Synthesis and Electrical and Gas Sensing Properties of Some 5-( Quinolinylmethylene )-2-thioxodihydropyrimidine-4 , 6 ( 1 H , 5 H )-dione and 5-( Quinolinylmethylene ) pyrimidine-2 , 4 , 6 ( 1 H , 3 H , 5 H )-trione Derivatives

1 Department of Chemistry, Faculty of Science andArt, Davutpasa Campus, Yildiz Technical University, Esenler, 34220 Istanbul, Turkey 2 Institute of Science and Technology, Yildiz Central Campus, Yildiz Technical University, Barbaros Bulvari, Besiktas, 34349 Istanbul, Turkey 3Department of Physics, Faculty of Science and Art, Davutpasa Campus, Yildiz Technical University, Esenler, 34220 Istanbul, Turkey


Introduction
Heterocyclic compounds and their derivatives, which are developing quite rapidly and becoming more important day by day, constitute a very important branch of organic chemistry.ese compounds containing nitrogen, sulfur, and oxygen as ring members are commonly used in various �elds of industry as analytical reagents, ligands, dyestuffs, pharmaceuticals, and bioindicators.In addition to these, barbituric and thiobarbituric acid derivatives are well known as antibacterials [1], sedatives [2], herbicides [3], fungicides [4], and antiviral agents with their speci�c properties [5].On the other hand, barbiturates having heterocyclic moieties have attracted considerable attention due to their potential pharmacological activity, and have become valuable alternatives in drug design [6,7].e electrical characterizations (d.c. and a.c.) of the materials are crucial in order to study relevant mechanisms of conduction and to realize its future applications.For example, to produce any device whose active layer is made of this kind of material, we need to know the effect of the temperature on both a.c. and d.c.electrical properties of the �lm, so as to carry out the accurate design.Measurements of d.c. and a.c.conductivity is also a reliable method to study the localised states near the band edges below the conduction and above the valance bands produced by the substitutional disorders which control many of the optoelectronic properties.Hence, a study of d.c. and a.c.conductivity in these materials will throw light on the nature of these levels.ere is an increasing demand for the detection of volatile organic solvents in the �eld of environmental analysis, industrial process control, and work-place monitoring.A number of materials, for example Pc [8,9], PVC blended lipid membrane [10],  functionalized alkanethiols [11], substituted azacyclophanes [12], and tetradentate dithioglyoximes [13] have been used as sensitive coating for the recognition of the organic solvent molecules.

Results and Discussion
2.1.Synthesis and Characterization.Structural assignments of the obtained products were based on their spectral properties and elemental analyses.e IR spectra of all compounds 1-8 showed the presence of the C=O stretching of N-C=O groups.In addition, the IR spectra of 5-8 also indicated the presence of C=O stretching of N-CO-N group of pyrimidinetrione ring.But 1-4 showed the absence of any C=O stretching absorption of N-CO-N, while they showed the presence of C=S group stretching of 2-thioxopyrimidine-4,6-dione.e molecular masses and the fragments determined for 1-8, rule out the possibility of the existence of the bispyrimidinylmethane structure and indicated that their formation was only accompanied by the elimination of one water molecule.
1 H NMR spectra of 1-4 revealed a triplet and a quartet for the protons of N-CH 2 CH 3 groups.Besides, the singlets for the N-CH 3 protons of 5-8 appeared at their spectra.On the other hand, 1 H NMR spectra of 3, 4, 6, and 7 showed a singlet for the 6 CH 3 group of the quinolinyl ring.Similarly, compound 4 revealed an additional singlet for the 4 CH 3 group of the ring.Moreover, PMR spectra of 1-8 indicated a singlet due to the methine proton.e 13 C NMR data were in accordance with the expected structures of the compounds and they showed the carbon atoms of the C=O groups and the carbon atom of the C=S group at their special regions.On the other hand, the signals for the N-CH 3 carbon atoms of the barbiturate structures 5-7 and the carbon atoms of N-CH 2 CH 3 groups of 2-thiobarbiturate moiety of the compounds appeared in their spectra.Furthermore, the 13 C NMR spectra of 1-8 showed the typical signals of vinylic carbon atoms.

