Near-Infrared Fluorescent\nThienothiadiazole Dyes with\nLarge Stokes Shifts and High Photostability

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