Combined ultrafast spectroscopic and TDDFT theoretical studies on dual fluorescence emissions promoted by ligand-to-metal charge transfer (LMCT) excited states of tungsten-containing organometallic complexes

Article abstract of DOI:10.1016/j.cplett.2020.137396

In this work, we first designed and synthesized two tungsten-containing organometallic complexes MeNOON and MeNOOO with novel dual fluorescence emissions. Moreover, the combined femtosecond ultrafast spectroscopy and time-dependent density functional theo

Full text of DOI:10.1016/j.cplett.2020.137396

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Research paper
Combined Ultrafast Spectroscopic and TDDFT Theoretical Studies on Dual
Fluorescence Emissions Promoted by Ligand-to-Metal Charge Transfer
(LMCT) Excited States of Tungsten-Containing Organometallic Complexes
Yue Liang, Yurong Guo, Ye Wang, Song Zhang, Yu Pan, Yanan Shi, Yanan
Wang, Lina Zhu, Bing Jin, Yan Sun, Bing Zhang, Xia Feng, Minghu Yuan,
Haiyuan Wang, Guangjiu Zhao
PII:
S0009-2614(20)30311-0
DOI:
https://doi.org/10.1016/j.cplett.2020.137396
CPLETT 137396
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Chemical Physics Letters
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13 January 2020
20 March 2020
21 March 2020
Please cite this article as: Y. Liang, Y. Guo, Y. Wang, S. Zhang, Y. Pan, Y. Shi, Y. Wang, L. Zhu, B. Jin, Y. Sun,
B. Zhang, X. Feng, M. Yuan, H. Wang, G. Zhao, Combined Ultrafast Spectroscopic and TDDFT Theoretical
Studies on Dual Fluorescence Emissions Promoted by Ligand-to-Metal Charge Transfer (LMCT) Excited States
of Tungsten-Containing Organometallic Complexes, Chemical Physics Letters (2020), doi: https://doi.org/
10.1016/j.cplett.2020.137396
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Combined Ultrafast Spectroscopic and TDDFT Theoretical Studies on Dual
Fluorescence Emissions Promoted by Ligand-to-Metal Charge Transfer
(LMCT) Excited States of Tungsten-Containing Organometallic Complexes
Yue Liang a, Yurong Guo a, Ye Wang c, Song Zhang c*, Yu Pan b*, Yanan Shi a, Yanan Wang a, Lina Zhu a, Bing
Jin c, Yan Sun a, Bing Zhang b, Xia Feng a, Minghu Yuan c, Haiyuan Wang a and Guangjiu Zhao a*
aTianjin Key Laboratory of Molecular Optoelectronic Sciences, National Demonstration Center for
Experimental Chemistry & Chemical engineering Education, National Virtual Simulation Experimental
Teaching Center for Chemistry & Chemical engineering Education, Department of Chemistry, School of
b
Science, Tianjin University, Tianjin 300354, China; State Key Laboratory of Fine Chemicals, School of
c
Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China; State Key
Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and
Mathematics, Chinese Academy of Science, Wuhan 430071, China.
ABSTRACT: In this work, we first designed and synthesized two tungsten-containing
organometallic complexes MeNOON and MeNOOO with novel dual fluorescence emissions.
Moreover, the combined femtosecond ultrafast spectroscopy and time-dependent density
functional theory (TDDFT) method has been performed to investigate the excited-state dynamics
of MeNOON and MeNOOO complexes. It is distinctly demonstrated that dual fluorescence
emissions of tungsten-containing organometallic complexes are originated from the localized-on-
ligand (LL) and ligand-to-metal charge transfer (LMCT) excited states, respectively. Furthermore,
the ultrafast LMCT process has been measured by the femtosecond spectroscopy.
KEYWORDS: Dual Fluorescence; Localized-on-Ligand; Ligand-to-Metal Charge Transfer;
Tungsten-Containing Organometallic; Excited State.
1

Introduction
OLED is a high-brightness [1], wide-visual [2], fully-cured light-emitting device and exhibits
unparalleled advantages: high luminous efficiency and high brightness [3-10]. Light-emitting
materials in OLEDs can be divided into three categories: organic small molecular [11],
organometallic complexes [12-16] and polymer [17-19]. Organometallic complexes have the
advantages of organic matter and well inorganic stability [20]. Organometallic chromophores,
which have potential utilities in optical and electronic applications, have been widely investigated
in recent years [21]. Photophysical investigations of these molecular chromophores are very
helpful for understanding of optical properties and further application in OLEDs of organometallic
materials [22-25].
There have been many reports about luminescent organometallic complexes with palladium,
platinum and iridium as the central metal atom. Tungsten is a transition metal like palladium,
platinum, and iridium, and its electron distribution outside the core is similar to iridium. Tungsten
oxides and sulfides have been widely used in light-emitting devices such as OLEDs due to their
excellent optical properties [26, 27]. It should be noted that tungsten has been used in incandescent
bulbs for decades. However, the tungsten-containing organometallic complexes with novel
luminescence properties have been rarely reported [28-30].
2