Electrical and Gas Sensing
e study of the temperature dependence of the electrical conductivity during heating may provide valuable information on process taking place in the �lm.In this connection, direct current (d.c.) conductivity studies were done on the �lm of 1-8.ese studies were carried out in the temperature range 295-440 K in vacuum of ≤10 −3 mbar to avoid contamination.Conductivity values were calculated from the measured current-voltage (-) characteristics by using the following relation ( 1): where  is the measured current,  is the bias voltage,  is the electrode spacing,  is the number of electrode �nger pairs,  is the overlap length of the electrode �ngers, and  is the thickness of the electrodes.Current-voltage measurements were carried out a�er the �lm deposition and as the temperature was increased, so that conduction arising from impurity levels are not expected.Figure 1 shows the variation of d.c.conductivity of compounds 1-8 as a function of reciprocal temperature in the temperature range 295-440 K. e order of electrical conductivities obtained is 7 > 3 > 8 > 4 > 6 > 2 > 1 > 5 for all temperatures investigated.e well de�ned straight line obtained from the Arrehenius plot suggests the presence of only one conduction mechanism, assuming that the dominant levels are the conduction and valence bands, in the temperature range of 295-440 K for all �lms.In this case, the temperature dependence of conductivity can be represented by the well known expression where   is the thermal activation energy of the electrical conduction,  is temperature,  is Boltzmann's constant, and  0 is a parameter depending on the semiconductor nature.e value of activation energy was derived from the slope of the ln ( d.c ) versus 1/ graph is 0.70, 0.74, 0.85, 0.82, 0.68, 0.77, 0.90, 0.84 eV for the compounds 1-8, respectively.It is known that the electrical conductivity of the compounds strongly depends on the interplanar spacing and the stacking direction of the molecules.e difference between the electrical behavior of compounds can be attributed to the differences in the stacking arrangements.e a.c.electrical conductivity measurements were also performed on thin �lms of compounds in the frequency range 40-10 5 Hz. at temperatures between 295 K and 440 K. Figure 2 shows the frequency dependence of the measured a.c.conductivity at 385 K for the �lms of 1-8.A considerable number of experimental data concerning the frequency dependent conductivity of a large variety of materials can be found in the literature.ere is a generally accepted view that the relation between the measured a.c.conductivity and frequency is given as where  is constant and  is the angular frequency.In spite of the absence of a satisfactory model describing a.c.transport in organic �lms, quantum mechanical tunnelling (QMT) and hopping models for the frequency response still appear to be adequate.In the QMT model an electron passes through a potential energy barrier without acquiring enough energy to pass over the top of the barrier.is model suggest  a temperature independent frequency exponent  which is given by On the other hand, the hopping model predicts a temperature dependent frequency exponent  which is given by where  OB is the optical band gap,  0 a characteristic relaxation time, and  Boltzman's constant.
To investigate the conduction mechanism involved the variation of the exponent s with temperature were examined.Values of the frequency exponent  were calculated from the straight-line �ts in the logarithmic conductivity versus frequency plot.e obtained results showed that  is de�nitely a function of temperature for all samples measured and shows a general tendency to increase with decreasing temperature.e temperature dependence of the calculated  values is an indication of hopping conduction.
In order to determine the optimum operating temperature and the additive amount, responses of the �lms of 1-8 sensors to VOC vapors in dry nitrogen were examined as a function of operating temperature.It is well known that the adsorption of the gas molecules on the surface of a �lm plays an important role in the change of physical or chemical properties of these materials.
e effect of the various concentrations (200 to 1000 ppm) of toluene vapors on the conductivity of the 6 coated interdigital transducer (IDT) is shown in Figure 3 at indicated temperatures.During the gas sensing F 2: Frequency dependent conductivity of the spin coated �lms of compounds at 3�5 K.
measurements, the �lm was exposed to toluene vapors repeatedly.Each cycle of exposure lasted for 10 min, followed by recovery in dry nitrogen for another 10 min.It is clearly seen that the current increased sharply in the initial doping stage for a few minutes and the rate of increase slowed down.Although a complete steady-state current cannot be approached during a period of 10 min, the increase in current is adequate to indicate the presence of toluene gas in the atmosphere.Aer several minutes' exposure to toluene vapors, purging with dry nitrogen leads to an initial fast decrease followed by a slow dri that the current reaches its initial value aer the toluene gas is turned off, and this proves that the adsorption processes are reversible.e obtained response characteristics of the �lm of 6 can be explained as follows.e adsorption of a toluene molecule on the �lm surface leads to the creation of acceptor level.ese states are located near the surface and extend into the bulk if diffusion takes place.ese acceptor states, which lie below the Fermi level at the initial stage of adsorption, make the trapping of valance electrons easy.When the number of the trapped electrons reaches to a sufficient value Fermi level shis toward the valence band.is shi in the Fermi level causes the reduction in the speed of trapping processes and the rate of increase in current slows down.When the sensor is then exposed to dry nitrogen it leads to desorption of adsorbed toluene molecules from the surface of the active sensing layer, decreasing the acceptor concentration and thus the current.It was found that the sensitivities of the �lms to toluene greatly depended on the operating temperature.For the �lm of 6, the highest sensitivities to toluene vapors were obtained at around 360 K. e sensitivity has been related to the surface area and the adsorption sites on the �lm surface.e present result indicates that the increase of temperature gives rise to enhanced active adsorption sites of the �lm surface.

