A green emitting phosphorescent copper(I) complex with tetrazole derived ligand for electroluminescence application

Article abstract of DOI:10.1016/j.saa.2014.04.008

In this paper, a tetrazole derived diamine ligand of 2-(1H-tetrazol-5-yl) pyridine (TP) owing electron-donors and short conjugation chain was synthesized to increase the band gap of its corresponding phosphorescent Cu(I) complex. This Cu(I) complex was ch

Full text of DOI:10.1016/j.saa.2014.04.008

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A green emitting phosphorescent copper(I) complex with tetrazole
derived ligand for electroluminescence application
⇑
Senmiao Tong, Donglin Yuan, Lita Yi
School of Forestry and Biotechnology, Zhejiang Agriculture & Forestry University, Linan 311300, Zhejiang, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
A tetrazole derived diamine ligand
with limited conjugation chain was
synthesized.
Its Cu(I) complex was studied by
theoretical calculation and
experimental data.
This Cu(I) complex owned increased
band gap caused by the tetrazole
group.
Green electroluminescence with
maximum luminance of 2860 cd/m2
was achieved.
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper,
a tetrazole derived diamine ligand of 2-(1H-tetrazol-5-yl)pyridine (TP) owing
Received 4 February 2014
Received in revised form 6 March 2014
Accepted 1 April 2014
electron-donors and short conjugation chain was synthesized to increase the band gap of its correspond-
ing phosphorescent Cu(I) complex. This Cu(I) complex was characterized in detail, including its single
crystal structure, singlet electronic transitions, photophysical parameters, thermal stability and
electrochemical property. Upon on photoexcitation, this Cu(I) complex emitted green emission peaking
Available online 16 April 2014
at 497 nm with biexponential decay pattern of
s1 = 5.5414 ls (A1 = 0.137) and s2 = 1.0679 ls
Keywords:
Copper(I) complex
Green emitting
Photoluminescence
Electroluminescence
Electron donor/acceptor
(A2 = 0.11503). Cyclic voltammerty experiment suggested that this Cu(I) complex owned HOMO and
LUMO energy levels of ꢁ5.79 eV and ꢁ2.39 eV. The thermal decomposition temperature was 170 °C as
indicated by thermogravimetric analysis. The optimal electroluminescence device constructed by
solution processed coating procedure showed green electroluminescence peaking at 525 nm, with
maximum luminance of 2860 cd/m2 and maximum current efficiency of 5.9 cd/A.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
electro-optical applications, numbers of transition metal com-
plexes have been designed and reported. Particularly, phosphores-
Owing to their favorable optoelectronic features, luminescent
transition metal complexes have presented their potential applica-
tions in electro-optical devices such as light-emitting devices, solar
cells and optical sensors [1,2]. To meet the requirement for
cent Cu(I) complexes have drawn much attention as a class of
novel electro-optical material owing to their virtues of abundance,
low cost and environment-friendliness [3].
Generally, the emission originated from charge-transfer (CT)
excited state of Cu(I) complexes is weak and short-lived since the
lowest excited state of d10 system includes the excitation of
metal–ligand dr⁄ orbital [4]. As a consequence, the ground state
usually takes a distorted tetrahedral coordination mode to meet
⇑
Corresponding author.
E-mail address: litayi887@163.com (L. Yi).
http://dx.doi.org/10.1016/j.saa.2014.04.008
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

S. Tong et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
281
closed-shell requirement, while the excited state tends to take a
tetragonally flattened one. This structural relaxation can reduce
the excited state energy and increase the non-radiative decay, as
suggested by McMillin [3].
Cu(I) complexes with general molecular formula of
[Cu(N–N)(P–P)]+ have been proved to be promising owing to their
long excited state lifetimes, where N–N means diamine ligand and
P–P means triphenylphosphane (or its derivative), respectively. In
[Cu(N–N)(P–P)]+ complexes, the geometric relaxation is effectively
repressed by the steric effect of P–P ligands [5–7]. Never the less,
the emission of these complexes usually centralizes in low energy
region [6–8], which limits their intensive application for electro-
optical devices.
