Copper(I) complexes of phenanthrolineimidazole ligands: Structures, photophysical properties, and quantum chemical studies

Article abstract of DOI:10.1007/s11243-015-9967-5

Three copper(I) complexes, [Cu(PimP)(POP)]PF₆ (1), [Cu(NimP)(POP)]PF₆ (2), and [Cu(AimP)(POP)]PF₆ (3) (PimP = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline, NimP = 2-(2-naphthyl-1H-imidazo[4,5-f]phenanthroline, AimP = 2-(9-anthryl)-1H-imidazo[4,5-f]phenanthroline, POP = bis[2-diphenylphosphino]-phenyl)ether) have been synthesized and characterized. Investigations into the effects of the varying substituents on the structural, absorption, and emission characteristics of the corresponding complexes are presented. Complexes 1 and 2 exhibit similar photophysical properties with the weaker lowest lying MLCT absorption at λ > 400 nm and the phosphorescence ³MLCT/³LLCT emission in the range of 570-590 nm, whereas complex 3 shows different absorption and emission properties, in which the LLCT absorption band is overlapped with the structured absorption bands belonging to the characteristic peaks of the anthracene moiety, and the emitting excited state is described as the ³LLCT character. Density functional theory and time-dependent density functional theory calculations were employed to rationalize the photophysical properties of the complexes.

Full text of DOI:10.1007/s11243-015-9967-5

Transition Met Chem (2015) 40:723–732
DOI 10.1007/s11243-015-9967-5
Copper(I) complexes of phenanthrolineimidazole ligands:
structures, photophysical properties, and quantum
chemical studies
Sheng-Xian Xu1 Jing-Lan Wang1 Feng Zhao1
•
•
•
Hong-Ying Xia1 Yi-bo Wang2
•
Received: 15 June 2015 / Accepted: 17 July 2015 / Published online: 26 July 2015
Ó Springer International Publishing Switzerland 2015
Abstract Three copper(I) complexes, [Cu(PimP)
(POP)]PF6 (1), [Cu(NimP)(POP)]PF6 (2), and [Cu(AimP)
(POP)]PF6 (3) (PimP = 2-phenyl-1H-imidazo[4,5-f][1,10]
phenanthroline, NimP = 2-(2-naphthyl-1H-imidazo[4,5-f]
phenanthroline, AimP = 2-(9-anthryl)-1H-imidazo[4,5-f]
phenanthroline, POP = bis[2-diphenylphosphino]-pheny-
l)ether) have been synthesized and characterized. Investi-
gations into the effects of the varying substituents on the
structural, absorption, and emission characteristics of the
corresponding complexes are presented. Complexes 1 and
2 exhibit similar photophysical properties with the weaker
lowest lying MLCT absorption at k [ 400 nm and the
employed to rationalize the photophysical properties of the
complexes.
Introduction
Copper(I) complexes have attracted considerable attention
since the pioneering work of McMillin et al. [1] on the
photoluminescence properties of copper diimine complexes
in the 1970s. Attractive features of Cu(I) complexes include
their low cost and generally low toxicities. Furthermore,
Cu(I) complexes show some photophysical properties sim-
ilar to those of ruthenium(II) complexes, which are widely
used for solar energy conversion, OLEDs, photocatalysis,
and so on [2–4]. The two types of Cu(I) complexes most
commonly investigated are the homoleptic [Cu(N^N)2]?
type (where N^N denotes a diimine ligand) and the heter-
leptic [Cu(N^N)(P^P)]? type (where P^P denotes a bis-
phosphine ligand). [Cu(N^N)2]? complexes often show poor
luminescent properties (low quantum yields and very short-
lived lifetimes) [5, 6] because of the distortion of the com-
plexes from tetrahedral (S0 state) to the square planar
geometry upon MLCT excitation, even when bulky sub-
stituents are introduced into the a positions relative to the
coordinating nitrogen atoms [7]. The longest excited-state
lifetimes reported to date for these complexes only range
from 1 to 2.8 ls, with moderate quantum yields of up to
6.3 % [8]. Compared to [Cu(N^N)2]? complexes,
[Cu(N^N)(P^P)]? complexes exhibit much superior photo-
luminescence properties. This is due to the introduction of the
P^P ligand [9–11], not only due to its greater rigidity but also
its ability to prevent solvent-induced exciplex quenching [12].
Among the most intensively studied complexes are the bis[(2-
3
phosphorescence MLCT/3LLCT emission in the range of
570–590 nm, whereas complex 3 shows different absorp-
tion and emission properties, in which the LLCT absorp-
tion band is overlapped with the structured absorption
bands belonging to the characteristic peaks of the anthra-
cene moiety, and the emitting excited state is described as
3
the LLCT character. Density functional theory and time-
dependent density functional theory calculations were
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-015-9967-5) contains supplementary
material, which is available to authorized users.
