Theoretical Study of the Effect of Substituting Nitrogen Atom in Tetracyanoquinodimethane (TCNQ), Using Density Functional Theory

The optimized geometrical structures and the electronic properties of the Tetracyanoquinodimethane molecule (TCNQ) and its derivative have been calculated using density functional theory with (B3LYP) and 6-31G** level. The aim of this work is to determine the effects of substituting the nitrogen atom in mentioned molecule with trivalent atoms (phosphorus P and arsenic As). The results show that the both HOMO and LUMO energy decrease whenever the trivalent atom substitute instead of nitrogen atom. The energy gap decrease when replace nitrogen atom with trivalent atom. With respect to electron affinity then any nitrogen atom has been replaced with trivalent atom, the formed molecule will has more positive electron affinity, this new molecule will be a good electron acceptor. :تصلاخلا ( خئَزجن خَُوشتكنلاا صاىخناو ًهثًنا خُسذُهنا تُكاشتنا ةبسح ىت TCNQ خناد و خفبثكنا خناد خَشظَ واذختسأث بهتبقتشيو ) ( B3LYP ,خَُوشتكنلاا طبجتسلااو لدبجتنا خقبط ةبسحن ) ( عىُنا ٍي لاود واذختسا ىت ذقف سبسلاا لاود بيا 6-31G** ٌا .) ( خئَزج ٍف ٍُجوشتُُنا حسر لاذجتسا شُثأت ذَذحت ىه خساسذنا فذه TCNQ و )خَُسزنا وا سىفسفنا( مثي ؤفبكتنا خُثلاث حسزث ) نا دبئَزجهن خفهتخًنا خَُوشتكنلاا دبقُجطتنا ٍف خثىغشي صاىخ ًهع لىظحنا خَُبكيا ٍنبتنبث ىت ٍتنا جئبتُنا دشهظأ . خَىضع ٍي حسر لاذجتسا ىت بًهك ضقبُتت مُطىتنا خيزح ٍف يىتسي أطواو ؤفبكتنا خيزح ٍف يىتسي ًهعا خقبط ٌا بهُهع لىظحنا داسر دذع حدبَزث ضقبُتت خسوسذًنا دبئَزجهن خقبطنا حىجف ٌا كنزك دبثبسحنا دشهظا . ؤفبكتنا خُثلاث حسزث ٍُجوشتُُنا داسر جوشتُُنا خفنا بهن ٌىكَ ٍُجوشتُُنا داسر لاذجتسا ٍي خَىكتًنا دبئَزجنا ٌأف خَُوشتكنلاا خفنلأن خجسُنبث بيا . خنذجتسًنا ٍُ لبجقتسا ًهع خُهثبقنا بهن ٌا ٌوشتكنا مجقتسي خفط خئَزجنا ِزه كهتًت ٍنبتنبثو ,بهن خقثبسنا خئَزجنا ٍي خُثبجَا شثكا خَُوشتكنا . دبَوشتكنلاا Introduction: The organic semiconductor molecules had been widely studied as they have been used in many electronic devices, such these devices are organic field effect transistors (OFET) which can be prepared either by vacuum evaporation, by solution-casting, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize lowcost, large-area electronic products and biodegradable electronics [1,2]. OFETs have been fabricated with various device geometries. The most commonly used device geometry is bottom gate with top drain and source electrodes, because this geometry is similar to the thin-film silicon transistor (TFT) using thermally grown SiO2 as gate dielectric. Organic polymers, such as polymethylmethacrylate (PMMA), can also be used as dielectric. [3,4]. While Organic light-emitting diodes (OLED) in which the emitting layer is a thin film of organic compound , when it subjected to electric current will emits light. This thin film of organic semiconductor is situated between two electrodes, at least one of these electrodes is transparent. There are two types of Organic lightemitting diodes: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical cell (LEC) which has a slightly different mode of operation [5,6]. In addition the organic semiconductors is used in organic photovoltaic cells is a type of polymer solar cell that uses organic electronics for light absorption and charge Journal of Kerbala University , Vol. 13 No.3 Scientific . 2015 303 transport to produce electric current from sunlight by the photovoltaic effect. [7,8]. These wide application of organic semiconductors because some of their suitable properties such as low cost, easy fabrication and mechanical flexibility [9]. Therefore, the develop of high-performance organic semiconductor materials is crucial for the development of organic electronics. (TCNQ) is the organic compound with the formula (NC)2CC6H4C(CN)2. This molecule is supposed as an electron acceptor that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. The typical oxidizing agents undergo constant chemical alteration through covalent or ionic reaction, resulting in the complete and irreversible transfer of one or more electrons. In many chemical Conditions, nevertheless, the transfer of electronic charge from an electron donor may be only fractional, that is mean an electron is not completely transferred, but results in an electron resonance between the donor and acceptor. This leads to the modeling of charge transfer complexes in which the components largely retain their chemical identities.The electron accepting power of an acceptor molecule is measured by its electron affinity which is the energy released when filling the lowest unoccupied molecular orbital (LUMO) [10]. Theoretical Details: In this study, it had been used the density functional theory (DFT) (Kohn Sham equations), where the electronic energy has the following form [11]: Where E T is the electrons kinetic energy term, E V is the potential energy of the nuclear attraction and the repulsion between nuclei, E J is the repulsion between electrons and E XC is the exchange correlation term. These terms are functions of electron density ( ), except the nuclear-nuclear repulsion. E is given by the following [11]: