Synthesis, structures and spectroscopic properties of new 1,2-bis[2-(4-methyl-7-acetylamino-1,8-naphthyridine)]ethylene ligand and its binuclear copper(I) complexes

Article abstract of DOI:10.1016/j.ica.2011.09.014

The synthesis, characterization, spectroscopic properties of a new ligand 1,2-bis[2-(4-methyl-7-acetylamino-1,8-naphthyridine)]ethylene (L) and its two binuclear Cu(I) complexes containing triphenylphosphine (PPh₃) or bis(diphenylphosphino)methane (dppm), [Cu₂(L)(PPh₃) ₄](BF₄)₂·2CH₂Cl₂ (1·2CH₂Cl₂) and [Cu₂(L)(dppm) ₂](BF₄)₂·4H₂O (2·4H₂O) are reported. The structural investigation of these compounds based on X-ray crystal analysis shows that the copper(I) centers adopt different coordination geometries, O(N)CuP₂⁺ and NCuP₂⁺ for complexes 1 and 2, respectively. Upon irradiation of a degassed organic solution of L at 365 nm, photoinduced isomerization reaction and an intramolecular proton transfer of ligand L were detailed studied by absorption spectral changes. A spectroscopic investigation involving time-dependent density functional theory calculations allows assignment of the excited states that relate to emission and transient absorption spectra. The observed lower-energy absorption bands appearing in the region of 413 and 418 nm for 1 and 2 in dichloromethane are assigned to ligand-to-ligand charge transfer (LLCT, phosphine → napy) transition in nature. Compared with well-structured solid-state emission originating from ππ transition of ligand L, complexes 1 and 2 exhibit intense room-temperature solid-state emissions with λ_max at 586 and 620 nm, respectively. The energy and the shape of the emission bands are clearly different from that of the ligand, indicating the emissions come from different excited states.

Full text of DOI:10.1016/j.ica.2011.09.014

Inorganica Chimica Acta 379 (2011) 7–15
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structures and spectroscopic properties of new
1,2-bis[2-(4-methyl-7-acetylamino-1,8-naphthyridine)]ethylene ligand
and its binuclear copper(I) complexes
Li Zhan-Xian a, Li Cong b, Mu Wei-Hua b, Xiong Shao-Xiang c, Fu Wen-Fu a,b,
⇑
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, HKU-CAS Joint Laboratory on New Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Peking 100190, PR China
b College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, PR China
c Beijing Mass Spectrometry Center, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 January 2011
Received in revised form 30 August 2011
Accepted 2 September 2011
Available online 22 September 2011
The synthesis, characterization, spectroscopic properties of a new ligand 1,2-bis[2-(4-methyl-7-acetyl-
amino-1,8-naphthyridine)]ethylene (L) and its two binuclear Cu(I) complexes containing triphenylphos-
phine (PPh3) or bis(diphenylphosphino)methane (dppm), [Cu2(L)(PPh3)4](BF4)2Á2CH2Cl2 (1Á2CH2Cl2) and
[Cu2(L)(dppm)2](BF4)2Á4H2O (2Á4H2O) are reported. The structural investigation of these compounds
based on X-ray crystal analysis shows that the copper(I) centers adopt different coordination geometries,
+
O(N)CuP2+ and NCuP2 for complexes 1 and 2, respectively. Upon irradiation of a degassed organic solu-
Keywords:
tion of L at 365 nm, photoinduced isomerization reaction and an intramolecular proton transfer of ligand
L were detailed studied by absorption spectral changes. A spectroscopic investigation involving time-
dependent density functional theory calculations allows assignment of the excited states that relate to
emission and transient absorption spectra. The observed lower-energy absorption bands appearing in
the region of 413 and 418 nm for 1 and 2 in dichloromethane are assigned to ligand-to-ligand charge
transfer (LLCT, phosphine ? napy) transition in nature. Compared with well-structured solid-state emis-
sion originating from pp⁄ transition of ligand L, complexes 1 and 2 exhibit intense room-temperature
solid-state emissions with kmax at 586 and 620 nm, respectively. The energy and the shape of the emis-
sion bands are clearly different from that of the ligand, indicating the emissions come from different
excited states.
1,8-Naphthyridine derivatives
Vinyl
Binuclear copper(I)
Crystal structures
Spectroscopic properties
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
derivatives [33–35]. In general, the copper(I) center in the class of
complexes is three- or four-coordinated with a distorted trigonal
The availability of 1,8-naphthyridine and its alkyl derivatives in
biological systems have been of considerable interest because of
their ability to form adducts containing multiple hydrogen bonds
with the nitrogenous bases of DNA [1–7]. Studies of ligand function-
alized with 1,8-naphthyridine have focused on examining the
behavior of this ring system [8–10], and intriguing coordination
modes and bonding properties of such compounds when they are
used as monodentate, bidentate and multidentate ligands [11–19].
