Facile Suzuki Coupling Strategy toward New Nile Red Derivatives with Improved Two-Photon Brightness

Chemical fine-tuning of fluorophores is a pivotal step towards development of next generation fluorescent dyes for microscopy. With the advent of high-resolution two-photon excitation fluorescence imaging, there is a growing demand for very sensitive laser dyes that can be efficiently excited using commercial Ti:sapphire laser sources in the first near-infrared window (NIR-I, 780–1020 nm). Using the fluorescent dye Nile Red as the lead structure, we report a robust and concise Suzuki coupling approach for the synthesis of 14 new Nile Red analogues that feature extended π ring systems and diverse functionalities. For this set, we gauged their two-photon excitation efficiency in NIR-I as well as evaluated their general fluorescent properties (emission wavelength, Stokes shift, quantum yield and solvatochromism). Several of the new fluorophores were found to display very favorable characteristics. In particular, the derivative featuring a 4-aminophenyl group in the 2-position of Nile Red exhibited extreme solvent sensitivity, and the thien-2-yl substituted Nile Red derivative showed significantly redshifted emission, large Stokes shift and high two-photon brightness.