A new calix[4]azacrown ether based boradiazaindacene (Bodipy): Selective fluorescence changes towards trivalent lanthanide ions

Article abstract of DOI:10.1016/j.dyepig.2013.05.025

We report on the synthesis and photophysical properties of a new calix[4]azacrown derivative, capped with two amide bridges, in the cone conformation bearing two boradiazaindacene (Bodipy) groups at the narrow rim. Its binding properties towards trivalent

Full text of DOI:10.1016/j.dyepig.2013.05.025

Accepted Manuscript
A new calix[4]azacrown ether based boradiazaindacene (Bodipy): selective
fluorescence changes towards trivalent lanthanide ions
Mevlü t Bayrakcı, Ahmed Nuri Kursunlu, Ersin Gü ler, Ş eref Ertul
PII:
S0143-7208(13)00196-4
10.1016/j.dyepig.2013.05.025
DYPI 3962
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 May 2013
Revised Date: 21 May 2013
Accepted Date: 24 May 2013
Please cite this article as: Bayrakcı M, Kursunlu AN, Gü ler E, Ertul Ş , A new calix[4]azacrown ether
based boradiazaindacene (Bodipy): selective fluorescence changes towards trivalent lanthanide ions,
Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.05.025.
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ACCEPTED MANUSCRIPT
A new calix[4]azacrown ether based boradiazaindacene (Bodipy): selective
fluorescence changes towards trivalent lanthanide ions
Mevlüt Bayrakcıa*, Ahmed Nuri Kursunlub, Ersin Gülerb and Şeref Ertulb
aNiğde University, Department of Chemistry, 51100 Campus Niğde/Turkey
bSelçuk University, Department of Chemistry, 42031 Campus Konya/Turkey
Abstract
We report on the synthesis and photophysical properties of a new calix[4]azacrown derivative,
capped with two amide bridges, in the cone conformation bearing two boradiazaindacene
(Bodipy) groups at the narrow rim. Its binding properties towards trivalent lanthanide ions such
as La3+, Gd3+, Tb3+, Dy3+, Er3+ and Yb3+ were investigated by using spectroscopic techniques.
With respect to fluorescence intensity changes upon trivalent lanthanide ion complexation,
calix[4]azacrown ether based Bodipy showed higher selectivity towards Yb3+ ion over other ions.
Presence of proximal two amide groups in calix[4]azacrown ether based Bodipy were observed to
play an important role in exhibiting its lanthanide ion binding.
Keywords : Calixcrown, Bodipy, Lanthanide ions, Fluorescence probe, Recognition,
Chemosensor.
* Corresponding author. Tel.: +90 388 2252106 fax: +90 388 2250180; e-mail: mevlutbayrakci@gmail.com
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1. Introduction
The design and synthesis of fluorescent chemosensors have received considerable interest in
supramolecular chemistry due to the simplicity and high detection limit of the fluorescent signals
[1]. Especially, the investigation of specific chemosensors for the efficient detection of lanthanide
ions is one of the most important issues because of their impacts on our environments and
chemical significances [2,3]. Lanthanide ions have been widely used in chemical and biological
applications as probes and labels [4-6]. Furthermore, these ions are very poor light absorbers,
thus, their luminescent states by direct excitation are inefficient. In order to overcome these
limitations, many lanthanide (III) chelates have been designed containing ‘antenna effect’ [7,8].
