Computational and spectroscopic studies on luminescence of [Ag(PPh 3)2(NMP)]NO3

Article abstract of DOI:10.1016/j.molstruc.2005.05.010

Silver compound [Ag(PPh₃)₂(NMP)]NO₃ (1) (NMP=2-(4-Dimethylaminophenyl)imidazo(4,5-f)(1,10)phenanthroline) has been synthesized and characterized by elemental analyses, IR and X-ray diffraction techniques. Two absorption bands (294 and 342 nm) have been observed in UV/vis absorption spectrum of 1. Compound 1 exhibits luminescence in solid state at room temperature and shows broad emission with maximum at 599 nm, which make red shift compared with free ligand NMP (λ_max=522 nm). Short lifetime (2.2 ns) indicates that emission of 1 does not come form triplet excited state and the intense spin-orbital coupling of Ag^I has little effect on the lowest singlet excited state. Excited state of 1 was simulated by using the time-dependent density functional theory, which gave a sound explanation for absorption spectrum and indicated that emission of 1 originates form the metal-perturbed intraligand charge-transfer.

Full text of DOI:10.1016/j.molstruc.2005.05.010

Journal of Molecular Structure 751 (2005) 133–138
www.elsevier.com/locate/molstruc
Computational and spectroscopic studies on luminescence
of [Ag(PPh3)2(NMP)]NO3
*
Yong-Qin Wei, Ke-Chen Wu , Bo-Tao Zhuang, Zhang-Feng Zhou
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on The Structure of Matter,
Chinese Academy of Sciences, Fujian 350002, China
Received 16 March 2005; revised 5 May 2005; accepted 5 May 2005
Available online 29 June 2005
Abstract
Silver compound [Ag(PPh3)2(NMP)]NO3 (1) (NMPZ2-(4-Dimethylaminophenyl)imidazo(4,5-f)(1,10)phenanthroline) has been
synthesized and characterized by elemental analyses, IR and X-ray diffraction techniques. Two absorption bands (294 and 342 nm) have
been observed in UV/vis absorption spectrum of 1. Compound 1 exhibits luminescence in solid state at room temperature and shows broad
emission with maximum at 599 nm, which make red shift compared with free ligand NMP (lmaxZ522 nm). Short lifetime (2.2 ns) indicates
that emission of 1 does not come form triplet excited state and the intense spin-orbital coupling of AgI has little effect on the lowest singlet
excited state. Excited state of 1 was simulated by using the time-dependent density functional theory, which gave a sound explanation for
absorption spectrum and indicated that emission of 1 originates form the metal-perturbed intraligand charge-transfer.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Silver; Crystal structure; Luminescence; TDDFT; Molecular orbital; Charge-transfer
1. Introduction
To further probe and rationalize luminescent property of
compound 1, quantum chemical approaches deserve to be
considered. The time-dependent density functional theory
(TDDFT) approach [20], which is usually recommended for
modling excited state, provides satisfactory molecular
orbital explanation for the electronic excitation. A number
of papers have recently shown that TDDFT approach offers
an efficient alternative to high-level ab initio techniques for
modling sound excited state [21–30]. In the case of metal
compounds of large size, the TDDFT approach is one of the
most suitable choices to calculate accurately the excited
energy because of its lower computational cost compared
with other high-level ab initio techniques and including the
effects of electron correlation at some reasonable level. In
this paper, we describe the synthesis, crystal structure and
luminescence of the silver compound 1. And TDDFT
calculation gave a sound explanation for absorption
spectrum and luminescence of 1.
The study of luminescent properties of d10 metal
compounds which exhibit intriguing photophysical and
photochemical properties has been receiving current
attention [1–10]. Among the d10 metal compounds of Au,
Ag and Cu, AgI compounds which exhibit luminescence at
room temperature are few [11–16] and most of luminescent
AgI compounds exhibit luminescence at low temperature
[17–19]. The main reason for this is that singlet excited state
of AgI compound is easily changed to triplet excited state by
the intense spin-orbital coupling of AgI and the energy gap
between triplet excited state and singlet ground state is very
small, which causes the luminescent qenching of AgI
compound. Interestingly, the silver compound [Ag(PPh3)2-
(NMP)]NO3 (1) which has been synthesized by us exhibits
remarkable luminescence at room temperature.
*
2. Experimental
Corresponding author. Tel.: C86 591 83792600; fax: C86 591
83714946.
E-mail address: wks@ms.fjirsm.ac.cn (K.-C. Wu).
2-(4-Dimethylaminophenyl)imidazo(4,5-f)(1,10)phe-
nanthroline (NMP, as shown in scheme 1) [31,32]
and Ag(PPh3)2NO3 [33] were synthesized as literatures.
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.05.010

