A Reusable Eu3+ Complex for Naked-Eye Discrimination of Methanol from Ethanol with a Ratiometric Fluorimetric Equilibrium in Methanol/Ethanol Mixtures

Article abstract of DOI:10.1002/ejic.201900843

Immediate naked eye distinction of methanol from ethanol can be performed by simple dissolution, in each of these solvents, of either [Na][EuFOD₄] (1) complex or in mixture of products from the reaction between [P_6,6,6,14][Eu(FOD)

Full text of DOI:10.1002/ejic.201900843

DOI: 10.1002/ejic.201900843
Full Paper
Luminescent Sensors
A Reusable Eu3+ Complex for Naked-Eye Discrimination of
Methanol from Ethanol with a Ratiometric Fluorimetric
Equilibrium in Methanol/Ethanol Mixtures
João P. Leal,[a,b] Filipe A. Almeida Paz,[c] Ricardo F. Mendes,[c] Tiago Moreira,[d] Mani Outis,[d]
César A. T. Laia,*[d] Bernardo Monteiro,*[a,b] and Cláudia C. L. Pereira*[d]
Abstract: Immediate naked eye distinction of methanol from quantification of methanol in mixtures with ethanol is de-
scribed. This method is based on the changes in the Eu3+ lumi-
nescence in the visible region due to the interaction of the
[P6,6,6,14][Eu(FOD)4] complex with these alcohols. A limit of de-
tection as low as 15 % (w/w) of methanol in mixtures of eth-
anol/methanol is discussed considering a linear calibration
curve.
ethanol can be performed by simple dissolution, in each of
these solvents, of either [Na][EuFOD4] (1) complex or in mixture
of products from the reaction between [P6,6,6,14][Eu(FOD)4] and
+
NaOPhMe3, referred as 2, ([P6,6,6,14
]
= trihexyltetradecylphos-
phonium cation, FOD– = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-
octane-4,6-dionate). Additionally, an easy, low-cost, efficient
and fast spectrofluorimetric method for the detection and
Introduction
red as the immersion time increased in ethanol, acetonitrile,
and diethyl ether. Moreover, these two MOFs presented solvato-
chromism in the presence of diethyl ether vapors. This allows
these porous structures to be used as ethanol, acetonitrile, and
diethyl ether sensors by the naked eye.[2]
Certain compounds present a variation of the absorption and
emission spectra depending on the solvent where they are dis-
solved. This effect is called solvatochromism and was previously
presented by Binnemans and co-workers as a new tool to dis-
tinguish structurally similar compounds. Strong solvent effects
were observed for lanthanide complexes containing the hemi-
cyanine chromophore. This effect was explained based on di-
polar interactions between the solvent and the complexes
when a series of n-alcohols are used as solvent.[1]
We have reported, among other studies, the solvatochromic
properties within the [P6,6,6,14][Ln(NTA)4] family (NTA
=
naphthoyltrifluoroacetonate and [P6,6,6,14]+ = trihexyltetradecyl-
phosphonium).[3,4] In the case of the Eu3+ ionic liquid, the li-
gand absorption spectra did not present a significant solvato-
chromic effect. However, comparing the pure Eu+ compound
(liquid state, at 65 °C) and when dissolved in cyclohexane,
methanol, and toluene a redshift and band broadening was
found for the pure compound. This result indicates aggregation
of ꢀ-diketonate ligands (NTA) in the ionic liquid form and is
responsible for the yellowish color of the pure liquid compound
while colorless in solution. The Gd3+ analogue presented the
same behavior and the Tb3+ had an irrelevant solvatochromic
effect (Δλmax ≈ 5 nm). However, in the case of the Dy analogue
a clear solvatochromic effect is observed (Δλmax ≈ 71 nm) with
a blueshift in non-alcohol solvents (cyclohexene and toluene)
when compared to methanol and 1-buthanol. Unlike what was
observed for the Eu3+ complex, in the case of the Dy3+ the
solvent interacts preferentially with the ligands through
hydrogen bonds, which weakens the ligand–metal bond lead-
ing to the observed solvatochromic effect.
Recently, Cui and co-workers reported that the color of two
isostructural Tb- and Eu-MOFs [with the ligand 5,5′,5′′-(1,3,5-
triazine2,4,6-triyl)tris(azanediyl)triisophthalate]
gradually
changed from colorless to golden, to dark orange and to dark
[a] Departamento de Engenharia e Ciências Nucleares, Centro de Ciências e
Tecnologias Nucleares (C2TN), Instituto Superior Técnico,
Estrada Nacional 10, 2695-066 Bobadela, Portugal
E-mail: bernardo.monteiro@ctn.tecnico.ulisboa.pt
[b] Departamento de Engenharia e Ciências Nucleares, Centro de Química
Estrutural (CQE), Instituto Superior Técnico,
Estrada Nacional 10, 2695-066 Bobadela, Portugal
E-mail: bernardo.monteiro@ctn.tecnico.ulisboa.pt
https://cqe.tecnico.ulisboa.pt/members/13317
[c] Departamento de Química, CICECO - Instituto de Materiais de Aveiro,
Universidade de Aveiro,
3810-193 Aveiro, Portugal
[d] Dr, Departamento de Química, Associated Laboratory for Sustainable
Chemistry-Clean Processes and Technologies - LAQV-REQUIMTE,
Universidade Nova de Lisboa,
Two-dimensional layered structures based on CdII and ZnII
exhibit multichromism such as solvatochromism, thermochro-
mism and piezochromism when subjected to various external
stimuli such as solvent, heat, and mechanical grinding or pres-
sure. Interestingly, these framework materials proved to be use-
ful for visual detection of DMF, DMA, DMSO, CH3CN, acetone
2829-516 Caparica, Portugal
E-mail: ccl.pereira@fct.unl.pt
catl@fct.unl.pt
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900843.
