A novel 2-butyloctyl (BO) substituted phthalimide (PI) end-capped fluorenone [PI-FN-PI (BO)] non-fullerene electron acceptor for organic photovoltaics has been designed and synthesized to study the effect of end-capping groups on the optoelectronic properties compared to our earlier reported naphthalimide (NAI) terminated fluorenone NAI-FN-NAI (BO) reference compound. The electron withdrawing terminal NAI groups were replaced by a PI group in order to evaluate the influence of electron affinity of such group on the lowest occupied molecular orbital (LUMO) energy level and other optoelectronic properties. The newly synthesized PI-FN-PI (BO) with a 2-butyloctyl alkyl chain is sparingly soluble in organic solvents due to its more planar structure and short alkyl chain. For organic solar cell devices, solubility of the active layer blend is extremely important so higher solubility is the key requirement. In order to enhance the solubility of the compound, we synthesized 2-decyltetradecyl (DT) substituted phthalimide (PI) end-capped fluorenone [PI-FN-PI (DT)]. Furthermore, another new compound NAI-FN-NAI (DT) was produced to compare with PI-FN-PI (DT) since both of these compounds have similar side alkyl chain and middle core but different end capping groups. It is clearly demonstrated that this simple chemical modification noticeably influences thermal, optical, electrochemical properties and significantly impacts the photovoltaic performance.