Computational and spectroscopic studies of the imidazole-fused phenanthroline derivatives containing phenyl, naphthyl, and anthryl groups

Article abstract of DOI:10.1016/j.molstruc.2015.11.082

Three N,N-bidentate ligands, 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (1), 2-(2-naphyl)-1H-imidazo[4,5-f]phenanthroline (2), and 2-(2-anthryl)-1H-imidazo[4,5-f]phenanthroline (3) have been synthesized and characterized. Effects of aryl subsistiuents (phenyl, naphthyl, and anthryl) on the photophysical properties of these ligands in solution have been studied. Ligand 1 exhibit the main absorption band at 283 nm with the shoulder bands at 300-350 nm and these bands are assigned as the typical π→π_(imPhen) state (imPhen = 1H-imidazo[4,5-f][1,10]phenanthroline). A similar absorption spectrum was also observed in the case of 2, in which the charge transfer (CT) state should be considered. 3 shows the slightly different absorption properties compared to that of 1 and 2. The highest-lying absorption band at 257 nm is assigned as a mixed π→π_(imPhen)/π→π_(Anth) (Anth = anthracene) state. Additionally, the characteristic absorption band of anthryl group with three vibronic bands at 352, 367, and 386 nm also observed in the visible region range from 340 to 400 nm. 1 shows the typical ligand-centered ¹π→πemission, while 2 and 3 emit from the mixed ¹π→π/CT states. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of these ligands studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states.

Full text of DOI:10.1016/j.molstruc.2015.11.082

Journal of Molecular Structure 1108 (2016) 46e53
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Computational and spectroscopic studies of the imidazole-fused
phenanthroline derivatives containing phenyl, naphthyl, and anthryl
groups
,
,
Jinglan Wang a, Shengxian Xu a, Feng Zhao a *, Hongying Xia a **, Yibo Wang b
a School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Fenglin Street, Nanchang, Jiangxi 330013, PR China
b Key Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry, Guizhou University, Guiyang 550025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three N,N-bidentate ligands, 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (1), 2-(2-naphyl)-1H-
imidazo[4,5-f]phenanthroline (2), and 2-(2-anthryl)-1H-imidazo[4,5-f]phenanthroline (3) have been
synthesized and characterized. Effects of aryl subsistiuents (phenyl, naphthyl, and anthryl) on the
photophysical properties of these ligands in solution have been studied. Ligand 1 exhibit the main ab-
sorption band at 283 nm with the shoulder bands at 300e350 nm and these bands are assigned as the
Received 27 July 2015
Received in revised form
25 November 2015
Accepted 30 November 2015
Available online 10 December 2015
typical
p
/
p
*
state (imPhen ¼ 1H-imidazo[4,5-f][1,10]phenanthroline). A similar absorption
(imPhen)
spectrum was also observed in the case of 2, in which the charge transfer (CT) state should be considered.
3 shows the slightly different absorption properties compared to that of 1 and 2. The highest-lying
Keywords:
Ligand
Imidazo[4,5-f][1,10]phenanthroline
Density functional theory
Charge transfer
absorption band at 257 nm is assigned as a mixed
state. Additionally, the characteristic absorption band of anthryl group with three vibronic bands at 352,
p
/
p
*
/
p
/
p
*
(Anth ¼ anthracene)
(imPhen)
(Anth)
367, and 386 nm also observed in the visible region range from 340 to 400 nm. 1 shows the typical
/p /p*/CT states. Density func-
ligand-centered 1p * emission, while 2 and 3 emit from the mixed 1p
Photophysical property
tional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize
the photophysical properties of these ligands studied. The theoretical data confirm the assignment of the
experimental absorption spectra and the nature of the emitting states.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
due to their rigid planar, hydrophobic, electro-poor heteroaromatic
system [13,14]. Additionally, the utilization of the functionalized
The transition metal complexes have received much current
interest owing to their unique photophysical properties such as the
MLCT visible absorption, relatively long-lived excited lifetimes, and
high luminescence efficiency, and so on [1e5]. Their photophysical
properties are sensitive to the coordinated ligand system and can
easily be modulated by an appropriate choice and combination of
ligands and ligand substituents. Until now, the number of ligands
with different structural and electronic properties have been
designed and synthesized in order to tune the photophysical
properties of these complexes [6e12]. Among them, the most
commonly used ligands are based on 1,10-phenanthroline (Phen)
Phen or its derivatives provides further options to tune the optical
and electrical properties [15e20].
