5-[4-(N,N-Dimethylamino)Phenyl]-2-(4-Pyridyl)-1,3-Oxazole as a fluorescent probe for monitoring microheterogeneous media

Article abstract of DOI:10.1007/s10593-014-1486-3

The N,N-dimethylamino-γ-pyridine analog of 2,5-diaryl-1,3-oxazole has been synthesized. Quantum-chemical modeling was carried out for the electronic absorption spectra and the effect of hydrogen bonding between the pyridine nitrogen atom and the water mol

Full text of DOI:10.1007/s10593-014-1486-3

Chemistry of Heterocyclic Compounds, Vol. 50, No. 3, June, 2014 (Russian Original Vol. 50, No. 3, March, 2014)
5-[4-(N,N-DIMETHYLAMINO)PHENYL]-2-(4-PYRIDYL)-
1,3-OXAZOLE AS A FLUORESCENT PROBE FOR
MONITORING MICROHETEROGENEOUS MEDIA
O. O. Borodin1, R. Yu. Il'yashenko1, and A. O. Doroshenko1*
The N,N-dimethylamino--pyridine analog of 2,5-diaryl-1,3-oxazole has been synthesized. Quantum-
chemical modeling was carried out for the electronic absorption spectra and the effect of hydrogen
bonding between the pyridine nitrogen atom and the water molecule on these spectra. The calculation
results were compared with the experimental data for the solvent effect on the spectra of this compound.
The feasibility of practical application of this derivative as a fluorescent probe was tested for
determining the critical micelle concentration in aqueous solutions of nonionic, zwitterionic, cationic,
and anionic surfactants.
Keywords: 2,5-diaryl-1,3-oxazole derivatives, Bader's AIM theory, critical micelle concentration, ESS
analysis of electronic excitations, fluorescent probe, quantum-chemical calculations, solvatochromism.
Fluorescent organic compounds, whose spectral characteristics are a function of both general and
specific intermolecular interactions with the solvent, have been used for many decades for probing
microheterogeneous media such as colloidal solutions, microemulsions, and biological systems [1-4]. Most
fluorescent probes are soluble to some extent in both water and lipophilic media and demonstrate a significant
increase in the fluorescence quantum yield and/or change in fluorescent color upon entering the hydrophobic
environment of surfactant micelles, cell membranes, protein globules, etc. [3, 4].
2,5-diaryloxazole derivatives are known to be efficient organic luminophores in the violet-blue spectral
range [5], but the fluorescent characteristics of most of these compounds are not highly dependent on their
microenvironment. Thus, their major application has been as liquid and plastic scintillation materials from the
mid-twentieth century until now [6, 7]. In the present work, we have discovered a 2,5-diaryloxazole derivative
serving as an efficient fluorescent probe and carried out a quantum-chemical analysis of its spectral and
physicochemical properties. This compound was also tested in microheterogeneous systems consisting of
aqueous colloidal solutions of various surfactants.
In order to make the 2,5-diaryloxazole derivative sensitive to the polar characteristics of the
microenvironment, its chemical structure was modified to provide considerable redistribution of electron density
in the electronically excited state: the benzene ring at position 2 was replaced by an electron-withdrawing
_______
*To whom correspondence should be addressed, e-mail: andrey.o.doroshenko@univer.kharkov.ua,
andrey.o.doroshenko@gmail.com.
1V. N. Karazin Kharkiv National University, 4 Svobody Sq., 61022 Kharkiv, Ukraine.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 413-423, March, 2014. Original
article submitted January 29, 2014.
0009-3122/14/5003-0379©2014 Springer Science+Business Media New York
379