The macrocyclic chelating agents including cryptands, [9] calixarene, [10] cyclodextrins [11] and
crown-ether derivatives, [12] are commonly used to sensitize luminescence towards lanthanide
ions. The fluorophore (signaling moiety) makes possible the detection of metal cations through a
light signal, whereas the chelating agents (ionophore) linked to the fluorophore is responsible for
the selectivity and cation binding efficiency of the chemosensor. Among the fluorophores,
boradiazaindacene (Bodipy) are well known fluorescent dyes that have applications in many
different areas such as, ion sensing and signaling, fluorescent labeling of biomolecules,
fluorescent stains, energy transfer cassettes, light harvesting systems due to their high quantum
yields, large extinction coefficients and narrow emission bands [13-15]. In the design of
chemosensors, attention should be paid to both recognition of ionophore and signaling of
fluorophore moieties. The signaling moiety converts the data into a signal expressed as the
differences in the photophysical characteristics of the chemosensor. These differences are due to
the perturbation by the bound cation of photochemical transactions such as electron, charge
and/or energy transfer, excimer or exciplex formation. Calixarenes are cyclic oligomers made of
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several phenolic units bounded with methylene bridges [16,17] and are regarded as the third
generation of host molecules because of their inclusion ability to cations, anions, and neutral
molecules [18-22]. Calix[4]arenes can be easily functionalized both at the phenolic OH groups
(lower rim) and, after partial removal of tert-butyl groups, at the para positions of the phenol
rings (upper rim) [23,24]. The vast majority of these modified calixarenes exist in the cone
conformation in which there is a cavity suitable for reception of different ionic and neutral
species [25]. Since the first report of calixcrown ether derivatives, [26] considerable interest has
been dedicated to the design and synthesis of calixcrowns and their use in some chemical
applications such as catalysis, ion selective electrodes and liquid supported membranes [27].
Although a large number of works exists on the crown ether and cryptand-based photo-induced
electron transfer (PET), those on calixazacrowns are limited [28,29]. Compared to the number of
reports on the binding of metal cations with calixazacrown fluoroionophores, reports on the
binding of trivalent lanthanide ions are still limited [30-32]. From this point of view
calixazacrown ionophore based boradiazaindacene (Bodipy) units is at the center of interest. With
this in mind, we set out to synthesis of the calixazacrown ionophore based boradiazaindacene
(Bodipy) and explore its metal binding properties towards La3+, Gd3+, Tb3+, Dy3+, Er3+ and Yb3+.
2. Experimental Section
2.1. Materials
All of the reagents used in this study were obtained from Merck (Germany) or Sigma-Aldrich
(USA) and used without further purification. CH2Cl2 was distilled from CaCl2 · MeOH over Mg
and stored over molecular sieves. Acetonitrile was dried from CaH2 and stored under N2 over
molecular sieves (4 A ˚ ). Other commercial grade solvents were distilled, and then stored over
molecular sieves (Aldrich; 4 A ˚ , 8–12 mesh). Thin layer chromatography (TLC) was performed
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using silica gel on glass TLC plates (silica gel H, type 60, Merck). All aqueous solutions were
prepared with deionized water that had been passed through a Millipore Milli-Q Plus water
purification system.
2.2. Apparatus
13
1H and C NMR spectra were obtained using a Varian 400 MHz spectrometer operating at 400
MHz. IR spectra were recorded on a Perkin–Elmer spectrum 100 FTIR spectrometer (ATR).
Absorbance spectra are collected by a Perkin Elmer Lambda 25 UV-vis spectrophotometer using
quartz cells of 1.0 cm path length. Fluorescence measurements are carried out in a PerkinElmer
LS 55 spectrofluorimeter. The fluorescence spectra are recorded in a 1 cm quartz cuvette at room
temperature. The excitation and emission slits are set at 5 nm. Elemental analyses were
performed using a Leco CHNS-932 analyzer. An Orion 410A+ pH meter was used for the pH
measurements. Melting points were determined on a Gallenkamp apparatus. Moreover, the
solutions of Bodipy-metal ion were mixed by Kudos Ultrasonic Cleaner 53 kHz.
