Theoretical Study of Substitution Effects on Reorganization Energy in Indeno[1,2-b]fluorene-6,12-dione-Based Molecular Semiconductors

The reorganization energy of an organic semiconductor can be modulated through molecular design. Density functional theory (DFT) calculations enable researchers to strategically design organic molecular semiconductors with low reorganization energy through chemical substitution. Herein, hole and electron reorganization energies of unsubstituted indeno[1,2-b]fluorene-6,12-dione (IFDK), and its 5,11- and 2,8-disubstituted derivatives were computed. Substituents positioned along the molecular long axes and short axes were employed to enhance understanding of how substitution position influences the molecular reorganization energy. Additionally, substituents with varying electron-withdrawing and electron-donating properties were also investigated to reveal the structure-property relationship in IFDKs. Based on DFT calculations, triphenylamine (TPA) substitution at the 5,11 and 2,8 positions exhibited the largest decreases in hole reorganization energy compared to the parent IFDK, reducing it from 240 meV to 80 meV and 110 meV, respectively. This suggest that more effective charge transfer is expected with TPA substitution in IFDKs due to lower reorganization energy. This work reveals the significant substitution effect on reorganization energy through the specific position and electronic characters (EWG vs EDG), providing a foundation for the future design of IFDK-based molecules with high charge transfer efficiency.