pyridine cycle, which provides for greater solubility of the resultant compound (the log P index calculated using
an additive scheme was 2.97 for the pyridine derivative, while it was 4.23 for the benzene analog), and a
dimethylamino group, which is a strong electron donor, was introduced in the benzene ring at position 5 of the
oxazole moiety. In order to achieve the greatest spectral effects in accord with our previous recommendations
[8], this molecule was constructed such that the direction of the redistribution of electron density between the
introduced electron-donating and electron-withdrawing sites corresponded to the direction of the shift in
electron density upon going to the excited state in the unsubstituted 2,5-diphenyloxazole molecule. 5-[4-(N,N-
Dimethylamino)phenyl]-2-(4-pyridyl)-1,3-oxazole (1), which was synthesized in accord with this principle, was
not found in the SciFinder [9] and Reaxys databases [10].
N
O
Me
N
N
Me
1
The structure of oxazole 1, its spectral characteristics, and its intermolecular interactions with proton
donors such as water were initially modeled using quantum-chemical approach by DFT methods, B3LYP/
cc-pVDZ calculation [11, 12] (TDDFT calculation for the excited molecules), the Bader's AIM theory [13-15],
and ESSA (excited state structural analysis) [16]. ESSA is an approach for estimating the participation of
individual atoms in electronic excitation and analyzing the redistribution of electron density in a molecule
related to its transition to an electronically excited state.
Fluorescent probe 1 should display sensitivity to proton-donating compounds since this molecule has
several nucleophilic sites, which are potential hydrogen bond acceptor sites, in addition to the polarity of the
microenvironment. Our calculations showed strong intermolecular hydrogen bonding between the pyridine
nitrogen atom in oxazole 1 and a water molecule; the calculated H···N bond length in the ground state was
1.97 Å. The hydrogen bond markedly contracts to 1.89 Å upon going to the lowest singlet ground state, which
indicates that it becomes stronger.
The energy of the above-mentioned hydrogen bond was calculated using the AIM theory and
semiempirical Espinosa approximation [17] based on an analysis of the electron density at its critical point {3, -1}
(EHB ~ ½VIR, Virial Field Function). According to our calculations, the energy of the hydrogen bond formed
by a water molecule with the pyridine nitrogen atom in oxazole 1 is enhanced by almost 25% in going to the
electronically excited state: from 5.9 (S0) to 7.3 kcal/mol (S1).
The effect of the hydrogen bond with proton-donating molecules on the electronic spectra was analyzed
for oxazole 1 using the ESSA approach [16], which presupposes calculation of special quantum-chemical
indices, namely electron excitation localization numbers (Li, %), which characterize the participation of
individual atoms or submolecular fragments in the formation of a specific electronic transition, and the charge
transfer numbers (lij, in % of electron charge), which give detailed picture of the redistribution of electron
density upon transition of the molecule to the excited state [18]. In order to make this analysis more meaningful
and avoid unnecessary details, the molecule of oxazole 1 was divided into four submolecular fragments,
namely, the pyridine, oxazole, and benzene rings and the dimethylamino group, for which the electronic
excitation localization numbers and charge transfer numbers were summed over all their atoms. The water
molecule acted as an additional fragment for the hydrogen-bonded complex oxazole 1···H2O. However, as
expected, no significant localization of electronic excitation was found on the atoms of the water molecule.
Similarly, by the way, no displacement of electron density to the water molecule from the fragments of the
major chromophore of oxazole 1 was found. Thus, the water molecule in the hydrogen-bonded complex acts
only as a perturbing factor, which does not fundamentally alter the nature of the electronic transitions (Table 1).
380

