We report molecular design, synthesis, and photophysical study of a series of near-infrared absorbing and fluorescing dyes, 3,5-difuranylvinyl-boradiaza-s-indacenes, bearing various 5-membered heteroaromatic heads at the 8-position (FBs). The correlation between the molecular structure and the spectral shift has been studied by quantum chemical calculations at various levels (B3LYP/6-31G*, HF/6-31G* and HF/PM3), which conclude that the planarity determined by the bulkiness of the head unit controls the optical bandgap of FBs, by energetically affecting the lowest unoccupied molecular orbital (LUMO) rather than the highest occupied molecular orbital (HOMO). We also show that incorporation of heavy atoms increased the capability of singlet oxygen generation as a result of enhanced intersystem crossing, which makes FBs potentially useful for near-infrared photodynamic therapy. In vitro near-infrared fluorescence imaging of live tumor cells has been demonstrated through a nanocarrier approach, by encapsulating one of the FB dyes in a stable aqueous formulation of organically modified silica nanoparticles which retains the fluorescence efficiency of the hydrophobic dye in water.