Eur. J. Inorg. Chem. 0000, 0–0
1
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
and Et3N, but not in case of exchange with water, ethanol or be attributed to the “crystal field effect” of the Eu3+ ion in a C1
methanol.[5]
symmetry and is constituted by five intense and well-defined
bands at 611, 612, 619, 623 and 633 nm. The Stark splitting of
the ground state level of F2 into “2J+1 = 5” sublevels with J =
Complexes of ZnII octa(carbazolyl)phthalocyanines revealed
a pronounced solvent effect presenting green color in DMF,
THF, pyridine, acetone, acetonitrile and DCM, a blue color in
hexane, cyclohexane, EtOH, 2-propanol, and 1- pentanol, and a
purple color in CHCl3, CCl4, benzene, toluene and p-xylene. This
color change was explained by the transparency shift region of
phthalocyanines in the 400–600 nm region, which is responsi-
ble for their common green color.[6]
7
2 demonstrates that the Eu3+ ions occupy the possible lowest
local site symmetry. In opposition, a lower number of J-splitting
in ethanol (see Supporting Information for details) represents a
higher site symmetry of the Eu3+ ions.[16]
Solvatochromic effects has been difficult to find between
ethanol and methanol no doubt due to their similarities in
terms of polarity, density and coordinating character.
Methanol is a highly toxic substance, but it is unfortunately
very difficult to differentiate from other alcohols (ex. ethanol)
without performing chemical analyses. Methanol quantification
is typically based on the use of advanced techniques such as
head space gas chromatography (GC),[7–10] GC-flame ioniza-
tion[11] detection, high-performance liquid chromatography
(HPLC), Raman[12,13] and infra-red spectroscopy.[14] A bioenzy-
matic analytical system was developed consisting of two bio-
sensors, one based on alcohol dehydrogenase (ADH) that re-
sponds only to the ethanol and the second one based on alco-
hol oxidase (AOX) that responds to both methanol and eth-
anol.[15]
Figure 1. Luminescence spectrum of [Na][Eu(FOD)4] with a concentration of
0.1 mM in methanol (black line) and ethanol (red dotted line) upon excitation
with λexcitation = 350 nm. Inset: Picture taken under 366 nm UV light that
This report summarizes our efforts to differentiate ethanol
from methanol by simple naked eye observation or under a
UV-light lamp. We developed an approach for a simple spectro-
fluorometric method to quantify methanol in the presence of a
large excess of ethanol with limit of detection of 15 %. The
method is based on the solvent displacement from an Eu3+
complex dissolved in ethanol which in the presence of meth-
anol produces an intense purple color, easily quantified and
even visible at the naked eye. Besides this and with an equip-
ment as simple as a UV-light lamp we propose a method to
distinguish ethanol from methanol using a stable luminescent
Eu3+ complex allowing its reutilization in several analysis.
Selectivity of this method for methanol detection in mixtures
with other n-alcohols is also discussed.
evidences a brighter orange emission in ethanol solution.
After several attempts, we were able to obtain poor quality
crystals of [Na][Eu(FOD)4](H2O) with the structure being un-
veiled by single-crystal X-ray diffraction studies as depicted in
Figure 2. The complex crystallizes in the centrosymmetric P21/n
space group, with the asymmetric unit being composed of four
independent FOD– anionic linkers connected to a Eu3+ cation,
plus one Na+ metal center interacting with three FOD– and one
disordered water molecule. The Eu3+ ion is octacoordinated,
Results and Discussion
[Na][Eu(FOD)4]
Strong solvatochromic effect was primarily observed for the
complex [Na][Eu(FOD)4] (1) as shown in the luminescence spec-
tra in Figure 1. The change in the coordination sphere, attrib-
uted to a decrease of symmetry caused by solvent change, is
observable by an increment in band intensity that corresponds
5
to the D0→7F0 strictly forbidden transition at 579 nm. The
5D0→7F0 band intensity of the [Eu(FOD)4]– moiety (FOD–
1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionate)
=
re-
sponds differently to ethanol and methanol. We associate this
effect with the higher coordination character of methanol,
which together with a labile Eu–O bond from one of the ꢀ-
diketonate ligands, leads to a more asymmetric Eu3+ structure.
Figure 2. Ball and stick representation of the [Na][Eu(FOD)4](H2O) complex
(1).