2. 3. Synthesis
Starting calixarene compounds 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrahydroxycalix[4]arene
(2), 25,26,27,28-tetrahydroxycalix[4]arene (3), 25,27,-di-(methoxycarbonylmethoxy)-26,28-
dihydroxycalix[4]arene (4) and calix[4]arene (ethyleneamido)crown (5) were synthesized
according to literature procedures [17,23,24]. 8-{4-(Chloromethyl)phenyl}-2,6-diethyl-4,4-
difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (1) was prepared by using modified
procedure as follow. Final product (6) was synthesized for the first time according to the
following described procedure:
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2.3.1. Synthesis of 8-{4-(Chloromethyl)phenyl}-2,6-diethyl-4,4-difluoro-1,3,5,7-tetramethyl-4-
bora-3a,4a-diaza-s-indacene (1)
To a stirred solution of 2,4-dimethyl-3-ethylpyrrole (2.5 mL) in dry dichloromethane (100 mL),
4-(chloromethyl)benzyl chloride (1.875 g, 10 mmol) was added drop wise at room temperature
and under N2. The solution was heated and stirred to 60° C for 2 h. After the cooling of the
solution, triethylamine (TEA) (5 equiv.) was added to the residual solid, the mixture was stirred
at room temperature for 30 min under N2, and boron trifuoride diethyl etherate (7 equiv.) was
then added. The solution was stirred at 60 °C for 2 h and the final residue was purified by column
chromatography (petrolium ether-EtOAc; in 8:1 ratio) and obtained to redden an adhesive
material. ¹H NMR [400 MHz, CDCl3]: δ (ppm) 7.41 (d, 2H, ArH), 7.18 (d, 2H, ArH) 4.63 (s, 2H,
CH2), 2.44 (s, 6H CH3) 2.21 (q, 4H, CH2) 1.27 (s, 6H, CH3) 0.89 (t, 6H, CH3).13C NMR [100
MHz, CDCl3]: δ (ppm); 153.81, 139.42, 138.42, 136.12, 135.81, 132.83, 130.62, 129.02, 128.78,
45.59, 17.07, 14.43, 12.43, 11.62. Anal.Calc. for (%) C24H28N2ClBF2: C, 67.23; H, 6.58; N, 6.53;
Found: C, 66.82; H, 6.98; N, 6.12.
2.3.2. Synthesis of Bodipy-calix[4]azacrown (6)
Calix[4]arene(ethyleneamido)crown (5) (0.5 mmol) were refluxed with NaH (2.1 mmol) in
anhydrous CH3CN (30 mL) for 2.5 h. To this mixture, the solution of 8-{4-
(Chloromethyl)phenyl}-2,6-diethyl-4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-
indacene (1) (1.5 mmol) in anhydrous mixture of CH3CN/DMF (20:5 mL) in the presence of
catalytic amount of NaI were added dropwise for 1 h. and then additionally refluxed for 72 h.
After filtration, the solution was evaporated to dryness. The residue was washed with water.
Then, the product was recrystallized from CH2Cl2 and MeOH. ¹H NMR [400 MHz, CDCl3]: δ
(ppm) 8.81 (t, 2H, NH), 7.71, (d, 2H, ArH), 7.42 (d, 2H, ArH), 7.17 (d, 4H, ArH), 7.03 (d, 4H,
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ArH), 6.82 (t, 2H, ArH), 6.77 (t, 2H, ArH), 5.41 (s, 4H, CH2), 4.57 (s, 4H, ArOCH2), 4.17 (d, 4H,
JAB= 13.3, ArCH2Ar), 3.50-3.55 (m, 8H, ArCH2Ar, NHCH2), 2.50 (s, 6H CH3) 2.22 (q, 4H, CH2)
13
1.20 (s, 6H, CH3) 0.86 (t, 6H, CH3). C NMR [100 MHz, CDCl3]: δ (ppm); 166.60, 152.77,
151.11, 149.79, 138.88, 138.12, 136.09, 135.13, 133.81, 131.90, 130.96, 129.27, 128.97, 128.11,
127.84, 127.33, 125.27, 73.21, 71.96, 46.61, 31.17., 18.11, 15.32, 12.57, 11.11.. FTIR (ATR)
(characteristic frequencies): 3352, 2964, 2931, 1688, 1459, 1251, 1095 cm-1; Anal.Calc. for (%)
C82H86N6O6B2F4: C, 73.00; H, 6.42; N, 6.23; Found: C, 73.09; H, 6.38; N, 6.17.
3. Results and Discussion
3.1. Synthesis and characterization
The synthesis of a new calix[4]arene derivative is given in Scheme 1. For the synthesis of
calix[4]azacrown containing two boradiazaindacene (Bodipy) functional units on the narrow rim
(6),
the
parent
calixarene
compounds
25,26,27,28-tetrahydroxycalix[4]arene
(4) and
5,11,17,23-Tetra-tert-butyl-25,26,27,28-
tetrahydroxycalix[4]arene
(2),
(3),
25,27,-di-
(methoxycarbonylmethoxy)-26,28-dihydroxycalix[4]arene
calix[4]arene
(ethyleneamido)crown (5) were prepared according to published literature procedures [17,23,24].