Efforts have been devoted to both theoretical analysis and
experimental synthesis, aiming at phosphorescent Cu(I) com-
plexes with controllable emitting wavelength [9–11]. It is found
that the highest occupied molecular orbital (HOMO) is mainly
composed of Cu d orbitals, while the lowest unoccupied molecu-
lar orbital (LUMO) is essentially p⁄ localized in diamine ligand.
The emission comes from the radiative decay of triplet state
metal-to-ligand-charge-transfer (MLCT). The chemical modifica-
tion on N–N and P–P ligands may thus result various emission
wavelengths.
Very recently, a novel green emitting phosphorescent Cu(I)
complex [Cu(TP)(PPh3)2]BF4 (TP = 2-(1H-tetrazol-5-yl)pyridine,
PPh3 = triphenylphosphane) has been reported [12], which
confirms the potential of realizing high energy emission from
phosphorescent Cu(I) complexes. Only photoluminescence (PL)
performance, however, is reported. In this paper, we decide to
explore its potential for electroluminescence (EL) application. The
correlation between geometric structure and high energy emission
is tentatively explained.
picolinonitrile, NaN3, Cu powder, polymethylmethacrylate (PMMA),
ZnCl2, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(shorted as PEDOT:PSS), 4,7-diphenyl-1,10-phenanthroline (BCP),
tris(8-hydroxy-quinoline)aluminum (Alq3) and Cu(BF4)2, were
bought from Shanghai Chemical Co. (China) and used without fur-
ther purifications. The organic solvents used in this work, including
CH2Cl2, CHCl3, N,N0-dimethylformamide (DMF) and MeCN, were
bought from Tianjin Chemical Co. (China) and purified using stan-
dard procedures.
1H NMR spectra were measured with a Varian INOVA 300 spec-
trometer. Elemental analysis was performed on a Carlo Erba 1106
elemental analyzer. UV–Vis absorption spectra were measured by
a HP 8453 UV–Vis–NIR diode array spectrophotometer. All photolu-
minescence (PL) spectra were measured with a Hitachi F-4500 fluo-
rescence spectrophotometer. PL quantum yield was determined in
PMMA film according a literature procedure [10]. Excited state life-
times were determined with a 355 nm light generated from the
third-harmonic-generator pump, using a pulsed Nd:yttrium alumi-
num garnet (YAG) laser as the excitation source. The Nd:YAG laser
possessed line width of 1.0 cmꢁ1, pulse duration of 10 ns and repe-
tition frequency of 10 Hz. Thermogravimetric analysis (TGA) data
were obtained from a thermal analysis instrument (SDT2960, TA
Instruments, New Castle, DE) with heating rate of 10 °C/min. Single
crystal data were collected by a Siemens P4 single-crystal X-ray dif-
fractometer with a Smart CCD-1000 detector and graphite-mono-
chromated Mo Ka radiation, operating at 50 kV and 30 A at 298 K.
All hydrogen atoms were calculated. Density functional theory
(DFT) and time dependent density functional theory (TD-DFT) cal-
culations were performed on [Cu(TP)(PPh3)2]+ at RB3LYP/SBKJC
level. The initial geometry was obtained from its crystal data. All
computations were finished by GAMESS. The graphical presenta-
tion was generated by wxMacmolplt software package with
contour value of 0.025. Cyclic voltammerty (CV) measurement
was measured with a CHI830b electrochemical workstation (CH
Instruments, Shanghai Chenhua Instrument Corporation, China) in
a three-electrode cell, using platinum-sheet working electrode,
platinum-wire counter electrode and saturated calomel electrode
(SCE) reference electrode. The operation was taken in CH3CN solu-
tion (ꢂ10ꢁ3 M) with tetrabutylammonium hexafluorophosphate
(TBAPF6) as the supporting electrolyte (0.1 M). The solution was
bubbled with nitrogen for 5 min to eliminate the dissolved
Experimental section
Chemicals and equipments
The synthetic route for the tetrazole derived ligand TP and its
corresponding Cu(I) complex [Cu(TP)(PPh3)2]BF4 is shown in
Scheme 1. The starting chemicals, including triphenylphosphane,
Scheme 1. The synthetic route for the tetrazole derived ligand TP and its corresponding Cu(I) complex [Cu(TP)(PPh3)2]BF4.

