Contribution of Energy Transfer from the Singlet State to the Sensitization of Eu3+and Tb3+Luminescence by Sulfonylamidophosphates

A Journal of

Accepted Article

Title: Energy transfer contribution from singlet state to the sensitization

of Eu3+ and Tb3+ luminescence by sulfonylamidophosphates

and the role of the 7F5 level of Tb3+ in this process

Authors: Ewa Kasprzycka, Victor A. Trush, Vladimir M. Amirkhanov,

Lucjan Jerzykiewicz, Oscar L. Malta, Janina Legendziewicz,

and Paula Gawryszewska

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content of this Accepted Article.

To be cited as: Chem. Eur. J. 10.1002/chem.201603767

Link to VoR: http://dx.doi.org/10.1002/chem.201603767

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Energy transfer contribution from singlet state to the

sensitization of Eu³⁺and Tb³⁺luminescence by

sulfonylamidophosphates and the role of the ⁷F₅level of Tb³⁺in

this process

Ewa Kasprzycka^[a], Victor A. Trush^[b], Vladimir M. Amirkhanov^[b], Lucjan Jerzykiewicz^[a], Oscar L.

Malta^[c], Janina Legendziewicz^[a], Paula Gawryszewska*^[a]

Abstract: A series of stable lanthanide complexes Na[Ln(L)₄] (Ln =

La³⁺, Eu³⁺, Gd³⁺, Tb³⁺, [L¹]^-, [L²]^-), with dimethyl(4-

visible or IR spectral regions, narrow emission bands and long

luminescence decay times (millisecond scale) ^[1,2]. Because of

the low value of the molar absorption coefficient, associated with

L

=

methylphenylsulfonyl)amidophosphate (HL¹) and dimethyl 2-

naphthylsulfonylamidophosphate (HL²), here denoted as 1Ln, 2Ln,

as well as sodium salts, NaL, denoted as 1Na, 2Na were

synthesized. The compounds were characterized by single-crystal X-

ray diffraction, IR, absorption and emission spectroscopies at 293

and 77 K. In contrast to the usual and well known dominant role of

the ligand triplet state in intramolecular energy transfer processes in

Ln complexes, in this particular new class of Ln compounds with

sulphonylamidophosphate ligands we discuss, for the first time, the

strong experimental and detailed theoretical evidences that suggest

a dominant role played by the ligand first excited singlet state. The

intraconfigurational 4f-4f transitions,

a strongly sensitized

lanthanide photoluminescence is achieved by excitation of a

chelating chromophore to its singlet state followed by efficient

energy transfer from the absorber to the metal ion excited state

(the so-called antenna effect) ^[1,2,4]. Due to the apparent Stokes

shift and their quite different characteristics between excitation

and emission, these types of compounds have been referred to

as light converting molecular devices (LCMDs) ^[5]

.

LCMDs put out different requirements on the range of absorption

and emission wavelength depending on their intended use.

However, all LCMDs must exhibit an effective sensitized

emission. To achieve this, three conditions must be fulfilled: (i)

efficient absorption process, (ii) efficient energy transfer from the

excited levels of the ligand to the emitting center and (iii) efficient

luminescence. Many studies on the lanthanide chelates apply to

one of these individual processes. However, the final 4f-4f

luminescence quantum yield depends on a balance between

them, including non-radiative decay channels, in the compound.

Correlation between experimental and theoretical results gives

the opportunity to test and to improve the functioning of

theoretical models, and to understand the mechanisms of

intramolecular energy transfer, which can be a tool for the

design of new compounds with functional optical properties.

The commonly observed sensitization process for luminescent

Eu³⁺and Tb³⁺complexes involves a triplet pathway, in which the

transfer of the energy absorbed by the ligand to the Ln³⁺ion

7

importance of the role played by the F₅level in the case of the Tb³⁺

compound in this process is shown. The theoretical approach for the

energy transfer rates was successfully applied to the rationalization

of the experimental data. The higher lying excited levels of Eu (⁵D_J,

⁵L_J, ⁵G_J) and Tb (⁵D_J, ⁵G_J, ⁵L_J, ⁵H_J, ⁵F_J, ⁵I_J) were included in the

calculations for the first time. Both the multipolar and exchange

mechanisms were taken into account. The experimental intensity

parameters (_), emission lifetimes (), radiative (A_rad) and non-

radiative (A_nrad) decay rates, quantum yields (theoretical and

experimental) were determined and discussed in detail.

Introduction

Lanthanide chelates are of great interest due to their special

luminescence properties and their various applications for

technological purposes ^[1-3]. Luminescence of these ions is

characterized by: large Stokes shift upon excitation in the ligand

states (in the UV) and subsequent 4f-4f emission in the near UV,

takes place from the ligand-centered triplet excited state (T₁) ^[6-8]

.

Among thousands of publications on lanthanide complexes, only

a few have included evidences of a dominant singlet (S₁)

pathway ^[9-22]. Dominant singlet energy transfer has seldom been

observed, in the sensitization of lanthanide ions, because

intersystem crossing (S₁→T₁) is usually very fast due to not only

the rather small S₁– T₁energy difference ( 5000 cm^-1), but also

to the external heavy atom effect induced by the lanthanide ion.

Particularly a few papers relate to Eu^{3+ [12-16]}and Tb^{3+ [23-26]}. In the

case of Tb³⁺chelates, the singlet pathway may be of relevance

particularly when the ligand triplet state lies below or very close

to the emitting ⁵D₄state, a situation in which luminescence

quenching is normally observed. The theoretical challenge of

describing intramolecular energy transfer processes that occur

between a ligand and a lanthanide ion in luminescent complexes,

including selection rules, was first treated in detail in references

^{[5, 27, 28]}. Expressions for transfer rates corresponding to the so-

called direct Coulomb and exchange mechanisms have been

[a]

E. Kasprzycka, Dr. L. Jerzykiewicz, Prof. Dr. J. Legendziewicz, Dr. P.

Gawryszewska

Faculty of Chemistry

University of Wroclaw

14 F. Joliot-Curie Str., 50-383 Wroclaw, Poland

E-mail: paula.gawryszewska@chem.uni.wroc.pl

Dr. V. A. Trush, Prof. Dr. V. M. Amirkhanov

Department of Chemistry

Taras Shevchenko National University of Kyiv

Volodymyrska str. 64, Kyiv 01601, Ukraine

Prof. Dr. O. L. Malta

[b]

[c]

Departamento de Química Fundamental

Universidade Federal de Pernambuco

Cidade Universitꢀria, Recife-PE, 50740-560, Brazil

Supporting information for this article is given via a link at the end of

the document.

obtained, from which selection rules could be derived ^[29]

.

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The literature related to the studies on the photophysical Results and Discussion

properties of Ln complexes with the antenna effect is very broad.

The most widely investigated group of complexes are the

compounds with -diketones ^[30-33], which are known as excellent

emission sensitizers in complexes with lanthanide ions. With that

in mind we undertook a study of a new class of lanthanide

complexes, sulfonylamidophosphates, which are the S, N, P

hetero analogues of β- diketones. The structure of the ligands

has been planned in such a way as to decrease the multiphonon

quenching of lanthanide emission in comparison to their

complexes with β- diketones. This has been achieved by a

replacement of C=O vibrations (1600 cm^-1) by the lower

energetic vibrations P=O (1250 cm^-1) and S=O (1350 cm^-1).