Recently, research has been extended to the chemistry of the
naphthyridine-based metal complexes [20–28], especially the lumi-
nescent multinuclear Zn(II) complexes which have potential appli-
cation in materials science [29–32]. We have been involved in the
study of the photochemical and photophysical properties of Cu(I)
and Zn(II) complexes containing flexible 1,8-naphthyridine
or tetrahedral configuration. 1,8-Naphthyridine-based ligands usu-
ally exhibit chelating and bridging coordination modes, which can
be ascribed to the limitation of their rigid frameworks. The investi-
gation of the photophysical properties of luminescent naphthyri-
dine derivatives and their copper(I) complexes demonstrated that
their emissive pp⁄ and metal-to-ligand charge-transfer (MLCT) ex-
cited states can be tuned by changing the environmental conditions
[36]. However, the study of the photochemical and photophysical
properties of copper(I) complexes with 1,8-naphthyridine-based
ligands using transient absorption and time-dependent density
functional theory (TD-DFT) calculations on copper(I) complexes is
still inadequate.
Herein, we report the synthesis and characterization of
1,2-bis[2-(4-methy-7-acetylamino-1,8-naphthyridine)]ethylene
(L) and its complexes [Cu2L(PPh3)4](BF4)2Á2CH2Cl2 (1Á2CH2Cl2) and
[Cu2(L)(dppm)2](BF4)2Á4H2O (2Á4H2O) (Scheme 1), as well as their
photophysical properties. Results from spectroscopic investiga-
tions involving transient absorption and TD-DFT calculations have
⇑
Corresponding author at: College of Chemistry and Chemical Engineering,
Yunnan Normal University, Kunming 650092, PR China.
E-mail address: fuwf@mail.ipc.ac.cn (W.-F. Fu).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.014

8
Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
2.3. 2,4-Dimethyl-7-acetylamino-1,8-naphthyridine
About 4 g (0.0231 mol) 2,4-dimethy-7-amino-1,8-naphthyri-
dine was added to 15 mL diacetyl oxide. The mixture was refluxed
with stirring for 1 h under a nitrogen atmosphere and concentrated
in vacuum to give the crude product. The product was recrystal-
lized from ethanol. Yield: 3.97 g (80%).
2.4. 7-Acetylamino-4-methyl-1,8-naphthyridine-2-aldehyde
About 4 g (0.0186 mol) 2,4-dimethyl-7-acetylamino-1,8-naph-
thyridine and 3.092 g (0.0278 mol) selenium dioxide were added
in 200 mL 1,4-dioxane, the mixture was refluxed with stirring for
4 h under a nitrogen atmosphere [38,39]. The resulting mixture
was filtered while hot, and the filtrate was concentrated down to
10 mL in vacuum. Dichloromethane (200 mL) was added and the
resulting solution was extracted with water (2 Â 80 mL). The
organic fraction was dried over Na2SO4 and the solvent was
evaporated to give the crude product. The product was purified
by column chromatography over silica gel column using dichloro-
methane/ethanol (30/1, v/v) as the eluent. Yield: 1.49 g (35%). 1H
Scheme 1. The chemical structures of ligand L and complexes 1 and 2.
NMR (CDCl3, 400 MHz)
d (ppm): 2.34–2.37 (s, 3H, COCH3),
2.83–2.87 (s, 3H, CH3), 7.87 (s, 1H, NH), 8.38–8.40 (m, 1H, Napy),
8.65–8.67 (d, 2H, Napy), 10.19 (s, 1H, CHO). ESI MS m/z (%): 229
(M+) (100%), 214, 186, 173, 160, 144, 131. Anal. Calc. for
important consequences for the photophysical behavior of
1,8-naphthyridine-based copper(I) complexes.
C12H11N3O2 (229.23): C, 62.87; H, 4.84; N, 18.33. Found: C, 62.85;
H 4.86; N, 18.31%.
2. Experimental
2.5. 1,2-Bis[2-(4-methy-7-acetylamino-1,8-naphthyridine)]ethylene
(L)
2.1. General comments
All reactions were performed under a nitrogen atmosphere. Re-
agent grade solvents were dried by the standard procedures and
freshly distilled prior to use. 1H NMR spectra were recorded on a
Bruker 400 spectrometer at 298 K with chemical shifts (d, ppm)
relative to tetramethylsilane (Me4Si) for 1H. Elemental analysis
was carried out on a Carlo Erba 1106 element analytical instru-
ment. The UV–Vis spectra were taken on a HITACHI U-3010 spec-
trophotometer, and the corrected emission spectra of solution
and solid were obtained on a HITACHI F-4500 fluorescence spec-
trophotometer adapted to a right-angle configuration at room tem-
perature. Emission lifetimes of solid or solution samples were
measured with Single Photon Count Technology on a FL920 Spec-
trometer (Edinburgh). Transient absorption spectra were recorded
on a Laser Flash Photolysis Spectrometer 920 (Edinburgh) after
excitation of the sample in degassed methanol or acetone with
8 ns laser pulse at 355 nm. The optical difference spectrum was
generated point by point by monitoring at individual wavelengths.
About 1.343 g (6.2 mmol) 2,4-dimethyl-7-acetylamino-1,
8-naphthyridine and 1.430 g (6.2 mmol) 7-acetylamino-4-methyl-
1,8-naphthyridine-2-aldehyde were mixed in 15 mL diacetyl oxide.