282
S. Tong et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
O2. The scan rate was 0.1 V/s. All transporting layers in electrolumi-
nescent devices were deposited onto pre-cleaned ITO substrates
(resistivity <30
X
, active area = 4 mm2) by resistive heating method
under chamber pressure of ꢂ3 ꢃ 10ꢁ4 Pa. The emitting layer was
prepared using solution processed coating procedure. Electrolumi-
nescence (EL) spectra were recorded by a PR650 spectrascan
spectrometer. The luminance–current–voltage characteristic of
the EL devices was measured simultaneously with a Keithley
2400 source meter. All measurements were carried out in the air
at room temperature without being specified.
Synthesis of TP ligand
The tetrazole derived ligand TP was synthesized according to a
literature procedure described as follows [13]. 10 mmol of picoli-
nonitrile and 20 mmol of NaN3 were slowly added into 50 mL of
redistilled DMF. Then 1 g of ZnCl2 was added. The solution was
firstly stirred at room temperature for 30 min and then heated to
80 °C overnight under N2 protection. After cooling, the solution
was poured into plenty of crushed ice. The crude product was fur-
ther purified on a silica gel column (n-hexane:CH2Cl2 = 50:1) to
give the desired ligand as white powder. 1H NMR (300 Hz, CDCl3,
25 °C): d 7.82 (1H, m), 8.12 (1H, m), 8.32 (1H, d, J = 6.0), 8.83
(1H, m). Anal. Calcd for C6H5N5: C, 48.98; H, 3.43; N, 47.60. Found:
C, 48.87, H, 3.56; N, 47.56.
Fig. 1A. Single crystal structure of [Cu(TP)(PPh3)2]BF4. All H atoms were omitted for
a clear view.
Synthesis of [Cu(TP)(PPh3)2]BF4
[Cu(TP)(PPh3)2]BF4 was synthesized according to a literature
procedure [7]. First, Cu(MeCN)4BF4 was obtained by refluxing the
solution of Cu(BF4)2 and Cu powder in MeCN [11]. 1 mmol of
Cu(MeCN)4BF4 and 2 mmol of PPh3 were dissolved in 10 mmol of
CH2Cl3 and stirred for 30 min. Then 1 mmol of TP was added. The
solution was stirred for another 30 min. Finally the solvent was
removed by rotary evaporation under reduced pressure to give
[Cu(TP)(PPh3)2]BF4 as yellow-green powder. 1H NMR (300 Hz,
CDCl3, 25 °C): d 8.24 (1H, t), 8.27 (1H, t), 8.35 (1H, d, J = 6.0), 8.87
(1H, d, J = 3.6), 7.32–7.39 (m, 10H), 7.74–7.86 (m, 20 H). Anal.
Calcd. for C42H35BCuF4N5P2: C, 61.36; H, 4.29; N, 8.52. Found: C,
61.42; H, 4.34; N, 8.39. This composition can be further confirmed
by its single crystal analysis (CCDC 924428).
Table 1
Selected geometric parameters of [Cu(TP)(PPh3)2]BF4.