As a Phen derivative, 1H-imidazo[4,5-f][1,10]phenanthroline
(imPhen) is an appealing ligand [21,22]. The fused imidazole ring
provides a more
p-delocalized molecular skeleton, and conse-
quently has a positive impact on the photophysical proprieties of
the corresponding complexes. Meanwhile, imPhen ligands can be
easily substituted with electron-withdrawing or edonating groups,
and these substitutions have an important effect on their HOMO
and LUMO energies and consequently on the corroding metal
complex emission. A large number of metal complexes of imPhen
such as those of Ru(II), Ir(III), Pt(I), Re(I), and Cu(I), have been re-
ported and shown interesting photophysical properties [23e30].
However, the electrical structures and the photophysical properties
of these ligands seem deficient compared to that of Phen [31,32]
and the origin of these spectroscopic properties remains unex-
* Corresponding author.
** Corresponding author.
E-mail addresses: zhf19752003@163.com (F. Zhao), xiahongying85@sohu.com
(H. Xia).
plored. Therefore,
a
deep understanding of fundamental
http://dx.doi.org/10.1016/j.molstruc.2015.11.082
0022-2860/© 2015 Elsevier B.V. All rights reserved.

J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
47
photophysical properties of imPhen ligand with new molecular
structures is desired to furthermore guide the future molecular
design of new complexes.
Herein, the photophysical properties of three imPhen de-
rivatives with phenyl, naphthyl, and anthryl groups (Chart 1) were
studied by exploiting their optical absorption and photo-
luminescent spectra. The emphasis is being given a detailed anal-
ysis of the structural and spectral properties of those ligands.
Meanwhile, the assignment of the electronic absorption and
emission spectra were also investigated using density functional
theory (DFT) and time-dependent density functional theory
(TDDFT), and the results are compared with the corresponding
experimental data.
2. Experimental
2.1. Materials and methods
1,10-phenanthroline (Phen), 2,9-dimethyl-1,10-phenanthroline,
Ethylenediamine, Potassium bromide were purchased from
Aladdin-reagent Co. and used without further purification.
Concentrated sulfuric and nitric acids were purchased from Sino-
pharm Chemical Reagent Co. The solvent dichloromethane used for
UVevis absorption and emission spectroscopy was obtained from
Shanghai Vita Chemical Reagent Co., Ltd (99.9%. HPLC grade). 1H
NMR spectra were performed in a Bruker AV400 MHz spectrom-
eter, using tetramethylsilane (TMS) as internal reference. DMSO-d6
was used as the solvents. UVevis absorption spectra were
measured using a Perkin Elmer Lambda-900 spectrophotometer.
Fluorescence spectra were determined with a Hitachi F-4500
fluorescence spectrophotometer. Photoluminescence (PL) quantum
yields were determined using a Hamamatsu system for absolute PL
quantum yield measurements (type C11347). Fluorescent lifetimes
were measured with a compact fluorescent lifetime spectrometer
(Hamamatsu, C11367, Japan).
Scheme 1. The synthetic pathways of ligands.
2.2.2. 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (1)
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 (13 mL) was refluxed for
4 h and then cooled to room temperature. It was diluted with water
and dropwise addition of concentrated aqueous ammonia gave a
yellow precipitate, which was collected, washed with water, and
dried. The crude product obtained was purified by chromatography
on silica gel, prepared in a 8:1 mixture of CH2Cl2 and methanol.
Yield: 96 mg (65%). 1H NMR (400 MHz, DMSO): 13.76 (s, 1H)9.02 (d,
J ¼ 8.0 Hz, 2H), 8.93 (d, J ¼ 8.0 Hz, 2H), 8.29 (d, J ¼ 7.9 Hz, 2H), 7.82
(dd, J ¼ 7.9, 4.4 Hz, 2H), 7.61 (t, J ¼ 7.5 Hz, 2H), 7.51 (t, J ¼ 7.2 Hz, 1H).