Corresponding 4-chloromethyl phenyl boradiazaindacene (Bodipy) derivative (1) was
synthesized according to modified literature method as mentioned Scheme 1 or follows [33]. In
order to access the cassette compound bearing Bodipy (1) synthetically, 4-(chloromethyl)benzoyl
chloride was condensed with 2,4-dimethyl-3-ethylpyrrole and was then treated with boron
trifluoride diethyl etherate in dry dichloromethane. The required starting compounds p-tert-
butylcalix[4]arene (2) and debutylated calix[4]arene (3) which was obtained from the
dealkylation of p-tert-butylcalix[4]arene was synthesized according to literature procedure [23]
and then 25,27,-di(methoxycarbonylmethoxy)-26,28-dihydroxycalix[4]arene (4) were obtained in
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satisfactory yields by refluxing debutylated calix[4]arene (3) with bromomethyl acetate with
K2CO3 in CH3CN for 48 h as described previously. When diester derivative of calixarene (4) was
interacted with ethylene diamine in CH2Cl2 at room temperature for 24 h, it was converted into
calix[4]arene (ethyleneamido)crown (5) which could be isolated by recrystallization from the
methanol-dichloromethane solvent mixture. The structures of (1) and (6) were established by the
analysis of their 1H and 13C NMR spectra as well as Fourier Transform Infrared FTIR (ATR) and
elemental analyses. In the 1H NMR spectrum, it was determined that the pair of doublets at δ 4.17
(JAB = 13.3 Hz) and 3.55 (JAB = 13.1 Hz) for ArCH2Ar protons that could be correlated with the
13C NMR signal at δ 31.17. The high field doublet around δ 3.55 was assigned to the equatorial
protons of methylene groups, whereas the low field signal around δ 4.17 was assigned to the axial
protons of calixarene scaffold for compound (6). These data suggested that compound (6)
possessed a cone structure. Calixazacrown based bodipy (6) exhibited prominent signals at δ 8.81
13
for amide protons, also this amide group could be correlated with the C NMR signal at δ
166.60, around 7.71 ppm and 7.42 ppm for aromatic protons of two bodipy fragments, while
signals for aromatic protons of calixarene scaffold appeared around δ 7.17-6.77 as two doublets
and two triplets. The ArOCH2- protons were seen around δ 4.57 while –NHCH2 protons were
hidden because of the overlapped peaks with the equatorial protons of methylene groups. In
addition to above signals calixazacrown based bodipy (6) also showed one singlet around δ 5.41,
δ 2.50 and δ 1.20 corresponding to the -OCH2 and -CH3 (2,4- position of pyrrole ring) protons of
bodipy fragments on the narrow rim, respectively. Furthermore, one quartet assigned to the
CH3CH2- and one triplet assigned to the CH3CH2- appeared at δ 2.22 and δ 0.86 for bodipy
fragments of compound (6), respectively. All other data were in agreement with the proposed
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structures of (6a-d). In the C NMR spectra of compound 6, it is obvious that compound 6 is
13
symmetrical and therefore the number of signals observed in the C NMR is lesser than the
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number of C atoms in the related compound 6. But in the 13C NMR for compound 6, two signals
are hidden because of the overlapped peaks. In the FTIR spectra, the formation of calixazacrown
based bodipy (6) have been confirmed by the appearance of the characteristic amide bands (N-
C=O) at about 1688 cm−1.
Please insert here Scheme 1
3.2. Lanthanide Recognition Studies
We investigated the recognition ability of receptor (6) for lanthanide metal ion such as
La3+, Gd3+, Tb3+, Dy3+, Er3+ and Yb3+. Firstly, the changes of color in solution were detected for
target molecules under Uv-lamp at 365 nm. When the colorless lanthanide solutions in methanol
were added to the turquoise solution of compound (6) in methanol, the color transformed to green
or the brightness of colour increased. However, there was no remarkable color change upon the
addition of La3+ metal ion. As shown in Fig. 1, the color change of compound (6) upon the
addition of lanthanide ions is visible to under Uv-lamp. These color variations may be attributed
to photophysical differences of target complex compounds such as luminescence (fluorescence or
phosphorescence). So, the fascinating optical properties of lanthanide ions are well-known due to
the different luminescent transitions of trivalent ions [34].
