Effect of Substituents on the Photophysical Properties and Bioimaging Application of BODIPY Derivatives with Triphenylamine Substituents

We investigated the intramolecular charge transfer characteristics in the S, state of boron-dipyrromethene (BODIPY) derivatives with triphenylamine (TPA) substituents, depending on the substituted position and the number of substituents. Based on the spectroscopic and theoretical results, the beta-substitution of TPA on BODIPY hybridizes locally excited and intramolecular charge transfer characteristics in the S, state because of strong coupling between the highest occupied molecular orbitals of BODIPY and TPA moieties, and consequently, the BODIPY derivatives with beta-substituted TPAs exhibit strong red-color fluorescence around 640 nm in nonpolar and moderately polar solvents. The TPA substituent with propeller-like nonplanar geometry could prevent H-type aggregation between neighboring BODIPY derivative units and induce aggregation-induced emission enhancement (AIEE) characteristics of the BODIPY derivatives with TPA substituents, which are helpful to maintain their emission efficiencies under highly concentrated and condensed conditions. Since the red-color emission and AIEE property of the BODIPY derivatives with beta-substituted TPAs are promising characteristics for a bioimaging application, we applied these derivatives to L-929 fibroblast cells for cellular imaging. The BODIPY derivative with a single beta-substituted TPA (compound 2) was effectively loaded into porous silica nanoparticles (SNPs). Consequently, we achieved good cellular uptake of 2-SNPs and good cellular imaging, which further confirmed the bioimaging ability of 2-SNPs.