134
Y.-Q. Wei et al. / Journal of Molecular Structure 751 (2005) 133–138
O
N
O
N
H
N
(H C) N
CHO
3
2
N
N
[O]
N(CH )
3 2
N
NH Ac/HAc
N
N
4
Scheme 1. The structure and synthetic route of NMP.
Elemental analyses were performed with a Vario EL III
CHNOS Elemental Analyzer. The infrared spectrum of KBr
pellet was recorded on a PerkinElmer Spectrum One FT-IR
Spectrometer. The UV/vis absorption spectrum was
recorded on a PerkinElmer Lambda900 UV/VIS/NIR
Spectrometer. The emission spectrum was recorded on a
FLS920 fluorescence spectrophotometer.
[Ag(PPh3)2(NMP)]C was obtained directly from the crystal
structure of compound 1.
3. Results and discussion
3.1. Synthesis and molecular structure
2.1. Preparation of [Ag(PPh3)2(NMP)]NO3$H20 (1)
The title compound 1 was easily obtained by reaction of
ligand NMP with Ag(PPh3)2NO3 at room temperature.
Fig. 1 shows perspective view of the cation in compound 1.
NMP chelates to Ag(I) by using N1 and N2 in phenanthro-
line derivative, which make Ag(I) four-coordinated. The
angle of P1–Ag–P2 (122.13(4)8) is quite larger than that of
N1–Ag–N2 (70.44(11)8), indicating a distorted tetrahedral
geometry around the silver atom. Nearly planar structure of
NMP favors charge-transfer along the long-conjugated
ligand.
Two mmol Ag(PPh3)2NO3 reacted with 2 mmol NMP in
20 ml CH3CN at room temperature for 1 h. The orange
crystals were obtained by natural volatilization. Yield 1.96 g,
93%. Anal. Calcd. For C57H49Ag1N6O4P2 (1051.87): C
65.09, H 4.70, N 7.99. found C 65.13, H 4.65, N 8.12. IR
(solid KBr pellet/cmK1): 3434 s, 1529 m, 1039 w, 816 w
(NMP); 1094 m (P–C). UV–vis lmax/nm (CH3CN): 294
(3/dm3molK1 cmK1 32468), 342 (39243).
2.2. X-ray crystallographic analysis
Table 1
Suitable single crystal of compound 1 with dimension of
0.45!0.45!0.10 was carefully selected and glued to thin
glass fiber with epoxy resin. Intensity data for single crystal
were collected at 293 K on a Rigaku Mercury CCD
diffractometer with graphite-monochromatized Mo Ka
Crystallographic data for 1
Formula
C57H49Ag1N6O4P2
Formula weight
Crystal size (mm3)
Crystal color
1051.87
0.45!0.45!0.10
Orange
˚
radiation (lZ0.71073 A). An empirical absorption correc-
Crystal system
Triclinic
ꢀ
P1
tion was applied using SADABS program [34]. The
structure was solved by direct method and refined on F2
by full-matrix least-squares technique using the SHELX97
program package [35,36]. Anisotropic thermal parameters
were applied to all non-hydrogen atoms. The crystallo-
graphic data are listed in Table 1.
Space group
˚
a (A)
˚
b (A)
10.971(4)
14.472(5)
20.053(6)
96.475(3)
97.895(2)
111.252(5)
2893.5(16)
2
˚
c (A)
a (8)
b (8)
g (8)
CCDC-247808 contains the supplementary crystallo-
graphic data for the compound 1. This data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: (internat.)C44
1223/336 033; e-mail: deposit@ccdc.cam.ac.uk].
3
˚
V (A )
Z
Dcaled. (g cmK3
)
1.204
F(000)
m (mmK1
1078
)
1.150
q for data collection (8)
Reflections collected
Unique reflections
(R(int))
2.08–27.48
21894
12802
[R(int)Z0.0219]
631
2.3. Calculational details
Parameters
GOF
1.014
TDDFT calculation on excited state of [Ag(PPh3)2-
(NMP)]C was performed with the GAUSSIAN 03 suite of
programs [37]. The contour plots of MOs were obtained
with the GaussianView program. The model of
R1, wR2 [IO2s(I)]
R1, wR2 (all data)
0.0726, 0.1991
0.0854, 0.2150
h
X
i0:5
:
X
X
X
2
2
R1Z
ðjFojKjFcjÞ=
jFoj; wR2Z
wðFo2 KFc2Þ =
wðFo2Þ