Moreover, in the complexes with sulfonylamidophosphates in

six-membered chelate ring created as a result of coordination

with the lanthanide ion, the high-frequency C-H vibrations

present in beta-diketonates do not occur. Additionally, the

presence of the phosphorus atom in our ligands raises the

possibility of bonding with the additional chromophore, in relation

to the carbon atom, which enables stronger sensitization of the

emission of the lanthanide ions. Furthermore, these complexes

constitute a new partition of coordination chemistry of this type

of ligands, creating simultaneously a new class of lanthanide

complexes with ligands possessing the structural fragment -

SO₂NHP(O)-. During the past few years, much effort has been

devoted to the investigation of the coordination behavior and

photophysical properties of these kinds of complexes with f-

metals ^[34-38]. Continuing our study of lanthanide complexes with

sulphonylamidophosphate ligands, the mechanisms of ligand-to-

metal energy transfer in the new europium and terbium

X-ray Analysis

Single-crystal X-ray diffraction data show that the structures of

1Eu and 2Eu are monoclinic with the space group C2c and C2,

respectively. The complexes of La³⁺, Gd³⁺, and Tb³⁺are

isostructural with the Eu³⁺ones. The crystallographic data of

1Tb, 1Eu and 2Eu is presented in Table S1. The molecular

structures of 1Eu and 2Eu containing the numbering scheme for

atoms are displayed in Figure 2 and 3, respectively.

complexes

with

dimethyl(4-

methylphenylsulfonyl)amidophosphate (HL¹) and dimethyl 2-

naphthylsulfonylamidophosphate (HL²) (Figure 1) were

examined for the first time in this class of complexes. For the

calculations of the energy transfer rates, an unprecedented

number of Eu and Tb excited levels was included, and the

important role played by the ⁷F₅level of Tb³⁺in the energy

transfer process was discussed. The reasons for unexpected

results that emerge, such as strong experimental and theoretical

evidences that sulfonylamidophosphates-to-Ln³⁺energy transfer

occurs mainly from the singlet states of our ligands were also

investigated.

Figure 2. The X-ray structure of 1Eu: view of crystal packing along the c-axis,

molecular structure of 1Eu (H atoms as well as dioxane molecules are omitted

for clarity), coordination polyhedron of the Eu ion.

Figure 1. Structural formulae of the ligands.

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geometry of the Na surrounding is observed for

Na[Nd(C₁₄H₂₁N₃O₅PS)₄] ^[40], Na[Tb(C₂₀H₁₉NO₅PS)₄] ^[41]

.

Intra- and intermolecular non-covalent interactions play a role in

the crystal packing and stabilize the structure. weak

A

intramolecular CH-π interaction (-OCH₃with naphthyl ring) within

each ligand exists for 2Eu. The CH hydrogen atoms tend to

point toward the center of the aromatic ring. The distances

C11H11C…Cp1 (Cp1: C13, C14, C15, C20, C21, C22) centroid

and C31H31B…Cp3 [1-x, y, 1-z] (Cp3: C33, C34, C35, C40,

C41, C42) equal 2.86 and 2.85 Å, respectively. Such

interactions are not found between the molecules in the chain or

the inter-chains. A number of short CH-π distances has been

previously shown in the crystal structures of organic compounds

and it was suggested that the CH-π interaction constitutes one

of the important factors in controlling the crystal packing of the

molecules ^[42]. Molecules of dioxane in the outer coordination

sphere of 2Eu were observed. These solvent molecules are

placed inside channels generated along the axis b. In turn, the

relative position of the planes of naphthyl rings eliminates the

existence of intermolecular - interactions in the chain and

between chains. The - interactions do not occur in the 1Eu

complex while there

are

weak

CH-π

interactions

(C12H12A…Cg(C13C14C15C16C17C18)

–

2.98

Å

and

C22H22C…Cg(C23C24C25C26C27C28) – 3.00 Å). Disordered

molecules of acetonitrile were observed in the outer coordination

sphere of 1Eu.

Figure 3. The X-ray structure of 2Eu: view of crystal packing along the b-axis,

molecular structure of 2Eu (H atoms as well as dioxane molecules are omitted

for clarity), coordination polyhedron of the Eu ion. Symmetry code: A 1-x, y, 1-

z; B 1-x, 1-y, 1-z; C x, 1+y, 1-z.

The measured Ln-Ln distances in both complexes were large,

equal to 11.232 (1Eu) and 11.268 Å (2Eu) within the chains and

12.058 (1Eu) and 13.424 Å (2Eu) between the chains. Eu³⁺is at

the two-fold axis and Na⁺is on the inversion center in 1Eu. In

2Eu the Eu³⁺and Na⁺ions are located on the twofold axis.

In both complexes, the Eu³⁺ions are eight-coordinated with the

primary coordination sphere made up of four deprotonated

ligands. One sulfonyl oxygen atom and one phosphoryl oxygen

atom of each ligand are involved in europium-ion coordination.

The six-membered chelating rings are formed, with the Ln-O

bond lengths falling within the expected range (see Table S2).

The Ln-O(S) distances are about 0.15 Å longer than the Ln-O(P)

distances for 1Eu and 2Eu. The Tb–O distances are shorter

than the corresponding Eu–O ones due to lanthanide

contraction. The coordination polyhedron of the lanthanide ions

can be described as a slightly distorted dodecahedron, based on

a criterion proposed by Porai-Koshits and Aslanov ^[39]. The value

of angles for ideal polyhedrons and for polyhedrons of 1Eu and

2Eu are shown in Tab. S3. The selected angles in 1Eu, 1Tb and

2Eu are presented in Tables S4a,b.

Spectroscopic Results

IR spectroscopy

The IR spectra of the Eu compounds and HL ligands were

recorded in the range of 50-4000 cm^-1(see Fig. S1). IR

spectroscopy for sodium salts (1Na, 2Na) and complexes (1Eu,

2Eu) shows the disappearance of the corresponding N-H

vibration band of the free ligand. This is due to the fact that the

ligands are deprotonated in the coordination compounds. The

vibration of the amide group of HL²appears in the region of

2450-2825 cm^-1, while for HL¹it is located between 2430 and

2847 cm^-1. Their maxima are at 2734 cm^-1and 2720 cm^-1,

respectively. In the spectra of the free ligands HL¹and HL², two

characteristic sharp bands are observed with the maxima at

1234, 1341 cm^-1and 1246, 1338 cm^-1. They are assigned to the

ν(P=O) and ν(S=O) vibrations. These bands appear for the 1Eu

and 2Eu complexes at lower wavenumbers because of the

coordination to the metal ion. The differences in frequencies for

these groups are 77 cm^-1(Δ_P=O), and 102 cm^-1(Δ_S=O) for 1Eu

and 61 cm^-1(Δ_P=O), and 89 cm^-1(Δ_S=O) for 2Eu. The O-Ln-O

fragment related to the chelate ring, formed by the coordination

is localized in the far infrared region. It is observed in the region

55-290 cm^-1as broad and middle intensity bands.

The Na ions connect the [Ln(L)₄]^-units in the complexes,

resulting in the creation of the polymeric chains along the c

(1Ln) and b (2Ln) axis. In 1Eu, the coordination number of Na

ions equals six, due to the bonding with two sulphonyl oxygen

atoms and one nitrogen atom from one complex anion, and

another two sulphonyl oxygen atoms and a nitrogen atom from

the neighboring ones. In 2Eu, the Na ion is tetra-coordinated by

sulphonyl oxygen atoms of four ligands, adopting tetrahedral

geometry. The coordination number equal to 4 is not typical for

sodium ion and may result from crystal packing. Similar

Electronic states of the ligands

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The room temperature absorption spectra of transparent KBr

tablets with 1Tb and 2Tb in the UV region are presented in

Figure 4. The absorption spectra in this region consist of three

bands in the ranges >212, 212-255, 255-325 and 200-255, 255-

305, 305-340 nm for 1Tb and 2Tb, respectively. These bands

are associated with   * transitions characteristic for phenyl

and naphthyl chromophores, for some of which vibronic

structure is seen. The barycenter of excited singlet states in the

complexes and their half-width determined on the basis of

Gaussian distributions of these absorption spectra amounts to

35088 cm^-1/6400 cm^-1and 31278 cm^-1/2000 cm^-1for 1Ln and

2Ln, respectively. Fitting of the spectra, the results of which are

presented in Fig. S2, was performed with Gaussian function

curves, yielding R²equal to 0.999 (1Tb) and 0.998 (2Tb).

