Visible light sensitized near-infrared luminescence of ytterbium: Via ILCT states in quadruple-stranded helicates

Article abstract of DOI:10.1039/c9dt00614a

Visible light excitation is especially important for the NIR emitting materials that are employed in biochemistry and cell biology, since living tissues are generally damaged under UV light irradiation. Here, two new bis-β-diketones, L¹ and L² (L¹ = 4,4′-di(4,4′,4′′-trifluoro-1,3-dioxobutyl)-triphenylamine; L² = 4-(N,N-dimethylamino)-4′,4′′-di(4,4′,4′′-trifluoro-1,3-dioxobutyl)-triphenylamine), featuring intra-ligand charge transfer (ILCT) excited states, have been designed and prepared for the synthesis of a series of anionic quadruple-stranded dinuclear helicates (HNEt₃)₂[M₂L¹₄] and (HNEt₃)₂[M₂L²₄], where M = Yb, Gd and Y. ¹H NMR and electrospray mass spectrometry confirm the formation of single complex species in solution. According to the comprehensive spectral characterization and in combination with quantum chemical modelling, the ILCT feature of L¹ and L² is confirmed. The presence of ILCT allows the use of lower energy excitation wavelengths in the visible spectral region to sensitize ytterbium NIR luminescence. In (HNEt₃)₂[Yb₂L²₄], the introduction of strong electron donating N,N-dimethyl leads to a remarkable red shift of the absorbance edge to 560 nm compared with that in (HNEt₃)₂[Yb₂L¹₄] at 450 nm. Upon excitation with blue light, the two ytterbium helicates show excellent NIR luminescence in the range of 900-1100 nm, with the luminescence quantum yields reaching 1.1% for (HNEt₃)₂[Yb₂L¹₄] and 1.5% for (HNEt₃)₂[Yb₂L²₄] in CH₃CN. Luminescence mechanism experiments show that the ILCT singlet path and classic triplet state path together participate in sensitizing Yb(iii) ion NIR luminescence.

Full text of DOI:10.1039/c9dt00614a

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DOI: 10.1039/C9DT00614A
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ARTICLE
Visible light sensitized near-infrared luminescence of ytterbium
via ILCT states in quadruple-stranded helicates
Zihan Zhang, Yanyan Zhou, Hongfeng Li,* Ting Gao and Pengfei Yan*
Received 00th January 20xx,
Accepted 00th January 20xx
The visible light excitation is especially important for the NIR emitting materials that are employed in biochemistry and cell
1
2
1
DOI: 10.1039/x0xx00000x
biology, since living tissues are generally damaged under UV light irritation. Here, two new bis-β-diketones, L and L (L =
2
4
,4'-di(4,4',4''trifluoro-1,3-dioxobutyl)-triphenylamine; L = 4-(N,N-dimethylamino)-4',4''-di(4,4',4''trifluoro-1,3-dioxobutyl)-
www.rsc.org/
triphenylamine ) featuring intra-ligand charge transfer (ILCT) excited state, have been designed and prepared for the
3 2 2 4 3 2 2 4
synthesis of a series of anionic quadruple-stranded dinuclear helicates (HNEt ) [M ] and (HNEt ) [M ], where M = Yb,
L1 L2
1
Gd and Y. The H NMR and electrospray mass spectrometry conform the formation of single complexes species in solution.
According to the comprehensive spectral characterizations, and in combination with quantum chemical modelling, the ILCT
1
2
feature of L and L are confirmed. The prescence of ILCT allows the use of lower energy excitation wavelength in the visible
spectral region to sensitize ytterbium NIR luminescence. In (HNEt [Yb ], the introduction of strong electron donating
L2
N,N-dimethyl leads to a remarkble red shift of the absorbance edge to 560 nm compared with that in (HNEt [Yb ] at 450
L1
nm. Upon excitation with blue light, the two ytterbium helicates show excellent NIR luminescence in the ranges of 900-1100
nm, with the luminescence quantum yields reach to 1.1% for (HNEt [Yb ] and 1.5% for (HNEt [Yb ] in CH CN.
L1 L2
3
)
2
2
4
3
)
2
2
4
3
)
2
2
4
3
)
2
2
4
3
Luminescence mechanism experiment show the ILCT singlet path and classic triplet state path together participate in
sensitizing Yb(III) ions NIR luminescence.
Recently, the direct lanthanide sensitization from intra-ligand
charge transfer (ILCT) excited state appears as an alternative
pathway resulting in a red-shift of excited wavelength far in the
Introduction
Near-infrared-luminescent lanthanide complexes have potential
applications in bioanalysis and cell image areas due to their
characteristic line-like emission and long excited state lifetime.1
Near-infrared luminescence has less photons scatter than visible
light emission and can penetrate deep into tissues in biological
systems, which can improve detection sensitivity of bioanalysis
visible.8 For instance, Ouahab et al. have shown that
tetrathiafulvalene (TTF) derivatives featuring strong ILCT
allowed the use of lower energy excitation wavelength in the
9
visible up to 600 nm for NIR emitters. However, a major
drawback of the complexes with ILCT character are their
instability in solution resulting from the strong electron-
withdrawing groups in the ligands, which could reduce the
charge density on the coordinated atoms.10 This decrease in
stability would significantly lower the luminescence quantum
yields of lanthanide complexes and limit their applications.