The formation of the hydrogen-bonded complex with the water molecule at the pyridine nitrogen atom
does not lead to a change in the nature of the electronic excitation localization relative to the free molecule of
oxazole 1. Only a slight increase was noted for the participation of the pyridine ring in the formation of the
S0–S1 electron transition. On the other hand, the hydrogen bond leads to a somewhat greater redistribution of
electron density in the molecule (see the charge transfer numbers lij and total charge changes on the selected
submolecular fragments) in going to its lowest singlet excited state responsible for subsequent fluorescence
emission. The vector difference of the dipole moments in the ground and excited states, which is a generally
accepted quantitative index, also indicates greater electron density redistribution. The vector  virtually
coincides in its direction in space with the axis traversing the pyridine ring and dimethylamino group nitrogen
atoms, which are the electron-withdrawing and electron-donating sites of oxazole 1. The rather large value of
 would lead us to expect significant solvatofluorochromism (change in the position of the emission band
upon change in the polarity of the surrounding medium) for the molecule studied. Furthermore, the above-
mentioned enhanced nucleophilicity of the pyridine ring nitrogen in oxazole 1 in the electronically excited state
should lead to an additional shift of the fluorescence band toward longer wavelengths in the presence of proton-
donating molecules such as hydrogen bond donor solvents and traces of moisture in aprotic media.
TABLE 1. ESS Analysis of the Long-Wavelength Transitions in the Calculated
Electronic Absorption Spectra of Oxazole 1 and Its H-Complex at the Pyridine
Nitrogen Atom with a Water Molecule*
S0–S1
transition
Electron density redistribution upon
transition to the excited state
Localization of electronic excitation
9
N
381 nm
13
N
26.4
26220 cm–1
f = 0.713
3
O
Me
12
N
O
Me
N
N
N
30.9
20
13
14.5
Me
28.3
Me
-0.444 -0.105 +0.304 +0.245
Δμ = 21.5 D
9
N
394 nm
13
26.0
N
25370 cm–1
f = 0.693
2
O
Me
H
12
N
O
Me
H
O
N
H
N
N
H
22
14
29.5
O
14.8
Me
Me
29.6
-0.478 -0.103 +0.325 +0.256
Δμ = 22.6 D
_______
*This table gives the calculated characteristics for the long-wavelength electronic
transition in the absorption spectra of oxazole 1 and its H-complex with a water
molecule (position in terms of wavelength and wavenumbers, oscillator strength),
localization of the electronic excitation in the structurally isolated submolecular
fragments (Li, % [16, 18]), and redistribution of electron density upon transition to
the excited state (the numbers in the arrows give the charge transfer numbers (lij
[16, 18]). The total change in electric charge on the selected submolecular
fragments and the vector difference in the dipole moments in the molecule of
oxazole 1 in the ground and excited states (D) are shown below the molecular
diagrams.
381

TABLE 2. Spectral and Fluorescent Properties of 5-[4-(N,N-Di-
methylamino)phenyl]-2-(4-pyridyl)-1,3-oxazole in Various Solvents*
N
Solvent
ET
λа, nm νа, cm–1 λf, nm νf, cm–1
ΔνST, cm–1
φf
Hexane
Toluene
Dioxane
Ethyl acetate
1,2-Dichloroethane
Dimethyl formamide
Acetonitrile
Ethanol
0.009
0.099
0.164
0.228
0.327
0.386
0.460
0.654
1.000
343
347
348
349
350
352
354
355
357
29160
28820
28740
28660
28580
28400
28240
28160
28020
412
422
446
456
458
547
542
552
542
24280
23700
22420
21920
21840
18280
18460
18120
18460
4880
5120
6320
6740
6740
10120
9780
10040
9560
0.47
0.52
0.44
0.41
0.46
0.18
0.22
0.17
0.03
Water
_______
*a, f, a, f are the positions of the absorption and fluorescence bands in
terms of wavelength and wavenumbers, ST is the Stokes shift, and f is
the fluorescence quantum yield. The polar properties of the solvent are
characterized qualitatively by the empirical Reichardt parameter ETN [22].
Very often a strong intramolecular charge transfer in the electronically excited state of organic
molecules results in quenching of their fluorescence in polar media [19-21]. Thus, a substantial increase in
fluorescence intensity at transition from the proton-donating polar aqueous medium to environment of low
polarity within the micelles, cell membranes, or protein globules, can be expected for compound 1.
Our theoretical analysis indicates that oxazole 1 satisfies the major set of requirements for a
fluorescence probe, namely, that it should display sensitivity to general and specific intermolecular interactions
by reacting to change in its immediate microenvironment by change in both the color and intensity of the
emitted fluorescence.
The spectral characteristics of oxazole 1 determined experimentally in a series of solvents differing in
polarity and proton-donating capacity (Table 2, Fig. 1, the data given in Fig. 1 are only for aprotic solvents), on
the whole, support our theoretical conclusions. The data obtained indicate that oxazole 1 does not show
significant solvatochromism, while its solvatofluorochromism is rather pronounced. The fluorescence Stokes
shift almost doubles in going from hexane to dimethyl formamide. High solvent polarity leads to a reduction in
the fluorescence quantum yield, especially pronounced for aqueous solutions, in which the rather strong
fluorescence of oxazole 1 found in nonpolar media is almost entirely quenched. Such a sharp change in the
spectral parameters of a fluorescent probe relative to the polar and proton-donating characteristics of its closest
molecular environment make it promising for fluorescent probing of microheterogeneous aqueous systems such
as micelles and biological media.
Since the acidity of the medium under conditions for the possible use of oxazole 1 as a probe may vary
in a rather broad range, it was also necessary to test this compound for sensitivity to pH. The results of the
spectrophotometric acid-base titration are shown in Figure 2.
In the first stage of protolytic interactions, a marked shift of the absorption spectrum toward longer
wavelengths is noted with a 30-35% drop in fluorescence intensity. This finding corresponds to protonation of
the pyridine nitrogen atom in oxazole 1, such that the intramolecular donor-acceptor interaction (charge transfer
upon excitation) is enhanced substantially. The latter circumstance is most likely to be the cause of a drop in the
fluorescence quantum yield of the monocation.
382