& Feng Zhao
zhf19752003@163.com
& Hong-Ying Xia
xiahongying85@sohu.com
1
School of Chemistry and Chemical Engineering, Jiangxi
Science and Technology Normal University, Fenglin Street,
Nanchang 330013, Jiangxi, People’s Republic of China
2
Key Laboratory of Guizhou High Performance
Computational Chemistry, Department of Chemistry,
Guizhou University, Guiyang 550025,
People’s Republic of China
diphenylphosphino)-phenyl]ether
(POP)
containing
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Transition Met Chem (2015) 40:723–732
[Cu(N^N)(POP)]? complexes [13–18]. Significant advances
in this area have been achieved through a judicious choice of
N^N ligands, in particular bulky phenanthroline [13, 14],
azolate [11, 15], and bipyridine ligands [16, 17].
Experimental
Materials and methods
Recently, a series of imidazole-fused phenanthroline
(Phenanthrolineimidazole, imPhen) ligands have been
developed [18, 19] because of their interesting electronic
properties and the relative simplicity of their synthesis. A
rich variety of functional groups with different electron-
donating or electron-accepting properties can be easily
functionalized on the imPhen framework. In our previous
paper, we reported a series of Re(I) complexes based on
imPhen ligands containing indolyl and thienyl moieties
[20, 21]. In this paper, three [Cu(N^N)(POP)]? complexes
incorporating imPhen derivatives and POP as ligands were
synthesized and characterized (Fig. 1). The addition of
phenyl, naphthyl, and anthryl moieties with various
degrees of p-conjugation and bulkiness on the imidazole
ring of the imPhen ligand, is expected to cause significant
changes in the photophysical properties of the corre-
sponding Cu(I) complexes. In this paper, we describe the
structural and spectral properties of these complexes. The
assignments of the experimental electronic absorption and
emission spectra were determined using density functional
theory (DFT) and time-dependent density functional theory
(TDDFT).
1,10-phenanthroline (Phen), potassium bromide, ben-
zaldehyde, aniline, ammonium acetate, [Cu(CH3CN)4]PF6,
and (bis[2-(diphenylphosphino)phenyl]ether) (POP) were
purchased from Aladdin Reagent Co. and used without
further purification. Concentrated sulfuric and nitric acids
were purchased from Sinopharm Chemical Reagent Co.
1H NMR spectra were recorded on
a Bruker
AV400 MHz spectrometer, using tetramethylsilane (TMS)
as internal reference. DMSO and CDCl3 were used as
solvents. UV–Vis absorption spectra were measured using
a PerkinElmer Lambda-900 spectrophotometer. Fluores-
cence spectra were determined with a Hitachi F-4500 flu-
orescence spectrophotometer. Photoluminescence quantum
yields (U) were measured in poly(methylmethacrylate)
(PMMA) films using an absolute photoluminescence yield
spectrometer (Quantaurus-QY, C11347, Hamamatsu Pho-
tonics, Japan). The excited-state lifetimes (s) were mea-
sured using a time-correlated single photon counting
(TCSPC) system on a Quantaurus-Tau instrument (Hama-
matsu Photonics, Japan).
Preparation of 1,10-phenanthroline-5,6-dione
1,10-Phenanthroline-5,6-dione was synthesized according
to a modified literature procedure. 1,10-Phenanthroline
monohydrate (10 g, 50.4 mmol) was dissolved in 60 mL of
concentrated sulfuric acid. Sodium bromide (5.19 g,
50.4 mmol) was then added, followed by 30 mL of 70 wt%
nitric acid. The mixture was heated under reflux for 6 h.
After 6 h, the temperature was reduced to 95 °C, and the
reflux condenser was removed to allow bromine vapors to
escape overnight. After cooling, the mixture was poured
onto 800 g of ice, carefully neutralized to pH 7 with 10 M
sodium hydroxide, and cooled to room temperature. The
resulting turbid solution was filtered. The aqueous phase
was extracted with CH2Cl2, and the organic phase was
dried over anhydrous magnesium sulfate and then evapo-
rated under reduced pressure. The crystalline residue was
recrystallized from 300 mL of methanol to give the pure
1
dione. Yield: 57 % (6.0 g). H NMR (400 MHz, CDCl3):
9.11–9.13 (dd, 2H), 8.50–8.52 (dd, 2H), 7.59–7.61 (dd,
2H).