5
The splitting of the D0→7F2 transition band in methanol can
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
2
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
{EuO8}, to all four FOD– ligands with the overall coordination dium 2,4,6-trimethylphenolate was added stoichiometrically
geometry resembling a distorted square antiprism. Remarkably, and without solvent to slightly heated [P6,6,6,14][Eu(FOD)4]. The
three FOD– ligands are orientated in the same direction, allow- viscous light-yellow material turned immediately to dark purple,
ing the first -CF2- groups to interact with the Na+ ion which lies showing color stability for several months after cooling to
in a “pocket” of the complex close to the Eu3+ ion. Experimental ambient temperature. The mixture of the reaction between
and simulated (from single-crystal data) powder X-ray diffrac- [P6,6,6,14][Eu(FOD)4] and NaOPhMe3 (hereafter coined as 2) was
tion patterns of compound [Na][Eu(FOD)4](H2O) agree well with characterized by ESI-MS using methanol and ethanol as sol-
the proposed structure (see Supporting Information for details). vents (Table S1 - Supporting Information). Remarkably, other
–
molecules such as azide (N3 ) or methoxide (CH3O–) only stabi-
lized the colored form for short periods, turning into a light-
yellow viscous liquid within just a few hours.
[P6,6,6,14][Eu(FOD)4] + NaOPhMe3
It is worth to mention that when using newly purchased
[P6,6,6,14]Cl reagent, it usually has a basic character and the
[P6,6,6,14][Eu(FOD)4] compound does not change color when
heated. It is necessary to either wash with water the phos-
phonium reagent, or in alternative the final complex, until
they reach a neutral pH in order to guarantee that no excess
of basic FOD– is present. Additionally, we have prepared the
[P6,6,6,14][Eu(FOD)3(DBM)] (DBM = dibenzoylmethanate)[20] and
another side comment that worth mention is that if even only
one FOD– ligand is substituted the thermochromism is lost.
We have previously observed that the reaction product be-
tween NaFOD and [P6,6,6,14]Cl yielded, when heated, in an irre-
versible way the purple P6,6,6,14FOD compound.[18] According to
the preformed solubility tests, this organic purple compound
was more stable (maintained the pinkish color) in MeOH than
in EtOH (although after ethanol evaporation the color was re-
gained again).[18] This behavior lead us to test the reaction
product 2 in these two solvents and the results obtained are
depicted in the equilibrium proposed in Scheme 1.
For the previously reported [P6,6,6,14][Eu(NTA)4], when dissolved
in protic solvents (e.g., 2-propanol or ethanol) a pronounced
interaction between NTA– and the solvent was evaluated.[3] The
emission spectra agrees with a high symmetry point group for
Eu3+ which breaks down if the compound is dissolved in polar
solvents (e.g., n-alcohols).[3,17] Also, we recently reported that
the [P6,6,6,14][Eu(FOD)4] ionic liquid undergoes a disturbance in
the coordination sphere of the Eu3+ ion, with a disruption of
the local symmetry when heated up to ca. 80 °C.[18] This can be
explained by the fact that the inclusion of fluorine substituents
in the organic ligand, with a high electron-withdrawing effect,
reduces the charge density on the oxygen atoms inducing par-
tial dissociation of one of the ligands.[19] The interaction of the
+
acidic [P6,6,6,14
]
counterion with the labile oxygen from one
FOD– ligand forms an electron delocalized six-membered ring
with Eu3+ with a red/purple color. Since this red colored com-
plex is only seven coordinated, we made several attempts to
block the reversibility of this process. The method used con-
sisted on coordinating an additional ligand to the red com-
pound in order to fill the coordination sphere, preventing the
labile oxygen to coordinate again to the Eu3+ center, and thus
obtaining a red eight-coordinated neutral Eu3+ complex. This
could not be achieved by neutral donor ligands like alcohols.
Color irreversibility for long periods could only be achieved
after the addition of anionic ligands with a highly electron
donor nature such as 2,4,6-trimethylphenolate (Scheme 1). So-
Like was previously observed for the purple P6,6,6,14FOD com-
pound, in ethanol mixture 2 dissociates giving a yellowish solu-
tion while in methanol it kept the purple/pinkish (Figure 3, left).
This means that the reaction with NaOPhMe3 is more extent
in methanol than in ethanol forming [Eu(FOD)3OPhMe3]– and
P6,6,6,14FOD (supported by ESI-MS). This can be explained by the
ability of the ethanol to interfere with the P–O bond of the
P
6,6,6,14FOD compound, yielding free FOD– that is able to re-
place [OPhMe3]– from de asymmetric [Eu(FOD)3OPhMe3]– com-
plex, ultimately increasing the concentration of the emissive
[P6,6,6,14][Eu(FOD)4] in 2. The mechanism behind this proposal is
Scheme 1. Solvent dependent equilibrium of 2. The colors used in the
scheme (except for NaOPhMe3, which is a white powder) are an approxima-
tion of the colors of the compounds {[P6,6,6,14][Eu(FOD)4] 2 is light yellow,
Na[P6,6,6,14][Eu(FOD)3OPhMe3] is most certainly light yellow and P6,6,6,14FOD
is purple}.
Figure 3. Picture under 366 nm UV light of 2 in methanol and ethanol with
the same concentration (1 m
Figure 4.