Fig. 1. Dependence of the position (from the top down) of the absorption spectrum, fluorescence spectrum, and
N
fluorescence Stokes shift of oxazole 1 on the Reichardt polarity (ET ) of a series of aprotic solvents.
Fig. 2. Spectrophotometric acid-base titration of oxazole 1 at pH from 7 to 0.5, changes in the absorption
spectrum with decreasing pH are shown by arrows: a) initial spectrum of the neutral form, b) spectrum at
intermediate pH values ~3 with a maximal contribution of the monocationic form, and c) spectrum of the final
solution with a high contribution of the dicationic form.
Further decrease in pH leads to a sharp shift of the absorption and fluorescence spectra toward shorter
wavelengths due to the exclusion of the lone electron pair of the dimethylamino group nitrogen atom from the
conjugation chain. The titration curves show two inflection points, corresponding to two stages of proton
addition upon increasing acidity of the medium. Mathematical treatment of these curves permits evaluation of
the equilibrium constants of the discussed protolytic reactions.
pK1 = 4.53 0.07
N
N
H+
–H+
H+
H+N
Me
O
Me
O
N
–H+
N
N
Me
Me
N
pK2 = 1.82 0.05
H+
+ Me
NH
H+N
O
–H+
Me
383

The basicity of the oxazole ring nitrogen atom is lower than for the pyridine nitrogen atom. Thus, the
third protonation stage should be observed for a more acidic medium at pH < 0 (in the region described by
acidity functions similar to the Hammett H0 function). Thus, this fluorophore will be in its neutral form in the
physiological pH range characteristic for most microheterogeneous biological systems.
The testing of oxazole 1 as a fluorescence probe for microheterogeneous media was carried out by
evaluating the critical micelle concentrations (CMC) for aqueous solutions of several surfactants of different
nature. Various methods are usually employed for determining this important colloid system index such as
conductometry, surface tension measurements, spectrophotometry, etc. [23, 24].
A significant bathochromic shift of ~2000 cm-1 was observed in the absorption spectra upon the titration
of aqueous solutions of oxazole 1 using the nonionic surfactant Brij-35 (lauryl ether of polyoxyethylene-23),
while the fluorescence intensity increases by more than an order of magnitude, which is also accompanied by a
slight shift of the emission band toward shorter wavelengths. These findings may be attributed to solubilization
of oxazole 1 by surfactant micelles into their nonpolar near-surface zone.
The changes in the absorption spectra for zwitterionic surfactant, cetyl dimethylammonium
propanesulfonate (CDAPS, the titration is presented as an example in Fig. 3) were similar to those for the
nonionic surfactant but a marked shift toward longer wavelengths was observed in the fluorescence spectra.
This result indicates greater polarity of the binding zone of oxazole 1 in the zwitterionic surfactant micelles.
Fig. 3. Spectrophotometric and fluorimetric titration of oxazole 1 in an aqueous solution of the zwitterionic
surfactant, CDAPS (concentration from 0 to 2·10-3 M). The changes in the spectra upon increasing the
surfactant concentration are indicated by arrows.
384