2.2.3. 2-(2-naphyl)-1H-imidazo[4,5-f]phenanthroline (2)
The synthetic procedure was the same as that for compound 1
except for that 106 mg (0.68 mmol) 2-naphthaldehyde was used for
the reaction. Yiedl: 95 mg (55%). 1H NMR (400 MHz, DMSO-d6):
13.76 (s, 1H) 9.10e9.02 (m, 2H), 8.98 (d, J ¼ 7.2 Hz, 2H), 8.83 (s, 1H),
8.45 (d, J ¼ 8.5 Hz, 1H), 8.14 (dd, J ¼ 16.6, 8.0 Hz, 2H), 8.03 (d,
J ¼ 8.4 Hz, 1H), 7.86 (dd, J ¼ 7.9, 4.2 Hz, 2H), 7.63 (p, J ¼ 8.4 Hz, 2H).
2.2. Synthetic procedure
2.2.1. 2,9-Dimethyl-1,10-Phenanthroline-5,6-dione
A
round-bottom
flask
containing
2,9-dimethyl-1,10-
phenanthroline (3.35 g, 16 mmol) and potassium bromide (19.0 g,
160 mmol) was cooled in an ice bath. Concentrated sulfuric acid
(60 mL) and concentrated nitric acid (30 mL) were added dropwise.
The reaction mixture was heated under reflux for 3 h, then cooled
to room temperature and the solution poured slowly into deionized
water (800 mL), neutralized with sodium bicarbonate and extrac-
ted with CH2Cl2. After washed with water and dried, the chloroform
was removed under reduced pressure to leave a yellow residue
which was recrystallized from dioxane. Yield: 78% (3.5 g). 1H NMR
(400 MHz, DMSO-d6): 8.20e8.22 (dd, 2H), 8.59e8.61 (dd, 2H), 2.82
(s,6H).
2.2.4. 2-(9-anthryl)-1H-imidazo[4,5-f]phenanthroline (3)
The synthetic procedure was the same as that for 1 except for
that 140.2 mg (0.68 mmol) 9-anthrylaldehyde was used for the
reaction and the reaction mixture was refluxed for 6 h. Yiedl: 89 mg
(45%). 1H NMR (400 MHz, DMSO-d6): 14.16 (s, 1H), 9.02 (d,
J ¼ 4.1 Hz, 2H), 8.90 (d, J ¼ 8.1 Hz,1H), 8.85 (s,1H), 8.72 (d, J ¼ 7.3 Hz,
1H), 8.20 (d, J ¼ 8.4 Hz, 2H), 7.79 (dd, J ¼ 8.0, 4.4 Hz, 2H), 7.74 (d,
J ¼ 8.7 Hz, 2H), 7.60e7.52 (m, 2H), 7.52e7.44 (m, 2H).
Chart 1. Molecular structures of the phenanthroline derivates studied in this study.

48
J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
Table 1
2.3. DFT calculations
Photophysical properties of 1, 2, and 3.
All calculations were performed using the Gaussian 09 [33]
program package. The B3LYP exchange-correlation function
[34,35] with 6-31G* basis set [36,37] was used to optimize the
ground state (S0 state) geometries and the excited state geometries
(S1 state) of ligands 1e3 using the polarized continuum model
(PCM) [38] in CH2Cl2 media. On the basis of the optimized ground
geometries, TDDFT method [39,40] associated with PCM in CH2Cl2
media were used to simulate the absorption spectra of compounds
1e3. The first 100 singlet vertical excitations were obtained form
the TDDFT output file to construct the calculated absorption
spectra. The calculated fluorescence emission energies of com-
pounds 1e3 were obtained by TDDFT calculations based on the
optimized singlet excited state geometries at the same level of
theory used for geometrical optimization. Calculated electronic
density plots for the frontier molecular orbitals were prepared
using Gauss View 4.1.2 software and the molecular orbital com-
positions are calculated using Multiwfn analyzer soft [41].
Absorption
Emission
labs/nm
lem/nm
t
/ns
F
/% kr/107sꢁ1 knr/108sꢁ1
1
2
3
282, 306, 322 (sh)
273, 332, 348 (sh)
257, 284 (sh), 352, 367, 386 510
423
430
5.7 2.9 0.51
6.1 5.5 0.90
1.7
1.5
0.8
11.1 7.0 0.63
transition is also present for this bands besides that of the imPhen-
based * transition. The shoulder at 284 nm is assigned to the
anthryl-centered * state with partial charge-transfer char-
p/p
p
/p
acter (CT) from the anthryl group to imPhen moiety. The assign-
ments of these bands are supported by TDDFT calculations (see
below). The characteristic absorption band of anthryl group with
three vibronic bands at 352, 367, and 386 nm also observed in the
visible region range from 340 to 400 nm in the spectrum of 3 (see
inset in Fig. 1), which is consistent with that reported in literature
[44,45].
