Highly luminescent SmIII complexes with intraligand charge-transfer sensitization and the effect of solvent polarity on their luminescent properties

Article abstract of DOI:10.1021/acs.inorgchem.5b00331

Samarium complexes with the highest quantum yields to date have been synthesized, and their luminescence properties were studied in 12 solvents. Sensitization via a nontriplet intraligand charge-transfer pathway was also successfully demonstrated in solut

Full text of DOI:10.1021/acs.inorgchem.5b00331

Communication
pubs.acs.org/IC
Highly Luminescent SmIII Complexes with Intraligand Charge-
Transfer Sensitization and the Effect of Solvent Polarity on Their
Luminescent Properties
Wai-Sum Lo, Junhui Zhang, Wing-Tak Wong,* and Ga-Lai Law*
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong SAR,
China
S
* Supporting Information
ABSTRACT: Samarium complexes with the highest
quantum yields to date have been synthesized, and their
luminescence properties were studied in 12 solvents.
Sensitization via a nontriplet intraligand charge-transfer
pathway was also successfully demonstrated in solution
states with good quantum yields.
ensitization via the antenna effect could be achieved
through different mutually nonexclusive pathways, depend-
S
ing on the electronic properties of the chromophore. The most
common pathway is the triplet-mediated energy-transfer
process.1 Upon photoexcitation of the organic antenna, its
singlet excited state undergoes intersystem crossing, where
energy is subsequently transferred from the long-lived triplet
excited state to the accepting state(s) of LnIII. Transition-metal
complexes serving as antennae by their triplet metal-to-ligand
charge-transfer transitions have been vastly researched as
well.2−4 Examples of energy transfer between LnIII ions are
also observed but are relatively less encountered.5 Last but not
least, sensitization via an intraligand charge-transfer (ILCT)
state has also garnered attention because they exhibit excellent
light-harvesting opportunities in the visible-light spectrum.6
We have previously observed,7 with time-resolved lumines-
cence experiments, direct energy transfer from the singlet
excited state of a 2-(N,N-diethylanilin-4-yl)-4,6-bis(pyrazol-1-
yl)-1,3,5-triazine ligand to EuIII, and such an ILCT state-
mediated energy-transfer pathway was further confirmed8 to
effectively quench (>98%) the excited-state energy.
Figure 1. Optimized structure of Sm-1 (left).
Supporting Information, SI), which is expected because the
methyl groups are situated relatively peripherally on the 3 and 5
positions of the pyrazole to exert influence on the coordination
environment.
The luminescence properties were studied in 12 solvents
with increasing dipole moments, including alcoholic, protic, and
nucleophilic solvents. The formation of a hydrogen bond with
and the protonation of the lone pair on the aniline would
hinder the ILCT transition, affecting the chromophore’s
excited-state energy, whereas that of the SmIII G5/2 level
4
would be quenched nonradiatively by resonating with the
vibrational overtones of O−H oscillators of the coordinated
solvent molecules (Figures 2 and 3).12
SmIII, on the other hand, suffers from an intrinsic
disadvantage of having a weaker luminescence intensity because
of a smaller energy gap between the emitting state and the next
lower energy level:9 ΔE(4G5/2→6F11/2) of ca. 7500 cm−1 of
SmIII versus ΔE(5D0→7F6) of ca. 12500 cm−1 of EuIII. The
luminescence quantum yields of SmIII complexes in solution
thus are normally quite weak, in which the highest quantum
yield value of visible emission, to our best knowledge, is 2.7% in
pyridine.10 However, SmIII offers a deeper-red emission (645
nm) than EuIII and is a dual-emitting lanthanide because it
emits in the near-IR (NIR) region as well; therefore; research
into enhancing the luminescence quantum yields of SmIII has
vital potential in a variety of applications.
Our studies show that the broad and structureless ILCT
band appears at different positions in the absorption spectra of
different solvents and, in general, exhibits a bathochromic shift
as the dipole moment of the solvent molecule increases. This
phenomenon arises because the solvent is able to stabilize the
ILCT ground state by dipole−dipole interaction with the
donor−acceptor backbone on the chromophore, leading to a
bigger energy gap.13 The absorption bands peaking at ca. 350
nm are attributed to 2-thenoyltrifluoroacetatonate (tta), where
the peak position and shape are not affected by the solvent
polarities.