Having compared the intensity ratios and shapes of the bands in

the range of 255-325 nm between 1La and 1Tb as well as in the

range of 260-340 between 2La and 2Tb, it was difficult to

exclude with certainty the presence of f-d transitions in Tb

complexes in mentioned energy ranges. Furthermore, no

additional band or widening of the absorption spectra of 1Eu and

2Eu complexes, as compared to 1La and 2La, could be seen. In

these cases, ligand-to-metal charge transfer states (LMCT)

could be covered by the bands of the ligands. According to the

theoretical calculations reported by Faustino and co-authors the

energy transfer rate S₁ LMCT for the same energy positions of

S₁and LMCT is about 10⁹s^{-1 [43]}. The LMCT state takes usually

part in quenching of sensitized luminescence. The relaxation to

the ground state via the crossover process to C-T states in Eu

complexes is very often strongly temperature dependent since

these processes are usually phonon-assisted.

phosphorescence observed in the range of 500 – 600 nm was

too weak for the measurement of lifetime to be possible using

our equipment. The phosphorescence of 2La is not observable

at an excitation wavelength of 290 nm, but can be observed for

_exc= 330 nm (see Fig. 5).The Na and La compounds reveal a

very similar intensity of the fluorescence, which is seen in Fig.6.

Moreover, fluorescence lifetimes of 1La and 2La are almost the

same as for 1Na and 2Na, respectively (see Tab. 1b).

Figure 5. The luminescence and excitation spectra of a) 1La (_exc=270 nm,

_mon=300nm); b) 2La ((_exc=290 nm, _mon=345nm) in solid state at 293 and

77K; c) the luminescence spectra of 2La at 293 and 77 K (_exc=330 nm). The

inset shows phosphorescence spectra of 1La after switching off the excitation

source (the fourth harmonic (266 nm) of an Nd:YAG pulsed laser) and 2La

(_exc=330 nm, xenon lamp).

Figure 4. The absorption spectra of 1Tb and 2Tb as KBr pellets at 293 K.

Figure 5 presents the luminescence spectra of 1La and 2La in

the solid state at 293 and 77 K and their excitation spectra. For

both types of complexes, the fluorescence (S₁S₀) at room

temperature, with maxima at 295.5 nm (1La), 342 nm (2La), and

the fluorescence and weak phosphorescence (T₁S₀) at 77 K

were measured. It is worth noting that for 2Na and 2La the

Figure 6. The dependence of the fluorescence intensity for a) 1Na, 1La, 1Eu,

1Tb (_exc=270 nm), b) 2Na, 2La, 2Eu, 2Tb (_exc=290 nm) at 293 K.

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(J = 0-4) and ⁵D₄⁷F_J(J = 0-6) transitions. For 2Tb, we see very

weak emission at 293 K because the ligand triplet state is below

the Tb³⁺emitting state (⁵D₄) and the energy difference between

T_1L2(barycenter) and ⁵D₄is 2614 cm^-1. The ⁵D₀⁷F₂and

⁵D₄⁷F₅transitions dominate the Eu and Tb spectra

respectively, which indicate that the lanthanide ions occupy sites

without an inversion center. The emission spectra of 1Eu and

2Eu as well as 1Tb and 2Tb, for each pair of ions, are very

similar in the profile, splitting and energy of the transitions. The

reason for this is the strong resemblance of inner coordination

sphere and coordination polyhedron (described as a slightly

distorted dodecahedron) of lanthanide ions in 1Ln and 2Ln (see

paragraph 2.1).

Table 1. a) Decay time of ⁵D₀, ⁵D₁and ⁵D₄emission for 1Eu (_exc= 300

nm), 1Tb (_exc= 280 nm), 2Eu, 2Tb (_exc= 320 nm) at 293 and 77K

.

1Eu

1Tb

2Eu

2Tb

(⁵D₀)

(⁵D₁)

(⁵D₄)

(⁵D₀)

(⁵D₁)

(⁵D₄)

 293 K

/ms

2.51

0.013

rise

0.0135

2.78

1.47

2.10

0.0094

rise

0.0096

0.0108

0.0274

time



/ms

77

K

2.51

2.79

1.73

2.22

1.08

2.27

For excitation at 464 nm, the decay time of ⁵D₀of 2Eu equals 1.77 ms.

The decay time values were estimated with an error of 3%

Table 1. b) Decay time of fluorescence for 1Na, 1La (_exc= 280nm,

_mon= 295 nm) and 2Na, 2La (_exc= 314 nm, _mon= 342 nm) at 293 K

1Na

1La

1Eu

1Tb

2Na

2La

2Eu

2Tb

 /ns

12.5

12

2.35

10

15

7.4

4.4

In the majority of lanthanide complexes, the ligands relax non-

radiatively to their triplet states and spin forbidden ¹*  ³*

intersystem crossing (ISC) is induced by spin-orbit coupling

enhanced through the presence of a heavy lanthanide. The

presence of fluorescence in 1La, 2La at 293 and 77 K and the

same values of fluorescence lifetime for La and Na compounds

indicate that the S₁S₀process of depopulation of the ligand

singlet state competes with the ISC, and that the investigated

complexes have low intrinsic ISC, i.e. the heavy-atom effect is

unnoticeable. The highly possible reason for this is the large

energy gap between the singlet state and the triplet state, which

is 11800 and 13300 cm^-1for the 1Ln and 2Ln complexes

respectively, and the low heavy atom effect due to the large

Figure 7. The emission spectra of 1Eu (_exc=300 nm) and 2Eu (_exc=320 nm)

at 293 (black line) and 77 K (red line).

distance (5.8Å) between the donor chromophore and the Ln ion.

The emission spectra at 77 K of 1Eu revealed a single

[44]

It was shown in ref

that ISC is maximized when the energy

5

component of D₀⁷F₀transition at 579 nm (17253 cm^-1) with a

difference between ¹* and ³* amounts to ca. 5000 cm^-1. In

turn, in ref. ^[10]the authors, apparently not observing the external

heavy-atom effect, reported that the presence of a tertiary

amine, which has an abnormal quadrupole moment, could be

responsible for the highly forbidden character of the S₁T₁

transition.

width at half-height equal to 9 cm^-1, which is consistent with only

one site being occupied by the Eu³⁺ions. On the other hand, the

⁵D₀⁷F₀band of the 2Eu site is two-fold (see inset in Fig.7). The

two components of this band were derived from a Gaussian

curve fitting procedure with the maxima at 579 nm (17260 cm^-1)

and 579.7 nm (17250 cm^-1). This probably results from the

disorder of the Eu³⁺ions, which are on a two-fold axis of

symmetry, because after the final refinement cycle for 2Eu

single crystal, large residual electron density peaks (2.0 and 1.6

e/Å³) remain near the Eu ion. This possibly is an unrecognized

twinning associated with the crystal data and a disordered with

respect to the Eu sublattice, which provides most of the resonant

contribution. However, the emission spectrum of 2Eu (similarly

to 1Eu) at 77 K shows three well separated electronic lines due

to the magnetic dipole ⁵D₀⁷F₁transition, suggesting the

presence of a single major chemical environment around the

Eu³⁺ion ^[45]. The analysis of the Stark components for the ⁵D₀

⁷F_Jtransitions at 77 K suggests that the Eu³⁺symmetry sites in

1Eu and 2Eu are close to D₂^[46]. This is in line with the

On the basis of the weak phosphorescence which accompanied

fluorescence (the inset in Fig.5), the barycenter of the triplet

states of the 1Ln (E_1T1) and 2Ln (E_1T2) complexes were

determined as 23260 and 18000 cm^-1. Decay curves of the 1Gd

phosphorescence are reproduced by two exponentials (₁= 1.1

ms and ₂

=

4.7 ms, R²

=

0.9997). Furthermore, the

phosphorescence decay contains

a

weak long lasting

component of the order of 20 ms, contributing a few percent to

the total decay time.

Photoluminescence of lanthanide complexes

Figs.7 and S3 present the high resolution emission spectra at

293 and 77 K of the 1Eu, 2Eu and 1Tb, 2Tb crystals

5

respectively. The bands correspond to the well-known D₀⁷F_J

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discussion about the coordination polyhedra in the 1Ln and 2Ln

excitation band of 1Tb is intense throughout the 240-325 nm

range. This phenomenon indicate differences between 1Eu and

1Tb intramolecular energy transfer mechanisms. The

participation of the 5d state in sensitiziation (range 240 – 275

nm) of the 1Tb luminescence cannot be ruled out. Furthermore,

we cannot completely exclude the participation of the LMCT

state from the quenching of the sensitized luminescence of 1Eu.

complexes.

