Excitation of SWIR-HD at 988 nm resulted in SWIR emission with a λem value of 1126 nm (QY = 0.01%, 138 nm Stokes shift) (Figure ). Of note, the entire emission spectrum of SWIR-HD fell within the SWIR window. Moreover, we observed a linear relationship between dye concentration and emission intensity up to at least 25 µM (Figures ). In the context of PA imaging, the best predictor of ultrasound intensity is an empirical term known as the PA Brightness Factor (PABF = molar extinction coefficient × (1 -fluorescent quantum yield)). Because the former term in this equation has a greater influence on the calculated PABF value, a measured ε value of 1.5 × 10 4 M -1 ⋅cm -1 for SWIR-HD almost certainly ensures the PA readout will be sufficiently strong for in vivo applications. To determine the experimental PA properties of SWIR-HD, we obtained a PA emission spectrum by irradiating the dye from 650 to 1100 nm (Figure ). Analysis of this spectrum reveal a λpa of 1035 nm along with a blueshifted shoulder at 945 nm. Next, we designed an experiment to predict whether SWIR could operate in a deep tissue context. This was accomplished by casting a tissue-mimicking phantom comprised of milk and agar that was three-fold thicker (3 cm) than phantoms routinely used to evaluate other photoacoustic dyes. Even with the substantial increase in phantom dimension, a strong PA signal could be readily observed at a dye concentration of 30 µM in both organic solvents and aqueous media (Figure ).