Therefore, it is still a challenge to design and synthesize ligands
that have strong ILCT character and can form high stable
complexes with lanthanide ions.
2
and biological imaging resolution. The forbidden nature of the
f-f transition in trivalent lanthanide ions results in a low
absorbance coefficients, and in order to achieve bright emission,
a suitable antenna is commonly used to absorb excitation light
3
and transfer energy to the lanthanide certers. However, one of
challenges for designing luminescent lanthanide complexes,
particularly those aimed at bio-assays and bioimaging, is to shift
excitation wavelength from UV to visible range, since living
4
tissues are usually damaged by UV light. Presently, several
approach have been proposed towards this goal, for example
5
excitation through d-block chromophores, by multiphoton
6
excitation, or via triplet states of ligands with low-energy π–π*
transition.7
a. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of
Education, P. R. China; School of Chemistry and Materials Science, Heilongjiang
University, Harbin 150080, P. R. China. E-mail: lhf4612@163.com;
yanpf@vip.sina.com
b. †Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI:10.1039/x0xx00000x
1
2
3 2 2 4
Scheme 1 Synthetic routes of L , L , and their complexes (HNEt ) [M ] and
L1
2
3 2 2 4
(HNEt ) [M L ].
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1
Metal helicates, a model complex of DNA, with two or more chosen as substitute for H NMR experiments. As shown in Fig.
View Article Online
metal centres wrapped by two to four ligand strands have been 1c, only one set of shifted resonances are DobOsI:e1r0v.1e0d3a9/nCd9DaTt0o0ta6l14oAf
extensively developed due to their glamorous structure and seven resonances match the C
2
symmetric feature of ligands L1
attractively optical, magnetic properties.11 Piguet has carried out and L and time-averaged C
2
symmetry of complexes 1 and 2 in
4
a plenty of work on lanthanide helicates, and revealed that acetonitrile, respectively. Additionally, the integral area ratios of
helicates generally show excellent thermodynamic stability in H in four stranded helicates to two counter cations (protonated
solution.12 Similarly, several lanthanide dinuclear helicates triethylamine) also support the formation of quadruple-stranded
based on bis-β-diketones reported by us also showed higher NIR helicates with the formula of (HNEt
luminescence quantum yields and relatively large absence of resonances of free ligands in the spectra of complexes
thermodynamic stability in solution.13
also reveal the stability and integrity of the helicates on the NMR
Herein, two C -symmetric bis-β-diketones L and L are time scale.
3 2 4
) [YL ]. Moreover, the
1
2
2
designed for building dinuclear quadruple-stranded helicates
with Yb(III) ion, which are comprised of rigid triphenylamine
core and two containing strong electron-withdrawing CF
3
substituent 1,3-diketones, respectively. The “push-pull”
character in molecules produces low energy ILCT electronic
states, where the triphenylamine units acts as an electron donor,
3
and the CF substituent 1,3-diketones as an electron acceptor. As
expect, the low energy ILCT states make the lowest energy
absorption edges of helicates extend to visible regions at 450 nm
L1 L2
for (HNEt ) [Yb ], and 560 nm for (HNEt ) [Yb ]. Upon
3 2 2 4 3 2 2 4
excitation with blue light, the helicates show higher
luminescence quantum yields, with the QY values are 1.1% for
1
3
and 1.5% for 2 in CH CN.
Results and discussion
Synthesis and characterization of the ligands and complexes
The synthesized procedures of the ligands and their
corresponding complexes are outlined in Scheme 1. First,
triphenylamine
or
4-(N,N-dimethylamino)phenyl
diphenylamine are reacted with acetic anhydride using standard
Friedel-Crafts reaction to give two diacetyl triphenylamine
intermediates, which are then converted to the final ligand L , L2
by a traditional Claisen condensation reaction with ethyl
1
1
trifluoroacetate. H NMR confirm the synthesis and purity of the
intermediates and ligands (Figures S1−S4). The Ln(III)
quadruple-stranded helicates are obtained by reaction of ligands
1
2
(
3
L , L ) with the corresponding LnCl salts (Ln = Yb, Gd and Y)
in a 2:1 ratio with triethylamine as base. ESI-TOF-MS analysis
show corresponding doubly charged peaks for the quadruple-
stranded helicates species [Yb
Interestingly, (HNEt [Yb
which correspond to three different species with [L − H] ,
2
L1
4 2 4
]2− or [Yb L2 ]2− (Figure 1d).