The greatest enhancement of the fluorescence of oxazole 1 by a factor of more than 50 was found for
solutions of the cationic surfactant, cetyl trimethylammonium bromide (CTAB). A significant shift of the
fluorescence band toward shorter wavelengths of about 1000 cm-1 was also observed. This finding is apparently
a result of specific solvation of rather nucleophilic oxazole 1 by CTAB cations such that this probe enters the
binding zone within the micelle with higher effective polarity than that for nonionic and zwitterionic surfactants.
Spectral changes fundamentally different from all the other cases examined were found for solutions of
the anionic surfactant, sodium dodecyl sulfate (SDS). Absorption bands for the mono- and dication appeared in
the absorption spectrum of oxazole 1 even before reaching the critical micelle concentration (Fig. 4), while the
TABLE 3. Monitoring of Various Microheterogeneous Micellar Media
Using 5-[4-(N,N-Dimethylamino)phenyl]-2-(4-pyridyl)-1,3-oxazole as the
Fluorescent Probe
Increase in the
fluorescence
intensity
Surfactant
f, (Csurfactant > CMC)
CMC, М
by a factor of
Brij-35
CDAPS
CTAB
SDS, pH 9.18 anionic
SDPS anionic
nonionic
zwitter-ionic
cationic
545
556
562
526
469
21
10
51
7
6.7·10–5 (6.2·10–5 [27])
3.6·10–3 (4.0·10–3 [28])
9.8·10–4 (9.2·10–4 [29])
8.8·10–3 (8.3·10–3 [29])
8.6·10–3 (8.0·10–3 [29])
5
absorption intensity of the monoprotonated form began to decrease above the CMC along with an increase in
the absorption in the range characteristic for the dicationic form of oxazole 1. The estimated value of the CMC
of SDS (9.4·10-3 M) under these conditions was obtained with an unsuitably high systematic deviation from the
literature data (Table 3).
Fig. 4. Spectrophotometric titration of oxazole 1 in an unbuffered aqueous solution by the anionic surfactant,
SDS (from 0 to 1.2·10-2 M, the surfactant concentration below the CMC is indicated by dotted lines, while the
surfactant concentration above the CMC is indicated by dashed lines). The changes in the spectra with
increasing surfactant concentration are indicated by arrows.
The most characteristic difference between SDS micelles from other types of surfactants is their high
surface density of negative charge [25, 26]. This increases the near-surface concentration of Na+ and H+
counterions, and consequently a substantial reduction of the local pH in the Stern layer, and thus to its
385

"acidification" compared with the intermicellar aqueous phase. Probe 1, falling on the surface of the micelles
SDS, undergoes protonation at the pyridine atom and stays there, without penetrating the hydrophobic regions
in the depth of the micelle. Therefore, in this experiment the enhancement of its fluorescence was not observed.
After achieving CMC, upon further increase in SDS concentration, acidity of the surface electrical double layer
of micelles decreases so much that the equilibrium monocation-dication increasingly shifts to the latter. Since
the probability of penetration of the dication form 1 to the interior of the micelles is even smaller than that of the
discussed above monocation it further does not increase the intensity of the fluorescence of the probe associated
with SDS micelles.
In order to check our conclusions concerning the reasons for the atypical behavior of oxazole 1 in
solutions of an anionic surfactant, this compound was titrated with another anionic surfactant, not providing
such high surface density of micellar negative charge as in the case of SDS, namely sodium
dodecylphenylsulfonate (SDPS). In the latter case, protonation of oxazole 1 on the micelle surface does not
occur and its fluorescence is enhanced upon solubilization though not to the same great extent as observed for
micelles of nonionic, zwitterionic, and cationic surfactants.
The SDS titration of oxazole 1 in alkaline buffer solution at pH 9.18 under conditions such that the
acidity of the micelle surface and the intermicellar aqueous phase differ only slightly leads to similar results.
The cationic forms of oxazole 1 were not appeared in this case. Thus, the neutral probe molecules are
solubilized within the micelles, which accompanied by a general increase in the fluorescence intensity. Under
the conditions for the measurements in the buffer system, our experimental value for the critical micelle
concentration of the anionic surfactant, SDS, proved much closer to the literature value than in the previous
case, in which this index was tried to estimate without buffer.
The experimental data on the titration of aqueous solutions of oxazole 1 by various surfactants were
mathematically treated to find the critical micelle concentration. Our quantitative results show rather satisfactory
agreement (deviations not exceeding 10%) with the literature data for the various surfactants obtained by other
methods [27-29] (Table 3).
Thus, we have synthesized a new 2,5-diaryloxazole fluorescent probe, namely, 5-[4-(N,N-dimethyl-
amino)phenyl]-2-(4-pyridyl)-1,3-oxazole, which was subjected to experimental testing and theoretical analysis.
This probe demonstrated its suitability for the quantitative monitoring of the physicochemical characteristics of
microheterogeneous colloidal systems.
EXPERIMENTAL
1H and 13C NMR spectra (DEPT-135) were recorded on a Varian Mercury VX-500 spectrometer (500
and 125 MHz, respectively) in DMSO-d6 with TMS as internal standard. Absorption spectra were recorded on a
Hitachi U3210 spectrophotometer, and fluorescence quantum yields were registered on a Hitachi F4010
spectrofluorimeter. Quinine bisulfate in 0.5 M aqueous sulfuric acid (f 0.546 [30]) was used as the standard for
measuring the quantum yields.
The protolytic reaction constants of oxazole 1 were determined by treating the data of the
spectrophotometric titration in 1:1 water–ethanol. pH of the water–ethanol solutions were measured using the
EV-74 universal ion meter calibrated relative to standard aqueous buffer solutions with a correction of 0.02 pH
unit [31].
In order to perform the titration using surfactant solutions, a solution of ~2·10-5 mol/l oxazole 1 was
prepared with the same content and concentration of the surfactant thrice greater than the CMC. The titration
was carried out in a standard 1-cm fluorimetric cell containing 2 ml initial aqueous solution of oxazole 1 and
from 0.005 to 0.2 ml surfactant solution added by previously calibrated dosing apparatus. The absorption and
fluorescence spectra were recorded after each addition.
386

