The development of deep red and near infrared emissive materials with high quantum yields is an important challenge. Several classes of squaraine dyes have demonstrated high quantum yields, but require significantly red-shifted absorptions to access the NIR window. Additionally, squaraine dyes have typically shown narrow Stokes shifts, which limits their use in living biological imaging applications due to dye emission interference with the light source. Through the incorporation of indolizine heterocycles we have synthesized novel indolizine squaraine dyes with increased Stokes shifts (up to >0.119 eV, >50 nm increase) and absorptions substantially further into the NIR region than an indoline squaraine benchmark (726 nm versus 659 nm absorption maxima). These materials have shown significantly enhanced water solubility, which is unique for squaraine dyes without water-solubilizing substituents. Absorption, electrochemical, computational, and fluorescence studies were undertaken and exceptional fluorescence quantum yields of up 12 % were observed with emission curves extending beyond 850 nm.

Indolizine squaraine dyes with impressive Stokes shifts and near infrared absorption spectra are introduced. Their significantly enhanced water solubility is unique for squaraine dyes and makes them good candidates for medical imaging applications.  Exceptional fluorescence quantum yields of up to 12% are also observed with emission curves extending beyond 850 nm. For more details, see the Full paper by J. H. Delcamp et al. on page 12494 ff.