An evaluation of the chalcogen atom effect on the mesomorphic and electronic properties in a new homologous series of chalcogeno esters

) tambem foram obtidos de maneira a se obter a diferenca de energia entre os orbitais moleculares HOMO-LUMO, para posterior comparacao com os resultados teoricos. Os valores calculados para o gap de HOMO-LUMO estao de acordo com os valores experimentais obtidos, onde a eletronegatividade do heteroatomo (O, S e Se) deve ser o principal fator para a localizacao do maximo de absorcao. Um estudo teorico foi realizado utilizando calculos de DFT/B3LYP com metodo hibrido de Becke aplicando uma correlacao funcional de Lee-Yang-Parr (B3LYP) com a base DGDZVP a fim de avaliar algumas propriedades fisico-quimicas relacionadas com o mesomorfismo e as propriedades eletronicas espectrais. Atraves da sintese destes compostos, a influencia do atomo de calcogenio nas propriedades fotofisicas e mesomorficas pode ser avaliada de uma forma mais precisa.


Introduction
Chalcogenide compounds have found such wide utility because their effects on an extraordinary number of very different reactions. 1 In addition, they have become attractive synthetic targets because of their chemo and regioselective reactions, 2 tolerating a large variety of functional groups, thus avoiding protection group chemistry. More recently, chalcogen containing compounds have been used as chiral catalysts or as chiral ligands in various stereoselective reactions. 3 Vol. 21, No. 11, 2010 The biological and medicinal properties of selenium and organoselenium compounds are increasingly appreciated, mainly due to their antioxidant, antitumor, antimicrobial and antiviral properties. 4 Besides, the synthesis of peptides containing selenocysteine is rapidly gaining interest with the discovery of an increasing number of proteins containing this aminoacid. 5 Chalcogeno esters (thiol, selenol and tellurol esters) are useful synthetic intermediates that have been employed, for example, as acylating reagents, building blocks for heterocyclic compounds, precursors of acyl radicals and anions, as well asymmetric aldol reactions. However, rare are the examples where these compounds are reported as liquid-crystalline materials despite its promising photophysical properties for optical device applications such as emissive LC (liquid crystals) displays, polarized organic lasers, anisotropic OLEDs and organic field-effect transistors (OFETs). 6 In the course of our work in the field of liquid-crystalline materials, we have reported selenoesters 1 (Figure 1), which exhibited liquid crystalline properties over a large range of temperature. 7 In addition, these selenoesters-LC are fluorescent in the blue region presenting a Stokes shift higher than 39 nm.
The general structure of selenoesters 1 is very attractive for several reasons. It can be synthesized in a straightforward manner which allows us to explore the effects of the chalcogen atom on the stability and packing order in the mesophase as well on their photophysical behavior. Most important, chalcogeno esters with high structural diversity can be readily generated, with is important for the systematic evaluation of the real influence of the chalcogen atom on the mesomorphic behavior. Earlier studies, shown for example that different chalcogen atoms in a series of compounds can induce changes in their photophysical properties. 8 In connection with our current interests in the study of the mesomorphic properties of some chalcogencontaining compounds, the objectives of this work were to investigate the photophysical and LC properties of the chalcogeno esters, focused on the chalcogen atom. In additions perform some theoretical studies using DFT/ B3LYP calculations with DGDZVP basis set.

Experimental
General 4-Bromophenol, 1-bromoalkanes, 2-methyl-3-butyn-2-ol (mebynol), KOH, triphenylphosphine (PPh 3 ), tributylphosphine (PBu 3 ), dimethylformamide (DMF), CuI, N,N'-dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), benzene, ethanol and 4-bromobenzoic acid were used without further purification from Aldrich. Triethylamine was distilled over KOH and isopropyl alcohol and dichloromethane distilled over CaH 2 under argon immediately prior to use. All other commercial solvents and reagents were used without further purification. The reactions were accompanied by analytical thin-layer chromatography (TLC), conducted on Merck aluminum plates with 0.2 mm of Silica Gel 60 F 254 . The melting points and mesophase transition temperatures and textures of the samples were measured on a Mettler Toledo FP82HT Hot Stage FP90 Central Processor and DSC Q2000 Series TA Instruments. Nuclear magnetic resonance spectra were obtained on a Varian 300 MHz instrument. Chemical shift are given in parts per million (d) and are referenced from tetramethylsilane (TMS). ATR/FTIR spectra were recorded on a Varian FT-IR-640 spectrometer. CHN analyses were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer. High resolution mass spectra were recorded on a Micromass Q-TOFmicro instrument. Spectroscopic grade dichloromethane was used to UV-Vis absorption spectroscopy measurements. UV-Vis absorption spectra were recorded in a Shimadzu UV-2450PC spectrometer. All experiments were performed at room temperature in a concentration of 10 -5 mol L -1 .