The mixture was refluxed and stirred for 24 h under a nitrogen
atmosphere. The resulting solution was added in 100 mL ice water
while hot, and then filtered after stirring for 1 h. The crude product
was recrystallized from ethanol, and then separated by column
chromatography over silica gel column using chloroform/ethanol
(20/1, v/v) as eluent. Yield: 0.4 g (15%). 1H NMR (400 MHz CDCl3)
d (ppm): 2.74–2.76 (s, 6H, COCH3), 2.97 (s, 6H, CH3), 8.41 (s, 2H,
NH), 8.52–8.54 (d, 2H, CH@CH), 8.69 (d, 2H, Napy), 8.76 (d, 2H,
Napy), 8.93–8.95 (d, 2H, Napy). ESI MS m/z (%): 426 (M+) (100%),
341, 173, 146. Anal. Calc. for C24H22N6O2 (426.47): C, 67.59; H,
5.20; N, 19.71. Found: C, 67.57; H, 5.21; N, 19.30%.
2.6. Cu2(L)(PPh3)4(BF4)2Á2CH2Cl2 (1Á2CH2Cl2)
Ligand L 0.085 g (0.2 mmol), Cu(CH3CN)4BF4 0.1258 g (0.4 mmol)
and PPh3 0.2096 g (0.8 mmol) were dissolved in the mixed solvents
of dichloromethane and methanol. After stirringfor 2 h at room tem-
perature, the mixture was evaporated to dryness in vacuum leaving
an orange solid. Recrystallization of the crude product from a dichlo-
romethane/diethyl ether solution afforded an orange crystal. Yield:
0.2336 g (60%). The crystalline solid was characterized by 1H NMR
and X-ray crystallography. 1H NMR (400 MHz CDCl3) d (ppm): 2.54
(s, 6H, COCH3), 2.92 (s, 6H, CH3), 7.31–7.41 (60H, Ph), 7.51–7.53 (d,
2H, CH@CH), 7.55–7.53 (d, 2H, Napy), 7.65–7.67 (d, 2H, Napy),
7.67–7.70 (d, 2H, Napy). ESI MS: m/z (%): 1690 [powders, MÀBF4À]+.
Anal. Calc. for C98H86B2Cl4Cu2F8N6O2P4 (crystals, 1946.18): C, 60.48;
H, 4.45; N, 4.32. Found: C, 60.25; H, 4.32; N, 4.57%.
2.2. 2,4-Dimethyl-7-amino-1,8-naphthyridine [37]
2,6-Diamino-pyridine (10.9 g, 0.1 mol) and acetyl acetone
(10.0 g, 0.1 mol) were mixed in 50 mL acetic acid, and 1.2 mL 96%
sulfuric acid was added. The mixture was refluxed with stirring
for 24 h under a nitrogen atmosphere. The resulting solution was
then cooled to room temperature, after which 150 mL sodium
hydroxide (6.67 M) solution was added slowly in an ice bath. The
precipitate was isolated by filtration, and washed with cool water
(3 Â 5 mL) and dried in vacuum. The product was recrystallized
from ethanol by slow evaporation. Yield: 5.2 g (30%); m.p.: 220–
221 °C; 1H NMR (400 MHz, CDCl3) d (ppm): 2.52 (s, 3H, CH3),
2.57 (s, 3H, CH3), 6.67 (d, J = 9.7 Hz, 1H, Naph-H), 6.89 (s, 1H,
Naph-H), 7.90 (d, J = 9.7 Hz, 1H, Naph-H); MS (EI): m/z (%): 173
(M+). Anal. Calc. for C10H11N3 (173.21): C, 69.34; H, 6.40; N,
24.26. Found: C, 69.17; H, 6.38; N, 24.41%.
2.7. Cu2(L)(dppm)2(BF4)2Á4H2O (2Á4H2O)
A mixture of ligand L 0.0852 g (0.2 mmol), Cu(CH3CN)4BF4
0.1258 g (0.4 mmol) and dppm 0.1538 g (0.4 mmol) in dichloro-

Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
9
Scheme 2. Synthetic route of L.
diffractometer using a graphite monochromator with MoK
a
radia-
Table 1
tion (k = 0.071073 nm) at 293 K, respectively. The program SAINT
was used for integration of the diffraction profiles. All structures
were solved by direct method and refined by full-matrix least
squares using SHELXL 97 program [40,41]. Metal atoms in each com-
plex were located from the E-maps and other non-hydrogen atoms
were located in successive different Fourier syntheses and refined
with anisotropic thermal parameters on F2. The hydrogen atoms of
the ligands were generated theoretically onto the specific atoms
and refined isotropically with fixed thermal factors. Despite
numerous attempts at growing better crystals and collecting better
data, diffraction records of 2Á4H2O remain poor, which are indi-
cated by the high Rint. Hydrogen atoms of water in 2Á4H2O could
not be localized, and remaining electronic density could not be as-
signed in a chemically meaningful way.
Summary of X-ray crystallographic data for 1 and 2.