M), with corresponding absorption spectra in
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
3
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
reinforced by the fact that methanol, with a higher coordination
ability to Eu3+ than ethanol, prevents any existing FOD– in the
mixture to coordinate to [Eu(FOD)3OPhMe3]–, thus stabilizing
the purple color (Figure 3, left).[21]
According to ESI-MS data, higher concentrations of
[Eu(FOD)3OPhMe3]–, and consequently lower concentrations of
[P6,6,6,14][Eu(FOD)4] (4.4 % in methanol vs. 1.9 % in ethanol) re-
sult in a less effective sensitization mechanism of the Eu3+ com-
plex and, consequently, a poorer energy transfer with the con-
comitant establishment of a P–O interaction between the free
FOD– moiety and the [P6,6,6,14]+ cation (Figure 3, right). This ex-
plains the ESI-MS result showing the detection of neutral
P6,6,6,14FOD which is exclusively found in methanol (16.7 % in
methanol vs. 0 % found in ethanol, see Table 1).
Table 1. ESI–MS analysis results in the negative mode. Molecular weight
(MW), percentage of peak area in methanol ethanol and the attributed
anionic species.
Figure 4. Normalized absorption spectra of 2 in methanol (red line, λmax. =
574 nm) in 1:1 mixture of ethanol:methanol (green line, λmax. = 576 nm ) and
ethanol (blue line, λmax. = 578 nm).
MW
CH3OH
C2H5OH
Attributed anion
295
3.7
0
0
1.9
100
FOD–
[P6,6,6,14FOD]FOD–·2CH3OH
Photoluminescence
1137
1173
1332
16.7
4.4
100
–
Eu(FOD)3OPhMe3
Eu(FOD)4
Figure 5. shows the room temperature excitation spectra of 2,
–
monitored within the intra-4f6, D0→7F2 transition observed at
5
612 nm. The spectrum displays a large broad band ascribed
to the excited states of the ligands (335–425 nm), with three
In summary, Figure 3 evidences the more intense purple/
pink color of mixture 2 in methanol as the P6,6,6,14FOD content
is higher. On the other hand, in ethanol the solution has a more
yellow/pink color, as the purple/pinkish P6,6,6,14FOD content is
lower. Under UV light, the solutions also have different emission
intensities, colors and brightness has the less emissive
[Eu(FOD)3OPhMe3]– moiety is more abundant in methanol and
the more emissive [P6,6,6,14][Eu(FOD)4] is more abundant in eth-
anol.
7
components peaking around 355, 395, 420 nm and the F0,1→
5D1,2 transitions of the Eu3+ ion.
Absorption Spectra
In the UV/Vis spectra of 2, the band with λmax. between 574–
578 nm corresponds to the phosphorane like compound
Figure 5. Excitation spectra of 2 at room temperature in methanol monitored
at 612 nm.
P
6,6,6,14FOD formed during the 2,4,6-trimethylphenolate addi-
tion to the complex [P6,6,6,14][Eu(FOD)4] (Scheme 1).[18] ESI–MS
The luminescence spectra of 2 was recorded by fixing the
5
confirms the presence of this neutral organic compound while excitation wavelength at 350 nm (Figure 6). The D0→7F0, and
Eu3+ is hepta-coordinated with three FOD– units plus one 2,4,6– 5D0→7F2 transitions (forbidden and hypersensitive electric and
trimethylphenolate unit. When 2 is dissolved in methanol the dipole transitions, respectively) are the ones most affected by
maximum absorption spectra in the visible region is centered local site symmetry. The spectra have significant differences for
at 574 nm to which corresponds a higher amount of the purple the ethanol and methanol solutions. This solvatochromic effect
P
6,6,6,14FOD compound. The final color of the solution results prompted us to further investigations with special attention to
from a balance of different amounts of Na[Eu(FOD)3OPhMe3] the 5D0→7F0 transition that systematically increases its intensity
(light yellow), [P6,6,6,14][Eu(FOD)4], (light yellow), NaOPHMe3 upon addition of methanol to an ethanolic solution, ranging
(white) and P6,6,6,14FOD (purple) in each of the alcohol solutions. from 0.02 to 0.50 molar fractions (Figure 1 and Figure 3 and
In ethanol, due to a lower amount of P6,6,6,14FOD (purple), and Supporting Information for additional details). The emission is
a relative higher amount of light yellow Eu3+ complexes, the distributed in the 577–625 nm spectral range, with lines associ-
solution has a slightly higher absorption wavelength (578 nm). ated with 4f–4f transitions of the 5D0 excited state to 7F0–2, with
In 1:1 mixture of ethanol and methanol the solution has a λmax. the strongest peak at 612 nm attributed to the 5D0→7F2 transi-
of 576 nm, exactly between the middle of 574 and 578 nm tion.[22–24] Addition of methanol favors the formation of
observed for pure methanol and ethanol, respectively (Fig- [Eu(FOD)3OPhMe3]– in a higher extent as reflected in the
ure 4).