Please insert here Fig. 1.
The cation binding properties of compound (6) were investigated by UV–vis, fluorescence, FTIR
1
and H NMR spectroscopy. The absorption spectra of the compound (6) are presented in Fig. 2.
Compound (6) shows an absorption spectrum with an absorption pattern typical for Bodipy
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chromophores. The absorption spectra of compound (6) (1×10−6 M) show typical absorption
bands at 255, 288, 331, 530 nm. These absorption bands correspond to π-π* transition of the
aromatic moieties and Bodipy fragments of calixarene backbone, respectively [31,35]. The
absorption spectra are similar to that of the compound (6) with one strong peak at 530 nm. Upon
addition of La3+, Gd3+, Tb3+, Dy3+, Er3+ and Yb3+ ions to the solution of compound (6), the
absorption spectra of (6) changed in response to all metal cations. Among the metal cations,
considerable an enhancement of absorbance for compound (6) which is attributed to the
formation of the (6)-Yb3+ or (6)-Dy3+ complexes were observed a peak at 390 nm in the form of
shoulder and a sharp peak 530nm upon addition of Dy3+ and Yb3+. When compound (6) is
complexed with Dy3+ or Yb3+, the original peak at 530 nm is increased and a new broad
absorption band appeared at 490-500 nm. The intense absorption band of conjugated system
around 330 nm is given a bathochromic shift in presence of Dy3+ or Yb3+ and the bands are
appeared around 390 nm. The conjugation between ligand and lanthanide ions is caused to charge
transfer to metal ion from the π- system that the changes in the absorption band may be due to
expansion of π-conjugated system by Dy3+ or Yb3+ and occurrence of donor–acceptor charge
transfer (CT) in the large π conjugate system with the conjugation of the Bodipy-
calix[4]azacrown ether units. On the other hand, the strong band of (6)-Gd (III) around 290 nm
can be ascribed to f-f intra-configurational electron transitions of gadolinium complexes or the
absorption spectra of the coordinated chromophoric ligands [34]. The new absorption band and
increasing original peak appeared at longer wavelengths indicating that Dy3+ or Yb3+ accept more
charge from the compound (6). Furthermore, the most intense band around 530 nm can be
attributed to a strong S0–S1 transition.
Please insert here Fig. 2.
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The fluorescence intensity of free ligand (6) and the changes in its fluorescence intensity in the
presence of the lanthanide cations are shown in Fig. 2. Upon addition of lanthanide cations to
solutions of compound (6), the maximum emission of compound (6) at 530 nm was enhanced by
all of the lanthanides and no shift of the fluorescence maximum was observed for lanthanide
cations except for Yb3+. However, slightly change between 490-500 nm and the largest increasing
at 530 nm in the fluorescence intensity were observed for Yb3+ (Figure 3). It might be anticipated
that this effect is related to the blocking of the carbonyl groups of two amide bridges by metal ion
complexation. This is not surprising result. Because, in literature trivalent cations as Yb3+, are
effectively bound by the amide groups [36,37]. Photoinduced electron transfer (PET) from the
electron-donating amide functional groups and oxygen atoms to the Bodipy units of the
calixarene is decreased or stopped. It known that lanthanide ions have a large lability and high
co-ordination number between 8 and 12 in solution medium. So, the multi-dentate chelating
agents contain anionic groups, amides, carboxylates, and/or O and N donor atoms are preferred
for lanthanide complexation due to ion-dipole bonding with little covalent character. Non-
bonding electrons of nitrogen atoms can be given to metal ion as more easy when compared with
oxygen due to its higher electronegativity. Non-bonding electrons of oxygen atoms can carry out
the weaker coordination bonds with metal ion. Although the non-bonding electrons of oxygen are
difficultly given to metal ions, it can make to co-ordination bonds with lanthanide ions owing to
their high co-ordination and hard nature of lanthanide ions [3,36-38]. Furthermore, it is known
that diamide derivatives of calix[4]arenes form stable complexes with lanthanide ions, alkali and
transition metals due to the presence of a cavity formed by donor atoms of phenol, ether and
amide groups [39,40]. Upon complexation with Yb3+, the carbonyl group of two amide bridges
donates its lone electron pairs to the empty orbital of the Yb3+ and a large chelation enhanced
fluorescence (CHEF) is observed because the chelation abrogates the PET process [3]. The
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increased affinity in complexation for Yb3+ can be explained by the fact that there is an important
role played by the carbonyl groups having π-electron system and electron-donor nitrogen atoms.
