Theoretical Study On Electronic Structure And Photophysical Properties Of 2,6-dithienyl Acene Oligomers

Organic semiconductors and π-conjugated oligomer or polymers are of wide currentinterest for applications in electronic and optoelectronic devices including organiclight-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and photovoltaiccells. Acenes and derivatives is one classic kind of these materials, which not only a kind ofconducting material but a fine kind of electroluminescent material. In the case of acenes,pentacene is known to exhibit the highest mobility, and is the mostly potential for preparationof the FET materials.In the present work, the electronic spectra properties and energy level structure of series2,6-dithienyl acene derivatives were investigated by quantum mechanics calculations. Theresults suggest new theoretical basis and direction for design of novel organic materials. Thegeometry structures of six selected model systems 2,6-dithienyl acene oligomer(nDTA,n=0-5)are optimized by means of DFT methods with B3LYP functional at 6-31G* basis set level.From the optimized geometry, the geometric relaxation focus on the side of phenyl ring nearthe terminal substituted group, whereas no change be found for thienyl ring. Next, we analyzethe effects of thienyl and acene repeat unit on molecular orbital and energy level. Thecalculation results show that HOMO energy level increase and LUMO energy level decreasewith increasing chain length, lead to the HOMO-LUMO energy gaps minish gradually.Compared with pentacene, HOMO energy level of thienyl substituted compound increasesappreciably, while decreases markedly for LUMO energy level, and the energy gaps diminish,furthermore, the effect extent become small with increasing chain length. We can find the finelinear relations between DFT energies gaps and excitation energies by TDDFT. Theexcitation energy calculated by TDDFT results indicated that, as the chain repeated unitincreased, the lowest vertical excitation energy decrease rapidly, and the oscillator strengthalso decreases. Combined with MO calculation, the lowest excitation can be assign ascontribution from acene unit, while the thienyl group has significant effect on high energyband. From the results of calculated reorganization energy (λ) and ionization potential (IP), λand IP decrease with increasing chain length;substitution thienyl increase λ and decreaseionization potential.