Quadrupling the PLQY of Tetraphenylethylene by Covalently Linking it with Isosteric Tetraarylaminoborane: A Potential Candidate for Multicolor Live Cell Imaging

Applications of organic luminophores depend on their photoluminescence quantum yield (PLQY). Several strategies have been developed to improve the PLQY of organic solids, and one such method is aggregation-induced emission (AIE). Herein, we disclose a comprehensive study of two molecularly engineered covalently linked isosteric AIEgens, BNTPE-1 and BNTPE-2. The independent isosteres tetraarylaminoborane (BN) and tetraphenylethylene (TPE) showed poor PLQY; however, the covalently linked BNTPE-1 and BNTPE-2 systems showed 4 times higher PLQY than the independent isosteres (∼78 and ∼92% for solids and aggregates, respectively). Detailed optical, structural, and computational studies revealed that BN and TPE moieties adopt more coplanarity and have stronger donor (-NPh2)-acceptor (BMes2) interactions in the covalently linked systems than do simple BN and TPE units. Despite having sterically demanding BMes2 units, these compounds are nonemissive in the solution state due to the presence of flexible TPE units. However, they are strongly emissive in condensed states, such as aggregates in solution and the solid state. The excited state structure analysis revealed that the TPE unit undergoes severe conformational distortion after photoexcitation, which activates nonradiative decay channels and consequently quenches the luminescence in the molecularly dispersed state. The bioimaging potential of BNTPE-1 and BNTPE-2 was also explored. These compounds showed high biocompatibility and stained the HeLa cells brighter than BN and TPE molecules.