Designing of DAD Type Small Semiconductor Molecules andInvestigation of Substituent Effect on Their Molecular, Electronic and Optical Properties: A DFT Study**

The design of multi-purpose semiconductor materials has become a major challenge in organic electronics. In this work, we designed DAD type 30 small molecules with thiophene as donor and phthalimide/benzimidazole/benzotriazole as acceptor groups and investigated their structural, electronic and optical properties via computational methods. E-HOMO, E-LUMO of the designed molecules were calculated with DFT and TD-DFT methods at HSEH1PBE functional with the 6-31+G** basis set. HOMO-LUMO gaps of the designed molecules were ranged between 2.25-3.41 eV. When an amino group is used as a substituent, the lowest HOMO-LUMO gaps (2.25-2.95 eV) were obtained with increasing HOMO level. Reorganization energies were calculated for hole or electron transport properties of the representative molecules. Benzotriazole based DAD compound, which is N-Phenyl bearing p-formyl substituted, has the smallest hole reorganization energy as a promising p-type organic semiconductor. High open-circuit voltages were obtained for aldehyde, carboxylic acid and ester substituted structures. This study presents a valuable insight to the experimental studies in designing new photovoltaics and electrochemical biosensors.