)
3 2
2 4
L ] exhibit three clusters of peaks
−
2 4 2 4
Yb L L + H] charge states. The assignments are
]2− and [Yb
−
[
verified by comparing the corresponding isotopic distributions
of the simulated with experimental results. The corresponding
1
2
. (a) 1H NMR assignment of ligands; (b)
Fig. 1 Self-assembly of L , L with LnCl
3
yttrium and gadolinium complexes, (HNEt
HNEt [Gd ], are also analyzed for comparison (Figure
S5−S8). Based on mass spectrometers (ESI-MS) analysis, it
3 2 2 4
) [Y L ] and
1
Sparkle/PM6 ground state geometries of the complexes 1 and 2; (c) H NMR (400
MHz) spectra of free ligands L , L and their self−assembled yttrium complexes
(
)
3 2
2 4
L
1
2
L1
L2
(HNEt
) [Y
3 2 2
4
] and (HNEt
3
)
2
[Y
2
4 3
] (CD CN); (d) ESI-TOF-MS of helicates 1 and 2
confirms the formation of quadruple-stranded helicates with with insets showing the observed (Obs.) and simulated (Sim.) isotopic patterns of
2 4
the anion [Y L
]2− peaks.
three kinds of lanthanide ions.
To examine the thermodynamic stability and the purity of the
1
complexes in solution, the H NMR spectroscopy are performed
To obtain better structural insight in this assembly, a
1
on the helicates. Because of the poor resolution of the H NMR molecular model is built with the parameters Sparkl/PM6 using
for Yb(III) complexes, the isostructural Y(III) complexes are MOPAC 2016 program implemented in the LUMPAC 3.0
2
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software, and this routine converges to the quadruple-stranded Based on these transition distributions, the low enerVgieywtrAartnicsleitOinolinnes
helicates shown in Fig. 1b. In a helicate, each Yb3+ ion is eight- are mainly attributed to an intramoleculaDrOIC: 1T0.1f0r3o9m/C9thDeT0d0o6n14oAr
coordinated to O atoms from four β-diketonate ligands. The TPA fragment to the acceptor β-diketone units. Finally, the
triphenylamine (TPA) cores adopts propeller-like conformation, highest-energy bands at 233 nm and 230 nm are attributed to the
resulting in two pendant diketone arms wrap two metal centres π−π* HOMO − 6 → LUMO, HOMO − 6 → LUMO intra-
forming helical twistification. Due to the absence of chiral diketone transitions and π–π* HOMO → LUMO + 6 intra-TPA
element in ligands, both helicate will present as two pairs of transitions.
racemes with homochiral Yb3+ centers, either a left-handed Λ, Λ
Similar orbits transitions can also be observed in L with a
strong electron-donor N,N-dimethyl group. The band (λ = 372
nm, f = 0.6718) is attributed to ILCT transition from the π−π*
HOMO (N,N-TPA) → LUMO (DK), while the transition (λ =
2
or right-handed ∆, ∆ configuration (Fig. S9).
Electronic Spectroscopy and TD-DFT Calculations
1
2
UV-Vis absorption spectra of L and L and their corresponding
Yb(III) complexes in CH CN are shown in Fig. 2. The absorption
330 nm, f = 0.3036;) is attributed to HOMO → LUMO + 1,
3
HOMO → LUMO CT transitions, respectively. The highest
energy bands at λ = 258 nm and λ = 229 nm are corresponding
to the HOMO − 7 → LUMO, HOMO − 6 → LUMO + 1 π−π*
transitions in β-diketone moieties. Thus, the theoretical
modelling prove that charge transfer does indeed take place
between triphenylamine units as an electron donating group and
the β-diketone moieties acting as the accepting group.
spectra exhibit one broad, intense absorption bands with a
maxima at around 390−400 nm and a shoulder peak at about 330
nm. It is noteworthy that the molar extinction coefficient ratios
(
ɛ
L
/ɛ
H
) of low energy band (ɛ
/ɛ
L
) to high energy band (ɛ
H
) show
H
= 1.5, 1.2 in L , L to 3.0, 1.7 for
1
2
significant increase from ɛ
L
that in 1 and 2, respectively. This obvious change indicates that
the coordination of Ln(III) ions heavily influence electron
transition distributions and probabilities in chromophores.
1
2
Fig. 2 UV−Vis absorption spectra of L , L and their corresponding complexes 1 and 2 in
CH
CN (c = 1.0 × 10−5 M for ligands; c = 2.5 × 10−6 M for complexes).
3
To gain further insight into the transition, the corresponding
electronic structure calculations using TD-DFT are also
performed using the Gaussian 09 package. For comparing with
the calculated results, the experimental absorption curves in the
range of 280−540 nm have been fitted into three bands by
Gaussian deconvolution (Figure S10). The calculated absorption
spectra reproduce the experimental curves (Figure S11). A
summary of calculation is given in Table 1, and their Fig. 3 Molecular orbital diagrams of L and L2
1
.
corresponding molecular orbitals are shown in Fig. 3. According
to the calculation, the lowest energy transitions band (λ = 358
Generally, the introduction of intense electron-donating group
1
nm, f = 0.2212 for L ) is attributed to π−π* charge transfer (CT) and/or electron-withdrawing group will facilitate the ILCT
corresponding essentially to the HOMO (TPA) → LUMO (DK), transition from donor to acceptor. Compared with the ligands,
HOMO − 3→LUMO + 1 transition and intra-diketone π−π* the absorption bands attributed to ILCT transitions of complexes
transition HOMO − 2 (DK) → LUMO (DK) (TPA = 1 and 2 show the larger intensity ratios of ɛ /ɛ , which can be
L
H
triphenylamine; DK = diketone units). The HOMO is a π orbital rationalized by Lewis acidity of Yb(III) ion. It helps increase
centered on the TPA fragment whereas the LUMO, LUMO + 1 electron withdrawing capacity of acceptor. In addition, the strong
is centered on the diketone moiety. The next absorption band at electron-donor N,N-dimethyl group red-shifts the maximum
3
50 nm (f = 0.5344) and 318 nm (f = 0.2785) are mainly absorption edges from 440 nm for 1 to 520 nm for 2.