The quantum-chemical calculations were carried out using the following program packages: Gaussian
09 [32] (optimization of the geometry of the ground and excited states) and NWChem 5.1 [33] (locally modified
version with the addition of program code for carrying out ESS analysis [16] and calculation of the absorption
spectra).
5-[4-(N,N-Dimethylamino)phenyl]-2-(4-pyridyl)-1,3-oxazole (1). A solution of 4-N,N-dimethyl-
amino--aminoacetophenone dihydrochloride (2.01 g, 8 mmol) and 4-pyridinecarboxylic acid (0.98 g, 8 mmol)
in POCl3 (30 ml) was heated at reflux for 1.5 h until HCl was no longer generated (tested on moist indicator
paper). The reaction mixture was then poured onto ice (0.5 kg) and made weakly alkaline by adding powdered
sodium carbonate. The precipitate formed was filtered off, washed with water until the wash water was neutral,
and dried in air. The product was purified by chromatography on an alumina column at elevated temperature,
close to the boiling point of the benzene eluent, and then recrystallized twice from water. The purity was
1
monitored by thin-layer chromatography. Yield 1.43 g (67%). Yellow needles. Mp 238-237°C. H NMR
spectrum, , ppm (J, Hz): 8.73 (2H, d, J = 6.1, H-2,6-Py); 7.93 (2H, d, J = 6.1, H-3,5-Py); 7.68 (2H, d, J = 9.0,
H-2,6 Ar); 7.66 (1H, s, H-4); 6.81 (2H, d, J = 9.0, H-3,5 Ar); 2.97 (6H, s, N(CH3)2). 13C NMR spectrum, ,
ppm: 156.6, 153.1, 150.6 (CH); 133.6, 125.6 (CH); 121.5 (CH); 119.2 (CH); 114.4, 112.1 (CH); 39.8
(N(CH3)2). Found, %: C 72.55; H 5.69; N 15.83. C16H15N3O. Calculated, %: C 72.43; H 5.70; N 15.84.
The authors express their sincere gratitude to Prof. N. A. Vodolazkaya (V. N. Karazin Kharkiv National
University) for supplying samples of the different surfactants and participating in a discussion of these results
and to Prof. Jerzy Błażejowski (Gdansk University, Poland) for cooperation in the measurement of the NMR
spectra.
This work was carried out using the computer resources of the joint computer cluster of the "Institute of
Single Crystals" and the Institute of Scintillation Materials, National Academy of Sciences of Ukraine, in
conjunction with the Ukrainian national grid.
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