Preparation of PimP
A
mixture of 1,10-phenanthroline-5,6-dione (104 mg,
0.50 mmol), benzaldehyde (72 mg, 0.68 mmol), ammonium
acetate (805 mg, 11.3 mmol), and glacial acetic acid
Fig. 1 Molecular structures of the Cu(I) complexes studied in this
study
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Transition Met Chem (2015) 40:723–732
725
Preparation of complex 2
(13 mL) was refluxed for 4 h and then cooled to room
temperature. It was diluted with water, and dropwise addi-
tion of concentrated aqueous ammonia gave a yellow pre-
cipitate, which was collected, washed with water, and dried.
The crude product was purified by chromatography on silica
gel, eluting with an 8:1 mixture of CH2Cl2 and methanol.
Yield of 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline
(PimP): 96 mg (65 %). 1H NMR (400 MHz, DMSO): 13.76
(s, 1H), 9.02 (d, J = 8.0 Hz, 2H), 8.93 (d, J = 8.0 Hz, 2H),
8.29 (d, J = 7.9 Hz, 2H), 7.82 (dd, J = 7.9, 4.4 Hz, 2H),
7.61 (t, J = 7.5 Hz, 2H), 7.51 (t, J = 7.2 Hz, 1H).
The synthetic procedure for complex 2 was the same as that
for complex 1. The complex was isolated as a yellow solid.
Yield: 37 mg (42 %).1H NMR (400 MHz, DMSO-d6):
9.11 (d, J = 8.3 Hz, 2H), 8.84 (d, J = 29.5 Hz, 3H), 8.50
(d, J = 8.0 Hz, 1H), 8.13 (s, 2H), 8.02 (d, J = 7.0 Hz,
1H), 7.88 (s, 2H), 7.62 (s, 2H), 7.45 (t, J = 7.4 Hz, 2H),
7.31 (t, J = 7.2 Hz, 4H), 7.26 (s, 1H), 7.20 (t, J = 7.4 Hz,
8H), 7.10 (t, J = 7.3 Hz, 3H), 6.99 (d, J = 5.9 Hz, 8H),
6.65 (s, 2H).
Preparation of complex 3
Preparation of NimP
The synthetic procedure for complex 3 was the same as that
for complex 1. The complex was isolated as a yellow solid.
Yield: 40 mg (42 %).1H NMR (400 MHz, DMSO) d 9.19
(s, 1H), 9.19 (s, 1H), 8.95 (s, 1H), 8.79 (s, 2H), 8.16 (s,
2H), 7.98 (s, 1H), 7.79 (s, 2H), 7.52 (dd, 10H), 7.30 (d,
14H), 7.07 (d, 8H), 6.70 (s, 2H).
The synthetic procedure for NimP was the same as that for
PimP except that 106 mg (0.68 mmol) 2-naphthaldehyde was
used for the reaction. Yield of 2-(2-naphthyl)-1H-imi-
1
dazo[4,5-f]phenanthroline (NimP): 95 mg (55 %). H NMR
(400 MHz, DMSO-d6): 13.76 (s, 1H) 9.10–9.02 (m, 2H), 8.98
(d, J = 7.2 Hz, 2H), 8.83 (s, 1H), 8.45 (d, J = 8.5 Hz, 1H),
8.14 (dd, J = 16.6, 8.0 Hz, 2H), 8.03 (d, J = 8.4 Hz, 1H),
7.86 (dd, J = 7.9, 4.2 Hz, 2H), 7.63 (p, J = 8.4 Hz, 2H).
DFT calculations
All calculations were performed using the Gaussian 09 [22]
program package. The B3LYP exchange-correlation func-
tion [23, 24] was used to optimize the ground-state
geometries of complexes 1–3 using the polarized contin-
uum model (PCM) in CH2Cl2 medium. The 6-31G* basis
set [25, 26] was used on the C, H, N, O, and P atoms, and
the LANL2DZ basis set [27] was adopted for the Cu atoms.
The unrestricted B3LYP method was used to optimize the
lowest triplet state geometries (T1) with the PCM solvent
method in toluene.
Preparation of AimP
The synthetic procedure for AimP was the same as that for
PimP except that 140.2 mg (0.68 mmol) 9-anthrylaldehyde
was used for the reaction, and the reaction mixture was
refluxed for 6 h. Yield of 2-(9-anthryl)-1H-imidazo[4,5-
f]phenanthroline(AimP): 89 mg (45 %). 1H NMR
(400 MHz, DMSO-d6): 14.16 (s, 1H), 9.02 (d, J = 4.1 Hz,
2H), 8.90 (d, J = 8.1 Hz, 1H), 8.85 (s, 1H), 8.72 (d,
J = 7.3 Hz, 1H), 8.20 (d, J = 8.4 Hz, 2H), 7.79 (dd,
J = 8.0, 4.4 Hz, 2H), 7.74 (d, J = 8.7 Hz, 2H), 7.60–7.52
(m, 2H), 7.52–7.44 (m, 2H).
