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vol.48 número4TAUTOMERISM OF XANTHINE AND ITS PAIRING WITH 2,6- DIAMINOPYRIMIDINE:AN ABINITIO STUDY IN THE GAS PHASE AND AQUEOUS SOLUTION índice de autoresíndice de materiabúsqueda de artículos
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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.4 Concepción dic. 2003

http://dx.doi.org/10.4067/S0717-97072003000400022 

J. Chil. Chem. Soc., 48, N 4 (2003) ISSN 0717-9324

ELECTROCHEMICAL BEHAVIOR OF 1,3-DITHIENYL AND DIFURYL PROPENONE DERIVATES.PART I

FRANCISCO BROVELLI1, BERNABÉ L. RIVAS1*, LUÍS BASAEZ2

1 Department of Polymers, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile. brivas@udec.cl
2 Department of Analytic and Inorganic Chemistry, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile.
(Received: August 5, 2003 - Accepted: September 29, 2003)

ABSTRACT

A series of heterocyclic a,b´-unsaturated ketones containing thiophene and furan rings has been synthesized. By ab-initio methods the geometries have been optimized and the electronic density distributions of neutral molecules were determined. To that the Hartree Fock (HF 6-31G*) calculations base were used. The electronic density distribution did not depend on the heteroatom type, however, there is a dependence of the group that acts as bridge. Moreover, the oxidation-reduction potentials of the molecules were determined by cyclic voltammetry on platinum electrodes. The electrochemical oxidation was irreversible and showed asymmetric peaks. This was explained by the simultaneous oxidation of the heterocyclic rings and the functional groups acting as bridge. The reduction of these species involves only the carbonyl group.

Keywords: a,b-unsaturated ketones, electrochemical oxidation-reduction, ab-initio calculations, electronic density, molecular orbitals

INTRODUCTION

There is a growing interest to study the electronic properties of conjugated organic molecules by theoretical calculations. These molecules are precursors of polymers with electrical properties. In the literature there are several studies about this type of systems. The extended Hückel calculations performed on dimers of selenophene, thiophene and furan [1] allowed to establish that the interconversion energetic barriers between the syn and anti form increase progressively. This implies that the conjugative interactions acquire a great importance, nevertheless in the case the free rotation is not excluded. By using the ab-initio methods [2] a model to study the internal rotations of dimer rings of thiophene and pyrrole was developed. The molecular structure and conformational behavior of oligoselenophenes (n = 2-4) have also been analyzed by the ab-initio methods and density functional theory (DFT) [3].

The Hartree Fock (HF) calculations predict a plane torsional potential which is determined by steric and non-bonding interactions allowing an easy interconversion between the rotamers, where the minimum energy corresponds to the anti-gauche conformation and the less stable conformation to the syn conformation. It was demonstrated that the plane and perpendicular conformations are really transitional states in these oligomers.

The theoretical calculations have not been restricted to heterocycles derived of sulfur and selenium [4], however a wide study was carried out from rings containing different heteroatoms [5]. The DFT calculations with a hybrid functional B3P86, showed that there exists a relation between the band gap energy and the nature of the heteroatom. It is increased by the donor strength of the heteroatom [5]. In general, it was established that the heteroatoms of the second period conduct to a higher band gap values than the superior analogues.

The experimental correlation is one of the most efficient methods to establish or corroborate the certainty of calculation. According to that, electrochemical properties of substituted mixed oligomers derived from (thiophene-pyrrole-thiophene) have been studied and compared with that corresponding to ter-thiophenes [6]. Ab-initio and DFT calculations indicate that the radical cation is twisted in comparison with the thiophene radical which is plane. This geometry should be the responsible of the variation of the oxidation potential in function of substituent position [6]. In a recent work [7] the theoretical and experimental values of a series of thiophene derivates have been compared. This allowed to explain the variation of the oxidation potential in function of the substituents.

Accordingly, the aim of this paper is to increase the theoretical and experimental study of conjugated organic systems as it is well known that some of them are precursors of polymers with electrical properties [8]. Thus, the synthesis of structures derived from a-b'-unsaturated heterocyclic ketones (see figure 1) was carried out and the study by computing calculations was made in order to determine the electronic density, molecular orbitals and their correlation with the oxidation and reduction potential.


Experimental: Synthesis and Characterization
Synthesis of 1,3 di-thienyl-2-yl-propenone (HX1)
[7].
Synthesis of 1,3 di-furyl-2-yl-propenone (HX2).