Complex
1
2
Formula
Formula
weight
T (K)
C
96H82B2Cu2F8N6O2P4Á2CH2Cl2 C74H66B2Cu2F8N6P4O2Á4H2O
1946.11
1568.03
293(2)
293(2)
monoclinic
Crystal
system
Space group
Crystal size
(mm)
triclinic
ꢀ
P2(1)
0.25 Â 0.10 Â 0.06
P1
0.22 Â 0.20 Â 0.18
a (Å)
b (Å)
c (Å)
12.106(2)
12.984(2)
16.320(3)
81.514(3)
72.605(3)
74.343(3)
2351.0(7)
1
14.1417(3)
17.0832(4)
16.6051(6)
90.00
106.3803(8)
90.00
3848.7(2)
2
1.351
1612
0.708
3.50–25.03
13518
a
(°)
b (°)
c
(°)
2.9. TD-DFT calculations
V (Å3)
Z
Dcalc (g cmÀ3
F(000)
)
1.375
1000
0.702
Time-dependent density functional theory (TD-DFT) calcula-
tions, employing B3LYP functional and 6-31G⁄ basis set, were car-
ried out with the GAUSSIAN 03 program to predict and verify the
absorption spectra of various species [42–44]. In order to simulate
real experimental conditions, the default polarized continuum
model (PCM/UA0) [42–44] in GAUSSIAN 03 program was also em-
ployed. So, all computational results presented here were carried
out at TD-DFT(SCRF(PCM/UA0)-B3LYP/6-31G⁄) level in CH2Cl2
solution.
l
(mmÀ1
)
h Range (°)
Unique
1.31–25.01
8248
reflections
Parameters
Rint
606
0.0242
1.049
908
0.1128
0.922
Goodness of
fit (GOF)
on F2
a
R1
0.0511
0.1330
0.0601
0.1417
0.799, À0.373
a
wR2
3. Results and discussion
Maximum,
minimum
peaks
0.839, À0.632
3.1. Synthesis and characterization
(e ÅÀ3
)
P
P
P
P
The synthetic procedure to obtain ligand L is depicted in
Scheme 2. 2,4-Dimethyl-7-acetylamino-1,8-naphthyridine was
obtained by modification of a literature method [37] and its yield
was improved by adjusting the pH 7–8. 7-Acetylamino-4-methyl-
1,8-naphthyridine-2-aldehyde was synthesized using selenium
dioxide in 1,4-dioxane as the oxidant. This was in contrast to the
general procedure, which involves first converting the amine to
an isoindole-1,3-dione group, brominating Ar–CH3, then hydrolyz-
ing. The isolated yield (35%) was not very high, but the oxidation
process was greatly simplified. L, [Cu(CH3CN)4]BF4 and PPh3 or
dppm were used as starting materials for the preparation of 1
and 2. Two different kinds of coordination modes were observed
for L, depending on the nature of the phosphine ligands. Reaction
a
2
I > 2r(I), R1
=
||Fo| À |Fc|| |Fo|. wR2 = { [w(Fo À Fc2)2]/ [w(Fo2)2]}1/2
.
methane and methanol was stirred at room temperature for 2 h.
The mixture color gradually changes from yellow to orange. The
solvent was removed in a vacuum leaving an orange solid. Orange
crystal of the complex suitable for an X-ray crystallograph struc-
ture determination was obtained by vapor diffusion of diethyl
ether into solution of crude product in dichloromethane. Yield:
0.1725 g (55%). The crystalline solid was characterized by 1H
NMR and X-ray crystallography. 1H NMR (400 MHz CDCl3) d
(ppm): 1.71 (s, 4H, CH2), 2.48 (s, 6H, CH3), 2.94 (s, 6H, COCH3),
9.27 (s, 2H, NH), 8.22 (d, 2H, CH@CH), 7.74 (d, 2H, Napy), 8.35 (d,
2H, Napy), 8.54 (d, 2H, Napy), 6.86–6.99 (40H, Ph). ESI MS: m/z
(%): 1409 [powders, MÀBF4À]+. Anal. Calc. For C74H74B2Cu2F8-
N6O6P4 (crystals, 1568.01): C, 56.68; H, 4.76; N, 5.36. Found: C,
56.49; H, 4.68; N, 5.56%.
+
of a mixture of [Cu(CH3CN)4](BF4) and L with dppm gave CuNP2
with a trigonal configuration, while copper(I) adopts a CuNOP2
+
tetrahedral configuration in complex 1 with PPh3. Both complexes
are air and moisture-stable at room temperature.
2.8. X-ray data collection and structure determinations
3.2. Crystal structures
X-ray single-crystal diffraction data for complexes 1 and 2 were
Crystal structures of complexes 1 and 2 were obtained by X-ray
collected on
a
Bruker Smart and Nonius KappaCCD X-ray
diffraction. A summary of the crystallographic parameters and data

10
Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
Fig. 1. Perspective drawing of Cu2(L)(PPh3)4(BF4)2, the thermal ellipsoids are drawn at 30% probability. The anions are omitted for clarity.