Using the optimized ground geometries, the TDDFT
method [28, 29] with PCM [30] in CH2Cl2 medium at the
same level of theory was used to simulate the absorption
spectra of complexes 1–3. The first 100 singlet vertical
excitations were obtained from the TDDFT output file to
construct the calculated absorption spectra. Calculated
electronic density plots for the frontier molecular orbitals
were prepared using Gauss View 4.1.2 software, and the
spin-density populations were calculated using Multiwfn
analyzer software [31].
Preparation of complex 1
A mixture of [Cu(CH3CN)4](PF6) (31 mg, 0.1 mmol), POP
(54 mg, 0.1 mmol), and nitrogen-saturated CH2Cl2
(10 mL) was stirred for 2 h under a N2 atmosphere. PimP
(30 mg, 0.1 mmol) was then added. The mixture turned
yellow instantaneously and then was stirred for 3 h at room
temperature. The mixture was filtered with kieselguhr, and
the clear red filtrate was evaporated to dryness. The
resulting yellow solid was recrystallized with diethyl ether
Results and discussion
Synthesis
1
from CH2Cl2. Yield: 50 mg (42 %). H NMR (400 MHz,
DMSO) 9.09 (d, J = 8.1 Hz, 2H), 8.78 (d, J = 8.0 Hz,
2H), 8.34 (d, J = 7.8 Hz, 2H), 7.91–7.82 (m, 2H), 7.61 (t,
J = 7.2 Hz, 2H), 7.52 (d, J = 7.3 Hz, 1H), 7.44 (t,
J = 7.5 Hz, 2H), 7.30 (t, J = 7.1 Hz, 5H), 7.19 (t,
J = 7.3 Hz, 9H), 7.09 (t, J = 7.3 Hz, 2H), 6.97 (d,
J = 5.7 Hz, 8H), 6.63 (s, 2H).
The synthetic pathways for the free ligands and their
Cu(I) complexes are shown in Scheme 1. 1,10-Phenan-
throline-5,6-dione was synthesized according to a modified
literature procedure [32]. The free ligands were prepared
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Transition Met Chem (2015) 40:723–732
by a condensation reaction according to the literature
procedure [33]. Complexes 1–3 were prepared by the
conventional reactions of [Cu(CH3CN)4]BF6 with one
equivalent of the POP ligand and one equivalent of the
corresponding imPhen derivative ligand.
UV–Vis and photoluminescence spectra
Figure 2 shows the UV–Vis spectra of complexes 1–3 in
CH2Cl2 solution at room temperature, and the absorption
band maxima are listed in Table 1. A high-lying absorption
band of complex 1 localized at 282 nm with a distinct
shoulder at around 306 nm is observed, which can be
provisionally assigned to the ligand-centered p ? p*
transitions of the imPhen and POP ligands. At longer
wavelengths (360–520 nm), weak and long tails centered at
approximately 409 nm are present, which are not visible in
the free ligand spectra (not shown). These low-energy
absorption bands are assigned to metal-to-ligand charge
transfer (MLCT) transitions [d(Cu) ? p*(imPhen)] mixed
with ligand-to-ligand charge transfer [LLCT, p(POP) ?
p*(imPhen)] transitions. In the case of complex 2, the
highest lying absorption band localized at 274 nm is much
broader than that of complex 1, indicating that this band
has a different transition character compared to complex 1.
This band may be assigned to an intra-ligand charge
transfer [ILCT, p(POP) ? p*(POP)] transition with large
MLCT character, which is supported by TDDFT calcula-
tions. In addition, a broad plateau is observed in the range
of 310–330 nm, assigned to the combined contributions of
multiconfigurational transitions involving the POP ligand,
imPhen ligand, and naphthyl moiety. The TDDFT calcu-
lations suggest that a large MLCT contribution is respon-
sible for these bands. A weak MLCT absorption band
centered at 420 nm is also observed (see inset Fig. 2). This
band is red-shifted by 11 nm compared to that of complex
1, owing to the decrease in HOMO–LUMO energy gap
resulting from the extended p-conjugation of the N^N
Fig. 2 Absorption spectra in dichloromethane at 298 K of complexes
1–3. (Inset) the region of 350–520 nm is magnified to allow easy
comparison
ligand due to the introduction of the naphthyl moiety. The
spectrum of complex 3 shows the highest lying band at
around 264 nm with a shoulder at 301 nm. The 264-nm
band is assigned to the ligand-centered p ? p* transitions
of the anthracene group, while the shoulder is more com-
plicated and arises from a mixture of transition characters.
