The Electronic and Optical Properties of ‎Pristine, Fluorinated and Chlorinated ‎Pentacene Molecules: An ab-initio Study

   In This research the effect of fluorine and chlorine substituents on the electronic and opticalproperties of pentacene molecule have been investigated based on density functional theory as implemented in SIESTA code. The results show thatthe full replacement of hydrogen atoms with fluorine and chlorine in pentacene molecule, leads to shrink the HOMO-LUMO gap by the value of 0.14 and 0.46 eV, respectively. Moreover, the cohesive energy of fluorinated (F-PENT) and chlorinated pentacene (Cl-PENT) follow F-PENT< PENT < Cl-PENT order with respect to the cohesive energy value of -7.54 eV corresponding to pristine pentacene.  Therefore F- PENT shows better stability than others. The results of optical properties demonstrate that fluorinated and chlorinated pentacene have greater dielectric constant and refractive index with respect to pristine pentacene. The reflectivity feature along the long axis of pentacene molecule undergoes a red shift and accordingly the violet color of pentacene changes to blue and green by the influence of fluorination and chlorination, respectively. These results can be utilized to improve molecular electronic and optical devices.