Fig. 2. Perspective drawing of Cu2(L)(dppm)2(BF4)2, the thermal ellipsoids are drawn at 30% probability. The anions are omitted for clarity.
is given in Table 1. Perspective drawings of their cations are
depicted in Figs. 1 and 2 and selected bond lengths and angles
are presented in Table 2. The coordination geometry of the Cu(I)
atom in complex 1 is a distorted tetrahedron with each Cu(I) atom
coordinated to the oxygen of a carbonyl group and a naphthyridyl
nitrogen atom as well as two PPh3 ligands. A crystallographic
inversion center is located at the vinylene group of L. The
Cu(1)–P(2) and Cu(1)–P(1) distances are 2.242(1) and 2.285(1) Å,
respectively. The different N(2)–Cu(1)–P(1) (106.93(9)°) and
N(2)–Cu(1)–P(2) (125.22(9)°) angles are typical of related Cu(I)
systems [33–35,45–47]. The Cu(1)–O(1) bond distance
(2.207(3) Å) falls in the range normally observed for analogous
compounds, while the Cu(1)–N(2) distance of 2.102(3) Å is similar
to those of typical four-coordinate copper(I) complexes [48,49].
The copper atom forms a six-membered ring with N(2)–Cu(1)–
O(1) and C(2)–N(1)–C(3) angles of 82.4(1)° and 130.8(4)°, respec-
tively. The six-membered ring and the adjacent naphthyridyl ring
are not coplanar with a dihedral angle of 2.6°.
Compared with the structure of 1, complex 2 containing biden-
tate ligand dppm exhibits two planar trigonal copper(I) centers and
has a strained bimetallic 11-membered ring. Three-coordinate
copper atoms are bonded to a nitrogen atom of ligand L and the

Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
11
Table 2
Selected bond lengths (Å) and bond angles (°) for 1 and 2.
Complex 1
Cu(1)–N(2)
Cu(1)–P(1)
P(1)–C(19)
P(2)–C(43)
N(2)–C(3)
N(3)–C(11)
N(2)–Cu(1)–O(1)
N(2)–Cu(1)–P(1)
C(13)–P(1)–C(25)
C(13)–P(1)–Cu(1)
C(37)–P(2)–C(31)
C(37)–P(2)–Cu(1)
C(2)–O(1)–Cu(1)
C(3)–N(2)–Cu(1)
2.102(3)
2.285(1)
1.834(4)
1.829(4)
1.318(5)
1.348(5)
82.4(1)
106.9(1)
103.7(2)
115.4(1)
101.9(2)
120.0(1)
130.1(3)
131.8(3)
Cu(1)–O(1)
P(1)–C(13)
P(2)–C(37)
O(1)–C(2)
N(2)–C(11)
2.207(3)
1.823(4)
1.823(4)
1.209(5)
1.375(5)
Cu(1)–P(2)
P(1)–C(25)
P(2)–C(31)
N(1)–C(3)
N(3)–C(10)
2.243(1)
1.824(4)
1.828(4)
1.391(5)
1.324(5)
N(2)–Cu(1)–P(2)
O(1)–Cu(1)–P(1)
C(13)–P(1)–C(19)
C(25)–P(1)–Cu(1)
C(37)–P(2)–C(43)
C(31)–P(2)–Cu(1)
C(2)–N(1)–C(3)
125.2(1)
O(1)–Cu(1)–P(2)
P(2)–Cu(1)–P(1)
C(25)–P(1)–C(19)
C(19)–P(1)–Cu(1)
C(31)–P(2)–C(43)
C(43)–P(2)–Cu(1)
C(3)–N(2)–C(11)
C(10)–N(3)–C(11)
109.2(1)
97.6(1)
102.0(2)
117.5(1)
101.8(2)
111.6(1)
130.8(4)
110.5(2)
123.18(4)
104.8(2)
111.8(1)
104.8(2)
114.8(1)
117.5(3)
117.5(3)
C(11)–N(2)–Cu(1)
Complex 2
Cu(1)–N(3)
Cu(2)–N(4)
N(1)–C(50)
2.083(6)
2.021(6)
1.38(1)
Cu(1)–P(3)
Cu(2)–P(2)
N(1)–C(51)
2.252(2)
2.223(2)
1.43(1)
Cu(1)–P(1)
Cu(2)–P(4)
O(1)–C(50)
2.259(2)
2.240(2)
1.22(1)
C(60)–C(61)
P(1)–C(7)
P(3)–C(31)
1.34(1)
N(3)–C(55)
P(1)–C(1)
P(3)–C(25)
1.355(9)
1.830(7)
1.836(8)
N(3)–C(59)
P(1)–C(73)
P(3)–C(74)
1.39(1)
1.815(7)
1.820(8)
116.0(2)
126.4(7)
126.0(8)
101.6(3)
111.1(2)
104.3(3)
105.9(2)
118.0(6)
1.848(6)
1.848(6)
128.06(8)
113.4(7)
123.7(10)
104.5(3)
117.4(3)
104.9(3)
118.7(2)
138.2(5)
N(3)–Cu(1)–P(3)
C(61)–C(60)–C(59)
C(50)–N(1)–C(51)
C(7)–P(1)–C(1)
C(7)–P(1)–Cu(1)
C(31)–P(3)–C(25)
C(31)–P(3)–Cu(1)
C(55)–N(3)–C(59)
N(3)–Cu(1)–P(1)
N(3)–C(55)–N(2)
O(1)–C(50)–N(1)
C(7)–P(1)–C(73)
C(1)–P(1)–Cu(1)
C(31)–P(3)–C(74)
C(25)–P(3)–Cu(1)
C(55)–N(3)–Cu(1)
115.9(2)
P(3)–Cu(1)–P(1)
N(2)–C(51)–N(1)
O(1)–C(50)–C(49)
C(1)–P(1)–C(73)
C(73)–P(1)–Cu(1)
C(25)–P(3)–C(74)
C(74)–P(3)–Cu(1)
C(59)–N(3)–Cu(1)
112.9(7)
121.8(10)
102.2(3)
117.8(2)
102.6(3)
118.4(3)
103.8(5)
Table 3
Spectroscopic and photophysical properties of 1 and 2.