Bond length
Å
Bond angle
°
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–P(1)
Cu(1)–P(2)
2.186
2.103
2.258
2.254
N(1)–Cu–N(2)
N(1)–Cu–P(1)
N(2)–Cu–P(1)
N(1)–Cu–P(2)
N(2)–Cu–P(2)
P(1)–Cu–P(2)
78.15
114.15
104.40
110.71
112.65
126.75
there is usually face-to-face
p–p attraction between N–N ligands
which makes their corresponding Cu(I) complex molecules take a
highly ordered array which can repress the MLCT excited state geo-
metric relaxation. There is, however, no such highly ordered array
in [Cu(TP)(PPh3)2]BF4 crystal, as shown by Fig. 1B. The limited con-
jugation plane should be responsible for the missing of face-to-face
Results and discussion
Molecular structure of [Cu(TP)(PPh3)2]BF4
p–p attraction.
Geometric structure
The successful synthesis of [Cu(TP)(PPh3)2]BF4, along with that
of TP ligand, can be identified by the single crystal structure of
[Cu(TP)(PPh3)2]BF4 shown in Fig. 1A. The central Cu(I) ion is sur-
rounded by a N atom from the pyridine ring, a N atom from the tet-
razole ring and two P atoms from two PPh3 ligands, respectively,
which means that it localizes at the center of a tetrahedral coordi-
nation field. The selected structural parameters of the coordination
sphere listed in Table 1 suggest that this tetrahedral coordination
field is distorted by the heterogeneous ligands, which is consistent
with literature case [9–14]. The two Cu–N bond length values of
[Cu(TP)(PPh3)2]BF4 are longer than literature values (ꢂ2.08 Å)
[11,14]. Similarly, the two Cu–P bonds are also longer than litera-
ture values (ꢂ2.27 Å) [11,14] The bond angles listed in Table 1 also
indicate the distortion of [Cu(TP)(PPh3)2]BF4 coordination sphere.
The N(1)–Cu–N(2) bite angle is smaller than literature values
(ꢂ80°) [11,14].
Electronic structure
To get a deeper understanding on the electronic structure and
electronic transitions of [Cu(TP)(PPh3)2]BF4, we decide to perform
a TD-DFT calculation on [Cu(TP)(PPh3)2]+ [11,14]. The percentage
It is observed that the pyridine ring is connected with the tetra-
zole ring with a
r bond, showing a large coplanar plane for coordi-
nation. According to the report by Zhang and coworkers [9,11],
Fig. 1B. The crystal stacking mode of [Cu(TP)(PPh3)2]BF4 molecules.

S. Tong et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
283
composition of the first ten singlet excitations and their related
frontier molecular orbitals (MOs) is summarized in Table 2. It
can be seen that the occupied frontier MOs of 128–126 and
116–113 are mainly composed of Cu and PPh3, while the occupied
frontier MOs of 125–118 are essentially PPh3. As for the unoccu-
pied frontier MOs of 130–129, they are composed of TP p⁄ orbital,
which is consistent with literature case [11,14]. The calculated sin-
glet excitations consist of electronic transitions from occupied
frontier MOs to unoccupied ones and are thus assigned as a mixed
character of metal-to-ligand-charge-transfer (MLCT [d(Cu) ?
p⁄(TP)]) and ligand-to-ligand-charge-transfer (LLCT). To get a
visual understanding, two representative frontier MOs of HOMO
and LUMO are plotted and shown as Fig. 2. On ground state, the
electron cloud mainly localizes on Cu center and PPh3 ligand. Upon
excitation, the electron cloud migrates to TP ring. As a conse-
quence, the onset electronic transition is confirmed to own a mixed
character of MLCT and LLCT. Clearly, the tetrazole group is involved
in those photoinduced electronic transitions. Compared with
literature reports on similar [Cu(N–N)(P–P)]+ complexes, the tran-
sition energy values of [Cu(TP)(PPh3)2]+ are found to be obviously
larger than literature values (ꢂ2 eV). The wide band gap in
[Cu(TP)(PPh3)2]+ is thus theoretically confirmed.
LLCT nature mixed with MLCT as revealed by TD-DFT calculation.
As for the broad low energy band extending to 425 nm, it is a
newly-generated one. After consulting the TD-DFT calculation
result, this absorption can be assigned to the onset electronic tran-
sition of MLCT and LLCT. It is observed that the onset electronic tran-
sition energy is obviously higher than those of literature ones
[7,9,11,14], even though their electronic natures are all MLCT/LLCT
ones. Here, the increased band gap is experimentally confirmed.
