Comparison of Vinyldimethylaniline and Indolizine Donor Groups on Si‐Substituted Xanthene Core Shortwave Infrared Fluorophores

Small organic molecules absorbing and emitting in the shortwave infrared (SWIR, 1000-2000 nm) region are desirable for biological imaging applications due to low auto-fluorescence, reduce photon scattering, and good tissue penetration depth which allows for in vivo imaging with high resolution and sensitivity. Si-substituted xanthene-based fluorophores with indolizine donors have demonstrated some of the longest wavelength absorption and emission from organic dyes. This work seeks to compare an indolizine heterocyclic nitrogen with dimethyl aniline nitrogen donors on otherwise identical Si-substituted xanthene fluorophores via optical spectroscopy, computational chemistry and electrochemistry. Three donors are compared including an indolizine donor, a ubiquitous dimethyl aniline donor, and a vinyl dimethyl aniline group that keeps the number of π-bonds consistent with indolizine. Significantly higher quantum yields and molar absorptivity are observed in these studies for a dimethylamine-based donor relative to a simple indolizine donor absorbing and emitting at similar wavelengths (~1312 nm emission). Substantially longer wavelengths are obtainable by appending aniline-based groups to the indolizine donor (~1700 nm) indicating longer wavelengths can be accessed with indolizine donors while stronger emitters can be accessed with anilines in place of indolizine.