Recently, density functional theory (DFT) and time-dependent density functional theory (TD-
DFT) based calculations have been important tools to understand or predict photophysical
properties of organometallic complexes [31-34]. Combining the theoretical methods with
experimental investigations could help to understand the correlation between the ligand and metal
as well as fluorescence properties for a rationalized application design of tungsten-containing
organometallic complexes [35-41]. According to the above principles, we have designed a series
of tungsten-containing organometallic complexes and synthesized two typical complexes
designated as ligand MeNOON and ligand MeNOOO using a symmetric phenol structural
compound containing an imine group as a ligand.
Experimental and Theoretical Methods
Synthesis of Ligand MeNOON. 2,4-dimethylphenol (2.95 g, 24.2 mmol), N,N-
dimethylethylenediamine (1.35 mL, 12.3 mmol) and aqueous solution with a formaldehyde
concentration of 36 (2.5 mL, 33.6mmol) were dissolved in methanol (10 mL) and stirred at room
temperature for 72 h. Put the mixture in the refrigerator overnight. Filtered the mixture and washed
thoroughly with ice cold methanol to give white solid. The solid was further purified by
recrystallization from methanol to afford ligand MeNOON as a white solid (2.73 g, yield 63%).
1H NMR (500 MHz, CDCl3) δ 6.85 (d, J = 20.2 Hz, 2H), 6.69 (s, 2H), 3.59 (s, 4H), 2.57 (s, 4H),
2.33 (s, 6H), 2.23 (d, J = 12.6 Hz, 12H).
3

Synthesis of Ligand MeNOOO. 2,4-dimethylphenol (1.95 g, 16.0 mmol), 2-
methoxyethylamine (0.52 mL, 6.00 mmol) and aqueous solution with a formaldehyde
concentration of 36 (1.2 mL, 16.0 mmol) were dissolved in methanol (6 mL) and refluxed at
75°C for 24 h. After completion of the reaction, the mixture was cooled at room temperature and
filtered. The solid was dissolved in methanol and refluxed at 75°C for 2 h. The mixture was cooled
at room temperature and filtered to give white solid. The solid was further purified by
recrystallization from methanol to afford ligand MeNOOO as a white solid (2.42 g, yield 58%).
1H NMR (500 MHz, CDCl3) δ 8.35 (s, 2H), 6.78 (s, 2H), 6.60 (s, 2H), 3.65 (s, 4H), 3.52 (dd, J =
11.2, 6.1 Hz, 2H), 3.40 (s, 3H), 2.63 (t, J = 5.1 Hz, 2H), 2.13 (d, J = 4.6 Hz, 12H).
Synthesis of Complex MeNOON. To a degassed solution of the intermediate ligand
MeNOON (0.97 g, 2.72 mmol) in dry toluene were added WO2Cl2 (0.80 g, 2.72 mmol) and
pyridine (0.45 mL, 5.92 mmol). The reaction mixture was refluxed for 24 h under N2 atmosphere.
The reaction mixture was evaporated and dissolved in dichloromethane. The reaction mixture was
filtered through sand and the filtrate was collected. The solvent was then removed under reduced
pressure and the yellow residue was dissolved in dichloromethane. The addition of methanol to
the dichloromethane solution led to the precipitation of a yellow solid. The solid was filtered and
dried under vacuum to afford complex MeNOON as a yellow solid (1.01 g, yield 65%). 1H NMR
(500 MHz, CDCl3): δ = 6.96 (d, J = 20.0 Hz, 2H), 6.66 (s, 2H), 4.75 (d, J = 13.8 Hz, 2H), 3.57 (d,
4