The normalized fluorescence emission spectra of 1e3 in CH2Cl2
solution are shown in Fig. 2. 1 displays the fluorescence emission at
423 nm with slightly vibrational structure at around 525 nm,
3. Results and discussion
3.1. Synthesis
clearly indicating an emitting state of the imPhen-centered p/p*
character. For 2, the main change on the shape of the emission
spectrum is that the vibrational structure vanishes and the reasons
for this behavior would be consistent with a somewhat greater
The synthetic pathways of the ligands 1e3 are shown in Scheme
1. 1,10-Phenanthroline-5,6-dione was synthesized according to a
modified literature procedure [42]. These ligands were prepared by
a condensation reaction according to the literature procedure [43].
Fig. 1 shows the UVeVis spectra of 1e3 in CH2Cl2 solution at
room temperature and the corresponding absorbance values of the
relevant bands are shown in Table 1. In both 1 and 2, the main
absorption bands between 250 and 300 nm are assigned to the
contribution of CT character into the p/p* emissive state. Further
confirmation of the emissive state is provide in the theoretical
description of 2 (see 3.5 section). For 3, the shape of the emission
spectrum resembles that of 2, but, the emission band maximum is
significantly red-shifted to 510 nm, meaning that 3 also have a
mixed
proportion of
p
/
p
*/CT character but with the different contribution
* and CT states. The different nature of the
p/p
* transitions involving in imPhen moiety. In addition, both
ligands also have low-energy bands (shoulder around
300e400 nm) that can be ascribed to the * states of imPhen
moiety. However, the low-energy bands of is red-shifted
compared to 1, attributing to the extended -conjugation in 2
p/p
emitting states is also reflected in the photoluminescence quantum
yields (PLQY) and excited-state lifetimes. 1, 2, and 3 show quantum
yields of 2.9, 5.5, and 7.0%, respectively, together with excited state
lifetimes of 5.7, 6.1, and 11.1 ns (see Table 1). The radiative rate
p
/p
2
p
constant, kr, of 1 (0.51 ꢀ 107
s
ꢁ1) is smaller than that of 2 and 3
due to the introduction of the naphthyl moiety. Ligand 3 displays
different absorption properties: a high-energy band localized at
257 nm is much narrower than that of ligands 1 and 2. This in-
(0.90 ꢀ 107 sꢁ1 and 0.63 ꢀ 107 sꢁ1, respectively), which is due to the
change of the nature of the emissive state from a p/p* transition
dicates that the contribution of the anthryl-centered
p/p*
for 1 to a mixed p/p*/CT transition for 2 and 3.
Fig. 1. Absorption spectra in dichloromethane at 298 K of ligands 1e3. (inset) the
region of 320e420 nm is magnified to allow easy comparison.
Fig. 2. Normalized emission spectra of 1e2 in dichloromethane solution.

J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
49
Fig. 3. Optimized ground state and the singlet excited state geometric structures of 1e3.
Table 4
Table 2
Frontier molecular orbital compositions (%) for 3 the ground state in CH2Cl2 solution
at the B3LYP/6-31G* level.
Frontier molecular orbital compositions (%) for 1 the ground state in CH2Cl2 solution
at the B3LYP/6-31G* level.