The excitation spectrum of [Cu(TP)(PPh3)2]BF4 is composed of a
broad band ranging from 300 nm to 500 nm, peaking at 387 nm.
The optimal excitation window localizes in the low energy region,
corresponding to MLCT electronic transitions whose absorption
intensity is rather weak as shown in Fig. 3. The high energy strong
absorption bands are all powerless to excite the emissive center.
This is because MLCT excited state energy can nearly be directly
transferred to the emissive center, compared with the indirect
energy-exhausting potential surface crossing (PSC) process from
the innerligand and LLCT electronic transitions [9–11,14].
PL spectrum, excited state lifetime and quantum yield
Fig. 4 shows the PL spectrum of [Cu(TP)(PPh3)2]BF4 measured in
PMMA film (10 wt%). The green emission peaks at 497 nm with full-
width-at-half-maximum (FWHM) value as large as 92 nm, showing
no vibronic progressions, which indicates the charge transfer nature
of the emissive state. This green emission owns higher energy than
most reported phosphorescent Cu(I) complexes do [7,9,11,14]. The
Stokes shift between its absorption edge (425 nm) and emission
peak (497 nm) is obtained as 3409 cmꢁ1. This value is found to be
comparable or even larger than literature ones [11,14–16], which
means that the MLCT excited state geometric relaxation is still
severe in [Cu(TP)(PPh3)2]BF4. Thus, the increased emission energy
can be simply attributed to the increased bang gap of
[Cu(TP)(PPh3)2]BF4 initiated by the tetrazole derived ligand.
Photophysical parameters of [Cu(TP)(PPh3)2]BF4
UV–Vis absorption and excitation spectra
The UV–Vis absorption and excitation spectra of [Cu(TP)
(PPh3)2]BF4 in CHCl3 solutions (10 lM) are shown in Fig. 3. The
absorption spectra of free TP and free PPh3 are also presented for
comparison. The absorption spectrum of [Cu(TP)(PPh3)2]BF4 is com-
posed of two high energy bands and a broad low energy band. The
first high energy absorption band peaking at 233 nm is nearly iden-
tical with the corresponding band of PPh3 ligand and thus is
assigned to the innerligand electronic transition. The other high
energy band peaking at 272 nm can be considered as the adduct
of PPh3 absorption and TP absorption, which is consistent with the
The excited state lifetime (s) of an emitting material is critical
to identify its phosphorescence nature [10,17]. The inset of Fig. 4
shows the emission decay dynamics of [Cu(TP)(PPh3)2]BF4
measured in PMMA film (10 wt%). The green emission follows
Table 2
[Cu(TP)(PPh3)2]+ percentage composition of first ten singlet transitions and their related frontier MOs calculated at RB3LYP/SBKJC level.