In there, the Bodipy-terminal of the calix-derivative behaves as an antenna unit capable of light
harvesting (by the binding of analyte). Metal cation binding to the potential chelate group
(calix[4]azacrown) are caused these changes in fluorescence spectra. Moreover, these
phenomena may reflect the “hard and soft acids and bases” concept introduced by Pearson [41].
According to hard soft acid base principle, lanthanide ions belong to hard acid groups, which
have strong affinity for electron-donor groups as carbonyl groups. As this environment exists due
to the presence of π-bonds containing functionalities, where cation-π interactions favor the
complexation with the more polarizable trivalent lanthanide ions. As mentioned above for
absorption spectra, the formation of large π conjugate system is beneficial to increasing the plane
structure of the molecule which could make the fluorescence emission easy.
Please insert here Fig. 3.
The excitation spectra of the lanthanide cation complexes are shown in Fig. 4, which matches its
absorption spectrum well. While excitation of compound (6) give a curve with four different
humps at 450 nm emission, the addition of Yb3+/Dy3+ gives graphs with more intensity excitation
due to energy transfer. This clearly indicated that the luminescence process was sensitized by the
two Bodipy units attached to distal position of phenyl rings. Excitation of compound (6) resulted
in a broad band centered on 288 nm, as has been observed earlier for similar calixarenes. These
excitation values are assigned to the fluorescence resulting from the π, π* singlet excited state of
the aromatic moieties. The origin of the fluorescence was established by recording excitation
spectra, which resembled the absorption spectra of the ligand.
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Please insert here Fig. 4.
To confirm calix[4]azacrown-Bodipy (6) as an ion-selective fluorescence receptor for
Yb3+ ions the effect of competing lanthanide ions was determined. Compound (6) (1 µM in
methanol) was treated with 20 equiv. Yb3+ in the presence of other lanthanide ions (20 equiv in
methanol), respectively. As shown in Fig. 5, the presence of other lanthanide ions on the
detection of Yb3+ ions had generally a minor effect or competing ions had practically no effect on
the emission peak at 530 nm.
Please insert here Fig. 5.
For complexation ratio between compound (6) and Yb3+ ion, we carried out Job’s plot experiment
by varying the concentrations of both compound (6) and Yb3+ ion (see Fig. 6). The maximum
point at the mole fraction of 䏀 0.49 indicates that a typical 1:1 (ligand:metal) complexation
performs in this case.
Please insert here Fig. 6.
To gain more information about the conformation of compound (6) and the binding mode of the
1
Yb3+ in complex formation, H NMR studies of the Yb3+ complex with compound (6) were
performed (Fig. 7). Compound (6) showed changes of the chemical shifts in DMSO-d6. The
symmetrical cone conformation of free compound (6) is preserved in the Yb3+ complex as
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indicated by the presence of two AB spin systems for the methylene protons (4.18 and 3.62 ppm)
of the calixarene skeleton. Compared to free compound (6), pronounced downfield shifts were
observed for methylene protons adjacent to the amide nitrogen and the methylene protons of
CH2-NHCO. And also, the amide protons of the two arms attached to opposite (distal) phenyl
rings were downfield shifted and broadened. This interaction was also supported by FTIR (ATR)
spectra. The amide (NHCO) stretching vibration in compound (6) was shifted from 1688 to 1744
cm-1 after the forming complex of compound (6) with Yb3+ in the FTIR (ATR) spectra of the
compound (6) and its Yb3+ complex. These data supported that the possible interaction between
crown linkage of calixarene and Yb3+ was occurred.