Orbital
Energy(ev)
Main bond type
Contribution (%)
Orbital
Energy(ev)
Main bond type
Contribution (%)
imPhen
Anth
imPhen
Ph
Lþ4
Lþ3
Lþ2
Lþ1
L
ꢁ0.54
ꢁ0.58
ꢁ1.27
ꢁ1.50
ꢁ2.02
p
p
p
p
p
*(Anth)þ
p
p
*(imPhen)
*(imPhen)
51.9
34.7
96.4
98.0
8.20
48.1
65.3
3.57
2.02
91.8
Lþ3
Lþ2
Lþ1
L
ꢁ0.23
ꢁ1.06
ꢁ1.41
ꢁ1.53
p
p
p
p
*(Ph)
1.50
77.2
83.4
89.2
98.5
22.8
16.6
10.8
*(Anth)þ
*(imPhen)
*(imPhen)
*(imPhen)
*(imPhen)
*(imPhen)
*(Anth)
HOMO-LUMO gap (4.14 ev)
H
H-1
H-2
H-3
HOMO-LUMO gap (3.37ev)
H
ꢁ5.67
ꢁ6.52
ꢁ6.80
ꢁ7.07
p(imPhen)
p(imPhen)
p(imPhen)
p(Ph)
81.9
98.7
99.9
4.7
19.1
ꢁ5.39
ꢁ6.06
ꢁ6.54
ꢁ6.74
ꢁ7.25
ꢁ7.59
p
p
p
p
p
p
(Anth)
22.9
75.2
98.1
0.8
98.6
95.5
78.1
24.8
1.93
99.2
1.37
4.53
1.33
0.04
95.3
H-1
H-2
H-3
H-6
H-7
(imPhen)
(imPhen)
(Anth)
(imPhen)
(imPhen)
Table 3
Frontier molecular orbital compositions (%) for 2 the ground state in CH2Cl2 solution
at the B3LYP/6-31G* level.
have been analyzed using Multiwfn analyzer soft [39] and are listed
in Tables 2e4. Fig. 4 shows the energy levels and plots of the main
molecular orbitals of interest in 1e3. For 1, the highest occupied
molecular orbitals (HOMO) largely resides on the imPhen moiety,
and the composition extends to the phenyl group with the pro-
portion of 19.1%. The HOMO of 2 has the similar distribution to 1
Orbital
Energy(ev)
Main bond type
Contribution (%)
imPhen
Naph
Lþ3
Lþ2
Lþ1
L
ꢁ0.84
ꢁ1.16
ꢁ1.48
ꢁ1.68
p*(Naph)þ
p*(imPhen)
p*(imPhen)
p*(Naph) þ
p
*(imPhen)
22.4
84.0
95.6
48.2
78.6
16.0
4.44
p
*(imPhen)
51.8
only with different orbital compositions. The extended
p-conju-
HOMO-LUMO gap (3.92ev)
gation across the imPhen moiety and the naphthyl ring pulls the
electron density moving toward the naphthyl ring with the
increasing proportion of up to 32.9%. For 3, the orbital compositions
of the HOMO change significantly compared with that in 1 and 2.
The electron densities are mainly localized in the anthryl group
(78.1%), rather than the imPhen moiety (22.9%). The main reason is
due to the torsion angle of 57.0ꢂ between the imPhen moiety and
H
ꢁ5.60
ꢁ6.23
ꢁ6.53
ꢁ6.80
p
p
p
p
(imPhen)þ
p
(Naph)
67.1
15.8
98.3
99.1
32.9
84.2
1.72
0.04
H-1
H-2
H-3
(Naph)
(imPhen)
(imPhen)
3.2. Ground state geometries and molecular orbital properties
the anthryl ring, which breaks the p-conjugation across the imPhen
The optimized ground state geometric structures (S0 state) of
the all ligands are shown in Fig. 3. Vibrational frequencies were
calculated based on these optimized geometries to verify that the
geometries represented a minimum on the potential energy sur-
face. The N1eC1eC2eC3 dihedral angles of 1 and 2 display small
moiety and the anthryl ring to a large extent and results in the shift
of the electron density from the imPhen moiety to the anthryl
group. The electron densities of the lowest unoccupied molecular
orbitals (LUMO) of 1e3 are greatly influenced by different aryl
groups. The contribution of aryl groups gradually increased from
the phenyl to anthryl group. The LUMO of 1 is composed of 89.2%
variability (
q
¼ 3.07ꢂ for 1,
q
¼ 2.95ꢂ for 2, see Fig. 3 for the atom
numbering). However, the torsion angle between anthryl group
p
p
*(imPhen) and 10.8%
*(imPhen) and 51.8%
p
*(Ph), while that of 2 is localized on 48.2%
and imidazole ring is larger for 3 than for 1 and 2 (
indicating the present of the steric repulsion between the H1
imidazole ring and H2 atom of the anthryl group.
q
¼ 57.0ꢂ for 3),
p*(Naph). But with respect to the LUMO of 3,
the contribution from the anthryl group is increased to 91.8%. These
results explain why the various aryl groups have effect on the
The frontier molecular orbital compositions for the all ligands

50
J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
Fig. 4. Energy levels and the energy values calculated for the molecular orbitals of 1e3 with representative contour plots of the molecular orbitals.
