Understanding Dichromic Fluorescence Manifested in Certain ICG Analogs.

Fluorescence has advanced our understanding in various aspects of biological processes. Fluorescence in the near infrared (NIR) region avoids background autofluorescence from biological samples leading to improved image quality. In searching for indocyanine green (ICG) analogs that can be attached to biomolecules, we observed that dichromic fluorescence manifested in some mono reactive-group functionalized ICG analogs. The two emission bands are distinctively separate from each other, making it a unique feature of fluorescent probes found in biological studies. We further demonstrated that the dichromism comes from the structure and is transferable from dye to its bioconjugates. In this paper, we used Resonance Theory and Molecular Orbital Theory to explain the fluorophore photochemistry in an effort to understand the general fluorescence feature of ICG analogs and provide understanding of the secondary emission band.