Please insert here Fig. 7.
4. Conclusion
In conclusion, the synthesis and trivalent lanthanide (La3+, Gd3+, Tb3+, Dy3+, Er3+ and Yb3+) ion
recognition abilities of novel calixazacrown ether appended Bodipy fluorescent probe (6) were
studied. The spectroscopic data indicated that the new compound was in the cone conformation.
The fluorescence of compound (6) was increased upon complexation of particular trivalent
lanthanide ios and this increasing is probably due to an intramolecular energy transfer process.
Furthermore, from the obtained fluorescence data, it was observed that compound (6) was
effective structure for Yb3+ lanthanide ion. Additionally, the selectivity experiments
demonstrated the Yb3+ recognition by compound (6) was not affected by the presence of other
metal cations such as La3+, Gd3+, Tb3+, Dy3+and Er3. These results also suggested that two
carbonyl groups of the crown ring may allow some degree of selectivity in sensing lanthanides
with very similar ionic radii from those with either larger or smaller radii. Furthermore, the
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complex mechanism could be easily changed depending on the concentration of fluorophore,
temperature of solution medium, pH, solvent, metal ion characteristic. Finally, this type of
receptors seems to be promising in design of chemical sensors, ion-binding processes, phase-
transfer catalysts, solid-state sensors, and etc.
Acknowledgements
The authors gratefully would like to thank Selcuk University and Nigde University Research
Foundation (BAP) for financial support. And also, the authors express their appreciation to Dr.
Ross W. Boyle for helpful discussions.
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Graphical Abstract
___________________________________________________________________________
A new calix[4]azacrown ether based boradiazaindacene (Bodipy): selective
fluorescence changes towards trivalent lanthanide ions
Mevlüt Bayrakcı* Ahmed Nuri Kursunlu, Ersin Güler and Şeref Ertul
___________________________________________________________________________
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RESEARCH HIGHLIGHT
► Calix[4]azacrown ether based boradiazaindacene (Bodipy) were synthesized.
► The spectroscopic data indicated that the new compound was in the cone conformation.
► Spectrofluorometric studies of both ligand and lanthanide ions were carried out.
► Lanthanides recognition ability of calix[4]azacrown were investigated.
Sincerely yours,
With Regards
M. BAYRAKCI

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FIGURE CAPTIONS
Scheme 1. Synthetic route of preparation of compound (1) and Bodipy-calix[4]azacrown (6) (i) Dry CH2Cl2,
60 °C, 2 h, under N2; (ii) Triethylamine, borontrifuoride diethyl etherate, 60 °C, 2 h under N2; (iii) AlCl3, phenol,
toluene, rt, 3 h; (iv) Bromomethylacetate, K2CO3, CH3CN, reflux, 48 h; (v) Ethylenediamine, CH2Cl2, rt, 24 h;
(vi) NaH, NaI, dry CH3CN, reflux, 72 h.

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Fig. 1 The increasing and change in the brightness of colour of compound (6) (1 µM) in
methanol induced by addition of 20 equiv. of lanthanide nitrate (Yb3+, Gd3+, Dy3+, Er3+, Tb3+
and La3+).

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Fig. 2. Absorption spectra of compound (6) (1·10-6 M) and its complexes (La3+, Gd3+, Tb3+,
Dy3+, Er3+ and Yb3+) in MeOH

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Fig. 3. Fluorescence spectra of the compound (6) in methanol (1 µM) in the absence and
presence of various lanthanide cations. Added cation ion concentrations were 20 µM.
Excitation: 400 nm

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Fig. 4. Excitation spectra of compound (6) in methanol (1 µM) in the absence and presence
of various lanthanide cations (emmission: 425 nm)

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Fig. 5. Compound (6) concentration was set at 1 µM and the concentration of Yb3+-competing
lanthanide cations was 20 µM, respectively. (The fluorescence intensity was at 530 nm)

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Fig. 6. Job’s plot for compound (6)/Yb3+, 1 µM in methanol

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Fig. 7. 1H NMR spectra of the complex and compounds. (a) Yb3+ complex; (b) compound (6);
(c) compound (1)