It was synthesized according to the literature [7,9-11]. The product was crystallized from ethanol/water (5:1) and dried under vacuum at 40° C for 24 h. Yield: 47%. Elemental analysis (%) (found/calcd.): 69.88/70.21(C); 4.11/4.29(H); 26.01/25.51(O).

The FT-IR spectrum was recorded in solid dispersion (KBr). The absorption signals were assigned to the following vibration modes [12-14]: 3134, 3102, 3083 (n CH heterocyclic ; n CH vinyl); 1655 (n C=O); 1603 (n C=C vinyl); 1559, 1464 (n C=C heterocyclic); 1394 (n C-C heterocyclic); 1322, 1282 (d C-H heterocyclic); 981 (d C=CH oop trans); 880, 848, 834 (d CH oop heteroc. subst-2); 762, 737 cm-1 (d CH oop heteroc. subst-2).

1H-NMR spectrum (200 MHz, CDCl3) shows the following signals d(ppm): 6.52 (dd; J=1.7; 3.6 Hz; 1H; H-4); 6.59 (dd; J=1.8; 3.4 Hz; 1H; H-11); 6.72 (d; J=15 Hz; 1H; H-7); 7.30 (dd; J=3.6; 1.0 Hz; 1H; H-3); 7.34 (d; J=3.2 Hz; 1H; H-10); 7.53 (d; J=1.4 Hz; 1H; H-12); 7.62 (d; J=15 Hz; 1H; H-6); 7.66 (dd; J=3.5; 2.9 Hz; 1H; H-5).

13C-NMR spectrum (50 MHz, CDCl3) shows the following signals d(ppm): 177.7 (C=O); 153.7 (C2); 151.6 (C9); 146.6 (C5); 145.0 (C12); 129.9 (C7); 118.9 (C6); 117.4 (C3); 116.4 (C10); 112.7 (C11), 112.5 (C4).

Synthesis of 1-furyl-2-yl-3-thienyl-2-yl-propenone (HX3)

It was synthesized according to the literature [7,9-11]. The product was crystallized from ethanol/water (5:1) and dried under vacuum at 40° C for 24 h. Yield: 69%. Elemental analysis (%) (found/calcd.): 64.65/64.69 (C); 3.70/3.95 (H); 15.43/15.67 (O); 16.22/15.70 (S).

The FT-IR spectrum was recorded in solid dispersion (KBr). The absorption signals were assigned to the following vibration modes [12-14]: 3115, 3090 (n CH heterocyclic ; n CH vinyl); 1650 (n C=O); 1597 (n C=C vinyl); 1557, 1463 (n C=C heterocyclic); 1426, 1396, 1365 (n C-C heterocyclic); 1286, 1253 (d C-H heterocyclic); 963 (d C=CH oop trans); 860, 831, 824 (d CH oop heteroc. subst-2); 776, 703 cm-1 (d CH oop heteroc. subst-2).

1H-NMR spectrum (200 MHz, CDCl3) shows the following signals d(ppm): 6.59 (dd; J=1.7; 3.5 Hz; 1H; H-11); 7.09 (dd; J=3.7; 5.3 Hz; 1H; H-4); 7.25 (d; J=15.5 Hz; 1H; H-7); 7.35 (m; 2H; H-3, H-10); 7.43 (d; J=5.0 Hz; 1H; H-5); 7.65 (d; J=1.1 Hz; 1H; H-12); 8.00 (d; J=15 Hz; 1H; H-6).

13C NMR spectrum (50 MHz, CDCl3) shows the following signals d(ppm): 177.6 (C=O); 153.6 (C9); 146.5 (C12); 140.3 (C2); 136.4 (C6); 132.2 (C3); 129.0 (C7); 128.4 (C5); 120.1 (C4); 117.4 (C10); 112.6 (C11).

Synthesis of 3-furyl-2-yl-thienyl-2-yl-propenone (HX4)

This product was synthesized according to the literature [7,9-11]. The product was crystallized from ethanol/water (5:1) and dried under vacuum at 40° C for 24 h. Yield: 42%. Elemental analysis (%) (found/calcd.): 63.95/64.69 (C); 3.75/3.95 (H); 16.16/15.67 (O); 16.14/15.70 (S).