attributed to charge transfer (CT) HOMO → LUMO, HOMO →
To further confirm the formation of the ILCT state, the
LUMO + 1 transition, but with a higher oscillation intensities. spectral dependence of ligand on solvent polarity is studied,
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because intramolecular charge transfer processes usually well known that solvatochromism is the charactVeierwisAtritcicleoOfnlainne
DOI: 10.1039/C9DT00614A
generate large Stokes’ shifts of the emission band in polar ILCT state. Moreover, a good correlation of ligand luminescence
14
1
2
solvents. The fluorescence spectra of L and L show red shifts maximum versus Reinhardt’s empirical solvent polarity
of 92 nm and 64 nm of emission maximum as solvent polarity parameter E
T
(30) also indicates the presence of ILCT transition
increases from n-Hexane to dichloroethane (Figure S12). It is (Figure S13).15
1
Table 1 Most Important TD−DFT Calculated Excitations Energies Associated with an Oscillator Strength f > 0.20 and Major Assignment of Electronic Transitions of L ,
2
and L .
Exptl. energy (nm)
426
Calcd. energy (nm)
358
Oscillator strengths
0.2212
Location of transition
Assignment
Nature of transitions
L1
ILCT
ILCT
DK
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
π → π*
HOMO → LUMO (0.3465)
HOMO – 3 → LUMO + 1 (0.2278)
HOMO – 2 → LUMO (0.3083)
HOMO → LUMO (0.6750)
3
3
2
82
31
68
350
318
233
0.5344
0.2785
0.3131
ILCT
ILCT
DK
HOMO → LUMO + 1 (0.9369)
HOMO – 6 → LUMO (0.3363)
HOMO → LUMO + 6 (0.2142)
HOMO – 6 → LUMO (0.2138)
HOMO – 4 → LUMO (0.2055)
HOMO – 6 → LUMO (0.3363)
HOMO → LUMO (0.2940)
TPA
DK
ILCT
DK
2
30
0.4228
ILCT
ILCT
ILCT
TPA
DK
L2
443
372
330
258
0.6719
0.3036
0.2424
0.5635
HOMO → LUMO (0.9643)
3
2
99
72
HOMO → LUMO + 1 (0.9672)
HOMO → LUMO + 4 (0.6425)
HOMO – 7 → LUMO (0.4963)
HOMO – 6 → LUMO + 1 (0.2470)
2
29
DK
Photoluminescence Properties of Complexes
The NIR-luminescent properties of the Yb(III) complexes are
investigated in CH CN at room temperature. The excitation
spectra of complexes are recorded by monitoring the maximum
emission at 1042 nm (Fig. 4). A broad bands in the range of 300–
Upon excitation with the maximum band at 400 nm (Fig. 4),
complexes 1 and 2 show the characteristic emission of Yb(III)
ions corresponding to the 2F5/2 → 2F7/2 transition at 900–1200 nm.
Interestingly, two complexes show similar spectra pattern, but
different luminescent intensity. Since the spectral splits are
related to symmetry of ligand fields,16 the consistence of the
splits indicates the Yb(III) ions in two complexes have similar
coordination environment and symmetry.
3
5
00 nm with the maxima at about 380 nm and 415 nm are
observed, which well match the corresponding absorption
spectra confirming that the energy transfer takes place from the
ligands to Yb(III) ions. To our delight, most parts of the
excitation bands localize in blue light region (420–500 nm) due
to the presence of ILCT state in ligands. From the excitation
wavelength dependent emission spectra, Yb(III) ion present
obvious NIR emission with the excitation wavelength in visible
region (Fig. 14). The visible light excitation is especially
important for the NIR emitting materials employed in
biochemistry and cell biology, since living tissues are sensitive
to UV irritation.
In the same concentration of complexes in CH
complex 2 displays about 1.3 times higher signal intensity than
complex 1. Taking the Yb(TTA) (H O)
as reference,17 the
values of emission quantum yields (Φtot) of two complexes in
CH CN are calculated to be 1.1% for 1 and 1.5% for 2,
3
CN, the
3
2
2
3
respectively. The values are obviously higher than the most of
Yb(III) complexes with H-containing ligands.18 As reported
previously by us, the helical structure can effectively depress the
nonradiative transition caused by high-energy oscillators in
ligands, and saturated eight coordination geometries can also
decrease the probability of solvent entering the metal centers.13b
In luminescent lanthanide complex, the luminescence quantum
yield (Φtot) is determined by two factors: (1) the efficiency of the
energy transfer (ηET); (2) and the intrinsic quantum yield (ΦLn
)
of the lanthanide luminescence (eqn. 1)
훷푡표푡 = 휂퐸푇훷퐿푛 (1)
According to this equation, the large Φtot value of lanthanide
center must originate from the improved ηET or ΦLn, or the both.