5D0→7F0 and 5D0→F2 band intensities and shapes. A gradual
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
4
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
decrease in the intensity of 5D0→7F2 is observed during the
addition of methanol (see the Supporting Information) because
of the gradual FOD– ligand replacement by phenolate. The line
splitting observed for higher concentrations of methanol indi-
cates a high level of asymmetry around the Eu3+ centre.[23]
A
single oxygen donor atom of the 2,4,6-trimethylphenolate is
less polarizable than the coordinating carbonyl groups of che-
lating ꢀ-diketonates, resulting in a decrease in the intensity of
the hypersensitive 5D0→7F2 transition.[23] The 5D0→7F0 band in-
tensity peaks reaches its maximum when the complex is solu-
bilized in methanol showing, for both solutions, a labile coordi-
nation between Eu3+ and the solvents, which grows stronger
with methanol. This process modifies the geometry of the com-
plexes to a more asymmetric structure around the Eu3+ center
(Figure 6a and Figure 6b). A similar behavior was found for
methanol/1-butanol and methanol/1-propanol mixtures allow-
ing this method to be used in other mixtures of methanol/n-
alcohol.
Figure 7. Sensing assay as the normalized luminescence response in the
5D0→7F0 transition range towards addition of different amounts (molar frac-
tion ꢀ) of ethanol to Eu3+ of 2 in methanol upon irradiation at 350 nm.
Chemical Characterization
The positive mass spectra, of the ESI-MS analysis, are identical
and show only one peak with m/z of 484 Da that corresponds
to the [P6,6,6,14]+ cation. In the negative mode, the most abun-
dant specie is the [Eu(FOD4)]– anion with m/z of 1332 Da, mean-
ing only that this anion is the most easily ionizable. In methanol
a peak appears with a 1137 Da mass that could be attributed to
[P6,6,6,14FOD2]– anion, [(P6,6,6,14FOD)–FOD]–, which was detected
with two solvent molecules. The MS2 spectra of this isolated
peak show the loss of 32-unit mass that corresponds to meth-
anol loss. It was not possible to see the loss of the second
methanol unit since the resulting peak was too small to per-
form MS3. Both negative spectra, with methanol and ethanol,
show a peak mass at 1173 that can be associated to the anionic
moiety [Eu(FOD)3OPhMe3]–. Considering in each case the peak
at 1332 as 100 %, the peak at 1173 has an intensity of 4.4 % in
methanol and 1.9 % in ethanol (Table 1). This difference is re-
lated with a higher concentration of this anion in methanol,
which is in accordance with the photoluminescence data.
2, when heated, present an incredibly high thermal stability
with decomposition temperature starting close to 300 °C,
Figure 6. Normalized luminescence spectrum (towards the most intense
5D0→7F2 band) of Scheme 1 reaction products; in mixtures methanol in eth-
anol with the methanol molar fractions ranging from 0 to 1, with λirradiation
=
350 nm. a) range 577–600 nm b) range 605–625 nm.
Luminescence of 2 shows that ethanol interferes (i.e.,
changes the Eu3+ symmetry) less than methanol, thus constitut-
ing an accurate and precise method for quantification of the
latter (Figure 7). The 5D0→7F1 transition reflects directly the
7
crystal-field splitting of the F1 level (see Supporting Informa-
tion for details). Asymmetry arises as a balance between the
concentrations of [Eu(FOD)4]– and [Eu(FOD)3OPhMe3]– com-
plexes. It is further directly dependent to the ratio of methanol
and ethanol in the solvent mixture.
Figure 8. Thermogravimetric analysis of [P6,6,6,14][Eu(FOD)4], products of reac-
tion of [P6,6,6,14][Eu(FOD)4] and NaOPhMe3, [P6,6,6,14][FOD] and P6,6,6,14OPhMe3.
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
5
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
almost 100 °C higher than the starting compound
[P6,6,6,14][Eu(FOD)4], Figure 8. This can be explained by the previ-
ously proved presence of [P6,6,6,14][FOD] in the products mixture
with temperature decomposition slightly lower than 300 °C
(blue line, Figure 7). The low weight loss up to 200 °C is ex-
pected for highly fluorinated complexes that usually presents a
minimum of adsorbed solvent retained in the structure. The
remaining mass residues are due to EuOF phases and sodium
oxide phases that are stable at 600 °C.
Sensing Assays
A ratiometric method was used to calculate the ratio of the
luminescence intensities in order to determine the methanol
concentration in methanol/ethanol mixtures. Calibration assays
were performed by adding different amounts of methanol to a
solution of 2 in ethanol and correlate the methanol concentra-
tion with the normalized intensity of the 5D0→7F0 transition
(λemision = 579 nm, Figure 6).
Figure 9. Calibration curves with linear behavior for methanol estimation in
ethanol/methanol mixtures. ꢀ molar fraction of methanol in ethanol.
measuring the I(5D0→7F0)/I(5D0→7F2) ratio after the addition of in-
creasing amounts of methanol to a solution of 2 in ethanol. 2
For these assays, 2 was solubilized in a quartz cuvette in was fully recovered after the removal of the solvents, thus
2 mL of ethanol. The calculated amounts of methanol were showing a good stability of the sensitizing molecule which al-
added to obtain a final molar fraction (ꢀ) of methanol in the lowed three consecutive calibration assays (see Supporting In-
solvent mixture of: 0.02, 0.04, 0.08, 0.08, 0.1, 0.2, 0.3, 0.4 and formation for details).
0.5 as represented in Figure 9.