energy levels of the HOMO and LUMO. Upon replacing the phenyl
group by the naphthyl group the HOMO energy of 2 is slightly
destabilized by 0.07 eV compared to that of 1, whereas the LUMO
energy is strongly stabilized by 0.15 eV since the LUMO of 2 has a
much larger amount of electron density on the naphthyl ring than
that of 1. With respect to 3, both the HOMO and LUMO have sub-
stantial electron density on the anthryl group and hence the
attachment of the anthryl groups to imPhen moiety has strong
influence on both the HOMO and LUMO. The HOMO energy of 3 is
destabilized by 0.28 eV, while the LUMO is stabilized by 0.49 eV
Table 5
Electronic absorptions of complexes 1e3 in CH2Cl2 based on TDDFT calculations at the (B3LYP)/6-31g* level, together with the experimental values.
Excited state
Transition
Coeff
E(eV)/(nm)
f
Assigna
Exptl/nm
1
2
2
3
7
1
H/Lþ1
H/Lþ2
H-1/L
0.63725(81.2%)
0.65698(86.3%)
0.58274(67.9%)
0.59761(71.4%)
ꢁ0.35113(24.7%)
0.59142(70.0%)
0.35822(25.7%)
0.50452(50.9%)
0.33708(22.7%)
ꢁ0.22073(9.74%)
0.70025(98.1%)
0.51668(53.4%)
ꢁ0.40634(33.0%)
0.48177(46.4%)
0.29305(17.2%)
ꢁ0.27480(15.1%)
0.36344(26.4%)
0.34747(24.1%)
0.28819(16.6%)
0.28100(15.8%)
0.36540(26.7%)
ꢁ0.35493(25.2%)
ꢁ0.28847(16.6%)
0.27118(14.7%)
0.40989(33.6%)
0.31838(20.3%)
ꢁ0.24952(12.5%)
0.24396(11.9%)
3.79/327
4.04/307
4.61/269
3.48/357
0.4934
0.3254
0.5322
0.5404
p
p
p
p
p
p
p
p
/
/
/
/
/
/
/
/
p
p
p
p
p
p
p
p
*
*
*
*
*
*
*
*
322
306
282
348
(imPhen)
(imPhen)
(imPhen)
H/L
(imPhen)/LLCT(imPhen / Naph)
H/Lþ1
H/Lþ1
H/L
H-2/Lþ1
H-1/Lþ2
H/Lþ2
H/L
H/Lþ4
H/Lþ3
H-2/Lþ1
H-1/Lþ2
H-6/Lþ1
H-3/L
H/Lþ3
H/Lþ4
H-1/Lþ3
H-1/Lþ4
H-1/Lþ3
H-7/L
(imPhen)/LLCT(Naph / imPhen)
(imPhen)/LLCT(Naph / imPhen)
(imPhen)/LLCT(imPhen / Naph)
2
3.58/346
4.55/273
0.3222
0.3631
332
273
10
(imPhen)
LLCT(Naph /imPhen)
p/
p/
p/
p/
p/
p/
p/
p/
p/
p/
p/
p/
p/
p
p
p
p
p
p
p
p
p
p
p
p
p
*
*
*
*
*
*
*
*
*
*
*
*
*
(imPhen)/LLCT(Naph / imPhen)
3
1
10
2.99/415
4.28/290
0.3067
0.0983
352,367,386
284
(Anth)
(Anth)/LLCT(Anth / imPhen)
(Anth)/LLCT(Anth / imPhen)
16
17
4.67/266
4.76/260
0.4240
0.7221
257
(imPhen)
(imPhen)
(imPhen)
(Anth)
(Anth)/LLCT(Anth / imPhen)
(Anth)/LLCT(Anth / imPhen)
(imPhen)/LLCT(imPhen / Anth)
(imPhen)/LLCT(imPhen / Anth)
(imPhen)/LLCT(imPhen / Anth)
23
26
5.07/245
5.13/242
0.6053
0.9814
LLCT(imPhen / Anth)
H-2/Lþ2
H-1/Lþ4
H-3/Lþ2
H-3/L
p/
p
p
*
*
(imPhen)
p/
(imPhen)/LLCT(imPhen / Anth)
LLCT(Anth / imPhen)
p/
p
p
*
*
(Anth)
H-1/Lþ3
p/
(imPhen)/LLCT(imPhen / Anth)
a
The actual percent contribution¼(configuration coefficient)2 ꢀ 2 ꢀ 100%.

