighton and Morse.29 Similar rigidiochro-
mism has been reported in the related Re(I) diimine system by
measuring the photoluminescence in a frozen glass at 77 K.30
By comparing our results with some work based on iridium
complexes,31 the stronger emission in solid than that in solu-
tion may be attributed to the excimeric interactions between
the benzene and pyridine of adjacent complexes in solid state
(Fig. 1, down). The strong π–π interactions elongate the overall
π-conjugation degree. As a result, the triplet energy level of the
charge transfer state from Re(I) to the interacting ligands is
reduced and the ligands participate in the excited state in the
solid state. In addition, the rotation of the free benzene groups
Fig. 3 (a) Excitation and emission spectra of the Re(I) complexes in CH2Cl2 solu-
tion (1 × 10−4 mol L−1) and (b) normalized emission spectra of the Re(I) com-
plexes in solid state at room temperature (the pictures of complexes ReL2
(CO)3Cl, ReL3(CO)3Cl and ReL4(CO)3Cl under 365 nm UV-light excitation were
inserted).
indicates that the introduction of carbazole and diphenyl-
amine groups improves the MLCT character of this class of
complexes.13,23
The excitation spectra of Re(I) complexes in solution,
measured at room temperature by monitoring the correspond-
ing emission maxima (Fig. 3a, left), exhibit two peaks, namely
π → π* (at ∼370 nm) and MLCT (at ∼445 nm).2c,j They differ
from the corresponding absorption profiles mainly for the
intensity ratio between the LC and MLCT bands since the UV
region (<300 nm) is known to be critical for the multiplier
response. For ReLn(CO)3Cl (n = 2–5), the MLCT band becomes
dominant after the introduction of hole-transporting units
because of the strong spin–orbit coupling imposed by the
heavy Re(I) ion that allows the upper excited state to relax into
the spin forbidden MLCT state with consequent light
emission.
By exciting either π → π* or MLCT absorption bands, the
emission spectra of ReLn(CO)3Cl (n = 2–5) (the emission of
2720 | Dalton Trans., 2013, 42, 2716–2723
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Table 2 Absorption and photoluminescence data of Re(I) complexes at room temperature
Emissionb
Excitationb
λ (nm)
Lifetimed, τP
μs (χ2)
kre
knr
HOMO/LUMO
eV
f
Absorption [λ (nm)
Complex
Medium (ε (103 M−1 cm−1))]
λ (nm)
ΦP (%)
103 s−1
105 s−1
ReL1(CO)3Cl
ReL2(CO)3Cl
ReL3(CO)3Cl
ReL4(CO)3Cl
ReL5(CO)3Cl
ReL6(CO)3Cl
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
220(19.3), 295(13.6), 378(2.6)
442
365
459
365
444
367
454
367
442
365
450
365
509
562
578
551
600
554
593
532
601
0.3
2.02 (0.989)
1.95 (0.983)
1.37 (0.988)
1.66 (0.987)
2.50 (0.992)
1.97 (0.987)
1.44 (0.987)
1.87 (0.987)
1.33 (0.985)
3.04 (0.972)
1.53 (0.989)
2.94 (0.993)
1.5
4.94
−4.93/−2.06
−5.72/−2.95
−5.76/−2.91
−5.63/−2.80
−5.73/−2.77
−5.75/−2.86
a
a
a
a
224(62.3), 277(25.9), 397(9.3)
0.3
2.2
7.28
a
a
a
a
219(31.4), 282(25.3), 380(8.0)
0.6
2.4
3.98
a
a
a
a
220(36.6), 256(29.4), 383(8.6)
1.3
9.0
6.85
a
a
a
a
220(45.5), 279(26.8), 378(6.9)
0.9
6.8
7.45
a
a
a
a
546
c
c
c
c
219(34.6), 255(20.9), 384(5.2)
a
a
a
a
551
a Not measured. b The excitation and emission spectra are corrected. c Not detected because of the weakness of the signal. d In the lifetime
analysis, the χ2 parameter indicates the discrepancy between the observed and the expected values for a certain fitting model. In principle, a χ2 =
1 is the best-fit for the given data and error bars. e Radiative rate constant calculated from kr = ΦP/τP. f Nonradiative rate constant calculated from
knr = 1/τP − kr.
was also limited. Therefore, strong emissions of the solid
samples were observed. High luminescence in immobilized
states is more beneficial because the OLEDs are based on the
solid state applications.