4´-[4-(Octyloxyphenyl)ethynyl]benzoic acid (4)
Deprotection and coupling reaction: 9 Potassium hydroxide (0.5 g, 9 mmol) and isopropyl alcohol (40 mL) were added to a round bottomed flask and heated at 50 o C for 15 min. Then, a solution of 4'-(4-octyloxyphenyl)-2-methylbut-3-yn-2-ol (8) (0.9 g, 3 mmol) in isopropyl alcohol (10 mL) was added. The mixture was heated under reflux for 2 h. The solvent was evaporated, the residue dissolved in CH 2 Cl 2 (50 mL), and washed with water (3 × 30 mL). The organic layer was dried over anhydrous sodium sulfate. The solvent was evaporated and a yellow oil was obtained in 99% yield. This oil was used to another Sonogashira coupling with methyl 4-bromobenzoate (3). After cooling of reaction test tube, the solid was filtered and washed with CH 2 Cl 2 (100 mL). The filtered mixture was evaporated, and the resulting dark yellow oil was dissolved in CH 2 Cl 2 (200 mL) and washed with water (3 × 80 mL), cold 5 mol L -1 hydrochloric acid (80 mL) and water (80 mL). The organic layer was dried over anhydrous sodium sulfate. The solvent was evaporated and the remaining solid was purified by chromatography and recrystallization from hexane. Yield: 1.79 g, 48%; mp 112.6 o C. 1  Hydrolysis reaction: 10 Methyl-4´-(4-octyloxyphenyl) ethynyl benzoate (9) (0.8 g, 2.3 mmol) was dissolved in THF (30 mL) and 1 mol L -1 aqueous KOH (12 mL) was added. After heating the reaction mixture at 60 o C for 24 h, the resulting solution was evaporated to dryness. The solid was dissolved in water (60 mL) and then the aqueous solution was acidified by conc. HCl (pH 1). White precipitate was filtered, washed with water, and then dried. The product was obtained by recrystallization from EtOH. Yield: 0.6 g, 68%; decomposition > 250 o C. 1