5

It should be noted that for both Eu complexes the D₁emission

is observed at room and low temperature, which is presented in

Fig. S4. In this case, the competition of two processes is

responsible for the emission from the ⁵D₁state: multiphonon

5

7

relaxation, which feeds the D₀state, and D₁ D_0,⁷F₀ F₃

cross-relaxation process. In both investigated compounds the

Eu-Eu distances are large (of the order 11 Å), suggesting the

inefficient ⁵D₁



⁵D_0,⁷F₀



⁷F₃cross-relaxation. As is known

5

from the literature, the D₁emission from europium complexes

with organic ligand is observed even when high energy

oscillations are present in the inner coordination sphere (e. g. 

1300 cm^-1, 1600 cm^-1,  2900 cm^-1or 3600 cm^-1) ^[47-50]

.

5

The values of emission lifetimes of the D₀, D₁and D₄levels

are presented in Tab. 1. The decay profiles of emission are

monoexponential for 1Eu (Fig. S5), 1Tb (Fig. S6), mono- or

biexponential for 2Eu (Fig. S7) and biexponential for 2Tb (Figs.

S8a, b). For the fit of the experimental data, coefficient of

determination R²equal to 0.999 was obtained. The spectra of

⁵D₀emissions were also fitted using a function containing

exponential rising and decaying parts. Lifetime values of the 1Eu

(⁵D₀) and 1Tb (⁵D₄) emissions are temperature and excitation

wavelength independent, and thus reflect the absence of

thermally activated non-radiative processes, either vibrational or

5

electronic in nature, including the emitting levels D₀and D₄. A

discussion of emission lifetimes of the ⁵D₀and ⁵D₄levels for 2Eu

and 2Tb is included in the section Analysis of energy

transfer.

Figure 8. The excitation spectra of 1Eu (red line, _mon=612.75 nm) and 1Tb

(green line, _em=546.5 nm) at 293 and 77 K.

5

In Table 1, rise times of the emitting D₀levels of 1Eu and 2Eu

are included. These luminescence rise times perfectly match the

decay times of ⁵D₁emission, which indicates that the ⁵D₀is

populated directly from the ⁵D₁level. Decay times of ⁵D₁

The energy transfer rates W_ETS(energy transfer rates from

S₁ff* levels) and W_ETT(T₁ff* levels) were calculated using a

[5, 25, 26]

theoretical model described in literature

(see paragraph

5

emission and rise time of D₀emission equal to 13 and 13.5 s

Theoretical Calculations). Table 2 presents the selected energy

transfer rates for 1Eu and 1Tb. In Tables S5-S8 U^(λ)squared

reduced matrix element values, as well as values of M, and

values of ff* energy transitions, and the energy difference

between the ligand state and the lanthanide are listed. The

energy transfer rate value correspond to the transitions between

levels shown in Fig. 9. The Cartesian coordinates are gathered

in Tab. S9 and corresponding to Figs. S9, S10, where the Ln ion

is in the center of the coordinate system.

(1Eu) and 9.6 and 9.4 s (2Eu) respectively, were measured at

293 K. The correlation between the temperature dependent rise

5

time of the D₀emitting level and decay time of the D₁excited

level in a europium complex has been discussed by Faustino et

al ^[51]

.

Analysis of energy transfer

The efficiency of ligand-to-metal energy transfer is reflected in

the excitation spectra. The excitation spectra at 293 and 77 K for

the 1Eu and 1Tb complexes are plotted in Fig. 8. They consist

of a broad band in the range 240 – 325 nm, arising from the

absorption transition to the ligand singlet state, and narrow lines

of the f-f transitions of the Eu³⁺and Tb³⁺ions. The broad band

dominates the excitation spectra of 1Tb, which proves a

relatively efficient ligand to metal energy transfer. In the spectra

of 1Eu, f-f transitions dominate, indicating less efficient ligand–

to-metal energy transfer. Efficiency of energy transfer in 1Eu

and 1Tb are nearly temperature independent, as evidenced by

the constant value of the intensity ratio of ⁷F₀^2S+1L_J/ *

transitions at 293 and 77K. However, the analysis of both

excitation spectra combined together shows differences in the

spectral range of 240-275 nm, where the 1Eu excitation band

has much lower intensity than in the range of 280-325, while the

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W_ETT1

-

2.34 x10³

2.20x10³

8.84x10²

7.69x10²

2.59 x10²

-

0.07

0.06

77

Table 2. Calculated values of total energy transfer rate from

⁵D₄→T₁

W_BETS1

⁵L₇→ S₁

W_BETT1

T₁→ ⁵D₄

2614

-

Exchange

singlet (W_ETS1) and triplet (W_ETT1) states, total back energy

transfer rate from singlet (W_BETS1) and triplet (W_BETT1) states,

selected energy transfer rates and % contribution of each in

overall energy transfer for 1Eu, 1Tb, 2Eu, 2Tb, the energy

difference between barycenter of ligands states and

lanthanide states ().

-

1746

-

Exchange

-

67

23

1Eu

Δ /cm^-1

W /s^-1

Mechanism

%

2614

Multipole

23

W_ETS1

-

1.28x10⁶

3.81x10⁵

2.21x10⁵

2.09x10⁵

1.61x10⁵

4.06x10²

2.47x10²

W/s^-1

-

S₁→ ⁵L₆

S₁→ ⁵G₃

S₁→ ⁵D₃

S₁→ ⁵G₆

W_ETT1

975

Multipole

-

30

8834

11045

8235

-

17

16

13

0.03

0.02

%

T₁→ ⁵D₁

1Tb

4216

Δ /cm^-1

Exchange

Mechanism

W_ETS1

-

7.53x10⁶

2.93x10⁶

1.36 x10⁶

1.15x10⁶

2.11x10³

7.98 x10²

7.20 x10²

-

S₁→ ⁵G₆

S₁→ ⁵G₅

S₁→ ⁵L₇

W_ETT1

8420

7132

5556

-

Both

-

39

Figure 9. The energy level diagram for 1Eu and 1Tb used in the analysis of

channels of emission sensitization.