The FT-IR spectrum was recorded in solid dispersion (KBr). The absorption signals were assigned to the following vibration modes [12-14]: 3143, 3121, 3097, 3076 (n CH heterocyclic ; n CH vinyl); 1648 (n C=O); 1587(n C=C vinyl); 1546, 1515, 1475, 1412 (n C=C heterocyclic); 1352, 1325 (n C-C heterocyclic); 1285, 1235 (d C-H heterocyclic); 975 (d C=CH oop trans); 837,823, 767 (d CH oop heteroc. subst-2); 740, 721, 695 cm-1 (d CH oop heteroc. subst-2).

1H-NMR spectrum (200 MHz,CDCl3) shows the following signals d(ppm): 6.52 (dd; J=1.8; 3.4 Hz; 1H; H-4); 6.73 (d; J=3.4 Hz; 1H; H-3); 7.17 (dd; J=3.8; 4.9 Hz; 1H; H-11); 7.33 (d; J=15 Hz; 1H; H-7); 7.53 (d; J=3.5 Hz; 1H; H-10); 7.60 (d; J=15 Hz; 1H; H-6); 7.67 (dd; J=1.1; 4.9 Hz; 1H; H-12); 7.85 (dd; J=1.1; 3.8 Hz; 1H; H-5).

13C NMR spectrum (50 MHz, CDCl3) shows the following signals d(ppm): 182.1 (C=O); 151.9 (C2); 146.1 (C9); 145.3 (C5); 134.1 (C7); 132.0 (C12); 130.3 (C10); 128.6 (C6); 119.6 (C11); 116.7 (C3); 113.1 (C4).

Electrochemical setup

The experimental method to carry out the potentiodynamic scans (CV) has been previously described [7,15]. As work electrode a polycrystalline platinum disc of 0.24 cm2 geometric area was used. The reference electrode was Ag/AgCl in solution of tetraethylammonium chloride (Et4NCl). The potential was adjusted with respect to the saturated calomelane electrode (SCE) [16]. As counter-electrode a spiral platinum was used in a separated compartment of work electrode by fritted glass. Before each experiment, the work electrode was polished with alumina slurry (particle size 0.3 m m). The data were recorded in a BAS CV-50W system coupled to a PC computer.

Anhydrous acetonitrile (Aldrich Chemical Co) as solvent was used. As support electrolyte tetrabutylammonium hexafluorphosfate (TBAPF6) (Aldrich Chemical Co) was used. It was dried under vacuum at 60 °C for 24 h.

All solutions were kept under nitrogen during 30 min before each experiment. The gas flux was inverted to keep an inert atmosphere while the electrochemical perturbation was applied.

Computing Calculations

These calculations were carried out by Mac Spartan Pro® [17]. The equilibrium geometries were determined by complete optimization of the conformations, employing as calculation base HF 6-31G* in the RHF level for neutral molecules.

The equilibrium geometries were determined by complete optimization of the conformations by using 6-31G* at RHF level for neutral molecules as a calculation method [7]. There is a very good agreement between the geometry values determined by this method and those experimental data [18].

RESULTS AND DISCUSSION COMPUTING CALCULATIONS

The electronic density maps of the HOMO and LUMO levels of each molecule were done. The electronic density distribution (EDD) of both levels is not influenced by the type of heteroatom (see figure 2). In the HOMO level (see figure 2a, c, e, and g) the EDD involves the heterocycles carbons and vinyl carbons, C2-C7. This result suggests that the oxidation of these molecules involves the ring and the vinyl group which acts as bridge [7]. The influence of the second ring on the EDD is not significant.

For the LUMO level (see figure 2b, d, f, and h), the electronic density maps indicate that these molecules may suffer a nucleophilic attack on the carbonyl carbon or an addition to a,b unsaturated double bonds.

By comparison of the electronic density values of the levels of HOMO for HX1 and HX3, HX2 and HX4, only a slight influence of the neighbor ring is observed on the carbonyl group (see Table 1). This effect can be explained by a loss of conjugation between the rings due to the presence of the carbonyl group and an increase of the dihedral angle (X1,C2,C6,C7) which implies a lower overlapping between the p electrons of the aromatic system and carbonyl group. The second factor allows to explain the differences of electronic density in both homologues. The electronic density of LUMO level for HX1 and HX3, HX2 and HX4 shows a similar behavior (see table 1). The influence of the heteroatom is evidenced by an increase of the positive charge on the carbonyl carbon when the neighbor carbon corresponds to furan.