The intrinsic quantum yields of Yb(III) ion luminescence could
be estimated using the equation 2, after the calculation of the
radiative lifetime (τrad) from the absorption spectra of Yb(III) ion
Fig. 4 Excitation (left) and emission (right) spectra of 1 and 2 in CH
Excitation spectra monitoring the 2F5/2 → 2F7/2 transition at 1042 nm).
CN (c = 2.5 × 10−6 M;
3
4
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in complexes (Figure S15) with a modified Einstein equation of a time delay (100 μs), the high energy bands inV4ie4w0A−rti5cl0e0Onnlimne
eqn. 3):19
for 1 disappears and is consequently assigned as arising from a
DOI: 10.1039/C9DT00614A
(
1
ππ* state. After eliminating this interference, the triplet state
푘
푟
휏
표푏푠
3
1
2
–
훷
퐿푛
=
=
(2)
energy levels ππ* of L and L are calculated to be 18 587 cm
푘
푟
+ 푘푛푟
휏
푟푎푑
1
and 16 313 cm–1 by referring to their maximum emission
2
2
푚
2
8
휋푐푛 푣 (2퐽 + 1)
wavelengths, which are both higher than the energy level of F5/2
,
1
=
푘
푟
= 2303 ×
∫휀(푣)푑푣
(3)
–1
3
2
10 250 cm of Yb(III) ion. The energy gap ΔE ( ππ*– F5/2) is 6063
cm–1 for 2 and 8337 cm–1 for 1. Generally, the lower energy gap is
favored to the energy transfer from high energy level to low energy
state.24 Thus, it is rationalized to explain the higher energy transfer
efficiency of 2 than that in 1.
휏
푟푎푑
푁
퐴
(2퐽’ + 1)
∫
푣 ∙ 휀(푣)푑푣
푚
푣 =
∫
휀(푣)푑푣
A
Where, N is Avogadro’s number, c is the speed of light in
Due to the presence of ILCT state in ligands, the participation of
ILCT states in sensitizing Yb(III) ions NIR luminescence are put into
consideration. To verify this, the fluorescence spectra of their
corresponding Gd(III) complexes are recorded and compared
with Yb(III) complexes 1 and 2 (Figure S18). The fluorescent
intensities of ligands in Yb(III) complexes show an obvious
decreases than that in Gd(III) complexes. Due to the same heavy
atom effect and the absence of energy transfer from ligand to
Gd(III) ions, the decreases in emitting intensities should attribute
to the energy transfer from the ILCT states to 2F5/2 energy level
of Yb(III) ions. In addition, it can be observed that the decrease
of emission intensity in 2 is more obvious than that in 1. Based
on the integration areas of emission peaks, the energy transfer
centimeters per second, J and J' are the quantum numbers for the
ground and excited states, respectively. ∫(푣)푑푣 is the integrated
spectrum of the f-f transition, 푣
transition, and n = 1.3456 for CH CN solution. In terms of these
푚
is the barycenter of the
3
equations, the radiative lifetime (τrad) are calculated to be 399 μs
and 499 μs for 1 and 2, respectively.
The observed lifetimes (τobs) are determined by monitoring
the emission decay curves within the 2F5/2 → 2F7/2 transition at
1
042 nm. Typical decay profiles of 1 and 2 are shown in Figure
S16. The two decay curves are well-reproduced by single-
exponential functions, indicating that only one species exists in
the excited state of the complexes. The lifetimes of 1 and 2 are
found to be 14.7 μs and 15.4 μs, respectively. While the lifetime
values increase to 14.7 μs and 15.4 μs, following the temperature
decrease to 77 K. This could be easily understood because the
low temperature decrease nonradiative transition caused by
thermal vibration of molecules. With the calculated radiative
lifetimes (τrad) and the observed lifetimes (τobs), the intrinsic
quantum yields (ΦLn, eqn.1) are found to be 3.7% and 3.1% for
1
efficiency from ππ* ILCT emission to Yb(III) excited state are
estimated to be 28% for 1 and 43% for 2. The higher energy
transfer efficiency for 2 should attribute to the lower ILCT state
1
–1
energy level ( ILCT*, 19 231 cm ) than that for 1 with 22 727
–1
cm , which are calculated from the maximum wavelengths of
UV-Vis absorbance edges of Yb(III) complexes.
On the basis of above experimental results, it can be
concluded that the ILCT state and triplet state together
participate in energy transfer pathway. The lower energy gap
1
and 2, respectively. With the observed and intrinsic quantum
yields, the energy transfer efficiencies (ηET, eqn. 1) are found to
be 30.6% and 47.6% for 1 and 2, respectively. From above
results, it can be observed that the higher luminescence quantum
yield of complex 2 are contributed from the larger sensitised
efficiency (ηET2, 47.6% > ηET1, 30.6%), because the intrinsic
quantum yields are not much difference between them. In
lanthanide luminescence complexes, the energy-level matching
between the excited states energy levels of ligands and metal ions
are key for realizing effective energy transfer. Several different
excited states of chromophores have been exploited for
populating the excited states of Ln(III) ions, such as singlet
states,20 triplet states,21 intra-ligand transfer state (ILCT), and
1
3
between excited states of ligands (regardless of ILCT* or ππ*)
and 2F5/2 energy level of Yb(III) ion are favored to improve the
sensitization efficiency and increase the luminescence quantum
yields of lanthanide complexes.