Results and Discussion
Scheme 1 outlines the synthesis of the key intermediate 4 starting with the preparation of the acetylene 2, which synthesis has been described previously. 9 Thus the alkylation of p-bromophenol with 1-bromooctane gave the corresponding alkylaryl ether (7) in 81% yield. Sonogashira reaction 11 of the alkylaryl ether with 2-methyl-3-butyn-2ol (mebynol), followed by deprotection using KOH and isopropyl alcohol furnished the acetylene 2. 12 Finally, through a second Sonogashira reaction of the acetylene derivative 2 and methyl 4-bromobenzoate (3), compound 4 was obtained after hydrolysis in satisfactory yields.
Having obtained the acid 4, we concentrated our efforts on the synthesis of the target molecules. In this way, the resulting acid 4 was converted into chalcogeno esters 6a and 6b by reaction with the corresponding phenol or thiol in classical conditions. Both final products were characterized by spectroscopic means and their structures are consistent with the spectral data.
The thermal transitions and enthalpies of the chalcogeno esters 1a, 6a, and 6b were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) and the results are tabulated in Table 1. The transition temperatures and enthalpy values were collected from the second heating scan. The texture of the mesophase was identified by microscopy studies and compared with pictures according reference 13. 13 All chalcogen-LC display the nematic phase which is expected for derivatives with short alkyl chains. 14 The presence of small polar methoxy group bonded to a conjugated core increases the effective length of the mesogen without affecting its breadth significantly. The DSC traces of chalcogeno esters 1a, 6a and 6b showed that all samples were thermally stable. The two reversible peaks observed in DSC traces are associated with Crystal (Cr) → Nematic (N) and Nematic (N) → Isotropic (I) transitions, respectively (Figure 2). On cooling from its  isotropic phase, the samples enter into nematic phase with a thread-like texture. An illustration of a nematic phase texture found in chalcogeno esters is inserted in Figure 2. This texture is usually observed in thin samples placed between two crossed polarizers. The dark lines, the so-called threads, are singularity lines which either connect two point defects or form closed loops. The planar thread-like texture is characteristic of liquidcrystalline nematic phases.
From Table 1 we can see that chalcogeno esters showed a wide phase transition temperature range for all compounds in this study. The enthalpy values for these LC compounds are in agreement with a less ordered nematic phase. The LC-6b has displays a widest temperature range melting at 116.9 °C to mesophase and to isotropic liquid at 248.8 °C (∆T = 131.1 °C), and in addition LC-6b have the highest enthalpy value in this series. The clearing temperatures increase in the order S > O ≅ Se, suggesting that the polarizability of chalcogen atom is an important factor in relation to the mesophase stability. However, the melting temperature of each chalcogeno ester is in the order: O > S > Se (Table 1). Probably, in the crystalline state the molecular interaction is more effective to the more electronegative chalcogen, which helps to stabilize the packing of molecules, favoring in this way the compound O-ester 6a.
To the liquid-crystalline state, the substitution of oxygen atom in this series by a chalcogen atom with superior atomic number having higher atomic radius and larger polarizability, enhances the intermolecular interaction after melting and thus stabilizes the mesophase. Our results are suggesting that enhancement of the clearing point of thioester 6b is attributed to the increasing of delocalization charge of the sulfur atom over benzene ring and to the carbonyl group; the values for the C=O stretching vibrations listed in Table 2 are indicative of substantial resonance interaction. 15 Considering our results listed in Table 1, we can see that the thermal stability of these chalcogeno esters does not follow only the Se > S > O polarizability order. 16 However, the clearing temperature for selenoester 1a is lower than expected. In addition, the formation of mesophase with low symmetry depends on the structural features of the molecular framework. In this sense, we have designed a proper molecular LC having a triple bond inserted between two aryl rings and at least one terminal short alkyl chain, such as a methoxy group.   To investigate the origin of this behavior a theoretical study was performed using a DFT/B3LYP calculations with Becke's three-parameter hybrid method 17 using the Lee-Yang-Parr correlation functional 18 (B3LYP) with DGDZVP 19 basis set. All quantum chemical calculations were performed with the Gaussian package. 20 All geometries were full optimized to a minimum, and, in order to check if there is any imaginary frequency, we have performed force constant calculations. Table 3 contains some selected data obtained by theoretical calculations and in Table 2 are compiled some spectral values collected from literature of IR and NMR spectrum in order to establish a comparative analysis. 21 The O=C-Y and Y-C sp2 bond distances of the chalcogeno esters O, S, Se and Te increase as the atomic radius increases in the order Te > Se > S > O. The calculated dipole moments also follow the same tendency in spite of the decreasing eletronegativity (in brackets) down the groups in the periodic table: O (3.44) > S (2.58) > Se (2.55) > Te (2.10). Interesting to note that the O=C-Y-C sp2 angle decreases with decreasing electronegativity of the chalcogen atom. The deviation angles observed for these compounds occur because of a slightly different hybridization of the chalcogen atom when bonded to two groups. So, as the O=C-Y-C sp2 angle decreases, the lone pairs on chalcogen atoms have more s character which contributes to increase the polarizability in the order Te > Se > S > O. The polarizability calculations show that the higher the chalcogen atomic number the greater will be the polarizability of the molecule. This information can be used to explain the chemical shift observed in Table 2 for these compounds.
The highest occupied and the lowest unoccupied molecular orbital (HOMO and LUMO, respectively) energies and energy band gap (LUMO-HOMO energy difference, ∆E) are given in Table 3. The energy band gaps close as the chalcogen atomic number increases. UV-Vis data in solution (ca. 10 -5 mol L -1 ) was applied in order to obtain the HOMO-LUMO gap for further comparison with the theoretical results. The calculated values to the HOMO-LUMO gap are in excellent agreement with the experimental results in which the electronegativity of the heteroatom (O, S and Se) seems to play a fundamental role in the absorption maxima.
The absorption maxima for 1a, 6b and 6a are observed at 341, 337 and 324 nm, respectively, which are clearly shifted to longer wavelengths (bathocromic effect) related with polarizability of the chalcogen atoms. The absorption maximum of selenol ester 1a is slightly red-shifted relative to 6b, but it is red-shifted at 17 nm relative to 6a. These data suggest that increasing the electronegativity of the heteroatom causes a hypsochromic effect indicating that the electrons are tighter in the molecular structure. Concerning the ε max values, the ester and thioester compounds present a similar planar structure in comparison with the selenoester which presents the higher one, as revealed by the chalcogen coefficients of the HOMO (Figure 3). Figure 3 shows the HOMO of the chalcogeno esters O, S, Se and Te. Both the HOMO and LUMO (Supplementary Information, SI) levels are of π nature and spread over the tolan group which is responsible for absorption of    ultraviolet radiation. It is interesting to note that both HOMO and LUMO orbitals present nodes along the long molecular axis. We can see that the aryl substitution directly on the chalcogen does not have a greater effect on the HOMO and LUMO energies, since the corresponding wave functions do not have contributions from the aryl substituents; hence, aryl substituents on the chalcogens are not expected to induce any significant alteration on the electronic properties of the chalcogen O, S, Se and Te. In contrast to the electronic properties, aryl substitution has an effect on the LC properties. Our results are suggesting that the thiol 6b is better than selenol 1a in stabilizing the LC mesophase. The smaller mesophase range of 1a compared to thiol 6b could be related with the reduced π-conjugation of chalcogen-benzene ring due to the high energy level of 4d orbital of the selenol 1a.

Conclusions
In summary, we have shown a practical and concise synthesis of an homologous series chalcogeno esters by an easy, straightforward and flexible synthetic route. The thermotropic liquid crystalline properties were investigated by DSC and POM analysis The final chalcogeno esters 1a, 6a and 6b exhibited a stable and large nematic mesophase range. In addition, the calculated values to the HOMO-LUMO gap are in excellent agreement with the experimental results in which the electronegativity of the heteroatom (O, S and Se) seems to play a fundamental role in the absorption maxima.

Supplementary Information
Supplementary data are available free of charge at http://jbcs.sbq.org.br, as pdf file.