Photosensitizers with absorption and emission in the near-infrared (near-IR) spectral region are of great importance for diverse applications including artificial photosynthesis, high-contrast bioimaging, optical recording, near-IR laser filter, near-IR photography, photodynamic therapy and solar cells.  For example, for solar cell application, sensitizers possessing light energy harvesting capability not only in the UV-visible region but also in the near-IR region where sunlight carries over 50% of its radiation are highly desired.  Additionally, the near-IR sensitizers show marked advantages over traditionally used visible sensitizers including their ability of low light scattering and background interference, and deeper light penetration in tissues, making them the highly sought out probes in bioimaging. However, these near-IR probes often suffer from shorter excited state lifetimes and lower fluorescence quantum yields.  Hence, there is a pressing need to develop near-IR probes exhibiting higher excited state lifetime and quantum yields, accompanied by better thermal and photostability.

In the present study, we have overcome this obstacle and report a multi-step syntheses of a series of peripherally fused azaBODIPY dimers revealing fluorescence in the near-IR region (see Scheme 1 for structures).  Additionally, owing to the imposed structural rigidity (as a result of utilization of two of the peripheral aryl substituents to fuse the azaBODIPY macrocycles), the fluorescence lifetimes and quantum yields of the azaBODIPY dimers are found to be higher than their corresponding monomer analogs; thus, offering their potential utility in a range of photonic/biophotonic applications.  Details of these findings are summarized below.