Conclusions
In summary, two new Yb(III) quadruple-stranded helicates
based on bis-β-diketone ligand are developed, which show
visible light sensitizing NIR luminescence upon excitation with
low-lying ILCT states of the ligands. TD-DFT calculation
combine with the polarity dependence of emission spectra
confirm the ILCT behavior in ligands. The existence of ILCT
state red-shift the absorption wavelength to the visible region,
22
2
3
metal-to-ligand charge transfer states (MLCT). The triplet state
sensitization is the usual pathway due to the heavy atom effects
of lanthanide ions, which will facilitate the intersystem crossing
from singlet to triplet state. The triplet state energy levels can be
easily estimated from the emission spectra of their Gd(III)
complexes at 77 K. Because the lowest excited energy level of
Gd3+ ion (6P7/2) is too high to accept energy from ligands, the
triplet state energy level of ligand is not significantly affected by
the Gd(III) ion. As shown in Figure S17, the complexes
2
especially in the case of L , the lowest energy absorption edge
1
further red-shift about 100 nm compared with that in L due to
the introduction of intense electron donor N,N-dimethyl. Upon
excitation with blue light at 420 nm, the two complexes show
high luminescence quantum yields of 1.1% for 1 and 1.5% for 2.
The helical structure and saturated eight coordination geometries
effectively depress the nonradiative transition caused by high-
energy oscillators in ligands, and preventing the solvent as H O
2
from coordinating to metal centers. Luminescence mechanism
experiment show the ILCT singlet and classic triplet state paths
3 2 2 4 3 2 2 4
HNEt ) [Gd ] and (HNEt ) [Gd ] display a broad
L1 L2
(
emission bands in the range of 450–720 nm at 77 K. To avoid
possible emission from residual fluorescence of ligands, the
time-resolution experiments are performed. Upon enforcement
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1−3 | 5
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Journal Name
are both responsible for sensitizing Yb(III) ions NIR 10 (to increase the SCF convergence criterion) aVniedw XArtYiclZe O(nflioner
DOI: 10.1039/C9DT00614A
luminescence. The enhanced luminescent quantum yield of 2 is Cartesian coordinates).
attributed to the larger energy transfer efficiency from ILCT state
to metal centers and lower energy gap between ππ* of L and
2F5/2 energy level of Yb(III) ion. In a word, the synthesis of 4,4'-diacetyl triphenylamine
3
2
Synthetic procedures
helicate based on ligand featuring intra-ligand charge transfer To a stirred solution of triphenylamine (0.98 g, 4.00 mmol) in 30
(ILCT) excited state is a valuable approach for enhancing the mL dry nitrobenzene, acetyl chloride (0.66 g, 8.40 mmol) and
lanthanide ions NIR luminescence with visible-light excitation.
3
anhydrous AlCl (1.12 g, 8.40 mmol) dissolved in 20 mL dry
nitrobenzene were added dropwisely. The reaction was carried
out at room temperature for 12 hours and then poured into 150
mL ice-water. The resulting organic layers were combined and
Experimental
dried by anhydrous Na
under reduced pressure to give the crude product, which was
2 4
SO . The solution was concentrated
Materials and instruments
The commercially available chemicals are analytical reagent purified by recrystallization from n-hexane to afford a yellow
grade and used without further purification.
bulk crystals (1.04 g, 79%). Anal. Calc. for C H NO (329.14):
Elemental analyses were performed on an Elementar Vario EL C, 80.22; H, 5.81; N, 4.25; O, 9.71. Found: C, 80.13; H, 5.88; N,
22
19
2
1
1
cube analyzer. The H NMR spectra are recorded on a Bruker 4.21; O, 9.78. H NMR (DMSO-d 400 MHz) δ 3.31 (s, 6H), 7.06
6
Avance III 400 MHz spectrometer. Electron ionization (EI) and (d, J = 8.8 Hz, 4H), 7.16 (d, J = 7.4 Hz, 2H), 7.27 (t, J = 7.4 Hz,
Electrospray time-of-flight (ESI-TOF) mass spectra were 2H), 7.44 (t, J = 8.9 Hz, 1H), 7.90 (d, J = 8.4 Hz, 4H). EI-MS
recorded on Agilent 5975N and Bruker maXis mass m/z 329.14 M
spectrometers, respectively. UV-Vis-NIR spectra were recorded 4,4'-di(4,4',4''trifluoro-1,3-dioxobutyl)-triphenylamine (L
+
.