Compound
Medium (298 K)
CH3OH
kabs/nm (
e
/104 dm3 molÀ1 cmÀ1
)
kem/nm (
s
/ns)
/
kabs/nm (s
/ns)a
em
1
375 (3.40), 382 (3.45)
408 (1.78)
423 (1.78)
405, 424
0.401
0.220
0.066
0.002
CH3CN
370 (1.77), 380 (1.83)
415 (0.77)
380 (2.85), 392 (2.75)
(CH3)2CO
CH2Cl2
405, 424
450 (250, 12)
620 (150, 15)
380 (2.34), 390 (2.52)
413 (1.39)
410, 430, 480 (4.21, 1.67)
solid
586 (6.5)
2
CH3OH
374 (1.64), 383 (1.62)
413 (1.03)
376 (2.21), 390 (1.96)
415 (0.45)
370 (1.75), 391 (2.21)
416 (1.41)
373 (1.28), 396 (2.40)
418 (2.38)
410 (0.99), 428 (0.99)
0.278
0.161
0.062
0.047
500 (284), 550 (301)
620 (246), 650 (266)
CH3CN
(CH3)2CO
CH2Cl2
solid
406, 425
404, 430
490 (266), 550 (380)
650 (348)
410 (0.77), 437 (0.77)
470 (3.57, 1.67)
620 (1.71)
L
CH3OH
CH3CN
CH2Cl2
solid
370, 388
365
380, 400
409, 422
403
410
0.39
471, 482, 514, 540
a
Decay lifetime of transient difference absorption signals.
phosphorus atoms of two dppm ligands in a Y-shaped geometry
with Cu(1)–P distances of 2.252(2) and 2.259(2) Å. The bond angles
around the copper atom are 116.0(2)°, 115.9(2)° and 128.06(8)°. An
interaction between the N(2) atom on the naphthyridyl ring and
Cu(1) is observed with N(2)Á Á ÁCu(1) distance of 2.501 Å, and the
N(2)–Cu(1)–N(3), N(2)–Cu(1)–P(1) and N(2)–Cu(1)–P(3) bond an-
gles are 58.32°, 99.57° and 105.24°, respectively. In contrast,
N(5)Á Á ÁCu(2) distance is 2.624 Å, and the N(4)–Cu(2)–N(5), N(5)–
Cu(2)–P(4) and N(5)–Cu(2)–P(2) bond angles are 55.61°, 101.97°
and 105.44°, respectively.
The average Cu–N(napy) distance of 2.052(6) Å in 2 is slightly
shorter than that in 1 (2.102(3) Å), and the dihedral angle between
the naphthyridyl planes is 10.4°. The dihedral angle between the
two trigonal planes (Cu(1), N(3), P(3), P(1) and Cu(2), N(4), P(2),
P(4)) is 36.9° and the CuÁ Á ÁCu distance of 3.637 Å is significantly
longer than the sum of the van der Waals radii of copper (2.8 Å)
[50]. The Cu(2) atom adopts slightly distorted Y-shaped trigonal
configuration with N(4)–Cu(2)–P(2), N(4)–Cu(2)–P(4) and P(2)–
Cu(2)–P(4) bond angles of 120.8(2)°, 114.9(2)° and 124.19(8)°,
respectively. The bond lengths of Cu(2)–N(4) (2.021(6) Å),

12
Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
Fig. 5. Changes of absorption spectra during 365 nm light irradiation of
acetonitrile or methanol (inset) at 298 K.
L in
Fig. 3. Absorption spectra changes during 365 nm light irradiation of L over a
period of 4 min in dichloromethane at 298 K. Inset: emissive spectral changes
accompanying the photochromism of L in dichloromethane with excitation at
320 nm.
and the naphthyridine moiety, respectively. The calculated energy
gaps of E406?417 and E416?417 for 1 are 3.3599 eV (369 nm) and
2.5631 eV (484 nm) and corresponding oscillator strength (f) val-
D
D
Cu(2)–P(2) (2.223(2) Å) and Cu(2)–P(4) (2.240(2) Å) are signifi-
cantly shorter than those around the Cu(1) atom.
ues are 0.8427 and 0.0124, respectively (Fig. S2), whereas those
of
DE337?343 and DE342?343 for 2 are 2.9888 eV (415 nm) and
2.2880 eV (542 nm), and with oscillator strength (f) values of
0.8905 and 0.0253, respectively (Fig. 4). Thus the observed low-
er-energy absorption bands with large extinction coefficients
(104 MÀ1 cmÀ1) for 1 and 2 are assigned to ligand-to-ligand charge
transfer (LLCT, phosphine ? napy) transition in nature.