18

15

In the case of both 1Eu and 1Tb, back-transfer rates were

insignificant due to a negative energy mismatch  (>2000 cm^-1),

0.03

0.01

−∆

푘

푇

퐵

which led to a small Boltzmann factor 푒 . In our analysis, we

have considered energy transfer channels from ligand singlet

state and ligand triplet state. The high lying excited levels of Eu

(⁵D_J, ⁵L_J, ⁵G_J) and Tb (⁵D_J, ⁵G_J, ⁵L_J, ⁵H_J, ⁵F_J, ⁵I_J) were included in

the calculations for the first time. The direct transfer rates from

S₁to the ⁵D₃, ⁵L₆, ⁵L₇, ⁵G₂, ⁵G₃, ⁵G₅, ⁵G₆levels for 1Eu were

calculated assuming a factor of thermal population equal to 0.33,

at 293 K, for the ⁷F₁manifold and an energy difference

=E(triplet) -[E(⁵D₀)-E(⁷F₁)]. According to the selection rules of

energy transfer, direct energy transfer to the ⁵D₀level is not

allowed. This rule is, however, relaxed due to J-mixing effects

and thermal population of the ⁷F₁level ^{[27, 28]}. Taking into account

T₁→ ⁵D₄

⁵G₆→ T₁

2642

3412

Both

2Eu

Δ /cm^-1

W /s^-1

Mechanism

%

W_ETS1

-

1.80 x10⁵

6.58x10⁴

5.07x10⁴

3.75x10⁴

1003.5

-

S₁→ ⁵G₆

S₁→ ⁵D₄

S₁→ ⁵G₃

W_ETT1

4425

3546

5043

-

Multipole

-

36

28

21

7

the selection rules and the F_0,1thermal populations at 293 K,

0.60

0.3

0.2

5

the intramolecular energy transfer from S₁to D₃, L₇, L_J, G₃,

⁵G₅levels becomes allowed through the multipolar (dipole-2^

pole and dipole-dipole) mechanisms (J-J’≤  ≤J+J’, J’ = J

⁵D₁→ T₁

T₁→ ⁵D₀

1040

747.6

577.77

Exchange

Multipole

376.02

5

= 0 are excluded) and the energy transfer to the D₀, D₂, G₂

′

2Tb

Δ /cm^-1

W /s^-1

Mechanism

%

|

=

becomes allowed through the exchange mechanism ( 퐽 − 퐽

0 표푟 1, J’ = J = 0 are excluded in the case of ligand-to-metal

energy transfer) in 1Eu. The main channels of energy transfer in

1Eu and their percent contribution to the overall energy transfer

from S₁are: S₁⁵G₆(13%), S₁⁵G₃(17%), S₁⁵D₃(16%),

S₁⁵L₆(30%) (see Tab. 2). As can be seen from the

calculations (Tables 2 and S5, S6), the sum of energy rates for

energy transfer from S_1L1and T_1L1in 1Eu (W_ETS1=1.28x10⁶s^-1,

W_ETT1=4.06x10²s^-1) is lower compared to that of 1Tb

(W_ETS1=7.53x10⁶s^-1, W_ETT1=2.11x10³s^-1). This is due to better

resonance conditions between the ligand states (S₁, T₁) and the

W_ETS1

-

3.60x10⁶

1.51x10⁶

4.66x10⁵

4.23x10⁵

4.07 x10⁵

-

S₁→ ⁵G₅

S₁→ ⁵L₇

S₁⁵L₉

S₁⁵L₈

3322

1746

2746

2030

Both

42

13

12

11

Exchange

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excited levels of Tb³⁺in comparison to the excited level of Eu³⁺,

as shown in the energy diagram in Fig. 9. The main channels of

energy transfer in 1Tb and their percent contribution to the

overall energy transfer from S₁are S₁⁵G₆(39%), S₁⁵G₅

biexponential (see Fig. S7). One component of the emission

decay is almost the same (2.43 and 2.10 ms) as the T_1L2⁵D₀

non-radiative energy transfer rate (2.53 ms), which suggests that

5

⁵D₀level is populated through T₁and obviously through the D₁

(18%) and S₁⁵L₇(15%). The T₁



⁵D₄pathway plays a

level. The contribution of this component in overall decay

process diminishes with decreasing temperature from 293 K to

77 K from about 50% to about 20%. The absence of this

component when the ff* levels are excited directly indicates that

the T₁state is populated as a results of intersystem crossing and

negligible role in 1Tb (797 s^-1). Due to a large distance between

donor and aceptor, taking as the distance between the

lanthanide ion and the center of the phenyl ring (5.8 Å), the

energy transfer rates are small for Eu³⁺and Tb³⁺levels for which

the exchange mechanism dominates (Tab. S5, S6).

5

not through the D₁ T_1L2(578 s^-1, 1.73 ms) transition, which is

Similar situation was observed for 2Eu and 2Tb (Tab. S7,

S8), where the Dexter energy transfer was disfavored by the

lack of orbital contact between the donor and the acceptor. The

main difference between the 1Ln and 2Ln complexes is a

different energy match between ligand and Ln³⁺, which we can

observe at the energy diagram (Figs. 9, 10). Moreover, the

too slow as compared to the ⁵D₁⁵D₀transition (9 s). The rise

5

time of the emitting D₀level has the same values either using

direct or indirect excitation.

Table 3. The values of the emission decay times of the ⁵D₀level

for the excitation wavelength for 2Eu at 293 and 77 K and the

goodness of fit (²).

5

barycenter of T_1L2is 2614 cm^-1below D₄(20614 cm^-1) level of

2Tb. Calculated values of the energy transfer rates also point at

the dominating singlet energy transfer in 2Eu and 2Tb, where

the sum of energy rates for energy transfer from S_1L2and T_1L2

equal W_ETS1=1.80x10⁵s^-1(2Eu), W_ETT1=1.00x10³s^-1(2Eu) and

W_ETS1=3,60x10⁶s^-1(2Tb), W_ETT1=2.34x10³s^-1(2Tb) (Tab. 2).

Basing on measurements of lifetimes and the calculations

of non-radiative energy transfer rates, the main path of energy

transfer is proposed for 2Eu as S₁ff* (mainly ⁵G₆, ⁵D₄and ⁵G₃)

⁵D₀and with a smaller contribution of S₁ T₁ ⁵D₁ ⁵D₀.

Excitation

wavelength /nm

Decay time

293 K /ms

²

Decay time

77 K /ms

²

291

322

1.63

2.43

1.015

1.005

1.76

2.34

1.130

1.159

1.47

2.10

1.73

2.22

394

1.77

1.78

1.199

1.026

1.030

1.75

1.76

1.77

1.123

1.002

1.003

464

535.5

In contrast to the 1Eu, the temperature dependent sensitized

emission is observed (Fig. 11) for 2Eu. A presence of back

5

energy transfer processes from D₀into T₁(E = 750 cm^-1) was

expected, but the values of lifetime at 293 and 77 K is almost the

same and the value of the back energy transfer (⁵D₀ T₁) rate

is low (W_BET2= 2.3 s^-1). As such, the contribution of the LMCT

state with energy a little higher than for S₁cannot be excluded.

Especially, that the participation of LMCT in the quenching of the

sensitized luminescence is

a process which is usually

temperature-dependent. The back energy transfer involving the

S₁state and ff* levels can be neglected (Tab. S7).

5

In the case of 2Tb, the back energy transfer L₇S_1L2and

5

T_1L2 D₄are present. The dominating singlet energy transfer

S₁⁵G₅(42%), S₁⁵L₇(13%), S1⁵L₉(12%) and S₁⁵L₈

Figure 10. The energy level diagram for 2Eu and 2Tb used in the analysis of

channels of emission sensitization.

5

(11%) (Tables 2 and S8) as well as competing processes D₄

T_1L2, T_1L2⁵D₄, ⁵D₄⁷F_Jand T_1L2S_0L2are responsible for weak

emission at room temperature with the lifetime of about 11 and

27 s (see Tab. 1a and Fig. S6).

The energy transfer process T_1L2⁵D₀in 2Eu, mediated by the

exchange mechanism, is slow (396 s^-1, 2.53 ms) but its

participation in the energy transfer process is confirmed by the

analysis of a dependence of the ⁵D₀lifetime on the excitation

wavelength (see Tab. 3). If ff* Eu³⁺levels are excited directly

using 394, 464 or 535.5 nm wavelength, the decays profiles of

⁵D₀emission are monoexponential (1.7 ms), which is proved by

respective residual plots. If the Eu³⁺ion is excited indirectly into

S₁state of L²ligand, the decay profiles of the emission are

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constants may lead to adverse simplification in the presentation

of the energy transfer mechanisms.

For 2Tb, the forward energy transfer rate ⁵D₄ T_1L2(9.4 

7

2.2x10³s^-1) is 200 times faster than involving only the F₆level.

At the same time, values of the back energy transfer rate,

T_1L2⁵D₄, are about 10⁷times larger (3.7x10^-52.6x10²s^-1).

Due to the very small values of the reduced matrix elements of

the unit tensor operators, 푼⁽²⁾, 푼⁽⁴⁾and 푼⁽⁶⁾, involved in the direct

Coulomb interaction energy transfer rates, the forward and back

energy transfer processes do not primarily involve the ground

⁷F₆level, what is also seen in our calculations (see Tab. S8).