Orbitals/transitionsa
Energy (eV)
Character
Percentage composition
Cu
TP
PPh3
130
129
128
127
126
125
124
123
122
118
116
115
114
113
ꢁ4.414
ꢁ5.222
ꢁ8.215
ꢁ8.468
ꢁ8.844
ꢁ8.928
ꢁ9.143
ꢁ9.233
ꢁ9.285
ꢁ9.453
ꢁ9.559
ꢁ9.600
ꢁ9.622
ꢁ9.660
TP
TP
Cu&PPh3
Cu&PPh3
Cu&TP
PPh3
PPh3
PPh3
PPh3
PPh3
1.1
2.2
25.2
37.2
58.6
0.6
0.2
6.3
1.8
3.1
85.7
91.4
3.5
13.5
24.7
0.1
0.3
1.5
1.5
2.3
13.2
6.3
71.2
49.4
16.7
99.3
99.5
92.2
96.7
94.5
72.2
66.4
36.2
23.4
Cu&PPh3
Cu&PPh3
Cu&PPh3
Cu&PPh3
24.9
31.8
59.9
72.7
2.9
1.8
3.9
3.9
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
S0 ? S6
S0 ? S7
S0 ? S8
S0 ? S9
S0 ? S10
2.4152
2.4876
2.6662
3.1900
3.2898
3.3642
3.4064
3.4626
3.4852
3.5398
MLCT&LLCT
MLCT&LLC
MLCT&LLCT
MLCT&LLC
LLCT
MLCT&LLCT
MLCT&LLC
MLCT&LLC
LLCT
128 ? 129 (97.2)
127 ? 129 (94.8)
126 ? 129 (95.7)
128 ? 130 (98.0)
125 ? 129 (98.5)
127 ? 130 (93.5)
114 ? 129 (44.3) & 115 ? 129 (20.3) & 116 ? 129 (12.6) & 120 ? 129 (9.7)
113 ? 129 (56.8) & 123 ? 129 (30.7)
124 ? 129 (94.3)
MLCT&LLC
123 ? 129 (50.5) & 113 ? 129 (20.0) & 118 ? 129 (14.2) & 122 ? 129 (6.0)
a
HOMO and LUMO are shown in bold.

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S. Tong et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
Fig. 3. The UV–Vis absorption and excitation spectra (kem = 497 nm) of
[Cu(TP)(PPh3)2]BF4 in CHCl3 solutions (10 M), along with the absorption spectra
of free TP and free PPh3 (in CHCl3, 5 M).
l
l
Fig. 4. The PL spectrum of [Cu(TP)(PPh3)2]BF4 measured in PMMA film (10 wt%).
Inset: The emission decay characteristic of [Cu(TP)(PPh3)2]BF4 (kex = 355 nm
generated from the third-harmonic-generator YAG laser, with line width of
1.0 cmꢁ1, pulse duration of 10 ns and repetition frequency of 10 Hz).
and non-radiative decay paths of the excited state, their values
are calculated by Formulas (1)–(3), where Kr and Knr suggest rada-
tive probability and non-emissive probability, respectively.
Kr
Kr þ Knr
U
¼
ð1Þ
ð2Þ
ð3Þ
1
s
¼ Kr þ Knr
Fig. 2. The graphical presentation of HOMO (up) and LUMO (down) of
[Cu(TP)(PPh3)2]+.
A1s2 þ A2s2
A1s11 þ A2s22
s
¼
s
s2)
biexponential decay pattern of y = y0 + A1e(ꢁx/ 1) + A2e(ꢁx/
with
With above PL quantum yield and excited state lifetime values
s
1 = 5.5414 (A1 = 0.137) and 2 = 1.0679 (A2 = 0.11503),
l
s
s
ls
on hand, Kr and Knr are calculated to be 4.3 ꢃ 104 sꢁ1 and
9.1 ꢃ 105 sꢁ1, respectively. It is observed that the dominant non-
radiative decay is responsible for the underdeveloped PL quantum
yield. Bearing the large Stokes shift in mind, the excited state
geometric relaxation should be blamed to be responsible for the
dominant non-radiative decay.
respectively. The former component can be assigned as the triplet
decay of MLCT, while the latter one is tentatively assigned as the
triplet decay of LLCT. This assignment is consistent with their
emissive nature [9,17]. The phosphorescence nature of the green
emission can thus be confirmed by the two long-lived components.
Combined with above TD-DFT result, the emissive state thus can be
assigned as a character of (ML&LLCT).
