This study explores the structure–property relationships controlling the aggregation-induced emission (AIE) behavior of parent isoindigo (1) and its p-phenylene (2) and m-phenylene (3) spaced derivatives. While all three chromophores exhibit excitation-dependent emission in dilute dimethyl sulfoxide (DMSO) solutions, only 2 and 3 display strong solid-state luminescence, accompanied by large bathochromic shifts, characteristic of AIE. The AIE phenomenon is further corroborated by steady-state and time-resolved emission studies in DMSO–benzene mixtures and theoretical calculations. Powder X-ray diffraction analysis reveals π–π intermolecular through-space interactions in 2 and 3, facilitated by phenylene-π-spacers and likely a key factor for the observed AIE. Cyclic voltammetry in dichloromethane (DCM) (oxidation) and dimethylformamide (DMF) (reduction) underscores the structure-dependent redox behavior, with significant variation in reduction potentials. Thermal analysis indicates reduced thermal stability in 2 and 3 compared to 1, possibly due to the incorporation of spacers. Furthermore, time-dependent density functional theory (TD-DFT) computations (CAM-B3LYP/6-31G) for S1 and T1 states for 1–3 show that the primary energetic criterion is fulfilled for singlet fission, which holds the promise to generate highly efficient solar cells.
Das et al. (Thu,) studied this question.