1
)
on a Perkin-Elmer Lambda 25 spectrometer. Excitation and A mixture of sodium methoxide (1.08 g, 20 mmol) and ethyl
emission spectra were measured with an Edinburgh FLS 920 trifluoroacetate (2.84 g, 20.00 mmol) in 40 mL dry DME
fluorescence spectrophotometer. Luminescence lifetimes were (ethylene glycol dimethyl ether) was stirred for 10 min, followed
recorded on a single photon counting spectrometer from by the addition of 4,4'-diacetyl triphenylamine (2.77 g, 8.40
Edinburgh Instrument (FLS 920) with a microsecond pulse lamp mmol), which was further stirred at room temperature for 24 h
as the excitation sources. The data were analyzed by software (Scheme 1). The resulting mixture was poured into 100 mL ice-
supplied by Edinburgh Instruments. The luminescence quantum water and acidified to pH = 2–3 using hydrochloric acid (2.0 M),
yields of the Yb(III) complexes were measured in CH
room temperature and cited relative to a reference solution of Recrystallization from acetone gave orange needle crystals (4.16
Yb(tta) (H O) ] (Φ = 0.35%), and are calculated according to g, 95%). Anal. Calc. for C H F N O (521.11): C, 59.89; H,
the well-known equation:
3
CN at the resulting orange precipitate was filtered and dried in vacuum.
[
3
2
2
26
22
6
2
4
3.29; N, 2.69; O, 12.27. Found: C, 59.84; H, 3.34; N, 2.66; O,
1
1
2.25. H NMR (DMSO-d
= 8.8 Hz 2H), 7.21(d, J = 7.6 Hz, 2H), 7.34 (t, J = 7.5Hz, 1H),
.46 (t, J = 7.8 Hz, 2H), 7.95 (d, J = 8.8 Hz, 4H), 15.4 (br, 2H).
ESI-MS m/z 522.11 [M + H]+
6
400 MHz) δ 6.71 (s, 2H), 7.15 (d, J
n2퐴푟푒푓퐼
∅
풐풗풆풓풂풍풍
=
∅풓풆풇
2
푛푟푒푓 퐴퐼푟푒푓
7
where n, A, and I denote the refractive index of solvent, and the
absorbance at the excitation wavelength, and the area of the
emission spectrum respectively. Φref represents the quantum
4
4
-(N,N-Dimethylamino)-4',4''-diacetyltriphenylamine
-(N,N-Dimethylamino)phenyl diphenylamine was firstly
prepared according to the process described in the literature. In
a three necked flask were placed 4-bromo-N,N-dimethylaniline
9.00 g, 0.045 mol), diphenylamine (7.61 g, 0.045 mol), sodium-
27
3 2 2
yield of the standard [Yb(TTA) (H O) ] solution. The subscript
ref denotes the reference, and the absence of a subscript implies
an unknown sample. The estimated errors are ±10% for the
quantum yields and ±5% for the lifetime determinations.
(
t-butoxide (6.48 g, 0.068 mol), Pd(OAc) (0.20 g, 0.90 mmol),
2
tri(tert-butyl)phosphine (0.55 g, 2.70 mmol) and 300 mL
toluene, and the mixture was stirred at 90 °C for 18 h. After
completion of the reaction, the mixture was cooled to room
Computational Details
DFT geometry optimizations and TD-DFT excitation energy
calculations of two ligands were carried out with the Gaussian
9 package.25 In both cases, the B3LYP/def2-TZVP basis set
was used. The first 50 monoelectronic excitations were
calculated. All calculations were done in the gas phase, and
geometry optimizations were performed with no symmetry
restraints.
The ground state geometries of two ytterbium complexes 1 and
were optimized by calculations using LUMPAC26 with a
Sparkle/PM6 model implemented in the MOPAC 2016 software.
The keywords used in the calculation reported here were PM6,
PRECISE, BFGS, GNORM = 0.25, GEO-OK, SCFCRT = 1.D-
temperature, then rinsed with saturated NH
resulting aqueous phase was extracted with CH
The organic layers were combined and dried over anhydrous
MgSO . The solution was concentrated under reduced pressure
4
Cl (2.0 mL). The
2
Cl (10 mL × 3).
2
0
4
to give the crude product, which was purified by column
chromatography (silica, 5% EtOAc in hexanes) to give the
product as a white solid (11.02 g, 85%).
4
-(N,N-Dimethylamino)-4',4''-diacetyltriphenylamine
was
2
synthesized according to the processes described for the isolation
of 4,4'-diacetyl triphenylamine by utilizing 1.15 g (4.00 mmol)
of 4-(N,N-Dimethylamino)phenyl diphenylamine, 1.02 g (10.00
3
mmol) of acetic anhydride and 2.67 g (20.00 mmol) of AlCl .
6
| J. Name., 2012, 00, 1−3
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
Purification by recrystallization from n-hexane afford a yellow
bulk crystals (1.22 g, 82%). Anal. Calc. for C24
372.18): C, 77.39; H, 6.50; N, 7.52; O, 8.59. Found: C, 77.34;
Conflicts of interest
There are no conflicts to declare.
View Article Online
DOI: 10.1039/C9DT00614A
24 2 2
H N O
(
1
H, 6.58; N, 7.58; O, 8.55. H NMR (CDCl
3
) δ 2.57(s, 6H), 3.01
(
(
[
s, 6H), 6.76(d, J = 9.0 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2H), 7.12
d, J = 8.8 Hz, 4H), 7.86 (d, J = 8.8 Hz, 4H). ESI-MS m/z 373.18
M + H]+.