It is worth noting that the additional anthracene absorption
band is also observed in the visible region range from 350
to 410 nm in the spectrum of complex 3 (see inset Fig. 2),
which retains its vibronic fine structure, implying only
weak conjugation between the imPhen ring and anthracene
group. This may be ascribed to the larger torsion angle
between these rings. Additionally, weak tails centered at
435 nm are observed, which can be assigned to LLCT
[p(Anth) ? p*(imPhen)] transitions.
The normalized emission spectra of complexes 1–3 are
shown in Fig. 3 as obtained from thin solid films of PMMA
in which 20 % by weight of the complex was dispersed.
The associated emission parameters are listed in Table 1.
Complexes 1 and 2 show similar emission properties with
broad and structureless features, suggesting that these
Scheme 1 Synthetic pathways
of the ligands and
Cu(I) complexes
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Transition Met Chem (2015) 40:723–732
727
Table 1 Photophysical
properties of complexes 1–3
Complex
kabs (nm)
kem (nm)
s (ls)
U (%)
kr (104 s-1
)
knr (104 s-1
)
1
2
3
401
420
435
570
593
552
4.24
5.04
1.53
4.3
3.5
0.2
1.01
0.69
1.30
22.6
19.1
65.2
3
emissions could be assigned to the triplet 3MLCT character
3
with some LLCT character. Complex 3 exhibits a larger
of the triplet state (T1) from a MLCT/3LLCT transition
3
(complexes 1 and 2) to a more LLCT transition.
blueshift emission, clearly indicating an emitting triplet
state of predominant 3LLCT character. The emission
maximum at 593 nm of complex 2 is red-shifted by 23 nm
compared to complex 1 (kmax 570 nm), which can be
rationalized by the larger degree of p-conjugation in
complex 2 compared to complex 1, revealing a good cor-
relation with respect to the lowest lying 1MLCT absorption
band of these complexes as discussed above.
Ground-state geometries and molecular orbital
properties
The optimized ground-state geometric structures for the
complexes are shown in Fig. 4, and selected bond lengths
and angles are summarized in Table 1S in the Supporting
Information. Vibrational frequencies were calculated based
on these optimized geometries to verify that the geometries
represented a minimum on the potential energy surface.
The angle between the N2Cu1N3 and P1Cu1P2 planes is
88.4°, 89.3°, and 86.8c for complexes 1, 2, and 3, respec-
tively, suggesting slightly distorted pseudotetrahedral
geometries. The N1–C1–C2–C3 dihedral angle displays
significant variability (10.1° for complex 1, 9.2° for com-
plex 2, and 58.4° for complex 3; see Fig. 4 for the atom
numbering). This can be explained by the steric repulsion
between the atom H1 of the imidazole ring and atom H2 of
the aryl substituent. These angular variations are probably
responsible for the difference in their photophysical
properties.
The different nature of the emitting states is also
reflected in the photoluminescence quantum yields (PLQY)
and in excited-state lifetimes. In PMMA films, complexes
1 and 2 give PLQYs of 4.3 and 3.5 %, respectively. These
values are much larger than that of complex 3 (PLQY
0.2 %). The excited-state lifetimes of complexes 1 and 2 in
PMMA films (4.24 and 5.04 ls, respectively) are much
longer than that of complex 3 (1.53 ls). The calculated
radiative decay rates (Kr) of complexes 1, 2, and 3 are
1.01 9 104, 0.69 9 104, 1.30 9 104, respectively, whereas
their calculated nonradiative decay rates (Knr) are
22.6 9 104, 19.1 9 104, 65.2 9 104, respectively. The Knr
value of complex 3 is two times larger than those of
complexes 1 and 2, which is due to the change in the nature
Figure 5 shows the energy levels and atomic orbital
compositions calculated for the HOMO and LUMO of
complexes 1–3 including solvent effects. It is clear that the
substituent (phenyl, naphthyl, or anthryl) at position C2 has
a significant effect on the composition of the HOMO. The
HOMO is mainly localized on Cu and the POP ligands,
with some contribution from the imPhen moieties for
complex 1. The HOMO of complex 2 is similar to that of
complex 1, but also includes a contribution from the
naphthyl moiety, which slightly destabilizes the HOMO of
complex 2 compared to that of complex 1. With respect to
complex 3, the HOMO is mainly localized on the anthryl
moiety. The main reason for this different composition
from complexes 1 to 3 is attributed to the torsion angle of
58.48 between the anthryl moiety and the imPhen ring,
which largely breaks p-conjugation between these ring
systems. On the other hand, the LUMO of all three com-
plexes is predominately localized on the imPhen group,
which is hardly influenced by different substituents on the
N^N ligands. Consequently, the LUMO appears at very
similar energies for 1 (-2.30 eV), 2 (-2.31 eV), and 3
(-2.32 eV). Additionally, the calculated HOMO–LUMO
energy gap follows the order: 1 [ 2 [ 3, in reasonably
Fig. 3 Normalized emission spectra of complexes 1–3 in PMMA
films
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Transition Met Chem (2015) 40:723–732
Fig. 4 Optimized ground-state
geometric structures of
complexes 1–3
Fig. 5 Energy diagram and the
energy values calculated for the
HOMO and LUMO of
complexes 1–3
good agreement with the red shift of the lowest energy
absorption maxima observed in the experiments.
