Very invisible: Bis(pyrrolopyrrole) cyanines are extended chromophores based on diketopyrrolopyrroles (general structure shown). The compounds are characterized by extremely high extinction coefficients, narrowband absorption in the near-IR range, and strong fluorescence around 1 μm. Since absorption in the visible region is negligible, they approach the ideal of selective NIR absorbers.
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Fischer et al. (2011) studied this question.
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