PL quantum yield (U) of [Cu(TP)(PPh3)2]BF4 is determined as
0.18 according to the literature procedures [9–11]. This value
seems to be underdeveloped compared with literature values,
though [15–17]. To get a further understanding on the radiative
Electrochemical feature and thermal stability of [Cu(TP)(PPh3)2]BF4
When designing optoelectronic device structure, it is important
to know the energy levels of functional materials. The energy levels

S. Tong et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
285
of [Cu(TP)(PPh3)2]BF4 are then determined by cyclic voltammerty
(CV) and shown as Fig. 5. [Cu(TP)(PPh3)2]BF4 owns a partially
reversible oxidation peak with potential of 1.05 V and a reversible
reduction peak with potential of ꢁ2.35 V. The partially reversible
oxidation peak can be attributed to metal center/PPh3 oxidation,
which means that the oxidation performance is compromised by
the mixed Cu/PPh3 character. While, the reversible reduction peak
can be attributed to the reduction of TP ligand, suggesting that TP
ligand owns good electron-accepting and electron-donating ability
due to its conjugation structure and tetrazole group. The energy
levels of HOMO (EHOMO) and LUMO (ELUMO) can be calculated as
ꢁ5.79 eV and ꢁ2.39 eV with Formulas (4) and (5), respectively,
where EOxd and ERed stand for the potential values of oxidation
and reduction peaks, respectively.
explore its potential for optoelectronic application. The increased
band gap and its poor thermal stability make it suitable for a solu-
tion-processed host–guest device structure. In this initial effort, we
decide to use a classic device structure of ITO/PEDOT:PSS (30 nm)/
PVK:[Cu(TP)(PPh3)2]BF4 (100 nm)/BCP (15 nm)/Alq3 (30 nm)/LiF/
Al, where PEDOT:PSS is used as the hole-injection layer,
PVK:[Cu(TP)(PPh3)2]BF4 is the emitting layer, BCP is the exciton-
blocking layer and Alq3 is the electron-transporting layer, respec-
tively. Here PVP is selected to be the host for [Cu(TP)(PPh3)2]BF4
owing to its proper energy levels of ꢁ5.8 eV for HOMO and
ꢁ2.3 eV for LUMO [7]. Four doping concentrations, 5 wt%, 7 wt%,
9 wt% and 11 wt%, are tried to find optimal device performance.
The EL spectra of the four devices at applied voltage of 12 V are
shown as Fig. 6. There are two bands, blue emission and green
emission, in the EL spectrum of the 5 wt% doped device. The dom-
inant green emission can be attributed to [Cu(TP)(PPh3)2]BF4. Upon
increasing the concentration, the emission shifts from 511 nm for
the 5 wt% doped device to 525 nm for the 12 wt% doped device,
which can be explained by the increased interaction between the
increasing dopant molecules [15–17]. In addition, the FWHM val-
ues are obtained as 120 nm for the 5 wt% doped device, 118 nm
for the 7 wt% doped device, 116 nm for the 9 wt% doped device
and 118 nm for the 11 wt% doped device, respectively. Those val-
ues are larger than that of PL emission. The representative PL spec-
trum of [Cu(TP)(PPh3)2]BF4 doped in PVK film (10 wt%) shown by
the inset of Fig. 6 also gives a large FWHM value of 116 nm. Thus
it can be concluded that [Cu(TP)(PPh3)2]BF4 excited state can be
stabilized in PVK host, resulting in the widened and red-shifted
spectra [18–21].
EHOMO ¼ ꢁðEOxd þ 4:74ÞeV
ð4Þ
ELUMO ¼ ꢁðERed þ 4:74ÞeV
ð5Þ
The band gap between HOMO and LUMO levels can be calcu-
lated as 3.4 eV, which is obviously higher than literature values
[7,9,11,14,17]. Thus, the increased band gap in [Cu(TP)(PPh3)2]BF4
can be further confirmed. EHOMO value is found to be slightly higher
than literature ones (ꢁ5.8 eV to ꢁ5.5 eV), while ELUMO is much
higher than literature values (lower than ꢁ2.5 eV), which means
that the increased band gap is mainly caused by the heightened
LUMO level. Here, the electronic effect of tetrazole on increasing
the band gap can be further confirmed.
Thermal stability is another key factor for optoelectronic mate-
rials since they need to be thermally stable enough to experience
optoelectronic device construction procedure. The thermal
degradation of [Cu(TP)(PPh3)2]BF4 thus should be investigated.
