Near-Infrared Fluorescent Thienothiadiazole Dyes with Large Stokes Shifts and High Photostability

A series of near-infrared (NIR) organic emissive materials were synthesized and the photophysical properties analyzed. The donor–acceptor–donor materials were designed with thienopyrazine and thienothiadiazole acceptor groups with thiophene-, furan-, and triphenylamine-based donor groups. The absorption and emission spectra were found to be widely tunable on the basis of the donor and acceptor groups selected. Computational analysis confirms these materials undergo an intramolecular charge-transfer event upon photoexcitation. Large Stokes shifts of ∼150 nm were observed and rationalized by computational analysis of geometry changes in the excited state. Fluorescence studies on the dye series reveal maximum peak emission wavelengths near 900 nm and a quantum yield exceeding 16% for 4,6-bis(2-thienyl)thieno[3,4-c][1,2,5]thiadiazole. Additionally, several dyes were found to have reasonable quantum yields within this NIR region (>1%), with emission wavelengths reaching 1000 nm at the emission curve onset. Photostability studies were conducted on these materials in an ambient oxygen environment, revealing excellent stability in the presence of oxygen from all the dyes studied relative to a benchmark cyanine dye (ICG) during photoexcitation with exceptional photostability from the 4,6-bis(5′-dodecyl-[2,2′-bithiophene]-5-yl)thieno[3,4-c][1,2,5]thiadiazole derivative.