Y.-Q. Wei et al. / Journal of Molecular Structure 751 (2005) 133–138
135
˚
Fig. 1. ORTEP drawing of the cation of 1 with 30% probability ellipsoids. Selected bond distances (A) and angles (8): Ag–N1 2.347(3), Ag–N2 2.414(3), Ag–
P1 2.4488(12), Ag–P2 2.4608(12), N1–Ag–N2 70.44(11), P1–Ag–P2 122.13(4), N1–Ag–P1 114.27(9), N1–Ag–P2 119.43(9), N2–Ag–P1 116.55(9), N2–Ag–
P2 100.58(9).
3.2. Luminescence
coupling of AgI would make effect, which causes
luminescent qenching. TDDFT calculation indicated that
the emission of 1 is neither MLCT nor LMCT in nature and
can probably be assigned to the metal-perturbed intraligand
fluorescent emission which make red shift compared with
free ligand NMP.
Fig. 2 shows emission spectra of 1 and ligand NMP in
solid state at room temperature. Compound 1 exhibits broad
emission with maximum at 599 nm, which make red
shift compared with free ligand NMP (lmaxZ522 nm).
Short lifetime (2.2 ns) indicates that emission of 1 does not
come from triplet excited state and the intense spin-orbital
coupling of AgI has little effect on the lowest singlet excited
state.
3.3. TDDFT calculation
We simulated 30 singlet excited states of [Ag(PPh3)2-
(NMP)]C by using TDDFT approach with an objective to
evaluate the electronic structure of excited states and
understand the origin of UV/vis absorption and lumines-
cence of 1. The method three-parameter hybrid functional
according to Becke with additional correlations due to Lee
et al. (B3LYP) [38,39] was adopted all along calculation
process.
Although NMP has strong electronic donor dimethyla-
mino, the lowest excited state of 1 with character of LMCT
is impossible. Because compound 1 has electron rich metal
Ag(I) (d(10)) and low-lying pp-acceptor orbitals of
phenanthroline derivative in the ligand NMP. If the lowest
excited state was MLCT in nature, the intense spin-orbital
1
Two calculational schemes were adopted. One was that
excited state of [Ag(PPh3)2(NMP)]C was directly calcu-
lated with basis set LanL2DZ upon unoptimized geomme-
try. In order to obtain more reliable result, the second
NMP
Table 2
The main optimized geometry parameters of [Ag(PPh3)2(NMP)]C
Parameter
Calculational
Experimental
˚
Bond length (A)
Ag–N1
2.400
2.430
2.514
2.509
2.347(3)
Ag–N2
2.414(3)
Ag–P1
2.4488(12)
2.4608(12)
Ag–P2
450 500 550 600 650 700 750 800 850
Wavelength (nm)
Bond angle (8)
N1–Ag–N2
P1–Ag–P2
69.8
70.44(11)
122.13(4)
120.7
Fig. 2. Emission spectra of 1 and NMP in solid state at room temperature.

