Red and NIR emitting ring-fused BODIPY/aza-BODIPY dyes

BODIPY dyes are well-known for their outstanding features, such as solubility in a wide range of solvents, useful photophysical properties, and good photochemical and thermal stability. However, the application of the BODIPY dyes is limited due to their narrow range of absorption and emission in the green-yellow-red region of the visible spectrum. Various ways for developing BODIPY dyes with photophysical characteristics in the near-infrared (NIR) region have evolved during the last few decades. The aromatic/heteroaromatic ring fusion and introduction of nitrogen atom at the meso position to get the fused BODIPYs and aza-BODIPYs respectively have emerged as the most promising approach towards red shifting the absorption/emission of the BODIPYs. This review summarizes the photophysical properties of the red and NIR absorbing/emitting fused BODIPY and aza-BODIPY dyes along with their synthesis schemes. The fusion of different aromatic and heteroaromatic rings, the length of conjugation, the effect of introducing various electron withdrawing and electron donating groups on the photophysical properties are discussed in detail. A separate section is dedicated to the discussion of the application of the fused BODIPYs and aza-BODIPYs in biomedical and optoelectronic fields.