Theoretical calculations of the energy transfer rates,

proving that the singlet energy transfer is the main pathway in

the sensitization of Eu³⁺and Tb³⁺luminescence in the

investigated complexes, correspond with the photophysical

behavior of the Na and La compounds (no detectable external

heavy-atom effect). The dependence of fluorescence intensity of

1Na, 1La, 1Eu, 1Tb and 2Na, 2La, 2Eu, 2Tb at 293 K is

presented in Fig. 6. The intensities of fluorescence decrease in

the following order: 1,2La>>1,2Tb>1,2Eu. The reduction of the

intensity is accompanied by a shortening of the fluorescence

lifetime from 12 ns (1La–monoexponential fit) to 0.3 and 4.7 ns

(1Tb-biexponential fit) for 1Ln and from 15 ns (2La-

monoexponential fit) to 0.03 and 7.4 ns (2Eu-biexponential fit)

for 2Ln (see Tab. 1).

Figure 11. The excitation spectra of 2Eu (red line, _mon=614.2 nm) and 2Tb

(green line, _exc=544.5 nm) at 293 and 77 K.

This is different from what has been observed by Souza et al. ^[52]

for the complex with thenoyltrifluoroacetonate with the energy of

barycenter of the ligand triplet state nearly the same (540 nm) as

in our 2Tb complex (555 nm). Differences in intensity of the

temperature dependent sensitized emission for these both

complexes may result from different mechanism of the energy

transfer. Moreover, the donor-acceptor distance plays an

important role. The presence of the Tb³⁺emission in 2Tb at 293

K additionally supports the occurrence of the S₁ff* energy

transfer. In the thenoyltrifluoroacetonate complex, ISC is

efficient, the triplet energy transfer dominates and as such the

Tb³⁺emission (⁵D₄⁷F₅) was observed at 60 K and lower

temperatures.

In the calculations of energy transfer rates (S_1L1ff* and

T_1L1ff*) for 1Tb and 2Tb, it was taken into account for the first

time that the Tb³⁺can be in its first excited electronic state (⁷F₅),

causing that new energy transfer paths become possible from S₁

to ff* levels and energy transfer from T₁to ⁵D₄level is facilitated

also by the exchange mechanism (Tabs.S6, S8). Our motivation

7

for this approach are the curiously long (from 2,8 to 22 ms) F₅

[53,54]

lifetimes, measured in glass and crystalline materials in

.

This level cannot be thermally populated (lying at 2185 cm^-1

above the ground ⁷F₆level). The value of W_ETS1, when the

population of F₅is taken into account, increases from 4.39x10⁶

7

to 7.53x10⁶s^-1for 1Tb. The energy transfer rate from T_1L1to ⁵D₄

level of Tb³⁺in 1Tb involving ⁷F₅(exchange mechanism) is

about 100 times larger than involving the ⁷F₆level (direct

Coulomb interaction), and additionally the following new energy

transfer paths become possible – from S₁to ⁵I₅(E = 1320 cm^-1),

5

⁵I₄(E = 1698 cm^-1), I₆(E = 1708 cm^-1), F₄(E = 3932 cm^-1),

⁵H₄(E = 4967 cm^-1). These transitions are allowed through the

exchange mechanism, so the 1Tb case gives

a small

contribution to the intermolecular energy transfer process

despite the good energy match between the S_1L1and the Tb³⁺

levels. This is an excellent example that sole consideration of

the resonance conditions without calculation of the rate

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Table 4. Experimental and calculated photophysical data for 1Eu, 1Tb, 2Eu: intensity parameters

Ω_2,4,6, radiative (A_rad), non-radiative (A_nrad) decay dates, intrinsic quantum yield (푄_퐿^퐿_푛^푛), overall

quantum yield (푄_퐿^퐿_푛), sensitization efficiency (_sen

)

[a]

[b]

₂(10^-20

₄(10^-20

)

₂(10^-20

₄(10^-20

₆(10^-20

calc

)

A_rad

/s^-1

A_rad

/s^-1

A_nrad

/s^-1

푄

푄_퐿^퐿_푛

exp

푄_퐿^퐿_푛

calc

(q)

_sens

퐿푛

[c]

/%

exp /cm^-2

/%

22

/%

/cm^-2

1Eu

6.80

4.83

6.51·

4.74·

0.977

304

281

299

94

74

16.6

16.8

1Tb

2Eu

284

307

76

79^[d]

55

36

47

20

8.19

2.75

7.74

2.67

1.41

255

10.8

11

exp

rad

Estimated error of 10% on the quantum yields 푄 and 푄_퐿^퐿_푛. [a] 1/

metal energy transfer _sens

= A_MD,0n³(I_tot/I_MD); A_MD,0=14.65 s^-1. [b]

푄

=

. [c] Sensitization efficiency of ligand-to-

퐿푛

rad

퐿푛

푄_퐿^퐿_푛

퐿푛

= _퐿푛. [d] 푄 was measured using an integrating sphere and _exc= 485.65 nm.

퐿푛

푄_퐿푛

Comparing theoretical and experimental results for 1Eu

and 2Eu, a very good correlation of emission quantum yields

and _parameters is obtained. The values of the overall

emission quantum yields (determined and calculated), A_rad, A_nrad

and _(experimental and theoretical) are collected in Table 4.

This article addresses some important photophysical

phenomena in lanthanide spectroscopy in the present class of

new complexes.

The mechanisms of ligand-to-metal energy transfer

processes occurring in this family of compounds were analyzed

on the basis of experimental data and theoretical results for the

first time. It was shown that the main pathway in the

sensitization of Eu³⁺and Tb³⁺luminescence is a very rare singlet

transfer. In our case this occurs mainly through the multipolar

mechanism once the ligand donor state electronic barycenter is

situated at a rather far distance from the lanthanide ion,

practically eliminating the energy transfer process by the

exchange mechanism.

Detailed, comparative studies of fluorescence intensity

S₁S₀and its lifetime for 1Na, 1La, 1Eu, 1Tb as well as 2Na,

2La, 2Eu, 2Tb showed for the first time that ISC contribution

resulting from the heavy-atom effect is not very operative in this

family of complexes.

A_radvalue was additionally calculated using the equation (A_rad

=

A_MD,0n³(I_tot/I_MD)) ^[55,56]. The values of ₂for 1Eu and 2Eu are

[5,30]

much smaller than for complexes with -diketones

and

reflect the higher local symmetry and smaller polarizability

compared to -diketonates.The emission quantum yield of 1Tb

is not very large and equals 36%. The reasons for this are the

unfulfilled resonance conditions between ligand singlet state and

ff* levels and also the large donor-acceptor distance. Worse

matching of the S₁state and the ff* Eu³⁺levels is responsible for

much lower emission quantum yield for 1Eu, which is also a

proof of the singlet energy transfer in the sensitization of Eu³⁺

and Tb³⁺luminescence by sulfonylamidophosphates ligands. In

the 1Ln complexes, the total decay rates (A_rad+ A_nrad) are largely

dominated by the radiative contribution (A_rad) which corresponds

to proper design of the structure of our ligands to decrease the

multiphonon quenching of lanthanide emission. In the 2Eu

complex, the total decay rates are also dominated by the

radiative contribution despite the low energy of ligand triplet

state. It should be emphasized that the systems with the efficient

energy transfer from the singlet state are very interesting

because of the possibility to shift the excitation range towards

lower energies.

Moreover, we are the first who took into account in the

theoretical calculations the higher lying excited levels of Eu (⁵D_J,

5

⁵L_J, G_J) and Tb (⁵D_J, G_J, L_J, H_J, F_J, I_J) and we have shown

their crucial role in the intramolecular energy transfer process in

the present case.

Furthermore, an important role of the F₅level of Tb³⁺in

7

the energy transfer process was shown. Namely new energy

transfer paths became possible from S₁to ff* levels as well as

energy transfer from T₁to ⁵D₄level was facilitated.

The knowledge of the energy transfer mechanism allows

us to choose sulphonylamidophosphates which have

chromophores, that will provide good resonance conditions

between the respective excited states of the donor and acceptor.

This in turn will allow for planning compounds - converters of UV

- Vis excitation energy with optimal luminescent properties.

Conclusions

The X-ray structure and spectroscopic properties of new

lanthanide complexes with sulphonylamidophosphates (1La,

1Eu, 1Tb, 2La, 2Eu, 2Tb) and sodium salts of the ligands (1Na,

2Na) have been studied in the solid state.

