The π-rich rings of conjugated polymers and molecular rotors shape their typical properties, allowing a variety of chemical and photoresponsive phenomena. Herein, we present a computational method in the framework of classical simulations to estimate the free-energy gap between ground and excited states of oligofluorenes, which is a class of molecular rotors widely used in optoelectronic devices, because of the inner torsional rotation triggered by light irradiation. We devised multiple sets of free-energy simulations in combination with free-energy perturbation theory to predict the free-energy gap between the ground state and the first excited state. The computed excitation energies show good agreement with experiments. The approach presented herein allows one to achieve comprehensive sampling of the conformational landscape and accurate estimates of the excited state free-energy landscapes at the same time.