conveniently discuss the character of the electronic tran-
sitions, the frontier molecular orbital compositions of the
complexes in CH2Cl2 solution, expressed in terms of the
four molecular fragments, are shown in Table 2–4S (see
Supporting Information). A comparison of calculated and
experimental absorption spectra for the complexes is pre-
sented in Fig. 6. In general, the experimental spectra are
well reproduced by the calculations.
Theoretical absorption spectra
To gain insight into the character of UV–Vis transitions of
these the complexes, TDDFT calculations were undertaken
to simulate their absorption spectra. Table 2 lists the
dominant singlet–singlet vertical excitations, their oscilla-
tor strengths, assignment configurations, and excitations
with maximum coefficients for each of the complexes. To
For complex 1, we calculate an intense feature at 275 nm,
together with a shoulder at 304 nm, in good agreement with
the 282 and 306 nm experimental values. The absorption
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Transition Met Chem (2015) 40:723–732
729
Table 2 Electronic absorptions of complexes 1–3 in CH2Cl2 based on TDDFT calculations at the (B3LYP)/6-31 g*/LANL2DZ level, together
with the experimental values
Complex Excited state Transition
Coeff.
E (eV/nm) Oscillator strength Assign
Exptl. (nm)
1
1
6
H ? L
0.69621 (96.9 %) 3.05/406
0.64004 (81.9 %) 3.66/338
0.1696
0.2182
0.6576
0.1532
MLCT/LLCT/ILCT 402
H - 1 ? L ? 1
H - 1 ? L ? 2
H - 2 ? L ? 3
LLCT/ILCT
ILCT/LLCT
MLCT/ILCT
MLCT/LLCT
ILCT
12
27
0.66186 (87.6 %) 4.08/304
0.44943 (40.4 %) 4.46/278
306
282
H - 3 ? L ? 3 -0.38967 (30.4 %)
30
H - 5 ? L ? 1
H - 10 ? L
H ? L
0.37488 (28.1 %) 4.51/275
0.2606
-0.34406 (23.7 %)
LLCT
2
1
6
0.65544 (86.0 %) 3.04/408
0.58492 (68.4 %) 3.51/353
-0.34405 (23.7 %)
0.2251
0.3691
MLCT/LLCT/ILCT 420
MLCT/LLCT
H - 1 ? L ? 1
H ? L ? 1
MLCT/LLCT/ILCT
ILCT/LLCT/MLCT 328
LLCT/MLCT
9
H ? L ? 2
0.65837 (86.7 %) 3.78/328
0.56153 (63.1 %) 3.91/317
0.30717 (18.9 %)
0.3659
0.3145
12
H - 1 ? L ? 2
H - 3 ? L ? 1
H - 2 ? L ? 3
H - 6 ? L ? 1
H - 12 ? L
H - 11 ? L
LLCT/ILCT
32
34
0.49175 (48.4 %) 4.48/277
0.21539 (13.1 %)
0.3780
0.2737
MLCT/ILCT
ILCT/LLCT
LLCT
274
0.31838 (20.3 %) 4.50/275
0.29394 (17.3 %)
LLCT
H - 4 ? L ? 3 -0.26483 (14.0 %)
MLCT/ILCT
LLCT
3
1
2
H ? L
0.68935 (95.0 %) 2.88/431
0.1198
0.3084
0.0746
435
301
264
H ? L ? 1
0.68319 (93.3 %) 3.03/410
0.46335 (42.9 %) 3.95/314
0.45076 (40.6 %)
LLCT
14
H ? L ? 5
LLCT
H ? L ? 4
LLCT
39
42
82
H - 3 ? L ? 3
H - 7 ? L ? 1
H - 2 ? L ? 3
H - 7 ? L
0.38878 (30.2 %) 4.46/278
0.32927 (21.7 %)
0.1116
0.1895
0.6709
MLCT/LLCT
ILCT
0.34071 (23.2 %) 4.50/275
-0.27625 (15.3 %)
MLCT/ILCT
ILCT
H ? L ? 9
0.35801 (25.6 %) 4.89/254
0.28093 (15.8 %)
ILCT
H - 6 ? L ? 1
ILCT
MLCT metal-to-ligand charge transfer, LLCT ligand-to-ligand charge transfer, ILCT intra-ligand charge transfer
peak at 275 nm mainly originates from the mixed transitions
of HOMO - 5 ? LUMO ? 1 (28.1 %) and HOMO -
10 ? LUMO (23.7 %). HOMO - 5 and HOMO - 10