3.3. Electronic absorption spectra
The spectral data of UV–Vis absorption for L, 1 and 2 in different
solvents are summarized in Table 3. The absorption spectrum of L
exhibits a broad band (350–400 nm) which can be assigned to a
spin-allowed
p ?
p⁄ transition (Fig. 3). The absorptions of L, 1
and 2 are sensitive to the solvent and exhibit a blue shift as the sol-
vent polarity increases (see the Supporting information Fig. S1).
Low energy absorption bands appear in the region of 400, 413
and 418 nm for L, 1 and 2 in dichloromethane, respectively. A
TD-DFT calculation of 1 and 2 demonstrated that their highest
occupied molecular orbital (HOMO) and lowest unoccupied molec-
ular orbital (LUMO) are mainly localized on the phosphine ligand
3.4. Photoinduced structural conversion
Upon irradiation of a degassed dichloromethane solution of L at
365 nm, a photoinduced structural conversion takes place, which is
accompanied by a change of the solution from colorless to
yellowish-green. The changes in the UV–Vis absorption and emis-
sion spectra recorded during irradiation at 365 nm over a period of
Fig. 4. Plots of the relevant HOMO and LUMO for complex 2.

Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
13
315 nm in protic and aprotic solvents imply that a photoinduced
isomerization reaction also occurs from the trans- to cis-
conformations.
Fig. 6 illustrates a typical sequence of spectra observed follow-
ing irradiation of 1 and 2 at 365 nm in dichloromethane solution.
The absorption spectra of 1 display a clear progression of the isom-
erization from trans to cis-conformations, and the peak in the ini-
tial spectrum with kmax at 393 nm exhibits a blue shift to 360 nm
during irradiation. Irradiation of 2 in dichloromethane affords
absorption spectra with isosbestic points at 310, 367 and 430 nm
as well as two new bands at 345 and 450 nm, and the absorption
bands at 373, 396 and 418 nm weaken. Overall, the spectral
changes for 2 are very different from those of 1 but low-energy
absorption bands are similar to those found for ligand L in dichlo-
romethane solution, indicating that considerable amounts of the
imino-tautomer (Scheme 3) [54,55]. The spectral observations of
1 and 2 also provide strong evidence for the mechanistic interpre-
tation of the photoinduced isomerization reaction and an intramo-
lecular proton transfer of ligand L in organic solvents.
Fig. 6. Absorption spectra changes during 365 nm light irradiation of 1 (inset) and 2
in dichloromethane at 298 K.
Generally, the steric hindrance inherent in a ligand or induced
by formation of a metal complex will increase the molecular rigid-
ity thereby preventing the intramolecular conformational conver-
sion. In contrast to ligand L, the markedly slow changes in the
low energy absorption bands of 1 upon irradiation at 365 nm indi-
cate that proton transfer is less facile because of the existence of
bonding interactions between the copper(I) center and nitrogen
and oxygen atoms. While the changes in the absorption spectra
of 2 are probably caused by the free acetylamino group allowing
1,3-hydrogen transfer to occur readily during irradiation.
4 min at room temperature are displayed in Fig. 3. The process is
characterized by isosbestic points in the absorption spectra at
324 and 404 nm. The absorption feature at 365 nm decreased with
a concomitant increase in the absorption at 310, 423 and 450 nm.
Deviation in the isosbestic point at 404 nm is observed over longer
irradiation times indicating subsequent photoisomerization of L.
During the period with irradiation at 365 nm, the band at
410 nm in the emission spectra of L rapidly weakened while four
new emission peaks appeared at 358, 375, 439 and 481 nm upon
excitation at 320 nm. Though the color changes in dichlorometh-
ane were largely reversible, the initial absorption profile was not
completely restored even after the solution was kept in the dark
for 4 days at room temperature (Fig. S3).
