Effects of Phenylene-π-Spacers on the Photophysics of Isoindigo: Aggregation-Induced Emission

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.