mainly consist of p(imPhen) (76.33 %) and p(POP)
(92.85 %), respectively, while the LUMO and LUMO ? 1
are predominantly composed of p*(imPhen) (see Supporting
Information). Thus, this band can be assigned to a combi-
nation of LLCT and ILCT. The 304-nm shoulder originates
from the transition of HOMO - 1 ? LUMO ? 2, which
can be ascribed to [p(imPhen) ? p(Ph)] ? [p*(im-
Phen) ? p*(Ph)] with LLCT/ILCT character. At longer
wavelengths, the calculated lowest lying absorption band
localized at 406 nm, corresponding to the weaker absorption
peak at 402 nm observed in the experimental spectrum,
should be assigned to the HOMO ? LUMO transition
originating
from
[d(Cu) ? p(POP) ? p(imPhen)] ?
[p*(imPhen)] with mixed MLCT/LLCT/ILCT character.
The highest lying absorption band of complex 2 calculated
at 277 nm, which corresponds well to the experimental value
of 274 nm, appears to mainly originate from the transition of
HOMO - 2 [d(Cu) ? p(POP)] ? LUMO ? 3 [p*(POP)]
(48.4 %) with MLCT/ILCT character. Additionally, mixed
transitions of HOMO - 4/HOMO - 11/HOMO - 12 ?
LUMO/LUMO ? 3 with LLCT/MLCT/ILCT character may
be responsible for this band. The excited state 9 originated
from the HOMO [d(Cu) ? p(POP) ? p(imPhen) ?
p(Naph)] ? LUMO ? 2 [p*(imPhen) ? p*(Naph)] transi-
tion with MLCT/LLCT/ILCT character and the excited state
12 at 317 nm originated from the HOMO - 1 [d(Cu) ? p
(POP)] ? LUMO ? 2 [p*(imPhen) ? p*(Naph)] with the
123

730
Transition Met Chem (2015) 40:723–732
Fig. 6 Comparison of the calculated (red line) and experimental (blue line) absorption spectra in CH2Cl2 solution for complex 1 (a); complex 2
(b); complex 3 (c). Red vertical lines correspond to oscillator strength of calculated singlet–singlet transitions. (Color figure online)
character of MLCT/LLCT responsible for the broad
shoulder centered at 327 nm, in good agreement with the
experimental value of 328 nm. The calculated lowest lying
absorption band localized at 408 nm, corresponding to the
weaker absorption peak at 420 nm observed in the exper-
imental spectrum, should be assigned to the HOMO [d(Cu) ?
p(POP) ? p(imPhen) ? p(Naph)] ? LUMO [p*(imPhen)]
transition with MLCT/LLCT/ILCT character.
HOMO - 2 [d(Cu) ? p(POP)] ? LUMO ? 3 [p*(POP)]
(MLCT/ILCT) mainly contributes to excited state 42. The
calculated absorption band with the largest oscillator
strength (f = 0.3084) is localized at 410 nm, which pre-
dominately originates from the HOMO [p(Anth)] ?
LUMO ? 1 [p*(Anth)] transition with ILCT character,
corresponding well to the experimental bands in the range of
350–410 nm considering the rather limited dimensions of
the basis set and the approximate nature of solvent model.
The weak absorption band centered at 435 nm in the
experimental spectrum, which was well overlapped with the
p–p* absorption of anthracene, can be assigned to the
transition configuration of HOMO [p(Anth)] ? LUMO
[p*(imPhen)] with LLCT character based on the calculated
absorption band at 431 nm.