For molar ratios below 0.2 we were able to observe an irregu-
There are numerous methods for the determination of limit lar increment on the 5D0→7F0 transition which did not allow an
of detection, many of which are described in a review by Belter accurate determination of the sensitivity and limit of detection.
et al.[25] A linear trend for the calibration curve of the molar From the calibration curve we can only speculate that this
fraction, ꢀ, of methanol in ethanol was then observed in the method has, still, some sensitivity for concentrations of meth-
0.2–1 range (Graphic 1). The calibration curve with ꢀ from 0.2 anol lower than 0.2 (15 % w/w).
exhibited a linear trend with, at least, R2 = 0.9993 with a limit
The methanol-sensing study of 2 demonstrates that the
characteristic luminescence intensity of Eu3+ (5D0→7F0,
of detection (LOD) of 0.207 (LOD = 3 × σ/m, σ is standard devia-
5
5D0→7F1 and D0→7F2) is modified immediately as the meth-
tion of noise and m is calibration curve slope; see Supporting
Information for details) and sensitivity of 0.0819.[26,27]
anol can efficiently stabilize the asymmetric ꢀ-diketonate com-
Accuracy assays were highly reproducible (in triplicate – see plex promoting a more pronounced purple color due to higher
Supporting Information for details) and were carried out by amounts of [P6,6,6,14][FOD]. In contrast, ethanol breaks the P–O
Figure 10. Normalized luminescence (upon excitation at 350 nm) of 2 in methanol (dark line), in ethanol (dashed blue line), in 1-propanol (red line) and in 1-
butanol (square dotted red line).
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
6
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
interaction of [P6,6,6,14][FOD], allowing coordination of the 4th
FOD– anionic ligand to Eu3+ and increasing local metal symme-
Microanalyses for C and H were carried using a Thermo Finnigan–
CE Instruments Flash EA 1112 CHNS series. FT–IR spectra (range
4000–400 cm–1) were collected using a drop of sample between
KBr round cell windows on a Thermo Scientific Nicolet iS50 FT–IR
spectrometer, by averaging 32 scans at a maximum resolution of
4 cm–1. TGA curves were obtained using a Thermal Analysis Ta
Q500–2207, with a scanning rate of 5 °C min–1, with samples
weighing around 6 mg in Aluminum crucibles. The calibration of
the TGA equipment was made following the recommendation de-
scribed in the manufacturer's manual. Electrospray Ionization Mass
Spectrometry (ESI–MS). ESI–MS was performed using a Bruker HCT
quadrupole ion trap mass spectrometer. Sample solutions approxi-
try. Methanol is a more strongly coordinating solvent to Eu3+
,
and with the simultaneous presence of a highly electron donor
ligands such as [OPhenMe3]–, FOD– is removed leading to a
5D0→7F0 band increase.
Selectivity assays were performed by testing other alcohols
like 1-propanol and 1-butanol as substituents of ethanol (Fig-
ure 10). For 2, the highest Eu3+ complex asymmetry was found
for methanol solutions, while addition of methanol, even in very
low concentrations to all the other solutions modified immedi-
ately the luminescence profile (Figure S7, see Supporting Infor-
mation for details). In opposition, addition of ethanol, 1-prop-
anol and 1-butanol to a methanol solution of 2 didn′t produce
significant changes in the luminescence spectrum of the emis-
sive specie.
mately 10–5
M in acetonitrile were introduced to the ESI source via
a syringe pump at a flow rate of 150 mL min–1. The heated capillary
temperature was set to 250 °C and the cover gas (N2) to a flow rate
of 2 L min–1. Both positive and negative modes were detected to
see the existing cations and anions. Spectroscopic Measurements.
UV/Vis absorbance spectra were performed using a UV/Vis-NIR Var-
ian Cary 5000 spectrophotometer within the spectral range 200–
800 nm. NMR studies were performed on a Bruker Avance III 400
using deuterated methanol and dichloromethane as solvents.
Conclusions
Sodium 2,4,6–trimethylphenoxide, NaOPhMe3, was synthesized
using standard Schlenk line and dry box techniques in an atmos-
phere of N2 to avoid hydrolysis. Small portions of freshly cut metal-
lic sodium was added to a THF solution of 2,4,6–trimethylphenol
(1 g) and the resulting mixture was left at room temperature whilst
stirring. When the evolution of H2 ended, the supernatant was de-
canted, and the solvent evaporated under reduced pressure yield-
ing NaPhO Me3 as a white powder.
Here, we observe unique photoluminescent solvatochromism
between methanol and ethanol, visible at naked eye.
This highly reproducible ratiometric methodology is based
in the changes of the intensities of the hypersensitive electric
dipole transition bands 5D0→7F0, and 5D0→7F2, which are
highly sensitive to the coordination geometry around Eu3+ and
can be rationalized by the I(5D0→7F0)/I(5D0→7F2) ratio as a function
of the methanol molar concentration in ethanol. The changes
observed in these bands could be explained by the presence
of [P6,6,6,14][Eu(FOD)4] and NaOPhMe3 in ethanol that, in the
presence of increasing amounts of methanol, change to a mix-
[P6,6,6,14][Eu(FOD)4] was prepared according to a procedure al-
ready reported by us. NaFOD(0.0767 g, 0.241 mmol) was added
stoichiometrically to a solution of Eu(FOD)3 (0.250 g, 0.241 mmol)
in methanol. After 2 hours of reaction at room temperature, 1 equiv-
ture of increasing amounts of [Eu(FOD)3OPhMe3]– and alent of P6,6,6,14Cl (0.117 g, 0.241 mmol) previously dissolved in a
minimum of CH2Cl2 was added dropwise to the solution and left
under magnetic stirring for one hour. The solvent was then re-
moved under reduced pressure and the resultant oily solid was dis-
solved in CH2Cl2. NaCl was removed by centrifugation and the
[P6,6,6,14][Eu(FOD)4] was recovered as a neat light yellow oil after
solvent evaporation under reduced pressure with an yield of 80 %.