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The importance of the analyzed problems lies also in the

possibility of their generalization and that they can be used to

study the complexes of lanthanides with other groups of ligands.

Among the investigated complexes, the 1Tb complex

exhibited the highest efficiency of the energy transfer despite the

long donor-acceptor distance (5.8 Å). The efficiency of the

energy transfer, the very strong metal-centred emission in

combination with resistance of the complexes to the UV

radiation could make this family of Tb complexes promising

candidates for effective UV-to-visible energy converters. This is

provided that the donor-acceptor distance is shortened. This can

be achieved by changing the position of the chromophore by

removal solvent molecules from the intermolecular space. These

investigations are currently in progress.

Scheme 1. Synthesis of HL².

Synthesis of sodium salt of HL¹and HL².

Experimental Section

The sodium salt of HL¹(1Na) was prepared by the reaction between

equimolar amounts of sodium methylate and HL¹. The metallic sodium

(0.09g, 4 mmol) was dissolved in methanol and added to 40 ml of a

stirred solution of HL¹(1.20g , 4 mmol) in MeOH. The resulting solution

was evaporated to obtain a white powder of 1Na. It was purified by

recrystallization from hot isopropanol.

General. Unless otherwise stated, commercially available reagent grade

chemicals (Sigma-Aldrich) were used as received.

Synthesis of ligands HL¹and HL²

Dimethyl(4-methylphenylsulfonyl)amidophosphate

–

HL¹-

was

Synthesis of sodium salt of HL²(2Na). Sodium carbonate (0.29 g, 2.7

mmol) was dissolved in water (5ml). Subsequently, it was added in a few

portions to 30 ml of a stirred solution of HL²(0.85g, 2.7 mmol) in MeOH

with a small amount of activated carbon. The resulting solution was

allowed to stir for 0.5 h at 55°C. The mixture was filtered. The filtrate was

evaporated to obtain a white powder of 2Na.

synthesized according to procedure described previously. ^{[57, 58]}

The process of obtaining dimethyl 2-naphthylsulfonyl-amidophosphate

(HL²) is a multistep reaction (Scheme 1.). First step was the dissolution

of the naphthalene-2-sulfonyl chloride (10.60 g, 46.8 mmol) in dioxane

(50 ml. AR, POCH). Resulting solution was added in portions to

concentrated liquid ammonia (200 ml). After stirring for 15 minutes, the

amide obtained in this reaction (1) was filtered off and dried in ambient

air. The yield of the reaction was 91.5%. The amide (8.86 g 42.8 mmol)

was refluxed with PCl₅(8.90 g, 42.7 mmol) and CCl₄(16 ml). The

reaction was allowed to stir for 4 hours at 77°C, until a clear solution was

obtained. Solvent was removed under reduced pressure. Intermediate (2)

was obtained in 95% yield. The compound (2) (7.0 g, 20.4 mmol) was

dissolved in dioxane (200 ml) and added dropwise to the solution of

sodium methoxide in 1:4 molar ratio. Sodium methoxide was prepared by

dissolving metallic sodium (1.88 g, 81.7 mmol) in methanol (100 ml). The

reaction mixture was cooled down to -5°C until the combination of the

substrates. The obtained solution was allowed to stir for 2 h at room

temperature. The solvents were removed under reduced pressure and

the resulting crude ester (3) was stirred and heated (80°C) with 100 ml of

solution of NaOH (20%) for about 20 minutes. The resulting solution was

cooled to the room temperature and filtered. The filtrate was acidified

with concentrated HCl (pH=2) to give a precipitate of the title compound

(HL²). After the filtration, the solid was purified by recrystallization from

hot isopropanol (AR, POCH). The amidophosphate (HL²) was isolated in

80% yield.

Synthesis of the complexes.

The complexes with lanthanum, europium, and terbium with both ligands

were synthesized in the same manner as the neodymium complex in the

earlier article ^[59]. Monocrystals suitable for X-ray investigations were

obtained by recrystallization from mixture of acetone and isopropanol in

ratio 1:1 (complexes with HL¹) or by vapor diffusion from dioxane against

toluene (AR, Chempur) (complexes with HL²).

1Eu: yield 85%, IR(nujol) ν_max= 2916, 1464, 1377, 1252, 1173, 1051,

865, 745, 666, 562, 444, 333, 297, 155, 100, 73, 56 cm^-1; IR(fluorinated

oil) ν_max= 3002, 2956, 2854, 2319, 1715, 1599, 1448, 1399, 1363 cm^-1.

2Eu: yield 67%, IR(nujol) ν_max= 2924, 1463, 1376, 1271, 1236 1166,

1108, 1036, 876, 838, 816, 750, 663, 645, 619, 553, 521, 479, 450, 375,

324, 280, 171 cm^-1; IR(fluorinated oil) ν_max= 2949, 2849, 1592, 1502,

1446, 1349 cm^-1.

¹H NMR (DMSO-d6, 400 MHz) δ 8.51 (s, 1H), 7.96-7.90 (m, 4H), 7.68-

7.59 (m, 2H), 7.27 (s, 1H), 3.67-3.65 (d, 6H)

Crystal structure determination. Preliminary examination and intensity

data were carried out on a Kuma KM4CCD -axis diffractometer with

graphite-monochromated MoK radiation (=0.71073Å). Crystals of 2Eu

were poorly shaped and weakly diffracting, giving low resolution data.

The data were corrected for Lorentz, polarization and absorption effects.

The structures were solved by direct methods and refined by the full-

matrix least-squares method on all F²data using the SHELXTL ^[60]. All

non-hydrogen atoms were refined anisotropically. The positions of

hydrogen atoms were calculated and treated as riding atoms with fixed

thermal parameters. For both complexes, 1Eu and 2Ln, the solvent

10.1002/chem.201603767

Chemistry - A European Journal

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molecules (CH₃CN and C₄H₄O₂) were disordered and could not be

obtained for the same excitation wavelength, geometry and experimental

conditions. The terms ΔQ_Lnand ΔQ_stare determined from the emission

spectra, by integrating the emission intensity over the total spectral range

for the sample and the standard phosphor. The Gd₂O₂S:3%Tb (GOS:Tb,

q_st= 100%), Gd₂O₂S:3%Eu (GOS:Eu, q_st= 100%) and Y₂O₃:3%Eu (YOX,

q_st= 90%) were used as the quantum yield standards. In order to have

absolute values for the reflected radiation, BaSO₄is used as reflectance

standard (r = 91%) ^[5]. The errors in the quantum yield values associated

with this technique were estimated to fall within 10%.

[61]

modelled properly and as such, program SQUEEZE

a part of the

PLATON ^[62]package of crystallographic software, was used to calculate

the solvent disorder area and remove its contribution to the overall

intensity data. Moreover, the highest residual electron density peaks in

2Eu are located near europium atoms and may be indicative of possible

structural unresolved disorder. Crystallographic data for the structures

reported in this paper have been deposited as supplementary publication

nos. CCDC 1497264 (1Tb), 1497265 (1Eu), 1497266 (2Eu) and

1496876 (2La). These data can be obtained free of charge from The

Cambridge

Crystallographic

Data

Centre

via

Three measurements were carried out for each sample.

www.ccdc.cam.ac.uk/data_request/cif.

Experimental Judd-Ofelt parameters. Based on the emission spectra

of the 1Eu and 2Eu complexes, the experimental intensity parameters Ω_λ

(λ = 2 and 4), radiative (A_rad) and non-radiative (A_nrad) rates, were

determined from the coefficients of spontaneous emission, according to

the following expression:

Methods. The ¹H spectra in DMSO-d6 solutions were obtained on an

AVANCE 400 Bruker NMR spectrometer at room temperature. Chemical

shifts are reported references to SiMe₄as interior standard.

3c³_0

4e²³ ⁷F_U^{() 5}D₀

(2)

_

The infrared spectra were recorded as nujol or fluorinated oil mull using

Bruker IFS66/S FITIR spectrometer in the 50 - 4000 cm^-1region.