For Sm-1−Sm-3, excitation at 390 nm gave the highest SmIII
luminescence intensity in nonpolar solvents (μ ≤ 1.6 D),
The optimized structure, by calculations using LUMPAC11
with a Sparkle/RM1 model, of Sm-1 is shown in Figure 1. Sm-
2 and Sm-3 are structurally very similar to Sm-1 (see the
Received: February 12, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00331
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
S41 in the SI). Further kinetics studies on their UV−vis
absorption properties revealed that the compound was not
stable and the original species already decayed within 5 min,
notably the diminishing of the ILCT band absorption (see the
SI) and the possible appearance of free ligand absorption. The
instability issue was observed in carbon tetrachloride as well, yet
the lifetimes were well-fitted to a monoexponential decay,
indicating slower deterioration. Again, the luminescence
lifetime behaviors are very similar for the analogues, and the
peculiar instability issues are being investigated concurrently
and will be followed up on in our future work.
Luminescence lifetimes in methanol and methanol-d4 were
measured and used to calculate the number of coordinated
solvent molecules (Table 1 and Figure 4). It was found that
Figure 2. Energy-level diagram depicting triplet sensitization by tta as
well as singlet sensitization by the ILCT state of the Sm complexes.
Table 1. Luminescence Lifetimes and Quantum Yields
(Estimated Error of 10% and 15%, Respectively) in the
Visible Region for Sm-1−Sm-3 in Different Solvents
a
b
c
lifetime (μs)
quantum yield (%)
Sm-1 Sm-2 Sm-3
solvent
μ (D)
Sm-1
Sm-2
Sm-3
CCl4
0
134
133
113
103
26.8
64
159
106
93.5
22.4
4.3
4.5
3.9
3.9
4.9
4.2
d
3.9
3.8
3.1
benzene
toluene
CH2Cl2
CHCl3
i-PrOH
THF
0
114
0.31
1.14
1.15
1.66
1.75
1.88
2.69
2.87
3.44
4.1
102
27.8
66.3
22.8
30.7
54.7
37.8
12.7
71.2
28.8
d
21.9
33.4
57.7
97.2
12.3
98.1
28.9
22.4
27.2
58.7
32
0.82
1.1
0.29
1.2
1.4
1.4
0.19
2.3
e
0.65
1.2
EA
1.4
1.4
acetone
CH3OH
CH3CN
DMSO
0.65
0.18
0.50
0.44
0.57
0.14
0.31
0.48
15.9
55.6
25.9
a
b
Reported data are mean values of triplicates. Emission lifetime of the
c
4G5/2 → 6H9/2 transition; λex = 350 nm. Relative to quinine sulfate in
d
0.1 M H2SO4 (λex = 350 nm; Φ = 0.577). Instability upon dilution,
e
inappropriate for measurement. SmIII luminescence dominated by
Figure 3. Emission spectrum in the visible region of Sm-2 in benzene;
ligand fluorescence.
λex = 350 nm. Inset: Integrated luminescence intensities of the 4G5/2
→
6HJ transition of Sm-2 in different solvents according to increasing
dipole moments.
about four methanol molecules coordinated to SmIII, rendering
the drastic difference between the lifetimes. This result may
except for isopropyl alcohol (μ = 1.56 D), which was able to
form hydrogen bonds. As the dipole moment of the solvent
further increased, excitation at 390 nm gave minimal
luminescence intensities and a broad ligand fluorescence
emission band was observed instead (whereas in acetonitrile
clearly recognizable SmIII emission profiles could be observed).
Nevertheless, excitation at 390 nm in nonpolar solvents, despite
displaying maximum SmIII luminescence, does not necessarily
indicate ideal energy transfer because the residual ligand
fluorescence, particularly for chloroform and dichloromethane,
was at intensities comparable to those of the SmIII f−f
transitions.