J = 13.8 Hz, 2H), 2.76  2.66 (m, 2H), 2.55 (s, 6H), 2.29 (dd, J = 13.5, 7.5 Hz, 2H), 2.19 (d, J =
3.8 Hz, 12H). MS (+ve, m/z): 571.18 [M+].
Synthesis of Complex MeNOOO. The procedure was similar to that described for the
preparation of complex MeNOON except intermediate ligand MeNOOO (0.93 g, 2.72 mmol) was
used instead of ligand MeNOON. Got complex MeNOOO through the above experiment as a
yellow solid (0.86 g, yield 59%). 1H NMR (500 MHz, CDCl3): δ = 6.69 (d, J = 16.6 Hz, 2H), 6.67
(s, 2H), 4.70 (d, J = 13.6 Hz, 2H), 3.77 (s, 3H), 3.66 (t, J = 16.8 Hz, 2H), 3.18 (t, J = 6.0 Hz, 2H),
2.81 (t, J = 6.0 Hz, 2H), 2.18 (t, J = 18.0 Hz, 12H). MS (+ve, m/z): 559.15 [M+].
Scheme 1.
UV-vis absorption spectra were obtained using an Ocean Optics PH-2000-BAL
spectrophotometer at 25°C. The test solvents were n-hexane and methanol. The steady-state
fluorescence was recorded using F-380 fluorescence spectrofluorometer at 25°C. The n-hexane
and methanol were used as the measurement solvents. Femtosecond transient absorption was
recorded using femtosecond transient absorption (Femtoframe Ⅱ) at 25°C.
All quantum chemical calculations were performed using Gaussian 09 program[42]. Ground-
state geometry and the excited states optimization was carried out by DFT method with the Cam-
b3lyp functional. The 6-31G basis set was chosen for nonmetallic elements and LANL2DZ basis
5

sets was selected for tungsten atoms. Geometry optimizations of both the ground state and excited
state were carried out using the DFT/TDDFT method without symmetry constraints [43-45].
Results and Discussion
As shown in Figure 1, the UV-Vis spectra of complexes exhibit absorptions in the UV region
(213–221 and 251–270 nm) with strong absorption intensity (ε ∼ 106 L mol−1 cm−1). One can note
that the position and intensity of the absorption peaks cannot be significantly affected by the
different solvents. In addition, both MeNOON and MeNOOO complexes have dual fluorescence
emissions in the ultraviolet and visible regions, respectively. Moreover, both the two emission
peak positions cannot shift at different excitation wavelength of 218 nm, 260 nm, 266 nm,
respectively. The relevant data are summarized in Table 1.
Figure 1.
Table 1.
By observing the UV-Vis absorption and fluorescence emission spectra of the two complexes,
it is found that the absorption and emission of the complexes MeNOON and MeNOOO are not
significantly different. The difference between the two complexes is that the amine branch of the
6

middle part of the ligand is respectively connected to an imino group and a methoxy group, so the
absorption spectra and emission spectra of two complexes are similar.
DFT/TDDFT theoretical calculations are performed to study the dual fluorescence emissions
mechanism [46]. The ground state configurations of the complexes MeNOON and MeNOOO are
optimized. The data of the two complexes are basically the same, indicating that the differences
caused by the imino group and methoxy group in the ligand have little effect on the configuration
of the complexes.
Figure 2.
The calculated electronic transition energies and corresponding oscillator strengths (f) of the
singlet excited states for complexes MeNOON and MeNOOO are listed in Table 2. It is shown
that the calculated wavelength absorptions for complexes MeNOON and MeNOOO are at 271,
263, 217 and 270, 266, 197 nm respectively, which are almost consistent with the experimental
results. As shown in Table 2, excited state S1, S3 and S14 of complex MeNOON correspond to the
contribution of the HOMO to the LUMO, the HOMO-4 to the LUMO+1 and the HOMO-5 to the
LUMO+1. Excited state S1, S2 and S19 of complex MeNOOO correspond to the contribution of the
HOMO to the LUMO, the HOMO-1 to the LUMO and the HOMO-2 to the LUMO. It can be
clearly noted from the Figure 2 that the charge population of HOMO and LUMO orbitals are
7

dominantly localized on the ligand and some other orbitals are relevant to both the ligand and
metal for complexes. So some excited states could be only localized on ligand (LL) while some
excited states will involve ligand to metal charge transfer (LMCT) character. The LL and LMCT
characters for some involved excited states are also listed in Table 2.
Table 2.
The high-energy fluorescence emission peak at about 292 nm is very close to the S1 state
absorption peak at 266 nm. Consequently, the Stokes shift for the high-energy emission is as small
as 26 nm for both MeNOON and MeNOOO complexes. The small Stokes shift confirms that the
high-energy emission and the S1 state absorption peak are originated from the same excited state
with the LL character. As a result, both the S1 state absorption and high-energy fluorescence
emission are mainly originated from the absorption and fluorescence emission of their ligand for
both MeNOON and MeNOOO complexes.
Figure 3.
It is worth noting that the low-energy fluorescence emission peak has a remarkable redshift
to about 570 nm and the Stokes shift is as large as 304 nm. Both the very large fluorescence spectral
redshift and Stokes shift propose that the low-energy fluorescence emission might be originated
from a new excited state with prominent charge transfer character between ligand and metal. We
8

have successfully optimized the excited state geometric conformation with TDDFT method for
both MeNOON and MeNOOO complexes in methanol solvent and presented the calculated
fluorescence emission value and relevant molecular orbitals in Figure 3a. One can see that the
calculated fluorescence emission value from this excited state is 535 nm and 575 nm, which is in
good agreement with the experimental low-energy fluorescence emission peak in methanol solvent
for MeNOON and MeNOOO, respectively. At the same time, it is distinct that the excited state is
obvious LMCT state from the analysis of relevant molecular orbitals. Therefore, it has been
demonstrated that the tungsten-containing organometallic complexes are directly photoexcited to
the S1 excited state of the LL character and hence could emit the LL fluorescence as shown in
Figure 3b. Subsequently, the ligand to metal charge transfer process will take place and finally
emit the LMCT fluorescence from the stable LMCT excited state.
Figure 4.
The femtosecond transient absorption (fs-TAS) spectroscopy has been performed to
investigate the ultrafast LMCT process upon excitation at 266 nm for MeNOON and MeNOOO
complexes [47]. Time-resolved spectra are recorded in the spectral range 360-660 nm. The
transient absorption spectra of complexes MeNOON and MeNOOO at different times after
excitation are shown in Figure 4. It can be seen from the Figure 4a) and c) that the complex
MeNOON has an S1 excited state absorption at about 365 nm. Combined with the calculated data,
9

it is known that this process is attributed to LMCT. Comparing the fluorescence emission spectra,
it is known that the complex MeNOON has stimulated emission at about 630 nm. For complex
MeNOOO, we can also see the excited state absorption at about 405 nm that process is attributed
to LMCT and the stimulated emission at about 630 nm from Figure 4b) and 4d). These
femtosecond transient absorption results are consistent with the DFT/TDDFT results. According
to Figure 4e) and f), the time constants of the LMCT process are experimentally measured as 453
ps and 959 ps for complexes MeNOON and MeNOOO, respectively.
Conclusion
In summary, the dual fluorescence emission mechanism of tungsten-containing
organometallic complexes MeNOON and MeNOOO was studied through steady-state and time-
resolved spectroscopy experiments and quantum chemical calculations. High-energy emission
peaks can be attributed to the LL state of the ligand. Low-energy emission peaks can be attributed
to the excited states formed by the LMCT process. At the same time, the femtosecond transient
absorption spectrum also confirmed the existence of LMCT process in the excited state. It is
conceivable that as we adjust and improve its ligand structure, the tungsten complex will be as
competitive as other luminescent complexes and have broad application prospects in the field of
OLEDs. At the same time, its emission in the deep ultraviolet region indicates its application
potential in the fields of water sterilization, disinfection and confidential communication.
10

Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 21573229,
21873068, 21422309). We also acknowledges financial support from Double First-Rate and
Peiyang Scholar Projects (Tianjin University), the Open Research Funds of State Key Laboratory
of Bioelectronics (Southeast University), the Frontier Science Project of the Knowledge
Innovation Program of Chinese Academy of Sciences (CAS), State Key Laboratory of Magnetic
Resonance and Atomic and Molecular Physics (Wuhan Institute of Physics and Mathematics of
Chinese Academy of Sciences) and the Project for Excellent Member of CAS Youth Innovation
Promotion Association.
*Corresponding
Authors.
E-mail:
gjzhao@tju.edu.cn;
ypan@dlut.edu.cn;
zhangsong@wipm.ac.cn
Note. The authors declare no competing financial interests.
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18