With respect to complex 3, the calculated highest lying
absorption band at 254 nm, corresponding to the experi-
mentally observed band at 264 nm, is dominated by the
HOMO [p(Anth)] ? LUMO ? 9 [p*(Anth)] transition
(25.6 %)
and
HOMO-6
[p(Anth)] ? LUMO ? 1
[p*(Anth)] (15.8 %) with ILCT character. The experimen-
tally observed shoulder at 304 nm originates from the
combined contributions of the excited state 39 at 278 nm
and excited state 42 at 275 nm. The HOMO - 3
[d(Cu) ? p(imPhen)] ? LUMO ? 3 [p*(POP)] (MLCT/
LLCT) and HOMO - 7 [p(imPhen)] ? LUMO ? 1
[p*(imPhen)] (ILCT) excitations are the main configuration
for excited state 39, while the transition configuration of
Triplet excited states and emission properties
To gain insight into the nature of the emission properties of
the complexes, their lowest lying triplet excited-state
geometries (T1) were optimized at the spin-unrestricted
123

Transition Met Chem (2015) 40:723–732
731
Fig. 7 Spin density distribution
contours (isovalue = 0.004) for
the lowest triplet state T1 of
complexes 1–3. The values of
the unpaired electron spin-
density population are depicted
together with the electronic
nature of the states
UB3LYP/6-31G*/LANL2DZ level with a spin multiplicity
of 3. The optimized lowest triplet state structures and
selected geometric parameters are provided in Figure 2S
and Table 1S in the Supporting Information. Where stated,
toluene was chosen as the solvent, since its polarity is
comparable with that of the doped PMMA films. A sub-
stantial decrease in the intersection angle of N2Cu1N3 and
P1Cu1P2 planes is observed for complexes 1 and 2 because
of the presence of Jahn–Teller distortion. In addition, the
with some contribution of the POP ligand to the imPhen
ligand. A similar spin-density distribution is observed for
complex 2 in the T1 state (Cu, 0.52e; POP, 0.30e; imPhen,
1.18e; Naph, 0.00e), and the 3MLCT/3LLCT characters are
responsible for the T1 state of complex 2. With respect to 3,
the spin-density distribution completely differs from that of
complexes 1 and 2. The spin densities calculated for T1 (Cu,
0.00e; POP, 0.00e; imPhen, 0.34e; Anth, 1.66e) undoubtedly
confirm the dominant 3LLCT nature from the imPhen ligand
to anthryl group.
˚
Cu–P bonds lengthen by about 0.06–0.1 A for complexes 1
and 2 compared to S0, whereas the Cu–N bonds shorten by
˚
about 0.135–0.158 A due to the electronic transfer from the
Cu and POP ligand to the imPhen ligand. Additionally, the
phenyl and naphthyl groups are coplanar with the
phenanthrolineimidazole ligand in the excited state. For
complex 3, the Cu–P and Cu–N bond lengths, as well as the
dihedral angle between the N2Cu1N3 and P1Cu1P2 planes
in the excited state are not significantly different to those in
the ground state, suggesting little to no contribution of the
Cu atom and POP ligand in the T1 state. On the other hand,
the coplanarity within the anthryl and imidazole rings is
substantially distorted because of the steric repulsion
between H1 of the imidazole ring and H2 of the anthryl
substituent.
Conclusions
Three copper(I) complexes, containing the imidazo[4,5-f]-
[1,10]-phenanthroline ligand with phenyl, naphthyl, and
anthryl groups, respectively, were synthesized and char-
acterized. The profound differences in absorption and
luminescence properties of the complexes are consistent
with the results of DFT and TDDFT calculations. These
experimental and theoretical insights should provide some
insights into the design and synthesis of efficient lumi-
nescent copper(I) complexes.
The nature of the emitting excited state of each complex
can be identified by the unpaired electron spin-density dis-
tribution, as shown in Fig. 7. For complex 1, the unpaired
electron spin-density distribution in the T1 state (Cu, 0.52e;
POP, 0.29e; imPhen, 1.18e; Ph, 0.01e) confirms the emission
Acknowledgments The work was supported by the National Nat-
ural Science Foundation of China (Nos. 21462020 and 21443010),
Jiangxi Science and Technology Normal University Key Laboratory
of Organic–Inorganic Composite Materials (Key training base), and
the Natural Science Foundation of Jiangxi Province (No.
20151BAB203006). The authors thank the Guizhou University High
Performance Computation Chemistry Laboratory (GHPCC) for help
with computational studies.
3
behavior to be of MLCT/3LLCT character and illustrates
the higher contribution of the d(Cu) to the imPhen ligand
123

732
Transition Met Chem (2015) 40:723–732
19. Cardinaels T, Ramaekers J, Nochemann P, Driesen K, Hecker
KV, Meervelt LV, Lei SB, Feyter SD, Guillon D, Donnio B,
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