Indolizine-Cyanine Dyes: Near Infrared Emissive Cyanine Dyes with Increased Stokes Shifts

Molecular
engineering strategies designed to red-shift cyanine dye absorptions
and emissions further into the near-infrared (NIR) spectral region
are explored. Through the use of a novel donor group, indolizine,
with varying cyanine bridge lengths, dye absorptions and emissions,
were shifted deeper into the NIR region than common indoline-cyanines.
Stokes shifts resulting from intramolecular steric interactions of
up to ∼60 nm in many cases were observed and explained computationally.
Molecular brightnesses of up to 5800 deep into the NIR region were
observed. Structure–property relationships are explored for
the six indolizine-cyanine dyes with varying cyanine bridge length
and indolizine substituents showing broad absorption and emission
tunability. The dyes are characterized by crystallography, and the
photophysical properties are probed by varying solvent for absorption
and emission studies. Computational data show involvement of the entire
indolizine π-system during light absorption, which suggests
these systems can be tunable even further into the NIR region through
select derivatizations.