2

where χ is the Lorentz local field correction term, given by:

The presence of sodium salt (1Na, 2Na) in the complexes (1La, 2La)

was excluded by determining the content of La³⁺in examined complexes

utilizing inductive coupled plasma atomic emission spectroscopy using a

spectrometer (ARL Model 3410 ICP).

푛(푛²+2)²

휒 =

(3)

9

⁷F_U ^(^{) 5}D₀

is a squared reduced matrix element whose value is

0.0032 for the ⁵D₀→ ⁷F₂transition and 0.0023 for the ⁵D₀→ ⁷F₄one ^[67]

.

In Eq.(2) e is the electron charge, ħ is Planck’s constant, n is the

reflective index, c – speed of light and ω that is the frequency of the

transition. The A_0λ(λ = 2 and 4), are spontaneous emission coefficients,

The high-resolution absorption spectra were recorded at room

temperature with Agilent Technologies Cary 5000 Series UV-Vis-NIR

Spectrophotometer.

calculated by taking the magnetic dipole transition ⁵D₀

→

⁷F₁as the

reference, as this transition is practically insensitive to the chemical

environment around the europium ion. The following expression was

The high-resolution emission spectra were measured with a SpectraPro

750 monochromator equipped with a Hamamatsu R928 photomultiplier

and 1200 Lmm^-1grating blazed at 500 nm. A Xe arc lamp (450W) was

used ^[5]

:

used as an excitation source, coupled with

a 275 cm excitation

푆_0휆

푆₀₁

퐴_0휆= 퐴₀₁

(

) (_^⁰¹) (4)

monochromator using a 1800 Lmm^-1grating blazed at 250 nm. These

emission spectra were not corrected for the instrument response. The

corrected emission spectra of Eu³⁺complexes used for the calculations

and corrected excitation spectra were recorded using an Edinburgh

Instruments FLSP 920 spectrofluorometer equipped with a 150 W Xe

0휆

where S₀₁and S_0λare the areas under the curves of the ⁵D₀→ ⁷F₁and

⁵D₀

→

⁷F_λtransitions, with ₀₁and _0λbeing their energy barycenters,

respectively. The coefficient of spontaneous emission, A₀₁, in equation

(4) is given by the relation 퐴₀₁= 0.31 ∙ 10⁻¹¹(푛 (₀₁) . Refractive

3

(

)

lamp and

a

red-sensitive photomultiplier (Hamamatsu R-928).

indexes were determined by the immersion method.

Fluorescence decay curves were recorded using a nF920 nanosecond

flashlamp, while the phosphorescence decay curves were recorded using

a μF920H 60W Xe flashlamp (Edinburgh instruments Ltd). Additionally,

the phosphorescence of 1La was excited by the 266 nm line of the

Nd:YAG pulsed laser and the spectra were collected using a CCD

OceanOptics SD-2000 spectrophotometer, immediately after switching

off the excitation source.

Theoretical calculations. The energy transfer between the ligand and

the Ln³⁺ion involves the singlet-triplet (spin - forbidden) and the singlet-

singlet (spin - allowed) bands of the ligand and the 훼^′퐽^′↔ 훼퐽 transition

lines of the Ln³⁺ion. This process was treated in terms of the direct

[27,28]

Coulomb and the exchange interactions

interaction the transfer rate is given by:

.

For the Coulomb

The luminescence measurements were performed at room temperature

and 77 K using a quartz Dewar cooled by liquid nitrogen.

2

ed

2

λ

2

λ

( )

2

〉

2

(

)〈 〉

〈

e

S

Ω

λ+1 r

3∥C ∥3

(1-σ )

λ

L

( )

λ

'

∑

〈

〉

W_Cl=

(

)

+

)| α J ∥U ∥αJ | (5)

2

λ=2,4,6

6

λ+2

(

G 2J+1

R

S

(R

)

L

where J and J’ represent the total angular momentum of the Ln³⁺ion

electronic states involved in energy transfer process. S_Lis the dipole

strength of the ligand transition, G stands for the degeneracy of the donor

Quantum yield measurements. The absolute emission quantum yield

was measured at room temperature according to the method developed

at the Philips Research Laboratories ^[63-66]. The overall emission quantum

yield, Q^L_Ln, defined as the ratio between the number of emitted and

absorbed photons, was determined according to: U^(λ)

state and Ω^edare the contributions of the forced electric dipole

λ

mechanism to the 4f – 4f transition intensity parameters (Judd – Ofelt

theory). <r^λ> are the 4f radial integrals, σ_λare the shielding factors due to

shielding effects produced by the filled 5s and 5p sub-shells, R_Lis the

distance from the Ln³⁺ion nucleus to the barycenter of the ligand

electronic state and U^(λ)are unit tensor operators. The exchange

ΔΦ_Ln

푄_퐿^퐿_푛= (^1−푟) (_ΔΦ) 푞_푠푡

(1)

푠푡

1−푟_퐿푛

st

where r_stand r_Lnare the amounts of exciting radiation reflected by the

standard and by the sample, respectively, and q_stis the quantum yield of

the standard phosphor. The values of r_st, r_Ln, ΔQ_Lnand ΔQ_stmust be

intramolecular energy transfer rate (W_Ex) is given by ^[27]

:

10.1002/chem.201603767

Chemistry - A European Journal

FULL PAPER

2

⟨

| ⟩⁴

4f L 8πe²

[4]

[5]

[6]

Y. Gerasymchuk, L. Tomachynski, M. Guzik, A. Koll, J. Jański, Y.

Guyot, W. Stręk, G. Boulon J. Legendziewicz, J. Photochem. Photobio.

A 2015, 309, 65–71.

' '

*

〈

〉

〈

| ∑ ( ) ( )| 〉

W_Ex

=

₄| α J ∥S∥αJ | | φ _iμ_zi s_mi φ | F (6)

(

)

2J+1 3ℏR_L

<4f│L> is the overlap integral between the 4f orbitals and ligand

eigenfunctions, S_mis a spherical component of the spin operator of

electron i in the ligand, μ_zis the z-component of its dipole operator, F is

the donor-acceptor spectral overlap that depends on the appropriate

energy mismatch conditions and S is the total spin operator of the Ln³⁺

ion. The selections rules on J are obtained using the reduced matrix

elements of the unit tensor operators U^(λ)and those for the total spin

operator S. From the above matrix elements, as far as J is considered a

good quantum number, the selection rules are │J-J’│=0 or 1, for the

exchange mechanism, and J’-J ≤ λ ≤ J+J’ for the Coulomb mechanism, in

both cases J’=J=0 excluded.

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1

Δ

√

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^ln(2)푒푥푝 [− (_ℏ훾) ln(2)]

(7)

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휋

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Boltzmann factor 푒^푘퐵

푇

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W_ETT

[(τ^-_T¹+W_ETT)τ_S]

q= (_σ^σ) A_radτ [

] [W_ETS

+

]

(8)

Em

τ^-_S¹+W_ETS

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Abs

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퐸푚

휎

퐴푏푠

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Acknowledgements

The authors wish to acknowledge support through the grant

of Minister of Science and Higher Education POIG.01.01.02-

02-006/09 and the grant Minister of Science and Higher

Education for young scientists 2432/M/WCH/14.

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The rarely observed singlet state energy transfer was proved to be a dominant

mechanism in the new class of lanthanide complexes. This is strongly evidenced by

theoretical calculations using the higher lying excited levels of Eu (⁵D_J, ⁵L_J, ⁵G_J) and

Tb (⁵D_J, ⁵G_J, ⁵L_J, ⁵H_J, ⁵F_J, ⁵I_J) for the first time. Photophysical properties make this

family of Tb complexes promising candidates for effective UV-to-visible energy

converters.

Article Doi

Contribution of Energy Transfer from the Singlet State to the Sensitization of Eu3+and Tb3+Luminescence by Sulfonylamidophosphates

DOI: 10.1002/chem.201603767

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Article abstract of DOI:10.1002/chem.201603767

Full text of DOI:10.1002/chem.201603767

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