A comparison of the spectral data for L in different solvents
indicates that the evolution of the photochromic process can be
tuned by varying the polarity of the solvent (Figs. 3 and 5). For L
in acetonitrile and dichloromethane solutions, upon irradiation at
365 nm, the changes observed in the low-energy absorption band
are tentatively attributed to a proton transfer from the amidocya-
nogen group to an adjacent naphthyridine–N atom resulting in an
intramolecular hydrogen bond with the oxygen atom of the car-
bonyl moiety [51,52]. Similar 1,3-hydrogen transfer has been re-
ported on 1,8-naphthyridyl derivatives and their crystal
structures were characterized [53]. Moreover, absorption spectra
of L in protic solvent methanol demonstrate that solvent polarity
has profound influence on the hydrogen atom transfer, and the
low-energy absorption band was not observed due to the interac-
tion between solvent protons and the naphthyridine–N atoms
(Fig. 5). Changes of the high-energy absorption band around
3.5. Emission spectra
Upon excitation at 380 nm, L exhibits well-structured emission
at 473, 512 and 545 nm in the solid state at room temperature, and
vibration progressions ranging from 1180 to 1610 cmÀ1 are close
to C@C and C@N stretching frequencies of L (Fig. 7). Emissive spec-
tral changes were recorded during 365 nm irradiation of L in ace-
tonitrile and methanol with excitation at 310 nm. Figs. 5 and 8
illustrate the emissive spectral changes accompanying the photo-
chromism of L in both acetonitrile and methanol. In acetonitrile
solution a new emission peak following irradiation (365 nm) ap-
peared at 490 nm, while in the case of methanol solution a charac-
terized emission peak at 410 nm for L weakened and blue-shifted
to 390 nm. This is in good agreement with the photoinduced struc-
tural conversion in different solvents mentioned above. Complexes
1 and 2 display intense room-temperature solid-state emissions
with kmax at 586 and 620 nm, respectively. The energy and the
shape of the emission bands are clearly different from that of the
ligand, and can be tentatively attributed to a mixture of MLCT
Scheme 3. Photoinduced intramolecular structural conversion of 1 and 2.

14
Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
Fig. 7. Room-temperature solid-state emission spectra of L (dashed dot line), 1
(solid line) and 2 (dot line) excitation at 380 nm at 298 K.
Fig. 9. Room temperature time-resolved difference absorption spectra of
2
(3.2 Â 10À5 M) recorded at 100 ns after excitation at 355 nm in degassed acetone
solution.
Fig. 8. Emissive spectral changes accompanying the photochromism of
L in
acetonitrile and methanol (inset) with excitation at 310 nm at 298 K.
Fig. 10. Room temperature time-resolved difference absorption spectra of
2
(3.0 Â 10À5 M) recorded at 100 ns after excitation at 355 nm in degassed methanol
solution. Inset: decay traces of transient difference absorption spectra of
monitored at 500 nm.
2
and LLCT origin. However, short emission lifetimes of
s < 10 ns do
not reflect the character of a typical 3MLCT excited state. In organic
solvents such as methanol, acetonitrile, dichloromethane and ace-
tone, L, 1 and 2 show strong emission ranging from 400 to 440 nm
(Fig. 3 and Fig. S4, Table 3). Shorter emission lifetimes of 1–2 ns
and the large overlap between emission and the lowest-energy
absorption band suggest that the emissions originate from intrali-
gand excited state in nature. However, upon excitation at 420 nm,
1 and 2 display weak, low-energy emission maxima at 480 and
470 nm with the emission lifetimes of <4 ns in degassed dichloro-
methane solution, respectively (Fig. S5). Observed dual fluorescent
phenomena indicate that two emissive species with considerable
amounts present in the solution. Attempts to obtain the com-
pounds were unsuccessful.
comparison of the lifetimes of the transient absorption signals for
1 and 2 with the corresponding emission lifetimes in methanol or
acetone implies that the transient absorptions probably originate
from
a new species generated upon laser flash photolysis
(Fig. S7) [56,57].
4. Conclusions
The new ligand 1,2-bis[2-(4-methyl-7-acetylamino-1,8-naph-
thyridine)]ethylene and two kinds of luminescent binuclear cop-
per(I) complexes containing L and PPh3 or dppm were prepared.
X-ray crystal analysis revealed that the different coordination
geometries of the copper(I) centers, O(N)CuP2+ and NCuP2+, depend
on the nature of the phosphine ligands. Upon irradiation at
365 nm, a photoinduced isomerization reaction from the trans- to
cis-conformations of L and complex 1 and intramolecular proton
transfer involving the free acetylamino group of L and complex 2
were observed through the combination of absorption and emis-
sion spectra in different solvents at room temperature. The parallel
processes depend on the solvent polarity and nature of phosphine
ligand. Spectroscopic investigations suggest that the intense solid-
state emissions of L and the complexes originate from pp⁄ and
3.6. Time-resolved absorption spectra
Time-resolved absorption spectra of 1 and 2 in methanol or ace-
tone solution were measured at room temperature following nano-
second-pulsed excitation at 355 nm. The transient absorption
spectra of 2 show intense absorption with kmax at 490–550 nm
and a tail beyond 650 nm in acetone solution (Fig. 9); in methanol
the absorption spectra display a blue shift to 450 nm (Fig. 10).
Complex 1 exhibits a weak transient absorption spectrum, with a
broad absorption band appearing from 350 to 700 nm (Fig. S6). A

Z.-X. Li et al. / Inorganica Chimica Acta 379 (2011) 7–15
15
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Acknowledgments
The work was supported by the National Natural Science Foun-
dation of China (NSFC Grant No. 21071123) and the foundation
(GJHZ200817) for Bureau of International Co-operation of Chinese
Academy of Science. We thank Program for Changjiang Scholars
and Innovative Research Team in University (IRT0979) for the
financial support.
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