J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
51
compared to that of 1. The calculated HOMO-LUMO energy gap
decreases in the order: 1(4.14 eV) > 2(3.92 eV) > 3(3.37 eV), in
reasonably good agreement with the red shift of the lowest energy
absorption maxima observed in the experiments.
3.3. Theoretical absorption spectra
To gain insight into the transition character of absorption
spectra of the all ligands, TDDFT calculations were undertaken to
simulate the absorption spectra. Table 5 lists the dominant energy
singletesinglet vertical excitations, their oscillator strengths (f),
assignment configurations, and excitations with maximum co-
efficients. A comparison of calculated and experimental absorption
spectra for 1e3 is presented in Fig. 5.
For 1, we calculate three intense excitation transitions at 269, 307,
and 327 nm (f ¼ 0.5322, 0.3254, and 0.4934), in good agreement with
the experimental values at 283, 306, and 322 nm, respectively. The
269 nm absorption band mainly originates form the transition of
HOMO-1/LUMO (67.9%). The HOMO-1 mainly consists of
p
(imPhen) (98.7%), whereas the LUMO are predominantly composed
of
*(imPhen)(89.2%) (seeTable 1). Thus, thisbandcan beassignedto
transition in character. The 307 nm band should be
p
p/p*
(imPhen)
assigned to HOMO [
with
similar
p
(imPhen)]/LUMOþ2 [
(imPhen) character. In addition, the 327 nm band has the
/ p* character, originating from the HOMO-
(imPhen)
p
*(imPhen)] transition
p
/ p*
p
/LUMOþ1 transition. The main spectral features in experiments are
well reproduced by our TDDFT calculation.
With respect to 2, the highest-lying absorption band calculated at
273 nm, which corresponds well to the experimental value of
272 nm, appears to mainly originate from the transition of HOMO-2
[
p
(imPhen)]/LUMOþ1 [
character, along with some contribution from the HOMO-1
*(imPhen)] (22.7%) with ligand-to ligand
p*(imPhen)] (50.9%) with p/p*
(imPhen)
[p
(Naph)] /LUMOþ2 [
p
charge transfer (LLCT) character. The calculated excitation transitions
at 346 nm originated from the HOMO/LUMOþ1 (70.0%)/LUMO
(25.7%) transition with the oscillator strength of f ¼ 0.3222 and
357 nm originated from the HOMO/LUMO (71.4%)/LUMOþ1
(24.7%) transition with the oscillator strength of f ¼ 0.5404 can be
related to the 332 and 351 nm features of the experimental spectrum
(Fig. 5b). These bands have mixed p/p*(imPhen) and LLCTcharacters.
For 3, the high-energy region is dominated by four primary
excitation transitions at 242, 245, 260, and 266 nm (f ¼ 0.9814,
0.6053, 0.7221, and 0.4240), to be correlated with the experimental
peak at 257 nm. These transitions can be described as mixed
p/p*
/p/p*
characters. The results support clearly
(imPhen)
(Anth)
our assignment of the high-energy absorption band of 3 made
above. It should be stressed that the some CT contribution between
imPhen moiety and anthryl group is also responsible for the high-
energy absorption bands as shown in Table 3. A lower intense
transition calculated at 290 nm originating from the HOMO-
/LUMOþ4 (53.4%)/LUMOþ3 (33.0%) with the oscillator strength
of f ¼ 0.0983 is the closest in energy to the experimental shoulder
at 284 nm and is assigned as p/p*(Anth) state with LLCT character
from anthryl group to imPhen moiety. The calculated absorption
band at 415 nm with the larger oscillator strength of f ¼ 0.3067 is
dominantly originated from the H [p(Anth)] / L [p*(Anth)] tran-
sition with
p/p
*
character, corresponding well to the
(Anth)
experimental absorption bands in the range of 350e400 nm
considering the rather limited dimensions of the basis set and the
approximate nature of solvent model, even though the calculated
spectra do not display vibronic fine structure of anthryl group.