The observed differences between HX2 and HX4 are higher than those in the anterior situation as in this case an increase of the dihedral angle is produced due to a repulsive interaction between the neighbor electron pair (O,O).


Table 1. Electronic density values of HOMO and LUMO levels of the a,b-unsaturated ketones obtained from the density maps. Contour value of density: 0.08e/A3.


CARBON

HX1

HX2

HX3

HX4


HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

C2

0.0421

-

0.0439

-

0.0441

-

0.0445

-

C3

0.0385

-

0.0397

-

0.0436

-

0.0382

-

C4

0.0310

-

0.0346

-

0.0315

-

0.0313

-

C5

0.0411

-

0.0444

-

0.0452

-

0.0429

-

C6

0.0311

0.0496

0.0273

0.0513

0.0331

0.0457

0.0314

0.0505

C7

0.0419

0.0417

0.0403

0.0467

0.0393

0.0391

0.0402

0.0476

C8
-
0.0552
-
0.0616
-
0.0528
-
0.0550

The conformations of these molecules are the result of the interactions between the electron pair of the oxygen from carbonyl group and the heteroatom of the neighbor ring, indicating a favored conformation. The molecules with the pair (O,S) show a dihedral angle near to 0° due to an attractive effect of the oxygen and sulfur [19-21]. However, for the molecules with pair (O,O) there is an increase of the angle which suggests an overlapping of the p system due to dihedral angle between the ring and the vinyl group. Other factor is related with the type of bonded groups at positions 2 and 9. Therefore, it is possible to expect that the reactivities of both rings may be strongly different. It has been previously established [7]. However, in a homologue series it would be expected that the heteroatom modifies the reactivity of the molecule, thus, the presence of the neighbor furan ring to vinyl group increases sharply the oxidation potential, but acting in contrary sense during the reduction. It can be summarized indicating that the type of heteroatom was influenced basically by the p donor strength over the electronegativity.

The conformations obtained by this calculation type are in agreement with the coupling constants of C6-C7 whose value is 15 Hz. These results are also in agreement with those of the literature [7]. The conformations of hetero aryl ketones in solid state and in solution indicate that in a solid state they are very similar to those conformations calculated with a low energetic content and the population of conformers is in agreement with the measurements in solution [22]. In the 2-thiophen-substituted, the sulfur atom is always placed at the same side of the carbonyl group. This is due to an attractive interaction between sulfur and oxygen atoms. The orientations (S,O) cis, (S,O) cis are kept with a 70%. The ring twisting angle respect to the plane of the carbonyl group is very similar to that found by the calculations.

Moreover, from ab-initio calculations the energy values of HOMO and LUMO of the molecules under study have been determined. The results are summarized in table 2. It is possible to observe more easily the effect of the heteroatom on the energy of the molecular orbitals. The molecules with furan ring neighbor to vinyl group yield an energy stabilization. However, with the change of the second ring a slight increase of the energy is produced. This can only be attributed to the increase of the p donor strength of the heteroatom.


Table 2. Energy values of the HOMO and LUMO levels of the molecules obtained by HF 6-31G* and oxidation-reduction potentials values obtained from voltammograms of the figures 3 and 4.


COMPOUND

HOMO

LUMO

Eox

Ered


HX1

0.311

-0.053

1.72

-1.32

HX2

0.305

-0.072

2.00

-1.47

HX4

0.306

-0.067

2.01

-1.29

HX3
0.313
-0.064
1.97
-1.43

Electrochemical characterization

The major advantage of the electrochemical methods is allowing to establish a fast correlation between the structure and the reactivity of a series of molecules.

The electronic effect allows to explain the variation of the oxidation-reduction potentials (Ep) that increase linearly with s p+ of Hammett-Brown. This constant s p+ that characterizes the atoms for its electronic effect in the stability of the intermediate of Wheland in aromatic electrophilic substitutions. Bipolaron, responsible specie of conducting properties in thiophene is similar to intermediate of Wheland [23].

Figure 3 a-d, shows the anodic voltammetric profiles of the molecules studied. During the anodic hemicycle and unsymmetrical peak appears demonstrating the presence of oxidation process which can be consecutive or competitive. The oxidation process of these molecules is not reversible.

