Fluorescent molecular systems based on carborane-perylenediimide conjugates†

This study presents the successful synthesis of two perylenediimide (PDI)-based ortho-carborane (o-carborane) derivatives, PDI–CB1 and PDI–CB2, through the insertion of decaborane into alkyne-terminated PDIs (PDI1 and PDI2). The introduction of o-carborane groups did not alter the optical properties of the PDI units in solution compared to their carborane-free counterparts, maintaining excellent fluorescence quantum yields of around 100% in various solvents. This was achieved by using a methylene linker to minimize electronic interaction between PDI and o-carborane, and by incorporating bulky o-carborane groups at imide- position to enhance solubility and prevent π–π stacking-induced aggregation. Aggregation studies demonstrated that PDI–CB1 and PDI–CB2 have greater solubility than PDI1 and PDI2 in both nonpolar and aqueous solvents. Despite the steric hindrance imparted by the o-carborane units, the solid state emission of PDI–CB1 and PDI–CB2 was affected by aggregation-caused fluorescence quenching. However, solid PDI–CB1 preserved bright red excimer-type emission, which persisted in water-dispersible nanoparticles, indicating potential for application as a theranostic agent combining fluorescence bioimaging with anticancer boron neutron capture therapy (BNCT) due to its high boron content.