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (Nos. 51773054 & 51872077). We
also thank the Key Laboratory of Functional Inorganic Material
Chemistry, Ministry of Education, P. R. China for supporting
this work.
4
-(N,N-dimethylamino)-4',4''-di(4,4',4''trifluoro-1,3-
dioxobutyl)-triphenylamine (L2)
A mixture of sodium methoxide (1.08 g, 20 mmol) and ethyl
trifluoroacetate (2.84 g, 20.00 mmol) in 40 mL dry DME
(ethylene glycol dimethyl ether) were stirred for 10 min,
followed by the addition of 4-(N,N-Dimethylamino)-4',4''-
diacetyltriphenylamine (3.13 g, 8.40 mmol), which was further
stirred at room temperature for 24 h. The resulting mixture was
poured into 100 mL ice-water and acidified to pH = 2–3 using
hydrochloric acid (2.0 M), the resulting orange precipitate was
filtered and dried in vacuum. Recrystallization from acetone
gave brown needle crystals (2.80 g, 59.0%). Anal. Calc. for
References
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) δ
3
9
1
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Synthesis of (HNEt
)
3 2
[M
L and L (0.18 mmol) and triethylamine (36.00 mg, 0.36 mmol)
were dissolved in 10 mL CH CN. To this solution cooled to room
temperature, MCl ·6H O (M = Yb, Gd and Y; 0.090 mmol) in
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mL), CH OH (2 × 10 mL) and diethyl ether (2 × 10 mL), then
dried under vacuum to give the desired products.
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L
] and (HNEt ) [M ]
L
1
2
3
3
2
1
3
4
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3
(HNEt
)
3 2
[Yb
2
(L1)
4
]
(1) Yield: 82%. Anal. Calc. for
4
(a) W. Huang, D. Wu, D. Guo, X. Zhu, C. He, Q. Meng and C.
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C
116
H
92
F
24
N
6
O
16Yb
2
(2627.50): C, 53.01; H, 3.53; N, 3.20; O,
9
1
.74. Found: C, 53.23; H, 3.66; N, 3.17; O, 9.71. ESI-MS: m/z =
212.1161 [Yb
[Yb
(L2)
(L1)
] .
2–
2 4
(c) J. Zhou, J. L. Leaño Jr., Z. Liu, D. Jin, K.-L. Wong, R.-S.
(HNEt
)
3 2
2
4
]
(2) Yield: 85%. Anal. Calc. For
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C
124
H
112
F
24
N
10
O
16Yb
2
(2799.67): C, 53.18; H, 4.03; N, 5.00; O,
9
1
.14. Found: C, 53.20; H, 4.15; N, 5.08; O, 9.13. ESI-MS: m/z =
(L2)
2–
298.2021 [Yb
2
4
] .
(3) Yield: 82%. Anal. Calc. For
116 92 24 6 2
H F N O16Gd (2599.47): C, 53.66; H, 3.57; N, 3.24; O,
5
6
(HNEt
)
3 2
[Gd ]
2
(L1)
4
C
9
1
.86. Found: C, 53.62; H, 3.61; N, 3.29; O, 9.81. ESI-MS: m/z =
195.4380 [Gd
[Gd
(L1)
(L2)
10
2–
2
4
] .
(4) Yield: 81%. Anal. Calc. For
(2771.64): C, 53.79; H, 4.08; N, 5.06; O,
(HNEt
)
3 2
2
4
]
C
124
H
112
F
24
N
O
16Gd
2
9
1
.25. Found: C, 53.65; H, 4.29; N, 5.15; O, 9.21. ESI-MS: m/z =
283.1941 [Gd
[Y
(L2)
+ H]2–.
2
4
7
8
(L1)
(HNEt
)
3 2
2
4
]
(5) Yield: 82%. Anal. Calc. For
(2459.43): C, 56.64; H, 3.77; N, 3.42; O,
C
116
H
92
F
24
N
6
O
16
Y
2
1
0.41. Found: C, 56.66; H, 3.82; N, 3.44; O, 10.35. ESI-MS: m/z
(L1)
] .
2–
=
1127.0832 [Y
2
4
(
C
HNEt [Y
)
3 2
2
(L2)
4
]
(6) Yield: 85%. Anal. Calc. For
124 112 24 10 16 2
H F N O Y (2631.60): C, 56.58; H, 4.29; N, 5.32; O,
(a) C.-L. Liu, R.-L. Zhang, C.-S. Lin, L.-P. Zhou, L.-X. Cai,
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9.73. Found: C, 56.62; H,4.33; N, 5.28; O, 9.68. ESI-MS: m/z =
(L2)
1213.1695 [Y ] .
2 4
2–
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Dalton Transactions
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DOI: 10.1039/C9DT00614A
Visible light sensitized near-infrared luminescence of ytterbium via
ILCT states in quadruple-stranded helicates
Zihan Zhang, Yanyan Zhou, Hongfeng Li,* Ting Gao and Pengfei Yan*
The quadruple-stranded helicates show visible light sensitized near-infrared luminescence of
ytterbium via ILCT states.