Anal. Calcd. for [PC32H68][Eu(C10H10O2F7)4]: C, 47.61; H, 5.99 %. Ex-
perimental; C, 47.69; H, 6,31.
[P6,6,6,14][FOD]. These modifications lead to a decrease in the
Eu3+ coordination symmetry, quantified by the I(5D0→7F0)
(5D0→7F2) ratio. This is further detectable by the naked eye due
to a color change in the solution because of the increasing
concentrations of [P6,6,6,14][FOD].
The methanol-sensing studies reported in this manuscript
show that the mixture of the reaction between
[P6,6,6,14][Eu(FOD)4] and NaOPhMe3 can be used as an approach
to a fast and low-cost sensitivity method to determine the
methanol content in methanol/ethanol mixtures from as low as
ꢀ of 0.2 that corresponds to 15 % (w/w). In terms of limit of
detection this method is still not competitive when compared
with other analytical methods that are currently used like Ra-
man spectroscopy or even available in bienzymatic disposable
kits.
/
I
[(P6,6,6,14)(FOD)] + Na[Eu(FOD)3(OPhMe3)] reaction mixture, (2),
was prepared by mixing in a flask under N2 stoichiometric amounts
of [P6,6,6,14][Eu(FOD)4] (200 mg; 1.1 mmol) and NaOPhMe3 (17 mg;
1.1 mmol). Within a few minutes, at room temperature, the pale-
yellow oil starts to turn to orange with red spots where the powder
of the NaOPhMe3 sticks to the walls. In order to get a uniform oil
this mixture was heated to 50 °C, whilst stirring, for 30 min forming
fluid purplish red oil. 1H–NMR (ppm): 6.14 (s, PhHOMe3–), 4.89 (s,
Hα-FOD), 3.04 [s, –PhO(CH3)3], 2.21 (t, +P6,6,6,14, Hα), 1.52–0.84 (m,
+P6,6,6,14), 0.97 (s, –CH3 FOD–). 13C-NMR (ppm): 199 (O=C-C(CH3)3,
FOD–). 31P-NMR (ppm): 33.37 (+P6,6,6,14).
In summary, we have successfully designed a new optical
method based on solvatochromic effects of a Eu3+ complex,
that allows quantification of methanol quantification in mix-
tures of ethanol/methanol.
CCDC 1874836 (for {for Na[Eu(FOD)4](H2O)} contai) contains the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre.
Experimental Section
Supporting Information (see footnote on the first page of this
article): Details of general procedures of the experiments, crystallo-
graphic, structure and tables refinement data of complex
Na[Eu(FOD)4], photoluminescence spectrum of complexes, 31P-NMR
Materials
Reagent grade chemicals were obtained from Aldrich and used
without further purification.
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
1
[3] C. C. L. Pereira, S. Dias, I. Coutinho, J. P. Leal, L. C. Branco, C. A. T. Laia,
Inorg. Chem. 2013, 52, 3755–3764.
[4] C. C. L. Pereira, J. T. Coutinho, L. C. J. Pereira, J. P. Leal, C. A. T. Laia, B.
Monteiro, Polyhedron 2015, 91, 42–46.
[5] A. Dey, A. Garai, V. Gude, K. Biradha, Cryst. Growth Des. 2018, 18, 6070–
6077.
and H-NMR spectrum of 1, electrospray-mass spectra of anions of
2 UV/Vis spectra of 2 in ethanol and methanol and 1:1 mixtures,
and calibration curve of sensor.
Conflict of interests
The authors declare no competing financial interests.
[6] S. A. Majeed, B. Ghazal, D. E. Nevonen, P. C. Goff, D. A. Blank, V. N. Nemy-
kin, S. Makhseed, Inorg. Chem. 2017, 56, 11640–11653.
[7] M. L. Wang, J. T. Wang, Y. M. Choongy, J. Food Compos. Anal. 2004, 17,
187–196.
[8] R. Caruso, G. L. Gambino, M. Scordino, Nat. Prod. Commun. 2011, 6,
1939–1943.