136
Y.-Q. Wei et al. / Journal of Molecular Structure 751 (2005) 133–138
Table 4
Excitation energy (l, nm), the corresponding oscillator strength (f) and
composition for the lowest excited states calculated with mixed basis sets
a
State
11A
l/f
Composition
Assignment
567.0/0.0555
0.70 (HOMO/LUMO)
Metal-per-
turbed ILCT
ILCT
b
21A
31A
440.9/0.1760
361.3/0.2259
0.68 (HOMO/LUMOC1)
0.68 (HOMO-1/LUMO)
K0.17 (HOMO/LUMOC2)
K0.18 (HOMO-1/LUMO)
C0.64 (HOMO/LUMOC2)
MLCT
Experiment
41A
349.6/0.6834
p/p*
NMP. HOMO-1 is mainly metal-localized with some
contribution of PPh3 group. LUMO and LUMOC1 are
mainly localized on the phenanthroline derivative in the
ligand NMP. But orbital analyses showed that orbital
composition of LUMO differs from that of LUMOC1.
Silver uses d orbital to take part in formation of LUMO and
imidazole ring has no contribution for LUMO, while
LUMOC1 is partly localized on pp orbital of imidazole
ring. LUMOC2 is p* orbital of NMP.
Low-energy absorption band is composed of excitation
from the ground state to optical allowed 31A and 41A state.
31A has a predominant metal to ligand charge-transfer
(MLCT) character, with a minor p/p* contribution. 41A
exhibits contrary multitransition character compared with
31A. Oscillator strength of 41A is higher than that of 31A,
which indicates that the low-energy absorption band mainly
derives from p/p* transfer and the information of MLCT
excited state is obscured. High-energy excited states relative
to high-energy absorption exhibit complicated multitransi-
tion character, which brings difficulty for assignment of
charge-transfer.
The lowest singlet excited state 11A denotes to
HOMO/LUMO transition and is assigned to metal-
perturbed intraligand transition excited state. Excited state
21A denotes to HOMO/LUMOC1 transition and is
assigned to intraligand charge-transfer (ILCT) excited
state. The two lowest excited states have low oscillator
strength and thus cannot be embodied in absorption
spectrum. The calculated energy of 11A is close to emission
energy. The emission of 1 originate from metal-perturbed
intraligand transition. If one considers that luminescence
commonly originates from the lowest excited state because
of vibrational relaxation and internal conversion between
excited states, the fact that ILCT excited state also cannot be
embodied in the emission spectrum is perfectly
understandable.
300
350
400
450
500
Wavelength (nm)
Fig. 3. Simulated UV/vis absorption spectrum of [Ag(PPh3)2(NMP)]C from
the LanL2DZ (a) and mixed basis sets (b) results. Each absorption peak has
been convoluted by using a Gaussian function of which the width at halve
maximum amounts to 30 nm.
scheme was that structure was optimized with the mixed
basis sets (Ag CEP-121G; C H 3-21G; N P 3-21G(D)) and
upon the optimized geometry excited state was calculated
with the same mixed basis sets. The main geometry
parameters optimized with the mixed basis sets are listed
in Table 2.
For excitation energy studies on small molecules, the
basis sets with diffuse functions need to be used to describe
the outer molecular region. However, for the system of such
large size under study, the main effects are related to
ordinary bound orbitals, which are well described with the
used basis sets. As shown in Fig. 3, two absorption peaks
also exist in the simulated UV/vis absorption spectrum of
[Ag(PPh3)2(NMP)]C, which is close to the experimental
absorption spectrum (294 nm and 342 nm). As listed in
Table 3 and 4, the two calculational schemes gave similar
results for excited states, especially for the four lowest
excited states and the contour plots of frontier molecular
orbitals relative to those excited states are also similar.
Energy and composition of MOs are a good tool to get a
first insight into the nature of excited states. As shown in
Fig. 4, electronic cloud of HOMO is distributed to
dimethylaminophenyl imidazole derivative in the ligand
Table 3
Excitation energy (l, nm), the corresponding oscillator strength (f) and
composition for the lowest excited states calculated with basis set
LANL2DZ
State
11A
l/f
Composition
Assignment
As shown in the plots of HOMO and LOMO involving in
charge-transfer of the lowest excited state 11A, silver only
make disturbed effect on charge-transfer of 11A. The intense
spin-orbital coupling of AgI have little effect on 11A and the
lowest singlet excited state is not easily changed into triplet
excited state. So the title compound exhibit character of
fluorescence at room temperature.
578.6/0.0157
0.70 (HOMO/LUMO)
Metal-per-
turbed ILCT
ILCT
21A
31A
479.7/0.0952
386.6/0.1506
0.69 (HOMO/LUMOC1)
0.68 (HOMO-1/LUMO)
K0.12 (HOMO/LUMOC2)
0.12 (HOMO-1/LUMO)
C0.66 (HOMO/LUMOC2)
MLCT
41A
369.8/0.6438
p/p*

Y.-Q. Wei et al. / Journal of Molecular Structure 751 (2005) 133–138
137
Fig. 4. Frontier molecular orbital diagrams related to the lowest excitations obtained with the second calculational scheme.
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