Due to that the monomer shows two rings with different reactivities, both anodic processes may be attributed to oxidation of the substituted ring in the position 2 by the vinyl group [7,24,25] and by the second peak due to oxidation of the vinyl group [26]. This mechanism has been proposed for the electro-oxidation of 2-vinyl thiophene [27], where the vinyl group is oxidized and initiates the propagation placing the rings as pendant groups, and then with a higher anodic potential a coupling 5,5' between the rings is produced yielding a crosslinked structure.


In systems which are polymerized by successive electrochemical scan oxidation [28-30] the non loss of conjugation shows reversible redox processes which favor the formation of polaron or bipolaron that are delocalized on a higher amount of units. Therefore, the absence of electrochemical reversibility would indicate that the doping level would be low.

Figure 4 shows the cathodic profiles obtained during the electrochemical reduction of these molecules. The reduction of these species is characterized by an irreversible process attributed to the reduction of the carbonyl group through free radical [31].


This mechanism has also been proposed when gold is employed as electrode substrate. In this case the voltammetric reduction of oligomers of 2-dithienylketone occurs through a mono-electronic reversible process. In general, the reduction mechanism of aldehydes in aprotic medium occurs through the formation of an anion radical that may be reduced again to produce a dianion which has been detected by EPR [32].

The mechanism of reduction of these species could be similar to that described for reduction of chalcones, where the process occurs by mono-electronic reduction with 2 to 3 waves where the initial radical can be stabilized due to the contribution of resonance structures formed with the b carbon with respect to the carbonyl group [33]. The third wave would correspond to the reduction of a species that does not present an unsaturated a,b bond as observed in the general mechanism (see figure 5). In the case of aprotic medium, the protonation of the carbonyl group corresponds to the slow step of the reaction .

For furfuryl acetophenones [34], each reduction is mono-electronic. In the initial step it is probable that a nucleophilic attack on the carbonyl group may occur yielding a free radical. For benzophenones the reduction process shows only one wave [34], that means that the free radical is unstable and it is fastly reduced to form the carbinol. Nevertheless the effect of the vinyl group in furfuryl and acetophenones [31,33] is attributed to the formation of a more stable free radical due to that a double bond reduces the electronic density on the carbonyl group.

Calculations with several derivatives of oligo 2-dithienyl ketones have demonstrated the localization of the properties of HOMO and LUMO of the dimer of 2,2'-dithienyl ketone. The HOMO level shows a disruption of conjugation between each carbonyl group, however an extra electron in the LUMO is delocalized in all the chains. That means that during the successive oxidation to produce the corresponding poly(2,2'-dithienyl ketone), the electrical conductivity would not be favored by the formation of a quinoide structure after doping [18].


The energy value of the LUMO level, indicates that the stabilization is higher than that expected according to the inductive effect of the carbonyl group, thus the electronic affinity in these compounds would be higher than that in the oligothiophenes that contain the same number of fragments [35].

The molecules with a ring of thiophene neighbor to vinyl group show a lower oxidation potential, meanwhile the substitution by a furan ring increases significantly such potential. This is attributed to two factors: p donor strength of the oxygen atom and the increase of the dihedral angle.

Table 2 shows that the potential peaks vary significantly due to the effect of heteroatom. In this case, the electronic effect modifies the oxidation-reduction potentials of the molecules. Other factor that could be affected in the potential variation is the torsion angle of the rings which would yield a decrease of overlapping of the p electrons, explaining the loss of electrochemical reversibility in solution. The energy values of the HOMO-LUMO levels predict qualitatively the order of the redox process. However, due to small differences in the energies of HX1 and HX3, HX2 and HX4, the order can be changed.

The materials that could be obtained by electrochemical polymerization of these molecules should be adequate for their use in electroluminescent diodes [8,36-38] as in these devices a very low doping level is required.

CONCLUSIONS

The distribution of electronic density depended on the type of heteroatom of the ring and on the equilibrium conformation. The effect of the heteroatom is observed basically in the local density value.

The higher electronic density is concentrated in the heterocycle ring, however, eventually the functional group may show a higher local density to that determined by any carbon atom in the ring.

The effect of the functional group in the calculations of the energy levels of HOMO and LUMO, allows to predict a qualitative order of the oxidation potentials for the studied molecules.

The voltammetric behavior of these species was characterized by irreversible processes. During the oxidation, it was not discarded the participation of more than one process. But during the reduction only one process which was attributed to the reduction of the carbonyl group was observed.

ACKNOWLEDGEMENTS

The authors thank to Fondecyt (Grant No 3020010) the financial support.

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