DFT calculations on conjugated organic molecules based on thienothiophene for electronic applications

We report theoretical studies on the optoelectronic structural properties of five thienothiophene (T) conjugated conjugates. The geometries, the prediction of the optoelectronic structural properties of the five compounds are studied by calculations of functional density theory (DFT). The absorption properties ( max, Etr, f) of molecules are gained by the (DFT) B3LYP / 6-31G (d) ZINDO method, so that the most occupied molecular orbitals (HOMO), the least molecular orbitals occupied (LUMO), the energy deficit being calculated using the factor Gaussian 09 and its GaussView 5.0.8 graphical interface.


INTRODUCTION
Organic photovoltaic tool have opened up the potential for producing light energy in a simple and economical way. The bilayer technology typically uses organic semiconductor cables intercalated between the anode and cathode electrodes, the first is an electron acceptor and the second is an electron donor [1][2]. In this work, we have based on unsubstituted thienothiophene ( fig.1a) as a ʌconjugated organic semiconductor. We mainly used DFT to study the optoelectronic and structural properties of the four molecules.
Thienothiophene often refers to all the structurally related thiophene derivatives with the given formula C6H4S2. As for importance, they are: thiophene thieno (3,2-b), thieno (2,3-b) thiophene and thieno (3,4-b) thiophene. The other isomers are characterized by S (IV) and are less stable [3]. Thieno (2,3-b) thiophene was the series' first member of the series to be isolated.
The thienothiophene conjugate compounds have extensive delocalization of n electrons along the molecular backbone, making them attractive for various optoelectronic applications [4][5].
Because of this applicative interest and to that these shirt systems can be used as model compounds for the parent polymer, they have been extensively studied [6][7]. Also, because of their controllable and precisely defined structure, physical properties can be correlated with the conjugation length and the side chains. Therefor implementing these molecular structures by functionalization at the terminal and side positions permit their application as molecular materials in organic field-effect transistors [8][9], light-emittingdevices [10,11,12,13],photovoltaic cells [14][15],or even as molecular wires for information storageor transfer [16][17].
Polymers and oligomers with low band gap are expected to show not only good intrinsic conductivity butalso nonlinear optical properties [18][19].For their successful design, it is vital to have a complete understanding of the relationship between electronic properties and the chemical structure of polymers [20][21]. Different routes are her followed for designing novel conducting polymers, one is provided by donor-acceptor polymers, based on the approach suggested for the first time by Havinga et al. [22].The study of conjugated oligomers is very attractive for finitesize systems can be achieved with a well-defined chemical structure and high purity. This opens the way for the investigation of electronic properties as a function of chain length and extent of the parent ʌelectron system.

Calculation methodology
The calculations done on the geometries of the four molecules were carried out under functional density theoretical theory (DFT) B3LYP and the set of bases 6-31G (d) [23]. The notation B3 indicates a parameter with three parameters of Becke [24] and LYP indicates the function Lee -Yang -Parr [25]. Calculations were given using the Gaussian 09 program. All structures are fully optimized by B3LYP/6-31G (d) without any constraint.

Geometric structure results
The optimized geometries of the five compounds (T1, T2, T3, T4 and T5) obtained at B3LYP / 6-31G    For the two systems (T2 and T3), we observe a great deference in the optimized binding angle ‫)1ڧ(‬ when we add thienothiophene (T) compound, but for T3, T4 and T5 the optimized binding angles ‫1ڧ(‬ and ‫)2ڧ‬ since they remain constant.

Electronic properties of the compounds examined
The electronic properties depend essentially on the fundamental and excited states. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). The gap energy is calculated by the difference between the LUMO and HOMO levels, their values for all the molecules examined are shown in Table 3. Calculations have been achieved by the method B3LYP / 6-31 (d).  In this result, we first observed that the synthesized molecules generally have a high gap energy, especially the molecule (T1) of 5.556 eV, but when we add a base (T) the band gap decreased slightly for all molecules, then the gap energy gap decrease of the molecule (T1) to (T5) from 5.556 eV to 3.944 eV.

Electronic structures of the compounds examined
In this section, we study the lowest virtual orbitals LUMO and highest occupied HOMO orbitals for these compounds, because the relative order of occupied and virtual orbitals give a reasonable qualitative indication of the excitation properties [26] and the capacity of electron transitions or whole transport. We plotted the contour curves of the LUMO and HOMO orbitals of five molecules (T1, We observe that in the HOMO orbital the electron density is mainly distributed throughout the chain of compounds. However, it moves completely to the acceptor unit in the case of LUMO. In the case of T5 the electron cloud are delocalized to the innermost ring in orbit LUMO.

Absorption spectra
The table 3 shows the vertical excitation energy Eex (eV), the maximum absorption Ȝmax (nm), and the oscillator strength (f) in all studied molecules. These properties are counted by the DFT-B3LYP/ 6-31G (d) ZINDO method. This indicates that these organic oligomers could absorbed the maximum amount of incident radiation light, especially T4 and T5 molecules. In the excitation state S1, it corresponds exclusively to the promotion of an electron from the HOMO to the LUMO. Moreover, the largest oscillation force (f <1) that comes from the S0 to S1 electronic transition.

CONCLUSION
The geometric parameters of the four Thienothiophene-based ʌ-conjugated organic compounds (T1, T2, T3, T4 and T5) were obtained by B3LYP / 6-31G (d) calculations. The gap energy calculated with the same method decreases when a base (thienothiophene) is added for all the molecules. Is basically due to the stabilization of the LUMO level and destabilization of the HOMO level of several compounds leads the reduction of energy gaps HOMO -LUMO. Regarding the T5 the reduction of the observed energy deficit is likely to guarantee the best electronic properties of the corresponding polymers. These results showed that the T5 is promising material for optoelectronic application.