Figure captions:
Table 1. Spectral data of complexes MeNOON and MeNOOO in n-hexane and methanol.
Table 2. Selected calculated electronic transition energies and correspond oscillator strengths of
the singlet excited state of complexes MeNOON and MeNOOOa.
Figure 1. UV-Vis absorption spectra in methanol, fluorescence emission spectra in n-hexane and
methanol and the structure of complexes MeNOON and MeNOOO.
Figure 2. Calculated frontier molecular orbitals for the singlet states of complexes MeNOON and
MeNOOO.
Figure 3. a) Frontal molecular orbitals of the excited state molecules and calculated and
experimental values of fluorescence emission wavelength of complexes MeNOON and complex
MeNOOO; a) Fluorescence emission mechanism, Abs = Absorbance, LL = Localized-on-Ligand,
LMCT = Ligand-to-Metal Charge Transfer.
Figure 4. Femtosecond transient absorption spectra of a) complex MeNOON and b) complex
MeNOOO; c) temporal evolution for complex MeNOON; d) temporal evolution for complex
MeNOOO; e) experimental (○) and fitted (lines) intensity decay profiles of fs-TAS for complex
MeNOON and f) experimental (○) and fitted (lines) intensity decay profiles of fs-TAS for complex
MeNOOO upon photoexcitation at λ = 266 nm. Solvent contribution was subtracted.
Scheme 1. Synthesis of ligand MeNOON, MeNOOO and complex MeNOON, MeNOOO,
reagents and conditions, ligand MeNOON: 2,4-dimethylphenol, N,N-dimethylethylenediamine,
formaldehyde and methanol, rt, yield: 63%; complex MeNOON: ligand MeNOON, toluene, 120°C;
yield: 65%; ligand MeNOOO: 2,4-dimethylphenol, 2-methoxyethylamine, formaldehyde and
methanol, 75°C, yield: 58%; complex MeNOOO: ligand MeNOOO, toluene, 120°C; yield: 59%.
19

Table 1.
흀풂풃풔.ퟏ
흀풂풃풔.ퟐ
(nm)
260
260
261
흀풂풃풔.ퟑ
(nm)
217
217
217
흀풆풎.ퟏ
(nm)
292
294
292
흀풆풎.ퟐ
(nm)
570
574
570
Complex
MeNOON
MeNOOO
Solution
(nm)
266
266
266
266
n-hexane
MeOH
n-hexane
MeOH
260
218
294
573
20

Table 2.
Absorption
Electronic
transition
Complex
MeNOON
MeNOOO
wavelength/nm
(energy/eV)
271(4.57)
f
Contrib
CI
Character
S0-S1
S0-S3
S0-S14
S0-S1
S0-S2
S0-S19
0.0971
0.1459
0.0516
0.1008
0.1057
0.0964
H→L
H-4→L+1
H-5→L+1
H→L
0.381
0.331
0.402
0.567
0.558
0.577
LL
263(4.72)
LMCT
LMCT
LL
LMCT
LMCT
217(5.71)
270(4.59)
266(4.66)
197(6.30)
H-1→L
H-2→L
aH means the highest occupied molecular orbital (HOMO), and L means the lowest
unoccupied molecular orbital (LUMO), LL = Localized-on-Ligand, LMCT = Ligand-to-
Metal Charge Transfer.
21

Figure 1.
22

Figure 2.
23

Figure 3.
24

Figure 4.
25

N
O
O
O
O
W
N
O
N
NH2
MeOH
72h
Toluene
24h
N
N
NMe2
H
H
WO2Cl2
OH OH
OH
Complex MeNOON
Ligand MeNOON
O
O
O
O
O
W
N
O
N
NH2
MeOH
24h
Toluene
24h
N
O
OMe
H
H
WO2Cl2
OH OH
OH
Complex MeNOOO
Ligand MeNOOO
Scheme 1.
26

Graphical abstract
27

HIGHLIGHT：
Novel tungsten-containing organometallic complexes are synthesized.
Dual fluorescence emissions are originated from localized-on-ligand (LL) and ligand-to-metal
charge transfer (LMCT) excited states.
Femtosecond transient absorption is used to explain charge transfer in organometallic complexes.
28

Credit Author Statement
Yue Liang: Conceptualization, Methodology, Formal analysis, Investigation, Data Curation,
Writing - Original Draft, Writing - Review & Editing, Visualization
Yurong Guo: Methodology, Software, Data Curation, Writing - Original Draft, Visualization
Ye Wang: Investigation, Data Curation, Validation, Resources, Methodology
Song Zhang: Resources, Supervision, Project administration, Funding acquisition, Methodology
Yu Pan: Investigation, Resources, Supervision, Project administration, Funding acquisition,
Methodology
Yanan Shi: Software, Validation, Formal analysis, Investigation, Methodology
Yanan Wang: Data Curation, Validation, Resources, Formal analysis, Data Curation
Lina Zhu: Supervision, Methodology, Project administration, Visualization
Bing Jin: Supervision, Methodology, Project administration, Visualization, Funding acquisition
Yan Sun: Methodology, Supervision, Project administration, Visualization
Bing Zhang: Resources, Supervision, Project administration, Funding acquisition
Xia Feng: Conceptualization, Methodology, Supervision, Project administration
Minghu Yuan: Investigation, Data Curation, Methodology, Resources
Haiyuan Wang: Visualization, Methodology, Supervision, Project administration
Guangjiu Zhao: Conceptualization, Writing - Original Draft, Writing - Review & Editing
Resources, Supervision, Project administration, Funding acquisition
29