Acknowledgments
This work was performed under the project “SunStorage – Har-
vesting and storage of solar energy”, with reference POCI-01-
0145-FEDER-016387, funded by European Regional Develop-
ment Fund (ERDF), through COMPETE 2020 - Operational Pro-
gramme for Competitiveness and Internationalization (OPCI),
and by national funds, through FCT -Fundação para a Ciância e
a Tecnologia I.P. This work was supported by the Associated
Laboratory for Sustainable Chemistry-Clean Processes and Tech-
nologies- LAQV which is financed by national funds from FCT/
MEC (UID/QUI/50006/2013) and co-financed by the ERDF under
the PT2020 Partnership Agreement (POCI-01-0145-FEDER –
007265) The NMR spectrometers are part of The National NMR
Facility, supported by Fundação para a Ciência e a Tecnologia
(RECI/BBB-BQB/0230/2012). Financial support from Fundação
para a Ciência e a Tecnologia and Portugal 2020 to the Portu-
guese Mass Spectrometry Network (LISBOA-01-0145-FEDER-
402-022125) is acknowledged. This work has been supported
by Fundação para a Ciância e a Tecnologia through the contract
n° IST-ID/077/2018 (Bernardo Monteiro), SFRH/BD/120985/2016
(Mani Otis) and PTDC/QEQ-QIN/3007/2014 (Tiago Moreira).
C2TN/IST authors gratefully acknowledge the FCT support
through the UID/Multi/04349/2013 project. Cláudia C. L. Pereira
thanks to Fundação para a Ciência e a Tecnologia, MCTES, for
the Norma transitória DL 57/2016 Program Contract.
[9] G. Ai, T. Sun, X. Dong, Rapid Commun. Mass Spectrom. 2014, 28, 1674–
1682.
[10] Y. C. Zhang, N. B. Lin, X. S. Chai, Z. Li, G. D. Barnes, Food Chem. 2015,
183, 169–172.
[11] See ref.[7]
.
[12] I. H. Boyaci, H. E. Genis, B. Guven, U. Tamer, N. Alper, J. Raman Spectrosc.
2012, 43, 1171–1176.
[13] D. I. Ellis, H. Muhamadali, Y. Xu, R. Eccles, I. Goodallc, R. Goodacre, Analyst
2019, 144, 324–330.
[14] P. Ramasami, S. Jhaumeer-Laulloo, F. Cadet, P. Rondeau, Y. Soophul, Int.
J. Food Sci. Nutr. 2005, 56, 177–183.
[15] B. Bucur, G. L. Radu, C. N. Toader, Eur. Food Res. Technol. 2008, 226, 1335–
1342.
[16] K. K. Kar (Ed.), Composite Materials: Processing, Applications, Characteriza-
tions. Springer Verlag, Berlin–Heidelberg, 2017.
[17] A.-V. Mudring, S. Tang, Eur. J. Inorg. Chem. 2010, 2569–2581.
[18] B. Monteiro, M. Outis, H. Cruz, J. P. Leal, C. A. T. Laia, C. C. L. Pereira,
Chem. Commun. 2017, 53, 850–853.
[19] E. Wolcan, L. Villata, A. L. Capparelli, M. R. Féliz, Photochem. Photobiol.
Sci. 2004, 3, 322–327.
[20]
J. P. Leal, M. Outis, M. H. Casimiro, L. M. Ferreira, F. Fernandes, B. Monteiro,
C. A. T. Laia, C. C. L. Pereira, Eur. J. Inorg. Chem. 2017, 3429–3434.
R. Diaz-Torres, S. Alvarez, Dalton Trans. 2011, 40, 10742–10750.
K. Binnemans, in: Handbook on the Physics and Chemistry of Rare Earths
(Eds.: K. A. Gschneidner Jr., J.-C. G. Bünzli, V. K. Pecharsky), Elsevier,
Amsterdam, 2005, Chapter 225.
[21]
[22]
[23]
[24]
[25]
[26]
K. Binnemans, Coord. Chem. Rev. 2015, 295, 1–45.
D. Prodius, A.-V. Mudring, Coord. Chem. Rev. 2018, 363, 1–16.
M. Belter, A. Sajnog, D. BaraIkiewicz, Talanta 2014, 129, 606–616.
Analytical Methods Committee, Recommendations for the definition,
estimation and use of the detection limit, Analyst 1987, 199–203.
G. L. Long, J. D. Winefordner, Anal. Chem. 1983, 712A–724A.
Keywords: Sensors · Europium · Fluorescence ·
Methanol · Ethanol · Ionic liquids
[27]
[1] K. Binnemans, C. Bex, A. Venard, H. De Leebeeck, C. Gorller-Walrand, J.
Mol. Liq. 1999, 83, 283–294.
[2] Z. Cui, X. Zhang, S. Liu, L. Zhou, W. Li, J. Zhang, Inorg. Chem. 2018, 57,
11463–11473.
Received: August 1, 2019
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Luminescent Sensors
J. P. Leal, F. A. A. Paz, R. F. Mendes,
T. Moreira, M. Outis, C. A. T. Laia,*
B. Monteiro,* C. C. L. Pereira* ......... 1–9
A Reusable Eu3+ Complex for Naked-
Eye Discrimination of Methanol
from Ethanol with a Ratiometric
Fluorimetric Equilibrium in Meth-
anol/Ethanol Mixtures
The reaction product between method for the detection and quantifi-
[P6,6,6,14][Eu(FOD)4]
and
NaOPh- cation of methanol in mixtures with
Me3 forms an equilibrium in solution ethanol based on the changes in the
that allows for an easy, low-cost, effi- Eu3+ luminescence in the visible re-
cient and fast spectrofluorimetric gion.
DOI: 10.1002/ejic.201900843
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim