Abstract
A series of novel near-infrared-absorbing aza boron dipyrromethenes (aza-BODIPYs) substituted with two electron-donating 4-(dimethylamino)phenylethynyl or 4-(diphenylamino)phenylethynyl groups at the 2- and 6-positions have been synthesized by palladium-catalyzed Sonogashira cross-coupling reactions. It has been found that the introduction of two dodecyloxy chains to the aza-BODIPY skeleton results in improved yield of the coupled products. These compounds have been fully characterized with various spectroscopic methods and elemental analysis. They display a strong Q-band absorption in the near-infrared region (up to 830 nm) and negligible fluorescence emission in toluene. Spectroscopic titration has also been performed for a dimethylamino derivative in chloroform. Upon addition of trifluoroacetic acid, which protonates the amino moieties, the compound shows remarkable ratiometric spectral changes. The Q-band absorption is blue-shifted from 830 to 736 nm and a new fluorescence emission at 756 nm appears, which can be attributed to inhibition of intramolecular charge transfer arising from the amino moieties. The color change from sky-blue to purple can be easily observed by direct visualization.
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