Conclusion
Elemental analyses results and IR, UV-Vis, 1 H NMR, 13 C NMR, and mass spectral data of the compounds con�rm the proposed structures.e d.c.results showed that the order of electrical conductivities obtained is 7 > 3 > 8 > 4 > 6 > 2 > 1 > 5 for all temperatures investigated.e response characteristics of the other �lm to VOC vapors were also investigated as a function of operating temperature.But we could not get any response to VOC vapors.In summary, all the observations demonstrate that the �lm of 6 would to be a promising sensor element for the detection of toluene due to high sensitivity, selectivity, and quick response characteristics.

Electrical and Gas Sensing
Measurements.e measurement electrodes used in the conductivity and gas sensing measurements consist of an interdigital array of metal electrodes photolithographically patterned on precleaned glass substrate.Glass substrates were throughly cleaned ultrasonically and then coated with 100 Å of chromium followed by 1200 Å of gold in an Edwards Auto 500 coater system.e �lm patterned photolithographically and etched to provide 10 �nger pairs of electrodes having a width of 100 m spaced 100 m from the adjacent electrodes.A thin �lm of compounds was prepared by spin coating method on gold interdigital electrodes (IDT).For the spin coating, coating solutions were prepared by dissolving of the compounds in appropriate solvents at concentration of 1.10 −3 M. Twenty microliters of such solutions were added with a glass pipette onto the IDT structure held onto spinner (Speciality Coatings Systems Inc., Model P6700 Series).e substrate spun at 2000 rpm for 30 s and the solvent had evaporated during this period, producing a homogeneous �lm of compounds.D.c. conductivity measurements were performed between 300 K and 440 K by using a Keithley 617 electrometer.A.c. conductivity measurements were carried out with a Keithley 3330 LCZ meter in the frequency range, 40-10 5 Hz, and in the temperature range from 295 K to 440 K.All the measurements were performed under 10 −3 mbar in the dark.e effect of the VOC vapors on the conductivity of the �lms was measured in a homemade te�on chamber implemented in our laboratory where dry nitrogen was used as carrier gas.Aer each compound sensor in the chamber was stable, it was exposed to �ve different concentrations of the target gases.e test gas was mixed with dry nitrogen using computer driven mass �ow controllers (MKS Inst.).A typical experiment consisted of exposure to test gases and subsequent purging with dry nitrogen.Both conductivity and gas sensing data were recorded using an IEEE 488 data acquisition system incorporated to a personal computer.

11 F 3 :
Response characteristics of 6 coated IDT on exposure to 5 different concentrations of toluene vapors.
Melting points were determined in a Gallenkamp melting point apparatus.Absorption spectra were obtained on an Unicam UV-Vis spectrophotometer.IR spectra were recorded on a Shimadzu IR 9300 spectrophotometer (KBr pellets). 1NMR and13C NMR spectra were recorded on Varian 200 MHz Gemini and Varian 400 MHz Mercury spectrophotometers and chemical shis are given in ppm down�eld from TMS as the internal standard.Mass spectra were acquired on a Shimadzu GC/MS QP 2000 A instrument. Elmental analyses were performed on a ermoplast Flash EA 1112 CHNS elemental analyzer.