Fig. 5. Comparison of the calculated (red line) and experimental (black line) absorp-
tion spectra in CH2Cl2 solution for 1 (a), 2 (b), and 3(c). Red vertical lines correspond to
oscillator strength of calculated singletesinglet transitions. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
3.4. Singlet excited state geometries and emission properties
According to Kasha's rule, the fluorescence of the molecules only

52
J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
Table 6
Plots of fluorescence emission transitions calculated for 1e3 at CH2Cl2 solution.
lem (eV)/(nm)
Transition(percent contributiona)
Assignment
Exptl (eV)/(nm)
1
2
3
3.16/392
3.08/402
2.20/562
LUMO/HOMO(94.4%)
LUMO/HOMO(97.2%)
LUMO/HOMO(100%)
p
p
p
/p
/p
/p
*
*
*
2.93/423
2.89/430
2.43/510
(imPhen)
(imPhen)/CT
(imPhen)/CT
a
The actual percent contribution¼(configuration coefficient)2 ꢀ 2 ꢀ 100%.
Fig. 6. Plots of emission transitions calculated in TDDFT level of 1e3.
emit from the lowest singlet excited states instead of the high-lying
Sn excited states. Thus, the singlet excited state geometries of 1e3
were optimized and the corresponding structures are also shown in
Fig. 3. For 1 and 2, the slight difference of the S1 structures
compared to S0 is the change of the dihedral angle between aryl
group and imPhen ring. That is, phenyl and naphthyl rings are more
planar with imPhen ring in the S1 state. However, the co-planarity
within the anthryl and imPhen rings is substantially distorted
because of the steric repulsions between the atom H1of the imPhen
ring and the atom H2 of anthryl groups. The remarkable geometry
difference between S0 and S1 states for 3 may be responsible for the
large red-shift of emission wavelength compared with that of 1 and
2 in the experiments. In order to gain insight into the nature of the
emissive excited states, the fluorescence emission energies of 1e3
were calculated using TDDFT method at their respective S1 geom-
etry. The results of the TDDFT calculations are listed in Table 6 and
the plots of frontier molecular orbitals related to emission of 1e3
are presented in Fig. 6. The fluorescence emission wavelengths
calculated at 392, 402, and 562 nm for 1e3 reasonably agree with
the experimental values of 423, 430, and 510 nm, respectively. For 1,
the emission at 392 nm (S1/S0) is contributed by the excitation
transition of LUMO/HOMO. The HOMO is largerly distributed on
the imPhen ligand (78.2%) with little contribution from phenyl
group (21.1%) in the S1 state (the distributions of the orbital com-
positions in the singlet excited state are provided in Table 1S,
p
*(imPhen)
(46.8%)]
/HOMO[
p
(imPhen)
(65.1%)]
with
p/p*
(imPhen)/CT character. The theoretical results support clearly
our assignment of the nature of the emitting state of 2 discussed
above. With respect to 3, the LUMO[ *(Anth) (78.8%)]/HOMO
(Anth) (63.4%)þ *(imPhen) (36.6%)] transition with
CTcharacter, in which the (Anth) state seem to predominate, is
p
[p
p
p/p
*
/
(Anth)
p
/p*
responsible for the emission calculated at 562 nm.
4. Conclusion
In this paper, three imidazole-fused phenanthroline derivatives,
2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (1), 2-(2-naphyl)-
1H-imidazo[4,5-f]phenanthroline (2), and 2-(2-anthryl)-1H-imi-
dazo[4,5-f]phenanthroline (3), were synthesized and character-
ized. The absorption and luminescence properties of these ligands
are investigated in the experiments and the assignments of the
absorption bands and the emitting states are interpreted in detail
with the aid of DFT and TDDFT calculations. The results show that
the main absorption bands in the UV for 1 are assigned as
transitions, while 2 and 3 exhibit the similar transition character
but with partial CT character. In addition, 1 shows 1
* emission,
whereas 2 and 3 emit from the mixed 1p
*/CT state. These
p/p*
p/p
/
p
experimental and theoretical insights should be expected to pro-
vide some guides for the design and synthesis of new ligands and
the corresponding complexes.
Supporting Information), while the LUMO is a
imPhen ligand (94.2%), thus the emission at 392 nm is mainly
ascribed as state. The fluorescence emission at
402 nm of 2 is originated from the LUMO[
p* orbital on the
Acknowledgments
p
/p*
(imPhen)
p
*(Naph) (53.2%)þ
The work was supported by the National Natural Science

J. Wang et al. / Journal of Molecular Structure 1108 (2016) 46e53
53
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