The luminescence properties do no change much for the
other analogues because modification of the tridentate ligand
has a minimal effect on the energy transfer from tta to SmIII and
does not play a role, if any, in the ILCT transitions.
4
6
The emission lifetimes of the G5/2 → H9/2 transition were
measured, and the lifetimes in nonpolar solvents were longer
than 100 μs, whereas in alcoholic and polar solvents, the
lifetimes were at ca. 30 μs. However, in chloroform,
dichloromethane, and acetonitrile, two distinctive luminescence
lifetimes were observed, i.e., biexponential decay (Figures S39−
Figure 4. Luminescence lifetimes of Sm-1−Sm-3 (left to right) in
selected solvents and their deuterated equivalents.
B
DOI: 10.1021/acs.inorgchem.5b00331
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
shed light into the elucidation of possible species responsible
for biexponential decay.
ASSOCIATED CONTENT
* Supporting Information
Experimental details, characterization, and full photophysical
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
In accordance with the lifetime measurements, the quantum
yields of the polar solvents are much lower than those of the
nonpolar solvents, implying that the radiative lifetime is in a
direct relationship with the overall quantum yield and further
suggesting that the sensitization efficiencies14 of the complex in
different solvents remained similar. Although energy dissipation
toward radiative transitions in the NIR region is not
investigated in this work, it is reasonable to assume that the
lower quantum yields in polar solvents could be attributed to
(1) the (hypsochromic) shift of the excited-state energy of the
AUTHOR INFORMATION
Corresponding Authors
*E-mail: wing-tak.wong@polyu.edu.hk.
*E-mail: ga-lai.law@polyu.edu.hk.
■
Notes
The authors declare no competing financial interest.
4
antenna, rendering less efficient population of G5/2 by
ACKNOWLEDGMENTS
This work is supported by Grant GUC08.
■
thermally promoted back energy transfer, (2) nonradiative
quenching by vibrational overtones of O−H and C−H
oscillators in proximity to the SmIII center, and (3) the loss
of the excited-state energy via dipole−dipole coupling of the
ligand and solvent molecules.
The long luminescence lifetimes and high quantum yields in
the visible region in nonpolar solvents are due to the following:
first, the absence of nucleophilic solvent molecules displacing
the tridentate ligand prevents inner-sphere quenching of
oscillators and, second, nonpolar solvent molecules lack
dipole−dipole interaction with the donor−acceptor structure
of the complexes but rather provide a relatively more rigid
solution-state environment to minimize vibrational quenching
processes.
The absolute quantum yield was also measured for
complexes that were excited at their ILCT band absorption
maximum, which excludes tta excitation, in order to explore
nontriplet sensitization of SmIII luminescence. Sm-2 again
exhibited in the highest quantum yield among all, with 4% in
toluene and 3% in benzene. Sm-1 and Sm-3 averaged a value of
2% in both solvents. It could be concluded that excitation solely
at the ILCT absorption band (410 nm) was able to sensitize
SmIII as well, with luminescence comparable to that of the
conventional triplet-mediated excited-state energy-transfer
mechanism.
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Three SmIII complexes have been synthesized with a
diethylamino moiety, which imparts an ILCT band onto the
chromophore. Proving previously that the ILCT band was able
to sensitize EuIII luminescence via a nontriplet energy-transfer
pathway, we were able to demonstrate in this work that SmIII
luminescence could also be sensitized likewise. High quantum
yields surpassing literature values were also recorded in solution
(highest at 4.9% in benzene). The luminescence lifetimes and
quantum yields were measured in 12 solvents for thorough
investigation and were found, in general, to decrease as the
dipole moment of the solvent increases, suggesting other
nonradiative deactivation dominating the decrease. This work
also showed the possibility of tuning the color of the SmIII
complexes in different polar environments for potential material
applications. Future work on NIR emissions sensitized by the
ILCT band on SmIII as well as NdIII and YbIII complexes is
ongoing, and the unexpected instability issue will also be
addressed in our future work.
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DOI: 10.1021/acs.inorgchem.5b00331
Inorg. Chem. XXXX, XXX, XXX−XXX