The thermogravimetric analysis (TGA) and the derivative thermo-
gravimetry (DTG) curves of [Cu(TP)(PPh3)2]BF4 suggest, however,
[Cu(TP)(PPh3)2]BF4 is not suitable for device construction through
thermal evaporation method (see Supporting Information for
details).
The blue emission peaking at 412 nm becomes weaker when the
doping concentration is as high as 7 wt%. It finally disappears upon
doping concentration of 9 wt% or higher, showing the dominant
green emission. Compared with literature report [7], this blue emis-
sion can be assigned to the emission from PVK, which means that
there are excess excitons in the emitting layer. When the doping
concentration is low, the excess excitons are trapped and aggre-
gated in the emitting layer by BCP exciton-blocking layer, resulting
in the blue emission. With the increasing doping concentration,
most excitons are trapped by the doping [Cu(TP)(PPh3)2]BF4,
leading to the absence of PVK emission.
Electroluminescence performance of [Cu(TP)(PPh3)2]BF4
Above analysis has suggested that [Cu(TP)(PPh3)2]BF4 is a
green-emitting phosphorescent material with acceptable PL quan-
tum yield and excited state lifetime. We thus intend to further
The EL luminance data of the four devices under various applied
voltages are shown as Fig. 7. It can be seen that the EL luminance of
each device increases with the increasing voltage. The 5 wt% doped
Fig. 5. CV curve of [Cu(TP)(PPh3)2]BF4 in CH3CN solution (ꢂ10ꢁ3 M) with scan rate
of 0.1 V/s, using platinum-sheet working electrode, platinum-wire counter elec-
trode and SCE reference electrode.
Fig. 6. The EL spectra of the four devices at applied voltage of 12 V. Inset: the PL
spectrum of [Cu(TP)(PPh3)2]BF4 doped in PVK film (10 wt%).

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S. Tong et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 280–286
There are usually two energy transfer paths in EL devices. One is
known as charge trapping upon a condition that dopant band gap
fully falls in that of host material. The other one is Förster energy
transfer mechanism when host emission overlaps with dopant
absorption. Considering that [Cu(TP)(PPh3)2]BF4 band gap is
smaller than that of PVK, while PVK emission (ꢂ410 nm) falls in
the optimal excitation window of [Cu(TP)(PPh3)2]BF4, both energy
transfer paths are possible. To identify the dominant one in those
EL devices, the current density variation against increasing driving
voltage of the four EL devices is shown by the inset of Fig. 8. The
current density increases with the increasing doping concentra-
tion, showing maxima of 28.5 mA/cm2 for the 5 wt% doped device,
89.8 mA/cm2 for the 7 wt% doped device, 270.5 mA/cm2 for the
9 wt% doped device and 399.7 mA/cm2 for the 11 wt% doped
device, respectively, which means that the increasing
[Cu(TP)(PPh3)2]BF4 concentration is positive to increase the charge
transportation in those devices. This observation suggests the pos-
sibility of charge trapping as the dominant energy transfer mecha-
nism. Förster energy transfer mechanism may also offer some
contribution owing to the good overlapping between PVK emission
and [Cu(TP)(PPh3)2]BF4 excitation.
Fig. 7. The EL luminance data of the four devices under various applied voltages.
Conclusion
In conclusion, we explored the PL and EL performance of a green
emitting phosphorescent Cu(I) complex. Green EL emission was
successfully obtained from [Cu(TP)(PPh3)2]BF4. However, the EL
parameters are to be further improved owing to the unsatisfactory
PL quantum yield and thermal stability. Further efforts should be
devoted to the improvement of PL quantum yield and thermal
stability, while he excited state lifetime should be decreased. A
potential solution might be the chemical modification on the active
H atom in TP ligand.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.04.008.
Fig. 8. The current efficiency variation against current density of the four EL
devices. Inset: the current density variation against increasing driving voltage of the
four EL devices.
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