The phosphorescence lifetime is a key factor in the TTA in
the OLEDs operation.32 The excited state lifetimes (τP) of Re(I)
complexes, measured in CH2Cl2 solution and solid state at
room temperature, support the hypothesis of the MLCT char-
acter of the emitting states. As indicated in Table 2, Fig. S1
and S2,† all the lifetime decays are monoexponential in the
range of sub-microsecond scale (1.33 − 3.04 μs) and can be
assigned to MLCT states.19,32–34 By using the equations ΦP
=
ΦISC{kr/(kr + knr)} and τp = (kr + knr)−1, it is possible to calculate
the radiative (kr) and nonradiative (knr) decay rates from ΦP
and τP, where the intersystem-crossing yield (ΦISC) is typically
assumed to be unitary (1.0) for metal phosphors with strong
heavy-atom effect. All the complexes have quite similar knr
values, while ReL4(CO)3Cl has a kr sensibly higher than those
of other complexes, which is comparable with the emission
intensity and quantum efficiency.
As discussed before, the reason for the emission red shift
in solution of ReLn(CO)3Cl (n = 2–5), relative to ReL1(CO)3Cl
can be attributed to the effect of the substituents on the MLCT
emission energy. To get a better insight on the observed behav-
iour, the triplet energy of the ligands L1–L6 has been deter-
mined from the phosphorescence spectra at 77 K in CH2Cl2
rigid matrix. The estimated values from Fig. 4 for L1–L6 are
24 090, 21 050, 20 100, 21 950, 19 400 and 19 850 cm−1, respec-
tively. These results, further suggest that the insertion of elec-
tron-donating moieties decreases the ligand energy levels
resulting in a lowering of the energy gap between d and π*
orbitals that reflects the luminescence behaviour of the corres-
ponding Re(I) complexes.
Fig. 4 Normalized phosphorescence spectra of the ligands at 77 K in CH2Cl2
rigid matrix. Applied delay: 100 μs, λex = 300 nm.
(0.1 M) using ferrocene as the internal standard at a sweep rate
of 0.1 V s−1 are shown in Fig. 3S and 4S.† All the free ligands
have chemical irreversible monoelectron reduction at negative
potentials. On the oxidation side, L1 has a single reversible
oxidation at −1.30 V, while L2–L6 have reversible oxidations at
more positive potentials (−1.19 to −0.87 V). The oxidation
potential decreases according to the electron-donating ability
sequence of the substituents of terpyridine ligands. All com-
plexes (Fig. S5†) show a couple of irreversible anodic waves vs.
SCE associated with a Re(I)-based oxidation process (ReI/ReII)
and a couple of quasi-reversible cathodic waves vs. SCE associ-
ated with a ligand-based reduction process [ReICl(CO)3(L)]/
[ReICl(CO)3(L˙)]−).35 On the basis of the potentials of the oxi-
dation and reduction couples of the Re(I) complexes, the
HOMO/LUMO energy levels of ReL2(CO)3Cl–ReL6(CO)3Cl were
estimated (Table 2), respectively, with regard to the energy
level of the ferrocene reference (4.8 eV below the vacuum
Electrochemical property
Cyclic voltammograms of L1–L6 and those of ReL2(CO)3Cl– level).36 Since the six complexes have the same central ion
ReL6(CO)3Cl (5 × 10−4 M) in CH2Cl2 solutions of (Bu4N)PF6 structure, the reduction potential has been affected slightly.
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The oxidation potential of the complexes decreases with the
similar sequence of the ligands contained.
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Conclusions
By introduction of the hole-transporting carbazole and
diphenylamine derivatives, we have successfully synthesized
five new ligands based on 2,2′:6′,2″-terpyridine and their corres-
ponding Re(I) complexes. The complexes ReL3(CO)3Cl–ReL5-
(CO)3Cl show dπ (Re) → π* (diimine) MLCT phosphorescence
around 533–611 nm and quantum efficiency higher than those
of ReL1(CO)3Cl without hole-transporting moiety, and the
(4-tert-butylphenyl)amine containing complex has the stron-
gest emission intensity of the series, followed by ReL5(CO)3Cl
(N-phenyl-naphthalen-1-amine) and ReL3(CO)3Cl (diphenyl-
amine), respectively. The long lifetime decay from the MLCT
state indicates that the potential surface crossing from the π–π*
to the MLCT states in the complexes is efficient. These obser-
vations imply that the modification of some of the diimine
rhenium(I) carbonyl complexes with suitable hole-transporting
moieties would lead to better phosphorescence performances.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant 20971067 and 21021062), the
Major State Basic Research Development Program
(2011CB808704, 2013CB922101) and the Italian CNR (PM.
P04.010, MACOL).
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